Scientific Bases for the Preparation of Heterogeneous Catalysts, Volume 175: Proceedings of the 10th International Symposium, Louvain-la-Neuve, Belgium, ... (Studies in Surface Science and Catalysis)

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Foreword This issue of Studies in Surface Science and Catalysis contains the Proceedings of the 10th International Symposium on the Scientific Bases for the Preparation of Heterogeneous Catalysts, held on the campus of the “Université catholique de Louvain” (UCL) in Louvain-la-Neuve, Belgium, on July 11-15, 2010. This series of symposia was initiated in 1975 on a regular 4-year interval basis. As for previous editions, this 10th Symposium was made possible thanks to the organizational skills of the members of the “Unité de catalyse et chimie des matériaux divisés” of UCL, benefiting from the assistance of the “Unité de chimie des matériaux inorganiques et organiques” [now merged together – with others- into the “Institute of Condensed Matter and Nanosciences” (IMCN Institute), UCL] and the “Centrum voor oppervlaktechemie en katalyse” of the Katholieke Universiteit Leuven (KULeuven). This symposium being the 10th in 2010, and 35 years after its foundation, was very special for the catalysis community: an occasion to reminisce on the progress in the field of heterogeneous catalysts’ preparation. With the years, the level of complexity attained by solid catalysts has been growing. The solids used as supports have welldefined textural characteristics, redox or acido-basic properties. The porosity can be tuned according to the application or even be bimodal. Diffusion challenges have been tackled. The active phase is now deposited in controlled ways in order to obtain the desired structure at the nanoscale. Progresses in nanotechnology have benefitted the subject. However, some old concepts have also been revisited, sometimes with new insight, sometimes giving old things a new name. Complex architectures are built on surfaces, with often a source of inspiration coming from Nature: bio-inspired or enzymes mimicks. Hybrid materials try to take the best from two worlds, for instance polymer science and inorganic chemistry or metal/organic in Metal Organic Frameworks (MOFs). This leads naturally to supported homogeneous catalysts, which is an active area of research since several years now. While some might have sought the ‘universal’ catalyst that would have been efficient for a multitude of applications, the inverse trend seems to have taken over, and specific materials are designed for specific target reactions or processes. Not only does the heavy chemical industry make use of catalysts but also fine chemicals syntheses for food additives, vitamins or drugs production. New areas have emerged where heterogeneous catalysts are important, and very often in relation with environmental issues: destruction of pollutants, photo-catalysis to use light as alternative source of energy, etc., with automotive car exhausts being now mostly equipped with catalytic converters, which was not the case 35 years ago. Catalysis has thus a bright future, at the forefront of science in various areas. Most research is now conducted by multidisciplinary teams, composed of engineers, chemists, materials scientists, physicists, or even bio-chemists, biologists or bio-engineers. Indeed, expertise in these fields is complementary and leads to real breakthroughs. Research in heterogeneous catalysis has also benefitted from recent developments in characterization methods. Tremendous advances in surface characterization and solids analysis has allowed a more precise picture of the active sites to be gained. Time resolved spectroscopy, pulse studies, and highly sophisticated methods once limited to ultra high vacuum operation that have ‘bridged the pressure gap’ have permitted to study catalysts at work. Important principles underlining the formation of the active phase, mechanistic aspects of the activity and mechanisms of deactivation have been

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unraveled, sometimes even visualized. This has confirmed statements made in the past based on indirect evidence, or brought new light on old technologies. This opens the door to tailor-made catalysts, and gives plenty of work in the area of catalysts preparation, with new challenges appearing every day. In this context, a symposium on the preparation of heterogeneous catalysts finds an easy justification, especially when bringing together delegates from academia and the industry. For this jubilee edition, the industrial and academic communities have more than ever shown a sustained interest in the event. More than 350 abstracts were submitted, constituting a record among all previous editions. In handling this huge success, the Organizing and Scientific Committees have preferred to maintain a human-sized Symposium with, in particular, a strong wish not to plan parallel sessions for oral communications. Therefore, a severe evaluation procedure was applied to select approximately 240 contributions. The criteria favored by the local Organizing Committee and the international Scientific Committee, exclusively constituted of delegates with an industrial appointment, were strongly focused on catalysts preparation aspects, privileging novelty and innovative procedures in the field, with the discussion of physico-chemical characteristics and catalytic properties being limited to the identification of the influence and control of the preparation parameters. The Symposium covered the following topics: scaling up, shaping and macrostructured catalysts, basic understanding and innovations in unit operations, nanostructured catalysts, hierarchical porous supports and hybrid catalysts and in situ spectroscopic follow-up of catalysts preparation. These topics served as guidelines for the sessions in the program of oral communications. Out of the selected papers, 40 contributions were presented orally, including 5 extended communications (one for each topic). The 193 other contributions were presented during two poster sessions. In addition, the opening invited lecture given by Professor Galen Stucky (University of California, Santa Barbara) addressed the question of the nanoscale integration of heterostructures in chemo- and bio-catalysis. The organizers are deeply indebted to the members of the Scientific Committee for their efforts in selecting the abstracts in order to maintain the high scientific level of the Symposium. In addition, all the papers included in this issue of Studies in Surface Science and Catalysis have been submitted to a systematic peer-reviewing procedure carried out by the Scientific Committee and senior members of the Organizing Committee’s laboratories, as well as of other Belgian laboratories involved in heterogeneous catalysis. The organizers thus wish to express their sincere gratitude to all of those who have participated in this outstanding work, guaranteeing, hopefully, a high quality level for the present volume. A special thought towards the post-docs of the Unité de catalyse: Victor Baldovino, M. Nawfal Ghazzal, Raquel Mateos, Robert Wojcieszak, who greatly helped the board in its editing work. The crisis has made that the sponsors were less numerous, but the organizers are indebted to some public and private sponsors who wished to perpetuate their financial support, without whom the organization of this Symposium would have been more difficult. The first companies to answer to our request this time were ExxonMobil, Johnson Matthey, Praxair and Micromeritics. The organizers are grateful to all of those who have contributed to the practical success of the event: secretaries and technical staff, trainees, Ph.D. students and postdocs of the co-organizing laboratories. The organizers wish to express their sincere gratitude to Professor B. Delvaux, Rector of UCL, Professor V. Yzerbyt, Prorector for research, Professor P. Bertrand, Vice rector for the Science and Technology sector, the

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“Service des auditoires” and the “Direction du développement institutionnel et culturel (DIC)”, for allowing the event to be patronized again by the University. Warm thanks are specifically due to Ms. Françoise Somers, Ms. Nathalie Blangenois and Ms. Jacqueline Boniver, for their constructive cooperation and decisive contribution to the conference organisation and editorial work of this volume.

The Editors.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Organizing committee Prof. M. DEVILLERS, Université catholique de Louvain Prof. E. GAIGNEAUX, Université catholique de Louvain Prof. S. HERMANS, Université catholique de Louvain Prof. P.A. JACOBS, Katholieke Universiteit Leuven Prof. J. MARTENS, Katholieke Universiteit Leuven Prof. P. RUIZ, Université catholique de Louvain

Honorary members Prof. B. DELMON, Université catholique de Louvain Dr G. PONCELET, Université catholique de Louvain

Scientific committee Dr S. ABDO, UOP, USA Dr M.P. ATKINS, Petronas Research, Malaysia Dr M. CLAREMBEAU, Ineos Services, Belgium Dr A. DE ANGELIS, ENI, Italy Dr M.P. DE FRUTOS, Repsol, Spain Prof. M. DEVILLERS, Université catholique de Louvain Prof. E. GAIGNEAUX, Université catholique de Louvain Dr J.J. HEISZWOLF, Albemarle, The Netherlands Prof. S. HERMANS, Université catholique de Louvain Prof. P.A. JACOBS, Katholieke Universiteit Leuven Dr K. JOHANSEN, Haldor Topsøe, Denmark Dr D. JOHNSON, Lucite Intl, U.K. Dr S. KASZTELAN, Institut Français du Pétrole, France Dr E. KRUISSINK, DSM Research, The Netherlands Dr A. LIEBENS, Solvay, Belgium Dr M. LOK, Avantium Technologies, The Netherlands Dr E. LOX, Umicore, Belgium Prof. J.A. MARTENS, Katholieke Universiteit Leuven Dr M. MERTENS, ExxonMobil Chemical Europe, Belgium Dr K. MÖBUS, Evonik/Degussa, Germany Dr B. REESINK, BASF, The Netherlands Dr M. RIGUTTO, Shell Global Solutions, The Netherlands Dr M. RUITENBEEK, DOW Benelux, The Netherlands Prof. P. RUIZ, Université catholique de Louvain Dr E. RYTTER, Statoil Hydro, Norway Dr F. SCHMIDT, Germany Dr M. TWIGG, Johnson Matthey, U.K. Dr W. VERMEIREN, Total Petrochemicals, Belgium

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

The nanoscale integration of heterostructures in chemo- and bio-catalysis Galen D. Stucky Department of Chemistry & Biochemistry and Materials Department, University of California, Santa Barbara, California 93106 USA

Abstract During the past twenty years improvements in synthesis and characterization capabilities have made possible the designed molecular assembly of complex materials with spatially distinct, multifunctional features that are hierarchically structured. These materials are systems in their own right, with property variables that can built in or used in a dynamic mode. This offers a challenging, but very real opportunity to control chemo- and bioprocesss systems. An example is given of the use of high-surface-area inorganic interfaces to control the catalytically driven bioprocesses of a biosystem of some complexity, followed by a selected overview of some recent strategies for the synthesis of multicompositional functional units and their use in controlling processes in chemo catalysis. Keywords: biosystems, catalytic systems, interfaces, bioprocess control, heterostructured materials

1. Introduction Living biosystems provide a sophisticated catalysis model that is currently well beyond the best bench-top and commercial efforts (Barnes 1986; Rich 2003; Litvin 2009; O’Driscoll 2007). In biogenesis, the components of the organized, integrated, multicomponent system are created and function via non-linear, frequently autocatalytic processing. The bioprocess is typically directed by entropy change and interface interactions in confined spaces with high fidelity selectivity at the atomic scale. The biosystem space/time assembly definition of structure and function makes possible the closely coupled organization and processing of integrated organic and inorganic domains. In their Introductory Perspective to a Special Feature of the Proceedings of the National Academy of Sciences on “Complex systems: From Chemistry to Systems Biology”, John Ross and Adam Arkin (Ross 2009) summarize it succinctly and well. “There is great interest in complex systems in chemistry, biology, engineering, physics, and gene networks, among others. The complexity comes from the fact that in many systems there are a large number of variables, many connections among the variables including feedback loops, and many, usually nonlinear, equations of motion, or kinetic and transport equations. ‘‘Many’’ is a relative term; a properly interacting system of just three variables can show deterministic chaos, a complex behavior indeed. For the natural scientist and the engineer, nearly all their systems are complex [emphasis added]. Many problems still resist the arguments of symmetry, averaging, time-scale separation, and covariation that often underlie complexity reductions.” Experimentally, from a combinatorial perspective of finding the “right catalyst” (e.g., Gobin 2008; Polshettiwar 2009) there is great flexibility for catalyst design but a major challenge in predicting the performance consequences of the catalysts that are

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created. The systems approach to composite materials and device assembly and design is an intriguing potential route for the control of bio and catalytic processes. Whether or not it will be a commercially successful approach remains to be determined, but there is little doubt but that it will provide a new perspective of complex system design and function. In the first part of the presentation, an example will be given of the use of highsurface-area inorganic interfaces to control the catalytically driven bioprocesses of a biosystem of some complexity. The latter part of the presentation is a selected overview of some recent strategies for the synthesis of multicompositional functional units and their use in controlling processes in chemo catalysis.

2. Inorganic interface control of bioprocesses This issue is of increasing interest because of the use of silica mesoporous agents as cardiovascular drug delivery agents for the treatment of cancer (Klichko 2009; Park 2009; Slowing 2006) and as gene transfection agents (Radu 2004). In the example presented here, the high-surface-area inorganic phase, which can include a zeolite, mesoporous structure or layered clay structure, is considered as a system in its own right, and is most effective as a porous heterostructure that is capable of acting as a delivery or uptake agent for heat, electrolytes, or large enzymatic biomolecules. The biosystem response is considered in the context of the new total system that is created by its interface with the original unperturbed biosystem. The inorganic interface is also used to probe the network nodes of the total biosystem by monitoring the bioprocess activity response to the interface upon selective depletion of the normal biosystem proteins. The biosystem of interest is the blood clotting cascade, which consists of 122 proteins, including enzymes, that form a system network with 278 known interactions that can be further complicated by anticoagulation agents such as Warfarin and heparin. The system is autocatalytic with the formation of thrombin, which then further catalyzes the activation of the clotting part of the cascade. It is also self-regulating with uncontrolled anti-coagulation catalysis resulting in coagulopathy and bleeding diathesis. Our in vitro studies (Ostomel 2006abc, 2007; Baker 2007, 2008) and in vivo studies carried out by UHUHS (Ahuja 2006; McCarron 2008) have shown that arterial hemorrhaging can be very effectively controlled to give close to 100% survivability (Kheirabadi 2009) by interfacing an appropriate inorganic material with the blood coagulation system and catalytically accelerating the blood coagulation process. The highest efficacy, at low therapeutic material dosage levels, is obtained when a pure silica mesoporous material is used with pore sizes above 24 nm. If the mesoporous material is also used as a delivery agent for thrombin, which can be readily loaded into its 3D cage structure, an even greater efficiency for blood clotting is obtained. The inorganic system variables evaluated in this research with different inorganic agents were composition, heterostructure, time to initiate coagulation, rate of coagulation, strength of the resulting fibrin network, heat transfer, local dehydration of the blood, electrolyte delivery or uptake, zeta potential in simulated body fluid, dissolution or exfoliation of the inorganic phase, accessible protein surface area, and biocompatibility. Studies of the blood clotting cascade system with and without the inorganic are still in progress, but inorganic variables that are highly correlated with the coagulation response of the biosystem, and most importantly from a mechanistic point of view, specific blood clotting factors, have been identified. The use of the inorganic interface to probe and better map out the processes of this biosystem is expected to continue for some time

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in the future, and will include microfluidic real time studies and system modeling. The in vivo validation of the in vitro studies carried out in our laboratories ultimately resulted in its adoption for military and civilian use (products commerically available from Z-Medica Inc.). From a pragmatic perspective, approaching this problem as a system problem was very effective. However, one important point must be made in this connection. The tie lines that connected benchtop research, scale-up (including in vitro to in vivo evaluation), formulating a commercial product, and even receiving critical evaluations from medical personnel regarding the pros and cons in field application were exceptional in terms of the short response time and openness of communication. This greatly expedited and facilitated the practical design of the most effective materials for this application. The most important characteristic of any complex system is the function for which it was intended to deliver. This determines the synthesis strategies that are used, the characterization techniques that are applied, and the on-going guidance of the direction of the research. The most serious limitation in systems analysis is the generation of sufficient experimental data to develop a meaningful understanding of the network interactions so that a predictive model can be generated.

3. Interfaces and the multicompositional, hierarchical assembly of functional units A desirable way to introduce coupled multiple subsystem functionalities is by the integrated, but spatially distinct, organization of domains with different composition. The size, composition and morphology of these heterostructure domains are dependent on the application. If 3D interconnected porous domains are introduced to control residence times in catalytic processes, pore or cage diameters on the order of 100 nm may be in order. For phonon Rayleigh scattering, smaller domain sizes may be appropriate. For oxidation-reduction, electron transport processes such as photo-catalysis (Yates 2009) and photovoltaic applications, two key challenges are electron-hole recombination and the existence of electron trap states at the domain interfaces as well as in the bulk. For photovoltaics, we have shown that the use of semimetal nanoscale heterostructures epitaxially implanted at the charge transfer interface of p-n semiconductor junctions resolves the problem of interface voltage bias and electron-hole recombination, giving ballistic electron transport and very close to the theoretical open circuit voltage (Zide 2006). This was a proof-of-principle exercise in that the synthesis was carried out using molecular beam epitaxial growth. The efficient use of electronhole states is equally important in catalytic redox processes. The use of heterostructured composite materials was also an exercise that grew out of earlier research (Yang 1998) in which single process chemical reactions were used to synthesize nanostructured composite materials made up of binary or ternary crystalline nanoparticles 4-8 nm in diameter that were organized into 3D periodic mesoporous structures. This was extended to the multicompositional, hierarchical assembly of functional units consisting of quantum dot chalcogenides with titanium dioxide nanoparticles into 3D mesostructures for visible light electron photoexcitation into the TiO2 conduction band (Bartl 2004). For example, PbS nanoparticles, which have an absorption spectrum profile that closely matches that of the solar spectrum and a conduction band at higher energy than that of anatase (TiO2), can be assembled in parallel and homogeneously integrated in situ with anatase nanocrystals into a highly ordered 3D mesostructured, flat thin film of macroscale dimensionality (Bartl 2004; Cha 2003). Thermal stabilization of the nanoscale anatase heterostructures in these 3D mesostructures is achieved by

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carbonization of the pores, which prevents formation of the rutile phase until 750°C (Tang 2004) and facilitates electron transport. We have recently extended this approach to a simple and widely applicable methodology for the synthesis of homogenous multicomponent nanoparticle arrays that are mesostructured (Fan 2006). This ability to use molecular assembly to synthesize high surface area porous 3D material systems with built-in multi-component functionalities and substructures has the potential of offering an exceptionally rich generic platform for catalyst fabrication. The synthesis procedure uses simple molecular precursors to generate a colloidal dispersion of metal oxo-acetate nanoparticles for an extensive list of metal species. Different amounts of nanoparticles made up of different compositions can be simultaneously generated with nanoparticle sizes in the range of 3 to 6 nm. This offers a mix-and-match opportunity to make variable composition nanoscale heterostructures by co-condensation of multicomponent nanoparticles and other ionic species in the reaction solution. The nanoparticles are quite stable and their growth can be readily controlled by the slow introduction of water from the ambient environment and esterfication of the acetic and acetate groups used in the preparation. This is in contrast to the diverse condensation and metal oxide particle growth behavior observed for metal alkoxides in ethanolic solutions. The multicomposition collection of nanoparticles that are not preformed, as is usually done for supported nanoparticle catalysts, but rather synthesized along with the support can be easily organized into a high surface area 3D mesoporous configuration, e.g. a cubic (Im3m) cage structure. As an example of a ternary phase, we have studied the phase segregation and crystallization behavior of the mesoporous NiO-2SiO2-2ZrO2 system in detail. A sample was made that had a solid phase composition determined to by EDX to be 1.19:1.96:2.00, which is close to the starting synthesis composition of 1:2:2. Between room temperature and 800°C the sample remains amorphous but with a well-ordered mesoporous structure. After heating to 900°C, the zirconia component in the mesophase crystallizes into a tetragonal phase; the other components, however, remain amorphous. The crystal size of tetragonal zirconia is ~5.3 nm based on WAXRD peak broadening by the Scherer formula. A further increase in the temperature to 1000°C does not cause additional crystal growth. SAXRD and TEM investigation confirm the preservation of mesoscopic ordering (p6m) even after heating to 1000°C. The zirconia nanoparticles retain a size of ~5 nm that are uniformly embedded in the one-dimensional channel walls, and in fact, the entire framework structure shows a homogeneous distribution of the zirconia nanoparticles and amorphous NiO and SiO2 phases. This material has a pore size of ~4.2 nm and a surface area of 102 m2/g. Both NiO and ZrO2 are particularly interesting for catalytic use in chemical and petrochemical processes because of their acidic, basic, and redox properties. (Mango 1996; Postula 1994) We believe that mesoporous multicomponent materials with other specifically chosen formulations, NiO-xSiO2-yZrO2, heterostructures are promising catalytic system candidates because of the homogeneous dispersion of each component, existence of tetragonal ZrO2 nanocrystals, and silica-stabilized mesoporous framework. We have demonstrated the ability to easily process the diverse multicomponent mesoporous metal oxides (in addition to that described above) that we have made in the form of thin films, free-standing membranes, and monoliths This presents substantial advantages over previously reported methods for commercial applications for heterogeneous catalysts, energy storage, photocatalysis, or nanostructured photovoltaics, where large quantities of material are required. The “single pot” approach that combines synthesis and processing is potentially environmentally friendly and cost-effective.

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The mesostructured wall thicknesses are of the order of 4 to 8 nm, depending on the synthesis methodology that is used so that active nanoparticle catalysts embedded in the wall matrix are easily accessible for gas or liquid phase reactions. To test this hypothesis the selective oxidation of benzyl alcohol to benzaldehyde was examined (Fan 2009). Using anatase as the primary nanoparticle matrix, nanoparticles of the first row transition metal element oxides integrated into the mesostructure walls were systematically examined using the one pot synthesis strategy described above, starting with molecular precursors. Copper and iron oxide nanoparticles embedded in the titania mesostructure walls were the most promising for the catalytic reaction carried out between 150 and 250°C. The concentration of copper oxide nanoparticles was then varied between 0 and 10 wt%. The best catalytic composition was determined to be 3K-Cu-50TiO2 which displays a stable benzaldehyde yield of >99% over a period of 50 hours for the benzyl alcohol-to-benzaldehyde selective oxidation transformation at 203°C. The best calcination temperature for this heterostructure catalyst composition before use was 350°C. The role of the host matrix, commonly called the support, is of considerable importance, and we believe that the nanostructured integration of the active catalytic species with the support is definitely a positive attribute for catalyst design and function. Several other strategies have been extensively considered for making use of nanoscale heterostructure properties. John Meurig Thomas has recently promoted the “sprinkle” methodology (Thomas 2009): ‘‘To design new solid catalysts take high-area nanoporous solids of the appropriate kind and ‘‘sprinkle’’ spatially isolated active centers over the entire (internal) surface area.’’ Tai and co-workers (Tai 2001) have functionalized Au and other metal nanoparticles with coordinating organothiol groups and have found that a homogeneous dispersion of metal nanoparticle catalyst sites is best obtained in a milder way. The approach is based on the general concept of utilizing relatively weak interactions between metal nanoparticles and the substrates in an aprotic solvent, which creates a homogeneous loading of the nanoparticles. The dispersion is then locked in by temperature treatment up to 400°C, depending on the metal and the support. Little aggregation or change in the size of metal nanoparticles is observed for a variety of metal oxide supports, e.g. in the catalytic selective hydrogenation of nitroaromatics (Shimizu 2009), oxidation of CO (Tai 2009), or oxidation of the polar ethanol molecule using supported gold nanoparticles at 200°C (Zheng 2006). In spite of its successes, the grafting, or “sprinkling”, of nanoparticles onto a substrate can have limitations with respect to long-term thermal and chemical stability under many catalytic reaction conditions. The integration of hetero/nano structures into the catalyst support matrix described above, or the covalent attachment (Margolese 2000; Nakazawa 2008) of the catalytic functionality to the support is a more robust solution, and has been shown to be effective for improving the yield and catalytic performance/stability of homogeneous catalysts (Terry 2007; Nakazawa 2008) and enzymes (Han 2002; Hartman 2010). The “ship in a bottle” (Herron 1985, 1986) heterostructure concept for packaging active catalyst species in a confined space has been extended to core/shell particle structures with porous walls containing nanoparticles with high chemical reactivity (Kustova 2008). The ability to encapsulate individual crystalline nanoparticles with chemically active crystal faces or edges and to prevent their aggregation or loss at temperatures under reaction conditions to 1000K make them attractive candidates (Huang 2009; Joo 2009). By appropriately mixing or otherwise spatially organizing core shell particles with different catalytic capabilities, independent catalytic processes could in principal be coupled together in a Kreb’s cycle or other contiguous system

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configuration. Some thoughts on this hierarchical heterostructure approach will be presented in the oral presentation as time permits.

References N. Ahuja, T. A. Ostomel, P. Rhee, G. D. Stucky, R. Conran, Z. Chen, G. Al-Bubarak, G. Velmahos, M. deMoya, and H. B. Alam, 2006, Testing of modified zeolite hemostatic dressings in a large animal model of lethal groin injury, J. Trauma 61, 1312-1320 M. Auffan, J. Rose, J-Y. Bottero, G. V. Lowry, J-P. Jolivet, and M. R. Wiesner, 2009, Towards a definition of inorganic nanoparticles from an environmental, health and safety perspective, Nature Nanotechnology 4, 641-641 S. E. Baker, A. M. Sawvel, N. Zheng, and G. D. Stucky, 2007, Controlling bioprocesses with inorganic surfaces: layered clay hemostatic agents, Chem. Mater. 19, 4390-4392 S. E. Baker, A. M. Sawvel, J. Fan, Q. Shi, N. Strandwitz, and G. D. Stucky, 2008, Blood clot initiation by mesocellular foams: dependence on nanopore size and enzyme immobilization, Langmuir 24, 14254-14260 S. J. Barnes and P. D. Weitzman, 1986, Organization of citric acid cycle enzymes into a multienzyme cluster, FEBS Lett. 201, 267-270 M. H. Bartl, S. P. Puls, J. Tang, H. C. Lichtenegger, and G. D. Stucky, 2004, Cubic mesoporous frameworks with a mixed semiconductor nanocrystalline wall structure and enhanced sensitivity to visible light, Angew. Chemie Intl Ed. 43, 3037-3040 J. N. Cha, M. H. Bartl, M. S. Wong, A. Popitsch, T. J. Deming, and G. D. Stucky, 2003, Microcavity lasing from block peptide hierarchically assembled quantum dot spherical resonators, Nano Letters 3, 907-911 J. Fan, S. W. Boettcher, and G. D. Stucky, 2006, Nanoparticle assembly of ordered multicomponent mesostructured metal oxides via a versatile sol-gel process, Chem. Mater. 18, 6391-6396 J. Fan, Y. Li, Y. Dai, N. Zheng, J. Guo, and G. D. Stucky, 2009, Low-temperature, highly selective, gas-phase oxidation of benzyl alcohol over mesoporous K-Cu-TiO2 with stable copper(I) oxidation state, J. Am. Chem. Soc. 131, 15568-15569 O. C. Gobin and F. Schüth, 2008, On the suitability of different representations of solid catalysts for combinatorial library design by genetic algorithms, J. Comb. Chem. 10, 835-846 Y-J. Han, J. T. Watson, G. D. Stucky and A. Butler, 2002, Catalytic activity of mesoporous silicate-immobilized chloroperoxidase, J. Molecular Catalysis B 17, 1-8 M. Hartmann and D. Jung, 2010, Biocatalysis with enzymes immobilized on mesoporous hosts: the status quo and future trends, J. Materials Chemistry 20, 844-852 N. Herron, G. D. Stucky, and C. A. Tolman, 1985, The reactivity of tetracarbonylnickel encapsulated in zeolite X. A case history of intrazeolite coordination chemistry, Inorganica Chimica Acta 100,135-40 N. Herron, 1986, A cobalt oxygen carrier in zeolite Y. A molecular “ship in a bottle”, Inorg. Chem. 25, 4714-4717 X. Huang, C. Guo, J. Zuo, N. Zheng, and G. D. Stucky, 2009, An assembly route to inorganic catalytic nanoreactors containing sub-10-nm gold nanoparticles with anti-aggregation properties, Small 5, 361-365 S. H. Joo, J. Y. Park, C-K. Tsung, Y. Yamada, P. Yang, and G. A. Somorjai, 2009, Thermally stable Pt/mesoporous silica core-shell nanocatalysts for high-temperature reactions, Nature Materials 8, 126-131 B. S. Kheirabadi, M. R. Scherer, J. S. Estep, M. A. Dubick, and J. B. Holcomb. 2009, Determination of efficacy of new hemostatic dressings in a model of extremity arterial hemorrhage in swine, J. Trauma 67, 450-460 Y. Klichko, M. Liong, E. Choi, S. Angelos, A. E. Nel, J. F. Stoddart, F. Tamanoi, and J. I. Zink, 2009, Mesostructured silica for optical functionality, nanomachines, and drug delivery, J. Am. Ceram. Soc. 92, S2-S10

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M. Kustova, M. S. Holm, C. H. Christensen, Y-H. Pan, P. Beato, T. V. W. Janssens, F. Joensen, and J. Jesper, 2008, Synthesis and characterization of mesoporous ZSM-5 core-shell particles for improved catalytic properties, in Studies in Surface Science and Catalysis 174A (Zeolites and Related Materials), 117-122 O. Litvin, H. C. Causto, B-J. Chen and D. Pe’er, 2009, Modularity and Interactions in the genetics of gene expression, Proc. Natl Acad. Sciences USA 106, 6441-6446 F. D. Mango, 1996, Transition metal catalysis in the generation of natural gas, Org. Geochem. 24, 977-984 D. I. Margolese, J. Melero, S. C. Christiansen, B. F. Chmelka, and G. D. Stucky, 2000, Direct syntheses of ordered SBA-15 mesoporous silica containing sulfonic acid groups, Chem. Mater. 12, 2448-2459 R. McCarron, 2008, Comparative testing of hemostatic dressings in a severe groin hemorrhage, presentation at Advanced Technology Applications for Combat Casualty Care (ATACCC) conference, St. Pete FL 10-13 Aug 2008 A. Miller, H. Reuter, and S. Dillinger, 1995, Supramolecular inorganic chemistry: Small guests in small and large hosts, Ang. Chem. Int. Ed. Engl. 34, 2328-2364 B. S. Moore and J. Piel, 2000, Engineering biodiversity with type II polyketide synthase genes, Antonie van Leeuwenhoek 78, 391-398 J. Nakazawa and T. D. P. Stack, 2008, Controlled loadings in a mesoporous material: click-on silica, J. Am. Chem. Soc. 130, 14360-14361 C. O’Driscoll, 2007, Chiral synthesis : reflective work, Chemistry & Industry 9, 22-25 T. A. Ostomel, P. K. Stoimenov, P. A. Holden, H. B. Alam, and G. D. Stucky, 2006a, Host-guest composites for induced hemostasis and therapeutic wound healing in traumatic injuries, J. Thrombosis and Thrombolysis 22, 55-67 T. A. Ostomel, Q. Shi, and G. D. Stucky, 2006b, Oxide hemostatic activity, J. Am. Chem. Soc. 128, 8384-8385 T. A. Ostomel, Q. Shi, C-K. Tsung, H. Liang, and G. D. Stucky, 2006c, Spherical bioactive glass with enhanced rates of hydroxyapatite deposition and hemostatic activity, Small 2, 12611265 T. A. Ostomel, Q. Shi, P. K. Stoimenov, and G. D. Stucky, 2007, Metal oxide surface charge mediated hemostasis, Langmuir 23, 11233-11238 J-H. Park, L. Gu, G. von Maltzahn, E. Ruoslahti, S. N. Bhatia, and M. J. Sailor, 2009, Biodegradable luminescent porous silicon nanoparticles for in vivo applications, Nature Materials 8, 331-336 V. Polshettiwar, B. Baruwati, and R. S. Varma. 2009, Self-assembly of metal oxides into threedimensional nanostructures: synthesis and application in catalysis, ACS Nano 3, 728-734 W. S. Postula, Z. T. Feng, C. V. Philip, A. Akgerman, and R. G. Anthony, 1994, Conversion of synthesis gas to isobutylene over zirconium dioxide based catalysts, J. Catal. 145, 126-131 D. R. Radu, C. Y. Lai, K. Jeftinija, E.W. Rowe, S. Jeftinija, and V. S. Y. Lin, 2004, A polyamidoamine dendrimer-capped mesoporous silica nanosphere-based gene transfection reagent, J. Am. Chem. Soc. 126, 13216-13217 P. R. Rich, 2003, The molecular machinery of Keilin's respiratory chain, Biochem. Soc. Trans. 31 (Pt 6), 1095-2105 J. Ross and A. P. Arkin, 2009, Complex systems: From chemistry to systems biology, Proc. Natl Acad. Sciences USA 106 (16), 6433-6434 J. C. Schon and M. Jansen, 1996, First step towards planning of syntheses in solid-state chemistry: determination of promising structure candidates by global optimization, Ang. Chem. Int. Ed. Engl. 35, 1286-1304 K. Shimizu, Y. Miyamoto, T. Kawasaki, T. Tanji, Y. Tai, and A. Satsuma, 2009, Chemoselective hydrogenation of nitroaromatics by supported gold catalysts: mechanistic reasons of sizeand support-dependent activity and selectivity, J. Phys. Chem. C 113, 17803-17810 I. Slowing, B. G. Trewyn, and V. S.-Y. Lin, 2006, Effect of surface functionalization of MCM41-type mesoporous silica nanoparticles on the endocytosis by human cancer cells, J. Am. Chem. Soc. 128, 14792-14793

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Y. Tai, M. Watanabe, K. Kaneko, S. Tanemura, T. Miki, J. Murakami, and K. Tajiri, 2001, Preparation of gold cluster/silica nanocomposite aerogel via spontaneous wet-gel formation, Adv. Mater. 13, 1611-1614 Y. Tai, W. Yamaguchi, K. Tajiri, and H. Kageyama, 2009, Structures and CO oxidation activities of size-selected Au nanoparticles in mesoporous titania-coated silica aerogels, Appl. Catal. A 364, 143-149 J. Tang, Y. Wu, E. W. McFarland, and G. D. Stucky, 2004, Synthesis and photocatalytic properties of highly crystalline and ordered mesoporous TiO2 thin films, Chem. Commun., 1670-1671 T. J. Terry, G. Dubois, A. Murphy, and T. D. P. Stack, 2007, Site isolation and epoxidation reactivity of a templated ferrous bis(phenanthroline) site in porous silica, Angew. Chem. Int. Ed. 46, 945-947 J. M. Thomas, 1994, Turning points in catalysis, Ang. Chem. Int. Ed. Engl. 33, 913-937 J. M. Thomas, J. C. Hernandez-Garrido, and R. G. Bell, 2009, A general strategy for the design of new solid catalysts for environmentally benign conversions”, Top. Catal. 52, 1630-1639 P. Yang, D. Zhao, D. I. Margolese, B. F. Chmelka, and G. D. Stucky, 1998, Generalized syntheses of large-pore mesoporous metal oxides with semicrystalline frameworks, Nature 396, 152-154 J. T. Yates Jr., 2009, Photochemistry on TiO2: Mechanisms behind the surface chemistry, Surface Science 603, 1605-1612 N. Zheng and G. D. Stucky, 2006, A general synthetic strategy for oxide-supported metal nanoparticle catalysts, J. Am. Chem. Soc. 128, 14278-14280 J. M. O. Zide, A. Kleiman-Shwarsctein, N. C. Strandwitz, J. D. Zimmerman, T. T. SteenblockSmith, A. C. Gossard, A. Forman, A. Ivanovskaya, and G. D. Stucky, 2006, Increased efficiency in multijunction solar cells through the incorporation of semimetallic ErAs nanoparticles into the tunnel junction, Appl. Phys. Lett. 88, 162103

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

How the manufacturing technology of industrial catalysts can influence their mechanical strength Nicola Perniconea, Tania Fantinelb, Valentina Trevisanb, Francesco Pinnab a

Consultant, Via Pansa 7, 28100 Novara, Italy. University of Venice, Dept. of Chemistry and Consorzio INSTM, 30123 Venice, Italy. Email address: [email protected]

b

Abstract The various physical properties characterizing the mechanical strength of catalysts are discussed: abrasion resistance, crush strength, attrition resistance. The related measurements have been performed mainly using ASTM standard methods with some improvements. It is shown how modifications of the manufacturing technology can improve the abrasion resistance of the traditional ammonia synthesis catalyst (oxidepromoted magnetite) and of the PTA catalyst (Pd on active carbon). As to crush strength, it is discussed how some preparation variables can be changed to improve the performance of catalysts for the production of styrene (Fe-K-Ca-Ce-Mo oxides), formaldehyde (Fe-Mo oxides), methanol (Cu-Zn-Al oxides). As to powders, it is shown that the attrition resistance of aluminas to be used for fluid bed catalysts can be improved by suitable fluorination. Finally, it is stressed that mechanical strength is a property not less important than activity and selectivity for industrial catalysts and that no catalyst must be loaded in an industrial reactor without previous check of its mechanical properties. Keywords: catalyst strength, attrition, abrasion, catalyst manufacture

1. Introduction Mechanical strength is a property of utmost importance for the industrial use of heterogeneous catalysts, consisting of either powders or pellets (cylinders, spheres, rings, granules, with size of few millimeters). For an effective industrial exploitation the pellets may not break nor abrade, while the powders may not generate fines by attrition. In other words, industrial catalysts must have a high mechanical strength. There are many examples of laboratory catalysts very successful as to activity and selectivity, but unsuitable to industrial development due to lack of mechanical strength, whose measurement is a key part of every quality control for any industrial catalyst. In practice, no catalyst should be loaded in an industrial reactor without having tested its mechanical properties. The practical consequences of the lack of mechanical strength are as follows: a) for fluid bed reactors Increase of catalyst consumption b) for axial fixed bed reactors Increase of pressure drop c) for radial fixed bed reactors Bypass formation with decrease of conversion d) for slurry reactors Problems in catalyst separation

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e) for trickle bed reactors Contamination of reaction product In all these cases a large increase of production costs is to be expected. The following three different physical properties characterize the mechanical strength of catalysts: 1) abrasion resistance (also called abrasion loss), for pellets 2) crush strength, also for pellets 3) attrition resistance, for powders As there is no straightforward procedure for the measurement of such properties, many arbitrary standard methods have been developed in different laboratories, with consequent problems in the comparison of data. Starting from the Seventies, ASTM was successful in the development of standard unified methods, which are widely used worldwide. Our work has been performed using such methods, sometimes with some modification. The literature relating the mechanical properties of catalysts with their manufacturing technology is very scanty. An old paper by one of the authors can be mentioned [1]. Some data about crush strength can be found in a recent paper [2]. It should be remarked that mechanical strength of catalysts is not a property amenable to be studied on the lab scale, because at least pilot-scale machines for catalyst forming are required. That means that such studies are usually performed during the scale-up of catalyst manufacture [3]. During this step of catalyst development mechanical strength is even a priority, as activity and selectivity have been already studied very deeply on the lab scale. In this report we will show how some variables of catalyst preparation can influence the final mechanical properties, starting in some cases from tests on different commercial catalysts.

2. Experimental techniques 2.1. Abrasion resistance For the measurement of the abrasion resistance the ASTM standard method D4058 has been used [4]. In practice it consists of determining the amount of fines formed after having rounded the catalyst in a strictly defined drum under specified conditions. The drum is equipped with a radial baffle so that in each revolution the catalyst falls from a height of about 20 cm. This test well simulates the abrasion suffered by the catalyst during reactor loading, much less that due to small displacements during reactor running. Nevertheless it is widely used worldwide. We have adopted the following experimental conditions: Catalyst mass 100 g Rotation rate 60 rpm Test duration 30 min Sieve size 0.85 mm The reproducibility of the test was within 5%.

2.2. Single-pellet crush strength First of all it should be remarked that the axial crush strength of cylindrical pellets and rings is completely useless for catalytic purposes as it is much higher than the radial one, therefore not relevant to catalyst behavior. No problem for spheres and extrudates, where only one crush strength exists. For extrudates, mostly not having constant length, the crush strength is frequently expressed in kg/mm, though making the measurement on pieces with the same length gives more reliable data.

How the manufacturing technology can influence their mechanical strength

11

This measurement is usually performed by a dynamometer. However most commercial dynamometers have been designed for high loads (hundreds and even thousands of kg), while most catalysts crush under few kg. Small-load dynamometers with efficient calibration system are required for reliable measurements on catalysts. In our machine the pellet is placed on a steel dish and the piston approaches it with a speed of 10 mm/min. The breaking force is monitored on a digital display and/or on the PC screen. It is convenient to make the measurement on dry samples. For small-size extrudates it is strongly recommended to follow the crush curve on the PC, as the detection of the breaking point is not easy with the display only. There are usually large differences in crush strength from pellet to pellet, so that a distribution curve must be obtained (see Fig. 1). The number of tests to be performed ranges from 20 to even 100 in the most adverse cases. The crush strength is expressed as the average value. Standard conditions for the measurement of single-pellet crush strength can be found in the ASTM methods D4179 [5] and D6175 [6]. 12

Count, step 0.1Kg

10 8 6 4 2 0 0

0.4

0.8

1.2

1.6

2

F, Kg

Fig.1. Crush strength distribution curve of an activated methanol synthesis catalyst.

2.3. Bulk crush strength For catalysts having shape of granules and also of irregular spheres and extrudates single-pellet measurements are unreliable. So bulk measurements have been developed consisting of applying a pressure on a catalyst volume and measuring the amount of fines formed during this treatment. There are two alternatives: a) measuring the pressure required to obtain a prefixed amount of fines (1% for ASTM D7084 [7]) b) measuring the amount of fines obtained under a prefixed pressure. We have used the latter procedure with the following operating conditions: Catalyst volume 12.5 ml Pressure 23 kg/cm2 Sieve size 0.85 mm It should be remarked that bulk crush strength simulates well the mechanical stress the catalyst may withstand in an industrial reactor.

2.4. Attrition resistance The attrition resistance of a fluid-bed catalyst can be evaluated by measuring the amount of very fine powder formed after an intensive attrition stress caused by several

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high-speed air jets. This procedure simulates in a few hours what happens in an industrial reactor during months. An example is given by the ASTM method D5757 [8]. We have adopted the same instrumentation and procedure, but with the following improvements: a) no obsolete dry or wet test meter, but mass flow controller b) no fines collection assembly, but exit gas to vent c) powder humidification only when indispensable d) previous elimination of fines below 45 micrometers e) duration of the air jet treatment: 1 hour f) after that the whole remaining powder is sieved again at 45 micrometers and the coarser fraction weighed to determine the amount of fines produced. Such modifications make the test faster and more reproducible.

3. Results and discussion 3.1. Abrasion resistance As a general rule, insufficient abrasion resistance sometimes occurs for granules, less for extrudates, more rarely for pellets. An undesired general problem is the formation of fines during reactor loading with obvious safety consequences. Furthermore, radial reactors can show bypass formation, while in trickle bed reactors the reaction product may be contaminated by catalyst fines. Two examples are discussed here, showing how to resolve the problem by modifying the preparation technology. The traditional ammonia synthesis catalyst (oxide-promoted magnetite) is frequently employed in its reduced-passivated form [9] to decrease the activation time in the reactor. As it consists of small irregular granules, large formation of fines frequently occurs during reactor loading. Furthermore also bypass formation sometimes occurs in the widely used radial reactors. The problem can be resolved by introducing a rounding step at the end of the preparation process, preferably before, but also after, prereduction. In fact, our experiments have shown that the abrasion loss of prereduced ammonia catalysts strongly decreases when the rounding time increases (Fig. 2).

Abrasion Loss, %

4

3

2

1

0

0

50

100

150

200

Rounding Time, min

Fig. 2. Prereduced ammonia synthesis catalyst. Increase of abrasion resistance with rounding time.

Of course other preparation variables can influence the abrasion resistance of this catalyst, but it is remarkable that, whichever level of abrasion resistance has been reached, the catalyst can be brought back to the specs by a final rounding treatment.

How the manufacturing technology can influence their mechanical strength

13

The catalyst employed for the purification of terephthalic acid (0.5% Pd on 2-4 mm active carbon granules, trickle bed reactor, [10]) can release black powder impairing the whiteness of the resulting PTA. This is due to small displacements of the catalyst granules in the running trickle bed. The problem is serious also for the connected loss of Pd (egg-shell catalyst). It cannot be easily resolved in the not unusual case of incorrect supporting of Pd. However, when the problem is caused by the low abrasion resistance of the active carbon, rounding the support before impregnation can be a good solution. Like for the ammonia synthesis catalyst, our experiments have shown that the abrasion loss of some poor active carbons decreases when the rounding time increases (Fig. 3). It can be immediately deduced that the catalyst so obtained, if properly manufactured, not only will give whiter PTA, but also will lose less Pd and have longer life. 3.0

Abrasion Loss, %

2.5 2.0 1.5 1.0 0.5

0

50

100

150

200

Rounding Time, min

Fig. 3. Active carbon granules 2-4 mm. Increase of abrasion resistance with rounding time.

3.2. Crush strength The crush strength is of special interest for the bottom catalyst layers and, for ringshaped catalysts, also during catalyst loading in the reactor. Average values of radial crush strength hardly exceed 10 kg in most industrial catalysts. Much lower values are usually found for ring-shaped catalysts. There are a lot of preparation variables influencing the crush strength of the final catalyst, depending on the formation procedure (tableting, extrusion, beading [11]). We will discuss here some examples. Styrene catalysts are formed by extrusion. A preferred composition is Fe-K-Ca-CeMo oxides [12,13]. The crush strength can be increased by increasing the calcination temperature, but this operation is very critical as to activity (Fig. 4). It results that a calcination temperature very close to 970°C should be adopted. It is interesting that small amounts of Mg compounds can strongly impair the crush strength (Fig. 4). Therefore the purity of the raw materials must be carefully checked.

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100

8

90

6

80

4

70 Mg 0.5%

2 0 800

60

Mg 1.0%

850

900

950

Conversion, %

Radial Crush Strength, Kg

10

1000

50 1050

Temperature, °C

Fig. 4. Styrene catalysts. Fe-K-Ca-Ce-Mo oxides. Extrudates 5x3 mm. Influence of calcination temperature on activity and crush strength.

Formaldehyde catalysts are ring-shaped (for instance 4x2x4 mm) and consist of FeMo oxides [10]. Their radial crush strength is very low (about 0.3 kg) and there is a definite need to increase it, in order to avoid extensive breaking during reactor loading. In fact the crush strength increases with the calcination temperature, but unfortunately there is at the same time a sudden decrease of surface area (Fig. 5), therefore of activity. A suitable compromise can be looked for. It should be remarked that temperatures higher than 700°C cannot be used, due to extensive shrinking of the rings. 5 4

0.45

3 0.40 2

2

Surface Area, m /g

Radial Crush Strength, Kg

0.50

0.35

0.30 450

1

500

550

600

650

700

0 750

Temperature,°C

Fig. 5. Ring-shaped Fe-Mo oxide catalysts. Influence of calcination temperature on surface area and radial crush strength.

Methanol synthesis catalysts consist of Cu-Zn-Al oxides and are activated in situ by reduction of CuO to Cu [14]. They are cylindrical pellets made by tableting. During activation, if the pellets have been completely dried, the catalyst loses about 10% of its mass as water coming from reduction of CuO by hydrogen. This makes the pellets more brittle, therefore decreasing the crush strength, which of course must be measured not on the fresh, but on the activated catalyst. It can be thought that, if the fresh catalyst contains additional water, its crush strength will be lower. This has been fully confirmed by our results (Fig. 6). Therefore the powder to be tableted must be brought to dryness, though this can make tableting more problematic.

How the manufacturing technology can influence their mechanical strength

15

Radial Crush Strength, Kg

14 12 10 8 6 4 2 0

0

2

4

6

8

10

Water Content, wt%

Fig. 6. Methanol synthesis catalysts. Influence of water content on crush strength of activated catalysts.

3.3. Attrition resistance Increasing the attrition resistance of the few fluid-bed catalysts brings to an appreciable reduction of catalyst costs due to a decreased consumption. High surface area alumina in its various forms (boehmite, bayerite, gamma, delta) and silica-alumina are the preferred supports for fluid bed catalysts. However in many cases their attrition resistance is not satisfactory, like, for instance, in the case of pure and silica containing boehmites. A remarkable improvement of the attrition resistance of these supports can be obtained by fluorination with HF (Table 1). Table 1. Attrition resistance of alumina supports for fluid bed catalysts. Sample

% fines formed

% fines formed after fluorination

C 1.5*

15.6

11.3

C 5*

15.0

11.2

CP

18.9

10.5

D

25.4

14.0

*SiO2 content

Of course fluorination can bring to modification of other properties, like surface area and acidity, which should be tested.

4. Conclusions Mechanical strength is a property of utmost importance for the industrial use of heterogeneous catalysts. Abrasion resistance and radial crush strength (for pellets) and attrition resistance (for powders) should be routinely measured for quality control of industrial catalysts before reactor loading or better before catalyst purchasing. Such measurements can be conveniently performed using the respective ASTM standard methods, whose possible improvements are suggested. Many variables of the catalyst preparation procedure can influence the mechanical properties of the final catalyst. It is remarked that this R-D step must be performed during the scale-up of catalyst production at the pilot scale. It is shown that the mechanical strength of the following catalysts can be improved in the following ways:

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Ammonia (oxide-promoted magnetite) Prerounding PTA (Pd on active carbon) Support prerounding Styrene (Fe-K-Ca-Ce-Mo oxides) Optimizing calcination temperature Formaldehyde (Fe-Mo oxides) Optimizing calcination temperature Methanol (Cu-Zn-Al oxides) Dryness before tableting Fluid bed aluminas Fluorination Finally, it is stressed that the mechanical properties of the fresh catalyst are completely useless when the catalyst has to be activated in the industrial reactor [15]. The activation conditions must of course be optimized to get a high mechanical strength of the working catalyst, which should be tested after activation and, if necessary, suitable passivation.

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]

[15]

F. Traina and N. Pernicone, Preparation techniques and their influence on the properties of the solid catalysts, Chim. Ind. (Milan), 52 (1970) 1. D. Wu, J. Zhou and Y. Li, Mechanical strength of solid catalysts. Recent developments and future prospects, AIChE J., 53 (2007) 2618. N. Pernicone, Scale-up of catalyst production, Catal. Today, 34 (1997) 535. Annual Book of ASTM Standards, ASTM International Publ., West Conshohocken, Pa., USA, (2009) p. 329. Annual Book of ASTM Standards, ASTM International Publ., West Conshohocken, Pa., USA, (2009) p. 334. Annual Book of ASTM Standards, ASTM International Publ., West Conshohocken, Pa., USA, (2009) p. 430. Annual Book of ASTM Standards, ASTM International Publ., West Conshohocken, Pa., USA, (2009) p. 438. Annual Book of ASTM Standards, ASTM International Publ., West Conshohocken, Pa., USA, (2009) p. 423. F. Pinna, T. Fantinel, G. Strukul, A. Benedetti and N. Pernicone, TPR and XRD study of ammonia synthesis catalysts, Appl. Catal A., 149 (1997) 341. N. Pernicone, Catalysis at the nanoscale level, CATTECH, 7 (2003) 196. N. Pernicone and F. Traina, Commercial catalyst preparation, in Applied Industrial Catalysis, Vol.3, Academic Press, (1984) p. 1. J.L. Smith, B.S. Masters and D.J. Smith, Dehydrogenation catalyst, U.S.Pat. 4,467,046 (1984) L.Forni and N.Pernicone, Catalyst for the dehydrogenation of ethylbenzene to styrene, Eur. Pat. Appl. 03714907.7-1211 (2003). C. Baltes, S. Vukoievic and F. Schuth, Correlations between synthesis precursor and catalyst structure and activity of a large set of CuO/ZnO/Al2O3 catalysts for methanol synthesis, J. Catal., 258 (2008) 334. N. Pernicone and F. Traina, Catalyst activation by reduction, in Preparation of Catalysts II, Elsevier (1978) p. 321.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Coating metallic foams and structured reactors by VOx/TiO2 oxidation catalyst: Application of RPECVD Adil Essakhia, Axel Löfberga, Philippe Supiotb, Brigitte Mutelb, Sébastien Paula, Véronique Le Courtoisa, Elisabeth Bordes-Richarda a

Université Lille Nord de France - Unité de Catalyse et de Chimie du Solide, UMR CNRS 8181, 59655 Villeneuve d’Ascq, France – [email protected] b Institut d’Electronique, Microelectronique et Nanotechnologie (IEMN) - UMR CNRS 8520, 59655 Villeneuve d’Ascq, France

Abstract VOx/TiO2, catalyst of oxidative dehydrogenation of propane, was immobilised on a SiO2 film coating stainless-steel (SS) plates and foams figuring out structured reactors. SiO2 is expected to act as a primer and a barrier against poisoning of VOx/TiO2 catalyst by elements of SS. The adhesive SiO2 layer was first coated on 2D- (plates) and 3D-SS substrates (foams) A good adhesion was obtained after polymerisation by RPECVD (Remote Plasma Enhanced Chemical Vapor Deposition) of a 6 µm-thick tetramethyldisiloxane polymer layer, followed by calcination (650°C) to obtain the SiO2 layer. A post-treatment in N2/1.5%O2 plasma afterglow was necessary to eliminate remaining carbon traces after calcination. The resulting SiO2/SS objects were dip-coated in TiO2–anatase aqueous suspension. Vanadium isopropoxide was grafted on calcined TiO2/SiO2/SS, yielding VOx polyvanadates after calcination at 450°C. The mechanical stability of the VOx/TiO2 catalyst immobilized onto SiO2/SS was examined by scratch test and ultrasonic bath experiment. The successive coated layers were studied by Raman spectroscopy, SEM-EDX, electron probe microanalysis and XPS. A special RPECVD reactor was designed to coat foams instead of plates. For the first time, a thin and homogeneous layer of silica could be deposited through the whole foam. The other steps were applied to obtain VOx/TiO2/SiO2/SS foams. XPS and Raman characteristics of deposits were the same than for coated plates and VOx/TiO2 powders. Keywords: RPECVD, catalytic coatings, SiO2, TiO2, metallic foam, structured reactor

1. Introduction Thin films of oxides as coating materials have several applications in the field of sensors, electronic and photonics devices, environmental purification, sterilization and deodorization, self-cleaning surfaces (textiles, windows…), biosensors, orthopaedics, etc. Although it was at first to disperse metals and oxides as aggregates on an oxidic support, another large application is heterogeneous catalysis, as far as shaped 3Dcarriers like monoliths, foams, or structured walls of reactors are concerned. Most reactions being strongly exothermic, it is important to get rid of hot spots which are present in the catalytic pellets in fixed bed reactors. Indeed, hot spots are responsible for structural damages and early deactivation of the catalyst, and generally they promote reactions leading to by-products, like carbon oxides in selective oxidation reactions. An alternative which was proposed for automotive pollution control more than 30 years ago is the use of 3D structures like monoliths. These opened structures which are covered by

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a thin layer of catalyst favour efficient heat and mass transfers between the gaseous reactants, the catalytic active phase and the wall of the reactors in which they are inserted. A larger and larger use of monoliths and foams varying by the substrates and shapes is now made. As in other applications the deposit must be mechanically, chemically and thermally stable. However in catalytic applications its surface has also to be strongly reactive and to be able to suffer high flowrates of reactants/products without attrition, erosion or collapsing. As far as reactive thin films of oxides are concerned, a large challenge is to succeed in coating metallic substrates of reactors or inserted carriers with thick and high surface area layers of active phase. For most applications, the oxidic coating may bear no chemical resemblance with the metallic (e.g. stainless steel) substrate. This makes more difficult to get a good holding of the coating and to avoid poisoning of the active phase by its elements. In former studies of a two-layers catalyst (VOx/TiO2) deposited on stainless steel (SS) plates figuring out the reactor walls, iron was shown to diffuse outwards and to make surface iron vanadates that decreased the selectivity to propene in the oxidative dehydrogenation of propane [1-3]. In another case, Co/SiO2 for Fisher-Tropsch reaction, a primer layer consisting in a thin layer of silica could be deposited by remote plasma enhanced chemical vapor deposition (RPECVD) [2-5]. This method was successful for coating cobalt on porous silica support onto silica-coated SS plates which were used in a specially designed reactor [2,3,6-8]. The methods of coating based on powder suspension and sol-gel preparation are well documented [9,10], and several others are known (magnetron sputtering, atomic layer deposition, chemical vapor deposition, reactive plasma, etc.) to make thin films. The use of PECVD is an alternative method worthwhile to be studied as shown by our previous results. In the preparation of thin films, PECVD has not received as high attention as methods using evaporation, though a wide range of experimental parameters can be varied to control the microstructure of the films [5,11-13]. The process we used is based on cold plasma assisted polymerization of tetramethyldisiloxane (TMDSO), the precursor of silica. The silica-like layer coated on stainless steel substrate is intended to ensure the mechanical steadiness [6-8], to act as a bonding layer for the deposition of the support of the active phase which is VOx/TiO2, and also to hinder Fe diffusion during the reaction. Successful results obtained on this catalyst/structured wall reactor or catalyst/3D-carriers system, the active phase of which is well-known for its applications in chemical industry (o-xylene oxidation to phthalic anhydride), as well as in pollution abatement (deNOx with ammonia, COV, etc.), could help to promote the use of new reactors with enhanced heat and mass transfers.

2. Experimental 2.1. Preparation of VOx/TiO2/SiO2/SS plates and foams

Three main steps are required to prepare the catalytic system VOx/TiO2/SiO2/SS. The stainless steel (AISI 316L) plates (50 mm × 20 mm × 0.5 mm) and foam (Porvair®, 40 ppi, density 5.4%) substrates were first sonicated with ethanol (30 min) to eliminate organic pollutants, and then twice in deionized water (30 min) before drying at 110°C for 3 h. 2.1.1. Coating of SS by SiO2 by RPECVD The new experimental set-up of the cold remote nitrogen plasma assisted polymerization reactor was adapted from [5,14]. Details can be found in [15]. Summarising, the nitrogen flow was excited by a microwave discharge (2450 MHz–200 W) in a fused silica tube. By continuous pumping (roots pump Pfeiffer), the reactive

Coating metallic foams and structured reactors by VOx/TiO2 oxidation catalyst

19

species, mainly atomic nitrogen in the ground electronic state N(4S), flowed from the discharge zone to the deposition zone located 1 m downstream. The TMDSO monomer (Sigma Aldrich, grade 97%), premixed with O2, was introduced in the remote nitrogen plasma through a coaxial injector. Flowrates of N2, O2 and TMDSO at 1800, 25, 5 sccm, respectively, were controlled at 550 Pa by means of MKS mass-flow controllers. The deposition rate was in situ measured by interferometry using He-Ne laser (λ = 632.8 nm) and a photodetector. The substrate was first pre-treated by remote nitrogen afterglow for 5 min for cleaning and then the monomer/O2 mixture was added without air exposure for the deposition step. Details about the reaction mechanism of decomposition of the monomer can be found in [14]. The TMDSO plasma polymer (ppTMDSO) coated substrate was then post-treated in a N2-1.5% O2 remote plasma during 5 min and the film was mineralized by thermal treatment in air to obtain the SiO2 layer. To remove carbon remaining traces shown by laser Raman spectroscopy (3000 cm-1 for CH3 and 1500 cm-1 for Si-C bonds), a N2/1.5%O2 remote plasma treatment was finally applied during 5 min. A special sample holder was designed to coat metallic foams which were cut in 1 cm × 1 cm × 0.7 cm blocks [15]. The same procedure than for plates was repeated. 2.1.2. Coating of SiO2/SS by TiO2 SiO2/SS plates or foams were dipped under stirring in an aqueous suspension containing 60 wt% TiO2 (Sigma-Aldrich) particles during 5 min, and withdrawn at 6 mm.s-1 [1,2]. The TiO2/SiO2/SS carriers were calcined in air flow at 110°C during 1 hour, and then at 700°C during 2 h (heating rate 80°C/min). 2.1.3. Coating of TiO2/SiO2/SS by polyvanadates The TiO2/SiO2/SS carriers were dip-coated in various amounts of VO(OPr)3 in dry ethanol, yielding VOx/TiO2/SiO2/SS coated plates and foams after calcination in air at 450°C for 4 h.

2.2. Methods of analysis Most analyses were performed at every stage of preparation. The silica films were analyzed using a Fourier Transform Infrared (FTIR) Perkin-Elmer spectrometer. Spectra were recorded in specular reflexion mode in the spectral range 4000- 400 cm-1 (resolution 4 cm-1). Laser Raman spectra (LRS) of TiO2/SiO2/SS carriers were recorded on LabRAM Infinity spectrometer (Jobin Yvon) equipped with a liquid nitrogen detector and a frequency-doubled Nd:YAG laser supplying the excitation line at 532 nm. The power applied on the sample was less than 5 mW. X-ray photoelectron spectroscopy (XPS) experiments were carried out on pieces of covered substrates using VG-Escalab 220 XL spectrometer. A monochromatic Al Kα X-ray source was used and electron energies were measured in the constant analyzer energy mode. The pass energy was 100 eV and 40 eV for the survey and single element spectra, respectively. XPS binding energies were referred to C 1s core level at 285 eV. Electron Probe Micro Analysis (EPMA) was performed on samples embedded into epoxy resin and polished with abrasive discs (2400 to 3 µm granulometry). A Bal-Tec SCD005 sputter coated allowed depositing a thin carbon film. Elemental analysis were made using a wavelength dispersive X-ray spectrometer (Cameca SX-100 microprobe analyser) working at 15 kV and 15 nA for back scattered electron (BSE) images and at 15 kV and 49 nA for Si, Ti and Fe Kα X-ray profiles and mapping using TAP, PET and LiF crystals respectively. The Scanning Electron Microscope (SEM) Hitachi 4100 S was equipped with micro-analyzed (EDS) and a Field Emission Gun (FEG). The working voltage was 15 KeV. The analyzed volume was estimated to be approximately 1 µm3.

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The wettability of the deposited silica layers was evaluated from contact angle with deionized water using a Krüss computer-controlled goniometer (± 1° accuracy). BET method and porosimetry was applied to TiO2 coatings on silica after scratching of the layer, confirming that the initial characteristics (10 m2/g, 99.8% anatase, average particle size 10 µm) of TiO2 remained the same after these operations.

3. Results and discussion 3.1. Coating of SS plate and foam by silica The rate of deposition of the ppTMDSO polymer film onto SS plates was evaluated by two methods depending on the thickness (t). In situ measurements by interferometry were made for t < 12 µm. For t > 5µm, EPMA-BSE images were obtained ex situ on samples after various deposition times. In the chosen experimental conditions the deposit rate was 1.0 ± 0.1 µm.min-1 with a very good linearity for thicknesses ranging from 1 to 35 µm (Figure 1). Figure 2 (a-c) shows SEM pictures of the SS plate (after corrosion by 30% sulfuric acid, 1 h). After coating, the thickness of the layer (t ≈ 30 μm) is quite regular on the whole plate (Figure 2 b). FTIR spectroscopy of ppTMDSO/SS confirmed the polymerisation process by, e.g., the presence of asymmetric Si-O-Si stretching at 1000 and 1200 cm-1. The intensity of the double band at 600 cm-1, which is characteristic of the asymmetric elongation of Si-O-Si bond in a polymeric conformation, was observed to increase. However at this stage, it showed also that the organic component of the siloxane was not removed completely during the decomposition of the precursor – and thus that polymerisation was not completed – since small bands assigned to the asymmetric Si-CH3 and symmetric Si-H stretchings modes were found at 2145 cm-1 [6-8,14], as well as the symmetric and asymmetric stretchings ones of Si-CH3 at ca. 2910 and 2960 cm-1, respectively. The mineralisation to SiO2 being required for our applications, the variation of four parameters (post-treatment or not in N2/O2 remote plasma, heating rate from 1 to 5°C/min, temperature of plateau 450 and 650°C, thickness of polymer layer from 5 to 15 µm) was examined using a matrix experiments. As shown by BES images in the example of a 15 µm-thick film/SS, post-treated (5 min) in N2/1.5%O2 remote plasma and calcined at 5°C/min up to 650°C (1 h), the film burst in pieces (Figure 2 c). XPS confirmed the presence of surface iron together with Si, C and O photopeaks. The peak at 710.7 eV (Fe2p3/2 binding energy) accounts for the formation of Fe2O3 which could be expected owing to the oxidising post-treatment. Two oxygen photopeaks are due to Si-O and Fe-O bonds at 532.9 and 529.7 eV [16]. Most carbon comes from contamination but a peak at 288.6 eV could correspond to remaining polymeric carbon. As the cross-cut-tape test also revealed that the coating was not adherent enough, it was decided to decrease the thickness and to modify the post-treatment. The best results were obtained for a 5 μm thick film/plate without N2/O2 plasma afterglow treatment and calcined at 1°C/min heating rate up to 650°C (1 h). XPS analysis indicated a low carbon level with surface composition SiC0.05O1.8. After the treatment the layer shrinked by 68% down to a 1.6 μm thickness. SEM (Figure 2 d,e) and EPMA (Figure 3) showed that the plate was covered homogeneously by the SiOx layer, all Fe being hidden even in the case of a stripe. The mechanical stability examined by both cross-cut-tape test and ultrasonic n-heptane bath was good. FTIR and Raman spectroscopies showed that the remaining carbon (νs and νas of Si-CH3 at 2910 and 2960 cm-1) was greatly reduced by a N2/O2 plasma afterglow treatment performed during 5 min [15], since the final surface composition obtained by XPS was SiC0.01O1.82.

Coating metallic foams and structured reactors by VOx/TiO2 oxidation catalyst

21

Figure 1. Linear relationship between the thickness of ppTDMSO layer and the time of deposition.

a

b

Resin

SS

c

d

e

SS

SiO2 Resin

Figure 2. Pictures obtained by SEM or EPMA of SS plates, before (a), after coating by ppDMSO polymer (ppDMSO/SS) (b); and of SiO2/SS after mineralisation (c-d) and optimised treatment (e).

a

b

Figure 3. EPMA of Si and Fe for well-coated SiO2 plate along (a), line “a”; (b), line “b” drawn on Figure (2e).

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The whole optimised coating process was applied to cover SS foam, using the sample holder specially designed for the plasma to convectively flow through the whole volume of the foam [15]. No acidic treatment was applied to the foam before coating as its microstructure favoured the anchoring of ppDMSO film (Figure 4 a,b). The foam was cut in several pieces and examined by EPMA. The homogeneity of the coating for an initial thickness t ≈ 5 μm was found to be good throughout the foam. A picture of a branch is shown Figure 4 c.

a

b

c

Figure 4. Pictures obtained by SEM or EPMA of foams before (a), after coating by ppDMSO polymer (ppDMSO) (b); SiO2/SS is obtained after mineralisation (c).

3.2. Coating of TiO2 on SiO2/SS

SS plates coated with 1.6 μm SiC0.01O1.82 (further called SiO2/SS) film were dip-coated in an aqueous suspension of 60 wt% TiO2 which had first been stabilised by stirring during one hour at room temperature. The optimal composition of the aqueous TiO2 suspension was determined after measuring the zeta potential. After being withdrawn at constant rate (6 mm.s-1) the TiO2/SiO2/SS plates were calcined at 700°C (2 h). However the titania coating was not homogeneous, as shown by EPMA on Figure 5.

Figure 5. Aggregates of TiO2 coating SiO2/SS plates without pretreatment and profiles of Si, Ti Fe (from top to bottom) along the arrow drawn on picture.

The thickness of TiO2 is ≈ 6 µm in grey areas (zone 1) and higher in aggregates (≈ 12 µm, zone 2) while the black colour (zone 3) accounts for uncovered SiO2. Moreover the mechanical stability was very poor. The contact angle was measured for SS (87°), ppTMDSO/SS (104°), and SiO1.8/SS (34°) (Figure 6 a-c). Several pretreatments of SiO2/SS were performed to decrease the hydrophobicity of the surface and favour further adhesion of titania [15]. The best contact angle was obtained by combined pouring in Brown solution (1 g NaOH in 4 mL ethanol and 3 mL H2O) followed by drying (1 h) at 110°C, and CNRP N2/O2 55 min treatement (Figure 6 d). The resulting coatings (ca. 6 μm) of titania onto SiO2/SS were reproducible, homogeneous and stable after the calcination step (Figure 7).

Coating metallic foams and structured reactors by VOx/TiO2 oxidation catalyst

a

b

c

23

d

Figure 6. Contact angles measured: (a) 87° for bare SS plate; (b) 104° for ppTMDSO/SS; (c) 34° for SiO1.8/SS; (d) 29° for Brown solution treated SiO1.8/SS.

SS (316L)

TiO2 SiOx

Resin

Figure 7. EPMA picture of TiO2 coating SiO2/SS plate and profiles of Si, Ti and Fe (from up to bottom) along arrow drawn.

a

b

c

d

Figure 8. Coating of TiO2 on SiO2/SS foams observed by EPMA before (a), (b), and after calcination (c),(d).

The same optimized treatment was applied to SiO2/SS foams, resulting on the covering by a uniform 15 μm thick layer of TiO2. EPMA profile image and X-ray mapping give evidence for the two successive TiO2 and SiO2 layers (Figure 8). After calcination at 700°C for 2 h, the layer did not resist well, it shrinked and cracked (Figure 8). Tries were done with thinner layers, and finally the highest thickness compatible with mechanical stability of both TiO2 layer and TiO2/SiO2/SS foam itself is

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6 μm (Figure 8). Finally, both TiO2/SiO2/SS plates and foams were impregnated by various amounts of vanadium, using vanadium isopropoxide to be grafted [1,2]. Amounts corresponding to a theoretical monolayer (ca. 3.5 wt% V2O5) and above (up to 20 wt% V2O5) were grafted. XPS analyses showed that the surface vanadium concentration was directly related to the initial concentration of VO(OPr)3 in ethanol solutions [2]. The vanadium concentration being very small, only XPS analysis could reveal its presence. The V2p3/2 photopeak at 517.3 eV means that the oxidation state is mostly V5+. These figures are similar to those found for VOx/TiO2 powders.

4. Conclusion RPECVD technique could be used with success to coat stainless-steel plates in a uniform way by a silica-like layer which is expected to protect the active phases from poisoning. After a proper treatment to enhance its wettability, this layer serves also as a carrier to TiO2-anatase, which is required to enhance the catalytic properties of the active polyvanadate phase. The optimised procedure could be applied to cover stainlesssteel foams for the first time. The catalytic properties of both plates and foams in the oxidative dehydrogenation of propane in specially designed reactors is in progress.

Acknowledgements The Agence Nationale de la Recherche (ANR-France) is aknowledged for its support to project Millicat (ANR-06-BLAN-0126).

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

16.

T. Giornelli, A. Löfberg, E. Bordes-Richard, 2005, Thin Solid Films, 479, 64-72. T. Giornelli, A. Löfberg, E. Bordes-Richard, 2006, Appl. Catal. A: General, 305, 197-203. T. Giornelli, A. Löfberg, L. Guillou, S. Paul, V. Le Courtois, E. Bordes-Richard, 2007, Catal. Today, 128, 201-209. A. Borrás, A. Yanguas-Gil, A. Barranco, J. Cotrino, A. R. González-Elipe, 2007, Phys. Rev. B 76 5303. P. Supiot, C. Vivien, A. Granier, A. Bousquet, A. Mackova, F. Boufayed, D. Escaich, P. Raynaud, Z. Stryhal, J. Pavlik, 2006, Plasma Proc. Polym., 3, 100. L. Guillou, V. Le Courtois, P. Supiot, 2005, Materiaux & Techniques, 93, 335-345. L. Guillou, D. Balloy, P. Supiot, V. Le Courtois, 2007, Appl. Catal. A: General, 324, 42-51. L. Guillou, P. Supiot, V. Le Courtois, Surf. Coat. Technol. 202 (2008) 4233. L.L.P. Lim, R.J. Lynch, S.-I. In, 2009, Appl. Catal. A: General, 365, 214. V. Meille, 2006, Appl. Catal. A: General 315, 1. A. Amassian, P. Desjardins, L. Martinu, 2004, Thin Solid Films 447, 40. S. Pongratz, A. Zoller, 1992, Annu. Rev. Mater. Sci., 22, 279. L. Martinu, D. Poitras, 2000, J. Vac. Sci. Technol. A 18, 2619. F. Callebert, P. Supiot, K. Asfardjani, O. Dessaux, P. Dhamelincourt, J. Laureyns, 1994, J. App. Polym. Sci., 52, 1595. A. Essakhi, A. Löfberg, P. Supiot, B. Mutel, S. Paul, V. Le Courtois, E. Bordes-Richard, Proc. 6th Int. Symp. Polyimides and Other High Temperature/High Performance Polymers: Synthesis, Characterization and Applications, Melbourne, Florida, USA, November 9-11, 2009. Submitted. http://www.lasurface.com

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Washcoating of metallic monoliths and microchannel reactors L. C. Almeida1, F. J. Echave1, O. Sanz1, M. A. Centeno2, J. A. Odriozola2, M. Montes1 1

Department of Applied Chemistry, University of the Basque Country, Paseo Manuel de Lardizabal, 3, ES-20018 San Sebastián, Spain. 2 Inorganic Chemistry Department and Institute of Material Sciences of Seville, University of Sevilla - CSIC, Avenida Américo Vespucio 49, 41092 Sevilla, Spain

Abstract The most important parameters controlling the washcoating of metallic structures from catalytic slurries are reviewed. The slurry must be stable with adequate rheological properties controlled by the solid content, particle size and additives. The metallic substrate must be pre-treated to obtain an adherent surface scale compatible with the coating and presenting appropriate surface roughness. The quality of the produced coating (homogeneity, specific load and adherence) depends essentially on slurry properties (viscosity and solid content) and on the technique used to remove slurry excess. Keywords: washcoating, catalyst preparation, structured catalysts, microchannel reactor

1. Introduction During the last decade, catalysts coated on metallic surfaces are reaching a prime importance [1]. Two types of structures justify this interest: metallic honeycombs and microchannel reactors. When pressure drop and thermal and mechanical resistance are key issues in catalytic processes, metallic honeycombs (monoliths) are the best option [2], while microchannel reactors are in the nucleus of the new tendency to process intensification [3]. The main step for manufacturing metal-based structured catalysts and reactors is the obtention of coatings with the required characteristics: amount loaded, homogeneity and adhesion. For this purpose, different techniques have been proposed [2,4-7], among them, in situ growing, electro-deposition, anodization, CVD, PVD…etc. However, the most popular and versatile is washcoating or dip coating using slurries or sols. It is possible to find in the literature works describing two steps procedures, which start with the coating of the catalytic support and finish with the impregnation of the active phase. In this communication, however, we will consider only washcoating procedures using slurries prepared from previously synthesized catalysts. The washcoating technique allows obtaining coatings on most metallic surfaces with the required homogeneity, amount loaded and adhesion provided that the metallic surface properties (alloy composition and pre-treatment conditions), the coating procedure including excess elimination and, in particular, the properties of the catalyst slurries are well controlled. This communication reviews the main parameters controlling the coating of metallic monoliths and microchannel reactors on the basis of our experience studying different systems: catalysts for Fischer-Tropsch synthesis [8], methane combustion

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L.C. Almeida et al.

[9,10], VOC elimination [11-14], CO [15-17] and phenol [18] oxidation, hydrogenation of sunflower oil [19], methanol and ethanol reforming, WGS and PROX. In these studies, we have tested different alloys, Al-alloyed ferritic steels, AISI 304, brass and aluminium, and different catalysts.

2. Results and discussion The first step to washcoat a metallic substrate is to prepare a stable slurry of the catalyst to be deposited. Next, nature and roughness of the substrate surface must be adequate in order to fix the catalyst coating. Finally, the metallic monolith is washcoated by immersing and withdrawing in the slurry followed by the elimination of the excess. In the following, we will discuss some of the most important variables controlling every step.

2.1. Slurry Stable slurries (non-settling) are obtained when the terminal velocity of the particles is very small. Small velocities are the result of cancelling the gravity force by the drag force. Particle settling is well described in the creeping flow regime that states for Newtonian fluids in which, the terminal velocity depends directly on the square of the particle size and the difference in density between the solid and the fluid, and inversely on the viscosity. Therefore, for a particular catalyst and liquid (usually water) to increase the stability it is convenient to reduce the particle size and to increase the viscosity of the medium. Let analyze these two parameters: particle size and viscosity. 2.1.1. Catalyst particle size Milling can reduce particle size, being ball milling the technique most frequently used in several studies about this parameter [20-23]. To obtain stable slurries of different solids particle size distributions below 10 µm have been proposed [24,25]. Nevertheless, the positive effect of decreasing particle size has a limit since very small particles induce flocculation. Indeed, the smaller the particles size the higher the surface to volume ratio, therefore, the interaction between particles that produces gelling is favoured. In a comparative study two different aluminas were used. Ball milled alumina, d4/3 = 2.8 µm, allowed preparing a slurry with 25% content in solids adequate to washcoat Fecralloy monoliths with 1 mg/cm2 load and 70% adherence. By contrast, Alumina Cabot FA 100, d4/3 = 0.34 µm, produce slurries with the same rheological properties having a maximum of 10% solid content resulting in washcoated monoliths with only 0.3 mg/cm2 load with even lower adherence, 38%. 2.1.2. Viscosity Assuming the Einstein model for the dilute dispersion of hard spheres the viscosity of ideal water slurries (non-interacting spheres) only depends on the solid content, being higher as the solid content increases [26]. Nevertheless, when considering surface interactions additives will play a fundamental role. First, we will consider pH modifiers since polarization of the surface of the oxide particles controls inter-particle interactions. In a first approach, pH values far enough from the pH value of the iso-electric point of the solid will assure the particles mutually repel each other and cannot aggregate, since they will bear electric charges of the same sign. However, the pH not only controls the stability of the slurry via peptization-flocculation, but also the viscosity because it controls the aggregate size. Other additives may be used in order to promote slurry stability, long-chain surfactants containing hydrophilic and hydrophobic groups, for instance, adsorbs on the catalyst surface leading to steric stabilization of the slurry [26]. Thickeners that increase the viscosity can be also used [2,27], inorganic colloids (alumina, silica...) or organic

Washcoating of metallic monoliths and microchannel reactors

27

compounds (polyvinyl alcohol, polyvinylpirrolidone, ammonium methacrylate...) are examples of these additives. However, it has to be considered that surfactants, thickeners and other additives may present competitive and synergic mechanisms making it difficult to a priori predict the behavior of such complex mixtures.

2.2. Washcoating procedure In these procedures, monoliths or, in general, metallic structures are dipped into suitable slurries, kept in the particle dispersion for a certain period of time and finally withdrawn. Once the metallic monolith is withdrawn it must be drained and the excess slurry eliminated. To form a thin oxide layer on the metal surface the metallic structure has to be dried and calcined to suitable temperatures. The procedure ends by evaluating the produced washcoated material by measuring three characteristics: the specific obtained load (mg/cm2), the homogeneity of the coating (by optical or electron microscopy) and the adherence (by the ultrasonic bath test). Relevant parameters controlling this procedure are discussed below. 2.2.1. Dipping and withdrawing velocity For the washcoating of ceramic monoliths this is a key factor in order to ensure the filter cake formation proposed by Kolb et al. [28]. However, for metallic substrates where the porosity of the oxide scale formed on the alloy surface is negligible no special care must be taken to control this velocity. Importantly, the dipping velocity and the time the monolith is kept in the dispersion must be long enough to allow the slurry filling completely the channels, these variables depending on the slurry viscosity. 2.2.2. Substrate surface Passive layer, protective oxide scale produced by the spontaneous oxidation in air or by a specific pre-treatment, covers all the non-noble metals and alloys. The chemical nature and physical appearance of this scale influences relevant aspects of the washcoating process. The first one is the contact angle with the slurry that is also affected by the surface roughness. For low contact angles, the capillary rise will allow a quick channel filling and the substrate is well coated by the slurry. On the contrary, if the contact angle is high slow channel filling must be expected, and at the limit, for contact angles higher than 90º, the substrate became hydrophobic, making it extremely difficult to fill the channels by dipping. The contact angle depends on the surface energies of both solid and liquid and hence, the surface nature and slurry additives modify surface energy of either the solid or the liquid. For instance surfactants, used as slurry additives, modify the liquid surface tension and therefore the contact angle with the metallic substrate. The chemical nature of the oxide scale, in particular the chemical composition and hydroxylation degree, determines the oxide scale surface energy and controls the contact angle with the slurry. These surface energy modifications may result in wetting (hydrophilic) or non-wetting character (hydrophobic) of the slurry. The metal substrate surface is of paramount importance in determining the adhesion of the coating. The formation of chemical bonds (chemical compatibility) between the oxide scale and the slurry may enhance the adhesion of the coating; however, as pointed out by Agrafiotis et al., the washcoating adhesion to the support takes primarily place through mechanical mechanisms such as “anchoring” and interlocking of the washcoat particles to the surface irregularities of the support and to a much lesser extent via chemical or affinity mechanisms [24]. Depending on the alloy nature and treatment of metallic substrates surface irregularities or roughness can be tailored [2]. Figure 1 shows two examples of proper ratio between the dimension of surface roughness and catalyst particle size: the catalyst particle must be smaller than the surface roughness to fit inside.

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a)

b)

Figure 1. a) 20%Co/Al2O3 on Fecralloy calcined in air 22 h at 900ºC. b) Pt/ZSM5 on aluminum anodized under cracking conditions [29].

2.2.3. Size and shape of the channels Capillary forces acting during coating and drying produce a certain accumulation of catalyst at the channel corners. As Figure 2 shows, the relative importance of these accumulations depending on size and shape (corner angle) of the channels.

Figure 2. Micrographs showing catalyst accumulation in metallic substrates (Fecralloy). Left side: 20%Co/Al2O3 on square (700 x 700 µm) microchannels. Right side: ZSM-5 deposited on micromonoliths (222 cells/cm2) [30].

The influence of the size of the channel on the coating adhesion was studied by means of different metallic substrates. Homemade Fecralloy monoliths of 350 cpsi (M sample) and 1180 cpsi (µM sample), Duocel aluminum foam of 10 ppi (E10 sample) and Fecralloy plates with square microchannels of 0.7 x 0.7 mm2 (µP sample) were coated with a steam reforming catalyst. The non-settling slurry contained 12.3% of catalyst (7.5%Ni/La-Al2O3), 3.8% polyvinyl alcohol and 8.5% colloidal alumina in water Nyacol AL 20); this formulation resulted in a viscosity of 9.6 mPa·s (measured at 3240 s-1). After coating, the excess of slurry was eliminated by centrifugation at 500 rpm for 2.5 min in the case of monoliths and foams, and by air blowing for the microchannels plates. Several successive coatings were carried out until the desired load was reached; a drying step (120ºC for 30 min) is always performed between successive coatings. Figure 3 presents the obtained specific load, and Table 1 the adhesion and the textural properties.

Washcoating of metallic monoliths and microchannel reactors

Specific load (mg/cm2)

4

3

μM - 6 μM - 7 M-5 M-6

29

E-5 E-6 μP - 3 μPS - 3

2

1

0 1

2

3

4

5

6

7

Number of coatings

Figure 3. Specific load vs. coating number obtained on different metallic substrates with a Ni/LaAl2O3 slurry.

The results in figure 3 clearly indicate that the coating procedure is very reproducible obtaining additive loads by repeated coatings. The specific load is similar for different geometries, and only slightly higher load were obtained on microchannel plates as a result of the different method used to eliminate the excess. The textural properties of the catalyst were preserved on the coating. The adhesion was very high for all the coating inside channels, increasing on decreasing the hydraulic diameter as a consequence of geometrical constraints. In the case of foams, the adhesion was significantly lower due to the external character of the coating around the foam struts (no geometrical constraints). Table 1. Textural properties and adherence of Ni/La-Al2O3 coatings on different substrates. The textural characteristics of the powder obtained after drying the slurry are shown for comparison. 2

SBET(m /g) Vp (cm3/g) Dp (nm) Hydraulic diameter (µm) Adherence (%)

Powder 51 0.26 16.9

-

μM-7 56 0.26 15.0

M-7 52 0.30 17.3

373

835

98

89

E10-5 -

μPS-3 700

51

93

2.2.4. Viscosity and additives The key parameter that controls the coating results is viscosity. Low viscosities allow to obtain highly adherent and homogeneous coatings but with low specific loads. Thus for obtaining the target loading numerous coating are required. On the contrary, high viscosity will allow high specific load per coating although the homogeneity is lower (accumulations and, at the limit, channels blocking) resulting in lesser adherent coatings. The optimal viscosity ranges between 5–30 mPa·s as proposed by several authors [3135]. However, the non-Newtonian character of the slurries with high solid contents makes difficult to compare the viscosity values obtained at different shear rates. The rheological properties are mainly controlled by the solid content of the slurry and the peptization step (pH and additives) [33]. But, as mentioned above, the role of additives (organics or inorganics) is complex producing synergic or competitive effects in the process variables. Table 2 resumes studies of the influence of these variables on

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the washcoat of Co catalysts on different supports over Fecralloy monoliths (350 cpsi, pretreated at 900ºC for 22 h). Polyvinyl alcohol, PVOH, and colloidal alumina, CA (Nyacol S20, 20% solid content) were used as additives with three different supports presenting different particle size: Titania Millenium G5, Alumina Cabot FA-100 and Alumina Spheralite SCS 505 milled to different particle size distributions. Slurries were prepared with the maximum allowable solid content reaching a viscosity that ranges between 5 and 15 mPa·s. The slurry excess was removed by centrifuging at 500 rpm for 5 min, except for TiO2 + CA slurry that presented high viscosity and required 1200 rpm to prevent channel blocking. The coating procedure was repeated five times with drying at 120ºC between coatings. Finally, coated monoliths were calcined at 500ºC for 4 h. Table 2. Effect of slurry composition on viscosity, specific load and adherence of washcoated Fecralloy monoliths. Slurry TiO2 (27.3%) TiO2 + PVOH (27.3 + 6.4%) TiO2 + CA (27.3 + 13.2%) TiO2 + PVOH+CA (27.3 + 6.6 + 6.4%) Al2O3CABOT (11.7%) Al2O3CABOT + PVOH (11.7 +1.3%) Al2O3CABOT + CA (11.7 +1.3%) Al2O3CABOT + PVOH+A.C. (11.7 +1.3 +1.3%) Al2O3SPHERALITE(1) + CA (17.7 + 4.3%) Al2O3SPHERALITE(2) + CA (17.7 + 4.3%) Al2O3SPHERALITE(3) + CA (17.7 + 4.3%)

D4,3 (µm) 1.6 1.6 1.6 1.6 0.34 0.34 0.34 0.34 2.8 6.6 13.3

Viscosity (mPa·2) 5.2 7.6 21.8 12.3 6.6 5.6 11.8 7.3 10.8 10.5 11.8

Specific load Adherence (mg/cm2) (%) 0.41 24 0.41 27 1.14 73 1.30 76 0.32 40 0.22 100 0.23 96 0.22 95 1.05 97 1.40 93 1.55 91

As discussed in section 3.1.1, the small particle size of the alumina Cabot did not allow solid contents higher than 11.7% without jellification. Therefore, low specific load were always obtained. On the other hand, alumina Spheralite shows a wide particle size distribution (Figure 4) allowing high specific load with moderate solid contents. Nijhuis et al. proposed a model in which the smaller particles are allocated between the bigger ones increasing the adherence [32].

10 8 6

Co/Al2O3 Spheralite (1) Co/Al2O3 Spheralite (2) Co/Al2O3 Spheralite (3) dpd4,3 = 6.6 μm

Volume (%)

12

dpd4,3 = 0.34 μm

14

Co/Al2O3 Cabot FA100 dpd4,3 = 2.8 μm

4

dpd4,3 = 13.3 μm

2 0 0.01

0.1

1

10

100

1000

Particle size (mm)

Figure 4. Particle size distributions (left) and schematic representation of the coating with alumina Spheralite presenting a wide particle size distribution.

Washcoating of metallic monoliths and microchannel reactors

31

PVOH slightly increased the viscosity of the titania slurry without changing either the specific load or the adherence. However, in the case of the alumina Cabot the viscosity decreased resulting in low specific loads, although the adherence was excellent which is coherent with the low load. Nevertheless, adherence change is very interesting taking into account that PVOH disappears during calcination. Colloidal alumina (CA) had a very positive effect in the titania slurry, increasing viscosity and thus load, but specially significantly increasing adherence. This is probably due to the increase in chemical compatibility between the alumina scale covering Fecralloy and the titania particles. The simultaneous use of PVOH and CA with titania allowed reducing the viscosity while increasing both load and adherence. With alumina Cabot the use of both additives, simultaneously or separately, resulted in lower loads, but the adherence increased dramatically. 2.2.5. Elimination of the slurry excess Usually, excess slurry is removed either by air blowing [2,4,24,28,32] or centrifuging [29,36]. In general, by gravitational draining or by applying some form of pressure or vacuum to clear the channels of the excess but the adhered catalyst layer. Air blowing is easier, in particular when big structured devices are considered, but should be carefully executed to prevent heterogeneity [37]. Figure 5 presents a comparison of the specific load obtained coating microchannel plates, µP (Fecralloy plates with 10 microchannels of 0.7 x 0.7 x 20 mm3) and Fecralloy micromonoliths, µM (1180 cpsi) using a slurry adjusted to pH 3 using HNO3 and containing 20% catalyst (20%Co-0.5%Re/Al2O3) and 6% colloidal alumina (Nyacol AL20). The loading obtained when the excess is removed by centrifugation is half the one obtained after eliminating the excess by air blowing, whatever the structure, microchannel or micromonoliths, considered. Menon et al. [36] has previously reported a similar trend.

Figure 5. Effect of procedure used to eliminate the slurry excess on the specific load.

2.2.6. Drying During drying of slurries and colloids, strong capillary forces are generated contracting the solid coating. This can produce cracks and detachments from the substrate surface. When this phenomenon reduces the coating adherence, two strategies may be used. The first one is the use of additives, like PVOH or surfactants, to reduce surface tension. The second approach eliminates the capillary forces in using freeze-drying, but special care must be taken during freezing to prevent movement of the liquid phase, the freeze-dryer may allow keeping the monolith horizontal while continuously rotating it around its axis [32].

32

L.C. Almeida et al.

3. Conclusions To washcoat metallic substrates slurries must be stable and present moderate viscosities (5–30 mPa·s). Particle size (usually around 1 to 10 µm), solid content (as high as possible) and additives control these properties. The protecting scale covering the metallic substrate must be adherent, compatible with the catalytic coating and presenting a surface roughness that allow the catalyst particles to fit inside surface grooves favoring mechanical anchoring of the catalyst to the scale. On washcoating, the obtained specific load depends on the viscosity, the solid content and the method used to remove the slurry excess. In general, to obtain the target load repeated coatings that generate thin layers resulted in more homogeneous layers. The coating adherence depends on the compatibility with the substrate surface (chemical and physical) and the use of additives like binders remaining after calcination or compounds allowing a soft drying to prevent cracks and detachments. Mechanical constraints increase with decreasing channel size improving adherence.

Acknowledgements Financial support by MEC (MAT2006-12386-C05), UPV/EHU (GUI 07/63) and Junta de Andalucía (P06-TEP-01965) is gratefully acknowledged.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.

A. Cybulski, J. A. Moulijn (Eds.), 2006, Structured Catalysts and Reactors, 2nd Edition, Marcel Dekker, Inc., New York. P. Avila, M. Montes, E. E. Miro, (2005) Chem Eng J 109, 11-36. K. Jahnisch, V. Hessel, H. Lowe, M. Baerns, (2004) Angewandte Chemie-International Edition 43, 406-446. V. Meille, (2006) Applied Catalysis A-General 315, 1-17. N. Burgos, M. A. Paulis, M. Montes, 2003, Journal of Materials Chemistry, 13, 6, 14581467. I. Barrio, I. Legorburu, M. Montes, M. I. Domínguez, M. A. Centeno, J. A. Odriozola, 2005, Catalysis Letters, 101, 3-4, 151-157. D. M. Frías, S. Nousir, I. Barrio, M. Montes, L. M. Martínez, M. A. Centeno, J. A. Odriozola, 2007, Applied Catalysis A-General, 325, 2, 205-212. L. C. Almeida, O. González, O. Sanz, A. Paúl, M. A. Centeno, J. A. Odriozola, M. Montes, 2007, Natural Gas Conversion VIII, 167, 79-84 E. Arendt, A. Maione, A. Klisinska, O. Sanz, M. Montes, S. Suarez, J. Blanco, P. Ruiz, 2008, Applied Catalysis A-General, 339, 1, 1-14. E. Arendt, A. Maione, A. Klisinska, O. Sanz, M. Montes, S. Suarez, J. Blanco, P. Ruiz, 2009, Journal of Physical Chemistry C, 113, 37, 16503-16516. N. Burgos, M. Paulis, J. Sambeth, J. A. Odriozola, M. Montes, 1998, Preparation of Catalysts VII, 118, 157-166. N. Burgos, M. Paulis, M. M. Antxustegi, M. Montes, 2002, Applied Catalysis BEnvironmental, 38, 4, 251-258. B. P. Barbero, L. Costa-Almeida, O. Sanz, M. R. Morales, L. E. Cadus, M. Montes, 2008, Chemical Engineering Journal, 139, 2, 430-435. O. Sanz, F. J. Echave, M. Sanchez, A. Monzon, M. Montes, 2008, Applied Catalysis AGeneral, 340, 1, 125-132. L. M. Martínez, D. M. Frías, M. A. Centeno, A. Paúl, M. Montes, J. A. Odriozola, 2008, Chemical Engineering Journal, 136, 2-3, 390-397. O. Sanz, L. M. Martínez, F. J. Echave, M. I. Domínguez, M. A. Centeno, J. A. Odriozola, M. Montes, 2009, Chemical Engineering Journal 151, 324-332.

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17. L. M. Martínez., O. Sanz, M. I. Domínguez, M. A. Centeno, J. A. Odriozola, 2009, Chemical Engineering Journal, 148, 1, 191-200. 18. L. M. Martínez , M. I. Domínguez, N. Sanabria, W. Y. Hernández, S. Moreno, R. Molina, J. A. Odriozola, M. A. Centeno, 2009, Applied Catalysis A-General, 364, 1-2, 166-173. 19. J. F. Sanchez, O. J. Gonzalez Bello, M. Montes, G.M. Tonetto, D.E. Damiani, 2009, Catalysis Communications, 10, 10, 1446-1449. 20. M. Z. He, Y. M. Wang, E. Forssberg, 2004, Powder Technology, 147, 1-3, 94-112. 21. Shi, F. N.; Napier-Munn, T. J., 2002, International Journal of Mineral Processing, 65, 3-4, 125-140. 22. He, M. Z.; Wang, Y. M.; Forssberg, 2006, E., Powder Technology, 161, 1, 10-21. 23. Tanaka, S.; Kato, Z.; Uchida, N.; Uematsu, 2003, K., American Ceramic Society Bulletin, 82, 8, 9301-9303. 24. C. Agrafiotis, A. Tsetsekou, A. Akonomakou, 1999, J. Mater Sci Lett, 18, 1421-1424. 25. J. R. González-Velasco, M. A. Gutiérrez-Ortiz, J. L. Marc, J. A. Botas, M. P. GonzálezMarcos, G. Blanchard, 2003, Ing. Eng. Chem. Res., 42, 311-317. 26. T. C. Patton, 1979, Paint Flow and Pigment Dispersions, 2nd ed. John Wiley, New York. 27. C. Agrafiotis, A. Tsetsekou, 2000, J Mater Sci, 35, 951-960. 28. W. B. Kolb, A. A. Papadimitriou, R. L. Cerro, D. D. Leavitt, J. C. Summers, 1993, Chemical Engineering Progress, 89, 61-67. 29. O. Sanz, L. C. Almeida, J. M. Zamaro, M. A. Ulla, E. E. Miro, M. Montes, 2008, Applied Catalysis B-Environmental, 78, 1-2, 166-175. 30. A. Eleta, P. Navarro, L. Costa, M. Montes, Microporous and Mesoporous Materials, 2009, 123, 113-122. 31. X. D. Xu, J. A. Moulijn, 1998, Preparation of Catalysts VII, 118, 845-854. 32. T. A. Nijhuis, A. E. W. Beers, T. Vergunst, I. Hoek, F. Kapteijn, J.A. Moulijn, 2001, Catal Rev, 43, 345-380. 33. D. -J. Liu, D. R. Winstead, N. Van Den Bussche, 2003, US Patent 6,540,843 B1. 34. V. Meille, S. Pallier, G.V.S.C. Bustamante, M. Roumanie, J.P. Reymond, 2005, Applied Catalysis A – General, 286, 232-238. 35. L. Giani, C. Cristiani, G. Groppi, E. Tronconi, 2006, Applied Catalysis B-Environmental 62, 121-131. 36. P. G. Menon, M. F. M. Zweinkels, E. M. Johansson, S. G. Jaras, 1998, Kinetics and Catalysis, 39[5], 670. 37. A. L. Tonkovich, B. L. Yang, T. Mazanec, F. P. Daly, S. P. Fitzgerald, R. Arora, D. Qiu, B. Yang, S. T. Perry, K. Jarosh, P. W. Neagle, D. J. Hesse, R. Taha, R. Long, J. Marco, T.D. Yuschak, J. J. Ramler, M. Marchiando, 2005, Tailored and uniform coatings in microchannel apparatus, US Patent 2005/0244304 A1.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Monolithic catalysts for the decomposition of energetic compounds Dan Amariei,a Rachid Amrousse,a Yann Batonneau,a Rachid Brahmi,a Charles Kappensteina and Bruno Cartoixab a

LACCO (Laboratoire de Catalyse en Chimie Organique), UMR CNRS 6503, University of Poitiers, Faculty of Sciences, 40 Avenue de Recteur Pineau, Poitiers 86022, France b CTI (Céramiques Techniques et Industrielles), F-30340 Salindres France.

Abstract Pellet-based catalysts have been developed more than 60 years ago for the decomposition of hydrogen peroxide and hydrazine for propulsion applications. Cellular ceramic supports are now proposed to replace such catalyst support for monopropellant decomposition or bipropellant ignition. Different honeycomb supports have been manufactured by CTI Company and used as catalyst support for lab-scale reactor as well as for full-scale application. The support parameters are the chemical nature (cordierite, mullite, mullite–zircone, SiC…), the channel shape and density. For fullscale reactors, dedicated apparatus have been developed to control the key parameters during the preparation of the catalysts: quality and homogeneity of the wash-coating layer, impregnation conditions to reach a high loading level of active phase. Keywords: cellular ceramics, honeycomb monoliths, propulsion, energetic compounds

1. Introduction The catalytic decomposition of energetic compounds is used for propulsion application (launcher, satellites and missiles) and gas generator (e.g. rescue systems) [1,2]. The role of the catalyst is to trigger the decomposition or the ignition of the propellant and the resulting hot gases are expelled through a converging-diverging nozzle generating thrust. Figure 1 shows the scheme of a catalytic engine for monopropellant and Figure 2 displays the 5 N engine used on the Pioneer 10 and 11 mission to the outer planets and beyond the solar system (1972-1997). The propellant is hydrazine N2H4 decomposed on Shell 405, an Al2O3-supported iridium catalyst. catalyst

thrust pressurized liquid monopropellant

valve

hot gases injector

nozzle

Figure 1. Scheme of a remotely controlled monopropellant catalytic engine.

The catalytic bed must be able to start the decomposition of the energetic propellant (H2O2, N2H4, N2O…) at low temperatures (to avoid preheating of the engine) with a short ignition delay (10 to 20 ms). It must present very good thermal and mechanical properties to resist and survive very severe conditions (frequent thermal and overpressure shocks at high flow rates) for long term use (up to 15 years or more) without performance degradation. The catalyst porosity must be adapted to very high reaction rates with good heat and mass transfer during the transformation into hot gases.

D. Amariei et al.

36

The most important catalyst is iridium supported on alumina [2]; it was developed for hydrazine decomposition about 60 years ago at the beginning of space exploration. Silver gauzes have been used too for the decomposition of hydrogen peroxide for bipropellant rockets using kerosene as a fuel.

Figure 2. Dual thruster MRE-1 (TRW). The ellipses show the decomposition chamber containing the catalyst.

The catalysts developed during the fifties and sixties were prepared on pellet substrates, displaying sphere or more irregular grain shape, and made of porous transition alumina; this alumina is generally manufactured by a dedicated preparation procedure to resist thermal shocks. The development of car exhaust catalysts leads to the use of monoliths as catalyst supports [3,4] and cellular ceramics are today easily commercially available. Recently, we proposed the use of cellular ceramic as catalyst support for the decomposition of hydrogen peroxide [5,6]. Compared to extrudates or pellets, honeycomb monoliths show many advantages: (i) lower pressure drop, (ii) better thermal shock and attrition resistance with limited fine formation, (iii) uniform flow distribution and mass/heat transfer conditions, (iv) shorter diffusion length, and (v) large heritage from cleaning of car exhaust gases. Therefore, the development of monolithic reactors for propulsion applications represent a very attractive alternative for macro- or micro-propulsion systems and the following propellants are currently under study: (i) H2O2 for monopropellant or hybrid engine; (ii) nitrous oxide; (iii) energetic ionic liquids like HAN (hydroxylammonium nitrate NH3OH+NO3-); and more recently (iv) cryogenic H2-O2 mixtures [7]. We present in this paper an overview of the current development of monolith-based catalysts at the lab scale (decomposition of hydrogen peroxide) as well as at full-scale (ignition of cold H2-O2 mixtures).

2. Geometric parameters of cellular ceramics

2.1. Honeycomb monoliths Honeycomb monoliths are described by simple geometric parameters defined in Figure 3, and determined from channel shape (square, circle, hexagon or triangle), channel dimensions and wall thickness. The mathematical relations are presented below and examples of typical values are given in Table 1. Channel density [mm-2]:

n = 1 / dch2

Channel density [cpsi]:

n(cpsi) = 25.42 / dch2

Geometric surface area [mm-1]:

GSA = 4n (dch – lch)

Open fraction area:

OFA = n (dch – lch)2

Monolithic catalysts for the decomposition of energetic compounds Hydraulic diameter [mm]:

Dh = 4 x OFA / GSA

Thermal integrity factor:

TIF = dch / lch

Mechanical integrity factor:

MIF = lch2 / dch (dch – lch)

Channel size dch /mm

37

Open fraction area (OFA)

Wall thickness

Geometric surface area (GSA) /mm-1

lch /mm

Figure 3. Geometric parameters for a honeycomb monolith support with square channels. Table 1. Values of the geometric parameters for cordierite monoliths with square channels; cpsi = channels per square inch. (Samples supplied by CTI Company). Channel density

dch /mm

Ich /mm

n /mm-2

n /cpsi

GSA /mm-1

OFA

Dh /mm

MIF

TIF

400 cpsi

1.30

0.35

0.592

382

2.25

0.534

0.95

0.099

3.71

100 cpsi

2.40

0.40

0.174

112

1.39

0.690

2.00

0.033

6.00

2.2. Foams The cell density in foams is defined in ppi (pores per inch) and the calculation of GSA is more complicated. The free volume of the foam can be described as a network of interconnected cells of complicated geometry. To overcome sophisticated calculations, the cells can be modelized as a set of partially open spheres with constant diameter; the open part of the sphere realizes thus the interconnection of the foam. From Figure 4-a, the average cell size (i.e. sphere diameter) is estimated about 1 mm for 20 ppi and 0.6 mm for 30 ppi, whereas the interconnection can be roughly estimated to represent about half the surface of the spheres. With these assumptions, it is possible to obtain a rough value of the GSA and to make a useful comparison with honeycomb monoliths. The calculated values are 2.33 mm-1 for 20 ppi foam and 3.24 mm-1 for 30 ppi foam; the first value compares well with the corresponding value for 400 cpsi monolith (2.25 mm-1).

3. Preparation of lab-scale cellular ceramic catalysts Honeycombs made of different materials (cordierite, mullite, mullite–zirconia, alumina, silicon carbide, yttrium-stabilized zirconia) have been supplied by CTI Company [8] as supports to prepare catalysts for lab-scale reactors. They present square channels and a total volume between 1 and 2 cm3. The external shape can be parallelepiped or cylindrical. The preparation of square-shaped monoliths for H2O2 decomposition has been presented at the previous Catalyst Preparation Symposium [5], focusing on the influence of the washcoat procedure and the nature of the active phase on the catalytic activity. Figure 4-b displays one example of such catalysts, as received and after washcoating, impregnation and reduction steps.

38

D. Amariei et al.

Figure 4. 20 ppi (left) and 30 ppi (right) alumina foams (a); lab-scale monolith before (b) and after (c) wash-coat, impregnation and reduction procedures (Ag-based catalyst).

Cylindrical monoliths are presented in Figure 5. They have been obtained by cutting a honeycomb block but suffer damaged channels (Figure 5-a) or directly manufactured; in this case, the external distorted channels are clogged to preserve only the square channels (Figure 5-b). For cylindrical foams with external irregularities (Figure 5-c) leading to preferential pathways, they can by wrapped with refractory fibers and blocked inside the decomposition chamber.

Figure 5. Cylindrical supports: cut in a honeycomb block (a); directly manufactured (b); cut in a foam block (c).

An important part of the current work is carried out in the field of the FP7-European project GRASP (Green Advanced Space Propulsion) [9]. As a GRASP partner, CTI Company manufactures tailored ceramic samples that differ in nature, channel shape (squares or triangles) and channel density (400 and 600 cpsi). One example of procedure to prepare the catalysts is as follows: Step 1, acidic treatment to clean the surface. The as-received monoliths are put for 1 h in a concentrated solution of nitric acid, then heated in a muffle furnace up to 300°C. Step 2, preparation of colloidal suspensions. Two suspensions (sol) have been prepared depending on the nature of the active phase: (i) Sol-A was obtained by mixing diluted nitric acid, pseudo-boehmite (AlOOH, Disperal P2, 200 m2.g-1, Sasol Compagny) and urea; (ii) Sol-B was made from aluminum chloride, aluminum powder and urea. They were mixed using a high-shear mixer (23 000 rpm). After drying and heating at 500°C in air, the resulting powders display the characteristic XRD profile of -alumina and the specific surface area is 100 m2 g-1 for Sol-A and 255 m2 g-1 for Sol-B. Step 3, monolith washcoating. The monoliths are put into the sol for 1 h with careful control of the temperature and viscosity. Then, they are retrieved and the channels are flushed with a smooth argon flow before drying and thermal treatment. Up to three successive washcoat stages can be carried out to get a given porous wash-coat content. Step 4, impregnation with the active phase precursor. The objective is to reach 10-20 wt.-% active phase based on the washcoat layer. For platinum-based catalysts, the washcoated samples were immersed in an aqueous solution of chloroplatinic acid under mechanical stirring, and then heated in a sand bath at 50-60°C until complete dryness. Finally, the samples were thermally treated in air and reduced under H2/He flow. The SEM view of the wash-coat layer deposited on cordierite from both colloidal suspensions shows that Sol-A leads to a more homogeneous layer than Sol-B (Figure 6).

Monolithic catalysts for the decomposition of energetic compounds

39

Figure 7 displays the XRD profile of Pt-based catalysts prepared on cordierite or silicon carbide monoliths. The formation of metallic platinum is clearly evidenced; from the profile of Pt diffraction peaks; the average size is estimated about 13 nm. Sol B

Sol A

Figure 6. SEM view of the wash-coat layer deposited on cordierite. 1000

Intensity /counts

1400

800

Intensity /counts

1200

Pt

1000

GC013 (+300)

600

Pt 400

800

Pt Pt

600 GS045 (+300) 400

200

GC023 200 GS056 0

0 30

35

40

45

50 2 theta

30

35

40

45

50 2 theta

Figure 7. XRD profile of Pt-based catalyst on cordierite (left) and SiC (right) monoliths. The platinum metal diffraction peaks are indicated. The other peaks origin from the support.

4. Preparation of full-scale catalysts

4.1. Objectives The catalytic ignition of cold hydrogen-oxygen mixtures is of current interest for the French Space Agency (CNES, Centre National d'Etudes Spatiales) [7]. The first application of this ignition system is a small upper stage thruster (class 10 to 100 N) using gaseous O2 and H2 in order to settle the liquid in the tanks during ballistic coast phases. A second, more prospective application could be the thrust chamber of a larger liquid O2 - liquid H2 cryogenic engine such as the VINCI engine developed by European companies. In both cases, catalytic ignition allows combustion initiation at low temperatures, 180 to 300 K, without the need of a spark delivered by high voltage electrical discharges. As the stoichiometric O2-H2 mixture leads to temperatures higher than 3000 K, both applications need to work in hydrogen excess (fuel rich) or oxygen excess (fuel lean), to control the maximum temperature and thus to avoid any thermal deterioration of the catalytic bed.

4.2. Supports The supports were specially manufactured by CTI Company; they are cylindrical honeycomb-type ceramic monoliths 50 mm diameter and 100 mm length, i.e. two orders of magnitude larger than the lab-scale catalysts (Figure 8-a, b) [10]. For all samples, the active phase is deposited after specific wash-coating procedures to increase the specific surface area of these supports from 0.5 m2 g-1 for the as-received monolith to 22 m2 g-1

40

D. Amariei et al.

for the wash-coated monoliths. The catalyst samples have been prepared by LACCO, which was in charge of optimizing the catalyst preparation method with regards to the functional objectives aimed at Air Liquide dedicated test bench.

100 mm 50 mm

a

b

c

d

Figure 8. View of a mullite 400 cpsi (a) and cordierite 100 cpsi (b) monoliths; furnace (c) and quartz reactor (d) for thermal treatments; the red ellipse shows the HCl trap.

4.3. Catalyst preparation Different active phases have been deposited by impregnation method onto the washcoated monoliths. Several preparation parameters have been tested: (i) the nature of the ceramic used for the base monolith supports (mullite: 3Al2O3·2SiO2 or cordierite: 2MgO·2Al2O3·5SiO2); (ii) the channel density (100 or 400 cpsi, i.e. channels per square inch); (iii) the nature of the active phase (Ir, Pt, Pd or Rh); (iv) the wash-coating procedure; and (iv) the content of the active phase (15 to 40 wt.-% of the washcoat mass or 10 to 40 g L-1). Table 2 gathers typical values of different representative catalyst samples. Table 2. Typical values of iridium catalysts supported on different monolithic carriers. support

Cordierite, 400 cpsi

Cordierite, 100 cpsi

Mullite, 400 cpsi

134.30

106.07

154.34

Volume /cm

196

196

196

Wash-coat mass /g

18.35

9.37

13.10

Iridium mass /g

4.43

2.57

4.25

Initial mass /g 3

For the wash-coating step, the monoliths are dipped into the colloidal suspension, in a specially lab-made double-wall beaker for a fine control of the temperature. The monoliths are periodically removed and turned over. The final unclogging of the channels is performed under weak flow of argon to remove excess colloidal solution. Beside the composition of the suspension, key parameters are the viscosity, the temperature and the duration of the wash-coating process which must be carefully

Monolithic catalysts for the decomposition of energetic compounds

41

controlled. The wash-coated monoliths are dried in a dedicated system allowing horizontal rotation to ensure a homogeneous distribution of the coating layer. Finally, thermal treatment of the coated monoliths at higher temperature (between 400°C and 700 °C) was carried out under air in a muffle furnace. The impregnation is performed generally from an aqueous solution of known concentration of the metal precursor. The procedure is to immerse overnight the coated monoliths into the precursor solution under mechanical agitation. The excess of the solution is then evaporated. When the precursor solution is completely evaporated, the impregnated monoliths are carefully dried before thermal treatment. This is carried out in a lab-made quartz reactor adapted to the size of the monolithic catalysts (Figure 8-c and d). For platinum, rhodium and iridium active phases, this treatment corresponds to a reduction under hydrogen flow diluted in helium. During the reduction of metal chloride precursors, the reaction produces quantitatively gaseous hydrogen chloride, e.g. in the case of platinum: H2PtCl6(s) + 2 H2(g) Pt(s) + 6 HCl(g) Therefore, the reduction extent can be easily controlled by determining the amount of produced HCl; this is done by a down-stream trap containing a basic solution; after the reduction, the rest of basic solution is titrated; key parameters are flow rate and composition of hydrogen/helium mixture to avoid redox decomposition of the precursor and formation of chlorine as it could be the case for platinum or iridium precursors H2PtCl6 or H2IrCl6.

4.4. Catalyst characterization Pre-tests and post-tests chemical analyses and characterizations have also been performed to verify the characteristics of catalysts using different methods [10]: - X-ray diffraction discloses the presence of metallic Pt, Rh or Ir particles; - metallic accessibility from hydrogen chemisorption leads to values in the range 30–33 % for iridium dispersion, (2.7 to 3.0 nm average crystallite size) which reduces to about 20% after tests; - transmission electron microscopy associated with EDX analysis to verify the presence of iridium in the wash-coat layer and the absence of impurities as chlorine; - specific surface area determination in the range 20 to 25 m2 g-catalyst-1; - elemental analyses of active phase by ICP-OES technique which permit to control the active phase variation along the monolith axis.

5. Conclusion One of the major drawbacks of honeycomb monolith for monopropellant decomposition is the initial laminar flow in the straight channels which delay the decomposition reaction. One way to overcome this difficulty is to increase the monolith length or to use foams instead of honeycombs to create turbulence but at the expense of higher pressure drop and lower mechanical strength. Thus, a comparison of catalytic activity between both monolith types would be very useful. Another way proposed by Robocasting Enterprises Company [11] is to create turbulence and preserve a periodical structure; this is done by manufacturing the monoliths from alternating rods as it can be seen in Figure 9.

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Figure 9. Monolith made of alternating rods (Robocasting Enterprises).

Acknowledgements We aim to thank CNES, Air-Liquide Company, and European Community (FP7 GRASP project) for funding different parts of this study. CTI Company is acknowledged for the supply of the different monoliths.

References R. W. Humble, G. N. Henry, W. J. Larson, Space propulsion analysis and design, MacGraw-Hill, New-York, 1995. 2. Y. Batonneau, C. Kappenstein, and W. Keim, “Catalytic decomposition of energetic compounds: gas generator, propulsion”, in Handbook of Heterogeneous Catalysis, G. Ertl, H. Knözinger, F. Schüth, and J. Weitkamp Eds, Vol. 5, Chapter 12.7, VCh-Wiley, Weinheim, Germany, 2008, pp. 2647-2676. 3. A. Cybulski, J. A. Mouljin, “Structured Catalysts and Reactors”, Marcel Dekker, 1997. 4. T. A. Nijhuis, A. E. W. Beers, T. Vergunst, I. Hoek, F. Kapteijn, J. A. Moulijn, “Preparation of monolithic catalysts”, Catalysis Reviews - Science and Engineering, 2001, 43 (4), 345-80. 5. R. Brahmi, Y. Batonneau, C. Kappenstein, P. Miotti, M. Tajmar, C. Scharlemann and M. Lang, “Ceramic catalysts for the decomposition of H2 O2”, Studies in Surface Science and Catalysis, 2006, 162, 649-656. 6. C. Kappenstein, R. Brahmi, D. Amariei, Y. Batonneau, S. Rossignol, J. P. Joulin, “Catalytic decomposition of energetic compounds-Influence of catalyst shape and ceramic substrate”, AIAA Papers 2006-4546. 7. P. Bravais, Y. Batonneau, D. Amariei, C. Kappenstein and M. Théron, “Experimental investigation of catalytic ignition of cold O2/H2 mixtures,” Space Propulsion 2008, Heraclion, Greece, May 2008, 3AF Publisher, Paper S51. 8. Website: http://www.ctisa.fr/ 9. Website: http://www.grasp-fp7.eu/grasp/ 10. R. Amrousse, R. Brahmi, Y. Batonneau, C. Kappenstein, M. Theron, and P. Bravais, “Catalytic Ignition of Cold H2/O2 Bipropellant Mixtures”, AIAA Paper 2009-5473. 11. Website: http://www.robocasting.net/ 1.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Glass fiber materials as a new generation of structured catalysts Bair S. Bal’zhinimaev, Evgenii A. Paukshtis, Olga B. Lapina, Alexey P. Suknev, Viktor L. Kirillov, Pavel E. Mikenin, Andrey N. Zagoriuko Boreskov Institute of Catalysis SB RAS, prospekt Akademika Lavrentieva, 5, 630090, Novosibirsk, Russia

Abstract Molecular structure of Zr-silicate glass fiber materials was studied to evaluate their potentiality in catalysis. Basing on NMR and IRS data the framework structure where Zr(IV) cations serve as a connectors linked with a few SiO4 tetrahedra was proposed. The effective ways of transition ions (Pt, Pd, Co) incorporation into the glassmatrix and their stabilization in highly dispersed state (clusters) were found. The obtained glass fiber based catalysts showed high activity and selectivity in oxidation of hydrocarbons and selective hydrogenation of acetylene-ethylene feedstock. The example of successful design of structured bed and commercialization of VOC removal process is presented.

Keywords: glass fiber catalysts, structure, clusters, oxidation, selective hydrogenation 1. Introduction The glass fiber materials of silicate origin are for a long time produced in industry and are widely used as perfect heat and electric insulators. At the same time these materials are much less known as catalyst supports despite their obvious advantages such as high thermal stability, high mechanical strength, improved hydrodynamic properties as well as the possibility to create the new types of structured catalyst beds and the catalytic reactors with new flexible designs. The glass fiber catalysts (GFCs) reveal unique catalytic performance in many oxidation reactions due to the ability of the glass to stabilize small nanoclusters or separate ions of transient metals in the bulk of fibers [1-3]. It results in high catalytic activity and high catalyst resistance to poisoning and deactivation in aggressive reaction media. Notably, excellent catalyst performance is achieved at very low noble metal content (0.01-0.02% wt.) thus providing quite reasonable pricing for the catalyst. The present paper devoted to study of molecular structure of promising Zr-silicate glass fiber materials, features of active component introduction into the bulk of glass matrix, as well as testing of obtained catalysts in deep oxidation of hydrocarbons and selective hydrogenation of acetylene.

2. Zr-silicate glass fiber materials The industrial production of these materials includes making of glass melt at high temperatures, manufacturing of separate fibers with typical diameter of 7-10 microns, twisting of these fibers into threads (0.3-1.0 mm in diameter) with final manufacturing of glass fiber fabric. The important stage is a leaching with inorganic acids resulting in complete removal of sodium. As follows from ICP data the leached Zr-silicate glass material used for preparation of GFCs contains 80.7% wt. SiO2, 16.5% wt. ZrO2, 2% wt. Al2O3 and the rest are oxides of Fe, Ca, Mg etc. It corresponds to atomic ratio of

44

B.S. Bal’zhinimaev et al.

main glass forming elements Zr/Si=0.1 wtat was confirmed with XPS technique. Specific surface area measured by BET technique is ca. 1 m2/g and corresponds well to geometrical surface of glass fibers.

3. Molecular structure of zirconium-silicate glass fiber materials The molecular structure of silicate glasses is not clear yet though some models were proposed [4,5]. In particular, it was found that SiO4 tetrahedra in the network are interconnected randomly with each other via bridge oxygens. However, this networks is not continuous, it is alternated by protons bound with non-bridging oxygens. In other words, more dense silicate layers alternate with less dense interspaces with H-bonded hydroxyl groups. In the present work, IR and NMR spectroscopies were used to reveal the structure of zirconium-containing glass fiber. Solid-state 29Si MAS NMR studies were performed using a Bruker AVANCE-400 (9.4 T) spectrometer at resonance frequencies 79.46 MHz for 29Si with rotation frequency 10-15 kHz, pulse duration of 7 μs (π/2) and pulse delay of 10-20 s. The chemical shift values were referenced to external reference tetramethylsilane (TMS). It is known that silicate glass may have five types of distinct silicon microstructures (denoted as Si(n) or Q(n), where n is the number of silicon atoms in the second coordination sphere, n = 0, 1, 2, 3, 4). The introduction of zirconia into silicon lattice leads to the formation of some possible silicon environments and can be considered within different Sin (mZr) units, where m (m≤n) is the number of zirconium atoms bound with the central silicon atom via bridging oxygen. Identification of different types of the oxygen environment of silicon Si(n) in 29Si MAS NMR spectra is based on the value of isotropic shift. The 29Si MAS NMR spectra of the leached zirconium-silicate fiber glass materials used for catalyst preparation show the lines with chemical shifts –93, –102, –109 and – 113 ppm. According to [6] and 1H-29Si CP MAS NMR data (present work), these lines are assigned to Q3 (1Zr, OH), Q3 (0Zr,OH), Q4 (1Zr) and Q4 (0Zr), respectively. (1H-29Si CP MAS NMR data allow to reveal the spectral lines corresponding to the Q3 type silicon bound with OH group). Table 1 presents NMR data and relative content for each type of silicon atoms in zirconium-silicate fiber glasses both as received (initial) and calcined at 700oC. Table 1. The ratio of lines in the 29Si MAS NMR spectra of zirconium-silicate material. Sample

Chem.shift, pm

Rel.content, %

Width, Hz

Type

Initial sample

-93

7

510

Q3(1Zr, OH)

-102

36

760

Q3(0Zr, OH)

-109

40

883

Q4(1Zr)

-113

17

713

Q4 (0Zr)

-109

57

1170

Q4 (1Zr)

-113

43

909

Q4 (0Zr)

Calcined at 700оС

It seen the fraction of silicon atoms bound with zirconium Q4 (1Zr) and Q3 (1Zr, OH) is rather high (above 47%). This ratio keeps after thermal treatment: 43% Q4 (0Zr) and 57% Q4 (1Zr). Taking into account that coordination number of zirconium with respect to oxygen in zirconium-silicate glasses is equal to six [7] and Zr-O-Zr bonds are

Glass fiber materials as a new generation of structured catalysts

45

absent we conclude that each zirconium atom is surrounded by six -O-Si bonds. Thus, taking into account the total ratio Zr/Si = 0.1, approximately up to 40% of Si atoms are not linked with Zr4+ cations, e.g. zirconium atoms are linked with each other via 3-4 silicon-oxygen tetrahedra. To satisfy the ratio between silicon atoms bound and not bound with zirconium (the ratio between Q4 (1Zr) + Q3 (1Zr, ОН) and Q4 (0Zr) + Q3 (0Zr, ОН) is close to 1), it is necessary to add a link of silicon-oxygen tetrahedra not bound with zirconium of type Q4 (0Zr) or Q3 (0Zr, ОН). The noncalcined sample comprises a large amount of hydroxyl groups (nearly 40% Q3 (0Zr, ОН)), i.e. approx. 2/3 of silicon-oxygen tetrahedra not bound with zirconium have a hydroxyl group, whereas the fraction of tetrahedra bound with zirconium and hydroxyl group is low (1/7 of all silicon atoms bound with zirconium), i.e. Q3 (1Zr, ОН) is present not in each structural unit. The formation of Zr-O-Si bonds is confirmed by IR studies in the region of SiO vibrations, as measured on an Alpha spectrometer (Bruker) by the ATR technique. The spectra show absorption bands at 1070 and 1170 cm–1 corresponding to SiO4 stretching vibrations, which are shifted to lower frequencies in comparison with the spectra of silica gel (1132 and 1250 cm–1). The low-frequency shift may result from an appearance of more heavy atom in the silicate network; in our case, it is obviously zirconium. Besides, a new band at 1030 cm–1, which is likely assigned to stretching vibrations of a Zr-O-Si fragment. Thus, the data of 29Si MAS NMR and IR spectroscopy unambiguously prove the incorporation of zirconium ions into the silicate network. The following facts: i) the coordination number of zirconium with respect to oxygen is equal to 6, ii) silicon atoms in the second coordination sphere have only one zirconium atom, hence all zirconium atoms are separated by at least two silicon-oxygen tetrahedra (see Table 1), iii) silicate species tend to form the cyclic structures (it is known from the chemistry of silicates), and iv) zirconium-silicate bonds are stable and do not break even at high temperatures (see Table 1) allows to conclude that zirconiumsilicon structure can be presented as a framework with six-coordinated zirconium atoms in the vertices (Fig. 1).

Fig. 1. The scheme of zirconium-silicate framework. Dashed line showes framework unit.

46

B.S. Bal’zhinimaev et al.

This gives a framework resembling the structure of metal-organic frameworks (MOF); the framework is formed by zirconium atoms, and the ligand cross-linking (three-dimensional in our case) is made by silicon-oxygen tetrahedra. It is clear that the structure is not regular and the length of ligands can be varied between 2 and 4. Besides, to maintain electroneutrality of the structure, protons should reside on two of six Zr-O-Si bridges.

4. Preparation of glass fiber catalysts The presence of bridge protons (Zr-OH-Si) is very important because the introduction of different cations of transition metals into the bulk of glass takes place namely via ion exchange with these protons. Introduction of transition metals, f.e. platinum, and its stabilization in the highly dispersed state is performed in two stages. First stage is a ion exchange of Pt amine complexes with protons of glass: 2 ZrOHSiglass + [Pt(NH3)4]2+solution↔ (ZrOSi)2 [Pt(NH3)4]glass + 2H+solution The second stage is a calcination of impregnated and dried sample at elevated temperatures to reduce Pt(II) cations into Pt0 with ammonia ligands: (ZrOSi)2 [Pt(NH3)4]+5/2O2→2(ZrOSi)H + Pt0 +2N2 + 5H2O Indeed, as follows from UV-Vis DRS studies the Pt amine complexes incorporate into glass matrix. UV-Vis spectra were recorded using Shimadzu 2501 spectrometer equipped with diffusion reflection attachment ISF-240. As seen from Fig. 2 (curves 1 and 2) the spectrum of impregnated and washed out sample is very close to that of Pt amine complex in solution (see characteristic absorption bands at 44600, 42000 and 35000 cm-1). It means, the environment of Pt(II) located inside glassmatrix is similar to that of in the tetra-ammonia complex in water solution.

Fig. 2. UV-Vis spectra of 0.02% Pt/FG samples: 1 – impregnated with further washing out and drying at 120oC; 2 – impregnating solution of [Pt(NH3)4]Cl2 in water; 3 – calcined in air at 300oC; 4 – calcined at 350oC but without previous washing and drying.

47

Glass fiber materials as a new generation of structured catalysts

In course of further sample calcination at 300oC the intensity of the band at 44600 cm sharply decrease and new band at 39800 cm-1 is simultaneously appeared. This band may be attributed to d-d transitions in small charged platinum clusters due to the following reasons. First, the electron microscopy data of GFC samples [1] showed the presence of Pt species with size not exceeding 1 nm and the absence of metal particles at the outer fiber surface. According to XPS data obtained in combination with ion etching the Pt clusters are localized in the upper layers of glass fibers at the depth up to 10 nm. Moreover, the UV-Vis DRS study of the sample prepared without washing and drying procedures (curve 4) shows the appearance of intensive band at 47800 cm-1, corresponding to metal particles of 5-8 nm in size located at the fiber surface (according to electron microscopy data [1]). Second, the position of absorbance bands, corresponding to d-d transitions is sensitive to ability of surrounding ligands to donate electron density towards cations. For example, for Pt(II) the replacement of oxygen ligands for ammonia ones results in band shift from 20000-25000 to 32000-36000 cm-1. Platinum atoms have higher electron donating ability than ammonia, therefore, the bands of these transitions must shift to higher frequency region. Therefore, the band at 38000-40000 cm-1 are quite probable caused by ions of two-valent platinum surrounded number of metal atoms. In other words, the band at 39800 cm-1 can be attributed to positively charged Pt clusters formed due to reduction of Pt(II) with ammonia. Indeed, as follows from TPO (temperature programmed oxidation) studies the formation of molecular nitrogen near 300oC takes place (Fig. 3). TPO profile was measured with continuous MS monitoring of gas phase composition using quadrupole mass-spectrometer VG Sensorlab 200D. The absorbed H2O and NH3 during impregnation (ability of glass fibers to absorb significant amounts of polar molecules was shown in [2]) are desorbed at T<300oC. The formation of nitrogen oxide and water at high temperatures accompanying by oxygen consumption is caused obviously by oxidation of dissolved ammonia with O2. -1

0,10

O2

Concentration, a.u.

0,08

0,06

NH3

H2O

0,04

NO 0,02

N2 0,00 0

100

200

300

400

500

0

Temperature, C

Fig. 3. TPO profile in 1% O2+He flow at temperature rate 10K/min, SV=7 l/hour and Pt/FG catalyst loading 0.66 g.

48

B.S. Bal’zhinimaev et al.

Similarly we introduced into the bulk of glassmatrix the cations of Pd and stabilize them in highly dispersed (clusters) state. In case of Ru and Co the redox treatments at elevated temperatures resulted in low coordinated oxide species.

5. Application of glass fiber materials in catalysis The glass fiber based catalysts containing transition metals in highly dispersed state (mostly, Pt and Pd) were tested in many catalytic reactions, such as deep oxidation of saturated hydrocarbons [1], SO2 oxidation [3], deNOx [8], selective hydrogenation of acetylene feedstock [2 ] etc. Despite of extremely low content of noble metals the GFCs showed high activity and thermal stability, low “ignition” temperature. Figure 4 demonstrates that Pt/GF performance in deep oxidation of ethylbenzene is much higher than that of commercial Pt/Al2O3 catalyst despite of Pt content in GFC is 30 times less than in commercial one (0.02% wt. in GFC vs 0.56% in reference). 100 90

Conversion, %

80

Pt/Al2O3 reference Pt/GFC

70 60 50 40 30 20 10 0 150

175

200

225

250

275

300

325

350

Temperature, C

Fig. 4. The TPO profile of ethylbenzene (EB) conversion to CO2 and H2O at temperature rate 10K/min. Feed gas (% vol.): 1,2 EB + 16% O2 + He for balance. GV=2,2 l/h, catalyst loading – 0,22 g.

Basing on glass fiber catalyst the efficient process of VOCs (mostly, isoprene, isobutylene, CO) removal from waste gases of synthetic rubber production was successfully commercialized in petrochemical company “Nizhnekamskneftekhim” (Nizhnekamsk, Russia). The industrial reactor with waste gas loading up to 15000 st.m3/hour was charged with 1 tonne of GFC, structured in the form of vertical spiral cartridges [9], alternating catalyst fabric and structuring metal gauze (Fig. 5). Such packing is characterized with highly efficient mass transfer in combination with low pressure drop. Despite of operation conditions were quite complicated (oxygen content as low as 2-3% vol., dusty flow, water vapour content around 80%) the abatement efficiency 99,599,9% was achieved. Continuous operation for more than 1.5 year has revealed no any decrease in catalyst activity. Another example is related to not only capability of glass matrix to stabilize small metal clusters but its capability to act as a specific membrane permeable for polar or polarizable molecules like acetylene [2]. In particular, it was showed that glass fibers absorb up to 2% wt. of acetylene. In case of hydrogenation of C2H2+C2H4 feedstock

Glass fiber materials as a new generation of structured catalysts

49

unlike ethylene molecules the acetylene ones will get predominantly the active sites (Pd clusters) located in the bulk of glass to be hydrogenated.

Fig. 5. Internal structure of the GFC spiral cartridge.

Indeed, as seen from Fig. 6 the GFC with low Pd content (0.02% wt.) showed high activity, so that at 50-60oC the 100%* conversion of acetylene was got. At the same time the selectivity decreases up to 40-50% but remains still high. It can be explained by the presence on the outer surface of fibers some amount of metallic Pd particles which catalyze the nonselective hydrogenation towards ethane. Obviously, the contributiuon of this undesirable reaction becomes noticeable at high conversions, e.g. in the absence of acetylene.

Fig. 6. The acetylene conversion and ethylene selectivity vs reaction temperature. Feed gas (% vol.): 0,60% C2H2 + 0,84% H2 + 97,56% C2H4 + 1% CH4 (balance marker). Ptotal=20 bar, GHSV=7000 h-1. *

-100% conversion means that residual concentration of acetylene is ≤ 1 ppm (GC threshold of sensitivity).

50

B.S. Bal’zhinimaev et al.

6. Conclusions 1. Molecular structure of Zr-silicate glass fibers material is studied by means of NMR and IRS. The high thermal-, and chemical stability are caused by formation of framework structure consisting of Zr(IV) ions (connectors) connected with SiO4 tertahedra. 2. The effective ways for introduction of transition ions (Pt, Pd) into the bulk of glass and further their stabilization in the form of small clusters are developed. It was showed that incorporation of metal cations proceeds via ion exchange with acid protons while formation of metal clusters proceeds as a result of intramolecular reduction of Pt(II) or Pd(II) with ammonia. 3. Despite of extremely low content of noble metals (~0.02% wt.) the prepared catalysts showed high performance in deep oxidation of hydrocarbons, as well as in selective hydrogenation of acetylene-ethylene feedstock. 4. GFCs was packed into structured cartridges with minimal mass transfer limitations and pressure drop was successfully comercialized in process of VOC removal.

References [1] L.G. Simonova, V.V. Barelko, A.V. Toktarev, V.I. Zaikovskii, V.I. Bukhtiarov, V.V.Kaichev, B.S.Balzhinimaev. Kinet. Catal. 42 (2001) 837. [2] B.S. Bal’zhinimaev, V.V. Barelko, А.P. Suknev, Е.А. Paukshtis, L.G. Simonova, V.B. Goncharov, V.L. Kirillov, А.V. Toktarev. Kinet. Catal. 43 (2002) 542. [3] B.S. Balzhinimaev, L.G. Simonova, V.V. Barelko, A.V. Toktarev, V.I. Zaikovskii, V.A.Chumachenko. Chem. Eng. J. 91 (2003) 175. [4[ G.N. Greaves. J.Non-Cryst.Solids 71 (1985) 203. [5] L.G. Simonova, V.V. Barelko, O.B. Lapina, E.A. Paukshtis, V.V. Terskikh, V.I. Zaikovskii, B.S. Bal’zhinimaev. Kinet. Catal. 42 (2001) 823. [6] J.-H. Choy, J.-B. Yoon, H.Jung and J.-H. Park. Journal of Porous Materials 11 (2004) 123. [7] L. Galoisy, E. lePelegrin, M.-A.Arrio, P.Ilefonse, G,Galas, D. Ghaleb, C. Fillet and F. Prasad. J.Am.Ceram.Soc. 82(8) (1999) 2219. [8] D.A. Arendarskii, A.N. Zagoruiko, B.S. Balzhinimaev. Glass Fiber Catalysts to Clear Diesel Engine Exhausts. Chemistry for Sustainable Development 6 (2005) 731. [9] Russian Patent # 66975, 2007.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

A novel electrochemical route for the catalytic coating of metallic supports Francesco Basile,a Patricia Benito,a Giuseppe Fornasari,a Marco Monti,b Erika Scavetta,b Domenica Tonelli,b Angelo Vaccaria a

Dipartimento di Chimica Industriale e dei Materiali, ALMA MATER STUDIORUM Università di Bologna, Viale Risorgimento 4, 40136 Bologna, Italy b Dipartimento di Chimica Fisica e Inorganica, ALMA MATER STUDIORUM Università di Bologna, Viale Risorgimento 4, 40136 Bologna, Italy

Abstract Ni- and Rh-based structured catalysts were prepared by electrosynthesis of hydrotalcitetype compounds on a FeCrAlY foam, followed by calcination. The effect of the electrosynthesis conditions on both the properties of the film coating and the catalytic activity was investigated. The coating growth and the morphology of the final catalytic layer were controlled by the applied potential and deposition time. By tuning the deposition parameters, it was possible to prepare Ni- and Rh-based catalysts which were very active in the steam reforming and catalytic partial oxidation of methane. Keywords: structured catalysts, electrosynthesis, hydrotalcite, catalytic partial oxidation, steam reforming

1. Introduction Catalysts based on an active coating of structured metallic supports (Al, stainless steel, FeCrAlY) [1] are significant alternatives to pelletized catalysts for highly endothermic or exothermic processes. Besides the advantages of low pressure drop and high mechanical stability of these catalysts, temperature gradients and hot spots can be avoided due to the thermal conductivity of the support [2,3]. However, when dealing with structured metallic supports, the adhesion of the catalytic layer to the metal is low, while the thermal expansion coefficients of catalysts and metallic supports are different, causing the detachment of the catalytic film and the lost of performances. FeCrAlloys are the best choice for high temperature applications, since an alumina scale is formed after calcination at high temperatures [4], thus protecting the support from corrosion and improving the adhesion of the catalytic coating. Several synthetic procedures have been reported to coat metallic supports [5-7]: washcoating is the most commonly used. However, when using this method, the properties of the slurry have to be carefully controlled [8] in order to guarantee a good reproducibility [9]. On the other hand, zeolites [10] and manganese oxides [11] were synthesized in situ on the support, thus increasing the adhesion and modifying textural properties. Electrodeposition is a well-known method to produce in situ metallic coatings by the action of an electric current on a conductive material immersed in a solution containing a salt of the metal to be deposited. Moreover, by controlling synthesis conditions, the electrochemical synthesis/deposition can be used to produce thin films of oxides and/or hydroxides on conductive materials [12]. The composition, morphology and texture of the film coating can be controlled by tuning the experimental parameters such as the potential, current density, deposition time, and plating solution composition. In

52

F. Basile et al.

particular, the electrogeneration of a base by the cathodic reduction of a solution containing nitrates has been reported to deposit hydrotalcite-type (HT) compounds on electrodes [13]. HT compounds are layered materials with the chemical formula [M2+1-xM3+x(OH)2]x+ [Ab-]x/b • nH2O [14]. Ni- and Rh-containing HT compounds are useful precursors of natural gas conversion catalysts [15], because well-dispersed metallic particles, highly stabilized in an oxidic matrix, are obtained after calcination at high temperatures and reduction. In this paper, while taking the conductivity of FeCrAlY supports into account, the electrochemical in situ synthesis/deposition of Ni- and noble-metal-containing HT compounds on a FeCrAlY foam was studied, with the aim of obtaining catalysts for H2 production processes - such as the endothermic steam methane reforming (SMR) [16,17] or the exothermic catalytic partial oxidation (CPO) of methane. The effect of the synthesis parameters on the properties of the film coating and catalytic activity were studied, while paying special attention to the influence of the composition of the HT precursor.

2. Experimental 2.1. Synthesis Ni/Al and Rh/Mg/Al HT precursors were electrochemically synthesised at room temperature (r.t.) in a three-electrode cell containing a 0.03 M solution of the nitrates of the elements and a 0.3 M solution of KNO3 as electrolyte. The working electrode was a FeCrAlY foam or plate, and a cathodic potential was applied. Before the electrochemical synthesis, the foam pellets were rinsed with ethanol and water. Electrode potentials were measured versus an aqueous saturated calomel electrode [SCE, i.e. reference electrode or RE]. A Pt gauze was used as the counter electrode (CE). The electrochemical deposition was carried out at different potentials vs SCE, for deposition times ranging from 600 to 1800 s (Table 1). After washing and drying at 40°C, the coated foam pellets were weighed. Catalysts were obtained by calcination at 900ºC for 12 h (heating rate 10ºC min-1). Table 1. Summary of the HT samples prepared by electrosynthesis. Composition

Atomic ratio

Potential vs SCE / V

Time / s

HT weight / %

Reaction

Ni/Al

75/25

- 0.9

600

nd

SMR

Ni/Al

75/25

- 0.9

1800

1.2

SMR

Ni/Al

75/25

- 1.2

600

1.5

SMR

Ni/Al

75/25

- 1.2

1000

2.2

SMR

Rh/Mg/Al

11/70/19

- 1.2

1000

<1

CPO

Rh/Mg/Al

11/70/19

- 1.3

1000

5

CPO

2.2. Characterisation SEM/EDS analyses were performed by using an EVO 50 Series Instrument (LEO ZEISS) equipped with both an INCAEnergy 350 EDS micro-analysis system and an INCASmartMap for imaging the spatial variation of elements in a sample (Oxford Instruments Analytical). The accelerating voltage was 25 kV, the beam current 1.5 nA, and the spectra collection time 100 s. X-ray diffraction (XRD) patterns were collected with CuKα radiation (λ= 1.5418 Å) by means of a X’PertPro PANalytical

53

A novel electrochemical route for the catalytic coating of metallic supports

diffractometer equipped with a fast X’Celerator detector. A 3 to 80° 2θ range was analysed, with steps of 0.07° and counting time of 120 s/step. Temperature Programmed Reduction (TPR) analyses were carried out with a H2/Ar (5/95 v/v) gas mixture (total flow rate 20 ml min-1) in the 100-950ºC range using a ThermoQuest CE Instruments TPDRO 1100. The pH near the foam pellets, as a function of the potential applied, was measured with selected acid-base indicators added to a colourless KNO3 solution.

2.3. Catalytic tests SMR tests were carried out in an Incoloy 800HT reactor (i.d. 12 mm). Six foam pellets (1.2 x 1 cm) were loaded in the isothermal zone of the reactor, filling the top and bottom sections with an inert material (beads of corundum or quartz pellets). SMR tests were carried out at P = 20 bar, Steam/Carbon = 1.7, Toven= 900°C and contact time (τ) = 4 s. CPO tests were carried out in a quartz reactor (i.d. 8 mm), filled with two foam pellets (0.8 x 1 cm). Catalytic tests were performed at Toven = 750°C, at two gas-hourly-spacevelocity (GHSV) values: 28,000 and 120,000 h-1, and feeding two gas mixtures: CH4/O2/He = 2/1/20 and 2/1/4 v/v. Before testing, catalysts were activated by in situ reduction at 900°C (Ni/Al) or 750ºC (Rh/Mg/Al) for 2 h with a H2/N2 equimolar flow (7 L h-1). GHSV values were calculated by using the total gross volume of the foam pellets. The products of the reaction were analysed on-line after water condensation by a Perkin Elmer Autosystem XL gas chromatograph, equipped with two thermal conductivity detectors (TCD) and two Carbosphere columns, using He as the carrier gas for the analysis of CH4, O2, CO and CO2 and N2 for the H2 analysis.

3. Results and discussion 3.1. Characterisation 3.1.1. Precursors After the application of a cathodic potential pulse to the metallic foam pellets immersed in a nitrate solution containing the wanted cations (Mn+), a series of reactions (consumption of H+, electrolysis of water and NO3- reduction) take place at the electrode/electrolyte interface [12]. Among them, the nitrate reduction (Eqs. 1 and 2) may be considered the most significant, yielding a steep increase of the pH near the electrode and the heterogeneous nucleation of the cation hydroxides (Eq. 3) on the surface of the FeCrAlY foam pellets. NO3- + H2O + 2e- → NO2- + 2OHNO3-+ 7H2O + 8e- → NH4+ + 10OHMn+ + nOH- → M(OH)n

E° = 0.01 V E° = -0.12 V

(1) (2) (3)

The amount and rate of OH- formation are determined by the potential applied. The pH in the vicinity of the foam pellets was estimated by using acid-base indicators (Table 2); the more negative the applied potential, the more basic is the medium close to the electrode. Table 2. pH values close to the metallic foam pellets as a function of the potential applied to a KNO3 solution with initial pH = 3.8. Potential applied vs SCE / V

-0.9

-1.2

-1.3

pH

6.5-7.5

8.7-9.6

9.0-11.0

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F. Basile et al.

The morphology of the film and coverage of the foam pellets vary as a function of both the potential applied and the synthesis time. Furthermore, some differences can be observed depending on the nature of the cations to be precipitated. SEM images and EDS analyses of Ni/Al samples prepared at low potential (-0.9 V vs SCE) and in a short time (600 s) show a coating made of small particles, mainly Alcontaining (Ni/Al < 0.6), with the solid preferably growing on the surface flaws of the foam pellets (or tips). When the synthesis time is increased to 1800 s it improves both the homogeneity of the coating and the thickness of the film (Fig. 1); moreover, a slight increase in the Ni content takes place, but Ni/Al values are lower than those in the solution. In agreement with the pH values reported in Table 1, at this potential the pH is not high enough to precipitate HT precursors, so hydroxides with lower solubility constants [Al(OH)3 or AlOOH] precipitate. The application of a higher cathodic potential (-1.2 V vs SCE) causes the co-precipitation of Ni2+ and Al3+ ions, since the conditions required to precipitate HT phases are achieved. However, a short deposition time (600 s) induces the formation of a not homogeneous HT film (mainly on the tips of the foam pellets); this makes it necessary to prolong the time up to 1000 s in order to cover the surface (Fig. 2), although some small cracks are present (probably formed during the drying step). Likewise, during the synthesis at high potential, while increasing the synthesis time more Ni2+ ions precipitate in the solid, from Ni/Al ≈ 2.1 at 600 s to Ni/Al ≈ 3.4 at 1000 s.

Figure 1. SEM images of the Ni/Al samples prepared at -0.9 V for 1800 s (left) and -1.2 V for 1000 s (right).

In order to synthesize Rh/Mg/Al HT compounds, it is worth noting first that a higher pH is required to precipitate pure HT phases in comparison to the Ni/Al HT ones. The synthesis was therefore performed at -1.2 V and -1.3 V vs SCE (Table 2). Moreover, since they contain easily Rh reducible species, special care has to be taken to perform the electrosynthesis under conditions which prevent the reduction of Rh3+ ions. The pH of the plating solution was therefore adjusted to a value of 3.8, since - according to Pourbaix’s diagram [18] - the formal potential of reduction of the Rh3+ ions is decreased by increasing the pH. Apart from these differences, a similar trend is observed in the chemical composition and morphology of the coating if compared to Ni/Al HT precursors. When the deposition potential is -1.2 V vs SCE, a thin HT film is deposited, which does not provide a homogeneous cover on the surface. Furthermore, the solid is richer in Al3+ and Rh3+ ions than Mg2+ ions, if compared to the concentration values in the original plating solution. Any further increase of the potential (-1.3 V vs SCE) leads to an increased pH, and therefore to a faster precipitation of the HT phase, as well as to a better coverage and a higher incorporation of Mg2+ ions in the solid.

A novel electrochemical route for the catalytic coating of metallic supports

55

However, deeper cracks than in Ni/Al samples are observed. Here, the thickness of the film seems to be greater and the formation of cracks more evident, which could be due to the shrinkage of the solid during the drying step. However, this behaviour may also be due to the evolution of H2 bubbles from the water electrolysis, due to the increase of the potential applied. For both Ni/Al and Rh/Mg/Al HT precursors, as a result of the homogeneous pH near the foam pellets, small particles are electrosynthesized, with an improved adhesion to the support [19]; in fact, it is accepted that at this step the interaction between the support and the coating layer is only mechanical, and is therefore favoured by small particles. Finally, it is noteworthy that even at the longest synthesis time, no blockage of the pores in the foam pellets occurs, despite the use of a 80 ppi foam with small pore sizes. No structural information can be obtained from HT precursors as made by conventional XRD analysis, because of both the low amount of solid deposited on the foam pellets, and the complex structure of the metallic supports. Therefore, the XRD + hydrotalcite + o Mg(OH) patterns of the HT phases synthesized on FeCrAlY plates under the same conditions o + o and scratched away from them were + + o o+ recorded (Fig. 3). The diffraction patterns Rh/Mg/Al - 1.3 V 1000 s confirm that a poorly-crystallized HT + phase is electrosynthesized. Whereas for + + the Ni/Al samples only the HT crystalline phase is detected, for the Rh/Mg/Al samples Ni/Al - 1.2 V 1000 s diffraction lines due to the Mg(OH)2 side 10 20 30 40 50 60 70 80 phase are also present. However, the 2θ (°CuKα) presence of α-Ni(OH)2 - which exhibits a diffraction pattern similar to that of HT Figure 3. XRD patterns of electrophases and may be easily electrosynthesized synthesized Ni/Al and Rh/Mg/Al HT phases on FeCrAlY plates. cannot be excluded. Detector signal (a.u.)

2

3.1.2. Catalysts During thermal treatment, HT precursors decompose evolving CO2, H2O and NOx [14], thus yielding a compact structure formed by oxide-type (MgO and NiO) and spinel-type (NiAl2O4 and MgAl2O4) phases. The shrinkage in volume may lead to the formation of cracks in the layer. On the other hand, the inward diffusion of O2 to the FeCrAlY foam pellets may take place, with the outward movement of Al3+ ions to form an alumina scale [4] that may improve the chemical compatibility between the catalytic layer and the metallic support surface. However, since in the first stages reactive γ-Al2O3 is formed, it may also act as Al3+ ions source and react with the cations of the HT phase to form additional amounts of spinel-type phase. The surface of the foams obtained by calcination of the HT precursors prepared at 0.9 V vs SCE is almost homogeneously covered by needle-like particles (Fig. 4) which are rich in Al, thus confirming the formation of alumina coming from both the deposited solid and the foam. During the calcination of the sample prepared at -1.2 V vs SCE for 600 s, thermal stresses result in cracking and spalling, leaving some parts of the metallic foam pellets uncovered. On the other hand, the surface of the sample prepared for 1000 s seems more compact than that of the HT precursor, with only small cracks (Fig. 4).

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Figure 4. SEM images of the calcined samples prepared at -0.9 V for 1800 s (left) and -1.2 V for 1000 s (right).

After the thermal treatment of the Rh/Mg/Al HT precursor synthesized at -1.2 V for 1000 s, the morphology of the solid does not change significantly, although some uncovered zones are observed where alumina coming from the oxidation of the FeCrAlY foam grows. Conversely, for the compound prepared at -1.3 V vs SCE, both surface flakings and outward diffusion of Al3+ ions are observed after calcination. SEM images show many wide cracks (Fig. 5), since the thicker the layer, the higher the thermal stresses. However, the oxide coatings obtained seem to stick well to the support. Together with the alumina scale segregated from the foam, EDS analyses reveal that Rh3+ ions are also present in the cracks. TPR profiles (not shown here) of Ni/Al catalysts obtained from HT precursors synthesized at -1.2 V vs SCE for 600 and 1000 s show two hydrogen consumption peaks at approximately 450 and 900ºC, which are attributable to the reduction of NiO and NiAl2O4 respectively. The Rh-based catalysts obtained from HT precursors show reduction profiles characteristic of wellstabilized Rh3+ species, which reduce at Figure 5. SEM image of the calcined temperatures higher than 450ºC. sample prepared at -1.3 V for 1000 s.

3.2. Catalytic activity The catalytic activity strongly depends on the synthesis conditions both in the SMR and CPO tests. Concerning the SMR tests carried out under industrial-type conditions (Table 3), the increase of both the cathodic potential and the synthesis time used to precipitate the HT precursors favours the initial methane conversion. However, the catalysts prepared in short times deactivate with time-on-stream. Catalytic performances close to those of a commercial-type catalyst are obtained for the catalyst obtained from the HT precursor prepared at -1.2 V vs SCE for 1000 s. By taking into account the morphological and chemical-physical properties of both fresh and used catalysts, performances (activity, selectivity, and stability) can be correlated to the number of Ni available sites as well as to the homogeneity and thickness of the catalytic film. A homogeneous film with dispersed Ni0 particles, although in small amounts (as obtained by calcination and reduction of the sample synthesized at -0.9 V vs SCE for 1800 s), yields stable but poor performances. Due to the higher Ni-content in the HT precursors,

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increasing the potential applied in the electrosynthesis also increases the activity; however, it is necessary to provide a homogeneous coating of the support in order to obtain stable performances. Table 3. Methane conversion and H2 yield for the Ni/Al samples in the SMR tests. Potential vs SCE / V

Time / s

Conversion CH4 / %

Yield H2 / %

2h

7.5 h

2h

7.5 h

- 0.9

600

34.1

21.7

30.8

20.3

- 0.9

1800

49.6

52.1

43.6

47.2

- 1.2

600

57

47.8

51.5

43

- 1.2

1000

64

63.5

57.9

56.8

%

The best catalyst in the SMR tests - i.e. that obtained from a HT precursor synthesized at -1.2 V vs SCE for 1000 s - was also tested in the CPO of methane. Surprisingly, the catalyst was not active in the CPO tests, probably due to the reoxidation of the Ni0 particles. On the other hand, Rh-containing catalysts show better activity in the CPO of CH4, because of the higher activity of the noble metal and its better stability under oxidant atmosphere. Methane conversion and syngas selectivity depend on the potential applied in the precursor synthesis. The catalyst prepared from the HT phase synthesized at -1.3 V vs SCE shows high conversion and selectivities values, regardless of both the GHSV value and the dilution factor of the gas mixture. On the contrary, when the synthesis of the precursor was performed at -1.2 V vs SCE, conversion and selectivity depend on the reaction conditions. By feeding the 2/1/20 v/v gas mixture (Fig. 6), the CH4 conversion improves as the GHSV increases, due to a higher surface temperature, which enhances the reforming reaction and syngas selectivity. Conversely, by feeding the 2/1/4 v/v mixture, although an improvement of CH4 conversion is observed at 28,000 h-1 100 with respect to the 2/1/20 v/v tests, as the 90 GHSV increases CH4 conversion and 80 selectivity in H2 decreases, whereas 70 selectivity in CO remains almost constant. 60 These results show that the available Rh 50 active sites are not sufficient to achieve 40 good performances. Finally, it should be 30 -1.2 V 1000 s 28000 h noted that the activity is not modified by -1.2 V 1000 s 120000 h 20 the reaction conditions, thus suggesting a -1.3 V 1000 s 28000 h 10 -1.3 V 1000 s 120000 h good stability of the film coating, which 0 is confirmed by the characterisation of Conv CH4 Sel CO Sel H2 the catalysts used. Figure 6. CPO activity of Rh/Mg/Al catalysts -1

-1

-1

-1

4. Conclusions

feeding the CH4/O2/He = 2/1/20 v/v gas mixture.

Ni- and Rh-containing HT phases are useful precursors of catalysts for SR and CPO of methane, and may be electrosynthesized in situ at r.t. on the surface of FeCrAlY foam pellets from nitrate solutions. Thermal treatment results in the formation of well adherent oxide coatings (rock-salt-type and spinel-type phases) containing active sites. The rate of base generation and electrosynthesis can be controlled by changing the potential applied, while the layer thickness is controlled by tuning the synthesis time.

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After calcination, well-coated samples do not show a high number of cracks, while in those either partially covered or formed by thin or too thick layers, larger cracks are observed. However, because of the formation of an intermediate alumina scale, the oxides show good adherence to the metallic support. By selecting optimum synthesis conditions, stable and active Ni- or Rh-based catalysts for the SR and CPO of methane, respectively, can be prepared. Finally, it is noteworthy that the method can be applied to the synthesis of further catalytic compositions and/or different metallic supports.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.

A. Cybulski, J.A. Moulijn (Ed.s.), 2005, Structured catalysts and reactors, CRC Taylor & Francis, Boca Raton. R. J. Farrauto, Y. Liu, W. Ruettinger, O. Ilinich, L. Shore, T. Giroux, 2007, Precious metal catalysts supported on ceramic and metal monolithic structures for the hydrogen economy, Catal. Rev. 49, 141–196. T. Giroux, S. Hwang, Y. Liu, W. Ruettinger, L. Shore, 2005, Monolithic structures as alternatives to particulate catalysts for the reforming of hydrocarbons for hydrogen generation, Appl. Catal. B 56, 95–110. R. Chegroune, E. Salhi, A. Crisci, Y. Wouters, A. Galerie, 2008, On the competitive growth of alpha and transient aluminas during the first stages of thermal oxidation of FeCrAl Alloys at intermediate temperatures, Oxid. Met. 70, 331–337. T. A. Nijhuis, A. E. W. Beers, T. Vergunst, I. Hoek, F. Kapteijn, J. A. Moulijn, 2001, Preparation of monolithic catalysts, Catal. Rev. 43, 345–380. P. Avila, M. Montes, E. E. Miro, 2005, Monolithic reactors for environmental applications. A review on preparation technologies, Chem. Eng. J. 109, 11–36. V. Meille, 2006, Review on methods to deposit catalysts on structured surfaces, Appl. Catal. A 315, 1–17. C. Cristiani, C. G. Visconti, E. Finocchio, P G. Stampino, P. Forzatti, 2009, Towards the rationalization of the washcoating process conditions, Catal. Today 147, S24–S29. V. Meille, S. Pallier, P. Rodriguez, 2009, Reproducibility in the preparation of alumina slurries for washcoat application. Role of temperature and particle size distribution, Colloids Surf. A 336, 104–109. A. Eleta, P. Navarro, L. Costa, M. Montes, 2009, Deposition of zeolitic coatings onto FeCrAlloy microchannels: washcoating vs. in situ growing, Micropor. Mesopor. Mater. 123, 113–122. D. M. Frías, S. Nousir, I. Barrio, M. Montes, T. L. M. Martınez, M. A. Centeno, J. A. Odriozola, 2007, Nucleation and growth of manganese oxides on metallic surfaces as a tool to prepare metallic monoliths, Appl. Catal. A 325, 205–212. G. H. A. Therese, P. V. Kamath, 2000, Electrochemical synthesis of metal oxides and hydroxides, Chem. Mater. 12, 1195–1204. E. Scavetta, A. Mignani, D. Prandstraller, D. Tonelli, 2007, Electrosynthesis of thin films of Ni, Al hydrotalcite like compounds, Chem. Mater. 19, 4523–4529. F. Cavani, F. Trifirò, A. Vaccari, 1991, Hydrotalcite-type anionic clays: preparation, properties and applications. Catal. Today 11, 173–301. F. Basile, P. Benito, G. Fornasari, A. Vaccari, 2009, Hydrotalcite-type precursors of active catalysts for hydrogen production, Appl. Clay Sci. doi:10.1016/j.clay.2009.11.027 F. Basile, P. Benito, P. Del Gallo, G. Fornasari, D. Gary, V. Rosetti, E. Scavetta, D. Tonelli, A. Vaccari, 2008, Highly conductive Ni steam reforming catalysts prepared by electrodeposition, Chem. Commun. 2008, 2917–2919. F. Basile, P. Benito, G. Fornasari, V. Rosetti, E. Scavetta, D. Tonelli, A. Vaccari, 2009, Electrochemical synthesis of novel structured catalysts for H2 production, Appl. Catal. B 91, 563–572. M. Pourbaix, 1963, Atlas d’equilibres thermodinamiques a 25°C, Gauthier-Villars, Paris. C. Agrafiotis, A. Tsetsekou, 2000, The effect of powder characteristics on wahscoat quality. Part I. Alumina washcoats, J. Eur. Ceram. Soc. 20, 815–824.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Solution Combustion Synthesis as intriguing technique to quickly produce performing catalysts for specific applications Stefania Specchia, Camilla Galletti, Vito Specchia Politecnico di Torino, Department of Materials Science and Chemical Engineering, Corso Duca degli Abruzzi 24, 10129 Torino, Italy

Abstract - rewritten Solution Combustion Synthesis (SCS) is becoming one of the most important ways to produce a wide range of advanced porous ceramic or metallic materials. These include ceramic oxide like nanostructured catalysts. SCS is an attractive alternative for the production of smart materials of high value compared to the more conventional and expensive preparation routes. SCS processes are characterized by relative medium heating oven temperatures (350-600°C), fast heating rates, short reaction times and very small residence time at high temperature. The final product is usually of high purity and well crystallized with nanometric size clusters. Furthermore, SCS, suitably adapted, is easily tunable to complex industrial supports to produce directly in situ structured catalysts. The present work deals with the analysis of low-environmental-impact premixed metal fiber burners (FeCrAlloy® fiber mat) for household applications. New catalytic burners based on Pd(LaMnO3·2ZrO2) catalyst were developed and tested in a partially modified commercial condensing boiler test rig fed with natural gas (NG). The catalytic burner presented a lower environmental impact, compared to the commercial bare burner, mainly in terms of CO. NO emissions were reduced, too, but in a slightly lower extent. The catalytic burner better stabilized the combustion process within the porous medium, maximizing the heat fraction transferred by radiation, cooling the flame temperature and enhancing the degree of completeness of NG combustion. Keywords: solution combustion synthesis, nanomaterials, catalysts, nitrate precursors, organic fuel, domestic appliances, natural gas combustion, CO/NO emissions

1. Introduction The chemical processing and synthesis of high performance technological catalysts requires the use of high purity precursors. Recently several attempts have been made to find out an intriguing technique to quickly produce performing catalysts for any kind of industrial application. In particular, methods based on “solution combustion synthesis” (SCS) have received a remarkable interest. These processes, in fact, make use of highly exothermic redox chemical reactions between metals and non-metals, to synthesize the desired ceramic nanomaterials [1-3]. SCS means the synthesis of compounds in a wave of chemical reaction (combustion) that propagates over starting reactive mixture owing to layer-by-layer heat transfer. Practically, it is an exothermic redox process in which the reaction between two or more reactants takes place in a self-sustaining regime leading to the formation of solid products of a higher value [1]. It is widely well-known in catalysis, that nanosized catalysts possess extremely high activity and selectivity [4,5]. Compared to other

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production methods, SCS allows effective low-cost production of nanomaterials with the desired phase compositions thanks to its relative medium heating temperatures (350600°C), fast heating rates and short reaction times, with the advantages of: 1. Use of relatively simple equipment; 2. Use of relatively cheap reactants (like nitrates); 3. Exothermic, fast and self-sustaining reaction; 4. Formation of high purity products with a variety of size and shape, tunable with the synthesis conditions; 5. Adaptability to a variety of structured substrates via in situ SCS. The high temperature gradients, combined with rapid cooling rates in the combustion wave, may form unique microstructures, which are not possible to achieve by conventional methods of powder metallurgy. Examples of SCS and in situ SCS for the production of structured catalysts in energy application, for combustion of natural gas in domestic appliances will be described.

2. Experimental 2.1. Theory Synthesizing virtually any oxide powder via SCS involves a relatively simple procedure. As first step, an aqueous solution containing suitable metal salts and an organic molecule that can properly work as fuel in the redox mixture must be prepared. When brought to temperatures in the range 300÷600°C, the solution reaches ebullition, becomes dry and in a matter of minutes the mixture ignites, thus setting off a highly exothermic, self-sustaining and fast chemical reaction, that results in a dry, usually crystalline, fine powder. Generally nitrates are chosen as metals precursors: not only they are fundamental for the method, the NO3− groups being the oxidizing agents, but also their high solubility in water allows a sufficiently high solution concentration. The fuel can be chosen among a variety of organic compounds, like urea, glycine, hydrazine or precursors containing a carboxylate anion. Urea seems to be the most convenient fuel to be employed, given that it is cheap and readily available commercially; therefore, it has received most of the attention. The organic fuels are a source of C and H, which on combustion form CO2 and H2O, releasing heat; moreover, they form complexes with metal ions facilitating homogeneous mixing of the cations in solution. The exothermicity of the redox reaction allows reaching peak temperatures that vary from 700 to 1500°C [6,7]. Depending upon the fuel used, the nature of combustion differs from flaming to non-flaming type. The overall SCS reaction by using for example a metal nitrate as oxidizer and glycine (CH2NH2COOH) as fuel can be written as follow: Mv(NO3)v + 5/9vΦC2H5O2N + 5/4v(Φ - 1)O2 → MvOv/2 + 10/9vΦCO2 + 25/18vΦH2O + 1/2v(5/9Φ + 1)N2 where Mν is a ν-valent metal and Φ is the so-called “elemental stoichiometric coefficient”, or, less properly, fuel-to-oxidizer ratio. Based on the propellant chemistry, CO, H2O and N2 are the most stable products of the SCS reaction with respect to other theoretically acceptable combinations that might be considered, including the formation of NOx, CO and so forth. Φ is the ratio between the total valences of fuel (glycine in the example) and the total valences of oxidizers (nitrates). Under stoichiometric conditions, Φ is equal to 1, i.e., the initial mixture does not require atmospheric O2 for complete oxidation of fuel. Φ < 1 means oxidant-rich (or fuel lean) conditions (O2 is not a reagent

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61

but a product), whereas Φ > 1 is the figure of fuel-rich conditions. Increasing the amount of fuel, Φ > 1, leads to an increase of not only the heat release but also in the gas-phase production, which is an important factor in controlling the product specific surface area and the spongy morphology of the primary obtained solid. It is possible to control the process by changing the Φ value, or using complex fuels and/or adding inert easily gasified precursors: combustion conditions depend in fact, on the chemical nature of the reactive solution formed [8-10]. For example, even if the systems have comparable energy for product formation, the activity of NH2 groups appears to be higher as compared to the OH group, which in turn is more active than COOH. This explains why, glycine, which contains the NH2 group, is a more reactive fuel than citric acid, which contains only OH and COOH groups [11]. What makes this process interesting for a potential application on a larger scale is that the energy (heat) necessary is basically provided by the exothermic reaction itself and hardly any external supply is required. Metal nitrates, in fact, can simply decompose upon calcination into metal oxides, by mere heating to or above their decomposition temperature. Subsequently the so-formed oxides may take part in other reactions to form other compounds. With such procedure, though, a continuous supply of heat from the outside should be necessary to maintain the system at the appropriate temperature, whereas the mixture of nitrates and organic molecule, suitable to serve as fuel, can be ignited at a relatively low temperature and the following reaction provides the heat required to complete the process. Notwithstanding the high temperature reached in the reacting mixture, the occurrence of sintering is reduced due to the very short residence time. Last but not least, SCS results a very interesting technique considering its simple adaptability for in situ catalysts deposition on structured supports, as ceramic or metallic monoliths, foams, tissues, mattresses, etc., as outcome of engineering industrialized or semi-industrialized processes. In fact, once prepared, the precursors solution can be deposited onto the structured supports by infusion, immersion, or spraying. The catalytic layer strictly anchored to the support can be easily obtained by placing the infused/immersed/sprayed support into an oven to start up the exothermic synthesis reactions. A series of continuous conveyor belts, ovens and infusion spraying nozzles can be, in fact, designed to realize a continuous industrial process. In view of the speediness of in situ SCS method for structured catalysts preparation and of its relatively low cost, in terms of starting materials and energetic expense, such a technique represents a very promising and cost-effective alternative to more traditional processes for catalytic systems preparation proposed in the recent past, as deep coating or wash-coating [13,14]. As a drawback of this technique, during the synthesis the formation of NOx is possible [12]: metal nitrates can undergo a partial thermal oxidation just before the main reaction is ignited, thus releasing nitrogen oxides, as well organic fuels containing nitrogen atom, like glycine and β-alanine, can decompose generating NOx. Apparently, being the process very fast, little time is given for thermal decomposition prior to ignition, and only small amounts of NOx are expected to be released. Doubtless, NOx emissions can anyhow become an issue when scaling up the process to the industrial level, where larger production is necessary. In that case a small NOx abatement reactor by selective catalytic reduction with ammonia might be envisaged.

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2.2. Catalyst synthesis and characterization The synthesis of LaMnO3·2ZrO2 catalyst, starting from metal nitrates of La, Mn and Zyrconyl and using glycine as fuel is reported as an example. The overall reactions in the general form can be written as: La(NO3)3 + Mn(NO3)3 + 2ZrO(NO3)2 + 44/9COOH-CH2-NH2 → LaMnO3·2ZrO2 + 88/9CO2 + 110/9H2O + 125/18N2 The precursors and fuel (Φ = 1), dosed in the stoichiometric amount, were dissolved in distilled water and the resulting solution, thoroughly stirred to ensure complete dissolution of all reagents, was then transferred in a ceramic dish and placed into an electric oven set at 450°C. After water evaporation and a significant increase in the system viscosity, the mixture frothed and swelled, until a fast and explosive reaction took off, and large amounts of gases evolved. The heat released in the fast reaction allowed the formation of the LaMnO3·2ZrO2 powder of a foamy structure, easy to be crumbled. The whole process was over after 5–6 min, but the time between the actual ignition and the end of the reaction was less than 10 s. Figure 1 shows a sequence of the SCS of LaMnO3·2ZrO2 powder: the foamy and spongy structure of the catalyst is visible.

Figure 1. Sequence of the SCS of LaMnO3·2ZrO2 powder.

Deeper investigations by FESEM enlightened such a structure, as shown in Fig. 2: perforated waffles with nanometric pores and complex network configuration were present. The BET specific surface area of the as-prepared sample was 132 m2 g-1 [15].

Figure 2. FESEM images of LaMnO3·2ZrO2 powder prepared by SCS.

The XRD spectrum for as-synthesized sample is presented in Fig. 3: weak diffraction peaks of tetragonal ZrO2 and orthorhombic LaMnO3 were detected [15], enlightening the simultaneous growth of both phases and the purity of the as-prepared sample. The quality of the ceramic oxide is very high. Moreover, the grain sizes of the two detected phases were calculated via the Scherrer equation: 45 nm for LaMnO3 and 10 nm for ZrO2 [15], pointing out how the SCS allowed reaching real nanoscale dimensions.

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Figure 3. XRD spectrum of LaMnO3·2ZrO2 powder prepared by SCS.

SCS allows the preparation of excellent materials also of true catalysts such as e.g. noble metal containing complex catalysts. Very high dispersion of the metal can be obtained. For example, the addition of Pd nitrate to the starting solution allows obtaining very fine Pd clusters on the support (Φ = 1): La(NO3)3 + Mn(NO3)2 + 2ZrO(NO3)2 + Pd(NO3)2 + 56/9C2H5O2N → Pd(LaMnO3·2ZrO2) + 112/9CO2 + 140/9H2O + 155/18N2 FT-IR spectroscopy analysis of a pure powder pressed disk of Pd(LaMnO3·2ZrO2) catalyst prepared by SCS, after outgassing at 500°C, revealed the small particle size of the powder, thanks to the very high transmission of the IR light. The low temperature adsorption of CO allowed enlightening a highly dispersed zerovalent Pd metal centers [16]: sign that a good dispersion of the noble metal can be obtained by SCS method. The as-prepared Pd(LaMnO3·2ZrO2) catalyst is very suitable for natural gas (NG) combustion [12,15]. The catalytic activity towards CH4 total oxidation was tested in a lab-scale fixed-bed reactor: 0.1 g of catalyst was mixed with 0.9 g of SiO2 (0.2–0.5 mm in size, to prevent the catalytic bed clogging), sandwiched between two quartz wool layers, and inserted in a quartz tube (4 mm ID). The obtained reactor was placed into a PID regulated electrical oven and fed with 50 Ncm3 min-1 of a gaseous mixture containing 2% CH4 and 16% O2 in He [12], which corresponded to a gas hourly space velocity (GHSV) of 6,000 h-1. The reactor temperature was measured by a Kthermocouple placed inside the catalytic bed. By feeding the reactive gaseous mixture, the catalytic bed was first heated up to 800°C at 50°C min-1, then the oven temperature was decreased at 2°C min-1 rate by monitoring the outlet CO2, CO, CH4 and O2 concentrations with a continuous analyzer (ABB Company), thus allowing the evaluation of CH4 conversion. The catalyst presented a T50 (50% CH4 conversion temperature) of 570°C, compared to T50 equal to 720°C of a pure SiO2 fixed bed tested in the same conditions [15].

3. Practical application In the light of the numerous stringent European regulations being proposed or adopted as regards NOx and CO emissions from domestic appliances fuelled with NG, a greater penetration of low-NOx burners into the global boilers market is expected in the next future. In the last decade pre-mixed combustion within porous media has been the object of extensive experimental and theoretical research [17,18], especially in the light of its remarkable potential in enhancing the efficiency of the heat transfer and reducing the impact on the environment related to pollutant emissions: CO2 and unburned hydrocarbons (HC) are well known as greenhouse gases, while CO and NOx are toxic, even in very small concentrations. A major goal and challenge for modern NG burners for domestic boiler applications is a wide power modulation range, which can satisfy the demands of

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different uses, ranging from hot sanitary water production, which requires on average 25 kW per apartment, down to 2-3 kW, that represent the necessary heating requirement of medium size apartments built with well efficient thermal insulation. At the same time, this would entail a decrease in the number of start-up and switch-off cycles, which can cause high energy loss, significant CO emissions and material stresses due to thermal shock. In this context, notwithstanding the great advantage coming with the low NOx emissions over the large modulation range, non catalytic premixed burners generally suffer from high and almost unacceptable CO and HC emissions at low Qs values (e.g. 200-400 kW m-2, corresponding to the lowest power values of the modulation range); the comparatively low flame temperatures occurring in the ‘weak’ radiant regime significantly affect the completeness of NG combustion. An improvement in the performance of the radiant premixed burners could be obtained by adopting perovskite-based catalysts, attractive because of their low cost, thermo-chemical stability at comparatively high temperature (900-1100 °C) and catalytic activity [19]: such catalysts increase the fuel flow rate fraction burnt within or just downstream the burner deck, thus maximizing the heat fraction transferred by radiation, cooling the flame temperature and improving the combustion completeness with lower CO, unburned HC and NOx levels. For such a purpose, a deposition technique based on in situ SCS was developed for the application of the catalyst on metal fiber burners. Based on the results previously obtained on powders [18,20,21], the most promising Pd(LaMnO3·2ZrO2) catalyst was employed. For its deposition on FeCrAlloy® fiber burner, optimum operating procedure to guarantee both good adherence of the catalytic layer to the metallic surface and high enough specific surface area of the deposited catalyst layer, was determined; the rapidity and low cost characteristics of the SCS route were preserved, too. Firstly, the FeCrAlloy® supports were kept at 1200°C for 10 min under O2 flow (0.5 vol % in N2) so as to favor the regular growth on the fiber surface of α-Al2O3 grains into a uniform protective layer, moreover characterized by an external surface morphology able to ensure a good adherence of the catalytic phase to be deposited on the metallic mat [22]. Subsequently, starting from a solution containing all the catalysts precursors (see reaction 2), an in situ spray-pyrolysis SCS was adopted to develop the catalyst on the metal fiber panels made of FeCrAlloy®, and therefore produce catalytic burners. The aqueous solution of the precursors was sprayed over the surface of the FeCrAlloy® panels, previously heated at 400°C. Due to in situ pyrolysis SCS occurring on the hot panel’s surface, catalyst formation was obtained. The panels were then placed back into the hot oven to stabilize the coating. The spray deposition cycle was repeated several times in order to achieve the desired catalyst load (namely, 2% w/w). For a further stabilization and complete crystallization of the catalytic phase, the burners were finally calcined at 900°C for 2 h in still air. SEM analyses were carried out on the as-prepared burners to evaluate the adherence quality of the catalytic layer and to verify whether the highly porous morphology observed on catalytic powders was maintained after the deposition on the metal fibers. As enlightened on Fig. 4, a highly corrugated and porous catalyst layer was formed, assuring an optimum gas-solid interaction for the heterogeneous catalysis; the thickness of the catalytic layer was about 2-3 μm.

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Figure 4. SEM images of FeCrAlloy® fibers catalyzed with Pd(LaMnO3·2ZrO2) by in situ SCS.

Tests under realistic operating conditions were performed on a partially modified commercial condensing boiler test rig for domestic application (Giannoni France), mounting a round flat catalytic burner. A bare burner was also tested as a reference counterpart to assess the effectiveness of catalyzed burner. CH4 was fed to a modulating electrovalve, able to vary its volumetric flow rate (max power output: 30 kW). Air coming from a blower was mixed with CH4 in a Venturi positioned so that a proper mixing was achieved before entering the burner. The cylindrical fiber-mat burner, fitted vertically in the combustion chamber, fired through the heat exchanger coils. The burner diameter was approximately 10 cm. Figures of the catalytic burner firing in two different combustion regimes are shown in Fig. 5.

Figure 5. Pictures of the FeCrAlloy® catalyzed burners at high (left) and low (right) power.

Tests were carried out over a wide range of operating conditions by varying the nominal power (Q) from 12 to 28 kW and the air excess (Ea) from 5 to 45% (i.e. λ from 1.05 to 1.45). The flue gases composition (O2, CO2, CO, and NO) was monitored by means of a multiple gas continuous analyzer (ABB Company). Figure 6 shows the CO and NO concentrations attained in the flue gases for both burners. When the λ approached stoichiometric condition, the non-catalytically assisted combustion was strongly penalized, given that the reduced O2 partial pressure can be a limiting factor for the conversion of CO into CO2, while in the presence of the catalyst those unacceptable CO emissions were lowered significantly. The beneficial effect of the catalyst was slightly less evident at higher both Ea and Q values. Considering the NO emissions, the contributions of the catalyst to the combustion was less evident, independently of Ea and Q: the NO emissions from the catalytic burner were only slightly lower compared to those of the bare counterpart.

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Figure 6. CO and NO emission vs Ea of air as a function Q.

4. Conclusions Solution Combustion Synthesis (SCS) is becoming one of the most important ways to produce a wide range of advanced porous ceramic, metallic materials and nanostructured catalysts, compared to the more conventional and expensive processes. SCS process is, in fact, characterized by exothermic, fast and self-sustaining reactions, formation of high purity products with a variety of size and shape, relatively easy procedures, use of relatively simple equipment and cheap reactants. Thanks to these main characteristics, SCS is easily tunable to complex systems to produce directly in situ structured catalysts. A successful example of in situ SCS was reported: a series of experimental tests on ad-hoc prepared catalytic premixed burner for household applications displayed the lower environmental impact mainly in terms of CO, compared to the bare counterpart, when a Pd(LaMnO3·2ZrO2) catalyst was properly deposited over the burner. The catalytic burner was able, in fact, to stabilize the combustion process within the porous medium in a greater extent, thus maximizing the heat fraction transferred by radiation (higher thermal efficiency), cooling the flame temperature (slightly lower NO emissions) and enhancing the degree of completeness of NG combustion (lower CO emissions).

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]

A. Varma, A.S. Rogachev, A.S. Mukasyan, S. Hwang, Adv. Chem. Eng. 24 (1998) 79. K.C. Patil, S.T. Aruna, S. Ekambaran, Curr. Op. Solid State Mat. Sci. 2 (1997) 158. K.C. Patil, S.T. Aruna, T. Mimani, Curr. Op. Solid State Mat. Sci. 6 (2002) 507. M.A. Keane, J. Mat. Sci. 38 (2003) 4661. M.A. Pena, J.G.L. Fierro, Chem. Rev. 101 (2001) 1981). J.J. Moore, H.J. Feng, Prog. Mat. Sci. 39 (1995) 243. J.J. Moore, H.J. Feng, Prog. Mat. Sci. 39 (1995) 275. A.S. Mukasyan, C. Costello, K.P. Sherlock, A. Varma, Sep. Pur. Tech. 25 (2001) 117. K. Deshpande, A.S. Mukasyan, A. Varma, Chem. Nanomat. 16 (2004) 4896. A. Mukasyan, P. Epstein, P. Dinka, Proc. Comb. Inst. 31 (2007) 1789. A.S. Mukasyan, P. Dinka, Int. J. Self-Prop. High-Temp. Synth. 16 (2007) 23. A. Civera, G. Negro, S. Specchia, G. Saracco, V. Specchia, Catal. Today 100 (2005) 275. S. Cimino, R. Pirone, L. Lisi, Appl. Catal. B: Environ. 35 (2002) 243. M. Valentini, G. Groppi, C. Cristiani, M. Levi, E. Tronconi, P. Forzatti, Catal. Today 69 (2001) 307. [15] S. Specchia, E. Finocchio, G. Busca, P. Palmisano, V. Specchia, J. Catal. 263 (2009) 134. [16] M. Daturi, G. Busca, R.J. Willey, Chem. Mat. 7 (1995) 2115. [17] S. Specchia, A. Civera, G. Saracco, Chem. Eng. Sci. 59 (2004) 5091.

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[18] S. Specchia, M.A. Ahumada Irribarra, P. Palmisano, G. Saracco, V. Specchia, Ind. Eng. Chem. Res. 46 (2007) 6666. [19] M.F.M. Zwinkels, S.G. Järâs, P. Govin Menon, T.A. Griffin, Catal. Rev. Sci. Eng. 35 (1993) 319. [20] P. Forzatti, G. Groppi, Catal. Today 54 (1999) 165. [21] S. Specchia, A. Civera, G. Saracco, V. Specchia, Catal. Today 117 (2006) 427. [22] D. Ugues, S. Specchia, G. Saracco, Ind. Eng. Chem. Res. 43 (2004) 1990.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V.

Impact of NO on the decomposition of supported metal nitrate catalyst precursors and the final metal oxide dispersion Mariska Wolters, Ignacio C. A. Contreras Andrade, Peter Munnik, Johannes H. Bitter, Petra E. de Jongh, Krijn P. de Jong Inorganic Chemistry and Catalysis, Debye Institute for Nanomaterials Science, Utrecht University, Sorbonnelaan 16, 3584 CA Utrecht, The Netherlands

Abstract In previous communications we have shown that the decomposition of nickel and cobalt nitrate in a flow of NO/He prevented agglomeration, yielding high nickel and cobalt oxide dispersions. We now report on the impact of NO on the decomposition of first row transition metal nitrates, i.e. Sc, Mn, Fe, Co, Ni, Cu, Zn, using thermal gravimetric analysis and mass spectrometry. It was found that NO decreased the temperature of decomposition significantly for all investigated metal nitrates. For cobalt, nickel and copper nitrate it was verified that decomposition in the presence of NO yielded high dispersions and narrow particle size distributions, whereas in Ar agglomeration resulted in broad particle size distributions. The beneficial effect of NO on the dispersion of Co, Ni and Cu coincided with a large difference in the decomposition profiles of these metal nitrates compared to that in Ar. It was found that NO induced fast and complete hydrolysis to highly dispersed cobalt, nickel and copper hydroxynitrates which decomposed to yield highly dispersed metal oxides. This is in contrast to literature reports that ascribe loss in dispersion to the formation of metal hydroxynitrate intermediates. Keywords: nitric oxide, calcination, copper, nickel, cobalt

1. Introduction Transition metal nitrate hydrates are industrially favored precursors for the preparation of supported metal (oxide) catalysts because of their high solubility and facile nitrate removal. The final phase and particle size depend on the experimental conditions, as reported for both supported and unsupported metal nitrates [1-3]. Several authors report that decreasing the water partial pressure during the decomposition of unsupported nickel nitrate hexahydrate, via vacuum or a high gas flow, increases the final NiO surface area [3, 4]. The low water partial pressure results in dehydration of the nickel nitrate hydrate to anhydrous nickel nitrate followed by decomposition to NiO. Decomposition at higher particle pressures, however, occurred through the formation of intermediate nickel hydroxynitrates prior to decomposition to NiO. Thus, NiO obtained via intermediate nickel hydroxynitrate species showed a poorer surface area (1 m2/g) compared to NiO obtained via anhydrous nickel nitrate species (10 m2/g) [4]. For supported zinc and copper nitrate a similar observation was reported. Louis et al. investigated the impact of the drying temperature on the copper and zinc oxide dispersion and found that drying at elevated temperatures (90-200°C) resulted in the formation of wide particle size distributions [5-7]. The loss in dispersion was ascribed to the formation of large copper and zinc hydroxynitate crystals during drying, which decompose to form large CuO and ZnO agglomerates. Highest dispersions were

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obtained by drying at room temperature, which prevents hydrolysis to the hydroxynitrate, followed by thermal decomposition in the presence of hydrogen. The latter was essential as decomposition in air yielded wide particle size distributions, irrespective of the drying treatment. In the case of supported cobalt and nickel nitrate agglomeration during drying is limited, but extensive during high temperature decomposition (240-350°C) [8, 9]. Again efficient decomposition gas removal positively affects the dispersion [8], however, agglomeration cannot fully be prevented in this way. Our group recently reported a facile method to prevent agglomeration via thermal decomposition of silica supported nickel or cobalt nitrate in a NO/He flow, reducing the average metal oxide particle size from 10-35 nm in air to 2-7 nm in 1%NO/He [10, 11]. We now report on the impact of NO on the low and high temperature decomposition steps of silica-supported first row d-metal nitrates, and the resulting metal oxide dispersions. It was found that NO significantly lowered the decomposition temperatures of all investigated metal nitrates and changed the decomposition pathways of cobalt, nickel and copper nitrate. For the latter metal nitrates it was verified that an improved dispersion was obtained after thermal treatment in the presence of NO as compared to Ar.

2. Experimental 2.1. Sample preparation Silica supported samples were typically prepared by impregnation, Davicat 1404 silica gel supplied by Grace Davidson (SA = 470 m2 g-1, PV = 0.9 ml g-1, PD = 7 nm), to incipient wetness with a 3 M solution of the appropriate metal nitrate. The metal nitrate solutions were made by dissolving the respective metal nitrate hydrate in a 0.1 M HNO3 solution. In view of the solubility of scandium nitrate a 2 M solution was prepared. After an equilibration period of 15 minutes the impregnates were dried for 48 h in a dessicator to remove most of the solvent water, and stored in closed containers in a dessicator. This approach was preferred over drying at elevated temperatures or no drying because it leaves the nitrate intact and facilitates handling and storage, as compared to wet samples. SBA-15 (SA = 600 m2 g-1, PV = 0.7 ml g-1, PD = 8 nm) samples were prepared similarly to the silica gel samples. Table 1 lists the used metal nitrates, intended metal loading and sample codes, where “SG” stands for silica gel and “SBA” for SBA-15. Table 1. Sample codes and metal loadings. Sample code

Metal nitrate

Metal loading (wt% )

SG-Sc / SBA-Sc

Sc(NO3)3 •4H2O

8/6

SG-Mn / SBA-Mn

Mn(NO3)2 •4H2O

14 / 10

SG-Fe / SBA-Fe

Fe(NO3)3 •9H2O

14 / 10

SG-Co / SBA-Co

Co(NO3)2 •6H2O

15 / 11

SG-Ni / SBA-Ni

Ni(NO3)2 •6H2O

15 /11

SG-Cu / SBA-Cu

Cu(NO3)2 •3H2O

16 /12

SG-Zn / SBA-Zn

Zn(NO3)2 •6H2O

16 /12

On the impact of NO on the metal nitrate decomposition

71

2.2. Thermal analysis Thermal gravimetric analysis (TGA) was performed with a Perkin-Elmer Pyris 1 apparatus. Typically 15 mg of impregnated silica gel was heated with a ramp of 5°C min-1 to 750°C in a 10 ml min-1 flow of Ar or 10% NO/Ar. In parallel evolved gas analysis was performed with a quadrupole Pfeiffer Omnistar mass spectrometer, which was connected to the outlet of the TGA apparatus. Ion currents were recorded for m/z values (m = molar mass of Xz+ ion, z = charge of the ion) of 14, 15, 16, 17, 18, 28, 30, 32, 44, 46, 62 and 63.

2.3. Ex situ thermal treatment and characterization

Typically 100 mg of SBA-15 impregnate was heated with a ramp of 1°C min-1 to 300500°C (depending on nitrate) in a 100 ml min-1 flow of air or 1%NO/Ar. Both air and Ar thermal treatment lead to significant agglomeration [9]. The resulting SBA-15 supported oxides were analyzed with powder X-ray diffraction (XRD) and transmission electron microscopy (TEM). XRD patterns ranging from 10 to 80°2θ were obtained at room temperature with a Bruker-AXS D8 Advance X-ray Diffractometer setup using Co-Kα1,2 radiation. The average metal oxide particle size was calculated using the Debye-Scherrer equation on the most intense diffraction lines. TEM images were obtained on a Tecnai 20 operating at 200 keV.

3. Results and discussion 3.1. Impact of NO on the temperature of decomposition In a previous communication we have shown that NO affects the decomposition temperature of nickel nitrate [11]. However, in this case a drying treatment at 120°C had been applied prior to the measurements, and hence the first hydrolysis step to nickel hydroxynitrate had already occurred. Here, we investigate the impact of NO on the temperature of decomposition of the metal nitrate hydrate, using TGA and MS. Figure 1 shows the TGA and MS results for the thermal treatment of Ni/SG in NO and Ar. The DTA plot, calculated from the TGA signal by taking the derivative, combines the weight-loss steps with the nature of the evolved gasses detected with MS. The temperature of decomposition (Td) was determined from the MS results by measuring the onset of the first NO/NO2 evolution peak. The NO (m/z=30) signal was used for the Ar thermal treatment because of its high intensity, but for the NO thermal treatment NO2 (m/z=46) was used because of the high NO background signal. In Figure 2 an overview is given of the Tds of the investigated nitrates. In general the Td values found in this study are lower than reported previously, which may be ascribed to the presence of the support [12]. In the Ar treatment a clear trend in the Td is observed. With the exception of scandium and iron nitrate, the Td gradually decreases going from the left to right in the periodic table, which can be explained by the decreasing radius of the divalent metal cations. Scandium and iron are trivalent cations and therefore have a smaller cation radius compared to the divalent cations, which decreases the pKa and thus facilitates hydrolysis to the metal hydroxynitrate. The hydrolysis reaction is depicted for copper nitrate in Equation 1. The presence of NO decreases the Td to below 100°C for all investigated metal nitrates. We propose the decrease in Td results from the facilitated decomposition of HNO3 formed during hydrolysis (Equation 1). HSC chemistry for windows 4.1 was used to calculate the reaction enthalpy for the decomposition of HNO3 in the absence or presence of NO (Equations 2 and 3, respectively) and showed that NO lowered the ΔHr0 from +65 kJ/mol in Ar to +36.5 kJ/mol in NO. This is also a possible explanation for the

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smaller differences in Td between different metal nitrates, as hydrolysis is in most cases feasible at such a low temperature that the dehydration rate becomes rate determining. 2Cu(NO3)2•3H2O Æ Cu2(OH)3NO3 + 3HNO3 + 3H2O 2HNO3 Æ 2NO2 + H2O + O2 2HNO3 + NO Æ 3NO2 + H2O

(1) (2) (3)

Figure 1. TGA (left, bottom), DTA (left, top) and MS (right) traces of nickel nitrate Ni/SG during thermal treatment in Ar (…) and NO (―).Gas evolution indicated as; I: H2O, II: H2O + NOx, III: NOx.

Figure 2. Temperature of the low temperature decomposition of SG supported metal nitrates during thermal treatment in 10% v/v NO/Ar (■) or Ar (●). Derived from the onset of the first NO/NO2 evolution peak.

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On the impact of NO on the metal nitrate decomposition

3.2. Impact of the gas atmosphere on the dispersion The ex situ thermally treated samples were characterized using XRD and TEM. SBA-15 was chosen as a support because in this case TEM analysis yields more information. Of all investigated metal oxides only cobalt, nickel and copper oxide showed diffraction lines, indicating the other oxides were either amorphous or the particles too small to detect. For SBA-Co, Ni and Cu large differences between the air and NO thermal treatments were observed both with XRD and TEM. Air calcination resulted in broad particle size distributions and large average crystallite sizes, whereas NO thermal treatment yielded small particles with narrow size distributions (Table 2). The positive effect of NO on the dispersion of cobalt and nickel oxide has been reported previously [10, 11], but the effect on copper we report here for the first time. Table 2. Metal oxide crystallite sizes as obtained from XRD. Sample

Particle size (nm)

x

Air

NO

SBA-Co

10

5

SBA-Ni

12

4

SBA-Cu

23

7

3.3. Impact of the gas atmosphere on the decomposition pattern 3.3.1. Scandium, manganese, iron and zinc nitrate The DTA curves of the decomposition of SG-Sc, SG-Mn, SG-Fe and SG-Zn are shown in Figure 3. The gas evolution, as determined from MS traces is indicated in the curve as follows; I: H2O, II: H2O + NOx and III: NOx. Manganese and iron nitrate decompose well below 200°C in Ar, where the first decomposition step involves only dehydration (I) and the second both dehydration and nitrate decomposition (II), in agreement with literature [2, 13]. The limited stability of iron nitrate may be ascribed to the small radius and high valency, resulting in destabilization of the nitrate anion (vide supra). However, manganese is comparable to cobalt and copper, but still decomposes at a significantly lower temperature. A possible explanation could be a difference in the extend of back donation. It is reported that transition metal nitrates decompose at lower temperatures than alkali metal nitrates with a similar charge density, because of back-donation of delectrons to empty π*-orbitals of the nitrate anion [14]. The presence of NO shifts the second decomposition step (II) to lower temperatures, now almost coinciding with the dehydraton step (I). In contrast, scandium and zinc nitrate show a three step decomposition. After partial dehydration (I), both dehydration and nitrate decomposition occurs in the 100200°C temperature range (II). The last decomposition step occurs much high temperatures (200-500°C) with only NOx evolution and no significant water evolution (III). All steps are broad and continuous such that no intermediates can be identified. For zinc nitrate hydrolysis to Zn(NO3)2·2Zn(OH)2 has been proposed as the first decomposition step [15], but the absence of water evolution at higher temperatures suggests anhydrous zinc nitrate was also formed. Moreover the significant evolution at higher temperatures suggests these species to be more stable than reported previously [12, 15, 16]. A likely explanation is the factor ten higher flow rate used in literature. A higher flow rate, i.e. faster removal of decomposition gasses, might accelerate decomposition.

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The decomposition of supported scandium nitrate hydrate has not been reported before, therefore no comparison to literature can be made. In general the decomposition patterns of scandium nitrate and zinc nitrate are more similar to that of main group metal nitrates such as potassium and calcium nitrate, which typically display decomposition over a very broad temperature range [14, 17]. The resemblance in behavior between scandium and zinc, and non d-metal nitrates is possibly be explained by their empty and full d-shell, respectively, resulting in a lower extent of back bonding between the metal ion and the nitrate. An additional explanation involves the fact that molecular oxygen has to evolve to complete the decomposition, and that the combination of two oxygen radicals to O2 is more efficiently catalyzed by oxides such as CuO, Mn2O3, Fe2O3 and NiO than ZnO and likely Sc2O3 [18]. As a consequence the decomposition is not accelerated by the increasing oxide formation. For both scandium and zinc nitrate the presence of NO during decomposition shifts the decomposition steps involving NOx evolution to lower temperatures, causing the first two decomposition steps in the 25-100°C temperature range to overlap. The high temperature (III) decomposition step is also shifted to lower temperatures, but stretches over a larger temperature range.

Figure 3. DTA curves of the decomposition of iron, manganese, zinc and scandium nitrate in Ar (…) and 10%NO/Ar (―). Gas evolution indicated as; I: H2O, II: H2O + NOx, III: NOx.

3.3.2. Cobalt, nickel and copper nitrate For this group of metal nitrates the difference between NO and Ar thermal treatment is much more apparent than for the other investigated metal nitrates (Figure 4). Decomposition in Ar results in all cases in a broad multi-step NO2 evolution ranging between 100-400°C. Several intermediates have been reported, including several dehydration stages and metal hydroxynitrates [4, 19, 20]. However, the broad pattern suggests multiple phases are present. Heat treatment in NO on the other hand results in rapid hydrolysis of the metal nitrate to its respective metal hydroxynitrate. This was confirmed with in situ XRD and IR (Figure 5), which show the presence of metal hydroxynitrates (a hexagonal layered structure, and a strong νOH band around 3600 cm-1). This phase is then stable over a certain temperature range before decomposing to the metal oxide in a single sharp step (with the exception of the two step decomposition of copper hydroxynitrate to CuO). Even though the decomposition is much faster in the presence of NO, in all cases the agglomeration is significantly reduced (Table 2). Metal hydroxynitrates bear close resemblance to metal hydroxides, which are generally less

On the impact of NO on the metal nitrate decomposition

75

prone to agglomeration than metal nitrate hydrates. It is therefore postulated that the formation of highly dispersed hydroxynitrates is the key to the high dispersion obtained in NO. In several previous reports hydroxynitrate formation resulted in the formation of very large copper oxide particles. An explanation for the apparent discrepancy is the high dispersion of the metal hydroxynitrate phase formed in NO (<10 nm crystal domains, Figure 5), in contrast to the large (>25 nm) copper hydroxynitrate crystal domains that are typically observed after drying at 120°C in stagnant air [5].

Figure 4. DTA curves of the decomposition of cobalt, nickel and copper nitrate in Ar (…) and 10%NO/Ar (―). Gas evolution indicated as; I: H2O, II: H2O + NOx, III: NOx.

Figure 5. XRD patterns (left) and IR spectra (right) of copper, nickel and cobalt hydroxynitrate formed at 150°C in 10%NO/Ar.

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4. Conclusions The impact of NO on the decomposition pattern and dispersion of first row transition metal nitrates was investigated using TGA-MS, XRD and TEM. The presence of nitric oxide lowered the temperature of decomposition significantly for all investigated metal nitrates. For cobalt, nickel and copper nitrate it was verified that the presence of NO during decomposition led to an improved metal oxide dispersion, where decomposition in Ar resulted in agglomeration. It was found that NO, in contrast to Ar, induced rapid hydrolysis of cobalt, nickel, and copper nitrate to highly dispersed metal hydroxynitrates, which was ascribed to a decrease in the decomposition temperature of nitric acid, the product of the hydrolysis. Hence, we show that NO affects the decomposition over the whole temperature range.

Acknowledgements The authors kindly thank Marjan Versluijs and Fred Broersma for their help with the TGA-MS experiments and and Cor van der Spek for the TEM analysis. Johnson Matthey Catalysts are acknowledged for the financial contribution to this work and Steve Pollington and John Casci for their scientific contributions.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.

F. Paulik, J. Paulik and M. Arnold, Thermochimica Acta, 121, 1987, 137 M.A.A. Elmasry, A. Gaber and E.M.H. Khater, J. Therm. Anal. Calorim., 52, 1998, 489 J. Estelle, P. Salagre, Y. Cesteros, M. Serra, F. Medina and J.E. Sueiras, Solid State Ionics, 156, 2003, 233 P.L. Llewellyn, V. Chevrota, J. Ragaib, O. Cerclier, J. Estienne and F. Rouquerol, Solid State lonics, 101-103 1997, 1293 T. Toupance, M. Kermarec and C. Louis, J. Phys. Chem. B, 104, 2000, 965 C. Chouillet, F. Villain, M. Kermarec, H. Lauron-Pernot and C. Louis, J. Phys. Chem. B, 107, 2003, 3565 S. Catillon-Mucherie, F. Ammari, J.M. Krafft, H. Lauron-Pernot, R. Touroude and C. Louis, J. Phys. Chem. C, 111, 2007, 11619 J. van de Loosdrecht, S. Barradas, E.A. Caricato, N.G. Ngwenya, P.S. Nkwanyana, M.A.S. Rawat, B.H. Sigwebela, P.J. van Berge and J.L. Visagie, Top. Catal., 26, 2003, 121 J.R.A. Sietsma, J.D. Meeldijk, M. Versluijs-Helder, A. Broersma, A.J. van Dillen, P.E. de Jongh and K.P. de Jong, Chem. Mater., 20, 2008, 2921 J.R.A. Sietsma, J.D. Meeldijk, J.P. den Breejen, M. Versluijs-Helder, A.J. van Dillen, P.E. de Jongh and K.P. de Jong, Angew. Chem. Int. Ed., 46, 2007, 4547 J.R.A. Sietsma, H. Friedrich, A. Broersma, M. Versluijs-Helder, A. Jos van Dillen, P.E. de Jongh and K.P. de Jong, J. Catal., 260, 2008, 227 T. Nissinena, M. Leskel, M. Gasika and J. Lamminen, Thermochimica Acta, 427, 2005, 155 K. Wieczorek-Ciurowa and A.J. Kozak, J. Therm. Anal. Calorim., 58, 1999, 647 S. Yuvaraj, L. Fan-Yuan, C. Tsong-Huei and Y. Chuin-Tih, J. Phys. Chem. B, 107, 2003, 1044 B. Malecka, R. Gajerski, A. Malecki, M. Wierzbicka and P. Olszewski, Thermochimica Acta, 404, 2003, 125 T. Cseri, Békássy, G. Kenessey, G. Liptay and F. Figueras, Thermochimica Acta, 288, 1996, 137 C. Ettarh and A.K. Galwey, Thermochim. Acta, 288, 1996, 203 P.G. Dickens and M.B. Sutcliffe, Trans. Faraday. Soc., 60, 1964, 1272 W. Brockner, C. Ehrhardt and M. Gjikaj, Thermochim. Acta, 456, 2007, 64 A. Malecki, R. Gajerski, S. Labus, B. Prochowska-Klisch and K.T. Wojciechowski, J. Therm. Anal., 60, 2000, 17

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

A novel approach to synthesize highly selective nickel silicide catalysts for phenylacetylene semihydrogenation Xiao Chen, Anqi Zhao, Zhengfeng Shao, Zhiqiang Ma, Changhai Liang* State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116012, China

Abstract Nickel silicides (NiSix) have been prepared by carbon template for nanostructured NiO and further silicidization with SiH4/H2 at relatively low temperature and atmospheric pressure. The results showed that the formation of nickel silicides involves the following sequence, Ni (cubic) → Ni2Si (orthorhombic) → NiSi (orthorhombic) → NiSi2 (cubic), with increasing temperatures. The as-prepared nickel silicides showed above 92% selectivity to styrene in the semihydrogenation of phenylacetylene due to the electronic and geometrical effects derived from the addition of Si into Ni particles. Keywords: nickel silicide, carbon template, silicidization, phenylacetylene semihydrogenation

1. Introduction The removal of phenylacetylene from styrene to low ppm range by semihydrogenation has become an important industrial process, because catalysts currently used for polystyrene production are extremely sensitive to phenylacetylene in styrene feed stocks. A number of catalysts including supported Pd, Ni, Pt, Pb, Ru, Rh, and Cu [1-6], have been developed and improved in the past tens of years. Palladium catalysts moderated by transition metals and /or by gaseous carbon monoxide in the feed, are now used to remove phenylacetylene from styrene [1, 2]. Supported Ni catalysts showed low activity and selectivity for phenylacetylene hydrogenation in the presence of styrene [3]. Novel catalytic materials for phenylacetylene semihydrogenation with high selectivity to styrene are highly desired under mild conditions. Transition metal silicides with unique physical and chemical properties, such as unusual structural, electronic, magnetic, and catalytic properties, have shown high selectivity in some hydrogenation reactions [7, 8]. Recently, Baiker et al. [7] indicated that amorphous Pd81Si19 catalyst in supercritical CO2 afforded high selectivity to styrene, which was usually necessary for high selectivity in “Lindlar-type” hydrogenations. Supported Ni silicides exhibited highly selectivity for the competitive dehydrogenation and hydrogenolysis of cyclohexane [9]. Supported Co silicides showed high activity and selectivity in selective hydrogenation of naphthalene [10]. However, conventional preparation methods, such as molten salt method, co-reduction route, and chemical vapor deposition inherited from the microelectronic industry resulted in a low surface area and a relatively low catalytic activity. Herein, we firstly demonstrate that carbon template method for nickel oxide and further SiH4/H2 silicidization to nickel silicides are of great potential in controlled synthesis of transition metal silicides. The as-prepared nickel silicides showed high styrene selectivity in the semihydrogenation of

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phenylacetylene. Formation mechanism of nickel silicides and effect of Si on catalytic properties of nickel silicides were studied in detail.

2. Experimental 2.1. Catalyst preparation Ultrahigh surface area carbon material [11] was prepared by a direct chemical activation route in which petroleum coke was reacted with excess KOH at 900oC to produce carbon materials containing potassium salts. These salts were removed by successive water washings. The surface area of the carbon materials measured by the BET method is about 3234 m2/g, the pore volume is about 1.78 cm3/g and the average pore size is about 2.2 nm. The carbon materials were impregnated at room temperature with a saturated aqueous solution of nickel nitrate. The slurry was then filtered and squeezed to remove the liquid on the carbon surface [12]. After drying in air at 60oC for 8 h, the resulting sample was transferred to a quartz reactor inside a tubular resistance furnace. The carbon template was removed by combustion at 500oC under a mixture gas of 20% O2 in Ar. Prior to reduction and silicidization, the reactor was purged with Ar to eliminate residual gases. NiO was reduced in a flow rate of 30 sccm H2 from room temperature to 450 oC, where it was held for 4 h. The reduced samples were cooled to the silicidization temperature under H2 atmosphere, and were silicidized with a 10% SiH4/H2 mixture for 15 min [9]. Then SiH4 was first stopped and the silicidized sample was cooled down to room temperature in H2 (30 sccm), passivated in 1% O2/Ar overnight. The obtained solids will be designated as T-NiSix, where T refers to the silicidization temperature. The exhaust gas was treated with water or alkali liquor during the silification. The reactions are as follows: SiH4 + 2H2O → SiO2 + 4H2 SiH4 + 2KOH + H2O → K2SiO3 + 4H2

2.2. Characterization TG/DTG experiments were performed in Mettler Toledo TGA/SDTA851e thermogravimetry to understand decomposition process of nickel nitrate and the removal temperature of carbon template. The nickel nitrate impregnated carbon was placed in the atmosphere of 80% Ar and 20% O2 and heated at 5 oC/min to the final temperature of 750oC. X-ray diffraction analysis of the samples was carried out using a Rigaku D/MaxRB diffractometer with Cu Kα monochromatized radiation source, operated at 40 KV and 100 mA. The average size of nickel silicide particles was evaluated by the Scherrer formula. The molar fraction Yi of NiSix in a Ni-Ni2Si-NiSi-NiSi2 mixture was calculated from XRD patterns as:

Yi =

Si S1 + S 2 + S 3 + S 4

Where S1, S2, S3, and S4 are the peak areas of the most intense reflections [13] of Ni, Ni2Si, NiSi, and NiSi2, respectively. Magnetic measurements were preformed on a JDM-13 vibrating sample magnetometer. M/H measurements were made with applied fields up to 4000 Oe at room temperature, and the complete hysteresis loops were recorded.

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A novel approach to synthesize highly selective nickel silicide catalysts

The particle size and distribution of the samples were analyzed by transmission electron microscopy (TEM) (Tecnai G220 S-Twin, 200 kV). Powder samples were ultrasonicated in ethanol and dispersed on copper grids covered with a porous carbon film. Energy dispersive X-ray spectroscopy was also performed in the same microscopy.

2.3. Hydrogenation activity measurements

Liquid-phase semihydrogenation of phenylacetylene was carried out in a 50 cm3 closed vessel at controlled temperature. The catalyst was always activated in an ultrapure hydrogen stream at 300oC for 1 h, followed by cooling to room temperature. Approximately 0.2 g of the catalyst was placed in the reactor with 10 mL of 1 M phenylacetylene-ethanol solution. The vessel was filled with H2 to 0.27 MPa and vented it three times so as to remove the air in the vessel. Then the reactor was filled with H2 to 0.41 MPa pressure. The reaction was carried out at 50 oC for 5 h with the stirring condition. The products were analyzed by 7890 gas chromatograph with FID detector.

3. Results and discussion 3.1. Synthesis of nanostructured NiO by the carbon template method 100

a

NiO

b

200

DTG

Intensity (a. u.)

Mass loss (%)

80

60

40

111 220

311 222

TG

20 100

200

300

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Fig.1. a TG/DTG curves of Ni(NO3)2-impregnated carbon; b XRD pattern of NiO prepared by the carbon template method.

Figure 1a shows TG/DTG curves of the Ni(NO3)2-impregnated carbon under 80% Ar and 20% O2. As can be seen in Fig. 1a, mass loss of about 24% from 60 to 160oC can be attributed to the removal of surface physisorbed water and partial dehydration of the precursor. The maximum rate of mass loss occurs at 260oC due to the decomposition of Ni(NO3)2 and the partial combustion of carbon template. A small peak at 330 oC may be due to the further loss of carbon template. The carbon combustion is completed at temperature higher than 400oC. It had been found that the presence of the metal nitrate can catalyze the combustion of the activated carbon [14], which results to sintering of the synthesized inorganic particles. Therefore, Ar was firstly passed over the Ni(NO3)2impregnated carbon before the oven was heated to 300oC in order to avoid severe combustion reaction, then Ar was switched to the 20% O2/Ar and heated to 500 oC for 200 min to remove carbon template completely in the preparation process. XRD pattern of nanostructured NiO by the carbon template method is shown in Fig. 1b. The (111), (200), (220), (311), and (222) reflections due to cubic phase NiO (JCPDS No. 47-1049) were clearly observed. The average size of the particles estimated by the Scherrer equation is about 20 nm. The peaks due to the carbon template cannot be observed, indicating the carbon template was completely removed in the process.

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3.2. Synthesis of nickel silicides by reduction and silicidization 100

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Fig. 2. a XRD patterns of Ni and NiSix; b The molar fraction Yi of NiSix in a Ni-Ni2Si-NiSi-NiSi2 mixture calculated from XRD patterns of NiSix.

Figure 2a shows XRD patterns of Ni and NiSix obtained by reduction with H2 and silicidization with SiH4/H2. The crystalline phases of the samples are identified according to the JCPDS files (Ni, cubic, No. 04-0850; Ni2Si, orthorhombic, No. 481339; NiSi, orthorhombic, No. 38-0844; NiSi2, cubic, No. 43-0989). XRD patterns of all samples have intensive diffraction peaks at 44.51, 51.85, and 76.37°, which are due to metallic nickel. The diffraction peaks due to Ni2Si at 32.48, 39.46, 42.38, 43.49, 45.50, 48.76, 53.34, and 69.00° are observed on the 300 - NiSix sample indicates that Ni2Si is formed by the silicidization of metallic nickel with SiH4. Meanwhile, weak peaks at 45.84 and 47.28° are also observed, and can be assigned to NiSi. When the temperature increases to 350°C, the diffraction peaks due to NiSi become sharper while the peaks of Ni become weaker. Further increasing silicidization temperature to 400 °C, new diffraction peaks at 28.60, 47.41, 56.33, and 76.59°, which can be attributed to NiSi2, are observed. When the silicidization temperature increases to 450°C, the peaks due to NiSi2 become sharper while the peaks of NiSi become weaker, indicating the transformation from NiSi to NiSi2. As shown in the inset in Fig. 2a, there is a slight diffraction peak shift from 45.50 to 45.84o with the increasing of silicidization temperature, which is due to the transformation of Ni2Si (121) to NiSi (112). In addition, the diffraction peak at 47.28o shifted to 47.41o with increasing of temperature, which is due to the transformation of NiSi (211) to NiSi2 (220). Fig. 2b shows that the molar fraction Yi of NiSix calculated from XRD patterns. It is clear that the formation of nickel silicides involves the following sequence, Ni→Ni2Si→NiSi→NiSi2, with the increase of silicidization temperatures. The phase transformation among metallic nickel and its silicides with the elevated silicidization temperatures was also observed by Foggiato et al. [15]. Figure 3 gives the crystal structures of Ni2Si, NiSi, and NiSi2. NiSi2 with a cubic cell belongs to the Fm3m, No. 225 space group. Surface on NiSi2 is characterized by alternating Si and Ni atoms, and has no direct Ni-Ni or Si-Si bonds. Its bonding type is different from metallic Ni and bulk Si [16]. Ni2Si with a orthorhombic cell belongs to the Pbnm, No. 62 space group, while NiSi with a simple orthorhombic primitive cell belongs to the Pnma, No. 62 (oP8 Pearson symbol) space group. The orthorhombic unit cell contains four nickel and four silicon atoms. Nickel atoms have six first silicon

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neighbors forming a strongly distorted octahedron, and silicon atoms have six first neighbors (nickel atoms), which form a distorted trigonal prism [17].

Fig. 3. The crystal structures of Ni2Si, NiSi, and NiSi2.

M (emu/g)

The formation of nickel silicides was also confirmed by magnetic measurements [18]. Fig. 4 shows the magnetization curves of the as-prepared Ni nanoparticles and the NiSix samples. All of the NiSix samples show certain coercivity at room temperature, which is characteristic for the ferromagnetic behavior of nanoparticles. The variability of coercivity is attributed to the formation of the NiSix phases. Their saturation magnetization values (Ms) at 4000 Oe drastically decrease after an amount of Si atoms adding into Ni. With increasing the silicidization temperatures, the metallic nickel was transformed into NiSi2 via Ni2Si and NiSi, while Ms values decrease firstly, and then increase. This may be due to different electronic and crystal structures of Ni2Si and NiSi2. Jarrige et al. [19] reported that the d states in Ni2Si had a strong Ni metal-like character, while NiSi was found to be non-ferromagnetic [17]. As shown in the inset in Fig. 4, the nickel silicide samples have magnetic character, indicating that the materials can easily be separated from reaction mixture in a magnetic field. 60

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Fig. 4. Hysteresis M/H loops at room temperatures corresponding to Ni and NiSix.

In order to investigate the structural properties and the composition distribution, TEM, HRTEM, and EDX were carried out. It can be seen that the particles of NiSix are aggregated to a certain extent. HRTEM image of the 350 - NiSix sample demonstrated that outer surface of nickel particles was covered with a thin layer of NiSix, i.e. Ni2Si

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[20] (the lattice spacing of 0.25 nm corresponds to (020) plane of Ni2Si) and NiSi [21] (the lattice spacing of 0.21 nm corresponds to (210) planes of NiSi). The EDX spectrum further confirmed the existence of Si element in Ni particles in the samples.

Fig. 5. A representative TEM image (a), HRTEM image (b), and EDX spectrum (c) of the 350NiSix catalyst.

3.3. Phenylacetylene semihydrogenation reaction Figure 6 shows the conversion and selectivity of the NiSix samples in the phenylacetylene semihydrogenation reaction. Phenylacetylene conversions are 100, 56.4, 14.6, 33.8, and 79.9%, respectively, over metallic Ni, 300 - NiSix, 350 - NiSix, 400 - NiSix, and 450 NiSix catalysts. The corresponding selectivities to styrene are 86.8, 90.5, 92.3, 89.4, and 87.7%, respectively. The conversions for the samples NiSix are lower than Ni while the selectivities are improved, which may be due to Si atoms residing in the interstitial sites between Ni atoms, changing the nickel unit cell lattice thereby influencing adsorption of styrene. This means that formation of NiSix prevents styrene overhydrogenation to ethylbenzene. The phenylacetylene conversion decreases initially and then increases, while the selectivity to styrene follows a parabolic curve with elevated silicidization temperatures. The 350 - NiSix sample shows the lowest conversion but the highest styrene selectivity. The reason is that the Ni2Si (3d8.74sp1.2) [22] with a stronger Ni (3d94s1) metal-like character is firstly formed with the elevated silicidized temperatures, and has similar chemical properties to metallic Ni. Subsequently, the NiSi (3d8.64sp1.3) is formed, and both its electronic structure and crystal structure are significantly changed, such that catalytic activity falls off. When the silicidization temperature reached 450oC, the catalyst particles are partly agglomerated, which further reduces the number of active sites, but the NiSi2 phase with similar crystal structure to metallic Ni appears, which may have similar chemical properties to metal Ni from the perspective of the geometrical effect, so that the phenylacetylene conversion increases again. It had been

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reported that Raney Ni catalysts were not suitable for selective hydrogenation of phenylacetylene due to their low activity and selectivity. However, the as-prepared NiSix catalysts show high activity and selectivity, which is attributed to the formation of metallic Ni particles modified by Si atoms using SiH4/H2 silicidization. These results indicate that the carbon template method for oxides and further SiH4/H2 silicidization to nickel silicides is a promising approach in the selective hydrogenation of phenylacetylene. 100

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350 - NiSix

400 - NiSix

450 - NiSix

Fig. 6. Results of the phenylacetylene semihydrogenation over the as-prepared NiSix catalysts.

4. Conclusions Nanoscale nickel silicides show excellent catalytic activity and high selectivity for phenylacetylene semihydrogenation and have been successfully synthesized using a carbon template method for oxides and further SiH4/H2 silicidization to silicides. XRD, magnetic measurements, HRTEM, and EDX confirm the formation of NiSix involved the following sequence, Ni→Ni2Si→NiSi→NiSi2, with the increasing silicidization temperatures. Si atoms reside into the interstitial sites between Ni atoms and change the nickel unit cell lattice, influencing catalytic activity and selectivity.

Acknowledgments We gratefully acknowledge the financial support provided by the National Natural Science Foundation of China (No. 20973029), the Program for New Century Excellent Talents in Universities of China (No. NCET-07-0133) and Doctoral Fund of Ministry of Education of China (No. 20070141048).

References 1. 2. 3.

S. Domínguez-Domínguez, Á. Berenguer-Murcia, Á. Linares-Solano, and D. CazorlaAmorós, 2008, Inorganic materials as supports for palladium nanoparticles: Application in the semi-hydrogenation of phenylacetylene, J. Catal., 257, 87-95 S. Domínguez-Domínguez, Á. Berenguer-Murcia, B. K. Pradhan, Á. Linares-Solano, and D. Cazorla-Amorós, 2008, Semihydrogenation of Phenylacetylene catalyzed by palladium nanoparticles supported on carbon material, J. Phys. Chem. C, 112, 3827-3834 F. M. Bautista, J. M. Campelo, A. Garcia, D. Luna, J. M. Marinas, R. A. Quiros, and A. A. Romero, 1998, Influence of surface support properties on the liquid-phase selective hydrogenation of phenylacetylene on supported nickel catalysts, Catal. Lett., 52, 205-213

84 4. 5. 6. 7.

8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22.

X. Chen et al. B. A. Wilhite, M. J. McCready, and A. Varma, 2002, Kinetics of phenylacetylene hydrogenation over Pt/γ - Al2O3 Catalyst, Ind. Eng. Chem. Res., 41, 3345-3350 R. D. Adams, B. Captain, and L. Zhu, 2006, The importance of cluster fragmentation in the catalytic hydrogenation of phenylacetylene by PtRu5 carbonyl cluster complexes, J. Organomet. Chem., 691, 3122-3128 J. Pellegatta, C. Blandy, V. Collière, R. Choukroun, B. Chaudret, and P. Cheng, 2002, Karine philippot catalytic investigation of rhodium nanoparticles in hydrogenation of benzene and phenylacetylene, J. Mol. Cata. A: Chem., 178, 55-61 R. Tschan, R. Wandeler, M. S. Schneider, M. M. Schubert, and A. Baiker, 2001, Continuous semihydrogenation of phenylacetylene over amorphous Pd81Si19 alloy in “supercritical” carbon dioxide: relation between catalytic performance and phase behavior, J. Catal., 204, 219-229 H. Thomas and II Maugh, 1984, A new route to intermetallics metal silicides and related intermetallic compounds with unusual properties are formed by exposing supported metals to volatile organometallics, Science, 225, 403 R. G. Nuzzo, L. H. Dubois, N. E. Bowles, and M. A. Trecoske, 1984, Derivatized, high surface area, supported nickel catalysts, J. Catal., 85, 267-271 C. Liang, A. Zhao, X. Zhang, Z. Ma, and R. Prins, 2009, CoSi particles on silica support as a highly active and selective catalyst for naphthalene hydrogenation, Chem. Commun., 20472049 G. C. Grunewald and R. S. Drago, 1991, Carbon molecular sieves as catalysts and catalyst supports, J. Am. Chem. Soc., 113, 1636-1639 C. Liang, Z. Ma, H. Lin, L. Ding, J. Qiu, W. Frandsen, and D. Su, 2009, Template preparation of nanoscale Ce xFe1-xO2 solid solutions and their catalytic properties for ethanol steam reforming, J. Mater. Chem., 19, 1417-1424 C. Liang, F.Tian, Z. Li, Z. Feng, Z. Wei, and C. Li, 2003, Preparation and adsorption properties for thiophene of nanostructured W2C on ultrahigh-surface-area carbon materials, Chem. Mater., 15, 4846-4853 F. Schüth, 2003, Endo- and exotemplating to create high-surface-area inorganic materials, Angew. Chem. Int. Ed., 42, 3604-3622 J. Foggiato, W. S. Yoo, M. Ouaknine, T. Murakami, and T. Fukada, 2004, Optimizing the formation of nickel silicide, Mater. Sci. Eng. B, 114-115, 56-60 R. G. Nuzzo and L. H. Dubois, 1984, The chemisorption and catalytic properties of nickel intermetallic compounds: studies of single crystalline and high surface area, suppourted materials, Appl. Surf. Sci., 19, 407-413 D. Connétable1 and O. Thomas, 2009, First-principles study of the structural, electronic, vibrational, and elastic properties of orthorhombic NiSi, Phys. Rev. B: Condens. Matter, 79, 094101 H. Praliaud and G. A. Martin, 1981, Evidence of a strong metal-support interaction and of Ni-Si alloy formation in silica-supported nickel catalysts, J. Catal., 72, 394-396 I. Jarrige, N. Capron, P. Jonnard, 2009, Electronic structure of Ni and Mo silicides investigated by x-ray emission spectroscopy and density functional theory, Phys. Rev. B: Condens. Matter, 79, 035117 X. Q. Yan, H. J. Yuan, J. X. Wang, D. F. Liu, Z. P. Zhou, Y. Gao, L. Song, L. F. Liu, W. Y. Zhou, G. Wang, and S. S. Xie, 2004, Synthesis and characterization of a large amount of branched Ni 2Si nanowires, Appl. Phys. A, 79, 1853-1856 C. J. Kim, K. Kang, Y. S. Woo, K. G. Ryu, H. Moon, J. M. Kim, D. S. Zang, and M. H. Jo, 2007, Spontaneous chemical vapor growth of NiSi nanowires and their metallic properties, Adv. Mater., 19, 3637-3642 A. Franciosi and J. H. Weaver, 1982, Electronic structure of nickel silicides Ni2Si, NiSi, and NiSi2, Phys. Rev. B: Condens. Matter, 26, 546-553

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Preparation of calcium titanate photocatalysts for hydrogen production Katsuya Shimura, Hiroyo Miyanaga and Hisao Yoshida* Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan, *[email protected]

Abstract Various CaTiO3 samples having different particle sizes, shapes, crystal defects and impurity phases were prepared by three methods, i.e., co-precipitation, homogeneous precipitation and solid-state reaction methods. The CaTiO3 samples were loaded with Pt co-catalyst (0.1 wt%) and examined for both the photocatalytic water decomposition (WD) and the photocatalytic steam reforming of methane (PSRM). The highest activities for the WD and the PSRM were obtained over the samples prepared by the solid-state reaction method from rutile and anatase TiO2, respectively. The controlling factors in their activity were discussed. Keywords: photocatalysis, hydrogen, water, methane, calcium titanate

1. Introduction The development of a hydrogen production method from renewable resources and natural energy would be important to realize a sustainable society. Since the 0 photocatalytic water decomposition (referred to as WD; H2O → H2 + 1/2O2, ΔG298 K =237 kJ/mol) would be one of the most desirable systems, various photocatalysts for it has been developed so far [1]. On the other hand, the photocatalytic hydrogen production from water and biomass such as ethanol [2], saccharides [3] and methane [4] is also valuable. In these systems, hydrogen could be obtained more efficiently than the WD due to having a low ∆G value, and the carbon dioxide formed from them would not influence the global warming in the carbon neutral concept. We found that some kinds of Pt-loaded semiconductors could efficiently produce hydrogen from water vapor and 0 methane ( CH4 + 2H2O → 4H2 + CO2, ΔG 298 K =113 kJ/mol) [4-7], which can be also interpreted as photocatalytic steam reforming of methane, thus referred to as PSRM. CaTiO3 has been known to show the photocatalytic activities. Pt-loaded CaTiO3 was reported to show photocatalytic activities for both the WD [8] and the PSRM [7] upon the UV light irradiation. Generally, important factors of the semiconductor photocatalyst in these reactions are considered to be some structural ones such as the crystallite size, the specific surface area and the band structure. In the present study, we prepared various CaTiO3 samples by some methods and examined their photocatalytic activities for both the WD and the PSRM. And, we discussed the important factors of CaTiO3 structure in the photocatalytic activity for the WD and the PSRM.

2. Experimental 2.1. Catalyst preparation CaTiO3 samples were prepared by three methods, i.e., co-precipitation method (CP), homogeneous precipitation method (HP) and solid-state reaction method (S).

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In the co-precipitation method, CaCl2·2H2O (Wako, 99.9%, 5.4 g) and TiCl4 HCl solution (Wako, 16-17% as Ti, 11.0 g) were dissolved in distilled water (100 ml). The whole solution was added at one time to an aqueous solution (300 ml) of (NH4)2C2O4·H2O (Kishida, 99.5%, 10.4 g), where white precipitation was soon obtained. After neutralization of the solution by an aqueous NH3 (Wako, 10%) the white precipitation was recovered by centrifugal separation and washed with distilled water several times. The obtained powder was dried at 353 K, followed by calcination in air at various temperatures (973-1273 K) for 10 h. In the homogeneous precipitation method, (COOH)2·2H2O (9.3 g), CaCl2·2H2O (5.4 g), and TiCl4 HCl solution (11.0 g) were added to distilled water (300 ml), followed by heating to 363 K. After adding (NH2)2CO (Kishida, 99.0%, 33.1 g) the solution was kept at 363 K until pH of the solution reached to 7 (it took about 5.5 h), where white precipitate was gradually obtained as hydrolysis of the urea proceeded. The precipitates were recovered and washed with distilled water. The obtained powder was dried at 353 K, followed by calcination in air at various temperatures (973-1273 K) for 10 h. In the solid-state reaction method, the starting materials were mixed by a wet ballmilling method. As the starting TiO2 material, employed were TiO2 (Kojundo, 99.9%, rutile, 2.1 m2/g), JRC-TiO-6 (Catalysis Society of Japan, rutile, 100 m2/g), JRC-TIO-7 (ibid, anatase, 270 m2/g) and JRC-TIO-7 pre-calcined in air at 673 K for 5 h (anatase, 130 m2/g), which referred to as R2, R100, A270 and A130, respectively. CaCO3 (Kojundo, 99.99%, 22.1 g), TiO2 (17.6 g), alumina balls (150 g, 1 cm in diameter) and acetone (80 ml) were put into a plastic bottle (300 ml) and they were mixed at room temperature (120 rpm, 24 h), followed by drying in an oven (343 K) overnight. The mixed powder was dried at 343 K overnight and calcined in air at various temperatures (1073-1473 K) for 10 h. The prepared CaTiO3 sample was referred to as CaTiO3(method, TiO2 source (if necessary), calcination temperature) such as CaTiO3(CP, 973) and CaTiO3(S, R2, 1073). Pt co-catalyst was loaded by photodeposition method, as the similar way to the previous study [8]. Loading amount was 0.1 wt%. The CaTiO3 sample (2.0 g) was dispersed in methanol aqueous solution (10 vol%, 400 ml) containing H2PtCl6 (Wako, 99.9%), followed by photoirradiation for 1 h using a 300 W xenon lamp. 2.2. Characterization Powder X-ray diffraction (XRD) patterns were recorded on a MiniFlex II/AP (Rigaku) using Ni-filtered Cu Kα radiation by using Si powder as an internal standard. Mean crystallite size of the samples was estimated from the diffraction line at 33.2 degree. Diffuse reflectance (DR) UV-visible spectra were recorded on a V-670 (JASCO) equipped with integrating sphere. The Brunauer–Emmett–Teller (BET) specific surface area was calculated from the amount of N2 adsorption at 77 K which was measured on a Monosorb (Quantachrome). SEM images were recorded by S-5200 (Hitachi). 2.3. Photocatalytic reaction test The reaction tests were carried out with a fixed-bed flow type reactor [4-7]. The catalysts were granulated to the size of 400-600 μm. The quartz cell (60 × 20 × 1 mm3) was filled with a mixture of the catalyst (0.8 g) and quartz granules (0-0.7 g). The reaction gas, water vapor (1.5%) or a mixture of water vapor (1.5%) and methane (50%), was introduced into the reactor at the flow rate of 40 ml/min and the reaction was carried out upon photoirradiation with the 300 W xenon lamp. The light intensity measured in the range of 230-280 nm and 310-400 nm were 42 mW/cm2 and 84 mW/cm2, respectively. The outlet gas was analyzed by on-line gas chromatography with a thermal conductivity detector.

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3. Results and discussion 3.1. Characterization of CaTiO3 samples prepared by the CP and HP methods Fig. 1 shows the SEM images of the representative CaTiO3 samples prepared by the CP and HP methods. For CaTiO3(CP, 1073), various sized particles (ca. 50-500 nm) were observed (Fig. 1a). When the calcination temperature increased to 1273 K, the size increased to ca. 0.1-5 μm and the shape was still irregular (Fig. 1b). For CaTiO3(HP, 1073), very small particles (ca. 50-200 nm) were observed and the size and shape of the particles were uniform in comparison with the CaTiO3(CP) samples (Fig. 1c). When the calcination temperature increased to 1273 K, the size slightly increased to ca. 200-500 nm (Fig. 1d).

(a)

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Fig. 1 SEM images of (a) CaTiO3(CP 1073), (b) CaTiO3(CP, 1273), (c) CaTiO3(HP, 1073) and (d) CaTiO3(HP, 1273).

The different morphology of these two series of CaTiO3 samples would be originated from the generation mechanism of the precursors. In the preparation of the CaTiO3(CP) samples, the white precipitation, i.e. CaTiO(C2O4)2·H2O, was produced quickly when the (NH2)2C2O4 aqueous solution was added to another solution containing Ca2+ and Ti4+ ions. Since the concentration of C2O42- ion in the solution was not homogeneous, the size and the shape of the particles in the precipitation and the calcined sample would not become homogeneous. On the other hand, for the CaTiO3(HP) samples, as the urea hydrolyzed, C2O42- ion was gradually produced in the solution and reacted with Ca2+ and Ti4+ ions to form CaTiO(C2O4)2·H2O. Since C2O42- ion was homogeneously produced in the solution, the size and the shape of the particles in the precursor and the calcined sample would be uniform. Fig. 2 shows the XRD patterns of the representative CaTiO3 samples. In the CaTiO3(CP) samples, single phase of CaTiO3 was obtained by calcination at 1073 K and higher temperatures (Fig. 2b). In the CaTiO3(HP) samples, CaTiO3 could be prepared at 1073 K and higher calcination temperatures, but impurities such as Ca(OH)2 were also existed (Fig 2d). In the HP method, the hydrolysis of urea forms both NH3 and CO2. Produced CO2 dissolved in water to form CO32- ion, which may precipitate Ca2+ ion as CaCO3. This may change the ratio of Ca2+ to Ti4+ in the calcined sample, which would provide the excess Ca specie as Ca(OH)2 in air.

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Fig. 3 shows the DR UV-vis spectra of the CaTiO3(CP) and CaTiO3(HP) samples Fig. 2 XRD patterns of (a) CaTiO3(CP, 973), calcined at 1073 and 1273 K. The (b) CaTiO3(CP, 1073), (c) CaTiO3(HP, 973) absorption edge of the CaTiO3(HP) samples and (d) CaTiO3(HP 1073). ○: CaTiO3, ▲: was slightly at the shorter wavelength than Ca(OH)2, ●: unknown and ■: Si for the that of CaTiO3(CP) samples calcined at angle correction. each temperature, and the edges shifted to longer wavelength when calcined at higher temperatures for each sample. The shift would be concerned with the variation of the crystallite size in the CaTiO3 samples. The band gap energy estimated from the adsorption edge of the spectra was 3.5-3.6 eV. The color of the CaTiO3(CP) sample calcined at 973 K was white, while that of the sample calcined at 1073 K was light pink and it became dark with increasing the calcination temperature. In the CaTiO3(HP) samples, even the sample calcined at 973 K looked pink and the color became dark with increasing the calcination temperature. These result consisted with the absorption at visible light region in the UV-vis spectra (Figs. 3b, 3c and 3d). The maximum of the band was around 500 nm. The formation of crystal defects such as Ti3+ sites and oxygen vacancies would be the reason for the coloration. The deeper color of the CaTiO3(HP) samples than that of the CaTiO3(CP) samples showed the existence of larger amount of the crystal defects. In (A) (B) addition, when Ti(SO4)2 was (a) (a) 30 20 used instead of TiCl4 in the CP method, the color of the CaTiO3 20 samples became much deeper. (b) From these results, it is 10 suggested that unreacted (b) 10 additives such as urea and the counter anions of the starting 0 0 materials such as Cl- would 973 1073 1173 1273 973 1073 1173 1273 adsorb on the surface of the Calcination temperature / K precursor and cause the formation of defects during the Fig. 4 (a) Crystallite size and (b) specific surface area of calcination. the CaTiO3 samples calcined at various temperatures prepared by (A) the CP method and (B) the HP method.

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Fig. 4 shows the crystallite size and the BET specific surface area of the CaTiO3(CP) and CaTiO3(HP) samples. As the calcination temperature increased, the crystallite size tended to increase though the CaTiO3(HP) samples calcined at high temperature (10731273 K) were exceptional (Fig. 4a). The BET specific surface areas of CaTiO3(CP, 973) and CaTiO3(HP, 973) samples were 18 and 24 m2/g, respectively, and they drastically decreased to less than 10 m2/g with increasing the calcination temperature (Fig. 4b). The crystallite size and the specific surface area of the samples calcined at the same temperature were almost the same values, which implied that they would be mainly influenced by not the preparation method but the calcination temperature. As a conclusion of this section, the CP method provided the single phase of CaTiO3 having irregular morphology, and the HP method gave the small CaTiO3 particles of a uniform shape, although the crystal defects and the impurity phase of Ca(OH)2 existed. 3.2. Characterization of CaTiO3 samples prepared by the S method Influence of calcination temperature was examined for the CaTiO3(S, R2) samples. In their SEM images, aggregated particles were observed, as the representative one was shown in Fig. 5a. The particle size (ca. 1 μm) did not depend on the calcination temperature and was almost the same as that of R2 used as the starting TiO2 material. In their XRD patterns, large diffraction lines assignable to unreacted TiO2 (rutile) and Ca(OH)2 were observed when the sample was calcined at 1073 K (Fig. 6a) and they were hardly observed for the sample calcined at 1273 K and higher temperatures (Fig. 6b). For CaTiO3(S, R2, 1073), a large absorption band assigned to TiO2 (rutile) was observed (Fig. 7a), while it quite decreased but still remained in CaTiO3(S, R2, 1273) sample (Fig. 7b). Result of XRD and UV-vis showed that higher temperature than 1273 K was required to obtain a pure CaTiO3 by the S method from this TiO2 samples. The color of the samples calcined at 1173 K and lower temperatures was white, while that of CaTiO3(S, R2, 1273) was light pink. In other words, these CaTiO3(S) samples had a smaller amount of crystal defects than CaTiO3(CP) and CaTiO3(HP) sample did, although high temperatures were required to obtain a single phase.

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Wavelength / nm Fig. 7 DR UV-vis spectra of (a) CaTiO3(S, R2, 1073), (b) CaTiO3(S, R2, 1273), (c) CaTiO3(S, R100, 1273), (d) CaTiO3(S, A130, 1273) and (e) TiO2 (rutile). CaTiO3(S, A130, 1273) showed the same spectra as (d).

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Other three kinds of TiO2 powders, R100, A130 and A270, were also examined as the starting materials for the preparation of CaTiO3 samples by the S method, followed by calcination at 1273 K. When TiO2 powders of higher surface area were used, the particle size of CaTiO3 samples became smaller as shown in SEM images (Figs. 5b-5d). From XRD, the formation of CaTiO3 was confirmed and no impurity phases were observed (not shown). From UV-vis spectra, the small absorption in the range of 350400 nm disappeared in the three CaTiO3(S, 1273) samples (Fig 7c and 7d). These results showed that no impurities were existed in the three CaTiO3(S, 1273) samples prepared from TiO2 particles of high surface area. The colors of all the CaTiO3(S, 1273) catalysts including CaTiO3(S, R2, 1273) were pale pink, showing that amount of crystal defects would be similarly small. The crystallite size did not depend on the kind of TiO2 (Fig. 8a), while the BET specific surface area of three CaTiO3(S, 1273) samples prepared from TiO2 particles of high surface area was larger than that of CaTiO3(S, R2, 1273) (Fig. 8Bb). The specific surface area of the two CaTiO3 samples prepared from anatase was smaller than that of CaTiO3(S, R100, 1273), suggesting that the specific surface area of 20 (B) (A) A130 and A270 would be (a) 30 reduced by sintering to be less than 100 m2/g (a) when they transformed 20 10 to rutile during the (b) calcination. 10 As a conclusion of this (b) section, it was found 0 0 that the CaTiO3 samples 1073 1173 1273 1373 1473 R R A A 2 100 130 270 of high surface area Calcination temperature / K Starting TiO2 material without impurities could be obtained by using TiO2 of high Fig. 8 (a) Crystallite size and (b) BET specific surface area of specific surface area as (A) CaTiO3(S, R2) samples calcined at various temperatures and (B) CaTiO3(S, 1273) prepared from various TiO2 raw materials. a starting material.

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3.3. Photocatalytic activities of the prepared CaTiO3 samples Fig. 9 shows the hydrogen production rate in the WD and the PSRM over Pt(0.1 wt%)/CaTiO3 samples. On almost all the samples, the hydrogen production rate in the PSRM was much higher than that in the WD on the present photocatalysts, where the loading amount of 0.1 wt% was employed to be suitable for the PSRM. This might be excess for the WD since Pt co-catalyst could promote the reverse reaction of the WD (H2 +1/2O2 → H2O) [9]. The hydrogen production rate in the WD on the Pt/CaTiO3(CP) and Pt/CaTiO3(S) samples tended to increase with decreasing the calcination temperature (Figs. 9Aa and 9Ca). In other words, it increased with increasing the specific surface area of the catalyst or decreasing the defects. Thus, one possibility is that the number of the surface Pt nano-particles per unit area decreased with the increase of the surface area and the reaction probability between the oxygen produced on the surface of CaTiO3 and the hydrogen produced over Pt might decrease. However, the activities of the Pt/CaTiO3(CP, 973) and Pt/CaTiO3(HP, 973) samples were low, although the specific surface area of them was high. This showed that a large crystallite size would be also important to promote the WD. The highest activity was obtained over Pt/CaTiO3(S, R2, 1073) having a high surface area and a relatively large crystallite size. Among the four Pt/CaTiO3(S, 1273) samples prepared from various TiO2 samples, Pt/CaTiO3(S, R100, 1273) with the largest surface area showed the highest activity for the WD (Fig. 9Da).

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Fig. 9 Hydrogen production rate on the Pt(0.1 wt%)/CaTiO3 samples in (a) the WD and (b) the PSRM; (A) the Pt/CaTiO3(CP) samples, (B) the Pt/CaTiO3(HP) samples, (C) the Pt/CaTiO3(S, R2) samples and (D) the Pt/CaTiO3(S, 1273) samples prepared from various TiO2 starting materials.

In the CaTiO3(CP) and CaTiO3(S, R2) samples, the activity for the PSRM first increased with increasing the calcination temperature (Figs. 9Ab and 9Cb), and the highest activity was obtained over CaTiO3(CP, 1073) and CaTiO3(S, R2, 1273) samples, respectively. The increase of the activity would be related to the increase of the crystallite size (Figs. 4 and 8), which would promote the smooth migration of photogenerated carriers in the conduction and valence bands to the surface. However, the activity decreased with further increasing the calcination temperature though the crystallite size increased. The color of CaTiO3(CP, 1073) and CaTiO3(S, R2, 1273) samples was pale pink, while that of CaTiO3(CP, 1173-1273) and CaTiO3(S, R2, 1473) samples was grayish pink. Therefore, these defects would decrease the activity. On the other hand, in the CaTiO3(HP) samples, the activity monotonously decreased with increasing the calcination temperature (Fig. 9Bb). As mentioned, in these samples, the color was pink even when calcined at 973 K and became dark with increasing the

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calcination temperature. Thus, the negative effect of the defects formation would become superior to the positive effect of the crystallite growth, resulting the decrease of the activity. These facts in Figs. 9A-9C show that the CaTiO3 samples with large crystallite sizes and few crystal defects would be effective for promoting the PSRM. Among the four CaTiO3(S, 1273) samples (Fig. 9D), the CaTiO3 samples prepared from anatase showed higher activity than the CaTiO3 samples prepared from rutile. The highest activity was obtained over CaTiO3(S, A130, 1273). Though the color of the CaTiO3(S, A130, 1273) and CaTiO3(S, A130, 1273) samples were similar to each other, the specific surface area and the crystallite size of the former were a little larger than those of the latter as shown in Fig. 8. These factors would improve the PSRM activity.

4. Conclusions We prepared CaTiO3 samples by three methods, i.e., co-precipitation (CP), homogeneous precipitation (HP) and solid-state reaction (S) methods, and examined their photocatalytic activities for the water decomposition (WD) and the steam reforming of methane (PSRM). By the CP method, large CaTiO3 particles without impurity phases could be obtained at 1073 K and higher temperatures though the particle size and shape were not homogeneous. By the HP method, small CaTiO3 particles with the regular shape could be obtained although the crystal defects formed even at low temperatures like 973 K and impurities such as Ca(OH)2 coexisted. By the S method, the crystallites with few defects could be produced, though high temperatures such as 1273 K or higher were required to obtain pure CaTiO3. When rutile TiO2 of large surface area was used as the start material for the S method, CaTiO3 of high specific surface area was obtained. CaTiO3(S, R2, 1073) and CaTiO3(S, R100, 1273) showed the highest activity for the WD and CaTiO3(S, A130, 1273) was the best for the PSRM. For the WD, the high surface area of CaTiO3 was more important than the large crystallite size of them, while, for the PSRM, the large crystallite size was more important. CaTiO3 samples with few defects showed high activities for both reactions.

References [1] A. Kudo and Y. Miseki, 2009, Heterogeneous photocatalyst materials for water splitting, Chem. Soc. Rev., 38, 253–278. [2] T.Sakata and T. Kawai, 1981, Heterogeneous photocatalytic production of hydrogen and methane from ethanol and water, Chem. Phys. Lett., 80, 341–344. [3] T. Kawai and T. Sakata, 1980, Conversion of carbohydrate into hydrogen fuel by a photocatalytic process, Nature, 286, 474–476. [4] H. Yoshida, S. Kato, K. Hirao, J. Nishimoto and T. Hattori, 2007, Photocatalytic steam reforming of methane over platinum-loaded smiconductors for hydrogen production, Chem. Lett., 36, 430–431. [5] H. Yoshida, K. Hirao, J. Nishimoto, K. Shimura, S. Kato, H. Itoh and T. Hattori, 2008, Hydrogen production from methane and water on platinum loaded titanium oxide photocatalysts, J. Phys. Chem. C, 112, 5542–5551. [6] K. Shimura, S. Kato, T. Yoshida, H. Itoh, T. Hattori and H. Yoshida, Photocatalytic steam reforming of methane over sodium tantalate, J. Phys. Chem. C, accepted. [7] K. Shimura and H. Yoshida, Energy Environ. Sci. in revision. [8] H. Mizoguchi, K. Ueda, M. Orita, S-C. Moon, K. Kajihara, M. Hirano and H. Hosono, 2002, Decomposition of water by a CaTiO3 photocatalyst under UV light irradiation, Mater. Res. Bull., 37, 2401–2406. [9] S. Sato and J. M. White, 1980, Photodecomposition of water over Pt/TiO2 catalysts, Chem. Phys. Lett., 72, 83–86.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

A new procedure to produce carbon-supported metal catalysts Jacco Hoekstra,a,b Peter H. Berben,b John W. Geus,a Leonardus W. Jenneskens*a a

Organic Chemistry & Catalysis, Debye Institute For Nanomaterials Science, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands b BASF Catalysts, Strijkviertel 67, 3454 PK De Meern, The Netherlands

Abstract Mechanically strong carbon support bodies of a narrow size distribution are produced from the renewable biomass resources cellulose and table sugar. To this end Micro Crystalline Cellulose (MCC) spheres or partially Carbonized Sucrose (CS) spheres prepared by hydrothermal treatment of a sucrose solution were used. Via wet impregnation these spheres are easily loaded with various base metal salt precursors. By keeping the loaded spheres in a stagnant inert nitrogen atmosphere at elevated temperatures (500°C-800°C) the metal salt precursors are reduced to the corresponding metals without an external hydrogen gas source. During pyrolysis the MCC/CS spheres provide the required reducing environment. Keywords: activated carbon, carbohydrates, hydrothermal, pyrolysis, reduction

1. Introduction Activated Carbon (AC) is frequently used as a support in heterogeneous catalysis. It is attractive due to inertness of carbon in acidic and basic environments, and because precious metals, such as platinum, can easily be reclaimed by combustion of the carbon.1 AC is generally produced from naturally occurring carbon sources, such as nutshells, wood or peat. By pyrolysis in the absence of air or treatment with steam a carbon support of a high specific surface area is obtained. However, with AC it is difficult to produce bodies of a desired size distribution that are mechanically strong and thus attrition resistant. Also the pore structure and the surface characteristics (hydrophilic vs. hydrophobic) are difficult to control. The reduction of base metal salt precursors to supported metal particles is often a challenge. Copper oxide, for instance, is difficult to reduce since its reduction with hydrogen gas is highly exothermic. To prevent sintering copper catalysts have to be reduced with a highly diluted hydrogen gas flow, the inert gas deals with the heat of reduction. Consequently, the reduction of copper catalysts involves a large period of time. With less noble metals, such as cobalt and especially iron, the reduction is also problematic. Water vapor strongly affects the rate of reduction (cobalt) or even prevents the reduction thermodynamically (iron). With the usual hydrophilic, highly porous supports, such as alumina and silica, it is difficult to decrease the water vapor pressure within the support bodies to a level where reduction to the metal can proceed smoothly. Usually, the reduction to metallic iron particles only proceeds at temperatures where the iron particles sinter appreciably. Hydrophobic supports, such as (graphitic) carbon facilitate the reduction of catalysts due to a more rapid transport of water vapor. Though hydrogen reduction of supported catalysts is common practice, operations with hydrogen

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are dangerous due to the extensive explosive limits. With carbon supports hydrogen reduction can lead to reaction of the carbon to methane. The objective of our work is to provide mechanically strong carbon support bodies of a narrow size distribution from renewable biomass resources. A second objective is to provide supported base metal catalysts without using hydrogen gas as a reducing agent. We start from hydrophilic bodies of carbohydrates, such as Micro Crystalline Cellulose (MCC) spheres or partially Carbonized Sucrose (CS) spheres prepared by hydrothermal treatment of an aqueous sucrose (common table sugar) solution. Impregnation of the MCC/CS spheres with aqueous solutions of base metal salt precursors can be readily executed. It has been found that during pyrolysis of the loaded MCC/CS spheres in a stagnant inert nitrogen gas atmosphere at elevated temperatures (500°C-800°C) the base metal salt precursors are rapidly reduced to the corresponding metals in the absence of an external hydrogen gas source. During pyrolysis the MCC/CS spheres provide the required reducing environment. A mechanically strong carbonaceous catalyst support of a narrow size distribution loaded with various base metal catalysts results.

2. Experimental 2.1. MCC-spheres Commercially available MCC spheres (Cellets, neutral pellets of Syntapharm GmbH, Mülheim an der Ruhr, Germany), employed for the slow release of drugs, with a size range of 100 µm-200 µm were used as received.

2.2. CS-spheres The CS spheres were obtained from a hydrothermal treatment of sucrose (table sugar) according to a modified literature procedure.2 A 1M aqueous solution of sucrose in demineralised water was placed in a Teflon-lined autoclave. The solution was kept at 160°C for 4h. Afterwards the solid product was separated by centrifugation and was washed with a mixture of ethanol, acetone and demineralised water until a colorless solution was obtained. The resulting black powder was dried at room temperature in vacuo to constant weight.

2.3. Wet impregnation with metal salt precursors The hydrophilic MCC and CS spheres were loaded via wet impregnation. The spheres were immersed in a 2M solution of Fe(NO3)3·9H2O, Co(NO3)2·6H2O, Ni(NO3)2·6H2O and Cu(NO3)2·2.5H2O, respectively. The mixtures were left for 24h. under occasional stirring. Next, the impregnated spheres were filtered using a Büchner funnel with glass filter. The isolated spheres were dried at room temperature in vacuo to constant weight.

2.4. Pyrolysis of loaded MCC/CS spheres The impregnated spheres were pyrolyzed under a stagnant inert nitrogen gas atmosphere in a quartz tube reactor (Thermolyne 21100 furnace). The heating rate was 5°C/min and the samples were treated for 3 h. at temperatures between 500°C and 800°C with consecutive 100°C increments.

2.5. Characterization of (pyrolyzed) MCC/CS spheres The morphology of the (pyrolyzed) MCC/CS spheres was examined with a Philips XL30 SFEG Scanning Electron Microscope (SEM). The samples were placed onto an aluminum stub coated with carbon tape.

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Both Transmission Electron Microscopy (TEM) and Scanning Transmission Electron Microscopy (STEM) were used to analyze the formed metal particles and the carbonized support. Samples were prepared by grinding and subsequently suspended in ethanol under ultrasonic treatment. One or two drops of the thus prepared sample were placed onto a holey carbon film on a copper (or nickel) grid. The samples were analyzed with a FEG-Technai-20 TEM apparatus operated at 200 KeV. Energy Dispersive X-ray (EDX) elemental analysis was performed with the installed TIA software. A High-Angle Annular Dark-Field detector (HAADF) provided images of the electron scattered over large angles, which are thus dominated by the heavier elements. Raman spectra were recorded using a Kaiser RXN spectrometer equipped with a 70 mW, 532 nm diode laser for excitation (data point resolution 2 cm-1).

3. Results and discussion 3.1. Structural properties of carbonized MCC 3.1.1. MCC loaded with Fe(NO3)3·9H2O SEM-images (Back-Scattered Electrons (BSE), indicative for heavier elements) show the typical morphology of the carbonized MCC-spheres in Figure 1. The surface of the spheres is quite rough. The color of the spheres has changed from white to black due to pyrolysis of the cellulose. As can be inferred from Figure 1 the resulting carbonized MCC spheres have a narrow size range (diameter ca. 100 μm).

Figure 1. SEM-images (BSE) of carbonized MCC spheres loaded with Fe(NO3)3·9H2O, pyrolyzed at 500°C (left) and 800°C (right). Scale bars 100 μm.

Whereas the distribution of iron is uniform after thermal treatment at 500°C, pyrolysis at 800°C brings about segregation and sintering of iron at the external edge of the carbon bodies. An important observation is that both the 500°C and 800°C samples are ferromagnetic, which indicates that reaction to either magnetite (Fe3O4) or metallic iron has proceeded. Figure 2 represents a HAADF STEM-image with EDX elemental analysis from the sample pyrolyzed at 500°C. The oxygen K-signal follows the iron K- and L-signals which indicate that the iron particles are oxidized. The STEM-image indicates that a high density of iron oxide particles is formed. The size of the iron oxide particles is 3 nm-10 nm. In Figure 3 a TEM lattice image of such a particle is shown.

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Figure 2. STEM-image (HAADF) with EDX elemental analysis along the line indicated of an MCC sphere loaded with Fe(NO3)3·9H2O and pyrolyzed at 500°C. The bright spots represent iron oxide particles. Scale bar 20 nm.

Figure 3. HR-TEM-image of an iron oxide particle with a size of ca. 6 nm (circle). Scale bar 2 nm.

When pyrolyzed at 800°C metallic iron particles are formed in larger clusters (20 nm – 20 μm, see also Figure 1). In the temperature regime between 500°C and 800°C the iron oxide particles are reduced to metallic iron,3 which sinters severely above 700°C. This is substantiated by TEM-analysis (Figure 4).

Figure 4. TEM-image (HAADF) of an MCC-sphere loaded with Fe(NO3)3·9H2O and pyrolyzed at 800°C with reduced particles of a larger size. Scale bar 500 nm.

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It is important to note that no external hydrogen gas source has been employed to reduce the base metal salt precursor. Clearly, MCC spheres during pyrolysis under a stagnant inert nitrogen atmosphere provide the reducing environment. Literature data indicate that during the pyrolysis of cellulose three kinds of species are formed, viz. char, tar (mainly levoglucosan) and light gases (mainly CO, H2 etc. released between 400°C and 800°C).4 It is anticipated here that CO and/or amorphous (carbonaceous) material acts as the reducing agent(s) of the iron precursors. 3.1.2. MCC loaded with Cu(NO3)2·2.5H2O Figure 5 represents SEM-images (BSE) of MCC spheres loaded with Cu(NO3)2·2.5H2O and subsequently pyrolyzed at 800°C under a stagnant nitrogen atmosphere. At the higher magnification (right) small supported copper particles can be observed. The primary difference between iron and copper is that the copper particles sinter appreciably less above 700°C. The copper particles have a size of about 100 nm.

Figure 5. SEM-images (BSE) of MCC-spheres loaded with Cu(NO3)2·2.5H2O and pyrolyzed at 800°C at different magnifications. Scale bar left 50 μm and right 5 μm.

In Figure 6 a STEM-image (HAADF) with EDX-analysis is given of the MCC supported copper particles after pyrolysis at 500°C. The copper particle size is 5-20 nm. From elemental analysis it is apparent that the copper catalysts are already in their reduced state, since the oxygen K-signal is very low in comparison to the copper L-signal. If the material was copper oxide, we expect a considerably more intense oxygen K-signal (compare Figure 2 for iron oxide). Therefore, it is concluded that copper particles are passivated. 4000,00

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Figure 6. STEM-image (HAADF) with EDX elemental analysis along the line indicated. The bright spots represent copper particles. Scale bar 50 nm.

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In Figure 7 a STEM-image (HAADF) is given for a sample heated at 800°C. Also here the oxygen K-signal is very low indicating that the copper catalysts have been reduced. Interestingly, the copper particle size is similar both at 500°C and 800 °C. The copper particles are thus much less prone to sinter at elevated temperatures than the iron particles. 600,00

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Figure 7. STEM-image (HAADF) with EDX elemental analysis along the line indicated. The bright spots represent copper particles. Scale bar 200 nm.

3.1.3. MCC loaded with Co(NO3)2·6H2O and Ni(NO3)2·6H2O The MCC-spheres loaded with cobalt- and nickel-salts behave more or less the same as the iron- and copper-loaded spheres. At 500°C nickel is already in its reduced state, whereas cobalt is still in an oxidic state. At 600°C cobalt is also fully reduced. The sintering characteristics of nickel and cobalt follow the behavior of copper. Some sintering occurs, but not as much as displayed by iron. The formation of the catalysts is believed to be as follows,5 at first the metal nitrates will be decomposed to the corresponding metal oxides (T < 240°C), then the carbohydrate bodies become pyrolyzed and transform into carbon (T = 300°C-600°C). At the lower temperatures the metal oxides are most likely reduced by the carbon monoxide that is released during the pyrolysis of the cellulose.

3.2. Raman spectroscopy To substantiate that the supports consist of (graphitic) carbon, Raman-spectra were recorded for various samples. An illustrative spectrum is shown for the iron loaded sample pyrolyzed at 800°C (Figure 8, see also Figure 1). The spectrum contains two strong peaks at 1344 cm–1 and 1584 cm–1. The peak at 1584 cm–1, called the G band, is due to in-plane bond stretching of pairs of sp2 hybridized C-atoms in graphitic planes. The D band at 1344 cm–1 stems from ring-breathing vibrations in benzene or condensed benzene rings in amorphous G band D band carbon. 5 Raman spectroscopy gives evidence that in the case of iron, cobalt or nickel loaded MCC spheres extensive graphitization occurs at temperatures above 700°C. With copper the extent of graphitization is substantially less which is apparent from the broadening of the G-band (not shown). 800 700 600

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Figure 8. Raman spectrum of MCC loaded with Fe(NO3)3·9H2O, pyrolyzed at 800°C.

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3.2. Structural properties of CS spheres Since the MCC spheres are used as an excipient in medicines, and thus have to be very pure, their price is quite high. To find an alternative for the MCC-spheres we considered hydrothermally treated sucrose spheres, which are much cheaper. Figure 9 (SEM + STEM) shows the morphology of the CS spheres after isolation.

Figure 9. SEM-image (left, BSE, scale bar 10 μm) and STEM-image (right, HAADF, scale bar 1 μm) showing the morphology of CS spheres hydrothermally treated at 160°C.

The size of the CS sphere is 2 μm-8 μm. Conglomerates of these spheres are also formed. From literature it is known that it is possible to tune the size of the spheres by adjusting the experimental conditions time, temperature and concentration of the sucrose solution. The CS sphere consist of a graphitic core with functional (-OH, -COOH) groups on the periphery, i.e. under hydrothermal conditions not all functional groups are lost. Due to these functional groups the CS spheres are hydrophilic and can easily be loaded with base metal salt precursors. 3.2.1. CS loaded with Ni(NO3)2·6H2O Figure 10 displays a SEM-image (BSE) of CS impregnated with Ni(NO3)2·6H2O pyrolyzed at 800°C. Finely divided nickel particles have formed onto the surface of the CS spheres.

Figure 10. SEM-image (BSE) of finely divided Nickel particles onto the surface of the CS spheres. Scale bar 1 μm.

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From the STEM-image (HAADF) and EDX-analysis (Figure 11) it is apparent that the nickel particles are in their reduced state. The oxygen (K-signal) level is very low. The results for the CS spheres were similar to those of the MCC spheres (vide supra). The metal nitrates were first converted to the corresponding oxides, and subsequently the metal oxide particles became reduced by the pyrolysis gases released from the further decomposing CS spheres. Thus, the CS spheres are a good and cheap alternative for the MCC spheres. 14000,00 12000,00

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Figure 11. STEM-image (HAADF) with EDX elemental analysis along the line indicated. The bright spots indicating the nickel particles. Scale bar 100 nm.

4. Conclusions Highly uniform carbonaceous catalyst supports have been synthesized with two carbohydrate precursors, Micro Crystalline Cellulose (MCC) spheres and partially carbonized sucrose (CS) spheres. Both hydrophilic carbon support precursors can be easily impregnated with a range of base metal catalyst precursors. A simple pyrolysis procedure reduces the metal salt precursors first to the corresponding metal oxides and ultimately to the corresponding metals. The thermally decomposing carbohydrate bodies provide the reducing environement (CO and/or amorphous (carbonaceous) material) for the reduction of the base metal precursors. With Raman, SEM and (S)TEM it is shown this procedure results in (graphitic) carbon-supported base metal catalysts.

References [1] F. Rodriguez-Reinos, 1998, The Role Of Carbon Materials In Heterogeneous Catalysis, Carbon, 36, 3, 159-175. [2] Q. Wang, H. Li, L. Chen, X. Huang, 2001, Monodispersed Hard Carbon Spherules With Uniform Nanopores, Angew. Chem. Int. Ed., 39, 2211-2214. [3] F. Gong, T. Ye, T. kan, Y. Torimoto, M. Yamamoto, Q. Li, 2009, Direct Reduction Of Iron Oxides Based On Steam Reforming Of Bio-Oil: A Highly Efficient Approach For Production Of DRI From Bio-oil And Iron Ores, Green Chem., 11, 2001-2012. [4] D.K. Shen, S. Gu, 2009, The Mechanism For Thermal Decomposition Of Cellulose And Its Main Products, Bioresource Technology, 100, 6496-6504. [5] M. Sevilla, C. Sanchis, T. Valdes-Solis, E. Morallon, A.B. Fuertes, 2008, Direct Synthesis Of Graphitic Carbon Nanostructures From Saccharides And Their Use As Electrocatalytic Supports, Carbon, 46, 6, 931-939.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Use of zeta potential measurements in catalyst preparation Stuart Soled,a William Wachter,b Hyung Woa a

ExxonMobil Research and Engineering Company, Corporate Strategic Research, Annandale, NJ 08801, USA b ExxonMobil Research and Engineering Company, ExxonMobil Process Research, Annandale, NJ 08801, USA

Abstract We illustrate two applications where zeta potential measurements have provided useful information for catalyst preparations. In the first case, we decribe how maximizing the electrostatic attraction between the active complex in an impregnating solution and the support leads to smaller metal particles upon reduction. This approach allowed synthesis of small platinum crystallites on a yttria-modifed amorphous silica-alumina support. The yttrium oxide not only titrates some acid sites but it provides more positively charged surface regions (at a given solution pH) on the support that better disperse the anionic chloroplatinate anion and the subsequently formed Pt crystallites. In the second application, we studied the attrition resistance of FCC (fluid cat cracking) catalysts, ~70 micron spray dried particles formed from micron-sized USY (ultrastable Y) zeolite crystals and submicron sol particles. The attrition is minimized when the larger zeolite particles are uncharged while the submicron-sized sol particles are highly charged. The results suggest that the stable colloid formed from nanocrystalline haloing provides an optimized dried and calcined agglomerate. Keywords: zeta potential, fluid cat cracking

1. Introduction Surface charging of small oxide particles can provide a useful tool to enhance catalyst syntheses. In this paper we describe two applications where surface charges are used advantageously- in the first, matching complementary charges on the support surface and metal impregnate complex are used to optimize Pt metal dispersion. This approach was described many years ago by Brunelle [1], and more recently expanded and refined by Regalbuto [2]. To determine the zero point of charge of a support, they measure the buffering action of the surface as it is exposed to solutions with different amounts of acid or base. Zeta potential measurements use a different approach to measure (near) surface charge but the concepts of optimizing support and impregnate electrostatic interactions remain the same. In the bifunctional catalyst briefly described here, a surface yttrium oxide partial monolayer is added to an amorphous silica-alumina support to temper its acidic properties and Pt is added to this modified support to allow bifunctional catalysis. In the second application, we discuss the use of surface charging concepts and zeta potential measurements to optimize the attrition resistance of fluid cat cracking (FCC) catalyst composite particles. The FCC particles consist of micron size zeolite particles held together by submicron sol particles to form 50-70μ composites. In this study, we start with a suite of USY zeolites of variable bulk Si-Al ratios and first determine their relative surface compositions using isoelectric points (IEP). We established an excellent

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correlation between the measured isoelectric points and the XPS-determined relative surface concentrations of Si and Al. We then show how appropriately matching the IEP of the zeolites with the properties of the sol particles can create an attrition-resistant composite. Small particle metal oxides lower their free energies by terminating their surfaces with the lowest possible charge, which normally means with oxygen. Such an atomic arrangement would create a stoichiometry richer in oxygen than the (crystallographically imposed) bulk stoichiometry and would generate a net negative charge on small oxide particles- this of course, does not occur. Instead, the surfaces normally terminate in hydroxyl groups rather than oxide anions. The hydroxyls effectively lower the surface anion charge from –2 to –1 and allow the particles to maintain a neutral total charge. Some of the hydroxyls may, on calcination, condense with release of water. This condensation creates bridged oxygen anions or coordinatively unsaturated metal cations but it maintains the neutral charge of the metal oxide (or more correctly, metal oxyhydroxide) particles. When small oxide (i.e. oxyhydroxide) particles contact liquid water they no longer have to remain electrically neutral; surface charges can develop because ions in solution are available to neutralize these charges by forming a classical double layer surrounding the particle. Consequently, surface hydroxyls on small oxide particles in aqueous suspension will ionize and their surfaces develop a net positive or negative charge, as a function of the pH (the protonation or deprotonation driving force) in the contacting solution. Zeta potential refers to the charge at the interface of the shear plane separating the tightly held compact layer and the more loosely held diffuse layer of the classical double layer. The isoelectric point represents the pH at which the zeta potential equals zero, and it reveals information about the near surface cation composition of the oxide support. At a specific pH for each solid, small but equal amounts of M(OH2)+ and MO– co-exist, with M(OH) being the most abundant surface species; this represents the isoelectric point. Consequently, the zeta potential is positive at pH values below the isoelectric point, where M(OH2)+ become the majority surface species as protons are donated from the hydronium ions, and negative at pH values above the isoelectric point. There is a difference between the zero point of charge and the IEP, but for most cases considered here (i.e. weak specific anion adsorptions), they are closely related. The IEP gives information about the chemical nature of the metal oxide support because oxides containing cations with high charge to radius ratio (for ex. Si+4) have low IEPs and oxides with lower charge-density cations have higher IEPs. This occurs because at high charge densities (e.g. for Si+4) a large driving force (low pH) is required to protonate the OH group as it is close to the small and already highly charged Si+4 cation. In other words, a protonated OH2+ on silica (if it were even to exist) would donate its proton to a solution at low pH. This sounds strange since silica classically acts as a non-acidic inert support in gaseous environments typical of most catalytic reactions, but in aqueous suspension it is very acidic. In contrast, Al+3 cations, because they have a lower charge and are larger than Si+4, are more easily protonated and therefore have a higher isoelectric point (~9). Ti+4 like Si+4 is tetravalent, but it has a larger cationic radius and thereby is more easily protonated: its isoelectric point is between 6 and 7 [3]. For mixed oxide supports the average surface population of the cations at the measured IEP reflects a molar average of the individual metal oxide IEP’s. Historically, electrophoresis measurements were used to measure isoelectric points, but fortunately, during the last couple of decades, simple and inexpensive laboratory instrumentation has become available to measure zeta potential even in relatively high

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concentration suspensions. These instruments measure acoustic signals created when the double layer around small particles distorts when these particles are placed in a megahertz ac electric field.

2. Experimental Table 1 lists the suite of ultrastable Y zeolites zeolites investigated in this study. Five grams of each zeolite was added to 200 cc of water and dispersed for 5 minutes using an ultrasonic dispersion probe and then measured with a Matec 8050 electrokinetic instrument. Zeta potentials were monitored during titrations using 1N HCl or 1N NaOH; titration with HCl if the initial zeta potential was negative and with NaOH if the initial zeta potential was positive. The crystallinity of the samples was determined by x-ray diffraction following ASTM Procedure D-3906-91. XPS measurements were performed on a Leybold-Heraeus ultra high vacuum system equipped with an Al Kα x-ray source (hυ=1486.6 eV) and a hemispherical energy analyzer. Photoemission spectra were obtained normal to the analyzed surface of pressed wafer samples with the electron analyzer operating at 50 eV pass energy. Surface areas were determined by a multipoint BET measurement after outgassing at 300C. The Davison Attrition Index (DI) uses 7.0 cc of sample catalyst which is screened to remove particles in the 0 to 20 micron range. The remaining particles are then contacted in a hardened steel jet cup having a precision bored orifice through which an air jet of humidified (60%) air is passed at 21 liter/minute for 1 hour. The DI is defined as the percent of 0-20 micron fines generated during the test relative to the amount of >20 micron material initially present, i.e., the DI = 100 x (wt% of 0 – 20 micron material formed during test)/ (wt% of original 20 microns or greater material before the test). The lower the DI, the more attrition resistant is the catalyst. The zeolites were obtained from an external source; consequently, the details regarding their modification are not known. Table 1. Properties of USY zeolites. Sample Label

% wt. Na

Bulk Si/Al

XPS Si(2p)/Al(2p)

BET Surface Area (m2/g)

% Crystallinity

A

0.86

4.62

8.82

543

93

B

1.5

3.54

4.6

590

98

C

0.43

2.99

3.76

569

79

D

0.15

2.66

1.73

560

67

E

0.03

6.55

3.56

642

88

F

0.14

2.74

1.58

543

66

G

1.5

2.38

1.65

490

67

H

0.11

2.57

0.93

498

83

I

0.15

2.69

0.86

593

91

J

0.65

2.71

0.86

667

98

K

2.8

2.61

1.45

605

99

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For the supported SiO2-Al2O3 catalysts, an amorphous silica alumina with a bulk concentration of 55%SiO2 and 45%Al2O3 was used. Aqueous yttrium nitrate was impregnated via incipient wetness impregnation onto the supports, and the samples were then dried and calcined at 450C. 0.3% wt. Pt was impregnated using a hexachlorplatinate precursor.

3. Results and discussion

7.4

60%

7

48%

6.6

36%

6.2

24%

5.8

12%

5.4 0

2

4

6

8

10

12

14

16

% Pt dispersion (H/M)

isoelectric point

Professor Brunelle in 1978 authored a masterful review describing a strategy for optimizing supported metal catalyst preparations by maximizing the electrostatic attraction between the precursor and support [1]. Others have followed and expanded on this protocol over the years, with many of the newer studies described by Regalbuto [2]. Our work is also based on enhancing electrostatic interactions but uses zeta potential measurements to chose the optimum preparation method for synthesizing Pt clusters on modified silica-alumina supports. In this study we were interested in tempering the acidity of amorphous silica-alumina by titrating with partial monolayers of yttrium oxide so that the residual acid sites would have strengths similar to chlorided or fluorided alumina [4]. The application involved bifunctional catalysis so optimizing platinum dispersion on the modified silica-alumina supports was important. Yttrium oxide is mildly basic and disperses readily onto the silica-alumina surface [5]. Since the isoelectic point of an oxide is related to the charge to radius ratio of its surface cations, the large trivalent yttrium cations have high isoelectric points- above 11 [3]. We determined that as the yttria surface population increases, the isoelectric point of the modified silica-alumina increases as does the hydrogen chemisorption uptakes (dual isotherm measurements at 40oC) of the reduced hexachloroplatinate anion (see Figure 1). The IEP represents an average surface state, so that below monolayer coverage we measure a contribution from regions of silica-alumina and regions of yttrium oxide. At a given pH, the regions of the support with a higher positive charge more strongly attract the dicholorplatinate anion and produce more dispersed Pt. It is reasonable to assume (although not proven here) that the Pt is preferentially located on the yttrium oxide.

0%

18

% Y2O3 in Y2O3/SiO2-Al2O3 Figure 1. Isoelectric Point and Strong Hydrogen Chemisorption of .3%Pt/Yttria Modified-Silica Alumina.

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The second problem we addressed concerns a common issue in catalyst synthesis, namely how to insure that larger particles formed from assembling smaller components maintain physical integrity. In the case of FCC catalysts, 50-70μ spray dried agglomerates were created from micron sized zeolites held together with submicron sol particles. The question we addressed is how to minimize the attrition of the composite. The suite of zeolites chosen consisted of ultrastable Y zeolites of 1-3 micron size with the properties shown in Table 1. The IEP of each of the USY samples was measured by titration with either HCl or NaOH. The fractional surface aluminum concentration for the suite of USY samples was also measured using XPS, and Figure 2 compares the two measurements.

isoelectric point

10 8 6 4 2 0

0

0.1

0.2

0.3

0.4

0.5

0.6

XPS surface Al/(Si + Al) Figure 2. Isoelectric point vs. XPS fractional aluminum content for suite of USY zeolites.

Although we expect XPS to sample to a penetration depth of ~40Å, the fractional aluminum content correlates well with the measured isoelectric points. The variation of isoelectric points from 2 to 9 suggests that for some of the USY samples the surface is silica-rich whereas other samples have predominantly alumina-rich surfaces. Because these samples were obtained from an outside source, the exact methodology used to surface enrich the USY samples is not know, although it would interesting to understand how to do this. Note that although the XPS ratios do correlate with the isoeletric points, the XPS ratios do not extend from aluminum contents of 0 to 1, suggesting that the subsurface layers are not as enriched in Si or Al as the surface. We did have concerns about the stability of the zeolites in acidic media, so we checked the time dependence of our measurements. When we changed the “soak” time in the acid from 15 to 150 seconds between measuring each data point, the zeta potential measurements did not change substantively. This probably results from the low concentration of acid present during titration (~5ml of 1 N HCl in 200 cc water) and the low reactivity at room temperature. Two different sols were used to bind the USY particles together. One was a SuperD silica sol prepared by reacting a sodium silicate solution with an aluminum sulfate/sulfuric acid solution under high shear to a pH 3.0 and the other was an aluminum chlorohydrol sol stable at pH 4.3. In Figure 3 we show schematically what we are attempting to achieve in the spray drying process, with the relative sizes of the USY zeolites and sol particles depicted.

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composite catalyst

+

(70μ)

submicron sol particles

zeolite, clay or alumina binder 1μ

Figure 3. Binding of components of an FCC catalyst.

The attrition of the composite FCC particles was measured using the Davison Attrition Test. Higher values of the index indicate weaker particles that attrit easier. A series of catalysts were prepared by spray drying the USY zeolites with one of the two sols and the attrition of the composite was measured. The results are shown in Figure 4.

Davison Attrition Index (increasing weakness) ==>

60 50 40 30 20 10 0

0

1

2

3

4

5

⏐sol pH - zeolite isoelectric point⏐

6

Figure 4. Correlation of FCC Agglomerate Strength and the difference between the absolute valuer of the sol pH and the zeolite IEP.

It is intriguing that the composite is strongest when the larger zeolite crystals have their IEP near to or at the pH of the sol. In the pH stability range of the small sol particles (2-4 for Super-D and 4-5 for alumina chlorohydrol) the sol nanoparticles are electrotatically stabilized with a high surface charge. The results in Figure 4 suggest that at the pH where the sol particles are highly charged, the zeolite should have no surface charge. This result may seem counterintuitive as one might think that the sol particles and the zeolites should be oppositely charged. However, this is not what this data suggests, so another phenomena is operating to create the stable agglomerate. A publication by Jennifer Lewis may help explain this phenomena [6]. She introduced the concept of nanoparticle haloing as a self-organizing process that imparts stability to naturally attractive particles by decorating their superficial areas with highly charged nanoparticles present at critical concentrations. She observed and calculated this effect for the case of micron sized silica particles decorated with a zirconia sol and found that the most stable arrangement occurred when the larger silica particles were

Use of zeta potential measurements in catalyst preparation

107

uncharged and the smaller zirconia nanoparticles had a high charge. Figure 5 represents this phenomena [7].

Figure 5. Representation of Nanoparticle Haloing (from 7).

We are hypothesizing that the FCC particles we have studied behave in an analogous way, with the zeolite crystallites replacing the silica spheres and the zirconia sol being replaced by either Super-D or alumina sol. This stable collodial suspension so formed by nanoparticle haloing has an optimized mixing of the sol and zeolite crystals. On drying and calcination, a porous but strong network forms to hold the agglomerates together. This approach has allowed formation of strong catalysts, even when using the Al chlorohydrol sol. The latter sol was not known to provide stable agglomerates.

4. Conclusion Zeta potential measurements have been used to successfully develop optimized metal dispersion on support oxides. They have also provided a useful tool for the design of attrition resistant FCC catalyst particles.

Acknowledgments The authors wish to thank Joe Baumgartner for help with some of these measurements.

References 1. 2. 3. 4. 5. 6. 7.

J. P. Brunelle, 1978, Pure Appl. Chem., 50, 1211. J. R. Regalbuto, 2009, in “Synthesis of Solid Catalysts”, (ed. K.P. de Jong) Wiley-VCH, p. 33. G. A. Parks and P.L. De Brnyn, 1962, J. Phys. Chem., 66, 967. S. Soled, G. B. McVicker, W.E. Gates and S. Miseo, 1995, US Pat. 5,457,253. S. L. Soled, G. McVicker, S. Miseo, W. Gates, and J. Baumgartner, 1996, Stud. Surf. Sci. Catal., 101A, 563-572 J.A. Lewis, 2001, Langmuir, 17, 8414. M. Jacoby, Jan. 7, 2002, Chemical and Engineering News, p. 11.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

The superior activity of the CoMo hydrotreating catalysts, prepared using citric acid: what’s the reason? A.V. Pashigreva, O.V. Klimov, G.A. Bukhtiyarova, M.A. Fedotov, D.I. Kochubey, Yu.A. Chesalov, V.I. Zaikovskii, I.P. Prosvirin, A.S. Noskov Boreskov Institute of Catalysis SB RAS, Novosibirsk,630090, Russia

Abstract It was demonstrated, that the main positive role of citric acid during the hydrotreating catalysts preparation is consist in the formation of bimetallic complex Co2[Mo4(C6H5O7)2O11]•nH2O, that is a good precursor for selective formation of catalyst active phase, so called Co-Mo-S phase type II. The preparation method for this bimetallic complex using different precursor is described. The catalysts prepared by the complex deposition onto alumina support were studied during the different stages of the catalyst genesis. Applicability of these catalysts for ultra low sulfur diesel production was shown. Keywords: hydrotreating catalysts, citric acid, bimetallic CoMo complex

1. Introduction The impregnated solutions containing cobalt and molybdenum compounds and citric acid (CA) is widely used for high active hydrotreating catalysts preparation, including industrial catalysts. An obvious positive effect of citric acid on the activity of catalysts is explained in variety of ways by different authors. Thus, Fujikawa et al. [1] prescribes the superior catalyst activity to the formation of cobalt citrate complexes, which are thermally stable at temperatures up to 200ºC and contributes the selective formation of the catalyst active component, so called Co-Mo-S phase of type II, described in [2]. Bergwerff et al. [3] reported the formation in a solution of molybdenum citrate complexes. Deposition of these Mo complexes onto Al2O3 support provides uniform molybdenum distribution along the carrier granule and ensures a high dispersion of MoS2 particles, that increases the relative activity of catalyst. In the cited and other papers of mentioned authors, the complex formation between CA and the individual components of the impregnating solution - cobalt or molybdenum was detected, while the formation of bimetallic Co-Mo-CA complexes was not revealed. Earlier Van Veen [4] reported that the positive role of chelating organic ligands consists in the formation of CoMoL compounds in the impregnating solution, whose structure remains unchanged after the drying of the catalyst; moreover the ligands screen metals from the interaction with the carrier. During the sulfidation stage of the CoMoL containing catalysts the Co-Mo-S phase type II is mainly formed, whereas in the catalysts prepared without using of chelating agents the low active Co-Mo-S phase type I is formed. Concerning our experimental results, the reasonable Van Veen’s conclusions should be supplemented by the followed statements: 1. Chelating ligands promotes the coordination of Co towards molybdenum containing anion forming a bimetallic compound with a close proximity of

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2.

cobalt and molybdenum, that further leads to the selective formation of bimetallic Co-Mo-S phase. Bimetallic complexes stabilized by chelating ligands, have relatively large size, that does not allow it to penetrate into the narrow pores of the carrier not available for heteroatomic organic molecules of the hydrotreated feedstock.

In the current paper it was demonstrated, that the use of labile bimetallic Co-Mo complexes with a close proximity between Co and Mo in combination with vacuum impregnation of Al2O3, low-temperature drying (<110ºC) and treatment in H2S at 400ºC, allows to obtain uniform catalysts containing fully sulfided particles, corresponding to Co-Mo-S phase of type II. The catalysts obtained reveal high activity in the hydrotreatment of diesel fuel [5]. As it was shown in [6] due to the interaction of ammonium heptamolybdate (AHM), CA, and cobalt acetate the bimetallic Co2[Mo4(C6H5O7)2O11]•nH2O (hereinafter CoMo4CA) complex is formed in aqueous solution. CoMo4CA is a good precursor for obtaining of high active hydrotreating catalysts. The linear sizes of this complex allow the localization of the active component predominantly in the wide, accessible for catalysis, pores of alumina [7]. The present paper describes the synthesis of (CoMo4CA) in solution from different initial compounds of Co and Mo, and the method for catalyst preparation, ensured the preservation of the (CoMo4CA) complex structure at the support surface. The properties of the catalysts obtained were studied.

2. Experimental The classical precursors for CoMo-impregnating solutions were used in this study for the catalysts preparation: (NH4)6Mo7O24 • 4H2O (AHM); MoO3; H2MoO4; cobalt acetate Co(CH3COO)2•4H2O (CoAcet); cobalt nitrate Co(NO3)2•6H2O (CoNitr); cobalt carbonate CoCO3•mCo(OH)2•nH2O (CoCarb). For the preparation of the solutions the components were taken in the molar ratio of Mo/CA/Co = 2/1.2/1, to prepare solutions without cobalt the ratio of Mo/CA=2/1 was used. pH of resulting solution lay in the range of 2.0-3.5. All solutions were prepared by dissolution of the components in water under reflux at 80ºC. The Mo concentration in aqueous solution being fixed (2 mol/liter). This concentration of the solution provides the obtaining of catalyst with 11.0% Mo and 3.5% Co loading. For the all samples the commercial γ-Al2O3 extudates (produced by CJSC Promyshlennye Katalizatory, Ryazan, Russia) with specific surface area of 285 m2/g, pore volume of 0.82 cm3/g and average pore diameter of 115 Å was used. The support particles have the trilobular shape with the circumscribed circle diameter of 1.5 mm and the length of 3-6 mm. The method of the catalyst preparation with vacuum impregnation of the carrier is described in [5]. The catalysts sulfidation procedure and catalysts testing are given in [8]. Nonsulfided and sulfided catalysts are further designated as CoMoCA/Al2O3 and Co-MoCA-S/Al2O3, respectively. NMR spectra at the natural isotope content were recorded in AVANCE-400 Bruker spectrometer at the frequencies 26.06 (95Mo), 100.4 (13C) and 54.24 (17O) MHz with accumulation rate of 45, 0.1 and 45 Hz, respectively. The chemical shifts CS (in ppm) were referenced to the external standards of H2O (17O), tetramethylsilane (13C) and 2M solution of Na2MoO4 (95Mo). The EXAFS spectra of the Mo-K edge were obtained at the EXAFS Station of the Siberian Synchrotron Radiation Center (Novosibirsk) under the conventional transmission mode [9]. The storage ring VEPP-3 with the electron beam energy of 2 GeV and the average stored current of 100 mA was used as the source of radiation.

The superior activity of CoMo hydrotreating catalysts

111

The spectrometer has the Si (111) cut-off crystal-monochromator and two proportional ionization chambers as detectors. For each sample the oscillating piece of EXAFS spectra (χ (k)) was treated in the form of k2χ (k) at the wave number interval of 2.5-14.0 Å–1.The EXAFS spectra simulations for retrieving of the structure data were performed by using the standard procedure by VIPER code [10]. The FEFF7 program was employed to fit the parameters of scattering [11]. Raman spectra were recorded at room temperature in the range of 3600-100 cm–1 using an FT-Raman spectrometer RFS 100/S BRUKER (Germany). The excitation source used was the 1064 nm line of a Nd-YAG laser operating at power level of 100 mW. Photoelectron spectra were recorded using SPECS spectrometer with PHOIBOS150 hemispherical energy analyzer and AlKα irradiation (hν = 1486.6 eV, 200 W). Binding energy scale was preliminarily calibrated by the position of the peaks of Au4f7/2 (84.0 eV) and Cu2p3/2 (932.67 eV) core levels. For spectra recording the samples were supported on conductive scotch tape. The method of the internal standard was used for correct calibration of photoelectron peaks. For this C1s peak (Eb = 284.8 eV) corresponding to the surface hydrocarbon-like deposits (CC and CH bonds) accumulated on the surface during the storage in the atmosphere was used. A low energy electron gun (FG-15/40, SPECS) was used for sample charge neutralization. HRTEM images were obtained on a JEM-2010 electron microscope (JEOL, Japan) with a lattice-fringe resolution of 0.14 nm at an accelerating voltage of 200 kV. Samples to be examined by HRTEM were prepared on a perforated carbon film mounted on a copper grid. Particle size distribution was evaluated by means of iTEM 5.0 software.

3. Results and discussion The complete dissolution of (AHM) powder in the aqueous solution of CA occurs within minutes forming a molybdenum citrate [Mo4(C6H5O7)2O11]4– (Mo4CA), with a structure described in [12]. The data of NMR, EXAFS, FTIR and Raman obtained for (Mo4CA) complex in the solution, in the solid form and on the surface of Al2O3 are given in [6]. Dissolution of other molybdenum compounds in the solution containing CA is rather more difficult, the complete dissolution of H2MoO4 occurs only in 24 hours. For MoO3 no more than 30% molybdenum passed in a solution in 12 hours. Solutions of H2MoO4 + CA and MoO3 + CA have a blue tint, due to the partial reduction of molybdenum by CA with the formation of molybdenum blue. In both cases, according to NMR 95Mo and 13C (Table 1) (Mo4CA) was formed in solution. It should note that 95Mo NMR spectra of H2MoO4 + CA and MoO3 + CA solutions are characterized by broad peak shapes compared with the data [6]; both solutions contain polymeric or colloidal particles, whose existence is confirmed by Tyndall effect when a laser beam passed through a solution. Co-Mo-CA solutions were prepared by dissolution of the joint mixture of Mo and Co compounds and CA. Complete dissolution of mixture containing (AHM), CA and (CoAcet) or (CoNitr) occurs within minutes. The mixtures of MoO3, CA, (CoAcet) or (CoCarb); (AHM), CA, (CoCarb); H2MoO4, CA, (CoAcet) dissolve more slower, but, the almost complete dissolution of the components with a small amount of suspension in solution was observed after 4-6 hours of stirring. After filtration the pink transparent solutions that do not contain colloidal particles were obtained. It should be noted, that in the presence of cobalt compounds the rapid and complete dissolution of MoO3 and H2MoO4 were achieved, that was not noticed in the solutions without cobalt. CoCarb and CA formed a slightly soluble cobalt citrate which also completely dissolved in the

112

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presence of molybdenum. Obviously, the readily soluble bimetallic Co-Mo complex with citric ligands is formed in the CA solutions with pH = 2.0-3.5 Table 1. 95Мо and 13С NMR data of aqueous solutions. Sample

δ 95Mo (W) [I]

δ 13C

0.5 M C5H8O7

—

177.8; 174.4; 74.3; 44.3

Mo4CA Ref. [5]

36(360)[1.0]; –54(380)[0.67]

184.7; 182.1; 173.9; 85.7; 43.7

H2MoO4+CA*

39(1500)[1.0]; –51(1200)[0.75]

184.9; 179.5; 174.9; 85.7; 43.4

MoO3+CA*

40(2600)[1.0]; –50(1900)[0.80]

184.8; 180.5; 175.2; 85.5; 43.1

δ – chemical shift, ppm; W – width of a line, Hertz; I – intensity, relative units. * – the lines in 13C NMR spectra derived from excess of CA are not given in the table. Raman and EXAFS data for the prepared Co-Mo solutions are shown in Fig. 1 and 2. Raman spectra of all solutions are identical and coincide with spectrum of (CoMo4CA) described in [6]. RDF Mo K-edge curves for all solutions agree well with each other and with the RDF curves for (CoMo4CA) from [6,13]. The analysis of these RDF Mo K-edge curves allowed the following distances to be distinguished : the peaks at 1.72 and 1.94 Å were assigned to Mo-O distances for the terminal and bridging oxygen atoms of the molybdenum containing anion [Mo4(C6H5O7)2O11]4–, respectively, the peaks located within 2.00-3.00 Å, corresponds the distances from molybdenum to oxygen atoms in citrate ligands; the broad peak at R-δ>3 Å contains Mo-Mo distance (3.36 Å) and Mo-Co one (3.41Å).

Raman intensity, arb. units

940

210 344

900

380

863

H2MoO4 + CoAcet MoO3 + CoAcet AHM + CoCarb MoO3 + CoCarb CoMo4CA

200

400

600

Raman shift, cm

800

1000

-1

Fig. 1 Laser Raman spectra of (CoMo 4CA) complex derived from the different precursors.

113

FT amplitude

The superior activity of CoMo hydrotreating catalysts

H2MoO4 + CoAcet MoO3 + CoAcet PMA + CoCarb MoO3 + CoCarb CoMo4CA 0

2

4

6

R-δ, A Fig: 2. Fourier transform of molybdenum K-edge EXAFS spectra for the (CoMo4CA) complex derived from the different precursors.

Thus, it can be concluded that bimetallic complex (CoMo4CA) with structure described in [6,13] can be obtained using any of the above specified compounds of Mo and Co in aqueous solutions containing components with the molar ratio of Mo/CA/ Co=2/1.2/1 and pH laying in the range of 2.0-3.5. The catalyst prepared by vacuum impregnation of Al2O3 with (CoMo4CA) solution and dried at 120 and 400ºC was studied by XPS. The Mo3d spectrum of Co-MoCA/ Al2O3-120 sample (Table 2) contains a peak of Mo3d5/2 with a binding energy of 232.3 eV. Although, this value corresponds to Mo6+ it differs significantly from the value of 232.6-232.9 eV characteristic of calcined Co-Mo catalysts [14]. The obtained values of the binding energy Mo3d5/2 agree well with 232.2 eV, corresponding to molybdenum complexes with polybasic organic acids in the nonsulfided hydrotreating catalysts [15]. In the C1s spectrum of the Co-MoCA/Al2O3-120 sample the peak of Eb = 284.8 eV corresponding to the surface hydrocarbon and the peak of 288.7 eV from carboxyl groups of CA was elucidated. The second peak disappears after drying of the catalyst at 400ºC, apparently, due to removal of citrate ligands. In the spectrum of Co 2p the increase of binding energy on 0,2 eV with increasing of drying temperature from 120 to 400ºC can be considered as experimental error or may be a consequence of removal of the citric ligands bonded with cobalt. Table 2. Binding energies (eV) measured by XPS. Sample

Mo 3d

Co 2p

C 1s

Al 2p

O1s

S2p

Co-MoCA/Al2O3-120

232.3

781.8

284.8; 288.7

74.7

531.6

—

Co-MoCA/Al2O3-400

232.8

782.0

284.8

74.7

531.6

—

Co-MoCA-S/Al2O3

228.6

778.8

284.8

74.7

531.6

161.8

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According to FTIR, Raman, XAS-spectroscopy data obtained for catalyst surface species the coordination of Co2+ cations towards the molybdenum containing anion through the carboxyl groups of citrate ligands and terminal oxygen atoms bonded with molybdenum, as well as the distance Co-Mo = 3.41 Å typical for (CoMo4CA) [6,13,16] remain unaltered. After the drying at temperatures up to 220ºC the citrate ligands (C6H5O7)2– turn into itaconate (C5H4O4)2–, but at the same time, although the distance Co-Mo is increased on 0,1 Å, the coordination of cobalt towards molybdenum anion is saved [16]. The treatment of the catalysts at temperature higher than 220ºC results in decomposition of the supported complex with the formation of undesirable surface compounds. Thus, the citrate ligands composing of (CoMo4CA) complex provide the stability of its structure in the solution and on the alumina surface and allow preservation of the coordination of cobalt to the molybdenum containing anion and close arrangement of Co and Mo atoms, in the case that catalysts were dried at temperature up to 220ºC. The detailed study of sulfidation behavior of the Co-MoCA/Al2O3 catalysts during the liquid phase sulfidation using straight-run gas oil spiked with dimethyldisulfide (DMDS) showed that the complete or partial conservation of initial structure of the complexes on the support surface after thermal treatment favor the formation of an active sulfide CoMo catalysts [8]. It was stated that the conservation of the complexes results in delaying the interaction of sulfiding agent with the surface Co-Mo compound due to stabilization by carboxylated ligands (citrate or itaconic). The appreciable sulfidation of the catalysts at the stage of low-temperature activation-sulfidation at 230°C began after the carboxylate ligands of the initial complex start decomposing to generate surface Co and Mo compounds which are capable of interacting with the sulfiding agent. As a result the longer period of time is needed to saturate the CoMo compound by sulfur during the sulfidation at 230-240°C. In this case the sulfidation of catalysts proceeds simultaneously with the decomposition of carboxilated ligands providing favorable conditions for the formation of the active Co-Mo-S phase type II, while Mo and Co atoms are situated in close proximity during sulfidation. According to XPS data for Co-MoCA-S/Al2O3 sample the sulfide components are at the atomic ratio S/Mo = 2.0, that indicates that prepared catalyst is fully sulfided. The XPS spectra (Table 2) contains narrow and intense peaks corresponded to CoMoS phase (778.8 eV Co2p3/2 and 228.6 eV Mo3d5/2) and do not contain any significant peaks that could be assigned to Co9S8 (778.1 eV) or to oxygen-containing compounds of Co2+ (781.7 ± 0.3 eV), as well as the presence of Mo5+ compounds (230.0 ± 0.1 eV) and Mo 6 + (232.5 ± 0.3 eV) is fully excluded [14,17]. Processing of the Mo and Co K-edge EXAFS data allows peaks related to distances Mo-S = 2.40 Å (c.n.=5.0), Mo-Mo = 2.60 Å (c.n.= 3.1), Co-S = 2.22 Å (c.n.= 4.0) and Co-Mo = 2.78 Å (c.n.= 0.8) to be identified in the curves. According to HRTEM, the prepared catalyst characterized with 11% Mo loading contains MoS2 particles with an average size of 31 Å, the mean stacking number of 1.72, and there are ca. 50 layers of MoS2 per 1000 nm2 of the catalyst surface. The set of data obtained with the Co-MoCA-S/Al2O3 catalysts lead us to conclude that the most all cobalt and molybdenum atoms here are constituents of Co-Mo-S phase of type II. The absence of appreciable quantities of oxygen-containing compounds in the catalyst sulfided at temperature not exceededing 340ºC allows the conclusion that the citrate ligands favor the avoidance of the strong interaction between the supported metals and carrier surface and the formation of compounds included Mo-O-Al fragments that can be fully sulfided only at the temperatures higher than 600ºC [2].

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The superior activity of CoMo hydrotreating catalysts Table 3. The results of SRGO hydrotreating activity of the Co-MoCA/Al2O3 catalysts.

Testing conditions: 3.5 MPa, liquid hourly space velocity of 1.0 h–1, H2/feed volume ratio of 300. Initial compounds for the synthesis of (CoMo4CA) complex

Temperature for 10 ppm S fuel production (оC)

(AHM), CA, (CoAcet)

351

(AHM), CA, (CoNitr)

353

MoO3, CA, (CoCarb)

348

MoO3, CA, (CoAcet)

350

The metals content are equal 11.0±0.2% Mo and 3.5±0.2% Сo based on sample calcined at 550оС. The Co-MoCA-S/Al2O3 catalysts obtained via bimetallic (CoMo4CA) complex that can be prepared from different initial compounds, after the drying at 120ºC and sulfidation with DMDS have shown the comparable activity in the hydrotreatment of diesel fuel (Table 3). All the prepared catalysts allow ultra low sulfur diesel fuel to be obtained.

4. Conclusions Thus, it is proved that much higher activity of the catalysts prepared using citric acid is due to: 1. CA provides the formation of stable tetrameric anion [Mo4O11(C6H5O7)2]4– in the studied intervals of the concentrations and pH. 2. CA enables the bimetallic CoMo complex to be obtained through the coordination of Co2+ cations towards Mo-containing anions. The CoMo-complex could be described by Co2[Mo4O11(C6H5O7)2]×xH2O formula. The existence of this complex within the impregnated solution ensures the formation of the oxide precursors with close proximity between Co and Mo during its deposition at an alumina surface and provides the formation of highly active disperse sulfide particles during the sulfiding step. So, CA assures the stabilisation of the vicinal arrangement of Co and Mo during the genesis of catalysts. 3. Citric ligands screen the metals from the strong interaction with support. 4. CA provides simultaneous sulfidation of Co and Mo favoring to the selective formation of the hydrotreating catalysts active sites.

References 1. 2. 3. 4. 5. 6. 7. 8. 9.

T. Fujikawa, M. Kato, T. Ebihara et al., J. Jpn. Petrol. Inst., 48(2) (2005) 114. H. Topsoe, Applied Catalysis A: General 322 (2007) 3. J.A. Bergwerff, M. Jansen, R.G. Leliveld et al., J. Catal. 243 (2006) 292. J.A.R. Van Veen, E. Gerkema, A.M. Van der Kraan, A. Knoestera, J.Chem.Soc., Chem. Commun., (1987) 1684. O.V. Klimov, A.V. Pashigreva, G.A. Bukhtiyarova et al., Catal. Today (2009), doi:10.1016/ j.cattod.2009.07.095 O.V. Klimov, A.V. Pashigreva, M.A. Fedotov et al., J.Mol.Catal. A, 2010, in press. A.V. Pashigreva, O.V. Klimov, G.A. Bukhtiyarova et al., Catal. Today (2009), doi:10.1016/ j.cattod.2009.07.096 A.V. Pashigreva, G.A. Bukhtiyarova, O.V. Klimov et al., Catal. Today 149 (2010) 19. D.I. Kochubey, EXAFS-Spectroscopy of the Catalysts, Science, Novosibirsk, 1992, p. 144.

116 10. 11. 12. 13. 14. 15. 16. 17.

A. Pashigreva et al. K.V. Klementev, J. Phys. D: Appl. Phys. 34 (2001) 209. J.J. Rehr, A.L. Ankudinov, Radiat. Phys. Chem. 70 (2004) 453. N.W. Alcock, M. Dudek, R. Grybos et al., J.Chem.Soc.,Dalton Trans. (1990) 707. O.V. Klimov, A.V. Pashigreva, D.I. Kochubey et al., Doklady Physical Chemistry, 424 (2009) 35. Y. Okamoto, T. Imanaka, S. Teranishi, J.Catal. 65 (1980) 448. L. Coulier, V.H.J. De Beer, J.A.R. Van Veen, J.W. Niemantsverdriet, J. Catal. 197 (2001) 26. G.A. Bukhtiyarova, O.V. Klimov, D.I. Kochubey et al., NIMA, 603 (2009) 119. A.D. Gandubert, E. Krebs, C. Legens et al., Catal. Today. 130 (2008) 149.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V.

Elucidation of the surface configuration of the Co(II) and Ni(II) aqua complexes and of the Cr(VI), Mo(VI) and W(VI) monomer and polymer oxo–species deposited on the titania surface during impregnation George D. Panagiotou,a Theano Petsi,a John Stavropoulos,a Christos S. Garoufalis,b Kyriakos Bourikas,c Christos Kordulis,a,d Alexis Lycourghiotis*a a

Department of Chemistry, University of Patras, GR–265 00 Patras, Greece Department of Physics, University of Patras, GR–265 00 Patras, Greece c School of Science and Technology, Hellenic Open University, 18 Parodos Aristotelous St., GR–26335, Patras, Greece d Institute of Chemical Engineering and High Temperature Chemical Processes, FORTH/ICE–HT, P.O. Box 1414, GR–265 00 Patras, Greece *corresponding author (e–mail: [email protected]) b

Abstract The structure of the precursor Co(II) and N(II) aqua complexes and the Cr(VI), Mo(VI) and W(VI) monomer and polymer oxo–species formed upon impregnation at the interface developed between the surface of the titania grains and the impregnating solution was thoroughly elucidated. Moreover, the interfacial speciation was determined for various surface concentrations of the precursor species regulated by adjusting the corresponding solution concentrations and the pH of the impregnating solution. Keywords: titania, catalysts preparation, interfacial chemistry, adsorption

1. Introduction In the preparation of supported catalysts we disperse “bi–dimensional” species or nano– particles of a catalytically active metal, oxide or sulphide on the surface of a rather limited number of supports with high specific surface area. Among these supports TiO2 is important. The impregnation step is critical for controlling the physicochemical characteristics of the precursor species and thus the characteristics and the catalytic behavior of the aforementioned “bi–dimensional” species or nano–particles. In order to obtain this control it is necessary to understand at molecular level the impregnation step [1,2]. This mainly concerns the “Equilibrium Deposition Filtration” and the “Homogeneous Deposition–Precipitation” techniques which favour deposition, upon the equilibration of the suspension, at the interface developed between the surface of the titania grains and the solution (interfacial deposition) and to a lesser extent the incipient wetness or wet impregnation techniques. In this communication we report on the structure and interfacial speciation of the precursor Co(II) and Ni(II) aqua complexes and the Cr(VI), Mo(VI) and W(VI) oxo–species formed at the interface developed between the surface of the TiO2 grains and the solution. The work is based on a recently development concise picture concerning the acid–base behavior of the TiO2 (hydr)oxo–groups, considered as the receptor sites for the deposition of the aforementioned species, and the structure of the “TiO2/electrolytic solution” interface as well [3].

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2. Experimental Several methodologies based on potentiometric titrations, microelectrophoresis and macroscopic adsorption measurements have been used in conjunction with Diffuse Reflectance, Raman and Electron Paramagnetic Resonance spectroscopy. Semi– empirical quantum mechanical calculations, stereochemical considerations and quantum mechanical calculations in the frame of the DFT are followed. The above are then used for developing a quantitative model for the interfacial deposition studied. Details concerning the combined application of these methodologies to obtain the interfacial structure and speciation of the aforementioned species have been reported elsewhere [4– 10]. The majority crystal terminations (1 0 1) and (1 0 0) of the anatase nanocrystals, comprised in the titania grains, were chosen to exemplify the interfacial structures. A titania rich in anatase (Degussa P25) has been used in all cases.

3. Results and discussion 3.1. Structure and interfacial speciation of the Cr(VI) oxo–species

A schematic representation of the local structures of the deposited species (CrO42–, HCrO4– and Cr2O72–) is illustrated in Figure 1 [8]. These species are electrostatically retained above the bridging hydroxyls forming ion pairs. Each CrO42– or HCrO4– ion is located above one bridging hydroxyl whereas each Cr2O72– ion is located above two bridging hydroxyls. The deposited species are located between plane 1 and plane 2 of the compact layer of the interface, with an equal distribution of their charge. Plane 2 Plane 1

Plane 0

Figure 1. Structures of the deposited CrO42–, HCrO4– and Cr2O72– ions on the anatase (1 0 1) crystal termination. Ti: gray; H: blue; O in TiO2 cluster: red; O in chromates: yellow; Cr: cyan. The dotted line indicates electrostatic bond between chromates and surface bridging hydroxyls.

At too low Cr(VI) surface concentrations (0–0.3 μmol Cr(VI) m–2) only the CrO42– and HCrO4– ions are deposited, with a preference of the titania surface for the first. At higher surface concentrations the Cr2O72– ions are, in addition, deposited. In the pH range 7.0–8.0 the electrostatic adsorption of the CrO42– ions on the bridging surface hydroxyls is the predominant deposition process. In contrast, at pH=4.5 the Cr(VI) deposition occurs mainly through the electrostatic adsorption of the HCrO4– ions on the bridging surface hydroxyls. In the intermediate pH range both electrostatic adsorptions contribute to the whole deposition process. The electrostatic adsorption of the Cr2O72– ions over two neighboring bridging surface hydroxyls becomes important at pH<6.

3.2. Structure and interfacial speciation of the Mo(VI) oxo – species The following Mo(VI) oxo–species are usually present in the aqueous solutions used for mounting molybdenum on a support surface: MoO42–, HMoO4–, MoO3(H2O)3, Mo7O246–,

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119

HMo7O245–, H2Mo6O214–, H3Mo8O285– and Mo8O264–. A schematic representation of the structures of the deposited Mo(VI) oxo–species is illustrated in Figure 2. At relatively low Mo(VI) surface concentration, only the monomer MoO42– ions are deposited on both the terminal oxygen atoms and bridging surface hydroxyls. In the first case, mono– substituted mononuclear inner sphere complexes (Ti–O–MoO3, fig. 2, left) are formed whereas in the second case the deposition takes place exclusively through formation of a hydrogen bond (Ti2OH…O–MoO3, fig. 2, right). Both species are located inside the compact layer of the interface. Specifically, in both configurations the Mo atom is situated between the surface plane and plane 1 whereas the solution oriented oxygen atoms are situated at or near the plane 1 of the compact layer. The charge of the deposited monomer species is distributed between the surface plane and plane 1. Plane 2

Plane 1

Plane 0

Figure 2. Structures of the adsorbed Mo(VI) monomers on the anatase (1 0 0) crystal termination. Ti: gray; H: blue; O in TiO2 cluster: red; O in molybdates: yellow; Mo: green. The dashed line indicates H–bond between the molybdates and the surface bridging hydroxyls.

Figure 3. Local structure of the electrostatically adsorbed Mo7O246– species on the (1 0 1) crystal termination of anatase. The notation is that described in fig. 2.

At relatively high Mo(VI) surface concentrations (obtained at pHs<5 and for initial Mo(VI) solution concentration ≥10–2 M) the polymeric Mo7O246– and HMo7O245– species are adsorbed through electrostatic forces, in addition to the aforementioned

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monomers. The adsorption sites involve 5 bridging and 5 terminal neighboring (hydr)oxo– groups. A schematic representation of the local structure of the deposited Mo7O246– species on the anatase (1 0 1) crystal termination is illustrated in Figure 3. The same structure is obtained for the HMo7O245– species. The deposited polymeric species are located in a range extended from plane 1 up to the first layers of the stagnant – diffuse layer close to plane 2 [3]. The charge of the deposited polymeric species is distributed between plane 1 and plane 2 with the greater portion situated at plane 2. The interfacial speciation at Mo(VI) solution concentration equal to 10–2 M and over a wide pH range is illustrated, as an example, in Figure 4. 1. 57 -

-2

[T i

2

OH

... O

-M

80

3

oO

]

CMo(VI), initial = 10 Μ

60 40

Ti O 3

20

35 0.

oO M

% Mo(VI) adsorbed

100

0.4+

[5(TiOH)5(Ti2OH)HMo7O24]

0.6-

[5(TiOH)5(Ti2OH)Mo7O24]

0 4

5

6

pH

7

8

9

Figure 4. Variation of the % Mo(VI) deposited as a certain Mo(VI) oxo–species with the pH of the impregnating solution. Initial Mo(VI) concentration in the solution: 10–2 M. Ionic strength: 0.1 M. The various Mo(VI) oxo–species are indicated.

3.3. Structure and interfacial speciation of the W(VI) oxo–species

The W(VI) oxo–species more probably present in the solution are: WO42–, HWO4–, WO3(H2O)3, H2W7O226–, W7O246–, HW7O245– and H2W12O4210–. A schematic representation of the local structures of the deposited species is illustrated in Figure 5. Plane 2

Plane 1

Plane 0

Figure 5. Structures of the deposited W(VI) monomer oxo–species on the anatase (1 0 0) crystal termination. Ti: gray; H: blue; O in TiO2 cluster: red; O in tungstates: yellow; W: black. The dashed line indicates H–bond between the tungstates and the surface bridging hydroxyls.

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The anatase (1 0 0) crystal termination has been chosen to exemplify these structures. At low W(VI) surface concentration, mono–substituted (fig. 5, left) and di– substituted (fig. 5, center) mononuclear inner sphere complexes are formed above the terminal oxygen atoms, whereas the deposition takes place exclusively through formation of a hydrogen bond above the bridging oxygens (fig. 5, right) [6]. It should be stressed that there was no way to describe the experimental data by considering formation of an inner sphere complex on the bridging oxygens. This is due to the fact that the oxygen atoms of this kind are already doubly coordinated by 2 Ti atoms. In all configurations the W atom is situated between the surface plane and plane 1 whereas the solution oriented oxygen atoms are situated at or near the plane 1 of the compact layer. The charge of the deposited monomer species is distributed between the surface plane and plane 1. At relatively high W(VI) surface concentrations the polymer W7O246–, HW7O245– and H2W12O4210– species are adsorbed through electrostatic forces, in addition to the monomer WO42– ions [6]. In the first case, the adsorption site involves 5 bridging and 5 terminal neighboring (hydr)oxo–groups, whereas in the second case the adsorption site involves 7 bridging and 7 terminal neighboring (hydr)oxo–groups. It is important to note that the deposited polymer species are located in a range extended from plane 1 up to the first layers of the stagnant – diffuse layer, close to plane 2 [3]. The charge of the deposited polymeric species is distributed between plane 1 and plane 2 with the greater portion situated at plane 2. One may imagine a picture similar to that illustrated in fig. 3 for the electrostatically adsorbed Mo7O246– species. It was, moreover, found that at solution W(VI) concentration ≤10–3 M all the mounted W(VI) is practically deposited as monomer WO42– species in the whole pH range. Concerning the monomer species, it was found that in the pH range 4–7 there is a preferential formation of the mono–substituded inner sphere complexes above the terminal surface oxygen atoms, while at pH>8 the deposition through hydrogen bonds above the bridging surface hydroxyls becomes more important. The contribution of the di–subtituted inner sphere complexes above the terminal surface oxygens is very small. -2

CW(VI), initial = 10 Μ

70

Ti OW

% W(VI), adsorbed

60

O 3

50

0.3 5-

0.4+

[5(TiOH)5(Ti2OH)HW7O24]

71.5

40

H. iO [T 2

30

] O3 -W ..O

0.6-

20

[5(TiOH)5(Ti2OH)W7O24]

10

[7(TiOH)7(Ti2OH)H2W12O42]

Ti2O2WO2

0 4

5

2.44-

1.3+

6

7

8

9

pH

Figure 6. Variation of the % W(VI) deposited as a certain W(VI) oxo–species with the pH of the impregnating solution. Initial W(VI) concentration in the solution: 10–2 M. Ionic strength: 0.1M. The various W(VI) oxo–species on the surface and their charges are indicated.

Inspection of fig. 6 shows that even at high W(VI) solution concentrations the monomer species are exclusively deposited in the pH range 7–9. The contribution of

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these species to the whole deposition is very important even in the pH range 4–7. This indicates the relatively high deposition selectivity of the surface for the monomer species with respect to the polymer ones. The interfacial speciation concerning the monomer species is similar to that described above for the lower W(VI) solution concentrations. The increase in the W(VI) solution concentration causes an increase in the relative amount of the deposited W(VI) through the electrostatically retained polymer species in the pH range 7–4. It may be seen that in this pH range the deposition of the W7O246– and HW7O245– species is favored with respect to the deposition of the H2W12O4210– species.

3.4. Structure and interfacial speciation of the [Co(H2O)6]2+ complexes

The sub–title aqua complex is practically the only Co(II) species present in the nitrate solutions frequently used for mounting cobalt on the surface of a catalytic support. Two different structures are the most probable at low and medium Co(II) surface concentrations (see Figure 7) [7].

Figure 7. Schematic representations of the mono–hydrolyzed (TiO–Ti2O) and the di–hydrolyzed (TiO–TiO) di–substituted configurations on the anatase (1 0 0) crystal face formed upon deposition of the [Co(H2O)6]2+ ions onto the titania surface, at low and medium Co(II) surface concentrations. Ti: gray; H: yellow; O: red; Co: pink.

In the one configuration (fig. 7, left) the [Co(H2O)6]2+ complex forms a mononuclear, di–substituted surface inner sphere complex with one terminal (TiO) and one bridging (Ti2O) oxo–group, by exchanging two water ligands with two surface oxygen atoms (TiO–Ti2O configuration). In this complex one of the remaining water ligands has been hydrolyzed. In the second configuration (fig. 7, right) the [Co(H2O)6]2+ complex forms a mononuclear, di–substituted surface inner sphere complex with two terminal (TiO) oxo–groups, by exchanging two water ligands with two surface oxygen atoms (TiO– TiO configuration). In this complex two of the free water ligands have been hydrolyzed. In the TiO–TiO configuration the Co atom and the two hydrolyzed ligands are located close to the plane 1 whereas the remainder two water ligands are located close to the plane 2. In the TiO–Ti2O configuration the Co atom, two water ligands and one hydrolyzed water ligand are located close to the plane 1, whereas the remainder water ligand at plane 2. In order to describe the experimental data in the whole Co(II) surface concentration range studied, it was necessary to assume, in addition, the formation of a binuclear and a three–nuclear inner sphere complex [7]. The best fitting was obtained assuming the aforementioned configurations as well as one binuclear (Ti2O–TiO–––TiO, fig.8, right) and one three–nuclear (Ti2O–TiO–––TiO–––TiO–Ti2O, fig. 8, left) configuration.

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Figure 8. Schematic representation of one binuclear (Ti2O–TiO–––TiO, right) and one three– nuclear (Ti2O–TiO–––TiO–––TiO–Ti2O, left) configuration on the anatase (1 0 0) crystal face formed upon deposition of the [Co(H2O)6]2+ ions onto the titania surface, at relatively high Co(II) surface concentrations. The notation is that described in fig. 7.

It was found that the interfacial speciation depends on the Co(II) surface concentration. This can be regulated by adjusting the Co(II) solution concentration and the impregnation pH. The formation of the mononuclear complexes, especially the formation of the TiO– TiO configuration, is largely favored. The increase of the Co(II) surface concentration increases the relative concentration of the bi– and three– nuclear complexes. Finally, it should be mentioned that in addition to the above mentioned octahedral configurations, tetrahedral configurations of Co(II) may be formed on titania surface, but in a too small extent, as it is implied by the spectroscopic analysis [7]. Plane 2 Plane 1 Plane 0

(a)

(b) Figure 9. Schematic representations of the mono–substituted (TiO, (a) left), di–substituted, (TiO– TiO, (a) right) and di–substituted (Ti2O–TiO (b)) configurations on the anatase (1 0 0) crystal face formed upon deposition of [Ni(H2O)6]2+ on titania surface. Ti: gray; H: yellow; O: red; Ni: green.

3.5. Structure and interfacial speciation of the [Ni(H2O)6]2+ complexes

Three mono–nuclear inner sphere complexes are formed at the compact layer of the “titania / electolytic solution” interface; one mono–substituted, di–hydrolyzed complex

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above the terminal oxo–groups, by exchanging one water ligand with a surface oxygen atom (TiO configuration, fig. 9a, right), a di–substituted, di–hydrolyzed complex above two terminal adjacent oxo–groups, by exchanging two water ligands with the two surface oxygen atoms (TiO–TiO configuration, fig.9a, left) and one di–substituted, non– hydrolyzed complex above one terminal and one bridging adjacent oxo–groups (Ti2O– TiO configuration, fig. 9b) [10]. In all these configurations the nickel atom and a number of free ligands are located near the plane 1 of the compact layer of the interface whereas the remainder three ligands near the plane 2. One binuclear and one three–nuclear inner sphere complexes are formed, in addition, upon the deposition of the [Ni(H2O)6]2+ ions at the compact layer of the “titania/electrolytic solution” interface (fig. 10). In the first case, the receptor site involves one bridging and two terminal oxo–groups (Ti2O–TiO–––TiO configuration, fig. 10, right) whereas in the second case the receptor site involves two bridging and three terminal oxo–groups (Ti2O–TiO–––TiO–––TiO–Ti2O configuration, fig. 10, left ).

Figure 10. Schematic representation of one binuclear (Ti2O–TiO–––TiO, right) and one three– nuclear (Ti2O–TiO–––TiO–––TiO–Ti2O, left) configurations on the anatase (1 0 0) crystal face formed upon deposition of [Ni(H2O)6]2+ on titania surface. The notation is that described in fig. 9.

The TiO configuration predominates in the whole range of the surface concentration studied. The contribution of the TiO–TiO and Ti2O–TiO configurations is also important at too low Ni(II) surface concentrations. But this contribution drastically decreases as the Ni(II) surface concentration increases. The relative surface concentrations of the Ti2O– TiO–––TiO and Ti2O–TiO–––TiO–––TiO–Ti2O configurations initially increase with the Ni(II) surface concentration and then remain practically constant. In conclusion, the present study revealed the configurations of the precursor Co(II), Ni(II), Mo(VI), W(VI) and Cr(VI) species on the titania surface formed upon impregnation and allowed the determination of the concentration of each configuration for given values of impregnation parameters. This allows designing severely the preparation procedure by controlling the interfacial structure and speciation. Such a control, obtained by regulating the impregnation parameters (pH, solution concentration of the active element), is expected to help the preparation of effective catalysts supported on titania. Finally, it should be noticed that the present work is located in the context of the application of concepts and methods of interfacial chemistry in catalysts preparation [e.g.1,2,6,7,11–16].

References 1. 2. 3.

K. Bourikas, Ch. Kordulis, A. Lycourghiotis, 2006, Catal. Rev., 48, 363. A. Lycourghiotis in “Synthesis of Solid Catalysts”, K.P. de Jong (Ed.), Wiley, 2009, ch. 2. G.D. Panagiotou, Th. Petsi, K. Bourikas, Ch.S. Garoufalis, A. Tsevis, N. Spanos, Ch. Kordulis, A. Lycourghiotis, 2008, Adv. Colloid Interface Sci., 142, 20.

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G.D. Panagiotou, Th. Petsi, J. Stavropoulos, K. Bourikas, Ch.S. Garoufalis, Ch. Kordulis, A. Lycourghiotis, 2006, Stud. Surf. Sci. Catal., 162, 809. K. Bourikas, G.D. Panagiotou, Th. Petsi, Ch. Kordulis, A. Lycourghiotis, 2006, Stud. Surf. Sci. Catal., 162, 251. G.D. Panagiotou, Th. Petsi, K. Bourikas, Ch. Kordulis, A. Lycourghiotis, 2009, J. Catal., 262, 266. Th. Petsi, G.D. Panagiotou, Ch.S. Garoufalis, Ch. Kordulis, P. Stathi, Y. Deligiannakis, A. Lycourghiotis, K. Bourikas, 2009, Chem. Eur. J., 15, 13090. Th. Petsi, PhD thesis, University of Patras, Patras, 2006. G.D. Panagiotou, PhD thesis, University of Patras, Patras, 2006. J. Stavropoulos, PhD thesis, University of Patras, to be submitted. L. Espinosa–Alonso, K.P. de Jong, B.M. Weckhuysen, 2008, J. Phys. Chem. C, 112, 7201. J.A. Bergwerff, T. Visser, B.M. Weckhuysen, 2008, Catal. Today, 130, 117. X. Carrier, E. Marceau, M. Che, 2006, Pure Appl. Chem., 78, 1039. X. Carrier, J–F. Lambert, S. Kuba, H. Knozinger, M. Che, 2003, J. Mol. Struct., 656, 231. L. Jiao, Y. Zha, X. Hao, J.R. Regalbuto, 2006, Stud. Surf. Sci. Catal., 162, 211. L. Jiao, J.R. Regalbuto, 2008, J. Catal., 260, 329.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved

Innovative characterizations and morphology control of γ–AlOOH boehmite nanoparticles: towards advanced tuning of γ–Al2O3 catalyst properties M. Digne,a R. Revel,a M. Boualleg,a D. Chiche,a B. Rebours,a M. Moreaud,a B. Celse,a C. Chanéac,b J.–P. Jolivetb a

IFP, IFP–Lyon, Rond–point de l’échangeur de Solaize BP 3, 69360 Solaize, France Chimie de la Matière Condensée, UMR 7574 CNRS, Université P. & M. Curie, 4 place Jussieu, 75252 Paris Cedex 05, France b

Abstract Alumina γ–Al2O3 is widely used as catalyst support in refining and petrochemistry and is often obtained by calcination of γ–AlOOH boehmite. Several crucial properties of γ– Al2O3, including particles size and shape, are fixed at the boehmite stage. For this reason, the control and the understanding of boehmite particles properties is a fundamental task to tune the catalyst support. Several complementary techniques have been developed to characterize the boehmite particles morphology and the γ–Al2O3 formation, taking into account the specificity of these materials. These methods have been applied to several samples, aiming at rationalizing the morphology observations and preparation conditions. Keywords: boehmite, γ–alumina, support, nanoparticles, morphology

1. Introduction Metallic and oxide nanocrystals with tailored sizes and shapes have attracted huge research interest in the past decade due to their particular morphology–dependent properties. In the heterogeneous catalysis domain, the research on new materials is not only limited to the design of optimized active nanoparticles but also to that of the catalytic support. Indeed, a synergetic effect of the active nanoparticles and the support was demonstrated in several systems. Most catalyst supports belong to the transition alumina group [1]. Specifically, gamma alumina is extensively used in the field of oil refining, petrochemicals and fine chemicals [2]. Theoretical studies based on particle surface modeling showed that γ–alumina acid–base and catalytic properties are strongly related to particle morphology, through the nature of exposed crystalline faces [3]. Experimentally, γ–alumina particles are obtained from the thermal dehydration of pristine AlOOH boehmite materials (> 450°C), from which are inherited particle morphology (topotactic transition [4]) and relevant surface properties. The control of boehmite synthesis is thus of paramount interest for catalytic properties enhancement. Given the importance of particle morphology as a key parameter in designing and controlling material properties, size and shape accurate characterization attracts a great deal of attention. In this goal, an approach using the analysis of transmission electron microscopy (TEM) images is developed to estimate the size of the boehmite nanoparticles. Complementary, for very small particles, a novel method is proposed for

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three–dimensional size and shape determination of nanoparticles based on calculations of X–Ray Diffraction (XRD) powder patterns with respect to nanoparticle crystal structure, size, and shape. Additionally, an automated method using XRD is developed to characterize alumina, allowing the detection of additional phases and the determination of the structural evolution of the gamma alumina phase. In term of boehmite synthesis, sol–gel routes, and more specifically precipitation in aqueous medium of metallic salts, are favored techniques for syntheses of inorganic particles with accurate size and shape control. Indeed, by adjusting a set of parameters, nanomaterials with finely tuned characteristics and properties can be obtained. We show that the addition of complexing species (polycarboxylates and polyols) in the reacting medium may play an important role in the shape and size of oxide nanoparticles.

2. Particles size and shape characterizations 2.1. Transmission electron microscopy TEM is a key technique to determine particles morphology. For boehmite, the direct image observation is often difficult because of high aggregation and overlapping between particles. To overcome such difficulties, an advanced TEM images analysis, efficient for high particles size and based on dilution model, is developed [5]. This method is illustrated on boehmite sample Disperal 40 (Sasol Germany GmbH). High resolution transmission electron micrographs (HR–TEM) are performed on a JEOL 2100F, at an acceleration voltage of 200.0 kV. The nanoparticles are ultrasonicated in ethanol and dispersed on carbon covered Cu–grids. Images are acquired with a resolution of 0.41nm/pixel. TEM images contain electronic noise and white diffraction artefacts localized on the edges of the boehmite nanoparticles. Image filters (median filter, bilateral filter [6] with Tukey’s biweight function and morphological opening by reconstruction [7]) are performed in order to improve without damage the image quality of the edge transitions and the grey level intensities corresponding to the nanoparticles (Figure 1). The observations are then modelized by means of a dilution model [5,8]. A dilution model is used to simulate situations with thick slices with mass accumulation over the thickness. A dilution random function (DRF) is constructed from primary function Z t' (x ) and from a Poisson point process P with intensity μ n (dx ) ⊗ θ (dt ) . The DRF is

given by : Z i (x ) =

∑ {Z it' (x − xk )}

( t k , x k )∈P

k

(1). This model is characterized by its centered

covariance function: C (h) = θ g(h) (2), with θ and g(h) denoting respectively the induced intensity in two dimensions of the 3D Poisson point process (θ = θ 3 e, θ 3 being the 3D intensity and e the thickness of the slice), and the transitive covariogram of the primary grains ie in our case the boehmite nanoparticles. For a primary grain function Z t' (x ) , the

{

}

transitive covariogram gt(h) is given by: g t (h) = ∫ E Z 't (x − y )Z 't (x + h − y ) dy (3). Raw R

information on the nanoparticles can be obtained by analysis of the covariance function. The range of this curve corresponds to the average size of the boehmite nanoparticles: a size of 35 nm is also deduced.

Innovative characterizations and morphology control

129

Figure 1. From left to right: initial experimental image; filtered image; realization of dilution model with fitted parameters.

More precise results can be obtained with an original approach by numerical calculation of transitive covariogram of a 3D geometric model of the boehmite nanoparticles. A geometrical model based on “thick parallelogram” is parameterized with three parameters L, l and e (Figure 2). The angle value, measured on TEM images, is fixed to 104°, indicating that the sample particles expose (101) planes. The 3D orientation of the nanoparticules follows a uniform law of distribution. For a set of fixed parameters, the covariogram is obtained by this way : 3D simulation of a nanoparticle with the three parameters (L, l,e,), 3D rotation following a uniform law, additive projection on a plane, calculation of the logarithm image, and measure of the covariance. The covariogram is obtained by taking the average covariance curve for typically 500 realisations. The size of the boehmite nanoparticles is obtained by estimation of L, l and e, which give the best fit between the numerical covariogram and the experimental centered covariance according to equation (2). The weakest mean square error is obtained for L = 35.5 nm, l = 36.0 nm and e = 5.5 nm (Figure 2). The values of L and l are very close, indicating a diamond–shaped morphology. Figure 1 presents an achievement of a dilution model with these parameters, for visual comparison with experimental images.

Figure 2. Left : 3D geometrical model of boehmite nanoparticules. Right : experimental centered covariance and numerical transitive covariogram for L=35.5 nm, l=36.0 nm and e=5.5 nm. Range of covariance curve is reached for a size of 35 nm.

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2.2. Powder x–ray diffraction 2.2.1. Principles and processing When the boehmite particle size becomes smaller than 10 nm, the morphology determination from TEM images becomes more difficult. Indeed, the identification of the primary particles shape is ambiguous, even using advanced image analysis. On the contrary, for such particles size, the XRD powder analysis becomes very sensitive to morphological effects. An efficient way to extract quantitative information about morphology from the XRD pattern is the use of the Debye formula. The Debye formula is a general law, allowing for any atoms cluster, the full calculation of diffraction pattern (ie the calculation of the peaks position, intensity and shape without further assumption). The principle of the developed method is presented elsewhere in detail [9]: it is assumed that the boehmite particles expose four different faces, namely the (100), (101), (001) and (101). These crystallographic orientations correspond to the four low surface energies planes [10]. As a consequence, a boehmite particle can be fully described by four distances, for instance the distances from the surface to the center of mass of the particle. Starting from the boehmite unit cell, a set of particles is generated by varying the particle edges lengths. For each particle, the simulated XRD pattern is calculated applying the Debye formula and stored. A database of morphology–dependent XRD patterns is also obtained. Once the sample experimental pattern is recorded, the particles size and shape are determined as follows: the experimental pattern is compared to all the simulated patterns of the database, by calculating the weighted profile R–factor, Rwp (as usually done for Rietveld refinements, for instance). The Rwp value is calculated between 21 and 80° (2θ) and three parameters are optimized to minimize the Rwp value: the scaling factor between the experimental and simulated patterns and two other parameters for the pattern background (linear). The particle leading to the lowest Rwp value corresponds to the most representative particle morphology. Important work is done to automate the method and improve its accuracy. An exhaustive database is generated in order to cover all the possible morphologies: first the particles volume is fixed to 4 nm3 (below this volume, the particles do not exhibit exploitable patterns). All the different shapes are generated by varying the percentage of the exposed surfaces, with an incremental step of 5% (the sum of the surface percentages remaining equal to 100%). This leads to 1326 shape configurations. Next, for each of these configurations, the particle is increased in volume by multiplying the initial volume by a √2 factor, in an iterative way. The iterations are stopped when the volume reaches about 1024 nm3: beyond this value, the XRD pattern becomes less sensitive to particles morphology, compared to the electronic microscopy methods. A sum of 22542 configurations is also generated and covers all the possible particles size and shape. With such a database, the determination of the optimized morphology takes less than five minutes, allowing a fast screening of boehmite samples. 2.2.2. Application examples The method is applied to three commercial samples, namely Disperal, Pural SB3 and Disperal P2 (Sasol Germany GmbH) and two home–made samples (Nano–A and Nano– B), which exhibit very small particles (see section 3 for the synthesis concept). Comparisons between the experimental and simulated patterns are given for the Pural SB3 and Nano–B samples on Figure 3. Whatever the sample, a rather good agreement is observed, except for the (020) and the (200) peaks. The (020) peak position is well simulated, but not its intensity. This is due to two main effects: first, the hydrogen

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atoms are not taken into account in the simulation and they significantly modify the intensity of this low–angle peak. Next, the linear model of the background is not satisfactory in this region (2θ < 21°). The high experimental intensity of the (200) peak is due to oriented aggregation effects that are not taken into this single particle model. a) (020)

(150) (002)

(021) (130)

(200) (151)

b)

2θ (°)

Figure 3. Experimental (gray line) and optimized simulated (black line) powder diffraction patterns of two boehmite samples : a) Pural SB3 and b) Nano–B.

Table 1 summarizes the size and shape parameters obtained for the five samples. The method allows an accurate determination of the nanoparticles sizes, ranking from 5.6 for the Disperal sample to 2.5 nm for the Nano–B sample. The particles also exhibit different shapes, arising from different synthesis conditions. The (100) surface appears as difficult to expose (A(100) = 0–5%), whereas the ratio between the (010) and the (101) surface area can be finely adjusted. Several samples exhibit a significant portion of exposed (001) surface (A(001)=20% for Nano–B sample). Table 1. Size and shape of boehmite nanoparticles determined by the XRD method. Sample

Rwp (%)

Area (nm2)

A(100) (%)

A(010) (%)

A(001) (%)

A(101) (%)

Diameter (nm)*

Disperal

21.2

296

0

30

10

60

5.6

Pural SB3

15.2

133

5

55

0

40

3.5

Disperal P2

13.4

98

5

50

5

40

3.1

Nano–A

14.1

96

5

35

0

60

3.1

Nano–B

13.6

58

0

30

20

50

2.5

* mean diameter of an hypothetical sphere exhibiting the same volume as the sample particles

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Some comments should be done about the Rwp values. They stand between 13 and 21%, which is remarkable, taking into account that only seven parameters are optimized (4 for the particle morphology and 3 for the Rwp calculation). The XRD patterns variability mainly arises from the differences between particles morphology. The model can be improved by adding other degrees of freedom. Preliminary results show that the introduction of a particle size distribution, instead of a single particle model, significantly decreases the Rwp values.

3. Synthesis of boehmite particles with tunable size and shape Boehmite nanoparticles with finely tuned morphologies can be obtained via soft chemistry routes, through precipitation of aluminum precursor in an aqueous solution. Previous studies have shown how the pH value of the reaction medium and the temperature may influence boehmite particle size and shape, whose variations were shown to be related to surface charges values (directly depending on pH) and thus to surface energies [11]. Complexing species such as polycarboxylates, polyols, sulfates, phosphates may be also used to tune particle properties. Indeed, adsorption of such species also modify oxide–solution interfaces, and thus affect the crystal growth and the particle morphology. A recent work focusing on boehmite syntheses performed in the presence of xylitol exclusively [12] shows how xylitol may influence particle size and shape. These effects are related to xylitol–surface interactions studied through adsorption isotherms and DFT calculations of surface energies. More generally, new boehmite morphologies and textures can be obtained through the use of low amounts of polyols [12,13] or polycarboxylates used as complexing agents during material synthesis. In comparison to usual particle morphologies, the important observed changes are strongly related to the complexing strength of the additive used. In the case of polyols, significant particle size decreases may occur, depending on polyol nature, and particularly on the carbon chain length and the number of OH groups. Some unusual morphologies obtained are presented here. Boehmite nanoparticles are synthesized in aqueous medium through precipitation of aluminum nitrate Al(NO3)3 in presence of polyols or polycarboxylates at pH=11.5. The pH is adjusted by addition of sodium hydroxide, NaOH and the resulting suspensions are then aged in a stove at 95°C for one week. Reference syntheses are also achieved without complexing species, according to the procedures described in [11]. Final pH values of the suspensions are equal to 11.5 in every case. Final aluminum and complexing species concentrations are respectively 0.07 mol.L–1 and 0.007 mol.L–1. Figure 4 shows TEM micrographs obtained for particles synthesized without complexing agent, in the presence of xylitol and of tartrate ions. The presence of xylitol in the synthesis medium strongly affects particle size and specific surface area (from 180 m2.g–1 without polyol to 270 m2.g–1 in the presence of xylitol). However particle morphology determination is complex starting from only TEM observations only. However, characterizations performed from XRD simulation evidenced a diamond– shaped morphology, similar to the one obtained without polyol but with higher (101) surfaces ratio increased to 63% of the total surface (vs. 47% for particle synthesized without polyols). Such a phenomenon is also observed using tartrate ions as complexing species. TEM picture shows diamond–shaped particles with intermediate size between those obtained without additive and in the presence of xylitol. Particles exhibit a 104° typical angle between lateral surfaces suggesting lateral faces to be (101) planes. Particle

Innovative characterizations and morphology control

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dimensions obtained from TEM and XRD show that (101) surface represent 64% of the total particle surface. Since γ–alumina is obtained from boehmite by a topotactic transformation [4], properties of the resulting materials are inherited from the boehmite precursor. Therefore, these methods will provide a promising way to control surface properties of γ–alumina.

Figure 4. TEM micrographs of boehmite nanoparticles synthesized (a) in standard conditions at pH=11.5 (without complexing agents), (b) in presence of xylitol, and (c) in presence of tartrate ions.

4. Monitoring of γ–alumina formation Regarding the phase of industrial interest, a calcination step is required to obtain γ– alumina from boehmite. The recovery of γ–alumina with respect to calcination conditions is often qualitatively checked by TGA and XRD measurements. Nevertheless, the XRD patterns of γ–alumina samples are often complex and unambiguous to interpret, because of the possible presence of several polymorphs and the fine evolution of diffraction peaks. In the same spirit as the boehmite case, an automated method is developed and validated, allowing the detection of additional phases and the determination of the structural evolution of γ–alumina phase in the analyzed sample. This method is based on the decomposition of the diffraction peaks of the DRX pattern and the automatic attribution of these peaks to a determined alumina phase. This attribution begins with the detection of the 3 characteristic peaks of α–alumina. For each signal, the position, the middle–height width and the intensity are analyzed. After these analyses (or if no α– alumina phase presence is detected), the characteristic peaks of the θ–alumina phase are then considered. Following the same methodology, the quantity of θ–alumina is characterized. Then the γ–alumina and δ–alumina phase amounts are determined. This method allows fast and systematic analysis of the presence of all alumina crystallographic phases. The quantification of their contribution to the XRD pattern is properly analyzed in an unequivocal and reproducible way (Figure 5).

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Figure 5. Principle of the automatic analyses of alumina XRD pattern.

5. Conclusions and perspectives A full range of characterization techniques has been developed to reveal the morphology of boehmite particles and to follow the formation of γ–alumina during calcination. These tools allow deep analysis of novel preparations of boehmite and γ–alumina: they offer a promising way for a better design of γ–alumina catalyst supports.

References [1] Schüth, F., Sing, K. and Weitkamp, J. (eds.), Handbook of Porous Solids, Wiley–VCH Verlag GmbH, Weinheim, Germany, 2002, vol. 3, pp. 1591. [2] Ertl, G., Knözinger H. and Weitkamp, J. (eds.), Handbook of Heterogenous Catalysis, VCH Verlag Gesellchaft, Weinheim, Germany, 1997, pp. 1802. [3] M. Digne, P. Sautet, P. Raybaud, P. Euzen and H. Toulhoat. J. Catal., 226 (2004) 54. [4] B. C. Lippens and J. H. de Boer, Acta Crystallogr., 17 (1964) 1312. [5] M. Moreaud, R. Revel, D. Jeulin and V. Morard, Image Anal. Stereol., 28 (2009) 187. [6] C. Tomasi, R. Manduchi, Proceddings IEEE , Bombay, India, (1998) 839. [7] J. Serra, Image analysis and mathematical morphology, Academic Press, London, United Kingdom, 1982. [8] D. Jeulin, Sci. Terre, 30 (1991) 225. [9] D. Chiche, M. Digne, R. Revel, C. Chanéac and J.–P. Jolivet, J. Phys. Chem. C, 112 (2008) 8524. [10] P. Raybaud, M. Digne, R. Iftimie, W. Wellens, P. Euzen and H. Toulhoat, J. Catal., 201 (2001) 236. [11] J.–P. Jolivet, C. Froidefond, A. Pottier, C. Chanéac, S. Cassaignon, E. Tronc and P. Euzen, J. Mater. Chem., 14 (2004) 3281. [12] D. Chiche, C. Chizallet, O. Durupthy, C. Chanéac, R. Revel, P. Raybaud and J.–P. Jolivet, Phy. Chem. Chem. Phys., 11 (2009) 11310. [13] D. Chiche, C. Chanéac, R. Revel and J.–P. Jolivet, Stud. Surf. Sci. Catal., 162 (2006) 393.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Highly active and selective precious metal catalysts by use of the reduction-deposition method Peter T. Witte,a Mariëtte de Groen,a Ralph M. de Rooij,a Pablo Bakermans,a Hans G. Donkervoort,a Peter H. Berben,a John W. Geus.b a b

BASF Nederland B.V., Strijkviertel 67, 3454 ZG De Meern, the Netherlands Utrecht University, Padaulaan 8, 3584 CH Utrecht, the Netherlands

Abstract New mono- and bimetallic precious metal catalysts are prepared by reduction-deposition, in a way that is suitable for large scale production. The preparation is done in water, without the use of any organic solvents and makes use of commercially available starting materials. The supported Pd catalysts are highly active lead-free alternatives for the well-known Lindlar catalyst in the semi-hydrogenation of substituted acetylenes. Impurities in the substrate cause deactivation after recycling, but this does not affect the catalysts selectivity. Keywords: heterogeneous catalysis, nanocatalyst, palladium, semi-hydrogenation.

1. Introduction Catalysts based on colloidal suspensions attracted much attention in recent years, both as supported[1] and as quasi-homogeneous[2] catalysts. These catalysts are prepared by the so-called reduction-deposition method, where a metal is first reduced in solution in the presence of a stabiliser before it is deposited on a heterogeneous support. By using the appropriate reaction conditions, metal crystallites of < 10 nm are available.[3] These catalysts are called nanocatalysts, although the metal crystallite size is not much different from commercial catalysts prepared by traditional methods. However, the metal crystallite size distribution of the nanocatalysts is narrower and they do not contain large metal crystallites that are sometimes observed in traditional catalysts (see right-hand side of Figure 1).

Figure 1. TEM image of 5%Pd on C prepared by standard methods.

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Large metal crystallites as observed in Figure 1 will only contribute marginally to the catalyst activity, while they contain a significant fraction of the total amount of metal. The catalyst activity can thus be boosted by preparing catalysts that do not contain these larger metal crystallites. For industrial applications the use of nanocatalysts prepared by reduction-deposition is hampered by their cumbersome preparation. In general low metal concentrations are employed,[4a] but also the use of low-boiling organic solvents,[4b] high temperatures,[4c] very fast addition of reagents,[4d] or the use of reagents that are expensive or not commercially available[4e] makes their production on an industrial scale difficult. Bönnemann et al. used ammonium borohydride salts, such as [NBu4][BEt3H], as combined reducing and stabilising agents.[4e] According to the authors[5] they can use much higher metal concentrations, because the reducing and stabilising functionalities are combined into one reagent. So when a metal is reduced by the hydride, there is always a quaternary ammonium moiety nearby to immediately stabilise the formed metal crystallite. However, these ammonium borohydrides are very sensitive towards water and can only be handled in a specialised organometallic lab.[6] Therefore their use in industrial applications is limited. In our work, we use the commercial hexadecyl(2hydroxyethyl)dimethyl ammonium dihydrogenphosphate (HHDMA, Figure 2) as a water-soluble stabiliser/reductant for the preparation of metal nanoparticles. We report here the preparation of precious metal catalysts by reduction-deposition, which meets all requirements for production on large scale. New Pd catalysts were tested in the semi-hydrogenation of substituted acetylenes. HO H2PO4 N

Figure 2. Hexadecyl(2-hydroxyethyl)dimethyl ammonium dihydrogenphosphate.

2. Results and discussion 2.1. Preparation of colloidal suspensions of Pd, Pt and bimetallic Pd-Pt Upon mixing HHDMA and Na2PdCl4 a colour change from yellow to red is observed. When a mixture of HHDMA and Na2PdCl4 is kept at room temperature, orange crystals form over a period of hours. A crystal structure determination[7] shows the hydrogen bonding of two units of the cationic moiety of HHDMA to a PdCl4 anion (Figure 3, Table 1). The alcohol functionality acts as a hydrogen bond donor, the metal-bound chlorine as acceptor. The Pd is located on an inversion center and the PdCl4 moiety is therefore exactly planar.

Figure 3. Molecular structure of (HHDMA)2PdCl4 in the crystal.

Highly active and selective PM catalysts by use of the reduction-deposition method 137 Table 1. Selected bond distances and angles in (HHDMA)2PdCl4. Bond (Ǻ) Pd-Cl(1) 2.3069(4) Pd-Cl(2) 2.3017(4)

Angle (°) Cl(1)-Pd-Cl(2) 90.13(2) Cl(1)-Pd-Cl(1a) 89.87(2)

Hydrogen bond distance (Ǻ) Cl(1)···H(1o) 2.47(3) Cl(1)···O(1) 3.1988(16)

At higher temperatures HHDMA is able to reduce Na2PdCl4, which is indicated by a colour change from red to dark brown. Figure 4 shows a TEM image of a Pd colloidal suspension (from hereon referred to as c-Pd) formed in the presence of 10 eq. HHDMA. The metal crystallite size of the formed nanoparticles is 4-8 nm. When c-Pd is formed in the presence of 2 eq. HHDMA, TEM analysis shows that large agglomerates of Pd(0) particles are formed. STEM (Scanning Transmission Electron Microscopy) measurements using a HAADF (High Angle Annular Dark Field) detector show that the agglomerates consist of Pd crystallites of the same size as those formed in the reaction with 10 eq. HHDMA. Apparently, 2 eq. HHDMA is sufficient for full reduction of Pd(II) to Pd(0), but not for stabilisation of the formed Pd nanoparticles. Attempts to determine the oxidation product of HHDMA in the reaction mixture by 13C NMR were unsuccessful, because of the low concentration of the product.

Figure 4. TEM image of c-Pd formed with 10 eq. HHDMA.

The metal concentration used for the preparation of c-Pd (0.75 g/L Pd in H2O) approaches the concentration used by Bönnemann et al. (3.5 g/L Pd in THF).[8] The concentration is more than 10 times higher than in the alcohol reduction method described by Toshima et al. (72 mg/L Pd in MeOH/H2O)[4b] and the citrate reduction method described by Turkevich et al. (33 mg/L Pd in H2O).[4a] These last two methods use a stabiliser that is not chemically bound to the reductor. Turkevich states that doubling the metal concentration in his preparation leads to a significant increase of the Pd crystallite size. Using the same metal concentrations as for Pd and only a higher reduction temperature, we were able to prepare colloidal suspensions of Pt(0), and by mixing the Pd and Pt starting materials bimetallic colloidal particles could be prepared (metal crystallite size in c-Pt ~2 nm; c-PdPt 4-8 nm). Agglomeration of metal crystallites is prevented by the steric bulk of the C16 alkyl chain of the organic moiety of HHDMA.[1,2,3] This is best illustrated by using choline (Me3N+CH2CH2OH) to reduce Na2PdCl4. Although the normal colour changes are observed upon addition and heating to 80°C (yellow to red to dark brown), a black precipitate is formed within minutes at 80°C. The much smaller organic moiety of choline does not have the ability to stabilise the Pd crystallites at this temperature, so agglomeration takes place resulting in the precipitation of Pd-black. The traditional way of describing a metal particle in a colloidal suspension, in which the polar ammonium functionality is directed towards the metal centre while the apolar alkyl chain is sticking out, does not explain why the metal particles formed with

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HHDMA are highly soluble in water and cannot be extracted into an organic solvent. It is more likely that these metal particles are stabilised by a double layer of HHDMA, where the outer shell of the colloidal particle exists of ammonium functionalities of the second HHDMA layer. An inverted monolayer, in which the polar functionalities stick out into the aqueous solution while the apolar alkyl chains interact with the metal surface, seems unlikely to us. Although a fully reduced metal surface in itself is not ionic, polarisation by ionic species is possible. In organic solvents, XANES studies on colloidal suspensions indicate the interaction of the reduced metal surface with the ionic moiety of alkyl ammonium stabilisers.[9]

2.2. Deposition of metal nanoparticles on a heterogeneous support Mixing c-Pd, c-Pt or c-PdPt with a heterogeneous support, like activated carbon (C) or titanium silicate (TiS), yields the supported metal particles (Figure 5).

Figure 5. TEM images: top left) c-Pd/TiS; top right) c-PdPt/TiS; bottom) c-Pt/TiS.

TEM imaging shows that at high metal loadings (~1%) the metal crystallites cluster to form large agglomerates. In these agglomerates not all colloidal particles are directly bound to the support and unsupported particles are often observed, probably formed during the ultrasonic pre-treatment of the TEM samples. This is never observed in TEM pictures of catalysts of low metal loadings, in which all colloidal particles are directly bound to the support. Apparently the colloid-support interaction is stronger than the colloid-colloid interaction. When free HHDMA is added to the support before metal deposition, the maximum metal loading is drastically decreased. This shows that HHDMA competes with c-Pd for active sites on the support, so it is likely that both bind through the same mechanism.

Highly active and selective PM catalysts by use of the reduction-deposition method 139 The bimetallic c-PdPt/TiS was analysed by STEM-EDX (Energy Dispersive X-ray analysis). The very high resolution of this analysis is unique (an electron beam with a diameter of 0.8 nm is used) and allows for the elemental analysis of individual metal crystallites. A scan over the surface of the material showed that all peaks of Pt and Pd coincide, so the metal crystallites are truly bimetallic (Figure 6). No monometallic metal crystallites, indicated by a peak of only Pt or only Pd, were observed. The IR spectrum of TiS support shows peaks at 3700 cm–1 from Ti- and Si-OH groups and at 1640 cm–1 from absorbed water. The water peak is also observed in the spectrum of c-Pd/TiS, but here the peak at 3700 cm–1 is not found. The Ti- and Si-OH groups are clearly involved in the bonding of the Pd colloids, probably by exchange with phosphate anions. IR spectroscopy shows that water is removed after heating c-Pd/TiS to 350°C (HHDMA remains bound to the catalyst), while heating a TiS sample to 250°C is sufficient to remove all water. This indicates that water is more strongly bound to c-Pd/TiS than to TiS, probably because of interaction with the hydrophilic HHDMA. This is consistent with TGA measurements that show that all water is removed from TiS at 250°C, while c-Pd/TiS needs to be heated to 400°C to remove all water.

Figure 6. STEM-EDX analysis of c-PdPt/TiS; Pt (---); Pd (___); distance A-B 110 nm.

2.3. Semi-hydrogenation of 3-hexyn-1-ol by c-Pd/TiS The catalytic semi-hydrogenation of substituted acetylenes is a well-known method to obtain cis olefins. However, overhydrogenation to the fully hydrogenated product and isomerisation to the trans olefin are known to occur. The hydrogenation of 3-hexyn-1-ol (Scheme 1) is an industrially relevant application, since cis-3-hexen-1-ol (leaf alcohol) is an important compound for the fragrance industry. OH

H2 OH

H2

OH

H2

OH

Scheme 1. Hydrogenation of 3-hexyn-1-ol to the cis-3-hexen-1-ol and formation of byproducts.

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We tested several catalysts for the hydrogenation of 3-hexyn-1-ol using the same amount of supported catalyst, although the metal loading differed significantly. In our test set up the hydrogen uptake was measured. Full hydrogenation to hexanol is achieved by 2.0 litre of H2. When a Pd/C catalyst prepared by standard methods is used, a fast uptake of 2 L of H2 is observed (Figure 7a). GC analysis shows full conversion to hexanol and the additional formation of hexane by C-O hydrogenolysis, which explains why the H2 uptake stops only after 2.1 L H2. When this Pd/C is treated with HHDMA, the H2 uptake curve shows that the catalyst activity is lower in the second part of the reaction, although this reaction is still taking place at a considerable rate. The H2 uptake stops after 2.0 L, and GC measurements show only formation of hexanol. Clearly, addition of HHDMA to Pd/C, makes this catalyst somewhat more selective.

Figure 7. H2 uptake curves for 3-hexyn-1-ol hydrogenation: a) Pd/C; b) Pd/C treated with HHDMA; c) c-Pd/TiS; d) Lindlar.

The H2 uptake curve of c-Pd/TiS shows a slower H2 uptake, which stops after 1.0 L H2 is consumed (Figure 7c), indicating a selective semi-hydrogenation to the olefin. When the reaction is stopped immediately after the consumption of 1.0 L H2, GC analysis shows a high selectivity towards the cis olefin. Since the unsupported c-Pd also turned out to be active and selective in this test reaction, hot filtration experiments were performed to see if the hydrogenation was quasi-homogeneous or purely heterogeneous. It was found that Pd leaching takes place at high metal loading, which is consistent with the weak bonding of metal agglomerates mentioned above. Table 2. Catalytic hydrogenation of acetylenic substrates: R1-C≡C-R2 Æ R1-CH=CH-R2. R1 R2 catalyst conversion (%) olefin (%) cis olefin (%) CH2CH3 CH2CH2OH c-Pd/TiS 97 99% 97% CH2CH3 CH2CH2OH Lindlar >99 99 97 H CMe2OH c-Pd/TiS 97 95 n.a. H CMe2OH Lindlar >99 96 n.a. H Ph c-Pd/TiS 93 94 n.a. 95 97 n.a. H Ph Lindlar a) Me Ph c-Pd/TiS a) 90 89 97 Me Ph Lindlar a) 48 b) 96 98 a) 5 times more catalyst used; b) reaction stops before full conversion is reached.

Highly active and selective PM catalysts by use of the reduction-deposition method 141 The catalyst most often used for the semi-hydrogenation of substituted acetylenes is the Lindlar catalyst (5%Pd + 2-3%Pb on CaCO3). Because of environmental reasons the use of Pb is not desirable. When the Lindlar catalyst is used in our test reaction, the H2 uptake curve shows that this hydrogenation also stops after 1.0 L H2 is consumed (Figure 7d). However, the exact end point of the reaction is more difficult to determine, since the reaction slows down significantly before full conversion is reached. Although the H2 consumption does not exceed 1.0 L, prolonged reaction times should be avoided since this leads to cis-trans isomerisation. GC analysis shows that the Lindlar catalyst has a high cis-selectivity (>99% after 40 min reaction time) at 95% conversion, while this has lowered to 97% at full conversion (100 min reaction time). When the hydrogenation using c-Pd/TiS is continued after uptake of 1.0 L H2, we observe very slow overhydrogenation, cis-trans isomerisation and isomerisation to 2- and 4-hexenol.

2.4. Recycling of c-Pd/TiS The stability of c-Pd/TiS in the 3-hexyn-1-ol hydrogenation was tested by recycling the catalyst 7 times. Although the selectivity remains constant, a linear decrease of the activity is observed (Figure 8). Before the start of run 8 a prehydrogenation step is performed to test if the deactivation is due to partial oxidation of the Pd. However, the drop in the reaction rate observed in run 8 is the same as in all other runs. 60

100 99

selectivity (%)

97

40

96 30

95 94

20

93 92

reaction rate (mL H2 per min)

50

98

10

91 90

0 run 1

2

3

4

5

6

7

8

Figure 8. Catalytic hydrogenation of 3-hexyn-1-ol: reaction rate (grey bars); olefin selectivity (___); cis selectivity (---); conversion ~96% for all runs.

Carbon analysis of the spend catalyst shows that the amount of HHDMA on the catalyst drops dramatically during the hydrogenations. The amount of catalyst used in the experiments was too low to also measure the metal content, but in a separate experiment it was shown that washing c-Pd/TiS with EtOH removes only the HHDMA and not the metal. This washed c-Pd/TiS has an identical activity and selectivity in a hydrogenation as the fresh catalyst Therefore it can be concluded that the catalyst deactivation observed in the recycling experiments is not caused by leaching of the metal or of the HHDMA. Most likely the deactivation is caused by impurities in the substrate. The high selectivity of c-Pd/TiS cannot be an effect of the presence of HHDMA (as in run b of Figure 5), since washed c-Pd/TiS contains almost no HHDMA and still has the same high selectivity. The selectivity of c-Pd/TiS is much higher than that of Pd/C prepared by standard methods, so it clearly is a direct effect of the reduction/deposition preparation method.

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3. Conclusions We developed a new method for preparing mono- and bimetallic precious metal catalysts through reduction-deposition that is suitable for large-scale production, since it does not use organic solvents or compounds that are not available on production scale. The newly developed Pd-catalysts are highly active and selective in the semihydrogenation of substituted acetylenes to the corresponding cis-olefins and are suitable as lead-free alternatives for the Lindlar catalyst. The catalyst remains highly selective after recycling, but the activity decreases because of impurities in the substrate.

4. Experimental section 4.1. Preparation of catalysts[10] c-Pd: A solution of 15 g HHDMA in 1 L water was heated to 60°C. A solution of 0.75 g Pd (as Na2PdCl4) in 10 mL water was added. The mixture was heated to 80°C and stirred at this temperature for 2 hours. c-Pt: As Pd, but using H2PtCl6 and a reaction temperature of 95°C. c-PdPt: As Pt, but using a mixture of equal amounts of Pd and Pt. c-Pd/TiS: A slurry of 75 g TiS powder in 750 mL water was stirred for 30 minutes at room temperature, after which c-Pd was added. After an additional 45 minutes of stirring, the catalyst was filtered off and washed. Analysis: 0.47% Pd, 6.1% C, 18% water. IR (cm-1): 1200 (TiS support), 1640 (water), 1410, 1470, 2855, 2925 + shoulder (HHDMA). TGA (weight loss): 6% 25-100°C, 4% 100-300°C, 6% 300-400°C, 0% 400650°C. PSD: d(0.1) 5.0 d(0.5) 22.4 d(0.9) 47.8. Catalyst used for recycling experiments: PSD: d(0.1) 13.7 d(0.5) 23.1 d(0.9) 53.9. Fresh: 4.6% C; spend: 0.6% C. c-Pd/C: As cPd/TiS, but using carbon powder. Analysis: 0.60% Pd, 63% water.

4.2. Hydrogenation of 3-hexyn-1-ol[10] Pd/C, c-Pd/TiS: A 250 mL stainless steel autoclave was charged with 50 mg catalyst (dry weight), 100 mL 96% ethanol, and 5 mL 3-hexyn-1-ol and the mixture was heated to 30°C. Without stirring the autoclave was flushed with hydrogen and pressurised with 3 bars of hydrogen. The reaction was started by starting the stirring (1500 rpm). Lindlar: As Pd/C and c-Pd/TiS, but with a 15 minute prehydrogenation step.

Acknowledgement We gratefully acknowledge Dr Guido Mul and Ana Rita Almeida of TU Delft for their help with IR and TGA measurements.

References [1] A. Roucoux, J. Schulz, H. Patin, Chem. Rev. 2002, 102, 3757-3778. [2] D. Astruc, F. Lu, J.R. Aranzaes, Angew. Chem. Int. Ed. 2005, 44, 7852-7872. [3] H. Bönnemann, K.S. Nagabhushana in Metal nanoclusters in catalysis and materials science: the issue of size control, Part 1, Chapter 2 (Eds.: B. Corain, G. Schmid, N. Toshima), Elsevier, 2007; and references therein. [4] a) J. Turkevich, G. Kim, Science 1970, 169, 873-879; b) H. Hirai, H. Chawanya, N. Toshima, React. Polym. 1985, 3, 127-141; c) F. Bonet, V. Delmas, S. Grugeon, R. Herrera Urbina, P-Y. Silvert, K. Tekaia-Elhsissen, Nanostruc. Mat. 1999, 11, 1277-1284; d) W. Lu, B. Wang, K. Wang, X. Wang, J.G Hou, Langmuir 2003, 19, 5887-5891: e) H. Bönnemann, W. Brijoux, R. Brinkmann, E. Dinjus, T. Joussen, B. Korall, Angew. Chem. Int. Ed. 1991, 30, 1312-1314. [5] H. Bönnemann, personal communication. [6] P.T. Witte, unpublished results.

Highly active and selective PM catalysts by use of the reduction-deposition method 143 [7] CCDC 754963 contains the supplementary crystallogrpahic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. [8] H. Bönnemann, R. Brinkmann, Appl. Organometal. Chem. 1994, 8, 361-378. [9] S. Bucher, J. Hormes, H. Modrow, R. Brinkmann, N. Waldöfner, H. Bönnemann, L. Beuermann, S. Krischok, W. Maus-Friedrichs, V. Kempter, Sur. Sci. 2002, 497, 321-332. [10] P.T. Witte, WO patent, 2009, 096783.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Investigation of the role of stabilizing agent molecules in the heterogeneous nucleation of rhodium(0) nanoparticles onto Al-SBA-15 supports R. Sassinea, E. Bilé-Guyonnetb, T. Onfroyc, A. Denicourtb, A. Roucouxb, F. Launaya* a

Laboratoire de Réactivité de Surface (LRS), UPMC Paris 06, UMR 7197 CNRS, 4 place Jussieu, 75252 Paris Cedex 05, France. [email protected] b Equipe “Chimie Organique et Supramoléculaire”, Ecole Nationale Supérieure de Chimie de Rennes, UMR 6226 CNRS, avenue du Général Leclerc, 35708 Rennes Cedex 7, France. c Laboratoire RMN des Matériaux Nanoporeux (RMN), UPMC Paris 06, FRE 3230 CNRS, 4 place Jussieu, 75252 Paris Cedex 05, France.

Abstract Interactions of stabilizing agent molecules (HEA16Cl) with Rh(0) particles and mesoporous aluminosilic supports were studied in rhodium(0) catalysts obtained by heterogeneous nucleation. HEA16Cl/Rh(0) proximity was indirectly demonstrated by monitoring the CO coverage of metal surface using FTIR spectroscopy as well as by the examination of the thermal decomposition profiles of HEA16Cl (TGA). Complementary work performed in the absence of Rh led us to identify electrostatic interactions between HEA+ ions and the support for surfactant concentrations in the range used for catalyst preparation. Keywords: stabilizing agent, mesoporous support, nanoparticles, interaction

1. Introduction Stabilizing agent assisted reduction of metal salts in solution leads to very reproducible particle size distributions of nanometer scale. Metal colloids thus obtained present specific physico-chemical properties particularly useful for various catalysis applications [1]. However, continuous flow processes may require the deposition of such active phase onto supports. Mesoporous materials are good candidates due to their large specific surface area, porosity and opportunities for particles confinement. Impregnation or one-pot [2,3] strategies are usually implemented in order to insert pre-formed colloidal particles. We recently developed another procedure based on the heterogeneous nucleation of rhodium colloids onto Na-Al-SBA-15 by the reduction of rhodium (III) chloride in the presence of N,N-dimethyl-N-cetyl-N-(2-hydroxyethyl) ammonium chloride (HEA16Cl), used as a stabilizing agent. Particles@Na-Al-SBA-15 were shown to be particularly active (R.T., atmospheric of pressure of H2) in the liquid phase hydrogenation of various aromatic derivatives [4]. A more thorough characterization of prepared materials is proposed in this work. The present study aims in particular to better understand the role of stabilizing agent molecules. Adsorption tests were carried out by contacting various amounts of stabilizing agent with the Na form of the support until equilibration. Isotherms have been built from the analysis of the recovered solids

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by thermogravimetric analyses (TGA) and filtrates by total organic carbon (TOC) determination as well as conductivity.

2. Experimental Chemicals in this study i.e. tetramethyl orthosilicate (TMOS, 99%, Fluka), Pluronic P123 triblock copolymer (EO20PO70EO20, Mw = 5800, Aldrich), aluminium isopropoxide (Al(OiPr)3, 98%, Aldrich), rhodium(III) chloride (RhCl3,xH2O, 38-40%, Strem) and sodium borohydride (NaBH4, 99%, Aldrich) were used as received.

2.1. Materials synthesis Al-SBA-15 mesoporous silica with a nominal Si/Al ratio = 10 was prepared by cohydrolysis and co-condensation of a mixture of Al(OiPr)3 and TMOS [5]. The resulting solid was characterized by a surface area (SBET) = 810 ± 50 m2 g-1, a total pore volume (VP) = 1.06 ± 0.04 cm3 g-1, an average pore diameter (DP) = 7.6 ± 0.2 nm and an experimental Si/Al ratio = 12. Na-Al-SBA-15 was obtained after stirring 3 g of AlSBA-15 solid in 300 mL of 4 M NaCl for 48 h at 80°C. The solid was then filtered out and washed with hot distilled water. Na-AlSBA-15 thus prepared was characterized by SBET = 750 ± 60 m2 g-1, VP = 1.06 ± 0.06 cm3 g-1, DP = 7.8 ± 0.1 nm.

2.2. Rhodium catalyst synthesis Rh(0) particles were obtained by reducing Rh(III) ions in the presence of Na-Al-SBA15 and a quaternary ammonium salt, HEA16Cl [6]. The reductant used was sodium borohydride. In a typical synthesis (Rh/Na-Al-SBA-15 sample), a weighted amount (0.5 g) of support was added to 8.5 mL of a HEA16Cl aqueous solution. The resulting suspension was stirred for 24 h at room temperature before addition of RhCl3.xH2O (0.012 g, 4.8 10-5 mol). After 2 h, NaBH4 (0.005 g, 1.3 10-4 mol) was introduced. Final volume was 12.5 mL. The solid was filtered 2 h later, washed with 50 mL of distilled water and dried at 60ºC for 24 h. Two other samples (Rh/Na-Al-SBA-15 (1/2) and Rh/Na-Al-SBA-15 (0)) were prepared similarly (Table 1). Table 1. Summary of samples codes. Samples

HEA16Cl (g)

Rh/Na-Al-SBA-15

0.0315

Rh/Na-Al-SBA-15 (1/2)

0.0158

Rh/Na-Al-SBA-15 (0)

0

2.3. Adsorption tests Na-Al-SBA-15 (0.2 g) was dispersed in 20 mL of water in the presence of varying amounts of HEA16Cl (from 0.0063 to 0.378 g). After 24 h, the suspension was centrifuged. The residual surfactant was determined in the supernatant using TOC analysis. Meanwhile, HEA16Cl adsorbed on the support was quantified by TGA after drying.

2.4. Characterization The physico-chemical properties of the solids were studied by different techniques. Nitrogen adsorption-desorption isotherms were measured at 77 K on a Micromeritics ASAP-2000 physisorption analyzer. SBET was determined according to the BET equation, whereas the pore size distribution in the mesopore region was obtained applying the BJH method to the desorption branch of the isotherms. VP was estimated from the nitrogen adsorption at P/P0 = 0.99. X-ray diffraction analyses (XRD) were

Investigation of the role of stabilising agent molecules

147

carried out at low angles on a Bruker D8 diffractometer using Cu Kα radiation (1.5418 Å). Transmission Electron Micrographs (TEM) were collected on a 200 kV JEOL JEM 2011 UHR (LaB6) microscope equipped with an Orius Gatan camera. This instrument was equipped with a system for EDX (PGT detector) with an analysis domain of 100 nm. The local distribution of Al, Si, Na or Rh was assessed by comparing surface areas of Si Kα, Al Kα, Na Kα and Rh Lα peaks. Transmission FTIR spectra of adsorbed CO were collected on a Bruker Vector 22 spectrometer using a DTGS detector (resolution 2 cm-1, 64 scans per spectrum). TGA of the samples were carried out on a TA Instruments – Waters LLC, SDT Q600 analyzer with a heating rate of 10°C min-1 in the presence of air flow (100 mL min-1). The amount of total organic carbon was determined on a Shimadzu TOC-VCSH analyzer.

3. Results and discussion Interactions of stabilizing agent molecules with Rh(0) particles and the mesoporous aluminosilic support were studied in rhodium(0) catalysts and alternately by sorption isotherms of the surfactant alone. The support used in the whole study is an aluminosilicic mesoporous material of the Al-SBA-15 type with Si/Al ≈ 12 treated with NaCl. Apart from a slight decrease in specific surface area (about 8%), textural properties of Na-AlSBA-15 are quite similar to those of Al-SBA-15 (Table 2). Na dispersion was shown to be homogeneous (Na/Al molar ratio = 0.7) throughout the whole samples. Assuming that all Al3+ cations are in tetrahedral environments (c.a. 1.3 mmol g-1 of Bronsted acid sites), it appears that H+/Na+ exchange is only partial.

3.1. Supported nanoparticles Two types of materials were obtained from Na-Al-SBA-15 either in the presence or in the absence of stabilizing agent molecules. They are denoted Rh/Na-Al-SBA-15, Rh/Na-Al-SBA-15(1/2) or Rh/Na-Al-SBA-15 (0), respectively. In the case of Rh/NaAl-SBA-15 and Rh/Na-Al-SBA-15(1/2), the aluminosilic solid was contacted with HEA16Cl for 24 h. Then, rhodium chloride (III) was introduced in order to obtain a 1 wt% Rh(0) catalyst after reduction. The amount of stabilizing agent molecules was halved in Rh/Na-Al-SBA-15(1/2). A reference material, Rh/Na-Al-SBA-15(0), was prepared in the same way but using H2O instead of an aqueous solution of HEA16Cl in the first step. In both cases, recovered filtrates were colorless which is consistent with a near-complete Rh incorporation. Nominal stabilizing agent molecules and rhodium quantities in Rh/Na-Al-SBA-15 sample (HEA16Cl/Rh ≈ 2 and NaBH4/Rh ≈ 2.7) were chosen in order to be approximately the same as for the preparation of aqueous colloidal dispersions [6]. Moreover, it has to be noted that HEA16Cl and Rh quantities introduced were well below the estimated ion-exchange capacity of the supports. Rh(0) nanoparticles were characterized by TEM (Figure 1). In the presence of stabilizing agent molecules (Rh(0)/Na-Al-SBA-15 and Rh/Na-Al-SBA-15(1/2) samples), particles are rather small. Counts conducted on 1700 and 508 particles in Rh(0)/Na-Al-SBA-15 and Rh/Na-Al-SBA-15(1/2) samples indicate that their average diameters are 2.5 ± 0.5 nm and 2.8 ± 0.7 nm, respectively. Halving of the amount of HEA16Cl has little noticeable effect on the size distribution. Rhodium particles are fairly well-dispersed across the grains of the support and, for some (Rh(0)/Na-Al-SBA-15 sample), partly located on the outer surface. Few agglomerates are visible on transmission electron micrographs. The magnitude of the particle size has been validated by high angle X-ray diffraction. In addition to poorly defined signals assigned to the support for 10 < 2θ < 90°, the diffractogram of Rh/Na-Al-SBA-15 sample is characterized by a broad 111 FCC peak centered ca. 41° [7]. In the absence of stabilizing agent molecules, particles

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are much less dispersed. Many rhodium aggregates are located outside the grains. Clearly, the size of individual particles is much larger than the diameter of the pores. The effect of HEA16Cl is obvious. In the presence of the stabilizing agent, the size distribution of particles is almost superimposable to that obtained in solution [4].

A

B D

C F

E Figure 1. TEM pictures of Rh/Na-Al-SBA-15(0) (A,B), Rh/Na-Al-SBA-15 (C) and Rh/Na-AlSBA-15(1/2) (E). Particle size histograms of Rh/Na-Al-SBA-15 (D) and Rh/Na-Al-SBA-15(1/2) (508 particles, E).

In situ formation of the particles more strongly affects the textural parameters than the H+/Na+ ion exchange step (Table 2). After reduction, the specific surface area has decreased by about 20%. Values of pore volume and average diameter are almost unchanged due to the low amount of rhodium incorporated (around 1 wt.%).

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Table 2. Evolution of the textural properties through the different steps of the catalyst preparation. SBET (m2 g-1)

Vp (cm3 g-1)

Dp (nm)

Al-SBA-15

827

1.1

7.7

Na-Al-SBA-15

759

1.05

7.8

Rh/Na-Al-SBA-15

595

0.95

7.45

Like in solution, above characterizations are in agreement with the involvement of stabilizing agent molecules in the formation of particles with a monodisperse size as the result of ammonium cations/particles interactions [6]. The amount of HEA16Cl embedded in Rh/Na-Al-SBA-15 sample was estimated by thermogravimetric analysis (Figure 2). Hence, Rh/Na-Al-SBA-15 is characterized by an initial weight loss due to physisorbed water (≈ 10-12 wt.%, maximum of the derivative weight at 54°C) and a second one, between 270 and 550°C (< 10 wt.%, maxima at 352°C (A) and 490°C (B)), due to the degradation of stabilizing agent molecules. Integration of the second signal allowed an estimation of the amount of residual HEA16Cl in Rh/Na-Al-SBA-15. This corresponds to almost all stabilizing agent molecules contacted with the support before rhodium incorporation. In addition to physisorbed water, it can be noted that the weight derivative of the thermogravimogram of pure HEA16Cl (not shown here) involves only one maximum (A) at 248°C under the same analysis conditions, i.e. 100°C lower than for HEA16Cl in Rh/Na-Al-SBA-15 sample. In the absence of rhodium (not shown here), three signals were observed at 303°C (A), 550°C (B) with shoulders at 200°C and 245°C. Shift of the (A) and (B) maximums clearly establish that stabilizing agent molecules interact with the solid in different ways depending on the presence of rhodium. 0,25 100 95 90 Loss weight (%)

Derivative weight (%/°C)

0,2

0,15

85 80 75

0,1 70 20

120

220

320

420

520

620

720

820

Temperature (°C)

0,05

0 20

120

220

320

420

520

620

720

820

Temperature (°C)

Figure 2. First derivatives and TGA curves (see insert) of Rh/Na-Al-SBA-15.

A more effective demonstration of the proximity between the surfactant and supported particles could be obtained by monitoring CO adsorption on Rh(0) by infrared spectroscopy. This technique is used to characterize the oxidation state of Rh and the nature of the probed atoms. Recently, in the case of Rh(0)-MCM-41 materials devoid of organic compounds [8], dispersion of Rh(0) was traced by comparing the number of metal atoms available for CO adsorption with their total number obtained by elemental analysis. The average size of particles could even be determined by this way assuming they are cubic objects. Results agreed with TEM. A similar analysis has been performed on Rh/Na-Al-SBA-15(1/2). Indeed, the two bands previously assigned to geminal CO species at ~ 2100 and 2040 cm-1 and the one characteristics of linear CO at

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2060 cm-1 are observed at equilibrium pressure [9,10] (Figure 3). However the intensity of these three bands are extremely low in comparison to the Rh loading (0.63 wt.%). As a result, completely erroneous estimations of the average diameter and Rh dispersion were obtained. Undoubtedly the strong difference between transmission electron microscopy and CO adsorption results can be considered as an indirect evidence of the interaction of the metal particle with stabilizing agent molecules. Moreover, problems of particles surfaces accessibility have been highlighted by the relatively slow kinetics of the establishment of equilibria (results not shown here).

Absorbance (a.u.)

0.001

2200

2150

2100 2050 Wavenumber (cm-1)

2000

1950

Figure 3. IR spectrum of Rh/Na-Al-SBA-15 (1/2) after adsorption of 5 Torr of CO at equilibrium pressure.

Further experiments, not detailed here, have been carried out in order to better understand the influence of synthesis parameters on the final dispersion of particles. They allowed to show the importance of the addition rank of the reagents. Thus, it was clear that the support must be contacted first and long enough with the stabilizing agent. Given the cationic nature of the surfactant and the ion exchange capacity of Na-AlSBA-15, HEA16Cl/Na-Al-SBA-15 interactions have been thoroughly studied (prior to Rh introduction). Results are detailed in the next part.

3.2. Surfactant adsorption The behaviour of stabilizing agent molecules toward the Na-Al-SBA-15 support has been studied from analyses of filtrates and solids recovered in several impregnation tests carried out with different amounts of surfactant. The concentration range, [HEA16Cl]0, tested was between 1 and 50 mmol L-1. In addition to thermogravimetric analyses of the centrifuged solids, supernatants were controlled by potentiometric, conductivity measurements and total organic carbon determination. For each stabilizing agent concentration studied, particular attention was paid to the extent of changes in conductivity (Δσ) between a HEA16Cl solution contacted with the support for 24 h and a blank solution (no support). These tests were performed routinely after a period of 24 h which is more than enough to reach a steady state as indicated by monitoring Δσ values vs time (not shown here). Largely positive and increasing Δσ values observed for 0.9 ≤ [HEA16Cl]0 ≤ 10.8 mmol L-1 have been attributed to the release of H+ (as evidenced by a pH decrease) and Na+ cations in solution. It can be noted that Δσ values of the supernatant measured under the same conditions are significantly smaller in the presence of SBA-15 silica. Some representatives graphs of the evolution of the weight loss (and its first derivative) for different solids recovered after 24 h adsorption tests are shown in Figure 4. Thermogravimetric analyses show that weight losses between 100 and 900°C increase

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with [HEA16Cl]0. Parallely, the amount of physisorbed water become less and less significant. A

B 100 NaAlSBA_1 NaAlSBA_4 NaAlSBA_6 NaAlSBA_20

90

Loss weight (%)

85 80 75

0,3

III

0,25 Derivative weight (%/°C)

95

0,2

NaAlSBA_1 NaAlSBA_4 NaAlSBA_6 NaAlSBA_20

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0,15

70

0,1

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0,05

II

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55

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20

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Temperature ( C)

Figure 4. TGA (A) and first derivatives curves (B) of Na-Al-SBA-15 samples recovered from adsorption tests in the presence of 1.8 (NaAlSBA_1), 7.2 (NaAlSBA_4), 10.8 (NaAlSBA_6) and 36 (NaAlSBA_20) mmol L-1 of HEA16Cl.

The adsorption isotherm of stabilizing agent molecules over Na-Al-SBA-15 could be plot directly from TGA measurements (weight losses between 100 and 900°C) or indirectly from TOC analyses. Both curves are displayed in Figure 5. For 0.9 ≤ [HEA16Cl]0 ≤ 10.8 mmol L-1, (i.e. 0.2 ≤ [HEA16Cl]éq ≤ 1.2 mmol L-1), the two approaches lead to superimposed curves. However, more significant differences could be observed for the “plateau” which is either at 1.4 or 1.2 mmol of HEA16Cl g-1 according to TOC and TGA analyses, respectively.

HEACl adsorbed (mmol g-1 support)

1,6 1,4 1,2 1,2

1

1

0,8

0,8

0,6

0,6 0,4

0,4

0,2

0,2

0 0

0 0

5

0,1

0,2

10

0,3

15

20

25

30

35

40

[HEACl] at equilibrium (mmol L-1)

Figure 5. HEA16Cl adsorption isotherms from TOC (S) and TGA (¡) analyses.

Thermal degradation of HEA16Cl is different from that obtained starting with pure HEA16Cl (vide supra). Moreover, the curve profile become more complex with increasing HEA16Cl concentrations. Besides the loss of H2O, three types of maxima (I, II and III) were detected on the first derivative curve. Only I (circa 340°C) and II (c.a. 500°C) are observed for 0.9 ≤ [HEA16Cl]0 ≤ 1.8 mmol L-1. For [HEA16Cl]0 ≥ 7.2 mmol L-1, another signal (III, c.a. 235°C) starts to emerge. The area of the latter is the main one for [HEA16Cl]0 ≥ 10.8 mmol L-1 (NaAlSBA_6). Signals I and II were shown to saturate for 10.8 ≤ [HEA16Cl]0 ≤ 30 mmol L-1. I and III are believed to be related to two forms of the stabilizing agent molecules differing in their interaction with the support. Peak I would correspond to molecules involved in a stronger association with the surface. Since I appears at the lowest HEA16Cl concentrations, that is to say, for

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tests leading to higher conductivity variations in solution, it is conceivable that the corresponding interaction is of electrostatic nature. This latter interpretation would mean that a certain amount of chloride ions corresponding to the portion of HEA+ linked to the surface by electrostatic interaction (peak I) is leached into solution. Analyses of similar materials obtained by deposition of HEA16Cl on H-Al-SBA-15 (instead of Na-Al-SBA-15) are not in agreement with this hypothesis. Indeed, values of the Cl/N molar ratio of the solid after adsorption are approximately equal to 1 and irrespective of the concentration of stabilizing agent molecules tested. Retention of chloride ions could be explained by the presence of ≡SiOH2+ groups on the surface. Indeed pH values measured after 24 h are between 3.85 and 4.41, i.e. below the PZC of the support.

4. Conclusion Like in aqueous solution, HEA16Cl molecules appear to be involved in the stabilization of the particles formed by the heterogeneous nucleation pathway. Besides similarities with aqueous Rh(0) colloids in terms of particle sizes distribution, HEA16Cl interaction with the metal surface was indirectly highlighted through adsorption measurements of CO followed by infrared spectroscopy and by some changes in the thermal decomposition profiles of HEA16Cl. The influence of the stabilizing agent was related to prior adsorption of the molecules to the support in the first stage of the Rh/Na-AlSBA-15 synthesis. Clearly, it was determined that a Na+ (or H+)/HEA+ exchange takes place. Further control of particle size and distribution could be the result of admicellization processes already described for different supports [11].

Acknowledgments The authors would like to thank Dr F. Lequeux and J. Marchal (Laboratoire Physicochimie des Polymères et des Milieux Dispersés, ESPCI ParisTech) for the use of the TOC analyzer. We gratefully acknowledge financial support from the ANR in the “Chimie pour le Développement Durable” Program (ANR-08-CP2D-14).

References [1] A. Roucoux, J. Schulz, H. Patin, Chem. Rev. 102 (2002) 3757. [2] R.M. Rioux, H. Song, J.D. Hoefelmeyer, P. Yang, G.A. Somorjai, J. Phys. Chem. B 109 (2005) 2192. [3] J.P.M. Niederer, A.B.J. Arnold, W.F. Hölderich, B. Spliethof, B. Tesche, M. Reetz, H. Bönnemann, Top. Catal. 18 (2002) 265. [4] M. Boutros, A. Denicourt-Nowicki, A. Roucoux, L. Gengembre, P. Beaunier, A. Gédéon, F. Launay, Chem. Commun. (2008) 2920. [5] B. Jarry, F. Launay, J.-P. Nogier, J.-L. Bonardet, Stud. Surface Sci. Catal. 158B (Molecular Sieves: From Basic Research to Industrial Applications) (2005) 1581. [6] J. Schulz, A. Roucoux, H. Patin, Adv. Synth. Catal. 345 (2003) 222. [7] S. Alayoglu, B. Eichhorn, J. Am. Chem. Soc. 130 (2008) 17479. [8] M. Boutros, F. Launay, A. Nowicki, T. Onfroy, V. Herlédan-Semmer, A. Roucoux, A. Gédéon, J. Mol. Catal. A : Chem. 259 (2006) 91. [9] C.A. Rice, S.D. Worley, C.W. Curtis, J.A. Guin, A.R. Tarrer, J. Chem. Phys. 74 (1981) 6487. [10] A.C. Yang, C.W. Garland, J. Phys. Chem. 61 (1957) 1504. [11] Z. Li, L. Gallus, Colloids Surfaces A 264 (2005) 61.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Preparation of the polymer-stabilized and supported nanostructured catalysts E. Sulmana, V. Matveevaa, V. Doludaa , L. Nikoshvilia, A. Bykova, G. Demidenkoa, L. Bronsteinb a b

Tver Technical University, A.Nikitina str., 22, Tver, 170026, Russia Indiana University, Bloomington, IN 47405, USA

Abstract In this work we report a comparative study of two types of nanoparticulate catalytic systems based on two amphiphilic block copolymers and a nanoporous polymer, hypercrosslinked polystyrene (HPS). Nanostructures in polymers (block copolymer micelle cores or nanopores) control nanoparticle (NP) formation and location while polymeric environment (functional groups) influences the catalytic performance. Catalytic properties of these nanocomposites were studied in selective hydrogenation of triple bond of dehydrolinalool and in direct selective oxidation of two monosaccharides: L-sorbose and D-glucose. In hydrogenation, the highest selectivity of 99% was achieved for Pd, PdZn, PdAu, and PdPt catalytic NPs in polystyrene-b-poly(4-vynyl pyridine) micelles due to a modifying influence of pyridine units, while the highest activity of 49.2 mol LN/(mol Pd . s) was observed for the PdPt NPs due to synergy of catalytic activity of both metals in hydrogenation. In oxidation of L-sorbose and D-glucose, the highest activities were observed for the Pt and Ru catalysts, respectively, based on HPS due to better access of catalytic sites of NPs. Keywords: metal nanoparticles, nanocomposites, hydrogenation, oxidation, monosaccharides

1. Introduction The demand for high quality vitamins, pharmaceuticals, and food supplements has been increasing throughout the world. With considerable medical evidence linking a diet and diet supplements to human health and with an attempt to facilitate a non-drug approach to combat common ailments, vitamins A, E, K, and C are increasingly being incorporated into functional foods [1,2]. Catalysis is a key methodology for the efficient industrial production of important biologically active compounds and pure optically active substances [3]. Thus the application of new methods and technologies in catalysis is very important [4]. Metal NPs have unique catalytic properties [5-7] due to high number of surface atoms, leading to a high number of reactive sites. Catalytic properties of NPs depend on their size, size distribution and environment [8]. The NP surface plays an important role in catalysis and determines the NP selectivity and activity. The studies of the last decade demonstrate that formation of NPs in nanostructured polymeric environment allows control over NP size and size distribution; in so doing, the stabilizing polymer may strongly influence the surface of NPs [9-12]. NPs formed in different types of nanostructured polymers (dendrimers, block copolymers, layer-by-layer polyelectrolyte structures, etc) were studied in various catalytic reactions [9-11]. Amphiphilic block copolymers are largely studied for NP

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formation. Controlled growth of metal NPs in a polymer matrix is also possible if it occurs in cavities or pores. In this case, the size of the growing particles can be limited to the cavity size. HPS is the first representative of a new class of cross-linked polymers characterized by unique topology and unusual properties. Due to its high crosslinking density, which can exceed 100%, HPS consists of nanosized rigid cavities of about 4 nm in size. HPS has a large inner surface area (usually nearly 1000 m2/g) and the ability to swell in any liquid medium including precipitating agents of the starting polymer. All of that makes it a promising matrix for nanoparticle formation. In this paper we present a comparative study of two types of nanoparticulate catalysts based on amphiphilic block copolymers (polystyrene-b-poly(4-vinyl pyridine) (PS-b-P4VP) and poly(ethylene oxide)-b-poly(2-vinyl pyridine) (PEO-b-P2VP)) and HPS. The catalytic properties of these nanocomposites were studied in selective hydrogenation of a triple bond of dehydrolinalool (DHL) and direct selective oxidation of monosaccharides: L-sorpbose and D-glucose.

2. Experimental 2.1. Materials PS-b-P4VP (Mn D19,400; Mw D22,500, and relative 4-VP content of 0.340) was prepared via living anionic polymerization and was a gift from Max Plank Institute of Colloids and Interfaces, Potsdam/Golm, Germany). PEO350-b-P2VP135 (MPEOn = 15,400, MP2VPn = 14,100, Mw/Mn = 1.04) was purchased from Polymer Source Inc., Canada, and used as received. The HPS was purchased from Purolite Int. (UK), as Macronet MN270/38600 type 2/100 (designated as HPS). HAuCl4×3H2O, Pd(CH3COO)2, Na2PdCl4, Zn(CH3COO)2, K[Pt(C2H4)Cl3]×H2O (Zeise salt), Ru(OH)Cl3, LiB(C2H5)3H (SH, 1M solution of LiB(C2H5)3H in THF)), NaBH4 were obtained from Aldrich and used as received. H2PtCl6×6H2O was obtained from Reakhim (Moscow, Russia). Dehydrolinalool (99% purity) was supplied by pharmaceutical company OAO “Belgorodvitaminy” (Belgorod, Russia) and distilled under vacuum (40–45°C at 50–60 kPa). D-glucose and L-sorbose were provided by Fluka. Reagent-grade THF was purchased from Aldrich. Isopropanol (i-PrOH) and toluene were obtained from Aldrich and distilled before use. KOH, NaOH, HCl, NaHCO3 and hydrogen (KhimMedService, Tver, Russia) were used as received.

2.2. Catalyst synthesis The synthesis of the catalysts was based on the formation of metal compound NPs or metal NPs after reduction of metal compounds in the cores of amphiphilic block copolymer micelles or in the pores of HPS [13-17]. 2.2.1. Catalysts on the base of amphiphilic block copolymers The synthesis of these catalytic systems is based on incorporation of metal compounds into the micelle core of amphiphilic block copolymer micelles followed by reduction with a formation of NPs. The core block contains functional groups which coordinate with metal compounds and the core serves as a nanoreactor for NP formation, while the corona block provides solubility in a selective solvent. Micellar catalysts in this work were prepared using PEO-b-P2VP and PS-b-P4VP. Mono- (Pd and Pt) and bimetallic (PdAu, PdPt and PdZn) catalysts were synthesized by solubilization of appropriate metal salts into the P4(2)VP micelle cores followed by reduction.

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155

2.2.2. Catalysts on the base of amphiphilic block copolymers HPS can control formation of Pt NPs within the pores. In this case HPS plays two roles: of nanostructured matrix for NP growth and the heterogeneous catalyst support. Incorporation of Pt, Pd, and Ru species in HPS was carried out via impregnation of the H2PtCl6 or Pd(CH3COO)2 solutions in THF and the Ru(OH)Cl3 solution in a complex solvent (THF-methanol-water) followed by reduction (for Pd, using NaBH4) or without any further treatment (for Pt and Ru). In this work we discuss catalytic samples HPS-Ru-1, HPS-Ru-2, HPS-Ru-3 containing 0.5, 0.9, and 2.9 wt.% Ru, respectively.

2.3. Catalytic testing Hydrogenation of a triple bond of dehydrolinalool was carried out at ambient pressure in a glass batch isothermal reactor installed in a shaker and connected to a gasometric burette. In the case of micellar catalysts based on amphiphilic block copolymers, different solvents providing the better swelling of micelle corona and access of the substrates to catalytic sites were used. For PS-b-P4VP based catalyst, toluene was used, while for PEO-b-P2VP based catalyst, a mixture of 30 vol. % water and 70 vol. % of isopropyl alcohol (i-PrOH) was emploied. The oxidation of monosaccharides was conducted batchwise at ambient pressure in PARR 4592 apparatus in water. To maintain pH of 6.0 – 7.5, NaHCO3 was added using automated feeder. The samples of the reaction mixtures were periodically removed and analyzed using gas chromatography (in hydrogenation) and HPLC (in oxidation).

2.4. Physicochemical characterization The catalysts studied were characterized using X-ray powder diffraction (XRD), X-ray fluorescence analysis (XFA), transmission electron microscopy (TEM), X-ray adsorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), and liquid nitrogen physisorption methods.

3. Results and discussion 3.1. Catalyst characterization 3.1.1. Catalytic NPs formed in the micelle cores of amphiphilic block copolymers XRD and TEM data for monometallic Pd NPs (PS-b-P4VP-Pd, PEO-b-P2VP-Pd) showed that in all the cases small particles with a diameter of 1.5-2.0 nm are formed, the diffraction patterns of which are typical for Pd(0) [15,16]. In the case of Pt NPs (PSb-P4VP-Pt, PEO-b-P2VP-Pt), the NP diameters are similar. The electronic properties of bimetallic (PdAu, PdPt, and PdZn) NPs were studied using XRD, TEM, XPS, and FTIR of the adsorbed CO. Bimetallic based catalysts contained 1.5-2 nm NPs with a narrow particle size distribution, but with different NP morphology: cluster-in-cluster for PdPt and PdZn and core-shell for PdAu [15]. Addition of a modifying metal (Au, Pt and Zn) leads to a change of the NP electronic properties as well. 3.1.2. Catalytic NPs stabilized in HPS By XPS Pt-containing HPS contains Pt in three forms: Pt(0), Pt(II), Pt(IV), thus the compound nanoparticles formed are expected to have a mixed composition. In this paper we compare three Pt catalysts: HPS-Pt-1, HPS-Pt-2, and HPS-Pt-3 containing 0.9, 2.9, and 4.9 wt.% Pt, respectively. Independently of the Pt loading, Pt NP size (by TEM) is in the range 1.8.-2.0 nm. HPS-Pd contains 0.05 wt.% Pd while NPs measure about 2 nm. Ru-containing HPS samples, studied by TEM and diffuse reflectance

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infrared Fourier transform spectroscopy of adsorbed CO, showed the presence of NPs of mixed composition (metal/metal oxide) with a mean diameter of 1.0–1.2 nm.

3.2. Catalytic testing 3.2.1. Selective hydrogenation of a triple bond of dehydrolinalool Palladium is well known to be the best metal for the selective hydrogenation of alkynes to alkenes. However, traditional catalysts often require modification to increase selectivity and prevent complete hydrogenation. Historically, Pd/CaCO3 and Pd/Al2O3 catalysts poisoned by lead acetate, quinoline and pyridine as additional modifiers are used [18]. Though the selectivity of these catalysts is rather high (about 95%), the use of modifiers leads to the decrease of the product quality [19]. Table 1 shows catalytic properties of the catalysts in dehydrolinalool DHL hydrogenation to linalool (LN) (Fig. 1). OH

OH

AcO

[H] Catalyst IP [H]

DHL

LN (target product) Catalyst

OH 3

O Vitamin E

OH

3

O

3 DiHL(side product)

O

Vtamin K1

Figure 1. Scheme of DHL hydrogenation (DiHL – dihydrolinalool).

All micellar catalysts based on PS-b-P4VP (# 1-4, Table 1) show outstanding selectivity (99.8% at 100% conversion) and high activity compared to other catalysts (# 5-6, Table 1). Comparison of the data presented in Table 2 (# 1-4) shows that catalytic activity is higher for bimetallic NP based samples than for the samples based on Pd NP. This can be explained by the modifying influence of gold, platinum, and zinc towards palladium. Moreover, the highest activity is observed for the PdPt-containing catalyst due to synergy of catalytic activity of both metals in hydrogenation, while Zn and Au are not catalytically active in this reaction. Thus the second metal can change the electronic properties of the catalyst, this in turn change can influence the energy of metal–hydrogen and metal–substrate bonds and the amount of hydrogen adsorbed. We believe that combination of these factors determines the change of catalytic activity of bimetallic catalysts, which is reflected in TOF values [20]. Formation of Pd NPs in the micelle cores of PS-b-P4VP and PEO-b-P2VP should be analogous and the particles sizes are alike. Moreover, the pyridine modifying groups are present in both cases. However, the activity of PEO-b-P2VP-Pd is only half of that of PS-b-P4VP-Pd. It is noteworthy that the latter form micelles in toluene, while the former form micelles in polar media [21], thus рН can strongly influence the micelle structure, NP formation and catalytic properties. For catalytic reactions with PEO-b-P2VP-Pd, we used a mixed solvent including water and i-PrOH. i-PrOH is a good solvent for DHL, while presence of 30 vol.% water keeps the PEO-b-P2VP-Pd micelles intact. It is known that alkaline medium (addition of KOH) modifies Pd catalytic systems [22] and leads to a selectivity increase in hydrogenation of alkyne alcohols. In our case, the optimal conditions for DHL hydrogenation with the PEO-b-P2VP-Pd catalyst are achieved at рН 13.0. However,

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157

even in these conditions the selectivity of PEO-b-P2VP-Pd (Table 1 (#5)) is slightly lower than that of PS-b-P4VP-Pd. We ascribe this decrease of selectivity to less efficient modification of the NP surface with 2VP units (nitrogen is located near the polymer chain) than with 4VP ones. As for the decrease of the catalytic activity of PEOb-P2VP-Pd compared with that of PS-b-P4VP-Pd, we tentatively ascribe it to the amphiphilic nature of DHL: presence of hydroxyl group at the third carbon atom and a different coordination on Pd NPs stabilized in different block copolymer micelles. We believe that in non-polar solvent (toluene) the DHL molecule tends to move inside the polar P4VP core due to OH group, then the terminal triple bond is likely to be situated near the Pd NP surface and hydrogenation is highly probable. In polar solvents (for PEO-b-P2VP-Pd), the amphiphilic substrate stays in the reaction solution and less actively penetrates the micelle cores containing Pd NPs, thus reaction rate decreases. In DHL hydrogenation with HPS-Pd, where Pd NPs are located in the pores of HPS, the catalytic activity is remarkably high, while selectivity is upsettingly low due to the absence of proper NP particle modification. Indeed, HPS contains no functional groups which might adsorb on the NP surface and modify the catalytic site formation. Table 1. The catalytic properties in DHL hydrogenation. #

Catalyst

Selectivitya, %

Pd loading, %

TOF, mol LN/

(wt.)

(mol Pd . s)b

0.04

18.5

99.0

1.

PS-b-P4VP-Pdc

2.

c

PS-b-P4VP-PdA u

0.04

36.9

99.0

3.

PS-b-P4VP-PdZn

c

0.04

34.4

98.5

4.

c

PS-b-P4VP-PdPt

0.04

49.2

98.5

5.

PEO-b-P2VP-Pdd

0.06

9.6

98.0

0.05

65.7

94.2

6.

f

HPS-Pd

a)

Selectivity is measured at 100% of DHL conversion Activity was calculated as the amount of moles of LN formed per second per Pd mole c) Reaction conditions: 90°C, 960 shaking/min (regime without diffusion limitations), toluene (30 ml), Co (substrate concentration) 0.44 mol/l, Cc (catalyst concentration) 2.3⋅10-5 mol Pd/l; d) Reaction conditions: 70°C, 960 shaking/min (regime without diffusion limitations), and solvent: ‘i-PrOH + water’ (30 ml), Co 0.4 mol/l, Cc 1.72⋅10-5 mol Pd/l; f) Reaction conditions: 90°C, 960 shaking/min (regime without diffusion limitations), toluene (30 ml), Co 0.4 mol/l, Cc 6.8⋅10-5 mol Pd/l. b)

3.2.2. Selective oxidation of monosaccharides Catalytic properties of NPs stabilized in polymeric matrices discussed above, were also studied in selective oxidation of monosaccharides. For L-sorbose, only the OH group situated at the 1st carbon atom should be oxidized, while for D-glucose, only the aldehyde group. There are several approaches to oxidation of monosaccharides: chemical, electrochemical, biotechnological, and catalytic. Advantages of direct catalytic oxidation include robustness, no need of group protection and deprotection and higher purity of the final products due to high selectivity of the metals used. Pt catalysts are generally used for catalytic L-sorbose oxidation. In our preceding work we developed highly efficient Pt-containing nanocatalysts based on HPS for L-sorbose oxidation by molecular oxygen in mild conditions [23]. According to literature, for catalytic oxidation of

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D-glucose Ru catalysts are mostly efficient. Earlier we demonstrated that Ru-containing catalysts based on HPS are the active catalysts in D-glucose oxidation [24]. Here we discuss Pt and Ru compound NPs (no reduction) formed in block copolymers and HPS in catalytic oxidation of L-sorbose and D-glucose (Fig. 2). Table 2 shows catalytic properties of these nanocomposites. COOH

O

H HOH2C H

OH

OH

C

O

HO

C

H

H

C

OH

HO

C

H

[O]

HO CH2OH

Catalyst

H

L - sorbose

CH2OH H H OH OH H

COOH

O H H

OH

[O] Catalyst

OH

D - glucose

CH2OH

2-keto-L-gulonic acid (target product) (a)

H

C

OH

HO

C

H

H

C

OH

H

C

OH

CH2OH

D-gluconic acid (target product) (b)

Figure 2. Scheme of L-sorbose (a) and D-glucose (b) oxidation.

The data presented in Table 2 show the lowest activity and selectivity for PS-b-P4VP-Pt (Table 2, # 1) due to the two factors. First, catalytic NPs are buried under the PS corona which is insoluble in water. Second, the substrate is too polar to penetrate the P4VP core. Indeed, for PEO-b-P2VP-Pt containing water soluble corona, the catalytic activity is higher (Table 2, # 2) by a factor of 2.0-2.5, but the selectivity of L-sorbose is still low. Ru-containing catalysts based on both amphiphilic block copolymers behave in a similar way (Table 2, # 3-4). On the other hand, Pt compound NPs formed in HPS pores (Table 2, #5-7), are remarkably active and selective in L-sorbose oxidation. We demonstrated that 2-ketogulonic acid (the target product) modifies the NP surface leading to its excellent stability and most likely high selectivity, while accessibility of the NPs in the pores allows exceptional activity. It is noteworthy that increase of the Pt content leads to the decrease of catalytic activity. According to nitrogen sorption experiments the increase of metal loading leads to a noticeable decrease of the BET surface area (by 30%), while the pore volume decreases only slightly, revealing blocking of a fraction of micropores. This can results in poor accessibility of some NPs and lower activity. As might be expected, Ru NPs formed in HPS showed highest catalytic activity in D-glucose oxidation (Table 2, # 8-10) while those formed in block copolymers demonstrate mediocre performance. In the HPS-Ru series, the best catalyst (highest activity and selectivity) also contains about 0.9 wt.% of active metal similar to the HPSPt series. It is noteworthy that HPS based catalysts show very high stability in oxidation of monosaccharides. Many catalysts are known to lose their activity after 2-3 repeated uses due to a loss of active metal or depositing the reaction products on the catalyst surface. We demonstrated that activity and selectivity of Pt- and Ru-containing catalysts based on HPS do not decrease even after 15 repeated uses, revealing the exceptional stability of these catalysts. We believe this stability is due to formation of NPs in the pores of a comparable size (micropores) and stabilization of the NP surface with the target molecules, preventing the loss of catalytic species, while macropores provide excellent mass transfer of reactants and products.

159

Preparation of the polymer-stabilized and supported nanostructured catalysts Table 2. Testing of the catalysts in L-sorbosea and D-glucoseb oxidation. #

Catalyst

Active

TOF, mol S/

metal

(mol Me . s) × 103

Selectivity, %

loading, % (wt.)

1.

PS-b-

L-sorbose

D-glucose

L-sorbose*

D-glucose**

0.9

1.7

0.8

85

97

0.9

4.7

1.5

88

98

1.0

0.5

1.2

67

97

1.0

0.8

2.4

71

98

P4VP-Pt 2.

PEO-bP2VP-Pt

3.

PS-bP4VP-Ru

4.

PEO-bP2VP-Ru

5.

HPS-Pt-1

0.9

8.1

1.0

95

98

6.

HPS-Pt-2

2.9

7.5

2.8

98

98

7.

HPS-Pt-3

4.9

6.8

5.0

92

98

8.

HPS-Ru-1

0.5

0.6

3.0

64

99

9.

HPS-Ru-2

0.9

1.4

8.0

65

99

10.

HPS-Ru-3

2.9

2.1

7.0

63

99

a)

-6

3

Reaction conditions: 70°C, water as a solvent, V(02) 14 · 10 m /s; stirring rate 1000 rpm; NaHCO3 is added in the equivalent amount to L-sorbose; C0 0.106 M; (C0/Cc ) 53.6 mol/mol Pt. b) Reaction conditions: 70°C, solvent water, V(02) 14 · 10-6 m3/s; stirring rate 1000 rpm; NaHCO3 is added in the equivalent amount D-glucose; C0 0.03 M; Cc 1.5⋅10-3 mol Ru/l. * Selectivity measured at 80% of L-sorbose conversion ** Selectivity measured at 95% of D-glucose conversion

4. Conclusions A comparative study of the catalytic properties of the systems based on two types of nanostructured polymers, amphiphilic block copolymers and nanoporous HPS, in selective hydrogenation of DHL and selective oxidation of monosaccharides demonstrated the following general trends. The highest selectivity in hydrogenation was achieved for NPs formed in the PS-b-P4VP micelles due to modification of the NP surface with 4VP units. Use of the PdPt NPs instead of Pd monometallic ones leads to a significant increase of activity due to synergy of activity of both catalytic metals. HPS-Pd shows high activity in the DHL hydrogenation due to high accessibility of NPs, but poor selectivity. In oxidation processes, where modification of Pt or Ru species with pyridine units is not needed, the highest activity and selectivity are observed for the HPS based catalysts. Here the successful modification (for enhanced selectivity and stability) occurs due to L-sorbose or D-glucose oxidation products.

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Acknowledgements This work has been supported by Federal Education Agency of Russian Federation (contract P 344), Federal Science and Innovations Agency of Russian Federation (02.552.11.7075), the NATO Science for Peace Program (grant SfP-981438) and by the 6th Framework Program project “NANOCAT” (contract number 506621-1). We also thank Prof. Dr. M. Antonietti for the PS-b-P4VP.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.

Weber, W., Nutrition 17 (2001) 880-887. Goss-Sampson, M.A., D.P.R. Muller, J.K. Lloyd, Journal of Human Nutrition and Dietetics 2 (2008) 145-150. Bonrath, W., T. Netscher, Appl. Catal. A 280 (2005) 55-73. Bonrath, W., M. Eggersdorfer, T. Netscher, Catal.Tod. 121 (2007) 45-57. Fendler, J.H., in, Nanoparticles and Nanostructured Films: Preparation, Characterization and Applications, Wiley-VCH, New-York, 1998. Wieckowski, A., E.R. Savinova, C.G. Vayenas, Catalysis and Electrocatalysis at Nanoparticle Surfaces, Inc., New York, N. Y., 2003. Schmid, G., Nanoparticles: From Theory to Application, Wiley-VCH Verlag GmbH & Co., Weinheim, 2004. Somorjai, G.A., A.M. Contreras, M. Montano, R.M. Rioux, Proc. Nat. Acad. Sci. 103 (2006) 10577-10583. Astruc, D., F. Lu, J.R. Aranzaes, Angew. Chem. Int. Ed. 44 (2005) 7852-7872. Mueller, C., M.G. Nijkamp, D. Vogt, European Journal of Inorganic Chemistry 20 (2005) 4011-4021. Bronstein, L.M., in: H.S. Nalwa (Ed.), Encyclopedia of Nanoscience and Nanotechnology, APS, Stevenson Ranch, CA, 2004, pp. 193-206. Bronstein, L.M., in: J.A. Schwarz, C.I. Contescu, K. Putyera (Eds.), Dekker Encyclopedia of Nanoscience and Nanotechnology, Marcel Dekker, Inc., New York, 2004, pp. 2903-2916. Bates, F.S., G.H. Fredrickson, Phys. Today 52 (1999) 32. Foerster, S., M. Antonietti, Adv. Mater. 10 (1998) 195-217. Mayer, A.B.R., J.E. Mark, Colloid Polym. Sci. 275 (1997) 333-340. Seregina, M.V., L.M. Bronstein, O.A. Platonova, D.M. Chernyshov, P.M. Valetsky, J. Hartmann, E. Wenz, M. Antonietti, Chem. Mater. 9 (1997) 923-931. Klingelhoefer, S., W. Heitz, A. Greiner, S. Oestreich, S. Förster, M. Antonietti, J. Am. Chem. Soc. 119 (1997) 10116. Lindlar, H., Helv. Chim. Acta 35 (1952) 446. Sulman, E.M., Russ. Chem. Rev. 63 (1994) 923-936. Bronstein, L.M., D.M. Chernyshov, I.O. Volkov, M.G. Ezernitskaya, P.M. Valetsky, V.G. Matveeva, E.M. Sulman, J. Catal. 196 (2000) 302-314. Semagina, N.V., A.V. Bykov, E.M. Sulman, V.G. Matveeva, S.N. Sidorov, L.V. Dubrovina, P.M. Valetsky, O.I. Kiselyova, A.R. Khokhlov, B. Stein, L.M. Bronstein, J. of Mol.Catal. A 208 (2004) 273-284. Sulman, E.M., Russ. Chem. Rev. 63 (1994) 923-936. Bronstein, L., G. Goerigk, M. Kostylev, M. Pink, I.A. Khotina, P.M. Valetsky, V.G. Matveeva, E.M. Sulman, M.G. Sulman, A.V. Bykov, N.V. Lakina, R.J. Spontak, J. Phys. Chem. B 108 (2004) 18234-18242. Sulman, E., V. Doluda, S. Dzwigaj, E. Marceau, L. Kustov, O. Tkachenko, A. Bykov, V. Matveeva, M. Sulman, N. Lakina, Journal of Molecular Catalysis A: 278 (2007) 112–119.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Carbon nanotube-supported sulfided Rh catalysts for the oxygen reduction reaction Chen Jina, Wei Xiaa, Junsong Guoab, Tharamani Chikka Nagaiahb, Michael Bronb, Wolfgang Schuhmannb, Martin Muhlera* a

Laboratory of Industrial Chemistry, Ruhr-University Bochum, D-44780 Bochum Elektroanalytik & Sensorik, Ruhr-Universität Bochum, D-44780 Bochum * Fax: 0049-234-32-14115 Email: [email protected]

b

Abstract Carbon nanotube (CNT) supported sulfided Rh catalysts were prepared applying three different routes: deposition-precipitation (DP), grafting of colloidal Rh nanoparticles, and polythiophene-assisted synthesis. The catalysts (1.4-1.8 wt%) prepared by DP were synthesized on CNTs from RhCl3 using hydrogen peroxide and subsequent exposure to on-line generated H2S followed by heat treatment. The Rh particles were found to be highly dispersed on the CNT surface. Alternatively, RhSx/Rh nanoparticles with four different loadings (4.3-21.9 wt%) grafted on carbon nanotubes were prepared through a functionalization of CNTs with short chain thiols and subsequent binding of colloidal Rh nanoparticles onto the thiolated CNTs. All steps of the synthesis were monitored by XPS. Finally, polythiophene/CNT composites were prepared and employed in the preparation of Rh17S15/Rh nanoparticles supported on CNTs. The CNTs with the highest polythiophene loading yielded the highest amount of Rh17S15 after Rh deposition and thermal treatment. The activity and stability of the prepared catalysts were studied towards the oxygen reduction reaction. Keywords: carbon nanotubes, RhSx/Rh, oxygen reduction reaction, hydrochloric acid electrolysis

1. Introduction In chlorine industry, the recovery of chlorine from electrolysis of aqueous HCl can be achieved by replacing the traditional hydrogen-evolving cathode by an oxygenconsuming cathode, a so called “oxygen-depolarized cathode” (ODC). The evolution of H2 at the cathode involves high energy consumption and safety problems, while the ODC process makes a lowering of the cell voltage by as much as 1 V possible, corresponding to a theoretical energy saving of ca. 700 kWh per ton of Cl2 (g) [1]. The oxygen reduction reaction (ORR) remains a major challenge in basic as well as applied electrochemistry. Platinum is reported to be the most active catalyst for the ORR in acidic media either for proton exchange membrane fuel cells or for electrolysers [2]. However, under the highly corrosive conditions of HCl electrolysis, Pt has serious durability problems. Adsorbed Cl− leads to the blockage of active sites, and even dissolution of Pt may occur. The voltage shift during an uncontrolled cell shut-down results in a significant loss of Pt [3], which poses significantly operating risks. Because of the intrinsic stability problems of Pt and Pt-based alloys in HCl electrolysis, tremendous efforts have been made to develop alternative catalysts. It was recently reported that transition metal chalcogenides of the general type RhSx exhibit a comparable catalytic activity and better stability as compared to Pt under the same

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corrosive conditions as mentioned above [4]. In spite of the fact that the modification of Rh-based catalysts with sulfur can reduce their ORR activity, for example, in aqueous H2SO4 [6], the Cl− contamination in the HCl electrolyte has a more negative effect than S on metallic Rh as site blocking agents [5]. Meanwhile, RhSx-type catalysts are not severely depolarized by the presence of Cl− and remain uninfluenced by a possible uncontrolled shut-down of the electrolysis cell. Therefore, RhSx-type catalysts are considered the most promising systems for ODC electrolysis of HCl. Different methods have been reported for the synthesis of transition metal chalcogenide catalysts of the RhSx type, including reaction of metals [5] and carbonyl precursors [6] with sulfur or selenium in different solvents, deposition-precipitation from solution with H2S [7], high temperature treatment with gas H2S [8] and post treatment with selenic acid [9]. However, the preparation of RhSx with well-defined properties still remains as a challenge. Furthermore, Rh is one of the most expensive noble metals. Consequently, optimizing the dispersion and loading is of vital importance. In this contribution, sulfided Rh catalysts supported on CNTs were prepared by different methods including deposition-precipitation (DP), grafting, and polymerassisted synthesis. The aim is to improve the dispersion of the active phase on CNTs, and enhance the catalytic activity and stability of the catalysts under highly corrosive conditions.

2. Experimental Multi-walled CNTs with inner diameters of 20-50 nm and outer diameters of 70-200 nm were obtained from Applied Sciences Inc. (Ohio, USA). Chemicals used in the syntheses include rhodium chloride (98%, Aldrich), hydrogen peroxide (30%, J. T. Baker), sulfur (99.5%, Riedel-de Haёn), MoS2 (99%, Aldrich), thionyl chloride (99%; Aldrich), 4-aminothiophenol (97%, Aldrich), sodium borohydride (98%, powder, ACROS), sodium citrate dehydrate (99% J. T. Baker), ferrocene (Merck, 98%), thiophene (Fluka, 98%), hydrochloric acid (J. T. Baker, 37-38%), nitric acid (65%, J. T. Baker) and toluene (J. T. Baker). Gases used in the synthesis include helium (99.9999%) and hydrogen (99.9999%)

2.1. Preparation of RhSx by deposition-precipitation (?) employing H2O2 and post-treatment with on-line generated H2S

The as-received CNTs were first treated at 800°C in helium to remove surface polyaromatics. The CNTs were subsequently treated with concentrated HNO3 at 120°C for 90 min to introduce oxygen-containing functional groups and to remove the residual Fe growth catalysts. For the synthesis of the RhSx catalysts, 100 mg of HNO3-treated CNTs were added to 10 ml of aqueous solution containing 87.2 mg of RhCl3 and the mixture was sonicated for 120 min. 20 mL of hydrogen peroxide were subsequently added dropwise to the suspension at a rate of 0.2 ml min-1. The resulting mixture was refluxed for 15 h at 75°C, filtrated after cooling, washed with deionized water, and dried at 60°C overnight. A parallel experiment was performed with Vulcan XC 72 carbon (Cabot) as support. The obtained CNT-supported rhodium oxide was treated at 200°C with H2S in a vertical quartz tube reactor (inner diameter 20 mm). H2S was generated online by passing a flow of H2 in He (20 sccm H2 in total 100 sccm) through 60 mg of molten sulfur and then through a MoS2 catalyst bed (50 mg of MoS2 in 500 mg of quartz powder) at 400°C. After the H2S treatment, the samples were treated in helium for 120 min at 400°C, 650°C, and 900°C.

Carbon nanotube-supported sulfided Rh catalysts for the oxygen reduction reaction 163

2.2. Preparation of Rh-RhSx nanoparticles grafted on carbon nanotubes 2.2.1. Preparation of Rh colloid 10 mL of a sodium citrate solution (0.25 mM) were first added to 90 mL of aqueous solutions of RhCl3 with the following concentrations: 0.015 mM, 0.0324 mM, 0.0729 mM and 0.125 mM. The mixtures were stirred for 3 min. Concurrently, 100 mL of 0.05 M NaBH4 solution were prepared by adding NaBH4 to 100 mL of ice-cold sodium citrate solution (0.025 M). A volume of 3 mL of the obtained solution was added dropwise to each RhCl3/sodium citrate solution and the mixtures were stirred for 10 min to obtain a dark colloid suspension. 2.2.2. Thiolation of CNTs The thiolation of the HNO3-treated CNTs was achieved by reaction with SOCl2 to form acyl chloride-functionalized CNTs, and then with 4-aminothiophenol (NH2C6H4SH). The detailed procedure is described elsewhere [10]. 2.2.3. Grafting of the Rh colloid onto the thiolated CNTs 30 mg of thiolated CNTs were added to the colloidal suspensions with different amounts of rhodium colloids. The mixtures were sonicated for 1 min, and then stirred for 30 min at room temperature. The suspensions were filtered, washed with deionized water, and dried at 60°C.

2.3. Polythiophene-assisted preparation of RhSx on CNTs 2.3.1. Deposition of iron oxide on CNTs Iron oxide was deposited on HNO3-treated CNTs by chemical vapor deposition (CVD) under oxidizing conditions using ferrocene as precursor [11]. The CVD was performed in a fixed-bed reactor to achieve a theoretical Fe loading of 10 wt%. The sample was reduced at 400°C for 2 h with a gas mixture of He (50 sccm) and H2 (50 sccm). 2.3.2. Coating of polythiophene on CNTs (polythiophene/CNT) A thin layer of iron oxides forms after exposing the reduced metallic Fe nanoparticles to air. These oxides, together with Fe, can function as oxidative initiator for the polymerization of thiophene. Different amounts of polythiophene on CNT were obtained by exposing the iron/iron oxide-coated CNTs to a vapor of HCl and thiophene, which was obtained by passing He through HCl (30 sccm He) and thiophene (20 sccm He) solution. The reaction was carried out for 5, 30, and 90 min. 2.3.3. Impregnation of polythiophene/CNTs with rhodium and subsequent thermal treatment (Rh-S/CNT). The three different polythiophene/CNT samples where impregnated with Rh employing an aqueous solution of RhCl3 to yield a theoretical Rh-loading of 30 wt%. Water was evaporated at 100 mbar and 60°C for 1 h with a BÜCHI Rotavapor R-114. The samples were subsequently dried at 50 mbar and 60°C for 2 h. The dried samples were treated at 650°C for 2 h in He. The catalyst samples obtained from the CNTs coated with polythiophene for 5, 30 and 90 min are labeled Rh-S/CNT-5, Rh-S/CNT-30 and Rh-S/CNT-90.

2.4. Characterization The morphology of the catalysts was studied by scanning electron microscopy (LEO Gemini 1530). Transmission electron microscopy (TEM) measurements were carried out with a Hitachi-H-8100 instrument. The mean particle size was obtained by analyzing ca. 100-220 particles with the iTEM software. Elemental analysis was performed using an Elementar Vario III atomic absorption spectrometer (AAS) by dissolving samples with aqua regia and H2SO4 at 300°C. X-ray diffraction (XRD) was carried out using a Philips

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X-Pert MPD system with Cu Kα radiation. X-ray photoelectron spectroscopy (XPS) measurements were carried out in an ultra-high vacuum set-up equipped with a Gammadata-Scienta SES 2002 analyzer. A flood gun was used to compensate for the charging effects. The binding energies were calibrated with the C 1s peak (284.5 eV). Thermogravimetry was performed with a Cahn TG-2131 thermobalance in pure O2 with a heating rate of 2 K/min.

2.5. Electrocatalytic ORR activity and stability test The ORR activity of the RhSx/CNT catalysts was characterized by rotating disc electrode (RDE) measurements at a scan rate of 5 mV s-1 in 0.4 M HCl at rotation rates of 100, 400 and 900 rpm. A single compartment electrochemical cell was used equipped with a Ag/AgCl/3 M KCl as reference electrode (RE), a Pt foil as counter electrode (CE), and a catalyst-coated glassy carbon electrode (GCE) as working electrode. Electrocatalytic stability tests were conducted in a flow cell applying a pulsed interruption mode with an operation time of 10 min at 0.3 V vs. Ag/AgCl/3 M KCl and an interruption time of 2 min, which is intended to simulate conditions occurring during cell shutdown in industrial HCl electrolysis. The currents were recorded, and the performance of the catalysts before and after a 24 h test period was studied by linear sweep voltammetry (LSV) at a scan rate of 5 mVs-1 from 0.6 V to –0.2 V. Further experimental details are described elsewhere [12].

3. Results and discussion The first experimental approach to prepare RhSx catalysts supported on CNTs consists in the deposition precipitation of RhCl3 using hydrogen peroxide and subsequent exposure to on-line generated H2S [13]. Vulcan XC72 carbon black was also employed as the support during the same preparation procedure, but a weight loss of more than 50 % was recorded after refluxing with H2O2 for 15 h. Thus, further experiments have not been carried out with this material. In the case of CNTs, the weight loss during refluxing was negligible. Rh oxide immobilized on CNTs was obtained after this DP process. A subsequent treatment with H2S at 200°C was carried out to initiate the formation of RhSx nanoparticles. The H2S was prepared online from S and H2 at 400°C catalyzed by MoS2. A continuous H2S supply monitored by online mass spectrometry of about 70 min was achieved using 60 mg S. The resulting sample was further treated at 400°C, 650°C, and 900°C in He.

Fig. 1 TEM images of CNT-supported sulfided rhodium catalysts treated in helium at (a) 400°C, (b) 650°C, and (c) 900°C.

Figure 1 displays TEM images of catalysts prepared via DP with H2O2, H2S treatment and subsequent heat treatment, as described in the experimental part. The average diameters of the samples treated at 400°C, 650°C, and 900°C are 1.0 ± 0.5 and 2.5 ± 1.0 nm, and 8.7 ± 4.7 nm, respectively. The Rh loadings determined by AAS were 1.4 wt% (400°C), 1.5 wt% (650°C), and 1.9 wt% (900°C). The slight increase of the

Carbon nanotube-supported sulfided Rh catalysts for the oxygen reduction reaction 165 loading can be due to the weight loss during the heat treatment from the decomposition of oxygen-containing functional groups on the CNT surfaces. The amount of anchoring sites (surface functionalities) and surface defects is crucial to achieve thermally stable, small particles at high metal loadings. On the specific type of CNTs employed by us, a significant improvement of loading based on the present H2O2 method can hardly be achieved without increasing the particle size. The XRD patterns of the samples heat treated at 400°C and 650°C (not shown) displayed only the characteristic peaks of hexagonal graphite from the CNTs. No contribution from Rh or any RhSx phase was detected, in agreement with the small particle size observed with TEM. The peaks corresponding to the fcc metallic Rh appeared after treatment at 900°C. Rh sulfide was not detected by XRD in all three samples. However, peaks located at 162 eV in XPS 2p spectra (not shown) confirm the presence of RhSx in the samples treated at 400°C and 650°C. The activity for electrocatalytic oxygen reduction has been evaluated with RDE measurements in 0.4 M HCl at room temperature (not shown). The activities of the investigated catalyst samples were found to be 650°C > 400°C > 900°C > Rh oxide. As it could be expected, the post-treatment with H2S significantly enhances the activity of the catalyst suggesting the formation of a RhSx surface layer. Among them, the sample treated at 650°C shows the best performance, which can be tentatively assigned to the formation of a stable RhSx layer.

Fig. 2 Linear sweep voltammograms of sulfided Rh catalysts before and after 24 h stability test in an O2 saturated flow cell at a scan rate of 5 mVs-1 from 0.6 V to –0.2 V at room temperature. 0.4 M HCl was used as electrolyte at a flow rate of 0.33 ml min-1.

The stability of the RhSx catalyst supported on CNTs under conditions relevant to industrial electrolysis was studied in a flow cell. Linear sweep voltammograms (LSV) of the samples recorded before and after operation for 24 h (including periodic interruptions as describe in the experimental part) are shown in Fig. 2. The performance before and after 24 h of operation shows the same activity sequence as the RDE results. The half wave potentials of the oxygen reduction shifted from 0.041 V in Rh-400, to 0.028 V in Rh-650, and to 0.019 V in Rh-900. The potential shift decreases with increasing treatment temperature which is attributed to the increasing particle size. In summary, synthesis of RhSx on CNTs via H2O2 precipitation and post treatment with H2S gives highly dispersed low loading catalysts. The catalyst sample with ca. 9 nm nanoparticles shows the best stability. In order to enhance the RhSx loading and stability, RhSx/Rh nanoparticles grafted on carbon nanotubes were prepared through the functionalization of CNTs with short chain thiols and subsequent binding of colloidal Rh nanoparticles onto the thiolated CNTs [14]. The carboxyl group is one of the major groups generated on CNTs by HNO3 treatment [14]. In the first step of the catalyst preparation, these carboxyl groups react

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with thionyl chloride to generate acyl chloride groups (Eqn. (1)). The thiolation of the CNTs was achieved by the reaction of the acyl chloride-functionalized CNTs with 4-aminothiophenol. The short spacer groups between the CNTs and the –SH groups were supposed to enhance the stability of the grafted RhSx/Rh nanoparticles while maintaining the electron transfer between the CNT support and the grafted colloidal Rh during the electrocatalysis. Colloidal rhodium nanoparticles were prepared from RhCl3 as a metal precursor, sodium citrate as a stabilizer, and NaBH4 as a reducing agent. The subsequent grafting of colloidal Rh onto the thiolated CNTs was achieved by adding the thiolated CNTs to four different concentrations of colloidal suspensions. The adsorption process was complete as indicated by a colorless filtrate. The Rh loadings determined by AAS were 4.3, 6.4, 16.1 and 21.9 wt%, corresponding to theoretical loadings of 5, 10, 20 and 30 wt%, respectively. The loss of Rh can be due to incomplete anchoring. (Eqn. 1.) All described steps of the modifications of the CNTs and grafting of colloidal Rh were monitored by XPS as shown in Fig. 3. In the XPS S 2p spectra, the three peaks located at binding energies of 168.6 eV, 163.7 eV, and 162.5 eV correspond to S=O, S−H, and Sδ- species, respectively (Fig. 3a) [15]. S=O species as indicated by the peak at 168.6 eV originate from the thionyl chloride treatment of the HNO3-treated CNTs in Fig. 3a. This contribution in trace (1) and (2) decreases upon grafting with the rhodium colloid in traces 3-6. The thiolation of the CNTs upon modification of the acyl chlorideterminated CNTs with 4-aminothiophenol is indicated by a strong increase of the peak at 163.7 eV, which is characteristic of thiol groups [16]. The signal in traces 3-6 at 162.5 eV appears after grafting with colloidal Rh, demonstrating the presence of Rh-S species [17].

Fig. 3 XPS S 2p (a) and N 1s (b) spectra of acyl chloride-functionalized CNTs (1), thiolated CNTs (2), and CNT-supported rhodium sulfide catalysts with Rh loadings of 4.3% (3), 6.4% (4), 16.1% (5), 21.9% (6).

The HNO3 treatment can form NOx species on the CNT surface, and the interaction between the NOx and SOCl2 can possibly account for the N 1s peak at 401.6 eV in Fig. 3b. The thiolation of the acyl chloride-terminated CNTs with 4-aminothiophenol (trace (2) in Fig. 3b) gave rise to the nitrogen peak typical for amides at 399.4 eV [18]. A shift

Carbon nanotube-supported sulfided Rh catalysts for the oxygen reduction reaction 167 of the 399.4 eV peak to lower binding energies is not observed after grafting with rhodium colloid (Fig. 3b, traces 3-6), thus excluding the formation of rhodium nitride. The TEM particle sizes (rtem) obtained for the different samples are quite similar (ca. 7 nm). No increase of the particle size was found in the highly loaded samples. The XRD particle sizes (rxrd) estimated from the Scherrer’s equation for all the samples were about 3 nm. The larger rtem than rxrd could be due to agglomeration and a sulfide/oxide layer outside a metallic Rh core. Based on the XRD and XPS results, the structure of the nanoparticles is assumed to consist of a metallic Rh core covered by a sulfide layer. RDE measurements disclosed that the 16.1% Rh catalyst was more active than the 4.3% and 6.4% samples. However, an even higher loading of 21.9% Rh did not lead to a further increase in performance presumably due to the lowered accessibility of the active sites at the aggregated RhSx/Rh nanoparticles. The stability for ORR of the 16.1% Rh catalyst was investigated under the same conditions as the sulfided Rh catalysts prepared by DP followed by H2S treatment. The 16.1% Rh catalyst showed a smaller half wave potential shift (0.012 V) than the Rh-900 sample (0.019 V) after a stability test for 24 h (Fig. 4) while with a more positive onset potential than the Rh-900. The results indicate that the grafted sample is more stable than the Rh-900 (rtem = 8.7 nm, rxrd = 8.0 nm) even with a much smaller particle size (rtem = 7nm, rxrd = 3 nm). Hence, we can conclude that grafting enhances the stability of the Rh catalysts.

Fig. 4 Linear sweep voltammograms of 16.1% Rh-S/CNTs and Rh-900 before and after 24 h stability test in an O2-saturated flow cell at a scan rate of 5 mVs-1 from 0.6 V to –0.2 V at room temperature. 0.4 M HCl was used as electrolyte at a flow rate of 0.33 ml min-1.

The sulfide species, although proven by XPS, were not detected by XRD in the above-mentioned samples. The following section focuses on enhancing both the amount and the dispersion of Rh sulfides on CNTs using polythiophene as sulfur source [19]. First, CNTs were coated by polythiophene synthesized by iron-catalyzed gas-phase polymerization. The iron catalysts were deposited on the CNTs by chemical vapor decomposition. Samples with three different polymerization times (5, 30, and 90 min) were prepared and the composition and loading of the polythiophene coating were analyzed by XPS and thermogravimetry (TGA). Increasing the polymerization time to 90 min led to the formation of a network of CNTs with pyrolysed polythiophene as linker. CNT-supported rhodium catalysts were obtained by impregnation of the polythiophene-coated CNT substrate with rhodium chloride, and subsequent thermal treatment in helium at 650°C. It was found that the degree of dispersion, the loading of the catalyst particles and the amount of Rh17S15 (determined by XRD) were enhanced by increasing the amount of polythiophene. A higher amount of polythiophene also led to a

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lower charge transfer resistance of the obtained catalysts, as evidenced by electrochemical impedance spectroscopy.

4. Conclusions Highly dispersed sulfided Rh catalysts supported on CNTs were prepared through deposition-precipitation using H2O2 resulting in low loadings. The sample treated at 650°C showed the highest activity, while treatment at 900°C led to the highest electrochemical stability. Grafting enhanced the loading and the stability. Finally, polythiophene-assisted syntheses were used to prepare Rh17S15/Rh catalysts supported on CNTs, and the dispersion, loading, and the amount of Rh17S15 increased with increasing amount of polythiophene.

Acknowledgements Chen Jin thanks the International Max Planck Research School Surface and Interface Engineering in Advanced Materials (SurMat) for a research grant. Dr. Tharamani Chikka Nagaiah is grateful to the Alexander von Humboldt Foundation for a postdoctoral fellowship.

References [1] F. Federico, G.N. Martelli, D. Pinter, in: J. Moorhouse (Eds.), Modern chlorine-alkali technology, Wiley, 2001. [2] J.M. Ziegelbauer, A.F. Gulla, C. O’Laoire, C. Urgeghe, R.J. Allen and S. Mukerjee, Electrochim. Acta, 52 (2007) 6282. [3] T.J. Schmidt, U.A. Paulus, J.A. Gasteiger and R.J. Behm, J. Electroanal. Chem., 508 (2001) 41. [4] A.F. Gulla, L. Gancs, R.J. Allen and S. Mukerjee, Appl. Catal. A:General, 326 (2007) 227. [5] D. Cao, A. Wieckowski, J. Inukai and N. Alonso-Vante, J. Electrochem. Soc., 153 (2006) A869. [6] M. Bron, P. Bogdanoff, S. Fiechter, M. Hilgendorff, J. Radnik, I. Dorbandt, H.Schulenburg and H. Tributsch, J. Electroanal. Chem., 517 (2001) 85. [7] A.V. Mashkina and T.S. Sukhareva, React. Kinet. Catal. Lett., 67 (1999) 103. [8] M. Lacroix, N. Boutarfa, C. Guillard, M. Vrinat and M. Breysse, J. Catal., 120 (1989) 473. [9] H. Schulenburg, M. Hilgendorff, J. Radnik, I. Dorbandt, P. Bogdanoff, S. Fiechter, M. Bron and H. Tributsch, J. Power Sources, 155 (2006) 47. [10] Y. Kim and T. Mitani, J. Catal., 238 (2006) 394. [11] W. Xia, D. Su, A. Birkner, L. Ruppel, Y. Wang, C. Wöll, J. Qian, C. Liang, G. Marginean, W. Brandl and M. Muhler, Chem. Mater., 17 (2005) 5737. [12] C. Jin, W. Xia, T.C. Nagaiah, J. Guo, X. Chen, N. Li, M. Bron, W. Schuhmann and M. Muhler, J. Mater. Chem., 2009, doi: 10.1039/b916192a. [13] C. Jin, W. Xia, T.C. Nagaiah, J. Guo, X. Chen, M. Bron, W. Schuhmann and M. Muhler, Electrochim. Acta, 54 (2009) 7186. [14] S. Kundu, Y. Wang, W. Xia and M. Muhler, J. Phys. Chem. C, 112 (2008) 16869. [15] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, in Handbook of X-ray Photoelectron Spectroscopy: A Reference Book of Standard Spectra for Identification and Interpretation of XPS Data, J. Chastain (eds.), Perkin-Elmer corporation, Minnesota, 1992. [16] N. Kocharova, T. Aaritalo, J. Leiro, J. Kankare and J. Lukkari, Langmuir, 23 (2007) 3363. [17] D. Grumelli, C. Vercat, G. Benitez, M. E. Vela and R. C. Salvarezza, J. Phys. Chem. C, 111 (2007) 7179. [18] D.N. Hendrickson, J.M. Hollander and W.L. Jolly, Inorg. Chem., 8 (1969) 2642. [19] C. Jin, T.C. Nagaiah, W. Xia, M. Bron, W. Schuhmann and M. Muhler, ChemCatChem, submitted.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Synthesis and characterization of highly loaded Pt/carbon xerogel catalysts prepared by the Strong Electrostatic Adsorption method Nathalie Job,a Frédéric Maillard,b Marian Chatenet,b Cédric J. Gommes,a Stéphanie Lambert,a Sophie Hermans,c John R. Regalbuto,d Jean-Paul Pirarda a

Laboratoire de Génie Chimique, Université de Liège, B6a, B-4000 Liège, Belgium LEPMI, UMR 5631 CNRS/Grenoble-INP/UJF, BP75, F-38402 St Martin d’Hères Cedex, France c Unité de Chimie des Matériaux Inorganiques et Organiques, Université Catholique de Louvain, Place Louis Pasteur 1/3, B-1348 Louvain-la-Neuve, Belgium d Department of Chemical Engineering, University of Illinois at Chicago, 810 S. Clinton Street, Chicago, IL 60607, USA b

Abstract In order to decrease the mass transport limitations reported in classical PEMFC electrodes, Pt/carbon xerogel catalysts have great potential to replace Pt/carbon black catalysts. These nanostructured materials with well defined pore texture allow for better gas/water diffusion and better contact between the platinum particles and the ionomer (Nafion®). Pt/carbon xerogel catalysts with high metal content (~ 25 wt.%) and high metal dispersion (nanoparticles ca. 2 nm in size) were prepared via the ‘Strong Electrostatic Adsorption’ method; the impregnation-drying-reduction step with H2PtCl6 was repeated until the desired metal loading was achieved. However, both physicochemical and electrochemical characterization show that the use of H2PtCl6 leads to Pt catalysts poisoned with chlorine, especially if the reduction temperature is lower than 450°C. This induces a dramatic decrease of the Pt utilization ratio in the final PEMFC catalytic layer. Keywords: carbon xerogels, Pt/C catalysts, PEM fuel cells, electrochemistry

1. Introduction Pt/C catalysts with high metal weight percentage are classically used in Proton Exchange Membrane Fuel Cells (PEMFCs) [1]. Indeed, minimization of ohmic and transport losses within the electrode, and compensation for both the sluggish oxygen reduction reaction rate and the usual lack of contact between Pt and the ionomer result from a compromise: (i) the thickness of the catalytic layer must be low, and (ii) the metal loading of the electrode must be high. In order to decrease the mass transport limitations encountered in PEMFC electrodes, which are prepared with Pt/carbon black catalysts, it was recently proposed to replace the classical carbon black support by carbon xerogels [2], i.e. nanostructured materials with well defined pore texture prepared by evaporative drying and pyrolysis of organic gels. Carbon xerogels allow for better gas/water diffusion within the pore texture of the electrode and better contact between the platinum particles and the ionomer (Nafion®).

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In this study, a carbon xerogel with selected pore size in the macropore range was used as Pt catalyst support. The catalyst was designed for use as oxygen reduction catalyst at the cathode of a PEM fuel cell. So as to obtain Pt/carbon xerogel catalysts with high metal content, the ‘Strong Electrostatic Adsorption’ (SEA) method [3, 4] was applied; H2PtCl6 aqueous solution was used as metal precursor. This method consists of maximizing the electrostatic interactions between the metal precursor and the support by adjusting the pH of the slurry. The latter depends on the surface chemistry of the carbon and on the chemical nature of the metal precursor. The goal was to obtain highly loaded Pt/carbon xerogel catalysts while preserving high metal dispersion: indeed, following the literature, the optimal Pt particle size for efficient oxygen reduction is ca. 2-3 nm [5]. The obtained catalyst was reduced under flowing hydrogen at various temperature conditions and characterized using both physico-chemical (TEM, XPS, CO chemisorption) and electrochemical (CO stripping voltammograms, ORR measurements) techniques. The samples were used as Pt/C catalyst at the cathode side of an air/H2 PEM fuel cell. The goal of the study was to demonstrate the effect of Pt chlorine poisoning, originating from the decomposition of H2PtCl6, and the importance of the catalyst reduction treatment on the cell performance. Finally, physico-chemical and electrochemical characterization allowed us to study the effect of poisoning on the metal availability and on the oxygen reduction kinetics.

2. Experimental 2.1. Catalyst preparation The carbon support chosen for this study was a micro-macroporous carbon xerogel with a macropore size ranging from 50 to 85 nm, a specific surface area of 640 m² g-1 and a total pore volume of about 2.1 cm³ g-1 (including 0.26 cm³ g-1 of micropores, i.e. pores smaller than 2 nm). Carbon xerogels are materials composed of interconnected spherical-like microporous nodules. So, they classically display a bimodal pore size distribution: micropores inside the carbon nodules, and larger pores identified as the voids located between the nodules [6]. The size of the nodules and that of the voids inbetween is controlled by the synthesis conditions of the material [6, 7]. The carbon support was synthesized by the drying and pyrolysis of a resorcinolformaldehyde aqueous gel, following a procedure developed in a previous study [8]: the R/C (resorcinol/sodium carbonate) molar ratio of the gel precursor solution was chosen equal to 1000, while all other synthesis variables (from gel preparation to pyrolysis conditions) were kept identical as in the above-mentioned study. In brief, resorcinol and sodium carbonate were solubilized in water and then formaldehyde was added. Gelation and ageing were performed at 85°C (72 h), and were followed by evaporative drying (60-150°C, 1 day) and pyrolysis (800°C, 2 h) under nitrogen flow. Pt/carbon xerogel catalysts were obtained via the ‘Strong Electrostatic adsorption’ (SEA) method. This consists of maximizing the electrostatic interactions between the metal precursor and the support by adjusting the pH of the slurry to the adequate value, which depends on the surface chemistry of the carbon and on the precursor chosen. Indeed, interaction between the support and the metal precursor depends on both the precursor nature (anion or cation, size, etc.) and on the carbon surface chemistry. At pH values lower than the Point of Zero Charge (PZC), i.e. the pH at which the surface is neutral in terms of charge, the surface is positively charged, and the adsorption of anions is favoured (Fig. 1a). At pH higher than the PZC, the surface is negatively charged, and the adsorption of cations is enhanced. The PZC of carbon xerogels after

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pyrolysis is about 9.5 [4], which leaves a large pH range for the adsorption of both Pt anions and cations. However, the Pt uptake is limited by steric effects: indeed, in the case of impregnation with H2PtCl6, for instance, the maximum surface density can be calculated as a close packed arrangement of chloroplatinic acid complexes which retain one hydration sheath [3]. As a result, the Pt uptake vs. final pH of the impregnation solution at equilibrium passes through a maximum (Fig. 1b): indeed, in the case of the impregnation of carbon xerogels with H2PtCl6 aqueous solutions (1000 ppmPt), the initial pH leading to the highest Pt uptake was found to be 2.5 (final pH at equilibrium ~ 3.0), and the corresponding maximum Pt surface density was found equal to 0.8-0.9 µmol m-2, which corresponds to a weight percentage ranging from 8 to 10 wt.% [4].

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OH2

Kads,an

Pt surface density (µmol m )

1

(a)

0.75

0.5

0.25

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Fig. 1. (a) Principles of the Strong Electrostatic Adsorption (SEA) method: depending on the PZC of the support and on the impregnation pH, the adsorption of ositively or negatively charged species is favoured; (b) due to steric effects, the Pt uptake is limited and the Pt surface density vs. pH curve presents a maximum (results from [4]).

The procedure used for preparing Pt/carbon xerogels via the SEA method is fully described in reference [9]. Briefly, the carbon support was contacted with the impregnation solution at the optimal pH value of 2.5 until the equilibrium was reached. The impregnated catalyst was then filtered, dried and reduced under flowing hydrogen. In order to increase the Pt weight percentage up to acceptable values for electrochemical applications, this impregnation-drying-reduction cycle was repeated up to three times using the same original catalyst batch. Note also that various final reduction temperatures (200, 350 and 450°C) were tested to evaluate the effect of Pt poisoning by chlorine.

2.2. Catalyst characterization 2.2.1. Physico-chemical characterization Catalysts were characterized using several complementary techniques. The Pt content was evaluated by ICP-AES after elimination of the carbon and solubilisation of the metal [10]. In order to measure the size of the metal particles, the catalysts were investigated by transmission electron microscopy, with a Jeol 2010 (200 kV) device (LaB6 filament). The samples were crushed and dispersed in ethanol and subsequently deposited onto a copper grid. Particle size distributions were obtained by image analysis performed on a set of at least 1000 particles: the procedure is described in [9]. The samples were analyzed by X-ray photoelectron spectroscopy (XPS), performed on an SSI-X-probe (SSX-100/206) spectrometer from Fisons. The samples were stuck onto troughs with double-sided adhesive tape then placed on an insulating home-made ceramic carrousel with a nickel grid 3 mm above the samples, to avoid differential

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charging effects. A floodgun set at 8 eV was used for charge stabilisation. The energy scale was calibrated by taking the Au 4f7/2 binding energy at 84 eV. The C1s binding energy of contamination carbon set at 284.8 eV was used as internal standard value. Data treatment was performed with the CasaXPS program (Casa Software Ltd). Finally, CO chemisorption was used to determine the accessible Pt surface, SPt-Chem. Isotherms were measured with a Fisons Sorptomatic 1990 equipped with a turbomolecular vacuum pump that allows vacuum of 10-3 Pa. The entire procedure, from the sample preparation to the adsorption measurement, is fully described elsewhere [10]. Briefly, a first CO adsorption isotherm was achieved so as to measure the total amount of adsorbed carbon monoxide (chemisorbed + physisorbed). The catalyst was then outgassed, and a second CO adsorption isotherm (physisorbed CO) was measured. The amount of CO forming the chemisorbed monolayer on surface Pt atoms, deduced by extrapolating the nearly horizontal difference curve to the uptake axis, was used to calculate SPt-Chem. 2.2.2. Electrochemical characterization All electrochemical measurements were carried out in sulphuric acid (1 M) at 25°C. The voltammetric experiments were performed using an Autolab-PGSTAT20 potentiostat with a three-electrode cell and a saturated calomel electrode (SCE) as a reference electrode (+0.245 V vs. normal hydrogen electrode, NHE). The catalyst sample was deposited on a rotating disk electrode (EDT 101, Tacussel), used as working electrode. All details, from sample preparation to experimental conditions, are extensively described in reference [9]. The electrochemically active Pt surface area of the catalysts, SPt-Strip, was determined by CO stripping. This electrochemical technique consists in the electrooxidation of a CO monolayer previously adsorbed on the Pt surface. It allows estimating the real Pt surface area, assuming that the electrooxidation of a COads monolayer requires 420μC per cm2 of Pt. Since CO stripping is performed in aqueous electrolyte, it implies 100% utilization of the Pt surface atoms and is influenced neither by contact problems between the metal and the electrolyte nor by mass-transport limitations. In addition, the electrooxidation of a COads monolayer is a structure-sensitive reaction and provides a wealth of information on the particle size distribution and the presence/absence of particle agglomeration [11, 12]. The CO stripping voltammograms were recorded at 0.02 V s-1 between +0.045 and +1.245 V vs. NHE, after saturation of the electrolyte by CO (6 min bubbling) and removal of the non-adsorbed CO from the cell by purging with Ar (39 min). The electrocatalytic activity for the ORR of the elaborated Pt/C nanoparticles was measured in O2-saturated liquid electrolyte. The quasi-steady-state voltammograms were recorded at 10-3 V s-1 from +1.095 to +0.245 V vs. NHE. To account for the reactants diffusion-convection in the liquid layer, the experiment was repeated at four RDE rotation speeds (42, 94, 168 and 262 rad s-1) [13]. 2.2.3. Fuel cell test The catalysts were tested as PEM fuel cell cathodic catalytic layers on a unit cell-test bench: 50 cm² Membrane-Electrode Assemblies (MEAs) were prepared by the decal method as described in reference [2]. The electrolyte was a Nafion® membrane, and the anode a commercial anode made from Pt-doped carbon black (40 wt.%, TKK) deposited by Paxitech onto a carbon felt (0.6 mgPt cm-2 mixed with Nafion®). The thickness of the cathode was kept constant by keeping constant the carbon mass in the catalytic layer. The Nafion®/carbon mass ratio of the ink used to prepare the MEAs was fixed at 0.5. After a standardized start-up procedure, polarization curves, i.e. the Ucell = f(jm) curves, were measured by setting the cell voltage at each desired value for 15 min, which

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assured the stabilization of the current. The current, jm, was normalized to the Pt metal loading of the cathode.

3. Results and discussion Figure 2 shows examples of TEM micrographs and the corresponding Pt particle size distributions obtained by image analysis of two Pt/carbon xerogel catalysts: the first one (Fig. 2a) was prepared by single impregnation of the support, and the second one (Fig. 2b) was obtained by three consecutive impregnation-drying-reduction cycles. Results show that repeating the impregnation with H2PtCl6 as Pt precursor yields an increase of the catalyst metal content up to 22.3 wt.% while keeping homogeneously dispersed Pt nanoparticles ca. 2 nm in size.

(a)

(b)

Fig. 2. TEM micrographs of Pt/carbon xerogel catalysts and particle size distributions obtained by image analysis. (a) Single impregnation, 7.5 wt.% Pt and (b) triple impregnation, 22.3 wt.%.

Table 1 regroups the characterization data of three catalysts, prepared from the same batch of double-impregnated sample (metal loading: 15.0 wt.%); the only difference is the temperature (200-450°C) and duration (1-5 h) of the reduction treatment, performed under flowing H2. The samples are denoted as follows: the letter ‘C’ is followed by the reduction temperature (in °C) and the reduction time (h). The average particle size, dTEM, obtained by image analysis, does not change when increasing the reduction temperature from 200°C (C-200-1) to 450°C (C-450-5). This agrees with a set of additional experiments performed under N2, which showed that Pt nanoparticles begin to sinter only at T ≥ 600°C. Interestingly, the Pt surface area detected by CO chemisorption, SCO-chem, increases from 92 (C-200-1) to 124 m² gPt-1 (C-450-5). In addition, chlorine is detected at the surface of every sample by XPS measurements. It seems thus that, when the reduction temperature is too low, the catalyst remains poisoned by chlorine coming from the metal precursor (H2PtCl6). The calculated Cl/Pt ratio decreases with increasing the reduction temperature, from 0.33 (C-200-1) to 0.07 (C-450-5): so, removing chlorine species completely from the catalyst proves difficult. One also notices that the CO chemisorption and XPS data do not match well. Indeed, the Cl/Pt ratio decreases by a factor 5 between C-200-1 and C-450-5 while the detected Pt surface shows an increase of 30% only; in addition, from sample C-350-3 to sample

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C-450-5, Cl/Pt is multiplied by 3 while SCO-chem remains almost constant (118 and 124 m² gPt-1, respectively). Table 1. Physico-chemical and electrochemical characterization of catalysts reduced under various conditions. Sample

C-200-1 C-350-3 C-450-5

TEM dTEM (nm) ± 0.2 1.8 1.8 1.8

CO chemisorption SCO-chem (m² gPt-1) ± 10% 92 118 124

XPS Cl/Pt (-) ± 10% 0.33 0.23 0.07

CO stripping SCO-strip (1) SCO-strip (7) (m² gPt-1) (m² gPt-1) ± 10% ± 10% 37 69 61 87 129 142

ORR b (V dec-1) ± 5% -0.074 -0.070 -0.066

SA90 (µA cm²Pt-1) ± 5% 10 11 12

CO stripping measurements allowed us to obtain complementary data about catalyst poisoning. Figure 3a shows the CO stripping voltammograms of two catalysts: C-200-1 and C-450-5, obtained by two different reduction treatments of the same initial batch (double impregnation, 15.0 wt.% metal). Globally, the surface of the peak detected between 0.7 and 1.0 V vs. ENH represents the current exchanged to electrooxidize the CO monolayer adsorbed on the Pt surface, and can be regarded as the electrochemically accessible Pt surface [9]. The curves in grey correspond to the voltammograms obtained on the fresh catalysts while the black curves are those measured after 7 consecutive CO strippings. One can observe that (i) the charge under the peak increases with the reduction temperature, (ii) the charge under the peak increases after several consecutive CO strippings, whatever the reduction temperature, and (iii) the peak is shifted toward lower potentials when the reduction temperature increases or when the number of consecutive CO strippings increases. These observations are in agreement with the hypothesis of catalyst poisoning. First, as already observed by CO chemisorption, the accessible Pt surface measured on the fresh catalyst increases with the reduction temperature: SCO-strip (1), increases from 37 (C-200-1) to 129 m² gPt-1 (C-450-5). Note that, contrary to CO chemisorption measurements, CO stripping data match perfectly the Cl/Pt ratios obtained by XPS: indeed, the relationship between Cl/Pt and SCO-strip (1) is close to linearity (not shown). Second, the detected surface increases after several CO strippings: this is due to the fact that chlorine species are progressively removed by repetitive adsorption/desorption processes. Indeed, Holscher and Sachtler [14] showed that CO is one of the strongest poisons adsorbed onto platinum: in the presence of CO, poisons originally adsorbed onto the Pt particle surface should be displaced. However, the kinetics of displacement may be too slow to be completed within a few minutes: this would explain why CO chemisorption in gaseous phase, during which equilibrium is reached prior to any further gas injection, leads to larger Pt surfaces than COads stripping in liquid phase; the differences in surface detected by CO chemisorption and CO stripping could then be due to slow Cl displacement kinetics. Finally, the positive shift of the onset of the CO electrooxidation peak may be ascribed to the competition between water and chloride species for the Pt adsorption sites. Indeed, previous studies [15, 16] have suggested that only a fixed number of active sites which are able to form OH species and to initiate the CO electrooxidation exist on the Pt surface. Competitive adsorption by Cl- species thus decreases artificially the number of active sites and shifts both the onset and the main CO electrooxidation peak towards positive potentials.

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1

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(b) 0.75 U cell (V)

-1

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Fig. 3. Electrochemical measurements on 15.0 wt.% Pt/C catalysts (double impregnation) reduced under H2 during 1h at 200°C, sample C-200-1 ({) and 5h at 450°C, sample C-450-5 (‘). (a) COads stripping voltammograms, measurements in H2SO4 1 M at 25°C, sweep rate of 0.02 V s-1. Curves in grey: first CO stripping; curves in black: voltammogram after 7 consecutive CO strippings. (b) Polarization curves at 70°C for Membrane-Electrode Assemblies with cathode processed with the same catalysts.

Oxygen Reduction Reaction (ORR) performed on the rotating disk electrode shows that the presence of chlorine has little effect on the intrinsic reactivity of the Pt sites. To evaluate the catalyst activity towards oxygen reduction, two parameters were evaluated: (i) the Tafel slope, b, and (ii) the specific activity at 0.90 V vs. NHE, SA90. This potential corresponds to 0.34 V ORR overpotential in 1 M sulphuric acid, a value classically monitored in a PEMFC cathode at low current densities, i.e. under kinetic control. Regarding samples C-200-1, C-350-3 and C-450-5, neither b nor SA90 changes significantly with the reduction treatment (Table 1), indicating that the reaction mechanism and kinetics remain the same. The only difference between the three samples is the accessible Pt surface, which decreases due to Cl poisoning when the reduction temperature is too low. Finally, Fig. 3b shows the impact of Cl poisoning on the functioning of an air/H2 fuel cell. The cathode of the Membrane-Electrode Assembly (MEA) was processed either with sample C-200-1, or with catalyst C-450-5. The catalyst poisoning dramatically decreases the current produced at a fixed potential. However, further calculation shows that the decrease of accessible Pt surface due to Cl coverage is not sufficient to explain the poor performance of catalyst C-200-1. Much probably, the presence of chlorine also hampers the contact between the Pt particles and the ionomer (i.e., Nafion®), and decreases thus further the amount of Pt atoms that are truly available for the oxygen reduction in the monocell. This was checked by in situ cyclic voltammetry: the detected Pt surface per mass unit of metal was lower in the processed MEA than in the initial catalyst powder.

4. Conclusions So as to obtain Pt/carbon xerogel catalysts with high metal content, the ‘Strong Electrostatic Adsorption’ method was applied. By repeating the impregnation-dryingreduction step with H2PtCl6 as Pt precursor, it was possible to increase the catalyst metal content up to 22.3 wt.% while keeping homogeneously dispersed Pt nanoparticles ca. 2 nm in size. However, both physico-chemical and electrochemical characterization

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show that the use of H2PtCl6, which is a very classical impregnation precursor, yields Pt catalysts poisoned with chlorine species. Chlorine coming from the metal precursor decomposition appears difficult to remove completely: even after reduction under H2 at 450°C for 5 h, Cl poisoning is still detected via electrochemical methods. The presence of chlorine on the Pt particles leads obviously to decreasing the active Pt surface, which can dramatically reduce the Pt utilization ratio in the PEMFC catalytic layer. Further work is in progress to extend the SEA method to Cl-free Pt precursors.

References [1] H.A. Gasteiger, S.S. Kocha, B. Sompalli, F.T. Wagner, 2005, Appl. Catal. B, 56 (1-2), 9-35. [2] N. Job, J. Marie, S. Lambert, S. Berthon-Fabry, P. Achard, 2008, Energ. Convers. Manage., 49 (9), 2461-2470. [3] J.R. Regalbuto, in: Catalyst Preparation: Science and Engineering, J.R. Regalbuto (ed.), CRC Press, Taylor & Francis Group, Boca Raton, 2007, p. 297. [4] S. Lambert, N. Job, L. D’Souza, M.F.R. Pereira, R. Pirard, J.L. Figueiredo, B. Heinrichs, J.P. Pirard, J.R. Regalbuto, 2009, J. Catal., 261 (1), 23-33. [5] K. Kinoshita, Electrochemical Oxygen Technology, Wiley, New York, 1992, p. 48. [6] N. Job, R. Pirard, J. Marien, J.-P. Pirard, 2004, Carbon, 42 (3), 619-628. [7] S.A. Al-Muhtaseb, J.A. Ritter, 2003, Adv. Mater., 15 (2) 101-114. [8] N. Job, A. Théry, R. Pirard, J. Marien, L. Kocon, J.-N. Rouzaud, F. Béguin, J.-P. Pirard, 2005, Carbon, 43 (12) 2481-2494. [9] N. Job , S. Lambert, M. Chatenet, C.J. Gommes, F. Maillard, S. Berthon-Fabry, J.R. Regalbuto, J.-P. Pirard, 2010, Catal. Today, in press. [10] N. Job, M.F.R. Pereira, S. Lambert, A. Cabiac, G. Delahay, J.-F. Colomer, J. Marien, J.L. Figueiredo, J.-P. Pirard, 2006, J. Catal., 240 (2), 160-171. [11] F. Maillard, M. Eikerling, O.V. Cherstiouk, S. Schreier, E. Savinova, U. Stimming, 2004, Faraday Discuss., 125, 357-377. [12] F. Maillard, S. Schreier, M. Hanzlik, E.R. Savinova, S. Weinkauf, U. Stimming, 2005, Phys. Chem. Chem. Phys., 7 (2) 385-393. [13] A.J. Bard, L.R. Faulkner, Electrochemical methods: fundamentals and applications, Wiley, New-York, 1992, p. 283. [14] H.H. Holscher, W.M.H. Sachtler, 1966, Discuss. Faraday Soc., 41, 29-42. [15] F. Maillard, E.R. Savinova, U. Stimming, 2007, J. Electroanalytical Chem., 599 (2), 221-232. [16] B. Andreaus, F. Maillard, J. Kocylo, E. R. Savinova, M. Eikerling, 2006, J. Phys. Chem. B, 110 (42), 21028-21040.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Catalytic wet air oxidation of succinic acid over monometallic and bimetallic gold based catalysts: Influence of the preparation method Radka Nedyalkova, Michèle Besson and Claude Descorme* Institut de recherches sur la catalyse et l’environnement de Lyon (IRCELYON), UMR 5256 CNRS – Université de Lyon, 2 avenue A. Einstein, 69626 Villeurbanne, France *E-mail : [email protected]

Abstract Different methods for the preparation of gold catalysts (mono and bimetallic) were used – the modified deposition-precipitation (MDP), the deposition-precipitation by ammonia (DPA) and the colloidal method (CM). The catalytic performances of all samples were evaluated in the catalytic wet air oxidation (CWAO) of succinic acid under mild conditions (190°C, 50 bar total pressure). The results showed that the preparation procedure and the addition of a second metal (Pt or Ru) clearly influence the catalytic activity and selectivity, depending on the size of the gold particles and the nature of the second metal. Keywords: Au-Pt(Ru) bimetallic catalysts, catalytic wet air oxidation (CWAO), organic compounds

1. Introduction Gold has long been disregarded for catalytic applications, due to its inert nature in the bulk state. Since Haruta’s [1,2] discovery of the remarkable activity of supported gold nanoparticles in oxidation reactions, different methods to prepare highly active gold catalysts have been developed. The same group has developed/adapted four different techniques that allow the deposition of gold nanoparticles on certain metal oxides: the co-precipitation (CP), the co-sputtering, the deposition-precipitation (DP) and the gasphase grafting [3]. Between all techniques developed so far, it appears that the deposition-precipitation is the most successful for preparing highly dispersed Au catalysts. The DP method has numerous variations, depending on the pH, the temperature of deposition, the precipitation agent and the state of the support (oxide or hydroxide). However, the DP method still cannot completely avoid the adsorption of gold hydroxyl chlorides species onto the support or the wrapping of chlorides in the precipitate, which may cause the deactivation of the Au catalysts. Grunwaldt et al. [4] first developed a two-stage method, based on Au colloids, for the preparation of Au/TiO2 and Au/ZrO2 catalysts employing tetrakis(hydroxymethyl)-phosphonium chloride (THPC) as the reducing and capping agent. Later, Porta et al. [5] employed poly(vinylalcohol) (PVA) as the protective agent and prepared Au/C and Au/TiO2 catalysts. The simplicity of the colloidal method and the possibility to prepare chlorine free catalysts are the main advantages, motivating his large application not only for the preparation of mono but also bimetallic catalysts. The interest towards bimetallic heterogeneous catalysts is increasing since the presence of a second metal can influence the catalytic properties, improving their activity, stability and/or selectivity. The presence of one less reducible component, which strongly interacts with the support,

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may stabilise the second, more noble metal, in the highly dispersed state. Recently, Pd and Pt were used for the preparation of colloidal bimetallic gold catalysts and apply in different oxidation reactions [6]. Furthermore, the catalytic properties of metal-oxidesupported gold catalysts strongly depend on the nature, the texture and the structure of the support. The support must present a defective surface available for strong interactions with the gold precursor. It is well known that reducible metal oxide supports (TiO2, Fe2O3, Co3O4), supplying reactive oxygen to the active gold sites, are more active in oxidation reactions than non reducible supports. Ceria has been regarded as one of the most important component in many catalytic systems due to its remarkable redox properties [7]. Recently, it has been shown that gold catalysts supported on ceria exhibit higher activity in the succinic acid wet air oxidation than Au/TiO2 [8]. Evidence was provided that the activity depended on the gold particle size. From the environmental point of view, the removal of toxic organic compounds from aqueous wastewaters is drawing a lot of attention and the wet air oxidation (WAO) is a suitable technology for that. The main disadvantage is that WAO requires high temperature and pressure (200-350°C, 70-230 bar), conditions that severely affect the economics of this technology. Using a catalyst, the operating conditions can be made significantly milder (120-220°C, 5-50bar). Since heterogeneous catalysts might easily be removed, their development and optimization has been the subject of several works in the recent decades. For the first time, Besson et al. [9] have reported that the Au/TiO2 catalyst is a promising candidate in the CWAO of succinic acid. The main disadvantage is that gold catalysts are not stable upon long term reactions and recycling. The important challenge is then to get stable gold catalysts in the CWAO. Deactivation may occur by sintering, poisoning of the active sites or “fouling” of the catalyst surface by adsorption of intermediate reaction products. Also, in hot acidic environments, the active components might be dissolved into the liquid phase (leaching). In order to reduce leaching and prevent the gold particles from sintering, the active phase might be incorporated into a catalyst support. On the other hand, the presence of a second metal may induce significant changes in both activity and stability. In our study, Pt and Ru were chosen as the second metal. Both concepts are the basis of the present study aiming to achieve an active and stable gold catalyst in the CWAO of succinic acid.

2. Experimental 2.1. Catalyst preparation A total of six gold catalysts were prepared by different methods. The total metal loading was fixed at 3wt.%. Two monometallic gold on ceria catalysts were prepared by the modified deposition precipitation method (MDP) and the deposition-precipitation by ammonia (DPA). First of all, the Ce(NO3)3.6H2O aqueous solution was precipitated with K2CO3 at 60°C and pH=9. In the case of the MDP, just after aging of the Ce(OH)4 precipitate for 1h at 45°C, the deposition of HAuCl4 was performed at pH=7. The resulting precipitates were aged for 1 h at 45°C, filtered and washed until no Cl - and NO3- could be detected. For the DPA method, the as prepared Ce(OH)4 precipitate was carefully washed to eliminate the K+ and NO3- ions and dried at 80°C. Noteworthy, to achieve a 3wt.% metal loading, a test was performed to determine the weight lost upon the transformation from Ce(OH)4 to CeO2 at 400°C for 2h. After that, the deposition of gold was performed as follows: the support was suspended in deionizer water, the gold precursor (HAuCl4.3H2O 6 10-4 M) was added and pH was adjusted to 11 with ammonia and maintained for 1 h. After washing and drying at 80°C under vacuum, the solid was

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calcined under flowing air (6 L h-1) at 400°C for 2h. Three bimetallic gold catalysts were synthesized using the MDP method: 2wt.%Au-1wt. %Pt/CeO2 MDP I, 2wt.%Au1wt.%Pt/CeO2 MDP II and 2wt.%Au-1wt.%Ru/CeO2 MDP II. I indicates that the two salts (HAuCl4 and Pt(NH3)4(NO3)2) were introduced simultaneously, while II indicates that Pt or Ru, respectively, was introduced first. As a precursor ruthenium nitrosil nitrate was used. After aging for 1h at 45°C, the received precipitate was washed carefully, until no Cl- and NO3- could be detected, and dried at 80°C under vacuum. The solids were calcined at 400°C for 2h under flowing air (6 L h-1). One bimetallic catalyst noted 2wt.%Au-1wt.%Ru/CeO2 CM was prepared via the colloidal method as follows: the gold precursor was dissolve in 400 mL deionised water in the presence of polyvinyl alcohol (PVA 2wt.% solution) under vigorously stirring. After that, the ruthenium precursor was added and the slurry was kept under stirring for 3 min. Then, the 0,1M NaBH4 solution, freshly prepared, was added to the solution to obtain a colloidal sol. Once the sol was obtained, the immobilisation on the Ce(OH)4 support was carried out for 2h. The resulting solid was centrifuged, carefully washed, dried at 80°C under vacuum and finally calcined at 400°C for 2h under flowing air. For the bimetallic catalysts, the thermal treatment is an important step, not only as far as the transformation of Ce(OH)4 into CeO2 is concerned but considering the interaction between the metal particles and the support. Before reaction all samples were reduced at 300°C for 2h under flowing H2 (12L h-1).

2.2. Characterization of the catalysts XRD patterns were obtained on a Siemens D5005 diffractometer (Cu Kα, 0.15406 nm). The metal concentration in the liquid phase after 8 h reaction was repeatedly measured by ICP-OES. The metal concentration in the solution was systematically lower than 0.5 ppm (detection limit), indicating that no leaching occurred.

2.3. Catalytic activity Experiments were carried out in a 300 mL autoclave made of Hastelloy C22 (model 4836, Parr Instrument Inc.). In a typical run, the autoclave was loaded with 150 mL succinic acid aqueous solution (5 g L-1, i.e. initial total organic carbon (TOC) = 2032 mg L-1) and 0.5 g catalyst. After the reactor was out gassed under argon, the mixture was heated to the reaction temperature (190°C) under stirring. Then, the stirrer was stopped and air was admitted into the reactor until the predefined pressure was reached (50 bar total). The reaction finally started when the stirrer was switched on again. This point was taken as ‘‘zero time’’ and one sample was withdrawn to measure the exact initial concentration of succinic acid. The liquid samples were periodically withdrawn from the reactor, centrifuged to remove any catalyst particle in the liquid sample and further analyzed. The substrate and the reaction intermediates (acetic acid and acrylic acid) were analyzed by HPLC (Shimadzu) using an ICSep Coregel 107H column. The mobile phase was a 0.005N H2SO4 aqueous solution (0.5 mL min-1). The HPLC system was equipped with a UV–vis detector set at 210 nm. The TOC in the liquid samples was measured with a Shimadzu 5050 TOC analyzer after subtraction of the inorganic carbon (IC) contribution from the total carbon (TC). Furthermore, the carbon mass balance in the liquid phase could be checked by comparing the TOC values with the total carbon concentrations in the liquid phase derived from the HPLC analysis.

3. Results and discussion Table 1 summarizes the metal loadings as measured by ICP. These results show a good agreement between the theoretical and experimental values for Au and Ru, especially

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for the catalysts prepared by DPA and CM, keeping in mind that the support was in the form of Ce(OH)4 and transformed into the oxide form during the calcination in air at 400°C. In the case of Pt, the experimental values are almost twice lower than the expected values. This unusually low platinum loading could indeed be connected with the nature of the precursor, whom precipitation at pH 7 and 45°C was not complete. Andreeva et al. [10] have applied the same method for the preparation of 3wt.%Au/CeO2 catalysts, but at higher temperature (60°C). They found that Ce3+ ion act as a reducing agent, converting Au3+ to Au0 during the preparation. In turn, Ce3+ ions are oxidized to Ce4+. Using XRD [10], the average gold particle size was estimated to be about 15 nm. In our study we decreased the temperature to 45°C in order to slow down the reduction process and decrease the gold particle size. At such low temperature and pH 7, the gold loading decreased slightly and part of the Pt was again lost upon washing since the Pt complex was not fully hydrolyzed. However, lowering the precipitation temperature, the average gold particle size decreased to 8 nm (Table 1). Table 1. Catalysts chemical composition measured by ICP-OES and average gold particle size derived from XRD measurements. Samples

Au , wt%

Pt, wt%

Ru, wt%

DAu a, nm

3wt.%Au/CeO2 MDP

2.7

-

-

8.0

3wt.%Au/CeO2 DPA

2.9

-

-

6.0

2wt.%Au-1wt.%Pt/CeO2 MDP I

1.74

0.45

-

25.0

2wt.%Au-1wt.%Pt/CeO2 MDP II

1.78

0.40

-

10.0

2wt.%Au-1wt.%Ru/CeO2 MDP II

1.82

-

0.85

10.0

2.0

-

0.8

6.0

2wt.%Au-1wt.%Ru/CeO2 CM a

derived from XRD diffractograms using Debye-Scherrer equation

Figure 1 compares the XRD patterns of mono and bimetallic catalysts. For all samples, the diffraction lines for CeO2 are typical of the cubic structure of fluorite type oxides. In the case of the monometallic catalysts, the main line characteristic for Au (2θ=38.2°) is more intense for the catalyst prepared by MDP than DPA, indicating that gold was better dispersed on the catalyst synthesized by DPA. In the case of bimetallic catalysts, although the diffraction lines characteristic for Au, Pt and Ru are very close in position, we could clearly see the difference between the MDP I and MDP II preparation routes. Bimetallics prepared by mixing the two salts led to catalysts with a lower dispersion (DAu=25 nm). On the opposite, when the salts were precipitated one after the other, the gold particles dispersion was improved (DAu=10 nm). Finally, gold particles of approximately 6 nm were obtained by the colloidal method, probably because of the presence of the protecting agent (PVA) that could stabilized the gold colloids at a higher dispersion state. It is noteworthy that, as far as the amount of Ru and Pt was 1wt.% and 0.5wt.%, respectively, that is below the detection limit for XRD, the dispersion of the second metal could not accurately be estimated from the XRD patterns.

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3 2

CeO2

CeO2 Au

Au Pt

CeO2

CeO2

CeO2 Au

CeO2

B

Intensity, [a. u.]

A

CeO2

Intensity, [a. u.]

CeO2

Catalytic wet oxidation of succinic acid

4 3 2 1

1 20

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2θ, deg

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Figure 1. XRD patterns for: A – (1) - pure support, (2) - 3Au/CeO2 MDP, (3) - 3Au/CeO2 DPA; B - (1) - 2Au-1Pt/CeO2 MDP I, (2) - 2Au-1Pt/CeO2 MDP II, (3) – 2Au-1Ru/CeO2 MDP II, (4) – 2Au-1Ru/CeO2 CM.

The catalytic performances of the monometallic gold catalysts in the catalytic wet air oxidation of succinic acid are presented on Fig. 2 and on Fig. 3 for the bimetallic Au-Pt and Au-Ru catalysts. As a preliminary test, a blank was performed to confirm that succinic acid is stable under the applied reaction conditions in the absence of catalyst. Furthermore, it is known that succinic acid might be intermediately degraded to acetic and acrylic acids or directly mineralized into CO2 and H2O [11]. In the presence of the ceria support 100% succinic acid conversion was achieved after 6h. Acrylic acid concentration was systematically very low and was not reported in the figures. 30

20

-1

without catalyst CeO2 3Au/CeO2 MDP 3Au/CeO2 DPA

30

Acetic acid, mmol L

Succinic acid, mmol L-1

40

10

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10

CeO2 3Au/CeO2 MDP 3Au/CeO2 DPA

0

0 0

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Figure 2. Evolution of the succinic and acetic acid concentrations as a function of time upon the CWAO of succinic acid over pure ceria and monometallic catalysts.

For the monometallic Au catalysts, 100% conversion was reached after only 3h for the catalyst prepared by DPA and 4h for the corresponding one prepared by MDP.

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30

Acetic acid, mmol L-1

Succinic acid, mmol L-1

40

without catalyst 2Au-1Pt/CeO2 MDP I 2Au-1Pt/CeO2 MDP II 2Au-1Ru/CeO2 MDP 2Au-1Ru/CeO2 CM

30

20

10

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10

2Au-1Pt/CeO2 MDP I 2Au-1Pt/CeO2 MDP II 2Au-1Ru/CeO2 MDP II 2Au-1Ru/CeO2 CM

0

0 0

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8

0

1

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4

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Time, h

Time, h

Figure 3. Evolution of the succinic and acetic acid concentrations as a function of time upon the CWAO of succinic acid over bimetallic catalysts.

The acetic acid concentration reached 26.8 and 29.0 mmol L-1 in the case of the DPA and MDP catalysts, respectively. In the case of the bimetallic catalysts, the highest activity was observed for the 2wt.%Au-1wt.%Ru/CeO2 MDP II catalyst. 1.0

0.6 0.4

CeO2

0.2 0.0 0.0

0.2

0.4

acetic acid acrylic acid mineralization

0.8

Yield to

Yield to

0.8

1.0

acetic acid acrylic acid mineralization

0.6

0.8

0.6 0.4 0.2

1.0

3Au/CeO2MDP

0.0 0.0

0.2

0.4

0.6

0.8

1.0

1.0

Yield to

0.8

acetic acid acrylic acid mineralization

0.6 0.4 0.2 0.0 0.0

3Au/CeO2DPA 0.2

0.4

0.6

0.8

1.0

Overall conversion Figure 4. Distribution of the reaction products as a function of the overall conversion.

Comparing the curves for the reaction product distribution as a function of the overall conversion (Fig. 4) for the ceria support and the monometallic gold catalysts, the direct mineralization pathway becomes predominant in the case of the catalysts. The faster direct mineralization for the DPA catalyst is certainly to be connected with the higher dispersion of the gold particles. These results are in good agreement with the results reported by Bond et al. [12] who showed that the catalytic activity rapidly increases as the gold particle size decreases. Another phenomenon concerns the reaction mechanism: acrylic acid was not formed as an intermediate product in the presence of highly dispersed gold particles. For the bimetallic catalysts, the reaction pathway

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depends of a nature of the second metal (Fig. 5). In the presence of Pt, no acrylic acid was detected, while for Ru the production of acrylic acid was much more significant. 1.0

Yield to

0.8

1.0

acetic acid acrylic acid mineralization

0.8

0.6

0.6

0.4

0.4

0.2

acetic acid acrylic acid mineralization

0.2

2Au-1Ru/CeO2 MDP II

2Au-1Pt/CeO2 MDP II 0.0 0.0

0.2

0.4

0.6

0.8

1.0

1.0

Yield to

0.8

0.0 0.0

0.2

0.4

acetic acid acrylic acid mineralization

0.8

0.6

0.6

0.4

0.4

0.2

0.8

1.0

acetic acid acrylic acid mineralization

0.2

2Au-1Ru/CeO2 CM

2Au-1Pt/CeO2 MDP I 0.0 0.0

0.6

1.0

0.2

0.4

0.6

Overall conversion

0.8

1.0

0.0 0.0

0.2

0.4

0.6

0.8

1.0

Overall conversion

Figure 5. Distribution of the reaction products as a function of the overall conversion.

The most active 2wt.%Au-1wt.%Ru/CeO2 MDP II catalyst was then submitted to a stability test (Fig. 6). Noteworthy, as the experiments were carried out in a batch reactor, the stability was studied by recycling the catalyst. For that reason, the first run was repeated three times in order to recover enough catalyst to perform a second run using the same amount of catalyst (0.5g). After every run, the catalyst was washed with cold water and dried overnight in air at 80°C. The results obtained upon three independent runs performed on the fresh catalyst showed a perfect reproducibility. Furthermore, deactivation was observed upon recycling. The total organic carbon (TOC) at the end of the 8 h run increased from 125 to 450 ppm. A similar deactivation was observed by Besson et al. [9] on a 2.2wt.%Au/TiO2 catalyst prepared by DP using NaOH and tested under the same reaction conditions. To get a better idea about the possible reasons for this deactivation, the catalyst was reduced again at 300°C for 2h under flowing H2 (12 L h-1) in between the two runs. The results showed that deactivation is partially reversible and might be connected with the metal particle surface re-oxidation. In that case, the TOC in the liquid phase after 8 h reaction reached 280 ppm. No Au or Ru leaching could be detected (< 0.1 ppm).

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1000 500 0

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3

4

5

6

7

8

Time, h Figure 6. Stability test of the 2Au-1Ru/CeO2 MDP II catalyst in the CWAO of succinic acid. Full symbols : first run (three independent tests); … dried only; 7 re-reduced

CeO2

CeO2

1500

CeO2

COT, mg L-1

2000

Au

only dried after direct reduced after direct direct direct direct

Intensity, [a.u.]

2500

CeO2

R. Nedyalkova et al.

2 1

20

30

40

50

2θ, deg

60

70

80

Figure 7. Comparison of the XRD patterns of the fresh (1) and re-reduced (2) 2wt.%Au1wt.%Ru/CeO2 MDP II catalyst

In Fig. 7 the XRD patterns of the fresh and used catalysts are compared. A slight increase in the gold particle size might be evidenced. As a conclusion, gold sintering is mainly responsible for the observed deactivation. This phenomenon is certainly related to the intrinsic instability of gold at elevated temperature which might somehow be related, in combination with particle size effects, to the lower melting point of bulk gold compared to other noble metals.

4. Conclusions The CWAO of succinic acid under mild reaction conditions over monometallic and bimetallic gold catalysts strongly depends on the applied preparation method. High dispersion of gold resulted in higher performances. The presence of a second metal has a beneficial effect on the catalytic activity and stability. The reaction product distribution was also affected by the nature of the second metal.

Acknowledgments We gratefully acknowledge the financial support from the Agence Nationale de la Recherche (Project ANR Blanc CatOxOr).

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

M. Haruta, N. Yamada, T. Kobayashi, S. Iijima, J. Catal. 115 (1989) 301. M. Haruta et al., J. Catal. 144 (1993) 175 M. Haruta, Cattech 6 (2002) 102 J.D. Grunwaldt, C. Kiener, C. Wögerbauer, A. Baiker, J. Catal. 181 (1999) 223 F. Porta, L. Prati, M. Rossi, G. Scari, J. Catal. 211 (2002) 464 N. Dimitratos, C. Messi, F. Porta, L. Prati, A. Villa, J. Mol. Catal A: Chem 256 (2006) 21 A. Trovarelli, Catal. Rew. Sci. Eng. 38 (1996) 439 N.D. Tran - PhD Thesis, IRCELYON (2008) M. Besson, A. Kallel, P. Gallezot, R. Zanella, C. Louis, Catal. Commun 4 (2003) 471 D. Andreeva et al., Appl. Catal. A: General 246 (2003) 29 J.C. Béziat, M. Besson, P. Gallezot, S. Durécu, Ind. Eng. Chem. Res. 38 (1999) 1310 G.C. Bond, C. Louis, D.T. Thompson, Catalysis by Gold, I.C. Press, London, 2006

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V.

Design of hierarchical functional porous mixed oxides from single precursors Arnaud Lemaire a and Bao-Lian Su a,b a

Laboratory of Inorganic Materials Chemistry (CMI), University of Namur (FUNDP), 61 rue de Bruxelles, 5000 Namur, Belgium. b State key Laboratory of Advanced technology for Materials Synthesisand Processing, 122 Luoshi Road, Wuhan, China.

Abstract Porous materials have been prepared via a single-source pathway. A one-step synthesis pathway has been developed for the design of hierarchically structured macromesoporous aluminosilicates with high tetrahedral aluminium content from a single molecular alkoxide precursor already containing Si-O-Al bonds (sec-BuO)2-Al-O-Si(OEt)3. The compensation of the cleavage of the intrinsic Al-O-Si linkage is successfully achieved by using highly alkaline media and the employment of reactive silica co-reactants, or aluminium selective chelating agents, leading to aluminosilicate materials with Si/Al ratios close to one and very high proportion of tetrahedral aluminium species. The macro-mesoporosity was spontaneously generated by the hydrodynamic flow of solvents released during the rapid hydrolysis and condensation processes of this double alkoxide. Secondly, recent advances in the conception of mesoporous zirconosilicate with homogeneous repartition of zirconium into the silicate structure (Si/Zr ~ 4), achieved by the use of Zr[OSi(OsBu)3]4 molecular precursor, are presented. Keywords: homogeneous mixed-oxides, self-formation phenomenon, aluminosilicates, zirconosilicates

1. Introduction Zeolites are successfully used in many industrial processes (such as the cracking of petroleum feedstocks), due to their various compositions and excellent textural characteristics. Nevertheless, present zeolites have a limited pore size (below 2 nm) which restricts their usefulness to processes involving small molecules. Mesoporous structures with broader pore size and high surface areas were then developed. However, this class of molecular sieves, mainly based on silica, exhibits a lack of active sites and is often limited to the use as catalyst supports. Consequently, many efforts are now devoted to the homogeneous insertion of the higher number of active sites inside the silica matrix, such as trivalent aluminium atoms, leading to acido-basic properties. This work reports first the synthesis of aluminosilicate materials almost constituted of Al-O-Si linkages and featuring a double sized-porosity, by the controlled polymerisation of a single molecular precursor. This part is followed by a brief report about the design of a mesoporous and homogeneous zirconosilicate material with an important amount Zr-O-Si bonds, by the similar use of a single molecular precursor.

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2. Macro-mesoporous aluminosilicate materials Integrating macroporosity in catalysis would permit better mass transfer to the active sites situated in the micro- and mesopores that are contained within the material’s walls, especially when large molecules are used. It is envisaged that these catalysts, with multiple porosity hierarchy, will allow in the near future the diffusion of the heavier oil fractions across the solids because of their macroporosity. Recently, a self-formation phenomenon of porous hierarchy which yields hierarchical macro-mesoporosity within oxide materials without the need of any physical templating agent, has been described [1]. The multiple porosity structure can be auto-generated by the brutal release of a porogen during the fast reactions of hydrolysis and polycondensation. The simultaneous polymerisation of two independent alkoxide precursors (alkoxisilane and aluminium alkoxide) leads to hierarchical macro-mesoporous aluminosilicate materials with interesting textural properties for catalytical applications [2]. Nevertheless, due to the very important differences of polymerisation rate between the two alkoxide precursors, inhomogeneous materials, constituted of separate oxides (Al2O3 and Si-O-Al in poor proportion) are obtained. The idea was to conserve the alkoxide ability to spontaneously generate macro-mesoporous structure, but to improve the incorporation of aluminium into the silicate framework. This was achieved by the use of the single molecular precursor (sec-BuO)2-Al-O-Si(OEt)3, which is containing both a pre-formed Al-O-Si linkage and alkoxy functions. This pre-formed Al-O-Si bond is very sensitive to hydrolysis, resulting in some ruptures and the preferential polymerisation through Al-O-Al linkages. Control of synthesis conditions is necessary to ensure the achievement of an only Al-O-Si linkages constituted material. Consequently, several strategies were employed such as the use chelating agents or inorganic silica co-reactant in high alkaline media.

2.1. Macro-mesoporous aluminosilicate materials from chelating agents An alkaline media, known to favours the conversion of aluminium precursors into monomeric (AlO4)- species, was combined with carboxylate anions as they are ideal chelating agents for aluminium atoms. The use of such anions as controlling agent of the polymerisation rate of aluminium in water has been well documented in the literature [3]. Upon hydrolysis, most of alkoxy groups are quickly substituted by water, while stronger complexing ligands will be less easily removed during hydrolysis steps. Thus, the polymerisation rate of the aluminium functionalities is reduced and prevents the cleavage of the Al-O-Si linkage. Some different carboxylate molecules were investigated (sodium acetate, sodium l-lactate, sodium oxalate, sodium citrate, sodium ethylenediaminetetraacetate, and a long alkyl chained carboxylate molecule; sodium caprylate). In these carboxylates, the number of carboxylate functions and the length of the hydrocarbon chains differ, which is impacting the aluminium incorporation into the silica framework. Among those synthesis, best results in terms of aluminium incorporation have been obtained with the sample prepared from a 1:1 (BusO)2-Al-O-Si(OEt)3/sodium citrate molar ratio. For sake of clearness, only this material, named CaCi13-1, was presented here. Ca-Ci13-1 material was prepared and characterised as follow : 4.3 g of sodium citrate dihydrate (99%) (chelating agent/Al = 1) were added in 60.0 ml of pH 13.0 sodium hydroxide solution (NaOH, 98%). After dissolution, 5.0 g of commercially available (BusO)2-Al-O-Si-(OEt)3 (Gelest, 95%) was slowly added dropwise into the above solution under very slow stirring. After 1 hour, the mixture was transferred into a Teflon-lined autoclave and heated at 80°C for 24 hours. The solid product was then filtered, washed, dried (40°C) and calcined (650°C, 10 hours, ambient atmosphere). Transmission electron microscopy (TEM) experiments were performed on a Philips

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Tecnaï-10 microscope at an acceleration voltage of 80 kV with powder samples embedded in an epoxy resin and ultramicrotomed. The N2 adsorption-desorption isotherms were measured at –196°C with a volumetric adsorption analyzer Micromeretics Tristar 3000. The macroporous array was studied using a JEOL FESEM scanning electronic microscope (SEM) with conventional sample preparation and imaging techniques. Mercury intrusion-extrusion curves and corresponding pore size distributions were collected with a Micromeritics Autopore IV. Finally, the environments of the Al and Si atoms were studied by means of 27Al and 29Si MAS NMR spectroscopies with a Bruker Avance 500 spectrometer and the Si/Al ratios were investigated using a Philips PU9200X atomic absorption spectrometer. SEM images in Fig. 1 (a and b) show Ca-Ci13-1 particles constituted of an irregular and open array of 3D interconnected macrochannels with openings ranging from 1 to 3 µm and separated by thick walls of about 3 µm large. Note that this macrostructure morphology is completely independent of the carboxylate nature of the added chelating ligand. The presence of this macrostructure inside the particles as well as the observation of a disordered mesoporosity (5–10 nm) inside macroporous walls is confirmed by cross-sectional TEM images presented in Fig. 1 (c and d).

3 µm

Fig. 1. SEM images (a and b) and TEM images (c and d) of Ca-Ci13-1 material.

As regards the development of macroporous structure, work previously carried out in our laboratory suggests a mechanism based on the synergy between the polymerisation kinetics of the inorganic precursors and the hydrodynamic flow of the solvent. The drop of (BusO)2-Al-O-Si-(OEt)3 added to the aqueous media polymerises quickly, which releases alcohol molecules. As the reaction progresses, more and more solvent molecules are generated, which in turn can produce microphase-separated domains of aluminosilicate-based nanoparticles and water/alcohol microdrops which will be converted in macrochannels. The solid structure would then grow around these channels until the single molecular precursor is depleted, resulting in a macroporous particle. The disordered mesoporosity arises from voids developing between the aluminosilicate nanoparticles as they begin to aggregate. These macropores are very different from those encountered for materials prepared from of two separate aluminium and silica alkoxide precursors (straight and parallel macrochannels) [2]. This observation could be explained by the presence of an alkoxysilane function that slows down the hydrolysis and polycondensation steps of the single molecular source. The inorganic phase surrounding water/alcohol channels harden more slowly, allowing a greater isotropy within the progression of solvents macrochannels. This may explain the non linear direction, the larger void volume and the thin partitions of these macrochannels.

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The coordination environments of Al atoms in the calcined material CaCi13-1 was characterised by 27Al MAS NMR. The spectrum showed at the Fig. 2(a) is presenting a very homogeneous aluminosilicate material only constituted of intra-framework aluminium species (59 ppm). As the Si/Al ratio is equal to 1.1, this material could be considered as an almost pure aluminosilicate material, only constituted of Al-O-Si linkages. For more accurate confirmations about the Al-O-Si repartition into the CaCi13-1 material, the chemical environment of Si was studied by 29Si MAS-NMR technique. The spectrum shows at the Fig. 2(b), in contrast to pure silica materials, evidence of a significant shift toward the lower field, such as very low silica content zeolites do, and assigned to silicate species surrounded by aluminium atoms [4]. This 29 Si MAS-NMR spectrum consist in a large peak, ranging from -75 ppm to -105 ppm, which could be decomposed into 3 peaks (Si(OAl)4 at -85 ppm, -Si(OAl)3 at -90 ppm and =Si(OAl)2 at -95 ppm), meaning that the aluminosilicate framework is constituted of a major part of Si(OAl)4 species. (a)

(b)

Fig. 2. (a) 27Al MAS NMR and (b) 29Si MAS NMR spectra of the Ca-Ci13-1 materials.

The textural property of the prepared CaCi13-1 material was assessed by N2 adsorption-desorption measurements. Fig. 3(a) is showing an isotherm which is relatively well matching to a type IV isotherm (according to the IUPAC classification), characteristic of mesoporous compounds. This one exhibits a capillary condensation step that can be seen at relative pressures (p/p0) of about 0.55-0.80 indicating the presence of mesopores. The specific surface areas have been calculated by the BET method giving value of 142 m²/g. The analysis of the pore size distributions (Fig. 3(a), inset), calculated by the BJH method from the adsorption branch of the isotherm, reveals a fairly broad distribution ranging from 5 to 10 nm, which is in good agreement with TEM observations. Log Differential Intrusion (ml/g)

Adsorbed volume (cm³/g - STP)

200

150

100

0

2

4

6

8

10 12 14 16 18 20

Pore size distribution (nm )

50

(a)

(b)

1 to 3 µm 5 to 10 nm

0 0 .0

0 .2

0 .4

0 .6

R e la tiv e p re s s u re (p /p 0 )

0 .8

1 .0

10

100

1000

Pore size distribution (nm)

10000

Fig. 3. (a) N2 adsorption-desorption isotherm and corresponding pore size distribution of Ca-Ci13-1 material, and (b) pore size distribution given by mercury intrusion analysis.

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Some more quantitative study of the macrostructure was performed via mercury intrusion-extrusion porosimetry. The pore size distribution obtained from this analysis, exposed at the Fig. 3(b), is clearly showing one peak located around 3 µm which could correspond to the macrostructure. This hypothesis is in good agreement with the previous SEM and TEM characterizations. The mercury extrusion curve appearance (data not shown) is the same than the intrusion curve, testifying of an interconnected macrostructure and a high pressure-resistant matrix.

2.2. Macro-mesoporous aluminosilicate material from inorganic silica co-reactants When a mixture of di-s-butoxyaluminoxytriethoxysilane and silica co-reactant is added to an aqueous media, the likelihood that polymerisation occurs between highly reactive, but less numerous aluminium sites and most widespread and less reactive silicon sites increase, favouring heterocondensation reactions (Al-O-Si) instead of Al-O-Al linkages. This was again combined with highly alkaline solutions. The added source of silica was the reactive tetramethoxysilane (TMOS). This co-precursor was intimately mixed in a 1:1 molar ratio with 5.0 g of (BusO)2-Al-O-Si-(OEt)3, and the mixture was added dropwise in 60.0 ml alkaline solutions (pH = 13.0 and 13.5). After 1 hour, mixtures were transferred into a Teflon-lined autoclave and heated at 80°C for 24 hours. The solid products were filtered, washed and dried (40°C). Materials were analyzed in a similar way that solids prepared at the point 2.1. These two materials, prepared from a molar ratio of (BusO)2-Al-O-Si-(OEt)3/TMOS = 1 at pH = 13.0 and pH = 13.5, are named TM13 and TM13.5 respectively. The comparison of those two materials prepared in quite identical synthetic conditions, but with a slight pH difference, illustrates the pH-dependence of both the macroporous array development and the aluminium incorporation into the tetrahedral silica network. Nevertheless, this point is not fully yet well understood and remains under investigations. The morphological structure of those materials was directly visualised by SEM analysis. The sample synthesised at pH 13.0 (TM13, SEM images at Fig. 4(a)), display material only constituted of highly spongy particles of ca. 10-20 µm which are fully comprised of very regular micrometre-sized macrovoids. These numerous regular 1 to 2 µm spherical voids are separated by thin walls and are found over the entire surface of the particle as well as within the particle. Fig. 4(b) represent SEM image of the material TM13.5 which shows a “reverse macrostructure”, consisting in the stacking of microsized (1-2 µm of diameter) hollow spheres. Image taken at higher resolution (Fig. 4(b), inset) exhibits fully independent hollow spheres, among plausible debris of destroyed hollow bubbles. The presence of those macrovoids within the particles is confirmed by TEM images (Fig. 5). The TEM images at Fig. 5(a) of the TM13 material highlight circular openings of ~ 2 µm large, surrounded by very thin walls of about 100 to 400 nm of thickness. A deeper look into the structure (Fig. 5(a), inset) reveals a vermicular mesoporosity contained into the walls separating macrovoids. Fig. 5(b) present the case of the TM13.5 material, prepared at a higher pH (13.5) value. The TEM images, taken at different level of magnifications, confirm that the macroporous material seems to be constructed by the stacking of independent and hollow microspheres of about 1 µm large, as it can be visualized by SEM at Fig. 4(b). Moreover, material prepared at pH 13.5 shows cavities between the stacked mesoporous aluminosilicate nanoparticles of 50 nm long, which could suggest a further interparticular porosity (Fig. 5(b) inset). Again, the morphology of the macrostructure is different from the one obtained by the use of two independent precursors or by chelating agents. An explanation could arise from the fact that, when pure metal alkoxides are carried into an aqueous solution

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by a dropwise addition, the very fast transformation of hydrophobic metal alkoxide into metal oxide is permitting the freezing of the droplet during the polymerisation process.

Small droplet shape generates important curved surface, which is applying some pressure on the water/alcohol microdroplets contained into the near surface of the metal alkoxide drop. This pressure should be the driving force responsible of the straight shaped macrostructure. Whereas, in the case of the mixture of di-s-butoxyaluminoxytriethoxysilane and silica inorganic co-reactant, even in the case of a very low dropwise addition, all the droplets of this mixture are gathering to form one unique homogeneous gelatinous cloud. In this “unique drop”, the surface/volume ratio is less important and no driving forces are applied onto the microdroplets of solvents, which are staying in static spherical shaped configuration during all the polymerisation process. Some investigations were realized by optical microscopy (data not shown), showing the apparition of those microbubbles of solvent during the polymerisation process. 27 Al MAS NMR spectra of the TM13 and TM13.5 materials are presented at Fig. 6(a and b). In presence of added TMOS, the final material possesses more important amount of tetrahedral aluminium relative to the octahedral proportion. When the pH of the solution increases to 13.5, the addition of TMOS to the single precursor allow the achievement of a very homogeneous material (Fig. 6(b)), only constituted of intraframework aluminium. 29Si MAS-NMR investigation of TM13 and TM13.5 is again showing evidence of the significant shift toward the lower field corresponding to very well mixed and enriched aluminosilicate materials. The 29Si NMR spectrum of the TM13.5 material is presented at the Fig. 6(c). The peak, midpointed around -87 ppm, seems to result in the contribution of two peaks, one centred on -85 ppm (Si(OAl)4) and the second on -90 ppm (-Si(OAl)3). This proposal is in good agreement with Si/Al ratio, provided by elemental analysis, which is close but not exactly equal to 1.0 (Si/Al = 1.3).

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(c) Si(Al)3

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Fig. 6. Al MAS NMR spectra of (a) the TM13 and (b) the TM13.5 materials, and (c) 29Si MAS NMR spectrum of the TM13.5 material.

Adsorbed Volume (cm³/g - STP)

The isotherm of TM13 material, exhibited in Fig. 7(a), is similar to type IV, characteristic of mesoporous compounds, with a capillary condensation step at p/p0 0.75. From these data sets, pore size distribution is calculated with maxima centred at ~ 5 nm (Fig. 7(a), inset). A high specific surface area of 315 m²/g is obtained. TM13.5 material presents a combination of type I and type II isotherms (Fig. 7(b)). This isotherm is characteristic of supermicroporous structures with a pore size inferior to 1.5 nm coupled with larger pores as is shown by the sharp increase in N2 adsorption located at high relative pressure (p/p0 ≥ 0.9). This confirms again the presence of a secondary porosity centred at ~ 30 nm for TM13.5 material (Fig. 7(b), inset), as it is observed by TEM imaging of the Fig. 5(b). Accessible surface area, provided by BET calculation, is about 80 m²/g. 450

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Intrusion-extrusion porosimetry measurements were done to characterize more correctly these macrostructures. Nevertheless, results are not well matching to the pore size distribution determined by SEM and TEM investigations. This is certainly due to the special morphology of those hollow shaped macrovoids. Indeed, since those macrovoids are not interconnected, mercury is not regularly intruded into pores, which is affecting the quality of results even if again, the appearance of the extrusion curve is well matching the intrusion curve aspect, meaning that those both materials are quite mechanically stable.

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3. Ordered mesoporous zirconosilicate High loading of zirconium into ordered mesoporous silica framework is still an amazing challenge but is difficult to achieve since the polymerisation rate of the two precursors are uneven. This is also affecting the interaction with the templating agent in the solution leading to weakly organized materials. Molecular precursor such as Zr[OSi(OsBu)3]4, prepared according to the literature [5], was converted, via the sol-gel process, into a zirconosilicate material with Si/Zr ~ 4 and SBET > 300 m²/g. Study of temperature and the pH during the process was required to obtain very homogeneous material with appropriate concordance of Si/Zr ratios and 29Si MAS NMR. Indeed, acidic solutions allow the hydrolysis of this single precursor, implying also too stable cationic complexes formation, leading to phase separated materials. A pH-adjustment step was required to favor heterocondensation reactions between these hydrolysed species. For a typical synthesis : 0.6 g of pure Zr[OSi(OsBu)3]4 was hydrolysed into a 10.0 ml acidic solution (pH = 0, HCl 37%) at 60°C. After a time period comprised between 4 to 12 hours, 10.0 ml of a NaOH solution is added to give a jelly solution with a pH of 12. After an ageing period of 5 days at 80°C, the material was filtrated, washed, dried at room temperature and finally characterized. Q2 The 29Si MAS NMR spectrum (Fig. 8) of this material is constituted of one large peak almost Q3 totally bared of Q4 silicon species attributed to pure silica phase (Si(OSi)4, -110 ppm). This observation, coupled with local and global Si/Zr ratios (Si/ZrEDX = 4.0, Si/ZrXPS = 4.93 and global Si/Zr = 3.91) can be interpreted as the synthesis of a very homogeneous -50 -60 -70 -80 -90 -100 -110 -120 -130 -140 δ Si (ppm) material, mainly constituted of Zr-O-Si linkages. 29

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Fig. 8. Si MAS NMR spectrum of homogeneous zirconosilicate material.

Hierarchically structured macro-mesoporous aluminosilicates containing a higher level of tetrahedral aluminium and Si/Al ratios close to one were successfully synthesised via a single-source molecular alkoxide precursor (sec-OBu)2-Al-O-Si-(OEt)3, in controlled conditions (chelating agent or silica co-reactants). Same concept is currently applied to the conception of highly ordered mesoporous zirconosilicate materials via the aqueous conversion of Zr[OSi(OsBu)3]4 into a homogeneous mixed oxide. Efforts are now devoted to the improvement of the mesostructuration by the use of templating agents.

References [1] Yuan, 2003, Surfactant-assisted synthesis of unprecedented hierarchical meso-macrostructured zirconia, Chem. Commun., 1558. [2] Léonard, 2004, A novel and template-free method for the spontaneous formation of aluminosilicate macro-channels with mesoporous walls, Chem. Commun., 1674. [3] van den Brand, 2004, Interaction of Anhydride and Carboxylic Acid Compounds with Aluminum Oxide Surfaces Studied Using Infrared Reflection Absorption Spectroscopy, Langmuir, 20, 15, 6308. [4] Lippmaa, 1981, Investigation of the Structure of Zeolites by Solid-state High-Resolution 29Si NMR Spectroscopy, J. Am. Chem. Soc., 103, 17, 4992. [5] Kriesel, 2001, Block Copolymer-Assisted Synthesis of Mesoporous, Multicomponent Oxides by Nonhydrolytic, Thermolytic Decomposition of Molecular Precursors in Nonpolar Media, Chem. Mater. 13, 10, 3554.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Hierarchical porous catalyst support: shaping, mechanical strength and catalytic performances S. Ould-Chikh,a S. Pavan,c A. Fecant,a E. Trela,a C. Verdon,a A. Gallard,a N. Crozet,a J-L. Loubet,c M. Hemati,b L. Rouleau,a a

IFP-Lyon, Direction Catalyse et Séparation, BP-3, 69360 Solaize, France, e-mail: [email protected], [email protected] b Laboratoire de Génie Chimique (LGC) de Toulouse - UMR5503, BP84232, 4 allée Emile Monso, 31432 Toulouse cedex 4, France, email: [email protected] c Laboratoire de Tribologie et Dynamique des Systèmes - UMR 5513, Ecole Centrale de Lyon, 36 avenue Guy de Collongue, 69134 Ecully cedex, France, email: [email protected]

Abstract Palladium layered catalysts were reported to have the best performances for selective hydrogenations. A new core/shell bimaterial consisting of low specific surface area α alumina beads coated by a high specific surface area γ alumina layer is proposed and is suitable for a controlled thickness deposition of metallic palladium particles. A coating process was developed for a pan granulator where γ alumina dry powder was added under a boehmite sol pulverisation onto α-Al2O3 beads. The material exhibits a uniform and non defective coating (20 µm), a strong resistance against general attrition and local mechanical properties of coating and interface measured by nanoindentation in the order of magnitude of conventional γ-Al2O3 beads. Metallic nano-particles deposited by incipient wetness impregnation of palladium nitrate solution are more preferentially located into the γ alumina shell. Activity and selectivity of Pd core/shell bimaterial catalyst are hugely improved compared to traditional catalyst (Pd deposited onto αAl2O3 beads) in selective hydrogenation of styrene/isoprene model mixture. This metallic core/shell catalysts are thus promising candidates for reactions sensitive to intra-particular diffusion limitations. Keywords: core/shell bimaterial, alumina support, indentation, Pd catalyst, selective hydrogenation

1. Introduction Monoenes, used in petrochemistry, are achieved by selective hydrogenation of a mixture of mono and polyunsaturated hydrocarbons produced invariably by conversion processes such as steam cracking of fluid catalytic cracking. It is well known that many heterogeneous metal catalysts exhibit high activity in the hydrogenation of the carbon– carbon double and triple bond. However, palladium is reported to be the most active and selective metal to achieve selective hydrogenations [1,2,3]. Recent developments have shown that hydrogenation selectivity and activity are very sensitive to palladium distribution on porous supports. Best performances were reported for egg-shell type catalysts where the palladium particles were deposited in crust [4,5]. This palladium distribution makes it possible to reduce intraparticular diffusion limitations. This kind of catalyst can be achieved for example by impregnation of metallic palladium nano-particles in a colloidal suspension onto porous alumina [6].

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The present study is the achievement of a broader one which aims at proposing a support ready for a controlled thickness deposition of metallic palladium particles by an easy incipient wetness impregnation of palladium nitrate solution. These new supports are spherical bi-materials whose core is a low specific surface area α alumina and the coating is a high specific surface area γ alumina. Indeed, γ alumina hydroxyls are protonated in contact with an acidic palladium solution. This leads to a pH shift of the solution and hence triggers the precipitation of palladium hydroxide nano-particles into the catalyst support [7,8]. The latter mechanism is made useful to locate palladium into the high specific surface area coating as the latter comprises the highest number of surface hydroxyls while keeping a catalyst with a globally low specific surface area which has been shown to be advantageous for selective hydrogenation [9]. We will describe here the elaboration of such a catalyst as well as the mechanical strength of such bi-material - crucial for an industrial application - and its catalytic performances.

2. Experimental section The core-shell bimaterials were prepared by coating of γ alumina powder (filler) and boehmite sol (binder) on α alumina beads and were analyzed by textural and mechanical characterization.

2.1. Coating procedure The binder was obtained by peptisation of boehmite (Pural SB3-Sasol) with nitric acid solution containing polyvinyl alcool (Carlo Erba). A dispersion (HNO3/AlOOH = 3.35 %(w/w), PVA/(PVA+AlOOH) = 2.5%(w/w), AlOOH/(AlOOH+H2O) = 3%(w/w)) was agitated during 2 h before removing the unpeptized boehmite by a centrifugation at 3800 g during 20 min. Coating device was a laboratory pan granulator GRELBEX P30 equipped with a cylindrical conical bowl. In the first step, 100 g of α alumina macroporous beads (1.6 mm, 9 m2/g, 64 nm, 0.24 mL.g-1) denoted Spheralite 537c (Spheralite 537–Axens calcined at 1273 K and sieved) were placed into the bowl under cascade state of flow at rotary speed of 40 rpm and 30° angle. Coating thickness was chosen to be 20 µm and corresponded to the use of 8.3 mL (6.4 g) of homemade mesoporous γ alumina powder (2 µm, 223 m2/g, 8 nm, 0.35 mL.g-1), considering the α−Al2O3 beads external surface and process efficiency. Coating procedure started with wetting of Spheralite 537c surface with the binder during 28 min with a volumetric flow rate of 1 mL.min-1. When the cascade state of flow was about to vanish, beads had received on their surface a liquid film large enough to collect efficiently the filler. Then, the filler volume was continuously added during 81 min with a volumetric flow rate of 1 mL.min-1 under the binder spraying and with an applied hot air flow (inlet temperature : 343 K). After adding the precursors, the coated cores were dried in a ventilated drying oven at 303 K during three days. Dried coated materials were calcined in a muffle furnace at 873 K for 2 hours in air with a heating rate of 3 K.min-1. Boehmite is then converted by a topotactic transformation into γ−Al2O3 and loose some structural water during this stage [10].

2.2. Textural characterization Pore size distributions of materials were determined Hg-porosimetry (Autopore 4Micromeretics). Specific surface areas and mesopore size distributions were calculated from nitrogen physisorption measurements (ASAP 2420-Micromeretics) by B.E.T. and B.J.H. mathematical treatment, respectively. Direct observations of raw materials and

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dried coated materials were performed by SEM (Supra 40-Zeiss). Microstructure was analyzed by SEM (Supra 40-Zeiss) on polished section of dried coated beads embedded inside an epoxy resin.

2.3. Mechanical characterization Ultralow load indentation, also known as nanoindentation, is a widely used tool for measuring the mechanical properties of thin films and small volumes of material. The principle is to pushing in a hard material tip called the indenter into the analyzed sample and to measure the curve load-penetration. A modified commercial nanoindenter (Nano indenter XP – MTS) was used to characterize coated materials. The device allows to measure the contact stiffness with superimposing a harmonic oscillation (small amplitude of 3 nm, constant frequency of 32 Hz) to the continuous penetration of the indenter into the sample. This specificity allows one to continually measure the elastic modulus and hardness according to the penetration depth. Loubet et al. demonstrated that reduced Young modulus and hardness for a Berkovich indenter with a dynamic measurement method could be deduced from the following equations [11]: E* =

S 2

H=

π

= 0,149.

Aind

(h

r'

S + h0 )

P P = Aind 35.36 × (hr ' + h0 )2

Equation 1 Equation 2

with E* is the reduced Young modulus (GPa), S the contact stiffness (N/m), Aind the indentation area for a Berkovich indenter (Aind = 35.36(hr’+h0)2), hr’ the plastic depth under loading (m), h0 the tip indenter defect (10-9 m), P the applied load (N). Coating and core characterization were respectively done with 100 mN and 450 mN maximum loads with a 3.10-2 s-1 loading rate. Seven indentations were performed on polished section of coated beads embedded inside a bakelite resin either for core and coating. An attempt for characterizing coating adhesion was undertaken with a diamond cube corner indenter. The principle relies on initiating and propagating a crack at the core/coating interface. To give a common parameter of adhesives properties, Chicot et al. proposed an apparent interfacial tenacity KIC (Pa.m-1/2) depending on the critical point load Pc (0.450 N), the length of propagated flaw c (m), a calibration parameter χV (0.015) and the Young modulus and hardness ratio (E/H)I of the interface [12]: 1/ 2

PC ⎛E⎞ K IC = χV ⎜ ⎟ 3/ 2 H c ⎝ ⎠I

Equation 3

As indentation is performed at coating/core interface (I), hardness and elastic properties of core (S) and coating (R) are concerned during loading. The following relation was proposed to take into account the latter mechanical properties: 1/ 2

1/ 2

⎛E⎞ ⎜ ⎟ ⎝ H ⎠I

1/ 2

⎛E⎞ ⎛E⎞ ⎜ ⎟ ⎜ ⎟ H ⎠S ⎝ ⎝ H ⎠R + = 1/ 2 1/ 2 ⎛H ⎞ ⎛ HS ⎞ ⎟⎟ 1 + ⎜⎜ 1 + ⎜⎜ R ⎟⎟ ⎝ HR ⎠ ⎝ HS ⎠

Equation 4

2.4. Catalysts preparation The preparation method used an aqueous solution of palladium nitrate (9.85%w/w, Engelhard). Incipient wetness impregnation was realized in a rotating beaker. 0.22 cm3 of palladium nitrate at adequate dilution were impregnated per gram of support corresponding to the total porous volume. After drying at 393 K, the catalysts were calcined under airflow at 473 K during 2 h. The palladium loading was 0.2 g.100 cm-3.

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2.5. Metallic dispersion Metallic dispersion of each catalyst was determined by measuring amount of CO chemisorbed at the metallic particle surfaces. Solids were previously reduced at 423 K under H2 flow, and return to room temperature was effected under helium flow. After 1h at 303 K under helium, CO pulses were operated and CO consumption was measured. Pd dispersion was then calculated assuming that the ratio between chemisorbed CO and metallic atoms is one. The following equation allows us to obtain metallic dispersion (percentage of surface atoms relative to total metal atoms) from CO consumption. D=

Va × M Pd % M × Vm × χ

Equation 5

where Va is adsorbed CO volume (mL.g-1), MPd molar weight of palladium (106.4 g.mol-1), Vm molar volume (24000 cm3 at 293 K), χ stoichiometric coefficient CO/Pd = 1, %M metallic weight.

2.6. Metal distribution Castaing micro-probe analysis was performed to determine the radial profile of Pd concentration along the diameter of the alumina beads. The preparation for the measurement includes an embedding in a metacrylate resin, polishing with SiC paper and coating with carbon black. Measurements were performed with a JEOL 8100 microprobe a semi-quantitative way.

2.7. Catalytic performances Mixture of styrene-isoprene in n-heptane was engaged in selective hydrogenation in liquid phase using a laboratory-scale stainless-steel and perfectly stirred batch reactor with variation of the concentration of reactants and products over time. 2 cm3 of shaped catalyst initially reduced under hydrogen flow at 423 K during 2 h, was transferred under Ar in a glove bag into the batch reactor filled with 210 mL of n-heptane. Catalyst beads were fixed in an annular basket located around the stirrer. The catalyst was then put into contact with about 34g of reactants (50%w isoprene, 50%w styrene) at 318 K under 35 bars of H2 and at a stirring velocity of 1600 rpm. A pressure gauge before the batch reactor maintains the pressure constant inside the reactor at 35 bars. The course of the reaction was followed by the loss of H2 pressure in the pressure gauge and by gas chromatography analysis (PONA column, split injector, FID detection). Experimental conditions were previously selected in order to avoid mass transfer limitations. Activities of catalysts were based on the rate of consumption of H2 for the hydrogenation of isoprene and styrene to 2-methylbutenes and ethylbenzene respectively before the formation of saturated product 2-methylbutane – formation of ethylcyclohexane is not detected in our conditions (Equation 6). Their selectivities in total hydrogenation were calculated in percentage relative to i-pentane formation (Equation 7). A(mol. min −1 .cm 3 ) =

VH 2 × PH 2 Vc × R × T

Equation 6

where VH2 is hydrogen volume (L), PH2 loss of pressure per minute (bar.min-1), VC catalyst volume (cm3), R molar gas constant (0.0829 L.bar.mol-1.K-1) and T temperature (K). S (%) =

% w(i − C5= ) × 100 % w(i − C5= ) + % w(i − C5 )

Equation 7

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where %w(i-C5=) 2-methylbutenes concentration (%) and %w(i-C5) 2-methylbutane concentration (%).

3. Results 3.1. Microstructural and textural characterization SEM micrographs indicate that dry deposited coating is continuous with a 20 µm homogenous thickness (Figure 1-a). The coating microstructure is granular as expected considering the coating formulation (Figure 1-b). The binder is homogenously dispersed between γ-Al2O3 grains and at γ-Al2O3 grains and α-Al2O3 core interfaces. Some very thin longitudinal cracks rise scarcely through the coating. The latter cracks are triggered by boehmite gel shrinkage during drying. a)

b)

Figure 1. SEM micrographs of dried coated Spheralite 537c : a) whole coated bead, b) coating microstructure.

The N2 physisorption and Hg porosimetry measurements of the calcined coated materials highlight a macroporosity (64 nm), and a mesoporosity (8 nm) brought by the core and the shell, respectively. The total specific surface area of the coated Spheralite 537c has increased up to 19 m2.g-1.

3.2. Mechanical characterization Figures 2-a and 2-b exhibit load-displacement curves obtained for core and coating. The spread of the results are due to the nature of our materials which belongs to porous ceramics. The average depth into the material is 3.4 µm. Even if the loading applied has been decreased to 100mN for the shell, the proximity of the indent and core induces a modification of the measured coating mechanical properties (Figure 2-d). This was taken into account in reduced Young modulus calculation by selecting first points obtained during loading (Figure 2-c). All the calculated mechanical properties are grouped in Table 1. Table 1. Mechanical properties of calcined coated Spheralite 537c (E was calculated with ν = 0.3). Localization Core Coating

Hmoy(GPa)

E*moy (GPa)

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59±5 9.4±1,5

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Ten core/coating interface loadings have produced only one crack initiation and propagation through the interface. This arises from the difficulty to accurately target the interface and from the limited applied strain by nanoindentation although a cube corner indenter was used. The measured crack length c is 43 µm. Using Equations 3 and 4, the apparent interface tenacity KIC was calculated to be ~0.135 MPa.m1/2. Courroyer et al. performed mechanical characterization of γ-Al2O3 beads commonly used in reforming operation [13]. Calculated mechanical properties were E=11.1±1.5 GPa, H=0.32±0.07 GPa, and KC = 0.178±0.021 MPa.m1/2. Comparing the mechanical properties of coating on Spheralite 537c with the latter values, it appears that Young modulus and hardness are in the same magnitude order. The apparent interface tenacity is also comparable to the bulk tenacity of γ-Al2O3 typical supports.

3.3. Catalytic performances Two Pd catalysts were prepared following the procedure indicated on experimental section, on γ-alumina coated Spheralite 537c (A) and on Spheralite 537c (B) as a reference. Properties of these materials are illustrated on Table 2. Both catalysts contain the same amount of palladium per volume. On Figure 3-a, solid exhibits a high palladium concentration into the 20-30 first micrometers from the external surface corresponding to the thickness of γ-Al2O3 coating. This means, as expected, that deposition of metal precursor was favored within the shell of high specific surface area γ alumina, whereas metal penetration occurred in much more amount along the whole diameter beads with catalyst on core Spheralite 537c (Figure 3-b). Besides, one can notice that catalyst A on core-shell bimaterial as support shows a higher metallic dispersion than on core Spheralite 537c. This could be explained by the preferential localization of metal for the first material in the outer shell of high specific surface area inducing stronger metal-support interactions during catalyst preparation causing then smaller particles than for catalyst B.

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Hierarchical porous catalyst support Table 2. Properties of Pd catalysts prepared on coated and non-coated Spheralite 537c. Catalyst A B

Carrier coated Spheralite 537c Spheralite 537c

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0.7 0.8 0.9 1.0 1.1

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Figure 3. radial profile of Pd concentration along the diameter of the alumina beads for catalyts prepared on a) coated Spheralite 537c and b) Spheralite 537c.

Performances of both catalysts were evaluated in terms of activities and selectivities. A twice higher activity was found for the catalyst A on core-shell bimaterial compared to catalyst B (Figure 4-a). A benefit in total hydrogenation selectivity was also measured especially from 60% to higher isoprene conversion (Figure 4-b). The preferred localization of metal into the outer layer for catalyst A may induced a decrease in internal mass transfer limitations. For the same solid, a higher metallic dispersion could allow more active sites to catalyze hydrogenation reactions. These two phenomenon could be at the origin of the gain in activity. Nevertheless, it seems difficult to figure out their respective contributions. Concerning selectivities, it is already known that selective hydrogenation of poly-unsaturated compounds are not favored with higher metallic dispersion catalysts (with small particles) [14]. Thus, the better selectivity observed with catalyst on coated Spheralite 537c is certainly due to lower intraparticular mass transfer limitations induced by metal profile of catalyst A. a) 8 b) 100 95

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Figure 4. catalytic performances in terms of a) activities and b) selectivities for catalysts prepared on coated and non-coated Spheralite 537c.

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4. Conclusion A bimaterial catalyst support was obtained in a pan granulator with a composite sol-gel formulation based on γ-Al2O3 filler, boehmite binder and α-Al2O3 beads. The resultant catalyst support shows a homogenous coating with a twenty micrometer thickness. Local mechanical properties of coating and interface are in the magnitude order of conventional γ-Al2O3 beads. Deposited metallic palladium nano-particles on this bimaterial are very preferentially located into the γ-Al2O3 coating as expected. Activity and selectivity of the bi-material catalyst show a huge improvement compared to the reference catalyst using conventional carrier. This study demonstrated that bi-material catalyst are promising candidate for all industrial catalytic reactions that present intraparticular diffusion limitations as mechanical properties and catalytic performances are very satisfactory. It should be planned in the future to extend the concept to multifunctional catalysis.

References [1] J.-P. Boitiaux, J. Cosyns, M. Derrien, G. Léger, 1995, Newest hydrogenation catalysts, Hydrocarbon Processing, 64, 3, 51-59. [2] G.C. Bond, P.B. Wells, 1964, Advances in catalysis, Academic Press, 15, 91-226 [3] J.-F. Le Page et al., 1978, Catalyse de contact. Conception, préparation et mise en oeuvre des catalyseurs industriels, Technip, 63-80. [4] R. Krishna, S.T. Sie, 1994, Strategies for multiphase reactor selection, Chemical Engineering Science, 49,24, 4029-4065. [5] T.-B. Lin, T.-C. Chou, 1994, Selective hydrogenation of isoprene on eggshell and uniform palladium profile catalysts, Applied Catalysis A: General, 108, 7-19. [6] D. Heineke, E. Schwab, M. Fischer, G. Schmid, M. Baeumle, 1998, Palladium clusters and their use as catalysts, EP 0 920 912 B1. [7] T. Pagès, 1998, PhD Thesis, Université Pierre et Marie Curie, Paris VI. [8] S. Verdier, 2001, PhD Thesis, Université Pierre et Marie Curie, Paris VI. [9] S. Asplund, 1996, Coke formation and its effect on internal mass transfer and selectivity in Pd-catalysed acetylene hydrogenation, Journal of Catalysis, 158, 267-278. [10] Handbook of porous solids Vol 3, 2002, Edited by F. Schüth, K.S.W. Sing, J. Weitkamp, Wiley-VCH, Chapter 4.7.2. [11] J.L. Loubet, M. Bauer, A.Tonck, S. Bec, B. Gauthier-Manuel, 1993, Mechanical Properties and Deformation Behavior of Materials Having Ultra-Fine Microstructures. Kluwer academic publishers, 429-447. [12] D. Chicot, P. Démarécaux, J. Lesage, 1996, Apparent Interface toughness of substrate and coating couples from indentation tests, Thin Solid Films, 283, 151-157. [13] C. Couroyer, M. Ghadiri, P. Laval, N. Brunard, F. Kolenda, 2000, Methodology for investigating the mechanical strength of reforming catalyst beads, Oil & Gas Science and Technology, 55, 1, 67-85. [14] J.P. Boitiaux, J. Cosyns, S. Vasudevan, 1983, Hydrogenation of highly unsaturated hydrocarbons over highly dispersed palladium catalyst. Part I : Behaviour of small particles, Applied Catalysis, 6, 41-51.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Catalytic property of carbon-supported Pt catalysts covered with organosilica layers on dehydrogenation of organic hydride Keizo Nakagawa,*a,b,c Yusuke Tanimoto,c Tetsuya Okayama,c Ken-Ichiro Sotowa,a,b,c Shigeru Sugiyama,a,b,c Toshihiro Morigaa,c a

Department of Advanced Materials, Institute of Technology and Science, The University of Tokushima, Minamijosanjima, Tokushima 770-8506 b Department of Geosphere Environment and Energy, Center for Frontier Research of Engineering, The University of Tokushima, Minamijosanjima, Tokushima 770-8506 c Department of Chemical Science and Technology, Faculty of Engineering, The University of Tokushima, Minamijosanjima, Tokushima 770-8506

Abstract Carbon-supported Pt metal nanoparticles were covered with a silica layer including phenyl or methyl groups using successive hydrolysis of 3-aminopropyl-triethoxysilane (APTES) and phenyltriethoxysilane (PhTES) or methyltriethoxysilane (MTES), followed by reduction with H2. Highly dispersed Pt nanoparticles could be produced in silicacoated Pt catalysts using each organosilane. The results of N2 adsorption showed that micropores were formed in the silica layer by introducing functional groups into the silica network. Because the microporous structure of silica layers which wrapped Pt metal particles increased the diffusion capability of cyclohexane, the Pt catalyst covered with silica layers containing functional groups showed higher activity in the cyclohexane dehydrogenation, compared with Pt catalysts covered with a silica layer containing no functional groups. Keywords: Pt metal particles, silica layer, organosilane, dehydrogenation of cyclohexane

1. Introduction Metals or metal oxides supported on supports are often used as catalysts for various catalytic reactions because the deposition of metal species on the supports results in the improvement of catalytic activity and selectivity, and/or in the inhibition of their sintering at high temperatures due to the chemical interaction between the metal species and the supports. Supported metal catalysts covered with silica layers have been studied [1-13]. Metal nanoparticles supported on supports were covered with silica layers of a few nanometers thickness by hydrolysis of silicon alkoxides such as tetraethoxysilane (TEOS). We demonstrated that the metal particles in these silica-coated catalysts showed good resistance for sintering, even at high temperatures, because the metal nanoparticles were covered with silica layers. In consequence, these silica-coated metal catalysts allowed preferential formation of carbon nanotubes or nanofibers with uniform diameters through ethylene decomposition, while the metal catalysts without silicacoating formed carbon nanotubes or nanofibers with various diameters because the metal particles aggregated severely during ethylene decomposition [6,7,11,12]. The silica-coated metal catalysts also showed a high stability for the repeated potential cycling experiment as a Pt electrocatalyst for a proton-exchange-membrane fuel cell

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[8,13]. Thus, the coverage of metal particles with silica layers is an effective method to enhance the stability of catalysts. Meanwhile, the metal particles in these silica-coated catalysts also showed specific reactant shape selectivity because the silica layer which wrapped Pt metal particles have porous structure. As a result, the silica-coated metal catalysts showed a specific performance for competitive oxidation of mixed hydrocarbons because the porous silica structure controlled the diffusion rate of reactant molecules [3,4]. The development of silica-coated catalysts to provide an increase of pore size or addition of functionality of the silica layer covering the metal particles, leads to an increase in their application for catalytic reactions. Organically functionalized materials have interesting effects on the porosity, adsorption and diffusion of reactants, and ultimately on the control of the surface reactivity [14,15]. These materials can be synthesized by using organoalkoxysilanes as precursors for the sol-gel reactions in which organic groups are introduced within an inorganic network through the Si-C bond. Thus, this synthesis method can be applied to silica-coated metal catalysts. In the present study, carbon-supported Pt catalysts covered with organosilica layers using phenyltriethoxysilane (PhTES) or methyltriethoxysilane (MTES) as the silica source were prepared. In addition, these catalysts were applied to the dehydrogenation of cyclohexane. We would report the specific catalytic performance of Pt catalysts covered with silica layers containing functional groups in the dehydrogenation of cyclohexane, compared with Pt catalysts covered with silica layers containing no functional groups. The effect of the amount of SiO2 of carbon-supported Pt catalysts covered with organosilica layers using MTES was also investigated.

2. Experimental Carbon black (CB) (Vulcan XC-72 supplied by Cabot Co.) was used as a support for Pt particles. CB was immersed in an aqueous solution containing H2PtCl6, and aqueous NH3 was added to the solution to deposit Pt metal precursors onto the CB. After this solution was filtered, the sample was dispersed in a solution containing aqueous NH3, and the successive hydrolysis of 3-aminopropyl-triethoxysilane (APTES) and other organosilanes, such as tetraethoxysilane (TEOS), phenyltriethoxysilane (PhTES) and methyltriethoxysilane (MTES) was performed at 333 K for 1.5 h to form the silica layer containing each functional group on the CB. The mole ratios of TEOS to Pt cations in the aqueous solution were changed in order to prepare the catalysts with different loadings of Pt. The obtained sample was dried at 333 K in air, then exposed to an atmosphere of H2 at 623 K for 3 h. Hereafter, the samples obtained are denoted as SiO2(each organosilane) /Pt/CB catalysts. For comparison, CB-supported Pt metal particles (Pt/CB) were prepared by conventional impregnation. Catalytic cyclohexane dehydrogenation was performed in a batch-wise reactor in the condition that catalytic dehydrogenation could be accomplished efficiently with carbon-supported Pt in the liquid-film state [16,17]. Cyclohexane dehydrogenation was performed under boiling and refluxing conditions by heating at 523 K and cooling at 278 K at atmospheric pressure. Cyclohexane (1.0 ml) and 0.3 g of catalyst were used in the catalytic reaction. The hydrogen that was evolved from the cyclohexane was collected in a gas buret and pursued volumetrically for 150 min. X-ray absorption spectra for the samples were measured at the Photon Factory in the Institute of Materials Structure Science for High Energy Accelerator Research Organization, Tsukuba, Japan, with a ring current of 2.5 GeV and a stored current of 250–450 mA. Pt LIII-edge EXAFS was measured at the beam line BL-7C and 9C

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equipped with Si(111) in transmission mode at room temperature (Proposal No.2006G343 and 2009G087). Analysis of EXAFS data was performed using an EXAFS analysis program, REX (Rigaku Co.). Inversely Fourier-transformed data for Fourier peaks were analyzed by a curve-fitting method, using phase-shift and amplitude functions estimated from EXAFS spectrum of Pt foil. The content of Pt, SiO2 and carbon in the CB-supported Pt metal nanoparticles covered with organosilica layers was evaluated by X-ray fluorescence spectroscopy (XRF) and elemental analysis. Transmission electron microscopy (TEM) images of the samples were recorded with a Hitachi H-800 instrument (Hitachi High-Technologies Co.). Specific surface areas were calculated from the adsorption isotherm obtained with a conventional BET nitrogen adsorption apparatus (BELSORP-18SP, Bell Japan Inc.). The exposed surface areas of the Pt metal particles in the silica-coated Pt/CB were evaluated by the CO adsorption method (BELCAT, BEL Japan Inc.) at 323 K, assuming an adsorption stoichiometry of 1:1 for CO/Pt. Before the measurement of CO adsorption, the samples were treated with hydrogen at 623 K for 30 min.

3. Results and discussion Table 1 presents the SiO2, Pt and carbon contents in Pt/CB and silica-coated Pt catalysts using different organosilanes. The Pt loading of all catalysts were about 1–2wt%. The SiO2 loading of silica-coated Pt catalysts were changed from about 30 to 55wt% by changing the organosilanes. Figure 1 shows TEM images of Pt/CB and silicacoated Pt catalysts using different organosilanes. CB and Pt metal nanoparticles were observed in all TEM images. The diameter of the Pt particles in Pt/CB ranged from 1 to 3 nm. However, some aggregated Pt metal particles with diameters of 8 to 10 nm were also observed in Pt/CB. In contrast, the diameter of the Pt particles in SiO2(TEOS)/Pt/CB, SiO2(PhTES)/Pt/CB and SiO2 (MTES)/Pt/CB ranged from 1 to 3 nm. Thus, highly dispersed Pt nanoparticles could be produced in silica-coated Pt catalysts using each organosilane.

Table 1. Contents of SiO2, Pt and C in Pt/CB and silicacoated Pt catalysts using different organosilanes. Sample Pt/CB SiO2(TEOS)/Pt/CB SiO2(PhTES)/Pt/CB SiO2(MTES)/Pt/CB

SiO2/wt%

Pt/wt%

55.1 31.4 41.1

1.3 1.8 1.9 0.8

C/wt % 98.7 43.1 66.7 54.2

(a)

(b)

(c)

(d)

Fig. 1 TEM images of (a) Pt/CB, (b) SiO2(TEOS)/ Pt/CB, (c) SiO2(PhTES)/Pt/CB, (d) SiO2(MTES)/Pt/CB.

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Absorbance / a.u.

Absorbance / a.u.

In order to confirm the (a) (b) existence of functional groups in 700 the silica layer of SiO2(PhTES)/ 740 1273 Pt/CB and SiO2(MTES)/Pt/CB, FT-IR measurement was performed. Si-C bond absorption derived from phenyl groups were observed at 700 and 740 cm-1 in SiO2(PhTES)/Pt/CB as shown in Fig. 2(a). Meanwhile, Si-C bond 850 800 750 700 650 600 550 1300 1200 1100 1000 900 Wavenumber / cm Wavenumber / cm absorption derived from methyl groups was observed at 1273 Fig. 2 FT-IR spectra of (a) SiO2(PhTES)/Pt/CB, (b) cm-1 in SiO2(MTES)/Pt/CB as SiO2(MTES)/Pt/CB. shown in Fig. 2(b). These peaks are in agreement with those from previous data [18]. In addition, ca. 20% weight loss in SiO2(PhTES)/Pt/CB and ca. 5 % weight loss in SiO2(MTES)/Pt/CB compared with the weight loss in SiO2(TEOS)/Pt/CB were observed in the results of thermogravimetric analysis (results not shown). These results strongly indicate that phenyl groups and methyl groups existed in the silica layers of SiO2(PhTES)/ Pt/CB and SiO2(MTES)/Pt/CB, respectively. Pt/CB covered with silica layers was prepared using the successive hydrolysis of APTES and organosilanes such as TEOS, PhTES and MTES in the presence of Pt/CB. In previous studies, APTES was adsorbed on CB supports through the interaction between graphene in CB and amino groups in APTES, which resulted in the coverage of Pt/CB with uniform silica layers of thickness (< 1 nm) [9,11]. The subsequent hydrolysis of organosilanes in the presence of Pt/CB covered with thin silica layers from APTES is expected to cover Pt/CB with silica layers of a few nanometers thickness. It should be noted that the Pt particles in silica20 coated Pt catalysts using each organosilane seem to be covered with a silica layer and were not 15 found on the outer surface of the silica layers, but in their bodies. These results suggest that the (a) surface of the Pt particles and 10 CB can be uniformly covered with silica layers by the succes(b) sive hydrolysis of APTES and 5 each organosilane. (c) Figure 3 shows Fourier trans(d) forms of Pt LIII-edge k3-weighted 0 EXAFS spectra (RSFs; radial 0 1 2 3 4 5 6 structural functions) for silicaR/ Å 3 coated Pt catalysts using dif- Fig. 3 Fourier transforms of Pt LIII-edge k -weighted ferent organosilanes. A strong EXAFS for (a) Pt foil, (b) SiO2(TEOS)/Pt/CB, (c) peak was observed at 2.7 Å in a SiO2(PhTES)/Pt/CB, (d) SiO2(MTES)/Pt/CB. The intensity RSF for Pt foil. This peak could of the peak for Pt foil was halved. be assigned to the presence of the neighbouring Pt atoms in Pt foil. In the RSFs for each silica-coated Pt catalyst using different organosilanes, a strong peak was observed at the same position as that for Pt |FT|

-1

-1

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3

Amount adsorbed [cm (STP)/g]

foil. In addition, the features of Table 2. Structural parameters estimated by the RSFs for silica-coated Pt cata- curve-fitting analysis for the Pt LIII-edge EXAFS lysts using different organosilanes spectra of each catalyst. were similar to each other. This Sample R/ Åa C.N.b result indicates that most Pt SiO2(TEOS)/Pt/CB 2.73 7.9 species in silica-coated Pt cataSiO2(PhTES)/Pt/CB 2.71 7.4 lysts using different organosilanes were present as Pt metal. The SiO2(MTES)/Pt/CB 2.70 7.7 intensity of the peak at 2.7 Å in aR, interatomic distance of Pt-Pt; bC.N., cothe RSFs of silica-coated Pt ordination number of Pt-Pt. catalysts using different organosilanes did not change. As for the RSFs of any metals, the peak intensity is sensitive to their crystallite size. These RSFs implied that the crystallite sizes of Pt metal in silicacoated Pt catalysts using different organosilanes were similar to each other. In order to confirm the structure of Pt species in silica-coated Pt catalysts using different organosilanes in detail, the curve-fitting analyses were performed for the Fourier transforms of Pt LIII EXAFS spectra shown in Fig. 3. The structural parameters estimated by the curve-fitting analysis for the EXAFS were listed in Table 2. The peak in the RSF was inversely Fourier-transformed in the ranged of R = 1.7–3.4 Å and the k3-weighted EXAFS spectra was fitted in the range of 4–15 Å by using amplitude function and phase sight extracted from the EXAFS spectra for Pt foil. All the EXAFS spectra for silica-coated Pt catalysts using different organosilanes could be fitted with a shell of Pt-Pt bond. The coordination number and interatomic distance of Pt-Pt bonds in silica-coated Pt catalysts using different organosilanes did not change very much. These results indicated that Pt metal particles with almost the same crystallite size could form silica-coated Pt catalysts using different organosilanes. Figure 4 shows the N2 adsorption isotherms at 77 K of silica-coated Pt catalysts using different organosilanes. An increase of the amount of N2 adsorption is observed below P/P0 = 0.1 for SiO2(PhTES)/Pt/CB and SiO2 140 SiO2(MTES)/Pt/CB (MTES)/Pt/CB as compared with SiO2(PhTES)/Pt/CB SiO2(TEOS)/ Pt/CB, suggesting 120 SiO2(TEOS)/Pt/CB the additional formation of micropores in the silica layers. 100 The specific surface area evaluated from the BET method was 80 208 m2/g for Pt/CB. On the other 60 hand, the specific surface area was 37 m2/g for SiO2(TEOS)/Pt/ 40 CB, 103 m2/g for SiO2(PhTES)/ 2 Pt/CB and 187 m /g for SiO2 20 (MTES)/Pt/CB. The specific surface areas of any silica-coated Pt 0.0 0.2 0.4 0.6 0.8 1.0 catalysts using different organRelative Pressure [-] osilanes were smaller than that of Pt/CB. The decrease of speci- Fig. 4 N2 adsorption measurements of silica-coated Pt fic surface area of silica-coated catalysts using different organosilanes. catalysts was because of the blocking of pores of CBs by a silica layer. Although it is difficult to measure the pore size or the specific surface area of only silica layer in the catalysts from N2 adsorption measurements because of the presence of the CB support,

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TOF / min

-1

Amount of hydrogen generation -2 / mmol m -Pt

these findings suggest that the increase of specific surface area was because of the formation of microporous silica layer which wrapped Pt metal particles by introducing phenyl or methyl groups into the silica network. Cyclohexane dehydrogenation was performed over Pt/CB and silica-coated Pt catalysts using different organosilanes. Figure 5 shows the time courses of amount of hydrogen generation from cyclohexane dehydrogenation with SiO2(TEOS)/Pt/CB, SiO2(PhTES)/Pt/CB and SiO2(MTES)/Pt/CB. In this batch reactor, the conversion of Pt/CB was about 80%. The amount of hydrogen generation of SiO2(TEOS)/Pt/CB was very low. In contrast, the amount of hydrogen generation of SiO2(PhTES)/Pt/CB and SiO2(MTES)/Pt/CB were significantly higher than that of SiO2(TEOS)/Pt/CB as shown in Fig. 5. It is assumed that this result was responsible for the diffusion of cyclohexane into the silica layer because Pt metal particles were similar with each other from the result of EXAFS spectra as shown in Fig. 3 and Table 2. Cyclohexane made contact with catalytically active Pt metal 25 particles on the CB after they difSiO2(MTES)/Pt/CB MTES 20 fused into the silica layer. The result SiO2(PhTES)/Pt/CB PhTES SiO2(TEOS)/Pt/CB of the low amount of hydrogen geneTEOS 15 ration for SiO2(TEOS)/Pt/CB implied that the pore size in the silica layer is 10 not large enough for the diffusion of cyclohexane in SiO2(TEOS)/Pt/CB. 5 On the other hand, higher amounts of hydrogen generation were obtained 0 for the SiO2(PhTES)/Pt/CB and SiO2 0 10 20 30 40 50 60 (MTES)/Pt/CB because diffusion time on stream / min capability of cyclohexane into the silica layer increased as a results of Fig. 5 Change of Amount of hydrogen generation formation of micropores in the silica with time on stream in the cyclohexane dehydrolayer considering from the results of genation over each catalyst. N2 adsorption. Thus, SiO2(PhTES)/ 0.3 Pt/CB and SiO2(MTES)/Pt/CB are effective catalyst for the catalytic reaction involved larger molecules 0.2 because higher activities were obtained nevertheless Pt metal particles are covered with silica layers. 0.1 We carried out the cyclohexane dehydrogenation over SiO2(MTES)/ Pt/CB with different SiO2 loadings 0.0 in order to examine the effects of 0 wt% 10.6 wt% 27.7 wt% 41.7 wt% SiO2 loading on catalytic performance. Figure 6 shows the turnover Fig. 6 Turnover frequency for the dehydrogenation frequency (TOF) estimated based on of cyclohexane over SiO2(MTES)/Pt/CB with difthe rate of hydrogen generation and ferent amount of SiO2. the number of Pt atoms at the surface of Pt metal particles which was evaluated by CO adsorption on SiO2(MTES)/Pt/CB. As shown in Fig. 6, the TOF for the cyclohexane dehydrogenation decreased with the higher SiO2 loading, i.e. with thickness of silica layers which wrapped the catalytically active Pt metal. It is likely that

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the TOF depends on the thickness of silica layers of SiO2(MTES)/Pt/CB because the diffusion rates of cyclohexane in silica layers affect the reaction rates over SiO2(MTES)/ Pt/CB. Although the activity of SiO2(MTES)/Pt/CB was not beyond that of Pt/CB, the activity of SiO2(MTES)/Pt/CB could be improved by decreasing the thickness of SiO2 layer which wrapped the catalytically active Pt metal. Thus, we developed Pt catalyst covered with microporous silica layers with higher catalytic activity for the cyclohexane dehydrogenation by introducing functional groups into silica layers.

4. Conclusion CB-supported Pt metal nanoparticles were covered with silica layers including phenyl groups or methyl groups using successive hydrolysis of APTES and PhTES or MTES. Micropores could be formed in the silica layers by introducing functional groups into the silica network. The Pt catalyst covered with silica layers containing functional groups showed higher activity in the cyclohexane dehydrogenation, compared with Pt catalysts covered with a silica layer containing no functional groups. This catalyst can be applied to various catalytic reactions involved larger molecules.

Acknowledgment This work was funded by a Grant-in-Aid for JST Research for Promoting Technological Seeds (2008) and a Grant-in-Aid for Young Scientists (B) KAKENHI 20750167 to K.N. The authors gratefully acknowledge Dr M. Tagami (Center for Technical Support, Institute of Technology and Science, The University of Tokushima) for his assistance with TEM experiments.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

M. Kishida, T. Tago, T. Hatsuta and K. Wakabayashi, Chem. Lett., 29 (2000) 1108. T. Tago, T. Hatsuta, K. Miyajima, M. Kishida, S. Tashiro and K. Wakabayashi, J. Am. Ceram. Soc., 85 (2002) 2188. S. Takenaka, K. Hori, H. Matsune, M. Kishida, Stud. Surf. Sci. Catal., 162 (2006) 585. K. Hori, H. Matsune, S. Takenaka, M. Kishida, Sci. Tech. Adv. Mater., 7 (2006) 678. S. Takenaka, H. Umebayashi, E. Tanabe, H. Matsune and M. Kishida, J. Catal. 245 (2007) 392. K. Nakagawa, S. Takenaka, S. Imagawa, H. Matsune and M. Kishida, Chem. Lett. 36 (2007) 252. S. Takenaka, Y. Orita, E. Tanabe, H. Matsune and M. Kishida, J. Phys. Chem. C 111 (2007) 7748. S. Takenaka, H. Matsumori, K. Nakagawa, H. Matsune, E. Tanabe and M. Kishida, J. Phys. Chem. C 111 (2007) 15133. S. Takenaka, T. Arike, K. Nakagawa, H. Matsune, E. Tanabe and M. Kishida, Carbon. 46 (2008) 365. S. Takenaka, T. Arike, H. Matsune, E. Tanabe and M. Kishida, J. Catal, 257 (2008) 345. S. Takenaka, T. Iguchi, E. Tanabe, H. Matsune and M. Kishida, Carbon. 47 (2009) 1251. T. Iguchi, S.Takenaka, K. Nakagawa, Y. Orita, H. Matsune and M. Kishida, Top. Catal. 52 (2009) 563. S. Takenaka, H. Matsumori, T. Arike, H. Matsune and M. Kishida, Top. Catal. 52 (2009) 731. J. Wen and G. L. Wilkes, Chem. Mater., 8 (1996) 1667 S. Tanaka, J. Kaihara, N. Nishiyama, Y. Oku, Y. Egashira and K. Ueyama, Langmuir, 20 (2004) 3780.

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16. C. Shinohara, S. Kawakami, T. Moriga, H. Hayashi, S. Hodoshima, Y. Saito and S. Sugiyama, Appl. Catal. A. Gen. 266 (2004) 251. 17. W. Ninomiya, Y. Tanabe, Y. Uehara, K-I. Sotowa and S. Sugiyama, Catal. Lett. 110 (2006) 191. 18. R. Al-Oweini and H. El-Rassy, J. Mol. Struct. 919 (2009) 140.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Molecular aspects of solid silica formation Istvan Halasz, Mukesh Agarwal, Robert E. Patterson PQ Corporation, R&D Center, 280 Cedar Grove Road, Conshohocken, PA 19428, USA

Abstract Raman spectroscopy indicates distinct differences between the molecular constitutions of amorphous silicas solidified from aqueous solutions at acidic and basic conditions which might have implications on the synthesis and properties of zeolites, mesostructured silicas and silica gels. The Qn connectivities of [SiO4] tetrahedra in the primary nanoparticles, which determine the ultimate molecular structure of gels, seem rarely to depend on the concentration, elemental composition, or molecular constitution of the dissolved alkaline silicate ingredients. Experimental and computational evidence support a surprisingly large volume of Qo silica monomers in many acid-set gels. Because of a good spectral resolution both in liquid and solid phases, robust and mobile instrumentation, low cost and ease of use, Raman spectroscopy is a preferred noninvasive analytical technique that allows one to follow in situ the complex solidification process of silicates. However, we found that FTIR spectroscopy is better suited in some cases, for example for studying the TEOS based synthesis of (Me4N)8Si8O20 x 65H2O, a starting material for the designed synthesis of double four ring (D4R) based nano structures. Keywords: silica gel, nano synthesis, zeolite, Raman, FTIR

1. Introduction Raman spectroscopy has been routinely used for identifying the Qn connectivity of [SiO4] tetrahedra in glasses and silica based minerals for decades [1, 2]. The usually well separated νs Si-O Raman bands near 850, 900, 980, 1060, and 1140 cm-1 (with about ±3% average tolerance) are considered to be characteristic of the presence and relative amount of Q0, Q1, Q2, Q3, and Q4 connected [SiO4] tetrahedra, respectively, where the superscripted numbers represent the number of Si-O-Si bonds connecting this unit with its neighboring [SiO4] building blocks [3]. Obviously, at < Q4 discontinuities must be present in the network of Si-O-Si connections and capping these “defect” points with protons or other cations is necessary to ensure charge neutrality. Since we have not seen any reason why this well established method would not work for other amorphous silica systems, its adoption for testing the structure of the sub-nano sized dissolved alkaline silicate molecules [3-8] and that of the variously prepared silica gels [9-11] seemed to be in order. Therefore, along with Raman-based ring identifications from the zeolite literature [3], we used this convenient in situ approach to exploring the formation and transformation of silica nanoparticles from dissolved alkaline silicate molecules, which is among the least understood hence least controlled steps in the synthesis of a wide variety of amorphous and crystalline silica products. While one can indeed identify marked differences between the Raman and FTIR spectra of these amorphous structures, we have found that the published peak assignments to siloxane rings and Qn connectivities scatter quite a bit which calls for their further theoretical study. Moreover, the dissociation in the aqueous environment and the presence of water were unexpectedly found to also affect the Si-O vibrations [3, 6].

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By investigating the effect of molecular structure, type of alkaline ion, alkaline/ silica ratio, concentration, and gelling method on the molecular structure of silica gels made from aqueous alkaline silicate solutions we demonstrated recently that the method of synthesis, i.e., drying or precipitating from alkaline or acidic solutions, has usually stronger impact on the Raman identified structural characteristics of products than the other parameters combined [9-11]. It has long been known that gelling can be performed both at acidic and at basic pH values and certain physical characteristics of such acid-set and base-set gels can characteristically differ from each other even when the same alkaline silicate ingredients are used [12-14]. However, the bulk molecular structures of differently made gels have virtually never been distinguished and, what is more, different molecular gel structures have rarely been assumed as a possibility. A notable exception is Iler’s more than half century old speculation about the possible linear siloxane-chain structure of acid-set gels versus a random 3D network of siloxane rings in base-set gels [13-15]. By systematically applying the electronegativity equalization principle, Livage [16] also concluded later that linear polymer chains should form from aqueous silicate solutions at acidic conditions and “branched species” at basic conditions. Recently the structure changing effect of pH was also implied for example in the success of template-free synthesis of RTH zeolite by excess NaOH [17]. In the course of our in situ vibrational spectroscopic studies we unexpectedly found that certain gelling processes, like drying or base setting of Na2SiO3 solutions [6, 9] or acid setting most alkaline silicates [10, 11], likely result in substantial Qo silica monomers in the solid. Here we focus on the clarification of this rather surprising issue also considering the above mentioned uncertainties of vibrational band assignments in the aqueous environment of silica. Further, we will show that the tetraethyl orthosilicate (TEOS) based non-aqueous synthesis of (Me4N)8Si8O20 x 65H2O, containing D4R (also named as cubic octamer or octa-silsesquioxane) siloxane entities [18-21], presumably proceeds without the explicit formation of H2SiO42- or similar hydrolyzed monomer ions from TEOS.

2. Experimental The dissolved alkaline silicate ingredients were commercial products from PQ Corporation. The solid Na2SiO3 x 9H2O and Na4SiO4 samples were purchased from Sigma and AlfaAesar, respectively. Forsterite (San Carlos, Arizona) was obtained from Ward’s Nat. Sci. Establishment, Inc., TEOS from Silbon, and TMA-OH from SAChem. For preparing base-set gels 3 M HCl solution was drop-wise added to the stirred silicate solution and the pH was measured with a Corning Scholar 425 pH meter equipped with an Accumet electrode. When the first visible gel particles appeared (usually near the electrode), the dosage of acid was stopped and we waited to see if the whole material gels within about 10 minutes or the visible particles dissolve again. In the former case the solidifying experiment was terminated and the vibrational spectrum of the fresh gel was measured. Acid-set gels were made by dosing the silicate solution into HCl solution following the same principles as for the base-set gels. Further experimental details including solution making and handling have been reported elsewhere [11]. For the synthesis of (Me4N)8Si8O20 x 65H2O we followed the Kuroda group’s method [21]. The crystal structure of product was identified by XRD. Raman spectra were obtained with 532 nm (180 mW) and 780 nm (400 mW) dispersive laser Raman spectrometers from Kaiser and Lambda Solutions, respectively. Both spectrometers are equipped with fiber optic connected sapphire sampling windows. FTIR spectra were measured with triple bounce diamond ATR on a Nicolet

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Magna 550 spectrometer. Details of instruments, techniques, and the concept of vibrational band assignments have been discussed elsewhere [3, 6, 7, 22, 23]. Model calculations were performed on the VAMP [24], DMOL3 [25, 26], and CASTEP [27] modules of the Materials Studio program package from Accelrys. Full geometry optimizations and vibrational frequency analyses were carried out in all electron approximation using in DMOL3 the BLYP [28, 29] functional in conjunction with the double-numeric-basis set with polarization functions (DNP) and the IR models were calculated from the Hessians [30]. In CASTEP the gradient-corrected (GGA) PBE [31] functional was selected for the density functional theory (DFT) computations with norm conserving and not spin polarized approach [32]. In the semi-empirical VAMP method we used the PM3 parameterization [33] from the modified neglect of diatomic differential overlap (NDDO) model to obtain the Hessians for vibrational spectrum models [30].

3. Results and discussion The Raman spectra in Fig. 1 illustrate that very different initial alkaline silicate solutions can result in quite similar Qn connectivity distributions in the acid- and baseset gels. A number of other Li, Na, and K silicates have shown similar trends although this phenomenon is not entirely universal [10, 11]. The specific pH values for gelling cannot be made uniform for each solution as “apple to apple comparison” type research logic would dictate since it has been long known that the gelling time might change

` Fig. 1 The Raman spectra of 3 M and 0.2 M aqueous solutions of commercial sodium (1/a) and potassium (1/b) silicates have substantially different molecular structures and compositions but similar Qn connectivity distributions when gelled at acidic (pH < 7) or basic (pH > 7) condition.

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Fig. 2 The mainly Q4 connected siliceous zeolites [22, 23] are poor Raman absorbers in the 700 to 1300 cm-1 νs and νas Si-O vibration range (2/a) and the 1140 cm-1 Raman band of a gel made from Kasil-1624 (K/Si ~ 0.76) [8, 10, 11] substantially decreases upon drying which makes its Q4 assignment dubious (2/b).

from 1 min to near infinite at a given pH simply by changing the concentration [12] because it also changes the dissociation, molweight, and constitution of dissolved silicate molecules [3, 7, 8]. Beside the predicted Q2 type Si-O-Si chains, all acid-set gels seem to contain mostly Q0 monomers and, also in line with Iler’s and Livage’s [13-16] conjecture, all base set gels seem to have predominantly a Q3/Q4 syloxane network. There are however some doubts about the validity of the glass-originated Qn assignments in Fig. 1. For example siliceous zeolites like those shown in Fig. 2/a do not have an intense Raman absorption near 1140 cm-1 although they are exclusively composed of Q4 type [SiO4] tetrahedra. We could not measure Raman spectra on many commercial silica gels either despite their assumed Q4 connected irregular siloxane rings in a 3D network. The cause of these phenomena are under investigation. Fig. 2/b suggests that the 1140 cm-1 band might also be water associated. There have been several experimental reasons [3-8] for assigning the ~780 cm-1 band to Q0 monomers in aqueous solutions in contrast to the corresponding ~850 cm-1 in glasses. Based upon this consideration we have also assigned the 750 cm-1 band in Figs. 2/c and 2/d to Q0. To clarify further the role of water with these Q0 associations of Raman bands, we collected Raman spectra of materials largely containing only monomer [SiO4] tetrahedra (proven by independent experimental methods) and also performed adequate molecular modeling calculations. Figure 3/a shows the Raman spectra of three sodium silicate solutions that have been identified in the literature as mostly monomer containing materials based on light diffraction, molybdate reactions, molweight measurements, and Si29 NMR data [6, 13, 34]. Most Raman bands overlap quite nicely hence the most intense 770 cm-1 band is reasonably characteristic for the monomer structure. The intensity differences of other bands might be associated with different levels of dissociation and perhaps also with some larger silicate impurity. It is not clear why the 850 cm-1 band appears only in the spectrum of Na/Si ~ 40 ratio silicate but not in the other spectra. It was recently demonstrated by various ab initio calculations [35] that for modeling realistic FTIR spectra of the dissolved monomers it is not enough to simulate the solvent with the usually applied conductor-like screening (COSMO) method [36, 37]: one has to explicitly include water molecules into the model, which significantly increases the computational power demand for modeling even small realistic silicate molecules. Figure 3/d shows that this is also valid for modeling the Raman spectra of H2SiO42- ions composing the bulk of dilute basic solutions of Na2SiO3 x 9H2O [6].

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Fig. 3 Raman spectra of independently verified monomer silicate solutions (3/a); model of Na4SiO4, the hydrolized variations of which compose the dilute basic monomer solutions (3/b); model of the H2Na2SiO4 molecule surrounded by 20 water molecules (3/c) and the computed and experimental Raman spectra of its dissociated H2SiO42- derivative likely present in dilute solutions of Na2SiO3 x 9H2O [6] (3/d).

In Fig. 4/a we compare the Raman spectra of some crystalline solids containing only monomer [SiO4] tetrahedra in their XRD verified structures [38, 39]. Their existence suggests that amorphous solids might also contain substantial amounts from [SiO4] monomers. There is a clear difference between the Q0 associated peak positions of the water-containing and water-free crystals: the latter ones tend to show these vibrations near 850 cm-1 like glasses while the 770 cm-1 vibration of the former one resembles that in spectra of dissolved silicates. Figure 4/b illustrates that the relative peak intensities (but not the positions!) in Raman spectra can substantially vary when measured with different wavelength lasers which has to be taken into consideration when one compares the experimental and model Raman spectra (the latter being more comprehensive). We carried out DFT calculations to model the Raman spectra of the water-free Forsterite and the aqueous Na2SiO3 x 9 H2O crystals. Their structures are shown in Figs. 5/a and 5/b. As Fig. 5/c illustrates, the model spectrum of Forsterite fits the experimental data extremely well. The more complex sodium silicate spectrum in Fig. 5/d is also a good fit but fails to be decisive exactly in the critical 750-850 cm-1 range. It is not clear yet if this is due to the above mentioned laser vawelength associated experimental artifact (this spectrum was measured with a 780 nm laser) or to the computational flaw that our system was not fully minimized before carrying out these initial vibrational calculations. These issues are currently being investigated involving also 29Si NMR studies on both these crystalline and the amorphous solids.

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Fig. 4 Raman spectra of XRD-verified aqueous and non-aqueous solid crystals containing only [SiO4] monomers (4/a) (see also Figs 5/a and 5/b); the relative intensity of Raman bands can change with the laser wavelength as these Fayalite spectra [38] demonstrate (4/b).

Fig. 5 DFT calculated and experimental Raman spectra of crystalline Forsterite (5/a and 5/c) and Na2SiO3 x 9 H2O; color assignments on 5/b are the same as those on Fig. 3 for the dissolved silicates (e.g., Figs 3/a and 3/d).

Since Raman spectroscopy can detect silicate structures both in solutions and in solids, it is well-suited to follow the poorly understood transformation of dissolved silicate molecules into the initial solid nanoparticles that ultimately agglomerate into the

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3D gels. We intended therefore to use this in situ technique to see how the monomer TEOS molecules convert into the D4R structures of the (Me4N)8Si8O20 x 65H2O crystals.

Fig. 6 In situ analysis of the synthesis of (Me4N)8Si8O20 x 65H2O; 6/a shows the structure of a D4R silicate building block presumed to form when the crystalline product in 6/b hydrolyzes; colors in 6/a have the same meaning as colors in Fig. 3 and colors in 6/b mean: red = oxygen, yellow = silicon, grey = carbon, blue = nitrogen; hydrogen atoms are not shown for clarity; the Raman spectra of the overall reaction mixtures at the beginning and at the end of reaction are largely identical owing to exact overlaps between the bands of reactants and product (6/c); additional bands have not appeared during reaction, suggesting organic reaction pathway instead of hydrolysis into [SiO4]4- type monomer ions; the FTIR spectra of reactants and product (marked as “solid D4R” for brevity) indicate that the development of a νas Si-O band near 1000 cm-1 can be a clear in situ indication for the progress of reaction (6/d)

Specifically our hope was to see the disappearance of the TEOS related Raman bands and the development of D4R related Q3 bands around 1050 cm-1 accompanied by new siloxane ring vibrations around 440 cm-1 [3]. As Fig. 6/c illustrates, experiment did not confirm this prediction. The weak but measurable (after magnifying electronically) TEOS band near 605 cm-1 disappeared after about 40 hours reaction time. This band appears in the pure material at 655 cm-1 but shifted in the reaction mixture. We could not see any other change in the Raman spectra in the course of the 65 hour reaction although spectra were frequently measured. This observation suggests that TEOS does not hydrolyze into reactive monomer ions or orthosilicic acid (H4SiO4) in the presence of alcohol as many researchers assume but rather reacts like an organic molecule breaking and making only one bond at a time. Raman bands in Fig. 6/c correspond to the typical bands of the pure organic ingredients (see list in Fig. 6/d) and the spectrum of solid D4R crystal totally overlaps with that of the TMA-OH. For clarity we do not show these overlapping spectra in Fig. 6/c. As 6/d indicates however, FTIR

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spectroscopy could be a good in situ method for following this reaction. It is possible that the 1100 cm-1 IR band is associated with Q3 connectivity [3] but regretfully it is not well resolved in the spectrum of the reactant/product mixture. The 1000 cm-1 band is well separated but we are not sure yet about its correct chemical bond assignment. References [1] B. O. Mysen, 1990, J. Geophys. Res., 95 (B10), 15733. [2] B. G. Parkinson, D. Holland, M. E. Smith, C. Larson, J. Doerr, M. Affatigato, S. A. Feller, A. P. Howes, C. R. Scales, 2008, J. Non-Crystalline Solids 354, 1936. [3] I. Halasz, M. Agarwal, R. Li, N. Miller, Microporous Mesoporous Materials, submitted. [4] I. Halasz, R. Li, M. Agarwal, N. Miller, 2005, 19th NAM, Philadelphia, USA, P-122. [5] I. Halasz, R. Li, M. Agarwal, N. Miller, 2007, Catal. Today, 126, 196. [6] I. Halasz, M. Agarwal, R. Li, N. Miller, 2007, Catal. Lett., 117, 34. [7] I. Halasz, R. Li, M. Agarwal, N. Miller, 2007, Stud. Surf. Sci. Cat., 170A, 800. [8] I. Halasz, M. Agarwal, R. Li, N. Miller, 2008, Stud. Surf. Sci. Cat., 174B, 787. [9] I. Halasz, R. Li, M. Agarwal, N. Miller, 2007, 20th NAM, Houston, USA, O-S2-04. [10] I. Halasz, 2008, IMMS, Namur, Belgium, P-037. [11] I. Halasz, M. Agarwal, R. Li, N. Miller, 2009, in “Characterisation of Porous Solids VIII”, ed. by S. Kaskel, P. Llewellyn, F. Rodriguez-Reinoso, N. A. Seaton, RSC Publ., pg. 416. [12] J. Vail, 1952, “Soluble Silicates”, Reinhold Publishing Co., New York. [13] R. K. Iler, 1979, “The chemistry of silica”, J. Wiley & Sons, New York. [14] R. E. Patterson, 2006, Surfactant Sci. Ser., 131, 779. [15] G. Alexander, 1967, “Silica and me”, Doubleday & Co., New York. [16] J. Livage, 1994, Stud. Surf. Sci. Catal., 85, 1. [17] T. Yokoi, M. Yoshioka, H. Imai, T. Tatsumi, 2009, Angew. Chem. Int. Ed., 48, 1. [18] Yu. I. Smolin, Yu. F. Shepelev, R. Pomes, D. Hoebbel, W. Wieker, 1979, Sov. Phys. Crystallogr., 24 (1), 19. [19] M. Wiebcke, M. Grube, H. Koller, G. Engelhardt, J. Felsche, 1993, Microp. Mater., 2, 55. [20] R. Goto, A. Shimojima, H. Kuge, K. Kuroda, 2008, Chem. Commun, 6152. [21] Y. Hagiwara, A. Shimojima, K. Kuroda, 2008, Chem. Mater., 20, 1147. [22] I. Halasz, M. Agarwal, E. Senderov, B. Marcus, W. Cormier, 2005, Stud. Surf. Sci. Catal., 158, 647 [23] I. Halasz, M. Agarwal, B. Marcus, W. Cormier, 2005, Microporous Mesoporous Materials, 84,318 [24] T. Clark, A. Alex, B. Beck, F. Burkhardt, J. Chandrasekhar, P. Gedeck, A. Horn, M. Hutter, B. Martin, G. Rauhut, W. Sauer, T. Schindler, T. Steinke, 2001, “VAMP Semi-Empirical Quantum Chemistry in Materials Studio”, Universität Erlangen. [25] B. Delley, 1990, J. Chem. Phys. 92, 508 . [26] B. Delley, 2000, J. Chem. Phys. 113, 7756. [27] S. J. Clark, M. D. Segall, C. J. Pickard, P. J. hasnip, M. J. Probert, K. Refson, M. C. Payne, 2005, Zeitschrift fur Kristallographie, 220(5-6), 567. [28] A. D. Becke, 1988, J. Chem. Phys., 88, 2547. [29] C. Lee, W. Yang, R. G. Parr, 1988, Phys. Rev. B, 37, 786. [30] E. B. Wilson, J. C. Decius, P. C. Cross, 1955, “Molecular Vibrations”, Dover, New York. [31] J. B. Perdew, K. Burke, M. Ernzerhof, 1996, Phys. Rev. Lett., 77, 3865. [32] D. Porezag, M. R. Pederson, 1996, Phys. Rev. B, 54, 7830. [33] J. J. P. Stewart, 1989, J. Comput. Chem. 10, 209 & 221. [34] G. Engelhardt, D. Michel, 1987, “High-Resolution Solid-State NMR of Silicates and Zeolites”, John Wiley & Sons, Chichester, NY, Brisbane, Toronto, Singapore [35] I. Halasz, A. Derecskei-Kovacs, 2008, Molecular Simulation, 34 (10-15), 937. [36] A. Klamt, G. Schüürmann, 1993, J. Chem. Soc., Perkin Trans. 2, 799. [37] B. Delley, 2006, Mol. Simul., 32, 117-123. [38] RRUFFTM database, http://rruff.info/. [39] F. Liebau, 1985, “Structural Chemistry of Silicates”, Springer Verl., Berlin, Heidelberg, NY. © 2009

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

A novel continuous approach for the synthesis and characterization of pure and mixed metal oxide systems applied in heterogeneous catalysis Stefan Kaluza,a Martin Muhlera a

Laboratory of Industrial Chemistry, Ruhr-University Bochum 44780 Bochum, Germany

Abstract An extensive set of characterization methods is required to study the processes occurring during the evolution of the initially amorphous precursor towards the complex Cu/ZnO/Al2O3 system. A novel preparation method was therefore developed that provides the possibility of a systematic study of all components in the different stages of the precipitation of the ternary catalyst. As a result, a continuously operating synthesis route was established as an alternative to the industrially applied process. Keywords: continuous precipitation, aging, Cu/ZnO/Al2O3 catalyst, methanol synthesis

1. Introduction Pure and mixed metal oxide systems are of immense importance in heterogeneous catalysis today. For instance, zinc oxide is an interesting material for a wide range of applications due to its unique electronic and optical properties. It is a wide-gap semiconductor that is also luminescent, thus being a promising candidate for optoelectronic applications. Because of the good conductivity and high transparency in the visible region, thin films of ZnO have been investigated as transparent electrodes for solar cells. Furthermore, zinc oxide nanoparticles have been used as white pigment or as gas sensors, for example, for detection of hydrogen or nitrogen oxide gases. Active aluminas are also interesting materials for a large range of applications in the field of heterogeneous catalysis. Similar to zinc oxide, they are catalytically active or are used as catalyst support in many processes of industrial importance. In recent years, the role of methanol as a basic chemical has strongly increased, and therefore further development of the ternary Cu/ZnO/Al2O3 catalyst for methanol synthesis has become more important. It is widely accepted that ZnO acts both as an electronic and structural promoter and exhibits a major influence on the catalytic activity, while alumina mainly increases the long-term stability of the ternary catalyst system. Thus, the interest in the binary Zn/Al as well as the ternary CuO/ZnO/Al2O3 system as catalytic materials is very high. In general, precipitation is one of the most frequently applied methods in terms of large-scale catalyst preparation since precipitation and coprecipitation processes provide a good dispersion and a high homogeneity of the components in the catalyst precursor. Nevertheless, in spite of its high industrial importance, precipitation is still a complex process, which is difficult to study, even with the highly advanced analytical tools currently available. Recently, a novel continuous precipitation process using a micromixer was developed, in which the continuous precipitation was immediately quenched by a subsequent spray drying process.[1] Thus, it was possible to investigate the formation

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mechanisms occurring during the first few seconds of the precipitation, which is hardly feasible in a batch operation mode. This novel method was successfully used for detailed studies on the synthesis and aging of single oxides (CuO, ZnO, and Al2O3) as well as mixed binary oxides (CuO/ZnO, ZnO/Al2O3, and CuO/Al2O3). The results were combined to a comprehensive systematic data set of chemical and structural information, which was used to learn more about the formation processes of the ternary catalyst. Additionally, a continuous aging device was developed that allows aging of the continuously formed precipitate with an exactly defined residence time. By this means, a time-resolved investigation of the phase transformations occurring during the aging process of the ternary catalyst precursor became possible.

2. Results and discussion 2.1. Quenching of the precipitation reaction The immediate quenching of the precipitation reaction by applying a subsequent spray drying process enabled us to investigate the initial precursor stage by means of solidstate characterization techniques. Considering the continuous precipitation of aqueous zinc nitrate solution with sodium carbonate, the initially formed metastable precipitation product is sodium zinc carbonate Na2Zn3(CO3)4. The same observation is made in the case of the precipitation of copper nitrate with sodium carbonate, which leads to the formation of sodium copper carbonate Na 2Cu(CO3) 2 (Fig. 1, left). By exposing those metastable phases to water during the subsequent washing step a phase transition into the thermodynamically more stable hydroxy carbonate phases, i.e., Zn5(OH)6(CO3)2 and Cu2(OH)2CO3, respectively, takes place (Fig. 1, right). In both cases, the phase transition taking place is an activated process due to mass transport phenomena, and its rate can be increased thermally, by intensive stirring, or by the use of ultra sound. washed/dried Zn-precursor Zn5(OH)6(CO3)2 Zn4(OH)6CO3

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Fig. 1 Diffraction patterns of the metastable initial precipitation products Na2Zn3(CO3)4 (left, top) and Na2Cu(CO3)2 (left, bottom), respectively, and their transformation into the corresponding thermodynamically stable hydroxy carbonate phase after exposure to water (right).

During the preparation of mixed ZnO/Al2O3 composites a highly X-ray amorphous binary Zn-Al phase is observed as the initial metastable coprecipitation product. Unfortunately, its structure could not be determined up to now as all attempts of thermal recrystallization result in the decomposition of this phase into ZnO and Zn/Alhydrotalcite. Nevertheless, the occurrence of this binary Zn/Al phase turns out to play a major role during the synthesis of the ternary Cu catalyst [2].

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2.2. The influence of the post-precipitation treatment After precipitation, the precursor obtained has to undergo several further steps to be transformed into the final oxidic material. The so called post-precipitation processes, i.e., washing, drying, and calcination, are often guided by intuition, and more attention should be paid to process control and reproducibility. However, the influence of the post-precipitation steps on the properties of the final material can be rather strong. By applying the novel quenching method developed in this work, the spray-dried precursors provide an excellent starting material to investigate the processes of posttreatment more detailed [3]. Considering the binary Zn/Al system, continuous precipitation followed by calcination, washing and freeze drying leads to the formation of Zn/Al-hydrotalcite, a material of high synthetic interest due to its layered structure (Fig. 2,left). By only slight changes of the post-precipitation sequence starting from the same precursor, the formation of the hydrotalcite is suppressed, and Al3+-incorporated ZnO with a specific surface area of up to 144 m2g-1 is obtained (Fig. 2,right) [2,4]. Zn(NO Zn(NO3)2 Al(NO3)3 calcined, washed and freeze-dried Zn/Al-precursor ZnO Zn6Al2(OH)16CO3

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Fig. 2 Schematic illustration of the continuous coprecipitation of a binary Zn/Al precursor. Different final materials were obtained by systematic process control.

This example out of many shows the high importance of every single preparation step in terms of sophisticated process control.

2.3. Continuous aging The immediate quenching of the continuous coprecipitation reaction of the ternary precursor leads to the formation of Na2Cu(CO3)2 separated from a binary Zn/Al phase. This result shows that in the initial stage of the reaction there is no coprecipitation of all three components occurring, but a competition taking place in favor of a mixed Zn/Al phase and a single Cu phase. However, previous results revealed that the ternary precursor consisted of binary Cu-Zn phases as well as a ternary hydrotalcite-like phase, when it was prepared by the usual batch process including 2 h aging of the precipitate in its mother liquid. Thus, during the aging process exchange reactions between the formerly separated single copper phase and the binary Zn/Al phase took place and led to a precursor with a much more homogeneous metal ion distribution. The continuous consecutive precipitation method provides the unique possibility to simulate the initial situation during the coprecipitation process, that is, the formation of a single Cu phase separated from a binary Zn/Al phase. By applying a novel continuous method, the exchange reactions of these metastable phases can be investigated as a function of aging time (Fig. 3).

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Fig. 3 Diffraction patterns of three continuously coprecipitated ternary precursors in comparison to a batchcoprecipitated precursor (bottom). t is the corresponding duration of aging.

The observed phase transitions proceed in an analogous manner to those described for the separated single copper and binary Zn/Al systems. Upon aging, the initially formed Na2Cu(CO3)2 was converted into malachite (Cu2(OH)2CO3). In the presence of the binary Zn/Al phase, the same transition took place and additionally, some zinc was incorporated into the structure to form zincian malachite (Cu,Zn)2(OH)2CO3 [5,6]. Furthermore, the amorphous Zn/Al phase transformed into a hydrotalcite-like structure during the aging process. The diffraction pattern of the continuously precipitated precursor that was continuously aged for 60 min is almost identical to that observed for the precursor coprecipitated and aged for 2 h in the usual batch process. Thus, it can be assumed that this precursor composition represents a thermodynamic minimum that all ternary precursors of the same molar ratios approach with increasing time, independent of the precipitation method. This is also reflected in the increasing crystallinity with increasing aging time compared to the initially X-ray amorphous precursor mixture.

3. Conclusions The continuous approach discussed in this work enabled us to study the precursor formation during the initial step of precipitation as well as the time-resolved transformations occurring during the process of aging. Moreover, better control of the process parameters allowing fast and reproducible parameter screening suggest that the continuous catalyst preparation is a promising alternative to the conventional batch process.

References 1. 2. 3. 4. 5. 6.

S. Kaluza, M.K. Schröter, R. Naumann d’Alnoncourt, T. Reinecke and M. Muhler, Adv. Funct. Mater., 18 (2008) 3670. S. Kaluza and M. Muhler, J. Mater. Chem., 19 (2009) 3914. S. Kaluza and M. Muhler, Catal. Lett., 129 (2009) 287. S. Miao, R. Naumann d’Alnoncourt, T. Reinecke, I. Kasatkin, M. Behrens, R. Schlögl and M. Muhler, Eur. J. Inorg. Chem., 7 (2009) 910. C. Baltes, S. Vukojevic and F. Schüth, J. Catal., 258 (2008) 334. M. Behrens, F. Girgsdies, A. Trunschke and R. Schlögl, Eur. J. Inorg. Chem., 10 (2009) 1347.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Innovative preparation of Au/C by replication of gold-containing mesoporous silica catalysts Fatmé Kerdi1, Valérie Caps1,2 and Alain Tuel1* 1

IRCELYON, UMR 5256 CNRS-Université de Lyon, 2 avenue A. Einstein, 69626 Villeurbanne Cedex, France. 2 KAUST Catalysis Center (KCC), 4700 King Abdullah University of Science and Technology, Thuwal 23955 – 6900, Kingdom of Saudi Arabia

Abstract A new strategy, based on the nanocasting concept, has been used to prepare gold nanoparticles (NPs) highly dispersed in meso-structured carbons. Gold is first introduced in various functionalized mesostructured silicas (MCM-48 and SBA-15) and particles are formed inside the porosity upon reduction of Au3+ cations. Silica pores are then impregnated with a carbon precursor and the composite material is heated at 900°C under vacuum. Silica is then removed by acid leaching, leading to partially encapsulated gold particles in mesoporous carbon. Carbon prevents aggregation of gold particles at high temperature, both the mean size and distribution being similar to those observed in silica. However, while Au@SiO2 exhibit significant catalytic activity in the aerobic oxidation of trans-stilbene in the liquid phase, its Au@C mesostructured replica is quite inactive. Keywords: mesostructured carbon, gold nanoparticles, catalysis, aerobic oxidation

1. Introduction Gold nanoparticles are efficient oxidation catalysts both in the gas and liquid phases. It is however essential to stabilize particles with diameters below a few nanometers [1]. This requires sophisticated chemical methods, which are usually support-specific. Another strategy consists in limiting particle aggregation via physical confinement. Gold particles synthesized within mesoporous titania-modified silicates exhibit significant activity for structure-sensitive CO oxidation [2]. However, aerobic epoxidations in the liquid phase require the use of low-polarity solvents [3], in which these conventional oxide-supported catalysts are poorly dispersed. The use of activated carbons as supports, which enhances mass-transfer, could be beneficial to the efficiency of the overall catalytic system. We have developed a new strategy, based on the nanocasting concept, to prepare gold nanoparticles highly dispersed but partly occluded in meso-structured carbons.

2. Experimental Calibrated gold nanoparticles were formed inside the porosity of mesostructured silicas of various pore size and architecture (MCM-48 and SBA-15) using two different routes. In both routes, silica was preliminarily functionalized before contacting with an aqueous HAuCl4.3H2O solution. The two routes essentially differed by the nature of the graft: Ntrimethoxysilylpropyl-N,N,N-trimethylammonium chloride (TPTAC) in Route 1 and 3mercaptopropyltrilmethoxysilane (MPTMS) in Route 2. Gold was then introduced in functionalized mesoporous silicas and particles were formed upon reduction of Au3+

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cations with sodium citrate and/or NaBH4. The gold content was limited to 1-1.5wt.% to favor a high metal dispersion inside the pores and prevent the formation of large particles upon heating. In the case of MPTMS, gold nanoparticles were not formed at room temperature and it was necessary to heat the sample in air at 300°C. The silica pores were then impregnated with a carbon precursor (sucrose in H2SO4) following the recipe of Ryoo et al. [4] and the composite material was heated at 900°C under vacuum. Silica was then removed by HF leaching, leading to partially encapsulated gold nanoparticles in mesoporous carbon (Scheme 1). Gold particles inside silica pores

Impregnation Carbonization

Carbonimpregnated silica

HF washing SiO2 removal

Mesoporous carbon with partially encapsulated particles

Scheme 1. The various steps in the synthesis of gold-containing mesostructured carbons [adapted from ref 5].

All modified silicas and gold-containing mesoporous carbons were characterized by X-ray diffraction, N2 adsorption and TEM. Au/silicas and the corresponding Au/C replicas were tested in the aerobic oxidation of trans-stilbene in the liquid phase under conditions previously described [6].

3. Results All silicas were synthesized following literature procedures. Their structural and textural characteristics are reported in Table 1. Moreover, carbon replicas were also prepared on pristine silicas to evaluate their characteristics in the absence of gold particles. Table 1. Textural properties of the various silicas and the corresponding carbon replicas. Support/replica SBA-15 CMK-3 MCM-48 CMK-1

SBET (m2/g) 624 1218 1318 1507

Pore diameter (nm) 7.3 4 2.6 2.9

As shown in the Table, all silica and carbon supports possess a high BET surface area, in excellent agreement with data reported in the literature for similar materials. Moreover, the regularity and ordering of the porous network was evidenced by intense and well-defined reflections in the corresponding X-ray diffraction patterns (Figure 1).

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2 theta (°)

Figure 1. XRD patterns of silicas (-) and the corresponding carbon replicas (-●-). SBA-15 (left) and MCM-48 (right).

The regularity and long-range ordering of the pore system was not affected by the different post-synthesis treatments: gold-containing silicas still show well defined XRD patterns, even after reduction of Au3+ cations. Both routes lead to well dispersed Au particles in silica (Fig. 2). However, a systematic study performed with TPTAC showed that the number of functional groups attached onto the silica surface is critical for the size and location of Au particles. At low coverage, TPTAC molecules are preferentially located on the surface of silica particles, leading to large Au crystals upon reduction. (a)

(b)

(c)

Figure 2. TEM pictures of Au/SBA-15 (0.3 wt. %) obtained with TPTAC (a) and MPTMS heated at 300°C (b) and Au/C obtained by replication of the sample prepared with MPTMS (c).

Au particles are significantly bigger with TPTAC (6.1 nm) than with MPTMS (3.8 nm). Moreover, for MPTMS-modified supports, the particle size is not strongly affected by temperature (3.8 and 4.2 nm at 300 and 560°C, respectively). As shown in Figure 3, the distribution of gold NPs sizes is retained after carbon pyrolysis and dissolution of silica. This is not the case for the catalytic properties of the materials: while Au/SBA-15 (MPTMS-300°C) is an efficient catalyst of the aerobic oxidation of trans-stilbene (Figure 4), the corresponding Au/C replica is essentially inactive. This is attributed to the lower accessibility to the gold nanoparticles partially embedded within the carbon walls. Ways to tune this nanocasting-based strategy towards active mesostructured Au/C catalysts are under investigation.

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1

2

3

4 5 6 Particle size (nm)

7

8

Figure 3. Distribution of gold particle sizes in SBA-15 (MPTMS-300°C, black) and the corresponding CMK-3 (grey).

Despite mass-transfer limitations, Au/SBA-15 (MPTMS-300°C) remains an interesting catalyst for this liquid phase reaction. Its selectivity is indeed markedly different from that displayed by gold nanoparticles supported on passivated high surface area silica [6]: deoxybenzoin (1,2-diphenyl-ethanone), not the epoxide, is the main reaction product, with a yield of 45% at full conversion (78 h).

Trans-stilbene Conv. %

100

trans-stilbene conversion (■), epoxide (●) and deoxybenzoin (∆) yields. Reaction conditions: trans-stilbene (1 mmol), methylcyclohexane (solvent, 20 mL), tert-butyl hydroperoxide (0.05 mmol / 7 μL of a 70% TBHP in water Aldrich solution), catalyst (91.7 mg / 2 μmol Au), 900 rpm, 80°C, air (atmospheric pressure).

80 60 40 20 0 0

10

20

30 40 50 Reaction time (h)

60

70

80

Figure 4. trans-stilbene conversion over 0.3%Au/SBA-15 (MPTMS-300°C)

4. Conclusion New Au@C materials have been obtained using a nanocasting method, starting form preliminary functionalized silicas. Excellent dispersions were obtained, with Au particles partially embedded in carbon walls. This route is very promising for the preparation of catalysts for the aerobic oxidation of olefins in the liquid phase.

References [1] M. Haruta, Gold Bull., 37 (2004) 27-36. [2] V. Caps, Y. Wang, J. Gajecki, B. Jouguet, F. Morfin, A. Tuel, J.-L. Rousset, , Stud. Surf. Sci. Catal. 162 (2006) 127-134. [3] P. Lignier, S. Mangematin, F. Morfin, J.-L. Rousset, V. Caps, Catal. Today, 138 (2008) 50-54. [4] R. Ryoo, S.H. Joo, S. Jun, J. Phys. Chem. B, 103 (1999) 7743-7746. [5] S.M. Holmes, P. Foran, E.P.L. Roberts, J.M. Newton, Chem. Commun., 14 (2005) 1912-1913. [6] D. Gajan, K. Guillois, P. Delichère, J.-M. Basset, J.-P. Candy, V. Caps, C. Copéret, A. Lesage, L. Emsley, J. Am. Chem. Soc., 131 (2009) 14667-14669.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

TiO2 photocatalysts prepared by thermohydrolysis of TiCl4 in aqueous solutions A. Di Paola, M. Bellardita, L. Palmisano “Schiavello-Grillone” Photocatalysis group, Dipartimento di Ingegneria Chimica dei Processi e dei Materiali, Università degli Studi di Palermo, Viale delle Scienze, 90128 Palermo, Italy

Abstract Nanostructured TiO2 photocatalysts were synthesized by thermohydrolysis of TiCl4 at 100 °C in various aqueous solutions. Anatase or rutile, binary mixtures of anatase and rutile or anatase and brookite, and ternary mixtures of anatase, brookite and rutile were obtained depending on the hydrolysis solution. The most efficient catalysts consisted of ternary mixtures of the three polymorphic TiO2 phases. Keywords: photocatalysis, TiO2, anatase, brookite, rutile

1. Introduction Heterogeneous photocatalysis by semiconductor oxides is a promising method for the removal of many organic and inorganic pollutants from water and air (Fujishima et al., 1999). TiO2 is the most reliable photocatalyst because of its low cost and (photo)stability under irradiation. TiO2 exists in three main crystallographic forms: anatase (tetragonal), brookite (orthorombic) and rutile (tetragonal). All three crystalline structures consist of deformed TiO6 octahedra connected differently by corners and edges. Anatase is generally accepted to be a photocatalyst more efficient than brookite and rutile but mixtures of different TiO2 phases have often revealed photocatalytic activities superior than those of the pure phases (Di Paola et al., 2008; Di Paola et al., 2009). Sol-gel techniques are usually employed to produce TiO2 catalysts. The resulting materials are generally amorphous or not well crystallised and, consequently, they must be subjected to calcination to obtain active samples. In this work we report on the preparation of polymorphic TiO2 nanoparticles obtained by thermohydrolysis of TiCl4 in various aqueous solutions at 100° C. The preparation method is very simple and does not require the use of expensive thermal or hydrothermal treatments. The content of anatase, brookite and rutile is easily tailored by varying the composition of the hydrolysis solution. The degradation of 4-nitrophenol was chosen as model reaction to evaluate the photoactivity of the various samples.

2. Experimental 2.1. Preparation of the samples 1 ml of TiCl4 (Fluka 98%) was slowly added to different volumes of distilled water or aqueous solutions at room temperature. The solutions obtained after continuous stirring were heated in closed bottles and aged at 100 °C in an oven for 48 h. The bottles were allowed to cool and the resultant solids were recovered using a vacuum pump at 55°C.

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2.2. Characterization X-ray diffraction patterns of the powders were collected by a powder diffractometer employing the CuKα radiation and a graphite monochromator in the diffracted beam. The crystal phase composition of the catalysts was determined using a modified Rietveld method (Lutterotti et al., 1998). The specific surface areas (SSA) were obtained by nitrogen physisorption experiments performed at the liquid nitrogen temperature. The band gap values of the samples were obtained by diffuse reflectance spectra measurements: BaSO4 was the reference sample and the spectra were recorded in the range 200–600 nm. The position of the flat band potentials of anatase, brookite and rutile was determined measuring the photovoltage as a function of the suspension pH (Roy et al., 1995).

2.3. Photoreactivity experiments A Pyrex batch photoreactor of cylindrical shape containing 0.5 L of aqueous suspension was used. A 125 W medium pressure Hg lamp was immersed within the photoreactor and the photon flux emitted by the lamp was Φi = 13.5 mWcm−2. O2 was continuously bubbled for ca. 0.5 h before switching on the lamp and throughout the occurrence of the photoreactivity experiments. The amount of catalyst was 0.6 g L-1 and the initial 4-nitrophenol (NP) concentration was 20 mg L-1. The quantitative determination of 4-NP was performed by measuring its absorption at 315 nm. The photoactivity of the various samples was compared to that of commercial TiO2 Degussa P25.

3. Results and Discussion The physical properties of the solids obtained by thermohydrolysis of TiCl4 in aqueous solutions are strongly influenced by the synthetic variables. In particular, acidity, presence (and nature) of anions, and titanium concentration govern the composition and the photoreactivity of the TiO2 photocatalysts (Cheng et al., 1995; Koelsch et al., 2004). Depending on the experimental conditions, rutile or anatase, binary mixtures of anatase and rutile or anatase and brookite, or ternary mixtures of anatase, brookite and rutile, can be obtained. Table 1 shows the crystal phase composition of some selected samples prepared under different experimental conditions. Table 1. Crystal phase composition, specific surface area and initial reaction rate (r0) values of TiO2 powders prepared under different experimental conditions.

[Ti] mol L-1 concentrated HCl diluted HCl NaCl solution H2O Degussa P25 a

0.34 0.15 0.22 0.12

Anatase %

70.9 64.5 80.0

Brookite %

Rutile %

SSA m2·g-1

ro x 109 mol L-1 s-1

73.6 16.2 28.3

100.0 26.4 12.8 7.2 20.0

29 141 189 216 51

12.5 16.5 21.6 76.1 43.0 a

The runs were carried out at pH=3.3, obtained by adjustment with HCl.

Titanium(IV) cations form octahedral aquo-hydroxo complexes in an acid or neutral medium (Jolivet, 2000). As a consequence of hydrothermic treatments, the octahedra link together by olation, through dehydration reactions between aquo and

TiO2 photocatalysts prepared by thermohydrolysis of TiCl4 in aqueous solutions

227

hydroxo ligands. Rutile type nuclei are developed if the octahedra combine by sharing equatorial edges, whereas anatase or brookite type nuclei form if the monomers combine by sharing apical edges. Further growth proceeds by formation of linear chains from the rutile type nuclei or of skewed chains from the anatase or brookite type nuclei. The exact nature of the titanium(IV) octahedral complexes depends on the acidity and type of ligand in solution (Zheng et al., 2001; Pottier, 2001). In concentrated HCl solutions only rutile crystallites were developed whereas in dilute HCl solutions both rutile and brookite crystallites can be obtained contemporaneously. In NaCl solutions, ternary mixtures of anatase, brookite and rutile were formed. The composition of the powders obtained by thermohydrolysis of TiCl4 in water depended on the TiCl4/H2O ratio and binary or ternary mixtures of the three polymorphs were prevalently produced. The average particle sizes of all the phases present in the various samples were in the range 2-10 nm. The photocatalytic activity of the samples, as well their composition, depended on the hydrolysis solution. Table 1 reports the values of the initial degradation rate of 4NP, r0, determined in the presence of the most active TiO2 samples. The mixed systems revealed an enhanced photoactivity compared with that of the pure TiO2 polymorphic phases and some samples were more active than Degussa P25. The most efficient samples consisted of a ternary mixture of anatase, brookite and rutile. The high photocatalytic activity can be explained by the presence of junctions among different polymorphic TiO2 phases. Figure 1 shows the relative positions of the energy bands of anatase, brookite and rutile (Di Paola et al., 2009).

-1 - 0.37 V

- 0.45 V

- 0.45 V

V

0

1

2.98 eV

3.05 eV

3.27 eV

2

3 Rutile

Anatase

Brookite

Figure 1. Electrochemical potentials (versus NHE) of the band edges of anatase, brookite, and rutile at pH = 7. The coupling of semiconductors possessing different redox energies for their corresponding conduction and valence bands allows the vectorial displacement of holes and electrons from one semiconductor to another and reduces the recombination of the photogenerated electron/hole pairs, enhancing the efficiency of the interfacial charge transfer to adsorbed substrates (Serpone et al., 1995). The contact among the different phases is very efficient due to the small sizes of the crystallites.

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4. Conclusion A facile way to prepare active TiO2 photocatalysts has been developed. The crystal phase composition of the samples can be easily tailored by simply varying the type of aqueous solution. The most efficient samples consisted of a ternary mixture of anatase, brookite and rutile. The presence of junctions among different polymorphic TiO2 phases favours the separation of the photogenerated electron-hole pairs, enhancing the catalyst activity.

Acknowledgments The authors wish to thank MIUR (Rome) for financial support.

References H. Cheng, J. Ma, Z. Zhao, L. Qi, 1995, Hydrothermal preparation of uniform nanosize rutile and anatase particles, Chem. Mater. 7, 4, 663-671. A. Di Paola, G. Cufalo, M. Addamo, M. Bellardita, R. Campostrini, M. Ischia, R. Ceccato, L. Palmisano, 2008, Photocatalytic activity of nanocrystalline TiO2 (brookite, rutile and brookitebased) powders prepared by thermohydrolysis of TiCl4 in aqueous chloride solutions, Colloid. Surf. A: Physicochem. Eng. Aspects, 317, 1-3, 366-376. A. Di Paola, M. Bellardita, R. Ceccato, L. Palmisano, F. Parrino, 2009, Highly active photocatalytic TiO2 powders obtained by thermohydrolysis of TiCl4 in water, J. Phys. Chem. C, 113, 34, 15166-15174. A. Fujishima, K. Hashimoto, T. Watanabe, 1999, TiO2 Photocatalysis. Fundamentals and applications, Bkc Inc., Tokyo. J.-P. Jolivet, 2000, Metal oxide chemistry and synthesis: from solution to solid state, Wiley, Chichester. M. Koelsch, S. Cassaignon, J.-P. Jolivet, 2004, Synthesis of nanometric TiO2 in aqueous solution by soft chemistry: obtaining of anatase, brookite and rutile with controlled shapes, Mater. Res. Soc. Symp. Proc., 822, 79-84. L. Lutterotti, R. Ceccato, R. Dal Maschio, E. Pagani, 1998, Quantitative analysis of silicate glass in ceramic materials by the Rietveld method, Mater. Sci. Forum, 278-281, 87-92. A. Pottier, C. Chanéac, E. Tronc, L. Mazerolles, J.-P. Jolivet, 2001, Synthesis of brookite TiO2 nanoparticles by thermolysis of TiCl4 in strongly acidic aqueous media, J. Mater. Chem.11, 4, 1116-1121. A.M. Roy, G.C. De, N. Sasmal, S.S. Bhattacharyya, 1995, Determination of the flat band potential of semiconductor particles in suspension by photovoltage measurement, Int. J. Hydrogen Energy, 20,8, 627-630. N. Serpone, P. Maruthamuthu, P. Pichat, E. Pelizzetti, H. Hidaka, 1995, Exploiting the interparticle electron transfer process in the photocatalysed oxidation of phenol, 2-chlorophenol and pentachlorophenol: chemical evidence for electron and hole transfer between coupled semiconductors, J. Photochem. Photobiol. A: Chem. 85, 3, 247-255. Y. Zheng, E. Shi, Z. Chen, W. Li, X. Hu, 2001, Influence of solution concentration on the hydrothermal preparation of titania crystallites, J. Mater. Chem. 11, 5, 1547-1551.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Metal complex-assisted polymerization of thermosetting resins: a convenient one-step procedure for the preparation of heterogeneous catalysts Ulrich Arnold, Manfred Döring ITC-CPV, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany

Abstract A series of molybdenum-doped materials based on thermosetting cyanate ester and epoxy resins was prepared and tested as catalysts for the epoxidation of cyclohexene, styrene, 1-octene and propylene with tert-butyl hydroperoxide as oxidant. Monomers with more than two functional groups yield highly stable catalysts that can be used in several consecutive reactions without any catalyst reconditioning step. Metal leaching strongly depends on the resin as well as the substrate. Keywords: epoxidation; cyanate ester resins; epoxy resins; molybdenum; polymersupported catalysts

1. Introduction Thermosetting resins such as epoxy or cyanate ester resins are valuable precursors for the preparation of high performance materials. Applications are manifold, e.g. in the coatings sector or the manufacture of composites for light-weight construction.(1) Recently, epoxy resins were polymerized using metal complexes as initiators. Thus, a variety of catalysts could be obtained by simple mixing of epoxy resins with small amounts of metal complexes, typically around 5%, followed by heating. The resulting metal-doped materials were shown to be useful catalysts for a variety of reactions, e.g. epoxidation, C-C coupling, hydroformylation and hydrogenation reactions.(2) In the meantime, this concept was extended to other thermosetting resins. Cyanate esters and cyanate ester/epoxy resin blends were polymerized in the presence of Mo(OEt)5 and the resulting materials were tested as catalysts for the epoxidation of alkenes.

2. Experimental 2.1. General

The cyanate ester resins PRIMASET® LECy, PRIMASET® BADCy and PRIMASET® PT30 were provided by Lonza. The epoxy resins TGAP and TGMDA were purchased from Aldrich and DGEBA (EPR 164) was obtained from Bakelite AG (now Hexion Specialty Chemicals GmbH). Structures of the resin monomers and oligomers are summarized in Table 1. The molybdenum alkoxide Mo(OEt)5 was obtained from Gelest. Anhydrous tert-butyl hydroperoxide (TBHP) in toluene was prepared by azeotropic drying of 70 wt% TBHP in water (T-HYDRO® solution from Aldrich). Differential Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TGA) were carried out under N2 with a heating rate of 10°Cmin−1.

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U. Arnold and M. Döring Table 1. Resin monomers.

Cyanate ester resin monomers

Epoxy resin monomers

LECY NCO

OCN

DGEBA O

O

O

O

NCO

OCN

OCN

OCN

O

O

BADCY

TGAP O

N

O

OCN

n

PT30

N

O O

N

O O

TGMDA

2.2. Preparation of the catalysts Catalysts were prepared by vigorous stirring of resin/Mo(OEt)5 mixtures followed by curing in an oven. Typically, molybdenum contents around 0.8% were adjusted. In the case of resin blends 50/50 wt% mixtures of the resins were employed. The catalyst PT30-TGMDA0.75%Mo was prepared by combining a solution of Mo(OEt)5 in acetone with a 1:1 mixture of PT30 and TGMDA followed by removal of the solvent at elevated temperatures. The hardened materials were ground and particle diameters between 20 and 300 μm were adjusted. Catalysts, curing temperatures and some catalyst features are summarized in Table 2.

2.3. Epoxidation procedures Typically, a mixture of alkene (10 mmol), a 37.5 wt% solution of TBHP in toluene (14 mmol) and 500 mg of catalyst was magnetically stirred at 90 °C for 24 h. The catalyst was separated by filtration (PTFE filters; pore width: 0.45 μm) and employed in the next run without reconditioning. The filtrate was analyzed by GC and atomic spectroscopy. Epoxidation of propylene was carried out in a 80-ml steel autoclave charged with 50 mmol TBHP (34.0 wt% in toluene) and 1 g of catalyst. The solution was saturated with propylene and a pressure of 8 bar was adjusted. The reaction mixture was stirred for 24 h at 90 °C (operating pressure: ca. 20 bar). Propylene oxide yields were based on peroxide consumption determined by iodometric titration and GC analyses.

3. Results and discussion 3.1. Catalyst preparation and characterization Hardening of the liquid or paste-like resin/Mo(OEt)5 mixtures was carried out in aluminum molds by raising the temperature stepwise up to 230 °C (Table 2). DSC measurements revealed high polymerization enthalpies thus indicating high crosslinking. Accordingly, glass transition temperatures Tg and TGA data revealed increasing thermal stability along with an increase of functional groups in the resin monomers.

3.2. Catalytic performance Initially, catalysts based on LECY, BADCY, DGEBA and TGAP were tested in the epoxidation of cyclohexene (Fig. 1a). Alkene conversions and epoxide selectivities were between 94 and 100% in five consecutive reactions. However, extensive metal leaching was observed. Metal leaching could be vastly reduced by use of resins with more than 2 functional groups. In the case of PT30-TGMDA0.75%Mo the catalyst metal content after 10 reactions was still 99.91% of the metal content originally loaded on the polymer

231

Metal complex-assisted polymerization of thermosetting resin

(Fig. 1b). Using this catalyst cyclohexene conversion was around 63% (epoxide selectivity ≥ 96%) and values between 90 and 100% for cyclohexene conversion and epoxide selectivity were observed employing other catalysts containing PT30. Table 2. Curing, DSC and TGA data of molybdenum-doped resins. Curinga

Resin LECY0.84%Mo BADCY-DGEBA0.80%Mo BADCY-TGAP0.75%Mo PT30-LECY0.47%Mo PT30-TGAP0.75%Mo PT300.78%Mo PT30-TGMDA0.75%Mo

Tonsetb (°C) 80 132 120 160 115 147

A A A B A B C

DSC data Tpeakc ΔHd (°C) (Jg−1) 168 842 166 711 150 782 202 599 150, 218 449+g 190 404 Not determined

TGA data T5%f (°C) 242 286 291 311 316 343 317

Tge (°C) 140 163 209 229 n.d.h 225 n.d.

a

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Method A: 80°C, 1h – 100°C, 1h – 120°C, 1h – 140°C, 1h – 160°C, 1h – 180°C, 1h – 200°C, 1h – 220°C, 1h – annealing at 230°C for 6h; Method B: 120°C, 1h – 140°C, 2h – 160°C, 1h – 180°C, 1h – 200°C, 1h – 220°C, 1h – annealing at 230°C for 6h; Method C: Solvent removal at 50°C for 12h – Method A. bOnset temperature of reaction peak. cPeak temperature. dPolymerization enthalpy. eGlass transition temperature of the resulting material. fTemperature at 5% weight loss. gThe value is higher but not determinable due to an unsteady baseline. hNot detected.

50 0.75% Mo

BADCY-TGAP 0.47% Mo PT30-LECY 0.75% Mo PT30-TGAP 0.78% Mo PT30 0.75% Mo PT30-TGMDA

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Fig. 1. Catalytic performance of molybdenum-doped cyanate ester/epoxy resin systems in the epoxidation of cyclohexene. Catalysts (a) without and (b) with PT30.

Catalytic performances of PT30-TGAP0.75%Mo, PT300.78%Mo and PT30-TGMDA0.75%Mo in the epoxidation of styrene and 1-octene were also investigated (Fig. 2). Epoxide selectivities were 100% but alkene conversions were moderate. Apart from significant metal leaching in initial reactions, metal leaching was very low and strongly depended not only on the polymer but also on the alkene. Promising results were obtained in the epoxidation of propylene catalyzed by PT30-TGAP0.75%Mo (Fig. 2a). Propylene oxide yields were around 76% and no byproducts were detected.

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U. Arnold and M. Döring

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Fig. 2. Catalytic performance of (a) PT30-TGAP0.75%Mo, (b) PT300.78%Mo and PT30-TGMDA0.75%Mo in epoxidation reactions.

4. Conclusion Polymerization of thermosetting cyanate esters and epoxy resins in the presence of catalytically active metal species represents a facile, time- and cost-saving one-step procedure for the preparation of various catalysts. The resulting polymers exhibit outstanding stabilities, superior to most other catalyst systems based on organic polymers. Such a strategy offers several tunable parameters, e.g. different metal species, resins and (inorganic) additives. Hence, a large potential for optimization is available.

Acknowledgment The authors thank Lonza for supplying the cyanate esters.

References (1) W. Schönthaler, 2005, Chapter Thermosets, in: Ullmann’s Encyclopedia of Industrial Chemistry: Electronic release; John Wiley & Sons Inc.. (2) J. Artner, H. Bautz, F. Fan, W. Habicht, O. Walter, M. Döring, U. Arnold, 2008, Metaldoped epoxy resins: Easily accessible, durable and highly versatile catalysts, J. Catal., 255, 180-189.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Synthesis and study of mesoporous WO3-ZrO2-SiO2 solid acid S.V. Prudius, O.V. Melezhyk, V.V. Brei Institute for Sorption and Problems of Endoecology, National Academy of Sciences of Ukraine, Naumova str., 13, Kyiv, 03164, Ukraine

Abstract Three methods for synthesis of mesoporous WO3-ZrO2-SiO2 oxide with high surface area (350 m2/g) are proposed. According to determined acid site strength distributions, WZrSi (H0 ≥ -11.4) occupies the position between superacid WO3/ZrO2 (H0 ≥ -14.5) and middle acidic ZrO2-SiO2 (H0 ≥ -8.2) oxides. It was found that WZrSi provides high yield of polytetramethylene ether (PTME) in tetrahydrofuran oligomerization reaction and demonstrates high activity in the process of isobutane-isobutanol transformation into branched hydrocarbons C8. Keywords: solid acid, hammett acidity, tungstated zirconia, PMTE

1. Introduction Tungstated zirconia (WZr) is known as stable solid superacid which demonstrates high catalytic activity in numerous reactions with a proton transfer, especially in the hydroisomerization of n-C4 – C7 alkanes and Friedal-Crafts acylation of aromatic compounds. Relatively low specific surface area (< 70 m2/g ) is a certain disadvantage of WZr. Also, on today the synthesis of solid acids which occupy intermediate position between zeolites with their acidity function values (Н0 ≥ -8) and superacids (Н0 ≤ -12) is of interest. The data on synthesis and catalytic study of mesoporous WO3-ZrO2-SiO2 (WZrSi) oxide are presented in this communication.

2. Experimental The WZrSi samples were synthesized by three procedures. Zirconyl chloride octahydrate ZrOCl2·8H2O, ammonium metatungstate (NH4)6H2W12O40·xH2O, potassium silicate K2Si2O5 or tetraethoxysilane (TEOS), nonionic surfactant Triton CF-10 (Dow Chemical), and carbamide were used as starting reagents. At first, water sol of silicic acid was obtained through treatment of potassium silicate with H-exchange resin KU-2 (sulfonated styrene and divinylbenzene copolymer). In order to synthesize mixed ZrO2SiO2 (ZrSi) oxide, the aqueous solution of zirconium oxychloride was added to 1 l of polysilicic acid solution to give a mole ratio of Zr:Si = 1:2 [1]. Then, 0.5 wt.% of carbamide and 0.1 wt.% of Triton CF-10 were added. Resulting solution was held at reflux for 2 h while stirring to transform sol into gel. Synthesis of WZrSi samples were performed in the same way, but solution of ammonium metatungstate was added to SiZr sol at atom ratio W:Zr = 0.2:1. The obtained gel was simultaneously aged and dried at 120°C, 48 h. The xerogel was separated in two batches and then was calcined with two different procedures: 1. Xerogel was heated (2°C/min) up to 500°C and kept at this temperature for 4 h. During the treatment, volatile and combustible compounds were eliminated. The product was calcined at 700°C, 2 h and is denoted as WZrSi-d;

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2. Xerogel was washed with water to eliminate chloride ions, dried, and calcined at 700°C, 2 h. This sample is denoted as WZrSi-w. The third sample WZrSi-s (Zr:Si = 1:2) was prepared using TEOS. In a glass reactor, 0.2 mole of TEOS was dissolved in 20 ml of ethanol and 2M HCl mixture. Ammonium metatungstate was mixed with zirconium oxychloride solution and quickly added with vigorous stirring to silica sol (W:Zr = 0.2:1). The resulting solution was boiled for 1 h under stirring to transform sol into gel and then aged at 100°C for 24 h. The semitransparent gel was washed with water, dried at 120oC, and calcined at 700°C, 2 h. For comparison, the WZr-ht sample was synthesized according to hydrothermal procedure described in [2]. Total acidity of the samples was determined by the reverse titration using nbutylamine solution in cyclohexane with bromthymol blue as an indicator. The strength of acid sites was estimated by direct titration with n-butylamine using Hammett indicators (Aldrich): benzalacetophenone (pKBH+ = -5.6), antraquinone (-8.2), 4nitrotoluene (-11.35), 1-chloro-3-nitrobenzene (-13.16), 2,4-dinitrotoluene (-13.75) and 2, 4-dinitro-1-fluorobenzene (-14.52). All samples were dried at 500°C, 1h before testing. XRD patterns of samples were recorded on DRON-4-07 diffractometer (CuKα radiation). Surface areas, pore size distributions and pore volumes were measured by N2 adsorption at 77 K using Nova 2200e Surface Area and Pore Size Analyzer. Before analysis, the samples were treated at 300oC under vacuum. The catalytic activity of the WZrSi, WZr and ZrSi oxides were tested in two reactions: tetrahydrofuran (THF) oligomerization and isobutanol dehydration in the presence of isobutane for producing i-C8 hydrocarbons. The experiments were performed using flow reactors with fixed bed of catalyst.

3. Results and discussion The formation of predominantly tetragonal phase of ZrO2 in framework of all WZrSi samples calcined at 700ºC is registered in the XRD patterns. The ZrO2 crystallite size calculated from peak half-width using Sherrer equation is 4-5 nm for WZrSi-w and 9-10 nm for WZrSi-d. Consequently, burning of the volatile templates without preliminary washing leads to the formation of larger ZrO2 crystals. The parameters of pore structure and acidity of WZrSi in comparison with prepared WZr and ZrSi samples are presented in Table 1. Table 1. Pore structure parameters and acidity of prepared WZrSi, WZr and ZrSi oxides.

Sample

WZrSi-d WZrSi-w WZrSi-s WZr-ht ZrSi

Specific Pore volume (BJH), surface area (SA), cm3/g m2/g 250 0.16 270 0.17 350 0.27 175 0.35 300 0.15

Pore radius (BJH), nm 2.2 2.0 3.6 5.2 2.8

Average pore radius, nm 1.8 1.4 3.2 7.7 1.6

Total acidity, mmol/g

H0

1.09 1.10 1.50 0.64 1.31

≥ -11.35 ≥ -11.35 ≥ -11.35 ≥ -14.52 ≥ -8.2

The SA values of prepared WZrSi samples are in the range of 250-350 m2/g, which is close to SA for ZrSi sample and ca. 4-5 times higher than for WО3/ZrО2 samples obtained by usual co-precipitation technique without hydrothermal treatment [3]. It should be noted that WZrSi-d possesses larger pores than WZrSi-w prepared from

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washed out xerogels. Perhaps, carbamide and ammonium chloride perform the role of templates and pore-forming substances. Total acidic site concentrations determined using n-butylamine adsorption consist 1.1–1.5 mmol/g for WZrSi samples calcined at 700°C (Table 1). The acid site strength distributions for synthesized oxides are presented on Fig. 1. For WZr-ht sample a wide range of acid strength is observed, from medium (-5.6 ≥ H0 ≥ -8.2) up to superacidic H0 = -14.5 (~5%). About 60% of the sites on the surface of ZrSi sample corresponds to H0 = -5.6. It has been found that WZrSi samples change their color from white to light yellow in the presence of 4-nitrotoluene (H0 = 11.35). However, WZrSi is a stronger solid acid than ZrSi because about 80% of its acid sites are characterized with Hammett acidity function value H0 = -8.2 (Fig. 1). C , m m ol/g 0 .8 0 .7 0 .6 0 .5 0 .4 0 .3 0 .2 0 .1 0 .0

− 1 3 ,7 5 ≤ Η 0 < − 1 4 ,5 2 − 5 ,6 ≤ Η 0 < − 8 ,2 − 1 1 ,3 5 ≤ Η 0 < − 1 3 ,1 6 − 1 3 ,1 6 ≤ Η 0 < − 1 3 ,7 5 − 8 ,2 ≤ Η 0 < − 1 1 ,3 5

Fig. 1. Concentration - strength acid site distributions for ZrSi ( ), WZrSi-w (■) and WZr-ht (□) samples.

The WZrSi, WZr and ZrSi oxides demonstrate high activity in liquid-phase THF oligomerization process (Fig. 2). However, the WZrSi catalyst provides higher yield of polytetramethylene ether (PMTE) at feed rates 8 - 13 mmol THF/gcаt/h in comparison with WZr and ZrSi oxides (Fig. 2). 50

Conversion, %

45 40 35 30

1

25

3

2

20 0

2

4

6

8

10

12

14

16

18

Feed rate, mmol THF/gcat/h Fig. 2. THF conversion over WZrSi-d (1), WZr-ht (2) and ZrSi (3) at different feed rates (Vcat=1 cm3, 60°C, THF: acetic anhydride = 8 : 1 mol).

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Typically PTME olygomers with Mn = 500-900 are formed. In the transformation of isobutanol – isobutane mixtures into high-octane hydrocarbons i-C8, WZrSi-w shows lower activity then WZr-ht (Table 2). This solid acid provides the yield of liquid phase of alkanes i-C8 and olefin i-C′8 at the level of 28 mol % (on expended alcohol) in comparison with 50 % for WZr-ht. The lower yield of alkanes i-C8 (13%) is observed for WZrSi-w also. Table 2. The content and yield of hydrocarbons i-C8 obtained over different catalysts1.

Catalyst

Т, oC

Р, МPа

WZr-ht WZrSi-w ZrSi

210 200 200

1.5 0,7 0,7

1)

Feed rate, mmol (iC4H9OН)/gcat/h 4 2 2

Yield, mol%

іC8H18,%

iC8H16,%

50 28 25

21 13 15

79 87 85

Vcat = 3 cm3, i-C4H10 : i-C4H9OН = 6 : 1 mol

It should been noted that WZrSi catalyst, as well as WZr, can be repeatedly (>20 times) regenerated by calcining at 600оС, 2 h without loss of it activity.

4. Conclusions Three synthetic procedures for preparation of mesoporous WZrSi oxide with high specific area have been proposed. The method utilizing TEOS allows WZrSi samples to be prepared with SA = 350 m2/g and concentration of acidic sites up to 1.5 mmol/g. Acid site strength distributions show WZrSi is strong solid acid which occupies the position between superacid WZr and middle acidic ZrSi oxides. WZrSi can be considered as promising catalyst for the production of PTME via THF oligomerization. Also WZrSi demonstrates high activity in the conversion of isobutane-isobutanol into high-octane hydrocarbons i-C8.

References [1] A. Tarafdar, A.B. Panda, P. Pramanik, Microporous and Mesoporous Materials, 84 (2005) 223. [2] M.A. Cortes-Jacome, J.A. Toledo, C. Angeles-Chavez, M. Aguilar, J.A. Wang, J. Phys. Chem. B, 109 (2005), 22730. [3] V.V. Brei, A.V. Melezhyk, S.V. Prudius, E.I. Oranskaya, Polish J. Chem., 83 (2009) 537.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Citral hydrogenation over Pt-M/CeO2 catalysts (M= Zn, Zr) M. Aoun a, b, M. Chaterb, P. Marecot c, C. Especel c, G. Lafaye c, a

Centre de Recherche scientifique et technique en Analyses Physico-Chimiques (C.R.A.P.C), Alger RP, 16004BP 248, Algérie b Laboratoire d’Etude Physico-Chimique des Matériaux et Application à l’Environnement, Faculté de Chimie, USTHB, El Alia, BabEzzouar, Alger B.P.32, 16111, Algérie. c Université de Poitiers, Laboratoire de Catalyse en Chimie Organique, UMR 6503, 86022 Poitiers, France

Abstract Bimetallic PtZn/CeO2 and PtZr/CeO2 catalysts prepared by impregnation were tested for the selective hydrogenation of citral. Samples with 5wt% Pt and atomic ratio Zn/Pt=Zr/Pt=5 were reduced at 450°C. The monometallic Pt/CeO2 sample was also prepared and studied for comparisons. Samples were analysed by TPR, H2-chemisorption and cyclohexane dehydrogenation. Their catalytic behaviour was evaluated in the citral hydrogenation reaction after reduction treatments in flowing hydrogen at 450°C. Results obtained show that the presence of Zn clearly promotes the hydrogenation of the carbonyl bond. Large differences in reducibility between catalysts were determined from the TPR results. A modification of the catalytic properties of platinum has been achieved by modifying the Pt/CeO2 catalyst by addition of Zn and Zr. Keywords: platinum catalysts; Pt/CeO2; Zn-promoted catalyst

1. Introduction Citral (3, 7-Dimethyl-2, 6-octadienal) is an α , ß-unsaturated aldehyde and the main component of the lemongrass oil [1]. As an unsaturated aldehyde it is a very attractive model molecule for hydrogenation from both scientific and industrial points of view [2]. The hydrogenation products of citral have all important uses not only in the synthesis of flavors but also in pharmaceutical and cosmetic industries. Citronellal and citronellol are especially interesting to the perfume industry because of their highly pleasant odors. The presence of 3,7-Dimethyl octanol and 3,7-Dimethyl octanal detracts from this valuable quality [3]. The reduction of citral can lead to a variety of products, the hydrogenation of the C=O bond produce geraniol and nerol. On the other hand, the hydrogenation of the conjugated C=C bond leads to the saturated aldehyde citronellal, which through cyclisation can form isopolegols. The hydrogenation of the isolated C=C bond leads to citronellol and finally to 3,7-diméthyloctanol. The purpose of the present work was to prepare active and selective ceria supported platinum catalysts for the hydrogenation of citral in order to produce unsaturated alcohols, namely nerol and geraniol. Furthermore, catalysts have been characterized by a number of techniques in order to correlate their surface characteristics with their behaviour in the reaction.

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2. Experimental The CeO2 support (99,995%, Aldrich) was calcined in flowing air for 4 h at 500°C. Prior to use, it was ground and then sieved to retain particles with sizes between 0,1-0,25 mm. 5Wt.% Pt/CeO2 catalyst was prepared by successive impregnation from Cl6H8N2Pt (44% Pt, Aldrich) precursor salt. The catalyst was dried overnight at 120°C and calcined at 500°C for 4h. Bimetallic PtZn/CeO2 and PtZr/CeO2 catalysts with Zn/Pt and Zr/Pt atomic ratio of 5 were prepared as the monometallic catalysts using ZnCl2 (99,999%, Aldrich) and ZrCl4 (98% purity, Aldrich) salt precursors. Catalysts description: PtCe: Pt/CeO2 catalyst; PtZnCe: PtZn/CeO2 catalyst, PtZrCe: PtZr/CeO2 catalyst The amount of H2 chemisorbed on the catalysts was measured in the same condition used by F. Benseradj and coll [4]. TPR experiments were carried out on a pulse chromatograph described else-where [5]. Cyclohexane dehydrogenation reaction was carried out following a procedure described elsewhere [6]. The hydrogenation of citral was performed in the liquid phase following a procedure described elsewhere [7] under the same conditions.

3. Results and discussion 3.1. H2 chemisorption

Selected supported platinum catalysts on CeO2 were characterized by H2 chemisorption after reduction at 500°C. Table 1 reports the results obtained. It was observed that the high hydrogen uptake of the monometallic catalyst reflects high platinum dispersion (69%). However, the metal dispersion is not sensitive to the presence of Zr. But the chemisorption values decreases with Zn addition, reflecting a loss in the number of surface sites, suggesting either the development of a SMSI (strong metal-support interactions) or a state of sintering of the metal particles. This lower ability for H2 chemisorption must be attributed to an increase in the support coverage by Zn species and to a poorer dispersion. These results are similar to those found by Silvestre-Albero and coll. [8] on Pt/TiO2 catalysts and by Aoun and coll. [9] on rhodium catalysts. Table 1. Characterization of samples, Hc: amount of hydrogen chemisorptions (μ mole/gcata), DHc: metal dispersion (%).

Catalysts PtCe PtZnCe PtZrCe

Hc (μ mole/gcata) 176,79 112.8 169.2

DHc(%) 69 44 66

3.2. Temperature programmed reduction Figure 1 summarized the TPR curves of all catalysts which were submitted to the same treatment before the TPR experiments. We noticed that the addition of zinc modifies the shape of the TPR curve of platinum. The observed evolutions are in accordance with previous work [10]. In brief, for PtCe catalyst, the first peak centred at 250°C which is assigned to the surface reduction of ceria in close contact with the metal, and may also include the reduction of platinum. The TPR profile obtained with PtZnCe catalyst shows two broad hydrogen consumptions. The first centred at 248°C can be correlated to the same peak observed above on the monometallic catalyst. Then, the second hydrogen consumption, appears at 494°C can be attributed to the reduction of platinum

Citral hydrogenation over Pt-M/CeO2 catalysts (M= Zn, Zr)

239

and/or zinc species. Nevertheless, the amount of hydrogen consumption is higher than that of other catalysts. A displacement of the maximum reduction temperature towards a relatively low temperature (200°C) is noted on PtZrCe catalyst.

3.3. Catalytic activity for cyclohexane dehydrogenation The different catalysts were tested for a structure insensitive reaction [11], the cyclohexane dehydrogenation, at 270°C and at atmospheric pressure. The evolution of the specific activity is shown in Figure 2. It can be seen that PtZnCe catalyst is not active in this reaction. This result suggests an interaction between the two metals (PtZn). Regarding hydrogen chemisorption, there is a decrease in the chemisorption values with the Zn addition. It has been reported in the literature that the zinc interacts strongly with platinum [10]. Moreover, the specific activity is practically not modified by Zr addition to PtCe. This result would indicate a negligible electronic change or a low alloy formation after Zr addition to Pt. Quantité d'hydrogène consommée (u,a)

1,0

PtC PtZnCe PtZrCe

0,8

0,6

0,4

0,2

0,0 0

100

200

300

400

500

600

700

Température (°C)

Fig. 1. TPR Profiles of PtCe, PtZnCe and PtZrCe catalysts.

Fig. 2. Specific activity evolution over catalysts.

3.4. Citral hydrogenation The catalytic behaviour of the samples is compared in the liquid phase hydrogenation of citral. Under the present experimental conditions, the products observed are citronellal, citronellol, unsaturated alcohols (geraniol and nerol). Figure 3 shows the results of their catalytic activity. A rapid hydrogenation occurs during the first few minutes, then the catalysts activity decreases. The same observations are noted by Lafaye and colleagues on Rh/Al2O3 catalysts. The explanation generally proposed is a decomposition of the citral or unsaturated alcohols yielding chemisorbed CO and carbonaceous species that accumulate on the catalysts surface and block a part of the active sites [12]. However, PtZnCe was the most active, reaching a citral conversion value of 52% after 30 min of reaction. This value is higher to that found by Malathi and colleagues [13]. This result could be due to a lower number of accessible platinum active sites for citral hydrogenation. From these results it can be concluded that the Zn addition to PtCe produces an important electronic modification of the metallic phase. The selectivity of the different samples is shown in Fig. 4. The selectiviy to nerol + geraniol is clearly enhanced by the Zn addition to PtCe. The modification of the catalytic activity in citral hydrogenation, when Zn is added to Pt, could be related to: 1) enhancing the selective reduction of the carbonyl group of citral leading to a production of nerol and geraniol, 2) Zn addition would inhibit the hydrogenation of -C=C-bonds, which is reflected in a lower formation of citronellal, citronellol and the saturated alcohol (3,7dimethyloctanol), the effect of Zn was in agreement with the literature [10]. The selective reduction or hydrogenation of the carbonyl group and the inhibition of the hydrogenation of -C=C-bonds of citral would require a particular structure of the metallic surface. By assuming that ionic Zn would enhance the polarization of the oxygen atom in the carbonyl group, increasing the positive charge on the carbon atom of the carbonyl group and favoring its reaction withhydrogen atoms dissociated in the neighbouring Pt atoms. According to the results of cyclohexane dehydrogenation reaction, the specific activity values for PtZnCe catalyst is different to that of the monometallic one, which indicate a high electronic effect between Pt and Zn, meaning a

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high concentration of PtZn alloy particles. Another topic related to the structure of the metallic phase is the formation of PtZn alloys [10]. Taking into account the results shown in Table 1, it can be observed that H2 chemisorption value decrease when Zn is added. This would mean that the size of these new ensembles after the Zn addition appears to be such as to inhibit the -C=C- hydrogenation. On the other hand, the value obtained on PtZrCe catalyst is very close to that of the monometallic one, which indicates a low electronic effect between Pt and Zr, meaning a low concentration of PtZr alloy particles. 100

100

PtCe PtZnCe PtZrCe

60

40

PtCe PtZnCe PtZrCe

80

Selectivity (%)

Conversion (%)

80

60

40

20

20

0

0 0

20

40

60

80

100

120

Time, min

Fig. 3. Citral hydrogenation as function as function of time

0

20

40

60

80

100

120

Time, min

Fig. 4. unsaturated alcohols selectivity as function of time

4. Conclusion Zn addition to PtCe modifies the selectivity in the citral hydrogenation. The modification of the selectivity to nerol + geraniol by the zinc addition to Pt can be associated to an important change in the structure of the metallic structure. On the basis of the test reaction results of the metallic phase, the catalytic behaviour in citral hydrogenation, H2 chemisorption experiments, PtZnCe catalyst can be described as having large metallic particles. A high concentration of Pt alloys appears to exist on the metallic surface of the particles to give any activity in the cyclohexane dehydrogenation. However, a low electronic effect takes place between Pt and Zr, giving a low concentration of PtZr alloy particles.

References [1] I.M.J. Vilella, S.R. Miguel, C.S.M. Lecea, A. Linares-Solano and O.A. Scelza, Appl. Catal. A: Gen. 281 (2004) 247. [2] M. Arvela, P. Tiainen, L.P. Lindblad, M. Demirkan, K. Kumar, N. Sjöholm, R. Ollonqvist, T. Väyrynen, J. Salmi and D. Yu. Murzin, Appl. Catal. A: Gen. 241 (2003) 271. [3] M.A. Aramendia, V. Borau, C. Jimenez, J.M. Marinas, A. Porras and F.J. Urbano, J. Catal 172 (1997) 46. [4] F. Benseradj, F. Sadi, M. Chater, J. Soc. Alg. Chim. 12 (2002) 99. [5] D. Duprez, F. Sadi, A. Miloudi, A. Percheron-Guegan, Stud. Surf. Sci. Catal. 71 (1991) 629. [6] T. Ekou, A. Vicente, G. Lafaye, C. Especel, P. Marecot, Appl. Catal. A: Gen: 314 (2006) 73. [7] K. Kouachi, G. Lafaye, C. Especel, O. Cherifi, P. Marecot, J. Mol. Catal. A: Chem. 280 (2008) 52. [8] J. Silvester-Albero, A. Sepulveda-Escribano, F. Rodriguez-Reinoso, J.A. Anderson, J. Catal. 223 (2004) 179. [9] M. Aoun, M. Chater, C.R. Chim. 10 (2007) 644. [10] J. Silvester-Albero, A. Sepulveda-Escribano, F. Rodriguez-Reinoso, J.A. Anderson, Appl. Catal A: Gen. 304 (2006) 159. [11] D.W. Blakely, G.A. Somorjai , J. Catal. 42 (1976) 181. [12] U.K. Sing, M.A. Vannice, J. Catal. 199 (2001) 73. [13] R. Malathi, R.P.Viswanath, Appl. Catal. A: Gen. 208 (2001) 323.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Foam-supported catalysts tailored for industrial steam reforming processes Raphaël Faurea, Francesco Basileb, Irene Bersanib, Thierry Chartiera, Aude Cunic, Mathieu Cornillacc, Pascal Del Galloc, Grégory Etchegoyend, Daniel Garyc, Fabrice Rossignola, Angelo Vaccarib a

SPCTS (Sciences des Porcédés Céramiques et de Traitments de Surfaces), UMR 6638 CNRS/ENSCI/Université de Limoges, Limoges, France bDipartimento di Chimica e Chimica Industriale, University of Bologna, Bologna, Italy c Air liquide CRCD Research Centre, Jouy-en-Josas, France d CTTC, Limoges, France

Abstract Alumina foams coated with Rh/MgAl2O4 spinel active phases have been produced to be used as catalysts in steam reforming processes with improved thermal transfers and limited pressure drops. Those foam-supported catalysts are here fully characterised before and after aging in water-rich atmosphere at elevated temperatures. It is shown that they are stable at any architectural scale: macro- (foam), micro- (coating) and nano(Rh active phase) structures. Such catalysts are then very promising catalytic loads to be further implemented in industrial units instead of standard loads. Keywords: foam-supported catalysts, Rh, spinel, steam reforming

1. Introduction The Steam Reforming of Methane (SMR) process is today widely used to produce hydrogen. However SMR also conducts to high amounts of CO2 releases. Moreover, the catalytic loads conventionally used in SMR processes are causing thermal transfer limitations and pressure drops. In order to decrease the impact of SR processes on the environment, other solutions that use bio-fuels as reactants are developed today, amongst which bio-ethanol [1]. Thermal transfer limitations and pressure drops can be partly solved by the use of highly porous catalysts. Such supported catalysts are already in development in many processes such as pollution abatement processes (catalytic converters) or conventional catalytic processes.[2, 3] Our work takes into account industrial requirements and existing issues of SR processes. Active phases suitable for both ESR and SMR are prepared and coated on alumina foams. The stability of the as prepared catalysts is tested in hydrothermal atmospheres (coating adherence, thermal stability, compressive strength of the foams, corrosion resistance). The catalysts produced are finally tested in SMR and ESR reactions at lab-scale and pilot-scale, before being implemented in industrial plants.

2. Experiments 2.1. Preparation of alumina foams First described in 1963, the fabrication of ceramic foams by impregnation of polymeric sponge-like templates has been widely studied.[4-6] For the need of our study, alumina foams are prepared by impregnating polyurethane-ester type foams cut into cylinders

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with an alumina slurry containing additives such as dispersing agents (polymethacrylate ammonium, 2wt%), binders (polyvinylacetate, 4wt%). Additional organics can be used to improve the coating of PU foams (anti-foaming agents and wetting-agents). Impregnated foams are successively dried (r.t., overnight), organics are pyrolized (650°C, 1°C.min-1 heating rate) and alumina is sintered at 1600°C for 2h. The monoliths thus obtained exhibit a densification of 99%.

2.2. Preparation of active phases and coating techniques Active phases are made of Rh dispersed on a magnesium aluminate spinel support. Two preparation techniques are reported here. The first preparation method consists in coating active phases on alumina foams. Commercial spinel powder is attrition-milled to break agglomerates. Attrition-milled MgAl2O4 spinel powder is dispersed in an aqueous solution of rhodium nitrates under stirring at room temperature, for 2h. The amount of Rh is calculated to produce 20wt% Rh loaded catalysts. Water is then evaporated from the slurry and residues are calcined in air (450°C, 4h). Powders obtained are used to prepare slurries which are later used to coat alumina foams. Another preparation method, described in details in the poster communication, can be used. It consists in using sodium polystyrene sulfonate (PSS) polymer to create negative charges on spinel that favour adsorption of Rh3+ precursor.

2.3. Aging tests Active phases, catalyst supports and supported-catalysts are aged in hydrothermal atmospheres figuring the most extreme conditions for SR processes. Powders or foams are introduced in a tubular furnace and heated at 900°C in water/nitrogen (3:1 in molar ratio) mixed atmosphere for durations ranging from 12 hours to 30 days. It must be noticed that active phases and supported catalysts are reduced in 5% H2/Ar before aging.

3. Results and discussions 3.1. Alumina foams characterisation Alumina foams are produced in a variety of pore sizes and shapes. Their porosity can be typically controlled to create graded catalytic beds that enable to improve the thermal profile in the reactor [7, 8]. The stability of alumina foams is attested in water rich atmosphere. TG analysis (Fig. 1.C) realised on alumina foams revealed that no weight losses can be observed upon aging at 900°C (90% relative humidity). However low surface corrosion is observed after 30 days of aging (Fig. 1.A&B). Micrographs from the bulk of alumina foams (not shown here) revealed no microstructural changes before and after aging. In order to point out surface modifications, alumina foams are polished and aged in water rich atmosphere.

Fig. 1 Evolution of alumina foams upon aging. Microstructure of the foam before aging (A) and after 30 days aging (B). TG analysis of alumina foam in water rich atmosphere (C).

Foam-supported catalysts tailored for industrial steam reforming processes

243

After 5 days of aging treatment, grain boundaries appeared on SEM micrographs. Surface modifications is assumed to be due to formation of volatile aluminium hydroxides, as expected by themodynamic modelling [9]. As the corrosion phenomenon remains limited to the surface, it is not affecting the stability of the foam in the bulk. As a proof, mechanical resistance of alumina foams is kept constant before and after aging: the compressive crush strength of 10 ppi alumina foams with 85% apparent porosity has been measured at 2 MPa.

3.2. Active phases 3.2.1. Bulk and surface characterisation Bulk (XRD, TDA-TG, TPR) and surface (XPS) analyses allow determining the Rh-O species present in Rh/MgAl2O4 catalysts. At low calcination temperatures, amorphous rhodium oxides are formed. Such species are reduced at low temperature (Fig. 2.B) and cannot be seen on HT-XRD (Fig. 2.A). Rh-O species crystallise in α-Rh2O3 at 720°C. Crystallite size increases between 720°C and 950°C, as a shift toward higher reduction temperatures can be observed on Fig. 2.B. At 950°C RhMgAl solid solution starts to form, as identified by the new line appearing on HT-XRD and by a large reduction peak from 800°C on TPR. Similar phases have already been described in the literature [10].

Fig. 2 HT-XRD (A) and TPR (B) characterization of 20wt% Rh/spinel catalysts calcined at 500°C (RhS500), 600°C (RhS600), 720°C (RhS720), 880°C (RhS880) or 950°C (RhS950).

3.2.2. Rh dispersion stability against coalescence Nice Rh dispersions of nano-sized Rh particles (1 to 3 nm, 75% metal dispersion) are obtained by reduction of fresh spinel-supported catalysts exhibiting amorphous Rh-O species (i.e. low initial calcinations temperature). Active phases calcined at higher temperatures (from 720°C to 1000°C) exhibit larger Rh particles. Nano-sized Rh particles observed on fresh RhS500 catalyst coalesce upon aging at 900°C in water-rich atmosphere: metal dispersion is decreased to 19%.

Fig. 3 TPR of aged Rh/spinel catalysts (A) and HR-TEM.

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HR-TEM micrographs reveal that particles of 1 to 5 nm coexist together with larger particles of 10 to 30 nm after aging (Fig. 3.B, RhS500). For samples calcined at higher temperatures (720°C, 880°C), lower Rh dispersions are maintained (Fig. 3.B, RhS720&880). On any aged samples, a solid solution is clearly evidenced at 600°C to1000°C by TPR (Fig. 3.A). Such interfacial solid solution would anchor Rh particles and limit their coalescence upon aging.

3.3. Foam-supported catalysts Foam-supported catalysts are first produced by washcoating the active phases on alumina foams. Coating thicknesses of 1 to 10 µm are produced. The homogeneity of the coating thickness is difficult to control on foams, but it is influenced by the spinel loading the slurry. Similar coatings are obtained by impregnating diluted Rh nitrate solutions on foams that are already washcoated with PSS-adsorbed spinel. It is demonstrated in the poster communication that sulfonate groups of PSS create negative charges on the spinel surface, thus preventing from inhomogeneous Rh dispersion that often occur during impregnation of spinel-coated alumina foams.

4. Conclusions Stable architectures dedicated to industrial SR processes are being developed. Alumina foams are produced by impregnation of polymeric sponge-like templates. Foams can exhibit a variety of porosities and shapes. Rh/spinel active phases are currently used. Active phases can be prepared separately and then coated on alumina foams, or catalysts can be directly prepared by impregnation of spinel-coated alumina foams. An interesting methodology is currently developed to favour the homogeneous Rh loading while impregnating spinel-coated alumina foams. It is shown that the best Rh dispersions on fresh catalysts are obtained after calcinations at low temperature, i.e. by reduction of amorphous Rh-O species. An interfacial RhMgAl solid solution, appearing upon aging, could be responsible for Rh stabilisation. The foam-supported catalysts are stable at 900°C in SR-like water-rich environments. These supported catalysts are currently giving promising results both in ESR and in SMR.

References [1] A. Haryanto, S. Fernando, N. Murali, S. Adhikari, Energy and Fuels 19 (2005) 2098-2106. [2] R.M. Heck, S. Gulati, R.J. Farrauto, Chemical Engineering Journal 82 (2001) 149-156. [3] M.V. Twigg, J.T. Richardson, Industrial and Engineering Chemistry Research 46 (2007) 4166-4177. [4] P. Colombo, Philosophical Transactions: Mathematical, Physical and Engineering Sciences (Series A) 364 (2006) 109-124. [5] J. Luyten, I. Thijs, W. Vandermeulen, S. Mullens, B. Wallaeys, R. Mortelmans, Advances in Applied Ceramics 104 (2005) 4-8. [6] K. Schwartzwalder, A.V. Somers, United States Patent Office 3,090,094 (1963). [7] P. Del Gallo, M. Cornillac, F. Rossignol, R. Faure, T. Chartier, D. Gary, EPO EP 2 123 618 A1 (2010). [8] P. Del Gallo, D. Gary, T. Chartier, M. Cornillac, R. Faure, F. Rossignol, EPO EP 2 141 140 A1 (2010). [9] E.J. Opila, D.L. Myers, Journal of the American Ceramic Society 87 (2004) 1701-1705. [10] F. Basile, G. Fornasari, M. Gazzano, A. Kiennemann, A. Vaccari, Journal of Catalysis 217 (2003) 245-252.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Synthesis of ordered nanostructured CuO-CeO2 catalysts by hard template method Petar Djinovića, Jurka Batistaa, Janez Leveca, Albin Pintarb a

Laboratory for Catalysis and Chemical Reaction Engineering, National Institute of Chemistry, Hajdrihova 19, SI-1001 Ljubljana, Slovenia b Laboratory for Environmental Sciences and Engineering, National Institute of Chemistry, Hajdrihova 19, SI-1001 Ljubljana, Slovenia

Abstract This study focuses on the hard template synthesis method, which was used to prepare CuO-CeO2 mixed oxide catalysts with 10, 15 and 20 mol % CuO content. KIT-6 silica template was synthesized using a TEOS/Pluronic P123 ratio of 60 and aging temperature of 100°C. After template removal by NaOH etching, CuO-CeO2 solids exhibited ordered mesoporous structure, which was identified by N2 adsorption/ desorption and XRD analyses as a cast of the KIT-6 silica mesostructure. BET surface area of tested materials was in the range of 147-166 m2/g and average CuO particle size between 1.3 and 1.9 nm. High activity and selectivity (over 99 %) of these solids was achieved during WGS reaction in the temperature range from 250 to 450°C. Keywords: hard template synthesis, CuO-CeO2 nanostructured catalysts, WGS reaction

1. Introduction CuO-CeO2 mixed oxides are very active and selective catalysts for preferential CO oxidation in excess H2 (CO PROX) [1] and water-gas shift (WGS) reaction [2]. Reducing particle size to nano scale and increasing surface area will provide a larger number of more active sites and consequently lead to higher activity in abovementioned reactions [3]. By using an inorganic template with high porosity and ordered arrays of mesopores, which is impregnated with a catalyst precursor solution and dissolved after their mineralization, catalysts with high surface area can be obtained, which cannot be usually prepared by conventional methods [4]. KIT-6 silica exhibits controllable pore size, interconnectivity of pores and high surface area, which can be tailored for practical applications by different synthetic pathways, thus making it an ideal template [5]. In this work, we report in detail on the synthesis and characterization of ordered CuO-CeO2 mesoporous mixed oxides (with 10, 15 and 20 mol % CuO content) by using a hard template method with KIT-6 silica acting as a template. The obtained CuO-CeO2 oxides were characterized by a variety of techniques, such as N2 adsorption/desorption, XRD, H2-TPR/TPD, H2-TPR/TPO/TPR, selective N2O chemisorption, and tested for performance in WGS reaction conducted in a continuous-flow fixed-bed reactor.

2. Experimental 2.1. Synthesis of KIT-6 silica template Mesoporous KIT-6 silica was prepared in aqueous solution by mixing 144 g of distilled water and 7.5 g of concentrated HCl (37 %, Merck). In this acidic solution, 4 g of Pluronic P123 (EO20PO70EO20 poly-(alkylene oxide) based triblock copolymer,

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MW=5800, Aldrich) as a structure directing agent was dissolved, while mixing on a magnetic stirrer. Afterwards, 4 g of butanol (absolute, p.a., Merck) was added under stirring and left for 1 h at 35º C. Finally, 8.6 g of TEOS (Si(OC2H5)4, 99.0 % purity, Fluka) was added and stirred for additional 24 h at the same temperature. Molar ratio of SiO2/P123 strongly influences the average pore size, pore volume and BET specific surface area of the prepared template [5]. At higher ratios lower pore volume, smaller pore size and lower BET specific surface area are usually obtained [6]. Furthermore, pore interconnectivity is reported to decrease when using SiO2/P123 ratios above 60, which could decrease the rigidity of the CuO-CeO2 mesostructure and cause its collapse during the subsequent process of template removal. During the synthesis of KIT-6 silica, we used SiO2/P123 ratio equal to 60, since it was reported as the best compromise between pore size (pore volume), silica wall thickness and ample 3D pore interconnectivity [5]. To obtain the desired cubic Ia3d phase, butanol must be added as a structure co-directing agent, together with Pluronic P123. In addition, HCl concentration has to be as low as possible. It was reported that cubic Ia3d phase is formed in the presence of butanol, when concentration of the acid catalyst (HCl) is 0.75 M or less [5]. On the other hand, higher acidity greatly accelerates the kinetics of mesophase formation, leading to smaller domains of ordered mesostructure and consequently greater deviance from the desired cubic Ia3d symmetry. The obtained gel was transferred into stainless steel autoclaves lined with teflon, and aged at 100ºC for 24 h under static conditions. This aging temperature was selected in order to obtain an average pore size of around 8 nm. This was reported as very favorable for subsequent impregnation with various metal oxide precursors [5]. The aged slurry was vacuum filtered and dried overnight at 100º C in a laboratory drier. The dried product was first mixed with 500 ml of ethanol (absolute, p.a., Riedelde Haën) and 30 ml of concentrated HCl and stirred on a magnetic stirrer at room temperature for 1 h. It was then vacuum filtered and washed with 250 ml of distilled water and 150 ml of ethanol. Finally, the product was dried at 60ºC overnight and calcined in air at 550ºC for 5 h to remove the polymer template.

2.2. Synthesis of ordered, mesoporous copper-cerium mixed oxides Three different CuO-CeO2 catalysts were synthesized with nominal 10, 15 and 20 mol % CuO content (they are referred to as CuCe10, CuCe15 and CuCe20, respectively). Appropriate amounts of Cu(NO3)2·3H2O (99.5 % purity, Merck) and Ce(NO3)3·6H2O (99 % purity, Aldrich) were dissolved in 25 ml of ethanol (absolute, p.a., Riedel-de Haën). The amounts of added metal salts were calculated to yield a total metal ion concentration of 0.7 M. Higher concentrations of metal ions are reported to induce formation of bulk metal oxide particles outside the pores of silica [7]. Into 15 ml of this solution, 1 g of KIT-6 was added and stirred at room temperature for 1 h in order to allow the solution to penetrate and fill the KIT-6 pore system completely. Afterwards, the solid was dried overnight at 60ºC. The obtained CuCe15 catalyst precursor was heated in a ceramic crucible in an oven at 400ºC for 3 h to completely decompose the nitrate species. CuCe10 and CuCe20 precursors were calcined at 550 and 450°C, respectively. The impregnation step was repeated with 10 ml of the ethanol-metal salt mixture. After overnight drying at 60ºC, CuCe15 precursor was again calcined at 400º C (CuCe10 at 550ºC and CuCe20 at 450ºC) for 3 h. Optimal calcination temperatures for catalysts with different CuO loadings, which were determined in our previous work [8], were employed in this study. KIT-6 silica template was removed from the resulting solids by leaching twice with 2 M NaOH (Merck) at 50ºC, while mixing the suspension on the magnetic stirrer.

Synthesis of ordered nanostructured CuO-CeO2 catalysts by hard template method 247 Traces of NaOH were removed by continuous washing with distilled water and centrifugation until pH value of the slurry reached 7. Finally, the mesoporous CuOCeO2 mixed oxide samples were dried overnight at 50ºC in a laboratory drier.

3. Results 3.1. Catalyst characterization CuO-CeO2 powders exhibited a well resolved crystalline FCC CeO2 nanostructure and Ia3d cubic symmetry (results not shown), as determined by both wide- and low-angle X-ray diffraction analyses using a PANalytical X`pert PRO diffractometer (Cu Kα radiation, λ=0.15406 nm). No characteristic peaks of copper-containing phases could be identified in any of examined solids, thus indicating very small size of copper entities. Table 1. BET surface area, total pore volume, average CeO2 crystallite size, average CuO particle size and partial CeO2 reduction of CuO-CeO2 catalyst samples. Sample

BET surface area,

Total pore volume, cm3/g

2

m /g

Average (111) CeO2 crystallite size,

Average CuO particle size,

Partial CeO2 reduction,

nm

nm

%

KIT-6 template

600

1.43

/

/

/

CuCe10

147

0.29

7.8

1.3

14.4

CuCe15

166

0.31

6.5

1.9

16.1

CuCe20

161

0.33

6.6

1.7

24.5

CuO phase dispersion at an extent of 28-40 % that was evaluated by selective N2O reduction on a Micromeritics AutoChem II 2920 catalyst characterization system at T=90°C, confirmed the above claim and revealed copper particle sizes, which are considerably smaller (Table 1) compared to chemically identical materials, prepared by either co-precipitation, sol-gel or other traditional methods [9]. BET surface area of synthesized CuO-CeO2 catalysts determined by N2 adsorption/desorption technique using a Micromeritics ASAP 2020 MP/C apparatus, disclosed values between 147 and 166 m2/g (Table 1). Decreasing surface area values follow the trend of increasing calcination temperatures. H2-TPR/TPD tests revealed facile reducibility of the CeO2 support, besides the complete reduction of CuO in the tested temperature range of -20 to 400°C (not shown). The extent of CeO2 reduction increased with CuO loading, indicating its positive influence on oxygen mobility and oxygen storage capacity in the CeO2 structure. Furthermore, H2-TPR/TPO/TPR cycling caused little change in pore size/volume and specific surface area of tested solids, demonstrating good thermal stability of the CeO2 skeletal structure under reductive and oxidative conditions.

3.2. Water-gas shift (WGS) reaction Nanostructured CuO-CeO2 catalysts were tested for WGS reaction performance in a stainless steel (9 mm I.D.) fixed-bed reactor with a 300 mg of catalyst loading. The gas feed composed of 30 % CO and 30 % H2O that was balanced with He and N2 (GHSV = 23600 h-1). Beside H2 and CO2, minute amounts of CH4 were detected throughout the tested temperature range, with the maximum value of 1500 ppm at 450°C. This implies that the selectivity of investigated CuO-CeO2 catalysts in excess of 99 % was measured. At the reaction temperature of 450°C, CO conversions of 62, 61 and 54 % were

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measured over CuCe10, CuCe15 and CuCe20 catalysts, respectively (see Fig. 1). This shows that the activity of nanocrystalline and ordered CuO-CeO2 solids in the stoichiometric WGS reaction is up to 100 % higher compared to chemically equivalent materials prepared by the co-precipitation method [10]. 0,90

CO conversion, /

0,75 0,60 0,45

CuCe10 CuCe15 CuCe20 Equilibrium

0,30 0,15 0,00 250

275

300

325

350

375

400

425

450

Reaction temperature, ° C

Figure 1. CO conversion as a function of reaction temperature obtained during the WGS reaction for CuCe10, CuCe15 and CuCe20 catalysts.

4. Conclusions Hard template preparation method enables the synthesis of highly porous and high surface area nanocrystalline CuO-CeO2 catalysts, which exhibit very small CuO entities (even at a CuO loading of 20 mol %) and strong interactions between both metal oxide phases that facilitate oxygen storage/mobility in the mesoporous CeO2 structure. Additionally, CuO-CeO2 powders proved to be very active and selective during WGS reaction in the middle-to-high temperature range.

References [1] G. Avgouropoulos, T. Ioannides, C. Papadopoulou, J. Batista, S. Hočevar and H.K. Matralis, Catal. Today, 75 (2002) 157. [2] Y. Li, Q. Fu and M. Flytzani-Stephanopoulos, Appl. Catal. B, 27 (2000) 179. [3] W. Shen, X. Doug, Y. Zhu, H. Chen and J. Shi, Microporous Mesoporous Mater., 85 (2005) 157. [4] S.C. Laha and R. Ryoo, Chem. Commun., (2003) 2138. [5] F. Kleitz, T.-W. Kim and R. Ryoo, Bull. Korean Chem. Soc., 26(11) (2005) 1653. [6] M. Choi, W. Heo, F. Kleitz and R. Ryoo, Chem. Commun., (2003) 1340. [7] A. Rumplecker, F. Kleitz, E.-L. Salabas and F. Schüth, Chem. Mater., 19(3) (2007) 485. [8] P. Djinović, J. Batista and A. Pintar, Appl. Catal. A, 347 (2008) 23. [9] A. Pintar, J. Batista and S. Hočevar, J. Colloid Interface Sci., 307 (2007) 145. [10] P. Djinović, J. Batista, J. Levec and A. Pintar, Appl. Catal. A, 364 (2009) 156.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Fine-tuning of Vanadium Oxide Nanotubes Jens Emmerich,a Marijn Dillen,a Christine E. A. Kirschhock,a Johan A. Martensa a

Centre for Surface Chemistry and Catalysis, KU Leuven, Department of Microbial and Molecular Systems, Kasteelpark Arenberg 23, B-3001 Heverlee, Belgium

Abstract The formation of vanadium oxide nanotubes (VOx-NTs) is sensitive to agitation during hydrothermal synthesis. The ordering of the VOx layers is lower and the interlayer distance increases. The pH range for VOx-NT synthesis could be extended to pH values below 5. At low pH, the VOx layers are better ordered and there is a trend towards shorter tubes. Below a critical pH around 4.5, other morphologies next to VOx-NTs appear. Only amine templates were found to be suitable while platelets and “star-like” VOx nanomaterials were obtained using alcohols and thiols. Keywords: vanadium oxide, nanotube, template, hydrothermal treatment

1. Introduction Vanadium oxide (VOx) is relevant to a multitude of catalytic reactions [1]. Amongst various open, metastable VOx phases containing organic species or metal cations, vanadium oxide nanotubes (VOx-NTs) and derivatives thereof can be prepared via hydrothermal synthesis. Thanks to their unique layered structure and the simultaneous presence of vanadium in various oxidation states, VOx-NTs are interesting with regard to catalysis, lithium intercalation in batteries and sensor applications [2,3,4]. Initially, carbon nanotubes (CNTs) were used as templates and coated with molten vanadium(V) oxide (V2O5) or VOx gel [5,6]. Alternatively, VOx gels or molten V2O5 were mixed with primary amines to yield VOx-NTs [7,8]. Recently, a safe and costefficient way to prepare VOx-NTs on a large scale from V2O5 and amine templates was published [9]. The possibility to roughly control the tube diameters, the number of VOx layers in the tube walls and the tube length by using different structure-directing amine agents has been shown [10]. However, for using VOx-NTs as heterogeneous catalyst or cathode material, better control of the NT formation is required. Therefore, we performed a systematic study of parameters affecting the formation of these nanomaterials. The preliminary results are summarized in this communication.

2. Experimental Vanadium(V) oxide (V2O5, Riedel-de Haën), dodecylamine (Acros), dodecanethiol (Aldrich), dodecanol (Fluka) and sulfuric acid (H2SO4, Merck KGaA) were used as purchased from the supplier without further purification. VOx-NTs were synthesized based on literature [9] and characterized using X-ray powder diffraction (XRD) and scanning electron microscopy (SEM): (a) Static vs. dynamic hydrothermal treatment (h.t.): V2O5 (15 mmole) and dodecylamine (15 mmole) were stirred for 2 days in a mixture of ethanol and water (20 mL) at 30°C. Ageing was followed by h.t. in teflon-lined autoclaves at 180°C for 7 days. One reaction was carried out in a stationary oven, a second batch was continuously stirred in an oven with a rotating axis for autoclaves. The final products were filtered, washed with ethanol/ hexane and dried in a vacuum oven (40°C, 24 h).

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(b) Synthesis of VOx-NTs at lower pH: The pH of a standard experiment containing V2O5 and dodecylamine (molar ratio 1/1, i.e. template/V = ½) was adjusted with sulfuric acid to pH values in the range from 4.3 – 4.7 by adding 1 – 3 mmole H2SO4. Ageing, static h.t. and product recovery were performed as described in (a). (c) Influence of the template on the formation of VOx nanomaterials: V2O5 (15 mmole) and template (dodecylamine, dodecanol or dodecanethiol: 15 mmole) were aged for 2 days in a mixture of ethanol and water (20 mL, 30°C). H.t., product filtration and drying followed the procedure outlined in (a).

3. Results and discussion Figure 1a provides a comparison of vanadium oxide nanotube (VOx-NT) XRD patterns upon synthesis under static and dynamic hydrothermal conditions. The interlayer distance is in good agreement with the original data in the former case while stirring results in a lower degree of VOx layer ordering and increased interlayer distances [10]. In Figure 1b, XRD patterns obtained for VOx materials prepared at pH = 4.3 – 4.7 are shown. While the tubular structure is well preserved above pH 4.5 (patterns 1 and 2), no pure VOx-NTs could be prepared at pH = 4.3 (pattern 3). (a)

(b)

Figure 1. (a) X-ray diffraction patterns of VOx-NTs hydrothermally treated at 180°C for 7 days without stirring and using an oven with a rotating axis for autoclaves to ensure a dynamic hydrothermal reaction (“stirring”). (b) XRD patterns of VOx materials upon pH adjustment with sulfuric acid (H2SO4) and hydrothermal treatment (180°C, 7 d). The numbers indicate the amount of H2SO4 used (in mmole).

3.1. Static vs. dynamic hydrothermal synthesis Niederberger et al. synthesized vanadium oxide nanotubes (VOx-NTs) without agitation [11]. In accordance with the original work, we observed that VOx-NTs have a tendency to stick together under this condition [4]. To overcome that problem, we synthesized the material under continuous stirring during the whole synthesis. The resulting XRD patterns of VOx-NTs obtained by different hydrothermal treatment (h.t.) are compared in Figure 1a. No significant differences between the materials concerning their morphology were observed with SEM. Thus, dissolution-precipitation and solidsolid transformation processes that occur during h.t. are only marginally affected [12]. No positive effect on VOx-NT dispersion could be identified. Besides, the sample prepared under dynamic conditions is of poorer quality regarding the ordering of VOx layers and the interlayer distance has shifted to higher values (d = 2.82 nm).

3.2. Synthesis of VOx-NTs at lower pH The influence of pH on the morphology of vanadium oxides (VOx) obtained from aqueous solutions is widely recognized [13]. In general, higher coordination numbers of vanadium(V) have been observed at pH values below 7 while tetrahedral coordination predominates with increasing pH. Around pH = 7, layered structures are formed in the

Fine-tuning of Vanadium Oxide Nanotubes

251

presence of organic cations which stabilize the negatively charged polyanions [14]. In a previous publication, the influence of the pH on the formation of vanadium oxide nanotubes (VOx-NTs) has been investigated for high pH values achieved by ammonia addition [15]. Bent and scrolled VOx layers but no typical VOx-NTs were obtained at pH = 9 – 10. Since agitation during the hydrothermal treatment (h.t.) did not result in a better dispersion of the VOx-NTs, we decided to explore the lower pH range to examine the influence on the polymerization of VOx and subsequent formation of nanotubular structures. The corresponding X-ray diffractograms are presented in Figure1b. The pH of the original experiment is around 5.9 while we succeeded to synthesize VOx-NTs at significantly lower pH around 4.5 – 4.7. Remarkably, the VOx-NTs synthesized at pH = 4.6 are shorter (data not shown here) and their crystallinity is even improved (Fig. 1b). Lowering the pH below 4.5 results in the formation of a second morphology next to VOx-NTs. Possibly, an increasing positive charge on the VOx polymers prevents strong interaction with the template which is protonated at this pH. Even though interactions between the amine template and the VOx layers might not be simply ionic in nature, the structure-directing properties of the ammonium species can be assumed to be significantly weaker at lower pH [16].

3.3. Functional groups SEM micrographs of vanadium oxide (VOx) nanomaterials obtained using dodecanol, dodecanthiol or dodecylamine as structure-directing template are shown in Figure 2.

Figure 2. SEM micrographs of (from left to right) vanadium oxide (VOx) platelets using dodecanol and V2O5 as vanadium source, “star-like” VOx obtained from the reaction of V2O5 with dodecanethiol and vanadium oxide nanotubes (VOx-NTs) synthesized according to ref. [9].

As becomes clear from the SEM images, vanadium oxide nanotubes (VOx-NTs) are only formed with the amine-functionalised template. However, next to individual, open tubular structures, agglomerates of VOx-NTs can be observed as already indicated in the original work on VOx-NTs [4]. Similar to a previous study, “star-like” VOx was obtained when dodecanethiol was used as template [17]. However, in our case, the platelets that are grown together to form the six-fold rotationally symmetric VOx nanostructures are thicker and the whole assembly is larger (0.5 – 1.0 µm). These differences most probably are due to different experimental parameters. The experiment was performed at significantly lower pH (pH = 3 – 4) compared to the VOx-NT synthesis, but neither alcohols nor amines resulted in similar morphologies, so assumedly the functional group is responsible for star formation. When changing the surfactant to dodecanol, nicely dispersed, tiny laths are obtained. In contrast to the very uniform “star-like” material synthesized with dodecanethiol, the particle size is less homogeneous, ranging from 1 – 10 µm in length but having very uniform widths. Similar as in case of the VOx/dodecanethiol hybrid materials, the pH of the reaction mixture was significantly lower compared to the reference using dodecylamine as template. It seems that both pH and especially the functional group influence the

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morphology by favoring a distinct VOx structure [14]. This hypothesis is supported by a control experiment with VOx/dodecanol mixtures where the pH had been adjusted to around 6 with KOH but no nanotubes were obtained (data not shown here).

4. Conclusions The formation of VOx-NTs is sensitive to agitation during hydrothermal treatment and the pH of the reaction medium. The ordering of the VOx layers is lower and the interlayer distance is increased when a dynamic hydrothermal treatment is applied. By careful pH adjustment with sulfuric acid, the pH range for VOx-NT synthesis could be extended to values below 5 where the VOx layers are better ordered and a trend towards shorter tubes is observed. Below a critical pH around 4.5, other morphologies next to VOx-NTs appear. Only amines were suitable templates while platelets and “star-like” VOx nanomaterials were obtained using alcohols and thiols.

5. Acknowledgments J.E. acknowledges financial support granted by IWT-Vlaanderen. C.E.A.K. and J.A.M. acknowledge the Flemish Government for long-term structural funding (Methusalem).

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]

I.E. Wachs, Y. Chen, J.-M. Jehng, L.E. Briand and T. Tanaka, Catal. Today, 78 (2003) 13 T. Chirayil, P.Y. Zavalij and M.S. Whittingham, Chem. Mater., 10 (1998) 2629 L. Krusin-Elbaum, D.M. Newns, H. Zeng, V. Derycke J.Z. Sun and R. Sandstrom, Nature, 431 (2004) 672 M.E. Spahr, P. Stoschitzki-Bitterli, R. Nesper, O. Haas and P. Novák, J. Electrochem. Soc., 146 (1999) 2780 P.M. Ajayan, O. Stephan, Ph. Redlich and C. Colliex, Nature, 375 (1995) 564 B.C. Satishkumar, A. Govindaraj, M. Nath and C.N.R. Rao, J. Mater. Res., 12 (1997) 604 G.T. Chandrappa, N. Steunou, S. Cassaignon, C. Bauvais, P.K. Biswas and J. Livage, J. Sol-Gel Sci. Technol., 26 (2003) 593 G.T. Chandrappa, N. Steunou, S. Cassaignon, C. Bauvais and J. Livage, Catal. Today, 78 (2003) 85 M. Niederberger, H.-J. Muhr, F. Krumeich, F. Bieri, D. Günther and R. Nesper, Chem. Mater., 12 (2000) 1995 F. Krumeich, H.-J. Muhr, M. Niederberger, F. Bieri, B. Schnyder and R. Nesper, J. Am. Chem. Soc., 121 (1999) 8324 M. Niederberger, Dissertation 13971, ETH Zürich A. Michailovski and G. R. Patzke, Chem. Eur. J., 12 (2006) 9122 J. Livage, Chem. Mater., 3 (1991) 578 J. Livage, Coord. Chem. Rev., 178-180 (1998) 999 K.S. Pillai, F. Krumeich, H.-J. Muhr, M. Niederberger and R. Nesper, Solid State Ionics, 141-142 (2001) 185 P. Liu, I.L. Moudrakovski, J. Liu and A. Sayari, Chem. Mater., 9 (1997) 2513 C. O’Dwyer, V. Lavayen, D. Fuenzalida, S.B. Newcomb, M.A. Santa Ana, E. Benavente, G. González and C.M. Sotomayor Torres, Phys. Stat. Sol. B, 244 (2007) 4157

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Plasma-assisted design of supported cobalt catalysts for Fischer-Tropsch synthesis Jingping Hong,a,b Wei Chu,a* Yongxiang Ying,a Petr A. Chernavskii,c Andrei Khodakovb* a

Department of Chemical Engineering, Sichuan University, Chengdu 610065, China Unité de Catalyse et de Chimie du Solide, UMR 8181 CNRS, Bât. C3, USTL-ENSCLEC Lille, Cite Scientifique, 59655 Villeneuve d’Ascq, France c Department of Chemistry, Moscow State University, 119992 Moscow, Russia b

Abstract Two CoIr-based catalysts were prepared via high frequency cold plasma jet following / or instead of thermal calcination, and studied using a wide range of characterization techniques (X-ray diffraction, X-ray absorption, temperature programmed reduction and in situ magnetic measurements). In the plasma assisted preparation process, the precursor was first treated by high frequency cold plasma jet for 10 min after drying. The catalyst prepared via a combination of the plasma treatment and calcination and which combined good cobalt reducibility and high cobalt dispersion exhibited an enhanced activity in Fisher-Tropsch synthesis. Keywords: high frequency cold plasma jet, plasma-assisted preparation, Fisher-Tropsch synthesis, cobalt

1. Introduction Fischer-Tropsch (FT) synthesis converts natural gas-, coal- and biomass-derived syngas into liquid hydrocarbon fuels which are totally free of sulfur- and nitrogen-containing compounds and have very low aromatic contents [1]. FT synthesis proceeds on cobalt metal sites, the overall number of cobalt metal sites on supported catalysts depends on both cobalt dispersion and reducibility. Decomposition of cobalt precursor is an important step in the catalyst preparation, which could significantly influence both cobalt dispersion and cobalt phase composition [2,3]. In our previous study, the effects of pretreatment with glow discharge plasma on cobalt FT catalysts were investigated. The glow discharge plasma was found to considerably enhance cobalt dispersion [3]. In this study, another type of plasma–high frequency cold plasma jet, which was previously found to have a great advantage in the preparation of Ni-based catalyst for methane reforming with CO2 [4], was employed for optimization of the iridium promoted Co/Al2O3 catalysts. Its effects on cobalt dispersion, reducibility and catalytic performance of CoIr-based FT catalysts are addressed in this paper.

2. Experiments 2.1. Catalyst preparation The 15 wt.% Co 0.3wt.% Ir/Al2O3 catalyst precursor used in this work was prepared by incipient wetness impregnation. The Al2O3 support was impregnated with an aqueous solution containing cobalt nitrate and iridium chloride, and dried at 383 K for 5h. The conventional prepared sample was obtained by thermal calcination (673 K for 6h) and

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labeled as CoIr/Al2O3-C. For plasma-assisted samples, the precursor was treated by the jet of plasma in a flow of 20 vol.% H2 and Ar. The catalyst prepared using plasma instead of thermal calcination is denominated as CoIr/Al2O3-P. CoIr/Al2O3-P+C sample corresponds to the sample first treating by plasma jet, and following by thermal calcination (673 K for 6h). Figure 1 shows the apparatus for the plasma treatment. An inside copper electrode was connected to a high voltage supply. The coaxial stainless steel cover served as the ground electrode, a mixture of 20 vol.% H2 and Ar was used as plasma-forming gas. When 20KHZ voltage was applied, the plasma gas was introduced into the catalyst bed. The plasma treatment was performed for 10 min.

Figure 1. Schema of the apparatus for atmospheric high frequency cold plasma jet.

2.2. Characterization X-ray powder diffraction patterns were recorded with a Siemens D5000 diffractometer and Cu Kα radiation. The average size of Co3O4 crystallites was determinated by the Sherrer equation using the Co3O4 diffraction peak at 2θ = 36.8o. X-ray absorption spectra at the Co K-edge were measured at DUBBLE beamline in ESRF (Grenoble, France). Characterization of calcined catalysts was performed using our X-ray absorption cell described in Ref. [5]. The Si (111) double-crystal monochromator was calibrated by setting the first inflection point of the K-edge spectrum of Co foil at 7709 eV. The temperature-programmed reduction profiles were obtained by passing 5% H2/Ar gas mixture through the catalyst while increasing the temperature at a linear rate. The amount of samples for all experiments was about 50 mg. The gas flow velocity was 30 ml/min, and the rate of temperature ramping was 3 oC /min. In situ magnetic measurements were performed using a Foner vibrating-sample magnetometer as described previously [6]. The experiments were conducted by passing pure H2 through the catalyst while increasing the temperature at a linear rate. The amount of samples for all measurements was around 20 mg. The appearance of metallic cobalt species in the samples was monitored in situ by a continuous increase in sample magnetization during the reduction.

3. Results Both CoIr/Al2O3-C and CoIr/Al2O3-P+C catalysts exhibited XRD patterns characteristic of Co3O4 spinel in addition to the patterns of γ-Al2O3, as shown in figure 2. Comparing with CoIr/Al2O3-C, the XRD peaks assigned to Co3O4 phase were broadened in plasma assisted CoIr/Al2O3-P+C catalyst. The particle size of Co3O4 crystallites based on the Sherrer equation, decreased from 14.7 nm in CoIr/Al2O3-C to 7.6 nm in CoIr/Al2O3-P+C. As for CoIr/Al2O3-P catalyst, all the XRD peaks were very broad, indicating very high cobalt dispersion. Since the plasma forming gas contained some concentration of

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Plasma-assisted preparation of CoIr/SiO2 catalyst using high frequency cold

hydrogen, new XRD peaks attributed to the reduced cobalt species (CoO and Co) were observed for this sample. The X-ray absorption results were in agreement with the above XRD findings. The XANES spectra of both CoIr/Al2O3-C and CoIr/Al2O3-P+C catalysts were almost identical, similar to those of the Co3O4 reference compound (Figure 3). While for CoIr/Al2O3-P catalyst, the XANES spectrum was rather different from those of the reference compounds (Co3O4, CoO and Co), which indicated the simultaneous presence of several cobalt phases. Linear combination fitting of XANES spectra showed that cobalt nitrate was completely decomposed during 10 min plasma jet treatment, 35.2 % of Co3O4, 59.1 % of CoO and 5.7 % of metallic Co were co-exist in plasma assisted CoIr/Al2O3-P sample.

CoIr/Al2O3-C CoIr/Al2O3-P CoIr/Al2O3-P+C

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CoIr/Al2O3-C

Norm. asorbance, a.u.

Intensity, a.u.

Co3O4

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CoIr/Al2O3-P CoIr/Al2O3-P+C CoO Co

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7720

2 Theta, degree

Figure 2. XRD patterns of CoIr-based Catalysts.

7740 7760 Energy, eV

7780

7800

Figure 3. XANES spectra of oxidized CoIrbased catalysts.

The reducibility of CoIr-based catalysts was investigated by TPR and in situ magnetic measurements. There was only one broad reduction peak in all three catalysts (Figure 4). The TPR peak was shifted to lower temperature for plasma assisted catalysts, and the hydrogen consumption was much higher in catalysts prepared with thermal calcination (CoIr/Al2O3-C and CoIr/Al2O3-P+C).

CoIr/Al2O3-P+C CoIr/Al2O3-P

CoIr/Al2O3

Magnetization, emu/g

Intensity, a.u.

18

500

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Temperature, K

Figure 4.TRP profiles of CoIr-based catalysts.

CoIr/Al2O3-P+C

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9 6 3 0

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Figure 5. In situ magnetization of CoIr-based Catalysts during the reduction in pure hydrogen.

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The effects of plasma treatment on cobalt reducibility in CoIr-based samples could be also seen from figure 5. Metallic cobalt was the only ferromagnetic phase present in the catalysts under the experimental conditions. This suggests that the magnetization in strong fields (saturation magnetization) is proportional to the concentration of the metallic cobalt phase. Plasma treatment resulted in a decrease in the temperature of appearance of the metallic cobalt phase. A much higher magnetization and thus a much higher concentration of cobalt metal phase were observed in calcined CoIr/Al2O3-C and CoIr/Al2O3-P+C catalysts. At 673 K, the saturation magnetization was 14.66 emu/g for CoIr/Al2O3-C, 13.26 emu/g for CoIr/Al2O3-P+C and only 6.11 emu/g for CoIr/Al2O3-P. This was consistent with the TPR results. The catalytic performances of the three CoIr-based catalysts were evaluated in a differential catalytic reactor under atmospheric pressure (Table 1). The catalyst assisted by plasma jet followed by thermal calcination (CoIr/Al2O3-P+C) showed a twice higher activity in FT synthesis than the conventional calcined sample (CoIr/Al2O3-C) and the catalyst (CoIr/Al2O3-P) which was prepared using plasma treatment instead of calcination. Table 1. Catalytic performance of conventional and plasma-assisted cobalt catalysts in FT synthesis. Selectivity, % CH4 C2-4-HC C5+-HC CoIr/Al2O3-C 8.52 11.68 13.30 75.02 CoIr/Al2O3-P 7.65 13.07 7.79 79.14 CoIr/Al2O3-P+C 17.50 20.13 21.95 57.92 Conditions: 0.5g catalyst, p = 1 bar, T=463 K, gas hourly space velocity (GHSV)=3000 ml/(g·h)-1, H2/CO = 2. Catalysts

CO conversion, %

4. Conclusion It was found that the catalysts assisted with plasma jet exhibited much higher cobalt dispersion than those prepared via conventional calcination. Smaller cobalt particles in plasma assisted CoIr/Al2O3-P catalyst displayed more difficult cobalt reducibility than cobalt particles in the catalysts prepared with thermal calcination. High catalytic acitivity of plasma assisted CoIr/Al2O3-P+C catalyst was attributed to the combination of relative high cobalt dispersion and good cobalt reducibility. Plasma jet seems to be a promising tool which can be used to control cobalt dispersion and improve catalytic performance of cobalt FT catalysts.

References 1 2 3 4

A.Y. Khodakov, W. Chu, P. Fongarland, 2007, Advances in the development of novel cobalt Fischer-Tropsch catalysts for synthesis of long-chain hydrocarbons and clean fuels, Chem. Rev., 107, 1692-1744. W. Chu, P.A. Chernavskii, L. Gengembre, G.V. Pankina, P. Fongarland, A.Y. Khodakov, 2007, Cobalt species in promoted cobalt alumina-supported Fischer-Tropsch catalysts, J. Catal., 252, 215-230. W. Chu, L. Wang, P.A. Chernavskii, A.Y. Khodakov, 2008, Glow-discharge plasma-assisted design of cobalt catalysts for Fischer-Tropsch synthesis, Angew. Chem. Int. Ed., 47, 5052-5055. Liu, Y. Li, W. Chu, X. Shi, X. Dai, Y. Yin, 2008, Plasma-assisted preparation of Ni/SiO2 catalyst using atmospheric high frequency cold plasma jet, Catal. Commun., 9, 1087-1091.

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J.-S. Girardon, A. Y. Khodakov, M. Capron, S. Cristol, C. Dujardin, F. Dhainaut, S. Nikitenko, F. Meneau, W. Bras, E. Payen, 2005, A new experimental cell for in situ and operando X-ray absorption measurements in heterogeneous catalysis, J. Synchrotron Radiat., 12, 680-684. P.A. Chernavskii, A. Y. Khodakov, G. V. Pankina, J.-S. Girardon, E. Quinet., 2006, In situ characterization of the genesis of cobalt metal particles in silica-supported Fischer-Tropsch catalysts using Foner Magnetic method, Appl. Catal. A Gen, 306, 108-119.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Chemical vapor deposition of Fe(CO)4(SiCl3)2 for the synthesis of hydrogenation catalyst made of highly dispersed iron silicide particles on silica Jingchao Guan, Anqi Zhao, Xiao Chen, Mingming Zhang, Changhai Liang * State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116012, China

Abstract New iron silicides based hydrogenation catalysts have been prepared by organometallic chemical vapor deposition of Fe(CO)4(SiCl3)2 precursor on silica support. Fe(CO)4(SiCl3)2 was synthesized from Fe3(CO)12 and SiHCl3 at 120 oC, as confirmed by FTIR, 13C and 29 Si NMR. The FeSi loadings have been varied by changing the amount of the precursor. XRD patterns only showed a diffraction peak due to silica, indicating that iron silicide particles were too small to be detected. TEM image showed that the lattice spacing of the particles was 0.2578 nm, which matched well with the lattice spacing 0.2591 nm of the FeSi (111) plane. TEM image showed that the size of iron silicide particles dispersed on the silica was about 3 nm. However, the so-prepared FeSi/SiO2 catalysts showed little catalytic activity in naphthalene hydrogenation. More studies would be needed to have a better understanding on how to improve the efficiency of these new catalysts. Keywords: Fe(CO)4(SiCl3)2; organometallic chemical vapor deposition; iron silicide; naphthalene hydrogenation

1. Introduction Transition metal silicides have unique physical and chemical properties, such as good electrical conductivity, high chemical inertness and thermal stability [1]. It has been shown that transition metal silicides can give excellent catalytic activity and stability in hydrogenation reactions, such as hydrodechlorination and hydrorefining, where they behave differently as conventional catalytic materials [2-4]. It was reported that the active phases of silicon tetrachloride hydrodechlorination catalyst were nickel silicide and copper silicide, but not metallic nickel or copper [2]. Panpranot et al. reported recently that formation of palladium silicide in Pd/SiO2 greatly improved the selectivity of phenylethylene in phenylacetylene hydrogenation [3]. Simultaneously, thermodynamic data also indicate that transition metal silicides have higher stability in the presence of hydrogen sulfide than other intermetallic compounds, such as transition metal nitrides, carbides and phosphides [5]. However, conventional preparation methods for silicides inherited from the microelectronic industry result in low surface area and poor catalytic activity. Chemical vapor deposition has been shown to be a powerful method for generating highly dispersed catalysts in a controlled and reproducible manner [6]. In this work, we report for the first time the metal organic chemical vapor deposition (MOCVD) of Fe(CO)4(SiCl3)2 as a single-source precursor to FeSi nanoparticles on silica support at atmospheric pressure in a fluidized bed reactor. The catalytic properties of the so-prepared FeSi/SiO2 catalysts were tested in the reaction of naphthalene hydrogenation.

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2. Experimental The precursor Fe(CO)4(SiCl3)2 was synthesized from Fe3(CO)12 and SiHCl3 at 120 oC in the absence of water and air following a procedure modified from literature [7,8]. Silica supported nanostructured iron silicides were prepared by a two-step chemical vapor deposition of Fe(CO)4(SiCl3)2 at atmospheric pressure in a fluidized bed reactor. In order to remove adsorbed water, silica was first calcined in air at 500 oC. The precursor was a moisture-sensitive solid and the adsorbed water would promote its hydrolysis. The precursor was sublimed at about 100 oC, carried downstream by argon and adsorbed on silica support. The adsorbed precursor was then treated at 420 oC in hydrogen at atmospheric pressure and a stable black sample was achieved. Especially, the iron silicide loadings could be controlled by changing the amount of the precursor. The samples were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Naphthalene hydrogenation was used as model reaction to test the hydrogenation activities of the so-prepared iron silicide catalysts. The reaction was carried out at 340 o C and 4.0 MPa in a continuous fixed-bed reactor, before which the catalysts (0.2 g, diluted with 2.0 g SiC) were activated in situ with H2 at 400 oC and 0.1 MPa for 4 h. The liquid reactant was composed of 1 mol% undecane (as internal standard for GC analysis), 5 mol% naphthalene reactant and varying amounts of decane (as solvent). The reaction product was analyzed by off-line gas chromatography.

3. Results and discussion 3.1. Characterization of the precursor Fe(CO)4(SiCl3)2

The IR spectrum of the synthesized precursor in CDCl3 solution revealed the characteristic peaks of carbonyl at 2129 (m) and 2060 (s) cm-1. The latter was so broad (from 2030 to 2090 cm-1) that it was difficult to find other peaks in this area. The results were in good agreement with the literature [7,8]. The 13C NMR spectrum of the precursor in CDCl3 solution showed two peaks at 197.30 and 199.44 ppm, which were attributed to the axial carbonyl group according to the literature [8]. The 29Si NMR spectrum of the precursor in CDCl3 solution showed two peaks at 39.6 and 44.4 ppm, which could be attributed to tetracoordination of the silicon atom in the isomers. This was different from 21.5 ppm reported by Novak et al. [8]. This may be due to the different isomers of Fe(CO)4(SiCl3)2, which needs further investigation. The crystalline product finally obtained was yellow or slightly green, which might correspond to the trans- and cis-isomer, and this resulted from isomerization that might occur above 90 oC during sublimation [7,8]. As we intended to do metal organic chemical vapor deposition, this point seemed to be of little importance due to their similar vapor pressure and their same composition. The reasons why Fe(CO)4(SiCl3)2 was chosen as the single-source precursor were as follows. Based on in situ UPS study and DFT calculation, it was reported that a 1:1 ratio of Fe and Si could be precisely delivered to the substrate surface according to a decomposition pathway of Fe(CO)4(SiCl3)2 with elimination of SiCl4 and formation of Fe=SiCl2(CO)4 which would decompose to FeSi [7,9]. For another, compared with conventional multi-source MOCVD, the single-source precursor approach allowed simpler and safer experimental setups because it avoids usage of highly hazardous liquid precursors (Fe(CO)5 typically for Fe and SiCl4 for Si) [7].

Chemical vapor deposition to synthesize highly dispersed FeSi/SiO2 catalysts

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3.2. Characterization of silica supported nanostructured iron silicides

Intensity (a.u.)

X-ray diffraction patterns of FeSi/SiO2 with 8.1, 13.9 and 18.8 wt% FeSi calculated loading only showed a broad diffraction peak due to silica at about 23.4 °, and did not exhibit any diffraction peak due to FeSi, Fe or FeCl3, indicating that iron silicide particles were too small to be detected (Figure 1). X-ray diffraction patterns of CoSi particles on silica support with similar loadings revealed the same results [10]. The FeSi/SiO2 samples were further determined by transmission electron microscopy (TEM) measurement. The high-resolution TEM image in Figure 2 showed that the lattice spacing of the particle was 0.2578 nm, which matched well with the reported value 0.2591 nm of the FeSi (111) plane. This confirmed the formation of iron silicide by chemical vapor deposition of Fe(CO)4(SiCl3)2 as the precursor. The TEM image also showed that the size of iron silicide particles dispersed on the silica was about 3 nm, which was in good agreement with the XRD results.

18.8%FeSi/SiO2 13.9%FeSi/SiO2 8.1% FeSi/SiO2 20

40

60

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2Theta (deg.) Figure 1. XRD patterns of FeSi/SiO2 with 8.1, 13.9 and 18.8 wt% FeSi loading from MOCVD.

Figure 2. HRTEM image of FeSi/SiO2 with 18.8 wt% FeSi loading from MOCVD.

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3.3. Catalytic properties of the FeSi/SiO2 catalysts in naphthalene hydrogenation

The reaction results showed little catalytic activity in the hydrogenation of naphthalene over the FeSi/SiO2 with 13.9 wt% FeSi calculated loading. This might be attributed to SiO2 covered the FeSi particles due to FeSi oxidation in air. Meanwhile, the actual loading of FeSi/SiO2 was too low to exhibit any activity in naphthalene hydrogenation. We also tested the catalytic activity of the sample in phenylacetylene hydrogenation. No activity was detected. Further work is necessary to clarify why the FeSi/SiO2 catalysts were not active and to understand how to improve the efficiency of the new catalysts.

4. Conclusions Nanostructured FeSi particles on silica support have been synthesized by MOCVD of Fe(CO)4(SiCl3)2 as the single-source precursor at atmospheric pressure in a fluidized bed reactor. The results indicate that the size of iron silicide particles dispersed on the silica is about 3 nm. The resulting iron silicide catalysts showed little catalytic activity in naphthalene hydrogenation and phenylacetylene hydrogenation. Nevertheless, it is believed that chemical vapor deposition method is of great potential in controlled synthesis of transition metal silicides, which may be applied to some specific reactions in synthesis of fine chemicals.

Acknowledgments We gratefully acknowledge the financial support provided by the National Natural Science Foundation of China (No. 20973029), the Program for New Century Excellent Talents in Universities of China (No. NCET-07-0133) and the Doctoral Fund of Ministry of Education of China (No. 20070141048).

References [1] A. H. Reader, A. H. V. Ommen, P. J. W. Weijs, R. A. M. Wolters, D. J. Oostra, 1993, Transition metal silicides in silicon technology, Rep. Prog. Phys., 56, 1397-1467. [2] H. Walter, G. Roewer, K. Bohmhammel, 1996, Mechanism of the silicide-catalysed hydrodehalogenation of silicon tetrachloride to trichlorsilane, J. Chem. Soc. Faraday Trans., 92, 22, 4605-4608. [3] J. Panpranot, K. Phandinthong, T. Sirikajorn, M. Arai, P. Praserthdam, 2007, Impact of palladium silicide formation on the catalytic properties of Pd/SiO2 catalysts in liquid-phase semihydrogenation of phenylacetylene, J. Mol. Catal. A, 261, 29-35. [4] W. Juszczyk, Z. Karpiński, D. Łomot, J. Pielaszek, 2003, Transformation of Pd/SiO2 into palladium silicide during reduction at 450 and 500 oC, J. Catal., 220, 299-308. [5] R. B. Levy, 1977, in Adv. Mater. Catal., Ed. J. J. Burton and R. L. Garten, New York, Academic Press, 101-127. [6] P. Serp, P. Kalck, 2002, Chemical vapor deposition methods for the controlled preparation of supported catalytic materials, Chem. Rev., 102, 9, 3085-3128. [7] A. L. Schmitt, M. J. Bierman, D. Schmeisser, F. J. Himpsel, S. Jin, 2006, Synthesis and properties of single-crystal FeSi nanowires, Nano. Lett., 6, 8, 1617-1621. [8] I. Novak, W. Huang, L. Luo, H. H. Huang, H. G. Ang, C. E. Zibill, 1997, UPS study of compounds with metal-silicon bonds: M(CO)nSiCl3 (M=Co, Mn; n=4, 5) and Fe(CO)4(SiCl3)2, Organometallics, 16, 1567-1572. [9] C. E. Zybill, W. Huang, 1999, Formation of FeSi and FeSi2 films from cis-Fe(SiCl3)2(CO)4 by MOCVD-precursor versus substrate control, Inorg. Chim. Acta, 291, 380-387. [10] C. H. Liang, A. Q. Zhao, X. F. Zhang, Z. Q. Ma, R. Prins, 2009, CoSi particles on silica support as a highly active and selective catalyst for naphthalene hydrogenation, Chem. Commun., 2047-2049.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Laser electrodispersion method for the preparation of self-assembled metal catalysts T.N. Rostovshchikovaa, S.A. Nikolaeva, E.S. Loktevaa, S.A. Gurevichb, V.M. Kozhevinb, D.A. Yavsinb, A.V. Ankudinovb a b

Lomonosov Moscow State University, Moscow, 119991, Russia Ioffe Physico-Technical Institute of RAS, St-Petersburg, 194021, Russia

Abstract Laser electrodispersion (LED) method makes possible to fabricate dense nanostructured catalysts with unique catalytic properties. In contrast to earlier laser ablation techniques, where nanoparticles were synthesized from vaporized matter, LED is based on the cascade fission of liquid metallic drops. Fabricated catalysts consist of ensembles of nanoparticles that are uniform in size and shape, amorphous and stable to coagulation. The catalytic activity of these self-assembled Pt, Ni, Pd, Au and Cu catalysts with extremely low metal content (<10-3mass.%) in hydrogenation and hydrodechlorination is several orders of magnitude higher compared to that for separated metal clusters, highly loaded metal films and supported catalysts prepared by usual methods. Keywords: laser electrodispersion, cascade fission of microdrops, amorphous nanoparticles

1. Main Text Nowadays, catalytic properties of metal nanoparticles are the subject of extensive studies. It has been found recently [1,2] that not only the particle size but also a distance between supported particles affects the properties of nanostructured catalytic systems. This effect opened up a new way to improve the catalytic properties of supported metal particles by the formation of coatings with the optimal surface particle density. However, most of the known preparative methods are unsuitable for fabrication of dense nanostructures because closely located crystalline particles coagulate into larger aggregates. For this reason, a search for methods that can enhance the coagulation stability of the metallic nanoparticles is the task whose accomplishment governs the development of new promising areas in catalysis. A new type of densely packed nanostructured catalysts was prepared using a technique of laser electrodispersion (LED) of metals developed at Ioffe PhysicoTechnical Institute, Russian Academy of Sciences [3]. This method is based on the ablation of a metallic target irradiated by a high-power pulsed laser. The laser ablation is widely used for deposition of metallic coatings. Usually parameters of deposition were chosen to minimize splashing of microdrops and convert target material primarily into the vapor phase. Accordingly, the coatings obtained on a substrate were either homogeneous metallic films, or thin island-type films. So, the microdrop splashing was regarded as an adverse effect. In contrast to that the formation of microdrops becomes a predominant process in our approach. For this purpose the regime of target irradiation when the initially generated liquid metallic microdrops become unstable and divide into a large number of nanometer-size droplets was chosen.

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The reason of the microdrop fission is that all liquid metal drops coming from the target surface become negatively charged entering laser torch plasma area. This plasma produced by optical breakdown (photoionization caused by the acceleration of electrons in electromagnetic wave) of the evaporated target material. If the drop charge is so high that Coulomb repulsion force exceeds the surface tension the drops become unstable and split into smaller droplets. The process of drop fission, which results from the development of a capillary instability, has been analyzed [4]. An estimate of the instability threshold, which makes it possible to determine the corresponding amount of charge, was first obtained by Rayleigh (Eq.1): Q≥8π(ε0αR3)1/2

(Eq.1)

In this expression, Q and R are the drop charge and radius, α is the surface tension of a molten material, and ε0 is the permittivity of free space. Regarding the screening effect, the drop charge in plasma can be estimated as (Eq.2) [5]: Q≈20πε0(κΤe/e)(R*(R+Rd)/Rd)

(Eq.2)

where k is the Boltzmann’s constant, Te is the plasma electron temperature, RD is Debye radius, and e is the electron charge. Taking this into account the Rayleigh condition can be rewritten in the form (Eq.3): kTe>0.4e(αR/ε0)1/2(Rd/(R+Rd))

(Eq.3)

which shows that Te should be raised to bring microdrops in the fission mode. Estimations made using (Eq.3) show that needed value of plasma temperature is about 20-30 eV. Laser power density required to heat laser torch plasma up to this value is more than 109 W/cm2. The development of the capillary instability of drops commonly involves two stages [4]. First, on exceeding the instability threshold, the drop loses its spherical shape, and the fission starts with a large number of finer (daughter) drops ejected from the prominence on the surface of the mother drop. Analysis shows that daughter drops are also unstable. Accordingly, the drop fission is a cascade process where the drop size decreases by approximately a factor of 10 at each stage of the cascade. The cascade fission stops suddenly when the daughter drops reach a nanometer size. As charged drops become smaller the electric field on their surface increases, that results in a dramatic increase in the field emission of electrons. After the size of the daughter drops decreases to several nanometers, the flow of electrons emerging from the drop surface becomes more than the electron flux coming in from the plasma. When the drops become discharged and stable, the fission terminates resulting in a tremendous number of nanometer drops with narrow size dispersion.

Fig. 1. Scheme of the nanoparticle deposition on granulated support.

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Figure 1. shows schematically the process of nanoparticle deposition on the surface of relatively large (1-3 mm) support grains. A laser pulse causes melting of the target surface and creates the laser torch plasma near the surface. Microdrops of molten metal, which escape from the target and arrive into the plasma, are charged and their fission occurs to give nanosize droplets. Divergent electric field concentrated near the substrate is applied to correct trajectories of charged nanoparticles. This opportunity is used to separate the nanoparticles from the residues of maternal microdrops. The electric field strength was chosen so as to direct nanosize particles to the substrate without disturbing the motion of larger drops. The support grains are placed on the oscillating piezoelectric plate, which is driven by AC source. This results in an intense vibration and random rotation of support grains. The nanodrops formed in the plasma fly apart at a velocity of ~104 cm/s, whereas the velocity of expansion of the plasma cloud exceeds 106 cm/s. Accordingly, at the final stage the expanding plasma moves away from the target and charged drops continue their movement to the substrate in a vacuum. They are cooled down to solidification and cover uniformly the substrate surface. The estimated cooling rate exceeds 106 K/s. That is why formed nanoparticles are amorphous. It is difficult to study directly a process of the drop fission in the laser torch plasma. The main obstacles are associated with the short duration of the fission process (less than 100 ns), the small size and high velocity of the particles. In addition, the charging and division of particles occur in high-density and hot laser torch plasma. The evidence for the process of microdrop fission was obtained in a special experiment when the substrate was mounted in a close vicinity of the target surface. As can be seen in Fig. 2., the surface of the substrate placed near the copper target is covered with submicrometer particles. Some particles have projections whose size is 10 times smaller than that of the main particles. The presence of such particles with projections can be interpreted as a consequence of rapid cooling and deposition onto the substrate of drops that are in the initial stage of fission. By now, nanostructured Cu, Ni, Pd, Pt and Au and Ni/Au catalysts on silicon (100), surface oxidized silicon SiO2/Si, γ-Al2O3 and carbon supports have been prepared by the LED technique. As one can see from the TEM image of Pd catalyst on γ-Al2O3 (Fig. 3.), the support grain is covered uniformly by small aggregates of separated particles. The relative particle size dispersion does not exceed 10%. Comparison of TEM micrographs of Cu, Ni, Pd, Pt and Au films shows that the average size of nanoparticles is only determined by the material of which the particles are composed, it is 5 nm for Cu particles and about 2-3 nm for other metals. In all cases, the electron diffraction patterns recorded directly in the TEM had the form of diffuse halos, which indicates that nanoparticles are in the amorphous state.

Fig. 2. Submicrometer Cu particles on the surface of a substrate near the target.

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The nanoparticles of all studied metals do not coagulate on coming in contact with each other. That may be a consequence of the amorphous state of the metal. This feature makes possible the fabrication of dense self-assembled catalysts. Unique catalytic properties of such catalysts are associated with the inter-grain electron tunneling in dense nanostructures [1,2]. Theoretical and experimental studies show a possibility of the formation of a significant amount of the charged particles due to charge redistribution within an ensemble of clusters on the dielectric support (SiO2/Si) or between supported clusters and conducting support (Si,C) [7]. These charged states provide unusually high catalytic activity (up to 105 mol(product)*mol(metal)-1*h-1) of self-assembled catalysts in hydrogenation as well as in chlorohydrocarbon conversions. This is several orders of magnitude higher compared to that for separated metal clusters, highly loaded metal films and usual supported catalysts (102 mol (product)*mol(metal)-1*h-1). Another important advantage of catalysts prepared by LED technique is their unusually high stability against oxidation and poisoning. For example, according to XPS data, the valence state of Pd remains unchanged during catalytic hydrodechlorination [6]. Catalysts consisting of ensembles of Pd and Ni nanoparticles deposited on sibunite and γ-Al2O3 with extremely low metal content (<10-3mass.%) prepared by means of laser electrodispersion were also extremely active in the hydrodechlorination of chlorinated compounds. This reductive process is ecologically more safe way of the toxic chlorinated wastes utilization in comparison with the oxidative method [8].

Fig. 3. TEM micrograph of Pd nanoparticles on Al2O3 support.

Acknowledgments This work was supported by RF State Contract № 02.740.11.0026.

References [1] [2] [3] [4] [5] [6] [7]

T.N. Rostovshchikova et al, Appl. Cat. A: General, 296 (2005) 70. T.N. Rostovshchikova et al, Catal. Tod., 105 (2005) 344. V.M. Kozhevin et al, J.Vac. Sci. Techn. B, 18 (2000) 1402. A.I. Grigor'ev, S. O. Shir'aeva, J.Phys.D:Appl.Phys., 23 (1990) 1361. Y. P. Raizer, J. E. Allen, V.I. Kisin, Gas Discharge Physics, Springer, the Netherlands, 1991. E.S. Lokteva et al, Kinet. and Catal., 49 (2008) 748. S.A. Gurevich et al, “Thin Films and Nano-structures. Physic-Chemical Phenomena in Thin Films and at Solid Surfaces”, Elsevier, Amsterdam, 2007, pp. 726-754. [8] V.V. Lunin et al, The Role of Ecological Chemistry in Pollution Research and Sustainable Development, Springer, the Netherlands, 2009, pp. 221-232.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Nitrogen doped TiO2 photocatalyst prepared by low energy N+ implantation technique Tomoko Yoshida,a and Eriko Kudab a

Division of Environmental Research, EcoTopia Science InstituteNagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan b Department of Materials, Physics and Energy Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan

Abstract A nitrogen doped TiO2 as a visible-light response photocatalyst was prepared by low energy N+ implantation technique. N + - implanted TiO 2 samples promoted the photocatalytic activity for degradation of methylene blue under visible-light irradiation. N 1s XPS and N K-edge XANES spectra of the photocatalytically active sample indicated that N replaces the O sites near the surface, whereas in the inactive samples NO species are formed. We also found that the nitrogen concentration of the sample implanted with small amount of N+ (1 x 1021 m-2) is a little higher than that with large amount of N+ (3 x 1021 m-2). In the low energy (5 keV) N+ implantation, the sputtering of the sample atoms would be one of the important processes for controlling both the concentration and chemical state of the doped nitrogen. Keywords: N-doped TiO2 photocatalyst, visible light response, ion implantation, XANES

1. Introduction Photocatalytic reactions at the surface of titanium dioxide (TiO2) under UV light irradiation have been attracting much attention in view of their practical applications to environmental cleaning such as self cleaning of tiles, glasses, and windows. Asahi et al. reported that the substitutional doping of N into TiO2 contributes to band gap narrowing to provide visible-light response [1]. Actually the doped nitrogen has been regarded as the catalytic active site generating the visible-light response in TiO2, since the optical absorbance in the visible-light region evolved with increasing nitrogen concentration. On the other hand, a discrepancy among the absorbance and photocatalytic activity has been pointed out [2], thus, it is important to understand the chemical environment and optimum concentration of N most effective for the photocatalytic response. To investigate these factors, we prepared visible-light response TiO2 photocatalysts by nitrogen implantation method, because we can control the depth and concentration of the nitrogen easily by changing their energy and fluence. In our previous study [3], 50 keV N+ ions were injected into TiO2 samples. We found that the photocatalytic active nitrogen species, i.e., nitrogen substituting for the O sites in TiO2, are preferentially formed in the samples implanted with a low N+ fluence (< 3 x 1021 m-2). In the samples prepared with a higher N+ fluence, the inactive N-O species generated to reduce photocatalytic activity. In the present study, a low energy, 5 keV N+ implantation was performed in order to inject nitrogen atoms near the surface of TiO2.

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2. Experimental The samples used in this study were TiO2 (1 0 0) single crystals (5 x 5 x 0.5 mm3), supplied by Furuuchi Kagaku, Japan. Mass analyzed 10 keV N2+ (5 keV N+) ions were injected into the samples at room temperature, perpendicular to the sample surface. After the ion implantation, parts of the samples were heat-treated at 573 K for two hours in air. As calculated by a Monte Carlo simulation using SRIM code (Fig.1), the implanted nitrogen atoms distribute up to ca. 25 nm, peaking around 10 nm in depth from the surface, and they would increase monotonously with N+ fluence. Thus, the 5keV N+ implantation enables to inject nitrogen atoms in the shallower region compared with the 50 keV N+ implantation in our previous study. -2

5keV N 6

+

50keV N

+

-1

-1

Atoms ( nm ion )

8x10

4 2 0 0

50

100 Depth (nm)

150

200

Fig. 1. Depth distributions of implanted nitrogen per incident N+ ion calculated by SRIM code for 5 and 50 keV N+ implantations.

A typical photocatalytic experiment consisted of placing the N+-implanted sample in 0.5 ml of aqueous methylene-blue (MB) solution (9.8 μmol/L) and subsequent exposure to visible-light using a 15 W Xe lamp with a cut-off filter permitting λ> 430 nm. The light absorbance at λ= 664 nm after exposure for 2 hours was measured to estimate the photocatalytic activity of the samples. UV-vis absorption spectra of the samples were recorded with JASCO V-550 in a transmission mode. XPS measurements were performed with a Shimadzu ESCA-3300 with Al Kα emission for the X-ray source, and the spectra of N 1s, O 1s and Ti 2p regions were recorded. Binding energy was calibrated using the C 1s peak at 284.6 eV. N K-edge X-ray absorption near edge structure (XANES) spectra of the samples were measured at the BL-8B1 station of UVSOR-II at the Institute for Molecular Science, Okazaki, Japan. Data were recorded at room temperature in total electron yield mode, and the X-ray energy dependence of the N Auger electron yield was monitored. Considering the escape depth of the Auger electrons, the spectra probe the sample from the surface up to a few nanometers in depth.

2. Results and discussion As shown in Fig. 2, in the UV-vis spectra for the as-implanted and 573 K annealed samples, the absorbance in the visible-light region from 420 nm to 540 nm increased with the fluence of nitrogen, suggesting the generation of visible light responsiveness.

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Visible-light response TiO2 photocatalyst prepared by nitrogen

0.50 21

-2

Absorbance

3 x 10 / m (heat treat.) 0.40 21

3 x 10 / m 21

0.30

-2

1 x 10 / m TiO 2

-2

0.20 440

480 520 Wavelength

560

Fig. 2. (left) UV-vis spectra for as-implanted and 573 K annealed samples.

Amount of MB decomp. (nmol)

The N+ fluence dependences of the visible-light responsive photocatalytic activity are shown in Fig. 3 for the as-implanted and 573 K annealed samples. The TiO2 and the sample implanted by 1 x 1021 m-2 (TiO2-N(1)) showed almost the same MB degradation rate under visible-light irradiation. The sample implanted by 3 x 1021 m-2 (TiO2-N(3)) clearly showed the photocatalytic activity, while this sample became almost photocatalytically inactive after the heat-treatment at 573 K (TiO2-N(3)-H). Thus, the visible light responsiveness was not proportional to the photoabsorbance, which is consistent with the previous reports [2]. 0.4 0.3 0.2 0.1

as implanted after heat treat.

0.0 0.0

1.0 2.0 3.0 + 21 2 N Fluence ( x 10 / cm )

Fig. 3. (right) Amount of decomposed MB after visible light irradiation for 2 h as a function of N+ fluence.

The chemical states of the implanted nitrogen were characterized by XPS, and the N 1s core-level spectra of the as-implanted, the 573 K annealed and TiN samples are shown in Fig. 4. In the spectrum of a TiN powder, a peak around 397 eV was observed. The N 1s peaks observed for as-implanted and the annealed samples can be divided into two groups, one at 396 eV and another at 402-403 eV. We assigned the N 1s peak around 396 eV to nitrogen replacing one of the O sites of TiO2 [1] and the peak at 402403 eV to nitrogen species bound to various surface oxygen sites (N-O like species) [1]. It is interesting to note that XPS spectrum for the photocatalytically active sample (TiO2N(3)) exhibits only one peak at 396 eV while those for the photocatalytically inactive samples (TiO2-N(1)) and the annealed sample (TiO2-N(3)-H) show two peaks. Figure 5 shows N K-edge XANES spectra of the N +-implanted TiO2 and TiN samples. Common XANES features in (b) and (d) suggest that N in TiO2-N(3) is in a chemical environment similar to that in TiN. Careful observation led us to notice that two peaks around 400 eV for the catalyst sample shift to lower energy side than those for the TiN sample, which was well reproduced by the theoretical prediction using FEFF code when N occupies one of the O sites of TiO2. On the other hand, the XANES spectra of TiO2N(1) and TiO2-N(3)-H showed a distinct single peak around 401 eV. This peak could be empirically attributed to formation of the species such as N−O bonds near the surface [4], and which also indicates that the active nitrogen formed in TiO2-N(3) changed to the inactive N-O species by oxidation at 573 K. Thus, both XPS and XANES spectra of TiO2-N(3) exhibited the generation of the photocatalytically active nitrogen (N replacing the O site of TiO2), and the similar spectra were measured for the sample implanted with the higher N+ fluence of 5 x 1021 m-2 (not shown here). The present XPS and XANES measurements also showed that the inactive N-O species dominates in TiO2-N(1). However, these results are in conflict

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with our previous study, in which the photocatalytically active nitrogen was preferentially produced by the implantation with the lower nitrogen concentration.

c)

b)

d) Absorbance (arb. unit)

Intensity (arb. unit)

d)

c)

b)

a)

a) 404

400

396

Binding energy (eV)

392

Fig. 4. (left) XPS spectra of the N 1s region for (a) N+-implanted at 1 x 1021 m-2, (b) N+implanted at 3 x 1021 m-2, (c) N+-implanted at 3 x 1021 m-2 followed by heating at 573 K for 2 h, and (d) TiN.

395

400

405

410

X-ray energy (eV)

415

Fig. 5. (right) N K-edge XANES spectra of (a) N+-implanted at 1 x 1021 m-2, (b) N+implanted at 3 x 1021 m-2, (c) N+-implanted at 3 x 1021 m-2 followed by heating at 573 K for 2 h, and (d) TiN.

Then, we roughly calculated the concentrations near the surface of the two samples TiO2-N(1) and TiO2-N(3) from their XPS spectra of N 1s, O1s and Ti 2p regions, and found that the nitrogen concentration of the former sample is a little higher (ca. 5.3 atom%) than that of the latter sample (ca. 4.6 atom%). It is unclear why the nitrogen concentration of the latter sample was so low in spite of the high dose N+ implantation. However, effects of the sputtering process would not be ignored for the present low energy N+ implantation, since a Monte Carlo calculation by SRIM code actually indicated that two atoms of the sample could be sputtered by one 5 keV N+ ion. The nitrogen concentration of the latter sample would not increase by the long time sputtering process, leading to produce the photocatalytically active nitrogen near the sample surface.

References [1] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki and Y. Taga, 2001, Visible-Light Photocatalysis in Nitrogen-Doped Titanium Oxides, Science, 293, 269-271.; R. Asahi and T. Morikawa, 2007, Nitrogen complex species and its chemical nature in TiO2 for visible-light sensitized photocatalysis, 2007, Chem. Phys. 339, 57-63. [2] H Irie, Y. Watanabe and K. Hashimoto, 2003, Nitrogen-Concentration Dependence on Photocatalytic Activity of TiO2-xNx Powders, J. Phys. Chem. B, 107, 5483-5486. [3] T. Yoshida, S. Muto and J. Wakabayashi, 2007, Depth-Resolved EELS and Chemical State Mapping of N+-Implanted TiO2 Photocatalyst, Mater. Trans., 48, 2580-2584. [4] J-H. Wang, P. K. Hopke, T. M. Hancewicz and S. L. Zhang, 2003, Application of modified alternating least squares regression to spectroscopic image analysis, Anal. Chim. Acta, 476, 93-109.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Preparation and characterization of shape-selective ZSM-5 catalyst for para-methyl ethylbenzene production with toluene and ethylene Binzuo Liu, Zhaoxiang Yu,Yongtao Meng, Luhao Cui, Zhirong Zhu* Department of Chemistry, Tongji University, Siping Road 1239, Shanghai 200092, China

Abstract In order to improve the shape-selective catalysis of ZSM-5 for para-methyl ethylbenzene (p-MEB) production with alkylation of toluene and ethylene, the acidic sites on the external surface of ZSM-5 crystal are completely eliminated by chemical liquid deposition of silica (SiO2-CLD) using polyphenylmethyl-siloxane (PPMS) as a modifier. So, orthomethyl ethylbenzene and meta-methyl ethylbenzene cannot be formed as by-products neither on the external surface nor in the channel of modified ZSM-5. The acidity of modified ZSM-5 can be further decreased by adding some metallic oxides (1.0 wt% of La2O3 or MgO). This may reduce side reactions like ethylene polymerization and coking deactivation of ZSM-5 catalyst. As a result, ZSM-5 modified by SiO2-CLD and metallic oxide loading shows both high alkylation efficiency (ethylene conversion over 98%) and high selectivity in para-methyl ethylbenzene (over 96%) with a good stability over time. Keywords: ZSM-5 zeolite; chemical modification; para-methyl ethylbenzene; alkylation; shape-selective catalysis

1. Introduction Para-vinyltoluene is a kind of important feedstock for producing quality polymers, which may replace styrene monomer [1]. Para-vinyltoluene is produced by the dehydrogenation of para-methyl ethylbenzene [2]. Para-methyl ethylbenzene may be obtained by alkylation of toluene with ethylene. The main reactions involved in this alkylation process are as follows:

C 2H 5

k1

k 3-1 k 3-2

CH 2 = C H 2 +

C 2H 5

k 4-1

k2

k 4-2 C 2H 5

Figure 1. Schema of the main reactions occuring during the alkylation of toluene.

As the isomerization reaction among three kinds of methyl ethylbenzene isomers (K3, K4) is much easier than alkylation reaction of toluene with ethylene (K1, K2), shown in Figure 1, the mixture of para-methyl ethylbenzene, orth-methyl ethylbenzene

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and meta-methyl ethylbenzene was produced over ordinary acidic catalysts. So, the acidic catalyst for alkylation of toluene with ethylene should be designed with the shape-selective property, to prevent formation of ortho-methyl ethylbenzene and metamethyl ethylbenzene. Zeolite ZSM-5 was often used for the shape-selective catalysis in the synthesis of the para-dialkylbenzene. ZSM-5 modified by chemical vapor deposition of silica (SiO2CVD) might improve para-xylene selectivity in toluene disproportionation or alkylation [3]. In preparation of the commercial catalyst for the shape-selective disproportionation of toluene to para-xylene, the pre-coking method has been replaced by SiO2-CLD [4]. Compared with SiO2-CVD, SiO2-CLD may be more easily transferred to an industrialscale preparation, such as SiO2-CLD modification of ZSM-5 with polysiloxane in Mobil’s MTPX Process [5]. Moreover, modification by loading MgO or phosphate was used to prepare zeolite catalysts for alkylation of EB with ethylene or ethanol, improve para-DEB selectivity over ZSM-5 [6, 7]. Though MgO modification was not as effective as SiO2-CLD for obtaining the high catalytic activity, it can decrease the formation of non-aromatic hydrocarbon and benzene over ZSM-5, by reducing the side reaction of dealkylation. In this work, modification of SiO2-CLD combined with loading metallic oxides was developed to prepare a shape-selective ZSM-5 catalyst for para-methyl ethylbenzene production by alkylation of toluene and ethylene.

2. Experimental 2.1. Preparation of modified ZSM-5 catalyst ZSM-5 with Si/Al 150 was hydrothermally synthesized according to the reported method [8]. NH4-form ZSM-5 was mixed with solution of polyphenylmethylsiloxane (PPMS), at 20 wt % PPMS to ZSM-5. After dried at 393 K, the impregnated ZSM-5 was heated to 823 K at a rate of 3 K/min, and the SiO2-CLD modified ZSM-5 (SiO2CLD/ZSM-5) was obtained. As a reference, the SiO2-CVD modified ZSM-5 (SiO2CVD/ZSM-5) was prepared with TEOS from NH4-form ZSM-5 according to the reported procedure [3]. The above SiO2-CLD/ZSM-5 was mixed with solution of Mg(NO3)2 or La(NO3)3, at the ratio of 1.0 wt % oxide to ZSM-5. After the above impregnated sample was dried at 393 K and calcined at 823 K for 2 h, the SiO2-CLD&MgO or La2O3 modified ZSM-5 (SiO2-CLD&MgO or La2O3/ZSM-5) was obtained.

2.2. Catalytic reaction The catalytic reaction for alkylation of toluene and ethylene was respectively carried out over the parent ZSM-5, SiO2-CLD/ZSM-5, SiO2-CVD/ZSM-5, SiO2-CLD& La2O3/ZSM-5 and SiO2-CLD&MgO/ZSM-5. The reaction was conducted in the fixedbed reactor with WHSV 8 h-1 and 1.2 Mpa (the molar ratio of ethylene / toluene 3.0) at 673 K. The products were determined by on-line gas chromatography with 50 m - 0.32 mm i.d. FFAP capillary column and FID.

3. Results and discussion 3.1. Zeolite materials for catalyzing alkylation of toluene Catalytic performance of a variety of acidic zeolites, with different structural and acidic properties, are compared in the methyl ethylbenzene (MEB) synthesis by alkylation of toluene. These zeolites are the MCM-22, Beta, SAPO-11, MOR and the ZSM-5 with different Si/Al ratio. Among these zeolites, ZSM-5 and SAPO-11 fall into the category

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of medium 10-membered ring, mordenite and Beta the category of large 12-membered ring. MCM-22 has its specific structure with 10-membered ring channels and large intracrystalline cages. Besides, there is a wide range of acidity among the investigated zeolites, from MOR with high-strength acidity to mid-strength acidity SAPO-11. Table 1. Catalytic performance for toluene alkylation over different zeolites.

Mordente ZSM-5 /48 ZSM-5 /150 ZSM-5 /220 MCM-22 SAPO-11 Beta

Toluene Conversion /% 30.9 30.5 29.6 27.7 31.0 27.3 29.2

Ethylene Conversion /% 98.2 97.5 95.7 90.8 97.6 89.5 96.4

Total MEB Selectivity /% 79.3 86.6 91.0 91.5 84.2 87.3 82.1

p-MEB / MEB para-Selectivity /% 63.1 71.2 74.5 76.8 62.0 73.4 68.1

According to the results shown in Table 1, zeolite ZSM-5, with the high Si/Al ratio 150, 10-membered ring channels and mid-strength acidity, shows both high selectivity and catalytic activity for alkylation of toluene with ethylene.

3.2. Modification of SiO2-CLD with loading metallic oxides

In order to improve the shape-selective catalysis of ZSM-5, SiO2-CLD using PPMS as a modifier was adopted to eliminate the acidic sites on the external surface of ZSM-5 crystals. As a result, orth-methyl ethylbenzene and meta-methyl ethylbenzene cannot be formed as by-products on the external surface of modified ZSM-5 as their molecular sizes are more than the pore size of modified ZSM-5. Comparatively, SiO2-CLD/ZSM-5 shows higher catalytic activity as SiO2-CVD/ZSM-5 loses more acidic sites in ZSM-5 channels during modification. However, the selectivity of total methyl ethylbenzene over SiO2-CLD/ZSM-5 is affected by both disproportionation of toluene and dealkylation of methyl ethylbenzene. Two side reactions are mainly caused by the stronger acidic sites of SiO2-CLD/ZSM-5. Table 2. Catalytic performance for toluene alkylation over modified ZSM-5 *. Samples

Toluene Ethylene Total MEB p-MEB / MEB Conversion Conversion Selectivity para-Selectivity /% /% /% /% unmodified ZSM-5 29.4 91.2 95.5 74.9 SiO2-CLD / ZSM-5 27.9 93.5 91.3 90.7 26.1 89.7 SiO2-CVD/ ZSM-5 91.0 87.2 SiO2-CLD&MgO /ZSM-5 25.0 (28.3) 90.7 (98.1) 95.1 (94.2) 98.6 (97.3) SiO2-CLD&La2O3 /ZSM-5 25.6 (28.8) 91.3 (98.5) 94.8 (93.9) 98.2 (96.8) Note: The data in the bracket were the reaction results after increasing reaction temperature 5 K.

In order to decrease ZSM-5 acidic strength, a little of metallic oxides, basic La2O3 or MgO, were loaded over SiO2-CLD/ZSM-5. This may effectively reduce the sidereactions of both disproportionation and dealkylation. Moreover, the amount of loaded La2O3 or MgO over ZSM-5 relates to catalytic activity and selectivity. The selectivity of para-methyl ethylbenzene and the selectivity of total methyl ethylbenzene increase with

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loaded La2O3 or MgO over ZSM-5, but the catalytic activity drops down. Taking both catalytic activity and selectivity into consideration, about 1.0 wt% La2O3 or MgO loading of over ZSM-5 is proper for the further modification of SiO2-CLD/ZSM-5. Table 3. Results of NH3-TPD characterization for the acidity of modified ZSM-5. Samples

unmodified ZSM-5 SiO2-CLD / ZSM-5 SiO2-CVD/ ZSM-5 SiO2-CLD&MgO /ZSM-5 SiO2-CLD&La2O3 /ZSM-5

Week acidic sites Temperature Desorbed NH3 /mmol/g /K 529 0.67 528 0.53 526 0.48 525 0.50 529 0.51

Strong acidic sites Temperature Desorbed NH3 /K /;mmol/g 778 0.31 775 0.25 773 0.20 750 0.11 754 0.12

Moreover, the further La2O3 or MgO modification reduces coking deactivation from ethylene polymerization over ZSM-5 catalyst, improving its stability. Therefore, SiO2-CLD& La2O3 or MgO/ZSM-5 may be considered as a promising catalyst for toluene alkylation to produce para-methyl ethylbenzene.

Acknowledgements This work is supported by China NSFC 20873091 and Shanghai 09JC141000.

References [1] [2] [3] [4] [5] [6] [7] [8]

C. Yu, C. Tan, J. of Supercritical Fluids, 44 (2008): 341. Z. Zhu, Q. Chen, Z. Xie, C. Li, 13th International Conference on Catalysis, Pairs, 2004: 114. B. Anand, Halgeri, Jagannath Das, Catal. Today, 73 (2002): 65. D. Rotman, Chemical Week, 30 (1995): 18. R. W. Weber, K. P. Moller, C. T. O’Conner, Micropor. Mesopor. Mater., 35 (2000) : 533. Z. Zhu, 8th International Conference on Mechanism of Catalytic Reaction, Russia, 2009: 82. N. Y. Chen, Stud. Sulf. Sci. Catal., 38 (1988): 153. Z. Zhu, Z. Xie, Q. Chen, W. Li, W. Yang, C. Li, Micropor. Mesopor. Mater., 2007(101): 169.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Microwave-assisted preparation of Mo2C/CNTs nanocomposites as an efficient support for electrocatalysts toward oxygen reduction reaction Min Pang, Ling Ding, Chuang Li, Changhai Liang* State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116012, China

Abstract Nanostructured Mo2C/CNTs has been synthesized by microwave-assisted thermolytic molecular precursor method. Pt nanoparticles were deposited on the as-prepared Mo2C/ CNTs by using the modified ethylene glycol method. The samples were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The electrocatalytic activity toward ORR was measured through a thin-film rotating disk electrode. The results showed the particles size of Mo2C and Pt ranged from 3 to 6 nm. The formation process of Mo2C followed the sequence: Mo(CO)6 → Mo → [Mo,O,C] → Mo2C → Mo3C2. The Pt-Mo2C/CNTs sample possessed higher ORR activity with a more positive onset potential in acid solution than that of Pt /CNTs under the same condition, which could be attributed to the synergistic effect among Pt, Mo2C and CNTs. Keywords: Mo2C/CNTs, microwave, thermolytic molecular precursor, oxygen reduction reaction

1. Introduction Proton exchange membrane fuel cells (PEMFCs) have recently attracted much attention from both a fundamental and an applied point of view for their future potential as clean and mobile power sources. High cost, low activity and poor durability are still major barriers to the commercialization of PEMFCs although lots of advances have been made within the past few decades. Therefore, an inexpensive, active and robust substitute for Pt-based catalyst is in urgent need. Transition metal carbides have received considerable attention for its exceptional noble metal-like activity in some reactions. It shows a good prospect in replacing or reducing the usage of the noble metal in manufacturing the electrode catalysts. However, there are difficulties in obtaining highly dispersed carbide nanoparticles according to the previous preparation methods. Microwave-assisted thermolytic molecular precursor has been proved to be a powerful method for generating highly dispersed tungsten carbide catalysts in a controlled and reproducible manner [1]. Here we report on the synthesis of evenly distributed Mo2C nanoparticles supported on carbon nanotubes by microwave-assisted thermolytic molecular precursor. The Mo2C/CNTs nanocomposite has shown the potential as an efficient electrocatalyst support for oxygen reduction reaction.

2. Experimental CNTs supported molybdenum carbides were prepared by the microwave-assisted thermolytic molecular precursor method. Briefly, CNTs and Mo(CO)6 were mixed in an agate mortar for 0.5 h. The mixture was placed in a quartz reactor and treated in 30

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mL/min argon flow for 2 h. The reactor was placed in a microwave oven operating at 2.45 GHz with a maximum power of 800 W. The duration of microwave exposure varied from 1 to 30 min in argon. Pt nanoparticles were deposited on the as-prepared Mo2C/CNTs by using the modified ethylene glycol method. The samples were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The electrocatalytic activity toward ORR was measured through a thin-film rotating disk electrode.

3. Results and discussion 3.1. Characterization of samples with different Mo loadings Figure 1 shows XRD patterns of the samples undergoing 15 min of microwave irradiation. No distinct peaks due to MoxC phase were detected for the 4.8 wt% sample, suggesting the highly dispersed MoxC on CNTs. The XRD pattern of the 13.0 wt% sample showed three typical diffraction peaks at 37.9, 39.4, and 61.5 °, which can be assigned to (002), (101), and (110) crystal face of β-Mo2C with hexagonal closedpacked structure. Further increasing the Mo loading to 16.7 wt%, the diffraction peaks became sharper, which could be ascribed to the growth of Mo2C particle. The average particle size of β-Mo2C estimated according to the Scherrer formula was 6.2 nm for 16.7 wt% samples, which was in good agreement with the value observed from TEM images (Figure 2). ♦

• Mo2C

* Mo3C2 Intensity (a.u.)

♦ CNTs

•*•

•

♦

♦

*

•

16.7 wt% 13.0 wt% 9.1 wt% 4.8 wt%

10

20

30

40

50

60

70

80

2 Theta (deg.)

Figure 1. XRD patterns of the Mo2C/CNTs samples with different Mo loadings.

Figure 2. TEM images of the Mo2C/CNTs sample with the 16.7 wt% Mo loading.

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Microwave-assisted preparation of Mo2C/CNTs

3.2. Formation process of Mo2C/CNTs

To understand the formation process of Mo2C on CNTs under microwave irradiation, the samples with different irradiation time were checked. Figure 3 shows the XRD patterns of the samples undergoing different duration of microwave irradiation. After 1 min of microwave irradiation, two clear diffraction peaks at 37.3 and 44.6° due to MoOxCy phase appeared, while a weak peak at 19.4° due to Mo2C also emerged. This was attributed to precipitation of O inside the MoOxCy and carburization of external surface of MoOxCy particles [2, 3]. With increasing reaction time to 5 min, the diffraction peaks due to MoOxCy phase became weaker and those due to Mo2C became sharper. Meanwhile the diffraction peaks at 36.4 and 46.4° assigned to Mo3C2 phase appeared. Further increasing reaction time to 15 min, the diffraction peaks due to Mo2C became much sharper while the diffraction peaks due to Mo3C2 phase also became clear. It was also found that the inner wall of the reactor was covered by a layer of metallic Mo when microwave irradiation time was 0.5 min. This was due to the decomposition of Mo(CO)6 precursor to metallic Mo in the initial step, which is in agreement with literature [4]. With the increase of microwave irradiation time, MoOxCy phase first was formed by the reaction between metallic Mo and active carbon species from CO. The MoOxCy phase was further carburized to Mo2C with the increase of microwave irradiation time. Accordingly, the formation process of Mo2C/CNT can be postulated as follows: Mo(CO)6→Mo→[Mo,O,C]→Mo2C→Mo3C2. • Mo2C ♣ Mo3C2

Intensity (a.u.)

♥ MoOxCy ♦ CNTs • • ♣•

15min

•

5min

♣

•

♦

•

♦

•

♥• •

♥ •

1min

•♦

♥

•

0 8

0 7

0 6

0 5

0 4

0 3

0 2

0 1

2 Theta (deg.)

Figure 3. XRD patterns of the MoxC/CNTs samples undergoing different duration of microwave irradiation.

3.3. Electrocatalytic activity of Mo2C/CNTs toward ORR

Figure 4 shows the HRTEM image of Pt-Mo2C/CNTs sample. All the particles were in sphere shape and no agglomerations were detected. The particle size varied from 3 to 6 nm. The Pt particles obtained through the reduction of ethylene glycol method were more likely to be closed with or in conglutination with the Mo2C particles, which was quite different from the Pt-WCx/CNTs samples gained through the same way. Figure 5 shows the linear sweep sweeping curves of the Pt/CNTs and Pt-Mo2C/CNTs samples in the O2 saturated 0.5 M H2SO4 solution. The Pt-Mo2C/CNTs catalyst had a more positive onset potential of 85 mV compared to Pt/CNTs catalyst with the same Pt loading, which could be attributed to a synergistic effect among Pt, Mo2C and CNTs. This performance implied that Mo2C/CNTs was an efficient support for electrocatalyst and possessed a good ability in cutting down the Pt usage.

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0.2 0.1

Current/mA

0.0

Pt-Mo2C/CNTs Pt/CNTs

-0.1 -0.2 -0.3

△ V = 85 mv

-0.4 -0.5 -0.6

0.0

0.2

0.4

0.6

0.8

1.0

Potential/V vs. SCE

Figure 4. HRTEM images of Pt-Mo2C/CNTs sample.

Figure 5. Linear sweeping curves of O2 at Pt/CNTs and Pt-Mo 2 C/CNTs in solution of o 0.5M H2SO 4 with scan rate of 5mV/s at 25 C.

4. Conclusions Mo2C particles with 3-6 nm has been successfully synthesized and well distributed on CNTs by microwave-assisted thermolytic molecular precursor method. The formation process of Mo2C followed the sequence: Mo(CO)6→Mo→[Mo,O,C]→Mo2C→Mo3C2. The Pt-Mo2C/CNTs which were prepared by the modified ethylene glycol method exhibited higher ORR activity with a more positive onset potential in acid solution than that of Pt /CNTs under the same condition, which was attributed to the synergistic effect among Pt, Mo2C and CNTs.

Acknowledgments We gratefully acknowledge the financial support provided by the Scientific and Technical Foundation of Educational Committee of Liaoning Province, Foundation for Returness of Ministry of Education of China.

References [1] C.H. Liang, L. Ding, A.Q. Wang, Z.Q. Ma, J.S. Qiu, T. Zhang, 2009, Microwave-assisted preparation and hydrazine decomposition properties of nanostructured tungsten carbides on carbon nanotubes. Ind. Eng. Chem. Res., 48, 3244. [2] D. Mordenti, D. Brodzki, G. Djéga-Mariadassou, 1998, New synthesis of Mo2C 14 nm in average size supported on a high specific surface area carbon material. J. Solid State Chem., 141, 114. [3] C.H. Liang, P.L. Ying, C. Li, 2002, Nanostructured β-Mo2C prepared by carbothermal hydrogen reduction on ultrahigh surface area carbon material. Chem. Mater., 14, 3148. [4] H.Y. Chen, L. Chen, L. Lu, Q. Hong, H.C. Chua, S.B. Tang, J. Lin, 2004, Synthesis, characterization and application of nano-structured Mo2C thin films. Catal. Today, 96, 161.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Laser-induced photocatalytic inactivation of coliform bacteria from water using pd-loaded nano-WO3 A. Bagabas,a* M. Gondal,b A. Khalil,b A. Dastageer,b Z. Yamani,b M. Ashameria a

Petroleum and Petrochemicals Research Institute (PAPRI), King Abdulaziz City for Science and Technology (KACST), P. O. Box 6086, Riyadh 11442, Saudi Arabia b Laser Group, Physics Department and Center of Excellence in Nanotechnology, King Fahd University of Petroleum & Minerals (KFUPM), Dhahran, 31261 Saudi Arabia

Abstract Nano palladium-loaded on nano tungsten trioxide (n-Pd/n-WO3), with 10% wt Pd loading, was prepared by the impregnation evaporation method. The n-WO3 support was prepared by dehydration of tungstic acid (H2WO4). The n-Pd/n-WO3 was characterized by Raman spectroscopy, X-ray powder diffraction (XRD) and transmission electron microscopy (TEM). This material was tested as a photocatalyst for inactivation and killing of coliform bacteria, by applying 355-nm pulsed UV laser radiations, generated from the third harmonic of Nd:YAG laser, to a model water sample, prepared using bacteria strains of Escherichia coli. The killing effect of n-Pd/nWO3 on coliform bacteria was characterized by means of selective culture media. The photocatalysis process did result in a very high irreversible injury (99%) under investigated conditions. This process is cost-effective because no bacteria re-growth was recorded under optimum environment conditions. The disinfection rate of water was estimated by exponential decay. The conventional titania (TiO2) semiconductor and commercially available WO3 display a lower decay rate than that for n-Pd/n-WO3. Keywords: Nano palladium-loaded on nano tungsten trioxide; n-Pd/n-WO3; E-Coliforms; water disinfiction; heterogeneous photocatalysis

1. Introduction The Word Health Organization (WHO) reports two million deaths worldwide annually due to consumption of infected water. The provision of clean water supplies is therefore a key issue for human health and environment. The increasing concern for pathogenic related water diseases has forced the world regulatory bodies to apply strong regulations on microbiological pollution of water to meet drinking water standards [1]. Due to these reasons, specific disinfection techniques must be developed for water treatment [2]. Various chemical processes based on activated carbon, coagulation and multimedia sand filtration have been applied for removing the microorganism [3]. Nevertheless, these conventional technologies only convert the contaminated substances from the treated water to another solid form, requiring further treatment and disposal. Chlorination is another cost-effective and efficient disinfectant method [4]. However, the residual chlorine in treated water is toxic. To overcome these disadvantages in current water disinfection methods, there is a need to develop efficient, cost-effective alternatives. Recently, TiO2 photocatalytic technology has been applied for water

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disinfection by using conventional UV lamps and solar radiation [5,6]. However, to enhance the efficiency of photocatalytic process, further improvements are required. In this work, n-Pd/n-WO3 was synthesized and was applied as a photocatalytic agent for the disinfection of water from coliforms by using 355-nm pulsed UV laser radiations, generated from the third harmonic of an Nd:YAG laser. The killing effect of n-Pd/n-WO3 on coliform bacteria was characterized by means of selective culture media.

2. Experimental 2.1. Catalyst preparation All the chemicals were commercially available (Sigma-Aldrich and Fluka) and were used without further purification. The n-WO3 support was prepared by dehydration of H2WO4 at 300oC for seven hours. The Pd was loaded on this support by the impregnation evaporation method using a sulfur-free benzene solution of palladium acetate in a rotary evaporator. The n-Pd/n-WO3 catalyst was obtained by reduction under H2 flow of 50 ml/min at 350o C for five hours.

2.2. Bacteria calturization and growth Escherichia coli K12 wild-type strain MG 1655 was grown overnight in nutrient broth at 37°C on a rotary shaker (160 rpm). Aliquots of the preculture were inoculated into a fresh medium and were incubated in the same conditions to an absorbance at 600 nm of 0.50±0.025. Cells were harvested by centrifugation at 4000 g for 10 min at 4°C, were washed twice with a sterile 0.9% NaCl solution at 4°C and were resuspended in the photocatalytical solution to a concentration of 2 × 107 CFU/ml. Culturable bacteria (tested bacteria with laser induced photocatalysis) were analyzed by plating on nutrient agar plates after serial dilution in 0.9% NaCl solution. Colonies were counted after 48 h incubation at 37°C.

2.3. Catalyst characterization Raman spectra was recorded on a Perkin Elmer NIR FT-Raman Spectrum GX spectrometer in the range of 4000-100 cm-1. The crystalline phase identification and crystallite size were determined by using a Bruker D8 Advance X-ray diffractometer, operated at 40 kV and 40 mA, using CuKα radiation, in the 2 theta range from 10 to 100 o. The particle size and morphology were determined by a Jeol JEM-2100F (HR) high resolution transmission electron microscope.

2.4. Photoreactor and photocatalytic inactivation experiments The Photocatalytic reactor, used in this study, has been described in detail in our earlier publications for hydrogen production, phenol degradation and other applications [7,8]. The contaminated water samples with bacteria (80 ml) were irradiated using Nd:YAG laser, at different incident laser energies, varying amounts of photocatalyst and for different times. To find the effect of the catalyst identity on the inactivation of the coliforms, micron-WO3 (μ-WO3), Pd-free n-WO3 and n-Pd/n-WO3 catalysts were used. Furthermore, to find out the effectiveness of either photocatalyst or laser radiation on removal of coliforms, photocatalysis was performed in the cases of photocatalyst without laser radiation as well as laser irradiation without a photocatalyst. During this study, the laser energy per pulse was kept at 100 mJ and the contaminated water samples were irradiated for 10 minutes. The treated water samples were collected at different time intervals to observe the removal.

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Laser-induced photocatalytic inactivation of coliform bacteria from water

3. Results and Discussion The Raman spectra are depicted in Fig. 1 for both doped and undoped n-WO3. In Fig. 1a different Raman active modes of vibrations of WO3 are marked. The most intense one (~808 cm-1) is for O-W-O stretching mode of vibration while the one at ~713 cm-1 is for O-W-O bending mode of vibration and the one at ~260 cm-1 is for W=O band. In Fig. 1b for n-Pd/n-WO3, there is a significant change in the relative intensities of the O-W-O stretching and O-W-O bending modes of vibrations, but there is a slight decrease in the intensity for W=O band. It is due to the clinging of Pd atom on WO3 in the doped material.

10% Pd/WO3

Intensity (a.u.)

WO3

20

30

40

50

60

70

80

o

2Θ ( )

Fig. 1. Raman spectra of (a) 10% n-Pd/n-WO3 and (b) undoped n-WO3.

Fig. 2. XRD spectra of 10% n-Pd/n-WO3 (black) and undoped n-WO3 (red).

The crystalline phase identification and crystallite size of the synthesized n-WO3 of both doped and undoped were estimated from the XRD study (Fig. 2). The undoped n-WO3 (spectrum in red), obtained from the dehydration of H2WO4 at 300oC, adopts the triclinic phase. Its crystallite size was calculated from peak broadening (in nm) using the Scherrer’s equation, resulting in an estimated average crystallite size of 8 nm. However, the XRD for n-Pd/n-WO3 (spectrum in black) shows that the phase of n-WO3 changed to tetragonal after the reduction of Pd acetate to Pd metal. The estimated average crystallite size of the doped n-WO3 is 14 nm. However, no characteristic patterns observed due to Pd metal. This observation is attributed to the highly dispersed [9] Pd metal nanoparticles (3-4 nm), supported on n-WO3, as confirmed by TEM study (Fig. 3).

Palladium nanoparticles

Fig. 3. TEM micrograph of 10% n-Pd/n-WO3.

The growth and decay of the bacterial population is exponential in nature. Hence, the rate constant of the bacterial decay was calculated from the slope of the curves, resulted by plotting ln(N/N0) versus the light exposure time. N0 is the initial normalized population (4×107 CFU/ml) and N is the diminishing number of bacterial population in CFU/ml. The threshold time of bacterial decay was estimated from the length of time

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when the decay process sets in after the exposure of the first laser pulse. Bacterial decay curves were established for μ-WO3, n-WO3, n-Pd/n-WO3, n-WO3 without UV laser radiation, and for the UV laser radiation without photocatalyst. When using 300 mg of photocatalyst and applying 100 mj laser energy pulse, μ-WO3 shows a decay constant of 0.645 min-1 and a threshold time of 5 min and 58 sec. On the other hand, n-WO3 demonstrates a decay rate constant of 0.945 min-1 and a threshold time of 0 sec, implying that the decay process is almost instantaneous. Furthermore, in the case of n- WO3 , the complete killing of bacteria occurs in 12 minutes, but, in the case of μ- WO3, it occurs in nearly 24 minutes. This substantial improvement in the catalytic process of antimicrobial activity is due to particle size differences. When applying n-WO3 without UV laser irradiation, no decay at all was observed. However, when UV laser irradiation was applied without photocatalyst, a much slower decay rate of bacteria was observed. Doping n-WO3 with 10% wt palladium metal improved the bacterial decay process. This activity enhancement is due to increasing the band gap energy, calculated from absoprtion spectra, from 2.71 eV for the undoped n-WO3 to 3.5 eV for n-Pd/n-WO3, corresponding to around 355 nm in wavelength. The irradiation wavelength used in this study is 355 nm, which is quite closer to the band gap of the doped material. Therefore, we could achieve a near resonance condition that enhanced the transfer of electron from the valence band to the conduction band and, in turn, increased the photocatalytic process. In the case of n-Pd/n-WO3, the decay constant is about 1.1 min-1 and the threshold time is zero sec when using 80 mg of n-Pd/n-WO3 and applying 80 mJ laser pulse energy.

References 1. 2. 3. 4. 5. 6. 7. 8. 9.

WHO/UNICEF, Report. 2000, Global water supply and sanitation assessment report, New York and Geneva. W.A. Yanko, 1993, Analysis of ten years of virus monitoring data from Los Angeles country treatment plants, Meeting on California Wastewater Reclamation Criteria. Water Environ Res., 66, 221-226. V. Lazarova, and J.C. Bourdelot, et. al., 1998, Advances in wastewater disinfection: technical and economic evaluation and large scale operation, Proceedings of the WEFTEC Asia’ 98, Singapore, March 8–11, 1998, 2, 129 -39. H. Arai, M. Arai, and A. Sakumoto, 1986, Exhaustive degradation of humic acid in water by simultaneous application of radiation and ozone. Water Res. 22, 123-126. R. Armon, N. Laot, and N. Narkis, 1998, Photocatalytic inactivation of different bacteria and bacteriophages in drinking water at different TiO2 concentration with or without exposure to O2, J. Adv. Oxid. Technol. 3, 145-150. P. K. J. Robertson, et. al., 2005, Photocatalytic detoxification of water and air, The Handbook of Environmental Chemistry; Volume 2M/2005: Environmental Photochemistry Part II, Springer, Berlin, Heidelberg, 367-423. M. A. Gondal, et al., 2008, Selective Laser Induced Photo-Catalytic Removal of Phenol from Water Using p-Type NiO Semiconductor Catalyst, J. Hazard. Mater. 155, 83-89. M.A. Gondal, et. al., 2009, Efficient Removal of Phenol from Water Using Fe2O3 Semiconductor Catalyst Under UV Laser Irradiation, J. Environ. Sci. Health Part A, 44, 515521. Z. Ma, F. Zaera, 2006, Characterization of Heterogeneous Catalysts, Surface and Nanomolecular Catalysis, Taylor and Francis, Boca Raton, FL, USA, 1-37.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Effect of the carbon nanotube basicity in Pd/N-CNT catalysts on the synthesis of R-1-phenyl ethyl acetate Serap Sahin a, Päivi Mäki-Arvela a, Jean-Philippe Tessonnier b, Alberto Villa b, Lidong Shao b, Dang Sheng Su b, Robert Schlögl b, Tapio Salmi a, Dmitry Yu. Murzin a a

Process Chemistry Centre, Åbo Akademi University, Turku, FI-20500, Finland Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradagweg 4-6, 14195 Berlin, Germany

b

Abstract Catalytic activities of palladium catalysts supported on activated carbon and carbon nanotubes were investigated in the one-pot synthesis of R-1-phenylethyl acetate in combination with an immobilized lipase in toluene. Palladium catalysts on carbon nanotubes with nitrogen-containing surface groups were prepared by incipient wetness impregnation. The basic N-CNT support was synthesized by post-treating oxidized CNTs in gaseous NH3 at high temperature, prior to Pd addition. The basic character of the support was adjusted by controlling the temperature of the post-treatment step. The results showed that the desired product yield was enhanced over palladium catalysts with the lowest basicity. Keywords: one-pot cascades, carbon nanotubes, immobilized lipase, acetophenone

1. Introduction Cascade methodology, implying several reactions in one reactor pot, has gained interest recently due to the savings in equipment and separation costs in particular for production of fine chemicals. One of the ways to utilize cascades efficiently is to combine biological and chemical (i.e. heterogeneous and homogeneous) catalysis [1]. Synthesis of the R-1-phenylethyl acetate, which is an important building block for the production of biologically active pharmaceuticals, was studied over a heterogeneous palladium (Pd) catalysts supported on carbon nanotubes (or activated carbon) in combination with an immobilized lipase in one-pot under mild reaction conditions. Pd supported catalysts on activated carbon (AC) have been widely studied as catalysts for hydrogenation, dehydrogenation and oxidation reactions for the production of fine chemicals [2]. Activated carbons as catalyst supports present several advantages being relatively inexpensive and inert materials [3]. However, they also exhibit a major drawback as their surface properties can vary from batch to batch. Furthermore, typically Pd/AC catalysts exhibit acidic surface groups, such as carbonyl, carboxylic, phenolic hydroxyl, lactone and quinone groups [4]. It was recently reported that Brønsted acid sites enhance the hydrogenolysis of secondary alcohols, such as 1-phenylethanol, in the model reaction [5]. Carbon nanotubes (CNTs), thanks to their unique properties such as high surface area, electrical properties, high mechanical stability, and adjustable surface properties [2]. Pd has been deposited on CNTs with various methods such as incipient wetness

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impregnation [6], homogeneous deposition precipitation [6, 7], ion exchange [6, 8] and organometallic grafting [6, 9]. Typically, the dispersion of the metal can be improved by pre-treating the carbon nanotubes in order to introduce functional groups (e.g. oxygencontaining surface groups) to the surface to enhance interactions between the support and the catalyst precursor [10, 11]. In the present work, the catalytic activity of palladium catalysts either on activated carbon or carbon nanotubes was studied in the one-pot synthesis of R-1-phenylethyl acetate. The influence of the acid-base properties of the support on the catalytic activity has been investigated by treating oxidized CNTs with NH3 at different temperatures in order to introduce various amounts of basic N-containing groups on the surface [12].

2. Experimental 2.1. Catalyst synthesis

CNTs were first oxidized with concentrated nitric acid at 100oC for 2h. After washing and drying, the oxidized CNTs were further treated with gaseous ammonia at 200oC, 400oC or 600oC, respectively [12]. 2 % (w/w) Pd/N-CNT catalysts were subsequently prepared by incipient impregnation using an aqueous solution of Pd (NO3)2.2H2O. After drying at room temperature for overnight, the samples were calcined in air at 350oC for 2h and reduced in hydrogen at 400oC for 2h. Part of the N-containing basic sites was lost during the calcination and reduction. For comparison with Pd/N-CNT catalysts, 5 % (w/w) Pd/AC (Degussa) was also tested.

2.2. Catalyst characterization The catalysts were characterized by nitrogen adsorption method (Sorptometer 1900, Carlo Erba Instruments). The catalysts were outgassed at 150oC for 3 hours prior to the specific surface area measurements calculated by using the BET equation. Hydrogen temperature programmed desorption was performed at 200oC for 120 min with a temperature ramp 5oC/min under H2 flow (Micromeritics, Autochem 2910). Palladium stoichiometry was taken as 2 [13]. The acid-base titrations were performed to characterize the surface chemistry of the N-CNT and AC supports. Typically, 100 mg of sample was dispersed in 50 mL of 10-3 M KCl solution and stirred for overnight. Prior to measurements, the mixture was degassed under Ar for at least 1h untill the pH value was constant. The titration was performed under Ar, using 10-2 M HCl solution. The initial pH (pHinitial) values of the solution were recorded. The pH of the Pd/AC was measured as in [14] for determination of the acidity.

2.3. Experimental procedure

Experiments were typically performed at 70oC in toluene (125 mL) in a glass reactor at atmospheric pressure under H2 flow (AGA 99.999, 295 mL/min). The initial reactant concentration was 0.02 mol/L. Ethyl acetate with the concentration of 0.06 mol/L was used as an acyl donor. The catalytic hydrogenation of acetophenone (Acros, 99%) was carried out over 2% (w/w) Pd/N-CNT (312.5 mg) and the formed R-1-phenylethanol was acylated in the same pot to R-1-phenylethyl acetate with an immobilized lipase (Novozym 435, lipase B from Candida antarctica) (62.5 mg). The supported Pd catalysts were pre-reduced at 200 oC prior to the experiment. The products were analysed by a gas chromatography equipped with a chiral column CP Chirasil Dex (250 μm × 0.250 μm × 25 m) and a flame ionization detector. The samples were analyzed by using the flowing temperature program 100 oC (1 min)0.30 oC/min-130 oC-15 oC/min-200 oC (10 min). The temperature of the injector and

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Carbon nano-tube basicity influence in R-1-phenyl ethyl acetate synthesis

split ratio were 280oC and 100:1, respectively. The products were identified with GC-MS (Agilent Technologies 6890N Network GC System, 5973 Network MS Detector).

3. Results and discussion 3.1. Catalyst characterization results Acid-base titrations showed that during the calcination and reduction processes many basic sites were lost, thus leading to lower catalyst pH values for the catalysts than for the starting N-CNT supports (Table 1). This might be due to the loss of carboxylic acid groups during the catalyst preparation. The low specific area of Pd/N-CNT is due to the wall thickness of the CNTs [15]. Pd/AC exhibited the lowest pH. Table 1. Catalyst characterization results. Entry Catalyst

1 2 3 4 a, b, c

Pd/N-CNTa Pd/N-CNTb Pd/N-CNTc Pd/C

Metal dispersion (%)

Metal cluster size (nm)

30 41 51 54

3.7 2.7 2.2 2.0

BET (m2/gPd) 43 43 43 949

pHinitial 7.0 5.6 4.6

CNT treated with NH3 at 200oC, 400oC, 600oC, respectively.

3.2. Catalytic activity results Although entries 3 and 4 had similar metal particle sizes, Pd/AC, being the most active, displayed the highest turnover frequency (TOF). The relation between the TOF and the pHinitial can be seen in Table 2. The dispersions of Entries 3 and 4 were alike while the pHinitial were different, leading to a conclusion that the catalytic activity is significantly influenced by the acid-base properties of the support. The differences in activities cannot be attributed to structure sensitivity. It is well known that the maximum catalyst dispersion is favored when the carbon material is acidic [16]. The highest acetophenone conversion was obtained over 5 % (w/w) Pd/AC (Entry 4). However, the yield of R-1-phenylethyl acetate (R-PEAc) was only 13 %, since ethyl benzene (EB) was formed as a major product (Entry 4) due to the acidic support (Table 2). The maximum yield of R-1-phenylethyl acetate over 2 % (w/w) Pd/N-CNT (Entry 3) was 26 % at 75 % conversion level of acetophenone corresponding to 34 % selectivity over 312.5 mg of 2 % (w/w) Pd/N-CNT catalyst in combination with 62.5 mg of immobilized lipase (Figure 1a). Furthermore, the yield of R-1-phenylethyl acetate as well as the conversion of acetophenone increased with an increased basicity of the support material. At the same conversion level, the most selective catalyst was Pd/NCNT (Entry 1) in which the support was treated at 200oC with NH3 prior to Pd addition exhibiting the lowest acidity of the three studied CNT-catalysts.

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S. Sahin et al. Table 2. Kinetic results using Pd catalysts reduced at 200 oC for 120 min under H2 flow.

Entry Catalyst 1 2 3 4

Pd/N-CNTa Pd/N-CNTb Pd/N-CNTc Pd/C

Initial hydrogenation rate (mmol/min/ghyd.cat.)

TOF (s-1)

Conversion after 480 min (%)

Selectivity to R-PEAc (%)

0.002 0.020 0.030 0.10

0.0001 0.0008 0.0009 0.005

26 66 75 96

41d 36e 34e 23e

a, b, c

CNT treated with NH3 at 200 oC, 400 oC, 600 oC, respectively, d selectivity to R-PEAc at 26 % conversion after 480 min, e selectivity to R-PEAc at 66 % conversion. a)

b) 50

0.003 o

o

2 wt% Pd/N-CNT treated at 400 C

0.002

o

2 wt% Pd/N-CNT treated at 600 C

30

5 wt% Pd/C

20

5 wt% Pd/C

0.0025

EB (M)

Selectivity to R-1-PEAc (%)

2 wt% Pd/N-CNT treated at 200 C 40

2 wt% Pd/N-CNT o treated at 600 C

0.0015 0.001

10

0.0005 0

0 0

20

40

60

Conversion (%)

80

100

0

40000

80000

120000

160000

Time x mg Pd (min x mg Pd)

Figure 1. a) Selectivity to R-PEAc as a function of acetophenone conversion, b) EB formation as a function of time.

4. Conclusion Pd/N-CNT catalysts with different surface acid/base properties were prepared. The basicity increased with increased support treatment temperature with NH3. In one-pot synthesis of R-1-phenylethyl acetate via hydrogenation of acetophenone over Pd/NCNT and acylation on immobilized lipase higher acetophenone conversion was obtained with higher metal dispersion and smaller metal particle size. The yield of the desired product increased with the decreased basicity of the support material.

References [1] P. Mäki-Arvela, S. Sahin, N. Kumar, J.P. Mikkola, K. Eränen, T. Salmi, D.Yu. Murzin, 2009, Catal. Today, 140, 70-73. [2] T. Harada, S. Ikeda, M. Miyazaki, T. Sakata, H. Mori, M. Matsumura, 2007, J Mol. Catal. A: Chem., 268, 59-64. [3] S. Wang, G.Q. Lu,1998, Carbon, 36, 283-292. [4] C.-C. Huang, H.-S. Li, C.-H. Chen, 2008, J Hazardous Mat., 159, 523-527. [5] P. Mäki-Arvela, S. Sahin, N. Kumar, T. Heikkilä, V.-P. Lehto, T. Salmi, D.Yu. Murzin, 2008, J Mol. Catal. A: Chem., 285, 132-141. [6] P. Serp, M. Corrias, P. Kalck, 2003, Appl. Catal. A: Gen., 253, 337-358. [7] M.L. Toebes, M.K. van der Lee, L.M. Tang, M.H.H. in’t Veld, J.H. Bitter, A.J. van Dillen, K.P. de Jong, 2004, J Phys. Chem. B, 108 , 31, 11611-11619. [8] M.L. Toebes, F.F. Prinsloo, J.H. Bitter, A.J. van Dillen, K.P. de Jong, 2003, J. Catal., 214, 78-87.

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[9] T.G. Ros, D.E. Keller, A.J. van Dillen, J.W. Geus, D. C. Koningsberger, 2002, J. Catal., 211, 85-102. [10] T.W. Ebbesen, H. Hiura, M.E. Bisher, M.M.J. Treacy, J. Shreeve-Keyer, R. Haushalter, 1996, Adv. Mater. 8, 2, 155. [11] A. Jung, A. Jess, T. Schubert, W. Schutz, 2009, Appl. Catal. A: Gen. 362, 95. [12] R. Arrigo, M. Hävecker, R. Schlögl, D.S. Su, 2008, Chem.Commun., 40, 4891-4893. [13] P. Canton, G. Fagherazzi, M. Battagliarin, F. Menegazzo, F. Pinna, N. Pernicone, 2002, Langmuir, 18, 6530. [14] H. Markus, P. Mäki-Arvela, N. Kumar, N.V. Kul’kova, P. Eklund, R. Sjöholm, B. Holmbom, T. Salmi, D.Yu. Murzin, 2005, Catal. Lett. 103, 125. [15] J.P. Tessonnier, D. Rosenthal, T.W. Hansen, C. Hess,M.E. Schuster, R. Blume, F. Girgsdies, N. Pfänder, O. Timpe, D.S. Su, R. Schlögl, 2009, Carbon, 47, 1779. [16] J.M. Solar, V.H.J. de Beer, F. Derbyshire, L.R. Radovic, 1991, J. Catal., 129, 330.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Metal-carbon nanocomposite systems as stable and active catalysts for chlorobenzene transformations Ekaterina Lokteva, a Alexey Erokhin, a Stanislav Kachevsky, a Anatoly Yermakov, b Mikhail Uimin, b Aleksey Mysik, b Elena Golubina, a Konstantin Zanaveskin, с Anara Turakulova, a and Valery Lunin a a

M.V. Lomonosov Moscow State University, Moscow, Russia Institute of Metal Physics, Ural Branch of RAS, Ekaterinburg, Russia с Karpov Institute of Physical Chemistry, Moscow, Russia e-mail: [email protected] b

Abstract Nanocomposites based on Pd and Ni encapsulated (@) in carbon have been prepared by condensation of nanoparticles in the flow of gas mixture (Ar and hydrocarbons) and characterized by TEM, TGA-MS, XRD spectroscopy and BET adsorption measurements. Ni@C, NiPd@C nanocomposites consist of metal core 3-10 nm in size covered by a few carbon layers; Pd particles are 10-15 nm in size, have no carbon shell and are joined in chains. Catalytic properties were investigated in hydrodechlorination (HDC) of chlorobenzene in gas phase and 1,2,4-trichlorobenzene in liquid phase. Totally carbon covered particles of Ni and Pd-Ni demonstrate high activity and stability in gasphase hydrodechlorination of chlorobenzene at 100-350°C and in liquid phase HDC of 1,2,4-trichlorobenzene at 130oC under middle pressure. Keywords: metal-carbon nanocomposites, hydrodechlorination, catalysis

1. Introduction Chlorinated organics are among the most significant and widespread toxic matters in the environment. The most environmentally friendly and usable method for its treatment is hydrodechlorination [1]. Useful products such as hydrocarbons can be produced without dioxins formation. Stability in reaction medium is the weak point of known catalytic systems, so the design of active and stable catalytic systems based on not-noble metals is still the problem to be solved. Different ways of nanoparticles stabilization were developed last years, including carbon coverage [2], but none of such systems were tested as HDC catalysts.

2. Experimental 2.1. Preparation of nanocomposites Nanocomposites Pd@C, Ni@C, and NiPd@C were produced in Institute of Metal Physics, Ural Branch of the RAS. The piece of Pd, Ni or Ni-Pd alloy was heated by induction levitation melting inside of two oppositely directed turns of inductive coil in closed system filled with hydrocarbon containing inert gas (Ar). Evaporation of strongly overheated (~ 2000°C) liquid metal drop was performed in the flow of Ar, containing butane or methane. The metal vapors were taken away by the flow of argon into the colder part of reactor where nucleation and condensation of nanoparticles occurred. This

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process was accompanied with hydrocarbon decomposition (or pyrolysis) on the surface of hot metal nanoparticles and resulted in the formation of the layered carbon coating and capsulation of metal particles. Encapsulated nanoparticles were collected in a bag house. The average particle size (from few up to about 20 nm) depends on the metal nature and can be controlled by metal drop temperature, argon pressure and its flow rate. The thickness of the coating can be controlled by hydrocarbon content in Ar.

2.2. Characterisation X-Ray diffraction (XRD) study was performed using DRON-6 diffractometer and Cr Kα1 or Cu Kα1 radiation. Thermogravimetric analysis (TGA-MS) was carried out with STA 409PC Luxx Netzsch and STA 449PC Jupiter Netzsch apparatus in a temperature range from 273 to 1273 K with heating rate 5%min in air flow (40 ml/min). Analysis by high resolution transmission electron microscopy (HR TEM) was done using JEOL JEM-2010 with lattice resolution 0.14 nm. SBET was measured by Quantochrom instrument using N2 adsorption measurements.

2.3. Catalytic experiment

Vapor-phase catalytic transformations of chlorobenzene were performed at 50–300oC, 0.1 MPa in a quartz fix-bed flow-type reactor. Chlorobenzene was fed to the reactor in H2 flow at molar ratio H2:C6H5Cl =55:1. Reaction products were analyzed by GC (Agilent 6890N; DB-WAX column 30 m, flame ionization detector). For each analysis a gas probe was taken directly after the reactor by syringe. Each point on conversion vs time curves is the average value for 4-5 measurements at the stable work period. Liquid phase HDC was investigated at 130°C in N2+H2, at H2 partial pressure of 1 Mpa and total pressure of 1.3 MPa in NaOH water solution.

3. Results and discussion 3.1. Catalysts characterization Hydrocarbons decomposition on the hot surface of metal particles in-situ in the condensation zone of the reactor leads to the nanocomposite particles formation. As it was demonstrated by HR TEM (Fig. 1) and XRD ([3]), metal core of nanoparticle is encapsulated in carbon shells composed of some graphene-like layers. According to XRD data no metal carbides formation was found in such-produced nanocomposites; this result is in strong contrast with literature data about preferential formation of carbon encapsulated metal carbides in arc-discharge (e.g. from Mo, and Ta [4]). TEM (Fig. 1) and XRD results demonstrate that in all prepared composites particles are enough uniform in size, crystalline in structure and weakly agglomerated so they could be easily dispersed by weak ultrasonic treatment. No alloy formation was found in PdNi@C by XRD; two separate phases formed were attributed to Ni and Pd and/or alpha-PdH. Perhaps, the driving force for phase decomposition is the growing role of surface chemical potential as the particle size is decreasing. Table 1. Properties of nanocomposites. SBET, m2/g Me, wt.%

Ni@C(pc) 68±4 85

Ni@C(dc) 168±9 72

PdNi@C (pc) 87±5 94.5 (95%Pd, 5%Ni)

C, wt.%

15

28

5.5

PdNi@C(dc) 115±7 68 (95%Pd, 5%Ni) 32

Pd@C 33±2 97 3

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Compositions of Ni, Pd and PdNi composites are presented in Table 1. Carbon content in the different nanocomposites varied from 5 to 35%wt., estimated by the DTA-TG data. Depending on flow rate and Ar pressure, hydrocarbons nature and its concentration in the gas mixture the more or less dense carbon shells can be formed on metal nanoparticles. Thus PdNi@C system was produced in Ar-butane mixtures having different composition: butane concentration in the first mixture was 4 times higher that in the second one. Carbon content in final nanocomposite produced in the first and second mixture was 30% and 5%, correspondingly; specific surface area also decreased, but the difference was not so prominent. The term “dc” means sample with fully dense carbon-covered particles, and “pc” means sample with poorly carbon covered particles. Also two different Ni@C samples were obtained in different synthesis conditions: hydrocarbon mixture was introduced in hot (dc) or cold (pc) condensation zone. Specific surface area of “dc” and “pc” samples was different (168 and 68 m2/g respectively).

Fig. 1. HR TEM for Ni@C(dc) (A) and Ni@C(pc) (B) and particle size distributions.

Thus, different types of nanocomposites can be produced by the method of levitation melting in Ar-hydrocarbon flow depending on the synthesis conditions. Surface properties of nanocomposites, including specific surface area and pore structure, are determined by the thickness and properties of surface carbon shell.

3.2. Catalytic hydrodechlorination (HDC) of chlorobenzene HDC of chlorobenzene (CB) proceeds according the following scheme (1) HDC reaction could be accompanied with benzene ring hydrogenation to cyclohexane. Figure 2 demonstrates the conversion of CB depending on temperature in the presence of nanocomposites, commercially available 5% Pd/C Fluka and pure carbon. In the presence of all composites HDC of CB proceeds at much lower temperatures than on carbon itself. The most active catalysts are Pd@C, Pd/C (Fluka) and PdNi@C; in the first and second catalysts Pd particles are available for reagents adsorption, but in the last one particles of metals are entirely carbon covered. It seems that encapsulating of metal particles by carbon doesn’t hinder catalytic reaction. The activity and specific surface of Ni@C (pc) both decrease in cyclic experiment, where temperature was first increased stepwise up to 350°C and then decreased. On the contrary, the activity and SBET of densely carbon covered Ni@C (dc) increase during the first and the second heating-cooling cycle (Table2); in the third cycle it begins slowly

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decrease but it is still higher than in the first cycle. SBET of Ni@C(dc) after 3rd cycle was the same as for fresh catalysts, for Ni@C (pc) after first cycle it was 15 m2/g. It means that the carbon encapsulation seems to protect active metal from deactivation under the influence of reaction mixture at high temperature. Table 2. Temperatures of 50% CB conversion on Ni@C at stepwise heating (↑) and cooling (↓) 1 cycle 2 cycle T50↑,°C T50↓,°C T50↑,°C T50↓,°C Ni@C(dc) 170 150 140 120 Ni@C(pc) 165 200 n.d.* n.d. *n.d.= not determined; catalyst was tested in one cycle only

3 cycle T50↑,°C 145 n.d.

T50↓,°C 165 n.d.

It is important to underline a high activity of Ni nanocomposites. In the presence of Ni@C(dc) T50 (temperature of 50% conversion) is about 180°C lower than on pure C. 100 90 80 70 60 50 40 30 20 10 0

Conversion , %

Ni@C PdNi@C T, °C

Pd/С (Fluka)

35 0

27 5

22 5

17 5

12 5

75

30

C

Fig. 2. Catalytic properties of nanocomposites in chlorobenzene reduction.

Liquid-phase HDC of 1,2,4-trichlorobenzene at increased pressure on Ni@C (dc) was also successive, conversion was about 70 g/g catalyst per hour.

4. Conclusion Totally carbon encapsulated nanocomposites of Ni and NiPd demonstrated good conversion of CB at the temperatures of almost 200°C less than pure C and very good stability in aggressive reaction medium; Ni@C composites are active in liquid-phase 1,2,4-trichlorobenzene transformations at middle pressure. Such prepared stable and active catalysts based on not-noble metal (Ni) could be promising for heavy chlorinated wastes processing or other hydrogenation reactions, in spite of high metal loading.

Acknowledgments The authors acknowledge financial support of Russian Ministry of Education and Science (02.513.11.3030 and 02.740.11.0026) and Russian Foundation of Basic Researches (07-03-01017a, 10-02-00323-а).

References [1] E. Lokteva, V. Lunin, 1996, Catalytic hydrodechlorination of organic compounds. Russ.Chem.Bull., issue 7, P. 1609 [2] P.Z Si. Z.D Zhang., D.Y Geng et al., // Carbon. 2003. V. 41. P. 247. [3] A. E. Yermakov, M. A. Uimin, E. S. Lokteva, 2009, Russ.J.Phys.Chem. A, Vol. 83, No. 7, p.1187 [4] F. Banhart, N. Grobert, M. Terrones et al., 2001, J. Modern Phys. B, V.15, issue 31, P.4037

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Development and design of Pd-containing supported catalysts for hydrodechlorination Elena V. Golubina, Ekaterina S. Lokteva, Stanislav A. Kachevsky, Anara O. Turakulova, Valery V. Lunin Department of Chemistry, M.V. Lomonosov Moscow State University, Leninskie Gory 1, build.3, Moscow, Russia

Abstract Hydrodechlorination (HDC) is a remarkable environment friendly and cost saving alterative to the traditional methods for utilization of chlorinated pollutants. The development of new catalysts and revelation of general approaches to catalysts design are discussed in present work. In this work several directions for catalyst design were considered: (1) change of support nature to influence on a formation of Pd-containing active site; (2) modification of active site by second metal addition; (3) varying of metal deposition method and reduction agent on an active site formation. Keywords: Pd catalyst, hydrodechlorination, ultradispersed diamond

1. Introduction Hydrodechlorination (HDC) is a remarkable environment friendly and cost saving alterative to the traditional methods for utilization of chlorinated pollutants. The development of new catalysts and revelation of general approaches to catalysts design are discussed in present work. Study of Pd, the most active in HDC, can give the basement for understanding an active site nature and to establishing general approaches in hydrodechlorination catalysts preparation. It was recently found that active site is dual in nature: Pd0 is responsible for hydrogen activation and Pdδ+ is responsible for substrate adsorption [1]. Moreover Pd0/Pdδ+ ratio should be close to 1. Additionally, possibility of substrate activation on support, hydrogen spillover and presence of adsorption centers should be taken into account. So, catalyst needs to be multifunctional to conform with all listed facts. In this work several directions for catalyst design were considered: (1) change of support nature to influence on a formation of Pd-containing active site; (2) modification of active site by second metal addition; (3) varying of metal deposition method and reduction agent on an active site formation.

2. Experimental 2.1. Catalyst preparation Catalysts with 0.5; 1; 2 and 5%wt. Pd loading were prepared by impregnation or deposition-precipitation from PdCl2 solution. Ultradispersed diamond (UDD, detonation nanodiamond, 260 m2/g, fraction 0,5-0.16 mm) [2], activated carbon (1212 m2/g, fraction 0,5-0.16 mm), ZrO2, Y2O3, Ga2O2 and ZrO2-M2O3 (M – Al, Y, Ga) were used as supports. Oxide supports were prepared by co-precipitation by ammonia. In modified zirconia the content of second oxide was 1; 5 and 10%.

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2.2. Hydrodechlorination HDC reaction was performed in gas phase or under multiphase conditions. Multiphase reaction conditions were following: 50°С, substrate solution in iso-octane, Aliquat 336 (tricaprylmethylammonium chloride, Aldrich), KOH (5% aqueous solution), catalyst, hydrogen flow 5 ml/min, nonane as the internal standard. Samples were taken from organic phase during the reaction at fixed intervals and analyzed by GC. Gas phase reaction was performed in flow-type fixed bed reactor. The fresh catalyst (100 mg) was put in reactor between quartz filter paper. The reactor was heated in hydrogen flow to the reaction temperature (100 - 350°С). Then a substrate–H2 mixture was passed through the reactor (hydrogen was bubbled through substrate). Gaseous products were continuously analyzed by GC. 2.2.1. Catalyst characterization Specific surface area and pore size distribution were measured by low temperature nitrogen adsorption on Quantachrome. Phase composition was studied by X-ray diffraction analysis. XRD were performed on STOE powder difractometer (CuKα radiation); 2 theta range 20-70° (scanning 0.05°). The reduction behaviour of samples was studied by temperature programmed reduction (TPR). About 50 mg of the sample was heated (12%min) in a flow of 36 ml/min а 5% hydrogen in Ar. Changes in a hydrogen concentration was measured by thermal conductivity detector. IR spectroscopy of adsorbed CO was performed on Bruker Equinox 55/s spectrometer. SEM images were obtained on scanning electron microscope “JEOL JSM – 6390LA” (Japan) combined by EDS.

3. Results and discussion Several directions for catalyst design were considered: (1) change of support nature to influence on a formation of Pd-containing active site; (2) modification of active site by second metal addition; (3) varying of metal deposition method and reduction agent on an active site formation. The activity of catalysts was tested in gas phase hydrodechlorination (HDC) of chlorobenzene and multiphase HDC of various chlorinated aromatic compounds: chlorobenzene; 1,3,5-trichlorobenzene; 2,4,8-trichlorodibenzofuran and hexachlorobenzene. First approach to directional catalyst synthesis is based on chemical interaction of Pd with support. In a series of Pd supported on oxides catalysts on modified zirconia with second oxide content of 1 and 5% were the most active. Complete trichlorobenzene dechlorination in liquid phase was achieved within 20 min. Moreover, catalysts on modified zirconia were more stable than catalysts on individual oxides. Total converted amount of trichlorobenzene was 500 mol per 1 mol of Pd. According to TPR and IR of adsorbed CO data both Pd0 and Pdδ+ are presented on the catalysts surface. Pdδ+ most likely to be a part of compounds like Pd-Zr-O. Second way to obtain Pd0/Pdδ+ in active site is use of ultradispersed diamond (UDD) as support. UDD is one of the new carbon cluster substances that may be produced in large amounts by the detonation method. UDD possesses high specific surface area, almost 300 m2/g, with several types of carbonyl functional groups predominant on the surface a highly defective structure, super hardness and chemical stability. It was shown by TEM data, palladium particles are well distributed on the surface of UDD and their size lies in relatively narrow range [3]. This fact provides high activity of catalysts supported on UDD in comparison with activity of activated carbon

Development and design of Pd-containing supported catalysts for HDC

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supported catalysts with the same Pd loading in HDC of CB. The catalysts supported on AC and UDD showed a significant difference in activity (Fig.1). Pd catalysts supported on UDD were highly active in trichlorobenzene hydrodechlorination as well. Complete TCB dechlorination was achieved within about 40 min. During the same time period TCB conversion in the presence of 5%Pd/C was only 9%. Activated carbon possesses an amorphous structure containing a mixture of carbon fibers, layers and single agglomerates of different size. This leads to agglomeration of supported Pd particles that makes them larger and decrease of active surface. Pd on activated carbon surface is dispersed without any significant order with broad particle size distribution. Texture characteristics were measured for Pd/UDD and Pd/C. Catalysts on UDD have mesoporous structure with pore size 13 nm. Part of micropores is less than 1%. At the same time activated carbon has microporous structure with average pore size 2 nm. Probably, some parts of Pd particles could be blocked in micropores of activated carbon. This fact was confirmed by TPR analysis. Mesoporous structure of ultradispersed diamond provides accessibility of most part of Pd particles. Consequently, Pd/UDD is more active in hydrodechlorination.

Fig. 1. 1,3,5-trichlorobenzene conversion in multiphase HDC in the presence of Pd supported on UDD, activated carbon and commercially available Pd/C (Fluca).

Another way to improve the catalytic activity is modification by second metal. In this work non-noble metals such as Fe, Ni, Co and Cu were used. All bimetallic catalysts were more active than Pd/C. Conversion of hexachlorobenzene in the presence of Pd/C and modified by different metals Pd-containing catalyst are shown on Fig. 2. It was found that activity of Pd-Fe/C and Pd-Ni/C (Me/Pd = 1:4 and 1:1) was similar in multiphase hydrodechlorination of 1,3,5-trichlorobenzene and hexachlorobenzene. In this case, it becomes possible to replace part of Pd by second non noble metal without significant decrease of activity that will result in reduction of cost of catalyst.

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Fig. 2. Conversion of hexachlorobenzene in multiphase HDC in the presence of mono- and bimetallic catalysts supported on activated carbon.

By TPR method combined with magnetic measurements the composition of metal particle in Pd-Fe/C was investigated. It was found that both Pd-enriched PdFe alloy and Fe2O3 are presented on catalyst surface. Alloy formation changes the electronic state of Pd in bimetallic catalysts. At the same time chlorine could be eliminated from the reaction mixture due to FeCl3 formation to prevent palladium deactivation as a result of PdCl2 formation.

4. Conclusions Several approaches to form Pd0/Pdδ+ in active site are investigated. Highly uniform distribution of Pd may serve as one of the reason of high activity of catalysts supported on UDD in HDC of chlorinated derivatives of benzene. Use of modified zirconia as support leads to formation of intermetallic oxide Pd-Zr-O, which results in active and stable hydrodechlorination catalyst. Addition of second non-noble metal improves catalytic activity by formation of bimetallic alloy and at the same time decreases Pd poisoning by chlorine. This work supported by Russian Foundation of Basic Research (№07-03-01017) and Russian Ministry of science and education (state contract № 02.740.11.0026).

References 1. 2. 3.

L. Ma.Gomez-Sainero, X.L. Seoane, J.L.G. Fierro, A.Arcoya, 2002, Liquid-Phase Hydrodechlorination of CCl4 to CHCl3 on Pd/Carbon Catalysts: Nature and Role of Pd Active Species, Journal of Catalysis 209, 279–288. S.A. Kachevskii, E.V. Golubina, E.S. Lokteva, V.V. Lunin, 2007, Palladium on Ultradisperse Diamond and Activated Carbon: the Relation between Structure and Activity in Hydrodechlorination, Zhurnal Fizicheskoi Khimii, 81 (6), 998–1005. E.V. Golubina, S.A. Kachevsky, E.S. Lokteva, V.V. Lunin, P.Canton, P.Tundo, 2009, TEM and XRD investigation of Pd on ultradispersed diamond, correlation with catalytic activity, Mendeleev Commun., 19, 133–135.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Role of deposition technique and support nature on the catalytic activity of supported gold clusters: experimental and theoretical study Elena V. Golubina, Daria A. Pichugina, Alexander G. Majouga, Sultan A. Aytekenov Department of Chemistry, M.V.Lomonosov Moscow State University,Leninskie Gory 1, build.3, 119991, Moscow, Russia

Abstract The properties of gold nanoparticles depend on their size, which is, in turn, determined by the type of a support and method used for their deposition. In this work Au particles were supported on oxides (ZrO2 modified with Ga2O3, CeO2, SiO2, and Al2O3) and ultradispersed diamond by traditional deposition-precipitation from HAuCl4 by ammonia or by proprietary deposition method from gold nanoparticles suspension. Supporting gold on modified zirconia results in formation of large metal particles even at low Au loading. Ultradispersed diamond was found to be promising support for gold catalysts. Electronic state of supported gold particles was found to be strongly depended on preparation method. Deposition-precipitation leads to formation of partially charged metal particles, while deposition from gold nanoparticles suspension results in Au0. Proposed CO oxidation mechanism on gold particles in the presence of different catalysts was corroborated by DFT calculations. Keywords: supported catalyst, Au nanoparticles, CO oxidation, DFT

1. Introduction In the early stages of the study of heterogeneous catalytic processes, it was observed that the course of some reactions depends on the size of the catalytic center. Thus promising catalysts should contain supported metal nanoparticles. The opening of opportunities for creating high-performance catalysts with new properties attracts the interest of chemists for such nano-scale systems. Catalysis by nanoparticles may be in demand in many industries, such as the use of nanoparticles in three way catalysts of exhaust gases processing, and it will allow both to increase the efficiency of the catalyst and reduce its cost. The problem of stabilization of nanosized particles, protection of such particles from aggregation, is of great importance. Moreover, sinthesis of nanoparticles with desired properties is of current importance as well. Small clusters are unstable and tend to be agglomerated. The stability against aggregation can be achieved by anchoring the particles with other solid compounds. The possibility of obtaining highly dispersed stable gold particles on various supports has opened up new opportunities for use of gold in catalytic and sorption processes. The surface of the support stabilizes a cluster, affects its structure and charge, and modifies it or creates active centers, with the adsorption and catalytic properties of the cluster changed. One of the promising support for catalyst is ultradispersed diamond (UDD). The small size of diamond particles and the presence of nonequilibrium defects on their surface make it possible to achieve a high dispersity of metal particles in their deposition. It has been found previously that

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using UDD as a support enables stabilization of partially oxidized palladium particles with sizes less than 12 nm [1,2]. Thus, UDD surface should favor to stabilization of nanoparticles of other metals. In this work influence of support nature and deposition tecnique on the size and electronic state of gold nanoparticles was studied. Catalytic activity was investigated in CO oxidation. Quantum chemical calculations were performed to corroborate reaction mechanism.

2. Experiment Catalysts were prepared by deposition-precipitation (DP) and deposition from suspension (DS). Deposition-precipitation of nanosize gold particles was performed by precipitation onto a support from a HAuCl4 solution with ammonia at pH 9. Suspension of gold nanoparticles with average size 11 nm was synthesized by Turkevich method [3], starting from gold precursor (HAuCl4) and sodium citrate. Then gold particles were deposited on support. UDD, ZrO2 modified with Ga2O3, CeO2, SiO2, and Al2O3 were used as supports. Modified ZrO2 and CeO2 were prepared by precipitation of corresponding nitrates was by ammonia at pH=10. Precipitated hydroxides were dried at 95 C and calcinated. Calcination temperature was chosen for each oxide on the base of DTA-TG analysis. The catalytic activity was examined in the reaction of CO oxidation. Reaction was performed by a pulsed microcatalytic technique. Impulses of (2% CO + 1%O2) in He were passed through the catalysts. Products were analysed at the output of reactor on 1 m packed column (Porapak Q) coupled with thermal conductivity detector. Impulses time interval was 5 min, because of reaction mixture analysis. Specific surface area was measured by low temperature nitrogen adsorption. Phase composition was studied by X-ray diffraction analysis. XRD were performed on STOE powder difractometer (CuKα radiation); 2 θ range 20-70° (scanning 0.05°). The reduction behaviour of samples was studied by temperature programmed reduction (TPR). About 50 mg of the sample was heated (12%min) in a flow of 36 ml/min а 5% hydrogen in Ar. Changes in a hydrogen concentration was measured by thermal conductivity detector. IR spectroscopy of adsorbed CO was performed on Bruker Equinox 55/s spectrometer. SEM images were obtained on scanning electron microscope “JEOL JSM – 6390LA” (Japan) combined by EDS. Quantum chemical calculations were performed by methods of density functional theory with PBE functional and gold pseudopotential with relativistic corrections included.

3. Results and discussion Comparison of the X-ray diffraction patterns of Au/UDDDP and Au/(5% Ga2O3–ZrO2)DP samples shows that the last sample is three-phase. The X-ray diffraction pattern contains peaks of zirconium oxide in a tetragonal (2θ = 30.3, 34.6, and 35.3°) and cubic crystalline modifications (2θ = 30.6 and 35.2°) and a peak corresponding to gold clusters at θ = 38.2°. All the peaks related to zirconium oxide are broadened, which indicates a high dispersity of the resulting oxide. The content of gold in the sample prepared is as low as 0.5%. At such a low content of the metal, it can be assumed that no reflections associated with the gold phase must be observed in the X-ray diffraction pattern. However, a minor peak of gold is well seen in the diffraction pattern at 2θ = 38.2°, which may be due to precipitation of gold clusters in the form of coarse crystals.

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The presence of coarse gold particles on the surface of modified zirconium oxide leads to its low catalytic activity: the maximum CO conversion is only achieved at T = 450– 500°C and is as low as 6%. Such a low degree of CO conversion is in all probability due to an inhomogeneous distribution of gold clusters over the support surface. So, modified zirconia is not favourable to gold nanoparticles stabilization. Catalysts on UDD prepared by DP and DS methods were studied by SEM. The size of gold particles supported by DS is in correspondence with particle size in suspension. Since gold particles in solution stabilized by organic ligands, metal surface is blocked for substrate access. Indeed, DS catalysts were not active in CO oxidation if they were used as prepared. CO conversion was less than 5% in all studied temperature range (150 – 450°C). So these catalysts were additionally treated on air at 450°C. CO conversion in the presence of treated catalysts is shown on Fig. 1. Au/SiO2 was the most active at higher temperatures (350 – 450°C). At temperatures below 350°C CO conversion was highest in the presence of Au/UDD.

Fig. 1. CO conversion in the presence of 2% Au supported on different supports by deposition from gold suspension.

Comparison of catalytic activity of catalysts prepared by deposition precipitation and deposition from suspension are shown on Fig. 2 by the example of Au/UDD. Ultradispersed diamond was not active in CO oxidation. Interesting result was obtained for 0,05% Au/UDDDP. This catalyst is more active at 250 and 300°C, though the gold content is lowest. At temperatures 350 and 400°C CO conversion is nearly the same in the presence of catalysts prepared by DP. Catalysts prepared by DS at these temperatures have lower activity; CO conversion was only 35%. Such behavior most likely related with electronic state of supported metal. Activity of gold catalysts in CO oxidation depends on metal charge [4]. Study of prepared catalysts by IR-spectroscopy of adsorbed CO shows that DS results in deposition of Au0, and the support influence in this case is minimal. On contrary, in the case of DP method both Au0 and Au+ are presented on surface. Thus, in this case nature of UDD surface strongly influence on supported metal.

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Fig. 2. CO conversion in the presence of Au/UDD catalysts.

To make obtained experimental data more clear theoretical study was performed. Density functional theory calculations with PBE functional and gold pseudopotential with relativistic corrections included show that an isolated Au8 and Au10 clusters should be able to catalyze the CO oxidation reaction even below room temperature. The disklike geometry is chosen on the base of STM data. Two possible reaction paths are considered: O2 dissociates on clusters or adsorbed O2 reacts directly with adsorbed CO. Both reactions are found to be extremely facile on Au10 cluster, with reaction barriers equals to 65,6 kJ/mol indicating that the reactions should be possible well below room temperature. Calculated value of activation energy is close to result obtained from experimental data (65,6 kJ/mol).

4. Conclusions UDD was found to be promising support for stabilization of small Au particles. Due to the presence of large amount of functional groups and defects deposition of gold on UDD surface results in partial oxidation of Au particles. The influence of support surface nature on supported metal particles could be reduced by use of DS method, which leads to deposition of Au0. This work is supported by “Russian Leading School” program (grant НШ428.2008.3) and President RF grant for young scientists (МК-158.2010.3).

References 1. 2. 3. 4.

E.V. Golubina, S.A. Kachevsky, E.S. Lokteva, V.V. Lunin, P.Canton, P.Tundo, 2009, TEM and XRD investigation of Pd on ultradispersed diamond, correlation with catalytic activity, Mendeleev Commun., 19, 133–135. S. A. Kachevskii, E. V. Golubina, E. S. Lokteva, and V. V. Lunin, 2007, Palladium on Ultradisperse Diamond and Activated Carbon: the Relation between Structure and Activity in Hydrodechlorination, Russ. J. Phys.Chem. A, 81, 866-873. B.V. Enustun, J. Turkevich, 1963, Coagulation of Colloidal gold, J. Am. Chem. Soc., 85, 3317-3328. M.S. Chen, D.W. Goodman, 2006, Structure–activity relationships in supported Au catalysts, Catalysis Today, 111, 22–33.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Nanosized nickel ferrite catalysts for CO2 reforming of methane at low temperature: effect of preparation method and acid-base properties R. Benrabaaa, H. Boukhlouf,a E. Bordes-Richard,b R. N. Vannier,bA. Barama a a

Laboratoire de Matériaux Catalytiques et Catalyse en Chimie Organique, Faculté de Chimie, USTHB, BP32, El- Alia, 16111 Bab Ezzouar, Alger, Algérie b Unité de Catalyse et de Chimie du Solide, UMR CNRS 8181, Université Lille Nord de France, Cité scientifique, 59655 Villeneuve d’Ascq, France

Abstract Activity and selectivity of nanosized nickel ferrites have been studied for hydrogen and syngas production via the CO2 reforming of methane (DRM). The catalysts were prepared by two different methods: (i) co-precipitation (CP) route using nitrates salts as precursors and (ii) hydrothermal (HT) method using chlorides as starting salts. The materials were characterized by several techniques: HT-XRD, TGA-DTA, XRD, BET, LRS, TPR, SEM. Surface acid-base measurements were performed by 2-propanol decomposition (IPA) and catalysts were tested in DRM reaction. A relationship is established between the method of preparation, the solid structure, the surface acid-base properties and the catalytic activity of iron-nickel solids in DRM reaction. The surface acid-base properties seem to play an important role in DRM reaction. Keywords: co-precipitation, hydrothermal, NiFe2O4, reforming, methane

1. Introduction Spinel ferrite nanoparticles have been intensively studied in the recent years, because of their typical ferromagnetic properties, low conductivity, high electrochemical stability and catalytic behavior. These materials are widely used in large-scale applications: (i) in electric and electronic devices, (ii) in H2O, CO2 and alcohols decomposition and in CO and CH4 oxidation [1, 2]. Several routes are used for the preparation of NiFe2O4 catalysts such as co-precipitation, hydrothermal, sol gel, combustion [3-6] etc. However, the structural and textural properties of ferrite spinel are strongly influenced by the preparation methodology used in their synthesis and may influence the catalytic activity of these materials when used as catalysts. Hence, the effect of the preparation method on the surface acid–basic properties and therefore on the catalytic activity is a very interesting subject. Methane is the cheapest and most available carbon source for the petrochemical industry, and steam reforming of methane is currently used industrially to produce hydrogen and syngas. In recent years, methane dry reforming process has received significant attention since it allows the production of syngas with a lower H2/CO ratio which is suitable for further use in the production of oxygenated compounds as well as Fischer–Tropsch synthesis for production of liquid hydrocarbons [7]. Another advantage of this reaction is that it consumes two greenhouse gases. However, one of the major drawbacks in this process is the coke deposition and sintering of active species, which deactivate the catalysts. According to the literature data [8] the active phase insertion into a well-defined structure, such as perovskites, spinels etc, increases

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the stability of the catalyst. The present study deals with the preparation, structural, textural and acid-base characters of spinel nickel ferrite nanoparticles prepared via (i) co-precipitation (CP) route using nitrates salts as precursors and (ii) hydrothermal (HT) method. A relationship between these parameters is established. The main aim of the present work is to analyze the influence of structure, texture and surface acid–basic sites properties of CP and HT catalysts on the activity and selectivity in DRM reaction.

2. Experimental The catalysts were synthesized by CP and HT methods. (i) The samples (noted CP-650, CP-750 and CP-850) were prepared by CP route using Ni(NO3)2, 6H2O (2.6M) and Fe(NO3)3, 9H2O (3.4M) aqueous solutions as precursors and NaOH (5.3M) as precipitating agent (pH=10). The obtained precursor (noted CP-80) was washed, dried at 80°C for 24h and annealed at various temperatures (650, 750 and 850°C) for 4h. (ii)The sample (noted HT-140) was obtained by HT process using NiCl2 (0.05M) and FeCl3 (0.07M) as the starting aqueous solutions. The mixture solution was put into a teflon-lined stainless autoclave and NaOH (1M) was slowly added under constant agitation until the final pH=10. The autoclave was put into an oven at 140°C for 12h then cooled down to room temperature. The product was washed with distilled water and absolute ethanol and dried at 60°C for 12h. XRD, FTIR, LRS, BET, TPR, IPA and catalytic testing were recorded as reported in the previous works [9-11].

3. Results and discussion XRD patterns of the precursor and catalysts are shown in Fig. 1. It shows that the precursor is nearly amorphous with only one peak which could be ascribed to FeO(OH). In contrast, NiFe2O4 (PDF 01-071-3850) is evidenced as the major crystalline phase in all catalysts. A pure phase is obtained for HT-140, while for the all CP-samples, besides NiFe2O4, additional peaks ascribed to Fe2O3 maghemite-c (PDF 00-039-1346) are to be noticed. However no trace of NiO, which should be in excess due to the presence of Fe2O3, is observed due probably to low crystallinity. XRD at variable temperatures was also carried out. It showed NiFe2O4 starts to form at 450°C and remains stable up to 1000°C. ♦

♦ NiFe 2 O 4

• FeO(OH)

♣ Fe 2 O 3

HT-140

♦

♦

CP-850

♣

CP-750

♣

CP-650

♣♣

♦

332

♣

20

30

696

479

CP-850

CP-750

CP-650

•

CP-80

10

♦ ♦

♦

♦ ♦

Intensity (a.u.)

Intensity (a.u)

HT-140

♦

40

50

60

2Theta (°)

Fig. 1. XRD patterns of NiFe2O4 catalysts.

70

200

400

600

800

1000

1200

1400

-1

Wavenumber (cm )

Fig. 2. Raman spectra of NiFe2O4 catalysts.

According to XRD data, the incorporation of iron in the NiFe2O4 structure is not complete at 850°C for the CP catalysts. These results are in agreement with those Rashad et al. results [12] who showed that the formation of pure phase NiFe2O4, using CP method, is observed around 1200°C.

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The LRS analysis (Fig. 2) is in agreement with XRD results. They revealed, for HT-140 and CP-850, the presence of three Raman bands at ca. 332, 479 and 696 cm-1 that have been assigned to NiFe2O4 [13]. Structural, surface and acid-base parameters are summarized in the table 1. The values of lattice constant (a) and the density (d) are in agreement with literature data [6]. The decrease of the unit cell parameter with annealing temperature is likely due to an evolution of the spinel composition which displays a solid solution domain. The surface area (SBET), for all CP samples, is very low (< 4m2/g) compared to the HT sample (37 m2/g). This difference is in agreement with the bigger size (Cs) of the CP-crystallites (20-50nm for CP-solids against 10nm for HT solid). These results are confirmed by SEM analysis which evidenced the highest size obviously for CP-850 (50 nm). Table 1. Structural, surface and acid-base parameters of CP and HT catalysts.

CP-650 CP-750 CP-850 HT-140

Structural and surface properties SBET Cs d a (m2/g) (nm) (g/cm3) (Å) 2 25 5.32 8.39 3 30 5.41 8.33 2 50 5.54 8.26 37 10 5.39 8.34

The catalyst reducibility was examined by H2-TPR. Results are depicted in Fig. 3. For all CP polyphasic samples, the TPR curves show one broad and asymmetric reduction peak between 559 and 579°C with a shoulder at higher temperatures (722-826°C). The first peak, whose position is function of the calcination temperature, is assigned to the reduction of both Ni2+ and Fe3+ species to Ni metallic and FeO respectively [14].

Acid-base properties at 250°C Con. IPA Sel. Pr Sel. Ac (%) (%) (%) 38 45 72 47

99 99 99 46

1 1 1 54

0,14 0,12

TCD response (a.u.)

Catalysts

CP-850

0,10 0,08 0,06

CP-650 CP-650°C HT-140

CP-750°C CP-750

0,04 0,02 0,00 -0,02

200

400

600

800

1000

Temperature (°C)

Fig. 3. TPR profiles of CP and HT samples.

The shoulder can be attributed to a deeper reduction leading to Fe°. For the HT monophasic solid, it can be seen two H2-consumption peaks at ca. 387 and 687°C. The peak, at lower temperature, could be due to the reduction of Ni2+ species and the one at higher temperature to Fe3+ into Fe2+ species and probably, in less extent, to the reduction of Fe2+ to the metallic oxidation state. The hydrogen consumption, during TPR, is similar for all samples (14-15 mmol.g-1). The HT-sample reduction, starting at lower temperature, is in agreement with the smallest grain size observed by SEM for this solid. The acid-base properties have been estimated using IPA decomposition in the temperatures range 200-350°C (table 1). In all case, propylene and acetone were detected. For the monophasic HT-sample, the IPA is mainly dehydrogenated into acetone and, in less extend, dehydrated to propylene. In contrast, for all polyphasic CPsamples, IPA is predominantly dehydrated into propylene (up to 99% of selectivity). The selectivity towards dehydrogenation and dehydration reactions was much affected by changing the reaction temperature and preparation method. The temperature of calcination did not influence markedly the products distribution.

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The CP and HT catalysts were tested in DRM reaction at 450-650°C. The results are gathered in the table 2. The HT-catalyst, exhibits the highest activity at low temperature (25% of CH4 conversion against 2-6% for CP-catalysts at 450°C). These results could be attributed to the better reducibility of this material and to its basic character and reasonable surface area. Table 2. Catalytic properties of CP and HT catalysts: CH4/CO2=1, Tr=450-650°C. catalysts CP-650 CP-750 CP-850 HT-140

Tr (°C) 450 550 650 450 550 650 450 550 650 450 550 650

% Con CH4 6 6 10 5 6 10 2 5 5 25 35 55

% Con CO2 8 9 9 10 10 10 10 12 12 18 17 35

% Yield H2 1 4 4 1 6 7 5 7 7 25 35 38

% Yield CO 2 5 5 2 4 8 3 7 8 18 37 41

4. Conclusions Nanosized NiFe2O4 was prepared by CP and HT methods. The HT method presented many advantages compared to the CP method. It led to the formation of a pure crystalline NiFe2O4 at 140°C, while a mixture of NiFe2O4 and Fe2O3 phases was evidenced for samples prepared by CP technique in the temperature range 650-850°C. In addition, the HT-sample presents the better SBET of 37m2/g. The IPA dehydration to propylene, which is an indication of acid character, predominate on CP samples; while, the dehydrogenation reaction to acetone, related to the contribution of redox (basic) site, is favoured on HT-sample. The pure NiFe2O4 solid exhibits interesting results in DRM reaction compared to CP-compound. The difference in catalytic behaviour of these materials could be explained in term of acid-base and redox properties.

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]

B. Baruwati, K. Reddy, S. Manorama, R. Singh and Om. Parkash, Appl. Phys. Lett. 85 (2004) 2833. M. M. Bucko and K. Haberko, Journal of European Ceramic Society 27 (2007) 723. S. Sreekumar and S. Sugunan, Journal of Molecular Catalysis A:Chemical 185 (2002) 259. J. Wang, Materials Science and Engineering B 127 (2006) 81. Dong-Hwang Chen and Xin-Rong He, Materials Research Bulletin, 36 (2001) 1369. S. Balaji, R. K. Selvan, L.J. Berchmans. S. Angappan, K. Subramanian and C.O. Augustin, Material sciences and Engineering B 119 (2005) 119. Ş. Özkara-Aydınoğlu, E. Özensoy and A E. Aksoylu. International Journal of Hydrogen Energy 34 (2009) 9711. T. Utaka, S.A. Al-Drees, J. Ueda, Y. Iwasa, T. Takeguchi, R. Kikuchi and K. Eguchi, Appl. Catal., A 247 (2003) 125. N. Haddad, E. Bordes-Richard, L. Hilaire and A. Barama, Catal. Today, 126 (2007) 256. H. Boukhlouf, R. Benrabaa, S. Barama and A. Barama, Mat Science Forum 609, (2009) 145. A. Djaidja, S. Libs, A. Kiennemann and A. Barama, Catal. Today 113 (2006) 194. M. M. Rashad and O. A. Fouad, Material Chemistry and Physics, 94 (2005) 365. Y. Shi, J. Ding, Z. X. Shen, W.X. Sun, L. Wang, Solid State Comm., 115(2000) 237. M. del Arco, P. Malet, R. Trujillano and V. Rives, Chem. Mater, 11 (1999) 624.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Hierarchical porous Ce-Zr materials for oxidation of diesel soot particulate Natalia V. Zaletova, Anara O. Turakulova, Valery V. Lunin Chemistry Department, M.V.Lomonosov MSU, Leninskie Gory, bld.1/3, Moscow, 119992, Russia

Abstract Hierarchical porous biomorphic catalyst Ce0.5Zr0.5O2 for oxidation of soot was prepared by calcination of sawdust, impregnated by solutions of Ce and Zr nitrates. SEM analysis revealed total reproducibility of biotemplate morphology by final oxide. Biomorphic Ce-Zr oxide has certain advantages over coprecipitated one: it possesses larger surface area, is thermally stable, has higher amount of mobile lattice oxygen and lower temperature of its release. Thanks to its filamentous-like morphology and improved redox properties biomorphic catalyst is more active in combustion of soot. Keywords: biomorphic Ce-Zr oxide, porous catalyst, soot oxidation

1. Introduction Hierarchical porous materials have attracted much interest in recent years due to their intensive use in different fields ranging from catalysis to ceramics [1,2]. In the present work an original environmentally friendly technique combining simple synthesis method with use of waste biomass was utilized for the production of Ce-Zr oxide, which can be used both as a support and as a catalyst in various processes. Thanks to its oxygen storage capacity (OSC) Ce-Zr oxide is the main component in three-way catalysts and catalytic filters for oxidation of soot emitted by diesel engines [3]. In the latter case the hierarchical porous structure of the systems is of great importance in order to capture the particulates of soot.

2. Synthesis of Ce-Zr catalysts In the present work Ce0.5Zr0.5O2 was prepared by the following method: pine sawdust (wood biomass with the approximate size of 0.63-1.25 mm) was impregnated by solution of Ce and Zr nitrates in the ratio sawdust:mixed oxide = 10:1 and the dried resultant system was calcined at 600°C during 4 hours to give, in a direct and simple manner, the material of desirable and predictable porosity, avoiding the use of additional chemicals in the process. The sample obtained with the help of sawdust is indicated as biomorphic. For comparison Ce-Zr oxide of the same composition was prepared by traditional coprecipitation method and calcined in the same conditions.

3. Physicochemical properties of the catalysts 3.1. SEM Analysis Reproducibility of initial biomaterial by biomorphic oxide is one of the advantages of such method of synthesis. This fact is confirmed by SEM analysis (fig. 1 a-d): biomorphic oxide completely repeats the structure of the cellulose component of the wood and is characterized by large pore distribution with significant contribution of

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macropores. Crystals of coprecipitated oxide are relatively smooth and vary greatly by dimensions (fig.1 e, f).

а)

b)

d)

e)

c)

f)

Fig. 1. SEM images of Ce0.5Zr0.5O2 a)-d) biomorphic; e), f) coprecipitated.

3.2. BET Analysis Compared to coprecipitated oxide biomorphic one possesses more developed surface area (72 over 54 m2/g). To examine the influence of initial texture on the thermal stability, both systems Ce0.5Zr0.5O2 were calcined at 1000°C, 2 hours. After calcination surface area of biomorphic oxide amounts to 21 m2/g whereas this value for coprecipitated oxide is 2 m2/g. As it is seen from pore size distribution diagrams (fig. 2), pore size of coprecipitated oxide doesn’t exceed 60Å (fig. 2a). Biomorphic oxide is characterized by larger pore distribution (fig. 2b), besides from SEM images (fig.1 a-d) the presence of macropores is evident. It is known [3], that systems characterized by wide pore distribution are less affected by high temperature treatment, that’s why we observe such a difference in surface area of calcined samples. Thus filamentous-like morphology of biomorphic mixed oxide leads to its high thermal stability, which is essential prerequisite for any catalytic application in order to maximize both the contact with the reactants and the durability of the catalyst. b)

2,1E-03 Pore volume, ml/g

2

54 -> 2 m /g 1,4E-03

7,0E-04

2,1E-03 2

Pore volume, ml/g

а)

72 -> 21 m /g 1,4E-03

7,0E-04

0,0E+00

0,0E+00 10

50 90 Pore diameter, А

130

10

50 90 Pore diameter, А

Fig. 2. Pore size distribution of Ce0.5Zr0.5O2 a) coprecipitated, b) biomorphic.

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Hierarchical porous Ce-Zr materials for oxidation of diesel soot particulate

307

3.3. Redox properties

Hydrogen consumption, a.u.

Redox properties of Ce0.5Zr0.5O2 systems were examined in temperature programmed reduction and oxidation. Hydrogen was acting as a reducing agent. Mobility of lattice oxygen is estimated by the temperature of maximal speed of lattice oxygen release. For biomorphic Ce0.5Zr0.5O2 this temperature is 513°C, for coprecipitated oxide there are two peaks at 535 and 730°C (fig. 3). Lower reduction temperature in case of biomorphic oxide is connected with its highly porous structure, which can be easily reached by gas. Besides, element analysis of biomorphic oxide revealed the presence of K+, Ca2+ and Mg2+ ions (0.58, 1.21 and 0.24 wt.% respectively). These elements exist in wood as inorganic components and are incorporated in the structure of biomorphic oxide during synthesis. These low-valent cations are supposed to create additional oxygen vacancies in biomorphic system and in this way enhance oxygen mobility. TPR profiles

Oxygen storage capacity, %

513

biomorphic biomorphic

535

80

coprecipitated

60 730

40

78

20 0

200

400 600 Temperature, 0C

800

coprecipitated

31

0

Fig. 3. TPR profiles and OSC values of Ce0.5Zr0.5O2.

Another important characteristic of redox properties is OSC, which is estimated as the quantity of oxygen absorbed by reduced system. OSC is calculated as a ratio of reduced cerium amount to total amount of cerium in the systems (ω(Ce3+)/ ω(Ce3++Ce4+)). In case of biomorphic material OSC is 78%, which is 2.5 times higher than in case of coprecipitated one (fig. 3). It is know [4] that OSC of Ce-Zr systems depends on its phase composition, homogeneous Ce0.5Zr0.5O2 solid solution is believed to exhibit the lowest temperature of lattice oxygen release. XDR analysis of initial oxides doesn’t allow to determine phase composition unambiguously. However phase composition can be estimated by implication of TPR profiles. The only peak on TRP profile of biomorphic oxide allows to propose the presence of phases characterized by close composition and properties. TPR profile of coprecipitated sample is characterized by additional peak in high temperature region. This fact is an evidence of presence of phases with considerably different properties. Thus inhomogeneity in phase composition of coprecipitated Ce0.5Zr0.5O2 explains its low OSC while high homogeneity of biomorphic oxide provided by the method of synthesis causes its high OSC.

3.4. Catalytical properties of Ce0.5Zr0.5O2 in oxidation of soot 3.4.1. Catalysis on initial biomorphic and coprecipitated systems Catalytic properties of biomorphic and coprecipitated systems were examined in the reaction of soot oxidation. Soot oxidation rate greatly depends on intensity of contact between soot and catalyst. Fibrous, highly porous structure of biomorphic oxide provides its better contact with soot particulates. Without catalyst soot is oxidized at 628°C. In presence of both catalytic systems temperature of soot combustion considerably decreases: biomorphic catalyst oxidizes soot at 415°C, whereas coprecipitated – at 490°C (tight contact in mortar). Lower temperature in the former case is due to both

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exo

initial

415 460

calcined

50

150

250

350

450

Temperature, 0С

coprecipitated Ce-Zr

DSC Signal, a.u.

exo

biomorphic Ce-Zr

DSC Signal, a.u.

improved redox properties of biomorphic material and its hierarchical porosity, which favours improving contact between soot and catalyst. 3.4.2. Catalysis after high temperature pretreatment of the catalysts During exploitation of the catalyst temperature inside diesel particulate filter sometimes can reach 1000°C as a result of local overheatings, that’s why influence of high temperature pretreatment on catalytic properties of Ce0.5Zr0.5O2 systems was investigated. As indicated above porous structure of biomorphic system provides its improved resistance to high temperatures (1000°C, 2 hours). This is extremely important in catalytic oxidation of soot because soot combustion being a reaction between two solid substances greatly depends on contact area between catalyst and substrate. Figure 4 shows catalytic tests after additional calcination of Ce-Zr oxides.

550

650

490

initial

516 calcined

50

150

250

350

450

550

650

Temperature, 0С

Fig. 4. DSC curves of soot combustion in presence of initial and calcined Ce0.5Zr0.5O2.

In both cases calcination of the catalysts shifts the temperature of soot combustion to higher temperature region: 460 and 516°C for calcined biomorphic and coprecipitated catalysts correspondingly. It is worth mentioning that even after calcination, soot combustion in presence of biomorphic system occurs at lower temperature than in presence of initial coprecipitated catalyst. In present work small-sized fraction of wood were used as a template and thus the catalyst was in the form of powder. However if we use wood monolith as a matrix, ceramic blocks characterized by unidirectional hierarchical pores can be obtained [1]. Besides, as final bioceramics totally reproduces the structure of initial wood it’s possible to regulate the diameter of pores and its structure by varying biomatrix. So biomorphic method of synthesis allows to obtain catalytic filters of desired porosity, which opens large perspectives for its application in combustion of soot emitted by diesel engines and in other processes where hierarchical porosity of the system is of great importance.

4. Conclusions Hierarchical porous Ce0.5Zr0.5O2 was prepared by using biomaterial as a matrix. Besides unique textural properties it possesses thermal stability, low temperature of oxygen release and high OSC value. Thanks to all these factors it exhibits high activity in soot oxidation.

References 1. 2.

Rambo C.R., Cao J., Sieber H. Preparation and properties of highly porous, biomorphic YSZ ceramics // Materials Chem. and Phys, 2004, V. 87, Iss. 2-3, P. 345-352. Vogli E., Sieber H., Greil P. Biomorphic SiC-ceramic prepared by Si-vapor phaseinfiltration of wood // J. Eur. Ceram. Soc, 2002, V. 22, Iss. 14-15, P. 2663-2668.

Hierarchical porous Ce-Zr materials for oxidation of diesel soot particulate 3. 4.

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Di Monte R., Kaspar J. Nanostructured CeO2-ZrO2 mixed oxides // J. Mater. Chem, 2005, V. 15, P. 633-648. Liotta L.F., Macaluso A., Longo A., Pantaleo G., Martorana A., Deganello G. Effects of redox treatments on the structural composition of a ceria–zirconia oxide for application in the three-way catalysis // Applied Catalysis A: General, 2003, V. 240, Iss. 1-2, P. 295-307.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

The role of organic additives in the synthesis of mesoporous aluminas and Ni/mesoporous alumina catalysts Faiza Bentaleb and Eric Marceau* Laboratoire de Réactivité de Surface, UMR 7197 CNRS, UPMC (Université Pierre et Marie Curie – Paris 6), 4 place Jussieu, 75252 Paris Cedex 05, France E-mail: [email protected]

Abstract Mesoporous aluminas can be prepared using organic additives as porogens even if these are not surfactants, as is the case for glucose. It is shown here that porosity arises from glucose entrapped in the material precipitated at the beginning of the synthesis, and subsequently caramelized. After impregnation by nickel(II) salts and calcination, more difficult-to-reduce nickel aluminate is detected on mesoporous aluminas than on commercial aluminas, because the surface area exposed to water is larger in the former case. The presence of citrates in the impregnating solution helps to lower the proportion of nickel aluminate, but may also lead to larger NiO particles. Keywords: alumina, nickel, glucose, citrate, temperature-programmed reduction

1. Introduction For almost 20 years now, syntheses of mesoporous aluminas combining a specific surface area higher than that of most commercial aluminas (> 250 m2.g-1), a high mesopore volume and a narrow pore size distribution have been intensively investigated [1]. Synthesis routes have often been patterned on the preparation of mesostructured silicas, with micelles of surfactants acting as porogens. In contrast, recent works have described the use in aqueous medium of non-surfactant, cheap organic additives such as glucose, as an easy way to obtain non-mesostructured, but thermally stable mesoporous aluminas satisfying the above-mentioned criteria [2, 3]. However, the role of the organic additive remains unclear and the stability of the alumina upon introduction of an active phase, such as nickel, can be questioned. For instance, Kim et al. have shown by temperature-programmed reduction (TPR) that on mesoporous aluminas, difficult-to reduce nickel aluminate phases are prominent compared to NiO [4]. The purpose of the present paper is to investigate how glucose acts as a porogen in the synthesis of aluminas and how organic additives can help to prevent the formation of mixed phases when Ni catalysts are prepared on these supports.

2. Experimental 2.1. Preparation and characteristics of aluminas The mesoporous alumina was prepared following ref. [2, 3]. 0.02 mole of glucose and 0.02 mol of Al(OiPr)3 were introduced into 54 mL of water. The pH was lowered to 5 by addition of HNO3, resulting in the precipitation of a solid. After 30 min under stirring and 5 h in static conditions, water was evaporated at 130°C in an oil bath. γ-Al2O3 was obtained by calcination of the solid in air at 600°C in a muffle oven.

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Ni/Al2O3 catalysts were prepared from this alumina (hereafter called “mesoporous alumina”) and from two commercial aluminas provided by the Institut Français du Pétrole and exhibiting pore size distributions centered on different pore diameters: a γAl2O3 sample (EC1285), and a η-Al2O3 sample presenting a higher specific surface area and a microporous contribution (EC08701) (Table 1). Table 1. Characteristics of the aluminas and Ni content of the catalysts measured after drying. Alumina

Specific surface area (m2.g-1) 350

Pore volume (cm3.g-1) 0.60

Mean pore diameter (nm) 5.6

Mesoporous γ-Al2O3 Commercial 200 0.60 8.8 (contribution > 15 nm) γ-Al2O3 Commercial 315 0.50 4.0 (contribution < 2 nm) η-Al2O3 * FWMH: full width at medium height of the pore size distribution

FWMH* (nm) 4.5

Ni wt% 6.5

4.0

8.0

2.0

7.2

2.2. Preparation of the Ni/Al2O3 catalysts

Three Ni/Al2O3 catalysts were prepared by incipient wetness impregnation of the aluminas described above, using 3.1 mol.L-1 aqueous solutions of nickel(II) nitrate ([Ni(H2O)6](NO3)2). The nickel contents after drying at room temperature are listed in Table 1. These nickel contents correspond to catalysts before calcination and the different values are linked to the different quantities of water retained in the supports. After calcination, the content in Ni for the three catalysts is 10 wt%. A fourth catalyst was synthesized by incipient wetness impregnation of mesoporous γ-Al2O3 using a 3.1 mol.L-1 aqueous solution of nickel(II) citrate (citrate/Ni = 1). 10 mL of this solution were prepared by heating under reflux a suspension of 2.91 g of Ni(OH)2 contacted with 6.63 g of citric acid monohydrate, till Ni(OH)2 was dissolved. The weight content in Ni of the dried catalyst is 7.2 wt%.

2.3. Characterization techniques The Ni and C contents of the dried catalysts were measured by ICP at the CNRS Vernaison Center of Chemical Analysis, and by catharometry after fast calcination at the UPMC microanalysis center, respectively. X-Ray Diffraction (XRD) analyses were carried out on a Siemens D500 diffractometer using Cu Kα radiation (1.5418 Å). Specific surface areas were determined by the BET method applied to N2 physisorption isotherms at –196°C, on samples outgassed at 250°C for 4 h prior to analysis, using an automatic Micromeritics ASAP 2010 instrument. Pore size distributions were calculated from the N2-adsorption curve (BJH method) due to ink-bottle shaped pores [3]. Calcination of the dried catalysts, purge and subsequent TPR were performed using an Autochem 2910 (Micromeritics), under air (25 cm3.min-1; heating rate 7.5°C.min-1 up to 500°C), Ar (25 cm3.min-1; down to 20°C) and 5% H2/Ar (25 cm3.min-1; heating rate 7.5°C.min-1 up to 900°C), respectively. H2 consumption was followed by catharometry.

3. Results and discussion The role of glucose in the formation of the alumina porous system was investigated by modifying the procedure of preparation described above (Table 2).

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The role of organic addtives in the synthesis of mesoporous aluminas Table 2. Influence of the preparation procedure on the porosity of aluminas. Procedure Centrifugation+calcination (no caramelization) Centrifugation+water +caramelization+calcination Prepared with 3.6g glucose (complete procedure) Prepared with 3.6g fructose (complete procedure)

Specific surface area (m2.g-1) 237

Mesopore vol.ume (cm3.g-1) 0.17

Mean pore ∅ (nm) 3.3

FWMH (nm) 2.2

325

0.45

4.6

5.0

350

0.55

5.6

4.5

200

0.22

5.0

4.0

The solid recovered by centrifugation after the 5h static step was identified by XRD and chemical analysis as a poorly crystallized boehmite containing 10% of the glucose introduced in solution. A direct calcination of this material at 600°C led to a γ-Al2O3 material exhibiting low specific surface area and pore volume, and small pores. In a second experiment, the centrifuged boehmite was placed back into 54 mL of water containing no glucose and the synthesis procedure was carried through to completion. During evaporation of water at 130°C, a change in color from white to yellow showed that glucose was caramelizing. Compared with the first experiment, a large increase in specific surface area, pore volume and mean pore size was observed after calcination.

675 760

(d) μ mol H / (°C.g ) 2 cat

400 593 790

(c)

645 785

(b) 586

10 790

(a)

100 200 300 400 500 600 700 800 900 Temperature (°C) Fig. 1. TPR profiles of calcined Ni catalysts supported on: (a) commercial γ-Al2O3; (b) mesoporous γ-Al2O3 (prepared from nickel nitrate); (c) mesoporous γ-Al2O3 (prepared from nickel citrate); (d) commercial η-Al2O3. H2 consumptions are expressed per g of dried catalysts.

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The formation of the alumina mesoporous system thus appears to be connected to the caramelization of glucose entrapped inside the precipitated boehmite. The difference with the material prepared following the complete procedure may be assigned to the diffusion of a fraction of glucose into water, as evidenced by a simple test with Fehling’s solution. It can be noted that the standard procedure carried out with a ketohexose, fructose, instead of glucose, led to a quasi non-porous γ-Al2O3, though the characteristics and content in sugar of the precipitated materials were similar. Ni/Al2O3 catalysts were synthesized from the mesoporous alumina prepared with glucose and from the two commercial aluminas mentioned above. The speciation of nickel was studied by TPR after calcination in air (Fig. 1). Three main reduction peaks were detected and attributed to: (i) larger NiO particles (Tred ≈ 400°C); (ii) smaller NiO particles interacting with the alumina surface (Tred = 500-700°C); (iii) a nickel aluminate phase originating from the migration of Ni2+ ions into the alumina surface layers (Tred > 750°C) [5-7]. On the mesoporous alumina (Fig. 1b) and η-Al2O3 (Fig. 1d), nickel is present as species (ii) and (iii). Compared with the commercial γ-Al2O3 (Fig. 1a), the proportion of nickel aluminate is higher and the reduction of the smaller NiO particles is shifted to higher temperatures (645°C instead of 586°C), despite a lower surface density of nickel ions. In contrast, and in line with earlier reports [8], protecting Ni2+ ions with citrate ligands in the impregnation solution helps to hinder the formation of mixed phases (Fig. 1c). The formation of nickel aluminate thus seems to be favoured by the high surface area of the support exposed to water during impregnation, with nickel ions penetrating in the hydrated surface layers of alumina. It should finally be noted that the citrate solution is viscous. Its difficult penetration into the ink-bottle pores of mesoporous Al2O3 may explain the high proportion of larger NiO particles reduced at 400°C. No XRD peaks assigned to NiO can be observed though.

4. Conclusions It is not glucose itself, but the caramelization of glucose entrapped in the boehmite precursor that creates the mesoporous system of the alumina. When mesoporous aluminas are used as supports for nickel catalysts, the high surface area exposed to water during impregnation is the cause for the formation of nickel aluminate. Protecting nickel(II) ions with citrate ligands helps to inhibit the formation of mixed phases, but the high viscosity of the solution may limit the penetration of the solution into the pores and as a result, larger particles of NiO may be formed after calcination.

References [1] C. Márquez-Alvarez, N. Žilková, J. Perez-Pariente and J. Čejka, Catal. Rev. Eng. Sci., 50 (2008) 222. [2] B. Xu, T. Xiao, Z. Yan, X. Sun, J. Slon, S.L. Gonzáles-Cortés, F. Alshahrani and M. L. H. Green, Micropor. Mesopor. Mater., 91 (2006) 293. [3] S. Handjani, J. Blanchard, E. Marceau, P. Beaunier and M. Che, Micropor. Mesopor. Mater., 116 (2008) 14. [4] Y. Kim, P. Kim, H. Kim, I. K. Song and J. Yi, J. Mol. Catal. A, 219 (2004) 87. [5] J. M. Rynkowski, T. Paryjczak and M. Lenik, Appl. Catal. A, 106 (1993) 73. [6] F. Negrier, E. Marceau, M. Che and D. de Caro, C. R. Chimie, 6 (2003) 231. [7] F. Négrier, E. Marceau, M. Che, J. M. Giraudon, L. Gengembre and A. Löfberg, J. Phys Chem. B, 109 (2005) 2836. [8] A. J. van Dillen, R.J.A.M. Terörde, D.J. Lensveld, J.W. Geus and K.P. de Jong, J. Catal., 216 (2003) 257.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Inverse replica of porous glass as catalyst support Sebastian Wohlrab,a Alexander Janz,a Marga-Martina Pohl,a Stefanie Kreft,a Dirk Enke,b Angela Koeckritz,a Andreas Martin,a Bernhard Lueckea a

Leibniz Institute for Catalysis at the University Rostock, Albert-Einstein-Str. 29a, D-18059 Rostock, Germany b Institute for Technical Chemistry, University of Leipzig, Linnéstr. 3-4, D-04103 Leipzig, Germany Dedicated to Professor Uwe Rosenthal on the occasion of his 60th birthday

Abstract Porous glass granules with a pore size of about 150 nm and a specific surface area of 44 m2/g were used as crystallization matrix for ceria. A complete pore filling revealed porous granules of CeO2 after removal of the glass matrix. The new material consists of primary crystallites with sizes between 8 to 20 nm and shows a surface area of 48 m2/g. It possesses a structure comparable to the former pores of the glass matrix detected by SEM. This inverse replica was used as a support for gold particles which could be deposited from a colloidal solution (dAu,colloid = 1–2 nm). Its catalytic performance during the oxidation of ethylene glycol was compared to a likewise impregnated porous glass, previously used as exotemplate. Keywords: VYCOR glass, nanocasting, template, catalytic oxidation, glycolic acid

1. Introduction Crystallization inside a protective solid matrix, called exotemplating [1], is a powerful tool for generating advanced materials. In general, scaffolds are used for sterical shielding, suppressing uncontrolled sintering and further crystal growth during the synthesis. Several materials have already been proved suitable and established successfully as matrices, for instance: activated carbon [2], in situ generated carbonaceous foams [3], porous polymers [4], silica aerogels [5], mesoporous glass [6] or colloidal silica particles [7]. For these examples, removal of matrices can be achieved either by thermal treatment of carbonaceous matrices or by simple dissolution of glassy exotemplates yielding nanopowders or inverse replica of former incorporated materials. Porous glasses are usually manufactured from phase separated alkali borosilicate glasses by extraction. The resulting extraction products strongly depend on the former phase composition. Such glasses can be synthesized with a narrow pore size distribution and with pore sizes ranging from 1 to 1000 nm. Pore volumes between 0.1 and 1.1 cm3/g and inner surfaces up to 500 m2/g can be provided in such materials [8]. These porous glasses are promising candidates for the application either as catalyst supports or as exotemplates to generate inverse replicas which themselves can be used as support materials. With inherent nanoscale domain morphologies (redox activation) and pores above the mesoscale range (molecular transport) such inverse replicas promise new developments in the field of heterogeneous catalysis.

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2. Experimental As described in [8], sodium borosilicate glasses were used as starting materials for the generation of porous glass. Granules of this glass were etched with 1 N hydrochloric acid for 1 h at 90°C, washed and dried at 20°C, followed by characterization via nitrogen adsorption using the five-point BET method (Asurf = 44 m2/g) and scanning electron microscopy (SEM). The synthesis of porous CeO2 within this glass was achieved by a process including multiple impregnation and calcination steps: The porous glass granules were thrice impregnated with 1 M aqueous Ce(NO3)3*6H2O (Acros Organics) until complete wet impregnation was achieved and dried at 60 °C. Afterwards the material was calcined at 450°C under air at a heating ramp of 5°C/min. The resulting material was reimpregnated and recalcined according to the above described method for another five times. After this impregnation and calcination process the CeO2/glass-composite was etched in refluxing 1 N NaOH for 90 minutes yielding CeO2 quantitatively which was characterized by nitrogen adsorption, X-ray diffraction, energy-dispersive X-ray spectroscopy (EDX) and SEM. Colloidal gold was prepared from 45.6 mg (0.12 mmol) HAuCl4*3H2O (from metal source) in 100 ml H2O containing 62 mg poly(vinylpyrrolidone) (PVP, Mw = 58 000 g/mol). Fast addition of 55.5 mg (1.5 mmol) NaBH4 (Fluka) in 10 ml H2O at 0°C yielded a brown solution which was concentrated to 4.5 ml by evaporation at 40°C under vacuum. This solution was characterized via transmission electron microscopy (TEM). The Au particles from 1 ml of the concentrated colloidal solution were deposited onto 650 mg of the porous glass granules as well as onto 750 mg of the porous CeO2 by drying at 75°C for 20 minutes. Fixation and polymer removal was achieved by adding 40 ml of 0.5 M aqueous AlCl3*6H2O (Merck) to the dried powders. After stirring for 5 minutes the materials were centrifuged and washed thrice with deionized water. After drying at 60°C the whole deposition process was repeated once more. The materials were characterized by TEM and inductively coupled plasma optical emission spectroscopy (ICP-OES). Catalytic testing was performed in stainless steel autoclaves equipped with glass inlets at 70°C and 5 bar O2 under strirring. The reaction solution consisted of 621 mg (10 mmol) ethylene glycol (Riedel-de-Häen), 20 ml of 0.5 M aqueous NaOH and 216 mg of Au/glass or 394 mg of Au/CeO2, respectively. Filtered reaction samples were analyzed via high performance liquid chromatography (HPLC).

3. Results and discussion A porous glass with a BET-surface of 44 m2/g and an average pore size of 150 nm (Figure 1a) was applied as exotemplate for the synthesis of ceria, CeO2. Approaches which use a low loading of the precursor cerium nitrate result in a nanocrystalline powder consisting of single particles and loose aggregates of CeO2. In order to generate an inverse replica of the porous glass the pores have to be completely filled. This status can be achieved by multiple impregnation and calcination. Alkaline etching of the so prepared glass quantitatively yields a nearly pure cerium oxide with traces of Si, detected via EDX. XRD-analysis revealed the cubic structure of CeO2 by comparison with the powder diffraction file 89-8436 (STOE, WinXPow) (Figure 1b).

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317

Figure 1. a) Outer surface of the porous glass used as exotemplate for the CeO2 synthesis: SEM; b) X-ray diffraction pattern of the isolated CeO2 after removal of the glass matrix.

The reflexes appear broad and point to apparent nanocrystalline domains. Using the Scherrer method (STOE, WinXPow) the averaged elongation of the CeO2 lattice planes was calculated with {111} = 13 nm, {200} = 19 nm, {220} = 13 nm and {311} = 12 nm. The obtained CeO2 was analyzed by SEM (Figure 2a). It appears similar in size and shape compared to the former porous glass exotemplate. Besides, some present smaller structures can be ascribed to an incomplete inverse replication due to an insufficient connectivity between the ceria nanoparticles. Figure 2b shows the regular structure mainly present in the formed inverse replica. Figure 2c shows a minor part of loose and anomalous pore structures which can be ascribed to irregularities within the porous glass powder, and insufficient impregnation of this template.

Figure 2. Microscopic structure of the inverse replica: SEM; a) granule structure; b) regular inverse replica structure; c) irregularities within the inverse replica.

The surface of the inverse replica was examined via nitrogen adsorption using the five-point BET method indicating a specific surface of 48 m2/g. Due to the higher density of the ceria compared to the glass it can be proposed that a certain porosity is present within the pore walls of the inverse replica. The two porous materials, glass and ceria, were used as support for gold particles. Therefore, a colloidal solution of Au stabilized by PVP (Mw = 58 000 g/mol) was prepared according to [9] in a slightly modified approach. The dispersion was concentrated by evaporation yielding Au particles of 1-2 nm in size as well as elongated structures in the nano-range (Figure 3a). The gold particles were deposited by impregnation and drying onto the porous glass (Figure 3b) as well as onto its inverse ceria replica with an obvious inherent mesoporosity (Figure 3c).

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Figure 3. Au particles [I]: TEM a) as prepared; and deposited onto supports [II]: b) porous glass; c) ceria.

It is important to note that the PVP has to be removed after particle deposition by a washing procedure. Multivalent ions are known for their destabilizing properties concerning colloids. A 0.5 M aqueous AlCl3 solution could be utilized to achieve polymer dissolution with a parallel physical stabilization of the catalyst onto glass or ceria. After this procedure, the aluminum chloride can completely be removed by washing with water. The main appearance of the gold onto both supports is polyhedral with a main particle size fraction ranging from 4 to 12 nm indicating a particle growth during deposition. CeO2 appears nanocrystalline in the size range which was calculated from the Scherrer method. The molar loading of the gold onto the supports was determined via ICP-OES with 0.53% and 1.33% for glass and ceria, respectively. Gold catalyzed oxidations of alcohols and diols possess great potential towards selectivity [10]. As example, chemical kinetics was measured during the catalytic oxidation of ethylene glycol based on the O2 consumption. Deposited Au on the porous glass shows no catalytic activity while the Au loaded ceria replica shows an initial activity of 53 mmol·gAu-1·min-1 within the first 10 minutes of the reaction. In the latter case, after 60 minutes a conversion of 59% was obtained. As main product glycolic acid was produced at a selectivity of 94%. Oxalic acid and glyoxylic acid were detected as side products.

4. Conclusion With the synthesis of porous CeO2 granules, a preparative method for generating porous oxides via porous glass exotemplates is introduced. This method is basically applicable to a sum of other oxidic systems which are stable under the conditions of acidic or basic template removal. In comparison to mesoporous systems an appropriate substance transport during catalytic reactions can be expected in such porous materials. Deposition of Au particles onto porous glass as well as of its inverse ceria replica was performed in order to compare the catalytic activity of these two materials during the oxidation of ethylene glycol. It was shown that the inverse transformation of the pore structure into a redox active material is a promising route with respect to the advanced catalytic performance.

References [1] [2]

F. Schüth, 2003, Endo- and exotemplating to create high-surface-area inorganic materials, Angewandte Chemie-International Edition, 42, 31, 3604-3622. M. Schwickardi, T. Johann, W. Schmidt, F. Schüth, 2002, High-surface-area oxides obtained by an activated carbon route, Chemistry of Materials, 14, 9, 3913-3919.

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A. B. Panda, A. Tarafdar, S. Sen, A. Pathak, P. Pramanik, 2004, Preparation of nanocrystalline SrBi2Ta2O9 powders using sucrose-PVA as the polymeric matrix, Journal of Materials Science, 39, 3739-3744. [4] S. Wohlrab, M. Weiss, H. C. Du, S. Kaskel, 2006, Synthesis of MNbO3 nanoparticles (M = Li, Na, K), Chemistry of Materials, 18, 18, 4227-4230. [5] D. Carta, G. Mountjoy, G. Navarra, M. F. Casula, D. Loche, S. Marras, A. Corrias, 2007, Xray Absorption Investigation of the Formation of Cobalt Ferrite Nanoparticles in an Aerogel Silica Matrix, The Journal of Physical Chemistry C, 111, 6308-6317. [6] W. B. Yue, W. Z. Zhou, 2008, Crystalline mesoporous metal oxide, Progress in Natural Science, 18, 11, 1329-1338. [7] N. C. Strandwitz, G. D. Stucky, 2009, Hollow Microporous Cerium Oxide Spheres Templated By Colloidal Silica, Chemistry of Materials, 21, 19, 4577-4582. [8] F. Janowski, D. Enke; F. Schüth, K. S. W. Sing (Editors) , J. Weitkamp, 2002, Handbook of Porous Solids, Wiley-VCH, 3, 1432. [9] H. Tsunoyama, H. Sakurai, N. Ichikuni, Y. Negishi, T. Tsukuda, 2004, Colloidal Gold Nanoparticles as Catalyst for Carbon-Carbon Bond Formation: Application to Aerobic Homocoupling of Phenylboronic Acid in Water, Langmuir, 20, 11293-11296. [10] A. S. K. Hashmi, G. J. Hutchings, 2006, Gold Catalysis, Angewandte Chemie International Edition, 45, 7896-7936.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

The use of small volume TOC analysis as complementary, indispensable tool in the evaluation of photocatalysts at lab-scale Stefan Ribbens,a Vera Meynen,a Koen Steert,b Koen Augustyns,b Pegie Coola a,

Laboratory of Adsorption and Catalysis University of Antwerpen (UA), Universiteitsplein 1, B-2610 Wilrijk, Belgium b Laboratory of Medicinal Chemistry, University of Antwerpen (UA),Universiteitsplein 1, B-2610 Wilrijk, Belgium

Abstract “Total Organic Carbon”-analysis (TOC) on micro volume (µV) liquids was applied for the first time by means of a special designed Shimadzu gas injection kit®. This way, it became possible to evaluate the efficiency of photocatalytic dye degradation in terms of CO2 conversion (photomineralization) simultaneously with classic UV-Vis (photobleaching) measurements within a small, lab-scale photocatalytic test setup. The possibility to allow multiple micro volume sampling in short time intervals during several hours without a substantial decrease in volume/catalyst ratio is of particular value in the evaluation of photocatalysts on lab-scale volumes (< 100 ml). By combining both complementary techniques (UV-Vis and µV TOC), indispensable, additional knowledge on the degradation process/mechanism and the catalyst efficiency can be obtained in a fast, inexpensive and easy way. In this study, the photocatalytic acitivity of mesoporous titania (EISA TNH4OH C450) and hydrogen trititanate nanotubes (H-TNT) towards the degradation of rhodamine 6G was investigated. It has been illustrated that the detailed TOC-plots add important, complementary information to the data obtained by UV-Vis analysis that can reveal rate limiting steps, surface adsorption effects and charge effects. This can lead to a better evaluation of the catalyst and improved insights in the various degradation mechanisms that can occur. Keywords: photocatalytic degradation, TOC, photomineralization

1. Introduction Various methods for assessing and characterizing the photoactivity of mesoporous titania based materials have been developed. Most of these techniques study the photobleaching process of dye molecules (e.g. methylene blue and rhodamine 6G) by measuring the decrease in concentration of the dye in function of time by applying UVVis analysis measured at only 1 wavelength, that of maximum absorption of the original dye. Although it is a particularly fast, non-destructive and inexpensive method, it only allows evaluating the decrease in concentration of the initial test molecule in function of time and not of possible existing intermediates. Even though, the photobleaching process can be studied in this way, it is not necessarily representative for the total degradation towards CO2. Therefore, in most cases, the UV-Vis study can only be correlated to the initial degradation steps (photobleaching) and not to the total process (photobleaching + photomineralization). Indeed, often photomineralization of organic compounds to carbon dioxide is a more slow and complex process [1]. The knowhow

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on both photobleaching and photomineralization processes is of great importance as it gives information on the global efficiency of the photocatalyst. “Total organic carbon”-analysis (TOC) allows measuring the total amount of organic carbon present in aqueous samples and is therefore a commonly used online technique in industry in order to evaluate wastewater streams. Nevertheless, the use of classic TOC-analysis in small, lab-scale photoreactors (volumes: 25-100 ml) can give rise to serious misinterpretations. Indeed, relatively large volumes of samples of at least 5 ml are needed for each sampling due to the substantial amount of external tubing towards the combustion tube, syringe, etc. that requires purging with part of the sample volume as well as the loss of sample in this dead volume. In case of correct evaluation of any photocatalytic experiment, the ratio of catalyst/dye solution should not be influenced to a great extent in order to avoid strongly altered reaction conditions and misinterpretations of the results due to progressively increasing catalysts concentration during sampling. The use of larger test setups is not opportune in the field of catalyst development and screening, studies of its degradation mechanism or other fundamental studies because this implies the need for a substantially increased amount of catalyst. Therefore, a method was developed to be able to inject microliter volumes in TOC. A Shimadzu® designed gas injection kit, in combination with a high precision syringe (Hamilton), was applied to inject a small quantity of sample directly on the combustion tube, therefore allowing the use of small volumes of a few microliters only (µl). This technique was used in combination with classical UV-Vis analysis in order to evaluate the photocatalytic activity of two different mesoporous photocatalysts.

2. Experimental section 2.1. Chemical reagents and synthesis All products were used as received without any modification or purification, unless stated otherwise. Ultrapure milli-Q water was used to prepare the 4.10-5 M rhodamine 6G solution. Trititanate nanotubes (TNT) were prepared using a template free, hydrothermal synthesis method identical to the one described by S. Ribbens et al [2]. Mesoporous titania was synthesized using the “Evaporation Induced Self-Assembly” method (EISA) followed by a post modification in ammonia to stabilize the structure. The surfactant was removed by calcination at 450°C [3].

2.2. Characterization The photocatalytic activity was tested by photodegradation of a cationic dye (rhodamine-6G) in aqueous solution. 16 mg of the catalyst was added to a solution of 50 ml 4*10-5 M rhodamine-6G and stirred for 30 minutes without UV irradiation to establish an adsorption-desorption equilibrium. The solution was then irradiated for 360 minutes with UV light (wavelength 365 nm) emitted by a 100 Watt Hg-lamp (Sylvania Par 38; 21.7 mW/cm² at 5 cm). During this illumination, samples with a volume of 5 ml were taken out of the suspension at fixed intervals (10 min) and analyzed using UV-VIS spectroscopy. After each measurement, the solution was returned to the initial solution to prevent large changes in volume/catalyst ratios. The absorbance was measured at 526 nm with water as a reference. A maximum of 0.5 ml of the sample volume of UV-VIS was used to analyze the photooxidation to CO2 by µV TOC. A minimum of three sequential injections of the obtained samples were injected by means of a high precision syringe (Hamilton 1725 gas-tight syringe, RN type 2). Standard deviation of the multiple injections in one measuring point is maximum 1%. This analysis was performed on a Shimadzu TOC-VCPH equipped with a manual injection kit.

The use of small volume TOC analysis as complementary, indispensable tool

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Conc (10^-5 M) R6G

TOC (ppm)

3,5 16 2.2.1. Results and discussion Figure 1 shows the results of the 14 3 photocatalytic test as obtained 12 by UV-Vis analysis. The 2,5 concentration of the initial 10 2 dye molecule is plotted as a 8 function of time. In Fig. 1 1,5 results of the TOC measure6 ments are shown. Here, the 1 4 total organic carbon present in the solution is plotted as a 0,5 2 function of time. In UV-Vis 0 0 analysis, it can be seen that 0 100 200 300 400 there is an immediate decrease Time (min) in concentration for both catalysts after the UV-light is Fig.1. TOC analysis : EISA TNH4OH C450 switched on. This indicates TOC analysis : H-TNT X UV-Vis analysis : EISA TNH4OH C450 that photocatalytic reactions UV-Vis analysis : H-TNT are initiated immediately at both catalysts, although the reaction rate of H-TNT is slow during the first 90 minutes. Both catalysts clearly cause (partial) degradation of the original dye molecule. If the analysis would be based solely on UV-VIS, one could conclude that both catalysts are very active, but the reaction rate of EISA TNH4OH C450 is three times higher compared to H-TNT. (kEISA:0.0073 and kH-TNT: 0.0025). However, an immediate decrease in carbon content is not apparent in TOC-analysis (Fig. 1B). The TOC amount remains constant during the first 60 minutes of UV irradiation, implying that only intermediate products are formed and no full degradation to CO2 has taken place. If the irradiation time is long enough (> 60 min), the intermediate degradation products can be photooxidized to very small molecules and further conversion to CO2 is possible. In case of EISA TNH4OH C450, an even longer irradiation time is needed for CO2 conversion and much less carbon has been removed from the solution compared to HTNT after 360 minutes (H-TNT: 74% removed carbon and EISA TNH4 OH C450: 39%). Therefore, H-TNT has clearly a better conversion rate to CO2 than EISA TNH4OH C450. This means that the lifetime of intermediates is much less in case of the H-TNT photocatalyst. This is of particular importance in photocatalytic degradation of pollutants since it will diminish the risk of creating harmful intermediates. Here, the importance of a complete evaluation of both photodegradation of the initial dye and photomineralization is clearly demonstrated. Using UV-Vis scans and LC/MS, the photocatalytic processes can be studied in more detail (not shown). UV-Vis scans of the dye solutions measured at time intervals of 20 minutes reveal that the degradation mechanism for both photocatalysts is completely different: for H-TNT a clear hypsochromic shift in absorption maximum can be observed, whereas for EISA TNH4OH C450 the absorption maximum of the dye solution just decreases in intensity in function of time. Furthermore, LC/MS results show that the N-deethylation of the dye in presence of H-TNT is more pronounced compared to the EISA TNH4OH C450. These results could imply that there is a stronger interaction between the cationic dye and the negativily charged surface of H-TNT compared to EISA TNH4OH C450: whereas strongly adsorbed molecules can be subjected to numerous photodegradation reactions, weakly adsorbed molecules can only interact with the catalyst surface for a short time and therefore the photooxidation of these molecules will be limited. This

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would explain the observations in Fig. 1A and Fig. 1B. Because of the weak interaction between the dye and EISA TNH4OH C450, dye molecules can easily adsorb and desorb on the surface of the catalyst. Therefore, a high photodegradation rate, as analysed in UV-Vis spectroscopy, can be observed for EISA TNH4OH C450. However, because of the short adsorption time of the dye/degradation products on the photocatalyst, the concentration of the initial dye and products will play an important role. Because the initial dye molecules are positively charged, they have a better interaction with the slightly negative charged surface of EISA TNH4OH C450 compared to the neutral intermediates. This implies that there is a competition between the initial dye molecules and the oxidation products. Therefore, oxidation to CO2 takes place after more than 200 minutes. Here, the competition between intial dye molecules and degradation products is seriously and degradation products can be further oxidized. If H-TNT is suspended in the dye solution, an oppisite effect can be observed. Due to the strong interaction between the dye and the photocatalyst, molecules are longer adsorped on the surface and will be further photooxidized. This will lead to a fast oxidation towards CO2, but slows down the photobleaching of the dye molecules. More work will be done to support this hypothesis.

3. Conclusion This study shows that a better evaluation of the catalyst and improved insights in the various degradation mechanisms can be obtained by performing UV-Vis and µV TOCanalysis simultaneously. It has been demonstrated that H-TNT has a slow photodegradation rate towards initial dye molecules, but oxidize the adsorbed dye molecules very efficiently into CO2. For EISA TNH4OH C450 opposite results are found. The small volumes (< 5µl) that are required in combination with the short analysis times (2-3 minutes) makes micro-volume TOC also suitable for other process that work with small analysis volumes such as high through put or pharmaceutical applications. The simple design of the manual injection kit could allow a further development towards auto sampling. In this way, fully automated analysis could become possible.

Acknowledgement V. Meynen is grateful to the FWO-Flanders for her postdoctoral research grant. This work has been done in the frame of the FWO project G.0237.09. The authors would like to thank Shimadzu for the technical support.

References 1.

2.

3.

R. Comparelli, E. Fanizza, M.L. Curri, P.D. Cozzoli, G. Mascolo, R. Passino, A. Agostiano, 2005, Photocatalytic degradation of azo dyes by organic-capped anatase TiO2 nanocrystals immobilized onto substrates, Applied Catalysis B : Environmental, Volume 55, issue 2, p. 81-91. S. Ribbens, V. Meynen, G. Van Tendeloo, X. Ke, M. Mertens, B.U.W. Maes, P. Cool, E.F Vansant, 2008, Development of photocatalytic efficient Ti-based nanotubes and nanoribbons by conventional and microwave assisted synthesis strategies, Microporous and Mesoporous Materials, 114, 1-3, p. 401-409. E. Beyers, P. Cool, E.F. Vansant, 2007, Stabilisation of mesoporous TiO2 by different bases inluencing the photocatalytic activity, Microporous and Mesoporous Materials, 99, 1-2, p. 112-117.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Enzymatic oxidation of phenols by immobilized oxidoreductases B. Tikhonov, A. Sidorov, E. Sulman, V. Matveeva Tver Technical University, A. Nikitina str., 22, Tver, 170026, Russia

Abstract 7 various cation-exchange resins on the basis of styrenedivynilbenzene were used as the carriers for immobilization of oxidoreductases (horseradish peroxidase and musroom tyrosinase). Ion exchangers were treated with sodium alginate, chitosan, glutaric dialdehyde and N-(3-dimethyl-aminopropyl)-N-ethylcarbodiimide hydrochloride. Synthesized biocatalytic systems on the basis of oxidoreductases were found to be highly active and stable in catalytic oxidation of phenols including sewage treatment and industrial waste products to harmless melanin-type polymers. Keywords: immobilization, oxidoreductases, waste water, phenol, oxidation

1. Introduction The use of oxidoreductases in the industrial catalysis has considerably increased recently . Their efficiency is proved in reactions of homogeneous oxidation of aromatic components, in particular, aniline and phenol in modeling solutions and waste waters [1,2]. Worldwide application of this method is limited to the high cost and poor stability of purified enzymes. These problems can be solved by the immobilization of enzymes from the aqueous extracts on inorganic or organic carriers with the obtaining as a result of a heterogeneous system [3,4]. One of the most prospective methods of enzymes immobilization is the covalent cross-linking of enzymes with the modified carrier which should be mechanically strong, water-insoluble, has high chemical and biological stability and low cost. Experimental selective oxidation of monosaccharides: L-sorpbose and D-glucose.

2. Experimental 2.1. Materials and methods In this work various biocatalytic systems were investigated. 7 various cation-exchange resins (Dowex 50WX, Dowex 50WX2, Lewatit CNP-105, Amberlite 200, Amberlite IR-120, Amberlite IRC-86, Ku 2-8) on the basis of styrenedivynilbenzene with SO3H or COOH active groups were used as the carriers. Ion exchangers were treated with sodium alginat, chitosan, glutaric dialdehyde and N-(3-dimethyl-aminopropyl)-Nethylcarbodiimide hydrochloride. Two methods of chitosan and activating agent deposition on ion exchange resin were studied. They are consecutive deposition and deposition of components mixture. The schemes of biocatalyst synthesis: (i) with primary support activation S + M Æ С-M;

S-M + A Æ S-M-A;

S-M-A + Е Æ S-M-A-Е;

B. Tikhonov et al.

326 (ii) with primary activation of the enzyme S + MÆ S-M;

A + Е Æ A-Е;

S-M + A-Е Æ S-M-A-Е.

where S – support; M – modifier; A – crosslinking agent; E – enzyme. Also catalytic efficiency of peroxidase and tyrosinase from various sources were investigated. The activity of biocatalysts in reactions of phenol and catechol oxidation to melanin-type polymers was found as a change of optical density of reaction mixture at 440 nm. Besides, to determine the kinetic parameters of the catalysts the chronometric method was used [5].

3. Results and discussion Experiments showed that the most efficient carriers are Ku 2-8 and Amberlite 200, which functional groups have high reactivity. Besides, they can be applied for biocatalysis by surface characteristics. It has been revealed that the scheme (i) is optimal for crosslinking agent glutaric dialdehyde, while the scheme (ii) – for N-(3-dimethyl-aminopropyl)-N-ethylcarbodiimide hydrochloride. During the measurements it was determined that consecutive deposition of chitosan and glutaric aldehyde on cation-exchanger provides the strongest and more stable bounding of enzyme with the carrier. It was shown that glutaric dialdehyde provides better stabilization of enzyme on the carrier to compare with N-(3-dimethylaminopropyl)-N-ethylcarbodiimide hydrochloride. For the investigated biocatalysts the optimal conditions of phenols oxidation process with the achievement of high degree of conversion (more than 95%) were found: temperature - 25°C, intensity of mixing - 300 min-1, pH 6.5 and 7 - for peroxidase and tyrosinase, respectively. The optimal ratio of the biocatalyst components was determined (see Table 1). Table 1. The optimal ratio of the components of the biocatalysts. Biocatalyst

Concentration of chitosan solution, %

Concentration of m(E)/m(S), % glutaric dialdehyde solution, % S-М-А-E1 0,1 25 8 S-М-А-E2 0,2 25 10 where S – cation exchanger; M – chitosan; A – glutaric dialdehyde; E1 – peroxidase; E2 tyrosinase.

Physicochemical investigations (FTIR spectroscopy, XPS, nitrogen physisorption) of optimal biocatalytic systems were carried out. The result of nitrogen physisorption for the biocatalyst components is shown in Fig. 1. Kinetic and physicochemical investigations showed that biopolymer (chitosan) is distributed on the surface of the carrier as separate molecules or bidimentional clasters without formation of 3D structures. Such distribution promotes minimization of intradiffusive limitation during the oxidation. The scheme of the optimal biocatalyst is shown in Fig. 2.

Enzymatic oxidation of phenols by immobilized oxidoreductases

327

Figure 1. The result of nitrogen physisorption.

Figure 2. The scheme of the optimal biocatalyst formation.

The representation of the surface of a biocatalytic system is shown in Fig. 3. Kinetic parameters of synthesized biocatalysts are shown in Table 2. Table 2. Kinetic parameters of synthesized biocatalysts. Biocatalyst Native Peroxidase S–C–A–E1

Substrate Phenol Catechol Phenol Catechol

Vm, mM s-1 0.069 0.156 0.024

Km, M 3.791 11.439 29.79

0.023

54.23

Native Tyrosinase Catechol 0.022 18.99 S–C–A–E2 Catechol 0.009 85.6 S – cation exchange resin Ku 2-8; C – chitosan; A – glutaric dialdehyde; E1 – horseradish peroxidise, E2 – mushroom tyrosinase

The decrease of immobilized peroxidase and tyrosinase activities are the consequence of heterogenization of enzymes and the influence of intradiffusive factors.

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Figure 3. Representation of a biocatalytic system surface (S – carrier, A – activator, E – enzyme).

However, biocatalysts are stable in more than 10 cycles, and heterogenization makes the system more technological. One more advantage of the developed biocatalytic systems is an essential depreciation of catalyst due to the obtaining of enzymes from vegetative raw material without expensive purification.

4. Conclusions Synthesized biocatalytic systems on the basis of horseradish peroxidase and mushroom tyrosinase were found to be highly active and stable in catalytic oxidations of phenols including sewage treatment and industrial waste products. The catalysts obtained can be used for sewage and industrial waste biocatalytic treatment as they allow transfering dangerous phenolic compounds to harmless melanin-type polymers.

Acknowledgements We sincerely thank Federal Education Agency of Russian Federation (contracts P 257 and P 1196) for the financial support.

References 1.

2.

3. 4. 5.

S. Ibrahim, H. I. Ali, K. E. Taylor., N. Biswas, J. K. Bewtra. Enzyme-catalyzed removal of phenol from refinery wastewater: Feasibility studies. Water Environ. Res. 73 (2001) 165172. M. A. Gilabert, L. G. Fenoll, F. Garcia-Molina, J. Tudela, F. Garcia-Canovas, J. N RodryguezLopez. Kinetic characterization of phenol and aniline derivates as substrates of peroxidase Biol. Chem., Vol. 385, (2004) 795–800. L. V. Bindhu, E.T. Abraham. Immobilization of Horseradish Peroxidase on Chitosan for Use in Nonaqueous Media. Inc. J. Appl. Polym. Sci. 88 (2003) 1456-1464. W. Tischer, F. Wedekind. Immobilized enzymes: methods and applications. Top. Curr. Chem., Vol. 200 (1999) 95–126. D. C. Goodwin, I. Yamazaki, S. D. Aust, and T. A. Grover, Determination of Rate Constants for Rapid Peroxidase Reactions, Anal. Bioch, 231, 333–338 (1995).

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V.

A coordinative saturated vanadium containing metal organic framework that shows a remarkable catalytic activity Karen Leus, a Ilke Muylaert, a Veronique Van Speybroeck, b Guy B. Marin, c and Pascal Van Der Voort, a a

Center for Ordered Materials, Organometallics and Catalysis (COMOC), Department of Inorganic and Physical Chemistry, Ghent University, Krijgslaan 281 (S3), 9000 Ghent, Belgium b Center for Molecular Modeling, Ghent University, Technologiepark 903, 9052 Zwijnaarde, Belgium c Laboratory for Chemical Technology, Ghent University, Krijgslaan 281 (S5), 9000 Ghent, Belgium

Abstract A completely saturated Metal Organic Framework, MIL-47 was synthesized and tested for its catalytic performance in the oxidation of cyclohexene with tert-butyl hydroperoxide as oxidant. The catalyst was compared to several reference catalysts: namely VAPO-5, supported VOx/SiO2 and the homogeneous catalyst VO(acac)2. MIL-47 shows a remarkable catalytic activity and preserves its crystalline structure and surface area after a catalytic run. Furthermore MIL-47 exhibits a very high activity in successive runs. Keywords: metal organic frameworks, vanadium, oxidation, liquid phase

1. Introduction Metal Organic Frameworks (MOFs) are crystalline porous solids composed of a threedimensional (3D) network of metal ions held in place by multidentate organic molecules [1,2]. In recent years, MOFs have received considerable attention as potentially valuable gas storage and catalyst materials [3-7]. MOFs possess several attractive features: a high micropore volume, crystallinity and a high metal content offering potentially valuable active sites. So far, only a few catalytic applications of Metal Organic Frameworks have been reported. Some of their potential applications were outlined recently in two excellent reviews [8,9]. All these reports deal with Metal Organic Frameworks that have unsaturated sites. However, to obtain insight into the real nature of the active sites, it is of a paramount importance to study saturated Metal Organic Frameworks. Therefore, a completely saturated, vanadium containing MOF was synthesized, namely MIL-47. This MOF is a porous terephthalate built from infinite chains of V4+O6 octahedra, held together by dicarboxylate groups of the terephthalate linkers and has a three-dimensional orthorhombic structure [10]. In the present work, we have tested MIL-47 for its catalytic performance in the oxidation of cyclohexene. Amongst the various oxidation products of cyclohexene, cyclohexane epoxide is a highly reactive and selective organic intermediate which is widely used in the synthesis of enantioselective drugs, epoxy paints and rubber promoters [11]. Furthermore the catalytic activity of MIL-47 is compared to VAPO-5, VOx/SiO2 and the homogeneous catalyst VO(acac)2.

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2. Experimental section The hydrothermal synthesis of MIL-47 is based on a literature procedure [10]. A mixture of VCl3, terephthalic acid and H2O (molar ratio 1/0.25/100) is brought into a Teflon lined steel autoclave, which is heated at 473 K for 4 days. In a next step, MIL47as is brought at 573 K for 22 h and 30 min to remove the excess of terephthalic acid in the pores. VAPO-5 is synthesized as described previously: a solution of oxovanadium (IV) sulphate-hydrate and a solution of H3PO4 are mixed together. While stirring, pseudo boehmite (from Sasol) and triethylamine are added. In a further step, the gel is brought into an autoclave and placed in an oven at 443 K for 2 days. By centrifugation, the solid is recovered. Furthermore the catalyst is dried and calcinated under a O2-flow [12]. For the synthesis of VOx/SiO2, Kieselgel 60 is stirred in a NH4VO3-solution at 338 K for 2 h. Afterwards, the solid is filtered and dried during 2 h at 373 K, followed by a calcination at 823 K during 5 h. After a catalytic run, the MIL-47 is regenerated by a treatment in a tubular furnace under a N2-flow at 523K. This is necessary to remove the organic compounds in the pores.

3. Results and discussion The oxidation of cyclohexene was carried out in a three neck flask under an inert atmosphere. To a solution of cyclohexene (0.05 mol), tert-butyl hydroperoxide (0,14 mol) and 1,2,4-trichlorobenzene (0.05 mol) (used as internal standard) in chloroform (0.38 mol) 0,1 g of the catalyst was added. The reaction mixture was stirred at 50°C. All the samples were analyzed with a Trace GC Ultra (Finnigan), fitted with an capillary column (10m, 0,1 mm, 0,4 µm) and an FID detector. Blanc reactions were performed without catalyst.

Fig. 1. Conversion curve of cyclohexene for (■) unsupported VO(acac)2 , (○) MIL-47, (▲) VOx/SiO2 and (▼) VAPO-5.

In Figure 1, the conversion curve of cyclohexene is presented in comparison with the three reference catalysts. As can be seen in Figure 1, VAPO-5 is catalytic inactive for the oxidation of cyclohexene, whereas the three other catalysts: MIL-47, the supported VOx/SiO2 and the homogeneous VO(acac)2 exhibit a very high catalytic activity.

A coordinative saturated vanadium containing Metal Organic Framework

331

Intensity/ a.u

The turn over number (TON) of MIL-47 is calculated, based on the amount of cyclohexene that is converted. The TON of MIL-47 was approximately 108 after eight hours of reaction. Thermal Gravimetric Analysis experiments (TGA) were performed on MIL-47 before and after a catalytic run to quantify the amount of leached vanadium. In comparison with the supported vanadium oxide catalyst, only a small amount of vanadium is leached. The leaching was less than 20% in the first run with MIL-47, whereas the VOx/SiO2 showed a leaching of more then 40%. Furthermore, the catalyst was recovered after a first catalytic run. The X-ray diffraction patterns of MIL-47 before and after regeneration are shown in Figure 2.

b

a 5

10

15

20

25

30

35

40

45

50

2 theta

Fig. 2. XRD patterns of MIL-47 (a) before and (b) after regeneration.

MIL-47 preserves its crystalline structure after regeneration, as can be seen from Figure 2. Moreover, the nitrogen adsorption experiments of MIL-47 before and after regeneration are presented in Figure 3. Note that the MIL-47 shows no loss at all of surface area and pore volume after regeneration.

Fig. 3. Nitrogen adsorption isotherms of MIL-47 (■) before and () after regeneration.

To evaluate the regeneration capacity of this novel catalyst, MIL-47 was tested for a second catalytic run and compared to the vanadium oxide catalyst. The conversion of cyclohexene for MIL-47 and the VOX/SiO2 catalyst in the first and second run is shown in Figure 4.

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Fig. 4. Conversion of cyclohexene for MIL-47 in its (■) first run, (●) second run and VOx/SiO2 (▲) first and (▼) second run.

MIL-47 still shows a high conversion of cylohexene, whereas the supported VOx/SiO2 shows no activity at all in its second run due to leaching of the vanadium centers. This observation indicates that MIL-47 acts as a truly heterogeneous catalyst. In conclusion, the saturated Metal Organic Framework, MIL-47, is investigated for its catalytic activity for the oxidation of cyclohexene and compared to three reference catalysts. MIL-47, containing saturated vanadium centres, shows a high catalytic conversion. X-ray diffraction measurements and nitrogen adsorption experiments prove the stability of this new catalyst under oxidation reactions. Furthermore MIL-47 exhibits a very high catalytic activity in successive runs.

References 1.

M. Eddaoudi et al, 2001, Modular chemistry: Secondary building units as a basis for the design of highly porous and robust metal organic carboxylate frameworks, Acc. Chem. Res., 34, 4, 319. 2. S.L. James, 2003, Metal Organic Frameworks, Chem. Soc. Rev., 32, 276. 3. C. Janiak, 2003, Engineering coordination polymers towards applications, Dalton Trans., 14, 2781. 4. S. Kitagawa et al, 2004, Functional porous coordination polymers,Angew.Chem. Int.Ed., 43, 18, 2334. 5. Y.M.A. Yamada, et al, 2006, Novel 3D coordination palladium-network complex: a recycable catalyst for Suzuki-Miyaura reaction, Org.Lett, 8, 19, 4259. 6. S.H. Cho et al, 2006, A metal organic framework material that functions as an enantioselective catalyst for olefin epoxidation, Chem. Commun., 24, 2563. 7. B. Gomez-Lor et al, 2005, Novel 2D and 3D indium metal organic frameworks : Topology and catalytic properties, Chem. Mater.,17, 10, 2568. 8. A.U. Czaja et al, 2009, Industrial applications of metal organic frameworks, Chemical Society Reviews,38, 1284. 9. U. Mueller et al, 2006, Metal organic frameworks- prospective industrial applications, J. of Mat. Chem., 16,7,626. 10. K. Barthelet et al, 2002, A Breating Hybrid Organic-Inorganic Solid with Very Large Pores and High Magnetic Characteristics, Angew. Chem. Int. Ed, 41, 2, 281. 11. M.R. Maurya et al, 2008, Immobilisation of oxovanadium (IV), dioxomolybdenum (VI) and copper (II) complexes on polymers for the oxidation of styrene, cyclohexene and ethylbenzene, App.Cat.A-General, 351, 2, 239. 12. M. J. Haanepen et al, 1997, VAPO as catalyst for liquid phase oxidation reactions. Part1: preparation, characterisation and catalytic performance, App.Cat.A-General, 152, 183.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Influence of preparation conditions on properties of gold loaded on the supports containing group five elements Izabela Sobczak*, Justyna Florek, Katarzyna Jagodzinska, Maria Ziolek A. Mickiewicz University, Faculty of Chemistry, Grunwaldzka 6, 60-780 Poznan, Poland

Abstract Gold catalysts based on MCM-41 modified with vanadium and niobium, SBA-3 and group V metal oxides were prepared by several methods. The properties of the materials obtained were characterised by nitrogen adsorption, XRD, TEM, UV-Vis, and test reactions (AcoAc cyclisation and methanol oxidation). The best dispersion of gold was reached when it was introduced during the synthesis of MCM-41 with the use of H2SO4 as pH adjusting agent and Nb source besides Na silicate. Acid/base properties were determined by the preparation methods and the presence of group five elements. Keywords: AuNbVMCM-41; AuSBA-3; Au/V,Nb,Ta-oxides; acidity/basicity

1. Introduction Following the breakthrough research results of Hutchings and Haruta, there has been a dramatic increase in the interest in gold catalysis [1]. It has been demonstrated that the physicochemical and catalytic properties of gold catalysts depend mainly on the type of support and the preparation method. Both parameters influence the size of Au clusters [1]. Interaction between gold and the metals localized in the support plays also an important role and can determine the catalytic activity of the catalysts. The idea of this work is to use two groups of materials, silicate or metalosilicate (Me=Nb, V) hexagonally ordered mesoporous molecular sieves and transition metal oxides (V2O5, Nb2O5, Ta2O5) as supports for gold. The first group of samples exhibits very high surface area and the presence of isolated metal species, whereas bulk metal oxides are characterized by smaller surface areas and much higher concentration of metals on the surface. The main focus of this study is the influence of the methods of support syntheses and gold loading and their effect on the physicochemical properties of materials prepared.

2. Experimental 2.1. Preparation of the catalysts SBA-3 material was synthesized following the procedure reported originally by Stucky et al. [2]. NbMCM-41 samples were synthesized by hydrothermal method [3] and modified with gold according to [4,5]. Au/MCM-41 (with gold loading of 1 wt%) and Au/SBA-3 (3wt % of Au) were prepared by incipient wetness impregnation (IMP) of the support with HAuCl4 (Johnson Matthey). The alternative, direct synthesis of AuSBA-3, AuMCM-41, AuVMCM-41 and AuVNbMCM-41 (COP- co-precipitation of all components in one pot synthesis) was performed in the same manner as conventional MCM-41 [6] and SBA-3 [2]. The only difference was the admission of HAuCl4,

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vanadium(IV) oxide sulphate hydrate and ammonium niobate(V) oxalate as the sources of Au, V or Nb, respectively and the use of HCl or H2SO4 as pH adjustment agent. Commercial oxides (V2O5,Ta2O5 –Aldrich, Nb2O5 –Alfa Aesar) were modified by goldsol method [7] with tetrakis(hydroksymethyl)phosphonium chloride(THPC) as reducing agent and HAuCl4 as a source of gold (1 wt.% of Au) and, additionally, by depositionprecipitation (DP) method with urea [7]. The prepared materials were calcined at 623 K.

2.2. Characterization The XRD patterns were obtained on a D8 Advance diffractometer (Bruker) using CuKα radiation (λ=0.154 nm). The surface area and pore volume of the samples were measured by nitrogen adsorption at 77 K, using the conventional procedure on a Micromeritics 2010 apparatus. The UV–visible spectra were recorded on a Cary 300Scan (Varian) spectrometer in the range from 800-180 nm. For transmission electron microscopy (TEM) measurements powders were deposited on a grid with a holey carbon film and transferred to JEOL 2000 electron microscope operating at 80 kV. The catalysts were tested in acetonylacetone (AcoAc) cyclisation at 623 K and methanol oxidation at 473 and 523 K as the probe reactions under conditions described in [4,5].

3. Results and discussion 3.1. AuMCM-41 and AuSBA-3 The main difference in the preparation of AuMCM-41 and AuSBA-3 in one pot synthesis is the use of an alkaline medium (pH=11) in the first one and an acidic medium (pH=1) in the second synthesis. Moreover, different sources of silicon (sodium silicate and TEOS, respectively) are applied. Results of this study have proved a significant difference in the surface properties of both materials containing metallic gold particles on the surface. Acetonylacetone (AcoAc) cyclisation allows us to evaluate acidity and basicity of the surface on the basis of the selectivity to methylcyclopentanone (MCP) and dimethyl furan (DMF) [8]. MCP/DMF<<1 indicates the acidic character of the surface and MCP/DMF>>1 – basic properties of the surface. Highly basic character of AuMCM-41 is demonstrated by MCP/DMF ratio equal 104, whereas it is only 0.11 for AuSBA-3 (Table 1). The acidity of AuSBA-3 is confirmed by dehydration activity (dimethyl ether formation) in methanol oxidation (Table 2). Interestingly, SBA-3 impregnated with gold (Au/SBA-3) reveals oxidative properties like AuMCM-41 demonstrated by selectivity to formaldehyde and methyl formate. It means that acidity of the surface is not generated during the synthesis of silicate SBA-3 and impregnation with chloroauric acid but it results from gelation of TEOS together with HAuCl4 in the presence of Pluronic and HCl at pH=1. Table 1. Texture properties of the catalysts and selectivity ratio in AcoAc cyclisation at 623 K. Catalyst AuSBA-3 AuMCM-41(HCl) Au/NbMCM-41(IMP) AuNbMCM-41(HCl) AuVMCM-41(HCl) AuVMCM-41(H2 SO4 ) AuVNbMCM-41(HCl) AuVNbMCM-41(H2SO4)

Average pore vol. BJH, Surface area (ads.) cm3g-1 BET, (ads.) m2g-1 996 0.45 886 0.81 900 1.00 870 0.86 813 0.80 1055 1.34 851 1.08 1042 1.05

* MCM = methylcyclopentenon; DMF = dimethylfuran

MCP/DMF* (AcoAc cyclisation) 0.11 104 0.23 9.00 22.0 0.19 16.0 0.05

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Influence of the preparation conditions on the properties of gold loaded

Participation of gold source in the formation of acidic centers is clearly deduced from these results. Table 2. The results of methanol oxidation at 473 K. Catalyst

MeOH conv., % 25 8 38

AuSBA-3 Au/SBA-3 AuMCM-41

HCOH sel., % 56 2

HCOOCH3 sel., % 91

CH3OCH3 sel., % 99.9 -

CO2 sel., % 0.1 44 7

3.2. MeMCM-41(Me=Nb, V) materials containing gold – effect of preparation method and synthesis conditions MCM-41 material has been chosen for the further study of the interaction of gold with group V metals located in the structure of mesoporous material. Gold containing NbMCM-41 materials were prepared by two manners: impregnation (Au/NbMCM-41) and co-precipitation (AuNbMCM-41). The introduction of gold during the synthesis leads to the catalysts with more disordered structure and lower surface area and pore volume compared to the material obtained by impregnation (Table 1). The Au-metal crystallites are present on both Au-catalysts (XRD, UV-Vis – Fig. 1). However, the peaks assigned to the metallic gold in the XRD patterns (at 2Θ = 38.2° and 44.8°) are sharper for the impregnated material indicating larger Au agglomerates. The introduction of Au during the synthesis strongly enhances the dispersion of Au. AuVNbMCM-41 (HCl)

Intensity, a.u.

10

AuVNbMCM-41 (H2SO4)

0.5

Au

AuVNbMCM-41 (HCl) AuVMCM-41 (HCl) 35

40

45

2Θ ,

50 o

55

60

F (R)

Au/NbMCM-41 (IMP) AuVNbMCM-41 (H2SO4)

0

AuVNbMCM-41 (H2SO4)

AuVMCM-41 (HCl)

AuVNbMCM-41 (HCl)

Au/MCM-41 (IMP) 300 400 500 600 700 800

Wavelenght, nm

Fig. 1. XRD patterns, TEM images and UV-Vis spectra of selected MCM-41 materials.

To obtain the catalysts with high gold dispersion, AuVMCM-41 and AuVNbMCM-41 materials were prepared by COP method with the use of HCl or H2SO4 as pH adjustment agent. As shown in Table 1, the use of HCl leads to the catalysts with lower surface area and pore volume when compared to those of AuVMCM-41 (H2SO4) and AuVNbMCM-41(H2SO4) materials. Moreover, MCM-41 prepared with HCl shows disordering of hexagonal structure. Considering the size and dispersion of gold, it was found on the basis of XRD patterns (Fig. 1) that bigger Au agglomerates are formed on the surface of Au(V,Nb)MCM-41 (HCl). TEM images (Fig. 1) confirm this conclusion. The average size of Au crystallites in AuVMCM-41(HCl) and AuVNbMCM-41(HCl) was 45-50 nm. The application of H2SO4 to adjust pH during the synthesis leads to much smaller gold particles (~20 nm). Moreover, Nb species located in MCM-41 samples plays the role of a structural promoter that decreases the agglomeration of gold. TEM images do not show the smaller Au particles located inside the channels. The use of HCl or H2SO4 during the synthesis of MCM-41 materials influences also the acid-base properties of the catalyst surface studied by AcoAc transformation (Table 1). AuVMCM-41(H2SO4) and AuVNbMCM-41(H2SO4) materials exhibit acidic properties

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(MCP/DMF << 1). The application of HCl during the synthesis significantly increases the basicity of the material (MCP/DMF >>1). There is no doubt that the presence of chlorine near Au species is responsible for a very high basicity of gold-MCM-41, as was indicated earlier for AuMCM-41 [9]. The introduction of V and mainly Nb into MCM-41 together with Au in one pot synthesis diminishes the basicity. The interaction between Au and group five elements in MCM-41 determines the surface properties also in methanol oxidation. Such interaction in AuVNbMCM-41 (H2SO4) results in the highest selectivity to formaldehyde because of the weaker chemisorption of HCHO, whereas bimetallic catalysts (AuVMCM-41(HCl) and AuNbMCM-41(HCl)) are the most active in CO2 formation because of their basicity [5].

3.3. Group V metals oxides - effect of preparation method

o

Au

Au/Nb2O5 (THPC)

Nb2O5 30

4. Conclusions

Au/Nb2O5 (DPU)

500

Intensity a.u.

Gold was introduced into group V metal oxides (V2O5, Nb2O5, Ta2O5) by two methods: via deposition-precipitation with urea and via gold-sol method with THPC as a reducing agent. In XRD patterns of all calcined materials the reflections characteristic of metallic gold are well visible on the catalysts prepared by DP method indicating bigger Au particles than that when gold sol method is used (Fig. 2 -example for Au/Nb2O5) as confirmed by TEM images. The average particle size in the Au/Nb2O5 prepared by DP method is about 20 nm, whereas in the sample prepared by gold-sol method it is of about 6 nm. THPC used during gold-sol method stabilizes the colloid gold solutions and that is why gold particle sizes are smaller and their dispersion is higher.

35

40

2Θ,o

45

Dispersion of gold is better when Au is introduced into Fig. 2. XRD patterns of Nb2O5 samples. ordered mesoporous material in one pot synthesis than in the case of using the impregnation method. Au crystallite size depends on the nature of acid used as pH adjusting agent in one pot synthesis (HCl promotes agglomeration of gold) and on the chemical composition of MCM-41 material (the presence of Nb decreases Au agglomeration because of strong Au-Nb interaction). Dispersion of Au on metal oxides is higher than on MeMCM-41 because of higher concentration of group V metals. Gold-sol method using for the modification of metal oxides gives rise to a higher gold dispersion than deposition-precipitation one. That is why the modification with Au using THPC as reducing agent is recommended. Acid/base properties of mesoporous gold – silica prepared in one pot strongly depends on pH of the synthesis medium. Acidity dominates for AuSBA-3, whereas basicity is characteristic of AuMCM-41. Oxidative properties of Au-MCM-41 catalysts are determined by the preparation methods and the presence of V and Nb.

Acknowledgements Polish Ministry of Science (Grant No. N N204 032536) is acknowledged for the partial financial support of this work. Acknowledge is made also to Johnson Matthey (UK) for supplying HAuCl4.

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References 1. 2. 3. 4. 5. 6. 7. 9.

G.C. Bond, C. Luis, D.T. Thompson, 2006, Catalysis by Gold, Imperial College Press Q. Huo, D.I. Margolese, U. Ciesla, D.G. Demuth, P. Feng, T.E. Gier, P. Sieger, A. Firouzi, B.F. Chmelka, F. Schüth, G.D. Stucky, 1994, Chem. Mater., 6, 1176 M. Ziolek, I. Nowak, 1997, Zeolites,18, 377 I. Sobczak, A. Kusior, J. Grams, M. Ziolek, 2007, Stud. Surf. Sci. Catal., 70, 1300 I. Sobczak, N. Kieronczyk, M. Trejda, M. Ziolek, 2008, Catal. Today 139, 188 C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, 1992, Nature, 359, 710 S. Demirel-Gulen, M. Lucas, P. Claus, 2005, Catal. Today 102–103, 1668. R.M. Dessau, 1990, Zeolites, 10, 205 I. Sobczak, A. Kusior, J. Grams, M. Ziolek, 2007, J. Catal., 245, 259

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

High loaded Ni/SiO2 catalyst for producing ultra-pure inert gas Jung Wha Son,a Songhun Yoon,a Hee Geun Oh,b Dong Young Shin,b Chul Wee Leea,g a b

Green Chemistry Division, KRICT, Daejeon 305-600, S. Korea Korea Pionics Co. Anseong-city, Gyeonggi-do 456-833, S. Korea

Abstract A simple synthesis protocol to prepare highly loaded Ni/SiO2 was developed, based on co-precipitation method using 200L batch type reactor, where kind of raw materials, precipitation rate, aging condition and calcinations/reduction conditions are important factors for determining the quality of catalyst. Coprecipitation method shows better results than impregnation. Their performance of removing impurities such as CO, O2, H2O and H2 in inert gas was evaluated. The physical properties, H2-TPR and chemisorption were attempted to understand the performance of Ni/SiO2. Keywords: Ni/SiO2, coprecipitation, ultra-pure inert gas, metal dispersion, H2-TPR

1. Introduction Large quantities of non-reactive gases such as He, N2, Ar are used for pharmaceutical and electronic industries, particularly during fabrication of semiconductors [1]. These inert gases must be as pure as possible and particularly, they must be substantially free from impurities such as O2, CO, H2O, CO2, H2 etc, which reduce the quality and performance of the semiconductors. A growing number of industries are now requiring gases having impurities concentration of sub ppb level. Generally Ni/SiO2 catalysts containing Ni in amount of higher than 50wt% are used for industrial hydrogenation processes, and it was reported that incorporation of Mg, as a promoter, into Ni/SiO2 can improve its catalytic activity [2]. The objectives of present study is to find optimum conditions for preparing Mg containing Ni/SiO2 with high loading and dispersion and its application for eliminating impurities from N2 gas at room temperature. The relationship between performance of Ni/SiO2 and its physical properties such as BET, H2-TPR, EDS and chemisorptions were discussed and compared. It was found that for preparing desired Ni/SiO2, kind of raw materials, composition of constituents and post treatment conditions such as calcination/reduction procedure are important factors.

2. Experimental 2.1. Sample preparation For preparing Ni/SiO2, nickel nitrate or nickel sulfate, supports such as silica, sodium silicate and precipitating agent, urea, were employed [3]. Typical preparation procedure of sample A is as follows. The reaction was carried out with 200L batch type reactor(BE630 model, Pfaudler Co.) 6510g of Ni(NO3)2x6H2O was dissolved in 90L of distilled water. 2870g of Mg(NO3)2x6H2O was dissolved in 10L of distilled water and it was mixed with the aq. nickel nitrate solution. Then 1220g of sodium silicate and 35L of distilled water was added successively for 1.5h and the solution temperature was

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heated up to 90°C while stirring. When the solution temperature was reached at the desired temperature 5380g of urea was added slowly. The solution temperature was maintained at least 90°C while stirring for 24h, followed by aging at 90°C for 10h without stirring then the solution temperature was cooled down to room temperature and the green precipitate was filtered and washed with hot water. The sample was dried in an oven at 110°C and calcined at 500°C for 5h. The product yield is about 90%. Sample B was prepared by the impregnation method. For preparing final product, the sample was reduced with two different ways. (1) 5%H2/He was passed through the bed at flow rate of 50cc/min at 400°C with ramping of 5°C/min for 3h, (2) 5%H2/He was passed through the bed at flow rate of 50cc/min at 150°C and 400°C for 3h, respectively. At the completion of reduction, the sample was outgassed in N2 for 0.5h at 20°C above the reduction temperature and it was cooled to ambient temperature with the inert gas flowing. For comparison, at least three different samples were prepared. Compositions and constituents of sample used in this study were summarized in Table 1. Table 1. Compositions and constituents of sample attempted in this study. sample

Ni (NO3)2(g)

MgNO3(g)

Urea(g)

support(g)

H2O(g)

pH(4)

(2)

A 6510 2870 5380 1220 135 4.6/7.8 B 6550 2920 5450 1200(3) 130 4.2/7.7 C 6500(1) 2700 5500 1200(2) 130 4.7/7.8 Raw materials of (1)NiSO4x6H2O, (2)sodium silicate and (3)SiO2 (BET 300m2/g, particle size of 5μm) were used. (4)Initial pH after mixing reaction mixture at 90°C /final pH after aging.

2.2. Characterization For elemental analysis, EDS was monitored by Bruker (Model Quantax 200). The BET surface area, pore volume and pore size distribution were measured by N2 adsorption/ desorption at 77K using Micromeritics ASAP 2000. The nickel surface area of the reduced sample was calculated from the amount of H2 chemisorbed on the sample by using Micromeritics ASAP 2010. TPR was measured by Belcat-M of Bel Japan Inc.

2.3. Performance evaluation Prior to testing the performance, the sample was reduced indicated in the text by H2 and the performance of the sample was evaluated by monitoring breakthrough curve for adsorbing impurity gases in N2 gas. The concentration of impurities such as O2, CO, H2O, CO2 was measured by Micro TCD-GC(HP5280), API-MS(Hitachi) and RGA3 (Trace Analytical), respectively.

3. Results and discussion In this study, depending on raw materials used, three different kinds of Ni/SiO2 were prepared. As indicated in Table 2, Ni loading is higher than 60wt%. After calcinations, elemental analysis by EDS indicates that C 1.65, 2.20, 2.31wt%, O 23.7, 26.5, 27.2wt%, Mg 1.28, 2.10, 1.95wt%, Si 1.84, 5.85, 4.60wt%, Ni 71.4, 63.4, 60.6wt% for sample A, B and C, respectively. Only for sample C, 2.57wt% of sulfur was detected due to NiSO4. The precipitation procedure is reproducible. After many synthesis attempts, Ni(NO3)2, Mg(NO3)2, urea, NaSiO3 seems to be a desirable staring materials with relative molar ratio with 5.2moles of Ni(NO3)2, 2.6moles of Mg(NO3)2, 20 moles urea, 1.0moles NaSiO3 and 2.0moles H2O. Increasing the concentration of urea and nickel nitrate resulted in increased nickel deposition, but leading to decreased dispersion. The BET surface area of Ni/SiO2 was in the range of 120~140m2/g which is in a good agreement with the previous work [4]. Pore volume is in the range of 0.22~0.28cc/g and average

High loaded Ni/SiO2 catalyst for producing ultra-pure inert gas

341

pore size is in the range of 5.8~8.0nm. H2O content was measured in the range of A 4.2~6.1 wt% by thermal analysis. Figure 1 shows H2-TPR curve of sample A, B and C. Sample A shows a reduction peak centered at 310°C, followed by a hump at 460°C. B Hydrogen uptakes centred at 450°C and 700°C for sample B and C, respectively, are 100 200 300 400 500 600 700 800 900 observed. This indicates that main reducTemperature ( C) tion temperature of Ni2+ to Nio is clearly different each other. High temperature Fig. 1. H2-TPR of three different Ni/SiO2. peak of sample C can be assigned to the Pretreatment: 450oC for 2h with He. reduction of NiO having strong interaction with support, i.e. SiO2 cause an increase in the reduction temperature of the NiO phase. However, when sodium silicate was employed as a support, the reduction temperature goes down clearly. TPR provides that reduction proceeds in one step, uniform phase composition of nickel and influenced by the source of NiO and supports [5]. Calcination in air prior to reduction has little effect on the resulting metal dispersion. This implies that the reduction precursor, presumably nickel hydrosilcicate, is not drastically altered by the air treatment. Chemisorption data such as metallic surface area and crystallite size were summarized in Table 2. Metallic surface area is in the range of 0.31~17.7 m2/g Ni-metal after calcination. Although reduction of calcined sample at 400°C shows only slight improvement on metallic surface area and crystallite size, however, as shown in Table 2, step-wise reduction with 5%H2/He at 200°C, followed by at 400°C for 3h shows a remarkable change. With this process, metallic surface area is increased by about 65% and crystallite size is reduced by about 60%. This indicates that reduction under mild conditions seems to be effective for improving metal dispersion. The basic catalytic reaction can be suggested as follows. For removal, Ni + CO ÆNiCO, 2Ni+O2 Æ 2NiO, NiO+H2Æ Ni+H2O and for regeneration, NiCO2 + 4H2 Æ CH4 + 2H2O + Ni, NiCO + 4H2 Æ CH4 + H2O + Ni

TCD (uV)

C

o

Table 2. Chemisorption data of various sample prepared in this work. crystallite size (nm) Ni loading metallic surface area (m2/g Ni) (wt%) calcined reduced* calcined reduced* A 17.7 18.5(1); 29.5(2) 38.1 36.4(1); 23.0(2) 62.8 B 2.73 2.77(1); 4.21(2) 246 232(1); 150(2) 63.5 C 0.31 0.22(1); 0.51(2) 1257 1170(1); 895(2) 61.3 *Reduction conditions: (1)5%H2/He was passed through the bed at flow rate of 50cc/min at 400°C for 3h, (2)5%H2/He was passed through the bed at flow rate of 50cc/min at 150°C and 400°C for 3h successively. sample

The reduction process generates almost 2 mol of water for every mole of nickel so that the presence of moisture may impede the thermodynamic equilibrium conversion of Ni. The reduction step may follow decomposition of the nickel silicate to the oxide, perhaps controlled by the same water removal. Varying the reduction condition was found to be a very sensitive and reproducible way of controlling the amount of reduced Ni and its dispersion. For screening the performance of the catalysts, model gas containing 50ppm of O2 in N2 gas was employed. Figure 2 shows the breakthrough point of sample A, B and C during adsorbing O2 impurities in N2 gas, which was determined by micro TCD-GC. Sample A shows the best performance indicating that 50ppm of O2 was reduced for 35h.

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Breakthrough point of 35h, 23h and 6h was detected for sample A, 0 .8 C C and B, respectively. Sample A shows the best performance 0 .6 B indicating that 50ppm of O2 was 0 .4 removed at least for 35h. This 0 .2 performance seems to be closely A related with metal dispersion and 0 .0 0 5 10 15 20 25 30 35 40 size. Highly dispersed shows higher T im e (h ) activity. Low activity of sample B is Fig. 2. Breakthrough curve for adsorbing O2 in N2 that NiO species is not fully reduced at RT with 50ppm of O2, WHSV=12,000h-1. to Nio and the active site is not fully Pretreatment: calcination under air at 500°C for 2h, developed under the reduction at reduction with 5%H2/Ar at 400°C for 3h. C1/C0 400°C. In order to evaluate the indicates O2 conc. ratio before and after reaction. performance of the sample in a real condition, the calcined powdered sample was mixed with stearic acid and graphite for making a cylindrical type pellet of 3mm diameter and 3mm length. As shown in Table 3, for sample A and B, impurities of CO, CO2, H2O and O2 of <100ppb, <900ppb, <100ppb, and <100ppb, respectively, was reduced to lower than 1ppb for 96h and 7h, respectively. However, in the case of sample C, the activity is very poor and a large amount of SO2 gas was detected which is closely related with NiSO4 as a starting material for sample C.

C1/C0

1 .0

Table 3. Concentration change of impurities before and after reaction over Ni/SiO2 and breakthrough points. sample

Concentration of impurities(ppb)* Breakthrough Before reaction After reaction point (h) CO CO2 H2O O2 CO CO2 H2O O2 A <1 <1 <1 <1 96 < 100 < 900 < 100 < 100 B <1 <1 <1 <1 7 C >100 >900 >200 >100 *determined by API-MS and RGA3 and performance was tested at room temperature.

4. Conclusions Obtaining high metallic surface area is the most critical factor for better Mg containing Ni/SiO2 catalyst, where Mg is known as a promoter, under the similar Ni loadings. SiO2 and NiSO4 can not be a desirable raw materials because it produce large Ni crystllite due to the agglomeration of Ni during synthesis and deposit of sulfur in Ni/SiO2. Successive reduction at 150°C and 400°C gives higher metallic surface area.

References [1] [2] [3] [4] [5]

H. Saxena et al., 2006, US Patent Application, US2006/0210454 A1. J.-K. Jeon et al., 2005, J. Ind. Eng. Chem. 11, 83-87. H. K. Oh et al., Korean Patent Application No. 10-2009-0035336; 10-2009-0097304. J.-K. Jeon et al., 2008, Chem. Eng. J. 140, 555-561. M. Pilar Gonzalez-Marcos et al., 1997, J. Mol. Catal. A. 120, 185-196.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

The effect of 3d-cation modification on the properties of cordierite-like catalysts E.F. Sutormina, L.A. Isupova, N.A. Kulikovskaya, A.V. Kuznetsova, E.I. Vovk Boreskov Institute of Catalysis, pr. Ac. Lavrentieva 5, Novosibirsk 630090, Russia

Abstract The MnOx-modified cordierite-like honeycomb catalysts 2(Mg1-x Mnx)O·2Al2O3·5SiO2 (x=0-1) were prepared by mechanochemical method from clay, talk, alumina and MnO2. Catalysts were characterized by BET, XRD, XPS, and H2-TPR. Catalytic activity was studied in ammonia oxidation to NO. Phase composition, texture and catalytic activity of prepared monoliths were shown to depend on the MnOx content and preparation conditions (preliminary mechanical treatment, time and calcination temperature). The fully substituted catalyst (2MnO·2Al2O3·5SiO2) was shown to have the highest selectivity to NO at 900°C, atmospheric pressure and millisecond contact time. Keywords: cordierite, 3d-cations, honeycomb catalysts, ammonia oxidation

1. Introduction Cordierite (2MgO·2Al2O3·5SiO2) is a widespread commercial material for high temperature catalyst applications due to very low coefficient of thermal expansion (~2*10-6 1/K), high resistance to thermal shock and good mechanical properties [1]. Advantages of this material are also its low cost and good rheological properties allowing manufacturing monoliths with various cell shapes and sizes [2]. Cordierite supports have been found to be the most appropriate ceramic material for combustion applications, for catalytic neutralization of automotive exhaust gases, for the selective catalytic reduction (SCR) of NOx and others [3]. The structure of cordierite-like materials is based on a frame of four- and sixmember rings composed of aluminum-silicon-oxygen tetrahedral. Mg cations are localized in octahedrons between tetrahedrons of six- and four-member rings. A frame structure allows to obtain modified materials with 3d cations instead of Mg2+ having similar ionic radiuses, for example, Mn2+, Co2+, Ni2+, Cu2+, Zn2+ [4]. The presence of transition elements is of interest because these elements may strongly influence on the cordierite properties and catalytic activity. This paper deals with preparation and characterization of frame structured cordieritelike honeycomb catalysts modified with manganese dioxide that is active in hightemperature NH3 oxidation to NO. We study the effect of 3d-cation addition on the formation of cordierite structure, texture, surface morphology and activity of modified cordierites in ammonia oxidation reaction. The effect of preparation condition (type and duration of mechanical treatment, calcination temperature) was studied as well.

2. Experimental A series of frame structured modified cordierite-like 2(Mg1-xMnx)O·2Al2O3·5SiO2 honeycomb catalysts was prepared from natural components (talc, kaolin, alumina) and MnO2. The mixture of raw materials taken in a stoichiometric ratio was treated in a disintegrator DEZI-15 characterized by productivity up to 200 kg/h at speed of rotation

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3000 rpm. Activation in planetary ball mill (PBM) or vibroball mill (VBM), described in [1], was used in some cases to study the effect of mechanical treatment. Particle size distribution was characterized by laser diffraction. The average particle size was measured to be about 12, 4.6, 18 μm for disintegrator, PBM and VBM activators, respectively. Then powders were placed in a Z-mixer (volume - 0.5 l, rotation speed 1370 rpm) and, after water adding (24–26% humidity) and mixing during 30–40 min, a uniform plastic pastes were formed. The plastic strength of the paste was determined using Rebinder plastometer and amounted to 0.56 MPa. Monoliths were prepared by extruding the pastes through hexahedral prism with side of 6 cm with a triangle channels side of 2.5 mm and wall thickness of 0.4 mm (channels density was about 200 channels per square inch). The monoliths were first dried at room temperature for 24 h, than at 350°C for 4 h and at 1050–1250°C for 4–8 h. The structural and textural features of frame structured cordierite-like catalysts were studied by XRD, XPS, TPR, thermal analysis and adsorption measurements. The X-ray diffraction (XPD) patterns were acquired with a URD-63 diffractometer using CuKα-radiation. The 2θ scan region was 5–70°. The degree of cordierite formation was controlled based on intensity of 2θ = 10.4° peak. The X-ray photoelectron spectra (XPS) were acquired in a SPECS spectrometer, which was calibrated with respect to the binding energies of the reference levels Au 4f1/2 (84.0 eV) and Cu 2p3/2 (932.7 eV) using AlKα radiation (hν = 1486.6 eV). H2-TPR experiments were carried out for samples pretreated in O2 at 500°C using feed containing 10 vol.% H2 in Ar at flow rate 40 ml/min and temperature ramp 10°C/min from 25°C to 900°C. Particle size was 0.25–0.5 mm; the catalyst weighed 100 mg. During the experiment H2O was frozen out at -80°C. The hydrogen concentration was determined using a thermal conductivity detector. The pore structure was characterized by the high pressure mercury porosimetry (HPM) using an Auto-Pore 9200 machine, and the specific surface area was determined by the routine BET procedure using the Ar thermal desorption data. Catalytic properties of micromonolith (samples 22 mm in diameter and 45 mm in length) were tested in ammonia oxidation process under atmospheric pressure at 900°C. The reaction gazes (5% ammonia in the air preheated at 450°C) were flowed through the catalyst with 8 l/min flow rate providing reaction time about 0.01 s. An on-line UV spectroscopic method has been used for quantitative analysis of reaction gazes (NH3, NO and NO2) described elsewhere [5]. N2 is proposed to be the main by-product.

3. Results and discussion o

o

o o 10,0

Intensity

XRD studies revealed that modification of cordierite with manganese oxide results in formation of mainly cordierite-like phase at calcination temperature about 1150°C, magnesium silicate, quartz and Mn2O3 being the minor phases (Figure 1). It should be mentioned that the usual ceramic preparation procedure does not lead to cordierite crystallization at this temperature. The preliminary mechanical treatment of the raw materials considerably reduces the temperature and time of

o - (Mg,Mn)2Al4Si5O18 + - Al6Si2O13 s - SiO2 (quartz) * - Mn2O3 Δ - Al2O3

o

3.1. Phase composition x=1

10,5

o

11,0

+oos

Δ

s

+

*

Δ

+

o oo * Δ

o o + oΔ * s

Δ

+

x=0.75 x=0.5 x=0.25 x=0 10

20

30

2 theta

40

50

60

Figure 1. XRD data for MnO2 -modified cordierites calcined at 1150°C (4 h) with different substitution degree (x).

The effect of 3d-cation modification on properties of cordierite-like catalysts

345

cordierite synthesis during further calcination probably due to not only disintegration (size decreasing) effect but also due to the effect of mechanochemical activation [6]. The increase of calcination time (from 4 to 8 h) or/and temperature (from 1050 to 1250°C) as far as the decrease of particle size during the mechanical treatment (DVBM > DDEZI > DPBM) leads to better cordierite crystallization. The modification of cordierite with manganese oxide (only up to substitution degree x=0.5) additionally promotes cordierite phase formation as presented in Table 1. A shift of diffraction peaks corresponding to the increase of cordierite unit cell volume was found (Figure 1) for these samples due to the possible isomorphic substitution of Mg2+ ions by Mn2+ ions having larger ionic radious [7].

3.2. Texture of frame structured oxides BET data for some prepared modified cordierites presented in Table 1 indicate that all samples have low specific surface area (0.02-4.9 m2/g). It was found that increase in calcination time and temperature as well as in MnOx contents results in lower specific surface area and internal pore volume that usually leads to better thermal shock resistance and high durability of monoliths. The prepared cordierite-like materials have low open porosity and the mean pore diameter does not exceed 1 μm. Table 1. Physicochemical properties of MnOx-modified cordierites (x is substitution degree) and its catalytic properties in ammonia oxidation reaction at 900°C. x 0

activator type DEZI

Tcal, °C 1150

0.25

DEZI

1150

0.5

DEZI

1050

0.5

DEZI

1150

0.5

DEZI

1250

0.75

DEZI

1150

1

VBM

1150

1

DEZI

1150

1

PBM

1150

phase composition Mg2Al4Si5O18 (41.3%), Al6Si2O13, SiO2 (α-Al2O3) (Mg, Mn)2Al4Si5O18 (68.9%), Al6Si2O13, SiO2, α-Al2O3, Mn2O3 (Mg, Mn)2Al4Si5O18 (20.5%), Al6Si2O13, SiO2, Mn2O3 (Mg, Mn)2Al4Si5O18 (73.3%), Al6Si2O13, SiO2, Mn2O3 (Mg, Mn)2Al4Si5O18 (91.1%), Al6Si2O13, SiO2, α-Al2O3 (Mg, Mn)2Al4Si5O18 (60.0%) Al6Si2O13, SiO2, Mn2O3 Mn2Al4Si5O18 (39.2%) Al6Si2O13, SiO2, Mn2O3, Mn7SiO12 Mn2Al4Si5O18 (45.0%) Al6Si2O13, SiO2, Mn2O3 Mn2Al4Si5O18 (49.0%) Al6Si2O13, Mn2O3

3.3. X-ray photoelectron spectroscopy and H2-TPR

SBET, m2/g 4.9

Vpore, cm3/g 0.194

XNH3 % 69.0

YNO % 12.3

3.6

0.181

93.7

28.8

4.3

0.280

94.1

49.5

2.1

0.174

96.4

42.7

0.6

0.073

85.2

36.1

1.2

0.156

95.2

52.2

1.2

-

93.0

41.8

0.4

0.145

95.0

65.4

0.02

-

93.4

77.4

X-ray photoelectron spectroscopy was applied for characterization of the surface concentrations and chemical states of manganese. XPS results indicate that the samples exhibit a systematic shift in the Mn 2p3/2 binding energy position (from 641.6 to 641.9 eV) with increasing of Mn content. This value is quite lower in comparison with that of the pure MnO2 which is generally between 642.2 - 642.6 eV [8]. According to the literature data, the values of the free width at half maximum (FWHM) of the Mn 2p3/2 emissions of the samples are higher than that of the manganese oxide having well defined single oxidation states. It is thus evident that manganese could be present in more than one oxidation state or that the same ions are located in different coordination

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environments. Based on X-ray data, the most probably that increase in MnOx content leads to increase both Mn+2oh (641.6 eV, cordierite) and Mn+3oh (641.9 ev, Mn2O3). XPS data revealed also that no noticeable surface Mn ions enrichment. A linear Mn 2p3/2/Al 2p ratio was found to be a function of Mn content. It was found that MnOx-modified cordierites did not significantly reduce under the action of hydrogen up to 900°C. The ratio H2/Mn was measured to be 0.03-0.1 (very low) for different samples. Thus, a significant interaction of manganese oxide with cordierite structure or its impenetrability for reduction may occur.

3.4. Catalytic activity The non-modified cordierite shows low NH3 conversion (XNH3) and NO yield (YNO) in ammonia oxidation reaction (Table 1). For modified cordierites, the ammonia conversion was found to be about 95% at 900°C that is in a good agreement with our calculations for ammonia oxidation occurring in diffusion limited regime. In this case the NH3 conversion on the monoliths depends mainly on their geometrical parameters under the same reaction conditions; therefore the NO yield could be taken as catalytic activity for comparison. NO yield was found to be not depending on cordierite crystalline degree and increase with increase of manganese oxide. The fully substituted catalysts (x=1) were found to have the highest NO yield at studied conditions, as well as modified monolith (x=0.5) calcined at lower temperature (1050°C). Hence the catalytic activity of the modified cordierites depends mainly on admixture of manganese oxide, while catalyst durability depends on the cordierite crystallinity.

4. Conclusion The series of MnOx-modified cordierite-like catalysts 2(Mg1-xMnx)O·2Al2O3·5SiO2 (x=0-1) was prepared by mechanochemical method. It was found that the increase of calcination time and temperature, the decrease of particle size during the mechanical treatment, the modification with manganese oxide (up to x=0.5) leads to better cordierite crystallization followed by reduction of specific surface area and internal pore volume of the samples. The fully substituted catalysts were shown to have the highest selectivity to NO at studied conditions, most probably, due to the higher content of Mn2O3 (active oxide) on the catalysts surface.

References 1. 2. 3. 4. 5. 6. 7. 8.

E.G. Avakumov, A.A. Gusev, 1999, Cordierite – an advanced ceramic material, Novosibirsk, SB RAS. [in russian]. L. Sheppard, in “Ceramic Transactions, Porous Materials,” edited by K. Ishizaki, L. Sheppard, S. Okada, T. Hamasaki and B. Huybrechts, 1993, 3. M.A. Keane, J. Mater. Science, 38 (2003) 4661. G. Pourroy, J.L. Guille, P. Poix, Chemistry of Materials, 2 (1990) 101. L.A. Isupova, E.F. Sutormina, N.A. Kulikovskaya, L.M. Plyasova, N.A. Rudina, I.A. Ovsyannikova, I.A. Zolotarskii, V.A. Sadykov, Catal. Today, 105 (2005) 429. E.T. Deviatkina, E.G. Avvakumov, N.V. Kosova, N.Z. Liakhov, Inorg. Mater. (Rus) 30 (1994) 237 [in russian]. R.D. Shannon, Acta Cryst. A32 (1976) 751. H.W. Nesbitt, D. Banerjee, American Mineralogist, 83 (1998) 305.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Large-scale synthesis of porous magnetic composites for catalytic applications Horacio Falcona, Pedro Tartajb, Aldo F. Rebolledob, Jose M. Campos-Martína, Jose L. G. Fierroa, Saeed M. Al-Zahranic a

Instituto de Catálisis y Petroleoquímica, CSIC, Marie Curie, 2 Cantoblanco, 28049 Madrid, Spain b Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco, 28049 Madrid, Spain c King Saud University, Riyadh, Saudi Arabia

Abstract We here report the large scale synthesis of porous magnetic composites that consist of superparamagnetic iron oxide nanorods coated with a porous silica shell. Essentially, the methodology combines a modified carbonate route with a layer-by-layer electrostatic self-assembly method and the controlled hydrolysis of alkoxides in the presence of porogens. By using this approach, we have prepared magnetic composites with a BET specific area as high as 500 m2g-1 and a magnetic moment high enough as to collect easily nanoparticles with a commercial magnet. This catalyst design appears particularly suited for liquid-phase reactions. Keywords: nanoparticles, magnetic composites, supermagnetic iron oxide, Fe3O4

1. Introduction The liquid-phase catalytic reactions of organic compounds constitutes a wide class of important processes in the chemical industry. In addition to the economic advantages such as high yield, improved selectivity and mild reaction conditions, these processes have been largely benefited by the design of new catalysts. Heterogenous catalysts used in these processes should have particle size small enough as to minimize the mass transportation limitations, which are especially important in liquid phase reactions. However, when the particle size is very low separation from the reaction medium is often difficult, if not impossible. Magnetically-driven separation makes the recovery of catalysts in a liquid-phase reaction much easier than by cross-flow filtration and centrifugation, especially when the catalysts are in the sub-micrometer size range [1-3] Developing methods for the large scale synthesis of magnetic composites with a large surface area and an accessible porosity is, therefore, highly attractive for the generalization of the magnetic separation technique to heterogenous catalysis. We here report the large scale synthesis of porous magnetic composites using a methodology that combines a modified carbonate route with a layer-by-layer (LBL) electrostatic self-assembly method and the controlled hydrolysis of alkoxides in the presence of porogens.

2. Experimental Section 2.1. Preparation of catalysts The synthetic pathway used for the preparation of porous magnetic composites is schematically represented in Fig. 1. In a typical synthesis, Na2CO3 0.45 M and FeSO4 0.075 M were stirred at 40°C under airflow. The formed precipitate (goethite) was

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centrifuged and dried. The solid obtained, was dispersed in water and HNO3 with a concentration of 6 g/L and pH = 4 (positively charged), then the solution was centrifuged and the solid was dried. Secondly, the solid obtained above was dispersed in a negatively charged polyelectrolyte (poly(styrenesulfonic acid-co-maleic acid) sodium salt, (PSS)) aqueous solution with a concentration 2 g/L, then ZrOCl2 were added to reach pH = 3, the solution was centrifuged and the solid was dried. Thirdly, the solid obtained, was dispersed in a aqueous solution of PDDA (poly(diallyldimethylammonium chloride)) at pH = 12, followed by addition of sodium silicate (2 g/L), the solid obtained was calcined at 300°C, obtaining hematite coated with ZrO2 and SiO2. Finally, these nanoparticles were dispersed in a solution of tetraethyl orthosilicate (TEOS) and octadecyltrimetoxysilane (C18TMS) (this agent is responsible of nanoparticle porosity after calcination), TEOS/C18TMS molar ratio = 5 in ethanol/water/ammonia, after that the solution was centrifuged and the solid was calcined at 400°C. Two samples with different content in silica FS50 and FS75 (50 and 75 wt.%) were obtained. + pH=4

+

+

+

+

goethite

PSS

-

-

- -

-

hematite

peptization

300 ºC

goethite

goethite

pH=3

PDDA

ZrO 2 ZrO 2

ZrO 2

ZrOCl2

goethite

silicate carb. bubbling

pH=12

goethite

ZrO 2

EtOH/TEOS/ C18TMS/400 ºC

H2

hematite

magnetite

ZrO 2 porous SiO 2

ZrO 2 porous SiO 2

Figure 1. Schematic representation of the synthetic pathway used for the preparation of porous magnetic composites.

2.2. Sample Characterization Morphology, particle size and EDX analyses of the obtained samples were studied with a TEM JEOL microscope working at 200 keV. Phase identification was performed by X-ray analysis. X-ray diffraction (XRD) patterns were collected from 5 to 70° (2θ) by using a Bruker D8 Advance instrument with CuKα radiation (λ = 0.15406 nm) and a SOLX detector operating at 40 kV and 30 mA. Nitrogen adsorption and desorption isotherms were performed at –196°C in a Micromeritics ASAP 2010 volumetric adsorption system. Temperature-programmed reduction (TPR) experiments were carried out on a semiautomatic Micromeritics TPD/TPR 2900 apparatus. Reduction profiles were obtained with 50 mg of sample and a of 50 mL/min flow of 5% H2/95% Ar. XPS were acquired with a VG Escalab 200R spectrometer equipped with a hemispherical electron analyzer and Al Kα (hν = 1486.6 eV) X-ray source. The powder samples were pressed into Al holders and then either degassed at 300°C. Magnetization curves were obtained by using a vibrating magnetometer (VSM, Oxford) up to 5 T.

3. Results and discussion The thermal reduction behavior of these two samples was followed by TPR (Fig. 2). Several components are present in the TPR profile, the first peak observed at 370°C corresponds to the reduction of Fe2O3 to magnetite (Fe3O4) [4]. The position of this peak

Large-Scale Synthesis of Porous Magnetic Composites for Catalytic Applications

349

does not depend on the silica content which suggests that the external highly accessible porous silica layer is preserved at these temperatures (porosity is not lost during the thermal treatment). Higher temperature reduction peaks correspond to further reduction likely of this phase to FeO and/or Fe2SiO4 (500-650°C) and to α-Fe (800-900°C) [4]. The different temperature at which these peaks appear and the change in intensity could simply reflect both the loss of porosity and the different probability for iron (II) silicate formation associated with the different content in silica. As we are interested to obtain air stable magnetic particle, it is better to transform the hematite into an air stable iron oxide spinels (Fe3O4/γ-Fe2O3) than the complete reduction to metallic iron. Thus, these nanoparticles have been reduced with hydrogen at 400°C. The formation of the iron oxide spinel was confirmed by XRD. Diffraction patterns do not show the presence of other iron oxide species. TEM pictures (Fig. 2) clearly show the formation of core-shell particles. The core (darker contrast) corresponds to the iron oxide spinel while the shell corresponds to the silica layer. We can also see that the sample with the lower content in silica still preserve the rod-like morphology (FS50) while the sample with the higher content consists in iron oxide nanorods encapsulated in silica spheres (FS75), due to the natural trend of silica to form spherical particles.

535 370

H2 consumption (au)

655

810

FS75

655 605

370

805

900

FS50

0

200

400

600

800

1000

Temperature (ºC)

Figure 2. TPR and TEM analysis of porous magnetic composites.

The full coverage of nanoparticles by silica layers was studied qualitatively by XPS. As the XPS technique has a high surface sensitivity and the electrons analyzed come only from surface atoms (1-2 nm), it implies that a low signal of iron means necessarily a good coverage of the nanoparticles. According to the synthesis approach undertaken in this contribution, the absence of iron signal in the samples synthesized is a clear indication of the complete coverage of the iron core by a thick, uniform silica layer. Textural properties of the samples were derived from the N2 absorption isotherms. Both samples have a high specific area and mesostructured porosity centered around

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4-5 nm, the pore size distribution were very sharp in both samples. The different specific surface area simply reflects the different content in porous silica. In any case, the textural properties clearly indicate that both samples are appropriate for use as catalyst support (Table 1). Table 1. Specific surface area and textural properties of porous magnetic composites. SBET (m2g-1)

Pore volume (cm3g-1)

Pore diameter (nm)

FS50

309

0.44

5.6

FS75

539

0.59

4.3

Finally magnetic properties of the two samples were measured (Fig. 3). Samples have an adequate magnetic moment (i.e. they can be guided by an external magnetic field) and show superparamagnetic-like behavior (zero coercivity) which is ideal for magnetic separation. The higher magnetic moment of sample FS50 is associated with its lower content in silica (i.e. higher content in magnetite).

Figure 3. Magnetization curves of samples as a function of the applied field. The inset shows a zoom at low fields. Samples clearly show superparamagnetic-like behavior (zero coercivity field).

4. Conclusions We have developed a method for the large scale synthesis of superparamagnetic composites. These superparamagnetic composites have a high specific surface area, a highly accessible porosity, a good magnetic response and present a surface easily to be functionalized. Thus, these composites are excellent candidates to develop recoverable catalysts employed in liquid-phase reactions.

Acknowledgments We thank to our research sponsor The King Saud University, Riyadh (Saudi Arabia).

References 1. 2. 3. 4.

P. Tartaj, Eur. J. Inorg. Chem. 2009, 333–343. A.F. Rebolledo, O. Bomatí-Miguel, J.F. Marco, P. Tartaj, Adv. Mater. 2008, 20, 1760–1765. W. Zhao, J. Gu, L. Zhang, H. Chen, J. Shi, J. Am. Chem. Soc. 2005, 127 (25), 8916–8917. H. H. P. Yiu, M. A. Keane, Z. A. D. Lethbridge, M. R. Lees, A. J. El-Haj and J. Dobson, Nanotechnology (2008), 19, 255606–255612.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Preparation of gallium oxide photocatalysts for reduction of carbon dioxide Hisao Yoshida* and Kazuki Maeda Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan, *[email protected]

Abstract A series of gallium oxide samples prepared by homogeneous precipitation method consisted of homogeneous granules of short rod-like morphology with similar size; 300500 nm in diameter and 1-2 μm in length. Calcination at various temperatures provided two polymorphs with various specific surface areas and crystallites sizes. The gallium oxide samples of low specific surface area photocatalytically promoted reduction of carbon dioxide by methane and non-oxidative coupling of methane, while those of high specific surface area additionally promoted consecutive photocatalytic reduction of carbon dioxide by produced hydrogen to give carbon monoxide selectively. Keywords: homogeneous precipitation method, gallium oxide, photocatalyst

1. Introduction The development of methodology for CO2 conversion is an important task for our sustainable future, and it is desired to reduce CO2 into other useful chemical compounds by using natural energy and renewable resources. Photocatalytic reduction of CO2 with methane (referred to as PRCM) to yield synthesis gas (CO and H2) has attracted our attention because it has a potential to reduce the CO2 by using solar energy and biomethane. Recently, it was found that gallium oxide could promote the PRCM upon UV light irradiation at moderate temperatures [1]. In the present study, we examined a homogeneous precipitation method to prepare a series of gallium oxide photocatalysts for the PRCM and discussed the merits of employing this preparation method.

2. Experimental Gallium oxide samples were prepared by a homogeneous precipitation method. Start reagents were gallium nitrate (Ga(NO3)3·nH2O, Kojundo, 99.999%) and urea (CO(NH2)2, Kishida, 99.0%). An aqueous solution (400 ml) of gallium nitrate (4 g) and urea (6 g) was gradually heated up to 353 K with magnetically stirring, and it was kept at the same temperature until the pH value became higher than 6 (it took about 5 h) to obtain precipitate, then it was cooled down to room temperature. The precipitate was filtered and washed by distilled water (total 500 ml), and then dried at 353 K overnight. The obtained powder was ground by a mortar and calcined at various temperatures (8231473 K) for 6 h in air to obtain gallium oxide samples. They are referred to as Ga2O3(x), where x indicates the calcination temperature. The samples were characterized by SEM (JASCO JSM-6330F, with Pt-coating), XRD (Rigaku RINT2500, Cu Kα) and N2-adsorption (Quantachrome, Monosorb). The photocatalytic reaction test was carried out in a closed system equipped with a vacuum system, in the similar way to the previous report [1]. Before the reaction test, the sample (0.2 g) was pretreated in O2 (13.3 kPa) and in vacuo at 673 K for 1 h each. Then, the

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sample was photoirradiated by a 300 W Xe lamp in the presence of CO2 and CH4 (200 μmol each) at 523 K for 3 h. The products were analyzed by GC-TCD and GC-FID.

3. Results and discussion 3.1. Characterization of catalysts SEM image for the Ga2O3(823) sample shows well-defined rod-like granules of similar size in the range of 300-500 nm in diameter and 1-2 μm in length (Fig. 1a) and of multilayers (Fig. 1a). This characteristic homogeneous morphology should be originated from the mechanism of the homogeneous precipitation method, in which the urea in the aqueous solution reacted with water to release NH3 and the pH would gradually and homogeneously increase to precipitate the uniformed precursors. This would be different from the cases of conventional methods by mixing aqueous solution of ammonia with that of gallium source. The calcination above 923 K formed many holes on the granules (e.g. Fig. 1b'), suggesting a structural change. However, a drastic change of the morphology was not observed. Even after the calcination at higher temperature such as 1373 K, the morphology of the granules maintained without a severe sintering (Fig. 1d).

(a')

(a)

100 nm

1 μm (b')

(b)

1 μm 100 nm (c')

(c)

1 μm

100 nm (d')

(d)

1 μm

100 nm

Fig. 1. SEM images of the representative Ga2O3 samples prepared by the homogeneous precipitation method; (a, a') Ga2O3(823), (b, b') Ga2O3(973), (c, c') Ga2O3(1173) and (d, d') Ga2O3(1373).

353

Preparation of gallium oxide photocatalysts for reduction of carbon dioxide

(c) β-phase

(b)

α-phase

(a) 10

20

30

40

50

60

2θ / degree

70

β-phase

30

80

(a)

(b)

60

20 40 10 20

0

773

973

1173

1373

Crystallites size (XRD) / nm

Intensity

(d)

Specific surface area (BET) / m2g-1

α-phase 2000 cps

0

Calcination temperature / K

Fig. 2. (left) XRD patterns of the representative Ga2O3 samples prepared by the homogeneous precipitation method; (a) Ga2O3(823), (b) Ga2O3(973), (c) Ga2O3(1173) and (d) Ga2O3(1373). Fig. 3. (right) Specific surface area (a) and average crystallites size (b) of the Ga2O3 samples calcined at various temperatures.

Figure 2 shows XRD profiles of the representative samples. The Ga2O3(823) and Ga2O3(873) samples were pure α-Ga2O3, while the samples calcined at 923 K and higher temperatures were pure β-Ga2O3. The formation of the holes observed by SEM would correspond to the transformation from α-phase to β-phase. With increasing the calcination temperature, the diffraction line intensity increased, indicating the growth of β-Ga2O3 crystallites. Figure 3 shows the BET specific surface area and the average crystallites size (calculated from XRD) of the samples calcined at various temperatures. The samples calcined at 923-1273 K exhibited the similar specific surface area to each other around 15-18 m2g-1. In the case of β-Ga2O3 prepared by a conventional reverse-strike method, where an aqueous solution of Ga(NO3)3 was added to an aqueous ammonia to yield the precipitation, the specific surface area of the obtained β-Ga2O3 drastically decreased from 17 m2g-1 to 12 m2g-1 with an increase of the calcination temperature from 1073 K to 1273 K, corresponding to 29% reduction [2]. In the present case, the corresponding reduction was 17% (from 18 m2g-1 to 15 m2g-1), which might be due to the stable rodlike multi-layered morphology. The heating at 1373 K and higher temperature drastically reduced the BET specific surface area and increased the crystallites size.

3.2. Photocatalytic reaction In the photocatalytic reaction over each sample, CO, H2 and hydrocarbons such as C2H6 and C2H4 were obtained. The representative results are shown in Table 1. The CO and H2 should be mainly produced through the photocatalytic reduction of CO2 with methane (the PRCM, eq. 1), and C2H6 and other hydrocarbons should be produced through the photocatalytic coupling of methane (PCM, eq. 2) and consecutive dehydrogenation, as previously reported [1]. CO2 + CH4 → 2CO + 2H2 2 CH4 → C2H6 + H2

(1) (2)

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Table 1. Results of the photocatalytic reaction at 523 K over the representative Ga2O3(x) samples. Entry

Sample

1

Yield of Product/ μmol

Reaction selectivity b (%)

CO

H2

HC a

α-Ga2O3(823)

3.5

0.20

0.041

50

3

47

2

β-Ga2O3(973)

5.1

1.3

0.42

48

15

37

3

β-Ga2O3(1173)

5.8

1.8

0.41

51

14

35

4

β-Ga2O3(1273)

3.9

4.6

0.50

79

21

0

5

β-Ga2O3(1473)

1.5

1.3

0.17

65

22

12

PRCM

PCM

PRCH

a Hydrocarbons such as C2H6, C2H4, C3H8, C3H6 and C4H10. The major product was C2H4 for entry 1 and C2H6 for entries 2-5. b Calculated from the product yields.

However, the amount of H2 was clearly less than that of CO especially on the samples calcined at lower temperatures (Table 1, entries 1-3). From the product distribution, it was proposed that the produced H2 consecutively reacted with CO2 to produce CO, i.e., photocatalytic reduction of CO2 with hydrogen (referred to as PRCH, eq. 3). In a separate experiment, we confirmed that CO could be photocatalytically produced from CO2 and H2 on the Ga2O3 sample, which was consistent with the literature [2]. CO2 + H2 → CO + H2O

(3)

The α-Ga2O3(823) sample of the high specific surface area showed a high CO selectivity, where almost all H2 that might be once produced via the PRCM with CO formation would be consumed for the PRCH to produced CO more. On the βGa2O3(973) and β-Ga2O3(1173) samples of the high surface area, the three reactions (eqs. 1-3) proceeded, while on other β-Ga2O3 samples calcined at higher temperatures the PRCH was suppressed. Thus, it is suggested that some kinds of the surface sites such as low-coordination sites or surface hydroxyl groups would act as the active sites for the consecutive PRCH. This consists with the fact that the PRCH was not observed on the commercially obtained β-Ga2O3 sample of low surface area (2 m2g-1) [1]. The high reaction selectivity for the PRCM was attained on the β-Ga2O3(1273) sample, which was higher than the best one reported on the commercial β-Ga2O3 sample of low specific surface area (2 m2g-1), due to the less activities for PRCH, PCM [1]. This also might be attributed to the present preparation method.

4. Conclusion Uniformed Ga2O3 granules of the rod-like multi-layered morphology were prepared by the homogeneous precipitation method. Calcination at various temperatures provided the α-Ga2O3 and β-Ga2O3 samples having various specific surface areas with the unique morphology. This preparation method provided the α-Ga2O3 photocatalyst producing CO with the high selectivity through the PRCM and the consecutive PRCH, and the β-Ga2O3 sample showing the high selectivity for the PRCM to produce both CO and H2.

References [1] L. Yuliati, H. Itoh, H. Yoshida, 2008, Photocatalytic dry reforming of methane over gallium oxide, Chem. Phys. Lett., 452, 178–182. [2] K. Teramura, H. Tsuneoka, T. Shishido, T. Tanaka, 2008, Effect of H2 gas as a reductant on photoreduction of CO2 over a Ga2O3 photocatalyst, Chem. Phys. Lett., 467, 191–194.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Catalytic combustion of methane on ferrites M.V. Bukhtiyarova, A.S. Ivanova, E.M. Slavinskaya, L.M. Plyasova, V.A. Rogov, V.V. Kaichev Boreskov Institute of Catalysis, Novosibirsk 630090, Russia Abstract Ferrites with the components ratio typical of hexaferrite prepared by co-precipitation and calcined at 700 – 1000°С were characterized by different methods. It has been shown that hexaferrite phase formed at 700°C is amorphous and its crystallization occurs at 800°С. Specific surface area of the samples calcined at 700°С is 27 – 59 m2/g. Temperatureprogrammed reduction of the samples with H2 proceeds in several steps, Fe(III) in the sample being partially reduced to Fe0. The main components on sample surface are in oxidized state: Fe3+ and Mn3+. Among the samples concerned SrMnxFe12-xO19 (0 ≤ x ≤ 2) are the most active catalysts in methane oxidation. Keywords: synthesis, properties, ferrite, methane, oxidation

1. Introduction High stability of methane needs relatively high temperatures for its oxidation. Moreover, high exothermicity of methane oxidation (ΔH298 = – 802,7 kJ/mol) [1] results in additional overheating of the catalysts applied for afterburning gas emissions in spite of low concentration of methane in exhaust feed. Therefore, the catalysts developed for methane oxidation should be highly active and thermally stable in the temperature range of 700 – 1000°С. Hexaaluminates and hexaferrites meet the demand of high stability. It was shown [2] that substituted hexaaluminates is applied for CH4 oxidation. These catalysts are prepared using precipitation of soluble nitrates of metals with NH4(CO3)2 at constant T (60°C) and pH (~7,5). For Mn(Fe)-substituted hexaaluminates the temperatures of 50% CH4 conversion are in the range of 560 – 776°C. Obtaining BaFe12O19 hexaferrite is accompanied by decrease in T50 to 533°С [3]. It can be believed that further T50 decrease can be achieved by using Mn-substituted hexaferrites.

2. Results 2.1. Experimental Ferrites with the components ratio typical for hexaferrite SrMnxFe12-xO19 (x = 0, 1, 2, 6), SrMn6Fe4Al2O19 and sample SrMn2Al10O19, marked as SF, SM1F, SM2F, SM6F, SM6FA2 and SMA, respectively, were prepared by precipitation method. 0.5M aqueous solutions of Sr(NO3)2, Fe(NO3)3, Mn(NO3)2 and Al(NO3)3 were prepared. The mixture of calculated amounts of nitrates was added slowly into a beaker with some distilled water, and the solution of NH4HCO3 as precipitating agent was dropped slowly into the beaker at the same time. The precipitation was carried out under vigorous stirring at constant temperature of 70°C and pH of 7.2 – 7.5 [3]. The formation of brown precipitate was observed. The slurry was aged at 70°C for 2 h under vigorous stirring and then filtered. The obtained precipitate was washed to remove nitrates and NH4HCO3 excess and dried in air. The solid was dried at 110°C for 12 – 14 h and then

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calcined at 700°C for 4 h in an air flow. The subsequent calcination of the samples was carried out at 800 and 1000°C for 4 h in a muffler. Structure and texture of the samples were characterized by X-ray diffraction (XRD), nitrogen adsorption, Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS) and H2 temperature programmed reduction (H2TPR). The catalytic properties were studied using the “llight-off ” test. Reaction mixture contained 0.1 vol.% CH4, 20 vol.% O2, 0.5 vol.% Ne and He as balance. The reactor was filled with 0.6 cm3 of catalyst (dp = 0.25 – 0.5 mm), space velocity was 50000 h-1.

2.2. Discussion

v

o

o oo o o o o vo v v o v o ov

v v s

+ +

SM1F-1000

v

+ s + +s

SF-1000 +

+

++

20

30

40

50

60

SF-800 SF-700

70

2 theta

Fig. 1. XRD patterns of the samples.

397

o

v o

859 705 698 592 548

o o

o o o

SF (700) SF (1000)

1456

o

Absorbance (a.e.)

Intensity

o

458;438

v - Fe2O3 ; s - SrFeO3 + - SrCO3 ; o - SrFe12O19

oo

552 470 376 377 330;324 323 360 304 232

2.2.1. Phase composition According to XRD, the samples calcined at 700°C contain mainly SrCO3 and α-Fe2O3 phases; whereas the SMA sample is amorphous [2]. Mn2O3 phase is also present in the SM6F sample. Raising the calcination temperature up to 800°C promotes crystallization for the SF sample with appearance of SrFe12O19 and SrFeO3-x phases. Further temperature increase to 1000°С leads to formation of practically single-phase hexaferrite. The SM2F and SM6F samples contain, except for SrFe12O19 and α-Fe2O3 phases, small amounts of Mn2O3 and SrMn3O6 ones (it is not shown on the Fig. 1). Distinctive feature of the SM6FA2 and SMA samples is the fact that hexaferrite (hexaaluminate) phase is not registered in patterns even after calcination at 1000°С. Moreover, SrAl2O4 phase is formed in the SMA sample (it is not shown on the Fig. 1).

2000

1750

1500

750

500

Wavenumber, cm

250

-1

Fig. 2. FTIR-spectra of the samples.

FTIR-spectra (Fig. 2) show absorbance bands (a.b.) at 288, 304, 359, 396, 438, 548, 591 and 779 cm-1, relating to SrFe12O19 [4], and a.b. at 323, 377, 458 and 799 cm-1, relating to α-Fe2O3 [5] for SF calcined at 1000°C. The a. b., which correspond to α-Fe2O3, SrCO3 and SrFe12O19 phases, are exhibited in the FTIR-spectrum of the SF sample calcined at 700°С. The reason of non-observation of hexaferrite phase (SrFe12O19) with XRD (Fig. 1) is probably its presence in highly dispersed state. Results of FTIRspectroscopy for Mn-containing samples agree in general with XRD. 2.2.2. Texture Together with formation of the phase composition, changes of specific surface area occur (SBET). It is seen (Table 1) that SBET of the samples calcined at 700°C is 30 – 60 m2/g. At the same time, addition of Al in the samples promotes increase in SBET. 2.2.3. Temperature-programmed reduction H2-TPR of the SF, SM2F, SM6FA2 and SMA samples calcined at 700°C was performed for estimation of their redox properties. Reduction of the samples proceeds in

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Catalytic combustion of methane on ferrite

several steps. H2-TPR spectra of the SF and SM2F samples show three peaks (Table 1). The first and second peaks (Table 1, columns 3, 4) correspond to step-by-step reduction: Fe(Mn)2O3 → Fe(Mn)3O4 → FeO + Fe0 [6]. Addition of Mn shifts reduction temperature to higher temperatures. The latter peak (~700ºC) is related to reduction of hexaferrite phase which is in an amorphous state (Table 1, column 5). Table 1. Specific surface area and influence of catalyst nature on its reduction.

Sample

SBET, m2/g

Temperature of the reduction step, °C

SF SM2F SM6FA2 SMA

30 38 59 145

400 422 381, 453 385, 466

523 539 552 -

711 700 810 895

H2 consumption, μmol Н2/g 60-900°С 12.5 13.7 9.1 2.0

Essential difference in Н2-TPR profile of SM6FA2 from the profile of SM2F (Table 1) seems to indicate that Al in the sample affects reduction behavior, in particular, shifting reduction temperatures of Mn2O3 and Fe2O3 to lower temperatures (Table 1, column 3). The peak at 816°C can be due to reduction of Fe(Mn) aluminates in strong-bonded state that agrees with reduction behavior of SMA. Thus, the feature of reduction is formation of Fe0, amount of which increases with introduction of Mn and Al to SrFe12O19. The amount of H2 consumed during the reduction of the involved Mn2O3 and/or Fe2O3 phases decreases in the order: SM2F > SF > SM6F > SMA (Table 1, column 6). It is seen that addition of Mn increases amount of H2 consumed during the reduction. Therefore, it can be supposed that the presence of manganese in the SM2F sample promotes the highest reduction of Fe2O3 to Fe0 in comparison with that for the SF sample. 2.2.4. X-ray photoelectron spectroscopy XPS data show that surface of the samples is enriched with strontium. Sr segregation is most probably explained by formation of surface carbonates. Nevertheless, part of Sr enters in hexaferrite structure. Iron and manganese on the surface are mainly in oxidized state: Fe3+ and Mn3+. 2.2.5. Catalytic methane combustion Figure 3 shows the tests in the methane oxidation for the catalysts calcined at 700ºС. One can see that temperature of 10%4 CH conversion increases in the order: 10)(Т T10, ºC

SM2F 340

<

SF < 343

SM1F 347

<

SM6F 365

<

SM6FA2 380

<

SMA 443

The amount of H2 consumed during the reduction of the samples decreases in the same order. Thus, the initial activity correlates with the amount of accessible oxygen. Temperature corresponding to 50% CH4 conversion (Т50) increases in the order: T50, ºC

SF < 410

SM1F 418

=

SM2F 418

<

SM6F 447

<

SM6FA2 < 463

SMA 538

Elevation of the amount of Mn in the sample correlates with increase in Т50. Similarly, introduction of Al3+ ions (SM6FA2) till total substitution of the Fe3+ ions with Al3+ is accompanied by drastic decrease in the activity. Т50 for the SMA sample is

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M.V. Bukhtiyarova et al.

higher by 120°C than that for the SM2F sample. The observed differences in the activity of the samples can be caused by variation of the phase composition. According to [7], activity in the methane oxidation is determined by the presence of red-ox sites in catalyst. Probably, for the ferrites active sites are Fe3+ and Mn3+. Their presence is confirmed by XPS data. The high catalytic activity can be caused by simultaneous presence of manganese and iron, which can change the oxidation state: 2Fe(Mn)3+ + Olattice2- ↔ 2Fe(Mn)2+ + ½ O2 facilitating the methane oxidation. 100

SF SM1F SM2F SM6F SM6FA2 SMA

Conversion CH4, %

90 80 70 60 50 40 30 20 10 0 100

200

300

400

500

600

о

Temperature, С

Fig. 3. CH4 conversion for hexaferrites catalysts calcined at 700°C.

It is worth to note that activity of the samples studied does not depend on SBET. In spite of higher value of SBET for the SM6FA2 and SMA samples compared to SBET of the remaining samples (59 and 145 vs. 30 – 39 m2/g), they are less active. Comparison of activity of the SF sample and Ba-hexaferrite [3] calcined at 700°C shows that T50% is 410 and about 530°C, respectively. Thus, obtained catalyst is more active even at lower concentration of methane in gas mixture and higher space velocity (0.1% vs. 1% CH4 and 50000 vs. 48000 h-1).

3. Conclusions • Ferrites calcined at 700°C are multiphase. Further temperature increase to 800 – 1000°C promotes formation of hexaferrite phase. • Specific surface area of the samples calcined at 700°C is 27 – 59 m2/g. • The main components on the surface are mainly in oxidized states Fe3+ and Mn3+. • The reduction of the samples proceeds in several steps: Fe2O3(Mn2O3) → Fe3O4(Mn3O4), Fe3O4(Mn3O4) → FeO(MnO) + Fe0. • The obtained catalysts are active in the methane oxidation.

References 1. 2. 3. 4.

P. Gelin, M. Primet, 2002, Complete oxidation of methane at low temperature over noble metal based catalysts: a review, Appl. Catal. B: Environ, V. 39, I. 1, p. 1 – 37. X. Ren, J. Zheng, Y. Song, P. Liu, 2008, Catalytic properties of Fe and Mn modified lanthanum hexaaluminates for catalytic combustion of methane, Catal. Commun., V. 9, p. 807 – 810. G. Groppi, C. Cristiani, P. Forzatti, 1997, BaFexAl(12−x)O19 system for high-temperature catalytic combustion: physico-chemical characterization and catalytic activity, J. Catal., V. 168, I. 1, p. 95 – 103. J. Jiang, L. Ai, L. Li, 2009, Multifunctional polypyrrole/strontium hexaferrite composite microspheres: preparation, characterization and properties, J. Phys. Chem. B V. 113, p. 1376 – 1380.

Catalytic combustion of methane on ferrite 5. 6. 7.

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S. Onari, T. Ari, K. Kudo, 1977, Infrared lattice vibrations and dielectric dispersion in α-Fe2O3, Phys.Rev. B, V. 16, I. 4, p. 1717 – 1721. L.C.A. Oliveira, J.D. Fabris, R.R.V.A. Rios, W.N. Mussel, R.M. Lago, 2004, Fe3−xMnxO4 catalysts: phase transformations and carbon monoxide oxidation, Appl. Catal. A V. 259, I. 2, p. 253 – 259. B.W.-L. Jang, R.M. Nelson, J.J. Spiveya, M. Ocal, R. Oukaci, G. Marcelin, 1999, Catalytic oxidation of methane over hexaaluminates and hexaaluminate-supported Pd catalysts, Catal. Today, V. 47, I. 1 – 4, p. 103 – 113.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Polymer-based nanocatalysts for phenol CWAO E. Sulmana,V. Doludaa , N. Lakinaa, A. Bykova, V. Matveevaa, L. Bronsteinb a b

Tver Technical University, A.Nikitina str., 22, Tver, 170026, Russia Indiana University, Bloomington, IN 47405, USA

Abstract Catalytic wet air oxidation (CWAO) of phenols is an important process of environmental catalysis, allowing one to reach nearly complete oxidation of phenols to non-hazardous compounds. Here we report the synthesis of nanocatalysts based on Pt-containing nanoparticles (NPs) formed in the pores of hypercrosslinked polystyrene (HPS) and their catalytic properties in the phenol CWAO under mild conditions. The Pt species were in corporated in HPS using wet impregnation of platinic acid in tetrahydrofuran followed by NaHCO3 treatment. The catalysts containing from 0.11 to 4.85 wt.% of Pt were studied by XFA, TEM, XAS, XPS, and liquid nitrogen physisorption methods. The NP sizes were found to be independent of the amount of platinic acid used for impregnation, but rather controlled by the pores of HPS. Three types of Pt species: Pt(0), Pt(II), and Pt(IV), constituted the NP composition. The effects of the phenol and catalyst initial concentrations and temperature were investigated in the phenol CWAO. Removal of 97% of the phenol with 94.2% selectivity to CO2 and H2O were observed for the most active catalyst containing 0.95 wt.% Pt. Keywords: platinum, nanoparticles, catalytic wet air oxidation, phenol

1. Introduction The purification of waste water from phenol compounds is one of the most essential tasks of “green chemistry” considering the hazardousness of phenols. Although numerous methods are known for the elimination of phenols from water, the majority of them are physical methods that preserve the phenol mass balance, i.e., lead to the pollutant redistribution/concentration without its transformation to non-hazardous substances. The ideal purification is a complete oxidation of phenols to CO2 and H2O. Catalytic conversion is considered to be the best solution to this problem. CWAO is one of the promising dephenolization methods. CWAO allows the sewage treatment at ambient temperatures and pressures. The use of the catalyst accelerates considerably the oxidation rate and reduces waste concentration to the level at which the sewage can be used for technological needs. The key to the successful application of the CWAO process is use of nanocatalysts. Although nanocatalysts based on NPs have been extensively studied, only a few studies have recently reported the CWAO of phenolic compounds with metal and metal oxide NPs. In the present paper we discuss on catalytic properties of Pt-containing NPs formed in the pores of HPS in the CWAO of phenol to non-toxic СО2 and Н2О (Fig. 1) and formation of quinone (I) and acid (II) intermediates.

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E. Sulman et al. O

OH OH

O

O OH

OH

OH HO catechol

o-benzoquinone

OH

O

phenol

OH

O

hydroquinone

p-benzoquinone

I

H C

C

HO C

O O O H maleic acid malonic acid HO HO C C O O O oxalic acid CH3 C OH HO acetic acid C H II O formic acid

CO 2 + H O 2

Figure 1. The scheme of phenol CWAO.

2. Experimental HPS was purchased from Purolite Int. (U.K.) as Macronet MN 270/3860. The granules of HPS were washed with acetone and water twice and dried under vacuum for 24 hrs. NaHCO3, H2PtCl6×6H2O, reagent-grade THF, sulfuric acid, and phenol were purchased from Sigma-Aldrich and used as received. The catalysts were prepared by impregnation of HPS with platinic acid in complex solvent (tetrahydrofuran + methanol + distilled water in volume ratio of 4:1:1). After the platinic acid impregnation the catalysts were treated with NaHCO3 to provide Ptcontaining NPs formation. The samples with 4.95%, 2.91%, 0.95% and 0.11% (wt.) of Pt content (the data of XFA) were synthesized (Table 1). The oxidation reaction was conducted batchwise in a PARR 4200 apparatus which provides independent control over parameters such as phenol, catalyst concentration, temperature, (pure) oxygen feed rate, oxygen pressure and stirring rate. A suspension of the catalyst and an aqueous solution of phenol (20 ml) prepared at a predetermined concentration were placed in the reactor. The rate of oxygen feed was controlled by a rotameter. Samples of the reaction mixture were periodically removed for analysis. The catalysts studied were characterized using XFA, TEM, XANES, EXAFS, XPS, and liquid nitrogen physisorption methods. The total organic carbon was detected by standard methodology of chemical oxygen demand (COD).

3. Results and discussion The data presented in Table 1 demonstrate that formation of Pt-containing NPs in HPS leads to a proportional decrease of both surface area and pore volume. HPS contains small mesopores (4 nm) which are responsible for 40% of the pore volume and larger mesopores (5-30 nm), the volume of which measures 56%. It was found that the size of the 4 nm mesopores does not change after the incorporation of Pt species, but the volume of the mesopores decreases with an increase of the Pt content, suggesting that Pt-containing nanoparticles are localized in these pores without changing their diameters. It is noteworthy, that the Pt-containing NP size (2.1-2.3 nm, see Fig. 2) is significantly lower than the pore size, thus preventing the pore blockage.

363

Polymer-based nanocatalysts for phenol CWAO

Figure 2. TEM images of HPS-Pt samples with 2.91(a), 0.95 (b) wt.% Pt.

XPS data showed that Pt(IV) is partially reduced to Pt(II) or even Pt(0) due to the oxidation of THF in the presence of water. For each HPS-Pt sample three components (Table 2) were revealed: Pt (0), Pt (II), and Pt(IV). Table 1. XPS characteristics of the catalysts. Catalyst notation

NP size, nm

BET surface area, m2·g-1

HPS-Pt-4.85%

2.2 ±0.4

968

HPS-Pt-2.91%

2.1 ± 0.5

1015

HPS-Pt-0.95%

2.3 ± 0.5

1156

Pt species Pt(0) Pt(II) Pt(IV) Pt(0) Pt(II) Pt(IV) Pt(0) Pt(II) Pt(IV)

Pt content, at.%, before (after) oxidation 1.9 (2.3) 48.3 (50.2) 51.7 (47.5) 6.9 (7.7) 57.9 (60.8) 35.2 (31.5) 5.2 (6.5) 57.8 (61.4) 37.0 (32.1)

In all the samples, except HPS-Pt-4.85%, the amount of Pt(II) species prevail while the amount of the Pt(0) species varies with the lowest (1.9 at. %) in HPS-Pt-4.85%. Considering that in all the samples both Pt(II) and Pt(IV) are present, we believe that mixed PtO2·2PtO and pure PtO2·H2O may form. To investigate the catalytic properties of these catalysts in phenol CWAO, we varied reaction conditions and the optimal conditions providing the best catalytic activity and selectivity (Table 2) were found: the catalyst concentration of 5.15·10-3 mol(Pt)/L, phenol concentration of 0.44 mol(Phenol)/L, temperature of 95°C, pressure of 0.1 MPa, time of 5 hrs, and the oxygen flow rate of 0.018 m3·h-1

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E. Sulman et al. Table 2. Catalytic activity and selectivity of the catalysts at optimal conditions. Catalyst

HPS-Pt-4.85% HPS-Pt-2.91% HPS-Pt-0.95% HPS-Pt-0.11%

TOF mol(Phen)/(mol Pt s) 1.6 . 10-3 4.6 . 10-3 7.3 . 10-3 5.8 . 10-3

COD removal % 54.3 64.1 94.2 85.4

Ea kJ/mol 61±4 65±4 58±4 61±4

Conversion of phenol, % 65 86 97 91

4. Conclusions It was demonstrated that the impregnation of HPS with platinic acid in THF followed by the NaHCO3 treatment leads to the formation of mixed Pt-containing NPs, the sizes of which are independent of the amount of incorporated Pt species and measure about 2.1-2.3 nm. The Pt species include Pt(0), Pt(II), and Pt(IV), the ratio of which varies insignificantly. Despite the similar NP sizes and composition, the catalysts containing different amount of Pt display very different catalytic properties in the phenol CWAO. The highest conversion, activity, and selectivity were obtained for the HPS based catalyst containing only 0.95 wt.% Pt, while the catalyst containing 4.85 wt.% Pt is least active and selective. These differences are explained by the shielding of the catalytic sites when the Pt load is too high.

Acknowledgements We sincerely thank Federal Education Agency of Russian Federation (contract P 344), Federal Science and Innovations Agency of Russian Federation (02.552.11.7075) and the NATO Science for Peace programme (SfP 981438) for the financial support. Authors also thank HASYLAB (DESY, Germany) for X-ray beam time (project I20060224 EC).

References A. Quintanilla, J. A. Casas, J. A. Zazo, A. F. Mohedano and J. J. Rodrigues, Applied Catalysis B: Environmental., 2006, 115 - 120. L. Chang, I.-P. Chen and S.-S. Lin, Chemosphere, 2005, 58, 485. S. Kim and S. Ihm, Topics in Catalysis, 2005, 33, 171 - 179. A. Schmid, F. Hollmann and B. Buehler, Enz. Catal. Org. Synth., 2002, 1170. F. d’Acunzo, C. Galli and B. Masci, Europ. J. Biochem., 2002, 269, 5330. F. Luck, Catal. Today, 1999, 53, 81 S. K. Mohapatra, F. Hussain and P. Selvam, Catal. Comm., 2003, 4, 57 S. H. Joo, A. J. Feitz and T. D. Waite, Environ. Sci. Technol., 2004, 38, 2242. S. H. Joo, A. J. Feitz, D. L. Sedlak and T. D. Waite, Environ. Sci. Technol. , 2005, 39, 1263. J. L. Gole and Z. L. Wang, Nano Letters, 2001, 1, 449.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

A new sulphonic acid functionalized periodic mesoporous organosilica as a suitable catalyst Els De Canck, Carl Vercaemst, Francis Verpoort, Pascal Van Der Voort* Centre for Ordered Materials, Organometallics and Catalysis (COMOC), Department of Inorganic and Physical Chemistry, Ghent University, Krijgslaan 281 – S3, 9000 Ghent, Belgium. [email protected], [email protected]

Abstract A new solid acid catalyst is developed by the direct sulphonation of the ethene bond of a pure trans ethene bridged Periodic Mesoporous Organosilica. The catalytic activity of this mesoporous material is evaluated in an esterification reaction and compared with ptoluenesulphonic acid. The sulphonated ethene PMO can compete with a homogeneous catalyst and maintains its porosity. Keywords: solid acid catalyst, periodic mesoporous organosilica, esterification

1. Introduction Esterifications are key reactions in the manufacturing of chemicals1 and mostly they require an acid catalyst2. Although homogeneous catalysts are still widely used, they exhibit a major disadvantage, as they cannot be easily separated from the reaction mixture, and are therefore not appropriate for continuous flow procedures. Heterogeneous catalysts are effortlessly recovered from the reaction mixture, but the recyclability of the catalyst is a very important additional parameter. Functionalized Periodic Mesoporous Organosilicas3 (PMOs) can act as a solid acid catalyst. PMOs exhibit large specific surface areas, pore diameters of 2-50 nm and narrow pore size distributions. They are mostly synthesized with bridged bis-silanes with a general structure (EtO)3-Si-R-Si-(OEt)3 (Figure 1)4. This bis-silane polycondensates around a template such as the surfactant P123 (EO)20(PO)70(EO)20. After formation of the PMO, the template is removed by an extraction to reveal the pores of the hybrid material5.

Figure 1. Synthesis of a Periodic Mesoporous Organosilica. R represents an organic functionality such as -CH=CH-.

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One of the most important advantages of functionalized ethene PMOs is the fact that the functionalization occurs by the addition and substitution to the ethene double bond, creating a C-C grafing as opposed to a Si-O-C grafing in the case of silica materials. The C-C grafting renders the functionalized materials very stable towards hydrolysis. In this case, the PMOs are modified to attain a sulphonic acid group and transform these materials into suitable solid acid catalysts. These catalysts can be straightforwardly removed from the reaction mixture and in addition, they can be regenerated with an acidic solution and reused in multiple catalytic cycles. Introducing the sulphonic acid functionality can be performed via different pathways. Pioneering work has been done by Nakajima and Kondo6. Via a Diels-Alder reaction with benzocyclobutene, they introduced a benzene moiety and afterwards they performed a sulphonation. They achieved an acidity of 1.44 mmol H+/g. Other reports7-9 describe the preparation of a solid catalyst by the co-condensation or grafting with 3-mercaptorpropyltriethoxysilane which is subsequently oxidized by H2O2, H2SO4 or HNO3. This study presents the synthesis of an ethene PMO functionalized with –SO3H moieties. The solid acid catalyst is prepared via a direct sulphonation of the ethene bond of a pure trans ethene PMO6 (Figure 2). This material is prepared with a homemade precursor8 described by our group before. After characterization, esterification experiments are performed to evaluate the catalytic activity of the solid acid catalyst.

ClSO3H Sulphonation

Figure 2. Sulphonation process of the trans ethene bridged PMO.

2. Experimental section The pure trans 1,2-bis(triethoxysilyl)ethene (E-BTSE) and pure trans ethene bridged PMO (E-ePMO) are prepared according to the procedures published by our group5, 10. The sulphonation of ePMO is performed with chlorosulphonic acid (E-ePMO-SO3H). In a typical synthesis, 20 ml of dry CH2Cl2 is added to the dried ePMO under an inert atmosphere and cooled with an ice bath. 4.5 ml of chlorosulphonic acid is added. The reaction is stirred for 24 hours. Subsequently, the mixture is poured out in H2O and the material is further washed until the pH of the filtrate is neutral. Finally, the sulphonated PMO (E-ePMO-SO3H) is washed with methanol and acetone before drying the sample. The acidity of the sulphonated PMO is determined by a titration with NaOH (0.1 M). The catalytic activity of the materials is evaluated with the esterification of n-propanol with acetic acid. In a typical experiment, 4.9 ml toluene, 3.4 ml n-propanol, 3.86 ml acetic acid and 1.0 ml butyl acetate are added to the catalyst (0.207 g). The mixture is heated to 135°C while stirring. During the catalytic reaction, the formed water is constantly removed by a Dean-Stark setup. Samples of the mixture are taken at several point in time which are analyzed by GC.

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A new sulphonic acid functionalized periodic mesoporous organosilica

3. Results and discussion The trans ethene bridged PMOs are sulphonated with chlorosulphonic acid and are characterized with nitrogen adsorption desorption experiments. The characteristics of the E-ePMO and the E-ePMO-SO3H are presented in Table 1. The sulphonation process has no great influence on the porosity of the material. Only a slight increase in SBET occurs after the treatment with chlorosulphonic acid. Table 1. Properties of the ethene bridged PMOs before and after the sulphonation. Sample

SBET (m²/g)[a]

Vt (cm³/g)[b]

Vµ (cm³/g)[c]

Dp (nm)[d]

E-ePMO

987

1.16

0.15

8.1

E-ePMO-SO3H[e]

1047

1.18

0.18

8.1

[a]

Surface area, [b] Total pore volume, [c] Micropore volume, [d] Pore diameter (calculated from adsorption isotherm with BJH method), [e] Sulphonation of the material for 24 h.

The acidity of the materials is determined with an acid base titration using sodium hydroxide. Approximately 0.69 mmol H+/gram material is observed when performing a sulphonation for 6 hours. A higher acidity of 1.08 mmol H+/gram E-ePMO-SO3H is observed when the reaction is performed for a longer period of time (24 hours). The solid acid catalyst is evaluated by performing an esterification reaction (Figure 3) with acetic acid and n-propanol. Its catalytic activity is compared with a homogeneous catalyst p-toluenesulphonic acid (p-TsOH).

E-ePMO-SO3H p-TsOH

Figure 3. The esterification reaction of acetic acid with n-propanol is examined for p-toluenesulphonic acid and E-ePMO-SO3H. The conversions of n-propanol for both catalysts are presented in the graph.

The catalytic performance of the E-ePMO-SO3H is shown in Figure 3. After 80 minutes, the E-ePMO-SO3H with a catalyst loading of 0.49% reaches complete conversion. The p-toluenesulphonic acid with a loading of 0.45% obtains a conversion

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of 100% after 40 minutes. Moreover, the turn-over-number (TON) of E-ePMO-SO3H (204) resemble that of the homogeneous catalyst (222) Therefore the PMO exhibits a comparable catalytic activity as the homogeneous catalyst. The stability of the heterogeneous catalyst is an important parameter. The porosity of the acidic PMO is investigated with nitrogen physisorption (Figure 4), where only a minor differences are observed. The specific surface area slightly decreases to 800 m²/g with a pore volume of 0.98 cm³/g and a pore size of 7.1 nm.

Figure 4. Nitrogen adsorption desorption isotherm of E-ePMO-SO3H before and after reaction.

These preliminary catalytic results already show the potential of this solid acid catalyst in esterification reactions. In depth studies on the recyclability of the catalyst and alternative sulphonation procedures are currently ongoing.

References 1.

Hoydonckx, H.E.; De Vos, D.E.; Chavan, S.A.; Jacobs, P.A. Topics in Catalysis 2004, 27, 14, 83. 2. Corma, A. and Garcia, H. Topics in Catalysis 2008, 48, 1-4, 8. 3. Van Der Voort, P.; Vercaemst, C.; Schaubroeck, D.; Verpoort, F. Phys. Chem. Chem. Phys. 2008, 10, 347. 4. Vercaemst, C.; Ide, M.; Friedrich, H.; de Jong, K.P.; Verpoort, F.; Van Der Voort, P. J. Mater. Chem. 2009, 19, 8839. 5. Vercaemst C.; Ide, M.; Allaert, B.; Ledoux, N.; Verpoort, F.; Van Der Voort, P. Chem. Commun. 2007, 22, 2261 6. Nakajima, K.; Tomita, I.; Hara, M.; Hayashi, S.; Domen, K.; Kondo, J.N. Adv. Mater. 2005, 17, 1839. 7. Yang, Q.; Liu, J.; Yang, J.; Kapoor, M.P., Inagaki, S. J. Catal. 2004, 265. 8. Yang, Q.; Kapoor, M.P.; Inagaki, S.; Shirokura, N.; Kondo, J.N.; Domen, K. J. Molecul. Catal. A Chem. 2005, 230, 85. 9. Hamoudi, S.; Kaliaguine, S. Micropor. Mesopor. Mater. 2003, 59, 195. 10. Vercaemst, C.; Ide, M.; Wiper, P.V.; Jones, J.T.A; Khimyak, Y.Z.; Verpoort, F.; Van Der Voort, P. Chem. Mater. 2009, 21, 24, 5792.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V.

Effect of the preparation procedure on the structural peculiarities and catalytic properties of Pt/(CeO2–TiO2) catalyst in CO oxidation Alexei A. Shutilova,b, Galina A. Zenkovetsa,b a

Boreskov Institute of Catalysis, Siberian Branch, Russian Academy of Sciences, pr. Lavrentieva 5, Novosibirsk, 630090, Russia b Novosibirsk State University, Novosibirsk, Pirogova 2, 630090, Russia

Abstract The procedure of the Pt/(CeO2–TiO2) catalyst preparation is crucial for the microstructure and catalytic properties in CO oxidation. Impregnation of ceria doped TiO2 with platinum nitrate solution followed heating in air at 500°C leads to the formation of ultra fine platinum particles 0.5-0.6 nm in size stabilized at interblock boundaries of the support formed by irregularly intergrown anatase particles. Calcination of the catalyst in hydrogen at 250°C leads to the formation of the platinum particles with 2-5 nm in size. The catalyst containing ultra fine platinum particles is much more active than the catalyst with particles of 2-5 nm in size. Infrared spectra of CO adsorbed on Pt revealed that high CO oxidation activity is exhibited by ultra fine Pt particles due to the high concentration of weakly bonded Pt0-CO complexes. Keywords: Pt/(CeO2–TiO2) catalysts, microstructure, oxidation of CO

1. Introduction Today the effect of noble metal particles size supported on oxide materials on catalytic activity still remains a real challenge in catalysis. The morphology of such types of catalysts is influenced by the support material and the procedure of the catalyst preparation. It is well known that the reduction of the metal particle size results in a change of electronic properties and strongly influences on the catalytic activity in CO oxidation [1].The study of the effect of the preparation procedure on the morphology of supported platinum nanoparticles and their catalytic activity in CO oxidation is essential for gaining a deeper insight into this problem and for development of new efficient catalysts. Here, we report a detailed investigation of the effect of the preparation procedure on the microstructure of Pt/(CeO2–TiO2) catalysts and their catalytic properties in CO oxidation.

2. Experimental Catalysts with formulation: 2 wt.% Pt/(CeO2–TiO2) with different microstructure were prepared as follows: for the preparation of Pt/(CeO2–TiO2) catalyst with ultra fine platinum clusters 0.5-0.6 nm in size the support (5 wt.% CeO2–95.wt % TiO2) was impregnated with an appropriated amount of platinum nitrate solution followed by drying and heating in air at 500°C; for the preparation of catalyst with platinum particles 2-5 nm in size the catalyst were additionally heated in flowing hydrogen at 250°C for 2h [2]. The catalysts obtained were investigated by XRD, TEM, XPS methods. Chemisorptions of CO by FTIR spectroscopy were investigated as well.

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X-ray diffraction patterns were obtained on URD-63 diffractometer with CuKα radiation. TEM investigations were carried out using JEM-2010 instrument with 0.14 nm resolution and an accelerating voltage of 200 kV. XPS spectra were registered on an ES-300 photoelectron spectrometer (Kratos Analytical) in the constant photoelectron pass energy mode using AlKα and MgKα primary radiation. Decomposition of difference Pt4f spectra into components were performed using Voigt functions as a sum of Gaussian and Lorentzian curves in accordance with [3]. FTIR spectra of CO adsorbed on Pt/(CeO2–TiO2) catalysts were recorded on a Shimadzu FTIR-8300 instrument with a resolution of 4 cm-1 for 50 signals accumulated. Adsorption of CO was conducted at room temperature and a pressure of 2.6 kPa. Catalytic properties of the catalysts (fraction of 0.25-0.5 mm) in CO oxidation were tested in a flow type reactor in accordance with Light-off test at the experimental conditions: the reaction gas composition: CO, 0.05 vol.%; H2O, 5 vol.%; O2, 6.7 vol.%; nitrogen, balance; GHSV, 180,000 h-1; catalyst volume, 1 cm3; heating rate 10°C/min. Catalytic activity was characterized by the temperature of 50% CO conversion, which was determined from the conversion versus temperature curve.

3. Results and discussion According to XRD and TEM data the structure of CeO2–TiO2 support is nanocrystalline and consists of incoherently grown anatase crystals (4-8 nm) with formation between them interblock boundaries where Се3+ ions are stabilized [4]. In supported Pt/(CeO2–TiO2) catalyst heated in air anatase is the only phase detected by XRD. At the same time fine platinum particles with size of 0.5-0.6 nm stabilized at interblock boundaries of support are observed by TEM (Fig. 1a). Under the reduction in hydrogen at 250°C microstructure of the Pt/(CeO2–TiO2) catalyst changed. TEM investigation reveals the formation of platinum particles 2-5 nm in size stabilized on the support surface (Fig. 1b). At the same time no changes in structure of the support were detected by TEM and XRD methods.

a

b

Fig. 1. TEM images of the Pt/CeO2–TiO2 catalysts with platinum particles in size of 0.5-0.6 nm (a) and 2-5 nm (b).

It can be seen (Fig. 2) that Pt/(CeO2 –TiO2 ) catalysts with different microstructures demonstrate different catalytic activity in CO oxidation. The support along shows a very low activity. The Pt/(CeO2–TiO2) catalyst containing ultra fine platinum particles is the most active. The conversion of CO equals 50% at 25°C, and catalyst operates for a long time without loss of activity. The activity of the Pt/(CeO2–TiO2) catalyst containing platinum particles 2-5 nm in size is lower, CO conversion of 50% is attained at 68°C.

371

СО conversion, %

Effect of the preparation procedure on the structural peculiarities

100 80 60

1

40

3

2

20 0 0

20

40

60

80

100 120 140 160 180 200 220 240 260 280 300

Temperature, 0C

Fig. 2. Conversion of CO as function of temperature on Pt/(CeO2–TiO2) catalyst containing platinum particles 0.5-0.6 nm in size (1), Pt/(CeO2–TiO2) catalyst containing platinum particles 2-5 nm in size (2), on CeO2–TiO2 support (3).

The XPS data of platinum particles for both catalysts are shown in Table 1. The decomposition of difference Pt4f spectra shows that spectra of both catalysts consist of three main doublets characterizing platinum atoms in different oxidation state. The content of different platinum species strongly depends on the catalyst microstructure. Table 1. Binding energy of Pt 4f7/2 (eV), and content of different platinum species in the catalysts. Binding energy of Pt4f7/2, eV 70.9 72.0 73.6 70.7 71.7 73.0

Catalyst 2% Pt/(СеO2-TiO2) with platinum particles of 0.5-0.6 nm in size 2% Pt/Се-TiO2 with platinum particles of 2-5 nm in size

-1

a

8

Pt0 Ptδ+ Pt2+ Pt0 Ptδ+ Pt2+

-1

-1

A b s o r b a n c e , c o u n ts g c m

-1

4

2100

A b s o rb a n c e , c o u n ts g c m

Content of different platinum species, % 34 47 19 54 27 19

Platinum species

b

6

2

1840

2145

1840

2

2100

2145

4

0

1800

1900

2000

2100

W a v e n u m b e r, с m

-1

1800

1900

2000

2100

W a v e n u m b e r, с m

-1

Fig. 3. FTIR spectra in the carbonyl region of CO adsorbed on the Pt/(CeO2–TiO2) catalysts with a platinum particle size of 0.5-0.6 nm (a) and 2-5 nm (b).

The FTIR spectrum of CO adsorbed on Pt particles 0.5-0.6 nm in size (Fig. 3 a) demonstrates following bands: the low-intensity absorption band at 1840 cm-1 assigned to the bridging form of adsorbed CO [5], a weak band at 2145 cm-1 related to CO adsorbed on ionic platinum species [6], and a very intensive symmetrical band at 2100 cm-1 assignable to the linear form of adsorbed CO, which is responsible for CO2 formation [6, 7]. The position of the last band points to rather weak bonding between the platinum particles and CO molecules in Pt0–CO complexes.

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The FTIR spectrum of CO adsorbed on the Pt particles 2-5 nm in size (Fig.3 b) also exhibits adsorption bands at 1840 cm-1, 2100 cm-1 and 2145 cm-1 but their intensities are essentially smaller than are observed for the previous catalyst. This is probably due to the change of the morphology of the platinum particles. From XRD, XPS and TEM data it can be seen that the nanocrystalline structure of support containing interblock boundaries between anatase crystals where Се3+ ions are localized is responsible for the stabilization of ultra fine platinum particles 0.5-0.6 nm in size in an oxidative atmosphere and under the reaction conditions. It is quite possible that the metal-support interface formed in the region of interblock boundaries are rather stable and small in size and stabilizes the small platinum particles. It results in the formation of rather big content of the Ptδ+ species connected closely with Pt0 species that provides the rather weak bonding between the platinum particles and CO molecules in Pt0–CO complexes. The high activity of the catalyst containing ultra fine platinum particles may be a result of high concentration of a weakly bonded Pt0–CO complexes. According to XPS data reduction of this catalyst by H2 leads to decreasing of the Ptδ+ species concentration in the region of metal-support interface and to increasing of the concentration of Pt0 species. The change of electronic state leads to agglomeration of ultra fine platinum particles into the particles with the size of 2-5 nm.

4. Conclusions The procedure of the Pt/(CeO2–TiO2) catalyst preparation is crucial for their microstructure and catalytic activity in CO oxidation. Nanocrystalline structure of ceria doped titania due to the stabilization of the ultra fine platinum particles (0.5-0.6 nm) in an oxidative atmosphere and under the reaction condition. Reduction of the catalyst in pure hydrogen at 250°C leads to increase of platinum particles to 2-5 nm in size. XPS data show the change of electronic state of platinum particles with increasing of their size. The microstructure of the catalysts has a significant effect on the catalytic activity in CO oxidation.

Acknowledgments This work was supported by the grant of ministry of Education and Science of RF № 2.1.1/729, government contract № P252/23.07.2009 and grant of SB RAS № 36. We wish to thank Dr. E. Paukshtis and Dr. A.Boronin for FTIR and XPS measurements and helpful discussions.

References 1. 2. 3. 4. 5. 6. 7.

B. Corine, G. Schmid, N. Toshima (Eds), 2008, Elsevier, Metal Nanoclasters in Catalysis and Materials Science: The Issue of Size Control, 458 pp. A.A. Shutilov, G.A. Zenkovets, G.N. Kruykova, V.Yu. Gavrilov, V.A. Paukshtis, A.I. Boronin, S.V. Koshcheev, S.V. Tsybulya. Kinetics and Catalysis, 2008,V. 49, N 2, p. 271. D. Briggs and M.P. Seach (Eds). Practical Surface Analysis by Auger and X-ray Photoelectron Spectroscopy. Chichester: Wiley, 1983. G.A. Zenkovets, A.A. Shutilov, V.Yu. Gavrilov, S.V. Tsybulya, G.N. Kruykova. Kinetics and Catalysis, 2007,V. 48, N 5, p. 742. K.I. Hadjiivanov, G.N.Vayssilov. Adv. Catal., 2002,V.47, p. 307. D.W. Daniel. J.Phys.Chem., 1988, V.92, p. 3891. O. Pozdnyakova, D. Teschner, A. Wootsch et al. J.Catal., 2006,V. 237, p. 1.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Study of the sorption of Cu (II) species on the “ TiO2/KNO3” interface A. Georgaka and N. Spanos School of Science and Technology, Hellenic Open University, Patras, Greece

Abstract In the present work the mechanism of the adsorption of the Cu2+ ions on the surface of TiO2 was studied. It was found that we can easily determine the pzc value and the surface charge of the support, by means of two potentiometric titration curves, one for the suspension and another one for the blank solution. The ionic strength did not affect the extent of the adsorption of Cu(II) onto the titania surface. Moreover, it was found that increase in the impregnating solution pH results in an increase of the adsorbed species concentration, in an increase in the adsorption constant and in a decrease in the lateral interaction energy. Keywords: supported catalysts, TiO2 /Cu(II), interface

1. Introduction The study on the electrical double layer (EDL) developed at the TiO2 /Cu2+ interface is of paramount importance for catalysis, especially photocatalysis, and heterogeneous catalysis [Matthews, 1986]. A huge number of studies refer to the photocatalytic oxidation of organic substances in titania suspensions, by means of solar or UV radiation (λ < 400nm) [Wold, 1993]. The photocatalytic activity increases with the presence of certain quantity of copper ions in the suspension [Fujihira et al, 1981]. Moreover, copper species existing in the suspension and adsorbed on the titania surface, are of crucial importance. For example, in the photocatalytic oxidation of acetate acid the presence of the species Cu(CH3COO+) increases the activity of the catalyst, whereas the presence of the species Cu(CH3COO)2 acts as a poison of the catalyst [Bideau, 1991]. It is obvious that the adsorption of various Cu(II) species on TiO2 surface, is of high importance as far as it concerns the removal of heavy metals and the photocatalytic oxidation of the organic substances existing in waste water. Copper catalysts supported on TiO2 are usually prepared by the dry impregnation method, resulting in relatively large crystallites which interact weakly with the substrate. Thus the obtained dispersion of the active phase is low. In recent decades, aiming at the improvement of the dispersion of the active phase, a new technique has been developed, which is known as Equilibrium Deposition Filtration (E.D.F.). According to this technique the preparation of supported catalysts takes place through the equilibration of the catalytic active precursor solution with the surface of the substrate [Lycourghiotis, 1994]. Unlike the classical method of impregnation, in E.D.F. the deposition of the catalytic active precursor species takes place exclusively through adsorption during the equilibration of the suspension. The properties of the final catalyst, prepared with E.D.F., depend on the active phase characteristics that are affected by the conditions of the impregnation. It is obvious that the elucidation of the detailed adsorption mechanism of the Cu(II) species on the TiO2 surface, is of paramount importance for the preparation of supported Cu2+/TiO2 catalysts with the desired properties.

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2. Experimental Titration experiments took place by adding suitable volume of KOH (10-2M) every time the change of the pH was lower than 0.017 pH units per minute, for different Cu2+ concentration solutions, both in the presence and in the absence of TiO2. Adsorption edges experiments were done at six different initial Cu(II) concentrations and a range of pH, at I=0.1M. The above procedure was repeated for two different values of ionic strength. Details concerning the experimental procedure can be found elsewhere [Bourikas et al]. At the adsorption isotherms experiments the change of the pH upon deposition was measured and then corrected with the addition of suitable volume of KOH (10-2M). The experiments were performed at four different initial pH values and a range of Cu(II) concentrations.

3. Results and discussion The experimental results concerning the choice of the most appropriate electrolyte between KCl and KNO3, showed that KNO3 was the most suitable one, due to the fact that the precipitation of Cu(II) species occurs at higher pH values (equilibrium equations 1, 2, 3, [NIST, 2001]). Cu2+ + OH- ↔ Cu(OH)+ 2+

-

2Cu + 3OH +

− NO 3

↔ Cu2(OH)3NO3(s)

2Cu2+ + 3OH- + Cl- ↔ Cu2(OH)3Cl(s)

(logk1 = 19.32)

(1)

(logk2 = 32.74)

(2)

(logk3 = 34.60)

(3)

As far as the experiments that include Cu(NO3)2/KNO3 solutions both in presence or in absence of TiO2 are concerned, it has been concluded that the maximum value of pH in which precipitation does not occur is 5.6 ( [Cu2+] = 10-2M). Potentiometric titrations experiments of two suspensions containing different solid mass, as well as of a blank solution, illustrated that the potentiometric titrations curves pass through the same point, which corresponds to the point of zero charge, pzc (fig. 1). The determination of pzc and surface charge, σο, can be easily obtained by two potentiometric titrations curves, one for the suspension and another for the blank solution. The mode of interfacial deposition may be investigated by inspecting the equilibrium deposition results (fig. 2). It may be seen that even in low pH values (pH < pzc), where the surface is positively charged, there is appreciable adsorption of the positively charged copper species. That indicates that copper adsorption takes place not only through electrostatic attractive interactions but through coordination bonds or hydrogen bonds as well.

Study of the sorption of Cu (II) species on the TiO2/KNO3 interface

375

Figure 1. Potentiometric titrations curves at 25oC, I = 0.1M KNO3. (a) 0.8g TiO2, (b) 0.4g TiO2 and (c) blank.

The study of the influence of ionic strength to the adsorption of Cu2+ on TiO2 surface showed that ionic strength does not affect the adsorption.

Figure 2. Surface concentration of the adsorbed Cu2+ species as a function of pH at 25oC, I= 0.1M KNO3 and various initial concentrations of copper.

The results of the adsorption isotherms experiments are illustrated in figure 3. It is obvious that by increasing pH, the adsorption increases as well. An increase in the pH value brings about an increase to the negatively charged groups of the surface, whose role is of great importance for the positively charged Cu2+ adsorption. These isotherms were adjusted by the following equation, 1/Γ = 1/Γm + 1/kΓmexp(λΓ/RT)Ceq

(4)

which can be derived from the Langmuir model taking into consideration the presence of lateral interactions. Hence, the values of the maximum surface concentration (Γm), the adsorption constants (K) and the lateral interaction energy (E), at different pH values were determined (table 1). It is obvious that when pH value increases, the value of the lateral interaction energy decreases, while the value of the adsorption constants increases. This is due to the fact that that at low pH values, the value of the adsorbed copper species density on to the surface is very low and as a result there are water dipoles interrupting and applying lateral interactions. While pH value increases the surface becomes less positive, increasing the density of the adsorbed copper species, a fact that leads to a

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huge decrease in the value of the lateral interaction energy, since the adsorbed copper species are closer to each other and are being repelled. The observed increment of the adsorption constant, by increasing the pH value, is due to the increasing attractive interactions between the adsorbing species (Cu2+) and the negatively charged surface. Table 1. Lateral interaction energy, adsorption constants and maximum surface concentration of the adsorbed copper species varying with pH. E / J mol-1 2554 211 79 15

pH 3 4 4.5 5

k / mol-1L 342 1344 1553 1989

Γm / μmol m-1 0.48 0.52 0.71 1.47

Figure 3. Adsorption isotherms curves at 25oC, I = 0.1M KNO3.

4. Conclusions We can easily measure the pzc value and the surface charge by means of two potentiometric titration curves, one for the suspension and another one for the blank solution. Ionic strength does not affect the adsorption of Cu(II) onto the titania surface. An increment at the pH value results in: a) an increase of the adsorbed species concentration, b) an increase in the adsorption constant and c) a decrease in the lateral interaction energy.

References R. W. Matthews, 1986, J. Catal., 97, 565. Α. Wold, 1993,Chem. Mater., 5, 280. M. Fujihira, Y. Satoh, T. Osa, 1981, Nature, 293, 206. M. Bideau, 1991,J. Photochem. Photobiol. A, 61, 269. Α. Lycourghiotis, 1994,in “Preparation of Catalysts VI”, Elsevier, Amsterdam, 95. K. Bourikas, T. Hiemstra, W.H. Van Riemsdijk, 2001, J. Phys. Chem. B, 105, 2393 NIST, 2001,Critically Selected Stability Constants of Metal Complexes Database, Version 6.0.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Hydrogenation/Hydrogenolysis of benzaldehyde over CaTiO3 based catalysts N. Sayada, A. Saadi*,a, S. Nemouchib, A. Taibi-Benziadab, C. Rabiaa a

Laboratoire de Chimie du Gaz Naturel, Faculté de Chimie, Université des Sciences et de la Technologie Houari Boumediene (U.S.T.H.B.), B.P: 32 El-Alia, 16111 Bab-Ezzouar, Algiers, ALGERIA. * Correspondind author email : [email protected]. b Laboratoire des Sciences des Matériaux, Faculté de Chimie, Université des Sciences et de la Technologie Houari Boumediene (U.S.T.H.B.), B.P: 32 El-Alia, 16111 Bab-Ezzouar, Algiers, ALGERIA

Abstract Ca1-xMgx(Ti1-xLix)O3-3xF3x perovskite-type with 0 ≤ x ≤ 0.15 were prepared from CaCO3, TiO2, and MgF2 as metallic salt precursors using the ceramic method. Their characterization was performed by BET, X-ray diffraction and FTIR spectroscopy. The acid-base and/or redox properties of catalysts were evaluated using the benzaldehyde hydrogenation as reaction test in gas phase at atmospheric pressure. Theses catalysts showed very different catalytic behaviours in terms of activity and selectivity of products. Keywords: perovskite, benzaldehyde, hydrogenation, hydrogenolysis

1. Introduction ABO3 perovskite-type mixed oxides with alkaline-earth (Ca,Ba) and/or rare earth (La, Ce, …) on A site and a transition metal (Co, Zr or Ni…) on the B site, are of important interest as catalysts for various reactions such as oxidation, oxidative dehydrogenation and combustion of light hydrocarbons. The studies of selective catalytic hydrogenation of benzaldehyde have been largely devoted to the liquid phase system whereas relatively few studies have been reported for the gas phase system. Such reactions are currently catalyzed by systems involving supported noble metals and other supported metal oxides, including transition metal oxides [1-6]. Some works on benzaldehyde hydrogenation by alkaline-earth oxides have been also reported [5]. It was found that the reactivity of catalysts strongly depended on both the reducibility and surface acid–base properties that depend on their composition and the treatment conditions [7,8]. It is admitted that benzaldehyde is reduced to benzyl alcohol or toluene by the Cannizzaro reaction which required hydroxyl groups and its hydrogenolysis to benzene on acid sites. In the present study, we report the results obtained in the considered reaction over various perovskites catalysts under atmospheric pressure at 310°C.

2. Experimental The perovskite catalysts were prepared by ceramic method. Materials with compositions Ca1-xMgx(Ti1-xLix)O3-3xF3x where 0 ≤ x ≤ 0.15 were obtained by solid state synthesis from CaCO3, TiO2 (rutile), LiF and MgF2 as starting materials in an appropriate ratio according the following chemical reactions:

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Ca CO 3 + TiO 2

Ca TiO 3

(1-x) Ca TiO 3 + x MgF 2 + x LiF

Ca 1-xMgx(Ti1-xLix)O 3-3xF 3x

After intimate and ground mixing, obtained powders were calcined at 850°C/8h for CaTiO3 and 900°C/2h for Ca1-xMgx(Ti1-xLix)O3-3xF3x. X-ray diffraction patterns (XRD) were recorded at room temperature with a Philips PW1710 diffractometer using Cu-Kα radiation (λ=1.54060A°). BET measurements were obtained using a Micrometrics ASAP 2010 apparatus. Infrared spectroscopy of KBr pellets were recorded on a Fourier Transform IR Nicolet 550 apparatus. Benzaldehyde hydrogenation reaction was carried out in a fixed-bed Pyrex reactor tube at atmospheric pressure with 200mg of samples and a total flow of 50ml/min. Before testing, the catalysts were in-situ reduced for 16 h at 350°C in a current of H2 (20ml/min). The gaseous reactant and products were analyzed on line by flame ionisation (FID) detector (Delsi ICG 121 Ml).

3. Results and discussion 3.1. Catalyst characterization - The obtained results showed that the specific surface areas of the solids were very low (1m2/g) due to the high temperature of calcination. Similar results were observed in the case of La 1-xSrx V3+1-xV4+xO3 [9]. - X-ray patterns of ground samples are quite similar and display strong reflections of perovskite type mixed oxides with orthorhombic structure (JCPDS 48-0123 and JCPDS 812124). This result confirms single phase formation. After reduction After reduction under hydrogen flow, the figure 1 shows in addition to Before reduction diffraction peaks corresponding to perovskite Fig. 1. X-ray patterns of CaTiO3 catalyst before and after phase, an intense line at reduction under hydrogen flow. 2θ=28° attributed to TiO2 and another lines at 2θ=39° and 2θ=55° assigned to CaO and titanium metal (Ti°), respectively. These results indicate a partial decomposition of perovskite. -The IR spectra of Ca1-xMgx(Ti1-xLix)O3-3xF3x show the characteristic bands of the perovskite structure [9]. Characteristic wavenumber of OH bands (3600-3200cm-1), residual carbonates species (2923-2917 cm-1) and metal-oxygen stretching (700-400 cm-1) are identified (Fig. 2A). After reaction over CaTiO3 catalyst, the figure 2B shows a decrease in the intensity of the metal-oxygen band at ca.600cm-1, the apparition of intense band at ca.1400cm-1 attributed to benzoates species adsorbed on the catalyst surface [10] and the disappearance of hydroxyl species situated in the 3600-3200cm-1 region. This result suggests that the benzoates are the intermediate species for benzene and toluene formation [10].

Hydrogenation/hydrogenolysis of benzaldehyde over CaTiO3. A)

95 90

379

B)

Before reduction

85 X=0,15

T r a n s m is sio n %

80 75 70

X=0 1

65 60

X=0 05

55 50

X=0

45 40 35 30 4000

3600

3200

2800

2400

2000 1800 1600 Wavenumber (cm-1)

1400

1200

1000

800

600 450

Fig. 2. FTIR spectra of : A) CaTiO3 and Ca1-x Mg x (Ti1-x Lix )O3-3x F3x catalysts (0 ≤ x ≤ 0.15). B) CaTiO3 perovskite before and after reaction.

3.2. Catalytic activity The conversion and selectivities to toluene and benzene were determined after 1h of reaction, when nearly steady-states were reached. Table 1 shows the influence of chemical composition on catalytic properties of CaTiO3 and substituted perovskites on benzaldehyde conversion reaction. Compared to CaTiO3, the substituted perovskites are less active under H2 flow and the benzaldehyde conversion decreases with the increasing of Ca substitution by Mg in the following order: CaTiO3 > Ca0.95 > Ca0.90 > Ca0.85. The use of dihydrogen flow, as the carrier gas can provoke not only the direct hydrogenation of benzaldehyde but it can also modify the surface properties [10]. Thus, the dihydrogen pretreatment is expected to affect the hydroxyl groups (confirmed by IR) and therefore, surface basicity decreases with hydroxyls surface number. - The substituted perovskites exhibited a high selectivity towards the benzaldehyde hydrogenation to toluene product. A significant enhancement in the selectivity to toluene is observed in presence of substituted perovskites. These results can be explained on the basis of an electronic effect produced by the Mg, Li, and F species that allow the polarization of the carbonyl group. Toluene product is favoured when benzaldehyde conversion decreases. Indeed, Ca0.85 perovskite is the least active but exhibits high selectivity to toluene whereas CaTiO3 is very active but less selective. In the chemical process of toluene formation, the metal cation and hydride entities interacted with oxygen and carbon atoms of the benzaldehyde molecule: C6H5CHOg + M-Hads C6H5CH2O-Msurf + 2H2

C6H5CH2O-Msurf C6H5CH3 + H2O + M-Hads

- Benzene is formed only on reduced CaTiO3, Ca095 and Ca0.90 perovskites and the selectivity decreases in the following order: CaTiO3 > Ca095 > Ca0.90 > Ca0.85 = 0. It is believed to be the product of the hydrogenolysis of the external C(aryl)-C bond of benzaldehyde [10,11]. The study of the activity as a function of time on stream shows that toluene and benzene appear from the start of the reaction. This result significantly shows that both products are formed by independent ways: Ph-CH3 + H2O Ph-CHO C6H6 + CO

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N. Sayad et al.

This mechanism is in good agreement with published kinetics data on benzaldehyde hydrogenation over supported nickel catalyst [11]. The same reaction scheme was proposed with other catalytic systems [4,11]. Carbon monoxide was identified as coproduct of benzene [10,12]. Table 1. Catalytic results for benzaldehyde reduction over Ca1-xMgx(Ti1-xLix)O3-3xF3x perovskites (pre-reduction: 2h/350°C/H2; Treaction =310°C). Catalysts

Conversion (%) 26

Sel. Tol. (%) 19

Sel. Ben. (%) 81

Ca0.95Mg0.05(Ti0.95Li0.05)O2.85F0.15 (Noted Ca0.95)

23

61

39

Ca0.90Mg0.1(Ti0.90Li0.1)O2.7F0.3 (Noted Ca0.90) Ca0.85Mg0.15(Ti0.85Li0.15)O2.55F0.45 (Noted Ca0.85)

20

71

29

10

100

0

CaTiO3

4. Conclusion Our results showed that the hydrogenation of benzaldehyde over perovskite catalysts depended on chemical compositions. CaTiO3 is more active than the substituted perovskites. Benzene, hydrogenolysis product, is the main product formed over CaTiO3 catalyst and toluene, hydrogenation product, over substituted perovskites. In conclusion, the perovkite oxides can be used as catalysts in reduction reactions and their catalytic properties depend of the subtitution degree of Ca, Ti and oxygen by Mg, Li and F respectively.

References [1] J. E. Miller, A. G. Sault, D. E. Trudell, T. M. Nenoff, S. T. Thoma, N. B. Jackson, 2000, Applied Catalysis A: General, 201, 45-54. [2] N. Tien-Thao, H. Alamdari, M.H. Zahedi-Niaki, S. Kaliaguine, 2006, Applied Catalysis A: General, 311, 204-212. [3] T. Hayakawa, H. Harihara, A.G. Andersen, K. Suzuki, H. Yasude, T. Tsunoda, S. Hamakawa, A.P.E. York, Y.S. Yoon, M. Shimizu, K. Takehira, 1997, Applied Catalysis A : General, 149, 391. [4] A. Saadi, Z. Rassoul, M.M. Bettahar, 2000, Journal of Molecular Catalysis A: Chemical, 164, 205–216. [5] A. Saadi, R. Merabti, Z. Rassoul, M.M. Bettahar, 2006, Journal of Molecular Catalysis A: Chemical, 253, 79–85. [6] H. Provendier, C. Petit, C. Estournès, S. Libs, A. Kiennemann, 1999, Applied Catalysis A: General, 180, 163. [7] A. Saadi, Z. Rassoul and M.M. Bettahar, 2006, Journal of Molecular Catalysis A: Chemical, 258, 59-67. [8] H. Adkins and H. R. Billica, 1948, Journal of American Chemical Society, 70, 695. [9] P. N. Trikalitis, P. J. Pomonis, 1995, Applied Catalysis A: General 131, 309-322. [10] D. Haffad, U. Kameswari, M. M. Bettahar,A. Chambellan and J. C. Lavalley, 1997, Journal of Catalysis, 172, 85–92. [11] M. A. Keane, 1997, Applied Catalysis A: General, 118, 261. [12] M. A. Vannice and D. Poondi, Journal of Catalysis, 169, 166.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

VSbOx phases formed on MCM-41 supports Hanna Golinska, Maria Ziolek* A. Mickiewicz University, Faculty of Chemistry, Grunwaldzka 6, 60-780 Poznan, Poland, [email protected]

Abstract Sb-V-Ox phases were loaded on MCM-41, AlMCM-41 and NbMCM-41 by different methods (step by step impregnation and one pot adsorption) and calcined at different temperatures (813 and 923 K). The main focus was on NbMCM-41 support as its Nb interacts with Sb-V-Ox phases and protects them against agglomeration. XRD, UV-vis, XPS, and SEM techniques allowed us to determine the following phases: SbVO4; SbVO5; V2O5; NaSb5O13. Relationship between them depending on the preparation procedure is discussed. Keywords: NbMCM-41, AlMCM-41, SbVO4, SbVO5, V2O5, NaSb5O13

1. Introduction SbVOx based catalysts are well known to be effective in propane ammoxidation [1-3] and have recently been applied to the new reaction of glycerol transformation to acrylonitrile [4]. It has been previously reported that the chemical and physical properties of the support modify the structure of SbVO4 and its catalytic behaviour [5-7]. Therefore, the use of proper support and formation of a mixed oxide phase desired is a crucial point in preparation of effective catalyst. The idea of this work was to obtain Sb, V and Nb containing catalysts supported on mesoporous materials. The previous study [8] indicated that MCM-41 mesoporous structure is promising for VSbOx loading. In this work the effects of different preparation methods and conditions are studied in detail. The applied strategy is based on the introduction of different amounts of Sb and V oxide species on NbMCM-41 material, application of different sequences of step by step impregnation, post synthesis one pot adsorption and different temperature treatment. To explain the influence of niobium additive on the properties of the SbVOx phases, the supports without Nb (silicate - MCM-41 and aluminosilicate - AlMCM-41) are also used.

2. Experimental 2.1. Preparation of the catalysts Ordered mesoporous NbMCM-41(Si/Nb=64), MCM-41, AlMCM-41 (Si/Al=32) were synthesized following the procedure reported originally by Kresge et al. [9]. They were modified by wetness impregnation with vanadium (NH4VO3 - BDH Chemicals Ltd.) and antimony ((CH3COO)3Sb - Aldrich) sources. The first procedure covered the sequenced impregnation starting from vanadium and next antimony sources, and with the atomic excess of vanadium (V/Sb = 1.25 and 3 wt% Sb). Finally the catalyst was dried at 353 K for 12 h and calcined in air at 923 K for 96 h. The sample obtained was labelled as 1.25SbV/NbMCM-41(I). The second procedure included the stepwise impregnation with antimony and vanadium precursors in the reverse sequence (first Sb and next V) using V/Sb atomic ratios of 1 or 0.5 and ~25 wt % of Sb (the samples were

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H. Golinska and M. Ziolek

labelled as 1VSb/NbMCM-41(II) and 0.5VSb/NbMCM-41(II), respectively). Drying and calcination conditions were the same as those in the first procedure. The third procedure included the one pot adsorption method for the simultaneous modification with Sb and V sources with the use of the same amounts of Sb and V as in method II, followed by the calcination at 923 (96 h) or 813 K (6 h) (indicated in the sample labels by (III-923) and (III-813), respectively).

2.2. Characterisation The prepared materials were characterised by XRD (D8 Advance, Bruker diffractometer, CuKα radiation (α = 0.154 nm)). The surface area and pore volume of the materials were estimated by nitrogen adsorption at 77 K using the conventional technique on a Micromeritics 2010 apparatus. SEM studies were performed on a Philips SEM 515 operating at 15 kV. Powdered samples were deposited on a grid with a holey carbon film before transferring to electron microscope. Photoemission spectra (XPS) were collected by a VSW Scientific Instrument spectrometer, equipped with a standard Al Kα excitation source. The binding energy (BE) scale was calibrated by measuring C 1s peak (BE = 285.1 eV). UV–vis spectra were registered using a Varian-Cary 300 Scan UV–visible spectrophotometer. Catalyst powders were placed into the cell equipped with a quartz window.

3. Results and discussion Table 1. Characterisation of the catalysts. Catalyst*

V/Sb atomic ratio

%Sb

%V

BET area (m2/g)

Dp (nm)

Pore volume (cm3/g)

Crystal phases **

A/B (from XRD)

M 1.25SbV/M(I) 0.5VSb/M(II) 1VSb/M(II) NbM 1.25SbV/NbM(I) 0.5VSb/NbM(II) 1VSb/NbM(II) 1VSb/NbM(III-813) 1VSb/NbM(III-923) AlM 0.12SbV/AlM(I) 0.5VSb/AlM(II) 1VSb/AlM(II)

1.25 0.5 1 1.25 0.5 1 1 1 1.25 0.5 1

3 25 25 3 25 25 25 25 3 25 25

1.5 5 10 1.5 5 10 10 10 1.5 5 10

823 483 35 15 1006 885 52 27 461 27 772 671 44 32

2.2 2.1 1.9 1.9 2.6 2.2 1.9 1.9 2.1 2.7 2.2 1.9 1.9

0.7 0.6 0.09 0.04 1.1 1.0 0.1 0.07 0.9 0.7 0.1 0.06

AB AB AB A ABCD ABCD A AB AB AB AB

2.40 2.52 2.84 2.50 3.43 8.47 1.16 1.21 1.43

* M=MCM-41

** Phases: A – Sb0.95V0.95O4 ; B - NaSb5O13 ; C - SbxVyO5 ; D – V2O5

The small-angle XRD patterns (not shown here) for the MCM-41-based catalysts, are characteristic of the mesostructured materials with ordered hexagonal arrangement (p6mm symmetry) as long as SbVOx loading is relatively low or at the higher loading but the lower calcination temperature (813 K). They are characterised by a single Bragg peak (100) at 2θ ~ 2o and up to three peaks in the region of 2θ ~ 3-8o. All of these peaks are very well resolved in the XRD patterns of the pristine MCM-41, AlMCM-41 and NbMCM-41 supports and after loading of 4.5 wt % of Sb+V. When the total loading increases to 30 and 35 wt% (Sb+V) the XRD peaks significantly decrease. The results

383

VSbOx phases created on MCM-41 supports

o x

A

B

o

Intensity, a.u.

o

o xx xx

x x (a)

o

o x x

x

(b)

o - NaSb 5 O 13

x(e)

(f)

(c)

x

100

x

o x

o

x x

(h)

(g) (d)

(d) 2000

2Θ,

x - Sb 0.95 V 0.95 O 4

1000

x

10 20 30 o40 50 60

o x

C

10 20 30 40 50 60

2Θ,

o

(i) 10

20

30

2Θ,

o

40

50

60

Fig. 1. Wide-angle XRD patterns: (a) 0.5VSb/NbM(II); (b) 1VSb/NbM(II); (c) SbVO5 ; (d) NaSb5O13 (JCPDS 01-076-1230); (e) 1.25SbV/M(I); (f) 1.25SbV/AlM(I); (g) 1.25SbV/NbM(I); (h) 1VSb/NbM(III-923); (i) 1VSb/NbM(III-813).

are confirmed by the textural parameters calculated from nitrogen adsorption isotherms (Table 1). SbVOx loading causes a significant decrease in the surface area and pore volume depending on the amount of binary oxide introduced. However, the pore size does not decrease so significantly. TEM images (not shown here) indicate the blocking of pores by large crystallites of loaded oxides. Two crystalline phases are identified in the wide-angle XRD patterns (Fig. 1), Sb0.95V0.95O4 rutile structure and NaSb5O13 . Only Sb0.95V0.95O4 is active in the ammoxidation of propane and glycerol and therefore the formation of NaSb5O13 should be reduced. NaSb5O13 is formed with the sodium cations remaining from the siliceous source (sodium silicate) used in the synthesis. In the stepby-step impregnation (procedures I and II) the sequence of admission of V and Sb sources does not influence the relation between the amount of both phases, which is deduced from the relative intensities of XRD peaks assigned to each phase (Fig. 1A, B and Table 1). The increase in the content of V to 10 wt % causes the increase in Sb0.95V0.95O4(A) / NaSb5O13(B) ratio. When one pot adsorption method (procedure III) is used for the modification of the mesoporous supports, rutile Sb0.95V0.95O4 dominates and the amount of NaSb5O13 is much lower giving rise to high A/B ratio (8.47) (Fig. 1C(h) and Table 1). Interestingly, the decrease in the calcination temperature from 923 to 813 K leads to the disappearance of NaSb5O13 from XRD pattern (Fig. 1C(i)). Thus, on 1VSb/NbMCM-41(III-813) only Sb0.95V0.95O4 rutile phase is present. In the highly loaded samples (25 and 5 wt.% of Sb and V, respectively) the formation of needle/stake Sb0.95V0.95O4 rutile crystals is confirmed by SEM images (Fig. 2C). The increase in V content to 10 wt.% gives rise to the domination of the plate shaped SbxVyO5 (SbVO5) phase. Besides, both phases described above, UV-Vis measurements also implied the presence of V2O5 on all Nb containing samples (Fig. 2A). XPS results indicate the presence of niobium species in the framework of the NbMCM-41 support at +5 oxidation state (BE 208.7 eV), which does not change after the modification with the Sb-V-Ox phase. Vanadium gives many components in the XPS spectrum in the range of 516 – 524 eV, which covers various oxidation states of vanadium species. It is in line with the detection of SbVO4, SbVO5 and V2O5 phases. The effect of composition of the support (silicate MCM-41, aluminosilicate AlMCM41 and niobosilicate NbMCM-41) on the dispersion of Sb0.95V0.95O4 phase is well demonstrated in Fig. 1 B (e-g). The same amount of the Sb+V loading (4.5 wt %) gives rise to the XRD peaks of various intensities, the highest when MCM-41 support is used

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Fig. 2. A, B: UV-Vis spectra; C: SEM images: (a) 0.5VSb/NbM(II); (b) 1VSb/NbM(II).

and the lowest for niobosilicate one. Such behaviour is caused by the interaction with the support, the strongest when Nb is located in its structure, which protects from the agglomeration of the crystal phase loaded. The agglomeration is also stimulated by the calcination temperature; the lower the calcination temperature (813 K instead of 923 K) the smaller the crystals (lower agglomeration). The design of desired Sb-V-oxide phases is an important task in the preparation of catalysts active in the insertion of nitrogen into propane or glycerol in the ammoxidation process. To be able to do this, the total effect of all phases identified in this work on the activity and selectivity in this reaction should be taken into regard. The SbVO4 rutile phase is selective for acrylonitrile formation, whereas SbVO5 and V2O5, with V5+, are able to crack the propane molecule and activate the insertion of nitrogen towards acetonitrile. The formation of NaSb5O13 diminishes the activity. The presence of niobium tunes the selectivity of nitrile formation.

4. Conclusions Depending on the chemical composition of the ordered mesoporous support and the method of Sb-V-O loading, different Sb-V-O structures (Sb0.95V0.95O4; SbVO5; V2O5; NaSb5O13 ) and dispersion can be obtained on MCM-41. Relationship between various structures depends on the preparation procedure. One pot adsorption method and lower calcination temperature favour the formation of Sb0.95V0.95O4 rutile structure. This knowledge allows one to design the catalysts active in the ammoxidation processes.

Acknowledgements Polish Ministry of Research and Higher Education (grant 118/COS/2007/03) and COST D36/0006/06 are to be acknowledged for a partial support of this work.

References 1. 2. 3. 4. 5. 6. 7. 8. 9.

R. Nilsson, T. Lindblad, A. Andersson, C. Song, S.Hansen, 1994, Stud. Surf. Sci. Catal., 82, 293 G. Centi, S. Perathoner, 1995, Appl Catal. A, 124, 317 M.O. Guerrero-Pérez, J.L.G. Fierro, M.A. Vicente, M.A. Bañares, 2002, J. Catal., 206, 339 M.O. Guerrero-Pérez, M.A. Bañares, 2008, ChemSusChem, 1, 511 M.O. Guerrero-Pérez, J.L.G. Fierro, M.A. Bañares, 2003, Catal. Today, 78, 387 M.O. Guerrero-Pérez, M.A. Bañares, 2007, Chem. Matter., 19, 6621 M.O. Guerrero-Pérez, J.S. Chang, D.Y.Hong, J.M. Lee, M.A. Bañares, 2008, Catal. Lett. 125, 192 H. Golinska, P. Decyk, M. Ziolek, J. Kujawa, E. Filipek, 2009, Catal. Today, 142, 175 C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, 1992, Nature 359, 710

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Influence of the preparation conditions of Ca doped Ni/olivine catalysts on the improvement of gas quality produced by biomass gasification Diana C. Cárdenas-Espinosaa, Julio C. Vargasa,* a

Departamento de Ingeniería Química y Ambiental, Universidad Nacional de Colombia, Ciudad Universitaria, Avenida Carrera 30 No. 45-03, Edificio 453, Bogotá, Colombia * [email protected]

Abstract The influence of the preparation conditions (impregnation and calcination temperatures, and calcium content) on the catalytic behaviour of Ni-CaO/olivine catalysts, prepared by wet impregnation method, in the elimination of non interesting by-products (CO2, CH4) produced from biomass gasification was studied. To select the preparation parameters and to understand their influence, several physicochemical analyses such as zetametry, DTA, XRD, TPR and SEM, were performed. The methane dry reforming and subsequently the modified dry reforming in presence of H2 and CO, reproducing the gas-effluent composition after air-wood gasification process, were carried out. Keywords: methane dry reforming, Ni-CaO/olivine catalysts, impregnation method

1. Introduction Among the different technologies of biomass valorisation, gasification is a thermochemical process that allows obtaining a mixture of CO and H2 (syngas), which is an intermediate route for energy generation and chemicals commodities. This process coproduces organic compounds (tars) and other gases (i.e. CH4 and CO2) that reduce quality and profitability [1]. However, the impurities could be eliminated by catalytic processes, such as methane dry reforming (MDR) in order to increase the syngas content. The Ni/olivine catalysts are an important system previously developed and showed excellent performance and good stability to tar reforming and steam gasification processes, as well as in CO2 and steam methane reforming reactions [2]. This system is usually prepared by the wet impregnation method [3] that, in spite of its simplicity, presents some interesting parameters that influence the catalytic activity, such as the impregnation and calcination temperatures, which should be selected from scientific basis (i.e.zetametry) [4]. This work contributes to the understanding of the influence of the preparation conditions, on the catalytic behaviour of calcium doped and non-doped Ni/olivine catalysts, determined using zetametry, in situ thermal XRD and DTA.

2. Experimental 2.1. Catalysts preparation The Ni/olivine and Ni-CaO/olivine catalysts loaded with 4 wt.% nickel and calcium content between 0.0 and 3.7 wt.% Ca, were prepared by successive wet impregnations of olivine (at 20 or 60ºC) using Ca(NO3)2•4H2O and Ni(NO3)2•6H2O precursors, dried over-night at 110ºC and calcined for 4h after each impregnation. The calcination after

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Ca-impregnation were carried out at 600ºC, whereas the calcination after Ni-impregnation at 900 or 1100ºC. The calcination temperatures were selected from DTA analysis and in-situ thermal XRD. The catalysts were designated using “Ni” and “Ca” to indicate the metal, the letter “M” represents the impregnation temperature and the letter “C” the calcination temperature.

2.2. Reactivity tests The catalytic tests were carried out in a fixed-bed “U” quartz-reactor, with 100 mg of catalyst in a total gas flow of 50 mL.min-1, at 800 °C and atmospheric pressure. The reaction products were analysed by TCD-gas chromatography (Carboxen1006 PLOT and MS 13X). The influence of Ca-content in methane dry reforming reaction (MDR), was determined using an gaseous mixture of CO2/CH4/Ar (2.5:2.5:45.0), and using catalysts activated by reduction under hydrogen atmosphere (5mL.min-1 of pure hydrogen, between 25 and 750°C with a heating rate of 2 C.min-1). In the first case, the temperature profile includes two cycles between 400 and 800°C and finally an isotherm at 800°C for 10 h, whereas for the tests with H2 reduction, after an Ar purge, the temperature profile test consist in an isotherm at 800°C for10 h. For the evaluation of H2 and/or CO presence, the feeding-mixture was rated to the composition of gaseous effluent obtained by biomass air-gasification including a CH4:CO2:H2:CO gas mixture (8:12:40:40 vol.), using the catalysts reduced under H2 flow.

2.3. Catalysts characterisation The Point of Zero Charge (PZC) of olivine was determined at 20ºC (Zeta-Meter System 3.0+ apparatus). The influence of the preparation parameter on the crystalline properties was determined by in situ thermal XRD (PANalytical X’Pert PRO MPD with a HTK-16 oven), obtaining the diffractograms between 10 and 90º2θ using a Cu Kα anode (1.5406 nm), and heating the samples at 10ºC.min-1 under inert atmosphere. The DTA analyses (TA Instruments 1600-DTA) were carried out heating the samples at 5ºC.min-1 between 20 and 1200ºC under oxidant and inert atmosphere and the TPR analyses (QUANTACHROME Chembet 3000 equipment) were performed on 150 mg of catalyst, under H2 (30% H2/Ar) flow with a heating rate of 10ºC.min-1 from ambient temperature to 950ºC. The morphology and chemical composition were determined by SEM-EDXS (JEOL JSM -6700F Field Emission Microscope).

3. Results and discussion In Fig. 1 and 2 are presented the Zeta potential curve and the influence of temperature on the Point of Zero Charge (PZC) for the olivine, respectively.

Fig. 1. Zeta potential curve and Point of Zero Charge (PZC) for olivine (Mg0.9Fe0.1)2SiO4.

Fig. 2. Influence of temperature over olivine Point of Zero Charge (PZC).

Considering that the pH of the nickel nitrate solution used in the catalyst preparation is 4.93 and is higher than pHPZC,20ºC, at the preparation conditions the olivine surface is

Influence of the preparation conditions of Ca doped Ni/olivine catalysts

387

negatively charged and the Ni2+ adsorption is favoured, increasing the probability of a higher interaction with the support during the calcination. Otherwise for the calcium, which solutions have a pH lower of the pHPZC, 20ºC (1.39 ≤ pH ≤ 2.06), the complex adsorption of Ca2+ is not favoured. These interactions tends to be strengthened with temperature increases (see Fig. 2), and are not solely physical since its involves complex mechanisms such as triple layer models, surface complexation and ion exchange [5]. The in situ thermal X-Ray diffractograms (Fig. 3) show the main diffraction peaks of olivine (not indicated) and the presence of α-Fe2O3 and NiO-MgO phases [1,2]. The first one due to the iron rejection from the olivine bulk at temperatures above 700ºC, whereas the second one is produced at temperatures above 1000ºC as a result of the interaction between the nickel and the olivine surface. Additionally, the formation of secondary olivine phases, such as enstatite (MgSiO3) and cubic spinel (MgFe2O4), are observed. The presence of the catalytic phases (α-Fe2O3 and NiO-MgO) is favoured between 900 and 1100 ºC, whereas the calcination at higher temperatures causes the nickel integration to olivine structure reflected in the reduction of NiO-MgO lines intensity. The presence of calcium is determined by detection of CaO and CaMgSiO4 phases, their contributions remains unchanging with temperature increase and not affect the lines associated with Ni-presence, however, other results (not presented) indicate that CaO intensity increase with calcium content whereas the CaMgSiO4 intensity just become substantial with the highest calcium contents (3.03 and 3.70 wt.%). This interaction are confirmed from the DTA results (Fig. 4), where the phase transitions are related with the nitrate-group decomposition, as well as the establishment of strong interactions between the metals (Ni and Ca) and the olivine, and related with the TPR results (Fig. 5), where is observed a more difficult reduction for the catalyst calcined at 1100 ºC, which is related with the presence of strong Ni-olivine interactions.

Fig. 3. In situ thermal X-Ray diffractograms for the calcination of NiCa1.68 catalyst.

Fig. 4. DTA results before calcinations of NiCa1.68 catalysts.

In Fig. 6 are presented the H2 and CO yields of Ni-CaO/olivine catalysts after reactant and hydrogen reducing atmosphere. For the catalyst activated by reactiveatmosphere reduction, H2-CO yield changes with Ca-content, decreasing until complete inactivity for the high Ca-loaded samples (3.03 and 3.70 wt.%). The H2-reduced catalysts (except the 3.70 wt.% Ca sample) reached H2-CO yields above 80%, obtaining the best results for 1.68 wt.% Ca sample. The 3.70 wt.% Ca sample with hydrogen reduction presents high selectivity towards CO but low H2 yield that along with the SEM results (Fig. 7) evidence the possible coverage of Ni-particles by the calcium oxide. At higher contents of calcium, it is observed the formation of ceramic structures of Ni-Ca, avoiding the interaction between the nickel and the olivine and decreasing the catalytic active phase formation.

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Fig. 5. TPR profiles for olivine, NiM60C900 and NiM60C1100 catalysts.

Fig. 6. H2 and CO yields in the MDR reaction (filled symbols: reduction under reactant flow, void symbols: reduction under H2 flow).

Fig. 7. SEM images of NiM60C1100 and NiCa3.70 catalysts – Calcium content effect.

Table 1 summarised the gas phase distribution of reaction products obtained for Ni/olivine and Ni-CaO/olivine (1.68 wt.%) catalysts using the CH4:CO2:H2:CO model feeding-mixture. It is observed that the calcium presence on Ni/olivine catalysts promotes the CH4 conversion and H2 production, but not affect the CO2 conversion significantly, reducing significantly the amount of gaseous impurities of the syngas. Table 1.Gas phase distribution of reactor effluent for Ni/olivine and Ni-CaO/olivine catalysts. Conversion/yield (%) Ca (wt.%) 0.00 1.68

CH4 75.62 77.72

CO2 87.20 83.36

H2 93.07 88.98

Gas phase distribution (%) CO 52.79 50.84

CH4 1.61 1.44

CO2 2.38 3.03

H2 52.64 53.38

CO 43.37 42.14

4. Conclusion The impregnation method conditions of Ni and Ni-CaO supported olivine catalyst have great influence in the system and catalytic behaviour. The temperature of impregnation affects the interaction between the metal and the support, whereas the calcination temperature allows obtaining catalytic stable species. The parameters selection on physicochemical basis allow obtaining an active catalyst. The presence of small amounts of calcium in Ni/olivine catalysts improves the conversion of CH4 and the selectivity of H2 in the reaction, independently of the presence or absence of H2 and CO.

References [1] [2] [3] [4] [5]

C. Courson, E. Makaga, C. Petit and A. Kiennemann, Catal. Today 63 (2000) 427. C. Courson, L. Udron, D. Swierczynski, C. Petit, A. Kiennemann, Catal. Today 76 (2002) 75. R. Zhang, Y. Wang, R. Brown, Energy Convers. Manag. 48 (2007) 68. P. Brunelle, Pure & Appl. Chem. 50 (1978) 1211. A. Lekhal, B. Glasser., J Khinast, Chem. Eng. Sci. 59 (2004) 1063.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Effect of ethylenediamine as chelating agent of cobalt species upon the cobalt-support interactions: application to the VOC catalytic removal Frédéric Wyrwalski,a,b,c Jean-Marc Giraudon,a,c,d Jean-François Lamoniera,c,d a

Univ Lille Nord de France, F-59000 Lille, France UArtois, Unité de Catalyse et de Chimie du Solide F-62400 Béthune, France c CNRS, UMR8181, France d USTL, Unité de Catalyse et de Chimie du Solide F-59652 Villeneuve d’Ascq, France b

Abstract The ethylenediamine presence was studied in the Co/CeO2 catalytic system. Using ethylenediamine, highly dispersed Co3O4 species were produced at the CeO2 surface. In the ethylenediamine absence (catalyst conventionally prepared), a synergistic coupling of the redox properties of CeO2 and Co3O4 was evidenced. Keywords: Co3O4, CeO2, ethylenediamine, VOC

1. Introduction Volatile Organic compounds (or VOCs) include a variety of substances, which may be either natural or anthropogenic origin. They are always composed of carbon element combined with other one such as hydrogen, halogen and oxygen. Their volatility gives them the ability to diffuse more or less away from their place of issue, causing thus direct and indirect impacts on human health and nature. Anthropogenic sources of VOCs are globally approximately 10 % of non-methane release but, in the industrialized regions, these sources are becoming majority. Over the last few years, environmental legislation has imposed increasingly stringent limits for VOCs emissions level. The treatment by catalytic oxidation is one of the most promising ways to reduce VOCs in the atmosphere since this technique allows operating at low temperatures (200-500°C) and thus leading to NOx formation in lower quantity. In most commercial applications, supported Pt, Pd, or mixed metal oxides [1,2] are usually employed. In recent times, efforts have been made to find first row transition metal catalysts, which are cheaper [3,4]. Among them bulk Co3O4 is a very active phase for hydrocarbon oxidation [5]. But the dispersion of cobalt species over a suitable support is a challenge since new chemical species can be generated depending on the precursor and support nature used and metal loading employed. Multiple impregnations with intermediate drying and calcination are usually performed to minimize the formation of large metal oxide particles [6]. In this study, Co(5wt%)/CeO2 solids have been synthesized using single impregnation in the presence or absence of ethylenediamine, a chelating agent of cobalt species.

2. Experimental 2.1. Sample preparation Cerium hydroxide was prepared by drop wise addition of an aqueous ammonium hydroxide solution (0.7 M) to a cerium nitrate Ce(NO3)3,6H2O solution 0.15 M keeping

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pH higher than 9. To obtain the CeO2, the precipitated hydroxide was filtered, washed and dried at 100°C before calcination in a flow of dried air (2 L/h) during 4 h at 600°C. The supported Co catalysts with 5 wt% cobalt were prepared by wet impregnation. Two different cobalt solutions were used to impregnate the supports. The first one was prepared by dissolution of the appropriate amount of cobalt nitrate in distilled water (samples named CoNit). The second impregnation solution was obtained by mixing an ethylenediamine–cobalt nitrate solution (1:1) with distilled water (samples named CoEn). The pH of the solution was 7.5. At this value [Co-en3]2+ and [Co-en3]2+ species are formed in the same proportion [7]. Consequently, using an “en/Co” ratio of 1, all the ethylenediamine used must be chelated with cobalt ions, whereas some cobalt species should be free of ethylenediamine. After evaporating the excess of solvent at 60°C under vacuum, the samples were dried at 100°C for 20 h followed by an activation treatment in a flow of dried air (2 L/h) during 4 h at 450°C.

2.2. Sample characterization The cobalt content was determined by atomic absorption at the ‘‘service central d’analyse du CNRS” (Vernaison, France). Differential thermal and thermo gravimetric analyses (DTA/TG) were conducted in flowing air (75 mL min-1) at a heating rate of 5°C min-1. Surface area analysis was performed using the BET method. N2 was selected as the adsorbent, and analysis was performed at liquid nitrogen temperature (–196°C). X-ray diffraction (XRD) measurements were made at room temperature (λ =1.5418 Å). The diffraction patterns have been indexed by comparison with the Joint Committee on Powder Diffraction Standards (JCPDS) files. Temperature Programmed Reduction (TPR) was performed using a 5% H2/Ar mixture (30 mL min-1 - 5°C min-1).

3. Results and discussion Thermal analyses have been performed on dried samples. According to the cobalt content in the sample, the experimental weight losses (Table 1) correspond to the cobalt precursor transformation into Co3O4 at 450°C. For CoNit sample, the small endothermic peak at around 180°C can be attributed to the nitrates decomposition (Figure 1). For CoEn sample, an exothermic peak at around 210°C is observed. A similar phenomenon was already observed during the calcination of cobalt acetate deposited on silica [8] and attributed to the carbon combustion of the precursor. The high exothermicity of the reaction can be explained by the autocatalytic effect due to the presence of cobalt oxide. Two peaks can be observed, the peak at higher temperature being attributed to Co free ethylenediamine combustion [5]. In this study, the absence of this peak indicates that ethylenediamine is mainly chelated with cobalt species. The shoulder in the exothermic peak at low temperature could be explained by the thermal decomposition of cobaltethylenediamine species close to the anionic vacancies of the ceria support [5]. After cobalt impregnation, a decrease in the specific surface area is observed (Table 1). This phenomenon is emphasized using ethylenediamine during the preparation. This strong decrease could be explained by the size of Co-en22+ and Co-en23+ complex ions probably present at the ceria surface [7]. After activation treatment at 450°C, the specific surface areas values are close to that of supports according to the previous thermogravimetric analyses which revealed a complete Co-precursor decomposition at this temperature. However the specific surface area of CoNit is lower than that of pure ceria and CoEn (Table 1). The modification of the surface by cobalt insertion into the ceria framework could explain this result. XRD patterns of calcined samples exhibit diffraction lines characteristics of the cubic structure of CeO2 without Co3O4 phase detection because Co species are well dispersed and/or are incorporated into the support lattice. Indeed, the

391

Effect of ethylenediamine as chelating agent of cobalt species

well cobalt species dispersion can be explained by the use of ethylenediamine: as the cobalt atom is surrounded by the ethylenediamine ligands, cobalt atoms are forced to be far apart one from another leading to highly dispersed Co3O4 undetectable by XRD. Moreover the cobalt incorporation into the CeO2 lattice forming homogeneous Ce-Co-O solid solutions is consistent with the smallest ionic radii of Co2+ and Co3+ in comparison with that of Ce4+. But due to the low cobalt content in the sample, a change in the lattice parameters of ceria in CoNit is probably not observable. Table 1. Characterization of ceria based samples. Samples

Co content (wt. %)

Weight loss (%)

Specific surface area (m2 g-1)

H2 consumption (μmol g-1)

CeO2

-

-

61

421

CoNit

4

8.25

45 (48)*

1900

CoEn

4.56

12.38

12 (57)*

1374

* after activation treatment at 450°C Cesurf + βB

CoNit

↓

↑ ↓

CoEn

Temperature (° C) 100

150

200

250

300

H2 consumption

DTA signal

αΑ

CoEn Cesurf + αA

100 350

Fig. 1. DTA signals obtained during the air treatment.

βΑ

αB

βΑ αB

βB

CoNit CeO2

300 500 700 900 Temperature (°C)

Fig. 2. TPR signals obtained during the H2 treatment.

H2-TPR measurements have been performed over calcined samples (Table 1 and Figure 2). For pure ceria the peak at 525°C is assigned to the reduction of surfacecapping oxygen of ceria whereas the peak at 810°C observed for the three samples is ascribed to the reduction of lattice oxygen in bulk CeO2 [9]. For CoEn sample, the four peaks at 210, 250, 310 and 440°C are assigned to the reduction of the species related to the Co3O4–CeO2 interaction. This interaction promotes the reduction of ceria at lower temperature (440°C) in comparison with that of individual CeO2 (525°C). Numerous studies have reported that the reduction temperature of metal oxide by H2 can be lowered by the addition of another metal [10]. In our case Co3O4 is probably first reduced to metallic cobalt at lower temperature and then H2 is dissociated over these Co atoms leading to activated hydrogen atoms which can reduce CeO2 more easily. If we postulate that the reduction of surface-capping oxygen of ceria takes place at around 440°C, then the reduction of Co3O4 into Co occurs into 4 steps αA (66 μmol H2 g-1), βA (202 μmol H2 g-1), αB (146 μmol H2 g-1) and βB (529 μmol H2 g-1). The Co3O4 reduction

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by hydrogen proceeds through the two following steps : Co3O4+H2→3CoO+H2O (α) and CoO+H2→Co0+H2O (β). The experimental ratio α/β well corresponds to the ratio of hydrogen consumptions of 1:3 involved in the both reactions. Therefore two different types of Co3O4 species “A” and “B” are probably dispersed on the ceria surface. Since ceria is known as oxide having anionic vacancies, the two species could be distinguished from other by the distance separating them from the vacancies according to the thermal analysis results. For CoNit sample, a H2-overconsumption is observed at 210°C (Figure 2) and can be attributed to the reduction of Ce4+ at low temperature [11]. Assuming the reduction temperature of surface-capping oxygen of ceria operates at 210°C, Co3O4 reduction into Co occurs in 4 steps αA (59 μmol H2 g-1), βA (195 μmol H2 g-1), αB (149 μmol H2 g-1) and βB (506 μmol H2 g-1). Then in CoNit solid, “A” and “B” species are also present. The extended reduction of ceria can be explained by the partial Co2+ incorporation into the ceria lattice, as suggested by BET and XRD analyses. Such a decrease of the reduction temperature of the host oxide to a greater extent than for pure CeO2 has already been observed in the Ni-Ce and Cu-Ce systems [12].

4. Conclusion Using cobalt nitrates as precursor, the possible cobalt insertion in the surface ceria lattice leads to a synergistic coupling of the redox properties of ceria and Co3O4. By the addition of ethylenediamine during the catalyst preparation, Co species do not enter in the ceria due to the preliminary formation of Co-ethylenediamine complex, but rather contribute to well dispersed Co3O4 phase after activation treatment. Consequently CoEn is more active than CoNit in the propylene (model molecule to mimic the total catalytic oxidation of a VOC of hydrocarbon type) total oxidation : the T50 is lowered by 20°C.

References [1]

M. Guillemot, 2007, Volatile organic compounds (VOCs) removal over dual functional adsorbent/catalyst system, Appl. Catal. B: Environ., 75, 249-255 [2] J.-F. Lamonier, 2007, Catalytic Removal of Toluene in Air over Co–Mn–Al Nano-oxides Synthesized by Hydrotalcite Route, Catal. Lett., 118, 165-172 [3] J. Carpentier, 2007, Synthesis and characterization of Cu–Co–Fe hydrotalcites and their calcined products, J. Porous Mat., 14, 103-110 [4] S.C. Kim, 2009, Properties and performance of Pd based catalysts for catalytic oxidation of volatile organic compounds, Appl. Catal. B: Environ., 92, 429-436 [5] F. Wyrwalski, 2007, Additional effects of cobalt precursor and zirconia support modifications for the design of efficient VOC oxidation catalysts, Appl. Catal. B: Environ., 70, 393-399 [6] L.A. Boot, 1996, Preparation, characterization and catalytic testing of cobalt oxide and manganese oxide catalysts supported on zirconia, Appl. Catal. A, 137, 69-86 [7] E. Norkus, 2001, Oxidation of cobalt(II) with air oxygen in aqueous ethylenediamine solutions, Trans. Metal Chem., 26, 465-472 [8] L. Poul, 2000, Layered Hydroxide Metal Acetates: Elaboration via Hydrolysis in Polyol Medium and Comparative Study, Chem. Mater., 12, 3123-3132 [9] H.C. Yao, 1984, Ceria in automotive exhaust catalysts: I. Oxygen storage, J. Catal., 86, 254265 [10] H. Zhu, 2004, Pd/CeO2-TiO2 catalyst for CO oxidation at low temperature: a TPR study with H2 and CO as reducing agents, J. Catal., 225, 267-277 [11] D. Terribile, 1999, Catalytic combustion of hydrocarbons with Mn and Cu-doped ceriazirconia solid solutions, Catal. Today, 47, 133-140 [12] G. Wrobel, 1996, Effect of incorporation of copper or nickel on hydrogen storage in ceria. Mechanism of reduction, J. Chem. Soc., Faraday Trans., 92, 2001-2009

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Influence of support on the ammoxidation activity of VPO catalysts V. N. Kalevaru*, B. Luecke, A. Martin Leibniz Institute for Catalysis at the University of Rostock, Albert-Einstein-Str. 29a, 18059 Rostock, Germany

Abstract Ammoxidation of 2,6-dichlorotoluene to 2,6-dichlorobenzonitrile was carried out at atmospheric pressure using VPO catalysts. Different catalyst supports were used and their influence on the catalytic performance has been evaluated. Nature of support has shown clear influence on the activity and selectivity behaviour of the catalysts. Higher activity (X-DCT > 90 %) and selectivity (S-DCBN > 75 %) along with good long-term stability could be successfully achieved over TiO2 (anatase) supported VPO catalyst. Keywords: ammoxidation, 2,6-dichlorotoluene, 2,6-dichlorobenzonitrile, VPO solids

1. Introduction Ammoxidation of alkyl aromatics and heteroaromatics is an interesting reaction for producing various industrially important nitriles in a more economic and environmental friendly way. Ammoxidation of 2,6-dichlorotoluene (DCT) in particular is of great importance because the target product 2,6-dichlorobenzonitrile (DCBN) has a wide range of applications starting from herbicides, pesticides to special kind of engineering plastics [1]. The most active and selective catalyst systems that are generally used for different ammoxidation processes are based on V-Ti-O, V-Mo-O, V-Sb-O, VOx/SiO2, VOx/Al2O3, Fe-Sb-O etc [e.g. 2]. On the other hand, vanadium phosphorus oxides (VPO) are special kind of solids that can also be applied as potential ammoxidation catalysts [e.g. 3]. Recently, we have explored the influence of P/V ratio on the catalytic performance of VPO catalysts for the ammoxidation of DCT to DCBN and found that the catalyst with a P/V ratio of 0.7 has exhibited the better performance [4]. Based on this observation, the work is further extended to investigate various aspects of VPOs in relation to their catalytic properties. In this contribution, we describe the catalyst preparation, basic characterisation and the influence of support on the catalytic performance of VPO materials with a special emphasis devoted to enhance the conversion of DCT and selectivity of DCBN along with good long-term stability.

2. Experimental 2.1. Catalyst preparation VPO precursor (P/V=0.7) is prepared through an organic route. Supported catalysts were prepared by solid-solid wetting method and calcined at 450°C for 3 h [5]. The content of VPO was kept constant at 25wt% and the supports used are TiO2 (anatase), γAl2O3 and β-AlF3. Catalysts were characterised by BET surface area, XRD, FTIR, bulk distribution of oxidation states by potentiometric titrations.

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2.2. Catalytic tests Catalytic tests were performed in a fixed bed tubular quartz reactor in the temperature range of 345 to 425°C. The products were analysed by off-line GC. However, the total oxidation products were determined on-line by non-dispersive IR analyser (Infralyte 40E, Germany).

3. Results and discussion 3.1. Catalyst characterisation 3.1.1. BET-surface areas and pore volumes of pure and supported VPO solids BET-surface areas (BET-SA) and pore volumes (PV) of pure and supported VPO samples are depicted in Table 1. It can be seen from the Table that the surface areas are clearly dependent on the nature of support applied. The pure VPO sample (P/V = 0.7) has exhibited a low surface area of 17.1 m2/g with a total pore volume of 0.043 cm3/g. Although both the pure supports (TiO2 and Al2O3) exhibited comparable surface areas, the final catalysts after VPO addition showed considerably different surface areas. This result implies that the nature of support has an influence on the surface areas as well as pore volumes, which might be due to varying active phase-support interaction. Table 1. Surface areas and pore volumes of pure and supported VPO solids (P/V=0.7). Catalyst 25% VPO/TiO2 25% VPO/γ-Al2O3 pure TiO2 pure γ-Al2O3 bulk VPO (0.7 P/V) SA = surface area; PV = pore volume

BET-SA (m2/g) 53.8 81.6 107.5 101.8 17.1

PV (cm3/g) 0.176 0.187 0.262 0.191 0.043

In addition, the samples were also characterised by XRD and FTIR. XRD showed the formation of VOHPO4*0.5 H2O phase in the precursor, while (VO)2P2O7 phase in the calcined samples, as expected. FTIR results lent good supporting evidence for the observations made from XRD. 3.1.2. Bulk distribution of oxidation states of vanadium in supported VPO catalysts The distribution of oxidation states of vanadium in the bulk and supported catalysts are presented in Table 2. Despite identical calcination conditions, the samples showed different distribution of vanadium oxidation states. This fact again suggests that the nature of support has a clear influence on the distribution of vanadium oxidation states. The bulk VPO solid and the TiO2 supported VPO catalysts are totally devoid of V+3 species, while γ-Al2O3 supported one showed certain amounts of V+3 species. Another common feature between bulk and TiO2 supported VPO is that they both exhibit smaller amounts of V+5 species and a comparable average oxidation state of vanadium. On the other hand, V+5 species are completely absent in VPO/γ-Al2O3 sample. Overall, an average oxidation state of vanadium in these samples is varied from 3.89 to 4.08 (Table 2). It has been observed earlier that the oxidation state of vanadium is an important factor in determining the activity and selectivity behaviour of the catalysts [6]. The results showed that the VPO catalysts with an average oxidation state in slight excess of 4 are found to exhibit better performance than the ones showing below 4.

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Table 2. Bulk distribution of oxidation states of vanadium in supported VPO catalysts with 0.7 P/V ratio. catalyst*

% V+3

% V+4

% V+5

25% VPO/anatase 0 8.16 0.65 0.90 7.21 0 25% VPO/γ-Al2O3 Bulk VPO (0.7 P/V) 0 31.49 2.60 * calcined at 450°C / 3h; AV = average oxidation state of vanadium

% Vtotal

AV

8.81 8.11 34.09

4.07 3.89 4.08

3.2. Catalytic tests

%

3.2.1. Influence of support on the catalytic performance of VPO solids Figure 1. illustrates the influence of support on the catalytic performance of VPO solids. DCBN is the desired product, while CO and CO2 are unavoidable by-products from the total oxidation. Even though the content of VPO is constant in all the catalysts (i.e. 25wt%), the catalytic activity 100 X-DCT S-DCBN and selectivity depended on the 80 type of support used. Among three different supports app60 lied, TiO2 (anatase) supported 40 one exhibited the best performance while the β-AlF3 sup20 ported catalyst gave the poor 0 performance. X-DCT of over TiO2 γ-Al2O3 β-AlF3 1 2 3 90 % with a DCBN selecsupport Fig. 1. Effect of support on the catalytic performance of tivity of > 75 % could be achieved over TiO2 supported 25wt%VPO catalysts (0.7 P/V). VPO solid. As a result, the DCBN yield of ca. 70 % is obtained in the best case. On the other hand, γ-Al2O3 supported one showed the highest conversion of DCT close to 100 %, but the selectivity of DCBN is less than TiO2 supported one. After obtaining such good performance, the long-term stability of the catalyst is tested and showed below. 3.2.2. Time-on-stream behaviour of 25wt%VPO/TiO2 (0.7 P/V) catalyst Time-on-stream behaviour of 25wt% 100 420 VPO/TiO2 solid is shown in Fig. 2. It is 90 temp evident that the catalyst has exhibited quite 80 70 consistent performance over a period of 380 60 90 hours without any deactivation. The 50 conversion of DCT is varied in the range 40 from 90% to almost 100%. DCBN is the 340 30 major product, however, considerable 20 amounts of total oxidation products such 10 0 300 as CO and CO2 can also be found in the 0 20 40 60 80 100 time, h product stream. The sum of total oxidaFig. 2. Time-on-stream behaviour of 25wt% tion products is varied from ca. 20% to a VPO/TiO2 catalyst. maximum of 30%. The reaction was performed at a temperature of 400 - 410°C. The carbon balances are found to vary in the range from 90 to 98 mol%. After such long term tests, the spent catalyst was again characterized by various techniques and the summary of such results is shown below. C-Bal conv. (2,6-DCT)

sel.(2,6-DCBN)

T,°C

%

yield(2,6-DCBN)

CO2

CO

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3.2.3. Comparison of fresh & spent 25wt%VPO/TiO2 (0.7 P/V) after 90 h-on-stream Comparison of changes observed between the fresh and the spent 25wt%VPO/TiO2 catalyst after 90 hours of long-term tests is presented below in Table 3. The results showed that the reaction conditions have brought considerable changes in the catalyst structure. The surface area and pore volume of fresh sample decreased from 53.8 to 36.3 m2/g and 0.176 to 0.156 cm3/g, respectively. In addition, some changes in the phase composition and average oxidation state of vanadium could also be noticed. Formation of (NH4)2(VO)3(P2O7)2 phase is observed in the spent sample, which might be the reason for decreased surface area of used catalyst. It is known that (VO)2P2O7 phase partly transforms into (NH4)2(VO)3(P2O7)2 phase due to the presence of NH3 in the reactant feed gas. However, the contents of phosphorus, vanadium and titanium are found to remain unchanged even after 90 hours of catalytic tests. This result indicates that leaching of catalyst components did not occur during the course of the reaction. Negligible amount of coke can also be found in the spent sample (Table 3). Table 3. Changes observed between the fresh & spent 25wt% (0.7) VPO/TiO2 catalyst after 90 h-on-stream studies. catalyst BET-SA PV phase composition ICP (m2/g) (cm3/g) %P %V %Ti fresh 53.8 0.176 (VO)2P2O7 4.56 8.78 43.79 + TiO2 (anatase) used 36.3 0.156 (NH4)2(VO)3(P2O7)2 4.56 8.80 42.63 + (VO)2P2O7 + TiO2 (anatase) PV = pore volume; AV = average oxidation state of vanadium

C H N analysis %C %H %N -

AV

0.07 0.27 1.2 3

4.26

4.07

4. Conclusions The catalytic properties of VPO catalysts are found to depend on the type of support used. Among different supports applied, TiO2 exhibited the best performance with good long-term stability. No leaching of catalyst components (V, P, Ti) is noticed in the spent solids. However, some changes in the phase composition and average oxidation state of vanadium could be observed after 90 hours-on-stream. In conclusion, ammoxidation is a valuable tool for producing various industrially important nitriles and the future of ammoxidation looks brighter; however new revolutionary ideas have yet to come.

References 1. 2. 3. 4. 5. 6.

Z. Qiong, H. Chi, X. Chongwen, H. Qiyong, 1997, CN 1166378 A. B. Luecke, K.V. Narayana, A. Martin, K. Jähnisch, 2004, Adv. Synth. Catal. 346, 1407. A. Martin, B. Lücke, 2000, Catal. Today, 57, 6. V.N. Kalevaru, B. Luecke, A. Martin, 2009, Cat. Today 142, 158. A. Martin, V.N. Kalevaru, B. Lücke, D. van Deynse, M. Belmans, F. Boers, 2003, WO 03/101939 A2 (Tessenderlo Chemie S.A., BE). K. V. Narayana, A. Martin, B. Lücke, M. Belmans, F. Boers, D. van Deynse, 2005, Z. Anorg. Allg. Chem. 631, 25.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Rationalization of the aqueous impregnation of molybdenum heteropolyanions on γ-alumina support J. Moreau,a,b O. Delpoux,a K. Marchand,a M. Digne,a S. Loridantb a b

IFP, IFP-Lyon, Rond-point de l'échangeur de Solaize BP 3, 69360 Solaize, France IRCELyon, UMR5256 CNRS-Université Claude Bernard Lyon 1, 2 avenue Albert Einstein, 69626 Villeurbanne, France

Abstract The aim of this work was to determine the physico-chemical parameters governing impregnation of H4Co2Mo10O386- Anderson-like heteropolyanion (HPA) solutions on γalumina. This species was first synthesized in solution and found to be stable in acidic medium and large molybdenum concentration range, as determined by Raman spectroscopy and Principal Component Analysis (PCA). Impregnation of various solutions on γ-alumina showed that H4Co2Mo10O386- dimers are kept only at high molybdenum concentration. Keywords: Impregnation, H4Co2Mo10O386- Anderson HPA, Raman spectroscopy, Principal Component Analysis (PCA), Speciation

Introduction The preparation of supported catalysts is a sequence of elementary steps. Among them, the impregnation consists in depositing the active phase precursors on the surface of a porous support [1]. This step, followed by the drying, is crucial for the catalytic activity: at this stage, the dispersion, the molecular structure of active phase precursors and, to some extent, their interactions with the support, are fixed. Few data are available to rationalize the different physical and chemical phenomena occurring during impregnation and drying: diffusion into pores, adsorption, acid-base reactions with the support, partial dissolution of the support… The aim of this work was to determine the physico-chemical parameters governing impregnation on γ-alumina of H4Co2Mo10O386- Anderson-like HPA whose catalytic interest was already proved [1-4]. Indeed this species has an optimum Co/Mo ratio of 0.5 with Co(II) counter-ions and the presence of molybdenum and cobalt into the same molecular structure must enable benefic interactions for the formation of the final (Co)MoS2 active phase of hydrotreatment catalysts. First the Mo speciation both in solution and on catalysts were determined using Raman spectroscopy combined with chemometric methods. Then, the nature of the species present in solution and the supported ones was correlated with respect to the impregnation conditions (pH, concentration).

1. Experimental 1.1 Solution chemistry

Starting from the synthesis protocol proposed in the literature [2], (3Co2+; H4Co2Mo10O386-) aqueous solutions were thermally treated in order to reach thermodynamic equilibrium and avoid side species (for instance H6CoMo6O243-). The molybdenum concentration was varied from 0.02 to 0.8 M and a 0.1 M (Na+ ; NO3-)

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solution was used as an internal standard for quantitative Raman analysis. Then, the pH of this solution was adjusted from 1 up to 9 adding concentrated HCl (5 M and 12 M) or NaOH (2 M and 10 M). After stabilization of the pH the prepared solutions were characterized by Raman spectroscopy.

1.1. Impregnations

The specific surface area of the γ-alumina support was 285±15 m².g-1 and its pore volume was about 0.8 mL.g-1. The point of zero charge (PZC) of this material was found at a pH of 8.1±0.3. Incipient wetness impregnations were carried out with 1 mL of impregnating solution poured on 1 g batches of powder manually stirred. As equilibrium was found to be reached at about 60 min after impregnation, the samples were characterized by Raman spectroscopy after 80 min.

1.2. Quantitative Raman analysis Raman spectra were recorded with a LabRam ARAMIS (HORIBA Jobin-Yvon) spectrometer using the exciting line at 532 nm of a diode Laser. The laser beam was focused with a 50x objective. The diffused light was dispersed with a 1800 lines.mm-1 grating. The laser power used was 1.5mW for solutions and was limited to 0.3 mW for impregnated samples to avoid local drying of the samples. All the measurements were achieved under ambient conditions (293 K). Care was taken to keep the samples in a wet state during measurements by using a sealed cell. PCA and quantification were achieved using NaNO3 as internal standard to normalize Raman spectra of solutions and the SIMPLISMA algorithm [5] implemented in the UV/IR Processor software (ACD Labs).

2. Results and discussion 2.1. Impregnation solutions In order to understand the physico-chemical phenomena occurring during the impregnation, we first determine the stability of H4Co2Mo10O386- anions in aqueous solutions with respect to the pH and molybdenum concentration, two relevant parameters to control the nature of molybdenum species after impregnation. 2.1.1. Influence of pH The H4Co2Mo10O386- dimer was found to be stable up to pH 5.5 at 0.8M in Mo as revealed by observation of Raman bands at 956, 918, 603, 570, 355 cm-1. Above pH 5.5, precipitation of Na[Co2(MoO4)2OH]·H2O occurs adding NaOH in these solutions (a similar phenomenon occurred using NH4OH). Simple experiments carried out by mixing solutions of sodium molybdate (0.8 M) and cobalt(II) chloride (0.4 M) proved that Na[Co2(MoO4)2OH]·H2O spontaneously precipitates in solutions containing sodium, cobalt(II) and molybdate ions. Additionally, MoO42- became the most stable molybdenum species in aqueous solutions when the pH was raised to 9, as evidenced by the increase in the intensity of Raman bands at 896, 840 and 320 cm-1 (Fig. 1a). Assuming that mainly H4Co2Mo10O386- and MoO42- were present in such solutions, the SIMPLISMA data treatment of the spectra enabled the quantification of molybdenum species. Then, the speciation diagram of molybdenum versus pH was plotted (Fig. 1b). Consequently, this experiment shows that the Anderson dimer is the main stable species in solution up to pH 7. Upwards, the monomeric species becomes stable according to the equation: H4Co2Mo10O386- + 8 OH- → 10 MoO42- + 2 Co3+ + 6 H2O. In a second step, MoO42- species could react with Co2+ counter-cations to form a cobalt-molybdenum precipitate. Consequently, the above equilibrium would be shift to the right, overestimating the quantity of MoO4 entities formed rising the pH.

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Rationalization of the aqueous impregnation of molybdenum heteropolyanions

[Mo] = 0.8 M 320 355

896 603

840

956

200 300 400 500 600 700 800 900 1000 1100

b) pH 8.1 7.1 6.9 6.7 6.5 6.3 6.1 5.9 5.7 5.5 4.5 1

100% H4Co2Mo10O386-

75% % Mo

Relative Intensity (a.u.)

a)

50% 25%

MoO42-

0% 3,5

5,5 pH

7,5

-1

Wavenumber (cm )

Figure 1. a) Raman spectra of solutions at various pH and b) Mo speciation diagram.

2.1.2. Influence of molybdenum concentration The impact of molybdenum concentration was also investigated. For that purpose, the Mo concentration was decreased down to 0.02 M by diluting a (3 Co2+ ; H4Co2Mo10O386-) solution 0.8 M in Mo. In spite of a slight increase, the pH did not exceed 4.5 after dilution. The Raman spectra of diluted solutions only evidenced the presence of H4Co2Mo10O386- species as far as a molybdenum concentration of 0.02 M evidencing the Anderson dimer is stable above this value. However below 0.02M, the incipient Raman band at 896 cm-1 revealed the beginning of H4Co2Mo10O386- deagglomeration to form MoO42- monomers.

2.2. Impregnation on γ-alumina

Impregnation solutions containing only H4Co2Mo10O386- dimers were selected (Fig. 2a) and impregnated on a γ-alumina support. It was shown that the nature of deposited Mo species strongly depends on the Mo concentration. Indeed, H4Co2Mo10O386- entities were successfully kept when the molybdenum concentration was 0.8 M (Fig. 2b) since only its typical bands at 955 and 602 cm-1 were observed on the Raman spectra of the wet impregnated support. The small bands shift compared to aqueous solutions suggests a coulombic interaction of Anderson dimers with the support. For [Mo] lower than 0.8M, formation of MoO42- in the support porosity was evidenced (Raman band at 896 cm-1) to the detriment of the HPA (Fig. 2c-e). Moreover, a band appeared around 920 cm-1 in all these impregnated catalysts and can be interpreted as MoO4 species in covalent interaction with γ-alumina [6]. No other molybdenum species were observed during the impregnation stage. In particular, no species resulting from the partial support dissolution (for instance, H6AlMo6O243-) was detected by Raman spectroscopy. The evolution of deposited species during the impregnation can be explained as follows: during interaction with an aqueous solution, the surface OH groups of an oxide tend to modify the solution pH to reach the PZC value. However, the PZC of γ-alumina is around 8 and it was shown from the study of solutions that the H4Co2Mo10O386- dimer is unstable above 7 and hence at this pH value. As a consequence, depending on the Mo concentration of the impregnation solution, the support will be more or less able to increase the pH value. For a concentration of 0.8 M, the limit value of 7 is not reached, meaning that H4Co2Mo10O386- is stable into the pores. On the contrary, for Mo concentration lower than 0.8 M, a final pH into the pores greater than 7 is obtained,

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Relative Intensity (a.u.)

leading to the main presence of monomeric MoO42- species in coulombic and covalent interactions with the support, as deduced from Raman spectra. 896

355

570 603 602

300

400

500

600

700

800

956 920 955

a b c d e

900 1000 1100

Wavenumber (cm -1 )

Figure 2. Raman spectra of a) the impregnation (3 Co2+ ; H4Co2Mo10O386-) solutions, impregnated Mo solutions of (b) 0.8 M, c) 0.2 M, d) 0.08 M and e) (2 Na+ ; MoO42-) solution.

3. Conclusion Aqueous CoMo solutions with a Co/Mo ratio of 0.5 containing only H4Co2Mo10O386anions were prepared and their speciation with respect to pH was determined by Raman spectroscopy from chemometric methods. It was shown that at molybdenum concentration of 0.8M, H4Co2Mo10O386- dimers remain the main species up to pH 7. The nature of Mo species deposited on γ-alumina from the prepared CoMo solutions strongly depends on the Mo concentration of impregnation solutions and was explained on the basis of thermodynamic criteria (the PZC value in the present case). From this study of physico-chemical parameters governing the impregnation step, H4Co2Mo10O386- dimers were specifically deposited using optimized conditions: it appeared that a Mo concentration no lower than 0.8 M should be reached. Moreover, solutions with an even higher Mo concentration should be prepared to obtain good final loading, taking care of the solubility limit of the studied species. Others experiments will be achieved to complete this approach. Impregnations on γ-alumina of CoMo solutions with various pH are planned. Moreover, a quantification of the H4Co2Mo10O386- and MoO42- species interacting with the support will be achieved to determine the associate apparent thermodynamic constants.

References [1] X. Carrier, J. F. Lambert, M. Che, Journal of the American Chemical Society, 1997, 119, p. 10137-10146. [2] E. Payen, D. Guillaume, C. Lamonier, K. Marchand, 2008, EP 1 892 038 A1. [3] C. Martin, C. Lamonier, M. Fournier, O. Mentré, V. Harlé, D. Guillaume, E. Payen, Chemical Materials, 2005, 17, p. 4438-4448. [4] J. Mazurelle, C. Lamonier, C. Lancelot, E. Payen, C. Pichon, D. Guillaume, Catalysis Today, 2008, 130, p. 41-49. [5] W. Windig, Chemometrics and Intelligent Laboratory Systems, 1997, 36, p. 3-16. [6] J. A. Bergwerff, T. Visser, B. R. G. Leliveld, B. D. Rossenaar, K. P. de Jong, B. M. Weckhuysen, Journal of American Chemical Society, 2004, 126, p. 14548-14556.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Mesoporous SBA-15 silica modified with cerium oxide: Effect of ceria loading on support modification L.F. Liottaa, G. Di Carlob, F. Puleob, G. Pantaleoa, G. Deganelloa,b a

Istituto per Lo Studio dei Materiali Nanostrutturati (ISMN)-CNR via Ugo La Malfa, 153, 90146 Palermo, Italy. E-mail: [email protected] b Dipartimento di Chimica Inorganica e Analitica “Stanislao Cannizzaro”, Università di Palermo, Parco d’Orleans II, Viale delle Scienze pad.17, Palermo 90128, Italy.

Abstract The present work investigates the effect of ceria loading on silica SBA-15. Five CeO2/SBA-15 samples with CeO2 content equal to 5, 10, 15, 20 and 30 wt% were prepared by wetness-impregnation of the support with cerium nitrate hexahydrate, as precursor, dissolved in ethanol. After drying at room temperature, the resulting samples were calcined at 400°C for 2h. Characterizations by BET surface area and pore-size distribution, XRD, NH3-TPD and H2-TPR were performed. Keywords: SBA-15, ceria impregnation, acidic and reduction properties, NH3-TPD, H2-TPR.

1. Introduction Since the discovery, in 1992, of mesoporous silica molecular sieves, such as MCM-41 [1], surfactant-templated synthetic procedures have been applied to the preparation of other mesoporous silicas, such as worm-like HMS and hexagonal well-ordered SBA-15, characterized by large surface area (> 700 m2/g) and uniform pores size ranging from 2 to 5.8 nm and from 5 to 30 nm, respectively [2,3]. In the recent years, such ordered mesoporous silicas have attracted worldwide attention as new supports for catalysts. A tailored porous structure is beneficial for catalytic applications because allows to control the optimum size of supported metal nanoparticles and/or metal oxides. Au and Pd nanoparticles dispersed on mesoporous silica are currently being explored for environmental catalysis, such as CO oxidation and CH4 combustion [4-7]. The textural properties of ordered mesoporous silicas, such as large specific surface area, a hexagonal array and uniform pore channels with size ≥ 5nm) meet the requirements for the achievement of the optimal size, between 2-5 nm, for Au nanoparticles [8]. However, due to the inertness of silica, a typical non-interacting support, sintering of noble metal particles during long time activity occurs [4,9]. Addition of modifiers, such as Cu, Fe, Nd, Ce, Ce1-xZrxO2 in appropriate concentration to mesoporous silicas is reported to modify the surface and acidic properties of the matrix and to stabilize against sintering the supported metal nanoparticles through interaction with the promoter [6,9-11]. On these grounds, the present work focus on the synthesis and characterization of ceria functionalized mesoporous SBA-15, as support for noble metals deposition. SBA15 with hexagonal porous structure was synthesized by using triblock co-polimer, as template and, then, impregnated with different amounts of cerium nitrate hexahydrate. Calcination at 400°C was performed to favor crystallization of the ceria fluorite structure, characterized by high mobility of lattice oxygen [12]. Characterizations of

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structural and morphological properties were performed by XRD, BET surface area and pore-size distribution measurements. Acidic and reduction properties were investigated by NH3-TPD and H2-TPR experiments.

2. Experimental Mesoporous SBA-15 was prepared starting from tetraethyl orthosilicate (TEOS, Aldrich 98%), as silica source and using a triblock poly (ethylene oxide)-poly(propylene oxide)poly(ethylene oxide) (EO20PO70EO20, Pluronic P123, Aldrich), as template, according to published procedure [3]. In a typical preparation, 8.1 g Pluronic P123 was dissolved in 146.8 g de-ionized water and 4.4 g of conc. HCl (37%) and stirred over night at 35°C in a 250 ml one neck flask. To this solution 16 g of TEOS was quickly added and stirred for 24 h at 35°C. The milky suspension was annealed at 100°C for 24 h in closed polypropylene bottle. The solid product was filtered, washed with an HCl/water-mixture and calcined at 550°C for 5h in air. Five CeO2/SBA-15 samples, with CeO2 content equal to 5, 10, 15, 20 and 30 wt%, were prepared by successive incipient wetness-impregnations. The SBA-15 was impregnated with 0.8 ml/g of an ethanolic solution of cerium nitrate hexahydrate, whose concentration was selected to obtain 5 wt% of CeO2. For higher ceria loadings, the impregnations were repeated until the target value. After drying at room temperature, the resulting samples calcined at 400°C for 2h were labeled as CexSBA, where x refers to the ceria weight content. Physico-chemical characterizations were performed on the finished ceria-doped silicas. Surface area measurements (BET) and mesopore size distribution (BJH) were carried out by means of Sorptomatic 1900 (Carlo Erba) instrument. X-ray diffraction patterns were recorded with a D 5005 X-Ray Diffractometer (SIEMENS) using Cu Kα radiation coupled with a graphite monochromator. The crystallite sizes of ceria phase were calculated from the line broadening of the most intense reflection using the Scherrer equation [13]. The surface acidity of the ceria-doped SBA-15 samples was studied by a temperature-programmed desorption of ammonia (NH3-TPD). The measurements were performed with a Micromeritics Autochem 2910 apparatus equipped with a thermal conductivity detector (TCD) and a mass quadrupole spectrometer (Thermostar, Balzers). Prior to the ammonia sorption, the samples (∼100 mg) were outgassed in a flow of O2 (5% in He) at 500°C for 1h, then, cooled to room temperature under He and saturated in a flow of NH3 (5% in He, 30 mL/min) for 1h. Subsequently, the catalysts were purged in a He flow at 100°C for 1h until a constant baseline level was reached. The ammonia desorption was carried out with a linear heating rate (10°C/min) up to 1050°C under a flow of He (30 mL/min). Calibration of the TCD were carried out in order to evaluate the ammonia desorption peaks. Temperature programmed reductions with hydrogen (H2-TPR) were carried out with the same Micromeritics Autochem 2910 apparatus. The samples (∼ 50 mg) were pre-treated with O2 (5% in He) at 600°C for 30 min, cooled in He and then H2 (5% in Ar, 50 mL/min) was flowed from room temperature to 1050°C (heating rate 10° C/min).

3. Results and discussion In Table 1 the morphological properties of CexSBA samples are listed. Doping silica with ceria (up to 15 wt%) resulted into a gradual reduction of the specific surface area and cumulative pore volume, indicating deposition of ceria nanoclusters inside pores. In addition, no evidences of crystalline ceria features were found by XRD. By further

Mesoporous SBA-15 silica modified with cerium oxide: Effect of ceria loading

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increasing the loading up to 30wt%, broad peaks indicative of fluorite-type cubic structure of ceria were detected. The calculated average particle size was of 5 nm. Surface area sintering and pore volume decrease were also observed, likely because of the crystallization process. Table 1. Morphological properties of CeO2-doped SBA-15 samples. Sample

BET (m2/g)

SBA-15 Ce5SBA Ce10SBA Ce15SBA Ce20SBA Ce30SBA

840 620 580 569 392 385

Pore size (BJH) (nm) 7.4 6.7 6.4 5.9 5.1 5.0

(a)

3

Volume adsorbed (cm STP/g)

(a) SBA-15 (b) Ce10SBA (c) Ce20SBA (d) Ce30SBA

(b) (c) (d)

0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure, P/P0

Fig. 1. Nitrogen adsorption/desorption isotherms for CexSBA samples.

(a) SBA-15 (b) Ce10SBA (c) Ce20SBA (d) Ce30SBA

TCD Signal (a.u.)

(a)

(b) (c)

(d)

50

100

150

200

250

300

350

Total pore volume (cm3/g) 0.81 0.70 0.68 0.55 0.48 0.34

In Fig. 1 the nitrogen adsorption/ desorption isotherms of CexSBa samples are displayed. A H1 hysteresis, which is typical of cylindrical mesopores and wide bottleshaped mesopores, was observed in any case, although the hysteresis shape slightly changes by increaseing ceria content and the relative pressure where capillary condensation step occurs shifts to lower P/Po values. No evidences of micropores were found. According to XRD results, these findings suggest that, at high loading, ceria crystallizes, partially obstructing the mesopores. The surface acidity of ceria-doped silica samples was studied NH3TPD. Desorption of ammonia from SBA-15 was detected with a peak centered at 105 °C. No additional desorption occurred at higher temperature. For CexSBA samples the quantity of chemisorbed NH3 increased from 0.09 mmol/g for pure SBA-15 to 0.35 mmol/g for Ce30 SBA. The observed values are in line with literature results [10].

400

Temperature (°C)

Fig. 2. NH3 -TPD curves for CexSBa samples.

Moreover, the desorption started at slightly higher temperature, around 120°C, with a broad and asymmetric peak ranging up to 350-400°C. Therefore, it seems that ceria-

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modification of silica resulted in an increased acidity, in terms of higher concentration and strength of surface acid sites. Accordingly, it has been reported that modification of mesoporous silica with transition metals considerably increases the surface acidity by the generation of Lewis acid sites, to which ammonia molecules are bonded by donoracceptor bond [10]. TPR experiments were carried out for studying the reduction properties of ceria in CexSBA oxides. For all samples high reducibility was observed, the reduction started at around 300°C and two peaks, generally, were observed at ~ 400°C corresponding to surface reduction and at 700°C due to bulk reduction of ceria. Stoichiometrically, 1g of ceria requires 2905 µmol H2/g for a complete reduction to Ce2O3, therefore, ceria in CexSBA samples was almost completely reduced, being for instance the overall consumption of 834 µmol H2/gCe30SBA for Ce30SBA and of 415 µmol H2/gCe20SBA for Ce20SBA. The presence of grain boundaries and defects in the small ceria crystallites may account for the enhanced reducibility [14], which is a very important property for catalytic application, in particular, CO oxidation reactions [12].

4. Conclusions Ceria-modified SBA-15 oxides with increased surface acidity and high reducibility were prepared. The specific surface area was ranging between 620-385 m2/g, and well dispersed ceria crystallites (mean diameter ≤5 nm) were obtained. The so far reported results suggest that CexSBA oxides, prepared by incipient wetness-impregnation approach, are suitable supports for preparation and stabilization of noble metal nanoparticles.

References [1] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710. [2] S.A. Bagshaw, E. Prouzet, T.J. Pinnavaia, Science 269 (1995) 1242. [3] D.Y. Zhao, J.L. Feng, Q.S. Huo, N. Melosh, G.H. Fredrickson, B.F. Chmelka, G.D. Stucky, Science 279 (1998) 548. [4] J.-H. Liu, Y.-S. Chi, H.-P. Lin, C.-Y. Mou, B.-Z. Wan, Catal. Today 93-95 (2004) 141. [5] M. Ruszel, B. Grzybowska, M. Łaniecki, M. Wójtowski, Catal. Comm. 8 (2007)1284. [6] A. Beck, A. Horváth, Gy. Stefler, R. Katona, O. Geszti, Gy. Tolnai, L. F. Liotta, L. Guczi, Catal. Today 139 (2008) 180. [7] A.M. Venezia, R. Murania, G. Pantaleo, G. Deganello, J. Catal. 251 (2007) 94. [8] M. Haruta, N. Yamada, T. Kobayashi, S. Iijima, J. Catal. 115 (1989) 301. [9] F. Yin, S. Ji, P. Wu, F. Zhao, C. Li, J. Catal. 257 (2008) 108. [10] L.Chmielarz, P. Kuśtrowski, R. Dziembaj, P. Cool, E.F. Vansant, Appl. Catal. B 62 (2006) 369. [11] J.A. Hernandez, S. Gómez, B. Pawelec, T.A. Zepeda, Appl. Catal. B 89 (2009) 128. [12] J. Kašpar, P. Fornasiero, M. Graziani, Catal. Today, 50 (1999) 285. [13] H.P. Klug, L.E. Alexander, X-ray Diffraction Procedures for Polycrystalline and Amorphous Materials, Wiley, New York, 1954. [14] H. Zhu, Z. Qin, W. Shan, W. Shen, J. Wang, J. Catal. 225 (2004) 267.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Synthesis and characterization of catalysts obtained by trifluoromethanesulfonic acid immobilization on zirconia Marina Gorsd, Mirta Blanco, Luis Pizzio Centro de Investigación y Desarrollo en Ciencias Aplicadas “Dr. J. J. Ronco” (CINDECA), Dto. de Química, Facultad de Ciencias Exactas, UNLP-CCT La Plata, CONICET, 47 N° 257, 1900 La Plata, Argentina

Abstract Mesoporous zirconia (zirconium oxide) materials containing mainly mesopores have been synthesized via sol-gel reactions from zirconium propoxide using urea as a template. The solid was dried, extracted with water to remove urea, calcined at different temperatures and impregnated with trifluoromethanesulfonic acid. The samples thus obtained were extracted with a mixture of dichloromethane and diethyl ether using a Soxhlet apparatus in order to remove the loosely adsorbed acid. The solids were characterized by FT-IR, XRD, DTA-TGA, and N2 adsorption-desorption measurements. The mean pore diameter of the support was higher than 3.7 nm, which increased with the increment of the thermal treatment temperature. At the same time, the specific surface area and the amount of triflic acid attached on the support decreased. The potentiometric titration with n-butylamine indicated that the catalysts present very strong acid sites. The catalytic activity of the prepared catalysts in the esterification of 4-hydroxybenzoic acid with propyl alcohol was evaluated. Keywords: zirconia, sol-gel, triflic acid, catalysts, esterification

1. Introduction The trifluoromethanesulfonic acid (CF3SO3H) has a highly acidic nature and excellent thermal stability; it also has good resistance to reductive and oxidative dissociation, with no generation of fluoride ions. The trifluoromethanesulfonic acid was used as an efficient homogeneous catalyst, but has environmental disadvantages because it generates high amount of wastes. An interesting alternative is the trifluoromethanesulfonic acid heterogeneization by immobilization on an adequate support. There are few works on this subject, though it can be mentioned its immobilization on titania and carbon [1, 2]. On the other hand, zirconia is an interesting material to be used as catalyst support due to its thermal stability in different atmospheres. The most common methods that can be used to obtain zirconia are the sol-gel method, the micellar technique or the mechanochemical synthesis [3]. Zirconia is frequently prepared by micellar method, while the sol-gel method from an alkoxide is less used [4]. Its acid properties can be modified by addition of cationic or anionic substances, such as sulfate or tungstate [5]. In the present work, trifluoromethanesulfonic acid was immobilized on mesoporous zirconia obtained by sol-gel method using urea as a low-cost template. The physicochemical and textural characteristics of the prepared catalysts are studied. In addition, the catalytic behavior of the synthesized catalysts in the esterification of 4-hydroxybenzoic acid with propyl alcohol was evaluated.

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2. Experimental 2.1. Support preparation Zirconium propoxide (Aldrich, 26.6 g) was mixed with absolute ethanol (Merck, 336.6 g) and stirred for 10 min to obtain a homogeneous solution under N2 at room temperature, then 0.47 cm3 of 0.28 M HCl aqueous solution was dropped slowly into the above mixture to catalyze the sol-gel reaction for 3 h. After that, an appropriate amount of urea-alcohol-water (1:5:1 weight ratio) solution was added to the hydrolyzed solution under vigorous stirring to act as template. The amount of added solution was fixed in order to obtain a template concentration of 10% by weight in the final material. The gel was kept in a beaker at room temperature till solidification. The solid was grounded into powder and extracted by distilled water for three periods of 24 h, in a system with continuous stirring to remove urea. Finally, it was calcined at 100, 200, 300, and 400°C for 24 h (ZrTX samples, where X is the calcination temperature).

2.2. Catalyst preparation Trifluoromethanesulfonic acid, CF3SO3H, (0.01 mol, Alfa Aesar, 99%) was added drop wise to a mixture of ZrTX (2 g) and toluene (20 cm3, Merck) at 90ºC under nitrogen atmosphere; then it was further refluxed for 2 h. Next, the sample was cooled, filtered, washed with acetone (Mallinckrot AR ) and dried at 100ºC for 24 h. The solids were extracted with a mixture of dichloromethane and diethyl ether (100 g of mixture per g of catalyst) for three periods of 8 h using a Soxhlet apparatus in order to remove the acid weakly attached to the support. Afterwards, they were dried again at 100ºC for 24 h. The samples were named TriZrT100, TriZrT200, TriZrT300, and TriZrT400. The amount of trifluoromethanesulfonic acid retained was determined by C and S elemental analysis with an EA1108 Elemental Analyzer (Carlo Erba Instruments).

2.3. Support and catalyst characterization The textural properties of the solids were determined from N2 adsorption-desorption isotherms at liquid-nitrogen temperature. They were obtained using Micromeritics ASAP 2020 equipment. The samples were previously degassed at 100ºC for 2h. Fourier transform infrared (FT-IR) spectra of the samples were recorded, using Bruker IFS 66 FT-IR equipment, pellets in BrK, and a measuring range of 400-1500 cm-1. X-ray diffraction (XRD) patterns of the solids were recorded with Philips PW-1732 equipment, using Cu Kα radiation, Ni filter, 30 mA and 40 kV in the high voltage source, 5-55°2θ scanning angle at a scanning rate of 1° per min. The thermogravimetric (TG) and differential scanning calorimetric (DSC) analysis were carried out using Shimadzu DT 50 thermal analyzer. The analysis were performed under argon, with 25-50 mg sample, heating rate 10°C/min, and temperature range 25600°C. The acidity of the solids was measured by means of potentiometric titration. A known mass of solid was suspended in acetonitrile and stirred for 3 h. Later, the suspension was titrated with 0.05 N n-butylamine in acetonitrile solution at 0.05 ml/min, measuring the electrode potential variation with a digital pH meter Hanna 211. The catalytic activity of the samples in the esterification of 4-hydroxybenzoic acid with n-propanol was evaluated. It was carried out in liquid phase at reflux temperature in a 50 ml glass reactor equipped with a condenser and a magnetic stirrer. The reagent mixture was heated under stirring and then the catalyst was added. An n-propanol:4hydroxybenzoic acid:catalyst molar ratio of 10:1:0.1 was used. Samples were taken periodically and analyzed by gas chromatography using dodecane as internal standard.

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Synthesis and characterization of catalysts

3. Results and discussion Mesoporous zirconia (zirconium oxide) materials containing mainly mesopores were obtained, with the total pore volume significantly higher than the micropore volume (Table 1). The mean pore diameter (DP) was higher than 3.7 nm, which increased with the thermal treatment temperature (Table 1). At the same time, the specific surface area (SBET) and the specific surface of micropores (SMicro) decreased. The textural properties of the catalysts were mainly the same as those of the supports. The amount of acid attached on the support (NTri) decreased with the increment of the thermal treatment temperature. This effect may be explained if the interaction is assumed to be of electrostatic type due to proton transfer to the -OH groups on the support surface. So, as a result of the support dehydroxylation during the thermal treatment, the amount of OH groups to be protonated decreases, and therefore NTri diminishes. Table 1. Support textural properties and CF3SO3H amount in the catalysts. Sample

SBET (m2/g)

SMicro (m2/g)

ZrT100 ZrT200 ZrT300 ZrT400

192 132 78 20

88 50 17 0

Total pore volume (cm3/g) 0.18 0.16 0.11 0.07

Micropore volume (cm3/g) 0.05 0.02 0.01 0

DP (nm)

Sample

3.7 4.7 5.5 14.2

TriZrT100 TriZrT200 TriZrT300 TriZrT400

NTri (mmol CF3SO3H/g) 0.91 0.53 0.41 0.10

By XRD it was observed that the samples have amorphous characteristics; there is not any line indicating the presence of crystalline phases. The DSC diagram of ZrT100 (Figure 1) showed two endothermic peaks at 69 and 171°C, attributed to the loss of physically adsorbed water, and the partial dehydroxylation of the solid respectively. The exothermic peak at 434ºC was assigned to the zirconia transformation from an amorphous to a metastable tetragonal phase. In the DSC diagram of TriZrT100 (Figure 1) TriZrT200, TriZrT300, and TriZrT400 samples, this peak appears at higher temperatures. Apparently, the trifluoromethanesulfonic acid retards the crystallization of zirconia. Additionally, another exothermic peak at 365ºC assigned to the elimination of acid was displayed, whose intensity decreases in parallel to the decrease of trifluoromethanesulfonic acid content. From TGA-DSC, it can be established that the catalysts are thermally stable up to 250ºC. The FT-IR spectrum of the TriZrT100 sample displayed bands at 1267, 1180, and 1040 cm-1, in addition to those present in the ZrT100 sample (Figure 2). The first two bands are ascribed to the S═O stretching mode of the adsorbed trifluoromethanesulfonic acid and the last one is assigned to the C-F stretching [6, 7]. These bands are also present in the spectra of TriZrT200, TriZrT300, and TriZrT400 catalysts, although their intensities are lower as a result of the lower amount of trifluoromethanesulfonic acid adsorbed on the support. The acidity measurements of the catalysts by means of potentiometric titration with n-butylamine let us to estimate the number of acid sites and their acid strength. As a criterion to interpret the obtained results, it was suggested that the initial electrode potential (Ei) indicates the maximum acid strength of the sites, and the value of meq amine/g solid where the plateau is reached indicates the total number of acid sites [8]. Nevertheless, the end point of the titration given by the inflexion point of the curve is a

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491

40

DSC (a.u.)

30

20 434

TriZrT100

365

10

-5

0

100

TriZr T100

TriZr T200 TriZr T300 TriZr T400

ZrT100

171

69

0

Transmittance (a.u.)

0

Zr T100

200

300

400

500

-10

600

Temperature (°C)

Fig. 1. DSC of ZrT100 and TriZrT100 samples.

4000

3000

2000

1000 -1

W avenumber (cm )

Fig. 2. FT-IR spectra of ZrT100, TriZrT100, TriZrT200, TriZrT300 and TriZrT400 samples.

good measure to carry out a comparison of the acidity of different samples. On the other hand, the acid strength of these sites may be classified according to the following scale [8]: Ei > 100 mV (very strong sites), 0 < Ei < 100 mV (strong sites), -100 < Ei < 0 (weak sites) and Ei < -100 mV (very weak sites). The potentiometric titration with n-butylamine indicated that the catalysts present very strong acid sites with potential between 100 and 700 mV. Ei values were in the range 700-650 mV, nearly independent of NTri, and the number of acid sites decreased with the thermal treatment temperature in an almost linear relation with the NTri decrease. The acidity of the supports is higher than that of a zirconia obtained by micellar technique from zirconium oxychloride and ammonia [8], which present very weak sites. The catalysts were tested in the esterification of 4-hydroxybenzoic acid with propyl alcohol. It was observed that the amount of 4-hydroxybenzoic acid converted into the ester increased continuously with the reaction time. The conversion depended strongly on the CF3SO3H content, decreasing from 80 to 7% when NTri decreased from 0.91 to 0.10 mmol CF3SO3H/g. The catalytic activity of the samples, expressed as moles of ester formed at 5 h/mol CF3SO3H in the catalyst, decreased slightly in the following order TriZrT100 (0.88) > TriZrT200 (0.86) > TriZrT300 (0.76) > TriZrT400 (0.71). On the other hand, the catalysts were reused several times without appreciable loss of catalytic activity. These results show that the prepared solids would be appropriate catalysts for their use in acid reactions employing a clean technology, to replace the classical acids used both in the laboratory and the industry.

References 1. 2. 3. 4. 5. 6. 7. 8.

L. Pizzio, Mater. Lett. 60 (2006) 3931. D.O. Bennardi, G.P. Romanelli, J.C. Autino, L.R. Pizzio, Catal. Commun. 10 (2009) 576. M. Fernández-García, A. Martínez-Arias, J.C. Hanson, J.A. Rodríguez, Chem. Rev. 104 (2004) 4063. X. Qu, Y. Guo, Ch. Hu, J. Molec. Catal. A 262 (2007) 128. G. D. Yadav, J.J. Nair, Micropor. Mesopor. Mater. 33 (1999) 1. Herzberg, Infrared and Raman Spectra of Polyatomic Molecules, Von Nostrand Reinhold, New York, 1945. L.J. Bellamy (ed.) The Infrared Spectra of Complex Molecules, Wiley, New York, 1960. L. Pizzio, P. Vázquez, C. Cáceres, M. Blanco, Catal. Lett. 77 (2001) 233.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Influence of precursor on the particle size and stability of colloidal gold nanoparticles A. Alshammari,a,b A. Köckritz ,a V.N. Kalevaru,a A. Martina a

Leibniz Institute for Catalysis at the University of Rostock, Albert-Einstein-Str. 29A, D-18059 Rostock, Germany b Petroleum and Petrochemicals Research Institute, King Abdulaziz City for Science and Technology, P.O. Box 6086, Riyadh 11442, Saudi Arabia

Abstract Particle size and stability of colloidal gold nanoparticles (AuNPs) have considerable importance in nanocatalysis due to their beneficial properties. Colloidal AuNPs of the present study were prepared in aqueous solution by one-step chemical reduction of various gold metal precursors. The nature of Au precursor showed a significant effect on the size and stability of colloidal AuNPs. Furthermore, the study describes the effect of different synthesis parameters such as temperature, pH-value, concentration of precursors and reducing agent on the size of the colloidal AuNPs. The obtained samples were characterized by means of UV-Vis spectroscopy, transmission electron microscopy, dynamic light scattering and zeta potential measurements. Keywords: gold metal precursor, colloidal AuNPs, size of AuNPs, stability, reaction parameters

1. Introduction The nano-scale level of metal nanoparticles is indeed an important parameter in the field of heterogeneous catalysis due to their unique and completely different catalytic properties compared to their bulk solids. For example, the catalytic properties of nanostructured gold catalyst are known to depend on the size of the gold particles [1]. It is known that metal reducibility, metal distribution, and particle size can be controlled by the preparation method. However, the nature of the metal precursor compound also shows significant influence on the particle size as well as stability. Particularly, to manufacture such catalysts in an efficient and reproducible way, it is important to gain the control over the parameters mentioned above. Therefore, efforts are being made by various researchers for the past few decades to find ways to stabilize and optimize the size of such nanoparticles in a desired way. A substantial number of studies have been published in recent times on the synthesis and use of supported AuNPs as catalysts for different reactions [e.g. 2]. Literature survey [e.g. 3] also reveals that the activity of catalyst depends not only on their composition but also on the kind of precursors used in the preparation method. However, investigations concerning the effect of precursor materials on the size of colloidal AuNPs are very rare in the literature. In the present study, we made attempts to prepare AuNPs using different gold precursors and ensure their affect on the size and stability. The aim is also to check the effect of temperature, concentration of reductants and pH-value of the reaction mixture on the size of AuNPs.

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2. Experimental 2.1. Samples preparation One-step chemical reduction of HAuCl4 solution was carried out using sodium thiocyanate (ST) as reducing agent in presence and absence of sodium citrate (SC) as stabilizing agent. In another set of experiments, SC was used as reductant. Three different gold precursors (Au-p) such as HAuCl4·3H2O (Au-pa, commercial), HAuCl4·3H2O (Au-pb, lab prepared) and NaAuCl4·2H2O (Au-pc, commercial) were used in the preparations. The syntheses were carried out similar to experiments described elsewhere [4]. The final samples are denoted as (a), (b) and (c). The reductant to Au+3 ratio was maintained at 4 : 1 (in case of syntheses with stabilizer, 7 mM of SC were used). The reactions were carried out at room temperature under stirring. In addition, the influence of increased reduction temperature (40-80°C), concentration of starting material and pH-value of reaction mixture on the size of AuNPs were also examined with sodium citrate (SC) as reductant.

2.2. Samples characterization ICP analyses were carried out to check the accurate concentration of the Au precursor compounds (Au-pa, Au-pb, Au-pc) in aqueous solution using an Optima 3000XL device (Perkin-Elmer). Optical properties (absorbance) of colloidal solutions were acquired with UV-Vis spectrometer (Avantes-2048). Particle size distribution and zeta potential values were obtained at room temperature from dynamic light scattering and zeta potential measurements, respectively, which were performed on a Malvern Instrument (ZS ZEN 4003). Size analysis (i.e. size, shape, morphology etc.) of colloidal AuNPs was further confirmed with HRTEM (JEM-2100F) at a voltage of 200 kV.

3. Results and discussion At first, ICP analyses showed comparable Au concentrations (0.045-0.05 mg/l). The corresponding changes in the morphology, size distribution and stability of the prepared colloidal AuNPs were characterized by different methods as described below.

3.1. Spectroscopic and microscopic investigations

Absorbance / a.u.

3.1.1. Ultraviolet-Visible spectroscopy (UV-Vis) In a first set of experiments colloidal AuNPs were obtained using ST as reductant and SC as stabilizer. UV-Vis spectroscopy is often used as quite sensitive technique for observing the formation of colloidal AuNPs 1.0 because they display an intense absorption (a) 0.8 band in the region of 500-550 nm, especially (b) (c) when the particle size exceeds ca. 5 nm [5]. 0.6 The position of those bands also depends on different factors (e.g. size, shape). 0.4 However, the colloidal samples prepared in b c the present study (Fig. 1) did not display a 0.2 such band in this range, which implies that the average particle size is ≤5 nm. Such 0.0 300 400 500 600 700 800 phenomenon can be explained as an Wavelength / nm indication of the quantum size effects (i.e. the loss of bulk Au character during the Fig. 1. UV-Vis spectra of colloidal AuNPs transition from bulky Au to small “quantum- using various gold precursors.

Influence of precursor on the particle size and stability of colloidal gold

411

sized” Au particles) [5]. Additionally, sample (b) reveals a slightly broadened spectrum compared to samples (a) and (c), which indirectly provides hints that bigger particles were formed with Au-pb precursor. 3.1.2. Transmission electron microscopy (TEM) The better evidence for the size and morphology of colloidal AuNPs can be easily obtained by TEM. Figure 2 (Au-pa, upper part) showed well-dispersed particles of colloidal AuNPs prepared using precursor Au-pa with average size of approx. 3 nm, while usage of Au-pb and Au-pc showed slightly bigger AuNPs. However, all obtained colloidal gold nanoparticles exhibited more or less spherical morphology. In addition, from HRTEM images (Fig. 2, lower part, final samples (a), (b) and (c)) one can clearly observe the crystal planes of gold. The distance between lattice plane fringes is estimated to be in the range from 0.19 to 0.25 nm, depending upon the type of crystal plane or precursors used. For instance, samples Fig. 2. Electron micrographs of colloidal AuNPs (a) and (c) showed (0.22 & 0.23 nm) and prepared using precursors Au-pa, -b & -c. (0.23 & 0.23 nm) that corresponds to Au(111) plane, while sample (b) displayed Au(200) besides Au(111). In sample (a), Au(111) is more exposed. 3.1.3. Dynamic light scattering (DLS) Supporting investigations regarding the effect of metal precursor compounds on the size and distribution of colloidal AuNPs were also accomplished by DLS. Figure 3 shows the result obtained from DLS using precursor Au-pa as a model. The average particle diameter was found to be 6.7 nm in this case. The other two gold precursors (Au-pb and Au-pc) gave somewhat bigger particles with the size of 13.9 and 10 nm, respectively. We have shown that DLS gives a bigger diameter value than those obtained by TEM. This phenomenon is due to the fact that DLS measures the hydrodynamic particle size in the suspension medium, where TEM shows the core particle size.

3.2. Stability of colloidal AuNPs In order to study the influence of precursor on the stability of colloidal AuNPs, the Au ions were reduced in absence of stabilizer. The expected particle growth was measured by their zeta potential (ζ), with the simultaneous intention to estimate their stability. The particles with zeta potential values ≥ +30 mV or ≤ -30 mV are usually Fig. 3. Size distribution of colloidal AuNPs considered stable. In the present study, prepared using precursors Au-pa. the ζ values varied in the range from 24 mV to -31 mV, depending upon the type of precursor used. Thus, the type of precursor used also affects the changes in the stability of the final colloidal AuNPs. Colloidal AuNPs obtained from precursor Au-pb displayed the lowest zeta potential (-24 mV) and

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hence less stable compared to other two precursors. Alternatively, the highest stability of AuNPs obtained from precursor Au-pc (i.e. NaAuCl4.2H2O) with a ζ value of -31.3 mV. Moreover, the stability of colloidal AuNPs can also be checked indirectly by a color change or precipitation time (τ). These results were in good agreement with spectroscopic results (zeta potential). The resulting suspension from precursor Au-pc neither lead to any change in color nor to appearance of agglomeration over a period of more than three weeks, whereas a precipitation of agglomerates using precursor compounds Au-pa and Au-pb was observed after two weeks and one week, respectively.

3.3. Influence of synthesis conditions on colloidal AuNPs Size Influence of the reaction parameters such as temperature, initial Au concentration, concentration of reducing agent and pH-value on the size, distribution and stability of colloidal AuNPs were studied in another row using SC as reducing agent instead of ST. Precursor Au-pa was selected as a model starting material. We observed that an optimum temperature of 80°C is suitable for the preparation of small AuNPs with reasonably good stability. However, the particles prepared at lower temperature (<60°C) seem to be less stable. Different concentrations of precursor Au-pa (i.e. 0.05-2.0 mM) were selected for reduction with 15 mM SC. Then the size of the final colloidal AuNPs was again monitored by various techniques such as UV-Vis, DLS and TEM. These techniques showed that the relationship between the initial concentration of gold precursor and the average particle size is not linear. The smallest AuNPs were achieved with a concentration of 1.0 mM of Au-pa precursor solution with an average size of ca. 12 nm (from TEM). Moreover, the pH-value of the reaction solution also influenced particle size. The particle size (from DLS) passed through a minimum of 14 nm at pH = 3.0. Lower pH-values of 1.5 and 2.5 led to the formation of larger AuNPs of 150-500 nm, whereas higher pH-values between 5.5 and 9.5 induced only a moderate increase of the size (40-67 nm) compared to pH = 3. This means, either too low or too high pH-values are not suitable for obtaining smaller AuNPs.

4. Conclusions Suitable method for colloidal preparation could enlarge the use of supported gold colloids in the chemical industrial applications. The investigations showed that the type of precursor compound plays a considerable role on the size and stability of colloidal AuNPs. The colloids Au particles prepared from precursor HAuCl4·3H2O (Au-pa) showed the smaller size compared to other two precursors. The most stable colloidal AuNPs were obtained using precursor compound NaAuCl4·2H2O (Au-pc). The preparation parameters (i.e. T, pH etc.) also revealed a significant effect on the size, distribution and stability of colloidal AuNPs. Further studies supporting the obtained colloids on oxidic carriers are in progress for the alcohol oxidation reaction.

References 1. 2. 3. 4. 5.

M. Haruta, 1997, Size and Support Dependency in the Catalysis by Gold,Catal. Today 36, 153. A.S. Hashmi, G.J. Hutchings, 2006, Gold Catalysis, Angew. Chem. Int. Ed. 45, 7896. R.S. Monteiro, L.C. Dieguez, M. Schmal, 2001, The role of Pd precursors in the oxidation of carbon monoxide over Pd/Al2O3 and Pd/CeO2/Al2O3 catalysts, Catal. Today 65, 77. D.A. Handley, Methods for synthesis of colloidal gold, in Colloidal Gold, vol. 1 (Hayat, M. A., ed.), Academic, New York, 13-33. M.M. Alvarez, J.T. Khoury, T.G. Schaaff, M.N. Shafigullin, I. Vezmar and R.L. Whetten, 1997, Optical absorption spectra of nanocrystal gold molecules, J. Phys. Chem. B 101, 3706.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

V-Mo-Nb-W-containing hydrotalcite-like materials as precursors of catalysts for oxidative dehydrogenation of hydrocarbons and alcohols Inna P. Belomestnykh,а Georgii V. Isaguliants, а Stanislav P. Kolesnikov, а Vjacheslav P. Danilov,b Оlga N. Krasnobaeva,b Tatyana A. Nosova,b Tatyana A. Еlisarova b а

N.D. Zelinsky Institute of Organic Chemistry RAS, Leninsky pr. 47, Moscow 119991, Russia b Kurnakov Institute of General and Inorganic Chemistry RAS, Leninsky pr. 31, Moscow 119991, Russia

Abstract The methods for preparation of precursors of the oxide catalysts in the form of hydrotalcite-like materials with the layered structure, containing V-Mo-Nb-W (decavanadate-, paramolybdate-, oxoniobate- and oxotungstate-iones) interlayers were developed. These precursors were applied for synthesis of oxide catalysts for oxidative dehydrogenation of hydrocarbons (ethane and ethylbenzene) and alcohols. The catalysts provide both high selectivity and the yields of the target products in transformations of ethane to ethylene, ethylbenzene to styrene and alcohols to carbonyl compounds as well in oxidative dehydrocyclization of n-octane to styrene. Strong dependence of the yield and reaction selectivity on the precursor loading and conditions of the heat treatment were observed. The results obtained were useful for synthesis of the proper catalytic systems for the oxidative dehydrogenation processes. Keywords: hydrotalcite-like materials (LDHs), oxidative dehydrogenation (ODH)

1. Introduction Layered double hydroxides (LDHs) known as hydrotalcite-like layered metal hydroxosalts, are extensively explored as precursors of various catalysts. Interest in LDHs is provided by the fact that two, three or more metal cations may be introduced in the brucite-like layers, besides various anions (e.g. decavanadate, paramolybdate, etc) may be incorporated into the interlayer space [1]. However information on the application of such materials as multi-component catalysts for oxidative dehydrogenation (ODH) of ethylbenzene (EB), light alkanes and alcohols is still scanty. We have performed an extensive investigation of the synthesis of the complex hydrotalcite-like hydroxosalts and their application as precursors of the catalysts. The performance of the latters was studied in ODH of ethylbenzene, light alkanes and alcohols.

2. Experimental 2.1. Catalyst preparation The multi-component LDHs (precursors) were prepared by reacting magnesium and aluminum nitrate solutions with a potassium hydroxide + potassium carbonate solution followed by anionic exchange to incorporate of decavanadate-, paramolybdate-, oxoniobate- and oxotungstate-iones into LDHs accordingly to experimental procedures

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developed previously [1, 2]. A special procedure has been used for introducing of niobium. To introduce Nb in the form of polyoxoniobate into composition of hydrotalcite-like hydroxosalt the method of anionic exchange was used. The solution of potassium polyoxoniobate was prepared by melting of Nb2O5 with K2CO3 followed by dissolving of melt into boiling water. The melting procedure was carried out in the Pt-cup firstly at 900oC (30 min) and then at 1050oC (40 min). Composition of polyoxoniobate corresponds to K8[Nb6O19].aq. To carry out anionic exchange of carbonate-ions of hydroxosalt for polyoxoniobate-ions [Nb6O19]8- the solution of potassium polyoxoniobate (pH 13) and Mg-Al hydroxocarbonate (hydrotalcite) were used. The anionic exchange was performed for 6 h at S:L = 1:4. After that the precipitate was collected on a filter and potassium and other ions were washed off with distilled water. To prepare precursors for the catalysts containing Nb as the hexaniobate as well decavanadate-, paramolybdate-, and oxotungstate-iones we used mixture of two isomorphic solid phase: LDHs, containing Mg, Al and Nb (I) and LDHs, involving Mg, Al, V, Mo and W (II), because I is formed at pH = 13, whereas II appears at pH = 4.5 – 5.0. (Table 1). The catalysts containing V, Mo, Nb and W were also prepared according to hydrothermal method [3]. The synthesized samples were dried at 100oC and heattreated at 500oC. The powders were pressed into the tablets and then crushed into particles of a required size. The particle fraction, phase and chemical compositions of the hydroxosalts and their thermolysis products were studied using such methods as photosedimentography, chemical analysis and X-ray powder diffraction. Table 1. Composition of the hydrotalcite precursors for ODH catalyst. Sample no. Precursors 1 [AlMg2(OH)6][(CO3)0,42(Nb6O19)0,02.nH2O] 2 [AlMg1,5(OH)5][(CO3)0,424(Nb6O19)0,004(Mo7O24)0,02.nH2O] 3 [AlMg1,8(OH)5,6][(CO3)0,33(Nb6O19)0.02(V10O28)0,03.nH2O] 4 [AlMg1,5(OH)5][(CO3)0,379(Nb6O19)0,004(V10O28)0,005(Mo7O24)0,03.nH2O] 5 [AlMg1,6(OH)5,2[(CO3)0.407(Nb6O19)0,003(Mo7O24)0.02(V10O28)0,005(H2W12O40)0,002.nH2O]

2.2. Catalyst testing The catalysts obtained were tested in a fixed bed quartz reactor provided with on-line GLC analysis. Experimental conditions (reaction temperature, space velocity of hydrocarbons, the acohols/oxygen mole ratio in the feed) were varied in wide ranges. Oxygen-nitrogen mixtures and CO2 were used as the oxidant in ODH of ethylbenzene, light alkanes and alcohols. The feed flow with 10% content of ethane (or propane) has been used. The catalytic performance of the catalysts in ODH of EB with oxygen-nitrogen mixtures and CO2 was studied in the 380-500°C temperature range.

2.3. Characterization 2.3.1. DTA-DTG analysis Measurements were performed in the air flow in the temperature range of 20-1000°C using MDTA 85 SETARAM apparatus. The heating rate was of 5°C/min. 2.3.2. Texture characterization The values of specific surface area were determined by N2 sorption at the temperature of liquid N2 or by benzene adsorption at 293°K, (in a vacuum setup supplied by a McBain balance) and calculated by the BET method. The pore size distribution was calculated by Kelvin equation using the cylindrical pore model with the adsorbed layer correction.

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2.3.2. X-ray diffraction analysis A Guinier-Hagg focusing camera Fr-552 and the powder X-ray “STADI-P” diffractometer with the imaging plate (CuKα radiation, 2θ range from 7 to 60o) have been used.

3. Results and discussion The synthesized LDHs showed well-crystallized hydrotalcite structure. The thermal treatment converted the precursors into mixed metal oxides (V, Mo, W, Nb). X-ray diffraction analysis indicates the formation of an orthorhombic Mo7,8V1,2NbO28,9 and MoO3 phase, the peaks are observed at 2θ = 22.1, 28.2,6.2,45.2,50.0° (3). The textural properties of the systems depend on the conditions of preparation of LDHs, in particular on the particle size of the sample. For example BET surface area increased simultaneously with the particle size (Table 2). Table 2. Effect of the particle size of the precursor (LDHs) on the properties of the VMoWNb-O samples. Particle size of the Catalyst textural properties hydroxo salt sample, µm BET surf. area, Average pore size, Å m2/g 5-6 116.0 250 10-12 142.0 360

Pore volume, cm3/g 0.067 0.215

Some results of the catalyst testing are given in Tables 3 and 4. Table 3. Results of ODH of ethane. Dilution Conv., % Yield,% Sample The catalysts T,oC no. С2Н6/О2 1 Nb-Al-Mg-O 500 1/0.35 12.0 10.8 2 Nb-Mo-Al-Mg-O 500 1/0.35 17.0 15.8 2 Nb-Mo-Al-Mg-O 700 1/0.5 57.3 41.4 3 Nb-V-Al-Mg-O 500 1/0.35 17.0 15.4 4 Nb-V-Mo-Al-Mg-O 450 1/0.30 19.0 17.1 Nb-V-Mo-Al-Mg-O 700 1/0.50 19.7 42.4 5 Nb-V-Mo-W-Al-Mg-O 450 1/0.30 58.3 19.0

Selectivity, % 92.0 92.8 72.3 92,0 94.0 74.3 97.0

It must be emphasized that ODH of ethane over the Nb-containing catalysts occurs at rather low temperature (Table 3). Selectivity to ethene achieved about 94-97% (58% conversion, 450°C) on the VMoWNb-O catalyst. The effect of modifying appeared to be much more poorly under conditions for ODH of propane. The VMoWNb-O catalysts exhibited high efficiency in oxidative dehydrogenation of ethylbenzene into styrene and dehydrocyclization of n-octane (to ethylbenzene and styrene) in the presence of oxygen or CO2. The yield of styrene from ethylbenzene was close to 80% at the 97-98% selectivity (500°C). In a scanty studied reaction of n-octane dehydrocyclization the yields of styrene and ethylbenzene reached 23-28% at the total selectivity of 90%. The efficiency in ODH of several alcohols over V-Mo-W-Nb-Mg-Al-O catalysts prepared using complex hydrotalcite-like layered hydroxosalts (LDHs) as precursors has been determined. The optimal reaction conditions for producing of carbonyl compounds with high yields and selectivity have been found. In the runs carried out at 320°C, the 85% yield of octan-2-one has been obtained while selectivity was close 97%. The catalyst operation in oxidative dehydrogenation was rather steady, without any reducing in the

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catalyst activity and selectivity towards octan-2-ol in the prolonged run. After 50 hours on stream the catalyst kept its initial activity. The catalyst has been studied in oxidative dehydrogenation of few alcohols having different structures: octan-2-ol, ethanol, butanol, propanol.

№ 1 2 3

Table 4. Results of ODH with the Nb-V-Mo-W-Al-Mg-O catalyst. Substrate T, oC Conversion, % Selectivity, % ethylbenzene 500 70.0 97.5 n-octane 500 28.1 90.0 butanol 320 45.0 97.0 340 55.0 96.8 360 70.0 96.0

According to the reactivity in oxidative dehydrogenation, the alcohols studied can be arranged in the following raw: octan-2-ol > ethanol > propanol > butanol. The results obtained show the ways for the further development of multi-component catalysts prepared on the basis of hydrotalcite-like LDHs and their application in oxidative transformations of EB, light alkanes and alcohols.

4. Conclusions The method of synthesis of multi-component hydrotalcite-like layered metal hydroxosalts (LDHs) was elaborated. The products obtained have been used as precursors of V-MoW-Nb-O catalysts for ODH of EB, ethane, propane and alcohols. Considerable increasing of EB, alkane and acohols conversion simultaneously with the selectivity to alkene, styrene and carbonyl compounds has been successively achieved using the purposeful progressive complication of the composition of the LDHs precursors. Investigations made it possible to formulate scientifically substantiated preparation and application of the V-Mo-W-Nb-O catalysts to obtain high activity and selectivity in the ODH processes.

References [1] O.N Krasnobaeva., I.P Belomestnykh., G.V Isagulyants., V.P Danilov., Rus. J. Inorg. Chem., 54 (2009) 485-499. [2] V.P Krasnobaeva, O.N., V. P Danilov, I.P Belomestnykh, G.V Isagulyants, Rus. J. Inorg. Chem. 52 (2007) 181. [3] P. Botella, J.M. Lopez Nieto and N. Solsona, Catal. Lett., 78 (2002) 383.

Dear Collegues, Now I send a manuscript corrected according to the remarks of the reviewer It is in the attach file Best regards, yours I. Belomestnykh

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Synthesis of high-surface area CeO2 through silica xerogel template: influence of cerium salt precursor L.F. Liotta a, G. Di Carlob, F. Puleob, G. Marcìc and G. Deganelloa,b a

Istituto per Lo Studio dei Materiali Nanostrutturati (ISMN)-CNR via Ugo La Malfa, 153, 90146 Palermo, Italy. E-mail: [email protected] b Dipartimento di Chimica Inorganica e Analitica “Stanislao Cannizzaro”, Università di Palermo, Parco d’Orleans II, Viale delle Scienze pad. 17, 90128 Palermo, Italy. c Dipartimento di Ingegneria Chimica dei Processi e dei Materiali, Università di Palermo, Parco d’Orleans II, Viale delle Scienze pad. 6, 90128 Palermo, Italy.

Abstract Ceria nanosized oxides with high surface area were synthesized by means of a templating approach, using a porous silica xerogel with surface area as high as 718 m2/g. After impregnation of the silica template with the cerium salt solution and further calcination at 600°C, the final ceria oxide was recovered by dissolving the silica framework in NaOH solution. The effect of cerium counteranion, nitrate or chloride, on the textural and reduction properties of the ceria oxide was examined. Characterizations by BET and pore size distribution, XRD, TPR and SEM/EDX techniques were performed. The silica xerogel templated approach resulted in the preparation of ceria with surface area of 205 m2/g and very small particle size (∼5 nm), when cerium chloride precursor was used. An enhanced reducibility, at temperature < 700°C, was also observed for the so obtained CeO2 sample. The results were discussed in terms of the influence of cerium choride and cerium nitrate thermal decomposition. Keywords: silica xerogel, cerium nitrate and chloride, incipient wetness impregnation, ceria nanostructure, reduction properties

1. Introduction Since the discovery of mesoporous silica (e.g. MCM-41, SBA-15) in the 1990s, highsurface metal oxides, such as titania, ceria, zirconia, alumina, have received worldwide attention for applications in catalysis, as chemical sensors and as electrodes in fuel cells. Various synthesis approaches, depending on the applications, are currently used such as sol-gel technique, deposition-precipitation, chemical vapour deposition, spray pyrolysis and micro-emulsion methods. Some of these procedures give materials with poor crystallinity and low thermal stability. Others are quite complicated and expensive. In recent years, a wide variety of porous materials have been obtained by means of template technique [1,2]. Recently, Fuertes [3] reported the use of an inexpensive porous silica xerogel as template for synthesis of various metal oxides. In particular, starting from a silica xerogel with specific surface area of 510 m2/g, prepared by sodium silicate in aqueous solution containing HCl, in appropriate molar ratio, the synthesis of ceria with specific surface area equal to 141 m2/g and a narrow pore size distribution centred at ∼4.6 nm was achieved. In the present work, we report the synthesis of nanosized ceria oxides, starting from cerium nitrate and cerium chloride, as precursors and using a high surface area silica xerogel, as template. After calcination of the template material impregnated with the

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cerium precursor, the ceria oxide was recovered by dissolving the silica matrix in NaOH solution. Characterizations by BET and pore size distribution, XRD, TPR and SEM/EDX techniques were performed.

2. Experimental The silica xerogel was synthesized according to a published procedure [3], opportunely modified, using sodium silicate solution, Na2O(SiO2)x⋅xH2O, (Sigma-Aldrich, with composition: Na2O ∼10.6% + SiO2 ∼26.5%), as silica source. In a typical preparation, an appropriate amount of sodium silicate solution was diluted (1:2) with distilled water and, then, was added under stirring to an aqueous solution of HCl. The final molar composition of the reagents was: sodium silicate/HCl/H2O = 1/6/194, accordingly to the literature [3]. The obtained transparent solution was stirred in the closed flask for 20h at room temperature. After that, the mixture, which turned from completely transparent to slightly opaque, was aged in a closed Teflon vessel at 100°C for 2 days. The obtained gel was filtered and washed several times with water. Finally, the solid was washed with acetone and diethyl ether and then dried at room temperature. Synthesis of ceria oxides was accomplished by incipient wetness impregnation steps of the so prepared silica xerogel with ethanol solution of cerium nitrate, Ce(NO3)3⋅6H2O, or cerium chloride, CeCl3⋅7H2O. In order to maximize pore filling and avoid the surface segregation of cerium salt, four successive impregnations were performed until a final loading of 40wt% of ceria. The resulting samples were dried at room temperature, calcined at 600°C for 4h and, in order to dissolve the silica matrix, treated with a 2M NaOH solution at 65°C. The final samples were labeled as CeO2-N (from cerium nitrate) and CeO2-Cl (from cerium chloride). As reference, a portion of silica xerogel was also calcined at 600°C for 4h. The resulting silica was labeled as SiO2-calc. Physico-chemical characterizations were performed during the different steps of ceria oxides preparation and on the finished samples. Surface area measurements (BET) and pore size distribution (BJH) were carried out by means of Sorptomatic 1900 (Carlo Erba) instrument. X-ray diffraction patterns were recorded with a D 5005 X-Ray Diffractometer (SIEMENS) using Cu Kα radiation coupled with a graphite monochromator. The crystallite sizes of ceria phase were calculated from the line broadening of the most intense reflection using the Scherrer equation [4]. Temperature programmed reductions (TPR) were carried out with a Micromeritics Autochem 2910 apparatus equipped with a thermal conductivity detector. The ceria samples (∼ 50 mg) were pre-treated with O2 (5% in He) at 600°C for 30 min, cooled in He and then H2 (5% in Ar, 50 ml/min) was flowed from room temperature to 1050°C (heating rate 10° C/min). Scanning electron microscopy (SEM) images and energy-dispersive X-ray (EDX) analyses were obtained using a FEI quanta 200 ESEM microscope, operating at 20 kV on specimens after being coated with a layer of gold.

3. Results and discusion The physical properties of silica xerogel and of the prepared ceria oxides are listed in Table 1. In Fig. 1 the XRD patterns of ceria samples are shown. The starting silica xerogel exhibits a surface area as high as 718 m2/g and a narrow pore size distribution (mean pore size of 8.2 nm) with a total pore volume of 1 cm3/g. After calcining the xerogel at 600°C for 4h, the surface area and pore volume were reduced, whilst the pore size increased only slightly. Maintaining good textural properties after calcination makes silica xerogel attractive as template. Rather high surface area (130 m2/g) was obtained for CeO2-N, showing pore size and pore volume values almost comparable

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with the literature results [3]. Crystallite sizes of 6.9 nm were calculated from XRD peak broadening. When cerium chloride was used as precursor, a ceria oxide with improved physico-chemical properties was achieved, displaying surface area of 204 m2/g and very small particle size (4.7 nm). Accordingly, for CeO2-Cl broad and weak peaks were detected as compared to CeO2-N (Fig. 1). It is likely that the exothermic decomposition of the nitrate precursor during calcination, by increasing much more the local temperature, induces a faster crystallite growth in CeO2-N. Table 1. Structural properties of silica xerogel and synthesized ceria oxides. Sample

Calcination temperature (°C)

BET (m2/g)

SiO2 xerogel SiO2-calc CeO2-N CeO2-Cl

No calcination 600 600 600

718 560 130 205

(a) CeO2-N

Intensity (A.U.)

(b) CeO2-Cl

(a) (b) 20

25

30

35

40

45

50

2θ (°)

Fig.1. XRD patterns in the angular range 20-50 2θ for ceria oxides obtained through template method . (a) CeO2-N (b) CeO2-Cl

3

Volume adsorbed (cm STP/g)

(a)

(b)

0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure, P/P0

Fig.2. Nitrogen adsorption/desorption isotherms for ceria oxides.

Mean pore size (BJH) (nm) 8.1 8.5 6.8 4.9

Total pore volume (cm3/g) 1.05 0.78 0.48 0.42

Crystallite size (nm) 6.9 4.7

In Fig. 2 the nitrogen sorption isothermes of the two samples, CeO2-N and CeO2-Cl are compared. The different shape of isotherms is indicative of different surface area and different pore size of structural mesopores. The isotherms obtained for CeO2-N shows a pronounced hysteresis in the range p/p0 ∼0.5-0.8, while in the case of CeO2-Cl the capillary condensation takes place at lower relative pressure, in the range p/p0∼0.4-0.6. Accordingly, CeO2-N shows bigger pore size than CeO2-Cl (Table1). From SEM and EDX investigation no significant differences were observed between the xerogels after impregnation with cerium nitrate or cerium chloride and drying at room temperature. Indeed the morphology and the percentage of Ce and Si in the two series of oxides were very similar. Interestingly some differences appeared after calcinations at 600°C, probably due to the different decomposition process of cerium counteranions. As far as final ceria samples are concerned, Fig. 3a,b shows two SEM images of CeO2-N and CeO2-Cl oxides. By the comparison of these pictures, it is evident that ceria from chloride shows a rougher surface with respect to the nitrate prepared one and the presence of smaller grains can be detected.

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Moreover, some interstices between the nanoparticles (textural pores in the size of macropores) can be detected. All these insights are in agreement with the higher surface area measured for the CeO2-Cl sample. The residual Si content, observed by EDX, was less than 10wt% in both ceria samples.

a

b

2μ

2μ

Fig. 3a,b. Comparison between SEM micrographs of two ceria samples: (a) CeO2-N, (b) CeO2-Cl. (a) CeO 2-N

H2 consumption (A.U.)

(b) CeO 2-Cl

(b)

(a) 200

300

400

500

600

700

800

900 1000

Temperature (°C)

Fig. 4. H2 -TPR profiles for ceria oxides obtained through template method.

It is know that the reduction profile of ceria depends on the surface area and crystallite size. H2-TPR measurements performed over the two ceria samples reflect different properties (Fig. 4). According to our previous results [5], the reduction profile of CeO2-N was characterized by an intense peak centered at 425°C (647 μmolH2/g) due to the surface reduction, while the broad peak at higher temperature, 700-800°C, was ascribed to the reduction of the bulk (446 μmol H2/g). Much more reducible appeared the CeO2-Cl sample showing the overall H2 consumption (1450 μmolH2/g) at temperature < 700°C, which accounts for almost 50% reduction of CeO2 to Ce2O3. The presence of grain boundaries and defects in such small ceria crystallites could explain the enhanced reducibility [6].

4. Conclusions Synthesis of mesoporous ceria oxides nanoparticles with surface area larger than those reported in literature for similar prepared materials can be successfully achieved by applying silica xerogel templating approach. The effect of cerium counteranion was significant, ceria from nitrate being more affected by the exothermic decomposition of the precursor. Enhanced bulk reducibility at temperature below 700°C was observed in nanosized ceria crystyallites prepared from chloride precursor, demonstrating the feasibility of this approach for the preparation of reducible oxides for catalytic purposes.

References [1] P.D. Yang, D.Y. Zhao, D.I. Margolese, B.F. Chmelka, G.D. Stucky, Chem. Mater. 11 (1999) 2813. [2] B.T. Holland, C.F. Blanford, T. Do, A. Stein, Chem. Mater. 11 (1999) 795. [3] A.B. Fuertes, J. Phys. Chem. Solids, 66 (2005) 741. [4] H.P. Klug, L.E. Alexander, X-ray Diffraction Procedures for Polycrystalline and Amorphous Materials, Wiley, New York, 1954. [5] A.M. Venezia, G. Pantaleo, A. Longo, G. Di Carlo, M.P. Casaletto, L.F. Liotta and G. Deganello, J. Phys. Chem. B 109 (2005) 2821. [6] H. Zhu, Z. Qin, W. Shan, W. Shen, J. Wang, J. Catal. 225 (2004) 267.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Iron based catalyst for hydrocarbons catalytic reforming: A metal-support interaction study to interpret reactivity data Luca Di Felicea,b Claire Coursona, Pier Ugo Foscolob and Alain Kiennemanna a

Laboratoire des Matériaux, Surface et Procédés pour la Catalyse, ECPM, UMR7515, 25 rue Becquerel, 67087, Strasbourg Cedex 2, France b Chemical Engineering Department, University of L’Aquila, 67040 Monteluco di Roio, L’Aquila, Italy

Abstract The addition of iron, a cheap and non toxic metal, to the natural minerals dolomite and related materials, CaO and MgO, has been investigated for biomass gasification applications. The Fe/CaO, Fe/MgO and Fe/dolomite systems have been prepared by impregnation following two preparation methods to generate Fe (+2) and/or Fe (+3) species, and carefully characterized. The improvement on tar conversion of CaO, MgO and dolomite by adding iron, has been investigated by using toluene as model tar compound in a microreactor rig. Keywords: iron catalyst, tar reforming, dolomite

1. Introduction The research in new materials with catalytic properties is an important stage on the development of the applicability of biomass gasification concepts, with the aim of overcoming problems of in-situ tar elimination for producing a clean gas suitable for feeding fuel cells. The aim of this work is to investigate synthesis methods, characterization and catalytic tests of new cheap and non toxic catalysts for gasification reactions, using toluene as model tar compound, in a fixed bed microreactor. Iron has been added to dolomite, CaO and MgO substrates, well known materials in biomass gasification processes [Delgado et al., 1997], in order to improve their catalytic activity.

2. Experimental 2.1. Catalyst preparation. The catalytic systems investigated in this work consists of pre-calcined natural dolomite (Ca,Mg)O, a pre-calcined natural lime (CaO) and magnesia (MgO) impregnated by 20% of iron by weight. There have been developed two preparation pathways: an oxidative one, focused on the evaluation of the Fe (+3)–substrate interaction (impregnation solvent: water; salt precursor: iron nitrate; samples named OxiCa, OxiMg and OxiDolo) and a neutral one to evaluate the Fe (+2, +2.5)–substrate interaction (impregnation solvent: ethanol; salt precursor: iron acetate; samples named NeuCa, NeuMg and Neu Dolo). For both preparation methods, the iron salt is solubilized in the impregnation solvent, then the substrate is added and stirred to obtain a suspension. The solvent is evaporated at the appropriated temperature and the solid recovered is dried (120°C, 5h) and crashed (80
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corresponding atmosphere at 850°C and 1100°C for 4 h, with a heating rate of 3°C/min, to investigate also the influence of thermal treatment in the crystalline phases obtained.

2.2. Catalyst characterization. Starting from iron acetate or iron nitrate salts and following the neutral and oxidative pathways, a separated analysis of iron (2+, 2.5+, 3+)-calcium oxide and iron (2+, 2.5+, 3+)-magnesium oxide will be useful also to clarify the nature of the iron-dolomite interaction. The Fe/substrate catalysts have been characterized by X-Ray diffraction (XRD), Temperature Programmed Reduction (TPR), Temperature Programmed Oxidation (TPO), and Mössbauer analysis. XRD and TPR experimental conditions, as well as the microreator used in catalytic tests, have been described previously [Di Felice et al., 2009], whereas Mössbauer spectroscopy and TPO are briefly illustrate here. The 57Fe Mössbauer spectra were recorded at room temperature (293 K) using a spectrometer with a triangular waveform with a 57Co source (50 mCi) dispersed in a rhodium matrix. From the obtained spectra, it has been determined the isomeric shifts in comparison with metallic iron standard at room temperature. To identify the different forms of iron present in the sample, it has been fitted the spectra with the NORMOS computer program. TPO (Temperature Programmed Oxidation) analyses have been carried out after catalytic tests in order to detect the amount of residual carbon from the amount of carbon-containing products (CO+CO2). The heating program of TPO permits to reach 1000°C with a rate of 15°C/min under a mixture of oxygen (10vol%) in helium.

3. Samples Characterization 3.1. Fe/MgO - oxidative and neutral pathway The XRD spectra of OxiMg has been compared with the raw materials incorporated in this sample, i.e. MgO and Fe2O3. MgFe2O4 (magnesioferrite) phase has been obtained, Free Fe2O3 oxides has not been detected. The same comparison procedure has been adopted for NeuMg sample and raw materials MgO and FeO. A strong shift of the MgO peaks toward FeO is observed in the NeuMg diffractogram; this experimental observation evidences that most of the iron is integrated in the MgO substrate as FeO-MgO solid solution. The average composition of the solid solution has been determined from Vegard law as Fe0.1MgO0.9. Mössbauer analysis has been carried out for the synthesized samples OxiMg and NeuMg calcined at 1100°C, confirming X-ray diffraction analysis. The samples calcined at 850°C shows the same general behavior, the same phases appearing in the X-ray diffraction analysis.

3.2. Fe/CaO - oxidative and neutral pathway For both NeuCa and OxiCa calcined at 850 and 1100°C, iron is detected by XRD only as Ca2Fe2O5 phase. Iron in the (2+, 2.5+) oxidation state, as well as free Fe2O3 iron oxides, are not detected. This evidence may be due to the possibility of brownmillerite structure to stabilize the oxygen defects [Hirabayashi et al., 2006] and to allow high oxygen mobility. These data are confirmed by Mössbauer analysis, not shown here.

3.3. Fe/dolomite - oxidative and neutral pathway From the study carried out on Fe/MgO and Fe/CaO, it is expected that Fe/dolomite interactions take place as Ca2Fe2O5 and MgFe2O4 phases in the oxidative pathway and as Ca2Fe2O5, FeOMgO and Fe3O4-Fe3-xMgxO4 in the so-called neutral pathway. Therefore, this study will proceed comparing these three phases for the respective preparation methods.

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XRD of OxiDolo was compared to XRD of Ca2Fe2O5 and MgFe2O4. It is evident that MgFe2O4 phase is not detected but that iron interacts strongly with CaO as Ca2Fe2O5. As no other MgO-iron oxide is evidenced, it appears that all Fe (3+) reacted with CaO. For the neutral pathway, X-ray diffraction shows once again a strong presence of Ca2Fe2O5, but a peak in the Fe3O4 zone is also clearly detected. A Mössbauer analysis has been carried out for both OxiDolo and NeuDolo calcined at 850°C; the former contains 88% of iron as brownmillerite-type Ca2Fe2O5, and the remaining 12% results in a Fe (3+) valence state representing paramagnetic phase of iron. The latter contains 43% of iron in the Ca2Fe2O5 phase, 25% of iron in the spinel phase, and a 32% of the iron representing sample in the paramagnetic state having iron in the mixed valence state Fe(2+) – Fe(3+). The spinel phase could be attributed to a Fe3O4-Fe3-xMgxO4 mixed phase [Stobb et al., 1991].

3.4. Reduction behaviour of iron 3.4.1. Fe/dolomite reduction As proceeded previously for XRD analysis, a first qualitative characterization of the reduction behaviour of the Fe/dolomite catalysts (calcined at 850 and 1100°C) may be performed from a superposition with his components. In particular, the OxiDolo sample evidence a good superposition with the OxiCa sample, confirming XRD and Mössbauer analyses. Also TPR of NeuDolo calcined at 850°C, is consistent with previous analyses, evidencing the presence of four main peaks that may be well superposed with Fe3O4 – Fe3-xMgxO4 → FeO (600-750°C) → Fe (0) (900°C), and Ca2Fe2O5→Fe (0) + CaO reduction pathways. A schematic model of the phases obtained in the OxiDolo and NeuDolo catalysts and their reduction behaviour is shown in Fig. 1. (CaO)2*Fe2O3

a)

Fe3O4 – Fe3-xMgxO4

reduction CaO

MgO

. . . .. .. . . . CaO

Fe0

(CaO)2*Fe2O3

reduction

b)

(FeO)x-(MgO)1-x

Fe0

CaO

MgO

Fe0

.. . .. MgO

MgO*FeO (traces)

. . . .. ... . . CaO

MgO

Figure 1:A schematic model of the phases obtained in the NeuDolo (a) and OxiDolo (b)catalysts, and their reduction behaviour

4. Catalytic activity The catalytic activity of iron supported on dolomite catalyst has been studied at the temperature of 850°C (Figure 2). The overall conversion of toluene may be expressed as a function of carbon-containing components in the gas-phase, Xt (Eq.5). Xt =

CO + CO 2 + CH 4 7 ⋅ C7 H 8

(5)

Catalysts have not been pre-reduced before test. The reactivity tests with calcined dolomite, lime and magnesia, confirm the reactivity trends obtained by other authors [Delgado et al., 1997]. OxiMg shows a slightly superior reactivity, which decreases during the 6 hours test from 70% to a value of about 60% of toluene conversion. A substantial improvement on toluene conversion, for Fe/dol, is not observed with respect

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to raw dolomite. OxiCa, however, is able to raise the catalytic toluene conversion of raw CaO, from values below 30% up to 45-50% of conversion. This result is very interesting because lime has been indicated as the most recommended additive for steam gasification [Corella et al., 2006], being less soft and therefore less susceptible to erosion erosion in a fluidized bed. The disadvantage of being less active than dolomite in tar elimination has been overcame in this study, demonstrating a useful improvement on catalytic activity by adding iron to this substrate. Similar trends are obtained for the samples prepared with the neutral pathway. TPO analyses after catalytic tests show that iron strongly limits the carbon deposited on the catalyst surface. 5% Fe/dol reduces by 1:7 the value of carbon obtained with raw dolomite. 5% Fe/CaO, reduces this amount of carbon by 1:15 compared with CaO, and 1:360 compared with dolomite. If carbon deposition represents a critical point of dolomite deactivation, as reported in literature, the use of iron may be of interest for further investigations in gasification processes. a)

80

80

b) 70

70

OxiMgO

60

dol

50

OxiCaO

50 %Xt

%Xt

60

40 30

40

OxiDolo

30 MgO

20

20

CaO

10

10 0

0 0

50

100

150

200 time (min)

250

300

350

400

0

50

100

150

200

250

300

350

400

time (min)

Figure 2: Toluene conversion, as defined in Eq. 11, as function of time for supports (a) and for the iron catalysts prepared in the oxidative pathway (b).

5. Conclusions From the interactions highlighted in this paper, it may be emphasized that in presence of dolomite, the Fe (3+)-CaO interaction is prevailing. Iron has been found to improve catalytic activity and carbon deposition resistance of CaO and MgO substrates, in order to render them more attracting for scale up applications.

References L. Di Felice, C. Courson, N. Jand, K. Gallucci, P.U. Foscolo, A. Kiennemann, 2009, Catalytic Biomass Gasification: Simultaneous Hydrocarbons Steam Reforming and CO2 Capture in a Fluidised Bed Reactor, Chemical Engineering Journal, accepted manuscript, doi:10.1016 /j .cej.2009.04.054. J. Corella, J.M. Toledo, G. Molina, 2006, Steam Gasification of Coal at Low-Medium (600-800 °C) Temperature with Simultaneous CO2 Capture in Fluidized Bed at Atmospheric Pressure: The Effect of Inorganic Species. 1., Ind. Eng. Chem. Res. 45, 6137-6146. J. Delgado, M.P. Aznar, J. Corella, 1997, Biomass Gasification with Steam in Fluidized Bed: Effectiveness of CaO, MgO, and CaO-MgO for Hot Raw Gas Cleaning, Ind. Eng. Chem. Res. 36, 1535-1543. D. Hirabayashi, T. Yoshikawa, K. Mochizuki, K. Suzuki, Y. Sakaib, 2006, Formation of brownmillerite type calcium ferrite (Ca2Fe2O5) and catalytic properties in propylene combustion, Catalysis Letters 110, 155-160. E.D. Stobbe, R. van Buren, 1991, Iron oxide dehydrogenation catalysts supported on magnesium oxide. Part 1.- Preparation and characterization, J. Chem. Soc. Faraday Trans. 87, 1623-1629.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Ecofriendly catalysts based on mixed xerogels for liquid phase oxidations by hydrogen peroxide M. Palacio, P. Villabrille, G. Romanelli, P. Vázquez, C. Cáceres Centro de Investigación y Desarrollo en Ciencias Aplicadas “Dr. Jorge J. Ronco” (CINDECA), CONICET CCT La Plata, Universidad Nacional de La Plata, Calle 47 Nº 257 (1900) La Plata, Argentina.

Abstract In this study TiO2-CeO2 and TiO2-V2O5 mixed oxide were synthesized by sol-gel technique. The mixed xerogels were prepared with different load of TiO2 and CeO2 or V2O5, using titanium isopropoxide and hexahydrated cerium nitrate or vanadyl acetylacetonate as precursors, and ethanol (99.8%) as solvent. The gels were dried in air at 50°C for 24 h and calcined in air at temperatures in the range 200-800°C for 4 h. Moreover, a sample without cerium, 100% (w/w) TiO2, was prepared to be kept as reference material. The prepared solids were characterized by means of XRD, FT-IR and TEM. Their textural properties were determined by adsorption-desorption isotherms of N2 at 77 K. The xerogels were tested as catalysts in the liquid phase oxidation of 2,6dimethylphenol to 2,6-dimethyl-p-benzoquinone at 20°C, using ethanol as solvent and aqueous hydrogen peroxide as a clean oxidizing agent. Keywords: mixed oxide, TiO2-CeO2, TiO2-V2O5, 2,6-dimethylphenol oxidation, H2O2

1. Introduction It is now widely accepted that there is an urgent need for more environmentally acceptable processes in the chemical industry. This trend towards what has become known as green chemistry necessitates a paradigm shift from traditional concepts of process efficiency, which focus only on chemical yield, to one that assigns economic value to eliminating waste and avoiding the use of toxic and/or hazardous substances. This factor is important in the production of fine chemicals and pharmaceuticals, partly because the production of the latter involves multi-step syntheses and partly because of the use of stoichiometric reagents rather than catalytic methodologies. The heterogeneous solid catalysts have the advantages of ease of recovery and recycling. Mixed metal oxides have interesting catalytic properties [1]. The presence of a foreign element in the matrix of a pure metal oxide can greatly modify its properties. Sol-gel process allows excellent control of the synthesised mixed oxide textural properties, such as surface area and pore volume. Within the sol-gel process, relative precursor reactivity can be used to control homogeneity [2]. The success of CeO2 based catalysts in oxidation reactions is mainly due to the unique combination of an elevated oxygen transport capacity coupled with the ability to shift easily between reduced and oxidised states [3]. On the other hand, the TiO2-V2O5 catalysts have been extensively studied for selective oxidation reactions of methanol, propane, and butane, among other compounds [4, 5]. Wet peroxide oxidation processes using hydrogen peroxide as oxidant have emerged as a viable alternative for the liquid phase oxidation of phenol. Hydrogen peroxide does not form any harmful by-products, and it is a non-toxic and ecological reactant [6]. The phenol oxidation to quinone is important since quinone derivatives play a key role in biosystems and are useful intermediates of fine organic synthesis [7].

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The objective of this study is to continue with the investigation on phenol oxidation in H2O2 solution using solid catalysts, in this case the mixed oxides (Ti-Ce, Ti-V).

2. Experimental 2.1. Materials and synthesis The TiO2-CeO2 mixed oxides were prepared by the sol-gel method using titanium isopropoxide (Aldrich, 97%) and hexahydrated cerium nitrate (Aldrich, 99%) as precursors. Absolute ethanol (Aldrich, 99.8%) was used as solvent for both reactants, with an alkoxide/solvent molar ratio of 1/16 and a 0.2 M concentration for the cerium salt. The mixture was reflowed with continuous stirring. The solution pH was fitted to 4 with a 0.01M HNO3 (Anedra, 40%) solution. Then, water was added dropwise for 4 h, using a water/alkoxide molar ratio of 8. The samples were named TiX-T°C for a nominal concentration X% w/w of TiO2 and (100-X) % w/w of CeO2 according to the drying or calcination temperature. Also, samples without cerium were prepared as reference material, following the same synthesis. They were named Ti100-T°C. The TiO2-V2O5 mixed oxides were prepared by the sol-gel method using titanium isopropoxide and vanadyl acetylacetonate (Fluka, 97%) as precursors. Absolute ethanol was used as solvent for both reactants, with an alkoxide/solvent molar ratio of 1/16, and keeping this ratio constant, the amount of ethanol required to solubilise the vanadium precursor was used. The alkoxide and solvent mixture was reflowed with continuous stirring. Then the hot solution of V precursor was added, and after 15 min a 0.01 M HNO3 solution was added up to pH=6. Then, water was added dropwise using a water/ alkoxide molar ratio of 8. The mixture was reflowed up to gel formation. The samples were named AcVTiX-T°C for a concentration X% w/w of TiO2 and (100-X) % w/w of V2O5 according to the drying or calcination temperature.

2.2. Solid characterisations The nitrogen adsorption/desorption isotherms at 77 K on solids were determined by using Micromeritics ASAP 2020 equipment. XRD patterns of solid samples were recorded by means of a Philips PW-1732 device with built-in recorder. Thermo Nicolet IR 200 equipment and pellets with BrK were used to obtain the FT-IR spectra of the solid samples. For the transmission electron microscopy (TEM and EDX) study, a JEOL transmission electron microscope, JEM-2010 model, was used.

2.3. Catalytic test The oxidation of 2,6-dimethylphenol (2,6-DMP) to 2,6-dimethyl-p-benzoquinone (2,6-DMBQ) was performed under vigorous stirring in a glass reactor at 20°C. The reactions were performed by adding 1 ml of H2O2 (60% (w/v) dropwise to a solution of 1 mmol of 2,6-DMP, 5 ml of ethanol (96%), and 0.1 mmol of catalyst. The reaction was followed by TLC (thin layer chromatography) up to total conversion of 2,6-DMP. TLC aluminium sheets (silica gel 60 F254, Merck), a commercial standard of 2,6-DMP and 2,6-DMBQ (99%) and CH2Cl2 as solvent were used. The reaction mixture was centrifuged and the catalyst was separated. The solution was diluted with 10 ml of distilled water and extracted with dichloromethane (2 x 5 ml). The organic extract was dried over anhydrous sodium sulphate and the solvent evaporated by a rotary evaporator. The product obtained (crude product) was analyzed by UV-vis spectroscopy in Perkin Elmer Lambda 35. For their analysis the samples were diluted in ethanol (30 ppm). The reaction product, 2,6-DMBQ, was identified by a MS-GC (Mass Spectrometer coupled to a Gas Chromatograph). The selectivity was calculated as the weight ratio between the

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obtained amount of 2,6-DMBQ (crude product) and the theoretical amount of product that would be formed.

3. Results and discussion

Intensidad (u.a.)

XRD patterns corresponding to the mixed oxides of different concentrations of Ce or V calcined at 200ºC are shown in Figure 1. In all samples, very wide bands without welldefined diffraction lines, the characteristic patterns of amorphous materials, are observed. However, the main peak of anatase begins to be formed, although it is hardly observed at 2θ=25.3º for CeTi85-200, CeTi90-200 and AcVTi95-200 On the other hand, in a previous work [8], it was observed that the mixed oxides TiO2-CeO2 calcined at 800 ºC exhibit the characteristic XRD patterns of crystalline solids showing a series of wellAcVTi85-200 defined peaks. The presence of anatase phase, as the main species is clearly identiCeTi85-200 fied. For the samples TiO2-V2O5, XRD studies showed a similar change of the AcVTi90-200 CeTi90-200 diffraction patterns as a function of the heating temperature increase. AcVTi95-200 CeTi95-200 The nitrogen adsorption–desorption Ti100-200 isotherms on mixed oxides TiO2-CeO2 and 60 70 80 90 100 TiO2-V2O5 calcined at 200ºC, with dif- 20 30 40 50 2 theta (°) ferent concentrations of Ce and V, were determined. The values of specific surface Figure 1. XRD patterns of Ti and Ce or V area (SBET), pore volume (VP) and mean mixed oxides. pore diameter (DP) obtained from the isotherms are shown in Table 1. The SBET and VP values significantly decreased with 5% w/w of CeO2 addition for the samples prepared and calcined at 200ºC (Table 1, entries 1 and 2). The values of both parameters decreased fundamentally with the increase of CeO2 concentration up to 10% w/w of CeO2 (Table 1, entries 2 and 3). However, the mean pore diameter (DP) for these samples slightly changed when Ce was added. The SBET, VP, and DP values change similarly with the increase of V concentration (Table 1, entries 5, 6 and 7). For the same nominal concentration of CeO2 and V2O5, the mixed oxides of V showed values of SBET and VP lower than those of Ce, although the DP values were only slightly lower than those of Ce. FT-IR spectra of the mixed and pure oxides, calcined at 200ºC, are shown in Fig. 2. The band around 600 cm-1 appears due to the Ti–O–Ti vibration bond, which results from the condensation reactions. This band is very intense for the pure Ti sample, but it is modified for Ti/Ce and Ti/V mixed oxides, which indicates the presence of Ti–O–Ce and Ti–O–V bonds in the mixed xerogels, respectively. In Figure 3 the values of 2,6-DMBQ selectivity determined in the oxidation of 2,6DMP using mixed oxides as catalysts were shown. These values resulted higher than those observed when 100% TiO2 was used as catalyst. The catalysts with 5% w/w of CeO2 or V2O5 were the most selective of the series Ce-Ti and V-Ti, respectively; but the catalyst CeTi95-200 showed more selectivity (99%) than the catalysts AcVTi95-200 (85%). These results could be explained taking into account that SBET of CeTi95 is higher than SBET of AcVTi95 and that it was observed, by TEM microscopy, that the VTi oxide was homogeneous while the CeTi oxide showed segregate clusters of CeO2 and TiO2 (9-11 nm in size).

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1- Ti100-200 2-CeTi95-200 3-CeTi90-200 4-CeTi85-200 5-AcVTi95-200 6-AcVTi90-200 7-AcVTi85-200

485 329 235 238 280 154 84

0.37 0.28 0.18 0.23 0.18 0.07 0.04

Ti100-200

Transmittance (a.u.)

Table 1. Textural properties of mixed oxides of Ce and V calcined at 200ºC. VP Sample SBET (m2/g) (cm3/g)

DP (Å)

30.26 33.74 30.84 38.83 25.07 19.43 19.98

AcVTi90-200

CeTi90-200

500

1000 1500 2000 2500 3000 3500 4000

Wavenumber (cm-1)

Figure 2. FT-IR spectra of Ti and Ce or V mixed oxides and Ti oxide.

Figure 3. 2,6-DMBQ selectivity using mixed oxides or Ti oxide as catalyst.

Selectivity (%)

100

Ce

80

AcV

60 40 20 0

4. Conclusions

85%

90%

95%

100%

TiO2 Nominal Composition

The mixed metal oxides synthesized by sol-gel technique were very active and selective for the oxidation in liquid phase at 20ºC of 2,6-DMP to 2,6-DMBQ, using H2O2, as clean oxidant and safer solvent (ethanol). It were obtained conversions of 100% and selectivity to 2,6-DMBQ of 59 to 99%. The last values were higher than that observed using pure titania as catalyst. This proves that the CeO2 or V2O5 addition to the catalyst has a positive influence in its performance in the studied reaction. The reaction time determined for to reach total conversion of 2,6-DMP was of 4 or 5 h. The catalysts most active, 4 h, were those with 5% w/w of CeO2 or V2O5, with similar activity to the pure TiO2 catalyst but with 2,6-DMBQ selectivity greater, 99% and 85%, respectively. It was found that the 2,6-DMBQ selectivity values of the mixed oxides tested were correlated with their SBET and their crystallinity. On the other hand, the tests of leaching and re-use carried out probe their stability. In short, the prepared xerogels of TiO2-CeO2 and TiO2-V2O5 resulted to be suitable as new heterogeneous catalysts to carry out the oxidation of organic compounds in an environmentally sustainable way.

Reference [1] T. Seiyama, 1978, “Metal Oxides and Their Catalytic Actions”, Kodansha, Tokyo. [2] C.J. Brinker and G.W. Scherer, 1990, Sol–Gel Science, Academic Press, New York. [3] A. Trovarelli, M. Boaro, E. Rocchini, C. De Leitenburg and G. Dolcetti, 2001, Journal of Alloys and Compounds, 323, 584. [4] H. Zhao, S. Bennici, J. Shen and A. Auroux, 2009, Applied Catalysis A: General, 356, 121. [5] F. Trifiró, 1998, Catalysis Today, 41, 21. [6] L. Liotta, M. Gruttadauria, G. Di Carlo, G. Perrinid and V. Librando, 2009, J. Hazardous Materials, 162 , 588. [7] O.A. Kholdeeva, N.N. Trukhan, M.P. Vanina, V.N. Romannikov, V.N. Parmon, J. MrowiecBiałon and A.B. Jarz˛ebski, 2002, Catalysis Today, 75, 203. [8] M. Palacio, P. Villabrille, G. Romanelli, P. Vázquez, C. Cáceres, 2009, Appl. Catal.,, 359, 62.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Preparation of MgF2-MgO supports with specified acid-base properties, and their influence on nickel catalyst activity in toluene hydrogenation Michał Zieliński*, Maria Wojciechowska Faculty of Chemistry, Adam Mickiewicz University, Grunwaldzka 6, 60-780 Poznań, Poland *e-mail: [email protected]

Abstract The use of mixed magnesium fluoride - magnesium oxide as a support for nickel yielded a catalyst of high activity for the hydrogenation of toluene under atmospheric pressure. The effect of the MgF2/MgO ratio and the acid-base properties of the support on catalyst activity was studied. Supports of MgF2-MgO with different levels of MgO, in addition to pure MgO and MgF2, were prepared by a one-step sol-gel method, and characterized by low-temperature nitrogen adsorption, thermo-gravimetric measurements and adsorption of CO2. Nickel catalysts with 5wt.% Ni were prepared by impregnation of the supports with a solution of nickel acetate. The catalysts were tested in the hydrogenation of toluene at temperatures ranging from 75 to 225oC. The best results were obtained at 175oC for a nickel catalyst supported on MgF2. Keywords: MgF2-MgO support, magnesium fluoride, magnesium oxide, nickel, toluene hydrogenation

1. Introduction Restrictions on the level of aromatics in diesel fuels have prompted the search for more active and selective catalysts for C=C bond hydrogenation [1]. Catalytic activity in hydrogenation significantly depends on the type and properties of the support used. The most important features of the support in this respect are its crystalline structure, surface properties and porous structure, which influence the dispersion of the active phase and determine its reducibility. The commonly used supports for nickel are: Al2O3 [2], SiO2 [3], TiO2 [4] or binary systems such as SiO2-TiO2 [5]. These supports have different surface properties (acidic, neutral, basic) affecting not only the development of the surface area of the phases deposited but also the type of metal-support interactions and thus their catalytic behaviour. A very interesting support is MgF2. Earlier studies on active phases supported on magnesium fluoride have provided active and selective catalysts in thiophene hydrodesulfurisation [6] or selective reduction of chloronitrobenzene to chloroaniline [7,8]. Interesting acid-base properties have been shown for MgF2-MgO mixed systems [9,10]. Nickel is one of the most active metals in hydrogenation. In view of the above, an intense search for new supports that improve catalyst performance has been undertaken. The present study was undertaken to examine the acid-base properties of mixed MgF2-MgO, MgO and MgF2 supports and their catalytic behaviour as Ni-catalysts in toluene hydrogenation.

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2. Experimental 2.1. Synthesis of MgF2-MgO, MgF2 and MgO samples

A series of MgF2-MgO samples of different MgO content were synthesized by the solgel method from magnesium methoxide and an aqueous solution of hydrofluoric acid. 120 cm3 of 0.5 M solution of freshly synthesized magnesium methoxide (from magnesium turnings for Grignard synthesis, MERCK) in methanol was added dropwise (20 cm3·h-1) at room temperature under intense stirring to an aqueous solution of hydrofluoride (40%, POCH – Polish Chemicals Reagents). The amount of hydrofluoride solution was chosen to ensure 30 and 60 mol% MgO in the samples. The resulting dense gels of MgF2-Mg(OH)2 were subjected to ageing for 40 h at RT, and then to drying at 80˚C for 3 h. The dried samples were calcined for 4 h at 400oC. The MgF2MgO samples were labelled as xMO, where x is the mol% of MgO. The MgF2 support was obtained by the sol-gel method from Mg(OCH3)2 and anhydrous HF (48.8% HF in methanol, Aldrich) in a way analogous to the above described synthesis for MgF2-MgO, but under strict anhydrous conditions (denoted as MF). MgO was obtained by the sol-gel method by hydrolysis of magnesium methoxide (120 cm3 of 0.5 M solution) in water and treated similarly to MgF2-MgO.

2.2. Synthesis of nickel catalysts

MgF2-MgO, MgF2 and MgO supports, calcined at 400oC were impregnated with an aqueous solution of nickel acetate (98%, Aldrich) in an amount ensuring 5 wt.% nickel in the catalysts. The samples were dried at 110oC for 24 h.

2.3. Physico-chemical characterization of support The low-temperature adsorption of nitrogen was performed using a Micromeritics ASAP 2010 sorptometer. Specific surface area was determined by using the BET method. Thermo-gravimetric analysis was carried out for samples dried at 80oC in the temperature range of 30-1000oC using a differential thermoanalyzer Setaram TGA, equipped with a TG measurement unit. The experiments were performed under nitrogen flow (99.995% purity) and a ramp rate of 5oC·min-1. The TPD-CO2 was performed on a Micromeritics ChemiSorb 2705 with TCD detector. Prior to CO2 adsorption, samples were pre-treated in situ in He (99.999%, Linde) at 400oC for 1 h to remove the molecules adsorbed on the surface. Carbon dioxide was adsorbed at room temperature during 15 min. TPD-CO2 analysis was carried out in the temperature range of 20-900oC using a ramp rate of 10oC·min-1.

2.4. Catalytic test 0.05 g of fresh catalyst was placed in the reactor and reduced in a flow of pure hydrogen (99.99 % purchased from Messer, flow rate = 100 cm3⋅min-1). Prior to the catalytic tests the catalysts were reduced in situ in hydrogen at 400oC for 2 h. Toluene hydrogenation was studied at atmospheric pressure using a fixed-bed flow reactor and H2 as carrier gas. The H2 flow passed through a saturator filled with toluene and equilibrated at 0oC (ptoluene = 0.9 kPa). The catalytic tests were carried out at various temperatures (75-225oC) over the same catalyst. The sample was heated or cooled at the rate of 10oC·min-1. The post reaction mixture was analyzed on a gas chromatograph equipped with a capillary column RESTEK - MXT – 1. The catalytic activity was represented as TOF.

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Preparation of MgF2-MgO supports with specified acid-base properties

3. Results and discussion Using the low-temperature nitrogen adsorption, the texture of MgF2, MgO and MgF2MgO supports after calcination at 400oC for 4 h was characterized. The symbol of the support, MgO content (mol.%), surface areas, average pore diameter and pore volume are presented in Table 1. Table 1. Support characterization after air pre-treatment (4 h, 400oC, 100 cm3⋅min-1).

MgF2 33%MgO-67%MgF2

BET surface area [m2⋅g-1] 32 144

Average pore diameter [nm] 15.5 17.8

Pore volume [cm3⋅g-1] 0.12 0.64

62%MgO-38%MgF2 MgO

208 152

8.2 10.5

0.43 0.39

Code

Support

MF 33MO 62MO MO

As follows from Table 1, the content of magnesium oxide has a considerable effect on the surface area of the MgF2-MgO systems. The surface area of the supports containing 33 and 62 mol.% MgO are 144 and 208 m2·g-1, respectively. The surface area of MgF2-MgO is seven times larger than that of pure magnesium fluoride and also larger than that of pure MgO. All the samples obtained were mesoporous with the mean pore diameters ranging from 8 to 18 nm. The size of the pores strongly depended on the composition of the samples and declined with increasing MgO content in MgF2-MgO samples. Table 2. Thermo-gravimetric data for supports dried at 80oC. Support MF 33MO 62MO MO

Weight loss I Weight loss, Temperature [%] range, [oC] 20-126 2.2 20-324 10.1 20-311 12.8 20-282 18.3

Weight loss II Temperature Weight loss, range, [oC] [%] 126-393 8.5 324-635 7.3 311-693 22.4 282-681 28.2

2

CO2 desorption, mmolCO ·g

-1

0.05

Thermo-gravimetric data obtained for MgF2-MgO, MgO and MgF2 show two processes accompanied by mass loss. The process at about 100oC is related to desorption of the physically adsorbed water. The corresponding mass loss is proportional to the amount of MgO in the sample (Tab. 2 – weight loss I), which is explained by the strong hygroscopic properties of MgO. weak medium strong The mass loss observed in the range 320600oC (weight loss II) for the materials obtained by the sol-gel method from metal alcoholates is related to the dehydroxyMO lation of magnesium hydroxide. In this range also a thermal decomposition of the 62MO organic remains of the precursor in the 33MO form of alkoxy groups [9] can take place. MF To establish the effect of the MgO 100 200 300 400 500 600 700 presence in MgF2-MgO samples on the Temperature, C nature of the basic sites, the samples were subjected to temperature programmed Figure 1. TPD-CO2 profiles of MgF2-MgO, MgO and MgF2 supports. desorption of CO2 (TPD-CO2) - Figure 1. o

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TOF, min

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For pure magnesium fluoride no desorption was recorded, which indicates the lack of basic sites on its surface. The curves recorded for the mixed systems MgF2-MgO, and those for pure MgO, reveal a few desorption signals, indicating the presence of basic sites of different nature and strength [11]: - weak sites – desorption signal below 160oC – corresponds to weak sites assigned to CO2 linearly bound with OH-; - medium strong sites – Table 3. Number of basic sites on MgF2-MgO, MgO desorption signal in the range and MgF2 surface. 160-380oC - assigned to CO2 -1 · g Number of basic centers, mmol bridged with a magnesium cation CO2 Support total weak medium strong and an oxygen anion; MF 0 0 0 0 - strong sites – desorption signal 33MO 5.57 0.23 0 5.51 above 380oC – corresponds to 62MO 7.02 0.24 0 6.78 strong sites assigned to CO2 MO 7.90 1.10 2.28 4.52 linearly bound to O2-. The TPD-CO2 results also permitted evaluation of the number of basis sites, Table 3. The total number of basic sites was found as the number of CO2 moles desorbed from 1 g of a sample studied in the range 50-750oC. Considerable influence of the content of magnesium oxide on the basicity of MgF2-MgO systems was noted. The total 0.5 Ni/MF number of basic centers varied from 5.74 mmolCO2·g-1 for the sample 0.4 containing 33MO to 7.90 mmolCO2·g-1 for MO. The increase in the basicity of 0.3 MgO-MgF2 systems is mainly due to the Ni/33MO increasing number of strong basic sites. 0.2 The support basicity was found to Ni/62MO 0.1 be the major factor influencing the Ni/MO nickel catalyst hydrogenation properties 0.0 0 1 2 3 4 5 6 7 8 9 (Fig. 2). The activity of nickel catalysts -1 Number of basic sites, mmolCO2·g in toluene hydrogenation decreased with increasing number of basic sites. Figure 2. Influence of the support basic properThe best results were obtained for a ties on nickel catalyst activity in toluene nickel catalyst supported on MgF2 hydrogenation at 175oC. having no basic sites at all.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

Worldwide Fuel Charter, Fourth Edition, September 2006 M.H. Peyrovi, M.R. Toosi, 2008, React. Kinet. Catal. Lett., 94, 115 M.A. Ermakova, D.Yu. Ermakov, 2003, Appl. Catal. A, 245, 277 K. Takanabe, K. Nagaoka, K. Nariai, K. Aika, 2005, J. Catal. 232, 268 J.R. Grzechowiak, I. Szyszka, A. Masalska, 2008, Catal. Today, 137, 433 M. Wojciechowska, M. Pietrowski, B. Czajka, 2001, Catal. Today, 65, 349 M. Pietrowski, M. Zieliński, M. Wojciechowska, 2009, Catal Lett, 128, 31 M. Pietrowski, M. Wojciechowska, 2009, Catal. Today, 142, 211 H.A. Prescott, Z.-J. Li, E. Kemnitz, J. Deutsch, H. Lieske, 2005, J. Mater. Chem., 15, 4616 M. Wojciechowska, A. Wajnert, I. Tomska-Foralewska, M. Zieliński, B. Czajka, 2009, Catal. Lett. 128, 77 11. Z. Liu, J.A. Cortés-Concepcion, M. Mustian, M.D. Amiridis, 2006, Appl. Catal. A, 302, 232.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Pd supported catalysts: Evolution of the support during Pd deposition and K doping Riccardo Pellegrini,a* Giuseppe Leofanti,a,b Giovanni Agostini,c Elena Groppo,c Michele R. Chierotti,c Roberto Gobetto,c Carlo Lambertic a

Chimet S.p.A - Catalyst Division, Via di Pescaiola 74, I-52041 Viciomaggio, Italy Consultant, Via Firenze 43, 20010 Canegrate (Milano,) Italy c Dipartimento di Chimica IFM and NIS, Università di Torino, Italy b

Abstract The changes in the support morphology, at nano- and micro-scale level, have been investigated along the two-step processes involved in the preparation of a Pd supported on SiO2-Al2O3 (SA) catalyst: Pd deposition and K2CO3 doping. During the latter step part of the support dissolves and re-precipitates (partially outside the pores) covering the Pd particles. XRPD shows a significant rearrangement of the support at the long range order scale, while 29Si and 27Al solid-state (SS) NMR indicate that the average local environment around both Si and Al atoms remains unaltered. Keywords: catalyst preparation; Pd/SiO2-Al2O3; K-doping; solid-state NMR; XRPD

1. Introduction In recent work1 adsorption of N2 at 77 K and scanning electron microscopy have been used to measure changes in the support morphology, at nano- and micro-scale level, during a three-step process involved in the preparation of a Pd supported on SiO2-Al2O3 (SA) catalyst: Pd deposition, doping and thermal treatment. The processes involved can be summarized as follows: (i) During Pd deposition the support itself is partially dissolved and removed as a result of both the basicity of the precipitating agent and the final washing. (ii) When the undoped sample is thermally treated up to 823 K, only modest phenomena are observed. (iii) Upon doping with K2CO3, the support dissolution continues, the greater the carbonate concentration the greater the dissolution extent. In this case the dissolved material is not removed, but re-precipitates (partially outside the pores), during the subsequent drying at 393 K. (iv) When doped samples are thermally treated, the reaction between K2CO3 and support causes the mobilization of the support itself, with sintering phenomena that can result in the total collapse of the porous structure. The starting temperature for pore collapsing decreases with increasing K2CO3 concentration. The support modification influences, directly or indirectly, the surface properties and the availability of Pd particles. These can be doped or even covered by materials from the support while the pore narrowing, widening or blocking change their accessibility. In two successive papers2,3 the evolution of the Pd nanoparticles dimension and of their surface availability was followed during the different catalyst preparation stages described above with TEM, Pd K-edge EXAFS, TPR, CO chemisorption and FTIR spectroscopy of adsorbed CO. Table 1 summarizes the major findings of previous papers.1-3 In the present contribution we will extend the characterization of the support modification along the catalyst preparation to two complementary techniques: XRPD, sensitive to the long range ordering of the material, and 29Si and 27Al SS NMR, sensitive to the local order around the selected atomic species. Both are bulk techniques, which

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allowed focusing our attention on the support structure rather than on the metal particles. Due to space limitation we report on the first two steps only: Pd deposition and K doping. Table 1. Effect of the preparation steps as observed by different techniques. (i) BET: surface area (As), pore volume (Vp) and mean pore size (Dp); (ii) TEM: mean Pd particle size () and its standard deviation σ; (iii) TPR: temperature of the maximum of the reduction peak (Tmax); (iv) CO chemisorption: metal particle dispersion arbitrarily defined as the CO/Pd atomic ratio;. Results are reported from Refs. [1-3]. Sample SA Pd/SA PdK20/SA

As (m2 g-1) 179 141 90

Vp (mm3 g-1) 719 827 854

Dp (Å) 160 235 381

(Å) 28 27

σ (Å) 9 6

Tmax (K) 300 343

CO/Pd 0.28 0.22

2. Experimental: Catalyst preparation and characterization techniques A sodium-neutralized SA with a SiO2/Al2O3 ratio of 5.7 has been used as support. Pd catalyst (Pd/SA in the following) was prepared by the deposition-precipitation method depositing palladium hydroxide by using Na2PdCl4 as precursor and Na2CO3 as basic agent.1 The sample was then water-washed until residual chlorides were removed and dried at 393 K overnight. The final Pd/SA catalyst contains 2.05% Pd and some impurities arising from SA (Na 2.64%, K 0.04%, Ca 0.05%, Mg 0.04%, S 0.08%, as determined by XRF technique). It has a surface area of 141 m2/g and a pore volume of 0.83 cm3/g, as evaluated by N2 adsorption measurements.1 K-doped catalyst with a K/Pd atomic ratio of 20.3, (hereafter PdK20/SA) was prepared by dry impregnation of the corresponding undoped catalyst with an aqueous solution of K2CO3, followed by drying in a static oven for 16 h at 393 K. XRPD data collection were performed with a PANalytical PW3040/60 X’Pert PRO MPD diffractometer equipped with a high power ceramic tube PW3373/10 LFF with Cu anode (λCu(Kα) = 1.5405 Å) and Ni filter and with a X’celerator detector. Samples were measured in the Bragg-Brentano geometry. SS NMR spectra were acquired on a Bruker Avance II instrument operating at 400.2, 101.6 and 104.3 MHz for the 1H, 29Si and 27Al nuclei, respectively. The samples were packed into 4mm ZrO2 rotors and spun at 12 kHz. Conditions for the 29Si CPMAS (cross-polarization magic angle spinning) spectra: CT=5 ms, recycle delay=5s, 1500 scans. Conditions for MAS experiments: 90°pulse=3.9μs (29Si) and 1.2μs (27Al), recycle delay=100s (29Si) and 0.5s (27Al), scans=800 (29Si) and 1200 (27Al). For the 27Al MQ MAS experiments a four-pulse sequence including zfiltering and double frequency sweep (DFS) were applied.4 After FT, the 2D spectra were sheared5 so that the orthogonal projection on the isotropic axes gave the onedimensional (1D) spectrum free of any anisotropic broadening. 29Si and 27Al chemical shifts were referenced relative to kaolinite (-91.2 ppm) and Al(NO3)3·3H2O, respectively. Results obtained with the TEM, BET, TPR and CO chemisorption are here just recalled to complete the characterization picture: experimental details on these techniques are reported elsewhere.1-3

3. Results and Discussion From BET data (Table 1), it is evident that the porous texture of the SA support is dramatically modified by the catalyst preparation steps, loosing 50% of the surface area, increasing the pore diameter by 130% while the pore volume is only slightly modified.1

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The last step (K-doping) is accompanied by a loss of metal particles dispersion of 20%.3 An accurate TEM analysis reveals that the dispersion loss was due to a partial coverage of the nanoparticles from the re-precipitation of the fraction of the support dissolved by K2CO3 rather then to a sintering process.3 This phase, easily visible in Fig. 1 around the Pd particles (evidenced by the dotted lines), is clearly crystalline, but highly instable under the electron beam, suggesting a low melting temperature. The presence of this “crust” explains the higher reduction temperature observed in the TPR runs (Table 1). BET and TEM show that the re-precipitated phase is insoluble in water.1,3 Consequently, the change at nanoscale morphology of the support upon doping is due to deep and irreversible changes in the support structure. To further investigate this aspect, we report here new results using two bulk-sensitive techniques: XRPD and SS NMR spectroscopy.

Fig. 1. Main figure: TEM micrograph of PdK20/SA, exhibiting two Pd particles covered by the re-precipitated phase dissolved during the doping procedure from K2CO3. Inset: XRPD pattern of, from top to bottom SA, Pd/SA and PdK20/SA. Vertical dotted lines mark the position of the maximum of the bump of the pattern reflecting the amorphous nature of the support.

The SA support (top curve in the inset of Fig. 1) exhibits an amorphous profile with some broad Bragg peaks due to the high fraction of impurities, see Experimental. This ill-defined ordered phase is removed in the Pd impregnation step. K-doping results in the appearance of the strong and sharp Bragg peaks of the carbonate phase, visible in SEM micrographs as needle-like crystals.1 Worth noting is that, during the catalyst preparation, the maximum of the amorphous bump progressively moves from 2θ = 26.0°, through 26.8°, to 28.2° (see vertical dotted lines), corresponding to a d-spacing of 3.42, 3.32 and, 3.16 Å, respectively. The important modification observed in the support texture by BET (Table 1) is fully justified on the basis of this XRPD study, testifying an important rearrangement of the pair distribution function in the long ordering range. Changes in the short-range structure of SA along the supported Pd catalyst preparation process have been investigated by means of multinuclear (29Si and 27Al) SS NMR. This technique is known to provide useful information concerning the local environment of nuclei. The 29Si MAS spectra of SA, Pd/SA and PdK20/SA samples (not reported) are very similar. They are characterized by a single broad resonance

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centred around -93.0 ppm typical of Q4(2Al) and Q4(3Al) species,6 as expected for a SiO2/Al2O3 ratio of 5.7. On the other hand, the 29Si CPMAS spectra show a highfrequency shift of about 3 ppm with respect to the MAS spectra indicating the presence of Q2 and Q3 sites at the catalyst surfaces. The signal width (FWHM = 1350 Hz) indicates the very low crystallinity of the samples in agreement with XRPD data. From both CPMAS and MAS experiments we can say that, after the dissolution, the Si atoms re-aggregate in a very similar local environment. 27Al MAS experiments and the MQ MAS NMR spectroscopy, which is designed to remove anisotropic second order quadrupolar interactions, were used to characterize the chemical environment of Al nuclei in the studied samples. The 27Al MAS and the 27Al MQ MAS NMR spectra (Fig. 2) show no differences either in the chemical shift or in the peak shapes again indicating that no relevant changes occur at short range level in both Pd-precipitation and Kdoping processes. The MQ MAS spectra are characterized by a single signal at 62 ppm arising from tetrahedral (T) aluminium. The peak consists of a sharp component. This narrow resonance of T-Al is centred close to the diagonal, indicating that the corresponding aluminium species experiences a relatively small quadrupolar coupling constant (i.e., δiso ≅ δF2 ), pointing to a rather symmetric environment.7

Fig. 2. Left: 27Al MAS spectra of SA, PdSA and PdK20/SA samples. Right: 27Al MQ MAS NMR spectrum of the sample PdK20/SA.

Summarizing, XRPD shows an important rearrangement of the support at the long range scale, supporting previous BET and SEM data,1 while 29Si and 27Al SS NMR indicate that the average local environment around both Si and Al atoms remains unaltered.

References [1] R. Pellegrini, G. Leofanti, G. Agostini, E. Groppo, M. Rivallain, C. Lamberti Langmuir, 25 (2009) 6476. [2] G. Agostini, R. Pellegrini, G. Leofanti, L. Bertinetti, S. Bertarione, E. Groppo, A. Zecchina, C. Lamberti, J. Phys Chem. C, 113 (2009) 10485. [3] R. Pellegrini, G. Agostini, G. Leofanti, L. Bertinetti, S. Bertarione, E. Groppo, A. Zecchina, C. Lamberti, J. Catal., 267 (2009) 40. [4] S.P. Brown, S.J. Heyes, and S. Wimperis, J. Magn. Reson. A, 119 (1996) 280. [5] A. Medek, J.S. Harwood, L. Frydman, .J. Am. Chem. Soc. 117, (1995), 12779. [6] K.J.D. MacKenzie, M.E. Smith in “Multinuclear Solid-state NMR of Inorganic Materials”, R.W. Cahn Ed., Pergamon Elsevier Science, Amsterdam, (2002). [7] M.F. Williams, B. Fonfé, C. Sievers, A. Abraham, et al., J. Catal., 251 (2007) 485.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Investigation of carbon and alumina supported Pd catalysts during catalyst preparation Riccardo Pellegrini,a Giuseppe Leofanti,a,b Giovanni Agostini,c Elena Groppo,c Carlo Lambertic* a

Chimet S.p.A - Catalyst Division, Via di Pescaiola 74, I-52041 Viciomaggio, Italy Consultant, Via Firenze 43, 20010 Canegrate (Milano) Italy c Dipartimento di Chimica IFM and NIS, Università di Torino, Italy b

Abstract The preparation by a deposition-precipitation method (using Na2PdCl4 as palladium precursor and Na2CO3 as basic agent) of Pd catalysts supported on γ-Al2O3 and on two different types of activated carbons has been studied by EXAFS and TPR as a function of Pd loading from 0.5 to 5.0 wt%. Independently from the support, neither reduced metal fraction nor crystalline phase has been observed. EXAFS shows that the Pd local environment of the final catalysts changes slightly as a function of the Pd loading from 0.5 to 2.0 wt%: at higher loadings no further modification has been observed. Keywords: catalyst preparation; Pd/C; deposition-precipitation; EXAFS; TPR

1. Introduction Pd-metal supported catalysts are widely used in hydrogenation reactions for the synthesis of fine chemicals (e.g. active pharmaceutical ingredients)1 and bulk chemicals (e.g. terephthalic acid).2 Their activity and selectivity toward different molecules are strongly related to the morphology and dispersion of the metal active phase and to its interaction with the support, that becomes a key parameter in the catalyst preparation. Also because of the cost of the metal itself, another key parameter is the metal loading. We report an EXAFS and TPR study on Pd catalysts supported on γ-Al2O3 and on two different types of activated carbons as a function of Pd loading from 0.5 to 5.0 wt%. This work represents the continuation of a study presented in 2006 at the 9th International Symposium Scientific Bases for the Preparation of Heterogeneous Catalysts.3

2. Experimental: catalyst preparation and characterization techniques Supported Pd samples have been prepared in the Chimet laboratories on γ-Al2O3 (surface area = 121 m2 g-1; pore volume = 0.43 cm3 g-1) and on two different activated carbons, based on wood (hereafter Cw: surface area = 980 m2 g-1; micropore volume = 0.62 cm3 g-1) and peat (hereafter Cp: surface area = 980 m2 g-1; micropore volume = 0.47 cm3 g-1) origin, following the deposition-precipitation method4 with Na2PdCl4 as palladium precursor and Na2CO3 as basic agent. For each support the following different Pd loading have been prepared: 0.5, 1.0, 2.0, 3.5, and 5.0 wt% Pd. With the nomenclature Pd/Al2O3, Pd/Cw and Pd/Cp we indicate the generic catalyst prepared on alumina, Cw and Cp, respectively. When we refer to a specific catalyst, its Pd loading, in wt%, is reported in brackets; i.e. Pd(1.0)/Al2O3 indicates 1.0 wt% Pd supported on alumina. As a reference, an unsupported sample has been prepared following a similar procedure but in absence of the support, hereafter Pduns. All catalysts and Pduns have been carefully washed till complete Cl- removal and kept in the wet state till

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measurement. A portion of Pduns sample has been dried at 773 K, hereafter called sample Pduns(773). TPR has been carried out in a Micromeritics Autochem 2910 instrument equipped with the CryoCooler system for reaching subambient temperature by means of liquid nitrogen. A molecular sieve trap has been put between the sample holder and the detector in order to absorb water that is formed during the reduction process. 100 mg of wet sample has been introduced in the sample holder, dried in situ by a nitrogen flow at 393 K for two h. Then the sample was cooled down to 153 K under Ar flow (50 cm3 min-1) after which, the flow was switched to 5% H2 in Ar (50 cm3 min-1) and maintained throughout the analysis. After a baseline had been established, the temperature was started at a rate of 5 K min-1 up to 623 K. A TPR experiment on sample Pduns reference was carried out on 10 mg of sample following the same procedure described above for the catalysts. X-ray Absorption experiments at the Pd K-edge (24350 eV) were performed at the BM26A beamline5 of the ESRF facility (Grenoble, F). The white beam was monochromatized using a Si(111) double crystal; harmonic rejection has been performed using Pt coated silicon mirrors. EXAFS part of the spectra was collected with a variable sampling step in energy, resulting in Δk = 0.05 Å-1, up to 20 Å-1, with an integration time that linearly increases with k from 4 to 25 s/point to account for the low signal-tonoise ratio at high k values. The extraction of the χ(k) function has been performed using Athena code.6 For each sample, 2 consecutive EXAFS spectra have been collected and corresponding χ(k) functions have been averaged.

3. Results and discussion The use of deposition-precipitation method to prepare supported palladium catalysts directly provides a palladium in an unreduced state (oxide/hydroxide) deposited into the pores of the carrier material. These unreduced catalysts can already be used in hydrogenation reactions without further treatment (e.g., for debenzylations7) or they can be reduced to Pd metal. In any case the final industrial catalyst, being either in the reduced or unreduced state, is almost always kept in the wet form for safety handling and storage, and it is loaded as such into the hydrogenation reactor. Therefore, we have characterized the catalysts by EXAFS in the original wet state and by TPR after drying in an inert feed to avoid changing the catalyst properties, first of all the reduction degree of Pd. The local environment of Pd in Pd/Cw, Pd/Cp and Pd/Al2O3 systems has been investigated as a function of Pd loading, from 0.5 up to 5.0%. Figure 1ab reports both moduli and imaginary parts of the k3-weighted, phase uncorrected FT of the EXAFS signal for Pd/Cw samples. The spectra of the Pduns and Pduns(773) are reported for comparison. The data reported in Fig. 1ab are almost equivalent showing the oxidic nature of the Pd precursor at every loading investigated. In particular, the signal is almost intermediate between that of the Pduns and that of the Pduns(773) model compounds corresponding to Pd(OH)2 and PdO species, respectively. The third shell contribution (centered around 3.2 Å) shows a small intensity increase upon increasing Pd loading, and saturates (already at 2.0 wt% Pd) at a value much lower than that observed for PdO heated at 773 K, indicating the dispersed nature of the oxidic Pd precursors on Cw support. Independent XRPD measurements display broad carbon bumps and did not show the presence of any crystalline phase. In the EXAFS spectra, no evidence of reduced Pd metal is appreciable. According to the sensitivity of the technique, we can confirm that if reduced Pd particles are present at this stage, their concentration is below 1-2%.

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Fig. 1. Part (a): Modulus of the k3-weighted, phase uncorrected FT of the EXAFS signal of samples with increasing Pd loading. Part (b): as part (a) for the imaginary part. The Pd(OH)2 and PdO heated at 773 K model compounds are also reported for comparison. Part (c): corresponding TPR curves. The TPR curve of Pd(0.5)/Cw sample is not reported, because the very low Pd concentration makes it doubtful. Labels in part (c) refer to all parts of the figure.

The effect of the nature of the support has been investigated using the EXAFS data reported in Fig. 2ab. At the lowest loadings (Fig. 2a), the differences in particle size of the amorphous PdO precursor is appreciable from the intensity of the higher shells signal at 2.3-3.5 Å: Pd/Cp < Pd/Cw < Pd/Al2O3. For the 5.0% Pd loaded samples (Fig. 2b) this trend holds, but becomes borderline within the sensitivity of the technique. All TPR profiles, see Fig. 1c and Fig. 2cd, show two main features: (i) a complex H2 consumption formed by a peak plus a shoulder in the 260-330 K region, due to the Pd2+ → Pd0 reduction and to the surface PdHx and bulk Pd-hydride phases formation; and (ii) a negative peak (due to the Pd-hydride decomposition). TPR results can be summarized as follows. (a) The TPR profile of the Pduns sample is markedly different from those of the supported catalysts, the same holds for the EXAFS spectra. (b) The starting temperature of reduction (Tstart) as well as peak temperature (Tpeak) of Pd/Cw samples decrease with increasing Pd concentration, more markedly at low Pd concentration and only slightly at high ones (Fig. 1c). This trend mirrors that observed in the FT of the EXAFS spectra (Fig. 1ab). TPR is more sensitive than EXAFS to Pd concentration because Tstart, and consequently Tpeak, depend on the surface of Pd crystals rather than on the bulk. (c) Conversely, samples supported on Al2O3 are characterized by TPR results that are much less dependent, as well as EXAFS ones, on the Pd loading: main peak temperature ranges from 270 to 273 K in Pd/Al2O3 samples and from 278 to 267 K in Pd/Cw samples. (d) The difference among the supports is clearly observed at low Pd loading (Fig. 2a,c), while it is much less visible at high loading (Fig. 2b,d): particularly Tpeak of Pd/Cw and Pd/Al2O3 samples differs by 8.5 K and 0.3 K for Pd(1.0) and Pd(5.0) samples, respectively. Analogies and differences observed among the different samples by EXAFS spectroscopy are fully supported by parallel TPR, the latter being more sensitive to support type and Pd concentration, because it is affected by Pd surface that is, in its turn, more sensitive to these characteristics.

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TCD signal (a.u.)

-4

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Pd(5.0)Cw Pd(5.0)Al2O3

Pd(1.0)Cw Pd(1.0)Al2O3 0.01

0.04

300 Temperature (K)

350

250

300 Temperature (K)

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Pd(5.0)Cw Pd(5.0)Cp Pd(5.0)Al2O3

2Å

-4

2Å

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2 R(Å)

| FT | (Å )

1

(b) Pd(0.5)Cw Pd(0.5)Cp Pd(0.5)Al2O3

-4

| FT | (Å )

(a)

3

TCD signal (a.u.)

2 R(Å)

1

350

Fig. 2. Part (a): Modulus of the k3-weighted, phase uncorrected modulus of the FT of the EXAFS signal of 0.5% Pd supported on Cw, Cp and Al2O3 supports. Part (b): as Part (a) for the 5.0% Pd supported catalysts. Part (c) TPR curves of 0.5% Pd supported on Cw and Al2O3. Part (d): as Part (c) for the 5.0% Pd supported catalysts.

Acknowledgments We acknowledge the staff of the BM26 beamline at the ESRF (Dr. S. Nikitenko in particular) for the important and competent support during Pd K-edge EXAFS data collection.

References [1] H.-U. Blaser, A. Indolese, A. Schnyder, H. Steiner and M. Studer, J. Mol. Catal. A: Chem., 173 (2001) 3. [2] N. Pernicone, M. Cerboni, G. Prelazzi, F. Pinna and G. Fagherazzi, Catal. Today, 44 (1998) 129. [3] F. Rotunno, C. Prestipino, S. Bertarione, E. Groppo, D. Scarano, A. Zecchina, R. Pellegrini, G. Leofanti and C. Lamberti In Stud. Surf. Sci. Catal.; E.M. Gaigneaux, M. Devillers, D.E. De Vos, S. Hermans, P.A. Jacobs, J.A. Martens and P. Ruiz, Eds.; Elsevier, 2006; Vol. 162, p. 721. [4] R. Pellegrini, G. Leofanti, G. Agostini, M. Rivallain, E. Groppo and C. Lamberti, Langmuir, 25 (2009) 6476. [5] S. Nikitenko, A.M. Beale, A.M.J. van der Eerden, S.D.M. Jacques, O. Leynaud, M.G. O’Brien, D. Detollenaere, R. Kaptein, B.M. Weckhuysen and W. Bras, J. Synchrot. Radiat., 15 (2008) 632. [6] B. Ravel and M. Newville, J. Synchrot. Radiat., 12 (2005) 537. [7] W.M. Pearlman, Tetrahedron Lett., 8 (1967) 1663.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Advanced photocatalytic activity using TiO2/ceramic fiber-based honeycomb Seong Moon Jung*, Ju Hyung Lee, Moon Suk Han, Jong Sik Choi, Sun Joo Kim, Joo Hwan Seo, Ho Yeon Lim. R&D LG Hausys Ltd. 104-1, Moonji-Dong Yuseong-gu Daejeon 305-380, Korea

Abstract A honeycomb-type substrate consisting of a ceramic fiber sheet was used as a practical photocatalyst for water purification. The ceramic substrate has a stable 3-dimensional pore structure, which has a high porosity of over 80% and possesses strong capillary forces due to the highly hydrophilic surface. When the TiO2/ceramic fiber based substrate was partially immersed in water, the photocatalyst, polluted water and UV light could easily be brought in contact within the thin layer of water surrounding the TiO2 . The effect of the substrate was evaluated using experiments with decomposable ink, methylene blue and bisphenol A. Especially, the photocatalytic activity of TiO2 on the ceramic fiber based substrate for the degradation of ink in the presence of water was several times higher than without water. This could be explained by synergism between the hydrophilicity of the ceramic media and the photocatalyst of TiO2. Keywords: photocatalyst, TiO2, ceramic fiber

1. Introduction The photocatalytic activity of TiO2 has been used to convert toxic and nonbiodegradable organics into CO2, H2O and inorganics [1]. To apply the photocatalyst for water purification, coating of titanium oxide on supports is most critical. Pozzo et al. proposed several properties for a good support material for titania as a photocatalyst [2]. It is also known that the photocatalytic performance of titanium oxide species loaded onto a substrate depends on the surface properties of the support, especially on the hydrophilic–hydrophobic balance [3]. Yamashita et al. reported that the formation of well-crystallized anatase TiO2 on Si3N4 and the hydrophobic surface of Si3N4 were found to be related to the efficient photocatalytic activity of TiO2/Si3N4 [3]. But for real engineering applications, the slurry process is not practical, because of the difficulty of transmitting UV light. In order to improve the transmission of UV light, we developed TiO2 photocatalysts loaded on a ceramic honeycomb structure having a hydrophilic surface and a three-dimensional pore structure using conventional dip-coating. The uptake of water by capillary force due to the small pore size and the hydrophilic surface created thin water layers surrounding the TiO2. As a result, the ceramic media could promote easy contact between photocatalyst, polluted water and UV light. We have monitored the photocatalytic activity via the degradation of Bisphenol A and Methylene blue under UV-C light.

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2. Materials and experimental 2.1. Materials TiO2 powder (P-25, Degussa) was used as a photocatalyst. The Al-sol contained 20wt% alumina (Nyacol Al-20), Si-Sol containing 15 wt% silica was from Ludox, Aluminum phosphate was used for ceramic fiber honeycomb coating. Methylene blue (Fluka) and Bisphenol A (Aldrich) were used as the organics for the photocatalytic test.

2.2. Experimental 2.2.1. Preparation of the ceramic fiber-based honeycomb Silica-alumina fiber having an average length of 300 µm was added to water, and dispersed by intensive stirring; pulp of a pine tree was added to the mixture in the amount of 25 wt% to the ceramic fiber, and an acryl binder for providing flexibility to ceramic paper was added to the mixture in the amount of 10 wt% to the ceramic fiber, after which 1% aqueous solution of ammonium aluminum sulfate with pH 3 was added to adjust the pH of the total slurry to about 5.5. Then, a ceramic green paper was prepared from the slurry by using a paper-making machine. The ceramic green paper prepared was shaped by using a wave shaping machine. The wave-shaped ceramic fiber sheets were wound to a diameter of 22 cm. These were dried in the oven at 180oC for 30 min. Then, the first coating was performed with silica sol by dip-coating. The second coating was carried out with aluminium phosphate to give the hydrophilicity on the surface of ceramic media using the same method as for the first coating. Drying was at 180 oC for about 1 h. The coated ceramic structure was calcined at 900, 950 and 1000oC. 2.2.2. Coating of TiO2 A solution was prepared by dispersing 10 wt% TiO2 powder in deionized water after which the mixture was stirred until it became homogeneous. Then 10 wt% ethanol was added. Finally, 10 wt% Al-Sol (Al-20) was dropped in. The TiO2/ceramic fiber based honeycomb was prepared by dip-coating with the TiO2 slurry. The coating thickness was controlled to be around 10µm. Excess coating was removed by air blowing. Thereafter, the ceramic honeycomb structure was dried in an oven at 130oC. It was annealed to remove impure organics and to improve the adherence between coating and ceramic honeycomb structure at around 450oC. 2.2.3. Photocatalytic decomposition & Analyses Irradiation under UV-C light was performed with a 20W mercury lamps (Germicidal lamp, Sankyo Denki Co.) having a wavelength of around 254 nm. Two lamps installed in parallel were fixed in the center of the ceiling of the reactor box made by black acrylic resin. Two identical beakers of 300 ml covered by a transparent vinyl sheet were placed in the reactor box. Each beaker included different organic compounds as Bisphenol A and Methylene blue. The concentration of each solution was different (Methylene blue 10ppm, Bisphenol A 300ppm). During the course of UV radiation, samples of 5 ml were withdrawn at different intervals for measurement of UV-Vis.

3. Results and discussion Figure 1 shows the Scanning Electron Micrograph (SEM) image of the ceramic fiber honeycomb structure before the TiO2 coating. The ceramic fibers covered with 3-phase binder are well-connected with a three dimensional structure and the pores are fully open without blocking by binders.

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(a)

(b)

(c)

Fig. 1. SEM image of ceramic structure calcined at different temperatures: (a) 1173K, (b) 1223K, (c) 1273K.

Upon increasing the temperature of calcination, the phase changed from a dual phase system of Al(PO3)3 and SiP2O7. to a single phase of AlSi2P3O12. This modification of phase improved the affinity to water and the surface stability, which enhanced the stability in water and the capillary force. The ceramic fiber based coupon showed a 5 times higher rate of water absorption than the polymer based one. The SEM micrograph of the ceramic fiber honeycomb structure after TiO2 powder deposition is shown in Fig 2. The TiO2 powder had aggregated and was dispersed between ceramic fibers. The TiO2 powder adhered strongly to the ceramic media, considering that it was not removed from the ceramic structure by rubbing and by cellophane tape.

T i O2 coat ed ar ea

Fig. 2. SEM image of TiO2 coated ceramic structure.

The photocatalytic performance for the degradation of organics with and without water was compared with the degradation of ink by the naked eye. Fig. 3. is the image of the TiO2 coated ceramic honeycomb-like structure after 1h under treatment of irradiation of UV light in the presence of water. The ink conversion rate in the presence of water was about 10 times higher than in the case of without water.

Fig. 3. The photo-oxidation effect with water.

The results for the TiO2 photocatalyst on the ceramic honeycomb structure are shown in Table 1. Firstly, methylene blue was chosen as the representative of the organic dyes and it was also used to estimate the efficiency of photocatalyst. It was 100% degraded in 24 h. Secondly, Bisphenol A, which is well-known as one of the

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most difficult chemicals to eliminate in wastewater, was chosen as a target material. 154mg/l of Bisphenol A in the water was eliminated in 48 hours. Table 1. The degradation of organics. Materials

The degree of decomposition (%)

Initial concentration (mg/L)

Time (h)

Methylene Blue

100

10

24

Bisphenol A

51.33

300

48

The results showed that the photocatalytic ceramic fiber-based honeycomb reactor whose inner space is partially coated with TiO2 was effective for purification of contaminated water. The significant water uptake of the ceramic honeycomb structure due to the capillary forces could play an important role in photocatalytic reaction by facilitating contact between photocatalyst, polluted water and UV light. This photocatalytic reactor is now expected to be applied for air and water purification..

References 1. 2. 3.

D. F. Ollis and H. Al-Ekabi (eds.), 1993, Photocatalytic Purification and Treatment of Water and Air, Elsevier, Amsterdam R. L. Pozzo, M. A. Baltanfis and A. E. Cassano, 1997, Supported titanium oxide as photocatalyst in water, Catal. Today, 39, 219 H. Yamashita, H. Nose, Y. Kuwahara, Y. Nishida, S. Yuan and K. Mori , 2008, TiO2 photocatalyst loaded on hydrophobic Si3N4 support for efficient degradation of organics diluted in water, Appl. Catal. A: 350, 2, 164

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Incorporation of group five elements into the faujasite structure Maciej Trejda,a Anna Wojtaszek,a Anna Floch,a Robert Wojcieszak,b Eric M. Gaigneaux,b Maria Ziolek,a a

Adam Mickiewicz University, Faculty of Chemistry, Grunwaldzka 6, 60-780 Poznan, Poland b Université catholique de Louvain, Unité de Catalyse et Chimie des Matériaux Divisés, Croix du Sud 2/17, 1348 Louvain-la-Neuve, Belgium

Abstract Niobium and tantalum were incorporated into the faujasite aluminosilicate structure in one-pot synthesis and also by post-synthesis method, i.e. solid-state ion exchange. Onepot synthesized zeolites exhibit Nb and Ta incorporated into the zeolite skeleton as evidenced by different methods. The efficiency of metal incorporation examined by XPS measurements was found to be higher for tantalum than for niobium. Keywords: Y zeolite, niobium, tantalum, one-pot synthesis, XPS

1. Introduction Zeolites are very known as heterogeneous catalysts, especially in petrochemical processes involving acidic centers. The surface properties of such catalysts can be modified via post-synthesis methods as well as by isomorphous substitution of silicium during gel formation in a so-called one-pot synthesis process. Among different metals which have been used for modification of zeolites, niobium and tantalum have been included into MFI and Beta zeolites (exhibiting high and medium Si/Al ratios) [e.g. 14]. Both types of zeolites are prepared in the presence of organic species as structure directing agents. The idea of our work was to include Nb and Ta into FAU zeolite characterized by a low Si/Al ratio and prepared without organic structure directing agents. As far as we know there are no reports on Nb and Ta incorporation into the framework of the FAU structure during the one-pot synthesis in the alkaline medium. The reference material was an Y-type zeolite modified by the post-synthesis method (solid-state ion exchange). Different techniques were applied for the characterization of the samples prepared: XRD, SEM, 27Al NMR, UV-Vis, XPS and FTIR.

2. Experimental 2.1. Preparation of the catalysts Preparation of Nb or Ta containing materials of the FAU structure was based on the modified two-steps procedure reported originally by Ginter et al. [5]. Firstly, the seed gel was prepared as follows. Sodium hydroxide (Chempur) and sodium aluminate (Riede-de Haën) were dissolved in H2O. Afterwards, the sodium silicate solution (Aldrich) was added and the mixture was stirred for about 10 minutes. The final solution composed of: 10.67 Na2O : Al2O3 : 10 SiO2 : 180 H2O was let to age at room temperature for one day. In the second step, sodium hydroxide and sodium aluminate were dissolved in H2O then sodium silicate was added upon vigorous stirring. The composition of the obtained feedstock gel was: 4.3 Na2O : Al2O3 : 10 SiO2 : 180 H2O.

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Afterwards, a part of the seed gel was added to the feedstock gel to get the final composition of: 4.62 Na2O : Al2O3 : 10 SiO2 : 180 H2O. The metal source (niobium or tantalum pentaethoxide - Aldrich) was added both to the seed and feedstock gel. The assumed Si/M ratio (M = Nb or Ta) was 64 or 32. Then after vigorous stirring the gel was put to a polypropylene bottle and heated in an oven for 5 hours at 373 K. The final product was washed with distilled water and dried at 383 K for 12 hours. For comparison the samples were also prepared by solid-state ion exchange using commercial sodium Y zeolite (Katalistiks) and group five metal oxides. Hydrogen form of zeolite and Nb2O5 or Ta2O5 were mechanically mixed and heated at 973 K for 8 h.

2.2. Characterization The XRD patterns were obtained on a D8 Advance diffractometer (Bruker) using CuKα radiation (λ=0.154 nm). Scanning Electron Microscopy (Philips SEM 515) operating at 15 kV was applied. 27Al NMR spectra were recorded using Bruker Avance DPX300 spectrometer. UV-Vis spectra were scanned by a Varian-Cary 300 Scan UV-Visible Spectrophotometer. The XPS analyses were performed with a SSI-X-probe (SSX-100/ 206) photoelectron spectrometer equipped with a monochromatic microfocused Al Kα X-ray source (1486.6 eV) from Surface Science Instruments. FTIR spectra were obtained on Bruker Vector 22 spectrometer.

3. Results and discussion 3.1. Materials structure

Intensity, a.u.

Intensity, a.u.

Niobium and tantalum containing Y zeolites prepared via the one-pot synthesis showed the FAU structure, as evidenced by XRD diffraction patterns (Fig. 1 – the numbers in the symbols indicate the assumed Si/M ratio). The zeolite structure is confirmed for the two zeolites of both Si/M ratios (64 and 32); hence the synthesis of faujasite samples was successful. Post=20000 =20000 synthesis modification of commercial Y zeolite by Nb and Ta elements via NbY-32 TaY-32 solid-state ion exchange NbY-64 TaY-64 did not change the zeolite structure. However, the 10 20 30 40 50 60 10 20 30 40 50 60 o 2 theta, 2 theta, latter technique leads to formation of metal oxide Fig. 1. XRD patterns. phases likely on the 221 material surface, which is evidenced by the peaks assigned to metal oxides in the XRD patterns (not shown here). Such peaks were not found for the materials prepared in the one-pot synthesis. For both, Nb and Ta containing zeolites no transition metal oxide phases were TaY-32 observed. However, for the samples prepared in the onepot synthesis, except for NbY-32, a small fraction of TaY-64 270 another crystalline phase was also observed (peaks at Ta2O5 12.5o and 25o). The lack of metal oxides in Nb and Ta Ta2O5/Y containing Y zeolites was also confirmed by UV-Vis spectroscopy. The UV-Vis spectra did not show the 200 300 400 500 600 Wavelength, nm characteristic band related to octahedrally coordinated Fig. 2. UV-Vis spectra. metal oxide phases, which is clearly seen for tantalum F(R)

o

447

Incorporation of group five elements into the faujasite structure 53.9

containing Y zeolite (Fig. 2). Further confirmation of the absence of extra framework oxide species was given by the 27Al NMR NbY-32 NbY-64 spectroscopy. The 27Al NMR spectra showed the tetrahedrally coordinated aluminum for 90 60 30 0 -30 90 60 30 0 -30 all Nb and Ta containing zeolites prepared 54.3 54.6 by one-pot synthesis and moreover, no octahedrally coordinated aluminum was TaY-64 observed (Fig. 3). This proved that all TaY-32 aluminum atoms are incorporated into the 90 60 30 0 -30 90 60 30 0 -30 δ, ppm δ, ppm zeolite lattice; hence the structure defects should not be expected. Moreover, the SEM 27 Fig. 3. Al NMR spectra. images of NbY and TaY materials show that the particles of zeolite are similar in size and shape. The same particle sizes for all samples were also inferred from XRD patterns, according to Scherrer equation. It points out that both the nature of the heteroatom (Nb or Ta) and the silica to metal ratio did not have any impact on the crystal morphology. 53.7

3.2. The metal state in the sample Location of transition metals in the framework and extra framework positions can be deduced from their coordination. Tetrahedrally coordinated species should be detected for framework position, whereas the octahedral coordination can point to the extra framework ones. The UV-Vis spectra of the samples prepared in the one-pot synthesis show the bands characteristic of tetrahedrally coordinated transition metals, especially for the tantalum containing samples in which the efficiency of metal incorporation was higher (see Table 1). Moreover, the spectra of these samples differ from those recorded for materials prepared via post-synthesis modification. Figure 2 presents the UV-Vis spectra of tantalum oxide, and tantalum incorporated into Y zeolite by synthesis and post-synthesis methods. It is clearly seen, that the latter technique leads to the formation of octahedrally coordinated species, which is likely to be bulk tantalum oxide. However, in the one-pot synthesis method all tantalum is characterized by a band at c.a. 221 nm, typical of tetrahedrally coordinated Ta species. Therefore, one can conclude about the location of Ta in the zeolite skeleton. Table 1. The XPS data of prepared samples. Catalyst NbY-64 NbY-32 TaY-64 TaY-32

Metal BE in the sample, eV 208.1 207.9 26.7 26.3

Metal BE in bulk oxide, eV 207.1 (Nb2O5) 207.1 (Nb2O5) 26.1 (Ta2O5) 26.1 (Ta2O5)

Metal concentration, at% 0.025 0.34 1.25 2.35

For further investigation of metal state in the materials prepared XPS spectroscopy was applied and the results obtained are shown in Table 1. The oxidation state of the metal incorporated was estimated as +5 for both niobium and tantalum. The presence of metal at +5 oxidation state in the zeolite framework should imply generation of Lewis acidity. Indeed, the pyridine adsorption followed by FTIR spectroscopy proved the presence of Lewis acid sites in all zeolites (spectra not shown here). As mentioned above, all aluminum is tetrahedrally coordinated; hence the origin of Lewis acidity should be associated with niobium and tantalum incorporated into the skeleton of zeolite. Nb5+ and Ta5+ in the zeolite framework forms positive charged tetrahedral species.

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The information concerning the metal location in the sample can be elicited also from its binding energy. Table 1 shows the metal binding energies obtained for the materials prepared by the one-pot synthesis as well as for bulk metal oxides. The binding energies observed for niobium and tantalum introduced into FAU zeolite were higher than those typical of bulk niobium oxide and tantalum oxide. The metal binding energy is the higher the lower the Si/M ratio. The increase in the binding energy can be attributed to different surroundings of the niobium or tantalum species, e.g. in the Si-ONb or Si-O-Ta link. Such links are formed when transition metal is incorporated into the zeolite framework, thus the XPS results presented support the location of metals in the zeolite framework.

3.3. Efficiency of metal incorporation The data obtained from XPS analysis allowed estimation of metal concentration on the surface of the zeolites prepared. For both niobium and tantalum it was found that the assumed metal amount was not reached in the final material. Moreover, it is clearly seen that tantalum was easier incorporated into the zeolite. This phenomenon seems to be surprising if one considers the atomic radii of both elements, the radius of tantalum is greater. However, the following two facts should be taken into account. Firstly, the solubility of the metal source in the reaction media. Corma et al. [4] showed that the fluoride media used for the one-pot synthesis of beta zeolite enhanced the availability of the metal precursor relative to the OH-media. In the basic medium the precursor is often precipitated as metal oxide that has to be redissolved to be incorporated into a zeolite framework. Another fact that should be considered is the role of the metal source (niobium and tantalum ethoxide in the case of this study) in the synthesis. The higher efficiency of tantalum incorporation can be explained by the role of metal pentaethoxide as a space-filling agent, which makes the contact of metal source with the forming zeolite crystal cavities much easier. The greater size of Ta than Nb containing molecules makes this effect more pronounced.

4. Conclusions According to our knowledge the Nb and Ta containing FAU zeolites have been successfully synthesized for the first time. This study confirmed the location of transition metals in the zeolite framework. All aluminum introduced was present in the framework positions. The presence of niobium and tantalum in the tetrahedrally coordinated positions indicating their incorporation into the zeolite skeleton. The binding energies obtained for niobium and tantalum allowed to conclude that these metals are located in the surrounding other than bulk oxides, i.e. in the zeolite skeleton. Their +5 oxidation state was supported by Lewis acidity of the samples. The efficiency of Ta inclusion into FAU zeolite is higher than that of Nb.

Acknowledgements Polish Ministry of Science and Higher Education (grants 118/COS/2007/03 and N N204 032536) and COST D36/0006/06 are acknowledged for a financial support.

References 1. 2. 3. 4. 5.

L. Kevan, A.M. Prakash, J. Am. Chem. Soc., 120 (1998) 13148. M. Hartmann, Chem. Lett., 5 (1999) 407. Y.S. Ko, W.S. Ahn, Microporous Mesoporous Mat., 30 (1999) 283. A. Corma, F.X. Llabres I Xamena, C. Prestipino, M. Renz, S. Valencia, J. Phys. Chem. C, 113 (2009) 11306. D.M. Ginter, A.T. Bell, C.J. Radke, in Synthesis of Microporous Materials, Vol. 1, Molecular Sieves, M.L. Occelli, H. E. Robson (eds.), Van Nostrand Reinhold, New York, 1992, p 6.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Glycerol conversion into H2 by steam reforming over Ni and PtNi catalysts supported on MgO modified γ-Al2O3 A. Iriondoa*, M.B. Güemeza, V.L. Barrioa, J.F. Cambraa, P.L. Ariasa, M.C. Sánchez-Sánchezb, R.M. Navarrob, J.L.G. Fierrob a

School of Engineering (UPV/EHU), c/ Alameda Urquijo s/n, 48013 Bilbao (Spain) Institute of Catalysis and Petrochemistry, c/ Marie Curie s/n, 28049 Madrid (Spain) *[email protected] b

Abstract The glycerol catalytic steam reforming over Ni and PtNi catalysts to produce H2 was studied. The activity results indicate that the catalyst with the lower content of MgO, the NiA1M, provides higher H2 molar ratios than the Ni catalyst. The behaviour of the NiA1M catalysts seems to be related to the Niº species stabilization by nickel-magnesia interactions, which are favoured by the presence of well dispersed MgAl2O4 spinels. The bimetallic catalyst, PtNiA3M named, reforms the intermediate products improving the activity of the Ni monometallic catalyst toward H2 production. The characterization results suggest that the Pt in the Ni monometallic catalyst enhances the Niº particles dispersion and the nickel species reducibility by H2 spillover. Keywords: glycerol, hydrogen, nickel, platinum, magnesia

1. Introduction In the last years, the attention has been focused on the H2 potential as clean energy vector. Moreover, this energy vector could be more attractive if it is produced from biomass-derived products such as glycerol. Glycerol, the main by-product in the biodiesel production, can be revalorised by its conversion into H2 using catalytic steam reforming. In this way, the impact on the total biodiesel manufacturing cost will be also reduced by valorization of this by-product [1]. The hydrocarbons steam reforming (SR) uses Al2O3-supported Ni catalysts [2] because these systems are more inexpensive than noble catalysts in industrial applications [3]. However, these catalytic systems suffer deactivation due to coke formation and/or metallic phase sintering related to the low thermal resistance of Al2O3. To improve the properties of this kind of catalysts and avoid its deactivation, the support can be modified with MgO.

2. Catalysts and reaction conditions 2.1. Materials and Methods The modified γ-Al2O3 supports were impregnated with different amounts of Mg(NO3)2 aqueous solutions and then were dried and calcined. Afterwards, the Ni was added to the supports by impregnation, drying and calcination, obtaining the catalysts designated as NiA1M, NiA3M, NiA15M, NiM and NiA. The bimetallic catalyst, PtNiA3M, was prepared by impregnation of the NiA3M catalysts with Pt, following by a drying and calcination. The theorical Ni and Pt content was around 13 and 2.6 wt%, respectively.

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The activity tests were carried out in fixed bed continuous catalytic reactor in which 200 mg of catalyst (0.42-0.5 mm) was diluted with inert CSi (1:9 wt). Prior to reaction, the catalysts were activated in situ with 75 mL(STP)/min of pure H2 at 0.1 MPa and 973 K during 2h. The reduced catalysts were tested under a space velocity of 7.7 gglycerol/ gcatalysth, 0.4 MPa of total pressure and temperatures of 773 K and 873 K using 10 wt% glycerol diluted in water as a feed stock. Gas chromatography was used to determine the type and amounts of reaction products.

2.2. Characterization techniques The calcined and reduced catalysts were characterized by temperature-programmed reduction (TPR), x-ray powder diffraction (XRD) and x-ray photoelectron spectroscopy (XPS). The reduction treatment to the catalysts was carried out ex-situ at 923 K under 100 mL/min of H2/N2 (1/9) gas mixture. For the XPS characterization is used a second reduction treatment in-situ under the same gas mixture flow and at 773 K. The TPR experiments were carried out with semiautomatic Micromeritics TPD/TPR 2900 apparatus equipped with a TCD detector. The TPR profiles were obtained by heating the samples from 298 to 1173 K. The XRD patterns were obtained with a computerised Seifert XRD 3000P vertical diffractometer (Cu Kα radiation, λ=0.15418 nm) equipped with a PW Bragg-Brentano θ/2θ goniometer. The samples were scanned with Bragg’s angles between 5º and 80º. X-ray photoelectron spectroscopy (XPS) data were obtained using a VG Escalab 200R electron spectrometer equipped with a Mg Kα X-ray source and a hemispherical electron analyser.

3. Experimental results and discussion 3.1. Characterization The TPR profiles for calcined Ni and PtNi catalysts are presented in the Fig 1. The NiM catalyst profile shows two reduction peaks at 524 and 670 K associated to isolated NiO [4] and NiO interacting with the MgO respectively. The profiles of the NiA, NiA1M, NiA3M and NiA15M catalysts show a contribution of three peaks attributed to the reduction of NiO weakly interacting with the Al2O3, NiO-Al2O3, to the nonstoichiometric Ni aluminate species, NiAlxOy, and to the stoichometric NiAl2O4 species [5] respectively.

944 812

575

500

1100 1067 1065

951

619 NiO

400

947

818

570

1053

NiA3M NiO-Al2O3 (694)

H2 consumption (a.u.)

H2 consumption (a.u.)

NiM NiA NiA1M NiA3M NiA15M NiO-Al2O3 851 NiAlxOy NiAl2O4 NiO-MgO 824 984

NiO-MgO 600

700

800

900

Temperature (K)

1000

1100

1200 400

NiAlxOy NiAl2O4 (814) (1045) NiO-Al2O3 (824) NiAlxOy (947) NiAl2O4 (1067)

PtOx-Al2O3 (474)

NiO-MgO (575) 500

600

PtNiA3M

700

800

900

1000

Temperature (K)

Figure 1. Reducibility profiles of calcined catalysts obtained from TPR.

1100

1200

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Glycerol conversion into Hydrogen by steam reforming

The NiA1M, NiA3M and NiA15M catalysts show additional peak ascribed to NiO interacting with the MgO. The PtNiA3M catalyst shows the same Ni2+ reducible species than the NiA3M catalyst. Nevertheless, the reduction temperatures decrease and the NiO-Al2O3 species contribution increases. These phenomena can be due to the H2 spillover generated by the Pt presence [6]. The XRD analysis was used to determinate the average particle size of Niº species on reduced catalysts. The average particle size of crystalline Niº species shown in the Table 1 was calculated using the Debye-Scherrer equation. The NiAM, NiA1M, NiA3M and NiA15M catalysts present Niº particle diameters very similar. The Niº particles size for the PtNiA3M catalyst is slightly higher than for NiA3M catalyst. As it is observed in the Table 1, the difference between surface atomic Mg/Al ratio of calcined and reduced catalysts is lower for the NiA1M catalyst than for the NiA3M and NiA15M catalysts. This observation suggests that the MgAl2O4 species (50.2 eV [7]), are better dispersed in the NiA1M catalyst. The good surface dispersion of MgAl2O4 species could hinder the surface Niº species diffusion on the alumina [7] and favouring the stabilization of Niº by the interaction with the magnesia. Hence, the binding energy showed by Niº in the NiA1M catalysts is similar to the NiM catalyst and higher than the NiA3M and NiA15M catalysts. The value of 48.7 eV is adscribed to surface MgO species [8]. Table 1. Different parameters determined by XPS and XRD for reduced catalysts. XPS Catalysts

Ni2p3/2 (eV) Pt 4d5/2 (eV) Niº Ptº Ni2+ NiA 856.4 852.7 NiM 856.3 852.3 NiA1M 856.4 852.3 NiA3M 856.4 851.4 NiA15M 856.3 851.3 PtNiA3M 856.1 852.8 314.3 (a → data for calcined catalysts)

Mg2p (eV)

Mg/Al

Ni/Al

48.7 50.1 50.4 49.4 50.2

0.03 (0.02)a 0.04 (0.05)a 0.18 (0.24)a 0.02 (0.02)a

0.03 (0.11)a 0.02 (0.17)a 0.02 (0.17)a 0.02 (0.20)a 0.02 (0.11 )a

XRD Niº (nm) 7 7 7 6 5

3.2. Catalytic Activity The glycerol conversion and the SR gas products molar ratios are shown in Figure 2A. All the catalysts provide the total glycerol conversion at 773 K, except the NiM catalyst. This suggests that the γ-Al2O3 support plays an important role in the glycerol conversion. Moreover, the H2 and the CO2 are produced as the main components of the gas when the catalysts supported on bare and MgO-modified Al2O3 are used. Regarding H2 molar ratio obtained with NiA, NiA1M, NiA3M and NiA15M, it can be observed that this ratio increases when NiA1M is used. When the MgO load is increased from 1M to 15M the H2 ratio decreases indicating that higher amounts of modifier are not effective to improve the H2 production. The low H2 production obtained with NiA, NiA3M and NiA15M is related to the higher formation of oxygenated hydrocarbons (OHC-s) which are collected in the liquid phase. The OHC-s are mainly acetaldehyde, acrolein, propionaldehyde and acetone. All the OCH-s are included as C wt% (OHC-s total amount in liquid phase given in dry basis) in the Figure 2B and they are mainly produced through dehydration/hydrogenation reactions in which the H2 is consumed. The different behaviour of the MgO-modified Ni catalysts seems to be related to the interaction between surface Ni metallic particles and the MgO. As it has been indicate the low amounts of MgO provokes a better stabilization of surface Ni metallic particles favoured by the well dispersed MgAl2O4 species. In the case of the NiA3M and

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NiA15M catalysts specially, the Niº-MgO interactions are weakened. This phenomenon could favour the formation of nickel-alumina interactions which could be promote by lower dispersion of MgAl2O3. The formation of nickel-alumina interactions could provide a similar activity for the NiA3M catalyst and lower activity for the NiA15M catalyst than the NiA catalyst.

60

3.0

40

2.0 1.0 0.0 NiA

NiA1M

NiA3M NiA15M

NiM

PtNiA3M Equil.

0.8 0.6 0.4

20

0.2

0

0.0

NiA15M

NiM PtNiA3M

4.0

NiA

5.0

B NiA3M

1.0

80

NiA1M

6.0

1.2

100

A Conversion (%)

Molar ratio (mol of product/ mol of fed glycerol)

7.0

C (wt%) (in liquid phase)

Figure 2. (A) Gas products molar ratios and conversion (■H2, ■CO2, ■CH4, ■CO, ●conversion), and (B) OHC-s amount in liquid phase given as C(wt%) in dry basis at steady state and 773 K.

The H2 production increases when Pt is added to NiA3M due to the bimetallic catalyst ability to transform OHC-s into H2, CO2, CH4 and CO. This behaviour could be associated to the Niº species dispersion determined by DRX, which is slightly higher than in the NiA3M catalysts, besides the metallic phases combination, where Pt promotes the H2 spillover increasing the reducibility of Ni species observed by TPR. After the measurements at 773 K, the catalysts were tested at 873 K. The results indicated that all the catalysts, except the NiM, were able to convert completely glycerol and OHC-s achieving activity data closed to the thermodynamic equilibrium. For NiM catalysts, the measured conversion and C wt% were 41.3% and 1.3 % respectively.

4. Conclusions The main conclusions of this study are: (a) the total glycerol conversion is obtained with catalyst supported on bare and Mg-modified Al2O3, (b) the formation of OHC-s liquid products has influence on H2 production, (c) low amounts of MgO are effective to improve H2 ratio due to surface Niº species stabilization (d) the Pt addition on Ni catalyst promotes the OHC-s transformation into H2 due to Pt favours Niº dispersion.

Acknowledgements The authors would like to thank financial support from the Ministry of Science and Innovation of Spain (ENE2007-67533-C02-02/ALT. HIREUS), the Regional Basque Government and the University of the Basque Country (UPV/EHU).

References [1] [2] [3] [4] [5] [6] [7] [8]

M.J. Haas, A.J. McAloon, W.C. Yee, T.A. Foglia, Biore. Tech., 97 (2006) 671. L.S. Carvalho, A.R. Martins, P. Reyes, M. Oportus, et al., 2009, Catal. Today, 142 (2009) 52. R.A. Meyers, 2003, “Handbook of petroleum refining procesess”, McGraw-Hill, 3rd edition. J.T. Richardson and M.V. Twigg, Appl. Catal. A, 167, (1998) 57. M.C. Sánchez-Sánchez, R.M. Navarro, J.L.G. Fierro, Int. J. Hydrogen Energy 32 (2007) 1462. Pawelec, S. Damyanova, K. Arishtirova, et al., Appl. Catal. A 323 (2007) 188. L.P.R. Profeti, E.A. Ticianelli, E.M. Assaf, Appl. Catal. A 360 (2009) 17. M.C.G. Albuquerque, J. Santamaría-González, et al., Appl. Catal. A 347 (2008) 162.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Butyraldehyde production by butanol oxidation over Ru and Cu catalysts supported on ZrO2, TiO2 and CeO2 A. Iriondoa*, M.B. Guemeza, J. Requiesa, V.L. Barrioa, J.F. Cambraa, P.L. Ariasa, J.L.G. Fierrob a School of Engineering (UPV/EHU), c/ Alameda Urquijo s/n, 48013 Bilbao (Spain) b Institute of Catalysis and Petrochemistry, c/ Marie Curie s/n, 28049 Madrid (Spain) *[email protected]

Abstract Ceria, titania, and zirconia supported ruthenium and copper catalysts were tested in the production of n-butanal by n-butanol oxidation. These catalysts were characterized by means of X-ray diffraction (XRD), N2 adsorption-desorption isotherms, temperatureprogrammed reduction (TPR), and X-ray photoelectron spectroscopy (XPS) techniques. The activity tests were performed in a fixed bed reactor at 0.1 MPa and 623 K and pure mixture of reactants, air and n-butanol, in stequiometric proportion was introduced to the reactor. The ruthenium catalysts showed a higher activity and stability than the copper catalysts, nevertheless the copper system showed a higher selectivity toward butyraldehyde production by n-butanol oxidation. Keywords: butyraldehyde, butanol, oxidation, copper, ruthenium

1. Introduction The bioalchols, such as bioethanol, biobutanol or bioglycerol can be converted to oxygenates fuel additives, acetals. The synthesis of acetal takes place by reacting one of these alcohols with and aldehyde, for example, n-butyraldehyde. Nowadays, aldehydes production takes place through hydroformylation reaction of alkenes, but other process such as bioalcohols oxidation can also be employed to produce aldehydes. Due to the origin of these bioalcohols, the production of the aldehydes by this new process is very attractive, therefore the butanol oxidation to butyraldehyde may be a way more consistent with the current environmental policies. Hence, the main objective of this work was to study the production of nbutyraldehyde by the oxidation in phase gas of reaction of n-butanol. For this purpose, different copper, ruthenium and ruthenium-copper catalysts supported on different oxides (ZrO2, CeO2 and TiO2) have been prepared.

2. Experimental 2.1. Catalysts preparation and characterization Ruthenium, copper, and ruthenium-copper catalysts were prepared by incipient wetness impregnation using these oxides: ZrO2, CeO2 and TiO2. The monometallic catalysts, designed as 5CuZr, 2RuZr, 2RuTi, 2RuCe, were prepared by a single-step impregnation, while the preparation of bimetallic catalyst, designed as 2Ru5CuZr, was carried out by impregnation of the 5CuZr catalyst with ruthenium. The fresh catalysts were characterized by XRD, XPS, TPR, ICP-AES and BET surface area.

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2.2. Acivity test The activity tests were carried out in a bench-scale unit equipped with a tubular stainless steel reactor (1.15 cm i.d. and 30 cm length) in which 200 mg of catalyst (0.42-0.5 mm) was diluted with inert CSi (1:9 wt). The reactor was electrically heated in a furnace to the reaction temperature, and the effluent stream was cooled to room temperature and the gaseous and liquid products were separated. Prior to reaction, the catalysts were activated in situ with 100 mL(STP)/min of 10 vol% H2/N2 mixture at 0.1 MPa and 523 K during 2h. The reduced catalysts were tested under the following conditions: 16.2 gBuOH·gcat-1·h-1, 0.1 MPa of total pressure and 623 K using 100wt% of butanol (BuOH) and 40.9 mL(STP)/min of O2. Gas chromatography was used to determinate the type and amount of reaction products. For a better understanding of catalytic activity and product distribution, the following parameters were calculated: i) n-butanol conversion:

BuOH conv (%) =

Feed Out FBuOH − FBuOH Feed FBuOH

× 100

(Eqn. 1)

ii) Product selectivity in the liquid phase:

S (%) =

(∑ F

Fiout

)

out Product of reaction free of H O and BuOH 2

× 100

(Eqn. 2)

3. Results and discussion The ruthenium catalysts showed higher conversion than the copper catalyst (Fig. 1A and B). The supported Ru catalysts present a similar activity toward BuOH conversion (see Fig. 1B) suggesting that the BuOH transformation into other oxygenated hydrocarbons is not affected by the support. On the contrary, the 5CuZr and the 2Ru5CuZr present different performance. The 5CuZr has shown the lower activity toward BuOH conversion (44.5%). Nevertheless when Ru is added to the mentioned catalyst the conversion is improved, reaching similar conversion data (≈ 62.2%) than 2RuZr catalyst. This indicates that the ruthenium presence in 2Ru5CuZr catalyst increase the catalyst activity. Hence, the catalytic activity of ruthenium is considerably greater than copper for n-butanol oxidation. For ruthenium and copper catalysts, the Ru 3d 5/2 level and the Cu 2p 3/2 level were analyzed. For the fresh ruthenium catalyst, the Ru 3d 5/2 level could takes different binding energy values, such as 279.9-280.7 eV corresponding to Ru0, 280.7-281.0 eV to RuO2, 281.7-282.6 eV, to RuO3 [1,2] and finally 282.6-283.3 to RuO4 [3]. Depending on the support, different species were observed in the ruthenium catalysts. For the fresh copper catalysts only Cu2+ were detected, all of the binding energies of the Cu 2p 3/2 were in a range of 933.9-934.2 eV [4,5], but after their use in the reaction only Cu0 was detected. Therefore, during the butanol oxidation no phenomenon of oxidation was observed in copper.

455

Butyraldehyde production by butanol oxidation

Figure 1A and B. Butanol conversion in oxidation process for supported ZrO2 catalysts and Ru catalysts (■ 2RuZr, ● 2Ru5CuZr, S 5CuZr, ‹ 2RuCe, ¢ 2RuTi) at 623 K.

The ruthenium content (ICP-AES, Table 1) and the fresh catalysts metal dispersion (XPS, Table 1) were higher in 2RuZr than in the 2RuCe. Unfortunately for 2RuTi it was not possible to measure the ruthenium content due to the incomplete dissolution of the TiO2 support, but it was possible to determinate the Ru surface dispersion of this 2RuTi, and the total amount of ruthenium on the support surface was lower than the ZrO2 and CeO2 supported catalysts. For the ruthenium copper bimetallic catalyst, the total amount of ruthenium in the catalyst was higher than in the 2RuCe and lower than 2RuZr, but the presence of copper on the catalyst surface increased the amount of ruthenium on the surface (Table 1, XPS). Nevertheless, this difference in the initial dispersion and ruthenium amount on the catalyst has not a high influence effect on n-butanol oxidation activity. Hence, it seems that the presence of a low ruthenium amount on the support surface is enough to reach high n-butanol conversion, and the nature of the support has not a high influence on the n-butanol oxidation for ruthenium catalysts. Table 1. A summary of the different characterization results of the copper and ruthenium catalysts.

Catalyst

Ru/Cu ICP- Diameter size AES (wt%) (nm, XRD)

BET (m2/g)

Ru/Cu/ M at Ru3d5/2 (eV)

Cu2p3/2 (eV)

2RuZr

1.26/-

168.93

0.058/-

282.1 (61)a 283.0 (39)a

2Ru5CuZr

1.12/5.85

161.60

0.097/0.114

281.0 (66)a 282.9 (34)a

5CuZr fresh

-/6.18

n.d.

171.80

-/0.185

934.0

5CuZr used

-/-

29.87

-/0.079

932.7

2RuCe

0.89/-

2RuTi

-/-

0.048/n.d.

0.024/-

281.2 (65)a 282.8 (35)a 280.4 (66)a 282.2 (34)a

(a → proportion of Ru species)

But for the product selectivity the support composition has a high influence (Fig. 2). For the ruthenium catalysts, the 2RuCe catalyst presented the best selectivity towards

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A. Iriondo et al.

1.0

70 60

0.8

50 0.6

40

0.4

30 20

0.2

Conversion (%)

Liquid products /Converted buthanol (mol/mol)

butyraldehyde, while the 2RuZr was the catalyst that has had the highest selectivity to the main by-product of the n-butanol oxidation: butyl-butyrate. Hence, the presence of ZrO2 has been favourable to the condensation product yield. The 2RuTi catalyst had an intermediate behaviour between the 2RuCe and 2RuZr catalysts. Regarding the copper catalysts, for the bimetallic catalyst the selectivity towards butyl-butyrate was higher than the 5CuZr. This behaviour was similar for 2RuZr. Therefore the presence of ruthenium on the support improves the butyraldehyde selectivity.

10

0.0

0 2RuZr

2Ru5CuZr

5CuZr

2RuCe

2RuTi

Figure 2. Mol of liquid products in dry basis per mol of converted butanol (■ butyraldehyde, ■ butyl butyrate, ● conversion) at steady state and 673 K.

4. Conclusions The main conclusions of this study are the following: (a) the metal active phase has influence on the BuOH conversion: the Ru seems to be more active toward BuOH conversion, while the type of the support has not a high influence on n-butanol conversion, (b) the type of support improves the selectivity toward the oxidation products: the CeO2 is the most selective.

Acknowledgements The authors gratefully acknowledge the financial support of this work by the Spanish Ministry of Science and Technology (ENE2007-67533-C02-02/ALT), and the University of the Basque Country.

References [1] L. Ma, D. He, Z. Li, (2008) Catalysis Communications 9 2489–2495. [2] F.M. John, F.S. William, E.S. Peter, D.B. Kenneth, 1995 Handbook of X-ray Photoelectron Spectroscopy, Physical Electronics, Inc., Minnesota USA. [3] H. Y. H. Chan,C. G. Takoudis, M. J. Weavery, journal of catalysis 176, 336–345 (1997). [4] I. Ritzkopf, S. Vukojevic, C. Weidenthaler, J-D. Grunwaldt, F. Schüth. (2006) Applied Catalysis A: General 302 215–223. [5] Lj. Kundakovic, M. Flytzani-Stephanopoulos, (1998) Applied Catalysis A: General 171 13–29.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Preparation of Au nanoparticles on Ce-Ti-O supports S.A.C. Carabineiro, a A.M.T. Silva, a G. Dražić, b J.L. Figueiredo a a

Laboratório de Catálise e Materiais (LCM), Laboratório Associado LSRE/LCM, Departamento de Engenharia Química, Faculdade de Engenharia, Universidade do Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal b Jozef Stefan Institute, Department of Nanostructured Materials, Jamova 39, SI-1000 Ljubljana, Slovenia

Abstract CeO2, TiO2 and Ce-Ti-O supports with different Ce/Ti molar ratios were synthesized by solvothermolysis. When titania was combined with ceria, a nanostructured architecture was produced, evidencing the strong influence of Ti on the support structure, so that the Ce-Ti-O supports obtained were mainly amorphous, with some crystalline nuclei. Addition of Ce to titania strongly increases the surface area and oxygen content of samples. Au was loaded on the supports by a double impregnation method. The obtained Au/CeO2, Au/TiO2 and Au/Ce-Ti-O materials were also tested for CO oxidation. The differences between samples can be explained by the Au nanoparticle size, which has been proved to be a crucial factor in the preparation of Au catalysts. Keywords: Ce-Ti-O materials, gold, solvothermal, double impregnation

1. Introduction Au/Ce-Ti-O materials have been scarcely used in catalysis [1-5]. In the few available reports on the synthesis of this kind of materials, the Ce-Ti-O supports were synthesized by a sol-gel method [1,2,4,5] or by incipient wetness impregnation of aqueous solution of cerium nitrate on titania [3]. In these works, Au was loaded by DepositionPrecipitation [1-5]. We have previously synthesised Ce-Ti-O supports with different Ce/Ti molar ratios by solvothermolysis [6]. In the present work, Ce-Ti-O supports, with higher Ce contents, were loaded with Au by a double impregnation method (DIM) [7], characterised by several techniques and tested for CO oxidation. To the best of our knowledge, this is the first report with the combination of the solvothermal method for Ce-Ti-O synthesis with the DIM approach for gold loading.

2. Experimental Ce-Ti-O materials with different mass percentages of cerium (5, 50 and 75% w/w) were prepared by solvothermolysis [6], using cerium(III) nitrate and titanium(IV) isopropoxide as precursors and methanol as solvent. The main role of the organic solvent is to act as an oxygen source for the metal oxide formation. In a typical synthesis procedure, 2.4 mL of titanium precursor was added to 75 mL of pure methanol, at room temperature under continuous stirring and in alkaline media adjusted by using KOH (3M). Subsequently, the proper amount of hydrated cerium nitrate was slowly dissolved in this solution, under stirring for 30 min, in order to obtain a homogeneous solution. The final solution was transferred to a 250 mL temperature controlled glass-container, immersed in an oil bath, heated up to 350 K under normal pressure, and kept at this temperature for 21 h,

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for solvothermal treatment. At the end of this period, the autoclave was cooled to room temperature. The precipitates obtained were separated from the liquor by centrifugation, washed thoroughly with deionized water and ethanol, dried at 275 K overnight, and calcined in air atmosphere at 675 K for 2 h, in an horizontal tubular oven. The solid was heated at 5 K/min, from room temperature to 425 K, kept at this temperature for 30 min and then heated again at 5 K/min up to 675 K. The absence of potassium in selected samples was confirmed by EDXS. The single TiO2 and CeO2 supports were also prepared using the same approach. Au was loaded on the ceria supports using HAuCl4·3H2O as the gold precursor (Alfa Aesar) in order to achieve 1% wt. content of Au, by the double impregnation method (DIM) [7]. This method consists in impregnating the support with an aqueous solution of the gold precursor and then with a solution of Na2CO3, with constant ultrasonic stirring, followed by washing with water and drying overnight at 395 K. The advantage of this method is that it removes chloride, well known to cause sintering of Au particles, thus turning them inactive [8,9]. Since CO oxidation is commonly used to test supported Au catalysts [7-9], the activities of the obtained Au/CeO2, Au/TiO2 and Au/Ce-Ti-O materials were compared for this reaction. Samples were characterised by adsorption of N2 at 77 K, high-resolution transmission electron microscopy (HTREM), selected area electron diffraction (SAED), energy-dispersive X-ray spectrometry (EDXS), high-angle annular dark-field imaging (HAADF), X-ray diffraction (XRD) and temperature programmed reduction (TPR). Further details of these techniques are described elsewhere [7].

3. Results and discussion 3.1. Characterisation of the supports It was shown by HRTEM, and confirmed by XRD, that CeO2 consists of agglomerated cubic particles ranging from ~3 to 8 nm (Figure 1a), while TiO2 showed to be anatase with some traces of rutile (Figure 1b). When titania was combined with ceria, a nanostructured architecture was produced, evidencing the strong influence of Ti on the support structure. Ce-Ti-O supports were found to be mostly amorphous with very rare nanocrystals with lattice fringes (Figures 2a and 3a). Addition of Ce to titania produced materials with a larger surface area (varying from 27 to 72 m2/g, with increasing Ce loading from 0 to 75% w/w). The activities of the obtained Ce-Ti-O materials were compared for CO oxidation reaction. It was found that the addition of Ce to titania strongly increases the catalytic activity. That can be explained by the increase in the surface area and oxygen content of samples.

3.2. Au loaded materials Samples loaded with gold showed Au nanoparticle sizes from 5 to 20 nm (Figures 2c-d and 3c-e). In these samples (particularly the Au/Ce-Ti 50-50) beside amorphous phase the anatase nanocrystals were already present at room temperature, which was confirmed also from SAED patterns (Figures 2b and 3b). The oxygen content of samples was improved as more ceria was present in the supports, as seen by TPR (Figure 4a). It is known from literature that a peak at ~875 K is assigned to lower temperature ceria surface shell reduction or reduction of surface oxygen species [7,10-12]. In addition, when a noble metal promoter is loaded, the surface shell reduction is facilitated. In the present case, the peak is significantly shifted to lower temperatures (~425 K), as it can be seen in Figure 4b (which shows Ce-Ti 5050 sample as an example). The lowering of the reduction temperature implies that the presence of Au improves the reducibility of the surface oxygen on the support, which

459

Preparation of Au nanoparticles on Ce-Ti-O supports

facilitates the oxygen transfer across the solid-gas interface during the reaction. The single peak visible in the spectrum for the Au loaded sample has an area exactly the same as the area of the corresponding peak of the Au-free sample, indicating that most gold is in the metallic state [7]. With addition of Au to the Ce-Ti-O samples the activity for CO oxidation increased, as expected, and the same trend was maintained. The catalytic behaviour can be explained based on the oxygen content and Au nanoparticle size, which has also been proved to be a crucial factor in the preparation of Au catalysts [1-5,7-12].

a

b

Traces of Rutile Rutile

CeO2

10 nm

TiO2 Anatase

Figure 1. HRTEM micrographs of CeO2 (a), TiO2 (b) with respective SAED on insets.

aa

5 nm

cc

25 nm

d

10 nm

b

Au

Ti

e

Ti Ce

Ce Ce

Ce

Au

Figure 2. HRTEM micrograph of Ce-Ti 50-50 (a) with respective SAED on inset (b). HAADF (c) and TEM (d) images of Au/Ce-Ti 50-50, with a EDXS spectrum of a Au nanoparticle (e).

a

20 nm 20 nm

b

5 nm

c d

5 nm

e

Figure 3. HRTEM micrograph of Ce-Ti 75-25 (a) with respective SAED on inset (b). TEM (c) and HAADF (d) images of Au/Ce-Ti 75-25, with a closer detail of an Au nanoparticle (e).

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S.A.C. Carabineiro et al.

1.3

TCD signal (a.u.)

TCD signal (a.u.)

b Au/Ce-Ti 50:50

1.5

0.7 0.4

1

Ce-Ti 50:50

0.5

Ce-Ti 50:50

0.1

2

a

Ce-Ti 75:25

1

0

-0.2

Ce-Ti 5:95

-0.5 650

700

750

800

850

900

Temperature (K)

-0.5

950

1000

1050

400

500

600

700

800

900

1000

1100

Temperature (K)

Figure 4. H2-TPR profiles of Ce-Ti-O samples (a) and of the Ce-Ti 50:50 sample with and without Au loading (b).

4. Conclusions CeO2, TiO2 and Ce-Ti-O supports with different Ce/Ti molar ratios were synthesized by solvothermolysis. The combination of titania with ceria produced a nanostructured architecture, evidencing the strong influence of Ti on the support structure. The materials obtained were mainly amorphous, with some crystalline nuclei. Addition of Ce to titania strongly increases the surface area and oxygen content of samples. Au was loaded by a double impregnation method onto the supports. The differences between samples obtained in CO oxidation can be explained based on the Au nanoparticle size.

Acknowledgments Authors acknowledge Fundação para a Ciência e Tecnologia, Portugal, and the Ministry of Higher Education, Science and Technology, Slovenia, for financial support from the Portugal-Slovenia Cooperation in Science and Technology (2008/2009). SAC and AMTS also acknowledge FCT for CIENCIA 2007 program and POCI/N010/2006 project. FCT and FEDER are also acknowledged for Programme POCI 2010 and Project REEQ/1106/EQU/2005. GD acknowledges the financial support of the Slovenian Research Agency.

References 1. 2. 3. 4. 5. 6. 7. 8. 9.

C. Gennequin, M. Lamallem, R. Cousin, S. Siffert, F. Aissi, A. Aboukais, 2007, Catal.Today, 122, 301. M. Lamallem, H. El Ayadi, C. Gennequin, R. Cousin, S. Siffert, F. Aissi, A. Aboukais, 2008, Catal. Today, 137, 367. P. Sangeetha, Y.W. Chen, 2009, Int. J.Hydrogen Ener., 34, 7342. M. Lamallem, R. Cousin, R. Thomas, S. Siffert, F. Aissi, A. Aboukais, 2009, Comptes Rendus Chimie, 12, 772. C. Gennequin, M. Lamallem, R. Cousin, S. Siffert, V. Idakiev, T. Tabakova, A. Aboukais B.L. Su, 2009, J: Mater. Sci., 44, 6654. A.M.T. Silva, C.G. Silva, G. Dražić, J.L. Faria, 2009, Catal. Today, 144, 13. S.A.C. Carabineiro, A.M.T. Silva, G. Dražić, P.B. Tavares, J.L. Figueiredo, 2010, in press, DOI 10.1016/j.cattod.2010.01.036. S.A.C. Carabineiro and D.T. Thompson, in Nanocatalysis, E.U. Heiz and U. Landman, Eds. 2007, Springer-Verlag: Berlin, Heidelberg, New York. p. 377. S.A.C. Carabineiro, D.T. Thompson, in Gold: Science and applications, C. Corti and R. Holliday, Eds. 2010, CRC Press Taylor & Francis Group, Boca Raton, London, New York. p. 89.

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10. D. Andreeva, V. Idakiev, T. Tabakova, L. Ilieva, P. Falaras, A. Bourlinos, A. Travlos, 2002, Catal Today, 72, 51. 11. A.M. Venezia, G. Pantaleo, A. Longo, G. Di Carlo, M.P. Casaletto, F.L. Liotta, G. Deganello, 2005, J. Phys. Chem. B, 109, 2821. 12. Q. Fu, W.L. Deng, H. Saltsburg, M. Flytzani-Stephanopoulos, 2005,Appl. Catal. B, 56, 57.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Preparation, active component and catalytic properties of supported vanadium catalysts in the reaction of formaldehyde oxidation to formic acid E. V. Danilevich, G. Ya. Popova, T. V. Andrushkevich, Yu. A. Chesalov, V. V. Kaichev, A. A. Saraev, L. M. Plyasova. Boreskov Institute of Catalysis, Ak. Lavrentieva st. 5, Novosibirsk 630090, Russia

Abstract The influence of the support nature was investigated with supported vanadium catalysts prepared by a wet impregnation method. SiO2, γ-Al2O3, ZrO2 and TiO2 (anatase) were used as supports. Two series of catalysts were prepared, the first one consisting of catalysts of composition ca. 20% wt. V2O5/80% wt. support (series 1) and the second one prepared by washing the series 1 samples with nitric acid (series 2). In the catalysts of series 1 (except 20% V2O5/80% SiO2), vanadium is represented by both monolayer species (monomeric and polymeric VOx) and crystalline V2O5 phase. When vanadium is supported on SiO2, only the crystalline V2O5 is formed. Washing the samples of series 1 with nitric acid removes crystalline V2O5 phase. Monomeric and polymeric vanadia species are more active in the reaction of formaldehyde oxidation to formic acid as compared to V2O5. Keywords: Supported vanadium catalysts; Formaldehyde oxidation; Vanadium species

1. Introduction Numerous studies of supported vanadium catalysts in different reactions have demonstrated that activity and selectivity depend on the chemical nature of support . In the present work we investigated the nature of vanadia species on different supports and their catalytic behavior in formaldehyde selective oxidation.

2. Experimental TiO2 (anatase) (catalog Alfa Aesar), SiO2 (airosil, A-200, Russia). γ-Al2O3 and ZrO2 were used as supports in this study. γ-Al2O3 was prepared by calcination of pseudoboehmite (“Industrial catalysts”, Russia) at 550ºС for 4 h. ZrO2 was prepared by precipitation from ZrOCl2 (Aldrich 99.9%) solution (0.4 mol/l) by the addition of a calculated amount of ammonium hydroxide solution at 50ºC and pH=8.5. A white precipitate thus formed was kept for aging for 5 h. The resultant cake was washed with distilled water till the pH reaches to 7. It was dried at 110ºC for 12 h and calcined at 400ºC for 4 h. Series 1. Catalysts containing ca. 20% wt. V2O5 were prepared by impregnation of supports with requisite amounts of V2O5 (> 99.6%, Reachim, Russia) dissolved in oxalic acid (> 97%, Reachim, Russia) solution. Ratio support/solution (g/ml) was equal to 1/1.3 (VTi, VAL); 1/1.8 (VZr); 1/6.3 (VSi) and determined by water-absorbing capacity of support. After impregnation the samples were dried at 110oC for 24 h and calcined in air flow (50 ml/min) at 400°C for 4 h. Series 2. The samples of series 1 were washed with a 10% water solution of nitric acid to remove the crystalline V2O5 phase. After washing, the samples were calcined

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once more at 400°C for 4 hours. The samples of series 1 and 2 are labeled as VMe and VMew, (Me = Si, Al, Zr, Ti), respectively. Catalysts were characterized by BET, the chemical analysis, Raman spectroscopy, XRD and XPS. Catalytic experiments were carried out in a flow-circulation setup with chromatographic analysis of reaction products. Composition of the initial mixture was as follows: 5% CH2O, 10% H2O, air balance. The reaction temperature was 120°C.

3. Results and discussion 3.1. Characterization of the catalysts The physicochemical properties of the catalysts are listed in Table 1. Table 1. Physicochemical properties of the catalysts. Sample

V2O5 (wt.%)

SBET, (m2/g)

N sa, V/nm2

[V/Me]

Phase XRD

Series 1 Support catalyst bulkb surfacec VSi 20.0 200 129 10.3 0.17 0.03 V2O5, SiO2 VAl 21.9 250 146 9.9 0.14 0.07 V2O5,γ-Al2O3 VZr 17.3 120 96 11.9 0.34 0.92 V2O5, ZrO2d VTi 19.6 350 111 11.7 0.22 0.28 V2O5, TiO2 Series 2 SBET, (m2/g)e VSiw 0.0 150 150 0.0 0.00 0.00 SiO2 1.7 250 233 0.5 0.01 0.01 γ-Al2O3 VAlw 3.4 120 122 1.9 0.05 0.07 ZrO2d VZrw VTiw 11.0 150 140 5.2 0.11 0.14 TiO2 a surface density (Ns, Vat/nm2); bthe bulk atomic ratio by the chemical analysis; cthe surface atomic ratio determined by XPS; dmonoclinic (85%) and cubic (15%) modification of ZrO2; e SBET after washing and calcination at 400oC.

An increase in specific surface area of the catalysts after washing is related to removal of crystalline V2O5, which makes its value close to the surface area of support after washing and calcination at 400oC. One can see that in the series 1 samples the bulk atomic ratios [V/Me]v and the surface atomic ratios [V/Me]s are very different. Low values of [V/Me]s for VSi and VAl samples indicate that only a small amount of vanadia in the catalysts belongs to the surface, while the main part of vanadia is in V2O5 crystallites. The excess of [V/Me]s over [V/Me]v in VTi and VZr samples can be attributed to high dispersion of V2O5 over ZrO2 and TiO2. Values of [V/Me]s and [V/Me]v in the washed samples virtually coincide, indicating the presence of surface vanadia species and absence of the crystalline V2O5 phase. The same conclusions follow from XRD patterns of the series 1 and 2 samples. XRD patterns (Fig. 1). of the series 1 samples show reflections of the crystalline phases of support and V2O5. In the spectra of series 2 samples only the reflections of support phase are observed. Complete removal of vanadium, as shown by the chemical analysis data, occurs after washing the VSi sample. It means that all vanadia in the VSi catalyst is represented by crystalline V2O5. After removal of crystalline V2O5, the VAlw, VZrw and VTiw samples still contain strongly bound insoluble vanadia species, their amount depending on the support nature. The nature of strongly bound vanadia species was studied by Raman spectroscopy (Fig. 2). The Raman spectrum of VTi sample shows an intense broad band with a maximum at 840 cm–1, which is attributed to stretching V-O-V vibrations in polymeric VOx species, and a low-intensity band of V2O5 (995 cm–1).

465

Preparation, active component and catalytic properties

* - V 2O

5

(V = O) 99 4

(V = O) 702 V Ti V T iw

*

* *

**

*

*

*

*

**

**

1 03 0

**

(V - O -V ) 8 40

VS i VS iw

*

*

V Zr

( V= O ) 990

VZ r

*

( V = O)

9 35

V Zrw *

*

*

(V -O-V ) 75 0

V Zrw

93 0 (V = O)

*

V Al

*

995

V Ti

V Alw V T iw 0

10

20

30

40

50

60

70

2 Θ

Fig. 1. XRD patterns of the catalysts.

7 00

80 0

90 0

R a m a n s h if t, c m

1 00 0 -1

Fig. 2. Raman spectra of the catalysts.

In the Raman spectrum of the washed sample only the bands of polymeric VOx species are observed. The presence of these species is confirmed by the data of chemical analysis giving a 5.2 V/nm2 density of VOx in VTiw samples, which is close to the monolayer surface coverage of polymerized vanadia species [1-4]. The Raman spectrum of VZr sample before washing shows low-intensity bands of polymeric (935 cm–1) and monomeric (1030 cm–1) VOx species and intense bands of crystalline V2O5 (994 cm–1). Similar bands are observed for the VAl sample (not shown). Washing of these samples results in disappearance of the bands from V2O5 and polymeric VOx. The density of VOx particles on the surface of the VZrw sample is 1.9 V/nm2 (Table 1), which is close to the monovanadate monolayer coverage [2]. Only the crystalline V2O5 bands are observed in the Raman spectra of VSi (not shown).

3. 2. Catalytic properties Table 2 compares the catalytic performance of the different supported vanadium catalysts with reference to the crystalline V2O5 and the supports without the addition of vanadium. The TiO2 and ZrO2 supports catalyze conversion of formaldehyde to methyl formate. Methyl formate and methanol are the main products of formaldehyde conversion on γ-Al2O3. SiO2 is inactive in both reactions. Crystalline V2O5 has low activity, but high selectivity (ca. 90%) in formaldehyde oxidation to formic acid. Some increase in activity (TOF to formic acid) is observed when vanadium is supported on SiO2, without significant changes of selectivity. Supported vanadium on γ-Al2O3 does not provide a notable increase in activity, which is explained by low dispersion of vanadium over the surface of support. Increasing activity of VAlw is related to the presence of monomeric VOx species in sample. Low selectivity to formic acid in the oxidation of formaldehyde on VAl sample is caused by a low coverage of the surface with vanadia species and by a large fraction of free areas of support that are active in formaldehyde conversion to methyl formate. The activity of vanadia sites for the formation of formic acid increases when vanadium is supported on ZrO2 и TiO2. The activity of the washed samples containing only the monomeric and polymeric vanadia species is considerably higher compared to that for the two-phase samples with prevailing crystalline V2O5. A higher activity of the monolayer samples in comparison with the two-phase those is caused by blocking a part

466

E.V. Danilevich et al.

of active VOx species by low-active crystalline V2O5 in the two-phase samples. The absence of methyl formate in the reaction products indicates a homogeneous coverage of the supports by VOx species. Table 2. Catalytic properties. Temperature – 120ºC. The composition of the reaction mixture (vol. %): 5% CH2О, 10% Н2О, air the balance. V=8.2 l/h. Catalyst

Weight, g

X,%a

Selectivity, %

rb,10-9 mol/m2·s

TOF,c 10-4s-1

FAd MFe Mf COx V 2O 5 10.5 6.1 91.8 6.1 0.0 2.1 6.6 0.02 VSi 2.9 6.6 87.7 10.0 0.0 2.3 0.4 0.20 TiO2 0.8 35.0 3.0 82.0 15.0 0.0 16.6 VTi 0.7 35.3 96.1 0.3 0.0 3.6 24.8 12.20 0.3 35.6 93.7 0.8 0.0 5.5 43.0 22.50 VTiw ZrO2 1.1 35.0 3.3 87.5 7.7 1.5 13.5 VZr 5.1 36.2 22.8 66.5 4.7 6.0 3.8 0.44 1.4 35.0 17.1 74.3 2.4 6.2 15.1 8.10 VZrw Al2O3 4.7 35.0 0.0 63.8 31.8 4.4 2.0 VAl 7.5 37.8 3.9 85.1 4.3 6.7 1.8 0.04 VAlw 4.5 35.0 2.0 64.5 29.9 3.6 1.7 0.42 a X- conversion of CH2O; b r - rate of СН2О transformation; c TOF – normalizing the rate of oxidation formaldehyde to formic acid per vanadium site, s-1; d FA - formic acid; e MF - methyl formate; f M- methanol.

The following order of activity for formic acid formation as a function of the nature of support was established: TOF to formic acid (10–4 s) (CH2O conversion ≈ 35%): Series 1: VTi (12.2) > VZr (0.44) > VSi (0.2) > VAl (0.04) ≈ V2O5 (0.02); Series 2: VTiw (22.5) > VZrw (8.1) > VAlw (0.42) > V2O5 (0.02).

4. Conclusions Activity of the samples with monomeric (VAlw, VZrw) and polymeric (VTiw) VOx species exceeds the activity of the samples, which contain too the crystalline V2O5. The presence of crystalline V2O5 in samples leads to a partial blocking of the active sites and hence decreases the catalyst activity. The lower formic acid selectivity over VAl and VZr catalysts was explained by the fact that VOx species do not fully cover the surface of support, and it results in formaldehyde transformation to methyl formate.

Acknowledgement The authors acknowledge Federal Agency Science Innovation for financial support.

References 1. 2. 3. 4.

G.C. Bond, S.F. Tahir, Appl. Catal. 71 №1 (1991) 1-31. A. Khodakov, B. Olthov, A.T. Bell, I. Iglesia, J. Catal. 181 №2 (1999) 205-216. G.Ya. Popova, T.V. Andrushkevich, E.V. Semionova, Yu.A. Chesalov, L.S. Dovlitova, V.A. Rogov, V.N. Parmon, J. Mol. Catal. A: Chemical. 283 (2008) 146-152. G.Ya. Popova, T.V. Andrushkevich, I.I. Zakharov, Yu. A. Chesalov Kinet. Catal. 46 (2005) 217-226.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Investigation of different preparation methods of PtIr, PtIrSn and PtIrGe catalysts Christophe Poupin, Camille La Fontaine, Laurence Pirault-Roy Laboratoire de Catalyse en Chimie Organique, Université de Poitiers, France

Abstract Addition of Ir to a Pt/Al2O3 catalyst by two different methods, successive impregnation and organometallic grafting resulted in catalysts with different structure: in the first case separate Pt and Ir sites can be suggested, while in the second case, Ir was exclusively deposited on Pt, forming very small Ir clusters on Pt particles. Addition of Ge or Sn to the Pt-Ir grafted catalyst led to third component deposit on the surface of mixed metals particles. On the other hand, when the promoter is added to PtIr-IS, Ge seems to be located on Ir particles while Sn modified Pt particles. Keywords: platinum, iridium, germanium, tin, organometallic grafting

1. Introduction Reforming of hydrocarbons is an important catalytic process for the production of highoctane gasoline, aromatics and hydrogen from naphtha. The reactions involved in this process as hydrogenation-dehydrogenation reactions occur over the metallic sites of the catalyst while isomerization and dehydrocyclization proceed mostly on bifunctional metal–support acid sites. The metallic function is usually provided by Pt in the form of very small particles dispersed on the surface of the catalyst. Its properties can be finetuned by the addition of another element. This metal function promoter can be another noble metal, e.g. Ir, or/and another element with the desired properties (Sn, Ge). It was observed that Ge or Sn adding could favorably replace the sulfidation step of Pt-Ir catalysts needed to decrease the high hydrogenolytic activity of the catalyst [1]. Our aim was to investigate bimetallic Pt-Ir catalysts prepared by two different methods, namely conventional successive impregnation and by organometallic grafting [2, 3] as well as to study the effect of third compound as Ge or Sn added in both cases by organometallic grafting method.

2. Experimental 2.1. Catalyst preparation 2.1.1. Support Alumina from Degussa (Aluminum Oxid C; δ-alumina; surface area of 100 m2.g-1) was used in powder form made of microspheres of about 100 Å. This powder was first wetted with water to prepare a slurry. Then it was dried overnight at 393 K in an oven, ground and sieved to collect the fraction between 0.1 and 0.25 mm, calcined in dry air flow (4 h, 773 K) and finally reduced in H2 flow (4 h, 773 K). 2.1.2. Monometallic parent catalysts The monometallic catalysts were prepared by wet impregnation of the treated alumina using a platinum salt [Pt(NO2)2(NH3)2] to obtain 1 wt.-Pt % loading and [Ir(C5H7O2)3] precursor to obtain 0.25 wt.-Ir %. The slurry was shaken at room temperature for 12 h and after drying in a sandbath at 353 K, the impregnated alumina was left overnight in

468

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an oven at 393 K. Then, the sample was calcined in dry air (4 h, 773 K) and reduced under pure H2 flow (4 h, 773 K). 2.1.3. Modified catalyst: multi-metallic catalysts Different bimetallic catalysts were prepared by using successive impregnation or the surface organometallic chemistry method [2, 3]. For the successive impregnation, the Pt parent was impregnated with the Ir precursor [Ir(C5H7O2)3]. Then it was calcined in dry air at 673 K for 3 h and reduced in N2 flow containing 20% H2 for 3 h at 673 K. The samples were denoted as PtIr(IS). For the organometallic grafting, 6.6 g of the parent Pt sample was pre-reduced (in H2 flow at T = 673 K for 2 h, heating rate 2 K/min), cooled in H2 to room temperature and kept at room temperature for 1 h (H2 adsorption). It was first immersed in 15 ml toluene, kept at room under argon flow (1 h) then 15 ml toluene solution of [Ir(C5H7O2)3] was added and the sample was kept for 6 h at 343 K in bubbling Ar. The amount of [Ir(C5H7O2)3] dissolved in toluene corresponded to nominal coverage of 1/2 Ir monolayer, as calculated for surface Pt atoms. The sample was washed with toluene, dried in Ar flow at 393 K for 1 h and finally reduced in H2 flow (473 K; 4 h). This reduction temperature was found to be sufficient for producing catalytically active and reproducible samples [3]. The sample was denoted as PtIr(GS). The tri-metallic catalysts were prepared by Ge or Sn grafting on Pt-Ir catalysts (IS or GS one) using organometallic route described before. A solution of Ge(nC4H9)4 or Sn(nC4H9)4 in heptane was used and the catalysts made were denoted PtIrGe(IS), PtIrGe(GS), PtIrSn(IS) and PtIrSn(GS).

2.2. Characterization methods The metallic accessibility was determined by two different techniques: hydrogen chemisorption and transmission electron microscopy (TEM). The volumetric hydrogen chemisorption was carried out on prereduced samples (473 K, p(H2)=75 kPa, 1 h) after evacuation at room temperature in an apparatus described previously [3]. TEM was performed with a Philips CM120 electron microscope operating at 120 kV with a theoretical resolution of 0.35 nm. Samples were included in a polymeric resin and cut into small sections (about 40 nm) using a diamond knife. Cuts were put onto copper TEM grids. The average particle size (average diameter) was determined on several pictures using the relation Σ nidi3/Σ nidi2. CO probe FTIR measurements were performed using a Nicolet Magna-750 spectrometer. The samples (about 20 mg) were pressed into pellets and reduced in situ in a dedicated cell at 473 K in H2 flow for 2 h, followed by outgassing and cooling to ambient temperature. CO adsorption was performed at room temperature by injecting pulses in the cell until the catalysts saturation. Then, the samples were evacuated at room temperature for 1 h. Difference spectra were obtained from the absorbances before and after adsorption of the probe molecule.

3. Results and discussion 3.1. Catalysts characterization by TEM and H2 chemisorption Good correlations between the values obtained by these two methods were pointed out for monometallic samples (see Table 1) that ensured the hypothesis, 1Irs or 1Pts = 1Hads, used for the chemisorption. By successive impregnation preparation, Pt and Ir species are expected to be separate and the dispersion should be near 50% as it was measured. The addition of a third component, on the PtIr(IS), decreased the metallic accessibility but the size of the particles measured by TEM remains the same. So Ge or Sn is grafted on particles surface and hindered the chemisorption of H2, but the amount of the addition is to low to increase the size of the particles. The PtIr(GS) accessibility

Investigation of different preparation methods of PtIr, PtIrSn and PtIrGe catalysts 469 measured was lower than expected. Ir poisoned the Pt particles and such monoatomic Ir or organometallic species don’t allow the dissociative chemisorption of H2. The addition of a third compound, on the PtIr(GS), does not change the poor metallic accessibility and the size of the particles. Table 1. Characterization of catalysts by H2 chemisorption and TEM. Metal. acc. (H2 Chem.);

Particles size

Estimated size (nm)

TEM (nm)

Pt

50%; 1.9

2.2

0.2%Ir/Al2O3

Ir

50%; 1.9

2.1

1%Pt-0.2%Ir/Al2O3 (IS)

PtIr(IS)

40%; 2.9

3.5

PtIrGe(IS)

7%; 16.3

3.4

PtIrSn(IS)

11%; 11.4

3.5

PtIr (GS)

10%; 10.4

3.0

PtIrGe(GS)

10%; 10.4

3.1

PtIrSn(GS)

10%; 10.4

3.2

Catalysts

Code

1%Pt/Al2O3

1%Pt-0.2%Ir-0.15%Ge/Al2O3 (Pt-Ir (IS) parent) 1%Pt-0.2%Ir-0.15%Sn/Al2O3 (Pt-Ir (IS) parent) 1%Pt-0.2%Ir/ Al2O3(GS) 1%Pt-0.2%Ir-0.2%Ge/Al2O3 (Pt-Ir (GS) parent) 1%Pt-0.2%Ir-0.15%Sn/Al2O3 (Pt-Ir (GS) parent)

3.2. Catalysts characterization by CO FTIR spectroscopy The left IR graph gathers the parents’ spectra as well as the bimetallic catalysts ones. For Ir/Al2O3, both Ir0 and Irδ+ specie exist. A major carbonyl band (2043 cm–1), is due to CO linearly adsorbed on fully reduced Ir sites, while an intense pair of bands observed at 2073 and 1976 cm–1 is assigned to gem-dicarbonyl species adsorbed on small Ir clusters or isolated Ir [4-7]. On Pt parent catalyst, the single band at 2075 cm–1 is due to linear CO species (LCO). PtIr (IS) catalyst exhibits two bands at 2075 cm–1 (LCO species on Pt) and 2066 cm–1 while on PtIr (GS) catalyst, three bands can be observed at 2075, 2063 and 2023 cm–1. The peak near 2060 cm–1 can be assigned to LCO species adsorbed on low coordinated Ir sites. The main difference between the two bimetallic samples is a band at ca. 2020 cm–1 observed only on the Pt-Ir grafted catalysts. It was recognized as LCO adsorbed on very small Ir clusters by Solymosi et al [6]. On the other hand, McVicker et al. [7] specifically assigned a peak around 2020 cm–1 to be LCO adsorbed on large Ir clusters on alumina support. As no gem-dicarbonyl species can be pointed out on the PtIr (GS) spectra, no large Ir cluster can exist. So, we assume that very small Ir clusters are present on the grafted catalyst. According to the preparation procedure, separate Pt and Ir sites can be suggested for PtIr (IS) catalysts, while for PtIr (GS) samples, Ir was exclusively deposited on Pt, forming very small Ir clusters on Pt particles. The right IR graph presents the spectra of bimetallic catalysts and an example of a sample modified by Ge. The presence of a peak at 2020 cm–1 for the PtIrGe (IS) suggests that Ge grafting occurred on Ir particles and divided the facets to create very small clusters. When tin is added, no peak appears but the Pt peak at 2075 cm–1

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470

drastically decreases. Addition of Ge or Sn to PtIr (GS) catalysts leads to a diminution of the band attributed to LCO species both on Ir and Pt. 2075 cm-1

2075 cm-1

2075 cm-1

2063

cm-1

0,07 0,06

Pt Ir PtIr (IS) PtIr (GS)

0,05 0,04

2023 cm-1

0,03 0,02

PtIr (GS) PtIr (IS) PtIrGe (IS)

0,070

2075 cm-1

2043

0,060 Absorbance (A.U.)

Absorbance (A.U.)

0,09 0,08

2063 cm-1

2066 cm-1

2066 cm-1

2075 cm-1

0,050 2075 cm-1

2023 cm-1

0,040 0,030

2020 cm-1

0,020

cm-1

2073 cm-1 0,010

1976 cm-1

0,01

-0,000

-0,00 2200

2000

1800

1600

wavenumber (cm-1)

2200

2000

1800

1600

wavenumber (cm-1)

Figure 1. Characterization of catalysts by FTIR CO spectroscopy.

4. Conclusion Addition of Ir to Pt catalysts by successive impregnation (IS) and organometallic grafting (GS) results in catalysts with different structure: in the case of PtIr-IS separate Pt and Ir particles on the alumina surface can be suggested while in the case of PtIr-GS, Ir is exclusively attached to Pt, as ensured by the organometallic grafting method. Addition of Ge or Sn to the Pt-Ir catalyst prepared by organometallic grafting leads to deposit on the surface of the bimetallic particles and blocks both. On the other hand, when Ge or Sn is added to PtIr-IS, Ge and Sn seem to be located on different sites: Ge mainly on Ir particles and Sn on Pt particles. Further experiments pointed out that PtIrGS catalyst showed very promising behavior in ring-opening reaction of MCP as resulting in ROPs at high selectivity even at high conversion.

References [1] P. Samoïla and al, 2007, “Influence of the pretreatment method on the properties of trimetallic Pt–Ir–Ge/Al2O3 prepared by catalytic reduction”, Appl. Cat. A : General, 332, 37. [2] A. Wootsch and al, 2006, “Characterization and catalytic study of Pt-Ge/Al2O3 catalysts prepared by organometallic grafting”, J. of Catal., 238, 67. [3] L. Pirault-Roy and al, 2003, “A new approach of selective Ge deposition for RhGe/Al2O3 catalysts: characterization and testing in 2,2,3-trimethylbutane hydrogenolysis”, Appl. Cat. A: General, 245, 15. [4] Y. M. Lopez-De Jesus and al , 2008, “Synthesis and Characterization of Dendrimer-Derived Supported Iridium Catalysts” J. Phys. Chem. C, 112, 13837-13845. [5] A.Bourane and al, 2002, “Heats of Adsorption of the Linear CO Species Adsorbed on a Ir/Al2O3 Catalyst Using in Situ FTIR Spectroscopy under Adsorption Equilibrium” J. Phys. Chem. B, 106, 2665-2671. [6] F.Solymosi and al. 1980, “CO and NO adsorption on alumina-supported iridium catalyst” J. Catal., 62, 253. [7] McVicker and al, 1980, “Chemisorption properties of iridium on alumina catalysts”, J.Catal., 65, 207.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Perovskite-type catalysts for the water-gasshift reaction Francesco Basilea, Giuseppe Brennaa, Giuseppe Fornasaria, Pascal Del Gallob, Daniel Garyb and Angelo Vaccaria a

Dipartimento di Chimica Industriale e dei Materiali, ALMA MATER STUDIORUM – Università di Bologna, Viale Risorgimento 4, 40136 Bologna (I) b Air Liquide, Centre Recherches Claude-Delorme, BP 126, 90124 Jouy-en-Josas (F)

Abstract Fe/La perovskite-type (PVK) catalysts - as such, or containing Cr, Ce or Cu - have been synthesized by the citrate method to be applied in the water-gas-shift reaction at moderate temperatures. A calcination temperature of 650°C made it possible to obtain stable PVK phases, with acceptable surface area values. TPR/TPO/TPR analyses evidenced the specific effects of the added cations or calcination parameters on the Fe3+ ions reducibility, which reflected on the catalytic behaviour of the final catalysts.

Keywords: water gas shift, perovskite, Fe, La, Ce, Cu , citrate method 1. Introduction Recent years have seen increasing interest towards the water-gas-shift-reaction (WGSR), CO + H2O → CO2 + H2, as an upgrading technique either to adjust the syngas (CO + H2) composition or to reduce the CO content and obtain a high H2 grade for application in low temperature fuel cells. Currently, the WGSR is performed in two steps: i) immediately after the steam reforming reactor, operating at about 350°C with stable Fe-based catalysts (HTS); and ii) operating at about 250°C with highly active Cubased catalysts (LTS) [1]. However, there is an increasing interest for new formulations able to operate at moderate temperatures (about 300°C, or MTS) with high activity, selectivity and stability with time-on-stream. Two paths may be explored: (i) to stabilize the LTS catalysts, thus avoiding methanation and sintering, or (ii) to improve the activity of HTS catalysts by introducing new active elements. The aim of this study was to develop new catalytic formulations which are active and selective in WGSR at moderate temperatures, and stable with time-on-stream and towards some poisons (sulphur, chlorine and silica) that may be present in the exit streams of steam reforming reactors. To obtain active catalysts and avoid interferences due to either structure dishomogeneity or phase segregation, reducible and flexible structures have to be selected; ABO3 perovskite-type (PVK) phases were therefore prepared, due to their capacity to cover broad composition ranges [2]. Furthermore, in order to improve this activity, it may be advantageous to prepare complex PVK formulations, with A and B substitution on the reference LaFeO3 PVK phase (considering the role of Fe-based HTS catalysts), by introducing Cr as a stabilizing element and Cu or Ce as activating elements [3,4].

2. Experimental PVK precursors have been synthesized by the citrate method [4,5], using metal nitrates as the starting materials, and regulating their amounts according to the nominal composition (Table 1). All reagents were obtained by Aldrich (≥ 99,0%). A citric

F. Basile et al.

472

acid/total metal nitrates molar ratio equal to 1.30 was used to avoid oxide or hydroxide segregation. The nitrate solution was added in drops to the citric acid solution, followed by evaporation to dryness at 90°C for 4h and gel decomposition at 180°C for 16h. Finally, the powder was calcined at different temperatures (450-950°C range) and times (2 or 12h). X-ray powder diffraction (XRPD) patterns were recorded using a Philips PW1050/81 diffractometer (Cu-Kα - Ni filtered, λ = 0.15418nm), investigating a 2θ range from 10° to 80°. Temperature-programmed reduction/oxidation/reduction analyses (TPR/TPO/TPR) were performed in a Thermo Quest CE TPDRO 1100 instrument; after a pre-heating step in He at 150°C for 15min to eliminate weaklyadsorbed species, samples were cooled and reduced (or oxidized) by heating from 60 to 650°C (10°C/min), with a final isothermal step of 1h. BET surface area was determined by a Carlo Erba Sorpty 1750 instrument, after a preliminary degassing step under vacuum at 200°C. Catalysts (30-40 mesh sizes) were charged in the reactor (INCOLOY 800HT) in the isothermal zone between two corundum layers. Before tests, catalysts were reduced for 2h, by progressively increasing both the temperature (to 300°C) and H2 content. The catalytic activity was investigated as a function of the temperature (250400°C), operating at 1.5MPa, with the steam / dry gas ratio equal to 0.55 (v/v) and a contact time (τ) of 0.50sec. Table 1. Composition of the investigated catalysts. Sample

La

Ce

Fe

Cr

Cu

Formula

LF LFC-Cu LFC-Ce

1 1 0.9

0.1

1 0.9 0.9

0.08 0.1

0.02 -

LaFeO3 LaFe0.9Cr0.08Cu0.02O3 La0.9Ce0.1Fe0.9Cr0.1O3

Figure 1. XRPD pattern of the LF sample calcined for 12h at different temperatures.

3. Results and discussion Figure 1 shows that an amorphous phase is also present after calcination up to 550°C; that phase disappears completely at 650°C without any further change in the pattern of sample calcined at 900°C. 650°C may therefore be considered a good compromise to obtain crystalline PVK phases [6], with acceptable surface area values (Table 2). All the investigated samples mainly contain mesopores (2-50nm) and the decrease in the surface area values is more evident for the LF sample, thus evidencing the thermal

Perovskite-type catalysts for the WGSR

473

stabilizing effect of Cr. Before and after the reaction, all the samples peak at the PVK phase (LaFeO3) [6], while those of Cu- or Ce-containing side phases were not detected, although their presence either as amorphous phases or as an undetectable amount cannot be excluded. The crystallite size calculated on the peak at about 32.3°, the peak corresponding to the crystallographic plane (121), indicates a high crystallinity (Table 2) in line with low surface area values. Moreover, the crystal size values do not show significant increases after reaction. Table 2. Bulk and surface data as a function of the calcination temperature. Sample LF-650 LF-450 LFC-Cu650 LFC-Ce650

Calcinat. temp (°C) 650 450 650 650

Phase Fresh PVK PVK PVK PVK

Crystal size (nm) Spent PVK PVK PVK PVK

Fresh 42 47 54 49

Spent 52 54 56 52

SBET (m2/g) Fresh 14.0 9.0 9.0 12.0

Spent 9.0 7.0 7.0 10.0

The properties of the PVK samples were investigated by TPR/TPO/TPR tests. The first TPR profile of the LF-650 sample shows two overlapped H2 consumption peaks, with maxima at ca. 360 and 540°C (Fig. 2). Since La3+ is not reducible in these conditions, these peaks may be ascribed to the reduction of Fe3+ ions, which are present respectively on the surface or in the bulk [7]. Following oxidation, it is possible to observe an intense reduction peak at 480°C with two shoulders at a lower temperature. The shoulder at 320°C may be attributed to the reduction of Fe4+ ions formed during the oxidation step [8], whereas the second one may be attributed to the reduction of surface Fe3+ ions. The reduction of bulk Fe3+ ions is shifted at 480°C, i.e. at a lower temperature than in the first TPR run. Finally, it must be noted that a decrease in both the calcination temperature (450°C) and the time (2h) (Fig. 2) significantly improves the reducibility of the Fe3+ ions.

Figure 2. TPR profiles of LF-650 and LF-450 samples, before or after oxidation.

In the LFC-Cu650 PVK sample (Fig. 3A), the first H2 consumption peak (ranging from 200-350°C) may be attributed to the reduction of Cu2+ ions, that occurs in two steps (Cu2+ → Cu+ and Cu+ → Cu0) with comparable rates. The reduction peaks of Fe3+ ions are shifted to lower temperatures (340 and 510°C) if compared to the LaFeO3 PVK, suggesting that Cu0 may promote the reduction of Fe3+ ions. Cr3+ ions are not reduced under the applied conditions, therefore the shoulder at 620°C ca. may be attributed to the reduction of well-dispersed La2CrO6 (formed in the synthesis) to La2O3 and stable LaCrO3; the latter does not oxidize in the oxidation step, since the peak disappears in the second reduction [9]. Finally, the LFC-Ce650 PVK phase (Fig. 3B) shows a first peak

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attributable to the reduction of Fe3+ ions present on the surface, whereas the peak at 490°C is related to the Ce4+ → Ce3+ reaction [10]. Finally, the broad peak with its maximum at 610°C may be attributed to the overlapping of the reduction of the bulk Fe3+ ions and chromates. After the oxidation step, a significant shift towards lower temperatures may be observed for all the reduction peaks.

Figure 3. TPR profiles of LFC-Cu650 (A) and LFC-Ce650 (B) samples, before or after oxidation.

The LF-650 sample demonstrates low activity at medium temperatures, reaching a CO conversion value of 30% ca. at 400°C, as was previously reported [3]. The partial substitution with Cr3+, Cu2+ or Ce4+ ions does not appear to have positive effects on catalytic performances, with CO conversion values ≤ 8%, also after a further reduction step, by raising the temperature to 500°C. The decrease in the calcination temperature (450°C) and time (2h) for the LF sample (i.e. starting from samples containing PVK and amorphous phases (LF-450)) significantly improves the sample’s reducibility, but does not seem to have any positive effect on the activity, with a CO conversion value of 15% at 400°C. Therefore, preliminarily, Fe-based PVK catalysts do not seem able to approach the interesting data previously reported for other PVK compositions [4].

4. References M.V. Twigg, 1989, The Water-gas Shift Reaction, Catalyst Handbook, 2nd ed., Wolfe, London, 283-339. [2] M.A. Peňa, J.L.G. Fierro, 2001, Chemical structure and performances of perovskite oxides, Chem. Rev. 101, 1981-2017. [3] M.J. Koponen, T. Venäläinen; M. Suvanto, K. Kallinen; T.-J.J. Kinnunen, M. Härkönen, T.A. Pakkanen, 2006, Water gas shift reaction studies on 2% Pd/AM1-xFexO3 (A= Ba, La, Pr; x= 0.4, 0.6) perovskites, Appl. Catal. A311, 79-85. [4] S.S. Maluf, E.M. Assaf, 2009, La2-xCexCu1-yZnyO4 perovskites for high temperature watergas shift reaction, J. Natur. Gas Chem. 18, 131-138. [5] Z. Liu, M.-F. Han, W.-T. Miao, 2007, Preparation and characterization of graded cathode La0.6Sr0.4Co0.2Fe0.8O3, J. Powder Sources 117, 837-841. [6] International Center for Diffraction Data, 1991, JCPDS Inorganic Files, Swarthmore, USA. [7] R. Zhang, H. Alamdari, S. Kaliaguine, 2006, Fe-based perovskites substituted by copper and palaldium for NO+CO reaction, J. Catal. 242, 241-253. [8] P. Ciambelli, S. Cimino, L. Lisi, M. Faticanti, G. Minelli, I. Petitti, P. Porta, 2001, La, Ca and Fe oxide perowskites: preparation, characterization and catalytic properties for methane combustion, Appl. Catal. B29, 239-250. [9] S. Ifrah, A. Kaddouri, P. Gelin, G. Bergeret, 2007, On the effect of La-Cr-O-phase composition on diesel soot catalytic combustion, Catal. Commun. 8, 2257-2262. [10] S. Ricote, G. Jacobs, M. Milling, Y. Ji, P.M. Patterson, B.H. Davis, 2006, Low temperature water-gas shift: Characterization and testing of binary mixed oxides of ceria and zirconia promoted with Pt, Appl. Catal. A303, 35-47. [1]

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Evaluation of different methods to prepare the Fe2O3/MoO3 catalyst used for selective oxidation of methanol to formaldehyde Karim H. Hassana,*, Philip C.H. Mitchellb a b

Department of Chemistry, College of Science, University of Diyala, Baqubq, Iraq School of Chemistry, University of Reading, Reading, RG6 6AD, UK

Abstract Different Fe2O3/MoO3 catalysts were prepared by kneading, precipitation and co-precipitation methods. Their activities and selectivities in the oxidation of methanol to formaldehyde were compared with those of a commercial catalyst. The iron(III) molybdate catalyst prepared by co-precipitation and filtration had a selectivity towards formaldehyde in methanol oxidation comparable with a commercial catalyst; maximum selectivity (82.3%) was obtained at 573 K when the conversion was 59.7%. Catalysts prepared by reacting iron(III) and molybdate by kneading or precipitation followed by evaporation, omitting a filtration stage, were less active and less selective. Keywords: Iron molybdate catalyst, Selective catalytic oxidation, Catalysts preparation

1. Introduction Formaldehyde, CH2O, is manufactured by the selective oxidation of methanol over a silver [1,2] or iron molybdate catalyst [3,,4]. Iron molybdate catalyst is a combination of the two oxides that produces the desired active and selective catalyst. Iron(III) oxide by itself is unselective producing carbon dioxide and water; molybdenum trioxide is selective but with low activity [5]. The overall reaction is CH3OH + 0.5O2 = CH2O + H2O. The oxidation reaction is exothermic (∆H=-159 kJ mol–1) and proceeds through reaction of methanol with the molybdate surface [6]. The technical catalyst composition is ca.(80% MoO3 and 20% Fe2O3), equivalent to an iron mole fraction 0.31. Iron may be partially replaced by a promoter, e.g. chromium. The active catalyst is considered to be Fe2(MoO4)3. The excess of MoO3 is said variously to be required to ensure the stability of the catalyst towards loss of MoO3, to maintain the active species and to enhance the surface area. It should be stressed that catalyst structure depends also on other parameters such as metal loading, and drying and calcination temperatures. The method of preparation appears to have a significant impact on the activity and selectivity of the catalyst. There are some studies of different preparations, for example sol-gel catalysts vs co-precipitated catalysts [7]. The aim of the present work is to evaluate different methods of preparation of the iron molybdate catalysts and test the activities and selectivities in the oxidation of methanol to formaldehyde by comparing with those of a commercial catalyst.

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2. Experimental 2.1. Catalyst preparation Ammonium heptamolybdate, (NH4)6Mo7O24.4H2O; and iron(III) nitrate nonahydrate, Fe(NO3)3 .9H2O, analytical grade (>99% purity), were used. 2.1.1. Kneading and evaporation: catalyst 1 Ammonium molybdate was added to the amount of water that is sufficient to obtain a homogeneous paste when added gradually to iron nitrate powder with continuous stirring. The paste was heated in an oven at 110oC for 2 h to evaporate water and calcined at 400–500°C in a current of air for 4 h. 2.1.2. Precipitation and concentration: catalyst 2 Solutions of ammonium heptamolybdate and iron (III) nitrate were mixed at pH of about 2. The precipitate formed was left to settle overnight at room temperature, the supernatant was decanted off and the precipitate dried and calcined as for catalyst 2. 2.1.3. Coprecipitation and filtration: catalyst 3 Solutions of ammonium heptamolybdate and iron(III) nitrate were prepared and mixed as in the preparation of catalyst 2. The precipitate was filtered off and washed several times with distilled water until the pH of the filtrate reached 7. The solid was dried and calcined as before. Pellets (or tablets) (7 mm diameter, 4 mm thick) were prepared in a tablet press at 2 atm. Using polyvinyl alcohol as binder and were calcined at 500oC .

2.2. Catalyst characterization and testing Iron and molybdenum were determined by standard atomic absorption methods. Pore volume, densities and hardness values were determined by the usual methods used in catalysts characterization. Activities and selectivities of the catalysts in the conversion of methanol to formaldehyde were determined in a continuous flow pilot plant described by Karim and Hummadi [8]. Test conditions were: reactor temperature 200 to 350°C (473–623 K); pressure, 10 atm (1013 kPa); flow rate, 15.858 cm3/s; methanol, 5.5% by volume in oxygen. Analysis of the reaction products was carried out periodically [9] after two hours of collection of the samples.

3. Results and discussion 3.1. Chemical composition and physical properties of the catalysts Catalyst 3 in its composition and physical properties is closest to the commercial catalyst. The most obvious difference between the different preparations is the excess MoO3, which is greatest for catalyst 3 (Table 1). Excess molybdenum (over the stoichiometric composition) appears to have little effect on the catalyst density. However, the two catalysts with the highest molybdenum (catalyst 3 and the commercial catalyst) have the greatest pore volumes and hardness.

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477

Table 1. Chemical composition and physical properties of the catalysts. Composition and Property

Catalyst Catalyst 1 Kneading and evaporation

Catalyst 2 Precipitation and concentration

Composition/wt. % Fe 17.2 15.6 Mo 50.2 51.7 Fe2O3 24.6 22.3 75.4 77.7 MoO3 MoO3 2.32 4.57 excess/wt.% Mo/Fe atomic 1.70 1.93 ratio Fe/(Fe + Mo) 0.371 0.341 mole fraction Colour Yellow green Yellow green Pellet size/cm 0.9×0.9 0.9×0.9 0.28 0.30 Pore volume/ cm3 g-1 0.53 0.50 Solid density/g cm-3 1.05 1.05 Bulk density/g cm-3 Hardness/105 1.70 1.63 dyne a Received from the Ministry of Industry of Iraq.

Catalyst 3 Coprecipitation and filtration

Commerciala

13.8 53.4 19.7 80.2 7.12

14.0 53.0 20.0 80.0 6.52

2.25

2.20

0.308

0.312

Yellow 0.9×0.9 0.40

Yellow 0.45×0.4 0.35

0.52

0.50

1.05

1.10

2.1

2.3

3.2. Catalytic properties: activities and selectivities in the oxidation of methanol to formaldehyde Activities and selectivities are shown plotted vs temperature in Fig. 1. The behavior of our co-precipitated catalyst (catalyst 3) is similar to that of the commercial catalyst. The activities of all catalysts rise with rising temperature and converge to roughly the same conversion at 598 K. The significant distinction between the catalysts is in the selectivity which passes through a maximum at 573 K with the commercial and the co-precipitated catalysts having the highest selectivities. Our catalytic results are consistent with the literature [3,4], with activities tending to the same value independently of the iron (or molybdenum) content of the catalyst and selectivities passing through a maximum with increasing reaction temperature. We discuss now how the activity and selectivity depend on the composition of the catalyst with reference to our results and literature data [10]. Generally the effect of composition has been discussed in terms of excess of MoO3. However, since MoO3 is in excess it would seem logical to express the variation of catalyst composition in terms of iron added (or not) to molybdenum, i.e. the Fe/Mo ratio or the Fe/(Fe+Mo) mole fraction as for other two-component catalysts, for example, the cobalt-promoted molybdenum disulfide based hydrodesulphurization catalyst. Unfortunately most researchers have not studied a wide range of Fe/Mo compositions (and we are no exception). However, we can combine certain patent literature data [10] with our data and thereby examine a wider range of compositions. Activities and selectivities values (figure and data can be obtained from authors). For

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the activities we see a typical volcano curve, the activity rising to a maximum value as iron is added to MoO3 and then dropping off. This behavior demonstrates synergy between iron and molybdenum. Beyond an iron mole fraction of 0.4 the activity begins to increase as Fe2O3 takes over. The selectivity to formaldehyde is more or less constant until an iron mole fraction of ca 0.3 is reached. The selectivity then drops as Fe2O3 becomes dominant. This behavior tells us that the selective catalyst is MoO3 and iron is an activity promoter. 65

0.85

0.75

55 selectivity

conversion/%

60

50

0.65 0.55

45 commercial catalyst catalyst 1 kneading catalyst 2 precipitation catalyst 3 co-precipitation

40 35 450

500

550 T /K

600

650

0.45

0.35 450

500

550 T /K

600

650

Fig. 1. Conversion (left) and selectivity (right) in conversion of methanol to formaldehyde.

References [1]

J.L. Li, W.L. Dai, K. Dong and J.F. Domg, (2000) “A new silver–containing ceramics for catalytic oxidation of methanol to formaldehyde”, Materials Letters,44 (3-4), 233-236. [2] I.E. Wachs, (2003) “Extending surface science studies to industrial reaction conditions; mechanism and kinetics of methanol oxidation over silver surface”, Surface Science, 544, 1-4. [3] A.P.V. Soares, M.F. Portela, A. Kiennemann, L. Hilaire and J.M.M. Millet (2001) “Iron molybdate catalysts for methanol to formaldehyde oxidation; effect of Mo excess on catalystic behaviour”, Applied catalysis, 206, 221-229. [4] K. Ivanov and I. Mitov, (2000) “Selective oxidation of methanol on Fe-Mo-W catalysts”, Journal of Alloys and Compounds, 309(14) 57-60. [5] C.T. Wang and R.J. Willey, (2001) “Mechanistic aspects of methanol partial oxidation over supported iron oxide aerogel”, Journal of Catalysis, 202(2)211-219. [6] E.M. McCarron and A.W. Sleight, in P.C.H. Mitchell and A.G. Sykes (eds.), The Chemistry and Uses of Molybdenum, Proceedings of the Climax Fifth International Conference, Polyhedron Symposia-, Number 2, Pergamon Press, Oxford, 1986, p.129. [7] A.P.V. Soares, M.F. Portela, and A. Kiennemann, Third World Congress on Oxidation Catalysis, By Grasselli, R.K; Oyama, S.T; Gaffney, A.M, Published by Elsevier, (1997), 110, 807-816. [8] K.H. Hassan and K.K. Hummadi, (2004), “Production of formaldehyde by catalytic conversion of methanol”, Iraqi Journal of Chemical and Petroleum Engineering, 5, 33-39. [9] D. Monti ,A. Reller and A. Baiker ,(1985) “Methanol oxidation on K2SO4- promoted vanadium pentoxide catalysts”, Journal of Catalysis, 93, 360-367. [10] I.E. Wachs and L.E. Briand, US Patent 6037290 (2000) to Lehigh University, Bethlehem, PA, USA.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Formation of active component of MoVTeNb oxide catalyst for selective oxidation and ammoxidation of propane and ethane E.V. Ischenko, T.V. Andrushkevich, G.Ya. Popova, V.M. Bondareva, Y.A. Chesalov, T.Yu. Kardash, L.M. Plyasova, L.S. Dovlitova, A.V. Ischenko Boreskov Institute of Catalysis SB RAS, Prosp. Ak. Lavrentieva 5, Novosibirsk, Russia

Abstract The effect of slurry pH on the formation of active component of MoVTeNbO catalyst for selective (amm)oxidation of ethane and propane has been studied. pH affects the nature and composition of the crude and dry precursors as well as chemical and phase composition of the final catalyst. The most effective catalyst is prepared at рН=3.0, which is characterized by a maximum content of M1 phase. Keywords: MoVTeNb mixed oxide; M1, M2 phase; ethane; propane (amm)oxidation

1. Introduction The most effective catalysts to date in the propane and ethane (amm)oxidation are MoVTeNbO ones reported by Ushikubo et al. [1]. Their catalytic properties are determined by the presence of orthorhombic M1 and hexagonal M2 phases [1]. The goal of the present work is to study the effect of slurry pH on the phase formation of MoVTeNbO catalyst.

2. Experimental Mo1V0.3Te0.23Nb0.12On catalysts were synthesized from aqueous slurry according to the patented procedure [1]. 34.267g ammonium heptamolybdate (NH4)6Mo7O24*4H2O, 6.811g ammonium metavanadate NH4VO3 (Reachem, Russia) and 10.247g telluric acid H6TeO6 (Aldrich) were dissolved in 300 ml of water under stirring at 80°C to obtain a uniform aqueous solution (pH ~ 6). Then, upon adding a 50.7 ml niobium oxalate solution (42.7 мг/мл Nb, pH ~ 1) to MoVTe solution at 30°C, the pH drops from 6 to 3, and a bright-orange gel forms. Niobium oxalate solution (C2O42–/Nb = 3/1) was synthesized by the interaction of oxalic acid and made-up niobium hydroxide that was prepared by alkaline hydrolysis of NbCl5 (Acros Organics, 99.8%). Thereafter a lab spray-dryer (Buchi-290, Tinlet = 220°C and Toutlet = 110°C) was used for fast drying of crude precursors. The resulting powders were calcined stepwise in air flow at 320°C shortly and then in He flow at 600°C for 2 h. pH of Mo1V0.3Te0.23Nb0.12On slurry was varied from 1 to 4 by adding HNO3 or NH4OH. The samples prepared with HNO3 were calcined in He flow only. XRD, IR, Raman and atomic absorption spectroscopy, HTEM and differential dissolution (DD) [2] methods were used for characterization of the samples. The propane (amm)oxidation and oxidative dehydrogenation of ethane were carried out in a fixed-bed tubular reactor with on-line chromatographic analysis. Experiments were performed at 380-420°C with the feed consisting of 5%C3H8, 30%H2O, 65% air, 5% C3H8, 6% NH3, 89% air and 30% C2H6, 30% O2, 40% N2 (% mol.).

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3. Results and discussion 3.1. Precursor characterization The following complexes were identified in the ternary MoVIVVTeVI solutions by NMR spectroscopy [3] at pH = 6: TeMo6O246– (the main one), Te-containing metavanadate derivatives with the average Te/V molar ratio of ca. 1 : 3, and small amounts of TeMo3V3O249–, MoO42– and telluric acid. Upon adding niobium oxalate to the MoVTe solution, the pH drops to 3 and a gel-like four-component material form. At that, MoO42– and mixed TeV and TeMoV complexes disappear, while a new TeMo3V5O275– complex forms. TeMo6O246– (HPA) remains the main complex in both the solution and gel, as evidenced by Raman spectroscopy data [4]. The solid precursor obtained upon drying is amorphous to X-rays (Fig. 1, pH=3-3.5). A FT-IR spectrum indicates the presence of an Anderson-type anion in the dried precursor [5] (Fig. 2). At pH = 4, the

pH=1

10

20

30

40

2 Theta,

o

50

60

Fig.1. Fig.1. XRD patter pattern of the pr preecurso ursor sors.

330

550

685

927 894

460

pH=3 375

pH=2

pH=4 pH=3.5 540

pH=3.5 pH=3

Absorbance

pH=4

930 898

x

795

x

934 904

1714 1670

x

x x

1408

x - (NH4)6TeMo6O24

x

670 620

x

2000 1600

1000

pH=2 pH=1

800

600 -1

400

Wavenumber,cm

Fig. 2. FT-IR FT-IR spec spectra of the pr preecurso ursors. so

(NH4)6TeMo6O24 phase is detected by XRD (Fig. 1) and FT-IR spectroscopy (Fig. 2). A decrease in the slurry pH leads to partial decomposition of HPA, which is indicated by decreasing intensity of the absorption bands assigned to heteropolyanion in the IR spectra of precursors (Fig. 2, pH= 1, 2). At that, a new peak at 2Θ = 22.2o appears in the XRD patterns of precursors synthesized at pH = 1 or 2 (Fig.1). According to DD method, more than 85% wt. of the dried precursor prepared at pH = 3 dissolves in water (DW), the composition being Mo1V0.28Te0.17Nb0.04On. With regard to XRD and FTIR data, we suppose HPA to be the main building block of DW. At pH = 4, along with DW (70%) there are also Mo1Te0.8 and a MoVNb oxide compound enriched with niobium. At pH below 3, the amount of DW decreases to 50% and formation of the MoVNb compound enriched with molybdenum is observed. Heat treatment at 320°C results in destruction of heteropolyanion and formation of nanosize particles with the structure similar to that of M1 and M2 phases (Fig. 3).

3.2. Catalysts characterization The transformation mechanism of amorphous precursors to M1 and M2 phases in 220600oC range was demonstrated previously [6]. Final phase compositions of the catalysts form upon heat treatment at 600°C. XRD patterns of the samples after high-temperature treatment are shown in Fig. 4. The content of constituent phases was calculated by Rietveld refinements of X-ray powder diffraction data (Table 1). Phase composition of the calcined catalysts is determined by the composition of solid precursors. The dependences of the amount of M1 phase and DW on the slurry pH are quite similar (Fig. 5, curve 4, 5). At that, their maximum amount is observed at pH = 3.

Formation of the active component of MoVTeNb oxide catalyst

481

Decreasing or increasing the pH value leads to a decrease in the content of M1 phase and appearance of Mo5–x(V/Nb)xO14 and TeMo5O16. Simultaneously Te content in the heat-treated catalysts varies. Note, minimal Te content is fixed at pH = 3, when maximal M1 phase content and maximal catalysts surface area are observed. o

x

o-phase M1 x-phase M2 Δ-Mo5-x(V/Nb)xO14

x

- TeMo5O16 oo

ΔΔ Δ

Δ Δ

Δ

x

Δ

o

x

x

pH = 4.0

ΔΔ

ooo Δ o oo

o

ooo

o

pH = 3.5 pH = 3.0 pH = 2.0

oo 0

10

pH = 1.0 20

30

2 Theta,

Fig. 3. TEM image of the sample (pH=3) calcined at 350°C in air flow.

o

40

50

60

Fig. 4. XRD patterns of the catalysts.

Table 1. Influence of pH slurry on chemical and phase composition of the catalysts. Chemical composition of Phase composition the catalysts M2b Mo5-x(V/Nb)xO14c TeMo5O16d M1a 1 Mo1V0.29Te0.21Nb013 40 42 18 2 Mo1V0.28Te0.22Nb013 53 48 80 15 3 2 3 Mo1V0.28Te0.12Nb011 3.5 Mo1V0.28Te0.16Nb011 58 30 9 3 4 Mo1V0.28Te0.16Nb011 24 45 17 13 a - [JCPDS 58-790], b - [JCPDS 57-1099], c - [JCPDS 58-788], d - [JCPDS 31-874] pH slurry

3.3. Catalytic performance

Content, % wt.

Conv ersion, %

The catalytic properties are presented in Fig. 5 and Table 2. Dependences of activity on pH slurry for propane oxidation (curve 2), propane ammoxidation (curve 3) and ethane oxidative dehydrogenation (curve 1) as well as content of M1 phase (curve 4) have the same shape. For all the reactions, the maximum activity is observed over the 80 80 5 samples obtained at pH = 3.0 and containing the greatest amount of orthorhombic phase 60 60 M1. Table 2 shows selectivities to main reaction products. Acrylonitrile or acrylic 40 40 acid and propylene are main products of 4 propane (amm)oxidation, selectivity to 20 20 3 sum of propylene and acrylonitrile or 2 1 0 acrylic acid is close to 80%. Changes in 0 1 2 3 4 the ratio of selectivities to propylene and pH slurry Fig. 5. Dependences of ethane (1) and acrylonitrile or acrylic acid are related propan e (2,3) conversion as well as co ntent with different conversion of propane over of M1 (4) phase and DW (5) on pH slurry. the catalysts under consideration and are

E.V. Ischenko et al.

482

caused by the consecutive formation of acrylonitrile or acrylic acid via intermediate propylene [4]. Ethylene is a sole product of selective conversion in ethane oxidative dehydrogenation. The studied catalytic system is most selective in this reaction. Table 2. Influence of pH slurry on catalytic performance. pH

1.0 2.0 3.0 3.5 4.0

S, m2/g 2.6 3.9 9.6 8.2 4.1

X a, %

S, %

Xb, %

S, %

S, %

X c, %

S, %

S, %

C2H6 17.0 16.9 46.8 23.9 2.0

C2H4 92.9 95.1 93.9 95.0 81.7

C3H8 8.2 13.1 60.0 27.5 6.1

C3H6 51.5 31.8 2.3 23.1 49.0

C3H4O2 30.1 52.0 65.0 48.3 21.4

C3H8 18.4 32.9 68.8 42.9 12.1

C3H6 20.7 10.6 6.3 7.7 33.4

C3H3N 48.8 59.2 68.8 66.0 41.1

a

30% C2H6, 30% O2, 40% N2,emperature 400°C, contact time 2.4 s; b 5%C3H8, 30%H2O, 65% air, temperature 380°C,contact time 2.2 s; c 5% C3H8, 6% NH3, 89% air; temperature 420°C, contact time 2.4 s. X – conversion degree, S - selectivity

4. Conclusion Formation of the active component of MoVTeNb oxide catalysts includes the following stages: (i) formation of compounds with heteropolyanion (HPA) structure at the step of initial solutions mixture, (ii) drying of the wet precursor with the HPA structure being retained, (iii) decomposition of the dried precursor upon low-temperature treatment leading to amorphous product that contains nanosize particles with the structure and composition close to those of M1 and M2 phases, and (iv) crystallization of M1 and M2 phases upon calcination in helium flow at 600°C. The slurry pH affects the nature and composition of the crude and dry precursors as well as the chemical composition and M1/M2 ratio of the final catalyst. The most effective catalyst is prepared at рН=3.0 when a maximum content of M1 phase is formed.

Acknowledgements The authors are grateful to the Federal Agency for Science and Innovations for financial support.

References 1. 2. 3. 4. 5. 6.

T. Ushikubo, A. Oshima, T. Ihara, H. Amatsu, 1995, US Patent 5.380.933. V.V. Malakhov, I.G. Vasilyeva, 2008, Russ. Chem. Rev., Vol. 77, P. 350-372. R.I. Maksimovskaya, V.M. Bondareva, G.A. Aleshina, 2008, Eur. J. Inorg. Chem., No 11, P. 4906-4914. G.Ya.Popova, T.V. Andrushkevich, Yu.A. Chesalov, L.M. Plyasova, L.S Dovlitova, E.V. Ischenko, G.I. Aleshina, M.I. Khramov, 2009, Catalysis Today, Vol. 144, p. 312-317. Evans, Jr.H.T. 1968 J. Am. Chem. Soc. V. 90. No 12. P. 3275-3276. G.Ya.Popova, T.V. Andrushkevich, L.S Dovlitova, G.I. Aleshina,Yu.A. Chesalov, A.V. Ischenko, E.V. Ischenko, L.M. Plyasova, V.V. Malahov 2009, Appl. Catal. A: Gen. Vol. 353, p. 249-257.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Functionalization of carbon nanofibers coated on cordierite monoliths by oxidative treatment Sabino Armenisea,b, Marcos Nebraa, Enrique García-Bordejéa, Antonio Monzónb a

Instituto de Carboquímica, C/Miguel Luesma Castan,4, 50018, Zaragoza, Spain. Departamento de Ingeniería Química y Medio Ambiente, Universidad de Zaragoza, C/Pedro Cerbuna, 50009, Zaragoza, Spain

b

Abstract Carbon nanofiber coating on cordierite monoliths has been functionalized by oxidation treatments conventionally used for carbon materials. The functionalized CNF-monoliths have been characterized to assess the impact of the different oxidizing treatments on the surface chemistry, texture and CNF coating adhesion. Keywords: carbon nanofibers, monoliths, functionalization, TPD, adhesion

1. Introduction Structured reactors such as monoliths have several obvious advantages over the traditional reactor randomly filled with catalyst particles [1]. They have lower pressure drop, uniform flow distributions, less hot-spot formation and uniform residence times. However, these catalytic systems have low surface areas, making it necessary to incorporate some phase like alumina or silica, which increases the surface area. New types of structured catalysts and reactors, which in addition to high surface area, have large pore volume (mesoporous range) and without the presence of micropores, based on carbon nanofibers (CNFs) are being investigated as catalytic support [2,3]. To anchor and disperse active metal phase on CNFs, usually the CNF surface is previously submitted to oxidation treatments which generates surface oxygen complexes. Most of the literature focuses on the study of the functionalization of unsupported carbon nanofibers and carbon nanotubes [4] but works reporting the functionalization of CNF grown on structured reactors such as cordierite monoliths are scarce [5]. Here we have functionalized CNF-coated cordierite monoliths using two conventional oxidation treatments, viz. HNO3 and H2O2, at different conditions of concentration, temperature and durations. We have characterized the monoliths after functionalization to assess the affect of oxidation treatment on the generated oxygen complexes and on the adhesion strength of nanocarbonaceous materials to the structured support. Work is underway to study other unconventional oxidative treatments and supporting Pd catalyst for hydrogenation in liquid phase reactions.

2. Experimental CNF on cordierite monoliths were prepared as reported elsewhere [6]. In brief, first cordierite monoliths were coated with γ-alumina by dipcoating with a sol. Subsequently, Ni was dispersed by ion exchange. Finally, the monolithic catalyst was reduced in a H2 flow at 823 K and CNF growth was carried out at 873 K using C2H6 as the carbon source.

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The CNF-monoliths were functionalized with HNO3 and H2O2 at different conditions. The identification of the most representative samples is displayed in Table 1. AD, AC and HP stands for treatment with diluted 1M HNO3, concentrated HNO3 and H2O2, respectively. T means that the functionalization has been carried out at boiling temperature. The number is the duration of the treatment in hours. The CNF-monoliths after functionalization were characterized by different techniques. Nitrogen adsorption was performed on a Micromeritics ASAP 2020 at 77 K, SEM study was performed with a Hitachi s4300 field emission microscope. TPD was carried out in a quartz microreactor and the gas analyzed with a Pfeiffer mass spectrometer. The adhesion was characterized by ultrasound (frequency 40 KHz) and the metal leaching by ICP-OES. Table1. Code sample identification, textural characterization CNF/Monolith composite and total amount of CO2 and CO calculated from the TPD for different oxidation treatments. Code Sample

Oxidation Treatments

S(BET) (m2/g carbon)

V(pore) (cm3/g carbon)

Amount evolved µmol /g carbon CO2 CO 144 640

CNF(untreated) 150,3 0,30 * AC-1T HNO3 (65%) Refluxed 1h to 353 K 152,8 0,31 326 1310 AC-5T HNO3 (65%) Refluxed 1h to 353 K 246,8 0,47 884 2002 AD-1T HNO3 (1M) Refluxed 1h to 353 K 151,7 0,41 650 1810 AD-5T HNO3 (1M) Refluxed 1h to 353 K 190,5 0,45 460 1296 AD-1 HNO3 (1M) 1h to RT 148,8 0,33 214 1214 AD-5 HNO3 (1M) 1h to RT 154,8 0,37 253 1775 HP-20 H2O2 (30%) 20h to RT 161,9 0,38 232 578 * Surface area and micropore volume calculated by t-plot method (10m2/g carbon and 0,008cm3/g carbon)

3. Results 3.1. Textural properties of the functionalized CNF-monoliths Textural properties of the samples were investigated by nitrogen adsorption and SEM. Table 1 shows the results of surface area and pore volume. Figure 1 shows SEM images of samples before (CNF-untreated) and after treatment with H2O2. Generally speaking, at the same magnification the samples after treatment show morphology more compact than before functionalization. Additionally, we can possibly see that the sample funcionalized with nitric acid presents a structure with greater porosity than the functionalized with hydrogen peroxide. The reason for that must be that the nitric acid is an oxidising agent more aggressive than hydrogen peroxide, which could eliminate any rest of ashes or non graphitic carbonaceous matter. Comparing all the treatments studied, it is apparent that as the severity of functionalization increases, the surface area increases, which is accompanied by an increase of the pore volume. The increase of the superficial area could be related to the generation of new pores or to the increase of the fibres roughness as reported by other authors [4].

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Figure 1. SEM images of carbon nanofibers. a) Untreated, b) treated with H2O2 (HP-20).

3.2. TPD characterization of surface chemistry TPD methodology and interpretation have followed previous results reported by Figueiredo et al. [7]. Figure 2 shows TPD traces of CNF/Monolith before and after some selected funcionalization treatments. The quantification of evolved CO and CO2 is displayed in table 1. CO2 spectra (Figure 2a) can be divided in two temperature ranges. First range at low temperature (373-673 K) exhibits two peaks. Those peaks are characteristic of carboxylic acid with different thermal stability. This difference in stability may be related to the groups surrounding the carboxylic carbon. Sample functionalized with hydrogen peroxide (HP-20) shows only one peak attributed to carboxylic acids evolving at slightly higher temperatures than for the nitric acid treated. The evolution of CO2 at higher temperature (750-1000 K) is attributed to carboxylic anhydride and lactone groups. Furthermore, from figure 2a we can observe that the increase of acid strength (AC-5T vs. AD-5) leads to increased formation of carboxylic acid groups and to the emergence of more basic groups ascribed to lactones. Figure 2b shows CO evolution which occurs in the temperature range between 750 K to 1173 K. It is possible to associate the peak around 750 K to anhydrides, the shoulder at 1050 K to phenol groups and finally last peak at 1170 K to carbonyl/quinone groups. The sample functionalized with hydrogen peroxide does not exhibit peaks referable to phenol but only one peak referable to quinone/carbonyl in almost similar amounts as for nitric acid treatment.

Figure 2. TPD spectra for the untreated CNF/Monolith and sample after functionalization treatment: (a) CO2 traces; (b) CO traces.

3.3. Testing of CNF coating adherence after functionalization by ultrasonication The impact of the different functionalization treatments on the adhesion strength of carbon nanofibers on ceramic monoliths has been investigated by means of ultrasonic treatment. Figure 3 a and b show the results of coating loss (% wt) of alumina, nickel and CNF, after 60 minutes of ultrasonic treatment. Figure 3a illustrates the impact of the

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temperature of functionalization on the adhesion strength of carbon nanofibers coating the monolith. After 60 minutes of treatment in ultrasound, approximately 30 wt% of the coating is removed in monoliths functionalized refluxed with 1M nitric acid during 1 h (AD-1T), whereas the functionalized at room temperature (AD-1) only entails a loss of 6 wt% of the coating weight. Figure 3b shows the effect of the type of oxidising agent and acid concentration on the coating adhesion. The samples treated with concentrated acid show a weight loss up to 42 wt% of the initial weight of the coating incorporated at the monolith. We can observe that after functionalization with hydrogen peroxide only 4% weight of coating is removed (figure 3b), similar to CNF without treatment. Possibly, only the outer fibers non-attached to the alumina are removed with the H2O2 treatment, whereas a deeper removal of nanofibers and support-alumina occurs when monoliths are functionalized with nitric acid. The leaching of metals from alumina coating and cordierite monoliths after the most aggressive oxidative treatment was confirmed by analysis of solution by ICP-OES.

40

50

a)

CNF (untreated) AD-1 AD-5 AD-1T AD-5T

45

(Al,Ni,CNF) Coating weight loss (%)

(Al,Ni,CNF) Coating weight loss (%)

50

35 30 25 20 15 10

b)

45

CNF (untreated) AD-1T AD-5T AC-1T AC-5T HP-20

40 35 30 25 20 15 10 5

5

0

0 0

10

20

30

40

Time in ultrasonic bath (min)

50

60

0

10

20

30

40

50

60

Time in ultrasonic bath (min)

Figure 3. Coating weight loss as a function of time of ultrasonication after different oxidation treatment. a) Effect of temperature. b) Effect of acid strength.

4. Conclusions CNF-coated cordierite monoliths have been functionalized with HNO3 and H2O2 at different conditions of concentration, time and temperature. All the tested treatments created oxygen functionalities on the CNFs as characterized by TPD. However, the treatment with concentrated HNO3 is always detrimental for the attachment of the coating. The same occurs when the treatment is carried out under reflux, irrespective of the oxidizing agent. The mechanical stability of CNF coating is preserved after the treatment with H2O2 or diluted HNO3 at room temperature.

Acknowledgement We acknowledge the European commission for financial support (contract 226347).

References [1] [2] [3] [4] [5]

A. Cybulski, J. A. Moulijn, Catal. Rev-Sci. Eng., 36(2) (1994) 179. K P. De Jong, J. W. Geus, Catal. Rev.-Sci. Eng., 42(4) (2000) 481. P. Serp, M. Corrias, P. Kalck, Applied Catal. A: General, 253 (2) (2003) 337. T.G. Ros, A.J. van Dillen, J. W. Geus, D. C. Koningsberger, Chem. Eur. J., 8 (5) (2002) 1151. S. Morales-Torres, A.F. Pérez-Cadenas, F. Kapteijn, F. Carrasco-Marín, F. J. MaldonadoHódar and J. A. Moulijn, Applied Catal. B: Environmental, 89 (3-4) (2009) 411. [6] E. Garcia-Bordeje, I. Kvande, D. Chen, M. Ronning, Advanced Materials, 18 (12) (2006) 1589. [7] J.L. Figueiredo, M.F.R. Pereira, M.M.A. Freitas, J. J. M. Órfão, Carbon 37 (9) (1999) 1379.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Synthesis of mesoporous silicas functionalized with trans (1R,2R)-diaminocyclohexane by sol-gel method F. Fakhfakh,a L. Baraket,a A. Ghorbela, J. M. Fraile,b J. A. Mayoralb a

Laboratoire de Chimie des Matériaux et Catalyse, Département de Chimie, Faculté des Sciences de Tunis, Campus universitaire, 2092 El Manar Tunis (Tunisie) b Departamento de Química Orgánica, Facultad de Ciencias. ICMA, Universidad de Zaragoza C.S.I.C. E-50009 Zaragoza (Spain)

Abstract N-[3-(Triethoxysilyl)propyl]-(-)-(1R,2R)-diaminocyclohexane was co-condensed with tetraethoxysilane (TEOS) under different synthesis conditions to obtain new functionalized hybrid silica materials. These materials were characterized by different spectroscopic methods. N2 sorption studies were used to confirm the mesoporous character of these functionalized materials. Keywords: trans-(1R,2R)-diaminocyclohexane, sol-gel, synthesis conditions

1. Introduction Recently the synthesis and the design of chiral mesoporous silica materials have attracted great interest because of their potential applications in heterogeneous asymmetric synthesis. The most important step in the preparation of these materials is the introduction of the chiral moiety in mesoporous silica. The co-condensation of two alkoxides is one of the most suitable methods, as it allows a homogeneous distribution of the organic groups on the materials. A wide variety of chiral groups has been incorporated into mesoporous silica, among them trans-(1R,2R)-diaminocyclohexane [1,2,3]. In this work, we report the synthesis of new trans-(1R,2R)-diaminocyclohexane functionalized mesoporous organosilica materials by sol-gel under different conditions.

2. Experimental 2.1. Synthesis of N-[3-(triethoxysilyl)propyl]-(-)-(1R,2R)-diaminocyclohexane (D) D was synthesized by reacting trans-(1R,2R)-diaminocyclohexane and (3-chloropropyl) triethoxysilane under microwave heating on a CEM Discover apparatus. The reaction was achieved at 140°C for 45min. The yield was 52%. The obtained chiral molecule was characterized by FT-IR, 1H and 13C NMR spectroscopies. The resulting spectra are in accordance with those found in previous works [1,2].

2.2. Synthesis of functionalized organosilica materials The synthesis were conducted at 35°C either in water or propanol as a solvent, with HCl, CH3COOH or C2H5COOH as a catalyst. In propanol, the molar composition of the mixture was TEOS:D:propanol:acid:water = 1:0.018:6.5:1:6. A solution of 0.04 mol of TEOS, 0.018 mol of D and 20 mL of propanol was stirred for 30 min followed by the addition of the acid. Two hours later, the desired amount of H2O was added. In water, the molar composition was TEOS:D:acid:water = 1:0.018:1:27. In a typical synthesis 0.04 mol of TEOS, 0.018 mol of D, 20 mL of water and the corresponding amount of

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acid were mixed together. In both cases the gel was kept in the reaction medium for an additional period of 24 h. After that, it was aged in a Teflon-lined autoclave at 120°C for 24 h, then oven dried at 120°C for 24h.

2.3. Characterization N2 physisorption, FT-IR, 29Si and 13C-CP-MAS-NMR spectroscopies were carried out as previously described [4]. 1H and 13C NMR spectra in solution were recorded on a Bruker AC-300 spectrometer.

3. Results and discussion 3.1. Preparation and textural characterization of silica materials containing trans-(1R,2R)-diaminocyclohexane The synthesis of hybrid organosilica containing trans-(1R,2R)-diaminocyclohexane was achieved D-S1 by sol-gel under different synthesis conditions. It was conducted either in water or in propanol as D-S2 solvent in the presence of an acid catalyst: HCl, CH3COOH or C2H5COOH. The choice D-S5 of propanol and a carboxylic acid was based on previous results dealing with the synthesis Relative pressure (P/P°) of ethylenediamine-functionalized mesoporous silica materials by the co-condensation method [4], showing that this combination favors the formation of functionalized materials with developed textural properties. The synthesis conditions D-S3 of the functionalized silica and their textural D-S4 properties are listed in Table 1. The N2 isotherms are represented in Figure 1. As can be seen, all Relative Pressure (P/P°) the trans-(1R,2R)-diaminocyclohexane functionFigure 1. N 2 isotherms of silica conalized materials exhibit a type IV isotherm sugtaining trans-(1R,2R)-diaminocyclogesting the formation of mesoporous materials. hexane. N2 isotherms of the samples D-S1, D-S2 and DS5 are similar. They show a H2 hysteresis loop type, indicative of the development of ink-bottle pores. 900

3 Adsorbed volume (cm /g)

800 700 600 500 400 300 200 100 0

0,0

0,2

0,4

0,6

0,8

0,0

0,2

0,4

0,6

0,8

1,0

1000

3

Adsorbed Volume (cm /g)

900 800 700 600 500 400 300 200 100

0

1,0

Table 1. Synthesis conditions and textural properties of the functionalized silicas. Sample

Solvent

Catalyst

SBET (m²/g)

Dmax (Å)b

D-S1

water

HCl

659

35

0

0.64

D-S2

propanol

HCl

663

35

0.05

0.45

D-S3

propanol

CH3COOH

124

n.d.a

0

0.49

252

a

D-S4 D-S5

propanol water

C2H5COOH C2H5COOH

466

n.d. 74

Vmic Vp (cm3/g)c (cm3/g)d

0.01

0.84

a

0.91

n.d.

a

n.d. :not determined. Maximum of the pore size distribution. c Microporous volume calculated by t-plot method. d Total pore volume at P/P0=0.95. b

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In the case of the samples D-S1 and D-S5 prepared in water with HCl and C2H5COOH respectively, the N2 isotherms show a large hysteresis loop which occurs under a wide range of relative pressure values. For D-S1, they are ranged between 0.37 and 0.89 and for D-S5 P/P° values varied between 0.55 and 0.94. As a consequence both solids show high total pore volume (0.64cm3/g for D-S1 and 0.91 for D-S5). In the case of the synthesis in propanol with HCl (sample D-S2), the total pore volume is reduced to 0.45cm3/g, confirmed by the narrow hysteresis loop, occurring under low relative pressure values (0.38-0.74). The pore size distribution of these solids (not shown) is narrow and monomodal with a maximum corresponding to 35Ǻ for D-S1 and D-S2 and 74Ǻ for D-S5. Table 1 reveals that the surface area for these solids is high (466 -663 m²/g). On the other hand, N2 isotherms of the samples D-S3 and D-S4 prepared in propanol, are similar. They exhibit a H1 hysteresis loop type, which occurs at high relative pressure values, indicating the formation of large and uniform cylindrical pores. The pore size distribution (not shown) reveals the formation of large mesopores which maximum could not be determined. Surface areas are lower than those of the samples prepared in water, 124 m²/g for D-S3 and 252 m²/g for D-S4. From these results, it can be seen that D-S1, D-S2 and D-S5 have similar textural properties, probably due to the similar mechanism of formation, as proposed by Brinker [5], consequence of the use of water as solvent or a strong mineral acid in alcoholic solution. Samples D-S3 and D-S4 show similar textural properties, but markedly different from D-S1, D-S2 and D-S5. The use of carboxylic acids in propanol might modify the mechanism of the sol-gel process. As reported before [4], carboxylic acids may modify the alkoxydes structure (TEOS and D), generating different types of precursors. The different hydrolysis rate of each precursor under sol-gel conditions would modify the textural properties. Jiang et al. [2] prepared the trans-(1R,2R)-diaminocyclohexane functionalized material by the co-condensation of TEOS and D under basic conditions, obtaining a porosity of 25Ǻ with a surface area of 890m²/g. Bied et al. [3] prepared the same kind of material by the co-condensation in water in presence of a primary amine. The chiral solids are generally microporous with a surface ranged between 600 and 1170m²/g. From the results of these works, it is worth to note that the porosities of the trans(1R,2R)-diaminocyclohexane functionalized materials are less developed than those prepared in this work. Thus the acidic conditions used here are favorable to obtain large porosities.

3.2. Spectroscopic characterization of the hybrid materials FT-IR spectra of hybrid organosilica D-S4 materials are collected in Figure 2. The spectra show the presence of typical D-S3 silica bands relative to the inorganic backbone. The vibration bands of D-S2 siloxane groups appear at about 464, D-S1 804 and 1080 cm–1. The bands at 960 –1 and 1650cm are respectively associated to the frequency of Si-OH bending and to the angular vibration of water mole4 00 0 35 0 0 30 0 0 2500 2000 1500 1000 500 -1 cules bonded to the framework. The W avenum bers (cm ) Figure 2. FT-IR spectra of chiral solids. large band centered at 3500 cm–1 is relative to OH stretching frequency of the silanol groups in the inorganic framework. The spectra show also vibrations at 2960 and 2850cm–1 which are assigned to C-H

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stretching of the organic moiety D. The -57 band at 1450cm–1 is the characteristic C-H vibration of cyclohexane of the -65 trans-(1R,2R)-diaminocyclohexane group. The N-H and N-H2 vibration bands overlap with ν(O-H) bands at 3500 cm–1 and 1630 cm–1. D-S1 NMR spectra of the different mateD-S4 rials are gathered in Figure 3 and 4. 29SiCP-MAS NMR spectra of the materials 0 -20 -40 -60 -80 -1 00 -1 20 -140 -16 0 -18 0 ppm D-S1 and D-S4 represented in Figure 3 29 show the presence of signals at −110, Figure 3. Si MAS NMR spectra of D-S1 and −101 and −92 ppm. These bands corres- D-S4. pond to Si(OSi)4 (Q4), (HO)Si(OSi)3 (Q3) 1, 4,7,8,9,10 and (HO)2Si(OSi)2 (Q2) silica species respectively. Moreover, the spectra show additional low-intensity signals at −65 and −57ppm. The signal at −65ppm is 2,5,6,11 3 assigned to CSi(OSi)3 (T3) species, whereas the signal at −57ppm is associated to C(OH)Si(OSi)2 (T2) species. The presence of these bands confirms that the chiral organic moiety is covalently bonded to (a) the silica framework. The 13C-CP-MAS70 60 50 40 30 20 10 0 NMR spectrum of D-S1, together with that ppm of the precursor in solution, are represented (b) in Figure 4. The spectrum of the solid shows prominent signals at 18 and 58 ppm, corresponding to Si-O-CH2-CH3 groups, not fully hydrolyzed in the sol-gel process. The signals in the range of 45-65 ppm, attributed to N-CH and N-CH2 groups, at 20-35 ppm corresponding to methylene 60 40 30 20 10 0 50 groups of diaminocyclohexane and propyl ppm backbones, and the signal at 10 ppm, 13 ascribed to CH2-Si, demonstrate the pre- Figure 4. C NMR spectra of (a) D-S1 and (b) the liquid precursor D. sence of the chiral moiety on the solid. Thus FT-IR and CP-MAS-NMR spectra confirm that the trans-(1R,2R)diaminocyclohexane moiety was successfully incorporated into the mesoporous silica framework. 5

7

6

4

8

9

11

10

3

NH

Si(OCH2CH3)3 2

1

NH2

4. Conclusion Trans-(1R,2R)-diaminocyclohexane functionalized silica materials were successfully prepared by sol-gel method through the co-condensation of TEOS and precursor D under acidic conditions. The use of propanol as a solvent and a carboxylic acid as a catalyst in the sol-gel process allows obtaining materials with large pores and moderate surface area, whereas the process in water leads to solids with high surface area and narrow pore distribution of around 35 Å. Due to their interesting textural properties,

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these mesoporous materials are promising as solid chiral ligands in enantioselective heterogeneous catalytic reactions.

References [1] A. Adima, J. J. E. Moreau, M. Wong Chi Man, Chirality 12 (2000) 411. [2] D. Jiang, Q. Yang, J. Yang, L. Zhang, G. Zhu, W. Su, C. Li, Chem. Mater. 17 (2005) 6154. [3] C. Bied, D. Gauthier, J. J. E. Moreau, M. Wong Chi Man, J. Sol-gel Sci. Technol. 20 (2001) 313. [4] F. Fakhfakh, L. Baraket, J. M. Fraile, J. A. Mayoral, A. Ghorbel, J. Sol-gel Sci. Technol. 52 (2009) 388. [5] C. J. Brinker, J. Non-Cryst. Solids 100 (1988) 31.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Physico-chemical and catalytic properties of effective nanostructured MnCeOx systems for environmental applications Francesco Arenaa,b,*, Giuseppe Trunfioa,§, Jacopo Negroa, Cettina Sajaa, Antonino Raneria, Lorenzo Spadaroa,b a

Dipartimento di Chimica Industriale e Ingegneria dei Materiali, Università degli Studi di Messina, V.le F. Stagno D’Alcontres 31, I-98166 Messina, ITALY b Istituto CNR-ITAE “Nicola Giordano”, Salita S. Lucia 5, I-98126 Messina, ITALY § current affiliation: Université de Franche-Comté, Chrono-environnement, UMR 6249 UFC/CNRS usc INRA, Place Leclerc 25030 Besançon Cedex, FRANCE

Abstract A synthesis route based on the occurrence of redox reactions between MnVII, MnII and CeIII precursors leads to nanostructured MnCeOx systems with high surface area (120250 m2/g) and a quasi-atomic dispersion of the active phase. Larger accessibility and higher oxidation state enhance the redox activity of the surface active Mn sites resulting in an improved mobility and availability of surface oxygen that greatly promotes the CO oxidation activity of the MnCeOx system at low temperature (20-150°C). Keywords: oxide catalyst; nanostructure; dispersion; oxygen mobility; redox activity

1. Introduction The increasing levels of industrial pollution is pressing worldwide a great research concern on catalytic technologies for the abatement of noxious organic pollutants in gas-exhausts and wastewaters, mostly based on total oxidation reactions. Although uncommon targets and reaction conditions hinder the assessment of general rules for catalysts requirements yet, according to principles of oxidation catalysis an enhanced mobility and availability of oxygen at the catalyst surface constitutes the basic condition for an effective conversion of organic substrates to carbon dioxide [1]. This explains the exploitation of noble-metal catalysts [1-3], whose high cost remains the main drawback before an extensive development of environmental catalytic processes. On this account, a great deal of research concern has been focused onto MnCeOx systems, as a viable less-costly alternative to noble-metal catalysts [1,2]. However, the preparation method is crucial for tuning the catalytic behaviour of the title system, since high surface exposure and higher oxidation state of the active Mn sites are the main catalyst requirements [1-5]. Therefore, in an attempt to improve the total oxidation catalytic performance, we designed an alternative synthesis route of the MnCeOx system based on redox reactions of suitable oxide precursors [1-5]. In fact, larger surface exposure and oxide dispersion promote the reducibility and the oxidative strength in comparison to the conventional co-precipitation method [1-5]. Therefore, this work shows some fundamental results documenting that the nanosized arrangement of the oxide domains in a wide range of the Mn loading (9-33 wt%) confers a superior electron and oxygen mobility to redox MnCeOx systems, enhancing the reactivity at low temperature (20-150°C) in the CO oxidation, taken as a “model” reaction.

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2. Experimental 2.1. Catalysts MnCeOx catalysts with different Mn-to-Ce atomic ratio (Mn/Ce) ranging between 0.33 and 2 were prepared via the “redox” route, consisting in the titration of a KMnO4 solution at ca. 60°C under stirring with an aqueous solution of Ce(NO3)3 and Mn(NO3)2 precursors at constant pH (8.0±0.2) [1-5]. The following main redox reactions: ⎧ MnO4− + 3e − + 2 H 2 O → MnO2 ⇓ +4 HO − (1), ⎪ 2+ − − (2), ⎨ Mn + 4 HO → MnO2 ⇓ +2e + 2 H 2 O ⎪ 3+ (3), Ce + 4 HO − → CeO2 ⇓ + e − + 2 H 2 O ⎩

account for the “sticking” of the MnOx and CeOx phases at a quasi-molecular level, due to the required contact of Mn and Ce ions for the electron-transfer prompting the precipitation of both oxide species [3,4]. A reference MnCeOx catalyst (Mn/Ce, 1.0) was obtained via the co-precipitation route of the MnCl2 and CeCl3 precursors [1-5]. All the solids were dried at 100°C and further calcined in air at 400°C (6h). The list of samples with the relative physico-chemical properties is given in Table 1. Table 1. List of the catalysts and main physico-chemical properties. Catalyst

prep. meth.

Mn/Ce

[Mn] (wt%)

SA (m2/g)

PV (cm3/g)

APD (nm)

M1C3-R M1C1-R M3C2-R M2C1-R M1C1-P

redox redox redox redox co-prec.

0.34 0.95 1.44 2.12 1.00

9.3 20.5 26.6 32.7 21.2

168 154 157 140 101

0.28 0.49 0.45 0.50 0.24

5.1 11.7 14.3 16.7 9.4

2.2. Methods The physico-chemical characterization was carried out by BET, XRD, XPS, H2-TPR (5% H2/Ar) and CO-TPR (5% CO/He) techniques, while the CO oxidation activity was probed in the range of 100-150°C under kinetic regime by feeding a CO/O2/He reaction mixture in the molar ratio of 2/1/22, at the rate of 0.1 stp L/min on powdered catalyst samples (0.035 g), diluted with granular SiC in the weight ratio of 1/10. 3.0

M1C3-R M1C1-P M1C1-R M3C2-R

B)

2.5

(Mn/Ce)XPS

Intensity (a. u.)

A)

2.0 REDOX 1.5 1.0 CO-PRECIPITATION

0.5

M2C1-R

0.0 20

30

40 50 60 2θ (degree)

70

0.0

0.5

1.0 1.5 2.0 (Mn/Ce)XRF

2.5

3.0

Figure 1. (A) XRD patterns and (B) surface chemical composition (XPS) of redox and co-precipitated MnCeOx catalysts.

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3. Results and discussion The physico-chemical characterization data in Table 1 show a strong enhancement in the texture of the redox catalysts, evidenced by surface are (SA) and pore volume (PV) values much larger than those of the co-precipitated one [1-5]. At variance of the coprecipitated system, showing the diffraction lines of cerianite and, to a lower extent of pirolusite [1,4], the XRD data (Fig. 1A) of the redox systems show analogous featureless patterns, irrespective of the composition. This indicates that the improved catalyst texture reflects the lack of any “long-range” crystalline order, as a consequence of the quasi-molecular mixing of the oxide phases, hindering the formation and growth of whatsoever crystalline domains [1,3]. This peculiar architecture also accounts for the quite regular pore size distribution [1-3] and the tinier average pore diameter (APD) of the redox systems (Table 1). Notably, further evidences on the homogeneity of the redox catalysts at microscopic level are provided by XPS data in Figure 1B. In spite of the high loading (9-33 wt%), the oxide dispersion keeps constant in the whole Mn/Ce ratio range, resulting even in a considerable surface enrichment of the active phase never observed for the co-precipitated systems [1]. This still depends on the singular characteristics of synthesis route that enables very effective reciprocal oxide dispersion [1-4] and, consequently, a strong synergism between ceria carrier and the active phase. Such an effective interaction promotes the electron transfer processes enhancing the oxygen mobility and the oxygen activation functionality at low temperature [1-3]. This is evident from TPR patterns under both H2 and CO, shown in Figure 2. The superior rate of H2 consumption at T<200°C (Fig. 2A) signals immediately the greater oxygen mobility and availability of redox catalysts [1-3]. Furthermore, a peak at ca. 130°C, not observed for the co-precipitated sample, parallels a larger intensity and downshift of the main reduction components. The superior reactivity of CO prompts the reduction of the redox systems already at 20°C, resulting in a systematic downshift of all the peaks (Fig. 2B), while the component at ca. 130°C observed under H2 (Fig. 2A) is no longer visible.

M1C1 - P

≈130°C

M2C1 - R

B)

rate of CO consumption (a. u.)

rate of H2 consumption (a. u.)

A)

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M2C1 - R

M3C2 - R M3C2 - R

M1C1 - R 0

100 200 300 400 500 600 700 800

Temperature (°C)

M1C1 - R 0

100 200 300 400 500 600 700 800

Temperature (°C)

Figure 2. TPR patterns of the catalysts under H2 (A) and CO (B) after a pre-treatment in situ at 400°C in flowing oxygen.

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This confirms that the latter component arises from very reactive electrophylic oxygen species, formed by the activation of molecular oxygen during the in situ pretreatment [1-4] and driving the CO oxidation at 20°C during the baseline stabilization. This enhanced reactivity toward CO at low temperature of the redox systems, paralleling the activity in the wet air oxidation of phenol [2], prompted the comparison of the oxidation behavior of the homologous M1C1-R and M1C1-P samples. The results of the catalytic tests are presented in Table 2 in terms of CO conversion (XCO), reaction rate, site time yield of Mn sites (styMn) and apparent activation energy. Table 2. Kinetic and integral CO oxidation data of the M1C1-R and M1C1-P catalysts. Catalyst

T (°C)

a

XCO (%)

reaction rate (μmol·gcat-1·s-1)

styMn (s-1)

Eapp (kJ/mol)

100 1.6 2.9 7.6E-4 130 3.9 6.9 1.9E-3 43±4 150 8.2 14.2 3.8E-3 150 97.1 * 130 0.3 0.9 2.4E-4 M1C1-P 140 0.7 1.3 3.4E-4 62±3 150 1.1 2.0 5.1E-4 150 37.6 * * integral conversion values obtained in the following conditions: wcat, 0.3 g; F, 0.05 stp L/min; CO/O2/He=1/5/94. M1C1-R

The superior reactivity of the redox system is immediately evident from systematically higher CO conversion values under kinetic and integral conditions. In fact, the M1C1-R catalyst features a noticeable activity in the range of 100-150°C with rate values rising from ca. 3 to 15 μmol⋅gcat⋅s-1, comparables with those of metal systems [1]. On the other hand, much lower conversion and rate values confirm the weaker oxidation strength of the M1C1-P catalyst, which cannot be related only to the lower availability of active Mn sites. Indeed, specific rate and styMn values much lower than the ca. threefold lower active phase dispersion (Fig. 1B) account for an Eapp value of 62 kJ/mol, considerably higher than the 43 kJ/mol value recorded for the M1C1-R catalyst. Such a difference in the value of the energetic barrier for CO oxidation is ascribable to the easier generation of active electrophilic oxygen species, as a result of the higher oxide dispersion and the consequent stronger MnOx-CeO2 interaction enhancing the electron-transfers process(es) at the catalyst surface [1,3]. Then the reactivity in the CO oxidation was probed under integral conversion conditions to address the suitability of the system for applicative purposes. The full CO conversion attained by the redox system at 150°C, in comparison to the much lower value (ca. 38%) of the M1C1-P sample, supports the fact that the redox MnCeOx system features an oxidative performance comparable with that of noble-metal catalysts [1], constituting thus a valuable alternative for environmental applications.

References 1. 2. 3. 4. 5.

F. Arena, G. Trunfio, B. Fazio, J. Negro, L. Spadaro, 2009, J. Phys. Chem. C 113(7), 2822. F. Arena, G. Trunfio, J. Negro, L. Spadaro, 2008, Appl. Catal B, 85, 40. F. Arena, G. Trunfio, J. Negro, B. Fazio, L. Spadaro, 2008, Chem. Mater., 19, 2269. F. Arena, G. Trunfio, J. Negro, L. Spadaro, 2008, Mater. Res. Bull., 46, 539. F. Arena, J. Negro, A. Parmaliana, L. Spadaro, G. Trunfio, 2007, Ind. Eng. Chem. Res., 46, 6724.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Novel method for doping of nano TiO2 photocatalysts by chemical vapor deposition Tran M. Cuong1, Vu A.Tuan1*, Bui H. Linh1, Dang T. Phuong1, Tran T. K. Hoa1, Nguyen D. Tuyen 1, Nguyen Q. Tuan 2and Hendrik Kosslick 3 1

Institute of Chemistry, Vietnamese Academy of Science and Technology (VAST), 18 Hoang Quoc Viet, Cau Giay, Hanoi, Vietnami 2 University of DaLat, 1 Phu Dong Thien Vuong, DaLat, Vietnam 3 Leibniz-Institute for Catalysis at the University of Rostock, Albert-Einstein-Str. 29a, 18059 Rostock, Germany * Corresponding author: Prof. Dr. Vu Anh Tuan, Tel: 04.8361145. E-mail: [email protected] ; [email protected]

Abstract A highly active photocatalyst based on Fe doped nano-sized TiO2 was successfully synthesized by chemical vapor deposition (CVD) using FeCl3 as iron source. The CVD was carried out by evaporating FeCl3 at 350oC in nitrogen flow. The samples were characterized by AAS, XRD, XPS, UV-vis and FTIR. The photocatalytic activity of samples with different Fe contents obtained after different treatment times were tested in the oxidation of i-propanol in liquid phase using visible light. Titania and high Fe loaded TiO2 samples showed very low photo catalytic activity. In contrast, low loaded Fe doped TiO2 sample exhibited high photo catalytic activity. The high catalytic activity of this sample could be related to the presence of lattice defects (Ti-OH groups). Keywords: nano TiO2, doped TiO2, photocatalytic activity, visible light

1. Introduction Photocatalysis with titanium dioxide is potentially a very interesting environmental technology. Nevertheless, it still suffers from some disadvantages like the low photo efficiency. Furthermore, the extension of the frequency range of usable light into the range of visible light (red shift) could improve its efficiency. Recently, doping with transitional metals has been proposed to be a facile tool to improve the above the photo catalytic performance of TiO2 catalysts [1]. Several attempts have been undertaken to dope titania with a variety of appropriate transition metals in order to study the influence on the photo catalytic activity of TiO2 [2,3]. It has been proposed that the incorporation of transitional metal ions leads to the formation of additional donor and/or acceptor levels in the wide forbidden band gap of TiO2. This allows lower energy photons to excite electrons into the conduction band. In this way, additional photocatalytically active electron–whole pairs may be formed under visible light radiation. Sol–gel synthesis and impregnation have been widely used to dope TiO2. The sol–gel process allows the formation of gels with very homogeneous distribution of elements. This way several transition metal doped TiO2 catalysts have been obtained. However, in the case of iron, the sol-gel method is not effective. This is due to the precipitation of Fe(OH)x3-x species already at pH > 6.5 which results in the formation of Fe2O3 on the surface of TiO2 during calcination.

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Recently, ion implantation by using magnetron sputtering has been reported to yield doped photo catalysts of improved photo catalytic activity, higher than that of corresponding pure TiO2. It has been suggested that doped metals located in substitution positions of the lattice of TiO2 enhance the photo catalytic activity. While catalysts with heterogeneous phase composition like extra-lattice transition metal species or oxides were less active [4]. Encouraged by these findings, a novel chemical vapour deposition doping method has been developed in this work specially with Fe doping. The photo catalytic performance of prepared Fe-doped TiO2 materials has been investigated. The improvement of photo catalytic activity of Fe-doped TiO2 under visible light could be proven.

2. Experimental The actual Fe content of doped TiO2 was determined by atomic absorption flame emission spectroscopy (Shimadzu AA-6400F). X-ray diffraction patterns of materials were measured with a Shimadzu XRD-6100 analyzer with Cu Kα radiation (1=1.5417Å). The X-ray photoelectron spectroscopic (XPS) investigations were carried out with a Shimadzu ESCA-3200 spectrometer in order to analyze the surface elemental composition and valence state of elements of the photo catalysts. Diffuse reflectance UV-vis spectra of the catalysts were measured using a Shimadzu UV-2200A and a Shimadzu UV-vis spectrophotometer. The FT-IR spectra of the samples were measured using KBr pellets (BIO-RAD FTS-3000). The photo catalytic activity of the TiO2 and Fe-doped TiO2 photo catalysts were investigated using the oxidation of i-propanol under visible light irradiation. The experiments were carried out at room temperature. The reaction mixture was irradiated with a Hg lamp through a colour glass filter (L-42, Asahi Techno Glass). The reaction was carried out in a Pyrex tube containing TiO2 powder (25 mg) and an aqueous solution of 2.5 x 10–3 M i-propanol (50 ml). The solution was bubbled with oxygen at a rate of 30 ml min−1 for 30 min. The amount of acetone formed during the course of reaction was determined on a Shimadzu GC-14A gas-chromatograph equipped with a PEG-1000 column.

3. Results and discussion 3.1. Catalyst preparation A selected amount of TiCl4 was added dropwise into i-propanol under stirring. The resulting solution was introduced into distilled water under a vigorous magnetic stirring. Thereby, the solution was cooled in an ice-water bath. Then the pH value of the obtained acidic solution was adjusted to 7 by adding an NH4OH solution. Thereby a white gel was formed [2]. The resulting gel was aged at room temperature for 24 hours under stirring. The obtained white precipitate was filtered and washed repeatedly with distilled water until the removal of chloride ions was complete. Thereafter, the precipitate was re-dispersed in water by treating in an ultrasonic batch. Then, a 30% aqueous solution of H2O2 was added dropwise into this mixture under stirring. The resulting yellow transparent solution was poured into an autoclave and heated at 100oC for 20 h. After hydrothermal treatment, the precipitate was removed, washed and dried at 100oC to get the TiO2 powders. Fe doping was carried out by high temperature chemical vapour deposition (CVD) using FeCl3 as Fe source. The reactor was a quartz tube (2cm x 25 cm) in which a selected amount of FeCl3, was introduced at one side. On the opposite side, separated by quartz filter, was placed a selected amount of C. A scheme of the experimental set up

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for titania doping using high temperature chemical vapour deposition and postdeposition thermal treatment in inert gas is shown in Fig. 1.

TiO2

FeCl

Quartz

N2 flow Quartz

Furnace Quartz

Figure 1. Scheme of TiO2 doping with Fe by CVD method.

For titania doping, FeCl3 was evaporated by heating at 350oC using N2 as carrier gas. The amount of deposited iron depends on the selected amount of used FeCl3 and titania, the temperature and the treatment time. After completing deposition, the samples were further heated at 500oC in N2 flow for 2h to remove all excess of chloride from samples. For comparison, additional Fe-doped TiO2 samples were prepared by impregnation of TiO2 with FeCl3 solution.

3.2. Characterization and photo catalytic testing The XRD patterns confirm that prepared titania and Fe-doped titania samples consist of pure anatase (Fig. 2). Other crystalline by-products have not been detected. This indicated that after TiO2 modification by doping with iron, the crystals structure of anatase still remained. After Fe-doping, no Fe2O3 crystalline phase was detected. This indicated that Fe has been well dispersed within the TiO2 matrix. 2500

2000

TiO2-Fe 1.82% 1500

TiO2-Fe 0.12%

1000

TiO2-Fe 0.12% (*) 500

TiO2 0 20

30

40

50

60

70

80

Figure 2. XRD patterns of TiO2 and Fe- doped TiO2 by CVD and impregnation method (*).

UV-vis absorption spectra showed the fundamental absorption edge of TiO2, appearing at about 385 nm. Iron doping leads to a red shift and increased absorbance in the visible light range with increasing doping content. This red shift may be attributed to a charge transfer transition between the iron d orbital and the TiO2 conduction or valence band [3]. XPS spectra (Fig. 3) of pure TiO2 and doped-TiO2 show that Ti 2p1/2 and Ti 2p3/2 peaks are located at binding energies of 464.2 and 458.5 eV, respectively, in excellent agreement with the values of Ti4+ in pure TiO2 [5].

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I n t e n s it y

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3 2 1 1100 1000

900

800

700

600

500

400

300

200

100

0

Binding energy/eV Figure 3. XPS spectra of TiO2 (1), TiO2-Fe 0.12% (2) and TiO2-Fe 1.82% (3) samples.

The FTIR lattice spectra reveal the formation of defect sites in the titania matrix after Fe doping at very low Fe content. Even these samples are photocatalytically active in the oxidation of i-propanol to acetone prior to all other catalysts. This finding tends to show that the improvement of photocatalytic activity might be related to the appearance of defect sites. After 10h of reaction, i-propanol conversion reached the value of 70%.

4. Conclusion The presented high temperature CVD procedure using metal chloride starting materials is an effective and easy-to-handle method for the preparation of improved metal doped photocatalytic materials. The role of defect sites in the catalytic performance needs further verification.

Acknowledgement This work was supported by the DAAD which is gratefully acknowledged.

References [1] A. Di Paola, G. Marci, L. Palmisano, M. Schiavello, K. Uosaki, S. Ikeda, B. Ohtani, Preparation of Polycrystalline TiO2 Photocatalysts Impregnated with Various Transition Metal Ions: Characterization and Photocatalytic Activity for the Degradation of 4-Nitrophenol. J. Phys. Chem. B 106 (2002) 637. [2] H. Kato, A. Kudo, Visible-Light-Response and Photocatalytic Activities of TiO2 and SrTiO3 Photocatalysts Codoped with Antimony and Chromium. J. Phys. Chem. B 106 (2002) 5029. [3] W. Choi, A. Termin, M.R. Hoffmann, The Role of Metal Ion Dopants in Quantum-Sized TiO2: Correlation between Photoreactivity and Charge Carrier Recombination Dynamics. J. Phys. Chem. 98 (1994) 13669. [4] M. Anpo, M. Takeuchi The design and development of highly reactive titanium oxide photocatalysts operating under visible light irradiation. J. Catal. 216 (2003) 505. [5] E. Borgarello, J. Kiwi, E. Pelizzetti, M. Visca, M. Grätzel, Hydrogen production from water by visible light using zinc porphyrin-sensitized platinized titanium dioxide. J. Am Chem. Soc. 103 (1981) 6324.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Study on the preparation of active support and multi-porous supported catalyst Vu A. Tuan*, Bui H. Linh, Dang T. Phuong, Tran T.K. Hoa, Nguyen T. Kien, Nguyen H. Hao, Hendrik Kosslick*a, Axel Schulza Institute of Chemistry, Vietnamese Academy of Science and Technology (VAST), 18 Hoang Quoc Viet, Cau Giay, Hanoi, Vietnam a

Leibniz Institute for Catalysis at the University of Rostock, Albert-Einstein-Str. 29a, Rostock 18059, Germany * Corresponding author. Prof. Dr. Vu Anh Tuan, Tel: 04.8361145. E-mail: [email protected]; Dr. Hendrik Kosslick, Tel. 0049381 4986384, E-mail [email protected]

Abstract The preparation of improved catalytic materials based on natural diatomite is reported. Aluminum incorporation by an atomic implantation methods yields supports with enhanced acidity. Nano-zeolite Y supported diatomite composite materials with a multimodaö distribution of interconnected micro- meso- and macro pores were obtained by in situ crystallization. Materials were characterized by XRD, TEM, NH3-TPD and textural studies using nitrogen adsorption / desorption measurements. The catalytic performance was tested in the cracking of a heavy petroleum residue. The results show, that diatomite due to its large pores is a superior support for the preparation of supported catalysts for the cracking of heavy oil fraction containing bulky molecules. Atomic implantation of aluminum yields catalysts of the same catalytic performance as loading with acidic nano-sized zeolite HY. Keywords: catalyst preparation, heterogeneous multi porous catalyst, zeolite composite, acidity, FCC

1. Introduction Due to the limited availability of fossil feedstock, the manufacture of high value fuel products from low value feedstock like heavy oil residues, oil sands or biomass has received great interest of research and manufacturing. Fluidized catalytic cracking (FCC) is one of the most important processes to produce gasoline and diesels. FCC catalysts consist of active components like zeolite Y, rare-earth modified zeolite-Y and USY or supported silica and /or aluminosilicates. However, they suffer from large crystal size (1-5µm) and narrow pore dimension (0.74 nm for zeolite Y) limiting access of active sites by large molecules such as polyaromatics. Improvement of catalytic performance and considering sustainability aspects needs the development of new generation of FCC catalysts. These catalysts should possess multi-modal pore structures of interconnected macro- meso- and micropores. They should allow easy access of heavy feedstock compartments to catalyst active sites. In this way large polyaromatics can be pre-cracked in the mesopores. Further cracking at acidic sites in the micropores converts the pre-cracked compounds into valuable fuel components and other valuable chemicals. Nano-sized crystals may improve the efficiency of catalyst by making use of the external surface [1-5]. Moreover, the catalyst supports are usually catalytically inactive or less active. The catalytic functionalization of the support with acidic

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aluminium site can additional improve the catalytic performance by synergetic effects between the active components and the matrix. In this paper, we report the recent results on the acidification of diatomite (Phu Yen-Vietnam) by atomic implantation aluminum and the preparation of nano-zeolite Y supported diatomite by in situ crystallization method.

2. Experimental Starting materials. Nano-zeolite Y was prepared by hydrothermal crystallization using the natural starting material kaolin. For this, the kaolin was calcined at 550°C. The Diatomite support (Phu Yen-Vietnam) was calcined at 500°C, treated with 1 M hydrochloric acid under reflux..

2.1. Characterization and catalytic testing of samples Samples were characterized by XRD (Siemens D5000 XRD spectrometer), FE-SEM (Field Emission Scanning Electron Microscope, Hitachi S-4800), textural properties were determined by N2 adsorption/desorption experiments) and NH3-TPD (temperatureprogrammed desorption of ammonia). Catalytic properties of samples were tested in the cracking of a Bach Hổ petroleum residue from Vietnam (370-500°C fraction) as heavy feedstock using a MAT 5000 micro activity testing system. The reaction conditions were 482°C, WVH=27, catalyst to feedstock ratio 3/1, reaction time 45 sec.

3. Results and discussion 3.1. Catalyst preparation 3.1.1. Acidification of diatomite Acidification of diatomite was carried out in a tubular reactor as presented in Fig. 1. Purified diatomite and excess of aluminum chloride, separated by aquartz filter, were placed into a tubular reactor. The reactor was heated from room temperature to the final temperature of up to 500° with a heating rate of 10K/min. The evaporated AlCl3 was passed through the diatomite with nitrogen carrier gas (N2). The flow rate was 50 mL/min. reaction time: 0.5- 2 h. Then the sample was calcined at 500°C to remove excess of chloride.

Fig. 1. Reactor set up for the acidification of diatomite by atomic implantation method.

3.1.2. Preparation of nano-zeolite Y and supported diatomite Nano-zeolite Y supported diatomite was synthesized according to a 2 steps procedure: Step 1: Preparation of nano-zeolite Y seeds and seeded diatomite. Nano-zeolite Y seeds were prepared according to the following: A solution of water glass and sodium hydroxide solution were added to meta-kaolin under vigorously stirring to form a homogeneous gel having the molar composition: 16NaOH:Al2O3:10 SiO2:720H2O. After aging at 25°C for 24 hours, the gel was transferred into an autoclave and crystallized at

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80°C for 20h under autogeneous pressure. The nano zeolite Y seeds were then mixed with purified diatomite assisted by ultrasonic agitation and at 100°C for 15h. Step 2: Nano-zeolite Y seeded diatomite from step 1 was introduced into same the gel that was used for the preparation of zeolite Y seeds under vigorous stirring to form a homogeneous mixture. The gel was aged one day at room temperature. The aged gel was transferred into an autoclave and crystallized at 90°C for 20 h under autogeneous pressure. The products were filtered, washed with deionised water and dried at 120°C over night. To obtain the acidic form, the products were ion exchanged 3 times with a 2 molar aqueous NH4NO3 solution. The obtained product was filtered off, dried and calcined at 500°C for 2 h.

3.2. Characterization and catalytic testing The crystallinty of as-synthesized zeolite Y nano particles and nano-sized supported HY has been checked by XRD. Zeolite synthesis using kaolin as starting material yields crystalline zeolite Y. The diatomite is x-ray amorphous. The seeded diatomite shows the XRD pattern of zeolite Y, however, the reflections are of low intensity. Based on the comparison of peak intensities with pure zeolite Y, the amount of nano-zeolite Y supported on diatomite was ca. 15%. Hence, diatomite allows a substantial loading with acidic active nano-sized zeolite Y components. This is obviously closely related to the macro-meso porous interconnected pore structure with large open void volumes. (c)

(a)

(b)

Fig. 2. FE-SEM photographs of diatomite (middle), nano-zeolite Y supported diatomite (right), and corresponding surface zoom (left).

The morphology and porosity of nano-zeolite Y supported diatomite was examined by FE-SEM (Fig. 2) The µm–sized diatomite support consists of cylindrically shaped tubes. The tubes have large-sized free internal pores. They are additionally accessible from the surface of the tubes by large mesopores (Fig. 2, middle). It was observed that nano-zeolite Y, providing micropores, was formed on the surface of diatomite. Additionally, voids of 20–50 nm size were formed. (Fig. 2, right). The nano-zeolite Y crystal size varied from 20 to 35 nm (Fig. 2, left). This result was also confirmed by N2 adsorption/ desorption measurement. In the NH3–TPD profiles of the samples, two peaks: a low temperature peak (150°C-180°C) and a high temperature pick (350°C-420°C) could be identified belonging to weak and strong acid sites were observed. For the Al-modified diatomite, a large amount of strong acid sites desorbing ammonia at 420oC (maximum) was noted. Besides strong sites, diatomite modified with nano-zeolite HY showed a large amount of weak acid sites (maximum desorption temperature of 185°C ).

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Catalytic testing reveals high cracking activity for Al-modified diatomite. In contrast, pure diatomite alone is not active. Under the same reaction conditions, acidified diatomite exhibited somewhat higher conversion and selectivity to LPG and gasoline compared to nano-zeolite Y supported diatomite (Tab. 1). This result is consistent with data obtained from NH3 –TPD that acidified diatomite has higher acidity compared to that of nano-zeolite Y supported diatomite. For both samples, the main products were gasoline, light and heavy cycle oil (LCO and HCO). Especially, very low yield of light gases was noted. Also, low yield of coke was observed. Table 1. Reaction conditions, conversion and products yields in cracking of Bach Ho residue over nano-zeolite Y supported diatomite and acidified diatomite. Catalyst Name Product Yield (wt%) H2 Coke Total C3 Total C4 LPG (C3,C4) Gasoline (25 oC ~216 oC) LCO (216oC~360 oC) HCO (360 oC)

Nano Y support. Diatomite

Acidified diatomite

1.45 3.14 0.01 0.10 0.21 16.60 20.37 57.78

2.20 3.72 0.03 0.34 0.65 18.60 20.30 53.52

4. Conclusion New catalytic materials based on Al-modifed diatomite was successfully prepared by using atomic implantation method. Acidity of this material is comparable to that of zeolite Y as evidenced by NH3-TPD measurements. This material is an active in hydrocarbon cracking. In addition, nano-zeolite Y supported diatomite was successfully synthesized by using a two step procedure: First step- Preparation of nano-zeolite Y seeds and seeded diatomite and the second step 2- Formation and deposition of nanozeolite Y seeds on diatomite by in-situ crystallization. The resulting supported material contains a multi modal pore structure. with interconnected macro-, meso- and micropores. Both the materials were active in the cracking of heavy petroleum residue, indicating its high application potential for FCC catalyst preparation.

References [1] Y. Liu, W. Zhang, T.J. Pinnavaia, Steam-stable aluminosilicate mesostructures assembled from zeolite type Y seeds, J. Am. Chem. Soc., 122 (2000) 8791. [2] Y. Liu, W Zhang, T.J. Pinnavaia, Steam-stable MSU-S aluminosilicate mesostructures assembled from zeolite ZSM-5 and zeolite Beta seeds, Angew. Chem. Int. Ed., 40 (2001) 1255. [3] K.S. Triantaflyllidis, T.J. Pinnavaia, A. Iosifidis, P.J. Pomonis. Specific surface area and I-Point evidence for microporosity in nanostructure MSU-S aluminosilicates assembled from zeolite seeds, Journal of Mater. Chem., 17 (2007) 3630. [4] Z. Jing, Hirotaka, K. Ioku, E. H. Ishida, Hydrothermal Synthesis of Mesoporous Materials from Diatomaceous Earth, J. AIChE, 53 (2007) 2114. [5] S.W. Rutherford, J.E. Coons, Water sorption in silicone foam containing diatomaceous, J. Colloid Interface Sci, 306 (2007) 228.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

The influence of preparation procedure on structural and surface properties of magnesium fluoride support and on the activity of ruthenium catalysts for selective hydrogenation of chloronitrobenzene Mariusz Pietrowski* and Maria Wojciechowska Adam Mickiewicz University, Faculty of Chemistry, Grunwaldzka 6, 60-780 Poznań, Poland; *e-mail: [email protected]

Abstract The effect of preparation conditions on structural and surface properties of magnesium fluoride was studied in the aspect of its use as a catalyst support. Amorphous and spherical polycrystalline MgF2 supports were prepared and characterised by BET, XRD, TEM, and FTIR (pyridine adsorption) techniques. The influence of MgF2 properties on the performance of Ru/MgF2 catalysts in selective reduction of ortho- and parachloronitrobenzene to respective chloroanilines is reported as well. Keywords: MgF2, ruthenium catalyst, hydrogenation of chloronitrobenzene

1. Introduction Recently, a growing interest of researchers in fluoride supports, including magnesium fluoride, is observed [1-15]. In many laboratories research on the development of new active and selective catalysts supported on MgF2 is carried out and this resulted in the creation of interesting catalysts for such processes as hydrodesulphurization [15], hydrodechlorination [16], ammoxidation [4], reduction of nitrogen oxides [3], Knoevenagel reaction [5], oxidation of CO [3], photodegradation of acetone [6] and recently, hydrogenation of chloronitrobenzene to chloroaniline [8-9]. A great advantage of magnesium fluoride is its easy preparation and availability as well as low cost of parent materials. The simplest way of MgF2 preparation is the reaction of hydrofluoric acid with MgCO3 [14] which enables to obtain magnesium fluoride with specific surface area up to 43 m2·g-1 after thermal treatment at 670 K. Results of studies on the preparation of MgF2 of a higher surface area were recently reported by Kemnitz and coworkers [2]. Monodispersive spherically-shaped powders of magnesium fluoride of particle size in the range 0.25-0.36 μm was obtained as well [12]. No comparative study on properties of magnesium fluoride obtained by using different preparation methods was published in the literature yet. This is why we have performed a comparison of MgF2 samples prepared by four different methods developed in our laboratory. The subject of the comparison were structural (XRD) and surface (FTIR) properties. Moreover, usefulness of magnesium fluoride obtained in different preparation ways was evaluated from the point of view of its application as a support of ruthenium catalyst for selective reduction of chloronitrobenzene to chloroaniline. It is worth to add that the latter compound is an important intermediate for the manufacture of a wide range of pharmaceuticals, herbicides and dyes [17].

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2. Experimental 2.1. Procedures for the preparation of MgF2 supports and Ru/MgF2 catalysts

Polycrystalline MgF2 was obtained using four methods listed below. (i) Reaction of HF with MgCO3 (carbonate sample - C): 40% solution of hydrofluoric acid was added dropwise to suspension of magnesium carbonate in water. The resulted precipitate of MgF2 was stirred for 48 h and then dried at 390 K. (ii) Reaction of NH4F with Mg(NO3)2 - amorphous MgF2 (nitrate sample - N): 0.1M solution of NH4F was slowly added, with the use of peristaltic pump, to 0.1M solution of Mg(NO3)2 at 330 K. The precipitate was filtered off and dried at 390 K. (iii) Reaction of NH4Fwith Mg(NO3)2 - spherical MgF2 prepared with the use of microwave radiation, (nitrate spherical - Ns): Cold 0.01M solution of NH4F was poured into 0.01M solution of Mg(NO3)2 followed by immediate placing in a microwave oven and heating within 30 s to a temperature close to boiling point. The precipitate was filtered off and dried at 390 K. (iv) Reaction of HF with magnesium alkoxide (alkoxide sample - Alk): A solution of freshly prepared magnesium methoxide was added dropwise to a 40% hydrofluoric acid solution in methanol on vigorous stirring. The process of gelation was conducted for 6h at room temperature. The gel was aged at room temperature for 40 h, then dried at 350 K. After drying, all MgF2 samples were calcined in air at 670 K for 4 h. 20 cm g Ruthenium was loaded onto MgF2 by conventional impregnation using methanolic solution of RuCl3·3H2O. Ruthenium content was 1 wt%. The catalysts were C dried at 350 K and reduced at 670 K under hydrogen flow. 3 -1

Volume adsorbed (cm g )

3 -1

2.2. Activity test

Alk

N Ns 0.0

0.2

0.4

0.6

0.8

1.0

p/p0

Figure 1. Isotherms of low-temperature nitrogen adsorption on MgF2.

Hydrogenation of ortho-, and parachloronitrobenzenes (o-; p-CNB) to orthoand para-chloroanilines (o-; p-CAN) was performed in liquid-phase at 350 K for 2 h at 4 MPa of hydrogen pressure in a 200 cm3 stainless steel autoclave with a magnetic stirrer. 0.05 g of a catalyst and 50 cm3 of 0.1M CNB solution in methanol were loaded to the autoclave. The reaction products were analysed on a gas chromatograph equipped with a capillary column RESTEK MXT-5.

3. Results and discussion 3.1. The effect of preparation on physico-chemical properties of fluoride supports Magnesium fluoride was prepared using four different methods, however, thermal treatment of all samples was identical and consisted in calcination at 670 K. Despite the same calcination temperature, MgF2 samples considerably differ in their porous

507

The influence of preparation procedure on structural and surface properties

structure which is reflected in the shape of hysteresis loop in the isotherm of lowtemperature nitrogen adsorption, specific surface area, pore size and pore volume (Table 1; Figure 1). The greatest surface area (43 m2·g-1) was obtained in the case of carbonate sample (C), whereas the smallest one (13 m2·g-1) in the case of spherical MgF2 prepared from magnesium nitrate (Ns). Adsorption-desorption isotherms of all samples are of type IV, however, they differ in the shape of their hysteresis loops. The latter belong to type H1 for samples C, Alk and N, which indicates the presence of cylindrical pores, whereas hysteresis loop of the sample Ns combines features of hysteresis loops of types H1 and H2, encountered when narrow-necked pores are present. The greatest pore size (r = 13 nm) and pore volume (0.250 cm3·g-1) were found in magnesium fluoride prepared from magnesium alkoxide (Alk). The discussed sample is also characterized by the lowest thermal stability as concluded from considerable increase in crystallite size and drastic reduction in surface area with the rise in calcination temperature (Table 1). The highest resistance to sintering and recrystallisation at high temperatures were shown by carbonate (C) and nitrate-spherical (Ns) samples. Table 1. Textural and structural properties of magnesium fluoride samples obtained by different methods. BET surface area (m2·g-1) after calcination at different temperatures

Crystallite size*(nm) after calcination at different temperatures

570 K

670 K

770 K

570 K

670 K

770 K

0.172

85

43

25

12

22

41

13

0.250

81

32

24

10

22

116

N

6

0.066

62

22

12

14

59

139

Ns

9

0.066

26

13

5

28

46

86

Average pore radius (nm)

Total pore volume (cm3·g-1)

C

8

Alk

Sample

* Determined by Hall method on the ground of broadening of XRD reflections

Infrared spectra of adsorbed pyridine have shown the presence of weak Lewis acid centres on surfaces of all MgF2 samples studied. The amount of these centres, as estimated on the ground of absorption band intensities, is small and decreases in the order: C ≥ N > Alk > Ns. No Brøensted acid sites were detected on MgF2 surface, irrespective of its preparation way. Summing up, we can say that magnesium fluoride of the greatest surface area (after calcination at 670 K) and the highest resistance to sintering was obtained by the reaction of magnesium carbonate with hydrofluoric acid. It should be stressed that the preparation of MgF2 by this method is cheap and is characterised by a excellent reproducibility.

3.2. Activity of Ru/MgF2 catalysts for selective reduction of chloronitrobenzene 1 wt.% of ruthenium was loaded onto surfaces of MgF2 samples by impregnation with ruthenium(III) chloride. The samples were dried at 350 K and reduced in hydrogen flow at 670 K for 4h. In spite of applying identical preparation conditions of Ru/MgF2 catalysts, different dispersions of the metal were obtained (Table 2). The highest dispersion (15%) was found in the case of Alk-Ru catalyst. Selectivities were somewhat higher in the case of reduction of p-CNB (~90%) compared to o-CNB (~85%). No significant effect of dispersion on the selectivity to chloroaniline was observed. In general, the order of catalytic activity is as follows: Ns > C > Alk > N, both in the case

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of ortho- and para-chloronitrobenzene. All magnesium fluoride supported catalysts were more selective than commercial 0.5%Ru/Al2O3 Engelhard catalyst. Table 2. Liquid-phase hydrogenation of o-, and p-CNB to o-, and p-CAN on Ru/MgF2 catalysts. Catalyst

Dispersion, p-CNB % Conversion, % Selectivity,%

o-CNB

Ns-Ru C-Ru Alk-Ru N-Ru

12.3 6.7 15.0 14.1

9.8 7.7 5.1 3.5

87.7 90.5 91.0 90.1

13.5 9.6 7.8 5.7

81.0 86.3 88.1 83.6

0.5%Ru/Al2O3 Engelhard

-

6.9

68.6

-

-

Conversion, % Selectivity,%

4. Conclusion Studies carried out recently proved that magnesium fluoride is a very good material for support in catalysis. Results reported in this paper have shown that by using different preparation methods it is possible to obtain MgF2 samples differing in their structural and surface properties. The most suitable preparation method seems to be the reaction between magnesium carbonate and hydrofluoric acid. Magnesium fluoride obtained by this method is characterised by higher surface area and higher thermal stability compared to samples prepared by other methods investigated. Simplicity of the preparation method and low cost of reagents used for synthesis are also of importance. Data obtained while studying the reaction of selective reduction of chloronitrobenzene to chloroaniline enable to draw the conclusion that preparation way and properties of MgF2 influence catalytic results. Differences in catalytic activity can reach as much as 300%.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

J.M. Winfield, 2009, J. Fluor. Chem., 130, 1069-1079. S. Wuttke, S.M. Coman, G. Scholz, H. Kirmse, A. Vimont, M. Daturi, S.L.M. Schroeder, E. Kemnitz, 2008, Chem. Eur. J., 14, 11488-11499. J. Haber, M. Wojciechowska, M. Zielinski, W. Przystajko, 2007, Catal. Lett., 113, 46-53. V.N. Kalevaru, B. David Raju, V. Venkat Rao, A. Martin, 2009, Appl. Catal. A, 352, 223233. R.M. Kumbhare, and M. Sridhar, 2008, Catal. Commun., 9, 403-405. F. Chen, T.H. Wu, and X.P. Zhou, 2008, Catal. Commun., 9, 1698-1703. J. Krishna Murthy, U. Gross, S. Rudiger, E. Unveren, W. Unger, E. Kemnitz, 2005, Appl. Catal. A, 282, 85-91. M. Pietrowski, M. Zieliński, M. Wojciechowska, 2009, Catal. Lett., 128, 31-35. M. Pietrowski, M. Wojciechowska, 2009, Catal. Today, 142, 211-214. M. Wojciechowska, A. Wajnert, I. Tomska-Foralewska, M. Zieliński, B. Czajka, 2009, Catal. Lett., 128, 77-82. M. Wojciechowska, W. Przystajko, M. Zielinski, 2007, Catal. Today, 119, 338-341. M. Pietrowski, M. Wojciechowska, 2007, J. Fluor. Chem., 128, 219-223. M. Wojciechowska, I. Tomska-Foralewska, W. Przystajko, M. Zielinski, 2005, Catal. Lett., 104, 121-128. M. Wojciechowska, M. Zielinski, M. Pietrowski, 2003, J. Fluor. Chem., 120, 1-11. M. Wojciechowska, M. Pietrowski, B. Czajka, 2001, Catal. Today, 65, 349-353. A. Malinowski, W. Juszczyk, J. Pielaszek, M. Bonarowska, M. Wojciechowska, Z. Karpinski, 1999, Chem. Commun., 685-686. X. Wang, M. Liang, J. Zhang, Y. Wang, 2007, Curr. Org. Chem. 11, 299-314.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Bimetallic Co-Mo-complexes with optimal localization on the support surface: A way for highly active hydrodesulfurization catalysts preparation for different petroleum distillates O.V. Klimov, A.V. Pashigreva, K.A. Leonova, G.A. Bukhtiyarova, S.V. Budukva, A.S. Noskov Boreskov Institute of Catalysis SB RAS, Pr. Lavrentieva 5, Novosibirsk,630090, Russia

Abstract The preparation method of the catalyst for the deep hydrotreatment of vacuum gas oil and gasoline is described. The method is based on vacuum impregnation of the carrier with required average pore diameter with the solution of bimetallic CoMo complexes. It was shown that the use of Co-Mo complexes, containing chelating ligands and having different molecule size, allows to obtain catalysts with the uniform distribution of the surface species, containing supported metals only in the form of Co-Mo-S phase type II that is located inside of the pores exposed to all reacting heteroatomic molecules of the feedstock. Keywords: hydrotreating catalyst, textural properties of carrier, catalyst active component localization

1. Introduction Nowadays, it is well known that the active sites of the hydrotreating reactions surface are the nanosized MoS2 particles with Co(Ni) atoms anchored at the edge plane[1]. From the other side, the textural properties of the supports that are optimal for different type of distillates are well distinguished [2]. The use of the pores with smaller size restricts the access of S- and N- containing compounds to the catalysts active sites. The use of pores with sizes larger than necessary results in a lower specific surface area and therefore in a decrease of the number of active sites per volume of catalyst. The catalysts designed for hydrotreating of the selected type of feedstock should contain the supported metals only in the form of the active sites, arranged in the pores with optimal size. The preparation of the carriers with required average pore diameter is easy-to-implement task. The catalysts containing Co and Mo only in the form of the active sites can be prepared using bimetallic complexes [3]. The selective synthesis of the active sites inside of the pores with the required size is much more difficult. The current contribution describes a hydrotreatment catalysts preparation method for different petroleum distillates – gasoline and vacuum gas oil (VGO). Since S- and N- containing compounds of these distillates characterized with a different molecule size, the nanoparticles of active phase have to be arranged in the pores with relatively small sizes for gasoline and with a large size for vacuum gas oil. The key factor in obtaining such catalysts is the size of the bimetallic complexes used for the catalysts preparation. Whereas the molecule size of the complex that will be supported determines in what pores the active sites will be arranged.

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The preparation route includes the following stages: - Synthesis of the bimetallic Co-Mo complexes with a different molecule size in aqueous solution with Co and Mo stoichiometry that is optimal for the active sites of hydrotreating catalysts; - Deposition of the bimetallic complexes onto support surface avoiding initial complexes decomposition. - The drying of the catalysts under conditions, excluding bimetallic complexes decomposition and migration of Co and Mo atoms through the support surface and subsequent sulfidation.

2. Experimental 2.1. Preparation of the complexes Two bimetallic Co-Mо complexes Co[Mo2O4(C2O4)2(H2O)2] (hereinafter CoMo2Ox) and Co2[Mo4(C6H5O7)2O11]•nH2O (hereinafter CoMo4CA) with the geometrical size (without its solvate sphere) of 4×4×7 Å and 7×10×12 Å, correspondingly, were used for hydrotreating catalysts preparation. Both complexes were synthesized by similar methods adding of Co(NO3)2•6H2O to the solution containing corresponding molybdenum containing anion, so that to obtain Mo/Co ratio in the resulting solution equaled 2. Initial (NH4)2[Mo2O4(C2O4)2(H2O)2] was obtained by dissolution of (NH4)2[Mo2O4(OH)4(H2O)2] (obtained by synthesis as in [4]) in aqueous solution of oxalic acid synthesized at stirring and heating at 50 ºC. The synthesis of (NH4)4[Mo4(C6H5O7)2O11]•nH2O is described in [5].

2.2. Preparation of the catalysts The catalysts were prepared by vacuum impregnation of the trilobe shaped carrier (Ø = 1.5 mm, l = 3-6 mm) by aqueous solutions of the complexes in accordance with the technique described in [6], followed by drying at 120ºC without calcination. Only for the determination of Co and Mo content the catalyst samples were calcined 550ºC 2 hours. To prepare catalyst for VGO hydrotreatment the alumina support developed deliberately by JSC Promyshlennye Katalizatory (Ryazan, Russia) was used. This support has specific surface area of 240 m2/g, pore volume of 0.75 cm3/g, average pore diameter of 120 Å (carrier 120). The concentration of CoMo4CA in the solution was chosen to prepare catalyst containing 10.5% Mo, 3.3% Co. The catalyst is further denoted CoMo4CA/Al2O3. To prepare gasoline hydrotreating catalyst the laboratory prepared carrier with specific surface area of 260 m2/g, pore volume of 0.50 cm3/g, average pore diameter of 70 Å (carrier 70) was used. The (CoMo2Ox) concentration in an impregnating solution corresponded to the content of 8.0% Mo, 2.4% Co in the catalyst. The catalyst is further denoted CoMo2Ox/Al2O3. The sulfidation of the catalysts was performed in H2S flow of 1000 ml/h during 2 hours at 230ºC and then 2 hours at 400ºC. The obtained samples are designated as CoMo2Ox-S/Al2O3 and CoMo4CA-S/Al2O3.

2.3. Complexes and catalysts characterization Equipment and techniques for NMR, FTIR, Raman, EXAFS and HRTEM characterization are described in [5,6]. Equipment and XPS characterization technique and the methods for textural properties study are given in [7].

Bimetallic Co-Mo-complexes with optimal localization on the support surface

511

3. Results and discussion The synthesized complexes of (CoMo2Ox) and (CoMo4CA) were obtained in the solid form by ethanol precipitation and were characterized by physical-chemical methods. FTIR, Raman and EXAFS data for complexes in the solution and in the solid form are fully agree with the data of [5,6]. That allows to conclude that structure of (CoMo2Ox) and (CoMo4CA) is identical to that described in [6]. The structure of the initial complexes is not changed after deposition onto support surface, that confirmed by followed methods: - In accordance with FTIR data the coordination of Co towards Mo-containing anions via the carboxylic group of oxalic and citric ligands, correspondingly, are saved; - Raman data show that Mo environment in the catalysts and in the initial complexes is the same; - According to EXAFS data the all distances of Mo-O, Mo-Mo and Mo-Co corresponded for the initial bimetallic complexes were also revealed for the catalysts; - The NMR analysis of the excess of impregnating solution drained out the catalysts showed the absence of any molybdenum containing compounds differed from the initial complexes. The results of the sulfide catalysts characterization study are presented in Table 1. Table 1. The results of sulfide catalysts characterization. Characterizatio n method Element analysis XPS EXAFS

CoMo2Ox-S/Al2O3

CoMo4CA-S/Al2O3

S/Mo=2,05

S/Mo=2,00

Mo3d 228,7 eV Mo K-edge

Co2p 778,8 eV Co K-edge

Mo3d 228,6 eV Mo K-edge

Co2p 778,9 eV Co K-edge

Mo-S=2,40Å

Co-S=2,22Å CoMo=2,78Å

Mo-S=2,42Å Mo-Mo=2,61Å

Co-S=2,22Å Co-Mo=2,80Å

Mo-Mo=2,60Å

HRTEM Raman Adsorption/ desorbtion N2

MoS2 particles Average length =30Å Average number of layers=2,35 MoS2 361;402 cm-1 No Mo-O compounds Carrier 70 Catalyst S=217 m2/g S=235 m2/g Vpore=0,43cm3/ Vpore=0,34cm3/g g Øpore=62Å Øpore=73Å

MoS2 particles Average length=40Å Average number of layers=1,95 MoS2 363;426 cm-1 No Mo-O compounds Carrier 120 Catalyst S=240 m2/g S=190 m2/g Vpore=0,73cm3/ Vpore=0,53cm3/g g Øpore=111Å Øpore=120Å

The catalysts characterization and element analysis data allow to conclude that both catalysts contain cobalt and molybdenum only in the form of Co-Mo-S phase type II, described in [8,9]. The decrease of wide pore volume in comparison with carrier was noticed for both catalysts, while, the narrow pore volume remained about the same (Fig.1). It means that Co-Mo-S phase type II is localized in the pores with diameter of 50-100Å in CoMo2Ox-S/Al2O3, and in pores with diameter of 70-130Å in CoMo4CA-S/Al2O3.

O.V. Klimov et al.

512

Figure 1. Pore size distribution in the carriers and in the catalysts.

Testing of CoMo4CA-S/Al2O3 sample in the hydrotreatment of VGO [7] showed that achieved hydrodesulfurization degree exceeds 99% and hydronitrogenation degree is more than 85%. Hydrotreatment of the straight run gasoline fraction over CoMo2Ox-S/Al2O3 catalyst allows to reach the product, which fully corresponds to the quality of reforming feed in terms of sulfur and nitrogen content at typical hydroprocessing conditions used in industry [10].

4. Conclusions Thus, the use of bimetallic Co-Mo complexes for the catalysts preparation allows to obtain high active hydrotreating catalysts, which mainly contain Co-Mo-S phase type II inside of the pores exposed to the reacting heteroatomic molecules of the feedstock. Depending on the molecules size of supported Co-Mo complex, the proposed method can be used for the preparation of hydrotreating catalysts both for light and heavy petroleum distillates.

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]

J.V. Lauritsen, J. Kibsgaard, G.H. Olesen et al., J. Catal. 249 (2007) 220. J. Ancheyta, M.S.Rana, E.Furimsky, Catal.Today, 109 (2005) 3. J. Mazurelle, C. Lamonier, Ch. Lancelot et al., Catal. Today, 130 (2008) 41. A.N. Startsev, O.V. Klimov, S.A. Shkuropat et al. Polyhedron 13 (1994) 505. O.V. Klimov, A.V. Pashigreva, M.A. Fedotov et al., J.Mol.Catal. A, 2010, in press. O.V. Klimov, A.V. Pashigreva, G.A. Bukhtiyarova et al., Catal. Today (2009), doi:10.1016/j.cattod.2009.07.095 A.V. Pashigreva, O.V. Klimov, G.A. Bukhtiyarova et al., Catal. Today (2009), doi:10.1016/j.cattod.2009.07.096 H. Topsoe, Applied Catalysis A: General 322 (2007) 3. S. Eijsbouts, L.C.A. Van den Oetelaar, R.R. Van Ruijenbroek, J. Catal. 229 (2005) 352. Song, Catal. Today 86 (2003) 211.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Mn, Mn-Cu and Mn-Co mixed oxides as catalysts synthesized from hydrotalcite type precursors for the total oxidation of ethanol Daniel Aguilera, Alejandro Perez, Rafael Molina, Sonia Moreno* Estado Sólido y Catálisis Ambiental, Departamento de Química, Facultad de Ciencias, Universidad Nacional de Colombia, AK 30 No. 45-03, Bogotá, Colombia. E-Mail address: *[email protected]

Abstract In this work, nanoparticulates of compounds precursors of LDHs for preparing Mn, Mn-Cu and Mn-Co mixed oxides were successfully synthesized by the reverse microemulsion method. It was observed that the precursor obtained from the above method had similar characteristics for preparing mixed oxide catalysts used in the oxidation of ethanol. This method was compared with the conventional co-precipitation method. Keywords: mixed oxides, VOCs, microemulsion

1. Introduction The total catalytic oxidation of volatile organic compounds (VOCs) is one of the most effective and economically attractive methods which can be applied in order to limit emissions. Metal oxides are very promising for the obtaining of active catalysts in the oxidation of VOCs. The metallic mixture may generate a cooperative effect that can promote oxygen mobility, stabilization of the most active species and generate a redox cycle [1]. Mixed oxides can be obtained by controlled decomposition of LDH compounds which show large surface areas, high metal dispersion and stability against sintering [2]. Nevertheless, the LDH structure depends on a great number of parameters. Co-precipitation methodology is the most common synthesis technique to obtain LDHs, however nucleation and the kinetics growth can not be controlled easily. An alternative method for the synthesis of nanomaterials is reverse microemulsion (waterin-oil) in which an aqueous phase is dispersed into an oil phase stabilized by a surfactant film. Microemulsions can be used as nanoreactors leading to homogeneous nanomaterials with a narrow particle size and better textural properties [3]. In this work, Mn and binary Mn-Cu, Mn-Co hydrotalcite-like precursors synthesized in reverse microemulsion and the effects of preparation methods on the performance of catalysts for deep oxidation of VOCs have been studied.

2. Experimental 2.1. Preparation of catalysts 2.1.1. Co-precipitation Mn–Mg–Al, Mn–Cu–Mg–Al and Mn–Co–Mg–Al hydrotalcites were synthesized from the nitrates of Mg 2+, Al3+, Mn2+ and Cu2+ or Co2+ with M2+/M3+ = 3 and (Mn + M)/ Mg = 0,36 ratios where M corresponds to Cu or Co [4, 5]. The M/Mn = 0.5 ratio was

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D. Aguilera et al.

selected to agree with preliminary tests. The nitrate solution was added drop wise to an aqueous solution of sodium carbonate at room temperature; the slurry was vigorously stirred while keeping the pH between 9 and 10 by the slow addition of a diluted solution of NaOH. After the complete addition of the metal nitrate solution, the suspension was stirred for 1 h, followed by ageing for 24 h without stirring. The solid was rinsed with deionized water and dried in air at 60°C for 24 h. The dried solid was ground into a fine powder (<250 μm) that was further calcined in air at 500°C for 16 h to obtain the mixed oxide [5]. 2.1.2. Reverse microemulsion method Reverse microemulsion LDH phases were prepared taking into account the same ratios used in the co-precipitation method. For each synthesis a solution nitrates was added into the carbonate microemulsion at room temperature; the solution also contained NaDDS, 1-butanol and isooctane which had been prepared by mixing the aqueous and the organic components previously prepared separately [6-8]. After mixing, the mixtures were tightly sealed. All the above mentioned mixing and reaction processes were carried out with continuous magnetic stirring. Gel-like solids were separated by centrifugation at 4500 rpm. In order to remove all the nitrate salts and unreacted NaDDS, the solids were first refluxed in an ethanol–water (1:1 in volume) mixture and then, in acetone for 8 h respectively. This procedure was repeated twice. For the generation of mixed oxides the same process as in the co-precipitation method was used [8]. Six precursors were obtained (HT-M) and 6 mixed oxides (OM-M and OM-M-ME) in which HT refers to hydrotalcite, OM mixed oxide, ME microemulsion and M can be either Mn, MnCu or MnCo.

2.2. Characterization X-ray diffraction (XRD) analysis was carried out on a Panalytical X’pert PRO X-ray diffractometer equipped with CuKα radiation (λ = 1.5418 Å). The particle size distribution of the nano-powder was measured by a laser particle size analyzer (Mastersizer 2000 Particle Analyzer, Malvern Instruments Ltd.). Particle size was reproducible within 20%. The specific surface area was measured by nitrogen adsorption using the BET method with a Micromeritics ASAP 2020 surface area analyzer.

2.3. Catalytic test: ethanol oxidation reaction

Catalytic testing was carried out with a total flow rate of 200cm3/min, 0.200g of the catalyst and ethanol concentration was 1000 ppm. Samples were pre-treated in airflow at 350°C for 2h. Then, the ignition curve was obtained by cooling at 1.0°C/min from 375 to 50°C. The conversion was calculated by measuring the ethanol disappearance and the water production by means of a mass spectrometer (Balzers Omnistar) and the CO2 production by an on-line IR detector (Sensotrans IR). Agreement between the three conversion curves thus obtained confirmed that no intermediate products were produced.

3. Results and discussion In figure 1 the XRD profiles of the hydrotalcite like precursors substituted by Mn and by binary Mn-Co and Mn-Cu mixes synthesized by the conventional coprecipitation method are shown. The profiles present a group of signals that are characteristic of the hydrotalcite structure (JCPDS No. 89-0460). However, the formation of a contaminant corresponding to rhodocrosite (MnCO3; JCPDS No. 44-1472), is identified. This is a crystalline phase which appears with a greater incident in the profiles of mixed binaries, suggesting that the addition of another metal favors the segregation of said phase.

515

Mn, Mn-Cu and Mn-Co mixed oxides as catalysts

The sample HT-Mn-ME synthesized by means of the microemulsion methodology reveals the formation of the hydrotalcite phase, with signals of lesser intensity and greater amplitude, which can be associated with a possible reduction of crystallinity. Signals also appear corresponding to Rhodocrosite and Hausmannita (Mn3O4; JCPDS No. 24-0734), this latter phase associated with the oxidation of the sample during the preparation, showing that the Mn+2 is partially oxidized [4]. Figure 1. XRD of hydrotalcites a) HT-Mn b) HT-MnCu c) HT-MnCo and d) HT-MnME.

The determination of the particle size distribution of the precursor solids showed diameters around 30 nm for the HT-Mn-ME with a homogeneous distribution, and of 120 nm for HT-Mn. This tendency (smaller particle size for the precursors obtained by microemulsion) is preserved in the mixed oxides. The textural analysis of the mixed oxides shows differences in the form of the hysteresis. The solids obtained by coprecipitation present type H1 hysteresis which corresponds to non interconnected agglomerated or compact pores, while the solids derived from microemulsion present hysteresis H2 characteristic of pores with narrow and wide sections with possible interconnected canals [9]. With regard to the BET area a direct correlation is not observed between the synthesis methodologies, and the values of both series of mixed oxides that oscillate between 75 and 115 m2g-1.

3.1. Catalytic activity The results of the catalytic activity for the total oxidation of ethanol are resumed in fig 2 and table 1. All the catalysts presented a total selectivity to CO2 and catalytic activity superior to the reference (1% Pt/Al2O3) which presented a more inclined sigmoid curve. According to T50 (temperature at which the production to CO2 reaches 50%) the catalysts based on the binary system of the active phases are more active than those in which only Mn is used (OM-MnCo is the most active- T50 the lowest). 100

C o n v e r s io n to C O 2 (% )

90

OM-Mn

80

Pt/Al2O3

70 60

OM-Mn-ME

OM-CuMn

50 40

OM-CoMn

30 20 10 0 50

100

150

200

250

300

350

Temperature °C

Figure 2. Conversion of CO2 vs. the reaction temperature.

516

D. Aguilera et al. Table 1. Temperature at which the conversion of CO2 reaches 50% (T50). Solid

T50 (ºC)

OM-Mn OM-Mn-ME OM-CoMn OM-CoMn-ME OM-CuMn OM-CuMn-ME Ref. 1% Pt/Al2O3

217.6 224.0 197.6 214.2 208.8 230.6 227.6

Regarding the solids obtained by means of the microemulsion methodology (in figure 2, the behavior of OM-Mn-ME was representative) the catalytic behavior is very similar to the solid obtained through the conventional methodology. However, the slight reduction in the T50 value can be associated to the formation of the contaminated hydrotalcite phase which generates segregated phases that can affect the dispersion and distribution of active metal in the final catalyst without being able to establish a direct correlation between the particle sizes (much less in the solids obtained by microemulsion) and the catalytic activity.

4. Conclusions Two different synthesis methodologies were used to prepare mixed oxides from a Mn hydrotalcite and binary mixes of Mn-Cu and Mn-Co. The samples in which the active Mn phase is accompanied with Co generates solids of low crystallinity and elevated superficial area values which are highly active in the oxidation of ethanol. The microemulsion methodology permitted the obtaining of hydrotalcite like precursor with a much smaller particle size and with different textural properties in comparison with those obtained by the conventional methodology. However, the conditions of synthesis did not permit the obtaining of solids with only one hydrotalcite phase which had repercussions in the catalytic behavior of the resulting oxide.

Acknowledgment Authors are grateful to the DIB-UN projects HERMES code 11166, 10864 and 10854 for partial financial support and to Professor Mario Montes Ramirez (Universidad del País Vasco - Facultad de Química - Donostia - San Sebastián) for the catalytic reaction.

References [1] M.R. Morales, B.P. Barbero and L.E. Cadús, Applied Catalysis B: Environmental, 67 (2006) 229. [2] A. Vaccari, Applied Clay Science, 14 (1999) 161. [3] S. Eriksson, U. Nylén, S. Rojas and M. Boutonnet, Appl. Catal., A, 265 (2004) 207. [4] S. Velu, N. Shah , T.M. Jyothi and S. Sivasanker, Microporous Mesoporous Mater., 33,(1999) 61. [5] C.E. Daza, J. Gallego, F. Mondragón, S. Moreno and R. Molina, Fuel, 89 (2010) 592. [6] J. He, B. Li, D.G. Evans and X. Duan, Colloids Surf., A, 251 (2004) 191. [7] G. Hu and D. O'Hare, Journal of the American Chemical Society, 127 (2005) 17808. [8] M.E. Pérez-Bernal, R.J. Ruano-Casero, F. Benito and V. Rives, Journal of Solid State Chemistry, 182 (2009) 1593. [9] J.B. Condon, Surface Area and Porosity Determinations by Physisorption, ElsevierScience, Amsterdam, 2006, p. 1.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Mesoporous manganese oxide catalysts for formaldehyde removal: influence of the cerium incorporation Jhon Quiroz -Torres, a,b,c Rémy Averlant, a,b,c Jean-Marc Giraudon,a,b,c Jean-François Lamoniera,b,c a

Univ Lille Nord de France, F-59000 Lille, France CNRS, UMR8181, France c USTL, Unité de Catalyse et de Chimie du Solide F-59652 Villeneuve d’Ascq, France b

Abstract Manganese oxide mesoporous materials were prepared by using template assisted method followed by an acidic treatment. The cerium addition to manganese oxide mesoporous structure was also studied. All the solids were characterized by XRD and specific surface area and pore size distributions were calculated from nitrogen sorption studies. XRD results suggested the formation of the MnOx-CeO2 solid solution with the fluorite-type structure. The BET surface area values and the pore size distributions allowed to conclude the important role of the surfactant by the creation of narrow mesopores in the material. Keywords: Manganese and cerium oxides, mesoporous materials, VOC

1. Introduction Formaldehyde (HCOH) is regarded as the major indoor air pollutant emitted from widely used building and decorative materials. Long-term exposure to indoor air containing even few ppb of HCOH may cause adverse effects on human health. Catalytic oxidation is one of the most promising technologies for controlling HCOH pollutant. For instance, noble metals supported catalysts have been reported to possess high activity for the complete oxidation of hundreds of ppm of HCOH into CO2 and H2O [C. Zhang and H. He]. However, the concentration of indoor HCOH emission is much lower (<1 ppm) and the corresponding catalytic treatment is energy consuming at this condition. Consequently, for this application it is crucial to develop a material combining high capacity adsorption and catalytic performances. Mixed-valent octahedral molecular sieves (OMS) of manganese oxides, which can have applications in energy storage, in acid catalysis and in ion-exchange processes, are extensively reported. The generation of mixed-valent manganese oxide mesoporous materials might lead to versatile system for oxidation catalysis [S.L. Suib]. Recently, high surface area manganese mesoporous material has been obtained through a surfactant-assisted wet-chemistry route [K. Sinha]. This mesoporous oxide material is able to eliminate VOCs at low temperature. With the possibility of multiple valencies for Mn species and the high efficient of redox couple (Ce4+/Ce3+) in oxidation reactions, the mixed MnOx-CeO2 samples are very interesting materials for further detailed investigations. In this work we present the strategies of incorporating cerium ions into the ordered phase of mesoporous manganese oxide catalysts using the surfactant assisted wet-chemistry route.

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2. Experimental 2.1. Sample preparation A precipitate formed by mixing an aqueous solution of Mn(NO3)·6H2O (16.7 g in 150 mL of H2O) with an aqueous solution of NaOH (4.8 g NaOH in 50 mL of H2O) was added to an aqueous solution of cetyltrimethylammonium bromide (67 g in 150 mL of H2O). The resulting mixture was heated to 75 °C and then stirred for 1 h. The final gel obtained in a sealed beaker was transferred to an oven and heated for 12 h at 75 °C. The solid residue was filtered, washed with water, dried in air and finally calcined at 500°C for 6 h (1°C min -1). The calcined sample (6.0 g) was treated with an aqueous solution of H2SO4 (120 mL - 10 mol L-1) by stirring in a beaker for 1 h. The final product was filtered, and the residue was washed with water and dried at 105 °C. Pure manganese oxide has been also prepared without surfactant. Three binary oxides (n)MnOx- (1-n)CeO2 (n = 0.25, 0.50, 0.75) were prepared by co-precipitation from aqueous solution of cerium and manganese nitrates using the same procedure as below.

2.2. Characterization X-ray diffraction (XRD) measurements were made at room temperature (λ =1.5418 Å). The diffraction patterns have been indexed by comparison with the Joint Committee on Powder Diffraction Standards (JCPDS) files. Nitrogen adsorption/desorption isotherms were obtained at -77K. BET and BJH analyses were used to determine the total specific surface area, pore volume and pore size distribution of the samples.

3. Results and discussion 3.1. XRD analyses a)

(3)

b)

(3)

(2)

(2)

(1)

0

10

20

30

40 2θ (°)

50

60

70

80

(1)

0

10

20

30

40

50

60

70

80

2θ (°)

Fig. 1. XRD patterns of pure manganese oxides a) with surfactant b) without surfactant (1) as made, (2) calcined and (3) acid treated.

The wide angle powder XRD patterns of the solids after each step of the synthesis, (1) solid dried in air, (2) calcined and (3) treated with sulfuric acid are shown in Fig. 1. The brown solid obtained (dried in air) showed diffraction peaks which could be attributed to the crystalline phase Mn3O4 (Hausmanite). After calcination the solid turned black and showed diffraction peaks due to the hausmanite phase and also peaks of lower intensity ascribed to the Mn5O8 phase. After acidic treatment the peaks due to the hausmanite phase disappeared and only peaks due to the monoclinic phase Mn5O8 were observed. In brief, the samples underwent the following phase transitions: Mn3O4 (dried in air), Mn3O4 + Mn5O8 (calcined) and Mn5O8 (H2SO4 treated). During the calcination step, the Mn3+ of the Mn3O4 was partially oxidized into Mn4+ to give Mn5O8

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which stabilizes Mn2+ species from the Mn3O4 phase [B. Gillot]. After the acidic treatment, the hausmanite phase was totally oxidized to generate exclusively the Mn5O8 monoclinic phase. The XRD patterns obtained after each synthesis step were the same for both preparations but a higher cristallinity of the pure manganese oxides was observed when surfactant was employed (Fig. 1a and 1b).

a)

b) n= 0 n= 0

n= 0.25

n= 0.25

n= 0.50

n= 0.50

n= 0.75

n= 0.75 46

n= 1 0

10

20

30

40

50

60

70

48

50

2θ (°)

80

2θ (°)

Fig. 2. Powder diffraction patterns of (n)MnOx-(1-n)-CeO2 after calcination at 500°C a) wide angle and b) expanded view of the (220) peaks.

The cerium incorporation to the MnOx was also studied by XRD analysis (Fig. 2). For samples with n values between 0 and 0.5 the main peaks in the patterns were those of a fluorite structure similar to that of pure ceria. But for samples with n = 0.25 and n = 0.50 the diffraction lines were broader. The diffraction profiles of samples with n≥0.75 showed the crystallization of Mn3O4 together with fluorite structure. Figure 2b (zoom of Fig. 2a) showed the incremental shift to higher angles of the (220) diffraction peak for the fluorite phase related to the formation of Ce-Mn-O solid solutions. The substitution of Ce4+ by Mn3+ in the fluorite structure seems to be possible when considering their structural similarity, the incremental shift of each diffraction line to higher angles is associated with the smallest ionic radius of Mn3+ (0.66 nm) in comparison with that of Ce4+ (0.94 nm) [M. Machida]. Considering the 2θ maximal value of the (220) diffraction peak (Fig. 2b), it seems that the solubility limit of manganese ions into the ceria structure is reached for the solid with n = 0.5.

3.2. Porous Properties Table 1. BET surface areas of (n)MnOx-(1-n)CeO2. n

Calcined samples (m2 g-1)

Acid treated samples (m2 g-1)

Surface area deviation (%)

1

20

82

76

0.75

91

160

43

0.50

84

98

8

0.25

130

137

5

1*

31

121

74

* Synthesized without surfactant

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According to the highest full width at half maximum of the diffraction peaks of calcined sample with n = 0.25, the surface specific area (SSA) of the corresponding solid was the highest (Table 1). The decrease in the SSA value related to the n increase could be due to the manganese oxide formation having low BET surface area. The acidic treatment had important effect on the SSA of pure manganese oxides (increase of 75%). However, this effect was strongly attenuated with the increase in cerium concentration in the sample (Table 1). Figure 3 shows the pore size distribution of the different oxides before and after the acidic treatment. For the calcined samples the pore size distribution was broader and centered between 10 and 20 nm. After the acidic treatment and for n = 1 and n = 0.75, a narrow mesoporous distribution centered at 3 nm and 2 nm, respectively, was observed. Based on Sinha works [K. Sinha], this result could be explained by the formation of interwoven nanofibrous aggregates with wormhole-like mesoporosity, however the formation of these narrow mesopores took place in a minor proportion for n = 0.75. Finally for n = 0.50 and 0.25, no significant effect of the H2SO4 treatment was observed. In the binary oxides, the mechanism involved in the mesoporosity creation is clearly related to the manganese oxide presence. Moreover the interaction of manganese species with surfactant at the beginning of the synthesis is crucial to generate narrow mesopores. Indeed without surfactant, pure manganese oxide after acidic treatment didn’t reveal such mesoporosity. n= 1

10

100 Pore Radius (A)

10

100 Pore Radius (A)

n= 0.25

n= 0.50

n= 0.75

10

100 Pore Radius (A)

10

100 Pore Radius (A)

Figure 3. Pore size distribution of (n)MnOx-(1-n)-CeO2 calcined (circle symbol) and acid treated (cross symbol).

4. Conclusion We have successfully synthesized pure and mixed manganese and cerium oxides using the surfactant assisted wet-chemistry route followed by acidic treatment. The XRD analyses of calcined samples reveal the substitution of Ce4+ by Mn species in the fluorite structure forming a solid solution with a solubility limit of 50% Mn. For all the solids, the acidic treatment has important effect in the BET surface area. For the binary oxides a narrow pore size distribution was created after H2SO4 treatment, with a major extension for solids having high Mn content.

References H. He, 2005, Perfect catalytic oxidation of formaldehyde over a Pt/TiO2 catalyst at room

temperature, 6, 211-214. S.L. Suib, 1997, Manganese Oxide Mesoporous Structures: Mixed-Valent Semiconducting Catalyst, Science, 276, 926-930. A.K. Sinha, 2008, Preparation and Characterization of Mesostructured δ-Manganese Oxide and Its Application to VOCs Elimination, J. Phys. Chem. C, 112, 16028-16035. B. Gillot, 2001, Particle size effects on the oxidation-reduction behavior of Mn3O4 hausmannite, Materials Chemistry and Physics, 70, 54-60. M. Machida, 2000, MnOx-CeO2 Binary Oxides for Catalytic NOx Sorption at Low Temperatures. Sorptive Removal of NOx, 12, 3158-3164.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Nickel nanoparticles with controlled morphologies application in selective hydrogenation catalysis Julie Aguilhona,b, Cédric Boissièreb, Olivier Durupthyb, Cécile Thomazeaua, Clément Sanchezb a

IFP Lyon, Rond-point de l’échangeur de Solaize, BP 3 - 69360 Solaize - France UPMC Univ Paris 06, UMR 7574, Laboratoire de Chimie de la Matière Condensée de Paris, Collège de France, Bâtiment C-D, 11 place Marcelin Berthelot, 75231 Paris Cedex 05, France

b

Abstract Nickel-based structures were obtained when alumina is directly added to the nanoparticles solution. Two morphologies were observed: whiskers and stacked platelets. They are constituted by a lamellar phase involving both nickel and aluminium. After a reducing treatment under H2 at 410°C, shape-controlled nickel nanoparticles were formed (MTP, tetrahedra, rods). The link between nanoparticles morphologies and catalytic activities and selectivities has been examined using a model reaction of selective hydrogenation. First results show an influence of the morphology on the adsorption of the reactants and a significant increase of the selectivity towards olefins. Keywords: nickel, nanoparticles, morphologies, catalysis

1. Introduction Nickel-based catalysts are widely used in petrochemistry for selective hydrogenation of poly-unsaturated compounds formed during steam cracking, such as dienes and/or alkynes. One of the most challenging tasks in catalysis by metals is the synthesis of metallic particles with narrow-sized distributions and homogeneous physico-chemical properties in order to establish clear reactivity–structure relationships[1]. Conventional metallic catalysts are constituted with supported nanoparticles, which are usually represented by a truncated cuboctahedra (Van Hardeveld and Hartog [2]). These nanoparticles present several different active sites: (111) and (100) facets, corners and edges, each one with different catalytic properties. So, the generation of nanoparticles with defined morphologies could favor one type of exposed crystallographic plane and adjust the ratio between atoms on corners or edges or facets. This will give the possibility to tune the activity and/or the selectivity of a catalytic system for a given reaction. Therefore, we proposed the synthesis of Ni(0) nanoparticles with controlled size and morphology, in order to expose mainly one type of site. This allowed us to study the influence of the exposed crystallographic plane on catalytic properties and thereby to improve our knowledge in nanodesign of metallic catalysts.

2. Experimental 2.1. Catalyst preparation Degazed water was used as a solvent to prevent further oxidation by oxygen. First, nickel chloride hexahydrate precursor (Fluka, 98%), a surfactant, hexadecylammonium bromide (Sigma, 98%) and the selected reducer (either sodium borohydride NaBH4 (Aldrich, 98%) or aqueous hydrazine 65% (Sigma-Aldrich, 98%)) were introduced in

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the reaction flask. The solution was then mechanically stirred during 1h at 30°C with NaBH4 (synthesis 1) or 3h at 70°C with N2H4 (synthesis 2) and afterward alumina support (δ-Al2O3 support (specific area 130 m2/g, porous volume: 1.04 cm3/g), previously crushed and sieved in the 63–100 mm range), was added. After three additional hours of stirring, suspensions were cooled down at room temperature and filtered. Catalysts were then dried at 30°C. The two catalysts prepared are designated as Ni-NaBH4 for synthesis 1 and Ni-N2H4 for synthesis 2.

2.2. Characterization techniques Nickel content in the catalyst was determined with a Varian AA1275 atomic absorption spectrophotometer. Transmission electron micrographs (TEM) were obtained on a Phillips CM20 STEM operating at 200kV. Samples were prepared by the deposition of a drop of the nanoparticles suspension on a carbon coated copper grid. X-ray diffraction patterns were recorded on a Philips PW 1820 diffractometer with a classical θ/2θ geometry using Cu-Kα radiation.

2.3. Catalytic test Styrene/isoprene hydrogenation was performed in liquid phase using a laboratory scale stainless-steel and perfectly stirred batch reactor. One gram of catalyst initially reduced during 16 h under H2 at 410°C K (ramp 5°C/min) was transferred under Ar in a glove bag into the batch reactor filled with 214 mL of n-heptane. The catalyst was then put into contact with 34 g of a styrene/isoprene solution at 70°C under 35 bars of H2. A pressure gauge maintains the pressure constant inside the reactor at 35 bars of H2. The rate of the reaction was followed by the loss of H2 pressure in the storage bottle. Samples are analysed by gas chromatography.

3. Results and discussion 3.1. Catalysts characterization after impregnation Nickel-based structures are observed after impregnation of the solutions on alumina. Whiskers are evidenced for the Ni-NaBH4 preparation and stacked platelets are observed for the Ni-N2H4 preparation. For the two syntheses, X-ray diffraction patterns indicate the presence of a lamellar phase probably involving both nickel and aluminium (Nickel Aluminium Carbonate Hydroxide NiAl(CO3)(OH)3 [ICDD 00-048-059]). When syntheses are carried out without support in solution, such structures and morphologies are not observed. This shows that the presence of alumina support is necessary to induce the formation of the observed structures and that a strong interaction should exist between nickel and alumina.

3.2. Catalysts characterization after treatment under H2

Well-faceted nanoparticles in the 20-50 nm size range were observed (Fig.1) for the two methods of preparation after treatment under H2. However X-Ray diffraction patterns indicate that two nickel species are evidenced on the support: metallic cubic facecentered nickel corresponding to the observed well-faceted nanoparticles and the initial lamellar phase still present on the support. From the analysis of frequencies, morphologies and mean sizes of the different nanoparticles, it appears that, among supported nanoparticles, icosahedra with a mean size of 26 nm are more often observed (frequency of 49%). Consequently, the two catalysts contains Ni(0) nanoparticles exposing mainly {111} faces.

523

Intensity (a.u.)

Nickel nanoparticles with controlled morphologies application

10

20

30

50 nm

40

50

60

70

2-theta (°)

Fig. 1. TEM image of the faceted nanoparticles on alumina after reducing treatment and XRD (Ni-N2H4 catalyst). : Nickel ° [ICDD 00-004-0850] X : Lamellar phase : Nickel Aluminium Carbonate Hydroxide NiAl(CO3)(OH)3 (hydrotalcite) [ICDD 00-048-0593] O : Alumina delta [ICDD 00-016-0394]

As the only carbon source, the amount of CTAB could be recalculated from the analysis of carbon content, after impregnation and drying. The amount obtained (3,44% or 3,84%) is in good agreement with the initial CTAB concentration solution (4,15% or 6,56%), taking into account that a part of CTAB is eliminated during filtration. The organic part of CTAB is removed during the reducing treatment since no carbon remains on the catalyst after this step. Nevertheless, the evolution of the amount of Br was shown to be independent from the CTAB’s one. Indeed, even after reduction, Br (1,17%/1,81%) still remains on the catalyst. The influence of this element on the catalytic properties could be significant and will be further investigated.

3.3. Catalytic properties The selective hydrogenation of a styrene/isoprene mixture was used as model reaction to evaluate the catalytic properties in selective hydrogenation of these catalysts. First, relative conversion of styrene and isoprene for both catalysts are compared with a commercial nickel catalyst (cuboctahedric nickel nanoparticles as metallic phase with an 8 nm mean size, 8% weight of nickel on alumina), Fig. 2 a). a)

b) 100

90 80

90

70

80

60 50

70

40

60

30

Selectivity (%)

Isoprene Conversion (%)

100

50

20 10 0

20

40

60

Styrene Conversion (%)

80

100

50

60

70

80

90

40 100

Isoprene Conversion (%)

Fig. 2. a) Evolution of isoprene conversion vs styrene conversion b) Evolution of catalysts selectivity vs isoprene conversion. ■ : Commercial nickel catalyst (reference) ▲ : Ni-NaBH4 Catalyst ♦ : Ni-N2H4 Catalyst

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Isoprene is converted more rapidly than styrene for the commercial catalyst. While, for the two catalysts with well-faceted Ni(0) nanoparticles, isoprene and styrene are converted simultaneously. This could be linked to a modification of the relative adsorption strengths of the two reactants for the well-faceted catalysts, towards quite similar adsorption properties. This modification of adsorption strengths is attributed to the well defined morphologies and thus to the modification of the exposed catalytic sites with mainly sites located on the {111} faces. The selectivity towards olefins (ratio Σ olefins Σ (olefins + paraffins) is also evaluated and reported in Fig. 2 b). For an isoprene conversion of 85%, the selectivity of the reference catalyst is 92% while it is 96% for both well-faceted catalysts. This clearly demonstrates that the modification of the exposed catalytic sites also induces an increase of the selectivity towards olefins formation.

4. Conclusion Supported nickel nanoparticles with well-defined morphologies (combination of icosahedra, rods, cubes, polyhedra) were obtained through optimized syntheses in presence of a nickel precursor, a surfactant, a strong reducer and the alumina support. Prepared catalysts are active and selective in the hydrogenation reaction of a styrene/ isoprene mixture. The modification of the exposed actives sites due to the well defined morphologies induces a modification of the adsorption properties of the reactants, thus leading to an improvement of the selectivity when compared to a conventional catalyst. A better understanding of the mechanisms involved in the formation of the wellfaceted nickel nanoparticles might be useful to control the morphology of the as prepared nanoparticles in order to select the type of exposed active sites. Consequently, it could be possible to determine the most active and selective sites, for a given reaction, depending on the experimental conditions of the reaction.

References 1. 2. 3.

G. Ertl, H. Knözinger, J. Weitkamp (Eds.), Handbook of Heterogeneous Catalysis, vol. 4, Wiley-VCH, Weinheim, (1997) 1560. R. Van Hardeveld, F. Hartog, Surface Science 15 (1969) 189. L.D. Marks, D.J. Smith, Journal of Crystal Growth 54 (1981) 425.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Behavior of NiMo(W)/Zr-SBA-15 deep hydrodesulfurization catalysts in presence of aromatic and nitrogen-containing compounds Alejandro Soriano, Pedro Roquero, Tatiana Klimova* Facultad de Química, Universidad Nacional Autónoma de México (UNAM), Cd. Universitaria, Coyoacán, México D.F., 04510, México

Abstract NiMo and NiW catalysts supported on Zr-SBA-15 and γ-alumina were characterized and tested in simultaneous hydrodesulfurization of DBT and 4,6-DMDBT in absence and presence of quinoline or naphthalene. NiMo and NiW catalysts supported on ZrSBA-15 showed higher HDS activity than those supported on γ-Al2O3. In presence of quinoline, all catalysts lost their activity for HDS of 4,6-DMDBT. Naphthalene addition had a small effect on NiMo/Zr-SBA-15 catalyst and almost none on NiW/Zr-SBA-15. Keywords: NiMo and NiW catalysts, Zr-SBA-15, deep hydrodesulfurization, dibenzothiophene, 4,6-Dimethyldibenzothiophene

1. Introduction Deep hydrodesulfurization (HDS) of diesel fuel has attracted much attention in recent years due to environmental requirements demanding significant improvement in S elimination. Different new catalysts have been reported in literature as highly active for the elimination of refractory S-containing aromatic compounds. In our group, interesting results were obtained with NiMo catalysts supported on ZrO2-modified SBA-15 materials [1]. Activity of these catalysts evaluated in a model reaction of 4,6dimethyldibenzothiophene (4,6-DMDBT) HDS was much higher than that of a conventional NiMo/γ-Al2O3 sample. However, it is not clear yet how these catalysts will behave in real conditions, in presence of different sulfur- and nitrogen-containing compounds and aromatics. The aim of the present work is to answer this question and to compare the behavior of NiMo and NiW catalysts supported on ZrO2-containing SBA15 with that of γ-Al2O3 supported counterparts.

2. Experimental The pure silica SBA-15 was synthesized according to literature [2] using the triblock copolymer Pluronic P123 as the structure-directing agent and TEOS as the silica source. SBA-15 support with 25 wt. % of ZrO2 (Zr-SBA-15) was prepared by incipient wetness impregnation of Zr(IV) propoxide solution in dry n-PrOH on the SBA-15 [1]. NiMo and NiW catalysts supported on Zr-SBA-15 and conventional γ-Al2O3 were prepared by incipient wetness impregnation of aqueous solutions of ammonium heptamolybdate, (NH4)6Mo7O24·4H2O, ammonium metatungstate, (NH4)6H2W12O40·xH2O, and nickel nitrate, Ni(NO3)2·6H2O. Mo (W) was impregnated first. After each impregnation, the catalysts were dried (100oC, 24 h) and calcined (500oC, 4 h). The nominal composition of the catalysts was 8×10–4 mol of MoO3 (WO3) and 4×10–4 mol of NiO per gram of catalyst. Catalysts were characterized by N2 physisorption, small- and wide-angle XRD, UV-Vis DRS, TPR and HRTEM. The catalytic activity tests were performed in a batch

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reactor at 300°C and 7.3 MPa total pressure for 8 h. Before the activity tests, the catalysts were sulfided ex situ at 400oC for 4 h in a stream of 15 vol. % of H2S in H2. Catalytic tests were performed for simultaneous HDS of DBT (1300 ppm of S) and 4,6DMDBT (500 ppm of S) in absence and presence of quinoline (QN, 300 ppm of N, 0.0166 mol/L) and naphthalene (NP, 0.0166 mol/L). Hexadecane was used as solvent.

3. Results and discussion 3.1. Catalyst characterization Results from textural characterization of the supports and catalysts (Table 1) indicate that the incorporation of Ni and Mo (W) species on the supports’ surface produced a decrease in the textural properties, which was stronger for the samples supported on ZrSBA-15. This decrease points out the possibility of some obstruction of the Zr-SBA-15 pores by deposited metal oxide species. Nevertheless, both NiMo and NiW catalysts supported on ZrO2-containing SBA-15 had surface areas twice larger than those of γAl2O3-supported samples. Table 1. Texture of NiMo and NiW catalysts supported on Zr-SBA-15 and γ-Al2O3. SBET (m2/g)*

Sample

Sμ (m2/g)

Vp (cm3/g)

Vμ (cm3/g)

Dads (Ǻ)

Zr-SBA-15 605 105 0.73 0.04 NiMo/Zr-SBA-15 387 37 0.53 0.01 NiW/Zr-SBA-15 396 47 0.51 0.02 γ-Al2O3 196 0.47 NiMo/γ-Al2O3 184 0.37 NiW/γ-Al2O3 169 0.35 * SBET, specific surface area; Sμ, micropore area; VP, total pore volume; Vμ, micropore Dads, pore diameter determined from the N2 adsorption isotherm by the BJH method.

60 60 61 103 97 90 volume;

Characterization of NiMo and NiW catalysts by UV-Vis DRS (Fig. 1) clearly shows the differences in the characteristics of Mo and W species supported on Zr-SBA15 and γ-Al2O3. ZrO2-containing SBA-15 support provides a better dispersion to W and Mo species in comparison to the γ-alumina. DRS spectra of all catalysts supported on γAl2O3 show broad absorption bands indicating the presence of complex mixtures of octahedral and tetrahedral Mo (W) species of different degrees of agglomeration.

30

35

(a)

NiW W NiMo Mo

F (R)

25 20

30

(b)

NiW W NiMo Mo

25

F (R)

35

15

20 15

10

10

5

5

0

0 200

400

600

λ (nm)

800

200

400

600

800

λ (nm)

Fig. 1. UV-Vis DRS spectra of unpromoted and Ni-promoted Mo and W catalysts supported on Zr-SBA-15 (a) and γ-Al2O3 (b).

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Behavior of NiMo(W)/Zr-SBA-15 deep HDS catalysts

TPR results (Figs. 2 and 3) also confirm that different types of Mo and W species are present in samples supported on Zr-SBA-15 and γ-Al2O3. On Zr-SBA-15 support, a higher proportion of dispersed octahedral Mo (W) oxide species easy to reduce was observed. The morphology of sulfided MoS2 (WS2) particles was also different on both supports (HRTEM). Shorter particles with larger number of stacked layers were found on Zr-SBA-15 (average length 35-36 Å, average stacking 2.0-2.2 layers), whereas on γAl2O3 they were larger (48 and 55 Å for MoS2 and WS2) and less stacked (~1.7 layers).

649 475

NiMo

669

Mo

379

Signal (a.u.)

Signal (a.u.)

(b)

(a)

354

541

427

770

NiMo 779

Mo .) (u D C lT a ñ e S

.) (u D C lT a ñ e S

200

400

600

800

Temperature (oC)

200

1000

400

600

800

1000

Temperature (oC)

Fig. 2. TPR profiles of Mo and NiMo catalysts supported on Zr-SBA-15 (a) and γ-Al2O3 (b).

(b)

763 839 853

NiW

398

503 621

747

631 .) (u D C lT a ñ e S

W

200

947

Signal (a.u.)

Signal (a.u.)

(a)

963

NiW .) (u D C lT a ñ e S

400

600

800

Temperature (oC)

1000

461

621

W

200

629

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Temperature (oC)

1000

Fig. 3. TPR profiles of W and NiW catalysts supported on Zr-SBA-15 (a) and γ-Al2O3 (b).

3.2. Catalytic behavior Catalytic activity tests (Table 2) showed that both NiMo and NiW catalysts supported on Zr-SBA-15 were more active in HDS than those supported on γ-alumina, especially in hydrodesulfurization of 4,6-DMDBT. This activity trends can be due to the particular characteristics of oxide and sulfided species on Zr-SBA-15, namely, the presence of a higher proportion of Mo (W) species easy to reduce and to sulfide, and to the particular morphology of the sulfided MoS2 (WS2) phases formed by shorter and more stacked slabs than on γ-alumina. Addition of quinoline in the reaction mixture slightly decreased the activity of all studied catalysts in HDS of DBT, but completely inhibited the 4,6-DMDBT transformations. In presence of quinoline, DBT elimination occurred almost completely through the direct desulfurization (DDS) route, indicating that quinoline poisoned the sites active for the hydrogenation pathway of the reaction. Since HDS of 4,6-DMDBT occurs preferentially through the hydrogenation route [3], it explains the complete loss of the catalysts’ activity for this reaction in presence of quinoline.

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Naphthalene addition in the reaction mixture had a smaller effect on the HDS activities of the tested NiMo catalysts. However, it can be noted that for the conventional NiMo/γ-Al2O3 catalyst, the retardant effect of naphthalene was more significant for hydrodesulfurization of 4,6-DMDBT than for that of DBT (Table 2). This result can be explained considering the competitive adsorption between naphthalene and sulfur compounds on the catalyst’s sites active for the hydrogenation pathway of HDS. In the same conditions, NiMo catalyst supported on Zr-SBA-15 showed only a slight decrease in both DBT and 4,6-DMDBT conversions. Probably, this is related with a higher hydrogenation ability of this catalyst induced by the Zr-SBA-15 support. HDS activity of the two NiW catalysts studied was almost not affected by the presence of naphthalene independently of the support used and the morphology of the WS2 active phase. Above results from the catalytic behavior of NiMo and NiW catalysts supported on Zr-SBA-15 and γ-alumina show that both the support used and, in a higher extent, the nature of the active phase affect the resistance of HDS catalysts to be inhibited by aromatics. ZrO2containing SBA-15 support seems to be appropriate for the preparation of NiW catalysts highly active for deep HDS with good resistance to the presence of aromatics. Table 2. Conversions (%) of DBT and 4,6-DMDBT obtained at 8 h reaction time. Catalyst

Without QN and NP

In presence of QN

In presence of NP

DBT

4,6-DMDBT

DBT

4,6-DMDBT

DBT

4,6-DMDBT

NiMo/Zr-SBA-15

96

88

90

0.0

92

84

NiW/Zr-SBA-15

91

79

84

0.4

90

78

NiMo/γ-Al2O3

93

56

87

1.6

85

28

NiW/γ-Al2O3

65

27

62

0.3

76

29

4. Conclusions NiMo and NiW catalysts supported on ZrO2-containing SBA-15 material showed higher activity in HDS of both DBT and 4,6-DMDBT than their γ-alumina-supported analogs. Quinoline added in the reaction mixture poisoned the catalyst’s sites active for hydrogenation. As a result, all catalysts lost their ability for HDS of 4,6-DMDBT. However, they were still able to desulfurize DBT via the direct desulfurization route. The presence of naphthalene slightly decreased the HDS activity of NiMo/Zr-SBA-15 catalyst, whereas a significant decrease in HDS of 4,6-DMDBT was observed for NiMo/γ-Al2O3. Both NiW catalysts studied were not affected by naphthalene addition.

Acknowledgements Financial support by DGAPA-UNAM, Mexico (grant IN-110609) is gratefully acknowledged. The authors thank I. Puente Lee, C. Salcedo Luna and M. Aguilar Franco for technical assistance with HRTEM and XRD characterizations.

References 1. 2. 3.

O.Y. Gutiérrez, F. Pérez, G.A. Fuentes, X. Bokhimi, T. Klimova, Catal. Today, 130 (2008) 292. D. Zhao, Q. Huo, J. Feng, B. Chmelka, G. Stucky, J. Am. Chem. Soc., 120 (1998) 6024. C. Song, X. Ma, Appl. Catal. B: Env., 41 (2003) 207.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Effect of citrate addition in NiMo/SBA-15 catalysts on selectivity of DBT hydrodesulfurization Diego Valencia,a Isidoro García-Cruz,b Tatiana Klimovaa* a

Facultad de Química, Universidad Nacional Autónoma de México (UNAM), Cd. Universitaria, Coyoacán, México D.F., 04510, México b Programa de Ingeniería Molecular, Instituto Mexicano del Petróleo, Eje Central Lázaro Cárdenas 152,Col. San Bartolo Atepehuacán, México D.F., 07730, México

Abstract NiMo catalysts supported on SBA-15 were prepared by coimpregnation and successive impregnation methods with the addition of citric acid in the impregnation solutions. Addition of citric acid resulted in an increase in both catalysts’ activity in dibenzothiophene HDS and selectivity towards the direct desulfurization route, which was due to an increase in the MoS2 dispersion and in the amount of Ni-Mo-S species. Keywords: NiMo catalysts, SBA-15, citric acid, hydrodesulfurization, dibenzothiophene

1. Introduction Nowadays, the need to improve the removal of sulfur from gasoline and diesel oil by means of deep hydrodesulfurization (HDS) is managed by new environmental legislations regarding fuel specifications. Most common HDS catalysts are molybdenum disulphide (MoS2) nanocrystallites, promoted by Co or Ni atoms and deposited on a high specific surface area support. In general, SiO2-supported catalysts show low HDS activity. However, recently we reported interesting results obtained with NiMo and NiW catalysts using SBA-15 silica [1,2]. The aim of the present work is to study the effect of the addition of citric acid (CA) during the preparation of NiMo catalysts supported on SBA15 on their activity and selectivity in hydrodesulfurization of dibenzothiophene (DBT).

2. Experimental NiMo catalysts were prepared by incipient wetness impregnation of SBA-15 with aqueous solutions of ammonium heptamolybdate and Ni(II) nitrate. Catalysts were prepared by two methods, coimpregnation of Mo and Ni in presence of citric acid (NiMoCA/SBA-15(C) catalyst) and successive impregnation of Mo (first) and then of Ni-citrate solution (NiMoCA/SBA-15(S) sample). In both cases, the molar ratio Ni:Mo:CA = 0.5:1:1 and pH = 9 were kept. After impregnation, NiMoCA/SBA-15(C) and NiMoCA/SBA-15(S) catalysts were air-dried at 100oC for 12 h without calcination. For comparison, three reference NiMo catalysts supported on SBA-15 and γ-alumina were prepared without CA using coimpregnation (C) or succesive impregnation (S) methods. Reference catalysts were air-dried (100oC, 12 h) and calcined (500oC, 4 h). Nominal metal charges in all catalysts were 12 wt. % MoO3 and 3 wt. % NiO. Prepared catalysts were characterized by N2 physisorption, small-angle and powder XRD, UVVis DRS, thermal analysis (TGA/DTG/DTA), TPR and HRTEM, and tested in the DBT HDS reaction. The catalytic activity tests were performed in a batch reactor at 300oC and 7.3 MPa total pressure for 8 h. Before the activity tests, the catalysts were sulfided ex situ in a tubular reactor at 400oC for 4 h in a stream of 15 vol. % of H2S in H2.

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3. Results and discussion 3.1. Catalyst characterization Results from textural characterization of the supports and NiMo catalysts (Table 1) indicate that the incorporation of Ni and Mo species on the supports’ surface produced a decrease in the textural properties, which was stronger for the samples prepared with the addition of citric acid. This result can be attributed to a higher loading of deposited species in dried NiMoCA catalysts compared with that in calcined NiMo samples. It can also be noted that NiMo catalysts prepared by coimpregnation method have better textural properties than their counterparts prepared by successive impregnation procedure. In addition, all catalysts supported on SBA-15 had a significantly higher surface area than that of the NiMo/γ-Al2O3 analog. Citric acid addition in NiMo/SBA15 catalysts almost did not affect the shape of the N2 adsorption-desorption isotherm characteristic for SBA-15 support (Fig. 1), as well as its pore arrangement (Fig. 2). Table 1. Textural characteristics* of supports and NiMo catalysts. SBET (m2/g) Sμ (m2/g) Vp (cm3/g) Vμ (cm3/g)

Sample

Dads (Ǻ)

SBA-15

850

140

1.09

0.056

85

NiMoCA/SBA15(C)

526

60

0.72

0.022

85

NiMoCA/SBA-15(S)

459

49

0.76

0.017

84

NiMo/SBA-15(C)

597

83

0.78

0.031

82

NiMo/SBA-15(S)

578

86

0.81

0.033

85

γ-Al2O3

200

-

0.48

-

115

NiMo/γ-Al2O3(S)

185

-

0.39

-

115

* SBET, specific surface area; Sμ, micropore area; VP, total pore volume; Vμ, micropore volume; Dads, pore diameter determined from the N2 adsorption isotherm by the BJH method. 800

(100)

600

(b) (c)

500 400 300

Intensity (a.u.)

Volume Adsorbed (cm3/g STP)

(a) 700

(110) (200)

200

(a) (b)

100

(c)

0.0

0.2

0.4

0.6

0.8

1.0

Relative Pressure (P/Po)

Fig. 1. N2 adsorption-desorption isotherms of (a) SBA-15; (b) NiMo/SBA-15(C); and (c) NiMoCA/SBA-15(C).

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

2Θ (o)

Fig. 2. Small-angle XRD patterns of (a) SBA-15; (b) NiMo/SBA-15(C); and (c) NiMoCA/SBA-15(C).

5.0

Effect of citrate addition in NiMo/SBA-15 catalysts on selectivity of DBT HDS 35

531

345

(a)

30

(b)

25

15

(d)

(a)

337

Signal (a.u.)

F(R)

(c) 20

(b)

400

10

(c) 376

5 0

(d) 200

250

300

350

λ (nm)

400

450

500

200

400

600

800

1000

Temperature (°C)

Fig. 3. UV-Vis DRS spectra of (a) NiMoCA/ SBA-15(S); (b) NiMoCA/SBA-15(C); (c) NiMo/ SBA-15(S); and (d) NiMo/SBA-15(C).

Fig. 4. TPR profiles of (a) NiMoCA/SBA15(S); (b) NiMoCA/SBA-15(C); (c) NiMo/ SBA-15(S); (d) NiMo/SBA-15(C).

Thermal analysis of CA-containing catalysts (not shown) revealed that CA decomposition occurs at about 200°C. Powder XRD patterns of NiMo and NiMoCA catalysts did not show the presence of any crystalline phase, pointing out a good dispersion of the deposited metal oxide species in all samples. However, DRS and TPR characterization results (Figs. 3 and 4) indicate that, as expected, citric acid addition increased the dispersion of oxidic Mo species on SBA-15 surface and made their reduction much easier and more complete. The dispersion of sulfided MoS2 particles was also improved by CA addition (Table 2). Table 2. Average length and layer number of the MoS2 crystallites determined by HRTEM and activity and selectivity of NiMo catalysts in HDS of DBT. Selectivity*

Morphology of MoS2 phase

Conversion (%)

Average length (Å)

Average stacking

4h

8h

NiMoCA/SBA-15(C)

32.1

2.96

54

96

2.8

NiMoCA/SBA-15(S)

33.4

3.18

46

83

1.5

NiMo/SBA-15(C)

36.0

3.43

40

76

0.9

NiMo/SBA-15(S)

37.6

3.37

43

75

0.6

NiMo/γ-Al2O3(S)

40 [3]

~2.0 [3]

55

95

2.0

Catalyst

*

BP/CHB ratio at 50% of DBT conversion; BP, biphenyl; CHB, cyclohexylbenzene.

3.2. Catalytic behavior Catalytic activity tests (Table 2) show that citric acid addition in NiMo/SBA-15 catalysts leads to an increase in the activity in DBT HDS and selectivity towards the direct desulfurization (DDS) route of the reaction. A comparison of the catalytic behavior of the NiMoCA/SBA-15 catalysts prepared by coimpregnation (C) and successive impregnation (S) methods indicates that the catalyst prepared by the C method resulted to be more active and more selective for the DDS route than its counterpart prepared by the S procedure. In general, activity and selectivity trends

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observed inside a series of the SBA-15-supported samples can be attributed to differences in the dispersion of oxide and sulfided Mo species. Results from textural characterization, DRS, TPR and HRTEM showed that citric acid addition in the impregnation solutions leads to a better dispersion of the active phase explaining its higher activity, and to a decrease in the stacking degree of MoS2 particles giving rise to an increase in the selectivity towards direct desulfurization of DBT. This conclusion is well in line with previous report [4] in which higher hydrogenation ability in DBT HDS was found for Mo catalysts with more stacked MoS2 particles and vice versa. In addition, another effect produced by CA addition can be noted when comparing catalytic behavior of the NiMoCA/SBA-15(C) catalyst, the best catalyst among SBA15-supported samples, and the NiMo/γ-Al2O3 reference. Both catalysts showed similar activity in HDS of DBT, but the selectivity for BP formation was higher for the catalyst prepared with CA, although the stacking degree of MoS2 particles in it was higher than in the alumina-supported reference (Table 2). The explanation for this fact can be made considering that not only the stacking degree of the MoS2 crystallites determines the catalyst’s selectivity for the direct desulfurization route. Also, the formation of Ni-Mo-S species is necessary to provide the catalyst ability for the cleavage of the C-S bond in the DBT molecule [5]. According to this, it can be supposed that citric acid addition during the impregnation of Ni and Mo species on SBA-15 surface also leads to an increase in the number of formed Ni-Mo-S active sites enhancing by this means the rate of the DDS route. Similar increase in the formation of the Co-Mo-S structure has been reported recently when citric acid was added to the impregnation solution during preparation of CoMo catalysts [6].

4. Conclusions Addition of citric acid in the impregnation solutions during preparation of NiMo catalysts supported on SBA-15 resulted in an increase in both catalyst activity in hydrodesulfurization of DBT and selectivity towards the direct desulfurization route. Coimpregnation of Ni and Mo species in presence of citric acid seems to be a better way for catalyst preparation than their successive impregnation. Addition of citric acid resulted in an increase in the MoS2 dispersion and formation of a larger amount of Ni-Mo-S species.

Acknowledgements Financial support by CONACyT-Mexico (grant 100945) is gratefully acknowledged. The authors thank I. Puente Lee, C. Salcedo Luna, M. Aguilar Franco and M. Portilla for technical assistance with HRTEM, XRD and TGA/DTG/DTA characterizations.

References 1. 2. 3. 4. 5. 6.

L. Lizama, J.C. Amezcua, R. Reséndiz, S. Guzmán, G.A. Fuentes, T. Klimova, Stud. Surf. Sci. Catal., 165 (2007) 799. L. Lizama and T. Klimova, Appl. Catal. B: Env., 82 (2008) 139. T. Klimova, D. Solís Casados, J. Ramírez, Catal. Today, 43 (1998) 135. E.J.M. Hensen, P.J. Kooyman, Y. van der Meer, A.M. van der Kraan, V.H.J. de Beer, J.A.R. van Veen, R.A. van Santen, J. Catal., 199 (2001) 224. F. Bataille, J.L. Lemberton, P. Michaud, G. Pérot, M. Vrinat, M. Lemaire, E. Schulz, M. Breysse, S. Kasztelan, J. Catal., 191 (2000) 409. T. Fujikawa, Top. Catal., 52 (2009) 872.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Investigation of the microwave heating techniques for the synthesis of LaMnO3+δ: influence of the starting materials Ramdame Kahia, Cécilia Menu, Jean-Marc Giraudon, Jean-François Lamonier Unité de Catalyse et de Chimie du Solide UMR 8181, Université des Sciences et Technologies de Lille, 59655 Villeneuve d’Ascq Cedex (France)

Abstract Lanthanum manganite were synthesized using microwave irradiation by two procedures differing from the nature of the starting materials which were the appropriate nitrate metal salts (nMn = nLa) with or without addition of citric acid (CA) (nCA/(nMn + nLa) = 1). For each procedure, the starting aqueous solutions were typically submitted to sequences of three successive microwave irradiation steps of increasing output power: 300, 600 and 1000W differing from the duration of one or two steps. The final grounded powders were characterized by XRD (X-Ray Diffraction), FT-IR spectroscopy and ASA (Apparent Surface Area) measurements. Keywords: microwave-assisted synthesis, perovskites

1. Introduction Potential application of pure and doped lanthanum manganese based systems as combustion catalyst, VOCs (Volatile Organic Compounds) and especially CVOCs (Chlorinated Volatile Organic Compounds) total oxidation catalysts, provides the motivation for the study of their synthesis. Emerging trends in material synthesis indicate that rapid synthetic routes are becoming increasingly important to achieve new compositions: thermo stable phases and materials with distinct physico-chemical properties. The concept of the microwave heating technique is based on the presence of microwave receptors at the 2450 MHz micro-wave frequency [1]. As the transformation is adiabatic, the heat absorbed by the system provides the energy for the formation of appropriate materials [2]. Among the numerous protocols of perovskite synthesis found in the literature, co-precipitation [3], Citrate and Pechini methods [4] as well as the reactive grinding route synthesis [5] are the most frequently used for catalytic purpose. Although if they lead to interesting specific surface areas they are typically multi-steps procedures which are time and cost consuming. This work considers the microwave-assisted synthesis of lanthanum manganese in a fast transformation reaction. The influence of the starting materials was particularly considered. For purpose, two experimental procedures were herein studied: the nitrate and the citrate micro-wave-assisted synthesis which differ from the nature of the starting materials which were the appropriate nitrate metal salts (samples labeled MH-Nx; MH stands for Microwave Heating, N for Nitrate precursors) with or without addition of citric acid (samples MH-Cx, C for addition of citric acid). For each procedure, the starting aqueous solution was typically submitted to three successive microwave irradiation sequences of increasing output power: 300, 600 and 1000W, each sequence duration was allowed to vary. The effects of the starting solution and for each procedure of the

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duration of the power sequences upon the structural and textural properties of the final material were discussed in light of the microwave receptors content.

1. Experimental 1.1. Catalyst preparation Experiments were carried out under microwave in a microwave-accelerated reaction system model Mars X which operates at 2.45 GHz with a maximum power supply of 1200W. Reagents La(NO3)3.9H2O (Fluka, 99%), Mn(NO3)2.5H2O (Alfa Aesar, 99%) and citric acid (Prolabo, 97%) were used as received. The resulting aqueous solution was transferred in a pyrex flask connected to a reflux set-up to collect the evaporated solvent. The step duration (min) of the microwave irradiation sequences were given in Table 1. For comparison a reference sample LaMnO3+δ was synthesized by the citrate method as in ref. [6] (calcination temperature: 700°C kept for 5h). Typically considering the MH-Nx sample preparation, the initial solution on the exposure to microwave irradiation (P=300W) boils to give a yellowish syrup. Increasing the power to 600W resulted in brown foam. At 1000W for short duration (1 to 2 min) the gel transforms into sticky brownish translucent particles. As the duration increases the particles explode. At that stage the product formation begin to take place. For the MH-Cx samples synthesis, irradiation of the solution at 300W allows the boiling of the solution which keeps uncolored. At 600W syrup was obtained which rapidly transformed into black foam when the output power was increased to 600W. Finally, by increasing the heating power (1000W) the powder obtained after grinding the foam, ignites with the appearance of a flame and become red hot in the product formation. Table 1. The duration (min) of the microwave irradiation step and the SSA for each sample. Samples MH-N1 MH-N2 MH-N3 MH-N4 MH-N5 MH-C1 MH-C2

300W 10 15 5 15 15 5 5

600W 10 15 15 15 15 10 10

1000W 1 2 5 7 30 0.5 0.33

SSA (m2/g) 11 12 13 18 5 2.9 3.6

1.2. Catalysts characterization 1.2.1. Powder X-ray diffraction pattern The powder X-ray diffraction patterns (XRD) of the samples were recorded on a D8 Advance-BRUKER diffractometer using Cu Kα radiation (40 kV and 100 mA). 1.2.2. Nitrogen adsorption/desorption The nitrogen adsorption and desorption isotherms were measured at -196°C on a Quanta Sorb Junior. The samples were out gassed at 200°C for 2 h before measurement. 1.2.2. FT-IR spectroscopy The spectra were performed with the sample diluted in KBr disk on a Nicolet Protege 460 apparatus.

Investigation of the microwave heating techniques

535

2. Results and discussion 2.1. XRD and IR analysis X-ray diffraction patterns of the MH-Nx recorded in the 2θ range of 20°–80° are shown in Fig. 1. The XRD patterns of MH-N1 (not shown here) and MH-N2 are characteristic of an amorphous material. MH-N3 and MH-N4 samples exhibit predominantly the diffraction peaks related to an orthorhombic lanthanum manganite oxide having a La0.99MnO3.03 or La0.92MnO2.88 stoichiometry according to JCPDS 01-087-2018 and JCPDS 01-087-2014 respectively. Additionally a broad peak at 29.3° is observed indicating the presence of lanthanum carbonate based species in accordance with the related IR results. MH-N5 shows diffraction peaks similar to those of the reference rhombohedral La0.95Mn0.95O3 according to JCFDS 01-089-8775 which is also referred to LaMnO3+δ. Considering the MH-Cx, only the peaks relative to LaMnO3+δ were observed.

Fig. 1. XRD patterns of the different preparations (*: LaMnO3.15 ; +: La0.99MnO3.03).

From the related sample FT-IR spectra (Fig. 2) the salient points are the following: (i): Considering the two first spectra, from both the detection of O-C-O (symmetric and asymmetric O-C-O bands at 1560 and 1425 cm–1 [7]) and of an ionic nitrate group (narrow line at 1385 cm–1 [7]) metal-citrate as well nitrate structures can be postulated. (ii): The rather similar spectra of MH-N2 and MH-N3 exhibited broad overlapping vibration bands below 700cm–1 related predominantly to lanthanum manganite. The two characteristic bands at 1480 cm–1 and 1380 cm–1 have been attributed to stretching vibrations relative to C-O from unidentate carbonates [7] resulting from citrate decomposition mainly linked to lanthanum.

Fig. 2. FT-IR of the MH-Nx samples.

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(iii): The spectrum of MH-N5 as well as those of the MH-Cx samples are similar to the reference one and exhibit vibration bands at 604 (I), 382 (I) and 503 (w), 283 (w) in accordance with a rhombohedral LaMnO3.15 [8] rid off impurities. These results show that formation of pure LaMnO3.15 at 1000W is faster with the nitrate procedure. Considering the citrate procedure, the sluggishness of the process at 1000W allows apprehending the different transformation steps to get the desired compound. At short times (1-2 min) decomposition of metal-citrate-nitrate based complexes occur. Increasing time exposure (5-7 min) allows the formation of an orthorhombic lanthanum manganite along with lanthanum carbonate based species. After 30 min LaMnO3,15 is formed free of impurity.

2.2. SSA of the samples The SSA relative to MH-Cx samples (Table 1) appear to be rather low compared to previous work [2] without explanation so far. For the MH-Nx samples, the SSA was found to be dependent on the time of exposure to microwave irradiation at 1000W. A Vulcano curve is obtained with a maximum SSA of 18 m2/g for MH-N4. A duration of 30 min has a negative effect on the SSA probably due to sintering [2], the SSA of 5 m2/g for MH-N5 is in accordance with that of the reference one synthesized by the citrate method of 7 m2/g.

3. Conclusion Microwave heating has herein proven successful for the preparation of pure LaMnO3.15. However it has been demonstrated that the nitrate procedure is a faster route to get the pure rhombohedral LaMnO3.15 compared to the citrate one. Adding CA allows to get different manganese related phases with time at 1000W. A lowering of the microwave radiation receptors content with citric acid readily account for the different results regarding the adopted procedure.

Acknowledgements We thank the European community through an Interreg 4 France-Wallonie-Flandre project REDUGAZ for financial supports.

References [1] D.R. Baghurt, 1988, Application of Microwave Heating Techniques for the Synthesis of Solid State Inorganic Compounds, J. Chem. Soc. Chem. Commun., 829-830. [2] A. Kaddouri, 2006, Microwave-assisted synthesis of La 1-xBxMnO3.15 (B = Sr, Ag;x = 0 or 0.2) via manganese oxides susceptors and their activity in methane combustion, Catal. Commun., 7, 109-113. [3] M.M. Savosta, 2004, Nuclear spin dynamics and magnetic structure of nanosized particles of La0.7Sr0.3MnO3, Phys. Rev. B 69 024413. [4] M.P. Pechini, US Patent, 1967, 330, 697. [5] S. Kaliaguine, Perovskite-type oxides synthesized by reactive grinding: Part I. Preparation and characterization, Appl. Catal. A: 209, 345-358. [6] J.-M. Giraudon, 2008, Catalytic oxidation of chlorobenzene over Pd/perovskites, Appl. Catal. B: 84, 251-261. [7] A.A. Davinov, 1990, Infrared Spectroscopy of Adsorbed Species on the Surface of Transition Metal Oxides, England, Chap.1. [8] M. Daturi, 1995, Surface and structure characterization of some perovskite-type powders to be used as combustion catalysts, Chem. Mater., 2115-2126.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

The novel route of preparation of the supported gold catalysts by deposition-precipitation O.A. Kirichenkoa, G.I. Kapustina, V.D. Nissenbauma, O.P. Tkachenkoa, V.A. Poluboyarovb, A.L. Tarasova, A.V. Kucherova, L.M. Kustova a

N.D. Zelinsky Institute of Organic Chemistry, RAS, 47 Leninsky prosp., Moscow, 119991, Russian Federation b Institute of Solid State Chemistry and Mechanochemistry, SB of RAS, 18, Kutateladze, Novosibirsk, 630128, Russian Federation

Abstract Au/SiO2 and Au/TiO2 catalysts were prepared by the original procedure of depositionprecipitation with ammonia. As compared with the previously published methods, the proposed procedure makes possible almost complete deposition (>99%) of gold from HAuCl4 solution on the SiO2 and TiO2 supports at the low ratio NH3:Au = 6.5-7 and pH<6. The genesis of supported gold species was studied with TG-DTA, TPR, TPD, XRD, TEM, (XANES + EXAFS) and FTIR of adsorbed CO. The samples prepared were catalytically active in the liquid phase oxidation of piperonyl alcohol to aldehyde and in the S-VOC removal at the relatively low temperatures. Keywords: catalyst preparation, gold

1. Introduction The expensive gold precursors and special equipment are required to prepare active silica supported gold catalysts that is not suitable for general application [1]. Impregnation of the supports with the solutions of the gold compounds resulted in the materials with a poor catalytic activity [1]. The widely used deposition–precipitation (DP) with NaOH or urea from HAuCl4 solution, which is the main gold chemical source, failed to prepare catalytically active Au/SiO2. The first attempts to deposit the Au precursor nanoparticles by addition of ammonia solution to HAuCl4 solution [2,3] were not successful. Recently the active Au/SiO2 catalysts were obtained by DP with ammonia solution [4]. However a huge excess of ammonia (NH3:Au = 1050) and the high pH≥10 were used that caused the simultaneous partial dissolution of the support and resulted in the incomplete (<80%) gold deposition. In this work, we have prepared the gold catalysts on SiO2 and TiO2 supports using the method of DP with an ammonia solution by variation in the preparation procedure.

2. Experimental The mesoporous MCM-41 [5] and nonporous SiO2 (Aerosil), TiO2 (Degussa P-25) were used as supports. HAuCl4 was prepared by dissolving the Au foil (99.99%) in aqua regia [6]. The DP with ammonia (DPNH3) was performed as followed. The initial solution was prepared by a slow dropwise addition of 1.1M NH3·H2O to 8.3·10–4 M HAuCl4 solution at vigorous stirring and permanent pH control maintaining pH<5.5 to avoid formation of the “fulminating gold” precipitate which is impact, attrition and heat sensitive explosive [1]. The solution prepared was stored for 1-7 days. A support was introduced in the initial solution, the slurry was kept under vigorous stirring at 20-22°C for 16 h and then

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for 3 h at 80°C. After filtering the sample was washed and dried at 60°C. The procedure without heating the slurry and sample is designed as DPNH31. The reference samples were prepared by well-known methods of DP with NaOH (DPNaOH) and with urea (DPurea) respectively [1]. The concentration of NH4+ ions was measured with ELIT-051 electrode and I-500 ion-meter. Chemical analysis on Au content was performed by the method of titration with Na2S2O3 solutions [6]. The amber glass bottles and the black polymer bags were used to protect the samples, slurries and solutions from light. The samples were studied with TG-DTA and TPR-TPD methods [7], DRIFT and X-Ray absorption spectroscopy [8]. X-ray diffraction patterns were recorded using DRON-2 diffractometer with Cu Kα radiation. The samples were examined by TEM with electron microscope JEM-2000FXII at 200 kV. Catalytic activity was tested in the reaction of oxidation of piperonyl alcohol with air [9] and in S-VOC removal [8].

3. Results and dicussion 3.1. Precursor deposition and characterization Almost complete transformation of the anion complex [AuCl4]- to the cation complex [Au(NH3)4]3+ occurred in the initial solution. During addition of the ammonia solution to the HAuCl4 solution at pH<5.5 the detected concentration of the NH4+ ions was two orders of magnitude lower than the concentration calculated from the amount of ammonia added that indicates strong ammonia bonding as a ligand in the complexes. It should be mentioned that at NH3/Au = 4 the equilibrating time was 16 hours. Titration curves did not exhibit the features of saturation, as well as no precipitate was formed. It is possible to avoid the precipitation and reduction of gold-ammonia compounds even at pH=8.1-8.7 by decreasing the Au concentration below 10–3 mol·L–1 [10]. Introduction of the support, as well as heating the slurries to 80°C induced the pH decrease. The additional amount of the ammonia solution was required to maintain the pH value of slurries about 5.5. Decrease in pH of the slurry with time, as well as pH of the slurry below 5.5 even at NH3 : Au = 7, can stem from a precursor precipitation. Supported nanoparticles are clearly visible on TEM images of the samples prepared by DPNH31 confirming that DP process followed adsorption even at room temperature. On keeping the slurry at room temperature, Au partly remained in solution (Table 1). The increase in temperature of the slurry up to 80°C resulted in complete withdrawal of Au from the solution (samples AuM-2, AuT-3). Table 1. Gold content in the solutions. Sample

Support

Procedure

AuM-2 AuT-3 AuM-1 AuS-6

МСМ-41 TiO2 МСМ-41 SiO2

DP NH3 DP NH3 DP NH3 1 DP NH3 1

Au in solution, μmol·L–1 initial after DP 830 <1 830 <1 830 150 830 130

Au deposited, %

H/Au (TPR)

>99.9 >99.9 82 84

1.5 1.9 0.85 -

The X-ray absorption (XANES + EXAFS) studies revealed Au3+ and no Au0 in the dried sample on MCM-41 [9]. The first coordination shell of Au atom contained O or N atoms instead of Cl atoms. FTIR spectra of the samples supported on MCM-41 exhibited no bands of CO adsorption. The reason can be coordinative saturation of Au species anchored to SiO2 surface. The precursor species on TiO2 seem to contain Au in the oxidation states +3, +1 and 0. The bands at 2179, 2193 and 2109 cm–1 were observed

The novel route of preparation of the supported gold catalysts

539

on FTIR spectra of the dried samples on TiO2. The first two bands can be attributed to CO adsorption on Aun+ whereas the last one is assigned to CO adsorbed on Au0 [11].

3.2. Reduction of precursors deposited and characterization of reduced samples The TPD revealed the weak hydrogen evolution (Fig. 1) indicating negligible autoreduction of precursor. The smooth TG curves and absence of any peaks on DTG and DTA curves confirm that supported Au species differ from “fulminating gold”, whose thermal stability varies between 200 and 230°C [12]. The TPR studies of dried samples indicate partial reduction of Au precursors under DPNH3. The hydrogen consumption values are lower than required for reduction of Au3+ to Au0 (Table 1). Since the low H/Au values were obtained even for the sample prepared by DPNH31, the reduction most probably took place in the slurry. The reduction behavior of the precursor species depends on a preparation procedure and the support nature (Fig. 1). At least 3 types of the gold precursor species are formed on MCM-41, one of which is formed also on TiO2. Au/MCM-2

80

9.97

40

9.96 9.95

20

9.94 0 100

200

300

400

500

600

-20

700

9.93 9.92 9.91

-40

AuT-3(TPD)

200 TCD response, a.u.

T C D res pons e, a.u.

AuT-3

9.98

0

100

9.99

Au/MCM-2(TPD)

60

300

10

Au/MCM-1

100

60

0

40 0

100

200

300

400

500

600

-100

9.9

80

700

800 20

Temperature, oC -200

Temperature, oC

0

Fig. 1. TPR and TPD profiles of samples prepared by the DPNH3 procedure on MCM-41 and TiO2.

The X-ray absorption studies revealed Au0 with average particle size 1.46 nm in the sample on MCM-41 reduced at 300°C. The particles of size 1-4 nm are clearly visible on TEM images, yet about 5% of particles are well shaped Au crystallites of size 12–21 nm. Very weak reflections of Au0 appeared on XRD patterns of samples with high Au loading, the estimated average particle size was 12-14 nm. These particles probably originate from aggregates observed seldomly by TEM in the dried sample. The catalytic activity in oxidation of piperonyl alcohol to piperonal varied depending on the preparation procedure, Au loading, and on the support nature (Table 2). Table 2. Catalytic activity data for the samples reduced at 300°C. Sample

Procedure

AuM-2 AuT-3 AuM-1 AuS-6 AuT-7 AuT-8

DP NH3 DP NH3 DP NH3 1 DP NH3 1 DP NaOH DP urea

Au, wt % 0.96 0.97 0.46 3.7 0.89 7.0

Alcohol conversion, % 4.8 8.6 9.0 1.3 7,6 6.2

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The high conversion was reached even at low Au loading on MCM-41 if DP and drying were performed at room temperature. The CH3SSCH3 degradation in air (1000 ppm) was detected at 40-130°C on AuT-3, whereas AuM-2 exhibited high activity at 290-320°C and decreased the residual content of all pollutants in outgoing gas below 3 ppm upon long-term testing at 500°C.

3.3. Mechanism of deposition-precipitation with ammonia solution During preparation of the initial solution ammonia acts mainly as complexing agent. The initial solution contains at least three Au complexes since the [Au(NH3)4]3+ hydrolyzes slowly to [Au(NH3)3OH]2+ in acid media and dissociates as a weak acid to [Au(NH3)3NH2]2+ at pH>5 [13]. Their adsorption and grafting on MCM-41surface can produce several types of Au complex species. Autoreduction of these complexes seems take place in the slurry. The numerous adsorbed Au complexes provide the basis for the nucleation on the surface resulting in the fine precursor particles. During depositionprecipitation the ammonia solution acts like both a base and reducing agent.

4. Conclusions The present studies exhibited the prospects of the method of deposition-precipitation with ammonia for the gold catalyst preparation using [Au(NH3)4]3+ synthesized from HAuCl4 solution. As compared with the previously published DPNH3 procedures, the proposed procedure makes possible almost complete deposition (>99%) of gold from HAuCl4 solution on the SiO2 and TiO2 supports at the low ratio NH3:Au = 6.5-7 and pH<6. The highly dispersed gold particles are catalytically active in the liquid phase oxidation of piperonyl alcohol to aldehyde and in the S-VOC removal.

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]

G.C. Bond, C. Louis, D.T. Thompson. Catalysis by Gold. Catalytic Science Series, V.6, Imperial College Press, 2006. N. Dimitratos, A. Villa, C.L. Bianchi, L. Prati, M. Makkee, Appl. Catal. A: 311 (2006) 185 K. Qian, Z. Jiang, W. Huang, J. Mol. Catal. A 264 (2007) 26. F. Somodi, I.Borbáth, M. Hegadus, A. Tompos, I.E. Sajó, A. Szegedi, S. Rojas, J.L.G. Fierro, J.L. Margitfalvi, Appl. Catal. A: Gen. 347 (2008) 216-222. A.V. Kucherov, A.N. Shigapov, A.V. Ivanov, T.N. Kucherova, L.M. Kustov, Catal. Today 110 (2005) 330. S.I. Ginzburg. The guide on chemical analysis of the platinum metals and gold. (In Russian) Nauka, Moscow, 1965. O.A. Kirichenko, V.D. Nissenbaum, G.I. Kapustin, L.M. Kustov, Thermochim. Acta 494 (2009) 35-39. A.V. Kucherov, O.P. Tkachenko, O.A. Kirichenko, G.I. Kapustin, I.V. Mishin, K.V. Klementiev, S. Ojala, L.M. Kustov, R. Keiski, Top. Catal. 52 (2009) 351-358. O.P. Tkachenko, L.M. Kustov, A. L. Tarasov, K.V. Klementiev, N. Kumar, D.Yu. Murzin. Appl. Catal. A: Gen. 359 (2009) 144-150. I.V. Mironov, Russian J. Inorg. Chem., 53 (2008) 711. D. Guillemot, V.Y. Borovkov, V.B. Kazansky, M. Polisset-Thfoin, J. Fraissard, J. Chem. Soc. Faraday Trans. 93 (1997) 3587. G. Steinhauser, J. Evers, S. Jacob, T.M. Klapötke, G. Oehlinger, Gold Bulletin 41, 4 (2008) 305. L.H. Skibsted, J. Bjerrum, Acta Chem. Scand. A 28 (1974) 740.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

A new approach for the dispersion of VOPO4.2H2O through exfoliation and its catalytic activity for the selective oxidation of cyclohexane Parijat Borah, Chandrashekar Pendem, Arunabha Datta* Indian Institute of Petroleum (CSIR), Dehra Dun 248005, India

e.mai : [email protected] Abstract The VOPO4.2H2O phase has been exfoliated in 2-propanol and the exfoliated solution dispersed on alumina with different VPO: alumina ratios. These dispersed samples were characterized by XRD, FTIR, and SEM studies and it has been shown that the dispersed VPO phases retain the original VOPO4.2H2O structure although the morphology, crystallinity, crystal size and surface areas of the samples are affected on exfoliation and subsequent dispersion on alumina with different VPO: alumina ratios. These phases have been shown to be active catalysts for the oxidation of cyclohexane preferentially to cyclohexanol with the activity varying quite markedly with the VPO: alumina fraction. The present work therefore not only represents a novel approach for preparing dispersed VPO phases but also demonstrates the efficiency of the VOPO4.2H2O phase in catalyzing the oxidation of cyclohexane selectively to cyclohexanol. Keywords: vanadium phosphate, exfoliation, dispersion on alumina, cyclohexane oxidation

1. Introduction Oxidation of the relatively inert C-H bond of hydrocarbons is one of the most desirable but challenging reactions from an academic and industrial point of view. Vanadium phosphorous oxide (VPO) catalysts are used commercially for the selective oxidation of n-butane to maleic anhydride [1] and it has also been shown that this catalyst can be effective for other selective oxidation reactions including the ammoxidation of propane, oxidation of pentane to phthalic anhydride and oxidation of toluene to benzaldehyde [2]. The VPO catalysts are very sensitive to their structural and morphological characteristics [3] and accordingly we have been working on the synthesis of new, modified and mesostructured VPO phases [4]. In this context we have recently demonstrated a novel approach involving exfoliation of the layered VPO phases in suitable solvents followed by their surfactant assisted organization into a mesostructured phase [5]. In the present work, the exfoliation concept has been used to prepare novel dispersed VPO phases by exfoliating VOPO4.2H2O in 2-propanol followed by its dispersion on alumina with different VPO: alumina ratios and evaluating the catalytic activity of these dispersed phases for the selective oxidation of cyclohexane.

2. Experimental 2.1. Synthesis A previously reported method was followed for the preparation of VOPO4.2H2O [4d]. The yellow solid obtained was identified as VOPO4.2H2O by XRD [3b] and FTIR studies [5] and is denoted as VPO-O.

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A suspension of powdered VPO-O (1 g) in isopropyl alcohol (50 cm3) was placed in a flask (100 cm3) equipped with a condenser and heated at 323 K for 30 minutes with stirring which leads to a homogeneous yellow colored liquid. To this yellow colored solution of exfoliated VOPO4.2H2O, 1 g of Al2O3 was added. The solvent was then evaporated by constant stirring to give a solid coded as VPO-50. In a similar fashion exfoliated VOPO4.2H2O was dispersed on alumina with VPO loadings of 25%, 35% and 75% by adding desired amount of alumina and these were denoted as VPO-25, VPO-35 and VPO-75 respectively.

2.2. Catalytic test The catalytic tests were carried out in a two necked round bottom flask. In a typical experiment 1.1 cm3(0.01 mol) of cyclohexane (SLR India, extra pure AR grad) was added to 10 cm3 of acetonitrile (Merk, GR grad) containing the VPO catalyst. In order to avoid immediate decomposition of H2O2 and strong effervescence, 50% H2O2 (Rankem,LR grad) was added slowly over a period of 6h. The resulting reaction mixture was stirred at 250 rpm at 333K for 24h. The reaction mixture was analyzed by GC-MS for identification of the reactants and products. The products were then quantified by a GC after determining the response factors for the reactants and products individually.

3. Results and discussion 3.1. XRD The XRD pattern (Fig. 1) of VOPO4.2H2O (VPO-O) shows an intense line at a d=7.4 Å (2θ = 11.9°) corresponding to the (001) plane along with other characteristic lines whereas VPO-25 shows lines of alumina only due to the low amount of VPO loading on alumina. The XRD pattern of VPO-35 shows two lines at d values of 7.35 Å and 3.1 Å (although with much reduced intensity compared to VPO-O) along with alumina lines. XRD patterns of VPO-50 and VPO-75 also show strong lines of VOPO4.2H2O. However the peaks of the dispersed VPO phases are broader compared to those of VPO-O indicating a smaller crystallite size and the d value of the (001) reflection is slightly lower than 7.4 Å for VPO-O. It is evident therefore that the solids dispersed on alumina contain the starting VPO-O phase although the crystallinity and crystallite size are reduced.

Fig 1. XRD patterns of VOPO4.2H2O and after exfoliation and dispersion on alumina with different VPO: alumina ratios.

Fig 2. FTIR spectra of VOPO4.2H2O and of dispersed VPO phases with different VPO: alumina ratios.

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3.2. FTIR FTIR spectra (Fig. 2) show that the spectra of the five samples are essentially similar. VOPO4.2H2O (VPO-O) shows characteristic P-O and V-O stretching frequencies in the region of 900 – 1200 cm–1 with bands centered at 947, 1038, 1088 and 1164 cm–1[6]. In the hydroxyl stretching region, two bands are observed at 3564 cm–1 and 3349 cm–1 which can be assigned to interlayer free structural water and to the water molecule coordinatively bound to the vanadium in the lattice opposite to the vanadyl oxygen respectively. The spectrum of VPO-25 shows the predominant band of Al2O3 at 1072 cm–1 with weak P-O and O-H bands of VPO. But VPO-35, VPO-50 and VPO-75 have spectra similar to the VPO-O indicating once again that these dispersed solids are similar to the parent VPO phase.

3.3. SEM SEM micrographs (Fig. 3) show that VPO-O has the typical platelet structure of VOPO4.2H2O with the platelet size ~6 – 10 µm. In the case of VPO-25 however the alumina particles are predominantly observed with ill-defined agglomerated platelets of the VPO phase present on them. A similar morphology is observed in the case of VPO35 with the VPO phase (now observed more prominently) being still ill-defined and agglomerated. On the other hand in the case of VPO-50 the alumina particles are not observed at all and though the platelet structure of the dispersed VPO phase is more well defined, it is agglomerated with smaller particles of ~3µm compared to VPO-O. Similarly in the case of VPO-75 the VPO phase has an agglomerated flower-petal like morphology with particles of ~1.5 – 2 µm. It is apparent therefore that the morphology of the dispersed phases is affected by the amount Fig 3. SEM of VOPO4.2H2O and after of the support on which it is dispersed. exfoliation dispersion on alumina with

3.4. Catalytic activity

different VPO: alumina ratios.

It is evident from the catalytic activity of different VPO phases (table 1) for the oxidation of cyclohexane, that in contrast to earlier work using vanadium pyrophosphate as the catalyst, wherein both cyclohexane and cyclohexanol are obtained in nearly equal quantities [7], the VPO-O phase gives cyclohexanol as the predominant product with a selectivity of 58% and only 1.6% for cyclohexanone at a cyclohexane conversion of 53.8%. The dispersed phases VPO-25 and VPO-35 however show lower conversion of 23.0% and 40.0% respectively with a low selectivity for cyclohexanol in spite of having a higher surface area than VPO-O. This could be due to the poor crystallinity and illdefined morphology of the VPO phase in these cases. On the other hand VPO-50 shows a very high cyclohexane conversion of 83.3% to give cyclohexanol preferentially with a cyclohexanol/cyclohexanone ratio of 71:1. However, there is quite a sharp decrease in the conversion of cyclohexane in the case of VPO-75 in spite of good crysallinity and a well defined morphology of the VPO phase, probably because of the lower surface area compared to VPO-50.

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In the oxidation of cyclohexane cyclohexylhydrogenpeoxide (CHHP) is an intermediate and its formation is the rate-determining step. This CHHP can undergo both heterolytic and homolytic decomposition to give cyclohexanone and cyclohexanol respectively. From our study it is evident that the VOPO4.2H2O phase facilitates preferentially the homolytic cleavage of the CHHP formed to give cyclohexanol with high selectivity. Table 1. Variation of cyclohexane conversion and selectivity of products with different VPO: alumina. Reaction conditions: H2O2: cyclohexane mole ratio is 5:1, temperature 333K, 24 h. S No

Code

Surface area (m2/g)

Cyclohexane conversion (%)

Selectivity (%) Cyclohexanol

CHHP

Cyclohexanone

1

VPO-O

5.0

53.8

58.0

40.4

1.6

2

VPO-25

139.8

23.0

16.3

54.3

29.4

3

VPO-35

108.4

40.0

35.7

54.8

9.5

4

VPO-50

101.0

83.3

42.8

56.6

0.6

5

VPO-75

56.4

59.0

49.2

36.0

14.8

4. Conclusions It is evident that the VOPO4.2H2O phase can be exfoliated in 2-propanol and dispersed on alumina with retention of the original structure. The crystallinity, crystallite size, and morphology, are dependent upon the VPO: alumina ratio and the activity of the different phases for the selective oxidation of cyclohexane varies accordingly. The present work therefore provides a novel approach for preparing dispersed VPO phases whose structural characteristics and consequently their catalytic activity can be controlled through the VPO: alumina ratio.

References [1] R.L.Bergmen, N.W.Frisch US Pat.3293268, 1996 assigned to Princeton chemical research. [2] Ursula Bentrup, 2000, Selective oxidation of p-substituted toluenes to the corresponding benzaldehydes over (VO)2P2O7: an in situ FTIR and EPR study, J.Mol.Catal. A 162, 391395. [3] (a) E.Bordes, 1993, Nature of the active and selective sites in vanadyl pyrophosphate,catalyst of oxidation of n-butane, butene and pentane to maleic anhydride, Catal.Today,16,1, 27-38. (b) P.A. Agaskar, L. DeCaul, R.K. Grasselli, 1994, A molecular level mechanism of nbutane oxidation to maleic anhydride over vanadyl pyrophosphate, Catal.Lett., 23, 3-4, 339351. [4] (a) A. Datta, M. Agarwal, S. Dasgupta, 2002, A novel procedure for the synthesis of NH3 incorporated VPO phases, J. Mater. Chem., 12, 6, 1892-1897. (b) S. Dasgupta, M. Agarwal, A. Datta, 2002, Long chain alkyl amine templated synthesis of a mesostructured lamellar vanadium phosphate phase, J. Mater. Chem., 12, 2, 162-164. (c) S. Dasgupta, M. Agarwal, A. Datta, 2004, Mesolamellar vanadium phosphate phases obtained by intercalation of a long chain alkylamine into different catalytically important VPO host lattices, J. Mol. Catal. A: Chemical, 223, 167-176. (d) A. Datta, S. Dasgupta, M. Agarwal, S.S. Ray, 2005, Mesolamellar VPO phases obtained by incorporating long chain alkyl amine surfactants into the layered vanadium phosphate dihydrate phase, Micropor. Mesopor. Mater.,83,1-3, 114124.

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[5] S. Dasgupta, M.Agarwal, A.Datta, 2004,Surfactant assisted organization of an exfoliated VOHPO4..5H2O to a mesostructured lamellar VPO phase, Micropor. Mesopor. Mater. 67, 229. [6] A.R. Antonio, R.L. Barbour, P.R. Bulm, 1987, Interlayer coordination environments of iron, cobalt, and nickel in VOPO4.2H2O, intercalation compounds, Inorg. Chem. 26, 8, 12351243. [7] Unnikrishnan Pillai, Endalkachew Sahle-Demessie, 2003, VPO as an efficient catalyst for hydrocarbon oxidations using hydrogen peroxide, New J. Chem.,27,3, 525-528.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Mesoporous CuO-Fe2O3 composite catalysts for complete n-hexane oxidation Silviya Todorovaa, Jian-Liang Caob, Daniela Panevaa, Krasimir Tencheva, Ivan Mitova, Georgi Kadinova, Zhong-Yong Yuanb, Vasko Idakieva a b

Institute of Catalysis, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria Institute of New Catalytic Materials Science, Nankai University, Tianjin 300071, China

Abstract A series of mesoporous CuO-Fe2O3 composite oxide catalysts with various CuO content have been synthesized by surfactant-assisted method of nanoparticle assembly. The prepared samples demonstrate high surface area and narrow mesopore size distribution. Mössbauer spectroscopy revealed ultra-dispersed hematite-like particles with super paramagnetic behavior of iron ions in all mesoporous CuO-Fe2O3 composites, though additional particles with the sizes above 10 nm were also observed in the pure mesoporous Fe2O3. The catalytic behavior of composite oxides in the reaction of complete n-hexane oxidation depends on the CuO loading and dispersion. Highly dispersed copper species are responsible for the better catalytic performance. Keywords: mesoporous CuO-Fe2O3, n-hexane oxidation

1. Introduction Catalytic combustion is a competitive technique for removal of volatile organic compounds (VOCs) especially when the organics cannot be recycled, sold, or present in low concentrations [1]. Compared with the thermal incineration, the catalytic oxidation occurs at lower temperatures and thus gives lower energy cost and no NOx emission [2]. Metal oxides are an alternative to noble metals as catalysts for total oxidation. Copper oxide is well known component of oxidation catalysts and has been considered as substitute for noble metal catalysts in emission control application due to its high activity, tolerance to sulphur and refractory nature [3]. Appropriate combination of oxides and supports may lead to the formation of highly active catalysts. The mesoporous support would give rise to well-dispersed and stable metal particles, due to its abundant pores and large surface area, thus possessing a great potential in further improvement of the catalytic performance. The main object of the present study is the relation between type of copper oxide species on mesoporous Fe2O3 and catalytic activity in complete n-hexane oxidation. Hexane has been chosen because it is a component of many products related to wastes of industry.

2. Experimental 2.1. Sample preparation and characterization High-surface-area mesoporous CuO- Fe2O3 catalysts with various CuO content (denoted as FeCuX, X=10, 15, 20, 33, 50 mol.%,) were prepared by a surfactant-assisted method of nanocrystalline particle assembly. The method is described in details in Ref. [4]. All samples were characterized by XRD, N2 adsorption, TEM, H2-TPR and Mössbauer spectroscopy. Mössbauer spectra (MS) were obtained with a Wissel electromechanical Mössbauer spectrometer (Wissenschaftliche Elektronik GmbH, Germany) working at a

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constant acceleration mode. A 57Co/Cr source (activity ≅ 10 mCi) and an α-Fe standard were used. The experimentally obtained spectra were treated using the least squares method.

2.2. Catalytic tests The tests for n-hexane oxidation were carried out in a flow type glass reactor at GHSV =14400 h−1. An inlet n-hexane concentration of 2.5 g/m3 in air was used. The reaction products were analyzed by a Varian 3700 gas chromatograph equipped with thermal conductivity detector, flame ionization detector and 3 m column with Porapak Q. Only CO2, H2O and hexane were identified in the reaction products.

3. Results and discussion Figure 1 presents the XRD patterns of all prepared samples. The pure Fe2O3 and the FeCu10 sample containing 10 mol.% CuO show reflections characteristic of rhombohedral hematite Fe2O3 structure. The mean particle size calculated from Scherrer equation is 36 nm for pure Fe2O3. The addition of 10 mol.% CuO shifts the XRD peaks of Fe2O3 to higher values, indicating changes into the lattice parameters. These changes suggest an incorporation of Cu species into the hematite Fe2O3 structure and formation of a solid solution. The average diameter of the large Fe2O3 particles is 69 nm for FeCu10 sample. When the copper content is increased to 15-25 mol.%, the respective XRD patterns are consistent with the presence of amorphous samples. Reflections characteristic for CuO appeared in the XRD patterns of the samples with Cu content of 33 and 50 mol.%. The mean diameter of CuO particles is 18 nm for FeCu33 and 26 nm for FeCu50. The N2 adsorption isotherms of these samples were of classical type IV, characteristic of mesoporous materials. The H2-type of the hysteresis loop was typical for wormholelike and hierarchical scaffold-like mesoporous structures [4].

Fig. 1. XRD patterns of samples calcined at 300ºC.

Fig. 2. Mössbauer spectra of mesoporous Fe2O3 and FeCu33 before and after catalytic test.

The surface area, pore volume and pore size of the synthesized FeCux samples decrease when CuO content increase from 15 to 50 mol.% [4]. The TEM micrographs of the samples (not shown here) demonstrate a disordered wormhole-like mesoporous structure, formed by the agglomeration of uniform nanoparticles. The accessible pores are connected randomly, lacking discernible long-range order in the pore arrangement among the small particles. According to Mössbauer data all iron ions in the pure mesoporous Fe2O3 have parameters characteristic for high-spin Fe3+ in an octahedral oxygen environment (Fig. 2). They are included in two type α-Fe2O3 particles: mainly

Mesoporous CuO-Fe2O3 composite catalysts for complete n-hexane oxidation

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such with size above 10 nm - sextet component (solid line) and ultra-dispersed particles with superparamagnetic behavior – doublet component (dashed line). FeCu33 sample exposed highest catalytic activity among all studied samples and was examined by Mössbauer spectroscopy before and after the catalytic test. The Mössbauer spectra of FeCu33 sample before and after catalytic test consist only of doublets revealing that iron ions are in ultra-dispersed hematite-like particles with super-paramagnetic behavior. TPR profile of the mesoporous Fe2O3 (Fig. 3) shows sharp reduction peak at 385ºC, attributed to the reduction of Fe2O3 to Fe3O4, and a broad peak in the temperature interval 423-700ºC, assigned to the subsequent reduction of Fe3O4 to FeO and Fe [5]. Three reduction peaks (170, 210 and above 500ºC) are visible in the TPR profile of copper modified samples. According to a number of TPR investigations of copper modified oxides, the peaks below 200ºC are generally attributed to the reduction of finely dispersed CuO and these above 200ºC – to the reduction of bulk CuO [6, 7]. As it was shown above there are no reflections of CuO in XRD patterns of the samples with CuO content up to 20 mol.%. By analogy with mesoporous CuO/Ce0.8Zr0.2O2, prepared by the same method [8], we could ascribe the peak at 170°C to the reduction of finely dispersed CuO species. The peak for reduction of Fe2O3 to Fe3O4 in copper containing catalysts is, most probably, shifted to the lower temperature and overlapped with the reduction peaks of other copper oxide species. Thus, the presence of two overlapping peaks with maximum at 210°C and 250°C is indication for the reduction of at least two different oxide phases (copper and/or iron oxide). The reduction of Cu+, incorporated in Fe2O3 (formation of solid solution CuO-Fe2O3 was suggested above discussing XRD data) and/or reduction of Fe2O3 to Fe3O4 could occur in this interval.

Fig. 3. TPR spectra of all samples after calcination at 300°C.

Fig. 4. Catalytic activity of the samples in the reaction of complete n-hexane oxidation.

The hydrogen consumption at 264°C in TPR profile of FeCu50 is ascribed to the reduction of bulk like CuO spices, formed on the support at high copper concentration [4]. The peak attributed to the reduction of Fe3O4 to FeO and Fe in copper-loaded samples is shifted to lower temperature. The higher copper content the larger is the shift, indicating that copper decreases the reduction temperature of Fe3O4 to FeO and Fe, as well. Figure 4 shows the n-hexane conversion vs. temperature on samples of various CuO content. The pure Fe2O3 is active in the complete n-hexane oxidation. The addition of CuO up to 20 mol.% results in displacement to the lower temperature of the conversion curves, indicating an increase in the catalytic activity. Further enhancement

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in CuO loading up to 50 mol.% CuO does not change significantly the catalytic activity. These results clearly show that there is an optimum in the copper oxide content. According to XRD results, finely dispersed copper species predominate in the samples at loadings up to 20 mol.% CuO. The catalysts containing more than 20 mol.% CuO exhibit the bulk CuO phase, which, most probably, does not influence the catalytic activity. Transition metal oxides operate in the oxidation reactions through a Mars and van Krevelen mechanism [9]. According to this mechanism the substrate is oxidized by the solid. The oxygen species introduced in the substrate come from the lattice. In this way the catalytic behavior can be correlated to the lattice oxygen mobility of the crystalline framework. The lattice oxygen mobility is associated with the catalyst reducibility. It is known that both copper and iron oxide are active in the combustion reactions. The increase in the catalytic activity of the bicomponent CuO-Fe2O3 in comparison with single component iron oxide can be explained by two factors: formation of finely divided CuO particles, reducible at low temperature and enhanced reducibility of the iron oxide, as it is shown by TPR.

4. Conclusion Mesoporous nanostructured CuO-Fe2O3 catalysts with high surface area and narrow mesopore size distributions have been prepared by the surfactant-assisted method of nanoparticle assembly. The iron ions in the pure Fe2O3 are included in two types of αFe2O3 particles: particles with size above 10 nm and ultra-dispersed particles with superparamagnetic behavior. The iron ions in CuO-Fe2O3 composite oxides are in ultradispersed hematite-like particles with superparamagnetic behavior. Catalytic activity measurements in the reaction of complete n-hexane oxidation showed that all catalysts were active. The catalytic behaviour depends on the CuO loading and the type of CuO species. The presence of finely divided copper species and the enhanced reducibility of iron oxide in the mesoporous CuO-Fe2O3 catalysts are responsible for the improved combustion activity.

Acknowledgements This work was supported by the Chinese-Bulgarian Project ДО/02-4-08 and the National Basic Research Program of China (No. 2009CB623502). The D. Paneva and I. Mitov are grateful to the project NSF ID-01-150/08.

References [1] A. Kołodziej, J. Łojewska, Catal. Today 105 (2005) 378. [2] J.J. Spivey, in: G.C. Bond, G. Webb (Senior Reporters), Complete Oxidation of Volatile Organics, Catalysis, Vol. 8, The Royal Society of Chemistry, Cambridge, 1989, p. 157. [3] P.O. Larsson, A.A. Andersson, Appl. Catal., B 24 (2000) 175. [4] J.-L. Cao, Y. Wang, X.-L. Yu, S.-R. Wang, S.-H. Wu, Z.-Y. Yuan, App. Catal., B 79 (2008) 26. [5] W.K. Jozwiak, E. Kaczmarek, T.P. Maniecki, W. Ignaczak, W. Maniukiewicz, Appl. Catal., A 326 (2007) 17. [6] G. Avgouropoulos, T. Ioannides, Appl. Catal., A 244 (2003) 155. [7] L. Ma, M.-F. Luo, S.-Y. Chen, Appl. Catal., A 242 (2003) 151. [8] J.-L. Cao, Y. Wang, T.-Y. Zhang, S.-H. Wu, Z.-Y. Yuan, Appl. Catal. B 78 (2008) 120. [9] J.C. Vedrine, G. Coudurier, J-M.M. Millet, Catal. Today 33 (1997) 3.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Preparation of PtRu/C electrocatalysts by hydrothermal carbonization using different carbon sources Marcelo M. Tusia, Michele Brandalisea, Roberto W.R.Verjúlio-Silvaa, Olandir V. Correaa, Juan C. Villalbab, Fauze J. Anaissib, Almir Oliveira Netoa, Marcelo Linardia and Estevam V. Spinacéa a

Instituto de Pesquisas Energéticas e Nucleares – IPEN/CNEN-SP Av. Prof. Lineu Prestes, 2242 – Cidade Universitária, 05508-900 São Paulo – SP, Brazil b Universidade Estadual do Centro-Oeste – UNICENTRO R. Simeão Varela de Sá, 03 - Vila Carli, 85040-080 Guarapuava – PR, Brazil

Abstract PtRu/C electrocatalysts were prepared by hydrothermal carbonization using glucose, starch or cellulose as carbon sources and reducing agents and H2PtCl6.6H2O and RuCl3.xH2O as metals source and catalysts of carbonization process. The obtained PtRu/C electrocatalysts were characterized by EDX, TGA, XRD and BET surface area and pore volume measurements. The electro-oxidation of methanol was studied by chronoamperometry. The PtRu/C electrocatalyst prepared using cellulose as carbon source showed the best performance for methanol electro-oxidation. The electrocatalytic activity of obtained materials seems to be related to the pore volume and mesoporous structure. Keywords: PtRu/C electrocatalyst, hydrothermal carbonization, methanol, fuel cell

1. Introduction Direct Methanol Fuel Cell (DMFC) is very attractive as energy source for portables, mobiles and stationary applications. PtRu/C electrocatalyst has been considered the best electrocatalyst and the catalytic activities depend on the preparation method [1]. Studies have been shown that the use of carbon nanotubes and mesoporous carbon as support increase the performance of the PtRu/C electrocatalysts, however, the synthesis of these supports are normally complex or involve harsh conditions. Recently, the synthesis of metal/carbon nanoarchitectures by a one-step and mild hydrothermal carbonization process was reported using starch or glucose and metals salts [2]. We have studied the synthesis of PtRu/C electrocatalysts by hydrothermal carbonization and focused especially on the effects of different carbon sources on the electrocatalytic performance for methanol oxidation.

2. Experimental 2.1. Preparation and characterization of PtRu/C electrocatalysts PtRu/C electrocatalysts (5 wt% of metal loading; Pt:Ru atomic ratio of 50:50) were prepared using glucose, starch or cellulose as carbon sources and H2PtCl6.6H2O and RuCl3.xH2O as metals sources. An aqueous solution of the carbon source was mixed with an amount of metals salts and then resulting mixture was submitted to hydrothermal

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treatment in a Teflon lined autoclave at 200ºC for a period of 6 h. The obtained solids were filtered, washed with ethanol and water and dried at 70ºC. After this, the materials were thermally treated under argon atmosphere at 900ºC. To calculate the carbonization yield (wt%) the mass of the as-synthesized material was divided by the mass of carbon atoms present in the molecule of the carbon source. The metal loading (wt%) was determined by thermogravimetric analysis (TGA) using a Shimadzu D-50 instrument and platinum pans. Heating rate of 5°C min-1 was employed under dry oxygen (30 mL min-1) The Pt:Ru atomic ratios were obtained by Energy dispersive X-ray analysis (EDX) using a scanning electron microscope (SEM) Phillips XL30 with a 20 keV electron beam and equipped with EDAX DX-4 microanaliser. X-ray diffraction (XRD) analyses were performed using a Rigaku diffractometer model Miniflex II using Cu Kα radiation source (λ = 0.15406 nm). The BET surface area and pore volume measurements were carried out by nitrogen adsorption at 77 K using a Micromeritics GEMINI V 2380 instrument.

2.2. Electro-oxidation of methanol Chronoamperometry experiments were carried out to examine the electrochemical activity and stability of the electrocatalysts. An amount of 20 mg of the electrocatalyst was added to 50 mL of water containing 3 drops of a 6% polytetrafluoroethylene (PTFE) suspension. The resulting mixture was transferred to the cavity of the working electrode. The reference electrode was a RHE and the counter electrode was a platinized Pt plate. Chronoamperometry experiments were performed at room temperature with a Microquimica (model MQPG01) potentiostat/galvanostat using 1.0 mol L-1 of methanol in 0.5 mol L-1 H2SO4 solution saturated with N2.

3. Results and discussion PtRu/C electrocatalysts were prepared by hydrothermal carbonization process using different carbon sources (Table 1). In the reaction conditions, the carbohydrates and/or their products of degradation can act as reducing agent of Pt(IV) and Ru(III) ions, which acts as catalysts of the carbonization process [2]. The carbonization yields of the assynthesized materials were in the range of 65-75 wt%. After thermal treatment at 900oC a weight loss of about 50-60 wt% was observed for all prepared materials and the Pt:Ru atomic ratios and the PtRu loadings were similar to the nominal values. Table 1. Properties of PtRu/C electrocatalysts prepared by hydrothermal carbonization using different carbon sources. Carbon source

Carbonization yield (wt%)

Weight loss (wt%)

PtRu load (wt%)

Pt:Ru atomic ratio

Crystallite size (nm)

glucose starch cellulose

75 71 66

51 51 59

4.3 4.8 6.0

58:42 51:49 56:44

10 10 7

BET surface area (m2 g-1) 50 117 76

Total pore volume (cm3 g-1) 0.048 0.084 0.260

The BET surface area of the material prepared using glucose was 50 m2 g-1 and this value increase to 117 m2 g-1 when starch was used as carbon source. Using cellulose the value decrease to 76 m2 g-1. However, the total pore volume of the material prepared using cellulose (0.260 cm3 g-1) was greater than the ones observed for starch (0.084 cm3 g-1) and glucose (0.048 cm3 g-1). Besides, the mesopore volume, calculated using BJH

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method, of the material prepared using cellulose (0,25 cm3 g-1) showed that it was formed almost exclusively by mesopores while the materials prepared using glucose (0,01 cm3 g-1) and starch (0,025 cm3 g-1) have a low volume of mesopores. The SEM micrographs of PtRu/C electrocatalysts after thermal treatment are shown in Figure 1.

a) b) c) Figure 1. SEM micrographs of PtRu/C electrocatalysts a) glucose, b) starch and c) cellulose.

The SEM micrograph of the PtRu/C electrocatalyst prepared using glucose (Fig. 1a) showed mainly a material with irregular morphology, however, some spherical shape were observed, while a spherical shape was predominant for the material prepared using starch (Fig. 1b). The use of cellulose as carbon source (Fig. 1c) leads to the formation of irregular agglomerates with porous structure. The X-ray diffractograms of PtRu/Carbon materials after thermal treatment were shown in Figure 2.

Figure 2. X-ray diffractograms of PtRu/C electrocatalysts prepared using different carbon sources after thermal treatment.

The diffractograms of PtRu/C materials showed a broad peak at about 2θ = 23° associated to the carbon material and five peaks at about 2θ = 40°, 47°, 67°, 82° and 87°characteristic of the face-centered cubic (fcc) structure of Pt [1]. All samples also presented a peak at about 2θ = 44º that was attributed to a separated hexagonal closepacked (hcp) phase of metallic ruthenium [1]. The (220) reflections of Pt (fcc) crystalline structure were used to calculate the average crystallite size according to Scherrer formula and the calculated values were in the range of 7-10 nm (Table 1). The chronoamperometric curves of PtRu/C electrocatalysts in 1 mol L-1 methanol in 0.5 mol L-1 H2SO4 at 0.5 V for 30 min are shown in Figure 3. The current values were normalized by gram of Pt, considering that methanol adsorption and dehydrogenation occur only on Pt sites at room temperature [1].

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Figure 3. Current-time curves at 0.5 V for PtRu/C electrocatalysts in 1.0 mol L-1 methanol in 0.5 mol L-1 H2SO4.

The following order of activity for methanol electro-oxidation was observed for PtRu/C electrocatalysts prepared with different carbon sources: cellulose > starch > glucose. Some studies have been shown that mesoporous carbons are very active as supports of electrocatalysts for Proton Exchange Membrane Fuel Cell [3,4].

4. Conclusions The hydrothermal carbonization showed to be a promising method for producing active PtRu/C electrocatalysts for methanol electro-oxidation. The electrocatalytic activity seems to be related to the pore volume and mesoporous structure of the obtained materials.

Acknowledgments CNPq, CAPES, FINEP-ProH2 and FAPESP for financial support.

References [1] A.O. Neto, R.R. Dias, M.M. Tusi, M. Linardi, E.V. Spinacé, 2007, Electro-oxidation of methanol and ethanol using PtRu/C, PtSn/C and PtSnRu/C electrocatalysts prepared by an alcohol-reduction process, J. Power Sources, 166 (1), 87-91. [2] S.-H. Yu, X. Cui, L. Li, K. Li, B. Yu, M. Antonietti, H. Cölfen, 2004, From starch to metal/carbon hydrid nanostructures: hydrothermal metal-catalyzed carbonization, Adv. Mater. 16 (18), 1636-1640. [3] S.H. Joo, C. Pak, D.J. You, S.-A. Lee, H.I. Lee, J.M. Kim, H. Chang, D. Seung, 2006, Ordered mesoporous carbons (OMC) as supports of electrocatalysts for direct methanol fuel cells (DMFC): Effect of carbon precursors of OMC on DMFC performances, Electrochim. Acta, 52, 1618-1626. [4] J.H. Kim, B. Fang, M. Kim, J.-S. Yu, 2009, Hollow spherical carbon with mesoporous shell as a superb anode catalyst support in proton exchange membrane fuel cell, Catal. Today, 146, 25-30.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Preparation of PtSn/C electrocatalysts using electron beam irradiation Dionísio F. Silva, Almir Oliveira Neto, Eddy S. Pino, Marcelo Linardi, Estevam V. Spinacé Instituto de Pesquisas Energéticas e Nucleares -IPEN / CNEN – SP, Av. Professor Lineu Prestes 2242, 05508-000 São Paulo, SP, Brazil

Abstract PtSn/C electrocatalyts are claimed to be active electrocatalysts for ethanol electrooxidation and their performances depends greatly on the preparation procedure and Pt:Sn atomic ratio. In this work, PtSn/C electrocatalysts with Pt:Sn atomic ratios of 9:1, 3:1, 1:1 and 1:3 were prepared in water/ethylene glycol using electron beam irradiation. The obtained materials were characterized by EDX and XRD and tested for ethanol electro-oxidation in acid medium using chronoamperometry. X-ray diffractograms of PtSn/C electrocatalysts showed typical face-centered cubic (fcc) structure of Pt with average crystallite size of 2 nm and the presence of a SnO2 phase (cassiterite). PtSn/C electrocatalysts prepared with Pt:Sn atomic ratios of 9:1 and 3:1 were more active for ethanol electro-oxidation than commercial PtSn/C BASF electrocatalyst. Keywords: PtSn/C electrocatalyst, electron beam irradiation, ethanol, fuel cell

1. Introduction Direct alcohol fuel cells (DAFC) are very attractive as power sources for mobile and portable applications. Methanol has been considered the most promising fuel because it is more efficiently oxidized than other alcohols. In Brazil ethanol is an attractive fuel as it is produced in large quantities from sugar cane and it is much less toxic than methanol, however, its complete oxidation to CO2 is more difficult than that of methanol due to the difficulty in C-C bond breaking and to the formation of COintermediates that poison the platinum anode catalysts. Thus, more active electrocatalysts are essential to enhance the ethanol electro-oxidation. PtSn/C has been considered the best electrocatalyst for ethanol electro-oxidation and the performance depends greatly on its preparation procedure and Pt:Sn atomic ratio [1,2]. Belloni et al. [3] prepared carbon-supported PtRu nanoparticles using electron beam irradiation and the obtained catalysts were found to be efficient for methanol electro-oxidation. In this work, PtSn/C electrocatalysts with different Pt:Sn atomic ratios were prepared in water/ethylene glycol using electron beam irradiation.

2. Experimental 2.1. Preparation and characterization of PtSn/C electrocatalysts PtSn/C electrocatalysts (20 wt% of metal loading) were prepared with different Pt:Sn atomic ratios using H2PtCl6.6H2O (Aldrich) and SnCl2.2H2O (Aldrich) as metal sources, which were dissolved in a water/ethylene glycol 25/75 (v/v) solution. After this, the Carbon Vulcan® XC72R, used as support, was dispersed in the solution using an ultrasonic bath. The resulting mixtures were submitted at room temperature under stirring to electron beam irradiation (Electron Accelerator’s Dynamitron Job

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188 – IPEN/CNEN – SP) and the total dose applied was 288 kGy (dose rate 1.6 kGy s-1, time 3 min). After electron beam irradiation, the mixtures were filtered and the solids (PtSn/C electrocatalysts) were washed with water and dried at 70oC for 2 h. The Pt:Sn atomic ratios were obtained by EDX analysis using a Philips XL30 scanning electron microscope with a 20 keV electron beam and provided with EDAX DX-4 microanaliser. The X-ray diffraction (XRD) analyses were carried out in a Miniflex II model Rigaku diffractometer using Cu Kα radiation (λ = 0.15406 nm). The diffractograms were recorded at 2Ө in the range 20° - 90° with step size of 0.05° and scan time of 2 s per step. The average crystallite size was calculated using Scherrer equation [2].

2.2. Electro-oxidation of ethanol Chronoamperometry experiments were carried out to examine the electrochemical activity and stability of the electrocatalysts. An amount of 20 mg of the electrocatalyst was added to 50 mL of water containing 3 drops of a 6% polytetrafluoroethylene (PTFE) suspension. The resulting mixture was treated in an ultrasound bath for 10 min, filtered and transferred to the cavity (0.30 mm deep and 0.36 cm2 area) of the working electrode. The reference electrode was a RHE and the counter electrode was a platinized Pt plate. Chronoamperometry experiments were performed with a Microquimica (model MQPG01) potentiostat/galvanostat using 1.0 mol L-1 of ethanol in 0.5 mol L-1 H2SO4 solution saturated with N2 at 0.5V and at room temperature. For comparative purposes a commercial PtSn/C BASF electrocatalyst (20 wt%, Pt:Sn atomic ratio of 3:1, alloy, Lot #F0930203) was used.

3. Results and discussion The electron beam irradiation causes the ionization and excitation of water molecules present in the reaction medium forming the species showed in Eq. 1 [3]. . .

H2O → eaq-, H+, H , OH, H2O2, H2 -

(1)

.

The aqueous solvated electrons, eaq , and H atoms are strong reducing agents and were able to reduce metal ions down to the zero-valent state (Eq. 2 and 3) M+ + eaq- → M0 . M+ + H → M0 + H+

(2) (3)

.

On the other hand, OH radicals could oxidize the ions or the atoms into a higher oxidation state and thus to counterbalance the reduction reactions (2) and (3). Thus, an . OH radical scavenger (ethylene glycol) is added to the reaction medium, which reacts with these radicals leading to the formation of radicals exhibiting reducing power that are also able to reduce metal ions (Eq. 4 and 5) [3]. .

(CH2OH)2 + OH → HOH2CĊHOH + H2O M + HOH2CĊHOH → M0 + HOH2CCHO + H+ +

(4) (5)

In this manner, the atoms produced by the reduction of metals ions progressively coalesce leading to the formation of metal nanoparticles. PtSn/C electrocatalysts were prepared with Pt:Sn atomic ratios of 9:1, 3:1, 1:1 and 1:3 (Table 1). The EDX analysis showed that the obtained materials have Pt:Sn atomic ratios very similar to the nominal ones.

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Preparation of PtSn/C electrocatalysts using electron beam irradiation Table 1. Pt:Sn atomic ratios and average crystallite sizes of PtSn/C electrocatalysts. Pt:Sn atomic ratio (nomimal)

Pt:Sn atomic ratio (EDX)

Crystallite size (nm)

9:1 3:1 1:1 1:3

9:1 2.8 : 1.2 1:1 1.2 : 2.8

<2 <2 2 2

The X-ray diffractograms of PtSn/C electrocatalysts are shown in Figure 1.

Fig. 1. X-ray diffractograms of PtSn/C electrocatalysts.

The XRD diffratograms of Pt/C and PtSn/C electrocatalysts showed a broad peak at about 25o, which was associated to the Vulcan XC72R support material, and five diffraction peaks at about 2θ = 40o, 47o, 67o, 82o e 87o, which are characteristic of the fcc structure of platinum and platinum alloys [2]. For comparative purposes it is shown in Figure 1 the diffractogram of the commercial PtSn/C BASF electrocatalyst (PtSn alloy), that showed a shift of the peaks relative to Pt(fcc) phase to lower angles compared to those of Pt/C electrocatalyst. On the other hand, this shift was not observed for all prepared PtSn/C electrocatalysts showing that no PtSn alloys were formed. However, two peaks at approximately 2θ = 34o and 52o were observed in the diffractograms of the PtSn/C electrocatalysts, which increase with the increase of tin content in the samples and were identified as a SnO2 phase (cassiterite) [2]. Henglein and Giersig [4] described the preparation of colloidal tin by radiolytic reduction of SnCl2 in water/2propanol. In this case, all process steps were performed under controlled argon atmosphere. In our case, the experiments were performed in open atmosphere and the SnO2 phase was probably formed through hydrolysis-oxidation of SnCl2 [5]. Thus, only Pt(IV) ions were reduced to the metallic state under the used conditions. The chronoamperometric curves of PtSn/C electrocatalysts in 1 mol L-1 ethanol in 0.5 mol L-1 H2SO4 at 0.5 V for 30 min are shown in Figure 2. The current values were normalized per gram of platinum, considering that ethanol adsorption and dehydrogenation occur only on platinum sites at room temperature [2].

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Fig. 2. Current-time curves at 0.5 V for PtSn/C electrocatalysts in 1.0 mol L-1 ethanol in 0.5 mol L-1 H2SO4.

In all current-time curves there is an initial current drop in the first minutes followed by a slower decay. PtSn/C electrocatalysts prepared with Pt:Sn atomic ratio of 9:1 and 3:1 showed a superior performance for ethanol oxidation compared to the commercial PtSn/C BASF electrocatalyt.

4. Conclusions In the preparation of PtSn/C electrocatalyts the electron beam irradiations were performed in open atmosphere which results only in the reduction of Pt(IV) ions to the metallic state. Sn(II) ions suffers hydrolysis-oxidation forming a SnO2 phase. Current studies have been shown that PtSn electrocatalysts containing Pt and SnO2 have good performances for ethanol electro-oxidation [2, 6].

Acknowledgments FAPESP (Proc. no 2007/08724-7), FINEP-ProH2 e CNPq for financial support.

References [1] E. Antolini, 2007, Catalysts for direct ethanol fuel cells, J. Power Sources , 170(1), 1-12. [2] A.O. Neto, R.R. Dias, M.M. Tusi, M. Linardi, E.V. Spinacé, 2007, Electro-oxidation of methanol and ethanol using PtRu/C, PtSn/C and PtSnRu/C electrocatalysts prepared by an alcohol-reduction process, J. Power Sources, 166 (1), 87-91. [3] J. Belloni, M. Mostafavi, H. Remita, J-L. Marignier, M-O. Delcourt, 1998, Radiationinduced synthesis of mono- and multi-metallic clusters and nanocoloids, New. J. Chem., 22 (11), 1239-1255. [4] A. Henglein, M. Giersig, 1994, Radiolytic formation of colloidal tin and tin-gold particles in aqueous solution, J. Phys. Chem., 98 (28), 6931-6935. [5] K. Ke, Y. Yamazaki, K. Waki, 2009, A simple method to controllably coat crystalline SnO2 nanoparticles on multiwalled carbon nanotubes, Nanosci. Nanotechno., 9 (1), 366-370. [6] A.Kowal, M. Li, M.Shao, K.Sasaki, M.B.Vukmirovic, J. Zhang, N.S. Marinkovic, P. Liu, A.I. Frenkel, R.R. Adzic, 2009, Ternary Pt/Rh/SnO2 electrocatalysts for oxidizing ethanol to CO2, Nature Mater. 8 (4) 325-330.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Preparation of PtSn/C skeletal-type electrocatalyst for ethanol oxidation Rudy Crisafulli, Almir Oliveira Neto, Marcelo Linardi, Estevam V. Spinacé Instituto de Pesquisas Energéticas e Nucleares-IPEN/CNEN – SP, Av. Professor Lineu Prestes 2242, 05508-000 São Paulo, SP, Brazil

Abstract PtSnCu/C electrocatalyst with Pt:Sn:Cu atomic ratio of 50:30:20 was prepared by borohydride reduction and the obtained material was further treated with nitric acid. The obtained material was characterized by EDX and XRD and tested for ethanol electrooxidation in acid medium using chronoamperometry. The X-ray diffractogram of as-synthesized PtSnCu/C electrocatalyst showed typical face-centered cubic (fcc) structure of Pt alloy and after acid treatment it was observed that the Pt (fcc) structure was preserved. PtSnCu/C electrocatalyst acid-treated was more active for ethanol electro-oxidation than as-synthesized PtSnCu/C and PtSn/C electrocatalyts. Keywords: PtSn/C electrocatalyst, skeletal-type, ethanol, fuel cell

1. Introduction Direct Alcohol Fuel Cells (DAFCs) are attractive as power sources for mobile and portable applications. Compared to hydrogen-fed fuel cells, which need a reforming system or have problems of hydrogen storage, DAFCs use a liquid fuel, thus simplifying the fuel system. Methanol has been considered the most promising fuel because it is more efficiently oxidized than other alcohols and PtRu/C electrocatalysts have been shown the best performance for anodic oxidation of methanol. However, ethanol offers an attractive alternative as fuel because it is produced in large quantities from biomass and it is much less toxic than methanol. On the other hand, its complete oxidation to CO2 and water is more difficult than that of methanol due to the difficulties in C-C bond breaking and to the formation of CO-intermediates that poison the platinum anode catalysts. In this manner, its complete oxidation remains a great challenge and the principal products formed are acetaldehyde and/or acetic acid [1]. PtSn/C electrocatalysts have been shown good performance for ethanol electrooxidation compared to PtRu/C electrocatalysts, however, their activities depends greatly on the preparation procedure [2]. Recently, Koh and Strasser [3] reported that after Cu dealloying from carbon-supported Pt-Cu alloy nanoparticle electrocatalyst the resulting material showed a significant activity enhancement for oxygen-reduction reaction in fuel cells compared to pure Pt. When a metal is leached from an alloy, electrochemically or chemically in acidic solutions, this results in a skeletal-type catalyst [4]. In this study, PtSnCu/C electrocatalyst (carbon-supported PtSnCu alloy) was prepared by borohydride reduction and the obtained material was treated with nitric acid to remove non-noble metals by chemical leaching in order to obtain a carbon-supported Pt alloy skeletal-type electrocatalyst. The obtained material was tested for ethanol electrooxidation.

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2. Experimental 2.1. Synthesis and characterization of the electrocatalysts Pt/C, PtSn/C (Pt:Sn atomic ratio of 50:50) and PtSnCu/C (Pt:Sn:Cu atomic ratio of 50:30:20) electrocatalysts were prepared with 20 wt% of metal loading by borohydride reduction. H2PtCl6.6H2O, SnCl2.2H2O and CuCl2.2H2O were dissolved in 2-propanol and the carbon support Vulcan XC72 was dispersed in the solution. A solution of sodium borohydride was added and the final mixture was kept under stirring for 40 min at room temperature. Finally, the mixture was filtered and the solid was washed with water and dried at 70oC for 2h. In a second step, the PtSnCu/C electrocatalytst was dispersed in concentrated nitric acid and it was kept for 70 min under stirring at room temperature. After this, the mixture was filtered and the solid was washed with water and dried at 70oC for 2h. The Pt:Sn:Cu atomic ratios were obtained by EDX analysis using a Philips XL30 scanning electron microscope with a 20 keV electron beam and provided with EDAX DX-4 microanaliser. The X-ray diffraction (XRD) analyses were carried out in a Miniflex II model Rigaku diffractometer using Cu Kα radiation (λ = 0.15406 nm). The diffractograms were recorded at 2Ө in the range 20° - 90° with step size of 0.05° and scan time of 2 s per step.

2.2. Electro-oxidation of ethanol Chronoamperometry experiments were carried out to examine the electrochemical activity and stability of the electrocatalysts. An amount of 20 mg of the electrocatalyst was added to 50 mL of water containing 3 drops of a 6% polytetrafluoroethylene (PTFE) suspension. The resulting mixture was treated in an ultrasound bath for 10 min, filtered and transferred to the cavity (0.30 mm deep and 0.36 cm2 area) of the working electrode. The reference electrode was a RHE and the counter electrode was a platinized Pt plate. Chronoamperometry experiments were performed with a Microquimica (model MQPG01) potentiostat/galvanostat using 1.0 mol L-1 of ethanol in 0.5 mol L-1 H2SO4 solution saturated with N2 at 0.5V and at room temperature.

3. Results and discussion Pt/C, PtSn/C and PtSnCu/C electrocatalysts were prepared by borohydride reduction (Table 1). Table 1. Pt:Sn and Pt:Sn:Cu atomic ratios and average crystallite sizes of the electrocatalysts. Electrocatalyst

Pt:Sn:Cu atomic ratio (nomimal)

Pt:Sn:Cu atomic ratio (EDX)

Crystallite size (nm)

Pt/C PtSn/C PtSnCu/C PtSnCu/C* *after acid treatment

– 50:50 50:30:20 –

– 53:47 52:29:19 68:21:11

5 3 2 2

The EDX analysis of the as-synthesized PtSn/C (50:50) and PtSnCu/C (50:30:20) electrocatalysts showed similar Pt:Sn and Pt:Sn:Cu atomic ratios to the nominal values. After acid treatment of the PtSnCu/C electrocatalyst the obtained Pt:Sn:Cu atomic ratio was 68:21:11 indicating that Cu and Sn atoms were partially removed. The X-ray diffractograms of Pt/C, PtSn/C and PtSnCu/C electrocatalysts are shown in Fig. 1. The

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XRD diffratogram of the Pt/C electrocatalyst showed a broad peak at about 25o, which was associated to the Vulcan XC72 support material, and five diffraction peaks at about 2θ = 40o, 47o, 67o, 82o e 87o, which are characteristic of the face-centered cubic (fcc) structure of Pt [5]. Peaks that could be attributed to Sn or Cu oxides were not observed in the diffractograms. However, for PtSn/C electrocatalysts it was observed a shift of the peaks relative to Pt(fcc) phase to lower angles compared to those of Pt/C electrocatalyst indicating an alloy formation between Pt and Sn. PtSnCu/C electrocatalyst showed a shift of the peaks relative to Pt(fcc) structure to higher angles compared to those of PtSn/C electrocatalyst, which could be attributed to the incorporation of copper atoms into the PtSn (fcc) lattice. After the acid treatment, the X-ray diffractogram of the PtSnCu/C electrocatalyst showed that the Pt (fcc) structure was preserved.

Figure 1. X-ray diffractograms of PtSn/C and PtSnCu/C electrocatalysts.

The average crystallite sizes were calculated using the Scherrer equation [5] (Table 1). A decrease of the crystallite size was observed for PtSn/C electrocatalyst (3 nm) compared to Pt/C electrocatalyst (5 nm). The addition of Cu to PtSn/C electrocatalyst also leads to a decrease of the crystallite size to 2 nm, while this value did not change after acid treatment of the PtSnCu/C electrocatalyst. The chronoamperometric curves of PtSn/C electrocatalysts in 1 mol L-1 ethanol in 0.5 mol L-1 H2SO4 at 0.5 V for 30 min are shown in Figure 2. The current values were normalized per gram of platinum, considering that ethanol adsorption and dehydrogenation occur only on platinum sites at room temperature [5].

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Figure 2. Current-time curves at 0.5 V for PtSn/C and PtSnCu/C electrocatalysts in 1.0 mol L-1 ethanol in 0.5 mol L-1 H2SO4.

The PtSnCu/C acid-treated electrocatalyst was more active for ethanol electrooxidation than the as-synthesized PtSn/C and PtSnCu/C electrocatalysts. The increase of electrocatalytic activity for ethanol oxidation could be attributed to the acid treatment that remove part of the non-noble metals of PtSnCu/C electrocatalyst creating new structural arrangements and/or more active crystallographic facets of Pt atoms at the nanoparticle surface [3].

4. Conclusions The preliminary results showed that the carbon-supported Pt alloy acid-treated (skeletaltype) electrocatalysts are very promissing for ethanol electro-oxidation. Further work is necessary to characterize the catalysts using different surface analysis techniques and to elucidate the mechanism of ethanol electro-oxidation. Also, it is necessary to perform experiments using these electrocatalysts in gas diffusion electrodes for tests in single direct ethanol fuel cells.

Acknowledgments FINEP-ProH2, CNPq and FAPESP for financial support.

References [1] E. Antolini, 2007, Catalysts for direct ethanol fuel cells, J. Power Sources , 170(1), 1-12. [2] E.V. Spinacé, L.A.I. do Vale, R.R. Dias, A.O. Neto, M. Linardi, 2006, PtSn/C electrocatalysts prepared by different methods for direct ethanol fuel cell, Stud. Surf. Sci. Catal., 162, 617-624. [3] S. Koh, P. Strasser, 2007, Electrocatalysis on bimetallic surfaces: modifying catalytic reactivity for oxygen reduction by voltammetric surface dealloying, J. Am. Chem. Soc., 129, 12624-12625. [4] C. Bock, H. Halvorsen, B. MacDougall, 2008, Catalyst Synthesis Techniques in PEM Fuel Cell Electrocatalysts and Catalyst Layers, J. Zhang (Ed.), Springer, p. 471. [5] A.O. Neto, R.R. Dias, M.M. Tusi, M. Linardi, E.V. Spinacé, 2007, Electro-oxidation of methanol and ethanol using PtRu/C, PtSn/C and PtSnRu/C electrocatalysts prepared by an alcohol-reduction process, J. Power Sources, 166 (1), 87-91.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Preparation of binary M/Mn (M = Co, Cu, Zn) oxide catalysts by thermal degradation of heterobimetallic complexes Valeriya G. Makhankova,a Oleksiy V. Khavryuchenko,a Vladyslav V. Lisnyak,a Vladimir N. Kokozaya a

Chemical Department, Kyiv National Taras Shevchenko University, 64 Volodymyrska str., UA-01601, Kyiv, Ukraine

Abstract Thermal degradation of heterobimetallic complexes [M2Mn(OAc)6(dipy)2] (M = Cu (1), Co (2), Zn (3); dipy = 2,2'-dipyridile) and freezly dried mixtures of corresponding metal acetates in stoichiometric ratio M(OAc)2:Mn(OAc)2 = 2:1 has been examined with TG/DTA (in dynamic air and under N2 atmosphere) and with thermo-programmed desorption mass-spectrometry. The products of 2 decomposition in air has been studied with SEM/EDX, indicating formation of highly dispersed oxide system. Keywords: heterobimetallic complex, thermal degradation, binary oxides

1. Introduction Preparation of catalytic binary oxide systems by thermal degradation of metal complexes gives an advantage of mild operational temperatures, extreme homogenity of resulting catalysts, easy deposition on the support and control over the preparation process [1]. However, the list of heterometallic complexes suitable for the thermolysis is still rather short, since the complexes should be cheap, acquirable with great yields, easily pyrolizing and giving no catalyst poisons as by-products. Direct synthesis method, being facile one-pot way to produce the desired compounds from metals with high yields [2], was used to obtain the complexes. In the present work we report preparation of three bimetallic complexes [M2Mn(OAc)6(dipy)2] (M = Cu (1), Co (2), Zn (3), dipy = 2,2'-dipyridile) by direct interaction of a powder metal M with Mn(OAc)2, NH4OAc and dipy in methanol solution. The compounds obtained have molecular trinuclear structure with bridging acetate groups, binding the metal centers. The peculiarities of the structure give a preposition for their application as precursors for binary metal oxides, since the ligands have low molecular weight, thermo-degrade easily and produce volatile non-poisonous products, while the metals are linked with oxygen bridges by default, which should assist formation of the mixed oxide frameworks. The thermal degradation of freezly dried mixtures of corresponding acetates in stoichiometric ratio M(OAc)2:Mn(OAc)2 = 2:1, where M = Cu (4), Co (5), Zn (6), was also examined.

2. Experimental The thermal degradation of the complexes has been examined with TG/DTA in dynamic air (Setaram Setsys 16/18) and under N2 atmosphere (Setaram Labsys 2000). Volatile

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species formed upon the dynamic thermal decomposition of 1–3 were examined by thermo-programmed desorption mass-spectrometry (МХ 7304 А). Thermal degradation of complexes 1–3 and the acetate mixtures 4–6 was performed in air (1–6-air) or in argon (in static atmosphere) (1–6-Ar) using a glass closed-bottom cylindrical reactor, stationed vertically in a self-constructed thermostated tube furnace at the temperature ~350°C during 6 h. The temperature on the sample, crucible and inside isothermal cell was monitored using Pt/Rh thermopiles. The residual solid products of the complexes and the acetate mixtures decomposition was studied by means of powder X-ray diffractometry (XRD) with a DRON–4–07 apparatus (Ni-filtered СuКα1–radiation, λ = 1.5405 Å) and scanning electron microscopy (SEM) using a Zeiss SEM Ultra60 instrument equipped with an Inca Wave 500 Oxford energy dispersive X-ray microanalysis (EDX) system.

3. Results and discussion The complexes 1–3 possess similar molecular trinuclear structure with a linear M–Mn– M [where M = Cu (1), Co (2), Zn (3)] arrangement of metal atoms, analogously to that reported in ref. [3]. Volatile species, which are formed during the dynamic thermal decomposition of 1–3, were studied by means of TPD MS in vacuum. The registered profiles for mass spectra lines m/z = 18 H2O+, 28 CO+, 44 CO2+, 58 (CH3)2CO+, 60 CH3COOH+, 78 C5H4N+, 156 C10H8N2+ were examined. At the first stage of complexes destruction the dipy desorbs in molecular form. One can thus consider further thermal processes as acetates fragment destruction. In the cases of 2 and 3 two consequent processes with maxima temperatures at c.a. 237 and 300ºC were detected in MS profiles of m/z = 18 H2O+, 28 CO+, 44 CO2+ and 58 (CH3)2CO+ belonging to acetone formation and confirming the following reactions [4, 5]: M(CH3COO)2 → MO + (CH3)2CO + CO2 (M = Mn, Co, Zn) In contrast to previous observation the destruction of acetates fragments of 1 passes through additional stage (peak at c.a. 215ºC, m/z = 60 CH3COOH+ on TPD MS curves) belonging to acetic acid formation and confirming the following reaction scheme [4]: 2Cu(CH3COO)2→2Cu + 3CH3COOH + CO2 + H2 + C TG/DTA curves (see Fig. 1 for 1 and 3) indicate that the thermal degradation of the complexes 1–3 with loss of two dipy molecules and six acetate residues decomposition occures in the range 220–370ºC (1), 210–345ºC (2) and 240–360ºC (3).

Fig. 1. TG/DTA curves for complexes 1 (a) and 3 (b) in air (top) and N2 atmosphere (lower).

The observed mass losses due to thermal degradation in air are: (1) 72.1% (calc. 2CuO·1/3Mn3O4 72.2%); (2) 72.6% (calc. for 2/3Co3O4·1/3Mn3O4 71.8%); (3) 71.3% (calc. for 2ZnO·1/2Mn2O3 71.6%). In the case of thermal degradation of the complexes

Preparation of binary M/Mn (M = Co, Cu, Zn) oxide catalysts

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in N2 atmosphere the mass losses are: (1) 72.9% (calc. 2CuO·MnO 72.9%); (2) 74.1% (calc. 2CoO·MnO 73.7%); (3) 72.8% (calc. 2ZnO·MnO 72.6%). According to XRD data (Fig. 2, b) 3 and the Zn/Mn acetate mixture 6 have similar composition products of thermal degradation in air (ZnO/ZnMn2O4) and in Ar (ZnO/MnO). In contrast to Zn-containing precursors different products depending on the nature of precursor are formed upon thermal degradation of Cu/Mn (1) or Co/Mn (2) complexes and of the metals acetates mixtures (4, 5). Thermal degradation of 1 in air gives mixture of crystalline Mn3O4 (hausmannite) and metallic Cu, while Mn3O4 and CuO+Cu2O are formed from 4. In argon 1 degrades to Cu and Mn3O4/MnO mixture, while Cu and MnO are formed from 4 (Fig. 2, a). In Ar only binary (Co,Mn)O oxides with different Mn to Co ratios are formed from 2 and from the mixture 5. Tetragonal (Co,Mn)Mn2O4 and cubic (Co,Mn)Co2O4 spinels are obtained if 2 degrades in air, but only the cubic (Co,Mn)Co2O4 spinel is registered in the XRD patterns of the degradation product for 5 (Fig. 3, a).

Fig. 2. XRD patterns of solid residues after complexes 1 (a), 3 (b) and acetate mixtures 4 (a), 6 (b) thermal degradation.

SEM/EDX was used to study the morphology and microstructure of the sample 2-air. The SEM micrographs are shown in Fig. 3, b-c. The EDX patterns support the presence of Mn, Co and O in the sample and show a broad distribution of Co and Mn content through the sample confirming formation of Co and Mn-based spinels (Co,Mn)Co2O4 and (Co,Mn)Mn2O4 particles. The Fig. 3, b-c indicates poor crystallinity of the sample. The powders obtained contain micron agglomerates and have a broad size distribution of particles within sizes range from 1 to 300 mkm. The morphology of the spinel nanoparticles is represented in the Fig. 3, b, a shape of the majority of the nanoparticles appear to be spherical with sizes ranging from 30 to 200 nm. The spherical shape of nanoparticles is observed due to smoothing of faceted grains of spinel crystallites initially having pronounced polyhedral morphology. As shown in the Fig. 3, b nanoscale crystallites tend to agglomerate because of the dipolar nature of each crystallite. Manganese-containing catalysts, namely, the samples prepared by thermal degradation in air, were tested in the oxidation H2 with O2 as a model reaction for the catalytic combustion of H2-containing gas mixtures. The conversion of H2 to H2O increases with temperature for all catalysts studied.

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Fig. 3. XRD patterns of solid residues after complex 2 and acetate mixture 4 (a) thermal degradation; SEM micrographs of 2-air (b, c).

The steady conversion is observed at certain temperature, when the saturation coverage was reached. The temperature at 100% conversion of H2 to H2O (t100%) is found to be 473 and 523 K for the most active catalysts 2-air and 1-air. The activity decreases in the following sequence (the content of spinel phase from XRD data is listed in brackets): 2-air (100%) > 1-air (85%) >> 5-air (100%) > 4-air (26%) > 3-air (57%) > 6-air (49%). If compare results of XRD study and values of t100%, one can conclude that the activity depends on not only the spinel phase content. The sample containing (Co,Mn)(Co,Mn)2O4 is characterized by the highest activity among the samples studied. So, one can suggest that the presence and red-ox behavior of Co2+/3+ – Mn3+/2+ pair sites in (Co,Mn)(Co,Mn)2O4 effect on the catalytic activity in H2 + O2 reaction the most.

4. Conclusions Application of heterobimetallic complexes [M2Mn(OAc)6(dipy)2] (M = Co, Cu, Zn) as precursors for thermal decomposition at relatively low temperatures (below 350ºC) leads to formation of highly dispersed oxide systems with significant content of spinel phases, which make them good candidates for preparation of heterogeneous catalysts for oxidation processes.

References [1] L. G. Hubert-Pfalzgraf, Inorg. Chem. Commun., 6 (2003) 102. [2] V. N. Kokozay and O. Yu. Vassilyeva, Trans. Met. Chem., 27 (2002) 693. [3] R. L. Rardin, P. Poganiuch, A. Bino, D. P. Goldberg, W. B. Tolman, Sh. Liu and S. J. Lippard, J. Am. Chem. Soc.,114 (1992) 5240. [4] M. Afzal, P. K. Butt and H. Ahrnad, J. Thermal Anal., 37 (1991) 1015. [5] K. Györyovfá and V. Balek, J. Thermal Anal., 40 (1993) 519.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V.

Preparation of highly active gas oil HDS catalyst by modification of conventional oxidic precursor with 1,5-pentanediol S. Herry,a O. Chassard,a P. Blanchard,a N. Frizi,a P. Baranek,a C. Lancelot,a E. Payen,a S. van Donk,b J P Dath,b M. Rebeilleau,b a

Unité de Catalyse et de Chimie du Solide, UMR 8181, Univ Lille Nord de France, USTL, Bâtiment C3, 59655 Villeneuve d’Ascq, France b Total Petrochemicals Research, Zone industrielle CB-7181, Feluy, Belgique

Abstract The performances of gasoil hydrodesulphurization catalysts are successfully improved by modification of CoMo/Al2O3 oxidic precursor with 1,5-pentanediol. This organic agent retards the sulfidation of the supported metals, leading to a simultaneous sulfidation of both Co and Mo atoms, which induces a better decoration of MoS2 slabs with smaller length. Keywords: hydrodesulfurization, organic agent, XPS, CoMoS phase

1. Introduction The new regulations concerning the reduction of sulfur content in diesel oil impose a drastic improvement of the hydrodesulfurization (HDS) of petroleum feedstock, which is a catalytic process most generally performed on CoMo/Al2O3 catalysts. The catalysts are obtained by sulfiding an oxidic precursor, the active phase being the so-called CoMoS phase that consists of well dispersed MoS2 nanocrystallites decorated with Co promotor atoms [1]. New methods of preparation of the oxidic precursors have been developed in order to improve the catalytic performances [2], including the use of organic agents in the impregnating solutions. A new approach for improving the performance of hydrotreating catalysts using various organic agents has been recently proposed [3, 4]. It consists in impregnating a CoMo/Al2O3 oxidic precursor with an organic agent and different class of organic agent were considered. With chelating agent such as thioglycolic acid we have shown that the increase in the catalytic performances was due to the complexation of both metals, which leads to an optimization of the active phase thanks to the simultaneous sulfidation of the Co and Mo atoms. In this study, we address the modification with non chelating agent such as 1,5-pentanediol that can however adsorb on the surface of the solid. The catalytic performance of the modified catalyst in HDS of straight run gas oil (SRGO) is evaluated and the detailed characterization of the genesis of the active phase is performed, which allows us to explain the role of the modifying agent.

2. Experimental The CoMo/Al2O3 catalyst used in this work, denoted hereafter CoMoRef, is a commercial one containing 18 wt % MoO3 and 3.5 wt % CoO. The CoMoRef was pore volume impregnated with an aqueous solution containing the desired amount of 1,5pentanediol (C5diol). After two hours of ageing, the so obtained modified solid was

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dried at 80°C under N2 during 15 hours. The C5diol/Mo molar ratio is here equal to 1, the optimum ratio as defined in a previous study [5]. The modified catalyst is denoted CoMo1C5diol. The liquid phase sulfidation procedure as well as the catalytic tests were performed in a high pressure up-flow micro reactor described in reference 3. The total sulfur contents were determined using an Antek 9000S sulfur analyzer (UV fluorescence) and sulfur compounds distribution was determined using a VARIAN CP-3800 gas chromatograph system equipped with a Sulfur Chemiluminescence Detector-SCD (Sievers model 355). To evidence keys steps of the metals sulfidation the gaseous effluents were analyzed during the activation using a rapid chromatograph coupled to the pilot unit and the genesis of the CoMoS phase was characterized at different steps of the activation using X-rays photoelectron spectroscopy (XPS). XPS sampling was performed under argon atmosphere in a glove box connected to the spectrometer (VG ESCALAB 220 XL). The binding energies (BE) of Mo3d, Co2p, C1s and S2p (sulfided samples) were determined by computer fitting of the measured spectra and referred to the Al2p photopic of the support at 74.6 eV. The surface atomic ratios IMo3d/IAl2p and ICo2p/IAl2p were calculated using the VG Eclipse software after subtracting the nonlinear Shirley background and the contribution of the S2s signal to the Mo3d signals. Decompositions of the spectra were performed to estimate the Mo sulfidation degree and to quantify the Co based entities (CoMoS and Co9S8) [6].

3. Results and discussion 3.1. SRGO HDS activity Figure 1 shows that the performance in SRGO HDS was successfully improved through the modification with 1,5-pentanediol (C5diol).

Figure 1. Catalytic performance of the CoMoRef and CoMo1C5diol solids in HDS of SRGO: Residual sulfur content in the desulfurized feed versus the reactor temperature.

Physico-chemical characterization of the CoMoRef and CoMo1C5diol catalysts was performed at each step of the genesis of the active phase in order to understand the exact role of this organic agent.

3.2. Characterization of the modified oxidic precursor Raman spectroscopy shows that after impregnation of the CoMoRef oxidic precursor with the 1,5-pentanediol solution no significant modifications of the Mo or Co species were evidenced. Whatever the solid, the features of bulk oxides such as MoO3 or

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CoMoO4 are not observed which evidence the good dispersion of the Mo species. The spectra (not reported here) exhibit the main lines at 360 and 950 cm-1 that are characteristic of the presence of well dispersed polymolybdate species [2]. The CoMoRef and CoMo1C5diol catalysts were characterized by XPS. The similarity of the Co and Mo XPS spectra of both solids shows that no reduction of the surfaces species occurs upon modification. Table 1 presents the XPS intensity ratios IMo3d/IAl2p and ICo2p/IAl2p that slightly increase upon modification, which could be assigned to a redissolution/redispersion phenomenon such as proposed by Costa et al. for triethyleneglycol [7]. Table 1. Dispersion of Mo and Co for CoMoRef and CoMo1C5diol solids as determined by XPS. Samples

IMo3d/IAl2p

ICo2p/IAl2p

CoMoRef

1.50

0.89

CoMo1C5diol

1.70

1.13

3.3. Characterization of the sulfided catalyst The evolution of Co and Mo during the sulfidation in liquid phase (Gasoil-DMDS-H2) has also been followed by XPS. The spectra (not reported here) mainly show that the sulfidation of the major part of the Co and Mo atoms occurs simultaneously at temperatures higher than 200°C for the modified catalysts whereas the available Co atoms were fully sulfided at 280°C before the MoS2 slabs formation in the case of the CoMoRef solid. Finally, XPS quantitative analysis clearly shows an increase of the Co sulfidation degree upon modification by the C5diol as reported in Table 2. Moreover the proportion of CoMoS phase is higher for the modified catalyst. Table 2 also presents the average stacking and length of the MoS2 slabs as determined by HRTEM analysis of these sulfided catalysts. Table 2. Mo sulfidation degree (% MoS2), % of Co atoms in the CoMoS (% CoMoS), average Stacking (N) and length (L) of the MoS2 slabs for the CoMoRef and CoMo1C5diol solids after liquid phase sulfidation. Samples

%MoS2

%CoMoS

N

L(Å)

CoMoRef

69

32

1.5

33

CoMo1C5diol

74

44

1.6

27

These results show that the simultaneous sulfidation induced by the use of 1,5pentanediol leads to an enhancement of the molybdenum sulfidation degree with a decrease of the length of the MoS2 slabs as indicated by HRTEM. Moreover the fraction of Co atoms involved in the CoMoS phase is higher for the modified catalyst. Both results suggest that a higher number of promoted MoS2 slabs is formed and accordingly a higher number of promoted active sites is generated on the modified catalysts, which explains the better HDS performances of the modified solid. The enhancement of the Mo sulfidation degree could be assigned to the increase of the dispersion of the Mo entities that is observed after modification of CoMoRef with C5diol while the increase of the amount of CoMoS phase has to be related to the simultaneous sulfidation of Co and Mo atoms as we proposed in a previous work on

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chelating agents [3, 4]. Such optimization of the activation process is therefore due to the delay of sulfidation of both metals. Analysis of the gas phase during the sulfidation enables to have information on the mechanism of decomposition of DMDS on the modified solid, which can proceed through two different ways according to Texier’s scheme [8]: (i) CH3-S-S-CH3 +2H2 = 2 CH3SH (ii) 2 CH3SH = CH3-S-CH3 + H2S with CH3-S-CH3 + 2H2 = 2CH4 + H2S or (iii) 2 CH3SH +2H2 = 2CH4 + 2H2S During the activation of CoMoRef, the decomposition of DMDS into CH3SH starts at about 100°C. DMS and CH4 are observed at about 140°C while H2S appears later (at about 200°C). Thus H2S produced from 140°C (simultaneously to DMS and CH4) is entirely consumed by the catalyst. This confirms the XPS results (not shown here) which show that the sulfidation of the CoMoRef starts at about 150°C. The activation process of the CoMo1C5diol is very different. Indeed, the decomposition of DMDS into CH3SH starts at about 150°C (much later than for the reference solid). Moreover DMS is observed at about 200°C while CH4 appears only during the stage at 220°C. H2S appears also during this stage but later than CH4 (one hour later). These results show that the sulfidation of the CoMoC5diol starts at about 200-220°C which is also confirmed by the aforementioned XPS results. Such delay of the DMDS decomposition was recently observed by Pashigreva et al. with citric acid as a modifying agent [9].

4. Conclusion Analysis of the gas phase explains the delay in the sulfidation of both Co and Mo atoms that are thus simultaneously sulfided at high temperature starting at 220°C. This is due to the previous adsorption 1,5-pentanediol that inhibits the decomposition of DMDS into H2S at low temperatures. This simultaneous sulfidation leads to the optimization of the nature and morphology of the active phase (decrease of the MoS2 slabs length and higher number of Co in decorating position) generating a higher number of promoted active sites which explains the better HDS performances of the modified solid.

References (1) H.Topsøe, 1996, Hydrotreating Catalysis, Springer, Berlin, p. 31. (2) P. Blanchard, 2007, New Insight in the preparation of alumina supported Hydrotreatment oxidic precursors: a Molecular Approach, Appl. Catal. A, 322, 33. (3) Frizi, N, Genesis of new HDS catalysts through a careful control of the sulfidation of both Co and Mo atoms: study of their activation under gaz phase, Catal. Today., 2008, 130, 272. (4) N. Frizi, 2008, Genesis of new gas oil HDS catalysts: study of their liquid phase sulfidation, Catal. Today., 130, 32. (5) N. Frizi, 2004, Amélioration des performances des catalyseurs en hydrodésulfuration des gazoles par modification du précurseur oxyde, PhD Thesis, Université de Lille, France, p. 186. (6) A. Gandubert, 2006, X-ray photoelectron spectroscopic surface quantification of sulfided CoMoP catalysts: Correlation with the toluene hydrogenation activity; Part I: Influence of the Co/Mo ratio, 38, p. 206. (7) V.Costa, 2008, New insights into the role of glycol-based additives in the improvement of hydrotreatment catalyst performances, Catal. Today., 130, 69. (8) S. Texier, 2004, Influence de la procédure de sulfuration sur la performance et la sélectivité des catalyseurs d’hydrotraitement, PhD Thesis, Université de Poitiers, France, p.187. (9) A.V. Pashigreva, 2008, Influence of the heat treatment conditions on the activity of the CoMo/Al2O3 catalyst for deep desulfurization of diesel fractions, Kinet. Catal., 49(6), 812.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Hierarchical meso-/macroporous phosphated and phosphonated titania nanocomposite materials with high photocatalytic activity Tian-Yi Ma, Xiu-Zhen Lin, Zhong-Yong Yuan* Institute of New Catalytic Materials Science, Engineering Research Center of Energy Storage and Conversion (Ministry of Education), College of Chemistry, Nankai University, Tianjin 300071, China. E-mail: [email protected]

Abstract Hierarchical meso-/macroporous phosphated and phosphonated titania nanocomposites were prepared by a simple template-free process with the use of precursor tetrabutyl titanate, phosphorus acid and phosphonic acid (1-hydroxy ethylidene-1,1-diphosphonic acid, ethylenediamine tetra(methylene phosphonic acid)), in which phosphate and organophosphonate groups were homogeneously incorporated in the network of the hierarchical porous titanias, respectively, exhibiting semicrystalline anatase phase with high surface areas. These synthesized hierarchical phosphated/phosphonated titanias exhibited significantly high photocatalytic activities in photodecomposition of Rhodamine B dye, in comparison with pure mesoporous titania materials, but the photoactivities of phosphonated titanias are better than that of phosphated titanias, due to the presence of novel nanorod-composed hierarchical porous nanostructure and the incorporation of phosphorus and organic groups within the titania framework. Keywords: hierarchical porosity, titania, phosphate, phosphonate, photocatalyst

1. Introduction Hierarchical porous phosphated oxides or metal phosphates have exhibited superior ability in catalysis, adsorption, ion exchange, etc., due to the homogenous incorporation of phosphorus into the material network [1]. Recently, the researching focus has been extended to organic-inorganic hybrid phosphonate-based porous materials, of which many metal phosphonate and metal oxide/organophosphonate hybrid porous materials have been synthesized. Some of the alkyl-monophosphonic acids and their derivatives (salts, esters) have been used as organophosphorus coupling molecules to modify metal oxide surfaces by grafting [2]. Mesoporous titania-phosphonate materials with organically bridged tetra- or penta-phosphonates were prepared in our previous work, in which higher photocatalytic activity could be observed of the hybrid materials than pure titania photocatalyst [3]. The high catalysis activity of these materials could also be attributed to the hierarchical meso-/macroporous structure, which is important for the increased mass transfer and reduced diffusion resistance. In this work, we report the designed preparation of nanostructured phosphated and phosphated titania materials with hierarchically meso-/macroporous architecture. The superior photocatalytic performance of the obtained phosphonated titania materials to that of phosphated titania and pure titania materials was highlighted, which could benefit from the elaborate nano-/porous structure and the large scaled micro-architecture and the homogeneous incorporation of phosphorus and carbon in the organic-inorganic network.

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2. Experimental 2.1. Material synthesis 0.04 mol of ethylenediamine tetra(methylene phosphonic acid) (EDTMP) or 1-hydroxy ethylidene-1,1-diphosphonic acid (HEDP) or H3PO4 was dissolved in a mixed solution of ethanol (30 ml) and distilled water (20 ml) with stirring, followed by the dropwise addition of 0.07 mol of tetrabutyl titanate. After stirring of 10 h, the mixture was sealed in a Teflon-lined autoclave and heated statically at 150ºC for 48 h. The product was filtered, washed with water, and dried at 110ºC, denoted as Ti-EDTMP or Ti-HEDP or PT. Mesoporous pure titania was also prepared by similar procedure for comparison.

2.2. Photocatalytic activity test In UV-photocatalysis experiment, 5.5 mg of the synthesized catalyst was placed into a tubular quartz reactor of Rhodamine B (RhB) aqueous solution (1×10-5 mol/L, 100 ml). A 125W UV lamp with maximum emission at 365 nm was located at 10 cm higher than the solution surrounded by a circulating water tube. The reaction mixture was stirred under UV-light irradiation. The mixture sampled at different time was centrifuged for 5 min to discard any sediment. The absorbance of reaction solutions was measured by a SP-722 spectrometer at λmax=554 nm. The visible-light photodecomposition of RhB was carried out with a household desktop lamp with a 40-watt tungsten bulb as the visible light source, of which the wavelength range is usually considered as 400-2500 nm.

3. Results and discussion By doping organophosphonic acid or phosphoric acid, typical hierarchically meso-/ macroporous structure were obtained, which was shown in Fig. 1. For the hybrid Ti-HEDP material, macropores were in channel-like shape with a uniform pore diameter distribution of 800-1500 nm (Fig. 1a). Interestingly, the macroporous frameworks of Ti-HEDP are composed of uniform nanorods of 40-80 nm in length and 10-15 nm in thickness, and “worm-eaten” disordered mesostructures are also observed in these nanorods (Fig. 1b), giving a novel hierarchical porous architecture. EDX analysis confirmed Ti-HEDP consisted of C, Ti, P and O, a chemical composition that agreed well with the phosphonated titania. Similar macro-channels and mesoporous nanorods could be observed in TiEDTMP (Fig. 1c,d). EDX confirmed Ti-EDTMP consisted of C, N, Ti, P and O. For PT with inorganic framework, macropores with the same size was also obtained, but composed of disordered mesoporous with diameter of around 4.2 nm (Fig. 1e). Thus the mesoporous nanorods appeared only for the organophosphonate coupled samples [4]. The XRD patterns of the synthesized Ti-EDTMP and Ti-HEDP present several very weak diffraction peaks, which could be identified as an anatase phase, while pure titania

Fig. 1 SEM (a) and TEM (b) images of Ti-HEDP; SEM (c) and TEM (d) images of Ti-EDTMP; (e) TEM image of PT.

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573

Fig. 2 (left) XRD patterns and (right) N2 adsorption-desorption isotherms and the corresponding BJH pore size distribution curves of the synthesized samples.

obtained in the absence of phosphonic acid shows the bicrystalline phases of anatase and brookite (Fig. 2left). This indicates that the hydrolysis of titanium alkoxide in thephosphonic acid solution resulted in the incorporation of organophosphonate into the titania network, leading to semicrystalline anatase nanoparticles of about 3 nm in size linked to each other by amorphous titanium-phosphonate nanoclusters [4]. The sample PT also possesses semicrystalline anatase framework with weak diffraction peaks, indicating the combination of titanium-phosphate and titania nanoparticles. Figure 2right shows the N2 adsorption-desorption isotherms and the corresponding pore size distribution curves of the samples. The isotherms of hybrid Ti-EDTMP and Ti-HEDP are of type IV, showing very steeply at high relative pressure (P/P0 > 0.8), which suggests the presence of an appreciable amount of secondary porosity of very large pores (macropores). The pore size distribution curve exhibits an asymmetric peak maximized at around 1.8 nm, which corresponds to the irregular mesostructure observed in titania–phosphonate hybrid nanorods (Fig. 1b,d). The BET surface areas of 240 and 238 m2/g with total pore volumes of 0.28 and 0.27 cm3/g were observed for Ti-EDTMP and Ti-HEDP, respectively. While the isotherms of TiO2 and PT are of type IV with H2 hysteresis loop and type II with H3 hysteresis loop respectively. The surface area of PT is 233 m2/g with a total pore volume of 0.28 cm3/g, while the surface area of pure TiO2 is 245 m2/g with a total pore volume of 0.27 cm3/g. This suggests that the synthesized PT and TiO2 have similar textural properties to that of the hybrid materials. The FT-IR spectra of Ti-EDTMP and Ti-HEDP exhibited strong band at 1048 cm-1 from the phosphonate P-O···Ti stretching vibrations. No peak at 928 cm-1 assigned to PO···H is observed suggesting the extensive condensation between Ti-OBu and P-OH groups to form Ti-O-P bridges. For Ti-EDTMP the bands at 1320 and 1435 cm-1, attributed to C-N and P-C stretching vibrations, respectively. For Ti-HEDP the bands at 1380 and 1450 cm-1, attributed to C-O and P-C stretching vibrations, respectively. The 31 P MAS NMR spectra of Ti-EDTMP and Ti-HEDP show one broad signal around 14.0 and 12 ppm, respectively, in the area characteristic of phosphonates [3]. It is thus deduced from the FT-IR and NMR spectroscopic results that no phase separation took place during the preparation of the hybrid materials and organophosphonate groups are homogeneously anchored into the hybrid solid [3,4]. Figure 3left presents the diffuse reflectance spectra of the samples, where a low reflectance means a high absorption in the corresponding wavelength. The onset wavelength of absorption (λonset) for Ti-EDTMP and Ti-HEDP is about 470 and 423 nm, respectively, which is larger than that of PT (about 408 nm) and pure TiO2 (about 400 nm). Compared with the Eg of pure TiO2 (3.10 eV), the bandgap narrowing observed in

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Fig. 3 (left) UV-vis. diffuse reflectance spectra and (right) the residual concentration of RhB solution after UV/Vis photodecomposition by Ti-EDTMP, Ti-HEDP, PT and TiO2.

Ti-EDTMP (2.61 eV), Ti-HEDP (2.93 eV) and PT (3.03 eV) should be the result of the homogeneous doping of phosphorus into the framework of the porous solids [1,3]. The photocatalytic activities of the synthesized hierarchical materials were evaluated by photodegradation of RhB under UV and visible-light irradiation (Fig. 3right). The hybrid samples exhibit superior photocatalytic activity than the others, whether under UV or visible-light irradiation, giving the following sequence: Ti-EDTMP > Ti-HEDP > PT > TiO2 > Self-degradation. The synthesized Ti-EDTMP, Ti-HEDP and PT exhibited higher photocatalytic activity than pure mesoporous titania mainly for two reasons. Firstly, titania photocatalysts doped P were reported to show their absorption edge red-shifted to lower energies (longer wavelengths), enhancing photocatalytic efficiencies in the visible light range [1,4]. Secondly, the micro/nanocomposite architecture could also increase the efficiency of photoabsorption and improving mass transfer, in which macroporous acted as light-transfer paths for the distribution of photon energy onto the inner surface of the mesoporous materials, making it a more efficient light harvester. On the other hand, the macroporous framework of the Ti-EDTMP and Ti-HEDP material was assembled by titania-tetraphosphonate nanorods with wormhole-like mesopores, while only titaniaphosphate nanoparticle assembling could be observed in PT material, giving disordered mesostructures (Fig. 1). The well-structured mesoporous nanorods would effectively prevent aggregation and thus maintain a large active surface area.

4. Conclusions Hierarchically meso-/macroporous phosphated and phosphonated titania nanocomposites were synthesized by a one-pot template-free strategy, which were further used for the photodegradation of RhB. The higher photocatalytic ability of Ti-EDTMP and Ti-HEDP than PT should be a result of well-structured hierarchical architecture constructed from mesoporous nanorods and the adding of organophosphonic acid EDTMP/HEDP during the preparation, which could offer an inorganic-organic hybrid framework with homogeneous incorporation of phosphorus and carbon.

References [1] [2] [3] [4]

Z. Y. Yuan, T. Z. Ren, A. Azioune, J. J. Pireaux, B. L. Su, Chem. Mater. 18 (2006) 1753. P. H. Mutin, G. Guerrero, A. Vioux, J. Mater. Chem. 15 (2005) 3761. X. J. Zhang, T. Y. Ma, Z. Y. Yuan, J. Mater. Chem. 18 (2008) 2003. X. J. Zhang, T. Y. Ma, Z. Y. Yuan, Eur. J. Inorg. Chem. (2008) 2721.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Gold and CuO nanocatalysts supported on hierarchical structured Ce-doped titanias for low temperature CO oxidation Tian-Yi Ma, Zhong-Yong Yuan* Institute of New Catalytic Materials Science, Engineering Research Center of Energy Storage and Conversion (Ministry of Education), College of Chemistry, Nankai University, Tianjin 300071, China. E-mail: [email protected]

Abstract A series of hierarchical nanostructured/porous titania materials doped with different content of cerium (Ce/TiO2) were synthesized by utilizing oil-in-water emulsion technique, and used as supports of gold and CuO nanocatalysts by the depositionprecipitation method. The synthesized Ce/TiO2 materials possess a hierarchical squamalike nanoarchitecture of high surface area, exhibiting pure anatase crystalline phase. The doped cerium exists mainly in the form of metal oxides with a mixture of Ce3+/4+ oxidation states, and also inter-substituting in each other’s crystal lattice occurred at the interface of titania and ceria. The gold and CuO nanoparticles were distributed homogeneously on the surfaces of the hierarchical Ce/TiO2 squamae, showing high activity in the catalytic oxidation of CO. Keywords: cerium doping; gold; CuO; titania; supported nanocatalyst; CO oxidation

1. Introduction Since the physical and chemical properties of materials depend not only on the chemical composition but crucially also on their porosity and shape, much effort has been focused on the rational design and synthesis of advanced nanostructured materials with controllable morphology, pore structure and diverse compositions. Nanostructured titania materials with different morphologies such as sphere, rod and diamond have been synthesized by various routes, due to their wide multifunctional applications [1]. It was also proved that the foreign-element doping of titania is one of the promising ways to enhance the catalytic behavior of the synthesized titania-based catalysts, because of the strong interactions between the doped elements and titania. Rare earth metal-doped TiO2 has been the object of many studies [2], such as cerium and lanthanum doping, but no interesting morphologies or hierarchical nanostructures were reported. Catalytic oxidation is an efficient way to control emission of CO that is very harmful to human health and environment. Many precious metal catalysts such as Pd/SnO2 and Au/MnOx have been demonstrated to have high activity for CO oxidation but with high cost and limited availability [3], while base metal catalysts give an alternative strategy to reduce the using of noble metal but with relatively low oxidation activity. These features of supported precious metal and base metal catalysts determine their individual use fields. We report herein the synthesis of gold and CuO nanocatalysts supported on squama-like micro/nanocomposite structured cerium-doped TiO2 (Ce/TiO2). The novel hierarchical structure and the cerium-doping in titania appear to have a cooperative contribution to make the synthesized Ce/TiO2 materials efficient supports for gold and CuO nanoparticles, which exhibited high activity in low-temperature CO oxidation.

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2. Experimental 3.0 g of Span-60 (sorbitan monostearate) was added into a mixture of 70 ml of deionized water and 30 ml of cyclohexane. The solution was kept at 65ºC in an oil bath, and allowed to adjust to pH=2 by H2SO4. Different amount of Ce(NO3)3·6H2O was added into the solution (Ce/Ti molar ratio: 0/100, 1/100, 2/100, 3/100), followed by dropwise addition of 3.404 g of tetrabutyl titanate under stirring. After a further stirring of 24 h, the obtained mixture was sealed in one Teflon-lined autoclave and aged statically at 100ºC for 24 h, followed by heating at 500ºC for 8 h to remove the surfactant species. The loading of gold and CuO was accomplished by adding the synthesized supports into HAuCl4 or Cu(NO3)2 solutions with calculated concentrations. The suspension was stirred overnight at room temperature, washed repeatedly with deionized water, and heated at 400ºC for 5h, which was marked as Au-CexTi100 or CuO-CexTi100, where x represents the content of Ce (0–3). Catalytic oxidation activity tests were performed in a continuous-flow fixed-bed microreactor, and analyzed online by a GC-900A gas chromatograph equipped with a thermal conductivity detector.

3. Results and discussion

Fig. 1 SEM (a) and TEM (b) images of the Au-Ce1Ti100 catalyst; SEM (c) and TEM (d) images of the CuO-Ce2Ti100 catalyst.

The squamalike Ce/TiO2 was synthesized in an oil-in-water emulsion, in which the nonionic surfactant Span-60 acts not only an emulsifier but also a structure directing agent for the mesostructure at the interface of water and oil phase. Afterwards, gold and CuO nanoparticles were loaded on the Ce/TiO2 support by the deposition-precipitation method, preserving the hierarchical architecture (Fig. 1). As shown in Fig. 1, the squamalike sheets of Au-Ce1Ti100 are hundreds nanometers in size with average thickness of 30-50 nm, which aggregated loosely, leaving disordered arrangement with plenty of interspaces between them (Fig. 1a). Each sheet is composed of accessible mesopores with a wormhole-like array that were formed by the assembly of the nanoparticles with the regular size of tens nanometers, and the lattice fringes can be seen in the images, reflecting the crystalline phase of the nanoparticles (Fig. 1b). For the CuO supported on Ce/TiO2 catalysts, the squamalike sheets of CuO-Ce2Ti100 could also be observed (Fig. 1c), composed of wormhole-like mesopores (Fig. 1d). Importantly, no obvious Au or CuO aggregates could be seen in the TEM images, indicating the high dispersion of the active components. The total Au and CuO loading contents were around 2.2 and 10.5 wt.%, respectively, confirmed by ICP analysis. All N2 sorption isotherms of the catalysts are of type IV with type H2 hysteresis, characteristic of mesoporous materials, and their textural properties are listed in Table 1. The pore size distributions present single peak around 7-14 nm, related to the organized aggregation of the catalyst nanoparticles

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arranged in a fairly uniform way. The pore sizes of either Au-CexTi100 or CuO-CexTi100 catalysts decrease dramatically with the increasing of cerium content, so do their pore volumes and surface areas. The surface areas mostly higher 110 m2/g were obtained after calcination at 500ºC for 8h, showing good thermal stability. Table 1. Physicochemical properties and CO oxidation abilities of the synthesized catalysts. Sample

SBET (m2/g)

DBJH-ads (nm)

Vpore (cm3/g)

Ce content (mass%)

Au or CuO content (mass%)

T100 (ºC)

Au-Ce0Ti100

144

13.5

0.44

0

2.16

35.0

Au-Ce1Ti100

140

9.8

0.44

1.70

2.15

22.5

Au-Ce2Ti100

138

8.9

0.42

3.29

2.15

20.0

Au-Ce3Ti100

113

7.4

0.28

4.65

2.16

25.0

CuO-Ce0Ti100

128

12.8

0.41

0

10.53

120.0

CuO-Ce1Ti100

126

9.0

0.40

1.56

10.53

100.0

CuO-Ce2Ti100

125

8.2

0.39

3.04

10.51

90.5

CuO-Ce3Ti100

96

6.7

0.24

4.31

10.52

115.5

All the samples exhibit pure anatase phase. None of the peaks of cerium oxide, CuO or Au structure are found, which may be attributed to the fact that the cerium species, and active components of CuO and Au are present as a highly dispersed state in TiO2. The crystallite sizes of the resultant titania phases are around 7-10 nm, and the crystal sizes of either Au-CexTi100 or CuO-CexTi100 catalysts increase with the cerium doping, corresponding to the decrease of surface areas. The values of lattice spacing (d101 of anatase) increase significantly with the doping degrees, which indicates that the large-radius rare earth ions (much larger than that of Ti4+) substitute for Ti and cause lattice distortion. The cerium-free sample shows a peak at 512 cm-1 in FT-IR spectra, which is the characteristic peak of titania, but in the case of cerium doped samples this peak is shifted to 488 cm-1, which may be due to the formation of Ti-O-Ce bond. Thus, Ce ions highly disperse on the surface of TiO2 mainly in the form of metal oxides, but inter-substituting in each other’s crystal lattice still happens at the interface of two types of metal oxides [2]. The high-resolution XPS O1s spectra indicated more oxygen adsorbed in the cerium doped catalysts than the cerium-free catalysts. Ce 3d spectra of all the synthesized catalysts basically denote a mixture of Ce3+/4+ oxidation states giving rise to a myriad of peaks, indicating the coexistence of Ce3+ and Ce4+ in CexTi100 and that the surface of the sample is not fully oxidized. Au 4f region of Au-CexTi100 exhibited doublet peaks of the catalyst located at 83.0 and 86.7 eV, assigned to the characteristic doublets of Au(0) loaded on the Ce/TiO2 support, suggesting that only elemental Au is formed on the surface of catalysts. For the Cu 2p spectrum of CuO-CexTi100, the peak centered at about 952.4 eV corresponds to Cu 2p1/2, and the peak at 932.5 eV with a shoulder at 933.7 eV corresponds to Cu 2p3/2. The presence of high Cu 2p3/2 binding energy (about 933.7 eV) and the shake-up peak (about 938–942 eV) indicate the existence of Cu2+ species in the catalysts. Meanwhile, the low Cu 2p3/2 binding energy (about 932.5 eV) is the characteristic of the reduced copper species. The formation of the reduced copper species may result from strong interaction of copper oxides with the

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high-surface area Ce/TiO2 support or the reduction of Cu2+ under the procedure of XPS measurement.

Fig. 2 (left) TPR profiles and (right) catalytic CO oxidation behavior of the synthesized catalysts.

Figure 2left shows the H2-TPR profiles of catalysts. For the naked support Ce2Ti100, two reduction peaks at around 450ºC and 580ºC were observed; the former peak could be attributed to the reduction of surface oxygen and cerium surface oxygen, and the latter be assigned to the reduction of bulk crystal lattice oxygen of titania and ceria. In the TPR curve of Au-Ce2Ti100, an additional strong peak at around 130ºC appeared, which has already been recorded for the previous reported Au/TiO2 catalysts, and this low-temperature peak was usually assigned to the reduction of oxygen species on the nanosized gold particles and to the Ti4+→Ti3+ reduction on the border with gold particles [2]. TPR curve of pure CuO shows a single peak of maximum hydrogen consumption at about 367ºC, while CuO-Ce2Ti100 shows a reduction peak at 224ºC, which is much lower than that of pure CuO, ascribed to the reduction of finely dispersed CuO species strongly interacting with the Ce/TiO2 support. The catalytic activity of the prepared catalysts for CO oxidation were investigated (Fig. 2right). All the AuCexTi100 catalysts exhibited higher activity than CuO-CexTi100 catalysts, showing the superiority of the noble metals (Table 1). The hierarchical Au-Ce/TiO2 catalyst has an obviously higher catalytic activity than TiO2-supported gold catalyst, which was also observed in the case of CuO supported catalysts. This suggests that the Ce-doping in the hierarchical titania supports benefits the catalytic oxidation activity of the resultant catalysts. The XPS spectra indicated large chemisorbed oxygen in the Ce-doped titania samples, supplying much reactive oxygen by releasing-uptaking oxygen through redox process involving the Ce4+/Ce3+ couple, which would further react with CO molecules adsorbed on Au or CuO nanoparticles to form CO2. Moreover, enough number of small crystallites of CeO2, preventing the TiO2 crystals from being in contact with each other, can be formed extensively, which leads to the stable surface area and crystal size of the supports and highly dispersed condition of Au or CuO nanoparticles. The 3D squamalike structure with plenty of interspaces between them and wormhole-like meso-channels could also supply pathways and enhance diffusion for reactant and production.

4. Conclusions Highly dispersed gold and CuO catalysts supported on hierarchical Ce/TiO2 materials have been prepared by deposition-precipitation method, showing high activity in the catalytic oxidation of CO, which could be attributed to the strong interactions between Au and CuO with the supports, the existence of Ce4+/Ce3+ redox couple and the hierarchical structure of the Ce/TiO2 supports with high surface area.

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References [1] C. C. Weng, C. P. Chen, C. H. Ting, K. H. Wei, Chem. Mater. 17 (2005) 3328. [2] T. Y. Ma, J. L. Cao, G. S. Shao, X. J. Zhang, Z. Y. Yuan, J. Phys. Chem. C 113 (2009) 16658. [3] S. R. Wang, J. Huang, Y. Q. Zhao, S. P. Wang, et al., J. Mol. Catal. A 259 (2006) 245.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Facile preparation of MoO3/SiO2-Al2O3 olefin metathesis catalysts by thermal spreading Damien P. Debecker,a Mariana Stoyanova,b Uwe Rodemerck,b Eric M. Gaigneauxa a

Université catholique de Louvain, unité de catalyse et chimie des matériaux divisés, Croix du Sud, 2/17, Louvain-la-Neuve 1348, Belgium b Leibniz Institute for Catalysis at University Rostock, Albert-Einstein-Strasse 29a, D-18059 Rostock, Germany

Abstract This paper reports a very straightforward preparation method producing active metathesis catalysts. The simple physical mixing of molybdenum oxide with a silicaalumina support followed by an adapted thermal treatment leads to the spreading of Mo oxide species at the surface of the silica-alumina. The catalysts are tested in the selfmetathesis of propene to butene and ethene and compared with samples prepared by classical wet impregnation. Characterization (XRD, in-situ XRD, Raman spectroscopy, XPS) shows that the spreading is particularly efficient at low loading and confirms the superior activity of well-spread Mo oxide species as compared to MoO3 crystallites. Keywords: propylene disproportionation, dispersion, heptamolybdate, sublimation

1. Introduction Light olefin metathesis is an attractive reaction to upgrade cheap alkenes into more demanded ones [1]. For the conversion of large amounts of light alkenes, robust and cheap heterogeneous catalysts are necessary. The preparation of MoO3-based metathesis catalysts is classically realized by impregnation of a support with a Mo precursor – usually ammonium heptamolybdate (AHM) – followed by calcination [2]. The control of the Mo surface species that form on the support is not easy. For example, increasing the Mo loading or the calcination temperature unavoidably leads to the formation of MoO3 crystallites, which are recognized as inactive in the metathesis reaction [3-5]. Recently, the direct thermal spreading (TS) of MoO3 onto an inorganic support has been proposed for the preparation of metathesis catalysts, allowing to get rid of the wet state step [4, 6]. Molybdenum trioxide can indeed be transported in the gas phase and spread at the surface of another solid if an appropriate thermal treatment is applied [7-9]. This communication depicts the thermal spreading of molybdenum oxide on a silica-alumina support and the test of the resulting catalysts in the metathesis of propene.

2. Experimental 2.1. Catalyst preparation The support is a silica-alumina from Aldrich (13 wt. % of alumina). The desired amount of MoO3 (Aldrich 99%) was mixed with the calcined (15 h at 500°C) support. The mechanical mixture was hand-ground in a mortar for 5-10 min and was then placed in a muffle furnace and heated at 500°C for 8 h under static air. Catalysts are denoted TSx

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where “x” is the MoO3 nominal weight loading (in %). Reference catalysts with loading in the same range were prepared by wet impregnation (WI) of AHM on the same support (see ref [3]) followed by calcination at 500°C (2 h) and are denoted WIx.

2.2. Catalyst characterization Table 1. MoO3 weight loading (ICP-AES) The weight content of Mo was measured by Inductively Coupled Plasma-Atomic and specific surface area (N2-physisorption). Emission Spectroscopy (ICP-AES, Table 1). MoO3 (%) SSA (m².g-1) The specific surface area (SSA) of the tested 492 catalysts was evaluated by N2-physisorption Support TS5 4.2 421 performed on a Micromeritics Tristar (Table TS10 8.9 367 1). X-ray diffraction (XRD) measurements TS14 11.5 343 were made with a Siemens D5000 diffracto16.5 335 meter using the Kα radiation of Cu (λ = TS20 1.5418 Å). For in situ experiments, a HTK10 furnace from Anton Parr was interfaced to the goniometer. The mechanical mixture was heated to 500°C under static air and diffractograms were recorded every hour and also at room temperature before and after that thermal treatment. Raman spectra were recorded on an InVia Raman microscope (Renishaw) equipped with a diode light (785 nm). Resolution was set to 4 cm-1. Acquisition time was 10 s and 10 scans were recorded and averaged for each sample. X-ray photoelectron spectroscopy (XPS) was performed on a Kratos Axis Ultra spectrometer [3]. The analyzed area was 700 µm x 300 µm. Charge stabilization was achieved by using the Kratos Axis device. Molar fractions (%) were calculated using peak areas normalized on the basis of acquisition parameters, experimental sensitivity factors and transmission factors provided by the manufacturer.

2.3. Catalyst evaluation

Intensity / a.u.

The evaluation of the catalysts was carried out in a multi-channel apparatus [10] with a capacity of treating of up to 15 samples under identical conditions. In each experiment, 200 mg of the analyzed samples was activated * * * h in parallel by heating up to 550°C with a * * ramp of 5°C.min-1 in N2 (14 ml.min-1 flow in g each reactor), kept at this temperature for 2 hours and then cooled down to the reaction f temperature (40°C) under the same medium. The propene flow (purity 99.95%) was e admitted (8 ml.min-1) sequentially in each reactor for one hour in order to measure the d initial metathesis activity of the catalyst. Analysis of the gas flow was made on an c Agilent 6890 GC [10]. The activity was calculated on the basis of metathesis products. b The selectivity to the metathesis products a (ethene and trans- and cis- butene) was high (typically 99%). Only traces of secondary 10 20 30 40 50 metathesis products (1-butene, pentenes, Bragg angle / ° hexenes) and isomerisation products Figure 1. XRD patterns obtained on (a) (isobutene) were detected. TS5, (b) TS10, (c) TS14, (d) TS20, (e)WI4, (f) WI8, (g) WI11 and (h) WI15. * = MoO3 crystals (JCPDS 05-0508).

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3. Results and discussion The MoO3 weight loading was always found to be fairly lower than the nominal one. Part of the MoO3 introduced in the preparation was presumably lost as a result of sublimation (typically ~15%). In the reference WI catalysts the Mo signal detected at the surface by XPS is high and increases linearly with the loading (Fig. 2 left). This has to be correlated to the accumulation of Mo oxide at the outer surface of the particles [11]. Bulky MoO3 crystals are indeed detected in XRD for about 8% MoO3 loading (Fig. 1). The formation of inactive crystals is recognized as the cause for the limit classically reached with such systems in the metathesis of light alkenes [3, 11-12]: the activity first increases with the loading and then reaches a plateau – as verified in Fig. 4 – or decreases. Intensity / a.u.

0.15

Mo/(Si+Al)

0.1

30°C 500°C - 1h 500°C - 2h 500°C - 3h 500°C - 4h 500°C - 5h 500°C - 6h 500°C - 7h 500°C - 8h 500°C - 9h 500°C - 10h 500°C - 11h 500°C - 12h 30°C

0.05

0 0

5

10 MoO3 loading / %

15

20

24

25

26

27

28

29

Bragg angle / °

Figure 2. Left: XPS Mo/(Si+Al) atomic ratio for (■) WI catalysts, (▲) TS catalyst and (∆) the 10% MoO3 : SiO2-Al2O3 mechanical mixture. Right: In-situ XRD analysis of a 10% MoO3 : SiO2Al2O3 mechanical mixture during thermal treatment air.

Intensity / a.u.

Thermal spreading coupled with in situ XRD (Fig. 2 right) evidence the progressive disappearance of the MoO3 crystals provoked by the thermal treatment. Consistently, no MoO3 crystals were detected in XRD, even for the catalyst with the highest MoO3 loading (Fig. 1). In XPS, the proportion of Mo detected at the surface of the 10 : 90 wt. % MoO3 : SiO2-Al2O3 mechanical mixture increases drastically after thermal spreading (Fig. 2 left). This evidences unambiguously the spreading of Mo oxide at the surface of the silica-alumina support. In Raman spectra, a e band attributed to 2D polymolybdates is d detected (around 950 cm-1), showing that MoO3 crystals are partly converted into amorphous Mo species during the thermal treatment (Fig. 3). In c parallel, the XPS Mo/(Si/Al) ratio increases less in TS samples than in WI. This can be b correlated to the better dispersion of Mo inside the pores of the support. At first sight the a thermal spreading seems to convert very efficiently pure MoO3 crystals into amorphous 600 700 800 900 1000 molybdenum oxide spread over the surface of Raman shift (cm ) the support. However, in contradiction with Figure 3. Raman spectra obtained on XRD and from ca. 8 wt. % Mo oxide loading, the (a) support, (b) TS5, (c) TS10, Raman spectroscopy enlightens the presence (d) TS14 and (e) TS20 catalysts. of MoO3 crystallites (bands at 666, 819 and -1

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-1

Activity / mmol.h .g

-1

995 cm-1). This result indicates that crystallites smaller than the XRD detection limit (~5 nm) actually remain in the mixture after thermal spreading. In other words, the spreading is not complete. At higher loading, the thermal spreading of MoO3 appears less effective, consistent with growing Raman signal for MoO3 crystallites. TS catalysts have shown a good initial 20 activity in the metathesis of propene (Fig. 4). At the lowest loading, for which complete 15 spreading of MoO3 is was measured, WI and TS catalysts perform the reaction similarly. Increasing the Mo oxide loading up to 8 wt. 10 % increases the activity of the WI sample and then a plateau is reached. The activity of the 5 TS catalysts levels off earlier and they are less active than WI catalysts. 0 So, not only the bulky MoO3 crystals 0 5 10 15 20 detected in XRD on the WI samples are MoO3 weight loading / % responsible for imposing a limit in the activity of MoO3/SiO2-Al2O3 catalysts. Also the very Figure 4. Specific activity of (■) WI and small (< 5 nm) crystallites left after thermal (▲) TS catalysts in the metathesis of spreading are inactive in the metathesis reaction. propene at 20 min time-on-stream.

4. Conclusion The direct thermal spreading of MoO3 on a silica-alumina support is a suitable and very convenient method for preparing active metathesis catalysts. Well-spread Mo surface species are produced by the TS method, especially at low loading and are identified as the precursors for the active centres. However, small MoO3 crystallites remain after thermal spreading if the loading is increased and these species are highlighted as limiting the activity of the catalysts.

Acknowledgments The authors acknowledge the UCLouvain and the FNRS of Belgium for the support and the position of D.P. Debecker. The authors acknowledge the Service public fédéral de programmation politique scientifique (Belgium) for its support via the “Inanomat” IUAP, and the European Science Foundation for its support in the Cost Action D41.

References 1. 2. 3. 4. 5. 6. 7. 8. 9.

K.J. Ivin and J.C. Mol, Olefin Metathesis and Metathesis Polymerization, ed. Academic Press. 1997, London. J. Handzlik, J. Ogonowski, J. Stoch, M. Mikolajczyk, Appl. Catal. A, 273 (2004) 99. D.P. Debecker, K. Bouchmella, C. Poleunis, P. Eloy, P. Bertrand, E.M. Gaigneaux, P.H. Mutin, Chem. Mater., 21 (2009) 2817. P. Topka, H. Balcar, J. Rathousky, N. Zilkova, F. Verpoort, J. Cejka, Microporous Mesoporous Mater., 96 (2006) 44. B. Zhang, N. Liu, Q. Lin, D. Jin, J. Mol. Catal., 65 (1991) 15. H. Balcar, P. Topka, N. Zilkova, J. Perez-Pariente, J. Cejka, Nanoporous Mater. IV, 156 (2005) 795. S. Braun, L.G. Appel, V.L. Camorim, M. Schmal, J. Phys. Chem. B, 104 (2000) 6584. F.C. Jentoft, H. Schmelz, H. Knözinger, Appl. Catal. A, 161 (1997) 167. S. Gunther, F. Esch, L. Gregoratti, A. Barinov, M. Kiskinova, E. Taglauer, H. Knozinger, J. Phys. Chem. B, 108 (2004) 14223.

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10. M. Stoyanova, U. Rodemerck, U. Bentrup, U. Dingerdissen, D. Linke, R.W. Mayer, H.G.J. Lansink Rotgerink, T. Tacke, Appl. Catal. A, 340 (2008) 242. 11. D.P. Debecker, M. Stoyanova, U. Rodemerck, E.M. Gaigneaux, (2010) In preparation. 12. S. Liu, S. Huang, W. Xin, J. Bai, S. Xie, L. Xu, Catal. Today, 93-95 (2004) 471.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Mesoporous TiO2-SBA15 composites used as supports for molybdenum-based hydrotreating catalysts M.T. Nguyen Dinh,a C. Lancelot,a P. Blanchard,a C. Lamonier,a M. Bonne,b S. Royer,b P. Marécot,b F. Dumeignil,a E. Payen,a a

Unité de Catalyse et de Chimie du Solide, UMR 8181, Univ Lille Nord de France, USTL, Bâtiment C3, 59655 Villeneuve d’Ascq, France. b Laboratoire de Catalyse en Chimie Organique, UMR CNRS 6503, Université de Poitiers, 86022 Poitiers Cedex, France.

Abstract Catalyst supports composed of titania deposited on SBA15 were prepared with titanium oxide loadings varying between 10 and 40 wt.%. Up to 30 wt.% of TiO2, titania was present as highly dispersed anatase nanocrystals in the silica pores, while some segregated titania particles were detected outside the silica network at higher loading (e.g., 40 wt.%). For all the solids, characterization evidenced the preservation of the mesoporous structure after titania deposition, with open porosity, high surface area, large pore size and pore volume. Mo-based oxidic precursors with molybdenum oxide contents of 20 wt.% were then prepared and characterized. Raman spectroscopy evidenced well dispersed polymolybdate species on all the solids whereas bulk MoO3 was also observed on the low TiO2 loading supports. This suggested that high TiO2 loading is necessary to maintain high molybdenum dispersion. A cobalt-molybdenum catalyst deposited on the composite containing 20 wt.% of TiO2 was then tested in thiophene hydrodesulfurization; its performance was found to be superior to those of catalysts based on pure TiO2 and pure SiO2, highlighting the beneficial effect of titanium oxide deposited in the form of nanocrystals inside the mesopores of a SBA15 support Keywords: HDS, thiophene, TiO2, SBA15, mesostructured sopports.

1. Introduction The active phase of conventional HDS catalysts consists of well dispersed nanometric MoS2 crystallites dispersed on a γ-Al2O3 support and promoted with cobalt or nickel atoms. The HDS performances strongly depend on the nature of the support. Among the solids considered as an alternative to alumina, titania showed interesting properties. Indeed, Mo-based catalysts supported on TiO2 were found several times more active than those supported on alumina. However, the low surface area usually developed by titania prevented its extensive use. On the other hand, mesostructured materials exhibit various advantages for catalytic applications such as a particularly high specific surface area with a pore size in the mesopores range. Mesostructured MCM41-based materials were extensively studied as HDS catalysts supports. SBA-15 material was much less investigated [1-4] but presents pore diameters significantly larger than those of MCM41 together with thicker pore walls and better hydrothermal stability. In the present work, we combined the advantages of the texture of SBA15 support together with the dispersive surface properties of titania by preparing supports composed of titania deposited on SBA15; the method used enables the impregnation of high amounts of

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TiO2 while preserving the open porosity of the prepared solids [5]. The obtained supports as well as the derived molybdenum-based catalysts were characterized and the catalytic performances of the corresponding CoMo catalysts were determined in thiophene HDS reaction and compared with those obtained for CoMo catalysts prepared on SBA15 and TiO2 supports taken as references.

2. Experimental section 2.1. Catalyst preparation The SBA15 support was prepared according to the procedure proposed by Roggenbuck et al. [6]. TiO2-SBA15 composites with TiO2 loadings ranging from 10 to 40 wt.% of TiO2 were prepared by impregnation in excess of solvent as described by Bonne et al. [6]. This method consists in the slow impregnation of a diluted solution of Ti(OiPr)4 in dry ethanol (volume ratio VTi(OiPr)4 / VEtOH = 0.2). A solution of Ti(OiPr)4 / ethanol (in which the volume of titanium isopropoxide is adjusted to obtain the desired TiO2 loading) is first prepared. A volume of solution depending on the desired titania loading is then slowly added to the silica support. The solvent is evaporated under stirring at room temperature, and the solid aged one day under ambient conditions. After further drying at 80°C overnight, the samples are then calcined at 400°C for 3 h (1°C.min-1). The Mo-based catalysts were prepared by incipient wetness impregnation of AHM [(NH4)6Mo7O24·4H2O] on the different supports with a fixed Mo content of 20 wt.% of MoO3. For the cobalt promoted catalysts, simultaneous impregnation of AHM and cobalt nitrate (Co(NO3)3·9H2O) was performed with a Co/(Co+Mo) ratio of 0.4. After impregnation and maturation for 2 h, the solids were dried overnight in an oven at 110°C and calcined at 500°C during 3 h under air flow. The prepared composites were named xTi-SBA and the corresponding catalysts were named Mo/xTi-SBA and CoMo/xTi-SBA, x being the TiO2 content in wt.%. For comparison purposes, two CoMo catalysts were prepared on pure SBA15 and on TiO2 (Alfa-Aesar, 200 m2/g).

2.2. Characterization techniques and thiophene HDS catalytic test N2 adsorption desorption isotherms were recorded at – 196°C using an automated ASAP2010 instrument from Micrometrics. The TEM pictures were taken on a Tecnai electron microscope operating at an accelerating voltage of 200 kV. The Raman spectra of the oxidic precursors were recorded at room temperature using an Infinity Raman microprobe from Jobin-Yvon, equipped with a photodiode array detector. The exciting laser source was the 532 nm line of a Nd:YAG laser. The catalytic tests were performed at 300°C under atmospheric pressure in a flowtype reactor. Before reaction, the oxidic precursor was sulfided during 2 h at 400°C under a flow composed of 10 vol.% of H2S in H2 (total flow rate = 60 mL min-1). Values of conversion are reported after one hour of catalyst activity stabilization.

3. Results and discussion 3.1. Solids characterization Table 1 presents the physical properties of Mo-based catalysts with various titania loadings. When increasing the Ti content on the composites, a decrease in the surface area, porous volume and pore diameter is observed. The physical properties evolution is coherent with the deposition of titania nanoclusters inside the pores of SBA15. Note that the small-angle X-ray diffraction patterns (not showed) always exhibit the (100) reflection suggesting the conservation of the hexagonal pore structure, even after Mo

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Mesoporous TiO2-SBA15 composites used as supports

impregnation. While some variations in the physical properties are observed between the samples when varying the titania loading, isotherms shape remains almost unchanged with respect to the silica parent support as shown in Figure 1. Hystereses of type H1-H2 are obtained for the different supports, which is characteristic of cylindrical pores, confirming the preservation of the well-organized pore structure of the SBA15. Observation of the different samples by TEM unambiguously confirms the maintaining of the hexagonal pore structure, and high magnification observation enables the detection of < 5 nm crystallized titania clusters inside the silica pores (Fig. 2). No external titania enrichment can be detected by TEM, except for the 40TiSBA15 composite for which a part of titania is found under the form of external aggregates. Table 1. Physical properties of the molybdenum-based catalysts. Sample name

SSA/ m2g-1

Vp/ cm3 g-1

dp/ nm

20Mo-10TiSBA15 20Mo-20TiSBA15 20Mo-30TiSBA15 20Mo-40TiSBA15

345 308 307 280

0.75 0.63 0.59 0.47

7.4 6.4 5.8 5.1

500

20Mo/10TiSBA15

Volume adsorbed (cm3g-1)

450

20Mo/20TiSBA15

400 350

20Mo/30TiSBA15

300

20Mo/40TiSBA15

250 200 150 100 50 0 0

0,2 0,4 0,6 0,8 Relative pressure (P/Po)

1

Figure 1: N2 adsorption–desorption isotherms for 20Mo/xTiSBA15 samples.

Figure 2: High magnification TEM image of 20TiSBA15 composite.

The X-ray diffraction patterns (not shown here) of the composites with 10, 20 and 30 wt.% of TiO2 does not show any feature indicating the presence of anatase, which suggests that titania remains amorphous or crystallized at a size below the detection limit of the instrument (‹ 4 nm). On the 40Ti-SBA, a small diffraction peak is present at 2θ = 25°, which is assigned to anatase structure, indicating for this sample the presence of larger crystallites of TiO2. These results are in good agreement with the observations made by TEM, with some segregation of titania outside the pores at high loading, while titania is observed under the form of small clusters inside the porosity up 30 wt.% loading. Raman spectra (Fig. 3) of the oxidic molybdenum catalysts exhibit large lines located at 950, 860, 560, 360, and 220 cm-1, whatever the titania loading. These bands are attributed to well-dispersed polymolybdate species. However, lines at 819 and 994 cm-1, attributed to bulk MoO3, are also observed, except for 40Ti-SBA and at a lower extent for 30Ti-SBA. Consequently, better molybdenum dispersion is achieved

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over composites having higher TiO2 loadings, which is in accordance with the dispersive character of titania.

Figure 3: Raman spectra of 20Mo/ xTi-SBA15 solids, x = a) 10, b) 20, c,d) 30, e) 40.

3.2. Catalytic activity Activity of the CoMo catalysts supported on SBA15, TiO2 and 20Ti-SBA with 20 wt.% of MoO3 were evaluated in thiophene HDS. Conversions obtained for these samples are respectively 9.0%, 16.0% and 19.5%. The lowest conversion is found for CoMo/ SBA15, which is in agreement with the poor dispersive properties of this silica. CoMo/20Ti-SBA exhibited the best performance with a conversion 1.2 times higher than that of CoMo/TiO2. Consequently, the presence of well-dispersed TiO2 nanoparticles inside the porous network of SBA15 seems to be beneficial to the thiophene HDS activity, probably by promoting a better dispersion of the MoS2 slabs.

4. Conclusion TiO2-SiO2 composites with variable titania loadings (10 to 40 wt.% of TiO2) were prepared by an optimized impregnation of SBA15. Titania was present as highly dispersed anatase nanocrystals in the silica pores, while larger TiO2 particles were observed outside the silica network at high loading. Characterization of the Mo-oxidic precursors evidenced well dispersed polymolybdate species on all the solids whereas bulk MoO3 was also observed at low TiO2 content, suggesting that the presence of TiO2 enabled an enhancement of the molybdenum dispersion. The performance of a cobalt molybdenum on 20Ti-SBA catalyst was evaluated in thiophene hydrodesulfurization; its conversion was found to be superior to that of catalysts based on pure TiO2 and SiO2, with respective conversions of 19.5%, 16% and 9%, highlighting the beneficial effect of TiO2 deposited in the form of nanocrystals in the mesopores of SBA15 support.

References [1] A. Sampieri, Hydrodesulfurization of dibenzothiophene on MoS2/MCM41 and MoS2/SBA15 catalysts prepared by thermal spreading of MoO3, 2005, Catal. Today, 108-108, 537-544. [2] G. M. Esquivel, HDS of 4,6-DMDBT over NiW/Al-SBA15 catalysts, 2009, Catal. Today, 148, 36-41. [3] R. Nava, CoMo/Ti-SBA-15 catalysts for dibenzothiophene desulfurization, 2007, Catal. Today, 127, 70-84. [4] O. Guttierez, Deep HDS over NiMo/Zr-SBA-15 catalysts with varying MoO3 loading, 2008, Catal. Today, 130, 292-301.

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[5] M. Bonne, 2009, Synthesis and characterization of high surface area TiO2/SiO2 mesostructured nanocomposite, Solid State Sci. doi:10.1016/ [6] J. Roggenbuck, Synthesis of Mesoporous Magnesium Oxide by CMK-3 Carbon Structure Replication, 2006, Chem. Mater., 18, 4151.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

p-Hydroxybenzoic acid degradation by Fe/Pd-HNT catalysts with in situ generated hydrogen peroxide Asma Turki,a Hafedh Kochkar,b* Gilles Berhault,c Abdelhamid Ghorbela a

Laboratoire de Chimie des Matériaux et Catalyse, Faculté des Sciences de Tunis (FST), Campus Universitaire El Manar, 2092, Tunisie. b Centre National de Recherche en Science des Matériaux (CNRSM), Technopôle de Borj-Cédria, 2050, Hammam-Lif, Tunisie. c Institut de Recherches sur la Catalyse et de l’Environnement de Lyon, CNRS – Université Lyon I, 69100 Villeurbanne, France.

Abstract Pd- and Fe/Pd-HNT were elaborated by successive ionic exchange and wet impregnation at room temperature of hydrogenotitanate nanotubes (HNT) nanomaterials. Hydrogen peroxide was generated in situ via reaction of oxygen and formic acid (FA) over Fe/PdHNT catalysts. A high p-hydroxybenzoic acid (p-HBz) mineralization was achieved (52%) using Fe/Pd-HNT/FA/O2/UV system compared to the simulated reaction using Fe/Pd-HNT/H2O2/UV system (34%). The rate of hydrogen peroxide formation over PdHNT catalyst, during in situ generation, probably plays a key role in controlling the kinetics of the p-HBz degradation. Keywords: hydrogenotitanate nanotubes, hydrothermal, p-hydroxybenzoic acid, photoFenton, formic acid

1. Introduction During the last decade, strong research effort was carried out to develop new chemical oxidation procedures (Advanced Oxidation Processes (AOPs)). These processes are interesting alternatives for the treatment of wastewater and the transformation of large ranges of organic refractory pollutants (lipids, polyphenols, etc.) present among other biodegradable molecules [1]. The chemical treatment with AOPs can drive to the total decomposition of organic pollutants in CO2 and in the case of halogen compounds to the formation of halogen ions. The main advantage of AOPs is the in situ generation of very reactive free radicals which have an elevated oxidizing power. Recently, Yalfani et al. [2, 3] described a simple and clean route to in situ produce H2O2 via the use of formic acid (substituting H2) and oxygen. The formation of H2O2 is achieved, with a remarkable productivity, under ambient conditions (room temperature and atmospheric pressure), in aqueous medium and using a Pd heterogeneous catalytic system. The produced H2O2 was used for the oxidation of phenol using an homogeneous Fenton reaction (Fe(II)/ Fe(III); H2O2). In the present work, a new heterogeneous bimetallic system is therefore developed by combining Fe and Pd over hydrogenotitanate nanotubes (HNT) to in situ produce H2O2 for direct degradation of p-hydroxybenzoic acid (p-HBz). To the best of our knowledge, this is the first study reporting the preparation and UV-visible light photocatalytic activity of Fe/Pd-doped titania nanotubes.

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2. Experimental 2.1. Catalysts synthesis HNT was elaborated using an alkaline hydrothermal method described by Kochkar et al. [4]. TiO2 P25 powder (Degussa-Hüls AG) was used as precursor. A part of HNT was calcined at 500°C for 2 h (2°C.min-1) under O2. This sample is named HNT-500. According to our previous study on Pd(II) adsorption over HNT [4] and to the work of Yalfani et al. [2, 3], we chose to dope HNT with 5 wt% Pd(II) and 10 wt% Fe(III). The elaboration of the different catalysts comprises several steps: (i) Pd(II) deposition on HNT by ionic exchange method : palladium nitrate (Pd(NO3)2, solution (5 mmol.L-1) was mixed with 0.5 g of HNT. Ionic exchange is set to 15 min at 20°C. The filtered material was then dried at 80°C for 24 h before to undergo a postthermal treatment at 500°C for 2 h (2°C.min-1) under O2 followed by a reduction under pure H2 flow (2 mL.min-1) at 300°C for 2 h. The black material obtained was named Pd-HNT catalyst, (ii) a part of the Pd-HNT material was also used as support for Fe(III) deposition using a wet impregnation method: the yellowish material was mixed with 10 mL of NH4+/NH3 buffer solution (pH ≈ 9.2). A suitable mass of Fe(NO3)3.9H2O was then added to this suspension. After 2 h of mixing, the suspension was dried at 80°C for 24 h. The obtained material had a brown color and was then treated at 500°C under O2 for 2 h (2°C.min-1). Color then changes to red-brick. This material was then reduced under pure H2 flow (2 mL.min-1) at 300°C for 2 h. The material then becomes black. This material was named Fe/Pd-HNT catalyst.

2.2. Characterization methods The identification of phases was determined by X-ray diffraction on a PANalytical X’Pert PRO apparatus using Cu-Kα radiation (λ = 1.5406 Å). The morphology of HNT was studied using transmission electron microscopy (TEM) images acquired with a JEOL-2010 microscope. Nitrogen adsorption-desorption isotherms were obtained on a Micromeritics ASAP 2000 nitrogen adsorption apparatus. The Brunauer-Emmet-Teller (BET) specific surface area was determined by a multi-point method using the adsorption data in the relative pressure (P/P0) range of 0.05-0.35. The Pd and Fe amounts were evaluated by ICP-AES measurements using a Horiba Jobin Yvon Activa apparatus.

2.3. p-Hydroxybenzoic acid (p-HBz) oxidation

p-HBz oxidation (10-3 mol.L-1) was carried out at room temperature in a 50 mL Pyrex reactor equipped with a magnetic stirrer in presence of UV light with a wavelength λ ≥ 345 nm (HPK 125W mercury lamp) and 100 mg of catalyst. The initial pH of the solution was 3.1. Before UV illumination, the suspension was stirred 30 min to reach the adsorption equilibrium. Formic acid (FA) was injected into the reactor volume with nFA:npHBz = 2:1 molar ratio. In the case of catalytic reaction using H2O2, the molar ratio was nH2O2:npHBz = 3:1. Oxygen was bubbled into the reaction medium. All reactions were performed in the dark to avoid any interfering effect of indoor visible light. p-HBz degradation and formic acid decomposition were monitored by sampling at regular time intervals and analyzing by high performance liquid chromatography (HPLC) using a HyperRez XP Carbohydrate (H+) column equipped with UV-visible detector. The remaining solutions after catalytic tests were collected for chemical oxygen demand (COD) determined using a hot sulphuric solution of potassium dichromate (K2Cr2O7) for 2 h at 150°C. H2O2 remaining in solution after reaction was detected through Na2SO3 titration with an excess of KI in acidic medium.

p-hydroxybenzoic acid degradation by Fe/Pd-HNT catalysts

595

3. Results and discussion 3.1. Samples characterization The XRD patterns of the as-synthesized HNT, HNT-500, Pd-HNT, and Fe/Pd-HNT samples are shown in Fig. 1. The crystal structure of HNT corresponds to the H2Ti2O5.H2O (or HxNa2-xTi2O5.H2O) phase. After calcination, HNT-500 presents a typical XRD pattern of the anatase phase except for several weak peaks of H2TiO2O5H2O. The intensity of the diffraction peak near 2θ~10°C was decreased and shifted towards a higher angle value indicating a decrease in the interlayer distance. This contraction is due to the release of water molecules adsorbed and present in the space between layers [5]. The powder XRD patterns of Pd and Fe/Pd doped materials (Fig. 1) revealed that the Pd-HNT sample is very well crystallized. Only anatase can be detected in plus of a single peak relative to metallic Pd. For Fe/Pd-HNT, besides anatase and metallic Pd, several additional peaks due to Fe2O3 and Fe3O4 are also observed. The good crystallization of the anatase phase could be related to the presence of metallic Pd in the structure. The coexistence of Fe2O3 and Fe3O4 in Fe/Pd doped materials is the consequence of the Fe(III) reduction into Fe(II) consecutively to the final H2 treatment of the sample at 300°C for 2 h.

Fig. 1. XRD patterns of: (a) HNT (b) HNT-500, (c) Pd-HNT, and (d) Fe/Pd-HNT.

TEM study of HNT samples clearly shows tubular structure with multiple layers (25 layers) and a layer spacing of about 0.25-0.35 nm (not shown). The tubes are hollow and open ended with an outside diameter of about 20 nm. HRTEM images of HNT-500 sample show that the nanotubular material has a very well crystalline wall. The distance between parallel planes is about 0.36 nm corresponding to the interreticular distance d110 of the anatase phase. Textural properties derived from nitrogen adsorption-desorption isotherm data are summarized in Table 1. The transformation of TiO2 (P25) into hydrogenotitanate nanotubes (HNT) was accompanied by a marked increase in surface area and pore volume. However, calcination of the HNT sample led to a decrease in surface area to 97 m2.g-1. This decrease may be explained by the disappearance of the multiwalled structure and partial agglomeration of nanotubes. The surface areas of Pd and Pd/Fe doped materials are also slightly lower than for HNT-500.

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A. Turki et al. Table 1. Textural parameters derived from N2 adsorption-desorption isotherms. Catalysts P25 HNT HNT-500 Pd-HNT Fe/Pd-HNT

SBET (m2.g-1) 50 258 97 71 82

Pore volume (cm3.g-1) 0 0.76 0.78 0.61 0.32

Average pore diameter (nm) 11 28 33 15

3.2. Catalytic tests Pd-HNT catalyst showed a remarkable efficiency to generate in situ hydrogen peroxide (0.12 mmol.min-1.g-1, 75% yield) by reaction of metallic palladium with formic acid in the presence of oxygen. But our main purpose was to in situ generate H2O2 for the subsequent oxidation reaction of p-HBz. Fe-doped Pd-HNT catalyst can be more eligible for the above purpose. The comparison between Fenton’s and Photo Fenton’s reactions by in situ generated H2O2 or by simulated reaction through external addition of H2O2 shows different p-HBz degradation behaviors. A maximum of 52% mineralization was achieved with Fe/Pd-HNT/FA/O2/UV system in comparison with the simulated reaction for which only 34% mineralization was achieved. For the Fe/Pd-HNT/FA/O2/UV system, the photo-Fenton process (Fe(II) or Fe(III)/H2O2/UV) seems to be the main factor for pHBz oxidation. The rate of hydrogen peroxide formation over Pd/HNT catalyst, during in situ generation, probably plays a key role in controlling the kinetics of the p-HBz degradation. These results highlight the advantage of an in situ approach in situ generating H2O2 for the p-HBz oxidation under ambient conditions. Finally, no leaching of Pd or Fe was detected through ICP measurements in the filtered reaction solutions showing that a heterogeneous catalytic system was working under our experimental conditions.

4. Conclusion In this study, hydrogenotitanate nanotubes (HNT) were elaborated using the alkaline hydrothermal method. Pd and Fe/Pd doped materials were elaborated by means of successive exchange ionic and wet impregnation reactions on HNT. The Pd-HNT catalyst showed a remarkable efficiency (75%) to generate hydrogen peroxide through heterogeneous reaction of metallic palladium in the presence of formic acid and pure oxygen at ambient temperature and atmospheric pressure. Photo-Fenton reaction using Fe/Pd-HNT/UV catalyst with in situ H2O2 generation opens a promising way for the organic molecule degradation (p-HBz) in aqueous phase under moderate conditions by reaction between formic acid and oxygen on metallic palladium.

References [1] K. Kestioğlu, T. Yonar, N. Azbar, 2005, Proc. Biochem., 40, 2409. [2] M. S. Yalfani, S. Contreras, F. Medina, J. Sueiras , 2009, Appl. Catal. B: Environmental, 89, 519. [3] M. S. Yalfani, S. Contreras, F. Medina, J. Sueiras, 2008, Chem. Comm., 3885. [4] H. Kochkar, A. Turki, L. Bergaoui, G. Berhault, A. Ghorbel, 2009, J. Coll. Int. Sci., 331, 27. [5] M. Qamar, C. R. Yoon, H. J. Oh, D. H. Kim, J. H. Jho, K. S. Lee, W. J. Lee, H. G. Lee, S. J. Kim, 2006, Nanotechnology, 17, 5922.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V.

Synthesis of ionic liquid templated zeolite like structures Amando Martín, Svetlana Ivanova*, Francisca Romero Sarria, Miguel Ángel Centeno, Jose Antonio Odriozola Departamento de Química Inorgánica and Instituto de Ciencias de Materiales de Sevilla, CSIC-US, Avda. Américo Vespucio 49, 41092, Sevilla, Spain. e-mail: [email protected]

Abstract This study describes the utilization of the alkyl substituted imidazolium based ionic liquid as a structure directing agent of the MFI type zeolite structure in water rich environment. The resulted zeolite like structure presents some similarities with the parent MFI structure, but grows preferentially in 1 D leading to zeolite like nanofibers. Keywords: ionic liquids, structure directing agent, MFI

1. Introduction The importance and extended use of the zeolites in catalysis, gas separation and adsorption and recently in medicine and electronic devices maintains the scientific interest in the development and application of this type of materials [1,2]. The synthetic technologies for these materials usually involve the use of structure directing agents (SDA), bulky organic molecules which can be removed only by a high temperature treatment thus increasing the price of the final compound and sometimes damaging the inorganic structure [3]. As well known, the ionic liquids are excellent solvents due to their negligible vapor presure. The later provokes some scientific interest devoted to the application of the ionic liquids in the “hydrothermally free” zeolite or molecular sieve synthesis [4-6] in which the IL plays a double role of solvent and template. However, the utilisation of the ILs as a template in water rich environment is poorly reported [7]. In aqueous media the Ils plays “only” the role of classical surfactant, however, with a very strong tendency towards self-organisation with high order [8]. This study reports the synthesis of a new zeolite like material starting from a known gel composition producing an MFI type zeolite and replacing the usual SDA by the imidazolium based ionic liquids.

2. Experimental 2.1. Synthesis The chemicals used for the synthesis were the commercials, tetraethoxysilane (TEOS, 99% (Assay) Alfa Aesar), sodium chloride (NaCl Rectapur ™ Prolabo), 1-Butyl-3methylimidazolium methane sulfonate (Bmim) synthesized as proposed by Cassol et al. [9], sodium hydroxide pellets (NaOH, Merck) and sodium aluminium oxide (Alfa Aesar). The gel containing the precursors (composition presented in Table 1) was aged for 4 hours at room temperature and then placed in a sealed stainless steel autoclave, partially filled with 100 mL of the reaction mixture. The hydrothermal synthesis was performed at 170ºC during 120 hours.

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A. Martín et al. Table 1. Gel composition in molar ratio.

Sample

Gel composition (molar ratios) SiO2 : NaCl : Bmim : NaOH : H2O : Al2O3

(A)

5.62 : 3.43 : 4.29 : 0.38 : 1000: 0

(B)

5.62 : 3.43 : 2.14 : 0.38 : 1000: 0

(C)

5.62 : 3.43 : 2.14 : 0.38 : 1000 : 0.13

(D)

5.62 : 3.43 : 0 : 0.38 : 1000 : 0

2.2. Characterization X-ray diffraction (XRD) analysis was performed on a Siemens diffractometer D500 over a 2θ-range of 5 to 50º. High resolution scanning electron micrographs (SEM) were taken using a HITACHI S 5200 microscope at 5 kV of the powdered samples. 29Si, 27Al and 1H single-pulse spectra were recorded on a Bruker DRX400 spectrometer with a magnetic field of 9.36 T.

3. Results and discussion 3.1. XRD

Intensity, a.u.

The XR diffraction patterns of the studied samples are presented in Figure 1. The sample D, prepared in the absence of IL presents the typical structure of amorphous silicon oxide. However, all the samples prepared in the presence of IL shows the existense of a second phase remembering the MFI structure. The appearance of one single broad diffraction peak at 2 Θ ∼ 8 for Si only samples (A and B) becomes two resolved peaks for Al containing sample (sample C). The group of diffractions at D 2 Θ ∼ 23, attributed to the MFI structure, are as well present, but slightly displaced. This shift could C be provoked by the exsistence of B the interrupted framework caused A by the presence of some unusual hydrogen bondings, as reported 5 10 15 20 25 30 35 40 45 50 2Θ by Cooper et al. [5]. The obtained structures are thermally instable Figure 1. XRD patterns of the studied samples and after calcination at 550ºC do not preserve its crystal structure. Similar behavior is characteristic for all the interrupted materials [5]. It is well known, that the template ability of the ionic liquid is strongly influenced by the nature of its anion, the change of which leads to different product phases [10]. The anion used in this study (methane sulfonate) presents low minelizer ability (not like F- containing anions) and high preference to form H-bond thus provoking the formation of the interrupted structure.

3.2. Solid state NMR

The 27Al NMR of the sample C shows a single and narrow signal at 51 ppm (not shown), which can unambiguously be attributed to tetrahedrally coordinated aluminum

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atoms in the zeolite framework. Furthermore, the absence of any signal at 0 ppm indicates the absence of any extraframework aluminum species. The 29Si NMR spectrum of the same sample shows one major peak at -112 ppm and a small shoulder centered at -105 ppm, signals assigned to Q3 and Q4 structure. However, the other IL containing samples (A and B) presents only one signal at -112 ppm attributed to the presence of SiO4 tetrahedra. For the samples A and C, the 1H NMR is performed and compared to the 1H spectra of the sample D. For the later, a broad signal between 4.5 and 5.5 ppm attributed to the presence of water in the sample is observed. The samples A and C C present similar signals, caused by the presence of IL (dashed lines indicates A the 1H NMR spectra of the bare IL). D Slight shift of the signals of n-butyl 11 10 9 8 7 6 5 4 3 2 1 0 and methyl groups are observed ppm attributed to the weak influence by 1 Figure 2. H NMR spectra of the studied samples. the neighbouring framework. The signal at 2.5 ppm, attributed to the methyl group of the IL anion, is shifted due to its participation in the H bonding with the formed frameworks through the π-π stacking mechanism, proposed by Zhou et al. [6]. However the biggest change is produced in the signals between 7.7 and 9.3 ppm attributed to the protons of imidazolium ring. As the ππ stacking interactions between the aromatic motifs and between the ring and the mineral atoms are not unexpected to occur, the signals displacement clearly show the formation of an extended net of hydrogen bonds, thus leading to natural selfassembling.

3.3. SEM The morphology of the as prepared materials have been studied by SEM ( Figure 3). Sample A presents nanocrystals, highly homogeneous in size. However, the sample does not present big characteristic prismatic MFI type crystals, despite of its XRD pattern concordance. The decrease of the IL amount during the synthesis (sample B) shows the appearence of spherical amorphous silica particles, which are the only resulting material when no IL is used in the preparation (sample D). However, the most interesting observation corresponds to the sample C. It seems, that the presence of Al in the starting gel not only increases the degree of crystallinity, as shown by XRD but promotes the formation of nanofibers structures. One can supposed, that the interrupted frameworks structures, provoked by the presence of the ionic liquids, are directed in lamellar structures by the presence of the Al. Once again, the calcination confirmed the presence of the lamellar phase by disappearing of the characteristic diffraction lines after thermal treatment.

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A

B

C

D

Figure 3. SEM images of A) sample A, B) sample B, C) sample C and D) sample D.

4. Conclusions This study confirms that the 1-butyl, 3-methyl imidazolium methanesulfonate can be successfully used as a structure directing agent for the synthesis of MFI type zeolites in water rich media. However, the resulted materials present more interrupted framework structure character, which is thermally instable. More detailled study of the synthetic parameters, such as the H2O/IL ratio and the nature of the employed IL anion is currently in progress.

References [1] A. Corma, 1995, Inorganic Solid Acids and Their Use in Acid-Catalyzed Hydrocarbon Reactions, Chem. Rev., 95 , 559-614. [2] J. Weitkamp, 2000, Zeolites and catalysis, Solid State Ionics, 131, 175-188. [3] C. S. Cundy, P. A. Cox, 2005,The hydrothermal synthesis of zeolites: Precursors, intermediates and reaction mechanism, Micropor. Mesopor. Mater., 82, 1-78. [4] H. Lee, S. I. Jones, M. E. Davis, 2003, A combustion-free methodology for synthesizing zeolites and zeolite-like materials, Nature, 425, 385-388. [5] E. R. Cooper, C. D. Andrews, P. S. Wheatley, P. B. Webb, P. Wormald, R. E. Morris, 2004, Ionic liquids and eutectic mixtures as solvent and template in synthesis of zeolite analogues, Nature, 430, 1012-1016. [6] Y. Zhou, J. H. Schattka, M. Antonietti, 2004, Room-temperature ionic liquids as template to monolithic mesoporous silica with wormlike pores via sol-gel nanocasting technique, Nanoletters, 4, 477-481. [7] C. J. Adams, A. E. Bradley, K. R. Seddon, The synthesis of mesoporous materials using novel ionic liquid templates in water, 2001, Aust. J. Chem., 54, 679-681. [8] M. Antonietti, D. Kuang, B. Smarsly, Y. Zhou, 2004, Ionic liquids for the convenient synthesis of functional nanoparticles and other inorganic nanostructures, Angew. Chem. Int. Ed. 43, 4988-4992. [9] C. C. Cassol, G. Ebeling, B. Ferrera, J. Dupont, 2006, A Simple and Practical Method for the Preparation and Purity Determination of Halide-Free Imidazolium Ionic Liquids, Adv. Synth. Catal., 348, 243-248. [10] R. E. Morris, 2008, Ionic liquids and microwaves-making zeolites for emerging applications, Angew. Chem. Int. Ed. 47, 442-444.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V.

New class of acid catalysts for methanol dehydration S. Ivanova 1*, X. Nitsch 2, F. Romero-Sarria1, B. Louis2, M. A. Centeno1, A.C. Roger2, J. A. Odriozola1 1

Departamento de Química Inorgánica and Instituto de Ciencias de Materiales de Sevilla, CSIC-US, Avda. Américo Vespucio 49, 41092, Sevilla, Spain. e-mail: [email protected] 2 Laboratoire de Materiaux, Surfaces et Procedes pour la Catalyse, 25, rue Becquerel, 67087 Strasbourg, France

Abstract Herein, a new class of acid catalysts is proposed for the reaction of methanol dehydration. The coupling of Keggin anion with organic cation protection results in a crystalline molecularly defined hybrid structures with a strongly hydrophobic character. The thermal stability and catalytic behaviour of the new materials strongly depend on the nature of the Keggin structure. Keywords: heteropoly acid, ionic liquid, hybrids, methanol dehydration

1. Introduction The heteropoly acids (HPAs) and related polyoxymetalate compounds present several advantages as catalysts, which make them economically and environmentally attractive and still provoke a huge interest in the scientific community. The HPAs possess a very strong Brönsted acidity, higher than conventional solid acids as aluminosilicates or zeolites, and are efficient oxidants exhibiting fast reversible multielectron redox transformations under mild conditions [1]. However, the application of HPAs remains limited by their extreme sensitivity to the presence of water. In this sense, the HPAs being soluble in water, an extensive and continuous leaching can occur during the dehydration reactions. An efficient and elegant way to overcome the stability problems and to increase the lifetime of the material is the modification of HPAs with organic groups. Combining heteropolyacids with room temperature ionic liquids (RTILs) should provide some interesting materials. The advantages of RTILs include good chemical and thermal stability, almost negligible vapor pressure, good electrical conductivity, and a wide electrochemical window [2]. This study presents the preparation and characterisation of organic-inorganic hybrids based on Keggin type phosphomolybdic (H3PMo12O40) and phosphotungstic acid (H3PW12O40) and imidazolium based ionic liquid 1-butyl 3-methyl imidazolium methanesulfonate (Bmim).

2. Experimental 2.1. Synthesis 1-butyl, 3-methyl imidazolium methanesulfonate was synthesized as proposed by Cassol et al. [3], and the HPAs produced as described by Wu [4]. The salt with molecular structure IL3PA (Figure 1) was synthesized according to Rao et al. [5].

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2.2. Characterizations X-ray diffraction (XRD) analysis was performed on a Siemens diffractometer D500 over a 2θ-range of 5 to 50º. TPO was performed in U-shaped quartz reactor under flow of oxygen 21% in He (40cm3.min-1), conditions repeating the activation procedure before the reaction. All the products formed were followed by mass spectrometry. Figure 1. IL3PA structure.

2.3. Catalytic reaction The methanol dehydration reaction was carried out at atmospheric pressure in a fixedbed configuration. The reaction temperature was fixed at 275ºC after activation in the reaction conditions at 400ºC for 1h, with a methanol injection rate of 0.235 ml h-1 in air.

3. Results and discussion 3.1. XRD The XR diffraction patterns of the studied samples, Bmim3PMo12O40 and Bmim3PW12O40 at room temperature and at 400ºC, measured in-situ in flowing air at room temperature and at 400ºC are presented in Figure 2.

Bmim3PMo12O40

Bmim3PW 12O40

400 ºC

400 ºC

30ºC

30ºC

10

20

30

40

50

10

20

30

40

50

2Θ Figure 2. XRDiffraction patterns of Bmim3PMo12O40 (left) and Bmim3PW12O40 (right) images.

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At room temperature, both samples exhibit a typical structure of HPAs organic hybrids with the molecular structure proposed by Rao et al. [5,6]. However, at high temperature in the presence of air, the stability of the hybrids depends strongly on the nature of the initial acid. The structure of Bmim3PMo12O40 is no longer available and a mixture of several compounds is observed – completely dehydrated H3PMo12O40 (JCPDS # 01-070-1705), molybdenum phosphate MoP2O7 (JCPDS # 00-039-0026) and molybdenum oxide MoO3 (JCPDS # 00-001-0706). The hybrids originated from H3PW12O40 appear to be much more stable and kept the structure at 400ºC. However, the appearence of a second phase, attributed to the phosphotungstic acid (JCPDS # 00050-0654) is observed.

3.2. Temperature programmed oxidation (TPO) The samples behaviour during oxidation at conditions close to those from the reaction are presented in Figure 3. Bmim3PW12O40 300

200

200

100

100

0

18 28 44

400

300

400

0 18 28 44

300

400 300

Temperature, ºC

Bmim3PMo12O40

57 68

400

Temperature

Intensity, a.u.

57 68

200

200

100

100

0 0

1000

2000

3000

4000

5000

Time, s

6000

7000

8000

0 9000

0

2000

4000

6000

8000

Time, s

Figure 3. Temperature programmed oxidation of the prepared samples.

The hybrid originated from phosphomolybdic acid starts to decompose at temperatures close to 300ºC showing a lower thermal stability than the Bmim3PW12O40, which is stable at the activation temperature of 400ºC at least 30 min. It is interesting to emphasize, that the two hybrids do not give the same oxidation products. In addition to the signals corresponding to CO, CO2 and H2O (m/z 18, 28, 44) the signals of unsubstituted imidazolium (m/z 68) and butyl radical (m/z 57) are observed in the MS spectra of the evolved products. The presence of both m/z signals at 57 and 68 for Bmim3PMo12O40 suggests the presence of acidic active centers able to break the butyl - imidazolium bond. However, the presence of just the butyl signals in the Bmim3PW12O40 case confirms the strong bonding of the imidazolium cation to the Keggin structure, as observed in the high temperature diffraction pattern. This different oxidation behavior suggests a different acid center composition and strength in the hybrids.

3.3. Catalytic test The results for methanol dehydration obtained after one hour activation at 400ºC in air flow are presented in Figure 4 as dimethyl ether yield versus time. The selectivity to dimethyl ether (DME) is close to 100% at low temperature (275ºC) for both hybrids. However, the hybrid based on phosphomolybdic acid presents higher activity confirming the TPO observation on the acid sites diversity in composition and strength. When the reaction is carried out at high temperature (400ºC) methanol is fully converted to CO2. This opens the possibility of employing these materials as oxidation catalyst. By comparing the catalytic activity of the hybrids with those obtained for the

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water insoluble acidic metal salt of HPAs (Cs2HPW12O40) the effect of the ionic liquid on the catalytic activity is studied. The results show that the ionic liquid not only play a role of acid sites stabilizing agent but also participate in the modification of the acid sites, producing more active catalysts for methanol dehydration to dimethyl ether at low temperatures. 100 90 80

DME yield, %

70 60 50 40 30 20 10 0 0

5

10

15

20

25

30

Time, h (Bmim3)PMo12O40 P t i l ((B i 3)PM 12O40)

(Bmim)3PW12O40 P t i l ((B i )3PW12O40)

Cs2HPW12O40 P t i l (C 2HPW1

Figure 4. DME yield for the prepared samples.

4. Conclusions The prepared hybrids present good thermal stability and excellent activity in the dehydration of methanol in the presence of air at low temperatures and reveal a potential as powerful oxidation catalysts at high temperatures. The nature and strength of the acid sites could be easily controlled by modification of the ionic liquid and/or Keggin anion.

References [1] I. V. Kozhevnikov, 1998, Catalysis by heteropoly acids and multicomponent polyoxometalates in liquid-phase reactions, Chem Rev., 98, 171-198. [2] Z. Li , Q. Zhang, H. Liu, P. He, X. Xu, J. Li, Organic–inorganic composites based on room temperature ionic liquid and 12-phosphotungstic acid salt with high assistant catalysis and proton conductivity, J. Pow. Sources 158 (2006) 103-109. [3] C. C. Cassol, G. Ebeling, B. Ferrera, J. Dupont, 2006, A Simple and Practical Method for the Preparation and Purity Determination of Halide-Free Imidazolium Ionic Liquids, Adv. Synth. Catal., 348, 243-248. [4] H. Wu, 1920, Contribution to the chemistry of phosphomolybdic acids, phosphotungstic acids, and allied substances, J. Bio. Chem., 63, 189-220. [5] G. Ranga Rao, T. Rajkumar, B. Varghese, 2009, Sythesis and characterization of 1-butyl 3methyl imidazolium phosphomolybdate molecular salt, Solid State Sci., 11, 36-42. [6] T. Rajkumar, G. Ranga Rao, 2008, Characterization of hybrid molecular material prepared by 1-butyl 3-methyl imidazolium bromide and phosphotungstic acid, Materials Letters 62 4134-4136.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

One-Pot deposition of palladium on hybrid TiO2 nanoparticles: application for the hydrogenation of cinnamaldehyde Afef Mehri,a Hafedh Kochkar,b* Stephane Daniele,c Violaine Mendez,c Gilles Berhault,c Abdelhamid Ghorbela a

Laboratoire de Chimie des Matériaux et Catalyse, Faculté des Sciences de Tunis, 2092, Tunisie. b Centre National de Recherche en Science des Matériaux, Technopôle de Borj-Cédria, 2050, Hammam-Lif, Tunisie c Institut de Recherches sur la Catalyse et l’Environnement de Lyon, CNRS – Université Lyon I, 69100 Villeurbanne, France.

Abstract One-pot deposition of Pd onto TiO2 has been achieved by direct impregnation in excess of Na2PdCl4 contacted with citrate-functionalized TiO2 support initially obtained by solgel process techniques using Ti(OiPr)4 and citric acid. Citrate groups act as functional moieties able to directly reduce the Pd salt avoiding any further reducing treatment. Various titanium to citrate (Ti/CA) ratios (20, 50, and 100) were used to prepare the hybrid TiO2 support in order to study the effect of citrate on the final catalytic properties of Pd/TiO2 catalysts. Characterizations were performed using N2 adsorption-desorption, ICP-AES, FTIR, XRD, XPS, and TEM. The as-obtained Pd/TiO2 catalysts were tested in the selective hydrogenation (HYD) of cinnamaldehyde. Using citrate-functionalized TiO2 support increases both the activity and the selectivity to saturated alcohol. Keywords: Palladium, citric acid, one-pot deposition, hydrogenation, cinnamaldehyde

1. Introduction In the last years, titanium oxide has been more and more widely used as support of metallic nanoparticles (NPs) for applications in both oxidation [1] and hydrogenation reactions [2]. However, the catalytic performances of metallic NPs for a given application are strongly influenced by the crystalline structure, the morphology and the size of these particles which very often depend on the method of preparation. Therefore, developing new synthetic methods for producing TiO2 in which size and crystal structure of the deposited metallic NPs can be controlled is of crucial importance. Among the different methods for synthesizing TiO2 powders, sol-gel process is a technique of choice since producing titanium oxide by hydrolysis of titanium alkoxides in soft conditions [3]. Control of metallic deposition can be achieved by a postfuntionalization route which is however difficult to control in terms of composition and surface chemistry. Recently, a controlled functionality processing of hybrid TiO2 was developed by some of us [4] using modified alkoxides allowing the formation of anatase particles at low temperature and with high surface areas. In the present work, Pd/TiO2 catalysts were obtained thanks to a one-pot impregnation method of Pd salt (Na2PdCl4) in the presence of a citrate-functionalized TiO2 hybrid support. Different Ti/citric acid (CA) ratios were herein used to determine the influence of citrate on the catalytic properties in the selective HYD of a α,β-unsaturated aldehyde, i.e. cinnamaldehyde.

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This reaction was chosen because of its structure-sensitivity. For example, selective hydrogenation of the C=O or of the C=C bond will depend on the types of crystallographic planes exposed by the Pd particles.

2. Experimental 2.1. Preparation of (TiO2)x(CA)1 (x=20, 50, and 100)

The hybrid (TiO2)x(CA)1 supports were prepared according to the method described in [4] using distilled Ti(OiPr)4 and citric acid with tetrahydrofuran (THF) as solvent under Ar. Typically, 0.36 ml of Ti(OiPr)4 (1.22 mmol) and 0.086 g of citric acid (0.41 mmol) (3:1 Ti:CA molar ratio) were added to 10 ml of THF and the medium was kept under vigorous stirring for 16 h. The resulting solution was added to a solution containing 10 ml of isopropanol and (x-3) eq.mol. of Ti(OiPr)4 (x=20, 50, and 100) and quickly added to 100 ml of refluxing deionized water containing 1eq. of tetrabutylammonium bromide (NBu4Br) referring to titanium. The suspensions were heated under reflux for 3 h at 373 K and were centrifuged to obtain a white solid. The different precipitates were washed twice with water and once with ethanol and dried overnight at 343 K.

2.2. Preparation of the Pd/TiO2 catalysts

In a typical synthesis, Pd/TiO2 catalysts were prepared by simply stirring the hybrid (TiO2)x(CA)1 (x=20, 50, and 100) NPs to 100 ml of Na2PdCl4 aqueous solution (2.55 ml, 0.01 mmol). The suspensions were heated at 373 K for 3 h. Darkening of the different solutions indicated reduction of the Pd ions. The suspensions were recovered by centrifugation and the grey solids were washed with deionized water and ethanol and dried overnight at 343 K.

2.3. Characterization methods N2 adsorption-desorption isotherms were performed at 77 K using a Micromeritics ASAP 2000 instrument. The BET equation was used to calculate the specific surface area. X-ray diffraction (XRD) patterns of the samples were obtained by a PANalytical XPERT MPD Pro diffractometer using Cu-Kα radiation (λ=1.542 Å) from 5 to 80° for investigating the crystallization behavior. Low magnification TEM images were taken at an accelerating voltage of 200 kV on a JEOL 2010 instrument equipped with EDX capabilities. Specimens for the TEM studies were prepared by depositing a drop of these aqueous suspension samples onto 300 mesh Cu grid coated with a lacey carbon film. FTIR spectra were recorded on a Perkin-Elmer Paragon 500 spectrometer. Inductively coupled plasma–atomic emission spectroscopy (ICP-AES) was performed on the different catalyst powders to quantify the amounts of palladium.

2.4. Catalytic test The hydrogenation of cinnamaldehyde in liquid phase was performed in a stirred autoclave. In a typical experiment, 3 mmol of cinnamaldehyde (99%) was dissolved into 150 ml of 2-propanol and purged with N2 twice. The Pd catalysts (Pd: 0.47-0.9 mg) were then introduced into the autoclave before a next N2 purge. The reaction was performed at 50°C under 10 bars H2 pressure at a stirring speed of 500 rpm. The duration of each test was 6 h. A small amount (0.3µl) of the reacting medium was withdrawn at regular times (every 30 min) and analyzed by gas chromatography (Varian 3380). Catalytic activity was measured in terms of initial rates and calculated from the slope of the curves conversion versus time (t) at t=0.

One-pot deposition of palladium on hybrid TiO2 nanoparticles

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3. Results and discussion 3.1. Characterization of the Pd/TiO2 catalysts

Specific surface areas of the Pd/TiO2 catalysts are reported in Table 1. Supports alone show quite high surface areas which increase strongly (from 298 to 442 m2/g) when increasing the CA/Ti ratios. After deposition of Pd, only a small decrease in surface areas is observed. ICP-AES measurements show a Pd loading around 0.3-0.4wt.% at Ti/CA ratios of 50-100. Table 1. Surface areas, % Pd (ICP-AES) and XPS results for the Pd/TiO2 catalysts. Ti/CA SBETa % Pd Pd 3d5/2 BE (XPS)b Pd/Ti (XPS) 2 0 molar ratio Pd PdOx (m /g) (ICP) 20 400 (442) 0.40 334.5 eV (75%) 335.9 eV (25%) 0.0023 50 0.36 334.4 eV (69%) 336.2 eV (31%) 0.0042 303 (353) 100 252 (298) 0.28 334.4 eV (70%) 336.2 eV (30%) 0.0046 a surface areas of the supports alone in parentheses, b relative proportions in parentheses.

FTIR spectra of the as-prepared catalysts exhibit similar features whatever the Ti/Ca ratio. Bands at 1520 and 1440 cm-1 were attributed to νas(CO2) and νs(CO2) stretching vibrations [5] respectively suggesting that some functional groups still persist on the TiO2 surface after Pd deposition. XRD analysis reveals that the introduction of Pd and citrate groups did not modify the crystallinity of the TiO2 materials (anatase). The chemical state of Pd and the relative proportions of species were determined by XPS and results are reported in Table 1. XPS spectra of Pd3d5/2 core level displayed two contributions at ca. 334.4 eV and 335.9 eV corresponding respectively to metallic Pd0 and to PdOx species (Pd2+) [6]. The relative proportions of metallic Pd appear quite similar whatever the Ti/CA ratio. Considering that no particular attention was paid when transferring samples to the XPS chamber, the different Pd/TiO2 catalysts appear relatively well reduced. Low magnification TEM image clearly shows that all the Pd NPs were located on the TiO2 surface (Figure 1). This can attributed to the role of citrate favoring the deposition of Pd on TiO2. Statistical measurements were performed on 200 particles for each sample. At Ti/CA ratios of 50 and 100, similar average sizes were obtained (3-4 nm) while bigger particles were obtained at Ti/CA ratio of 20 (6.0 nm) (Table 2). This result is in agreement with Pd/Ti ratios measured by XPS confirming a slightly smaller dispersion of Pd NPs on TiO2 at the Ti/CA ratio Fig. 1. TEM image of the Pd/TiO2 of 20 catalyst with a Ti/CA molar ratio of 50.

3.2. Effect of citrate on the catalytic properties The Pd/TiO2 catalysts prepared at the different Ti/CA ratios of 20, 50, and 100 were evaluated in the selective HYD of cinnamaldehyde. Increasing the relative proportion of citric acid in the preparation of the hybrid TiO2 support strongly influences in fine both activity and selectivity properties. The activity increases with increasing relative proportion of CA going from 3.9 mmol/gPd.s at Ti/CA = 100 to 6.3 mmol/gPd.s at Ti/CA = 50 while the activity remains relatively constant at higher relative proportion of CA.

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Taking into account the average sizes of Pd NPs, determination of TOF values even emphasizes the beneficial role of CA showing a continuous increase of turnover values up to the Ti/CA ratio of 20. These results show a quite beneficial effect of using increasing quantities of CA in the preparation of Pd supported on hybrid TiO2. The selectivity into saturated alcohol also increases when increasing the proportion of CA going from 58 to 68% when going from Ti/Ca = 100 to Ti/CA = 50. At the Ti/CA ratio of 20, however, no more increase in selectivity was observed. This effect is even more significant when considering that similar experiments performed on citrate-free Pd/TiO2 showed a selectivity of only 24% in saturated alcohol. Therefore, citrate seems to strongly influence the selectivity in the HYD of cinnamaldehyde. Previous experiments [7] have shown that the preferential exposition of (111) sites increases the selectivity to saturated alcohol. Citrate was found to strongly stabilize (111) facets of Pd during the synthesis of Pd nanocrystals [8]. Such a stabilizing effect favoring the preferential exposition of (111) facets and shifting selectivity to saturated alcohol can also be expected here. Table 2. Pd particle size and catalytic results of the Pd/TiO2 catalysts. Ti/CA molar ratio Particle size (TEM) (nm) Activity (mmol/gPd.s) Selectivity to saturated alcohol (%) TOF (s-1)

20 6.0 5.9 68 3.0

50 3.1 6.3 68 1.6

100 4.1 3.9 58 1.3

4. Conclusion Citrate-functionalized TiO2 was used to one-pot deposit Pd NPs without further reducing treatment. This method allows to obtain quite well dispersed Pd NPs on TiO2 with a minimal average size of 3.1 nm at Ti/CA ratio of 50. The effect of citrate on catalytic properties was analyzed in the HYD of cinnamaldehyde. Results showed a strong beneficial effect of CA on the activity and a selectivity shift to saturated alcohol when increasing the relative proportions of CA.

References [1] [2] [3] [4] [5]

C. Zhang, H. He, 2007, Catal. Today, 126, 345. J. Hong, W. Chu, M. Chen, X. Wang, T. Zhang, 2007, Catal. Comm., 8, 593. H. Choi, E. Stathatos, D.D. Dionysios, 2006, Thin Solid Films, 510, 107. G. Goutailler, C. Guillard, S. Daniele, L.G. Hubert-Pfalzgraf, 2003, J. Mater. Chem., 13, 342. G.E. Tobon-Zapata, O.E. Piro, S.B. Etcheverry, E.J. Baran, 1998, Z. Anorg. Allg. Chem., 624, 721. [6] N. Iwasa, S. Masuda, N. Ogawa, N. Takezawa, 1995, Appl. Catal. A, 125, 146. [7] G. Berhault, L. Piccolo, A. Valcarcel, M. Bausach, C. Thomazeau, D. Uzio, 21th NAM Meeting San Francisco, USA, 06-12/06/09/09. [8] Y. Xiong, J.M. McLellan, Y. Yin, Y. Xia, 2007, Angew. Chem. Int. Ed., 46, 790.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Catalytic activity of nanostructured Pd catalysts supported on hydrogenotitanate nanotubes Khaled Jabou,a Hafedh Kochkar,b Gilles Berhault,c* Abdelhamid Ghorbela a

Laboratoire de Chimie des Matériaux et Catalyse, Faculté des Sciences de Tunis, 2092, Tunisie. b Centre National de Recherche en Science des Matériaux (CNRSM), Technopôle de Borj-Cédria, 2050, Hammam-Lif, Tunisie. c Institut de Recherches sur la Catalyse et de l’Environnement de Lyon, UMR 5256 CNRS – Université Lyon I, 69100 Villeurbanne, France.

Abstract Palladium nanoparticles (NPs) with well-controlled morphologies (cubes, rods, icosahedra) and with preferential crystallographic planes were prepared through a seed-mediated colloidal approach. Pd NPs were then impregnated onto high surface area hydrogenotitanate nanotubes before being evaluated in the selective hydrogenation of a α,βunsaturated aldehyde, i.e. cinnamaldehyde. Their preferential exposition of crystallographic planes (mainly (100) and (111) sites) offers the unique ability to determine the influence of the structure-sensitivity of this selective hydrogenation reaction in real experimental conditions. Results show that the selectivity to saturated alcohol increases with increasing proportion of Pd(111) sites. Keywords: hydrogenotitanate nanotubes, nanocrystal, palladium, cinnamaldehyde

1. Introduction Over the last decade, a growing interest for nanostructured materials with dimensions in the nanometric range has induced an important research effort, mainly focused on the synthesis and characterization of oxide and metallic nanoparticles with narrow size distribution. A new challenging task is now to control the shape of such nanoparticles. Shape of metallic nanoparticles is controlled in solution mainly by selective adsorption of ions, surfactants, or polymers during the growth of the particles. The role of surfactants or polymers is in fact complex. They can induce a preferential stabilization of specific crystallographic faces and then inhibit the growth of particles along a given crystallographic direction [1] but surfactants can also modulate the growth regime of metallic NPs shifting it from a kinetic to a thermodynamic control favoring respectively 1D (or 2D) (rods, plates, ...) or 3D (icosahedra) morphologies. The formation of such shapecontrolled NPs (exposing preferential crystallographic planes) also provides new interesting alternatives for studying structure-sensitive reactions like selective hydrogenations (HYD) under real experimental conditions [2, 3]. Shape control was also extensively developed in the last years for producing well-defined oxides. For example, hydrogenotitanate (HNT) nanotubes were elaborated by alkaline hydrothermal reaction [4]. These new supports could also exhibit high surface areas making them quite interesting as supports for heterogeneous catalysis. In the present work, shape-controlled Pd NPs have been synthesized and deposited onto HNT using two different impregnation procedures. The resulting catalysts with various proportions of (100) or (111) sites were then evaluated in the selective HYD of cinnamaldehyde.

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2. Experimental 2.1. Catalysts synthesis The HNT support was elaborated by an hydrothermal treatment of P25 in the presence of a high concentration of NaOH (11.25 M) at 130°C for 20 h [4]. Its surface area and pore volume are respectively 269 m2/g and 0.67 cm3/g. XRD revealed the formation of a H2Ti2O5.H2O phase [4]. The impregnation of the Pd particles onto the HNT support was carried out using two different procedures: 1) an ex situ method in which the Pd nanocrystals were formed through a seed-mediated approach based on the injection of Pd seeds into a growth solution (see below) or 2) an in situ method based on the incipient wetness impregnation of Pd seeds onto HNT before contacting the as-obtained solid with the growth solution. For both approaches, Pd seeds were obtained following a method developed by Nikoobakht et al. [5] and Berhault et al. [6]. An aqueous solution containing 25 ml of Na2PdCl4 (5.10-4 M) and 50 ml of CTAB (0.3 M) was prepared. Next, 6 mL of ice-cold 0.1 M freshly prepared NaBH4 solution was injected into the solution all at once while stirring vigorously. The color of solution turned dark immediately indicating the formation of small Pd NPs or “seeds” (3-4 nm). The growth solution is obtained by mixing 50 ml of CTAB (0.24 M) with 50 mL of Na2PdCl4 (3.10-3 M) and 2.8 mL of freshly prepared ascorbic acid (0.08 M). If ex situ prepared, 120 µL of seed solution was added to induce the growth of the Pd NPs. This step is stopped after 48 h. After impregnation, the solids were washed with ethanol before being dried at 303 K overnight. The ex situ and in situ prepared catalysts are named Pdex/HNT and Pdin/HNT. Their metallic contents are 2.1 and 1.1 wt% Pd respectively.

2.2. Characterization methods Transmission electron microscopy studies of the supported catalysts were performed in a JEOL JEM 2010 operating at 200 kV equipped with EDS capabilities. The microscope is equipped with an ultrahigh resolution polar piece (point resolution: 1.9 Å). Specimens for the TEM studies were prepared by depositing a drop of aqueous suspension onto 300 mesh Cu grid coated with a lacey carbon film. The Pd loadings of the different catalysts were determined by ICP-AES.

2.3. Hydrogenation of cinnamaldehyde The hydrogenation of cinnamaldehyde in liquid phase was performed in a stirred autoclave. In a typical experiment, 3 mmol of cinnamaldehyde (99%) was dissolved into 150 ml of 2-propanol and purged with N2 twice. The Pd catalysts (Pd: 0.47-0.9 mg) were then introduced into the autoclave before a next N2 purge. The reaction was performed at 50°C under 10 bars H2 at a stirring speed of 500 rpm. The duration of each test was 6 h. A small amount (0.3 µl) of the reacting medium was withdrawn at regular times (every 30 min) and analyzed by gas chromatography. Catalytic activity was measured in terms of initial rates and calculated from the slope of the curves conversion versus time (t) at t=0.

3. Results and discussion 2.4. Samples characterization As mentioned in section 1 the evaluation of the structure/activity relationships of metallic heterogeneous catalysts requires a very accurate geometric and electronic description of the metallic nanoparticles. These particles are usually described by simple geometric structures. In our work, the Pd NPs are non-conventional in size (higher than 15 nm) and offer the possibility to establish a well-defined geometric model for each

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Catalytic Activity of Nanostructured Pd Catalysts supported

kind of nanoparticles. Thus, electronic microscopy characterization was firstly used to determine the proportion of the different Pd nanocrystals formed for each catalyst. Figure 1 shows TEM images representative of various Pd NPs formed through the seedmediated growth techniques. Mainly rods, cubes, and polyhedra particles were observed. The morphology and types of crystallographic planes exposed by each type of nanocrystals were already determined in [6]. While cubic particles exposes exclusively (100) crystallographic planes, nanorods are formed by the assembly of five elongated tetrahedrons producing a pentagonal prism with five (100) faces on lateral sides and five (111) faces on each extremity. Polyhedra present multiply twin planes (MTP) and are formed by the assembly of regular tetrahedral units joined through (111) planes forming an icosahedral structure. These observations indicate that seed-mediated growth of nanoparticles favors shapes with low index exposed planes. Frequency distributions deduced from TEM measurements on Pd NPs are presented in Table 1. Fig. 1: TEM images of different Pd nanostructured particles Table 1. Frequency distributions (%) and average size of well faceted Pd NPs for the two impregnation procedures. Methods Ex situ

In situ

Types of morphology Icosahedra Rods Cubes Triangular plates Hexagonal plates Bipyramids Icosahedra Bipyramids Rods

Frequency (%) 43 22 20 8 5 2 93 5 2

Size (nm) 67 ± 3 L = 101 ± 6, D = 44 ± 2 29 ± 2 71 ± 6 72 ± 6 53 ± 5 22 ± 2 26 ± 6 L = 65 ± 6, D = 27 ± 2

Results clearly emphasized a strong influence of the experimental conditions of impregnation on the final morphology of the Pd nanocrystals. For the ex situ prepared catalyst, besides icosahedra (43%), manly rods and cubes were observed. For the in situ prepared catalyst, icosahedra are selectively obtained. Therefore, taking into account the size and proportion of each morphology, Pdex/HNT would be composed of 2/3 of (111) sites and 1/3 of (100) sites while for Pdin/HNT, ∼90% of (111) sites would be exposed..

2.5. Catalytic tests The different Pd/HNT catalysts were tested in the selective HYD of cinnamaldehyde. The reaction scheme of the cinnamaldehyde HYD proceeds via parallel and consecutive pathways that involve hydrogenation of C=O and C=C groups forming cinnamyl alcohol (CA), hydrocinnamaldehyde (HCALD), and 3-phenylpropanol (PP). On our Pd catalysts, the reaction scheme is formed of two parallel pathways, one corresponding to a preferential C=C bond hydrogenation of cinnamaldehyde into hydrocinamaldehyde and another one leading through the initial hydrogenation of the C=O bond to cinnamyl alcohol followed by the rapid hydrogenation into 3-phenylpropanol. Activity and selectivity results are reported in Table 2 for both catalysts. Similar activity results were found in both cases at 0.20-0.21 mmol/gPd.s. However, due to the smaller size of Pd NPs

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formed through the in situ approach, a higher dispersion value of 6.1% was found for Pdin/HNT compared to Pdex/HNT (2.5 %). A 2.5 times higher TOF is therefore expected on the Pdex/HNT catalyst (0.89 s-1 vs 0.35 s-1). Since Pdex/HNT exposes more (100) sites, this results is in agreement with previous studies showing that the exposition of Pd(100) sites leads to a higher intrinsic HYD activity [3]. Selectivity to saturated alcohol also tends to increase with increasing proportion of (111) sites confirming previous DFT results [7]. Finally, comparison to previous results acquired for Pd NPs supported on αAl2O3 [8] showed a strong increase of the selectivity to HCALD on HNT (65-73% vs 35-54% with Pd/α-Al2O3). Since α-Al2O3 is a weak interacting support contrary to titanate [9], a possible (and unexpected due to the size of the Pd NPs) electronic effect on Pd induced by HNT seems to strongly modify the selectivity properties in the HYD of cinnamaldehyde. Future studies will be centered on this aspect. Table 2. Activity and selectivity results in the HYD of cinnamaldehyde. Catalysts Pdex/HNT Pdin/HNT

Activity (mmol/gPd.s) 0.21 0.20

Selectivity HCALD (%) 73 65

Selectivity PP (%) 22 32

Selectivity CA (%) 6 3

4. Conclusion In this study, well faceted palladium NPs (rods, cubes, icosahedra…) were synthesized by a seed-mediated growth technique. These Pd NPs were impregnated on HNT. TEM characterization was used to determine the relative proportions in each type of Pd morphology and then calculate the corresponding contribution in (100) and (111) exposed planes. Selectivity to saturated alcohol was found to increase with the proportion of (111) facets while an intrinsic activity of (100) sites was observed. Finally, comparison with similar systems supported on α-Al2O3 suggests a strong electronic effect of HNT on Pd shifting its selectivity properties.

References [1] T.K. Sau, C.J. Murphy, 2004, J. Am. Chem. Soc, 126, 8648. [2] G. Berhault, L. Bisson, C. Thomazeau, C. Verdon, D. Uzio, 2007, Appl. Catal. A: Gen, 327, 32. [3] L. Piccolo, A. Valcarcel, M. Bausach; C. Thomazeau, D. Uzio, G. Berhault, 2008, Phys.Chem.Chem.Phys., 10, 5504. [4] H. Kochkar, N. Lakhdhar, G. Berhault, M. Bausach, A. Ghorbel, 2009, J. Phys. Chem. C, 113, 1672. [5] B. Nikoobakht M.A. El-Sayed, 2003, Chem. Mater, 15, 1957 [6] G. Berhault, M. Bausach, L. Bisson, L. Becerra, C. Thomazeau, D. Uzio, 2007, J. Phys. Chem. C, 111, 5915. [7] F. Delbecq, P, Sautet, 1995, J. Catal, 152, 217. [8] G. Berhault, L. Piccolo, A. Valcarcel, M. Bausach, C.T. Thomazeau, 21th NAM Meeting San Francisco, USA, DUZIO, 06-12/06/09 [9] C-Y. Su, T-C. Chiu, M-H. Shih, W-J. Tsai, W-Y. Chen, C-H. Lin, 2010, J. Phys. Chem. C, 114, 4502.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Temperature – dependent evolution of molecular configurations of oxomolybdenum species on MoO3/TiO2 catalysts monitored by in situ Raman spectroscopy George Tsilomelekis,a,c Antonios Tribalis,a,c Angelos G. Kalampounias,a,c Soghomon Boghosian,*,a,c George D. Panagiotou,b Kyriakos Bourikas,b Christos Kordulis,b,c and Alexis Lycourghiotisb a

Department of Chemical Engineering, University of Patras, GR-26500 Patras, Greece Department of Chemistry, University of Patras, GR-26500 Patras, Greece c Institute of Chemical Engineering and High Temperature Chemical Processes, Foundation of Research and Technology-Hellas (FORTH/ICE-HT), GR-26500 Patras, Greece b

Abstract The evolution of the molecular structure of molybdena/titania catalysts prepared by the equilibrium–deposition–filtration (EDF) method is monitored by in situ Raman spectroscopy in the temperature range 25–430oC. The study is based on a recently proposed picture for the interfacial speciation of Mo(VI) oxo-species deposited at the “titania/electrolytic solution” interface of “wet” catalyst pastes and on the temperature– dependent features of in situ Raman “snap–shots” obtained with increasing temperature. The configurations of the deposited Mo(VI) oxo-species on “wet” (i.e. prior to drying) samples conform to the theoretically previewed speciation for the respective pH’s and concentrations of the precursor electrolyte and impregnating solutions; for initial Mo(VI) concentrations up to 2.5×10-2 M the predominant deposited species evolve from monomeric MoO42- ions retained above a bridging surface hydroxyl through a hydrogen bond [Ti2OH…O–MoO3] (for solution pH=9) to a mixture with inner sphere mononuclear mono-substituted (with the terminal surface oxygen of titania) complex [Ti–OMoO3] (for solution pH=6) and to a simultaneous occurrence of the former monomeric species with Mo7O246- and HMo7O245- (for solution pH=4). A gradual transformation of the complex configurations is monitored with rising temperature by in situ Raman snap–shots and the spectra of the final calcined samples exhibit the features of dispersed mono-oxo (Ti–O)3Mo=O species. A low presence of associated species (possessing Mo–O–Mo linkages) is evident for the higher loaded samples. Keywords: molybdena catalysts; titania; in situ Raman; oxomolybdenum species; temperature evolution

1. Introduction Oxomolybdenum species supported on titania constitute a class of materials that serve as catalysts for several reactions. The literature abounds in studies pertaining to characterization of MoO3/TiO2 catalysts, though in most cases the various catalyst properties are investigated after drying and mainly after calcination. To the best of our knowledge very little attention is directed on the impregnation of the titania support grains with the solution containing the molybdenum ionic species, although the

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understanding of this interfacial process at the molecular level constitutes an essential prerequisite for controlling the whole preparative sequence. A comprehensive report addressing the relation between the species formed upon impregnation and the features of the final catalytic phase and behavior is available [1]. To this end, it is of direct relevance to investigate the structural evolution of the deposited Mo(VI) oxospecies with rising temperature, which has not been addressed to date. This reports is part of a general study aiming to an understanding (at the molecular level) of the interfacial deposition processes involved in the preparation of MoO3/TiO2 catalysts by the EDF method. In situ Raman spectroscopy is used for probing the structure of Mo(VI) oxospecies deposited on fresh Mo/TiO2 catalyst pastes and for monitoring the temperature–dependent structural evolution of the deposited species.

2. Experimental 2.1. Catalyst preparation The electrolyte and impregnating solutions used for the deposition were prepared by mixing appropriate amounts of (NH4)6Mo7O24⋅4H2O solutions with NH4NO3 solutions (Merck analytical grade). Industrial TiO2 (Degussa P25) was used as support. At each experiment 2 g of TiO2 were immersed in 100 mL impregnating solution. The resulting final Mo(VI) concentrations, pH values (as regulated during deposition), wt% Mo loadings, specific surface areas and Mo surface densities are summarised in Table 1. Catalysts were dried overnight and calcined in air at 480oC. The samples are denoted by xMoTi with x being the Mo wt% loading. Table 1. Catalyst preparation conditions and properties. 0.3MoTi 2.2MoTi 3.2MoTi

CMo(VI) (M)

pH

wt% Mo

SBET (m2/g)

ns (Mo/nm2)

10-2 1.5×10-2 2.5×10-2

9 6 4

0.26 2.21 3.18

53.7 53.5 51.8

0.3 2.6 3.9

2.2. Raman spectra For recording the temperature–dependent in situ Raman spectra of the wet catalyst pastes (i.e. prior to drying), the pastes were puttied inside the cavity of a U-shaped quartz holder, which was mounted on the sample supporter of the in situ Raman cell [2]. For the calcined samples, approximately 120 mg of each sample were pressed into a wafer and mounted on the sample supporter of the Raman cell.

3. Results and discussion 3.1. Raman spectra of “wet” catalyst pastes Figure 1A shows the Raman spectra of the fresh catalyst pastes, obtained under flowing O2 (15 cm3/min) at 25oC. It is seen that the main observed Raman band undergoes a blue shift with decreasing pH of the electrolyte and impregnating solution (from 918 to 939 and 950 cm-1 for respective pH of 9, 6 and 4). Alongside, in Fig. 1B, the contribution of TiO2 is subtracted and the spectra are fitted into sets of Gaussian bandshapes and the resulting band positions and assignments are summarised in Table 2. It appears that at high pH (where only the structural oxygens of TiO2 are protonated) Mo(VI) is deposited as monomeric MoO42- retained above a bridging surface hydroxyl through a H–bond [Ti2OH…O–MoO3], thus justifying the blue shift of ν s and ν as of the H-bonded O– MoO3 tetrahedron (observed at 924 and ~870 cm-1, Table 2), relative to the respective

Temperature dependent evolution of Mo(VI) oxo-species on MoO3/TiO2 catalysts

615

897 and 837 cm-1 values for the free MoO42- ion [3]. With gradual lowering of the pH (pH=6) more terminal oxygens are protonated and mononuclear mono-substitution [Ti– O–MoO3] is favored, as hydroxyls constitute good “leaving groups”. This kind of bonding “pulls further away” the anchoring oxygen from the Mo atom, thereby strengthening further the bonds within the MoO3 moiety of the coordinated molybdate ion (ν s and ν as observed at 939 and ~875 cm-1, Table 2). With further decrease of the pH of the impregnating solution, polymeric Mo7O246- (and/or) HMo7O245- (bands at 955 and ~908 cm-1 for the ν s and ν as , Table 2) coexist with the monomeric complexes. MoO3/TiO2 EDF pastes

pH=4

(A)

T=25°C Under O2 flow

(B)

(*)

(*) (*)

~950 -

NO3

Intensity, a.u.

3.18 wt% (pH = 4)

(*)

pH=6

~939

(*)

(*)

(*)

2.21 wt% (pH = 6) ~918

pH=9 (*) (*)

0.26 wt% (pH = 9)

1100

(*): vitreous quartz

1000

900

Raman Shift, cm

800 -1

1100

1000

900

Raman Shift, cm

800 -1

Figure 1: (A) Raman spectra of fresh “wet” catalyst pastes. (B) Deconvolution of spectra into Gaussian shapes, following subtraction of the TiO2 spectrum. Asterisks mark bands due to the Ushaped quartz holder. λ 0 =488 nm; laser power, w=40 mW; time constant, 1 s; resolution, 7 cm-1.

Based on the surface and interfacial structure and an exploitation of potentiometric titration, microelectrophoretic mobility and macroscopic adsorption data a modelling of the interfacial deposition process is able to provide an integrated picture of the interfacial chemistry for the deposition process [1,4]. The observed band wavenumbers and structural properties for the species deposited on the pastes conform to the previewed [4] respective speciation; an exploitation of normalised Raman intensities of the main representative bands due to the three species participating in the speciation is also in full conformity with the model prediction [4].

3.2. Temperature dependent evolution of molecular configurations and structures Figure 2 shows selected in situ Raman snap–shots (under flowing O2) of the temperature– evolution of the spectral features obtained for the lowest and highest Mo loaded samples. The observed features are indicative of gradual structural changes that can be accounted for by a progressive anchoring of the initially mono-substituted species [Ti– O–MoO3] to bi–substituted [(Ti–O)2–Mo(=O)2] and eventually to trianchored units [(Ti–O)3–Mo=O] with one terminal Mo=O bond seen at 992–995 cm-1 (Fig. 2, Table 2).

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Table 2: Observed Raman wavenumbers (cm-1) and assignments for “wet” and calcined catalysts. 0.3MoTi (0.3 Mo/nm2) Paste Calcined

2.2MoTi (2.6 Mo/nm2) Paste Calcined

3.2MoTi (3.9 Mo/nm2) Paste Calcined

995

995

992

Assignment Moderef ν ( Mo = O) ν ( Mo = O)

Speciesref

5

(Ti - O)3 Mo = O, (MoOx ) n

5

(Ti - O) 3 Mo = O

955 940 924

924

920 w,br

Mo7 O 24 , HMo7 O 24

νs νs

920 br

~875

ν (O − Mo − O)

5

Ti 2 - OH ⋅ ⋅ ⋅ OMoO3 (MoOx ) n

5

6−

Mo7 O 24 , HMo7 O 24

~875

6−

Ti - O - MoO3

908 ~870

5

6−

940 924 w

5

6−

Ti - O - MoO3 , Ti 2 - OH ⋅ ⋅ ⋅ OMoO3

ν as

abbreviations: w= weak; br=broad 0.3MoTi O2 (g) 430°C

3.2MoTi O2 (g)

(A)

νMo=O,(Ti-O)3Mo=O

(B) νMo-O-Mo ~920

992

430°C

2

(0.3 Mo/nm )

Raman Intensity, a.u.

νMo=O 995

2

(3.9 Mo/nm )

300°C

300°C

200°C

200°C

150°C

150°C

120°C

120°C

100°C 100°C 80°C 80°C 25°C

25°C 1100

6-

950, Mo7O24

918 ,Ti-OH...OMoO(=O)2

1000

900

Raman Shift, cm

800 -1

1100

1000

900

Raman Shift, cm

800 -1

Figure 2. Temperature dependent evolution of Raman spectra obtained under flowing O2 for the 0.3MoTi and 3.2MoTi catalysts. λ 0 =488 nm; laser power, 40 mW; resolution, 7 cm-1.

The proposed progressive anchoring caused by heating, justifies the strengthening of the terminal Mo=O bond. A low presence of associated species (possessing Mo–O– Mo linkages, band at ~920 cm-1) is evident for the highest loaded sample.

References [1] K. Bourikas, Ch. Kordulis, A. Lycourghiotis, Catal. Rev. 48 (2006) 363. [2] A. Christodoulakis, S. Boghosian, J. Catal. 215 (2003) 139. [3] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, 6th ed., Wiley, New York, 2009. [4] G. D. Panagiotou, T. Petsi, K. Bourikas, A. G. Kalampounias, S. Boghosian, Ch. Kordulis, A. Lycourghiotis, submitted. [5] H. Hu, I. E. Wachs, S. R. Bare, J. Phys. Chem. 99 (1995) 10897.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Preparation of nanosized bimetallic Ni-Sn and Ni-Au/SiO2 catalysts by SOMC/M. Correlation between structure and catalytic properties in styrene hydrogenation Layane Deghedi,a Jean-Marie Basset,a Gérard Bergeret,b Jean-Pierre Candy,a Manuel Corral Valero,c Jean-Alain Dalmon,b Aimery De Mallmann,a AnneClaire Dubreuil,c Lars Fischerc a

Université de Lyon, Institut de Chimie de Lyon, UMR C2P2-CNRS-ESCPE Lyon 1, Equipe COMS-43, Bd du 11 Novembre 1918, F-69616 Villeurbanne, France b Université de Lyon, Institut de Chimie de Lyon, UMR 5256 CNRS-Université de Lyon 1, IRCELyon, 2 av. A. Einstein, F-69616 Villeurbanne, France c IFP-Lyon, BP 3, F-69360 Solaize, France

Abstract The aim of this study is to prepare silica-supported Ni-X bimetallic catalysts, to characterize them and to compare their catalytic activity in the hydrogenation of styrene, as well as their selectivity in the hydrogenation of the styrene’s olefinic double bond instead of the hydrogenation of the aromatic ring. The element X is grafted in a controlled way on the supported nickel particles, and is chosen according to its electronegativity, which is either equivalent (Sn) or higher (Au) than the electronegativity of Ni, in order to study the geometrical and/or electronic effects due to the doping of nickel. Among the prepared samples, the Ni-Au/SiO2 catalyst has exhibited high activity and high selectivity in the hydrogenation of styrene into ethylbenzene, suggesting that a combination of geometric and electro-attractor effects are involved. Keywords: selective hydrogenation, bimetallic catalyst, Ni-Au, Ni-Sn

1. Introduction Traditional catalysts for hydrogenation of diolefins and aromatics are Ni-based catalysts. Addition of sulphur is generally needed to perform selective hydrogenation of diolefins and alkenylaromatics rather than hydrogenation of aromatics [1]. However, sulphur deposition is not so easy to control at industrial scale. Insufficient passivation by sulphur leads to hydrogenation of aromatics (and thus dramatic increase of the reactor temperature), whereas excessive passivation leads to formation of Ni3S2 phase which is completely inactive [2]. It is generally accepted that the observed selectivity in the hydrogenation of C=C double bond against the hydrogenation of an aromatic ring could be due to a geometric effect of sulphur deposition on the nickel surface. But, since sulphur is clearly more electronegative than nickel (2.5 for S against 1.8 for Ni according to the Pauling classification), an electronic effect cannot be rejected. The aim of the present work was therefore to replace sulphur by another compound either equivalent to nickel (1.8 for Sn) or more electronegative than nickel (2.4 for Au) in order to explore the possible roles of electronic and/or geometric effects in selective hydrogenation.

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A parent catalyst constituted of small nickel particles supported on silica with a very narrow size distribution - around 3.5 nm - was first prepared. The nickel catalyst was then modified with organometallic compounds of tin (SnBu4) or gold (AuCN) using the SOMC/M route [3]. The solids thus prepared were fully characterized by electron microscopy, X-ray diffraction, EXAFS, magnetic and volumetric measurements. Their catalytic behavior was tested during the hydrogenation of styrene first into ethylbenzene, then into ethylcyclohexane, in order to evaluate the selectivity in hydrogenation of an olefinic double bond against an aromatic ring. This eventually allows us to explore the possible correlation between the catalysts’ structures and catalytic properties.

2. Bimetallic catalysts, preparation and characterization Ni/SiO2 parent catalyst is prepared by the ion exchange procedure starting with hexaammine nickel nitrate and silica Aerosil 200 from Degussa. The preparation and characterization of this sample is fully described in a previous work [4]. The Ni/SiO2 parent catalyst is reduced at 400°C under flowing H2. Organometallic compounds of tin (SnBu4) or gold (Au(CN)) react with the reduced surface of the nickel parent catalyst to form grafted organometallic fragments which are fully decomposed after heating at 400°C under hydrogen. To prepare the Ni-Sn catalysts, the desired amount of pure SnBu4 is introduced at room temperature, using a syringe, via a septum, into the reactor containing the reduced parent catalyst. The reaction is conducted under 50 mbar of hydrogen, at temperature increasing from 25 to 400°C. Butane evolved in the meanwhile is condensed into a cold trap, as described in an earlier procedure [5]. The samples thus obtained are stored under air, then reduced at 400°C under flowing H2 before further utilization. Three samples with increasing Sn/Ni atomic ratios were prepared (Sn/Ni= 0.03; 0.3 and 0.8 from elemental analysis).

Figure 1. TEM and XRD analysis of the 0.8 Sn/Ni sample.

TEM-EDS analysis of the 0.8 Sn/Ni sample, reported on Fig. 1, shows the presence of bimetallic particles with an average diameter of 6.2 nm and an average composition (from EDS) of Ni3Sn2. XRD analysis of this sample acquired under flowing hydrogen at 400°C exhibits a crystalline Ni3Sn monoclinic structure. Alloy phases close to Ni7Sn are detected by XRD on the 0.3 Sn/Ni sample after treatment at 400°C under flowing hydrogen. EXAFS measurements were performed at the Sn-K edge, on line X1 (RÖMO II) at DORIS III in Hasylab (Hambourg). The

Preparation of nanosized bimetallic Ni-Sn and Ni-Au/SiO2 catalysts

619

presence of ca. 7 first Ni neighbours at 2.61 Å away from Sn is suggested. This result is in good correlation with a SnaNib solid solution, with most probably a Sn enrichment at the surface of the metal particles. With the 0.03 Sn/Ni sample, the tin loading being too low, XRD patterns show only the Ni phase. The magnetic isotherm obtained on this sample is compared to the one obtained on the pure Ni/SiO2 sample. Both samples were previously reduced under hydrogen at 400°C, then treated under vacuum at 350°C during 3 hours in order to remove adsorbed hydrogen. The magnetization at saturation decreases from 1.33.10 -3 uemcgs/g for the Ni sample to 1.09.10 -3 uemcgs/g for the NiSn0.03 sample. These values correspond to a loss of magnetization of 5.4 Ni atoms per Sn atom introduced, thus suggesting the formation of a SnaNib solid solution. To prepare the Ni-Au catalysts, the reduced parent Ni/SiO2 catalyst is introduced at room temperature, without contact with air, into a closed reactor containing a suspension of the desired amount of AuCN in n-heptane, under atmospheric pressure of hydrogen. After one night of interaction at room temperature, the solid is washed with pure n-heptane and dried under vacuum. It is then introduced into the glove box under argon to be transferred into the decomposition reactor to be finally heated at 400°C under flowing hydrogen. The NiAux/SiO2 sample thus obtained is stored under argon into the glove box to avoid any contact with air until further utilization. According to elemental analysis, the prepared catalyst contains 7.6 wt% Au and 9.2 wt% Ni i.e. NiAu0.25/SiO2. Electron microscopy coupled with EDS (Fig. 2) shows bimetallic particles sized from 2 to 10 nm with a mean diameter of 5.1 nm. From EDS measurements, compositions of the bimetallic particles vary from 0.1 to 3.9 Au/Ni. There is not any visible trace of atomically dispersed Ni or Au on the support.

Figure 2. TEM analysis of NiAu0.25.SiO2 sample.

XRD analysis of the sample reduced at 400°C under flowing hydrogen indicates the presence of Ni and Au separate phases, but no solid solution of Au in Ni or Ni in Au. It is well admitted that under our working conditions, gold and nickel do not form solid solutions. This observation is confirmed by EXAFS measurements at the Au-LIII edge. Ni neighbours could not be detected around Au due to the presence of large gold particles where each Au atom is surrounded by an average of ca. 12 other Au atoms. However, for lower gold loadings (e.g. NiAu0.05/SiO2 sample), the presence of some Au adatoms anchored on the surface of Ni particles, with a Au-Ni distance of 2.56 Å can be evidenced, together with large gold particles. Since TEM-EDS analysis show the

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simultaneous presence of Au and Ni within a particle, we can suggest that the prepared N-Au catalyst consists of supported gold particles and supported nickel particles covered with gold adatoms, as previously suggested by Molenbroek et al [6].

3. Styrene hydrogenation Styrene hydrogenation was conducted in a well stirred stainless steel autoclave (Parr Instrument, 100 mL, magnetically driven stirrer) following the procedure described earlier [4]. The initial reaction rates of formation of ethylbenzene (r1) and ethylcyclohexane (r2) are compared to the reaction rates obtained with the parent monometallic catalyst (r1ref and r2ref). For the bimetallic catalysts, the relative reaction rates are defined by the ratios r1/r1ref and r2/r2ref; the relative selectivity is given by S/Sref=(r1/r2)/(r1ref/r2ref)=(r1/r1ref)/(r2/r2ref). Values are reported in Table 1. Table 1. Relative reaction rates of formation of ethylbenzene (r1/r1ref) and ethylcyclohexane (r2/r2ref), and relative selectivities (S/Sref) evaluated for the bimetallic samples. Sample

Sn/Ni=0.03

Sn/Ni=0.8

Au/Ni=0.25

0.55

0.2

0.59

0.73

0.65

0.5

0.1

2.8

0.8

0.4

5.9

ref

2.0

ref

ref

r1/r1 r2/r2 S/S

Sn/Ni=0.3

A small amount of tin (0.03 Sn/Ni) clearly increases the rate of the C=C bond hydrogenation and slightly decreases the rate of the aromatic ring hydrogenation, leading to a rather good enhancement of selectivity. Increasing the amount of tin decreases the rate of both hydrogenation steps, leading to a loss of selectivity. Addition of gold to the parent nickel catalyst slightly decreases the rate of the C=C bond hydrogenation while it drastically decreases the rate of the aromatic ring hydrogenation, leading consequently to a great improvement of selectivity.

4. Conclusion Doping nickel catalysts with tin at low loadings can improve selectivity by a 2.8 factor in hydrogenation of C=C bonds against aromatic rings. Tin being “electronically neutral” towards nickel, this phenomenon can only involve geometric effects. More significantly, addition of gold on nickel catalysts increased selectivity by a 5.9 factor! Gold being more electronegative than nickel, we suggest that a combination of geometric and electro-attractor effects are involved in this case.

References (1) (2) (3) (4)

D. Lumbroso, 1979, Pétrole Information, 46-47 J.R. Rostrup-Nielsen, 1968, J. Catal., 11, 220-227 J.P. Candy, B. Didillon, E.L. Smith, T.B. Shay, J.M. Basset, 1994, J. Mol. Catal., 86, 179-204 L. Deghedi, J.M. Basset, J.P. Candy, J.A. Dalmon, A.C. Dubreuil, L. Fischer, 2009, Chem. Eng. Trans., 17, 31-36 (5) P. Lesage, O. Clause, P. Moral, B. Didillon, J.P. Candy, J.M. Basset, 1995, J. Catal. 155, 238-248 (6) A.M. Molenbroek, J.K. Norskov, B.S. Clausen, 2001, J. Phys. Chem. B, 105, 5450-5458

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Microwave-assisted synthesis of Au, Ag and Au-Ag nanoparticles and their catalytic activities for the reduction of nitrophenol S. Albonettia, M. Blosib, F. Gattia, A. Migliorid, L. Ortolanid, V. Morandid, G. Baldic, A. Barzantic, M. Dondib a

Department of Industrial Chemistry and Materials, INSTM, Research Unit of Bologna, Viale Risorgimento 4, 40136 Bologna University, Italy b ISTEC-CNR, Institute of Science and Technology for Ceramics, National Research Council, Via Granarolo 64, 48018, Faenza, Italy c CERICOL, Via Pietramarina 123, 50053 Sovigliana Vinci, Firenze, Italy d IMM-CNR Sezione di Bologna - Via Gobetti, 101, 40129 Bologna, Italy

Abstract A simple, microwave-assisted, strategy for producing Au/Ag concentrated sols by glucose reduction in water was developed. Ag-Au bimetallic nanoparticles stabilized by polyvinylpyrrolidone (PVP) were characterized and their catalytic activity was studied in the reduction of 4-nitrophenol with NaBH4 as a probe reaction. The Ag-Au nanoparticles were prepared by first optimizing the synthesis of Au colloid and then carrying out the deposition of a silver shell. Microwave heating has been shown to provide more homogeneous particle nucleation and shorter synthesis time than traditional heating. Prepared Au, Ag and Au/Ag nanocrystals function as effective catalysts for the reduction of p-nitrophenol in the presence of NaBH4, otherwise unfeasible if only the strong reducing agent NaBH4 is employed. Keywords: Au, Ag, microwave, bimetallic sols, nanoparticles, water media

1. Introduction Colloidal suspension of different metals has found applications in various fields, including catalysis, because a large fraction of the catalytically active metal sites in this case is exposed to the reactants [1-2]. In particular, alloy and core-shell nanoparticles have received special attention due to the possibility of tuning the electronic (and thus catalytic) properties over a broad range by simply varying the composition [3].

2. Experimental procedure 2.1. Bimetallic synthesis All the chemical reagents used in this experiment were analytical grade (Sigma Aldrich). Au/Ag core-shell nanoparticles were prepared by a two-step method, characterized by the shell synthesis on the preformed core used as seeds of nucleation (Scheme 1). Au-core and Ag-core sols, exploited as seeds, were obtained separately through the reduction of HAuCl4 or AgNO3 by glucose in alkaline water. PVP-coated metal seeds (nPVP/nMetal=5.5) were synthesized in 5 minutes at 70°C (for Ag) or at 90°C (for Au) by using microwave heating and following a patented procedure [4]. For each metal, the glucose amount and the solution pH were carefully optimized (nGlucose/nMetal=2; nNaOH/nMetal=8). Table 1 shows the characteristics corresponding

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to the synthesized samples, each metal is followed by its molar concentration and core metals are in brackets. Core sols (Au or Ag)

PVP

Glucose

NaOH

Water solution MW heating Metal Precursor (Shell)

Scheme 1 – Synthesis flow chart

Bimetallic suspension

Table 1. Characteristics of prepared bimetallic nanoparticles. Sample Au (Au)67Ag33 (Au)40Ag60 (Au)18Ag82 Ag (Ag)67Au33 (Ag)40Au60 (Ag)18Au82

Series AuCore AuCore AuCore AgCore AgCore AgCore

%Au (mol/mol) 100 67 40 18 0 67 40 18

%Ag (mol/mol) 0 33 60 82 100 33 60 82

∅DLS (nm) 16 29 32 36 65 62 66 67

Kinetic constant (s-1) 1.75x10-2 0.71x10-2 0.48x10-2 0.31x10-2 0.17x10-2 -

2.2. Catalytic reaction The catalytic reduction of 4-nitrophenol by NaBH4 was studied at room temperature of 25°C in a standard quartz cuvette with 1 cm path length and about 3 mL volume. All the prepared samples were properly diluted with distilled water in order to achieve a metal concentration of 1.1x10-2 mM. Thus 10 mL of diluted suspensions were mixed with 5 mL of a 4-nitrophenol solution (9.0x10-2 mM) and with 1 mL of a freshly prepared NaBH4 aqueous solution (0.72 M). An aliquot of the solution was poured into the quartz cuvette and the absorption spectra were collected by a Lambda 35 spectrophotometer (Perkin Elmer, USA) in the range between 250 and 500 nm. The rate constants of the reduction process were determined by measuring the change in absorbance at 400 nm, corresponding to 4-NP, as a function of time.

2.3. Analytical characterization UV-VIS extinction spectra were measured with a Lambda 35 spectrophotometer (Perkin Elmer, USA). Particle size distribution, based on hydrodynamic diameter, was evaluated by Nano S (Malvern, UK), a dynamic light scattering analyzer (DLS). Unreacted metal cations, extracted from the sample by a semi-permeable osmotic membrane (Visking tube), were detected by ICP-AES quantitative analysis (Liberty 200, Varian, Australia) in order to determine the reaction yield. Suspensions were dropped and dried on a copper grid, then observed by high resolution transmission electron microscopy (HRTEM) (Tecnai F20) and by the STEM mode with microanalysis EDX.

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3. Results and discussion 3.1. Nanoparticles synthesis Figure 1a shows the UV-VIS spectra collected for the Ag-core series. While the pure Au nanoparticles solution has a characteristic resonance peak at 520 nm, Au-Ag samples showed a significant blue-shift of the plasmon resonance band increasing the silver content. The plasmon resonance shifting confirmed the formation of multicomponent nanostructures; in fact, for a physical mixing, two distinct bands would be observed [5]. a) b)

Figure 1. a) Extinction spectra of Ag-core series; b) Surface plasmon resonance shift in Ag-core series depending on particle composition.

The prepared sols show excellent stability up to several months of storage, indicating that no nanoparticles aggregation occurred. Hydrodynamic diameter (HD) measured by DLS evidenced that Au nanoparticles have a smaller size with respect to Ag nanoparticles and samples containing Ag exhibited a progressive increase in particle size (Table 1). HRTEM analysis generally indicated a nanoparticle diameter lower than DLS (Fig. 2a), highlighting the difference between hydrodynamic diameter, comprehensive of coordination sphere, and real size. a)

b)

5 nm

Figure 2. a) HRTEM analysis of sample (Au40)Ag60; b) EDX analysis of a particle profile.

Moreover, HRTEM indicated that nanoparticles are typically spherical and polycrystalline and confirmed that samples containing higher shell element concentration

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exhibited a slight increase in the diameter, indicating that reduction/deposition rather than other process dominates the coating process. EDX-STEM line scanning across the nanoparticles was carried out to analyze the distribution of the chemical composition. For example in the Au core systems, especially the samples with high Ag/Au ratio, the maximum of the Au signal is in the center of nanoparticles, supporting the formation of Au-core Ag-shell nanostructures (Fig. 2b). On the contrary, samples with low amount of Ag exhibit a typical Au-Ag alloy behavior. Prepared Au, Ag and AuAg nanocrystals act as effective catalyst for the reduction of p-nitrophenol in the presence of NaBH4. a)

b)

Figure 3. a) UV-Vis spectra of 4-NP reduction for sample Au; b) plot of the absorbance ln(At/A0) vs time.

The Uv-vis peak of 4-NP at 400 nm decreased, while at 290 nm a new peak, assigned to 4-AP, appeared (Fig. 3a). The pseudo-first-order rate constants, reported in table 2, were calculated by the decreasing of the band of 4-NP, considering the slope of the ln(At/A0) as a function of time (Fig. 3b). Increasing the silver amount, a lower catalytic activity was observed, confirming the lower activity of Ag in this reaction. 4. Conclusions Stable bimetallic nanoparticles were synthesized with a total yield by a microwave assisted eco-friendly method. Both UV-VIS spectroscopy and microscopy data confirmed the formation of bimetallic nanostructures, in form of core-shell or alloy. Catalytic tests showed results in good agreement with literature data, with a decreased activity for higher silver content. Studies on applying these nanoparticles for the preparation of catalysts supported on polymer and nanotube are underway. Moreover, this synthesis technique will be extended to the preparation of other bimetallic nanoclusters.

References [1] F. W. Hou, N. A. Dehm, R. W.J. Scott, J. Catal. 253 (2008) 22. [2] S. Carregal-Romero, J. Prez-Juste, P. Hervs, L. M. Liz-Marzn, P. Mulvaney, Langmuir 26 (2010) 1271. [3] S. Rojluechai, S. Chavadej, J. W. Schwank, V. Meeyoo, Catal. Comm. 8 (2007) 57. [4] M. Blosi, S. Albonetti, M. Dondi, G. Baldi, A. Barzanti. Patent : FI2009A000034 (2009). [5] Rivas, S. Sanchez-Cortes, J.V. Garcia-Ramos, G. Morcillo, Langmuir, 20 (2000) 9722.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V.

A new composite micro/meso porous material used as the support of catalyst for polyaromatic compound hydrogenation Jialin Yu, Ye Tian, Xiaoling Ma, Yongdan Li Tianjin Key Laboratory of Applied Catalysis Science and Technology and State Key Laboratory for Chemical Engineering (Tianjin University), School of Chemical Engineering, Tianjin University, Tianjin 300072, China

Abstract A composite micro/meso porous material possessing well-ordered hexagonal structure, uniform pore size, strong acidity and high hydrothermal stability has been examined. Compared to the catalysts using zeolite Y, USY, MSU-SFAU as supports, the Pd-Pt catalyst in this work shows high activity for the hydrogenation of naphthalene in the absence and presence of 4,6-dimethyldibenzothohene, and also high activity for pyrene hydrogenation. Keywords: composite micro/meso porous material, Pd-Pt catalyst, naphthalene hydrogenation, pyrene, sulfur tolerance

1. Introduction Aromatic compounds in diesel lower its cetane number and increase undesirable emissions such as soot and hydrocarbons [1]. Nevertheless, deep hydrodearomatization and desulfurization of diesel is a challenging task because it contains bulky aromatics and sulfur-containing compounds. Song [2,3] proposed a bimodal pore system for metal based deep hydrotreating catalyst. Meng et al. [4] introduced mesopores into zeolite Y by silicon tetrachloride treatment followed by steam dealumination. Jin et al. [5] made mesopores into ZSM-5 by desilication in alkaline medium. Zhang et al. [6] prepared a composite zeolite Y overgrown with a thin layer of MCM-41. Zhang and Li [7] reported a Beta/MCM-41 composite with a silica-alumina source originated from alkaline treatment of zeolite Beta. Liu et al. [8] assembled a hydrothermally stable and ordered mesostructured material called MSU-SFAU using a pre-crystallized zeolite Y seed colloidal. In this work, a secondary hydrothermal treatment is applied on MSU-SFAU to get a stable micro/meso composite structure. The new material has a strong acidity and high surface area. When it is used as the support of the Pd-Pt catalyst, the catalyst shows a good activity in the hydrogenation of pyrene and a good sulfur tolerance during naphthalene hydrogenation.

2. Experimental 2.1. Synthesis of material A MSU-SFAU sample was prepared from Y seed colloidal as described in ref. [8]. However, the ratio of the starting materials was modified as SiO2: 0.1 Al2O3: Na2O: 0.25 CTAB: 60 H2O at pH 10. The material was treated in an autoclave with deionized water at 120oC for 4 days. Then a sample A-s was obtained and the Na-type was

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converted into acidic form via ion exchange using 1mol/L solution of NH4NO3 at 50oC for 8 h, followed by calcination at 550oC for 10 h.

2.2. Preparation of catalyst The Pd-Pt catalyst was prepared by the ion-exchange method. A mixed solution of Pd(NH3)4Cl2 and Pt(NH3)4Cl2 with 0.01 M metal and a Pd/Pt = 4 ratio was added dropwise to a slurry of the acidic support. After keeping the material at room temperature for 24 h, it was filtrated and washed, and dried at 80oC for 12 h. The powder below 270 meshes was used and was calcined in oxygen flow (180 mL/min, STP) at 450oC for 2 h with a ramping 0.5oC/min. As comparison, a Pd-Pt/HY and a PdPt/HUSY catalyst were prepared as in [2]. MSU-SFAU with a similar Si/Al ratio was prepared as in [8], and a catalyst was prepared. The catalysts are listed in Table 1. Table 1. Samples used in this work. Supports Y USY MSUSFAU A-s

Specific surface area /m2.g-1 671 688

Micro pore volume /cm3.g-1 0.370 0.150

Pd-Pt/HY Pd-Pt/HUSY

492

0

Pd-Pt/HMSU-SFAU

880

0.0437

Pd-Pt/H-A-s

Metal content (wt%) Catalyst

Pd

Pt

Total

0.731 0.685

0.326 0.320

1.06 1.00

0.628

0.313

0.941

0.831

0.381

1.21

2.3. Catalytic activity 0.3 g catalyst was reduced in hydrogen flow at 300oC for 2 h and the reaction was carried out with a 300 ml Parr stirred tank reactor at stirring speed 650 rpm. 120 mL tridecane and 9 g naphthalene or 4.5 g pyrene was prefilled. In some cases 1200 ppm dimethyldibenzothiophene (DMDBT) was added. The reaction was at 250oC and 6 MPa. An Agilent 6890N GC with a HP-PONA Methyl Siloxane capillary column and a FID and another GC (same model) with an MS detector were employed. 0

3. Results and discussion

2

4

6

8

10

2 Theta (deg.)

3.1. Material characterization 0

10

20

30

40

50

60

2 Theta (deg.)

Fig. 1. XRD patterns of the material.

HY NH3Signal (a.u.)

The XRD pattern of the calcined material is presented in Fig. 1. In the low 2θ region, i.e. 1-10 o, the sample displays three distinct diffraction peaks indexed as (100), (110) and (200), which is characteristic for the hexagonally symmetrical MCM-41 mesoporous structure. In the high 2θ region, that is, 7-60o, all of the major peaks of zeolite Y can be identified. The N2 adsorptiondesorption isotherm exhibits a combination of type I and IV isotherm, The isotherm has a steep rise at the range P/Po= 0.28-0.42, and a narrow mesopore size distribution around 2.7 nm. The textural and structural properties of the sample are listing in the table 1. As shown, the zeolite structure in the composite contributes a pore

100

H-USY H-A-S H-MSU-SFAU

200

300

400

500

600

o

Temperature( C)

Fig. 2. NH3-TPD of the supports.

Composite micro/meso porous material and its performance volume 0.0437 cm3g-1. The mesopore surface area is around 773 m2 g-1. Figure 2 plots the curves of NH3-TPD measured with samples with similar Si/Al ratio. As shown, the amount of acid sites on sample A-s is much larger than that on sample MSU-SFAU, is comparable with that on sample USY, and is much less than that on zeolite Y.

627

Naph. conversion/%

100 Pd-Pt/HY Pd-Pt/H-A-s Pd-Pt/HMSU-SFAU Pd-Pt/HUSY

80 60 40 20 0

3.2. Catalytic activity

0 50 100 150 200 250 300 350 Figure 3 illustrates the time dependence of Reaction time/min naphthalene conversion over the catalysts Fig. 3. Naphthalene conversion versus examined. Notably, both the values of Pdreaction time in the clean feed. Pt/H-A-s and Pd-Pt/HUSY are much higher than those of Pd-Pt/HY and Pd-Pt/HMSUSFAU. After 140 min, the conversions are 100% over Pd-Pt/HUSY and 98% over Pd-Pt/H-A-s, but only 15% over Pd-Pt/HY and 34% over Pd-Pt/HMSU-SFAU.

Table 2. Pseudo-first-order kinetic rate constants for the consecutive hydrogenation of naphthalene over different catalysts in the absence and presence of 1200 ppm DMDBT. Catalysts Pd-Pt/HUSY Pd-Pt/H-A-s

k1/L·molM-1·h-1 k1c k1s 9029 7678 4895 4025

k2/L·molM-1·h-1 k2c k2s 657 55 135 36

K1s/k1c 0.850 0.822

K2s/k2c 0.084 0.27

In this work, naphthalene hydrogenation can be considered as irreversible pseudofirst order consecutive reactions, with the second ring saturation being a slow step [9]. The rate constants for the hydrogenation of the first and the second rings in the absence (k1c, k2c) and presence of 4,6-DMDBT (k1s, k2s) and the sulfur-tolerance defined as the ratio of k1s/k1c, k2s/k2c are presented in Table 2. The values for the second ring saturation are obtained by fitting the data after the conversion is higher than 97% and with byproducts less than 3%. The constants are normalized to per mole of metal atoms. As shown, the hydrogenation rates of both the two rings on Pd-Pt/HUSY are higher than those on Pd-Pt/H-A-s for with and without DMDBT. Pd-Pt/H-A-s shows similar thioresistance with Pd-Pt/HUSY for the first ring and higher thioresistance than PdPt/HUSY for the second ring. Pd-Pt/H-A-s Pd-Pt/HUSY

80 70

50

Content /%

Pyrene converson (%)

Pd-Pt/H-A-s Pd-Pt/HUSY

60

90

60 50 40

40

30

20

30 20

10

10 0

0

A 0

50 100 150 200 250 300 350

B

C

D

E

Product type

Reaction time/min

Fig. 4. Pyrene conversion versus reaction time.

B C E D A Fig. 5. Products distribution of pyrene hydrogenation over Pd-Pt/HUSY and Pd- Pt/H-A-s after 340 min.

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Figure 4 presents the time dependence of pyrene conversion over Pd-Pt/H-A-s and Pd-Pt/HUSY. After reaction for 340 min, the conversion of pyrene over Pd-Pt/HUSY gets 38%, while that over Pd-Pt/H-A-s reaches 80%. The difference of the product distribution from pyrene hydrogenation is displayed in Fig. 5. After 340 min, the content of unreacted pyrene over Pd-Pt/HUSY is much higher than that over Pd-Pt/HA-s. Furthermore, the amount of deep hydrogenated products, especially that of tetrahydropyrene (E), is much higher over Pd-Pt/H-A-s than that over Pd-Pt/HUSY.

4. Conclusion A composite micro/meso porous material has been prepared by a secondary hydrothermal treatment. The material shows bimodal pore system due to the formation of zeolite Y in mesoporous framework. When used as the support of a Pd-Pt catalyst for hydrogenation of naphthalene in the presence and absence of 4,6-DMDBT, it demonstrates that the catalyst has an enhanced activity and sulfur tolerance. When pyrene is hydrogenated, the material shows a remarkable enhancement over the USY supported catalyst.

Acknowledgements This work has been supported by the Natural Science Foundation of China under contract number 20425619. The work has been also supported by the Program of Introducing Talents to the University Disciplines under file number B06006, and the CheungKong Scholar Program for Innovative Teams of the Ministry of Education under file number IRT0641.

References [1] A. Stanislaus, B.H. Cooper. Aromatic hydrogenation catalysis: a review. Catal. Rev.-Sci. Eng., 1994, 36, 75-123. [2] C.S. Song. Designing sulfur-resistant, noble-metal hydrotreating catalysts. Chemtech, 1999, 29, 26-30. [3] C.S. Song. Sulfur-resistant noble metal catalysts based on shape-selective exclusion and hydrogen spillover. A C S. Symp. Ser. 2000, 738, 381-390. [4] X.C. Meng, Y.X. Wu, Y.D. Li. Tailoring the pore size of zeolite Y as the support of diesel aromatic hydrogenation. J. Porous Mater. 2006, 13, 365-371. [5] F. Jin, Y.G. Cui, Y.D. Li. Effect of alkaline and atom-planting treatment on the catalytic performance of ZSM-5 catalyst in pyridine and picolines synthesis. Appl. Catal. A: Gen. 2008, 350, 71-78. [6] H.J. Zhang, X.C. Meng, Y.D. Li, Y.S. Lin. MCM-41 overgrown on Y composite zeolite as support of Pd-Pt catalyst for hydrogenation of polyaromatic compounds. Ind. Eng. Chem. Res. 2007, 46, 4186-4192. [7] H.J. Zhang, Y.D. Li. Preparation and characterization of Beta/MCM-41 composite zeolite with a stepwise-distributed pore structure. Powder Technology, 2008, 183, 73-78. [8] Y. Liu, W. Zhang, T.J. Pinnavaia. Steam-stable aluminosilicate mesostructures assembled from zeolite type Y seeds. J. Am. Chem. Soc. 2000, 122, 8791-8792. [9] P.L. Song, J.J. Bian, X.C. Meng, D.P. Liu, Y.D. Li. Naphthalene hydrogenation and sulfur tolerance of Pd-Pt catalyst supported on modified MCM-41 zeolite. Acta Pet. Sin. 2004, 20, 40.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Photodeposition of Au and Pt on ZnO and TiO2 S.A.C. Carabineiro,a B.F. Machado,a G. Dražić,b R.R. Bacsa,c P. Serp,c J.L. Figueiredo,a J.L. Fariaa a

Laboratory of Catalysis and Materials (LCM), Associate Laboratory LSRE/LCM, Faculty of Engineering, University of Porto, 4200-465 Porto, Portugal b Jozef Stefan Institute, Department of Nanostructured Materials, Jamova 39, SI-1000 Ljubljana, Slovenia c Laboratoire de Chimie de Coordination UPR8241 CNRS, composante ENSIACET, Toulouse University, 118 Route de Narbonne, F-31077 Toulouse Cedex, France

Abstract Au and Pt were loaded (1 wt.%) on ZnO and TiO2 supports from different sources using a photodeposition method. The catalytic activity of the prepared materials was tested in CO oxidation. It is shown that Pt gives better results for ZnO supported catalysts, while Au is pronouncedly more active in the case of TiO2. This might be related to the smaller particle size of Au when supported on TiO2 and the stronger metal-support interaction effect, which are important factors in catalysis by Au. In the case of Pt catalysts, the dependence on the support is not so marked, which may be attributed to the reaction taking place predominantly on the bare metal surface. Keywords: Photodeposition, gold, platinum, ZnO, TiO2

1. Introduction Photochemical deposition (PD) is nowadays gaining importance as an alternative method for preparing heterogeneous catalysts. It allows metal deposition over semiconductor materials, with simultaneous reduction of metal ions by the electrons of the conduction band. This process can be enhanced by addition of “sacrificial electron donors” (such as formaldehyde, methanol or 2-propanol) that can supply an almost unlimited amount of electrons. PD takes place at, or near, the photoexcited sites, leading to an enhanced dispersion. This method has been scarcely used, particularly for Au, and the few publications found in literature deal only with TiO2 supports [1-4]. Reports on photodeposited Pt catalysts are more common [5,6]; however, to the best of our knowledge, no Pt or Au photodeposition studies were performed so far on ZnO. In the present work, Au and Pt were photodeposited on TiO2 and ZnO using methanol as a sacrificial electron donor. Different ZnO samples were used: two commercial (from Evonik and Strem), and one prepared by chemical vapour deposition (CVD) [7,8]. Commercial TiO2 from Evonik (P25) was also used for comparison purposes. CO oxidation was used as a test reaction to evaluate the catalytic activity of the materials.

2. Experimental 2.1. Support preparation The ZnO prepared by CVD (ZnOCVD) was synthesised according to a previously described procedure [7,8]. The Zn metal powder was allowed to melt under argon

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atmosphere at 900ºC before air was introduced. The resulting small white flakes of ZnO were then collected using a liquid nitrogen trap, and used without further purification. Two commercial ZnO samples were also used: one from Strem Chemicals (ZnOSC; 85-95% ZnO, 3-7% Al2O3) and another from Evonik (ZnOEV; AdNano VP 20). Commercial TiO2 from Evonik (P25) was also used for comparison purposes.

2.2. Au and Pt photodeposition The necessary amount of Au or Pt precursor (HAuCl4 and H2PtCl6, respectively) was dissolved in water and methanol (300 mL, 15:1 ratio), to obtain a 1 wt. % metal load. Afterwards, the ZnO or the TiO2 support was added, and the mixture sonicated for 30 min, to improve dispersion. The photodeposition process was carried out under irradiation with a UV lamp Heraeus TNN 15/32, with a strong emission line at 253.7 nm (ca. 3 W of radiant flux), in 2 h cycles. A series of tests were carried out for the ZnOEV and P25 using different pH adjusted values (from 1 to 12), before photodeposition. The best results for PD of Au were obtained without pH adjustment (pH~5.5) for ZnOEV, the other Au/ZnO samples were prepared in these conditions. The best results for TiO2 were obtained at pH=10.

2.3. Catalyst characterization Samples were characterized by N2 adsorption-desorption isotherms at 77 K, scanning electron microscopy (SEM), transmission electron microscopy (TEM), energydispersive X-ray spectrometry (EDXS), selected area electron diffraction (SAED) and X-ray diffraction (XRD). Further details of the experimental procedures are described elsewhere [9].

2.4. Catalytic tests The obtained catalysts were tested in the oxidation of CO (CO + ½O2 → CO2), since this is an established model reaction to evaluate the activity of Au catalysts, not just due to its simplicity, but because it can have important applications [10,11]. A continuousflow reactor was used. The feed gas (5% CO, 10% O2 in He) was passed through the catalytic bed at a total flow rate of 50 mL min-1 (STP) and 0.2 g of catalyst sample was used. The composition of the outgoing gas stream was determined by gas chromatography. Further details can be found elsewhere [9].

3. Results and discussion 3.1. Characterisation of the supports The BET specific surface areas of ZnO samples were obtained by N2 adsorption isotherms at 77 K and were found to vary from 17 m2 g-1 for ZnOCVD to 30 m2 g-1 for sample ZnOSC. An intermediate result was obtained for ZnOEV with 26 m2 g-1. The surface area of P25 was 51 m2 g-1. The XRD patterns of the ZnO supports showed the presence of a hexagonal structure for all the ZnO samples. XRD of P25 revealed 81% anatase (particles around 21 nm) and 19% rutile (28 nm particle size). This was also confirmed by SAED. SEM and TEM results showed that ZnOEV and ZnOSC samples consist mainly of cylindricalshaped particles, whereas ZnOCVD (Figure 1b) is composed by tetrapod-like structures, where needles grow from a faceted seed particle [7,8].

3.2. Au and Pt loaded materials Au particle sizes for ZnO materials varied from 4-20 nm (Figure 1b) while Pt samples showed Pt sizes between 2-25 nm (Figure 1a). For P25 materials, Pt showed a particle

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size of 2-5 nm (Figure 2a) and Au of 3-11 nm (Figure 2b). The presence of Au and Pt was confirmed by EDXS and XRD. a

50 nm

b

100 nm

Figure 1 - TEM images of Pt (a) and Au (b) nanoparticles photodeposited on ZnOCVD.

a

b

50 nm

20 nm

Figure 2 - TEM images of Pt (a) and Au (b) nanoparticles photodeposited on TiO2.

3.3. Catalytic tests Neat P25 was found not to convert CO, but nearly full conversion was obtained at 200ºC when loaded with Au, both for pH 10 and pH 7 adjustments, the first value being slightly better. Not adjusting pH, or adjusting it to values higher than 10, yielded less active catalysts. In the Au/TiO2 PD studies reported in the literature, some authors report pH adjustment to 6 [5], 7 [1] or alkaline values (8-10) [2]. In fact, a pH above the isoelectric point (~6.2) causes the surface to be negatively charged which might increase hydroxylation of the Au precursor and elimination of chlorine, producing better dispersion. Au is much more active than Pt, especially at lower temperatures (full conversion was also obtained at 200ºC for Pt/P25). The higher activity of Au at lower temperatures is most likely related to its lower particle size on P25 (3-11 nm, Figure 2b) when compared to ZnO (4-20 nm, Figure 1b), as it is well known that the catalytic activity for Au catalysts is strongly related to the particle size (usually, the smaller, the better) [10,11]. The ZnOCVD support showed the best results among the samples without Au, achieving full CO conversion at 450ºC, in comparison with the other two ZnO samples that only attained it at ~600ºC. This might be related with its interesting tetrapod like structure (Figure 1b). As expected, loading the ZnO samples with Au caused CO conversion to occur at much lower temperatures (~300ºC), without pH adjustment (i.e., pH~5.5), since adjusting the pH to more acidic or basic values produces less active samples. The different catalysts showed different performances, which in case of Au are not so marked and can be explained in terms of the particle size. However, Pt catalysts were much more active, (full conversion at 200ºC), regardless of the ZnO support.

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Again, this could be related to the larger size of the Au particles on ZnO, when compared to Pt particles on the corresponding supports. The reason why Pt loaded materials have similar behaviour, independently of the support, can be related to the fact that CO oxidation on Pt follows a LangmuirHinshelwood mechanism that consists in the adsorption of both CO and O2 (that dissociates into atomic O) on the Pt surface itself. [CO]ads then reacts with [O]ads to give [CO2]ads which is then released as CO2 [12]. For Au catalysts, the Bond-Thompson mechanism takes place [10,11,13]. This suggests that it is the lattice oxygen of the support that reacts with CO, and that the O2 provided is only needed to restore the support surface. Therefore, the reaction greatly depends on the strength of interaction with the support [1]. Since TiO2 has a stronger metal-support interaction (SMSI) effect [6,14], that can also explain the better catalytic behavior of Au/TiO2 catalysts.

4. Conclusions The PD method provides a simple and effective route to produce supported Au and Pt catalysts efficient in CO oxidation. With P25 supported catalysts, the best results were obtained for Au at pH 10, while for ZnO supported catalysts the most active were those prepared with no pH adjustment. On P25, Au is more active than Pt, while Pt provides better results for ZnO. This might be related to the smaller particle size of Au when supported on P25 and a stronger SMSI effect, which are important factors in the BondThompson mechanism preferred by Au, which requires a significant participation of the support lattice oxygen. With Pt catalysts, the dependence on the support is not so high, reflecting the preference for a Langmuir-Hinshelwood mechanism, which involves primarily the bare active metal centre.

Acknowledgements Fundação para a Ciência e Tecnologia (FCT, Portugal) and the Ministry of Higher Education, Science and Technology (Slovenia) for financial support (Portugal-Slovenia Cooperation in Science and Technology 2008/2009). SAC acknowledges FCT for financing (CIENCIA 2007 program). RB and PS gratefully acknowledge research funding from Agence Nationale de Recherche, France (RNMP05-PRONANOX). Authors are grateful to Evonik (Spain) for the free ZnO (VP AdNano 20) sample provided. This work was partially supported by ACENET ERA-NET (project SIPROHYM-ACENET/0001/2007) and FEDER (POCI/FEDER/2010).

References 1. 2. 3. 4. 5. 6. 7.

G.R. Bamwenda, S. Tsubota, T. Nakamura, M. Haruta, 1997, Catal. Lett. 44, 83-87. C.-y. Wang, C.-y. Liu, X. Zheng, J. Chen, T. Shen, 1998, Coll. Surf. A, 131, 271-280. R. Kydd, K. Chiang, J. Scott, R. Amal, 2007, Photochem. Photobiol. Sci., 829-832. L.-H. Chang, Y.-L. Yeh, Y.-W. Chen, 2008, Int. J. Hydrogen Ener. 33, 1965-1974. Z.B. Zhang, C.C. Wang, R. Zakaria, J.Y. Ying, 1998, J. Phys. Chem. B 102, 10871-10878. B.F. Machado, 2009, Ph.Thesis, University of Porto, and references therein. R. Bacsa, J. Dexpert-Ghys, M. Verelst, A. Falqui, B. Machado, W. Bacsa, P.Chen, S. Zakeeruddin, M. Graetzel, P. Serp, 2009, Adv. Funct. Mater. 19, 875-886. 8. R. Bacsa, Y. Kihn, M. Verelst, J. Dexpert, W. Bacsa, P. Serp, 2007, Surf. Coat. Technol. 201, 9200-9204. 9. S.A.C. Carabineiro, B.F. Machado, R.R. Bacsa, P. Serp, G. Dražić, J.L. Faria, J.L. Figueiredo, 2010, submitted. 10. S.A.C. Carabineiro, D.T. Thompson, In: Nanocatalysis, Springer-Verlag, Berlin, Heidelberg, New York, 2007, pp. 377-489, and references therein.

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11. S.A.C. Carabineiro, D.T. Thompson, In: Gold: Science and Applications, CRC Press, Taylor and Francis Group, Boca Raton, London, New York, 2010, pp.89-122, and references therein. 12. T.A. Nijhuis, M. Makkee, A.D. Langeveld, J.A. Moulijn, 1997, Appl. Catal. A 164, 237-249. 13. G.C. Bond, D.T. Thompson, 2000, Gold Bull. 33, 41-51. 14. S.J. Tauster, S.C. Fung, R.L. Garten, 1978, J. Am. Chem. Soc. 100, 170-175.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Cellulose-templated materials for partial oxidation of methane: effect of template and calcination parameters on catalytic performance Claudia Berger-Karin, Evgenii V. Kondratenko* Leibniz-Institute for Catalysis at the University of Rostock, Albert-Einstein-Str. 29A, D-18059 Rostock, Germany (*e-mail:[email protected])

Abstract Commercial ashless cellulose fibers were applied as template materials to synthesize Ni(10wt.%)-Al2O3 catalysts for partial oxidation of methane (POM) to synthesis gas. It was demonstrated that using small-sized cellulose fibers (3 µm) in combination with their flash combustion at 1173 K resulted in a highly active and thermally stable catalyst in the target reaction. Its POM performance was superior compared to that of Ni(10wt.%)/Al2O3 prepared via a conventional impregnation route. Keywords: cellulose template, preparation procedure, NiOx-Al2O3, POM

1. Introduction Partial oxidation of methane (POM) to synthesis gas over nickel and noble metal supported catalysts has been intensively investigated as an alternative for industrial steam reforming of methane [1]. From an economical point of view, nickel-based catalysts are more preferable than those containing precious metals but suffer from deactivation via coking [2]. This deactivation is retarded over catalysts possessing highly dispersed small nickel species. However, the POM reaction is performed at high temperatures favoring sintering of these species and support materials or phase transformations. All these processes contribute to loss of catalyst resistance against coking [3, 4]. Therefore, it is challenging to prepare thermally stable materials possessing high specific surface area and nickel dispersion coinciding with environmentally friendly and cost-efficient preparation methods. The present contribution reports on a simple cellulose-templated method for synthesizing NiOx-Al2O3 materials according to the procedure originally described by Shigapov et al. [5] for preparation of high-surface area CeO2-based materials. Special attention has been paid to the influence of cellulose template, and calcination parameters on surface morphology, redox properties, phase composition, and catalytic performance of NiOx-Al2O3 materials in the POM reaction.

2. Experimental 2.1. Catalyst preparation W542 (pore size of 3 µm) and W541 (pore size of 22 µm) cellulose fibers provided by Whatman® were used as templates for catalyst preparation. According to this method, an aqueous solution of aluminum nitrate (Acros Organics) with nickel nitrate (Merck) was fully absorbed by the template at 300 K. Metals concentration was adjusted to obtain Ni(10 wt.%)-Al2O3 catalysts. Two strategies for the removal of the organic fibers were applied: i) the impregnated wet fiber was dried at 353 K overnight followed by its calcination at 1173 K for 2 h in a muffle furnace (slow); ii) the impregnated wet fiber

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was directly transferred into a muffle furnace preheated to 1173 K and calcined for 2 h (flash). For comparison, a commercial γ-Al2O3 (SASOL) was impregnated by an aqueous solution of nickel nitrate via a conventional incipient wetness method (iw) and calcined at 1073 K for 4 h. The catalytic materials as well as their selected physicochemical and catalytic properties are summarized in Table 1.

2.2. Catalyst characterization Fresh NiOx-Al2O3 materials and those used in the POM reaction at 1073 K for 150 hours on stream were characterized by XRD, N2-physisorption (BET, BJH), and H2TPR. Morphology of the powder particles was studied by scanning (SEM) and transmission (TEM) electron microscopies. SEM images were taken using JSM-7401F operated at a voltage of 4-5 kV. TEM investigations were performed on a FEI, CM20 STWIN (LaB6). The chemical composition was determined by Energy Dispersive Xray analysis (EDXS) on a Noran six from Thermo Fisher Scientific. BET specific surface areas (SBET) were determined on a Micrometrics ASAP 2010 by computing the BET equation of N2 physisorbed at 77 K. The method of Barret, Joyner and Halenda (BJH) was employed to determine the pore size distribution. Temperature-programmed reduction (H2-TPR) measurements were carried out in a six-channel reactor by heating 100 mg of sample with 10 K·min-1 up to 1073 K in a gas mixture of hydrogen (10 % H2 in N2) with a total flow of 30 cm3·min-1 taking into account the criterion of Monti and Baiker [6]. A 48-channel reactor was employed for POM experiment at 1073 K and ambient pressure. 30 mg of catalyst were placed in a quartz-tube reactor (i.d. 4 mm) within the isothermal zone of the oven. Methane and air in a stoichiometric CH4/O2 ratio of 2 were fed to each channel with a GHSV of 29160 cm3·gcat-1·h-1. No catalyst reduction before the catalytic tests was performed. The products and feed components were analyzed using an on-line 2-dimensional GC (Agilent 7890) equipped with four columns: FFAP and Al/S for the quantification of CH4, C2H4, and C2H6 as well as Plot Q and Molsieve for CO2, O2, N2, CO, and H2 acquisition.

3. Results 3.1. Morphology of NiOx-Al2O3 materials Table 1 demonstrates that the method of catalyst preparation influenced BET surface areas, nickel reducibility and catalytic POM performance. The low SBET of 100 m2·g-1 of the conventionally prepared NiOx/Al2O3_iw material clearly stands out. NiOxAl2O3_W542_slow and NiOx-Al2O3_W541_flash possessed similar surface areas of 111 and 133 m2·g-1, respectively. By far the highest SBET was determined for NiOxAl2O3_W542_flash and amounted to 187 m2·g-1. N2-isotherms of all cellulose-templated catalysts showed type IV hysteresis loops, which is typical for mesoporous solids. Pore size distribution was between 6.6 and 10.3 nm. Table 1. Selected physicochemical and catalytic properties (reaction conditions as in section 2.2). Catalysts NiOx-Al2O3_W542_slow NOx-Al2O3_W542_flash NiOx-Al2O3_W541_flash NiOx/Al2O3_iw

SBET / m2·g-1 111 187 133 100

BJH / nm 6.6 8.3 7.6 10.3

Ni reduced /% 54.0 72.5 49.2 51.3

X(CH4) /% 29.6 96.2 31.5 31.4

S(CO) /% 39.7 92.9 38.4 34.0

CO/H2 no H2 0.70 no H2 0.76

Cellulose-Templated NiOx-Al2O3 Materials for Partial Oxidation of Methane

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It is important to highlight that the procedure of template removal is an essential parameter influencing catalyst stability against sintering. For example, the BET values of NiOx-Al2O3_W542_flash and NiOx-Al2O3_W542_slow decrease by 11 % and 23 %, respectively, after 150 hours on stream in the POM reaction at 1073 K. With other words, the fast removal of cellulose template appears to be favorable for preparing thermally stable materials. Figure 1 shows SEM images of four NiOx-Al2O3 materials. The micrographs evidence the effect of the preparation method on the morphology of catalyst particles. Conventionally prepared NiOx/Al2O3_iw consisted of 100-500 µm-scale particles (Fig. 1 (d)). Their surface looks like a moonscape exhibiting hardly any edges or curbs. The appearance and size of particles of cellulose-templated samples depended on the template and the calcination procedure. Among these materials, the biggest catalyst particles were formed, when the cellulose fiber with the largest pores (22 µm) was used as a template. These particles represent cast-to-mould structure of the cellulose fibers (see inset in Fig. 1 (c)). In contrast, particles of the materials prepared using the W542 template possessing 3 µm pores are significantly smaller. NiOx-Al2O3_W542_flash consisted of 1 µm-range particles exhibiting a frayed surface (Fig. 1 (a)). When the cellulose template was slowly burnt out, bigger and angular particles were formed (Fig. 1 (b)). Further insights into the surface morphology of catalysts were derived from TEM measurements coupled with EDXS technique. Neither Ni nor NiO particles were detected by TEM on the surface of fresh samples, while surface Ni/Al ratios were equal for all the samples. This means that either Ni is well dispersed for example in a thin layer of NiAl2O4, or Ni particles are smaller than 3 nm being below Fig. 1. SEM micrographs of a) NiOx-Al2O3_W542_flash, b) NiOx-Al2O3_W542_slow, c) NiOx-Al2O3_W541_flash, the spatial EDXS resolution. and d) NiOx/Al2O3_iw.

3.2. Redox and Catalytic Properties H2-TPR tests evidenced that catalyst reducibility is also influenced by the preparation method. For all the materials, the reduction started at ca. 950 K and was not completed at 1073 K (end temperature of the measurements). However, the H2-TPR profile of NiOx-Al2O3_W542_flash is characterized by an additional peak of H2 consumption at ca. 800 K. This can be explained taking into account the results of XRD analysis. Before the POM reaction, all the investigated catalysts contained crystalline NiAl2O4. However, NiOx-Al2O3_W542_flash possessed as well NiO. The high-temperature H2 consumption over all the materials should be related to the reduction of hardly reducible NiAl2O4, while the presence of NiO in NiOx-Al2O3_W542_flash is responsible for the low-temperature H2 consumption [7]. Quantifying the H2-TPR experiments enabled us to calculate the reduction degree of nickel species, which amounted to around 50 % for

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NiOx-Al2O3_W542_slow, NiOx-Al2O3_W541_flash, and NiOx/Al2O3_iw, while 72.5 % of the loaded Ni was reduced in NiOx-Al2O3_W542_flash. In order to estimate the potential of cellulose-templated materials for the POM reaction, we performed catalytic tests at 1073 K over 150 hours on stream. For all the catalysts, neither CH4 conversion nor CO selectivity were decreased during these experiments indicating that Ni sintering and deactivation by carbon deposition did not proceed in an appreciable extent. However, the catalysts differed in their performance (Table 1). The NiOx-Al2O3_W542_flash catalyst showed the highest CH4 conversion and CO selectivity of 96.2 and 92.9 %, respectively. The conversion was close to the thermodynamic one. Compared to this catalyst, other cellulose-templated catalysts (NiOx-Al2O3_W542_slow and NiOx-Al2O3_W541_flash) and the conventionally prepared NiOx/Al2O3_iw resulted in significantly lower CH4 conversion and CO selectivity of 30 and 40 %, respectively. The superior performance of NiOx-Al2O3_W542_flash is related to its easier reducibility compared to the other materials (Table 1). Intense reflections corresponding to metallic nickel were visible in the XRD-patterns of this catalyst after the POM reaction. It is well accepted that Ni0 is an active species in this reaction [8, 9]. In summary, comparing the POM performance, redox properties and surface morphology of our catalysts (Table 1, Fig. 1) revealed that cellulose fibers with tiny pores (3µm) and their fast combustion are two crucial parameters for preparing small catalyst particles possessing easily reducible nickel species. This may be due to the fact that the flash combustion results in the formation of big amounts of gases due to water evaporation causing stress on the emerging solid. In contrast, during drying phase in the slow combustion method, Ni and Al nitrates may agglomerate to larger particles and form hardly reducible and low active NiAl2O4.

4. Conclusion Cellulose-templated method for preparation of mixed-metal oxide materials is characterized by absence of any metal-containing wastes, because all components are directly attached in resulting catalysts. Using same sample composition and reagents but different templates and preparation conditions results in catalytic materials with distinctly different physicochemical and catalytic characteristics. For the POM reaction, using a small-pore sized cellulose template in combination with its flash combustion is required to prepare active and thermally stable Ni-containing materials. These two synthesis characteristics are important for introducing a high degree of stress and tension, which force the emerging solid to form frayed particles providing a large and thermally stable SBET and easily reducible nickel species being potential active sites.

References [1] B.C. Enger, R. Lodeng, A. Holmen, Appl. Catal. A-Gen., 346 (2008) 1. [2] E.V. Kondratenko, M. Baerns, Encyclopedia of Catalysis, I. Horvath (Ed.), John Wiley and Sons, 6 (2003) 424. [3] J.B. Claridge, M.L.H. Green, S.C. Tsang, A.P.E. York, A.T. Ashcroft, P.D. Battle, Catal. Lett., 22 (1993) 299. [4] J. Sehested, J.A.P. Gelten, S. Helveg, Appl. Catal. A-Gen., 309 (2006) 237. [5] A.N. Shigapov, G.W. Graham, R.W. McCabe, H.K. Plummer, Appl. Catal. A-Gen., 210 (2001) 287. [6] D.A.M. Monti, A. Baiker, J. Catal., 83 (1983) 323. [7] C. Li, Y.W. Chen, Thermochim. Acta, 256 (1995) 457. [8] A.T. Ashcroft, A.K. Cheetham, J.S. Foord, M.L.H. Green, C.P. Grey, A.J. Murrell, P.D.F. Vernon, Nature. 344 (1990) 319. [9] G.R. Gavalas, C. Phichitkul, G.E. Voecks, J. Catal., 88 (1984) 54.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Highly porous hydrotalcite-like film growth on anodised aluminium monoliths F. Javier Echavea, Oihane Sanzb,*, Luciano C. Almeidaa, José Antonio Odriozolab, Mario Montes a a

Applied Chemistry Department, University of the Basque Country (UPV-EHU), Paseo Manuel de Lardizabal 3, 20018 San Sebastián, Spain b Inorganic Chemistry Department and Instituto de Ciencia de Materiales de Sevilla (Centro Mixto US-CSIC), Avd. Américo Vespucio 49, 41092 Sevilla, Spain Corresponding author: [email protected]

Abstract Zinc-aluminum hydrotalcite-like thin films were prepared by direct precipitation on the surface of Al2O3/Al monoliths. The mesopores of anodic alumina provided channels and nano-sized wall-edges for supplying Al3+ and highly active reaction sites. In this work, the influence of temperature, time and Zn:NH3 molar ratio in the obtention of highly homogeneous and adherent hydrotalcite films is studied. We show that the amount loaded, integrity and textural properties of the Zn-Al hydrotalcite-like film are strongly influenced by the preparation conditions. The prepared monoliths were tested for VOC abatement (total oxidation of ethanol and ethyl acetate) showing good activity. Keywords: Zn-Al hydrotalcite-like , Al2O3/Al monolith, VOC catalytic combustion

1. Introduction High-pressure drop and random and structural maldistribution in packed-bed reactors have driven the development of structured catalysts and reactors. Monolithic catalysts, although predominantly used in environmental applications, have become the most commonly used sort of chemical reactors finding relevant and economically significant applications in industrial catalysis so far. Thin walls (lower pressure drops), thermal conductivity and mechanical properties of metals are key properties for choosing metallic monoliths. The formation of α-Al2O3 overlayers upon oxidation has led to use mainly Al-alloyed ferritic steels for designing metallic monoliths. However, when the working temperature is not as demanding as in automotive exhausts, aluminium can be an excellent structural material that upon anodization is coated a high surface area alumina layer, while offering good mechanical and thermal properties. On the other hand, hydrotalcite materials, one of the most useful layered inorganic compounds, are used in the preparation of catalyst, as environmental materials, or as matrixes for hydrotalcite-based nanocomposite films. The objective of this work is to study the growth of porous hydrotalcite-like film on anodized aluminum monoliths for obtaining Cu-Zn/Al2O3-Al monoliths a suitable catalyst in many industrial processes.

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2. Experimental 2.1. Preparation of CuO-Zn/Al2O3-Al monoliths The Zn/Al2O3-Al structures were prepared on cylindrical (3 cm height, 1.6 cm diameter) Al2O3/Al monoliths having 350 cpsi obtained by anodizing Al foils [1] in oxalic acid (40ºC, 2Adm-2 and 40min) and subsequent post-treatment by leaving the monoliths in the electrolytic bath at 40ºC for 40min without electric current [2]. The resulting monoliths were dried at 60ºC for 1h and calcined at 500ºC for 2h. The Zn–Al hydrotalcite-like film was prepared by direct precipitation based on the method proposed by Gao et al. [3]. Zinc acetate, 1.3761 g, was dissolved in 500 mL of deionized water; to this solution ammonia was added until reaching the desired Zn/ammonia molar ratio. This dissolution was heated and stirred using two monoliths fixed to the stirrer blades ensuring the liquid flow lines passed through the monolith channels. Temperature, time and Zn/ammonia molar ratios are shown in Table 1. After synthesis, the monoliths were washed and dried at 100ºC for 12 h and then calcined at 400º C for 4 h. Cupper was deposited the same way as zinc, but in this case ammonia was added to form copper ammonia complex.

2.2. Characterization Nitrogen adsorption is used to determine the textural properties using a Micromeritics ASAP 2020 with a homemade cell accepting the entire 6 cm3 monoliths. FESEM was used to study the morphology (HITACHI 5200). Adherence of the coating was measured by the ultrasound test [4]. Zinc and cupper amounts were determined by atomic absorption spectroscopy (GBC Avanta Σ). The catalytic activity of the prepared monoliths was measured for the complete oxidation of ethyl acetate and ethanol in air. Ignition curves were obtained by heating up to 400°C at 1.5°C/min the monoliths in a 500 ml/min air stream containing VOCs, 1000 mgC/Nm3. VOCs conversion was calculated by measuring the ethyl acetate or ethanol disappearance by GC-TCD (HP 5890) and the CO2 production with an on line IR detector (Vaisala GMT 220).

3. Results and discussion The highly porous structure of the Al2O3-Al monolith provides multiple channels for supplying Al3+ species (Fig.1 A and B). Mesopore walls and edges provide sites at which the Zn-Al hydrotalcite-like film successfully grows up [5].

Fig. 1. FESEM images. (A) Top and (B) lateral view of anodic alumina; (C) top and (D) view of alumina layer coated with Zn-Al hydrotalcite-like.

Table 1 shows the amount loaded and the textural properties of the monoliths prepared at different conditions. On increasing the amount of added ammonia, temperature and stirring time, the amount loaded increases, as well as the total surface area and porosity of the monolith. However, the pore size decreased drastically (Fig. 2) and short whiskers appeared on the alumina pore walls (Fig. 1D). These whiskers are

Highly porous hydrotalcite-like film growth on anodised aluminium monoliths

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mainly alumina hardly containing Zn as shown by EDX analysis. To confirm that it was just a morphological change of the alumina coating, monoliths were prepared using ammonium acetate instead of zinc acetate. These monoliths showed similar textural changes and whiskers growth on the alumina pores. Wefers [6] observed the formation of this type of structure, composed of aluminum hydroxide and bohemite, by immersing the anodized aluminium in boiling deionized water (sealing by hydrotreatment). This phenomenon is attributed to swelling of the alumina due to hydration. Spooner et al. [7] suggested that the sealing is produced by the dissolution of the oxide on the pore walls and their precipitation near the pore mouth. The FESEM image in Figure 1C shows the alumina layer covered with platelet-like nanostructures with >100nm in thickness, resulting in a highly porous surface. The morphology of the nanostructures was not affected by synthesis conditions, but the thickness of the layer that increases with the Zn/NH3 ratio, temperature and time. The growth rate is considerably higher than those reported on Al substrate [8] and Al2O3 films [3], and similar to values reported on Al2O3 membranes [5]. Consequently, this result suggests that porous anodic alumina provides more sites for formation of Zn-Al hydrotalcite-like films [3]. Table 1. Properties of monoliths. Preparation conditions T (ºC)

t (h)

50 50 50 50 50 50 100

Al2O3/Al 3 1:0.5 3 1:2 3 1:3 3 1:6 1 1:3 6 1:3 3 1:3

50

3

50

3

Zn:NH3

NH4Ac/NH3

1:3 1:3

Loaded hydrolatcite-like oxide mg/monolith Tickness Adherence (%) (μm) Zn Cu

Textural properties SBET (m2/monolith)

VP (cm3/monolith)

DP (nm)

53 52 58 65 47 73 70

-

3.8 3.8 3.9 4.3 3.4 5.8 6.0

100 70 70 70 65 75 65 65

17 89 94 101 110 96 157 160

0.074 0.235 0.241 0.235 0.246 0.203 0.247 0.228

17.2 6.7 6.7 6.0 5.8 6.4 4.4 4.0

-

-

-

-

132

0.208

4.3

52

85

3.8

70

228

0.258

3.8

The adhesion of the washcoating was evaluated measuring the weight loss after the ultrasound test. In general the adherence is very poor and depends on the amount of ZnAl hydrotalcite-like film loaded. The higher the amount loaded, the lower the adherence of the coating. However, the catalyst adherence was better than that reported by Sanz et al. [9] for Pt-ZSM5/Al2O3-Al monolith prepared by washcoating. This in situ preparation strategy allows to deposit a large amount of catalyst in an adherent form due to chemical bond although needs a proper metal surface preparation. The best results for homogeneity and adherence of the Zn-Al hydrotalcite-like films were obtained for synthesis carried our at 50ºC, 3 h and Zn/NH3 1:3, and therefore The CuO impregnation was performed on these monoliths. CuO impregnation increased the specific surface area and porosity but decreased the pore size with respect to the ZnAl/Al2O3-Al monoliths (Table 1 and Fig. 2). Furthermore, the morphology of the anodized alumina did not change as observed by SEM. The combustion of ethyl acetate and ethanol were used to test the prepared catalytic devices. The monoliths active for VOCs combustion (Fig. 3) and only total oxidation

F J. Echave et al.

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products, CO2 and H2O, were detected under the experimental conditions employed in this study. The conversion of VOCs starts at 200-250ºC and reaches 100% conversion below 300-360ºC. These results show that CuO addition enhances the VOCs combustion activity of Zn-Al hydrotalcite-like monoliths, reducing the total conversion of VOCs by ca. 75ºC. On the other hand, the activity of the catalysts depends on the VOCs nature. Oxidation of ethanol is easier than that of ethyl acetate. This behaviour also was observed with both noble metal as transition metal [10,11]. Among several metal oxides, Rajesh and Ozkan [12] and Yao [13] found CuO/Al2O3 to be the most active catalyst for the complete oxidation of ethanol. 1 Zn/NH =1/0.5

0.8

2

2

Zn/NH =1/3 3

Zn/NH =1/6 3

Al O /Al 2

3

CuO-Zn/Al/Al O -Al

0.4

2

3

0.2 0 1

3

Ethanol_CuO-Zn/Al O -Al

3

0.6

Ethanol_Zn/Al O -Al 2

3

Zn/NH =1/2

10

Pore Width (nm)

Fig. 2. Pore size distribution.

100

Conversion to CO

3

Pore Volume (cm /monolith)

1

0.8

3

Ethyl acetate_Zn/Al O -Al 2

3

Ethyl acetate_CuO-Zn/Al O -Al 2

3

0.6 0.4 0.2 0 100

150

200

250

300

350

400

Temperature (¼C)

Fig. 3. VOCs ignition curves.

4. Conclusion Zn-Al hydrotalcite-like film has been directly grown on the surface of porous anodic alumina monoliths. The highly porous structure of the Al2O3-Al monolith provided multiple channels for supplying Al3+ species and promoted a chemical anchoring of the coating. The CuO was successfully impregnated on the hydrotalcite surface. In this way, active catalytic structures have been prepared and tested in total oxidation of ethanol and ethyl acetate.

Acknowledgments Financial support by MEC (MAT2006-12386-C05 and FPU fellowship to F.J.E.) and UPV/EHU (GUI 07/63) are gratefully acknowledged.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

N. Burgos, M.A. Paulis, M. Montes, 2003, J. Mater. Chem., 13, 1458-1467 P. Hoyer, K. Nishio, H. Masuda, 1996, Thin Solid Films, 286, 88-91 Y.F. Gao, M. Nagai, Y. Masuda, F. Sato, W.S. Seo, K. Koumoto, 2006, Langmuir, 22, 35213527 S. Yasaki, Y. Yoshino, K. Ihara, K. Ohkubo, US Patent 5,208,206 (1993) F. Yang, B.Y. Xie, J.Z. Sun, J.K. Jin, M. Wang, 2008, Materials Letters, 62, 1302-1304 K. Wefers, 1973, Aluminium, 49, 8-9, 553-622 R.C. Spooner, D. J. Forsyth, 1970, Aluminum, 46, 165-69 J.A. Gursky, S.D. Blough, C. Luna, C. Gomez, A.N. Luevano, E.A. Gardner, 2006, J. Am. Chem. Soc., 128, 8376-8377 O. Sanz, L.C. Almeida, J.M. Zamaro, M.A. Ulla, E.E. Miro, M. Montes, 2008, App. Catal. B, 78, 166-175 N. Burgos, M. Paulis, M.M. Antxustegi, M. Montes, 2002, App. Catal. B, 38, 251-258 D. Delimaris, T. Ioannides, 2008, App. Catal. B, 84, 303-312 H. Rajesh, U.S. Ozkan, 1993, Ind. Eng. Chem. Res., 32, 1622-1630 Y.-F. Yu Yao, 1984, Ind. Eng. Chem. Process Des. Dev., 23, 60-67

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V.

The influence of impregnation temperature on the pzc of titania and the loading of Ni upon preparation of Ni/TiO2 catalysts John Kyriakopoulos,a George Panagiotou, Theano Petsi,a Kyriakos Bourikas,*b Christos Kordulis,a,c Alexis Lycourghiotisa a

Department of Chemistry, University of Patras, GR-265 00 Patras, Greece School of Science and Technology, Hellenic Open University, 18 Parodos Aristotelous St., GR-26335, Patras, Greece c Institute of Chemical Engineering and High Temperature Chemical Processes, FORTH/ICE-HT, P.O. Box 1414, GR-265 00 Patras, Greece *corresponding author (e-mail: [email protected]) b

Abstract The pzc of titania (anatase) was regulated by changing the suspension temperature. A decrease of about 1.5 pH unit was achieved, when the temperature increases from 10 to 75oC. This decrease in pzc causes an increase of the negatively charged surface hydroxyls of titania, which act as receptor sites for the deposition of the cationic [Ni(H2O)6]2+ species, upon the preparation of Ni/TiO2 catalysts following the equilibrium adsorption methodology. Thus, a considerable increase of well dispersed Ni species on titania surface was obtained by regulating the temperature of the impregnation suspension. Keywords: Ni/titania catalysts, impregnation temperature, pzc, equilibrium adsorption

1. Introduction Ni/TiO2 catalyst materials are known to be important in various reactions, including Fischer-Tropsch (CO hydrogenation) and CH4/CO2 reforming [1,2]. The catalytic behavior of a supported catalyst depends largely on the physicochemical characteristics of the supported nanoparticles of active phase. Suitable characteristics may be obtained by controlling the preparation procedure, mainly the initial impregnation step, and thus imposing a given deposition mode [3]. The first critical choice is between bulk and interfacial deposition. The first, realized by using incipient wetness impregnation and in some extent wet impregnation, usually results to catalysts with relatively large supported nanoparticles and thus to catalysts with relatively low activity [3]. The second, realized using equilibrium deposition filtration (EDF) (otherwise called equilibrium adsorption) [3] or homogeneous deposition precipitation (HDP) [4], frequently results to relatively small supported nanoparticles and thus to catalysts with relatively high dispersion [3,5]. On the other hand, the active surface depends on both, dispersion and loading of the active phase. Thus, the maximization of the active surface requires, in addition, the achievement of a high active phase loading. Following EDF the extent of deposition of the precursor species, i.e. the [Ni(H2O)6]2+ species in our study, depends mainly on the pH and the initial Ni concentration of the impregnation solution. In order to achieve high Ni loading, deposition of Ni should take place at pH values higher than the point of zero charge (pzc) of the titania support, where its surface is negatively charged and the adsorption of the positively charged [Ni(H2O)6]2+ species

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is favored. However, at high pH values nickel precipitates in the bulk solution, favoring thus the bulk deposition (which results at low dispersion) instead of the interfacial one. In the present study we attempt to overcome this difficulty by decreasing the pzc of the titania support. This would increase the number of the receptor sites (negatively charged hydroxyls) on its surface and thus would allow an increase of the deposited Ni [3]. The change of suspension temperature is perhaps an effective tool for regulating the pzc and the concentration of the aforementioned receptor sites, at constant pH. Since controversial studies dealing with the influence of temperature on the pzc of titania had appeared in the literature [6-9] (most of them concern titania in the rutile form), we first studied systematically the variation of pzc, with temperature, of various titanias in the anatase polymorph used as catalytic supports. Then, we studied systematically the deposition of Ni species on titania surface at various temperatures and constant pH values at each temperature. The ultimate goal of our study is the maximization of the extent of interfacial deposition of the [Ni(H2O)6]2+ species on the titania surface.

2. Experimental 2.1. Substances, supports and textural analysis NaNO3, Ni(NO3)2·6H2O were used for the preparation, respectively, of the electrolyte and impregnating solutions necessary for the potentiometric mass titrations and deposition experiments. The TiO2 supports used are given in table 1. The lab prepared titania was synthesized by hydrolysis of titanium isopropoxide. The specific surface area (SSA) and the porosity were calculated from the physical adsorption–desorption isotherms of N2 at −196 °C, obtained using a Micromeritics TriStar 3000 instrument. Table 1. Physicochemical characteristics of the titania supports used in this study. Support (notation) P25-1 P25-2 A.A. A.E.

Origin

Crystal structure

Degussa (1st batch) Degussa (2nd batch) Alfa Aesar lab prepared

80% anatase, 20% rutile 80% anatase, 20% rutile anatase anatase

Porosity (cm3/g) 0.14 0.14 0.37 0.15

SSA (m2/g) 50 50 150 50

2.2. Potentiomertic mass titrations The recently reported [10] potentiometric mass titration (PMT) technique was used for determining the pzc of titania suspensions, in the temperature range 10oC – 75oC.

2.3. Adsorption isotherms Adsorption isotherms were obtained at three temperatures: 25oC (pHs 6.3, 6.9 and 7.5), 50oC (pHs 6.3 and 6.9) and 75oC (pH 6.3). The initial Ni solution concentration vary in the range 3×10-4–2.5×10-2 M. Details about the experimental procedure can be found elsewhere [11]. In all adsorption experiments the surface concentration of Ni on TiO2 (ΓNi / μmol m-2) was determined by the difference between the initial Ni concentration of the impregnation suspension and the equilibrium one.

3. Results and discusion 3.1. Influence of temperature on pzc of titania Table 2 compiles the pzc values of the titania supports of table 1, as determined using the PMT technique, for various values of the suspension temperature (T).

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Table 2. pzc values of the titania supports at various temperatures. Support (notation) P25-1 P25-2 A.A. A.E.

pzc 10oC 4.8 5.0 5.7 6.6

25oC 4.5 3.7 5.6 6.4

50oC 4.2 3.5 4.8 6.0

75oC 3.4 3.1 4.2 5.5

A variation of 3.7 units in the pzc, at 25oC, between the various titanias is observed. This is an indication that the pzc of TiO2 depends on the preparation conditions. Moreover, it can be observed that an increase in the suspension temperature causes a decrease in the pzc. This observation is independent of the origin or the preparation procedure of the material and it seems to be of general validity. This general trend is reported for the first time in the literature concerning anatase. The decrease of pzc means that the concentration of the negatively charged surface hydroxyls of titania increases, as T increases, at the expense of the positively charged ones. Taking into account that the first act as receptor sites for the deposition of the cationic [Ni(H2O)6]2+ species, one could predict that the increase of impregnation temperature is an easy way to increase the Ni amount on titania surface, deposited through interfacial deposition. Thus, the next step of our study was the verification of this prediction.

3.2. Influence of temperature on the Ni loading on titania Anatase A.A. was selected for this study due to its higher SSA (table 1). In figure 1 it is shown a representative adsorption isotherm (surface Ni concentration vs initial Ni concentration) at two temperatures of the impregnation suspension. 2.5

ΓNi/μmol.m

-2

2.0

o

50 C o 25 C

pH=6,3

1.5 1.0 0.5 0.0 0.000

0.005

0.010

0.015

0.020

-1

CNi init./mol.L

Figure 1. Adsorption isotherms of Ni on TiO2 at 25οC and 50οC (impregnation pH=6.3).

It can be observed that at a constant pH, the plateau of the isotherm increases with T. This plateau corresponds to the maximum uptake of Ni achieved through interfacial deposition. The same trend with T was found in all adsorption isotherms. Thus, a considerable increase of well dispersed Ni species on titania surface was obtained by regulating the temperature of the impregnation suspension. The observed increase in Ni is explained as follows: as T increases, the pzc of titania decreases and the difference ΔpH=pH-pzc also increases. As we move from pzc to higher pH values, the titania surface becomes more negative and the deposition of the positive [Ni(H2O)6]2+ species is favored. In view of the above, we could say that

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following EDF methodology and making a proper selection of the impregnation parameters (pH, T, initial concentration of Ni) we could result to a controlled preparation of Ni/TiO2 catalysts with a high active surface. The increase of the Ni loading by increasing the temperature suspension is a useful finding from the catalytic viewpoint. On the other hand, an open question is whether the deposition is endothermic, as implied by the adsorption isotherms (figure 1), or exothermic, as anticipated for an adsorption process [12,13]. In order to examine this point we should eliminate the influence of the factor ΔpH=pH-pzc on the Ni uptake. For this reason in figure 2 we have plotted the variation of the Ni uptake with the difference ΔpH=pH-pzc, at three temperatures of the impregnation suspension. We may observe that an increase of T brings about a decrease in the Ni uptake, for a constant value of the ΔpH=pH-pzc. This is a direct proof that the adsorption process is exothermic. 2.4

CNi,initial = 0,005 M

ΓNi/μmol.m

-2

0

2.2 2.0

T=25 C 0 T=50 C 0 T=75 C

1.8 1.6 1.4 1.2 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 pH-PZC

Figure 2. Variation of surface concentration of Ni on TiO2 with ΔpH=pH-pzc, for three temperatures of the impregnation suspension. The initial Ni concentration is indicated.

4. Conclusions A considerable increase of well dispersed Ni species on titania surface was obtained by regulating the temperature of the impregnation suspension. This is due to the decrease of the factor ΔpH=pH-pzc, which renders the TiO2 surface more negative. However, the extent of the increase is restricted by the exothermic character of the adsorption process.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

P.C. Das, N.C. Pradhan, A.K. Dalai, N.N. Bakhshi, 2004, Fuel Proc. Techn., 85, 1487. K. Takanabe, K. Nagaoka, K. Nariai, K-I Aika, 2005, J. Catal., 232, 268. K. Bourikas, Ch. Kordulis, A. Lycourghiotis, 2006, Catal. Rev., 48, 363. G.L. Bezemer, P.B. Radstake, V. Koot, A.J. van Dillen, J.W. Geus, K.P. de Jong, 2006, J. Catal., 237, 291. L. Jiao, Y. Zha, X. Hao, J.R. Regalbuto, 2006, Stud. Surf. Sci. Catal., 162, 211. K. Akratopulu, Ch. Kordulis, A. Lycourghiotis, 1990, J. Chem. Soc. Faraday Trans., 86, 3437. V.G. Berube, P.L. De Bruyn., 1968, J. Colloid Interface Sci. 27, 305. S. Subramanian, J.A. Schwarz, Z. Hejase, 1989, J. Catal. 117, 512. M.L. Machesky, D.J. Wesolowski, D.A. Palmer, K. Ichiro-Hayashi, 1998, J. Colloid Interface Sci. 200, 298. K. Bourikas, J. Vakros, Ch. Kordulis, A. Lycourghiotis, 2003, J. Phys. Chem. B, 107, 9441. L. Karakonstantis, Ch. Kordulis, A. Lycourghiotis, 1992, Langmuir, 8, 1318. P. H. Tewari, W. Lee, J. Colloid Interface Sci. 52 (1975) 77. L.G.J. Fokkink, A.G. Rhebergen, A. de Keizer, J. Lyklema, 1992, J. Electr. Chem., 329, 187.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Immobilization of homogeneous catalysts in nanostructured carbon xerogels Cristina C. Gheorghiu,a María Pérez-Cadenas,a M. Carmen Román-Martínez,a Concepción Salinas-Martínez de Lecea,a Nathalie Jobb a

Department of Inorganic Chemistry, University of Alicante, Ap. 99, Alicante, 03080, Spain b University of Liège, Department of Chemical Engineering, B6a, Sart-Tilman, Liège, B4000, Belgium

Abstract A Rh diamine complex has been successfully immobilized by anchorage on the surface of carbon xerogels. The catalysts are active and fully recyclable for cyclohexene hydrogenation, with conversion higher than 80% kept in four catalytic runs. TEM analysis reveals the presence of Rh particles in the used catalysts, meaning that partial reduction of the metal complex takes place under reaction conditions. The XPS data show that in the used catalysts Rh is present as Rh(I) (anchored complex) and Rh(0) metallic particles (about 30%). Keywords: Rh diamine complex, carbon xerogels, hydrogenation

1. Introduction The immobilization of homogeneous catalysts (metal complexes) on solid supports produces hybrid catalyst, combining the advantages of the homogenous and heterogeneous catalysts (high activity and selectivity, and easy recovery, respectively) (F.R. Hartley, 1985). Carbon materials, adequate as catalyst supports, can be interesting solids for this purpose (C. Freire, 2009). In particular, nanostructured carbon materials are preferred because of the potential confinement effect, as the involved molecules (active species, reagents and products) have nanometric dimensions. The immobilization could NH2 take place by the creation of a siloxane type Rh covalent bond between the metal complex and the carbon surface. This kind of bonding, frequently NH reported for the anchorage of metal complexes on silica and related materials (H. Gao, 1999), was also successfully used for the immobilization of metal complexes on carbon materials (L. Lemus, 2008). The methodology implies the use of a Si complex that contains a trimethoxysilane functionality H3C O O O and the creation of phenol type oxygen groups on CH3 CH3 the carbon surface. A model of the complex used in this work anchored to the support surface by a Figure 1.- Model of the Rh(NN)Si siloxane bond in shown in Figure 1. complex anchored to the support.

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2. Experimental 2.1. Supports Two carbon xerogels, CX1 and CX2, prepared at the Department of Chemical Engineering of the University of Liège (Belgium) have been used as supports. The original samples were grinded and sieved in order to have a particle size between 1.0 and 1.4 mm. The samples were oxidized by treatment with air (40mL/min, 350°C, 3h). Oxidized samples, CX1Ox and CX2Ox, were characterized using the following techniques: Gas adsorption (N2 at 77K and CO2 at 273K), Temperature Programmed Desorption (TPD) and Transmission Electron Microscopy (TEM).

2.2. Synthesis and characterization of the complex Rh(NN)Si The complex [Rh(COD)NH2(CH2)2NH(CH2)3Si(OCH3)3]BF4, named here Rh(NN)Si, was prepared following the reported procedure (L.Lemus, 2008). The complex was characterized using the following techniques: FTIR-spectroscopy, Elemental Analysis and XPS. Results show that the desired complex was obtained.

2.3. Preparation and characterization of hybrid catalysts The procedure used for the preparation of the hybrid catalysts can be briefly described as follows: 200 mg of the oxidized support were put in contact with a methanol solution (60 ml) of complex Rh(NN)Si (20mg, 0.038 mmol). This implies that the maximum amount of Rh loaded in the catalyst can be 2 wt.%. The mixture is kept under reflux and stirring for 21h and then, the hybrid catalyst is recovered by filtration. In order to remove weakly adsorbed species, the samples are submitted to Soxhlet treatment with methanol for 24h. Analysis of the solution shows that the amount of complex removed, corresponds to about 0.1 wt.% Rh in the catalyst. Finally, the samples were vacuum dried at room temperature for 24h. The hybrid catalysts are named CX1Ox-Rh and CX2Ox-Rh and they have been characterized using the following techniques: ICP-OES, Gas adsorption (N2 at 77K and CO2 at 273K), XPS and TEM.

2.4. Cyclohexene hydrogenation Catalytic activity tests for cyclohexene hydrogenation were carried out for 3 hours at 60 °C in a Parr reactor (40 mL), magnetically stirred (1100 rpm), using about 30 mg of the hybrid catalyst, 10 mL of a 5 vol.% of a cyclohexene methanol solution and 10 bar H2. In the case of the homogeneous test, 0.8 mg (2 μmol) of the Rh(NN)Si complex was used. Product analysis was performed by Gas Chromatography (HP6890 Series II, capillary column HP-1 Methyl Siloxane, 30 m x 250 μm x 0.25 μm, FID detector) after each catalytic run. In order to study the reutilization of the hybrid catalysts, after each reaction run, they were filtered in air, washed with fresh dissolvent, and placed in a new reaction. Hydrogen uptake was, also, monitored.

3. Results and discussion 3.1. Characterization of supports and hybrid catalysts The surface area of the supports is about 600 m2/g, but they have different pore size distribution. Figure 2 shows the pore volume distribution of the supports and catalysts. It can be observed that the support CX1Ox has a higher volume of mesopores. Moreover, in a previous study (N. Job, 2005), the maximum pore diameter of samples CX1Ox and CX2Ox was estimated as 26 nm and 10 nm, respectively. Regarding the consequences of the complex anchorage, data of Figure 2 show that there is a partial blockage of the microporosity, while the mesoporosity remains

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cm3/g

almost unchanged. This means that the complex is likely located at the entrance of 0.3 the micropores. TPD results show that both supports have 0.2 only slight differences in surface chemistry, 0.1 and the deconvolution of the CO - TPD profile indicates that the amount of phenol 0 type groups is 780 and 1200 µmol/g in Vµt Vuµ Vmeso supports CX1Ox and CX2Ox, respectively. CX1Ox CX1Ox-Rh Analysis of the Rh content in the two CX2Ox CX2Ox-Rh hybrid catalysts shows that the amount of complex loaded is 78 and 68 µmol/g (0.8 and Figure 2. Pore volume distribution of 0.7 wt.% Rh) in catalysts CX1Ox-Rh and supports and catalysts. CX2Ox-Rh, respectively. The amount of Rh is considerable lower than the maximum available, meaning that only part of the phenol type groups allow the anchorage. Data of the XPS analysis carried out in the fresh supported catalysts showed the binding energy characteristic of Rh(I) (B.E. (Rh 3d3/2)≈ 310 eV) and amine nitrogen (B.E. (N1s)≈ 400,5 eV).

3.2. Catalytic properties of the supported catalysts Figure 3 shows conversion data in five consecutive catalytic runs. 100.00

100

In the first catalytic run, after 3 70.00 70 hours, cyclohexene 60.00 60 50.00 50 conversion is 11% 40.00 40 (TOF= 0.02s-1) and 30.00 30 36% (TOF=0.07s-1) 20.00 20 10.00 10 for samples CX1Ox0.00 0 Rh and CX2Ox-Rh, 1 2 3 4 5 1 2 3 4 5 respectively. For the Catalytic run (3 hours) Catalytic run (3 hours) Rh complex in Figure 3. Conversion data in consecutive runs of cyclohexene homogeneous phase, hydrogenation. conversion is 37% (TOF= 0.08s-1). It is, however, striking that in the second and 120 Cycle 2 consecutive runs the heterogeneous catalysts Cycle 3 100 are very active. Conversion is almost 100% Cycle 1 for catalyst CX1Ox-Rh and above 80% for 80 catalyst CX2Ox-Rh. This means that the 60 complex has suffered an important modification during the first catalytic run that 40 makes it much more active, and a high 20 conversion is kept in four consecutive catalytic runs. 0 This behavior is also proved by the 0 20 40 60 80 100 120 140 160 180 200 Time (min) hydrogen uptake curves. Figure 4 shows the hydrogen uptake corresponding to the three first catalytic runs, carried out with Figure 4. Hydrogen uptake curves obtained for 90

90.00

CX2 Ox-Rh

80.00

Accumulated volume H2

Conversion (%)

80

CX1 Ox-Rh

CX2Ox-Rh

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catalyst CX2Ox-Rh. It can be seen that hydrogenation is very slow in the first run, while it is noticeably faster in the second and third runs. Determination of the Rh content in the used catalysts shows that leaching in samples CX1Ox-Rh and CX2Ox-Rh is 50% and 20%, respectively. A TEM analysis of the used catalysts reveals the presence of small (< 5nm) and dispersed Rh particles in both of them. Figures 5a and 5b show, as an example, TEM images corresponding to used catalysts (5 catalytic runs) CX1Ox-Rh and CX2Ox-Rh, respectively.

Figure 5. TEM image of (a) the CX1Ox-Rh and CX2Ox-Rh catalyst.

XPS analysis of the used catalysts reveals, also, the presence of Rh(I), in a proportion around 70%. This means that the used catalysts contain both, anchored complex molecules and Rh(0) particles. Thus, the high increase in the catalytic activity could be, at least partially, related with the development of metallic Rh particles. Similar catalysts prepared with different carbon materials did not show metallic Rh (L.Lemus, 2008). Additional experiments were carried out at a shorter reaction time, 30 minutes. In the first and consecutive runs, conversion was low, around 5%, without an increasing trend. Thus, it seems that a long first run is needed to produce the modification that makes more active catalysts. TEM analysis of these used catalysts did not show Rh particles. The behavior of both heterogeneous catalysts is, in general terms, similar. The slightly higher conversion, achieved in second and consecutive runs, with catalyst CX1Ox-Rh is probably related to the more open porous structure. In both cases, the heterogenized complex is partially transformed, with reaction time, into a highly dispersed heterogeneous catalyst, which has shown to be very active and recyclable.

4. Conclusions A Rh diamine complex has been anchored on two carbon xerogels of different porous texture. The reduction of the metal complex takes place under reaction conditions and the used catalysts contain the anchored metal complex and Rh particles. The catalysts are very active and fully recyclable for cyclohexene hydrogenation. The supports favor the reduction of the active phase and dispersed metallic particles have been obtained.

References C. Freire, 2009, Carbon-anchored metal complex catalysts in “Carbon Materials for Catalysis”, ed. P. Serp and J.L. Figueiredo, Wiley. H. Gao, 1999, Rhodium-amine complexes tethered on silica-supported metal catalysts. Highly active catalysts for the hydrogenation of arenes, New J. of Chem. 23, 6, 633-640 F.R Hartley, 1985, Supported Metal Complexes, D. Reidel Publ. Co. Dordrecht

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N. Job, 2005, Carbon aerogels, cryogels and xerogels: Influence of the drying method on the textural properties of porous carbon materials, Carbon 43, 12, 2481-2494. L. Lemus, 2008, Effects of confinement in hybrid diamine-Rh complex-carbon catalysts used for hydrogenation reactions, Microporous and Mesoporous Materials 109, 1-3, 305-316.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Coating method for Ni/MgAl2O4 deposition on metallic foams Cinzia Cristiania, Carlo Giorgio Viscontib, Saverio Latorrataa, Enrico Bianchib, Enrico Tronconib, Gianpiero Groppib, Paolo Polleselc a

Politecnico di Milano, Dipartimento di Chimica, Materiali e Ingegneria Chimica “G. Natta”, 20133 Milano, Italy b Politecnico di Milano, Dipartimento di Energia, 20133 Milano, Italy c eni Spa, Divisione Refining&Marketing, Via Maritano 26, 20097 San Donato Milanese, Italy

Abstract A new method to deposit active washcoats of Ni/MgAl2O4 steam-reforming catalysts over FeCrAlloy foams is reported in this work. The MgAl2O4 powdered support was prepared via a co-precipitation method, and Ni was dry-impregnated on it. The deposition of the catalyst over the foams was performed by dip-coating followed by airblowing (dip-blowing). The resulting washcoat layers were quite homogeneous and uniform, and well adherent to the metallic support. Prototype catalyst samples, tested at the lab-scale, were found to be active and selective in the steam reforming process. Keywords: coating, foams, reforming, structured catalysts, preparation

1. Introduction While ceramic honeycomb catalysts are nowadays widely used in a variety of applications (Moulijn and Cybulski, 2006), there is growing interest in the adoption of metallic monoliths and metallic open-pore foams as catalyst support due to their low density, high surface area per unit volume, good heat and mass transfer properties and excellent mechanical strength. The steam reforming of methane (SRM) is an endothermic process and high temperatures are required to activate the hydrocarbon. Elements of group VIII of the periodic system are active for the process in their metallic state. However, supported Ni catalysts are the only materials to be commercially used in this process because Ni is economical compared with noble metals, metallic iron is oxidized at the reaction conditions and metallic cobalt is unstable due to the high H2O/H2 ratios in the reaction mixture (Rostrup-Nielsen, 1984). In particular, Ni/MgAl2O4 has been reported to be a good catalyst for the process (Roh et al., 2007). Industrial reformers consist of a series of tubes, containing pelletized catalysts, located in a furnace. In this configuration, temperature gradients inside the reactor tubes are strong both longitudinally and radially (Basile et al., 2009). Structured catalysts, with conductive (metallic) supports have been proposed for this reason as an alternative to conventional reactors. In a previous paper the preparation of Pd/Al2O3-based catalysts washcoated onto metallic foams was reported by some of us (Giani et al., 2006), using a sol-gel-type procedure. In this work, the development of a simpler method to deposit active washcoats of Ni/MgAl2O4 catalysts over FeCrAlloy (Fe 73 wt. %, Cr 20 wt. %, Al 5 wt. % and Y 2 wt. %) foams is described.

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2. Experimental The MgAl2O4 support was prepared via a co-precipitation method. In a typical experiment the nitrate salts of the constituents (magnesium and aluminum) were dissolved in water (pH≈2). The coprecipitation reaction was performed by pouring this solution into an aqueous solution containing stoichiometric ammonium carbonate (the precipitating agent), stirring and heating at 333 K for 3 h (ageing time). The pH is constant at about 7.5 for all the ageing time. The precipitated material was then filtrated, washed, dried overnight at 373 K and calcined at 1173 K for 10 h (heating rate 2 K/min). Further details are reported elsewhere (Groppi et al., 1994). Ni/MgAl2O4 (Ni = 10% w/w) was prepared according to a classical dry impregnation technique, using an aqueous solution of nickel nitrate. After impregnation, the sample was calcined at 1173K. Adopted FeCrAlloy foams, supplied by Porvair, had a nominal cell size of 10 pores per inch (ppi) and a nominal relative density of 0.05. Before the coating-process, the foams were pretreated to make the surface suitable for the deposition. First the foams were calcined in air at 1173 K for 10 h (heating rate 2 K/min) to allow the migration of α-Al2O3 to the surface (Giani et al., 2006); then, their surface was covered by a thin layer of Al2O3, deposited via vacuum percolation using a quasi-Newtonian dispersion, prepared under magnetic stirring and aged for 72 h, containing 20 wt. % of a fully dispersible pseudobohemite (Disperal®, Sasol) and 0.9 wt. % of HNO3; finally the deposited layer was flash-dried at 553 K for 5 minutes and calcined at 1173 K for 10 h, to form a crystalline layer of γ-Al2O3 on the surface. The washcoat was deposited onto the FeCrAlloy foams by dip-blowing (Nijhuis et al., 2001). Slurries with rheological behavior appropriate for the dip-blowing process were prepared by ball-milling of the Ni/MgAl2O4 powders using HNO3 as dispersant (HNO3/powder = 2.27 mmol/g), water as diluent (added water = 110% of the powder pore volume), ZrO2 balls as grinding bodies (ZrO2/powder = 10 g/g) and following the experimental procedures previously developed in our laboratories (Valentini et al., 2001). The optimal amount of dispersant was determined by titration to evaluate the maximum surface charging of the powder, while the water content took into account the pore volume of the powder (Cristiani et al., 2009). The washcoat was deposited onto the foams by dip-blowing, with an air flow at 5 bar for 10 seconds. Coated foams were flash dried at 553 K for 5 min and then calcined at 1173 K for 10 h. Activity tests of the washcoated samples were run in a 10 mm inner diameter quartz tubular microreactor, which was externally heated by a three zone Carbolite oven. Space velocity was kept at 113000 Ncm3/h/gcat and pressure at atmospheric pressure. The temperature of the catalyst was measured by means of a thermocouple located at the foams inlet. Before the activity tests, loaded foams were reduced in a 5% (v/v) H2 flow in N2, adopting a space velocity of 138750 Ncm3/h/gcat. During the reduction, the temperature was increased up to 1123 K (heating rate 5 K/min), kept at this level for 3h, and then decreased at 293 K (Xu et al., 1989). Reforming tests were carried out following this protocol: pre-reduced catalyst was heated up from room temperature to 923 K in a 5% (v/v) H2 flow in N2, the reacting mixture (CH4 = 1 % (v/v), H2O = 3 % (v/v), N2 complement) was fed, the catalyst was gradually heated up to 1123 K and from here the temperature was gradually decreased until 673 K, measuring the composition of the outlet stream at constant T-intervals (50 K). Products composition was measured by means of an on-line micro-GC (3000 A, Agilent Technologies) equipped with 2 TCD connected to a Molecular Sieve 5 Å column (453 K, Ar as carrier gas) and a Plot Q column (433 K, N2 as carrier gas) for the identification of N2, H2, O2, CH4, CO and CO2, H2O, respectively.

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Coating method for Ni/MgAl2O4 deposition on metallic foams

3. Results 3.1. Catalyst preparation Figure 1 shows the X-ray diffraction pattern of the prepared MgAl2O4, after calcination. The sample is crystalline, and its surface area, calculated according to the BET method in the range of relative pressure 0.05-0.35, is 144 m2/g. Figure 2 shows the flow curves of the dispersions containing the bohemite primer, as function of the ageing time. Its viscosity increases with time, and the rheological behavior changes from non-Newtonian to quasi-Newtonian in the first 72 hours after the preparation. The primer loading, calculated by weighting the foams before and after the impregnation, was found to be always between 2.6 and 3.8 wt. %. Figure 3 shows the flow curve of the slurry after ball-milling for 24 hours. The slurry is a non-Newtonian fluid with a viscosity of 0.05 Pa·s at 10 s-1, appropriate for dip-blowing applications. Upon calcination, a Ni/MgAl2O4 load of 6% w/w was obtained over the foams. The resulting washcoat layers were quite homogeneous and uniform (Figure 4) and well adherent to the support, being the weight loss after 30 min sonication limited to 10% w/w. 1

Viscosity (Pa*s)

72 h 48 h 24 h

0.1

0.01

1E-3 10

20

30

40

50

60

70

80

1

10

100 -1

2θ

Shear rate (s )

Figure 1. XRD pattern of the prepared MgAl2O4

.

Figure 2. Flow curves of the bohemite dispersion.

Viscosity (Pa*s)

1

0.1

0.01

1E-3 1

10

100 -1

Shear rate (s )

Figure 3. Flow curve for the Ni/MgAl2O4 dispersion.

Figure 4. Optical microscopy of the coated foam.

3.2. Catalytic activity tests Measured CH4 conversions and CO selectivities are shown in Figures 5 and 6, respectively. The prepared catalyst is active at temperatures higher then 673 K and, at the adopted process conditions, it allows to reach quantitative CH4 conversion at 873 K. CO selectivity increases with increasing temperature, as a result of the increased reforming rate and the decreased water gas shift rate, and approaches the equilibrium values. Such performances are in-line with those of similar catalysts reported by other groups (Xu, 1989). The stability of the washcoated foams was tested at constant process

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conditions for 5 hours. No significant variations of either the CH4 conversion or the CO selectivity as a function of time on stream were observed during this period, indicating a substantial stability of the prepared sample. 0.8

1.0

0.7 0.6

CO selectivity (-)

CH4 conversion (-)

0.8 0.6 0.4 0.2 0.0 573

Experimental Data Equilibrium

673

773

873

973

1073

1173

Temperature (K)

Figure 5. Measured CH4 conversion and theoretical equilibrium data as a function of the temperature.

0.5 0.4 0.3 0.2 Experimental Data Equilibrium

0.1 0.0 573

673

773

873

973

1073

1173

Temperature (K)

Figure 6. Measured CO selectivity and theoretical equilibrium data as a function of the temperature.

4. Conclusions A steam reforming structured catalyst, consisting of a layer of Ni/MgAl2O4 supported over a FeCrAlloy foam, has been prepared according to a new simple procedure involving the dip-coating of a 10 ppi calcined and primerized foams with a slurry containing a Ni/MgAl2O4 powder. The so prepared catalysts were found to be active and selective in the steam reforming process, having performances similar to those reported in the literature for state-of-the-art Ni/MgAl2O4 powdered catalysts, prepared according to different procedures. Complete CH4 conversion was measured over 1023 K, with CO selectivity values close to equilibrium (at 1023 K measured CO sel. = 58%, equilibrium CO sel. = 65%). Future work will be devoted to investigate the adoption of such catalysts in the steam reforming process, in order to explore the potential associated with improved heat transfer rates and increased catalyst effectiveness factors.

References F. Basile, P. Benito, G. Fornasari, V. Rosetti, E. Scavetta, D. Tonelli, A. Vaccari, 2009, Applied Catalysis B: Environmental, 91, 563–572. C. Cristiani, C.G. Visconti, E. Finocchio, P. Gallo Stampino, P. Forzatti, 2009, Catalysis Today, 147, S24–S29. L. Giani, C. Cristiani, G. Groppi, E. Tronconi, 2006, Applied Catalysis B: Environmental, 62, 121–131. G. Groppi, M. Bellotto, C. Cristiani, P. Forzatti, P.L. Villa, 1999, Journal of Material Science, 34, 2609–2620. G. Groppi, M. Bellotto, C. Cristiani, P. Forzatti, 1994, Journal of Material Science, 29, 3441–3450. J. Moulijn, A. Cybulski (Eds.), 2006, Structured Catalysts and Reactors, 2nd Ed., Taylor & Francis, Boca Raton, FL (USA). T.A. Nijhuis, A.E.W. Beers, T. Vergunst, I. Hoek, F. Kapteijn, J.A. Moulijn, 2001, Catalysis Review Science and Engineering, 43, 345–380. H.S. Roh, K.Y. Koo, J.H. Jeong, Y.T. Seo, D.J. Seo, Y.S. Seo, W.L. Yoon, S.B. Park, 2007, Catalysis Letters, 117, 85–90. J.R. Rostrup-Nielsen, 1984, Catalysis, Science and Technology, Vol. 5, J.R. Anderson and M. Boudart Eds., Springer, Berlin, 3–117. M. Valentini, G. Groppi, C. Cristiani, M. Levi, E. Tronconi, P. Forzatti, 2001, Catalysis Today, 69, 307–314. J. Xu, G.F. Froment, 1989, AIChE Journal, 35, 88–96.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Use of commercial carbons as template for the preparation of high specific surface area perovskites Rita K. C. de Limaa, Edilene D. da Silvab, Ernesto A. Urquieta-Gonzálezb* a

Department of Chemical Engineering – Federal University of Ceara – Campus do Pici, Bloco 709 – Fortaleza – CE – CEP 60455-760 – Brazil b Laboratory of Applied Catalysis – Department of Chemical Engineering – Federal University of Sao Carlos – C. Postal 676 – Sao Carlos – SP – CEP 13565-905 – Brazil

Abstract LaFe1-xCoxO3 (x = 0 or 0.4) perovskites were obtained by nanocasting using Fluka 05120 and Black Pearls 2000 commercial carbons, which act as hard template during the gel formation and subsequently limiting the particle growing during the perovskite formation. Perovskites with the same nominal composition were also conventionally prepared. The results clearly showed that both commercial carbons was efficient to prepare perovskites with nanosized nature and specific surface area substantially improved. In addition, catalytic tests in the reduction of NO with CO evidenced the better activity of the nanocast perovskites. Keywords: Perovskites, Nanocasting, Commercial Carbons, NO reduction, CO

1. Introduction In the last decades, great efforts have been done in order to obtain perovskite-type mixed metal oxides [1] with improved textural properties [2-3]. However, due to the synthesis of these materials be carried out through solid state reactions at high temperatures, the obtained binary, ternary or multinary oxides present lower specific surface areas. At high calcination temperatures severe sintering occurs, producing large particles, while lower temperatures lead to the formation of other phases besides perovskite [4]. As described in our recent paper [5], the preparation of perovskites with better textural and catalytic properties can be successfully achieved by the use of the nanocasting technique. In that work, LaFeO3 and LaFe0.6Co0.4O3 nanoperovskites were synthesized using a porous carbon as hard template, which was obtained from Aerosil 200 pyrogenic silica as cast. It was found that the nanocasting synthesis route resulted in the formation of nanoperovskites with specific surface areas substantially higher than those obtained conventionally. In addition, catalytic tests revealed that the reduction of NO to N2 over such nanoperovskites was significantly improved. In this work, we show that the synthesis of nanoperovskites with similar properties is also possible using commercial carbons as hard template. The application of these carbons is of great interest since they are commercially available in a large variety of structural and textural characteristics, with accessible cost.

2. Experimental Nanosized LaFeO3 and LaFe0.6Co0.4O3 perovskites were obtained by nanocasting using the citrate method [6] and Fluka 05120 activated carbon (Fluka) or Black Pearls 2000

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black carbon (Carbot Corporation) as hard template. A precursor solution was prepared using stoichiometric amounts of La(NO3)3⋅6H2O (Fluka, 99.0%), Co(NO3)2⋅6H2O (Riedel-de Haën, 99.0%) and Fe(NO3)3⋅9H2O (Fluka, 97.0%) salts, deionised water and equimolar amounts of citric acid (Vetec, 95.0 %) and ethylene glycol (Synth, 99.5 %). Subsequently, it was added a pre-established amount of carbon and the suspension stirred for 1 h. The excess of water was evaporated at 60°C for 20 h and in order to convert the obtained gel into the perovskite phase, the carbon-gel composite was thermally treated in static air at 550ºC for 3 h and then at 800ºC for 5 h (4 ºC.min-1). Finally, to ensure the complete carbon removal, the resulting solids were thermal treated under air flow at 550ºC for 6 h (10ºC.min-1). Reference perovskites with the same composition were also conventionally prepared. The solids were characterized by XRD, N2 sorption, TG, SEM, TEM and the catalytic activity evaluated in the reduction of NO with CO between 150 to 700°C using a continues feed of 5 000 ppm of NO and 5 000 ppm of CO diluted in He (V/V) at a GHSV of 15 000 h-1.

3. Results and discussion Figure 1 shows the X-ray diffractograms of LaFeO3 and LaFe0,6Co0,4O3 perovskites conventionally or by nanoscating prepared. From this Figure it is evident that the perovskite structure is the main phase, but low intensity peaks of the La2O3 oxide are present in all the diffractograms, with the ternary nanocast LaFe0,6Co0,4O3 perovskite also showing low intensity peaks of Co3O4 and Fe2O3. Since the most intense diffraction line is a single peak for all the samples (insets in Fig. 1), the orthorhombic symmetry of the conventionally prepared perovskites was the same for the nanocast solids. The wellresolved and intense XRD peaks of the conventional prepared perovskites indicate the formation of very crystalline solids. In contrast, the nanocast solids showed XRD patterns with weak diffraction intensities and enlarged peaks that indicate that these solids are constituted by particles with sizes on the nanometer scale. Application of the Sherrer’s formula suggests crystallite sizes of about 7 nm. According to the textural data showed in Table 1, the nanocasting route resulted in the formation of perovskites with specific surface areas substantially higher, evidencing the efficiency of both carbons as template. However, the Black Pearls 2000 carbon (1500 m2/g) was more effective than the Fluka 05120 carbon (1100 m2/g). In spite of the progresses pointed, the resulting nanocast perovskites synthesized from the procedure applied here still have relatively low surface area when compared to other nanocast oxides obtained at lower temperatures. As verified by TG analyses (not shown), during the thermal treatment at 800°C under static air occurs the transformation of the inorganic precursors together with carbon removal by oxidation, this facilitating the sintering of the formed perovskite particles, diminishing, at least partially, the carbon template effect and preventing to achieve higher specific areas. As can be seen in the SEM images (Figure 2), the conventionally perovskites are formed by monodisperse particles, with irregular morphology and homogeneous size distribution in the range of 0.1 to 0.3 μm. On the other hand, TEM images (Figure 3) confirm the nanosize characteristics of the nanocast perovskites, which consist of agglomerates smaller than 100 nm, which is coherent with the N2 sorption data. The selected-area electron diffraction (SAED) patterns (insets in Fig. 3) consist of single spots superimposed on diffuse rings, indicating that the crystallites making part of the agglomerates are quite small, which is in agreement with the XRD data.

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(c)

LL

Intensity [a. u]

Intensity [a. u]

LL

(b)

LL

L

(f) O

F

O

F

(e)

(a)

20

30

40

50

60

70

80

(d)

90

2 theta degree

20

30

40

50

60

70

80

90

2 theta degree

Figure 1. XRD patterns of: (a) LaFeO3 and (d) LaFe0.6Co0.4O3 conventionally perovskites; (b) LaFeO3 and (e) LaFe0.6Co0.4O3 perovskites nanocasted from Fluka 05120 carbon; (c) LaFeO3 and (f) LaFe0.6Co0.4O3 perovskites nanocasted from Black Pearls 2000 carbon. The capital letters indicate the observed phases as following: (L) La2O3; (O) Co3O4; (F) Fe2O3. Table 1. Textural properties of the prepared perovskites. Sample

Preparation method

Carbon

Symmetry

SBETa (m2/g)

Vpb (cm3/g)

LaFeO3

Conventional

-

Orthorhombic

5.6

0.007

LaFeO3

Nanocasting

Fluka

Orthorhombic

27.9

0.118

LaFeO3

Nanocasting

Black Pearls

Orthorhombic

39.7

0.083

LaFe0.6Co0.4O3

Conventional

-

Orthorhombic

3.6

0.004

LaFe0.6Co0.4O3

Nanocasting

Fluka

Orthorhombic

24.8

0.126

Nanocasting

Black Pearls

Orthorhombic

33.3

0.145

LaFe0.6Co0.4O3 a

b

BET surface area.; Pore volume.

(a)

(b)

Figure 2. SEM images of: (a) LaFeO3 and (b) LaFe0.6Co0.4O3 perovskites prepared by the citrate method. Magnification: 20,000x.

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The catalytic tests (not shown) revealed that the nanocast perovskites were more active in the reduction of NO with CO than those conventionally prepared. Considering that the catalyst tests were carried out using the same contact time and that nanocast and uncast LaFeO3 perovskites have the same composition, the observed higher activity of the former must be related with a higher number of accessible active sites and therefore with its higher specific surface area.

(a)

(b)

(c)

(d)

Figure 3. TEM images of (a) LaFeO3 and (c) LaFe0.6Co0.4O3 perovskites nanocasted from Fluka 05120 carbon; (b) LaFeO3 and (d) LaFe0.6Co0.4O3 perovskites nanocasted from Black Pearls 2000 carbon.

4. Conclusions Both activated and black commercial carbons were efficient to produce the templating effect necessary to prepare nanosized perovskites with specific surface area substantially improved that led to an enhance in their catalytic properties. As expected, the textural characteristics of the employed carbon had a substantial influence on the quality and performance of the produced nanoperovskites.

Acknowledgments The authors acknowledge the financial support provided by CNPq and CAPES, Brazil.

References [1] L.G. Tejuca; J.LG. Fierro, J.D. Tascón, 1989, Structure and reactivity of perovskite-type oxide, Adv. Catal., 36, 237-328. [2] A.E. Giannakas, A.A. Leontiou, A.K. Ladavos, P.J. Pomonis, 2006, Characterization and catalytic investigation of NO + CO reaction on perovskites of the general formula LaxM1-xFeO3 (M = Sr and/or Ce) prepared via a reverse micelles microemulsion route, Appl. Catal A: Gen., 309, 2, 254-262. [3] J. Kirchenova, D. Kwana, J. Vaillancourt, J. Chaouki, 1993, Evaluation of some cobalt and nickel based perovskites prepared by freeze-drying as combustion catalysts, Catal. Lett., 21, 1-2, 77-87. [4] M. Schwickardi, T. Johann, W. Schmidt, F. Schüth, 2002, High-surface-area oxides obtained by an activated carbon route, Chem. Mater., 14, 9, 3913-3919. [5] R.K.C. de Lima, M.S. Batista, M. Wallau, E.A. Sanches, Y.P. Mascarenhas, E.A. UrquietaGonzález, 2009, High specific surface area LaFeCo perovskites - Synthesis by nanocasting and catalytic behavior in the reduction of NO with CO, Appl. Catal. B: Environ., 90, 3-4, 441-450. [6] K. Takehira, T. Hayakawa, H. Harihara, A.G. Andersen, K. Suzuki, M. Shimizu, 1995, Partial oxidation of methane to synthesis gas over (Ca,Sr) (Ti,Ni) oxides, Catal. Today, 24, 3, 237-242.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Ethyl acetate combustion catalyzed by oxidized brass micromonoliths Oihane Sanza,*, Sylvia A. Cruza, Juan Carlos Millána, Mario Montesb, Jose Antonio Odriozolaa a

Inorganic Chemistry Department and Instituto de Ciencia de Materiales de Sevilla (Centro Mixto US-CSIC), Avd. Américo Vespucio 49, 41092 Sevilla, Spain. b Applied Chemistry Department,University of the Basque Country (UPV-EHU), Paseo Manuel de Lardizabal 3, 20018 San Sebastian, Spain. Email: [email protected]

Abstract The thermal treatments of brass micromonoliths were studied in order to generate a highly homogeneous oxide scale strongly attached to the base alloy. In this work it has been shown that the morphology, integrity and homogeneity of the scale are strongly influenced by the temperature and time of treatment. The prepared Cu/ZnO micromonolith were characterized by XRD, GD-OES and SEM, and tested in ethyl acetate total oxidation reaction. The results show that brass oxidation conditions forms different scales being most active when large nano-sheets scale is formed. Keywords: brass oxidation, VOCs oxidation, micromonolith

1. Introduction The catalytic oxidation is the most attractive way to eliminate volatile organic compounds (VOCs) at low concentration in industrial gaseous effluent. For practical applications, the catalyst should be supported on a structured support to treat large gas flows with low pressure drop. Structured catalysts and reactors became one of the most relevant and economically significant applications of catalytic reactor engineering and industrial catalysis so far, mostly due to the commercial success of well-known environmental catalytic processes. Metallic monoliths have excellent mechanical and thermal properties, allow high cell density and are easy to produce. Typically, stainless steels are used, mostly Al-alloyed ferritic, although corrosion and/or embrittlement may result when used at moderate temperatures or in corrosive environments. The use of micrometerscale reaction space allows the precise control of diffusion, heat exchange, retention time and flow patterns in chemical reaction [1]. For low temperature processes the use of brass as structural material of structured support may be an alternative due to its high strength, excellent hot-working properties and corrosion properties. The metallic monolithic structures are usually coated with a catalytic support layer to improve the catalytic activity [1]. This coating is usually obtained by washcoating. The monolith is dipped in a slurry prepared with the catalytic material. In this work we study the oxidation of a brass alloy and its further use as micromonolith for VOCs abatement in order to test the ability of this material for being used as structured catalytic support.

2. Experimental 2.1. Preparation of micromonilths A commercial brass sheet (Cu0.66Zn0.34, 50 μm thick) was used as raw material. Prior to any treatment, the brass sheets were washed with water and soap, cleaned with water and then carefully rinsed with acetone to remove the organic impurities. Micromonoliths

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were prepared by rolling around a spindle alternate flat (360 mm × 30 mm) and corrugated (420 mm × 30 mm) sheets [2]. The final microreactor is a cylinder of 30 mm height; 16mm diameter, a cell density of 1180 cpsi and a hydraulic diameter of 400 mm. Thermal treatments were used for generating an adherent Cu/Zn oxide scale adequate to be used for VOCs abatement. Micromonoliths were oxidized in an 1100 W horizontal furnace. Using a 10 ml min−1 flow of synthetic air from room temperature to the target temperature (500-600°C) at 10 C min−1, maintained for the scheduled time at the target temperature (1-24 h) and then cooled down to room temperature under the same atmosphere.

2.2. Characterization XRD measurements were carried out in a Phillips PW 1729–1820 diffractometer. SEMEDX was used to study the morphology and the profiles of distribution of the principal elements in the samples. Experiments were carried out in a JEOL 5400 equipment to which an energy dispersive spectrometer (OXFORD LINK) was coupled. Adherence of the coating was measured by the ultrasound test [3]. In-depth compositional analysis of both the oxide scale and the catalytic layer were determined by Glow Discharge Optical Emission Spectroscopy (GD-OES) experiments using a LECO GDS 750A spectrometer. The catalytic activity of the prepared monoliths was measured for the complete oxidation of ethyl acetate in air. Ignition curves were obtained by heating up to 400°C at 1.5°C/min the monoliths in a 500 ml/min air stream containing 467 ppm of ethyl acetate (1000 mgC/Nm3).

3. Results and discussion 3.1. Brass oxidation The influence of time and temperature on weight gain (ΔW) and properties of the oxidized layer, upon air oxidation, are presented in Table 1. When increasing temperature and oxidation time, ΔW and scale thicknesses are increased. At both temperatures as expected, ΔW shows a pseudoparabolic behaviour for the formation of a protective oxide scale. In every cases, the adherence of these scales is very high (always higher than 95%) slightly decreasing with time and temperature increase. This decrease, resulting in scale cracking and spalling, arises as a result of the mismatch between thermal expansion coefficients of the oxide layer and the metal. The oxide coefficient is lower for all relevant temperatures [3,4]. Table 1. Oxidized brass micromonolith properties. Oxidation conditions

T (ºC)

T (h)

Catalytic activity

Scale properties ΔW mg/monolith

Thickness Aa (μm)

1 23.8 0.6 2 29.7 1.0 500 4 37.5 1.4 12 54.9 1.8 24 57.7 1.9 1 46.0 2.3 2 104.1 3.0 600 4 143.4 3.6 12 205.0 4.0 24 275.8 4.3 a The total scale thickness measured by GD-OES b The outer scale thickness measured by GD-OES

Thickness Bb (μm)

Adherence (%)

T50c (ºC)

T100d (ºC)

0.18 100 0.20 100 0.28 100 0.17 99 0.15 98 0.12 98 0.05 97 0.03 96 0.00 95 0.00 95 c 50% conversion temperature d 100% conversion temperature

323 318 315 322 325 322 336 355 356 355

346 340 335 353 354 358 364 388 388 490

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Ethyl acetate combustion catalyzed by oxidized brass micromonoliths

The XRD patterns of oxidized monoliths (see Fig.1) present diffraction lines corresponding to ZnO and CuZn alloy. Diffraction lines that would be associated to copper or other alloy elements containing phases are completely absent. The tendency to form ZnO in oxygen atmospheres is explained by zinc and copper diffusivities as well as by the differences in the free energy for ZnO and CuxO formation [5]. The oxidation treatment has a strong influence on the relative intensity of the diffraction lines. At 500 ºC, the ratio between (002) plane and (100) or (101) is c.a. 2.8. Although, at 600 ºC the ratio is c.a. 0.2. Therefore, the measured intensity ratio clearly indicate that the (002) plane is preferentially exposed at 500 ºC. This is in agreement with the morphological modification observed in the SEM images (Fig. 3). C uZn ( 200)

C uZ n (111)

Z nO (101)

ZnO (100) ZnO ( 002)

ZnO ( 110)

Z nO (102)

600º C-24h

Intensity

6 00ºC-4h

6 00ºC-1h 500 ºC-24h

500º C-4h 500ºC-1 h

30

35

40

45

2θ

50

55

60

Fig. 1. XRD spectra of oxidized brass at different conditions.

A

B

Fig. 2. GD-OES depth profile of brass after different oxidation conditions.

E nanosheets

C

D

Compact scale Brass

Fig. 3. SEM micrograph of brass foil treated at different conditions: top view (A) 500ºC-1 h, (B) 500ºC-4 h, (C) 600ºC-1 h and (D) 600ºC-4 h; lateral view (E) 500ºC-4 h

The morphology of the oxide scale formed upon oxidation at 500 ºC consists in fine ZnO nano-sheets pointing outward the alloy surface. These nano-sheets grow when increasing the oxidation time (Fig. 3A and 3B). At 600ºC (Fig. 3C) a more dense oxide scale is formed consisting in almost equiaxial ZnO crystals that grow in size with oxidation time (Fig. 3D). The scales are formed by two oxide layers as clearly seen in the cross-section of the scale formed upon air oxidation at 500ºC for 4 h (Fig. 3E). The inner scale is dense and featureless and the outer one consists in fine ZnO nano-sheets pointing outward the alloy surface. Chen et al. [5] have reported modifications of the morphology of ZnO crystals grown in the alloy surface as a function of temperature, time and atmosphere. GD-OES experiments provide information on the in-depth composition of the oxide scale. In Fig. 2 a selection of the in-depth profiles illustrates the influence of time and temperature in structure and composition of the oxide scale. A complex structured oxide scale formed by an outer layer of thickness B (Table 1) consisting of Zn, Cu and O, in

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most cases, and an inner layer of thickness A (Table 1) formed almost exclusively by ZnO is observed. This observation is in agreement with the SEM data.

3.2. Catalytic activity The combustion of ethyl acetate is used to test the prepared catalytic structures. The micromonolith showed good activity in the ethyl acetate combustion (Table 1) despite the BET surface area of the oxidized monoliths is too low to be measured the ASAP 2020 apparatus. Under the experimental conditions of this study, only the total oxidation products, CO2 and H2O, were detected. The conversion of ethyl acetate starts at 240-260ºC and is completed below 350-390ºC. The results show that the ethyl acetate combustion depends on the formed oxide structure on brass. The micromonolith covered by nano-sheets presented higher activity. The most active one was obtained by oxidizing brass at 500ºC for 4 h. That corresponds to the larger nano-sheets scale formation and to the higher Cu content in the outer scale surface (Table 1 and Fig. 2). In previous works, our group has study the preparation of steel monoliths for ethyl acetate combustion in the same reaction conditions [6,7]. The oxidized steel monoliths, AISI 304 and FeCrAlloy, did not show activity in VOCs combustion. Although, manganese oxide catalysts either washcoated or grown in situ on steel monoliths showed 100% conversion of ethyl acetate at much lower temperatures than the brass micromonolith, ca. 250ºC. The use of brass as structured catalyst is promising since it is easier to prepare it with very high adherence and is not necessary to coat another catalyst because the oxide scale formed on brass show activity in VOCs combustion. Moreover, it is commercially available at low cost and may to resist chloride-containing atmospheres.

4. Conclusion When using brass for catalytic purposes, it is necessary to generate an oxide scale highly homogeneous and strongly adherent to the base alloy. In this study, it has been shown that the morphology, integrity and homogeneity of the scale are strongly influenced by temperature and time of the treatment. The formed scale is composed of ZnO containing variable amounts of CuxO as detected by GD-OES. The oxidized micromonolithic reactors are active in the total oxidation of ethyl acetate even considering their negligible surface area. The maximum activity is observed for the monolith oxidized at 500ºC for 4h. This monolith present a surface formed by ZnO crystals exposing the (002) plane and has the maximum copper content in the outer surface layer.

Acknowledgments Financial support by MEC (MAT2006-12386-C05) and Junta de Andalucía (P06-TEP01965) is gratefully acknowledged.

References 1. 2. 3. 4. 5. 6. 7.

A. Gavriilidis, P. Angeli, E. Cao, 2002, Chem. Eng. Research & Design 80, 3-30 P. Avila, M. Montes, E.E. Miro, 2005, Chem. Eng. J. 109, 11-36 L.M Martinez, O. Sanz, M.I. Domínguez, M.A. Centeno, J.A. Odriozola, 2009, Chem. Eng. J., 1, 191-200 A. Paúl, J.A. Odriozola, 2001, Mater. Sci. Eng. A ,300, 22-33 W.J. Chen, W.L. Liub, S.H. Hsieh, T.K. Tsai, 2007, App. Surf. Sci., 253, 6749-6753 D.M. Frías, S. Nousir, I. Barrio, M. Montes, L.M. Martínez T, M.A. Centeno, J.A. Odriozola, 2007, Applied Catalysis A: General, 325, 205-212 B.P. Barbero, L.C. Almeida, O. Sanz, M. R. Morales, L. E. Cadus, M. Montes, 2008, Chem. Eng. J., 139, 430-435

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Preparation of CMI-1 supported H3+xPMo12−xVxO40 for the selective oxidation of propylene Siham Benadji,a,b,c Pierre Eloy,c Alexandre Léonard,d Bao–Lian Su,d Chérifa Rabia,a Eric M. Gaigneaux,c a

Laboratoire de Chimie du Gaz Naturel, Faculté de Chimie, Université des Sciences et de la Technologie Houari Boumediene (U.S.T.H.B.), B.P: 32 El-Alia, 16111 Bab-Ezzouar, Alger, Algeria b Centre de Recherche Scientifique et Technique en Analyses Physico-Chimiques (C.R.A.P.C.), B.P 248 Alger RP, 16004, Algeria c Unité de catalyse et de chimie des matériaux divisés, Croix du Sud 2/17, Université catholique de Louvain, B-1348 Louvain-la-Neuve, Belgium d Laboratoire de Chimie des Matériaux Inorganiques (CMI), I.S.I.S, Facultés Universitaires Notre-Dame de la Paix, 61 rue de Bruxelles, B-5000 Namur, Belgium

Abstract Catalysts, comprising 30 wt.% of heteropolyacids (HPAs) H3+xPMo12−xVxO40 (x = 0−3) supported on a mesoporous material CMI-1 by dry impregnation were characterized by several techniques. Their catalytic performances were compared to those of bulk HPAs in the propylene oxidation by molecular oxygen at 350 °C. The supported HPAs exhibit higher catalytic activity in propylene oxidation than the bulk ones and favoured the formation of acrolein, acetaldehyde and acetic acid, compared to the mother catalysts which lead only to the formation of COx. The enhanced oxidation catalytic activity of supported systems was attributed to the fine dispersion of H3+xPMo12−xVxO40 species on the CMI-1 mesoporous material via physical adsorption, together with the fact that the structure of both CMI-1 and HPAs remained intact during the impregnation. Keywords: HPA; mesoporous silica; CMI-1; dry impregnation; propylene oxidation

1. Introduction In Keggin type phosphovanadomolybdate acids (HPAs), vanadium appeared to be the most crucial element for oxidative processes and their strong acidity is known to be favourable to functionalization of alkanes [1]. However, the major drawback of this type of materials is their low surface area (< 10 m2/g). In order to overcome this disadvantage, a possible solution is to support them on materials with large surface area. The mesoporous silicates are materials which possess large surface area and pore volume with uniform mesopores and high thermal stability [2, 3]. Among them, the most studied is MCM-41. CMI-1 has the same characteristic as MCM-41. It can be prepared using a non toxic surfactant and can thus be an attractive support in catalysis. In the present investigation, H3+xPMo12−xVxO40 with x=0−3 were chosen for their high acidity and high oxidative power and CMI-1 for its large surface area, as catalytic systems towards selective oxidation of propylene. HPAs were supported on CMI-1 at 30 wt.% by dry impregnation and characterized by different techniques. The catalytic performances of supported HPAs were compared to those of bulk ones in the propylene oxidation by molecular oxygen at 350°C.

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2. Experimental 2.1. Sample preparation CMI-1 and H3+xPMo12−xVxO40 (x=0−3) were prepared as reported by Léonard et al. [4, 5] and Tsigdinos et al. [6, 7], respectively. The HPAs were denoted by V0, V1, V2 and V3, for x=0, 1, 2 and 3, respectively. The 30 wt.% HPA/CMI-1 were prepared by dry impregnation of CMI-1 with an aqueous solution of HPA (pH 0.75) followed by drying at 50 °C for 20 h in air and were noted: V0CMI-1, V1CMI-1, V2CMI-1 and V3CMI-1.

2.2. Characterization and catalytic measurements ICP analyses were measured on an Iris Advantage apparatus from Jarrell Ash Corporation. BET measurements were performed using a Micromeritics Tristar 3000 equipment. Powder XRD patterns were recorded in the 2θ range between 0.5° and 10° using a Panalytical X’pert Pro and between 10° and 70° on a Siemens D5000 diffractometer. SEM analysis was performed with a LEO 983 GEMINI microscope. Transmission and diffuse reflectance (DR) FT−IR spectra were recorded at room temperature with a Equinox 55 (Bruker) spectrometer. TG and DTA were carried out in a 100 mL min–1 air stream with a Mettler Toledo TGA/SDTA apparatus. The propylene oxidation was performed at atmospheric pressure in a fixed-bed reactor.

3. Results and discussion 3.1. Characterization The results of chemical analysis of HPAs adjusted considering 12, 11, 10 or 9 atoms of molybdenum per Keggin unit, were found in good agreement with desired stoichiometries for P, Mo and V. For the impregnated samples, the experimental P/Mo/V/Si molar ratio was very close (< 2 wt.% experimental errors) to that in the formula for each HPAcontaining sample, indicating that HPAs did not collapse after anchoring on the CMI-1 and that the quantities of metals used were preserved [3]. Table 1 shows that BET surface area (ca. 1100 m2/g) and pore volume (0.92 cm3/g) of CMI-1 decrease strongly after impregnation of 30 wt.% HPAs of a factor of 2 while the other parameters (∅BJH , d100, a0 and WTt) remain practically the same. This suggests that the mesoporous structure of the support was preserved in all synthesized materials. The decrease of surface area and pore volume of mesoporous supports has been attributed either to the blockage of the mesopores by HPAs or to the partly collapse of the walls of mesopores [3]. The sorption isotherms of pure CMI-1 along with HPA/CMI-1 have the shape of the type IV isotherm with an H1 hysteresis [4, 5]. The capillary condensation occurs at similar P/P0 values (ca. 0.4), suggestive of constant pore sizes. The pore size distributions are centered between 28.4 and 30.8 Å. XRD patterns typical of triclinic system were observed for all bulk heteropolyacids. At small angles, for CMI-1, in addition to a sharp strong peak (1 0 0) at 2θ = 1.8°, two weaker peaks (1 1 0) and (2 0 0) at 2θ = 3.1° and 2θ = 3.6° are detected (Fig. 1(A)). The presence of these two last peaks indicates a hexagonal organization of the channels [4, 5]. According to Bragg’s rule, the unit cell dimension (a0) can be deduced and is about 56 Å. With impregnation of HPAs, the position of the sharp peak remains almost constant (Fig. 1(A)), indicating that a0 does not vary. Nevertheless, the three reflections become less resolved and their intensity decreases after impregnation. These observations indicate that the framework structure of the CMI-1 was preserved with a partial collapse having occurred upon incorporation of HPAs [3]. At high angles (Fig. 1(B)), the XRD pattern of CMI-1 presents a broad line between 2θ = 15 and

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40° assigned to the amorphous silica [3]. No HPA crystal phase corresponding to their secondary structure is observed, which indicate that the heteropolyanionic species do not form aggregates with a sufficient size to exhibit intense diffraction peaks but are rather finely and molecularly dispersed on the CMI-1 surface [3]. Table 1. Physical properties of various samples. H2Oa (wt.%) (a) (b) 1.71 0.92 4.33 3.84 8.23 3.64 3 8.5 3.64 3 9.8 3.74

Material CMI-1 V0CMI-1 V1CMI-1 V2CMI-1 V3CMI-1

Pore volumee (cm3/g) 0.92 0.49 0.45 0.47 0.48

∅BJH d (Å) 30.8 28.4 28.8 28.7 28.5

SBET (m2/g) 1114c 634b 610b 627b 622b

a0f (Å) 56.1 56.1 55.7 55.7 55.7

d100 (Å) 48.6 48.6 48.2 48.2 48.2

WTtg (Å) 25.3 27.7 26.9 27 27.2

a Weight loss from TGA: (a) 1 from 25 to 550 and (b) 2 from 550 to 900; (a) 3 from 25 to 150 and (b) 4 from 150 to 600 °C; b BET surface area measured after evacuation at 130 and c at 350°C; d Mean pore diameter determined from BJH desorption dV/dD pore volume; e Single-point adsorption total pore volume at P/P0 = 0.98−0.99; f Unit cell parameter a0 = 2d100 /√3; g Average thickness of walls WTt = a0 - ∅BJH.

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Fig. 1. (A) Low and (B) high-angle X-ray patterns of (a) CMI-1, (b) V0CMI-1, (c) V1CMI-1, (d) V2CMI-1 and (e) V3CMI-1.

4400 4200 4000 3800 3600 3400 3200 3000 2800 2600 2400 2200 2000 1800 1600 1400 1200 1000 800

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Fig. 2. DRIFT spectra of (a) pure V1, (b) CMI-1, (c) V0CMI-1, (d) V1CMI-1, (e) V2CMI-1 and (f) V3CMI-1.

From SEM measurements, gyroids, toroids and ropes of 5 μm are the main observed morphologies for CMI-1 [5] and are retained upon the impregnation procedure. The IR spectra of bulk HPAs are characteristic of the Keggin structure [8]. The IR spectrum of CMI-1 exhibits vibration bands in region 3700−400 cm−1 corresponding to (Si−O−Si), SiO4 and surface OH groups [3]. The IR spectra of all supported samples are similar, all vibration bands corresponding to Keggin structure were partially or fully overlapped with those of the support in the 1200−400 cm−1 region excepted that corresponding to νas(Mo=Od) and νas(Mo−O−Mo) at ca. 960 and ca. 870 + ca. 800 cm−1, respectively, indicating that the Keggin structure is preserved [3]. In DRIFT (Fig. 2), the spectra of HPAs show the characteristic bands of the Keggin structure [3]. The spectrum of CMI-1 shows vibration bands in the 3750−800 cm−1 region assigned to the isolated terminal silanol groups, silanol groups inside the channels, to Si−O−Si, Si−OH and Si−O− groups [3]. For supported samples, the band at 3750 cm−1 of the support strongly decreases in intensity and is shifted of 4 cm−1 toward lower wavenumbers, suggesting a high dispersion of HPAs at the outer surface of the support with a non-homogeneous vibration of the isolated silanol groups induced by the presence of H2+xPMo12V3−xO40−

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anion [3]. In the 1800−400 cm−1 region, all vibration bands corresponding to Keggin structure are fully overlapped by those of the support excepted those corresponding to metal−oxygen bands (ca. 870, ca. 960 and ca. 800 cm−1), which are still present at the same positions showing that the Keggin structure is not affected by the silica support. The TG curves of HPAs/CMI-1 are similar to that of bulk HPAs with a gradual weight loss up to 600°C, attributed to the departure of water molecules of HPAs and probably to remaining traces of the organic surfactant used in the material synthesis. The exothermic peaks assigned to the decomposition of HPAs are observed at a temperature higher (560−590°C) than that corresponding to bulk HPAs (405−460°C). This suggests that the CMI-1 support stabilizes the HPAs likely by the formation of (≡SiOH2+)(H2+xPMo12−xVxO40−) surface species more stable than the free acid form [3].

3.2. Reactivity in propylene oxidation No product was formed in the absence of catalyst. In all samples, carbon is not in balance that is attributed to the acrylic acid polymerization. Bulk HPAs and CMI-1 (Table 2) show a low conversion (Conv. < 5%) and COx are the only products observed. For HPAs/CMI-1, the propylene conversion increases strongly up to ca. 21−26% and the formation of oxygenated compounds (acetaldehyde (Act), acrolein (Acr) and acetic acid (AcA)) that seems to be independent on the vanadium number in the acid, is favored. This can be attributed to the accessibility of active sites further to the fine and homogeneous dispersion of HPAs on the high surface CMI-1 [3]. The improved production of oxygenated compounds can be explained by the fact that mesoporous materials stabilize the HPAs, preserving their more selective catalytic behaviour and delay their degradation to MoO3, known to be less selective. Table 2. Oxidation of propylene by O2 over all catalysts at 350 °C after 5 h on stream a. S. Act (%) S. Acr (%) S. AcA (%) Catalyst Conv. (%) S. COxe (%) CMI-1b 4.1 1.5 0.0 0.0 0.0 V1c 3.2 5.4 0.0 0.0 0.0 V0CMI-1d 26.1 50.2 6.1 2.8 2.5 V1CMI-1d 24.5 42.7 5.9 3.4 2.8 V2CMI-1d 21.7 44.5 6.5 3.4 2.5 V3CMI-1d 20.9 42.1 5.3 4.0 2.9 a Feed gas: C3H6: 3 mL min−1, O2: 6 mL min−1, He: 21 mL min−1; b Catalyst mass; 210 mg; c Catalyst mass; 90 mg; d Catalyst mass; 300 mg; e COx; CO+CO2; S. : selectivity.

4. Conclusion In this work, characterization results showed that dry impregnation of H3+xPMo12−xVxO40 on CMI-1 stabilizes the HPA structure and leads to systems more active and selective in propylene oxidation. These results are promising in the use of supported HPAs on mesoporous materials for the oxidative reactions.

References [1] A. Predoeva, S. Damyanova, E.M. Gaigneaux, L. Petrov, App. Cat. A 319 (2007) 14−24. [2] G. Satish Kumar, M. Vishnuvarthan, M. Palanichamy, V. Murugesan, J. Mol. Cat. A 260 (2006) 49−55. [3] S. Benadji, P. Eloy, A. Léonard, B.L. Su, K. Bachari, C. Rabia, E.M. Gaigneaux, Micro. Meso. Mat. 130 (2010) 103−114. [4] J. L. Blin, A. Léonard, B. L. Su, Chem. Mater. 13 (2001) 3542−3553. [5] A. Léonard, J. L. Blin, M. Robert, P. A. Jacobs, A. K. Cheetham, B. L. Su, Langmuir 19 (2003) 5484−5490. [6] G.A. Tsigdinos, Ind. Eng. Chem., Prod. Res. Dev. 13 (1974) 267−274.

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[5] A. Léonard, J. L. Blin, M. Robert, P. A. Jacobs, A. K. Cheetham, B. L. Su, Langmuir 19 (2003) 5484−5490. [6] G.A. Tsigdinos, Ind. Eng. Chem., Prod. Res. Dev. 13 (1974) 267−274. [7] G.A. Tsigdinos and C.J. Hallada, Inorg. Chem. 7, 3 (1968) 437–441. [8] C. Rocchiccioli-Deltcheff, M. Fournier, R. Frank, R. Thouvenot, Inorg. Chem. 22 (1983) 207–216.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Direct addition of the precursor salts of Mo, Co or Ni oxides during the sol formation of γ-Al2O3 and ZrO2 – The effect on metal dispersion Eduardo Prado Bastona, Ernesto A. Urquieta-Gonzaleza a

Federal University of São Carlos, Department of Chemical Engeneering, C. postal 676, CEP 13565-905, São Carlos-SP, Brazil

Abstract Mo, CoMo and NiMo supported oxides were successful prepared by direct addition of the precursors salts during the γ-Al2O3 and ZrO2 sol-gel synthesis procedure. Thermogravimetric analysis (TGA), X-ray diffraction (XRD), nitrogen sorption and hydrogen temperature programmed reduction (H2-TPR) were used to characterize the gels, the supports and the incorporated Mo-, CoMo- and NiMo-oxides. High specific surface area mesoporous γ-Al2O3 and ZrO2 were obtained with the metallic Mo, Co and Ni species highly distributed. As expected Co and Ni species promoted the reduction of Mo at lower temperatures. Nevertheless, the Co and Ni promoting effect in the reduction of Mo was more pronounced when the Mo species are supported on ZrO2. This behavior makes ZrO2 especially attractive for the preparation of hydrodesulfurization (HDS) catalysts. Keywords: HDS catalysts; Mo-Co-Ni; γ-Al2O3, ZrO2, in situ addition

1. Introduction Due to the increase of the SOx emissions, the environmental regulations have strongly diminished the limits of the sulfur content in fuels. Catalytic hydrodesulfurization of petroleum feedstocks has been a strategic process to produce clean fuels [1] and the requirement to improve the catalyst performance becomes the most important issue. In this sense, the researches are devoted to study the nature of the support and the active species [1-4]. Traditional HDS catalysts are Al2O3 supported Mo or W sulfides promoted by Co or Ni sulfides. Supports like ZrO2 [1-3], TiO2 [1,3] or mesoporous molecular sieves [4] or ZrO2-Al2O3 mixed oxides have also been proposed [5]. In the present work, Mo-, CoMo- and NiMo-oxides were prepared introducing the precursor metallic salts directly during the sol preparation (in situ addition) of γ-Al2O3 and ZrO2 supports under controlled pH conditions.

2. Experimental section 2.1. Preparation of γ-Al2O3 and ZrO2 and in situ Mo-, CoMo- and NiMosupported oxides via the sol-gel procedure The supports and the metallic supported oxides were prepared using the procedure described by Lebihan et al. (1994) [6]. Firstly, aluminum tri-sec-butoxide (ATSB, Aldrich 97%) or zirconium propoxide (PZr, Aldrich 70%) and 2-butanol (Aldrich, ≥ 99%) were mixed at 85°C under stirring for 30 min. Then, an appropriate amount of 1,3-butanediol (Aldrich, 99+%) was added to the solution and left under stirring for 1 h. Subsequently, the hydrolysis step was done by mixing deionized water and maintaining

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the stirring for 1 h. Finally, the stirring was stopped and the gel aged at room temperature for 1 h. The suspension was dried in a rotary evaporator under reduced pressure at 45°C during 48 h. The molar composition of the gel was: 1 ATSB or PZr : 8.5 2-butanol: 5 1,3-butanediol : 10 H2O. In the in situ addition of the Mo, Co or Ni salts done during the hydrolysis step was firstly added to the sol a solution of ammonium heptamolybdate tetrahydrate having pH=9 and then added a solution of cobalt (II) nitrate hexahydrate or nickel (II) nitrate hexahydrate. The resulting gel suspension was filtered, the separated gel washed, dried overnight at 100°C and finally calcined at 500°C for 4 h. The samples were prepared with a 12 wt % of MoO3 and 3 wt % of CoO or NiO.

2.2. Characterization Thermogravimetric analyses (TGA) of the powders were carried out on a DSC-TGA TA Instruments, model SDT 2960, with a heating rate of 10°C.min-1 until 1000°C under air flow. X-ray diffraction (XRD) were obtained on a Rigaku Multiflex Diffractometer operating with CuKα (λ=1.5406 Ǻ) radiation. The specific surface area and mean pore diameter were measured from N2 sorption/desorption isotherms at 77 K in a Quantachrome Corporation Nova-1200 apparatus. The H2-TPR analyses were carried out on a SAMP3 equipment having a thermal conductivity detector (TCD) using 60 mg of sample under 25 mL.min-1 of H2 (5 %) in N2 flow with a heating rate of 10°C.min-1.

3. Results and discussion 3.1. Thermogravimetric Analysis (TGA) The TGA curves (not shown) of the zirconia and alumina supports show between 25 and 200°C a weight loss attributed to dehydration and other two events at higher temperatures mainly due to decomposition and combustion of organic compounds [6-7]. The TGA for samples containing the precursor of the metallic salt of Mo, Co and Ni incorporated in situ no significant in weight loss profile was observed.

3.2. X-ray diffraction The XRD patterns of the Mo, CoMo and NiMo oxides supported on ZrO2 or γ-Al2O3 are shown in Figure 1. It can be observed for the ZrO2 support (Figure 1a) the presence of the monoclinic and the predominant tetragonal phases. In the case of solids containing the Mo, Co and Ni oxides supported on ZrO2, the diffractograms show only the tetragonal phase (Figures 1b, 1c and 1d, respectively). This behavior could suggest that with the incorporation of the precursor salts in the sol occurs a substantially decrease in the specific surface free energy of the zirconia favoring the formation of the tetragonal phase [8-10]. The diffractogram of γ-Al2O3 (Figure 1e) confirm that the synthesis procedure allowed the preparation of γ-Al2O3, whose peak intensities diminishes after incorporation of Mo, Co or Ni species (Figures 1f, 1g and 1h, respectively). This intensity loss can be mainly attributed to the strong X-ray absorption of Mo [11]. No diffraction peaks attributed to Mo, Co or Ni species are observed in the diffractograms of Figure 1, that could suggest that the Mo species are well dispersed on the studied supports and constituted by small crystals amorphous to the X-ray radiation [3-4].

3.3. N2 sorption

The textural characteristics of the synthesized materials are given in Table 1. It can be seen that ZrO2 shows smaller specific surface area and pore volume than γ-Al2O3, which is in accordance with the literature where these materials presented specific surface areas in the range of 60 to 100 m2.g-1 and of about 400 to 490 m2.g-1 for ZrO2 and γ-Al2O3, respectively [1,2,4,11,12]. On the other hand, ZrO2 shows higher mean pore diameter (DBJH), which would have the advantage to facilitate the diffusion of bulk

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molecules. From Table 1 it can also be observed that the in situ prepared Mo, Co or Ni supported oxides presented similar or a slightly higher specific surface area than the pure supports. This behavior was attributed to the pH control of the solution of ammonium heptamolybdate tetrahydrate. * * * (d)

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Figure 1. X-ray diffractograms of: (a) ZrO2; (b) Mo/ZrO2; (c) CoMo/ZrO2; (d) NiMo/ZrO2; (e) γ-Al2O3; (f) Μο/γ-Al2O3; (g) CoΜο/γ-Al2O3; (h) NiMo/γ-Al2O3. Table 1 – Textural properties of the prepared solids*. Catalyst γ-Al2O3 Mo/γ-Al2O3 CoMo/γ-Al2O3 NiMo/γ-Al2O3 ZrO2 Mo/ZrO2 CoMo/ZrO2 NiMo/ZrO2

SBET (m2.g-1) 451 488 428 460 82 83 136 115

DBJH (nm) 5.0 5.4 5.1 4.8 11.2 8.0 7.6 9.1

Vt (cc.g-1) 0.74 0.67 0.58 0.57 0.23 0.12 0.26 0.16

*DBJH: mean pore diameter; SBET: specific surface area; Vt: total pore volume

3.4. H2 Temperature Programmed Reduction (H2-TPR)

The H2-TPR profiles of Mo, CoMo and NiMo supported oxides are shown in Figure 2. The profiles exhibit two main H2 consumption areas attributed to the reduction of Mo species in the sequence from low to higher temperatures: Mo+6 → Mo+4 → Mo0 [2,4,11]. The H2-TPR profiles of the bimetallic CoMo and NiMo supported oxides (Figures 2b, 2c, 2e and 2f) are similar to the pure Mo supported oxide, however, a decrease in the reduction temperature of Mo is observed due to the promoter effect of Co or Ni [13]. From Figure 2 it is also seen that the H2-TPR profiles of the Mo, CoMo and NiMo species incorporated in ZrO2 show reduction peaks at lower temperatures.

4. Conclusions Mo, CoMo and NiMo supported oxides were successful prepared by in situ addition during the sol-gel preparation of γ-Al2O3 and ZrO2 with the simultaneous adjusting of pH during the hydrolysis step . High specific surface area mesoporous γ-Al2O3 and ZrO2 were obtained with the metallic Mo, Co and Ni species highly distributed. As expected Co and Ni species promoted the reduction of Mo at lower temperatures. Nevertheless,

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H2 consumption (a.u.)

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the Co and Ni promoting effect in the reduction of Mo was more pronounced when the Mo species are supported on ZrO2. This behavior makes ZrO2 especially attractive for the preparation of HDS catalysts.

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Figure 2 – H2-TPR profiles of: (a) Mo/ZrO2; (b) CoMo/ZrO2; (c) NiMo/ZrO2; (d) Mo/γ-Al2O3; (e) CoMo/γ-Al2O3; (f) NiMo/γ-Al2O3.

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]

Y. Okamoto, K. Ochiai, M. Kawano, K. Kobayashi, T. Kubota, 2002, Effects of support on the activity of Co–Mo sulfide model catalysts, Appl. Catal. A, 226, 115-127. L. Kaluza, M. Zdrazil, 2007, Preparation of zirconia-supported hydrodesulphurisation catalysts by water-assisted spreading, Appl. Catal. A, 329, 58-67. J. Escobar, M.C. Barrera, J.A. Reyes, J.A. Toledo, V. Santes, J.A. Colín, 2008, Effect of chelating ligands on Ni–Mo impregnation over wide-pore ZrO2–TiO2, J. Mol. Catal.A,287, 33-40. S. Garg, K. Soni, G.M. Kumaran, M. Kumar, J.K. Gupta, L.D. Sharma, G.M. Dhar, 2008, Effect of Zr-SBA-15 support on catalytic functionalities of Mo, CoMo, NiMo hydrotreating catalysts, Catal. Today, 130, 302-308. G. Li, W. Li, M. Zhang, K. Tao, 2004, Morphology and hydrodesulfurization activity of CoMo sulfide supported on amorphous ZrO2 nanoparticles combined with Al2O3, Appl.Catal. A, 273, 233-238. L. Lebihan, C. Mauchaussi, L. Duhamel, J. Grimblot, E. Payen, 1994, Genesis and Activity of Mo-Based Hydrotreating Catalysts Prepared by a Sol-Gel Method, J. Sol-Gel Sci. Tech., 2, 837-842. V. Santos, M. Zeni, C.P. Bergmann, J.M. Hohemberger, 2008, Correlation Between Thermal Treatment and Tetragonal/Moniclinic Nanostructured Zirconia Powder Obtained by Sol-Gel Process, Reviews Adv. Mat. Sci., 17, 62-70. B.H. Davis, 1984, J. Am. Ceramic Soc., 67, C-168. J.H. Adair R.P . Denkewicz, 1990, Ceramic Powder Science, 3, 25. B.M. Reddy, V. Angala, R. Reddy, 2000, Influence of SO42- , Cr2O3, MoO3 and WO3 on the stability of ZrO2-tetragonal phase, J. Mat. Sci. Letters, 19, 763- 765. O.Y. Gutiérrez, D. Valencia, G.A. Fuentes, T. Klimova, 2007, Mo and NiMo catalysts supported on SBA-15 modified by grafted ZrO2 species: Synthesis, characterization and evaluation in 4,6-dimethyldibenzothiophene hydrodesulfurization, J. Catal., 249, 140-153. F. Dumeignil, K. Sato, M. Imamura, N. Matsubayashi, E. Payen, H. Shimada, 2003, Modification of structural and acidic properties of sol–gel-prepared alumina powders by changing the hydrolysis ratio, Appl. Catal. A, 241, 319-329. T. Klimova, M. Calderón, J. Ramírez, 2003, Ni and Mo interaction with Al-containing MCM-41 support and its effect on the catalytic behavior in DBT hydrodesulfurization, Appl. Catal. A, 240, 29-40.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Glycothermal synthesis as a method of obtaining high surface area supports for noble metal catalysts W. Walerczyk, M. Zawadzki, J. Okal Institute of Low Temperature and Structure Research, Polish Academy of Sciences, P.O. Box 1410, 50-950 Wrocław, Poland

Abstract The catalysts for total VOC oxidation, among others noble metals supported on various materials, are still examining and all research are going to improve their catalytic properties. It is many possibilities to enhance catalyst activity and stability, for example using as supports nanocrystalline spinel type oxides with high specific surface area. Solvothermal syntheses, with their advantages in terms of high reactivity of reactants and easy control of solution reactions, are one of the methods of obtaining catalytic materials with desired properties. This work presents the microwave-assisted (MW) glycothermal synthesis as a method of obtaining nanosized mixed metal oxides with spinel structure, on the example of zinc aluminate ZnAl2O4, used as support for Pt or Pd combustion catalysts. Keywords: solvothermal synthesis, VOC combustion, zinc aluminate

1. Introduction Glycothermal synthesis seems to be very attractive method for catalytic materials preparation, especially when microwave heating was applied. Research carry on this field indicate on many advantages of this method as short synthesis time, phase purity with better yield, homogeneity and high reproducibility [1-3]. Various glycols may be used as reaction medium, and different reaction parameters (temperature, pressure, time, and others) can be applied what has influence on the properties of obtained materials [4]. As a result, it provides direct and effective one step route to prepare nanomaterials of well-controlled properties, which can be used as catalyst or catalyst support. Mixed metal oxides with spinel structure, like ZnAl2O4, are interesting materials from catalytic point of view because of their properties like high thermal and chemical stability, high mechanical resistance and good metal dispersion capacity [5]. The main disadvantage of zinc aluminate spinel, prepared by the conventional methods like the ceramic, sol-gel or coprecipitation method, is its low surface area as compared with traditional supports such as γ-Al2O3 or SiO2 [6]. Recently, it was found that solvothermal route allows to obtain nanocrystalline mixed metal oxides with high specific surface area, including ZnAl2O4 and other spinel type oxides [7,8]. The aim of the current work was to prepare and characterise ZnAl2O4 using microwave-assisted glycothermal method, and test Pt and Pd catalysts supported on it in total oxidation of diluted isobutane, selected as a VOC molecule. Reaction parameters were optimized to obtain low crystalline zinc aluminate spinel with high specific surface area.

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2. Experiment 2.1. Sample preparation To prepare ZnAl2O4 under MW glycothermal conditions were used: zinc acetate (Zn(CH3COO)2 *2H2O, POCH Poland) and aluminum isopropoxide (Al[OCH(CH3)2]3, Alfa Aesar) in molar ratio 1:2 as a zinc and aluminum precursor and water solution (75 vol %) of 1,4-butanediol as reaction medium. Syntheses were performed in teflon vessel placed in microwave accelerated reaction system (MW Reactor Model 02-02, ERTEC Poland). Reaction time was optimized as 30 min, and temperature and pressure as 200oC and 25 bar, respectively. Reaction product was washing several times to remove organic compounds using acetone and water and then extruded into a wires, air-dried, crushed and finally calcined at 550oC. Support obtained in this way was impregnated through conventional wet impregnation method with water solution of platinum hydrochloric acid to obtain 1 and 3 wt% Pt concentration on ZnAl2O4; samples assigned as Pt1 and Pt3, respectively. The same procedure was employed to obtain 1 and 3 wt% Pd/ZnAl2O4; samples denoted as Pd1 and Pd3, respectively. All catalysts were reduced in hydrogen atmosphere and finally were washing with boiling water to remove Cl- ions.

2.2. Experimental methods All samples were studied using X-ray powder diffraction (XRD) to phase identification and average grain size calculation; XRD patterns, including Small Angel X-Ray Scattering (SAXS), were detected using X’Pert-Pro PANalytical instrument. Elemental analysis and the impurity identification were done by using energy dispersive X-ray (EDS) analysis, EDAX9800. High resolution transmission electron microscopy (HRTEM) images and selected area electron diffraction (SAED) patterns were done with Philips CM20 SuperTwin microscope to study morphology and microstructure of the samples. Nitrogen adsorption-desorption isotherms at –196ºC were collected using an Autosorb-1 Quantachrome to examine samples textural properties. Metal dispersion in catalysts was found from H2 chemisorption measurements performed on home-made volumetric glass apparatus equipped with a high vacuum system. Catalytic activity tests were performed in a fixed-bed flow reactor, at atmospheric pressure with volume ratio isobutane/air=1:500; reaction products were analyzed by gas chromatography.

3. Results and discussion XRD diffractograms, shown in Fig. 1 for selected samples, confirmed spinel structure a- Pd3 and low crystallinity of zinc aluminate: b- Pt3 as-prepared and after catalysts preparation. c- ZnAl O as prepared Average grain size calculated using d- pattern of ZnAl O Scherrer equation amount to 3 nm for the as-prepared ZnAl2O4 support, and 5-6 nm for the catalyst samples. SAXS method, employed only to the a b as-prepared sample, shown that crystallite c size is in the range 2.08 - 3.42 nm with d 20 30 40 50 60 70 80 average crystallite size amount 2.75 nm 2 θ, degree what is in good agreement with results of Fig. 1. XRD patterns of Pd3, Pt3 and ascalculation using Scherrer equation. XRD prepared ZnAl2O4 samples. patterns confirm also phase purity of support. Unfortunately, the diffraction peaks due to Pt or Pd crystallites are not Intensity, a.u.

2

4

2

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appeared for all catalysts except Pd3 sample, for which one week and broad peak at 2Θ=40,8 is observed what can be assigned to Pd presence. This result indicate on low crystallinity and good metal dispersion for all Pt and Pd catalysts supported on zinc aluminate spinel prepared using MW glycothermal method. It is very promising because high dispersion of active metal phase usually provide to her lower concentration at retaining the same catalytic activity. Selected HRTEM images of prepared samples are shown in Fig. 2 indicating on low crystallinity of bare support and good dispersion of Pt and Pd particles what was additionally confirmed by H2 chemisorption measurements, which revealed very high dispersion close to 90 % for low loaded (1wt%) catalysts while with increasing metal loading to 3 wt.% the dispersion decreased to about 60 %. For all catalysts, it was difficult to identify Fig. 2. HRTEM images of ZnAl2O4 heated at 550 oC and Pt1, Pt3 metal particles by and Pd3 samples. HRTEM images or SAED patterns since they are too small to exhibit lattice fringes or visible diffraction features. We only assume that round and dark spots with sizes in the range of 0.8-1.6 nm for Pt1, and 1.11.7 nm for Pt3 catalyst are metal crystallites, though we have no positive evidence for that (except close correspondence of these sizes with data obtained from chemisorption: mean Pt particle size was 1.3 and 2 nm, respectively for Pt1 and Pt3). Similar correlation was found for Pd catalysts but it could be noticed that when palladium was supported on ZnAl2O4 slightly higher metal particle sizes were formed but there are still small and rather thin. N2 adsorption-desorption measurements showed that ZnAl2O4 prepared under MW glycothermal conditions is mesoporous material with very high specific surface area SBET, which amount to 400 m2/g. After heat treatment at 550°C and impregnation process, considerable decrease of surface area was observed and SBET amount to 140 and 100-130 m2/g, respectively. It suggests that sintering occurs during the heating process and some pore blockage by metal particles during the impregnation process. Nevertheless, it could be noticed that catalysts supported on ZnAl2O4 as spinel type oxide prepared in this way show much better textural properties as compared to that ones prepared by conventional methods [6].

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Isobutane conversion, %

The combustion of isobutane was studied by obtaining light-off curves and results are reported in Fig. 3, indicating on good catalytic properties of all samples. The low temperature required for 50% of isobutane conversion (T50) exhibit Pt1 and Pd3 sample while a little higher T50 was observed in the case of Pt3 and Pd1sample. Lower activity of Pd1 and Pt3 as compared to Pd3 and Pt1, respectively, may result from lower palladium concentration and platinum dispersion, respectively. Moreover, the 100 lower activity of some samples may be Pd1 also caused by incomplete chlorine ion Pd3 80 removing. EDS analysis showed the Pt1 presence of Cl in all samples but the 60 Pt3 highest (2.53 at%.) and the lowest (0.35 40 at.%) content of chlorine was found in Pt3 (the least active) and Pt1 (the most active) 20 sample, respectively. The evolution of the catalytic activity for isobutane oxidation 0 reaction as a function of time on stream 100 150 200 250 300 350 400 450 500 o was also measured for all samples, at Temperature, C constant temperature. It was observed that Fig. 3. Isobutane conversion over: Pt1, isobutane conversion has not changed Pt3 Pd1 and Pd3 after 6 h time on line indicating on high stability of the catalyst under reaction conditions.

4. Conclusion Obtained results showed that microwave-assisted glycothermal method provides direct and effective route to prepare nanosized zinc aluminate with spinel structure and enhanced textural properties like high specific surface area and mesoporous structure. The surface area of ZnAl2O4 considerably decreases after further heat treatment and impregnation process but still remains significant. Based on our catalytic and characterization data it was revealed that Pt or Pd supported on ZnAl2O4 prepared under MW glycothermal conditions may be appropriate as catalysts for combustion of diluted isobutane and probably other light alkanes. MW glycothermal method could be also used to prepare other spinel type oxides to improve their properties for catalytic applications.

Acknowledgments This work is financially supported by Polish Committee for Scientific Research (Grant No. N N507 500738).

References 1. 2. 3. 4.

M. Inoue, 2004, Glycothermal synthesis of metal oxides, J. Phys-Condens. Mat., 16, S1291S1303. J. Panpranot, N. Taochaiyaphum, P. Praserthdam, 2005, Glycothermal synthesis of nanocrystalline zirconia and their applications as cobalt catalyst supports, Mater. Chem. Phys., 94, 2-3, 207-212. S. Cho, J. Noh, S. Park , D. Lim, S. Choi, 2007, Morphological control of Fe3O4 particles via glycothermal process, J. Mater. Sci., 42, 13, 4877-4886. M. Inoue, H. Kominami, T. Inui, 1994, Synthesis of large pore-size and large pore-volume aluminas by glycothermal treatment of aluminium alkoxide and subsequent calcination, J. Mater. Sci., 29, 9, 2459-2466.

Glycothermal synthesis as a method of obtaining high surface area 5. 6. 7. 8.

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M. Inoue, H. Otsu, H. Kominami, T. Inui, 1991, Synthesis of Double Oxides Having Spinel Structure (ZnAl2O4, ZnGa2O4) by the Glycothermal Method, J. Chem. Soc. Jpn.,7, 10361038. M. Valenezuela, P. Bosch, S. Reijne, B. Zapata, H. Brongersma, 1997, The influence of the preparation method on the surface structure of ZnAl2O4, Appl. Catal. A, 148, 315-324. M. Zawadzki, 2007, Pd and ZnAl2O4 nanoparticles prepared by microwave-solvothermal method as catalyst precursors, J. Alloy Compd. 439, 312-320. M. Takesada, M. Osada, T. Isobe, 20 09, Characterization of ZnGa2O4:Mn2+ nanophosphor synthesized by the solvothermal method in 1,4-butanediol–water system, J. Phys. Chem. Solids 70, 281-285.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Synthesis and characterization of cok-12 ordered mesoporous silica at room temperature under buffered quasi neutral pH Jasper Jammaer,a Alexander Aertsa, Jan D’Haenb, Jin Won Seoc, Johan A. Martensa a

Catholic University of Leuven, Centre for Surface Chemistry and Catalysis, Kasteelpark Arenberg 23, B-3001 Heverlee, Belgium b Hasselt University, Institute for Materials Research, Wetenschapspark 1, B-3590 Diepenbeek, Belgium c Catholic University of Leuven, Department of Metallurgy and Materials Engineering, Kasteelpark Arenberg 44, B-3001 Heverlee, Belgium

Abstract A procedure to synthesize ordered mesoporous silica denoted COK-12 under mild conditions is presented. A P6m ordered mesoporous silica with uniform pores is synthesized at room temperature and quasi neutral pH. The synthesis makes use of P123 triblock copolymer in an aqueous citric acid / sodium citrate buffer solution and sodium silicate as Si precursor. Synthesis examples of COK-12 materials are presented and the materials characterized using small angle X-ray scattering, nitrogen sorption, electron microscopy, 29 Si MAS NMR and thermogravimetric analysis. Keywords: Ordered mesoporous silica, COK-12, buffer, room temperature synthesis

1. Introduction Ordered mesoporous silica materials are of interest to various application fields including e.g. catalysis, molecular sieving, optics and drug delivery [1-5]. The increasing interest in these materials explains why research efforts have been paid to develop convenient synthesis procedures. The popular MCM-41 and SBA-15 materials typically are synthesized using quaternary ammonium and nonionic polyalkylene block copolymer surfactants as supramolecular structure directing agents, respectively [1, 6-7]. MCM-41 and SBA-15 syntheses usually are performed under hydrothermal conditions in alkaline and strongly acidic aqueous solution, respectively. Several investigations on ways to simplify the synthesis procedure have been published [8-11]. The expensive TEOS as Si source has been successfully replaced by sodium silicate solution [12]. A synthesis at quasi neutral pH has been achieved by neutralizing an acidic surfactant solution with an equivalent amount of hydroxide anions introduced with an alkaline sodium silicate solution [13]. Such acid-base neutralization procedure, however, may be difficult to realize at large scale. Syntheses carried out at room temperature tend to yield disordered mesoporous silica material [13]. Recently we reported a convenient procedure which enables synthesis of high quality ordered mesoporous silica at room temperature and stable quasi neutral pH using a buffered solution and a cheap silicon source [14]. Here we present more examples and discuss the influence of synthesis parameters.

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2. Experimental 2.1. Synthesis procedure A standard synthesis comprises dissolution of 4.0 g of triblock copolymer P123 (BASF, Belgium) in 107.5 g H2O. The buffer is prepared by adding 3.68 g citric acid monohydrate (Riedel-de Haen, Germany) and 2.54 g trisodium citrate (UCB, Belgium). The solution is stirred overnight. 10.4 g sodium silicate solution (10 %wt NaOH, 27 %wt Merck, Germany) is further diluted with 30.0 g of water. This solution is added to the surfactant solution under stirring at 150 rpm with a mechanical mixer. Stirring is stopped after 5 min. and the resultant mixture kept at room temperature for 24 h. The synthesis is performed in PP bottles and all solutions stored at room temperature prior to mixing. The product is recovered by vacuum filtration and washed with 300 ml distilled water. The material is dried at 60°C and calcined at 300°C for 8h and 550°C for another 8h with heating ramps of 1°C/min.

2.2. Characterization 2D-Small angle x-ray scattering (SAXS) patterns were recorded at the European Synchrotron Radiation Facility (ESRF, Grenoble, France) on Beamline BM-26B. The detector (2D gasfilled) distance was 2.50 m and the X-ray wavelength 1.03 Å. The scattering vector q-scale was calibrated using Ag behenate crystals. Nitrogen adsorption isotherms were recorded at -196.8°C on a Micromeritics Tristar 3000. Degassing was performed at 200°C under nitrogen flow. The mesopore size was determined using BJH method on the adsorption branch of the isotherm. TEM was performed on a Philips CM 200 FEG; SEM on a Philips XL 30 FEG. 29Si MAS NMR spectra were recorded on a Bruker AMX 300 spectrometer operating at a 29Si resonance frequency of 100 MHz under conditions of magic-angle spinning at 5 kHz. 4000 scans were accumulated with a recycle delay of 60 s. The 29Si MAS spectra are referenced to Si(CH3)4.

3. Results and discussion The use of a buffer was inspired by the need to prevent a pH shifting upon addition of alkaline sodium silicate solution to triblock copolymer solution. Citric acid/citrate buffer was selected because of the three pKa values of citric acid in the near neutral pH range and the high buffering capacity. The initial pH of the buffer was selected between 4 and 6. Thanks to the buffer the pH change upon mixing silicate source and surfactant solution was limited to 1 pH unit. Typical values of the synthesis parameters including pH, synthesis temperature and buffer strength, and sodium/silicate ratios are reported in Table 1. SAXS patterns (Fig 1.A) showed strong reflections indicating a high degree of ordering of the meso structure. The reflections could be indexed according to p6m hexagonal symmetry. The d10 spacing was in the range 8.0-8.7 nm for COK-12 materials synthesized at room temperature. An ageing step at 90°C caused an increase of d10 to over 10 nm (Table 1). The template was removed from COK-12 material by calcination. TGA revealed a 30-45%wt weight loss in the region below 200°C ascribed to the decomposition of the block copolymer. Figure 1.B illustrates an N2 adsorption isotherm of calcined COK-12(5) synthesized at room temperature, and of COK-12(8) that underwent an ageing step at 90°C in the synthesis. Both isotherms are of type IV with an H1 hysteresis loop, typical for ordered mesoporous material. The steep adsorption and desorption branches in the nitrogen adsorption isotherm evidence the narrow pore size distribution (inset in Figure 1). Materials synthesized at room temperature typically exhibited pore sizes around 4.3 – 6.3 nm; ageing at 90°C caused an enlargement of the mesopores to a diameter of 9.5-10.4 nm (Table 1). The wall thickness was estimated from pore diameters according to nitrogen adsorption and d10 –

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spacings from SAXS (Table 1). The hexagonal ordering, the pore diameters and wall thicknesses were confirmed using TEM (Fig. 2). The hexagonal ordering on the mesoscale was also reflected in the particle morphology. Table 1. Synthesis parameters and structural properties of COK-12 materials.

a c

Samples

pHa

1 2 3 4 5 6 7c 8c

5 3.5 4.6 5.1 5.6 6.0 4.6 5.6

Buffer strength M 0.17 0.26 0.26 0.26 0.26 0.26 0.26 0.26

[Na+]/ [SiO2] mol/mol 1.13 0.76 1.50 1.83 2.14 2.31 1.52 2.15

d10b nm

dp nm

Wall nm

BET m² g-1

Vp cm³ g-1

8.7 8.0 8.4 8.6 8.6 8.4 10.3 10.2

5.8 4.3 5.0 5.2 5.9 6.3 9.4 10.5

4.3 4.9 4.7 4.8 4.1 3.5 2.5 1.3

410 305 398 467 323 429 536 350

0.45 0.30 0.41 0.45 0.40 0.50 0.88 0.85

pH measured 30 s after addition of sodium silicate; b d10 spacing of the calcined materials; synthesis was performed at room temperature followed by a ageing step at 90°C.

A

B

Fig. 1 (A) SAXS patterns of calcined COK-12(5) (black) and COK-12(8) (grey); (B) Nitrogen adsoption („) – desorption (†) isotherms of calcined COK-12(5) (black) and COK-12(8) (grey). Inset: mesopore size distribution.

In these syntheses a change of pH of the buffer was associated with a change of the sodium concentration. The pH value and/or the sodium content of the synthesis mixture had an influence on the pore size (Table 1). In the sample series COK-12(2)-COK12(6), the pH was increased from 3.5 to 6.0 and the [Na+]/[SiO2] ratio from 0.76 to 2.31. Upon changing these parameters the pore size increased from 4.3 nm to 6.3 nm. In parallel with this evolution the wall thickness decreased from 4.9 nm to 3.5 nm. The presently observed widening of the mesopores upon increasing the temperature to 90°C (COK-12(7) and (8), Table 1) has been observed with other ordered mesoporous materials making use of triblock copolymers.[15] Since the COK-12 material is readily formed at room temperature, the swelling of the triblock copolymer micelles upon heating likely occurs after the formation of an initial mesoporous silica material. The enlargement of the mesopores was accompanied by a thinning of the mesopore walls supporting this interpretation.

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Interestingly, in all COK-12 samples of Table 1 the micropore volume was lower than 0.08 cm³g-1. The rather thick walls of COK-12 synthesized at room temperature render COK-12 a high stability. 29Si MAS NMR spectroscopy revealed a high condensation degree of the silica composing the pore walls. The spectra of COK-12(1) showed three signals at ca. -92 ppm, -101 ppm and -110 ppm, assigned to Q2, Q3 and Q4, respectively, with an intensity distributions of 3%, 36%, and 61%. The high amount of fully condensed Si Q4 species suggest a high stability of the silicate structure. Fig 2. TEM perpendicular to the mesopores of COK-12(5) (left); and parallel with mesopores of COK-12(7) (right).

4. Conclusions Ordered mesoporous silica COK-12 can be prepared at room temperature and quasi neutral pH, making use of sodium silicate. The use of a citric acid/sodium citrate buffer ensures a stable pH. The mesopore size is dependent of the pH and the [Na+]/[SiO2] ratio. Swelling of the triblock copolymer micelles in the pores upon heating at 90°C causes an increase of pore size and thinning of the mesopore walls. The COK-12 pore walls contain little micropores and the silicate network is highly condensed.

5. Aknowledgements AA acknowledges FWO for a postdoctoral fellowship. JAM acknowledges the Belgian Government for supporting an IAP-PAI network and the Flemish government for a concerted research action (GOA) and long-term structural funding (Methusalem).

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature, 359 (1992) 710. A. Corma, Chem. Rev., 97 (1997) 2373. M. Vallet-Regí, F. Balas, D. Arcos, Angew. Chem., Int. Ed., 46 (2007) 7548-7558. B.J. Scott, G. Wirnsberger, G.D. Stucky, Chem. Mater., 13 (2001) 3140-3150. R. Mellaerts, C.A. Aerts, J. Van Humbeeck, P. Augustijns, G. Van den Mooter, J.A. Martens, Chem Commun, (2007) 1375. D. Zhao, J. Feng, Q. Huo, N. Melosh, G.H. Fredrickson, B.F. Chmelka, G.D. Stucky, Science, 279 (1998) 548. D. Zhao, Q. Huo, J. Feng, B.F. Chmelka, G.D. Stucky, J. Am. Chem. Soc., 120 (1998) 6024. E.B. Celer, M. Jaroniec, J. Am. Chem. Soc., 128 (2006) 14408. Z. Jin, X. Wang, X. Cui, Colloids Surf., A, 316 (2008) 27. A. Sayari, B.-H. Han, Y. Yang, J. Am. Chem. Soc., 126 (2004) 14348. J.M. Kim, G.D. Stucky, Chem Commun, (2000) 1159. K. Kosuge, T. Sato, N. Kikukawa, M. Takemori, Chem. Mater., 16 (2004) 899. S.S. Kim, A. Karkamkar, T.J. Pinnavaia, M. Kruk, M. Jaroniec, J. Phys. Chem. B, 105 (2001) 7663. J. Jammaer, A. Aerts, J. D'Haen, J.W. Seo, J.A. Martens, J. Mater. Chem., 19 (2009) 8290. E. Prouzet, T.J. Pinnavaia, Angew. Chem., Int. Ed., 36 (1997) 516.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Spray drying of porous alumina support for Fischer-Tropsch catalysis Anna Linda, Rune Myrstada, Sigrid Erib, Torild Hulsund Skagsethb, Erling Rytterb, Anders Holmenc a

SINTEF Materials and Chemistry, NO-7465 Trondheim, Norway Statoil R&D, Research Centre, Postuttak, NO-7005, Trondheim, Norway c Department of Chemical Engineering, Norwegian University of Science and Technology (NTNU), NO-7491 Trondheim, Norway b

Abstract Porous alumina particles, that are to be used as support in Fischer-Tropsch catalysis, have been produced by spray drying of spherical alumina primary particles, where the material is formed by agglomeration of the primary particles. We have investigated the influence of both instrumental parameters, as well as the condition of the suspension of primary particles on the porosity, as well as the particle size and morphology of the materials. Some of the materials have also been impregnated with cobalt and rhenium by the incipient wetness method and tested in a fixed bed reactor.

1. Introduction Spray drying is an established method that produces solid particles by spraying a suspension of droplets, followed by a drying process. As a result of spraying a liquid into very small droplets in the order of tens to hundreds of micrometers there is a large surface area available for heat and mass transfer, and this makes it a very efficient drying method [1]. Porous materials can be prepared by agglomerating nanoparticles by spray drying. Agglomeration of silica nanoparticles to form porous particles, where the porosity originates from interparticular voids, has been reported on previously [2-4]. Lind et al. have reported on spherical mesoporous silica particles with a hierarchical porosity by spray drying of two different types of mesoporous materials [5, 6]. A highly interconnected bimodal pore network is obtained due to the intra- and inter-particle pore systems within the spherical agglomerates. The production of porous silica materials with an organized porosity by spray drying a mixture of silica nanoparticles and polystyrene latex beads has been reported on by Iskandar et al. [7-9] and Lind et al. [10]. In this paper the preparation of porous alumina supports for Fischer-Tropsch catalysis was studied by spray drying of various starting materials, such as 1) differently sized non-porous alumina particles and 2) non-porous alumina particles together with polystyrene latex particles.

2. Experimental The materials were prepared according to the following procedures: the primary particles were weighed and dispersed in de-ionized water by stirring for minimum 60 minutes prior to spray drying. The agglomeration of the materials was preformed using a Labplant SD 05 spray dryer. Nitrogen sorption measurements were preformed on a Beckman Coulter SA 3100 to determine the BET specific surface area, pore volume, and average pore size. The particle size measurements and attrition tests were performed on a Malvern Scirocco 2000 instrument. The particle size is determined by laser diffraction,

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and by applying different pressures on the material during the measurement the stability is also measured. The morphology of the particles was studied by a Zeiss Ultra 55 scanning electron microscope. The catalytic performance of the catalyst was tested in a fixed bed reactor. The experimental setup and methods for the fixed bed testing are described by Borg et al. [11].

2.1. Agglomeration of non-porous alumina primary particles The formation of porous alumina materials has been studied by agglomeration of nonporous alumina particles, using three different alumina materials from SASOL, with the following particle sizes: 25, 120 and 240 nm. To study the effect on particle size, porosity and morphology, the samples were prepared in the concentration range 5-40 wt% using three different nozzle sizes (0.5, 1.0 and 2.0 mm) for all three starting materials. Various types of binders were also tested for some materials. These binders were 1) aluminum nitrate added at low concentration (0.5 and 1.0 wt%), 2) zinc nitrate added at a concentration of 5% Zn, and 3) 6 nm silica particles added at 1.0 wt%. The following parameters of the spray dryer were kept constant for all the experiments: pump speed 10 rpm (equals 8.1 ml/min), drying temperature 200°C, and fan speed 50 (maximum speed, equals 4.3 m/s). After spray drying the materials were calcined in 550ºC for 6 h, with a heating ramp of 1ºC/min.

2.2. Agglomeration of non-porous alumina particles with polystyrene latex particles An other type of alumina material was prepared by spray drying polystyrene latex particles of 260 nm from Magsphere Inc. together with two different alumina powders, respectively. The alumina powders used were Aeroxide Alu C fumed alumina with a diameter of 13 nm from Degussa, and a γ-alumina with a diameter of 20 nm from Alfa Aesar. To study the effect on the pore structure and morphology various ratios of alumina/polystyrene latex was investigated (1:3, 2:3, 1:1, and 3:2), the total concentration was increased from 2 wt% to 10 wt%, and instrumental parameters, such as pump speed, air flow, drying temperature and nozzle size were varied. After spray drying, the materials were calcined in 650ºC for 5 h, with a heating ramp of 1ºC/min, in order to burn out the polystyrene latex particles, and consequently induce the porous structure.

3. Results and discussion 3.1. Agglomeration of non-porous alumina primary particles Particle size distributions for three various materials measured at 0.15 bar and 3 bar are shown in Figure 1. The results show that independent of the concentration of solid, nozzle size, or addition of aluminum nitrate, zinc nitrate, or silica particles, the mean particle size of the agglomerates is between 5-10 µm, with a small amount of large particles (>100 µm). The results also indicate that the material is quite stable, only the large particles will break at higher pressure during attrition testing. Nitrogen sorption measurements were preformed to determine the BET surface area, the average pore size, and the pore volume. Some of the materials were impregnated with 20% cobalt and 0.5% rhenium by the incipient wetness method using cobalt nitrate hexahydrate and perrhenic acid, and tested in a fixed bed reactor. The results from the N2-sorption measurements as well as the fixed bed testing of three selected materials are listed in Table 1. The data from the fixed bed testing are reported as CO reaction rate (rCO) and selectivity for C5+ hydrocarbons. These results are in agreement with previously reported data [12].

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Spray drying of porous alumina support for Fischer-Tropsch catalysis Particle Size Distribution

13

10

9 8.5 8

10 9.5 9 8.5 8

7.5 7

6

Volume (%)

Volume (%)

6.5

5.5 5 4.5 4 3.5 3 2.5

7.5 7 6.5 6 5.5 5 4.5 4 3.5 3 2.5 2 1.5

2 1.5 1 0.5 0 0.1

Particle Size Distribution

12.5 12 11.5 11 10.5

9.5

1

10

100

600

1 0.5 0 0.1

1

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P article S ize (µm )

P article S iz e (µm )

35% Dispal, 3.0, 1. juli 2009 11:40:13 20% Dispal + 1/2% B inder, 3,0 bar, 1. juli 2009 11:47:24 10% Dispal + 5% Zn, 3,0B ar, 1. juli 2009 11:59:23

35% Dispal, 0,15, 1. juli 2009 11:38:33 20% Dispal + 1/2% B inder, 0,15bar, 1. juli 2009 11:49:55 10% Dispal + 5% Zn, 0,15B ar, 1. juli 2009 11:57:11

Figure 1. Particle size distributions for materials prepared from 1) 35% alumina (120 nm), 2) 20% alumina (120 nm) + 0.5% Al(NO3)3, and 3) 10% alumina (120 nm) + 5% Zn, all spray dried with a 1.0 mm nozzle measured at 0.15 and 3.0 bar, respectively. Table 1. Results from N2-sorption measurements and fixed bed testing for three different materials spray dried with a 1.0 mm nozzle and water as balance. The fixed bed data were measured at 210°C, 20 bars and 45% CO conversion. 35% Alumina (120 nm)

20% Alumina (120 nm) + 0.5% Al(NO3)3

10% Alumina (120 nm) + 5% Zn(NO3)2

Pore Volume (ml/g)a 2

BET Surface Area (m /g)

0.50

0.55

0.53

a

138

137

128

a

15-20

15-20

15-20

0.051

0.051

0.039

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82.5

Mean Pore Diameter (nm) rCO (molCO/gCo*h)

b b

C5+ Selectivity (%) a

b

N2-sorption data; Fixed bed data

The materials were furthermore characterized by scanning electron microscopy to study the morphology. The SEM study shows that all the various materials prepared will consist of densely packed spherical particles.

3.2. Agglomeration of non-porous alumina particles with polystyrene latex particles From scanning electron microscopy it was seen that increasing the alumina/polystyrene latex ratio a denser material is obtained, and that the particle size can be increased by increasing the total concentration of solid and by increasing the nozzle size. Varying the other instrumental parameters (pump speed, air flow, drying temperature) did not change the particle morphology or pore structure, but they were important to insure efficient drying and a higher yield. The material was fluffy, and there was no welldefined pore structure originating from the latex particles, as seen in a previous study on agglomeration of silica particles with polystyrene latex [10]. Two typical SEM images of a) a spray dried and b) spray dried and calcined material are shown in Figure 2. These materials are still to be tested in the fixed bed reactor.

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a)

b)

1 µm

1 µm

Figure 2. SEM images of agglomerates of 13 nm alumina primary particles together with 260 nm polystyrene latex particles, before (a) and after calcination (b).

4. Conclusions Porous alumina supports for Fischer-Tropsch catalysis have been prepared by spray drying of various primary particles. By agglomeration of differently sized non-porous alumina particles a porous material is obtained, where the porosity originates from voids between the primary particles. These particles are densely packed, with a specific pore size, and a spherical morphology. By agglomeration of non-porous alumina particles together with polystyrene latex particles, a porous material is obtained, where the porosity originates from voids between the alumina particles, as well as from annihilation of the latex particles. These particles have a spherical morphology, and as they are quite loosely packed, they have a broad pore size distribution. The catalytic performance correspond well to what has previously been reported in the litterature.

References [1] [2]

M. Fareed, Chem. Eng. Sci., 58 (2003) 2985. F. Iskandar, I.W. Lenggoro, T.-O. Kim, N. Nakao, M. Shimada, K. Okuyama, J. Chem. Eng. Jpn., 34 (2001) 1285. [3] F. Iskandar, I.W. Lenggoro, B. Xia, K. Okuyama, J. Nanoparticle Res., 3 (2001) 263. [4] C. du Fresne von Hochenesche, K. K. Unger, T. Eberle, J. Mol. Catal. Chem., 221 (2004) 185. [5] C. du Fresne von Hochenesche, V. Stathopoulos, K.K. Unger, A. Lind, M. Lindén, Stud. Surf. Sci. Catal., 144 (2002) 339. [6] A. Lind, C. du Fresne von Hochenesche, J.-H. Smått, M. Lindén, K.K. Unger, Microporous Mesoporous Mat., 66 (2003) 219. [7] F. Iskandar, Mikrajuddin, K. Okuyama, Nano Lett., 1 (2001) 231. [8] F. Iskandar, Mikrajuddin, K. Okuyama, Nano Lett., 2 (2002) 389. [9] F. Iskandar, Mikrajuddin, K. Okuyama, Encyclopedia of Nanoscience and Nanotechnology, 8 (2004) 259. [10] A. Lind, C. du Fresne von Hohenesche, D. Schmidt, V. Schädler, K.K. Unger, submitted. [11] Ø. Borg, S. Eri, E.A. Blekkan, S. Storsæter, H. Wigum, E. Rytter, A. Holmen, J. Catal., 248 (2007) 89. [12] Ø. Borg, S. Eri, E. Rytter, A. Holmen, Prepr.Pap.-Am. Chem. Soc., Div. Fuel Chem., 51 (2006) 699.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Ni/SiO2 fiber catalyst prepared by electrospinning technique for glycerol reforming to synthesis gas Prasert Reubroycharoen,a,b Nattida Tangkanaporn,a Chaiyan Chaiyac a

Department of Chemical Technology, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand, E-mail: [email protected] b Center for Petroleum, Petrochemicals and Advanced Materials, Bangkok 10330, Thailand c Division of Chemical Engineering, Faculty of Engineering, Rajamangala University of Technology Krungthep, Bangkok 10120, Thailand.

Abstract The Ni/SiO2 fiber catalysts is successfully prepared for the first time by sol-gel and electrospinning techniques and used as a reforming catalyst. Nickel acetate and tetraethyl orthosilicate are used as a source of nickel and silica at different Ni loading (5, 10, and 20%wt). The effect of spinning voltage on the morphology of the SiO2 fiber is studied. The Ni/SiO2 fiber catalyst is prepared by impregnation technique and characterized by SEM-EDS, XRD, and TPR. SEM results show that the average diameter of the SiO2 fibers ranged from 1.28 μm to 930 nm. The amount of Ni metal measured by EDS technique is close to that of Ni loading. The reaction test shows that the activity of the fiber catalyst is higher than that of a conventional Ni/SiO2 porous catalyst, and the synthesis gas with H2/CO ratio of 2, a raw material for Fischer-Tropsch synthesis, is obtained by using the fiber catalyst. Keywords: Ni/SiO2, fiber catalyst, electrospinning, glycerol, reforming

1. Introduction Ni-based catalysts have been widely used in the steam reforming of methane, ethanol, and glycerol [1-3] because of their appreciable catalytic activity, good stability, and low price. Although these are much cheaper than Ru- and Rh-based catalysts, they require high reaction temperature and an excess steam to prevent the sintering of Ni particles and the deposition of carbon on the catalyst surface [4]. Moreover, the deposited carbon on the catalyst surface leading to rapid catalyst deactivation which is contributed by the catalyst pore blockage [5]. Thus, a non-porous catalyst such as a fiber-like structure catalyst could avoid the pore blockage, reduce pressure drop, and increase the mass transfer rate at the same time due to no diffusion in the catalyst pore [6]. An effective production technique of the fiber catalysts is electrospinning technique commonly used for producing metal oxide and polymer fibers [7]. Previously, there were no works on the preparation and performance of the fiber catalyst for the syngas production from a glycerol steam reforming. In this study, the electrospinning is used to prepare Ni/SiO2 fiber catalyst which is then used as a glycerol steam reforming catalyst for syngas production. The fiber catalyst is characterized by SEM-EDS, XRD, and TPR.

2. Experimental The Ni/SiO2 fiber catalyst is prepared via sol-gel and electrospinning techniques following by impregnation method. Nickel acetate and tetraethyl orthosilicate (TEOS)

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are selected as a source of nickel and silica. SiO2 fiber is prepared by sol-gel incorporated with electrospinning technique. Then, Ni is impregnated on the SiO2 fiber at different metal loading percentage.

2.1. Silica sol preparation via sol-gel process The silica sol is prepared from TEOS, ethanol, distilled water, and HCl at molar ratio of 1:2:2:0.01, respectively. Firstly, TEOS is mixed with distilled water in a beaker under vigorous stirring for 5 min. Then, HCl and ethanol are added to the solution under stirring for another 5 min. Finally, the solution is heated to 55oC and stirred at 55oC for 50 min before it is cooled down to room temperature.

2.2. Silica fiber preparation via electrospinning process The electrospinning apparatus consists of a high voltage generator with metal collector and a precision syringe pump shown in Fig. 1. The silica sol is filled into a disposable syringe equipped with 0.6 mm-diameter needle and placed on the syringe pump. The silica sol is electrospun and transformed to SiO2 fiber. The standard electrospinning condition is as followings: feeding rate of 10 μl/h, applied voltage of 15 kV, and the tipto-collector distance (TCD) of 15 cm. The electrospun fibers are collected and dried at 110oC overnight, then calcined at 500oC for 2 h.

Fig. 1. The electrospinning apparatus.

2.3. Catalyst impregnation Ni/SiO2 fiber catalyst is prepared by an impregnation of the fiber with different nickel acetate solution. The nickel acetate solution is prepared by dissolving a nickel acetate in the solution of glycerol/ethanol (volume ratio of glycerol and ethanol is 1:9). The solution of nickel acetate is impregnated onto the SiO2 fiber. The catalyst is dried at 120oC for 12 h and calcined at 500oC for 2 h.

2.4. Catalyst characterization The fiber catalyst is characterized by SEM-EDS, XRD, and TPR. TPR is performed in the Micromeritics II 2920. The TPR is performed by flowing Ar (50 mL/min) over 60 mg of catalyst. The reducing gas (10%H2 in Ar at 50 mL/min) is passed over the catalyst with the heating rate of 10oC/min until 600oC is reached. XRD (Philips model X’Pert) equipped with CuKα is used to investigate crystallite size. The crystalline average size is calculated by L = Kλ/Δ(2θ)cos θ0, where L is the crystalline size, K is a constant (K = 0.9–1.1), λ is the wavelength of X-ray (CuKα = 0.154 nm), and Δ(2θ) is the width of the peak at half height. SEM, JOEL JSM-6480LV, is used to investigate morphology of the fiber catalyst. The average diameter of the fiber is analyzed by a SemAfore program. EDS is used to determine catalyst composition.

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2.5. Catalyst performance on a glycerol steam reforming Catalyst performance on a glycerol steam reforming is carried out in a fixed-bed quartz tube reactor. The reaction conditions are Ptotal = 1 atm, PN2 = 0.7 atm, Pwater/glycerol = 0.3 atm, T = 550oC, catalyst = 0.1 g, reaction time = 6 h, water/glycerol = 9:1 (molar ratio), and feed rate of water/glycerol = 0.01 mL/min. The effluent gas from the reactor is analyzed by on-line TCD-GC equipped with Unibead-C column.

3. Results and discussion

Fig. 2. SEM images of SiO2 fibers with various applied voltage (a) 15 kV, (b) 20 kV, and (c) 25 kV at TCD of 15 cm.

The average diameters of SiO2 fiber prepared at 15 kV, 20 kV, and 25 kV are 1.28 μm, 1.27 μm, and 930 nm, respectively. It is shown in Fig. 2 that the shape and diameter of the electrospun fiber from 15 kV applied voltage are more uniform than those of 20 and 25 kV.

Fig. 3. SEM images of Ni/SiO2 fiber catalysts (15 kV, TCD: 15cm.) at Ni loading of (a) 5%wt, (b) 10%wt, and (c) 20%wt.

As shown in Fig. 3, the roughness of the catalyst surface depended upon Ni loading. When Ni loading increases, the roughness of the catalyst surface increases. The rough surface as confirmed by EDS is Ni particles depositing on the SiO2 fiber. Large Ni particles and non-uniform particle distribution are obtained when Ni loading percentage increases. The Ni loading percentage on the fiber analyzed by EDS is shown in Table 1. It is obvious that the actual %Ni depositing on the fiber is close to %Ni loading when the %Ni loading are 5%, and 10%. However, Ni deposited on the fiber catalyst is lower (only 14.42%) when the %Ni loading is 20% implying that 15% Ni loading is maximum percentage loading. The crystallite size calculate from L = Kλ/Δ(2θ) cos θo is indicated in Table 1. The Ni particle sizes range from 11.31 to 13.95 nm. The slight increase in crystallite of 20%Ni/SiO2 fiber catalyst derives from the agglomeration of Ni on the surface of fiber catalyst.

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%Ni deposited on the fiber*

Crystallite size (nm)**

70

a

H2 consumption (a.u.).

%Glycerol conversion .

5 4.53 10 9.35 20 14.42 *Analyzed by EDS, **Calculated from L = Kλ/Δ(2θ) cos θo. Fiber catalyst

60 Porous catalyst

50

11.31 11.37 13.95 271 o C

b

Fiber

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Porous

40 2.5

3

3.5

4 4.5 T ime (h)

5

5.5

6

50

150 250

350 450 T ( o C)

550 650

Fig. 4. (a) Time on stream of glycerol steam reforming and (b) TPR profiles of 10%Ni/SiO2 fiber and 10%Ni/SiO2 porous catalysts.

Figure 4 (a) shows that the glycerol conversion of the fiber catalyst is higher than that of the porous catalyst. Moreover, the conversion of the fiber catalyst becomes stable faster than that of the porous catalyst. Syngas produced from the porous catalyst has H2:CO molar ratio of 7.5, while that produced from the fiber catalyst gives H2:CO molar ratio of 1.9. It could be conclude that the fiber catalyst is very selective for produce syngas from glycerol. In general, the activity of catalysts is related to the reduction peak in TPR profiles. In Fig. 4 (b), both catalysts exhibit two reduction peaks at low and high temperature, corresponding to NiO bulk with weak interaction with SiO2 and NiO with fairly strong interaction with SiO2. Ni/SiO2 porous catalyst shows the reduction peaks at 341oC and 435oC while Ni/SiO2 fiber catalyst exhibits the reduction peaks at 271oC and 428oC. It is clear that the first reduction peak of fiber catalyst shifts to lower temperature and the intensity of second reduction peak decreases. This indicates that the fiber catalyst can be easily reduced at lower temperature implying its higher activity compared to the porous one.

4. Conclusion Ni/SiO2 fiber catalyst is successfully produced by electrospinning technique. The fiber shows the average diameter of 1.28 μm–930 nm at TCD of 15 cm. The SEM-EDS results show that Ni is deposits on the surface of SiO2 fiber. Compared with a conventional porous catalyst, the fiber catalyst exhibits higher glycerol steam reforming activity.

Acknowledgement This work is financially supported by Energy Policy and Planning Office, Ministry of Energy, Thailand Research Fund (TRF) and Center for Petroleum Petrochemicals and Advanced Materials, Chulalongkorn University.

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References [1] M.C. Sanchez, R.M. Navarro and J.L.G Fierro, Int. J. Hydrogen Energy, 32 (2007) 1462. [2] S. Adhikari, S. Fernando and A. Haryanto, Renewable energy, 33 (2008) 1097. [3] S. Adhikari, S. Fernando, S.D. To, R. Bricka, P. Steele and A. Haryanto, Energy Fuels, 22 (2008) 1220. [4] J. N. Amor, Appl. Catal. A 176 (1999) 159. [5] B. Zhang, X. Tang, Y. Li, Y. Xu, and W. Shen, Int. J. Hydrogen Energy, 32 (2007) 2367. [6] A. K. Neyestanaki, P. M. Arvela, H. Backman, H. Karhu, T. Salmi, J. Vyrynen, and D. Yu. Murzin, Ind. Eng. Chem. Res., 42 (2003) 3230 [7] S.W. Lee, Y.U. Kim, S.S. Choi, T.Y. Park, Y.L. Joo, S.G. Lee, Mater. Lett. 61 (2007) 889.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Selective preparation of β-MoO3 and silicomolybdic acid(SMA) on MCM-41 from molybdic acid precursor and their partial oxidation performances Tran M. Huong, Nguyen H.H. Phuc, Hironobu Ohkita, Takanori Mizushima, Noriyoshi Kakuta* Department of Materials Science, Toyohashi University of Technology, Tempaku, Toyohashi, Aichi-pref. 441-8580, Japan

Abstract SMA(Mo/MCM-41Imp) and β-MoO3(Mo/MCM-41 Evap) were successfully prepared on MCM-41 using the molybdic acid solution. The molybdic acid solution is the effective precursor to generate selectively either SMA or β-MoO3, and those species are anchored on MCM-41 through the formation of SMA. The Mo/MCM-41Imp catalyst showed a good catalytic performance for partial oxidation of methane to formaldehyde while the Mo/MCM-41 Evap is a promising catalyst for partial oxidation of methanol to formaldehyde. Keywords: SMA, β-MoO3, MCM-41, methane, methanol

1. Introduction Mizushima et al. successfully synthesized β-MoO3 through a simple evaporation of a molybdic acid solution which prepared by a cation exchange of an aqueous solution of Na2MoO4.2H2O [1]. The key was an addition of a small amount of nitric acid before the evaporation. A heating the dried molybdic acid at 573K in an oxygen stream led to a formation of a bright yellow powder, which was confirmed to be β-MoO3 and almost free from α-MoO3 by XRD and Raman analyses. Deltcheff et al. pointed out that the molybdic acid was an effective precursor for the synthesis of SMA [2]. This suggests that the formation of anchored SMA species can be expected by the reaction with Si-O species as a new SMA catalyst preparation method. Huong et al. reported the formation of the desired Mo species, either SMA or β-MoO3, on SiO2 and MCM-41 by use of the molybdic acid solution [3]. In this study, we investigated the behavior of both β-MoO3 and SMA species supported on MCM-41 catalysts and their catalytic activities were evaluated through partial oxidation of methane and methanol.

2. Experimental MCM-41(SA=1328m2g-1) was synthesized using a modified procedure reported by Grun et al. [4]. A molybdic acid precursor was prepared by a cation exchange of 1M Na2MoO4.2H2O solution with resin through a one meter long glass column in order to remove Na+ ion [1a]. Either SMA or β-MoO3 supported on MCM-41 catalyst was prepared according to the previous paper [3a]; two methods were employed.

*

Corresponding author, e-mail: [email protected].

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Impregnation method: MCM-41 powder (1.6g) was immersed in the molybdic acid precursor solution(120 mL). The impregnated powder was dried at 383K for 24h. Evaporating dry method: The molybdic acid precursor (120 mL) was mixed with 79mL of 1% HNO3 solution and MCM-41 powder (1.6g) in a flask, and then was submitted to the vacuum evaporation at 323K for 1h. The obtained powder was dried in desiccator for 12h followed by calcination at 573K for 1h under an oxygen stream. Mo concentrations in the prepared catalysts were 20wt%. They were named as 20%Mo/MCM-41Imp and 20%Mo/MCM-41Evap, respectively. Structural analysis was carried out by a JASCO Laser Raman spectrometer(NR-1800). Partial oxidation of methane (POM) [3b] was carried out in a continuous stainless steel fixed-bed reactor at 873K. Partial oxidation of methanol [1b] was also performed in a fixed-bed flow 10mm i.d. Pyrex reactor operating at atmospheric pressure, where air was bubbled in a methanol solution to maintain a constant flow rate of the liquid vapor.

3. Results and discussion 3.1. Characterization of 20%Mo/MCM-41 catalysts

Intensity

Figure 1 displays Raman spectra of catalysts. The spectra of reference materials are also added for α -MoO3 comparison. The 20%Mo/MCM-41Imp catalyst gives typical Raman bands which are corresponding to the bands of SMA. Four characteristic Mo/MCM-41 Evap frequency ranges, 160-290, 340-467, 504-680, and 900-1000 cm-1, are assigned to Mo-O-Mo deformation mode, Mo=O bending mode, symβ -MoO3 metric Mo-O-Mo stretching mode, and symmetric Mo=O terminal stretching mode, respectively [5]. Two bands at 1043 and 1057 cm-1, which Mo/MCM-41 Imp have been seen in the 20%Mo/MCM-41 catalysts, were attributed to background bands of Ar SMA laser. 200 400 600 800 1000 On the other hand, Raman spectrum of the Raman shift (cm-1) 20% Mo/MCM-41Evap catalyst is clearly Fig. 1 Raman spectra of Mo/MCM-41 different from that of the 20%Mo/MCM-41Imp catalysts. catalyst and the spectrum pattern is the same as that of β-MoO3 but not of α-MoO3. As the site symmetry of the molybdenum atoms of β-MoO3 is not known in detail, we could differentiate Raman bands of β-MoO3 from αMoO3 by the absence of the band at 997 cm-1(α-MoO3) and the presence of Mo-O-Mo symmetric stretch band at 777 cm-1(β-MoO3) [6].

3.2. Catalytic performances of SMA catalysts prepared from fresh SMA and from molybdic acid solution Since Mo phases of Mo/MCM-41Imp and Mo/MCM-41Evap catalysts were assigned to SMA and β-MoO3, respectively, partial oxidation of methane is an effective reaction for evaluating the activity of β-MoO3 in the presence of water vapor. This was reported previously that the β−MoO3catalyst(Mo/MCM-41Evap) showed the high selectivity to HCHO and the SMA catalyst(Mo/MCM-41Imp) showed the high conversion of methane [3]. It means that the SMA catalyst is effective for the activation of methane and the β−MoO3 catalyst is active for the selective oxidation to HCHO from activated methane. In order to elucidate in detail the activity of SMA on MCM-41, the SMA

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catalyst was prepared by a conventional impregnation method for comparison. A series of SMA/MCM-41 catalysts were prepared with Mo loading ranged from 1 to 20wt%. The performance of these catalysts in POM is exhibited in Figure 2. Mo/MCM-41Imp catalysts give higher activities in POM at all the ranges of Mo loading. However, at low loading of Mo (1, 5 wt%) the differences was not definitely observed both in conversion of methane and HCHO yield. The 10%Mo/MCM-41Imp catalyst has shown the highest catalytic activity with 4.2% HCHO yield in 11% conversion of methane. A distinct difference was found in the performance of the 20% Mo/MCM-41 Imp and the 20%SMA/MCM-41. The formation of SMA on MCM-41 is expected by the following reaction when the molybdic acid solution is used: SiO2 +12MoO3+2H2O = H4SiMo12O40

12 11

CO2

Yield %

10 The reaction of Si-O of MCMCO 9 HCHO 41 with the molybdic acid under 8 7 excess SiO2 leads to the highly 6 dispersed and anchored SMA 5 species on the MCM-41 surface, 4 3 resulting in the high activity even at 2 20% Mo loading. However, as SMA 1 0 species on the 20%SMA/MCM-41 20% 10% 5% 1% 20% 10% 5% 1% catalyst are unstable, the aggregation SMA/MCM-41 Mo/MCM-41 Imp and decomposition might occur in Fig. 2 Performances of SMA/MCM–41 and Mo/ the reaction condition, resulting in MCM–41 Imp catalysts. the sharp decrease in activity. Consequently, the catalyst preparation using the molybdic acid solution is the effective method for the stabilization of SMA species at higher SMA loadings. Raman spectra of the catalysts after POM demonstrate the reason of good performance of the Mo/MCM-41Imp catalyst. The presence of SMA species was revealed on 1, 5, 10%SMA/MCM-41 catalysts while 1, 5 and 10% Mo/MCM-41Imp catalysts showed both β-MoO3 and SMA after POM. As we reported previously that βMoO3 assists in structural transformation of SMA as well as in the production of formaldehyde, the both species thus are attributable to the higher activity of Mo/MCM41Imp catalyst. At 20% Mo loading, the low activity of SMA/MCM-41 may be due to the appearance of α-MoO3 peak with relatively high intensity. In contrast, the coexistence of SMA, α-MoO3 and β-MoO3 were detected in the spectrum of Mo/MCM41Imp catalyst after POM. This probably makes POM more active because of the predominant β-MoO3 phase.

3.3. Catalytic activity of 20%Mo/MCM-41Imp (SMA) and 20%Mo/MCM-41Evap (β-MoO3) catalysts in methanol oxidation

Methanol oxidation(MO) is an important chemical reaction that produces valuable intermediates used in chemical industry. The production of formaldehyde for the synthesis of phenolic resin from methanol is largely preferred. Compared with other oxidation reactions, MO has a wide selectivity pattern and set of reaction mechanisms [7]. The activities of β-MoO3(20%Mo/MCM-41Evap) and SMA (20%Mo/MCM41Imp) species in MO were investigated. The result is shown in Figure 3. Dimethyl ether (DME) and formaldehyde were main products. At low temperature (523K), DME was generated, and the yield reached up to about 20% of total products but this yield rapidly decreased to 6% at 573K and to 0% at 623K in both catalysts. This means that the dehydrogenation reaction of methanol was performed at low temperature regardless of Mo species. However, there were

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remarkable differences in the HCHO yield and in the conversion of methanol. The HCHO yields of both catalysts reached to maximum at the temperature of 623K, 39% and 79% for the 20%Mo/MCM-41Imp and the 20%Mo/MCM-41Evap, respectively. In counterpoint to the role of the catalysts in POM, β-MoO3 supported on MCM-41 demonstrated a high activity in the oxidation of methanol to formaldehyde. The conversion of methanol also exhibited excellent results, 90% at 623K and almost 100% at 723K, respectively. β -MoO3(20%Mo/MCM-41Evap)

SMA(20%Mo/MCM-41Imp)

100

100

80

80

60

60 Conversion Formaldehyde 40 DME

%

%

Conv ersion

40

HCHO DME

20

20

0

0 523

573

623

673

723

523

573

623

673

723

Te mpe rature /K

Tempe rature /K

Fig. 3 Catalytic performances of Mo/MCM–41 catalysts in MO.

10

Since the HCHO yield of bare β-MoO3 was a maximum of 30% at 573K[1], the high dispersion of β-MoO3 species on the large surface area of MCM-41, where βMoO3 was stabilized through SMA formed on the interface of MCM-41[3], may α -MoO contribute in enhancement of its catalytic β -MoO activity. On the other hand, the Mo/MCM41Imp catalyst exhibited poor activity even β -MoO (20% Mo/MCM-41Evap) in comparison with fresh SMA catalyst. Figure 4 shows Raman spectra of MCM41 supported catalysts after MO at 623K. No peaks corresponding to SMA and β-MoO3 SMA(20% Mo/MCM-41Imp) were observed in the Mo/MCM-41 Imp 100 200 300 400 500 600 700 800 900 1000 1100 catalyst. This suggests that the low catalyric Raman shift (cm ) activity of Mo/MCM-41Imp is due to the Fig. 4 Raman spectra of 20%Mo/MCM–41 structural transformation of SMA to α-MoO3. catalysts after methanol oxidation at 623 K. In addition, SMA species even anchored on MCM-41 are decomposed easily at 623 K when water was absent in the feed. In contrast, β-MoO3 species act as active species for the formaldehyde formation although some of β-MoO3 was transformed to α-phase. Consequently, it is necessary to find the optimum conditions (temperature, oxygenmethanol ratio, etc.) for the stabilization of β-MoO3 species anchored on MCM-41. 3

Intensity

3

3

-1

References [1] a) T. Mizushima, K. Fukushima, T. M. Huong, H. Ohkita, N. Kakuta, Chem. Lett., 34(2005)986. b) T. Mizushima,, K. Fukushima, H. Ohkita, N. Kakuta, Appl. Catal. A, 326 (2007)106. [2] C. R. Deltcheff, M. Fournier, R. Franck and R. Thouvenot, Inorg. Chem., 22 (1983)207. [3] a) T. M. Huong, K. Fukushima, H. Ohkita, T. Mizushima, N. Kakuta, Catal. Commun., 7(2006)127. b) M. T. Huong, H. Ohikita, T. Mizushima, N. Kakuta, Appl. Catal. A, 287(2005)129. [4] M. Grun, K. K.Unger, A. Matsumoto, K. Tsutsumi, Character. of porous solid, Vol. IV, 81-89. [5] G. Mestl and T.K.K. Srinivasan, Cata. Rev.-Sci. Eng., 40 (1998)451. [6] S. Kasztelan, E. Payen, J.B. Moffat, J. Catal., 112 (1988)320. [7] J.M. Tatibouët, Appl. Catal. A: Gen., 148 (1997)213.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Functionalization of carbon xerogels for the preparation of Pd/C catalysts by grafting of Pd complex Chantal Diverchy,a Sophie Hermans,a Nathalie Job,b Jean-Paul Pirard,b Michel Devillersa a

Université catholique de Louvain, Unité de Chimie des Matériaux Inorganiques et Organiques, Place L. Pasteur 1, Louvain-la-Neuve B-1348, Belgium. b Université de Liège, Laboratoire de Génie Chimique, Sart Tilman B6a, Liège B-4000, Belgium.

Abstract A mesoporous carbon xerogel was functionalized by treatments with nitric acid in order to introduce oxygenated functions at its surface. The carbon supports were characterized by Boehm’s titrations, XPS and nitrogen adsorption, and the results were compared with those obtained with a microporous activated carbon. It was shown that carbons with high oxygen content were obtained. The oxygenated functions were then used as anchors for the grafting of a palladium complex. High Pd surface concentrations are observed by XPS but it appears that the Pd particles are not homogeneous in size and repartition. Keywords: carbon xerogel, palladium, functionalization

1. Introduction Carbon materials are used extensively as supports in heterogeneous catalysis because they present interesting properties such as inertness, cheapness… Moreover, it is possible to modify their surface chemistry by adding functional groups. Among these surface groups, oxygenated functions are by far the most often studied. It has been well documented that these modifications affect, amongst other parameters, their behaviour when used as catalyst supports [1]. However, because the spectroscopic characterization of carbon materials remains a challenge, the study of their surface chemistry is not easy and many developments are still to come. This work consists in the preparation of Pd catalysts supported on carbon xerogel. This kind of carbons was used because their pore texture and surface chemistry are well controlled compared to traditional activated carbons [3]. The aim is to take advantage of oxygenated functions introduced onto the surface of the carbon support in order to improve the interaction between the metallic precursors and the support. The precursor chosen in this study is a water-soluble coordination compound containing carboxylate ligands, which were found to exchange easily at least one ligand to allow metal grafting onto oxygenated surface functions [2].

2. Experimental 2.1. Support synthesis The carbon support used in this study is a mesoporous xerogel X25 (maximum mesopore size ca. 25 nm, SBET ~ 660 m².g-1), obtained by evaporative drying and subsequent pyrolysis of an aqueous resorcinol-formaldehyde gel prepared under well-

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defined conditions [3]. This carbon xerogel was submitted to oxidative treatments using HNO3 as follows: 4 g of crushed xerogel (granulometry: 50-100 µm) was stirred in 100 mL nitric acid 0.2 or 2.5 mol.L-1 under reflux. After 24 h, the solid was recovered by filtration and washed with water on a Soxhlet apparatus. The samples were then dried under vacuum at 50°C.

2.2. Catalyst preparation The synthesis of [Pd(OAc)2(Et2NH)2] was described elsewhere [2]. The preparation of the catalysts was carried out by mixing 0.1742 g of complex and 0.95 g of carbon in 50 mL of water for 24 h. The amount of complex introduced was calculated to obtain a theoretical loading of 5 wt.% of Pd in the final catalysts (after activation). After this treatment, the solution was filtered out and the solid sample was dried under vacuum at 50°C. It was then activated in a CARBOLITE tubular oven at 200°C under N2 flow during 4 h.

2.3. Characterization Boehm’s titration: This method was used to evaluate the amount of carboxylic acid, lactone and phenol groups present at the surface of carbon samples, by stirring 0.5 g of carbon in 50 mL of NaHCO3, Na2CO3 or NaOH [2]. The carbon was filtered out after 24 h and the filtrate was titrated with hydrochloric acid. All solutions were prepared using freshly distilled decarbonated water and maintained under nitrogen. XPS: X-ray photoelectron spectroscopy was carried out on a SSI-X-probe (SSX100/206) Fisons spectrometer. The binding energies were set up by fixing the C1s peak (C-(C,H) component) at 284.8 eV. Three photopeaks (C1s, O1s and N1s) were systematically analyzed for the carbonaceous materials, the Pd3d peak was added for the catalysts. The XPS results were decomposed with the CasaXPS software, using a sum of Gaussian/Lorentzian (85/15) after subtraction of a Shirley-type baseline. The constraints used for decomposition of the Pd3d peaks were as follows: imposing an area ratio Pd3d5/2 / Pd3d3/2 of 1.5, a difference in the binding energies (Pd3d3/2 - Pd3d5/2) of 5.26 eV and a FWHM ratio (for the Pd3d5/2 / Pd3d3/2 peaks) of 1. Nitrogen adsorption: The specific surface areas were measured on an ASAP 2000 Micromeritics instrument by nitrogen adsorption at -196°C. Before analysis, the sample (0.1 g) was outgassed during several hours at 150°C under a pressure of 500 Pa. Data were analyzed using the classical BET theory in order to calculate the specific surface area, SBET. SEM: Topographic SEM images were obtained using a FEG Digital Scanning (DMS 982 Gemini LEO) electron microscope fitted with an EDAX analyzer (Phoenix CDU LEAP). Atomic absorption (AA): Atomic absorption was carried out on a PERKINELMER 3110 spectrometer in order to determine the possible residual metal amounts in the filtrates.

3. Results and discussion The mesoporous carbon xerogel support (X25) was submitted to an oxidative treatment using HNO3 in order to increase the amount of oxygenated surface functions. Such treatments were previously studied using a microporous activated carbon (SX+) supplied by NORIT (SBET ~ 920 m².g-1) [2].

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Table 1. Characterization of the carbon xerogel supports compared to the SX+ carbon supports. Notation C0 C1 C2

Treatment Carbon used CHNO3 (mol.L-1) Xerogel SX+ Xerogel 0.2 SX+ Xerogel 2.5 SX+

O/C (XPS) 0.04 0.04 0.14 0.10 0.27 0.19

Total acidity (mmol.(100 gC)-1) 14 27 149 110 424 310

SBET (m².g-1) 624 922 576 913 478 756

It is found, as previously for the SX+ activated carbon [2], that the acidity or the oxygen content of the mesoporous sample increases with the HNO3 treatment. The rise is, however, more pronounced when using the xerogel than SX+ (Table 1). Boehm’s titrations indicate that this increase in acidity is mainly due to the formation of carboxylic acids and phenols (Figure 1). A decrease of the BET surface area is also observed after functionalization in both cases (Table 1).

Acidity (mmol/100g C)

400 350

X25

300

SX+

250 200 150 100 50 0 Carboxylic acid

Lactone

Phenol

T otal

Figure 1. Distribution of acid functions for the most functionalized carbon supports (C2).

The oxygenated functions introduced on the supports were then used as anchors for the grafting of [Pd(OAc)2(Et2NH)2]. It appears that the amount of incorporated Pd depends on the degree of oxidation of the carbon xerogel support whereas the Pd loading is maximum for the SX+ supports (Table 2). Moreover, Pd atomic % (XPS) measured before activation are higher for samples prepared on the mesoporous carbon xerogel support but are not related to the surface acidity. The higher Pd atomic % obtained on xerogels supports may be due to a stronger hydrophobicity of these samples compared to SX+ supports. As a result, the Pd complex (dissolved in water) may not easily penetrate into the pores and so might react preferentially on the external surface. Moreover, it seems that when the loading increases on xerogels, the Pd atomic % decreases. For example, the most functionalized support, which contains the highest quantity of Pd, presents the lowest Pd atomic %. This may be the result of an agglomeration of Pd, linked to the higher amount of Pd introduced combined with a loss of specific surface area. The grafted samples were then activated and characterized after activation by XPS and SEM. Results show that the surface Pd atomic % decreases after activation but remains higher on xerogels than on SX+ supports (data not shown). SEM images show that the samples obtained are inhomogeneous in terms of particles size and repartition. Moreover, big particles are observed in all cases, the smallest ones being obtained with the intermediate functionalized support (Figure 2). Various hypotheses may be put

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forward to explain these results. One explanation may be that the intermediate support contains an optimal number of oxygenated functions: the most functionalized support with a lot of thermally unstable functions may lead to agglomeration during the activation process, while for the non-modified support, the grafting process may be not optimal due to the smaller amount of functions. Finally, it can be seen from Table 2 and Figure 2 that the particle size and the atomic Pd % follow the same evolution, meaning that this trend was already present before activation. Table 2. Catalyst characterization by XPS before activation and Pd loading obtained by analysing the filtrate by atomic absorption spectrometry (AA). Treatment C0 C1 C2

Pd atomic % (XPS) Xerogel SX+ 2.62 0.73 3.21 0.78 1.46 0.78

Pd loading (AA) (wt.%) Xerogel SX+ 3.90 4.97 3.52 4.97 5.00 5.00

2 µm

(xerogel C0)

(xerogel C1)

(xerogel C2)

Figure 2. SEM images of the Pd/C catalysts obtained on xerogel carbon supports with variable degrees of functionalization.

4. Conclusion The functionalization of a carbon xerogel sample was carried out with success and led to higher oxygen content. These functionalized carbons were used as supports for the grafting of a palladium complex. It was shown that, by opposition with a commercial microporous activated carbon support (SX+), the degree of functionalization influenced the amount of introduced palladium.

Acknowledgement The authors wish to thank the PAI INANOMAT, and FNRS for financial support and the NORIT firm for supplying the SX+ carbon.

References [1] P. E. Fanning and M. A. Vannice, 1993, Carbon, 31, 721. [2] S. Hermans, C. Diverchy, O. Demoulin, V. Dubois, E.M. Gaigneaux and M. Devillers, 2006, J. Catal., 243, 239. [3] N. Job, R. Pirard, J. Marien and J.-P. Pirard, 2004, Carbon, 42, 619. [4] A. Deffernez, S. Hermans and M. Devillers, 2005, Appl. Catal. A-Gen., 282, 303.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Preparation of Pd-Bi catalysts by grafting of coordination compounds onto functionalized carbon supports Chantal Diverchy, Sophie Hermans, Michel Devillers Université catholique de Louvain, Unité de Chimie des Matériaux Inorganiques et Organiques, Place L. Pasteur 1, Louvain-la-Neuve B-1348, Belgium

Abstract HNO3 functionalized carbons are used as supports for the preparation of bimetallic PdBi/C catalysts. The aim is to take advantage of the oxygenated surface functions introduced on the carbon, which are expected to act as anchoring sites. Two neutral coordination complexes were selected to incorporate the two metals onto differently functionalized carbon supports. The grafted samples are then submitted to an activation treatment and characterized by SEM, XRD and XPS. It appears that the order of incorporation strongly influences the surface properties of the catalysts and thus modifies their activity in the oxidation of glucose. Keywords: carbon, palladium, bismuth, functionalization, grafting

1. Introduction Activated carbons are widely used as supports for heterogeneous catalysis. They are especially suited for applications carried out in the liquid phase because they are robust, cheap and easy to recover. However, as carbonaceous materials are difficult to characterize spectroscopically, the study of their surface chemistry remains a challenge. In this study, carbon-supported catalysts were synthesized using the following strategy [1]: (i) a chemical functionalization of the carbon surface with HNO3 to allow (ii) the grafting of selected metallic precursors. The present work deals with the preparation of Pd-Bi/C catalysts because of the interest of such an association in selective oxidation processes involving renewables like carbohydrates [2-4].

2. Experimental 2.1. Support functionalization The carbon used in this study is an activated carbon SX+ supplied by Norit (C0). It was submitted to oxidative treatment by stirring 6 g of carbon in 150 mL of nitric acid 0.2 (C1) or 2.5 (C2) mol.L-1 under reflux. After 24 h, the carbon was recovered by filtration and washed with water on a Soxhlet apparatus. The samples were then dried under vacuum at 50°C.

2.2. Catalysts preparation The complex [Pd(OAc)2(Et2NH)2] was synthesized as described elsewhere [1] while [Bi(dpm)3] (dpm = dipivaloylmethanate) is commercially available (Acros). The amount of complex introduced was calculated to obtained 5 wt.% in both metals in the final catalysts (after activation), i.e. 0.1742 g for [Pd(OAc)2(Et2NH)2] and 0.1818 g for [Bi(dpm)3] with 0.95 g of C for the grafting of the first metal; the amount of second metal grafted was determined on the basis of grafting models, assuming that the grafted

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parts were ‘Pd(OAc)(Et2NH)’ and ‘Bi(dpm)2’. The syntheses were realized in 50 mL of solvent. After 24 h of treatment, the samples were systematically recovered by filtration, washed with water and dried under vacuum at 50°C. Monometallic samples were first prepared under different grafting temperatures (room temperature (RT), 50°C and reflux) to determine the optimal grafting conditions. The solvents were chosen depending on the solubility of the metallic complexes (water and toluene for the Pd complex, toluene only for the Bi complex). Following this, bimetallic samples were prepared by consecutive grafting of the complexes, by grafting first the Pd complex (in water at RT) or the Bi complex (in toluene at RT). Other bimetallic samples were also obtained by grafting the Pd complex as described previously followed or preceded by deposition of bismuth oxoacetate in heptane, as described in the literature [4]. All bimetallic samples obtained were activated by thermal treatment for 8 h at 500°C under a N2 flow.

2.3. Characterization Boehm’s titration: This method was used to evaluate the amount of carboxylic acid, lactone and phenol groups present at the surface of carbon samples, by stirring 0.5 g of carbon in 50 mL of NaHCO3, Na2CO3 or NaOH. The carbon was filtered out after 24 h and the filtrate was titrated with hydrochloric acid. All solutions were prepared using freshly distilled decarbonated water and maintained under nitrogen. XPS: X-ray photoelectron spectroscopy was carried out on a SSI-X-probe (SSX100/206) Fisons spectrometer. The binding energies were set up by fixing the C1s peak (C-(C,H) component) at 284.8 eV. The XPS results were decomposed, with the CasaXPS software, using a sum of Gaussian/Lorentzian (85/15) after subtraction of a Shirley-type baseline. The constraints used for decomposition of the Pd3d peaks were as follows: imposing an area ratio Pd3d5/2 / Pd3d3/2 of 1.50, a difference in the binding energies (Pd3d3/2 - Pd3d5/2) of 5.26 eV and a FWHM ratio of 1. The constraints used for decomposition of the Bi4f peak were as follows: imposing an area ratio Bi4f7/2 / Bi4f5/2 of 1.33, a difference in the binding energies of 5.31 eV and a FWHM ratio of 1. XRD: Diffractograms were recorded on a SIEMENS D5000 diffractometer. Phases were identified with reference to the JCPDS database. Atomic absorption (AA): Atomic absorption was carried out on a PERKINELMER 3110 spectrometer. Catalytic tests: The catalysts were tested in the oxidation of glucose into gluconic acid, as described elsewhere [4]. The tests were carried out at 50°C during 4 h with 54 mg of catalyst. Catalytic results are expressed as glucose conversion.

3. Results and discussion The treatments with HNO3 caused an increase in the number of oxygenated (acid) functions on the surface of the carbon support. Figure 1 shows the quantification of the different acidic functions in the initial and the two functionalized supports, as determined by Boehm’s titration [5].

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Lactones

200

150

100

50

Acidité (mmol/100 gC

Phenols

Acidity (mmol/100gC)

Carboxylic acids

0

C0

C1

C2

Figure 1. Distribution of acid functions in the non-modified and functionalized supports.

Bimetallic catalysts were synthesized using [Pd(OAc)2(Et2NH)2] combined with [Bi(dpm)3]. These complexes were chosen because they were thought to exchange easily at least one ligand for surface oxygenated functions. Table 1. Grafting of [Pd(OAc)2(Et2NH)2] in toluene and in water on non-modified and functionalized carbon supports.

Toluene

Solvent

Water

Carbon C0 C2 C0 C2 C0 C2 C0 C2

T RT RT 50°C 50°C Reflux Reflux RT RT

Pd atomic % (XPS) 0.90 0.75 0.18 0.72 3.84 1.10 0.68 0.81

Pd % on C (AA) 1.25 4.73 1.64 4.97 4.73 5.00 4.97 5.00

Monometallic samples were first prepared. By varying the grafting temperature and the solvent used, optimal grafting conditions were determined (Table 1). The monometallic samples obtained were analysed by XPS and the amount of metal grafted was determined by measuring the residual metal amounts in the filtrates by atomic absorption spectroscopy. As shown in Table 1, the results depend strongly on the solvent and the type of support. When working with toluene, with the non-functionalized support (C0), the amount of grafted Pd complex is highly temperature-dependent, and approaches the expected 5 wt. % only under reflux. The Pd grafting yield is in any case higher with the functionalized support (C2), and reaches 5 wt. % under reflux. Grafting from aqueous solution appears to be more efficient at RT even for the non-treated support. Water was thus selected for the preparation of bimetallic catalysts even if higher surface Pd atomic % could be obtained when using toluene. On the contrary, the Bi complex was quantitatively grafted at room temperature in toluene. Following these results, bimetallic samples were prepared by a consecutive grafting procedure at RT using soft conditions in water for the Pd complex and in toluene for the Bi one. Samples are noted PdBi when the Pd complex was grafted first and BiPd when the Bi complex was grafted first. Other bimetallic samples were also obtained by grafting of the Pd complex followed or preceded by deposition of bismuth oxoacetate in heptane. The obtained samples were then activated, characterized by XPS, SEM and XRD, and tested in the reaction of glucose into gluconic acid.

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Table 2. Characterization of the samples obtained by grafting of Pd and Bi (after activation). Support PdBi BiPd

C0 C1 C2 C0 C1 C2

Pd % 0.54 0.53 0.41 0.66 0.36 0.37

XPS Bi % 0.56 0.51 0.35 2.54 0.30 0.25

Pd/Bi 0.96 1.05 1.18 0.26 1.22 1.48

Activity (%/mgPd) 12.1 11.6 9.60 0.38 5.23 7.75

All the bimetallic samples prepared in this study contained 5 wt.% of Pd and 5 wt.% of Bi, while the surface Pd and Bi atomic % (XPS) decreased with the degree of functionalization when grafting both metals (Table 2). Pd/Bi XPS ratios reached a unique value of 1 for activated PdBi samples whereas this ratio increased with the degree of functionalization for activated BiPd samples. These experimental surface Pd/Bi atomic ratios appear to be systematically lower than the theoretical value of 1.96 calculated from the bulk composition, independently from the order of incorporation. This reflects the lower surface energy of Bi with respect to Pd, leading to surface enrichment in this element, as always observed [3]. Various PdxBiy intermetallics were suspected by XRD. The activity decreased with the functionalization in PdBi samples while it increased in BiPd samples (Table 2 and Table 3). These catalytic results are linked to the amount of Pd and Bi on the surface and also to their surface ratio. Table 3. Characterization of the activated samples obtained by the grafting-deposition method. Support PdBi BiPd

C0 C2 C0 C2

Pd % 0.53 0.40 0.26 0.32

XPS Bi % 0.62 0.35 0.29 0.27

Pd/Bi 0.84 1.14 0.90 1.18

Activity (%/mgPd) 10.2 5.39 7.22 10.3

4. Conclusion Bimetallic Pd-Bi/C catalysts were prepared on carbon supports bearing different amounts of oxygenated groups. The grafting of Pd and Bi complexes was carried out in different solvents and at variable temperatures, allowing the determination of optimal conditions. Bimetallic catalysts were then prepared either by grafting Pd before Bi or the opposite. Other bimetallic samples were synthesized by a mixed process (grafting for Pd and deposition for Bi). It was observed that the properties of these catalysts were influenced by the degree of functionalization of the support but also by the order of incorporation of the metals. Their activity in the oxidation of glucose seemed to be linked to the amount of surface metal but also to their surface ratio.

References [1] S. Hermans, C. Diverchy, O. Demoulin, V. Dubois, E.M. Gaigneaux and M. Devillers, 2006, J. Catal., 243, 239. [2] P. Gallezot, 2007, Catal. Today, 121, 76. [3] M. Wenkin, R. Touillaux, P. Ruiz, B. Delmon and M. Devillers, 1996, Appl. Catal. A-Gen., 148, 181. [4] M. Wenkin, P. Ruiz, B. Delmon and M. Devillers, 2002, J. Mol. Catal. A-Chem, 180, 141. [5] C. Diverchy, S. Hermans and M. Devillers, 2006, Stud. Surf. Sci. Catal., 162, 569.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Novel dicarboxylate heteroaromatic metal organic frameworks as the catalyst supports for the hydrogenation reaction Vera I. Isaeva,*a Olga P. Tkachenko,a Igor V. Mishin,a Elena V. Afonina,a Gennady I. Kapustin,a Ludmila. M. Kozlova,a Wolfgang Grünert,b and Leonid M. Kustov a

N. D. Zelinsky Institute of Organic Chemistry RAS, Leninsky pr. 47, Moscow 119991, Russia b Lehrstuhl Technische Chemie, Ruhr-University Bochum, D-44780 Bochum, Germany

Abstract The novel Zn-derived heteroaromatic metal organic frameworks (MOFs) based on 2(5)pyridinedicarboxylate and 2(5)-pyrazinedicarboxylate ligands were synthesized. In order to elucidate the framework nature effect, a reference sample of aromatic MOF-5 derived from Zn4O clusters and benzene-1,4-dicarboxylate linkers was prepared. The variation of the preparation procedure parameters in respect to the MOF texture (porosity, surface area) was accomplished. The synthesized metal organic frameworks were characterized by the combination of the physicochemical methods: XRD, volumetric N2 adsorption/desorption, DRIFT, and XAS. The catalytic activity of the Pd-containing MOFs in the liquid-phase hydrogenation of cyclohexene (20°C, PH2 1atm) was higher than that of Pd on activated carbon. Keywords: Metal-organic framework, characterization, hydrogenation

1. Introduction The synthesis of metal organic frameworks (MOFs) is a realization of an idea of constructing new materials with tunable physical and chemical properties. However, most of these synthesized systems are being explored from the point of view of its application in adsorption and separation processes with a relatively few citations of the use of MOF materials in catalytic reactions [1-3]. The situation has changed during the recent years. The research work number of the application of the metal organic framework in the catalysis field significantly arose. The phenylenecarboxylate MOFs present a most wide studied group of these materials due to its rigid three dimensional structure characterized by permanent porosity. As a rule these systems are used for the catalysis purposes as the traditional carriers like activated carbons and zeolites. The present contribution deals with the synthesis of the novel porous heteroaromatic dicarboxylate Zn-based MOFs, and its utilization as the supports for the Pd–containing catalysts for the liquid-phase hydrogenation of cyclohexene under mild conditions. Two alternative procedures were used for the synthesis: solvothermal (100°C) and direct mixing methods (20-80°C). In order to compare the MOF texture characteristics as well as the catalytic performance in the hydrogenation tests, a sample of the reference aromatic metal organic framework (MOF-5) derived from Zn4O clusters and benzene1,4-dicarboxylate linkers was prepared.

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2. Experimental 2.1. Synthesis Two alternative procedures were used for the MOF sample preparation: solvothermal and direct mixing methods (Table 1). The syntheses of the samples 2 and 3 were carried out according to the procedure developed by us for the preparation of MOFs based on 2(5)-pyridinedicarboxylic acid and 2(5)-pyrazinedicarboxylic acid respectively. Sample 1. Zn(NO3)2 6H2O (0.350 g, 1.177 mmol) and 2(5)-pyridinedicarboxylic acid (0.197 g, 1.179 mmol) were dissolved in a mixture of N,N’-dimethylformamide (DMF) (9 ml) and toluene (35 ml). The reaction mixture was homogenized (1 h) for solvothermal treatment. The solution was heated in a Teflon-line stain-less autoclave (95°C, 20 h), cooled to room temperature and filtered off in Ar flow. The resulted pale crystals were washed with DMF. The material was evacuated for 6 h (10-3 Hg, 150°C). Sample 2. Zn(NO3)2 6H2O (1.21 g, 4.07 mmol), 2(5)-pyridinedicarboxylic acid (0.33 g, 3.1 mmol), DMF (40 ml) were stirred (80°C, 1.5 ч). The resulting solid was centrifuged repeatedly, washed with DMF (3 x 20 mL) and dried under vacuum (10-3 Hg, 200°C). Sample 3. Zn(NO3)2 6H2O (2.3 g, 7.81 mmol), 2(5)-pyrazinedicarboxylic acid (0.623 g, 3.1 mmol), DMF (85 ml) were stirred (80°C, 1.5 ч). The resulting solid was centrifuged repeatedly, washed with DMF (3 x 20 mL) and dried under vacuum (10-3 Hg, 60°C). Sample 4 was synthesized according to [4]. Table 1. Preparation procedure and composition of MOFs and Pd/MOFs. Sample 1 2 3 4 2a, 3a, 4a

Sample composition Zn, 2(5)-pyridinedicarboxylate Zn, 2(5)-pyridinedicarboxylate Zn, 2(5)-pyrazinedicarboxylate Zn, 1,4-benzenedicarboxylate Pd-containing MOF samples 2, 3, 4

Preparation method solvothermal direct mixing direct mixing direct mixing incipient wetness impregnation

2.2. Catalyst preparation and catalytic performance Pd-containing MOFs were prepared by impregnation of the parent frameworks with a Pd(OAc)2 solution in dry chloroform (1% wt Pd) analogously to Pd(acac)2 deposition [1]. The solution of Pd(OAc)2 (0.032 - 0.050 g) dissolved in chloroform (0.30 ml) was slowly added to evacuated MOF samples (0.5 g) with formation a light orange paste. The solvent was evaporated under continuous stirring. The Pd(OAc)2/MOFs were dried under reduced pressure (20°C, 4 h). The samples of Pd/heteroaromatic MOFs (2a and 3a) were obtained by heating under vacuum (140°C, 4 h) and Pd/aromatic MOF-5 reference sample (4a) was obtained at 200°C for 4 h. The hydrogenation tests were carried out in absolute 1,4-dioxane (20°C, PH2 1atm, catalyst 0.05 g, cyclohexene 0.2 mL). The reaction products were analyzed by GLC equipped with a FID detector and a capillary column.

2.3. Characterization N2 adsorption data were obtained at -196°C by a volumetric method. Specific surface areas were calculated according to the BET equation. Powder XRD patterns were recorded with a DRON 3M diffractometer using Cu Kα radiation in Bragg-Brentano reflecting and Debye–Sherrer transmission geometry (λ=1.54 Å). The DRIFT spectra were recorded at room temperature with Nicolet 460 Protégé spectrometer with a diffuse reflectance attachment. Before IR study samples were evacuated at 200°C for 1 h. The CD3CN was adsorbed at 20°C and saturated vapour pressure. X-ray absorption spectra

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Novel dicarboxylate heteroaromatic metal organic frameworks

(Zn K edge at 9659 eV) were measured at the Hasylab X1. The spectra were recorded in the transmission mode at -190°C. The spectrum of a metal foil was registered simultaneously between the second and third ionization chambers for energy calibration. The EXAFS data analysis was performed using the software package VIPER. Reference spectra were taken using standard reference compounds: ZnO and Zn-foil. The fitting was done in the k- and r-spaces.

3. Results and discussion 3.1. Characterization All synthesized MOF samples are characterized by proper crystallinity. The synthesis procedure does not remarkably influence the textural parameters of the resulted metal organic framework. The specific surface areas for the 2(5)-pyridinedicarboxylate samples 1 and 2 are 270-300 m2/g. These values are lower, than for 1,4-benzenedirboxylate (MOF-5) sample 4 (1000 m2/g). Despite the crystallinity retention after evacuation, the 2(5)-pyrazinedicarboxylate sample 3 has no surface area. Tentatively, such difference in surface areas could be explained by lower micropore volume for the samples 1 and 2 or lake the permanent porosity for the 2(5)-pyrazinedicarboxylate framework (sample 3). Figure 1 shows the vibration bands in the region 1400 and 1700 cm-1 (a) corresponding symmetric and asymmetric vibrations of C=O bond in carboxylate ion and aromatic ring, while a few bands in the region ~ 3030 сm-1 (b) belong to valent and combination C-H vibrations of aromatic systems (sample 1 - 4, 4a). These data confirm the retention of heterocyclic or phenylenedicarboxylate bridge fragments in the MOF structures.

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Figure 2. DRIFT spectra of CD3CN.

The DRIFT spectra of adsorbed acetonitrile-d3 on the samples 3, 4, 4a (Figure 2) show the presence of strong Zn2+ Lewis acid sites at the MOF surface. The C≡N stretching vibrations frequency shift relative to the gas phase of acetonitrile-d3 (2253 cm-1) is 45-55 cm-1. The XANES (Figure 3a) evidence that zinc exists as Zn2+ ions in all synthesized organic frameworks. EXAFS spectra exhibit pronounced differences (Figure 3b) in the position and the intensity of the second peak. The analysis of the EXAFS oscillations shows that the nearest neighbors of the central Zn atom in all synthesized frameworks are O atoms with an average coordination number (CN) 3-4 and with Zn-O real distance ~ 1.95-2.04 Ǻ. Next neighbors in the sample 4 (MOF-5) are Zn atoms with average coordination numbers of 3 and 3.20-3.22 Ǻ real distance. The presence of Zn neighboring atoms in this sample suggests the presence in our samples of some Zn species and/or MOF frameworks interpenetrating each other like that observed in [5]. The introduction of Pd in this sample results in the increase of the amount of interweaved cells, the average Zn-Zn coordination number in 4a sample is 4.

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3.2. Hydrogenation reaction As it was mentioned above (see Experimental section) incipient wetness impregnation was used for the preparation of Pd-containing MOF samples. It should be noted that the solvent quantity needed for impregnation is lower, than in case of the reference sample 4 (MOF-5). Probably it could be connected with the different framework dimensionality (2D and 3D) and various pore openings of the synthesized heteroaromatic (sample 2, 3) and aromatic (sample 4) MOFs. Despite the differences in specific surface areas and framework nature the catalytic activities of synthesized frameworks in cyclohexene hydrogenation are similar. The specific surface area does not influence in the activity that indicates the localization of Pd mainly in outer surface of MOF microcrystals, i.e. all Pd species are accessible to cyclohexene. The leaching of Pd from the metal organic support is not detected. The XRD patterns indicate the retention of the crystallinity of Pd/MOFs systems both in the impregnation course and during the reaction. Using Pd/MOFs this reaction proceeds much faster, than on 5%Pd/C (Figure 4). Cyclohexene is hydrogenated selectively to cyclohexane over Pd/MOFs. Any traces of benzene due to disproportionation reaction into benzene and cyclohexane are found in hydrogenation course unlike the hydrogenation reaction on Pd/C. Probably above mentioned observations indicate the advantage of MOFs as the highly ordered and crystalline catalytic systems as compared to activated carbon with an irregular structure [1]. 100

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4. Conclusions The novel Zn-derived metal organic frameworks based on heterocyclic 2(5)pyridinedicarboxylate and 2(5)-pyrazinedicarboxylate bridging ligands were synthesized. The heteroaromatic MOFs were utilized as the supports of the Pd catalysts for the liquid-phase hydrogenation of cyclohexene. Pd/MOF systems showed the higher catalytic activity in this reaction in comparison with Pd supported on activated carbon.

References [1] Sabo M., Henschel A., Frode H., Klemm E., Kaskel S., J. Mater. Chem., 17(2007) 38273832. [2] Xamena, F. X. L. i.; Abad, A.; Corma, A.; Garcia, H J. Catal., 250 (2)(2007) 294-298. [3] Opelt S., Turk S., Dietzsch E., Sabo M., Henschel A., Kaskel S., Klemm E., Catal. Commun., 9(2008)1286-1290. [4] Isaeva V.I., Tkachenko O.P., Mishin I.V., Kostin A.A., Brueva T.R., Klementiev K.V., Kustov L.M., Topics in Chemistry and Materials Science. Advanced Micro and Mesoporous Materials, 1(2007) 155-162. [5] J. Havicovic, M. Bjorgen, U. Olsbye, P.D.C. Dietzel, S. Bordiga, C. Prestipino, C. Lamberti and K.-P. Lillerud J., Am. Chem. Soc., 129 (2007) 3612-3620.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Monitoring of the state of silver in porous oxides during catalyst preparation Elie Sayah, Dalil Brouri, Anne Davidson, Pascale Massiani Laboratoire de Réactivité de surface, UPMC-Université Pierre et Marie Curie, CNRS-UMR 7609, 4 place Jussieu casier 178, 75252 Paris cedex 05, France Tel.: +33 1 44274917, Fax: +33 1 44274917 email: [email protected]

Abstract Silver-supported porous oxides are promising heterogeneous catalysts for air treatment. Classical ways of preparation are incipient wet impregnation or ion exchange (in case of zeolites) of a support with silver nitrate. Nevertheless, this precursor is known for its photosensitivity. The latter can lead to preparations that are not well controlled yet. In this contribution, the influence of the nature of the oxide support and of the thermal activation conditions towards the state and the dispersion of silver in the catalyst is investigated. Complementary characterization techniques are used, i.e. Temperature Programmed Reduction (TPR) coupled with mass spectrometry (MS), UV-Visible spectroscopy (UV-Vis.) and Transmission Electron Microscopy (TEM). Keywords: zeolites, mesoporous oxide, silver cations, TEM microscopy, nanoparticles

1. Introduction Silver supported on porous oxides in the form of isolated species or nanoparticles have gained considerable attention in the last years. They can be potential catalysts for air treatment in industrial and automotive processes such as deNOx [1] and VOC's decomposition [2]. Actually, the use of silver, alone or combined to base metals [3], would represent a cheaper alternative to classic noble metals such as Pt and Pd used for this kind of reactions. Different types of active silver phases have been proposed, depending on the work. Small metallic silver nanoparticles have been reported for the SCR of NOx by methane [4]. Small charged clusters have been proposed for the SCR of NOx by propane [5]. Finally, dispersed silver oxide clusters have been considered in the case of the catalytic oxidation of methyl ethyl ketone [6]. In addition, it has been reported that the active phase may be a combination of the above species. For instance, the presence of both metallic silver and silver oxide species has been shown to enhance the catalytic oxidation of acrylonitryle [7]. These various examples illustrate the complexity of the silver-based catalytic systems and it points to the need for a deeper understanding of the evolution of the silver state under reaction conditions. Several studies were carried out on Ag-based catalysts supported on different porous oxides but no general consensus concerning the nature and the evolution of the silver species under thermal/gas treatment is reached yet. In this contribution, the influence of the oxide support, the silver deposition method (exchange or impregnation) and the activation conditions on the state of silver are investigated. The final size, the dispersion and the state of the nanoparticles are evaluated by transmission electron microscopy (TEM).

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2. Experimental Ag/13X(IWI) and Ag/SBA-15(IWI) catalysts with 3 wt.% Ag were prepared by incipient wet impregnation method [8] using an aqueous solution of silver nitrate in a rotary evaporator at 60°C. After impregnation, vacuum was applied at the same temperature to remove the excess water. The resulting powder was left to dry in air. A silver 3 wt.% exchanged faujasite Ag/13X(E) was prepared by exchanging the zeolite with the adequate amount of a diluted solution of silver nitrate in the dark [9,10]. All samples were calcined in air up to 500°C (5°C/min) then temperature programmed reduction (TPR) was performed under flowing 5%H2/Ar (7°C/min) and followed by both catharometry (TCD, Autochem 2910, Micromeritics) and mass spectrometry (MS, HPR20/DSMS). UV-Visible spectroscopy (UV-Vis.) was carried out on a Varian Cary 5000 spectrophotometer. For TEM experiments, performed after TPR at 700°C, samples powders weare deposited on Cu grid covered with a carbon film. Observations were made on a JEOL 2011 UHR (200kV accelerating voltage and LaB6 emission) microscope equipped with an Orius Gatan camera.

3. Results Transmission electron microscopy was carried out to compare the silver dispersion on the three different samples after TPR. Representative micrographs and their related histograms of particle sizes are shown in Figure 1. On Ag/13X(E), small particles are present with a mean diameter of 7 nm. Much bigger particles with a mean diameter of 22 nm are formed on Ag/SBA-15(IWI). In the case of the silver impregnated zeolite Ag/13X(IWI), two populations with a mean particle diameter of 7 and 24 nm are found.

(a)

(b)

(c)

(d)

2.8 Ǻ (oxide) 2.36 Ǻ (metallic) Figure 1. Representative micrographs of Ag/13X(IWI) (a), Ag/SBA-15(IWI) (b) and Ag/13X(E) (c) and corresponding histograms of particle diameters (in arbitrary unit). High Resolution Electron Microscopy (HREM) micrograph of Ag/13X(E) after calcinations; both metallic silver and silver oxide inter-reticular distances are found in the image: 2.4 Å and 2.8 Å characteristic of (111) fcc metallic silver and (111) cubic silver oxide (d).

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For a better understanding of the different particle sizes, TPR/MS and UV-Vis. spectroscopy were conducted after reduction of the samples at different temperatures. Water production, detected by mass spectrometry (m/e=17,18), enables to differentiate whether the metal reduction involves oxide or cationic species. Figure 2 shows the results of TPR followed by simultaneous TCD and MS measurements for the silver exchanged faujasite Ag/13X(E) sample that gives the best dispersion. Three temperature ranges are detected: I (80-200°C), II (260-400°C), III (423-540°C). Some authors attributed the low temperature zone (I) to the reduction of silver cations into charged clusters on Ag/MFI [4,5,12]. Other authors attributed this peak to the reduction of silver oxides on Ag/HY [6]. At higher temperatures (zone III), according to Baek et al. [6], reduction of cations into metallic silver occurs. Nevertheless, other authors attributed this high temperature peak to a reduction of silver clusters [4,5,12]. From our data, only the second temperature range involves a water production, being thus assignable to the reduction of oxide species as was already proposed [13]. The two other low and high temperature ranges (I and III) unambiguously correspond to the reduction of cationic species. Concerning Ag/SBA-15(IWI), no significant TCD signal is detected upon sample reduction. This suggests that the AgNO3 impregnation on this support followed by calcination led mainly to reduced Ag particles with low dispersion as is seen by TEM (Figure 1b). The formation of significantly smaller particles on the Ag/13X(E) sample is related to the reduction of exchanged silver cations stabilised by the zeolite support. Diffuse reflectance UV-Visible spectroscopy was carried out to further identify the transformations of the cationic species in the exchanged zeolite after reduction, at different temperatures (Figure 3). This allows a better comprehension of the silver reduction mechanism. In freshly calcined Ag/13X(E), cationic silver (band at 235nm) and silver clusters (246-280 nm) with a slight contribution of metallic silver were detected (>290 nm) [11]. The decrease of the cationic silver band contribution when the temperature increases to 700°C, and the simultaneous increase of the metallic and clusters bands, suggest a two step formation of the silver particles in the exchanged silver zeolite as follows: Ag+ ÆAgnm+ ÆAg.

Figure 2. H2 -TPR profile of Ag/13X(E) followed by mass spectrometry.

Figure 3. UV-Visible spectra of the Ag/13X(E) after TPO at 500 (a) followed by TPR at 350°C (b), 700°C (c).

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Finally, in addition to the cationic and oxide silver species seen above by combined TPR/MS and UV-Vis. data, the presence of a metallic phase is also found by TEM on Ag/13X(E) freshly calcined. Therefore, the coexistence of either oxidized or reduced Ag particles is deduced from the measurement of two distinct inter-reticular distances at 2.4 and 2.8 Å corresponding to oxide and metallic silver, respectively (Figure 1d).

4. Conclusion In this work, the state of silver, the formation and the final size and dispersion of the particles are investigated. The combination of TPR/MS and UV-Vis. spectroscopy allows a better comprehension of the transformations occurring during thermal/gas treatment In addition, TEM shows that the exchanged zeolite exhibits the smaller particles with best dispersion that can be attributed to the stabilization, and next, to the reduction of exchanged ions

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]

M. Boutros et al, 2009, Appl. Catal. B, 91, 640-648. E. Cordi et al, 1997, Appl. Catal. A, 151, 179-191. N. Luo et al, 2009, Mater. Lett., 63, 154-156. C. Shi et al, 2004, , Appl Catal B, 51, 171-181. J. Shibata et al, 2004, J. Catal, 227, 367-374. S-W. Baek et al, 2004, Catal Today, 93-95, 575-581. T. Namba et al, 2008, J. Catal, 259, 250-259. Y. Li et al, 2009, Appl. Catal. B, 89, 659-664. J. R. Morton et al, 1987, Zeolites, 7, 2-4. T. Sun et al, 1994, Chem. Rev., 94, 4, 857-870. A. Satsuma et al, 2005, Catal. Surv. Asia, 9, 2, 75-85. J. Shibata et al, 2004, Appl. Catal. B: Environ, 54, 137-144. W. Gac et al, 2007, J. Mol. Catal. A: Chem, 268, 15-23.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V.

Strong electrostatic adsorption for the preparation of Pt/Co/C and Pd/Co/C bimetallic electrocatalysts L. D’Souza and J. R. Regalbuto* ([email protected]) Dept. of Chemical Engineering, University of Illinois at Chicago 810 S. Clinton, Chicago, IL 60607

Abstract The method of “strong electrostatic adsorption” (SEA) can be extended to the rational synthesis of bimetallic catalysts. In this study it is demonstrated that cationic ammine complexes of palladium or platinum selectively adsorb onto the cobalt oxide particles of a cobalt oxide/carbon surface. This done at an equilibrium pH of 11, where the carbon surface is negligibly charged and the cobalt oxide surface is deprotonoated and negatively charged. Reduction at high temperature leads to homogeneously alloyed particles while lower temperature reduction leads to core-shell morphologies with a core of cobalt. Keywords: bimetallic catalyst synthesis, electrostatic adsorption

1. Introduction The seminal paper of Brunelle (1978) outlined the rational method of catalyst synthesis whereby charged metal coordination complexes such as hexachloroplatinate ([PtCl6]-2) or platinum tetraammine ([(NH3)4Pt]+2) can be electrostatically adsorbed onto oxide surfaces which contain naturally occurring hydroxyl groups (-OH) that are either protonated and positively charged (--OH2+) or deprotonated or negatively charged (-O-), depending on the solution pH. We have employed this method to synthesize highly dispersed, highly loaded noble and base metals on silica, alumina, and carbon supports (Regalbuto 2007). Electrostatic control of metal complex adsorption might also be achieved at the nanoscale over surfaces containing two oxides for a scientific method to prepare a wide range of bimetallic catalysts and promoted catalysts. The idea is illustrated in Figure 1 in the simulation of surface potential versus pH for a surface consisting of a carbon with

Figure 1. Electrostatic adsorption preparation strategy for bimetallics.

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a PZC of 9, which supports particles of cobalt oxide, which has a PZC of about 7. At a pH of 8, the cobalt oxide phase will be deprotonated and negatively charged, while the carbon surface will be protonated and positively charged. Tetraammine cations of Pt and Pd should then adsorb selectively onto the cobalt oxide particles. Subsequent reduction in H2 can then be used to form bimetallic PtCo or PdCo particles.

2. Experimental The details of our experimental procedures for studying metal adsorption and characterizing synthesized materials are found elsewhere (D’Souza 2008). With a bimetallic system the idea is to study adsorption of a particular metal complex over the individual components (in this case, Vulcan XC72 carbon black with a PZC about 9 and 254 m2/g and cobalt (II,III) oxide, synthesized by calcining cobalt nitrate at 400°C, PZC = 7 and 60 m2/g). From these experiments the pH to maximize adsorption selectivity is determined. Next, adsorption is conducted at this optimal pH over a physical mixture of the carbon and cobalt oxide to permit facile characterization by electron microscopy. When the selectivity of adsorption has been confirmed in this way, the carbon supported Co3O4 sample is impregnated with the metal amine and the bimetallic nature of the material is characterized by single particle analysis in the electron microscope (single point and line EDXS and EELS scans) and other methods such as EXAFS and TPR.

3. Results and discussion The results of adsorbing platinum and palladium tetraammine (PTA and PdTA) over Vulcan XC72 and over Co3O4 are shown in Figure 2. Neither metal adsorbs to an appreciable extent over the carbon support. In another publication we have surmised that the chargeable groups on high PZC carbon, likely the pi bonds of aromatic rings, can only be protonated and positively charged, and not deprotonated; that is, virgin carbon blacks with high PZC can only adsorb anions at low pH, and not cations at high pH (Hao, Barnes, and Regalbuto, manuscript in preparation). On the other hand, both the PTA and the PdTA exhibit the volcano shape of uptake versus pH characteristic of strong electrostatic adsorption (Regalbuto 2007). Since the carbon does not adsorb either noble metal to an appreciable extent in the high pH range, a pH of 11 was employed for adsorption onto physical mixtures and onto the carbon supported cobalt oxide.

Figure 2. PTA and PdTA uptake versus pH on Co3O4 and Vulcan XC72 (200 ppm metal, 1000 m2/l surface loading.

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For adsorption over the physical mixtures, 200 ppm solutions of PTA and PdTA were contacted with cobalt oxide and carbon each with 1000 m2/l in solution. The dried materials were reduced at 150°C in hydrogen for one hour to form Pt particles (TPR results, not shown, reveal that this temperature is not sufficient to reduce the cobalt oxide). Representative micrographs of PTA adsorption are shown in Figure 3. The Pt particles predominate over the cobalt oxide fraction of the sample. From analysis of several dozen areas and hundreds of particles, the Pt and Pd (not shown) are about 80 percent partitioned onto the cobalt oxide phase. All particles are in the size range of 1 to 3 nm, typical of SEA preparations.

Co3O4

Vulcan XC72

Figure 3. PTA uptake at pH 11 over a physical mixture of Co3O4 and Vulcan XC72 (200 ppm metal, 1000 m2/l each surface loading).

The preparation of highly dispersed cobalt oxide on carbon is detailed in another paper (D’Souza et al., 2007). A 10 wt% Co/Vulcan XC72 sample is shown in Figure 4, before and after adsorption of PTA at a pH 11. The PtCo sample was reduced at 400°C. The mass ratio of the Pt to Co adsorbed is 1 to 1.5. The particles are somewhat larger, on the order of 2-6 nm. Single spot analysis of many dozens of particles has revealed the virtually complete contacting of platinum with cobalt. The appearance of the PdCo sample was similar, with a similar essentially complete contacting of the Pd with Co.

Co3O4/VXC

PtCo1.5/VXC

Figure 4. The 10 wt% cobalt oxide/carbon substrate, onto which was adsorbed PTA at pH 11.

Analysis of single particles was also done with line scans using either EDXS or EELS. An EDXS line scan of a series of PtCo particles reduced at 500°C (Figure 5) shows a coincidence of Pt and Co profiles. When the PdCo material was reduced at the lower temperature of 200°C, a core-shell morphology is apparent both from the image (bottom left of Figure 5) and the EELS line scan profiles of Pd and Co.

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PtCo1.5/VXC

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Co Pd

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0.01 µm

Figure 5. Line scans of PtCo (with EDXS) and PdCo (with EELS) particles.

4. Conclusions A rational approach for synthesizing bimetallic catalysts can be based on electrostatic adsorption by exploiting the PZC differences between the catalyst support and a supported metal oxide of one of the catalyst metals. The catalyst support is chosen with a PZC different from the metal oxide, and the second metal catalyst precursor is made to selectively adsorb onto the first metal oxide by appropriate control of solution pH. Carbon is a particularly good support for this type of synthesis as it can be oxidized and its PZC can be controlled (Hao 2006). As demonstrated in this work for PtCo and PdCo on carbon, the severity of reduction can be used to produce either alloyed particles or core shell structures.

References J.P Brunelle, 1978, Preparation of catalysts by metallic complex adsorption on mineral oxides, Pure Appl. Chem. 50, 1211. L. D'Souza, J.R. Regalbuto, J.T. Miller, 2008, Preparation of carbon supported cobalt by electrostatic adsorption of [Co(NH3)6]Cl3, J. Catal., 254, 157. X. Hao, L. Quach, J. Korah, W. A. Spieker, J. R. Regalbuto, 2004, The Control of Platinum Impregnation by PZC Alteration of Oxides and Carbon, J. Mol. Catal. A: Chem., 219, 97. J.R. Regalbuto, 2007, Strong Electrostatic Adsorption of Metals onto Catalyst Supports, in Catalyst Preparation: Science and Engineering, J.R. Regalbuto, ed., Taylor and Francis/CRC Press, 2007

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Preparation of gold catalysts supported on SiO2-TiO2 for the CO PROX reaction L. Gonzalo-Chacón,a B. Bachiller-Baeza,a A. Guerrero-Ruiz,b I. Rodríguez-Ramosa a

Instituto de Catálisis y Petroleoquímica, CSIC, C/ Marie Curie, 2, Cantoblanco, 28049 Madrid b Dpto. Química Inorgánica y Técnica, Facultad de Ciencias, UNED, Senda del Rey, 9. 28040 Madrid

Abstract A titania-coated silica, as prepared and calcined, was used as support to prepare two Au catalysts. The catalysts were characterized by several techniques, X-ray photoelectron spectroscopy (XPS), scanning transmission electron microscopy (STEM), and tested in the CO Preferential Oxidation (CO PROX) reaction. The dispersion of Ti onto the silica was very homogeneous. On the other hand, the incorporation of Au was limited to less than 1 % and the particles size varied in the range 3-10 nm in both samples. Reaction studies were carried out on a fix bed reactor and on a Temporal Analysis of Products (TAP) reactor. The results reveal the participation of the surface -OH groups on the mechanism of the selective CO oxidation. Keywords: titania-coated silica, Au catalyst, deposition-precipitation method

1. Introduction The preferential CO oxidation in H2-rich streams is a reaction of great relevance due to its application in the purification of feeds for hydrogen fuel cells, and because of the scientific interest. This reaction is known to be very sensitive to catalytic surface structures and to the pretreatments. Au catalysts supported on metal oxides with high metal dispersions have been demonstrated as very effective in this PROX reaction [1]. However, the studies carried out over these systems have allowed us to conclude that several factors affect the performance of the catalyst, such as particle sizes, preparation method, supports, etc. Nevertheless, there is still some controversy with respect to the nature of the active site or about the mechanism of reaction. In the present communication and with the aim to obtain further information to elucidate these unresolved points a study on the role of the support in the CO PROX mechanism, particularly emphasizing in the aspects related to the preparation of Au-supported catalysts is presented. The use of a TAP (Temporal Analysis of Products) reactor is also applied to reveal elementary processes that are taking place on the surface under reaction conditions, since this reactor system allows the detection of reactants and products with a submillisecond time resolution [2].

2. Experimental The support material used is a titania-coated silica (Ti-Si). This titania-coated silica is prepared by deposition-anchoring of TiO2 from ethanolic solutions of Ti[O(CH2)3CH3]4 alkoxide over SiO2. The silica support (aerosil, SBET=482 m2·g-1) is suspended in ethanol (250 ml/g support) and the amount of alkoxide to reach a Ti/-OH ratio of 3 is added.

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The mixture is stirred under He flow and the temperature is increased to 343 K. Then, water is added in small doses at 30 minutes intervals; the water to Ti(OR)4 ratio being of 30. After that, the mixture is kept under reaction for 4 h. The sample is filtered, washed with ethanol and dried at 383 K overnight. The support prepared using this procedure is denoted as Si-Ti and is characterized by thermal analysis (TG). An aliquot of this support is calcined at 723 K for 24 h leading to Si-Ti-C support. Fourier Transform Infrared Spectroscopy (FTIR) is carried out on self-supported wafers. The spectra are recorded on a Nicolet 5 ZDX spectrophotometer equipped with an MCT (mercurycadmium-telluride) detector with a resolution of 4 cm-1. The Au catalysts are prepared by the deposition-precipitation method in basic medium using HAuCl4. The pH of the aqueous solution of HAuCl4 is adjusted between 10 and 11 with a 0.2 M NaOH aqueous solution, then the support is added and the pH is again corrected to 10-11. The mixture is stirred for 18 h and the solid is filtered, washed and dried overnight at 383 K. The prepared catalysts are denoted as Au-Si-Ti and AuSi-Ti-C for the uncalcined and calcined support respectively. The catalysts are characterized by XPS before and after reaction, and by STEM and STEM mapping after reaction. XPS spectra are obtained on a ESCAPROBE P spectrometer from OMNICROM equipped with a EA-125 hemispherical multichannel Electronics analyzer. The pressure in the analysis chamber is kept below 10-9 Pa. The excitation source is the Mg Kα line (hν=1253.6 eV, 300 W). The binding energy is referenced to the C 1s line at 284.6 eV. Samples for examination by STEM are prepared by dispersing the catalyst powder in high-purity ethanol, then allowing a drop of the suspension to evaporate on a holey carbon film supported by a copper grid. High-angle annular dark-field (HAADF) imaging is carried out on a JEOL microscope model JEM 2100F, which is also equipped with an Oxford Instruments Inca software package for X-ray energy dispersive spectroscopic (XEDS) mapping. The CO oxidation reaction is studied in a fix-bed reactor under atmospheric pressure and temperatures in the range 333-413 K. A mixture 1% CO, 1% O2, 40% H2 (He balance) is passed through the catalytic bed. Transient experiments are carried out in a TAP-2 reactor where single pulses or simultaneous pulsing of reactants are performed at 473 K after the sample is outgassed in vacuum at the same temperature for 1 h.

3. Results and discussion The comparison of the FTIR spectra of a self-supported wafer of the support, before and after in situ calcination, and of the original silica support is presented in Figure 1. The spectrum obtained for Si-Ti present a group of bands in the range 2850-2970 cm-1 which are assigned to the C-H stretching vibration of ligands anchored to the silica surface during the preparation process. These bands are absent in the spectrum after calcination confirming the removal of the hydrocarbon fragments. It is also important to notice some modifications occurring in the range 4000–3000 cm-1 related to stretching vibrations of hydroxyl groups. The sharp single peak at 3740 cm-1 arise from isolated surface hydroxyl groups (–Si–OH), and is typical for highly hydrophilic samples [3]. The increase in this signal for the Si-Ti sample can be related to changes in the proportion between H-bridging hydroxyl groups (–Si–OH. . .O–Si–), associated with the band at 3590 cm-1, and isolated silanol groups due to reaction between adsorbed titanium alkoxide molecules and surface OH, producing Ti-O-Si bonds. A comparison of curves also confirms the removal of a considerable amount of adsorbed water, showed by the broad band at 3350 cm-1, during the in situ calcination process. The XPS spectra of the fresh Au catalysts (Figure 2) show the characteristic peaks of Ti, Si and O, and of Na due to residual species coming from the preparation method.

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However, the Au signal cannot be detected probably due to the is 0.21 and 0.12 for Au-Si-Ti and Au-Si-Ti-C respectively. The Ti 2p3/2 binding energy for the TiO2-grafted SiO2 (about 460 eV) is shifted to higher values compared to that for TiO2 (458.5 eV) . This fact reflects an intimate association of TiO2 and silica producing Ti–O–Si bonds where Ti4+ occupies tetrahedral coordination sites similar to Si in SiO2. The O 1s peak presents two components at 533.5 and 530.5 eV. The first is typical of SiO2 while the second component can be reasonably assigned to oxygen in Si–O–Ti bonds at the surface [3]. The presence of residual hydrocarbon fragments on the non calcined support is confirmed again after analysis of the C1s core level. Using the annular dark field image (Figure 3), which gives strong atomic number (Z) contrast, it is found that for these catalysts the metal particles range from 3 to 10 nm in diameter for both catalysts. STEM-XEDS mapping also shows that the Si-L1 and Ti-L1 signals are spatially coincident, indicating that the Ti coating is very homogeneous. However, a higher concentration of Ti is observed in some small areas which could reveal the existence of some TiO2 surface segregation (see the arrow in Ti map).

Figure 1. FTIR spectra of the support before and after calcination.

Figure 2. XP spectra of the Au catalysts.

20 nm

Au

Si

Ti

Figure 3. HAADF image showing small Au particles in the Au-Si-Ti-C sample (left) and corresponding STEM-XEDS maps of the Au-L1, Si-L1 and Ti-L1 signals.

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As far as the oxidation of CO reaction is concerned, both Au catalysts are active for the CO oxidation without any pretreatment (Figure 4). Comparing the fresh and the calcined support, it can be observed that the catalysts prepared on the calcined support give higher CO conversions at lower temperatures. Therefore, it is suggested that the OH groups are involved in the reaction mechanism affecting the catalyst performance. By pulsing CO and O2 over the catalysts on the TAP reactor, it is revealed that both molecules are adsorbed reversibly on the surface. The lower conversions obtained when CO, O2 and H2 are simultaneously pulsed over the catalysts compared with those obtained in the fix bed reactor, are due to thermal effects and heat transfer differences in both types of experiments. In spite of that, a shift to longer times is observed for the maximum of the CO2 peak signal compared to the transient responses of reactants. This shift suggests that H2O derived species, which are formed by oxidation of H2, and bicarbonate species, which are related to the –OH of the support, are involved on the reaction mechanism.

Figure 4. Results of CO oxidation in the fixed bed reactor over fresh and calcined supports.

4. Conclusions Therefore, all the characterization techniques indicated that the preparation method of the titania-coated silica support is very effective and that multilayer coatings of TiO2 are obtained on the SiO2 after calcination. More precisely, the grafting of titanium onto silica is produced by reaction between alkoxide precursor and surface OH groups. Additionally, the support post-treatment, although not influencing the particle size distribution significatively, determines the Au catalyst performance.

Acknowledgment Authors recognize financial support from MICINN (CTQ2008-06839-C03-01/PPQ and -03/PPQ).

References A Stephen, K. Hashmi, G.J. Hutchings, 2006, Gold Catalysis, Angew. Chem. Int. Ed., 45, 7896. G.S. Yablonsky, M. Olea, G.B. Marin, 2003, Temporal analysis of products: basic principles, applications, and theory, J. Catal., 216, 120. C.U.I. Odenbrand, S. L. T. Andersson, L.A.H. Andersson, J.G.M. Brandin, G. Busca, 1990, Characterization of Silica-Titania Mixed Oxides, J. Catal., 125, 541.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

A method of preparation of active TiO2-SiO2 photocatalysts for water purification M.P. Fedotovaa, G.A. Voronovaa,b, E.Yu. Emelyanovaa, O.V. Vodyankinaa a b

Tomsk State University, 36, Lenin str., 634050 Tomsk, Russia Tomsk Polytechnical University, 30, Lenin str., 634050 Tomsk, Russia

Abstract A new method of synthesis of highly active TiO2-containing systems to purify wastewater by photodecomposition of organic substances has been proposed. A number of catalysts with titania nanoparticles homogeneously distributed on the silica support surface have been prepared and investigated. It is shown that the photoactivity for both unpromoted and Au-containing samples increase not only under action of UVirradiation. The significant growth of kinetic constant of methylene blue degradation is observed under the action of visible light, compared to Degussa P 25. Keywords: photocatalysis, titania, supported catalysts, gold nanoparticles

1. Introduction This work deals with the preparation of active titania photo-catalysts. To obtain effective semiconductor photo-catalysts it is necessary to note that different interfacial electron processes with participation of e- and h+ must compete effectively with the main deactivation processes of e-–h+ recombination. Recombination of e-–h+ pair may occur in the bulk or on the surface. To form an active photo-catalyst it is necessary to distribute active TiO2 species homogeneously on the surface of support, and create conditions for prolongation of lifetime of charge carriers. This can be achieved by deposition of titania on a surface of suitable support and/or addition of promoter such as noble metal nanoparticles (mainly, Pd, Pt, Ag, Au). A new method of synthesis of highly active TiO2-containing systems to purify wastewater by photodecomposition of organic substances is developed in the present work. The main idea of the developed preparation method is the controlled hydrolysis of organic precursor of titania by surface hydroxyl group of the silica support in dehydrated organic solvent.

2. Experimental 2.1. Catalyst preparation The TiO2 supported catalysts are prepared by the hydrolysis of titanium tetraisopropoxide (TTIP, Merck) on the surface of silica support in the dehydrated toluene. SiO2 aerogel (Ssp = 100 m2/g) is used as a support; it is prepared as recommended in [1]. The required amount of the support is added into the solvent, continuously bubbled by dry gas (nitrogen or argon) flow. TTIP is partially added at constant temperature of 110 oC into reaction vessel. After addition of each portion of precursor the reaction mixture is thermostated for 30 min. Finally, the obtained samples are dried in vacuum at 25 oC for 2 h and then heated in inert gas flow at 550 oC for 5 h. Several TiO2/SiO2 supported samples containing 3–15 wt % of TiO2 are synthesized to study their physicochemical properties and catalytic characteristics.

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Gold deposition on the surface of the prepared TiO2/SiO2 catalysts is performed by deposition-presipitation technique using water solution of HAuCl4 (Acros) in the presence of urea according to [2]. 1 g of catalyst is added to 100 mL of an aqueous solution containing HAuCl4 (4.2 × 10−3 M) and urea (0.42 M). The initial pH is 2. The suspension is thermostated at 80 oC and vigorously stirred for 16 h. Urea decomposition leads to gradual rise in pH from 2 to 7. The solids are gathered by centrifugation (12,000 rpm for 10 min), washed in 100 mL of distilled water under stirring for 10 min at 50 oC, and then centrifuged. The sequence of operation is repeated several times. The solids are dried under vacuum at room temperature for 16 h and then at 150 oC for 2 h in air.

2.2. Photocatalytic tests The photocatalytic activity of the prepared systems is studied in the well known methylene blue (MB) dye decomposition reaction under stationary conditions [3]. The source of UV-radiation is a DRSh-250 mercury lamp; the source of visible-radiation is a Sylvania lamp (60 mW); the radiation is not filtered. An aqueous solution of MB and the required amount of the catalyst (5 or 8 mg) are placed into reactor (volume 50 ml) with a quartz window. The suspension is treated in ultrasonic bath for 10 min for homogenization. During irradiation, the suspension is thoroughly stirred using magnetic stirrer. Air is passed through the suspension at a constant rate for saturation with oxygen and more effective stirring. The activity of the catalysts is estimated from a decrease in the MB concentration. The MB concentration on solutions is determined spectrophotometrically using SF-256 spectrophotometer at equal time intervals (samples were preliminarily centrifuged for 5 min on an OPN-12 centrifuge with a 8000 rpm rotation rate). The photoactivity of the prepared samples (the degree of MB transformation in a certain time interval) is compared to the activity of titania Degussa P25 (45 m2 g−1, nonporous, 70 % anatase and 30 % rutile, purity > 99.5%). Table 1. Chemical composition and several features of prepared catalysts. Sample composition

Ssp, m2/g

Average pore diameter, nm

1%Au / 3%TiO2 / 96%SiO2

85,5

35,7

1%Au / 6%TiO2 / 93%SiO2

89,7

31

1%Au / 8%TiO2 / 91%SiO2

74,6

20

1%Au / 15%TiO2 / 84%SiO2

36,6

20,3

Phase composition, % mol. Au SG 225 Amorphous Au SG 225 Anatase Amorphous Au SG 225 Anatase Amorphous Au SG 225 Anatase Amorphous

0,5 99,5 0,5 4,5 95 <0,5 4,5 95 0,5 19,5 80

Coherent scattering region, mn 9 9 9 9 -

2.3. Catalyst characterization The specific surface area of the samples is measured by the single-point Brunauer– Emmett–Teller method (nitrogen adsorption). Measurements are taken on a TriStar 3020 analyzer (Micromeretics). The X-ray diffraction patterns of the samples are obtained on a Shimadzu XRD-6000 diffractometer. The structure and size of particles are studied using a Philips CM 30 transmission electron microscope with an accelerating voltage of 200 kV. The chemical composition of the systems is studied by FTIR spectroscopy on a Nicolet 5700 spectrometer.

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3. Results and discussion The chemical, phase composition and structural characteristics of supported catalysts are represented in Table 1. It is shown that addition of Au does not practically influence the phase composition and structural features of supported catalysts. According to TEM and electron microdiffraction data the active component (titania) is uniformly distributed along the support surface as 9-10 nm nanoparticles of two basic phases: TiO and TiO2 (tetragonal anatase). Gold nanoparticles are not observed in TEM image. IR spectra obtained from Au promoted catalysts are presented in Fig.1. The chemical composition of the silica aerogel is characterized by the following set of absorption bands: 470, 808, 1100, and 1640 cm–1. Bond vibrations at 1100 cm–1 are Si–O–Si asymmetric stretching vibrations. Absorption bands at 808 and 470 cm–1 can be assigned to bending vibrations of Si–O–Si and O–Si–O groups. A weak absorption band at 1640 cm–1 appears because of the presence of adsorbed water on the surface of the samples. The increase in TiO2 content in the sample composition leads to the appearance of a new line at 940-960 cm-1 which may be associated with vibration of Ti–O–Si groups. Thus, the support stabilizes titanium dioxide at the expense of chemical interaction with the Ti–O–Si bond formation. The UV-VIS absorption spectra of the investigated photocatalysts before and after promoter addition are presented in Fig. 2. The unpromoted supported catalysts exhibit a UV-vis adsorption below 400 nm, i.e. blue shift of absorbance is observed because of the reduction of titania particle size to up to less than 10 nm. This fact corresponds well to XRD and TEM data (Table 1).

The presence of additional absorbance band at 400-425 nm is observed for the supported catalysts with TiO2 content not more than 8 % wt. This band is related to the presence of a defect TiO phase in the investigated samples (similar data are obtained by an electron microdiffraction method). The Au-promoted catalysts display the plasmonic band at 520-550 nm in the visible region (fig. 2b) which is typical for gold nanoparticles

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(~10 nm) [4]. Additional absorbance at 420 nm may be associated with the presence of highly dispersed Au clusters on the catalyst surface. The photocatalytic activity of the prepared sample is studied in the well known reaction of MB degradation under the action of UV- and visible light. The results are represented in Table 2. All the prepared catalysts (unpromoted and promoted) exhibit higher photoactivity in comparison with Degussa P 25. Moreover, the photoactivity increases not only under UV-irradiation action. The significant growth of the kinetic constant of the MB degradation is observed under the action of visible light. It is shown that the Au promoter distribution plays a key role in the photoactivity enhancement. The amount of Au promoter for all prepared samples is constant (1 % wt). In the catalyst with low TiO2 content (3-6 % wt) a part of Au-containing additive is distributed on silica support surface. Such particles are not able to participate in the photoprocesses. The increase of TiO2 content up to 8-15 % wt allows to distribute Au promoter over titania surface predominantly and provide its participation in the MB photodegradation. Table 2. Catalytic activity of Au promoted TiO2/SiO2 catalysts. Sample composition

Adsorption MB, а×106, mol/m2

Kinetic constants for photocatalytic degradation, k×104, s−1 UV

VIS

1%Au/3%TiO2/96%SiO2

10,1

3,72

4,85

1%Au/6%TiO2/93%SiO2

9,9

4,4

4,03

1%Au/8%TiO2/91%SiO2

6,7

5,48

6,11

1%Au/15%TiO2/84%SiO2

4,4

6,2

6,08

TiO2 Degussa P25

0,27

2,33

0,58

4. Conclusions Supported TiO2-SiO2 catalysts are prepared by the heterogeneous supporting technique. The photoactivity of the prepared samples is investigation in the reaction of MB photodegradaton. The kinetic rate constant of TiO2-SiO2 systems is higher, compared with Degussa P 25. It is shown that the photoactivity of TiO2-SiO2 systems modified by Au nanoparticles is more effective in comparison with the catalysts mentioned above under UV and visible light illumination.

Acknowledgement This work was supported by the Russian Federal Program “Scientific and scientificeducational professional community of innovated Russia”.

References 1. 2. 3. 4.

T. I. Izaak, O. V. Babkina, O. V. Magaev, A. S. Knyazev, et al., Nanotekhnika, 2007, Issue 4, Pages 34-47. R. Zanella, S. Giorgio, Shin Chae-Ho, C.R. Henry, C. Louis, Journal of Catalysis, 2004, Volume 222, Pages 357–367. M. P. Fedotova, G. A. Voronova, E. Yu. Emel’yanova, N. I. Radishevskaya, O. V. Vodyankina, Russian Journal of Physical Chemistry A, 2009, Volume 83, Issue 8, Pages 1371–1375. G. L. Chiarello, E. Selli, L. Forni, Appl. Catal. B: Envir., 2008, Volume 84, Pages 332-339.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

n-Heptane hydroconversion on bifunctional hierarchical catalyst derived from zeolite MCM-22 Márton Kollár, Magdolna R. Mihályi, József Valyon Institute of Nanochemistry and Catalysis, Chemical Research Center, Hungarian Academy of Sciences, Pusztaszeri út 59-67, Budapest, Hungary 1025

Abstract MCM-22/MCM-41 composite, having hierarchical micro/mesoporous pore structure, was prepared using two-step synthesis method. Accordingly, mesoporous MCM-41 silica was synthesized in the presence of a delaminated MCM-22 zeolite precursor (dl-MCM-22). The structure, texture, and the acidity of the composite were compared to the corresponding properties of the MCM-22 zeolite. The hydroconversion of n-heptane was studied over the Ni,H-forms of these preparations at 240°C and 5 bar total pressure. At equal space times, regardless of the Ni-loading (Ni/H+ = ~0.25 or 0.5), the rate of cracking was significantly lower over the composite than over the zeolite catalysts. The lower cracking activity was attributed to the faster transport of the reactants and products within the composite particles. At similar conversions similar selectivities were obtained over the compared catalysts. Keywords: n-heptane hydroconversion, MCM-22/MCM-41 composite, hierarchical pore structure

1. Introduction The catalytic potential of zeolites often can not be fully exploited because of the hindered mass transport of the reactants and/or products within the microporous zeolite particles. If slow transport governs the reaction rate the catalytic efficiency can be enhanced by decreasing the diffusion hindrance. Obviously, mesopores in the microporous zeolite particles promote diffusion and the utilization of the active sites in the micropores. The discovery of new routes for obtaining hierarchically structured micro/mesoporous zeolites, and the understanding of the surface and catalytic properties of the obtained materials is a great research challenge [1,2]. Many papers relate to zeolite ZSM-5, Y, and mordenite but only few concerns zeolite MCM-22, having hierarchical pore system [3]. Zeolite MCM-22 is obtained by calcination of a synthetic layered-structure zeolite precursor. Prior to calcination the precursor can be delaminated. Calcination of the delaminated material (dl-MCM-22) gives fixed disordered aggregations of aluminosilicate layers. This material is usually referred to as ITQ-2 [4]. A single sheet comprises twodimensional ten-membered ring (10-MR) sinusoidal channels and large 12-MR cups (7.1 Å diameter, 7.0 Å depth) on both sides of the sheet. There are no pores for molecular transport between the channels and the cups. The zeolite MCM-22 has cups on the crystal surface and two independent channel systems within the crystals: 10-MR sinusoidal channels and supercages formed by two cups, facing each other. The supercages are interconnected and accessible through 10-MR apertures. The MCM-41 material is a representative of the micelle-templated silica (MTS) materials, having parallel hexagonally ordered cylindrical mesopores of about 1.6-10 nm diameter [5]. Despite of their favorable mass transport properties the use of these

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materials as catalysts is limited due to the difficulties of generating high stability strong acid sites within the pores. The integration of the favorable acidic and diffusion properties of the MCM-22 zeolite and the MCM-41 material, respectively, promised a composite, which is better catalyst than the components alone. This study deals with the synthesis of micro/ mesoporous MCM-22/MCM-41 aluminosilicate composite material, and with the catalytic behavior of the Ni,H-form composite and zeolite in the hydroconversion of n-heptane (n-C7).

2. Experimental Hexamethyleneimine (HMI) was applied as structure directing agent and a stirred autoclave was used to get small MCM-22 precursor crystals. The obtained precursor was calcined in air at 550 °C for 5 h to get the zeolite MCM-22. The dl-MCM-22/MCM-41 composite of the present study was prepared by the method described in ref. [3]. First the layers of the MCM-22 precursor were separated. The procedure included swelling the crystals in basic medium using cetyltrimethylammonium bromide (CTAB), washing and ultrasonic treatment of the solid. Then, MCM-41 material was synthesized in presence of the swollen and delaminated MCM-22 material after adding of aluminum-free water glass and CTAB to the mixture. The amount of added Si was equal with the Si content of the dl-MCM-22. After washing and drying the composite material was calcined in air at 550 °C for 5 h. The NH4- and the Ni,NH4-forms of the materials were prepared by conventional ion exchange at room temperature. The preparations were characterized by ion exchange capacity (IEC), Al and Ni content, X-ray powder diffractogram (XRD), acidity, specific surface area and porosity. A fixed-bed flow-through microreactor (12 mm ID) was used for the catalytic measurements at 5 bar pressure and 240 °C temperature. The Ni,H-form catalysts were obtained by thermal deammoniation of the Ni,NH4-forms in situ in the catalytic reactor. The H2/n-C7 molar ratio was 19.6. The reactor effluent was analyzed by an on-line GC.

3. Results and discussion

500

dl-MCM-22/MCM-41 102

002 100

110 200

MCM-41 Intensity, counts

The composite material consists of two crystalline phases, (i) a mesoporous MCM41 phase evidenced by the appearance of its characteristic reflections (100, 110 and 200) in the range of below 2Θ ≈ 5°, and (ii) a delaminated zeolite MCM-22 component. Delamination is confirmed by the disappearance of the characteristic 002 reflection at 2Θ = 6.5°, and the broadening of the 100 and 102 reflections of the MCM-22 precursor (Fig. 1). The micropore volumes of the samples suggested that the zeolite content of the composite was about 65-70 % (Table 1).

100

3.1. Structure and acidity

MCM-22 precursor zeolite MCM-22

5

10

15 20 2 Θ, degree

25

30

Fig. 1. X-ray (Cu Kα) diffraction patterns of preparations.

n-Heptane hydroconversion on bifunctional hierarchical catalyst

1622 cm

-1

0.1

1454 cm -1

Absorbance, a.u.

1545 cm

-1

dl-MCM-22/MCM-41

zeolite MCM-22

1700

1600 1500-1 Wavenumbers, cm

1400

Fig. 2. FT-IR spectra obtained from adsorption of Py. The catalysts were activated in high vacuum at 400 °C for 1 h before Py adsorption at 0.6 kPa at 200 °C. Spectra were recorded at room temperature after 1h evacuation at 200 °C.

729

Upon pyridine (Py) adsorption the band of the bridging hydroxyls at 3610 cm-1 disappeared and a band appeared at 1545 cm-1, indicating that pyridinium ion (HPy+) was formed (Fig. 2). The intensities of the HPy+ bands suggest that the composite contains about half as much strong Brønsted acid site than the zeolite sample. This data are in conflict with the NH4+ IEC of the samples that is quite similar (Table 1). It was substantiated that delamination generates tricoordinated Al atoms, which are virtually absent in the zeolites. The tricoordinated Al atoms retain NH4+ ions but, unlike to Al T-atoms, do not generate strong Brønsted acid sites [6].

Table 1. Characterization of the zeolite and the composite samplesa. Sample +

NH4 IEC

b

Al,

mol/g

mol/g

MCM-22e

0.96

0.97

dl-MCM-22 /MCM-41

0.86

0.90

H-form Micro-pore Specific volume, d surface m3/g area, 2 m /g 518 565

0.14 0.09

Ni,H-form Mesopore volume, d m3/g 0.46

Ni,

Ni/IEC

mol/g 0.24

0.25

0.48

0.50

0.25

0.28

a

All data were related to 1 g of sample, calcined at 1000 °C. b The ion exchange capacity (IEC) of the sample was obtained as the amount of ammonia, evolved between 180 and 650 °C from the NH4+-form in a temperature-programmed deammoniation measurement. d Calculated from the alpha-s plot of the N2 adsorption isotherms. e The MCM-22 sample was used to prepare two catalysts of different Ni-loadings.

3.2. n-Heptane hydroconversion Isoalkanes are clean high-octane fuels that can be produced from straight chain alkanes by hydroisomerization over catalysts having both acid and metal active sites. The activity of the Ni,H-form preparations, characterized in Table 1, were compared in the hydroisomerization of n-C7. The yield vs. space time plots permit to distinguish primary and secondary products. The formation of dimethylpentanes was found to speed up as space time was increased (not shown). At low conversions (space times) the formation rates of the mono-branched C7 isomers and the cracking products (propane and iso-butane) were independent of the space time as shown by the linear plots in Fig. 3. These results suggest that multi-branched C7 isomers were obtained from the primary product methylhexanes and 3-ethylpenthane [7]. Neither hydrogenolysis products, such as methane and ethane, nor >C7 alkanes, were formed under the applied conditions. The metal to acid site ratio strongly influences the hydroconversion activity and selectivity [8]. TEM and XRD results suggested that Ni has high dispersion in each of our catalyst preparation. By doubling the Ni-content of the Ni,H-MCM-22 more than twofold increase was obtained in the rate of mono-branched C7 formation (Fig. 3A), whereas the rate of cracking decreased to a small extent only (Fig. 3B). Regarding the

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rate of the mono-branched C7 formation the activity of the Ni/H-dl-MCM-22/MCM-41 catalyst was between the activities of the Ni,H-MCM-22 catalysts (Fig. 3). An important finding of the present study is that the composite catalyst has a significantly lower cracking activity than any of the reference Ni,H-zeolite catalysts (Fig. 3B). The Ni/H+ ratio of the composite must be higher than the Ni/IEC ratio. It can be near to 0.5. Nevertheless, the low cracking activity of the composite catalyst can neither be accounted for its lower zeolite content nor for its higher Ni/H+ ratio. 25

60

Ni/H-MCM-22 + (Ni/H =0.50)

A

50 Ni/H-dl-MCM-22/MCM-41 (Ni/IEC=0.28)

40 30

Ni/H-MCM-22 + (Ni/H =0.25)

20 10 0 0.0

0.2

0.4 0.6 0.8 -1 1.0 Space time, gCATg C7h

1.2

1.4

B 20 Crack yield, wt%

Mono-branched isomer yield, wt%

70

Ni/H-MCM-22 + (Ni/H =0.25)

15 Ni/H-MCM-22 + (Ni/H =0.50)

10 5 0 0.0

Ni/H-dl-MCM-22/MCM-41 (Ni/IEC=0.28)

0.2

0.4 0.6 0.8 -1 1.0 Space time, gCATg C7h

1.2

1.4

Fig. 3. Yields of mono-branched isomers (A) and cracking products (B) as a function of space time. To get the Ni/H-catalysts the Ni,H-forms were pre-reduced in situ in the reactor in a 150 cm3 min-1 H2 flow of at 450°C for 2 hours.

The lower rate of cracking in the micro/mesoporous composite relative to the microporous zeolite can be explained by the shorter residence time of the C7+ carbenium ions and C7 products in the mesoporous particles having lower diffusion resistance. If the residence time was equalized by adjusting the space time to get similar conversions the selectivity difference of the catalysts disappeared.

4. Conclusions Hierarchical micro/mesopore structure can be obtained by forming composite from delaminated layered-structure zeolite and a mesoporous MTS material. At the same space time the bifunctional zeolite catalyst, having hierarchical micro/mesoporous structure, show lower n-C7 hydroconversion activity and higher hydroisomerization selectivity than the corresponding microporous zeolite catalyst.

Acknowledgement The authors thank for the financial support of the OTKA (No. K 68537), and of the National Office for Research and Technology (NKTH No. GVOP-3.2.1. 2004-04-0277/3.0).

References [1] M. Kollár, M.R. Mihályi, J. Valyon in I. Halasz (ed.), Silica and Silicates in Modern Catalysis, Transworld Research Network, India, Kerala, 2010, pp. 171. [2] J. Čejka, S. Mintova, Catal. Rev., 49 (2007) 457. [3] M. Kollár, R.M. Mihályi, G. Pál-Borbély, J. Valyon, Micropor. Mesopor. Mat., 99 (2007) 37. [4] A. Corma, V. Fornes, S.B. Pergher, Th.L. Maesen, J.G. Buglass, Nature, 396 (1998) 353. [5] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature, 359 (1992) 710. [6] B. Onida, L. Borello, B. Bonelli, F. Geobaldo, E. Garrone, J. Catal., 214 (2003) 191. [7] J.A. Martens, P.A. Jacobs, J. Weitkamp, Appl. Catal., 20 (1986), 283. [8] A. Lugstein, A. Jentys, H. Vinek, Appl. Catal., A: Gen., 166 (1998) 29.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Preparation and characterization of nanocrystallines Mn-Ce-Zr mixed oxide catalysts by sol-gel method : application to the complete oxidation of n-butanol Saïd Azalim,a,b,c,d Rachid Brahmi,d Mohammed Bensitel,d Jean-Marc Giraudon,a,b,c Jean-François Lamonier a,b,c a

Univ Lille Nord de France, F-59000 Lille, France CNRS, UMR8181, France c USTL, Unité de Catalyse et de Chimie du Solide F-59652 Villeneuve d’Ascq, France d UCD, Laboratoire de Catalyse et de Corrosion des Matériaux, Faculté des Sciences, 24000 El Jadida, Morocco b

Abstract A series of Zr(0.4)Ce(0.6-x)Mn(x)O2 mixed oxides catalysts with different compositions (x = 0; 0.12; 0.24; 0.36; 0.48) were prepared by a sol–gel method. The samples calcined at 500°C were characterized by X-ray diffraction (XRD), surface specific areas (SSA) and H2-TPR measurements and tested in the butanol oxidation. Using a sol-gel method very high SSA, small crystallite sizes and high redox properties are obtained especially when manganese content increased in the Zr-Ce-Mn-O system. The butanol complete oxidation is easier with Mn content increasing. Keywords: VOC, butanol, sol-gel, Zr-Ce-Mn oxide

1. Introduction Volatile Organic Compounds (or VOCs) are major air pollutants and the treatment by catalytic oxidation is one of the most promising ways to reduce these pollutants in the atmosphere since this technique allows operating at low temperatures (200-500°C) and thus leading to NOx formation in lower quantity. Noble metal (Pt, Pd) catalysts supported on alumina or other oxides are usually employed for VOC oxidation [1]. But metal oxides catalysts are also studied as cheaper alternatives to noble metals. Among them manganese oxides are the most active catalyst in VOC oxidation [2]. The Mn-CeO catalytic system has been the subject of a number of studies [3-5] due to the unusual redox behavior of ceria and its high oxygen storage capacity (OSC) [6]. It was further showed that the MnOx-CeO2 mixed oxides had much higher catalytic activity than those of pure MnOx and CeO2 owing to the formation of the solid solution between manganese and cerium oxides [7]. Besides it is well known that formation of mixed oxides of ceria with Zr4+ enhanced oxygen storage properties of ceria and the so-formed mixed oxides exhibited good thermal stability [8]. The present work describes different Zr-Ce-Mn-O catalytic systems synthesized using a sol-gel method. The effect of Mn amount is particularly studied in order to optimize the performances of the Zr-Ce-Mn oxides for the complete oxidation of n-butanol. Butanol has been chosen as VOC model molecule, because oxygenated VOC concentration in the atmosphere is increasing.

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2. Experimental 2.1. Catalyst preparation The mixed metal oxides catalysts Zr(0.4)Ce(0.6-x)Mn(x)O2 (x = 0; 0.12; 0.24; 0.36; 0.48) were prepared using a sol–gel method. The Ce(NO3)3.6H2O, Zr(NO3)2.5H2O and Mn(NO3)2.5H2O (0.5 mol/L) nitrates were separately dissolved in ethanol and added together in order to get the different molar ratio of Zr:Ce:Mn. To the resulting solution heated at 80°C was added deionized water (5 vol. % of ethanol) under constant stirring. The resulting gel was gradually formed after few minutes and the temperature was maintained for 1h30. After that the gel was allowed to mature overnight at room temperature (RT) before to be heated at 80°C and 100°C respectively in order to remove ethanol and water excess. After grounding, the resulting powders were submitted to calcination from RT to 300°C (2 h) and from 300°C to 500°C (2 h) in flowing air.

2.2. Catalyst characterization The X-ray diffraction (XRD) patterns were collected with a D8 Advance-BRUKER diffractometer using Cu Kα radiation. The crystallite size was determined from the Scherrer equation. The lattice parameter was estimated using FullProf software. The textural properties were evaluated at -196 °C from the nitrogen isotherms (Micromeritics ASAP 2010). The samples were previously outgassed at 160°C for 4 h. The specific surface area (SSA) was calculated using the BET model. Temperature programmed reduction (H2-TPR) was investigated (Micromeretics Autochem II) by heating Zr(0.4) Ce(0.6-x)Mn(x)O2 samples (50 mg) in H2 (5 vol.%)/Ar flow (50 mL min-1) at a heating rate of 5°C min-1 from 20 to 900°C.

2.3. Catalytic tests The activity of the catalysts (200 mg) was measured in a continuous flow system on a fixed bed reactor at atmospheric pressure. The flow of the reactant gases (0.1 mL min-1 of butanol and 99.9 mL min-1 of air) was adjusted by a Calibrage PUL 010 and DGM 110 apparatus constituted of a saturator and three mass flow controllers. The reactor temperature was increased from RT to 400°C (0.5°C min-1). The outflow gases were analyzed by a VARIAN 3800 gas chromatograph.

3. Results and discussion XRD patterns of Zr(0.4)Ce(0.6-x)Mn(x)O2 samples are displayed in Fig.1. The pattern of Ce0.6Zr0.4O2 is rather similar to that of a reference cerianite (JCDS 81-0792) suggesting the incorporation of Zr ions in the cubic lattice to form an homogeneous Ce–Zr–O solid solution, in accordance with the size of the ionic radius of Zr4+ ion (0.84Å) which is smaller than that of Ce4+ (0.97Å). Indeed a decrease in the lattice parameter a from 5.4120 Å for pure CeO2 [9] to 5.315 (±0.001) Å is observed on the fresh Ce0.6Zr0.4O2 sample. When adding small amounts of manganese, the fluorine-type structure is preserved. No manganese and zirconium oxide phases were detected on Zr0.4Ce0.12Mn0.48O2 solid. The absence of such phases suggests that Mn and Zr related species may be incorporated into the CeO2 lattice forming solid solutions [4]. Again the formation of a Mn–Ce–Zr–O solid solution is in line with the lower value of the lattice constant a of Ce0.48Zr0.4Mn0.12O2 sample which is of 5.303 (±0.001) Å according to the low ionic radius of Mnn+ (Mn2+ = 0.83Å, Mn3+ = 0.64Å, and Mn4+ = 0.53Å). Only a broad asymmetric peak in the 2θ range of 25–35° is observed for the Zr0.4Ce0.36Mn0.24O2 sample. Considering the Zr0.4Ce0.24Mn0.36O2 sample the dominant peaks of the cerianite are observed but they appear significantly enlarged. Finally the most enriched Mn

Preparation and characterization of nanocrystallines Mn-Ce-Zr mixed oxide

733

sample is totally amorphous. Hence with increasing content of manganese, amorphisation of the samples is enhanced. Similar qualitative observations have been already observed on Mn-Ce-O composites elsewhere [4,5]. These authors conclude to the occurrence of more defective fluorite like lattices having a lower degree of crystallinity and a smaller particle size as the sample is Mn enriched. Then comparing with Ce-Mn-O systems addition of zirconium herein retards the crystallization of the samples and/or allows forming some small oxide related crystallites. Adding manganese to the Zr-Ce-O system has a positive effect on the SSA which doubles from the free Mn sample to the higher Mn loaded one as well as on the average crystallite size which decreases from 4.5 nm for x= 0 to 1.2 nm for x= 0.36. Hence the effect of Mn in the presence of Zr is to retard the crystallization of the sample likely due the existence of Zr-Mn-Ce-O solid solution. The direct consequences are a substantial increase of the SSA concomitant with a crystallite size lowering with increasing the Mn content as observed in the MnCe-O system [4,5] but in a much more amplified manner in our case. Table1. Surface area, particle size, average crystallite size, and T50 of samples. average crystallite size (nm)

SSA (m2 g-1)

Zr0.4Ce0.6O2

4.5

98

960

216

Zr0.4Ce0.48Mn0.12O2

2.8

110

1700

184

Zr0.4Ce0.36Mn0.24O2

1.2

157

2420

176

Zr0.4Ce0.24Mn0.36O2

1.2

163

2880

172

Zr0.4Ce0.12Mn0.48O2

-

199

3470

162

Samples

H2 Consumption

T50* (°C)

-1

(µmol g )

* Temperature at 50% of butanol conversion

Fig. 1. XRD patterns of calcined samples

Fig. 2. H2-TPR profiles of calcined samples

° The H2-TPR profiles of the Zr(0.4)Ce(0.6-x)Mn(x)O2 samples are presented in Fig. 2. The onset of Ce0.6Zr0.4O2 reduction arises at 295°C. The reduction of oxygen from surface and bulk solid may account for the TPR profile of Ce0.6Zr0.4O2. When adding Mn the most interesting points are the following : (i) easily reducible species are formed as the onset of reduction is dramatically reduce all the more than the Mn content is increased ; (ii) the development of a broad complex envelop in the 100-500°C temperature range in line with the different Mnn+ whose shape is quasi-similar considering the three higher Mn loaded samples ; (iii) the H2 consumption linearly increases with Mn content (Table 1). Considering both Mnn+ to be reduced completely to Mn2+, Zr4+ not reduced in the temperature range studied, and Ce3+/Ce4+ molar ratio unchanged, an average oxidation number (AON) of manganese of circa 3.1 is obtained for each sample. The increase of

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the H2 uptake is only due to an increase of reducible sites in accordance with the SSA increase having a similar AON. Hence the rate increase of the different sites appears to be the same with Mn content increase in accordance with the similarities of the different H2-TPR envelops. The results of the catalytic activity in butanol oxidation are summarized in Table 1 through the T50 (temperature at 50% of butanol conversion) values : the butanol complete oxidation is easier all the more than the Mn content increases.

4. Conclusion Zr(0.4)Ce(0.6-x)Mn(x)O2 solid solutions were successfully synthesized by a sol-gel method, characterized and tested for the total oxidation of butanol. The butanol complete oxidation is easier with Mn content increasing. Here the results may be explained by the SSA and crystallite size measurements as observed previously in similar systems but for lower Mn content [10].

Acknowledgements The authors thank the European Community for financial supports through an Interreg IV France-Wallonie-Flandre project named REDUGAZ.

References [1] K. Okumura, 1998, Appl. Catal. B 15, 75-84. [2] C. Lahousse, 1998, J. Catal., 178, 214-222. [3] S. Imamura, 1996, Effect of cerium on the mobility of oxygen on manganese oxides, Appl. Catal. A: Gen., 142, 279–288. [4] G. Picasso, 2007, Preparation and characterization of Ce-Zr and Ce-Mn based oxides for n-hexane combustion: Application to catalytic membrane reactors, Chemical Engineering Journal, 126, 119–130. [5] H. Chen, 2001, Composition-activity effects of Mn-Ce-O composites on phenol catalytic wet oxidation, Appl. Catal. B: Environ., 32,195–204. [6] A. Trovarelli, 1999, The utilization of ceria in industrial catalysis, Catal. Today, 50, 353– 367. [7] A.M.T. Silva, 2004, App. Catal. B., 47, 269-279. [8] C.E. Hori, 1998, Thermal stability of oxygen storage properties in a mixed CeO2-ZrO2 system, Appl. Catal. B: Environ., 16, 105–117. [9] M. Wolcyrz, 1992, Rietveld refinement of the structure of CeOCI formed in Pd/CeO2 catalyst: Notes on the existence of a stabilized tetragonal phase of La2O3 in La-Pd-O system, J. Solid State Chem., 99, 409-413. [10] T. Rao, 2007, Oxidation of ethanol over Mn-Ce-O and Mn-Ce-Zr-O complex compounds synthesized by sol–gel method, Catal. Comm., 8, 1743–1747.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

SCR activity of conformed CuOX/ZrO2-SO4 catalysts S.B. Rasmussen,a* M. Yatesa, J. Due-Hansenb , P. Ávilaa R. Fehrmannb a

Instituto de Catálisis y Petroleoquímica (ICP), Consejo Superior de Investigaciones Científicas (CSIC), Calle Marie Curie 2, Cantoblanco, 28049 Madrid, Spain b Centre for Catalysis and Sustainable Chemistry (CSC), Department of Chemistry, Technical University of Denmark (DTU), Bygn. 207, Kemitorvet, DK-2800 Kgs. Lyngby, Denmark

Abstract CuOX/ZrO2-SO4 catalysts have been synthesised as conformed materials with the use of sepiolite as agglomerant and the performance in the NH3-SCR reaction with relation to biomass fired boiler units have been studied. The optimal Cu-loading of the catalysts is 3 wt.% CuO, both in terms of activity and selectivity. This catalyst constitutes a possible solution for NO removal in biomass-related applications, since it posses mainly Lewis acid sites, and therefore might be less subjected to deactivation by potassium containing fly ash particle produced during biomass combustion Keywords: CuOX, sepiolite, biomass, NH3-SCR, ZrO2-SO4

1. Introduction The use of biomass in fossil fuel based power plants is of increasing interest, since it is considered as a CO2 neutral fuel, having zero human impact on the carbon release to the atmosphere. Simultaneously, continuous efficient selective catalytic reduction of NO with ammonia (NH3-SCR) remains a very important condition for the implementation of biomass fuel as a sustainable alternative for energy production. However, the NH3SCR catalyst suffers from a number of deactivation phenomena when installed in boiler units based on biomass combustion, due to exposure to potassium containing fly ash[1]. The active V=O and V-OH sites on the commercially used V2O5-WO3/TiO2 based catalyst reacts with the potassium salts and form inactive alkali vanadates, which are unable to adsorb ammonia. Therefore new NH3-SCR catalysts more resistant to deactivation by potassium salts metals are needed. One of the possible ways to increase catalyst resistance to alkaline poisons is the use of supports revealing high or super-acidic properties, which would interact more strongly with potassium than vanadium species. Potassium oxide affects the Brønsted acid sites of the catalyst to a much larger extent than Lewis sites. Therefore, another possible solution for NO removal in biomass-related applications is the use of other metal oxides as active components, which posses mainly Lewis acidity [2,3]. In this context we have synthesised and studied CuOX/ZrO2-SO4 catalysts with respect to the performance of NH3-SCR in biomass fired boiler units.

S.B. Rasmussen et al.

736

CuO

Sepiolite

ZrO2

Starch

H2O

(NH4)2SO4

IWI Extrusion Drying (150°C) Calcination (600°C) Figure 1. “One pot” synthesis route used for the synthesis of sepiolite supported CuOX-applied for the production of scalable pelletised catalysts.

2. Experimental 2.1. Catalyst preparation The zirconia source was freshly precipitated Zr(OH)4 from Mel Chemicals. The αsepiolite employed (Pansil 100) was supplied by Tolsa S.A. Ammonium sulphate and the Cu-precursor, Cu(NO3)2 were from Panreac, ( > 99%). The sulphated zirconia was prepared by dissolving (NH4)2SO4, and Cu(NO3)2 into a part of the water. Thereafter the solution was mixed with the homogenised powder blend of the oxides and pore generating agent (PGA, starch), and extra water was added in order to obtain a paste of adequate rheology. After digestion/drying in ambient atmosphere for 2 hours with occasional kneading, the paste was extruded from 20 ml plastic syringes with 2 mm orifices. The samples were then dried at 150°C for overnight. Calcination of the samples was performed at 600°C for 4 hours in air. Finally the extruded materials were chopped into 3-5mm cylindrical pellets. The synthesis procedure is outlined in Figure 1.

2.2. Catalyst characterisation The thermal gravimetric analysis of the samples were carried out on a Netzsch 409 EP Simultaneous Thermal Analysis device. The DSC curves were measured using approximately 20-30 mg of powered sample which were heated in an air flow of 75 ml·min-1 at a rate of 5 ºC·min-1 from room temperature to 1000 ºC, using α-alumina as reference. The specific surface areas, SBET, were obtained from nitrogen adsorption at −196 ◦C using a Micromeritics Tri-Star apparatus, after application of the BET equation at relative pressures in the range 0.05–0.35 p/p◦. Table 1. Textural characteristic of conformed catalysts. Sample

SBET (m2g-1)

SExtern (m2g-1)

Vmeso (cm3g-1)

Vmacro(cm3g-1)

1% CuO

110

104

0.20

0.71

2% CuO

80

68

0.19

0.68

3% CuO

79

70

0.20

0.73

5% CuO

84

77

0.19

0.74

Prior to N2 adsorption the samples were outgassed overnight at 150 ◦C to a vacuum of <10−4 Pa to ensure a dry clean surface, free from loosely held adsorbed species.

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737

2.3. NO SCR with ammonia Activity measurements of the cylindrical pellet shaped catalysts were carried out in a continuous tubular glass reactor that operated at an integral regimen close to an isothermal axial profile. The reactor was 75 cm long and had an internal diameter of 2.54 cm. Ammonia was fed directly into the reactor bed to avoid the formation of ammonia salts [4]. Pellet beds 10 cm long were used. The inlet and outlet NO and NO2 concentrations were determined by chemiluminescence with a Signal NO + NO2 analyser (Series 4000). Analysis of N2O and NH3 was carried out by IR spectroscopy with a Signal 7000FT GFC Analyser and with an A.D.C. Double Beam Luft Type Infrared Gas Analyser, respectively.

3. Results and discussion 3.1. Characterisation

DSC signal (mV)

The characteristic data from the pore analyses of the CuOX/ZrO2-SO4 catalysts are collated in Table 1. The BET surface areas are around 84 m2g-1, except for the 1% CuOX, which exhibits 110 m2g-1. The samples exhibit high external surface areas, close to the BET surface areas, which suggests that no micro porosity remains. A thermo gravimetric analysis was carried out in order to determine how the sepiolite, copper oxide, PGA and sulphation modify the phase transitions of the zirconia hydroxide gel (Fig. 2). The phase change from amorphous zirconia to the monoclinic phase was observed to occur at around 440◦C for pure zirconia hydroxide gel. By introducing the sulphate as stabilizer for zirconia instead the transformation into the tetragonal phase is achieved at around 580oC, which appeared to be unaltered by the presence of the binder, and thus treatment at 600◦C was valid to ensure its formation. Furthermore we tested if the PGA and the introduction of Copper salts had influence on the transformations. It was observed that introduction of CuOX lead to lower combustion temperatures for the starch, but the transition temperature for formation of the tetragonal zirconia phase remained constant. Based on these data 600oC was chosen as the temperature for calcinations. 60 50 40 30 20 10 0 -10 -20 100

X 0.1

Cu(NO3)2 Starch combustion

A B Sepiolite

Tetragonal

C D

Amorpheous – monclinic

200

300

400

500

600

700

800

900

1000

1100

Temperature (°C)

Figure 2. A) CuOX/ZrO2-SO4 sepiolite catalyst B) ZrO2-SO4 sepiolite carrier material with PGA. C) ZrO2-SO4 sepiolite carrier material without PGA. D) Pure ZrO2.

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70

Conversion (%

60 50 40

5% 3% 2% 1% 0%

30 20 10

CuO CuO CuO CuO CuO

0 200

250 Temperature (°C)

300

90 (%) 80 70 60 50 40 30 20 10 0

NO conversion (%) 260°C Molar consumption ratio (NO/NH3) 250°C 0

1

2 3 4 5 CuO content (%)

6

Figure 3. SCR activity of composite catalyst before and after 248 hours of potassium-exposure at 350°C.

3.2. Catalytic activity Varying the CuOX content on the CuOX/ZrO-SO4/sepiolite catalysts has some effects on the catalytic activities and selectivities. The NH3-SCR activity is steadily increasing with the CuO content until the catalyst contains 3% CuO. Thereafter formation of crystalline CuO sets in according to complementary powder XRD data. Thereafter, any additional copper is deposited as crystalline CuO, which doesn’t contribute to the catalytic activity, since the conversion remains stable. However, this also implies that the crystalline copper formed is merely a spectator species- hence there is no significant decline in activity nor in selectivity. However, the slight decrease in NO/NH3 consumption ratio observed for the catalysts with 3% compared to 5% CuO could be interpreted as a slight loss in selectivity suggesting that for this catalytic system 3% CuO provides the optimal loading of the active species.

References 1. 2.

3.

4.

H. Zhou, A.D. Jensen, P. Glarborg, A. Kavaliauskas, 2006, “Formation and reduction of nitric oxide in fixed-bed combustion of straw Fuel”, 85, 5-6, 705-716. A.L. Kustov, S.B. Rasmussen, R. Fehrmann, P. Simonsen, 2007, “Activity and deactivation of sulphated TiO2- and ZrO2-based V, Cu, and Fe oxide catalysts for NO abatement in alkali containing flue gases”, Appl. Catal. B: Environmental, 76, 1-2, 9-14. D. Pietrogiacomi, A. Magliano, D. Sannino, M.C. Campa, P. Ciambelli, V. Indovina, 2005, “In situ sulphated CuOX/ZrO2 and CuOX/sulphated-ZrO2 as catalysts for the reduction of NOX with NH3 in the presence of excess O2”, Appl. Catal. B: Environmental, 60, 1-2, 83-92. J. Blanco, P. Avila, C. Barthelemy, A. Bahamonde, J.A. Odriozola, J.F. Garcia de la Banda, 1989, “Influence of phosphorus in vanadium-containing catalysts for NOx removal”, Appl. Catal. 55, 1, 151-164.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Pore design of pelletised VOX/ZrO2-SO4/Sepiolite composite catalysts Søren B. Rasmussena, Johannes Due-Hansenb, MalcolmYatesa*, Mirza Villaroelc, F. Javier Gil Llambíasc, Rasmus Fehrmannb, Pedro Ávilaa a

Instituto de Catálisis y Petroleoquímica (ICP), Consejo Superior de Investigaciones Científicas (CSIC), Calle Marie Curie 2, Cantoblanco, 28049 Madrid, Spain b Centre for Catalysis and Sustainable Chemistry (CSC), Department of Chemistry, Technical University of Denmark (DTU), Bygn. 207, Kemitorvet, 2800 Kgs. Lyngby, Denmark c Facultad de Química y Biología, Universidad de Santiago de Chile (USACH), Casilla 40, Correo 33, Santiago, Chile

Abstract The NH3-SCR activities of a series of extruded and calcined VOX/ZrO2-SO4 - sepiolite catalysts were determined. The pore structures were heavily influenced by the clay content with macropore sizes ranging from 50 to >1000 nm. Mechanical strength and SCR activity measurements suggested that 25% w/w sepiolite is the optimal catalyst composition. Keywords: Composite, extrusion, biomass, NH3-SCR, ZrO2-SO4, sepiolite

1. Introduction The NH3-SCR process is established as a robust and useful technique for the elimination of NOX from off-gases. Commercially used V2O5-WO3/TiO2 catalysts reduce NO selectively to N2 and H2O by the following reaction: O2 + 4NO + 4NH3 Æ 4N2 + 6H2O

(1)

The reaction involves two cycles, an acid cycle and a redox cycle [1,2]. Thus, the redox capacity of V(V) and V(IV) is of key importance, but also it is crucial to have sufficient acidic surface sites in order to chemisorb and administer NH3 for the SCR reaction [3]. To comply with the Kyoto protocol, substitution of fossil fuels with biomass constitutes a practical, economical and environmentally viable solution, as biomass (straw, wood chips, saw dust etc.) attains its carbon from air during photosynthesis. Thus, biomass can be regarded as a CO2 neutral fuel, and making attempts to increase the biomass content in energy production is of great interest. Though biomass combustion technology is relatively easy to implement in coal and oil-fired power plants, there are drawbacks. Among others, potassium fly ash particles, originating from the firing of biomass, poison the traditional SCR catalysts [4]. Thus, there is an urgent need for alternative potassium fly ash resistant catalysts. We report herein some special features of composite VOX-ZrO2-SO4/sepiolite catalysts. The catalyst exhibit increased surface acidity as well as a shielding effect of the working catalyst, induced by the sepiolite clay enhancing the durability of the SCR catalyst while working under exposure to fly ash that contains potassium.

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740

(NH4)2SO4

ZrO2

H2O

Drying (RT, 48 hrs) Drying (150°C, 24 hrs)

Calcination (500°C, 4 hrs) Sepiolite

VOX/ZrO2-SO4

VO2+

Calcination (500°C, 4 hrs)

(aq)

VOX/ZrO2-Sepiolite catalyst

Extrusion

Figure 1. Synthesis route employed to produce scalable pelletised catalysts.

2. Experimental 2.1. Catalyst preparation Melcat Zr(OH)4 with a particle size d50 = 15μm was used as the zirconia source and the ammonium sulphate was from Panreac ( > 99%). The sulphated zirconia was prepared by impregnating 101.6 g of the zirconia hydroxide with 8.3 g (NH4)2SO4 by the incipient wetness method. After digestion/drying in ambient conditions for 2 hours the samples were dried at 150°C for 3 hours. Calcination of the samples at 500°C for 4 hours in air was then performed by placing the material directly into a pre-heated furnace, in order to facilitate the preferred formation of metastable tetragonal phases. The catalysts were prepared by wet impregnation of VOSO4 onto the sulphated zirconia, to produce a slurry that was allowed to settle for an hour under stirring. Thereafter sepiolite, α-sepiolite Pansil 100 supplied by Tolsa S.A. and water were added in the desired amounts to obtain a homogeneous paste with an adequate viscosity for extrusion from a 20 ml syringe with a 2 mm orifice. The extrudates were allowed to dry slowly, sealed in a wet atmosphere for 48 hours then dried overnight at 150°C in air. Calcination was carried out at 500°C for 4 hours in air. Finally the extruded material was broken into 3-5mm cylindrical pellets. The synthesis procedure is outlined in Figure 1.

2.2. Catalyst characterisation The specific surface areas (SBET), were obtained from the corresponding nitrogen adsorption isotherms at −196°C using a Micromeritics Tristar apparatus, after application of the BET equation in the relative pressure range 0.05–0.35 p/p°. Prior to N2 adsorption, the samples were outgassed overnight at 150°C to a vacuum of <10−4 Pa to ensure a dry clean surface, free from any loosely held adsorbed species. The determination of the surface area of the external (Aextern), the cumulative pore volumes (VP), and the macropore porediameters (Dmacro) were complemented by mercury intrusion porosimetry (MIP) using a CE Instruments Pascal 140/240 and applying the Washburn equation for cylindrical pores [5], with the values recommended by the IUPAC of 141° and 484 mNm−1, for the contact angle and surface tension of mercury. Table 1. Textural characteristic of conformed catalysts. Sample Sepiolite (%) Aextern (m2/g) SBET (m2/g) Vp (cm3/g) Dmacro (nm)

8/8

7/8

6/8

5/8

4/8

3/8

2/8

1/8

0/8

100 118 130 0.325 26

87.5 128 136 0.305 40

75 140 140 0.291 52

62.5 146 145 0.278 82

50 146 148 0.260 130

37.5 144 147 0.234 248

25 143 146 0.215 320

12.5 143 145 0.195 767

0 152 156 0.201 911

Pore design of pelletised VOX/ZrO2-SO4/Sepiolite composite catalysts

741

2.3. NO SCR with ammonia The SCR activity measurements were carried out on crushed samples sieved to obtain fractions of 0.18–0.300 mm. These measurements were carried out in a fixed-bed reactor, with 10 mg of the catalyst loaded between two layers of inert quartz wool. The reactant gas composition was 1000 ppm NO, 1100 ppm NH3, 3.5% O2, 2.3% H2O and He balance. The total flow rate was 300 mL/min (ambient conditions), with a continuous monitoring of the NO and NH3 concentration with a Thermo Electron model 17C chemiluminescent NO–NOx gas analyser. The catalytic activity, represented as a first-order rate constant (k), can be calculated from the NO conversion, X, as Eq. 2:

k=−

F0 ln(1 − X ) [NO]0 Vcat

(2)

where F0 is the molar inflow of NO, [NO]0 is the initial molar concentration of NO, and Vcat is the catalyst bed volume.

2.4. KCl poisoning To simulate poisoning by potassium in the flue gas stream, the sample pellets were submerged in fine particles of KCl (<200 nm) and placed in a furnace for 248 hrs at 350°C in a water saturated air flow. Hereby the temperature is above the Tamman temperature for KCl (m.p. = 771ºC, TTamman = 258.5ºC). Thus, the salt is surface mobile, and will diffuse into the catalyst pores deactivating any accessible active sites

3. Results and discussion 3.1. Catalyst pore system The characteristic data from the pore analyses are collated in Table 1. The SBET areas were maintained close to 150 m2g-1 for samples with a sepiolite content below 62.5% w/w, whereupon the surface area gradually reduces to a minimum of 130 m2/g - the surface area of pure sepiolite. Since the catalysts were treated at 500°C, little microporosity remained although all exhibited high external surface areas, useful for a catalytic system. The N2 adsorption-desorption isotherms (VN2 versus P/P0) and MIP cumulative pore volumes (VP) are shown in Figure 2 for samples with 0-50% w/w sepiolite content. 200

50%

A

175

0.4

150

0% 12.50% 25% 37.50% 50%

B

25%

0.3 Vp (ml/g)

VN2 (ml.g-1)

0%

125 100

0.2

75 50

0.1

25 0

0.0

-

0.2

0.4

0.6 0

P/P

0.8

1.0

1

10

100

1000

10000

Pore diameter [nm]

Figure 2. A) Nitrogen adsorption-desorption isotherms of composite catalysts with 0, 25, 50% w/w sepiolite content measured as the volume of N2 (VN2) versus the relative pressure (P/P0). B) MIP cumulative pore volumes (VP) for 0, 12.5, 25, 37.5 and 50% w/w sepiolite.

S.B. Rasmussen et al.

742 300

1000

KCl treated

Crush strength

k-value (ml/g.s)

250

800

200 600 150 400 100 200

50 0

Crush strentgh (ton/m2)

Fresh catalyst

0 12.50%

25%

37.50%

50%

Sepiolite content (%)

Figure 3. SCR activities of composite catalysts before and after 248 hours of hydrothermal treatment at 350°C in the presence of potassium chloride (left axis). On the right axis the crush strength measurements are shown.

Varying the sepiolite content of the VOX/ZrO-SO4/sepiolite composites has a marked effect on the observed pore volumes and macropore diameter. For instance, by introducing 25% w/w of sepiolite the characteristic interparticular macropore diameter decreased from 911 nm (0/8, pure VOX-ZrO2-SO4) to 320 nm.

3.2. Catalytic activity and relation to the pore system and crushing strength Comparing the activity data in Figure 3, before and after KCl exposure, it may be appreciated that almost no deactivation was observed. An extruded reference catalyst (3% w/w) V2O5-WO3/TiO2) under the same conditions suffered an 11% deactivation. It therefore seems that the sepiolite provides a protective effect of the catalytically active centres, or at least delays the deactivation by potassium salts. Furthermore, the effect is present even for the sample with only 12.5% w/w sepiolite, suggesting that the sulfated zirconia itself might have some chemical affinity for the potassium species, which enhance the resistance to deactivation. The crush strengths of the catalysts are also provided in Figure 3. The optimal composition of the composite system, regarding both activity and crushing strength is deduced to be a catalyst consisting of 25% w/w sepiolite.

References 1. 2.

3. 4.

5.

H. Bosch, F.J.J.G. Janssen, 1988, Catalytic Reduction of Nitrogen Oxides, Catal. Today, 2, 4, 369-529. G. Busca, L. Lietti, G. Ramis, F. Berti, 1998, Chemical and mechanistic aspects of the selective catalytic reduction of NOx by ammonia over oxide catalysts: A review, Appl. Catal. B: Environ., 18, 1-2, 1-26. Q. Liu, Z. Liu, C. Li, 2006, Adsorption and Activation of NH3 during Selective Catalytic Reduction of NO by NH3, Chin. J. Catal., 27, 636-646. Y. Zheng, A.D. Jensen, J.E. Johansson, J.R. Thøgersen, 2008, Deactivation of V2O5-WO3TiO2 SCR catalyst at biomass fired power plants: Elucidation of mechanisms by lab- and pilot-scale experiments, Appl. Catal. B: Environ., 83,186-194. E.W. Washburn, 1921, The Dynamics of Capillary Flow, Physical Review, 17, 3, 273 - 283.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Titanium oxide nanotubes as supports of Au or Pd nano-sized catalysts for total oxidation of VOCs Haingomalala Lucette Tidahya, Tarek Barakata, Renaud Cousina, Cédric Gennequina, Vasko Idakievb, Tatyana Tabakovab, Zhong-Yong Yuanc, Bao-Lian Sud, Stéphane Sifferta* a

Univ Lille Nord de France, ULCO, UCEIV, 145 avenue Maurice Schumann 59140 Dunkerque, France. *[email protected] b Institute of Catalysis, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria c Institute of New Catalytic Materials Science, Nankai University, Tianjin 300071, China d Laboratory of Inorganic Materials Chemistry, University of Namur (FUNDP) 5000 Namur, Belgium

Abstract Titanium oxide nanotubes (TNTs) have been synthesized via laboratory-made TiO2 A and commercial TiO2 P25. 1.5wt% gold or palladium were deposited on TNTA and TNTP25 supports and the catalytic activity was evaluated in propene, methyl ethyl ketone (MEK) and toluene total oxidation. The catalytic properties of supported TNT were correlated with the structural peculiarity and the nature of the TNT, but also with the nature of the noble metal and the kind of the VOC. Keywords: titanium oxide nanotubes; gold; palladium; VOCs oxidation

1. Introduction Volatile Organic Compounds (VOCs) are hazardous to human health and the environment. The deep catalytic oxidation of these pollutants to carbon dioxide and water is identified as one of the most efficient ways to destroy VOCs at low concentrations. In this paper, we report a study on the oxidation of propene, methyl ethyl ketone (MEK) and toluene in air over catalysts supported on TNTs. Those VOCs are commonly used as solvent in chemical industries and are then often found in industrial exhausts. Palladium catalysts are well known for high activity in oxidation reactions [1-4]. Moreover in the last decade gold catalysts show rapidly a large interest because it is possible to prepare gold nanoparticles deposited on metal oxide supports which exhibit high catalytic activity towards oxidation reactions [5-7]. Furthermore, it is found that the support plays an important role to obtain highly dispersed noble metal particles and catalysts with high performance. Various metal oxides including TiO2 are investigated as gold and/or palladium supports in VOCs complete oxidation [1,6-8]. The potential of using the TNT materials as supports should be an interesting choice. The main goal of this work is then to study the oxidation of some different VOCs over gold or palladium catalysts supported on TNTs.

2. Experimental 2.1. Synthesis of TiOx nanotubes and catalysts preparation Titanium oxide nanotubes TNTA and TNTP25 are synthesized using a laboratory made and a commercial titania (TiO2 A and TiO2 P25). The preparations of materials are

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processed by the method described elsewhere [9]. After calcination of nanotubes in air at 400°C, gold or palladium are loaded. Gold is deposited on nanotubes by depositionprecipitation method, the preparation is the same as described in the previous paper [9]. Pd supported by TNTA and TNTP25 are prepared by aqueous impregnating method using palladium nitrate (Pd(NO3)2.xH2O). The impregnated powders are dried at 100°C overnight. The catalysts are then denoted ATNTA, Pd/TNTA, ATNTP25 and Pd/TNTP25 with 1.5wt% Au and Pd contents after calcination in air at 400 °C for 4 h.

2.2. Sample characterization The structures of catalysts are analyzed by powder X-ray diffraction (XRD) technique at room temperature with a D8 Bruker diffractometer using Cu K∝ radiation. The specific surface areas of solids are determined by the BET method using a Quantasorb Junior apparatus, and the gas adsorbed at -196°C is pure nitrogen.

2.3. Catalytic activity measurement The activity of the catalysts (100 mg) is measured in a continuous flow system on a fixed bed reactor at atmospheric pressure. Before each test, the catalyst is reactivated in flowing air (2 L.h-1) at 400°C for 4 hours. The flow of the reactant gases (100 mL.min-1 with 1000 ppm of toluene or MEK or propene in air) is adjusted by a Calibrage apparatus and mass flow controllers. The feed and the reactor outflow gases are analyzed on line by a Varian CP4900 µGC. The volume hourly space velocity (VHSV), calculated at ambient temperature and atmospheric pressure, is 60 000h-1.

3. Results and discussion Table 1. BET specific area of calcined catalysts and T50 value. Catalyst ATNTA ATNTP25 Pd/TNTA Pd/TNTP25

SBET (m2/g) 132 213 117 165

Propene 332 326 186 192

T50 (°C) MEK 310 283 256 251

Toluene 395 356 261 255

Previous results [9] using TEM show that titania as prepared possesses a large quantity of tubular materials with nearly uniform diameters (around 8-10 nm) and calcination treatment does not change the shape of titania nanotubes. Fig. 1 shows the X-ray powder diffraction patterns of calcined samples. All catalysts show some broadened diffraction peaks due to the nanometer size of the tube and the bending of some atom planes of the tubes [9]. Furthermore TNTP25 samples show anatase phase of titania and samples based on TNTA show a mixture of anatase and rutile phases (JCPDS 21-1272 and 21-1276) with higher proportion of rutile. The higher crystallization of TNTA after the calcination can explain the low surface area of the sample (Table 1). There are no significant differences in the XRD patterns of the samples after deposition of gold (ATNTA and ATNTP25) or palladium (Pd/TNTA and Pd/TNTP25) by comparison with titania. Gold catalysts show typical lines of Au at 2θ = 38.2° (JCPDS 04-0784) and palladium catalysts present a broad band attributed to PdO phase (JCPDS 41-1107). The ignition curves for propene, MEK and toluene total oxidation over gold and palladium catalysts are shown in Figs. 2 (A, B, C). For propene oxidation the observed products are only carbon dioxide and water, indicating complete combustion occurring during the reaction. However it is found that toluene and MEK oxidations to CO2 are

Titanium oxide nanotubes as supports of Au or Pd nano-sized catalysts

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+ *

+ +

Intensity (a.u.)

•

*

+

+

+

++

++

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•

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Pd/TNTA TNTP25

• •

ATNTP25

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Pd/TNTP25 20

30

40

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60

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Fig. 1. XRD patterns of calcined samples: (+) rutile; (*) anatase; (•) Au; (♦) PdO.

accompanied with formation of by products, like benzene during toluene oxidation and partially oxidized products during MEK oxidation, mainly acetaldehyde but also methyl vinyl ketone. According to several authors acid base character of the catalyst plays an important role in the oxidation of MEK. Acid catalysts facilitate oxidative scission reaction giving C2 by-products, principally acetaldehyde and basic catalysts lead to the formation of by-products with C4 hydrocarbons like methyl vinyl ketone [3,10]. Byproducts are observed only at temperatures where the conversion of the VOC is still incomplete. The activity for both propene, MEK and toluene oxidation given by T50 value (Table 1) follows the same order Pd/TNTP25 ≥ Pd/TNTA > ATNTP25 > ATNTA. This clearly shows that Pd catalysts are more active than gold catalysts. The highest activity of palladium based catalysts can be related to the easiest accessibility and reducibility of palladium particles (PdO) [1], which are probably outer the surface. However gold particles (Au) are more difficult to access and in our case some of them are inserted into the tube hollows [9]. The nature of the support plays also a key role in the reaction thus catalysts prepared on TNTP25 give higher performance than catalysts prepared on TNTA. This result can be related to the high surface area of TNTP25 (Table 1), leading to well dispersed palladium and gold particles. The trend for the rate of oxidation of VOCs over palladium catalysts is propene > MEK > toluene: the more the molecule is small the more it is easier to oxidize. Whereas with gold catalysts, the trend appears to be different: MEK > propene > toluene. The reactivity of oxygenated molecules seems to be more important with the gold catalysts and VOC oxidation is governed by the polarity of VOC molecule. Minico et al. [11] attributed this behaviour to the ability of highly dispersed gold to activate the oxygen in which oxygenated VOC was strongly adsorbed. Consequently alcohols and ketones showed higher reactivity than aromatics during reactions investigated on Au/Fe2O3 samples [11]. It can be deduced that oxidation mechanisms over palladium and gold catalysts are quite different.

4. Conclusion Propene, MEK and toluene total oxidation are studied with palladium or gold based catalysts (1.5wt%) supported on titanium oxide nanotubes (TNTA and TNTP25) with high surface areas. Palladium catalysts are more active than gold catalysts. The trend for the rate of oxidation of VOCs over palladium catalysts is propene > MEK > toluene: the more the molecule is small the more it is easier to oxidize. Whereas with gold catalysts, MEK is easier to oxidize, the reactivity of oxygenated molecules seems to be more

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important with the gold catalysts. The nature of the support plays also a key role in the reaction, thus catalysts prepared on TNTP25 give higher performance than catalysts prepared on TNTA. 100

100 ATNTA

propene conversion (%)

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Fig. 2. Light-off curves for propene (A), MEK (B) and toluene (C) total oxidation in air over catalysts supported on titania nanotubes.

100 ATNTA 90 80

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Pd/TNT P25 70

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This work was supported by IRENI and Interreg IV “Redugaz” projects and the European Community (European Regional Development Fund).

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References [1] H.L. Tidahy, S. Siffert, J.-F. Lamonier, E.A. Zhilinskaya, A. Aboukaïs, Z.-Y. Yuan, A. Vantomme, B.-L. Su, X. Canet, G. De Weireld, M. Frère, T.B. N’Guyen, J.-M. Giraudon, G. Leclercq, Appl. Catal. A 310 (2006) 61 [2] H.L. Tidahy, M. Hosseini, S. Siffert, R. Cousin, J.-F. Lamonier, A. Aboukaïs, Catal. Today 137 (2008) 335 [3] G. Arzamendi, V.A. de la Peña O'Shea, M.C. Álvarez-Galván, J.L.G. Fierro, P.L. Arias, L.M. Gandía, J. Catal. 261 (2009) 50 [4] M. Paulis, L.M. Gandia, A. Gil, J. Sambeth, J.A. Odriozola, M. Montes, Appl. Catal. B 26 (2000) 37 [5] S. Ivanova, C. Petit, V. Pitchon, Appl. Catal. A 267 (2004) 191 [6] M. Hosseini, S. Siffert, H. L. Tidahy, R. Cousin, J.-F. Lamonier, A. Aboukaïs, A. Vantomme, B.-L. Su, Catal. Today 122 (2007) 391 [7] C. Gennequin, M. Lamallem, R. Cousin, S. Siffert, F. Aïssi, A. Aboukaïs, Catal. Today 122 (2007) 301 [8] V. Idakiev, L. Ilieva, D. Andreeva, J.L. Blin, L. Gigot, B.L. Su, Appl. Catal. A 243 (2003) 25 [9] V. Idakiev, Z.-Y Yuan, T. Tabakova, B.-L Su, Appl. Catal. A 281 (2005) 149 [10] Y. Takita, K. Inokuchi, O. Kobayashi, F. Hori, N. Yamazoe, T. Seiyama, J. Catal. 90 (1984) 232 [11] S. Minicò, S. Scirè, C. Crisafulli, R. Maggiore, S. Galvagno, Appl. Catal. B 28 (2000) 245

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Preparation of Alkali-M/ZrO2 (M = Co or Cu) for VOCs oxidation in the presence of NOx or carbonaceous particles Aissa Aissata,b, Stéphane Siffert*,a,b, Dominique Courcota,b a

Univ Lille Nord de France, F-59000 Lille, France ULCO, UCEIV, F-59140 Dunkerque, France *Corresponding author. E-mail address: [email protected]

b

Abstract Total oxidation of toluene is investigated on catalysts with alkali metals (Na or Cs) alone or with Co or Cu impregnated on ZrO2. The sample Cs/ZrO2 is more active than Na/ZrO2. In the presence of NOx or carbonaceous particles, the addition of Co to Cs/ZrO2 improves toluene oxidation, especially for the catalyst with a low Cs/Co ratio. Keywords: Toluene oxidation; NOx ; Carbonaceous particles; Alkali metals; Transition metals

1. Introduction The increasing environmental awareness in the last two decades has prompted the emergence of stricter regulations for industrial activities. Among these, the reduction of volatile organic compounds (VOCs) emissions is important because these molecules represent a serious environmental problem. Complete VOCs oxidation needs highly active catalysts at low temperatures [1]. Several industries are faced to the problem of simultaneous release of VOCs and NOx. A solution to this environmental problem could be a catalyst with high activity for NO oxidation to NO2 and high activity for NO2 reduction to N2 with hydrocarbons [2]. Zirconia has shown good catalytic activity, especially in hydrocarbons oxidation reactions owing to its ability to convey oxygen species [3]. The introduction of alkali metals in catalysts developed for VOCs oxidation in the presence of NOx is expected to provide interesting effects. In some cases, alkali metals are known to be promoters for the oxidation of carbon black (CB) as well as for NOx reduction [4]. Previous works of our laboratory [5,6] presented this effect of alkali metals in the oxidation of soot. Moreover, Co and Cu are interesting active phases for toluene oxidation [7]. The aim of this work is focused on the preparation of catalysts with alkali metal (Na or Cs) alone or impregnated with Co or Cu on ZrO2. The effect of alkali/Co ratio on the catalyst performance for toluene oxidation in presence or without NOx or CB is especially studied.

2. Experimental 2.1. Catalysts preparation ZrO2 support is prepared by a precipitation method, adding dropwise an aqueous solution of zirconyl (IV) chloride to an ammonia solution under continuous stirring. The precipitate is filtered and washed to remove remaining chloride ions [8]. This solid is dried at 115°C for 24 h and subsequently calcined under air flow (2 L h-1) at 300°C for

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748 4 h [5,6]. The alkali/ZrO2 systems are obtained by impregnation of aqueous alkali carbonates on a ZrO2 support (201 m2 g-1). Cs-Co/ZrO2 catalysts are prepared by co-impregnation of CoCO3 and Cs2CO3 salts onto ZrO2. CsCu/ZrO2 catalysts are prepared with the same method (using CuCO3·nCu(OH)2). After drying, the samples are calcined under air flow at 600°C for 4 h. The as-obtained solids are denoted Csx-M0.1/ZrO2, where M corresponds to the transition metal, x and 0.1 to alkali/Zr and M/Zr atomic ratio respectively.

2.2. Catalysts characterization The chemical composition of the samples is determined by ICP-MS (Varian 820 MS). BET surface areas are measured by N2 adsorption at 77 K (Thermo Electron Qsurf M1). XRD measurements are obtained by a Bruker D8 Fig. 1. XRD patterns of catalysts calcined Advance diffractometer. TG-DTA experiments at 600°C (A: ZrO2, B: Alkali0.15/ZrO2 are performed in air flow (75 mL min-1, -1 catalysts “a: Na0.15/ZrO2, b: Cs0.15/ZrO2”, 5°C min , Netzsch STA 409). C: Cs0.15-M0.1/ZrO2 catalysts “a: Cs0.15Toluene oxidation is carried out in a Cu0.1/ZrO2, b: Cs0.15-Co0.1/ZrO2”, continuous flow reactor with a fixed bed at (•) monoclinic phase, (♦) tetragonal phase, atmospheric pressure (1000 ppm toluene and -1 (□) CuO phase, (○) Co3O4 cubic phase). 10% O2 in N2, total flow rate: 100 mL min ). Before each activity test, the catalyst (100 mg) is dried in air (2 L h-1) at 500°C (1°C min-1) for 4 h [3]. Toluene conversion is checked using a Varian CP-4900 µGC. The effect of the presence of NOx (NO2 + NO) in the feed gas (1000 ppm of NO in N2) or CB (100 mg of catalyst + 6 wt.% of CB) is also studied. The products (NO, NO2, CO and CO2) are measured with Servomex Xentra 4900C analyzer. The volume hourly space velocity (VHSV) is 105 000 h-1.

3. Results 3.1. Characterization The theoretical alkali metals content in the catalysts is almost confirmed (Table 1). However, the content of transition metals and Cs are both lower than that expected in the case of coimpregnated samples. A XRD analysis of the different alkali0.15/ZrO2 and Cs0.15-M0.1/ZrO2 solids calcined at 600°C is carried out (Fig. 1). After calcination at 600°C, ZrO2 is a mixture of tetragonal

Table 1. Chemical composition, specific areas and mean crystallite size of tetragonal ZrO2 (2θ = 30.4° for the (111) reflex) of alkali0.15/ZrO2 and Csx-M0.1/ZrO2 solids after calcination at 600°C for 4 h. Catalyst ZrO2 Na0.15/ZrO2 Cs0.15/ZrO2 Cs0.15-Co0.1/ZrO2 Cs0.015-Co0.1/ZrO2 Cs0.15-Cu0.1/ZrO2

Experimenta l molar ratio 0.130 0.132 0.095 (Cs) 0.073 (Co) 0.013 (Cs) 0.089 (Co) 0.100 (Cs) 0.074 (Cu)

BET surface area (m2 g-1) 84 34 21

Particle size (nm) 8±3 23 ± 3 14 ± 3

34

18 ± 3

85

11 ± 3

17

21 ± 3

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(JCPDS 50.1089) and monoclinic (JCPDS 65.1023) phases (Fig. 1.A.). When Co or Cu salt is impregnated with Cs onto the ZrO2 carrier (Fig. 1.C), the XRD patterns of the corresponding solids (Cs0.15-M0.1/ZrO2) calcined at 600°C show only the presence of the tetragonal phase revealing that Co or Cu favors the stabilization of this crystalline phase. Similar observations are obtained for Na0.15/ZrO2 (Fig. 1.B.a). In the case of Cs0.15/ZrO2 (Fig. 1.B.b), both tetragonal and monoclinic phases are detected. Nevertheless, the presence of the transition metal in Cs0.15-M0.1/ZrO2 leads to the stabilization of the tetragonal ZrO2 phase evidenced by the proportion of monoclinic phase which is extremely low after calcination at 600°C. Moreover, differences in middle height width are detected for the different solids. Mean crystallite size for the tetragonal phase (2θ = 30.4° for the (111) reflex) is estimated using the Scherrer equation (Table 1). No significant difference in mean crystallite size is obtained for ZrO2 and M0.1/ZrO2. On the contrary; it appears that tetragonal ZrO2 crystallites possess a bigger size in the presence of alkali containing solids and particularly in the presence of Na. ZrO2 possesses a high specific area value (Table 1), but the presence of alkali metal (Na or Cs) leads to a strong decrease of this area. These phenomena can be explained considering the increase of mean crystallite size of tetragonal ZrO2 in the presence of alkali. Moreover, the strong coverage of Cs species on ZrO2 surface could explain the low specific areas of Cs0.15/ZrO2 and Cs0.15-M0.1/ZrO2. TG-DTA analysis is performed on dried alkali0.15/ZrO2 and alkali0.15-M0.1/ZrO2 solids and the obtained DTA curves are displayed in Fig. 2. For ZrO2 solid, the exothermic peak detected at 428°C is attributed to the crystallisation of tetragonal ZrO2 [1]. DTA curves show the influence of alkali species on the ZrO2 crystallisation and its tetragonal-monoclinic transformation. Broad exothermic peaks are detected at 505°C and 564°C in Na0.15/ZrO2 and Cs0.15/ZrO2 respectively. Furthermore, additional shifts of this exothermic peak in Cs0.15-M0.1/ZrO2 versus alkali/ZrO2 seem to indicate that Cs and the transition metal have cumulative effects hindering the tetragonal ZrO2 crystallisation. An exothermic peak at 910°C for Na0.15/ZrO2 solid ascribed to a rapid tetragonal-monoclinic transformation [9] is detected. Recall that Na0.15/ZrO2 contains only a well-crystallized tetragonal ZrO2 after calcination at 600°C (Fig. 1.B.a). The absence of exothermic peak in Cs-containing solids could be explained by a different transformation kinetic in these solids. In the case of alkali-Co catalysts, an endothermic peak detected at 930°C corresponds to Co3O4 decomposition to CoO [10].

3.2. Catalytic test Between the catalysts promoted by alkali alone, the oxidation of toluene is only complete at 500°C for Cs0.15/ZrO2 (Table 2). Co and Cu impregnations on Cs0.15/ZrO2 lead to a powerful catalyst for Cs0.15Co0.1/ZrO2 but a less active solid for Cs0.15Cu0.1/ZrO2. However, the solid with a low amount of Cs (Cs0.015-Co0.1/ZrO2) allows to reach a T50 = 296°C instead for Cs0.15Co0.1/ZrO2.

Fig. 2 DTA curves of dried solids (a: ZrO2, b: Na0.15/ZrO2, c: Cs0.15/ZrO2, d: Cs0.15Co0.1/ZrO2, e: Cs0.15-Cu0.1/ZrO2).

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Therefore, the presence of a high amount of alkali metal (interesting for CB Toluene Toluene Toluene oxidation [5]) decreases the Catalyst + O2 + O2 + CB + O2+ NO activity of the Co3O4 phase for toluene oxidation, and in Na0.15/ZrO2 > 500 (283) the presence of CB, T50 is not Cs0.15/ZrO2 443 (283) 451 (276) 490 (308) enhanced, but Ti is slightly Cs0.15-Cu0.1/ZrO2 487 (300) enhanced. This is due to the Cs0.15-Co0.1/ZrO2 371 (260) 423 (220) 399 (250) oxidation of CB proceeding Cs0.015-Co0.1/ZrO2 296 (260) 269 (208) 279 (210) before that of toluene. For Cs0.015-Co0.1/ZrO2, T50 and Ti are both enhanced in the presence of CB. In this case, adsorption of toluene on CB could explain this result. Toluene oxidation is not enhanced by the presence of NOx in the case of catalysts with a high Cs content. This is due to the adsorption of NOx species by the catalyst with a high alkali amount, leading to the formation of nitrates [11]. A lower amount of Cs permits to both enhance NOx conversion and toluene oxidation compared to the same oxidation test performed without NOx. The oxidation of toluene is then very efficient in the presence of NOx but also in the presence of CB over Cs0.015Co0.1/ZrO2. Table 2. T50: temperature at which 50% of toluene is oxidized, (Ti): ignition temperature (°C).

4. Conclusion The addition of transition metals (Co or Cu) to Cs0.15/ZrO2 stabilizes tetragonal ZrO2 phase. The presence of CB or NOx leads to improve toluene oxidation, especially with the catalyst Cs0.015-Co0.1/ZrO2.

Ackowledgements The “Nord-Pas de Calais” Region, the “Syndicat Mixte de la Côte d’Opale” and European Union via Interreg IV “Redugaz project” are gratefully acknowledged for financial support.

References [1]

M. Labaki, S. Siffert, J.-F. Lamonier, E.A. Zhilinskaya, A. Aboukaïs, Appl. Catal. B 43 (2003) 261. [2] T. Holma, A. Palmqvist, M. Skoglundh, E. Jobson, Appl. Catal. B 48 (2004) 95. [3] J.-F. Lamonier, M. Labaki, F. Wyrwalski, S. Siffert, A. Aboukaïs, J. Anal. Appl. Pyrolysis 81 (2008) 20. [4] A. Bueno-Lopez, J. Soriano-Mora, A. Garcia-Garcia, Catal. Commun. 7 (2006) 678. [5] D. Hleis, M. Labaki, H. Laversin, D. Courcot, A. Aboukaïs, Colloids Surf. A 330 (2008) 193. [6] H. Laversin, D. Courcot, E. Zhilinskaya, R. Cousin, A. Aboukaïs, J. Catal. 241 (2006) 456. [7] F. Wyrwalski, J.-F. Lamonier, S. Siffert, A. Aboukaïs, Appl. Catal., B 70 (2007) 393. [8] J. Matta, J.-F. Lamonier, E. Abi-aad, E. Zhilinskaya, A. Aboukaïs, Phys. Chem. Chem. Phys. 1 (1999) 4975. [9] S. Liu, S. Huang, L. Guan, J. Li, N. Zhao, W. Wei, Y. Sun, Microporous Mesoporous Mater. 102 (2007) 304. [10] G. Gürdağ, I. Boz, S. Ebiller, M. Gürkaynak, React. Kinet. Catal. Lett. 83 (2004) 47. [11] I. Matsukuma, S. Kikuyama, R. Kikuchi, K. Sasaki, K. Eguchi, Appl. Catal. B 37 (2002) 107.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Design of appropriate surface sites for rutheniumceria catalysts supported on graphite by controlled preparation method J. Álvarez-Rodríguez, A. Maroto-Valiente* , M. Soria-Sánchez, V. Muñoz-Andres, A. Guerrero-Ruiz Departamento de Química Inorgánica y Química Técnica, Facultad de Ciencias, UNED, Pº Senda del Rey, 9, 28040 Madrid, Spain.

Abstract Ru-Ce supported catalyst properties were studied with the aim of improving catalytic performance in phenol abatement from aqueous solutions by investigating the effect of different thermal pretreatments under a flow of helium. Characterization using TGA, XRD, TEM and XPS shows that ruthenium acetylacetonate was more highly dispersed than CeO2. Ru-Ce/HSAG samples show increased Catalytic Wet Oxidation (CWO) of phenol compared to Ru/CeO2, Ru/HSAG and Ce/HSAG. The Ru-Ce sites generated during the preparation and activation of the Ru-Ce/HSAG catalyst at 473 K, result in higher activity, conversion and mineralization values in the phenol CWO. Keywords: phenol, CWO, Ce, Ru, graphite

1. Introduction Due to their unique properties, cerium oxides are currently one of the most employed components in the preparation of catalysts, i.e. as support for metallic heterogeneous catalysts, as stabilizers of dispersed components or as promoter ingredients [M. Boaro (2003), A. Trovarelli (2002)]. Its redox properties, oxygen storage capacity, high mechanical strength and ultraviolet absorption should be related to the formation of oxygen vacancies and oxygen mobility, which means the Ce(IV)-Ce(III) relation can play a critical role in catalytic reactions (fuel cells, three-way catalysts, CWO, etc). Moreover, it is well known that catalyst preparation methods are important to the final surface properties. It should therefore be interesting to explore the possibility of using different thermal activation pretreatments which could modify the Ru-Ce interactions, and then evaluate the effects of these modifications in the required processes (oxidation hydrogenation, decomposition, etc.) to expand and improve the properties of the catalyst. The Ru/CeO2 system is known for its efficient performance in CWO processes, which are employed to remove pollutants in wastewater [S. Imamura (1988), L. Oliviero (2000)]. It is expected that the catalytic performance could be improved if optimized Ru species are well dispersed on CeO2. Acting cooperatively they can act as active centers when deposited over a carbon material. Thus the purpose of this work is to study if it is possible to optimize a thermal pretreatment method and analyze the decomposition process of the metal precursor in order to improve Ru-CeO2 aggregates built on graphitic surface for phenol CWO.

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2. Experimental 2.1. Preparation Graphite (HSAG Lonza) was employed as support after pretreatment in a tubular reactor at 1173 K under helium flow. It was then activated by impregnation of a aqueous solution of Ce(NO3)3·6H2O (Fluka) and subsequently treated under nitrogen flow at 773 K for 2 h (5 wt.% of Ce content). The sample obtained was labeled as Ce/HSAG. Supported Ru catalysts were prepared by excess-impregnation solution (MeOH:H2O = 1:1) of Ru(acac)3 (Alfa-Aesar), over Ce/HSAG and CeO2 (Rhone-Poulenc), labeled as RuCe/HSAG and Ru/CeO2 respectively (2 wt.% of Ru content). All these samples were dried at 373 K and then stored in a desiccator.

2.2. Characterization Thermogravimetric Analysis (TGA) was carried out using a SDTQ600 5200 TA System. 10 mg samples were pretreated at room temperature for 30 min under helium atmosphere (flow rate = 100 mL min−1) and heated to 973 K, with a heating rate of 10 K min−1. The X-ray diffraction (XRD) patterns were recorded in an X-ray diffractometer (Seifert Model XRD 3000P), using Cu-Kα radiation and a graphite monochromator. Transmission Electron Microscopy (TEM) and X-Ray Energy Dispersive Spectroscopy (XEDS) studies were carried out in a JEOL JEM-2000 FX microscope at 200 kV. The samples were prepared by grinding and ultrasonic dispersal in an acetone solution before being placed on a copper TEM grid and the solvent evaporated. X-ray photoelectron spectra (XPS) were recorded with an Omicron spectrometer equipped with an EA-125 hemispherical electron multichannel analyzer and an unmonochromatized Mg Kα X-ray source having radiation energy of 1253.6 eV at 150 W and a pass energy of 50 eV. The spectral data was analyzed with CasaXPS software and RSF database by peak fitting after Shirley background correction.

2.3. Reaction test Oxidation reaction experiments were performed in a 300 mL stainless-steel high pressure reactor vessel (Parr Instruments Co., USA, 5521) operated under isothermal batch mode at 413 K, 2 MPa of oxygen pressure and stirred at 500 rpm to optimize the mass transfer in the liquid phase. For every run a fresh feed of aqueous phenol solution of 20 mmol L−1 and 4 g L−1 of the catalyst was introduced to the reaction vessel. The liquid phase was analyzed by HPLC on a Pursuit XRs 5 C18 150 while the gas phase was analyzed in a GC equipped with a Porapak Q packed column.

3. Results and discussion 3.1. Characterization TGA of the Ru-Ce/HSAG sample indicates that metal precursors, Ru(acac)2 and Ce(NO3)3, interact with the support and display three contributions to a total weight loss of up to 5%. There is a first weight decrease around 460 K, a second one close to 550 K and last weight loss at around 920 K. Derivate profiles (D-TGA), displayed in Figure 1, reveal, with accuracy, the three peaks positions are centered at 375 K, 553 K and 898 K. The first should be related to water desorption, close to 9% of weight lost. The main peak, at 553 K, can be assigned to ruthenium precursor decomposition and represents 78% of weight lost, and finally, the peak at 886 K, may be attributed to oxygen loss from the CeO2. In fact, when Ce/HSAG is studied, only the peak at 886 K was obtained. The shape of the main peak of Ru-Ce/HSAG seems to contain different contributions. So in order to identify them, three fresh samples were pretreated for 2 h in helium at 473 K, 503 K and 553 K, respectively. The TGs of these samples show that three

Design of appropriate surface sites for ruthenium-ceria catalysts supported on graphite 753 contributions can be distinguished in the main DTG peak of the Ru-Ce/HSAG sample; at 513 K (18 wt.% lost), at 563 K (54 wt.% lost) and at 617 K (7 wt.% lost). These three meta-species are attributed to multi-step decomposition of ruthenium precursor and/or with different surface site distribution of the adsorbed ruthenium acetylacetonate, namely over ceria or over graphite sites/planes.

%w / K

D-TGA

400

600

800

Temperature (K)

Figure 1. D-TGA of Ru-Ce/HSAG as made(black), pretreated under He flow at 473 K (red) and 503 K (blue).

Ce/HSAG and Ru-Ce/HSAG XRD patterns show a main diffraction peak at 2θ = 26° (0 0 2) due to graphitic carbon structures and CeO2 crystallites, which exhibit a cubic fluorite-type structure, displaying characteristic diffraction lines of CeO2 at 2θ = 28.5° (1 1 1), 33.2° (2 0 0) and 47.5° (2 2 0). The presence of crystalline ruthenium species, RuO2 and Ru0, is not confirmed at characteristic 2θ values. However, in all samples with ruthenium content, Ru-Ce/HSAG and Ru/CeO2, an increase of the diffraction peak at 56.3º (3 1 1) of CeO2 was observed, thus the presence of some crystalline Ru species was not discarded. Information concerning particle size and component distribution in these catalysts was obtained by Transmission Electron Microscopy (TEM). For the Ce/HSAG sample, dispersed CeO2 particles were observed and identified by XEDS analysis with heterogeneous sizes between 20-100 nm. Likewise, CeO2 particles were detected for Ru-Ce/HSAG, but no RuO2 or Ru0 crystallites were observed by TEM. This probably means that the ruthenium species are highly dispersed. Careful XEDS analysis was used to gain information about the Ru distribution, and Ru was detected whenever the sample was studied. Everywhere that CeO2 particles exist, ruthenium is also present. However, in the positions where ruthenium appears, CeO2 is not always detected. Following the Luo 2009 processing method, analysis of X-ray photoelectron spectra of samples showed the Ce 3d region was composed of 10 contributions, named U1, U0, V1 and V0 for Ce3+ species, and U3, U2, U, V3, V2 and V for Ce4+ species. The peak areas were then determined and the oxidation states were calculated using the following equations: Ce3+ = U1 + U0 + V1 + V0 Ce4+ = U3 + U2 + U + V3 + V2 + V Percentage of Ce3+ = Ce3+ / (Ce3+ + Ce4+)

Eq. (1) Eq. (2) Eq. (3)

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After solving this percentage equation, ratios found for Ce3+/Ce4+ were 36/64 for Ce/HSAG and 27/73 for Ru-Ce/HSAG. This suggests that the fraction of ruthenium anchored over ceria particles is preferred at those sites with Ce3+. Note that this fraction of ruthenia is similar to that fraction of first TGA-peak identified and could suggest a Ru-Ce3+ site.

3.2. CWO reaction test Phenol is commonly employed as a reference molecule for aqueous oxidation tests of heterogeneous catalysts in order to gain knowledge about its performance for abatement of aromatic compounds. Phenol CWO over our catalysts yields hydroquinone, benzoquinone, cathecol, acetic acid, formic acid, oxalic acid, fumaric acid, maleic acid, malonic acid and carbon dioxide, but with different distribution. After 5 h of reaction, Ce/HSAG and Ru/CeO2 samples shows conversion values approximately of 30%, but higher phenol transformation of up to 100%, is achieved with Ru-Ce/HSAG. However, depending on the pretreatment, total conversion is achieved in differing time periods. Similar conversion is observed after 180 min and 220 min for catalysts pretreated at 473 K and 503 K respectively. Mineralization (phenol transformed to CO2) tendencies were observed and it was seen that Ru/CeO2 < Ce/HSAG << Ru-Ce/HSAG. Initial activities were also determined and they increased in the order Ru/CeO2 < Ce/HSAG << Ru-Ce/HSAG (without thermal stabilization) << Ru-Ce/HSAG (pretreated at 503 K) < Ru-Ce/HSAG (pretreated at 473 K). These values are higher than those previously reported by Oliviero 2000 (245 mmol h−1gRu-1) for a similar catalytic system (carbon supported Ru and Ru–Ce), and by Castillejos-López 2009 employing Ru/HSAG (188 mmol h−1gRu-1).

4. Conclusions The preparation and activation methods of Ru-Ce supported catalysts should be carefully chosen, considering that the higher the optimized interaction between active species, the higher catalytic performances. The Ru-Ce/HSAG exhibits improved properties due to the multicomponent structure of the surface active sites and their distribution on carbon surface, which results in increased catalytic performance in phenol abatement compared Ru/CeO2, Ru/HSAG and Ce/HSAG. It was also found that the Ru-Ce sites generated during the preparation and activation step at 473 K drive the higher activity and performance for phenol CWO compared to other methods.

References M. Boaro, M. Vicario, C.D. Leitenburg, G. Dolcetti, A. Trovarelli, 2003, The use of temperatureprogrammed and dynamic/transient methods in catalysis: characterization of ceria-based, model three-way catalysts, Catalysis Today 77, 44, 407-417. A. Trovarelli (Ed.), Catalysis by Ceria and Related Materials, Imperial College Press, 2002. S. Imamura, I. Fukuda, S. Ishida, 1988, Wet oxidation catalyzed by ruthenium supported on cerium(IV) oxides, Industrial & Engineering Chemistry Research 27, 4, 718-721. L. Oliviero, J. Barbier Jr., D. Duprez, A. Guerrero-Ruiz, B. Bachiller-Baeza, 2000, Catalytic wet air oxidation of phenol and acrylic acid over Ru/C and Ru–CeO2/C catalysts, Applied Catalysis: B Environmental, 25, 4, 267-275. X.J. Luo, R. Wang, J. Ni, J.X. Lin, B.Y. Lin, X.M. Xu, K.M. Wei, 2009, Effect of La2O3 on Ru/CeO2-La2O3 Catalyst for Ammonia Synthesis, Catalysis Letters, 133, 382-387. E. Castillejos-López, A. Maroto-Valiente, D.M. Nevskaia, V. Muñoz, I. Rodríguez-Ramos, A. Guerrero-Ruiz, 2009, Comparative study of support effects in ruthenium catalysts applied for wet air oxidation of aromatic compounds, Catalysis Today, 143, 355–363.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Preparation of monolithic catalysts for space propulsion applications Rachid Amrousse,a Rachid Brahmi,a Yann Batonneau,a Charles Kappenstein,a Marie Théron,b Patrick Bravaisc a

LACCO (Laboratoire de Catalyse en Chimie Organique), 40 Avenue de Recteur Pineau, Poitiers 86022,France b CNES (Centre National d’Etudes Spatiales), Rond-Point de l’Espace, EvryCourcouronnes 91023, France c DTA (Division des Techniques Avancées), Rue de Clemencière, Sassenage 38360, France

Abstract Cellular ceramics (honeycomb monoliths and foams) are attractive alternatives to conventional catalyst carriers for propulsion applications. Conventional procedures to prepare the corresponding catalysts, however, cannot be simply transferred to monolithic catalysts. The procedure is discussed focusing on the quality of the washcoat layer deposited on the internal surface of the monolith. The deposition of active phases like noble metals on a wash-coated monolith is carried out in a similar manner as with catalyst pellets using impregnation with an excess of solvent. The concentration of active phase precursor in the impregnation solution is linked to the porous volume of the wash-coat layer and the desired final metal loading. The phase compositions of monolithic catalysts were investigated using powder X-ray diffraction. The morphology and active phase distribution in the wash-coat materials are studied by scanning electron microscopy (SEM) equipped with an energy X-ray dispersive spectrometer (EDS) and the particle size is determined from transmission electron microscopy (TEM). Keywords: cellular ceramic, wash-coating, impregnation

1. Introduction Cellular ceramic supports are proposed as an attractive alternative to conventional carriers (generally spheres, extrudates or pellets) for solid catalysts. They are of two types: (i) monolith honeycomb made of small parallel squared channels (about 1 mm size) displaying a catalytic wall (400 channels per square inch or cpsi) or (ii) monolith foam presenting small blocks of pores (30 pores per inch or ppi). These materials are specifically manufactured by CTI Company (Céramiques Techniques et Industrielles, Salindres, France) or Corning Company. The largest application of monoliths is in the automotive industry for the cleanup of exhaust gases and in the selective catalytic reduction of off-gases released by power stations [1]. The use of monolithic supports for space propulsion applications is currently limited to the decomposition of monopropellants as hydrazine substitutes in the field of the GRASP European project (H2O2, energetic ionic liquids, nitrous oxide) and to the ignition of cold H2 + O2 mixtures [2] which is the current application for this paper. In multiphase reactions, monolithic reactors have clear advantages (energy input, efficiency, low pressure drop, safety and catalyst separation) over the generally used pellet bed reactors.

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Conventional procedures to prepare catalysts, however, can not be simply applied to monolith-based catalysts. The wash-coating procedure focuses on the quality of the layer deposited on the monolith to increase the specific surface area and to ensure a better dispersion of the metallic phase; two wash-coating procedures have been used but only one is described in this paper. The deposition of active phase like platinum, iridium or rhodium on a wash-coated monolith is carried out in a similar manner as with pellet supports using impregnation with an excess of impregnation solution. The concentration of active phase precursor in the impregnation solution takes the porous volume of the wash-coat layer [3], the solubility limit and the desired final metal loading into account.

2. Experimental 2.1. Wash-coating of monolithic supports To increase the internal surface area, the monoliths were wash-coated using sol-gel procedure. The preparation of colloidal solution was carried out at room temperature by mixing a known volume of nitric acid (0.3 mol.L-1), Disperal boehmite (Sasol, SBET = 260 m2 g-1) and urea (OC(NH2)2). Urea, which is in the form of white pellets, is added to the beaker containing nitric acid. The urea is dispersed at high velocity (6200 rpm) for 5 min. The addition of boehmite (very fine white powder) is then performed in small quantity for 20 min under high mixing speed (17600, 21700 and 23000 rpm) using a Ultra Turax T25 mixer. This step aims to develop a porous layer thus promoting a better dispersion of the metallic active phase. The principle of the coating step consists in immersing the monoliths in the prepared sol. The unclogging of the channels is then performed under a weak argon flow to remove excess colloidal solution. Beside the composition of the sol dispersion, key parameters are the temperature, the viscosity of the sol and the duration of the wash-coating process which must be carefully controlled. Then, the wash-coated monoliths are dried in a dedicated system allowing horizontal rotation to ensure a homogeneous distribution of the coating layer. Finally, thermal treatment of the coated monoliths at higher temperature (500°C) was carried out under air in a muffle furnace.

2.2. Impregnation of coated monolithic supports The technique used to deposit the active phase into the porosity of the coating layer is the impregnation of the coated substrate with an active phase precursor (i.e. hexachloroiridic acid H2IrCl6 for Ir-based catalysts). As the porous layer remains thin (few tens of micrometers), the wet impregnation remains the simplest procedure. It is generally performed from an aqueous solution of the metal salt precursor. The coated monoliths are immersed overnight into the precursor solution under mechanical agitation. The excess of solution is then evaporated. When the precursor solution is completely evaporated, the impregnated monoliths are carefully dried before thermal treatment. The last thermal treatment is performed to form the active phase from the dried active phase precursor. This is carried out in a dedicated quartz reactor adapted to the size of the monolithic catalysts. Generally, this treatment corresponds to a reduction under hydrogen flow diluted in helium (to reduce the thermal effects of the reduction) of the metal precursor to oxidation state zero. These treatments are specific to each active phase. The reduction of chloride metal precursors produces quantitatively gaseous hydrogen chloride, e.g. in the case of iridium active phase: H2IrCl6(s) + 2 H2(g) Î Ir(s) + 6 HCl(g) Therefore, the reduction reaction extent can be easily controlled by using an online trap containing a basic solution. After each step, the monoliths are carefully weighed to control the reproducibility and the variability of the catalyst preparation.

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3. Results and discussions Lab-scale (about 1 g) and large-scale catalyst samples (about 150 g) have been prepared. They are wash-coated with γ-alumina and impregnated by iridium precursor. Figure 1 presents the cross section of samples based on 400 cpsi and 30 ppi monoliths.

Figure 1. Front view of a 400 cpsi honeycomb cordierite monolith: (a) as received (length 100 mm, diameter 50 mm), (b) after wash-coating by γ-alumina and impregnation by iridium and (c) front view of a 30 ppi foam cordierite monolith.

The reduction completion is controlled through the titration of the basic solution of the trap after the reduction; this leads to the determination of a HCl recovery factor nHCl(1)/nHCl(2) where nHCl(1) is the experimental amount of trapped hydrogen chloride and nHCl(2) the calculated amount. For the first set of catalysts, a recovery factor of about 0.5 was obtained, meaning that a part of chloride was not trapped or remained onto the catalyst surface; the second assumption is in agreement with the detected presence of excess chloride by EDS measurements (vide infra). For the second set of catalysts, the hydrogen flow rate was increased from 20 to 40 mL.min-1, thus leading to a recovery factor of 0.92.

3.1. Characterization of the catalysts The γ-alumina wash-coat layer and the filling of the wash-coat porosity by the iridium active phase were followed by SEM in order to see the quality of the coating layer and the active phase impregnation. Figure 2 shows that the improvement of the wash-coating procedure leads, as expected, to a homogeneous wash-coat layer whereas the iridium impregnation shows slight transformations of this layer.

Figure 2. Photo of a channel of a 400 cpsi cordierite monolith; (a) coated by γ-alumina and (b) impregnated by iridium (first catalyst set). EDS spectrum of the circle inside.

EDS Spectrum of the part inside the circle (Figure 2) shows the characteristic peaks of the elements that constitute the wash-coat layer (Al and O) and the active phase (Ir) as well as the remaining chlorine atoms from the H2IrCl6 precursor. Iridium-based monolithic catalysts were characterized by powder X-ray diffraction. Diffractogram (Figure 3) of the catalyst highlight the presence of cordierite (PDF file 13-0294) as a support, and iridium metal (PDF file 06-0598) as the active phase.

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Figure 3. Diffractogram of iridium based catalysts.

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Figure 4. TEM image of iridium active phase deposited on cordierite.

Photo taken by TEM for Ir-based monolithic catalyst are presented in Figure 4: the particle size is uniform and varies between 2 and 4 nm. The metallic phase is in the form of particles mostly distributed on the alumina wash-coat layer.

Conclusion Preparation of catalysts for propulsion application needs improvement of the catalytic bed and careful control of the different steps and their corresponding parameters: - Wash-coating step: ceramic monoliths require a specific wash-coating step to increase the specific surface area of the support. The parameters to be controlled are: nature and viscosity of the colloidal suspension, temperature, procedure duration, drying and thermal treatment conditions, mass percentage, thickness and homogeneity of the coating layer. This washcoat layer must be resistant to thermal shocks. - Impregnation step: the active phase is deposited inside the porosity of the coating layer by impregnation of the active phase precursor followed by drying, calcination and/or reduction. The parameters to be controlled are: nature and concentration of active phase precursor, drying, calcination and reduction conditions, loading level of active phase. Current studies at LACCO aim at improving catalyst efficiency and verifying the impact of these different parameters by characterization and evaluation of the catalysts.

Acknowledgments This study was supported financially by French Spatial Agency (CNES, Centre National d’Études Spatiales) and Air Liquide Society.

References R. M. Hecka, S. Gulati, 2000, The application of monoliths for gas phase catalytic reactions, Chemical Engineering Journal, 82, 3, 149-156. T. C. Tien, J. S. T'Ien, 1991, Catalytic ignition model in monolithic reactor with in-depth reaction, Aerothermodynamics in combustors; IUTAM Symposium, National Taiwan Univ., Taipei, Selected Papers (A93-51626 22-25), 231-44. T. A. Nijhuis, A. E. W. Beers, T. Vergunst, I. Hoek, F. Kapteijn, J. A. Moulijn, 2001, monolithic Catalysts as Efficient Three-Phase Reactors, Catalysis Reviews - Science and Engineering, 43, 4, 345-380.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Synthesis of mixed zirconium-silver phosphates and formation of active catalyst surface for the ethylene glycol oxidation process N.V. Dorofeevaa,b, O.V. Vodyankinaa, O.S. Pavlovaa, G.V. Mamontova a b

Tomsk State University, 36, Lenin Str., 634050, Tomsk, Russia Tomsk Polytechnic University, 30, Lenin Str., 634050, Tomsk, Russia

Abstract In the present work the catalytic activity, structure, physical and chemical properties of silver-containing zirconium phosphates, containing different amounts of silver, and synthesized using sol-gel, co-precipitation and ion exchange methods have been studied. It was shown that the method of phosphate synthesis determines the final composition, structure and catalytic properties. The phosphates, treated in reducing media, exhibit catalytic activity in the process of ethylene glycol oxidation into glyoxal. Keywords: catalyst, silver, zirconium phosphates, glyoxal

1. Introduction One of the main tasks in the field of catalysis is a purposeful organization of catalyst structure and stability of active sites. From this point of view, NASICON-type zirconium phosphates mixed with different metals, providing properties such as ionic conductivity, thermal and chemical stability, etc., are of particular interest. The ionic conductivity of the phosphates under specific conditions allows forming the active component particles with a given size on the matrix surface, and also providing their stability in the catalytic process. At the same time, the composition and structure of materials depend on the synthesis technique. The aim of the present work is to study the influence of the methods of synthesis of silver-containing zirconium phosphates on the composition, structure and catalytic activity of materials in the process of ethylene glycol oxidation, and formation of active sites (Ag nanoparticles) on the surface of zirconium–phosphate matrix.

2. Experimental 2.1. Catalysts preparation In the present work silver-containing zirconium phosphates AgZr2(PO4)3 (18.75 wt % of Ag) was prepared using two different methods: sol-gel method (SG) and coprecipitation (CP). Sodium-containing zirconium phosphates NaZr2(PO4)3 was prepared using sol-gel method. Ion exchange Na+ by Ag+ in sodium-containing zirconium phosphates was used to prepare the catalyst containing 10 wt % of Ag (IE). Coprecipitation of mixed phosphates with the composition АgZr2(PO4)3 was performed by mixing of aqueous solutions of phosphoric acid and metal nitrates under stirring at room temperature. The obtained precipitate was filtered. Sol-gel method used in this work is the one originally developed for synthesis of metal oxide nanopowders [1]. The method is based on the formation of metal complexes with complexing agent such as citric acid. Then polyatomic alcohols such as ethylene

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glycol are added. The polyesterification reaction takes place between complexes and alcohol. Silver (or sodium) and zirconium nitrates is dissolved in the citric acid (CA) solution at 343 K. Then in the solution of metal complexes was added the solution of ammonium hydrophosphate and ethylene glycol (EG) and the temperature of reaction solution was increased to 363 K. Silver- and sodium-containing zirconium phosphates were synthesized by this method. All silver-containing phosphates were heated up to 973 K, sodium-containing sample – up to 1123 K. Ion-exchange properties of sodiumcontaining zirconium phosphates were used for preparation of catalyst with 10 wt % Ag. The calculated amount of sodium-containing zirconium phosphates was placed into the 0,04 М silver nitrate solution during 4 days, and then filtered. All samples were treated by hydrogen in identical condition (873 K, 10 vol% H2 in N2 at a total flow rate 200 cm3min-1).

2.2. Characterization of catalysts Phosphates were characterized before and after treatment in hydrogen by several physical-chemical methods. The X-ray diffraction patterns of the samples were obtained on a Shimadzu XRD-6000 diffractometer with CuKα-irradiation. The structure and size of particles were studied using Philips SM30 transmission electron microscope with an accelerating voltage of 200 kV. The chemical composition of the systems was studied by IR spectroscopy on a Nicolet 5700 FTIR spectrometer. Diffuse reflectance spectra were recorded in the 200–800 nm range on a Evolution 600 spectrometer equipped with an integrating sphere. The amount of Ag+ in the mother liquid of the samples CP and IE was established by the ICP spectroscopy method (iCAP 6300). Size and form of original particles and also structure elements were characterized by scanning electron microscopy with EDAX detector (LMU2). The specific surface area of the samples was measured by the one-point Brunauer-Emmett-Teller method (nitrogen adsorption). Measurements were taken on a TriStar 3020 analyzer. Temperature programmed oxidation/temperature programmed reduction experiments were carried out on ChemiSorb 2750, 10 vol% O2 in He and 10 vol% H2 in Ar at a total flow 20cm3min-1. Catalytic measurements were carried out as previously described [2].

3. Results and discussion The amount of Ag introduced in the synthesis and its content in the phosphates depend on conditions and method of synthesis. Coprecipitation of phosphates occurs with the loss of the silver, the residual Ag remains in the mother liquid. Mass concentration of silver in the solid is 18,5 %. The concentration of Ag+ in the mother liquid of phosphate IE is minimal, and effectiveness of ion-exchange is no less than 99%. For SG sample all the silver, introduced in the synthesis, distributes in a volume of phosphate, because the synthesized viscous gel dries and heats without filtering (the loss of the silver is excluded). Phase composition of CP sample differs from other samples (SG and IE) by the presence of the cubic ZrP2O7 and rhombohedral Zr2,25(PO4)3 impurities. The presence of the ZrP2O7 was verified by X-ray diffraction and IR spectroscopy by the presence of absorption band at 725 cm-1 assigned to P–O–P vibration. One of the reasons of the impurity phase formation is the loss of silver on the synthesis. However, this imperfection of system can be removed by the synthesis of phosphates from concentrated solution and increasing time of heating.

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High temperature treatment of SG phosphate led to formation of Zr2,25(PO4)3 impurity and trace amount of Ag as established by X-ray diffraction. The presence of Ag in the sample was confirmed by the UV-VIS absorption spectrum. The reduction of Ag+ in SG sample can be explained by the presence of great amount of organic compounds in the synthesis procedure. TEM measurements of all samples showed that they have a microcrystalline structure. It was found that specific surface area of phosphates synthesized using sol-gel method depends on the CA/EG ratio as for metal oxides. Thus, for AgZr2(PO4)3 prepared by this method at the ratio CA/EG = 1,8 surface area is 4,3 m2g-1 and at CA/EG = 3,6 it is equal to 10,1 m2g-1. The specific surface area of CP sample was 4,8 m2g-1. The catalytic activity of silver-containing phosphates was investigated in the process of ethylene glycol oxidation into glyoxal. It was found that the main products of this reaction are СО2 and СО. For the purpose of phosphate activation they were treated in reducing media at 873 К prior to catalyst tests. This temperature was selected on the basis of oxidation conditions of ethylene glycol into glyoxal (673–923 K). The XRD diagrams of silver-containing phosphates treated by hydrogen-containing flow are represented in Fig. 1. The diagrams of all samples have reflexes of Zr2,25(PO4)3, ZrP2O7 and Ag which are formed under the reducing treatment, and also attend reflexes of skeleton structures - AgZr2(PO4)3 and NaZr2(PO4)3. Probably, formation of zirconium phosphates and Ag begins from the step formation of Ag1-xHxZr2(PO4)3 to exchange Ag+ in main structure to H+ as described earlier in [3] and decomposition of Ag1-xHxZr2(PO4)3 at 823 K. The associated reactions may be expressed as follows: AgZr2(PO4)3 + x/2 H2 → xAg0 + Ag1-xHxZr2(PO4)3, 2Ag1-xHxZr2(PO4)3 → 2(1-x)Ag0 + ZrP2O7 +Zr3(PO4)4 + xH2O.

(1) (2)

It was shown in Fig. 1, that content of Ag (accordingly intensities reflection Ag (111) in the sample CP and IE more than in sample SG. It may be explained by distribution of Ag+ in the SG phosphate and more regular structure of the latter. Formation of Ag1-xHxZr2(PO4)3 and the reversible oxidation / reduction of silver at subsurface region was studied by TPO/TPR methods. In Fig. 2 the TPR curves SG phosphate after oxidation are represented. In the first TPR cycle up to 11 wt% of total amount of silver can be reduced. The curve consists of two peaks with Tmax equal to 424 K and 482 K. Both peaks are associated with reduction of silver. In the second TPR cycle new peak appears with Tmax = 350 K. This peak is associated with formation of surface oxides Ag2sO, which are formed on the surface of Ag particles obtained during the first TPR cycle.It correlates with XRD of hydrogen-treated phosphates, where the destruction of AgZr2(PO4)3 with formation of zirconium phosphates and Ag was found. Crystallization of ZrP2O7 may prevent reversible oxidation/reduction of silver at subsurface region.

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In Figure. 3 the UV-Vis spectra reducing phosphate are represented. Under the reducing conditions a part of Ag+ transforms to Ag0 (Eq. 2). The UV-Vis spectra contain absorption bands at 294 and 427 nm assigned to silver clusters Ag4+ and silver nanoparticles, respectively [4]. Fig. 3 shows that catalysts CP and SG (curves a, b) are characterized by narrow size distribution of silver particles. The size of silver particles in CP and SG samples are 10 – 50 nm. The IE catalyst sample contains less silver than CP and SG, but has wide size distribution of silver particles (curves c), and, probably, contains bigger silver particles. It may be explained by irregular distribution of Ag+ in original sample and AgZr2(PO4)3 distortion. Catalytic activity of silver-containing phosphates treated in reducing media was investigated in the process of ethylene glycol oxidation into glyoxal. The conversion of glycol for all catalysts increases simultaneously with oxygen content in the feed, and at O2/EG=1 it amounts to 95%. However, CO and CO2 yields also increase. The maximal selectivity with respect to glyoxal is observed at temperature below 773 K and also depends on the O2/EG ratio. Thus, the maximal selectivities for CP and SG catalysts are 53 and 41%, respectively, at the O2/EG = 0,6. The maximal selectivity with respect to glyoxal for IE catalyst (53%) is observed at O2/EG = 1 at the alcohol conversion of 96%.

4. Conclusion The analysis of physical-chemical data shows that both phosphate structure and method of synthesis determine the amount of reduced silver. The determinative factor in silver reduction process with narrow size distribution is a uniform distribution of Ag+ and structure units in phosphates during synthesis. The silver-containing phosphates treated in reducing media show the catalytic activity in the process of ethylene glycol oxidation into glyoxal, which depends on the catalyst composition, structure and amount of reduced Ag. This work was supported by the Russian Federal Program “Scientific and scientificeducational professional community of innovated Russia”.

References 1. 2. 3. 4.

B. L. Cushing, V.L. Kolesnichenko, C.J. O’Connor, 2004, Recent Advances in the Liquid Phase Syntheses of Inorganic Nanoparticles, Chem. Rev., 104, 3893-3946. O.V. Magaev, A.S. Knyazev , O.V. Vodyankina, N.V. Dorofeeva, A.N. Salanov, A.I. Boronin, 2008, Appl. Cat., 344, 142-149. S. Arsalane, M. Ziyad, G. Coudurier, J.C. Vedrine, 1996, Silver-cluster Formation on AgZr2(PO4)3 and Catalytic Decomposition of Butan-2-ol, J. Catal., 159, 162-169. B.G. Ershov, E.V. Abkhalimov, 2007, Nucleation of Silver upon the Reduction by Hydrogen in Aqueous Polyphosphate-Containing Solutions: Formation of Clusters and Nanoparticles, Coll.J., 69, 5, 579-584.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Characterization of cobalt nanoparticles on different supports for Fischer-Tropsch synthesis Maria do Carmo Rangela, Andrei Khodakovb, Francisco J. Cadete Santos Airesc, Michèle O. de Souzad, Jean-Guillaume Eone, Lilian M. dos Santosa, Alexilda O. de Souzaf, Anne G. Constantb a

GECCAT, Instituto de Química, Universidade Federal da Bahia. Federação, 40170290, Salvador, Bahia, Brazil, E-mail: [email protected] b Unité de catalyse et de chimie du solide, Université de Lille 1, 59655 Villeneuve d’Ascq, France, E-mail : [email protected] c Institut de Recherches sur la Catalyse et l´Environnement de Lyon (UMR 5256 CNRS/Université Lyon I),2, Av. Albert Einstein,F 69626 Villeurbanne Cedex, France d Instituto de Química, Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves, 9500, 91501-970, Porto Alegre, RS, Brazil e Instituto de Química. Universidade Federal do Rio de Janeiro, Av.Brigadeiro Trompovski. Ilha do Fundão. 21945-970 - Rio de Janeiro, RJ. Brazil f DEBI-UESB, Praça Primavera 40, Primavera, 45700-000, Itapetinga, Bahia, Brazil

Abstract The structure of cobalt nanoparticles on alumina and carbon nanotube supports, prepared by a simple and fast method, is investigated using a wide range of characterization techniques (TG, DTA, XRD, nitrogen adsorption, XPS, EXAFS and (HR)TEM). The samples were obtained by several cobalt impregnation steps and are expected to be efficient catalyts for Fischer-Tropsch synthesis. Cobalt nanoparticles ranging from 3 to 5 nm, forming small clusters of 10 to 20 nm, were successfully prepared on alumina and carbon nanotube, the morphology depending on the support. Keywords: Fischer-Tropsch, cobalt, alumina, carbon nanotube, nanoparticles

1. Introduction Cobalt-based catalysts have found several applications in different reactions, such as Fischer-Tropsch synthesis and natural gas reforming, among others [1, 2]. In all cases, the particle size is an important feature to take into account when one aims to get very efficient catalysts for different purposes. In Fischer-Tropsch synthesis, cobalt-based catalysts are recognized as a commercially attractive option. Cobalt shows high activity and selectivity for long-chain hydrocarbons, lower water gas shift reaction activity than iron and has lower price in comparison to noble metals such as ruthenium [3]. Several studies [4, 5] have shown that the activity of cobalt catalysts for FischerTropsch synthesis is structure-insensitive; the number of active sites depends on the cobalt particle size, loading amount and reduction degree. Also, it is known that the catalyst performance can be significantly increased by cobalt dispersions at the particle range of 3-5 nm [4]. It is thus very important to control the metal size particle. However, cobalt crystallites dispersions above 5-6 nm are difficult to prepare [4]. The present work, focuses on the detailed investigation of the structure of cobalt nanoparticles on alumina and carbon nanotubes, prepared by a simple and fast method, to be used in Fischer-Tropsch synthesis.

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2. Experimental A cobalt nitrate solution (4.93g/14.6mL of water) was gently added to a commercial alumina in order to obtain catalysts with 10% (w/w) Co. The material was kept for 1 h at room temperature and dried in an oven at 90ºC overnight. Then, the solid was heated (2ºC.min-1) up to 300ºC for 3 h, under nitrogen flow (100 mL.min-1). The solid with 20% Co was prepared by another successive impregnation as described. In the preparation of carbon nanotube-supported cobalt, the commercial support was previously refluxed with nitric acid 54% for 3 h at 80ºC, washed with water up to neutral pH and the solid was dried in an oven for 16 h. The cobalt impregnation was carried out as described, but the solid was heated at 220ºC. Thermogravimetry (TG) and differential thermal analysis (DTA) were carried out in a Mettler Toledo model TGA/SDTA851 equipment, under air flow from room temperature up to 1000°C. X-ray diffraction patterns (XRD) were obtained using a Shimadzu model XD3A equipment, with Cu-Kα radiation (λ=1.5420 Å) and a nickel filter. Specific surface areas and porosity measurements were performed using a Micromeritics model ASAP 2020 equipment. Surface analyses (XPS) were carried out with a Leybold Hereaus spectrophotometer using the Al-Kα radiation at 1486.6 eV. Binding energies were corrected relatively to 2p signal of Al in Al2O3 at 74.6 eV. EXAFS data at Co K-edge (7.709 keV) were acquired at the Italian synchrotron Elettra (Trieste). In situ analyses were performed under hydrogen flow, between ambient temperature and 400ºC in a home-made boron nitride cell. EXAFS oscillations were extracted using the program Athena; simulation of the spectra was performed using the program Artemis. The photoelectron amplitude and phase functions associated to the scattering paths used in the simulation were determined by the program FEFF6. These programs belong to the Ifeffit package developed by M. Newville [6]. Morphological and composition analyses of the supported catalyst were performed, respectively, by TEM and high spatial resolution EDX-S in the TEM associated with a 2D-FFT analysis of HRTEM micrographs. A JEOL JEM 2010 microscope operating at 200 kV and equipped with a LaB6 tip, a high resolution pole-piece and a Pentafet-LinK ISIS EDS-X spectrometer (Oxford Insts.) was used. The samples were grounded and dispersed ultrasonically in ethanol. A small drop was then deposited on holey-carbon film covering an electron microscopy copper grid (200 mesh, 3.05 mm).

3. Results and discussion It was found that all supports and catalysts were mesoporous solids with specific surface area ranging from 110 to 180 m2.g-1 and with almost no micropores. The normal spinel structure of cobalt oxide (Co3O4) was found by XRD for all samples. This was in agreement with the XPS analysis, whose spectra revealed a contribution of Co2+ species, evidenced by satellite peaks; the quantitative analysis indicates a lower surface ratio Co/(Co+M) (M=Al or C) on as compared to the bulk. Figure 1a shows the magnitude of the Fourier Transform of the EXAFS spectrum at Co K-edge for alumina-supported cobalt (10%) and fitted spectrum using the Co3O4 structure. The first peak corresponds to the overlapping of simple scattering paths due to four oxygen neighbors at 1.99 Å from Co2+ and six oxygen neighbors at 1.90 Å from Co3+. The second peak is exclusively due to Co3+ and corresponds to six Co3+ ions at 2.86 Å. The third peak was attributed to a common coordination sphere to both kinds of cations since it is associated to the simple scattering path Co2+-Co3+, of length 3.35 Å. It is known that Co2+ species in the spinel structure occupy tetrahedral coordination sites while Co3+ ions occupy octahedral coordination sites in a ratio 1:2, respectively. The

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Co2+ / (Co2+ + Co3+) ratio was fitted to 0.35 ± 0.04, suggesting that the stoichiometry of the catalyst corresponds to Co3O4 compound, in agreement with the XRD and XPS results. The EXAFS-fitted cell parameter was also equal to the crystallographic value, with an error of one per thousand. On the other hand, the Debye-Waller factors were low, indicating that the materials were well crystallized. The quality of simulation (Rfactor close to 1.0%) corroborates the interpretation. No change in the EXAFS spectrum was observed after in situ reduction by hydrogen up to 200ºC. At 300ºC, the oxide reduced to CoO; some structural disorder was noted. 16 14 1

Normalized spectra

10 8

3

FT[k .chi(k)]

12

6 4

0

2 0 0

1

2

3

4

5

7680

7700

7720

7740

7760

7780

Energy (eV)

Radial distance (Å)

(a)

(b)

Figure 1. (a) Magnitude of the Fourier transform of the EXAFS spectrum at the Co K-edge of (open circles) the 10%Co/Al2O3 catalyst and, (black line) fitting curve using Co3O4; (b) XANES spectra at the Co K-edge of (open circles) the 10%Co/Al2O3 solid after reduction under hydrogen at 400ºC and cooling at ambient temperature, (full line) linear fitting using the spectra of (dashes) Co metal, (dash-dots) CoO and, (dots) residual.

After reduction at 400ºC, the XANES spectrum (Fig. 1b) displayed an intense white line, indicating a high average oxidation degree of cobalt; indeed, linear fitting using a combination of Co and CoO spectra indicated only an amount of 22% (±3%) of metallic cobalt in the sample. This value was corroborated by simulation of the EXAFS spectrum. In contrast to cobalt alumina catalysts, almost complete cobalt reduction to metallic phase was observed by XANES in all carbon nanotube supported samples.

(a)

(b)

Figure 2. Morphology of (a) alumina-supported cobalt and (b) carbon nanotube-supported cobalt as seen by transmission microscopy.

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The images obtained from transmission microscopy ((HR)TEM) showed a high amount of cobalt clusters on alumina, with an irregular distribution as a result of different stages of aggregation. Small particles on the support are observed, in some cases, a very thin coverage or thin plates, as shown in Fig. 2a. In the case of carbonsupported cobalt (Fig. 2b), cobalt particles with poorly-defined shapes, like layers of plates with around 10 nm and 1 nm width were found. The difficulty to obtain diffraction images for a large amount of cobalt aggregates confirms their nanometer-order thickness. For all samples Co3O4 phase was related to cobalt aggregates by 2D-FFT analysis of HRTEM images.

4. Conclusions Cobalt nanoparticles on alumina and carbon nanotube with a with thin-plate shape from 3 to 5 nm, in small clusters of 10 to 20 nm, were successfully prepared and characterized using a wide ragne of techniques. Theier strcture, morphology and reduction behavior depend on the support. These solids have suitable characteristics which make them potential candidates as catalysts for Fischer-Tropsch reaction.

References 1. 2. 3. 4. 5. 6.

H. Zhang, C. Lancelot, W. Chu, J. Hong, A. Y. Khodakov, P. A. Chernavskii, J. Zheng, D. Tong, 2009, The nature of cobalt species in carbon nanotubes and their catalytic performance in Fischer-Tropsch reaction, J. Mater. Chem., 19, 9241-9249. G. C. de Araújo, S. Lima, V. La Pagola, M. A. Peña, J. L. G. Fierro, M. C. Rangel, 2005, Characterization of precursors and reactivity of LaNi1-XCoxO3 for the partial oxidation of methane. Catal. Today, 107, 906-912. M. E. Dry, 2002, The Fischer–Tropsch process: 1950–2000, Catal. Today, 71, 227-241. E. Iglesias, S. L. Soled, R. A. Fiato, 1992, Fischer-Tropsch synthesis on cobalt and ruthenium. Metal dispersion and support effects on reaction rate and selectivity, J. Catal., 137, 212-224. B. G. Johnson, C. H. Bartolomew, D. W. Goodman, 1991, The role of surface structure and dispersion in CO hydrogenation on cobalt, J. Catal., 128, 231-247. M. Newville, 2001, IFEFFIT : interative XAFS analysis and FEFF fitting, J. Synch. Rad., 8, 322-324.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Enhanced dibenzothiophene desulfurization over NiMo catalysts simultaneously impregnated with saccharose José Escobar,* José A. Toledo, Ana W. Gutiérrez, María C. Barrera, María A. Cortés, Carlos Angeles, Leonardo Díaz Instituto Mexicano del Petróleo, Eje Central L. Cárdenas 152, San Bartolo A., G. A. Madero, México, D.F., 07730, México

Abstract High surface area Al2O3 (Sg=307 m2 g-1) was impregnated by pore-filling with a solution prepared from MoO3 digestion in presence of H3PO4. Nickel hydroxycarbonate was further added resulting in Ni and P concentrations corresponding to Ni/(Ni+Mo)=0.29 and P2O5/(NiO+MoO3)=0.1, respectively. Saccharose (SA) at various SA/Ni molar ratios (1, 2 and 3) was further dissolved in impregnating solutions. TPR profiles of noncalcined precursors suggested that highly stable Mo-complexes were formed in SAmodified impregnating solutions. Hydrotreating catalysts obtained by sulfiding precursors at 400 °C were tested in dibenzothiophene hydrodesulfurization which was enhanced by the organic modifier. However, that beneficial effect depended on SA concentration, maximum activity being observed at SA/Ni=1. By HR-TEM, SA addition resulted in MoS2 particles of lower stacking but slightly increased length. Keywords: saccharose, NiMo catalyst, hydrodesulfurization, organic additive

1. Introduction Methodology of impregnation of Co-Mo or Ni-Mo phases plays a major role on dispersion, sulfidability and promotion degree in final sulfided hydrodesulfurization (HDS) catalysts. Various effects have been related to using either chelating [1] or nonchelating [2] organic agents, due to their interaction with metals to deposit. In formulations prepared in presence of those additives the mechanism provoking increased “M1M2S” (where M1= Ni or Co and M2= Mo4+ or W4+) formation is not completely unveiled. Promoter chelation by organic additives could contribute to delayed sulfidation of those species which could be then properly integrated to already formed MoS2 crystals [1]. Also, due to their interaction with the carrier non-chelating additives could contribute to preserve (during support impregnation) species where Mo and promoter atoms could coexist, that fact resulting in enhanced formation of either CoMoS or NiMoS phases after sulfiding [2,3]. However, other factors (carbon deposition, type II sites formation, etc.) could result in catalysts of increased HDS activity. Thus, deeper studies are pertinent. In this regard, we prepared P-doped NiMo/Al2O3 catalyts modified by saccharose (SA) at various SA/Ni ratios. Materials were studied by TPR and HR-TEM and tested in dibenzothiophene (DBT) HDS.

2. Experimental 2.1. Catalyst synthesis High surface area alumina was obtained by calcining (under static air) a commercial boehmite (Versal 200, Euro Support) at 500 ºC (5 h). Al2O3 textural properties were

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Sg=307 m2 g-1, Vp=0.9 cm3 g-1 and average pore diameter (from 4×Vp/Sg) of ~12 nm. Pore-filling simultaneous impregnation (of support previously dried at 120ºC, 2 h) was carried out by an aqueous solution (pH ~1.9) prepared from digestion (at ~80ºC) of MoO3 (99.5 wt % PQM) in presence of H3PO4 (85.3 wt% Tecsiquim). After 2 h, a yellow transparent solution was observed. 2NiCO3·3Ni(OH)2·4H2O (Sigma-Aldrich) was then added, heating being maintained (2 h). Ni and P concentration corresponded to Ni/(Ni+Mo)=0.29 and P2O5/(NiO+MoO3)=0.1 (mass ratio), respectively [2]. Transparent emerald green solutions were thus obtained. Saccharose (SA, J.T. Baker) at various SA/Ni molar ratio (1, 2 and 3) was added to impregnating solutions which pH was essentially unaltered (pH ~2.0) after organic agent dissolution. After impregnation materials were dried at 120ºC (2 h), calcining being avoided to preserve organic additive integrity. Nominal Mo, Ni and P loadings corresponded to 12, 3 and 1.6 wt% in final catalyst, respectively. Samples were identified by the SA(x) key where “x” stands for SA/Ni ratio. A reference material with no organic additive was also synthesized (SA(0). Sulfided catalysts were obtained by submitting impregnated precursors to treatment at 400°C (heating rate 6ºC/min) under H2/H2S (Praxair) at 50/6 (ml/min)/ (ml/min) during a 2 h period of time.

2.2. Materials characterization Temperature-programmed reduction experiments were conducted in an Altamira 2000 equipment. As-made (dried) Ni–Mo–P/Al2O3 samples, either with or without glycol, were put in a quartz reactor. Circa 50 mg of materials ground at particle size that passed through U.S. Mesh 80 (178 μm) were heated from 30 to 850°C (at 5°C min-1, heating rate) under a 30 ml min-1 flow of an Ar/H2 90/10 (vol/vol) mixture. High resolution transmission electron microscopy studies of sulfided catalysts were performed in a JEM-2200FS at 200 kV accelerating voltage. The apparatus was equipped with a Schottky-type field emission gun and an ultra-high resolution (UHR) configuration (Cs = 0.5 mm; Cc = 1.1 mm; point-to-point resolution, 0.19 nm) and in-column omega-type energy filter. Samples to be analyzed were ground, suspended in isopropanol and dispersed with ultrasonic agitation. Then, some drops of the suspension were deposited on a 3 mm diameter lacey carbon copper grid. GATANTM software was used during statistical determination of slab length and stacking of supported MoS2 particles.

2.3. HDS reaction test HDS activity of synthesized catalysts was studied in a tri-phasic slurry batch reactor (Parr 4575). The reaction mixture was prepared by adding ∼ 0.3 g of dibenzothiophene (99 mass %, from Aldrich) and ∼0.2 g of sieved catalyst (80-100 U.S. mesh) in 100 cm3 of n-hexadecane (99 mass %, from Aldrich) Operating conditions, carefully chosen to avoid external diffusion limitations, were P= 5.59 ± 0.03 MPa, T= 320 ± 3°C and 1000 RPM. Samples taken periodically were analyzed by gas chromatography (Agilent 6890N, flame ionization detector and Econocap-5 capillary column (from Alltech). HDS kinetic constants were calculated assuming a pseudo-first order model referred to organo-S compound concentration and zero order with respect to excess H2.

3. Results and discussion 3.1. HDS activity test It was observed that original emerald green impregnating solutions changed to cobaltblue after aging. This phenomenon was function of saccharose concentration, the change in coloring taking place in about two days (at room temperature) for solution at SA/Ni=3 and longer time for the others. From Figure 1, it was clear that saccharose

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Intensity (a.u.)

-4

-1

-1

k × 10 (L g s )

addition resulted in catalysts of increased HDS activity. Notably, this effect was much more pronounced when cobalt-blue solutions were used during impregnation.

2.5 2.0 1.5 1.0

SA(3) SA(2) SA(1)

SA(0)

0.5 0.0

SA(0)g SA(1)g SA(1) SA(2) SA(3)

Catalyst Figure 1. DBT HDS pseudo first order kinetic constants for catalysts tested. Subindex “g” indicates materials impregnated with green solution.

200

400

600

800

Temperature (°C) Figure 2. TPR profiles corresponding to alumina-supported NiMo samples prepared with saccharose as additive.

The organic additive could be possibly converted to an chelating compound during aging in the impregnating solution. It is known that saccharose could be transformed to saccharic acid in presence of some ions [4]. Saccharose in aqueous solution decomposes in two monosaccharides, glucose and fructose. Then, these cyclic compounds could be oxidized (by ions present in the impregnating solution), generating linear carboxylic acids [5]. These species could effectively chelate Ni2+ or Mo6+ cations, that fact being possibly related to the color change observed. Interestingly, during separate experiments where Ni nitrate was dissolved in H3PO4 solution, folllowed by SA addition (at SA/Ni=1) no color change (due to blue-shifted Ni2+ d-d transitions bands) was observed. This indicated that water molecules (in Ni[(H2O)6]2+) were not substituted by organic moieties (stronger ligands in spectroscopic series). Nevertheless, when a sulfur-yellow solution obtained by MoO3 digestion in H3PO4 (Mo, P and SA added in similar ratio to that in SA(1)) was left aging for some days, a drastic color change to cobalt-blue was registered. That suggested that phosphomolybdates originally present in solution [2] could be complexated by organic moieties. However, Ni2+ complexes formation in our SA-modified impregnating solutions could not be ruled out. In any case, it seemed that Mo chelation could be partially responsible for HDS activity trend in Figure 1.

3.2. TPR and high resolution TEM From TPR profiles (Figure 2 and Table 1) of non-calcined impregnated precursors it could be determined that the signal related to reduction of Mo6+ species to Mo4+ shifted to higher temperature in samples prepared with saccharose, that effect being more remarkable in materials of augmented organic agent content. Similar phenomenon was observed for the peak related to Mo4+ reduction to Mo0. Those facts suggested the existence of highly stable Mo-complexes originally formed in impregnating solutions. On the other hand, Ni2+ reduction seemed to be shifted to lower temperature in materials prepared with saccharose as additive. Similarly to the case where ethyleneglycol was used as organic modifier, the corresponding signal appeared to merge with that related to Mo6+ reduction [2]. According to those results and in agreement with that previously reported regarding ZrO2-TiO2-supported NiMo catalysts synthesized with citric acid [6],

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presence of Ni2+-complexes of lower stability than those of Mo6+ is strongly suggested. By statistical analysis of HR-TEM micrographs of sulfided catalysts prepared, SAaddition resulted in MoS2 slabs of lower stacking but slightly increased size. Table 1. Temperature of maximum in TPR profiles of alumina-supported NiMo samples prepared with saccharose as additive. n.d.: not determined. Sample

SA(0) SA(1) SA(2) SA(3)

Tmax (ºC) Mo6+ → Mo4+ 400 418 425 460

Tmax (ºC) Ni2+ → Ni0 500 n.d. n.d. n.d.

Tmax (ºC) Mo4+ → Mo0 576 591 613 640

Stacking

Slab length (nm)

7 4 3 4

3 5 4 4

Finally, strongly diminished HDS activity in material prepared at higher SA concentration (SA/Ni=3) remains to be explained. We hypothesize that some surface carbon could remain after organics decomposition (during sulfiding). Enhanced amount of carbonaceous deposits could be formed in solids of increased SA concentration. Presumably, that C could provoke partial plugging of porous network, hindering access of reactant molecules to surface active sites. Clarification of this point is clearly needed.

4. Conclusions In uncalcined P-doped NiMo/alumina precursors modified by saccharose addition existence of highly stable Mo-complexes is strongly suggested. Corresponding sulfided hydrotreating catalysts had enhanced activity in dibenzothiophene conversion. Nevertheless, that beneficial effect depended on SA concentration, maximum activity being registered for materials prepared at equimolar SA/Ni ratio. SA addition resulted in MoS2 particles of lower stacking but slightly increased length.

References [1] A.J. van Dillen, R.J.A.M. Terörde, D.J. Lensveld, J.W. Geus, K.P. de Jong, 2003, Synthesis of supported catalysts by impregnation and drying using aqueous chelated metal complexes, J. Catal. 216, 257-264. [2] J. Escobar, M.C. Barrera, J.A. Toledo, M.A. Cortés-Jácome, C. Angeles-Chávez, S. Núñez, V. Santes, E. Gómez, L. Díaz, E. Romero, J.G. Pacheco, 2009, Effect of ethyleneglycol addition on the properties of P-doped NiMo/Al2O3 HDS catalysts: Part I. Materials preparation and characterization, Appl. Catal. B. 88, 564-575. [3] D. Nicosia, R. Prins, 2005, The effect of glycol on phosphate-doped CoMo/Al2O3 hydrotreating catalysts, J.Catal., 229, 424-438. [4] J.-S. Girardon, E. Quinet, A. Griboval-Constant, P.A. Chernavskii, L. Gengembre, A.Y. Khodakov, 2007, Cobalt dispersion, reducibility, and surface sites in promoted silicasupported Fischer–Tropsch catalysts, J. Catal. 248, 143-157. [5] E.A. Souza, J.G.S. Duque, L. Kubota, C.T. Meneses, 2007, Synthesis and characterization of NiO and NiFe2O4 nanoparticles obtained by a sucrose-based route, J. Phys. Chem. Solids 68, 591-599. [6] J. Escobar, M.C. Barrera, J.A. De los Reyes, J.A. Toledo, V. Santes, J.A. Colín, 2008, Effect of chelating ligands on Ni-Mo impregnation over wide-pore ZrO2-TiO2, J. Molec. Catal. A 287, 33-40.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Preparation of Pt on NaY zeolite catalysts for conversion of glycerol into 1,2-propanediol Stijn Van de Vyver, Els D’Hondt, Bert F. Sels, Pierre A. Jacobs Center for Surface Chemistry and Catalysis, K.U. Leuven, Kasteelpark Arenberg 23, 3001 Heverlee, Belgium, [email protected]

Abstract High-yield catalytic conversion of glycerol into 1,2-propanediol without using an external H2 supply is possible on Pt impregnated NaY zeolites, characterized by extrazeolitic metal clusters combined with traces of Brønsted acidity. A comparison is made with NaY zeolite supported Pt catalysts prepared by a conventional ion-exchange procedure. Both materials were characterized using SEM, XRD and CO chemisorption. Keywords: glycerol, Pt/NaY, impregnation, hydrogenolysis, aqueous-phase reforming

1. Introduction Glycerol, considered as one of the platform chemicals for future biorefinery processes, can be converted into 1,2-propanediol (PDO), a valuable bulk intermediate. Whereas the present industrial route for manufacturing PDO involves hydrolysis of propylene oxide, glycerol of renewable origin seems to be an attractive source for a sustainable future PDO production. A large number of papers deals with the one-pot hydrogenolysis of glycerol into PDO via bifunctional heterogeneous catalysis [1-24]. Unfortunately, literature results imply the need of expensive bio- or petrochemical hydrogen. The latter, originating from reforming of natural gas or petroleum fractions, makes the processes dependent on fossil carbon. The innovative aspect of the catalytic system discussed below is the use of biogenic, in situ produced H2 as a second reagent, allowing for the unprecedented catalytic transformation of glycerol into 1,2-propanediol without using an external hydrogen supply [25, 26]. In a recent communication, we presented a consistent reaction mechanism derived for the hydrogenolysis of glycerol in presence of NaY zeolite supported Pt catalysts [26]. The action of this bifunctional catalyst is based on consecutive dehydration of glycerol into hydroxyacetone and hydrogenation of the latter into PDO. Preliminary results with glycerol under inert atmosphere are reported to yield up to 55% 1,2propanediol at 85% conversion, ethanol, n-propanol and hydroxyaceton being the main side-products. The aqueous-phase reforming of glycerol is identified as the source of H2 for the hydrogenation of the hydroxyacetone intermediate. Aqueous-phase reforming of oxygenated hydrocarbons is well-established now [27, 28]. From the influence of the gaseous side-products supported by quantitative GC analysis of the gas phase, it follows that initially formed CO2 is at the origin of solution acidity and subsequently of suitable zeolite Brønsted acidity, catalyzing dehydration of glycerol into hydroxyacetone [26]. The elucidation of a general reaction mechanism offers a rational approach to the optimization of the catalytic process. More specifically, fine-tuning of the balance between the two catalytic functions as well as a suitable acid strength/concentration offers unique possibilities to control the product distribution. Therefore, the aim of the present study is to discuss the impact of the catalyst preparation procedure on the

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catalytic properties and performance of NaY zeolite supported Pt catalysts in the onepot hydrogenolysis of glycerol into PDO in absence of added hydrogen.

2. Preparation and characterization of NaY zeolite supported Pt catalysts The preparation procedure of the catalyst evidently is a key factor, determining metal dispersion as well as support acidity. In the present work, commercial NaY (Zeocat, Si/Al ratio = 2.7) was used as the parent zeolite. Generally, there exist two different routes to load the zeolite matrix with a platinum metal-containing precursor. First incipient wetness impregnation of NaY with an aqueous Pt(NH3)4Cl2 salt solution can be performed. Alternatively, the zeolite can be subjected to a conventional ion-exchange procedure. Each catalyst sample was dried overnight at 110°C, calcined for 30 min at 400°C (heating rate 5°C/min), and subsequently reduced under a hydrogen flow for 1 h at 400°C. Special attention should be paid to the generation of zeolite Brønsted acid sites upon precursor loading of both samples. In contrast to the exchanged zeolite, the impregnated catalyst contains no strong zeolite acidity after metal reduction [26]. Examination of the as-prepared catalysts by scanning electron microscopy (SEM) is shown in Figure 1. It can be clearly seen that the impregnated Pt NaY, denoted as Pt/NaY, contains a significant amount of large extra-framework Pt particles. In contrast, the Pt ion-exchanged NaY sample, further denoted as Pt-NaY, shows absence of large Pt particles, suggesting that such clusters are predominantly located in the intracrystalline space or cages, in accordance with previous reports [29].

Fig. 1. SEM images of the impregnated Pt/NaY (a-b) and ion-exchanged Pt-NaY zeolite (c).

X-ray diffraction data were collected on a STOE STADI P Combi diffractometer. The diffracted intensity of CuKα radiation (wavelength of 0.154 nm) was measured in a range between 35 and 70 degrees of 2θ. Figure 2 shows two XRD profiles taken of the impregnated and ion-exchanged samples after an oxidation-reduction cycle. The XRD signals of impregnated Pt/NaY having the large Pt particles reveal three sharp and intense peaks at 39.7; 46.1 and 67.4 degrees 2θ; coinciding with the (111), (200) and (220) lattice planes of crystalline Pt. Using the simplified Scherrer equation, the average particle size is estimated at about 42 nm, [(Pt0)n]. On the other hand, Pt-NaY shows only broad flat reflections due to much smaller Pt nanoclusters. Thus, the XRD data in Figure 2 confirm prior findings: (i) impregnation results in the dispersion of large Pt particles on the external NaY zeolite surface, and (ii) the ion-exchanged NaY zeolite contains exclusively small intra-framework Pt, [(Pt0)x]. In addition, when comparing CO chemisorption on the Pt surface of the impregnated and ion-exchanged zeolites, distinct dispersions are obtained of 25 and 385 mmol CO per mol Pt, respectively, in agreement with the higher Pt dispersion for the exchanged sample.

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Preparation of catalysts for conversion of glycerol into 1,2-propanediol

Fig. 2. X-ray diffraction patterns of the impregnated Pt/NaY and ion-exchanged Pt-NaY zeolite.

3. Catalytic conversion of glycerol into 1,2-propanediol Batch experiments with glycerol were applied to compare the catalytic behaviour of Pt impregnated and ion-exchanged NaY zeolite catalysts. The process was carried out in high pressure autoclaves with aqueous 20% solutions of glycerol at 230°C under inert atmosphere, thus devoid of any added H2. Product analysis (from liquid and gas phase) was performed using common GC techniques. Table 1 shows a comparison of the carbon selectivity at short reaction time and relatively low glycerol conversion for the ion-exchanged and impregnated samples. Table 1. Conversion of glycerol over Pt on NaY zeolite catalysts.

Si (%) Sample

X (%)

mbL (%)

PDO

EtOH

PrOH

HOAcON

Pt/NaY Pt-NaY

18.1 18.4

54.7 29.4

25.0 10.8

7.0 6.6

6.6 6.5

10.0 3.2

100 ml Parr reactor under inert atmosphere; 40 ml 20 wt% aqueous glycerol at 230°C; 0.22 mmol Pt in added catalyst; X = conversion, mbL = mass balance in the liquid phase, Si = selectivity for 1,2-propanediol (PDO), ethanol (EtOH), n-propanol (PrOH), hydroxyacetone (HOAcON).

As seen from these data, the mass balance in the liquid phase and the 1,2-propanediol selectivity for the impregnated Pt/NaY catalyst are substantially higher than those for the exchanged Pt-NaY catalyst. It should be noted that all samples have the same amount of platinum loaded, and such an effect can be explained tentatively by assuming that the small intracrystalline Pt clusters in Pt-NaY are the basis of extensive gas formation via the aqueous-phase reforming process. In contrast, on the large extraframework Pt particles of the impregnated Pt/NaY sample, the hydrogenolysis reaction seems to be more selective, yielding less gaseous side-products, and hence more 1,2propanediol via hydrogenation of the hydroxyacetone intermediate.

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4. Conclusion The impregnation and ion-exchange preparation of Pt-tetrammine salt on NaY zeolites, after an activation pretreatment in oxygen and hydrogen, results in different materials with respect to the size and dispersion of the Pt nanoclusters, as was evidenced by SEM, XRD and CO chemisorption. The impregnated samples are the most suitable catalysts for the selective one-pot hydrogenolysis of glycerol into 1,2-propanediol in absence of added H2. The present results have demonstrated that the size of the Pt particles is crucial at the start of the reaction network, as highly dispersed intrazeolitic Pt compared to extra-framework large Pt particles favors aqueous-phase reforming, thus reducing 1,2-propanediol selectivity.

Acknowledgements This work was performed in the framework of an IAP-PAI network from BELSPO, GOA and CECAT. S.V.d.V. acknowledges financial support from FWO Flanders.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29.

B.F. Sels, E. D’Hondt and P.A. Jacobs, (eds. G. Centi and R. A. van Santen), Catalysis for Renewables, Wiley-VCH, Weinheim, 2007, ch. 11, pp. 223–256. B. Casale and A.M. Gomez, US Patent No. 5 276 181 (1994). C. Montassier, J. M. Dumas, P. Granger and J. Barbier, Appl. Catal., A, 121 (1995) 231. L. Schuster and M. Eggersdorfer, US Patent No. 5 616 817 (1997). J. W. Wabe and D. Eit, International Patent No. WO 9905085 (1999). J. Chaminand, L. Djakovitch, P. Gallezot, P. Marion, C. Pinel and C. Rosier, Green Chem., 6 (2004) 359. M.A. Dasari, P.P. Kiatsimkul, W.R. Sutterlin and G.J. Suppes, Appl. Catal., A, 281 (2005) 225. A. Perosa and P. Tundo, Ind. Eng. Chem. Res., 44 (2005) 8535. C.W. Chiu, M.A. Dasari and G.J. Suppes, Am. Inst. Chem. Eng., 52 (2006) 3543. I. Furikado, T. Miyazawa, S. Koso, A. Shimao, K. Kunimori and K. Tomishige, Green Chem., 9 (2007) 582. T. Miyazawa, S. Koso, K. Kunimori and K. Tomishige, Appl. Catal., A, 329 (2007) 30. S. Wang and H.C. Liu, Catal. Lett., 117 (2007), 62. J. Feng, J.B. Wang, Y.F. Zhou, H.Y. Fu, H. Chen and X.J. Li, Chem. Lett., 36 (2007), 1274. O. Franke and A. Stankowiak, International Patent No. WO 2008049470 (2008). A. Westfechtel, N. Klein and T. Alexandre, International Patent No. WO 2008/020077 (2008). M. Balaraju, V. Rekha, P.S. Sai Prasad, R.B.N. Prasad and N. Lingaiah, Catal. Lett., 126 (2008) 119. D.J. Miller, J.E. Jackson and S. Marincean, US Patent No. 2008/0242898 A1 (2008). A. Behr, J. Eilting, K. Irawadi, J. Leschinski and F. Lindner, Green Chem., 10 (2008), 13. A. Alhanash, E.F. Kozhevnikova, I.V. Kozhevnikov, Catal. Lett., 120 (2008) 307. L.C. Meher, R. Gopinath, S.N. Naik and A.K. Dalai, Ind. Eng. Chem. Res., 48 (2009), 1840. A. Marinoiu, G. Ionita, C.L. Gaspar, C. Cobzaru and S. Oprea, React. Kinet. Catal. Lett., 97 (2009) 315. J. Wang, S. Shen, B. Li, H. Lin and Y. Yuan, Chem. Lett., 38 (2009) 572. T. Jiang, Y. Zhou, S. Liang, H. Liu and B. Han, Green Chem., 11 (2009) 1000. X. Guoa, Y. Lia, R. Shia, Q. Liua, E. Zhana and W. Shen, Appl. Catal., A, 371 (2009) 108. E. D’Hondt, P.A. Jacobs and B.F. Sels, International Patent No. WO 2008077205 (2008). E. D’Hondt, S. Van de Vyver, B.F. Sels and P.A. Jacobs, Chem. Commun., (2008) 6011. R.D. Cortright, R.R. Davda and J.A. Dumesic, Nature, 418 (2002) 964. J.W. Shabaker, G.W. Huber and J.A. Dumesic, J. Catal., 222 (2004) 180. P. Gallezot, A. Alarcon-Diaz, J.A. Dalmon, A.J. Renouprez and B. Imelik, J. Catal. 39 (1975) 334.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Alkali metal supported on mesoporous alumina as basic catalysts for fatty acid methyl esters preparation Roxana M. Botaa, Kristof Houthoofda, Piet J. Grobeta, Pierre A. Jacobsa a

K.U. Leuven, COK, Kasteelpark Arenberg 23, B-3001 Leuven, Belgium; [email protected]

Abstract Transesterification of rapeseed oil with methanol into fatty acid methyl esters (biodiesel) was successful using CH3ONa loaded on mesoporous γ- Al2O3 as heterogeneous catalyst. The experimental results showed this catalyst had a better stability than NaOH/MSU-γ and NaN3/MSU-γ catalysts in continuous operation. Via Na MAS NMR this could be correlated with the presence of a specific narrow Na-oxide line at -20 ppm. Keywords: biodiesel, mesoporous alumina, sodium, continuous bed-reactor

1. Introduction Numerous types of basic heterogeneous catalysts, such as alkaline earth metal oxide, anion exchange resins and alkali metal compounds supported on alumina or zeolite can catalyze various chemical reactions such as isomerization, aldol, Michael, and Knoevenagel condensation, oxidation and transesterification [1]. Today considerable attention is devoted to the production of biodiesel (FAMEs) as an alternative for petroleum-derived diesel fuel. Biodiesel is synthesized by direct transesterification of vegetable oil or animal fat with a short-chain alcohol, viz. methanol, ethanol, and isopropanol in presence of an acid, base or enzymatic catalyst [2]. Considering the advantages of solid base catalysts, for easy separation and recovery, reduced corrosion and environmental acceptance [1], many studies have been conducted on basic heterogeneous catalysts development for biodiesel production [3-13]. Presently, we are focusing on the preparation of new basic heterogeneous catalysts consisting of alkali metals, derived particularly from CH3ONa, supported on mesoporous alumina. As a model reaction, the catalytic transesterification of rapeseed oil with methanol was used. The fate of adsorbed Na methoxide during catalyst preparation and use was followed using 23Na MAS NMR. As solid basic catalysts might be very susceptible to leaching in polar medium, the catalytic biodiesel synthesis was done in continuous operation.

2. Experimental 2.1. Catalyst preparation The mesoporous γ-alumina support (MSU-γ) was prepared according to the method of Zhang et al. [14]. Nitrogen adsorption shows that the obtained mesoporous gamma alumina has a BET surface area of 298 m2/g, a pore volume of 0.98 cm3/g and an average pore size of 8.85 nm.

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The standard catalyst, denoted as CH3ONa/MSU-γ, was prepared by wet impregnation with a methanol solution of CH3ONa on a mesoporous aluminium oxide support. After impregnation, the wet material was dried in air at 60°C, compressed and sieved to form pellets with a diameter ranging from 0.5 to 1 mm. Before use in transesterification of rapesed oil with methanol, in situ activation of the catalysts was done at 200°C under nitrogen for 3 and 6 h, respectively. The NaOH/MSU-γ and NaN3/MSU-γ catalysts were prepared by impregnation of mesoporous γ-alumina with an aqueous solution of NaOH and with a slurry of NaN3 in methanol, respectively. After drying at 60°C in air, the powder was compressed and sieved yielding pellets of 0.5-1 mm. Prior to use, the air dried catalysts were transferred in the reaction glass U tube and activated in situ for 1 h using a heating rate of approximately 3°C/min in a continuous flow of nitrogen The NaOH/MSU-γ catalyst was activated at 400°C and NaN3/MSU-γ at 380°C respectively. The amount of alkali metal precursor used corresponded to a metal on support loading of 15 wt%.

2.2. 23Na MAS NMR characterization of catalysts

The catalysts were also characterized by means of 23Na MAS NMR spectroscopy. The activated catalysts were packed under inert atmosphere in 2.5 mm Zr rotors. The closed rotors were air-tight as confirmed by the constant signal intensity over time. The 23Na MAS NMR spectra were recorded on a Bruker Avance 400 spectrometer, at a magnetic field strength of 9.4 T. 1700 scans were accumulated with a recycle delay of 1 s. The spinning frequency of the rotor was 20 kHz.

2.3. Transesterification of rapeseed oil Synthesis of fatty acid methyl esters from rapeseed oil was carried out in a fixed bed continuous flow reactor at 60°C with 1 g of catalyst pellets, a molar methanol to oil (triglyceride) ratio of 40 and at liquid hourly space velocity of 6.5 h-1. On-line sampling was done every 15 min, followed by evaporation of residual MeOH. Samples containing 0.1 g of reaction product were dissolved into 1 ml of nhexane solution containing 0.1% n-C17H36 as standard for GC analysis. Such samples were injected into a HP 6890 GC with injection port at 250°C, equipped with a polar BPX-70 SGE column of 60.0 m with an internal diameter of 320 μm and a film thickness of 0.25 μm. The FID-detector was at 280°C. The biodiesel or methyl ester yield, Y, was calculated according to the following formula: Y (%) = 100

mIS * AFAMEs * fIS AIS * msample * fFAMEs

with IS, n-heptadecane (n-C17H36) used as a chromatographic internal standard; f, the response factor; A, the peak area; m, the mass (g) of sample or of IS. The values of fIS and fFAMEs are 0.81 and 1.12, respectively. As fatty acid residues from the oil triglycerides were only transformed into fatty acid methyl esters (FAMEs), the reaction conversion, X, and the ester yield, YFAMEs, are identical.

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3. Results and discussion 3.1. Characterization of the catalysts (d)

(c)

(b)

(a)

1250

1150

1050

950

850

(ppm)

750

650

550

350

250

150

50

0

-50

(ppm)

-150

-250

-350

Fig. 1. 23Na MAS NMR spectrum of (a) NaN3/MSU-γ, (b) NaOH/MSU-γ, (c) CH3ONa/MSU-γ activated for 6 h, and (d) CH3ONa/MSU-γ activated for 3 h.

In Figure 1, the 23Na MAS NMR spectra of the prepared Na species on mesoporous alumina are shown. The 23Na MAS NMR spectrum of NaN3/MSU-γ shows a broad signal of sodium oxide around – 20 ppm and two Na metal signals, viz. a narrow signal at 1132 ppm ascribed to large metallic sodium particles and a broad one at 1090 ppm corresponding to smaller sodium metal particles, tentatively assigned to those located in the mesopores of the support (Fig. 1 (a)). The former line corresponds to the Knight shift of bulk sodium metal [16]. Residual sodium azide with sharp resonance line at – 11 ppm, was not detected on the catalyst. The spectrum of NaOH/MSU-γ catalyst (Fig. 1 (b)) shows three distinct peaks. The signal at -20 ppm coresponding to sodium oxide and the signals at -4.7 ppm and 3.6 ppm which can be assigned to residual sodium hydroxide, in a different chemical environment. For the CH3ONa/MSU-γ catalysts, next to the presence of sodium oxide at - 20 ppm, residual sodium methoxide is found to be present at - 5 ppm (Fig. 1 (c,d)). The sample heated for 6 h shows a narrower signal of sodium oxide at -20 ppm and a decrease of the residual sodium methoxide signal compared with the sample heated for 3 h.

3.2. Reaction The catalytic activity of NaOH/MSU-γ, NaN3/MSU-γ and CH3ONa/MSU-γ catalysts for transesterification of rapeseed oil with methanol in continuous flow operation was investigated. During the reaction with NaN3/MSU-γ, the color of the catalyst was changing from black to white due to the oxidation of Na metal (Fig. 1 (a)) and the Na-soap was the major reaction product. NaOH/ MSU-γ gave almost 65% biodiesel yield at the very beginning of the reaction, but deactivated rapidly to 5% after 120 min time-on-stream. The reason for their deactivation is assume to the leaching out of the sodium respectively. Indeed, 23 Na MAS NMR shows that after reaction, the Na-oxide species show decreased intensities (spectra not shown). Under the same reaction conditions, CH3ONa/ MSU-γ was the most stable and active catalyst although the 23Na MAS spectra (Fig. 1, (c), (d)) showed residual CH3ONa even for the catalyst activated for 6 h. For CH3ONa/ MSU-γ, the biodiesel

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yield varied around 50% for the catalyst activated 3 h and 60% for the catalyst activated 6h respectively.The enhanced activity after 6 h activation is in line with a relative reduction of the line assigned to Na methoxide and an increase of the Naoxide line. At this stage, the exact nature of this species thus generated cannot be identified unambiguously.

4. Conclusions CH3ONa/MSU-γ, NaOH/MSU-γ, NaN3/MSU-γ heterogeneous base catalysts were prepared and evaluated in biodiesel production. Excepting the NaN3/MSU-γ catalyst, the thermodynamic equilibrium of about 60% was reached by each catalyst at the very beginning of the reaction. The catalytic activity of NaOH/MSU-γ decrese rapidly, while for CH3ONa/ MSU-γ remained almost constant after 120 min time-on stream. Also, the most active and stable basic catalyst shows the most narrow resonance line at -20 ppm attributed to supported sodium oxide, which remains unaltered after reaction.

Acknowledgements RMB acknowledges fellowships from IWT (STWW) and IAP. The authors acknowledge sponsoring from IWT STWW and BELSPO IAP-PAI programs.

References 1. H. Hattori, 2001, Solid base catalysts: generation of basic sites and application to organic synthesis, Appl. Catal. 222, 247–259. 2. F. Ma, 1999, Biodiesel production: a review, Bioresour. Technol. 70, 1-15. 3. G.J. Suppes, 2001, Calcium carbonate catalyzed alcoholysis of fats and oils, J. Am. Oil Chem. Soc. 78, 139–146. 4. H.J. Kim, 2004, Transesterification of vegetable oil to biodiesel using heterogeneous base catalyst, Catal.Today 93–95, 315–320. 5. T. Ebiura, 2005, Selective transesterification of triolein with methanol to methyl oleate and glycerol using alumina loaded with alkali metal salt as a solid-base catalyst, Appl. Catal. A 283, 111–116. 6. W. Xie, 2006, Transesterification of soybean oil catalyzed by potassium loaded on alumina as a solid-base catalyst, Appl. Catal. A 300, 67–74. 7. W. Xie, 2006, Alumina-supported potassium iodide as a heterogeneous catalyst for biodiesel production from soybean oil, J. Mol. Catal. A 255, 1–9. 8. L.C. Meher, 2006, Transesterification of karanja (Pongamia pinnata) oil by solid basic catalysts. Eur. J. Lipid Sci. Technol. 108, 389–397. 9. A. Kawashima, 2009, Acceleration of catalytic activity of calcium oxide for biodiesel production, Bioresource Technology 100, 696–700. 10. S. Benjapornkulaphong, 2009, Al2O3-supported alkali and alkali earth metal oxides for transesterification of palm kernel oil and coconut oil, Chemical Engineering Journal 145, 468–474. 11. G.J. Suppes, 2004, Tranesterification of soybean oil with zeolite and metal catalysts, Appl. Catal. A 257, 213–223. 12. D.G. Cantrell, 2005, Structure-reactivity correlations in MgAl hydrotalcite catalysts for biodiesel synthesis. Appl. Catal. A: Gen. 287,183–190. 13. N. Shibasaki-Kitakawa, 2007, Biodiesel production using anionic ion-exchange resin as heterogeneous catalyst. Bioresour. Technol. 98, 416–421. 14. Z. Zhang, 2002, Mesostructured Forms of γ-Al2O3, J. Am. Chem. Soc. 8124, 1592–1593. 15. D.G.B. Boocock, 1996, Phase diagrams for oil/methanol/ether mixtures, J. Am. Oil. Chem. Soc. 73, 1247–1251. 16. P.A. Anderson, 1992, Magnetic resonance study of the inclusion compounds of sodium in zeolites: beyond the metal particles model, J. Am. Chem. Soc. 114, 10608–10618.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Modifications of porous stainless steel previous to the synthesis of Pd membranes C. Mateos-Pedreroa,*, M. A. Soriaa, I. Rodríguez-Ramosa, A. Guerrero-Ruizb. a

Instituto de Catálisis y Petroleoquímica, CSIC, C/ Marie Curie, 2, Cantoblanco, 28049 Madrid b Dpto. Química Inorgánica yTécnica, Facultad de Ciencias, UNED, Senda del Rey, 9. 28040 Madrid

Abstract Two Pd composite membranes were prepared by electroless plating over Porous Stainless Steel (PSS) support. In order to avoid the intermetallic diffusion between the Pd film and the PSS and also to permit the Pd layer to be thinner an intermediate layer was required. With this aim, the surface of PSS was modified by oxidation or coating with different refractory metal oxides. The modified-PSS substrates and the Pd composite membranes were characterized by SEM/EDX, XRD, Hg porosimetry, gravimetric analysis and permeation measurements. The formed Pd film was thinner when oxide coating was used to create the intermediate layer (in particular ZrO2-coating) in comparison with the oxidized-PSS. In addition for ZrO2-coated PSS a lower number of plating cycles were necessary to get a dense Pd membrane. Keywords: palladium membrane, hydrogen separation, surface modification

1. Introduction H2 separation technologies are of great importance due to the role of H2 as an alternative, clean, energy-efficient carrier. Membrane related processes are considered to be one of the most promising routes in the production of high purity H2. Pd membranes are well known for their application in H2 separation and purification due to their high chemical permeability and perfect selectivity to hydrogen [1]. Since the permeation rate through a Pd membrane is often inversely proportional to its thickness [2], thin membranes are always preferable. In addition, thin membranes save expensive Pd. To form a thin continuous Pd membrane without defects, the support surface should be smooth and the external pore size small. PSS are promising substrates due to their good mechanical strength, operation at high pressures, etc [1]. However, the pore size of conventional PSS tubes can be very large, rendering the coverage of all pores very difficult. Moreover, direct deposition of Pd onto PSS would cause intermetallic diffusion at high temperature, decreasing the stability of the membrane. To reduce the surface roughness and the external pore size gradually intermediate layers are necessary prior to Pd deposition. The creation of different intermetallic diffusion barriers: metallic [3] or ceramic [1, 4] has been reported in the past. The present contribution is aimed at preparing sufficiently thin and stable Pd membranes on modified PSS supports. In this sense, two techniques, oxidation or coating, to produce barrier layers against intermetallic diffusion are studied.

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2. Experimental 2.1. Preparation of PSS modified supports In this work, Pd composite membranes were prepared by deposition of Pd onto a porous substrate. Media grade 0.5 μm 316L PSS tubes from Mott Corporation were used as substrates. Two types of oxides were used as intermetallic diffusion barriers: those obtained after heating PSS at different temperatures (metallic oxides by surface oxidation) and those obtained after coating PSS with different oxides (ceramic oxides by coating). All PSS supports were first cleaned ultrasonically (at 60°C) with an alkaline solution (45 g/l NaOH, 65 g/l Na2CO3, 45g/l Na3PO4.12H2O, 5ml/l of an industrial detergent), deionised water and isopropanol and then dried at 120°C overnight. After cleaning, the supports were submitted to oxidation or coating. 2.1.1. Oxidation The supports were oxidized in stagnant air at the desired temperature (600, 700 or 800ºC; with heating ramp rate of 2°C/min) for 12h. Table 1. List of the modified-PSS substrates studied in this work. Sample name OX-600 OX-700 OX-800 SiO2 Al2O3 ZrO2

Modification of PSS support

He permeance* (m3/m2⋅h)

Oxidation at 600°C for 12h Oxidation at 700°C for 12h Oxidation at 800°C for 12h Coating with SiO2 from TEOS Coating with Al2O3 from Al isopropoxide

470.8 372.3 272.1 187.6 457.3 280.3

Coating with ZrO2 from Zr tetrabutoxide

*He flux at 25ºC and ΔP= 1 bar. He flux of the cleaned PSS at 25ºC and ΔP of 1 bar was 587.6 (m3/m2⋅h).

2.1.2. Coating The supports were modified through the deposition of a layer of a ceramic oxide (Al2O3, SiO2 or ZrO2) by the coating technique. The washcoating was performed with: Al-isopropoxide, TEOS and Zr-tetrabutoxide for Al2O3, SiO2 and ZrO2, respectively. The coating solution was prepared by the sol-gel method. The non-porous parts of the PSS support were covered with a Teflon tape and both sides of the PSS tube were sealed with Teflon caps in order to avoid the coating of the inner part of the PSS tube. Then the PSS was placed vertically in a vessel containing the synthesis gel for a few min. After coating, the substrate was calcined in air at 650ºC for 5h (for SiO2 and Al2O3) or 500ºC for 2h (for ZrO2). Table 1 summarizes the modified PSS substrates studied in this work.

2.2. Preparation of Pd dense membranes The Electroless Plating technique (EPD) [5] was used to obtain dense Pd layers on various PSS substrates. From the characterization results obtained for modified PSS supports, two of them were set as substrates for the synthesis of Pd composite membranes by EPD. The first support was obtained after oxidation at 700ºC for 12h and the second one was obtained by coating a thin layer of ZrO2. The PSS substrates were activated prior to Pd deposition. The activation process was performed using sequential dipping in SnCl2-HCl (1 g/l, pH 2) and PdCl2-HCl (0.1 g/l, pH 2) solutions. The plating bath used for EPD contained a solution of Pd(NH3)4Cl2·H2O (4 g/l), NH4OH (28%, 198 ml/l), Na2EDTA.2H2O (40 g/l) and N2H4 (1M, 5.6 ml/l). The nonporous stainless steel parts were covered with a Teflon tape, and then the PSS substrate

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781

was immersed in the plating bath for 90 min in an ultrasonic bath at 50ºC. The activation-plating procedure was repeated until the composite Pd membrane became dense (He permeance <10-4 m3/m2⋅h⋅bar; 25ºC; ΔP= 1 bar). Once the plating was finished the membrane was thoroughly rinsed with deionised water and dried overnight at 120ºC. Table 2 summarizes the Pd composite membranes prepared for this study. Table 2. List of the Pd composite membranes studied in this work. Sample name

He permeance* (m3/m2⋅h)

Modification of PSS support

Pd thickness Gravimetric (μm)

MB-003

dense

Coating with ZrO2

18

MB-004

dense

Oxidation-700°C; 12h

20

*He flux measured at 25ºC and ΔP= 1 bar

2.3. Characterization The samples were characterized by SEM/EDX, XRD, Hg porosimetry, gravimetric analysis and He permeation measurements.

3. Results and discussion For the modified PSS carriers, He permeance decreased after oxidation or coating. For the oxidized PSS this decrease is more marked at higher temperature. In the case of the coated PSS, the permeation depends on the kind of oxide, decreasing in the order: γAl2O3> ZrO2>> SiO2. It is observed that, in general, coating leads to a larger reduction of the He permeance (Table 1), which is in good agreement with results reported by other authors [1, 4] For the Pd composite membranes, the He flux gradually decreases while increasing the amount of plated Pd, as expected. Moreover a lower number of plating steps was necessary in the case of the ZrO2-coated membrane in order to get a dense Pd membrane. This indicates that the coating with ZrO2 leads to an increase in the plating effectiveness. This information has not been published before to the best of our knowledge. Hg porosimetry measurements for the coated PSS substrate shown the maximum pore size (external porosity) greatly decreases after coating whereas the mean pore size remains unchanged. It is important to mention that, the decrease in the external porosity follows the same trend as permeation. For the oxidized PSS substrates the maximum and mean pore size tends to decrease while increasing the calcination temperature, which matches previous studies [4]. After Pd deposition, a dramatic decrease in porosity was observed but both Pd composite membranes (MB-003 and MB-004) present similar values. The sample oxidized at 600ºC shows the same XRD pattern as for the original PSS and only the main peaks of PSS (43.8º, 51.0º and 74.9º) are observed. However, for the samples oxidized at higher temperatures together with the PSS lines additional features are observable. These new reflection lines, even though they are difficult to be assigned (little is said in literature about the application of XRD to the characterization of PSS and modified PSS substrates), could be attributed to oxides of the other elements of the stainless steel (Fe, Ni, Cr, etc.). In the case of the coated samples, no significant differences are observed with respect to the original PSS. The XRD patterns of these samples show the characteristic lines of PSS. For the MB-004 and MB-003 membranes only the reflection features of metallic Pd (40.1º, 46.7º, 68.1° and 82.1°) are observed, and no peaks from the modified PSS support were detected.

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The surface and cross-sectional morphological characteristics of the modified PSS and Pd composite membranes were examined by SEM (Fig 1). The SEM picture of the original PSS is also shown for comparison. It appears that the surface becomes smoother after oxidation or coating. This effect is even more marked in the case of the coated samples (Fig. 1), which indicates that the bigger pores are being closed and the surface becomes smoother after coating, whatever the oxide. For these samples, it is clear that the surface morphology and roughness change with the kind of oxide. For example the coating with SiO2 allows to obtain the higher surface coverage, and although some cracks are appreciable in the micrograph of this sample (Fig.1 G). The composition on the outer surface of the various samples was checked by means of EDX. For the oxidized samples the amount of Fe on the surface decreases as the calcination temperature increases from 600 to 700ºC. On the contrary, for the substrate calcined at 800ºC the Fe/Cr at. ratio (EDX) increased dramatically indicating the formation of an Fe-rich oxide layer (it is about 6 μm thick) on the outermost layer. This is in line with studies reported by other authors [4]. The major component on the external surface of the samples after coating was the corresponding oxide; anyway Fe and Cr were still observable, which is likely due to the partial coverage of PSS or to the thinness of the deposited oxide layer. The SEM image of the cross section of the fresh membrane (MB003) is shown in Fig. 1. The mean thickness of the Pd layer is around 17 and 20 µm for MB003 and MB004 respectively. This is consistent with the layer thickness assessed by gravimetric analysis for both membranes (Table 2), and indicates that slightly thinner Pd layer is formed for the ZrO2-coated PSS than for the oxidized one. It is also observed that a dense Pd film is present without the appearance of cracks. Leak test confirmed the absence of detectable defects in both membranes.

A 100 μm E 100 μm

B 100 μm F 100 μm

C 100 μm G 100 μm

D 100 μm H Pd layer B C PSS substrate 100 μm

Fig. 1. SEM images of: (A) PSS, (B) oxidation at 600ºC, (C) oxid. at 700ºC, (D) oxid. at 800ºC, (E) γ-Al2O3 coating, (F) ZrO2 coating, (G) SiO2 coating and (H) cross section after Pd plating.

4. Conclusions The modification of PSS by coating results in a better substrate for Pd deposition by EPD. So the ZrO2-coated PSS allows a more effective plating process as well as the deposition of thinner Pd layers. In order to achieve more stable and thinner Pd membranes, further research will be focused on the synthesis conditions to be used for ZrO2 coating.

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Acknowledgments The authors wish to acknowledge the financial support received from MICINN (CTQ 2008-03068-E/PPQ). This work has been realized in the frame of the ACENET project (ACE.07.012) “Methane activation as a route to CO2 remediation: the integration of dry reforming into Fischer-Tropsch fuel production plants”

References [1] M. L. Bosko, F. Ojeda, E. A. Lombardo, L. M. Cornaglia, 2009, NaA zeolite as an effective barrier in composite Pd/PSS membranes, J. Membr. Sci., 331, 57. [2] T. L. Ward, T. Dao, 1999, Model of hydrogen permeation behavior in palladium membranes, J. Membr. Sci., 153, 211. [3] S. Nam, K. Lee, 2001, Hydrogen separation by Pd alloy composite membranes: Introduction of diffusion barrier, J. Membr. Sci., 192, 177. [4] Y.H. Ma, B. Ceylan Akis, M. Engin Ayturk, F. Guazzone, E.E. Engwall, I.P. Mardilovich, 2004, Characterization of Intermetallic diffusion barrier and alloy formation for Pd/Cu and Pd/Ag porous stainless steel composite membranes, Ind. Eng. Chem. Res., 43, 2936. [5] US Patent 6152987, (28 November, 2000).

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Design of nano-sized FeOx and Au/FeOx catalysts for total oxidation of VOC and preferential oxidation of CO S. Albonettia, R. Bonellia, R. Delaiglec, E.M. Gaigneauxc, C. Femonib, P.M. Riccobened, S. Scirèd, C. Tiozzob, S. Zacchinib, F. Trifiròa. a

Dip. Chimica industriale e dei Materiali, Viale Risorgimento 4, 40136 Bologna, Italy INSTM, Research Unit of Bologna, Italy b Dip. Chimica Fisica ed Inorganica, Viale Risorgimento 4, 40136 Bologna, Italy. c Unité de Catalyse et Chimie des Matériaux Divisés, Université catholique de Louvain (UCL), Croix du Sud 2/17, 1348 Louvain-la-Neuve (Belgium). d Dipartimento di Scienze Chimiche, Università di Catania, Viale A. Doria 6, 95125 Catania, Italy.

Abstract A novel preparation method was developed for the preparation of iron and gold/iron supported catalysts using metallic carbonyl clusters as precursors of highly dispersed nanoparticles over TiO2 and CeO2. A series of catalysts with different metal loadings were prepared and tested in the complete oxidation of methanol and the preferential oxidation of CO in the presence of H2 (PROX) as model reactions. The characterization by BET, XRD, TEM, H2-TPR, ICP-AES and XPS spotlights the interaction between Au and Fe and their influence on the catalytic activity.

1. Introduction It has been recently reported that gold/iron catalysts exhibit high performances in the total oxidation of different molecules. Moreover, it was proven that the addition of iron to supported gold catalysts leads to improved activity and stability during CO oxidation [1,2]. Since it is well known that the catalytic behavior of multi-component supported catalysts is strongly influenced by the size of the metal particles and by their reciprocal interaction, we tried to develop a methodology for the preparation of iron stabilized gold nanoparticles with controlled size and an intimate contact between the active phases starting from bimetallic carbonyl clusters. In this paper we report the study on the preparation of FeOx and Au/FeOx catalysts supported on TiO2 and CeO2 utilizing homoand bimetallic carbonyl cluster salts, i.e. [NEt4][HFe3(CO)11] and [NEt4][AuFe4(CO)16], and their activity in the total oxidation of methanol and PROX reaction.

2. Experimental 2.1. Catalysts preparation Au/FeOx catalysts have been prepared by impregnation of the bimetallic carbonyl cluster salt [NEt4][AuFe4(CO)16] on the support, TiO2 (DT51 Millennium powder) or CeO2 (VP AdNano Ceria 90 Evonik). The required amount of the carbonyl cluster was dissolved in degassed acetone (20-40 mL) under nitrogen and added dropwise over a period of 1 hour to an acetone suspension of the support (10-15 g), previously degassed and stored under nitrogen. The resulting suspension was allowed to stir overnight then the solvent was removed in vacuum at room temperature. Following a similar procedure,

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catalysts containing iron dispersed on TiO2 and CeO2 were prepared using [NEt4] [HFe3(CO)11] as precursor [3]. In this case the catalysts were dried at 100°C for 2 hr and calcined at 400°C in air because the atmosphere utilized during calcination does not influence catalytic properties. On the contrary, Au/FeOx samples were stored in air at ambient temperature, dried at 100°C for 2 hr in air and thermal treated in flowing N2. During this last treatment, the temperature was ramped at a rate of 10°C/min from room temperature to 400°C in nitrogen, in order to fully decompose the carbonyl cluster and the ammonium cations but avoiding the Au sintering. In fact, the rapid oxidation of iron that takes place in air can lead to uncontrolled temperature rise so inducing a gold nanoparticles dimensions increase, as demonstrated in our previous work [4].

2.2. Characterization Surface areas were measured by N2 physisorption with a Sorpty 1750 CE instrument and the metal content by ICP-AES analysis with a Fision 3410+ instrument. TEM observations have been carried out with a Fei Tecnai F20 and XPS analyses were performed on a Kratos Axis Ultra spectrometer. XRD measurements were carried out with a Bragg/Brentano diffractometer (X’pertPro Panalytical) equipped with a fast X’Celerator detector; Au particle size values were calculated by the Scherrer equation. Prepared catalysts and some characterization data are reported in Table 1. Table 1. Prepared catalysts and characterization. Fe loading (wt. %)

Au loading (wt. %)

Au particle size *

Nominal

Measured (ICP)

Nominal

Measured (ICP)

Surface Area (m2/g)

Fe2.3Au2-Ti Fe4.3Au4-Ti

2.3 4.3

1.1 2.9

2.0 4.0

2.0 4.8

78 67

<3 6.9

Fe2.3-Ti Fe4.3-Ti

2.3 4.5

1.4 2.5

0 0

0 0

79 78

-

Fe2.3Au2-Ce Fe4.3Au4-Ce Fe2.3-Ce Fe4.3-Ce

2.3 4.3 2.3 4.3

1.4 3.2 1.7 2.6

2.0 4.0 0 0

1.5 2.7 0 0

73 65 76 72

<3 <3 -

Catalyst

(nm)

* Mean diameters calculated from XRD patterns of fresh samples by the Scherrer equation

Methanol combustion catalytic tests were performed in a continuous-flow fixed-bed reactor, using 0.1 g of catalyst diluted with an inert glass powder. A reactant mixture of 0.7 vol. % methanol, 10 vol. % O2, the rest being helium and a gas hour space velocity of 7.6×10-3 mol h-1 gcat-1 were used. PROX reaction was carried out in a continuous-flow microreactor filled with catalyst (0.05 g) diluted with an inert glass powder. The gas composition (total flow rate: 80 ml/min)was 1% of CO, 1% of O2, the rest being H2.

3. Results and discussion Surface area data indicate that gold/iron deposition did not result in a significant change in the total surface area of titania and ceria (Table 1). A small decrease in the area is indeed observed at high Fe-Au content. XRD and TEM studies indicated that gold is present as small nanoparticles whose dimensions are within 2 – 20 nm for TiO2 based catalysts (Figure 1) and lower than 3 nm for CeO2 systems. Moreover, TEM analysis indicated that iron species are highly dispersed on the samples surface. Detailed selected

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area diffraction studies suggest that iron oxides are not in a crystalline form and metallic Au was found. X-ray diffraction analyses confirm these data, excluding the formation of any kind of crystalline iron species while small, broad peaks attributed to metallic gold were detected in some samples, mainly supported on TiO2.

Figure 1. STEM image of Fe4.3Au4-Ti catalyst, with evidences of metallic gold (details with brighter contrast) and size distribution of the Au nanoparticles.

X-ray photoelectron data reported in Table 2 confirmed the presence of metallic gold as well as Fe2+ and Fe3+ species are the only iron species. Moreover, the Au/Fe ratio at the surface was found to be extremely low respect to the bulk ratio suggesting that part of gold is encapsulated by iron oxide. Table 2. XPS data for Au/FeOx based catalysts. Atomic concentration ratios Catalysts

Au/Ti-Au/Ce Au/Fe

Au/Fe

Binding Energy (eV) Au 4f

Fe 2p

0.52

83.9

710.5

0.07

0.47

84.0

710.6

0.012

0.04

0.30

83.8

710.4

0.006

0.03

0.24

83.8

710.4

XPS

XPS

ICP

Fe2.3Au2-Ti

0.003

0.12

Fe4.3Au4-Ti

0.014

Fe2.3Au2-Ce Fe4.3Au4-Ce

This possibility was confirmed by H2-TPR analysis (not reported) which suggested a strong interaction between gold and iron species, denoted by a strong decrease of Fe2O3 reduction temperature. The positive effect of gold on iron species reducibility well matches the catalytic performances of Au/FeOx systems in methanol combustion (Figure 2). The presence of gold has been found to enhance the activity of iron oxide towards the complete oxidation of methanol, strongly improving the selectivity with respect to iron based catalysts: no formation of partial oxidation products was observed for any of the bimetallic tested materials, CO2 being the only product formed at any conversion value. The extent of this effect is remarkably bigger for Au/FeOx/CeO2 catalysts at low metal loading. A similar trend has been obtained in the PROX reaction where ceria supported bimetallic systems showed higher activity than titania ones. This behavior has been attributed to the higher reducibility of ceria catalysts (confirmed by H2-TPR) and to the smaller Au particle dimensions of Au/FeOx/CeO2 catalysts, which results in a

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higher gold/oxide interaction degree and therefore higher activity towards the combustion of methanol and PROX reaction. a)

FeOx/TiO2

350

Au/FeOx/TiO2 Temp 50% MeOH conversion (°C)

Temp 50% MeOH conversion (°C)

350 300 250 200 150 100 50

b)

FeOx/CeO2

Au/FeOx/CeO2

300 250 200 150 100 50 0

0 0

2,3 Fe loading (% wt)

4,3

0

2,3 Fe loading (% wt)

4,3

Figure 2. Temperature of 50% of methanol conversion as a function of the Fe content for TiO2 (a) and CeO2 (b) catalysts.

4. Conclusions The used novel preparation method by means of bimetallic carbonyl clusters resulted in small gold nanoparticles and highly dispersed iron oxide species on different supports. Au dispersion achieved is higher on CeO2 than on TiO2 and a strong interaction between gold and iron oxide was observed. The extent of this effect combined with Au particle size can explain the different catalytic performances of bimetallic ceria catalysts compared with titania ones in the total oxidation of methanol and in the PROX reaction.

References 1. S. Carrettin, Y. Hao, V. Aguilar-Guerrero, B. C. Gates, S. Trasobares, J. Calvino, A. Corma Chem. Eur. J. 13 (2007) 7771. 2. F. Moreau, G. C. Bond Top. Catal. 44 (2007) 95. 3. S. Albonetti, R. Bonelli, R. Delaigle, C. Femoni, E. M. Gaigneaux, V. Morandi, L. Ortolani, C.Tiozzo, S. Zacchini, F. Trifirò Appl. Catal. A 372 (2010) 138. 4. S. Albonetti, R. Bonelli, J. Epoupa Mengou, C. Femoni, C. Tiozzo, S. Zacchini, F. Trifirò Catal. Today 137 (2008) 483.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Supported Pd nanoparticles prepared by a modified water-in-oil microemulsion method Robert Wojcieszaka, Michel J. Genetb, Pierre Eloya, Eric M. Gaigneauxa, Patricio Ruiza a

Université catholique de Louvain la Neuve, Unité de Catalyse et Chimie des Matériaux Divisés, Croix du Sud 2/17, B-1348 Louvain-la-Neuve (Belgium) b Université catholique de Louvain la Neuve, Unité de Chimie des Interfaces, Croix du Sud 2/18, B-1348 Louvain-la-Neuve (Belgium)

Abstract Supported Pd nanoparticles (1.6 wt. %) with different diameters were synthesized by the modified water-in-oil microemulsion method using hydrazine as reducing agent. The size of palladium nanoparticles was investigated by varying the nature of the organic surfactant and solvent. The catalysts were characterized by XRD, XPS, ICP, and TEM. Supported palladium nanoparticles (1-8 nm) were obtained. The results confirmed the dependence of the particle size on the nature of organic surfactants. Smaller particles were obtained with organic solvents and anionic surfactants. Keywords: palladium nanoparticles, microemulsion, hydrazine reduction, XPS

1. Introduction Metal nanoparticles research has recently become the focus of intense work due to their unusual properties compared to bulk metal. After the pioneer work on preparation of Pt, Pd, Rh and Ir nanoparticles done by Boutonnet et al. using the water-in-oil microemulsion method [1], several other metallic nanoparticles were synthesized [2]. The classical reverse micelles method allows the preparation of colloidal suspensions of metallic nanoparticles. In this method the use of appropriate reducing agent such as hydrazine or NaBH4 is necessary. Two solutions containing reducing agent (first microemulsion) and metal precursor (second microemulsion) are mixed to form the colloidal suspension of metallic nanoparticles. These metallic particles could be then supported on an inert support. The deposition of metallic nanoparticles on a support can be done by: i) direct synthesis of support in the microemulsion using an appropriate organometallic precursor, ii) impregnation of support with the colloidal suspension of reduced metallic nanoparticles, and iii) precipitation of metal on the support during the reduction step. The aim of this work was to obtain a set of supported Pd particles in the nanometer size range. The reverse micelle method was simplified as compared to the classical one [1]. Firstly, in the classical method the reductant is introduced as a second microemulsion while in our case the direct injection of pure reducing agent to the microemulsion formed was done. Secondly, the support (TiO2) was introduced before the microemulsion formation what resolved the problem of the particle transfer on the support. Several parameters influencing the size of metal nanoparticles such as nature of the surfactant and solvent were studied.

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2. Experimental Palladium (IV) chloride (from Aldrich, purity) was used as the Pd precursor and TiO2 oxide (Merck, purity) as the support. The organic surfactants used in this work were purchased from Sigma (sodium bis(2-ethylhexyl, purity)sulfocuccinate (AOT, purity) and ethylene glycol monolauryl ether (Brij30), purity). Organic solvents (cyclohexane, 1-butanol, 1-octanol, and n-octane, purity) and hydrazine hydrate solution (80%) were purchased from Fluka. All reagents were used without further purification. All catalysts were prepared as follows: appropriate quantity of PdCl2 (origin, purity) was dissolved in 5 ml of distilled water in the presence of NaCl (purity) (NaCl/PdCl2 weight ratio of 0.75). Then the solution was evaporated at 60°C on a hot plate until volume of 1 ml. The microemulsion was then formed using 50 ml of organic solvent, 1 ml of precursor solution and appropriate quantity of organic surfactant. This microemulsion was then heated to 50°C and then 2 g of TiO2 was incorporated to the reactant flask under magnetic stirring. After 30 minutes the appropriate quantity of hydrazine was injected. The reactant solution changed colour from light red to black indicating the palladium reduction and nanoparticles formation. The solution was then filtered, purged with acetone(purity) and water for 1h and drying at 100°C for 30 minutes. The reduction was carried out in a thermostated water bath at 50°C. The time of reduction of 30 min was measured from the hydrazine injection to the microemulsion. The reaction was carried out under nitrogen atmosphere. ICP-OES analysis was used to determinate the chemical composition of the prepared catalysts and it was performed on a Thermo Jarre ASH IRIS Advantage analyzer. X-ray diffraction study was carried out on a Siemens D5000 diffractometer using the Kα radiation of Cu (1.5418 Å). The 2Θ range was scanned between 2 and 65° at a rate of 0.01°s-1. The identification of the phases was achieved by using the ICDDJCPDS database. The electron microscopy images were obtained with a TEM LEO 922 Omega microscope after placing a drop of the catalyst suspension on the carbon coated copper grid. X-ray photoelectron spectroscopy (XPS) was performed on a Kratos Axis Ultra (Kratos Analytical, Manchester, UK) spectrometer as described elsewhere [3]. XPS spectroscopy was also used to estimate the size of the palladium nanoparticles.

3. Results and discussion The hydrazine was chosen as appropriate reducing agent because it can be used at low temperature and it decomposes to NH3 and/or N2 after reduction takes place. The reduction with hydrazine depends on several parameters such as the nature of the metal, hydrazine concentration, pH, time of reduction and reduction temperature. In case of the noble metals such as palladium, this reaction occurred very easily at low temperature. In our case the hydrazine to palladium molar ratio was 100 (particle size is independent on hydrazine concentration). Under this condition the particle size should be mainly governed by the nature of solvent and surfactant. Moreover, in our study, the water to surfactant molar ratio was 6 which let us to suppose the presence of bounded and trapped water only. This is expected to result in very small size of palladium nanoparticles [4]. ICP analysis enables to determinate the chemical composition of the catalyst after preparation. The results, given in Table 1, showed, that almost all palladium atoms were deposited on the support in the case of AOT/n-octane catalyst (about 2 wt.%). Contrary,

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the amounts of palladium in other samples were smaller (1.6-1.7 wt.%) than expected (2 wt.%). This could be explained by loss of palladium during filtration. Nanometric particles can, indeed, pass more easily through the filter used. This is also valuable for support because a loss of weight of about 8% was observed for all catalysts after filtration and drying. However, the reasonable results obtained from ICP are comparable with others methods such as precipitation and ion-exchange. In addition, the specific surface area did not change significantly after chemical reduction as showed by the BET results in Table 1. BET surface area of TiO2 is 7.6 [m2g-1]. Figure 1 showed the TEM images of Pd/TiO2. The palladium particle size depends on the chemical composition of the primary microemulsion. Monodisperse size of the supported palladium nanoparticles has been obtained (Table 1). The palladium particle size increased (from 1 to 8 nm) with decreasing the carbon number of the n-alcohol (1-octanol and 1-butanol respectively) when such compounds are used as solvents. The smallest particles were obtained when n-octane, 1-octanol or cyclohexane were used as organic solvent. The size of palladium nanoparticles depends also on the organic surfactant. The smallest particles (1-2 nm) were obtained when the anionic bis2-ethylhexylsulfosuccinate (AOT) surfactant was used, as compared to the nonionic Brij30 surfactant.

AOT/cyclohexane

AOT/1-butanol

Brij30/1-decanol

Figure 1. TEM images of Pd/TiO2 catalysts prepared by the microemulsion method under three different synthesis conditions. Table1. ICP, BET and particle size for Pd/TiO2 catalysts. Catalysts

Pd wt.%

AOT/cyclohexane AOT/1-butanol AOT/1-octanol AOT/n-octane Brij30/1-decanol TiO2

1.7 1.8 1.6 2.2 1.6 -

Surface area [m2g-1] 8.3 8.0 7.9 8.4 8.0 7.6

Particle size TEM [nm]

Particle size XPS [nm]

< 2.0 ~ 8.0 < 2.0 < 2.0 3-10 -

1.3 7.7 1.0 1.5 3.9 -

This is in good accordance with the literature data [4]. Compared to cationic surfactants, smaller palladium nanoparticles were obtained using anionic ones. The differences in the adsorption of these surfactants molecules on the newly formed nanoparticles surface should play a leading role in the mechanism of particle growth. The change in the particle size for the AOT/1-butanol (about 8 nm) catalyst could be explained by the different polarity of the 1-butanol as compared to the higher alcohols (1-octanol) and other solvents (n-octane and cyclohexane). As the polarity of an n-

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alcohol becomes stronger with decreasing of the carbon number, 1-butanol had a stronger affinity for the positively charged palladium particles. The stabilization of the Pd nanoparticles would be little affected by the AOT surfactant [4]. The choice of surfactant is crucial for the final size of metal particles. The surfactants used represented two groups: non-ionic (AOT) and anionic (Brij30). One could expect that different mechanisms of stabilization of the nanoparticles are responsible for differences in the final particle size. Different effects would be observed in case of the ionic (AOT) or non-ionic (Brij30) surfactants [4]. In case of AOT two opposite effect are present. The polar head group of AOT has an increased charged on its oxygen in a similar way that Brij30, but the counterion Na+ from AOT exerts an opposite effect. The interaction between Pd and these surfactants are different. The balance between both effects in AOT results in a lower hydration of metal cations and a higher rate constant for the complex formation in AOT than in Brij30 microemulsions [5]. Moreover, it could be expected that positively charged palladium particles would be more strongly stabilized by electrostatic forces in case of anionic surfactant. This electrostatic interaction will be lower in case of non-ionic surfactant adsorbed on the palladium by negatively charged oxygen from hydrophilic group. This results in smaller Pd particle size obtained from AOT microemulsion [1, 4].

4. Conclusions Supported Pd nanoparticles were obtained using simplified microemulsion method. The results suggest that one step reduction-precipitation method allows obtaining supported Pd nanoparticles with small average size. The average palladium nanoparticle size can be controlled by changing the chemical composition of the microemulsion. The smaller nanoparticles were obtained with AOT surfactant. The differences in the particles size were ascribed to the different ionicities of the surfactants or solvent polarities.

Acknowledgements The financial support of the Interuniversity Attraction Pole (IAP), INANOMAT, Belgian Science Policy is gratefully acknowledged.

References [1] M. Boutonnet, J. Kizling, P. Stenius, G. Maire, 1982, The preparation of monodisperse colloidal metal particles from microemulsions, Colloids and Surfaces, 5, 209-225 [2] S. Eriksson, U. Nylén, S. Rojas, M. Boutonnet, 2004, Preparation of catalysts from microemulsions and their applications in heterogeneous catalysis, Applied Catalysis A: General, 265, 207-219 [3] D.P. Debecker, Ch. Faure, M-E. Meyre, A. Derré, E.M. Geigneaux, 2008, A new bioinspired route to metal-nanoparticle-based heterogeneous catalysts, Small, 10, 1806-1812 [4] T. Hanaoka, T. Hatsuta, T. Tago, M. Kishida, K. Wakabayashi, 2000, Control of the rhodium particle size of the silica-supported catalysts by using microemulsion, Applied Catalysis A General, 190, 1-2, 291-296 [5] C. Cabaleiro-Lago, L. Garcıa-Rıo, P. Herves, J. Perez-Juste, 2007, Nonionic microemulsions. Effects of the interface on metal–ligand reactions, Colloids and Surfaces A, 309, 286-291

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Preparation of silica-coated Pt-Ni alloy nanoparticles using microemulsions and formation of carbon nanofibers by ethylene decomposition Keizo Nakagawa,*a,b,c Sakae Takenaka,d Hideki Matsune,d Masahiro Kishidad a

Department of Advanced Materials, Institute of Technology and Science, The University of Tokushima, Minamijosanjima, Tokushima 770-8506 b Department of Geosphere Environment and Energy, Center for Frontier Research of Engineering, The University of Tokushima, Minamijosanjima, Tokushima 770-8506 c Department of Chemical Science and Technology, Faculty of Engineering, The University of Tokushima, Minamijosanjima, Tokushima 770-8506 d Department of Chemical Engineering, Graduate School of Engineering, Kyushu University, 744 Moto-oka, Nishi-ku, Fukuoka 819-0395

Abstract Silica-coated Pt-Ni alloys were prepared using a water-in-oil-type microemulsion. The silica-coated Pt-Ni alloys prepared without thermal treatment decomposed ethylene to form nanocomposites of carbon nanofibers.

Keywords: silica-coated metal catalyst, Pt-Ni alloy, carbon nanofiber, ethylene decomposition

1. Introduction Highly dispersed precious metal particles on supports have been shown to be active for various catalytic reactions such as oxygen reduction at the cathode or hydrogen oxidation at the anode in a proton exchange membrane fuel cell (PEMFC), and dehydrogenation of alkanes [1,2]. However, the active metal particles agglomerate easily at high temperatures because they are supported on the outer surface of the supports, which results in a decrease in catalytic activity. Thus, supported metal catalysts with high resistance to sintering at high temperatures are required. We have previously studied the preparation of silica-coated metal catalysts using microemulsions [3-6]. Using these methods, metal particles such as Ni, Co and Pt can be uniformly covered with silica layers. The metal particles in these catalysts show high resistance to sintering at high temperatures because each particle is covered with silica layers. In our previous study, these silica-coated metal catalysts allowed the selective formation of carbon nanotubes (CNTs) or carbon nanofibers (CNFs) with uniform diameters through ethylene decomposition while the metal catalysts without silicacoating formed CNTs or CNFs with various diameters because the metal particles aggregated severely during ethylene decomposition at 973 K [4,5]. Thus, silica-coated metal catalysts are effective catalysts for the production of nanoscale carbon structures. The catalytic performance of precious metals is frequently modified by the addition of other metal species. For example, the tolerance to CO and the durability of Pt electrocatalysts in PEMFCs are improved by the addition of metal species such as Ni,

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Co, Mo and Pd [7]. In addition, the activity of the Pd catalyst for the oxidative dehydrogenation of sodium lactate to pyruvate was improved by the addition of Te [8]. Recently, silica-coated Pt-Co alloys and Pt-Pd alloys covered with silica layers were prepared and used as catalysts for ethylene decomposition to form CNTs [6]. The structure of the graphene that forms the walls of the CNTs was different for the two catalysts. This result indicates that the nanoscale carbon structure formed by hydrocarbon decomposition was influenced by the type of metal species in the silicacoated metal alloys. Therefore, further research on the hydrocarbon decomposition of silica-coated alloys using other metal species is needed because it is possible to develop nanocomposites using the silica-coated metal alloy catalyst and nanoscale carbon structures. Supported Ni is known to be an effective catalyst for hydrocarbon decomposition [9]. In this study, silica-coated Pt-Ni alloys were prepared using a microemulsion and they were used as catalysts for ethylene decomposition to form nanoscale carbon structures. The influence of silica-coated Pt-Ni alloy thermal treatment on the formation of the nanoscale carbon structures was also investigated.

2. Experimental Silica-coated Pt-Ni alloys (denoted as coated Pt-Ni) were prepared in a water-in-oiltype microemulsion [3-6]. Mixed aqueous solutions of H2PtCl6 and Ni(NO3)2 were used for the preparation of coated Pt-Ni. The microemulsion system was prepared by adding aqueous solutions containing the metal cations described above into a surfactant solution in cyclohexane. Polyoxyethylene (n = 15) cetyl ether was used as a surfactant. Nanoparticles containing the metal species were formed by the addition of aqueous NH3 into the microemulsion. Hydrolysis of tetraethoxysilane (TEOS) was carried out in the microemulsion by the addition of TEOS and aqueous NH3. After filtration, the samples were calcined at 773 K for 2 h in air. The calcined samples were washed with aqua regia at room temperature to remove the metal species not covered with silica layers. The metal loading was evaluated using XRF spectra and found to be 2.0 wt% Pt and 0.2 wt% Ni in the coated Pt-Ni. Ethylene decomposition to form the nanoscale carbon structures was performed at 973 K with a conventional gas-flow system over a fixed catalyst bed. Before ethylene decomposition and measurement by TEM and EXAFS, the catalysts were reduced with hydrogen at 623 K. To confirm the influence of the alloying degree of the Pt-Ni alloy on the formation of nanoscale carbon structures by ethylene decomposition, thermal treatment at 973 K in an atmosphere of Ar was performed before ethylene decomposition. X-ray absorption spectra for the samples were obtained at the Photon Factory in the Institute of Materials Structure Science for High Energy Accelerator Research Organization, Tsukuba, Japan. Pt LIII-edge EXAFS was measured at the beam line BL-7C and 9C equipped with Si(111) in transmission mode at room temperature (Proposal No.2006G343). Analysis of the EXAFS data was performed using an EXAFS analysis program, REX (Rigaku Co.). Inversely Fourier-transformed data for the Fourier peaks were analyzed by a curve-fitting method using the phase-shift and amplitude functions derived from FEFF 8.0 [10].

3. Results and discussion Figure 1 shows TEM images for coated Pt-Ni prepared with or without thermal treatment. In both the TEM images, particles with diameters of ca. 40 nm were observed. Judging by the contrast of the TEM images, the particles are mainly composed of silica.

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|FT|

Small particles with dia(b) (a) meters of a few nanometers were also observed in the TEM images. It should be noted that the small particles were not observed on the surfaces of the silica particles but in their bodies. Although the small particles grow a little bigger by thermal treatment, the sintering of the Fig. 1. TEM images of (a) coated Pt-Ni, (b) coated Pt-Ni metal particles was suppressed, prepared with thermal treatment. as shown in Fig. 1 (b). These results strongly suggest that the 70 small particles on the coated PtNi are uniformly covered with silica layers. 60 (c) Figure 2 shows Fourier transforms of Pt LIII-edge k3weighted EXAFS spectra 50 (RSFs; radial structural functions) for coated Pt-Ni with 40 (b) or without thermal treatment as well as for Pt foil. In a RSF for Pt foil, a strong peak due to Pt30 (a) Pt bonds is observed at around 2.7 Å. In contrast, two peaks 0 1 2 3 4 5 6 were observed in the R range R/Å from 1.5 to 3.0 Å in the RSFs Fig. 2. Fourier transforms of Pt L -edge k3-weighted III for the coated Pt-Ni suggesting weighted EXAFS for (a) coated Pt-Ni, (b) coated the formation of alloys between Pt-Ni prepared with thermal treatment, (c) Pt foil. Pt and Ni. The peaks in the R Table 1. Structural parameters estimated by curve-fitting range from 1.5 to 3.0 Å for the the EXAFS spectra of coated Pt-Ni. coated Pt-Ni were analyzed by C.N.b Sample Shell R/ Åa a curve-fitting method. The 2.60 1.03 Pt-Ni structural parameters that were Coated Pt-Ni Pt-Pt 2.69 6.66 evaluated by curve-fitting are summarized in Table 1. The 2.64 1.58 Pt-Ni Coated Pt-Ni with peaks for coated Pt-Ni thermal treatment Pt-Pt 2.70 7.38 prepared with or without a R, interatomic distance. bC.N., coordination number. thermal treatment were fitted using Pt-Pt and Pt-Ni bonds. The interatomic distances for the bonds in the samples were very similar to those of the corresponding alloys in previous reports [11]. Thus, the metal species in the coated Pt-Ni are present as alloys. The coordination number of the Pt-Ni bond increased with thermal treatment at 973 K, indicating better alloying between Pt and Ni in the coated Pt-Ni prepared with thermal treatment. Figure 3 shows TEM images for the coated Pt-Ni prepared with or without thermal treatment after ethylene decomposition at 973 K. The formation of nanoscale carbon structures on the coated Pt-Ni prepared with thermal treatment was not observed in the TEM images, as shown in Fig. 3 (b). In contrast, the coated Pt-Ni prepared without thermal treatment formed CNFs with diameters of ca. 10 nm by ethylene decomposition

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at 973 K as shown in Fig. 3 (a). (a) (b) The amount of carbons deposited on the coated Pt-Ni were estimated to be 55 wt%. In our previous work, CNTs were obtained over silica-coated Pt-Co alloys and silica-coated Pt-Pd alloys by ethylene decomposition [6]. Thus, the nanoscale carbon structures formed by ethylene Fig. 3. TEM images of after ethylene decomposition, (a) coated Pt-Ni, (b) coated Pt-Ni prepared with thermal decomposition are strongly treatment. dependant on the type of metal alloy in the silica-coated catalysts. It is well accepted that Ni also decomposes hydrocarbons to form carbon nanofibers [9]. However, the Ni species in the silicacoated Pt-Ni alloys were mainly present as alloys. Considering the results of the EXAFS spectra as shown in Table 1, the heterogeneous alloyed parts with slightly higher Ni concentration are present in the coated Pt-Ni prepared without thermal treatment. It is assumed that these parts are active sites for the formation of carbon nanofibers by ethylene decomposition. Therefore, the Pt-Ni alloys prepared without thermal treatment decompose ethylene to form carbon nanofibers. No metal particles were found at the tip or in the body of CNFs in the TEM images of the coated Pt-Ni after ethylene decomposition. The Pt-Ni alloy particles stabilized in the silica layers decompose ethylene to form the CNFs, i.e., the base-growth model as previously reported [4-6].

4. Conclusions Pt-Ni alloy particles covered with silica layers were prepared using microemulsion systems. The metal species in the coated Pt-Ni were present as alloys. Carbon nanofibers formed over the silica-coated Pt-Ni alloys prepared without thermal treatment during ethylene decomposition. These nanocomposites composed of coated Pt-Ni and the CNFs are useful in various fields because of the electronic and chemical properties of the CNFs as well as the silica-coated Pt-Ni alloy nanoparticles.

References S-H. Liu, W-Y. Yu, C-H. Chen, A-Y. Lo, B-J. Hwang, S-H. Chien and S-B. Liu, Chem. Mater., 20 (2008) 1622. 2. L. Bednarova, C. E. Lyman, E. Rytter and A. Holmen, J. Catal., 211 (2002) 335. 3. T. Tago, T. Hatsuta, K. Miyajima, M. Kishida, S. Tashiro and K. Wakabayashi, J. Am. Ceram. Soc., 85 (2002) 2188. 4. S. Takenaka, Y. Orita, H. Matsune and M. Kishida, J. Phys. Chem. C 111 (2007) 7748. 5. K. Nakagawa, S. Takenaka, S. Imagawa, H. Matsune and M. Kishida, Chem. Lett., 36 (2007) 252. 6. S. Takenaka, Y. Orita, T. Arike, H. Matsune, E. Tanabe and M. Kishida, Chem. Lett., 36 (2007) 1250. 7. J. Wee and K. Lee, J. Power Sources. 157, 128 (2006). 8. S. Sugiyama, T. Kikumoto, H. Tanaka, K. Nakagawa, K-I. Sotowa, K. Maehara, Y. Himeno and W. Ninomiya, Catal. Lett., 131 (2009) 129. 9. S. Takenaka, S. Kobayashi, H. Ogihara and K. Otsuka, J. Catal., 217 (2003) 79. 10. A. L. Ankudinov, B. Ravel, J. J. Rehr and S. D. Conradson, Phys. Rev. B 58 (1998) 7565. 11. Y. Shu, L. E. Murillo, J. P. Bosco, W. Huang , A. I. Frenkel and J. G. Chen., Appl. Catal. A 339, 169 (2008). 1.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Sol-gel synthesis combined with solid exchange method, a new alternative process to prepare improved Pd/ZrO2-Al2O3-SiO2 catalysts Shemseddine Fessi,a Abdelhamid Ghorbel,a Alain Rivesb a

Laboratoire de Chimie des Matériaux et Catalyse, Département de Chimie, Faculté des Sciences de Tunis, Campus Universtaire 2092 Tunis, Tunisie b Unité de Catalyse et de Chimie du Solide, UMR CNRS 8181, Université des sciences et technologies de Lille, Bâtiment C3, 59655 Villneuve d’Ascq, France.

Abstract The Pd/ZrO2-SiO2-Al2O3 catalysts are prepared by sol-gel synthesis (SG) and by combining sol-gel and solid exchange methods (SG-SE), with variable Si and Zr loadings. N2-physisorption, NH3-TPD and CH4-TPR are the main techniques used to characterise these catalysts. Furthermore, the total methane oxidation is used to test their catalytic activity. Compared to the SG sample, better BET surface area, higher pore volume and larger average pore diameter are obtained on the the SG-SE catalyst. In addition, similar acidity and PdO reducibility are obtained on these two solids. Moreover, the slightly higher activity observed on the SG catalyst seems to be due to its better PdO dispersion. However, the significant differences of texture, acidity and PdO reducibility should be the reasons for the catalytic activity variations on the catalysts prepared by SG-SE method with variable Si and Zr loadings. Keywords: Pd/ZrO2-SiO2-Al2O3 catalysts, combined sol-gel and solid exchange methods

1. Introduction The sol-gel synthesis is a successful method to prepare homogenous mixed oxides [1-14]. Nevertheless, some difficulties such as: (i) the difference between hydrolysis rates of metal alkoxides, especially when Si alkoxides are used [1-5, 11,12], (ii) the restricted choice of chemical reagents (solvent, complexing agent, …) which are suitable for the all used alkoxides [3,7], (iii) the limited alternative of drying conditions which are favourable for the all mixed metals [3]. In this work, an attempt to combine the sol-gel synthesis and the solid exchange method is tried in order to avoid these described disadvantages and to prepare improved Pd/ZrO2-SiO2-Al2O3 catalysts. In effect, this seems to be probable when each oxide is synthesised separately with the adequate conditions, then an effective combination of the obtained solids is carried out.

2. Experimental 2.1. Catalyst synthesis 2.1.1. Sol-gel method (SG) Al2O3 xerogel: a mixture of aluminium-sec-butoxide (AsB, 97%) and sec-butanol (sB, 99%) is stirred at 40°C for 20 min, ([AsB] =1M). Then, acetic acid (AcA, 99.8%) is added with a molar ratio nAcA/nAsB = 6. The formed gel is aged in air for 24 h and then dried in oven at 70°C for 24 h.

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SiO2 aerogel: a mixture of tetraethoxysilane (TEOS, 97%), deionised water and acetic acid (nAcA/nTEOS = 0.05 and nH2O/nTEOS = 25) is stirred at 70°C for about 1h. The formed gel is aged in air for 24 h and then dried in autoclave with the hypercritical conditions of sec-butanol (262.2°C and 41.4 atm). Pd/ZrO2-SiO2-Al2O3 catalyst (SG): a mixture of TEOS, deionised water and AcA (nAcA/nTEOS = 0.05 and nH2O/nTEOS = 25) stirred at 70°C for about 55 min is added to a mixture of AsB, palladium acetylacetonate (Pd(acac)2, 35% Pd) and sB ([AsB] =1M) stirred at 40°C for 2h. Then, AcA is added with a molar ratio nAcA/nAsB = 6. The formed gel is aged in air for 24 h and dried in oven at 70°C for 24 h. The obtained solid is heated in flowing oxygen (30 ml/min) at 2°C/min up to 500°C and kept at this temperature for 2h. The precursors amounts are fixed to have designed loadings of Zr = 8 wt.%, Si = 30 wt.% and Pd = 2 wt.%. 2.1.2. Combined sol-gel and solid exchange method (SG-SE) Pd/ZrO2-SiO2-Al2O3 catalysts: Appropriate amounts of Al2O3 xerogel, SiO2 aerogel, zirconium acetylacetonate (Zr(acac)2, 99,7%) and Pd(acac)2, are mixed mechanically for 10 min in a mortar. The resulting solid is then heated in flowing oxygen (30 ml/min) at 2°C/min up to 500°C and kept for 2h. The designed loadings of Zr = y wt.%, Si = x wt.% and Pd = 2 wt.%.

2.2. Methods BET surface area, total pore volume and average pore diameter are determined from the N2 adsorption-desorption measurements using an automatic Micrometrics ASAP 2000. Methane temperature programmed reduction (CH4-TPR) is carried out on 0.1 g of precalcined sample, under 2% CH 4/He flow of 40 ml/min. The temperature is ramped from 30 to 450°C at 5°C/min. The consumed methane and the formed product amounts are determined continuously with a quadrupole mass spectrometer QMC 200 (Pfeiffer). For the NH3TPD measurements, the sample is first flushed with Ar (40 ml/min) for 30 min, at 475°C and then cooled to 30°C. After NH3 adsorption (10% NH3/He, for 30 min), the catalyst bed is flushed with Ar (40 ml/min) for 2h. NH3 TPD is performed by heating the sample in Ar (40 ml/min) at a rate of 5°C/min. Catalytic activity for methane combustion is determined over the calcined sample (0.1g) in a dynamic micro reactor. The flow of 1 vol.-% methane, 4 vol.-% oxygen and balance helium are mixed, regulated at a total flow of 100 ml/min and admitted at 250°C. The reactor effluent is then analysed at different reaction temperatures (from 250 to 350°C) by a mass spectrometer detector QMC 200 (Pfeiffer).

3. Results and discussion The N2 adsorption-desorption results (see Table 1) show significant modifications of the Pd/ZrO2-SiO2-Al2O3 texture, when the preparation parameters are varied. In fact, better BET surface area, higher pore volume and larger average pore diameter are obtained when the SG-SE method is used instead of the SG synthesis and when Si amount is increased from 0 to 100%. Nevertheless, the increase of Zr loading from 8 to 16% (respectively Si30 and Zr16 catalysts) decreases the BET surface area, the pore volume and the average pore diameter. According to the obtained NH3 desorption temperatures and desorbed NH3 amounts (see Table 1 and fig.1), practically the same acid site number and the same acidity strength are obtained when the SG-SE and the SG methods are used. The NH3 desorption temperatures are respectively 147°C and 145°C. When Si amount is increased from 0 to 100%, the acidity strength diminishes and the acid site number augments significantly on the Si30 sample compared to that on the Si0 and on the

799

Sol-gel synthesis combined with solid exchange method

Si100 catalysts. These results suggest that new acid sites are created due to SiO2 and Al2O3 interaction. Similar results were observed in the literature when mixed SiO2Al2O3 oxides are prepared [12-15]. Moreover, when Zr loading increases from 8 to 16%, the acidity strength falls but the acid site number is preserved practically the same. The last result believes that the created new acid sites due to SiO2 and Al2O3 interaction are modified by the enhancement of Zr loading. In fact, the Zr16 and the Si30 catalysts are prepared both with 30% of Si and have practically the same acid site number but not the same acidity strength. Consequently, this leads to advance that important interactions are occurred between Al, Si and Zr precursors during the preparation of the Pd/ZrO2-SiO2-Al2O3 catalysts by SG-SE method. Table 1. BET surface area (SBET), total pore volume (Vp), average pore diameter (Dp), NH3 desorption temperature (TDNH3), CH4 reduction temperature (TRCH4) and activity in methane combustion at 325°C of the Pd/ZrO2-SiO2-Al2O3 catalysts calcined at 500°C (Ac). TRCH4 (°C)

Ac x105 (mole CH4.h-1.g-1)

150

301

1.42

145

145

267

2.84

2.44

152

115

231

7.30

294

1.32

143

131

261

1.93

269

0.54

98

147

260

4.66

Sample

Preparation method

Si wt.%

Zr wt.%

SBET m2/g

Vp cm3/g

Dp TDNH3 Å (°C)

Si0

SG-SE

0

8

210

0.38

57

Si30

SG-SE

30

8

360

1.63

Si100

SG-SE

100

8

518

Zr16

SG-SE

30

16

SG

SG

30

8

Furthermore, the CH4-TPR results (see table 1) show different PdO reduction temperatures (TRCH4) on the prepared catalysts. Small difference is observed between the PdO reduction temperatures of the SG-SE and the SG samples (respectively 267°C and 260°C), which traduces the similar PdO reducibility on these two catalysts. Then, an amplified PdO reducibility is observed when Si loading is increased. However, no considerable variation of the reduction temperature is observed with the Zr amount increases. These results advise that considerable interactions are happened between Pd(acac)2, alumina xerogel and silica aerogel but not between Pd(acac)2 and Zr(acac)2. This show the better reactivity of the derived sol-gel products (xerogels and aerogels) in this preparation process. Accordingly, the addition of zirconia aerogel or zirconia xerogel should be more efficient for the modification of the PdO reducibility. The catalytic activity measured in methane combustion at 325°C show that slightly higher activity is observed on the SG catalysts than that on the SG-SE sample (see Table 1) and that a significant activity increase is observed when Si content rises from 0 to 100% or when Zr loadings falls from 16 to 8%. According to the obtained results, the catalyst prepared by SG-SE method have an improved texture compared to that prepared by the SG synthesis and practically similar acidity and PdO reducibility. Consequently, the slightly higher activity observed on the SG catalyst seems to be due to its better PdO dispersion. In effect, since the Pd(acac)2 is introduced in the solution mixture during the SG catalyst synthesis, the palladium dispersion should be more favoured that on the SG-SE sample on which, Pd(acac)2 is introduced from the solid state. For this reason, such parameter should be studied and improved on the SG-SE solids. Moreover, the significant differences of texture, acidity and PdO reducibility

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obtained on the catalyst prepared by SG-SE method when Si or Zr loading is varied should be the reasons for the catalytic activity variations on these solids [12-14,16].

NH3 amount (a.u)

7.000E-09 6.000E-09

Si0

5.000E-09

Si30

4.000E-09

Si100

3.000E-09

Zr16

2.000E-09

Sg

1.000E-09 0.000E+00 0

200

400

600

T(°C) Figure 1. NH3 TPD profiles of the Pd/ZrO2-SiO2-Al2O3 catalysts.

4. Conclusion As a conclusion, the properties variation of the Pd/ZrO2-SiO2-Al2O3 catalysts prepared by SG-SE method seems to be caused by the extensive interactions between the metal precursors during preparation. Which suggest that the SG-SE process proposed in this paper is enough efficient and enough sensitive to the variation of the preparative parameters as the sol-gel method. This allows to prepare several homogenous mixed oxides with a wide range of physicochemical properties without the difficulties found during the preparation of such solids by SG method. Consequently, the SG-SE process seems to be a promising alternative to prepare improved catalysts.

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]

R.K. Iler, The Chemistry of Silica, Wiley, New York, 1979. C.J. Brinker and G.W. Scherer, Sol–Gel Science: The Physics and Chemistry of Sol–Gel Processing, Academic Press, Boston, MA, 1990. D. A. Loy, Sol-gel processing, Encyclopedia of Physical Science and Technology (2004) 257. J. Livage, Inorganic Materials, Sol–Gel Synthesis of, Encyclopedia of Materials Science and technologie (2008) 4105. J. A. Crayston, Sol–Gel, Comprehensive Coordination Chemistry II (2004) 711. J.B. Miller and E.I. Ko, Catal. Today 35 (1997) 269. Q. Yang, C. Xie, Z. Xua, Z. Gaob, Z. Li, D. Wang and Y. Dua, J. Mol. Catal. A 239 (2005) 144. J. Klein, C. Lettmann and W. F. Maier, J. Non-Cryst. Solids 282 (2001) 203. R.G. Rodrı´guez Avendan˜o, J.A. De Los Reyes, T. Viveros and J.A. Montoya De La Fuente, Catal. Today 148 (2009) 12. J.B. Miller and E. I. Ko, Catal. Today 35 (1997) 269. E. Pab_on, J. Retuert, R. Quijada and A. Zarate, Micropor. Mesopor. Mater. 67 (2004) 195. V. Lafond, P.H. Mutin and A. Vioux, J. Mol. Catal. A 182 (2002) 81. C. E. Volckmar, M, Brona, U. Bentrup, A. Martinb and P. Clausa, J. catal. 26 (2009) 1. A.M. Venezia,V. La Parola, B. Pawelec and J.L.G. Fierro, Appl. Catal. A 264 (2004) 43. P.R. Aravind, P. Mukundan, P. Krishna Pillai and K.G.K. Warrier, Micropor. Mesopor. Mater. 96 (2006) 14. M. Guemini and Y. Rezgui, Appl. Catal. A 345 (2008) 164.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Sol-gel synthesis of micro- and mesoporous silica in strong mineral acid Anouschka Depla,a Christine Kirschhock,a Johan Martens a a

Centre for Surface Chemistry and Catalysis, K.U. Leuven, Kasteelpark Arenberg 23, 3001 Heverlee (Leuven), Belgium

Abstract Acid catalyzed silica sol-gel syntheses were performed using TEOS and TMOS, the corresponding alcohol solvent, a high concentration of mineral acid and a low hydrolysis ratio. The molar Si:H2O:solvent:acid ratios of the synthesis mixture were 1:2:3:0.35. The obtained silica materials were calcined to remove residual alkoxide groups. The nature of the mineral acid, the use of TEOS versus TMOS, and the temperature of the polymerization had a strong influence on the textural properties of the obtained silica. Distinct microporous and mesoporous materials were obtained. Keywords: amorphous silica, TEOS, TMOS, microporosity, mesoporosity

1. Introduction The synthesis of ordered micro- and mesoporous silica materials often relies on the use of organic templates that are sacrified in order to evacuate the pores. Inspired by the synthesis of amorphous microporous silica materials by Maier et al.[1] applicable in molecular shape selective catalysis [2] and controlled release [3, 4] in this work we continued the exploration of silica synthesis under strongly acidic conditions. We observed the outcome of the synthesis to be strongly dependent of the Si-source, the type of mineral acid and the synthesis temperature. In this paper the synthesis of silica materials with a porosity ranging from micro- to mesoporous is demonstrated. This approach of synthesis of silica with tunable nanopores not involving sacrificial templates will be convenient for many applications.

2. Experimental Appropriate amounts of Si-alkoxide (TEOS or TMOS), technical solvent (Ethanol or methanol), acid (HCl, HNO3 or H2SO4) and water were mixed at molar Si:H2O:solvent:acid ratios of 1:2:3:0.35. The synthesis code names and procedures are listed in Table 1. In the standard procedure a sol is prepared by stirring during 1 day (250 rpm, Variomag Multipoint 15). Subsequent heating at 55°C provokes gel formation. Alternatively, the sol-gel process can be conducted upon heating at 85°C under reflux. Synthesis mixtures filled in autoclaves were heated at 55°C or 100°C under agitation through tumbling of the autoclaves. Elimination of residual alkoxide groups and evacuation of pores was performed through calcination in air at 350°C. To reach that temperature a slow heating rate of 0.1°C min−1 was used. Occasionally calcination was done under O2-flow. Porosity was characterized using nitrogen adsorption isotherms at −196°C, recorded on a Tristar instrument (Micromeritics). Prior to analysis, samples were outgassed for 4h at 250°C.

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3. Results and discussion 3.1. Syntheses using TEOS The standard synthesis conditions using HCl according to literature leads to formation of microporous material.[1] The type I N2-physisorption isotherm with important N2uptake at low relative pressure confirms the formation of microporous material (Fig. 1A). Substituting hydrochloric acid with nitric or sulfuric acid doesn’t alter the pore type but leads to a lower BET value (Table 1), viz. 416 m2/g with HCl to 328 m2/g with HNO3 and 349 m2/g with H2SO4. A distinction can be made between ultramicropores narrower than 1.5 nm and supermicropores (1.5 to 2 nm).[5] Ultramicroporous materials reach the plateau at P/P° values of ca. 10-1. Based on this criterion, the synthesis in HCl and HNO3 are classified as ultramicroporous, and in H2SO4 as supermicroporous (Fig. 1A). All gels were transparent to white, except for the gel prepared with H2SO4 which colored black. The black color was ascribed to the formation of coke upon calcinations catalyzed by the sulfuric acid.[6] To reduce coke formation the calcination was performed under oxygen flow. A white supermicroporous powder was obtained with increased BET surface area (433 m2/g). Synthesis under reflux improved the BET surface area to 503 m2/g (HCl) and 561 2 m /g (HNO3). The use of both acids led to formation of supermicroporous structures. Using the reflux procedure in combination with H2SO4 a different porosity was obtained (Fig. 1A). The isotherm shape with hysteresis was characteristic of a mesoporous material (Fig. 1A). The shape of the hysteresis loop, expanded over a wide range of P/P° values, points at a variety of pore diameters. Such isotherm is e.g. encountered with materials having wedge pore shape. This mesoporous material prepared using H2SO4 had a much lower BET surface area of 156 m2/g. The mesopore volume estimated using BJH model was 0.23 cm3/g. When the synthesis was performed with H2SO4 acid in an autoclave at temperatures of 55°C an ultramicroporous material with BET surface area of 340 m2/g was formed.

3.2. Syntheses using TMOS Standard synthesis conditions led to microporous materials including supermicropores with different BET surface areas depending on the acid (Table 1). The BET surface area increased in the order HCl (432 m2/g) - HNO3 (478 m2/g) - H2SO4 (545 m2/g) (Table 2). When the synthesis was performed in an autoclave at room temperature the microporosity was less well developed as reflected in the BET surface area of 365 m2/g only. Increasing the temperature to 55°C or 100°C significantly improved the BET surface area. The highest BET surface area of 800 m2/g was obtained using HCl and synthesis at a temperature of 55°C. These materials present a combination of micropores and mesopores (Fig. 1B). The hysteresis loop of the sample prepared using HCl is characteristic of wedge-like mesopores. The texture of the silica obtained using H2SO4 and the autoclave at 55°C based on the N2 adsorption isotherm is different. The hysteresis loop shows abrupt closure at a P/P° around 0.45 owing to the tensile strength effect. Such behavior is typical of mesopores with narrow openings. The mesopore volume of this sample was ca. 36 cm3/g, which is much smaller than the 1.3 cm3/g reached using HCl under otherwise identical conditions. When using the reflux setup, microporous materials were obtained using H2SO4 and HNO3. The BET surface areas were 390 m2/g and 363 m2/g, which is lower than obtained using the standard method. In the reflux setup the use of HCl instead of H2SO4 or HNO3 resulted in a material with both micropores and mesopores with narrow openings, similar to the autoclave type using H2SO4 at 55°C (Fig. 1B c). The mesopore volume according to the BJH method was 0.31 cm3/g. The material synthesized using HCl under reflux had a high BET

Sol-gel synthesis of micro- and mesoporous silica in strong mineral acid

803

surface area (733 m2/g). In another experiment using H2SO4, the sol-gel transformation was allowed to occur at room temperature following the standard procedure. Subsequently the gel was heated under reflux. A mesoporous silica was obtained presenting a combination of all features observed in the other methods. Based on the N2 adsorption isotherm this material contains micropores, wedge-type mesopores and mesopores with narrow pore openings (Fig. 1B). The BET surface area was 680 m2/g and the BJH mesopore volume 0.3 cm3/g. A silica material with similar texture was also obtained using H2SO4 acid in an autoclave at 100°C. Table 1. N2-physisorption BET-values and pore classification for TEOS samples, listed according to synthesis conditions and catalysts. Synthesis method

Time (days)

Temp

Reflux

a

4

55

BET

Pore type

2

(°C)

Standard

Autoclave

Acid

(m /g) HCl

416

ultramicropores

HNO3

328

ultramicropores

H2SO4

349

ultra + supermicropores

H2SO4

a

433

ultra + supermicropores

H2SO4

340

ultramicropores

10

85

HCl

503

ultra + supermicropores

10

85

H2SO4

156

mesopores-wedge shape

5

85

HNO3

561

ultramicropores

Calcination under O2-flow

Table 2. N2-physisorption BET-values and pore classification for TMOS samples, listed according to synthesis conditions and catalysts. Synthesis method

Time

Temp

(days)

(°C)

Standard

Autoclave

Reflux

Gel prior to Reflux

Acid

BET

Pore type

(m2/g) HCl

432

ultra + supermicropores

HNO3

478

ultra + supermicropores

H2SO4

545

ultra + supermicropores

20

RT

H2SO4

365

ultra + supermicropores

1

55

H2SO4

605

mesopores- narrow openings

1

100

H2SO4

655

mesopores-wedge shape and narrow openings

9

55

HCl

800

mesopores-wedge shape

6

85

HCl

733

mesopores-wedge shape

5

85

H2SO4

390

ultra + supermicropores

5

85

HNO3

363

ultra + supermicropores

9

85

H2SO4

680

mesopores- wedge shape and narrow openings

A. Depla et al.

A

200

a b c

150 100 50 0 0.0

0.2

0.4

0.6

0.8 0

Relative Pressure (P/P )

1.0

Adsorbed volume (cm 3 STP/g)

3

Adsorbed volume (cm STP/g)

804 B

1000

a

800 600 400

b

200

c d e

0 0.0

0.2

0.4

0.6

0.8

1.0

0

Relative Pressure (P/P )

Figure 1. N2- adsorption isotherms of materials synthesized: (A) from TEOS and EtOH, with acid and method: (a) H2SO4-Reflux, (b) HCl-Standard, (c) H2SO4-Standard and (B) from TMOS and MeOH, with acid and method: (a) HCl-Autoclave 55°C (shifted by 120 cm3 STP/g), (b) H2SO4Gellation prior to reflux (shifted by 120 cm3 STP/g), (c) H2SO4-Autoclave 55°C (shifted by 20 cm3 STP/g), (d) H2SO4-Standard, (e) HCl-Standard.

4. Conclusion Generally, significantly higher specific surface areas were obtained using TMOS compared to TEOS. The highest BET surface area of 800 m2/g was obtained using TMOS in combination with HCl in an autoclave at 55°C. When comparing TEOS and TMOS silicon sources in the standard synthesis method, with TMOS independent of the type of mineral acid systematically higher BET-surface areas were reached. The order of the acids with respect to the BET surface areas was opposite for both Si-sources. Using TEOS, HCl was the best acid, while with TMOS the highest BET value was obtained using H2SO4. Another difference was that using TMOS the contribution of supermicroporosity was higher than using TEOS. The development of mesoporosity frequently occurred using TMOS. Using TEOS only in one instance mesoporosity was achieved, namely when using H2SO4 in the reflux method. A polyporous material presenting ultra- and supermicropores, wedge-type mesopores and mesopores with narrow entrances was obtained when the sol-gel transformation starting from TMOS and H2SO4 was performed at room temperature and the gel was subsequently heated at 85°C under reflux.

Acknowledgements This work was supported by FWO Vlaanderen. J.A.M. acknowledges the Flemish Government for long-term structural funding.

References [1] W. F. Maier, I. C. Tilgner, M. Wiedorn, H. C. Ko, Adv. Mater, 5 (1993), p. 726. [2] W. F. Maier, J. A. Martens, S. Klein, J. Heilmann, R. Parton, K. Vercruysse, P. A. Jacobs, Angew. Chem., Int. Ed. Engl. 35 (1996), p. 180. [3] C. A. Aerts, E. Verraedt, R. Mellaerts, A. Depla, P. Augustijns, J. Van Humbeeck, G. Van den Mooter, J. A. Martens, J. Phys. Chem. C 111 (2007), p. 13404. [4] E. Verraedt, M. Pendela, E. Adams, J. Hoogmartens, J. A. Martens, Journal of Controlled Release 142 (2010), p. 47. [5] F. Rouquerol, J. Rouquerol, K. Sing, Adsorption by Powders and Porous Solids: Principles, Methodology and Applications, (1999). [6] J. Li, G. Xiong, Z. Feng, Z. Liu, Q. Xin, C. Li, Micropor. Mesopor. Mater. 39 (2000), p. 275.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Ag-V2O5/TiO2 total oxidation catalyst: autocatalytic removal of the surfactant and synergy between silver and vanadia Damien P. Debecker,a Romain Delaigle,a Mélissa M. F. Joseph,a Chrystel Faure,b Eric M. Gaigneauxa a

Université catholique de Louvain, Unité de catalyse et chimie des matériaux divisés, Croix du Sud, 2/17, 1348 Louvain-la-Neuve, Belgium b Université Bordeaux 1, Centre de Recherche Paul Pascal (CNRS), Av. Dr. Schweitzer, 33600 Pessac, France

Abstract Silver nanoparticles prepared at room temperature in multilayer organic vesicles are deposited onto a V2O5/TiO2 support to produce a bifunctional oxidation catalyst performing very well in the total oxidation of benzene. The synergy between Ag and V2O5 takes place only when the surfactant is eliminated by in situ calcination. This paper shows that vanadium oxide autocatalytically promotes the combustion of this organic matter. This effect allows treating the catalyst at lower temperature which leads to an enhanced doping effect of the small tailored Ag nanoparticles, presumably because sintering is avoided. Keywords: amphiphilic surfactant, synergy, VOC abatement, air pollution, metal

1. Introduction The preparation of catalysts with small metal nanoparticles (NPs) is of primary importance for many chemical reactions [1]. The main limitation of classical preparation methods is the difficulty to control the size of the NPs [2]. Indeed, the formation of the metal phase often relies on a treatment at high temperature (e.g. reduction) during which sintering phenomena are hardly controlled. In turn, sintering is the main cause of deactivation of many metal nanoparticle-based catalysts [3]. (CH2-CH2-O)x-H N (CH2-CH2-O)y-H

Scheme 1. Chemical formula of the Genamin T020 surfactant (x+y = 2).

A new route for the preparation of supported Ag NPs catalysts was proposed recently [4]. Silver NPs are first prepared by the spontaneous reduction of Ag+ inside organic microreactors called “onions” [5]. Onions are organic vesicles made of closelypacked concentric bilayers of surfactant (Scheme 1). The formation of silver nanoparticles takes place at room temperature under the sole reducing effect of the surfactant. The vesicles loaded with ~5 nm sized Ag NPs can then be transferred onto a support (TiO2 or V2O5/TiO2) by a simple impregnation step followed by evaporation and drying. The catalysts were used as such in the total oxidation of benzene. It was evidenced [4] that Ag NPs take part in the catalytic reaction, either as active phase (when deposited on TiO2) or as a promoting agent for the already very active V2O5/TiO2

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catalyst [6]. The presence of the surfactant however hinders the access to the surface. So high performances were only observed once the surfactant was removed in situ by increasing the reaction temperature over the ignition temperature of the surfactant. However, by doing so, silver nanoparticles could presumably sinter, thereby losing the benefits of their tailored preparation. Here, the removal of the surfactant is inspected. The temperature at which the removal of the surfactant occurs is determined. The effect of vanadium oxide on this process is investigated and the impact of the treatment on the V2O5-Ag synergy is assessed.

2. Experimental 2.1. Catalysts preparation 150 mg of GT020 was mixed with 150 mg of AgNO3 solution (0.44 M) and immediately sheared with a spatula in a glass vial until a TiO2 V2O5 Ag (%) homogeneous paste was obtained. The paste was allowed to incubate for 40 min (at ~25°C). A (%) (%) precise amount of this paste (~100 mg) was TiO2 100.0 dispersed into 2 ml of water to obtain a colloidal AgTiO2 96.4 0.3 suspension. Impregnation was performed by magnetically stirring for 2 hours 1g of TiO2 95.5 4.5 V2O5-TiO2 (Degussa P25) or V2O5/TiO2 (prepared as in [7]) 4.0 0.3 Ag V2O5-TiO2 90.7 in 100 ml of water after the addition of the suspension of silver nanoparticles-loaded onions. Water was then evaporated in a rotavapor (50°C; ~100 mbar) and the material was dried for one night at 80°C. Table 1. Composition of the investigated materials as measured by ICP-AES.

2.2. Catalysts characterization The thermogravimetric analysis (TGA) were carried under a 100 ml flow of dry air on a Mettler Toledo TGA/SDTA 851 apparatus connected online with a mass spectrometer (MS) from Pfeiffer Vacuum (Thermostar) during a 10°C/min temperature program.

2.3. Catalysts evaluation Catalytic tests were performed in a fixed-bed micro-reactor [7] operating at atmospheric pressure with 200 mg of catalyst powder diluted in 800 mg of inactive glass spheres. The catalysts was brought to a “pre-treatment” temperature (PT) for 5h under a 200 ml.min-1 flow (VVH = 37000 h-1) of 80:20 vol % of He (Praxair; 99.996%) and O2 (Praxair; 99.995%). Then the reactor was cooled to the reaction temperature and 100 ppm of benzene (Praxair) was added to the flow. The catalyst was stabilized for at least 60 min before determining the conversion by on-line gas chromatography [7].

3. Results and discussion The composition of the materials is given in Table 1. The mass balance reached 100±1% for TiO2 and V2O5-TiO2. For catalysts impregnated with onions, the amount of surfactant can be evaluated by the mass needed to complete the mass balance (~4%). TGA coupled with a MS allowed following the evolvement of water, carbon dioxide and nitrogen dioxide. Figure 1 (left A) shows that the main weight loss occurs around 285°C. A shoulder is noticed around 415°C. No such peak is observed on TiO2. Figure 1 (left B) unambiguously links this weight loss to the combustion of the surfactant. All combustion products are detected in parallel with the weight loss. The

Ag-V2O5 /TiO2 total oxidation catalyst

807

top of the peaks of CO2 and NO2 corresponds to 283°C and their shapes closely fit the shape of the weight loss derivative peak. The NO2 peak is less resolved because the amount of nitrogen in the sample is much lower than the amount of carbon (22 atoms of C for 1 atom of N; Scheme 1). Evolvement of water starts at a slightly lower temperature (267°C). Water not fully eliminated by the drying step (only 80°C) could be released at the early stage of surfactant burning in addition to the production of water through the oxidation of the organic molecule. No peak of CO2, H2O or NO2 was observed when the same experiment was carried out with the bare TiO2 support (results not shown).

Figure 1. TGA-MS experiments on (left) Ag-TiO2 and (right) Ag-V2O5-TiO2. (A) Thermogravimetric analysis (TGA) and derivative TGA of the catalysts. Arrows indicate the ordinate axis. The dotted line is the derivative TGA obtained with TiO2 or V2O5-TiO2. (B) CO2 (m/z = 44), H2O (m/z = 18) and NO2 (m/z = 46) evolvement measured from Ag-promoted catalysts during TGA by online MS (signal magnification is indicated in brackets).

This ascertains that the combustion of the surfactant mainly occurs at 285°C. Below this temperature, the silver nanoparticles are not exposed to the reactive gas phase because the surfactant still covers them. Actually, Ag NPs proved to be active only from ~350°C [4]. Importantly, a fraction of the surfactant is only removed at higher temperature, since a peak is also observed around 415°C. So further “cleaning” of the surface at higher temperature could likely lead to higher accessibility of the NPs and higher activity. Conversely, it was noticed that the activity of the NPs at 350°C decreased after that the catalyst was brought to 400°C [4]. Sintering is a plausible explanation to this observation. The control on the size and the shape of the particles – that is one of the main interest of the synthesis via onion vesicles [5] – would then be lost. In the Ag-V2O5/TiO2 catalyst (Fig. 1 right), the weight loss falls at a much lower temperature than in the case of Ag-TiO2. Here the combustion of the surfactant mainly occurs at 230°C: a downward shift of more than 50°C is noticed. The second weight loss peak is less marked and shifted downwards by about 70°C (down to ~345°C). This result indicates that the surfactant burns at lower temperature when deposited onto the V-containing catalyst than onto the bare TiO2. This is confirmed by the MS data. The peaks of the three main combustion products appear centered at a temperature

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Benzene conversion (%) -

systematically lowered by about 55°C as compared to the V-free formulation. Both weight loss peaks and combustion products evolvement peaks are sharper and the shoulder appearing at higher temperature is reduced. The presence of V speeds up the removal of the surfactant by calcination. The same organic molecule burns at different temperatures depending on the nature of support onto which it is deposited. This effect of vanadia can be linked to its well-known ability to catalytically oxidize organics. V2O5 exhibits labile surface oxygen atoms which act as oxidant for organic compounds. Reduced V centers are then re-oxidized by gaseous molecular oxygen, thus completing the so called Mars & van Krevelen catalytic cycle [8]. This is why V-based catalysts are widely used as total oxidation catalysts for volatile organic compounds [6]. It was earlier observed that an organic deposit formed on V2O5/TiO2 catalysts had an ignition temperature around 250°C [7]. The V2O5/TiO2 and Ag-V2O5/TiO2 catalysts were tested at 250°C after a pretreatment (PT) at 450°C or 350°C (Figure 2). Even if Ag NPs are themselves inactive at 250°C, they were shown to promote the activity of the vanadia catalyst (synergy) [4]. 450°C would have been chosen to ensure complete removal of the surfactant if one would not be aware of the 40 effect of vanadia. From our TGA experiments, it appears that a treatment at 350°C is sufficient to remove completely the surfactant from the 20 Ag-V2O5/TiO2 catalyst. The reference V2O5/TiO2 catalyst treated at 350°C performs as expected from previous studies [6]. When treated at 450°C, 0 V2O5/TiO2 is slightly more active (presumably PT @ 450°C PT @ 350°C better oxidized). The Ag-promoted catalyst is in any case more active. Importantly, when the Figure 2. Benzene conversion measured at 250°C with (grey columns) V2O5/TiO2 and PT is done at lower temperature (350°C) the extent of the doping effect is significantly (black column) Ag-V2O5/TiO2. higher.

4. Conclusion Easier removal of the surfactant – as autocatalytically favored by V2O5 – leads to a facilitated access to the catalyst surface and to the nanoparticles. Upon a moderate thermal treatment, Ag-V2O5/TiO2 catalysts exhibit better performances. This effect is likely attributable to the fact that small NPs prepared via the “onions” do not sinter during the pre-treatment procedure and optimally develop the synergy with V2O5.

Acknowledgments The authors acknowledge the UCLouvain and the FNRS of Belgium for the support and the position of D.P. Debecker. The authors acknowledge the Service public fédéral de programmation politique scientifique (Belgium) for its support via the “Inanomat” IUAP, and the European Science Foundation for its support in the Cost Action D41.

References 1. 2. 3.

R.J. White, R. Luque, V.L. Budarin, J.H. Clark, D.J. Macquarrie, Chem. Soc. Rev., 38 (2009) 481. S. Eriksson, U. Nylén, S. Rojas, M. Boutonnet, Appl. Catal. A, 265 (2004) 207. G. Boskovic, N. Dropka, D. Wolf, A. Bruckner, M. Baerns, J. Catal., 226 (2004) 334.

Ag-V2O5 /TiO2 total oxidation catalyst 4. 5. 6. 7. 8.

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D.P. Debecker, C. Faure, M.E. Meyre, A. Derre, E.M. Gaigneaux, Small, 4 (2008) 1806. C. Faure, A. Derre, W. Neri, J. Phys. Chem. B, 107 (2003) 4738. R. Delaigle, D.P. Debecker, F. Bertinchamps, E.M. Gaigneaux, Top. Catal., 52 (2009) 501. D.P. Debecker, R. Delaigle, P. Eloy, E.M. Gaigneaux, J. Mol. Catal. A, 289 (2008) 38. J.C. Vedrine, G. Coudurier, J.M.M. Millet, Catal. Today, 33 (1997) 3.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Controlled synthesis of porous heteropolysalts used as catalysts supports Sébastien Paula,b,e, Andres Miňob,c,e, Benjamin Katryniokb,c,e, Elisabeth BordesRichardb,d,e, Franck Dumeignilb,c,e a

ECLille, UCCS, F-59650 Villeneuve d’Ascq, France Univ Lille Nord de France, F-59000 Lille, France c USTL, UCCS, F-59650 Villeneuve d’Ascq, France d ENSCL, UCCS, F-59652 Villeuneuve d’Ascq, France e CNRS, UMR 8181, F-59650 Villeneuve d’Ascq, France b

Abstract The preparation conditions of heteropolycompounds (HPC) dramatically influence the morphology of the crystals and therefore the heteropolysalts textural properties. When HPC are used as supports it is likely to have a strong influence on the catalytic performances. In this work we try to clarify this point using a perfectly stirred instrumented semi-batch reactor in order to prepare a series of Cs3PMo12O40 Keggin heteropolysalts. Preparation conditions (namely reactants concentrations, reaction and maturation temperature, addition rate and maturation time) have been varied following an experimental design (Hadamard matrix). The aim was to rule out the influence of the preparation conditions on the textural properties of the HPC, which were determined by nitrogen adsorption using the BET and BJH methods. The results show the excellent reproducibility of the supports preparation procedure, as well as the strong effect of the preparation conditions on the textural properties of HPC. Keywords: heteropolycompounds, textural properties, preparation conditions, controlled synthesis

1. Introduction Previous studies have shown that a strict control of the preparation conditions is necessary to control the crystals morphology of heteropolycompounds (HPC) and therefore their textural properties (i.e. specific area, porous volume and pore size distribution) [1,2]. In this work we try to clarify this point using a perfectly stirred instrumented semi-batch reactor in order to prepare a series of Cs3PMo12O40 Keggin heteropolysalts starting from aqueous solutions of H3PMo12O40 and Cs2CO3.

2. Experimental 2.1. HPC preparation The synthesis of Cs3PMo12O40 consists of a simple ionic exchange between the protons of the heteropolyacid H3PMo12O40 and Cs2CO3. The heteropolyanionic Cs salt is insoluble in water and precipitates as follows: 2 H3PMo12O40 + 3 Cs2CO3 → 2 Cs3PMo12O40 + 3 H2O + 3 CO2

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These syntheses were carried out in a specially designed perfectly-mixed instrumented reactor permitting a constant and controlled addition rate of the reactants. The reactants solutions were both thermostated at reaction temperature (one in the reactor and the added reactant in a separate vessel) before starting the addition which rate was controlled by a peristaltic pump. During the maturation, the reactor was kept under constant stirring and at constant temperature (the same as during the reaction). Preparation conditions have been varied following an experimental design (Hadamard matrix) as shown in Table 1. Table 1. Experimental design. Support reference

Reactants concentrations (mol/L)

Reaction and maturation temperature (°C)

Addition rate (mL/min)

Maturation time (min)

S1

0.1

45

55

30

S2

0.05

45

55

120

S3

0.05

30

55

120

S4

0.1

30

25

120

S5

0.05

45

25

30

S6

0.1

30

55

30

S7

0.1

45

25

120

S8

0.05

30

25

30

In this work the cesium carbonate solution was added to the heteropolyacid solution; stoichiometric conditions were used and the following thermal treatment was applied to the supports under static air: 200°C for 2h, then the temperature was increased to 300°C following a ramp of 100°C/h and kept at 300°C for 3h. The oven was then switched off and the temperature decreased during the night. The treated product was recovered the next day. The supports were labeled Si as stated in Table 1.

2.2. Characterization The textural properties were determined from N2 adsorption-desorption isotherm at 77K using ASAP 2010 Micromeritics apparatus after degassing the solids at 200°C for 2 hours. Surface areas, and porous volumes, and pore size distribution, were obtained by BET [3] and BJH [4] methods, respectively.

3. Results and discussion Table 2 presents the textural properties of the prepared supports. As expected the specific surface area, the porous volume and the pore size distributions varied in broad ranges depending on the preparation conditions used. To test the reproducibility of the procedure, several supports were prepared twice. As an example illustrating the excellent reproducibility obtained the results for S4 and S4’ are given in Table 3.

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Controlled synthesis of porous heteropolysalts used as catalysts supports Table 2. Textural properties of the supports. Pore size distribution (in % of the porous volume)*

Support reference

SBET (m2/g)

Porous volume (mL/g)

Mean pore diameter (nm)

Dp<5

5
10
Dp>20

S1

59

0.172

11.2

14

15

20

51

S2

110

0.182

7.5

31

6

12

51

S3

82

0.161

7.4

32

6

13

49

S4

63

0.188

9.7

23

10

28

39

S5

92

0.144

7.1

30

6

14

50

S6

122

0.163

5.4

42

11

45

2

S7

57

0.149

12.7

7

18

14

62

S8

103

0.159

7.2

31

7

17

45

* Pores diameters Dp in nm Table 3. Reproducibility tests. Pore size distribution (in % of the porous volume)*

Support reference

SBET (m2/g)

Porous volume (mL/g)

Mean pore diameter (nm)

Dp<5

5
10
Dp>20

S4

63

0.188

9.7

23

10

28

39

S4’

62

0.142

9.0

21

8

23

48

* Pores diameters Dp in nm

The effects of the preparation conditions studied over the textural properties of the HPC were calculated from the results of the experimental design. They are listed in Tables 4 and 5. These values represent the half of the mean change observed in a given response (i.e. a particular textural property) when a considered preparation condition is moved from its lower (-1) to its upper level (+1). For instance, using 0.1M reactants concentrations instead of 0.05M leads to a mean decrease of 26.8 m2/g of the surface area. Table 4. Effects of the preparation conditions on the textural properties. Level

Effects

-1

+1

SBET (m2/g)

Porous volume (mL/g)

Pore mean diameter (nm)

Reactants concentrations

0.05 mol/L

0.1 mol/L

-13.4

+0.003

+0.7

Reaction and maturation temperature

30°C

45°C

-3.7

-0.003

+0.5

Addition rate

25 mL/min

55 mL/min

+4.5

+0.005

-0.3

Maturation time

30 min

120 min

-5.1

+0.005

+0.3

Preparation conditions

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S. Paul et al.

The reactants concentrations is the more influent parameter on the specific surface. Using 0.05M instead of 0.1M leads to a mean increase of 26.8 m2/g of the specific surface. To increase the surface it is preferable to work with a short maturation time, high addition rate and low reaction and maturation temperature. The effects of the preparation conditions studied on the total porous volume are not significant. However using high reactants concentrations and reaction and maturation temperature will increase the mean pore diameter. Table 5. Effects of the preparation conditions on the pore size distribution. Preparation conditions

Effects on the pore size distribution (in % of the porous volume)*

Level -1

+1

Dp<5

5
10
Dp>20

Reactants concentrations

0.05 mol/L

0.1 mol/L

-6

-5

+3

-2

Reaction and maturation temperature

30 °C

45 °C

+3

+2

0

0

Addition rate

25 mL/min

55 mL/min

+6

+2

+2

-4

Maturation time

30 min

120 min

-4

+9

+5

+5

* Pores diameters Dp in nm

It is clear from Table 5 that changing the preparation conditions can notably influenced the HPC pore size distribution. For instance increasing maturation time increases the pore diameters whereas increasing the addition rate favors the microporosity. This is in good agreement with our previous results [1].

4. Conclusions This work leads to the conclusion that using low reactants concentrations is important to get a support with a high surface area. The total porous volume is not significantly influenced by the preparation conditions studied. However it possible to get a mesoporous support by choosing low reactants concentrations, high reaction and maturation temperature, maturation time and and addition rate. This corresponds to S2 which has a 110 m2/g specific surface, 50% of the porous volume provided by pores of Dp>20 nm and 18% by pores of Dp in the range 5 to 20 nm. The reproducibility of the preparation procedure is excellent.

References 1. 2. 3. 4.

S. Paul, V. Dubromez, D. Vanhove, 2002, Stud. Surf. Sci. Catal., 143, 481 D. Lapham, J. B. Moffat, 1991, Langmuir, 7, 2273 S. Brunauer, P. H. Emmet, E. Teller, 1938, J. Am. Chem. Soc., 60, 309 E. P. Barret, L. G. Joyner, P. O. Halenda, 1951, J. Am. Chem. Soc., 73, 373

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Influence of the sodium-based precipitants on the properties of aluminum-doped hematite catalysts for ethylbenzene dehydrogenation Alleyrand Sérgio Ramos Medeiros, Maria do Carmo Rangel a

GECCAT, Instituto de Química, Universidade Federal da Bahia,Campus Universitário de Ondina, 40170-290, Salvador, Bahia, Brazil. E-mail: [email protected]

Abstract Different sodium-based precipitants (sodium carbonate and sodium hydroxide) were compared to ammonium hydroxide, during the preparation of aluminum-doped hematite, in order to avoid ammonium hydroxide, which is not allowed in industrial preparations, due to its toxicity to humans and to the environment. The study aims to find alternative dopants to replace chromium in commercial catalysts for ethylbenzene dehydrogenation to produce styrene, a high value chemical. The different precipitants did not change the kind of phase produced (hematite) but led to textural and catalytic changes. Sodium hydroxide is the most suitable precipitant to prepare the solids, leading to the catalyst with the highest specific surface area during reaction and the highest activity and selectivity, among the samples studied. Keywords: aluminum, hematite, sodium hydroxide, styrene, ethylbenzene

1. Introduction Among the several routes to obtain styrene, ethylbenzene dehydrogenation in the presence of steam is by far the most important one. This reaction is responsible for the global production of more than 90% of this monomer, used as precursor of various resins and polymers [1]. In industrial processes, the reaction is carried out over hematite-based catalysts doped with chromium and potassium oxides which are active and selective but deactivates with time [2], besides being toxic and harmful to the environment. Therefore, a lot of work has been devoted to find alternative dopants to replace chromium [2-7]. In a previous work [7], we have found that aluminum is a convenient dopant to replace chromium in hematite-based catalyst for ethylbenzene dehydrogenation. In order to improve the preparation of this solid, a comparison of the effect of sodium carbonate and sodium hydroxide with ammonium hydroxide on the properties of hematite-based catalysts is done in the present work. The study intends to avoid ammonium hydroxide which is normally used in laboratory preparations but is not allowed in commercial preparations due to its toxicity to human and to the environment.

2. Experimental Samples were prepared by adding iron nitrate (0.25 M) and aluminum nitrate (0.025 M) solutions simultaneously with a sodium hydroxide (AN sample), sodium carbonate (ANC) or ammonium hydroxide (A) solution (25%) to a beaker with water at room temperature. After rinsing with water and drying at 120°C, the samples were calcined at 700°C under nitrogen flow, for 2 h. Also, aluminum-free samples were prepared (F, FN, FNC samples) for comparison.

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The iron and aluminum contents were determined by inductively coupled plasma atomic emission spectroscopy in an Arl 3410 model machine. X-ray diffractograms (XRD) were recorded at room temperature with a Shimadzu model XD3A instrument using CuKα radiation generated at 30 kV and 20 mA. The specific surface areas were measured in a Micromeritics model ASAP 2020C equipment on samples previously heated under nitrogen (150°C, 2 h). The temperature programmed reduction (TPR) experiments were performed in a Micromeritics model TPD/TPO 2900 equipment, using a 5% H2/N2 mixture. The catalysts performance was evaluated using a fixed-bed microreactor, providing there is no diffusion effect. The experiments were carried out at 480°C and 1 atm, employing a steam to ethylbenzene molar ratio of 10. The reactor, containing the catalyst, was heated under nitrogen flow (60 ml.s-1) up to the reaction temperature. Then the feed was interrupted and the reaction mixture was introduced. The reaction mixture was obtained by passing a nitrogen stream by a saturator with ethylbenzene and then by a chamber where it was mixed with steam. The gaseous effluent was collected in a condenser and the organic phase was analyzed by gas chromatography, using a CG-35 instrument. The spent catalysts were characterized by XRD and specific surface area measurements.

3. Results and discussion For all samples, the aluminum to iron molar ratio was close to the expected one (0.1). From the X-ray diffractograms (not shown) hematite (α-Fe2O3) was detected for all catalysts, indicating that the different precipitants did not affect the kind of phase produced. During the ethylbenzene dehydrogenation, the catalysts went on phase transition and magnetite (Fe3O4) was produced. The same behavior was noted for the aluminum-free samples. No aluminum-containing phase was detected. As aluminum has ionic radius similar to the iron atom, it is expected to go into the iron oxide lattice [8] rather than to segregate as another phase. The specific surface areas are shown in Table 1. One can note that both aluminum and sodium increases these values, aluminum being more efficient. Sodium carbonate was the most efficient precipitant to increase the specific surface area, followed by sodium hydroxide, regardless of the presence of aluminum. Also, aluminum increased these values independently of the precipitants. The sample with the highest specific surface area was obtained with sodium carbonate in the presence of aluminum. These finding can be assigned to the role of aluminum as textural promoter, as found previously in several catalysts [7, 9, 10]. This action has been related to a surface phenomenon in which aluminum acts as a spacer, keeping the particles apart from each Table 1. Specific surface area before (Sg) and after reaction (Sg*), activity (a), intrinsic activity (a/Sg) and selectivity of the catalysts to styrene (S) and the drop in ethylbenzene conversion (ΔX) during ethylbenzene dehydrogenation performed at 480°C and 1 atm. Samples Sg (m2.g-1) Sg* (m2.g-1) a.107 (mol.g-1. s-1) a/Sg.108(mol.s-1.m-2) ΔX (%) S (%)

F 5.4 5.3 1.9 3.6 13.8 97

FN 6.0 6.2 1.9 3.1 3.7 92

FNC 9.0 10 2.0 2.0 2.6 87

A 7.2 7.9 1.7 2.2 10.7 96

AN 9.0 9.3 5.7 6.1 19.1 96

ANC 11 6.0 2.9 4.8 7.1 89

Influence of the sodium-based precipitants

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other [10]. However, it can also be associated with aluminum inside the iron particles as an occluded phase, which causes strains in the lattice and shifts the equilibrium size particle toward smaller particles, since the ratio of strain to the surface effects becomes greater for larger particles [9]. Sodium seems to play a similar role. During ethylbenzene dehydrogenation, the specific surface area was not changed except for the catalyst with aluminum prepared with sodium carbonate, which was more susceptible to sinterization. The kind of precipitant also changed the reducibility of the samples, as inferred by the temperature programmed reduction curves (not shown). The aluminum-free sample prepared with ammonium hydroxide (F) showed a peak centered at 480°C, which is assigned to the reduction of Fe3+ to Fe2+ species and two high temperature peaks above 540°C, related to the reduction of Fe2+ species to produce metallic iron on the surface and in the bulk [11]. The first peak was not affected by changing ammonium hydroxide by sodium hydroxide while the use of sodium carbonate shifted it to higher temperature, showing the role of this precipitant in delaying reduction. A similar behavior was noted for the high temperature peaks. In the presence of aluminum, however, the effect of the precipitants was quite different. For the sample prepared with ammonium hydroxide, aluminum shifted the peaks to higher temperatures, indicating its role in delaying reduction, as found in a previous work [7]. However, the use of sodium carbonate and sodium hydroxide caused a shift to lower temperatures, indicating that sodium makes the reduction easier. Also, the reduction of Fe2+ species to metallic iron was easier for the samples prepared with sodium-containing precipitants. The activities of the catalysts after 7 h of reaction are shown in Table 1. One can see that the kind of precipitant did not affect the activity of the free-aluminum samples but decreased the intrinsic activity which was compensated by the increase of specific surface area. On the other hand, for the sample prepared with ammonium hydroxide aluminum decreased the intrinsic activity and increased the specific surface area, resulting in an increase of activity. For the samples with aluminum, the use of sodiumcontaining precipitants led to an increase of both intrinsic activity and specific surface areas, resulting in more active catalysts; sodium hydroxide is the most efficient precipitant which produced the most active solid. The stability of the catalysts is expressed as the drop in conversion calculated as the difference between the initial and final conversion. It can be noted that the use of sodium-containing precipitants largely increased the stability of the catalysts for the aluminum-free samples. However, in the aluminum presence, only sodium carbonate is able to decrease the conversion drop. The selectivities to styrene are also shown in Table 1. One can see that aluminum did not affect these values for the sample prepared with ammonium hydroxide. The same occurred with the solid with aluminum obtained with sodium hydroxide. However, sodium carbonate is harmful to the catalyst selectivity regardless the presence of aluminum. These results show that sodium hydroxide is the most suitable precipitant to prepare aluminum-doped hematite for ethylbenzene dehydrogenation to produce styrene. The catalyst obtained has the highest activity and selectivity to styrene among the samples studied, a fact which can be related to its highest intrinsic activity as well as to its highest specific surface area during reaction.

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4. Conclusions The use of sodium hydroxide and sodium carbonate, instead of ammonium hydroxide to obtain aluminum-doped hematite, led to the production of solids made of hematite containing aluminum. They have different textural and catalytic properties, as well as different resistance against reduction. Sodium hydroxide is the most suitable precipitant to prepare the solids, leading to the catalyst with the highest activity and selectivity to styrene among the samples studied. This can be related to its highest intrinsic activity and to its highest specific surface area.

References 1.

F. Cavani, F. Trifiro, 1995, Alternative processes for the production of styrene, Appl. Catal. A: Gen., 133, 219-239. 2. Lee E.H., 1973, Iron oxide catalysts for dehydrogenation ethylbenzene in the presence of steam, Catal. Rev.- Sci. and Tech., 8, 285-305. 3. M. S. Ramos, M.de S.Santos, L. P. Gomes, A. Albornoz, M. C. Rangel, 2008, The influence of dopants on the catalytic activity of hematite in the ethylbenzene dehydrogenation, Appl.Catal. A: Gen., 341, 12-17. 4. M. S. Santos, A. Albornoz, M. C. Rangel, 2006, The Influence of the preparation method on the catalytic properties of lanthanum-doped hematite in the ethylbenzene dehydrogenation. Stud. Surf. Sci. Catal., 162, 753-760. 5. M. de S. Santos, S. Marchetti, A. Albornoz, M. C. Rangel, 2008, Effect of lanthanum addition on the properties of potassium-free catalysts for ethylbenzene dehydrogenation. Catal. Today , 133-35, 160-167. 6. H. E. L. Bonfim, A. C. Oliveira, M. C. Rangel, 2003, The effect of zinc on the catalytic activity of hematite in ethylbenzene dehydrogenation. React.Kinet. Catal. Lett., 80, 359-364. 7. A. C. Oliveira, A. Valentini, P. S. S. Nobre, J. L. G. Fierro, M. C. Rangel, 2003, Non toxic Fe-based catalysts for styrene synthesis. The effect of salt precursor and aluminum promoter on the catalytic properties. Catal. Today , 85, 49-573. 8. M. E. Dry, L.C. Ferreira, 1967, The distribution of promoters in magnetite catalysts, 7, 352-358 9. H. Topsoe, J. A. Dumesic, M. Boudart, 1973, Alumina as a textural promoter of iron synthetic ammonia catalysts, J. Catal., 28, 477-488. 10. J. Ladebeck and K. Kochloefl, in G. Poncelet et al., (editors), Scientific Bases for Preparation of Heterogeneous Catalysts. Elsevier Science B. V., Amsterdam. 1995. p. 1079. 11. H.-Y. Lin, Y.-W. Chen, C. Li, 2003, The mechanism of reduction of iron oxide by hydrogen, Thermochim. Acta, 400, 61-67.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Effect of the preparation method on the properties of hematite-based catalysts with lanthanum for styrene production Manuela de S. Santosa, Sérgio G. Marchettib, Alberto Albornozc, Maria do Carmo Rangela a

GECCAT-Universidade Federal da Bahia, Instituto de Química, Campus Universitário de Ondina, 40155-290, Salvador, Bahia, Brazil. e-mail: [email protected] b CINDECA, Facultad de Ciencias Exactas, Universidad Nacional de La Plata, 1900, 47 y 115, La Plata, Argentina c Instituto Venezolano de Investigaciones Científicas, Apartado 21 827, Caracas 1920-A Venezuela

Abstract The influence of the preparation method on the properties of lanthanum-doped hematite prepared with potassium carbonate was studied aiming to get catalysts for styrene production from ethylbenzene dehydrogenation. The most active catalyst was obtained by adding the reactants on water. This solid has the highest intrinsic activity, the highest resistance against reduction and the lowest conversion drop. These properties were related to the presence of potassium compounds on the surface and to the presence of Fe+3 species (active phase) stabilized in lanthanum oxide lattice. Keywords: lanthanum, iron oxide, potassium carbonate, styrene, ethylbenzene

1. Introduction The main commercial route to produce styrene, a high-value chemical, is the ethylbenzene dehydrogenation in steam presence. The industrial catalysts are hematite doped with potassium and chromium oxides, which have low price and are active and selective. However, they quickly deactivate with time [1, 2] and there is a need for investigating new systems, concerning formulation and textural and catalytic properties, the last ones closely related to the preparation methods. In previous works [3, 4] we have found that hematite with lanthanum, obtained with ammonium hydroxide, sodium carbonate and sodium hydroxide, is promising to ethylbenzene dehydrogenation. Ammonium hydroxide produced the best catalysts, but its use in industry is not allowed due to the environment and human health restrictions. Threfore, in the present work, we continue this investigation by studying the effect of the order of mixing the reactants on the properties of hematite with lanthanum,using potassium carbonate as precipitant.

2. Experimental Samples were prepared by the sol-gel method by hydrolysis of iron and lanthanum nitrate with a potassium carbonate solution (6 mol.L-1) to get solids with lanthanum to iron molar ratio of 0.1. The solids were then calcined under nitrogen flow at 600°C, for 2 h. The sample named CK was obtained by the addition of the reactants solutions on a beaker with water, under stirring. Other two samples were prepared by changing the order of mixing the reactants: the first one (CKM sample) was obtained by adding the

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precipitant on the solution of iron and lanthanum nitrate and the second one (MCK) by the inverse procedure. A reference sample was also prepared using ammonium hydroxide instead of potassium carbonate (LF sample). The X-ray diffraction (XRD) experiments were performed with a Shimadzu model XD3A instrument using CuKα radiation generated at 30 kV and 20 mA, using a nickel filter. The specific surface areas were measured in a Micromeritics model ASAP 2020C equipment on samples previously heated under nitrogen (150°C, 2 h). The temperature programmed reduction (TPR) was performed in a Micromeritics model TPD/TPO 2900 equipment, using a 5% H2/N2 mixture. X-ray photoelectron spectra (XPS) were obtained with a VG ESCALAB 220i-XL spectrometer with a MgKα X-ray radiation source (hν= 1253.6 eV) and a hemispherical electron analyzer, at 400 W. The Mössbauer spectra were obtained in transmission geometry with a 512-channel constant acceleration spectrometer. A source of 57Co in Rh matrix of nominally 50 mCi was used. Velocity calibration was performed against a 6 μm-thick α-Fe foil. All isomer shifts (δ) mentioned in this paper are referred to this standard. The catalysts were evaluated in a fixed-bed microreactor at 530°C and 1 atm, using a steam to ethylbenzene molar ratio of 10. The gaseous effluent was collected in a condenser and the organic phase was analyzed by a CG-35 chromatograph. The spent catalysts were characterized by XRD, specific surface area measurements and Mössbauer spectroscopy. The coke amount in spent catalysts was measured in a CS-200 LECO model equipment.

3. Results and discussion The use of potassium carbonate instead of ammonium hydroxide led to the production of different solids. While the sample prepared with ammonium hydroxide showed only hematite, those obtained with potassium carbonate showed several phases, whose identification by XRD was complicated by the coincidence of the peaks of the different phases. The catalyst prepared by adding the reactants on water (CK) was made off hematite and lanthanum oxide (La2O3) but the presence of a mixed oxide (FeLaO3), potassium ferrite (KFeO2) and potassium oxide (K2O) could not be discarded. On the other hand, La2O3 and KFeO2 were found in CKM sample and hematite and KFeO2 were detected in MCK sample; the other phases could not be confirmed. During ethylbenzene dehydrogenation, a phase transition occurs and magnetite was produced; however, it was not possible to distinguish the presence of the other phases. The use of potassium carbonate favored the production of solids with lower specific surface area, in comparison with that prepared with ammonium hydroxide (Table 1). The addition of the reactants to water (CK sample) led to the production of the solid Table 1. Specific surface area before (Sg) and after reaction (Sg*), activity (a), intrinsic activity (a/Sg), conversion drop (ΔX), selectivity to styrene (SS), benzene (SB) and toluene (ST) and coke deposited on the catalysts during ethylbenzene dehydrogenation. Samples Sg (m2.g-1) Sg* (m2.g-1) a.107 (mol.g-1. s-1) a/Sg.108(mol.s-1.m-2) ΔX (%) SS (%) SB (%) ST (%) Coke (%)

LF 115 10 8.9 8.9 1.5 96 0.74 2.7 1.09

CK 1.0 4.0 11 27 1.7 100 --3.81

CKM 3.2 4.0 6.1 15 5.0 100 --3.73

MCK 3.0 4.3 5.2 12 9.2 100 --2.32

Commercial --6.4 -3.6 97 0.8 1.8 1.36

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with the lowest specific surface area while no significant difference was found between the samples prepared by the other methods. After reaction, the solids showed higher values, indicating that the phase changes led to the formation of pores and/or particles of smaller sizes. The order of mixing the reactants also changed the reducibility of lanthanum-doped hematite, as inferred by the TPR curves (not shown). The samples prepared by adding the metallic precursors on the precipitant (MCK) or by the inverse procedure (CKM) showed similar profiles, with several reduction peaks; the last sample was the most susceptible to reduction. The low temperature peaks, in the range of 360 to 563°C, are related to the reduction of Fe3+ to Fe2+ species, as pointed out early [2-5]. In these samples, the multiplicity of peaks suggests the presence of Fe3+ species in different compounds, in accordance with the X-ray diffraction results. The two high temperature peaks occurred in the range of 600 to 850°C and are associated to the reduction of Fe2+ to Fe0 species on the surface and in the bulk, respectively [2-5]. On the other hand, the sample prepared by adding the reactants on water (CK) showed the most resistance against reduction with a single peak at 683°C with a shoulder at around 700°C and another peak at 886°C, showing that the Fe3+ species are stabilized in this solid. The first peak can be related to the reduction of Fe3+ species stabilized by lanthanum oxide La2O3, as inferred by Mössbauer spectroscopy. In fact, the spectrum of the CK sample showed two sextuplets, with hyperfine parameters typical of α-Fe2O3, the second one assigned to smaller particles. A central doublet was also noted, associated to superparamagnetic α-Fe2O3 and/or to paramagnetic ferric ions in La2O3 lattice. Taking into account the hyperfine parameters values, the presence of FeLaO3 and KFeO2 was discarded [6, 7]. The activity and selectivity of the catalysts were also affected by the order of mixing the reactants as well as by the presence of potassium. As shown in Table 1, potassium increased the activity and this can be related to an increase of the intrinsic activity (a/Sg). Also, the selectivity achieved 100% due to potassium and no benzene and toluene were produced. However, they produced higher amounts of coke than the potassium-free sample, indicating that lanthanum is much more able to prevent coke in the absence of potassium which, in turn, is less efficient than lanthanum. However, this coke seems not to be harmful to the catalysts, since there is no relationship between coke and the conversion drop. The catalyst prepared by adding the reactants on water (CK) was the most active one and this can be related to its highest intrinsic activity and also to the its highest resistance against reduction, due to the stabilization of Fe3+ species, which are responsible for the activity of the catalysts in the reaction. The XPS spectra of this sample revealed the presence of lanthanum (La/Fe= 0.884) on the surface as well as a high concentration of potassium (K/Fe= 12.229), which can be associated to the production of potassium oxide, which is believed to increase the activity of iron in ethylbenzene dehydrogenation [1, 8]. This catalyst was more active and selective than a commercial one (Table 1) which makes it a candidate for industrial applications.

4. Conclusions The addition of potassium increased the intrinsic activity of lanthanum-containing hematite, but its action depends on the preparation method. Concerning the order of mixing the reactants, the best method consists in adding the metallic precursor and potassium carbonate on water. The solid produced has the highest intrinsic activity, the highest resistance against reduction and the lowest conversion drop in ethylbenzene dehydrogenation to produce styrene. These properties were related to the presence of

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potassium compounds on the surface as well as to the stabilization of Fe3+ species, which are believed to be the active phase.

References 1. 2. 3. 4. 5. 6. 7. 8.

F. Cavani, F. Trifiro, 1995, Alternative processes for the production of styrene, Appl. Catal. A: Gen., 133, 219-239. M.S. Ramos, M. de S.Santos, L.P. Gomes, A. Albornoz, M.C. Rangel, 2008, The influence of dopants on the catalytic activity of hematite in the ethylbenzene dehydrogenation, Appl.Catal. A: Gen., 341, 12-17. M.S. Santos, A. Albornoz, M.C. Rangel, 2006, The influence of the preparation method on the catalytic properties of lanthanum-doped hematite in the ethylbenzene dehydrogenation. Stud. Surf. Sci. Catal., 162, 753-760. M. de S. Santos, S. Marchetti, A. Albornoz, M.C. Rangel, 2008, Effect of lanthanum addition on the properties of potassium-free catalysts for ethylbenzene dehydrogenation. Catal.Today , 133-35, 160-167. H.-Y. Lin, Y.-W. Chen, C. Li, 2003, The mechanism of reduction of iron oxide by hydrogen, Thermochim. Acta, 400, 61-67. A. Delmastro, D. Mazza, S. Ronchetti, M. Vallino, R. Spinicci, P. Brovetto, M. Salis, 2001, Synthesis and characterization of non-stoichiometric LaFeO3 perovskite, Mat. Sci. and Eng. B79 140-145. L.A. Boot, A.J. van Dillen, J.W. Geus, A.M. van der Kraan, A.A. van der Horst, F.R. van Buren, 1996, Mössbauer spectroscopic investigations of supported iron oxide dehydrogenation catalysts, Appl. Catal. A: Gen., 145, 389-405. G. Kettles, G. Ranke, R. Schlögl, 2002, Potassium-promoted iron oxide model catalyst films for the dehydrogenation of ethylbenzene: an example for complex model systems, J. Catal., 212, 104-111.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Low-organics method to synthesize silver nanoparticles in an aqueous medium N. Ballarini,a F. Cavani,a E. Degli Esposti1, Z. Sobalik2, J. Dedecek2 a

Dipartimento di Chimica Industriale e dei Materiali, Università di Bologna, Viale Risorgimento 4, 40136 Bologna, Italy. INSTM, Research Unit of Bologna: a partner of NoE Idecat, FP6 of the EU. b J. Heyrovský Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejškova 3, 182 23 Prague 8, Czech Republic.

Abstract In this report we describe the preparation of Ag nanoparticles (AgNP) in water by means of a modified (low-organics) Turkevich method, with the aim of verifying the feasibility of this procedure for the development of stable colloidal sols. The latter may be used for the deposition of AgNP over supports. Keywords: silver nanoparticles, nanoparticles quantification

1. Introduction The wet chemical reduction is one of the most common methods for making NPs [1]. The first reproducible synthesis was obtained by Turkevich, who prepared 20 nm Au particles by citrate reduction of [AuCl4]− [2]. In this approach the reducing agent is mixed with the metal precursor salt in the presence of stabilizing agents (ligands, polymers or surfactants), with the aim of preventing the agglomeration and formation of metal powder. Many factors affect the size of NPs, including the type of reducing agent, solvent, concentration, and temperature. The advantage of this procedure - over other techniques such as electrochemical synthesis, reduction of organic ligands in organometallic precursors or metal vapor chemistry - is its simplicity and reproducibility. Here we report on a preliminary study on the feasibility of a modified Turkevich procedure for the synthesis of an AgNPs colloid, a precursor for the preparation of supported catalysts.

2. Experimental 2.1. Ag colloid synthesis Silver nitrate (99.98%), sodium citrate (99%) and sodium ascorbic acid (99%), all supplied by Sigma Aldrich), were used without further purification. Colloidal solutions were prepared by following a modified Turkevich method [2,3]: an AgNO3 aqueous solution was heated to boiling point and organic reductants were added until the desired reductant / Ag molar ratio was obtained. All solutions became colored in few minutes after the addition of the reductant, ranging from dark green/ brown-red to gold/ochre. After the change of color, the solutions were kept boiling for 30 minutes. All solutions were then centrifuged (4000 rpm, 30 min) in order to separate the grayish suspension of Ag0. A trisodium citrate solution was used as mild organic stabilizer/reductant. In two preparations, a small amount of disodium ascorbate was also added in order to tune the reducing power.

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Two different concentrations of Ag+ ions were used - (0.3 mM and 1.0 mM) - and correspondingly two different quantities of reductant. Moreover, in order to explore different ways to control the characteristics of the particles, we repeated the synthesis using a higher amount of reductant at 283 K. In this case we kept the colloidal solution at 283K under stirring for 30 min, after observing the change in color. Table 1 shows the samples prepared, and the main parameters used for their synthesis. UV-vis absorption spectra were recorded on a Perkin-Elmer UV-Vis-NIR spectrometer Lambda 950, in a 1 cm optical path cuvette, with a data interval equal to 1 nm and a Scan Speed of 20 nm/min. Samples for SEM examinations were prepared by filtering a small amount of the colloidal solution on Anodisk™ membranes with a pore size of 20 nm. SEM micrographies of membranes loaded with AgNP were taken with an EVO ZEISS microscope, with an EHT of 25 kV. In order to limit filtering-related aggregation phenomena, after the preparation of samples a and b, we reduced the quantity filtered thus obtaining a better dispersion.

Synthesis code

Table 1. Main synthesis parameters of AgNPs colloids. [Ag+ ] (mM) Citrate / Ag + Ascorbate / Ag+

T (K)

a b c d e f

(mol/mol) 2.00 2.00 7.50 7.50 7.50 7.50

373 373 373 373 283 283

0.30 0.30 1.00 1.00 1.00 1.00

(mol/mol) 0.00 0.30 0.00 2.00 0.00 2.00

3. Results and discussion Figure 1 shows the UV-VIS spectra of the Ag colloid in the range 325 nm – 700 nm. The absorption band in a visible light region (350 nm – 550 nm, plasmon peak at around 400 nm) is typical for spherical silver NPs [4]. The plasmon peak position and the FWHM depend on particle mean diameter and on the extent of colloid aggregation [5].

Figure 1. UV-Vis spectra of samples a-d (see Table 1 for a description of the preparation procedure).

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In order to monitor the stability of silver colloids, we measured the UV-vis absorption of the colloids over several months. Colloidal solutions were stored at room temperature, with no exposure to light, and over a period of three months there was no change in absorbance and peak position. As the particles increased in size, the absorption peak generally shifted toward the red wavelengths [6]; the increased absorption indicates an increased amount of silver NPs. An unchanged peak position indicates that particles do not undergo aggregation or coalescence phenomena, thus confirming the stabilization power of citrate. Moreover, an unchanged maximum absorption indicates an absence of nucleation and after-synthesis growth phenomena, thus leading to stable and predictable materials. SEM micrographs (Figure 2, reporting images for selected samples) showed pseudospherical NPs with a mean diameter of 50 nm for synthesis a and b and around 20 nm for synthesis c and d; low-temperature synthesis e end f showed a wider dispersion in diameters, ranging from 20 to 50 nm.

Figure 2. SEM images of samples b (left) and e (right), after deposition over alumina membranes.

TEM pictures (Figure 3) made it clear that NPs with a diameter of around 50 nm are actually made up of an aggregation of smaller particles with diameter of around 10 nm. Therefore, citrate can play a good stabilizing role in medium range interactions, while preventing the interactions of aggregates during and after synthesis. However, at the molar ratio that we used, it lacks the capability of stabilizing NPs during the growth. During synthesis, some Ag was lost at the centrifugation step, so we developed a method to quantify the yield in colloidal silver. We used the free software MiePlot [7] to calculate the single AgNP attenuation efficiency factor Qext in a water medium; then we calculated the number and density of particles and finally the weight of Ag0 in colloidal form that was available in solution (Figure 4) [8]. Step 1 in this procedure (mean diameter evaluation by SEM) may be skipped if synthesis is routinely performed with a well-established method and a calibration has been made between UV spectra and mean diameter obtained. In this case, this quantification method may be an attractive alternative to TEM analysis as “quality control” routine. Table 2 summarizes the calculated yield for each synthesis procedure. It is shown that the citrate-only synthesis led to better yields, thanks to the milder reducing power of citrate, whereas the synthesis conducted with an additional small amount of ascorbate showed lower yields in AgNP, probably due to a reduction rate which was too fast and was not matched by the citrate stabilization rate.

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Figure 3. TEM pictures of sample b. Table 2. Calculated yields for AgNP produced by the various preparation methods. Synthesis code

AgNP yield, % 52 36 57 28 10 16

A B C D E F

AgNP synthesis

1. Filtration on Anodisk 2. UV spectrum

Colloidal Ag 3. MiePlot

SEM analysis

Mean diameter calculation

Peak position Absorbance (max)

Qext calculation

Ag separated by centrifugation 4. Colloidal Ag quantification

Figure 4. Conceptual scheme of AgNP quantification method.

References [1] L. Durán Pachón, G. Rothenberg, 2008, Appl. Organomet. Chem. 22, 288-299. [2] J. Turkevich, P. C. Stevenson, J. Hillier, 1951, Discuss. Faraday Soc., 11, 55. [3] J. Kimling, M. Maier, B. Okenve, V. Kotaidis, H. Ballot, A. Plech, 2006, J. Phys. Chem. B, 110, 15700-15707. [4] R. He, X. Qian, J. Yin, Z. Zhu, 2002, J. Mater. Chem., 12, 3783. [5] S. Yamamoto, K. Fujiwara, H.S. Watari, 2004, Anal. Sci. 20, 1347-1352. [6] Y. Xia, N.J. Halas, 2005, MRS Bull., 30, 338-343. [7] The “MiePlot” Software is available at: www.philiplaven.com/mieplot.htm. [8] M.M. Maye, L. Han, N.N. Kariuki, N.K. Ly, W.B. Chan, J. Luo, C.-J. Zhong , 2003, Anal. Chim. Acta 496, 17-27.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Clusters as precursors of nanoparticles supported on carbon nanofibers Deborah Vidick, Sophie Hermans and Michel Devillersa a

Université catholique de Louvain, Unité de chimie des matériaux inorganiques et organiques, Place L. Pasteur 1, 1348 Louvain-la-Neuve, Belgium [email protected]

Abstract Carbon nanofibers were functionalized following two distinct strategies, in order to introduce anchors for the grafting of clusters at their surface. In the first case, chelating phosphine groups were built using a multi-step synthesis, while in the second case, ammonium groups were introduced to create positive charges at the surface. To prove the success of the functionalization, a [Ru5PtC(CO)14(COD)] cluster was covalently grafted onto CNF-PPh2 while a negatively charged (NEt4)[FeCo3(CO)12] cluster was incorporated on CNF-NMe3+. Nanometer-sized RuPt and FeCo particles were evidenced by TEM. Keywords: carbon nanofibers, molecular clusters, functionalization, grafting

1. Introduction In recent years, the use of carbonyl clusters as sub-colloïdal defined metallic entities has received much interest [1]. These molecules may act as models for heterogeneous catalysts by virtue of the ‘cluster-surface analogy’ and make the connection between the molecular level and the bulk. Indeed, a cluster is defined as a multi-center transition metal complex including at least three metal atoms, linked by a minimum of two metalmetal bonds, and stabilized by a layer of organic ligands, usually CO. Clusters can be used as precursors for the preparation of heterogeneous nano-structured catalysts, by incorporation on a support and thermal activation [2]. To improve the control on the catalysts synthesis, the incorporation of the cluster on the support can be optimized through surface functionalization. The ligands sheath can then be removed selectively to yield supported nanoparticles of controlled size and composition.

2. Experimental section All manipulations were carried out under nitrogen by using Schlenk techniques. The solvents were distilled before use and stored under nitrogen on molecular sieves, and the obtained products were stored under Ar. [Ru5PtC(CO)14(COD)] (1) and (NEt4)[FeCo3(CO)12] (2) were prepared according to published procedures [3,4]. All other mentioned reactants were commercially available and used as received. The support was carbon nanofibers of the type PR24-XT-LHT-OX (noted CNFox) from Applied Sciences Inc.

2.1. Functionalization with chelating phosphines [5] The acidity of this support was estimated to be ~200 mmol/100g from XPS analysis. In a 100 mL Schlenk flask, 1g of CNFox was introduced with 5 ml SOCl2 and 40 ml of toluene. The mixture was refluxed (120°C) for five hours and filtrated. The obtained powder (CNF-Cl) was extensively washed with toluene and dried under vacuum. In

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the second step, 900 mg of CNF-Cl was refluxed (120°C) with 1.3 equivalents of ethylenediamine in 30 mL toluene for four hours. Then, the mixture was filtrated, and the obtained powder (CNF-NH2) was extensively washed with toluene and dried under vacuum. In the third step, 2.5 equivalents of CH2O and HPPh2 were introduced in a 100 mL Schlenk flask together with 7.5 mL of methanol. The mixture was stirred at 70°C for ten minutes and was then cooled to room temperature. 800 mg of CNF-NH2 were concurrently placed in a 100 mL Schlenk flask with 12.5 ml of methanol. When the first mixture reached room temperature, it was added to the CNF-NH2 suspension and stirred for fifteen minutes at room temperature. Then, 25 mL of toluene were added and the mixture was stirred at 70°C for 24h. Finally, the mixture was filtrated and the resulting powder (CNF-PPh2) was extensively washed with methanol and dried under vacuum.

2.2. Functionalization with ammonium groups The procedure was identical to the one described in section 2.1 for the first two steps except that N,N-dimethylethylenediamine (1.3 equivalents) was used instead of ethylenediamine. In the third step, 800 mg of CNF-NMe2 were placed in a 100 mL Schlenk flask with 30 ml of acetone and 5 equivalents of methyl trifluoromethanesulfonate (0.91 mL). The mixture was stirred at room temperature for 24 hours. Finally, the mixture was filtrated and the resulting powder (CNF-NMe3+) was extensively washed with acetone and dried under vacuum.

2.3. Grafting of metal complexes The amount of cluster engaged in each grafting experiment corresponds to a theoretical 5 wt.% metal loading on the support after ligands removal. In a typical experiment, 8.7 mg of cluster 1 was stirred with 95 mg of CNF-PPh2 in 10 mL of toluene and 10 mL of dichloromethane at room temperature, in the dark, for five days. The solid was filtrated, washed with dichloromethane and dried at room temperature under vaccum. The same procedure was used to graft cluster 2 on CNF-NMe3+ (using 15 mg of 2 and 95 mg of support with acetone/toluene 1:1 v/v).

2.4. Physico-chemical methods of characterization Atomic absorption measurements were carried out on a Perkin Elmer atomic absorption spectrometer 3110. XPS analyses were performed on a Kratos Axis Ultra spectrometer (Kratos Analytical – Manchester – UK) equipped with a monochromatized aluminium X-ray source (powered at 10 mA and 15 kV) and an eight channeltrons detector. The sample powders were compacted with a spatula into small stainless steel troughs of inner diameter 4 mm and 0.5 mm depth. Charge stabilisation was achieved by using the Kratos Axis device. Spectra were decomposed with the CasaXPS program (Casa Software Ltd., UK). TEM images were obtained with a LEO 922 OMEGA energy filter transmission electron microscope. The samples were suspended in hexane under ultrasonic treatment; a drop of the supernatant was then deposited on a holey carbon film supported on a copper grid, which was dried overnight under vacuum at room temperature before analysis.

3. Results and discussion The aim of support functionalization is to introduce anchors for the grafting of clusters at its surface. Two different functionalization strategies of carbon nanofibers were envisaged (Figure 1). In the first case, chelating phosphine groups were introduced at the surface of nanofibers in three steps. This strategy allows a covalent grafting of

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Clusters as precursors of nanoparticles supported on carbon nanofibers

clusters through ligand exchange. The other functionalization strategy was used in order to introduce positive charges at the surface of the support when dealing with negatively charged clusters. It is realised through formation of a pending arm ending with an ammonium group. Table 1 shows the XPS results for functionalization samples. At each step, an increase of surface concentration of the heteroatom of interest was observed. R

O

Cl

CNF

O H2N(CH2)2 N(R)2

N

F3 CSO2 O-

R

N

NH CNF

R = H, CH3

O F3CSO2OCH3

NH CNF

SOCl2

O

OH

PPh2 PPh2 N

CNF

HPPh2/CH2O O

NH

CNF

Figure 1. Functionalization of CNFox. Table 1. XPS results for functionalization samples. Surf. At. Ratios

CNFox

CNF-Cl

CNF-NH2

CNF-NMe2

CNF-PPh2

CNF-NMe3+

O/C Cl/C N/C P/C S/C F/C F/N

0.121 0.009 0.010 -

0.114 0.023 0.013 0.010 -

0.104 0.011 0.060 0.008 -

0.081 0.010 0.052 0.004 -

0.066 0.005 0.031 0.013 0.002 -

0.111 0.005 0.031 0.020 0.053 1.688

To prove the success of the functionalization, a [Ru5PtC(CO)14(COD)] cluster was covalently grafted onto CNF-PPh2 while a negatively charged (NEt4)[FeCo3(CO)12] cluster was incorporated on CNF-NMe3+. Table 2 shows the results of metal loading and XPS characterization of these samples. We can see that experimental values for Ru/C, Pt/C and Co/C ratios are higher than calculated values, which means that, a priori, clusters form small and well-dispersed particles on the support. The samples were characterized by TEM to visualize particles sizes (Figure 2). The size of particles observed was about two nanometers for the Ru5Pt cluster and ten nanometers for the FeCo3 cluster.

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Table 2. Loading and XPS results for the incorporation of clusters on functionalized nanofibers. Samples Cluste r

Support

Ru5Pt

CNF-PPh2

Atomic absorption Metal Grafting loading yield (%) (wt.%) 44

2.2

FeCo3 CNF-NMe3+ 40 2 calculated value corresponds to bulk molar ratios.

M/Ccalca (Ru) 0.002 (Pt) 0.0004 (Co) 0.003

XPS M/Cexp before activation 0.013 0.004 0.011

M/Cexp after activation 0.010 0.003 0.015

(a)

(a)

(b)

Figure 2. TEM image of (a) [Ru5PtC(CO)14(COD)] grafted on CNF-PPh2 and (b) (NEt4)[FeCo3(CO)12] incorporated on CNF-NMe3+.

4. Conclusion The goal of this study was to covalently graft potential coordination sites at the surface of carbon nanofibers to allow the incorporation of clusters. Two functionalization strategies were investigated : the incorporation of chelating phosphine groups and the incorporation of ammonium groups. To prove the success of the functionalization, two organometallic compounds were used - [Ru5PtC(CO)14(COD)] and (NEt4)[FeCo3(CO)12] as precursors of supported bimetallic nanoparticles. Atomic absorption allowed us to confirm the incorporation of the metals at the surface of the functionalized supports. Small metallic particles were observed by TEM. In the case of [Ru5PtC(CO)14(COD)], the particles were approximately 2 nm in size, while in the case of (NEt4)[FeCo3(CO)12] they were, on average, about 10 nm in diameter.

Acknowledgement The authors gratefully acknowledge the FNRS, FRIA and PAI Inanomat for funding.

References [1] W. Eberhardt, 2002, Surf. Sci., 500, 242-270. [2] P. Braunstein, L. A. Oro, P. R. Raithby, 1999, Metal Clusters in Chemistry, Wiley-VCH, Weinheim. [3] S. Hermans, T. Khimyak, B. F. G. Johnson, 2001, J. Chem. Soc., Dalton Trans., 3295-3302. [4] P. Chini, L. Colli, M. Peraldo, 1960, Gazz. Chim. Ital., 90, 1005-1020. [5] C. Willocq, S. Hermans, M. Devillers, 2008, J. Phys. Chem. C, 112, 5533-5541.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

X-ray photoelectron spectroscopy study of nitrided zeolites Mondher Srasra, Stéphanie Delsarte, Eric. M. Gaigneaux* Unité de catalyse et de chimie des matériaux divisés, Croix de Sud 2/17, 1348 Louvain-La-Neuve, Belgium (*Email :[email protected])

Abstract X-ray photoelectron spectroscopy (XPS) was used to observe the changes in the nitrogen, silicon and aluminum local environments occurring upon nitridation of ultrastable Y zeolite (Si/Al = 13) at different temperatures. In the case of nitrogen an identification and quantification of incorporated species was possible. The substitution of oxygen by nitrogen in the immediate vicinity of both silicon and aluminum atoms was also demonstrated. The extent of this substitution was more important at high nitridation temperature. Keywords: nitridation, zeolite, XPS, Auger peak

1. Introduction The base-catalyzed reactions play a more and more important role in modern fine chemistry. Nevertheless reports about solid base catalysts are still far fewer than those about solid acid catalysts [1-3]. Therefore, new solid base catalysts are imminently desirable to meet the fine chemical industry needs [4]. In the beginning of the nineties, it was found that nitrogen incorporation is an effective way to synthesize new solid basic materials [5-7]. Through treating various amorphous oxides with flowing ammonia at high temperatures “nitridation treatment”, a new family of heterogeneous basic catalysts was made [8-12]. In previous papers we demonstrated that the same nitridation approach can be also successfully applied for the incorporation of nitrogen into the framework of different Y zeolites, this making possible the preparation of porous basic catalysts active in the Knoevenagel condensation reaction [13-14]. The present work was undertaken in order to understand the modifications induced by nitridation and to provide a picture of the chemical rearrangements that occur upon nitrogen incorporation into ultrastable Y zeolite (Si/Al ratio of 13). Since catalysis is a surface phenomenon we choose to characterize the local environments of nitrogen, silicon and aluminum by X-ray photoelectron spectroscopy (XPS). A clear identification of these modifications is essential to allow a control of the preparation parameters for more efficient basic catalysts.

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2. Experimental 2.1. Materials The commercially available ultrastable Y zeolite CBV720 (Si/Al = 13) was provided by Zeolyst, and used as received. The zeolite was placed in a fixed bed quartz microreactor, and heated under flowing ammonia (Indugas 99.9%) for 48 h at different temperatures. At the end of the treatment, the samples were cooled down to room temperature under a pure and dry nitrogen flow (Indugas 99.999%). All the samples will be referred to as 13USYx00N; where 13 stand for the Si/Al ratio and x00N gives the nitridation temperature in °C. Reference samples were also treated in the same way except that ammonia was replaced by helium (Indugas 99.999%); they will be referred as 13USYx00He.

2.2. XPS

3. Results and discussion 3.1. N1s peaks

As reported in Figure 1, an increase of the nitridation temperature induces an increase of the amount of surface nitrogen from 0.6 wt% in the parent zeolite to 10.7 wt% in the sample nitrided at 900°C (for clarity the intensity of the different spectra was normalized in order to have the same peak height). In addition to the increase of the N1s peak intensity a shift of the complete envelope of the peak to lower binding energies was observed. This shift is indicative of the chemical form of nitrogen modification.

Arbitrary Units

The XPS analyses were performed with a Kratos Axis Ultra spectrometer (Kratos Analytical, UK). The residual pressure in the analysis chamber was lower than 10-6 Pa. The nonmonochromatized Mg X-ray source (hν = 1253.6 eV) was powered at 15 kV and 10 mA. The charge stabilization was achieved by using an electron source mounted co-axially to the electrostatic lens column and a charge balance plate used to reflect electrons back towards the sample. The electron source was operated at a 1.8 A filament current and a bias of -1.1 eV. The charge balance plate was set at -2.8 eV. The analyzed area was 700 μm x 300 μm and the pass energy of the analyzer was set at 160 eV for the survey scan and 40 eV for narrow scans. In the latter conditions, the full width at half maximum (FWHM) of the Ag 3d5/2 peak of a freshly sputtered silver standard was about 1.1 eV. The powdered samples were placed into stainless steel throughs of 4 mm diameter and gently pressed. The samples were introduced into the spectrometer then outgassed overnight and analyzed at room temperature. For each sample, a survey spectrum was recorded, followed by narrow scans on C1s, O1s, N1s, Al2p, AlKL23L23, SiKL23L23 and C1s again. The binding energies were determined by fixing the C-(C, H) contribution of the C1s adventitious contamination at 284.8 eV. The peaks were decomposed using the least-squares best fitting routine of the Casa XPS program (Casa Software Ltd, UK) with Gaussian /Lorentzian (70/30) product function and 13USY900N 10.7 (wt %) after subtraction of a linear background. 13USY800N

6.0 (wt %)

13USY600N

0.7 (wt %)

13USY50 0.6 (wt %)

13USY 410

408

406

404

402

400

398

396

394

392

Binding Energy (eV)

Figure 1. N1s peak evolution as a result of nitridation and the concentration of surface nitrogen.

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X-ray photoelectron spectroscopy study of nitrided zeolites

Arbitrary Units

By comparison with the data given in the literature for different nitrogen-containing solids the following nitrogen species incorporated in nitrided zeolites were identified and quantified [14]: NH4+ (402.0-402.8 eV), adsorbed ammonia (400.0-400.9 eV), NHx where x can be equal to 1 or 2 (397.9-398.7 eV) and nitride species >N- (397.1-397.6 eV). The order of appearance of these species depends mainly on the nitridation temperature. 13USY900N Firstly, at the nitridation temperature of 600°C XPS evidenced three components related to NH4+ present also in the parent zeolite, adsorbed 13USY800N ammonia and NHx groups. A further increase of the nitridation 13USY600N temperature to 800°C induced the disappearance of the peak related to NH4+ species and a remarkable increase of the peak assigned to 13USY NHx species. Finally, at the nitridation Kinetic Energy (eV) temperature of 900°C an additional increase of Figure 2. SiKLL peaks evolution as a the intensity of the peak of NHx species was result of nitridation. revealed with the appearance of a lower energy component related to >N- species. 1600

1610

3.2. SiKLL peaks

Arbitrary Units

For silicon and aluminum the Auger peaks were used instead of XPS peaks. Indeed the Auger peak is more sensitive to environment modifications compared to the XPS peak since its larger relaxation energy due to the double-hole final state associated with Auger emission [15, 16]. The SiKLL region for the starting zeolite and the different nitrided samples can be seen in Figure 2. In addition to the shift and broadening of the SiKLL Auger peak, an important change of the shape was observed when the nitridation temperature was increased. The SiKLL peak was decomposed in two components. The FWHM of the SiKLL Auger peak of the zeolite before nitridation was imposed for component 1; this component at 1608.2 eV was attributed to silicon bound to four oxygen atoms in SiO4 tetrahedra. The component 2 assigned to silicon in an environment modified by the presence of nitrogen. This component shifts from 1609.3 to 1610.9 eV with the increasing of the nitridation temperature. Knowing that the 13USY900N treatment with helium instead of ammonia did not provoke any significant modification of SiKLL peak, it is confirmed the effective replacement of oxygen atoms by nitrogen 13USY800N atoms in the first coordination sphere of silicon was confirmed. This replacement is 13USY600N becoming progressively important with the treatment temperature.

3.3. AlKLL peaks

The AlKLL peak of the parent zeolite shows a complex shape with two components. This shape was due to the presence of aluminum in two coordination states Al (IV) and Al (VI) as demonstrated by 27Al MAS-NMR

13USY 1374

1376

1378

1380

1382

1384

1386

1388

1390

1392

Kinetic Energy (eV)

Figure 3. AlKLL peak evolution as a result of nitridation.

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analysis [17]. Nitridation induced a progressive shift of the whole envelope of the AlKLL peak to higher kinetic energies. An important change of the peak shape was also observed already at 600°C. Owing to the complexity of the AlKLL peak profile, its evolution was quantified by subtracting the peak profile of the native zeolite (dotted component) from that of the treated sample. The samples heated under helium flow show only negligible changes. For the nitrided samples, the intensity and the width of the resulting component were relatively important, becoming significant already at 600°C. Its position shifted from 1385.6 to 1386.4 eV with the nitridation temperature. This argues in favor of the occurrence of the substitution of oxygen by the less electronegative nitrogen in the first coordination sphere of aluminum. It remains to be clarified whether ammonia reacts with all aluminum species Al(IV) or Al(VI) in the same manner or reacts preferentially with certain aluminum species.

4. Conclusion The investigation of the surface of nitrided zeolites by X-ray photoelectron spectroscopy yields several information. In addition to the amount of incorporated nitrogen the nitridation temperature influences also the nature of nitrogen species. These nitrogen species are clearly identified by comparison with data given by literature on nitrogen containing solids. Both silicon and aluminum environments are affected by nitridation. An effective substitution of oxygen atoms by nitrogen atoms in the immediate vicinity of silicon and aluminum was evidenced by SiKLL and AlKLL peaks. The extent of the oxygen-tonitrogen substitution in the silicon and aluminum environments is much more significant at higher nitridation temperature.

References [1] [2]

W. F. Hölderich, Catal. Today. 62 (2000) 115. M. Harmer, in Handbook of Green Chemistry and Technology, J. Clark, D. Macquarrie (Eds.), Blackwell, Oxford, (2002) 86. [3] R. A. Sheldon, H. van Bekkum (Eds.), Fine Chemicals through Heterogeneous Catalysis, Wiley-VCH, Weinheim, 2001. [4] K. Tanabe, W. F. Holderich, Appl. Cata. A Gen. 181 (1999) 399. [5] R. W. Chorley. P. W. Lednor, Adv. Mater. 3 (1991) 474. [6] P. W. Lednor, R. de Ruiter, J. Chem. Soc. Chem. Commun. (1991) 1625. [7] P. W. Lednor, R. de Ruiter, K. A. Emeis, Mater. Res. Soc. Symp. Proc. 271 (1992) 801. [8 A. Massinon, E. Gueguen, R. Conanec, R. Marchand, Y. Laurent and P. Grange, Stud. Surf. Sci. and Catal. 101 (1996) 77. [9] P. Grange, P. Bastians, R. Conanec, R. Marchand, Y. Laurent, Appl. Catal. A., 114 (1994) L191. [10] P. Grange, P. Bastians, R. Conanec, R. Marchand, Y. Laurent, L. Gandia, M. Montes, J. Fernandez, J. Odriozola, Stud. Surf. Sci. Catal., 91 (1994) 381. [11] A. Massinon, J. Odriozola, P. Bastians, R. Conanec, R. Marchand, Y. Laurent, P. Grange, Appl. Catal. A. 137 (1996) 9. [12] E. Guegeun, S. Delsarte, R. Marchand, V. Peltier, R. Conanec, R. Marchand, Y. Laurent, P. Grange, J. Eur. Ceram. Soc. 17 (1997) 2007. [13] M. Srasra, G. Poncelet, P. Grange, S. Delsarte, Stud. Surf. Sci. Catal.158 (2005) 888. [14] M. Srasra, S. Delsarte. E. M. Gaigneaux, Top. Catal. 52 (2009) 1541. [15] S. Kohiki, S. Ozaki, T. Hamada, K. Taniguchi, J. Electron. Spectrosc. Relat. Phenom. 28 (1987) 103. [16] C. Wagner, A. Joshi, J. Electron. Spectrosc. Relat. Phenom. 47 (1988) 283. [17] M. Srasra, These “Preparation of basic catalysts by nitridation of Y zeolites”, Université Catholique de Louvain, Louvain-La-Neuve, Belgium, 2008.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V.

Development of a modified co-precipitation route for thermally resistant, high surface area ceria-zirconia based solid solutions Alfonsina Pappacenaa, Karl Schermanzb, Amod Sagarb, Eleonora Aneggia, Alessandro Trovarellia a b

Dip. Scienze e Tecnologie Chimiche, Università di Udine, Udine, 33100, Italy Treibacher Industrie AG, 9330 Althofen, Austria

Abstract In this work a modified co-precipitation route for ceria-zirconia based material with a high surface area, high thermal stability and enhanced OSC properties has been developed and the importance of the addition of surfactants and H2O2 in order to promote the stability of these materials at high temperatures has been developed. Keywords: ceria-zirconia, TWC, OSC, surface area, surfactant

1. Introduction The use of ceria-based materials in catalytic science is well established [1-2]. Ceria is presently used in a large number of industrial processes and it accounts for a large part of the rare earth oxide market. Undoubtedly its major commercial application is in the treatment of emissions from internal combustion engines where ceria-based materials have been used in the past 30 years [3]. Its more important action in TWCs is to take up and release oxygen following variations in the stoichiometric composition of the feedstream. Typical materials currently used for the manufacture of TWC washcoats are mixtures based on aluminium oxide combined with Ce or Zr rich CeO2-ZrO2 plus additional rare earth dopants. Within the automotive exhaust gas purification there is estimated to be a future huge demand on TW catalysts when more stringent emission standards will be introduced to the market within the next years. Particularly, countries which do not have any emission standards yet are considered to give a significant contribution to the growing demand in the materials. The severe conditions to which catalysts are subjected during operations require materials with high thermal stability and this forced the researcher to develop new synthesis route in order to achieve formulations responding to the market requests for the new generation of washcoat materials [4-5]. The aim of this work is the design of highly stable ceria-zirconia based materials with particular attention to surface and oxygen storage properties, understanding the effect of composition and of addition of surfactant agents, the effect of pre and post treatments using H2O2 and the effect of any variation in coprecipitation procedure.

2. Experimental procedures Ceria-zirconia solid solutions were prepared by co-precipitation starting from nitrate salts precursors (Treibacher Industrie AG) with ammonium hydroxide (Aldrich) in a glass reactor (1L) equipped with a temperature-controlled chamber and four automatized pumps for adding reagents and controlling pH. A typical composition contains 20% wt.

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of CeO2, 73% ZrO2, and two dopants from the rare earth family (5% Nd2O5 and 2% La2O3). Precipitates were filtered and washed three times with 0.5L of de-mineralized water and the resulting cakes were dried at 393K and calcined at 773K (fresh), 1273 K (soft aging) and 1373K (severe aging) for 4 h and they were extensively characterized by conventional techniques (XRD, TGA, BET) for checking phase purity, textural and oxygen exchange properties. Textural properties and porosity were measured, respectively, according to the B.E.T. and BJH methods, by nitrogen adsorption/desorption at 77K, using a Tristar 3000 gas adsorption analyzer (Micromeritics). Structural features of the catalysts were characterized by X-ray diffraction (XRD). XRD patterns were recorded on a Philips X’Pert diffractometer operated at 40 kV and 40 mA using nickel-filtered Cu-Kα radiation. Spectra were collected using a step size of 0.02° and a counting time of 40s per angular abscissa in the range 20°–145°. The mean crystalline size was estimated from the full width at the half maximum (FWHM) of the X-ray diffraction peak using the Scherrer equation with a correction for instrument line broadening. The OSC properties of the powders are determined in static conditions with a TGA method similar to those reported in Mamontov et al [6]. The oxidized powder is treated with 5%H2/Ar at 923K for 70 min and the observed weight loss, due to oxygen removal by H2 to form water, can be associated to total oxygen storage capacity at that temperature.

3. Results and discussion The optimization of synthesis parameters was done by working on pre and post treatment variables such as introduction of reflux, addition of H2O2 [5,7-8] and/or surfactant and by varying pH during precipitation. The synthesis route was modified step by step evaluating the effect of each variable on the surface area, porosity and OSC of materials. The sample prepared by the traditional co-precipitation (i.e. without reflux and with no addition of H2O2 and surfactant) is identified as CZ. Table 1. Textural morphological and OSC properties of materials. average pore size (nm) Ageing

SABET (m2/g)

OSC923K (µmol-O2/g-CeO2)

crystallite size (Å)

773

773

1273

1373

773

1273

1373

773 1273

1373

CZ

26

72

0

0

1059

822

699

54

133

168

rCZ

30

112

7

0

957

841

175

55

137

265

hCZ

43

93

19

1

1002

987

943

71

147

196

r,hCZ

46

111

18

1

1027

958

638

55

142

242

temperature (K)

The reflux post-treatment for 20h (rCZ) induces an increase of the surface area of the material and its role decreases by increasing the temperature of thermal treatment (table 1). On the contrary, the pretreatment with H2O2 (hCZ, using [H2O2]/[metal]=3) shows an important role (correlated with its capability to induce the precipitation of ceria at acid pH [7]), enhancing the surface area up to 19 m2/g at 1273K. It is interesting to observe that the reflux positively affects the surface area of fresh samples, while the pretreatment with H2O2 significantly improves the thermal stability at 1273K and a combination of the two treatments (r,hCZ) produces a material showing high surface area under fresh and aged conditions. The calcination at 1373K induces a dramatic drop in surface area for all the samples, but nevertheless the use of H2O2 improves the

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oxygen storage capacity also after thermal treatment. From table 1 it appears that while the reflux post-treatment slightly affects the properties of the samples, the pre-treatment with H2O2 is a key parameter to increase the surface area and the OSC and for these reasons in the optimization of the synthesis only the H2O2 treatment was maintained. The synthesis procedure was further modified with the introduction of a surfactant post-precipitation treatment adding the lauric acid (Aldrich), directly in solid form to the batch, as “soft templating” agent [9] (commonly used in the preparation of mesoporous structures [10]); the resulting solution is kept under stirring 4h, then the slurry is filtered and washed as describe above. The role of the surfactant agent is to modify the surface morphology of the material inducing a mesoporous structure. Varying the total metal ions/ surfactant molar ratio ([metal]/[lauric acid]) affects the pore size distribution of samples and, as shown in figure 1, the thermal stability after aging at 1373K is strictly correlated with the average pore size. The higher the average pore size, the higher the surface area after severe aging treatment (the better results is obtained for the molar ratio of 1/0.25 with a surface area of 14 m2/g). Summarizing, the post treatment with lauric acid results a key parameter in order to obtain materials with high surface area and coupling the pre-treatment with H2O2 and the post-treatment with lauric acid the thermal stability was dramatically improved. A very interesting result is obtained by studying the effect of the pH on the final properties of the materials. The precipitation was interrupted at different pH values, from 6 to 11, and then the precipitate was treated with lauric acid (all the samples were pretreated with H2O2). The value of pH affects the interaction between the surfactant and the precipitate, influencing not only the surface area, but also the porosity of materials; higher values of the pH of precipitation are correlated with higher porosity of the catalysts and enhanced thermal stability (figure 2). This effect is probably due to the ζ-potential of the precipitate that drives the interaction with the amphiphilic molecules [10,11]. The porosity results to be very important in order to maintain elevated surface area, indeed our study pointed out that fresh samples with higher pore size show higher thermal stability. 0.6

30

1/0.1

0.5

1/0.75 0

12

0.3

1

0.2

5 0.1 0

1

10 pore diameter (nm)

100

Figure 1. pore size distribution of fresh materials prepared with H2O2 and surfactant with different [M+]/[C12H10O2] molar ratio (in the label the SA values for samples calcined at 1373K).

10 20

8

15

6 4

10

2

5

0

5

6

7

8

9

10

11

SA (m2/g)

dV/dlog(w) (cc/g)

25

14

1/0.5

0.4

12

1 average pore size (nm)

1/0.25

0 12

pH

Figure 2. dependence of average pore size (c, fresh samples) and surface area (■, aged samples) against pH (surfactant ratio 1/0.25).

Considering TWC catalysts, the deterioration of the OSC [4], due to the aging processes, is another critical property to evaluate. Figure 3 shows the dependence of OSC deterioration (% drop of OSC following thermal treatments) against pH; the graph assumes an inverse volcano plot profile with the best results obtained at pH 9.5.

838

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45

1400

40

1200

35

OSC 1373K

35

30

1000

30

25

800

OSC (μmol

25 20 15 10 5 0

40

SA 1373K OSC 773K

5

6

7

8

9

10

11

12

20 600

15

400

10

200 0

SA (m2/g)

OSC deterioration (%)

The interaction time of the surfactant with the precipitate (1 or 4 hour) influenced in different way the thermal stability and the OSC of the materials; indeed, lower interaction time affects positively the surface area (figure 4), while higher times of interaction are correlated to a better OSC. The best result was obtained for pH 10.5 with 1 hour of interaction being the surface area of the sample 22 m2/g and with almost no deterioration of OSC (2%).

5 pH 9,5-1h

pH 9,5-4h

pH 10,5-1h

pH 10,5-4h

0

pH

Figure 3. deterioration of OSC due to thermal ageing.

Figure 4. OSC properties and surface area of materials prepared with different interaction time with the templating agent.

4. Conclusion In this work a modified co-precipitation route for ceria-zirconia based material with a high surface, high thermal stability and enhanced OSC properties has been developed. While the reflux post-treatment slightly affects the properties of the samples, the pre-treatment with H2O2 and the post-treatment with lauric acid improved dramatically the thermal stability after ageing up to 1373K for 4 hours with a surface area of 22 m2/g against ca. 1-2 m2/g for the unmodified synthesis. Another important variable in order to achieve highly stable materials is the pH of precipitation; an increase of the pH affects the porosity of the materials, with important effects on stability and oxygen storage/ release properties. In summary, it seems that modified coprecipitation with addition of surfactants and H2O2 strongly promotes stability of these materials at high temperatures.

References [1]

A. Trovarelli (Ed.), Catalysis by Ceria and Related Materials, Imperial College Press, London, 2002, pp. 1-528. [2] S. Bernal, J. Kaspar, A. Trovarelli (Eds.), Catal. Today 50 (2) (1999)173-443. [3] J. Kaspar, P. Fornasiero, J. Solid State Chem. 171 (2003) 19. [4] T. Wakita, A Kohara, Y. Kann, H. Omoto, Daiichi Kigenso and Chemcat Corp, Eur. Pat. Appl. EP 1801074 (A1), 2007. [5] O. Larcher, D. Monin, E. Rohart, Rhodia Elect & Catalysis, Pat. Appl. FR 2852596(A1), 2009. [6] E. Mamontov, R. Brezny, M. Koranne, T. Egami, J. Phys. Chem. B 107 (2003) 13007. [7] B. Djuričić, S. Pickering, J. Europ. Ceram. Soc. 19 (1999) 1925-1934. [8] P. Yu, S. A. Hayes, T. O’Keefe et al., J. Electrochem. Soc. 153 (2006) C74-C79. [9] Q. Zang, W. Wang, J. Goebl, Y. Yin, Nano Today 4 (2009) 494-507. [10] D. Terribile, A. Trovarelli, J. Llorca, C. de Leitenburg, G. Dolcetti, Catal. Today 43 (1998) 79-88. [11] M. Ozawa, M. Hattori, J. Alloys Compd, 412 (2006) 560-562.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Deposition of gold clusters onto porous coordination polymers by solid grinding Tamao Ishida,a,c Noriko Kawakita, a,c Tomoki Akita,b,c and Masatake Haruta,a,c a

Tokyo Metropolitan University, 1-1 Minami-osawa, Hachioji, Tokyo 192-0397, Japan National Institute of Advanced Industrial Science and Technology, 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan c Japan Science and Technology Agency, CREST, 4-1-8 Hon-cho, Kawaguchi, Saitama 332-0012, Japan b

Abstract Gold could be deposited on porous coordination polymers (PCPs) as clusters and nanoparticles smaller than 5 nm by grinding dimethyl Au(III) acetylacetonate with PCPs. In particular, on Al-containing PCP, the mean diameter of Au was minimized to 1.6 nm. These tiny Au clusters exhibit markedly high catalytic activity and selectivity in N-alkylation of amine with alcohol under N2. Keywords: gold clusters, gold catalysts, porous coordination polymers, N-alkylation

1. Introduction Gold exhibits unique catalytic properties for a number of vapor and liquid phase reactions when it is deposited as nanoparticles (NPs) smaller than 10 nm on base metal oxides [1]. Gold NPs supported on or stabilized by organic polymers, on the other hand, have recently been prepared and have shown that they are catalytically active for liquid phase oxidations [2]. Since the size sensitivity of the catalysis by Au NPs is stronger over polymer supports than over metal oxides, it often happens that the critical diameter of Au NPs is around 2 nm, below which catalytic capability dramatically changes. Porous coordination polymers (PCPs), having highly ordered nanometer-sized porous structures, have recently been attracting interests as supports for metal clusters [3]. However, it was difficult to deposit Au as clusters on or in PCPs by conventional chemical vapor deposition (CVD). In this work, we demonstrated the direct deposition of Au as clusters onto PCPs by a solid grinding method and investigated their catalytic performance for one-pot N-alkylation of amine with alcohol.

2. Experimental 2.1. Catalyst Preparation Porous coordination polymers, Al-MIL53 ([Al(OH)(bdc)]n, bdc = benzene-1,4dicarboxylate) [4] and MOF-5 ([Zn4O(bdc)3]n) [5] were supplied from BASF, and CPL-2 ([Cu2(pzdc)2(bpy)]n, pzdc= pyrazine-2,3-dicarboxylate, bpy = 4,4’-bipyridine) were synthesized according to the literature [6]. A support (300 mg) and slightly volatile organogold complex, Me2Au(acac) (2 wt% of Au loading), were ground in an agate mortar in air at room temperature for 20 min. The adsorbed Au(III) species were reduced in a stream of 10 vol% H2 in N2 (50 mL/min) at 120 °C for 2 h.

2.2. Catalytic Test A mixture of benzylalcohol (0.5 mmol), aniline (0.5 mmol), Au catalyst (Au 1.5 mol%), Cs2CO3 (0.5 mmol), and toluene (3 mL) was stirred under N2 atmosphere at 110 °C for

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22 h. The reaction mixture was washed with H2O, extracted with Et2O, dried over Na2SO4, filtered, and anaylzed by GC using anisole as an internal standard.

3. Results and discussion 3.1. Particle size of gold

Figure 1. HAADF-STEM image of 2 wt% Au/Al-MIL53 prepared by CVD.

Fisher and his co-workers reported that Au could be deposited onto MOF-5 by CVD, however, the particles size was large in the range of 5-20 nm [3]. When Au was deposited onto Al-MIL53 by CVD, Au clusters smaller than 2 nm were observed with a high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) together with Au NPs in the range of 3-10 nm (Figure 1). It appeared that Al-MIL53 could stabilize small Au NPs more efficiently than MOF-5. On the contrary, solid grinding method enabled us to deposit Au NPs in diameters below 5 nm with narrower size distributions as compared to CVD for all the three PCPs (Figure 2 and Table 1).

Figure 2. HAADF-STEM image (a) and the size distribution (b) of 2 wt% Au/Al-MIL53 prepared by SG.

Figure 3. Nitrogen adsorption isotherms of Al-MIL53 and 2 wt% Au/Al-MIL53 prepared by SG.

The mean diameters of Au particles in Au/MOF-5, Au/CPL-2, and Au/Al-MIL53 were estimated to be 3.3±2.3, 2.4±1.0, and 1.6±1.0 nm, respectively (Table 1). On AlMIL53, in particular, 80% of Au particles could be deposited as clusters smaller than 2 nm (Figure 2a, b). The specific surface area obtained by the N2 adsorption isotherms of 2 wt% Au/AlMIL53 (SG) was reduced to 630 m2 g-1 from 770 m2 g-1 of supplied Al-MIL53 (Figure 3), implying that the pore blocking by Au particles and/or the parial degradation of porus structures. However, the X-ray diffraction (XRD) patterns of Al-MIL53 did not change

Deposition of gold clusters onto porous coordination polymers

841

after the deposition of Au. These results indicated that the most of porous structures remained after depositing Au. The size of Au particles deposited on PCPs were still larger than the pore size even on Al-MIL53, indicating that Au particles were mainly placed on the outer surfaces of PCPs. However, the deposition of Au clusters at the outer PCP surfaces is not discouraging but beneficial in catalytic applications owing to the rapid diffusion of substrates to the Au catalytic sites. In general, complete vaporization and uniform vapor diffusion of volatile organogold complex through support powder are difficult during CVD experiments due to long reaction time which causes the agglomeration of Au precursors. In the SG method, the sublimation of Me2Au(acac) might occur during grinding and grinding facilitates faster diffusion and adsorption of Me2Au(acac) on support surfaces than in CVD, yielding highly dispersed small Au clusters with narrower size distributions [8].

3.2. Catalytic performance of Au/PCPs We have investigated the one-pot N-alkylation of aniline (2) with benzylalcohol (1) to produce secondary amine, N-phenylbenzylamine (4) via the formation of imine (3) under inert atmosphere (Scheme 1).

Scheme 1. N-Alkylation of aniline (2) with benzylalcohol (1) over Au/PCPs.

As can be seen in Table 1, imine (3) was formed without O2 in the presence of base over all Au/PCP catalysts, indicating that the dehydrogenation of 1 to produce benzaldehyde occurred over Au catalysts. Furthermore, Au/Al-MIL53 gave 4 without H2 (entry 3), while Au/CPL-2 and Au/MOF-5 did not (entries 1,2) [9]. It is likely that Au clusters on MIL-53 could form Au-H species by the dehydrogenation of 1 and Au-H species is used for the hydrogenation of 3 to give 4 (Scheme 1). The optimization of reaction conditions improved both the conversions and selectivity to 4 (entry 4). Gold on Al-MIL53 prepared by CVD yielded lower conversions and selectivity due to a certain amount of large Au NPs (entry 5). Neither the three PCP supports alone showed catalytic activity in N-alkylation, thus Au is responsible to the catalytically active sites. Recently, N-alkylation over Au/Al2O3 was reported but the selectivity to secondary amine was low such as 29% eventhough Lewis acid and metal-support interaction of Au/Al2O3 were employed [10]. The hydrogen transfer reaction of alcohol to ketone over Au/TiO2 and Au/C was also reported [11]. However, Au/C was catalytically much less active for this reaction than Au/TiO2 in spite of similar size of Au NPs. It indicated that the minimizing the size of Au particles is important especially for inert supports such as carbons and polymers. Although the support effect of Al-MIL53 can not be fully excluded, the existence of Au clusters as the majority of Au on PCPs is critical to the hydrogen transfer efficiency. After 24 h, Au/Al-MIL53 catalyst was removed by filtration at 29% yield of 4. After the catalyst removal, N-alkylation did not proceed to give the corresponding secondary amine (4), suggesting that this catalytic reaction occurrs heterogeneously. Although Au particles did not leach into the reaction solution, its recycled use was difficult due to the aggregation of Au clusters to form larger NPs (>10 nm) after the reactions.

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Table 1. One-pot N-alkylation of aniline (2) with benzylalcohol (1) to N-phenylbenzylamine (4).a Au size

Conversion (%)c

Yield (%)c

Catalyst

Entry

(nm)b

1

2

3

4

1

Au/MOF-5

3.5±2.6

89

62

62

trace

2

Au/CPL-2

3.4±1.4

14

18

12

0

3

Au/Al-MIL53

1.6±1.0

62

42

19

22

4d

Au/Al-MIL53

1.6±1.0

67

63

11

48

5d,e

Au/Al-MIL53

−

55

38

9

19

a

Reaction conditions: benzylalcohol (0.5 mmol), aniline (0.5 mmol), 2 wt% Au catalyst (Au 1.5 mol%), Cs2CO3 (0.5 mmol), toluene (3 mL), N2 atmosphere, 110 °C for 22 h. b Observed by HAADF-STEM. c GC conversions and yields obtained by using anisole as an internal standard. d Au 2 mol%, toluene 2 mL, 130 °C for 48 h. e Au/Al-MIL53 was prepared by CVD.

4. Conclusions Gold clusters smaller than 2 nm in diameter could be deposited onto Al-MIL53 by solid grinding. Gold clusters on Al-MIL-53 enabled N-alkylation of aniline with benzylalcohol to produce the secondary amine in one-pot under N2 atmosphere. The dehydrogenation of alcohol took place over Au clusters on Al-MIL-53 to form Au-H species, which was utilized for the hydrogenation of imine, while it did not over Au NPs on MOF-5 and CPL-2.

Acknowledgements We thank Dr. U. Müller and Mr. N. Takenaka of BASF for supplying PCP samaples. This work was financially supported by JST-CREST and a Grant-in-Aid for Young Scientists (B) (no. 21750160) from MEXT, Japan.

References [1] M. Haruta, Chem. Rec. 3 (2003) 75. [2] T. Ishida , M. Haruta, Angew. Chem. Int. Ed. 46 (2007) 7154, and references therein. [3] S. Hermes, M.-K. Schröter, R. Schmid, L. Khodeir, M. Muhler, A. Tissler, R. W. Fischer, R. A. Fischer, Angew. Chem. Int. Ed. 44 (2005) 6237. [4] N. A. Ramsahye, G. Maurin, S. Bourrelly, P. Llewellyn, T. Louiseau, G. Férey, Phys. Chem. Chem. Phys. 9 (2007) 1059. [5] H. Li, M. Eddaoudi, M. O’Keeffe, O. M. Yaghi, Nature 111 (1999) 190. [6] M. Kondo, T. Okubo, A. Asami, S.-I. Noro, T. Yoshitomi, S. Kitagawa, Angew. Chem. Int. Ed. 38 (1999) 140. [7] J. Guzman, B.C. Gates, Langmuir 19 (2003) 3897. [8] T. Ishida, M. Nagaoka, T. Akita, M. Haruta, Chem. Eur. J. 14 (2008) 8456. [9] T. Ishida, N. Kawakita, T. Akita, M. Haruta, Gold Bull. 42 (2009) 267. [10] K.-I. Shimizu, M. Nishimura, A. Satsuma, ChemCatChem 1 (2009) 497. [11] F. Z. Su, L. He, J. Ni, Y. Cao, H. Y. He, K. N. Fan, Chem. Commun. (2008) 3531.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Influence of the preparation methods for Pt/CeO2 and Au/CeO2 catalysts in CO oxidation Satoshi Shimada,a, Takashi Takei, a, Tomoki Akita,b Seiji Takeda, c Masatake Haruta,a a

Graduate School of Urban Environmental Sciences, Tokyo Metropolitan University, 1-1 Minami-osawa, Hachioji, Tokyo 192-0397, Japan b AIST, 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan c Faculty of Science, Osaka University, 1-1 Yamadaoka, Suita, Osaka 565-0871, Japan

Abstract The influence of the preparation methods (IP:impregnation, DP:deposition- precipitation, SG:solid grinding) for Pt/CeO2 and Au/CeO2 was studied on the size and shape of metal particles and the catalytic activity for CO oxidation. The fine structures of these catalysts were examined by HR-TEM and were correlated to their catalytic properties. The size of metal particles depended on the preparation methods and increased in the order of DP<SG
1. Introduction Supported noble metal catalysts are most widely used among heterogeneous catalysts because they are active for hydrogenation and dehydrogenation reactions as well as for complete and selective oxidation reactions (1). Although they are usually prepared via impregnation, the influence of preparation method on their fine structure and catalytic properties has not been fully understood yet. In this work, the activity of CeO2 supported Pt and Au catalysts was compared. In order to change the size of metal particles and their contact structures with the support, Pt/CeO2 and Au/CeO2 catalysts were prepared by three different methods (IP, DP, and SG). In the conventional IP method using metal precursors containing Cl-, chloride ion remains on the surfaces without washing or high temperature calcination. We have paid attention to this aspect. To avoid this, chloride contamination was minimized in the evaluation of catalytic activities. Deposition-precipitation is an efficient method to yield stronger interaction of catalytic metals with the metal oxide supports because noble metal hydroxides are chemically adjoined with the surface hydroxide layers of the support (2). Solid grinding is a new method with uses slightly volatile organic metal complexes and can produce halogen-free supported metal catalysts (3). This method is applicable to a wide range of materials as supports including acidic metal oxides, carbons, and polymers.

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2. Experimental 2.1. Impregnation (IP) method H2PtCl6 or HAuCl4 was dissolved in a small amount of water, to which CeO2 (DAIICHI KIGENSO KAGAKU KOGYO CO., LTD., specific surface area 20 m2/g) was added. The slurry was stirred for 30min, and then water was evaporated. The solid precursors were freeze dried, calcined or reduced, and the sample was washed with water to remove chloride ions. This operation was repeated until the pH reached a steady value. For Pt/CeO2, the sample was reduced by H2 before catalytic tests.

2.2. Deposition precipitation (DP) method The aqueous solution of H2PtCl6 (1×10-2 M) or HAuCl4 (1×10-3 M) was adjusted to pH of 7 by adding NaOH and heated to 70°C. CeO2 was added and the suspension was stirred at pH=7 and at 70°C for 1h. The suspension was centrifuged and washed to remove chlorine and sodium ions until the pH reached a steady value. The sample was filtered and dried for over night, and finally calcined in air or reduced in H2 containing gas stream.

2.3. Solid grinding (SG) method Mixture of organometallic complex (Pt(C5H7O2)2 or (CH3)2Au(C5H7O2)) and CeO2 were ground in a mortar for 20min. The sample was calcined in air or reduced in H2 containing gas stream. All the samples of Pt/CeO2 were reduced in H2 containing gas stream before catalytic tests. Thermal treatment conditions were summarized in Table 1. Table 1. Thermal treatment conditions. Preparation method 5wt%Pt/CeO2

5wt%Au/CeO2

Gas for catalyst preparation atmosphere / Temperature (°C) / Time (h) IP

Air / 450°C / 4

→

H2:N2=1:9 / 450°C / 2

DP

Air / 450°C / 4

→

H2:N2=1:9 / 450°C / 2

SG

Air / 450°C / 4

→

H2:N2=1:9 / 450°C / 2

IP

H2:N2=1:9 / 150°C / 4

DP

Air / 300°C / 4

SG

Air / 300°C / 4

3. Results and discussion 3.1. Metal particle size Table 2 shows that the size of Au particles markedly changes from 4 nm to 53 nm depending on the preparation method, whereas the size of Pt particles is always smaller than 3 nm. For both the metals, the mean DP 1.6 ± 0.7 4.1 ± 1.3 diameter increased in the order of SG 1.8 ± 0.7 10.6 ± 3.1 DP<SG
Table 2. Mean diameters of metal particles for 5wt%-Pt/CeO2 and 5wt%-Au/CeO2. Preparation dPt / nm dAu/ nm method IP 2.6 ± 0.8 53 (XRD)

Influence of the preparation methods for Pt/CeO2 and Au/CeO2

845

shows that size distribution of Au is wider than that of Pt. This difference may be a reflection of weaker interaction of Au with the metal oxide supports. In DP method metal precursor is deposited in the form of hydroxide on the metal oxide surfaces with strong adhesion. This leads to the smallest metal particles by DP method among the three. Another reason why the mean diameter of Pt tends to be smaller than that of Au is the difference in the melting points of the two metals, 1064°C for Au and 1769°C for Pt. (a) Pt/CeO2-SG

300

dPt=1.8± 0.7 nm

250

Counts

Counts

100

15 10

50

5

0

0 0

2

4

6

dAu=10.6± 0.7 nm

20

200 150

(b) Au/CeO2-SG

25

8 10 12 14 16 18 20 Diameter (nm)

0

2

4

6

8 10 12 14 16 18 20 Diameter (nm)

Fig. 1. Size distributions of metal particles for (a) Pt/CeO2-SG and (b) Au/CeP2-SG.

3.2. Structure of metal particles HR-TEM observations revealed that the shape of Pt particles was hemispherical irrespective of the preparation method. In contrast, the shape of Au particles changed depending on the preparation methods. IP and SG methods formed spherical and DP method yielded hemispherical particles. Platinum is thermodynamically oxidizable and forms Pt oxides on the surfaces of CeO2 during calcination, yielding hemispherical particles by the reduction with H2. On the other hand, Au does not form oxides and the interaction with CeO2 is weak. Therefore, Au was deposited on CeO2 as hemispherical particles only by DP method (Fig. 2(b)). Over CeO2 Pt is deposited by DP as patches with raft structure (Fig. 2(a)). (a) Pt/CeO2-DP

(b) Au/CeO2-DP

Fig. 2. TEM images of (a) Pt/CeO2-DP and (b) Au/CeO2-DP.

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846

3.3. Catalytic activity for CO oxidation Table 3 summarizes the values of metal time yield (MTY) of Pt/CeO2 and Au/CeO2 for CO oxidation at 303 K and apparent activation energies. It is surprising that the apparent activation energies for CO oxidation over both metal catalysts were similar. In particular, the activation energies for Pt/CeO2 are appreciably smaller than those for conventional Pt catalysts reported so far (170kJ/mol). Table 3. Metal time yield and activation energy for CO oxidation over Pt/CeO2 and Au/CeO2.

MTY (s-1・ COmol・metalmol-1) at 30ºC

Catalyst

MTY* (s-1COmolMetalmol-1) IP

DP -4

Ea (kJ/mol)

SG -4

IP

DP

SG

-4

Pt/CeO2

2.0x10

1.7x10

7.9x10

54

56

53

Au/CeO2 * at 303 K

1.4x10-3

8.1x10-2

4.3x10-3

46

53

48

-0

10 1.E+00

-1

1.E-10 01

10-2

1.E- 02

-3

1.E-10 03

-4

1.E-10 04

-5

1.E-10 05

0 10 1

1 10 102 10 100 Diameter (nm)

103

1000

Fig. 3. Dependence of MTY for CO oxidation over Pt/CeO2 and Au/CeO2 on the mean diameter of metal particles.

Figure 3 shows the relationship between MTY and the mean diameter of metal particles. Catalysts were prepared by different methods and conditions. The MTY of Au/CeO2 increased with a decrease in a particle size, whereas the MTY of Pt/CeO2 decreased with a decrease in a particle size. The result suggests that perimeter interfaces between gold nano-particles and the oxide support are important for CO oxidation over Au/CeO2. Accordingly, a decrease in the size of Au particles brings longer perimeter distance in proportion to the reverse second power. The decrease in catalytic activity of Pt/CeO2 in the small particle size region can be ascribed to the electronic change and/or the oxidation of Pt rafts by the strong interaction with the CeO2 support.

4. Conclusions In order to clarify how the size and shape of metal particles affect the catalytic properties, three different preparation methods were applied to deposit Pt and Au on CeO2. The conclusions were: 1) The mean diameter of metal particles appreciably changed depending on the preparation methods and increased in the order of DP<SG
Influence of the preparation methods for Pt/CeO2 and Au/CeO2

847

Acknowledgment This work was supported by Grants-in-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture of Japan (No. 19001005).

References 1. G. P. Chiusoli, P. M. Maitlis, 2008, “Metal-catalysis in Industrial Organic Processes”, RSC. 2. M. Haruta, 2003, Chem. Record, 3, 75. 3. T. Ishida, M. Nagaoka, T. Akita, M. Haruta, 2008, Chem. Eur. J., 14, 8456.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Author index Aboukaïs A., 743 Aerts A., 681 Afonina E.F., 707 Agarwal M., 210 Agostini G., 433, 437 Aguilera O., 513 Aguilhon J., 521 Aires F.C.S., 763 Aissat A., 747 Akita T., 839 , 849 Albonetti S., 621, 785 Albornoz A., 819 Aleshina G.I., 479 Almeida L.C., 639 Alshammari A., 409 Alvarez-Rodriguez J., 751 Al-Zahrani S.M., 347 Amariei D., 35 Amrousse R., 35, 755 Andrushkevich T.V., 463, 479 Aneggi E., 835 Angeles C., 767 Ankudinov A.V., 263 Anqi Zhao, 77, 259 Aoun M., 237 Arena F., 49 3 Arias P.L., 449 , 453 Armenise S.A., 483 Arnold U., 229 Ashameri M., 279 Augustyns K., 321 Averlant R., 517 Avila P., 735, 739 Aytekenov S.A., 29 7 Azalim S., 731 Bachiller-Baeza B., 719 Bacsa R., 629 Bagabas A., 279 Bakermans P., 135 Baldi G., 621 Ballarini N., 823 Bal’zhinimaev B.S., 43 Baraket L., 487 Barama A., 301 Baranek P., 567

Barrera M.C., 767 Barrio V.L., 449 , 453 Basile F., 51, 241, 471 Basset J.-M., 617 Batista J., 245 Batonneau Y., 35 Bellardita M., 225 Belomestnykh I.P., 413 Benadji S., 665 Benito P., 51 Benrabaa R., 301 Bensitel M., 731 Bentaleb F., 311 Berben P.H., 9 3, 135 Bergeret G., 617, Berger-Karin Cl., 635 Berhault G., 59 3, 605, 609 Bersani I., 241 Besson M., 177 Bianchi E., 653 Bilé-Guyonnet E., 145 Binzuo Liu, 271 Blanchard P., 567, 587 Blanco M., 405 Blosi M., 621 Boghosian S., 613 Boissière C., 521 Bondareva V.M., 479 Bonelli R., 785 Bonne M., 587 Borah P., 541 Bordes-Richard E., 17, 301, 811 Bota R.M., 775 Boualleg M., 127 Boukhlouf H., 301 Bourikas K., 117, 613, 643 Brahmi R., 35, 731, 755 Brandalise M., 551 Bravais P., 755 Brei V.V., 233 Brenna G., 471 Bron M., 161 Bronstein L., 153, 361 Brouri D., 711 Budukva S.V., 509 Bui H. Linh, 49 7, 501

850 Bukhtiyarova G.A., 109, 509 Bukhtiyarova M.V., 355 Bykov A., 153, 361 Caceres C.V., 425 Cambra J.F., 449, 453 Campos-Martin J.M., 347 Candy J.-P., 617 Cao J.L., 547 Caps V., 221 Carabineiro S.A.C, 457, 629 Cardenas-Espinosa D.C., 385 Cartoixa B., 35 Cavani F., 823 Celse B., 127 Centeno M.A., 25 Chaiya C., 689 Chanéac C., 127 Changhai Liang 77, 259, 275 Chartier Th., 241 Chassard O., 567 Chatenet M., 169 Chater M., 237 Chen Jin, 161 Chernavkii P.A., 253 Chesalov Yu. A., 109, 463, 479 Chiche D., 127 Chierotti M., 433 Chuang Li., 275 Chul Wee Lee, 339 Constant A.G., 763 Contreras Andrade I.A.C., 69 Cool P., 321 Cornillac M., 241 Corral Valero M., 617 Correa O.V., 551 Cortes M.A., 767 Courcot D., 747 Courson C., 421 Cousin R., 743 Crisafulli R., 559 Cristiani C., 653 Crozet N., 193 Cruz S.A., 529, 661 Cuni A., 241 da Silva E.D., 657 Dalmon J.-A., 617 Dang Sheng Su, 283 Dang T. Phuong, 497, 501

Author index Daniele S., 605 Danilevich E.V., 463 Danilov V.P., 413 Dastageer A., 279 Dath J.-P., 567 Datta A., 541 Davidson A., 711 De Canck E., 365 de Groen M., 135 de Jong K.P., 69 de Jongh P.E., 69 de Lima R.K.C., 657 de Rooij R.M., 135 de S. Santos M., 819 de Souza A.O., 763 de Souza M.O., 763 Debecker D.P., 581, 805 Dedecek J., 823 Deganello G., 401, 417 Deghedi L., 617 Degli Esposti E., 823 Del Gallo P., 241, 471 Delaigle R., 785, 805 Delpoux O., 497 Delsarte S., 831 Demallmann A., 617 Demidenko G., 153 Denicourt A., 145 Depla A., 801 Descorme Cl., 177 Devillers M., 327, 699, 703 D’Haen J., 681 Dhondt E., 771 Di Carlo G., 401, 417 Di Felice L. 421 Di Paola A., 225 Diaz L., 767 Digne M., 127, 497 Dillen M., 127, 497 Diverchy C., 699, 703 Djinovic P., 245 do Carmo Rangel M., 763, 815, 819 Doluda V., 153, 361 Dondi M., 621 Dong Young Shin, 339 Donkervoort H.G., 135 Döring M., 229 Dorofeeva N.V., 759 dos Santos L.M., 763 Dovlitova L.S., 463, 479

851

Author index Drazic G., 457, 629 D’Souza L., 715 Dubreuil A.-C., 617 Due-Hansen J., 735, 739 Dumeignil F., 587, 811 Durupthy O., 521 Echave F.J., 25, 639 Elisarova T.A., 413 Eloy P., 665, 789 Emelyanova E.Yu., 723 Emmerich J., 249 Enke D., 315 Eon J.G., 763 Eri S., 685 Erokhin A.V., 289 Escobar J., 767 Especel C., 767 Essakhi A., 17 Etchegoyen G., 241 F. Wyrwalski F., 389 Fakhfakh F., 487 Falcon H., 347 Fantinel T., 9 Faria J.L., 629 Faure C., 805 Faure R., 241 Fecant A., 193 Fedotov M.A., 109, 723 Fedotova M.P., 723 Fehrmann R., 735, 739 Femoni C., 785 Fessi S., 797 Fierro J.L.G., 347, 449, 453 Figueiredo J.L., 457, 629 Fischer L., 617 Floch A., 445 Florek J., 333 Fornasari G., 51, 471 Foscolo P.U., 421 Fraile J.M., 487 Frizi N., 567 Gaigneaux E.M., 445, 581, 665, 785, 805, 831 Gallard A., 193 Galletti C., 59 Garcia-Bordejé E., 483 Garcia-Cruz I., 529 Garoufalis Ch. S., 117

Gary D., 241, 471 Gatti F., 621 Genet M., 789 Georgaka A., 373 Geus J.W., 93, 135 Gheorghiu C.C., 647 Ghorbel A., 487, 593, 605, 609, 797 Gil Llambias F.J., 739 Giraudon J.-M., 389, 517, 533, 731 Gobetto R., 433 Golinska H., 381 Golubina E.V., 289, 293, 297 Gommes C.J., 169 Gondal M., 279 Gonzalo-Chacon L., 719 Gorsd M., 405 Grobet P.J., 775 Groppi G., 653 Groppo E., 433, 437 Grünert W., 707 Güemez M.B., 449, 453 Guerrero-Ruiz A., 719, 751, 779 Gurevich S.A., 263 Gutierrez A.W., 767 Halasz I., 209 Haruta M., 839, 843 Hassan K.H., 475 Hee Geun Oh, 339 Hemati M., 193 Hermans S., 169, 699, 703, 827 Herry S., 567 Ho Yeon Lim, 441 Hoekstra J., 93 Holmen A., 685 Houthoofd K., 775 Hui Zhang, 253 Hulsund Skagseth T., 685 Huong T.M., 695 Idakiev V., 547, 743 Iriondo A., 449, 453 Isaeva V.I., 707 Isaguliants G.V., 413 Ischenko A.V., 479 Ischenko E.V., 479 Ishida T., 839 Isupova L.A., 343 Ivanova A.S., 355 Ivanova S., 597, 601

852 Jabou K., 609 Jacobs P.A., 771, 775 Jagodzinska K., 333 Jammaer J., 681 Janz A., 315 Jenneskens L.W., 93 Jialin Yu, 625 Jin Won Seo, 681 Jingchao Guan, 259 Jingping Hong, 253 Job N., 169, 647, 699 Jolivet J.-P., 127 Jong Sik Choi, 441 Joo Hwan Seo, 441 Joseph M., 805 Ju Hyung Lee, 441 Jung Wha Son, 339 Junsong Guo, 161 Kachevsky S.A., 289, 293 Kadinov G., 547 Kahia R., 533 Kaichev V.V., 355, 463 Kakuta N., 695 Kalampounias A.G., 613 Kalevaru V.N., 393, 409 Kaluza S., 217 Kappenstein Ch., 35, 755 Kapustin G.I., 537, 707 Kardash T.Yu., 479 Katryniok B., 811 Kawakita N., 839 Kerdi F., 221 Khalil A., 279 Khavryuchenko O.V., 563 Khodakov A., 763 Khodakov A.Y. 253 Kiennemann A., 421 Kirichenko O.A., 537 Kirillov V.L., 43 Kirschhock C., 801 Kirschhock C.E.A., 249 Kishida M., 793 Klimov O.V., 109, 509 Klimova T., 525, 529 Kochkar H., 593, 605, 609 Kochubey D.I., 109 Köckritz A., 409

Author index Kolesnikov S.P., 413 Kollar M., 727 Kondratenko E.V., 635 Kordulis C., 117, 613, 643 Kosslick H., 497, 501 Kozhevin V.M., 263 Kozlova L.M., 707 Krasnobaeva O.N., 413 Kreft S., 315 Kucherov A.V., 537 Kuda E., 267 Kulikovskaya N.A., 343 Kustov L.M., 537, 707 Kuznetsova A.V., 343 Kyriakopoulos J., 643 La Fontaine C., 467 Lafaye G., 237 Lakina N., 361 Lambert S., 169 Lamberti C., 433, 437 Lamonier C., 587 Lamonier J.-F., 517, 533, 731 Lancelot C., 567, 587 Lapina O.B., 43 Latorrata S., 653 Launay F., 145 Le Courtois V., 17 Lemaire A., 185 Leofanti G., 433, 437 Léonard A., 665 Leonova K.A., 509 Leus K., 329 Levec J., 245 Lidong Shao, 283 Linardi M., 551 Lind A. 685 Ling Ding, 275 Liotta L.F., 401, 417 Lisnyak V.V., 563 Löfberg A., 17 Lokteva E.S., 263, 289, 293 Loridant S., 497 Loubet J.-L., 193 Louis B., 601 Luecke B., 315 Luhao Cui, 271 Lunin V.V., 289, 293, 305 Lycourghiotis A., 117, 613, 643

853

Author index Machado B.F., 629 Maeda K., 351 Maillard F., 169 Majouga A.G., 297 Makhankova V.G., 563 Mäki-Arvela P., 283 Mamontov G.V., 759 Marceau E., 311 Marchand K., 497 Marchetti S.G., 819 Marci G., 417 Marecot P. 237 Marécot P., 587 Marin G.B., 329 Maroto-Valiente A., 751 Martens J.A., 249, 681 Martin A., 315, 393, 409, 597 Massiani P., 711 Mateos-Pedrero C., 779 Matsune H., 793 Matveeva V., 153, 325, 361 Mayoral J.A., 487 Mehri A., 605 Melezhyk A.V., 233 Mendez V., 605 Menu C., 533 Meynen V., 321 Migliori A., 621 Mihalyi M.R., 727 Mikenin P.E., 43 Millan J.C., 661 Min Pang, 275 Mingming Zhang, 259 Mino A., 811 Mishin I.V., 707 Mitchell P.C.H., 475 Mitov I., 547 Miyanaga H., 85 Mizushima T., 695 Molina R., 513 Montes M., 25, 639, 661 Monti M., 51 Monzon A., 483 Moon Suk Han, 441 Morandi V., 621 Moreau J., 497 Moreaud M., 127 Moreno S., 513 Moriga T., 201 Muhler M., 161, 217

Munoz-Andres V., 751 Murzin D. Yu., 283 Mutel B., 17 Muylaert I., 329 Myrstad R., 685 Mysik A.A., 289 Nakagawa K., 201, 793 Navarro R.M., 449 Nebra M., 483 Nedyalkova R., 177 Negro J., 493 Nemouchi S., 377 Nguyen D. Tuyen, 497 Nguyen Dinh M.T., 587 Nguyen H. Hao, 501 Nguyen Q. Tuan, 497 Nguyen T. Kien, 501 Nikolaev S.A., 263 Nikoshvili L., 153 Nisenbaum V.D., 537 Nitsch X., 601 Noskov A.S., 109, 509 Nosova T.A., 413 Odriozola J.A., 25, 597, 601, 661 Ohkita H., 695 Okal J., 675 Okayama T., 201 Oliveira Neto A., 551, 555, 559 Onfroy T., 145 Ortolani L., 621 Ould-Chikh S., 193 P.G. Vazquez P.G., 425 P.I. Villabrille P.I., 425 Palacio M., 425 Palmisano L., 225 Panagiotou G.D., 117, 613, 643 Paneva D., 547 Pantaleo G., 401 Pappacena A., 835 Pashigreva A.V., 109, 509 Patterson R.E., 209 Paukshtis E.A., 43 Paul S., 17, 811 Pavan S., 193 Pavlova O.S., 759 Payen E., 567 Pellegrini R., 433, 437

854

Pendem C., 541 Perez A., 513 Perez-Cadenas M., 647 Pernicone N., 9 Petsi Th., 117, 643 Phuc N.H.H., 695 Pichugina D.A., 297 Pietrowski M., 505 Pinna F., 9 Pino E.S., 555 Pintar A., 245 Pirard J.-P., 699 Pirault-Roy L., 467 Pizzio L., 405 Plyasova L.M., 355, 479 Pohl M., 315 Pollesel P., 653 Poluboyarov V.A., 537 Popova G. Ya, 463, 479 Poupin C., 467 Prado Baston E., 671 Prosvirin I.P., 109 Prudius S.V., 233 Puleo F., 401, 417 Quiroz-Torres J., 517 Rabia C., 377, 665 Ramos Medeiros A.S., 815 Raneri A., 493 Rasmussen S.B., 735, 739 Rebeilleau M., 567 Rebolledo A.F., 347 Rebours B., 127 Regalbuto J.R., 169, 715 Requies J., 453 Reubroycharoen P., 689 Revel R., 127 Ribbens S., 321 Riccobene P.M., 785 Rives A., 797 Rodemerck U., 581 Rodriguez-Ramos I., 719, 779 Roger A.C., 601 Rogov V.A., 355, 463 Romanelli G.P., 425 Roman-Martinez M.C., 647 Romero-Sarria F., 597, 601 Roquero P., 525 Rossignol F., 241

Author index Rostovshchikova T.N., 263 Roucoux A., 145 Rouleau L., 193 Royer S., 587 Ruiz P., 789 Rytter E., 685 S.M. Jung, 441 Saadi A., 377 Sagar A., 835 Sahin S., 283 Saja C., 493 Salinas-Martinez de Lecea C., 647 Salmi T., 283 Sanchez C., 521 Sanchez-Sanchez M.C., 449, 751 Sanz O., 25, 639, 661 Sassine R. 145 Sayad N., 377 Sayah E., 711 Scavetta E., 51 Schermanz K., 835 Schlögl R., 283 Schuhmann W., 161 Schulz A., 501 Scirè S., 785 Sels B., 771 Serp P., 629 Shimada S., 843 Shimura K., 85 Shutilov A.A., 369 Sidorov A., 325 Siffert S., 743, 747 Silva A.M.T., 457 Silva D.F., 555 Slavinskaya E.M., 355 Sobalik Z., 823 Sobczak I., 333 Soled S., 101 Songhun Yoon, 339 Soria M.A., 779 Soriano A., 525 Soria-Sanchez M., 751 Sotowa K.-I., 201 Spadaro L., 493 Spanos N., 373 Specchia S., 59 Specchia V., 59 Spinacé E.V., 551 Srasra M., 831

855

Author index Stavropoulos J., 117 Stucky G.D., 1 Steert K., 321 Stoyanova M., 581 Su B.L., 185, 665, 743 Sugiyama S., 201 Suknev A.P., 43 Sulman E., 153, 325, 361 Sun Joo Kim, 441 Supiot Ph., 17 Sutormina E.F., 343 Tabakova T., 743 Taibi-Benziada A., 377 Takeda S., 849 Takei T., 843 Takenada S., 793 Tangkanaporn N., 689 Tanimoto Y., 201 Tarasov A.L., 537 Tartaj P., 347 Tenchev K., 547 Tessonnier J.-Ph., 283 Tharamani Chikka Nagaiah, 161 Théron M., 755 Thomazeau C., 521 Tian-Yi Ma, 571 Tidahy H.L., 743 Tikhonov B., 325 Tiozzo C., 785 Tkachenko O.P., 537, 707 Todorova S., 547 Toledo J.A., 767 Tonelli D., 51 Tran M. Cuong, 497 Tran T.K. Hoa, 497, 501 Trejda M., 445 Trela E., 193 Trevisan V., 9 Tribalis A., 613 Trifiro F., 785 Tronconi E., 653 Trovarelli A., 835 Trunfio G. 493 Tsilomelekis G., 613 Tuel A., 221 Turakulova A., 289 Turakulova A.O., 293, 305

Turki A. 593 Tusi M.M., 551 Uimin M.A., 289 Urquieta-Gonzalez E.A., 657, 671 Vaccari A., 51, 241, 471 Valencia D., 529 Valyon J., 727 Van de Vyver S., 771 Van Der Voort P., 329, 365 van Donck S., 567 Van Speybroeck V., 329 Vannier R.N., 301 Vargas J.C., 385 Vercaemst C., 365 Verjulio-Silva R.W.R., 551 Verpoort F., 365 Vidick D., 827 Villa A., 283 Villalba J.C., 551 Villaroel M., 739 Visconti C.G., 653 Vodyankina O.V., 723, 759 Voronova G.A., 723 Vovk E.I., 343 Vu A. Tuan, 497, 501 Wachter W., 101 Walerczyk W., 675 Wei Chu, 253 Wei Xia, 161 Witte P.T., 135 Wohlrab S., 315 Wojciechowska M., 429, 505 Wojcieszak R., 445, 789 Wojtaszek A., 445 Wolters M., 69 Woo H., 101 Xiao Chen, 77 Xiaoling Ma, 625 Xiu-Zhen Lin, 571 Yamani Z., 279 Yates M., 735, 739 Yavsin D.A., 263 Ye Tian, 625

856 Yermakov A. Yu., 289 Yongdan Li, 625 Yongtao Meng, 271 Yoshida H., 85 Yoshida T., 85 Yuan Z.-Y., 547 Zacchini S., 785 Zagoriuko A.N., 43 Zaikovskii V.I., 109

Author index Zaletova N., 305 Zanaveskin K., 289 Zawadzki M., 675 Zenkovets G.A., 369 Zhaoxiang Yu, 271 Zhengfeng Shao, 77 Zhiqiang Ma, 77 Zhirong Zhu, 271 Zielinski M., 429 Ziolek M., 333