Advances in Catalysis, Volume 36

ADVANCES IN CATALYSIS VOLUME 36

Advisory Board

M. BOUDART Stanford. Calijomia

V. B. KAZANSKY Moscow, USSR.

G. A. SOMORJAI Berkeley. Calijornia

G. ERTL Berlid!hlem. F.R.G.

A. OZAKI Ilbkya Japan

W. 0. HAAG Princeton, New Jersey

W. M. H. SACHTLER Evanston, Illinoh

J. M. THOMAS London, UK.

ADVANCES IN CATALYSIS VOLUME 36

Edited by D. D. ELEY The University Nottingham. England

PAULB. WEISZ

HERMAN PINES Northwestern University Evanston, Illinois

University of &nnsylvania Philadelphia, &nnsylvania

ACADEMIC PRESS, INC. Hmourt Brace Jovnnovich, Publishers

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Contents CONTRIBUTORS ............................................................... PREFACE ....................................................................

vii ix

Studies of Model Catalysts with Well-Defined Surfaces Combining Ultrahigh Vacuum Surface Characterization with Medium- and High-pressure Kinetics

.

CHARLES T CAMPBELL

I. I1 I11. IV

. .

Introduction ....................................................... Experimental Design Considerations................................... Elucidation of Structure-Function Relationships in Catalysis ............. Conclusion ......................................................... References .........................................................

2 4 14 47 49

The influence of Particle Size on the Catalytic Properties of Supported Metals

.

MICHELCHE AND CARROLL 0 BENNETT I. I1 I11.

.

. . VI . VII. IV V

. . .

I I1. I11 IV V. VI .

Introduction ....................................................... Preparation of Supported Metal Particles .............................. Background for Evaluating the Catalytic Properties of Small Supported Particles ......................................... Characterization of Small Supported Metal Particles .................... Effect of Particle Size on lhrnover Frequency and Selectivity: Presentation and Comparison of the Data ............................. Possible Explanations of Particle Size Effects: Experiments versus Models ........................................... Conclusions and Future Directions .................................... References .........................................................

Metal-Support interaction: Group Vill Metals and Reducible Oxides GARYL. HALLER AND DANIEL E. RESASCO Introduction ....................................................... Titania-Supported Catalysts .......................................... Model Studies ...................................................... A Comparison of Rh/TiOl and Pt/TiO, ...............................

55 59 72 91 109 140 155 160

173 179 203

209 Current Explanations of the Promoting Effect of TiO, on Catalytic Activity ......................................... 214 Bonding and Charge 'Ikansfer in Group VIII-Ti Systems ................. 221 V

vi

CONTENTS

VII. VIII

.

I.

. Ill . IV. V. VI. VII . VIII . IX. 11

Metal-Support Interactions in Other Oxide Supports and Related Phenomena ............................................. Conclusions ........................................................ References .........................................................

226 229 230

Structure and Reactivity of Perovskite-Type Oxides LUISG. TEJUCA.Jost LUISG. FIERRO. AND JUANM . D. TASC6N Introduction ....................................................... Structure .......................................................... Preparation ........................................................ Nonstoichiometry ................................................... Stability in a Reducing Atmosphere ................................... Adsorption Studies .................................................. Perovskites in Catalysis .............................................. Miscellaneous ...................................................... Prospective Lines of Research ........................................ References .........................................................

237 240 244 254 258 270 279 315 318 319

New Catalytic Materials from Amorphous Metal Alloys ARPADMOLNAR.GERARD v . SMITH.

. 111. I I1.

IV.

AND

MIHALYB A R T ~ K

Introduction ....................................................... Preparation and Characterization of Amorphous Alloys ................. Amorphous Alloys in Catalytic 'Ifansformations ........................ Conclusions ........................................................ References .........................................................

329 330 336 374 317

ADDENDUM TO STRUCTURE AND REACTIVITY OF PEROVSKITE-TYPE OXIDES ............ 385 INDEX...................................................................... 387

Contributors Numbers in parentheses indicate the pages on which the authors' contributions begin.

MrnALY BART~K, Department of Organic Chemistry,J b m f Attila University, Szeged, Hungary (329) CARROLL 0. BENNETT,Department of Chemical Engineering, University of Connecticut, Storm, Connecticut 06268 (55) CHARLES T. CAMPBELL ChemistryDepartment, Indiana University,Bloomington, Indiana 47405 (1) MICHELCHE, Laboratoire de Rkactivitk de Surface et Structure, UA 1106, CNRS, Universitk Pierre et Marie Curie (Ihris Vl). 75252 Aris Cedex 05,

Fmnce ( 5 5 ) JOSELUISG. FIERRO, Instituto de Catdlisisy €ktmleoqulmica. CS.I.C, Sermno 119, 28006 Madrid, Spain (237) GARYL. HALLER Department of Chemical Engineering, Yale University,Nav Haven, Connecticut 06520 (173) ARPADM O L N ~ Department , of Organic Chemistry,Jbzsef Attila University, Szeged, Hungary (329) DANIELE. RESASCO,Department of Chemical Engineering, Yale University, New Haven, Connecticut 06520 (173) GERARDV. SMITH,Department of Chemistry and the Molecular Science h g m m , Southern Illinois University, Carbondale, Illinois 62901 (329) JUANM. D.TAXON,Instituto Nacional del Carbdn y sus Brivados, C S I C , Apartado 73, 33080 Oviedo, Spain (237) LUISG. TEJUCA, Instituto de Cbtdlisisy &tmleoquulmica, CSIC, Sermno 119, 28006 Madrid, Spain (237)

vii

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We live in the midst of an information explosion. Publications are mushrooming. Even journals dedicated to any one “field” like catalysis are multiplying. Furthermore, when we look at catalysis, we deal with a diversity of basic ingredients and interests which can be equally at home in many disciplines (chemistry, solid state physics, materials science, enzymology, chemical engineering, electro-chemistry, crystallography, to name but a few). How do we keep informed when the proliferation of reporting is so massive and scattered over a wide spectrum of the scientific information media? The answer is, and always will be, “with difficulty.” But progress in science is dependent on two equal partners in symbiotic coexistence: (1) the generation of knowledge and (2) its successful transfer. The latter has been the legitimate driving force behind the tradition of publishing. But printing alone does not lead to a successful implant in the body of knowledge In fact, the more that gets offered in print, the more difficult it becomes for that body to choose the essentials for its growth. Mere quantity of reported data, theories, and concepts does not contribute to the progress of science. It takes a continuing process of sieving, identifying, and formulating the probable key ingredients. In good science, the key concepts, observations, and theories are those that produce the most consistency within the broader experience of the field, as well as within the basic sciences in general. That characterizes the environment, challenge, and task of the editors and authors of a series such as Advances in Catalysis. It is an objective which is not served by the daily stream of publications. Nor is it served by books with subject titles of interest to us, but which are collections of diverse and unconnected symposium papers. Instead, we seek to fill the gap between the papers that “report” and the textbooks that “teach.” We look for the competent sketch of the current state of knowledge that will help us grasp, teach, or use the current “total picture”: its key elements, their meaning and implications, and the associated challenges and remaining inadequacies. Writing our traditional day-to-day publications has a large element of “look what I have done now,” with comments on what it does to the field of knowledge. On the other hand, our authors must first focus on the field of knowledge, with appropriate comments on individual contributions of course Such a task is not easy. There is not enough space to discuss all contributions, and in their choices and analyses authors cannot be infallible predictors of the ultimate truth. If that were the case, the final chapter would have been written, not in a series of Advances, but of Arrivals. We are fortunate to have authors who have dedicated their efforts to ix

X

PREFACE

recording progress. This volume of Advances could well be considered a textbook on the current state of knowledge concerning structure and catalysis of metals and metal oxide particles, old and new. Campbell addresses the basic and broad problems of what the catalytically relevant surface structures of metals are, where we stand in techniques capable of attacking this problem, and what the current state of knowledge is. The focus is on a long-standing, important, and central problem of general investigative methodology and strategy: the “pressure gap” is created by the fact that the best techniques of surface analysis require high-vacuum conditions, while “useful” catalysis is confined to conditions of near ambient or higher pressures. Che and Bennett review the basic question of the influence of particle size on catalytic behavior of metal particles which involves questions of the basic sciences as much as practical considerations of catalyst design and use. There are so many aspects involved that this chapter is a unique resource of information on preparatory techniques, analytical technology, and methods of characterization of these materials, as well as a discussion of the many phenomena, mechanisms, and catalytic effects that deal with the question of the dependence on particle size. Haller and Resasco provide an in-depth examination of another basic phenomenon, the existence and nature of interaction between metals and their oxide supports. Their approach focuses on an exhaustive study of titania as a support. That consideration then becomes a useful and central model from which to view the behavior of other supports. Tejuca, Fierro, and Tascbn review the perovskite class of metal oxides. Many challenging concepts have been associated with these. They called early attention to the relationship of catalytic activity to electronic properties; they have been seen as potential substitutes of platinum; and more recently the discovery of superconductivity has rekindled the interest in the “electronic connection” of oxide catalysts. Smith, Molnir, and Bartbk introduce us to amorphous metal alloys, recent newcomers to the scene of materials science, and catalysis. They offer new variants of metal structures to research and application. The authors point to work (some 90 publications) of only about the last 7 years, to remaining questions, and to potentials barely touched. Reading these topics in the same volume, one discovers existing and potential relationships between them; one experiences some sparks of recognition of further relationships and of matters one might connect to develop new knowledge or innovations. We hope that this is an additional attribute of the role and manner of reporting in Advances.

PAULB. WEISZ

ADVANCES IN CATALYSIS, VOLUME 36

Studies of Model Catalysts with Well-Defined Surfaces Combining Ultrahigh Vacuum Surface Characterization with Medium- and High-pressure Kinetics CHARLES T. CAMPBELL Chemistry Department Indianu University Bloomington, lndiunu 47405

The development over the past two decades of many powerful techniques for surface characterization promised to open the area of heterogeneous catalysis for a more fundamental understanding of the relationships between atomic-level surface structure and catalytic activity. The realization of this promise has been hampered by a “pressure-gap’’ problem: the reactions are generally performed at pressures above - 100 torr, whereas surface analysis is accomplishable only in ultrahigh vacuum. To overcome this problem, many researchers have recently developed equipment that combines ultrahigh vacuum surface analytical instrumentation (AES, XPS, LEED, TDS, ISS, etc.) with a medium- or high-pressure microreactor in a single apparatus, where the catalyst can be transferred between these pressure regimes without intermediate exposure to air or other gases. A “model catalyst” approach has generally been adopted, whereby well-defined surfaces are prepared (usually starting from a single crystal) that are very homogeneous in surface structure and chemical composition. Through comparison of the activity and selectivity of a series of such model catalysts with controlled changes in detailed surface structure, a fundamental understanding of structure-function relationships in the surface chemistry of catalysis is beginning to evolve. The purposes of this paper are to (1) review the apparatus design considerations for experiments of this type, (2) review the results that have been obtained thus far using this model catalyst approach in combined presI Copyright 0 1989 by Academic Press. Inc. All rights of reproduction in any form reserved.

2

CHARLES T. CAMPBELL

sure-vacuum experiments, and (3) discuss the future of this area of research. The results so far have given exciting but not yet completely satisfying insights into many catalytic phenomena including structural sensitivity, surface modification, poisons, alkali promoters, ensemble and electronic effects at bimetallic surfaces, and strong-metal support interactions. I. Introduction

It has long been a goal of the catalytic chemist to fundamentally understand the relationship between the atomic level structure of a catalyst surface and the materials’ performance as a catalyst (Lea,its activity and selectivity). Much beautiful work has been dedicated to this effort, especially since development of modern surface analytical techniques such as the many electron spectroscopies. A major stumbling block to progress in this direction has been the fact that, while our most powerful surface analytical techniques operate only under the pristine conditions of ultrahigh vacuum (UHV), most catalytic reactions of commercial importance occur with measurable rates only at pressures of a few torr and above. Thus, in order to inspect the surface of the working catalyst, one is faced with the problem of decreasing the pressure above the catalyst by some ten orders of magnitude while at the same time maintaining its surface chemical integrity. This is, of course, an impossible task since the weakly adsorbed species (i.e., those with short residence times) are in equilibrium with the gas phase and removed almost as rapidly. Nevertheless, much effort has been devoted to developing techniques that allow for UHV inspection of catalyst surfaces that retain, as well as possible, their high-pressure structural characteristics and thereby “bridge” this “pressure gap.” In 1974 the Somojai group published a pioneering study that combined catalytic kinetic measurements at pressures up to 1 atm with pre- and postreaction surface analysis in UHV in the same apparatus (I). The group soon developed new systems that extended the kinetic regime up to -100 atm. (2, 3). A few years later, the group of Yates et al. had developed and proved a system that allowed a catalyst sample to be transferred relatively rapidly (a few minutes) between a vessel at high-pressure reaction conditions and a UHV chamber for surface analysis (4, 5 ) . At about the same time, the groups of Bonze1 et al. (6, 7) and Polizzotti and Schwarz (8)developed similar systems, each with its own relative advantages. Since then, many applications of these and related systems have been published (see below), and numerous improvements to the original

PRESSURE-VACUUM ANALYSIS OF MODEL CATALYSTS

3

designs have appeared (9-12). It is safe to say that this combination of high-pressure reaction kinetics or sample treatment with UHV surface analyses is now a more or less routine method of the surface scientist’s trade, and it is finding its way into many application areas such as corrosion science (1.3-15) and electrochemistry (14-18). A most enlightening utilization of these high-pressure-UHV methods involves what is referred to as the “model catalyst” approach, whereby a surface derived from a clean and well-defined, single-crystal plane is used to model a particular site or set of sites expected to exist on practical highsurface-area catalysts. Commercial catalysts are exceedingly complex materials, the surface analysis of which would undoubtedly lead to clear identification of eight or more different elements and countless structurally distinct surface moieties. The catalytic surface chemist wants to know which of these elements and surface structures are responsible for catalyst activity and selectivity, and how they participate in the catalytic reaction. Therefore, model surface structures of one or several elements are synthesized, usually starting from a polished and well-oriented singlecrystal surface. In doing this, the chemist takes advantage of “preparatory surface chemistry in UHV.” Thus, a model catalyst is created, the surface of which is now extremely homogeneous in a certain structural moiety of interest. This catalyst can then be transferred to the highpressure reaction conditions for kinetic characterization, where the activity and selectivity of the model catalyst are measured. After reaction, the chemist reexamines the surface in UHV to ensure that the structural details that were tuned into the starting material have not changed as a consequence of the reaction, or, if so, to determine exactly what is the structure of the working catalyst surface that has been kinetically characterized. Because the model catalyst is based on a single-crystal surface, the whole arsenal of surface analytical techniques can be brought to bear on these problems, including those techniques that rely on long-range surface order or smoothness. The purpose of the present article is to review the existing literature with respect to model catalyst studies in which attempts have been made to obtain and compare high- or medium-pressure kinetics over well-characterized, homogeneous surfaces. (For the purposes of this article I will define the medium-to-high pressure range as -0.1 torr to -200 atm.) I start with a thorough discussion of the design criteria that should be considered in constructing an apparatus and preparing samples for such studies. I then summarize the results of such studies and show that structure-function relationships in the surface chemistry of catalysis can, indeed, be determined and understood by using this approach. Some attempt has been made here to present a relatively exhaustive bibliography

4

CHARLES T. CAMPBELL

of these types of studies, at least of those using model catalysts based on single-crystal surfaces. That is not to say that every article has been included here, and I apologize for any inadvertent omissions. I also should point out that in several cases I have omitted papers for brevity that are reviewed thoroughly in those articles that I do discuss, so that they should be relatively easy for the reader to locate. In this presentation I also attempt to discuss the articles within the more global perspective of this review format, in some cases pointing out contradictions in results or potential problems in interpretation. For most cases, only a few of the major features of a given article are presented. There is a vast amount of information in these articles that I could not cover in the limited space of this review. The reader is encouraged to seek out these articles, for they are rich in results and insightful in discussion. Finally, I conclude by attempting to critically evaluate the field in general, its impact thus far on catalytic concepts, and its potential for fundamental contributions to our understanding in the future. II. Experimental Design Considerations In general, the types of studies to be discussed here utilize a single apparatus that interfaces a microreactor for high-pressure kinetic studies with an ultrahigh vacuum (UHV)chamber for surface analysis and adsorption-desorption studies. In a typical experiment, a model catalyst is prepared by orienting, cutting, and polishing a single-crystalsurface. This sample is then mounted in the UHV chamber and the desired surface structure is prepared by using what can be called “preparatory surface chemistry.” This can include such steps as sputter cleaning with an argon ion beam, high-temperature chemical cleaning, annealing to establish surface order, vapor deposition or chemical vapor deposition of other elements (e.g., oxidation) to modify the surface composition, ion-beam surface modification, and molecular-beam epitaxy . The chemical treatments are sometimes even performed at higher pressures in the microreactor or in an atmospheric load-lock position (e.g., liquid droplets of ultrapure solutions). The surface is then characterized with an array of UHV surface analytical techniques. Typical methods of choice include Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), ionscattering spectroscopy (ISS), secondary ion mass spectroscopy (SIMS), high-resolution electron energy loss spectroscopy (EELS), and many others. Since the samples usually have smooth single-crystal surfaces, methods such as low-energy electron diffraction (LEED) and other surface crystallographic methods that rely on long-range surface order can

PRESSURE-VACUUM ANALYSIS OF MODEL CATALYSTS

5

also be used here. The surface analytical methods themselves will not be discussed here since they have been excellently reviewed elsewhere (1922). Following surface characterization, the sample is enclosed in the microreactor; it is pressurized with ultrapure reactant gases, and a catalytic reaction is initiated by heating the sample to the desired reaction temperature. The rate of the reaction is determined from product buildup versus time by monitoring the composition of the gaseous reaction mixture using gas chromatography, mass spectroscopy, or other methods. After reaction, the sample is again evacuated and the surface is structurally and compositionally analyzed in UHV to determine the characteristics of the working catalyst’s surface. Extreme caution must, of course, be exercised here to ensure that the surface structures observed in postreaction surface analysis are indeed those characteristic of the working catalyst under high-pressure conditions. As noted above, this is, strictly speaking, an impossible task since weakly adsorbed species with short surface residence times (<1 s) are always pumped away almost immediately as the supporting gas phase is removed. However, to assess structure-function relationships in catalysis, it is really those more permanent aspects of the surface structure that are of most interest here. For an alloy catalyst, what is the elemental composition and metal distribution at the surface? For an oxide catalyst, what is the oxidation state or stoichiometry at the working catalyst’s surface? Often, lattice mobility is slow enough at reaction temperature (compared to the transfer time into UHV) that questions such as these can, indeed, be answered. Thus, the concern is not to maintain the adsorbate coverage of all reaction intermediates exactly as they were at high pressures, but only to maintain the surface structural features of the catalytic material itself. One is, therefore, concerned with impurity problems during the transfer into UHV, such as (1) the deposition of impurity elements onto the catalyst surface from background gases, pump oils, or vapors from the vessel walls and (2) the reduction of surface oxides by reactions with background gases such as CO or H2, which are ubiquitous in vacuum technology. Once inside UHV, these problems are well under control and the model catalyst can (in principle) be rapidly cooled to minimize thermally induced changes in the catalyst surface such as, for example, lattice diffusion in an alloy or oxide. It is clear that the best experimental designs for addressing structurefunction relationships in catalysis are those that minimize exposure of the catalyst surface to undesired vapors between high-pressure kinetic measurements and surface analysis. Thus one aims for a system where the transfer to UHV is as rapid and as clean as possible, and where the sample can be cooled as rapidly as possible once inside UHV.

6

CHARLES T. CAMPBELL

Very stringent cleanliness requirements are vital within the microreactor itself, for the sample surface must not be violated by foreign contaminants deposited during high-pressure reaction. Thus, to the extent possible, the microreactor and sample transfer device must be constructed of low-surface-area, nonporous, and bakeable materials. It is generally desirable that the microreactor itself be able to achieve UHV with the sample in position. The design should also ensure that, when heating the sample at high-pressure reaction conditions, very little surface area other than the sample itself becomes warm. This minimizes outgassing of contaminants and undesired side reactions and catalytic processes not due to the sample itself. Typical sample mountings are shown in Fig. 1 , where thin (0.010-0.020 in.) heating wires are spot-welded or otherwise attached to the back or sides of the single crystal. The choice of wire depends on the reaction and sample to be studied, but tantalum is often a good inert choice. (Other metals such as tungsten should be chosen if high hydrogen pressures will be used, since Ta embrittles under Hz.) These wires are then attached across two relatively massive electrically insulated conductors which themselves are in good thermal contact with a room-temperature thermal bath, such as an air-cooled transfer rod. When a voltage is externally applied across the conductors, current flows through the heating wires (and the sample) and the wires start to heat. Because the sample is in good thermal contact with these wires, it also becomes hot. A thermocouple spot-welded or otherwise attached to the sample can be used to monitor its temperature, as shown. The thermocouple wires should be thin (0.0030.005 in.) and inert to minimize side reactions on them. The lengths of the heating wires between the points where they contact the sample and the conductors should be -2-3 mm for O.OlO-in.-diameterheating wires. This is long enough to ensure sufficient heating without excessive current (< 35 A) but short enough to ensure that the sample will cool rapidly back toward room temperature when the heating current is removed. In this design, only the sample and the heating wires and a short length of thermocouple wire are heated significantly above room temperature during sample heating in the microreactor. It is also useful to note that, since the mean free path of a typical gas even at 1 torr is only -0.05 mm, the sample is usually surrounded by a thin film of gas that is at the same temperature as the sample under reaction conditions. Thus, all gas-surface collisions can be described by a single temperature, equal to the sample temperature as recorded by the thermocouple. A visual check that the thermocouple is properly attached should be performed periodically during use. According to Davis et al. (23), the threshold temperature for observing visible emission from the sample in a completely dark laboratory is 785 & 10 K

7

PRESSURE-VACUUM ANALYSIS OF MODEL CATALYSTS

a CRYSTAL TRANSFER ROD

MODEL

ELECTRICAL FEEDTHROUGH

SLOT FOR WIRES

b

I

5 CM

I

THERMOCOUPLE -5% Re / W-26% Re)

PLE

THERMOCOUPLE FEEDTHROUGH (CERAMASEAL 80780572-1)

MOLYBDENUM SAMPLE HOLDERS

FIG.I . (a) Details of the sample mounting on the transfer rod for resistive heating (via Ta wires) and liquid nitrogen cooling (via conduction down the Cu feedthrough rods). From Ref. 10. (b) Sample head and sample mounting configuration. From Ref. 176.

(at least for metal samples). (This agrees with our experiences for Cu, Ag, and Pt samples.) Care must be exercised in all choices of material. For example, the deposition of Ni onto the model catalyst surface has been observed under reaction conditions when using nickel alloy thermocouples. It is the responsibility of the experimenter to prove that the crystal surface probed by the surface analytical methods truly represents that probed by the high-pressure kinetics. For example, in assessment of structural sensitivity, kinetics on several different single crystals with

8

CHARLES T . CAMPBELL

different surface orientations will be compared. These crystals are typically polished wafers where both faces are of the desired orientation. However, the surface orientations of the edges of these wafers are ill defined and not probed by surface analysis. Similarly, when surface additives are used to modify reactivity, these are typically dosed only to the front surface of the crystal under study. For these reasons, it is generally convenient to somehow passivate the edges and backs of the crystal. In our experience, if the front surface is cleaned by means of Ar+ sputter cleaning and care is taken to ensure that only this face is sputtered, a passivating film of impurities will naturally develop on the remainder of the crystal (by surface segregation and deposition from the gas phase). This can be verified once reproducible kinetics have been established by dosing a poison to the front surface alone and proving that this completely quenches the observed catalytic rate. Depending on the reaction studied, the front surface can be passivated for this background experiment by using a directed sulfur or chlorine doser (24, 25) or vapor-depositing an inert metal overlayer such as Au or Bi onto the front surface. It is important following such a poisoning experiment to show that the original activity of the sample can be readily regenerated by a brief sputter cleaning of the front face of the sample. Another confidence-building test is to restart the high-pressure reaction after surface analysis to ensure that the same rate is again immediately achieved. Another important consideration is the volume and compactness of the microreactor. The reactants and products must be well mixed and at observable concentrations for accurate kinetic measurements. Using a typical gas diffusivity at atmospheric pressure and room temperature D = 0.1 cm2/s, the characteristic diffusion length (0 during t = 100 s is only approximately 3 crn ( I = f i t = 3 cm). Because of convection currents due to sample heating, the actual mixing of products into a batch microreactor is considerably faster than this. Nevertheless, it should not be expected that the products will be efficiently mixed into remote parts of the microreactor that are further than -5 cm from the sample during time periods of less than 100 s. For this reason, the microreactor should be either a compact design, such as in Ref. f0, or include a recirculating mixer, such as in Ref. 2. The mixer and associated tubing should also be designed to avoid impurities or wall reaction problems. For this and several other reasons, we (10) and others (5) have preferred a compact batch reactor design. Self-mixing has always been sufficiently rapid in our experiments ( < I atrn), but it could undoubtedly be problematically slow at very high pressures. We have also considered mounting a small bellows on the side of the microreactor that could be manually pumped periodically to mix the gases.

PRESSURE-VACUUM

ANALYSIS OF MODEL CATALYSTS

9

Single-crystal catalysts are very low in surface area (-1 cm2). For molecules/site/s), the overall amount typical catalytic reaction rates ( of product produced in 100 s is rather small: Product = =

molecules/site/s)( 1015 sites/cm2)(1 cm2)(100 s) lOI5 molecules

If the microreactor is at I-atm pressure and has a volume of 1 liter, this corresponds to only about 40 ppb for detection. Since typical gas chromatographic (GC)detection limits are between 0.1 to 1 ppm, it is advantageous to reduce the microreactor volume by at least a factor of 10 to 100 ml or less. There are some trade-offs here because, as the reactor volume gets smaller, it becomes more difficult to take small-volume aliquots for GC analysis without significantly decreasing the reactor pressure. Similarly, it becomes more difficult with faster reactions to ensure, for example, that you are operating in a region that can be considered the limit of low conversion (i.e., no further reactions or inhibition by the products). However, yet another factor favors a small-volume microreactor. This relates to the purity requirements on reactant gases. At I atm of pressure, a trace impurity at the 0.1-ppm level serves as a potential source for about one full monolayer of impurity on the model catalyst for every 0.33 liter of reactor volume. We have chosen a reactor volume of only -30 ml (101, which greatly diminishes the purity requirements placed on reactants compared to larger designs. In our case, every I-ml aliquot extracted for GC sampling diminishes the reactor pressure by -3%. In practice, the first aliquot must always be discarded due to poor gas mixing in the valve body. In some cases, the microreactor is operated as a continuous flow device ( 6 ) , but this method suffers the same disadvantages as a largevolume batch reactor. Because of the difficulty of building a valving system capable of fully utilizing the gas aliquot extracted for analysis, we have developed a method that we call the carrier gas piston for signal enhancement in GC sampling. This is always useful when the sample pressure is below the GC carrier gas pressure. In such cases, a rapid compression of the reactor aliquot into the GC sampling loop is accomplished immediately prior to injection by using a reservoir of the carrier gas effectively as a piston for driving the gas. Details of the method, which can give increases in sensitivity by factors up to 20, can be found elsewhere (26). Cold fingers for trapping condensable product are also useful in this respect. The means of interfacing the microreactor to the UHV chamber also requires considerable thought, for, as noted above, the transition between high-pressure reaction conditions and UHV must be as rapid and as clean as possible. The two basic designs employed almost exclusively utilize

10

CHARLES T. CAMPBELL

either of the following: (1) a movable cup that seals around the sample and its holder in a standard UHV chamber and therefore acts as a highpressure cell (1-3, 12) or (2) a transfer rod on which the sample is mounted for heating and by which the sample can be translated between physically isolated UHV and high-pressure vessels (5-11). Examples of these types of designs that have seen extensive successful use are shown in Figs. 2-4. Any parts of the apparatus that see the high-pressure reac-

-

-CARRIER GAS

1

818 -E I

I

w rn

'L

SS WELDED BELLOWS

1

MANIFOLD

CIRCULATION PUMP

b FIG. 2. (a) Schematic diagram of a one-tier HPLP system with high-pressure cell closed. (b) Detail with high-pressurecell open. (From Ref. 9 . )

FIG.3. Ultrahigh vacuum apparatus for studying single-crystalcatalysts before and after operation at high pressure in catalytic reactor. Position I : crystal is in position for Auger electron spectroscopy study of surface composition or of ultraviolet photoemission spectrum of surface species. Position 2: crystal is in position for deposition of a known coverage of poisons or promoters for a study of their influence on the rate of a catalytic reaction. Position 3: crystal is in position for a study of catalytic reaction rate at elevated pressures ( 5 2 atm). Gas at high pressure may be circulated by using a pump; mass spectrometric-gas chromatographic analysis of the reactants and products is carried out by sampling the catalytic chamber. From Ref. 5 .

B

A SURFACE ANA LY S IS

C MICROREACTOR

FIG.4. Apparatus combining UHV surface analytical chambers (stages A and B) with a microreactor (stage C) capable of high-pressure kinetic measurements on model, low-surface-area ( 1 cm2)catalysts. Rapid (I7 s) transfer of the crystal between the stages is accomplished by a transfer rod on which the sample is mounted. From Ref. 10.

12

CHARLES T. CAMPBELL

tion mixture are likely to outgas once inside U H V and cause vacuum problems. One disadvantage of the “pressure cup” design is that the entire surface area of the microreactor is outgassing to UHV. In some “transfer rod” systems, only the sample itself and a small section of transfer rod outgas into U H V following high-pressure reactions (10). Another disadvantage of the “pressure cup” design is the necessity to have the gas handling system and GC sampling system physically separated by some distance from the microreactor itself, because of the presence of the vacuum chamber and surface analysis components surrounding the highpressure cell. This generally has led to the use of a gas recirculating pump in a batch reactor design (2, 9, f2),as shown in Fig. 2. This increases the volume of gas involved and greatly increases the amount of foreign surface area in intimate communication with the sample surface. Both of these factors exacerbate problems with surface cleanliness and reliable product detectability. Another disadvantage related to the greatly increased surface area is that these surfaces tend to outgas once evacuated, and times on the order of 15-30 min are required to achieve U H V following reaction (2, 12). A huge advantage of the “cup” design is that it can easily achieve reactor pressures as high as 120 atm (2, 12), which is critical for some reactions. The transfer-rod designs rely either on a long bellows for rod movement (Fig. 3) or on differentially pumped sliding gaskets that seal on the outer diameter of the rod (Fig. 4; see also Refs. 10 and 11). A great advantage of the bellows design is that it can be entirely constructed of bakeable metal parts. A disadvantage of the bellows design is that this entire bellows contributes to the volume and potential outgassing surface area of the microreactor. Restricted conductance along the surface of the transfer rod between the bellows and the microreactor could potentially eliminate this problem. In this case the bellows system begins to resemble one with sliding seals. Advantages of the sliding seal design are (1) its potential for a low-volume and low-surface-area microreactor and (2) the ability to rotate the sample on the transfer rod to access more analytical probes and even perform angularly resolved surface analyses once inside UHV. In this design, that part of the rod that extends outside the differential pumping stages is usually enshrouded in dry N2 to prevent its contamination during high-pressure reaction. The bellows is usually limited to less than a few atm. maximum reactor pressure. In transfer-rod designs, the sample can either be located at the end of the rod ( 4 , 5 , 8 ) ,or within a cutout in the rod (6, 7, 10, If) as was shown in Fig. I . When located at the end of the rod, an all-metal valve is used to isolate the high-pressure cell from U H V during reaction measurements (4, 5 ) . In this case, the microreactor must be evacuated with a separate pump before opening the valve for transfer back to UHV. Since this pump

PRESSURE-VACUUM ANALYSIS OF MODEL CATALYSTS

13

generally joins the microreactor via a relatively small valve (to minimize reactor volume), this evacuation takes some time and sets a lower limit on the time between high pressure and UHV of at least some tens of seconds. For end-mounted sample rods, it is useful to include a section of tubulation between the microreactor and UHV that is only slightly larger than the rod diameter so that a conduction limitation is established to minimize outgassing from the microreactor into the UHV chamber when the rod tip is in UHV. When the sample is mounted in a cutout on the transfer rod as in Fig. 1 , differentially pumped sliding seals are used to isolate the high-pressure cell from UHV. Typically, two stages of differential pumping allow for reactor pressures up to 10 atm with no significant change in the UHV base pressure (10). More stages can be added to increase the maximum reaction pressure. A distinct disadvantage of the sliding seals is that the reaction gas slowly bleeds off through the seals. In a typical design where the reactor volume is only 30 ml, the reactor pressure decreases by -5% for every 5 min of reaction time (10). The time scale can be expanded proportional to the reactor volume, or a large ballast volume of reactants can be attached to the microreactor to maintain constant pressure. This ballast volume need not contribute greatly to problems generally associated with large-volume reactors (such as stringent gas purity and lower product concentrations), since it can be diffusionally isolated from the microreactor. It is straightforward to correct for the influence of product bleed-off in calculating reaction rates from the measurements of product concentration versus time. A ballast volume also eliminates the problem of decreasing reactor pressure due to GC sampling, mentioned above. A great advantage of this “sample cutout” design is that, once inside UHV, only a small portion of the sample transfer rod is outgassing because it experienced the high-pressure reaction mixture. For this reason, UHV can be achieved extremely rapidly following high-pressure reaction. One such system (10) has proved to achieve UHV (< torr and rapidly dropping) within 17 s after reaction at 1 atm of total pressure. We have proposed one design that should be capable of achieving UHV within -1 s following reaction (27). For the transfer of powdered catalysts, systems already exist that allow evacuation from high-pressure, high-temperature reaction conditions down to UHV in fractions of a second (28). It should be emphasized that transfer times on the order of 51 s are a very important goal, since typical turnover frequencies for a catalytic reaction are 10-3-1 molecule/site/s. Thus, transfer times are now potentially achievable that are a small fraction of the average time it takes a site to produce a product molecule. If the rate of reaction is limited by the conversion of some surface intermediate, then rapid transfer has the potential for capturing that intermediate for postreaction analysis.

14

CHARLES T. CAMPBELL

Undoubtedly, the future will see cleaner and cleaner transfer systems that are designed to maintain as much surface structural integrity as possible during evacuation from process conditions. With these advances, our ability to make definitive assessments of structure-function relationships in catalysis will certainly improve. At the same time, we should see many new developments of in situ surface analytical methods that allow inspection of the catalyst surface while actually in the high-pressure reaction environment. Recent applications of reflection-absorption Fourier-transform infrared (FTIR) (29) and extended X-ray absorption fine structure (EXAFS) methods (30-32) show that this is already beginning to occur. Such in situ techniques will eventually render high-pressure-UHV transfer methods obsolete. However, the present author strongly doubts that broad-based in situ surface analytical methods will be developed so soon that the transfer techniques described above will not hold a very important place in the science of catalysis and many other surface-related technologies. One of the important advantages of the model catalyst approach is that the changes in catalyst activity and selectivity with surface structure or composition can be correlated with the influences of these surface modifications on the rates of some of the individual steps in the reaction mechanism. The adsorption kinetics of the reactants and products are typically probed in UHV by measuring surface coverage versus exposure. The reactant and product desorption kinetics are generally determined by using thermal desorption mass spectroscopy (TDS) or some measurement of surface coverage versus temperature (thermal conversion spectroscopy). The power of TDS and thermal conversion spectroscopy for determining detailed kinetic parameters of desorption and other elementary surface reactions has been carefully reviewed elsewhere (33-36). Sometimes, the kinetics of the formation or removal of some adsorbed intermediate can even be determined by timed exposures at higher pressures in the microreactor. Correlating changes in the overall reaction kinetics that are due to surface structural changes with changes in the rates of elementary reaction steps can give key insight into the rate-determining step in the catalytic process, the reaction mechanism, and the role of the various surface structural moieties in converting the reactants to products. 111.

Elucidation of Structure-Function Relationships in Catalysis

Over the past 10 years, a number of groups have become active in applying these experimental techniques to clarify the relationships between atomic-level surface structural-compositional properties and the

PRESSURE-VACUUM

ANALYSIS OF MODEL CATALYSTS

15

activity or selectivity of that surface for a particular catalytic reaction. In this section, we will review the results of those studies and highlight particularly illustrative examples of how they have provided unique information regarding the microscopic details of catalytic processes. A. STRUCTURAL SENSITIVITY: THEDEPENDENCE OF CATALYTIC ACTIVITY A N D SELECTIVITY ON CRYSTAL-SURFACE ORIENTATION It has long been known that catalytic reaction rates and selectivity can depend sensitively on the size of catalyst particles (37-39). Such “structural sensitivity” has generally been explained by models whereby the activity or selectivity of the reaction was assumed to vary markedly with the local geometry of the surface sites. Using the methods outlined above, it has now been unequivocally proved that, indeed, many steady-state high-pressure catalytic reactions depend sensitively on the crystal-surface orientation of the model catalysts, and that others do not. This subject has been recently reviewed by Boudart (40), who points out the potential utility of single-crystal surfaces as standards against which to compare industrial catalysts. One of the earliest reported attempts to determine the structural sensitivity of catalytic reactions over single-crystal surfaces at medium pressures was an investigation of the selective oxidation of ethylene over several different Ag single-crystal surfaces by Kummer in 1956 (41). His studies were not supported by surface analysis as in modern systems, so the state of cleanliness and crystal orientation of the sample surfaces could not be verified. A few years later, Wilson et al. (42) extended this experiment to include postreaction analysis of the silver “surfaces” using (high-energy) electron diffraction. This experiment was a true precursor to the modern approach that combines reaction rate measurements with surface analysis. The samples started out as oriented silver films, but after reaction no preferential orientation remained. Many years later, Campbell (43) repeated these types of experiments with bulk single crystals of silver and the added advantage of a surface analytical chamber attached directly to the medium-pressure catalytic reactor. The results proved that oriented Ag crystal surfaces could be maintained under reaction and showed that no strong structural sensitivity existed for this reaction. The latter conclusion was also suggested by Kummer’s original study (41), but his specific activities (rates of reaction per surface Ag atom) were significantly below those obtained on the verifiably clean Ag surfaces by Campbell (43, 44). This suggests that Kummer’s surfaces may have been largely poisoned by surface impurities. Results on high-surface-areaAg catalysts have some-

16

CHARLES T. CAMPBELL

times indicated structural sensitivity for this reaction. These results, together with the recent single-crystal results, have been critically reviewed by Sajkowski and Boudart (451, who conclude that, indeed, this is a structurally insensitive reaction. This reaction further illustrates one of the advantages and one of the potential pitfalls of the pressure transfer-model catalyst approach. As noted above, Campbell found that Ag( 110) and Ag( 1 11) displayed very similar kinetics and selectivity for the epoxidation of ethylene even when the reaction was clearly rate-limited by oxygen supply. Yet, at reaction temperature, the Ag(ll0) surface was some 50-fold more active than Ag( 11 1) in the dissociative adsorption of 0 2 to form oxygen adatoms (at least for coverages below 00 = 0.5, which was the saturation coverage achievable in the UHV adsorption experiments). From this lack of correlation, he concluded that molecularly adsorbed O2 (rather than oxygen adatoms) must be the actual oxidizing agent involved in both ethylene epoxide and CO2 production (43). Recent isotopic tracer evidence supporting a mechanism involving atomically adsorbed oxygen (46, 47) suggests that the surface reaction may be occurring on oxygen islands of very high local coverage (0, > 0.5), where the activity differences between Ag(ll0) and Ag( 11 1) for 0-0 bond cleavage may not exist (45, 48). Because of their higher reactivity with background gases during transfer to UHV (49), these very high coverage oxygen states were possibly not maintained for postreaction surface analysis. This highlights the need for improved transfer methods in order to maintain truer surface integrity during evacuation. 1. Hydrocarbon Conversion Reactions over Pt

An early champion of the single-crystalapproach to studying structural sensitivity in catalysis was Gabor Somoqai, who, together with his group, has performed numerous comparative studies of hydrocarbon conversion reactions over different oriented Pt single-crystal surfaces (49-53; see also refs. cited below). The 1974 study of the hydrogenolysis of cyclopropane over a stepped Pt single crystal by Kahn et al. ( I ) was the first application of a combined atmospheric-pressure reactor with a UHV surface-analysis chamber for addressing the structural sensitivity of a catalytic reaction. In that case, a favorable comparison of the reaction kinetics with those from supported Pt catalysts suggested that this reaction was relatively insensitive to surface geometric structure. An excellent example of marked structural sensitivity in hydrocarbon conversion reactions is shown in Fig. 5, which compares the accumulation of benzene versus time in a reaction mixture of 20 tom of n-hexane and 200 torr of H2 over

PRESSURE-VACUUM ANALYSIS OF MODEL CATALYSTS

0

30 60 90 120 Reoction Time (minutes)

17

150

FIG.5 . Product accumulation curves measured as a function of reaction time at 573 K for n-hexane aromatization catalyzed over platinum single-crystal surfaces. (0).Pt( 10,8.7); (O), Pt(ll1); (A),Pt(100); (A),Pt(l3,l,l). HJHC = 10. P,,, = 220 torr. From Ref. 54.

four different Pt faces at 573 K (54). As can be seen, the rate of benzene production increased in the order (10,8,7) < (111) < (100) = (13,1,1). Other important processes occurring in this reaction mixture included cyclization to methylcyclopentene, hydrogenolysis to lower alkanes and isomerization to 2- and 3-methylpentane. These processes showed little structural sensitivity, consistent with the conclusion based on studies over supported Pt catalysts (54). Interestingly, the overall initial rates of n-hexane skeletal rearrangement over Pt single crystals was about 10-fold faster than initial rates for the same reaction over powdered and supported Pt catalysts (54). This may be due to the greater degree of surface cleanliness of the starting catalyst in the case of the single-crystal studies, where surface cleanliness was directly verified. The structural sensitivity in aromatization was even more pronounced in reactions with n-heptane and H2 over Pt crystals, where the Pt(557) surface was some sevenfold more active than Pt(ll1) in toluene production (53,55). The rate was maximized on surfaces where there are atomic steps separated by flat, five-atom-wide hexagonal terraces (53,55). Since the isomerization of n-alkenes (six carbon atoms or longer) is believed to involve a methylcyclopentane (MCP)intermediate in C6 iso-

18

CHARLES T. CAMPBELL

mer production, Zaera et al. studied the reactions of MCP over four Pt crystal surfaces (56). In contrast to results on supported Pt catalysts, aromatization was much slower than that with the use of n-hexane as a reactant. The authors concluded that Pt alone cannot catalyze the ring expansion of MCP. Dauscher et al. (57) followed up these studies with a detailed comparison of the reactions of 2-methylpentane, n-hexane, and methylcyclopentane with Hz over several Pt crystal surfaces, a polycrystalline Pt foil, and two PtlAltO, catalysts. These studies involved the use of '3C-labeled methylpentanes to track its reaction mechanism. The authors concluded that the Bs site configuration is responsible for bond-shift isomerization mechanisms and cracking reactions, and that kink and step sites are necessary for cyclic isomerization mechanisms. Garin et al. (58) also compared the contact reactions of C,j hydrocarbons over several Pt single crystals, foils, and supported catalysts. They found that isomerization by bond-shift or cyclic mechanisms and hydrogenolysis over crystalline Pt surfaces simulate well data for supported Pt of large Pt cluster size, and that isomerization by bond shift is more important when the surface is stepped. Davis et al. (2.3) also studied the isomerization and hydrogenolysis of isobutane, n-butane, and neopentane over flat, stepped, and kinked Pt single-crystal surfaces. The rates and selectivities of butane isomerization and consecutive rearrangements were maximized on (100) portions of the surfaces. Competing hydrogenolysis reactions were most rapid on surfaces containing the greatest step- and kink-site densities. The dehydrogenation and hydrogenolysis of cyclohexane has been compared over several Pt single-crystal surfaces by Herz et al. (59). By far the major product was benzene, with minor amounts of cyclohexene, n-hexane, and alkanes of less than six carbon atoms. The activity for benzene production increased by -3.5-fold in the order (1 11) < (557)< (10,8,7) < (25,10,7),whereas the strength of benzene inhibition increased in the opposite order. This suggests that the variation in activity may be related only to the ability of the specific surface to maintain active Pt sites for the reaction. Indeed, even on Pt( 11 I), adsorbed benzene is produced well below room temperature with high probability (>67%) from adsorbed cyclohexane if the surface is kept free of contaminants and other adsorbed species that block Pt sites necessary for hydrogen abstraction (60). The mechanism of benzene production from cyclohexane on Pt(ll1) is now fairly well understood, as is the ensemble requirement for this reaction (60). Other mechanisms may be important on Pt catalysts containing step and/or kink sites, or support surfaces. It is interesting to point out that significant inverse kinetic isotope effects (RDIRH= 1.3-3.3) for the initial rates of a number of alkane conver-

PRESSURE-VACUUM ANALYSIS OF MODEL CATALYSTS

19

sion reactions have been observed by Davis et al. ( 6 0 , where the unlabeled hydrocarbon was reacted in the presence of H2 or D2 gas over a variety of Pt surfaces. They attributed this to the fact that the coverage of atomically absorbed deuterium should be lower than that of hydrogen under identical reaction conditions, and that the deuterium would block fewer Pt sites and therefore inhibit the reaction less severely than hydrogen. In the case of Dz gas, the products were extensively deuterated, which suggests that the adsorbed intermediates were also largely deuterated. Given the recent observations of strong normal isotope effects (RHI RD = 4-15) in C-H bond cleavage steps on Pt surfaces (60, 62), the inverse kinetic isotope effect in the overall catalytic reaction might instead be due to a lower extent of dehydrogenation in the adsorbed hydrocarbon layer when deuterated. Since severely dehydrogenated hydrocarbons are known to irreversibly poison Pt surfaces (52, 6.9, deuterated hydrocarbons should show a lower degree of poisoning of surface sites by carbonaceous residue. Since the overall rate of reactant conversion is positive-order with respect to H2 pressure (61),the original explanation based on inhibition of the reactions by adsorbed hydrogen (or deuterium) seems less satisfactory than a model where the main species poisoning the reaction is partially dehydrogenated hydrocarbon residue. Very high coverages of such residue are generally observed in surface analysis following hydrocarbon reactions over Pt (63). The reaction rates for hydrocarbon conversion on Pt generally decrease with reaction time, and this has been correlated with the deposition of irreversibly adsorbed carbonaceous species (55, 63). The reactions of hydrogen and cyclohexene over Pt single crystals have been studied by Davis and Somorjai (64)over ten orders of magnitude in pressure. Although benzene was the major product in vacuum conditions, when the reactions were carried out near atmospheric pressure, mainly cyclohexane was obtained. This higher pressure reaction was structurally insensitive, which was attributed to the presence of a near monolayer of carbonaceous species. Massardier et al. (65) studied the competition between hydrogenation of benzene versus toluene over several Pt crystal surfaces and compared the results to those of high-area Pt catalysts. The hydrogenation of ethylene to ethane has also been studied over Pt single crystals by the Somorjai group (66).This reaction was found to be structurally insensitive as well, a fact that again was attributed to the presence of a monolayer of carbonaceous fragments under the high-pressure reaction conditions. In this case, there is strong evidence that most of these fragments are adsorbed ethylidyne species (=C-CHd. On the basis of measurements made in the presence of D2 gas, the authors pro-

20

CHARLES T. CAMPBELL

posed a reaction mechanism whereby the ethylidyne species serve as a deuterium transfer agent between the Pt surface (which cleaves the D-D bond) and ethylene molecules that are adsorbed uboue the ethylidyne adlayer. Radiotracer studies with I4C showed that the ethylidyne species are readily hydrogenated and removed from the surface at atmospheric hydrogen pressure and temperatures above 350 K, but this reaction occurs much more slowly than does ethylene hydrogenation under similar conditions (66). Similar studies (67,68) of the hydrogenation of ethylidyne on Rh( 111) and H / D exchange over well-defined ethylidyne adlayers suggest that the reaction on rhodium surfaces is very similar. Very high coverages of strongly adsorbed carbonaceous deposits are a ubiquitous occurrence in high-pressure hydrocarbon conversion reactions over Pt. The uncovered-site concentration (measured by postreaction CO adsorption-TDS) has been correlated with the total surface carbon coverage (as measured by AES) (6.3, which, in turn, has been correlated with the catalytic activity and selectivity for several reactions (55, 63). The major roles of the deposit are as a nonselective site-blocking agent and as a pool for the storage and rapid exchange of surface hydrogen (63). The ease or reversibility with which these deposits can be removed from the surface decreases with increasing temperature of their deposition or treatment and increases with their H/C atomic ratio (63). Given the constant presence of these carbonaceous deposits, it is somewhat surprising that any structural sensitivity is ever seen in hydrocarbon catalysis over Pt. The fact that some is seen is probably related to the fact that the underlying Pt structure controls the structure of this adlayer and the concentration of the few free sites found within this adlayer, which, in turn, can control the catalytic reaction rates. It is useful in this respect to point out that reaction probabilities (per collision of hydrocarbon molecule with the surface) are many orders of magnitude lower at high-pressure reaction conditions than in UHV where the surface is partially clean (64).Therefore, only a tiny (immeasurably small) fraction of free Pt sites are necessary to explain the overall observed rates of catalysis at high pressures. Oudar et ul. (69) studied the hydrogenation of butadiene over Pt( I lo), paying special attention to the mechanism of carbon deposition that deactivates the surface. At high hydrogen pressures the rate was first-order in H2 and zero-order in butadiene, but at low HZpressures the rate became second-order in H2. For high H2, they argued that the surface was covered with one butadiene molecule per two surface Pt atoms and that the sites for butadiene adsorption are different from those for H2 dissociation. At low H 2 , the sites for H2 dissociation were said to be poisoned by an additional hydrocarbon species. Above a critical temperature, irreversible

PRESSURE-VACUUM ANALYSIS OF MODEL CATALYSTS

21

deactivation by carbon occurs. This latter was attributed to a simple siteblocking effect, since the rates of both butadiene hydrogenation and H2/ D2exchange were proportional to the remaining carbon-free area, and the activation energy was unchanged.

2. Hydrocarbon Conversion over Other Metals Goodman has compared the hydrogenolysis of ethane, cyclopropane, and butane over Ni( 100) and Ni( 1 11) (70-73). At similar pressure conditions (H2 = 100 torr, C2H6 = 1 torr) the Ni(100) surface is considerably more active and has a lower overall activation energy then Ni(ll1) for methane production from ethane (25 versus 46 kcal/mol) (72).The steadystate kinetics for Ni( 1 11) compare favorably with those for SiO2-supported Ni catalysts, which suggests that Ni(ll1) facets dominate on the supported catalyst surfaces. Indeed, this is the most thermodynamically stable facet (in vacuum, at least). The hydrogenolysis of cyclopropane to methane, propane, and ethane also proceeds much faster on Ni(100) than on Ni(ll1) (73),as can be seen in Fig. 6 by the much lower temperatures required for comparable rates on Ni( 100). The hydrogenolysis of butane to methane occurs far more rapidly over Ni(100) than over Ni(l1 l), although in this case the activation energies may be very similar (70, 71).At low temperatures (400K)large amounts of propane and ethane are also produced. Interestingly, the absolute rate and apparent activation energy for hydrogenolysis of butane to methane over Ni( 100) was very similar to that of ethane hydrogenolysis and CO methanation (70).This indicated a common rate-determining step for all three methane production reactions. This step was thought to be removal of surface carbide (or a partially dehydrogenated adsorbed methane precursor) by hydrogenation with H2 (70). Also consistent with this picture is the fact that alkane dissociation proceeds on clean Ni(100) with rates that are one to two orders of magnitude faster than the steady-state rate of their hydrogenolysis (74).These catalytic reactions proceed with steady-state rates on Ni surfaces with the buildup of only submonolayer quantities of carbonaceous residue (70, 72) and were not slowly poisoned by carbon buildup as frequently occurs for Pt (see above). Engstrom et al. (75-78) have compared the reactions of several small hydrocarbons with H2 over different single-crystal surfaces of iridium. For ethane, propane, n-butane, and neopentane, the dominant reaction pathway on both Ir( 111) and Ir( 1 10)-(1 x 2) was cleavage of a single C-C bond, except for the case of n-butane or Ir(ll1) (77). The major reaction channels involved demethylization of the parent hydrocarbon, except for butane on Ir( 1lo)-( 1 X 2), which made mostly ethane (75).This

22

CHARLES T . CAMPBELL 1.o

-

A

I

u)

i k

d 6F

10’’

8 LL

10-1

1 I

I

I

I

I

I

I

I

I

I

1.7

1.8

1.9

2.0

2.1

2.2

2.9

2.4

2.5

TEMPERATURE (I/K

x lo3

FIG.6. Product formation from the reaction of cyclopropane with hydrogen over a Ni(l1l) surface (open symbols) and a Ni(100) surface (filled symbols) (total pressure = 100 tom; H2/cyclopropane = 100). (0, O), Methane; (A, A), propane; ( 0 ,H), ethane. From Ref. 73.

production of ethane was attributed to the stabilization of a special intermediate, suggested to be a metallacycle pentane, by the low-coordination C, sites present on Ir( 110) but not found on Ir(ll1). This structural sensitivity in ethane selectivity correlates well with results for supported Ir catalysts, where the selectivity increases markedly with decreasing average size in the supported Ir particles (75). At low temperatures, ethane hydrogenolysis proceeds much more rapidly on Ir( 1 1 1) than on Ir( 110)( 1 x 2), but both propane and neopentane gave comparable rates on these surfaces (77). The activation energies vary depending on the temperature and the surface structure, so distinct activity comparisons do not hold over the whole range of reaction conditions. A full analysis of the kinetic

PRESSURE-VACUUM ANALYSIS OF MODEL CATALYSTS

23

dependencies on temperature and reactant partial pressures has been performed, and the mechanistic implications have been discussed (77). The reaction of cyclopropane and hydrogen over Ir gave both hydrogenation to propane and hydrogenolysis to methane and ethane, with the former dominating below 500 K (78). For both products, the Ir(ll0)(1 x 2) surface was more active by a factor of -3.5 than Ir(ll1) (78). Again, this difference was attributed to the presence of C7sites on Ir(l10). The hydrogenation of methylcyclopropane on both surfaces was dominated by production of n-butane, although a pathway for isobutate production was also competitive at low temperatures and had a much lower activation energy. The Ir( 1 lo)-( 1 x 2) surface has a greater activity for hydrogenation of propylene, and generally superior hydrogenation capability, than does Ir( 1 11). This finding was correlated with the presence of higher adsorption energy states for Ha on the former surface, which suggests a higher hydrogen coverage under reaction conditions (78). As with Ni, steady-state reaction rates for these hydrocarbon reactions were achieved over Ir single crystals with the buildup of only submonolayer quantities of carbon (& < 0.5) (77, 78). Wax et al. have studied the hydrogenolysis of ethane over W(100) (79). This surface is itself not active in this reaction until a monolayer of surface carbide is formed, after which an active and very stable model catalyst results. In general, the activity of cleaned metal single crystals for this reaction increased in the order W < Ni < Ru < Ir at the same conditions (573 K, 1 torr ethane, 100 torr H2)(72, 75, 79, 80). The cyclotrimerization of acetylene to benzene has been studied by Rucker et al. (81) over Pd( 11I), (loo), and (1 10) at pressure near 1 atm. The (1 10) surface was four-fold less active than Pd(ll1) or (loo), which contrasts with their relative selectivities during TDS under UHV conditions. The activity at high pressures was correlated with the fraction of the various surfaces that exposed clean Pd atoms, as probed by postreaction CO adsorption-desorption. In all cases, most of the surface was covered with a carbonaceous residue. The authors stated that the reaction rate is first-order in acetylene pressure for all three surfaces. The extensive data on the Pd(ll1) surface clearly indicate first-order kinetics in that case. However, the limited data presented for Pd(ll0) seem (to the present author) to be better fitted by an order of -2.5, which is closer to the value of three suggested by the overall stoichiometry. The structure of carbonaceous deposits formed from medium pressure exposure of ethylene between 325 and 800 K have been compared for initially clean surfaces of Ir, Pt, and Rh by Niemantsverdriet and van Langeveld (82) using SIMS and AES. On all surfaces, the hydrogen con-

24

CHARLES T. CAMPBELL

tent (H :C ratio) decreased with increasing reaction temperature. The hydrogen content also decreased in the order Ir > Pt > Rh. The deposits were, however, most graphitic in nature on Pt surfaces. The authors point out that the tendency of Pt to graphitize carbon may explain the relatively low hydrogenolysis activity of Pt. 3 . Ammonia Synthesis (N2

+ 3H2 --* 2NH3)

The synthesis of ammonia over both Fe and Re single-crystal surfaces has been studied by the Somorjai group and found to be extremely structural sensitive (83-85). The activity of iron increases in the order (1 10) 4 (210) = (100) 4 (221) = (1 1 1) (83,84).Because dissociative N2adsorption is thought to be the rate-determining step, this order has been correlated (83) with increasing relative activity and decreasing activation energy for dissociative N2 adsorption, as measured by the Ertl group (86, 87, 136). They attributed the higher activity of the more open crystal planes to the more aggressive behavior of low-coordinated Fe atoms. However, Strongin et al. (84) attribute the high activity of very open Fe(ll1) and (211) surfaces to their exposure of highly coordinated C7 sites at the surface for chemisorptive bond formation. As shown in Fig. 7, Re surfaces showed amazing structural sensitivity in this reaction, with relative activitiesof(0001):(10~0):(11~0):(11~1) = 1 :94:920:2820(85). That is, the specific activity increases dramatically as the atomic roughness or openness of the surfaces increases. Again, this catalytic activity correlates well with the ability of the surface for dissociative N2 chemisorption, which is thought to be the rate-determining step (88). Strongin et al. (84) argue that the surface roughness is important only to the extent that it can again expose highly coordinated surface atoms (C,, and C ~inOFig. 7) that are present in the second and third layers. They support this argument with theoretical calculations by Falicov and Somorjai (89) (which do not consider steric hindrance). 4.

Carbon Monoxide Oxidation (CO + b Reduction (NO + CO + C02 + #N2)

0 2 4

C02) and Nitric Oxide

The oxidation of CO by either O2or NO was studied by Peden et al. and Oh et al. over Rh, Pd, Pt,and Ir single crystals (90-92). The CO + 0 2 reaction was relatively insensitive to the atomic structure of the surface, and the specific activities and kinetic parameters agreed for both crystal surfaces and for alumina-supported catalysts. The Rh surfaces deactivated at high 0 2 pressures due to the formation of a near-surface oxide (91, 92). On the other hand, the CO + NO reaction was very sensitive to

PRESSURE-VACUUM ANALYSIS OF MODEL CATALYSTS

TF

C6(SOLID)

I4

25

C, (SOLID)

C,o(DOT TED)

f 9

I

I

FIG.7. Structure sensitivity in the ammonia synthesis over rhenium single-crystal faces. The turnover frequencies (TF) are given as NH, molecules/cm2/s (PtOtd= 20 atm, H2 : N2 = 3 : 1, TcWstd = 870 K).Schematics of the atomic structure of each surface are given above each bar. From Ref. 85.

surface geometry (90).A mechanism and kinetic model for these reactions has been successfully developed that quantitatively fits the medium-pressure kinetic data, but that was parameterized using the results of UHV adsorption/desorption kinetic studies (92). This contrasts with an earlier kinetic study of the NO + CO reaction over Rh(100) by Hendershot and Hansen ( 9 3 , who modeled their data with a somewhat different mechanism that included adjustable kinetic parameters. It is difficult to directly compare their actual data with that of Peden et al. (90) because of differences in the reactant pressures and temperatures used.

26

CHARLES T. CAMPBELL

The CO + 0 2 reaction also appears to be structurally insensitive over Ru, since kinetic data for Ru(0001) and silica-supported Ru are fairly similar (94). The highest activity occurs when the Ru is covered with a monolayer of oxygen. Since the reaction is, instead, inhibited by an oxygen-covered surface for the case of Rh, the higher activity of Ru compared to Rh at medium-pressure conditions could be explained (94). The oscillatory kinetics of CO oxidation over Pt single crystals has been studied at atmospheric pressure by Yeates et al. (95). They present a model to explain rate oscillations that relies on the oscillatory formation of surface platinum oxide, which was observed in postreaction analysis and was related to the presence of silicon impurity.

5 . Hydrogenation of CO The methanation reaction (3H2 + CO --$ CH4 + H2O) has been thoroughly studied by Goodman and co-workers (4,5, 71,96) over Ni single crystals. Since the specific rates, activation energies, and pressure dependencies are very similar over Ni( loo), Ni( 1 1 l), and A1203-supportedNi, the reaction is structure insensitive (71,96). Transient kinetic studies at medium pressures combined with postreaction AES analysis on Ni( 100) have identified a carbidic form of adsorbed carbon as the reaction intermediate, and graphitic carbon as a poison formed at higher temperatures (71,96). The hydrogenation of CO was studied by Castner et al. over Rh( 1 1 1 ) and polycrystalline Rh foils at 6-atm pressure (97). These model catalysts produced mainly (90%) methane and showed similar specific rates, again suggesting a lack of structural sensitivity. When the Rh surface was oxidized, the initial rates were much larger, the activation energy changed, and the selectivity increased for heavier hydrocarbons and oxygenated products such as methanol, ethanol, and acetaldehyde. Logan et al. compared C O hydrogenation over Mo(100) and polycrystalline Mo foil, which gave similar rates and, therefore, apparent structural insensitivity (98). Here, the major product was methane, but ethene and propene were also significant products. The hydrogenation of CO was observed by Udovic et al. (99) and Szuromi et al. (ZOO) to proceed readily over W(llo), with methane as the dominant product at steady state. The surface was covered with a monolayer of carbidic carbon under reaction conditions, as well as some surface oxygen. The activation energy for methanation over W( 110) was only about half that seen for Ni surfaces. The synthesis of methanol from CO and H2 has been studied over Pd( 110) at medium pressures (500-1800 torr) by Berlowitz and Goodman

PRESSURE-VACUUM ANALYSIS OF MODEL CATALYSTS

27

(101). The (extrapolated) specific rates and activation energy agree well with results for Pd dispersed on noninteracting supports such as Si02and some basic supports such as ZnO and MgO. Palladium on highly interactive and acidic supports such as zeolite, alumina, or lanthana give rates and selectivities (i.e., significant dimethyl ether production) substantially different from those of Pd(l10). The synthesis of methanol from CO(+CO2) and H2 was also attempted by Campbell and co-workers (102) over a Cu(ll1) surface and over welldefined Cu overlayers on a ZnO(OO0i) single-crystal surface. They could set an upper limit of < 2 x 1OI2 molecules/cm2/son the methanol production rate at temperatures up to 600 K and pressures up to 1500 torr. These limits are consistent with expectations based on (extrapolated) kinetics for high-surface-area Cu/ZnO catalysts. Their results also showed that C02 has extremely low reactivity for oxidation of the clean Cu(ll1) surface (102), and they argue that it is very unlikely that significant oxygen concentrations exist on the Cu(1 1 1)-like surfaces of practical catalysts under methanol synthesis conditions (102, 103).

6 . Other Reactions Thiophene HDS Activities have been reported by Bussell et al. over three low Miller index single-crystal surfaces of Mo and four of Re (104106). The reaction over Mo was insensitive to the surface structure. The Re(0001) surface showed about the same activity as the Mo surfaces, but Re( 1121) was twice as active, and the (1 120) and (lOi0) surfaces were approximately sixfold more active (106). While Mo surfaces were covered with a near monolayer of partially hydrogenated carbon after reaction, the Re was not moderated by a carbon overlayer. The product distribution over Mo(100) was similar to that reported for powder MoS2 catalysts, although the single-crystal surface of pure Mo was much more active (104). Measurements of the rate of hydrogenation of 35Son Mo( 100) suggest that sulfur adatoms are not intermediates in thiophene HDS (104). The water-gas shift reaction (CO + H20 + C02 + H2) has been studied in detail over a Cu( 111) surface at pressures up to 15 torr CO and 200 torr HzO by Campbell and co-workers (26). The specific activity, activation energy, and reaction orders are very similar to those extrapolated from kinetics at somewhat higher pressures over Cu/ZnO and Cu/ZnO/A1203 catalysts. Similarly, doping of the Cu(ll1) surface with ZnO, had no distinct effect on the observed activity. These results suggested that there is no specific Cu-ZnO interaction necessary for an active water-gas shift catalyst, the essential ingredient of which is metallic Cu suiface. Kinetic analysis using kinetic parameters obtained from both UHV and medium-

28

CHARLES T . CAMPBELL

pressure adsorption-desorption experiments indicated that the rate-determining step was the dissociative adsorption of water. Since there is significant structural sensitivity for this step (lo?'), the catalytic activity may depend on crystal orientation. A word of caution should be remembered concerning the general concept of structural sensitivity for catalytic reactions, since this can depend in some cases on the reaction conditions chosen. This is because, as the temperature or reactant pressures are changed, the rate-determining step can change from one that is structurally sensitive to one that is not. Similarly, optimum practical catalytic conditions are often not those where a single elementary step is rate-determining, but where a delicate balance of kinetic competition exists between steps in the mechanism. Thus, global interpretation of the above results should be used only with caution and proper qualification.

B. THEROLEOF CATALYST ADDITIVES: SURFACE MODIFICATION Another major contribution of the pressure-transfer-model-catalyst studies has been to clarify the role of catalyst additives and impurities in controlling catalytic activity and selectivity. Particularly important here has been the controlled study of the influence of surface chemical modification on the medium- or high-pressure reaction kinetics. Typically, the kinetics are followed as a function of the surface coverage of an added element or complex, and the changes in global kinetics are then correlated with the influence of the additive on the rates of individual steps in the reaction mechanism. This type of data has been found to be extremely useful in identifying the rate-determining step and in clarifying the role of surface modifiers such as poisons or promoters, or the presence of a second metal in bimetallic or alloy catalysts. (The subject of bimetallic catalysts will be discussed in a following section, as will metal-support interactions.) 1.

Electronegative and Electroneutral Elements as Additives

With a few exceptions, electronegative elements have been observed to inhibit the reaction rates of catalyzed reactions when added to well-defined transition-metal surfaces. The mechanism of this poisoning, however, can vary from a simple, steric site-blocking effect to a longer range, electronic interaction acting either through the orbitals of the metal surface or through space. Sometimes selective poisoning of one competing reaction in a branching mechanism can actually lead to a desirable increase in selectivity.

PRESSURE-VACUUM ANALYSIS OF MODEL CATALYSTS

SULFUR ON "100) AT 0.25 MONOLAYERS

1

29

SULFUR ON Ni(100 AT 0.5 MONOLAYERS

\

P

10

!i

E 10'

0

0.1

0.2

0.3

0.4

i

ADATOM COVERAGE IN MONOLAYERS

FIG.8. Rate of the methanation reaction (CO + 3H2 -+ CH4 + HzO) as a function of sulfur (0) and phosphorus (0)coverage on a Ni(100) surface (pressure = 120 tom, H2: CO = 4 : 1 , reaction temperature = 600 K). From Ref. 71. Reprinted with permission from Acc. Chem. Res. 17, 194. Copyright 1984 American Chemical Society.

The influence of sulfur or phosphorous addition on the catalytic activity of a Ni(100) surface in CO methanation has been thoroughly studied by Goodman and co-workers (71, 96, 108). As shown in Fig. 8, the rate decays very nonlinearly with the coverage of S or P. The initial slope of the curve for sulfur has been interpreted to suggest that 210 Ni surface sites are deactivated by each sulfur adatom (108).Phosphorous, which is slightly larger but considerably less electronegative than sulfur, poisons the rate much less rapidly than sulfur, so the authors concluded that longrange electronic effects were playing the dominant role in catalyst poisoning here. According to their model, a phosphorous adatom sits in a fourfold hollow site and poisons only its four nearest neighbors, while the more electronegative sulfur adsorption leads to deactivation also in the second shell of neighboring Ni atoms. This model relies on LEED studies which suggests that both sulfur and phosphorous spread uniformly over

30

CHARLES T. CAMPBELL

the surface in the low-coverage range (80,109). Interestingly, sulfur addition did not alter the activation energy below 600 K (fog),which suggests that a given Ni site is either completely poisoned or not affected at all, depending on its distance from a sulfur atom. Very similar effects of sulfur were also reported on Ni/A120~catalysts (108).Nakamura et al. (109a) have shown that the rate of H2-D2 scrambling at 12 torr over Ni(100) decays linearly with sulfur coverage to zero at 0s = 0.45. Similar effects of sulfur have been observed by Goodman and coworkers for CO hydrogenation over Ru(0001) and Rh( 111) model catalysts (80) and C02 hydrogenation over Ni(100) (110).In the case of CO2 hydrogenation, the reverse water-gas shift reaction (C02 + H2 CO + H20) apparently establishes a rapid equilibrium, and then methane is produced from the resulting CO (110).The hydrogenation of CO over sulfur-dosed Mo(100) was studied by Logan et al. (981,who found that whereas methane production was poisoned, the effect was much less dramatic than on the surfaces outlined above. In addition, the amount of ethylene product became comparable to that of methane upon sulfur poisoning. The effect of adsorbed sulfur upon the rate of CO methanation over W(110) has been studied by Szuromi et al. (100, 111). The rate decayed much more slowly with sulfur than on Ni surfaces, and the activation energy was unchanged. At high coverages, the rate and the saturation hydrogen coverage decayed in a similar way. These authors also showed that low levels of 0 2 torr) completely poison the surface with oxygen. The rate of hydrogenolysis of ethane to methane over Ru(0001) is much more slowly poisoned by sulfur than is methanation (80).The hydrogenolysis of cyclopropane to methane, ethane, and propane has been studied by Goodman over sulfur-dosed Ni( 111) and Ni( 100) (7.3). The production of methane and ethane on Ni( 11 I ) and methane on Ni( 100) is dramatically poisoned by low sulfur coverages. The yield of propane plus propylene decreases much more slowly. In contrast to the clean Ni(lI1) surface, ethylene production increases, becoming a significant product, after sulfur addition. Overall, the addition of sulfur causes a reduction in the ability of the Ni surface to cleave C-C bonds. Oudar et al. (112) have studied the influence of sulfur on the hydrogenation of 1,3-butadiene and H2-Dz equilibration over Pt(ll0). The rates decayed linearly with sulfur coverage, so that each sulfur atom poisons one dissociation site for hydrogen without influencing the activation energies or mechanism. The authors established the first isotherm for sulfur adsorption under actual catalytic reaction conditions. The adsorbed hydrocarbons influenced the equilibrium coverage of sulfur on the Pt surface. The thermodynamics of adsorbed sulfur on several metal single crystal surfaces have been presented by Bernard et al. (114).

*

PRESSURE-VACUUM ANALYSIS OF MODEL CATALYSTS

31

The general subject of sulfur poisoning of metal catalysts has been reviewed by Bartholomew et al. (115), who discussed results mostly on high-surface-area catalysts. The hydrodesulfurization of thiophane over Mo( 100) is deactivated by sulfur adsorption, although the rate for nearly a monolayer of sulfur is still almost half that measured in the absence of sulfur (104). Excessive sulfur exposure leads to formation of a MoS2 layer and severe deactivation of Mo(100)(104). Campbell and Koel have studied the deposition kinetics of sulfur from H2S onto Cu(ll1) and the resulting influence of sulfur adatoms on the kinetics of the water-gas shift reaction (H20 + CO + H2 + COZ)(24). Although H2S deposits sulfur adatoms on the surface and liberates H2 gas (H2S+ Hz + S,) with a reaction probability of -0.03 per H2S collision with the surface at 500 K, the reverse reaction proceeds only very slowly, with a reaction probability of 5 per H2collision. This kinetic discrepancy suggests that a few ppm of H2S will be sufficient to deposit almost a full monolayer of sulfur onto Cu surfaces under catalytic reaction conditions. The rate of the water-gas shift reaction decreases linearly to zero with sulfur coverage over the coverage range between zero and a saturated, close-packed monolayer. Thus, it appears that in this case each sulfur adatom simply poisons a fraction of the Cu surface equivalent to that which it can sterically block (-2.6 Cu atoms), and that only one or maybe two Cu atoms are required for the rate-determining step. The reaction is positive-order with respect to H2O adsorption (26), a fact that does not change on sulfur addition. This suggests that only one or two Cu surface atoms are needed to cleave the 0-H bond in H20. The stability of adsorbed sulfur and its influence on the water-gas shift kinetics helps to explain the well-known sulfur intolerance of Cu-based catalysts for this reaction (24). The influence of adsorbed Si, P, S, and C1 on the medium-pressure cyclotrimerization of acetylene to benzene over Pd(l1 l), (loo), and (1 10) has been studied by Logan et al. (113). Whereas both sulfur and chlorine decrease the activity, silicon increases the activity. The effect of phosphorous depends on the crystal face. According to their work function measurements, sulfur withdraws electron density from the Pd surface (as is also expected for chlorine), whereas Si donates electron density, and P has the least effect on the work function. Thus, the qualitative influences on catalytic activities correlate with the influences of the additives on the electronic character of the surface. In addition, Si decreases the carbon coverage seen in postreaction AES from -82 to -70% of a monolayer, whereas sulfur and chlorine increase the amount of carbonaceous residue. The authors interpreted these results by suggesting that the electrondonating ligands keep the Pd surface cleaner for the desirable reaction by

32

CHARLES T. CAMPBELL

preventing surface polymerization. Inconsistent with this model, potassium, which should have the greatest extent of charge transfer of all the ligands studied, caused a decrease in the catalytic activity of Pd(llO), although it did double the rate on the other two surfaces. Garfunkel et al. (116) studied the influence of preoxidation of Re and Fe polycrystalline surfaces on their activity in CO hydrogenation. Oxidation usually led to higher methane selectivity and less carbon buildup on the surface. The activation energy for methanation was changed by oxidation of Fe(poly), which suggests either an electronic effect or a change in the rate-determining step. In contrast, Castner et al. (97) found that Rh surfaces produced a greater fraction of higher-molecular-weight hydrocarbons from CO + H2 mixtures when preoxidized, and that the selectivity for oxygenated products was substantial on oxidized Rh but insignificant on the metal alone. Hasenberg and Schmidt (119) studied the synthesis of HCN from CH, + NH3 mixtures at 0.01 to 10 tom over a polycrystalline Pt surface that was initially clean. Under reaction conditions the surface was covered with nearly a monolayer of carbon. Addition of 0 2 to the reaction mixture reduced the carbon coverage, decreasing the HCN production rate at the expense of NO production. A kinetic model was developed that fit the data well. Campbell and co-workers (48, 117, 118) have studied the influence of chlorine adatoms on the selective oxidation of ethylene to ethylene epoxide over Ag( 110) and Ag( 1 1 1) surfaces. As also occurs when trace levels of chlorinated hydrocarbon are added to the reactant feed over aluminasupported Ag catalysts, the deposition of chlorine adatoms on Ag single crystals led to a decrease in the steady-state rates of both epoxide production and full combustion (COz + H20). The latter was poisoned more rapidly than the former, and this lead to a marked improvement in selectivity, to a limiting value of 80-85%. This proved that the true promoter is atomically adsorbed chlorine, and that no support effects are required to understand (in first order) the role of the promoter. The selectivity is improved at significantly lower chlorine coverages on Ag( 111) than on Ag(I10) (48). In both cases the reaction rates decrease only at considerably larger chlorine coverages than those required to markedly suppress dissociative 0 2 adsorption, even under conditions where the reaction rate is limited by 0 2 supply. This was originally interpreted to indicate that molecularly, rather than dissociatively, adsorbed 0 2 was the true oxidizing agent (48, 117). The relative importance of molecularly versus dissociatively adsorbed oxygen in the reaction mechanism is, however, still a subject of some controversy (46). Figure 9 summarizes the results of a series of experiments in which the

PRESSURE-VACUUM ANALYSIS OF MODEL CATALYSTS

c

,,

0.9

0

0.1 I

02

0.3

04

I

I

I

05 I

06 1

33

1

FIG.9. Influence of chlorine coverage on the kinetic parameters for selective ethylene oxidation over a Ag( 110) surface. Parameters for both the production of ethylene epoxide (EtO, circles) and the undesired side reaction to full combustion (COz, squares) are presented. Steady-state reaction orders in Ps and PE,and activation energies E, versus chlorine coverage near 563 K, PE,= 20 torr, and Po2 = 150 torr. From Ref. 118.

influence of chlorine coverage on the steady-state kinetics of ethylene epoxidation was determined over a range of temperature and partial pressure conditions on Ag(l10) (fZ8).The ability demonstrated here to monitor directly the changes in kinetic parameters as a function of modifier coverage highlights the strength of the surface science approach in addressing the influence of surface additives. The most obvious feature here is that the kinetic parameters for full combustion (COz) essentially track those for epoxidation (EtO), which strongly suggests that a common adsorbed intermediate is formed in the rate-determining step of both path-

34

CHARLES T. CAMPBELL

ways, and that the selectivity is determined only subsequent to this step by the branching ratio of two relatively rapid competing reactions available to this intermediate. Campbell and co-workers have analyzed these chlorine-induced changes in the kinetic parameters using the results of UHV adsorption-desorption experiments (kinetics and energetics) measured for ethylene and oxygen as a function of chlorine coverage (118). For example, low chlorine coverages increase the heat of adsorption (and therefore steady-state coverage) of ethylene. As a consequence, under conditions in which the reaction rate is positive-order in ethylene pressure, chlorine addition leads to an increase in epoxidation rate and a decrease in the reaction order with respect to ethylene pressure. Tan et al. (120-122) have also studied the influence of chlorine on the epoxidation of ethylene over Ag( 111) in detail. The Lambert group (123126) has also performed numerous other mechanistic investigations of this reaction over Ag(ll1). These and other mechanistic studies of this reaction are excellently reviewed in a recent article by van Santen and Kuipers (46).

2. Alkali Promoters Catalysts of various types are frequently treated with solutions of alkali salts in order to improve their subsequent activity or selectivity in specific applications. A fundamental understanding of the role of alkali promoters has long been a goal of the catalysis community. In 1982, Campbell and Goodman (127) investigated the role of potassium promoters in nickel catalysts for CO hydrogenation by using model catalysts synthesized by the vapor deposition of well-defined K overlayers on a clean Ni(100) surface. As with K impregnation of supported Ni catalysts, the steady-state rate of catalytic methane production decreased and that of higher hydrocarbons and olefins increased (see Fig. 10). This result showed that the dominant role of the alkali in this case is unrelated to interactions with the catalyst support material. The activation barrier for CO dissociation on Ni(100) was shown to be greatly reduced by K addition; therefore, the steady-state carbide coverage under reaction conditions increased markedly. Within the accepted “carbide” mechanism, this was expected to cause an increase in the C :H ratio in the surface layer and, therefore, a decrease in the probability for hydrogenation of carbon and an increase in the probability for chain growth (C-C bond formation) in the adsorbed intermediates. Thus, less CH4 and more olefins and higher hydrocarbons were seen in the products, and the graphitization of the surface occurs at a lower temperature. Since that study, the coadsorption of CO and alkalis on transition-metal surfaces has been

PRESSURE-VACUUM ANALYSIS OF MODEL CATALYSTS

METHANE

ETHYLENE

35

ETHANE

FIG. 10. A comparison of the product distributions (weight percent) over a Ni(100) catalyst observed during CO hydrogenation over clean (solid bars) and K-doped (crosshatched bars) catalysts at T = 500 K, H2: CO = 4 : 1, and a total pressure of 120 torr. Potassium coverage = 0.10 monolayers (ML).From Ref. 127.

extensively studied, and numerous short- and long-range electronic interactions have been shown to operate in the coadsorbed layer, which can facilitate C-0 bond cleavage (see Ref. 128 and refs. cited therein). It should be noted, however, that the K on Ni(100) was not atomically absorbed under reaction conditions, but instead stabilized by some oxygen-containing species, perhaps in a CO, H20, or OH complex (127). Similar results have been obtained for K-promoted Fe foils and surfacecharacterized Fe powders by Bonze1 and co-workers (129-132) and by Dwyer and Hardenbergh (133, 134). Many of these results over Fe have been discussed in detail in a recent paper by Wesner et al. (132). On Fe foil, the chain-growth factor (Y in the Fischer-Tropsch reaction increased by -30% on K addition. Dwyer and Hardenbergh (134) observed a new surface-carbon species (neither carbidic nor graphitic) in postreaction XPS analysis of K-promoted Fe powders and suggested this species to be “polymethylene-like.” This same K-induced species was later seen on Fe foils by Wesner et al. (132) and shown to be kinetically active in hydrogenation, and its presence was well correlated with the enhancement in chain growth. In general, postreaction surface analysis indicates much higher carbon coverages on Fe compared to Ni surfaces, both with and

36

CHARLES T. CAMPBELL

without K. As on Ni, the carbon graphitizes at high temperatures. The graphitization temperature is considerably higher on Fe powders than on Fe foil (133). Similar structural and kinetic effects were observed for vapor-deposited and solution-deposited K on polycrystalline Fe foil (132), suggesting that the catalytic reaction conditions rather than the method of deposition controlled the structure of the alkali on the working catalyst’s surface. Evidence from XPS measurements suggested a surface KOH complex for this structure (131, 132). Similar influences of Na have been reported in CO hydrogenation over Re foils (116), where the selectivity for longer-chain hydrocarbons increased and the overall rate declined. Wesner et al. (135) studied K promotion of Co foils in the FischerTropsch reaction. Again, quite similar results were obtained. Like Ni, Co showed much less surface carbon following reaction than on Fe, although on Co the carbon is mostly graphitic in nature. However, after K addition, the carbon is much more carbidic. The reactivity of carbidic carbon with H2 on both Fe and Co is reduced in the presence of K. On Mo(loo), somewhat similar results were also obtained in CO hydrogenation by Logan et al. (98),where the selectivity for higher hydrocarbons and olefins increased on K addition. In this case, the overall rate of reaction also increased, which may be related to the unusual positive CO pressure dependence for this surface. The hydrogenation of C02 over K-promoted Ni(100) was studied by Peebles et al. (110), who showed a strong enhancement in both Co and methane production rates but no changes in activation energies. They also showed that K could be used as an antidote to low-level sulfur poisoning, where the combined effects of K + S was almost a coverage-weighted average of their individual effects. The addition of potassium to industrial Fe catalysts leads to an increase in activity for ammonia synthesis (Nz + 3H2 --.* 3NH3)(136).This promotion effect has been the subject of considerable attention from the surface science community, particularly with regard to the coadsorption of K or K + 0 and N2 (136-139). Ertl and co-workers have shown that potassium addition to single-crystal Fe surfaces can lead to a 10- to 100-fold enhancement in the rate of dissociative N2 adsorption, which is thought to be the rate-determining step in NH3 synthesis (136-139). However, Bare et al. (140) were unable to promote the activity of Fe( 1 1 I ) , (loo), or (1 10) surfaces for this reaction at 20-atm pressure with either K, K + 0, or K + AIO, addition. They interpreted this result to indicate that the promotional role of K in industrial catalysis may be cooperation with other promoters, such as the support material, to cause structural rather than electronic promotion. These results were for very low conversions, however, so that the product (NHj) partial pressure was low. Strongin and

PRESSURE-VACUUM

ANALYSIS OF MODEL CATALYSTS

37

Somojai (183) later found that potassium does promote the activity of Fe( 1 11) and (100) at higher conversions, and that the effect increases with increasing conversion. They interpreted this in terms of the poisoning of surface sites by adsorbed ammonia and a decrease in the coverage of NH3 due to the presence of potassium. They also found that potassium decreases the heat of NH3 adsorption. Asscher er al. (85) also saw at best only a very slight increase in the NH3 production rate over Re singlecrystal surfaces after potassium addition. For both Fe and Re, surface oxygen was required to stabilize the surface alkali under reaction conditions. Stolze and Norskov (181, 182) have modeled reasonably accurately the kinetics of high-pressure ammonia synthesis over Fe/AI2O3and commercial K/Fe-based catalysts using kinetic parameters for elementary reaction steps taken from UHV studies over clean and K-dosed Fe single crystals (86, 87, 136-139). Zaera and Somojai (141) studied the influence of potassium on the reactions of n-hexane and H2 over Pt( 1 11) near atmospheric pressure. The overall reaction rate decreased dramatically with as little as 2% of a monolayer of K, and the coverage of carbonaceous residue also increased slightly in this K coverage range. At higher K coverages, the rates continued to decrease but the carbon coverage also decreased. The results were explained in terms of inhibition by K of the dehydrogenation of adsorbed hydrocarbons. Campbell and Koel (142, 143) have studied cesium promotion of the medium-pressure water-gas shift reaction (H2O + CO + H2 + C02) over a well-defined Cu(ll1) surface under conditions in which the rate is limited by dissociative H2O adsorption. As shown in Fig. 1 1, they observed a 15-fold enhancement in catalytic activity at 8cs = 0.15, with no major change in reaction order with respect to H20 and only a slight increase in the overall activation energy (17 to 20 kcal/mol). Under reaction conditions, the Cs was stabilized on the surface in an oxidic or hydroxidic complex of approximate ratio Cs : 0 = 1 : 1, which formed, at high coverages, islands of ap(2 x 2) overlayer structure. The maximum promotional effect was achieved at a Cs coverage just below that required to nucleate these p(2 X 2) islands. They attribute the promotional effect to direct participation of the surface CsOH complex in H20 bond cleavage. Again, coadsorption of S poison and Cs promoter led to nearly directly additive individual effects. Campbell (144, 145) has also studied Cs promotion of the selective oxidation of ethylene over Ag( 11 1) at low conversions. Under mediumpressure reaction conditions, the Cs is stabilized as a surface cesium oxide of approximate stoichiometry Cs : 0 = 1 : 3, where bonding of the oxygen atoms both to the surface Ag atoms and to at least one Cs atom is

38

C H A R L E S T. C A M P B E L L

1.6 1.4

3

1.2

u)

s; 1.0 W

4

i , -1

o*8

0 0.6

I

v

0.4

Cs/Cu

AES RATIO / ( l / l O O )

FIG.1 I . Variation of the water-gas shift reaction rate with cesium coverage on Cu( I 1 1) at 612 K, 26-torr CO, and 10-tom H20. The open data point represents dosing of pure water (no CsOH). From Ref. 142.

suggested both by this stoichiometry and by XPS,AES, and TDS results. This C s / O complex coalesces into islands of (2 fi x 2 f i ) R 3 0 " structure according to postreaction LEED analysis. This same cesium structure results either by running the reaction over a pure Cs overlayer on atomically clean Ag( 11I), or by dosing Cs from ultrapure aqueous solutions of cesium hydroxide, carbonate, or nitrate onto Ag( 11 1) using procedures very similar to those used in preparing Cs-doped industrial catalysts. This result lays to rest fears that the vapor-deposition method chosen for alkali promotion by surface scientists yields a final alkali structure any different from what would be expected from the more industrial approach of impregnation from solution. Small coverages of Cs led to a decrease in both rate and selectivity, whereas very high coverages continued to poison the rate but led to only a very small improvement in selectivity compared to alkali-free Ag(ll1). Although these minor effects cannot be neglected, the major promotional effect in industrial catalysis appears to be related to other factors such as interactions with the support or other promoters and/or further oxidation of the product ethylene epoxide.

PRESSURE-VACUUM

ANALYSIS OF MODEL CATALYSTS

39

Lambert and co-workers (121-123, 125) have addressed this issue of further oxidation and other features of Cs promotion using model catalysts based on Cs/Ag(11 1). Their results indicate that the simultaneous presence of Cs and COz lead to marked enhancements in selectivity (125). In contrast to Campbell’s results, they report that Cs enhances the rate of epoxidation at low conversion (122). They also show that, although Ag( 11 1) is active in the isomerization of the product (ethylene epoxide) to acetaldehyde, its activity for this side reaction can be suppressed by surface Cs addition (122, 123). Since this isomerization may lead to further oxidation of the epoxide, this may help to explain the enhancement in selectivity derived from Cs addition in industrial catalysis. C. BIMETALLIC A N D ALLOYCATALYSTS The subject of bimetallic or alloy catalysis has long been of deep interest to the catalytic chemist. Addition of a second metal to a moderately good catalyst can frequently improve either the activity, the selectivity, or the lifetime of the catalyst. This improvement can arise from any combination of effects, including those that are basically electronic in nature, whereby the metal-metal bonding changes their electronic structure, or those that are basically geometric in nature, where the size, shape, and availability of the ensembles or groups of active metal sites can be altered by the presence of a second, frequently more inert metal. These subjects have been excellently reviewed in a number of recent works by Sachtler and van Santen (146, 143, Ponec (39), and Sinfelt and Cusumano (148,149).The goal of the surface science experiment here has been to carefully distinguish these two effects in certain reaction systems, and even to quantify the surface electronic, structural, or ensemble characteristics required for an effective catalyst. This hope is, of course, spurred by a knowledge that these properties can be carefully controlled and quantitatively characterized by use of the modern spectroscopic and structural tools of surface science combined with model catalysts based on single crystals. Here, the second metal is either present as an alloy in the bulk crystal itself or is vapor-deposited (or chemically deposited) onto the surface under the pristine conditions of UHV. In the latter case it has been repeatedly demonstrated that well-ordered overlayers or surface alloys of the second metal can often be generated that present the entire range of surface compositions of the two metals (see Refs. 150-154 and refs. cited therein). Although UHV adsorption-desorption studies on these types of model catalyst have been pursued extensively for some

40

CHARLES T. CAMPBELL

time, actual catalytic kinetics at realistic pressures has been only a recent development. In 1983 Sachtler and Somorjai (155) studied the reactions of n-hexane + H2 at -230 torr over a Pt( 1 1 1) single-crystal surface that had been carefully modified with vapor-deposited Au overlayers and surface alloys generated by annealing these overlayers. Although Au itself is very inert for these reactions, the surface alloys were found to be more active than pure Pt( 1 1 1). Large increases in the isomerization rate and simultaneous exponential decreases in the rates of hydrogenolysis and aromatization with increasing Au concentration resulted in high isomerization selectivity. The authors attributed this result to an ensemble effect, where the probability for finding large groups of Pt sites, required for hydrogenolysis, aromatization, and possibly deactivation by carbon buildup, decreases as Au dilutes the Pt alloy. These authors soon extended this study to include cyclohexane dehydrogenation (156), where they found a large enhancement in the rate of benzene production, which maximized at a surface composition of -50% Au and declined thereafter. Gold addition also induced cyclohexene production, which occurred most readily at 90% Au surface concentration. Again, the results were explained by an ensemble effect, where a decrease in the sizes of available Pt sites reduced the extent of site poisoning by the benzene product. More recently, the authors have further extended this work to include a comparison of the Au-Pt( 1 1 1) versus Au-Pt( 100) alloy surfaces in nhexane reactions (157). As shown in Fig. 12, the same general decreases in reaction rates and increases in isomerization selectivity with Au addition were seen for both Pt surface orientations, except for the fact that, in contrast to Pt( loo), the rate of methylpentane production actually increases with Au addition. Other more subtle differences were also observed, demonstrating clearly the existence of structural sensitivity in alloy catalysis. Again, ensemble effects were used to explain the results. Unfortunately, the sizes and shapes of available Pt atom ensembles could not be determined in any of the preceding studies even for the starting alloys. This is due to the difficulty of determining the lateral distribution of elements in bimetallic surfaces, even when they give good LEED patterns as in these cases. This prevents any quantitative assessment of ensemble requirements for the various reactions and clearly highlights a glaring need for the development of surface analytical tools that determine the lateral distributions of elements on multimetallic surfaces. Recently, Campbell and co-workers (158) have demonstrated new methodology that may allow for semi-quantitative assessment of ensemble effects. This method relies on the inertness of bismuth adatoms and their

PRESSURE-VACUUM

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41

ANALYSIS OF MODEL CATALYSTS

.\

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FIG. 12. A plot of the initial turnover frequencies for n-hexane conversion over singlecrystal surface alloys of Pt plus Au at T = 570 K,H2/HC = 10, and P,o, = 220 torr. (a), AuPt(100); (b), Au-Pt( 1 11). Rates are plotted as molecules converted per surface atom (platinum and gold) versus the surface atom fraction of gold. From Ref. 157.

potential as nearly ideal site-blocking agents that have well-defined lateral distributions on transition metal surfaces. The bimetallic Cu/Ru system does not alloy, but supported Cu/Ru catalysts have been well studied because Cu addition very selectively suppresses hydrogenolysis reactions compared to simple dehydrogenation over Ru (148, 149). Recently, Peden and Goodman (159-163) have investigated this system using well-defined adlayers of Cu on a Ru(0001) surface. In this case, Cu adatoms clusters into two-dimensional islands on the surface. The addition of submonolayer Cu causes a dramatic (eightfold) increase in the rate of cyclohexane dehydrogenation to benzene at 100 torr (159, 160, 163) and a linear decrease in the rate of ethane hydrogenolysis to methane (159, 161). Ultimately this leads to the same marked

42

CHARLES T. CAMPBELL

increase in selectivity for dehydrogenation over hydrogenolysis as seen for supported Cu/Ru (159). However, the enhanced activity for dehydrogenation has not been observed on the supported catalysts. In Fig. 13, Peden and Goodman compare the supported and single-crystal catalysts in terms of their specific activity (now on a per surface Ru atom basis) for these two reactions (159). In the case of the unsupported catalysts, Peden and Goodman (see Ref. 159 and refs. cited therein) have seen substantial spillover of hydrogen chemisorption from Ru to Cu sites on their model catalysts. Because the Ru surface atom density in the experiments with supported catalysts was measured by HZchemisorption, they conclude that there may have been an overestimation of the Ru surface concentration in those experiments (159). This correction would bring the data of Fig. 13 for the two types of catalyst at least into qualitative agreement. This subject, however, remains controversial. Peden and Goodman attribute the enhancement in cyclohexane dehydrogenation to the altered

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FIG.13. Relative Ru-specific activity as a function of Cu coverage (ML)on Ru(0001) (dashed lines) and Cu atomic ratio on silica-supported Cu/Ru catalysts (points and solid curves) for the ethane hydrogenolysis (a) and cyclohexane dehydrogenation (b) reactions. Note that the atomic ratio reported for the supported system probably underestimates the surface coverage of Cu since Cu resides predominantly on the Ru surface in these catalysts. From Ref. 159. Data for silica-supported catalysts taken from Ref. 184.

PRESSURE-VACUUM ANALYSIS OF MODEL CATALYSTS

43

geometric and electronic properties of the surface sites, or perhaps to a cooperative effect such as they see for hydrogen spillover from Ru to Cu. Ensemble effects may certainly be at work here since carbonaceous residue and Cu both suppress hydrogenolysis on the Ru surface in a similar way (163). Goodman and co-workers have also studied the kinetics of methanation and cyclohexane hydrogenolysis on their Cu/Ru(0001)model catalysts. In these cases, as with ethane hydrogenolysis, Cu merely serves as an inactive diluent, suppressing the rates on a simple one-to-one site-blocking basis (161, 162). Greenlief et al. (164) have studied CO methanation and ethane hydrogenolysis over submonolayer Ni films on W(110) and W(100). For these reactions, W alone is very inactive. They have compared the kinetics with that seen for pure Ni crystals. When compared on a per surface Ni atom basis, the turnover frequencies and activation energies for CO methanation are very similar for pure Ni crystals and for the whole range of coverages of Ni on W surfaces. This highlights the extreme structural insensitivity of this reaction on Ni. Although the activation energies for ethane hydrogenolysis over Ni/W( 110) and Ni/W(100) were very similar, the rate (per Ni atom) on Ni/W(l 10) decreased with increasing Ni coverage.

D. METAL-SUPPORT INTERACTIONS The oxidic support material (e.g., SiOz, Al203, TiO2, ZnO, ...) for a catalytically active metal can favorably or adversely influence the performance of that metal in a particular catalytic process. This has been a subject of considerable study for many years, and the general picture has evolved that the catalytic kinetics can be affected by electronic influences on the active metal atoms due to their specific bonding to the support, structural effects where the dispersion of the metal phase or its atomic geometry can be controlled by the underlying support, and cooperative effects were adsorbed intermediates bond to both the metal and the oxidic surface sites, or where certain necessary or undesirable steps in the reaction mechanism take place on the surface of the oxidic support. However, the specific details of these effects have been clarified in only a few cases. Recently, surface science methods have been applied to the study of these interactions, which are referred to as “SMSI” effects (strong metalsupport interaction) when particularly noticeable. The general subject of SMSI effects has recently been reviewed in detail by Tauster (165), who discusses results for both practical and model catalysts.

44

CHARLES T. CAMPBELL

In 1982 Kao et al. (166) reported on the CO hydrogenation activity of nickel overlayers on the TiOz(100) surface as a model SMSI catalyst. They found that an average Ni thickness of -5 8, gave optimum activity, displaying a rate that was 3.3-3.7 times that for an equivalent size sample of pure Ni( 11 I ) , with an accompanying increase in the selectivity for higher hydrocarbons. As shown in Fig. 14, the activation energy for this model SMSI catalyst was unchanged from Ni(ll1). Later, Chung and coworkers (167) studied the activity of a Ni(ll1) surface containing controlled amounts of reduced titania. This catalyst was optimally promoted at a titania coverage of -0.1 monolayer, displaying an activity and product distribution similar to Ni/Ti02(100) and essentially identical to the practical Ni/TiO2 catalysts. The model Ni/TiO2 catalysts prepared by different methods showed similar surface Ti3+ concentrations. These results, together with their companion study (168) of the surface composition of model Ni/Ti02catalysts that had been given the high-temperature SMSI reduction, suggested that partially reduced titania species migrate up from the support material to partially cover the Ni surface. Since this reaction is known to be structurally insensitive (see above), these species must improve the catalysts by some favorable electronic influence on the Ni surface atoms or else by direct participation in the reaction mechanism. Demmin et al. (169-1 72) studied methanation and CO and H2 chemisorption on model SMSI catalysts prepared by deposition of titania onto T(C)

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PRESSURE-VACUUM ANALYSIS OF MODEL CATALYSTS

700600500 400

45

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FIG. 15. Methanation rates for 100-torr CO and 400-torr H2 on a Pt foil. The turnover is in units of molecules/site/second and is based on 1 x 1OIs sites/cm2for the number (hiCH4) foil. The dashed lines indicate rates reported by Vannice and Twu (185) for Pt/TiO, (--4, Pt/Al2O3(---), and Pt/SiO, (-----). The rates for the niobia- and titania-covered foils were measured with an oxide coverage sufficient to completely suppress CO and H2adsorption. (x), Clean Pt; (O),TiOJPt; (O), NbOJPt. From Ref. 172.

Pt, Pd, and Rh foils and niobia onto a Pt foil. In all cases, a partial oxide formed uniform layers on the metal surfaces, suppressing chemisorption of H2 and CO. Nevertheless, the rate of methanation increased substantially and its activation energy decreased, as can be seen in Fig. 15. These three results are almost identical to those reported for the well-known “SMSI state” of high-surface-area niobia- and titania-supported metals. On the basis of these results, Demmin and co-workers attribute the SMSI effect to the migration of a titania or niobia species onto the metal crystallites. Demmin and Gorte (170) also showed that, while clean Pt foil showed no surface carbon in postreaction AES, the titania-dosed foil showed significant quantities of reactive carbon following reactions at low H2/CO pressure ratios, suggesting that CO dissociation on Pt is enhanced by the presence of titania species at the surface. Levin et al. (173) studied the surface structure and chemisorption properties of Ti0,-dosed Rh foil following high-temperature reduction in medium pressures of H2. Again, CO adsorption was inhibited. Later, they

46

CHARLES T. CAMPBELL

showed that an optimum coverage of TiO, of -0.15 monolayers led to a threefold enhancement in activity for CO hydrogenation at atmospheric pressure, and a higher selectivity for olefins, a lower activation energy, and higher reaction orders with respect to H2 and CO (174, 186). These results are very similar to those for Ni and Pt (above). The authors argued that surface Ti3+participates in CO dissociation. In contrast to titania and niobia, alumina is a support that does not display the classic SMSI behavior outlined above, at least for practical catalysts (173, 175). Bischke et al. (176) studied CO methanation over model Ni/A1203 catalysts prepared by evaporation of Al onto a W foil, subsequent oxidation of the Al, and finally vapor deposition of Ni onto the thin A1203film. Specific reaction rates (per Ni surface atom) and the activation energy were similar to those found for pure Ni crystals and practical Ni/A1203 catalysts. This agrees with the classical picture of A1203 as a more or less inert support for this catalyst and the known structural insensitivity of this reaction over Ni (see above). Submonolayer quantities of oxidized aluminum were shown by Levin et al. (186) to only decrease the rate of methanation over a Rh foil, in proportion to the fraction of Rh sites covered by the oxide film. Lee et al. (17) prepared model Fe/MgO catalysts by deposition of Fe onto MgO(100). These surfaces were then characterized in detail following Hz + CO treatments at -0.1 torr by application of a powerful array of surface analytical methods. For Fe layers less than four atoms thick, the Fe was oxidized after reaction with CO + H2. For higher coverages, the Fe was carburized. Carbon deposition was seen at all Fe thicknesses. They saw at least two carbon species, with C-Fe distances of 1.78 and 2.06 A. A strong synergy between Cu and ZnO has been reported for highsurface-area Cu/ZnO catalysts in methanol synthesis (CO + H2 + CH20H) by Klier (see Refs. 178 and 179 and refs. cited therein). To study the pertinent Cu/ZnO interactions, Campbell et al. (102) prepared and characterized model catalysts consisting of well-defined Cu overlayers on the oxygen-terminated ZnO(0M) single-crystal surface and others consisting of ZnO, films on Cu( 111). They attempted to produce methanol on these catalysts at temperatures of 500-600 K and H2 + CO(+COz) total pressures of up to 1500 torr. Although they were unable to observe any methanol production, they were able to set an upper limit on the activities of these model catalysts at < 2 X molecules/site/s. This limit is consistent with the rates expected for high-area Cu/ZnO catalysts (extrapolated from a somewhat higher pressure regime). Campbell and Daube (26) also studied the kinetics of the water-gas shift reaction over a

PRESSURE-VACUUM ANALYSIS OF MODEL CATALYSTS

47

Cu( 11 1) surface and saw no major changes in the rate due to the addition of ZnO, to the surface. Strongin et al. (180) studied metal-support effects in Fe-catalyzed ammonia synthesis by depositing aluminum oxide onto the Fe(l1 l), (loo), and (1 10) surfaces and observing the resulting influences on the kinetics of NH3 production from N2 + Hz.Treatment of the model catalysts in 20torr H 2 0 caused a lasting restructuring of the alumina-dosed Fe(l10) and (100) surfaces. This restructuring resulted in a marked increase in catalytic activity for these Fe surfaces, to rates near those for the (already very active) Fe(ll1) plane (see above). This was explained in terms of an alumina-induced geometric rearrangement of the Fe surface atoms to a local geometry more favorable for participation in NH3 production. IV. Conclusion It is safe to say that the combination of UHV surface analyses with medium- and high-pressure kinetics over well-defined planar surfaces has already added considerably to our understanding of structure-function relationships in catalysis. This is certainly nor to say that experiments using this approach are all that will be required to develop a thorough understanding of the relationships between catalyst performance and atomic-level surface structure. Indeed, experiments with high-surfacearea catalysts will play an equally important role. Studies of either type are certainly most meaningful when they can be directly compared with results using the other approach. We are slowly approaching a happy symbiosis between careful experiments over practical, high-surface-area catalysts and model, single-crystal catalysts. Because of the inherent surface heterogeneity of practical catalysts, many types of experiment are preferably performed over the homogeneous surfaces of model catalysts such as those reviewed here. However, present technology limits many of the kinds of experiment that can be performed on well-defined planar surfaces. For example, the amount of gaseous product is small and difficult to analyze accurately. Therefore, transient kinetics and isotope-labeling experiments designed to elucidate the reaction mechanism are currently most conveniently performed with powdered or supported catalysts. It is notable that even state-state kinetics over single-crystal metal oxides have never been reported, partly because the specific rates in oxide catalysis are very low. Similarly, the surface area of planar model catalysts is small, so that competing side reactions over undefined reactor surfaces can lead to significant problems

48

CHARLES T. CAMPBELL

that are difficult to detect and avoid. In addition, recipes for preparing truly homogeneous, well-defined planar samples presently exist for only a very limited range of materials. The preparation of well-defined surfaces of more complex materials remains as one of the major challenges of “UHV preparatory surface chemistry. ” Catalysis over biphasic materials, such as is thought to be the case in many redox catalysts for selective oxidation (e.g., copper and copper oxide mixtures), represents a class of reaction where study over a single homogeneous surface may well be impossible at steady state. However, transient kinetics of elementary steps over homogeneous surfaces should provide useful insight into the relative roles of the separate phases even in this case. In any case, many types of experiments will continue to be most preferably performed over powdered and supported high-surface-area catalysts. Indeed, these catalysts certainly are much closer in structure to those of commercial import, and many methods of surface analysis can be performed only with higharea samples. It should be pointed out that this field of catalysis on well-defined, planar surfaces is still in its infancy. As in the early days of vacuum surface physics, the results should generally be taken “with a grain of salt” until they are corroborated over another sample by another group. Complications in surface preparation and cleanliness are very difficult to avoid and are more common than expected by the “high-surface-area” catalysis community. The quality of kinetic results over homogeneous surfaces is presently limited by the quality of sample preparation and mounting. There is a tendency to see the sophisticated instrumentation involved and trust implicitly any results from so impressive an apparatus. Scientifically speaking, this can be as dangerous as the reactionary response of discarding all kinetic results from the surface science approach after a few (or several) contradictions are discovered. At the same time, surface scientists must make a dedicated effort to familiarize themselves with the literature of high-area catalysis, as a knowledge of the experimental complications in that field can help them to understand and appreciate seemingly contradictory results found there. Clearly, some highsurface-area catalysts, however heterogeneous, have been better characterized than others. In the future we will see the development of more and more sophisticated surface analytical techniques capable of analyzing a variety of surface types under high-pressure reaction conditions. Eventually this will eliminate the complications of sample transfer into UHV described here. At the same time, more sensitive detection schemes for gaseous products will appear, as will reproducible recipes for preparation of homogeneous surfaces of complex materials. All these developments will facilitate the

PRESSURE-VACUUM ANALYSIS OF MODEL CATALYSTS

49

study of accurate transient and steady-state catalytic kinetics over welldefined, planar surfaces. They will also aid in the quality of structural and kinetic characterization of high-surface-area catalysts resembling those used industrially. The present author is optimistic that investigations using these two different approaches, when coupled with proper theoretical input, will eventually lead to a relatively complete understanding of structure-function relationships in the surface chemistry of catalysis. ACKNOWLEDGMENTS

The author gratefully acknowledges support for this research by the Department of Energy, Office of Basic Energy Sciences, Chemical Science Division. I thank D. W. Goodman and G. A . Somojai for helpful discussions, for communication of results prior to publication, and for contribution of figures. I am also indebted to R. J. Gorte and Y.-W. Chung for contributions of figures and Elizabeth McGaw for typing of the manuscript.

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ANALYSIS OF MODEL CATALYSTS

51

63. Davis. S . M., Zaera, F., and Somojai, G. A., J . Caral. 77,439 (1982). 64. Davis, S. M., and Somojai, G. A., J . Catal. 65, 78 (1980). 65. Massardier, J., Bertolini, J. C., Tri, T. M., Gallezot, P., and Imelik, B., Bull. Soc. Chim. Fr. 3, 333 (1985). 66. Zaera, F., and Somojai, G. A., J . Am. Chem. SOC. 106, 2288 (1984). 67. Davis, S. M., Zaera, F., Gordon, B. E., and Somojai, G. A., J. Card. 92,240 (1985). 68. Koel, B. E., Bent, B. E., and Somojai, G. A., Sutf Sci. 146, 211 (1984). 69. Oudar, J., Pinol, S., and Berthier, Y., J . C a r d 107, 434 (1987). 70. Goodman, D. W., Proc. inr. Congr. Caral., 8rh, Berlin 4, 3 (1984). 71. Goodman, D. W., Acc. Chem. Res. 17, 194 (1984). 72. Goodman, D. W., Surf Sci. l23, L679 (1982). 73. Goodman, D. W., J. Vac. Sci. Technol., A 2, 873 (1984). 74. Sault, A. G., and Goodman, D. W., J. Chern. Phys. 88, 7232 (1988). 75. Engstrom, J. R., Goodman, D. W., and Weinberg, W. H., J . Am. Chem. Soc. 108, 4653 (1986). 76. Engstrom, J. R., Goodman, D. W., and Weinberg, W. H., J. Vac. Sci. Technol., A 5 , 825 (1987). 77. Engstrom, J. R.. Goodman, D. W., and Weinberg, W. H., J . Am. Chem. SOC. 110, 8305 (1988). 78. Engstrom, J. R., Goodman, D. W., and Weinberg, W.H., J . Am. Chem. Soc. (in

preparation). 79. Wax, M. J., Kelley, R. D., and Madey, T. E., J. Card. 98,487 (1986). 80. Peden, C. H. F., and Goodman, D. W., ACS Symp. Ser. No. 288, 185 (1985). 81. Rucker, T. G . , Logan, M. A., Gentle, T. M.. Muetterties, E. L.,and Somotjai, G. A., J. Phys. Chem. 90,2703 (1986). 82. Niemantsverdriet, J. W., and van Langeveld, A. D., in "Studies in Surface Science and Catalysis: Catalysis 1987" (J. W. Ward, ed.), Vol. 38, p. 769. Elsevier, Amsterdam, 1988. 83. Spencer, N. D., Schoonmaker, R. C., and Somojai, G. A., J. Carol. 74, 129 (1982). 84. Strongin, D. R., Carrazza, J., Bare, S. R., and Somojai, G. A., J. C a d . 103, 213 ( 1987). 85. Asscher, M., Carrazza, J., Khan, M. M., Lewis, K. B., and Somojai. G . A., J. Catal. 98, 277 (1986). 86. Bozso, F., Ertl, G., Grunze, M., and Weiss, M., J. C a d . 49, 18 (1977). 87. Bozso, F., Ertl, G., and Weiss, M., J. Carol. 50, 519 (1977). 88. Haase, G., and Asscher, M., Surf. Sci. (in press). 89. Falicov, L. M., and Somojai, G. A., Proc. Narl. Acad. Sci. USA 82,2207 (1985). 90. Peden, C. H. F., Goodman, D. W.,Blair, D. S., Berlowitz, P. J., Fisher, G. B., and Oh, S. H., J. Phys. Chem. 92, 1563 and 5213 (1988); Goodman, D. W., and Peden, C. H. F., J. Phys. Chem. 90,4839 (1986). 91. Oh, S. H.. Fisher, G. B., Carpenter, J. E., and Goodman, D. W., J. Carol. 100, 360 (1986). 92. Peden, C. H. F., Berlowitz, P. J., and Goodman, D. W., Proc. Inr. Congr. Catal., 9th p. 1214 (1988). 93. Hendershot, R. E., and Hansen, R. S., J . Card. 98, 150 (1986). 94. Goodman, D. W., and Peden, C. H. F., J . Phys. Chem. 90,4839 (1986); correction, J. Phys. Chem. 91, 5832 (1987). 95. Yeates, R. C., Turner, J. E., Gellman, A. J., and Somojai, G. A., Surf. Sci. 149, 175 ( 1985). 96. Goodman, D. W., and Houston, J. E., Science 236, 403 (1987).

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97. Castner, D. G., Blackadar, R. L., and Somojai, G. A., J . Card. 66, 257 (1980). 98. Logan, M., Gellman, A., and Somojai, G. A., J. Caral. 94, 60 (1985). 99. Udovic, T. J., Kelley, R. D., and Madey. T. E., Surf. Sci. 150, L71 (1985). 100. Szuromi, P. D., Kelley, R. D., and Madey, T. E., J. Phys. Chem. 90, 6499 (1986). 101. Berlowitz, P. J., and Goodman, D. W., J. Catal. 108, 364 (1987). 102. Campbell, C. T., Daube, K. A., and White, J. M., Surf. Sci. 182, 458 (1987). 103. Campbell, C. T., Appl. Card. 32, 367 (1987). 104. Gellman, A. J., Neiman. D., and Somojai, G. A., J. Catal. 107, 92, 103 (1987). 105. Bussell, M. E., and Somojai, G. A., J. Card. 106, 93 (1987). 106. Bussell, M. E., Gellman, A. J., and Somojai, G. A., J . Catal. 110, 423 (1988). 107. Spitzer, A., and Luth, H., Surf. Sci. 160, 353 (1985). 108. Goodman, D. W., Appl. Surf, Sci. 19, 1 (1984). 109. Goodman, D. W., and Kiskinova, M., Surf. Sci. 105, L265 (1981). 109a. Nakamura, J., Yamada, T., and Tanaka, K., Surf. Sci. 185, L515 (1987). 110. Peebles, D. E., Goodman, D. W., and White, J. M., J . Phys. Chem. 87,4378 (1983). 111. Szuromi, P. D., Kelley, R. D., and Madey, T. E., J . Vac. Sci. Technol., A 5, 867 (1987). 112. Oudar, J . , Pinol, S.. Pradier, C. M., and Berthier, Y., J. Catal. 107, 445 (1987). 113. Logan, M. A., Rucker, T. G . , Gentle, T. M., Muetterties, E. L., and Somojai, G. A., J . Phys. Chem. 90, 2709 (1986). 114. Bernard, J., Oudar, J., Barbouth, N., Margot, E.. and Berthier, Y . , Surf. Sci. 88, L35 (1979). 115. Bartholomew, C. H., Agrawal, P. K., and Katzer, J. R., Adu. C a d . 31, 135 (1982). 116. Garfunkel, E. L., Parmenter, J., Naasz, B. M., and Somojai, G. A., Langmuir 2, 105 (1986). 117. Campbell, C. T., and Paffett, M. T., Appl. Surf, Sci. 19, 28 (1984). 118. Campbell, C. T., and Koel, B. E., J . C a r d 92, 272 (1985). 119. Hasenberg, D., and Schmidt, L. D., J . Catal. 104, 441 (1987). 120. Tan, S. A., Grant, R. B., and Lamberg, R. M., J . Caral. 100, 383 (1986). 121. Tan, S. A., Grant, R. B., and Lambert, R. M., J. Catal. 106, 54 (1987). 122. Grant, R. B., Harbach. C., Lambert, R. M., andTan, S. A., J. Chem. SOC.,Faraday Trans 1 , 83, 2305 (1987). 123. Grant, R. B., and Lambert, R. M., in “Catalysis on the Energy Scene” (S. Kaliaguine and A. Mahay, eds.), p. 251. Elsevier, Amsterdam, 1984. 124. Grant, R. B., and Lambert, R. M., J . C a d . 92, 364 (1985). 125. Tan, S. A., Grant, R. B., and Lambert, R. M., Appl. Caral. 31, 159 (1987). 126. Tan, S. A.. Grant, R. B., and Lambert, R. M., J. C a d . 104, 156 (1987). 127. Campbell, C. T., and Goodman, D. W., Surf. Sci. l23, 413 (1982). 128. Rodriguez, J . A., and Campbell, C. T., J . Phys. Chem. 91, 2161 (1987). 129. Bonzel, H. P., and Krebs, H. J., Sutf Sci. 109, L527 (1981). 130. Bonzel, H. P., and Krebs, H. J., Surf. Sci. 117, 639 (1982). 131. Bonzel, H. P., Broden, G., and Krebs, H. J., Appl. Surf. Sci. 16, 373 (1983). 132. Wesner, D. A., Coenen, F. P., and Bonzel, H. P., Langmuir 1, 478 (1985). 133. Dwyer, D. J., and Hardenbergh, J. H., J. Caral. 87, 66 (1984). 134. Dwyer, D. J., and Hardenbergh, J. H., Appl. Surf. Sci. 19, 14 (1984). 135. Wesner, D. A., Linden, G., and Bonzel, H. P.. Appl. Surf. Sci. 26, 335 (1986). 136. Ertl, G . , Catul. Rev.-Sci. Eng. 21, 201 (1980). 137. Lee, S. B., Weiss, M., and Ertle, G., Surf Sci. 108, 357 (1981). 138. Paal, Z., Ertl, G., and Lee, S. B., Appl. Surf. Sci. 8, 231 (1981). 139. Ertl, G., Weiss, M., and Lee, S. B., Chem. Phys. Lett. 60, 391 (1979).

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Bare, S . R., Strongin, D. R., and Somojai, G. A., J. Phys. Chem. 90,4726 (1986). Zaera, F., and Somojai, G. A., J. Curd. 84, 375 (1983). Campbell, C T , and Koel, B. E., Surf. Sci. 186, 393 (1987). Campbell, C. T., Koel, B. E., and Daube, K. A,, J . Vuc. Sci. Techno/., A 5, 810 ( 1987). 144. Campbell, C. T., J. Phys. Chem. 89, 5789 (1985). 145. Campbell, C. T., and Daube, K. A., J. Curd. 106, 301 (1987). 146. Sachtler, W. M. H., CHEMTECH July, p. 434 (1983). 147. Sachtler, W. M. H., and van Santen, R. A.. Adu. Curd. 26,69 (1977). 148. Sinfelt, J. H., Sci. A m . 253(3), 90 (1985). 149. Sinfelt. J. H., and Cusumano. J. A., “Advanced Materials in Catalysis,” p. 1. Academic Press, New York, 1977. 150. Bauer, E . , in “Chemical Physics of Solid Surfaces and Heterogeneous Catalysis” (D. A. King and D. P. Woodruff, eds.), Vol. 3B, p. I . Elsevier, Amsterdam, 1984. 151. Biberian, J. P., and Somojai, G. A., J. Vuc. Sci. Techno/. 16, 2073 (1979). 152. Weissman-Wenocur, D. L., and Spicer. W. E., Surf. Sci. 133, 499 (1983). 153. Campbell, C. T., Surf. Sci. 167, L181 (1986). 154. Campbell, C. T., Paffett, M. T., and Voter, A. F., J . Vuc. Sci. Techno/.,A 4, 1342 (1986). 155. Sachtler, J. W. A., and Somojai, G. A., J. Curd. 81, 77 (1983). 156. Sachtler, J . W. A., and Somojai, G. A., J. Curd. 89, 35 (1984). 157. Sachtler, J. W. A., and Somojai, G. A., J . Card. 103, 208 (1987). 158. Campbell, C. T., Campbell, J. M., Dalton, P. J., Henn, F. C., Rodriguez, J. A., and Seimanides, S. G., J . Phys. Chem. (in press) (1989). 159. Peden, C . H. F., and Goodman, D. W., J . Curd. 104, 347 (1987). 160. Peden, C. H. F., and Goodman, D. W., J . Card. 100, 520 (1986). 161. Goodman, D. W., and Peden, C. H. F., 1nd. Eng. Chem. Fundum. 25, 58 (1986). 162. Goodman, D. W., Houston, J. E., and Peden, C. H. F., J. Vuc. Sci. Techno/., A 5 , 823 (1987). 163. Goodman, D. W., and Peden, C. H. F., J.C.S. Furuduy 1 8 3 , 1967 (1987). 164. Greenlief, C. M., Berlowitz. P. J., Goodman, D. W., and White, J. M., J . Phys. Chem. 91, 6669 (1987). 165. Tauster, S . J., A r c . Chem. Res. 20, 389 (1987). 166. Kao, C.-C., Tsai, S.-C., and Chung, Y.-W., J. Card. 73, 136 (1982). 167. Chung, Y.-W., Xiong, G., and Kao, C.-C., J. Curd. 85, 237 (1984). 168. Takatani, S., and Chung, Y.-W., Appl. Surf. Sci. 19, 341 (1984). 169. Demmin, R. A., KO, C. S., and Gorte, R. J., ACS Symp. Ser. No. 298, 48 (1986). 170. Demmin, R. A., and Gorte, R. J., J . Curd. 105, 373 (1987). 171. Demmin, R. A., KO, C. S., and Gorte, R. J., J. Phys. Chem. 89, 1151 (1985). 172. Demmin, R. A., and Gorte, R. J., J . Curd. 98, 577 (1986). 173. Levin, M. E., Salmeron, M., Bell, A. T., and Somojai, G. A., Surf. Sci. 169, 123 ( 1986). 174. Levin, M. E., Salmeron, M., Bell, A. T., and Somojai, G. A,, J. Card. 106, 401 140. 141. 142. 143.

(mn.

175. 176. 177. 178. 179. 180.

Tauster, S . J., Fung, S. C., and Garten, R. L., J . Am. Chem. Soc. 100, 170 (1978). Bischke, S . D., Goodman, D. W., and Falconer, J. L., Surf. Sci. 150, 351 (1985). Lee, Y. C., Tong, P., and Montano, P. A., Surf. Sci. 181, 559 (1987). Klier, K . , Adu. Curd 31, 243 (1982). Klier, K., Appl. Surf. Sci. 19, 267 (1984). Strongin, D. R., Bare, S. R., and Somojai, G. A., J . Cutal. 103, 289 (1987).

54 181. 182. 183. 184. 185. 186.

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Stoke, P., Phys. Scr. 36, 824 (1987). Stoke, P., and Norskov, J. K., J . Card. 110, 1 (1988). Strongin. D. R., and Somorjai, G . A., J . Caral. 109, 51 (1988). Sinfelt, J . H., J . Cural. 29, 308 (1973). Vannice, M. A.. and Twu. C. C., J . Catal. 82, 213 (1983). Levin, M. E.,Salmeron, M., Bell, A. T., and Somojai, G . A,, J.C.S. Faraduy I 83, 2061 (1987).

ADVANCES IN CATALYSIS, VOLUME 36

The Influence of Particle Size on the Catalytic Properties of Supported Metals MICHEL CHE Laboratoire de RPactivitP de Surface et Structure, UA 1106, CNRS Universiti Pierre et Marie Curie (Paris V I ) 75252 Paris Cedex 05, France

AND

CARROLL 0. BENNETT Department of Chemical Engineering University of Connecticut Storrs, Connecticut 06268

1.

Introduction

Metals constitute a wide class of catalysts, and because catalysis occurs on the surface, there is an economic incentive, especially for precious metals, to obtain catalysts in the form of small metal particles. This, however, raises two main problems. One is fundamental in nature and addresses the question as to below which particle size the metallic properties are lost. The other is more practical and concerns the preparation and characterization of very small particles and their catalytic activity. Let us consider a large metal particle that has electronic levels so close that they actually form bands. The spacing between adjacent levels is approximately expressed as (l a , b ): 6

z

&FIN

where eF is the Fermi level energy and N is the number of atoms in the particle. As the spacing between the levels becomes larger than the thermal energy kT, the levels begin to behave individually and the particle may lose its metallic properties. The critical size of the particle can be calculated by the above formula. At room temperature 6 = 2.5 x eV, 55 Copyright 0 1989 by Academic Press, Inc. All rights of reproduction in any form reserved.

56

MICHEL CHE A N D CARROLL 0. BENNETT

and with eFof the order of 10 eV, N is calculated to be approximately 400, which corresponds to a diameter of about 2 nm. This estimation is, of course, very approximate, but a lot of data confirm that the physical and catalytic properties start to change near this size and that catalysis is one of the most sensitive means to probe the surface of such particles. The other problem concerns the practical ways of producing metals with particle sizes around the critical value calculated above. The easiest and most practical way to achieve this goal is to deposit the metal in low concentration on a high-surface-area support. The main purpose of using a carrier is, of course, to achieve a high dispersion of the metal and to stabilize it against sintering. In a number of reactions, however, the metal support is not inert and the overall process is actually a combination of two functions: that of the metal and that of the catalytically active support. Thus, if an intrinsic size effect on catalytic properties is to be evidenced, much care should be taken to choose as much as possible a neutral nonactive support and to avoid using conditions that could possibly induce any metal-support interaction. These precautions are often not sufficient, and the preparation methods, as we shall see, are also likely to affect the final properties of the particles obtained. In the last decade, remarkable progress has been made in the preparation methods of supported metal catalysts. This has stimulated interest in the study of the relation between the catalytic behavior of metal particles and their mode of preparation with the aim of achieving the goal indicated above. Many workers have investigated the dependence of reaction rate on particle size. Early attempts to approach this problem were made by Kobosev and co-workers in the 1930s from the viewpoint of atomic dispersion and active ensembles. They studied the behavior of catalysts containing very small amounts of supported metal and were able to derive the number of atoms within the ensembles that were active for specific reactions (one atom for SO2 oxidation, two atoms for benzene hydrogenation, three atoms for ammonia synthesis, four atoms for acetylene oligomerization) (2a-c). These results as well as later ones have been reviewed by Gil’debrand (3). Boreskov et al. (4, 5 ) were the first to complete a systematic investigation of the relationship between particle size and catalytic activity, after their development of a technique for measuring the surface area of platinum catalysts by means of the selective chemisorption of hydrogen (6). They showed that the specific activity of platinum in the oxidation of sulfur dioxide ( 4 ) and of hydrogen (5) varied by less than one order of magnitude for catalyst samples differing in platinum surface area by four orders of magnitude. A few years later, Kobosev’s ideas were further

METALS: PARTICLE SIZE AND CATALYTIC PROPERTIES

57

investigated by Poltorak and Boronin on highly dispersed metals prepared by adsorption techniques (7u-d). Their studies led to many of the conclusions reached independently by Boudart et al. (8) for the hydrogenolysis of cyclopropane over supported Pt catalysts. One of the most important conclusions was that the rate per unit surface area of metal seemed for certain catalytic reactions very insensitive to the degree of dispersion of the metal. Boudart called these reactions facile (8)or structure-insensitive (9). The term structure refers to the coordination number of surface atoms that can be varied by exposing different crystallographic planes and making them imperfect by means of steps and kinks or by varying the particle size as can be done with supported metals (10). In the early work of Boreskov et ul. an apparent exception to this lack of sensitivity of activity to platinum particle size was observed for the H2DZexchange at low temperature (11). Other examples of the sensitivity of the specific activity to particle size were reported later on. Thus hydrogenolysis of ethane was found to depend on particle size for the Ni/AI2O3Si02 (12) and Rh/SiOz (13) systems. In the former case, the catalytic activity for unit surface area decreased for increasing particle size, whereas in the latter, an optimum particle size was associated with a maximum specific activity. These reactions were called “demanding” (8) or “structure-sensitive” (9) by Boudart, who drew attention to the difficulty of differentiating between metal-support interactions, the role of impurities, and mass-transfer limitations. This author proposed to select a molecule that can react along two parallel paths and to follow the change of selectivity with the particle size or the mode of preparation of the metal. The reaction of neopentane in the presence of hydrogen is a typical example (14). Reactions can now be classified into four categories (15) depending on how the turnover frequency TOF (the rate of the reaction expressed in moles per exposed metal atom and per unit time) varies as a function of the particle size d or of the fraction exposed (Fig. 1): the TOF of structure-insensitive or facile reactions does not depend on the particle size (curve 1). The TOF of structure-sensitive or demanding reactions may vary in two opposite ways: it may decrease when the particle size decreases, i.e., larger particles are more active than small ones; this is termed a negative particle size effect or antipathetic structure sensitivity (curve 2); or it may increase for decreasing particle sizes, i.e., smaller particles are more active than larger ones; this is called a positive particle size effect or sympathetic structure sensitivity (curve 4). The TOF may go through a maximum, for instance, if small particles exhibit a negative effect and larger ones a positive effect. In this case, those of an intermediate size will have maximum specific activity (curve 3). We have chosen

58

MICHEL C H E A N D CARROLL 0. BENNETT

d,nm

0

3

c I

Ln

LL

0

I-

9 ; 1

c

c

\

\

2

\

0

FE

1

FIG.1. Some of the ways in which turnover frequency has been found to vary with fraction exposed and particle size (see text for a discussion of the curves).

this numbering because it appears that only curves 1-3 can be reasonably understood as explained at the end of this review. The number of papers dealing with metal particles is extremely large: a computer survey based on volumes 66-101 of Chemical Abstracts gave the following numbers of references for the Group VIII metals. Fe, 213,818; Cu, 167,896;Ni, 140,550; Co, 83,827; Ag, 59,960; Pt, 39,268; Au, 36,444; Pd, 30,150; Rh, 14,761; Ru, 13,015; Ir, 7390; and Os, 4976. The same search restricted to particle size effects still gave 3,541 references. Our coverage had to be more limited and has been restricted to those papers in which curves of the types described above were presented. Unfortunately, the catalytic results are not always fully comparable. In fact, most works do not distinguish initial rate results (on clean surfaces) from steady-state rates (on semipoisoned surfaces), even though most refer to low conversions. Further, if a true particle size effect is to be evidenced, the metal particles must correspond as much as possible to the following criteria: they must be ( 1 ) monodisperse, (2) fully reduced, (3) unpoisoned (by liquid solvents or adventitious species), (4) unperturbed (by the carrier), and (5) unpromoted (by Ca, K, etc.). This goal is most clearly approached by methods based on vaporization of a metal by laser pulses, leading to a cloud of clean clusters of bare metal containing as few as one atom. Catalytic activity of clusters as a function of number of atoms can be obtained by time-of-flight mass spectrometry (16-18). Reviews have appeared in the past on particle size effects (3, 9, 10, 15, 19-24), but in the present one we attempt to give a comprehensive and

METALS: PARTICLE SIZE AND CATALYTIC PROPERTIES

59

comparative study of particle size effects, particularly with regard to catalytic properties. Although this review is limited to the intrinsic particle size effect, as exhibited by either model or practical systems, it is clear that activity or selectivity in a useful catalyst may be altered favorably (or unfavorably) by support interaction (25a), addition of promoters (25b,c), and bifunctionality, matters that were discussed earlier. Particle size effects are also important in related fields such as electrocatalysis (25d-31). For example, the reduction of dioxygen over Pt/C shows antipathetic structure sensitivity, leading to an optimal particle size of about 4 nm for a working electrode (31). However, work on electrochemistry is not included in this review. II. Preparation of Supported Metal Particles

A. INTRODUCTION Recent years have witnessed marked progress in the preparation of small metal particles. This has been achieved by the choice of a suitable support, the selection of the appropriate preparation method, or the combination of both. The selection of the carrier is relatively simple. It may be imposed by the type of reaction to be promoted. For instance, if the latter requires a bifunctional catalyst (metal + acid functions), acidic supports such as silica-aluminas, zeolites, or chlorinated aluminas, will be used. On the other hand, if the reaction occurs only on the metal, a more inert support such as silica will be used. In certain cases, other requirements (shock resistance, thermal conductivity, crush resistance, and flow characteristics) may dominate and structural supports (monoliths) have to be used. For the purpose of obtaining small metal particles, the use of zeolites has turned out to be an effective means to control their size. However, the problem of accessibility and acidity appearing on reduction may mask the evidence of the effect of metal particle size on the catalytic properties. The choice of the method is more complicated and depends on whether an industrial or a model catalyst is prepared. The most common industrial preparation methods involve several steps: (1) the distribution of a precursor compound over a carrier, (2) the drying and calcining of the resulting solid, and (3) the reduction of the precursor compound, which may, after steps 1 and/or 2, no longer be identical to the initial precursor, to obtain the active final metallic state. Rather than describe the numerous methods available in the literature, we shall in the following emphasize the advantages and disadvantages of each method with respect to the goal

60

MICHEL CHE A N D CARROLL 0. BENNETT

to be achieved (i.e., the preparation of monodispersed metal particles). The design and preparation of supported catalysts have been the subject of several recent reviews (Ib, 3, 32-39, 40a-c).

B. THEVARIOUSPREPARATION METHODSFOR DEPOSITING THE METALPRECURSOR OR THE METAL This review begins with the more complex methods and moves on to the simpler ones. In this context, complexity is defined as the number of species involved before finally obtaining the highly dispersed metal particles. Table I summarizes the principal features of the different methods used to prepare the metal particles on a support, which is the common feature of all the methods and thus is not included in the table. They are subdivided into groups when the principles are similar. We could have ordered the preparation methods differently, for instance, using a chemical approach starting from the oxidation state and nuclearity of the metal in the precursor compound, but this would not have served our purpose, specifically, to show the parameters that can possibly influence the behavior of the final metallic state. 1.

Coprecipitation

Most methods deal with the formation of metal particles on a support that is preformed since this leads to simpler preparation processes. There is an important route, however, typically used for metal-SiOz and metalA1203 catalysts, which involves (Table I) the coprecipitation in a precursor form (hydroxides, nitrates, carbonates, silicates, etc.) of both the support and the active phase from a solution (37a,b, 38, 41). The advantage is to produce an intimate mixing of metal precursor and support. The precipitate leads on calcination to a support with the active component dispersed throughout the bulk as well as at the surface. After reduction to the final catalyst, it is difficult to obtain metal crystallites of uniform size (42,43) because of the presence of both the oxides (of the support and of the active metal) and other intermediate compounds [e.g., nickel aluminate or silicate for the Ni/A1203 (42) and Ni/Si02 (43) systems, respectively] that have different reducibilities. The coprecipitation can be improved in various ways so as to avoid any heterogeneity during the process. This is achieved in the superhomogeneous coprecipitation method, which consists of two steps. In the first, layers of the salt solutions are superposed and then mixed into a supersaturated solution instantaneously. The second step consists of forming a

TABLE I The Various Preparation Methods of Supported Metal Particles Further activation steps" Washing, hydrolysis, Solvent

Metal precursor

Inorganic

Salt

Inorganic Inorganic Inorganic

Solvated cation Complex anion Complex cation

Inorganic

Complex cation

Inorganic

Complex anion

organic

Metal cluster compound

Organic Organic

-

-

Metal cluster compound (solid or vapor) Metal colloid

Other species present

or Example

evacuation

Support precursor Precipitating agent Counteranion Countercation Counteranion Counteranion Competing cation Countercation Competing anion

-

Pd colloids/Si02

Atoms (metal vapor) Ions Atoms (metal vapor)

+ or - indicates whether or not the corresponding activation step has been performed. The method number, as referred to in the text, is indicated in parenthesis. TMI, transition metal ion.

Drying

Calcination

Reduction

Methodb

+

+

Coprecipitation (1)

+ + +

+

+ +

Impregnation (2)

+

+

+

+

Simple ion exchange (3) Competitive ion exchange (3)

+

+

-

+

+

-

+

-

-

-

-

Decomposition of metal cluster compound (4)

Chemical deposition ( 5 )

Ion implantation (6) Vapor-phase deposition (7)

62

MICHEL CHE A N D CARROLL 0. BENNETT

homogeneous precipitate from this supersaturated solution ( 3 7 ~ )Some . authors produce the coprecipitation of the components in an organic solvent that is then evacuated under supercritical conditions. This leads to particularly high-surface-area materials (44, 45). The disadvantages of the coprecipitation method are that the precursor ions are also distributed within the bulk of the support oxide and that the pore structure of the final catalyst is more difficult to control than when one starts from a separately produced carrier. The next type of preparation method avoids these difficulties. 2. Impregnation

This method is begun by impregnating a preformed support with a solution of a metal salt. The solvent is removed by evaporation during drying (40a,b). When the amount of solution corresponds to the pore volume of the support, the method is more precisely referred to as the incipient-wetness impregnation. This method is used when there is little or no interaction between the precursor and the support. The advantages are technical simplicity, low cost, and reproducible metal loadings. If the metal content is limited by the solubility of the metal compound, multiple impregnation steps can be used. When the pores contain only air, the impregnation is referred to as capillary, whereas when they are filled by a solvent, the impregnation is called diffusional (46a). In some cases, precipitation of an active precursor onto a separately prepared carrier is also used, and this is known as the deposition-precipitation method (46b). Here the active precursor is precipitated onto the support by means of a chemical reaction, whereas in the classical impregnation method, it is “physically” deposited onto the carrier through removal of the solvent as described above. Recently, a new type of “dry” impregnation has been presented (46c,d),which consists of heating a mechanical mixture of the active precursor and the carrier. This leads to the spontaneous dispersion of the active precursor onto the support. This solid-solid adsorption is a spontaneous entropydriven process since, in contrast to what happens in gas-solid or liquidsolid adsorption, there is a change from an ordered (three-dimensional)to a less ordered (two-dimensional) structure of the active precursor, and this is a process of increasing entropy (46e). This type of solid-solid adsorption will be the subject of a forthcoming review (46f). In the case of impregnation, the subsequent step of drying (40a,b)is an important operation in the distribution of the active phase at the surface of the support. When a pellet with a uniform pore system is heated at a fast rate, a temperature gradient is established between its external surface

METALS: PARTICLE SIZE A N D CATALYTIC PROPERTIES

63

and its interior. Evaporation starts at the external surface, and the gasliquid interface moves toward the interior. The precursor concentration increases at the menisci, and the active phase is deposited on the pore walls. After reduction, the metal particles will be rather homogeneous in size. By contrast, if the drying rate is low, the active phase, which interacts little with the support, remains in the liquid phase so that the concentration in the active phase steadily increases. When the drying is finished, the active phase is found at the bottom of the pores. After reduction, the metal particles obtained will, therefore, depend on the volumes of the pores (47). If the latter are not uniform in size, the situation is more complex and there will be a correspondingly broad distribution of particle sizes in the finished catalysts. It should be noted that there is no washing step after impregnation (Table I) because this method involves little or no interaction between the precursor and the support. Thus, any washing would eliminate the precursor phase. During the calcination step, occurring between drying and reduction, two phenomena generally occur: (1) the precursor oxide is formed by decomposition of the impregnated salt, and (2) some chemical bonding is established between the precursor oxide and the support (48). As seen in Table I, the impregnation method may involve a precursor in the form of either a complex cation or a complex anion, with the disadvantage that counterions are always present. During the calcination step, such ions may lead to uncontrolled side reactions producing strongly held species, such as nitrogen-containing species (NO, N02, NO;) for samples prepared from nitrates (49) or ammonium compounds (50). The next preparation method avoids this problem to a large extent.

3. Zon Exchange This technique is used when the precursor interacts with the support. The interactions are basically controlled (52) by (1) the type of support and state of the surface (number and nature of the functional groups, their acid-base properties) and (2) the impregnating solution (pH, type and concentration of the metal precursor, and presence of competing ions). In this method, also called dipping impregnation, wet soaking impregnation, impregnation with adsorption, or adsorption from solution, the support is immersed in a solution of the metal compound. The slurry is stirred for a given time, filtered, and possibly washed. The resulting product is then dried. The concentration of the precursor solution may be readjusted and recycled. This technique is widely used in the preparation of laboratory and industrial catalysts. Depending on the strength and conditions of adsorption of the precursor species, the concentration of the

64

MICHEL CHE A N D CARROLL 0. BENNETT

active phase may be varied. This method thus leads to a greater control over the dispersion and distribution of the active species within a catalyst pellet. Because the precursor interacts with the support, it is not eliminated easily by washing. The metal loading is governed by the concentration of adsorption sites on the support oxide. As a consequence, the weight of the active component that can be incorporated is limited. One can increase the metal loading by successive impregnations, but this is not recommended if a careful control of the physical parameters is to be achieved. Another disadvantage is the high cost of the process because of lower productivity and complex precursor solution recycling in the case of costly precursors. a. Surface Charge of Oxides in Suspension. Most oxides are amphoteric and when immersed in an aqueous solution present surface charges that are controlled by the equilibria S-OH;

tH+ -OH7 S-OH = +OH- S-0-

+ H20

where S-OH represents a surface adsorption site. The addition of an acid shifts the equilibrium to the left, increasing the number of positive charges on the surface: the oxide behaves as an anion exchanger. Conversely, the addition of a base shifts the equilibrium to the right, increasing the number of negative charges: the oxide behaves as a cation exchanger. When the number of positive and negative charges at the oxide surface is equal, the corresponding pH is the isoelectric point of the solid (IEPS) or the zero point of charge (ZPC). The isoelectric point can be determined by electrophoresis measurements. The, values of the IEPS, which are known for a number of oxides, reflect the chemical composition of the solid and the electrolyte in which it is immersed (52, 53). Structural or adsorbed anionic impurities shift the IEPS to more acidic values, and cationic impurities shift it to more basic values or toward the IEPS characteristic of the impurity oxide (53). Above the IEPS value, the oxides will behave as cation exchangers and as anion exchangers below that value. The further one gets from this value, the larger the surface charge and the exchange capacity (54). b. Simple Zon Exchange. Among the supports, zeolites, Si02-AI203, and Si02 adsorb cations; A1203,Ti02,and Cr2O3, which are amphoteric, adsorb anions in acidic and cations in basic solutions; MgO, La203,and ZnO adsorb only anions (36, 41, 53). The adsorption equilibria can be represented by the following equations (36): S-OH

+ C+ =S-0-C+ + H+

cation adsorption

METALS: PARTICLE SIZE A N D CATALYTIC PROPERTIES

S-OH

+ A- + H+ =S-0H;A-

65

anion adsorption

A very interesting feature of the ion-exchange method is that the counterion can be easily removed by washing (Table I) because it is repelled from the surface that possesses charges of the same sign. This is an important advantage over the impregnation method, where washing cannot be used because of the weak interaction between the precursor and the surface. In the ion-exchange method, the cations C+ or the anions A- tend to saturate the first adsorption sites so that most of the compound adsorbs near the pore mouth, and a large concentration gradient develops within the pellet pore. There are several ways to obtain a more uniform distribution: (1) a large supply of compound is used so as to saturate every adsorption site, (2) the support may be left for a long time in contact with the solution, and (3) competing ions are added to the solution, which will adsorb on the same sites as the ions of the metal precursor. This is the subject of the method discussed next. c . Competitive Zon Exchange. This method, also called chromarographic adsorption, involves the competition between two types of ion for the same adsorption sites (Table I). The competitive exchange has long been known for anions (55), whereas the competitive cation exchange has been developed more recently (56). The competitive ion-exchange equilibria can be represented by the following: S-0-C+

S-0H;A-

+ D+ =S-O-D+ + C+ + B- =S-OH;B- + A-

competitive cation exchange competitive anion exchange

A typical example is illustrated by the following reactions: Si-O--H+ 2Si-0-NH:

+ NH3

+ [Pt(NH3)4]2+

Si-0-NH; (Si-0-)2[Pt(NH3)4]2+

+ 2NH:

where the NH: and [Pt(NH3)4]2+ ions are competing for the same surface Si-0- sites. By varying the concentration of NH: competing ions, it is possible to shift the equilibrium of the second reaction presented in the typical example above and to control the deposition of the precursor metal species onto the oxide surface to obtain a homogeneous distribution. The adsorption of the precursor metal species by oxide surfaces can be accompanied by a ligand reaction (57a) between a surface site and a ligand on the metal complex, with the result that the surface becomes a mono- or polydentate ligand within the metal ion coordination sphere (57b-e):

66

MICHEL CHE A N D CARROLL 0. BENNETT

(Si-O-)2[Ni(NH,),]2+

=[(Si-O)2Ni(NH3)4] + 2NH3

This stronger interaction between the metal precursor and the surface and its homogeneous distribution leads, after reduction, to highly dispersed metal particles. Very often, though, because of their strong interaction with the surface, the precursor ions are difficult to reduce at the temperatures where metal aggregation is not important. The final state thus consists of highly dispersed metal particles anchored onto the surface by means of a “chemical glue” composed of partially reduced or unreduced ions (58). In order to avoid this problem of incomplete reduction, which becomes all the more important as the particle size decreases (59), a number of attempts have been made to bypass the reduction step and to deposit the metal directly in its zero-valent state. There are different methods for this, and they are discussed separately below. 4.

Decomposition of Metal Cluster Compounds

Ever since Parkyns (60) prepared nickel particles on oxide (A1203,Si02) surfaces by decomposition of nickel carbonyl, metal cluster compounds (alkoxides, carbonyls, organometallics, etc.) have been used increasingly for the production of laboratory metal catalysts (Table I), and several reviews have appeared on this subject (61-65a,b). Metal cluster compounds were initially employed for the two following purposes. The first concerned the formation of heterogeneous analogs of homogeneous active catalysts, and the second dealt with the production of small metal particles. The major advantage of the latter approach is that the temperatures usually required to decompose the metal cluster compounds and produce small metal particles are substantially lower than those observed for the reduction of metal salts (nitrates, chlorides) involved in conventional methods so that the number of atoms initially present in the complex can be preserved, e.g., Ni3 if one starts from trinuclear nickel clusters (66). However, the results using this approach have not been always as promising as originally believed (64). Metal cluster compounds are usually anchored onto an oxide surface via the two following routes: 1.

reaction involving surface hydroxyl groups S-OH: nS-OH

+ MX,

[S-O].MX,-,

+ nHX

The metal content is controlled by the concentration of surface S-OH groups, the stoichiometry of the reaction, and the number of metal atoms within the cluster.

METALS: PARTICLE SIZE A N D CATALYTIC PROPERTIES

67

2. reaction first to produce functional anchoring groups at the oxide support surface and then to anchor the metal cluster compound [A number of functions can be selected as well as their further reactions, and this subject has been extensively reviewed (62)l.

From recent literature data, there are many cases in which the resulting surface products, after anchoring, are not in a zero-valent state but rather form a mixture of species so that the metal is both in zero-valent and partially oxidized states. Most studies on functionalized supports have been made on hydroxylated oxide surfaces. The metal particles produced on such supports are not of an unusually high degree of dispersion. The nature of the decomposition process on dehydroxylated supports has not been as thoroughly studied as on hydroxylated supports. Preliminary results, however, indicate that formation of highly dispersed zero-valent metal particles may be obtained (64). The other disadvantage of the metal cluster compound decomposition is that the ligands, which have to be removed to produce the metal particle, and the organic solvent used to impregnate the support by the precursor compound may remain on the surface and thus interfere later in the catalytic reaction. The problem of the solvent can sometimes be avoided by using volatile metal cluster compounds (6%). The next method based on depositing colloidal metal particles onto a support removes the problem of incomplete reduction, i.e., the presence of the mixture of reduced and unreduced species, but does not remove that of the solvent. 5 . Chemical Deposition from a Metal Colloid Dispersion

In this method, the preformed metal is adsorbed as individual particles onto a carrier, both suspended in a solvent (Table I) (67,68).The size and structure of the colloids are not influenced by the support (69), but the liquid medium may contain species that are by-products of the preparation and that may complicate the situation unless removed by dialysis. In various attempts to use this method the metal particle size, which depends on the metal and the preparation conditions, is usually larger than that obtained by the preceding methods. A more sophisticated technique, the so-called solvated metal atom dispersion technique, has been developed by Klabunde and Tanaka (70). It involves the evaporation of metal atoms in the presence of complexing solvents to yield “solvated” metal atoms. The solution is brought into contact with the support at low temperatures and the slurry obtained is warmed to room temperature. The excess solvent is removed by further

68

MlCHEL CHE A N D CARROLL 0. B E N N E T T

evacuation. Although attractive, this method presents the disadvantage that the small crystallites are surrounded by organic fragments. Those fragments may affect the catalytic and magnetic properties of the metal particles (71). Several solutions have been proposed to avoid these organic fragments, and we shall review two of those, which can, however, produce only model catalysts. In these methods, only the metal is deposited on the support; one concerns the deposition of the metal in its oxidized state and the other, in its zero-valent state. 6 . Ion Implantation

The principle of this method is to select by mass spectroscopy a positively charged ion, for example l9W+,which is implanted by ion bombardment of a given support (Table I) (72). The ion-implantation technique is simpler when single crystals are used, for instance, Mg(100) (73). Since electrons are readily available in the ion implanter, the ion is quickly neutralized and the neutralized form of the element implanted can be finally found either as a gas (e.g., Xe) or as a metal (e.g., Pt) in the outer layers of the oxide exposed to ion implantation (74). The advantage of this method is that, in contrast to all previous methods, it involves only the metal and the support and that metal-support interactions concern only a single face of the support (75). The disadvantage is that the metal is not located only at the oxide surface but also within the oxide and the concentration profile is critically dependent on the energy of the implanted ions. The other problem is that a number of defects are produced during the bombardment that may interfere later in catalysis. However, this effect can be evaluated (75) if ions of similar weight but that are catalytically inactive (e.g., Xe') are implanted under the same conditions (energy, angle, and time of ion implantation). The next method avoids the problems caused by the implantation profile and the presence of defects.

7 . Metal Vapor Deposition This method (Table I), like the preceding one, eliminates the problem of any spurious species (counterion, ligand, solvent, etc.) and has the advantage that it does not alter physically the support surface. There are a number of other advantages, which can be listed as follows: 1. When they come to land on the surface, the atoms are in their zerovalent state, except for those atoms that may be reoxidized by surface OH groups by reaction such as (61) Ni" + OH =Ni2+ + 0'-+ 1H2

METALS: PARTICLE SIZE

A N D CATALYTIC PROPERTIES

69

2. There is thus no thermal reduction step involved, and this suppresses both any sintering process and the formation of some strongly adsorbed hydrogen phase, which, for small particles, can decrease or even eliminate the catalytic activity (58, 76, 77). The “reduction” degree is thus much larger than that obtained for similar sizes with conventional methods. 3. The dispersion can be controlled by monitoring only the time of evaporation (78). For small times of evaporation, atomic dispersion can be obtained all the more easily because the system does not need to be heated as in other methods for thermal reduction. The surface atom diffusion is thus drastically slowed down. 4. Finally, heterogeneous nucleation theory can be applied (79), and it affords an independent way of estimating the particle size, particularly difficult if not impossible to measure close to atomic dispersion by conventional methods because of the low metal content of such model catalysts.

There are, of course, some limitations. An obvious one is that this method cannot be applied yet to the preparation of industrial catalysts, another one is its cost because ultra-high-vacuum (UHV) equipment is required. This drawback explains why this method is usually coupled to surface techniques such as XPS, UPS, RHEED, and AES, which also require UHV. The last disadvantage is that the best suited supports are those that are flat, i.e., oxide single-crystal faces, oxides produced by oxidation of a metal single crystal, or compressed powder oxides. There have been several examples where the preparation chamber also serves as sample chamber for surface techniques and is coupled to a catalytic reactor. Whereas there are a number of works using this approach for bulk metals (80),there are, by contrast, few studies dealing with metals supported on either single crystals (81) or polycrystalline supports (78,79,82, 83). The latter type of system appears to be the model catalysts closest to the real catalyst.

C. FURTHER ACTIVATION STEPS The preceding paragraphs have introduced the various preparation methods leading eventually to supported metal particles. These methods fall obviously into two categories depending on whether the metal is basically in its zerovalent state (decomposition of metal cluster compound, chemical deposition, ion implantation, and vapor-phase deposition) or in an oxidized state (coprecipitation, impregnation, and ion exchange)(Table I).

70

MICHEL CHE A N D CARROLL 0. BENNETT

For the latter methods, it is necessary to reduce the ions to obtain the metal. In some cases, a calcination is performed before the reduction. We shall now concentrate on these two stages.

I. Calcination Very often, calcination is carried out in air or oxygen and leads to the following transformations: 1. formation of the oxide system on decomposition of the precursor compound and subsequent bonding of the formed oxide with the support (typically observed for catalysts prepared by impregnation), 2. removal of some of the elements introduced during the preparation by formation of volatile compounds ( S 0 2 , NO2, COZ, etc.), 3. decomposition of the precursor ionic complex and further ligand exchange reactions between surface groups (OH, 0 2 -and ) ligands bound to the metal ion, resulting in stronger precursor-support interactions (typical of catalysts prepared by ion exchange), 4. elimination of carbonaceous impurities possibly introduced during the preparation, and 5 . sintering of the precursor compound or of the formed oxide system.

Consequently, calcination may have a pronounced effect on the reducibility, dispersion, and distribution of the metal in the final catalyst. For instance, high-temperature treatment may lead to very stable solid solutions or to large oxide crystals, particularly for catalysts prepared by impregnation. In the case of solid solutions, the metal is difficult to reduce because of strong precursor-support interactions. This can be overcome by higher reduction temperatures. Because of these strong precursorsupport interactions, high dispersion of the metal can be achieved in the final catalysts (84). By contrast, large oxide crystals will lead, after reduction, to low dispersions of the metal. For catalysts prepared by impregnation, it is often observed that the final particle size distribution reflects the different types of precursor-support bonding obtained after calcination, and this depends on the metal loading. Thus, for Ir-Ti02 systems prepared by impregnation (83,it has been observed that two types of particles were formed after calcination in air and reduction by hydrogen. One corresponded to small particles issued from precursors ion exchanged on the support surface and in amount never in excess of the monolayer; the other corresponded to large particles issued from precursors impregnating the support and observed for metal loading exceeding the monolayer. There have been relatively few studies on the influence of treatment

METALS: PARTICLE SIZE A N D CATALYTIC PROPERTIES

71

conditions on metal dispersion, and most concern platinum. Kubo et ul. (86) reported that the temperature of calcination in air before hydrogen reduction was the key parameter determining the platinum dispersion in RNaY and RNH4Y zeolites. The best dispersion was obtained for air calcination at 300°C. In many cases, the calcination in air is used to carefully control the decomposition of ion complexes used as precursors and to develop strong interactions with the support so that after reduction, small metal particles are formed (58, 87u,b).

2. Reduction The conditions of reduction are particularly important since they can strongly affect the final distribution of the metal particles (88). In studies of nucleation and film growth from metal vapor, it is generally observed that the crystallite size is determined by the saturation density of nuclei, which is found to increase with decreasing substrate temperature due to shorter adatom migration distances (89).In the case of nucleation in metal oxide reduction, an opposite trend is found. Higher temperatures will favor more rapid initial reduction of the metal oxide and higher density of nuclei but also coalescence or sintering of particles. An intermediate temperature must be found that leads to the maximum nuclei concentration without any appreciable coalescence. The parameters probably effecting this compromise are the presence of nucleation sites that may correspond to surface defects (90) and the presence of water vapor, which usually leads to an increased mass transfer (92).It is thus anticipated (88)that the smaller particles should be obtained (1) by using starting materials that will maximize the defect concentration in the final support, (2) thoroughly drying the supports at the lower temperatures to avoid changing the defect concentration, and (3) using a high flow rate of hydrogen in the reduction process while keeping the temperature low. This high flow rate of hydrogen lowers the partial pressure of water vapor and increases the rate of reduction at lower temperatures (92).Once nucleation has started, hydrogen can dissociatively chemisorb on the metal nuclei that are formed. The hydrogen atoms are mobile and can migrate to the metal particle-metal oxide interface and be used in reduction of the oxide. This causes a much more rapid reduction since hydrogen atoms react with either the metal ion (93) or the oxide ion 02-(94) more readily than does molecular hydrogen at the same temperature. The technique of hydrogen atom beams produced by a microwave discharge has been used to produce small metal particles (58, 93). The microwave discharge technique has also been used to decompose metal carbonyl compounds and small metal particles have been obtained (95).

72 111.

MICHEL CHE A N D CARROLL 0. BENNETT

Background for Evaluating the Catalytic Properties of Small Supported Particles

A. DEFINITIONS In his interesting article on particle size effects, Bond (21) gives the results of model calculations based on spheres and on cubes with five faces exposed, for Ni, Pt, and Pd. For these simple systems, the fraction of total metal atoms exposed, FE, is related to the diameter or length of a side d (in the figures, d is written with a bar above it to highlight the fact that it is an average value) by the approximate formula Bld,

FE

= 11.0,

d > 1 nm d < 1 nm

where d is in nanometers and B is about 1 nm. In discussing the literature on structure sensitivity we shall use B = 1 .O nm in Eq. ( I ) , realizing that B varies from one metal to another or as the morphology changes with d for a given metal. We define the atomic rate of reaction AR as the rate of production of a given product, with the units S - I , or moles of product per mole of total metal atoms per second. This is the rate that we strive to maximize for economic reasons, as far as catalyst development is concerned. However, it is convenient to decompose the atomic rate AR into two factors of a more fundamental nature:

AR = (TOF)(FE) (2) where TOF is the turnover frequency, s - I (per second), a rate expressed as moles of product per mole of surface metal atoms per second. In general, TOF is a function of d ; such systems are structure-sensitive. For certain systems TOF is not a function of d , and these cases are called structure-insensitive. In this work we review the behavior of TOF(d) for various metals and supports and for various reactions. The essence of this behavior was presented elegantly by Boudart (10). However, it is now clear that other factors in addition to d commonly affect the turnover frequency. We prefer the term fraction-exposed (96) to dispersion, since the latter conveys an idea of distribution in space that is not directly related to particle size and shape. The quantity FE is commonly measured by hydrogen chemisorption at room temperature, using the equation H/M FE = H/M,

(3)

METALS: PARTICLE SIZE A N D CATALYTIC PROPERTIES

73

The quantity H/M, i.e., H atoms irreversibly adsorbed divided by total metal atoms, is measured experimentally. H/M,, where M, refers to surface metal atoms, may be known on the basis of surface science studies [e.g., low-energy electron diffraction (LEED)] and is usually assumed equal to unity. However, we shall discuss some papers that show that H/M, is also a function of d and other variables. In addition, reversibly adsorbed hydrogen may also be important (97,98). For interpreting most data, nevertheless, we can only assume that FE = HIM. It seems reasonable to use surface metal atoms as a basis, and not surface sites. The number of surface atoms required to form a site is in many cases a subject of debate, whereas H/M is measured experimentally. Turnover frequency is preferred in the IUPAC report (96) to turnover number TON, for it has the correct units. A number implies a dimensionless quantity, whose magnitude is independent of the system of units. TON also seems to suggest an aura of a physical constant, referring to a property of a particular catalyst system. Since TOF is a rate, a function of temperature, pressure, and fluid composition in addition to d, etc., turnover rate had been suggested as a more appropriate name (99). These ideas are clearly discussed by Boudart and Djega-Mariadassou (100, 101), where rate is expressed as vitesse de rotation or turnover rate. We have defined TOF in terms of moles of a certain compound, relating to the idea of a yield. Thus, for methanation, we have TOFco and TOFcH,, etc. Ichikawa et al. ( 1 0 2 ~have ) used the term site-timeyield (STY), but we do not propose such a departure from ingrained habits. Often rates are expressed per unit mass of supported catalyst, and we can define the activity A, moles of product per second per kilogram of supported catalyst, as A = (AR)(C)/AW

(4)

where C is the catalyst loading, kilogram (metal)/kilogram (metal + support), and AW is the atomic weight of the metal. In line with the usual meaning of “specific,” A might be called a specific rate, but this term has, unfortunately, become associated with a rate per unit area of metal. This term seems unnecessary, because for well-defined surfaces TOF is interpreted as molecules reacted per surface metal atoms per second. Because of these difficulties, the latest version of the IUPAC manual (102b) expresses activity as mass-specific rate, rate per unit area as area-specific rate, and volumetric rate (see below) as volume-specific rate. For reactor design, the usual volumetric rate is convenient, written as r = ApB

(5)

74

MICHEL CHE A N D CARROLL 0. BENNETT

where r is moles per second per cubic meter of bed and p~ is the bed density expressed in kilograms of catalyst per cubic meter of bed. We have taken pains to present these definitions because it seems that the organization and understanding of catalytic results is hampered by lack of agreement on these simple details. Rates may be expressed as turnover frequency, atomic rate, activity, or volumetric rate, provided the authors determine also H/M, C, and p ~ and , defend their choice of H/M,. Of course, d should be measured independently if possible, as should the morphology of the metal particles. We emphasize the simplicity of Eqs. (1-5). It would seem desirable to express all kinetic results in terms of these rates. From a fundamental point of view, results on TOF(d) furnish important data for testing models of catalysis. We shall discuss how such results may be explained in terms of the geometric and the electronic properties of supported metals. From a practical point of view, the form of Eq. (2) shows that there may be a value of d that maximizes the value of AR, depending on how TOF varies with d.

B. POSSIBLE EXPLANATIONS OF PARTICLE SIZEEFFECTS 1. Electronic Effects

As the number of atoms in an isolated metal particle is reduced, the differences in energy levels of the valence electrons will eventually become appreciable with respect to kT. In the limit the discrete levels associated with an isolated atom would prevail. Thus, we expect a variation in the catalytic properties of small particles because of this electronic effect, which applies to all the atoms of the particle. However, the proportion of the total atoms present on the surface and thus in various states of coordination with respect to the other metal atoms will also change. The electronic configuration of these surface atoms is changed, leading to changes in the catalytic properties. This second electronic effect may be viewed as having a geometric origin, so that the usual division of size effects into electronic and geometric is somewhat arbitrary. Some attempts have been made to measure the electronic properties of small particles by X-ray photoelectron spectroscopy (XPS). The preparation of samples of isolated small metal particles is not easy. The most successful methods are either vapor deposition of noble metals (Pt or Pd) on carbon or silica, or ion exchange used to prepare metals in Y zeolite. For the noble metals and inert supports used, it is assumed that the metal particles are isolated from each other. The results of several studies are presented in Fig. 2, in which the

METALS: PARTICLE SIZE AND CATALYTIC PROPERTIES

1 I

3.0

10

i.nm L O 2.0

1.0

7s

0.L

ZD > al

Y

m

a in

FIG. 2. Binding energy shifts ABE of metal core levels measured-by photoelectron spectroscopy (XPS) versus fraction exposed F E and mean particle size d for various supported metal systems (see Table I1 for details of the studies).

TABLE I1 Details of the Supported Metal Systems Studies Presented in Fig. 2 Curve (Fig. 2)

Metal and electron

BEu for bulk (eV) 335.0 335.0 335.0 71.5 314.5 314.0 71.3 314.6

~~~

support

Referenceb

C Y-Zeolite Si02 Y-Zeolite C A1203

Si02 A1203

~~

BE = binding energy; ABE = BE(observed) - BE(bu1k). References: (a) Takasu, Y., Akimaru, T., Kasahara, K., and Matsuda, Y., J . Am. Chem. SOC. 104,5249 (1982); (b) Vedrine, J. C., Dufaux, M., Naccache, C., and Imelik, B., J.C.S. Faraday f74,440 (1978); (c) Takasu, Y., Unwin, R., Tesche, B., Bradshaw, A. M., and Grunze, M., Surf. Sci. 77,219 (1978); (d) Mason, M. G., Gerenser, L. J., and Lee, S.-T., Phys. Rev. Lett. 39, 288 (1977); (e) Huizinga, T., van? Blick, H. F. J., Vis. J. C., and Prins, R., Surf. Sci. 135, 580 (1983); (f) Masson, A,, Bellamy, B., Colomer, G., M'bedi, M., Rabette, P., and Che, M., Proc. f n t . Congr. Catal., 8th 4, 333 (1984); (g) Masson, A., Bellarny, B., Hadj Romdhane, Y., Che, M., Roulet, H., and Dufour, G., Su$. Sci. 173,479 a

(1986).

76

MICHEL CHE A N D CARROLL 0. BENNETT

appropriate electron energies are shown to change with metal particle size. It appears that for particles of diameter greater than about 5 nm, the energy levels are those of the bulk metal. For particles of 5 nm, the fraction exposed is about 0.2, and the number of atoms varies from 3000 to 6000, depending on the metal and its morphology. For Pd, the energies stop changing below about 1.O nm, where all the atoms are exposed. For Pt, the changes continue until even smaller particles are reached. The three energies of Fig. 2 in the small-particle limit for Pd show considerable differences. Is this so because of the three different supports used? This explanation would imply that the particles are not really isolated from their supports. 2. Geometric Effects

a . Mathematical Models Based on Regular Small Crystallites. van Hardeveld and Hartog (20) have published the results of many calculations describing the arrangements of the atoms in small metal particles. Many graphs and sketches of small crystallites are presented. As an example, we show in Fig. 3a an octahedron made up of atoms in the facecentered cubic (fcc) system. The three types of surface atom are indicated by the symbols C9, face atoms; C7, edge atoms; and C4,corner atoms. A bulk atom is Clz. The subscripts on C indicate the coordination of a particular atom. The superscripts of Cj refer to a system for identifying which of the 8 atoms are missing in C4, for example. The original work should be consulted for details. For the various possible small octahedrons, the ratio N(C,)IN, has been calculated (20), where N(Cj) is the number of atoms of coordination Cj and N, is the number of surface atoms. These quantities vary with the total number of atoms, which, in turn, can be related to d for a given metal. Figure 3b shows, as a function of d for Ni, the fraction of the surface atoms that are made up of each of the three typical atoms of Fig. 3a. The fraction of face atoms C9 in Fig. 3b becomes about 0.9 at about 6 nm and falls to zero at about 0.6 nm, where all the atoms are at edges and corners. The effect of surface atom coordination makes itself felt in general for particles smaller than 4 nm. These changes are, of course, linked to changes in electronic characteristics also. Supposedly the catalytic properties of the metal are linked to N(Cj)IN, of Fig. 3b, and it is implied that one-atom sites are involved. For example, if catalysis is favored by atoms of low coordination, small particles would give higher rates. If catalysis is favored by face atoms, large particles

77

b

I

1

10

4.0- 2.0 1.0

0.L

d,nrn(Ni)

FIG.3. (a) Face-centered cubic (fcc) octahedron, m = 9 ( m is the number of atoms alon_g an edge). From Ref. 20. (b) Coordination of surface atoms versus the mean particle size d for nickel, assuming that the particle is a face-centered cubic (fcc) octahedron. The symbols cprt,. refer to atoms identified in (a).

would give higher rates (10). The former case corresponds to sympathetic structure sensitivity and the latter, to antipathetic structure sensitivity. Since ordinary supported catalysts exhibit a distribution of crystal sizes, it is clear that the effects may be more diffuse than those suggested by Fig. 3b. This distribution can be measured and controlled in model catalysts (79).

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MICHEL CHE A N D CARROLL 0. BENNETT

As a model of a small crystallite is built up by adding atoms one by one (103), a complete octahedron or other regular shape is reached only periodically ( 1 0 4 , for instance, at particles with a number of atoms of 6, 19,

44,etc. (20) for an octahedron. Adding the 20th, 21st, etc. atom starts a new layer and creates atoms of different coordination. At the edges of added layers, a particularly interesting ensemble of five atoms is formed, called a Bs site (20). Crystallites are considered to be fcc octahedra with extra layers of atoms on each face. If the extra layers are added in such a way as to produce the maximum total number of BS sites, the metallic particle illustrated in Fig. 4a is obtained. There are now some atoms in coordination states that did not exist in Fig. 3a. Figure 4b is based on calculations for a shape similar to that in Fig. 4a (20). For this more complicated arrangement, the fraction of face atoms CS falls to 0.9 at about 20 nm and goes to zero at about d = 2.5 nm. Geometric effects thus persist to higher d than in Fig. 3b. It is probable that catalysis for certain reactions requires a site made up of a particular ensemble of surface atoms. The number of adjacent atoms’ required may increase from 2 for the dissociative adsorption of oxygen or hydrogen to something like 12 for ethane hydrogenolysis (105). The Bs sites seem to have particular importance in catalysis, and it is claimed that they are necessary for the appearance of infrared-active adsorption of nitrogen (106). The concentration of BS sites is very sensitive to the shape of the crystallite. For various crystals containing 683 atoms, van Hardeveld and Hartog (20) calculate a number of Bs sites ranging from 76 to 0; for example, 36 for a sphere, 76 for a cubooctahedron, 13 for a cube, and 0 for many other possible arrangements. For a nickel crystal of octahedral shape van Hardeveld and van Montfoort (106) calculate that there are no BS sites for particles of d below 1.2 nm. Then the curve for the fraction of Bs sites on the surface passes through a distinct maximum of 0.17 at 1.8 nm and then falls so that it is 0.072 at 4.4 nm (4009 atoms). Sites made up of two atoms or more are thought to be important in various catalytic systems. For example, Dalmon and Martin (105) proposed that 12 atoms are needed for a site for the hydrogenolysis of ethane. At the present moment, the BS sites serve as a popular explanation of structure sensitivity, for curves of TOF versus d for a number of reactions pass through a maximum in the range of around 2-3 nm (107-112). Further discussion of this matter is reserved until after a presentation of the results from the literature. From the foregoing, it is clear that geometric effects may be important out to much larger particles than would be deduced from Fig. 2b. One lesson to be learned from these geometric considerations, which have

METALS: PARTICLE SIZE A N D CATALYTIC PROPERTIES

I

100

,

#

I

1

1

79

,

LO 2 0 -10 4.0 21) 1.0 d,nm(Ni)

FIG.4. (a) Face-centered cubic (fcc) octahedron-max B5.From Ref. 20. The shaded atoms represent one of the possible B5 sites. (b) Coordination of surface atoms versus the mean particle size a for nickel, assuming that the particle has a face-centered cubic (fcc) octahedron-max B5 structure. The symbols Cip.+‘,.-refer to atoms identified in (a).

been available for some years, is that fundamental studies on structure sensitivity must include data on the crystallite shapes. Modern computeraided electron microscopy should make this possible (113, 114). As time passes it should also become more practical to check the plausibility of models by quantum-mechanical calculations. Other calculations and experimental work (104, 114) have shown that the situation is probably in reality more complicated than that described

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MICHEL CHE A N D CARROLL 0. BENNETT

above. Most small fcc metal particles ( d = 10 nm) crystallize in a cubooctahedral arrangement (14 faces) illustrated by Fig. 5a. For even smaller crystallites, an icosahedral non-fcc arrangement may become more stable (22, 114-117); such a shape is shown in Fig. 5b and c. The differences in energy between the smaller icosahedrons and cubooctahedrons are small, so that the d below which the former are stable varies considerably from one metal to another. For more details and references, the reviews by Anderson (22), Gillet (116), and Hoare and Pal (117) are useful. It seems plausible that the catalytic activity of small metal particles would be influenced by the crystal structure. Yacaman et al. (If@ have studied pentane hydrogenolysis over Rhly-AlzO3. Rh/Si02, Rh/C, Rh/Ti02, and Rh/MgO. The support and preparation method, all for particles of d < 5 nrn, determined whether cubooctahedrons or icosahedrons were formed, but the catalytic properties depended more on d than on crystal type. b. Alloying Effects. The behavior of binary metal alloys of supported metals has been investigated by many researchers. Examples of reviews on this subject are those of Sinfelt (1 19), Sachtler and van Santen (120),

FIG.5. (a) Face-centered cubic (fcc) cubooctahedron (20). (b,c) Icosahedra. From Ref. 117.

METALS: PARTICLE SIZE A N D CATALYTIC PROPERTIES

81

b

FIG.5 . (continued)

Clarke and Creaner (121), and Ponec (122). Alloys are sometimes used as practical catalysts because of their selectivity or resistance to poisoning. Here we mention this kind of research because of the similarity of the effect of alloying a relatively inert Group Ib metal with a Group VIII metal

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MICHEL CHE A N D CARROLL 0. BENNETT

to the effect of decreasing the metal particle size. This similarity arises because in the present view the valence electrons of each metal in these alloys retain their identities, so that relatively inert Cu can be considered a diluent in an active Ni particle, for instance. The physical basis for this notion that each constituent in an alloy retains its own atomic character was first obtained through photoelectron spectroscopy in the period 19681970, notably by Seib and Spicer ( 1 2 3 ~ )Evidence . based on catalytic results led to similar conclusions (120). A geometric interpretation has been applied to kinetic measurements on the hydrogenolysis of cyclopentane which indicates that a mixed PtRe ensemble is much more active than either pure metal (123b). Thus in our discussion of the role of the multiatom sites (ensembles) usually considered important in antipathetic structure sensitivity we shall have occasion to refer to related results on catalysis over alloys. For example, the hydrogenolysis of ethane over Ni is inhibited as Cu is added and the necessary ensembles are broken up. c. Experimental Models via Stepped Single Crystals. The understanding of adsorption and catalysis has been advanced by studies on well-defined surfaces, to which it is possible to apply a variety of electron spectroscopic and electron diffraction methods. Often it is found that the TOF varies from one face to another of a single crystal. Thus, changes in TOF for supported particles may be related to changes in the relative surfaces of the faces exposed as a function of d , method of preparation, or catalyst history. In order to apply the powerful methods of surface science to atoms of low coordination, such as those encountered on small particles, the use of stepped surfaces of single crystals has been developed. Davis and Somorjai (124) have reviewed this method. If a single crystal is cut at a slight angle from that required to produce a principal face, steps or kinked steps are formed. The heights and orientations can be changed, as expressed for example by the Miller indices shown in Fig. 6. Atoms at the edges of the steps and at the corners of the kinks can be assimilated to the low-coordination C7 and C4 atoms of Fig. 3. Thus we have a geometric model for some of the effects produced by small supported particles. Of course, the low-coordination atoms are still surrounded by many other metal atoms. Somorjai and his collaborators have studied a number of such systems [Blakely and Somorjai (125); Smith et al. (126); Davis and Somorjai (127)l. The data shown in Fig. 6 for cyclohexane dehydrogenation imply that this reaction should be structure-sensitive. Some results from the older literature [e.g., Cusumano et al. (128)] seem to indicate that this is a structure-insensitive reaction over Pt/A120, and Pt/silica-alumina. A

METALS: PARTICLE SIZE A N D CATALYTIC PROPERTIES 10

1

1

I

I

1

1

I

83

I

15 torr Cyclohexone 100 torr Hvdroaen

Time (minutes)

FIG.6 . Dehydrogenationof cyclohexane. Benzene accumulation in the gas phase at 573 K and I atm in a closed (batch) reactor containing a Pt crystal of known exposed surface. The crystals expose the flat Pt(l11) (M), stepped Pt(557) (V),kinked Pt (25, 10, 7) (O),and Pt(l0,8,7)(A) surfaces. Initial rates are proportional to the slopes at zero time. From Herz, R. K., Gillespie, W. D., Petersen, E. E., and Somojai, G . A.. J . C u d . 67, 371 (1981).

number of possible reasons for such differences will be discussed in later sections. C.

MODELCATALYSTS V I A CONTROLLED DEPOSITION OF SMALL PARTICLES

In discussing electronic effects and Fig. 2, a number of references have been made to model catalysts formed by deposition of metal from the vapor phase onto an inert support. An attempt is made to simulate the surface of a real catalyst so that it can be better studied by XPS and electron microscopy. In addition, Masson et al. (78, 79) have developed techniques to control and make almost uniform the size of the particles. Rutherford backscattering is used to find the total number of atoms deposited, and the number of nuclei that grows into discrete metal particles is found by electron microscopy. From these measurements the average number of atoms per particle is obtained. Longer deposition times give larger particles. With such a model catalyst (79)it is possible to measure both electronic and geometric characteristics of the metal particles, along with their reactivity, as the particle size is changed. It is of interest to apply this method not only to particles smaller than 4.0 nm, for which electronic effects may

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MICHEL CHE A N D CARROLL 0. BENNETT

be important, but also to particles up to 50 nm. For the latter, catalytic effects must be related to the identities of the exposed faces of the metal crystallites, and these are particularly accessible because of the method of sample preparation. In this way the structure sensitivity shown by many systems above d = 4 nm may be better understood. D. EXPERIMENTAL DIFFICULTIES In the preceding sections we have discussed how observations on the effect of d on catalysis might be explained by electronic and geometric factors. Usually it has been assumed that one is dealing with isolated crystallites of completely reduced metal. Often such may not be the case, and we now consider some common situations in which complicating factors may arise. 1. Metal Bonding with the Support

The atoms of a metal that are in contact with the support may undergo some electron transfer with the support, leading to some ionic or covalent bonding. Such effects would influence only a small fraction of the atoms of a large particle, but the fraction of atoms affected can be expected to increase as the particle size decreases. Thus measured variations in catalytic behavior may be due to a combination of intrinsic size effects and support effects. Strong metal support effects are often observed even for particles larger than 4 nm, after reduction at temperatures above 450°C. These results on supports such as Ti02 have been explained as arising from a large increase in metal-support contact made possible by the decoration of the metal by a reduced form of the oxide support [Resasco and Haller (129); Tatarchuk and Dumesic (130); Sadeghi and Henrich (131a)l. Clearly the decoration also permits a geometric interpretation (129) similar to that invoked to explain the effects of alloying. In any event, most studies of the effect of particle size do not involve the consequences of high-temperature reduction on reducible oxides.

2. Incomplete Reduction of Metal What may seem to be an intrinsic decrease in TOF with particle size may sometimes be caused by the decreasing extent of reduction of smaller particles. Thus, for nickel, small particles are not completely reduced, and the degree of reduction must be considered in evaluating the effect of particle size on TOF (59). Attempts to produce small clusters of iron

METALS: PARTICLE SIZE A N D CATALYTIC YKOPERTIES

85

supported on alumina by the decomposition of iron carbonyls led mostly to iron oxides (64). 3 . Changes in Crystal Structure Figures 3 and 4 are based on the maintenance of the same crystal shape as the size varies. For small particles, needles or even two-dimensional shapes may predominate, as discussed by Bond (21). More subtle changes may occur, such as the replacement of cubooctahedrons by icosahedrons for crystals of d < 2 nm. For large crystals kinetic effects during the preparation may cause deviations from the expected equilibrium distribution of faces, often assumed to be equal portions of the (1 1I), (loo), and (1 10) planes. In their measurement of the TPD of CO on small Pt particles on A1203, Altman and Gorte (13Zb) explain their results by proposing that (1 1 1)-type facets are preferred on the larger particles. 4.

Changes in Deactivation with Particle Size

Deactivation may be structure-sensitive. Thus Lankhorst et al. (132) have shown that variations of TOF with d for hexane reforming on Pt were caused more by increases in carbon deposition with d than by decreases in the intrinsic rate of reaction with d. For model stepped-surface single crystals of platinum, Somorjai and Blakely (133) showed that carbon deposition was favored on terraces over edges and corners, so that the reforming reactions favored by the latter sites are poisoned less rapidly than those occurring mostly on the flat surfaces. Thus small particles are expected to be poisoned less rapidly than large ones, for this example.

5 . Metal Reconstruction It has long been suspected that chemisorption and catalysis may actually cause major changes in the shapes of small crystallites from those observed in the absence of adsorption. The reality of such changes has been demonstrated dramatically by van’t Blik et al. (134) for the chemisorption of CO on well-reduced crystallites of 6-10 Rh atoms supported on alumina. On adsorption the particles are broken up to form Rh(C0): bonded to the oxygens of the support. In the absence of CO, a raft structure for such Rh particles has been observed ( 1 3 5 ~ )However, . in the presence of CO this structure is destroyed and the Rh atoms oxidized, on alumina. For nickel, a dramatic change in particle size may occur by another process. When kinetically and thermodynamically favored, gaseous

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MICHEL CHE A N D CARROLL 0. BENNETT

Ni(C0)4 may be formed in the presence of CO so that there is transport of Ni from small particles to large ones (135b). Under certain conditions, usually to be avoided, all the nickel may be removed from the support and deposited on cold surfaces downstream of the reactor. OF STRUCTURE SENSITIVITY E. SOMECONSEQUENCES

Before considering some of the details of structure sensitivity in catalysis, it is of interest to present a few examples. In this way some of the practical consequences of structure sensitivity are illustrated, although it must be kept in mind that there are many steps between the laboratory results we show and the development of an industrial process.

I . Structure Sensitivity and Reaction Rates Results that illustrate three examples of structure sensitivity are presented in Fig. 7. Curve 1 represents the TOF for the structure-insensitive reaction of benzene hydrogenation on Pt/SiOz (136). The TOF is about 0.22 s - I and is not a function of particle size over the range studied. Also plotted for this system is the atomic rate AR, calculated from TOF by Eq. (2). As already mentioned, the atomic rate is an approximate measure of

u) l[O

Y

i,nm

2,5

1,0

AR

2;5

1p

0;C

I

FIG.7. Turnover frejuency (rate) TOF and atomic rate AR versus fraction exposed FE and mean particle size d for a selection of reactions and metal systems (see Table Ill for details of the studies).

87

METALS: PARTICLE SIZE A N D CATALYTIC PROPERTIES

TABLE 111 Details of the Structure Sensitivity Studies of Metal Systems Presented in Fig. 7 Curve (Fig. 7)

Metal

1

Pt

Si02

2

Rh

Si02

3

Ru

AI2O3,SOz, etc.

Support

Temperature, pressure

Reaction

Reference

50°C, 1 atm, Hz/C6H6 = 12.75 253"C, I atm, H*/C2H6 = 6.67 250°C I atm, HJCO = 2

C & ,-k 3H2 --* C6Htz C2H6 f H2-* 2CH4 co -F 3H2+ CH4 + H20

136 13 137

the rate of product produced per unit cost of metal in the catalyst. Thus, it is of interest to maximize the atomic rate, and for the present example this reasoning leads to using a catalyst of the highest FE that is practical. The dotted parts of the curves are extrapolated to FE = 1, where TOF = AR. Naturally, processing costs may become prohibitive as FE increases, and this factor is not included in the above reasoning. Curve 2 of Fig. 7 shows TOF and AR for a sympathetic structuresensitive system, the hydrogenolysis of ethane over Rh/Si02 (13). The TOF rises with FE and then falls for very small particles. The authors expressed some reservations about this behavior, but in our review of the literature we shall see that such behavior is often observed. The curve for AR goes through a maximum in the region of d around 1.5 nm. Thus, for this system there is an optimal value of d, close to the smallest sizes that are practical. Finally, curve 3 of Fig. 7 shows some results for an antipathetic structure-sensitive system, the hydrogenation of carbon monoxide over supported ruthenium (137). For such systems there is usually a maximum in the curve of AR versus d, and this occurs at about 5 nm for the present case. For particles smaller than 5 nm, the decrease of TOF is the predominating effect, and for particles larger than 5 nm, the decrease in FE is the predominating effect. We shall see that CO hydrogenation over almost all systems studied shows an antipathetic structure sensitivity, so that there is no economic incentive to decrease the crystallite size below a fairly large size, 5 nm for the data shown. This reasoning is based on a consideration of the reaction rate, but the selectivity is also important, and we discuss this matter presently. It should be noted that in constructing Fig. 7 and the other figures in this article we use Eq. (1) with B = 1 nm. Actually, B may vary between 0.7 and 1.0 nrn, depending on the metal and the shape of the crystallites.

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MICHEL CHE A N D CARROLL 0. BENNETT

However, in view of the precision of the data and the facility of presentation with only one value of B, we have retained B = 1 nm for all figures. It should also be clear that the relation between d and FE loses its meaning as FE approaches I .O. From the geometric point of view, the properties of the surface atoms of a small cubooctahedron are different from those of the same atoms arranged as a one-dimensional raft. From Figs. 3a and 3b, it is deduced that structure sensitivity should appear only for particles with d less than about 5 nm. It is convenient to call systems that actually exhibit this behavior systems with limited structure sensitivity. On the other hand, if geometric effects are governed by ensembles of several atoms, then structure sensitivity should extend to diameters considerably greater than 5 nm. This behavior we call extended structure sensitivity. In other words, with limited structure sensitivity, the system is structure insensitive ford > 5 nm. A glance at the figures in the rest of this review shows that many actual systems present extended structure sensitivity. 2. Structure Sensitivity and Selectivity

The key to the practical exploitation of a catalyst is often its selectivity, so it behooves us to see how the selectivity may vary with d . Curve 1 in Fig. 8 sketches the variation of the molecular weight of the product of the Fischer-Tropsch synthesis over supported cobalt as a function of FE (1.38). Large particles favor a higher production of hydro-

1

100 - 1.0 1

25

i,nm 10

25

10 L

0 L

al

>

a

FE FIG.8. Selectivity ad ! average carbon number of product versus fraction exposed FE and mean particle size d for a selection of reactions and metal systems (see Table IV for details of the studies).

TABLE IV Details of the Reactions and Conditions of the Metal Systems Studies Presented in Fig. 8 Curve (Fig. 8)

Metal

Support

1

co

2

Pt

A1203, Ti02, Si02, C Si02

3

Ag

Si02

Temperature, pressure 225"C, 1 atm, H-JCO = 2 240-375"C, 1 atm, HJC5 = 10.5 220°C, 1 atm, 02/C2H4 = 1.0

Reaction

CO + H2hydrocarbons nCsH,2 + H2 + isomerization, hydrogenoly sis C2H4 + 0 2 -, C2H40. c02

Reference

Comments Fischer-Tropsch synthesis Selectivity = (isomerization)/ (hydrogenolysis isomerization) Selectivity = (C2H4OV ( 0 2 + C2H4O)

138 140

+ 141

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MICHEL CHE A N D CARROLL 0. BENNETT

carbons in the gasoline range, one of the original goals of the process. Since the atomic rate is also favored by relatively large particles, the use of such particles is doubly indicated. This is in accord with the old literature, which indicated that highly dispersed supported catalysts were not desirable. Precipitated or fused (iron) catalysts were preferred (139). Curve 2 of Fig. 8 concerns the competitive isomerization and hydrogenolysis of normal pentane as a function of particle size over Pt/Si02 catalysts (140). Isomerization is favored over large particles and hydrogenolysis over small particles. It is clear again that the best catalyst probably corresponds to an intermediate particle size. For the partial oxidation of ethylene over Ag/SiOz catalysts (141), curve 3 shows that the selectivity toward ethylene oxide (versus COz) is favored by the largest crystallites studied. However, both the production of CzH40 and of COz show sympathetic structure sensitivity. Thus, there must exist an optimal crystallite size for the most effective catalyst. It should be noted here that the rates and selectivities usually reported are initial rates, found in a differential reactor. It is clear that there exists a complicated optimization problem in the design of the industrial reactor and catalyst. The best values of crystallite size, conversion, and other operating conditions must be sought. Often these various searches are carried out by separate groups, at separate times, so that the attainmknt of the optimal process is difficult to achieve. It is worth noting here that the metal loading on the catalyst [C in Eq. (4)] is an important variable. Crystallite size can be manipulated by variation of C and of the pretreatment (sintering) temperature and gas-phase composition. For instance, a given d may be obtained from a low loading and a high sintering temperature, or from a high loading and a low sintering temperature. Most authors find that the TOF is about the same for two or more such catalysts of the same d (or FE) and different combinations of loading and sintering temperature. Thus we express results with d as independent variable rather than C. An early review actually used metal loading as the independent variable, reporting catalytic activities as a function of metal loading without reference to FE or d (3). For Ti02 and other reducible supports, a high reducing temperature may induce a strong metal-support interaction (SMSI). In this case TOF becomes a function of sintering (reduction) temperature at a given d. SMSI is absent in most of the literature we shall discuss. However, in the observed variation of TOF with d, the explanation of the observations may be sought as arising from electronic effects, ensemble effects, support effects, reconstruction effects, etc. We shall see that which explanation, or combination thereof, is valid is not evident for many of the literature studies.

METALS: PARTICLE SIZE AND CATALYTIC PROPERTIES

91

3 . Structure Sensitivity and Chemisorption In our discussion of the influence of structure on the turnover rate our understanding is frequently hampered by lack of information on the ratedetermining step and the most abundant surface intermediate. It would be logical to consider the structure sensitivity of the rate of an elementary step, such as the desorption of a chemisorbed gas. Results on temperature programmed desorption as a function of particle size might be simpler to interpret than those of global reactions consisting of a sequence of steps. However, few such data are available. There are, however, a number of papers that treat the equilibrium relations in chemisorption, and we shall discuss some of these. Such measurements can lead to information on the structure and binding of adsorbed species, and also to the characterization of metal catalysts. For example, CO adsorbed on PtO shows a lower stretching frequency in the infrared than when adsorbed on oxidized platinum (142). Another reason to consider chemisorption in this review is that it is often used as the basis for estimating d or FE and thus also influences TOF via Eq. (2). As already mentioned, hydrogen is usually used for measuring FE and in most cases the best value of H/M, seems to be close to 1.0. However there are a number of complications, and we discuss them in the following section on characterization.

IV. Characterization of Small Supported Metal Particles

A. INTRODUCTION In order to obtain data on structure sensitivity, a reliable measure of the metal fraction exposed is needed. The simplest method of measuring FE is via chemisorption of hydrogen; carbon monoxide and oxygen are also sometimes used. After a discussion of chemisorption, we consider later in this section other methods of characterization. Chemisorption leads to an estimate of the average particle size; by electron microscopy and electron microdiffraction, information can be obtained on the particle size distribution and in favorable circumstances on the morphology of the small particles. It is also important to know the chemical state (degree of reduction) of the particles, and to this end electron paramagnetic resonance (EPR), X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS),Auger electron spectroscopy (AES), and infrared spectroscopy (IR) are useful. These and other methods will be discussed in what follows. Our goal in the discussion is not to review the methods

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MICHEL CHE AND CARROLL 0 . BENNETT

considered but to show how they are used to further the study of small particles. In order to determine the effect of d or FE on TOF,evidence is required that the metal particles are indeed approximately monodisperse, fully reduced, unpoisoned, little influenced by the support, and unpromoted. These matters have been reviewed in the previous section under the heading “Experimental Difficulties.” Thus we shall discuss how FE can be measured and then consider other methods of characterization that will furnish more details about the geometric and chemical states of the small particles. We shall, however, not consider metal-support interactions since this aspect is covered in a separate review (25a).

B. PARTICLE SIZEA N D PARTICLE SIZEDISTRIBUTION 1. DeJinitions

Although the expression “particle size” is very often used in the literature, it is very seldom clearly defined, mostly because many definitions are possible and none is unambiguous except if crystallites are all-cubic or all-spherical. This, in practice, never happens and it is necessary to define arbitrarily some quantity as the particle size. Matyi et al. ( M a ) propose that the simplest way to do this is to equate “size” with “diameter,” where the diameter is defined as any straight line passing through the center of mass of the particle and terminating at the particle boundary. The authors present other definitions and show that the most suitable choice depends on both the experimental technique used and the nature of the system being examined. As discussed in Section V, we do not need be too concerned about this aspect, as important as it might be, since there are other more important difficulties to resolve before any rationalization of the data becomes possible. The same type of problem is encountered in the definition of particle size distribution since the starting assumption is that all particles have the same shape. In this case, the particle size distribution is defined as the relationship between a given particle size and the frequency, or number of particles, with that certain diameter or size. Matyi et al. ( 1 4 3 ~discuss ) in detail for a given particle size distribution the several features that can be interpreted as the average size. We refer the reader to this review article.

2. Chemisorption a . Hydrogen. The use of the chemisorption of hydrogen to characterize metal catalysts has been discussed in a useful review by Scholten et al. (14%). Usually an isotherm is measured at room temperature, and mono-

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layer coverage (H/M, = 1.0) is assumed to correspond to irreversibly adsorbed hydrogen. The latter is determined by some arbitrary definition. In the best cases, the actual average particle size and morphology have been determined by some reliable physical methods, so that the chemisorption method is calibrated. Following a certain recipe of waiting and pumping, the amount of H2 adsorbed at 25°C and a given pressure are said to correspond to a certain H/M, ratio, preferably unity. Sometimes the appropriate amount of H2 is taken as that found by extrapolating the highpressure part of the isotherm (supposedly linear) to zero pressure. In some cases the isotherm of total adsorption is obtained, the reversibly adsorbed H2 is removed according to a recipe, and then the reversibly adsorbed isotherm is measured. The irreversibly adsorbed H2 isotherm is obtained by difference, and then the monolayer is deduced from this isotherm. For details we refer the reader to Scholten et al. (143b) and appropriate textbooks (e.g., 1b). We next discuss a number of points to keep in mind concerning the use of H2 chemisorption as a method of characterization. 1. Chemical adsorption on metals is usually considered as nonactivated. For H2 on Ni/A1203, however, the isobars pass through a maximum at about 100°C (144), meaning that at 25°C kinetic factors (adsorption time) have an important influence on the amount of H2 adsorbed. For Rh/SiOz (144) the amount of H2 adsorbed is found to depend very little on time; the isobar descends smoothly as the temperature is increased. Thus each metal has its own behavior, which must be taken into account. The thermodynamics and kinetics of adsorption and desorption are discussed in the literature (145, 146). 2. As discussed in the interesting article of Crucq et al. (145),for most chemisorptions the AH of adsorption (exothermic) decreases owing to lateral repulsive interactions as coverage increases. Often the relation is approximately linear, leading to a Temkin isotherm. The change in AH may arise because of a distribution of metallic sites and/or because of increasing interactions between adsorbed H a s the coverage increases. Thus the fraction of adsorbed gas that can be removed depends on temperature, time, and arrangement of the equipment; the activation energy for desorption obviously also decreases with increasing coverage. Thus the term reversible adsorption has only an artificial meaning and might better be replaced by weak (and strong) adsorption (147). Sayari et al. (98) have studied the weak adsorption of H2 on various supported ruthenium catalysts of 0.9 < d < 12.4 nm. They express their results in terms of the ratio HwIHT,where the subscripts W and T, stand for weak and total adsorption. At 0.9 nm, HwIHTis about 0.1, passes

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through a maximum of 0.32 at 1.5 nm, and drops to zero above 2.5 nm. The authors (98) associate the weak adsorption with special sites, invoking in particular the Bs sites. Although these sites do reach a maximum concentration at about the right d, their decline for higher d is gradual (106) and extends far above 2.5 nm. It is interesting to recall that Aben et al. (97) have found that the hydrogenation of benzene is structure-insensitive if the TOF is based on the FE on the basis of the adsorption of weakly bound hydrogen only. In Fig. 13 (Section V,B,5), the TOF (curve 4) is based on strongly adsorbed hydrogen and shows a maximum at about 2.0 nm. 3. Although a calibration of H2 chemisorption may be desirable, there must exist another reliable measure of particle size, and such has not been the case for d < 3 nm. In recent years, however, improved techniques in electron microscopy have permitted the measurement of particle sizes down into the range 0.3-1.0 nm. Even though the resolution of modern instruments may be of the order of 0.3 nm, it is often difficult to obtain enough contrast to distinguish the metal from the support. By careful sample preparation McVicker et al. (148) were able to see particles of Ir on A1203of about 0.6 nm. They found that for particles in this range HIM, is about 2, whereas it drops to 1.O for larger particles (strongly adsorbed H2). The contrast in electron micrographs can be remarkably enhanced by computer-aided image processing. An example is the recent work of Fuentes et al. (149) for Rh/Ti02,but they did not report results for hydrogen chemisorption. It is also possible to use extended X-ray absorption fine-structure (EXAFS) spectroscopy to deduce particle sizes, and this method has been used, along with hydrogen chemisorption, to study Rh, Pt, and Ir supported on Si02 and A1203 (147, 150). The average coordination number of the metal atoms is calculated from the EXAFS results. A model implemented by computer is then used to estimate the particle size from the coordination number. Metal support interaction can be included in the model. Kip et al. (147) have used total adsorbed hydrogen in reporting chemisorption results. For Pt they find 0.6 C H/M, C 1.2; for Rh (FE always > 0.8), 1.5 < HIM, < 2.0; and for Ir, 2.5 < HIM, < 3.0 for 0.2 < FE < 0.9. In these data the higher H/M, refers to FE close to 1 .O and the lower one, to the samples of lowest dispersion measured. Although there is some scatter in the results, it is clear that H/M, is a function of FE for a given metal, and that its range changes from one metal to another. Roughly, at FE = 1.0, it seems that H/Pt, = 1, H/Rh, = 2, and H/Irs = 3, where H/M has been obtained by extrapolating HT/M to zero pressure.

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It should be noted that hydrogen held by refractory supports such as SiO2 and A1203 usually has a negligible effect on the results, either via physisorption or spillover (147, 148, 149a) at 25°C. Clearly, metal-support interactions are important, for instance, in the formation of rafts of rhodium or the location of groups of only one or two atoms (149b).However, on the flat Rh particles found on a Ti02 support, there should be at least two H atoms adsorbed per Rh atom (147, 148). For hydrogen adsorption on reducible oxides, there may be an apparent slow adsorption that actually arises from the reduction of the support. For Rh/TiOz it has been found that at 22°C there is rapid chemisorption of Hz corresponding to H on Rh followed by further slow disappearance of H2 corresponding to the gradual creation of Ti3+by spilled-over hydrogen (151). The resulting lines representing Hz adsorbed versus In (time) have slopes and intercepts that are functions of reduction temperature (SMSI) and subsequent oxygen treatments. Oxygen adsorption is also measured and a coherent picture of the various processes is presented (151). It should be noted that the preparation of catalysts of low metal loading (<0.5%) leads to particular difficulties. For instance, chemisorption by hydrogen may be falsified by reaction with oxide or other impurities or actual adsorption on the support. Also, the metal particles may attract trace impurities from the support that surrounds them in great excess. b. Carbon Monoxide. Since carbon monoxide may dissociate and/or adsorb on support materials, it is not used as much as HZfor catalyst characterization by chemisorption. Sometimes volatile carbonyls are formed [e.g., Ni(C0)4]. However, unlike H2, it is convenient to study adsorbed CO by infrared spectroscopy, so that there is a large literature on CO adsorption on supported metals. Discussion of this matter is deferred until the section on the structure sensitivity of chemisorption. 4.

3 . Electron Microscopy The most important advantage of electron microscopy is that the experimentalist can test the effectiveness of the preparation method by directly observing the metal particles. One can obtain the particle size distribution, derive the average particle size, and determine whether the particles are evenly distributed or packed up in larger aggregates. If the particles are sufficiently large, their shape can be distinguished and their crystal structure determined. The most obvious difficulty, however, is that the electron micrographs used must be representative of the sample under investigation. This can be achieved only if a number of electron micrographs are taken so that

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one can estimate whether they are representative of the sample, but this is a painful and time consuming task. Another difficulty is that the observation of small metal particles may be affected by the support microcrystallinity (58, 152), orientation of particles and imaging conditions (153). It appears now that bright-field techniques are increasingly unreliable for particles below 2 nm and that other contrast methods-hollow-cone (154) and other dark-field methods (155u,b),the z contrast technique (156)-are useful for extremely small particles (36). There are cases where the contrast is so poor that the support has to be removed by dissolution as in the case of the Pt/Ce02 (157). In this instance, some information is lost, such as spatial distributions of the metal and the metal-support interaction, and even some metal particles can be removed during the dissolution process. As stated earlier (Section IV,B,l,a), the contrast can be enhanced by computer-aided image processing (149). Other difficulties include electron beam effects, crystallite agglomeration, and the selection of the diameter when examining irregularly shaped crystallites (158). Despite the problems mentioned above, modern atomic resolution TEM is now available in many university and industrial research laboratories and successfully applied to characterize microstructure of a wide range of materials at resolution on, or approaching, the atomic scale (159163). Recently, controlled atmosphere electron microscopy (CAEM) or in situ TEM has allowed in situ observation of gas-solid reactions at a resolution of about 25 A. The principal feature of the technique is that a relatively high gas pressure is obtained at the specimen while preserving the low pressure needed for operation of the microscope. Various types of cell and differentially pumped systems have been described (164). The applications of CAEM to catalysis have been reviewed with special emphasis on carbon deposition, carbon gasification, and sintering studies (165). The electron microscopes can be divided into two types (166): scanning electron microscopes (SEM), which use a 10-nm electron beam at the specimen surface, and transmission electron microscopes (TEM). With current TEMs, resolution of about 0.2 nm can be achieved, provided very thin (<20 nm) samples are available. With conventional inorganic oxidesupported metal catalysts, particles of approximately I nm can be detected. Scanning transmission electron microscopes (STEM) use a high brightness dark-field emission gun to produce a probe about 0.3 nm in diameter and combine the techniques of SEM and TEM. Further experimental and theoretical aspects of electron microscopy applied to catalysis have been reviewed recently (113, 167-169).

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4. X-Ray Diffraction Techniques

There are two very general X-ray techniques for study of the metal particle size distribution: the line-broadening analysis (LBA) and the small angle X-ray scattering (SAXS) (Ib, 170). Other methods include the radial electron distribution (RED) and the extended X-ray absorption fine structure (EXAFS), which are aimed primarily at studying the structure of catalysts (Section IV,G). a . Line-Broadening Analysis. The breadth of X-ray reflections does not depend on the crystallite size for dimensions above approximately 500 nm. For smaller crystals and in presence of defects, the breadth is increased as a result of the limited number of reflecting planes (crystallite size broadening) or to the perturbation of interreticular spacings (stain or deformation broadening). LBA can be performed in two ways. The first is to use the Scherrer equation (171), which states that the breadth of X-ray reflections is inversely related to the dimensions of the crystals, assuming that the broadening is due to crystallite size only. The other way is to perform a complete line-profile analysis, including a correction for instrumental broadening, the separation of crystallite size broadening, and the calculation of particle size distribution. There are, however, a number of difficulties that restrict the use of LBA. The reflections from the different solid phases present in the systems might be superimposed. Neighboring reflections such as the (1 11) and (200) lines in fcc metals will overlap when the particles become very small. For supported metals, the metal reflection must be intense enough to give a measurable signal above the background contributed by the support. As the amount of metal decreases, the wings of the line profile become too weak and the smaller particles are not taken into account. LBA becomes less and less accurate, resulting in a mean size that is overestimated and a size distribution displaced toward the larger size. LBA is believed to be applicable to particles in the range 100 nm to a value around 1.5 nm (170). b. Small-Angle X-Ray Scattering. This technique uses information contained in radiation scattered, within a few degrees of the primary X-ray beam, by centers whose scattering arises from differences in electron densities. The intensity of X-ray scattering depends on the size and shape of the scattering centers, which in a three-phase system involve the metallic phase, the support, and the pores (voids) themselves. The scattering of the voids can be eliminated either by compression to destroy the pore structure or by impregnating the catalyst with a liquid (CH212 is commonly used for SiO2 and A1203) that has the same electron density as the support, so that the small metal particles are left as the only main

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scattering centers. Pore masking is not always easy in a SAXS experiment because certain supports have such high electron densities that it is impossible to find a fluid that can match them (172). In such cases, neutron small-angle scattering can be particularly useful, since it is relatively easy to find a suitable fluid with the same neutron scattering length density as a chosen catalyst support (173). Important SAXS parameters are always derived from an analysis of the profile of the SAXS curve that gives the intensity Z(s) of the X-ray scattering as a function of the scattering vector s. From the so-called Porod surface extracted from the wings of the SAXS curve, a mean surfaceweighted particle diameter can be easily calculated by assuming a given shape for the distribution as reported by Whyte et al. (174,175).When the support is nonporous and in the form of spherical particles that give weak scattering, the distribution of metal particle diameters can be obtained without the use of pore-masking fluids (176). The SAXS technique is interesting and more powerful than LBA. The main advantages are that particles as small as 0.5 nm can be observed and that the particle sizes of poorly crystallized or even amorphous solids can still be measured. With both LBA and S A X S , the catalysts can be studied in situ during heat treatments or catalytic reactions. Finally, X-ray diffraction methods have the advantage over gas adsorption measurements that they are insensitive to catalyst contamination (175). In gas adsorption techniques, some of the metal surface may be inaccessible to adsorbate, leading to overestimated particle diameters. This lack of accessibility may be due to surface contamination or to the presence of micropores that are too small to allow adsorbate entrance. 5 . Magnetic Methods There are two magnetic techniques that can lead to particle size or particle size distribution; one is a static method whereas the other is a resonance one. a. Static Magnetic Measurements. When a bulk ferromagnetic metal is in the form of very small particles, there is a drastic change in the magnetic properties that can be used to derive the particle size and distribution profile. If a particle of a normally ferromagnetic metal has a size that is smaller than a ferromagnetic domain (30 nm for nickel), it will act as a paramagnetic substance with a very large magnetic moment N p o , where N is the number of atoms within the particle and p, is the magnetic moment of one atom. When subjected to a large external magnetic field H, the magnetic moment of the particle will tend to align itself with H but this orientation

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is counteracted by the thermal agitation kT. This phenomenon is called “collective paramagnetism or superparamagnetism” (177). The resulting magnetization in the direction of the applied field H is given by the Langevin equation, valid for large values of Npo: M/M, = L ( N p o H / k T ) with L(a) = coth a - l/a, where a represents the function Np,H/kT. M, is the saturation magnetization (maximum field strength after which the magnetization becomes field independent). Two types of approximation are used: the Langevin high-field method applies to high-field strength [ 10 Oe (oersteds)] and low temperature (-4.2 K) to which the smallest particles (small values of N) are most sensitive, whereas the Langevin lowfield method refers to low-field strength (SI Oe) and higher temperature (-300 K) and concerns the larger particles (larger values of N ) . By performing measurements at different temperatures, it is possible to obtain the distribution profile. When the distribution of the particle size is described by means of a simple analytical function, it is possible to determine the latter accurately by optimization (178). The preceding equation applies to monodomains in thermal equilibrium. However, for large or elongated particles, low temperature, or short observation time, the magnetic moments remain in the facile magnetization direction, leading to a remanent magnetization that is proportional to the total volume of the particles. The method based on particle size determination from relaxation time measurements after the magnetizing field is removed is referred to as the NCel method (179). The variation of the remanent magnetization with temperature for a given observation time allows the derivation of the particle size distribution. This is the Weil method (180). Another technique (the frequency method) exists, which consists in modifying the observation time by applying a variable frequency alternating field (181). Among these various methods, the low-field Langevin approach is the most commonly used because low field strength and easily accessible temperatures are involved. It is important to realize that the magnetic properties are strongly affected by chemisorption, requiring clean particle surfaces. This has been exploited by Martin and Imelik (182) to determine the number of covalent bonds between adsorbed hydrocarbons and the metal in the Ni/Si02 system. This approach indicates that the magnetic methods are well adapted for in situ studies, although these are limited to ferromagnetic metals or their alloys. There is some promise that metals with Pauli paramagnetism can also be investigated ( 1 8 3 ~ The ) . static magnetic methods applied to chemisorption and to the determination of particle size and particle size distribution have been reviewed recently (183b, 184). b. Ferromagnetic Resonance. Ferromagnetic resonance (FMR) (185, 186) is based on the same principle as electron paramagnetic resonance

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(EPR). In a FMR experiment, a microwave field is used to induce electronic transitions between Zeeman levels produced by the external applied magnetic field. However, there are two important differences. First, FMR deals with strong magnetic moments associated to the metal particles rather than with weak magnetic moments due to one or a few unpaired electrons associated typically to a radical containing a few atoms or to a transition-metal ion. Second, in a ferromagnetic substance, by contrast to a paramagnetic one, there exist strong magnetic anisotropies due to the crystalline structure, to the form of the material and to the constraints it undergoes (58, 89, 187). These anisotropies correspond to strong anisotropic magnetic fields H a , which can reach 6000 Oe and vary with temperature (89, 285). The effective magnetic field is thus the sum of those local fields and of the external applied magnetic field. The FMR signal is characterized by an intensity which is proportional to magnetization and by a value H , of the external magnetic field at the resonance which depends on the nature of the anisotropy and of its variation as a function of temperature. The latter follows the Langevin law as seen above, so that it becomes possible by recording the FMR signal intensity as a function of temperature to derive the particle size (186). It is also possible from the variation of the shape of the FMR signal as a function of temperature and of the microwave frequency to obtain information on the nature of the main anisotropy (185,186).This may lead to the form of the metal particles and to the constraints they undergo at the metal-support or metal-gas interface. The latter information allows detection of the presence of any chemisorbed phase (Section IV,D). The FMR technique presents the same advantages as EPR: in situ experiments are possible on minute quantities of samples that are not destroyed. The interpretation of the spectra is, however, rather involved and the method is limited to ferromagnetic compounds containing essentially Fe, Co, Ni, or their alloys. 6 . Miscellaneous

Several techniques have been reported to yield particle size and distribution profiles, but their use has not been generalized yet, either because the technique is not straightforward or limited to specific cases or because the method used to process the spectroscopic data is too involved. This can be exemplified by the work of Dalla Betta and Boudart (188a), who determined by infrared (IR) the particle size of platinum encaged in CaY zeolite. The size was deduced from the fraction of OH groups exchanged with deuterium, and it was assumed that the isotopic exchange at low temperature is rapid only in the immediate vicinity of the particle. This

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assumption was substantiated in a later paper (188b).Correlation between the stretching frequency of an adsorbed probe molecule and particle size has sometimes been made. Pnmet et al. (189) showed that the smaller the Pt particle, the higher the U N O frequency of adsorbed NO. Another example can be found in the use of xenon probe molecules to measure by 129Xe nuclear magnetic resonance (NMR) the size of Pt particles in zeolites (190) or on conventional supports (191). In some cases the technique is not easily available, and this can be represented by the case of IssPt NMR (192). In a comprehensive study of 195PtNMR lineshapes ( 1 9 3 , relaxation phenomena (194) and microscopic variations in Knight shifts ( 1 9 3 , Schlichter and co-workers have demonstrated the power of 195PtNMR to investigate the particle size, the condition of Pt surfaces (i.e., whether metallic or nonmetallic), and the chemical nature of surface species. The IssPt NMR is, however, very time-consuming (192). Some attempts have been made to relate X-ray photoelectron spectroscopy (XPS) dispersed phase-support intensity ratios to the size of the supported particles, and examples are given by Moss (158). Recently, the binding energy shift in XPS spectra has been proposed for measurement of the particle size of supported platinum prepared by vapodeposition in ultrahigh vacuum (78). This method has the advantage that it can be employed in situ and for sizes that cannot be seen by electron microscopy. Some other techniques have been applied to particle size determination such as Mossbauer (196). A certain number of catalytically important metals are Mossbauer-active. However, certain isotopes are short lived and in low natural abundance (197-199) thus increasing the experimental cost. It is possible to bypass this problem by the introduction of small amounts of easily available Mossbauer-active isotopes (e.g., "Fe) in catalytic systems (200). The magnetic dipole splitting is sensitive to ferromagnetic ordering in Fe and its alloys, so that Mossbauer spectroscopy is particle-size-sensitive. The applications of Mossbauer spectroscopy to catalyst problems have been recently reviewed (197-201). Some other techniques, including EXAFS (147,150)and EPR (202), have been proposed to estimate particle size but they have been applied to a few cases only. C. DEGREE OF REDUCTION A N D PRESENCE OF UNREDUCED IONS There are two ways to follow the reduction of catalyst precursors to yield the metal. One is quantitative in nature; the other is qualitative and both may prove useful. Most quantitative methods refer to conventional techniques, whereas the qualitative ones essentially refer to spectroscopies (optical, magnetic etc.). We review them briefly here.

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1. Conventional Methods

The principle is to measure the oxygen consumption necessary to oxidize the metal particles and from the knowledge of the metal content to deduce the reduction degree. Alternatively, one can conduct a complementary reduction of the ions that have not been reduced during the preparation of the metal particles. This may require high temperatures. The hydrogen consumption and the knowledge of the metal content lead again to the reduction degree. The amounts of gas needed in either case (oxidation or complementary reduction) can be obtained by means of a microbalance or a volumetric apparatus (203, 204). Another useful method (35, 205-209), called temperature-programmed reduction (TPR), can also lead to the reduction degree. In this technique, by following the change in hydrogen concentration in a nitrogen gas carrier as it passes over the catalyst sample, the latter’s rate of reduction can be continuously recorded. When oxygen is used instead of hydrogen, the corresponding technique is referred to as temperature-programmed oxidation (TPO). 2 . Optical Methods a . Infrared Spectroscopy. Infrared is certainly the spectroscopy that has found widest application in catalysis ever since Eischens and Pliskin (210) adapted this technique to the study of supported metal catalysts and extended it to a broad range of adsorption systems relevant to catalysis. In the early days, the applications of IR spectroscopy were limited by its low sensitivity leading to slow data acquisition and consequently sample heating. There were also problems due to low transmittance of the catalysts. These difficulties have now been overcome by the advent of the laser and the computer. Fourier transform infrared (FTIR) methods are capable of studying intensely colored samples (e.g., transition metal based catalysts) and a number of other kinds of solid (211). Most IR studies concern metals highly dispersed on carriers rather transparent to IR radiation such as SiO2 and A1203. This dispersion is required since metal particles larger than about 100 8, create severe energy dispersion by scattering (212). Although IR gives fairly direct evidence on many surface groups (hydroxyls, sulfates, carbonates, etc.), the study of supported metal through the use of probe molecules generally provides no direct evidence on the oxidation state or even on the chemical identity of the absorbing entity. When the chemical composition of the sample is known, certain correlations exist that can lead to the identification of certain types of bonding and certain oxidation states of the adsorbing metal center.

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Several types of molecule have been used to probe the state of the supported metal (fraction exposed, structure, support-metal or metalmetal interactions) and the presence of unreduced ions. However, some IR-active species such as the hydrocarbons exhibit weak absorptions. This is one reason why most IR studies of chemisorption have been limited to strongly IR absorbing species such as carbon monoxide. Experience and analogies with spectra of known compounds have shown that bands at frequencies above or below 2000 cm-’ could be assigned to adsorbed CO in a linear (i.e., bonded by one end to one metal atom) or bridged (i.e., attached to two or more metal atoms) form, respectively (213,214). Bands observed above 2100 cm-I are generally assigned to CO adsorbed on metal atoms in an oxidation state greater than zero (215). It will be seen that CO can also be used to study the geometric and the electronic factors related to the small particles. b. Diffuse Reflectance Spectroscopy. A diffuse reflectance UV-visible spectrum is composed of several bands, the frequency of which depends on the energy involved in the transition of an electron from the ground state to an excited state. For a given unreduced ion, the number and energy of the electronic levels depends, in turn, on the symmetry of the ion environment. The lower this symmetry, the lower the degeneracy of the electronic levels and the larger the number of possible electronic transitions. The energy levels are designated by terms that indicate in which way the corresponding orbital transforms in a given symmetry operation of the group considered and the spin multiplicity of the system. When a transition ion is engaged in a solid, it experiences a crystal field whose symmetry and strength depend on the number, location, and nature of the ligands. For a given electronic configuration, the TanabeSugano diagrams, established for a particular symmetry, give the energy change of the various electronic terms as a function of D,lB, where D, is the crystal field parameter and B is the electron repulsion Racah parameter. The electronic transitions that are allowed must satisfy selection rules based on two criteria, the spin and the orbital (Laporte rule) of the excited and ground states. Diffuse reflectance spectroscopy has found wide application for the characterization of supported transition-metal ions that are the normal precursors of supported metals. It, therefore, can be used to detect unreduced transition metal ions (57a,c, 186, 216-221). 3 . Magnetic Methods

a . Ferromagnetic Resonance. The principle of ferromagnetic resonance has been given in Section IV,B,S,b. It was mentioned that the FMR

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signal is characterized by its intensity and a resonance field. There is, in addition, a very important parameter, called the “Curie temperature above which the FMR signal disappears.” This parameter is a constant for a given metal and is equal to 63 1 K in the case of nickel. In some cases, the Curie temperature is lower than expected, and this lowering has been assigned to a partial reoxidation of the particles and can be accentuated by oxygen chemisorption (58,89, 186,222). It thus appears that the measure of the Curie temperature is indicative of the presence of unreduced ions. b. Electron Paramagnetic Resonance. The FMR and EPR techniques operate on the same basic principle (Section IV,B,S,b). The EPR technique, however, concerns paramagnetic species, i.e., those that possess one or several unpaired electrons and that, for solids, are located in the bulk or at the surface (223-225). The nature of the information gained from EPR may vary from the simple confirmation of the presence of a given paramagnetic species within a particular solid (226,227)to the more sophisticated and detailed description of the coordination sphere of a particular ion supported on a carrier (57a,b,228-230). The extreme sensitivity of EPR, as compared to usual spectroscopic techniques, is perhaps its most acknowledged advantage and makes it best suited to investigate and characterize unreduced ions of supported metal catalysts (58).Those ions must however be paramagnetic. 4. Photoelectron Spectroscopy

This “surface” technique is based on the relation established by Einstein (231)that the kinetic energy E k of an electron ejected by the photoemission process can be calculated as the difference between the energy of the photon hv and the binding energy Et, of the electron in the target. With the development of high-resolution spectrometers capable of measuring Ek with a high precision (232) and the Einstein relation, Siegbahn and co-workers measured binding energies of core electrons released on subjecting the sample target to X-ray photons. The discovery that these energy levels shift by several electron volts when the chemical state of the atom studied changes (233) led to the technique now referred to as electron spectroscopy for chemical analysis (ESCA) (234)or X-ray photoelectron spectroscopy (XPS). A parallel development was made by Turner and co-workers with the ultraviolet photoelectron spectroscopy (UPS) to study valence electrons (235). However, only XPS has been widely applied to supported catalyst systems. Since the excitation process in XPS concerns core electrons, the technique is element-specific and elements can be identified on the basis

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of the binding energy values. The latter reflect the chemical environment (oxidation state, ligands) of atoms and make XPS a powerful technique to follow chemical changes during various treatment procedures (236). Shifts of about 1 eV per change in formal oxidation state are typical for catalyst applications and can be used to monitor such phenomena as the reduction of ions to prepare supported metal particles (237) and detect the presence of unreduced ions. In addition, the technique is highly surface sensitive since the investigated sample depth is limited by the escape depth of the ejected core electron, usually around 2.0 nm, and this property can be used to detect the migration of ions from or into the bulk of the support (238).The main limitation of XPS is that experiments are carried out in ultrahigh vacuum so that pretreatment procedures must be performed in separate high pressure chambers. Several reviews on the application of XPS to catalytic problems have appeared (236-239). 5. Extended X-Ray Absorption Fine Structure

Extended X-ray absorption fine structure spectroscopy (EXAFS) has been increasingly applied to the study of catalysts. This technique involves the absorption of X-rays by a catalyst sample as a function of photon energy (240).Once the energy of the absorption edge is exceeded, a photoelectron is ejected that is scattered by the neighbors of the central atom. The scattered electron wave can interfere constructively or destructively with the ejected photoelectron wave, producing oscillations in the X-ray absorption coefficient. The latter rises sharply at the absorption edge, and modulation of this coefficient is observed beyond the edge. The frequency of the oscillations is inversely proportional to the distance of neighboring atoms, and the amplitude is proportional to the number of neighbors. Furthermore, the area under the curve near the edge (called the “white-line area”) and the position of the edge may provide information about the chemical state of the absorbing atom and thus allow the detection of unreduced ions. The advantages of EXAFS are (23) (1) measurements can be made in situ in different gaseous environments, (2) short-range order around a particular species of atom can be determined and the metal need not possess long-range order, (3) bond lengths can be measured, (4) average coordination numbers can be calculated, and ( 5 ) the electronic structure of the atoms and the number of d electrons can be determined. The main disadvantage of EXAFS is that it is an expensive technique that is not easily available. Reviews on the applications of EXAFS to catalytic phenomena have been published recently (241, 242a,b ) .

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6 . Miscellaneous

Apart from those described above, there are several other techniques for detection of the presence of unreduced ions. They are presented here because they have not been used as much as the preceding ones or are not as general. These include, for instance, Mossbauer spectroscopy. This technique is based on the recoil-free emission of gamma rays by a radioactive source and the resonant absorption of these gamma rays by Mossbauer nuclei in the sample under investigation. The resonant absorption induces nuclear transitions that are strongly influenced by the chemical (oxidation) state and/or environment of the Mossbauer nucleus affording the possibility to follow reduction processes and detect the presence of unreduced ions (197-201). Chemical methods have also been used to probe the completeness of reduction. Thus Martin et al. (243) selectively dissolved the metallic nickel obtained after reduction by a bromine methanol solution and titrated it by complexometry. For the same nickel catalysts, it was possible to measure the reduction degree from saturation magnetization data at low temperatures, and a good agreement was found between the two estimations of the reduction degrees (244).

D. PRESENCE OF POISONS A N D OF PROMOTERS As indicated in Section 11, undesired species are often found on catalysts at the end of the preparation process. It is thus advisable to probe the purity of the catalysts, and this is considered below. In some cases, these species can also act as promoters. Promoters added deliberately to the catalysts will not be considered here since this section deals basically with the detection of undesired species that would modify the intrinsic properties of the metal and compete with the true particle size effect. 1 . Temperature-Programmed Desorption

The temperature-programmeddesorption (TPD) technique provides information about the binding of adsorbed species to the catalyst surface. In a thermal desorption experiment, the adsorbent (catalyst) is given the desired exposure to the adsorbate (or reacting mixture) and then the sample is heated in a programmed manner: desorbing gases cause an increase in the measured partial pressure and the recorded pressure-time (or sample temperature) curve is known as the desorption spectrum (245). The desorption of the adsorbed species can be followed by various techniques (gas chromatography, mass spectrometry, etc.). There are several different ways to induce desorption of the adsorbed layer, which include

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thermal desorption, electron impact desorption, field desorption, ion impact desorption, and photon- and phonon-induced desorption. The information that can be obtained from desorption methods includes surface analysis, i.e., surface concentration and stoichiometry, and sometimes the nature of the surface species (245). Because of the widespread use of molecular hydrogen as a reducing agent to produce the metal particles, the TPD of hydrogen from metal catalysts has been of course largely investigated (14.36).In many systems, the presence of several desorption peaks revealed species with different adsorption strengths. For very small nickel particles ( d below about 1 nm) it has been found that hydrogen was so strongly held that it could act as a poison and suppress catalytic activity for hydrogen01ysis (58). In some cases, specific desorption techniques like secondary-ion mass spectroscopy (SIMS) has been very fruitful. This method employs mass analysis of both negative and positive secondary ions emitted by a surface subjected to ion bombardment (245). For instance, the negative secondary-ion (SI) mass spectrum, determined by Benninghoven (246, 247), of a commercial silver catalyst used in the oxidation of ethylene indicates the presence of hydroxide, cyanide, chloride, sulfate, nitrate and many fatty acids in the uppermost monolayer. Since the low primary-ion current perturbs the surface only weakly, the SI mass spectrum can be continuously monitored while the surface is heat treated. The results of Benninghoven showed that during heat treatment, impurities could reach the surface by diffusion from within the catalyst. This aspect is very important and must always be kept in mind particularly in the context of particle size effect. The application of TPD methods to catalytic problems has been reviewed earlier (248-251). A different approach can be adopted to detect surface poisons which consists in involving those poisons in simple reactions such as combustion and to detect the products by gas chromatography or mass spectrometry. This has been performed very often in TPO experiments particularly for samples contaminated by carbonaceous impurities (252).

2. Other Analytical and Spectroscopic Methods Many techniques have been used to detect the presence of poisons on metal surfaces, particularly by chemical analysis, but the effectiveness decreases with the poison content and special techniques become necessary for traces. For instance, special analytical (gas chromatography) capabilities to measure S concentrations below 5 ppb and CH, below 1 ppm must be on-line (253). The sensitivity of spectroscopic methods becomes in those cases an important advantage. IR has often been employed to detect surface and/or bulk species by means of their character-

108

MICHEL CHE A N D CARROLL 0.BENNETT

istic vibration frequencies and a number of reports have been given on many surface groups such as hydroxyls, sulfates, sulfides, carbonates, nitrates etc. (204,211,214, 218), which can strongly influence the chemical state of a metal particle. It is known, for instance, that hydroxyl groups are capable of oxidizing metal atoms (61) (Section II,B,7); some others, such as pseudocarbonates, are capable of functioning as electron reservoirs to reduce ions into metal atoms (218). EPR, which is very sensitive, has also been used to detect surface and/or bulk species that are paramagnetic such as nitrogen-, sulfur-, and particularly carbon-containing species (50, 223, 254,255). In the same way, nonresonance magnetic methods have shown that the residues of the solvent used during preparation could strongly modify the properties of supported metal particles (70). The preceding spectroscopic methods detect both surface and bulk species. By contrast, electron spectroscopies such as XPS or Auger electron spectroscopy (AES) have been very useful to detect and identify species located in the outermost layers of the catalyst. A N D ELECTRONIC PROPERTIES OF SMALL E. STRUCTURE METALPARTICLES

1.

Structure

We have already mentioned that some of the previously described techniques can be used to study the shape of the metal particles. In view of their relevance to catalytic problems, it is also important to obtain information on the lattice parameters. The techniques that have proved most useful for this purpose are electron microscopy, EXAFS, and X-ray diffraction, which have been presented in the preceding sections. A number of papers have been published that report unusual symmetries for small particles such as atoms in abnormal positions, raft-like structures, close-packed structures different from the normal bulk structures and different textures (23). Among those unusual arrangements, there is a strong emphasis on the pentagonal symmetry. The conditions under which the different types of structure tend to be formed have been summarized by Yacaman et al. (256). The EXAFS results indicate that the lattice parameter of small particles is contracted as compared with that of the bulk metal (23). This fact is confirmed by radial electron distribution data obtained from X-ray experiments (257a,b) and also by electron diffraction measurements ( 2 5 7 ~ ) . Both this contraction and the anomalous pentagonal symmetry can be eliminated in the case of Pt particles onto which hydrogen is adsorbed. These “relaxation” effects, which are found to be reversible, will not be

METALS: PARTICLE SIZE A N D CATALYTIC PROPERTIES

I09

observed if studies are not performed under catalytic reaction. This is possible with EXAFS and small-angle X-ray scattering, which, as mentioned earlier, can both be used in situ in contrast to conventional electron microscopy (TEM, STEM, etc.) techniques.

2. Electronic Properties The techniques that have been most employed for investigating the electronic properties of small particles are photoemission (UPS, XPS), soft X-ray spectroscopy, EXAFS, photoionization mass spectrometry, and AES (23, 111, 240, 257d,e). While there is some controversy from theoretical work about the minimum particle size required to give bulk properties-from 10 (258) to several hundred atoms (259)-there seems to be a consensus that a cluster of about 150 atoms or more is required to observe a photoemission spectrum similar to that of the corresponding bulk metal (23, 260). When other properties are considered (ionization potential, density of states, valence bandwidth, etc.), the agreement is less satisfactory between the results obtained with different techniques (231. V.

Effect of Particle Size on Turnover Frequency and Selectivity: Presentation and Comparison of the Data

A.

INTRODUCTION

In presenting data on the effect of metal crystallite size we shall show mostly the results of studies in which the authors themselves have measured rates as a function of d . We do not take results from different works, each using a different d, and construct a graph of TOF. Some other studies have incorporated this approach, and we do report some such data. This selective approach means that only a small fraction of the results on the kinetics for any given reaction are reported in our survey. The rate data are reported at the temperature, pressure, and gas composition used by the authors. However, some authors have themselves converted their results to a given set of conditions, by means of activation energies and reaction orders that they consider appropriate. However, these parameters themselves may be structure sensitive, so that the results at the conditions actually used are of the most fundamental interest. When rates or dispersion data have been given in units other than TOF and d or FE, we have converted them to these units via a few simple

110

MICHEL CHE A N D CARROLL 0. BENNETT

calculations. More weight should be given to the shapes of the curves shown than to the numerical values referred to the indicated axes. The data are presented according to reaction type, not chronologically or by structure sensitivity type.

B. HYDROGENATION OF HYDROCARBONS The kinetics of these reactions has been studied extensively, but the effect of particle size has been investigated mainly for benzene hydrogenation. In general, the reactions are considered to be structure insensitive, but we shall see that there are a number of exceptions. 1. Hydrogenation of Ethylene

This reaction is usually taken as structure insensitive, following the work of Schlatter and Boudart (261). These authors collected data from many sources on the rates over various supported catalysts of differing FE. These results, along with some of their own, were plotted on an Arrhenius diagram. To a certain level of approximation the rates, when properly interpreted, lay near a single line. Thus it appeared that d and the nature of the support were irrelevant in determining TOF. Masson et al. (79) have used vapodeposited Pt to study this reaction over Pt/Si02 and their results are shown in Fig. 9. There is a clear maxi-

1.5 c

-

la -

IA

i

+ 0 CT

0.5

0

0.1

0.25 0.5 1.0 FE

FIG.9. Rate of hydrogenation of ethylene at 250°C and 1 atm on Pt/SiO, prepared by vapocondensation. (0)Atomic rate (AR), s-I; (0)turnover frequency (or rate) TOF, s-I. From Ref. 79.

METALS: PARTICLE SIZE A N D CATALYTIC PROPERTIES

111

mum in the rate at about 0.6 nm, and separate atoms do not appear to act catalytically. Masson et al. (78)have studied ethylene hydrogenation on Pt/AI,O3 also prepared by vapodeposition. The shape of the TOF/FE curve is identical to that shown in Fig. 9. The maximum TOF is also at about 0.6 nm. The XPS spectra of the Pt particles have been recorded as a function of d (78,79),and the results are among those presented in Fig. I . Most of the change in binding energy occurs below 2 nm. However, as already explained, the curves of ABE (Binding Energy) versus d appear to be about the same for all metals so that measuring ABE can be a way of estimating d as suggested by Masson et al. (78).At any rate there are important electronic effects in the region of the maximum TOF of Fig. 9, and it seems unlikely that there is any geometric ensemble that has a maximum surface concentration for particles of d near 0.6 nm. 2. Hydrogenation of Acetylene

We note here the work of Kojima et al. (262), although the effect of particle size has not been studied in their article. By using field-emission microscopy they were able to observe the surface structure of a tip of platinum. In an annealed condition a regular (1 10) plane was exposed, and the surface was relatively inert as a catalyst for the hydrogenation of C2H2.After argon ion bombardment the (1 10) surface was disrupted, and its catalytic activity greatly increased. This experiment demonstrates a form of structure sensitivity, induced in this case not by changing d but by changing the concentration of surface defects. The structure sensitivity of the selective hydrogenation of C2H2 to C2H4 over Pdlcr-Al203 catalysts has been studied by Gigola et al. (263). Industrial conditions of pressure and gas composition were used. For differential conversions, a strong antipathetic structure sensitivity was found as the FE was increased above that used for the industrial-type catalysts. Little ethylene was consumed. As the temperature and conversions were increased, however, the high FE catalysts lost their selectivity, probably through thermal effects. The low FE, industrial-type catalysts retained their selectivity (little ethylene consumption) at high conversion. 3. Hydrogenation of Cyclopropane, Propene, and 1-Hexene

In Fig. 10, curves I and 3 represent the hydrogenation of cyclopropane over Pt. The shapes are similar, but curve 2, for nickel, shows a maximum TOF at a d of about 1.2 nm, the lowest value used for the platinum data.

d ,nm 100

I

0.01

25

1

0.04

10

,

0.1

I

I

0.4

1.0

FE

FIG. 10. Turnover frequency TOF versus fraction exposed FE and mean particle size 2 for the hydrogenation of propene, 1-hexene, and cyclopropanes on various metal systems (see Table V for details of the studies).

TABLE V Details of the Metal Systems Studies Presented in Fig. 10 Curve (Fig. 10)

Symbol (Fig. 10)

Metal

I

4

Pt

Si02

2

A

Ni

Si02

3

0

Pt

AI2O1,Si02

4

0

Pt

SiOl

5

0

Pt

Si02

6

0

Ni

AIPOI

Support

Temperature, pressure 0°C. 1 atm, Hz/CP = 14 5"C, 1 atm, HJCP = ? O"C, 1 atm, H*/CP = 4.7 OT, 1 atm, H2/MCP = 14 -57°C 1 atm, HJC] = 19 25°C 4.1 atm, H2/C* = >>I

Reactant

Reference"

Cyclopropane

a

Cyclopropane

b

Cyclopropane

C

Methylcyclopropane Propene

a

I-Hexene

d

a

References: (a) Otero-Schipper, P. H., Wachter, W. A.. Butt, J. 9..Burwell, R. L., and Cohen, J. B.,J . Catal. 50,494 (1977);(b) Coenen, J. W.E., Schats, W. M. T. M., and Van Meerten, R. Z. C., Bull. SOC. Chim. Eelg. 88,435 (1979); (c) Boudart, M.. Aldag, A,, Benson, J. E., Dougharty, N. A., and Harkins Girvin, C., J . Catal. 6,92 (1966); (d) Campelo, J. M.,Garcia, A., Guttierez, J. M.,Luna, D., and Marinas, J. M., Appl. Card. 7, 307 (1983).

METALS: PARTICLE SIZE A N D CATALYTIC PROPERTIES

I I3

Methyl cyclopropane and propene hydrogenation over Pt, curves 4 and 5 , show the same trend as the other curves over Pt. Curve 6 for the hydrogenation of 1-hexene is similar to the others; to a first approximation the reaction is structure insensitive. The hydrogenation of propene has also been studied on Pt/Si02and Pd/ Si02 having fractions exposed in the range 0.06-0.86 (264). For Pt/Si02 the TOF increases by about 50% as the FE increases from 0.06 to 0.86. For Pd/SiOz there is a maximum TOF at about FE = 0.75. The magnitudes of the TOF were considerably influenced by the temperature at which the catalysts were treated in pure H2 prior to the reaction experiments. 4. Hydrogenation of C4Hydrocarbons

Boitiaux et al. (265, 266) have studied the hydrogenation of 1-butyne, I-butene, and 1,3-butadiene over Pd/A1203and Pd/SiOz, and some results are presented in Figs. 11 and 12. This catalyst system is remarkable in that the selectivity of the hydrogenation of 1-butyne to 1-butene remains almost 100% until most of the butyne is consumed. Then l-butene readily reacts, but only if almost all of the I-butyne is gone. The graphs show that 1-butyne and 1,3-butadiene have a similar structure sensitivity; as d decreases, so does TOF. However, the hydrogenation of 1-butene is structure insensitive, as shown in Fig. 12. This behavior is consistent with the idea that 1-butene is excluded from the surface by 1-butyne. When I-butene is the feed (no I-butyne present), then the rate-determining step and surface composition are different, and the structure sensitivity disappears.

i

nm

5.0 2.5 1.67 1.25 1. 80

FE

FIO. 1 I . Turnover frequency TOF versus fraction exposed FE and mean particle size 2 for the hydrogenation of 1-butyne on Pd/AI2O3at 20 "C and 20 atm in liquid n-heptane. From Ref. 266.

114

MICHEL CHE A N D CARROLL 0. BENNETT

i. nm 1.67 125

5.0

2.5

0.20

0.40

1.0

I

& 100 I-

75

25 0

FE

0.60

0.80

1.00

a

FIG.12. Turnover frequency TOF versus fraction exposed FE and mean particle size for the hydrogenation of olefins on Pd/AI2O3at 20°C and 20 atm in liquid n-heptane. The triangles represent the isornerization of I-butene to 2-butenes (cis and trans), whereas the circles refer to the sum of isomerization and hydrogenation reactions. The squares refer to the partial hydrogenation of 1,3-butadieneto butenes. From Ref. 265.

5. Hydrogenation of Benzene

This reaction has usually been classified as structure insensitive [Boudart (10); Bond (2Z)I. Figure 13 does show some systems that are structure insensitive, but it is immediately evident that generalizations are risky in this field. Curves 1,2, and 3 are for Ni/Si02, Ni/SiOz, and Ni/MgO, respectively. Curve 1 shows a behavior that might be understood on the basis of Figs. 2 and 3. Perhaps the low-coordinated atoms are the more active ones, but then as d decreases further, Ni progressively loses its metallic character and ceases to be such a good catalyst as d falls below 1.2 nm. However, it may also be argued that as d decreases the support interaction becomes stronger and the Ni is partly oxidized. Curve 2 shows a very different behavior. Why is there such strong size sensitivity above 6 nm, where electronic effects should be negligible? Are the faces exposed sensitive to d for these large particles, which could be studied by electron microscopy and microdiffraction techniques? Coenen et al. (112) probably thought that TOF would be constant for d above 5 nm. However, it is too bad that the authors of the later studies, curves 2 and 3, did not investigate the region below 2.5 nm. Curves 4-7 refer to Pt,and these older results form most of the basis for considering the hydrogenation of CaH6 to be structure insensitive. If curve 5 represents structure insensitivity, can we really say that curves 4 and 6 represent the same behavior? Barbier and Marecot (267) also report structure-insensitive behavior for Pt/A1203and Ir/A1203. If curve 9 is accepted as structure insensitive for Pd/A1203, should curve 10 for Rh/ A1203,measured in the same laboratory, be considered to show the same behavior? Apparently Rh is much more active than Pd; note the tempera-

METALS: PARTICLE SIZE AND CATALYTIC PROPERTIES

10

0

25 1

10

d,nm

I

2.5

1.0

I

1

0.4

1.0

115

0.4

1.c

10

F

I,lO L L

0 b-

10

10 1

0.04 0.1 0

FE

FIG.13. Turnover frequency TOF versus fraction exposed FE and mean particle size d for the hydrogenation of benzene on various metal systems (see Table VI for details of the studies).

ture for curves 9 and 10. The expenditure of mental effort to explain why a system is structure insensitive should probably wait until we know whether Ni behaves as in curve 1 or 2, and Pt as in curve 4 or 5 . 6 . Dehydrogenation of Cyclohexane

This reaction has been studied by Kraft and Spindler (268), who find that it is structure insensitive over Pt/AIzO,, at 350°C and 1 atm, with a H2/C6HI2ratio of 3. This finding is at variance with the idea that the reaction should be structure sensitive that follows from Fig. 6. However,

TABLE VI Details of the Studies Presented in Fig. 13 on the Hydrogenation of Benzene on Metal Systems ~~

Curve (Fig. 13)

Symbol (Fig. 13)

I

@

Ni

Si02

2

Ei

Ni

Si02

Ni

MgO

Pt

A1201

Pt, Ir

AI2O3

Pt

A1201 Si02, 70 m2/g 950 m2/g SiO2-Al2O1 A1201 Charcoal

3 4

0

Metal

5

0

6

0

Pt

0

Pt

Support

7

8 63

8

A

Pt Pt Pd

9

X

Pd

MgO. A1201

Pd

Si02

10

A

Rh

Temperature, pressure, H&H6 25T, 0.88 atm, H2/C6H6 = 8.57 30°C 0.79 atm, H2/C& = 150 40T, 1 atm, H2/C& = 9 5OoC, 1.5 atm, Hz/C&6 = 20.4

50°C. 1 atm,

Comments

Reference“ a, b C

d Note influence of reversible H2 adsorption Structure insensitive on Pt and Ir, d = 1.4 to 12 nm Structure insensitive

e

f

g h

High rate

I

j k

Structure insensitive, d = 1.3 to 13.4-nm rate = %/mz

1

m

References: (a) Coenen, J. W. E., Schats, W. M. T. M., and Van Meerten, R. Z. C., Bull. SOC. Chim. Belg. 88,435 (1979); (b) Coenen, J. W. E., Van Meerten, R. Z. C., and Rijnten, H. T., Proc. Int. Congr. Catal., 5th 1, 671 (1972); (c) Martin, G. A., and Dalmon, J. A., J. Catal. 73, 233 (1982); (d) Nikolajenko, V., Bosacek, V., and Danes, V. L., J . Catal. 2,127 (1963);(e) Aben, P. C., van der Eijk, H., and Oelderik, J. M., Proc. Inr. Congr. Cafal., 5fh 1,717 (1972); (f) Barbier, J . , and Marecot, P., NOULJ. J. Chim. 5 , 393 (1981); (g) Basset, J . M., Dalmai-Imelik, G.,Primet, M., and Mutin, R., J . Cafal. 37,22 (1975); (h) Dorling, T. A., and Moss, R. L.. J . Catal. 5, 1 I 1 (1966); (i) Ratnasamy, P., J . Cafal. 31, 466 (1973); (i) Benedetti, A., Cocco, G., Enzo, S., Pinna, F., and Schimni, L., J . Chim. Phys. 78, 877 (1981); (k) Figueras, F., Fuentes, S., and Leclercq, C., in “Growth and Properties of Metal Clusters” (J. Bourdon, ed.), p. 525. Elsevier, Amsterdam, 1980; (I) Moss, R. L., Pope, D., Davis, B. J., and Edwards, D. H., J. Catal. 58,206 (1979);(m) Fuentes, S., and Figueras, F., J. Caral. 61, 443 (1980).

METALS: PARTICLE SIZE AND CATALYTIC PROPERTIES

117

the range of d used by Kraft and Spindler was only 0.77-1.4 nm, so their results are not conclusive. Cusumano et al. (128) studied the reaction over Pt on alumina and on silica supports and concluded that the TOF was about the same for both catalysts, which did show quite different atomic rates AR. The later work of Sinfelt et al. (269)on reactions over copper-nickel alloys led also to the suggestion that cyclohexane dehydrogenation over Ni does not require a large ensemble of surface atoms and thus may be structure insensitive on a geometric basis. For ethane hydrogenolysis studied on the same CuNi alloys, it was found that the activity decreased much more rapidly than did the fraction of Ni atoms on the surface of the alloys. This implies that ethane hydrogenolysis requires an ensemble of surface atoms and should show antipathetic structure sensitivity. This reaction will be discussed in connection with Fig. 15 (below). Returning to the dehydrogenation of cyclohexane, this reaction has been studied over Ni/SiO2 at 25°C and 1 atm using a H1/C6H12ratio of 6.7 (270).It is found in the range 4 nm < d < 8 nm that the dehydrogenation shows antipathetic structure sensitivity, and hydrogenolysis (of cyclohexane) shows sympathetic structure sensitivity. These results are not consistent with those of Sinfelt et al. (269).For Pt and a reaction pressure of about torr it was found (125, 126) that the rate of cyclohexane dehydrogenation was about the same on the (1 11) plane as on planes showing more step and kinks, e.g., (557), (271-273). However, a later report from Somorjai’s group (274)presents the result shown in Fig. 6: the equivalent of sympathetic structure sensitivity. In this case the kinetics were measured at l atm. Among the explanations proposed (274)for the differences between the low- and high-pressure results is that at 1 atm the planes are quickly covered with carbon and that reaction takes place largely at kinks and edges. It is clear that the surface composition is different at the two pressures. The apparent activation energies are also quite different.

C. HYDROGENOLYSIS OF HYDROCARBONS These reactions are structure sensitive for the most part, and this effect is generally explained by geometric reasoning. Since C-C bonds are broken, a site probably consists of several surface atoms, so that the site density is strongly affected by d. 1.

Hydrogenolysis of Ethane

The hydrogenolysis of ethane is generally considered to be structure sensitive, and this belief is supported by Fig. 14. However, this figure

118

MICHEL CHE A N D CARROLL 0. BENNETT

1.0

100

25

10

2.5

1.0

0.4

I

I

1

I

I

( 7)

-

10-1-

I

ln 4

I U

L

10-2-

0 L

LL

0

c

10-3-

104 0.01

1

1

0.04 010

0.4

1.0

FIG.14. Turnover frequency TOF versus fraction exposed FE and mean particle size for the hydrogenolysis of ethane on various metal systems (see Table VII for details of the studies).

permits no generalization about the direction of the change of TOF with d. Curves 1 and 2 refer to Rh, and they seem to agree that for Rh/Si02 and for Rh/TiO2 in the normal state, TOF increases as d decreases. For Rh/ Ti02in the SMSI condition, the behavior is reversed. Resasco and Haller (129) explain this in terms of delocalized (charge transfer from Rh to Ti02 with charge deficiency delocalized over the whole metal particle) and localized (charge transfer from Ti02 to Rh localized at the metal-oxide interface) electronic effects. Since the hydrogenolysis of ethane is thought to require a relatively large ensemble of atoms, it is difficult to explain the curves of Fig. 14 for which TOF increases as d decreases. For Ni, curves 4 and 6 indicate a maximum rate at about 4 nm. If the fall in rate for small d is explained on the basis of the ensemble effect, how is the opposite behavior for d greater than 4 nm to be explained? Curve 3, if displaced a little to the left, agrees roughly with 4 and 6. However, curve 5 shows a completely different behavior; is this caused by the low H~/C~HC,ratio?

METALS: PARTICLE SIZE AND CATALYTIC PROPERTIES

119

TABLE VII Details of Studies Presented in Fig. 14 on the Hydrogenolysis of Ethane Curve (Fig. 14)

Symbol (Fig. 14)

I

Metal

Support

Rh

Si02

A

Rh

0

Rh

3

0

Ni

T i 0 2 , reduced at 250°C T i 0 2 , reduced at 500°C SiO2-AI2O3

4

X

Ni

SOz

5

0

Ni

AI2O3

Ni

Si02

Pt Ir

A1203 AI2O3

2

6 7

A 8

Temperature, pressure, H 2 / C 0 or C 0 2 253"C, I atm, H2/C2H6 = 6.7 250°C, 1 atm, pulse reactor 350°C. 1 atm, pulse reactor 267"C, 1 atm, HJC2H6 = 6.7 220"C, 0.8 atm, H2/CzH6 = 20 200"C, 0.13 atm, HJC2H6 = 1.0 234°C. 0.26 atm, Hz/C2H6 = 6 275"C, 1 atm H,/CzHs = 9

Comments

Reference" a

Normal state

b

SMSl state

b C

d e

( I 1 I ) planes not active

f g

References: (a) Yates, D. J. C., and Sinfelt, J. H., J . Caral. 8,348 (1967); (b) Resasco, D. E., and Haller, G . L.. J . Catal. 82, 279 (1983); (c) Carter, J. L., Cusurnano, J. A., and Sinfelt, J. H., J . f h y s . Chem. 70, 2257 (1966); (d) Sarkany, A., and Tetenyi, P., React. Kinet. Catal. Lett. 12, 297 (1979); (e) Ryndin, Y. A., Kuznetsov, B. N., and Yermakov, Y.I., React. Kinet. Caral. Lerr. 7 , 105 (1977); (f) Martin, G. A., and Dalmon, J. A., C.R. Acad. Sci., Ser. C 286, 127 (1978); (g) Barbier, J., and Marecot, P.. N o w . J. Chim. 5 , 393 (1981).

Curves 7, for Pt and for Ir, show a different intrinsic activity but have the same shape. Barbier and Marecot (267) have suggested that since Pt and Ir have almost identical crystal structural parameters, the similarity in the curves 7 must mean that the structure sensitivity arises from geometric factors. The monotonic nature and the direction of the variation support this explanation also. In connection with curve 6, Martin and Dalmon (275) have shown that the (1 11) planes of Ni are relatively inactive for the hydrogenolysis of ethane. Crystallites exposing principally (1 11) planes have been prepared by the reduction of antigorite, and the resulting crystallites give a TOF of 5.4 x s-' at the conditions of curve 6, the same as the value at the left end of this curve. The crystallite size was varied from 4 to 10 nm. For the usual supported catalyst, the increase in TOF as d reduces to about 5 nm would come from the decreasing fraction of (111) planes. For smaller crystallites, the classical ensemble effect (Fig. 3) would explain the de-

120

MICHEL CHE A N D CARROLL 0. BENNETT

crease in TOF. This behavior agrees with results on Cu-Ni alloys, indicating that an ensemble of atoms is needed for ethane hydrogenolysis (105, 269). 2. Hydrogenolysis of Propane and n-Butane

The hydrogenolysis of propane has been studied over Ni/Si02 for a particle size range of 2.5 to 21 nm (276).The curve for TOF versus FE is quite similar to curve 6 of Fig. 14, for ethane hydrogenolysis on the same catalyst (275).The selectivities toward ethane or methane did not change with fraction exposed. Namizek (109)has studied the rate of n-butane hydrogenolysis over Ni/ A1203for Ni crystallites of d varying between 1 .O and 10 nm. A maximum rate is found at about 2.5 nm, so that those results resemble closely those for C3Hsand for C2H6.The maximum in the rate curve occurs at a d that corresponds roughly to the maximum surface concentration of Bg sites, as measured by the method of van Hardeveld and van Montfoort (106). In later work, Namizek and Ryczkowski (110) measured rates of propane and n-butane hydrogenolysis over Ni/A1203 catalysts of various FE. They found maximum values of TOF for propane at d = 3-4 nm and for butane at 2-3.5 nm. Masson et al. ( 1 1 1 ) have studied n-butane hydrogenolysis on well-characterized Ni vapodeposited on silica, prepared as already discussed (78, 79). The TOF rises as d is decreased, reaches a maximum at about 2 nm, and then tends toward zero as d goes toward zero (FE = 1.0).

3 . Hydrogenolysis of Methylcyclopropane Results for the turnover frequency for the production of i-butane from this reaction at 0°C have been reported as a function of fraction exposed over both Pt/SiO2 and Pt/A1203 (277). For the alumina support the reaction is structure insensitive, whereas for the silica support the TOF for FE = 0.9 is about four times that at low dispersion. For both catalysts the activity is reduced as the solid is treated at higher temperatures (370"C, 480°C) in H2, followed by cooling in H2 to 0°C.Such differences arising from varying a pretreatment probably persist at 0°C but are less likely to do so for other reactions that occur at 200-400°C. Methylcyclopropane hydrogenolysis has also been studied over Pd/ Si02 and the TOF passes through a flat maximum at about FE = 0.65 (278).Over Rh/SiO:!, this reaction is found to show a marked sympathetic structure sensitivity (279).

METALS: PARTICLE SIZE A N D CATALYTIC PROPERTIES

4.

121

Skeletal Reactions of Hydrocarbons

The hydrogenation of various unsaturated hydrocarbons has already been described in the preceding sections. These reactions involve C-H bonds only. We have also considered hydrogenolysis of C2-C4 hydrocarbons (a C-C bond is broken). It is now logical to proceed to the hydrogenolysis of larger molecules. Reactions involving 5- and 6-carbon alkanes are important as models for refining processes such as catalytic reforming, in which isomerization and dehydrocyclization are to be promoted and hydrogenolysis suppressed. The reactions of even moderately large molecules like 2-methylpentane or methylcyclopentanes are complex, and much effort has been devoted to understanding the kinetics and mechanisms of such reactions. The recent review by Anderson (22) and the encyclopedic article by Gault (280) should be consulted for anything more than a superficial idea of this vast subject. Figure 15 presents the effect of particle size on the turnover rate in the hydrogenolysis of cyclopentane and methylcyclopentane. Anderson and Shimoyama (281)studied methylcyclopentane, and their results are represented by curves I and 2. Curve 1 represents rates over a clean catalyst and shows higher values than curve 2, which represents rates on a cata-

( 2 1 ---- 9.' -_-----1

0.01

0.04 0.10

, 1

1

0.4

1.0

FE

FIG.15. Turnover frequency TOF versus fraction exposed FE and mean particle size 2 for the hydrogenolysis of cyclopentane (-) and methyl-cyclopentane (---) on various metal systems (see Table VIII for details of the studies).

122

MICHEL CHE A N D CARROLL 0. BENNETT

TABLE VIIl Derails of the Studies Presented in Fig. 15 on the Hydrogenolysis of Cyclopenrane and Methylcyclopentane

Curve (Fig. 15)

Symbol (Fig. IS)

Metal

Support

I

X

Pt

2

0

Pt

Pyrex glass, silica glass, mica Pyrex glass, silica glass, mica

Ir Rh Pd Rh H

Rh

273"C, 0.13 atm, H?/HC = 10 273"C, 0.13 atm, H2IHC = 10 3WC, 1 atm, HJHC = 9 17OoC, 1 atm, HJHC = 9 225°C 1 atm, H2/HC = 6.6 290"C, 1 atm, HZ/HC = 6.6 220"C, I atm

Pt

8

Temperature, pressure

Si02

220°C, I atm

Comments

Reference

Methylcyclopentane; initial rate Methylcyclopentane; steady-state rate Cyclopentane; initial rate Cyclopentane; initial rate Cyclopentane; initial rate Cyclopentane ; initial rate Methylcyclopentane; no deactivation Methylcyclopentane; no deactivation

281 281

267 267 282 282 282 282

lyst partly deactivated by the deposition of a carbonaceous layer. The Pt catalysts used were vapodeposited on glass and mica supports. Both curves show limited sympathetic structure sensitivity. However, curve 3 for the hydrogenolysis of cyclopentane on Pt/A1203 (267) shows the opposite effect of particle size, as does curve 7 for methylcyclopentane over Rh/A1203 (282). Curve 4, for Ir/AI203,was reported in the same article as curve 3, for Pt/A1203 [Barbier and Marecot (267)l; over Ir the hydrogenolysis of cyclopentane is structure insensitive. Since curves 3 and 4 are so different for the two metals of almost identical crystal parameters, Barbier and Marecot reason that the observed changes arise from the variation of the electronic factor with particle size. Curves 5-8 are based on the data of Del Angel et al. (282). For the hydrogenolysis of cyclopentane, Rh/A1203 is structure sensitive and Pd/A1203 is essentially structure insensitive. For the hydrogenolysis of methylcyclopentane, Rh/A1203, as already mentioned, gives the unusual result shown by curve 8, which is difficult to explain. Note also the point

METALS: PARTICLE SIZE A N D CATALYTIC PROPERTIES

I23

for curve 5 for the lowest FE, which has not been connected to the rest of curve 5 . Recent work of Ponec and co-workers (283) is pertinent to the discussion, although the results are not in a form convenient to plot in Fig. 15. Lankhorst et al. (132) have measured the hexane-H2 reaction over a series of Pt/Si02catalysts for which the particle size varied from 1.5 to 8 nm. For the clean catalysts, with the reaction occurring at 210"C, a slight structure sensitivity was observed. The catalyst did not deactivate appreciably. Then the reaction was run at 450°C for 30 min, and carbon deposition and deactivation occurred. After this treatment the variation of TOF with d was once more measured at lower temperatures. The deactivation was such that temperatures greater than 210°C were needed to obtain useful conversions. By varying the temperature at each particle size and using the Arrhenius relation, the temperature increase AT over 2 10°C needed to produce a given TOF were found for each particle size. For these partially deactivated surfaces, AT increased strongly with d. This is equivalent to a strong sympathetic structure sensitivity, whereas the clean surface showed little structure sensitivity. The large particles are self-poisoned to a much greater degree than are the small particles. Manogue and Katzer (284) have proposed that structure sensitivity that arises from reaction-induced surface changes rather than from the intrinsic properties of the underlying metal be called secondary structure sensitivity. For the oxidation of NH3over Pt/AI,03 (285),it was observed that small particles deactivated faster than did large particles, enhancing the original antipathetic structure sensitivity of this system. Barbier et al., (286, 287) have studied the structure sensitivity of coke formation on Pt/A1203during the hydrogenolysis of cyclopentane. All the catalysts contained a similar concentration of chlorine. The coke formed on the metal was distinguished from that on the alumina by temperatureprogrammed oxidation. A low-temperature peak represents coke on Pt, and a second peak at higher temperature represents coke on Al203. It was found that the total coking rate (metal + support) was structure insensitive to metal d but that the rate of formation of coke on the R was enhanced for particles of high d. The results were explained in terms of a metal support effect that renders the smaller particles electron-deficient. However, the data show little change in the region 0.5 < FE < 1 .O, where the electronic effect might apply. We recall that others (132, 133,288,289) ascribe the sensitivity of coking to geometric effects. Returning to Fig. 15, the sympathetic structure sensitivity of curves 1 and 2 is slightly enhanced for the partly deactivated surface (curve 2), in agreement with the higher resistance to carbon deposition of the smaller particles for this system.

124

MICHEL CHE A N D CARROLL 0. BENNETT

It is quite well accepted that during hydrogenolysis dehydrogenated intermediates are an essential part of the sequence of steps. Such intermediates would form multiple bonds with the metal involved, and a correlation has been found (283) between the activity of a metal for the hydrogenolysis of cyclopentane and the propensity of the metal to form multiple bonds with adsorbed methane or cyclopentane. It has also been found that small particles form fewer multiple bonds, have a lower activity in hydrogenolysis, and poison less rapidly through carbon deposition (288, 289). Thus these studies are in accord with the idea that hydrogenolysis requires an ensemble of surface atoms, resulting in limited antipathetic structure sensitivity, which can be explained by geometric reasoning. We turn next to the important problem of selectivity, illustrated in Fig. 16 by the hydrogenolysis of methylcyclopentane. Mostly six-carbon products are obtained. However, the five-membered ring may open at the bond next to the methyl group, at one bond removed, or at two bonds removed. Considering the symmetry of the reactant and a random breaking of the C-C bonds of the ring, simple reasoning leads to the product distribution 40% n-hexane, 40% 2-methyl pentane, and 20% 3-methyl pentane. This is nonselective hydrogenolysis (280) and in the catalytic reforming of naphthas we would like to suppress the formation of non-

1t - 0 1

oai

aok

0.10

0.40 1.0

0.1

FE FIG.16. Hydrogenolysis of methylcyclopentane. Selectivity and mechanistic effects as a function of fraction exposed FE and particle size 2 (see Table IX for details of the studies).

125

METALS: PARTICLE SIZE A N D CATALYTIC PROPERTIES

TABLE IX Details of the Selectivity Studies Presented in Fig. 16 in the Hydrogenolysis of Methylcyclopentane Curve (Fig. 16)

Symbol (Fig. 16)

1

n

2 3 4

0

0 0

Metal

support

Pt Rh Rh Pt

A1203 Si02 A1203 A1203

Temperature, pressure 220°C 220°C 220"C, 220°C,

Reference

1 atrn I atm I atm 1 atm

290 291 291 290

branched products. For the example of methylcyclopentane, we then would have selective hydrogenolysis if the formation of hexane were suppressed. Figure 16 shows that for Pt, hexane is suppressed by the use of large particle sizes (290). For Rh the selective product distribution is favored, with little effect of particle size (291). More data for the same reaction on Pt have been gathered in Fig. 17 (292). The samples used by Glass1 et al. (292) were prepared so as to

8

*

0

0

0

8 8

0 0

*

.# 8

I lot

A

*

I

0

1

2

3

4 5 6 20 aD mean particle diameter, nm

FIG.17. Percentage of normal hexane in the six-carbon products as a function of mean particle diameter for the hydrogenolysis of methylcyclopentane over supported Pt catalysts (292) (see Table X for details of the studies).

126

MICHEL CHE A N D CARROLL 0. BENNETT

TABLE X Details of the Studies Presented in Fig. 17 on the Hydrogenolysis of Methylcyclopentane Symbol (Fig. 17)

Metal

~~

~~~

0

Pt Pt

A

Pt

0 A

R Pt Pt

*

(I

support

AI2O3,Pt on glass Anodically oxidized AI,O,/Pt AI2O3and none (foil) A1201 A1203 S i 0 2 , AlzOl

Temperature, pressure

Reference

~

237"C, 1 atm 237°C. 1 atm 237"C, I atm 260°C. 1 atm 273"C, 0.13 atm

292 292 292 2w 281 Brandenberger et al."

Brandenberger, S. G . , Callender, W. L., and Meerbott, W. K . , J . Coral. 42,282 (1976).

facilitate the use of electron microscopy to measure metal particle sizes, and the supports used had a considerable influence on their data (filled points). However, all the data of Fig. 17 confirm that hexane formation is suppressed for large particles. In order to discuss the effect of particle size on selectivity we must consider the mechanisms that have been proposed to explain the isomerization of 2-methylpentane to 3-methylpentane and the hydrogenolysis of methylcyclopentane. These matters have been extensively studied (22, 24, 200, 280), and we discuss here only the elementary aspects of the problem. Gault and his group (280) have proposed the following sequence, in which the reactant is 2-methylpentane-2J3C;the labeled carbon is indicated by the black dot.

The nature of the bonding and the extent of dehydrogenation of the intermediates are not known at the moment. We have shown them so that they agree with the explanation proposed by Anderson (22). Path B has been called the bond-shift mechanism; it passes through a 3C intermediate. Path C has been called the cyclic mechanism; it passes through a 5-C intermediate. Obviously the mechanisms cannot ordinarily

METALS: PARTICLE SIZE A N D CATALYTIC PROPERTIES

127

be discerned by their products unless one of the carbon atoms is labeled as shown. Then the products can be distinguished by microwave spectroscopy (280). The results of these experiments show that the 5-C intermediate is favored for d near 1 nm, whereas the 3-C intermediate is favored above 3 nm. Anderson (22) relates these results to increasing abundance of adjacent sites, which should form the 3-C intermediates, on larger particles. Gault (280) has objected to this explanation because the effect on selectivity is confined to a smaller range of d than would be expected from Fig. 3b. However, at the moment this geometric explanation remains plausible. For the hydrogenation of methylcyclopentane, it is reasonable to expect the 5-C intermediate to predominate. Supposedly the ring will be broken at the bond next to the metal, and the steric effects of the methyl group would cause adsorption as shown in path B on the flat faces of large particles. For atoms at edges and corners, orientation of the 5-C intermediate becomes random and the proportion of hexane in the products rises to 40%, as shown in Fig. 17. The preceding explanations are certainly speculative. What is needed, as usual, are data on the orientation and bonding of the intermediates during reaction and their behavior when the composition of the surface is perturbed. Simultaneously, the (changing) state of the metal surface should be determined. The skeletal rearrangements that have been considered in our discussion are all supposed to occur on the metal only, without participation of the support and/or chlorine that might originate in the metallic salt used to prepare the catalyst. This matter was brought up by Dautzenberg and Platteeuw (293), who claimed the apparent structure sensitivity of Pt/ A1203systems really arose from dual-function catalysis. Higher loadings, leading to higher chlorine contents were suggested as the origin of the effects; the use of a pulse technique by Gault’s group was also questioned (29.3). These objections appear to have been rebutted by showing that the particle size effects are not changed by using a different source of Pt (294) or by using a differential reactor rather than a pulse reactor (295). The hydrogenolysis of methylcyclopentane has also been studied by Kramer and Zuegg (296).The catalysts were prepared by the evaporation of Pt and its condensation onto amorphous alumina. Particle sizes (2.5 nm) were measured by electron microscopy. In accord with Fig. 17, the nonselective production of hexane was enhanced for small particles. When additional A1203was then deposited on top of the Pt particles, more hexane was favored. Since the Pt crystallite size was not altered, a mechanism involving catalysis by the phase boundary, Pt-AI203, was proposed (296). However, the decoration of the Pt surface by A1203could

128

MICHEL CHE A N D CARROLL 0. BENNETT

also reduce the size of ensembles available for the selective mechanism, as has been proposed in another context for Rh-TiOZ (129). The (1 1 1) face of Pt for the conversion of hexane has been investigated by Sachtler and Somorjai (297). Covering part of the Pt surface by gold changed the selectivity of the reaction to some extent, but when enough gold was added and subsequently annealed so that a surface alloy was formed, the lack of ensembles had a large effect (isomerization favored versus hydrogenolysis). As implied in the foregoing, geometric, not electronic, effects are invoked to explain the results (297). Foger and Anderson (298) report that for the reaction H2/neopentane, hydrogenolysis is favored for small d, and isomerization is favored for large d. In the range 1 < d < 20 nm, the percent isomerization is about the same as the percent (1 11) faces on cubooctahedrons. The success of the geometric explanation is quite striking. However, the question remains open as to why small particles of Pt favor hydrogenolysis, whereas for most other active metals small particles suppress hydrogenolysis. The effect may be related to secondary structure sensitivity. We note briefly that, over Ir/A1203 (299), the selectivity toward many of the skeletal reactions is not affected by particle size, whereas, for Pt/ A1203, the same reactions (e.g., Figs 16 and 17) are affected by d. Following the reasoning of Barbier and Marecot invoked earlier (267), it would be concluded that the observed structure sensitivity for Pt arises from electronic effects. This conclusion then is at odds with much of our previous discussion, which ascribes observed differences to geometric effects.

D. HYDROGENATION OF CARBON MONOXIDE The structure sensitivity of the methanation reaction is shown in Fig. 18. In general the TOF decreases for increasing FE for all the metals studied. The decrease in TOF for particles smaller than 5 nm is usually explained in terms of the decrease in surface concentration of ensembles of sufficient size to dissociate adsorbed CO, but why this effect should persist out to 100 nm for some metals is little discussed. Curve 1 for nickel shows a maximum at about 4 nm, and curve 2 may also be at a maximum at around 10 nm. Recent data of Richardson and Koveal(300) (not shown in Fig. 18) also exhibit a maximum turnover rate at about 10 nm. They studied Ni/Si02 catalyst in the size range d of about 2.5 to 25 nm, and the Ni particle sizes were measured by magnetic methods. Also reported are results on HZchemisorption (H/M). As d and M, were known from the magnetic measurements, it was shown that H/M,

METALS: PARTICLE SIZE A N D CATALYTIC PROPERTIES

100

25

I

I

d,nm

10 I

25

1.0

0,4

I

I

I

I29

FIG. 18. Turnover frequency TOF versus fraction exposed FE and mean particle size a for the hydrogenation of CO on various metal systems (see Table XI for details of the studies).

decreased from about 0.5 to 0.2 as d increased from 2 to 16 nm. This may be why curve 2 shows a maximum at 4 nm rather than at 10 nm. Even more important is the apparent lack of agreement between the two methods of particle size measurement. Kao et al. (81)also found a maximum TOF for the CO/H2 reaction over Ni/Ti02(100). The Ni particle sizes were not estimated, but the maximum seems to occur for a d of about 1 nm. The variations in this region are explained by XPS results (301), which indicate electron transfer from Ti02to Ni, a metal support effect. Huizinga et al. (302) have not observed any appreciable negative charge on Pt or Rh supported on either A1203 or TiO2.Variations in the XPS spectra are related rather to intrinsic particle size effects (303).

130

MICHEL CHE A N D CARROLL 0. BENNETT

TABLE XI Details of the Studies Presented in Fig. 18 on the Hydrogenation of Carbon Monoxide

Curve (Fig. 18)

Symbol (Fig. 18)

Metal

0

Ni

0

Ni

0

Ni

X

Ru

A

Ru

0

Ru

Solid line

Co

Support Si02

Pd, Pt Ru 8

A

Ru Fe

A1203 Carbon

Temperature, pressure, H21C0 or C02 245"C, 1 atm, Hz/CO = 10 252"C, 0.07 atm, H2/CO = 4 275°C. 1 atm, HZ/CO = 3 25OoC, 4 atm, H2/CO = 2 205"C, 1 atm, Hz/CO = 2 225°C. 10 atm, HJCO = 3 225"C, I atm, H2/CO = 2 275"C, 1 atm, H2/CO = 3 280°C, 0.79 atm, HJCO = 3 280°C. 0.8 atm 275"C, 1 atrn, H2/CO = 3

Comments

Reference" a

Average behavior

b C

d e e Co poorly reduced; average behavior No clear effect of d

g

Structure insensitive

h

Structure insensitive

f

i

j

References: (a) Van Meerten, R. Z. C., Beaumont, A. H. G. M., Van Nisselrooij, P. F. M. T., and Coenen, J. W. E., Surf. Sci. l35,565 (1983);(b) Bartholomew, C. H., Pannell, R. B., and Butler, J., J . Caral. 65,335 (1980); (c) Bhatia, S.,Bakhshi, N. N., and Mathews, J. F., Can. J. Chem. Eng. 56, 575 (1978); (d) King, D. L., J . Card. 51, 386 (1978); (e) Kellner, C. S . , and Bell, A. T., J . Card. 75, 251 (1982);(0 Reuel, R. C., and Bartholomew, C. H., J . Catal. 85,78 (1984);(g) Vannice, M. A., J . Carol. 40, 129 (1975); (h) Elliott, D. J., and Lunsford, J. H., J . Caral. 57, 11 (1979); (i) Dalla Betta, R. A., Piken, A. G., and Shelef, M., J. Catal. 35, 54 (1974); (i)Jung, H. J., Walker, P. L., and Vannice, M. A., J. Catal. 75,416 (1982).

Curves 4-6 for ruthenium all show antipathetic structure sensitivity, but the difference in the magnitudes of TOF is difficult to explain. Curves 4, 7, and 8 all show extensive structure sensitivity, which may imply that a large ensemble of atoms is needed to form a site. Recent results of Rieck and Bell (303b) for methanation on Pd/SiOz lie between curves 5 and 6. Limited antipathetic structure sensitivity seems indicated since there is a plateau of constant TOF for FE C 0.4. A satisfying explanation of the results is presented, based on an estimation of the proportions of (100) and (1 11) faces obtained by measurement of the proportions of linearly and bridged bonded CO on the Pd.

METALS: PARTICLE SIZE A N D CATALYTIC PROPERTIES

131

Jacobs has reviewed the work of his group (304),with particular emphasis on the role played by zeolites in altering the product distribution from its normal form obtained on A1203. For RuY zeolites, antipathetic behavior is found similar to that shown by curve 5 . Guczi et al. (305) also find antipathetic structure sensitivity for the CO/H2reaction over Ru/A1203and Ru/SiOl. Alloying with iron has little effect on TOF but increases selectivity toward olefins. In contrast to some previous reports (128, 306), it has been found (307) that the product carbon number distribution is sensitive to particle size. Osmium catalysts prepared principally from osmium carbonyls, supported on alumina, have also been found to show antipathetic structure sensitivity (308). The results shown in Fig. 18 inspire some confidence that methanation is an antipathetic structure sensitive reaction, even though the explanations of this behavior, as usual, are not very convincing. Structure sensitivity for H*/CO has been reviewed by Boudart and McDonald (308b). E. SYNTHESIS OF AMMONIA The reaction of Hz and N2 to form ammonia is a reaction for which an enormous literature exists, dating to the beginning of the century. Recent reviews are those of Boudart (309) and Ertl (310).Industrial fused iron catalysts present iron particles of 25 nm or higher, and it has not been possible to prepare well-reduced small iron particles (1-10 nm) on conventional silica and alumina supports. Dumesic e? al. ( 3 1 1 ~ - c )were able to prepare iron crystallites of diameters from 1.5 to 30 nm by using MgO as the support. The state of the iron was monitored by Mossbauer-effect spectroscopy. The synthesis rates were measured from 300 to 405°C at atmospheric pressure; the conversion was small enough so that the reverse reaction could be neglected. It was found that the reaction showed antipathetic structure sensitivity. Dumesic et al. ( 3 1 1 ~ - c )deduced that large particles are more active than small ones because their surface has a higher fraction of 7-coordinated Fe atoms (C7) than does the surface of small particles. This explanation is consistent with later results of Spencer et al. (312), who measured rates of ammonia synthesis at 525°C and 20 atm on low-index planes of iron. They find that the relative rates are 418 :25 : I for the ( 1 1 l ) , (loo), and ( 1 10) faces of iron. For the body-centered cubic lattice of iron, only the (1 1 1 ) surface exposes a large concentration of C7 atoms. On this face the TOF is about 18 s-' (312). It is found that the rate of reaction slowly increases with time on stream, especially for the smaller particles ( 3 1 1 ~ - c ) .It is argued (311a-c,

I32

MICHEL C H E A N D CARROLL 0. BENNETT

312) that this change comes about through surface reconstruction, probably by formation of an iron nitride, perhaps Fe4N as postulated by Ertl (310) and his group. These investigators worked at low pressure so that they could not detect ammonia formation, but they did measure rates of dissociative Nz adsorption and found them to occur in the ratio 60: 3 : 1 for the ( I l l ) , (IOO), and (110) faces of iron at 277°C. For ammonia synthesis it is believed that the dissociative adsorption of NZis the rate-determining step (309), and this idea is supported by the above results (22, 311a-c).

F. OXIDATION REACTIONS These reactions tend to be structure sensitive and are often complicated by the strong affinity of oxygen for certain metals. Surface reconstruction and the formation of bulk oxides may occur. 1.

Oxidation of Hydrogen

The influence of the fraction exposed on the turnover frequency for the oxidation of HZon Pt/SiOz catalyst is shown in Fig. 19. Hanson and Boudart (313) found structure-sensitive behavior when the reaction mixture contained excess H2 and structure insensitivity in mixtures with excess 0 2 . This reaction (TOF = 10 s-') is very fast and must be studied at

,

d,nm

2;O

1,O $0

1?5

[l 0.0 2

20

c I

c I

In

ul

Y

0 I-

0.0 1

10

E

".

s

0I

0

0.2

0.4

0.6

0.8

1.0

FE

FIG.19. Turnover frequency TOF versus fraction exposed FE and mean particle size for the H2 + O2 reaction on Pt/SiO2 systems (see Table XI1 for details of the studies).

METALS: PARTICLE SIZE AND CATALYTIC PROPERTIES

133

TABLE XI1 Details of the Studies Presented in Fig. 19 on the Oxidation of Hydrogen Curve

Temperature, pressure

Metal

Support

1

Pt

SO2

25°C 1 atm, H 2 / 0 2 = 3

2

Pt

Si02

2YC, 1 atm, H 2 / 0 2 = 0.14

Pt Pt

Si02 Si02

O’C, 1 atm, H2/02 = 10 O’C, I atm, HJ02 = 0.16

Comments

Reference

Experimental points not shown Experimental points not shown r = kSR ( 0 2 ) r = kSR ( 0 2 )

314

~~~

A

3 4

0

314 313 313

room temperature or below in order to avoid the intrusion of mass-transfer effects. Marshneva et al. (314) studied the same system later and found structure insensitivity for an excess of either reactant. They explain this difference from the previous work (313) by the fact that they used a pretreatment in oxygen-helium at 573 K before reaction, whereas Hanson and Boudart exposed their catalyst to oxygen only at ambient temperature. Since this reacting system may exhibit multiple steady states, it is probable that a more complete kinetic study of the system would be of interest. The lack of structure sensitivity in excess O2 is explained (313) by the suggestion that the corrosive adsorption of oxygen leads to a surface that is defined by the presence of an oxygen surface layer and not by the underlying crystal structure of platinum. In excess hydrogen, when the rate is first-order in oxygen concentration, it may be presumed that the dissociative adsorption of oxygen is favored by the Pt atoms of low coordination present on the smaller particles, even though two or more adjacent atoms may be considered necessary for this process. In order to reason about these matters with more assurance, the sequence of steps (detailed kinetics) needs to be measured as a function of d. We note that during CO oxidation the surface structure of Pt has been measured by low-energy electron diffraction during reaction, and the surfaces does reconstruct in passing from a CO-rich to an oxygen-rich surface, and vice versa (315). 2.

Oxidation of Carbon Monoxide

The results of a number of studies on the variation of TOF with FE for the CO oxidation reaction are shown in Fig. 20. Herskowitz et al. (316) used for their work the thoroughly studied Pt/SiOz catalysts prepared and

134

MlCHEL C H E A N D CARROLL 0. B E N N E T T

I

0.01

0.04 0.10

FE

0.4

1.0

a

FIG.20. Turnover frequency TOF versus fraction exposed FE and mean particle size for the oxidation of carbon monoxide on various metal systems (see Table XI11 for details of the studies).

characterized at Northwestern University (317). No less than 265 data points were measured for a range of temperatures (380-450 K), partial pressures, and four catalysts (FE = 0.062,0.40,0.63,0.81).It was practical to collect so much data because the apparatus was completely computer controlled. No deactivation of the catalyst was observed. The reaction was found to be slightly structure sensitive; TOF decreased as the fraction exposed increased. Somewhat earlier, Cant (318) had studied a similar system and found essentially structure-insensitive behavior, although his catalysts deactivated with time on stream. McCarthy et al. (319) have studied the structure sensitivity of the C0/02 reaction as the fraction of CO is increased in 02-rich reaction mixtures. When CO is present at a concentration exceeding 1%, TOF remains about constant as the particle size varies. As the CO concentration becomes very small, the structure sensitivity is that shown by curve 3 of Fig. 20. As is well known, for a given particle size the rates as a function of CO concentration increases to a maximum (at 0.3% CO) and then decreases to an almost constant level for CO concentration above

135

METALS: PARTICLE SIZE A N D CATALYTIC PROPERTIES

TABLE XI11 Details of (he Studies Presented in Fig. 20 on the Oxidation of Carbon Monoxide

Curve (Fig. 20)

Symbol (Fig. 20)

Metal

I

0

Pt

Si02

2

X

Pt

Si02

3

8

Pt

A1203

4

0

Pd

A1203

5

A

Pd

A1203

6

0

Ir

Si02

Support

Temperature, pressure 164°C. 1 atm, COIO2 = 2 177°C. I atm. C 0 / 0 2 = 2.5 T = ?, 1 atm, co/oz = 0.0002

Comments

Reference 316

318

CO/Oz = 0.01-0.02 TOF = 40 s - I (structure insensitive)

p = 2.5 x lo-‘ Pa, CO/Oz = 1.0, T = 172°C p = 2.5 x Pa, CO/O2 = 1.0, T = 245°C 177°C. I atm. co/o*= 2

319

32I

32I

Cant el a/.“

Cant, N. W., Hicks, P. C . , and Lennon, B. S., J . Catal. 54, 372 (1978).

1.5% (319). The authors propose that the Pt surface is increasingly deactivated by platinum oxide as the crystallite size is reduced. At higher CO concentrations the “low-rate’’ region is encountered, and crystals of all sizes are supposed to be covered almost completely by CO. Recent work (142) has confirmed that the formation of platinum oxide at 220°C does occur on small particles of FE = 0.6. For the H2/02 reaction it was found that structure insensitivity occurs for low H2/02 ratios (313), whereas for the C0/02reaction it occurs (319) for relatively high C0/02 ratios. However, CO competes with 02 for surface sites more strongly than H2. We recall the already-mentioned observed surface reconstruction of Pt as the CO/O2ratio on the surface changes (315). This behavior, as well as the related formation of bulk and surface Pt02, is connected to observed multiple steady states and isothermal oscillations (320). Ladas et al. (321) have studied the oxidation of carbon monoxide over model Pd/A1203 catalysts formed by vacuum deposition of Pd on an aAlz03single-crystal support. Little structure sensitivity is found (curves 4 and 5 of Fig. 20), and the rates for low FE agree with those reported for Pd

136

MICHEL CHE A N D CARROLL 0. BENNETT

(111) by Engel and Ertl (322).It has been suggested that these relatively high TOF observed at 2.5 x Pa are influenced by the collision rate calculated from the kinetic theory of gases. This idea has been carried further (321, 323) to explain the rise in curve 5 as d decreases. The collision rate on small three-dimensionalparticles is proposed to be higher than that on a flat surface. Curve 6 of Fig. 20 is for Ir/SiOz, and it has the same general shape as curve 3. This similarity recalls the arguments of Barbier and Marecot (267),applied to ethane hydrogenolysis over these two metals; from their point of view, the effect of d for a given metal would arise through changes in geometric, not electronic, effects.

3.

Total Oxidation of Propylene

This oxidation has been studied by Carballo and Wolf (324) over PtlyA1203crystallites of 1.1, 6.0, and 14.4 nm and 100-130°C. The results for rate as a function of propylene concentration, which was varied from 0 to 2% in oxygen, produce curves of shapes similar to those for CO oxidation (319).However, the rate is structure-sensitive for all the C3H6concentrations studied, and TOF increases with increasing d , as found for most CO systems in Fig. 20. 4. Partial Oxidation of Ethylene

The production of ethylene oxide from ethylene over silver is an important industrial catalytic process, and its success depends on achieving a high selectivity for C2H40 rather than for C02, the principal by-product. Figure 21 shows the effect of silver particle size on both TOF and selectivity defined as (ethylene reacted to C2H40)/(totalethylene reacted). The results of industrial experience influenced the catalyst type and reaction conditions chosen by Verykios et al. ( 3 2 3 , in particular. Thus, these authors used low surface area a-A1203supports, with areas in the range 0.63-3.03 m2/g and silver loadings from 1 to 14 wt%. The resulting catalysts exhibit silver particle sizes mostly in the range 30-170 nm. Four different aluminas were used, with BET areas 0.20, 0.63, 0.97, and 3.03 m2/g,and for each support the silver particle size was varied by sintering. The other studies shown on Fig. 20 were done on silver on relatively higharea supports, such as Cab-0-Sil. Each group found structure-sensitive behavior, although the variation of TOF in any one study is much less than the variations among the groups. In addition, for a given silver particle size, Verykios et al. (325)found differences among the four supported catalysts that could be correlated as a function of the surface area of the

METALS: PARTICLE SIZE A N D CATALYTIC PROPERTIES

l.ol,D3

d,nm 2,0

Z?O l ? O 5,O

1,O 5;0

2;O

137

I

lo-l

1.0

.-+> .c

U

al al

m

0.5

1

0.00 1

0

ao 1

-

0.1

3 0

I

FE

FIG. 21. Turnover frequency TOF and selectivity (fraction of ethylene reacted that forms ethylene oxide) for the partial oxidation of ethylene over supported silver catalysts, presented as a function of fraction exposed FE and mean particle size d (see Table XIV for details of the studies).

TABLE XIV Details of the Studies Presented in Fig. 21 on the Partial Oxidation of Ethylene Curve (Fig. 21)

Symbol (Fig. 21)

support

Temperature, pressure

I

A

a-A1203,0.63 m2/g

240°C. I5 atm

2

A

a-A1203,0.63 m2/g

240"C, I5 atm

3

0

S O2 , 172 m2/g

220°C. 1.32 atm

4

0

S O2 , 172 mVg

220°C, 1.32 atm

5

Q

Si02, 300 m2/g

200"C, 1 atm

6

0

Si02, 300 mZ/g

200"C, I atm

Partial pressures

C2H4, 0.45 atm; 02, 2.25 atm CzH4, 0.45 atm; 02. 2.25 atm CZH4, 0.26 atm; 02, 0.26 atm C2H4, 0.26 atm; 02, 0.26 atm C2H4, 0.21 atm; 02, 0. I26 atm C2H4, 0.21 atm; 0 2 , 0. I26 atm

Reference 325 325 141 141

326 326

138

MICHEL CHE A N D CARROLL 0.BENNETT

Curves 1 and 2 are for just one of the four supports studied (325). Another support (0.97 m2/g)shows the TOF going through a minimum at about 70 nm, and there is some evidence that the selectivity shows a maximum at a similar fraction exposed. Note that Jarjoui er al. (326) show a maximum in TOF at about 20 nm; the data of Cheng and Clearfield (327) (not shown on Fig. 21) for silver on zirconium phosphate also exhibit a maximum at about 20 nm, but the TOF is about lo-) s-I, two orders of magnitude smaller than that of Jarjoui et al. (326). It is interesting that for the epoxidation of ethylene over silver it is the single-crystal surface that gives the highest TOF. At 217"C, Campbell (328,329) has found that TOF = 2 s-I for the Ag (1 10) surface and about 1 s-I for the Ag (1 1 I) surface (330). The activity and selectivity of the single-crystal surfaces is modified by chlorine (330) and by cesium (331), for example, in much the same way as are those of supported silver catalysts. These data lead us to the conclusion that the usual explanations for structure sensitivity, which are valid in particular for particles of less than 5 nm, do not apply for these silver systems. The great sensitivity of the catalyst to the nature of the support, chlorine, and alkaline earth elements makes it clear that the chemical state of silver is greatly varied in these different environments. The morphology of the silver particles has been invoked as an explanation of its unusual behavior (325, 327), but the data are not convincing. Studies on model catalysts by modern methods of electron microscopy and microdiffraction would be of value for this particular system. The extreme sensitivity of silver to its environment is to be contrasted with the essentially insensitive behavior of platinum for the hydrogenation of ethylene. For this case, the TOF varies little among single-crystal planes, small particles on various supports and polycrystalline films and wires. a-AI203 used.

G. STRUCTURE SENSITIVITY OF CHEMISORPTION The disproportionation of CO has been measured as a function of particle size for a number of supported metals: Ni/mica (332, 333), Pd/mica (332), Pd/SiO2 (102), and Fe/C (334).For all these systems the rate of CO desorption decreases with increasing d, and also the rate of carbon deposition decreases with increasing d. This sympathetic structure sensitivity extends over the range of d from 1 to about 7 nm. We recall from Fig. 18 that over Ni and Ru on several supports the methanation reaction shows extended antipathetic structure sensitivity. However, Pd is not such a

METALS: PARTICLE SIZE A N D CATALYTIC PROPERTIES

139

good catalyst as Ni for CO dissociation and CH4 formation. Ichikawa et al. (102)found that the TOF for methane formation was the highest for the

smallest particles of Pd on silica (sympathetic structure sensitivity). No methanol is formed at 262"C, 5.3 atm, H2/CO = 6, but as d is increased, considerable methanol appears in the product when d reaches 2.5 nm (102).On supported Ni and Ru, although carbon is an active intermediate, so much is formed and partially converted to graphite on smaller particles that the CO/H2reaction is inhibited. On Pd, only on the small particles is carbon formed fast enough to lead to methane. The structure sensitivity for carbon deposition of different crystal planes of Ni has been measured in an interesting way by Caracciolo and Schmidt (335, 336). In a UHV apparatus they expose to CO or C2H4 a clean curved surface cut from a single crystal so that (loo), (1 1 I ) , (1 lo), and higher-indexed zones are exposed in different areas. The carbon deposition is then measured by Auger electron spectroscopy, which gives a measure of C/Ni near the surface. At about 324"C, the carbon formation is a maximum on the (1 11) planes from C2H4 and a minimum for the same orientation from CO. The orientation specificity is much higher for C2H4 than for CO. In a later paper Lee et al. have confirmed the antipathic structure sensitivity of methanation over Ni/SiO2 (337).They associate this result with the previous finding (335,336) that the high-index planes of Ni tend to accumulate carbon that can transform to graphite, a poison for the CO/H2 reaction. Similar results have been found by Doering et al. (338) for Pd/mica. On Pt/mica, there is no CO dissociation (339). Herz and McCready (340) have measured temperature-programmed desorption profiles for CO over Pt/A1203, and they find that at FE = 1.0 there are an intense narrow peak at 82°C and a broad low peak extending out to about 480°C. As FE is reduced, however, the first peak diminishes, and below FE = 0.4, only the broad desorption peak remains, as typically observed for this system by Foger and Anderson (341). Thus small particles seem to be responsible for the unusual weakly bonded CO. Doering et al. (339) have studied CO chemisorption at 60°C over Pt/ mica catalysts prepared by vapodeposition. For discontinuous films of d = 1.6 nm, two peaks were found by TPD, one at 120°C and one at 175°C. For larger particles the peak at 175°C disappears, leading to the conclusion that the more strongly adsorbed CO is held on the small particles. It is not clear whether the contradiction between these results (339) and those discussed above (340, 341) is due to the different supports. Ladas et al. (321) have measured TPD curves for CO adsorbed on Pd/a-AI2O3 single crystal at 37°C. The small particles, 1.5 < d < 8 nm, have been prepared by vapodeposition and characterized by electron mi-

140

MICHEL C H E A N D CARROLL 0. BENNETT

croscopy. For 1.5-nm particles, the higher of the two observed unresolved peaks is at about 100°C and the lower one at about 180°C. For the 4.9-nm particles, the weakly bound CO (I00"C) has shrunk so that it is much smaller than the higher-temperature peak, which has been little affected by particle size. These results are quite similar to those of Herz and McCready (340)for Pt on A1203powder. Recalling the results already cited for CO decomposition, it seems that the weakly bound CO, adsorbed on edges and corners, may lead more readily to C formation than the CO bound to the higher coordinated metal atoms that are relatively more abundant on large particles. Ponec and associates (342, 343) have measured the IR spectra of CO adsorbed on Pt, Ir, and Cu supported on silica and/or alumina. When the results are properly normalized to the same coverage, it is found that for Pt and Ir the C-0 vibration frequency increases with particle size. Difficulties in interpretation because of island formation and changes in dipole-dipole coupling related to surface coverage are avoided by observing the spectra of suitable mixtures of T O and I3CO. Iridium films have also been studied (344).The results can be interpreted as meaning that CO is bound more strongly to the smaller (rougher) particles for Ir and Pt. Altman and Gorte (1316) have reported that there is no change in the position of the TPD peaks for CO on Pt/AhO3 as the particle size is varied. The Pt particles were prepared by vapodeposition. As the FE increases, the ratio of the height of the higher-temperature peak to that of the lower-temperature peak increases. This supports the view that CO is more strongly bound to small particles. Since the peak positions do not change, it is argued that the variation is caused by a change in the proportion of crystal faces exposed as FE changes. For iron, Topsoe et al. (345) find that the amount of weakly bound CO on Fe/MgO increases as FE increases. Recall that this change in FE leads to lower TOF (antipathetic behavior). VI. Possible Explanations of Particle Size Effects: Experiments versus Models

A.

INTRODUCTION

A coherent organization of the welter of data presented in the graphs of this article is no easy task. However, the attempt must be made, knowing that future results will challenge some of the ideas set forth. The generally accepted behavior of the reaction classes is maintained; hydrogenations are less structure-sensitive than are most other reactions. For any one

METALS: PARTICLE SIZE A N D CATALYTIC PROPERTIES

141

study, TOF varies at most by about two orders of magnitude (H2/C0 reactions). Among the systems presented in Figs. 9-12, only a few (see Table XV) show structure insensitivity, i.e., no variation of TOF with FE within the apparent precision of the data. This definition is narrower than that used by Boudart (346d). We think that the experimental methods now in use permit significance to be attached to changes of TOF of much less than an order of magnitude. Of course, reference is to the relative values (shapes of curves) for a given study. In what follows we shall emphasize certain ideas that arise from our study of the literature: 1. Studies on both supported small metal particles and naked metal clusters indicate that the TOF tends to zero as the number of atoms per particle tends to unity. This amounts to saying that, in the limit of the smallest particles, all systems show antipathetic structure sensitivity. We have used “particles” as a general term. They may be crystallites, but as the number of atoms per particle decreases their organization departs in general from that of a macroscopic crystal. These small particles (<1000 atoms; d = 2.5 nm) are called clusters in the literature on naked metal particles (347).However, organometallic chemists may reserve the term cluster to describe a structure of metal atoms at least partly terminated by various ligands. 2. Some reactions require an ensemble of atoms located on the principal crystal faces of particles, and this leads also to antipathetic struc-

TABLE XV Some Systems Showing Structure Insensirivity

Reaction type Hydrogenation C6H6 + H2 C6Hn f H2 cd%f H2 C6H6 + H2 CO + H2 I-CdHs + H2 Oxidations H2 + 0 2 H2 + 0 2

co + 0 2 co + 0 2

Catalyst

Reference

Year

Graphical representation

Pt/AI,O, Pd/MgO or A1203 Pd/Si02 Pd, Ir/AI2O3 R~/A1203 Pd/A1203

136 346a 346b 267 346c 265

1975 I980 I979 1981 1974 1983

Fig. 13, curve 5 Fig. 13, curve 9

Pt/Si02 Pt/Si02 Pd/A1203 Pt/AI,O,

314 313 321 319

1982 1978 1981 1975

Fig. 19, curve 2 Fig. 19, curve 4 Fig. 20, curve 4

Fig. 12

142

MICHEL C H E A N D CARROLL 0. BENNETT

ture sensitivity. The larger the ensemble required, the lower the FE to which sensitivity persists. Although all systems should approach structure insensitivity as FE tends to zero, some puzzling exceptions exist. 3. Systems often show a maximum in the curve of TOF versus FE. The following two reasons will be considered: (1) special multiatom sites, such as Bs sites, are required, and these sites exist only at crystallite edges-thus they are sparse for large particles, increase, and then disappear as the cluster size goes to zero; and (2) especially coordinated atoms, at edges, corners, and kinks are required, and this induces sympathetic structure sensitivity. However, as the number of atoms per particle gets small enough, antipathetic behavior takes over. 4. Certain systems show structure insensitivity over a region of surface states for which it is claimed that coverage of the rate-determining intermediate is high or for which the reaction takes place on an overlayer. We now turn to a more detailed discussion of these matters.

B. METALPARTICLES WITH FE 1.

ABOVE ABOUT

0.5

Unsupported Metal Clusters

For the more volatile (alkali) metals it has been possible since the late < n < 100, where n is the number of atoms per cluster (348,349). Metal vapor is produced in an oven and then cooled by expansion. The clusters are separated in a timeof-flight mass spectrometer (TOFMS). Later it became possible to handle less volatile (transition) metals through vaporization by a laser beam (347, 350-352). For the mass analysis the neutral clusters are photoionized by a suitable UV laser. a . Size Distribution. As the clusters form in the supersonic expansion nozzle, aggregates of a certain size distribution are formed. This distribution is by no means monotonic. A striking example is formed by carbon clusters, as reported by Rohlfing et al. (353). Figure 22 shows the remarkable power of the laser generation technique combined with photoionization time-of-flight mass spectrometry. For C,+(1 < n C 30), a complicated distribution results, with every value of n represented. Then for larger clusters, only even-numbered entities are found. Any understanding by a theoretical calculation is made especially difficult because of the important role that must be played by the kinetics of the cluster formation. Results such as those of Fig. 22 exist for many other systems; experimental and theoretical study in this field is a subject of intense activity. For more details the reader is referred to reviews by Whetten et al. (347) and Burch (23). 1970s to create a beam of cold, naked clusters of I

143

METALS: PARTICLE SIZE A N D CATALYTIC PROPERTIES

11

0

x10

15

1

1s

1

0

70

I

20

1

40

I

I

1

60

1

80

I

I

100

1

1

Cluster Size (Atoms)

FIG.22. Size distribution of carbon clusters obtained by laser vaporization and measurement by photoionization time-of-flight mass spectroscopy. From Ref. 353.

b. Electronic Properties. As the photoionization energy directed at a beam of clusters entering the photoionization time-of-flight mass spectrometer is increased, a threshold is reached above which ions are generated and detected. For iron, a nonmonotonic variation is found, ranging from about 6.25 eV for Fez to about 5.10 eV for Fe25. These values are below the ionization potential of Fe (7.87 eV) and generally decrease, as n increases, toward the value of the work function, 4.80 eV (347). The oscillations observed between Fez and Fezo are not explained by any model at this time. Although there is considerable disagreement among the detailed results predicted by the Hartree-Fock, the SCF-X,-SW, and the extended Huckel methods, theory in general agrees with experiment in limiting electronic effects to particles smaller than a few hundred atoms (FE 0.5) (23).

144

MICHEL C H E A N D CARROLL 0. B E N N E T T

c . Morphology. In Section III,B,2,a and the accompanying figures we have mentioned the possibility that small clusters may favor certain pentagonal structures such as icosahedrons. Many of the contrasting theoretical predictions concerning structure are reviewed by Burch (23). However, as proposed some time ago by Hoare and Pal (103), and recently emphasized by Simonetta (354),the energy differences among various competing structures are predicted to be less than 0.1 eV for Pt,, and for Ptlzor Pt13,the configurations were calculated to be almost degenerate. Thus when such a cluster interacts with an adsorbate, it is quite probable that its configuration changes. These ideas are related to the concept that the melting point is greatly depressed for small clusters (23, 347). The surface atoms at the usual temperatures for catalysis are probably mobile and ready to accommodate to adsorbing gases. From the lability of the surface and from the definite presence of electronic effects for clusters of n < 100, it seems logical to attribute structure sensitivity in this size range to electronic factors. d . Chemical Properties. For the moment, the data on the reactivity of bare metal clusters are limited to rates of chemisorption. For example, H2 is added to the helium gas, which carries the iron clusters just before quenching by expansion into vacuum with formation of the molecular beam (17).Time of contact can be varied by adding reactant at various rates, leading to Fig. 23. The rate constant represents a first-order decay in the signal for the appropriate bare cluster as the flow rate of H2 injected is increased. Thus Fig. 23 shows the reactivity of each cluster size, independent of the abundance of that cluster. A model to explain the enormous difference in reactivity shown in Fig. 23 is not available. However, the ionization potential (IP) results for these clusters have been discussed in a previous section (347).It is found that clusters with a high IP have low reactivities and vice versa (17). The donation of electrons from the metal to the u* antibonding orbital of H2 is invoked to explain the observations. In short, the electronic factor is apparently important, and we have seen that the probable lability of the geometric arrangement does not support an important role for that effect. Finally, the overall trend is toward antipathetic structure sensitivity; iron atoms (n = l), which have the highest IP, are not active (17). Of course, for supported small particles the size distribution would not ordinarily permit the observation of the detailed oscillations of Fig. 23. Other results for HZchemisorption on bare cobalt and niobium clusters have been reported by Geusic et al. (16), and the reaction with 0 2 and H2S has been studied by Whetten et al. (3551).For the latter, the oscillations as the reactivity increases with n are much smaller than those of Fig. 23. Fayet et ul. (3556) have deposited size-selected ions containing one to nine atoms of Ag onto binder-free AgBr microcrystals. Particles of three

METALS: PARTICLE SIZE A N D CATALYTIC PROPERTIES

145

1000:

100:

10:

1-

+ l . 0

n FIG.23. Reaction rate constants for the adsorption of hydrogen on iron clusters of size n (number of atoms). From Ref. f7.

or less atoms of silver did not catalyze the development (reduction) of AgBr, whereas particles of four or more atoms catalyzed the process effectively. In general, it appears that the results on clusters indicate that their intrinsic behavior is antipathetic. It is probably best explained by electronic factors. 2 . Supported Metal Clusters

We consider now highly divided (FE > 0.5) supported metals, which may be model or actual working catalysts. Clusters supported by frozen inert gas matrices are not discussed here. Possible interactions with the support now complicate the interpretation of experiments, but we shall see that for silica and alumina the behavior of supported and unsupported clusters seem to be similar. a . Electronic Effects. We note that in Fig. 2 the variation of binding energy with FE is mostly limited to the region FE > 0.5. A similar behavior is found for other XPS data reported in the references on which Fig. 2 is based. For example, Cheung (356, 357) has observed the lineshape assymmetry and the linewidth broadening as well as the shift in core-level XPS peaks. All these effects die out as FE falls below about 0.5. We

146

MICHEL CHE A N D CARROLL 0. BENNETT

have explained the shifts in Fig. 2 on the basis of an intrinsic particle size effect rather than as evidence of electron exchange with the support, supposedly absent for supports such as SiOz and A1203 and for low pretreatment temperatures. For all but the smallest clusters (n < lo), it is not likely that electron transfer per atom of metal would be sufficient to explain the results. Although it is not clear how the XPS data are related to catalytic activity, the observation is that electronic effects are limited to the systems for which FE is greater than 0.5. 6 . Structure Sensitivity Results. It is interesting to study the behavior of TOF as FE approaches 1.0, as illustrated by our graphs. In general, TOF is decreasing in this region. This is true for Figs. 9, 1 1 , 12, and 18, for all curves. In Fig. 13, TOF decreases as FE tends to 1.0 for five of the curves that extend to this region, but not for four others, two of which exhibit structure insensitivity. In Fig. 14, TOF decreases for six curves and rises for two, one of which is for Rh in the SMSI state. In Fig. 15, all but one of the pertinent curves decreases as FE tends to 1.0. In Figs. 19 and 20, about half of the curves decrease, and in Fig. 21 there are no data for FE > 0.5. The tendency toward decreased TOF as FE goes to 1.0 is least pronounced for the hydrogenations of Figs. 10 and 13. Sometimes the results of two investigators on the same metal-support system show the opposite trends. Much careful experimental work remains to be done to rationalize this situation. However, it does seem that for supported clusters, for almost all reactions, TOF decreases as FE tends to unity. c. Morphology and Support Effects. Supported clusters in some cases are altered by interaction with the support, and this has an effect on their shapes. For example, as FE tends to 1.0, Rh is known to form twodimensional rafts rather than three-dimensionalforms ( 1 3 5 ~ )The . geometry of small metal particles can be greatly influenced by adsorbed species also (134). This is consistent with the lability of bare clusters which we have already discussed. Raft-like structures may expose a higher proportion of higher coordinated atoms than a small polyhedron with the same number of surface atoms. In addition, more atoms could be close to the support, leading to an alteration of their electronic states. These influences change from one metal-support system to another, so that the precise behavior as FE tends to 1.0 is different for each system. C. SYSTEMS EXHIBITING ANTIPATHETIC BEHAVIOR OVER A WIDERANGEOF FE 1. Review of Ensemble Theory

If a reaction requires a site consisting of several atoms on the face of a crystallite, then we have seen that antipathetic behavior is to be expected

METALS: PARTICLE SIZE A N D CATALYTIC PROPERTIES

I47

over all FE. As FE tends to zero, the TOF should approach that corresponding to the average of the rates on the crystal faces exposed. These latter are measurable from single-crystal studies, assuming no important support effects. As FE tends to unity, the ensembles disappear. Thus above an FE of about 0.5, both the ensemble and the electronic effects reinforce the antipathetic behavior. If the ensembles involved are not on the faces but are associated with edges (Bs sites), then a different behavior is to be expected. There is a maximum in the curve of TOF versus FE, as will be discussed.

2. Effect of Surface Coverage As already mentioned, the effect of structure of the metal surface on kinetics depends on the details of those kinetics, for instance, the nature of the rate-determining step. Martin et al. (358a) have considered the situation in which the formation of an adsorbed species determines the rate, and that this process is governed by the ensemble effect. For a given ensemble size X,the rate will be greatly slowed down as the fraction of unoccupied sites decreases. Based on the statistics of Miyazaki and Yasumori (358b),equations are developed from which it is argued ( 3 5 8 ~that ) for low-surface coverage, with little accumulation of intermediate, the crystallite size will have little effect, and the TOF will not decrease much below what it would be for an infinite crystal. Figure 24 shows the effect of particle size ( 3 5 8 ~for ) a large ensemble, X = 16. The probability of reaction is roughly proportional to ( 1 - W ,so that the larger the ensemble, the more striking is the effect of the surface coverage 8. As the surface coverage increases, the figure predicts that the structure sensitivity becomes more pronounced. In a review of the ensemble model, with particular emphasis on the work of his group, Martin ( 3 5 8 ~has ) proposed that a large ensemble size may be necessary to explain experimental results. For the example of ethane hydrogenolysis on supported nickel, an ensemble of X = 12 atoms is proposed. The kinetic data fit a model based on rate control by adsorption of ethane, with surface H playing the role of blocking (“poisoning”) individual atoms of surface nickel. Because the surface is mostly covered by H, the rate is expected to be proportional to (1 and indeed X is found to be 12 ( 3 5 8 ~ )In. addition, magnetism adsorption measurements lead to a nuclearity for ethane adsorption of Ni of 12 ( 3 5 8 ~ )Finally, . kinetic results on Cu-Ni alloys, assuming that Cu acts merely as a diluent of the surface nickel, also lead to an ensemble size of 12 atoms ( 3 5 8 ~ ) . A more detailed model of the poisoning of ensembles on metal surfaces has been published by Andersen et al. (3584 and Alstrup and Andersen (358e). For a given crystal face, in addition to the number of atoms in an

148

MICHEL CHE A N D CARROLL 0. BENNETT

FIG.24. Reaction rate as a function of surface coverage 0 and particle size 2 for an intermediate ensemble of 16 atoms. PIP, is the r$io of the probability per unit surface area of finding a ldatom site on a particle of a given d to that on an infinite surface. From Ref. 358.

ensemble X and the number of atoms on the face (particle size), good results require the consideration of the following points: 1 . The detailed structure of the crystal face (square, hexagonal, etc.) and the shape as well as the size of the ensembles influence the reaction probability. 2. The location of the potential sites of poison adsorption or, for ethane hydrogenolysis, hydrogen adsorption, must be specified. For single crystals, such information may be available from LEED studies. The poisoning entities may occupy a sublattice relative to the metal atoms. For instance, H may be adsorbed on the hollow sites centered among 4 atoms in a square lattice. 3. The action of the passivating entity may be limited to the place where it is located, so that metal atoms are simply blocked. Copper diluting the nickel surface would have such an effect. However, the poisoning action may extend to neighboring atoms within a certain distance of R interatomic units. For alloying, R is zero, but for poisoning by sulfur, R is probably greater than zero (358d,e).

In view of the above ideas, the ethane hydrogenolysis might be modeled by X = 12 and R = 0 or, for example, by X = 4, square lattice, and

METALS: PARTICLE SIZE A N D CATALYTIC PROPERTIES

149

R = 1. C2H6landing on the surface may need to form many bonds to accommodate the C and H produced, as observed by the magnetic measurements. However, the size of the ensemble needed to accept the incoming molecule may be much smaller. The H atoms, for example, probably diffuse rapidly away from the initial landing place to become adsorbed on farther individual metal atoms with no particular geometric requirement. Those metal atoms have no other role than to park the H atoms and this can be viewed as the “parking lot effect” (358f). Andersen et al. (358d)approach the problem from the solid side, invoking a sort of ligand effect from a poisoned metal atom to a neighboring one (R > 0). On the other hand, Frennet et al. (359) picture a hydrocarbon molecule, because of its size, as deactivating a number of sites around the small number of atoms to which it is actually chemically bonded. This interesting paper also has data on the effect of multiatom sites on the CH4/D2exchange on rhodium. The results are discussed in terms of ensembles. The trends of the effect of coverage and particle size on reaction probability of Fig. 24 have been challenged by Alstrup and Andersen (358e). According to their model, deactivating entities near edges and corners are less effective in eliminating ensembles than those far from the boundaries. Thus in their view, at equal coverages of poison, large particles would be deactivated more than small ones. This is a possible explanation of the observed secondary sympathetic structure sensitivity of systems that might be expected to show antipathetic structure sensitivity because ensembles are involved (132, 362, 284). Boudart (346d) argues that at high-surface coverage the reaction rate (decomposition of intermediate) becomes governed by lateral interactions and that the structure of the underlying surface becomes irrelevant. However, it can be argued that as particle size is reduced, the rate of intermediate formation (adsorption) may be reduced, as is the case for FE > 0.5 as discussed previously. This change would result in the reduction of the surface coverage as FE decreases. This discussion clearly shows the importance of measuring the kinetic details along with structure sensitivity. 3 . Some Experimental Examples

a . Hydrogenation of Carbon Monoxide. Figure 18 and other data show that almost always the H2-CO reaction presents antipathetic structure sensitivity. This behavior agrees with the idea that several atoms in a face are required for the dissociation of CO. The effect may be enhanced if the active surface is decorated with inactive carbon deposits, and this

150

MICHEL C H E A N D CARROLL 0. B E N N E T T

may be an explanation of the extended structure sensitivity observed for this reaction. b. Hydrogenolysis. The hydrogenolysis of ethane and other small hydrocarbons is often cited as requiring multiatom sites. However, a glance at Fig. 14 and Table XVI shows that most of the curves pass through a maximum, and this behavior requires a different explanation, which will be considered in the next section. c. Ammonia Synthesis. We have seen that the Hz/N2 reaction over iron catalysts shows a strong antipathetic structure sensitivity (3ZZa-c, 345). Measurements on the single-crystal faces of iron have also been made, and they show that the (111) face is much more active than the others (312). Thus the explanation offered for the antipathetic behavior of the particles is that the fraction of the surface exposed as (111) faces increases as the crystal size increases ( 3 1 1 ~ - c ,345). However, the morphology of the particles was not actually measured. Falicov and Somorjai (360),in agreement with Dumesic et al. (311a-c), propose that it is the abundance of C7atoms on the open (1 11) face that TABLE XVI Some Sysrems Showing a Maximum in TOF versus FE

Reaction type

Catalyst Pt/Si02 Pt/AI,O, Ni/Si02 Ni1SiO2 Ni/Si02 Pt/A1203 Rh/AI203 Rh/Si02 Ru/Si02 Ru/Si02 Pdlmica Rh/Si02 Ni/Si02 Ni/Si02 Ni/AI2O3 Ni/A1203 Ni1SiO2

FE at maximum TOF I .O (0.6 nm) 1.O (0.6 nm) 0.9 0.9

0.2 0.5 0.3 0.6 0.4 0.3 0.8 0.9 0.4 0.3 0.3 0.4 0.5

Reference

Year

79 78 59 112 361b 97 361c 361d 108 108 107

1984 1986 1979 1972 1982 1972 1980 1981 I986 1986 1984

I3 361e 2 75 110 109

1%7 1979 1978 1986 I984 1986

Ill

Graphical representation Fig. 9 Fig. Fig. Fig. Fig. Fig.

10, curve 2 13, curve I 13, curve 2 13, curve 4 13, curve 10

Fig. 14, curve I Fig. 14, curve 4 Fig. 14, curve 6

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151

favors the reaction. The former (360) suggest that highly coordinated but exposed atoms in the second layer show a particular activity because of increased low-energy electronic fluctuations. This idea is also extended to the evaluation of the activity of atoms at steps. Rather than the lowcoordinated atoms at the apex of a step, it is those at the inside of the step that are particularly active (360). Data on H2-Dz exchange seem to support this concept ( 3 6 1 ~ ) . We may note that if the above ideas are generalized to small particles, which expose mostly low-coordinated atoms, the TOF would logically decrease as FE increases, as is usually observed. However, the electronic environment of an atom in a cluster is quite different from that of an atom at a step or kink on a large crystal. D. SYSTEMS SHOWING A MAXIMUM I N TOF VERSUS FE 1. Some Experimental Examples

In recent years, many studies of the hydrogenation and the hydrogenolysis of hydrocarbons have shown a maximum in the TOF-FE curve. The data are presented in Table XVI.The shapes of the curves are similar to the TOF curve of Fig. 9, but the maximum TOF for the other data of the table are in the range 0.2 < FE < 1.0 and are often of the order of 0.4-0.5. Let us consider how this behavior may be explained.

2. Sympathetic Behavior for FE below about 0.5 a . Role of Low-Coordination Atoms. If antipathetic behavior is linked to the role of ensembles on crystal faces, then sympathetic behavior may be linked to the role of low-coordination atoms at edges and corners. This is the original point of view of van Hardeveld and Hartog (20). If this picture prevails, TOF rises as FE rises, but eventually the particles become small enough (FE > 0.5) so that the antipathetic behavior associated with small clusters sets in, and TOF descends toward zero as FE tends to unity. In connection with the reasoning presented above, the work of Ponec and his associates (132,362)should be recalled. Even though hydrogenolysis probably requires a multiply bound intermediate and should thus be favored on faces made up of high-coordination atoms, some systems show sympathetic behavior. This is explained by the preferential poisoning by carbon deposits on the faces as compared to the edges, so that after

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some time on stream the reaction is poisoned on the faces and proceeds near or on the edges. This phenomenon has been called secondary structure sensitivity (284). b. Role of& Sites. Systems that need certain multiatom sites associated with crystallite edges may show sympathetic behavior, even though self-poisoning is not important. For instance, near the edges of a regular geometric figure (cubooctahedron, etc.), for which there are incomplete layers of atoms on the faces, special multiatom ensembles can occur (20). The Bs sites have been discussed in connection with Fig. 5 . We have mentioned that the ratio of B5 sites to total surface atoms when plotted against FE results in a curve such as that in Fig. 9, but with a maximum near FE = 0.5. From that point, there is a continued decrease down to FE = 0.2 or less (106). Of course, B2 or B3 sites may be important, but they present a maximum that is not dramatic and occurs at a FE close to 1.0 (206).On the other hand, sites of more than five atoms, if such arrangements are stable, should show a maximum at values of FE lower than 0.5. Thus we see that the results of Table XVI can be reconciled with the geometric effect of multiatom sites associated with crystallite edges. For FE > 0.5, both the geometric and the cluster-likeelectronic effects lead to a rapid decrease in TOF to zero. c. Role of the Distribution of Crystal Faces. For the ammonia synthesis the antipathetic behavior was explained by the increasing proportion of the open, active ( I 1 I ) faces [body-centered cubic (bcc)] of iron as FE decreases (311a-c). If the kinetics of crystallite formation are such that reduction at relatively low temperature (450°C) does not favor the growth of ( I 1 1) faces, then particle growth obtained by sintering at higher temperatures should lead to higher proportions of the thermodynamically favored ( I I I ) faces and to antipathetic structure sensitivity. Another case for which the effect of the single-crystal faces is known is for alkane hydrogenolysis over nickel (fcc). Here the open face is the (100) plane, and Goodman found it to be more active than the closepacked ( I 11) plane (363,364). A geometric explanation is presented (364). Here also it is reasonable to assume that sintering the supported catalysts at successively higher temperatures to form larger particles leads to an increase in the proportion of (1 1 1) faces as FE decreases. Now for ethane hydrogenolysis sympathetic structure sensitivity results. Note that the structure sensitivity is extensive, persisting to d = 17 nm (FE = 0.06). As FE increases, a maximum TOF is reached at FE = 0.3 (275). Further increase in FE now leads to smaller TOF, as cluster-like behavior is reached. Martin and Dalmon (275) have reinforced the preceding arguments by using a Ni catalyst obtained by reducing antigorite. This leads to Ni

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crystallites rich in (111) faces. TOF is now reduced to about the same value as that found at the lowest FE (0.06)for the ordinary Ni/Si02 catalysts; also, the antigorite-based catalyst leads to approximately structure insensitive behavior. Ethane hydrogenolysis has also been studied by Lee and Schmidt (365), using Rh/SiOz catalysts. By a series of reversible oxidations and reductions, they are able to change the morphology of the 10-nm crystals, as observed by transmission electron microscopy. Although the total metal surface, as measured by H2 chemisorption, changes little, Rh2O3 reduced at 300°C is 50 times as active for ethane hydrogenolysis as when reduced at 600T.Rhodium is 100% reduced in both cases, as deduced from XPS measurements. The high-activity crystallites expose a roughened surface that consists of a much smaller proportion of the (1 11) faces that are preponderant on the low-activity crystallites. Similar conclusions have been reached by Altman and Gorte (131b), who have measured the TPD of CO on Pt/A1203as a function of particle size. The weaker binding of CO to Pt for larger particles is explained by suggesting a preponderance of (1 11) faces on the latter.

E. STRUCTURE-INSENSITIVE SYSTEMS 1. Some Experimental Examples

Table XV lists the systems that we have defined as structure insensitive. These reactions constitute only a small fraction of those considered in this review. A comparison of Tables XV and XVI gives the impression that as time passes and experimental techniques improve, structure insensitivity is becoming rarer. An inspection of a number of the curves referred to in Table XV shows that TOF is reported as constant all the way to FE = 1.O. In view of the results on clusters and the propensity of H/M, to rise above 1.0 for the smallest particles, the reported behavior seems questionable. The explanation of reputed structure insensitivity has been the subject of much speculation, which we next discuss briefly. 2. Explanation via Ensembles

It is difficult to discern a consistent theory based on geometric effects. If the surface is relatively bare, and adsorption controls, then the rate of adsorption (and reaction) should be structure sensitive also. If surface reaction controls (346),then we expect a relatively high surface coverage, and now it is plausible that the lateral interactions between adsorbed species may control, leading to structure insensitivity.

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It is clearly possible that as FE is changed, the surface coverage during reaction may change. Then it is not correct to reason on the basis of a varying FE with a constant 8. In order to resolve these difficulties, it seems important to study all the kinetics (sequence of steps), including surface states, as FE is varied. This type of experiment does not seem to have been done. In discussing this matter, one is confronted with the reasonableness of structure sensitivity and the conceptual difficulty of explaining structure insensitivity. 3 . Explanation via Overlayers Zaera et al. (366) have emphasized that Pt and Rh single-crystal surfaces during ethylene hydrogenation are largely covered with ethylidyne species (CH3-C). Considering this reaction to be structure insensitive (however, see Fig. 9), they then explain this behavior by proposing that the catalytic reaction takes place on top of this overlayer. Thus the reaction should be insensitive to the structure of the underlying metal. However, the nature of the overlayer itself may be structure sensitive, especially for FE > 0.5. More experimental proof is needed before this explanation can be accepted, even as it is for the other explanations already discussed. ON F. REMARKS

THE

DEFINITION OF TOF

We recall here that the definition of turnover rate used in this review is based on the number (or moles) of exposed surface atoms. Thus TOF is an alternative but particularly expressive way of representing the rate (refer to Section 111,A). However, it is clear that the reciprocal of the TOF is not the residence time of an active intermediate on the surface. To know this quantity, it would be necessary to know the active sites per surface atom for the particular system. Usually, this quantity is not at present measurable by any general type of experiment. However, it is interesting that in two references the variation of TOF with d has been used to estimate a further characteristic of the active ensemble of atoms. Topsoe et al. (345) have reasoned that since N2 adsorption seems to be rate-determining for the synthesis of NH3 on Fe/MgO, it would be logical to measure the surface site concentration by Nz chemisorption. When a new TOF is defined in this way, it is found to be insensitive to particle size. Recall that we have classified this system as showing antipathetic structure sensitivity, with FE measured as usual on H2 chemisorption.

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In a similar vein, Aben et al. (97) have found that a TOF based on reversible (weak) hydrogen chemisorption is invariant with particle size for the hydrogenation of benzene over Pt/A1203.This system appears in Table XVI with TOF based as usual on strong hydrogen chemisorption. We note that the use of N2 chemisorption on iron, for example, really amounts to measuring the atoms per active site as a function of FE. This procedure is merely an alternative to the one we have adopted, in which we reason about the number of sites involving a given configuration of atoms per unit of exposed atoms. This quantity has been called the Taylor fraction (367). Similar concepts have been discussed recently by Carberry (368).

G. SECONDARY STRUCTURE SENSITIVITY The trend of the variation of TOF with FE can be reversed by secondary effects such as carbon deposition or restructurization by oxygen (284). Clearly, the surface composition of the catalyst during reaction must be measured in order to evaluate these important effects. VII. Conclusions and Future Directions

A. CLASSIFICATION OF STRUCTURE-SENSITIVE EFFECTS

We summarize here what seem at this moment to be the ways in which turnover frequency may vary with fraction exposed. We note, however, that in the figures presenting the results there is often disagreement between studies on a given system concerning the shapes of the curves. Perhaps this discussion will encourage the future experimental resolution of some of these problems. 1. Antipathetic Behavior over Most FE

Reactions favored by principal crystal planes may be antipathetic, and we think that most systems should be antipathetic for FE > 0.5. Thus an explicable behavior is that of the monotonic curve 2 of Fig. 25. 2 . Structure Insensitivity

For FE < 0.5, structure insensitivity is conceivable, but we think that curve 1 of Fig. 25 is the expected behavior, with a decrease in TOF for FE > 0.5.

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01 0.1

1

I

I

0.25

0.5

1

FE

b

FIG.25. Schematic curves of types of structure sensitivity in accord with the rationale of this review: ( I ) structure insensitivity; (2) antipathetic structure sensitivity; (3) systems with a maximum TOF.

3. Systems with a Maximum TOF Reactions favored by special sites near edges (Bs sites) may show sympathetic behavior for FE < 0.5, but at higher FE antipathetic behavior sets in for electronic reasons and sometimes also for geometric reasons. The result is represented by curve 3 of Fig. 25. Through comparison of Fig. 25 with Fig. 1 and experimental curves giving TOF versus FE it is not known whether differences are caused by incomplete understanding or by incomplete experimental results. Farin and Avnir (369, 370) have published a phenomenological expression related to the geometry of fractals, which they write as follows: a a (d)DR (6) where a is the reaction rate in moles per second per particle, and DR is an empirical quantity called the reaction dimension. Equation (6) can be converted to the following equation for the rate in moles per second per surface atom:

TOF a (FE)2-D~ (7) Clearly, there is sympathetic structure sensitivity for DR< 2, structure insensitivity for DR = 2, and antipathetic structure sensitivity for DR > 2. According to Eq. (7), the log TOF versus log FE should be a straight line. However, reference to our figures shows that the real behavior is more complicated in many instances. Naturally, Eq. (6) was established on geometric principles; no electronic effects are included. Equation (7) in a different form has been suggested also by Carberry (368).

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TABLE XVII Some Characteristics of Small Supported Metal Particles Catalytic properties as a function of particle size Possible optimal TOF (Fig. 25) Possible optimal AR (Fig. 7) Possible optimal yield Variable resistance to deactivation, e.g., coking Smallest particles least reactive Variable metal-support interaction Change in physico_chemical propertiesa for particles with FE > 0.5 (d < 2 nm) Shorter interatomic distance Unusual crystal structure (e.g., pentagonal symmetry) Labile surface atoms Lower melting point Larger ionization potential Unusual metallic electronic structure (electron deficiency)

With respect to those of bulk metals.

Some characteristic properties of small supported metal particles are summarized in Table XVII.

B. POSSIBLE FUTURE STUDIES Consider Fig. 13, representing the reputed structure-insensitive reaction, the hydrogenation of benzene, and Fig. 14, representing the structure-sensitive reaction, the hydrogenolysis of ethane. It does not seem productive to continue studies using the same techniques as those underlying these figures. The results may lead to more lines going in all directions. Therefore, in this section we attempt to point out the ameliorations in procedures that may give rise to progress. 1. Importance of the Precise Preparation of Small Supported Particles

It has been mentioned several times that it is difficult to interpret results obtained over poorly defined catalysts, and the preparation of model catalysts has been discussed in the appropriate sections. What appear to be the most promising methods are covered in what follows. a . Low-Area Catalysts. Vapodeposition methods seem to lead to the best defined catalysts. For SiOz and AlzO3, support effects seem to be absent. Would it be possible to narrow the particle size distribution to the point where effects similar to those seen on small clusters (Fig. 23) are

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observed, or would the curves be smoothed owing to support effects? Perhaps naked particles could be deposited on a support by using it as a target inside a large magnetic sector mass spectrometer. A disadvantage of such low-area samples is that reactions at atmospheric pressure must be run in a batch mode. Thus we are deprived of some of the transient and isotopic methods that lead to the in situ measurement of the surface coverage of active species and/or inert species. b. High-Area Catalysts. As discussed in previous section, it may be possible to graft metal salts or complexes to certain supports and to reduce them gently via H atoms. It may be difficult to achieve the narrowness of size distribution and the high degree of reduction of vapodeposited catalysts, but this method represents a considerable improvement over simple impregnation and other similar methods. Some characterization methods are sacrificed in dealing with high-area catalyst (LEED), but most others remain. However, these catalysts have the great advantage that they can be studied in flow reactors suitable for in situ transientisotopic kinetic and spectroscopic methods. 2. Importance of Quantitative Characterization The catalyst used must be characterized at the various particle sizes by a number of chemical and physical methods. For example, if TOF is supposed to increase because the fraction of (1 10)faces exposed is said to increase, then the shapes of the particles should be studied by appropriate electron microscopy and microdiffraction techniques, and perhaps by EXAFS. Further comments follow. a . Chemisorption. We emphasize the importance of calibrating Hz adsorption techniques. For instance, if H/Irs really approaches 3 as FE approaches 1.0, an observed value of H/Ir of 1.0 then represents a FE only somewhat greater than 0.33. In this way the fall of TOF proposed as FE goes to unity would not be observed. b. Electron Microscopy. Often an article presents micrographs with black, gray, and light blobs that are claimed by the authors to represent metal and support particles. How this is known is sometimes not clear. Calibration against known samples is rare. This is a difficult subject, and sample preparation is complicated. High resolution, computer processing of images, dark-field methods, and microdiffraction techniques should be used whenever possible. These techniques may be the source of much future progress in catalysis. c. EXAFS. Like electron microscopy, this technique is complex and expensive. Data are interpreted through large computer programs. However, this method is needed in order to obtain a quantitative estimation of metal-support bonds, and it can be used to measure particle size.

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d. Chemical Properties of a Site. Whenever possible, UV-VIS (ultraviolet-visible spectroscopy), IR, EPR, XPS, UPS, NMR, and other spectroscopies should be used to identify the nature of the coordination sphere of the surface atom, its valence, and other properties. 3 . Importance of Quantitative Kinetic Studies This matter has been mentioned repeatedly. The composition of the surface during reaction, the sequence of steps, and the rate parameters of the steps should be measured as a function of particle size and structure. It is not sufficient just to measure TOF. 4. Importance of Single-Crystal Studies

We have seen how studies on the various faces of Fe and Ni have contributed to the understanding of the structure sensitivity of the ammonia synthesis and of the hydrogenolysis of alkanes. Clearly, the effect of crystal faces, steps, and kinks on other reactions should be pursued. In some cases, kinetics can be measured at vacuum conditions, permitting the use of transient methods. The example of CO oxidation is striking (315).

5 . Importance of Naked Metal Cluster Studies This subject has already been emphasized, and it is sure to be a popular research topic. It is the only present method that permits the measurement of kinetics on particles of a precisely known number of atoms. 6 . Multiproduct Studies: Selectivity Besides the obvious practical interest of such investigations, they are important because two or more reactions can be studied on identical metal particles. We owe much to the work done to this point on hydrocarbon refining reactions.

7. The Way to More Progress Unfortunately for the individual investigator, it appears that the way to progress in this field is through complicated preparation methods, the use of heavy techniques such as TEM and EXAFS, and detailed kinetic (mechanistic) studies on each catalyst. Ideally, single-crystal and metal cluster studies would be done on the same systems. The holders of purse strings will have to decide whether the added understanding resulting from these expensive studies is justified in a field in which empiricism has

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ADVANCES IN CATALYSIS, VOLUME 36

Metal-Support Interaction: Group Vlll Metals and Reducible Oxides GARY L. HALLER AND DANIEL E. RESASCO* Department of Chemical Engineering Yale University New Haven, Connecticut 06520

I. Introduction

Our goal is to assess our current understanding of the interaction of small metal particles with reducible oxide supports. Our focus will be on Group VIII (or Group 8-10) metals supported on Ti02 because of the rather large volume of literature that has recently been produced on these systems. A survey of results for other reducible oxide supports, for example, V203,will be given where appropriate, and we will attempt to extrapolate or generalize our conclusions to oxides such as Si02 and Al203, which are not usually considered reducible. This is one of the scientific justifications for the widespread investigation of Ti02 support; that is, Ti02 as a support for metal catalysts has not found much practical application, but its effects on metal catalysis are so large and easily studied that it can be presumed that a complete understanding of this support will provide a basis for searching for similar but much more subtle effects of the more inert oxide supports. Moreover, whereas the direct use of Ti02 (or other reducible transition oxides) may never find large-scale applications as supports, they are increasingly added to catalysts as promoters. As will be discussed below, there is little chemical distinction between Ti02 (or other reducible transition-metal oxides) as supports as opposed to promoters. Thus it should be understood that much of what can be concluded about metal-reducible oxide support interaction can be extended to the use of these same compounds as promoters. The use of a transition metal as a promoter, cosupported on a conventional support, takes advantage of the high area and stability provided by conventional

* Present address: Instituto de Investigaciones en Ciencia y Tecnologia de Materiales, Facultad de Ingenieria UNMDP, (7600)Mar del Plata, Argentina 173 Copyright 8 1989 by Academic Pres, Inc. All rights of reproduction in any form reserved.

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supports such as Si02and Alz03.The disadvantage of this mode of metalreducible oxide interaction is the degree of interaction, which generally is not as good as when the metal is supported directly on the reducible oxide; however, this shortcoming can be circumvented. As we begin to contemplate the structure of this review, it seems appropriate to start by stating some disclaimers and by revealing some of the authors’ prejudices with respect to “electronic effects” of metal-support interactions. It is understood that ultimately all catalytic effects must be electronic in the sense that chemical bonds are being formed and broken by the catalyst, and these bonds are electronic by nature. The obvious corollary to this statement is that any metal-support interaction must be electronic even if it does not result in chemical bonds between the metal particles and the support; in other words, even van der Waals bonds that result from a combination of factors such as dipole-dipole interactions, dipole-polarization interactions, and quantum-mechanical dispersion forces are basically electronic. However, it does not follow that all synergies between a metal particle and a support are the direct consequence of electronic interactions at the interface. The obvious example is bifunctional catalysis, e.g., reforming of alkanes on Pt/AI2O3when an olefinic intermediate is formed on the metal and further reacts on acidic sites of the A1203. In this case it is not even necessary that the metal be in physical contact with the “support,” although this becomes a practical necessity in order to overcome transport limitation of an intermediate of very low concentration (1). The real question that we pose, but do not intend to answer definitively, asks whether the electronic interaction between a metal particle and a support can alter the chemical properties of the metal particle. This possibility was first formulated by Herbo (2) in a rather simplistic picture and has been extensively investigated by Schwab et al. (3, 4 ) and Szabo and Solymosi (5, 6). Our prejudice, which will become more apparent as we proceed, is that an electronic interaction does occur at the interface between a metal particle and a support, and it may be physically detected, e.g., by X-ray absorption near edge structure (XANES) or X-ray photoelectron spectroscopy (XPS) (7-9). However, it does not significantly alter the chemical and catalytic properties of the metal particle except for those metal atoms at the interface or directly contiguous with the support. What we are admitting is a local or ligand effect well known in inorganic complex chemistry and introduced to catalysis by Sachtler as a way of thinking about metal-metal interactions (10). We believe that this is also a good way to conceptionalize some aspects of metal-support interactions. Boudart and Djega-Mariadassou (11) and Bond and Burch (12) have emphasized the need to distinguish between apparent (or indirect) and

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real metal-support effects. In the former category, there are inhibition of reduction of the metal by the support, specific particle size effects (cluster size, epitaxial growth of metal on support or morphology), bifunctionality, spillover, and effects of support action as a source or sink of catalytic poisons. Only if all these and related complications can be eliminated is it possible to study real metal-support interactions that may be attributed to geometric and electronic effects. Such an elimination process was used in the original work of Tauster et al. (13), who reported that H2 and CO chemisorption was greatly reduced for metals supported on TiOt and reduced at 773 K but normal after a 473 K reduction. Bifunctionality and hydrogen spillover could be eliminated by virtue of the negative effect on chemisorption, and questions of support as a source of poisons could be ruled out because the effect was not different for the rather impure Degussa P-25 (purity >97%) and the high purity Cabot Cab-0-Ti (99.8%) titanias. Significant changes in particle size were shown not to occur by X-ray diffraction and electron microscopy. Total encapsulation seemed an unlikely possibility because there was very little change in the BET surface area that accompanied 773 K reduction, and a low-surface-area Ti02 formed by prereduction at 973 K gave similar results. They concluded that the decrease in the chemisorption was a result of bonding between the metal particles and Ti of the support, although they left open the question of the degree of oxidation of both Ti and the metal atom of the cluster to which it was bonded. Tauster et al. (13)were not, of course, the first group to use TiOz as a support for metal catalysts. A careful reading of a paper by Nehring and Dreyer (14) published in 1960 would have revealed that the competitive dehydrogenation versus hydrogenolysis of cyclohexane was greatly favored over Pt/Ti02 compared to P t / A l 2 0 3 , Pt/MgO, Pt/SiOz, or Pt/C. Shortly thereafter Szabo and Solymosi (5) reported on a study of Ni supported on Ti02, the electronic properties of which had been altered by doping. These papers did not garner much interest, perhaps because they did not clearly rule out the apparent support effects mentioned above. Another factor may have been the neglect of metal surface areas, which, at the time, were not routinely measured to properly normalize rates. The work of Szabo and Solymosi (5) undoubtedly suffered because they chose to interpret their results in terms of bulk electronic properties of the support, which were reflected in the altered properties of the metal particle. This was just at the time when work on alloys was beginning to cause the catalytic community to consider abandoning the idea that bulk electronic properties could have a dominant effect in surface catalysis. Why, then, did the paper of Tauster et aL(Z3)on “strong metal-support interactions,” almost two decades later, attract so much attention and

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stimulate so much research? The careful elimination of artifacts was important, but the demonstration that the effect of high-temperature reduction could be reversed signaled that the systems of noble metaVTiO2 were amenable to investigation in a way that previous systems were not. One should not ignore the fact that the effect of high-temperature reduction of noble metal/Ti02 has a dramatic effect on chemisorption and that this is a simple experiment accessible to any catalytic laboratory. Perhaps some small credit should also be given to the clever labeling of the effect as a strong metal-support interaction which quickly fell to the acronym “SMSI” and became the catchall for all kinds of effects and artifacts, many of which had nothing in common with the interaction that occurs between metals and Ti02 when reduced at high temperature. How, then, will we use the term SMSI? We will return to this question after we briefly describe the physical properties of the metal-Ti02 interaction. The groundwork for understanding the nature of the metal-Ti02 interaction was layed at an international symposium entitled “Metal-Support and Metal-Additive Effects in Catalysis” held in September 1982 (15). The papers of the written proceedings mostly interpret results in purely electronic terms, and, indeed, our own contribution on Rh/Ti02 is no exception (16). There were participants such as Ponec (13,who argued against a measurable influence of electron transfer of charge from the TiOz support to the metal particle on theoretical grounds, and others such as Huizinga and Prins (18), who presented XPS evidence against such a charge transfer for the Pt/TiOz system. In contrast to the written papers that were published, the oral exchange of ideas at this meeting resulted in a growing consensus that high-temperature reduction caused a TiO, species to move over and partially cover supported metal particles. In retrospect, the concluding statement in the paper of Meriaudeau et al. (19) exhibits extraordinary insight. Their discussion conforms to the prevailing prejudice for an electronic explanation, but having paid service to this concept, they state that “Although not experimentally supported, one may add another explanation for SMSI on Pt-Ti02: at high temperature hydrogen treatment and due to Pt, Ti0 suboxide would form which may cover platinum metal. . . . If this latter suggestion prevails the so called SMSI for high temperature reduced Ti02-supported noble metals would have a pure geometric origin.” This interpretation had, in fact, already been advanced by von Engels et al. (20) the previous year in order to understand the observed selectivity of Ni/TiO2 catalysts. Like Nehring and Dreyer (14) before them, von Engels et al. reported that Ni/Ti02 catalysts are characterized by small hydrogenolysis activities and high dehydrogenation selectivities and suggested that this resulted from a partial poisoning of the Ni surface by Ti cations.

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Experimental evidence for this picture for other Group VIII metal catalysts on high-area Ti02 began to build quickly in 1983. Santos et al. (21) studied Fe/TiOz and observed that the bulk metallic properties of Fe (as measured by Mossbauer spectroscopy) were not changed by high-temperature reduction where both CO chemisorption and ammonia synthesis were decreased. Because these effects were evident even on very large particles, they suggested that a TiO, species must have spread over the entire surface because interaction only at the interface between metal and oxide could not account for their results. We continued our investigation of Rh/Ti02and observed a strong parallel between the effect of a Group Ib (or Group 11) metal on the relative activity for hydrogenolysis and dehydrogenation;the former was strongly suppressed, and the similar effect of increasing Rh-Ti02 interaction was induced by thermal treatment (22). We also noted that the kinetics of the metal-oxide interaction followed a square root of time dependence consistent with a diffusion step and that the degree of SMSI correlated with particle perimeter but not interface area. Direct physical evidence for migration of a titania species was reported at about the same time in two studies of Pt/TiO2. Cairns et al. (23) used Rutherford (very high energy, e.g., several mega-electron volts) backscattering of He+ from a rather thick (20oO-A) Pt film deposited on singlecrystal Ti02(rutile) reduced at very high temperature (1 173 K) to demonstrate interdiffusion between the metal and the support. Kelley et al. (24) used low-energy (2 keV) ion scattering of 4He+and "Ne+, which gives much higher surface sensitivity than does Rutherford backscattering. Moreover, they used a highly dispersed, low-loading Pt on anatase powder to prepare a conventional catalyst. Their results show that Pt particles are less visible to ion scattering after 673K reduction than after 473 K reduction, a result clearly consistent with covering of the Pt particles by a TiO, species. However, they chose instead to invoke other explanations such as a change in the neutralization probability of the Pt particles by interaction of Ti02 with the particles after high-temperature reduction. This could result if there were a major change in the charge on Pt, but even their own X-ray absorption spectroscopy provided no evidence for a net gain or loss of electrons by Pt following reduction at 673 K (25).There followed in 1984 a series of papers on model systems of thin metal films on Ti02 single crystals or oxidized Ti foils that provided unequivocal evidence for migration of a TiO, species onto the surface of metal particles during reduction (26-30a). It is now generally agreed that Ti interacting with metal particles is a cation of lower oxidation state, that is, Ti3+or Ti2+.In addition, it has been claimed that direct metal-Ti bonding occurs in the Rh/TiOz system (31). The degree of cationic character of those

178

GARY L. HALLER A N D DANIEL E. RESASCO

metal atoms in the particle directly bonded to the Ti cation remains an open question, but it seems reasonable that they should acquire some cationic character . Clearly, the operational definition for SMSI that one would glean from the original paper of Tauster et al. (13) requires metal-oxide interaction between a metal particle and Ti02,which results in suppression of H2 and CO chemisorption. From our subsequent understanding of the physical cause of this suppression, that is, the migration and bonding of a suboxide of the support onto the metal particle induced by reduction, the same acronym should apply to other systems in which a similar phenomenon occurs. Indeed, by use of just such an operational definition, SMSI was shown to be exhibited by Ir supported on TaZOs, V203, and Nb205 (32). It seems clear that SMSI should also imply a large relative effect on the rates of structure-sensitive reactions (e.g., hydrogenolysis) and a relatively small effect on structure-insensitive reactions (e.g., dehydrogenation) when these reactions occur under reducing conditions in which the SMSI will be maintained (16, 33). This behavior may be used to extend our operational definition of SMSI and, indeed, was one of the driving forces behind the original Exxon work as evidenced by a patent on improved reforming selectivity over Ir/TiOz compared to Ir/AI2O3 (34). SMSI suppresses the undesired hydrogenolysis side reaction in preference to the dehydrogenation, aromatization reactions that are desired in reforming. For very high dispersion prepared by ion exchange, the suppression of hydrogenolysis is so great that alkane isomerization and dehydrogenation can become important competing reactions on Rh and Ir (35). These reactions are normally significant only on Pt when noninteracting supports are used. The narrow definition of SMSI given above would exclude the effect of Ti02 support on CO hydrogenation activity, selectivity, and activity maintenance because it is now known that the bonding between the migrated species, Tior, is broken by reaction with the H20 and C02 byproducts of this reaction (36). Moreover, there is no need to prereduce at high temperature to observe the advantages of Ti02 relative to SiOz or A1203 supports for CO hydrogenation (37). The Exxon patents on Ru/TiOz (38) and Ni/TiOz (39), catalysts for CO hydrogenation, predate the Exxon SMSI paper of Tauster et al. (13) and in their subsequent paper, on CO/H2 synthesis reactions over Ni/TiOz, Vannice and Garten (40) neither directly connected their work to that of Tauster et al. (13)nor referred to it as a strong metal-support interaction. However, in the note that followed on Rh/TiO2, Vannice and Garten (41)did attribute alteration of catalytic properties to strong metal-support interaction and use the SMSI acronym. Unfortunately, all the effects of Ti02 on chemisorption and activity have come to be lumped together under this acronym, and

METAL-SUPPORT

INTERACTION

179

such use is not likely to be quickly reversed by either reviewing the history or distinguishing the different physical origin of hydrogenolysis suppression and CO hydrogenation increased activity on, for example, Rh/TiO2 (36). One way out of this morass is to adopt the terminology of Boudart and Djega-Mariadassou (ZZ), who have suggested that the acronym SMSI be used to mean Schwab metal-support interaction to recognize the pioneering work of Schwab on metal-support interactions. Boudart and DjegaMariadassou have distinguished two different Schwab support effects: (1) the Schwab effect formulated first, involving the concept of altered catalytic activity of a semiconductor in contact (or supported) on a metal (42) and (2) an inverse effect in which the catalytic activity of a metal is modified by its contact with a nonmetallic support (3). It is the latter to which Boudart applies the acronym SMSI, and it would then include all such real metal-support effects. In this review we will retain the narrow definition of SMSI as originally defined operationally by Tauster et al. (13). We will, of course, review the other kinds of interaction that result in increased activity for CO hydrogenation of Group VIII metals supported on Ti02 but will simply call this a metal-oxide interaction, reserving specific labeling until the specific physical origin is more completely understood.

II. Titania-Supported Catalysts

A. PREPARATION OF TiO2-SUPPORTED CATALYSTS Properties of Ti02

Titanium dioxide (TiOz), titania, exists in three crystallographic forms: rutile, anatase, and brookite. The Ti02 used as a support for catalytic studies usually contains phases of anatase and rutile. For example, Degussa P25 Ti02 is approximately 85% anatase and 15% rutile. In both of these crystallographic phases the bulk Ti cations are surrounded by six oxygen neighbors in an octahedral configuration. The difference between the two crystalline modifications is due to the different packing of these octahedra (43). Different perfect Ti02 surfaces can exhibit six-, five-, or fourfold coordinated Ti cations (44). Rutile is the thermodynamically most stable Ti02 crystal structure, and the transformation from anatase to rutile takes place at about 1300 K. This temperature can be much lower in the presence of foreign atoms, which can catalyze the transformation. In a series of studies of the properties of titania, Shanon and Pask (4.5) postulated that the transformation to rutile

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GARY L. HALLER A N D DANIEL E. RESASCO

is accelerated by processes that increase the removal of oxygen. Therefore, this transformation may, in fact, occur under high-temperaturereduction (HTR; usually 773 K) conditions of catalysts. For example, Fig. 1 (46) shows X-ray diffraction patterns of various Ti02 samples. The HTR causes no change in the rutile/anatase ratio of bare Ti02 or when 2 wt% Rh is present (Fig. lb and c). However, reduction at 1013 K , well above the standard 773 K HTR, of a 3.2-wt% Rh/Ti02 catalyst has been found to enhance the anatase-to-rutile transformation (47). In contrast to 2-wt% Rh/TiOz catalysts, the HTR of 5-wt% Ni/TiOz catalyst causes a dramatic change in the ruti1e:anatase ratio (see Fig. Id). A change in the peak positions is also observed. This would indicate a rather extensive degree of Ti02 reduction. After HTR, both Rh/TiOz and Ni/TiO2 exhibited the characteristic features of SMSI, but only the Ni-containing catalyst caused a significant alteration of the crystallographic structure of the support. Thus, it appears that this crystallographic transformation has little, if any, direct effect on the metal-support interactions. ANATME

A BUTILE

FIG. 1. X-Ray-diffraction patterns of the Ti02 support: (a) Ti02 (P25 Degussa) as received; (b) 2-wt% Rh/Ti02reduced at 473 K; (c) 2-wt% Rh/Ti02reduced at 773 K; (d) 5-wt% Ni/Ti02 reduced at 773 K. (After Ref. 46.)

METAL-SUPPORT INTERACTION

181

In stoichiometric Ti02, all cations are Ti4+,are octahedrally coordinated, and fill half of the octahedral sites of the oxide lattice to form the rutile structure. On reduction, the Magnelli (48, 49) series of phases of stoichiometry Ti,,Oz,-l, with varying ratios of Ti4+and Ti3+,are formed until the end of the series Ti203is reached. In Ti203, all cations are Ti3+, are octahedrally coordinated, and fill two-thirds of the octahedral sites of the oxide lattice to form the corundum structure. Titanium(II1) is a d' system, but Ti203 is diamagnetic as a result of spin pairing (Ti-Ti bonding) across the shared faces of octahedra. The structure of the Magnelli phases can be constructed of rutile layers with varying thickness where the Ti on the surface of the layers are all Ti3+and the layers are bound together by spin pairing in a plane with the structure of Ti203. Crystallographically, this spin pairing is possible by sliding the rutile layers with respect to each other by half a lattice spacing, i.e., a crystallographic shear plane. The simplest of these phases is Ti407, which has a structure composed of layers four Ti06units thick; the center two units are formally Ti4+and the face units, Ti3+.All the Ti3+can be paired across the shear plane; thus, one expects and finds that this material is diamagnetic. One may also predict that the nonstoichiometrictitanias that lie between Ti02 and Ti407 will be paramagnetic and there will exist a maximum density of unpaired spins at a stoichiometry halfway between, that is, at Ti6O11. This expectation is borne out by experiment. This is illustrated in Fig. 2, which shows the relative number of unpaired spins, as measured by EPR at 77 K,as a function of composition x of TiO, (43). It is clear that, starting from stoichiometric Ti02, for which no EPR signal is detected, the number of unpaired spins increases, reaching a maximum for x = 1.85 (i.e., about Ti7013). At increasing x values, this number sharply decreases, becoming zero for a composition equivalent to T407. The incipient-wetness impregnation is the preparation method most commonly used to introduce the metal precursors in the catalysts. Some authors have also used alternative preparation procedures such as the decomposition of organometallic complexes (21) or ion-exchange methods (50). In some of our own work (16,35) we have used an ion-exchange preparation technique that renders high metal dispersions and low metal loadings. Through this preparation technique we were able to achieve a high degree of metal-support interaction. For example, Ir/Ti02 and Rh/TiO2 catalysts resulting from ion exchange exhibited unusual activities for isomerization and dehydrogenation of alkanes after reduction at 773 K, which were not observed on more inert supports or on similar TiO2-supported catalysts prepared by impregnation. With the use of this preparation technique, the initial metal precursor-support interaction is a function of the composition of the support and the species present in

182

GARY L. HALLER A N D DANIEL E. RESASCO 124

158

450

OK

136

(D

0, E

W

Ti20,

Ti02

X--c

FIG.2. Relative numbers of unpaired spins versus composition x of TiO, at 77, 80, and 140 K as measured from the EPR signal and magnetic susceptibility. The dashed curve (curve 1) corresponds to one unpaired spin per Ti3+ion not spin-paired at a shear plane of the Ti407phase. (After Ref. 43.)

solution. The extent and nature of the ion exchange strongly depends on the pH of the solution. The details of the ion-exchange preparation procedure have recently been reviewed (52). In our particular case, for the preparation of Rh/Ti02 catalysts, we adjusted the support pretreatment [washing with NI&(OH)], the type of rhodium salt [Rh(N03h], and the pH of the solution (>7) to achieve a cation exchange. Analogously, we have also effected anion exchange by working at low pH values (lower than the isoelectric point of Ti02, i.e., a pH of 5-6) and using metal chloride solutions. Under these conditions, anion complexes (A-) of the type [Mn+Clx](x-")-are present in solution. They can be adsorbed onto the support by the following mechanism: S-OH

t A- t H+

S-OHiA-

(1)

On the other hand, the impregnation process involves a combination of both adsorption of metal complex ions and deposition of the solute as the solvent is evaporated. Thus, the resulting metal dispersion depends on the fraction of metal ions undergoing exchange with support species relative

183

METAL-SUPPORT INTERACTION

to the fraction of metal precursors deposited on the support as occluded solute. We prepared a Rh/TiOz catalyst series by varying the pH of the Rh(N03)3 impregnating solution from 3 to 11 (46). As shown in Fig. 3, as the pH was increased, the cation-exchange capacity of the support increased, resulting in a higher dispersion. At excessively high pH values, precipitation of rhodium can occur, resulting in a lower dispersion. In an electron microscopy study of a cation exchanged Rh/TiOz catalyst, Fuentes et al. (52) used image processing techniques to filter the noise and enhance the contrast. They observed two-dimensional structures of Rh with about 1-nm cross section and one-dimensional rows of Rh along the [Ool] direction of rutile (110) planes. We have recently rationalized this structure in terms of the genesis of the catalyst (53). Considering the surface of the Ti02 powder during the impregnation as that of a fully hydroxylated TiOz(ll0) plane, we expect to find both bridging hydroxyls bound to two Ti4+cations and terminal hydroxyls bound to one. In the former case, the bridging oxygen will be more acidic and, therefore, will preferentially act as cation-exchange sites. We can then expect that during the first treatment with NHXOH) those bridging sites will preferentially hold NH:, which will subsequently be exchanged by the cationic metal complex (probably [Rh(H20)4(OH)2]+). The chemistry

\

0

/

A

E E

0.25

\

-

3

6

9

pH OF IMPREGNATING

12 SOLUTION

FIG.3. Metal dispersion (H/M) as measured by H2chemisorptionafter reduction at 473 K of a series of 2-wt%Rh/Ti02 catalysts as a function of pH of the solution used during the impregnation step in the catalyst preparation. (After Ref. 46.)

184

GARY L. HALLER A N D DANIEL E. RESASCO

by which the metal precursor loses water, adsorbs H2, and reduces to the metal is difficult to imagine in detail, but formally we can write 3/2& + [Rh(H20)3(0H)2(0Ti)]3+ SH20 + [Rh(HOTi)13+ (2)

-

As written, only the Rh has been reduced and all the titanium cations remain Ti4+. This is a reasonable assumption for the low-temperature reduction (LTR; usually 473 K) case. When the reduction temperature increases, surface oxygen vacancies may be created, causing the reduction of Ti4+to Ti3+.In that case, the HTR step may be written as [Rh(HOTi)]’+ + 1/2H2 [RhTi]’+ + H 2 0 (3)

-

In both cases, the resulting structure would be one-dimensional rows of Rh with the spacing of the Ti rows, as observed experimentally (52).

B. EFFECTOF REDUCTION TEMPERATURE ON THE CHEMISORPTIVE A N D CATALYTIC PROPERTIES OF TiO2-SUPPORTED CATALYSTS Following the original work of Tauster et al. (13), every Group VIII metal has been supported on Ti02 and investigated as a SMSI catalyst. Among these catalysts, Pt/TiOz and Rh/Ti02 stand out as the most thoroughly investigated systems with regard to this phenomenon. Perhaps platinum, which is more widely used in petroleum refining catalytic processes, has received more attention than rhodium. Nonetheless, a number of particularly illustrative studies have been carried out on Rh/Ti02. Our own investigations have concentrated on the Rh/TiOz system because Rh has the advantage over Pt of having a much higher activity for some structure sensitive reactions, e.g., alkane hydrogenolysis. Thus, it is possible to measure the catalytic activity at temperatures below 473 K, which is used for the LTR. This is not as practical for either ethane hydrogenolysis or CO hydrogenation on Pt because of its relative inactivity compared to Rh. The first-row Group VIII metals have the added disadvantage that one cannot always be certain of complete reduction to the metal after a LTR. Because of the rich literature and our own interests, we will concentrate our discussion of the effects of reduction temperature on Rh/Ti02 and Pt/TiO2 catalysts. We will briefly discuss the case of Ni catalysts because this metal exhibits a behavior rather different from that of other Group VIII metals and has also been widely studied. 1.

H2

and CO Chemisorption

Tauster et al. (13) first identified what has become a distinctive feature of the so-called SMSI catalysts, the decrease in H2 and CO chemisorption

METAL-SUPPORT INTERACTION

185

capacity as the reduction temperature is increased. It is now well established that the chemisorption suppression after HTR is a common characteristic of all Group VIII metals supported on TiO2, and the extent of suppression is usually taken as an indicator of the extent of metal-support interaction. Even though a direct comparison of data from different laboratories is not straightforward, in part because of the different preparation procedures and pretreatment conditions, a general trend may be ascertained. This is illustrated in Fig. 4, which includes data from several laboratories (13, 16, 22, 34, 54-63). There exists a correlation for TiOzsupported catalysts between the loss of adsorption capacity after HTR (723-773 K) and the original metal dispersion, as measured after LTR (473-523 K). This trend was first pointed out by us for the case of Rh/Ti02 catalysts (16). We explained it in terms of the electronic description prevailing at that time and speculated that the effect of an electron transfer should be more noticeable on the smaller particles. Later, we realized that this trend could also be interpreted in terms of the extent of interfacial perimeter around the metal particle, which increases as the particle size decreases. Thus, a localized interaction could be used to explain the observed correlation. As mentioned above, the idea of localized interac-

El

I

?

0.15-

0.1.

0.05.

FIG.4. Hydrogen uptake (H/M) after reduction at 773 K over a series of Group VIII metals supported on Ti02 as a function of the uptake (H/M) after reduction at 473 K. These measurements have been performed in several different laboratories (13,16,22,34,54-63).

186

GARY L . HALLER A N D DANIEL E. RESASCO

tion lead us to our current TiO, decoration model. Most of the data in Fig. 4 lie approximately on the same curve. Those corresponding to Ir and Pd

catalysts appear to follow a different trend. In the case of Ir this difference may be due to a different H/M stoichiometry. It is generally believed that for Ir the adsorption stoichiometry H/M is about 2 (64). In that case, the Ir points would also lie on the lower curve which assumes a H/M = 1. It is more difficult to explain the different behavior of Pd, for which there are no reports indicating H/Pd stoichiometries greater than one. However, the larger particles of Pd can form bulk hydrides that could increase the resulting H/Pd ratio. The effect of reduction temperature on the chemisorption of CO on Rh/Ti02catalysts has been studied by IR spectroscopy (36,37).The lowtemperature reduced catalysts evidence the presence of the three typical adsorption forms of CO on Rh, i.e., linear (2070 cm-I), dicarbonyl (2030 and 2100 cm-I), and bridged (1800 cm-I). As the metal dispersion increases, the proportion of dicarbonyl form, characteristic of small particles (65) or nonzero valent Rh ions (66), increases. As shown in Fig. 5 (36), an increase in reduction temperature to 517 K causes a more pronounced decrease in the amount of multisite bridged form. At higher reduction temperatures (623 K)there is a substantial decrease of both the

WCIVENUHOERS

wvmmocns

FIG.5 . Infrared absorption spectra of CO adsorbed on a 2-wt% Rh/Ti02 catalyst after saturation at room temperature following reduction at (A) 473 K for 1 h; (B) 517 K for 2 h; (C) 623 K for 2 h; (D) 773 K for 1 h; (E) 773 K for 3 h. (After Ref. 36.)

METAL-SUPPORT INTERACTION

I87

linear and the dicarbonyl forms, although the latter is clearly more affected. In the case of highly dispersed, ion-exchanged Rh/TiO2 catalysts (36,37), only the gem-dicarbonyl form is observed after LTR. However, increasing the reduction temperature in this case causes the appearance of the linear form as the dicarbonyl form disappears. Even though this conversion could be partially explained by particle growth during the HTR it must be pointed out that it is reversed by oxidation followed by LTR (36). This might indicate that a non-zero-valence species, i.e., Rhl+, exists, which is responsible for the CO dicarbonyl form and these are reduced to Rho during HTR. van’t Blik et al. (67) have ruled out this possibility for Rh/AI2O3catalysts by TPR, EPR, and EXAFS experiments. They have proposed that all the Rh is in the zero-valence state but that the CO adsorption itself causes the rupture of the Rh particle and the oxidation to the + I valence state. This concept had previously been proposed by Primet (68) from IR and XPS data. Similar effects on chemisorptive properties were observed for Pt catalysts. In a combined IR and kinetic study (69)it was demonstrated that the HTR of Pt/TiO2 may, indeed, change the ability of H2 and CO to compete for adsorption sites. It was observed that after HTR the CO IR, bands appear to be significantly reduced and at slightly higher frequency, particularly the band corresponding to the bridged form. The latter feature was considered as evidence of a weakening of the surface bond. More recent TPD studies have shown that after HTR the CO becomes more weakly adsorbed (30).Chemisorption studies indeed show that irreversible chemisorption of CO no longer occurs on Pt particles highly dispersed on Ti02after HTR (69a).In addition, under CO hydrogenation reaction conditions, the IR bands of CO could not be detected on Pt/TiO2 catalysts, but they were still present when other supports, i.e., Si02 or A1203, were used. The question is, however, whether such bands are kinetically important. A more significant finding (70)is perhaps the fact that, even at room temperature, the presence of hydrogen readily decreases the intensity of the CO bands. Unfortunately, this experiment has not been reproduced by Robbins in his IR investigation of Pt/Ti02 catalysts (71).This effect was not observed on Pt catalysts supported on other oxides. If this effect is real, the important consequence would be that hydrogen may more effectively compete with CO for the available adsorption sites on Pt when supported on Ti02 compared to less reducible oxides. The alteration of heats of adsorption by SMSI is still a controversial matter. From differential scanning calorimetry measurements Vannice ef al. (72)originally reported an increase in the heat of adsorption of H2 with Pt particle size, but not much effect of the support or reduction temperature. More recently (73)they have corrected the earlier work to conclude

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GARY L. HALLER A N D DANIEL E. RESASCO

that neither particle size nor support affects the heat of adsorption of H2 on Pt. Herrmann et al. (74)made more accurate measurements in a microcalorimeter and reported that the initial heat of adsorption decreases from 22.2 to 19.1 kcal/mol and the average heat from 14.9 to 12.9 kcal/mol for LTR and HTR, respectively. In the case of CO, the initial heat is not changed but the average heat of adsorption decreases from 25.2 to 20.6 kcal/mol for LTR and HTR reduction, respectively. One characteristic of the SMSI originally reported by Tauster et al. (13) was the restoration of normal chemisorption capacities on oxidation at 673 K followed by reduction at 473 K. Table I gives an example of this effect for the case of Rh/TiOz catalysts (46). It is observed that the chemisorption capacity is partially restored when the oxidation treatment is carried out at room temperature. An interesting feature was observed for the chemisorption of H2 and CO on Pd/Ti02 catalysts (75). The HTR does not have a significant influence on the distribution of hydrogen adsorption states, but causes important changes in CO adsorption states. The four CO desorption peaks observed by TPD were attributed to different adsorption structures. The TPD peaks appearing at 638 and 773 K for a 1.7% Pd/TiOz catalyst are ascribed to bridged bonded CO adsorbed on Pd( 100) and Pd( 11 1) planes, respectively. The low-temperature peaks (400 and 488 K)are attributed to linearly adsorbed CO. As shown in Fig. 6, the attenuation of the intensity of the peak at 638 K at increasing reduction temperatures is much more pronounced than the other three peaks. According to the assignment of TPD peaks, this trend would suggest that the bridge-bonding sites on Pd(100) planes are deactivated first. Generally speaking, geometric arguments alone, independent of the surface structure, suggest that bridged sites will be blocked before linear sites. In this case, however, there is a preferential deactivation of a particular type of site. This observation can be related to a recently reported field-emission microscopic study (76) of a TABLE I The Reversibility of SMSI in RhlTi02 by 298 and 673 K Oxidation" Treatment

COIRh

H/Rh

HTR 02 (673 K) + LTR HTR 02 (298 K) + LTR

0.01 0.85 0.01 0.18

0.02 0.76 0.01 0.15

From Ref. 46. Measured at 523 K .

Ethane hydrogenolysis rateb (molecules1Rh atom min) 0.36 <0.001

I .9

189

METAL-SUPPORT INTERACTION

CO/CO,

1.7% W/Ti02

3.5

e& eco em --s 573 0.63 0.56 0.07 T , ( ~K t

A

6 C

Q

I .5

623 673 773

0.42 0.42 0.32 0.32 0.03 0.03

0 0 0

c

300

400

500

600 700 T (K)

800

900

FIG.6. Effect of reduction temperature on the CO TPD spectra for 1.7% Pd/Ti02. (After Ref. 75.)

Ti0,- covered Pt tip. In that case, it was observed that as the temperature was increased, the migration of titanium suboxide initially occurred over (100) and (110) planes, leaving (1 11) planes uncovered. At higher temperatures, a more massive transport of titanium oxide occurred, covering all the planes. Rieck and Bell (75) have also studied the effect of titanium oxide on the dissociation of the CO molecule. They observed that the addition of Ti02 to Pd catalysts facilitates the disproportionation of CO and ascribed this effect to a decrease in the activation energy and/or an increase in the number of sites of CO dissociation. The proposed mechanism for this promoting effect contemplates an interaction between the oxygen of the adsorbed CO molecule with oxygen-deficient TiO, moieties residing on the Pd surface, as depicted in the following scheme: HO

0

HO

0

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GARY L. HALLER A N D DANIEL E. RESASCO

This mechanism is consistent with a recent EPR study of Pt/Ti02 catalysts in which only one Ti3+species (which is assumed to be close to the metal particle) of four different species characterized could be reversibly titrated by CO (76a). Compared to other Group VIII-Ti02 catalysts, Ni/Ti02 (as well as Fe/TiO2) have an additional complication, which is the difficulty in achieving complete reduction to metallic Ni (or Fe) at relatively low temperatures. Nickel forms a stable oxide that resists reduction up to temperatures around 723 K. At these temperatures the SMSI effects would certainly be present. Therefore, unlike with other Ti02-supported catalysts, it is not possible to refer the HTR catalyst (SMSI state) to the LTR catalyst (“normal” state). Most of the comparisons have been done by referring to Si02-supportedNi catalysts. Thus, it is not possible to distinguish between intrinsic support effects and treatment-induced effects in these studies. It would be interesting to perform a LTR of Ni/TiO2 catalysts with hydrogen atoms produced by a microwave discharge. This procedure would probably yield metallic nickel catalysts avoiding HTR, and it would be possible to have a LTR reference state for HTR Ni/Ti02 catalysts. Surprisingly, Burch and Flambard (77)have used as reference a Ni/TiO2 catalyst reduced at 723 K, which, according to these authors, exhibited “normal” H2 chemisorption properties. According to the authors, the “normal” state of Ni was corroborated by catalytic activity measurements. However, they observed that the activity of a 8.5% Ni/Ti02 catalyst decreased by less than only two orders of magnitude after reduction at 923 K, compared to that of the same catalyst reduced at 723 K, which appears to be a rather small effect in comparison with the large activity decreases observed for hydrogenolysis reactions on &/Ti02 catalysts (16, 22). This discrepancy would indicate either a difference in the nature of metal-support interactions of the two metals or, more probably, that after reduction at 723 K, the activity of the Ni/Ti02 catalyst was already affected by the SMSI.

2. Hydrocarbon Reactions In our study (16,22)of the effects of reduction temperature on catalytic properties we investigated several hydrocarbon reactions: hydrogenoly sis of ethane, n-butane, and cyclohexane and dehydrogenation of cyclohexane over a series of Rh/TiOz catalysts with varying Rh particle size. We found that, for all the catalysts investigated, the HTR caused a strong activity suppression, i.e., several orders of magnitude, for the hydrogenolysis reactions (Figs. 7 and 8a) but only a modest suppression (a factor of 2) for the dehydrogenation reaction (Fig. 8b). In the case of hydrogenoly-

METAL-SUPPORT INTERACTION

METAL

DISPERSION

191

(H/Rb)

FIG.7. Rates of ethane hydrogenolysis measured in a pulse reactor over a series of 2wt% Rh/Ti02 catalysts as a function of metal dispersion (H/Rh) as measured by H2 chemisorption after reduction at 473 K . (After Ref. 46.)

sis reactions, two clear trends with varying metal dispersion were observed: after HTR the activity suppression was more pronounced as the metal dispersion increased, whereas after LTR the activity slightly increased with metal dispersion. Other laboratories have also observed much more profound effects on hydrogenolysis reactions than on isomerization, hydrogenation, or dehydrogenation reactions (77-82). It became evident that the HTR had a great influence on structure-sensitive reactions but only a minor effect on structure insensitive reactions. This behavior is quite analogous to the effect of adding a Group Ib metal to Group VIII metal catalysts. As illustrated in Fig. 9a, a striking similarity is observed when comparing the effects of adding Cu to Ni catalysts on ethane hydrogenolysis and cyclohexane dehydrogenation reactions (83), with the effect of increasing the reduction temperature on Rh/TiOz catalysts. By studying a series of Rh/Ti02 catalysts of varying metal dispersion, we noticed that the deactivating effect of HTR for ethane hydrogenolysis could be better correlated with the extent of interfacial perimeter around the metal particle than with the extent of interfacial metal-support contact area. In addition, we found that the kinetics of deactivation followed a square root of time dependence, which is generally observed for diffu-

192

GARY L. HALLER A N D DANIEL E. RESASCO

0.2

0.4 METAL

0.6 DIBPCBBION

0.8

1.0

(Hlnh)

P

0.2

0.4

0.0

0.8

MCTAL

DIBPCM101(

(H/.h)

1.0

FIG.8. Rates of (a) hydrogenolysis and (b) dehydrogenation of cyclohexane measured in a pulse reactor over a series of 2-wt% Rh/TiOZcatalysts as a function of metal dispersion (HIRh) as measured by Hzchemisorption after reduction at 473 K. (After Ref. 46.)

sional phenomena (22). From these three experimental observations, we postulated that a surface migration of a support species onto the metal particle would rationalize the observed effects. This migration process would be initiated at the interfacial perimeter around the particle and

193

METAL-SUPPORT INTERACTION

a

RH/TIO~,48%D

NI-CU

0.01

473

0 2 0 4 0 6 0 8 0 ATCUIC % Cu

573 673 REWCTION 1, K

773

FIG.9a. Ethane hydrogenolysis and cyclohexane dehydrogenation on Ni-Cu catalysts as a function of Cu content and a 2-wt% Rh/Ti02 catalyst (H/Rh = 0.48) as a function of reduction temperature. (After Ref. 22.) b

-9

2-

h

E

-8 -7

1-

-6

-5

0-

-4 -1: 400

’

I

500

-

I

600

’

I

700

. :3 800

REDUCTION TEMPERATURE

FIG.9b. Hydrogenolysis of methylcyclopentane over ion-exchanged Rh/Ti02 catalyst. Variation of total conversion at 423 K and percentage of n-hexane in products as a function of reduction temperature after oxidation at 673 K.

would result’in a “decoration” of the metal surface. The presence of these species on the surface would physically block active sites, preferentially affecting those reactions that require a large ensemble of atoms to constitute the active sites. The SMSI effect would then be essentially geometric in nature. However, we did not rule out the influence of localized electronic effects.

194

GARY L. HALLER A N D DANIEL E. RESASCO

We also demonstrated that for this process to occur, it was necessary to have simultaneously a sufficiently high temperature and a source of H atoms (22). Both conditions are encountered on Group VIII metal catalysts during HTR. They could be studied separately by first treating a Rh/TiOz catalyst at low temperature with H atoms, produced by a microwave discharge, and subsequently heating the catalyst in vacuum at 773 K. As shown in Table 11, the effects on activity and chemisorption capacity were negligible compared with those observed after HTR. It has been shown (84) that H atoms produced by microwave discharge are able to reduce Ti02. However, to start the surface migration, a Ti3+/latticeoxide vacancy must be created close to the metal particle, and, in addition, the metal atoms must have the necessary mobility to get to the oxygen vacancy, or alternatively the Ti3+must move to the metal atoms. The two steps, support reduction and transport, cannot be achieved separately because the diffusion of oxygen ions from the bulk is apparently a more rapid process than the surface diffusion process, which allows the Ti3+ and the metal particles to get together. The loss of catalytic activity exhibited by these catalysts in the SMSI state can be restored by mild oxidation followed by LTR, in the same way as their chemisorption capacity is restored (85, 86). This similarity is illustrated in Table I, which shows the rate of ethane hydrogenolysis over RhlTiO2 after various pretreatments together with the changes in chemisorption capacity. It has been observed (87) that even the level of oxidants ( 0 2 or H20) found in cylinder gases as impurities is enough to reverse, at least partially, the SMSI state. This is probably the reason why some investigators have observed only minor differences in the reaction rates of structure-sensitive reactions after HTR relative to LTR. A particular example is the early work of KO and Garten (88),who studied ethane hydrogenolysis in a steady-state reactor over a number of Group VIII TABLE 11 Hydrogen Chemisorption and n-Butane Hydrogenolysis Activity 4fter Reduction by H Atoms"

Treatment LTR HTR H atoms. 300 K H atoms, 300 K

+ LTR

+ vac. at 773 K

From Ref. 22.

Rate (moleculeslsurface Rh atom min) 20 9 x 10-4 15 9

HIRh 0.45 0.04 -

0.40

METAL-SUPPORT INTERACTION

195

metals supported on Ti02 and Si02 after HTR. For some catalysts, the observed differences between the two supports were less than an order of magnitude, which is much smaller than differences reported in later work. Fenoglio et al. (88a), using an anion-exchanged Rh/TiOz catalyst, have analyzed the methylcyclopentane hydrogenolysis activity and selectivity as a function of reduction temperature following oxidation at 673 K. As illustrated in Fig. 9b, they have observed that the total activity varies with reduction temperature, exhibiting a maximum at about 530 K. As indicated by these authors, the decreasing rate observed in the higher-temperature region can be attributed to the SMSI effect. However, the initial increase in activity observed as the reduction temperature increases from 450 to 520 K must be explained by a different mechanism. As demonstrated by TPR, the hydrogen consumption stops at significantly lower temperatures than 450 K. Therefore, the activity change cannot be ascribed to a reduction process. Instead, they have explained this change in terms of a reconstruction of the metal particles associated with the oxidation-reduction cycle. They speculate that during the reduction process, these well-dispersed rhodium oxide particles may become highly dispersed metal clusters. Consequently, the increase in reduction temperature beyond the point at which the hydrogen consumption is complete may result in a coalescence of very small metal clusters into larger particles. This idea is in agreement with the XANES analysis described in Section VI, which shows that the ion-exchanged Rh/TiOz catalyst exhibits a significant particle size growth from LTR (473 K)to MTR (623 K), but little change between MTR and HTR (773 K). On the other hand, the change in selectivity shown in Fig. 9b as a function of reduction temperature can also be explained in similar terms. It must be noted that, although the percentage of n-hexane in the products is higher at both ends of the reduction temperature range and lower at about 533 K, the rate of formation of all three products increases with reduction temperature between 450 and 520 K, while it decreases thereafter. The higher n-hexane percentages after reduction at 450 and 773 K are not due to an increase in the amount of hexane produced but rather to a more pronounced decrease in the production of 2-methylcyclopentane (2MP). If, as proposed above, the increase in reduction temperature from 450 to 533 K causes the coalescence of small Rh clusters into larger aggregates, the increase in the rate of formation of the three products would indicate that all of them require an ensemble of several atoms. The more marked change observed for 2MP would suggest that the ensemble required to produce this molecule would be larger than for the others. Similarly, when the reduction temperature is high enough to cause the

I96

GARY L. HALLER A N D DANIEL E. RESASCO

migration of TiO, species onto the metal particles, the increase in the hexane12MP ratio can be ascribed to a disruption of the larger ensembles, required for 2MP formation, by the presence of the support species. The similarities exhibited by catalysts in the SMSI state and Group VIIIGroup Ib metal catalysts can be evidenced once more here as we compare the increase in the hexane/2MP ratio as the reduction temperature increases with that observed when Pt catalysts are alloyed with Au (88b). 3. COIH2 Reactions Almost simultaneously with the discovery of the unusual chemisorptive characteristics of the Ti02-supportedcatalysts, a very attractive feature was reported by Vannice and Garten (38-41) for the Ni/Ti02 catalysts. They found that these catalysts, reduced at 723 K, exhibited a significantly higher activity for the CO hydrogenation reaction than Ni/A120,, Ni/SiOz, or Ni/graphite catalysts. Even on a per-gram basis, the Ti02supported catalysts were about one order of magnitude more active than catalysts using other supports. In addition, these catalysts exhibited an enhanced selectivity toward higher hydrocarbons. Vannice and Garten also observed that these catalysts had a superior resistance to deactivation. Unfortunately, as the catalytic performance of these catalysts was compared to that Ni catalysts on other supports rather than to the same catalyst after reduction at lower temperatures, the enhanced activity for the CO/H2reaction was correlated to the SMSI effect, i.e., the loss in H2 and CO chemisorption capacity. This activity enhancement observed for Ni/TiO2 catalysts has been responsible for a good deal of confusion concerning SMSI for all the Group VIII metals. The experimental fact is that the catalytic activity of Ti02-supported Group VIII metals is significantly higher than that obtained when other supports are used, e.g., Si02 or Al203. However, this activity enhancement is observed even after LTR. Therefore, the SMSI effect, as originally defined, i.e., the change in properties of the catalyst reduced at high temperatures relative to the same catalyst reduced at low temperatures, does not play an essential role in the promotion of this reaction. For instance, as shown in Table 111, the activity of Rh/TiO2 catalysts after HTR is somewhat lower than after LTR. This trend has also been observed by numerous authors (37, 89-95), for several Ti02supported systems. These observations do not agree with previous reports, which indicated an increase in activity after HTR (69). Table IV shows that by varying the support the catalytic activity of lowtemperature-reduced catalysts can be significantly varied. Even though the HTR does not cause any promoting effect, the results in Table IV

197

METAL-SUPPORT INTERACTION

TABLE 111 Change in Rate of CO Hydrogenation with Reduction Temperature" Product formation rate Catalyst

H/Rh

2% Rh/TiOz

0.45

2% Rh/Ti02

0.76

3% Rh/Si02

0.85

Reduction temperature (K)

CH4

Cz+

Alcohols

413 113 413 713 113

40.0 35.0 30.0 20.0 2.0

1.1 1.0

2.4 2.1 I .8 1.3 0.01

2.1 2.0 0.0

Turnover frequencies (sec-I) based on H/Rh values as measured after LTR, steady-state reactor at I atm, 473 K, H2/C0 = 3. From Ref. 36.

suggest that the highest activities are usually observed when reducible oxides are used as supports. It is well known that this type of oxide support causes losses in chemisorption capacity after HTR. This is probably why some authors have tried to correlate the enhancement in CO/H2 reaction rate with the decrease in HZand CO adsorption capacity. The analogy of the effect of HTR on Rh/TiOz catalysts with the addition of a Group Ib metal to Group VIII metal catalysts observed for hydrocarbon reactions would appear not to hold for CO hydrogenation. For example, the CO/H2 reaction is strongly retarded when Cu is added to Ni catalysts (96), whereas little deactivation is detected when comparing a Rh/Ti02catalyst reduced at 773 and 473 K. As first explained by Morris et al. ( 9 3 , the reason for this discrepancy is that the SMSI effect is reversed by the CO/H2 reaction itself, particularly by atomic oxygen and water, generated during the reaction. In the case of Ni-Cu catalysts, the activity drops by more than an order of magnitude by the addition of 10% Cu. This TABLE IV Effect of Varying the Support on COIHz Reaction Rate and Selectivity over R k Product formation rateb Catalyst

CH,

CH3OH

C2HsOH

0.95% Rh/Ti02 2.4% Rh/A1203 1.6% Rh/Ce02 0.89% Rh/Si02

39.6 I .6 0.40 0.16

7.0 1.1 I .o 0.30

3.4 0.04 0.060 0.0018

From Ref. 37. Turnover frequency x lW.

Activation energy (kl/mol) 134 k 4 134 k 4

-

134 k 4

198

GARY L. HALLER A N D DANIEL E. RESASCO

has been explained by geometric arguments, considering that Cu can effectively block CO dissociation sites. In the case of Rh/Ti02 catalysts, those sites are kept clean of foreign species by the reaction products. This process has recently been demonstrated in our laboratory (94) in a pulse reactor experiment. As shown in Fig. 10, the initial activity of the hightemperature-reduced Rh/Ti02 catalyst is much lower than the corresponding catalyst after LTR. However, subsequent pulses gradually restore the catalytic activity, eventually reaching the same level of activity a

LTR

\-

I

1

I

2

I

3

I

4

I

6

' ,J

6

FLOW (20min)

NUMBER OF PULSES FIG. 10. Amounts of (A) H20, (B)CHI, and (C) C 0 2 produced from consecutive pulses of CO over a Rh/Ti02 catalyst in a H2 flow following reduction at (a) 473 K and (b) 773 K . For comparison, the amounts produced in a steady-state flow reactor are included. (After Ref. 94.)

199

METAL-SUPPORT INTERACTION

of the low-temperature-reduced catalyst. A similar result has been obtained by Kunimori et al. (95). Therefore, when analyzing the metalsupport interaction effects on the CO hydrogenation reaction we must distinguish between those characteristics that are intrinsic to the particular oxide support and those that can be generated by a given thermal treatment. This consideration will be important when we discuss the relevance of model studies. Even though the promoting effect of Ti02on the CO/H2 reaction should not be related to the HTR-induced SMSI effect, we do not rule out the possibility that it may still be associated with the initial stages of support reduction, which begin at rather low temperatures or a steady-state degree of reduction under reaction conditions. b

HTR

.

\\

NUMBER OF PULSES

FIG.10.

(continued)

200

GARY L. HALLER A N D DANIEL E. RESASCO

4. Other Reactions

Pande and Bell (97-99) studied the promoting action of Ti02 on Rh/ Si02 catalysts for the reduction of NO by H2 or CO. They observed an enhancement of the activity after the Ti02addition. The activity level and the kinetic parameters obtained on these TiO2-promoted catalysts were very similar to those obtained on Rh/TiOz catalysts. The authors explained the promoting effect of Ti02 in terms of the creation of new sites, i.e., partially reduced titania species, which would be active for the NO reduction reaction. The promoting effect of Ti02 addition was found to be less pronounced when CO was used instead of HZin the NO reduction. Pande and Bell ascribed this difference to the formation of isocyanate species that would block the TiO, centers. As in the case of CO hydrogenation reaction, the promoting effect of Ti02 is not related to the hightemperature reduction. In fact, the HTR causes a decrease in the activity of these Ti02-promoted catalysts compared to LTR. Therefore, the enhancement in activity cannot be ascribed to the presence of TiO, species on the metal surface. These authors have also studied the effect of Ti02 on the chemisorption of NO on Rh. They have proposed that the titania species transported onto the metal surface during the HTR block NO adsorption sites, but NO still is able to adsorb on anionic defects on portions of Ti02 adjacent to Rh particles. Only modest differences in activity were observed when the decomposition (36) or synthesis (21) of ammonia over TiOz-supported catalysts were compared after LTR and HTR. Similarly, small effects of reduction temperature were observed for the self-hydrogenation of C2H4 (100). On the other hand, the decomposition of formic acid appears to be favored over Rh/Ti02 reduced at high temperature (101). This reaction, like CO hydrogenation and NO reduction, takes place under partially oxidizing conditions that can, at least partially, reverse the SMSI state. C. EFFECTS OF IMPURITIESON THE EXTENT OF METAL-SUPPORT INTERACTIONS 1. Alkali Metals

Chen and White (102), in an attempt to demonstrate the influence of electron transfer on metal-support interactions, showed that the addition of small amounts of K to Pt/TiOz catalysts suppressed H2 chemisorption on low-temperature reduced catalysts. They explained this result in terms of a charge transfer from K to Pt and suggested that the loss in chemisorption capacity occurring after HTR on Ti02-supported catalysts could be

METAL-SUPPORT INTERACTION

20 1

related as well to an electron transfer. Several years before that (103), we had used a similar argument to explain the unusual capacity of hightemperature-reduced Rh/TiOz catalysts to chemisorb nitrogen, drawing an analogy with K-promoted rhodium catalysts. As mentioned above, at the present we believe that delocalized electron transfers such as those previously proposed may not have significant catalytic consequences. In fact, in the same system in which we observed the high-temperatureinduced nitrogen chemisorption, we looked for an increase in the rate of ammonia decomposition of the same kind as observed on K-promoted Rh, but the observed rate on high-temperature-reducedRh/TiO2 was even lower than that on the low-temperature-reduced Rh/TiOz or on Rh/A1203 or RhlSiOt (36). Recently, Spencer (104) has criticized Chen and White’s explanation pointed out that even though a transfer of electrons from K to Pt may, indeed, occur, the increase in electron density in platinum would be restricted to those atoms adjacent to K, with the electron transfer on the order of 0 . 4 per ~ K atom (105). Thus, it seems unlikely that such a small transfer can have the reported effects on catalytic properties. According to Chen and White (102),a K : Pt ratio of 0.001 in a 25% dispersed catalyst would cause a significant decrease in hydrogen chemisorption, which would indicate that every K atom would be affecting 250 surface atoms. As Spencer has proposed (W), these results could rather be explained in terms of the migration model of titanium suboxide species onto the metal particles. In this particular case, K would form a surface compound, e.g., K2Ti03,which might be mobile at rather low temperatures, e.g., 473 K (LTR). In this way it might promote the redistribution of titania species on the metal surface acting as a “flux” for the oxide transport.

2. Chloride The promoting effect of impurities for the migration of oxide support species over the metal surface might be a rather general phenomenon. For instance, van den Boogert et al. (106) have recently proposed that the presence of chloride may, indeed, favor the migration of vanadium oxide over rhodium particles. Similarly, Orita et al. (107) have found that the CO hydrogenation activity suppression over Rh/TiO2 catalysts was strongly enhanced by the presence of chlorides. van den Boogert et al. (106) have explained this trend in terms of a chloride-assisted TiO, migration over the metal. Bond et al. (108) have proposed that chloride may play just the opposite role in the Ru/Ti02 system, i.e., inhibition of SMSI. Using hydrogenolysis test reactions, they observed that 893 K reduction produced a catalyst

202

GARY L. HALLER A N D DANIEL E. RESASCO

with higher activity than did a 758 K reduction. Oxidation at 623 K and rereduction at 433 K increased the activity about two orders of magnitude for either initial reduction temperature. The second high-temperature reduction gave substantially lower rates than did the first. It is unfortunate that hydrogen chemisorption was not reported for these catalysts in order to make a more direct comparison to other noble metals. It is worth noting that in the study performed by KO and Garten (88) on ethane hydrogenolysis over the nine Group VIII metals, high-temperature-reduced Ru/ Ti02 did not exhibit a substantial decrease compared to the Si02-supported catalyst.

3. Group Ib Metals The effect of impurities assisting the migration of titania species over the metal particles or the formation of new compounds that can act as a flux for such a process becomes reminiscent of what we called (58) “indirect SMSI effect” occurring on Rh-AgITiOz catalysts. Studying the evolution of activity for ethane hydrogenolysis over Rh-Ag/TiOz catalysts as a function of reduction temperature, we observed a more pronounced deactivation on these catalysts than on pure Rh/TiO2. We ascribed this effect to a modified capacity of Ag on these particular catalysts to spread on the Rh surface. We speculated that such spreading could occur only via an interaction with titania since it was not observed on SiO2-supported Rh-Ag catalysts. More recently, in an EXAFS investigation that analyzed the changes in Debye-Waller terms, we confirmed that, indeed, there is a greater degree of Rh-Ag interaction when Ti02 is used as support than when Si02 is used (209,110).The EXAFS results are consistent with a nearly homogeneous mixing of Rh and Ag in clusters supported on Ti02 while these metals are practically immiscible in the bulk. This suggests that TiO, migration on metal particles may extend to migration into metallic clusters and may result in effective compound formation. In an electron-microscopystudy of Ti02-supportedPt and Ag particles, Baker et al. (121) found that when the HTR treatment is carried out in the presence of Pt, the Ag particles adopt a flat pillbox-like structure, almost identical to that observed for Pt particles. On the other hand, if Pt is not present, the Ag particles become large, dense, and globular in shape. These authors attributed to Pt the capacity to indirectly effect a Agsupport interaction by reducing the Ti02 substrate to T407. Otherwise, the Ag particles, which are unable to dissociate Hz, would evidence the normal behavior observed on more inert supports, e.g., Si02 or graphite. If, as proposed by these workers, the only function of Pt in this Ag-Ti02

METAL-SUPPORT INTERACTION

203

interaction is to provide a source of H atoms, it would be possible to observe a similar behavior by independently supplying H atoms from an external source, such as a microwave discharge cavity. Such an experiment has not yet been performed, but evidence of this type would be necessary before ruling out the role of metal-metal interaction. Baker et al. concluded that metal-metal interactions may not have an important influence on the observed morphology changes. In an experiment in which they deposited Pt on half of the specimen surface they observed that, after the thermal treatment, even those Ag particles that were remote from Pt exhibited the characteristic pillbox structure. In addition, some fractions of the Ti02 in that area had been reduced to T407. They took this as evidence of H spillover from the Pt particles to the region of the substrate where Pt was not present. However, the mobility of the Pt particles [or a Pt-titania compound (Z12)]was not taken into account, and the presence of small amounts of Pt in that area may not be completely excluded. 111. Model Studies

Direct physical evidence supporting the hypothesis of the migration of a titanium suboxide species onto the metal during high-temperature reduction has come from surface science studies on low-surface-area (- 1 cm2) unsupported model catalysts. Several model studies have been carried out on Ti02 single crystals onto which a metal film was evaporated (2729). Alternatively, the metal was deposited on previously oxidized polycrystalline titanium foils (30).The advantage of these model studies derives from a better characterization of the structural and electronic state of the surface. In these studies the samples have been analyzed by one or more of the following surface science techniques: Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS),UV photoelectron spectroscopy (UPS), electron energy-loss spectroscopy (EELS), ion-scattering spectroscopy (ISS), or secondary ion mass spectrometry (SIMS). Sadeghi and Henrich (27)prepared their metal/Ti02 model system by evaporating rhodium at room temperature in an ultrahigh-vacuum (UHV) apparatus from a heated rhodium filament over a well-characterized Ti02 (1 10) single crystal. In this case, the Ti02 samples were annealed to produce nearly perfect surfaces with a low density of Ti3+cations. Subsequently, one to four monolayers equivalent of rhodium were deposited and the effects of high-temperature treatment were analyzed by LEED, AES, XPS, and UPS. Likewise, Chung et al. (29, lZ3) prepared various

204

GARY L . HALLER A N D D A N I E L E. RESASCO

Ni/Ti02 (100) systems by varying the thermal treatment of the Ti02 single crystal before and after Ni deposition. One question regarding these model studies that must be addressed is the extent to which they reflect the interactions occurring in real systems. To approximate the high-temperature-reduced real catalysts, these model systems have generally been either reduced in low hydrogen pressures or in vacuum. Sadeghi and Henrich (27) noted that the same effects occur after high-temperature treatments in both H2 and UHV. They gave two possible interpretations of this fact: (1) H2 is not necessary for the migration of Ti oxides onto the metal (2) or there are enough interstitial H atoms under UHV conditions present in the Ti02 support to cause the surface reduction. KO and Gorte (114) have argued that the high-temperature treatment in vacuum for these systems would be equivalent to the HTR conditions in H2 used for TiO2-supported metal catalysts. This is very likely the case with regard to reduction of TiO2, but H2 may play a role other than reduction, e.g., associate with the TiO, species and affect its rate of migration. This point has been made by Munuera et al. (115), who present evidence that the species HTiO, has high mobility, i.e., that H2 is necessary for both reduction and induction of mobility. More recent work has concentrated on the study of model systems in which the characteristic metal-support interaction effects were reproduced by depositing controlled amounts of titania species on metal surfaces (216-124). To ascertain the chemisorption and catalytic consequences of the TiO, overlayer, the evaporation has usually been carried out under 10-7-10-6 tom of oxygen. In this section we analyze the more relevant metal/Ti02and TiOJmetal model studies and their impact on our understanding of the metal-support interactions taking place on real supported catalysts. In their original experiment, Sadeghi and Henrich (27) monitored the surface composition of Rh-covered Ti02(110) single crystals by measuring Auger peak amplitudes as a function of sputtering depth after various thermal treatments. The resulting profiles are shown in Fig. 11. They unequivocally demonstrate the presence of a TiO, layer over the Rh after reduction in H2 at 773 K that is not present on the unreduced sample. Whereas the unreduced sample showed a monotonic decrease in Rh signal and increases in Ti and 0 signals as the Rh layer was removed, the high-temperature-reduced sample showed that the Rh peak initially increased with sputtering depth and, at the same time, the Ti and 0 signals decreased. After reaching a maximum intensity, the Rh peak began to decrease as the Ti and 0 peaks increased. At longer sputtering times the profiles of the unreduced and reduced samples became identical. Simultaneously, they observed that the normal CO chemisorption was sup-

205

METAL-SUPPORT INTERACTION

-

- - - unreduced a t 613 K -reduced

al

01 0

10

20

30

Sputtering T i m e

40

I

D

(min)

FIG.1 1 . Auger sputter profiles (Auger amplitude vs. ion-bombardment time) for unreduced (open points, dashed curves) and reduced (solid points and curves) Rh/(single crystal) Ti02 model catalysts. Curves for Ti and 0 have been shifted up for clarity. (After Ref. 27.)

pressed by the high-temperature treatment, but it was restored after sputtering the TiO, overlayer away from the Rh surface to produce the maximum Rh Auger signal. Qualitatively, the Auger O/Ti ratio could be seen to be lower than on clean titania, but the authors did not estimate the TiO, stoichiometry, except to say that x < 2. In a similar experiment, Belton et al. (30)deposited an Rh overlayer on a thin film of Ti02 (supported on metallic Ti) and annealed it at 773 K. By TPD they observed a significant loss in H2 chemisorption. Subsequent argon sputtering caused a decrease in the Ti/Rh ratio as detected by AES accompanied by an increase in the Ht uptake. This behavior was taken as evidence of the presence of a titania species on the Rh surface. In agreement with Sadeghi and Henrich (27), Belton et d . (30) observed that the Ti/O ratio indicated that the migrating species was a reduced oxide of Ti.

206

GARY L. HALLER AND DANIEL E. RESASCO

Tamura et al. (119a) have reported an angular-resolved XPS study of a Pt/TiOz (100) model system. After deposition of a relatively thick Pt film (about 20 A), annealing in vacuum at 823 K and reduction in 1 atm H2 at 773 K, the Ti 2p signal evidenced, in addition to the Ti4+ states, the presence of.Ti3+and Ti2+.As shown in Table V, the relative intensity of the lower-oxidation-state species increases for collection angles increasing toward grazing. This trend indicates that such reduced species are preferentially located on the surface. In addition, the increase in the (Ti2+ + Ti3+)/Ptratio as a function of the take-off angle is considered as evidence that the reduced titanium oxide species are covering the Pt surface. In one of the first reports of catalytic activity measurements on this type of model system, Chung ef al. (113)compared activities of several Ni-covered TiOz(100) surfaces after various thermal treatments. They reported that the CO hydrogenation activity of these samples depends on the thermal treatment performed before or after the Ni deposition, but the C2H4/CH4ratio produced is independent of the treatment. The important point is that the activity of any of these Ti02-supported samples was more than one order of magnitude higher than that of a Ni(ll1) single crystal. Likewise, the resulting C2H4/CH4 ratio was twice as high as that obtained over unsupported Ni. Analogously, they found that small amounts of TiO, deposited on a Ni(ll1) single crystal caused increases in the catalytic activity and in the C2H4/CH4 ratio similar to those obtained on Ti02( 100)supported samples. The maximum increase was about fourfold and occurred for a deposited amount of TiO, equivalent to 10% of a monolayer. Investigations of model systems have been very helpful and convincing with respect to the role that migration plays in the SMSI phenomenon. However, they are not always unambiguous and a very significant difference in the interpretation of almost identical systems has developed between KO and Gorte (126) and Levin ef al. (117) concerning the relationship between TiO, coverage of Rh and its suppression of CO chemisorption. Basically, KO and Gorte conclude that there is a simple TABLE V Presence of Titanium Oxidation-State Species in a PtITi02( 100) System as Determined by Angular-Resolved XPS

Relative intensity, in %

Angle from surface normal

Ti4+

Ti3+

Ti2+

(Ti3++ TiZ+)/Pt

0 50 70

82 70 62

5 9 16

13 22 22

0.0277 0.0281 0.0336

F]

207

METAL- .SUPPORT INTERACTION 0 (508)/Pt(238)

0 (506)/Pd(330)

0 (508)/Rh (302)

0.0

e

0.5

0.00

&A

0.0 0.0

0.6

1.6

0.0

Ti (365Wt(238)

0.1

0.2

Ti (385)/Rh(302)

0.0

0.1

0.2

Ti (385)/Pd(330)

FIG.12. Saturation coverages for CO and H2as a function of titania coverage on Pt, Rh, and Pd. The coverage of titania that completely suppressed the adsorption of CO and H I was about 1 x I0lJ molecules/cm2 for each metal. (After Ref. 114.)

linear relationship between TiO, coverage and CO chemisorption suppression as shown in Fig. 12, whereas Levin er al. conclude that the correlation is very nonlinear, with most of the suppression occurring between 0-0.5 monolayer of TiO, as shown in Fig. 13. It is possible that the difference is real and attributable to the difference in sample preparation. Both groups use polycrystalline Rh foil as the substrate for their samples but use quite different procedures to produce the TiO, overlayer. KO and Gorte prepare their surface TiO, by evaporation of Ti in a back2 followed by oxidization in O2and reduction in Hz at 650 K ground of 0 and anneal the film in vacuum at 1400 K. The last step is used to produce

0

0.20

0.40 0.60 0.80 1.00 'Tiox

FIG.13. Effect of Tioxcoverage on the amount of CO adsorbed on Rh foil. Solid line indicates profile expected for physical blockage of adsorption sites by TiO,. (After Ref. 117.)

208

GARY L. HALLER A N D DANIEL E. RESASCO

an even overlayer of submonolayer coverage, but it also drives some TiO, into the bulk. Levin et al. attempt to minimize conditions that would form RhTi alloys or RhTiO, compounds by vapor depositing metallic Ti onto Rh followed by oxidization in 50-150-torr 0 2 at 420 K for 5 min. The primary calibration of TiO, coverage used by Levin et al. (117) relies on the identification of a break in the normalized Auger peak intensities for Rh, Ti, and 0, which signals the completion of a monolayer of TiO, and the onset of the growth of a second layer. If one removes the lines and considers the raw data objectively, it must be admitted that there is room for some doubt about whether there is a clear break, although Levin et al. (118) have recently repeated their experiment with A1203 overlayers on Rh and conclude that this system behaves differently, i.e., that there is a linear suppression of CO as A1203 coverage is increased. On the other hand, KO and Gorte (116) rely on the O(508 eV)/ Rh(302 eV) ratio from 0 2 chemisorption at 90 K, which is assumed to form a characteristic saturated O(2 x 2) structure corresponding to a surface coverage of 0.4 x lOI5 0 atoms/cm2. The O(508 eV)/Rh(302 eV) ratio in the presence of TiO, is then converted into an apparent 0 coverage using this calibration point. The complication here is that not all the oxygen may be on the surface and there is an uncertainty about the orientation of the TiO, entity, i.e., Ti-0 parallel to the surface, Ti-0 perpendicular with Ti or 0 end out, all of which would result in quite different O(508 eV): Rh(302 eV) ratios. Both groups use AES, but this technique is probably not surface sensitive enough to decide which of the two groups are closer to the truth. Ion-scattering spectroscopy (ISS) is considerably more surface sensitive than AES and should be capable of resolving the argument, but now we must switch to Pt foil modified by TiO, where there is some available data (119). A plot of exposed Pt sites as measured by ISS against the total amount of Ti deposited on the surface as measured by XPS is very nonlinear, a result that is undoubtedly due to nucleation and three-dimensional growth of Ti02 on the Pt surface. Thus XPS cannot be used to measure the true coverage of TiO, on the Pt surface and AES would be no better. This result does not necessarily speak to the Rh case because not only is a different metal involved but the procedure for adding the TiO, was different (Ti metal was evaporated onto an oxygen monolayer on Pt at 120 K followed by annealing at 775 K). When the amount of CO chemisorbed as measured by TPD is plotted against exposed Pt measured by ISS, the plot is convincingly linear and makes the case for simple site blocking. Recently Altman and co-workers (120) have used ISS in conjunction with their preparation procedure to come to the same conclusion as Dwyer et al. (119), i.e., TiO, overlayers on Pt foil lead to simple site blocking for

METAL-SUPPORT INTERACTION

209

CO. Since Altman and co-workers obtain the same result using their original AES calibration or ISS on a similar preparation, the problems with their work mentioned above may not be serious. Recently, Badyal et al. ( 1 2 0 ~have ) emphasized the strong influence of preparation method and pretreatment on model systems. They have prepared Ru-Ti02 model catalysts by depositing controlled amounts of TiO, on a Ru (0001) single-crystal and studied their growth morphology and adsorptive properties as a function of oxide loading and temperature. Depositing Ti in a torr 0 2 at 300 K, they observed a layer-by-layer growth of TiO,. Under these conditions, the TiO, species on the Ru surface merely blocks sites for CO chemisorption in a geometric manner. The relationship between the TiO, coverage and the amount of CO adsorbed is linear whereas the TPD curves do not show any significant change in peak position as a function of TiO, coverage. On the other hand, if the sample containing TiO, above a certain critical value is heated to 800-1 100 K, diffusion of TiO, into the bulk of Ru occurs, and, at the same time, a monolayer of TiO, spreads over the Ru surface. This new situation has pronounced effects on CO chemisorption. It is observed that the TiO, dependence on the amount of adsorbed CO becomes highly nonlinear, while significant shifts to higher temperatures are observed in the TPD curves as the TiO, loading increases. These results, however, do not explain the differences in the findings of Altman et al. and Levin et al. because the first group, which observes linear CO-TiO, relationships, anneal the sample at high temperature, whereas the second group, which observes nonlinear CO-TiO, relationships, does not follow this procedure. IV. A Comparison of Rh/Ti02 and PtlTiO2

Once it has been accepted that the nature of SMSI is best understood in terms of chemical bonding, albeit bonds between metal atoms on the surface of a particle with cations of an oxide, it follows that the details of such bonding and its consequences will be a strong function of the partners. That is, the details of the metal-oxide interaction will change when two different noble metals supported on the same reducible oxide are compared or when the same metal on two different reducible oxides, e.g., Ti02 and V203,are compared. We will make this point by comparing metal-oxide interactions in the Rh/Ti02 or Pt/Ti02 systems. Using our operational definition of SMSI, i.e., inhibition of hydrogen and CO chemisorption following high-temperature reduction, Rh/Ti02 and Pt/TiOz do appear to behave in an analogous manner. However,

210

GARY L. HALLER A N D DANIEL E. RESASCO

when one considers a macroscopic property that can be directly observed by electron microscopy, the effect of SMSI on particle morphology, it is clear that Rh and Pt respond quite differently to interaction with TiO2. Very early in the literature on SMSI, Baker et al. (125,126) reported that Pt particles spread out to form thin pillbox particles during reduction. Meriaudeau et al. (79) and Singh et al. (127) performed similar electronmicroscopic investigations of Rh/TiO2 and reported that there was no observed change in Rh particle shape with reduction. Comparison between benzene hydrogenation on Rh/Ti02and Pt/TiO2 reveals an observable difference of the effect of metal-oxide interaction, which can be rationalized in terms of the different morphology of the metal particles after HTR (228). When the rate of benzene hydrogenation is plotted against the reciprocal of the temperature, there is a maximum in the rate that is understood in terms of a Langmuir-Hinshelwood mechanism of a surface bimolecular mechanism (129). On Rh/Ti02 the maximum rate occurs at the same reaction temperature on both LTR and HTR catalysts, but, of course, the rates are depressed on the HTR catalyst because some of the sites are blocked by TiO, species on the surface of the Rh particles. The observations on Pt/TiO2 are similar, except the maximum rate on HTR catalyst is shifted to high reaction temperature compared to LTR Pt/TiO2. Temperature-programed desorption (TPD) of benzene demonstrates that the heat of adsorption of benzene on Rh/TiOz is not a function of the reduction temperature, but that benzene is more strongly bound to Pt particles supported on Ti02 after HTR, i.e., the shift in the temperature of maximum rate can be correlated with an increase in benzene coverage on HTR Pt/Ti02 compared to LTR Pt/TiO2. Independent experiments show that a comparable shift of the temperature of maximum rate to higher temperature occurs when the Pt particles are made larger, which suggests that the morphology change to a pillbox structure that has been observed by electron microscopy (125) may cause preferential exposure of the same crystal planes (presumably of low index) that predominate on large crystals and that these planes bond benzene more strongly. The recent field-emission microscopy of TiO, migration over a Pt tip provides an alternative explanation (76). From field emission one concludes that TiO, preferentially covers high Miller index planes, and, in particular, the (1 11) plane is resistant to covering by migration of TiO, . Thus, if the low index planes, and (1 11) in particular, have a higher binding energy for benzene, one could rationalize the shift of the maximum rate of benzene hydrogenation when HTR Pt/TiO, is compared to LTR TiO2. The change in morphology that small Pt particles supported on Ti02 undergo may also explain the apparent contradiction in the findings of

METAL-SUPPORT INTERACTION

21 1

Belton et al. (30) and KO and Gorte (114).Both research groups agree that the temperature of maximum rate of CO and HZdesorption in the TPD are not affected by the reduction temperature of Rh/TiOz (although the amount of adsorption is decreased by HTR), but Belton et al. find that the TPD peak shifts to lower temperatures for CO and H2 on Pt/TiOz while KO and Gorte find no change. However, KOand Gorte were investigating a Pt foil decorated with TiO2, a situation in which morphology change is less likely than for the small particles formed by Pt evaporation on the surface of oxidized Ti foil. Direct evidence for this interpretation can be found in the work of Dwyer et al. (130) who have investigated the TPD of CO from a thick film of Ti02 onto which Pt had been evaporated in a thin film (reduced for various times) and from a Pt foil with various coverages of TiO, . With the latter model system there was no shift of the TPD peaks and a linear decrease in CO adsorption relative to TiO, coverage measured by ISS. On the other model system where small Pt particles were formed there was a modest shift in the TPD peaks to lower temperature. The TPD for these two model systems are compared in Fig. 14. Note also that in the TPD of CO from Pt particles the lower-temperature peak around 400 K, which, if due to desorption from Pt(l1 I), is preferentially retained after 30-min reduction at 875 K relative to the high-temperature peak due to desorption from steps and kinks (131). This is consistent with the Vanselow and Mundschau field-emission results, which show that the (1 11) planes resist coverage by TiO, migration (76). It is interesting to note that the reduced titania phase Tho7 has been identified by electron diffraction of samples of Pt/Ti02 (125), but this phase is not observed on reduced Rh/Ti02 (127). This may be simply a result of different handling procedures in the sequence of steps used to obtain the electron diffraction of a reduced sample, although this seems unlikely since the samples were exposed to air between reduction and electron microscopy in both cases. It is clear from a wide variety of studies that there is reduction of a portion of the Ti02 to Ti3+in both the FWTi02 and Rh/TiOz systems. Moreover, there is always a thermodynamic driving force to form the Ti407 phase because isolated Ti3+in a TiOzmatrix must always be associated with anion vacancies and have less than a full octahedral coordination. As described above, the Ti407structure can be viewed as layers of Ti02 (rutile structure) sandwiched between layers of Ti203 (corundum structure), the Ti ions being in a full octahedral coordination in both layers. Because the coalescence of Ti3+ into Ti407domains is thermodynamically favorable, it must occur in Pt/ Ti02 and not in Rh/Ti02 for kinetic reasons. It has been proposed (112) that a very mobile Pt-Ti double oxide is formed under high-temperature reduction. The movement of this species across the surface may provide a

(K)

B

T1 = 120 K

CO Saturated

P((foll) + TiO,

I

I .

I

I

I

I

I

I

I

200

300

400

500

600

700

800

Temperature (K)

FIG.14. (A) TPD results obtained from a Pt thin film supported on a TiOz film/Ti metal model catalyst as a function of reduction time at 875 K in IO%nbar H2after 0 min (curve I), IS min (curve 2). and 30 min (curve 3). (B)TPD of CO from Pt polycrystal as a function of TiOz-, coverage. ISS ratio of Pt to clean Pt foil, Pt/& equals 1.0 (curve I), 0.27 (curve 2), and 0.07 (curve 3). (After Ref. 130.)

213

METAL-SUPPORT INTERACTION

mechanism by which Ti3+can coalesce to form T407. This would imply that either a similar Rh-Ti double oxide is not formed or is not mobile in the Rh/Ti02 system. Perhaps the most direct proof that the interaction between Rh and Ti02 is different from the Pt and Ti02 interaction comes from experiments that probe the stoichiometry of the metal-oxide interaction chemistry. The hydrogen consumed to produce interaction has been measured by Miessner et al. (132) for Rh/TiOz and by Kunimori and Uchijima for Pt/TiO2 (60). We have measured the oxygen consumption to reverse the interaction on both catalysts, using alternating H2 and 0 2 pulses at 423 K and monitoring the consumption of 02, which did not result in H20 as a product (94). In all chemisorption experiments the H2 or 0 2 consumption has been normalized to the H2 chemisorption after LTR, assuming that the stoichiometry here is HIM,,, = 1 and that this is an approximate measure of the fraction of the metal on the surface. These results, along with some Auger O/Ti ratios for TiO, on the two foils measured by KO and Gorte (114), are collected in Table VI. The expected ratio of HZconsumed to produce the SMSI state to 0 2 consumed to reverse it would be 2, of course, if the reduction produced M,,,TiO, and (2 - x) H20.Obviously this ratio is low for Rh and high for Pt, but one must keep in mind that the reactions were not done on the same samples, that the H2 chemisorption is not an exact measure of exposed surface metal atoms, and that the percentage exposed will change if a change in morphology accompanies SMSI, as it almost certainly does for Pt. It can be concluded from these experiments that the SMSI surface complex, M,,,TiO,, is likely to be more oxygen-rich on Rh than on Pt, and this is confirmed by the O/Ti Auger ratio for TiO, on the surface of Rh and Pt foils. It should be added that this is not likely a simple compound, e.g., RhTiO, but a mixture of stoichiometric surface compounds and may even contain a component of the intermetallic compounds M3Ti and MTi (133).Note also that the stoichiometry of 0 2 conTABLE VI HydrogenlOxygen Stoichiometry to Produce or Reverse SMSI"

H2 or O2consumption

Rh

Pt

H2 consumed to produce interaction O2consumed to reverse interaction (02/H2titration) H2/02from the two rows above O2consumed to reverse interaction (CO/H2 titration) Auger 0 (508 eV)/Ti (385 eV) ratio

1.7 1.2 I .42 1.1 1.1

5.2 2.2 2.36

a

Data from Refs. 60, 94,and 114.

-

0.6

214

GARY L. HALLER A N D DANIEL E. RESASCO

sumed to Rh,,, is independent of the source; i.e., we obtain the same stoichiometry when CO is the oxidant (alternate H2 and CO pulses producing CH4 and H20 after complete oxidation of TiO,) or when 0 2 is the oxidant (see Table VI) (94). Anderson et al. (134) have performed similar experiments where hydrogen and oxygen consumed in chemisorptive titration (as opposed to oxygen consumed to form and break the SMSI state) was measured on Rh/TiOz and Pt/TiO,. These results cannot be directly compared to those presented in Table VI because they used a higher LTR (573 K instead of 423 K) than did Huang (94), but they come to complementary conclusions. They find that Pt enters into the SMSI state more easily (at a lower temperature or in a shorter time) than does Rh and that the SMSI state for Pt/TiO2 is more difficult to reverse than Rh/Ti02 is. V. Current Explanations of the Promoting Effect of Ti02 on Catalytic Activity

As mentioned above, the promoting effect of Ti02 on the CO hydrogenation reaction should not be ascribed to the phenomena occurring after HTR because the promoting effect is also observed after LTR. In this respect, the model studies themselves may have been rather misleading. It has been frequently shown that when small amounts of TiO, species are deposited on a metal surface, the CO hydrogenation activity, indeed, increases. Such a Ti0,-containing metal surface was believed to simulate a high-temperature-reduced catalyst. However, the bare metal surface usually taken as a reference almost surely does not represent the lowtemperature-reduced catalyst; i.e., there is no physical or chemical evidence for TiO, coverage after LTR, but there is evidence for an increase in CO hydrogenation relative to other supports. Therefore, the promoting effect may originate in an interaction that does not require migration and decoration of metal particles by TiO, . Some authors (113, 117, 118) have observed a sharp maxima in CO hydrogenation activity over Ni and Rh surfaces decorated with titanium suboxides species occurring at very low TiO, coverages. According to Levin et al. (117, 118) (see Section I11 for a discussion on the calibration of TiO, coverage), the maximum in the CO hydrogenation rate occurs at 0.15 monolayer of TiO, . The reaction rate as a function of TiO, coverage can be kinetically modeled if one assumes that it involves pair sites consisting of a Rh site at the periphery of TiO, islands and an adjacent Rh site unaffected by TiO,. By contrast, other workers (114, 116, 130) have reported that Pt surfaces almost completely covered (i.e., 95%) by thin

METAL-SUPPORT INTERACTION

215

layers of titanium oxide (Ti0 or TiO2) are, indeed, much more active than the bare metal. KOand Gorte (116) have observed that the reaction kinetics and carbon buildup exhibited by these surfaces are very different from those of the bare metal. These authors propose that the methanation reaction takes place over that oxide-covered surface. As explained above, there has been some controversy regarding the actual degree of TiO, coverage. Even assuming that most of the surface is covered by a thin layer of TiO, and that the surface is active for methanation, however, it is difficult to accept that this picture faithfully represents the promotion effect observed on low-temperature-reduced conventional catalysts. The promoting effect of TiO, has been rationalized in terms of an alteration of the energetics of CO and H2 adsorption. In a series of particularly elegant papers, Raupp and Dumesic (121-123) have systematically studied the effects of varying the TiO, coverage on Ni foils, the Ni coverage on oxidized Ti foils, the oxidation state of Ti, and other effects on the chemisorptive properties of Ni. They have carefully investigated the adsorption of CO, H2, and mixtures of both and have shown that the presence of TiO, species on the surface of polycrystalline Ni blocks CO adsorption and weakens the CO adsorption strength whereas hydrogen adsorption becomes activated. Using TPD, they demonstrated that the effects of titania species on the Ni surface are short-ranged. Figure 15 shows the change in TPD spectra obtained from saturated CO surfaces on the addition of TiO,. The gradual disappearance of an a1 (Ni) peak, accompanied by the appearance of a lower-temperature (YI (TiOJNi) peak, which does not shift with the addition of TiO, lead the authors to postulate that long-range interactions do not have an important effect on CO adsorption. If long-range interactions dominated, the gradual addition of TiO, would have caused a continuous shift to lower desorption temperatures. From the small desorption temperature shift observed by the addition of 0.1 monolayer of TiO, ,they concluded that long-range interactions can account for only a modest decrease in the heat of adsorption, about 1.4 kcal/mol. By contrast, the presence of the titania species caused an increase in the strength of hydrogen adsorption and in the hydrogen coverage based on available Ni sites. The contrasting features of CO and H2 chemisorption observed on these Ti0,-containing nickel surfaces may have implications for the increased catalytic activity reported for Ti02-supported Ni catalysts. The variation of hydrogen and CO coverages as a function of temperature on coadsorption of a H2/CO (4: 1 ratio) mixture on clean Ni and TiO,/Ni surfaces is illustrated in Fig. 16. The presence of TiO, on the surface allows hydrogen to compete more effectively for adsorption sites. On the basis of this result and assuming a mechanism in which CO dissociation

216

GARY L. HALLER A N D DANIEL E. RESASCO

-3 3

z a

Y

X

3

J LL

1

Ti

8

Coverage (ML) 0

r

loo

1

I

I

I

200

300

400

500

TEMPERATURE (K 1

0.10 0.19 0.69°.20

600

700

FIG.15. Temperature-programmeddesorption of carbon monoxide from nickel surfaces containing titania. (After Ref. 122.)

and surface carbon hydrogenation are balanced at the steady state, Raupp and Dumesic (123) have calculated CO hydrogenation rates. They found that, as a result of a higher hydrogen adatom coverage, the reaction rate increases on Ti0,-containing surfaces by one to two orders of magnitude. These authors have drawn an analogy between the effect of TiO, species in weakening the Ni-CO bond strength and that of electronegative species (e.g., chlorine) over metal surfaces. However, they also point out that changes in CO desorption temperature may not necessarily imply chemical perturbations of the Ni atoms. In the presence of foreign species, as a result of geometric constraints, the structure of adsorbed CO may change from strongly bound to less strongly bound forms. This argument would imply that the electronic effects, although present, may be of secondary importance. In fact, the unusual increase in the strength of Hz adsorption observed by these workers weakens the hypothesis of electronic effects since the presence of TiO, species causes the opposite effect observed when electronegative species are added to a metal surface, i.e., a decrease in the adsorption strength. Raupp and Dumesic have preferred to explain the observed effects either by the formation of direct bonds

217

METAL-SUPPORT INTERACTION

co

r

100

1

2dO

I

300

1

LOO 500 TEMPERATURE iKl

I

I

600

700

FIG. 16. Desorption flux of H2and CO coadsorbed on titania-containing Ni (hi0.2 monolayer).Solid curves represent 10 L H2adsorption at 140 K followed by (a) background, (b) 7, (c) 10, and (d) 10 L CO. Dashed lines correspond to desorption of a saturation layer of each gas adsorbed separately. L = one Langmuir exposure. (After Ref. 123.)

between H adatoms and the 0 associated with Ti or by a steric hindrance to H adatoms to migrate and recombine in the desorption step. Even though this interpretation appears to be internally consistent and may be a good explanation for the promoting effect of TiO, species on Ni, it may not be generalized to other Ti0,-Group VIII metal systems or TiO2-supported catalysts. For instance, it does not take into account the fact that the H2 adsorption strength in other Ti0,-Group VIII metal system has been observed to decrease on TiO, addition (74).Therefore, the hypothesis of enhanced H adatom coverage cannot be generalized to these systems. It has also been argued by Anderson e? al. (54) that the promoting effect of Ti02 in supported catalysts can be ascribed to special sites created in the interfacial perimeter around the metal particle. According to this hypothesis, the catalytic activity should increase with the interfacial perimeter, which, in turn, is roughly proportional to the second power of the metal dispersion (22). However, as shown in Fig. 17, the activity for CO hydrogenation on LTR Rh/TiOz catalysts does not show this dependence.

218

GARY L. HALLER A N D DANIEL E. RESASCO

2

1.0-

4

el Y

-

F

:0.6-

[,,/:

,

,

'

0.2 0.2

~

0.6 PLLATIVL

1.0

PEBIYLTLP

Ra. 17. Relative rates of (a) CH4, (b) alcohols, and (c) CI+production at steady state (CO :H1 = 4, 30 min) on a series of Rh/Ti02 catalysts of different metal dispersions as a function of the relative metal-support contact perimeter. (After Ref. 36.)

As discussed previously, it seems unlikely that long-range interactions have a significant effect in either the conventional catalyst or the models. Therefore, we end up with a very reduced number of alternatives. If we accept that the promoting effect must be localized, associated with the support, but not restricted to the interfacial perimeter, one plausible explanation would be to ascribe the observed activity promotion to the presence of support species on the surface of the metal. Under LTR, however, these species do not have enough mobility to achieve a significant transport over the metal, unless they were already there before the LTR. This possibility was first discussed by Santos et a/. (21), who suggested that during the impregnation procedure, a partial dissolution of the support into the acidic impregnating solution may take place. Subsequently, a redeposition of support species over the metal precursors may follow. These species may even remain on the metal particle after reduction. As will be discussed below, this dissolution-redeposition mechanism can be important for other oxides, such as vanadia. In the case of Ti02 , however, its solubility in acidic impregnating solutions is probably too low to have a significant effect. For example, on the basis of an mol/liter, which estimated TiO(OH)+concentration of about 3.15 X would result when Ti02, is placed in a pH = 0.2 solution, Santos et al.

METAL-SUPPORT INTERACTION

219

(21) have calculated, for a 5% Fe/TiO2 catalyst with average particle size of 20 nm, a maximum Ti :Fe ratio of 0.07. This value would be much lower when the impregnation is carried out at higher pH values. However, as shown in Fig. 18, our own data on Rh/TiOz catalysts prepared at various pH, show that the activity enhancement compared to Rh/Si02 catalyst occurs even at high pH values. In addition, a comparison of data from several laboratories for various Group VIII-Ti02 catalysts indicates that the promoting effect of Ti02 for CO hydrogenation reaction does not depend on the preparation procedure (37, 75, 135). Similarly, catalysts prepared by impregnation or ion-exchange methods exhibit the same activity enhancement. It may be recalled that in the ion-exchange preparation procedure the supernatant liquid is washed out and this step would greatly reduce the redeposition of any dissolved support species. Therefore, it would appear that the dissolution-redeposition mechanism is not determinant in the promoting effect of TiO2. Assuming that the formation of methane proceeds via the dissociation of CO to form an active surface carbon species that is subsequently hydrogenated, Rieck and Bell (75) have pointed out that the promoting effect of TiO, could then be explained in terms of a higher rate of CO dissociation. As described in the previous section, from their TPD spectra they concluded that the CO dissociation was facilitated by Ti02 promo40

8l

/

20

0/

pH OF IMPREGNATING SOLUTION

FIG. 18. Turnover frequencies for CO hydrogenation over a series of Rh/Ti02 catalysts as a function of the pH of the solution used in the impregnation step in the preparation; following reduction at 473 K (solid points); following reduction at 773 K (open points). (Data taken from Ref. 46.)

220

GARY L. HALLER A N D DANIEL E. RESASCO

tion. It would then appear that the titanium oxide species particularly affect the bonding of the CO molecule. However, these authors have also noted that when H2 is present, the CO dissociation appears to proceed more readily on both the Ti02-promotedand TiO2-unpromoted Pd catalysts. A possible scheme to illustrate how hydrogen might assist the promotion of the CO dissociation in Ti02-containingcatalysts is HO

0

HO

OH

Therefore, the promoting effect of titanium oxide species might be described as a synergistic effect on both CO and H2. In the next section we discuss the role of hydrogen in the metal-support interaction, which perhaps may be related to its role in the promotion of the CO-H2 reaction. ROLEOF HYDROGEN I N THE METAL-SUPPORT INTERACTION The correlation between the support reducibility and the extent of metal-support interaction seems rather well established (32). Therefore, the obvious role of hydrogen would be the reduction of the support, probably involving spillover of atomic hydrogen from the metal ( 1 I I , 125). In fact, the SMSI effects have been observed only on those metal systems that dissociate H2 or in systems where dissociated Hz is provided by a second metal, e.g., Ag/TiOz in contact with PtlTiO2 (111). Conesa et al. (136) and Sanz et al. (136a) have investigated Rh/TiOz with NMR, EPR, and quantitative adsorption techniques. Among their findings is the observation that the EPR signal of Ti3+passes through a maximum as Rh/TiOz is reduced at progressively higher temperatures. They suggest that this is the result of migration of hydrogen into the bulk that becomes stabilized at the previously generated anionic vacancies V O , giving rise to a diamagnetic hydride-like species as follows Ti3+...Vo+ Rhn-H,

-

(Ti-H)’+

+ Rh,

(6)

Annealing at high temperatures in vacuum reverses Eq. (6) reforming Ti3+...Vo,the bulk Ti3+and associated vacancy that is detected by EPR and surface hydrogen on a metal particle, Rh,-H,, which is detected by NMR. In very recent work ( 1 1 3 , this group has added quantitative XPS to the techniques used to study the effect of hydrogen on SMSI. They demonstrate that annealing at high temperature in the presence of Ti3+is not sufficient to cause migration, at least not to significantly change the intensity of the XPS Rh signal. Heating to the same temperature in the

METAL-SUPPORT INTERACTION

22 1

presence of gas-phase hydrogen does cause a decrease in the Rh XPS signal, from which they conclude that the migrating species is HTiO,. What is curious in these experiments is the fact that reduced Ti formed by ion bombardment is not reoxidized by oxide ion transport from the bulk when heated in vacuum at 773 K. Certainly single-crystal Ti02 rendered nonstoichiometric by ion bombardment is annealed back to a stoichiometric Ti02 surface at this temperature (1366). We had previously observed that Ti3+formed by H atom reduction at 300 K did not migrate to cover Rh particles under vacuum at 773 K (22) (see Table I1 in Section 11,B,2). However, we had rationalized this negative experiment on the assumption that effective transport of reduced Ti into the bulk (oxide ion transport to the surface) was fast compared to transport on the surface over Rh particles as suggested by experiments on single crystals (1366). Thus, the question as to whether the reduction of the support is the only requisite for the migration of the TiO, species still remains unanswered. VI. Bonding and Charge Transfer in Group VIII-Ti Systems

The bulk structure and electronic properties of the Group VIII-Ti alloys have been studied for some time (137).Among the stable intermetallics formed, the stoichiometric compounds M3Ti and MTi, e.g., Pt3Tiand PtTi, have been widely investigated. Both theory (138) and experiment (139-142) indicate that there is d-d-bonding between the Group VIII metal and Ti with net transfer of charge from Ti to the Group VIII metal. One must ask to what extent the bonding situation in the intermetallic compounds may carry over to metals supported on TiOz. Recent investigations would indicate that there is a strong similarity between the electronic interactions taking place in the intermetallic compounds and those in the strongly reduced Ti02-supported metal samples. Ocal and Ferrer (143) have analyzed XPS linewidths after depositing thin Pt films on both clean and oxidized Ti(0001) followed by annealing. In both cases a narrowing of about 0.2 eV in the width of the Pt 4.h2 line was observed on annealing to 800 K, but this may be only a result of sintering of Pt particles. This was interpreted in terms of electron transfer between Pt and Ti. In agreement with previous work, the CO uptake per Pt atom was significantly suppressed by the Pt interaction with TiOz induced by the hightemperature annealing. Our own X-ray absorption measurements of Rh-Ti compounds exhibit suggestive similarities with the reduced Rh/Ti02 catalysts. First of all, as described above, the EXAFS analysis of high-temperature-reduced Rh/TiOz(100% dispersed) catalyst provides evidence for the formation of

222

GARY L. HALLER A N D DANIEL E. RESASCO

Rh-Ti bonds (32). An inspection of the near-edge structure (XANES) of the K edge of Rh reveals interesting features (9). As illustrated in Fig. 19, the XANES of Rh foil, Rh3Ti, and RhTi show significant variation in the sizes of the peaks located at 10 and 35 eV above the edge. It is observed that both peaks decrease as Rh is gradually replaced by Ti around each Rh atom when going from pure Rh to Rh-Ti. The first peak can be related to symmetry-allowed electronic transitions from the core level 1s to empty states above the Fermi level (244). Thus, the decrease in the size of that peak from Rh to RhTi can be explained in terms of a filling of Rh states resulting from the formation of Rh-Ti bonds. The second peak after the edge reflects multiple scattering processes of the outgoing electron (144). In agreement with multiple scattering calculations done for different cluster sizes (245,146) we have observed that this second peak increases with metal particle size (number of ligands). Figure 19 shows that it also varies with the kind of ligand because it decreases as Rh scatterers are replaced by Ti. In the case of the highly dispersed Rh/TiOzcatalyst (see Fig. 201, it was observed that as the reduction temperature was increased the size of the first peak gradually decreased, very much like for the series Rh, Rh3Ti, RhTi. This would strongly suggest that the same type of Rh-Ti interactions occurring for the Rh-Ti alloys, i.e., hybridization of d-orbitals and electron transfer from Ti to Rh, does occur for the high-tempera-

0

50 ENERGY (eV)

100

FIG. 19. Near-edge X-ray absorptiondifference spectra of (a) Rh foil, (b) Rh,Ti, and (c) RhTi. The difference spectra are obtained by subtraction of an arctangent function constrained to pass through the inflection point (taken as zero energy) of the Rh Kedge and the second minimum in the near-edge structure, which occurs about 50 eV above the edge. (After Ref. 9.)

223

METAL-SUPPORT INTERACTION 1.0 v)

E

a

-4 6 i

0.5

tG

z I-

I 0 0

50 ENERGY (eV)

100

FIG.20. Near-edge X-ray absorption difference spectra (defined in Fig. 19) of a highly dispersed Rh/TiOl catalyst following reduction at (a) 494 K; (b) 628 K; and (c) 775 K. (After Ref. 9 . )

lure-reduced Rh/Ti02 catalysts. It is worth repeating that this is very likely bonding between a Rh particle and Ti cations and thus is not identical to the bonding in the intermetallic compounds. We would be remiss not to mention an opposing view of Martens et al. ( 2 4 6 ~who ) have repeated the EXAFS analysis of Rh/Ti02 at low (473 K) and high (723 K) reduction temperatures and have come to quite different conclusions. They claim that there is no evidence for the covering of Rh particles by a suboxide of Ti02 or the formation of Rh-Ti bonds. Instead, they have assigned the peak on the low r side of Rh-Rh in the magnitude of the Fourier transform to a long Rh-0 bond at 2.61 A. Because they do not believe any covering exists, they are forced to conclude that the anomalous properties of small Rh metal particles supported on Ti02 in the SMSI state are the result of electronic perturbation, but do not venture to suggest what kind of electronic perturbation might be involved. They do report two Rh-Ti interactions at long distances (i.e., 3.41 and 4.39 A). Because the bonding distance in the intermetallic compounds is about 2.67 A, it is difficult to see how interaction at such a long distance could have a significant electronic perturbation. Moreover, the long Rh-0 bond is unlikely to produce an electronic perturbation because it is already present after the LTR and, if it were the result of the electronic perturbation, it would give a change in the XANES in the opposite sense to that shown in Figs. 19 and 20 (i.e., an increase in the near-edge intensity). There is no way to rectify the opposing EXAFS analyses at the present time. However, the overwhelming evidence from reaction studies of structure-sensitive reactions and model studies supports the decoration model for Rh/TiO2; we will continue to assume that

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covering of metal particles has a dominant effect in SMSI in the following discussion. Assuming that the electronic interactions in SMSI are similar to those in the intermetallic compounds, we must then ask whether there are any significant effects on chemisorption or catalysis other than the effects of site blocking and ensemble breakup, which have clearly been demonstrated to affect chemisorption capacity and activity for structure-sensitive reactions, respectively. Sadeghi and Henrich have addressed this question by preparing Rh particles on a TiOz surface effectively reduced to Ti203(147). The samples were prepared at low temperature to avoid migration and emphasize electronic interactions. Using several different electron spectroscopies, they conclude that there is electron transfer for the titania surface to the Rh particles. However, they find no evidence for an effect of this electron transfer on either the amount or kind of CO chemisorption as detected by UPS. This is not a very sensitive probe compared to other methods of investigation of chemisorption, e.g., TPD, or compared to catalytic probes. In the 9th International Congress on Catalysis, a series of remarkably resolved electron micrographs of Pt/Ti02were presented by a group from China ( 1 4 7 ~ ) Small . Pt particles, partly covered with Ti oxide species after HTR, were neatly shown. It was observed that depending on the pretreatment temperature, the oxide layer was more or less mobile. The higher the reduction temperature the less mobile became the oxide layer over the metal particle. A more striking finding of this group was the identification of Pt-Ti intermetallic compounds after HTR. By diffraction and image simulation techniques, they have identified the formation of cubic PtTi, hexagonal Pt3Ti, and cubic Pt3Ti. The chemisorption properties of Pt3Ti have been reported (148-150). Both the structure of and CO chemisorption on the (1 11) plane of Pt3Ti have been investigated by Bardi et al. (148)and Paul et al. (150), but those authors reached quite different conclusions. Bardi et al. find a surface composition that is near that predicted from the bulk, i.e., about 25% Ti, and considerable CO dissociation, whereas Paul et al. conclude that the first atomic layer is quasi-pure Pt and that there is negligible CO chemisorption. Both groups used samples from the same Pt3Ticrystal but significantly different cleaning and annealing procedures, so that the differences might be real. Because CO dissociation is known to occur on Ti, the finding of CO dissociation on a surface containing Ti but none on the quasi-pure Pt surface would be consistent. Both groups agree that the TPD of the nondissociative CO is similar to Pt(ll1) but shifted toward lower temperature by 50 K, equivalent to a 4-kcal/mol decrease in the CO adsorption energy on (1 11) Pt3Ti relative to (1 11) Pt assuming the same

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frequency factor. This is in very good agreement with the microcalorimetric measurement of Herrmann et al., who report a 4.56-kcal/mol decrease in the average heat of adsorption of CO on HTR Pt/TiO2 compared to LTR Pt/Ti02 (74).(Herrmann et al. report an analogous decrease for H2 average heat of adsorption equal to 1.98 kcal/mol.) In both cases of CO chemisorption on Pt3Ti, the TPD shift can be interpreted as a ligand effect of Pt-Ti bonding, between the Pt-enriched first atomic layer and a presumed Ti-enriched second atomic layer by Paul et al. (150) and the regular truncated ligand field of Pt3Ti by Bardi et a!. (148).It appears that the (1 11) Pt3Tisurface suppresses CO chemisorption by a factor of about 2 relative to (1 I I ) Pt according to Bardi et al. Chemisorption of 0 2 causes TiOz islands to form and draws Ti to the surface at temperatures above 650 K (150). Electron energy loss spectroscopy (EELS) demonstrates that the Ti4+ in Ti02islands formed on Pt3Ti by 0 2 chemisorption at 650 K (which may contain some Ti3+ defects) can be reduced by CO desorption at 450 K (150). This is a very important observation because it suggests that the Ti3+detected by XPS or Auger electron spectroscopy (AES) on catalysts subjected to CO/H2 reaction mixtures and evacuated at the reaction temperature may not have had detectable Ti3+under reaction conditions, but it was formed by the desorption of CO above 450 K. This may be the case in the work of Dwyer et al. (130),who report that Ti3+is formed on TiOzpromoted Pt black by H2 reduction at 450 K and is also detected by XPS on the same catalyst after 16 h of reaction at 625 K in a 1-atm 3 : 1 mixture of H2:CO. We note that the surface of single crystal Ti203 is readily oxidized even at ambient temperature by O2 (151). Even HzO will dissociatively adsorb on defect Ti203 to partially oxidize the surface to Ti4+at room temperature, although perfect (047) Ti203 only associatively adsorbs H2O with accompanying transfer of electrons from the Ti alg band (152). Because the mechanism of CO hydrogenation to CH4 is generally believed to involve dissociative adsorption of CO ( I S ) , it would seem that the oxygen atoms formed would oxidize any Ti3+ under reaction conditions. This picture is in accord with pulse experiments we have performed at 535 K using alternating CO and Hz pulses; i.e., CO pulses are adsorbed and react with a subsequent pulse of H2to produce CH4,but no H20 is produced until one 0 atom per surface Rh atom (measured by H2 chemisorption after LTR) has been consumed (see Table V) (94). There will always exist some Ti3+at steady-state (or equilibrium) conditions, which will depend on the H2 : H 2 0and/or CO :C 0 2ratios and temperature. Unfortunately, we do not have an in situ measurement of the Ti3+concentration under reaction conditions, and our prejudice is that its concentration is unlikely to be significant.

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VII. Metal-Support Interactions in Other Oxide Supports and Related Phenomena A vaguely defined “metal-support interaction” has frequently been used to describe modifications of metal properties observed when oxidesupported catalysts are thermally treated. After the original report by Tauster ef al. (13) on the SMSI effect in TiOz-supported catalysts, the same authors (32) extended their operational definition to other reducible oxides. Consequently, several investigations were conducted using reducible oxide supports such as vanadia (87,106, 154-156), niobia (157162), or ceria (95). In general, the same characteristic features of Ti02 were obtained for these oxides, i.e., suppression in H2and CO chemisorption capacity, suppression of catalytic activity for several reactions, promotion of the CO/H2 reaction, and reversibility by oxidation. In this section we will discuss only those features that have not been contemplated for Ti02 or that are substantially different. Meriaudeau et al. (19) first observed that when a Pt/CeOZ is reduced at 773 K it loses catalytic activity and chemisorption capacity such as Pt/Ti02. However, it exhibits some characteristics not observed in TiO2. For example, on Pt/TiOz the IR intensity of CO was about 80% suppressed when HTR was compared to LTR but there was no change in frequency, whereas the suppression was only about 30% on Pt/CeO2 but was accompanied by a frequency shift of 12 cm-I. The relative HZsuppression on Pt/Ce02 compared to Pt/Ti02 was even less than that for CO. Some important differences relative to titania-supported catalysts were observed for vanadia (V203)-supportedcatalysts. As a result of a higher solubility, the degree of dissolution followed by redeposition during the preparation step is more pronounced on V2O3 than on Ti02 (154). This effect causes a partial loss of exposed metal area, even before any HTR. Hydrogen spillover onto and into the support during chemisorption at room temperature is a more prominent feature of vanadia than for titania (154). In addition, it appears that the chemical interaction between VZO~ and the metal is different from that between Ti02 and the metal. This difference is evident in the stability of the SMSI state as measured by the loss in catalytic activity (see Fig. 21). In the case of the high-temperaturereduced Rh/TiOZ catalyst, the hydrogenolysis activity increases very rapidly when it is placed in contact with unpurified gases that contain oxidizers. In contrast, the activity of the Rh/V2O3 catalysts under the same conditions increases very little. A significant decrease in the activation energy for hydrogenolysis reactions on Rh/V203 catalysts after HTR, contrasting with the increase observed for TiO2-supported catalysts, would suggest the presence of ligand effects that have different conse-

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loo

8

0.1

FIG.21. Evolution of conversion for n-butane hydrogenolysis at 623 K as a function of reaction time. Circles represent Rh/Ti02 catalysts; squares, RhlV2O3. Full symbols represent tank purity reactants; open symbols, high-purity reactants. (After Ref. 87.)

quences in the two systems. Substantial increases in activation energy had previously been observed (3.5) for n-butane hydrogenolysis on highly dispersed Rh and Ir/Ti02 catalysts, which were ascribed to a change in mechanism. This change was evidenced by the appearance of isomerization products as the reduction temperature was increased. On the other hand, when the dispersion was lower, no isomerization products were observed and the activation energy was only slightly increased from LTR to HTR. In the case of V203, however, isomerization products are obtained after HTR, even with poorly dispersed catalysts but accompanied by a decrease in activation energy. Niobia (Nb2O5) appears to have a SMSI behavior similar to that of Ti02 support (157-162). In general, the observed effects have been explained in terms of the same migration model developed for TiO2. KO et al. (158) have used magnetic studies to demonstrate that, when Ni/Nb205 is reduced at high temperature, it loses metallic nickel and the apparent particle size decreases. They have interpreted these changes in terms of the formation of a surface compound. Kunimori et al. have compared the SMSI behavior of Rh/NbzO5 and Rh/Ti02 for both CO hydrogenation (162) and ethane hydrogenolysis (162) and have noted differences in the

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nature of SMSI. Particularly noteworthy is the suppression in the CO hydrogenation activity on Rh/Nb205 after HTR. As in the case of V2 0 3 support, this may reflect only the stronger Rh-NbzOs interaction compared to Rh-Ti02 similar to that mentioned above for Rh-V203 (see Fig. 20). In a recent international conference, Kunimori et al. (162a) reported that the Rh-Nb205 interaction increases with the temperature of calcination as evidenced by the formation of a RhNb04 phase, identified by X-ray diffraction, when Nb205-promotedRh/Si02 catalysts are calcined at 973 K.Subsequent TPR studies indicate that as the Rh-NbzOj interaction increases, the Rh becomes more difficult to reduce. There is a series of other phenomena that sometimes have been called “SMSI” but are probably due to totally different effects. For example, chemisorption losses observed after HTR and attributed to metal-support interactions have been reported for nonreducible oxides such as Si02 ( 1 6 3 , A1203 (164, 165), or MgO (166). It seems unlikely, however, that the observed effects in these cases could be related to partial reduction of the support. Nevertheless, Dautzenberg et al. (164, 165) attributed the observed decrease in the H2 chemisorption capacity of a high-temperature-reduced PtlAI20, catalyst to the formation of a Pt-AI alloy. Similar to the TiO2-supported catalysts, the high-temperature-reduced Pt/A1203 catalysts were restored to their original state by a 773-K oxidation followed by LTR. Thus, several authors have used this example as evidence of SMSI in Al2Oj-supported catalysts. More recent studies (167-169) have shown a strong correlation between the suppression of chemisorption capacity and the sulfur content in the support. Kunimori and Uchijima (168) have proposed that sulfur might catalyze the reduction of the alumina leading to alloy formation. However, recent investigations of Apesteguia et al. (169) attributed the observed effects to a redox reaction involving sulfur impurities, Likewise, sulfur impurities were shown (170) to be responsible for the effects observed on high-temperature-reduced MgO supported catalysts that previously had been related to metal-Mg bond formation (166). A growing interest in the possibility of metal-cation and metal-metal bond formation is evident in the recent literature. One application of this concept is in the stabilization of species on a given support. For example, it has been demonstrated that Rh (171) can be effectively “anchored” to a NaY support by the use of transition-metal ions such as Fe2+or Cr3+.The observed high metal dispersions and high stabilities under HTR conditions were attributed to the formation of direct Rh-Crn+ cation bonds. It has been suggested that this bonding would involve an electron transfer from Rh to Cr”+ ions. A similar metal-cation bonding was proposed by

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Yermakov (172) to account for the stability of PtMo/SiO2 catalysts prepared by sequential decomposition of allylic complexes. Moreover, it has been shown that in presence of Ce3+cations, welldispersed nickel particles (7-13 8, in diameter) could be obtained by reduction of nickel exchanged in acidic X zeolite. A direct electronic interaction Ce3+---* Ni" leading to the electron enrichment of the small nickel particles has been proposed based on X-ray emission spectroscopy. This electron enrichment of the metal has been rationalized by the electron donating properties and the polarizing power of the Ce3+cation (171a,b). VIII. Conclusions

The strong suppression in catalytic activity and chemisorption capacity observed on TiO2-supported catalysts after reduction at high temperatures (ca. 770 K) can be, at least partially, explained by a geometric blocking of sites by TiO, species. The dominant mechanism of formation of TiO, species on metal particles is migration during the high-temperature reduction, but some deposition of metal precursors during preparation may also occur. This picture finds an analog in the structure of the bimetallic clusters observed in Group VIII-Group Ib catalysts, in the sense that the structure sensitive reactions, e.g., alkane hydrogenolysis, are most affected whereas only modest effects are observed for structureinsensitive reactions, e.g., hydrogenation-dehydrogenation. The migration of reduced species from the support is accompanied by the formation of metal-Ti bonds, which provide the thermodynamic driving force for the migration. The metal-support bonding is not identical to that in the intermetallic compounds because of the associated oxygen that imparts cationic character to the Ti and Group VIII metal partner; in other words, it may be analogous to the Group VIII-Ti bonding that persists on intermetallic compounds surfaces after oxygen chemisorption. In fact, strong similarities exist between the high-temperature-reduced TiOz-supported Group VIII catalysts and the intermetallic compounds of Ti and the same Group VIII metal. Therefore, ligand effects of the type observed in the surfaces of intermetallic compounds that can modify the chemical properties of the metal cannot be ignored and probably play a role in some catalytic reactions. On the other hand, the promoting effect of TiOz for CO hydrogenation may not be directly related to the presence of TiO, species on the metal surface because the enhancement in activity relative to other supports is observed even after reduction at low temperatures where there is no evidence for migration. However, there is evidence that deposition of

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TiO, on the surface of a bulk metal can increase the rate of CO hydrogenation and selectivity of products relative to the clean metal surface. In the case of Ni, the effects of surface TiO, on CO hydrogenation rate can be rationalized in terms of the perturbation in the relative competition between CO and H2 for chemisorption sites. This explanation cannot be generalized to other Group VIII metals. There is circumstantial evidence that a Ti species may be directly involved in the formation of the site on which reaction occurs on other Group VIII metals, but definitive proof and elaboration of the mechanism await future work. Most of the conclusions reached for the Ti02 support can be extended to other reducible oxides (e.g., V ~ 0 3 ,CeOz, Nb205). However, differences in the extent of the interaction or other secondary effects such as degree of dissolution during the preparation and formation of hydrogen bronzes during reduction can modify the observed effects. It is less likely that migration of support species or formation of short metal-cation bonds can occur at normal reduction temperatures in less reducible oxides such as A1203 or Si02. ACKNOWLEDGMENTS We are grateful to many of our colleagues who provided us with reprints and preprints of their work on metal-support interactions and particularly wish to acknowledge Stevenson el a/. (173) and Tauster (174). Neither of the latter preprints are explicitly referenced in the text, but both were important in sharpening our thoughts on the subject of SMSI. A special thanks for his critical reading of the manuscript is due to Sam Tauster, who noted both our English and conceptional errors; we lay claim to all errors of interpretation that remain. We gratefully acknowledge support from NSF under ENG-7813314. CONICET (NSF U S Argentina Cooperative Science Program),and the Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-AC02-81ER10829. REFERENCES 1 . Weisz, P. B., Adv. Catal. 13, 137 (1962). 2. Herbo, C., J . Chim. Phys. Phys.-Chim. Biol. 47, 454 (1950). 3. Schwab, G. M., Block, J., Muller, W., and Schultze, E., Natunvissenschqften 44,582

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ADVANCES IN CATALYSIS, VOLUME 36

Structure and Reactivity of Perovskite-Type Oxides LUIS G. TEJUCA AND JOSE LUIS G. FIERRO Instituto de Catdisis y Patroleoquimica C.S.I.C. Serrano 1 19, 28006 Madrid, Spain AND

JUAN M. D. TASCdN Instituto Nacional del Carbdn y sus Derivados C.S.I.C. Apartado 73, 33080 Oviedo, Spain

1.

Introduction

Perovskite-type oxides have the general formula AB03 (A, cation of larger size) and are structurally similar to CaTiO, , the mineral that gave its name to that group of compounds. These materials were first studied because of their important physical properties such as ferro-, piezo-, and pyroelectricity, magnetism and electrooptic effects. The earliest works where perovskites were used as catalysts were conducted in 1952 and 1953 by Parravano (1, 2), who observed that the rate of the catalytic oxidation of carbon monoxide is affected by ferroelectric transitions in NaNbO,, KNb03, and LaFe03 ( 1 ) . This may be interpreted as evidence supporting an electronic mechanism for this reaction. A similar effect in the rate of CO oxidation was observed in the neighborhood of the ferromagnetic transition in Lao.ssSro.3sMnOj (2). Years later, Dickens and Whittingham (3) reported data on the recombination of oxygen atoms on the surface of alkali metal tungsten bronzes M,W03, where M = Li, Na, or K. The catalytic activities were found to be closely related to the electronic properties of the bronzes. Galasso ( 4 ) reported a study carried out by Epperly et al. ( 5 ) on the application of A(BB’)O3 oxides as electrodes for fuel cells; B and B’ cations were selected, respectively, for 231 Copyright 0 1989 by Academic Press, Inc. All rights of reproduction in any form reserved.

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their catalytic properties and for the corrosion resistance they impart to the perovskite. Later on Meadowcroft (6) suggested the application of LaCo03 as an oxygen electrode for fuel cells. Libby (7)and Pedersen and Libby (a), prompted by Meadowcroft’s report, carried out a study on the hydrogenation activity of Cd hydrocarbons of rare earth cobalt perovskites and pointed out the potential application of these oxides as automobile exhaust oxidation catalysts. Following these studies, Voorhoeve et al. (9) and Bauerle et al. (10) reported the high catalytic activity of rare earth perovskites and copper tungsten bronzes for CO oxidation and NO reduction, respectively. Although Schlatter et al. (11) indicated the limitations of Voorhoeve et al.’s study for drawing valid conclusions on the suitability of these oxides as substitutes of platinum as catalysts for the removal of automotive pollutants, a continued and increasing number of papers appeared from 1971 up to the present referring to the preparation of massive and supported perovskite oxides of a moderate or high specific surface area, their bulk and surface properties and their role in heterogeneous catalysis. This is illustrated in Fig. 1, where the relative number of publications relevant to surface science and catalysis is represented from the early work of Parravano (I) up to 1986. This representation clearly shows the considerable impact of the publication of Nobel Laureate W. F. Libby (7),which greatly favored the development and rapid increase that the scientific research in these fields underwent in the following years. The ample diversity of properties that these compounds exhibit, is derived from the fact that over 90% of the natural metallic elements of the periodic table are known to be stable in a perovskite oxide structure and also from the possibility of synthesis of multicomponent perovskites by partial substitution of cations in positions A and B giving rise to cornpounds of formula (AxAf,-x)(ByBf,-y)Oj. This accounts for the variety of reactions in which they have been used as catalysts. Other interesting characteristics of perovskites are related to the stability of mixed oxidation states or unusual oxidation states in the structure. In this respect, the studies of Michel et al. (12) on a new metallic Cu2+-Cu3+mixed-valence Ba-La-Cu oxide greatly favored the development of perovskites exhibiting superconductivityabove liquid N2 temperature (13). In addition, these isomorphic compounds, because of their controllable physical and chemical properties, were used as model systems for basic research (14). The most numerous and most interesting compounds with the perovskite structure are oxides. Some hydrides, carbides, halides, and nitrides also crystallize with this structure (4). This review will refer only to the study of oxides and their behavior in the gas-solid interface and in heterogeneous catalysis. It will not cover, however, electric, magnetic, and optical properties of perovskites. Comprehensive studies on these

PEROVSKITE-TYPE OXIDES

239

FIG. 1. Trend of publications in Surface Science and Catalysis on perovskite oxides appearing before and after 1971.

latter topics can be found in Galasso (4), Voorhoeve (M), Goodenough and Longo ( 1 3 , Goodenough (16), and Nomura (17). In Section I1 the perovskite and related structures are briefly introduced. In Section I11 the methods most frequently used for perovskites preparation are described comparatively. Sections IV, V, and VI refer to the bulk and surface properties of perovskites. Some of these properties will facilitate understanding of the catalytic action of these compounds. Section VII includes a review of the reactions where perovskite oxides were used as catalysts. Some of them are described separately (Sections VI1,A-H). These include reactions that were more extensively studied or reactions that may have an increasing interest in the near future. Section VI1,I includes less studied reactions such as oxygen homomolecular exchange, hydrogen and NH3 oxidations, NtO decomposition and dehydro-

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genation, and dehydration of 2-propanol. Potential applications for coal liquefaction and removal of carbonaceous deposits from coked catalysts are also considered. Section VII,J refers to SO2 adsorption on A, B, and oxygen sites and its effect in the catalytic performance of perovskites for CO and hydrocarbon oxidation. Section VIII describes other applications of perovskites such as in actinide storage in radioactive waste, in solidstate chemical sensors and in superconductivity at high temperatures. Finally, in Section IX some prospective lines of research are suggested. II. Structure

A. THEPEROVSKITE STRUCTURE The ideal perovskite-type structure is cubic, with space group Pm3mO),. Its unit formula is ABX3, where A is the larger cation, B is the smaller cation, and X is an anion. In this structure, the B cation is in a six-fold coordination and the A cation is in a twelvefold coordination with the anions. Figure 2a shows the corner-sharing octahedral units that form the skeleton of the structure (stabilized by the Madelung energy), whose body-center position is occupied by the A cation. Alternatively, the structure can be viewed with the B cation in the center, as shown in Fig. 2b. The perovskite structure is thus a superstructure with a Re03-typeframework, formed by the introduction of A cations into the BX3 octahedra building. Raveau (18) has recently pointed out the important role of the Re03 framework as a host structure for deriving numerous structures of complex oxides.

0 0 .9 A X FIG.2. ABXl ideal perovskite structure: (a), cation A, or (b). cation B, at the center of the unit cell. (Reprinted by permission from Ref. 15.)

24 1

PEROVSKITE-TYPE OXIDES

The lower limits for cationic radii are rA> 0.09 nm, and rB > 0.051 nm in the case of oxides. Goldschmidt (19), on the basis of geometric considerations, defined the tolerance limits of the size of ions through a tolerance factor t = ( r A + rx)/*(r~ + r x ) , where r A , TB ,and rx are the radii of the respective ions; t would be equal to one for the ideal cubic structure (Fig. 2). In fact, the perovskite structure exists in oxides only between the limits 0.75 < t < 1.0 with t between 0.8 and 0.9 in most cases. For t > 1 the calcite and aragonite structures are prevalent, whereas for t < 0.75 the stable structure is ilmenite. Roth (20)has classified the limits of the existence of these competing structures according to the ionic radii values. Besides the ionic radii requirements, the other condition to be fulfilled is electroneutrality, i.e., that the sum of charges of A and B ions equals the total charge of X anions. This is attained in the case of oxides by means of charge distribution of the form A'+B5+03,A2+B4+03,or A 3 + B 3 + 0 3Moreover, . partial substitution of A and B ions giving rise to complex oxides is possible while keeping the perovskite structure. Figure 3, elaborated from some comprehensive compilations of data on the structure and properties of this type of compound (4,15,17,21,22), shows that almost all the stable elements have been included in the perovskite framework, many of them in both the A and B positions. In what follows we will A X H

I X Er

A

B A

B A B

A U

Po

Tb

B

B A B

B

A

A Pu

B

A Eu

B Np

B

A Sm

h

Nd

B Am

B

A

A

B Cm

A

Cf

A Er

Ho

B

B EC

B

A Dy

Tb

Gd

B Es

A Tm

B Fm

A Yb

B Md

Kr

Lu

B No

. Lw

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limit the structural description of perovskites to the case of X = oxygen, for the reasons indicated in Section VI1,I. The ideal perovskite structure appears only in a few cases for tolerance factors very close to 1 and at high temperatures. In other conditions different distortions of the perovskite structure will appear. The compound CaTi03 was originally thought to be cubic, but the true symmetry was later shown to be orthorhombic (23). Distorted structures with orthorhombic, rhombohedral, tetragonal, monoclinic, and triclinic symmetries are known, but the last three types are very rare and poorly characterized (21), so we will only describe the orthorhombic and rhombohedral distortions. For t values in the range of 0.75 < t < 0.90 a cooperative buckling of corner-shared octahedra takes place, leading to the orthorhombic distortion. This network, sometimes typified as GdFe03 structure, has a space group Pbnm,and its relationship to the perovskite structure is as shown in Fig. 4a. It is obtained by tilting oxygen octahedra in such a way that the A atoms are displaced along (1, 1, 0) pseudocubic directions or (0, 1, 0) directions. The true orthorhombic cell is usually referred to as “0orthorhombic,” characterized by a lattice parameter ratio (cla) > fi,to be distinguished from the 0‘-orthorhombic structure, with (cla) < fi. This latter form is the result of a superimposed Jahn-Teller distortion of the perovskite structure. When there is no octahedra buckling, a small deformation from cubic to rhombohedral symmetry may take place. This occurs for tolerance factors in the range of 0.9 < t < 1 .O. The rhombohe-

ox,, (3x1 @ A FIG.4. Orthorhombic (a) and rhombohedral (b) distortions of the perovskite structure. (Reprinted by permission from Ref. IS.)

PEROVSKITE-TYPE OXIDES

243

dral distortion, sometimes referred to as “LaAI03 structure,” has the RSc-D% symmetry. Its relationship to an ideal perovskite is shown in Fig. 4b. In most cases the anions are displaced, thus requiring a large unit cell; in the most general situation the anion displacements may be decomposed into R?c and R3m components. Nonstoichiometry in perovskites can arise from either cation deficiency (in the A or B site), anion deficiency, or anion excess. This subject has been widely discussed in some excellent reviews (24, 25); thus, we will focus only on general features of the different types of nonstoichiometry. Because of the stability of the BO, groups, A cations can be missing without collapse of the perovskite network. The ReO3-type structure is thus the limiting case of A-site vacancy nonstoichiometry. The most typical example is that of tungsten bronzes, &W03. The question as to whether A-site atoms and vacancies are ordered is not fully resolved (24). Because of the large formal charge and the small size of the B cations in perovskites, B-site vacancies are not energetically favored; B-B interactions, which may be a compensating factor, are favored by hexagonal stacking of A03 layers. Accordingly, a number of hexagonal perovskites exhibiting B-site vacancies have been described (24). Normally, these vacancies are ordered between h-h layers where the B 0 6 octahedra share faces, in agreement with Pauling’s rules for the sharing of coordination polyhedra. Anion vacancy in perovskites is more common than cation vacancy. Unlike the well-known case of W 0 3 , anion-deficient nonstoichiometry is not accommodated by the crystallographic shear mechanism, but by assimilation of vacancies into the structure, resulting in supercells of the basic network. The review by Rao et al. (24) contains numerous examples of this kind of behavior. Anion excess has been described in a more limited number of systems. Structural details of this type of compounds can be found in Rao et al. (24) and Smyth (25). B. RELATEDSTRUCTURES The strong bonding between B-site ions and oxygen ions leads to retention of the BO3 grouping even when a three-dimensional network is no longer permitted by stoichiometry. The simplest case is that of the SnF4type structure, consisting of the superposition of octahedra nets, with the free peaks of octahedra of one layer lying in the holes formed between the peaks of octahedra in the neighboring layers. The peaks of octahedra are sunk so deeply into these holes that an almost planar structure is obtained. In the K2NiF4-typecompounds, slices of the perovskite structure

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one unit cell thick are displaced relative to one another along the c axis. Two neighboring K-F layers are positioned over one another as in the rock-salt-type structure. Compounds such as La2Ni04having this type of structure are important in relation to perovskite transformation in mild reduction conditions. If the slices of the perovskite structure have the thickness of n unit cells, the composition of the crystal is An+,Bn03,,+, . The compounds Sr3Ti*O7and Sr4Ti3010 (26) are typical members of this family. This type of compound shows disordered intergrowth (27). A further variation results from a combination of the above-described structures with those of the tetragonal PbO-type structure. A typical example of the resulting class of compounds., referred to as Aurivillius phases, is BitNbzOsF, where the bismuth atoms combine with the oxygen atoms to form a highly altered polymeric network (28). Other examples of both ordered and disordered intergrowth phenomena from Aurivillius phases have been given by Rao et al. (24). 111. Preparation

A number of methods have been used in the synthesis of perovskites; the choice of a particular one depends mostly on the expected use for these oxides. Obviously, no attention has been paid to textural characteristics of samples whose uses are based on their electric or magnetic properties. However, application of perovskites in the field of catalysis requires solids with a well-developed porous network. As "the present review is concerned particularly with the surface and catalytic properties of perovskites, we will place special emphasis on preparation methods leading to a high surface : volume ratio. Also, methods yielding homogeneous solids will be discussed because of the important effect that inhomogeneities may play in heterogeneous catalysis. A primary characterization of perovskite-type oxides must include textural analysis and X-ray identification of the phase(s) present. For a more detailed characterization, structural analysis for establishing the lattice position of cations and surface analysis (by means of techniques such as XPS) for defining the surface concentration and oxidation states of cations are desirable. Consequently, information provided by these techniques will furnish the essential criteria for comparing the different preparation methods. For convenience, we will classify the methods used to date for the preparation of pure perovskite phases according to the scheme proposed by Courty and Marcilly (29) for the whole field of mixed oxides. Table I gives a survey of methods used as a function of the phenomena on which they are based.

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TABLE1 Preparation Methods for Perovskite-Type Oxides

Method Reaction Solid-solid Liquid-solid

Physical

Chemical

-

Ceramic Crystallization Coprecipitation Single compounds Mixtures Complexation

Dry evaporation Explosion Spray-drying Free-drying

A. METHODSBASEDON SOLID-SOLID REACTIONS

Solid-solid reactions are the basis of the most frequently used procedures for preparing mixed oxides, especially when the surface areas of the resulting solids are not an important parameter. Indeed, these high-temperature methods are essential for preparing perovskites with special morphologies, such as monocrystals or thin layers. Because this kind of method is most frequently used for the preparation of ceramic materials, it is usually referred to as the “ceramic method.” The solid-solid methods offer the advantage of their simplicity. It suffices to calcine physical mixtures of the single-oxide components or other adequate precursors. Very high temperatures (usually > lO00”C) are required for a complete reaction between the single phases to take place, thus leading to a drastic loss of surface area by sintering. Consequently, surface areas of perovskites that have been prepared by this method are very low. As an example, Voorhoeve et al. (30) prepared by the ceramic method a series of 20 perovskite oxides, almost all of them having surface areas smaller than 1 m2/g. Thus, in spite of its simplicity, this method is not very attractive for preparing perovskite catalysts. A further drawback is the lack of homogeneity of the solids obtained, because of an incomplete reaction between the mixed precursors. Several approaches have been made toward gaining a better degree of mixing and, if possible, lowering the temperature necessary for complete reaction. Besides repeating grinding and firing (a usual procedure in solid-state reactions), joint grinding of precursors in a liquid medium (3I),pelleting of powder mixtures (32), and high-pressure calcination (33) have been tried, this latter method being especially efficient; moreover, use of high oxygen pressures in a belt-type equipment (33, 34) has permitted stabilization of high oxidation states of several elements in the perovskite structure.

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More recently ( 3 3 , a new method using peroxides as oxidizing agents has been reported to obviate the need of high oxygen pressures. In other cases the nature of the precursors was changed in order to generate in situ single-oxide phases with small particle sizes and highly reactive surfaces. Precursors whose anions can be easily eliminated, such as hydroxides, nitrates, carbonates, acetates, and oxalates, have been preferred for perovskite preparation. High-temperature solid-state methods deserve particular attention in the preparation of perovskite phases with special morphologies such as thin films or single crystals. Thin films of perovskites, used in microcircuitry, are seldom used in catalysis, and their preparation will not be discussed here. Indeed, this subject was reviewed by Galasso (41, who concluded that vacuum evaporation provided the best results in the case of BaTiO3 thin layers. Single crystals of perovskites have been, however, widely used for fundamental studies in catalysis. Among the various preparation techniques also reviewed by Galasso (4,growing from a flux has been the most commonly used. Voorhoeve et al. (36) prepared Lal-,Pb,Mn03 by growing from a lead borate flux heated at 1250°C. Minute flux inclusions were found to significantly alter the surface composition, but they were readily removed by leaching with diluted acid (37). Etching with a more concentrated acid has permitted elimination of residues of unreacted oxides and, in some cases, was used for increasing the surface area of perovskite phases (38,39). The main disadvantage of the flux growing technique is the high-temperature reaction with crucible material, which is then incorporated into the perovskite as an impurity. This has led to undesired presence of platinum in perovskites which, as will be discussed in Section VII, brought about misleading results of catalytic activity for these materials.

B. METHODS BASEDON LIQUID-SOLID REACTIONS The simplest physical method based on a liquid-solid reaction is dry evaporation. It has much in common with the ceramic method, as the homogeneity of the solution is not preserved during evaporation. Thus, a heterogeneous solid is formed that resembles the solid mixtures used as starting material for solid-state reactions, and consequently, inhomogeneous materials with low surface areas are obtained. Arai et al. (40) obtained LaM03 (M = Cr, Mn, Fe, Co, Ni, Cu) perovskites with surface areas in the range of 0.6-4.8 m2/g.However, Gysling et al. (41) reported that LaRhO3 prepared by dry evaporation was much more homogeneous than the same perovskite obtained by the ceramic method. The explosion

PEROVSKITE-TYPE OXIDES

247

method proposed by Wachowski er al. (42,43)uses the explosive properties of ammonium nitrate added to a precursor solution of perovskite, which is then dry-evaporated. Surface areas in the order of 30 m2/gwere obtained by carrying out a preliminary vacuum decomposition at 300°C and then calcination in oxygen at 500°C. This method offers the advantage of using simple laboratory equipment and requiring a relatively low expenditure of work. Another possibility for preparing perovskites from a solution consists in increasing the rate of evaporation by powdering the liquid to give a mist that is subsequently spray-dried. Using the two-stage method proposed by de Lau (44),Johnson, Jr. et al. (45) prepared LaMnO3-substituted perovskites with surface areas in the range of 9-17 m2/g. Later on, Imai and Orito (46), using a three-stage method, prepared Lac003 with 12 m2/g. Imai et al. (47) have used several additives dissolved in the starting solution to increase the surface area of the final sample. The best result was obtained with NH4Cl, in which case a surface area of approximately 50 m2/g was attained. More recently, Murphy and King (48) have used spray-drying for preparing L~.sSro.zCOo.sNio.z03, which was subsequently plasma-sprayed on nickel grids (see Section II1,D) to be used as an electrocatalyst. Freeze-drying is probably the physical method that best preserves the homogeneity originally present in the solution. This results in a homogeneous precursor that needs a rather low temperature for complete transformation into perovskite. Tseung and Bevan (49) first prepared Lac003 by means of this method, with a surface area of 38 m2/g. Johnson, Jr. et al. (45) and Wachowski et al. (42,43)have also obtained perovskites having surface areas greater than 30 m2/g. However, in other cases (50,51) high calcination temperatures were needed, thus resulting in surface areas smaller than 4 m2/g. Crystallization of complexes from a liquid phase is, a priori, a method expected to produce extremely homogeneous mixed oxide precursors, although, as Courty and Marcilly pointed out (29),the difficulty in preparing complexes containing the stoichiometric ratio required for the catalyst severely restricts its applicability. Crystallization of oxalate complexes La[M(C204),] * 9H20 (M = Fe, Co) constitutes a typical application of this procedure (52, 53). However, the high temperatures necessary for complete decomposition of these complexes (640°C for Fe and 800°C for Co) reduce the surface areas of the corresponding perovskites. Nag and Roy (52,53)studied in detail the decomposition of precursors. Usha et al. (54) obtained some further insight into these thermal decomposition reactions. Coprecipitation constitutes the most widely used chemical procedure for separating a precursor from the solution. However, two different

248

LUIS G . TWUCA

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cases should be distinguished. In the first one, the cations to be present in the mixed oxide are separated from the solution in the form of a single mixed compound. This method, which has many characteristics in common with that of crystallization, is limited by the stoichiometric ratio of cations in the precursor and in the final mixed oxide. A typical application to perovskites, first reported by Gallagher (55), is the precipitation of cyanide complexes such as L~[CO(CN)~] * 5H20, which are then calcined to LaCoO3. Gallagher (SS), Gallagher and Schrey (56),and Tascdn et al. (57, 58) have studied in detail the thermal decomposition of cyanides leading to the formation of CaCo03, LaFe03, and similar perovskites. Three reaction steps were found: dehydration of the pentahydrate, oxidation of the anhydrous cyanide, and final reaction to yield the mixed oxide (58). Because of the low tepperatures necessary for obtaining the perovskite phases (450°C for Lac003 ; 330°C for LaFe03), surface areas of 37.5 m21g (LaCo03) and 9.5 m2/g (LaFe03)were obtained (57). Other applications of this method to perovskites have included precipitation of Ba[Ti(OH)6](59) or barium titanium citrate as precursors for BaTi03 (60) apd strontium titanyl oxalate, as a precursor for SrTiO3 (61).No surface area data were given for these preparations. The second case refers to the joint precipitation of at least two different compounds. O~viously,this will result ie a very heterogeneous precursor; however, there are no constraints imposed by stoichiometry. For the preparation, of perovskites, coprecipitation as hydroxides, carbonates, and oxalates have been very frequently used. In the case of hydroxides, the precipitating agents were KOH (62),NH40H ( 6 3 , or tetraethylammonium hydroxide (64).Coprecipitation as carbonates was performed by means of (NH4)&03(45,62),K2C03(62),or NH4HC03+ NH40H (65). Oxalic acid (42,57)or ammonium oxalate (52) was used for the coprecipitation of oxalates. In principle, precipitation by means of oxalic acid Seems to be the most adequate method because (1) the presence of alkaline ions, which could contaminate the precipitate, is avoided and (2) the absevce of ammonia or their derivatives prevents a possible selective dissolution of one of the precipitates because of the well-known ability of transition metal ions to form ammoniacal complexes. Gallagher et al, (62) have compared the different coprecipitation methods and indicated that carbonates required higher temperatures than did hydroxides to form the perovskite phase. Oxdates and carbonates required similar temperatures as oxalates form carbonates as intermediates in their decomposition. Tasc6n et al. (57, 58) found Co304, La202CO3, and La203 as intermediate compounds in the formation of LaCo03 from mixed oxalates. Small differences were observed amongst surface areas of perovskites obtained by coprecipitation, which are generally lower

PBROVSKITE-TYPE OXIDES

249

than 10 m2/g(42,45,57,62,63,65), irrespective of the precipitating agent used. An exception was found in a sample of SrTiO3 with a surface area of 64 m2/g (64),, whose precursor had been totally decomposed at 550°C. Complexation in the form of aniorphous compounds with a vitreous structure is another chemical procedure for separating from the solution a solid precursor while preserving as much as possible the homogeneity present in the solution. A method uses the complexing properties of hydroxiacids, in particular citric acid (66,67). It was originally employed for preparing perovskites such as LaCr03(66,67), and LnA103(Ln = Y, La, Sm) (67); no surface area data are available in these cases. Later, Tasc6n et al. (57) prepared the oxides LaM03 (M = V, Cr, Mn, Fe, Co, Ni) by this method, with surface areas ranging from 3.5 m2/g (LaCr03)to 55 mz/g (LaVO,), showing the influence of the transition-metal ion on the decomposition temperature of the corresponding citrate complexes. The mechanistic features of citrate decomposition have been analyzed by Courty er al. (67), Delmon and Droguest (68) and Tasc6n et al. (57), who found three decomposition steps corresponding to dehydration, first decomposition, and pyrolysis; the latter was catalyzed by the presence of cations whose oxides were oxidation catalysts. This may explain the different behavior observed for the first-row transition series of metal ions (57). C. COMPARISON OF METHODS As we have outlined in the preceding discussion, the presence of different metal ions leads to strong differences in the final heating temperature for obtaining a single perovskite phase. Consequently, comparison of the performance of different preparation methods is possible only when data are available concerning the preparation of a specific mixed oxide by different methods. Unfortunately, scarce examples exist in the literature in this respect. In the following, some of these will be discussed. Berndt et al. (69) compared the ceramic and coprecipitation methods for the preparation of interlanthanide perovskites. Although the objective of their work was not to prepare catalysts and, consequently, no surface areas were measured, clear differences were observed among the temperatures necessary with the different methods for obtaining pure perovskite phases. The solid-state reaction of mixtures of separately precipitated hydroxides proceeded much faster than that of physical mixtures of single oxides (ceramic method). This is a clear indication of the influence of particle size in solid-solid reactions. The spray-drying, freeze-drying, and coprecipitation methods were compared in a study carried out by Johnson, Jr. et al. ( 4 3 , concerning the preparation of La,-,M,Mn03 (M = Sr,

250

LUIS G . TEJUCA

et al.

Pb, K, Ce, Co, Ni, Mg, Li) perovskites. Coprecipitation produced wellcrystallized oxides, with surface areas smaller than 10 m2/g. Spray-drying led to solids with some compositional segregation; their surface areas were below 17 m2/g. Freeze-drying permitted the synthesis of perovskites at temperatures low enough to preserve larger surface areas (14-32 m2/g). The relative order of catalytic activity per unit surface area was freezedrying > spray-drying > coprecipitation; according to the authors, this could be due, in part, to the differences in the calcination temperature (i.e., merely changing the calcination temperature would change the surface properties of these oxides). According to Delmon and De Keyzer (701,the chemical liquid-solid methods are used in the great majority of preparations of mixed oxides at both laboratory and industrial scales. Consequently, these should provide the essential guidelines for the preparation of practical mixed oxide catalysts. Tascdn et al. (57) have carried out a comparative study of coprecipitation and complexation methods for the preparation of LaMO3 (M = V, Cr, Mn, Fe, Co, Ni) catalysts. Coprecipitation as a single cyanide compound, or as a mixture of coprecipitated oxalates, as well as the citrate method, was used. Heating temperatures for obtaining a single perovskite phase were higher for oxalate decomposition (900-1000°C) than for cyanide or citrate decomposition. Surface areas varied in the ranges of 2-6 m2/g (oxalates), 10-40 m2/g (cyanides), and 4-55 m2/g (citrates), with a clear influence of the cation in position B of the structure. Wachowski et al. (42,43) have compared the surface areas of a series of eight perovskites prepared by different methods: ceramic (<2.4 m2/g), coprecipitation as oxalates (4.5-1 1 m2/g), explosion (21-37 m2/g), and freeze-drying (22-39 m2/g). Again, surface areas clearly depended on the minimum temperature necessary for complete reaction. The greatest losses in surface area by sintering were observed in the temperature range 7OO-93O0C. Wachowski (43) and Tascdn et al. (57) have reported a detailed textural analysis of perovskites obtained by different methods. Hysteresis loops of adsorption isotherms measured on samples prepared by freeze-drying and explosion methods (43) are of the A type according to de Boer's classification (71), while adsorption isotherms of samples prepared by ceramic and coprecipitation methods give type B loops. This clear differentiation proves that the porous texture of the samples is strongly dependent on the preparation method. The pore structure of these perovskites is very close to that previously suggested by Tascdn et al. (57) for oxides of similar composition prepared by other methods, although, as pointed out by Wachowski ( 4 3 , the absolute values cannot be compared because of the different histories of the samples investigated. It seems worth mentioning

PEROVSKITE-TYPE OXIDES

25 1

that some perovskite samples showed stepwise adsorption isotherms. Since adsorption of different probe molecules on these oxides pointed to the existence of heterogeneous surfaces, it was concluded (57) that step formation in these systems must be due to lateral interactions of adsorbed nitrogen molecules rather than to the presence of energetically homogeneous surfaces, in agreement with Nicholson and Silvester’s criterion

(72). It can be concluded that, except for the explosion method, homogeneous perovskites with high surface areas are obtained only by those procedures requiring specialized equipment, such as freeze-drying. The chemical liquid-solid methods yield lower surface areas but require much simpler laboratory equipment. Moreover, close control of the factors that influence the specific surface area is feasible with these latter methods; in some cases it is possible to obtain values greater than 40 m2/g.Finally, the methods based on solid-solid reactions are extremely simple, but the very low surface area values and the lack of chemical homogeneity they yield render these procedures inadequate for preparing perovskite catalysts. The ceramic method must be restricted to perovskite oxides whose textural characteristics are not important for their use, and, even in this case, care should be taken to check the purity and degree of homogeneity of the samples.

D. SUPPORTED PEROWKITES Since the earliest studies of perovskites as potential anticontamination catalysts, it was believed that the most adequate support for these oxides would be an inert ceramic material such as cordierite (2A1203- 5Si02 2Mg0), which had been previously used for metallic catalysts in the treatment of automobile exhaust gases. In the first attempts (62) for supporting perovskite oxides on cordierite monoliths, the single perovskite phase was synthesized by any of the conventional methods. An aqueous slurry of this powdered material was prepared, and then the ceramic support was dipped into the slurry. Finally, the wetted catalyst was fired in order to bind the active phase to the support. In other cases (73)the perovskite and an alumina support were physically mixed and then pelleted together with the help of stearic acid. Obviously, these simple methods could not adequately fulfil the objectives of supporting oxide catalysts, namely, to increase the exposed surface of the active species and to stabilize this species toward sintering. By means of the impregnation methods, first used by Johnson, Jr. et al. (74), the perovskite is synthesized directly on the support. In order to

-

252

LUIS

c . TWUCA et a/.

minimize migration of the solution during the drying step (which would lead to a macroscopic inhomogeneity in the distribution of the catalyst on the support), special techniques such as freeze-drying were used. Indeed, catalysts prepared in this way showed higher catalytic activities for CO oxidation than did those prepared by conventional drying, thus showing that the more homogeneous distribution led to a higher degree of dispersion of the active phase. The presence of an alumina wash coat on monolithic supports caused only minor changes in catalytic activity. On the other hand, when several supports were used for dispersing the oxide Lao.sPbo.sMnO3, the order of catalytic activity was low surface area aalumina 3 high surface area y-alumina > hollow mullite > molecular sieve. Again catalytic activity, used as a fingerprint for the degree of active-phase dispersion, provided evidence of an insensitivity to support surface area. It seems, however, that the decrease in catalytic activity of perovskites supported on high-surface-area alumina and molecular sieve may be due, in part, to a reaction of the active phase with the support. Thus, Johnson, Jr. et al. (74) indicated that no perovskite formation on these supports was evident by X-ray diffraction. Also, the formation of a spinel by reaction of transition-metal cations with the surface of y-alumina is a well-known drawback in the preparation of other supported mixed oxide catalysts. Difficulties due to this phenomenon seem to have been overcome by Jin-Fang (75), who recently reported the preparation of Lao,,Sro.3MnO3(23 wt%) supported on alumina of high oxidation activity. An approach recently used by Mizuno et al. (76)to avoid reaction of the active phase, L a o . ~ S r ~ . ~ Cwith o 0 ~a, high-surface-area support consists in precoating the cordierite with a lanthanum oxide layer. When La203 was supported bn cordierite, two different phases (hexagonal and monoc~inic) were formed. In Fig. 5 , the fraction of monoclinic La203 and also the rate for propene oxidation at 400°C are plotted as a function of the total amount of La203 precoated. Maxima were obtained for a 18 wt% of La203 in both cases. The evident parallelism of these representations suggests that La&ro.2Co03 is preferentially formed on the surface of monoclinic lanthanum oxide. This is certainly an important result for establishing the scientific bases for the preparation of supported perovskite catalysts. Recently, Nudel et al. (77) have impregnated Co(NO3)z on La203 and studied the solid transformations leading to the formation of LaCo03 supported on excess La2O3. In these catalysts, the solid-state reaction between the single lanthanum and cobalt oxides was observed to take place at preferential sites (potential nucleation centers). Since these catalysts were prepared from massive La2O3, whose expected structure is hexagonal (the most stable form), it is not surprising to find that, in this case, a poorly dispersed perovskite oxide was formed. Nudel et al. (78)

PEROVSKITE-TYPE OXIDES

253

Amount of Lb, 0, precwted/wt%

FIG.5 . Catalytic activities for propene oxidation and percentages of monoclinic La&), in

L~,8Sro.2CoOl/,La201/cordierite catalysts as a function of the amount of LazOl precoated. Loading amount of La,.,.8Sro.2Co03,3.8 wt%. (Reprinted by permission from Ref. 76.)

later used as a support a 25-m2/galumina previously stabilized by impregnation with La(NO3)3. Subsequent impregnation (incipient wetness technique) with an equimolecular solution of cobalt and lanthanum nitrates, followed by firing at 900°C, gave rise to a complex solid containing the following phases: y-alumina, LaA103, CoA1204, and LaCo03. On the basis of XPS, diffuse reflectance spectroscopy, temperature-programmed reduction, and catalytic activity data, the authors proposed a model for the solid-state reactions occurring during the genesis of this catalyst. Although the surface area of the solid was only 18 m2/g, its catalytic activity for hydrogenation (the Lac003 phase carrying the active centers) was much higher than that of bulk LaCoO3. This suggests either a high dispersion of this phase within the multicomponent oxide or a synergetic effect due to interaction with other phase(s) present. Some less common approaches were also tested for deposition of perovskites on supports. For example, Murphy and King (48) used plasma-spraying for preparing coatings of Lao &3r02C00 8Nio.203 onto nickel grids. The resulting coatings showed excellent resistance to corrosion and mechanical breakdown and enhanced the electrocatalytic properties for oxygen evolution in strong hot alkaline solutions. It can be concluded that, although data are still scarce, the recent appearance of relevant work in the literature indicates that an effort is being made to

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er al.

develop new methods for the preparation of efficiently supported perovskites. IV. Nonstoichiometry

The defect perovskites that have been more studied in heterogeneous catalysis were those having in position A an alkaline, alkaline-earth, or lanthanide element and in position B a first-row transition metal. We will discuss here some examples of nonstoichiometric perovskites, paying attention preferentially to the concentration and type of defects that are formed. The influence of these defects in the catalytic performance of these oxides has been clearly established in a number of cases. Some relevant examples will be discussed in Section VII. The better characterized perovskite exhibiting oxidative nonstoichiometry has been the lanthanum manganite. Tofield and Scott (79) prepared LaMnO3.12 by firing the simple oxides in oxygen at 1200°C. Similar stoichiometries were reported by Voorhoeve et al. (80),Vogel et al. ( 8 0 , Nakamura et al. (82) and Kamegashira et al. (83).Tofield and Scott (79) showed by powder neutron diffraction that the nonstoichiometry in this perovskite involves cation vacancies mainly on the A sites and to a lesser extent on the B sites. Their calculations indicated that a model involving interstitial oxide is very unlikely. Oxygen excess has also been observed in La-doped BaTi03 and La-doped PbTi03. As for LaMn03.,2, the extra oxygen in these systems may be accommodated over a wide range of nonstoichiometry by the introduction of defects in both metal lattices. In these perovskites having B-site vacancies, the higher oxidation state ion responsible for the nonstoichiometry is significantly smaller than the host cation. This is seen in Table 11, where some relative ion sizes are given ( 7 9 , M ) . On the contrary, such nonstoichiometry should not be expected, when partial substitution of the A or B sites is effected with ions of larger radius than that of the substituted ion, e.g., Ti4+by Nb5+. The oxygen content of LaMn03+Amay be changed by varying the 02 partial pressure in contact with the oxide in a reducing atmosphere at high temperatures. Thus Kamata et al. (8.5)reported that a sample of starting composition LaMnO3.m at 1200°C and 10S-Pa 02 loses oxygen and becomes a stoichiometricperovskite at 10-3.79-Pa0 2 and a perovskite exhibiting reductive nonstoichiometry (LaMn02.95) at 10-6.65Pa02. Nakamura et al. (82) reported also a similar evolution for LaMn03.1z.However, Kamegashira et al. (83) did not find the reductive nonstoichiometry phases in similar conditions. In a different kind of experiment, Voorhoeve et al. (80) obtained LaMnO3.0, in a nearly stoichiometric orthorhombic

255

PEROVSKITE-TYPE OXIDES

TABLE I1 Some Ionic Radii Relevant to Oxidative Nonstoichiometry in Perovskiles*c ~~

~~

~~

Lower-oxidationstate ion

Higher-oxidationstate ion

Decrease in radius (%)

X11Ba2+ 1.60 v1Mn3+0.645 V111E~2+ 1 .2Sd X11Pb2+ 1.49 v1Cr3+0.615 "lTi3+0.67

X11La3+ .I .32 v1Mn4+0.540 v11rEu3+ 1.07 X11La3+ 1.32 "lCfl+ 0.55 V1Ti4+ 0.605

17.5 16.3 14.4 11.4 10.6 9.7 7.8 25.0 -5.8

w 3 +

0.640

v1Cr2+0.82 v1Ti4+0.605

W4+0.59 v1Cr3+0.615 VINbs+0.64

Values in angstroms (1 A = 0.1 nm). Values taken from Shannon and Prewitt (84).Table reprinted by permission from Tofield and Scott (79). The coordination numbers are indicated by Roman-numeral superscripts. The appropriate coordination number for europium is XII, but radii are tabulated only up to coordination number VIII. (1

state and LaMn03.1sin an oxygen-rich rhombohedral state by firing the simple oxides in N2 at 1200°C and in O2 at !MOT, respectively. Kamegashira et al. (83)showed that the degree of oxidative nonstoichiometry in LaMn03+Ais highly dependent on the temperature and duration of the firing and on the oxygen partial pressure. The nonstoichiometry in LaMn03+Acan also be controlled by partial substitution of the A and B cations. Thus Chengxian et al. (65) obtained samples of Ca,Lal-,Mn03+A,where A decreases with increasing values of x . For total substitution ( x = I) the value of A(-0.06) is very near to that found by Kamata et al. (85) at Po* = 10-6.65Pa (see above). The charge compensation is achieved not only through the oxygen nonstoichiometry (A) but also through the concentration of Mn4+, which increases with increasing x . This is illustrated in Fig. 6, where it is observed that both A and the weight percent of Mn4+vary linearly with the calcium content. By XPS it was shown that lattice oxygen is predominant over the surface of the oxides exhibiting oxidative nonstoichiometry whereas adsorbed oxygen is predominant over CaMn03-A.In a similar series of perovskites, specifically, M,Lal-,Mn03(M = Sr, Ca), the concentration of Mn4+ was found to rise slowly, once a level of approximately 50% Mn4+ had been reached (86, 87). Further introduction of M leads to formation of oxygen

256

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I

I

I

0.0

0.5 X

i.o

1

FIG.6. Relationship between x. A, and Mn4+ content in Ca,La,-,Mn03+A.(Reprinted by permission from Ref. 65.)

vacancies. In K,Lal-,Mn03 perovskites with a Mn4+content comparable (40% ~ O ~Mn4+) to that of LaMn03.1s (30% Mn4+) and K O . ~ L ~ , , . ~ M Voorhoeve et al. (80) found that the nature of the charge-compensating defect determines the catalytic activity for NO reduction. When this is a cation vacancy (LaMnO~.ls), the activity is higher than when it is an alkali ion. The same effects, although less pronounced, have been observed by Vrieland (87) for LaMnO3+*and CaxLal-,Mn03 samples in ammonia oxidation. Gallagher et al. (88) and Vogel et al. (81) reported that substitution of Mn by Cu in LaMn03 gives rise to oxygen-deficient perovskites LaMnl-,Cux03-r, where A increases with increasing copper content. The main oxidation states of the B cations in these compounds appear to be Mn4+ and Cu2+(89). Other manganites LnMn03, where Ln = Nd, Sm, Dy, Y, Er, were found to exhibit oxidative nonstoichiometry (90, 91). Wachowski et al. (92) prepared a perovskite of starting composition LaFe03.22.However, Tofield and Scott (79) did not find nonstoichiometry in LaFeO3. The different behavior of these compounds should be related to the significant difference in the temperatures used for the final heat treatment in air (500OC for LaFe03.22; ll0OoC for the stoichiometric compound). This

257

PEROVSKITE-TYPE OXIDES

would be in agreement with the decrease of A in LaFe03+,with increasing heating temperature reported by Voorhoeve et al. (30). By means of thermogravimetric analysis, Patil et al. (93) showed that perovskites Ba,Lnl-,C3oO3 (Ln = La, Nd) lose lattice oxygen easily, yielding oxygen-deficient compounds. The oxygen loss in air is higher than in an 02 atmosphere and increases with increasing x values and with temperature. This is illustrated in Fig. 7, where the percent mass loss is plotted as a function of the heating temperature for several values of x. The reductive nonstoichiometry is, therefore, dependent on the surroundin'g oxygen partial pressure, the Ba content, and temperature. In agreed ment with these results, Yamazoe et al. (94) and Nakamura et al. (95, 96) found that the desorbed oxygen measured by TPD from Sr,Lal-,Co03-A samples and A both increased with the Sr2+content. According to Jonker and van Santen (97), in this mixed oxide, Co4+is formed without the appearance of oxygen vacancies for x e 0.4. However, oxygen vacancies besides Co4+are formed for x > 0.4. This is due to the instability of Co4+, which tends to be reduced by a simultaneous release of oxygen. The reductive nonstoichiometry,which increases with increasing x, was found to have a remarkable effect in the reducibility of these oxides (see Section V,B) and in their activity for oxidation (95, 96). Gibb et al. (98) studied the series of substituted perovskites SrFe,RU~-,O~ by- ~Mossbauer spectroscopy. The incorporation of Fe in the structure takes place exclusively as Fe3+.Substitution of Ru4+by Fe3+for x < 0.3 leads to the appearance of oxygen deficiency. For x > 0.3 there is an increasing proportion of Ru5+ and a parallel decrease in oxygen deficiency. Thus for x = 0.5 the reductive nonstoichiometry appears to be I

X

0.2 0.3

0.4 0.5 I

400

1

I

I

500

I

600

I

I

1

700

T,OC

FIG.7. Percent mass loss as a function of the temperature for the compositions Ba,La,-,CoO,. (Reprinted by permission from Ref. 93.)

258

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minimal. It seems that this system is unable to tolerate more than approximately 4% oxygen deficiency and, as a result, partial oxidation of Ru4+ occurs. Other oxygen-deficientperovskites were described and characterized. Thus, rather unstable SrVO3-,, (A =z 0.1) oxides have been synthesized by Dougier et al. (99). MacChesney et al. (ZOO) prepared a series of SrFeO3-,, oxides. By decreasing the equilibration temperature from 1400 to 550°C and increasing the oxygen pressure from 2.03 x 104 to 3.41 x lo7 Pa, these authors obtained perovskites with increasing oxygen content from SrFe02.72to the stoichiometric compound. The tendency of this oxide to reductive nonstoichiometry as opposed to the behavior of the lanthanum ferrite (92) seems to be justified by the charge-compensating factor, resulting in an oxygen loss, introduced by the presence of Sr2+in the lattice. Wachowski et al. (92), using the explosion method, obtained the slightly oxygen deficient perovskite LaNi02.9B.Soderholm et al. (101) found that the X-ray diffraction lines of BaCe03 broaden and shift to lower angles on substitution of Ce4+by Dy3+,This has been attributed to the incorporation of oxygen vacancies into the lattice as charge compensators. Then this compound can be formulated as BaDy,Cel-,03-,,. Voorhoeve et al. (30) reported oxidative nonstoichiometry for the perovskites L ~ M O J(M + ~= Cr, Mn, Fe) and reductive nonstoichiometry for Lacoo,-,,. XPS measurements carried out on a similar series of LaMO3 oxides after heat treatment in air at 900°C indicated a surface nonstoichiometry that changes from oxidative (M = Cr, Mn) to reductive (M = Fe, Co, Ni, Rh) (102).This nonstoichiometry was found to be much more marked than that observed in the bulk. The more oxygen deficient perovskites were those that are more easily reducible (M = Co, Ni, Rh). Tabata et al. (103)found also significant differences between the chemical composition of the surface (determined by XPS) and of the bulk (determined by X-ray fluorescence spectroscopy) in a series of SrxLal-,Coo3 oxides. These results indicate a very different behavior of the surface with respect to the bulk in these compounds. This is an important factor to be considered when trying to correlate the composition of a perovskite with its catalytic performance. V. Stability in a Reducing Atmosphere

A. TEMPERATURE PROGRAMMED AND ISOTHERMAL REDUCTION Temperature-programmed reduction (TPR) experiments were performed mainly on LnMO3 oxides, where Ln and M are, respectively, a rare earth and a transition element. Reduction of LaRh03 with CO (4"C/

PEROVSKITE-TYPE OXIDES

259

min) (104) resulted in a weight loss equivalent to a reduction of about 75% of Rh3+to Rho at 580°C (reduction up to 1000°C in H2 or CO affects only the metal M; reduction of the rare earth cation requires higher temperatures). After this the sample weight increased with temperature because of carbonate formation or carbon deposits. Formation of carbonaceous species has been observed in the reduction of Lal-,Sr,Fe03-A with CO (105). To avoid this effect, the reduction is carried out most frequently in a hydrogen atmosphere. Fierro et al. (106) and Tascdn et al. (107) studied the series of LaM03 oxides (M = Cr, Mn, Fe, Ni, Rh) (reduction took place in H2 at a heating rate of 4"C/min unless otherwise specified). LaC1-03 ( 1 0 6 ~exhibited ) the highest stability. At 1000°C this oxide undere- per molecule (3e- per molecule went a reduction of only 1.3 x would amount to full reduction of M3+to MO).LaMn03 (106b) showed a single reduction step of le- per molecule at 800°C (temperature where the weight loss corresponding to the indicated reduction degree is attained). Vogel et al. (10% H2-N2,2OoC/min) (81) reported, however, a first reduction step of LaMn03.],to the stoichiometric compound at 550°C and then a second reduction of le- per molecule (to Mn2+)at 1050°C. TPR data with reduction of Mn3+to Mn2+in the partially substituted perovskites LaMno.5C&.503and L ~ o . & . ~ M ~were O ~ also given by Vogel et al. (81) and Vogel and Johnson (108). LaFe03 (1074 reduced to 3e- per molecule at 1OOO"C. This temperature is practically coincident with those found by Wachowski et al. (19% H2-He, 20"C/min) (92) and Carreiro et al. (15% Hz-Ar, 0.4"C/min) (109) for full reduction of this perovskite. The total weight loss observed was higher than that expected for reduction of Fe3+ to metallic Fe (92, 1074, indicating that the lanthanum and iron oxide exhibit oxidative nonstoichiometry (LaFe03.20).In addition, Wachowski et al. (92) found a plateau for a weight loss of 4.2%, close to that that would be expected for reduction of Fe3+to Fe2+.Reduction of YFe03 (15% H2-Ar, 0.4"Clmin) (109) to 3e- per molecule took place at a substantially lower temperature (740°C). LaCo03 (D2,3"C/min) (64)and LaNi03 ( 1 0 6 ~both ) exhibited two reduction steps of le- and 3e- per molecule at 450 and 625°C (LaCo03)and 325 and 475°C (LaNi03). A similar reduction behavior has been reported by Wachowski et al. (92) and by Futai et al. (110) for LaCo03, LaNi03, and LnCo03 perovskites. Crespin et al. (111) and Levitz et al. (112) detected, however, three reduction steps for LaNi03 (H2, O.OS"C/min) of 1, 2, and 3e- per molecule at 300, 370, and 455"C, respectively. LaRh03 (107b) reduced in a single step to 3e- per molecule at 600°C. Carreiro et al. (109) and Gysling et al. (41) recorded lower temperatures for reduction (15% H2-Ar, 0.4"Clmin (109);H2, lO"C/ min (41))of LaRh03 (430"C), and YRh03(400°C). No intermediate reduction state has been detected for rhodium in these perovskites. Rh(1) has

260

LUIS G . TEJUCA

et al.

been found in the reduction by heating in vacuum of Rh-Y zeolites (113, 1 14). The ease of reduction increases, therefore, from Cr3+to Ni3+ in the series of LaM03 perovskites. The same trend has been found for the simple oxides of Fe, Co, and Ni (115). On the other hand, the mixed oxides LaM03 (M = Fe, Co, Ni) were found to be more stable in a H2 atmosphere than the simple oxides NiO, Fe203, and Co304(92); this shows the increased stability of transition-metal cations in a perovskite structure. The TPR experiments also show that the stability of perovskite oxides increases with increasing size of the A ion. Different TPR steps correspond to different reduction mechanisms. These can be studied by kinetic experiments of reduction in isothermal conditions (116). As illustration, reduction data of PrCo03(117) are given in Fig. 8. The TPR diagram (Fig. 8a)shows reduction of Co3+to Co2+ (step a) at 400°C and of Co2+to metallic Co (step b) at 525°C. The plots of (I! (reduction degree) versus t (time) for step a in Fig. 8b show that the reduction rate decreases continuously with time, indicating that the reduction occurs according to the contracting-sphere model. The process starts with a very fast nucleation, which results in a total coverage of the PrCoO3 grains by a thin layer of the reduced phase. This causes a continuous decrease in the rate of the interfacial reaction as the grains of the starting oxide are consumed in the reaction. The reduction process is described by the Mampel intermediate law ( I 18)

1 - (1 - a ) " n

=

kt

+b

(1)

where cy is the reduction degree; k and b are temperature-dependent constants; and n = 2 would correspond to a two-dimensional growth and n = 3, to a! three-dimensional growth: This equation holds over the middle range of (I! values. Kinetic runs in step b in Fig. 8c started with a very fast reduction of approximately le- per molecule, after which a slow reductioh took place, yielding sigmoidalJreductioncurves. This. indicates that reduction of Co2+ to Coo is controlled by the formation and slow growth of reduction nuclei of metallio cobalt on the surface of the reduced phase in step a (nucleation model). Initially, the reduction rate increases because of the growth of nuclei already formed and the appearance of new ones.'At a certain point the reduction nuclei start to ovedap; at the inflection point, the interface of ,the oxidized and reduced phases and the reduction rate both begin to decrease. Reduction of this type i$ described by the Avrami-Erofeev equation (1 18) 1 - (I! = e-kr' (2)

26 1

PEROVSKITE-TYPE OXIDES

1.o

0.8 0.6 0.4

0.2 0.0

I

1

0

20

I

40

I

I

60

8U

I

too

t,min Reduction of PrCoO, : (a) temperature-programmed reduction; (b, c) isothermal reduction in the first (to le- per molecule) and second (to 3e- per molecule) steps, respectively, of diagram (a) (90cmVmin H j flow rate;4"C/min heating rate). (Reprinted by permission of the publisher, Chapman & Hall, from Ref. 117.)

262

LUIS G. THUCA

et al.

[where a,k , and n have the same meaning as in Eq. (l)], which holds over a values of 0.05-0.90. Kinetic studies were done with other perovskites. For example, LaNiO3 ( 1 0 6 ~ exhibited ) a behavior similar to that of PrCoO3; LaFe03 (107a)and LaRh03 (107b)reduced to the metal according to the contracting-sphere model. It should be noted, however, that the distinction between the two reduction mechanisms is somewhat arbitrary because the contracting-sphere model starts with a very rapid nucleation, and the nucleation mechanism ends according to a contracting-sphere model (note that reduction after the inflection point of the kinetic curves in step b in Fig. 8c is similar to reduction in step a in Fig. 8b. B. EFFECTOF CATIONS IN POSITIONS A

AND

B

Arakawa et al. (119) showed that the extent of reduction in H2 of LnCoO3 oxides increased from LaCo03to EuCo03. In the same direction the energy of the metal-oxygen bond decreased. Futai et al. (110) performed TPR experiments for a series of LnCo03oxides and found that the temperature for maximum consumption of H2 (Tmax)for the reduction steps to 1 and 3e- per molecule decreased from Lac003 to EuCo03; however, T,,, increased of remained constant for the perovskites of Gd, Tb, and Dy. Carreiro et al. (109) observed a higher stability in H2 of LaFe03 and LaRh03 than the corresponding yttrium perovskites. In concurrence with these results, Sakai et al. (120) found that the strength of the V-0-V interactions in LnVO3 oxides is weakened by the substitution of La3+with heavier lanthanide ions. On the other hand, Sugihara et al. (121) and Katsura et al. (122) calculated the Gibbs free energy of formation AG of a series of LnFe03 oxides from metallic iron, LnzO3, and oxygen at 1200-1297°C and found a linear relationship between AG and the tolerance factor t , as defined by Goldschmidt (19). This is illustrated in Fig. 9, where it is observed that AG becomes more negative and the change in entropy (AS)decreases for increasing values of t . A similar relationship holds for the Gibbs free energy of formation of LnFeO3 oxides from Fez03 and LnzO3 (12.3). However, in this case the plots AG versus t were found to be independent of the temperature from 877 to 1024°C. These data are consistent with the TPR results and suggest that the stability (or reducibility) of perovskite oxides increases (or decreases) with increasing size of the A ion. This conclusion would be consistent with the preferred occupancy of the larger Ln3+ ions in the 12- coordinated A sites. Partial substitution of the A ion by an ion of different charge may also cause significant changes in reducibility; for instance, when

263

PEROVSKITE-TYPE OXIDES

1

I 1200oc

- -30 h

--28

?

Q)

v

--26

-55

Yb '

Dy Tb Gd Eu Sm 0,93

Nd 0,94

2

La 0,95

t FIG.9. The Gibbs free energy of formation (AG) of a series of LnFeOl oxides from Ln203,metallic iron, and oxygen (solid lines) and the change of entropy (AS, dashed lines) as a function of the tolerance factor t . (Reprinted by permission from Ref. 122.)

La3+is partially replaced by Sr2+in LaCoO,, the charge compensation is accomplished by oxidation of Co3+to Co4+and by the formation of an oxygen-deficient perovskite, Lal-,Sr,C003-~ (see Section IV). The increasing concentration of both Co4+ (unstable) and oxygen vacancies (which facilitates the diffusion of lattice oxygen from bulk to surface) accounts for the increased ease in the reduction of this oxide with increasing strontium content (95, 96). Nakamura et al. (82) studied the structural changes undergone by LaM03oxides in a reducing atmosphere at I bar pressure (1 bar = lo5 Pa) and 1000°C as a function of the O2partial pressure, Po,. Their results (Fig. 10) are qualitatively in agreement with those discussed above (Section V,A). La2C020s,La2Ni205,and LaNi02are unstable at high temperatures (111,112) and thus were not observed. Instead, the dismutation products La2M04were found. Neither has the formation of the La2Mn04 phase been observed, since it is unstable above 925°C (108). The order of stability expressed in terms of the -log Po, values appears to be LaV03 = LaCr03 > LaFe03 > LaMn03 > LaCo03 > LaNi03. This is the order of the Madelung constant except for Lac003 and LaNi03, which have higher constants. According to Nakamura et al. (82), a cause for this

264

et al.

LUIS G . TUUCA

-2

-3

0

1

1

1

1

1

1

1

1

1

10

5

1

1

1

1

15

20

- log Po*

La R O3

-3

-o\" o

-4

n

-7 -5 -6

l

,

3 -8 ' -9 3 -10

l ,+Fe

a

*

-1 2

-log Po2 FIG.10. Weight and structural changes undergone by LaMOl oxides in a reducing atmosphere at lo-' Pa and 1OOO"C. (Reprinted by permission of the publisher, Pergamon Journals, Ltd., from Ref. 82.)

PEROVSKITE-TYPE OXIDES

265

discrepancy may be the existence of stable compounds La2M04for Co and Ni. These results indicate the same stability order found by means of TPR. It should be noted, however, that both sets of data would indicate a higher stability of LaMn03 relative to LaFeO3 if reduction of M3+to Mo is considered. Carreiro er al. (109) discussed the higher stability toward reduction of LaFe03 and YFe03 as compared with the corresponding rhodium perovskites in terms of the standard Gibbs free energy of formation of the corresponding sesquioxides Fe2O3 (- 177 kcal/mol) and Rhz03(-49 kcal/ mol): the less negative the value of AGO, the greater the tendency for dissociation to the metal and oxygen. The above stabilities could be explained considering that similar relative values in AGO can be expected for the mixed oxides LnFe03 and LnRh03. ON REDUCTIONA N D OXIDATION C. PHASETRANSFORMATIONS

Perovskites of titanium and chromium such as SrTi03,BaTi03( 6 4 , and LaCr03 (106a) remained virtually unchanged even after reduction at 1OOO"C. Other perovskites of transition metals underwent substantial changes after less severe treatments. Reduction of LaMn03 to le- per molecule by isothermal heating in H2at 720°C (this temperature is, of course, somewhat lower than that needed to attain the same reduction degree in a TPR experiment) yielded MnO and La203 (106b).By reduction of alkali-substituted LaMn03 above this temperature, Vogel and Johnson (108) reported a breakup of the perovskite structure with formation of La2Mn04.However, when the heating is carried out below 700"C, the initial structure can be preserved; e.g., LaMno,5Cuo.502,93 after reduction to LaMno.sC~.S02.26 (containing Mn2+ and Cu') showed X-ray diffraction (XRD) lines of a perovskite phase only (81). After reduction of LaFe03.1sto approximately 1.2e- per molecule, the perovskite structure was partially preserved ( 1 0 7 ~ )besides, ; lines of a-Fe and Laz03 were observed. Further reduction led to the disappearance of the perovskite. Sis et al. (124), by means of thermogravimetric, calorimetric, XRD, and magnetic measurements, showed that Lac003 does not reduce directly to Co and Laz03but rather through the formation of intermediate oxygendeficient structures. Studies carried out by Wachowski et al. (92) on reduced LaM03 (M = Fe, Co, Ni) and by Arakawa et al. (119, 125) on reduced LnCo03 oxides (Ln = La, Nd, Sm, Eu) point to the same conclusion. In the initial weight-loss region of LaCo03 (0-3.25 wt% or a reduction lower than le- per molecule) the oxygen-deficient compound LaCo,03-, (monoclinic)and metallic cobalt are formed. At a weight loss

266

LUIS G . TEJUCA

er al.

of about 3.25 wt% ( l e - per molecule) the original rhombohedra1 LaCo03 with a loss of lanthanum and cobalt appears. At 5.5-6.5 wt% loss (about 2e- per molecule), Coo and La2Co04 are formed (124). This latter compound was previously synthesized and characterized by Janecek and Wirtz (126). Crespin and Hall (64)found that the perovskite structure of LaCo03was slightly modified by reduction to le- per molecule. The XRD pattern of the sample reduced to 3e- per molecule (at 400°C) showed only La203 peaks, indicating that the metallic cobalt was highly dispersed; when the sample was heated in He at 800"C,the expected Coo lines appeared. These results are consistent with those of Sis er al. (124). The reduction step of le- per molecule in Ref. 64 would correspond to the formation of L ~ C O ,in~Ref. ~.~ 124, which is then transformed according to Eq. (3):

-

L~CO,O~.~

+ $ C03,-2 + Q La20,

f LaCoO,

(3)

According to Crespin et al. (111, 112), reduction of LaNi03 to le- per molecule at low temperatures (300°C) occurs according to the following transformation : 2LaNiO3 $. H2

-

La2Ni203+ H20

(4)

A comparison between the observed distances obtained from XRD data of the reduced sample and the distances calculated from the proposed unit cell demonstrated the existence of a single phase with the stoichiometry of LazNi~Os.Similar compounds of Co and Ni were prepared and described by Vidyasagar er al. (127). Reduction of LaNiO3 to 2e- per molecule above 300°C also yielded LazNi20~besides metallic Ni and La2O3. However, when the reduction is effected below 300"C, LaNiO2 is formed. Its structure has been described using XRD and EXAFS techniques. Both La2Ni20~and LaNi02 are stable in air at room temperature. However, after heating at 1000°C in helium a dismutation reaction occurs with formation of La2NiO4, NiO, and NiO. These results are in agreement with those of Fierro et al. ( 1 0 6 ~ 1who , found formation of La2Ni04after reduction of LaNiO3 to le- per molecule at 240°C and heating in helium at 800°C. La203 is the only phase that is detected after reduction of LaNi03 to 3e- per molecule at 300°C. Similar to that observed for LaCo03 (M), the lines of Nio appear in the XRD pattern only after heating at 800°C in an inert atmosphere (1 1 I). Perovskites may undergo reversible reduction-oxidation cycles when these are carried out at temperatures where sintering of the oxidized or reduced species does not occur. Thus, reoxidation at 400°C of LaCoO, reduced to 3e- per molecule fully restores the perovskite structure. However, reduction to 3e- per molecule, heating in He at 800°C and reoxida-

PEROVSKITE-TYPE OXIDES

267

tion as above did not produce a perovskite phase. Instead, Co304appeared (64).This irreversibility in the redox cycle is caused mainly by the drastic increase that the particle size of the metal undergoes in the sintering process. This increase for metallic nickel (after reduction of LaNi03at 430°C and heating in He at 900°C) was found to be larger than an order of magnitude ( 1 0 6 ~ )The . reduction products (up to le- and 2e- per molecule) of LaNi03 (LaZNizOs and LaNiOz; see above) were fully reoxidized in O2at 180°C (111). Vidyasagar et al. (127) reported a somewhat higher reoxidation temperature (325°C) for LazNizOs.In the reoxidation at 180°C of the fully reduced (3e- per molecule) LaNi03 a strongly exothermal reaction was observed and a mixture of LazNi04 and NiO was obtained (111). This result is similar to that observed for reduction at 500°C of LaCo03 to Coo and La203 and reoxidation at 400°C with pure 0 2 (64). In these conditions substantial amounts of Co304 plus La203 besides perovskite were obtained. The formation of simple oxides may be avoided by carrying out the reoxidation at a slower rate, using diluted oxygen in an inert gas or air as an oxidizing agent. Reversible reduction-oxidation processes were also reported for LaRh03 (107b) and PrCo03 (117). In these cases the particle sizes of the regenerated perovskites were found to be smaller than those of the starting samples. Reller et al. (128)presented evidence by XRD, high-resolution electron microscopy, and selected area electron diffraction of CaRu03 reduction to Ruo and reoxidation at low temperatures to form a pure perovskite phase. When these redox processes were repeated, the size of the Ruo crystallites decreased beyond the detection limit of X-ray diffraction. This and other examples mentioned above [Lac003 (64),LaNiO3 (111)l show that by reduction treatments or redox cycles, the metal in position B is in a highly disperse state on a matrix composed by the oxide of the metal in position A. Considering the importance of dispersed metals in heterogeneous catalysis, the reduction or reduction-oxidation of perovskites in controlled conditions may represent a promising pathway for the preparation of highly active catalysts. D. XPS STUDIES The surface reduction of perovskites (mainly cobaltites) has been studied by examining the evolution of the XPS photolines of the metal in position B and oxygen. Lombard0 et al. (129) reported a multiplet splitting of the C02p photolines in oxidized and reduced LaCo03 of 15.4 eV (Co2p3l2at 779.9 - 779.3 eV and C02p,/~at 795.5 - 795.0 eV for Co3+ ions), which is in excellent agreement with that given previously by Ichi-

268

LUIS G . TFJUCA

f?t

Ul.

mura et al. (63) for unreduced LaCo03. Formation of Co2+on reduction can be detected by the appearance of two shake-up satellite peaks that are situated at about 7 eV upscale from the main C02p peaks (63) [8.4 eV upscale in SrCo03-, (130)].These satellites are characteristic of Co2+and can be considered as the fingerprints of this ion since the lines of the multiplets for both Co3+and Co2+ions appear very close in the spectrum. The C02p photolines for metallic cobalt are situated at 1.5-2.0 eV downscale from those of Co3+.Following the evolution of the C02p photolines, Marcos et al. (51) studied the surface reduction of pure and Sr- or Thsubstituted LaCo03as a function of the bulk reduction (Fig. 11). At a low extent of reduction these oxides are homogeneously reduced. However, for a bulk reduction of 1.5e- per molecule, the surface is reduced to approximately 3e- per molecule. At this point the maximum difference between bulk and surface reduction occurs. It is observed that the surface of the Sr-substituted perovskite is more easily reducible than that of LaCo03, in agreement with Nakamura et al.'s data for bulk reduction of Lal-,Sr,Co03-k (95, 96). Likewise, Gysling et ul. (41) observed that

"

0

I .5

3.0

3.6

Bulk reduction (e-/mol ) FIG. 1 I . Correlation between surface reduction, determined by XPS and bulk reduction, calculated from the hydrogen uptake in a gas recirculating system. (Reprinted by permission from Refs. 51 (0)and 129 ( O ) . )

269

PEROVSKITE-TYPE OXIDES

whereas the bulk of LaRh03 underwent reduction to Lat03and Rh in H2 at 430"C, the surface reached the same extent of reduction at 300°C. These results illustrate the higher reactivity of the surface relative to the bulk in these perovskites. The XPS spectra of perovskites present two 0 1 s photolines that correspond to two different oxygen species (63,94,103,129,131).Yamazoe et al. (94), by means of studies on the evolution of the 0 1 s photolines with the outgassing temperature and also with x in the perovskite La,-,Sr,Coo3 in combination with a TPD study of oxygen, associated the 0 1 s photolines with lattice (lower binding energy B E ) and absorbed (higher B E ) oxygen, although this latter fraction should also contain adsorbed oxygen. Some representative examples of BE values of these photolines and their assignation for some perovskites (51, 63, 94, 102, 117) and simple oxides (132) are given in Table 111. Marcos et al. (51) and Fierro and Tejuca (102) reported an intensity increase of the 0 1 s line at higher BE on reduction of Lac003 and PrC003 at increasing temperatures (1500°C). This was assigned to the formation of hydroxyls in the presence of the water generated in the reduction process. After reduction of Tho.2Lao.sCo03 at 300-500°C no distinguishable OH- signal was detected. However, after reduction of Sro.4Lao.6Co03 at temperatures as low as 250°C the lattice oxygen signal is lost under the tail of the OH- signal, i.e., TABLE 111 0 1 s Photoemission Lines in Oxides ~~~

~

Compound LaCoO, LaCoOJ SrxLal-,CoOl LaMO,

PrCoO, c0104

NiO

a

Adsorbed.

* Lattice.

Absorbed.

~

~

BE (eV)

Assignation

Reference

531.9 528.3 532.2 529.7 530.2 - 531.4 528.2 531.4 - 532.6 529.1 - 530.0 530.9 528.4 531.0 529.6 53 1.4 529.4

Oxygen" Oxygenb Hydroxyl anion" Oxygenb Oxygen" Oxygenb Oxygen or hydroxyl aniona Oxygenb Oxygen or hydroxyl anion4 Oxygenb Water or hydroxyl anion" Oxygenb Water or hydroxyl anion" Oxygenb

63 51

94

102 117

132 132

270

LUIS G . TUUCA

et al.

the hydroxyl concentration increases or decreases in the reduced cobalt perovskite where La3+has been partially substituted by Sr2+or Th4+ions (51). This effect is in concurrence with those of Yamazoe et al. (94) and Tabata et al. (103), who observed an increase of the 01s signal at higher BE with increasing values of x in LaI-,Sr,Co03. The above results are consistent with the higher reducibility of the Sr-substituted perovskites as compared with Lac003 (51, 95, 96) and also with the higher basicity of Sr2+with respect to La3+or Th4+and, therefore, with its higher tendency for hydroxide formation. VI. Adsorption Studies

This section refers to adsorption studies that have been directed to the characterization of perovskites or to the determination of the role of adsorbed species on the catalytic activity. These include mainly equilibrium and kinetics of adsorption, successive or simultaneous adsorption of two gases, infrared spectroscopy (IR), and temperature-programmed desorption (TPD). Physical adsorption has been discussed in Section I11 because of the dependence of the textural characteristics of these oxides on the preparation method.

A. HYDROGEN AND OXYGEN Crespin et al. (133) found associative adsorption of hydrogen at -5°C on LaNi03 reduced to an extent of le- per molecule; the coverage reached was lower than that reported by Tejuca (134) on LaNi03 reduced at 300°C. Ichimura et al. (135) reported TPD spectra after adsorption of H2 on LaAI03, LaCrO3, and LaCoO3 at 25°C showing superposed desorption peaks. The species that desorb above 70°C were considered responsible for C2Hs hydrogenolysis and C2H4 and C2H2 hydrogenation. However, TPD spectra reported after H2 adsorption at 25°C on reduced LaM03 oxides showed one peak at 65-105"C, except for the system H2-LaNiO,, which showed also a second desorption peak at 335-340°C (134). The intensity increase of these peaks with the reduction temperature of the oxide suggests that hydrogen adsorbs on reduced transition metal ions. The interaction of oxygen with perovskites has been studied mainly because of the importance of these materials as oxidation-reduction catalysts. Data of oxygen adsorption on LaMO3 (M = Cr, Mn, Fe, Co, Ni) oxides were reported by KremeniC et al. (136). The adsorption profile at 25°C showed two maxima for Mn and Co (Fig. 12) that coincide with the maxima observed by Iwamoto et al. (137)for the respective simple oxides

PEROVSKITE-TYPE OXIDES

27 1

FIG.12. Total (open symbols) and reversible (filled symbols) adsorption profiles of O2on LaMOl oxides on a clean surface (circles) or on a surface with preadsorbed isobutene (triangles) ( P= 2 x l(r Pa; T = 25°C). (Reprinted by permission from Ref. 136.)

of transition metals and also for the catalytic activity of oxidation of this series of perovskites (136) (see Section VI1,B). The reversibly adsorbed oxygen represents a small fraction of the total adsorption, in agreement with Nakamura et d ’ s results (95). The oxygen-adsorption heat for Lac003 (138) was found to be higher than the adsorption heat for LaCr03 ( 1 0 6 ~ that ) showed a lower oxidation activity. Shimizu (139) found a clear correlation between the oxygen adsorption and the metaloxygen binding energy in this series of LaM03 oxides. Yamazoe et al. (94)and Seiyama et al. (140) reported the appearance of two oxygen desorption peaks after oxygen adsorption on Lal-,Sr,Co03 at 800°C. The low-temperature peak (a)can be ascribed to absorbed and adsorbed oxygen, whereas the high-temperature peak ( p ) was assigned to lattice oxygen. These oxygen species were also detected by XPS. The influence of x in Lal-,SrxM03 (M = Mn, Fe, Co) on the intensity of the low-temperature peak (a)has been explained on the basis of the nonstoichiometry and defect structure of these oxides (94, 95, 140-142) (Fig. 13a). Thus, for M = Fe and Co, the Sr substitution increased oxygen desorption (a).Since A-site substitution with a divalent ion is expected to lead to the formation of oxygen vacancies in these oxides, this result suggests that the low-temperature species is associated with oxygen va-

272

LUIS G. TUUCA

et al.

i"

9

!\

-'cn

3.0-

>

b

E

i2.0-

t In

2

Temperature pC

1.0-

Temperature ,"C

FIG.13. Temperature-programmeddesorption of oxygen from La,-,Sr,MO,+, (M = Co, Fe, Mn) (a) and from LaMO, (M = Cr, Mn. Fe. Co, Ni) (b). Oxygen preadsorption, 80025"C, 1.33 x 104 Pa. (Reprinted by permission of the American Chemical Society, from Ref. 140.)

cancies. In LaMn03+A the partial substitution of La3+by Sr2+for x = 0.2 decreases both the cation vacancies and Mn4+concentration without the formation of oxygen vacancies (see Section IV). Therefore, a desorption is not promoted. For x = 0.4, oxygen vacancies are formed and a small a desorption appears (140).The high-temperature peak ( p )were found to be more specifically associated to the B cation, although it is also affected by A-site substitution. This is illustrated in Fig. 13b, where peaks of oxygen desorption from unsubstituted LaM03 perovskites are shown. Oxygen adsorption and desorption were found to be larger for the perovskites, showing a higher catalytic activity for total oxidation, that is, perovskites of manganese, cobalt, and nickel (Fig. 12) (136,140). Seiyama et al. (140) associated the onset temperature for these large desorption peaks with the decomposition temperatures of the respective cgmponent oxides. George et al. (143) found that LaCoO, did not adsorb CO or 02 separately but adsorbed both gases from their mixtures. Enhanced adsorption of oxygen after CO-02 (138,144) and isobutene-02 (136,145) successive adsorption on this perovskite has also been observed by several authors. Some of thcse results are illustrated in Fig. 12, which shows that the enhanced adsorption measured after isobutene preadsorption is greater for the oxides (LaFeO3 and LaCo03) adsorbiqg larger amounts of the

PEROVSKITE-TYPE OXIDES

273

hydrocarbon (136). This seems to indicate that the increase in 02 adsorption is directly related to the amount of preadsorbed hydrocarbon. This effect causes the maxima for LaMn03 and LaCoO, on a clean surface to become attenuated by isobutene preadsorption. This behavior has also been described for other systems (146). According to Weller (146), enhanced adsorption of one reactant in the presence of a second reactant accounts for the negative adsorption constants, when kinetic data are analyzed by using rate equations of the Langmuir-Hinshelwood type. The opposite effect has been observed for COz preadsorption, which moderately decreased both the irreversible and reversible subsequent adsorption of 0 2 on LaCo03 (138). Since it was shown that 02 and C02 adsorb on different surface centers (138, 147) this result was ascribed to steric hindrance. Other techniques have been used to study adsorbed or lattice oxygen in perovskites. Thus, XPS permitted to follow the changes undergone by adsorbed oxygen before and after CO adsorption (148)or on outgassing at increasing temperatures (94). In addition, this technique provided evidence of large departures from stoichiometry at the perovskite surfaces (65,102). Electron spin resonance (ESR) has been used to identify oxygen species on Lal-,Ca,Mn03+k (65) and Lac003 (138).After adsorption of oxygen on this latter perovskite at 150°C and lowering the temperature to -196"C, a signal attributed to 0; was found that changed rapidly into a diamagnetic species. Conductivity changes as a function of 02 pressure were associated to a fast adsorption process followed by transformation of adsorbed oxygen into lattice oxygen (149).DTA has been used to study oxygen uptake on cobalt perovskites in order to obtain a relative measurement of the degree of nonstoichiometry (150).

B. NITRICOXIDEA N D CARBON MONOXIDE Comparison of NO adsorption at 25°C on the series of LaM03 oxides revealed a profile similar to that found for 02 showing maxima for Mn and Co (151). NO adsorption was found to be independent of temperature for some perovskites such as LaFe03 (152) and LaNi03 (151) for a wide temperature range (0-400°C). This suggests that the surface sites for NO on these oxides did not change substantially in character with the temperature. For LaCrO3 (153), LaMn03 (154), and LaRh03 (107b) a narrower interval was found, where NO adsorption changes moderately ( ~ 2 0 % ) with temperature. Coverages were of the same order of magnitude as those reported for simple oxides (155).

214

LUIS G . THUCA

et al.

The infrared spectrum obtained after adsorbing NO on LaMn03 above room temperature included bands at 1910 cm-l of dinitrosyl species; at 1610, 1485, 1135, and 1045 cm-* of bidentate and monodentate nitrates; and at 1300 cm-I of nitrite structures (154). The intensity of these bands increased with temperature, showing a progressive chemisorption character for the adsorption process. Similar species were found for LaFeO3 (152); for this oxide, however, formation of N2O at 100°C and higher temperatures was observed, suggesting that NO adsorbs in dissociative as well as molecular forms. Voorhoeve et al. (156)found also a TPD peak at 100-250°C assigned to nitrosyl groups on low-valence metal ions after NO adsorption on potassium- and ruthenium-substituted LaMn03 perovskites. These results indicate that NO interacts with both cations and anions on the surface of these perovskites. Quantitative measurements of the successive NO-CO and CO-NO adsorption on LaMO3 oxides at 25°C showed an inhibiting effect of NO on subsequent CO adsorption, which was found to be larger than the inhibiting effect of CO on NO adsorption (151, 152). Thus, NO appears to be more strongly adsorbed than CO on the perovskite surface, in agreement with previous results of Shelef et al. (155) for other transition-metal oxides. Indeed, a comparative analysis of the adsorption energetics of NO and CO on LaCr03 (153) showed higher adsorption heats and lower adsorption entropies for NO. Supporting these results, infrared bands of carbonates formed after CO adsorption at 25°C on LaMn03, which contained preadsorbed NO, were eliminated by a thermal treatment at 300°C (154). However, carbonates formed after CO adsorption on a fresh perovskite sample showed a higher stability (50). Infrared spectra recorded after simultaneous adsorption of NO + CO on LaMnO3, LaFeO3, and LaCoO, at 300 and 500°C provided evidence of the presence of N 2 0 , isocyanate species, and NO adsorbed with a donortype or coordinative bond (151). An additional band of nitrosyl groups was detected on LaCo03. These results may provide some clues for the mechanism of NO + CO reaction on these oxides, as N20 and isocyanate species have been previously suggested as intermediates for this reaction on simple oxides (157). The chemisorption of NO seems to play an important role in this reaction catalyzed on perovskites. Thus, Chien et al. (73) observed a higher NO adsorption rate for activated (reduced) than for unactivated LaCo03 and L%.~~B%.&oO~. Also, a higher adsorption rate was found for the Ba-doped oxide, which exhibited a higher degree of catalytic activity. The relative constancy of NO adsorption with temperature and the strength of its bond with perovskite surfaces have suggested the use of this molecule over CO for determining surface metallic centers (107b,

PEROVSKITE-TYPE OXIDES

275

151-154). However, the IR evidence indicates that NO does not show any particular specificity for adsorption on metallic or oxide ions (151, 152, 154); therefore, the assumption of a 1 : 1 correspondence between adsorbed NO molecules and adsorption centers, as that reported for simple oxides (155) may not give a proper estimate of the number of surface transition-metal ions. Ulla et al. (258) used the poisoning effect of NO adsorption in ethylene hydrogenation at -20°C for the estimation of metallic centers in reduced LaCoO3. Active-site concentration was found to be lower by one order of magnitude than the theoretical concentration of metallic cobalt. This was assumed to be due to the fact that only a small fraction of the metallic sites are active for hydrogenation. The adsorption profile for CO on the surface of LaMO3 oxides at 25°C showed a maximum for LaFe03 (151) resembling that found for isobutene adsorption (136). The extent of CO adsorption was found to be substantially lower than that of NO adsorption (151). Data on the kinetics of CO adsorption on LaCr03 fitted satisfactorily the integral Elovich equation (159). Analysis of plots of time versus the reciprocal of the adsorption rate suggests the existence of a precursor state previous to the adsorption process. Experiments of CO2-CO successive adsorption on LaCoO, strongly suggest that these molecules adsorb on the same center (160). On the contrary, CO adsorption on a surface with preadsorbed oxygen was practically equal to that measured on a clean surface showing the noncompetitive character of the adsorption of these molecules (160). The IR and TPD spectra obtained after CO adsorption on LaM03 oxides in the temperature interval 25-500°C showed the presence of different types of carbonate besides linear and bridged CO (50, 151, 154, 160162). Thus CO, as NO, interacts with surface oxygen and metallic ions. Carbonate formation increased for increasing adsorption temperatures. This was particularly so for LaMnO3 (50), where large increases in the intensity of the infrared bands were observed for adsorption temperatures above 200°C. Adsorption heats determined from the TPD peak temperatures showed that CO is slightly more stable on the Rh3+ cations of the LaRh03 than on metallic rhodium (163, 164).

C. CARBON DIOXIDE, SULFUR DIOXIDE, WATER,AND PYRIDINE Studies on equilibrium and energetics of C 0 2 adsorption have been carried out on LaCr03, LaFeO3, and LaCo03 in the zone -78,500"C by Tejuca et al. (147,165-167). Surface coverages followed the general trend LaCo03 > LaFe03 > LaCrO3. The adsorption isobars on LaCr03 and LaCo03 showed activated adsorption above approximately 150"C,

276

LUIS G . TEJUCA

et al.

whereas on LaFeO3 the coverage decreased continuously with temperature. In these systems the Freundlich model of adsorption, which assumes an exponential decrease of isosteric heat with the coverage, was obeyed. Results of adsorption of C02 and other molecules have led to propose the Freundlich equation for the estimation of surface areas of oxide catalysts (167). Experiments on successive 02-CO2 and CO-CO2 adsorption on LaFeO, (166) and Lacoo, (147) at 25°C were in agreement with previous results of 0 2 and CO adsorption; that is, they showed noncompetitive and competitive adsorption, respectively. Isobutene preadsorption did not affect in a significant way the subsequent C02 adsorption on LaCo03 (145). Kawai et al. (168) found IR bands of monodentate and bidentate carbonates on C02 adsorption on BaTiO3. Likewise, IR spectra after C02 adsorption on LaMO3 oxides in the interval 25-500°C contained bands of free carbonates and monodentate, bidentate, and bridged carbonates (50, 147,165,166). In some cases, transformation of monodentate into bidentate carbonates was observed with increasing temperature (166). The width of the IR bands, together with the fitting of the equilibrium results to the Freundlich model, was interpreted as indicating heterogeneity at the perovskite surface. TPD spectra after C02 adsorption at 25°C on oxidized and reduced LaMn03 (161) and LaCo03 (162) provided further evidence of the formation of different types of carbonate. Whereas CO and C02 desorption peaks were observed for reduced LaMnO,, no CO peaks were found for reduced LaCoO3. This may be associated to the higher catalytic activity of this latter compound for CO oxidation. Adsorption of SO2 on La,,,sSro.SMn03has been studied by Minming et al. (169), who took advantage of the poisoning effect of this molecule on the oxidation activity of the perovskite, in order to determine active center concentration on the surface. The adsorption at monolayer coverage suggested that the (1, 1, 0) face is the most frequently exposed. Infrared spectroscopic data of SO2 adsorbed on Lao.aSro.4Col -xM,O, were reported by Wan et al. (270).These are discussed in Section VII,J. Crespin and Hall (64)determined hydroxyl concentrations between 13.8 and 0.3 x lOI4 OH cm-2 on the surface of BaTi03, LaCo03, and SrTiO, perovskites previously equilibrated with water vapor at room temperature and then heated from 25 to 600°C. These concentrations were similar to those found by Fierro and Tejuca (106a) after adsorbing water vapor on LaCrO3 at 150°C and pumping from 125 to 525°C. The infrared spectra yielded by this latter adsorbent-adsorbate system showed bands at 3680 and 3550 cm-I. The adsorption and dissociation of water was assumed to take place on pairs of surface acid-base centers, anion va-

277

PEROVSKITE-TYPE OXIDES

cancy-02-, yielding an acidic OH on an anion vacancy placed between coordinatively unsaturated La3+and a reduced transition-metal ion (band at lower wavenumber) and a basic OH on a terminal oxide ion. Infrared spectra of pyridine adsorbed at 25°C on LaM03 (M = Cr, Mn, Fe, Co) oxides previously outgassed at 500°C are given in Fig. 14. For LaMn03 and LaFe03 (Fig. 14a) (50), they include Lewis bands at 1595, 1490, and 1440 cm-I whose intensity remained constant on H2O adsorption. For LaMn03 a shoulder at 1540 cm-' was also observed. A similar spectrum was found for pyridine adsorbed on LaCr03 (165). Pyridine adsorbed on the reduced (500"C, H2) and outgassed (as above) LaM03 oxides yielded a spectrum similar to that of the unreduced samples and also a weak Bronsted band at 1540-1545 cm-I whose intensity increased on H20 adsorption. The appearance of Bronsted acidity (Fig. 14b, Q-Q) may arise from the heterolytic dissociative adsorption of H2 on co-

I

1600

I

I

1400

3 /cm-'

I

1

I

1600

I

I

1400

3/

C d

FIG.14. (a) Infrared spectra after pyridine adsorption on LaMnO] (al) and LaFeOl (a2) and after subsequent H20adsorption (bl, b2). (b) Spectra after pyridine adsorption on reduced (HI, 500°C) LaMOl [M = Co (al), Fe (a2),Mn (a3),Cr (a,)] and after subsequent H20 adsorption (bl , b2, b3, b,). (Reprinted by permission from Ref. 50.)

278

LUIS G. TEJUCA el al.

ordinatively unsaturated M3+ and 02-ions, as occurs on some simple oxides: -0-M-0

+ H2

-

H

I

0-M-OH

The increase in Bronsted acidity (Fig. 14b, 61-b4) may be accounted for by the increase in anion vacancy concentration produced in the reduction process and the dissociative adsorption of the H20 molecule as indicated above. These results show the rather low acidic character of these perovskites. AND ALCOHOLS D. HYDROCARBONS

The adsorption of C2H6 and C2H4 at -5°C on reduced LaNiO3 was studied by Crespin ef al. (133). Whereas C2H6 adsorption followed Henry’s law, C2H4 adsorption obeyed the Langmuir isotherm for dissociative adsorption. The adsorption plateau for C2H4 was found to be close to the total number of Nit+ ions on the surface. Min and Peiyan (171) identified by IR spectroscopy the species -C2H4(ads) and -C2Hs(ads) after adsorption of C2H6 on Ndo.7Sro.3MnO3 at room temperature. Ichimura ef al. (135) found TPD peaks of CzH6, C2H4, C2H2, and CH4 after adsorption of C2H6 and C2H4 on Lac003 at 27°C. However, the TPD spectra obtained after adsorbing C2H6 and C2H4 on LaAI03 and LaFe03 contained only peaks of the undissociated molecules. These results, together with XPS data, emphasize the contribution of cobalt ions in the perovskite structure for the C-C bond scission. Tascon and Tejuca (172) and KremeniC ef al. (136) reported data on propene and isobutene adsorption on LaCrO, and LaMn03. These authors found physisorption on LaMnO3 below 0°C.A wide IR band centered at 2330 cm-I of COZwas observed on adsorption of these hydrocarbons above 25°C; its intensity was higher on the oxide with the higher oxidation activity (LaMn03) (136). In the system propene-LaMn03 bands of carbonate species were also found. Isobutene adsorption on LaM03 oxides at 25°C showed a maximum for the oxide exhibiting the lowest catalytic activity for CO and hydrocarbon oxidation (LaFe03) (136). Results of successive adsorption of 02-isobutene on LaM03oxides and C02-isobutene on LaCo03 pointed to noncompetitive adsorption (136, 145).

Madhok (173) found that the sorptive capacity of LaCo03 for alcohol vapors decreased in the order methanol > ethanol > n-propanol > nbutanol. No data were reported for aromatic alcohols. However, this

PEROVSKITE-TYPE OXIDES

279

oxide showed catalytic activity for vapor-phase oxidation of benzyl alcohol to benzaldehyde. VII. Perovskites in Catalysis

A. CO OXIDATION The oxidation of carbon monoxide over perovskite-type oxides has been widely studied. Voorhoeve et al. (30) brought forward new ideas in explaining the role of defect chemistry of perovskites such as cobaltites, manganites, chromites, and ruthenates. They suggested that two different oxidation processes should be distinguished; (1) the catalyst participates in the reaction as a reagent, being partially consumed and regenerated in a continuous cycle, and (2) the catalyst provides the atomic orbitals of the proper symmetry and energy to activate the reactant molecules. These two alternatives were termed reagent or intrafacial catalysis and template or suprafacial catalysis, respectively. The oxidation of CO in the middle temperature range ( 1OO-3OO"C) has been suggested as a suprafacial catalytic process where it is expected to observe important effects of the ferroelectric and magnetic order of surface spins and of semiconductivity on the catalytic reaction. Some of these will be reviewed below. The first study on the CO oxidation using perovskites was conducted by Parravano (2,2) (see Section VI1,I). Later on, Kawai et al. (168)found a discontinuity in the activation energy for the oxidation of CO by 0 2 or N 2 0 near the ferroelectric Curie temperature (T, = 120°C) of BaTiO3. The surface properties of these ferroelectrics are particularly different from those of the bulk. Alkaline-earth niobates and BaTi03 display charged surface layers and polarization reversal at the surface that are influenced by the charge layer. The reaction rate is very slow in the temperature range 100-200°C and under steady-state conditions was found to be limited by the rate of desorption of C 0 2 . The activation energy is 10 kcal/mol above T, and only 1.8 kcal/mol at lower temperatures. This suggested a large dipole-dipole contribution to the bond of COZas carbonate above T,. The low-rate and low-activation energy observed suggest that the oxidation reaction proceeds via surface defects. LnM03 perovskites in which the lanthanide (Ln) ions are essentially inactive in catalysis and the active transition-metal (M)ions are placed at relatively large distances (ca. 0.4 nm) from each other are excellent catalytic models for study of the interaction of CO and 0 2 on single surface sites. It must be stressed, however, that idealized correlations between catalytic activity that is confined to the surface, and a single collective

280

LUIS G . TEJUCA

et al.

parameter, (conductivity, ferromagnetism, etc.) should not be taken as conclusive. Voorhoeve et al. (14,30, 174), Shimizu (175), and Tasc6n and Tejuca (I76) have shown a suggestive correlation between the activity data, using mixtures of CO and 0 2 at atmospheric pressure, and the electronic configuration of the transition-metal ion. In Fig. 15 the catalytic activity is plotted either as the reciprocal of the reaction temperature at which the activity is lo+' mol of CO converted per square meter of catalyst per second (Fig. 15a) or as the rate of mole CO converted per unit area and unit time (Fig. 15b) versus the occupancy of the d levels for the transition M3+ion. It is known that the octahedral environment of the B ions splits the d-orbitals into two levels; the lower (tZe) contains orbitals that are repulsed less by negative point charges than are the orbitals in the higher (e,) level. It is observed that the maximum activity is attained in both --5 I

c

In n I

E

-48

P

--I -B .-c

- -8 - -9

FIG.15. Activities of first-row transition-metal oxide perovskites for CO oxidation in a 2 : I mixture of CO and O2at atmospheric pressure (a) or in a 1 : I mixture of CO and O2 at 227°C at atmospheric pressure (b). The activities of vanadates (B), chromates (O), manganates (A), ferrates (O), cobaltates (01,and nickelates (A)are plotted at the appropriate dorbital occupation corresponding to the average valence of the transition-metal ion. (Redrawn by permission from Refs. 14 and 176.)

PEROVSKITE-TYPE OXIDES

28 1

cases (Fig. 15a,b) for an occupation of the eg levels of less than one electron being the t2plevels half-filled or totally filled. This is in agreement with the data of Sazonov et al. (177)for the homomolecular 0 2 exchange on perovskites (see Section VIIJ and also with those of Boreskov (178) for the methane and HZoxidation and homomolecular 0 2 exchange reactions on several spinels. Voorhoeve et al. (14,301 have also stressed that the catalytic activity of perovskites is influenced by their stoichiometry. A simple way of varying the oxidation state of the ion at the position B is by substitution of the A ion by a different ion with an oxidation state other than 3. This method has been used by several authors (9,62,88, 96, 179-181) to understand the role of the 3d-orbital occupancy in the LaM03 series on the catalytic oxidation of CO. For M = Co the appearance of Cot+ions by introduction of Ce4+in position A enhances the rate of oxidation of CO, whereas the presence of Co4+ ions by substitution with Sr2+reduces the rate. The explanation for this behavior has been given by assuming that CO is bonded to the transition-metal ion as a carbonyl, as occurs on metals (182), with donation of the carbon lone pair into the empty 3 4 2 orbital of M to form a a-bond accompanied by back-donation of the tzgelectrons of the metal to the antibonding .rr-orbital of CO. It should be noted that the dz2orbital is the lowest e, level for the M3+ions at the surface, and in order to have a partially empty dzzlevel, the occupation of all the e, levels must be below unity. It must be also emphasized that for either nonsubstituted LnCoO, (Ln = La, Sm, Nd, Dy) or substituted Ndl-,Ba,CoO3 perovskites, two Co ions in oxidation states differing by one, bonded by an oxygen ion, can exchange an electron through the 02p orbital (183-185): C03+-O-C04+

C04+-O-C03+

(5)

This exchange, known as Zener double exchange, accounts for an average oxidation state of Co ions (183) at least on the time scale of the reaction of CO oxidation and reaches a maximum for an angle Co-0-Co of rr-radians (184),i.e., for the cubic structures. It has been suggested that such an average oxidation state of Co facilitates the adsorption-desorption of the reactants, whereas individual Co3+ions in their discrete spin and oxidation states lead to an adsorption process that is too strong or too weak for the catalytic reaction to take place (183). The kinetics of CO oxidation on Lac003 has been studied by Tasc6n et al. (186). In the temperature range 120-155°C these authors found a strong C02 inhibition and proposed a kinetic equation by assuming the controlling step to be the surface reaction between adsorbed CO and

282

et a/.

LUIS G . THUCA

dissociatively adsorbed oxygen:

and COz, where PCO, P O , , and Pco2 are the partial pressures of CO, 02, respectively; bo, is the adsorption coefficient of oxygen and k' = k K (k is the specific rate constant, and K is the equilibrium adsorption constant). Experimental data according to the linear form of Eq. (61, ( Pq/r)1/2versus Pbt and l / r versus Pco2 for variable partial pressure of 0 2 and CO2, are given in Fig. 16. From Arrhenius plots, these authors calculated an activation energy of I5 kcal/mol similar to that found by Rao and Chakrabarty (150) and lower than that reported by Yao (282). From these results and from IR spectroscopic data (see Section VI) the following scheme for the mechanism of CO oxidation on LaCoO3 was proposed:

coladsl

-

+ 2oladsl

Co31adrl

CO2ldr) + Olads)

(9) (10)

COU,,

(11)

C03(sdn)

C021adr1-

(slow)

where equation (9) should be rate-controlling. Oxygen is adsorbed as molecular 0 2 species on Co ions of low oxidation state, which subseadsorbed on the same quently dissociates yielding atomic oxygen (0-) 6-

b

ul

\ N

-2

N

3.0-

v

a 125 11 13511

P

\

N O ' 04

'

'

0.8

'

'

12

'

'

1.6

I

0

2

6

4

1/2

P i /mmHg

pcrJz/""

Hg

FIG. 16. Linear plots of experimental data for oxidation of CO according to Eq. (6) for variable pressure of O2 (a) and C02 (b) [Pco = 6.75 x 102 Pa (a); POI = 3.37 x 102 Pa (b)]. (Reprinted by permission from Ref. 186.)

PEROVSKITE-TYPE OXIDES

283

center. CO adsorbs on surface 02-ions, producing a labile species that interacts with adsorbed atomic oxygen, producing a carbonate that then decomposes, yielding adsorbed C02 and oxygen. Previous work strongly suggested that the activation centers for oxygen and CO are cobalt ions and oxide ions, respectively, whereas the La3+ions are catalytically inactive (138, 160). A slightly different mechanism has been proposed by Gunasekaran et al. (187) for the oxidation of CO on La2M04(M = Ni, Cu) oxides. On the basis of electrical conductivity changes in different atmospheres, these authors found that the rate of 0 2 adsorption was independent of the state of the surface (freshly evacuated or with preadsorbed CO); however, the rate of CO adsorption underwent a remarkable decrease by the presence of preadsorbed 0 2 . Because the rates of CO and 0 2 adsorption were faster than the rate of the oxidation reaction, they concluded that the surface reaction is the rate-determining step. Viswanathan and George (149) studied the oxidation of CO on LnCo03 (Ln = La, Nd, Sm, Gd) and found important gradient changes in the Arrhenius plots in the temperature range 190-220°C, where the adsorption isobars of O2and CO showed a maximum or a minimum. Tascon and Tejuca (138,160) and Chakrabarty et al. (148), using IR and XP spectroscopies, have explained this behavior by assuming that the adsorption of oxygen and CO occurs as described above. On the basis of conductivity measurements (149), a mechanism similar to that depicted by Eqs. (7)(1 1) was proposed, although other possible reactions involving formation of carbonates, namely:

-

CObds)

+ 02(ads)

Cokis)

+ 20%~)

COiiadsl

(12) (13)

C0,ads)

+ 2OL)

CO:i,dsi + 2e-

(14)

coiiadsl

have been advocated. In this reaction sequence any of the reaction steps of Eqs. (12)-( 14) between the adsorbed species may be rate-controlling. B. OXIDATION OF HYDROCARBONS AND OXYGENATED COMPOUNDS Libby ( 7 ) and Pedersen and Libby (8) were the first investigators to suggest the potential application of perovskites as oxidation catalysts. In the research work that ensued, the particular behavior of these materials was explained in terms of the relative ease with which oxygen species can be released from the catalyst surface. To confirm these ideas, work concerning the oxidation of paraffins (40,131,140-142,171,188-191), olefins (136,140,181,192),aromatics (193,194),and oxygenates (140,193,195199) have been carried out.

284

LUIS G . T U U C A ('1

d.

The oxidation of light paraffins, as methane (40, 140, 190), propane (141, 142, 189), and n-butane (140) has been frequently taken as a test reaction for perovskite oxides. Arai et al. (40) studied the catalytic combustion of methane over LaM03 (M = Cr, Mn, Fe, Co, Ni, Cu) and partially substituted Lal-,A,MO3 (M = Mn, Fe, Co; A = Sr, Ca, Ba, Ce; 0 e x =e0.4) perovskites and compared these oxides with Pt/A1203 catalysts. The Sr-substituted La-Mn, La-Fe, and La-Co oxide systems showed high oxidation activities; Lao.6Sro.4Mn03 was almost as active as Pt/A1203for a conversion level below 80%. These authors examined the dependence of the oxidation rate on the partial pressures of CH4 and 0 2 and proposed an empirical rate equation to describe their experimental data: r = k * P&,

*

P&

(15)

where r is the oxidation rate of methane and m and n are the reaction orders of methane and oxygen, respectively. The m and n values of two substituted La,,&o.2MO3 (M = Co, Mn) perovskites and a reference of 1 wt% Pt/A1203for methane conversions below 20% in the temperature range 450-650°C are collected in Table IV. The results clearly indicate a first-order kinetics with respect to the partial pressure of methane. Although Eq. (15) describes quantitatively the kinetic data, it is desirable to deduce formal rate equations on the basis of the precise knowledge of the reactivity of oxygen. Two kinds of oxygen species, adsorbed and lattice oxygen, with different bond strengths are present at the surface of perovskite-type oxides (see Section V,D). The adsorbed oxygen is believed to become active and to react with hydrocarbons at lower temperaTABLE IV Dependence of Methane Oxidation Rate ru on the Partial Pressure of Methane and Oxygenb

Catalyst L~(~.RS~O,ZCOOI Lao,&~MnOl 1 Wt% Pt/A120]

Temperature ("C) 450 550 650 450 550 650 450 650

Reaction order m

n

I .o

0.6 0.4 0.3 0.5 0.3

1.o 0.9 1.1 0.9 I .o 0.9 1.1

Oxidation rate, r = kP&, P & . Reprinted by permission from Arai et al. (40).

0.2 -0.5 -0.3

285

PEROVSKITE-TYPE OXIDES

tures than the lattice oxygen. At low temperatures, the oxidation rate depends largely on both CH4 and 0 2 partial pressures, and it may be expressed by the Rideal-Eley (R-E) mechanism, which assumes reaction between adsorbed O2 and gaseous CH4:

However, this expression does not apply in a large temperature range since the experimental n values (Table IV) decrease with increasing temperature. To explain this behavior, two active oxygen species should be taken into account. At low temperatures, the methane combustion is governed mainly by molecularly adsorbed 0 2 ; the contribution of lattice oxygen is negligible. At higher temperatures the coverage of molecular O2 decreases and the lattice oxygen becomes reactive. The methane combustion at high temperatures over metallic oxides can be described by a redox mechanism. The rate of oxygen incorporation to the lattice is much faster than the oxygen consumption; hence the rate is zero-order with respect to the oxygen partial pressure: (17)

r = kl * PCH4

The decrease of n with increasing temperature can be attributed to the change of the oxygen species that participate in the reaction, specifically, from molecularly adsorbed to lattice oxygen. The extent of reaction of molecular oxygen largely depends on its coverage; that is, it decreases with decreasing oxygen partial pressure and with increasing temperature. In an intermediate temperature range the contribution of both oxygen species to the rate equation can be equally important. The complex temperature dependence of the CH4combustion on a Lal-,Sr,Mn03 ( x = 0 . 2 , 0.4) (Fig. 17a) points to the participation of both oxygen species. On the basis of these considerations, the overall rate of CH4 combustion in the whole temperature range can be described by a combination of Eqs. (16) and (17):

At high temperatures Eq. (18) can be simplified because the coverage of molecularly adsorbed oxygen is small [I >> (KO, PO,)^/^]:

-

r = r,

+ tj = k,

*

PcH, * (KO,* PO,)^'^

+ kl

*

PCH,

(19)

The contributions of adsorbed and lattice oxygen to the overall combustion rate can be estimated by means of plots of the experimental rate versus (Fig. 17b). The intercepts of the straight lines represent the contribution of lattice oxygen, which, as expected, increases with in-

LUIS G . TWUCA

et al.

a

4c

!OO

400

600

T("C1

800

t 0d v a t m

FIG.17. (a) Rate of methane combustion on La&ro.rMn03. Circles, experimental data; dashed line, rate expected for oxidation by lattice oxygen; pointed line, rate expected for oxidation by adsorbed molecular oxygen. (b) Rate for methane oxidation versus Pod; intercepts represent contributions of lattice oxygen. (Reprinted by permission from Ref. 40.)

creasing reaction temperature. These considerations are consistent with Voorhoeve et al. 's classification of oxidation reactions on perovskites in suprafacial and intrafacial processes that occur at low and high temperatures, respectively (14, 30). Conner et al. (188) studied the oxidation of propane on Bal,85 Bio.~Oo.os(Bi&I v3Te)06(0,vacancy). This defect oxide containing Bi and vacancies on both A and B positions showed 25% propane conversion at 4Oo0C, yielding partial oxidation and cracking products. Other nonsubstituted perovskites containing Bi in only the B site were tested and found to be inactive. Propane oxidation has been also used by Nakamura et al. (95) and Nitadori and Misono (142) to study the effects of Sr2+ and Ce4+ substitution in LaM03 (M = Fe, Co) perovskites and more recently by Nitadori et al. (289) to investigate the effects of Sr2+,Ce4+,and Hf4+ substitution in LaMn03. In general, the activity of Lal-,Sr,MnO3 varied in a way parallel to the rate of isotopic equilibration of oxygen, the reducibility of the catalyst surface, and the amount of reversibly adsorbed oxygen (measured by volumetric adsorption). Because these properties

PEROVSKITE-TYPE OXIDES

287

pertain exclusively to the surface oxygen it is inferred that the activity is directly related to the nonstoichiometric character of the surface. The absence of any correlation between activity data and the oxygen peak that appears at high temperatures in the TPD spectra, which is known to arise from bulk lattice oxygen (Fig. 13), supports that assumption. Yao (181) investigated the combustion of C2H4 over LaCo03, BaCo03, LaxSrl-,MnO3, and L*.7Pbo.3MnO3 and found lower reaction rates for the cobaltites and manganites than for Co304. On the other hand, Jhaveri et al. (192) studied the oxidation of propylene on several cobaltites, CuC0204, and Ce02-Co304and fitted the kinetic data to the empirical rate equation r = k Pfic. At 400°C the catalytic activity increased in the order cobaltite < substituted cobaltites < Ce02-Co304< CuC0204. KremeniC et al. (136) studied the catalytic activity for the oxidation of C3 and C4 hydrocarbons on a series of LaM03 (M = Cr, Mn, Fe, Co, Ni) oxides and found remarkable differences among these compounds. The catalytic activity for total oxidation of propylene and isobutene at 300°C and for molar ratios HC : O2 = 1 : 4 and 2 : 1 are given in Fig. 18. The rates for

-

FIG. 18. Catalytic activity profiles of LaMO, oxides at 300°C in propene (open symbols) and isobutene (filled symbols) oxidation. Molar ratio HC:02= 0.25: 1 (circles) or 2 : 1 (triangles). (Reprinted by permission from Ref. 136.)

288

LUIS G. TEJUCA E f

a/.

isobutene were found to be higher than those for propylene. In both cases two pronounced maxima that are coincident with those found for oxygen adsorption (LaMn03 and LaCo03, Fig. 12) were observed. The partial oxidation of isobutene (propylene yielded only carbon oxides) followed the reverse sequence to that found for total oxidation. A similar pattern was found by Seiyama ef al. (140) for the propylene oxidation on LaM03oxides. Twin peak patterns were also found by Dowden et al. (200) and Dowden and Wells (201) in reactions involving hydrogen catalyzed by first-row transition-metal oxides and by Boreskov (178) in oxidation reactions catalyzed by a series of spinel-type oxides. This general behavior has usually been explained in terms of the change in crystal field stabilization energy (CFSE) as a result of the change in coordination of the M3+ ion, which shows two maxima when passing from dn to d9 configurations. Although the position of the experimental peaks either in the catalytic profiles or in the oxygen adsorption pattern does not exactly match with the predictions of the CFSE theory because other effects such as surface defects may be operative, these results provide further support to the ideas of Dowden and Wells (201) on the relationship between the local symmetry of M3+ions and adsorption and catalysis. On the other hand, the close parallelism between oxygen adsorption and catalytic activity for total oxidation of propene and isobutene indicates that these reactions occur through a suprafacial catalysis mechanism in which adsorbed oxygen is the dominant species participating in the reaction. A somewhat different application of perovskites is the oxidative dehydrogenation of olefins. Kehl et al. (202) reported that Lao,&ro.,jsFen,5503 can be used for butene dehydrogenation to butadiene and also for the conversion of other monoolefins containing at least four C atoms, such as the production of isoprene from isoamylenes. The oxidation of benzene on SrV03(193) in the temperature range 280400°C yielded only CO2 and small amounts of maleic anhydride at the higher temperature studied. The catalyst became completely oxidized to Sr2Vz07after one run. Because of the instability of V4+ in the perovskite structure, the highly exothermic deep oxidation caused the eventual thermal decomposition of the catalyst. The vapor-phase oxidation of toluene to benzaldehyde on LaCo03in the temperature range 350-600°C has been studied by Madhok (194).The kinetic analysis showed the oxidation to be first-order with respect to the hydrocarbon. The activity increased with the temperature below 500°C and decreased at higher temperatures. The activity increase was ascribed to removal of surface contaminants and to the generation of structural defects such as anion vacancies. The activity

289

PEROVSKITE-TYPEOXIDES

decrease above 500°C was assumed to be due to a decrease in surface disorder. Perovskites have also been studied as model compounds in the oxidation of oxygenated compounds. Arakawa et al. (197, 198) reported activity profiles for the oxidation of methanol on LnFe03 (Ln = La-Gd) and LnCo03(Ln = La-Eu) oxides. In the former series the activity decreased in the order Gd > Eu > Sm > Nd > Pr > La. From the analysis of the binding energy of the Fe2p31zphotoemission these authors found that the activity increased with decreasing covalence of the Fe-0 bond. Shimizu (199)found the oxides Lal-,Sr,Fe03 and LaMO3 (M = Mn, Fe, Co, Ni) to be active for oxidation of ethanol to acetaldehyde in the temperature range 2OO-45O"C. However, complete oxidation proceeded above 350°C and at high O2 partial pressure. The catalytic activity of Lal-,Sr,Fe03 increased with x up to x = 0.2 and decreased for higher x values. The activity of LaM03 oxides decreased in the sequence Co > Mn > Ni > Fe. C. NO REDUCTION Reduction of NO with CO or H2 was found to be an interesting example of intrafacial catalytic process (30). If this reaction is conducted over a transition-metal oxide, the reaction rate appears to be related primarily to the thermodynamic stability of the oxygen vacancies adjacent to a transition metal ion. Associative as well as dissociative adsorption of NO have been reported on perovskite oxides (14, 22, 80, 174) (see also Section VI,B); the adsorption on the reduced oxides is stronger than in the oxidic compounds. Dissociative adsorption takes place at moderate temperatures as in NO reduction over Lao.~sBao.lsCo03 at 100°C with the subsequent formation of N2 and N2O (73). To account for the observed products in the NO reduction over several perovskite oxides, the reaction scheme, which includes the participation of molecular and dissociative chemisorption of NO, has been proposed (22, 30, 80): M-0-M 2e-

+ NO + 0 + NO

-

M-0-M

+ 1 Nz

02(lattice)

+ Nad,

(20)

( 2 1a)

where 0 is an oxygen vacancy. The relation of the reduction of the catalyst with the conversion of NO was first demonstrated by Bauerle et

290

L U I S G . TWUCA

et al.

al. (203) and subsequently by Sorenson et al. (204), who found a close parallelism between the onset of NO reduction and the onset of bulk reduction of LaCo03. It was determined that the activity for NO reduction was associated with the formation of an oxygen-deficient structure having the general formula LaCo1-,03-,, where 0 C x S 0.08 and 0 < y S 0.5. Chemisorption of NO took place readily in this compound because of the presence of anion vacancies and low spin configuration of the Co3+ ions. From these considerations it is expected that perovskites that can easily release oxygen will be active for NO decomposition. Voorhoeve et al. (30, 80, 174, 179) demonstrated that manganites satisfy this requirement fairly well. In these perovskites the bond energy of the surface oxygen may be varied by introduction of La3+vacancies that change the electronic configuration of the Moo6octahedra, viz., lowering the binding energy of oxygen in Mn-0 and increasing the Mn4+content. Thus, these authors (14,80)compared the rate of reduction of NO over stoichiometric LaMn03 and L~.90’0,1Mn03 perovskites. They found the latter perovskite to be substantially more active than the former. In these compounds the binding energy of oxygen may be affected by the average oxidation state of Mn. To know whether the large improvement in the rate of reduction of NO was due to the Mn4+ions or to the La vacancies present, perovskites (AyAt1-,)Mn:?,Mnjt03 (where A and A’ represent one or more of the following entities: La, O’, Bi, Pb, Sr, Na, K, Rb; O’, cation vacancy) containing approximately constant proportions of Mn3+ and Mn4+ions but different A-0 binding energies, were tested for this reaction. In the (1 ,O,O) plane the lattice oxygen is coordinated by two Mn nearest neighbors at a distance of approximately 0.2 nm and by two A positions: A’

/ Mn-0-Mn / A

The heat of formation of an oxygen vacancy should be related to the sum of the A-0 and A‘-0 binding energies, which are given by the expression

A(A-0) =

AHf - m AHs - (n/2) DO 12 m

where AHf, AH,, and DOare the enthalpy of formation of 1 mol of A,O,, the enthalpy of sublimation of A, and the dissociation energy of oxygen, respectively. For A,A;-,Mni,?,Mn$+O3 perovskites, Eq. (22) gave values for the binding energy of oxygen decreasing in the order Bi, K > La, 0’> La, Rb = La, K = La, Na > La, Pb > La, Sr. This trend is closely related

PEROVSKITE-TYPE OXIDES

29 I

to the rate of NO reduction (N2 + N20 yield). These data clearly reflect the importance of the binding energy of surface oxygen, which determines the number of active centers (anion vacancies). The cation vacancies provide weakly bound oxygen to the surface and, therefore, favor both the formation of anion vacancies and the NO reduction. D. POLLUTION CONTROL The removal of CO, unburned hydrocarbons (HC), and NO from automotive exhaust requires catalytic devices in which these pollutants are eliminated. The catalysts that are being used for this purpose are supported Pt, Pd, and Rh (205). These present a number of desirable characteristics such as high activity, stability, and resistance to sulfur poisoning. However, their use may be constrained by the limited availability of noble metals. The metal oxides constitute an alternative that should be further explored. Among this latter category, perovskite-type oxides of Co, Mn, and Ru have been investigated as catalysts for such devices. Two designs of converters have been prominent: ( I ) in the dual bed, NO is reduced in the first catalyst bed by excess of CO and HC, and then the unreacted CO and HC are oxidized over a second catalyst bed; and (2) the three-way design uses a single catalyst bed to convert NO, CO, and HC simultaneously. Experimental tests were carried out with engine exhaust gases as well as with synthetic laboratory gas mixtures. The reduction of NO with mixtures of CO and Hz (dual-bed system) has been studied over several compounds. The La0.900.1MnO3-deficient perovskite yielded 100% selectivity to Nz;however, the catalyst became progressively deactivated by reduction with the CO-HZ mixture. The substituted manganite (Lao.s~Bia.onKo.o,)MnO3 showed a high activity but a poor stability because of the presence of low-melting-point components (256). Cobaltites showed a high activity and selectivity; their activity was strongly dependent on the oxidation state of the catalyst (204, 206). Ruthenium supported on alumina proved to be unsuitable for exhaust gas purification since the noble metal is oxidized to the highly volatile R u 0 3 oxide and thus is eventually lost. To overcome this problem Gandhi et ul. (207) and Dalla Betta et al. (208) proposed the stabilization of Ru3+ or Ru4+ ions in the matrix of a perovskite structure. Additionally, the dispersion of Ru in the B sites renders the catalyst more resistant to losses in exposed atoms caused by sintering. Thus the perovskites SrRu03 and LaRuO, and the substituted (Lao.8K0.2)(Mno.94Ru0,~)0~ and (Lql.xSr0.2) ( C O ~ . ~ R Uoxides ~.~)O were ~ studied by using laboratory gas mixtures (NOCO-H2) or engine exhaust gases (256,206,208,209). The most relevant

292

LUIS G . TUUCA

et a / .

feature of the above preparations was a high activity and a high selectivity to N2. Especially the latter perovskite supported on a Torvex alumina honeycomb has been reported to undergo only minimal changes in the level of NO conversion for extended periods of time (206). This is illustrated in Fig. 19a, where conversions of NO and CO are given as a function of 0 2 or CO excess. For the stoichiometric composition, the NO conversion appears to be approximately 90%. In general, the selectivity to N2 of Ru perovskites was found to be comparable to that of Ru metal (209-2Zf). Sorenson et al. (204) proposed LaCo03 as a potential catalyst for the simultaneous reduction of NO and the oxidation of CO and unburned hydrocarbons (three-way design). These authors found this perovskite to be very effective with NO conversions of approximately 95% and CO and HC conversions of up to 85%. The perovskite (La,-,.&o.4)(Coo.NPto.o3R~o.o3)O3 has been reported to be particularly suited for this purpose. For the selective reduction of NO to N2 at the reducing side and an efficient oxidation of CO and HC at the oxidizing side of the stoichio-

I

I

L-1

O

Excess of O2(O/0)

Excess of CO(%)

;

I

I

I

0

1

2

Excess of (&(YO) Excess of CO (YO)

FIG.19. Performance of noble-metal-substituted perovskites in the treatment of exhaust from single-cylinder engines. (a) (La,,8Sro.2)( C O ~ , ~ RO3~ supported ~.~) on a Torvex alumina honeycomb after 1000-h exposure to exhaust from leaded fuel in the first bed of a dual-bed catalytic system. (b) ( L h 6Sro.4)( C O ~ . ~ ~ P O3 ~ . on ~ ~a R Torvex ~ ~ , alumina ~ ~ ) support after 800-h exposure to exhaust from unleaded fuel in a three-way catalytic system. (Redrawn by permission from Ref. 206.)

PEROVSKITE-TYPE OXIDES

293

metric composition, this multicomponent oxide appears to provide an operating window in which the removal of all three pollutants was about 80% or higher (Fig. 19b). Other perovskites have been also used for CO and HC oxidation in both the dual-bed or the three-way systems. Cobaltites were investigated with laboratory gas mixtures (174, 181, 192) and with the exhaust gases of a single cylinder engine (204), whereas manganites were studied only with synthetic gas mixtures (36, 179, 181). The conversion of CO on LaCo03(204) was higher than those of HC in engine exhaust gases with excess air and substantially higher than that of propylene on La,,,Pbo.3Mn03 in synthetic automobile exhaust mixtures (212). The activity of the catalyst was found to be highly dependent on the oxidation state of the surface and, particularly at high temperatures, on the ratio of the partial pressures of oxidants and HC. At low temperatures and high partial pressures of CO and 02,LnCoO3 (Ln = La, Pr) and La,,7Pbo.3Mn03 and its homologues with Ba, Sr, Pr, and Nd at the A site showed an activity for CO oxidation similar to that of Pt. The activity of cobaltites, and especially of manganites, decreased with decreasing partial pressure of CO. On the other hand, platinum substitution was shown to have an important effect in the performance of perovskites. Thus, Lauder (206) reported that L~.sSro.~Coo.9pto.103 supported on monolithic alumina, besides displaying a high activity, showed a high resistance to Pb poisoning in single engine and automotive tests. The durability of these materials enhanced when used with near stoichiometric mixtures. The problem of sulfur poisoning has not been considered here. In Section VI1,J some aspects related with the interactions of sulfur dioxide with perovskite oxides as the adsorption of SO2 on B and oxygen sites, the effect of SO2 on Pt-doped oxides and in CO and hydrocarbon oxidation will be reviewed. In the light of these studies, some considerations on the present state of research on this topic are made.

E. HYDROGENATION AND HYDROGENOLYSIS OF HYDROCARBONS Libby (7) and Pedersen and Libby (8)were the first authors to report on the activity of LnCo03 perovskites (Ln = La, Nd, Dy) for cis-Zbutene hydrogenation; hydrogenolysis became important above 200°C. Later on, Lombard0 et al. (77,158,213-217) studied the hydrogenation and hydrogenolysis of several hydrocarbons on unsupported or LazO3-supported LaCoO,. These authors found a sharp activity peak in ethene hydrogenation at -20°C for a reduction of the catalyst of 1 . 5 - per molecule. Ad-

294

LUIS G . THUCA

et al.

sorbed CO presents a peak that coincides with that of hydrogenation (213-215). These results are illustrated in Fig. 20, where activity and

adsorption data are plotted as a function of the reduction degree of the oxide. Cyclopropane conversion (hydrogenation plus hydrogenolysis) at 200°C also reaches a maximum for Lac003 reduced between le- and 1.5e- per molecule (158). The selectivity for hydrogenolysis is constant up to a reduction degree of 1.5e- per molecule and then drops sharply. These results, together with data on deactivation, self-hydrogenation of ethene in the presence of the reduced perovskite, and tracer experiments with deuterium (the ethene-deuterium reaction yields multiplyexchanged ethene and ethane) (214, 215), seem to indicate that metallic cobalt is responsible for both hydrogenation and hydrogenolysis activity. The increase in catalytic activity with the oxide reduction and the observed product distribution in the hydrogenation of 1,3-butadiene on Lac003 and LaCo03 deposited on La203 support that assumption (77, 217). However, other oxidation states of cobalt, such as Co2+,may also participate in these catalytic reactions. Thus, Crespin et al. (133) showed that LaNi03, a perovskite very similar to LaCo03, is active for ethene hydrogenation after reduction to le- (La2Ni205)and 3e- (Nio/La203)per molecule. On the other hand, the adsorption plateau of C2H4 on La2NizOs appears to be close to the total number of Ni2+cations on the surface. This finding also indicates the involvement of NiZ+ions in the adsorption and presumably in the hydrogenation process. Ichimura et al. (63, 135, 218) on the basis of kinetic, tracer, and XPS studies, concluded that Co3+plays an important role in the rupture of the C-C bond whereas La3+and 02-ions contribute mainly to the dissociative adsorption of hydrogen in the hydrogenation and hydrogenolysis of C2-C5alkenes and alkanes. However, it must be noted that in the experimental conditions used (25-350°C and excess H2) some reduction of Co3+ to Co2+and Coois to be expected (64, 129). These reduced cobalt species may, therefore, be responsible for the activity. However, the possibility of the lanthanum and oxygen ion pairs acting as hydrogen atoms suppliers in hydrogenation and hydrogenolysis should not be discarded. This is supported by the fact that La203 catalyzes the H2-D2 equilibration as well as the ethene hydrogenation, thus showing the ability of this oxide for hydrogen dissociation (63). Other perovskite oxides containing Re(VI1) and cation vacancies in position B, such as BazBl,30~3Re06(B = Y,Sm), were found to exhibit hydrogenation activity of ethyl acetate to ethanol (2Z9). On the basis of the increased conversion to ethanol and the change of the catalyst color to black with time of operation, it is assumed that the activity is due to reduced Re in a highly disperse state.

z.

a

w

0

c c S T P of CO/g

d

i

O r + - -

m mol of C2H,/m2 min

lo3

296

LUIS G. TWUCA et al.

F. CO AND C02 HYDROGENATION Recently, special interest has arisen for the production of oxygenated chemicals from syngas (CO + H2) and C02 + H2. Supported rhodium appears to be one of the most promising catalysts for this purpose (220); however, it leads to very different product distributions. To understand the origin of these differences among nominally similar catalysts, model perovskite compounds have been used. Somorjai and co-workers (163, 221) have studied the activity of clean and oxidized Rh foils and of LaRhO, at different temperatures, under H2 :CO mixtures, and thus at different levels of reduction. Rh2O3 produces large concentrations of CZ and C3 products. Furthermore, addition of C2H4 to the CO + HZ feed yielded propionaldehyde, showing the carbonylation ability of Rh2O3. Under similar conditions over Rho, C2H4 was quantitatively hydrogenated to C2H6.Thus, higher oxidation states of rhodium seem to be necessary to produce the oxygenated organic molecules. The product distribution over LaRh03is highly temperature dependent and can be accounted for by a mechanism that involves associative adsorption of CO to form methanol and a dissociative mechanism for other products (163,221). The variation of selectivity with temperature is due to competing processes of hydrogenation and carbonylation and variable concentrations of molecularly and dissociatively adsorbed CO and hydrogen on the surface. Thus, below 225°C methanol production through hydrogenation of chemisorbed CO predominates. Between 225 and 350°C dissociative adsorption of CO occurs, yielding CH, species that can then undergo either CO insertion to produce oxygenates or hydrogenation to methane. At these temperatures CO insertion competes favorably with hydrogenation, producing mainly acetaldehyde and ethanol. Above 350°C CO adsorbs dissociatively; therefore, methanol production is severely depressed whereas methane is the main product. The active catalyst has been reported to contain rhodium as Rhl+and a small fraction as reduced Rho. The product distribution (hydrocarbons plus oxygenates) of CO + H2 reaction over lanthanum rhodate at temperatures of 200-350°C has been analyzed by Gysling et al. (41) and Monnier and Apai (222) by means of Schulz-Flory plots (Fig. 21). The observed monotonic decrease in the rates of hydrocarbon formation indicates an essentially common mechanism for the C-C bond formation in c2-C~hydrocarbons. However, the rate of formation of C2-oxygenates in the interval 300-350°C is greater than that of CH3OH, indicating that these compounds are formed through two different mechanisms. This finding is consistent with the results of Watson and Somorjai (163) and Somorjai and Davis (221).

PEROVSKITE-TYPE OXIDES

297

c2

Hydrocarbons

c4

Oxygeno tes

FIG.21. Fischer-Tropsch activity of LaRh03at different temperatures, as Schulz-Flory plots. (Reprinted by permission from Ref. 41.)

Gysling et al. (41) have clearly indicated the need for careful attention to the preparation method of LaRh03 perovskites, which are to be evaluated as catalysts for the CO hydrogenation and the usefulness of XPS for monitoring the surface of such materials. They showed that under typical hydrogen reduction conditions the surface of this oxide is fully reduced to Rho. If the catalyst is contacted with a 2 : 1 H2 :CO mixture at 1 atm and 300°C the Rh3d levels can be deconvoluted into Rho and Rh3+contributions, with no Rhf species detected. On the other hand, the trends observed in the Schulz-Flory plots of Fig. 21 on LaRh03 are essentially equivalent to those observed on Rh/Si02, suggesting the same catalytically active Rh species for both catalysts. It appears, therefore, that the formation of hydrocarbons and oxygenates occurs on Rho centers. This conclusion would be in agreement with gravimetric results, indicating that the lanthanum rhodate reduces in H2 directly to metallic rhodium (107b). Conversion of syngas to oxygenate organic chemicals over perovskitetype compounds has been also studied by Broussard and Wade (223). These authors examined several perovskites with La, Ce, Nd, Sr, and Ba

298

LUIS G. TUUCA

et al.

in position A and transition metals in position B. The selectivities for oxygenates obtained with LaM03 (M = Co, Ni, Fe) oxides were in the range 24-33%. These LaMO3 perovskites, partially substituted with Ru, yielded much lower selectivities (3-14%). This is an expected result since Ru has been used in the Fischer-Tropsch synthesis for the production of higher hydrocarbons (224). Copper-substitutedperovskites such as LaCuo.5Ti0.503and L ~ C U ~ , ~ M Q yielded , . ~ Omethanol ~ selectivities of 3738%. This high selectivity suggests that copper, presumably in the reduced form, is the catalytic agent. To the authors' knowledge the influence of the nonstoichiometric (oxygen-deficient) character of these perovskites (see Section IV) has not been studied in this type of reaction. It would be interesting to investigate how the concentration of oxygen vacancies in these oxides affects the selectivity for oxygenated compounds. The highest selectivities (almost exclusively methanol) were found for noble metal perovskites as BaRh03 and BaPtO, (62 and 54%, respectively) (223). The catalytic activity for C02 hydrogenation over LaCo03and partially substituted Lal-,M,Co03 (M = Sr, Th) perovskites was recently studied by Ulla et al. (225). These authors used a recirculation system with a ratio H2 :C02 = 4 : I at 280°C and a total pressure of 2.13 x 104 Pa. The C02 conversion to oxygenates plus hydrocarbons over LaCo03 is little affected by the extent of reduction of the catalyst; however, the rates of formation of hydrocarbons were highly sensitive to this parameter. Reduced Lao.sTh0.2CoO3displayed the highest initial conversion of C02 and a high methane selectivity. L~.6Sro,4Co03 prereduced at 300°C showed a sharp maximum in activity and a high selectivity to methane; however, at higher reduction temperatures the initial conversion of COz decreased whereas the selectivity to higher hydrocarbons increased. Metallic Coo sites are assumed to be the active sites for CO2 hydrogenation; their specific. activity and selectivity are strongly influenced by the presence of Sr and Th promoters. It seems that the matrix effect of the perovskite, which permits a high dispersion of Co in the reduced materials, plays an important role in obtaining a high selectivity toward higher hydrocarbons. G. SO2 REDUCTION The reduction of sulfur dioxide by CO to elemental sulfur can be effected over perovskites of transition metals (22, 226-228). However, in addition to the desired reaction

2co + so2

-

2c02 + 4s

(23)

PEROVSKITE-TYPE OXIDES

299

a second reaction occurs between CO and the elemental sulfur produced during the course of the reaction:

co + 4s

-

cos

(24)

Carbonyl sulfide production usually proceeds to a substantial extent over conventional iron oxide (229) and supported-copper catalysts (230-232). This compound, which is highly toxic and undesirable as an effluent contaminant, seems to be produced on the sulfide species of these catalysts. Happel et al. (22, 228) reported that COS production can be greatly decreased with a catalyst consisting essentially of the perovskite LaTi03by means of which high conversion of SO2 at space velocities of up to 40.000 h-' were obtained. This improved performance is thought to be related to the high resistance of LaTiO3 to form sulfides with the elemental sulfur produced by the reaction given by Eq. (23). The oxide-versus-sulfide stability has been measured by the difference between the heats of formation of simple oxides and sulfides for several metals. This difference was found to be a maximum for titanium (22). Kinetic data obtained in a flow reactor with several CO + SO2 mixtures and LaTi03 as catalyst indicated that the reaction is first-order with respect to SO2 partial pressure and zero-order for CO partial pressure. These results were, therefore, correlated by the rate expression r = k .Psa. The reduction of SO2 with CO on Lac003 has been investigated by Bazes et al. (226) in the temperature range 284-465°C and CO concentrations between one and three times the stoichiometric amount for complete reduction. For temperatures above 380°C and a CO concentration of 2.5 times the stoichiometric amount a reduction of SO2 of 90% or higher was obtained. However, substantial amounts of COS were produced at high temperatures and large excess of CO. The formation of COS was lessened by decreasing CO : SO2 initial concentrations to the stoichiometric ratio. These authors also'found that the kinetic data correlated with the power rate law equation: r = k Pio0,* P$o. From Arrhenius-type plots (In r vs. 1 / T ) as well as from effectiveness factor values they concluded that LaCo03 exhibited important pore diffusion effects, particularly at high temperatures. Hibbert and Tseung (227) investigated the effect of water and oxygen on the reduction of SO2 over La&ro.&oO3 as well as the changes in chemical composition undergone by the catalyst during the reduction reaction. Unlike alumina-supported metal catalysts, L ~ . 5 S r o . ~ C tolero03 ated high levels of oxygen in the gas stream, provided sufficient SO2 was present to react with the oxygen. Water vapor ( ~ 2 %did ) not adversely affect the reaction; however, hydrogen sulfide was detected, although its concentration was at least an order of magnitude lower than that of COS.

300

L U I S G . TUUCA ef at.

Tracer studies with I4C and 35S also provided information about the mechanism of the overall reaction given by Eq. (23) (233). Experiments employing reaction mixtures containing all four species or only carbon or sulfur compounds, with steam as a diluent, were carried out with labeled 14C02and %. It was possible to measure the reverse 14Ctransfer when sulfur compounds were removed from the reaction chamber. Thus the exchange reaction

+ co

1 4 ~ 0 ~

-

cot + 14co

(25)

furnished evidence of transfer of I4C from C02 to CO. Carbon-14 exchange still occurred in the presence of sulfur, although at a slightly lower rate. On the other hand, the exchange reaction

so2+ 3 5 s

-

+s

3 5 ~ 0 ~

(26)

took place at a lesser extent in the absence of CO and C 0 2 than when these compounds were present. The low rates of I4C exchange, coupled with the zero-order reaction in CO concentration, suggested that the rate of C02 desorption may be important in controlling the rate of the overall reaction. The formation of carbonate structures and their stability under reaction conditions could, therefore, be an important factor influencing the catalyst performance.

H. ELECTROCATALYSIS AND PHOTOCATALYSIS The cathodic reduction of oxygen is one of the most important processes in energy conversion systems. Metallic oxides such as Pt-doped tungsten bronzes (234-238), LaNi03 (239-242), and partially substituted Lnl-,Sr,O3 (Ln = lanthanide element, mainly La) perovskites (6, 242247) were reported to have properties suitable for this reaction. The catalytic performance of these materials for the electroiytic evolution of oxygen has also been investigated (246, 247). The Tafel slopes of oxygen evolution were dependent on the perovskite-type electrode; however, a common mechanism seemed to be operative, i.e., the electrochemical adsorption of OH- followed by the electrochemical desorption of OH (rate-determining step) yielding H202as an intermediate that then undergoes catalytic decomposition to 02. The reduction of oxygen has been investigated over either unsubstituted or partially substituted perovskites (6, 2.34-247). Matsumoto et al. (240) used a rotating ring-disk electrode that consisted of a LaNiO3 disc and a Pt-ring electrode. These authors proposed the following reactional

PEROVSKITE-TYPE OXIDES

scheme: 0 2

-

02fnd.)

e- + O Z W ~ ) Hz0 e-

+ Olw

Oldr)

-

HOzfadr) + OH-

+ HO~adr,

HOiads,

30 1

(27) (28) (29) (30)

The rate-controlling step was found to be that represented by Eq. (30) when the Tafel relation holds (low polarization region) or by Eq. (27) for the limiting current (high polarization region) (241). The subsequent steps to (30) must conform to the experimental result of nondetection of HO; ion in the electrolyte. The rearrangement of adsorbed HOT on the electrode surface with formation of OH- in a manner such as represented by Eq. (31) has been proposed (240): 0

O-h!l-O-OH

-

PH.

0-M-b

(31)

where 0 denotes an 02-vacancy on the perovskite surface. Equation (3 1) should be fast since no dissolved HOT was detected at the ring electrode. Such a mechanism with an electron transfer from the electrode to the cr* orbital of the adsorbed HOT should involve the break of the cr bond of this species with the subsequent formation of OH-. The surface composition of the La and Ni perovskite may be represented as LaNi03-h,where A = 0.2-0.3 in the oxygen-reduction potential. Bockris and McHardy (234) found that NaxW03bronzes containing traces of Pt exhibited a catalytic activity for oxygen reduction that was higher than that of Pt metal, although other studies (236, 237) failed to confirm this finding. Na,W03 and other tungsten bronzes also showed activity for the electrochemical reduction of H202. For this reaction, Randin (248,249) found an inflection in the current-potential curves that was attributed to an inhibition phenomenon. The fact that the relative position of these curves undergoes a regular shift as the H202 concentration changes suggests that at a given potential the same fraction of surface sites are blocked independently of the Hz02 concentration. This situation can arise from a parallel reaction with incorporation of hydrogen to the bronze NaxW03t nH+ t ne-

-

Na,HnW03

(32)

This reaction has been shown to occur in the potential range where the inhibition region starts, i.e, about 0.2 V. Because the activity of Na,H,W03 for the reduction of H202 is lower than that of Na,W03, the number of active sites should decrease as the reaction progresses.

302

LUIS G . TEJUCA

et al.

The oxygen evolution has been chosen by several authors for kinetic studies that involve rate-determining step evaluation. This reaction has been studied on metals as well as on oxide electrodes. Within this latter category, RuO2 has been most frequently investigated. On this oxide, the reaction has been suggested to occur with participation of lattice oxygen (250). Oxygen evolution on perovskites has been studied in alkaline solution (251-253). Bockris and Otagawa (246,247) carried out a careful and systematic investigation on several substituted perovskites in an attempt to correlate their electrocatalytic properties with their structure, laying particular emphasis on the bonding of oxygenated intermediates at the surface. These authors showed that the most likely mechanism involves a rate determining M-OH desorption (where M is the transition element in LnMO3). A careful analysis showed that differing M-OH bond strengths among the perovskites give rise to different adsorption isotherms in the liquid-solid interface. The mechanism found most consistent with the kinetic data (253) is Mz + OHMz - OH (H@Z)(phyr)

(H202hphys)

+

+ OH+ OH-

(HOThphys)

=Mz - OH + e-

-

M L H 2 O 2 + e-

(HoThphys)

+ H20

H20 + OH-

0 2

(33) (34) (35) (36)

A model based on the MO theory has been proposed for the active (0,0,1) plane of the perovskite surface, viz., the MOj cluster (Fig. 22a), in which the 3d (eg, rzg) levels will split further because of the lower symmetry at the surface. Wolfram and co-workers’ (254) calculations for SrTi03 showed that the eg levels of the M06 cluster (bulk) split into the d,~,2and d,l levels, with the latter laying below the former at the surface. Also, the t2g levels split into the double degenerated d,, and d,, states and the singlet dJystate. These energy levels are schematically represented in Fig. 22b. Assuming that these calculations also hold for other perovskites it is possible to assign the d-electron configuration of the M3+ ion at the surface as shown in Fig. 22b. The 4 2 orbital will be occupied by an electron only in M = Ni and Co (high spin configuration). On the other hand, the electronic structure of OH may be represented as follows: Is2 2s2 (2p,+ 1s ) 2pz ~ 2p:, in which the bonding state is an 02p, orbital o-bonded to a HI s state. In the interaction between an M3+ ion at the surface of the perovskite and an OH species, the d22 orbital will overlap with the (2p, + 1s) MO of an OH forming c+-typeorbitals whereas the d,,(d,,) orbital will interact with the 2px(2py)of an OH, giving r-type orbitals. On the basis of the d-electron configuration, MO diagrams for the MZOH bonding at the surface of LnM03 oxides (M = Mn, Ni) were con-

PEROVSKITE-TYPE OXIDES

OHSolution

OH-

OH-

303

y-

OH- OH- OH- OH OH-

0 Transition metol Mz

8

:oOXni::-ion

FIG.22. Schematic details of the perovskite surface in electrocatalysis:(a) model for the active surface; (b) d-electron configuration of M3+ ions; (c) MO diagrams for the M*+-OH (M = Mn, Ni) bonding. (Reprinted by permission of the publisher, The Electrochemical Society, Inc., from Ref. 247.)

structed (Fig. 22c). The following observations can be derived from these diagrams: (1) electrons in the tiz* orbital, e.g., Ni3+and high spin Co3+ ions, may play an important role in achieving high reaction rates, since they tend to occupy (+*-antibondingorbitals of MZ-OH, resulting in a weaker bond; (2) since OH is a saturated ligand, the contribution of rbondings, i.e. back-bonding to MZ-OH, would be negligible, especially in Co3+and Ni3+;and (3) the poor catalytic activity of V3+and Cr3+ions may be due to a higher oxidation state easily formed on those perovskites. Matsumoto et al. (252,253,255) have made an interpretation of the relative reaction rates in terms of geometrical factors of the surface. They consider the overlap integrals between the sp, orbital of 0 and the eg orbital of the transition-metal ion that are assumed to be related to the o * character of the bonds in the perovskite. In the same line of electrocatalytic applications, some perovskites have been shown to be suitable cathode materials in high-temperaturefuel cells (256, 257). Successful and economical operation of a high-temperature zirconia fuel cell has been achieved by means of porous PrCo03cathodes. With hydrogen as the fuel and oxygen as the oxidant, power densities of

304

LUIS G . TEJUCA

et ul.

about 0.3 kWlcm2 at 1000°C for periods in excess of 5000 h have been generated (256). Such cells, however, do not survive thermal cycling because the thermal expansion coefficient of &Coo3 is 2.5-3.0 times larger than that of zirconia, which leads to interfacial stresses during cooling. Perovskite-type oxides have also found application in several photoassisted reactions. Among these, the photolysis of water over BaTiO3 electrodes (258), SrTi03 powders (259), and NiO-coated SrTiO3 powders (260), the ammonia synthesis over metal oxide-coated MTiO3 (M = Sr, Ba) powders (261), the oxidation of CO over LaCo03, and MTiO3 (M = Sr, Ba) (262). have been investigated. Kennedy and Frese (258) have measured photocurrent efficiencies and the distribution of potential in semiconducting BaTi03used for the water-splitting. By applying the standard techniques for studying single-crystal electrodes to polycrystalline sintered BaTiOs, these authors have demonstrated a temperature effect of the photocurrent efficiency and used the semiconductor depletion layer theory to interpret the efficiency data in terms of solid-state properties. Later on, Lehn et al. (263) observed that SrTiO3 particles alone cannot catalyze the photodecomposition of water; rhodium coating was essential for the production of H2 and 0 2 , with Rh(II1) as the only active species. The preparation of the same type of catalyst from ultrafine powder SrTi03 (259) did not enhance the photocatalytic activity in the water-splitting reaction. Other surface modifications such as NiO coating where Ni metal is said to exist at the NiO-SrTiO3 interface have been attempted (260). The existence of Ni metal at this interface suggested the transfer of electrons between both materials. The photocatalytic synthesis of ammonia from N2 and H2O was studied by Li et al. (262) over SrTi03-and BaTi03-basedcatalysts. These authors demonstrated that the thermodynamically unfavored reaction can proceed at 50°C using band-gap illumination of a RuO2-NiO doubly coated MTi03 (M = Sr, Ba) catalyst covered with liquid water. The catalytic effect was interpreted in terms of the ability of Ru for N2 activation and of Ni for H2evolution. The reverse reaction (photodecomposition of ammonia), which was coupled with photodecomposition of water, became important when ammonia was accumulated in the reactor. A strong photocatalytic effect was observed by van Damme and Hall (262) for CO oxidation on MTiOJ (M = Sr, Ba). In addition, they found a very similar behavior related to the ferroelectric properties of the catalyst for CO oxidation over BaTiO3 in the dark. The effect of the light was discussed in terms of band-to-band transitions followed by hole capture, whereas ferroelectricity would arise from the effect of polarized surface layers on the shape of the bands.

PEROVSKITE-TYPE OXIDES

305

I. OTHERREACTIONS

-

The homomolecular exchange of oxygen (HEO) I60

+

I80

16Ol.90

(37)

on LaM03 (M = Cr, Mn, Fe, Co, Ni) and LnCo03 (Ln = Nd, Sm) oxides has been investigated, in a broad range of temperatures, by Sazonov et al. (277).Kinetic results for the H E 0 and for the electrochemical reduction of oxygen over a partially substituted La&ho.~Co03perovskite were also reported by Hibbert and Tseung (264). The rate of H E 0 on samples that have been equilibrated with an isotopic (I602 + I8O2) gas mixture is given by C" - co In C" - C' = kt where Co,C", and Cf are the initial, equilibrium, and current concentrations, respectively, of l6O1*0molecules and k is the rate constant for the H E 0 reaction (265). Values of the rate constant k were determined from Eq. (38) at several temperatures, and the number of oxygen molecules reacting per second and per unit of surface area Z was calculated by

where NR is the total number of oxygen molecules in the reactor and S is the surface area of the catalyst. The activation energies of the exchange of oxygen for the LaM03 compounds in the high-temperature region (>300"C) were calculated from Arrhenius plots log Z versus 1/T. These, together with the activation energies for reaction (37) and the energy of the oxygen bond (M-0) for the simple oxides of the transition elements, are depicted in Fig. 23. As can be seen, the activation energy of the H E 0 for LaM03 and for the corresponding simple oxides follow almost the same trend. This behavior has been explained considering that the catalytic activity of the LaM03 oxides in the H E 0 is determined by the M3+ ions with no significant contribution of the La3+ions in the compounds. Such an explanation is supported by the important differences observed in the activation energies (AE,) and rates of the HE0 between nickel compounds where the transition metal is in + 3 or +2 oxidation states, e.g., LaNiO3 and NiO. These differences were found to be 20 kcal/mol for AE, (Fig. 23) and more than three orders of magnitude for the rate of exchange, respectively. However, the similarity of the temperature dependence of the exchange for LnM03and the corresponding LnzO3(Ln =

306

LUIS G . TEJUCA

et af.

50 t C903 Mn%

F e 3 Co304 NIO

P

40

LaCr4

I

LaFe4

LaMnO3

I

LaNi4

Lac003

FIG.23. Comparison of the activation energy of oxygen exchange for h M 0 3 oxides (squares) with the activation energy of exchange (circles) and the bond energy of oxygen (triangles) for the corresponding simple oxides. (Reprinted by permission from Ref. 177.)

La, Nd, Sm) oxides in the low-temperature region (<2OO0C) shows that, in these conditions, the H E 0 is due to the Ln3+cations. These results strongly suggest that a clear separation of the functions of the M3+ and Ln3+ ions exists in the perovskites studied, with predominance of the former in the exchange at high temperatures and of the latter at low temperatures. Hibbert and Tseung (264) calculated from Arrhenius plots the activation energies for the gas-phase oxygen exchange and for the reversible electrochemical reduction of oxygen on L Q . S S ~ ~ . ~ CThese O O ~were . found to be 16 and 13 kcal/mol. The similarity of these values were taken as indicative of that the chernisorption of oxygen is the rate-determining step for both processes. Van Damme and Hall (262) studied the oxidation of H2 over LaCoO,, either in the dark or while MTiO3 (M = Sr, Ba), and Ba(Feo.33Tio.67)02.677 being irradiated with band-gap light. No effect of light on the rate of the oxidation was found with any of these materials. These authors described this reaction as a process dominated by charge transfer in the boundary layer of the catalysts. More recently, Futai et al. (266) studied the activity

PEROVSKITE-TYPE OXIDES

307

of LnCoO3 (Ln = La, Pr, Nd, Sm, Eu, Tb, Dy) perovskites in the same reaction. With the exception of TbC003, the activity decreased with increasing atomic number of Ln. These authors found the same sequential order for the surface oxygen binding energy and the activation energy of the reaction. They concluded that the surface lattice oxygen participates in the oxidation of Hz.The varying size of the rare earth ion changes the cell parameters and the binding energy of the surface ions, thus modifying the catalytic activity of the perovskite. The series of partially substituted Ca,Lal-,Mn03 (x ranging from 0 to 1) perovskites was studied by Vrieland (87)and Chengxian et al. (65) in NH3oxidation. Although this reaction is probably more complex than the commonly used CO oxidation or the N20 decomposition, it has the distinct advantage that selectivity to three products, Nz, N20, and NO, can be examined. Working in the temperature range 350-400"C, Vrieland (87) found two types of catalytic behavior. Below x = 0.3, Ca,Lal-,MnOs produced large amounts of N20 and Nz, whereas above x = 0.4 appreciable amounts of NO were also formed. Since NzO results from an atomic adsorbed species of oxygen or lattice oxygen and NO from a molecularly adsorbed species, this author concluded that the lanthanum-rich region of this catalyst series had a high and constant concentration of atomic oxygen on their surface. By contrast, in the calcium-rich region the surface may act as an electron acceptor rather than donor; therefore, oxygen is not adsorbed in atomic form and the lattice oxygen is not reactive. Chengxian et al. (65) found that the NO selectivity at 800°C increased with increasing reductive nonstoichiometry and, therefore, with increasing calcium substitution (see Section IV). These results are qualitatively consistent with those of Vrieland (87) and were interpreted to indicate that the lattice oxygen is not directly involved in the reaction. It seems that the main function of this type of catalyst in the calcium-rich region is to provide suitable sites for the activation of NH3 and 0 2 molecules. Therefore, in that region the reaction would be of a suprafacial type according to Voorhoeve and co-workers classification (24,30). The N2O decomposition over perovskites ABO3 (A = La, Ba, Sr, Ca; B = Ti, Mn) or perovskite-related compounds in the range 280-550°C has been studied by Nagasubramanian et al. (267-270). Muralidhar et al. (271), Srinivasan et al. (2721, Raj et al. ( 2 7 3 , Rao and Srinivasan (274), and Kameswari and Swamy (275). This reaction was found to be firstorder with respect to N20 below 6.67 x lo3 Pa, whereas the reaction orders were, respectively, 1 and -0.5 for N 2 0 and 02, above 2.67 x lo4 Pa. On the basis of kinetic, electrical conductivity, and thermoelectric power data, the following mechanism, first advanced by Winter (276)for

308

LUIS G . TEJUCA

et al.

The decomposition rate is strongly inhibited by oxygen for initial pressures above 2.67 X 104 Pa. Raj et al. (273) also found a weak oxygen inhibition in the low-pressure region. These authors showed that adsorbed oxygen species arising from N20 decomposition over LaMnO, behave differently from those obtained from the adsorption of gas-phase oxygen. They found that oxygen inhibits or accelerates the N2O decomposition depending on the nature of the adsorbed species. The surface of this perovskite should contain a spectrum of sites capable of promoting different types of adsorbed oxygen. This finding was supported by the observed compensation effect between the frequency factor and the activation energy for various pretreatments of the catalyst (273). The kinetic analysis of N20 decomposition at higher pressures revealed that the rate is controlled by oxygen desorption [equation (44)] (267-275). If this step is rate-controlling,the reaction rate should be a function of the lattice parameter of the catalyst, because the M-0 bond length and bond energy should influence the M-O,d, bond. Such a correlation between the activation energy for N20 decomposition at 2.67 X lo4 Pa and the lattice parameter has been shown by Nagasubramanian et al. (270) for the series of MTiO3 (M = Mn, Co, Ni) oxides with the ilmenite structure and by Muralidhar et al. (271) and Srinivasan et al. (272) for the series of perovskites LaM03 (M = Cr, Mn, Fe, Co, Ni). On the other hand, Muralidhar et al. (271) found a twin-peak pattern (maxima for LaMnO, and LaCo03) in plots of activity for NzO decomposition versus d-orbital occupancy in the series of LaMO3 oxides and concluded that LaM03 structures containing low-spin M3+ ions were highly active for the decomposition reaction. As commented previously for CO and hydrocarbon oxidation, this behavior is characteristic of the occurrence of crystal field effects (see Section VI1,B). A twin-peak pattern has been also observed for the oxygen isotope exchange for the same series of perovskites (177). Because the exchange has been shown to be controlled by oxygen desorption (276), the similarity in activity patterns between these two reactions supports the conclusions based on kinetic data for N20 decomposition. On the other hand, Nagasubramanian et al. (267) showed that the activi-

PEROVSKITE-TYPE OXIDES

309

ties of the ATiO3 (A = Mn, Mg,Ca, Sr, Ba) oxides can be rationalized with the standard heat of formation on the basis of a volcano relationship. This finding is consistent with the mechanism proposed for the N2O decomposition. The factors brought forward in the literature as exerting an important influence on the selectivity of metal oxide surfaces in the dehydrogenation or dehydration of aliphatic alcohols have been reviewed by Cunningham et al. (277). 2-Propanol decomposition over simple or complex oxide systems has been frequently chosen as a model reaction as it provides information about both the activity and the selectivity and is free from undesired side products. The double perovskites La2MnM06 (278), Ln2MnNi06 (279,280),and La2TiMO6 (281) and the unsubstituted LaM03 (M = Fe, Co, Ni) or partially substituted L ~ O . B ~ M ~ . (M I ~ C=OCa, O ~Sr, Ba) oxides ( 2 8 2 ~were ) studied for their catalytic performance in the decomposition of this alcohol. This reaction has also been investigated over tungsten bronzes M,WOs (M = alkaline or rare earth metal) or suboxides WOp, ( y < 0.20) (283). Radha and Swamy (278) proposed a possible mechanism for the dehydrogenation of 2-propanol over La2MnM06 (M = Co, Ni, Cu). These authors found that admission of H2, together with the alcohol, does not have any influence on the reaction rate; however, admission of acetone with 2-propanol decreases the reaction rate at all partial pressures. It can be inferred that H2 acts as a mere diluent whereas acetone has an inhibiting effect that may be due to its slow desorption. They also measured the conductivity changes of the catalyst in the presence of the reactants or products of the dehydrogenation. As a result of these studies it was concluded that the catalyst surface is covered predominantly with acetone under reaction conditions. Because acetone adsorbs by a donor-type mechanism, as shown by the decrease of the conductivity on its adsorption, its desorption involving electron transfer from the p-type semiconductor catalyst to the adsorbed species can be expected to be the slow process. Radha and Swamy (279, 280) also studied the effect of the lanthanide ion in 2-propanol dehydrogenation (only acetone and H2 were found in the products) over a series of perovskites similar to that described above, i.e., LnzMnNiO6 (Ln = La, Nd, Sm, Gd). A plot of the logarithm of the frequency factor A versus the activation energy for this reaction [E,(-H2)] gave a straight line (Fig. 24, circles), which would indicate the existence of a compensating effect, and this implies that energetically different active sites are involved in the reaction. This effect has been assumed to be due to the influence of the Ln3+ ions on the catalytic activity of the transition metal ions because the rare earth ions have been

3 10

L U I S G . TWUCA

et al.

FIG. 24. The frequency factor as a function of the activation energy for 2-propanol dehydrogenation over Ln2MnNi06(Ln = La,Nd, Sm, Gd) (circles)and La2TiM06(M = Ni, Cu,Zn) (triangles). (Redrawn by permission from Refs. 279 and 281.)

shown to exert only a modifying effect in perovskites (174). The higher activation energies found for Ln2O3 oxides with respect to those reported for LnlMnNiOB perovskites support this assumption (279). On the other hand, the activation energy for the dehydrogenation was found to increase linearly with decreasing lattice parameter, i.e., with decreasing radii of the Ln3+ cations. This result was accounted for by a parallel decrease of the electron density around the transition-metal ion. Thus, the slow step of the reaction, viz., desorption of acetone with electron transfer from the catalyst to the adsorbed species, would be less facile for increasing atomic number of the rare earth element. Details of 2-propanol dehydrogenation over La2TiMO3 (M = Cu,Ni, Zn) perovskites have been reported by Ramanujachary et al. (281). The kinetic parameters were evaluated from the initial reaction rates and plotted in Fig. 24 (triangles). Contrary to that observed for the series of LnzMnNiOb oxides, the absence of a linear relationship between log A and E,(-H2) seems to exclude the possibility of occurrence of a compensation effect. On the other hand, if lanthanum and/or titanium ions (common to the compounds studied) were to be active sites for the decomposi-

PEROVSKITE-TYPE OXIDES

31 I

tion reaction, the activation energy would be expected to vary linearly with the lattice parameter. The absence of any such relationship, together with the fact that no compensation effect was observed, suggests that the Cu2+, Ni2+, and Zn2+ ions, which differ inherently in their electronic densities, are the active sites for the reaction. On the basis of the preceding data and electric conductivity measurements the desorption of acetone was proposed as the rate limiting step. Data presented by Wachowski et al. (282a) showed that perovskite oxides can also be catalytically active for reactions proceeding according to a carbonium ion mechanism as 2-propanol dehydration. This fact suggests the presence of acid and base centers on the surface of these compounds. These authors found that LaNi03 was particularly active for dehydration (propylene formation) and also for dehydrogenation whereas LaFe03and LaCo03were active exclusively for dehydrogenation. This is in agreement with the low acidity found in these latter compounds by IR spectroscopy of adsorbed pyridine. No information was obtained about the acidic properties of LaNiO3 because of the low transmittance of the sample (see Section V1,C) (50). The incorporation of Ca into LaCo03 brought about the appearance of dehydration activity at the level of that of LaNiO3, whereas the incorporation of Sr or Ba caused still a higher increase in dehydration activity. In these partially substituted perovskites a part of the cobalt must be in the form of Co4+as charge compensator. The rearrangement of charge distribution described above should produce coordinatively unsaturated cobalt ions with acceptor properties, i.e., Lewis acid centers. Perovskites such as SrTi03have been proposed as selective hydrogenating materials in the hydrocracking reactions of polynuclear aromatics (284). Zeolitic catalysts are commonly used for such purposes; however, the modification of the acidity of the perovskite by addition of a second acidic oxide such as silica or alumina yields materials with balanced hydrogenating-cracking functions that may be effective for selective hydrocracking. Barium zirconates were found to be materials particularly suited for use in catalytic hydrocracking of residua, especially for their ability for removal of carbonaceous deposits from the coked catalysts (285). Perovskite oxides were used as catalysts in other less studied reactions where H20 appears as reactant or as product, such as H 2 0 dissociation, H202 decomposition, and water gas shift. Reduced Lal-,M,Co03 (M = Ca, Sr, Ba, Ce) oxides were shown to be active for H 2 0 dissociation at 600°C by Wachowski and Laniecki (282b).Disperse metallic cobalt particles were assumed to be involved in this process. Brookes et al. ( 2 8 2 ~ ) reported XPS data indicating SrTiO3(1,O,O) step sites as active centers for

312

LUIS G . TUUCA

et al.

H 2 0 dissociation. The enhanced reactivity of step sites seems to be due to the low coordination of titanium centers where the 3d-orbital occupation will be maximized. The 0-H2 bonds would be weakened by interaction of occupied 3d orbitals with H2O antibonding orbitals. Seiyama et al. (140)found La, -xSrxFe~.&o,-,.403-Ato be more active for H202 decomposition at 80°C than Lal-xSrxCo03-Awith a single kind of B cation. This higher activity was accompanied by a higher oxygen desorption after oxygen adsorption on the former sample at 800°C. Wachowski and Laniecki (282b) found cobalt perovskites to be active for water-gas shift reaction at 345°C. LaCoO3 and L ~ O . & ~ ~ . ~showed ~ C O an O activity ~ which was significantly higher than that of Lal-,MxCo03(M = Ca, Sr, Ba), i.e., substitution leading to Co2+or Co4+increased or decreased, respectively, the catalytic activity. This behavior is parallel to that observed for CO oxidation (14). Water added to the reactant mixture in CO oxidation at 225°C caused a loss in activity for Lac003 and the Ce-substituted oxide much more marked than that observed for the alkaline-earth perovskites. This was assumed to be due to a weaker adsorption of water on the latter compounds. The higher catalytic activity in water-gas shift found for Lal-,Ce,Co03 is then accounted for by a stronger adsorption of water on these compounds as compared with that on Ca, Sr, or Ba substituted oxides. J. THEEFFECTOF SULFUR DIOXIDE

Thermal aging, SO2 poisoning, and reaction of the active phase with the support are factors that are known to be important in the deactivation of solid catalysts. Noble metals are usually poisoned by lead, whereas base metal oxide catalysts are more susceptible to poisoning by sulfur (88,286289). Indeed, the deactivation of oxides when used in oxidation or reduction processes and particularly as catalysts for exhaust gas purification has been attributed to a large extent to SO2 (14,174,290).In this section, some aspects of the SO2 poisoning effect and the nature of the interactions of SO2 with perovskite oxides are reviewed. Wan et al. (170)carried out an IR spectroscopic study of the adsorption of SO2 on partially substituted lanthanum and cobalt perovskites. The spectrum of L~.aSr~.4Co~-,Mx03 after poisoning with SO2 at 200°C included bands at 1133 and 994 cm-' that are assigned to a bridging structure of adsorbed SO2 (species I): 0

B

M- -M

A I

0

M-S

4-

3. 0

I1

PEROVSKITE-TYPE OXIDES

313

After poisoning in these conditions, La0.&0.4Co03 showed IR bands at 1294 and 1128 cm-I that are assigned to adsorbed SO2 on a metallic site (species 11). Species I1 may be converted into species I by calcining the sample in air at 400°C. After exposure of La203 to SO2 at room temperature no bands of adsorbed SO2 were detected. However, Coj04contacted with SO2 at room temperature gave a spectrum with bands at wavenumbers near to those of species I and 11. Minming et al. (169) observed that the SO2poisoning of La&3ro.sMnO3for CO oxidation occurs through both a fast process and a slow process. These authors detected two adsorbed species of S02: -SO2 and -OS02. On the basis of these facts they assume that there are two different kinds of active center, Mn- and MnO-. Sulfate formation was also detected. On the other hand, Trimble (210) reported that the most important effect of SO2 on NO reduction by CO over reduced L~.8Ko.2Mno.~Ruo.o03 and SrRu03 below 40O0C, apart from reduction of the activity, is the lower NH3 and higher N20 yields obtained when sulfur dioxide is added to the reacting mixture. Because the oxidation state of Ru affects NH3 formation over Ru-containing perovskites (291), the interaction of SO2 with Ru on the reduced surface should be responsible for the lower NH3 yield. Supporting this interpretation, Unland (292) observed formation of NCO- groups after exposing Ru/A1203 to a gas mixture of NO, CO, and N2. However, these species were not detected in the presence of SO*. Assuming that NCO- is an intermediate in the formation of NH3 over Ru surfaces, poisoning of the active sites where its formation takes place would explain the lower yield of ammonia. These results show that the poisoning effect of SO2 on these perovskites takes place through adsorption of this molecule on B sites or oxygen anions that are the centers responsible for the catalytic action in these compounds. SO2 may also interact with cations in position A, but this process does not result in the deactivation of the catalyst (see below). Some perovskites were reported to be highly resistant to poisoning by lead compounds (39). In less resistant oxides Pb poisoning may be reduced or avoided by introducing this element in position A of the structure (14,174,206).The addition of small amounts of Pt to lead perovskites results also in an improvement of their oxidation activity in the presence of SO2. Thus, Yao (181) observed that the catalytic activity for C2H4 oxidation of L ~ ~ . . I P ~ containing ~ . ~ M ~ 30 O~ ppm of Pt increased remarkably after admitting 0.1% SO2 to the reacting mixture at 500°C. This high activity did not change on removal of SO2 from the gaseous phase. A similar SO2-induced effect in CO oxidation was reported by Gallagher et al. (293) for the above-mentioned perovskite doped with 570 ppm of Pt. This enhanced activity is accounted for by assuming that Pt is bonded to or covered by Pb, and SO2 frees the Pt through formation of PbS04. Water favors this process by increasing the diffusion of lead sulfate from

3 14

LUIS G . TEJUCA

et al.

the vicinity of Pt (181). This interpretation is consistent with the findings of Sachtler et al. (294), who detected by X-ray diffraction the formation of PbSO4 in a P t / A l 2 0 3 sample previously poisoned by PbBr2 and reactivated with SO2 at 500°C. However, the fact that the activity enhancement caused by SO2 has been observed, although to a lesser degree, for compounds with no lead content suggests that an additional mechanism is operative. For example, Johnson, Jr. et al. (287) observed in supported La&ro.sMn03 and L ~ C U O . ~ M that~ Pt , ~contributed O~ to a low extent to the total activity or even acted as a poison (because of the formation of a Pt-Cu alloy) in CO oxidation in the absence of SO2. Addition of 50 ppm of SO2 to the reacting gas mixture gave to both oxides an activity proportional to the Pt content. An analogous effect-an increased activity for alkane oxidation caused by the presence of S02-has been reported by Kummer (205) and Sachtler et al. (294) for Pb-free Pt/Al2O3 and has been related to the presence of sulfate groups on the alumina surface that increased the chemisorption of the alkane. Yao (181) studied the SO2 effect on the oxidation of CO on MCo03 (M = La, Ba), La,Srl-,MnO,, and L a ~ . , P b ~ . ~ Mand n oon ~ the C2Hd oxidation on MCo03 at 450-500°C. The total amount of SO2 needed to cause a reduction in rate for CO oxidation higher than 90% was found to be approximately one monolayer (Sso, = 0.3 nm2)except for BaCoO3, indicating a strong chemisorption of SO2on the surface of the catalyst. However, for C2H4 oxidation an equilibrium between the degree of poisoning and the concentration of SO2 in the gas phase is established so that at a constant SO2 concentration the oxidation rate decreases with time to a constant value; further introduction of SO2 produced no further poisoning until its concentration was increased. Therefore, the amount of SO2 passed over the surface is much larger than that required for monolayer coverage. On the other hand, Wan et al. (170) found that the poisoning at 180-200°C is effect of SOz for CO oxidation on L~C,.~S~~.~CO~-,M,O~ much more pronounced than that for CH4 oxidation at 460°C. This behavior, i.e., SO2 affecting the carbon monoxide oxidation to a considerably larger extent than the hydrocarbon oxidation, has been reported for other oxides (290,295). Particularly relevant is the work of Farrauto and Wedding (296). These authors observed that the SO2-induced decrease in activity for CO oxidation on copper-chromite at 500°C is severe and fast, whereas the activity decrease for hexane oxidation is less severe and slow. Isothermal adsorption measurements of SO2 on CuCr204at 500°C showed a fast adsorption process followed by a slow one, which occur at carbonyl (metallic) and carbonate (oxygen) sites, respectively. The spinel interacts with SO2 via strong chemisorption with no evidence of sulfate formation.

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315

Since a higher resistance to SO2 poisoning of hydrocarbon oxidation as compared to CO oxidation was found for different perovskite and other metal oxide catalysts, the different poisoning effects for CO and hydrocarbon oxidation should come from the different oxidation mechanisms and therefore from the different active sites involved. The results mentioned above, together with those of Wan et d . (170) for SO2 adsorption on L~.aSro.4Col-,M,03and Minming et al. (269) for SO2 poisoning of L&.&-o.sMn03(see above), seem to indicate that the poisoning of the perovskite oxides studied takes place mainly by fast adsorption of SO2 onto metallic sites in position B of the structure (responsible for CO oxidation) and by slow adsorption onto oxygen sites (responsible for hydrocarbon oxidation), although some participation of poisoning via sulfate formation, as suggested by the results due to Minming and co-workers (169), may also occur. The perovskite oxides appear to have some advantages over other metal oxides as anticontamination catalysts. A factor to consider is the higher stability in a Hz atmosphere shown by some metallic cations (Fe3+, Co3+,Ni3+,Rh3+)in a perovskite structure relative to that in the corresponding simple oxides (92,209).It is also known that Ru is stabilized in a perovskite structure (209,220,297)and is less susceptible to poisoning by SO2 (220). This higher stability may explain the lower reactivity of some perovskites with SO2 and with materials frequently used as supports as compared with that of simple oxides (222). Although some progress has been attained in the preparation of highly active perovskites for CO and hydrocarbon oxidation and NO reduction by the incorporation of noble metals (Pt and Ru) into the structure (24,274),the problem of SO2 poisoning remains basically unsolved. From measurements of catalytic activity and reducibility on fresh and SO2-poisonedperovskites, Wan et al. (270) concluded that a high mobility of the lattice oxygen and the ability to form weaker bonds with sulfur are essential factors for improving the SO2 resistance of perovskites in oxidation processes. For this to be achieved, further spectroscopic studies will be needed, in order to gain a better insight into the nature of the interactions of SO2 with the surface of these compounds. VIII. Miscellaneous

Perovskite oxides have been considered for actinide storage in radioactive waste. According to Penneman and Eller (298), the trivalent or tetravalent oxidation state should be chosen for this purpose. Valences higher than 4 are not appropriate, as the actinides could leach as acid-

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soluble aquocations. In the case of uranium, the oxidation of U(IV) by dissolved 0 2 may result in the dissolution or complexation of the UO:' ion (299). Actinide(II1) and (IV) ions can be incorporated into the perovskite structure, which provides cavities of the proper size, such as those in position B. Some actinide(1V) oxides, which have been described, are BaM03 (M = Th through Cf), SrM03(M = Pa through Pu), and EuMO3 (M = U, Np) (299).However, Penneman and Eller (298)have pointed out that the use of perovskites is limited because the large tetravalent actinides are coordinated to only six oxygens and hence are vulnerable to oxide addition and oxidation. Notwithstanding, Williams et al. (299) suggested that this undesirable oxidation in nuclear waste may be inhibited by the incorporation of reducing agents or oxidation-reduction buffers to maintain a very low oxygen partial pressure. Arakawa et al. (196-198,300-302) studied the physicochemical properties of LnMO3 (Ln, La through Gd; M, Cr through Co) perovskites that are relevant to their use as solid-state chemical sensors, mainly for methanol. These measure the gas concentration in terms of the changes in electrical conductivity induced by adsorption or reaction of gases on the surface of semiconducting oxides. The authors determined the temperature dependence of the response ratio (R - RolRo) 100 (where R - ROis the maximum change in resistance after adsorption of methanol and Ro the resistance before adsorption) for the above-mentioned compounds. In general, the activity for methanol sensing (as measured by the temperature at which a given response ratio is attained) increased as the radius of the rare earth ion decreased for LnCr03, LnFe03, and LnCoO3 (196-198, 302), whereas the reverse trend was observed for LnMn03 oxides (300). The highest activity for methanol sensing was found for EuCoO3 and SmCo03(302).The sequence of sensing activities for SmCo03was found to be CH3OH > Hz > CO. On the other hand, Arakawa et al. (301,302) calculated the binding energy of oxygen AH and also AE (= Ec - E , , where Ec and E, are the energy of the conduction band and the energy of the surface state, respectively, i.e., A E is a measure of the energy needed to promote an electron from a conducting to a nonconducting state on adsorption of methanol) as a function of the electronic configuration of the M ion in LnMO3 oxides. They found that both A H and A E follow the same pattern. A similar pattern had been previously found for the catalytic oxidation of CO on LaMO3 oxides (14).In the light of these results it may be concluded that the strength of the metal-oxygen bond in these compounds is an important factor for methanol sensing. Other oxides, such as oxygen-deficient copper perovskites, were reported to exhibit high oxygen mobility at elevated temperatures, which alter their electrical behav-

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3 I7

ior. These properties suggest possible applications as oxygen sensors for these compounds (13). A novel and intriguing application of perovskites relates to superconductivity at high temperatures (13). These oxides were reported to be a class of materials, where improvements in the superconducting transition temperature T, beyond that of known compounds appeared to be possible. Transition at temperatures around 13 K was observed in the mixed valence perovskite BaPbl-,Bi,03 (303).The search for new oxides was first directed to nickel- and then to copper-containingperovskites, such as LaCu03, which has a partially filled valence band that gives rise to its metallic behavior. Bednorz and Muller (304)observed superconductivity at 35 K in a La-Ba-Cu oxide, which was found to be a mixture of several phases. They identified the superconducting phase as a layered perovskite of the K2NiF4type. Substitution of La by a smaller isoelectronic ion such as Y made it possible to raise T, above 90 K in Yj.2B~.&u0,(305).Other compositional variations, such as YBa2Cu309-,,presented a transition to zero resistance at 95 K. Although the triple cubic perovskite unit cell can accommodate nine oxygen atoms, this is not possible in this compound because of valence considerations. In the superconductor YBa2Cu3O.l the plane, where Y is situated, is devoid of oxygen ions, and this accounts for one oxygen vacancy. The second oxygen vacancy is split between the two Cu basal planes at the end of the triple unit cell (13). It has been established that the superconducting properties of YBa2c1.130, are critically dependent on the amount and ordering of oxygen in the basal planes of the unit cell that give rise to a network of Cu-0 ribbons that are perpendicular to Cu-0 sheets. This contributes to the orthorhombic character of the unit cell, which appears to be essential for superconductivity to occur. To ensure the proper ordering and amount of oxygen in the Cu-0 ribbons, the solid-state reaction of Y203, BaC03, and CuO must be carried out in an oxygen atmosphere at approximately 950°C using a gradual heating through 500-600°C and a slow cooling rate from 950°C to room temperature. A fast quench or reaction under a low partial pressure of oxygen would yield a unit cell with a more tetragonal character and a perovskite with a lower T , . These changes are due to a disruption of the Cu-0 ribbons caused by the decrease in oxygen content and an increased oxygen disorder in the basal planes. Preparation of this compound under an inert atmosphere completely suppresses superconductivity. The critical importance of the oxygen content in YBazCu30, appears to be related to the level of Cu2+-Cu3+mixed valence. Recent evidence suggests that T, is directly related to the average oxidation state of Cu in these superconductors (306).Thus, when x = 6.5, Cu is in a +2 oxidation

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state, and an insulating compound is obtained. For x = 7 the oxidation state of Cu is +2.33, and superconductivity appears. Superconductivity at temperatures above 90 K has been observed also in perovskites where Y has been replaced by all the rare earth elements except for Ce, Pr, and Pm. These isostructural derivatives of YBazCu30,, although they do not possess higher transition temperatures, may exhibit other important alternative properties. These superconducting characteristics will certainly extend the use of perovskites to many fields, including integrated electronics and electromagnetic applications. IX. Prospective Lines of Research

Much of the research published in the 1970s on perovskite oxides focused on their application as catalysts for removal of atmospheric contaminants. However, this initial impetus has slowed down as a result, to a large extent, of their lower resistance to poisoning by sulfur as compared with noble metals. To date very few studies at the spectroscopic level have been carried out on the SO2 interactions with the surface of these oxides, and most of the conclusions regarding the behavior of perovskites in an SO2 environment have been reached on the basis of the indirect evidence furnished by the available data on SO2 interactions with simple oxides. This situation certainly warrants further research on this subject. A problem, different in nature, that needs additional attention refers to the characterization of the active centers involved in adsorption and catalytic processes and particularly to the estimation of the number of metallic centers and the exposed surface in supported and unsupported perovskites. A number of chemical and physical methods have been used for metals and oxides, and those based on selective chemisorption of probe molecules seem to be the most promising for this purpose (307).However, while considerable progress has been made for supported metals, no method has been accepted for oxides. This has been caused by the comparatively complex nature of these latter compounds where oxide ions and metal ions of different oxidation states may be present. As probe molecules, 02,CO, and NO were the most frequently used (307);the O2 chemisorption presents the problems inherent to any method based on gas adsorption at low temperatures (a large fraction of physisorbed gas accompanying the chemisorption). On the other hand, its symmetric character renders this molecule unamenable to study by IR spectroscopy. Nonetheless, this method has been used with some success by Weller et al. (308410) on simple oxides, and its possible application to perovskites and other mixed oxides should be explored. Previous chemisorption work

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3 19

on carbon monoxide and nitric oxide suggested that NO is the molecule that appears to fulfill better the desirable prerequisites for perovskite characterization (see Section V1,B). Furthermore, NO has the property of chemisorbing strongly on transition-metal oxides but weakly on the materials (silica, alumina) usually employed as supports. Quantitative measurements of the NO adsorption on samples of unsupported oxide with different specific surface areas should allow the determination of a factor of adsorbed NO per surface unit. This factor could be applied to NO adsorption on the supported perovskite in the same way as this is done for 02 adsorption (307-310). Care should be taken to use a range of temperatures where NO is not reduced to N20. This transformation can be easily detected by IR spectroscopy (152). A specific point of interest would be to ascertain whether the NO chemisorption can be correlated to perovskite loading in highly dispersed catalysts (less than one monolayer deposited on the support) as well as to catalytic activity. An important characteristic of perovskites, mentioned in the preceding sections, is their susceptibility of partial substitution in both A and B positions. This provides a wealth of isomorphic compounds that can easily be synthesized. Given the extensive range of possibilities in the tailoring of their chemical and physical properties, there is no doubt that new reactions will be studied, where these oxides can participate as catalytic agents. An addendum to this chapter appears on page 385. REFERENCES 1. 2. 3. 4.

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ADVANCES IN CATALYSIS, VOLUME 36

New Catalytic Materials from Amorphous Metal Alloys ARPAD MOLNAR,* GERARD v. SMITH,? AND MIHALY BART6K* * Deparimeni

of Organic Chemistry J6zsef Aiiila Universiiy Szeged. Hungary and

‘IDepartment of Chemistry and The Molecular Science Program Souihern Illinois University Carbondale. Illinois 62901

1.

Introduction

Since the introduction of the technique of rapidly quenching melts ( I ) for producing metallic alloys with glassy structures, these metastable, amorphous materials (alloys) have attracted the attention of metallurgists, physicists, and, recently, chemists because of their exceptional properties (easy magnetization, superior corrosion resistance, high mechanical toughness, interesting electronic properties). In fact, from analogy they already have industrial applications. The term metallic glasses refers to metallic alloys prepared by the melt quenching methods. These materials have no long-range order, but they are not totally amorphous: they have short-range ordering. Metallic glass can, therefore, be a name for the structure as well. The terms amorphous metal alloy and metallic glass are used as synonyms in this chapter. Amorphous materials have properties that are of interest in terms of catalysis. Amorphous alloys can be produced with wide composition ranges not available in crystalline form, which permits the continuous control of their electronic properties. Their single-phase character and possible lack of surface segregation of the alloying elements ensure that the active species are in a uniform dispersion in a chemically homoge329

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neous environment. There are indications that they have a high concentration of coordinatively highly unsaturated sites that makes them easier to study and characterize than ordinary catalysts. Their structure is nonporous, so diffusion limitations, often a problem in traditional heterogeneous catalysis, do not affect the surface reaction. All these features make amorphous alloys attractive materials in heterogeneous catalytic studies. Additionally, metallic glasses are a kind of missing link in the study of the nature of active sites in catalysis. The common thread running through theories of active sites (from Kobozev, through Balandin, up to the current “ensemble” ideas) is the concept of a small cluster of atoms being responsible for catalytic activity. Because metallic glasses have no long-range order, they offer the opportunity to examine reactions requiring little cluster order in the absence of those requiring high order. Since the publication of the first result using metallic glasses as catalysts ( 2 ) about 90 more publications have appeared, along with several short overviews of limited scope emphasizing mainly corrosion behavior and electrocatalysis (3-11). Newer review articles deal mainly with the possible advantages and prospects of their future applications (f2-19). When compared to their crystalline counterparts, these materials sometimes exhibit superior catalytic properties, higher activity, better selectivity, and greater stability. Recently, the topic of several papers has been the search for the correlation between the catalytic properties, the activity and selectivity, and the surface structure, combining catalytic studies with surface analysis and characterization (20-27). These efforts have already led to a model of the working catalyst of the Fe-(Ni)-B alloy system (24-26, 28). Since most papers report studies of the FischerTropsch synthesis, chemisorption and decomposition of CO and H2 have also been studied; however, these are only limited studies (24, 29-35). II. Preparation and Characterization of Amorphous Alloys

A. MATERIALS

Although many elements of the periodic table can form a wide variety of alloys with glassy structures, only a few elements and certain compositions have been more popular than others for catalytic studies. Most of the available data are for Group VIII and Group Ib metals. These metals in the crystalline state are known to catalyze many reactions, so studying their amorphous alloys allows for comparison of their activities in both the crystalline and amorphous states. Of the different possible composi-

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33 I

tional varieties the so-called readily glass-forming alloy systems are mainly used in catalytic investigations, but there are numerous other examples as well. One readily glass-forming system includes zirconium as a second metal in two-component metal-metal alloys. The active component of the alloys is frequently nickel, iron, palladium, or copper. The activities of other metal-metal alloys, Ni-Ti, Cu-Ti, and Ni-AI, also have been investigated. Another readily glass-forming system involves metal-metalloid alloys. The most common ones use iron or nickel with phosphorus or boron. Their frequent use results from the catalytic properties of the metals for the widely studied hydrogenation of CO. Alloys prepared with all possible combinations of these four elements in different compositions have been studied. Additionally, some of these alloys, Ni-B, Ni-P, and Fe-Ni(Cr)-P-B, as well as Pd-Si and Pd-Ge alloys, have been studied in the hydrogenation of hydrocarbons containing multiple bonds. Because of the high activity of palladium in electrocatalysis, palladiumbased alloys, which contain one or more platinum-group metals along with titanium as well as boron and/or silicon, have been thoroughly investigated.

B. METHODSOF PREPARATION The melt-quenching method, which ensures a cooling rate of at least 105-106K S K I has , made amorphous alloys easily available in large quantities. Specimens prepared by using variations of this technique are used in most investigations. The melt-spinning or strip-casting methods (single roll or rotation wheel, planar flow and disk) bring the molten alloy in contact with a rapidly moving heat sink. The products are ribbons from several millimeters to several centimeters wide with a thickness of several nanometers. Flakes are produced by the “gun” splat-quenching (shocktube) method, and thin wafers are produced from the hammer-and-anvil (piston-and-anvilor arc-hammer) method. The former method blasts molten alloy through a nozzle onto a curved copper plate, and the latter shoots a copper column (hammer or piston) onto a molten slug of melt resting on a massive copper plate (anvil). Specimens prepared by other, less widely used techniques, such as ion sputtering, flash evaporization, gas atomization, chemical reduction, electrodeposition, and laser surface melting-self-quenching, have also been studied as catalysts. More detailed descriptions of the various sample preparation methods are summarized in monographs (e.g., 36).

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C. CHARACTERIZATION The most important methods used for the characterization of heterogeneous metal catalysts are applied also to the characterization of amorphous alloy catalysts. The basic and most commonly used method for identifying the amorphous structure is X-ray diffraction (XRD). The diffraction patterns consist of broad bands instead of discrete peaks because of the lack of longrange atomic order of the amorphous state. This method is used to follow the changes in the amorphous structure that occur during pretreatment (37, 38), heat treatment (21, 23, 39-44), and reaction (22, 45-49). XRD permits the identification of different intermediate metastable crystalline phases (2,39,40,50-52). Also, changes in the surface chemical composition induced by catalytic transformation are detected by XRD (46,53,54). Finally, X-ray line broadening is used to determine the mean crystallite size (21, 53). Differential thermal analysis (DTA) and differential scanning calorimetry (DSC) are also useful methods for structure determination. These methods can detect crystallization of amorphous alloy catalysts as a result of heat treatment (21, 23, 41-44) or as a result of the action of reacting gases, such as in the case of hydrogenation of carbon monoxide (53) or ammonia synthesis (22). Determination of surface areas by Brunauer-Emmett-Teller (BET) isotherms is also an important method for the characterization of amorphous catalysts. For example, the effects of different pretreatments have been followed by measuring BET surface areas (37, 38, 43). BET measurements revealed a dramatic increase in surface areas concomitant with a substantial increase in catalytic activities for Zr-containing amorphous alloys ( 4 8 , 5 3 4 6 ) during the early period of hydrogenation of CO. Data more relevant to understanding catalytic activity can be obtained by chemisorption measurements. The number of surface metal atoms of Ni and Cu alloys have been determined by chemisorption of hydrogen ( 2 3 , 3 7 , 4 1 , 5 7 ,58) and of carbon monoxide ( 3 8 , 5 7 , 5 9 ) . By disclosing surface morphology electron microscopy permits qualitative characterization of catalytic surfaces and helps to interpret the phenomena occurring during pretreatment and reaction. For example, scanning electron microscopy (SEM) revealed that HF activation of Zr-containing amorphous alloys results in the formation of a rough, Raney-type porous surface (38,60-62). Formation of a similar, porous surface layer generated by a special activation procedure (Zn deposition, heat treatment, Zn leaching) also was revealed by electron microscopy (63,64). SEM can not only give useful information about the differences

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in surface morphology of amorphous and crystalline samples (43,58, 65, 66)but also reveal changes in surface structure in heat-treated (21,23,43, 67)and used samples (22,49,53,56, 65). The most valuable information about the surface of catalysts can be obtained by the different ESCA methods (electron spectroscopy for chemical analysis). Besides ultraviolet photoelectron spectroscopy (UPS) and Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS) is used extensively. Details of results from these methods will be discussed in subsequent subchapters. Other modem instrumental techniques, specifically, Mossbauer spectroscopy (20) and ion-scattering spectroscopy (ISS) (43, 67), as well as temperature-programmed reduction (TPR) (57) and temperature-programmed desorption (TPD) (24,25), have been used for characterization. Simple measurements of electrical resistivity can give useful information as well (23). An additional, newly developed method for the characterization of metal surfaces is the use of sterically complex organic probe molecules. Comparisons of the transformations of cis-cyclododecene( 2 , 5 1 )and (+)apopinene (52,68, 69) on glassy and crystalline metals distinguish certain surface features of the catalysts (see details in Section III,C,l). D. THEPROBLEM OF ACTIVATION In many cases the as-prepared virgin alloys have only minimal or no catalytic activity. For this reason different, sometimes rather drastic special pretreatments are necessary to activate amorphous metal catalysts. In most cases these activation procedures are neither thoroughly studied nor well understood. As is usual in hydrogenation reactions metal catalysts are pretreated in hydrogen or deuterium at elevated temperatures. Similarly, the amorphous alloys employed in the hydrogenation of carbon monoxide are pretreated with hydrogen or with the feed gas. During this latter case, in the absence of pretreatment, an induction period followed by an activity increase is observed. In the case of the metal-zirconium alloys it is thought that an inhibiting surface oxide film is removed and a high-surface-area porous layer is formed during the reaction (48, 54, 55). Treatment with HN03 followed by oxidation with oxygen and reduction with hydrogen at elevated temperatures was found to be an effective procedure for activating Ni-containing alloys for the hydrogenation of C=C double bonds (57). Surprisingly, pulverization of a Ni-B alloy ribbon has the same effect (37).Etching with HF also proved to be success-

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ful for the activation of Zr-containing alloys used mainly in electrocatalysis (38,60-62). HCI and NaOH are effective in activation (26, 70, 7l),Just as is cathodic polarization (70). Catalytic activities of certain electrodes (mainly Pd-containing) can be enhanced by a special surface activation. This consists of either electrodeposition (63,64)or diffusion (72) of zinc onto the amorphous materials followed by heat treatment, and, finally, partial dissolution of the zinc. This activation results in electrodes with increased surface area and high activity in brine electrolysis and in fuel cells. An activity increase can also result from the hydrogenation of terminal alkynes. Some amorphous Pd-Si and Pd-Ge alloys that otherwise were inactive for the hydrogenation of olefins could be activated by first using them for the hydrogenation of a terminal acetylene (phenylacetylene, I-octyne) (73). The authors suspect that a part of the alloys dissolved by complexation with the terminal alkyne (the reaction mixture turned gray and turbid) and that the remaining bulk metal underwent comminution. The gray turbidity disappeared exactly at complete consumption of the starting acetylene compounds. After that a black deposit appeared on the walls of the reaction flask.

E. STABILITY A N D CATALYTIC APPLICATION The use of amorphous metals as catalysts is limited by the fact that the amorphous state is thermodynamically unstable; metallic glasses are prone to crystallize. Crystallization can occur at any temperature under isothermal conditions depending on time, and it can occur at definite temperatures during dynamic heat treatment depending on the heating rate. During temperature ramp a metallic glass starts to transform to other, nonequilibrium metastable phases at the glass transition temperature (Tb,then at the crystallization temperature (T,) it transforms to the equilibrium crystalline phase or phases. Fortunately, many metallic glasses maintain the amorphous structure for a prolonged practical period, provided they are used at temperatures well below T,. However, this is not a firm rule since some glasses crystallize at 100-200 K below their Tc under reaction conditions. On the other hand, stabilities of metallic glasses can be increased. In general, alloying components of different atomic sizes and different chemical characters enhances the glass forming ability; that is, both Tg and Tc increase. For many metal-metalloid glasses, T, has a maximum value around the eutectic composition. Although not all factors and correlation have been discovered in detail, the

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above-mentioned possibilities permit improvement in the thermal stabilities of amorphous alloys when required for catalytic applications. For example, the introduction of La into a Ni-P glass increases T, by 100 K (59).Another metallic glass of potential catalytic applications is RuMZrS4, for which T, is over lo00 K (74). In terms of catalysis it is important to know the stability of the amorphous catalysts at elevated temperatures in the reacting atmosphere. Although not all papers dealing with amorphous catalysis clarify this point, a number of observations show that catalytic transformations do not change some amorphous structure, while other observations indicate that certain catalyst compositions are rather sensitive to the presence of chemicals and do change. One paper specifically concerned with this problem revealed that 20 torr of cyclohexane causes Pd80Si2~ (subscripts represent atomic percent) to crystallize not only about 200 K lower than in vacuum aging but also by a new path (75). It appears that hydrogen has a similar effect on Fe-B alloys at high temperatures. Partial crystallization of the surface (20) or the bulk (22) of different Fe-B alloys was detected in the presence of hydrogen at temperatures below T, . Crystallization and loss of catalytic activity for amorphous FegoBzowere observed during the hydrogenation of CO at a temperature 132 K lower than T, (45). The sensitivity of the amorphous structure was also observed in M-Zr metallic glasses. Crystallization by the action of hydrogen on CusoZrsoand NisoZrso takes place at 530 and 310 K, respectively (the corresponding T, values are 720 K and 730 K in inert atmosphere) (76). In contrast, NiuZrw remains amorphous in the bulk after 100 absorption-desorption cycles, but in the surface layers formation of crystalline Ni and Zr02 can be detected (77).The sensitivity of Ni63Zrj7to Hz, Oz, and CO in contrast to NinZra7was also reported (78). A mixture of CO and Hz (53)or H2O and N2 (79) induces the fast oxidation of and surprisingly, the same happens to Zr70Au30,even at room temperature, from the action of a mixture of air and water vapor (80). However, there are observations indicating that the properties of metallic glasses can be improved by a small amount of crystallinity. For example, Zr-containing alloys that are used mainly in the hydrogenation of carbon monoxide exhibit increasing activity during reaction. This increase is attributed to the formation of high-surface-area fine metal particles in a Zr02matrix (53,54,56). Highly active and porous Pd-Zr and NiZr methanation catalysts have been prepared in this way ( 4 6 4 8 ) . Similarly, continuous activity increases as well as structural and chemical changes in Fe-Zr and Ni-Zr alloys were observed during the synthesis of ammonia (22,49).Likewise, certain careful, well-defined heat treatments

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of amorphous metal alloy precursors can lead to catalysts with outstanding catalytic properties because of the presence of controlled amounts of crystallinity (23, 41, 43, 67). Although the working catalysts are only partially amorphous in all the above-mentioned cases, it is the originally amorphous structure that ensures the subsequent formation of the highly active or selective state.

111. Amorphous Alloys in Catalytic Transformations A. ELECTROCATALYSIS 1.

Water Electrolysis

Hydrogen is an excellent candidate as an efficient and inexpensive energy carrier in the future because it is recyclable, nonpolluting, and available in practically unlimited supply. Since hydrogen can be produced by the electrolysis of water, the search for suitable electrodes is important. Because amorphous alloys possess high mechanical strength and superior corrosion resistance, as well as a defect-free homogeneous structure, they are attractive as electrode materials. Moreover, the rapid cooling techniques used for the preparation of amorphous alloys allow the preparation of metal compositions not available from ordinary techniques. This is important in electrocatalysis because the Brewer-Engel theory predicts high electrocatalytic activities in the hydrogen electrode reaction for certain combinations of transition metals. Left-lying transition elements, with empty or half-filled d orbitals, alloyed with right-lying elements, with internally paired d-electrons not available for bonding in the pure metal, should exhibit synergism and, therefore, higher activity than either component alone. (See good summaries of this problem in Refs. 81 and 82.) Studies of the possibilities for using amorphous alloys in the hydrogen electrode reaction were carried out by Enyo and co-workers (60-62,8386), who found that both Pd-Zr and Ni-Zr alloys (61, 84, 85) prepared from their melt by the rotating-wheel method have low activity in the asquenched form in O S M HzS04 or 1M NaOH solutions at 303 K. However, the activity for the cathodic hydrogen evolution increased dramatically after treatment with HF (I M solution, treatment for several seconds or minutes). For example, the treatment of Ni-Ti alloys resulted in electrodes with activities comparable to those of smooth Pt-Pt electrodes, particularly at low current densities (Fig. I). The same behavior was observed for Cu-Zr and Cu-Ti alloys (62). Also, amorphous NiSLa and

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d

FIG. 1. Cathodic polarization behaviors of the total overpotential 7) on amorphous (0) and crystalline (0)Ni33Ti67 alloy electrodes and on platinized Pt electrode (0)(1MNaOH, 303 K).(Reprinted from Ref. 85 with permission of The Chemical Society of Japan.)

Ni-Ti films prepared by the flash evaporation technique showed similar electrodic characteristics (86). High roughness factors for the Ni and Cu amorphous alloys were determined from the double-layer capacitances [2-300 (61, 84, 85) and 102-103 (62), respectively]. And lower activities were found for the crystalline samples (Fig. l), but the Pd-Zr specimen did not exhibit these differences (60). The electrocatalytic activity per true unit area was practically the same as that of pure Ni and Cu electrodes (62, 85). Scanning electron micrographs reveals (60-62, 84) that HF treatment results in rapid dissolution of the surface. The as-quenched Ni and Cu alloys contain furrows that become cracked [Ni-Zr (61)] and contain pores [Cu-Zr, Cu-Ti (62)]or pits [Pd-Zr (60)].The surface of Ni-Ti is probably too smooth to detect the presence of the cracks (61). X-Ray photoelectron spectroscopy (XPS)reveals that the surfaces of amorphous samples after preparation are covered with a layer of Zr02or TiOz(60-62,84,85), which results in their very low electrocatalytic activity (Fig. 2a and c). These inert oxide layers are removed by the acid

FIG.2. XPS spectra of the amorphous NiI3Th7and Ni33Zr67 alloys. (The X-ray sources respectively; a and c, nontreated specimens; are A1 K (Ifor Ni33Ti67 and Mg K (Ifor b and d, HF-treated specimens). (Reprinted from Ref. 85 with permission of The Chemical Society of Japan.)

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treatment (HF) and a Pd-, Ni-, or Cu-rich porous layer with a large surface area is formed on the alloy surface (Fig. 2b and d). This is much thicker on the amorphous alloys and is formed much more readily than on the crystalline samples (Fig. 3). The HF treatment was shown to be analogous to the effect of alkaline dissolution of aluminium or zinc from Ni-A1 and Ni-Zn alloys in the preparation of Raney Ni. Raney Ni electrodes prepared from crystalline Ni-A1 by leaching in either 3M NaOH or 1M HF gave the same high electrocatalytic activities because of their high roughness factors (87). However, the current densities of HF-treated amorphous Ni-Ti and NiZr alloys per true unit surface area are much larger, indicating a synergistic effect between Ni and Ti and Ni and Zr. Another possible factor contributing to the superiority of electrodes prepared from the amorphous precursors is their homogeneous amorphous structure, which permits the formation of a surface layer with more highly dispersed active sites. Nonactivated Fe-Zr, Ni-Zr, and Co-Zr amorphous alloys behave as simple, ideal mixtures of the two pure metal components for hydrogen evolution in H2S04 (88). Added zirconium improves the corrosion resistance of the pure metals and simultaneously increases the overpotential for hydrogen evolution, because zirconium is inactive in the process. NiNb and Co-Nb exhibited higher overpotentials, whereas CujoTi70was the only alloy that exhibited significantly low overpotential in the hydrogen evolution reaction. There is a known correlation between the activity of a metal for hydrogen evolution and the number of its d-electrons (89). In the long periods of the periodic table the activity of the elements increases with the gradual filling up of the d-orbital and decreases sharply with its completion. The high activity of the Cu-Ti alloys suggests electron transfer from copper to titanium resulting in d-band deficiency of the copper. Iron-, nickel-, and cobalt-based multicomponent alloys, some of which

FIG.3. Depth profiles of the atomic ratio ratio [Ni]/[Zr] for amorphous and crystalline Ni)jZr67before and after treatment with HF (0,untreated amorphous alloy; 0 , HF-treated amorphous alloy: A, untreated crystalline alloy; A, HF-treated crystalline alloy). (Reprinted from Ref. 61 with permission of Elsevier Sequoia, S.A.)

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are the commercial Vitrovac metallic glasses, were tested not only for hydrogen but also for oxygen evolution in alkaline solution (1M KOH) (90). FemCo20SiloBlo exhibited the lowest overvoltage and the highest activity for hydrogen evolution and proved to be better than either polycrystalline Pt or Ni. Synergistic behavior was observed on both this alloy and FemN&~Bzo.All (Fe, Ni, Co)-(Si, B) alloys and NimPdmP20 samples used as anodes showed high rates of oxygen evolution and higher activities than did crystalline Ni. The best combination of electrodes studied was FemCo20SiloBlo as the cathode and Co5oNi25Sil~Bloas the anode. These compounds provided an energy saving of 10% compared to Ni/Ni. Cyclic voltammograms indicate high corrosion stability for all Ni-containing alloys and the formation of either mixed oxide/hydroxide or spinel surface layers during oxygen evolution. These surface layers impart their high electrocatalytic activity and corrosion resistance. The behavior of the amorphous alloys under anodic polarization conditions indicates the presence of higher surface oxides and suggests that the formation of lower oxides is inhibited in the amorphous state.

2. Electrolysis of Sodium Chloride The aim of studying amorphous alloys in the chlor-alkali electrolysis process is to find electrodes with improved properties. The practical anode material used at present is a mixture of Ru02 and TiO2. The main disadvantage of this mixture is its low overvoltage for oxygen evolution competing with chlorine evolution. As a result, the chlorine generated becomes contaminated with oxygen, which can be a problem in further applications. In this respect palladium is superior to all other platinum group metals, but it cannot be used because of its fast dissolution under working conditions. On the other hand, it has been known that certain amorphous alloys have extremely high corrosion resistance (8). These facts led to the studies of amorphous palladium-based alloys as anode materials in the electrolysis of soda. Two-component palladium alloys (PdaoSi20,Pd&g) showed no improvement in their corrosion resistance compared with pure palladium (92-93). At the same time, Pd-Ti-P amorphous alloys exhibited extremely good corrosion properties during anodic polarization, although their catalytic activities were lower than that of pure crystalline palladium (experimental conditions: 2 or 4M NaCl solution, pH 4, room temperature) (94, 95). The best composition proved to be Pd73TiRP19, which showed a high overvoltage for oxygen evolution as well. There is no great difference between the catalytic activities of the amorphous and the crys-

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talline materials. XPS measurements revealed (94) that during polarization a thick surface film rich in titanium is formed. It has a maximum amount of palladium and a minimum of titanium as well as a maximum film thickness at 1.3 V, where gas evolution is most active (Fig. 4). It is interesting to note that at higher potentials (above 1.6 V) gas evolution starts again, even though the surface film consists mainly of Ti. This behavior is explained by the presence of micropores in the film through which the chlorine evolution takes place directly on the underlying alloy surface, which consists exclusively of Pd (94). Besides their lower activities, the other disadvantage of the Pd-Ti-P alloys is their low corrosion resistance under industrial conditions (pH 1.5, 353 K)which causes them to undergo active dissolution without chlorine evolution. Further studies include the application of Ru, Rh, Pt, and Ir as second metallic alloying elements (65, 91-93, 96-99). The Pd411r40P19 amorphous alloy is the best composition, which has higher catalytic activity for chlorine evolution and lower activity for oxygen evolution than the commonly used RuOz/Ti (Table I). The catalytic activity of Pd-Ir-P can be further improved by adding a small amount of a third metal sometimes together with titanium (Ti and Rh or Ti and Ru) (Table I) (92, 96). X-Ray and SEM measurements indicate that during anodic polarization there is a preferential, fast dissolution of the palladium-rich phases from the crystalline alloys whereas the surfaces of the amorphous alloys remain unchanged (65).Detailed XPS studies revealed that the amorphous alloys are passivated by the formation of a thick, passive film on the alloy surface (65, 96). This film is enriched in the ions of the second metallic element (65, 96, 98, 99), and the activities of the alloys increase almost linearly with the concentrations of the platinum group cations in this surface layer (98),suggesting that these ions are the active sites in chlorine evolution. The fact that the surface film is formed in the gas evolution

FIG. 4. Atomic fractions of cations in surface film and thickness of film formed on amorphous Pd73TiBP19 alloy by polarization for 2 h in 2M NaCl at pH 4 as a function of polarization potential. (Reprinted from Ref. 94 with permission of Pergamon Press, Ltd.)

34 I

N E W CATALYTIC MATERIALS TABLE I Current Densities qf Electrodes for Chlorine and Oxygen Euolutiona.b Current density ( A h 2 ) Electrode composition

4M NaCl

IM Na2S04

1700

340

2000 3000 2200 860 700 2600 2000

5 2 28 17 4 15 15

a Reprinted from Ref. 96 with permission of NorthHolland Physics Publishing. 1.15 V, saturated calomel electrode (SCE), pH 4, 353 K .

region indicates that chlorine evolves at the film-solution interface (93, 98, 99). The lower activity of the alloys, which contain Pt and Rh compared to Pd-Ir-P, is attributed not only to the presence of a large amount of neutral chlorine on the surface that retards chlorine evolution but also to a high ratio of Pt4+ions in the film, which decreases the electronic conductivity of the Pd-Pt-P samples (99). Beside the beneficial effect of the addition alloying metallic elements that contribute to the increased corrosion resistance, the amorphous structure itself is also responsible for the very low corrosion. For example, crystalline alloys with the same composition exhibit high rates of dissolution. The chemically homogeneous, single-phase nature of amorphous alloys is believed to account for their corrosion resistance (8, 100, 101). This also allows for the formation of a uniform, protective film on the surface of amorphous alloy electrodes. Similar problems arise in connection with the electrolysis of dilute NaCl solutions (seawater). NaOCl produced by electrolysis is used to kill marine life in cooling systems in industrial plants using seawater. Interestingly, amorphous Pd-Ir-P alloys have lower activity in dilute solutions, and Pd-Rh-P samples prove to be more suitable under these conditions (102,103).In order to further increase their activity, surface activation by the Zn deposition-heat treatment-Zn leaching method was used (63).

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This treatment increases the surface area. Furthermore, addition of other alloying metals and the use of both phosphorus and silicon as metalloids lead to electrodes with high activity for chlorine and low activity for oxygen evolution. Pd.&h2~Ti5Ru5P~oSi9amorphous alloy, surface activated at 573 K is the best of all samples and exhibits a current density of 1300 A/m2at 1.15 V, which is much higher than the 750 A/m2 of Pt-Ir-Ti, currently the best electrode (102). A surface-activated nine-component alloy exhibits an even higher value (1450 A/m2) (104). The demands of practical applications led to attempts to overcome the high electric resistance of thin ribbons by a new technical solution of laser-induced surface vitrification (105, 106). First an amorphous alloy ribbon was adhered uniformly to a nickel plate by heat treatment. Subsequently, this surface alloy layer was transformed to the amorphous structure by laser surface melting and self-quenching (107). A sample consisting of P d ~ ~ R h ~ 5 Padhered ~ ~ S i 9 to bulk crystalline nickel exhibited anodic characteristics very similar to those of the melt-spun amorphous ribbon (102). Clearly, similar improvements forced by practical demands will be a part of the future use of amorphous alloys. 3. Amorphous Alloys in Fuel Cells

Amorphous alloys have also been tested as electrodes in fuel cells for the oxidation of CI compounds. The as-quenched ribbons showed very low electrocatalytic activities, but high activities were attained by different activation techniques. The surface activation consisting of zinc deposition, heat treatment, and subsequent leaching of zinc (63, 64) was applied to different amorphous iron-, cobalt-, nickel-, and palladium-based alloys (63, 64). SEM measurements indicated the formation of a porous surface layer. Cyclic voltammetric examinations suggested an increase of surface area by about two orders of magnitude. Heat treatments at higher temperatures resulted in thicker, more porous surface layers and higher electrocatalytic activities (Table 11). Palladium-phosphorus alloys with Ni, Pt, Ru, or Rh proved to be the best specimens. Pd-Ni-P with 5% Ni, after treatment at 573 K, exhibited even higher activity than that of the Pt-Pt electrode (Table 11). These amorphous alloy electrodes were active in the oxidation of methanol, formaldehyde, and sodium formate. Electron probe microanalysis (EPMA) results of Pd76NiSP19 alloy revealed that the quantity of Pd and Ni decreased somewhat and phosphorus was entirely lost from the surface layer as a result of the Zn impregnation-leaching surface activation, but about 30% zinc remained on the surface. The higher activities of amorphous samples compared to

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N E W CATALYTIC MATERIALS TABLE I1 Current Densities of Electrodes for Oxidation of Methanolasb Electrode composition

Temperature of heat treatment (K)

Current density (mA/cm*)

-

120

Pt-Pt

92 12 12 90 134

80 112 106 a

Data from Ref. 63.

* 1M KOH + 6 M CHIOH, -0.5

V (SCE), 333 K.

the crystalline ones can be attributed to the lack of long-range periodicity of the amorphous structure, ensuring the uniform diffusion of zinc into the bulk and, therefore, higher surface areas after leaching. Copper-based amorphous alloys also proved to be active in the oxidation of formaldehyde (108, 109). As it was reported earlier in connection with the hydrogen evolution reaction (62) (see Section III,A,I), HF treatment leads to the formation of a copper-rich porous surface layer. As a result, electrodes with very high electrocatalytic activity for anodic formaldehyde oxidation could be prepared. It was found that the rate-determining step is a one-electron transfer and the oxidation proceeds via the hydroxymethanolate ion HOCHzO-. However, it is not clear whether the catalytically active copper species is Cuoor Cu+.It would be interesting if either Cuo or Cu+ could be stabilized in amorphous alloys. The HF-treated Cu33Zr67 alloy exhibits higher activity than a similarly treated CujSTh5sample showing characteristics similar to those of a copper plate. XPS measurements revealed (109) that Ti was almost lost from the Cu-Ti surface after the HF treatment whereas a noticeable amount of Zr still remained on the Cu-Zr. Therefore, the higher activity may be due to the presence of Zr in the surface layer. Since the HF-corroded Cu-Zr exhibited characteristics similar to a Cu plate, it seems unlikely that its activity is due to some peculiar ensemble of Cu atoms emphasized by surrounding Zr atoms. The oxidation taking place on the copper-based amorphous electrode is incomplete because formate ion is formed and gaseous hydrogen is liber-

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10-2

E3

I

0

.

a

S

.

1

50 75 100 x Iatornic%)

FIG.5. Activity of HF-treated (Culao-xPdx)33Zr67 amorphous alloys in formaldehyde and fromate ion electrooxidation as a function of palladium concentration [0, 0.3M H2CO;a, 0.3M HCOONa (1.OM NaOH, 0.3 V) (reference: reversible hydrogen electrode), 303 K]. (Reprinted from Ref. 110 with permission of Elsevier Sequoia, S.A.)

ated during the process. Improvement of the electrodes requires their combination with electrocatalysts active in the oxidation of HCOO- and H2 to C02 and HzO, respectively. Cu-Pd-Zr electrodes fulfill these requirements (110). With increasing palladium concentrations the activity for the formaldehyde electrooxidation decreases while the oxidation of formate ion sharply increases (Fig. 5 ) . In parallel, the quantity of hydrogen evolved exhibits a sudden decrease, and no hydrogen evolution could be detected on alloys with palladium excess relative to copper. The optimum alloy composition with similar activities for both formaldehyde and formate electrooxidation is (CuzjPd7j)3,Zr,j7. The Raney-type porous surface layers are also formed after HF treatment on Pt-Zr alloys doped with Sn or Ru (66). The dopants are necessary to ensure high electrocatalytic activity. In contrast to tin, neither HF activation nor longer periods of polarization caused ruthenium to dissolve from the alloy matrix. As a result, the PtloRul&rsoamorphous electrode exhibited a high and stable activity in the oxidation of methanol in acidic solution.

B. FISCHER-TROPSCH A N D METHANOL SYNTHESIS 1.

The Interaction of CO with Alloys

Despite the numerous papers dealing with the hydrogenation of carbon monoxide, there are very few studies on the interaction of CO with surfaces of amorphous catalysts. The first experiments with Ni-Zr and Fe-B glasses revealed a substantial weight gain by thermogravimetric analysis indicating CO dissociation (30). More details have been provided by recent UPS studies on Ni-Zr

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alloys. This work started from the observation that there is an increasing tendency for dissociative chemisorption of carbon monoxide on transition metals as their numbers of d-electrons decreases (111). For example, nickel, a late transition metal, brings about molecular adsorption, while zirconium, an early transition metal, brings about dissociative chemisorption. Changing the chemical composition of the Ni-Zr amorphous alloys permits continuous alteration of the d character and allows the study of its effects on chemisorption. As it turns out, the Ni-Zr amorphous alloys exhibit increasing dissociative chemisorption of CO with increasing Zr content and with increasing temperature (31-33, 122). However, the ratio of molecular to dissociative chemisorption is very different from the ratio expected from the alloy composition (32, 33). For example, on NiglZrg only dissociative chemisorption occurs at 423 K despite the very low zirconium content. The experimental data indicate that the interaction between CO and glassy metals is controlled by electronic effects. Alloying Ni with Zr changes the local electronic structure at the Ni site and increases the electron density in the antibonding 27r molecular orbital of the adsorbed CO. This occurs through back-donation from the Ni 3d band and weakens the C-0 bond. Thus, the potential barrier for CO dissociation is lowered. On CusOZr5o,CO dissociates at all exposures and only Zr is oxidized (35). Similar dissociative chemisorption was observed on Ni76B12Si12 and FeNNi3aMo4B metallic glasses (34, 35). Surface analyses revealed the absence of molecular adsorption on the Ni-B-Si glass and only negligible molecular adsorption on the Fe-Ni-Mo-B glass and the formation of oxide and carbide layers due to CO decomposition. Apparently, this oxidation of boron and silicon results in the formation of a protective surface oxide layer, preventing the oxidation of the metals. In contrast, on the partially crystallized Fe-Ni-Mo-B sample, clear evidence for molecular adsorption was found. So disruption of the amorphous structure frees Ni back to its ordinary crystalline state, in which it can adsorb CO molecularly. One might speculate about whether this results only from electronic effects or also from the long-range ordering in the crystalline state of Ni. Among the amorphous, partially crystallized, and crystalline FesoBzoand Fe40Ni40B20 alloys, again only the partially crystallized sample of the FeNNi40B20 exhibits molecular adsorption of CO at 300 K, as shown by XPS, UPS, and thermal desorption spectroscopy (24, 29). The dissociative adsorption results mainly in carbidic species on the amorphous and partially crystallized samples because, according to the authors, small metal ensembles hinder the transformation of this carbon into graphitic species. In contrast, on the fully crystalline alloys mainly bulk carbides exist.

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346 2. Ni(Fe)-P(B) Alloys

Early results (39,40, 113) on the hydrogenation of carbon monoxide on Ni, Fe, and mixed Ni-Fe amorphous alloys with P and/or B as metalloids are summarized in Ref. 45. Experiments with 15 different amorphous catalysts and their crystallized derivatives in a flow reactor, at atmospheric pressure, and in the temperature range of 493-643 K led to the following main findings: 1. With one exception, N b F e d z o , the stable catalytic activities of the amorphous specimens are as much as several hundred times higher than those of the crystallized samples (Table 111). 2. The activity of the alloys varies irregularly with composition. NiaFe20P20 exhibits the highest activity and NbFe.,,,Pz0,the lowest. 3. Both the amorphous and crystalline catalysts exhibit the same activation energy (100 k 4 kJ/mol) and obey the same rate law ( r = kPH);that is, the rates are independent of the partial pressure of CO. 4. The catalysts show an initial transient period during which either activation or deactivation can occur (Fig. 6). After about 10 h on stream, all amorphous catalysts approach the same constant activity value. 5 . Some of the amorphous catalysts show crystallization during reaction at temperatures lower than their ordinary crystallization temperatures, and as a result they lose their high catalytic activity. TABLE 111 Correlation between Structure and Activity of Fe40Ni4~P,~B4 in Hydrogenation of COa Activity XRD pattern a

b

A

c

Structure

(% conversion, H/CO = 29, 533 K)

Amorphous

0.14

Metastable 11

0.032

Stable crystalline

0.058

1 0 5 0 6 0 7 0 2 Bldegreel a Reprinted from Ref. 45 with permission of Academic Press, Inc. Data from Ref. 39.

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347

12

0

10

20 Reaction time (hr)

FIG.6. The change of activity of amorphous FeaNiaPI6B4 as a function of time in hydrogenation of carbon monoxide. (Pretreatments and reactions at 593 K. Without pretreatment: -A-. Pretreatment in hydrogen: --0--1 h, --El--2 h. Pretreatment in CO for 2 h: V....) (Reprinted from Ref. 45 with permission of Academic Press, Inc.)

6. Further data published later (56, 114-118) showed that the amorphous catalysts have different selectivities for the product distribution. For the formation of c2-C~products, amorphous Ni60Fe20P20, studied in detail, gives higher selectivities than does the crystalline alloy, which produces methane as the main product. The amorphous catalyst, in contrast to the crystalline one, does not obey the Schulz-Flory distribution (geometric series) law (119, 120), and the product distribution is strongly dependent on the conversion of CO. With increasing conversions higher selectivities are observed (65% of C2-C5 hydrocarbons at 40% conversion).

Observations 1 and 3 indicate that the active sites of the catalysts are of similar nature but their numbers are different in the amorphous and crystalline states. The lack of chemisorption data renders it impossible to make an exact comparison of activities in terms of turnover frequencies. Nevertheless, the similarity of BET surface areas of the amorphous and crystalline samples points to the presence of more active sites per unit surface area on the amorphous catalysts than on the crystalline ones. However, as the observations cited below indicate, other factors also contribute to the activity difference between the amorphous and crystalline states. Detailed XPS measurements with N ~ B ~ -alloys ~ F (121,122) ~ ~ P ~showed ~ a decrease in the binding energy of P(2p) electrons. This indicates electron transfer from Fe and Ni to P and suggests the formation of electrondeficient nickel and iron species as the probable active sites. As a result of

348

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crystallization and subsequent hydrogen treatment, a significant iron enrichment in the surface film was observed (222). This film, composed mainly of a-Fe and FejP, had almost the same composition as the surface of FeEaP20 after similar treatment. Nickel showed lower binding energy in the crystalline samples, which indicates less electronegative character and, therefore, less electron transfer. These observations suggest that both the smaller number of active sites resulting from aggregation and the lower electronegativity of the active sites resulting from weaker interaction between nickel and other species can account for the lower activity of the crystalline state. XPS measurements made with the aim of the interpretation of observation 2 above revealed a linear correlation between the composition of the alloy and both the composition of the alloy surface and the binding energies. That is, these two factors do not correlate with the irregular activity changes (122). The study of Ni~~Pl&a3 and Ni&9 alloys activated by the nitric acidoxidation-hydrogenation treatment supported the above results to some extent and revealed interesting differences between the two alloys (59). Detailed studies of NislPl9in the hydrogenation of the olefinic bond already revealed (57) that the activity of the catalysts depends on the temperature of the oxygen pretreatment. This treatment increases the number of active surface sites and changes their electronic states (see Section III,C,l). XPS, chemisorption, and rate measurements also led to the same conclusion in the hydrogenation of carbon monoxide (59). The turnover frequencies were higher on Ni-P-La than on Ni-P, and the oxidation had less effect on the activity of the former. The amorphous state was more stable in the case of the lanthanum-containing alloy (the crystallization temperatures are 743 and 643 K for Ni-P-La and Ni-P, respectively). These observations show that lanthanum has advantageous effects on both the activity and the stability of the amorphous state. For example, it has already been proven that oxidation leads to the formation of NiO, which facilitates the diffusion of electron-deficient Ni in the bulk to the surface (57). The presence of lanthanum, however, depresses both oxidation of Ni and its migration to the surface during the hydrogenation of CO. Lanthanum seems to depress the formation of discontinuities in the surface structure, the structural changes resulting from diffusion in the surface layer, and surface reconstruction leading to overoxidation. As a result, the formation of surface nickel atoms with favorable electronic states takes place and ensures the high activity of the amorphous Ni-PLa alloy. Both the Ni78P&a3 and NislPl9alloys exhibit the usual large activity drop after crystallization. XPS spectra indicated much deeper oxidation

NEW CATALYTIC MATERIALS

349

of Ni, P, and La in the bulk of the crystallized samples in comparison with the amorphous samples, while XRD detected aggregation and separation of nickel species during crystallization. These transformations lead to weaker interactions between Ni and other species and result in the formation of less electronegative Ni atoms. It is also interesting to note that the Ni-P alloy exhibited negative order with respect to the partial pressure of CO, in contrast to earlier observations (45). This negative order indicates that the highly electron deficient nickel suppresses the cleavage of the C-0 bond in carbon monoxide by limited electron backdonation to an antibonding orbital of the adsorbed CO. In contrast, the moderately electronegative nickel atoms on the surface of the Ni-P-La alloy are effective at rupturing the C-0 bond as indicated by the change in order to zero-order kinetics. It is well known that nickel has an outstanding activity for methanation. Thus, it is worth mentioning the observation that in CO hydrogenation an amorphous NisoNbso alloy produced C2-C4 hydrocarbons with a selectivity of 46% (39% ethene and propene) together with only 40% methane (523 K, H :CO = 1) (55). Superior activities of the amorphous structures, high selectivities in olefin formation, and formation of crystalline a-iron have been observed for amorphous iron-boron alloys (20, 21). Although XRD and transmission Mossbauer spectroscopy, which is sensitive to bulk properties, showed no sign of crystallinity of Fes2.2B17.s after a 133-h reaction time at 543 K, conversion electron Mossbauer spectroscopy and UPS indicated the formation of crystalline a-iron on the surface (20). Coexistence of both amorphous and crystalline structures on the surface was revealed by UPS even after a 14-h hydrogen treatment at 570 K. A commercial FealB13.sSi3.sCz alloy (Allied Chemical Corp.) suffered more serious changes during a I-h heat treatment in hydrogen (21). Even XRD indicated the presence of a-iron (d 25 nm) at 723 K (30 K below the crystallization temperature, Tc). In the completely crystalline alloy, annealed at 773 K, mainly a-iron (d > 50 nm) and several different Fe-B compounds were detected. Concomitantly, with the growth of the a-iron particles during crystallization, segregation of Si and B occurred in the top surface layer. In the fully crystalline alloy, the next three layers detected by AES, were in descending order, mixed silicon oxide-boron oxide, crystalline iron carbide, and crystalline iron boride mixed with crystalline a-iron. The activities of the heat-treated, partially or totally crystallized samples were smaller than that of the completely amorphous sample. This was probably the result of the increasing particle size produced by annealing. The hydrogenation of carbon monoxide also brought about similar suri=

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350

face boron enrichment of both the amorphous and the crystalline Fe-B alloys (20). The amorphous Fe-B specimen showed a higher surface boron content than did the crystalline specimen and a sharper decrease of boron concentration with depth. It is the boron that is considered to have a beneficial effect on the activity of the amorphous sample. Boron is believed to govern the formation of ensembles of the proper size for producing the higher activity of the amorphous state. When present in the form of boron oxide, under reaction conditions it is assumed to initiate the crystallization of a-iron into the crystalline size range for maximum activity (123). At the same time, it prevents the formation of large crystallites; that is, it stabilizes small iron ensembles by separating them. Apparently, on these small ensembles a suitable concentration of reactive carbon can exist, yet the formation of stable iron carbide, which would lead to deactivation, does not occur. Recently Guczi et al. published newer results from detailed studies of the effects of different pretreatments on the surface structure and activity of Fe-B and mixed Fe-Ni-B alloys (25,26,28, 70, 124). They observed that the two sides of Fe-B alloy ribbons exhibit different surface structures. The dull side, which was in contact with the rotating wheel [also called the rapidly quenched (RQ) side] is covered with a porous, deeply oxidized layer, depleted in boron, consisting of mainly iron oxide. In contrast, on the shiny side [or free-surface (FS) side] they observed the enrichment of boron and the formation of a more compact boron oxide layer (25, 26) (Fig. 7). Both the structure (morphology and composition) and the catalytic activity were affected by changes in the cooling rate, which result from altering the rotation of the spinning disk of the meltspinning process. Higher cooling rates produce smaller difference between the two sides of the ribbon. With increasing revolution the reaction rate for CO hydrogenation changed from 250 to 30 nmol s-' m&! (25, 70, 124). The authors interpreted this to mean that at low cooling rates a more porous, more deeply oxidized layer is formed on the dull side and that this ensures a higher number of accessible metallic iron sites after activation (Fig. 7). &on

oxide

shiny side

omqhous ollqr tirm

dull side

ion oxidwborm d

a

e

b

FIG.7. Model for the surface structure of Fes2Blsalloy ribbon [a, as-received; b, after treatment at 543 K with hydrogen or the reacting gas mixture (H:CO = 2)]. (Reprinted from Ref. 25 with permission of Elsevier Science Publishers, B.V.)

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35 I

Mechanical polishing, anodic polarization, and HNO3 etching proved to be ineffective for activation, while cathodic polarization and HCI pretreatment were effective and increased the catalytic activity (26, 70). In the as-received state FesoBzohas an activity about twice as high as that of FeNNbBZO (26). After HCI etching, however, the activity of Fe-B decreases significantly whereas the activity of Fe-Ni-B increases sixfold, accompanied by a parallel decrease of selectivities to higher hydrocarbons and olefins. The activity decrease of Fe-B was interpreted as a deep oxidation to total inactivity on the shiny side and a transformation of the originally porous oxide layer to a compact one on the dull side. Both processes lead to decreases in the accessible zero-valent iron sites on the surfaces and result in decreased activities. In contrast, both surfaces of the Fe-Ni-B ribbon are similar, a phenomenon that the authors could not interpret (26). Moreover, nickel is practically missing from the surface of the as-received Fe-Ni-B ribbon, in contrast to another observation (125), and this leads to a somewhat lower but similar activity to that of the Fe-B ribbon. Complex surface processes during HCI etching (acidic dissolution of oxides, electrochemical oxidation) lead to the formation of a porous, chloride-containing iron oxide layer while nickel remains in the zerovalent state. Subsequent reduction, facilitated also by hydrogen atoms formed on nickel sites, results in an increased number of surface iron and nickel atoms and an enhanced catalytic activity. The larger concentration of atomic hydrogen on the surface and the presence of surface Ni are observations that are supported by the decreased selectivity of olefin formation. An activity increase was also observed on a partially crystallized FeNi-B alloy (24). Both XPS and UPS revealed reactive surface carbides formed from dissociative CO chemisorption. It was suggested that the proper ensemble size of the partially crystallized alloy stabilizes the reactive surface carbide and establishes an optimum equilibrium between it and the molecular form of CO, which results in the unique, high catalytic activity. Several other amorphous alloys, Fe-Co-Si, Ni-Al-Cr, Ni-Co (126), and Fe-Ni-P, prepared by ion sputtering (127), also showed high activities in the CO hydrogenation reaction. Surprisingly, heat-treated Fe4NkPl6B4exhibited high reaction rates as well as high selectivities for the formation of ethane and propane (128). The authors suspect that the surface layer still maintained the amorphous character despite crystallization of the bulk, which contradicts other observations (20).

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352

3 . Metal-Zirconium Alloys

Results characteristic of the majority of zirconium-containingalloys in the hydrogenation of carbon monoxide were first reported for iron- and nickel-based materials. Ni63Z1-37(55, 56, 117) and a mixed Ni~jFeloZr37 alloy (55, 56) exhibited remarkable activity increases during reaction. In parallel with this, substantial increases in BET surface areas occurred. SEM revealed the formation of a cracked, rough surface while XRD indicated that the Ni-Zr alloy became crystalline after 40-60 h on stream at 531 K, far below its crystallization temperature (800 K) (56). In contrast, the increase of the BET surface of FewZrlowas less pronounced (55, 56, 114). This alloy showed a stable activity during reaction, and neither crystallization nor change of surface texture was observed (55,56, 114, 115, 117). As discussed earlier (Section II,E), it is clear that T, alone is not necessarily a criterion for stability under reaction conditions. Another difference between Fe-Zr and Ni-Zr was that the former showed high activity for C2-C4 olefin formation at low conversion, while the latter gave mainly methane, which is a characteristic of Ni (Table IV). In fact, the estimated activity of the amorphous Ni63Zr37 was comparable to that of highly active methanation catalysts (56). The amorphous Fe&rlo alloy exhibited only small deviations from the Schulz-Flory distribution, and the product distribution changed only slightly with conversion (56, 114, 117). This was in contrast to the amorphous NimFeaoPzo alloy mentioned earlier, which did not exhibit the Schulz-Flory distribution and did produce a change in product distribution during conversion (113-115, 117). Finally, crystallization of the FewZr10alloy caused a rapid decrease of activity (55,56, 114). TABLE IV Comparison of Iron- and Nickel-Based Zirconium Alloys in Hydrogenation of COO

FewZrlo Increase of BET surface Change of activity during reaction Crystallization during reaction Increase of surface roughness Product distribution

Fel~Nidr37

Nidr37

3.6 times

8.5 times

50-70 times

Stable

Increasing

No

No

Increasing, stabilized after 40-60 h Yes

No

Yes

Yes

C2-C4 olefin: 55% (H/CO = 1, 521 K)

Data from Refs. 55 and 56.

CH4 :80% (HICO = 5, 531 K)

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353

A comparison of all the information for the three catalysts (Table IV) clearly shows that replacement of Ni by Fe suppresses the crystallization otherwise taking place in Ni-Zr, decreases the cracking of the surface, and curbs the increase of the BET surface area (56). Experiments with Ni-Zr alloys of different compositions, mainly with Ni67Z1-33, led to an interpretation of some of the above phenomena (48, 129). On Ni6,Zra and NisoZrJo,the reaction rate based on unit catalyst weight increased as a function of elapsed time until it finally reached a steady activity (Fig. 8) (129). However, the specific activity, calculated by taking into account the actual increased BET surface areas of the alloys, showed the opposite trend. The specific activity remained nearly constant during the first 20 h and then decreased markedly as a result of the drastic increase of BET surface (Table V) (48). XRD proved that the catalyst retained its original amorphous structure in this early period of stable activity but, subsequently, gradually crystallized to yield Ni and Zr02and then, after 115 h, segregated to crystalline Ni/ZrOa with a surface area of more than three orders of magnitude higher than that of the virgin alloy. The drastic changes of the amorphous alloy surface during reaction are presumed to take place because of the partial removal by hydrogenation of surface Zr02 formed during preparation. This exposes the underlying Ni-Zr alloy to the further action of hydrogen (129). Exploiting the above phenomenon, a technique for preparation of a highly active, high-surface-area, amorphous methanation catalyst was successfully developed (48). The high-surface-area, porous, but still amorphous catalyst structure was created by a short, 3-h hydrogenation of CO at 538 K on Ni67Zr33 (Table V,b). After this treatment, ensuring the controlled enlargement of the catalyst surface, the actual reaction was carried out at lower temperatures (453-493 K). Under this condition no further change of either the activity or the surface structure of the catalyst

Reaction timehl

FIG.8. The change of activity of Ni-Zr alloys as a function of time in hydrogenation of carbon monoxide (-Ni6,Zr3,, ---Ni&d. (Reprinted from Ref. 129 with permission of The Society of Chemical Engineers, Japan.)

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354

TABLE V Change of Catalyst Structure and Activity of Ni6,Zrj3 in Hydrogenation of COa.b

XRD pattern

L

L

L

Elapsed time in reaction

BET surface (m2/g)

As quenched

0.05

Specific activity ( x lo-’ nmol/mVs)

3h

5.3

390

10 h

7.8

450

30

~~

Reprinted from Ref. 48 with permission of The Chemical Society of Japan. H:CO = 4.538 K.

was observed even after I5 h. An activation energy of 100.6 kJ/mol was determined. This value was also reported for supported Ni catalysts (130) and amorphous Ni-P(B) alloys (45). It is worth mentioning that CO hydrogenation experiments using different Ni intermetallic compounds, including NijTh, NisZr, Ni5Si2, and Ni2Si, also resulted in decomposition to Ni and the corresponding oxide as well as nickel enrichment of the surfaces. These results account for the high methanation activity of these in situ prepared “supported” catalysts (131, 132).

Results on VIII2 and VIII3 as well as Ib metals were reported recently (46, 47, 53-55, 117, 128). RhZSZr75, OsuZr75, I ~ ~ J Zand T ~P ~t 2, ~ Z alloys r~~ showed hardly any activity below 600 K and exhibited no change of specific surface areas under reaction conditions (653-670 K, 6 MPa, H : CO = 2) (54). In contrast, Pd35Zra5,Pd2sZr7~,Au25Zr75, and C u ~ 5 Z r ~ ~

were active at much lower temperatures. They showed a continuous increase of activity, reaching a stationary state after several 10-h time periods on stream. Note the large difference in selectivity between Pd25Zr7s (54) and Pd33Zrs5 (46,47,55,117) (Table VI). This seems surprising even after taking into account the different reaction conditions. Also, with the exception of Pd35Zr65, all the catalysts exhibited the usual enlargement of specific surface areas that are characteristic of zirconium-containing amorphous alloys in the hydrogenation of carbon monoxide. It was found that on the Pd-containing alloys the spent catalysts were changed to a crystalline unknown oxide (46). The XRD patterns of the

TABLE VI Characteristics of Pd-, Cu-. and Au-Zr Amorphous Alloys in Hydrogenation of Carbon Monoxide Elapsed time Selectivity (%) Steady-state until steady conversions state reached Higher H:CO T ( K ) P(MPa) (%) (h) CH, MeOH MeOMe hydrocarbons COz Reaction conditions

AUoy Pd&rT5

2 4 pd35zr65 3 C U ~ W 2 2 AusZr7S

478 523 548 500 523

6 0.1 0.1 6 6

21.2 38 17 8.4 33

48

60-70 30 32

15

79

1.1

86 2.2 49.5

94.4 3

3.4 0.3

0.9

3.9

14 47.1

Surface area before/after (m2/g) I 136 0.12l0.13

Ref.

54 55 Slight - increase 46. 47 1/31-67 53 1/51 54

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356

oxidized catalyst and of the alloy used in hydrogenation of CO showed striking similarities (Fig. 9). The available data all point to the presence of a weakly bound Pd-Zr-0-type complex oxide as the possible active species (47). This oxide is formed during the transient early period of the reaction from the oxygen and water either produced during methanation or present as contaminants. This active species, formed in situ from the amorphous Pd-Zr alloy, is more active than the one prepared under identical conditions from the heat-treated, crystalline sample, which exhibits more complete oxidation by EPMA (47). These data indicate that palladium exists in a nonzero oxidation state despite the highly reductive reaction conditions. At higher zirconium content (PdloZrw),XRD shows the presence of ZrO,, which causes deactivation of the catalyst. On the other hand, an alloy with excess Pd (Pd75Zr25)is unable to form the active oxide species (47). Experiments with silicon-containing alloys yielded evidence supporting the preceding findings. With a small amount of Si (Pd35Zr63Si~) the usual change of catalytic activity can be observed, but the steady activity is one order of magnitude lower than that of Pd35Zr65(Fig. 1Oa and b) (47). The sample proved to be amorphous by XRD after the reaction. The other two Si-containing alloys (Pd80Si18Zrzand Pd80Si20) showed hardly any activity changes (Fig. 10c and d), exhibited activities two orders of magnitude lower than that of Pd35Zr65 and retained their original amorphous structures. The results indicate that Si stabilizes the amorphous structure and hampers the formation of the oxidized active species. The behavior of amorphous Cu70Zr30(53)and Au25Zr75 (54) is very similar to that of Pd-Zr. In both cases an increase of activity and surface area were observed and a phase change, resulting from the decomposition

amorphous (as quenched) Pd-Zr-0 cchplex (formed i n hydrooenation of CO at 533 K , after an hr, H:CO= 4 )

Pd-Zr-0 conplex (formed in oxygen at 533 K , after 50 hr)

30

40 50 2 9(degreel

FIG.9. The change of XRD pattern of Pd,,ZrsS. (Reprinted from Ref. 46 with permission of The Chemical Society of Japan.)

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357

I . . . . . . . . 10 20 30 40 50 60 10 80 Reaction time( hr)

FIG.10. The change of activity of Pd-Zr-Si alloys as a function of reaction time (H: CO 513 K , space velocity = 6.1 X lo-' h; c-Pd&iI8Zr2, 513 K, space velocity = 5.9 x lo1h-I; d-PdmSi2,,, 533 K, space velocity = I .4 X 104 h-l). (Reprinted from Ref. 47 with permission of NorthHolland Physics Publishing.) = 4) (a-Pd35Zr65, 533 K. space velocity = 1 . 1 x I@ h-I; b-Pd&&i,,

of the amorphous structure to Cu or Au particles dispersed in Zr02, was detected by XRD. SEM showed the transformation of the smooth Cu70Zr~~ ribbon surface to a cracked surface which is responsible for the increased surface area. The observed phase change starts with the absorption of hydrogen in the amorphous Cu-Zr phase, which brings about the cracking of the ZrOz layer covering the ribbon surface. These cracks permit the penetration of the reactant gases to the underlying Cu-Zr phase, where the methanation reaction takes place. The oxygen and water present oxidize the zirconium, which, in turn, destabilizes the amorphous phase by decreasing the zirconium content and leading to the formation of fine copper particles dispersed in ZrOz (53). Recently, the high activity of a Cu-on-ZrOz catalyst in steam reforming of methanol has been interpreted as an interaction between copper and zirconia (133).A similar interaction might also contribute to the activity of the foregoing catalyst formed from the amorphous alloy. It is interesting to note that amorphous C U ~ ~ exhibits . ~ H ~ activity ~ ~ and . ~ selectivity similar to that of Cu-Zr. In this case there is also an increase in surface area and an analogous phase change leading to the formation of dispersed copper in HfOz. In contrast, under identical conditions, CumTia converted to copper, Cu-Ti metallic compounds, and titanium hydride, all of which were inactive for the hydrogenation of carbon monoxide even at 600 K (45).

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358

c.

HYDROGENATION OF ORGANIC FUNCTIONAL GROUPS

1. Hydrogenation of Alkenes The first results of the use of metallic glasses as catalyst were reported by Smith et al. (2,52), who found that the hydrogenation-deuteriumation of cis-cyclododecene on PdmSi20 and on Pd77Ge2, glassy alloys results in higher ratios of isomerization to addition than on crystalline Pd; that is, they produce more trans-isomer. Also, the glasses show higher activity for deuterium exchange. Relative to palladium powder, both Pd-Si and Pd-Ge glasses produce more monodeutero-trans-cyclododeceneand more dideuterocyclododecane (Table VII). Because no differences in surface electronic structure were found by AES and UPS (234) between the glassy and crystallized Pd-Si alloys, reactivity differences are explained on the basis of differences in surface topography. Later Giessen and coworkers found no significant differences between glassy and crystalline PdmSizo for catalytic selectivity in either the cistrans isomerization and double-bond migration or the stereochemistry of addition from the hydrogenations of n-hexenes, a-pinene, and cis- and trans-cyclododecene(135). However, they did find that deuteriumation of 1-hexene over Pd powder gave more rapid isomerization relative to addition than did either the Pd-Si glass or the crystallized glass (Table VIII). Only minor differences were observed between the amorphous and crystalline alloys in the deuterium exchange with hydrocarbons (135, 236). Further detailed examinations were camed out by Smith and coworkers using the probe molecule, (+)-apopinene (6,ddimethyl- lR,SR-bicyclo[3.1.l]heptene-2), which is sensitive to catalytic sites of differing surface coordination. (+)-Apopinene was developed to differentiate between different catalyst preparations and to trace minor alterations in the morphology of metallic catalysts. The Pd-Si and Pd-Ge alloys examined show the lowest ratios of isomerization (double-bond migration) to addiTABLE VII Deuieriumation of cis-Cyclododecenen,b Addition Alloy

(%)

Trans-cyclododecene-dl

Cyclododecane-d2

PdwSia PdnGez3 Pd powder

14.2

52.6 54.7 40.6

31.7

10.9

14.5

Data from Ref. 51. Room temperature, 1 atm.

35.3

24.1

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TABLE VIII Hydrogenation of I-Hexenea.b Compound Hexane I-Hexene Cis-2-Hexene Trans-2-Hexene Trans-3-Hexene Additionlisomerization a

PdaoSim (amorphous)

PdaaSi20 (crystalline)

18 36 27 17 2 18/46

13 57 12 17 1

13/30

Pd powder 13 2 20 56 9 13/85

Data from Ref. 135. 323 K, 1.3 atm.

tion among many different palladium catalysts in deuteriumation experiments (68,69).The ratio of relative rates of double-bond migration (isomerization) and addition, kilk,', are as low as 0.68 for Pd80Si20 and 0.59 for Pd77Ge23. Interestingly, pure Pd prepared by the rapid quenching as well as Pd foil produce very similar values, despite the fact that both are obviously crystalline, whereas kilk, ratios of all other polycrystalline catalysts are in the range of 1.9-10.7. Different kilk, values were recorded for catalysts prepared by two different rapid-quenching methods; that is, minor alterations of the surface structure caused by slightly differing cooling rates can also be detected (52). The shock-tube technique produces catalysts with lower kilk, values (0.77) than does the hammer-and-anvilmethod (0.92) (Fig. 11). Heat treatment of the alloys results in a relative increase of isomerization (higher kil k,). The comparison of electron microscopy and chemisorption data as well as the results of deuteriumation experiments lead to the conclusion that the glassy alloys possess few sites of low coordinative unsaturation and many sites of high coordinative unsaturation. The former may be the 'M and the latter, the 3MSiegel-type sites (137).Heat treatment of the alloys results in catalysts with higher isomerization activities. This fact suggests that the more highly coordinatively unsaturated sites occur on protuberances that may crystallize into staircases with medium coordinative unsaturation (2M). These new 2M sites are different from the former 2M atoms in that they are involved in long-range order, whereas the former 2M sites were merely isolated surface Pd atoms of intermediate coordinative unsaturaI Calculated from In(l-2Iso) = In(l-Add) 2ki/k,PH,where Is0 = mole fraction of doublebond migration (isomerization), Add = mole fraction of hydrogenation (69).

ARPAD MOLNAR et al.

360

51 -

hl

-I

c C I

0.1

0.2

0.3

-In( 1-Add I

FIG. 1 I . Plots of In(l-2lso) versus In(l-Add) for PdwSi20and Pd77Ge23alloys. Slopes of the lines are 2ki/k,PHfor the catalysts (ST = shock tube, HA = hammer and anvil, HT = heat treatment). (Data from Ref. 52.)

tion. Since these new 2M sites catalyze double-bond migration, we conclude that some special ordering involving edge (2M) sites is required for the more rapid double-bond migration. Additional evidence for the differences between the two types of 2Mtype site comes from the fact that hydrogenation does occur, so hydrogen must be dissociated on the glassy surfaces. Since edge (2M) sites have been identified with the rapid dissociation of hydrogen (138), we assume that there are 2M sites on the glassy surfaces, that dissociate hydrogen, and that they do not catalyze double-bond migration. When 2M sites are ordered into staircases (edges), however, they do catalyze double-bond migration. Experiments with (+)-apopinene purified with different methods also lead to interesting observations (69). After only a simple gas-chromatographic (GC) purification an induction period and a higher ratio of kilk, occurs (1.7); however, when the GC purification is followed by percolation through activated alumina, there is no induction period and a lower ratio of kilk, occurs (0.81). The small amount of unknown poison still present after only the GC purification apparently inhibits addition sites and, therefore, increases the relative rate of isomerization. This can be understood if we adopt the explanation for hydrogenation of Tanaka (139), specifically, that hydrogen dissociation occurs on edge sites (*M-typesites on glasses) and the dissociated hydrogen atoms migrate to other sites where addition occurs. If we further assume that a slow double-bond migration (classical Horiuti-Polanyi half-hydrogenated-state type) occurs concomitantly on the same sites as addition [the

36 I

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more rapid ~-allylAnderson-type isomerization (140) does not occur because of the absence of ordered edge sites], then the results of the poison can be explained as follows. The poison adsorbs on *Msites, lowering the number of dissociated hydrogen atoms able to migrate to the additionisomerization sites. Since addition requires more hydrogen, isomerization increases relative to addition and, therefore, kilk, increases. Another remarkable example of the sensitivity of (+)-apopinene to the changes of surface morphology is the detection of the effects of terminal alkynes. As was discussed earlier in connection with the problem of activation (Section II,D), terminal acetylenes (phenylacetylene, I-octyne) comminute palladium catalysts. These dissolution and fragmentation processes lead to substantial changes in both the activity and the selectivity of splat-cooled palladium catalysts for alkene reactions (73).In every case, following the hydrogenation of a terminal alkyne both addition and isomerization of (+)-apopinene increased. But differences between the catalysts exist. For example, Pd or Pt foils and reduced palladium oxide exhibit comparable increases of both reactions; consequently, the ratio of the two rates remain almost the same. In contrast, however, the splatcooled catalysts show higher rate increases for isomerization than for addition (Table IX). It is suggested that for the splat-cooled materials, comminution exposes new surfaces that are different from the old, These TABLE IX Changes of k,lk. Ratio in (+)-Apopinene Hydrogenation"

After hydrogenation of terminal acetylene

Alloy

Original

Pd80Si20 (hammer and anvil) Pd77Ge2, (shock tube) Pure Pd (hammer and anvil) Pd foil Pt foil reduced Pd oxide

1.oo

1.30'

0.59

0.97d

1 .oo'

I .09

1 .39d

1.33f

0.96 0.09 1.91

0.84d 0.09d 1.92

Reprinted from Ref. 7.3 with permission of Academic Press, Inc. Experimental error 4%. After hydrogenation of PhCCH and I-octyne. After one hydrogenation of PhCCH. After five runs of hydrogenation of PhCCH. f After additional hydrogenation of I-octyne. (I

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new surfaces exhibit greater order than do the old because fractures occur at regions where disorder is greatest and, hence, the surfaces produced zxhibit greater order. This is accentuated in the splat-cooled pure Pd, which, being crystalline, has many internal grain boundaries (disordered interfaces) between well-ordered surfaces. NielPlg and Nb2B3sglasses also can be used as catalysts for the hydrogenation of olefins (ethene, propene, butenes) and dienes (1,3-butadiene, isoprene) (37, 41,57, 141, 142). The virgin alloy ribbons prepared by the rapid-quenching method showed no activity for hydrogenation of olefins even after a high-temperature hydrogen pretreatment (573 K, 6 h), but successive treatments with HN03, oxygen, and hydrogen brought about high catalytic activity for both hydrogenation and isomerization of C=C double bonds (141, 142). Crystallization by heat treatment of the Ni-P alloy caused decreased catalytic activity whereas crystallization of the Ni-B alloy caused hardly any change in activity, even though the BET surface areas of the amorphous and crystalline samples were very similar. Kinetic analysis suggested that the activities of the amorphous alloys depend on the amount and electronic states of active surface sites (142). Hydrogen and olefins are not competing for adsorption on the same sites, but hydrogen is activated on electron-deficient nickel atoms. Detailed ESCA measurements on Ni-P supported this observation (57). It was shown that HN03 is effective for removing surface oxides and that this results in a decrease in concentration of surface nickel. The oxygen treatment brings about oxidation of not only the surface but also the inside of the alloy, which increases the concentration of Ni species in the surface layer. The treatment with hydrogen reduces NiO to Ni, but metalloid oxides remain unchanged. With changes in the temperature and time of both oxygen and hydrogen pretreatments, substantial changes in the activities of the catalysts were observed. Both overoxidation and overreduction decrease the catalytic activities, suggesting that an optimum ratio of both oxidized and reduced species on the surfaces is required for highest activity. The facts that the catalytic activity of Ni-P increases when unreduced NiO is present and that the activity does not correspond to the number of surface Ni atoms indicate that the electronic state of nickel determines the catalytic activity. Electron transfers from Ni to phosphorus species and to NiO, indicated by the shift of the binding energies, are important for the generation of surface Ni species with electronic states responsible for high catalytic activity (57). The turnover frequencies of Ni-P for the hydrogenation of ethene increased monotonously with the increase of the temperature of oxygen treatment. This increase is correlated with the increase in the strength of

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the Ni-0 bonds of nickel oxides. ESCA and TPR also indicate increasing electron transfer from nickel to nickel oxides and, consequently, decreasing electron density on the nickel atoms (57). Pulverization of the amorphous Ni-B alloy ribbon in a vibratory roll mill was also found to be a useful method for activation (37). Data show increasing BET surface areas, increasing amounts of surface Ni, and increasing initial rates of ethene hydrogenation with decreasing particle sizes. The method is effective for removing the stable surface oxides and mildly oxidizing the surface species; that is, the effect of pulverization is the same as that of the HNO3-02-H2 treatment. Additional oxidation after pulverization enhances these effects by increasing the surface concentration of electron-deficient nickel species. Changes in the composition and structure of the surface as a result of crystallization were also observed by ESCA (142). In Ni-B the formation of stable boron-oxygen compounds was detected. In the Ni-P alloy the nickel was transferred from the surface to the interior, leading to a smaller Ni concentration on the surface. Because of the lower homogeneity of the crystalline alloy, oxidation proceeds more deeply inside the bulk and the interactions between nickel and the other species become weaker. This was indicated by the lower binding energies of Ni (57). These changes may explain the decreased activity of the crystallized alloys and indicate that caution should be taken to compare the activities of amorphous and heat-treated, crystalline alloys because changes other than simple crystallization also occur during thermal treatments. Here again, the amorphous structure ensures more homogeneous distributions and higher concentrations of active species as well as stronger interactions between different surface species during pretreatment. Because of these factors, the resulting catalysts exhibit much higher activities for catalytic transformations than do their crystalline counterparts. Very detailed studies of the effects of thermal treatment on the structure and activity of the Ni62B3samorphous alloy permitted the observation of minute structural changes and the establishment of correlations between activity and structure (23,42).During stepwise heat treatment, first the catalytic activity in hydrogenation of ethene and isoprene increased with increasing temperature up to 623 K (Fig. 12). This is the region in which structural changes are accompanied by the largest heat evolution (Fig. 13). A marked activity drop was observed at higher temperatures (Fig. 12). In parallel, both the calorific value and the number of surface nickel atoms determined by hydrogen chemisorption showed a similar decrease (Fig. 13). XRD did not show the presence of any crystalline structure in the alloy heat-treated at 623 K; however, intense peaks assigned to crystalline Ni2B and B204 were detected in samples treated at

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FIG. 12. Effect of thermal treatment on the catalytic activity of amorphous Ni62B38in hydrogenation (A, ethene; A, isoprene). (Reprinted from Ref. 41 with permission of The Royal Society of Chemistry.)

653 and 683 K even though they were still partially amorphous. The alloy heated at 783 K exhibited no signs of the amorphous state. In the region of increasing activity (heat treatment below 623 K), XRD and calorific value do not indicate any changes in this so-called precrystallization state; however, electrical conductivity does show stepwise changes. These changes are thought to be connected to structural relaxation. The form of this relaxation is short-range ordering, which alters the chemical bonding between the components of the alloy. ESCA results show increased binding energies of nickel and boron oxide in the alloy treated at 623 K and indicate that there is increased interaction between Ni and B and 0. As a result, there is more electron-deficient nickel produced, which is effective in the dissociation of hydrogen. The activity drop at higher temperatures is partially connected with the decreasing number of surface nickel atoms. Beside hydrogen chemisorption, other observations also show the inverse correlation between tem-

613 I73 Tempemture(K1

FIG.13. Changes of the amount of surface nickel atoms and calorific value of Ni62B3son heat treatment. (From Ref. 23 with permission of The Royal Society of Chemistry.)

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365

perature of heat treatment and number of active surface sites. SEM images reveal the growth of large crystalline compounds, whereas ESCA spectra show the increase of the amount of surface boron oxide; both these observations point to decreasing surface nickel. (See further discussion on this point in Section III,D,l). The authors argue that the decreasing number of surface Ni atoms cannot solely explain the difference in catalytic activities between the amorphous and crystallized alloys, as was proposed earlier in connection with the hydrogenation of CO (45). Although their reasoning must be true, the available data do not seem to support their explanation. Moreover, in their work on the hydrogenation of CO on Ni-P (59), the weakening interaction between Ni and other species induced by aggregation during crystallization (heating) is given as an explanation for decreased activity. At the present instant, however, the binding energy of Ni in the crystalline sample shifts back to the same value as that of the starting amorphous alloy, while the binding energy for B in the crystalline sample is as high as it is in the sample with the highest activity (23). Thus, binding energy arguments do not seem to be important in this case. Nevertheless, the data published show without a doubt that the structure in the precrystallization state has outstanding catalytic activity for the hydrogenation of olefins. Still, further evidence is needed to explain the details of the observed phenomena. Nickel-boron films prepared by glow discharge (143) with boron content above 15% are also amorphous and active without any special pretreatment for the hydrogenation of 1,3-butadiene (144). Product distributions show maximum butane yield at 50% boron content, which can be attributed to the increasing electron density of the nickel. XPS analyses show that the d-bands of nickel are filled by electron transfer from the boron. At higher boron contents trans-2-butene becomes the main product of the transformation. This increasing selectivity for semihydrogenation may be explained by a decrease in the numbers of adsorption sites for hydrogenation that result from an increase in the numbers of isolated nickel atoms. Although the investigations of nickel-metalloid alloys for the hydrogenation of double bonds led to interesting observations, many features of their behavior have not yet been clarified. For example, the higher activity of crystalline Ni-B was correlated with the increased electron density of nickel compared to the decreased electron density of nickel in the less active Ni-P. However, the same electron-deficient nickel in the amorphous Ni-P was identified as the active species, causing the higher activity of that alloy for the hydrogenation of ethene and propene, whereas the activites of the two alloys for diene hydrogenation were very similar (141, 142). On the other hand, the effect of pulverization of the amorphous Ni-

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B ribbon was explained as increasing the concentration of electron-deficient nickel species (37). Moreover, as other studies revealed, the electronic state of nickel in Ni-P alloys depends on the ratio of the alloying elements (245). In alloys with 75% or more nickel, P donates electrons to nickel, whereas with lower percentages of nickel, P accepts electrons and forms electron-deficient nickel. This demonstrates that changes in the local surface concentrations that result from pretreatment can alter the electronic states of the active species. Similar changes might take place also as a result of the action of the reactant. Full interpretation of data different from previous results on X-ray amorphous and crystalline Ni-B (246-248) and Ni-P (146-150) as well as the correlation of observations on amorphous Ni-B prepared by different methods (242, Z42, 244) requires further examination. The much higher activity of Ni-B amorphous alloy powder than that of Ni-P, both prepared by chemical reductions, was observed in the transformations of cyclohexene and the isomerization of ally1 alcohol. Stepwise heat treatments caused neither an increase of catalytic activity nor a substantial difference in activity between the amorphous and crystallized samples (44). Amorphous Ni-P foil deposited by electrolysis had similar characteristics. These properties are quite different from those of Ni alloys prepared by the rapid quenching; moreover, alloys prepared by chemical reduction and electrolysis are active without any special pretreatment. Olefin hydrogenations also were carried out on binary metal-zirconium alloys (38, 43, 67, 72, 252, 152). A pulverized Cu62Zr38 amorphous catalyst has a BET surface 20 times higher than the ribbon (38). Pulverization brings about activity increase for the hydrogenation of ethene and isoprene similar to that found on Ni62B38. Further treatment of pulverized Cu-Zr with HF results in an additional increase of BET surface area and a large increase in catalytic activity. After hydrogen pretreatment, XRD showed partial, short-range crystallization taking place as a result of hydrogen occlusion and long-range crystallization after treatment with HF. The observation that the size of copper particles is smaller in the amorphous powder than in the crystalline alloy correlates with higher frequency of active sites in the former. SEM measurements indicate the formation of a Raney-type copper surface as a result of the selective extraction of zirconium by the HF. However, as XPS data show, unextracted Zr affects the electronic state and stability of the porous copper structure by accepting an electron transfer from Cu to Zr. On the basis of these XPS data it is assumed that Cu+ is the active site for the hydrogenation reaction. The hydrogenation of ethene was studied on Cu70Zr30alloys as well (43,

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367

152). Activities of the originally amorphous, partially amorphous (meta-

stable), and crystalline ribbons prepared by different quenching rates were not tested; rather, measurements were carried out after annealing these materials at 473 K for 16 h in 180 kPa of.hydrogen (43). It is the completely amorphous precursor that exhibits the most drastic changes during heat treatment. This precursor exhibits an increase of BET surface area from 0.015 to 0.56 m2/g and large changes in surface morphology. Both ISS and XRD indicated that the copper segregated onto the surface, forming small crystalline particles. Nevertheless, the catalyst prepared from the precursor of the metastable structure showed an activity one order of magnitude higher than both of the other two samples as well as the copper powder. XRD and ISS measurements reveal that during heat treatment the crystalline Cu10Zr7 compound and Cu are formed embedded in the amorphous matrix. Although the nature of the active sites is not assigned, comparison of the activation energies and preexponential factors implies that the density of hydrogenation sites is highest on the catalyst made from the metastable precursor. In the hydrogenation of 1,3-butadiene, the sample prepared from the amorphous precursor showed much higher activity than did the crystalline alloy (initial rates at 403 K are 29.9 and 3.2 pmol m-2 s-l, respectively); moreover, it exhibited different selectivities (67). Much less Ibutene versus 2-butenes were produced on the catalyst prepared from the precursor, whereas the ratio of cis- to trans-2-butene was slightly higher than on either the crystalline alloy or copper powder. Removal of 1,3-butadiene by selective hydrogenation from industrial olefin feedstocks before hydroformylation or polymerization is an important process in preventing catalyst deactivation. Amorphous Cu70Zr30 exhibits an excellent ability to do this (152). At 348 K a mixture of butenes containing 3% 1,3-butadiene could be converted to a diene-free product with only 1.63% butane. This catalyst also hydrogenates 1,3-butadiene in ethene with a selectivity of 95% with no hydrogenation of ethene. One hundred percent selectivity for the hydrogenation of acrolein to propanal was exhibited by Ni28Ti72(71). In contrast, nonalloyed nickel catalysts (ultrafine nickel particles and Ni on alumina) were active for the hydrogenation of both C=C and C=O double bonds and, therefore, for catalyzing the formation of 1-propanol. The amorphous Ni28Ti72 showed higher activity than the crystallized alloy (35% and 11% conversion at 383 K, respectively) but the same high selectivities as the crystallized alloy, indicating that it is the alloying effect of titanium and not the amorphous structure that is responsible for the outstanding selectivity. Both alloys were inactive for the hydrogenation of propanal under the reaction conditions up to 473 K, which is evidence used to rule out the possibility of an

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ensemble effect and to favor the ligand effect as the explanation of the observed selectivity.

2. Hydrogenation of Alkynes Selective hydrogenation of acetylenic bonds to olefinic bonds is an important transformation for both practical and theoretical reasons. The early results on the use of metallic glasses for this reaction indicate remarkable characteristics. Amorphous and crystalline Fe-Ni-P-B alloys show fairly high activity for the hydrogenation of ethyne (42). The activity and selectivity of the amorphous alloy is greater than those of the crystalline alloy. Added chromium results in decreased activity, but in this case it is the crystalline samples that exhibit slightly higher activity. The differences are rather small; for example, selectivities lie in the range of 85-91%. The main feature of Cr revealed by DSC is stabilization against crystallization. Excellent selectivity was observed in the liquid-phase hydrogenation of phenylacetylene on Pd,&3iz0(135). Similarly, both amorphous and crystalline catalysts showed highly selective syn-addition in deuteriumation experiments (97 and 95%, respectively) (135). Detailed studies on the hydrogenation-deuteriumation of different acetylenic compounds on amorphous and crystalline palladium catalysts supported the above findings (73). All rapidly cooled catalysts (Pd80Si20 and Pd77Ge23 formed by the shock-tube and hammer-and-anvil methods) exhibit similar high semihydrogenation selectivity (94-98%) for the hydrogenation of all compounds studied and similar high stereoselectivity for the hydrogenation of 4-octyne (Table X).Neither crystallization of the amorphous catalysts nor the use of hydrogen instead of deuterium change selectivities. On the basis of the preceding results as well as the observations of other authors, who hydrogenated acetylenic compounds on different supported, highly dispersed and on unsupported palladium catalysts (153-155). it is suggested that the high selectivities of the rapidly cooled alloys is due to sites of very high coordinative unsaturation such as disordered Pd atoms and isolated Pd atoms. These kinds of sites may be the 3M and/or 3MH Siege1 sites (237),the presence of which on Pd-Si and Pd-Ge glasses was suggested earlier (68, 69). During the hydrogenation of both phenylacetylene and 1-octyne, comminution of the rapidly cooled samples and of the Pd foil was observed. In contrast to (+)-apopinene, which revealed significant changes of surface morphology, no changes could be detected in acetylene hydrogenation selectivities. As discussed earlier (see Section III,C,l), the effect of the terminal alkynes on the rapidly cooled materials is to expose new sites of

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TABLE X Selectivities of Semihydrogenation of Alkynesa,b PhCECH selectivityc

Alloy PdmSi20

Pd7Gez3

1.60 1.62 1.62 + HT' HAPS 2' 1.01 1.01

1.01 I .01 5.1

I-Octyne selectivity

4-Octyne selectivity cltd

93.9 97.7 97.7 %.6 %.9

95.3 96.3(H)8

97.7h 94.5(H)B 95.7

96.0

98.7 97.0 97.9 100.O(H)8

90110 87113 87/13 93/7

Data from Ref. 7.3.

* 298 K, atmospheric pressure, solvent = heptane. % ene x 100 at 1.0 mol hydrogen uptake. % ene % ane Ratio of cis-4-octene to trans4octene at 1 .O mol hydrogen uptake. Heat treatment = 788 K, 24 h, in Ar.

Selectivity =

+

'All catalysts were formed by the shock-tube method, except HAPS 2, which was prepared by the hammer-and-anvil technique. 8 Reactions in hydrogen instead of deuterium. The fifth consecutive reaction on the same catalyst sample.

greater order and lower coordinative unsaturation. These sites do not influence alkyne reactions but do influence the rates of different alkene reactions. The most remarkable result of terminal acetylene treatment is the activation of inactive palladium catalysts (see Section 11,D). 3 . Hydrogenation of Other Organic Compounds

Brooks et al. (58) studied a special method for improving the hydrogenation activity of Raney-type nickel catalysts. It is known that A13Ni enrichment enhances the catalytic activity of Raney Ni, and it has been suggested that the so-called heat-soak treatments of Ni-A1 alloys, precursors in the preparation of Raney Ni, lead to enrichment of the A13Ni phase (156, 157). Therefore, they conducted experiments that revealed an inverse correlation between the AI3Ni primary dendrite spacings and the cooling rate. Alloy powders were prepared by rapid solidification in helium (RSR atomization) with a cooling rate of about lo5 K s-'. Heat-soak treatment ( 1 120 K, 10 h, fluidized bed) led to enrichment of an A13Ni phase in the alloy without crystallite size enlargement. As a result, a more efficient aluminium removal could be reached by caustic extraction acti-

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TABLE XI Reaction Rates (mol g-' min-I) for Hydrogenation of Organic Compounds over Different Raney Ni Catalysts" Alloy

Acetoneb

Nitrobenzeneb

Butyronitrile*

DextroseC ~~~~

Commercial, 49 wlo Ni RSR, 28.5 wlo Ni RSR, 42 wlo Ni

Toluene' ~

0.18

0.066

0.01 I

0.271

0.475

0.226

0.239

0.242

1.246

0.64

0.258

0.277

Data from Ref. 58. 299 K , 87 kPa constant overpressure. 353 K, 405 kPa initial pressure.

vation, a crucial point in Raney Ni preparation. The catalysts thus prepared were used for the hydrogenation of acetone, nitrobenzene, itaconic acid, butyronitrile, toluene, and dextrose. A sample containing 28.5 w/o Ni exhibited superior reaction rates over commercial bulk-cast Raney Ni by factors ranging from 2 to 20 for the reactions given above. The greatest specific activity was observed on an RSR, heat-soaked catalyst with 42 w/o Ni, which is near the peritectic composition and produces a 100% AbNi phase (Table XI). A Japanese patent (126) discloses the preparation of amorphous materials with diverse compositions and their resulting activities for different transformations. One of these specimens, a Ni49AIS&rI alloy, is active in the hydrogenation of thymol to menthols at high pressure (10.1 MPa, 403 K). Treatment with hydrogen (523 K, 1 h) increases the selectivity of the formation of 1-menthol (71 versus 54%), whereas a 2-h heat treatment at 873 K decreases markedly the activity (96.7-10.2%).

D. OTHERTRANSFORMATIONS 1. Hydrogenolysis The Otsuka Chemical Company patent (226)also contains data on the use of amorphous metals in hydrogenolyses. Ni-Cr on K2C03plus Ti02 and Ni-Co on SiO2 catalysts, prepared by ion sputtering, exhibited 65.6 and 96.4% conversions, respectively, in the reaction of 1-hexanol to nhexane. The Ni-Cr alloy showed almost no activity after annealing at 1273 K. The hydrogen-treated Ni49AI&rl catalyst brought about total conversion of methylamine to methane and ammonia (373 K, 20 atm, methylamine : hydrogen = 1 : I), but its activity decreased to 14.3% after the usual heat treatment.

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573

37 1

TI3 Ternpemture ( K 1

613

FIG.14. The effect of heat treatment on the catalytic activity of Ni6>Bnin hydrogenolysis (0,ethane; 0, cyclopropane). (Reprinted from Ref. 23 with permission of The Royal Society of Chemistry.)

The effect of stepwise heat treatments on the activity of NiszB38 was studied in the hydrogenolysis of ethane and cyclopropane (23, 41). The reaction rates of the hydrogenolysis of both compounds show maxima on samples heat-treated at 623 K and sudden decreases on samples heattreated above 623 K (Fig. 14). The explanation for the changing activities has already been given in connection with the hydrogenation of olefins (see Section III,C, 1). For hydrogenolysis the formation of more electrondeficient nickel species was found to be responsible for the increasing activity. However, the decreasing activity occurring on catalysts heattreated above 623 K was correlated with the decreasing number of active species as shown by hydrogen chemisorption, SEM,and ESCA. Additionally, the authors point out that the relative catalytic activity of hydrogenolysis to hydrogenation shows a large increase at the higher heattreatment temperatures (Fig. 15); that is, hydrogenolysis is less affected by heat treatment.

m

673 773 Temperature ( K I

FIG.IS. Change of relative rates of hydrogenolysis of ethane (0)and cyclopropane (0) against the rate of hydrogenation of ethene as a function of thermal treatment of Ni6?B3*. (Reprinted from Ref. 23 with permission of The Royal Society of Chemistry.)

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This latter was accepted as evidence for possible aggregation, since some literature data indicate that hydrogenolysis, being a structure-sensitive reaction, involves multiple adsorption (158-161). However, two recent review articles on particle size effects on metal catalysts (162, 163) emphasize the inconsistency of the rather scarce data on nickel and warn of the difficulties connected with this problem. These data, although scarce, support the conclusion that the structure in the precrystallization state is the most favorable for catalytic activity, but detailed understanding of the phenomenon requires further clarification. 2. Transformations of NO

In the decomposition of NO Nigo-,Fe,Pzo amorphous alloy ribbons showed a high initial activity followed by a decrease and establishment of a steady state (264). The process turned out to be an oxidative dissociation with the rate-limiting step being the oxidation of the amorphous surface. During NO decomposition the BET surface area of FegoPzo increased from 0.2 to 0.8 m2 g-'. With increasing temperatures NiaFezoPzo showed a sudden decrease of activity at 636 K, which is the glass transition temperature of the alloy. A sample crystallized in helium at 773 K exhibited activites similar to the alloy crystallized in situ. The data do not permit one to decide whether the low activity of the crystalline materials is due to the slower diffusion of oxygen or the lower activity of the surface for inducing dissociation. The same catalysts were studied in the reduction by hydrogen of NO to Nz and NH3 as well (125, 164). The most active NigoPzo alloy showed an activity 10 times higher than the crystalline catalyst and exhibited interesting changes in activity. The initial high activity first decreased markedly, then increased, and finally, after about 2.5 h at 523 K, reached a steady-state activity as high as the initial activity. Although a satisfactory explanation is not given, the presence of a relatively large amount of N 2 0 in the product mixture during the period of activity change might indicate partial oxidation of the surface. Similar product compositions (90% Nz plus N20 and NH3) with high (>80%) conversions were attained on Ni-Fe-Zr alloys. Pt70Z1-30was also shown to be active in the reduction of NO with CO at 523 K, giving a 9 : I N2-N20 mixture at 93% conversion (55). 3 . Ammonia Synthesis The alloys studied in the ammonia synthesis are Ni-Zr and FeglZr9(22, 49,112, 252). With these catalysts, marked increase in BET surface areas

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and continuous activity increases were observed during the reaction (22, 49). This behavior also is characteristic of zirconium-containing amorphous metal alloys in the hydrogenation of CO (see Section III,B,3). In the case of the ammonia synthesis the activation process continued until about 1000 h on stream. In contrast, the crystalline FeglZr9alloy showed a continuous decrease of activity, reaching a steady-state activity similar to that of the amorphous material (112) whereas a steady deactivation was observed on the crystalline Ni-Zr alloys (22). The results of long experiments (several thousand hours) indicate that both the amorphous and crystalline Fe-Zr alloys transform to a similar active surface state under the influence of the reacting gas mixture. The turnover rates of the catalysts prepared from the amorphous Fe-Zr precursor after pretreatments with different hydrogen-nitrogen mixtures at 653-723 K were one order of magnitude higher than those of a polycrystalline iron ribbon prepared by melt-spinning. XRD patterns taken at different reaction times revealed the simultaneous formation of both large and small iron particles during the activation period (49). SEM showed the segregation of irou on the surface in the form of small iron particles that were found to be related to high activity. This segregated iron resulted in an increase in BET surface area and was considered to be stabilized by the zirconium oxide present. Gradual increase of crystallinity was also observed on amorphous Ni24Zr76 and Ni64Zr36(22). The catalyst bed after reaction contained two distinct zones. Large Ni crystallites (XRD) and surface Zr02(XPS) were detected in a black, inactive bed entrance zone, but no zirconium nitride and almost no metallic zirconium were found. In contrast, an active, golden zone showed only traces of crystallinity (XRD) and had a ragged surface (SEM). It contained small, strongly disordered Ni particles, NiO, zirconium oxide, and zirconium nitride. Nonstoichiometric Z I O - ~was prevalent on the surface, whereas the subsurface region contained metallic Zr as well. The amount of ZrN increased with depth. The high activity was attributed to the presence of the small Ni particles embedded in the zirconium oxide-zirconium nitride matrix. Nitrogen adsorption measurements indicated the coexistence of molecularly and dissociatively adsorbed nitrogen, the latter reacting with added hydrogen to form ammonia. These observations, together with kinetic data, prove that dissociatively adsorbed nitrogen is the most important reaction intermediate in ammonia synthesis. A remarkable new observation is also attributed to the presence of the amorphous structure in connection with the ammonia synthesis (165). In situ X-ray diffractograms showed the presence of a-iron in catalysts prepared from unpromoted Fe304after treatment with hydrogen at 723 K. In contrast, however, promoted industrial (BASF and ICI) ammonia synthe-

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sis catalyst precursors produce almost fully reduced catalysts with very broad diffraction peaks as a result of the actior, of either hydrogen or the reacting gas mixture at about 700 K. The XRD patterns are very different from those of finely divided and paracrystalline a-iron but show remarkable similarity to those of certain iron-containing glassy alloys. These data, as well as other unpublished data (Mossbauer spectroscopy, adsorption and kinetic studies, electron microscopy), indicate that the catalytically active phase of these reduced, promoted catalysts is amorphous iron, interspersed with microcrystalline promoter phases (e.g., FeA103). However, further study will be required to elucidate the exact nature of noncrystallinity in the active catalysts. 4. Decomposition of Formic Acid

Because of the milder reaction conditions (lower temperature) the decomposition of formic acid is found to be more sensitive to catalyst structure than the hydrogenation of carbon monoxide (20). Although increasing cooling rate (increasing disk velocity) has almost no effect on catalyst performance, opposite selectivites were observed on amorphous and crystalline FewBzoalloys (27). On the amorphous catalyst the main transformation is dehydrogenation of formic acid to form C 0 2 , whereas on crystallized samples excellent selectivities are exhibited for dehydrogenation. (At 458 K, CO is formed with 100% selectivity.) But during this latter process it is not clear whether HzO is also detected, a feature that seems characteristic only of Ni (166). XPS revealed substantial differences between the structure of the amorphous and the crystalline Fe-B alloys. The amorphous Fe-B, in which small iron ensembles are separated and stabilized by iron and boron oxides, can be considered an oxide-supported, highly dispersed iron catalyst. In contrast, on the crystalline alloy large iron particles exist. At present there is no explanation for the structure-selectivity correlation; also, in a similar manner, the variables influencing the formation of C02 and CO on Fe(100) were not determined in earlier studies (167). IV. Conclusions

A. MAIN ACHIEVEMENTS Despite the rather short period since the attention of catalytic chemists turned to metallic glasses as catalysts, there have already been a remarkable number of publications, and there are already some rather well-studied reactions and catalysts. These studies led to the identification of the probable active phases of certain transformations, disclosed the way they

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form, and permitted explanations of how they act in the catalytic process. Certain correlations could also be established between activity, selectivity, and stability, and in some reactions, the superiority of the amorphous state over the crystalline catalysts could be explained. In electrocatalysis the use of metal-zirconium alloys in water electrolysis is a good example. It seems to be well established that the very high electrocatalytic activity of the HF-treated amorphous Ni and Cu electrodes is related to the presence of a rough, Raney-type surface structure. The beneficial effect of the amorphous structure is that it ensures the ready formation of a thick, stable, active surface layer. Another thoroughly and widely studied field is the hydrogenation of carbon monoxide. Although a large number of investigations have been performed on many different catalyst compositions, only the zirconiumcontaining alloys and the Fe-(Ni)-B system yield definite and general conclusions. The continuous activity increase of the zirconium-containing alloys during reaction is correlated with the formation of a porous, highly active surface. The active phase for the CO hydrogenation reaction proved to be fine metal particles that are embedded in the ZrOz matrix. Results on the Fe-(Ni)-B system also indicate that the amorphous alloy as a precursor ensures the formation of quasi-crystalline (near-crystalline) structures. These otherwise nonexisting phases are stabilized by the amorphous matrix and may have very specific catalytic properties. A similar phenomenon turned out to be the case in the synthesis of ammonia catalysed by Ni-Zr and Fe-Zr. The advantage of these catalysts is that the amorphous structure is the precursor for generating a controlled, uniform dispersion of active species in high concentration in a chemically homogeneous environment. Progress has been made in overcoming some of the structural and stability problems inherent in metallic glasses. The low-surface-area problem has been attached by several chemical and physical roughening techniques. The problem of shaping the glass has been circumvented by adhering a ribbon of the crystalline alloy to a Ni plate and then converting the alloy surface to glass by the laser melting-self-quenching technique. Moreover, the stability problem is receding as more glasses are examined under reaction conditions and some are found to be remarkably stable. Disconcertingly, others are unexpectedly unstable under reaction conditions even though their T, are relatively high. B. FUTURE PROSPECTS One drawback of amorphous catalysts is their metastable character, which can hinder their high-temperature use. There are methods, how-

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ever, which are suitable for improving the thermal stability of amorphous materials. Alloying of properly selected components can result in amorphous alloys with either increased T, values or increased activity, which permit low-temperature applications. Overcoming the stability problem can increase the possibilities for the use of these materials as catalysts. Clearly, this is an important subject for future research. Another subject for future exploration is the surface area of metallic glasses. Ribbons produced by rapid quenching have surface areas of little larger than the geometric surface; that is, the specific surface (surface per weight) is very low for industrial applications. This problem may be overcome by some amorphous alloys prepared by new methods developed recently. In turn, different atomization techniques (gas, liquid, centrifugal, electrohydrodynamic atomizations) and comminution methods (milling and chopping) produce amorphous powders with higher specific surfaces. Powders or films prepared by either chemical deposition or electrodeposition also deserve higher attention. Although results on chemically deposited Ni-B and Co-B amorphous alloys are available (168-171), clarification of the exact nature of their structure and further examination of their catalytic activity are needed to gain insight into the structure-activity relationship and to compare the effect of preparation methods of amorphous alloys on their catalytic properties. Chemical deposition of alloys onto suitable carriers might produce supported amorphous alloys similar to the traditional, high-surface-area supported catalysts. The development of amorphous alloys with compositions specially designed for catalytic applications is another field for future work. Most amorphous alloys studied in catalysis have been those that were prepared because of their outstanding physical properties for which they might even have had technical applications as such. However, a carefully planned and executed project could result in amorphous catalysts with the proper combination of alloying elements in suitable ratios to ensure high thermal and chemical stabilities as well as high catalytic activity and selectivity for a given transformation. A good illustration of this strategy is the development of the multicomponent Pd-Ir-Ti-Rh(Ru)-P alloys, electrode materials for the electrolysis of sodium chloride. Investigations started with Pd-P; then, with the addition of other components, gradual improvements of different properties were attained, eventually reaching compositions with excellent corrosion resistance and an activity surpassing that of the traditional RuOz/Ti electrode. Continuing studies using surface characterization techniques are necessary for better understanding of amorphous catalysis. The comparison of amorphous and heat-treated catalysts can reveal what changes other than

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crystallization (changes of nominal composition, phase separation, segregation, chemical transformations) can possibly give rise to the large differences in activities. Analysis of the catalyst surface before and after reaction can not only give information about the stability of the amorphous state but can also detect the exact nature of changes taking place on the action of the reacting chemicals. The interpretation of the continuous activation or deactivation of certain alloys during reaction is naturally intriguing, but a search for minute alterations of the surface structure and the recognition of their effect on catalytic properties are essential for understanding such unstable systems as amorphous alloys. Similar studies on catalysts that require activation can disclose the formation of the activated state and can reveal the nature of active sites formed. Besides the modern physical methods, the use of sterically complex organic probe molecules is also an excellent method for characterizing amorphous alloys, and, in turn, the amorphous structures are likely to be useful for modeling catalytic surfaces with high concentrations of unorganized coordinatively unsaturated sites. For example, the idea of active sites consisting of clusters of atoms (172-177)may be examined on metallic glasses. First it was Kobozev who suggested that the catalytically active centers are atomic in nature. The active centers consist of amorphous ensembles of a few metal atoms that do not form a crystal lattice. It follows from this observation that the ensemble theory is in serious contrast with those theories that ascribe the catalysis to the crystal phase. Similar to the Kobozev adsorption catalysts and alloy catalysts, in which the active metal is present in very low concentration, metallic glasses seem to contain the active centers in ensemble form; they are, therefore, intermediate between the molecular and the crystalline systems, that is, between homogeneous and heterogeneous catalysts in the classical sense. Finally, it seems reasonable that amorphous alloys will find applications in a broader range of catalytic reactions. In fact, their unique activities may render them suitable catalysts for reactions heretofore not ordinarily considered in the realm of catalysis. ACKNOWLEDGMENTS We gratefully acknowledge National Science Foundation Grant INT-8403357 and Hungarian Academy of Sciences grant 319/82/1.3 for support of our collaboration. REFERENCES

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167. Benzinger, J. B., and Madix, R. J . , J . Cafal.65, 49 (1980). 168. Wade, R. C., Holah, D. G . , Hughes, A. N., and Hui, B. C., Carol. Rev.-Sci. Eng. 14, 211 (1976). 169. Schreifels, J. A., Maybury, P. C., and Schwartz, W. E., Jr., J . Card. 65, 195 (1980). 170. Uken, A. H . , and Bartholomew, C. H., J . Cafal.65, 402 (1980). 171. Rosier, D . , Dallons, J.-L., James, G . , and Puttemans, J.-P., A c f a Chim. Hung. W, 57 ( 1987). 172. Kobozev, N. I., Zh. Fiz. Khim. W , 1 (1939). 173. Kobozev, N. I., and Klyachko-Gurvich, L. L., Zh. Fiz. Khim. 13, 27 (1939). 174. Kobozev, N. I., U p . Khim. 25, 545 (1956). 175. Balandin, A. A . , Adv. Cafal. 10, 96 (1958). 176. Balandin, A. A , , Adv. Caral. 19, I (1969). 177. Sachtler, W. M. H . , Faraday Discuss. Chem. SOC.72, 7 (1981).

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ADVANCES IN CATALYSIS, VOLUME 36

Addendum to Structure and Reactivity of Perovskite-Type Oxides LUIS G. TEJUCA AND JOSE LUIS G. FIERRO lnstituto de Catdlisis y Petroleoquimica C.S.I.C. Serrano 119, 28006 Madrid, Spain

AND

JUAN M. D. TASC6N Instituto Nacional del Carbdn y sus Derivados C.S.I.C. Apartado 73, 33080 Oviedo, Spain

Recently, some interesting publications appeared on catalytic properties of perovskite oxides, including perovskite superconductors, in oxidation and reduction processes. Hibbert and Campbell (311) studied the interactions of SO2 and CO with Lal-,Sr,Co03 and used these perovskites for the catalytic removal of SO2 by CO in the interval 500-600°C. In the used catalyst, sulphides of each metal predominate, with no perovskite oxides or perovskite sulphides being detected. The catalyst with x = 0.3 gave the optimum removal of sulphur with no COS formation. France et al. (312) used a series of Lal-,A,Mn03 (A, an alkaline metal or a vacancy) oxides at 820°C as catalysts for the oxidative coupling of methane. Higher binding energies for oxygen were correlated with higher selectivities for C2 hydrocarbons. Nagamoto ef al. (313) studied AB03 oxides (A, alkaline earth metal; B = Ti, Zr, Ce) at 750°C for this transformation. The catalytic activity for C2 products was found to decrease in the order Ba > Sr > Ca and increase with increasing values of a parameter (Ad) which is a measure of the deviation of cations and anions from the ideal equilibrium distances. YBa2Cu30, (x = 6.88) was reported to retain large amounts of NO at 300°C (314). This effect was explained by absorption into the solid and/or 385

Copyright 8 1989 by Academic Press. Inc. All rights of reproduction in any form reserved.

386

LUIS G . TEJUCA

et al.

reaction with the bulk to form metal nitrates. In addition, this superconductor catalyzes the NO decomposition in the presence of oxygen (315). The oxygen deficiency of the sample seemed to influence strongly its catalytic activity. A similar superconductor (6.5 Ix I6.9) was used by Mizuno et al. (316) for the reduction of NO by CO at 300°C. Hansen et al. (317) reported data for toluene oxidation in the presence of oxygen and ammonia on YBazCu@6+, at 400°C. At low fa the catalyst is very active for mild oxidation to benzonitrile, whereas at high Po, deep oxidation to COz is preferentially catalyzed. This transition in product selectivity is reversible and occurs at Pa = 8.65 x lo3Pa. At transition conditions, the catalyst composition was found to be close to YBazCu306 (x = 0). An increase in the content of lattice oxygen (x > 0) makes the catalyst selective for COZ formation. REFERENCES 311. Hibbert, D. B., and Campbell, R. H., Appl. Card. 41, 273, 289 (1988). 312. France, J. E., Shamsi, A., and Ahsan, M. Q., Energy Fuels 2,235 (1988). 313. Nagamoto. H., Amanuma, K., Nobutomo, H., and Inoue, H., Chem. Lett. p. 237 (1988). 314. Tabata, K., Fukuda, H., Kohiki, S., Mizuno, N., and Misono, M., Chem. Letr. p. 799 ( 1988). 315. Tabata, K., and J. Mater, Sci. Lett. 7, 147 (1988). 316. Mizuno, N., Yamato, M., and Misono, M., Chem. Commun. p. 887 (1988). 317. Hansen, S., Otamiri, J., Bovin, J.-O., and Anderson, A., Nature (London) 334, 143 ( 1988).

A

ammonia synthesis, 372-374 characterization methods, 332-333 composition, 329-331 crystallization and activity, temperature effects, 334-336 electrocatalysis as electrodes in fuel cells, 342-344 sodium electrolysis, 339-342 water electrolysis, 336-339 formic acid decomposition, 374 future prospects, 375-377 hydrogenation catalysis Of alkynes, 368-369 of (+)-apopinene, 358-362 of 1.3-butadiens 365, 367 of carbon monoxide dissociative chemisorption, 344-345 over nickel(iron)-phosphorus(b0ron) catalysts, 346-351 over zirconiumcontaining catalysts,

Acetons hydrogenation over Rnney-type nickel catalysts, 370 Acetylene conversion to benzene over palladium, 23 hydrogenation over platinum, particle size and, 111 Acrolein, hydrogenation over amorphous nickel-titanium, 367 Actinides, incorporation into perovskites, 315-316

AES,see Auger electron spectroscopy Alcohols, conversion over perovskites adsorption, 278-279 dehydrogenation, 309-311 oxidation, 289 Alkylens. hydrogenation over amorphous and crystalline alloys, 368-369 Alloys amorphous, see Amorphous metal alloys singlecrystal surface cyclohaane dehydrogenation over copper-ruthenium, 41-43 n-hexane hydrogenation over goldplatinum, 40-41 metal particle size effects, 80-82 properties. 39-40 Alumim-~~pported catalysts, 46,228 Ammonia, oxidation over perovskites, 307 Ammonia synthesis over amorphous metal alloys, 372-374 over iron, 24-25 on alumina support, 47 antipathetic behavior, 150, 152 particle size and, 131-132 promotion by potassium, 36-37 photocatalysis over perovskites, 304 over rhenium, 24-25 promotion by potassium, 37 Amorphous metal alloys, see a h specifi alloys activation methods. 333-334

333, 352-357, 375

of cfs-cyclododecene, 358 of 1-huene, 358, 359 of olefins, 362-367 Raney-type nickel catalyst activities, 369-370

hydrogenolysis catalysis, heat treatment and, 370-372 preparation methods, 331 (+)Apopinenenq hydrogenation over amorphous palladium alloys catalyst preparation methods and, 358-360 catalyst purification methods and, 360-361 terminal alkyne effects. 361-362 AR, see Atomic rate of reaction Atomic rate (AR) of reaction definition, 72-73 structure sensitivity and, 86-87 Auger electron spectroscopy (AES) small metal particles. 108, 109 titania-supported catalysts, 203-205, 208-209

UHV analysis and, I, 4, 11 387

388

INDEX

B Benzene hydrogenation metal particle size and, 114-116 TPD on rhodium/titania and platinum/titania. 210 oxidation over perovskite, 288 production from cyclohexane over platinum, 18 Bimetallic catalysts, see Alloys Boron in iron-boron amorphous alloys, 344, 349-350

in nickel-boron amorphous alloys, 365-366

Brunauer-Emmet-Bller (BET)surface, amorphous alloy catalysts, 332 ammonia synthesis and, 372-373 carbon monoxide hydrogenation and, 352-354

nitric oxide decomposition and, 372 1,3dutadiene, hydrogenation over amorphous alloys copper-zirconium activity, 367 nickel-boron activity, 365 over platinum, 20-21 nlutane, hydrogenolysis, metal particle size effect, 120 1-Butene, hydrogenation over palladium/alumina, TOF, 113-114 1-Butyne, hydrogenation over palladium/alumina, TOF, 113 Butyronitrile, hydrogenation over b e y type nickel catalysts, 370 C

Calcination, in precursor-support interaction, 70-71 Carbon dioxide, adsorption on perovskites, 273, 275-276

Carbon monoxide adsorption on perovskites, 273-375 chemisorption on supported metals, 95 IR spectroscopy, 103 structure sensitivity, 138-140 titania coverage studies, 206-209 TPD studies, titania-supported metals, 186-190, 209-212, 224-225

hydrogenation. see also Fischer-ltopsch reactions, methanation reaction

over amorphous metal alloys dissociative chemisorption, 344-345 nickel(iron)-phosphorus(boron) activities, 346-351 zirconium-containing alloy activities, 333, 352-357, 375

over pemskites, 2%-298 over single crystals, 26-27 sulfur addition and, 30 over supported metals chloride effect, 201-202 ensemble effect, 149-150 metal-support interaction, 44-46 nickel-copper catalysts, 197-198 particle size and, 128-131 rhodiudtitania, reduction temperature effect, 196-200 ruthenium-based catalysts, 87 titania coverage effect, explanations, 214-220

oxidation over perovskites catalytic activity, 279-281 kinetics on lanthanum cobaltate, 281-283

mechanism of, 282-283 pollution control and, 291-293 sulfur dioxide poisoning effect, 313-315 over single crystals, 24-26 over supported metals, particle size and, 133-136 sulfur dioxide reduction over perovskites, 298-300

Carbonyl sulfide, production over pemskites, 299 Ceramic method, perovskite preparation, 245-246

Ceric oxide-supported catalysts, 226, 229 Cesium, metal catalyst promotion, 37-39 Chemisorption, on supported metals of carbon monoxide, 95 structure sensitivity, 138-140 of hydrogen, structure sensitivity. 91-95 Chlorides, effects on titania-supported catalysis, 201-202 Chlorine electrolytic evolution on amorphous metal alloys, 339-342 metal catalyst activity and, 31-34 Chromium, effect on amorphous and crystalline alloys. 368

389

INDEX Cobalt catalysts Fischer-Tropsch reaction, potassium effect, 36 supported, selectivity. 88 Copper-based amorphous alloys formaldehyde oxidation in fuel cells,

Dextrose, hydrogenation over Raney-type nickel catalysts. 370 Diffuse reflectance spectroscopy. unreduced ion detection in metal catalysts, 103 Dry evaporation, perovskite preparation. 246-247

343-344

zirconium-containing, hydrogenation catalysis, 366-367 Copper catalysts deposition on zinc oxide, SMSI, 46-47 effect on nickel activity, 197-198 effect on ruthenium activity, 41-43 single crystals, 27 poisoning by sulfur, 31 promotion by cesium, 37-38 Copper/zinc oxide catalysts, SMSI. 46-47 Coprecipitation. perovskite preparation, 247-250

Cordierite-supportedperovskites. preparation impregnation, 251-253 plasma spraying, 253 Cubooctahedron, small supported particle model, 80-81 replacement by icosahedron, 85 Cis-Cyclododecene, hydrogenation-deuteriumation over amorphous alloys, 358 Cyclohexane, dehydrogenation benzene accumulation over platinum, 18 stepped surfaces and. 82-83 metal particle size and, 115. 117 over rhodium/titania. reduction temperature and, 191-193 over ruthenium, copper addition and, 41-43 Cyclopentane, hydrogenolysis, metal particle size and. 121-124 Cyclopropane conversion over perovskites, 294 hydrogenation over metal systems, particle size and, 111-112 hydrogenolysis over nickel, 21-22 over nickel-boron amorphous alloys, heat treatment, 371 over iridium, 23

D Deuteriumation, of hydrocarbons over metal alloys. 358 over platinum catalysts, 19-20

E Electrocatalysis amorphous metal alloy activities as electrodes in fuel cells, 342-344 sodium chloride electrolysis, 339-342 water electrolysis, 336-339 perovskite activities oxygen cathodic reduction, 300-301 oxygen evolution, 302-303 Electron microscopy, small metal particle assay, 95-96 Electron paramagnetic resonance (EPR) metal particle size distribution, 99-100, 104 oxygen adsorption on perovskites, 273 poisons on metal surface, 108 Electron spin resonance (ESR), see Electron paramagnetic resonance Ensemble effect carbon monoxide hydrogenation and, 149-150

definition, 146-147 hydrogenolysis of small hydrocarbons and, 150 structure insensitivity and, 153-154 EPR. see Electron paramagnetic resonance ESR, see Electron spin resonance Ethane adsorption on perovskites, 278 hydrogen01ysis over iridium, 21-22 metal particle size effect, 117-120 over nickel catalysts boron effect in amorphous alloys, heat treatment and, 371 TOF maximum versus FE, 152-153 on tungsten support, 43 over rhodium/silica, 153 over rhodium/titania. SMSI, 194 over tungsten, 23 Ethene, see Ethylene Ethylene adsorption on perovskites, 278 combustion over perovskites, 287

390

INDEX

hydrogenation over amorphous metal alloys copper-zirconium activity, 366-367 nickel-phosphorus activity, 362-364 over perovskites, 293-295 over platinum/silica, particle size and, 110 oxidation over silver catalysts, 15-16 poisoning by chlorine, 32-34 promotion by cesium, 37-39 support effects, 90, 136-138 Ethylidine, hydrogenation over rhodium, 20 EXAFS, see Extended X-ray absorption fine structure Exhaust gas, purification by perovskites, 291-293

Explosion, perovskite preparation, 250 Extended X-ray absorption fine structure (EXAFS) analysis, 14 rhodium-titanium interaction, reduction temperature effect, 221,223 small metal particle assay size calculation from, 94 Structure, 108-109

F FE, see Fraction exposed, of total metal atoms Ferromagnetic resonance (FMR), metal particle size distribution, 99-100, 104 Fischer-ltopsch reaction over lanthanum rhodate, temperature effects, 297-298 potassium-promoted over cobalt catalysts, 36 over iron catalysts, 35 over supported cobalt, selectivity, 88 FMR, see Ferromagnetic monance Formaldehyde, oxidation on amorphous alloy electrodes, 343-344 Formic acid, decompoiition over amorphous metal alloys, 374 Fraction exposed (FE), of total metal atoms definition, 72-73 effects on turnover frequency in hydrocarbon conversion hydrogenation, 110-115 hydrogenolysis, 118-123 higher than 0.5, in metal clusters, 142-146 lower than 0.5, sympathetic behavior and, 151-152

selectivity as function of, 88, 124-127

wide range, antipathetic behavior and, 146-151 Freeze-drying, perovskite preparation, 247, 249-251 G Gas chromatography (OC), in UHV surface analds, 9-13 OC, see Gas chromatography

Oibbs free energy, perovskite formation and, 262-263, 265 Oold, platinum activity changes by. 40-41

H n-Hcxane, conversion over platinum aromatization, 16-18 gold addition and, 40-41 particle sue and, 85 1-Haanol, hydrogenolysis over amorphous alloys, 370 I-Harene, hydrogenation over amorphous and crystalline alloys, 358, 359 High-temperature reduction (HTR), effects on titania-supportedmetals, 176177, 180 carbon monoxide hydrogenation, 196-200 chemisorption of carbon monoxide and hydrogen, 184-190, 224-225 hydracarbon hydrogenation, 190-196 titania migration onto metal, 203-206 Homomolecular exchange of oxygen, over perovskit-, 305-306 HTR, see High-temperature reduction Hydrocarbons, see o h specific compoundr adsorption on perovskites, 278 conversion over single crystals iridium and, 21-23 nickel and, 21-22 platinum and, 16-21 rhodium and, 20 silver and, 15-16 tungsten and, 23 hydrogenation metal particle size effects, 110-117 over perovskites, 293-295 hydrogen01ysis ensemble effect, 150 metal particle size effects, 117-128 over perovskites, 293-294 OVCT rhodium/titania, after HTR, 190-196 oxidation over perovskites, 283-289

391

INDEX Hydrocracking reactions, over perovskites, 311 Hydrogen adsorption on perovskites, TPD and, 270 chemisorption on iron clusters, rate constants. 144-145 as method of catalyst characterization, 91-95

on titania-supported catalysts after HTR. 184-186, 194 TPD assay, 209-211, 214-217 consumption by metals, TPR and, 102 effect on SMSI, 220-221 electrolytic evolution on amorphous metal alloys, 336-338 oxidation m d particle size and, 132-133 over perovskites, 306-307 Hydrogen chloride etching, iron-boron amorphous activities and, 351 Hydrogen fluoride, amorphous alloy treatment, 333-334 formaldehyde oxidation in fuel cells, 343-344 water electrolysis and, 336-338

I Impregnation metals on supports, 62-63 perovskites on supports, 251-253 titania-supported catalyst preparation, 181-183

Infrared (IR) spectroscopy perovskitc adsorption studies, 274-278 sulfur dioxide effects, 312-313 poison detection on metal surface, 107-108

support-metal interaction, 102-103 Ion exchange, in supported metal preparation, 63-66 competitive. 65-66 oxide surface charge in suspension and, 64 simple, 64-65 titania as support, 182-184 Ionization potential, metal clusters, 144 Iridium/alumina catalysts, hydrogen chemisorption, 94 Iridium catalysts, hydrocarbon conversion, 21-24

Iridiudtitania catalysts, preparation, 70 Irodalumina catalysts, SMSI, 47

Iron-boron amorphous alloys carbon monoxide hydrogenation, 349-350 formic acid decomposition, 374 Iron catalysts ammonia synthesis, 24 antipathetic behavior, 150, 152 particle size and, 131-132 promotion by potassium, 36-37 clusters, hydrogen chemisorption, 144-145 deposition on magnesium oxide, SMSI, 46 foils, potassium-promoted, 35-36 preoxidation effect, 32 Iron/magnesia catalysts, SMSI, 46 Ironltitania catalysts, HTR, 177 Isobutene, oxidation over perovskites, 287-288

L Lanthanum cobaltate catalysts carbon monoxide oxidation, kinetics, 281-283

exhaust gas purification, 292-293 hydrocarbon hydrogenation, 294-295 reduction-oxidation, 266-267 XPS Studies. 267-270 toluene oxidation, 288-289 Lanthanum manganite catalysts, oxidative nonstoichiometry, 254-256, 258 Lanthanum oxide, in cordierite-supported perovskites, 252-253 Lanthanum rhodate catalysts, carbon monoxide hydrogenation, 296-297 Fischer-Ttopsch reaction, temperature effect, 297-298 Lead, perovskite activities and, 313, 314 Line-broadening analysis, metal particle size distribution, 97-98 Low-temperature reduction (LTR), effects on titania-supported metals benzene dehydrogenation, 210 carbon monoxide dehydrogenation. 1%. 198 chemisorption of carbon monoxide and hydrogen, 185, 187-188, 190 hydrocarbon dehydrogenation, 191, 194 LTR, see Low-temperature reduction

M Metal clusters decomposition, small particle preparation, 66-67 definition, 141

392

INDEX

supported, FE > 0, 5 electronic effects, 145 morphology, 146 structure sensitivity, 146 unsupported, FE > 0, 5 chemical properties, 144-145 electronic properties, 143 morphology, 144 size distribution, 142-143 Metallic glasses, see Amorphous metal alloys Metal-support interaction, see also

spec@ catalysts apparent, 174 electronic effects, 174-175 HTR and, see High-temperature reduction real, 175 strong, see Strong metal-support interaction TPD and, see Temperature-programed desorption Methanation reaction over amorphous alloys, 352, 354. 357 antipatheticsmcture sensitivity, 128-131,139 over nickel catalysts, 26 phosphorus effect, 29 sulfur effect, 29-30 tungsten support effect, 43 SMSI and, 44-45 over tungsten catalysts, 26 Methane, oxidation over perovskites, 284-286

Methanol oxidation on amorphous alloy electrodes, 342-343

sensing by perovskites, 316 Methylcyclopentane aromatization over platinum, 17-18 hydrogenation, metal particle size and, 127 hydrogen01ysis metal particle size and, 121-122 over rhodium/titania. reduction temperature and, 193, 195 selectivity as function of FE, 124-127 Microreactor, in UHV analysis compact design, 8 GC analysis, 9-13 sample mounting, 4-8 small-volume design, 9 Model catalysts, see aLr0 specflc catalysts alloy catalysts, kinetics, 39-43 definition, 1-2

high-pressure-UHV methods, 2-4 metal-support interactions, 43-47 structural sensitivity, 15-28 ammonia synthesis, 24 carbon monoxide hydrogenation, 26-27 carbon monoxide oxidation, 24-26 hydrocarbon conversions, 16-24 surface modification by additives alkali promoters, 34-39 electronegative and electroneutral elements, 28-34 UHV surface analysis, apparatus designs, 4-14; see also Ultrahigh vacuum surface analysis Molybdenum catalysts, 27 poisoning by sulfur, 31 Massbauer spectroscopy, metal catalyst assay, 101. 106, 177

N Nickel/alumina catalysts, SMSI, 46 Nickel-boron amorphous alloys, hydrogenolysis catalytic activity, 371 Nickel catalysts cluster compound decomposition. 66 cyclohexane dehydrogenation, particle size and, 117 deposition on alumina, lack of SMSI, 46 ethane hydrogenolysis particle size effect, 119-120 turnover frequency maximum versus

FE, 152-153 hydrocarbon conversion, 21, 22 methanation reactions, 26 phosphorus addition and, 29 sulfur addition, 29-30 potassium effects, 34-36 propane hydrogenolysis, particle size and, 119-120

Raney-type, organic compound hydrogenation, 369-370 Nickel-copper catalysts, hydrocarbon hydrogenation. HTR and, 191, 193 Nickel-iron amorphous alloys, with phosphorus or boron, carbon monoxide hydrogenation, 346-347 Nickel-metalloid amorphous alloys, hydrogenation catalysis boron and phosphorus role comparison, 365-366

393

INDEX of I,J-butadiene, 365 of olefins. heating effect, 362-365 Nickel-phosphorus amorphous alloys, carbon monoxide hydrogenation, 348-349

Nickel/titania catalysts carbon monoxide hydrogenation, 129 copper effect, 1%-197 SMSI, 44, 1% chemisorption of carbon monoxide and hydrogen, 215-217 reduction temperature effects, 190, 1% selectivity, 176 surface composition, 206 Nickel-titanium amorphous alloys, acrolein hydrogenation, 367 Niobia-supported catalysts, 227-228 Nitric oxide adsorption on perovskites, 273-275 decomposition over amorphous metal alloys, 372 reaction with carbon monoxide over rhodium catalysts, 24-25 reduction over perovskites, 289-291 pollution control and, 291-292 Nitrobenzene, hydrogenation over h e y type nickel catalysts, 370 Nitrous oxide, decomposition over peravskites, 307-309 NMR, see Nuclear magnetic resonance Nuclear magnetic resonance (NMR), metal particle size distribution, 101 0

Octahedron, small supported particle model, 76-79 I-Octyne, hydrogenation over amorphous alloys. 368-369 4-Octyng hydrogenation over amorphous

alloys. 368-369 Oxygen adsorption on perovskites. TPD and, 270-272

cathodic reduction over perovskites, 300-301

electrolytic evolution on amorphous metal alloys. 339 on perovskites, 302-303 homomolecular exchange, over perovskites, 305-306

P Palladium-based amorphous alloys hydrogenation catalysts of (+)-apopinene, 358-362 of carbon monoxide, 354-357 of ck-cyclododecene, 358 of I-hexene, 358. 359 phosphorus-containing, methanol oxidation in fuel cells, 342-343 sodium chloride electrolysis and, 339-342 Palladium catalysts acetylene conversion to benzene, 23 hydrocarbon hydrogenation, particle size and. 111. 113, 114 methanol synthesis, 26-27 poisoning by potassium, 32 Palladiudtitania catalysts, chernisorption of carbon monoxide and hydrogen, 188-189

Pentane, hydrogenolysis over platinum/silica, 90 Perovskite-type oxides, see also s p @ c lanthanum-based caralysa actinide storage in radioactive waste, 315-316

adsorption studies, temperature effects, 270-279

ammonia oxidation, 307 deposition on supports impregnation, 251-253 plasma spraying. 253 electrocatalysis, 300-303 history, 237-239 homomolecular exchange of oxygen, 305-306

hydrocarbon conversion, 293-295 hydrocracking reactions, 311 hydrogen oxidation, 306-307 methanol sensing, 316 nitric oxide reduction, 289-291 nitrous oxide decomposition, 307-309 nonstoichiometry oxidative, 254-256 oxygen deficiency and, 257-258 reductive, 254, 257-258 oxidation catalysis of alcohols, 289 of carbon monoxide. 279-283 of hydrocarbons, 283-289 photocatalysis, 304

394

INDEX

poisoning by lead, 313-314 by sulfur dioxide, 312-315 pollution control and, 291-293 preparation methods, 244-254 liquid-solid reactions coprecipitation, 247-250 dry evaporation, 246-247 explosion, 250 freeze-drying, 247, 249-251 Spraydrying, 247, 249-250 solid-solid reactions, 245-246, 249, 251 2-propanol conversion dehydration, 311 dehydrogenation, 309-311 prospective research, 318-319 reduction isothermal, 260-2452 phase transformation, 265-266 reversible reduction-oxidation cycles, 265-267

temperature-programed, 258-261 StNCtUd Changes and, 262-265 XPS Studies, 267-270 structure cubic, 240 electroneutrality, 241 nonstoichiometry, 243, 254-258 orthorhombic distortion, 242 of related compounds, 243-244 sulfur dioxide reduction by carbon monoxide, 298-300 superconductivity at high temperature, 317-318

water-gas shift reaction, 311-312 Phenylacetylene, hydrogenation over amorphous alloys. 368-369 Phosphorus mctal catalyst poisoning, 29, 31 in perovskite.catalyst, 346-349, 251 Photocatalysis, over perovskites, 304 Plasma spraying. pe~wskitcson supports, 253 Platinum catalysts particle size, effect on hydrocarbon conversion activities hydrogenation and, 110-117 hydrogenolysis and, 119-120 skeletal reactions and, 122-123 singlecrystal surface carbon monoxide oxidation, 26 gold addition effect, 40-41

hydrocarbon conversion, 16-21 potassium effect, 37 sulfur poisoning, 30 supported, hydrogen chcmisorption, 94 Platinum/ceric oxide catalysts, SMSI,226 Platinumlsilica catalysts, pentane hydrogenolysis, particle size and, 90 Platinumltitania catalysts bonding and charge transfer, 224-225 HTR, 176-177 potassium effects, 200-201 silver effects, 202-203

SMSI,44-45 comparisonwith rhodium/titania, 20!3-214 surface composition, 206-208 Poisons, detection on metal surface EPR, 108 IR spectroscopy, 107-108 secondary-ion mass spectrometry, 107 TPD technique, 106-107 Pollution control, with perovskite catalysts, 291-293

Potassium, effect on catalysts iron, 35-37 nickel, 34-36 palladium, 32 platinum, 37 rhenium, 37 titania-supported, 200-201 Praseodymium cobaltate, reduction, temperature effect, 260, 261 Propane hydrogenolysis, metal particle size and, 120 oxidation over perovskites, 286 2-Propanol, conversion over perovskites dehydration, 311 dehydrogenation, 309-31 1 Propene, oxidation over perovskites, 287-288 Propylme, oxidation metal particle size and, 136 over perovskites, 287-288 Pyridine, adsorption on perovskites, 277

R Ranw-tw nickel catalysts organic compound hydrogenation, 369-370

Reduction of catalyst precursors, methods, 101-106 isothermal, of perovskites, 260-267

395

INDEX metal-support particle preparation and, 70 temperature-programed, see 'Rmperatureprogramed reduction Rhenium catalysts, ammonia synthesis, 24-25 promotion by potassium, 37 Rhodium/alumina catalysts, carbon monoxide chemisorption, 85 Rhodium catalysts carbon monoxide conversion hydrogenation, 26 pnoxidation and. 32 sulfur addition and, 30 oxidation, 24-26 ethane hydrogenolysin, particle size and, 118 ethylidine hydrogenation, 20, 23-24 supported, hydrogen chemisorption, 94-95 Rhodium/niobia catalysts, SMSI, 227-228 Rhodium/silica catalysts ethane hydrogenolysis, 153 titania-promoted activity, 200 Rhodiumltitania catalysts bonding and charge transfer, 221-223 carbon monoxide hydrogenation. 217-218 potassium effects, 201 preparation. 181-184 in UHV apparatus, 203 reduction temperature effects carbon monoxide hydrogenation, 196-200 chemisorption of carbon monoxide and hydrogen, 184-187 EXAFS analysis, 221,223 hydrocarbon hydrogenolysis, 190-196 XANES study, 222-223 silver effects, 202 SMSI, 45-46 comparison with platinum/titania, 209-214

surface composition, 204-208 Rhodium/vanadia catalysts, SMSI, 226-227 Ruthenium catalysts carbon monoxide oxidation, 26 copper addition and, 41-43 sulfur effects, 30 supported hydrogen chemisorption. 93 structure sensitivity, 87 S

Schulz-Flory plots, lanthanum rhodate catalytic activity, 297-298

Secondary-ion mass spectroscopy, poison detection on metal surface, 107 Selectivity alkyne hydrogenation over palladium alloy catalysts, 368-369 ethylene oxidation over supported silver. 90, 136-137

methylcyclopropane hydrogenolysis over metals, 124-127 structure sensitivity and, 88-90 Silicon. metal catalyst activity and, 31 Silver catalysts effect on titania-supported catalysts, 202-203

ethylene oxidation poisoning by chlorine, 32-34 promotion by cesium, 37-39 singlecrystal surface orientation and, 15-16

over supported catalysts, particle sue and, 90, 136-138 Singlacrystal surface catalysts, see Model catalysts Small-angle X-ray scattering, metal particle size distribution, 97-98 SMSI, see Strong metal-support interaction Sodium chloride, electrolysis by amorphous alloys, 339-342 Spray-drying, perovskite preparation, 247. 249-250

Static magnetic methods, metal particle size distribution, 98-99 Stepped single crystals, in supported metal assay, 82-83 Strong metal-support interaction (SMSI) in alumina-supported catalysts, 228 carbon monoxide hydrogenation and, 196-200

chemisorption of carbon monoxide and hydrogen, 184, 187-188, 190 chemisorption loss after HTR and, 228 comparison between rhodium/titania and platinum/titania, 209-210, 213-214 hydrocarbon hydrogenolysis and, 190-196 hydrogen effect, 220-221 in model catalysts, 43-47 in niobia-supported catalysts, 227-228 in platinum/ceric oxide catalysts, 226 vanadia-supported catalysts, 226-227 Structure insensitivity ensemble effects and, 153-154

INDEX experimental examples, 141-142 overlayer effects, 154 Structure sensitivity antipathetic behavior, 147, 149, 150, 152, 155-156

of carbon monoxide chemisorption, 138-140

of carbon monoxide oxidation, 134-135 definition, 57 of hydrogen chemisorption. 91-95 limited and extended, 88 in metal clusters, supported, 146 secondary, 128, 149, 155 selectivity and, 88-90 single-crystal surface orientation effect in platinum catalysts, 16-21 in silver catalysts, 15-16 sympathetic behavior, 151-152, 155-156 TOF assay, see 'hnover frequency Sulfur metal catalyst poisoning, 29-30 in oxide-supported catalysts, chemisorption loss after HTR and, 228 Sulfur dioxide effects on perovskites adsorption, IR study, 312-313 carbon monoxide oxidation poisoning, 313-315

reduction by carbon monoxide over perovskites, 298-300 Superconductivity,yttrium role in perovskites, 317-3 18 Supported metals, small particles. see o h specflc catalysts activation by calcination and reduction. 70-71

alloying effects, 80-82 AR, definition, 72-73; see also Atomic rate of reaction bonding with support, 84 characteristics, 157 (table) crystal structure change, 85 electronic properties, 74-76. 109 FE. definition, 72-73; see ulso Fraction exposed, of total metal atoms future studies, 157-160 mathematical models, 76-81 model catalysts and, 83-84 particle size catalytic activity and. 56-59 critical, calculation, 55-56

distribution chemisorption and. 91-95 electron microscopy, 95-96 magnetic methods, 98-100 X-ray diffraction, 97-98 poisons, detection methods, 106-108 preparation methods, 61 (table) carrier selection, 59 chemical deposition, from metal colloid dispersion, 67-78 coprecipitation, 60, 62 decomposition of metal cluster compounds, 66-67 impregnation. 62-63 ion exchange, 63-66 ion implantation, 68 vapor phase deposition, 68-69, 83 reconstruction, 85-86 reduced ions, detection methods, 102-106 stepped single crystals. 82-83 structure, methods of assay, 108-109 structure sensitivity, 86-91 TOF, definition, 57-58. 72-74, 154-155; see ulso 'hrnover frequency

T Temperature-programeddesorption (TPD) benzene hydrogenation on titaniasupported metals, 210 carbon monoxide chemisorption on platinum/titania, 187-189, 224-225 comparison with rhodiudtitania, 209-212

titania coverage and, 208-209 carbon monoxide from nickel surface, 215-217

hydrogen chemisorption on titaniasupported metals. 209-211, 213-214 poison detection on metal surface, 106-107

Temperature-programedreduction (TPR) hydrogen consumption by metals and, 102 of perovskites, 258-261 comparison with isothermal reduction, 260-262

oxygen adsorption, 270-272 structural changes during, 262-265 Thermal desorption mass spectroscopy, 1, 14 Timeof-flight mass spectrometer, metal cluster assay, 142, 143

397

INDEX Titania comparative effects on platinum and rhodium activities, 209-214 coverage on metal, carbon monoxide chemisorption and, 206-209 migration onto metal during HTR, 189, 192, 1%. 201-207 nickel deposition on, SMSI. 44 platinum deposition on, SMSI,44-45 promoting effect on catalytic activity explanations, 214-221 on rhodium/silica, 200 rhodium deposition on, SMSI,45-46 Titania-supported catalysts, see also spec@ catalysts

bonding and charge transfer, 221-225 HTR. see High-temperature reduction hydrogen effect, 220-221 metal-support interactions, 173, 175-179 preparation impregnation, 181-183 ion exchange, 182-184 reduction temperature effects carbon monoxide chemisorption, 186-190 carbon monoxide hydrogenation. 196-200 hydrocarbon hydrogenation, 190-196 hydrogen chemisorption, 184-186 silver effect, 202-203 "OF, see 'hrnover frequency Toluene hydrogenation over Raney-type nickel catalysts, 370 oxidation to benzaldehyde over lanthanum cobdtate, 288-289 TPD, see 'Rmperature-programed desorption TPR, see 'Rmperature-programed reduction 'hngsten catalysts carbon monoxide hydrogenation, 26 sulfur effect, 30 ethane hydrogenolysis, 23 nickel submonolayer fiim effect, 43 F) lbrnover frequency O ammonia synthesis, 131 antipathetic behavior and, 147, 149-150, 152, 156 carbon monoxide hydrogenation, 128-130 definition, 57-58, 72-74, 84-87, 154-155

hydrocarbon hydrogenation, 110-117 hydrocarbon hydrogenolysis, 118-123 maximum versus FE, 151-153, 155-156 oxidation reactions, 132-138 structure sensitivity and, 86-87, 90-92, 94 sympathetic behavior and, 151-153, 156 unchanged with FE, 141-142

U Ultrahigh vacuum surface analysis (UHV) apparatus, 9-14 high pressure cell and, 9-11, 13 microreactor, 4-14 presssure cup design, 10-12 rhodium/titania preparation, 203-204 transfer rod design, 12-13 future assays, 48-49 -high pressure transfer, 2, 3 cleanliness requirements, 6 impurity problems, 5 homogenous surface preparation, 47-48 metal vapor deposition, 69

V Vanadia-supported catalysts, 226-227

W Water adsorption on perovskites, 276-278 electrolysis using amorphous metal alloys, 336-338. 375 photolysis over perovskites. 304 Water-gas shift reaction over copper, 27 poisoning by sulfur, 31 promotion by cesium. 37-38 over perovskites, 311-312 Wolfram, see Tungsten X XANES. see X-ray absorption near-edge

structure XPS, see X-ray photoelectron spectroscopy X-ray absorption near-edge structure (XANES), metal-support interaction. 174 in rhodium/titania catalysts, 221-223 X-ray diffraction (XRD) amorphous metal alloys. 332 during ammonia synthesis, 373-374

398

INDEX

during carbon monoxide hydrogenation nickel-iron alloys, 346 zirconium-containing alloys, 352-354,

UHV analysis and, 1.4 XRD, see X-ray diffraction

356-357

metal particle size distribution, 97-98 perovskite reduction-oxidation, 265-267 X-ray photoelectron spectroscopy (XPS) amorphous metal alloys carbon monoxide hydrogenation and,

Y Yttrium, in perovskites. superconductivity and. 317-318

347, 348, 351

hydrogen fluoride effect. 337-338 during sodium chloride evolution. 340 hydrogen role in SMSI,220-221 metal-support interaction, 174 in titania-supported catalysts, 203, 206,208

perovskite studies oxygen adsorption, 273 reduction, 267-270 platinum spectra. as function of FE,110-111 small supported particle assay, 74-75, 83 particle size distribution, 101, 104-105, 109

Z

Zinc oxide, copper deposition on, SMSI, 46-47

Zirconium-containing amorphous metal alloys ammonia synthesis, 372-373 hydrogenation catalysis of 1,3-butadiene, 367 of carbon monoxide, 333, 352-357. 375 of olefins, 366-367 hydrogen evolution, 338 nickel-based, carbon monoxide chemisorption, 344-345