Studies in Surface Science and Catalysis 95
CATALYSIS BY METALS AND ALLOYS
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Studies in Surface Science and Catalysis Advisory Editors: B. Delmon and J.T. Yates Vol. 95
C ATA LY S I S BY METALS AND ALLOYS
Vladimir Ponec
Leiden Institute of Chemistry, Leiden University, Leiden, The Nether~ands
Geoffrey C. Bond
Department of Chemistry, Brunel University, London, United Kingdom
ELSEVIER Amsterdam - Lausanne- New York- Oxford - Shannon - Singapore- Tokyo
ELSEVIER SCIENCE B.V. Sara Burgerhartstraat 25 P.O. Box 211, 1000 AE Amsterdam, The Netherlands
First printing: 1995 Second impression: 1998
ISBN 0-444-89796-8 9
1995, ELSEVIER SCIENCE B.V. All rights reserved.
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Contents
PROLOGUE Chapter 1 STRUCTURE AND PROPERTIES OF METALS AND ALLOYS 1.1 A microscopic theory of solids 1.1.1 The quantum theory of pure metals 1.1.2 Pauling's theory of pure metals 1.1.3 The Engel-Brewer theory of metals and alloys 1.1.4 The Miedema theory of stability of alloys 1.1.5 The quantum theory of alloys 1.2 Some results of the theory of chemisorption on metals and alloys 1.2.1 General features of chemisorption 1.2.1.1 Chemisorption of atoms 1.2.1.2 Chemisorption of undissociated molecules 1.2.1.3 Adsorption of molecular fragments 1.2.1.4 Semi-empirical approach to the problem of chemisorption 1.3 Adsorption of molecules and radicals which are intermediates in catalytic reactions 50 1.3.1 Hydrocarbons 1.3.2 Other molecules 1.4 Macroscopic thermodynamic theory of alloys 1.4.1 A short introduction to the statistical thermodynamic description of alloys, as random solutions 1.4.2 Phase composition of some catalytically interesting alloys Chapter 2 EXPERIMENTAL TECHNIQUES OF SOLID STATE PHYSICS, RELEVANT TO RESEARCH ON ALLOYS 2.1 Photoelectron Spectroscopy (PES) 2.1.1 Instrumentation 2.1.2 Basic principles and phenomena in PES 2.1.2.1 Ionization and relaxation effects on the binding energy of electrons in atoms and molecules 2.1.2.2 Relaxation effects in solids, particularly metals 2.1.2.3 Relaxation effects in adsorbed atoms and molecules 2.1.2.4 Theoretical and semi-empirical calculation of binding energies in atoms, molecules, metals and alloys 2.1.2.5 Integrated and angle-resolved spectra of valence band electrons in metals and alloys
7 7 7 19 23 25 26 35 35 35 41 46
48 50 53 54 55 59
73 74 74 76
77 80
81
84 94
vi
2.2
2.3
Contents
Auger 2.2.1 2.2.2 Other 2.3.1 2.3.2 2.3.3 2.3.4 2.3.5 2.3.6 2.3.7 2.3.8
2.1.2.6 Quantitative analysis by XPS spectroscopy Basic principles Quantitative analysis by AES methods Ion scattering techniques (LEIS) Medium and high energy ion scattering Ion neutralisation spectroscopy (INS) by slow ions Secondary Ion Mass Spectroscopy (SIMS) High-Field Emission Techniques Microscopes with atomic resolution Work function measurements Extended X-ray Absorption Fine Structure (EXAFS)
Chapter 3 THE ELECTRONIC STRUCTURE OF ALLOYS; EXPERIMENTAL RESULTS 3.1 Magnetic measurements 3.2 The M~Jssbauer effect 3.3 Photoemission Spectroscopy (PES) 3.4 Soft X-ray emission and absorption 3.5 Conclusions Chapter 4 SURFACE COMPOSITION OF ALLOYS 4.1 General remarks on surfaces of metals 4.2 Binary systems with surface segregation 4.2.1 Chemical approach, kinetic and thermodynamic description of equilibrium 4.2.2 Simple thermodynamics of segregation 4.2.3 Broken-bond model of alloy surfaces 4.2.4 Regular solution model for systems with components of different molar volume 4.2.5 Monte-Carlo calculations on surface segregation 4.2.6 Metal-on-metal layers 4.3 Surface segregation in catalytically interesting alloys 4.3.1 Nickel-copper alloys 4.3.2 Palladium alloys 4.3.3 Platinum alloys
Chapter 5 PHYSICAL PROPERTIES AND STRUCTURES OF SMALL METAL AND ALLOY PARTICLES 5.1 The electronic structure of small free metal particles 5.2 Equilibrium shape and thermal properties of small metal particles
98 102 102 106 113 113 115 117 118 119 122 124 127
143 143 148 150 167 169
175 175 181 181 183 184 192 194 196 202 202 206 208
219 219 224
Contents
5.3 5.4 5.5
Adsorption sites on small metal particles Reactivity of small metal particles Polarization and charging of small metal particles by a support
vii
227 229 234
Chapter 6 THE CATALYTIC CYCLE 6.1 Prelude 6.2 The role of reaction mechanism in catalytic research 6.3 Methods of investigating catalysed reactions 6.3.1 Heat transfer and mass-transport limitation 6.3.2 Methods for conducting reactions in gas-solid systems 6.3.3 Transient kinetics and temporal analysis of products (TAP) 6.3.4 Ways of performing reactions in gas-liquid-solid systems 6.3.5 Catalysis under UHV conditions 6.3.6 Scaling-up 6.4 Kinetics of heterogeneously catalysed reactions 6.4.1 Ways of expressing the rate of a catalysed reaction 6.4.2 Introduction to Langmuir-Hinshelwood kinetics 6.4.3 Activation energy 6.4.4 The compensation phenomenon 6.4.5 Selectivity 6.4.6 Epilogue 6.5 Structure sensitivity and particle size effects 6.5.1 Principles and concepts 6.5.2 Hydrogenation of multiple carbon-carbon bonds 6.5.3 Hydrogenolysis of the C-C bond 6.5.4 Hydrogenation of carbon monoxide 6.5.5 Ammonia synthesis 6.5.6 Oxidation reactions 6.6 Catalytic consequence of metal-support interactions
247 247 248 250 250 253 258 261 264 264 266 266 268 273 276 277 279 280 280 283 284 285 286 288 288
Chapter 7 PREPARATION AND CHARACTERIZATION OF METAL AND ALLOY CATALYSTS 7.1 Macroscopic materials 7.1.1 Polycrystalline materials 7.1.2 Metal films 7.1.3 Other methods of comminution 7.1.4 Single crystals 7.1.5 Two-dimensional alloys 7.1.6 Intermetallic compounds 7.1.7 Interstitial alloys 7.1.8 Amorphous alloys 7.2 Small unsupported metal particles 7.2.1 Metal 'blacks' by chemical reduction 7.2.2 Colloidal metals
299 300 300 303 308 309 312 313 313 314 315 315 316
viii
7.3
7.4
Contents
7.2.3 Reduction of binary oxides 7.2.4 Raney alloys Supported metal catalysts 7.3.0 Introduction 7.3.1 Supports 7.3.2 Use of precursors in positive oxidation states 7.3.3 Use of zero-valent compounds and atoms 7.3.4 Methods specific to alloys 7.3.5 Reduction to the active form Characterization of alloy catalysts 7.4.0 Introduction 7.4.1 Characterization of the physical structure of supported metal catalysts 7.4.2 Physical methods of characterizing small alloy particles 7.4.3 Characterization of supported metals and alloys by selective gas chemisorption
Chapter 8 ADSORPTION ON ALLOYS 8.1 Ensemble size and composition effects 8.2 Ensemble statistics and the extent of adsorption 8.3 Adsorption as a probe of active sites on alloys 8.4 Adsorption of simple gases on alloys 8.4.1 Adsorption of hydrogen 8.4.2 Carbon monoxide adsorption as a probe 8.4.3 Adsorption of hydrocarbons and of some other gases 8.4.4 Adsorption on incomplete layers of alkali metals on transition metals Chapter 9 CATALYSIS BY ALLOYS - GENERAL FEATURES 9.1 Basic problems 9.2 Investigation of electronic structure effects by means of catalytic reactions 9.3 Important side effects of alloying Chapter 10 REACTIONS OF HYDROGEN AND ALKANE-DEUTERIUM EXCHANGE 10.1 Reactions involving only hydrogen and its analogues 10.1.1 Reactions involving hydrogen atoms 10.2 The equilibrium of hydrogen + deuterium and parahydrogen conversion 10.3 Exchange of alkanes with deuterium Chapter 11 CATALYTIC HYDROGENATION AND DEHYDROGENATION
318 319 320 320 321 330 344 348 350 356 356 357 361 370
393 394 394 398 404 404 409 424 430
437 437 441 444
449 449 452 456 464
477
Contents
11.1
11.2
11.3
11.4
11.5
11.6
Hydrogenation of alkenes
11.1.1 General principles 11.1.2 Carbon deposition 11.1.3 Kinetics and mechanism of hydrogenation 11.1.4 Hydrogenation of alkenes by alloys 11.1.5 Hydrogenation of the cyclopropane ring Hydrogenation of alkynes and alkadienes 11.2.1 General principles 11.2.2 Kinetics and mechanism of alkyne hydrogenation 11.2.3 Hydrogenation of alkynes by alloys 11.2.4 Hydrogenation of alkadienes Hydrogenation of aromatic compounds 11.3.1 General principles 11.3.2 Hydrogenation and exchange of aromatics on pure metals 506 11.3.3 Hydrogenation and exchange of aromatics on alloys Hydrogenation of other unsaturated groups 11.4.1 The problem of diffusion limitation 11.4.2 Reactions and catalysts 11.4.3 Alloy catalysts in liquid-phase hydrogenation Dehydrogenation 11.5.1 Dehydrogenation of alkanes and cycloalkanes 11.5.2 Decomposition of formic (methanoic) acid 11.5.3 The decomposition of hydrogen peroxide 11.5.4 Decomposition of alcohols Hydrogenation of diatomic molecules: oxygen and nitrogen
ix
477 477 484 486 488 490 491 491 494 498 500 504 504 508 511 511 514 516 518 518 520 525 526 527
Chapter 12
OXIDATION REACTIONS 12.1 Fundamentals - chemisorption of the reactants 12.2 Selected information on simple oxidation reactions on metals 12.3 Oxidations on alloys 12.3.1 Oxidation of carbon monoxide 12.3.2 Oxidation of hydrogen 12.3.3 Epoxidation and other reactions of alkenes 12.4 Practical applications of oxidation reactions on metals and alloys 12.4.1 Three-way catalysts 12.4.2 Oxidation of ammonia 12.4.30xirane (ethylene oxide, EO) production 12.4.4 Electrocatalytic oxidations
541 541 546 555 555 561 564 568 568 571 572 573
Chapter 13 REACTIONS OF ALKANES AND REFORMING OF NAPHTHA 13.1 Fundamentals 13.1.1 Adsorption of hydrocarbons under reaction conditions 13.1.2 Kinetics of skeletal reactions
583 583 583 592
x
13.2
13.3 13.4
13.5
13.6
Contents
13.1.3 Model reactions of alkanes on metal catalysts 13.1.4 Reactions on supported metal catalysts Model reactions of alkanes on alloys 13.2.1 Nickel, cobalt and iron alloys with Group 11 elements 13.2.2 Palladium alloys 13.2.3 Ruthenium alloys 13.2.4 Platinum in combination with Group 11 elements 13.2.5 Alloys containing rhodium, iridium and Group 11 metals 13.2.6 Platinum-rhenium model catalysts 13.2.7 Platinum-rhenium on alumina (sulfur-free catalysts) Fundamental studies on reforming catalysts Various combinations containing two transition metals 13.4.1 Combinations containing platinum 13.4.2 Transition metal alloys without platinum 13.4.3 Multimetallic cocktails Platinum-tin and other related catalysts 13.5.1 Platinum-tin catalysts 13.5.2 Other catalysts containing tin and related elements Reforming of naphtha
Chapter 14 SYNGAS REACTIONS 14.1 Fundamentals 14.1.1 Historical introduction 14.1.2 Present ideas concerning the mechanisms 14.1.3 The role of promoters 14.1.4 Some of the ideas behind the work with alloys 14.2 Alloys in Fischer-Tropsch synthesis: combinations of active and inactive metals 14.3 Alloys in Fischer-Tropsch synthesis: combinations of two active metals 14.3.1 Iron-ruthenium alloys 14.3.2 Iron-cobalt, iron-nickel and other iron-containing alloys 14.3.3 Cobalt-containing alloy catalysts 14.3.4 Other alloys and pseudo-alloys 14.3.5 Intermetallic compounds as precursors of catalysts for syngas reactions 14.3.6 Promoted and alloyed copper catalysts 14.3.7 Interstitial compounds of iron and cobalt 14.4 Industrial processes with syngas
596 604 604 604 612 614 619 623 628 631 639 650 650 658 658 659 659 662 663
679 679 679 681 689 692 693 695 695 695 698 698 701 703 703 704
EPILOGUE
717
SUBJECT INDEX
723
PROLOGUE The phenomenon of catalysis is as old as life itself: indeed in a very real sense the existence of life is due to enzymatic catalysis. All living organisms are complex catalytic reactors. Our interest in catalysis is however more directed towards things that can be made with its help. The catalytic action of the enzymes in the bloom on grapes was known to the Patriarchs - Noah was clearly aware of their ability to convert monosaccharides into ethanol, and was the first recorded person to experience its effects, which can be pleasant and unpleasant. The Saints did not despise alcohol either: St Paul advised us 'to take a little wine for your stomach's sake'. The fruit of the wine has indeed been a source of comfort and inspiration through the ages: many centuries ago Omar Khayyam was caused to '. .... wonder what the vintner buys One half so precious as the goods he sells'. However in this book we shall be principally concerned with catalysis by inorganic solids, that is to say, with heterogeneous catalysis, especially that involving metals and alloys. It was the task of one of the Founding Fathers of Chemistry, Jons Jacob Berzelius [1,2], to recognize the significance of a number of early observations indicating that small traces of chemical substances could markedly increase the rates of some reactions. Robertson [3] gives credit to Sir Humphry Davy for being the first to realize that a chemical reaction between two gases can occur on the surface of a metal without its being visibly changed. L.J.Thenard observed the decomposition of ammonia over iron, copper, silver, gold and platinum [2], while Davy and Michael Faraday noted that, while platinum and palladium catalysed the oxidation of hydrogen, copper, silver, gold and iron did not [3]. This must be the earliest recorded pattern of catalytic activity. Another contributor to the early work on metal catalysis was Johann Dobereiner [4], who not only made the first 'supported' metal catalyst by mixing platinum with clay, but also devised a catalytic cigarlighter in which platinum ignited a small hydrogen flame [5]. Other practical applications were not far behind. In 1831 Peregrine Phillips was granted British Patent as 6096 for the platinum-catalysed process of sulfur dioxide oxidation to manufacture sulfuric acid. Little is known about Phillips, and this was his only contribution to chemical technology. It is symptomatic of this field of chemistry that practice has usually anticipated understanding: after all, it is the effect that makes money, and knowing how it works only provides intellectual satisfaction. The first suggested explanation (i.e. the first really scientific one) for catalysis was made in 1834 by Faraday [6], who thought it was the result of the attractive force exerted by the solid on the gaseous reactant; in modern terms he was advocating adsorption as being responsible. However to J.J.Berzelius belongs the honour of having first employed the name catalysis
2
Prologue
to describe the phenomena [7]: 'I shall, using a derivation well-known in chemistry, call it catalytic power of the substances and the decomposition by means of this power catalysis, just as we use the word analysis to denote the separation of the component parts of bodies by means of ordinary chemical forces. Catalytic power actually means that substances are able to awaken affinities which are asleep at this temperature by their mere presence and not by their own affinity'. If this statement lacks the lucidity of later scientists such as Ostwald and Arrhenius, it is of course because chemical theory was at that time only at a rudimentary stage of development. As the understanding of chemical principles grew, it became clear that Liebig's negative comment to the effect that 'the creation of a new force by a new word explained nothing, since it prevented further research' [2] was unjustified: although there was a long period through most of the 19th century when catalysis was not much employed in chemical manufacture, the great men of physical chemistry were able before that century ended to propose a definition that has stood the test of time. Ostwald [2] recognized that the thermodynamic parameters of a chemical reaction could not be changed by the assistance of a catalyst, otherwise there would be the creation of a
perpetuum mobile. Thus the statement that a catalyst is substance that increases the rate of which a chemical system attains equilibrium is broadly satisfactory: catalysis is the phenomenon of a catalyst at work. However the great thing about a catalyst is that it is not consumed in the reaction it catalyses; indeed in many major industrial processes it is expected that it will continue to operate, with if necessary occasional resuscitation, for many years. We must therefore add to our definition a phrase such as 'without being consumed in the process' or 'without suffering chemical change' (not always strictly true) or 'without appearing in the stoichiometric equation for the process'. It is however important for pedagogic purposes to avoid believing the species that can initiate chain reactions, explosions, and polymerizations are catalysts, because they frequently are to be found in the products, and the term initiator is better reserved for such species. For further clarification one may recognize a catalysed reaction as one that '. .... proceeds by repetition of the catalytic cycle or chain, with the catalytic species remaining unchanged at the end' [8]. We should not however strive too hard to find a form of words to meet all circumstances. As someone once remarked, 'I cannot define an elephant, but I'm sure if I saw one I should recognize it'. We may pause to wonder why Berzelius selected the word 'catalysis' as an
omnium gatherum for the accelerating effects of trace amounts of substances not permanently caught up in the reaction. The word literally means 'a breaking down': the prefix cata- occurs in many words (catastrophe, catalepsy, catatonic) and the verb lysein, to break or split, is familiar to scientists in such terms as hydrogenolysis, photolysis etc. In ancient times the word had been used to signify a loss of social cohesion (e.g. a riot) or a moral lapse, and it has been used over the centuries in this or a similar connotation. The first scientific use of the word may have been due to A.Libavicius, who used it in the sense of
Prologue
3
'decomposition'. We may suppose that Berzelius felt that when catalysis occurred, the laws of chemical combination were in some way violated or broken down. From a slightly different point of view, it is forces that restrain reactions from taking place that are overcome in catalysis. In this way we can understand that the popular use of the word catalyst, for example by journalists to describe someone who may bring conflicting parties into agreement, is not so very different from the idea that it removes barriers. The Chinese and Japanese use the same ideograph for 'catalyst' and 'marriage-broker': this is perfectly logical. In chemical reactions the barrier is of course the activation energy [9]. We have seen that metals played a leading role in the development of both the theory and the practice of catalysis: this continued into the present century with the work of Sabatier, Senderens and Ipatieff in exploring catalytic hydrogenation at atmospheric and at high pressure, and of Haber and his associates with their discovery of the means of synthesising ammonia. It was in fact the need to find a means of stabilizing the platinum gauzes used for ammonia oxidation that first drew notice to the potentially beneficial effects of alloys. Between the First and Second World Wars, academic scientists began systematic studies of the catalytic properties of alloys. Tammann made an early study of the hydrogen-oxygen reaction on palladium-gold alloys [10], and Gunther Rien~icker began a systematic series of investigations into the effect of the ordering of alloys on their catalytic activity [11-13]. Georg-Maria Schwab examined a great many Hume-Rothery alloys for their activities in simple reactions such as formic acid decomposition and sought their dependence on the electron/atom ratio, i.e. the filling of the Brillouin zone [14]. This work together with the important concepts explicated by Dennis Dowden following the Second World War [15-17] and the inspired experimental work of Dan Eley and his colleagues [18,19] formed the basis of what came to be known as the electronic theory of
catalysis. The link between chemistry and solid state physics was forged, and has remained unbroken. Dowden's ideas were developed from those of Nyrop, who in a short but important publication [20] stated that molecules are activated for the process of adsorption by either accepting or releasing an electron. Dowden then explained the conditions under which such transfer of electrons at the surface of a metal is easiest: (i) when a metal has unoccupied levels (i.e. holes) in its d-band; or (ii) when its density of states at the Fermi surface is high [15-17]. These matters will be discussed further in chapter 1. Both conditions are best fulfilled by metals at the end of the Transition Series, in agreement with the clearly evident experimental observation that the Group 10 metals have exceptional catalytic activity. It then followed from the Rigid Band Theory, to which all solid state physicists then subscribed, that the number of d-band holes could be decreased by alloying with a metal containing more valence electrons, as a result of electron transfer from one to the other. Although this simple and attractive hypothesis has failed to stand the test of time (see chapter 3), its falsification in no way diminishes the enormously important role that
4
Prologue
Dowden's paper played in the development of post-war catalysis. The ideas of Dowden [15-17] and Eley [18,19] dominated theoretical thinking concerning catalysis by metals and alloys until the 1960's, but with the development of new and powerful experimental techniques, especially electron spectroscopy, it gradually became clear that their revision was unavoidable, and that a new formulation of the theory of bond formation in chemisorption at metal surfaces would be required. It became evident that it is covalent bond formation, not ionisation or transfer of electrons, that chiefly operates in chemisorption; that there is practically no transfer of electrons between the components of an alloy; and that although there may be some small changes in the electronic structures in the components of substitutional and some ordered alloys, this probably has little siginificance for chemisorption or catalysis. As we shall see as the story develops, the concepts of the importance of ensemble size and composition (chapter 8 and 9) are claiming progressively more attention as a way of explaning the catalytic behaviour of alloys. It is desirable to come to an understanding at the outset concerning the terminology to be used when speaking of alloys. After extensive inspection of the literature we find it is general practice to use the term alloy to describe any metallic system containing two or
more components, irrespective of their intimacy of mixing or the precise manner in which their atoms are disposed. We follow this practice here: as Humpty-Dumpty said: 'When I use a word, it means just what I choose it to mean - neither more nor less'. We can and do extend the use of the word alloy to cover interstital alloys such as are formed between a metal and a clearly non-metallic element such as boron, carbon, nitrogen or silicon. Stoichiometric hydrides are not however thought of as alloys, and we hesitate to use the term (as some have done) to describe physical mixtures of metals where there is no mixing on the atomic level. Intermetallic compounds of fixed composition do however fall within our definition. Substitutional alloys are more straightforward: provided the size of the two components does not differ by more than about 10-15%, if their electron/atom ratios are not too different and if the metals have the same crystallographic structures, they may form a continuous series of solid solutions over the whole concentration range. These conditions governing mutual solubility are sometimes known as the Hume-Rothery Rules. Thus for example palladium and silver and palladium and platinum form such solid solutions of monophasic alloys. Some combinations only form a monophasic alloy in some restricted temperature range: thus with nickel and copper this only occurs above about 473K, since at lower temperatures it is possible for two phases of different composition to co-exist, and thus to behave as a biphasic alloy. It is thermochemical considerations that determine what type of alloy system will be formed between two metals. Combinations such as palladium and silver form almost ideal solutions, because the enthalpy of mixing is very small. With pairs such as nickel and copper, or platinum and gold, alloys are formed endothermically, so that mutual solubility increases with temperature, until the critical solution temperature is reached,
Prologue
5
above which each component is freely soluble in the other. When the enthalpy of mixing is negative (i.e. -AHmi x < 0) or has only a small positive value, it is not surprising to find many experimental techniques pointing to a mutual perturbation of the electronic structures -AHmi x of the components that is only weak (chapter 3). As -AHmixbecomes progressively more positive, the first effect observed is ordering, as in the platinum and copper system; and after that one encounters intermetallic compounds of fixed stoichiometry, as in the platinum, tin and zirconium systems. When the enthalpy change of compound formation is very large, one finally has compounds such as oxides and sulfides, where electron transfer has quite evidently occurred.
References
10 11 12 13 14 15 16 17 18 19 20
J.Trofast, Chem.Britain (1990) 432 J.Trofast, 'J.J.Berzelius and the Concept of Catalysis' in "Perspectives in Catalysis" (editors: R.Larrson, C.W.K.Gleerup) Liberaromodel, Lund, 1981 A.J.B.Robertson, Plat.Met.Rev. 19 (1975) 64 D.McDonald, L.B.Hunt, "A History of Platinum", Johnson Matthey, London, 1982 G.C.Bond in "Chemistry of the Platinum Group Metals", (editor: F.R.Hartley) Elsevier, Amsterdam, 1991, p.32 M.Faraday, Phil.Trans. Roy.Soc. 124 (1834) 55 J.J.Berzelius, Anals.Chim.Phys. 61 (1836) 146 S.T.Oyama, G.A.Somorjai, J.Chem.Educ. 65 (1988) 765 G.C.Bond, "Heterogeneous Catalysis: Principles and Applications", Oxford U.P., 2nd edn., 1987 G.Tammann, Z.Anorg.Chem. 111 (1920) 90 G.Rien~icker, Z.Anorg.Chem. 227 (1936) 353 G.Rien~icker, G.Wessing, G.Trautman, Z.Anorg.Chem. 236 (1938) 251 G.Rien~icker, H.Hildebrandt, Z.Anorg.Chem. 248 (1941) 52 G.M.Schwab, Disc.Faraday Soc. 8 (1950) 166 D.A.Dowden, J.Chem.Soc. (1950) 242 D.A.Dowden, Ind.Eng.Chem. 44 (1952) 977 D.A.Dowden, P.W.Reynolds, Disc.Faraday Soc. 8 (1950) 184 D.D.Eley, A.Couper, Disc.Faraday Soc. 8 (1950) 172 D.D.Eley, Z.Elektrochem. 60 (1956) 397 J.E.Nyrop, "The Catalytic Action of Surfaces", Levin Munksgaard, Copenhagen, 1937
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Chapter 1
S T R U C T U R E AND P R O P E R T I E S OF METALS AND ALLOYS
1.1
A microscopic theory of solids
1.1.1 The quantum theory of pure metals It is impossible to present in a single chapter an exact theory of metals and alloys, or of phenomena, such as those forming the basis of electron-spectroscopies, that are used to study and to establish the electronic structure of metallic catalysts. However, it is felt that a book on catalysis by alloys should at least introduce some of the important terms (band structure, density of states, photoemission from the valence band, etc.) on basis of some very simple theoretical considerations; it is not our ambition to achieve more than that. All modern books on undergraduate physical chemistry [1-3] offer an introduction to quantum mechanics, which is the basis of the chemical bond theory. The reader is thus expected to be familiar with terms such as the wave or state function (e.g. Z or ~t), the Hamiltonian or total energy operator It/and the Schr6dinger equation: (/-~ - E) z =0
(1)
where E is the steady state total energy of the system, the state of which is described by function Z. The total energy operator 121 can be split into two parts, the kinetic energy operator T and the potential energy operator V. Operator T is a differential operator and thus equation 1 is a differential equation of second order. The text books [1-3] offer also an introduction to the form of functions Z for hydrogen atom, for the hydrogen-like atoms (lithium, sodium, potassium, etc.) and for functions (orbitals) of some other atoms. With metals and alloys we are interested in the form of the solid or crystal orbitals, ~. Let us summarize some of their basic features [4]. We shall mostly be interested in metals or alloys in a crystalline form. Such bodies distinguish themselves by having a periodic potential V, so if we consider a linear one dimensional system of an infinite length and with a periodicity, i.e. lattice constant, a, the electron density p, which is proportional to the probability of finding an electron in a unit volume, i.e. gt*gt where gt* is the complex conjugated form of ~, will be the same at all places differing by a; therefore we state that
8
chapter 1
(2)
p =~*(x +a)~(x+a) = ~*(x)~(x) This means that ~(x+a) and ~(x) differ in such a way that ~(x+a)
= X, ~(x+2a)
,(x)
= xz .....
(3)
,(x)
and ~,*~, equals 1. The factor ~, is either unity, which is a trivial case, or it is a complex number, which is the general case. If we assume, following the idea of Born and Karman, that in a crystalline solid when passing a region of N atoms, then we find a point where not only the densities but also the wave functions themselves are equal, that is ~(x+Na) = ~(a), then the most general form of ~,, which is the one we are seeking, is ~= exp (i 2n
g/N)
(4)
= exp (i 2r~ galL)
where g is 0,1,2... N-1 and can be regarded as the quantum number describing the different ~ ' s. The symbol L stands for Na. Thus X=
~(x+a) ,(x)
= exp
(i 2zga). L
(5)
We are not interested in the ratio X, but in the most general form of ~(x). Then we have to realize that this must contain a factor like the exponential term in equation 5 and there may be a function U(x) having the property that U(x+a) = U(x). A function with such property would cancel in the ratio ~, in equation 5. With a very large value of a, U(x) is the atomic orbital. The Bloch theorem states that the most general form of the function ~t(x) is (6)
, ( x ) = oxp (th). U(x)
and, indeed this form, called the Bloch function, fulfils all the conditions which we have put on ~(x) above. Various trial forms of equation 6 can be used and various degrees of approximation accordingly achieved. When U(x) is put equal to a constant, ~(x) = A exp (ikx), and for three dimensions we write: ~(r) = A exp(ikr). The function ~ ( r ) is a plane wave and this is the
free electron approximation. When U(x) nearly-free electron
is a series of exponentials, i.e. a Fourier-like series, we have the
approximation.
Some other higher approximations will be listed below, but first we shall
turn our attention to the 'one electron approximation' which chemists know well as the linear combination of atomic orbitals (LCAO) and we shall apply it to our one-dimensional solid chain of atoms.
Structure and properties of metals and alloys
9
We substitute the crystal orbital by a linear combination of atomic orbitals I~ n with n indicating their place Vrr = L.C.A.O. =
•n Cn *n
(7)
in the Schr6dinger equation 1 and read it as:
2 n C n (fl- E) On -" 0
(8)
To convert equation 8, which is a differential equation, into an algebraic relation between numerical values, we multiply it by 0*e and integrate over the space of N atoms. In this way, by writing t7t in an extended form (the usefulness of it will be seen immediately), we obtain the relation
]~n Cn {I **e (T "1- Vcrystal+ Uat- Uat) 0Pnd~ - g I **e *n d'l:} = 0
(9)
T + Uat is H ~ the Hamiltonian of a free atom of the element of the chain and d'c the element of the space. Substituting this expression for H ~ and calling Vcryst- Uat "- AV, we obtain
]~n Cn {I 0*e (H~ AV) *n d'l~- g I 0*e On d'c} = 0
(10)
We now assume that the overlap between *n and **e is zero when n is not equal to e (n and e denote different positions of atoms), but is unity when e equals n. Thus I 0*e *n d'c = o and I **n *n d'c = 1
(11)
Further, in our approximation we keep only the following terms of equation 10
II~e H~
-- Eat
(e=n) =
= 0
(ee:n)
Cn d1:
(12)
= 13 (e=n_+l)
I *e mv *n
= 0 (e=n) = 0 (e>n_+l)
(13)
10
chapter 1
The second term (e=n) is zero because of the definition of the 'zero' potential energy. If we choose n, somewhere in the chain, we are then left with a general equation for Cn: (14) since all other terms vanish in our approximation defined by equations 11 to 13. Then a + an+ 1 + an_ 1 [3 Cn
E=
(15)
The Bloch theorem can be fulfilled by taking C,, = exp (ikx)
(16)
since
0 = ~ e ~kx,, gO,,=eikx .~e rex"-x) gO(x-x,,) n
(17)
n
The term represented by the summation has the properties of the function U(x). By substituting the expression for Cn in equation 15 the energy E is
E(k)=a + [3(e-~+e +U'a)=a +213coska
(18)
Several features of this equation are very interesting. There are N different values of k, thus there are N different crystal orbitals ~k and N different energy levels. The levels E(k) form, for a large Ns, a quasi-continuous band of energies between Emax and Emin, and from equation 18 the band width is
Emax-Em~n=4p
(19)
In other words, the band width is proportional to the overlap or hopping integral B. The (n-1)d orbitals (n being the principal quantum number of valence electrons) overlap less than do the ns orbitals. In the rough approximation which we have considered, the bands are separated and the (n-l) d-band is narrow and the (n) s-band is broad. The d-band has then 5N levels, but the s-band only N levels. The density of states between E and E+dE is as a consequence higher in the d-band than in the s-band. These are the main pieces of information which one needs in order to understand chapters 2 and 3.
Structure and properties of metals and alloys
11
E(k)
l
>N(E)
>N(E) -l't O
0
k
!
O
figure 1 left: Energy as a function of the wave-vector k, for a hypothetical one-dimensional 'crystal' (a chain of atoms) with a lattice constant a. right: Density of states curve, corresponding to E(k) shown in the left part.
The function cos (k a) is defined over the interval k from -rt/a to +rt/a and in figure 1 6 has a negative value, and tx is taken as the arbitrary zero. Where the slope of the function is steep, there are only a few states (i.e. few k's) in a given range of energy, but where the slope is low, as in the neighbourhood of g/a, there are many. In other words, the density of states N(E) is a function which increases with (dE/dk) -1, as is seen in figure 2. We shall meet the term density of states at many places in this book. In the free electron approximation and for an one dimensional solid chain, ~k is equal to A e ikx and the Schr6dinger equation is used with V equal to zero. It reads
d21~k 8n2m d~ 2
+ ~Eq~ h2
k =0
(20)
Substitution of ~k by the function of free electrons in the Schr6dinger equation produces
12
chapter 1
E -
kZh 2
(21)
8~;2m which can be compared with the Newtonian relation which says that E equals pZ/2m, with p being the momentum. Indeed, for free electrons the momentum p equals h k or in other words, k is a momentum in units of h.
Elk)
a 2 - 2r~ 1 figure 2 E(k), dispersion functions, f o r a hypothetical
a2
al + 2 -n G
k
0
k
dimensional
crystal (atomic chain) with two
a2 + 2132 al-2~
one
1
orbitals,
corresponding
energies
(z 1 and
(z2 on
to
the each
atom.
j(31, j(32 < O;
I~11< Ij%l
[31
n t3
The interval -n/a to n/a forms a k-space into which all possible non-equivalent k's are placed; it is called the first Brillouin zone. We have seen through our discussion of the Bloch theorem that k equals 2ng/Na, or in other words k is related to the reciprocal lattice constant, a -1. The interval -n/a to n/a is thus reciprocal with respect to real space. Because p equals hk, this interval is also a momentum space, into which all possible states, each characterized by its energy and k-number or momentum, are placed. In higher approximations than that corresponding to the free electron model, the momentum is not equal to hk. However, with regard to various forces F, hk still behaves like a momentum, since F is always equal to hk'. Therefore, k can be called a pseudo-momentum, in units of h. This is an important point for understanding the angle-resolved valence-band photoemission, which is discussed in chapters 2 and 3. Let us now make the step from chains of atoms to two-dimensional flat arrays. Now, for a square lattice
Structure and properties of metals and alloys
~,
= P'
13
(22)
Cr,~ ~,.,~
r,s
with
Or,s
=
eXp
(ikrXr+ iksYs)
(23)
and the energy is E(k)
= a + 213(cosk~a
(24)
+ coskya)
and it forms a plane in the E(kx, ky) space (see figure 3, under a) on the left) The first Brillouin zone shown under b) in fig.3 is now two dimensional, as is also the whole k-space. It is often useful to show the function E(k) in a more simple way. Then
E(k) is calculated for selected values of k,
for example along certain lines in the
Brillouin zone, such as, from the point k(0,0) to the point k0t/a; 0), etc., as is shown in figure 3 left under c). The density of states corresponding to the energies between ~ + 413 and ot - 4g is shown in the lower right comer [4]. In three dimensions, with E (k x, but not conceptually very different.
ky, kz)
the pictures are slightly more complicated
The mathematical theory of groups teaches us that in each space having translational symmetry and in which Born-Karman conditions hold, there are always 14 different Bravais lattices. Both the real and the reciprocal or k space are spaces with a lattice, i.e. translational symmetry, and this means that each lattice type in one space must have a counterpart in the other (i.e. reciprocal) space. For example, it follows that bcc (real space) translates into fcc (reciprocal) and fcc (real) translates into bcc (reciprocal), etc. Further, it follows that the vector k and the pseudo-momentum p have in a crystal of rectangular form the same directions in the real and in the reciprocal space. This enables us to indicate the real movement of electrons by movements of k-states in the reciprocal space. This is again an important statement for the description and understanding of the angleresolved electron photoemission. If electrons proceed in the k-space derived, for example, from a cubic crystal in a certain direction, they do the same in the real crystal too, and at the surface they continue to pass into vacuum without refraction because the kx,ky-components are preserved. Figure 4 shows a Brillouin zone of an fcc real-space lattice. The function E(k), called often the dispersion law, is usually theoretically calculated for certain selected values of k, for example, for k's along well chosen lines interconnecting important points on the Brillouin zones. These points are denoted by letters F, X, K, W etc. ( see also chapters 2 and 3). The typical form of such E(k)-sections are shown in figure 5a by results for copper. When the k-points corresponding to the highest energy levels still occupied by electrons are interconnected by a plane, the so called Fermi surface
14
chapter 1
is created (figure 5b).
ky (a)
(b)
bl[/ a
~/a w -
-
kl
t~/a
k, -- :~/a
'a
k~
(c)
=-4~ Energy
o{
~+4~ (0,0)
(n,a, O) (n/a,n/a) X
M
(0,0)
NIE)
F
figure 3. A hypothetical two dimensional crystal. Three representations of E(k) for the s band. (a) Energy surface for one quarter of the Brillouin zone. (b) Constant-energy contours, illustrating the symmetry of the zone. (c) Energy plotted over a triangular path of k values, showing minimum and maximum energies, and density of states. (for symbols F, X and M see figure 4)
Structure and properties of metals and alloys
15
kz
ky
figure 4. Brillouin zone in a reciprocal space with b.c.c. lattice, corresponding to fc.c.
1
lattice in the
real space.
tdl
J figure 5. Band structure (a) and
Energy
[
''
Fermi surface (b) for copper. In (a) the Cu 3d bands are labelled; the dashed curve shows the 4s band predicted without any mixing with the d band [4]
F
X
{b)
W
,,,,~ I"
C"
K
3'
16
chapter 1
The occupation of E(k)-levels at temperatures above absolute zero is governed by the Fermi-Dirac distribution function [1-4]. f(E)
= [ 1 + exp (
E-Ev)] kT
-1
(25)
Equation 25 states that as the temperature approaches zero, all levels below EF, the Fermi energy, become occupied (i.e. f(E) tends to unity) and all levels above E v become vacant. Thus for metals having a pseudo-continuous band, E F is the highest occupied level at the absolute zero. By using equation 25 in statistical thermodynamics, one can derive that E F is the total free energy of the electrons, per electron, i.e. it is the electrons' chemical potential. At equilibrium there is only a single value of EF for the whole system. The Fermi energy is a total energy, i.e. it includes also the electrostatic potential energy, such as that due to the contact potential between metals, and other similar terms. It is therefore almost always necessary to take EF as the zero reference level, because it is always difficult and usually impossible to establish the exact position of EF with regard to the vacuum level Eva c. The work function of the metal, O, is only approximately (0-2 eV) equal to Evac-EF. If one connects by a continuous surface in k-space all k's corresponding to EF, the so-called Fermi surface arises. For free electrons, as can be seen from equation 21, this surface is a sphere (EF - k2). For other, higher approximations this sphere is deformed; an example of a Fermi surface which has been established experimentally as well as by theoretical calculations is shown in figure 5b. The geometric form of the Fermi surfaces is already known for most metals [7]. The main techniques to establish the form of the Fermi surface are those associated with the so-called de Haas and van Alphen effect and the skin effect [7]. An important feature of a metal or an alloy is the density of states at the Fermi level N(Ev); this value can be determined by measurements of the magnetic susceptibility and the heat capacity at low temperature [7]. In the discussion on the electronic structure of metals and alloys and its relation to electron spectroscopies, the most important quantity is most probably the density of states, N(E). This is because a simple relation exists between the distribution of the photoemitted electrons I(E), and the integral density of states taken over all angles N(E): to a good approximation
I(E) = const.M~i.N(E) ininarN(E)f ~ 2
(26)
where Mf, i is ~ffinal H' ~initial dx and H' is the perturbation causing the transition from the initial into the final state. The density of states for the free electron in the final state is proportional to ~/E, so that for high energies it changes comparatively little over the
Structure and properties of metals and alloys
17
energy range of interest. This means that the distribution I(E) measured at the detector is only a slightly deformed density of states for the system before ionisation,
N(E)initiav
In the approximation of 'nearly free electrons', the density of states function resembles that shown in figure 6. This simple form already reflects the main features observed experimentally, and therefore in theoretical discussions the N(E) function is often
schematically pictured as in figure 6 (see chapters 2 and 3). Figure 5b shows the Fermi surface of copper. If this were a metal which could be exactly described by a free electron model, the Fermi surface would be perfectly spherical. The "necks" sticking out towards the (111) faces are caused by the periodic crystal potential V. NIE)
i
figure 6. Density of states in a band
I.
/
(Ema~ Emin)for a model of 'nearly free' electrons
i
s I
\
\
I
I I
I I ,,
E min
,/E m a x
-E
J
E max
E
The volume of the space enclosed by the Fermi surface depends on the total number of electrons n in the system; for free electrons, the surface area is proportional to n~'3. Knowing that, we shall now make a hypothetical experiment: we replace some copper atoms by atoms with more than one valence electron, for example, by zinc or aluminum. This increases the volume under the E F surface and since the sphere cannot in general case continue to grow into the higher Brillouin zone, because of a gap in energy on the Brillouin zone face, the sphere-like form will probably be deformed to fill up the states near to and just under the Brillouin zone faces. However, it is also possible that if the alloy could have another crystallographic structure than that of copper, and as a consequence to have another form of Brillouin zone, the additional electrons could be better accommodated at lower energies, for example, in a more sphere-like body. Thus, the average number of electrons per atom in such cases will dictate the crystallographic structure of the alloy. Hume-Rothery has formulated several very useful rules relating the most stable structure to the average number of electrons [8], and although some details of his theory are not longer valid, the basic idea is probably sound. There are also some papers which try to relate the Hume-Rothery's structural changes in alloys to the changes
18
chapter 1
in the catalytic activity [9]. Three approximations in the description of the behaviour of electrons in a periodic potential have so far been mentioned: (i) a model of free electrons; (ii) a model of nearly free electrons; (iii) a model of electrons tightly bound to the atoms (tight binding approximation, with L.C.A.O. used as a trial function). These approximations are useful to elucidate the terms of which the theory and an experimentalist make use and to identify the phenomena typical for systems with a periodic potential, but all three approximations mentioned are unsuitable for quantitative predictions. Approximations i and ii exaggerate the de-localization of electrons, while approximation iii considers the electrons as too strongly bound and too much localized on the individual atoms. Since all three are one-electron approximations and take the electronelectron interactions (Coulombic and exchange interactions) implicitly in the average potential, they do not treat this particular aspect properly. The higher approximations try to improve on this situation. Of many ways of doing it that are described in the literature, we shall mention only the following ones [10-16]: (1) Augmented Plane Wave (APW) and related theories, [10,1] and the Korringa-Kohn-Rostoker (KKR) approximations [12], which both attempt to improve the construction of the wave function; (2) electron density method (Kohn, Sham, Lang [13,14]) which explicitly treats the electron-electron interactions. Just a few remarks about these theories follow. One possibility for improving the constructed wave function is to cut the crystal in a space where the electrons behave as essentially free and in a space where they behave as being bound tightly to the nuclei: this is what the APW (Augmented Plane Wave) theory does. The Schr6dinger equation is then solved inside a spherical potential wall of a radius R. The electrostatic potential of the nucleus is hypothetically contained in this sphere, being zero outside. The solution for the sphere resembles that for free atoms, being a linear combination of products of the radial functions and spherical harmonics. The coefficients of the linear combination are then chosen in such a way that the solutions match smoothly, on the surface of the sphere, the plane waves which describe the behaviour of electrons outside the sphere [ 10, 11 ]. Another technique for constructing a wave function or crystal orbital which would describe properly the delocalized character of the electrons in the metal is the theory suggested by Korringa and by Kohn and Rostoker (KKR) [12]. In the KKR theory the atomic spheres are again considered. We can then imagine that at the surface of a certain atomic sphere there is a solution which we shall call the outgoing function ~out, and the same holds for all other atomic spheres. As a consequence at the surface of our first chosen sphere a combination
CI)in of all other waves exists. The two functions CI)in and Oout
are put equal on the surface of the atomic sphere, and they are mutually related as scattered and incident waves, with scattering depending on the potential inside the atomic spheres.
Structure and properties of metals and alloys
19
Both the APW and the KKR techniques describe the potential inside the spheres as an artificial potential, which has no components outside the sphere; it is called muffin-tin potential. Both techniques have also been applied to alloys (see below). Another successful approach to the problems of the description of the solid state has been suggested [13-16]. The authors of these papers have shown that the system of many electrons can be totally described by the electron density n(r), and have introduced a function E(n(r)), by the following equation [13,14]
E (n(r)) = T [n(r)] -
(27)
N n(r) e2 n(r)n(r//) E Z e 2 f Ir_RMI dr + f ir_--~ii + E,o._,o" + E.xch (n(r)) M---1 2 In this equation T stands for the kinetic energy of the non-interacting electrons with density n(r), the second term is for nuclei-electron interactions with the nuclei at the positions RM in the lattice, the third term is the mutual Coulombic interaction of electrons, is for Coulombic repulsion of nuclei and the last term is the exchange energy. The function E(n(r)) has a minimum when n(r) corresponds to the ground state density and the
Eion_io n
minimal energy is then taken as the ground state energy of the system. The practical approximation is to write down the equation for one electron functions with an effective potential and with the exchange term written in the so called local-density approximation. The simplest form of the whole theory is formulated for a model with an uniform continuous positive background, with electrons as discrete charges on it. This is the so called Jellium model. Many problems of chemisorption and promoter effects have been successfully attacked by this theory and many important conclusions derived [15,16]. To our knowledge it has not been used for alloys, for which it is not well suited. 1.1.2 Pauling's theory of pure metals This theory was formulated [17,18] at a time when the chemical bonding was usually described in terms of electron pairs and resonance structures, with the real structure somewhere in between them. Physicists never responded to Pauling's idea's with much enthusiasm, but all his ideas, including the theory of metals, are extremely popular among chemists. That is the main reason why they are presented and analyzed below. The other reason is to demonstrate that in reality it is hardly possible to avoid a more difficult theory [4-16] by accepting one such as that of Pauling. It is not possible to use semiempirical approaches based on vague reasoning and yet be able to make reliable predictions.
20
chapter 1
Pauling analyzed the crystallographic structures and distances between atoms for various metallic elements. In order to be able to compare structures with various coordination numbers, CN (CN is 8 for bcc, 12 for fcc) and with various numbers of electrons available for bonding (that is the valency, v). Pauling introduced the so called single bond radius, R(1) for all elements, defined as R(n) = R(1) - 0.600 In n
(28)
where R is in A and n is the bond order. For metals the latter is n = (v/f.N.)
(29)
The analytical form of equation 28 and the value of 0.6,h, for the prelogarithmic constant were derived from R(1), R(2) and R(3) of ethane, ethene and ethyne. With molecules used for this calibration the order n was thus always greater than one, but for metals it is always less than one. However, Pauling assumed the same equation to hold for both cases, the constant being only slightly adjusted, from 0.7A for carbon-carbon bonds to 0.6/k for all other bonds. The system of single bond-radii is shown in figure 7 [ 17].
Z.5
The first 10ng period 2.0 .
-"-%
\
-
~o
So~
z'\o._
=
T,-,o _ _o=o=g~ ..o,-~-~.o V"~,-O-o_o_o_O C.,r J Fe I NiCU I I ASs;"~r
1.0 -
Mn
0.5
0.0
The second long period
o
co~ 1.5
o
t
18 A
t
20
Co
Zn
Ge
o
Single-bond
o
Octahedral
A
Tetrahedral
I
30
-o'-o=_~o"~
Nb ,~176 Aq J I Tr ! Rh I Cd Mo ~ Pd Ru
metallic
I~"...
J 5Oj n Sn ie I
radii
radii radii
,1
36 Kr
1
40
1
50
t
54 Xe
figure 7 Single bond-radii as calculated by Pauling for the indicated metals and semiconducting elements [17].
Structure and properties of metals and alloys
21
In the same figure the tetrahedral radii are also plotted; they are real for s,pelements and fictitious for the transition metals. The straight line of the first period is described by: R(1) = Rl(SP 3) = 1.825 - 0.043
z
where z is the number of electrons
outside the argon shell. Transition metals show a contraction in R(1) and according to Pauling [17,18] this is due to the participation of d-orbitals in the metallic bond. He therefore introduced the concept of d-character 8(in %), a quantity expressing exactly how much of the bonding is due to the d-electrons. With this 8, he wrote the empirical equation for single bond radius R(1): R(1) = Rl(5,z) = 1.825-0.043z - (1.600-0.100z) 8
(30)
The form of equation 30 was chosen to describe the results and to fit the curves of R(1) vs z shown above only for the first row of transition metals. To understand how Pauling obtained the points necessary to derive the absolute values of the constants in equation 30 we must look to his treatment of the electronic structure of the magnetic elements iron, cobalt and nickel. Pauling speculated that each metal has three types of orbital: (i) atomic orbitals into which unpaired as well as paired electrons can be placed; (ii) valence orbitals into which electrons which form the metallic bonds are placed; (iii) metallic orbitals which are unoccupied and which mediate "unhindered resonance". To be able to explain the use of fractional numbers of electrons when describing the bonding, Pauling assumed that a metal can have several imaginary extreme structures, which are mixed in certain proportions to give the real structure. The real structure is that which results in the experimentally-found magnetic moments. This is illustrated by table 1 [17]. It is assumed that nickel has two structures which are mixed in the proportions 30% magnetic nickel and 70% nonmagnetic, in which all electrons are paired. This mixture leads to the experimentally found magnetic moment per atom of 0.6 Bohr magnetons, and in a similar way the structures of cobalt and iron are mixed to produce the experimentally determined value of the magnetic moment per atom. By constructing such hypothetical structures and mixing them in the indicated way, Pauling also calculated the 8% character, (the last column of table 1). He calculated values for the d-character for iron, cobalt and nickel and with them he created equation 30 for R(1). This equation has to fit all points which have been calculated from R(n)'s of individual metals. Then, he used the R(1) values to calculate 8 for non-magnetic metals and produced the table of valencies and % d-band character (see table 2), values of which soon became very popular amongst chemists. There have also been attempts [21] to apply the 8 values to explain results on alloys and even on sulphides.
22
chapter 1
table 1 Percentage d-character of cobalt, nickel and copper (Pauling theory) (brackets indicate bonding orbitals)
Outer electrons Metal 3d
4s I 4p
Co(B)
~ T T ~
Co(B)
T, T
Ni(A)
T$ T T ' ~ - ~ ~ .
Ni(B)
T~ T~
I/ i-
Cu(A)
~
Cu(B)
T~ T~T~
P~esonance ratio
Percentage d-character
35
35~oo X z~ + 6~o o X 3/~ = 39.5%
~
65
9
--o
].
I1-~
30
30~00 X 2/~ -Jl- 70~00 X 3/~ _-400-/0
70 25
25/~00 X 3/~ _Jr- 75/~00 X 2/~ -- 35.7%
75
table 2 Percentage d-character (d%) and valency (v) of elements in the first series of transition metals V
d%
Sc Ti
3 4
20 27
V Cr Mn Fe Co Ni Cu
5 6.3 6.4 5.78 6 6 5.5
35 39 40.1 39.7 39.5 40.0 36
However, the question is whether the popularity of the 5 values is justified. They have been derived from hypothetical electronic structures using empirical equations for
Structure and properties of metals and alloys
23
R(1)'s. It is doubtful whether equation 28 from which the argument starts and which holds for C-C bonds and the bond order n greater than one, can be applied to metal-metal bonds and n less than one. Hume-Rothery [20] collected some results which contradicted Pauling's statements on this point. However, even if equation 28 were of general applicability (as some modern authors assume [21]) a very mildly critical reader would still find many questionable steps in the procedure leading to the table of valencies and 8 values. 1.1.3
The Engel-Brewer theory of metals and alloys
This theory has a number of features that are similar to the ideas of Pauling: directed valencies, an important role of hybridization of orbitals on atoms constituting the metal, widely changing valencies and the omnipresent electron pairs. Brewer illustrates his theory with the example of tungsten [26], The configuration of tungsten in a free atom ground state is d4s2. However, the two s-electrons form, according to Brewer, a closed shell, which is non-binding and which in the solid state causes repulsion of other tungsten atoms. However, the configuration dSs is only 33,5 kJ/mol (8 kcal/mol) above the ground state, this difference being called promotion energy of the d 5 s configuration, and the d4sp configuration is 230kJ/mol (55 kcal/mol) above the ground state. Upon forming the metal, the energy of the das 2 configuration is supposed to be lowered by 569kJ/mol (136 kcal/mol), the dSs configuration by 890kJ/mol (211 kcal/mol) and dnsp configuration by 569kJ/mol (136 kcal/mol). We shall now examine the procedure by which the numerical values are obtained. Following Hume-Rothery, Engel [24] associated crystallographic structures with numbers of valence electrons in certain orbitals, i.e. with certain electronic configurations. Having in mind the elements: sodium (bcc), magnesium (hcp) and aluminum (fcc), with one, two and three valence electrons respectively, he suggested that the transition elements with the configuration dn-ls should have a bcc structure, with dn-2sp they should have the hexagonal close-packed structure and with dn-3sp2 the fcc-structure where n is number of valence electrons. Of course, some small deviations in n (for example alloys) are tolerated. Vice versa, knowing the crystallographic structure one can determine the number and distribution of the valence electrons over the orbitals. The authors of the theory [24-27] assumed further that the contribution per s or p electron is given by the interpolation line, which connects the points for metals having no binding by d-electrons, and serves as a calibration (see figure 8). The contribution to the binding strength by d-electrons is calculated in the following way. The promotion energy is subtracted from the sublimation energy: the former is fixed by the crystallographic structure of the metal in question. The structure determines, namely, how many electrons should be in the s and p orbitals.
24
chapter 1
Co
Sc
Ti
V
Cr
Mn
Ire
Co
Ni
Cu
I
I
I
l
i
1
1
I
I
5
1 4
1 3
t 2
[ I
Zn
60
figure 8 Brewer-Engel theory of metals Bonding energy (kcal/mole electron) of the indicated electrons (4 s,p or
*~ 50 -3 E
c 40 0 -~
3d, resp.) as a function of the position ~
XX
30
X
in the periodic table. Elements of the first long period are shown.
-a 20 u
3d
ue
E,p (the upper curve) is estimated by inter~extrapolation. E a calculated as described in the text.
IO o
F / 0
o
1. I
l 2
l 3
1 4
No. of unpaired electrons
per
=
-
0
atom
The total contribution by s, p bonding is then subtracted, values being taken from graphs such as that in figure 8, and the rest of the binding energy is divided by the number of unpaired d-electrons. For example, hcp cobalt is expected to have the configuration dTsp. From the sum of all d-orbitals, two should be occupied by pairs of electrons and three by unpaired electrons. The maximum possible number of unpaired electrons is considered as the ground state configuration. As can be seen from figure 8, while the contribution to the binding energy by s,p orbitals increases monotonically with atomic number, the contribution by unpaired d-electrons decreases. By circular argument, the authors [24-27] rationalize the crystallographic patterns in the periodic table of elements, using values such as those shown in figure 8. Sometimes the assignment of the most stable configuration appears to be easy, as with molybdenum and tungsten, but in other cases various configurations lead to very similar energies and thus to uncertainties, such as is the case with yttrium and zirconium. The Engel-Brewer theory has also been applied to problems of the stability and crystallographic structure of alloys, in particular to structures of some intermetallic compounds. Such compounds are formed when a metal on the left-hand side of the periodic table (i.e. a metal with almost empty d-orbitals) is combined with a metal on the right-hand side, where elements have several d-orbitals with paired d-electrons. Brewer stated [26] that "the use of empty orbitals of hafnium and tantalum by the non-bonding (i.e. paired) electrons of osmium or platinum could optimize the use of available orbitals and electrons, and approach the optimal binding achieved by tungsten". Using the example of Hflr 3 Brewer illustrated how difficult it is to make quantitative predictions of heats of alloy (compound) formation, that is, to go beyond qualitative predictions. Nevertheless, the number of cases of binary and ternary alloys where the predictions are satisfying is
Structure and properties of metals and alloys
25
respectable. Although successful as a semi-empirical approach, the theory [24-27] gave rise to some serious criticism. The obvious problem [28] is how to believe any correlation based on three outer electrons in fcc structures, when this is so far from the final description we need for noble metals? The Fermi surfaces of copper, silver and gold clearly show the presence of one sp electron per atom. Further, some low temperature structures are probably different [28] from those suggested [24-27]. The most important problem is the explanation of the structure of some magnetic elements and, on the other hand, the absence of magnetism in configurations like that of copper [29]. Some conclusions concerning alloys and structures stable at high pressures have also been criticized [29]. Modem experimental techniques (see chapter 3) have also made the assumption concerning extended charge transfer in alloys such as HfPtx doubtful. The Engel-Brewer theory has however been appreciated by some chemists [30]. The basis of the application was the idea that, by varying the composition of some alloys, one can go from one crystallographic structure to another. For example, one can start with pure molybdenum of the bcc structure, dissolve increasing amounts of iridium in it, until at a concentration known from the phase diagram, the structure switches over into the fcc structure of iridium. It means that in the state before alloying the elements had to have one sp-electron on the Mo-rich side, and three valence sp-electrons on the fcc side of the phase diagram. However, the change in the number of the sp-electrons leads according to the Engel-Brewer theory to a change in the number of d-electrons. Thus, it was expected [30], that the number of d-electrons could be varied by changing the composition of the alloys. It was not appreciated in this approach that the Engel-Brewer theory makes an assumption about the electronic configuration of the free atoms from which the alloy is made. The Engel-Brewer theory does not draw any conclusion about the electronic structure of the solid alloys and of course says nothing about the surface composition of alloys. 1.1.4
The Miedema theory of stability of alloys
This theory starts with a cellular model of solids [31]. A crystal of an alloy is divided into cells by planes which bisect the distances between the nearest neighbours and form the so-called Wigner-Seitz cells, which are analogous to the Brillouin zones in the reciprocal space, as discussed in 1.1.1. In an alloy, cells around elements of different electronegativity have different densities of electrons on their boundaries. Miedema suggested [32] that the enthalpy of formation of an alloy can be calculated by an empirical equation: AH = f(c) [- P.e (Ate*)2 + Q (Anws)2]
(31)
26
chapter 1
where P and Q are constants, f(c) is a symmetrical function of the molar ratios, and for an alloy AB forming a solid solution it is XA(1-XA), XA being the mole fraction of component A. A~)* is the difference in the values of ~* for the two elements, ~* being to a first approximation the work function ~; Anws is the difference in the values of electron densities in the Wigner-Seitz cells, all corresponding to A and B, respectively. Miedema showed that a more self-consistent system of enthalpies of formation, in better agreement with values known from experiment, can be obtained if one uses the adjusted ~* values tabulated by the author. The difference between ~ and ~* is small for platinum (5.55 vs 5.65 V), but somewhat large for some other elements (for Zr, 3.15 vs 4.05 V). Miedema suggested calculating the densities nws by using compressibility and V m the molar volume.
(B/Vm)v~, where
B is the bulk modulus of
The idea behind equation 31 is that electrons are transferred from atoms of a metal of lower electronegativity to atoms of a metal of higher electronegativity. According to Miedema [32], the charge transferred per a t o m mT~a can be calculated by AZ A = 1.2 (1-XA) A~*
(32)
This means that in Hflr 3 about 0.7 of an electron per hafnium atom is transferred from hafnium to iridium. The Engel-Brewer theory, which also explains the high stability of this compound (see 1.1.3) , assumes an opposite electron transfer. The experimental results, e.g. core level shifts, on various compounds of this type indicate that most likely there is no electron transfer at all (see chapter 3), but formation of strong partially-localized bonds takes place between unlike elements (see chapter 2). The practical success of this theory is indisputable. It is almost impossible to check the stability experimentally and to make some predictions concerning phase diagrams of all alloys of potential interest for material sciences. Miedema's theory, however, offers a certain tool for making rough but useful predictions, where experimental results are lacking. The theoretical background of the theory is however weak. It is too strongly associated with the assumed charge transfer between the components of alloys, and moreover, while the work function ~ is indeed a measure of the electronegativity of metal surfaces, a substantial contribution to ~ is made by the surface dipole, which is not present in the bulk at the Wigner-Seitz cell boundaries, where the charge transfer should take place. 1.1.5
The quantum theory of alloys
Quantum mechanical calculations on small organic molecules can achieve a very high accuracy, which is impossible to achieve with large systems of interacting particles,
Structure and properties of metals and alloys
27
such as solid crystals. Yet the fact that the potential in the solid can be taken as periodic, and Born-Karman conditions can be assumed to be fullfilled (see 1.1.1), allows us to be somewhat precise when treating the properties of large single crystals of metallic elements (see for example a comparison of calculated band structures with those derived from electron photoemission in chapter 3). However, when a random alloy is formed with elements A and B, the mole fractions being x A and xB, the periodicity of the potential is abolished and the degree of sophistication which is needed for a description of the same accuracy is considerably enhanced. Faulkner summarized the early development of the quantum theory of alloys in a paper [33] which we shall follow. The potential in the alloy A-B at a point r can be written as a sum of contributions from different lattice sites (Rn): V(r) :
•n Wn ( r - Rn)
(33)
V n is V A or VB according to the atom on site n, but the fact that in random alloys V is no
longer periodic is a problem in the description of alloys. There are several ways of coping with this difficulty, but we shall mention only three of them. (1) I n the Rigid Band Theory (RBT) one neglects the difference between A and B and assumes that the only consequence of substituting A for B is that the common band is occupied to a higher or lower degree, viz. E F is shifted, just by adding electrons to or extracting them from the pool of electrons under the Fermi surface [34,35]. A consequence of this model is that charge is freely transferred from one component to another, for example, from copper to nickel. The RBT was for a long time the basis of early theories of catalysis by alloys [9,19,36], but the total failure of this theory, and of ideas behind it, to explain the photoemission results (see chapters 2 and 3) stopped its application after about 1968, when the papers by Spicer appeared [37]. (2) The next level of approximation is a model of a virtual crystal with an average potential VAV [38,39] on each lattice point: WAy = XAVA(r) + XBVB(r)
(34)
The difference V Av(r) - Vo(r), where Vo(r ) is the ideal periodic potential, can be treated as a perturbation and it leads to small deviations from the Eo(k) function for the periodic potential. It has been shown that this is also a rather poor approximation. (3) Higher approximations stem from the theory of multiple scattering phenomena. This is appropriate, because the crystal orbital ~ is, in the context of these theories, constructed in such a way that the delocalization of electrons outside the atomic spheres is formally described by wave functions which look like a combination of "incoming" waves with
28
chapter 1
waves "scattered" by surrounding atoms. Atoms A and B are in this way considered as unlike scatterers converting the incoming function into different scattered functions, by the operation of potentials V A and VB. The operator which relates the incoming and scattered waves is t, there being different values tA and tB for each type of atoms. In early attempts an averaged scattering operator was used. tav =
XA tA + x B tB
(35)
but the results were even worse than with the approximation of the virtual crystal. The break through came when Soven [40,41] suggested using the following picture. A virtual crystal is constructed which has an initially undetermined coherent potential W(r) on each site. The scattering is caused by local deviations from this potential, so that the scattering operators are: tA = (VA- W) + (VA- W) CJ tA tB = (VB - W ) + (V 8 - W ) CJ tB
In equation 36, further below.
(36)
stands for the so-called Green operator, which will be briefly discussed
The reader is already familiar with the Schr6dinger equation" [-h2/2m) V 2 + V (r)] ~ = E ~
(37)
which in the operator form reads as" 12I~ = E ~
or ( E - 121)~ = 0
(38)
The Green operator is defined by an analogous operator equation: ( E - 121) G = 1
(39)
and it is very useful in describing scattering phenomena or other quantum mechanical problems, such as the construction of wave functions for crystals, which have a similar structure. For example, in the formalism and the language of the multiple scattering, the solution of equation 37 is written as [33]:
V-- r -I" (~Jo ]~n tn ~]/~n
(40)
Structure and properties of metals and alloys
29
where ~n is constructed in the form of
~n = (~ + Go
]~m,ntm 1]/im
(41)
The function ~) represents the incoming wave on the system and /I/in is the total incoming wave on the site n, that is, including the scattered waves coming from other sites m, and G Othe Green operator of the free propagation of the wave ~. The procedure of the Coherent Potential Approximation (CPA) is to determine the potential W from the condition that
XAta + XB tB = 0
(42)
This condition means, in the language of scattering, that the electron propagates in the alloy as in the virtual crystal and experiences scattering by V A and VB (equation 36), but scattering effects cancel on average (equation 42). After W is determined, the properties of the alloy can be found by manipulations of Green operators and its matrix elements. The most important of these manipulations is the calculation of the density of states function from the equation N(E) = - lm. Z m Im < Gmm> In this equation,
(43)
Gmm is the matrix element of the operator G, with orthogonalized
functions localized on site m. The term Im indicates that only the imaginary parts of the matrix elements are used in equation 43. By studying various restricted averages of the matrix elements of the Green function, one can determine various local densities of states, for example, the average state density localized around a particular type of atom [41]. The results of such calculations for some interesting systems are shown in figures 9 and 10 [42,33]. By comparing the results of calculations with the experimental photoemission results [33,42] in figs.9 and 10, we can see that the CPA theory successfully describes their main features. The basis for success lies in the theory properly acknowledging that sites A and B can differ substantially not only in number of valence electrons but also in respect of V A and VB. When inspecting figs. 9 and 10 one can easily arrive at the conclusion that, when a molecule like carbon monoxide interacts with a specific site on the surface, it feels whether that site is a nickel or a copper atom. According to the Rigid Band Theory or the virtual crystal theory, it would not be so, because in those models the individual atoms would be indistinguishable.
30
chapter 1
5
--1 4O |
77% Cu, 23% N,
35
>~
3O E o
o?
,/ ,/
._
zf
i/
hx \ I -\\ \-,~,
/,'
-
25
_
20~
~ o
"\_
i---
~5 ~ LL
I0
o
>-
/
o
i
-0.7
-0.6
-0.5
5
-0.4
...... -0.3
r- ..... -0.2
figure 9 Density of states for a Cu-Ni alloy and the projected densities of states for the Cu and Ni sites.
"l
-0.I
~Z
0
0
Notice: the UPS experimental results are at lowest energies deformed by the artefacts of the experimental techniques but the region round EI is correctly probed [44b1.
ENERGY BELOW E f (rydbergs)
I0
figure 10 Comparison of XPS valence-band spectrum for an evaporated Cu-Ni alloy sample with a density of states for Cuo.6-Nio.4 calculated in CPA. (from ref 42)
!
!
Q
i
I
.~ /
0
-2
/
.1
0.5
#
o
0
8
6
1
9
4
2
BINDING
ENERGY
( eV )
The Coherent Potential Approximation (CPA) has been worked out in great detail [43,44] and further modified and extended to ordered alloys. It goes much beyond the scope of this book to discuss these developments, but we refer the interested reader to some selected papers [45]. An extended comparison of experimental and theoretical UPS and XPS intensities has been presented [46].
Structure and properties of metals and alloys
31
Theoretical calculations using the CPA [47] and experimental XPS results [48] have been compared and found to agree for the Pd-Hx system (foreign atom is placed interstitially); the experimentally found hydrogen-induced states centered at 5.4 eV below the Fermi level strongly support the idea that hydrogen is present as atoms and not as protons which have injected their electrons into the d-holes of palladium. Interstitial alloys behave probably in a similar way. In relation to the various semi-empirical theories of alloys (see sections 1.1.3 and 1.1.4) we note that the alloys of s-metals have been also analyzed by CPA theory [49]. For these alloys the CPA theory predicts the existence of various localized states in the whole range of energies of valence electrons. There are also some other sophisticated methods which either generally or for some special cases are still better suited for exact calculations than the original form of the CPA theory [50,51 ]. Alloying is known to cause some redistribution of electrons on the individual atoms of alloy components. For example, palladium in silver has a narrower d-band than in pure palladium. In consequence, from a certain dilution up (about 60% silver), the whole narrowed band of states localized around palladium atoms falls below the Fermi level. This band therefore becomes fully occupied at the expense of the s-band. The narrowing of the d-band results from the diminished overlap of the palladium orbitals (see section 1.1.1 for the relation between overlap and band width), and from the suppressing of the d-d electron repulsion by dilution. These and similar effects should also exist in some other alloys and one has always to consider the possibility of a change in electron configuration caused by alloying. More intriguing is the question to what extent charge transfer between alloy components occurs. In chapters 2 and 3 we will discuss some results dealing with this point, but first we consider attempts to deal theoretically with this problem. Kfibler et al. [52] used the Augmented Spherical Wave method and the local functional density theory for self-consistent calculations; they calculated the total and partial density of states, that is, expressed per component and per orbital of a given symmetry, and from that they determined the occupation of s, p and d orbitals of individual components. They concluded, for example, that in Zr3Pd the palladium atoms receive 0.6 s,p electrons per atom from the zirconium atoms which each lose 0.2 s,p electrons per atom. The intra-atomic transfer of s to d electrons on palladium is calculated to be nearly zero. A closer inspection of calculations made by the same technique [52] reveals that the configurations for pure metals do not agree with the experimental results (e.g. copper has 9.5 d-electrons, instead of 10), so that the predicted charge transfers in alloys might be doubted. Predictions for core level shifts are also presented [52]. Early studies of the M6ssbauer effect on alloys of transition metals revealed somewhat large isomer shifts which were at first explained solely by charge transfer
32
chapter 1
between the alloy components (see chapter 3). However, later thorough theoretical work revealed that the original straightforward explanation needed serious corrections [53]. When an atom A with a spatially extended p- or d-orbital is squeezed in a lattice of another element B without that particular feature, electrons on the far reaching p-orbitals simulate in the space around B atoms a charge transfer. An analysis shows that indeed the p-like charge increases on B, but that this is mainly due to the tailing of the p-orbitals from A into the B atomic spheres and not to an increase of p-electrons on the on site orbitals of B, or to other bonding effects. Attempts were made to subtract the tailing effect or pseudo-charge transfer from the total charge transfer, the latter being calculated by integrating the density of electrons within the Wigner-Seitz or atomic spheres. Results of these very delicate calculations are shown in figure 11.
ol
' ' ' ' ' '
Tail Only
o.o , ............
'
_
/
I
. . . .
Mod Mulhken
'
i
't<
1
I .............
-01
-02
_
i
0.2
figure 11 Residual charge transfer at Ir, Pt and Au and the other atomic sites in compounds of the CsCI structure compounds after the
Au Compounds o
It. Pt,and Au Sites ri,.~..,
o,
\\
-
Pt Compounds Ir Compounds
[] ,,
_
j
effects of tailing were subtracted. This involves taking the total charge transfer and substracting of the tailing charge. The right hand column shows the results for the residual char-
ge when the modified Mulliken scheme is used to estimate the -0,1 + tailing. The left hand column shows the results when overlap Hf Ta W Re Os Ir Pt HI Ta W Re Os Ir Pt contributions to the tailing are neglected and is shown to provide some sense of how sensitive the results are to the treatment of the overlap. The open symbols were obtained using elemental volumes, however, when hafnium compounds have volumes a few percent smaller than the sums of the elemental volumes and the solid symbols indicate the consequences if this volume contraction is assigned to the hafnium site. ~rom ref 53) O0
Structure and properties of metals and alloys
33
For catalytically interesting alloys, for example platinum-gold, the charge transfer to platinum is +0.2 electrons when correction for tailing is not made, but with this correction it is -0.1. The difference in calculated charge transfer values, which is only the result of the use of different methods of calculations can be as large as 0.5 electrons. Conclusions are therefore very cautiously drawn [53]; too much should not be read into our figure 11 and perhaps of greatest interest is that once charge tailing is accounted for, the remaining changes in electron counts are consistent with a picture where electronegativities increase as one traverses the 5d row from hafnium to the elements to its right, with gold however being less electronegative than platinum and perhaps iridium as well. While consistent with some notions this is inconsistent with many notions concerning the chemistry of gold". The difficulty in assessing theoretically the extent of the charge transfer is clearly and simply demonstrated by this: one has to calculate charge transfer of the order of 0.1 electrons per atom with up to 10 electrons being involved, not knowing exactly over what space one has to count the electrons belonging to each component. In chapter 3, several electron density contour maps are presented (figures 13,21,22) which also touch the problem of the charge transfer. Actually they do not show much that one would call charge transfer. The reader will also find in chapter 3 other views and preliminary conclusions on the problem of charge transfer as derived from the totality of all results presented by that chapter. Finally, we mention the electronic theory of ordering and segregation in catalytically less interesting alloys such as these formed between simple metals, for example, alkali and noble metals [54]. An important step is made by using microscopic quantum mechanical calculations to predict macroscopic thermodynamic behaviour. Values for charge transfer between alloy components are derived, for example, for the 1:1 alloys: Li-Cs, 0.25 electron/atom from Cs to Li; Na-Cr, 0.27 electron/at from Cs to Na; Cu-Au, 0.11 electron per/at from Cu to Au; Ag-Au, 0.13 electron/at from Ag to Au. We have seen above how difficult it is to predict, if only in a qualitative way, the main features of alloys; use of the coherent potential approximation (CPA) theory was the first real breakhtrough. To predict the properties quantitatively, e.g. the amount of charge transfer, is again a task an order of magnitude more difficult. However, catalytic chemists want to know the composition and the electronic structure of alloy surfaces as well as their properties in chemisorption and catalysis. It is an extremely demanding task, but some progress has already been made in this direction [55-57]. For example a prediction has been made concerning the binding energy on a surface of an nickel-copper alloy [56]. Atomic adsorption on the hollow square of four atoms is considered and the conclusion is reached that the binding energy on this cluster is so much influenced by the average composition of the alloy that it drops by a factor of two when going from pure nickel to very dilute nickel-copper alloys. However an effect of this size seems to be at variance with the experimental results mentioned in other parts of this book.
34
chapter 1
The problem of adsorption on alloys has been also approached theoretically by other authors, for example, in the book by van Santen [59], whose treatment is simpler than that in [56] although it goes further in applying the theory. Van Santen concludes that there is an electronic structure effect on the binding energy of hydrogen atoms on an alloy cluster where the average number of valence electrons in the alloy is a variable. The nickel-copper system is a favourite subject for calculations, since it concerns somewhat light elements, and also many of the experimental results relate to it. Castellani [60] studied it by Extended Htickel Theory and concluded that there should be a charge transfer and an effect of copper on adsorption properties of nickel. Placing of nickel into a copper matrix causes according [60] a decrease of about 30 kJ/mol in the heat of adsorption of carbon monoxide, when compared with nickel in a nickel matrix. An even more pronounced charge transfer from copper to nickel was found in [61 ]. Simple LCAO theories are good enough to explore new phenomena [59-63] such as adsorption on multicomponent systems, and to introduce the terms necessary for the description of the experimental results, but they are not very reliable for making quantitative predictions. However, some pioneering work in higher approximation has already been carried out too. Muscat [64] has studied hydrogen adsorption on the (111) face of copper, in the surface of which a nickel atom impurity is placed. Binding energy was first calculated for a cluster using 19 muffin-tin potentials of either pure copper or with one nickel atom instead of one copper atom (see figure 12). figure 12 Model clusters HCu19 or HCu18Ni used in calculations by Muscat. Circles-upmost layer, triangles - the layer under it, square - an atom in the next lower layer. Cross indicates the position of H. Placing Ni in position 1 changes the one electron energy of the system by 0.40 a.u. (in other words the ensemble NiCu2 of nearest atoms behaves very differently from the ensemble Cu3). However placing of Ni in position 2, 3 or 4 causes an effect 4-5 times smaller and with Ni in 5 and 6 the effect is zero (this is a negligible interaction through the metal).
In the following step, the energy change was calculated due to the cluster being embedded into an effective medium consisting of a homogeneous electron gas. The results were quite interesting: the effect of introducing a nickel atom in a copper matrix is only important when a nickel atom is in the position 1 (see figure 12). In position 2 it is five times
Structure and properties of metals and alloys
35
smaller and in position 3 it is small and of the opposite sign; in position 5 it has no influence at all. Obviously, hardly any of the effect of nickel is through-the-lattice, viz. through the collective metal properties; it is clearly a short range, chemical bond effect. We can extrapolate this conclusion and say that on the (111) surface of copper-nickel alloys one can expect four types of tri-atomic cluster each showing a distinctly different binding energy towards hydrogen: Ni 3, Ni2Cu, NiCu2, Cu3 , the last showing very weak binding; we may then try to explain chemisorption and catalytic results by this model, in which the ensemble size (Ni 3, Ni2 ...) plays the most important role. We shall turn to this point in chapter 9 and shall see that this approach can be successful. The results for the calculated charge transfer and ligand effects show an interesting and clear picture: the simpler the theory is, the more pronounced is the charge transfer. The reader will find further discussion on this point in chapter 3 and elsewhere in this book.
1.2
Some results of the theory of chemisorption on metals and alloys
1.2.1 General features of chemisorption - a qualitative picture based on quantum chemical calculations
1.2.1.1 Chemisorption of atoms The reader will be familiar with the quantum mechanical theory of bonding in a molecule A-B. The simplest case is that of two atoms each having one valence electron in one atomic orbital (see [1-4] or, more advanced text [65]). The main terms and results of such a theory are summarized in figure 13 and the information contains the first ingredient needed to build up a qualitative picture of a chemisorption theory. A solid is actually a giant molecule. Due to the mutual interaction of all atoms in the solid, the molecular energy levels form a whole band of levels (see 1.1.1 and figure 14 below). In analogy with diatomic molecules, the lower part of the band is called bonding and the upper one antibonding. We have presented a simple theory of band formation in section 1.1.1 from which we know that the band width is proportional to the matrix element 13 (1.1.1, equation 19). In principle, both the metal atoms, on left-hand side of figure 15, and the adsorbed atoms or molecules at high surface coverages can form a band of energy levels [66]. However, let us start with the simplest case. A single atom, with one electron in a single valence orbital on a single energy level, interacts with a solid, the electrons of which occupy a band of energies (fig.15, left-hand side) The interaction of an atom with a solid can be described by one of the two limiting cases: (i) the interaction is weak and leads to a broadening of level A into a virtual band
36
chapter 1
inside the metal band; (ii)
the interaction is strong and can be described as the formation of a pseudomolecule from atom A and one or several surface atoms of the surface; by interaction with the solid, the energy levels of the pseudomolecules are broadened again into a narrow band.
Before
bond formation
Before
ab EAB / ~abantibonding M.O. \ \
bond formation
I
/
I
\
I
/
Ea,~ a
/ / /
I
/
b
.E
b
metal
~
/
AE
\
, I
/
/
adsorbate
atom
/ /
/
I
/ \
atom
\\
/
/
I
/
z
Eb B
~ B ' b o n d i n g M.O.
after
amplitudes
"
b
a
~a b "
b
a
~B
bond
formation'
figure 13 Left and right: energy levels of atoms b and a, before a molecule AB has been formed. In the middle, energy levels (bonding-B, antibonding ab) corresponding to the molecule AB are formed by the interactions of electrons on the level b and a. In the crudest approximation: 2 89 A E = [4V2,b,a + (Eb-Ea) } , with Vo,a = ~dOo n ~ E a = ],/2(Eb+Ea) + {V2b, a + 1/4 (Eb-Eo)2} 89
dr,
37
Structure and properties of metals and alloys
a)
b)
c)
----~N (E)
l
o~
N=8
N=
oo
N=
oo
figure 14 One dimensional chain of atoms, with indicated numbers of atoms N a) b) - energy bands c)
- density of states curve, corresponding to b)
The theory for both limiting cases was developed in the 60's, and in some earlier pioneering papers [67]. What follows is based on some of the original papers [68-71] and some reviews [15,16,59]. Let us start the discussion with the weak-bond limit.
figure 15 Weak bond limit in the formation of a bond between atom A (single orbital, single electron) and atoms of metals. Metal electrons occupy the energy band up to Fermi Energy. By interaction, level A broadens and becomes partly occupied. The interaction of an electron in a n E A level (figure 15, right) with electrons in the band (figure 15, left) leads to two effects on the E n level: (i)
the E A level is shifted on the energy scale, and
38
chapter 1
(ii)
by the interaction with atoms, the electrons of which form the band, it is broadened.
The broadened level E A can be fully or partially occupied by electrons or be completely empty, in which case it represents an A + ion. When the energy band for the metal is narrow, the broadening of the adsorption level is less pronounced than when the band is broad. Let us now make a step to the strong-bond limit and consider the so-called pseudomolecules. The strong pseudomolecular interaction splits the E A level into two broadened levels: a bonding and an antibonding one, with a gap in between. This is similar to the situation with the molecule AB in figure 13. The antibonding band can be either above or below the Fermi level E F, or it can be split by EF into an occupied and unoccupied part. This last case is shown in figure 16. E
--EF~ d
- E A- m e t a l ,
EA
ontibonding EA- m e t a l ,
1
S
bonding before
N(E)
interaction
figure 16 Strong chemisorption (pseudomolecules) limit. Chemisorption of atom A on a transition metal with s and d bands. Left - before interaction, right - after interaction. We shall now investigate whether and how the simple theory can explain the trends in the chemisorption bond strength for adsorbed atoms when metals of the periodic table are compared. Theoretical analysis [15,16, 59, 68-71] shows that we can make the following simple statements. (i) The position of the E A levels (bands) and their separation from each other are mainly due to the interaction of the electron in the EA levels with those in the orbitals of the solid. It is mainly a group of d-orbitals on the metal atoms adjacent the A atom which have, moreover, suitable symmetry that are involved in forming a bond with A [72,73]. These d-orbitals are sometimes called 'group' orbitals.
Structure and properties of metals and alloys
39
(ii) The interaction of the E A electrons with those of the sp-band is mainly responsible for the level-broadening, while its influence on the downward movement of EA is less important. The latter aspect should be particularly kept in mind when the metals of one period are compared with each other: the variations in the number of the sp-electrons ns are smaller than variations in the n d, the number of d-electrons. The exact position of the broadened levels EA also depends on the strength of the electron-electron interaction in the given orbitals. These considerations are sometimes thought to be sufficient to explain the trends in chemisorption bond strength along the periods [15,16,59,68-71]. When going from left to right in the Periodic Table, the band width and its occupation changes as is roughly illustrated in figure 17.
Ti
Fe I I
Cu I I
I
F
"~Z
f
figure 17 Position and width of the d-band as a function of the atomic number Z, within the first metal period of the Periodic Table (the occupied part is indicated). The trend in figure 17 is caused by the effect of nuclear charge (Z e): a higher nuclear charge contracts the orbitals and lowers the overlap, so that a lower band width results; the band narrowing and decrease in the energy of the band occur in the consequence. Certainly, the greater the nuclear charge, the lower is the energy of the electrons in the bands. A similar trend is observed in the positions of both the antibonding and bonding E A levels (bands). The antibonding band is almost empty for metals on the left of the Periodic Table, but becomes increasingly occupied as Z increases. The more electrons that occupy the antibonding bands, the higher is the total energy and the lower is the chemisorption bond strength. This explains why the heats of dissociative adsorptions of hydrogen and oxygen, and so the respective atomic chemisorption bond strengths, decrease
40
chapter 1
on passing from left to right across each period [68,69]. The trends are shown in figure 18.
~176 1
figure 18 Experimentally measured
-
chemisorption energies
>
f o r hydrogen and oxygen
"-"
on the 3d transition metals.
>(_9 CV_
(selection by [68,691). The trends are the same in the 4d and 5d series. The theoretical results shown are f r o m a model calculation within the effective medium theory
2.0
9
-4.0
I
I
I
1
I
~
~
I
!
9
1
I
w Z
tu
0.0
9THEORY u EXPERIMENT (poly)
[OXYGEN]
z o a_ -4.0
!"1
t2s
o
rl
t.t3
~_ I11
[68,691.
9THEORY a EXPERIMENT
[HYDROGEN l
I
-8.0-
(D rl
-12.0~ T - -
Sc
'
i
Ti
I
V
i
I
Cr Mn
I
Fe
i
Co
I
Ni
i
Cu
The theory predicts that, in all cases of adsorption of atoms, the most likely position of an adsorbed atom is in the valley between several surface metal atoms. The higher the coordination of the adsorbed atom, the greater the energy lowering by adsorption. Adsorbed atoms prefer to occupy the sites at which the crystal would grow by accretion of further metal atoms. With non-dissociating molecules the situation is different, as we shall see below. Several authors have addressed the problem of the localization of the chemisorption bond. Does an adsorbed atom on one site influence the adsorption of another atom on an adjacent site by an interaction through the metal? This problem also exists with regard to the promoters or supports interacting strongly with metal particle. The answer is in the affirmative; there is indeed an interaction through-the-metal [74], but the size of this interaction requires discussion. Schrieffer approached this problem by analysis of experimental results concerning the strength of the interatomic interaction of chemisorbed atoms. He concluded [75] that the total of through-the-vacuum and through-the-metal interaction
Structure and properties of metals and alloys
41
energy amounted to about 5% of the energy of the chemisorption bond formation. It is not known how much of this is due to the through-the-metal interaction, but in any case 5% of the adsorption energy would be the limit. This all indicates that any through-the-metal interaction is of a very short range character. This conclusion is also supported by figure 19 [76], where we can see that the change in the electron density due to an adsorbed hydrogen atom is very limited in space.
1 IIII;/; ;/IIIII~
8
4
11
-8
-4
0
4
ii!11
0
8
figure 19 Total electron density map, H-atom on 7ellium'-metal (parameter rs = 2) Haas is in its equilibrium position of 1.1 a.u. from the jellium edge. The shaded area indicates the positive background of 7ellium'. Distances in atomic units (a.u. = 0.053 nm). For comparison the lattice constant of Ni is 6.66 a.u.; the parameter rs = 2 describes the somewhat high density of the positive charge, for example in a metal such as Al or Pb.
1.2.1.2 Chemisorption of undissociated molecules
The main features of the theory dealing with this problem can be best understood by taking adsorption of carbon monoxide as an example. Metals can be classified into the following groups. (i)
The block of metals between Sc to La (on the left side) and Cr to W (on the right side). Adsorption of carbon monoxide is dissociative at low temperature and the resulting oxides are stable against reduction by hydrogen or carbon monoxide; Mn and Re probably behave similarly.
42
chapter 1
(ii)
Fe, Co, Ni, Ru, Rh. These metals dissociate CO at room temperature such as Fe or slightly above 400 K such as Ni. Oxides are reducible by CO above 500 K.
(iii)
Pd, Ir, Pt. Carbon monoxide is adsorbed strongly, but non-dissociatively, unless the surface is highly defective [77].
(iv) Cu, Ag, Au, Zn, Cd and Hg adsorb CO very weakly. We shall discuss below the non-dissociative adsorption of CO, but many features of this adsorption are also common to other molecules: NO, ethene, alkenes in general, benzene, etc. In the usual approximation, orbitals of free atoms which constitute the molecule combine to form molecular orbitals. A correlation diagram showing which atomic orbitals form which molecular orbitals in the case of CO is presented in figure 20. Even the simplest theories give us a reasonably accurate idea about the form of the electron densities, viz. the shape of particular molecular orbitals. A very useful collection of shapes of molecular orbitals has been presented by Jorgensen and Salem [78]. The shape of those orbitals which are most relevant for chemisorption of CO (Frontier orbitals, including the highest occupied and lowest unoccupied orbitals, i.e. HOMO, LUMO) are shown in figure 21 [78]. Electrons of the metal interact through the orbitals of the surface atoms (in particular the group orbitals, see above) with the bonding and the lowest-lying antibonding molecular orbitals of the molecules. The situation for CO is as in figure 22. Such picture was first used by Dewar to explain bonding in n-complexes of ethene, and was later adopted by Blyholder to describe the chemisorption of CO [79]. The bonding due to shifts of electrons indicated by the arrows occurs through what is called a d o n a t i o n - b a c k d o n a t i on
mechanism. While delocalization of the 5cy-electrons in the HOMO stabilizes the C-O
bond and frequency of the stretching vibration increases, partial occupation of the antibonding orbitals by electrons destabilize it. The delocalization of the 5or-electrons is often away from the M-C region as found e.g. for palladium and copper [80]. Occupation of the antibonding rt-orbitals of adsorbed CO is increased when an positive ion is placed near to oxygen atom or the centre of the C-O bond. This is an electrostatic and not a binding effect, since placing a point charge without any orbital has the same effect [81]. When temperature is increased, more electrons can be raised into the rt*-antibonding orbitals. A higher temperature also activates the tilt of the C-O axis [82] away from the most stable orientation [83] and towards the metal surface. The perpendicular orientation the most stable on (100) or (111) fcc faces, can also be perturbed by CO-CO interactions, as is clearly the case on (110)fcc planes.
Structure and properties of metals and alloys
43
;~ff--W_,"2:;;
"h 2 , . ' . . . .
2/7, E:
O. 1268
,',, - . . . . . . . . . . . .
.s;,. .~:.~-'-.-
1,
~ . ,"
E=-0.6395
-,
.4
"k.L. -'---;-~"
-'r . . . .
;'
-..-i-50
E=-O.
,, .;};'_-i./...o /,.., ,._..,_.._.-,x,,..
V.; I - - 7 - -
5544
-,.
,'..".-"
40 E=-O. 8038
30
figure 20 Shapes of selected MO's of CO [78].
F=-1.5210
1 IT E = - O ,
6395
44
chapter 1
6o-"
\\
/
\\
i1~/ I I
\ \ \\ \\
II II
\\ \\
I I
\ \
I I
/ 2p
/
\ \
I / / //
\\\ \ \ \ \
27r ~
/.~
--e%-~L__ ,~,
">--'4~- ~
/ .-~ .... \
,,.
a 1= ~"+'r<\
.x4~-=.->~" "\ ~"
\, \\. \\ \ \\\ \
\ \ \\\
"-~,/\ \ I
,,~ ~, 1
\\\
' * '*
\ \ 2S
i
3n
C
CO
1,
~ ~ 0 ~ ~ 0 _.
2%
_ ~
,% 10"
g
~ -----------
.........
..........
0
....--Q~~O
so.
~
,0.
@ "" "- :
N2
-
~o
"" --- -.. _ . . .
10"
Q
CO
figure 21 Correlation of orbital diagrams Upper part: relation between atomic (C,O) and molecular (CO) orbitals, only orbitals of higher energies shown Lower part: a comparison of homo-atomic (N2) with hetero-atomic (CO) molecule. N.B. The vertical position is related to the total energy, but is not on scale/
Structure and properties of metals and alloys
figure 22 A schematic picture indicating the main contributions to the M-CO bonding.
45
Ik
\\V ~ ii When the oxygen atom comes near to the surface, the molecule can dissociate. Whether or not the dissociation takes place depends on the activation energy of the tilt and the thermodynamics of the C and O adsorption [84-86]. These atoms have to be bound to several contiguous surface atoms, that is to say to an ensemble, as has been established both experimentally [87] and theoretically [88]. The theoretical calculation revealed that various pathways of dissociation are often possible, but that they differ in activation energy. Several of these pathways are shown in figure 23 [88]. v('(" ( 111 )
v('( (10())
44 kcal
36 kcal
/
moi-l
mol-~
figure 23 Minimum dissociation energy paths of CO on large clusters of Rh. The activation energy on the (111) surface is predicted to be 8 kcal moU (33 kJ mol 1) higher than on the (100) surface.
Adsorption of alkenes is very similar to that of carbon monoxide, but the donation mechanism is more important. With other multiple bonds some caution is necessary. For example, one often sees a schematic picture for the bonding of ketones and aldehydes in the following form:
46
chapter 1
\c o J
"
M
However, the position of the HUMO's and LUMO's of molecules having a C=O bond [78] shows us that the donating HUMO orbital is localized on the atom O; in a classical chemical terminology, it is the lone pair. It is then better to write schematically C=O~M. The geometry of this complex is not favourable for the back-donation. Within the metals of groups 8-10, the heat of adsorption of carbon monoxide does not vary very much, but, as with other molecules or atoms, it decreases when going from the left to the right in each period [89]. There are clearly observable differences between the various crystallographic planes of the same metal and we must remember this when analyzing trends in chemisorption bond strengths. Calorimetric in measurements of adsorption heats with molecular beams and single crystal planes [90] has shown that each of the various surface structures, characterized by LEED and other means of surface crystallography, has its characteristic adsorption enthalpy. Older calorimetric results, obtained with polycrystalline materials such as powders and films, and quoted elsewhere in this book, agree reasonably with an averaged value at least in this case. Thus, old results on films and powders should be used with caution, but in the absence of a better alternative they can be used for correlations with other physical properties of metals. It is difficult to predict theoretically the optimal location for CO molecules to adsorb. The differences between various potential sites are not large and the optimal position is a result of various factors such as [59] (i) optimal binding with various group orbitals, (ii) minimum repulsion interactions between the fully occupied orbitals and (iii) mutual interaction of adsorbed molecules [83,91]. The above mechanism of dissociation, i.e. first, a partial occupation of the antibonding orbitals of the molecule and then bonding of the individual atoms of the molecule to the surface, is probably operating in many other cases (e.g. N2, NO, 02). Some molecules, CO being one example, require at least two sites to become dissociated. Two sites each consisting of several atoms means that quite a large ensemble is required for dissociation of such a molecule. However, some other molecules can also possibly be dissociated on one site or perhaps even on one atom. Such is the dissociation of the H-H bond in hydrogen or dissociation of C-H bonds in hydrocarbons.
1.2.1.3 Adsorption of molecular fragments Not surprisingly, hydrocarbons have attracted much attention. A simple quantum mechanical calculation has confirmed, for example, what the intuition would suggest [92]:
47
Structure and properties of metals and alloys
a CH 3
radical with one electron in
an
sp 3 orbital binds to the top position,
a CH 2
radical
binds to two metal atoms and a CH radical to three metal atoms on the (111) face of an fcc metal. However, this example can immediately serve as a warning: the reality is sometimes more complicated than the intuitive picture. Calculations involving very high approximations have revealed that for CH 3 the optimal site is that offering the
highest
coordination [93,94]. The first result seems [92] to be an artifact caused by the narrow basis of orbitals employed. The orbital of NH 3 which causes the binding of the molecule to the metal is the doubly-occupied lone pair orbital of E-symmetry on the N atom. With nickel, where the Fermi energy is low and well below the vacuum level, the highest coordination site is the optimal one. However, with a metal such as copper, which has a lower work function, the balance of various factors leads to a different result: the optimal site is on the top of the copper atom [95]. The situation may be similar with the CH 3 radical, which on platinum may be optimally bonded to a single atom [71]. However, exact theory or an experiment must solve this problem. Other fragments, for example, on the PF3, PF2, PF series [96] have been studied, as has the formation of ethylidyne (- C-CH3) [97]. The problem of dissociation of chemical bonds on the surfaces of metals, as well as the recombination of fragments and atoms, is of great relevance for catalysis. Examples include hydrogen dissociation [98]; C-H dissociation and recombination [98, 99], dissociation of N 2 [100] and of CO [88]. It would be extremely helpful if, in future, the theory could also supply information on the expected behaviour of alloys. Lennard-Jones [101] suggested a very useful picture of dissociation more than sixty years ago. This was a general form of potential energy curves which would hold for dissociations of simple diatomic molecules A 2. The idea is as follows. If there were no help from the surface, the dissociation would have a potential barrier as high as the dissociation energy D(A-A). However, if the A 2 molecule is first bound to the surface by a weak chemisorption or by physisorption, its approach from
infinity towards surface,
which leads finally to dissociation, would not encounter a barrier nearly as great as the full D(A-A) energy. The activation energy can indeed even be zero (or < 2 kJ/mol ~) with the mediation by weakly chemisorbed intermediates, as with H 2 on transition metals. This situation is shown in figure 24. A similar picture should also hold for dissociation of various other bonds, such as for example CH3-H, etc. However, recent work on molecular adsorption dynamics has shown that, although this picture is still useful, it is a very simplified one [15,70]. It does not account for the possibility that a molecule need not approach the surface in a straightforward way: it can for example be repeatedly reflected by potential walls. Furthermore, it is also difficult to express by such pictures the role of the energy in the particular degrees of freedom and the tunnelling effects, etc.
48
chapter 1
E
O (AA)
Distance,
r
Metal -A 2 . phys. k
ads
chemis
A. ads
Metal - A
figure 24 Potential energy curves used to describe the process of A2 dissociation (Lennard-Jones):
A 2, gas (r ---) ~) ---) A2, phys.ads. ---) A2 weak chemis. ~ A, ads.atoms. While the direct transition gas ---) Aaa~ requires a very high activation energy (D(AA)), weakly bound forms allow a transition via a much lower barrier (zero, with weak chemisorption).
1.2.1.4 Semi-empirical approach to the problem of chemisorption An important initial step in the theory was taken by Eley [102], who suggested applying to chemisorption an empirical formula due to Pauling [17]. Pauling was the first to make an estimate of the bond strength of a molecule AB from data on A 2 and B 2 molecules. According to him the bond strength E(A-B) is an average of bond strengths in A 2 and B2, corrected by a term containing the electronegativities x. The form of the correction as well as the values of x were chosen to make values of E(A-B) fit a collection of experimental results. Eley, by analogy to Pauling's approach, suggested that, for example, for H atom chemisorption: E(M-H) = 1/'2 {E(M-M) + E(H-H)} + 23.06 (Xmet-XH)2
(44)
where E(M-M) is the sublimation energy divided by the coordination number of atoms in the metal M. For the difference in electronegativities, the value of the bond dipole of the chemisorption bond can be taken, this being determined from measurements of work function changes upon adsorption. For other molecules and radicals, similar equations have been suggested [102]. Estimated bond strengths cannot be expected to be very accurate and the theory is
Structure and properties of metals and alloys
49
not straightforwardly applicable to our topic, adsorption by alloys. However, at the time it was suggested , it was extremely important that it had been shown, that the adsorption bond is the same kind of bond as that in free molecules. Several other semi-empirical approaches have been suggested, but we shall confine ourselves to mentioning only the most general one: the bond-order-conservation theory [103]. This is based on two postulates: the first postulates that the energy of a pair of atoms forming a chemical bond depends on the distance between the nuclei r, according to the Morse curve:
E(r) Qo 2 e x p ( - ( r - r ~ a
_2(r_ro) ] a
-exp(~)
(45)
This can be rewritten as with x substituting the exponentials
E(r) = -Qo(2X-X 2)
(46)
where the exponential function x is called the bond order. For a diatomic molecule, when r equals ro, E is equal to - Qo and the bond order is unity. If an atom A is bound to an ensemble of several metal atoms, for example, A-M2, the individual pair-wise bonds A-M then have a fractional order [103]. However, the
second postulate of this theory requires that the sum of the individual orders must be unity:
•
x~ (A-M i ) - 1
(47)
i=1
The main components of the theory are thus the two observables ro and Qo, which have to be taken from experimental results, and the two foregoing postulates. With this theory it is then possible to make predictions regarding pathways for dissociation, recombination, or migration, for various catalytic reactions. An example of the application of this theory is the prediction concerning the selectivity of various metals in syngas reactions [104]. The reader have probably noticed that the relation between the bond order and bond length is formally the same as that proposed by Pauling's equation (see eq. 28). Thus, the same criticism which has been expressed above concerning his theory applies here too. Further, this theory assumes that the energy of a bond is a function of only its length, being independent of the angle which this bond makes to other pair of atoms bonds. For the adsorption of species which make a directed bond to the surface (for
example, ,CH3, CO) and with surfaces which bind the adsorbates by spatially-oriented orbitals, this is not a very helpful restriction. On the other hand, this theory is somewhat flexible and suited to make the zero-approximation estimates. It has already been applied to promoters [103] and application to alloys should be possible.
50
chapter 1
1.3
Adsorption of molecules and radicals which are intermediates in catalytic reactions The understanding of results obtained in catalysis by alloys requires some
knowledge of reaction intermediates. Many have been postulated, and it is important to realise that many are species which are very well established experimentally. Kinetic and spectroscopic evidence exists for some of them and only the most important will be described below. For more detailed information, the reader must consult the original papers. The theory concerning adsorption of H2, 02 and CO has already been briefly discussed and we shall now concentrate our attention on other molecules. However, now we have to rely more on experiments than on theory alone. Some aspects of the carbon multiple pictures shown below as rt-bonding, metal-carbon multiple bonding etc., have been analysed theoretically, but other aspects of our picture are from experiments. 1.3.1 Hydrocarbons Hydrocarbons form a large variety of adsorbed intermediates on the surface of metals and alloys. Burwell [ 105], who made the first inventory of species for the existence of which good evidence was available, called this collection an organometallic zoo. The most important species are presented in figure 25; a comment and an explanation follow. Alkyl radicals (1) have been observed spectroscopically (for example, by EELS and IR) but their existence, doubted ever by theoreticians and many organic chemists, was first extablished by the very easy D2/alkane exchange [106]. The product distributions suggest (see chapter 10) that the exchange proceeds via one of the following three species: (i) alkyl radicals (see 1). (1) as been observed spectroscopically (for example by EELS and IR), but their existence doubted ever by theoreticians and many organic chemists, was first established by the very easy alkane/D2-exchange [106]. The product distribution suggest strongly that the exchange proceeds via one of the following three species. (i) Alkyl radicals (see (1), with these species substitution of H for D occurs in a stepwise fashion and the product distribution has throughout a binomial character. (ii) Species multiply bound to the surface such as 2 and 3 for which species there is now also EEL-spectral evidence too [107-109]. (iii) Species bound as in 4 and 5 for which spectroscopic evidence is also available [107-109]. Multiply-bound species (ii) and (iii) are responsible for multiple exchange, i.e. the cases where more than one H atom is exchanged during one sojourn of a molecule on the surface, in consequence of which the initial product distribution deviates from the equilibrium (binomial) one. The deviation can be even so pronounced that the fully exchanged alkanes appear while the products with one or two D atoms do not desorb into the gas phase.
51
Structure and properties of metals and alloys
It is important that this chemical evidence, as just described, does exist for the various reaction intermediates, since some sceptics doubt whether the spectroscopies (EELS, IR) can detect reactive intermediates at all, observing instead only unreactive spectators of the reaction.
R I CH2 I
(b
I
R i CH II
/\
@
|
--c--c~
I ~i
-X
9 CH 2 ii CH
/i\
-X-
@
~
@
C
a'[
\C / /, )~
\C
/
l\ )~
|
-X-
|
\C /
\C / / \
Y
I
-X- -~ -~
a~
|
CH2 ~ CH2
R I c
i
\C / / \
R I CH
| ,C ,C,_-C C,." " '~.:t-t:: "~ ~
@
C C I I C--C--C--C , , c c I I
@
figure 25 Various surface complexes formed by chemisorption of hydrocarbons. Only those species for which a solid documentation exists are presented here. Spectroscopic evidence [107,108] exists to show that species 4 is preferred by palladium and 5 by platinum; and that both can be converted at higher temperature and low H2 pressure into species with a higher degree of C-H bond dissociation [109-112]. The latter species (2, 6, 7) exhibit a lower reactivity in hydrogenation than those mentioned above. Good evidence exists too for associatively adsorbed (horizontal oriented) benzene (8) [113]. This all is in compliance with the theory discussed in 1.2.1.3.
52
chapter 1
For all the species 9 to 12, evidence comes only from kinetic and isotopic labelling studies, but it is fairly solid. Carbon-labelled iso-hexanes have for example been used [114] to show that two mechanisms operate on platinum in skeletal isomerization, one with a C3-cyclic intermediate and one with a Cs-intermediate. Both these intermediates are indicated in figure 26.
4-_ C l', iI
C
II
I
C-C~C-C-C
c
I~ C-C - C - C - C
C-C-C-C-C
30/
(1)
5
{2) C-C-C I
-.-t:I
C C
C i
~- C - C - C ~ C - C
7'(
figure 26 Isomerisation of a CU-labelled 2-methyl-pentane can proceed by formation of complexes in which either 3 or 5 carbon atoms are involved. The two indicated pathways produce differently labelled 3-methyl-pentanes. The two distinct intermediates can thus both produce one and the same product 3-methylpentane, but the products of the two pathways differ in the positions of the label. The contributions by individual intermediates to the product distribution can be derived from the concentration of products labelled in different places. While the arguments for the existence of the C 3 and C 5 intermediates are strong, the technique does not tell us how the species are bound to the surface. Indirect information suggests that species resembling 9 or 10 may be involved or an analogous species in which carbon atoms 1 and 5 are bound to the surface. By using variously substituted pentanes, a very likely intermediate for the C 5 pathway has been shown to be species 11 [115]. Also for benzene formation a
rt-
complexed C6-poly-alkene seems to be a well-documented intermediate [116,117]. Species 11 is somewhat exceptional but in c~a1313tetrasubstituted alkanes it is probably this species which leads to the splitting of the central C-C bond [118]. The ease with which the
Structure and properties of metals and alloys
53
variously bound species are formed decreases in the following sequence: o~B>o~T~o~8. Metals with a high activity for hydrogenolysis of C-C bonds are all effective in forming multiple bonds [106] and the o~B-bound species (for review see [117,119]). This fission of bond is efficiently suppressed by diminishing the C-C particle size or by some types of alloying. Platinum seems to be a more complicated case and here most likely the o~ single site bound species (8) contribute to hydrogenolytic splitting [119] of hydrocarbons. Figure 25 presents species for which quite solid experimental evidence already exists. A theoretical analysis of these species comprising the prediction of their reactivity and behaviour in the possible surface reaction is mostly missing. For example, next to nothing can be said, on the basis of a theory, about the mechanism of the C-H or C-C bond splitting in various species shown in figure 25. Of course, even less is known about the predicted influence of alloying, particle size effects, etc. 1.3.2
Other molecules
Various spectroscopies (IR, EELS, XPS) and other techniques (exchange reactions, TPD, LEED) have already yielded valuable information on surface species generated by adsorption. As examples, we mention cyano-compounds [120], pyridine [121], pyrrole [121], azomethane [122], trifluorophosphine [123], phenol [124] and variously substituted benzene rings [125]. These molecules have mainly been adsorbed at lower temperatures and, in contrast to hydrocarbons, much less is known about those species which might be intermediates in their catalytic reactions at elevated temperatures. Species arising from adsorption of alcohols, aldehydes, ketones and acids are summarized in figure 27. Alcoholate-like structures have been postulated on the basis of a very rapid exchange of the hydroxylic H-atom [126]. The O-coordinated, di-cy coordinated and dissociatively adsorbed species have been seen by EELS [127] as well as structures, both mono- and bidentate, derived from acids [128]. Although details of the surface reactions can only be speculated on, it is likely that some if not all the intermediates shown in figure 26 participate in conversions of alcohols to aldehydes to acids, and the reverse processes. When overlooking this concise inventory of species, one sees the following conclusions emerge: (i) much information exists on various intermediates, but most of them have never been analyzed by quantum theory; (ii) most of the information relates to metals, but very little to alloys; some will however be presented in chapter 8.
54
chapter 1
R I C
R
I 0 I
0
R I C 0
I
t
*
0
\
*
0
% /
/
0
C-R
0 I monodentate
bidentate
R R--CH=O
R--HC *
O- coordinated
! C = 0
sigma
--X-
~ di-
,e
-- 0
le
I
bound
dissociatively adsorbed
figure 27 Chemisorption complexes observed upon adsorption of oxygen-containing molecules.
1.4
Macroscopic thermodynamic theory of alloys An important thermodynamic parameter for metals is the enthalpy of sublimation
or atomization. Figure 28 shows the values. The form of the curves can be understood in the following terms. There are five d-orbitals and one s orbital available for bonding and therefore the bonding is optimal when half of the valence band (see 1.1.1) is just occupied as at tungsten. With fewer valence electrons than six the metallic cohesion is weaker than the maximum possible. When there are more than six valence electrons, the antibonding part of the valence bond is successively occupied and the cohesion is again
weaker than
at the maximum. With elements that are smaller in size, the interactions between the d-delectrons play an increasing role and diminish the cohesion too. On the other hand, the same phenomenon contributes to the para- and ferro- magnetism of the element. When we pass from group 8 to group 11, not only the metallic cohesion but also the chemisorption bond strength decreases (see 1.2). The enthalpies of adsorption of H2, CO, N2 and ethene are very well correlated with the average metal-oxygen bond strength [129] in the highest oxide of the metal, which in its turn is correlated with the average enthalpy of adsorption of oxygen on the same metals.
55
Structure and properties of metals and alloys
250
200
~176176176
E t~ ,r v t~O
O
~176176
"'0 ........... O.
,~176176 ~ .,,
150
,
~ s."
.'"i" 's ~"
" "
",, " " " " ~I
~t~,
""0.... ~
"'... ~"
ss -
~N
E
"~176176176176176176
.0""
O
"r
9
..
"
0
~176176176176176
100
O
-.~
"1"
50
6
2
I 4
. . . .
I 6
I 8
1 10
12
Elements Sc...Cu
Y.~g_
Yb...Au
figure 28 Heats of atomization of various transition metals.
1.4.1
A short introduction to the statistical thermodynamic description of alloys, as random solutions
We shall confine ourselves to binary solutions and follow the literature [130] in their description. Metallic mixtures of interest in catalysis form homogeneous liquid solutions at sufficiently high temperatures; however, liquid alloys are in the main uninteresting for catalysis, although some work has been done on metallic liquids, for example, on amalgams; we have to look principally at solids formed from liquid alloys. Several cases can be distinguished. First, a solid solution may be formed upon solidification and be homogeneous almost down to the atomic dimensions. Alternatively, clusters or even microscrystals with distinctly different compositions may appear in the equilibrated solid at a temperature lower than the melting point of the mixture. Let us analyse which factors determine the phase equilibria in alloys.
56
chapter 1
Consider a solid solution of A and B with mole fractions x a and x B. We shall call the number of nearest neighbours of a given atom s; this is twelve for fcc structure, eight for bcc, etc. If there are a total of N atoms, then in a random solution the following pairs have the indicated populations. 2
N.s x a AA:
(48)
N . s ( 1 - x a )2 BB:
AB: N.s ( 1 - x A ) x a
2
2
These expressions are the results of very simple statistics. To form a pair AA we need to occupy one position (any of N) by A, the probability to find that is Nxa. From the s neighbours of that atom are s.x a again of the A-type. Not to count the pairs twice, two appears in the denominator. Then we ascribe to the bond between each pair a dissociation energy -EAA, -EBB , or -EAB respectively, and if we set EtotaI at zero when all atoms are infinitively separated from each other, the total energy of our alloy is at 0 K: Etotal - N.s [E s4 xJ + EBB
2
(49)
(1-XA) 2 + 2X(1-XA) E ~ ]
After algebraic rearrangement and after adding the Cp-term for the effect of the difference between temperature T and absolute zero, the internal energy is: T
+ f % aT
(50)
o
There are two terms in the entropy: (i) the thermal or internal entropy So, which for pure components is I(Cp/T)dT and (ii) the configurational entropy of mixing which is Sco,r = - N k [x4 In xA + (1-xA) In (1-xA)]
(51)
k being the Boltzman constant. The Gibbs free energy is therefore: G = U-
TS = U -
TS ~ + N k T [ x
a In x A + (1 -x A) In (1 -;CA)]
(52)
To illustrate the results we shall analyse three cases characterized by the parameter Z: (i) ideal solution alloys, for which
Z
-
eaA + EBB 2 -0
(53)
(ii) exothermically formed alloys (Z < 0) and (iii) the endothermically formed alloys (Z > 0). Figure 29 shows the Gibbs energy graphically, for the three indicated cases.
Structure and properties of metals and alloys
57
-U I I I I I I I i
,,
0
X
A
1
---->
0
X(1) A
X(2) A
1
figure 29 Free energy G as a function of the alloy composition x a. left: ideal solutions shown and (weakly) exothermically formed alloys show a similar curve.
right: alloys formed endothermically upper stipped line: a physical mixture of pure unalloyed elements lower stipped line: a physical mixture of two alloys with composition XA(1) and XA(2), respectively. The graph indicates that formation of a mixture of two alloys ('1', '2')from the frozen solution (AG") or from the physical mixture of elements (AG'), is thermodynamically favoured.
-T i
figure 30 Phase diagram, showing the limits of the (co-) existence of phases. An alloy with compo-
sol
I I I
,Tcrit
sition x m is a liquid solu-
'B' rich
r ch
Xm
XA
>1
tion at Tsol, but at T < Tcrit it should decompose (if the kinetics of phase separation allows it) into two solutions, 'A' and 'B'-rich ones.
58
chapter 1
In the ideal case (left), the form of the curve is dictated by changes in the
Sconf. When
the
solution is formed weakly exothermically, and no compounds are formed, the curve has a similar form, but the contribution of enthalpy of mixing makes the curve form a deeper valley. In the endothermic case (right), G always decreases for small concentrations of A in B or B in A. This is due to the logarithmic form of the configurational entropy. However, when x A is 0.5, the configurational term is small (near to zero) and the endothermic enthalpy of mixing causes G to rise. The straight lines in the figures represent the free energy of a physical mixture of pure components. Let us now inspect the case at the right of figure 29 more closely. Suppose we make a phase with a composition XA~l~ and another one with XA~2~. By physically mixing these two phases, we can obtain a mixture with any composition between the indicated ones. The free energy of these mixtures is always on the straight line connecting G1 and G 2 . The mutual tangent connecting the points G l and G 2 at constant temperature and pressure fulfills the condition
OG
OG
or in other words, the chemical potentials are equal. This therefore represents the equilibrium situation. If we prepare a frozen solution of a composition x m and allow it to equilibrate, it will rearrange until it is converted into two phases with compositions x ~ and x ~2~. Repeating this analysis for different temperatures, we obtain a graph in which we can indicate the limits of existence (T, XA) of individual phases, as for example in figure 30. When the state of an alloy approaches the limits (T, x) of solution phases, clustering appears leading to the nuclei formation of the new phases. When the attractive interaction between the components is strong, ordering occurs in the alloy. For example, within the group of catalytically interesting alloys , it is observed with the Pt-Cu system. For the ordered alloys, there are theories of different degrees of approximation and sophistication [130]. The simplest is the approximation of regular solutions (Bragg-Williams approximation). In this case U and S ~ are corrected for ordering, but the term for the configurational entropy is kept in the form valid for ideal solution (Z = 0). This approximation has been also widely applied in dealing with the problems of surface segregation (see chapter 4) and adsorption of gases.
Structure and properties of metals and alloys
59
With a large piece of a metal or an alloy, the surface energy, that is the energy needed to form the surface when we cut such a piece from an infinite large block, is negligible. However, most catalysts contain small metal particles. A frozen solution comprising such particles might not decompose into the equilibrium phases because the consequential energy gain could be insufficient to compensate for the energy necessary to form new surfaces or interfaces. Some calculations illustrating this point have been published [ 131 ]. For the catalytic chemist it is important to realize that, when a solution of metallic elements is prepared by chemical means, it can survive equilibration and annealing as a frozen solution, when the alloy is in the form of very small particles. It has been seen experimentally that making the particles smaller also suppresses the ordering. We shall also see in chapter 4 that the surface segregation is less likely to occur in small particles. 1.4.2
Phase composition of some catalytically interesting alloys
Nickel-Copper The early literature took for granted that this system formed a continuous series of solutions. However, Sachtler et al. [132] showed that if this alloy is formed endothermically, it should decompose into two phases below a critical temperature Tcrit (fig.30). With data available at that time they [132] predicted a critical temperature of about 1100K. Another author [133] predicted the critical temperature of 450K. An experimental study [ 134] with equilibrated co-evaporated nickel-copper films showed that the critical temperature was between 440 and 490K. Once the equilibrated solution had been formed and the structure did not contain many defects, annealing below Tcrit did not lead to a reverse decomposition of the solution alloy into two phases. This means that alloys used in the early literature were probably frozen solutions when they were prepared from carbonates by thermal decomposition and reduction, or by other procedures favouring good mixing at elevated temperatures. On the other hand, the complicated process of slow diffusion in solids, the necessity of demixing below Tcrit and the effects of gas induced segregation (see chapter 4) could have been the reason why the results in the early literature were sometimes badly reproducible and surprisingly irregular [135]. Nickel-copper alloys also exemplify other phenomena mentioned in section 1.4.1. For example, in one-phase solutions above Tcrit, clustering of the magnetic moments of nickel into giant moments occurs. This has been seen by magnetic measurements [136] and by neutron diffraction [137]. The latter study in particular offers extended information on the kinetics of the process of nickel clustering in nickel-copper alloys and on the ensuing steady state. For catalysis it is important to note that alloys which appear macroscopically (e.g. by X-ray diffraction) to be homogeneous, are not really so on the atomic scale. Thus, when comparing alloys palladium-silver and nickel-copper, one of the
60
chapter 1
differences could be the larger tendency to form clusters in the latter than in the former system. This can also play a role in valence band electron photoemission spectra (chapters 2 and 3). Sachtler [132] and Burton [131] have pointed out that, when alloys equilibrate into a mixture of two phases, the phase with the lower surface energy may surround the other when the particles are small, i.e. about 20 nm or less. Above that, the surface segregation effects can also take place in the external shell.
Selected palladium alloys The phase diagram of the palladium-silver alloys in figure 31 shows that at a given temperature, the compositions of the liquid and solid phases are not exactly equal, but that in the whole range of composition [138] solid solutions are formed. These solutions do not deviate greatly from ideal solutions. The same holds also for palladium-gold alloys, although in this case some authors mention a slight tendency to ordering. The palladium-nickel system is also interesting. While the alloys mentioned above can be used in catalysis to investigate or to exploit practically the consequences of the dilution of an active metal (Pd) in an inactive one (Ag, Au), nickel-palladium alloys combine two metals both of which are active, but which differ substantially in their catalytic behaviour. For example, nickel readily forms multiple bonds with hydrocarbons but palladium does not. Nickel is an excellent element for hydrogenolysis, but palladium is very bad; nickel dissociates carbon monoxide at only slightly elevated temperatures, while palladium does not do that. The palladium-copper system does not form solid solutions in the whole range, but only in a somewhat broad range of composition (10-100%) [138]. Palladium forms also a series of solutions with platinum [139].
Some platinum alloys Platinum is one of the most important elements in catalysis. Since its physical properties can be widely manipulated by alloying with other metals, there was a hope that this would hold also for the catalytic properties. A great deal of catalytic work has therefore been carried out with platinum alloys. Platinum is not an element which forms solutions so easily as nickel or palladium. One of the exceptions is however the platinum-copper combination which forms solutions, but with some tendency for ordering. At high temperatures continuous solid solutions are formed but long annealing produces ordered superlattices which appear at Cu4Pt, Cu3Pt and CuPt. The tendency to ordering is stronger than with palladium-copper, although real intermetallic compounds are not formed [140,141 ]. The most important alloy for catalytic processes in the industry is probably platinum-rhenium. Platinum-rhenium solutions have been prepared [142] by arc melting
61
Structure and properties of metals and alloys
and homogenization at about 2200K and at about 1300K the solubility of rhenium in platinum was found to be about 40% wt. On the other side of the diagram, about 40% wt of platinum is soluble in rhenium to form a hcp structure (see figure 32).
At. % O
C
20
1600
Pd
40
80
60
(1541 ~)
Liq
1400 '
f
I.So~
Liq§
/
f
i
/
/
i t
J
f
1200
J
/
1000
/
/
,
i"
Solid
Solution
i /
960.5 0
20
40
60
80
100
W t.% Pd Ag-
Pd
figure 31 Phase diagram showing the (co-)existence of liguid and solid Ag-Pd solutions.
From a fundamental point of view, the platinum-gold combination is also interesting. It also represents the dilution of an active by an inactive metal, whereby platinum and gold most likely tend to form clusters in solutions. The phase diagram shown in figure 33 demonstrates that for catalysts prepared at temperatures that are interesting for catalysts, the solution limit on the platinum rich side is low, but on the gold rich side about 20% at of platinum dissolve in gold. This is quite useful for fundamental studies. De Jongste et al. [145] have shown that, when frozen solutions that are not truly homogeneous are prepared by coreduction from solutions of precursors, the solid solutions equilibrate upon reduction at about 700K into a mixture of two phases with lattice contsants (shown as upper and lower broken line in figure 33), corresponding with the phase diagram. This is all indicated in figure 33. The triangles show the values for the initial frozen solutions.
62
chapter 1
C
in
, x
P! - R e
,~o
2
OlR
kX
Lattice constants o f solid soluti-
4,3o
ons Pt-Re;
, ,,o
(Pt- fcc; Re- hcp)
3.910
O
3.900
2.760
3.890
)2.756
el
figure 32
I0
20
30
40
5'0
60
"/;0 80
90
RI
%Re
Delhez and Mittemeijer [146] have carried out a full position, width and peakshape analysis with the above mentioned materials and concluded that chemically-prepared alloy particles of about 200 nm size contain a platinum-rich kernel and a gold shell. Alloys prepared by this procedure obviously look like Sachtler's cherries mentioned above: a kernel and a shell both of different compositions. Platinum-tin catalysts are used quite widely in naphtha reforming. Platinum and tin form a series of well defined intermetallic compounds having narrow ranges for solubility of an excess element, as can be seen in figure 34. [140,147]. It is therefore most likely that chemically prepared mixtures of platinum and tin
will tend to convert themselves
upon annealing into mixtures of various compounds, amongst which Pt 3 Sn and PtSn will dominate. Closing remark. It is not our ambition to present a full treatment of an essentially metallurgical topic discussed by this paragraph. However, we hope that with the presented information as a base, the reader will find it easier to consult the original or reviewing literature [138,140,148,149] when necessary. This literature contains also all information which has been quoted throughout this chapter.
Structure and properties of metals and alloys
63
1400T (~ 1200
1000
800
AI.%Sn i0 20 30 40 50 60 70 80 90 __ _~_.._,~jl~_.__~l__l~
=C
600
1800
400
I
I
I
I
I
I
I
i
~
,
,
L
I
4.10 I
il IF\J/!i \]
k\
4.05
i
\
'~176 \
4.00
\
700
\ \
:
i ~tt
o
-*
~-
-'~
\ \o
\
3.95
0
I
I
20
I
4'0 % Pt
l
6'0
I
8'0
l
I O0
~J
I0
r
JO 4 0
50
Wr. %
60
Sn
7b
80
90100
Pt-Sn
figure 33 Upper part: phase diagram of the Pt-Au alloy system Lower part: alloy lattice constants of indicated composition Full line: thermally annealed alloys Stipped line (triangles)frozen solutions prepared by chemical co-reduction. figure 34 (on the right) Phase diagram for Pt-Sn alloys showing the regions of existence and coexistence of various intermetallic compounds.
64
chapter 1
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66
67
68 69 70 71 72 73 74
75 76 77
Structure and properties of metals and alloys 78 79
80 81 82
83
84 85 86 87 88 89 90 91
92 93 94 95 96
69
W.L.Jorgensen, L.Salem, "The Organic Chemist's Book of Orbitals", Academic Press, London (1973) J.Blyholder, J.Phys.Chem. 68 (1964) 2772 idem J.Phys.Chem. 79 (1975) 756 F.A.Cotton, G.Wilkinson, "Advanced Inorg.Chem." J.Wiley, N.Y.(1986) 97 J.Koutecky, G.Pacchioni, P.Fantucci, Chem.Phys. 99 (1985) 87 G.Pacchioni, P.S.Bagus, J.Chem.Phys. 93 (1990) 1209 G.Pacchioni, P.S.Bagus, Phys.Rev.B 40 (1989) 6003 V.Bonacic-Koutecky, J.Koutecky, P.Fantucci, V.Ponec, J.Catal. 111 (1988) 409 N.V.Richardson, A.M.Bradshaw, Surf.Sci. 88 (1979) 255 J.N.Allison, W.A.Goddard, Surf.Sci. 115 (1982) 553 N.D.Shinn, T.E.Madey, J.Chem.Phys. 83 (1985) 5928 C.L.Allyn, T.Gustafsson, E.W.Plummer, Chem.Phys.Lett. 47 (1977) 127 W.R.Riedel, D.Menzel, Surf.Sci. 163 (1985) 39 P.M.Williams, P.Butcher, J.Wood, K.Jacobi, Phys.Rev.B 14 (1976) 325 S.R.Bare, K.Griffiths, P.Hoffmann, D.A.King, G.L.Nyberg, N.V.Richardson, Surf.Sci. 120 (1982) 367 P.Hoffmann, S.R.Bare, D.A.King, Surf.Sci. 117 (1982) 245 C.W.Bauschlicher, Chem.Phys.Lett. 115 (1985) 535 V.Ponec in "Catalysis, Specialist Reports" 5 (1982) 48 V.Ponec, W.A.van Barneveld, IEC, Product Res.Dev. 18 (1979) 268 A.de Koster, A.P.J.Jansen, R.A.van Santen, Faraday Disc.Chem.Soc. 87 (1989) 263 J.B.Banziger, Appl.Surf.Sci. 6 (1980) 105 M.Araki, V.Ponec, J.Catal. 44 (1976) 430 R.A.van Santen, A.de Koster, Stud.Surf.Sci.& Catal. 64 (1991) 1 I.Toyoshima, G.A.Somorjai, Cat.rev.Sci.Eng. 19 (1979) 105 D.A.King, Proc. 10th Int.Congr.on Catal., Budapest, 1992, Elsevier, 1993, plenary lecture (not printed) H.A.C.M.Hendrickx, C.des Bouvrie, V.Ponec, J.Catal. 109 (1988) 120 P.Gelin, J.T.Yates jr., Surf.Sci. 136 (1984) L1 H.Pfnur, D.Menzel, Surf.Sci. 148 (1984) 411 M.Ban, M.A.van Hove, G.A.Somorjai, Surf.Sci. 185 (1987) 355 C.Minot, M.A.van Hove, G.A.Somorjai, Surf.Sci. 127 (1982) 441 J.Schule, P.Siegbahn, U.Wahlgren, J.Chem.Phys. 89 (1988) 6982 H.Yang, J.L.Whitten, J.Chem.Phys. 91 (1989) 126 H.Yang, J.L.Whitten, Surf.Sci. 255 (1991) 193 W.Biemolt, G.J.C.S.Kerkhof, P.J.Davies, A.P.J.Jansen, R.A.van Santen, Chem.Phys.Lett. 188 (1992) 477 A.W.E.Chan, R.Hoffmann, J.Chem.Phys. 92 (1990) 699
70
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A.Gavezotti, M.Simonetta, Surf.Sci. 99 (1980) 453 P.E.M.Siegbahn, M.Blomberg, C.W.Bauschlicher, J.Chem.Phys. 81 (1984) 2103 P.Cremaschi, J.L.Whitten, Phys.Rev.Lett. 46 (1981) 1242 J.Y.Saillard, R.Hoffmann, J.Am.Chem.Soc. 105-6 (1984) 2006 C.Zheng, Y.Apeloig, R.Hoffmann, J.Am.Chem.Soc. 110 (1988) 749 E.Ortoleva, M.Simonetta, J.Mol.Struct. (Theochem) 149 (1987) 161 J.E.Lennard-Jones, Trans Faraday Soc. 28 (1932) 333 D.D.Eley, Disc.Faraday Soc. 8 (1950) 34 E.Shustorovich, Adv.Catal. 37 (1990) 101 E.Shustorovich, Surf.Sci.Reports 6 (1986) 1 E.Shustorovich, A.T.Bell, Surf.Sci. 253 (1991) 386 R.L.Burwell jr., J.B.Peri, Ann.Rev.Phys.Chem. 15 (1964) 131 C.Kemball, Proc.Roy.Soc.London A 207 (1951) 541 J.R.Anderson, C.Kemball, Proc.Roy.Soc.London A 223 (1954) 361 M.A.Chesters, C.de la Cruz, P.Gardner, M.McCash, P.Pudney, G.Shahid, N.Sheppard, J.Chem.Soc.Faraday Trans. 86 (1990) 2757 D.J.Bandy et al, Phil.Trans Roy.Soc.London A 318 (1986) 141 J.C.Bertolini, J.Massardier in "The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis", (editors: D.A.King, D.P.Woodruff) Elsevier, 1984, Vol.3B, p. 107 L.L.Kesmodel, L.H.Dubois, G.A.Somorjai, J.Chem.Phys. 70 (1979) 2180 H.Ibach, D.L.Mills, "Electron Energy Loss Spectroscopy and Surface Vibrations", Academic Press, N.Y. (1982) S.B.Mohsin, M.Trenary, H.J.Robota, J.Phys.Chem. 92 (1988) 5229 G.H.Hatzikes, R.I.Masel, Surf.Sci. 185 (1983) 479 J.A.Gates, L.L.Kesmodel, Surf.Sci. 124 (1983) 68 N.R.M.Sassen, Ph.D.Thesis, Leiden University, The Netherlands (1989) J.F.M.Aarts, N.R.M.Sassen, Surf.Sci. 214 (1989) 257 F.G.Gault, Adv.Catal. 301 (1989) 1 O.E.Finlayson, J.K.A.Clarke, J.J.Rooney, J.Chem.Soc.Faraday Trans.I, 90 (1984) 191 Z.Paal, P.Tetenyi, Raect.Kinet.Catal.Lett. 12 (1979) 131 Z.Paal, Adv.Catal. 29 (1980) 273 V.Ponec in "Chemical Physics of Solid Surfaces and Heterogeneous Catalysis" (editors: D.A.King, D.P.Woodruff) Elsevier (1982) Vol. 1, p.365 G.Leclercq, J.Leclercq, R.Maurel, J.Catal. 50 (1979) 87 A.J.den Hartog, A.G.T.M.Bastein, V.Ponec, J.Molec.Catal. 52 (1989) 129 W.Hoffman, E.Bertel, F.P.Netzer, J.Catal. 60 (1979) 316 M.E.Kordesh, W.Steuzeland, H.Conrad, Surf.Sci. 205 (1988) 100
99 100 101 102 103 104 105 106 107 108
109 110 111 112
113 114 115 116 117 118 119 120
Structure and properties of metals and alloys
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122 123 124 125 126 127
128
129 130 131
132
133 134 135 136
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T.Szilagys, Appl.Surf.Sci. 35 (1988) 19 G.Mizutami, S.Ushioda, J.Chem.Phys. 91 (1989) 598 J.G.Searfin, C.M.Friend, J.Phys.Chem. 93 (1989) 1998 F.P.Netzer, Vacuum 41 (1990) 49 F .P.Netzer, E.Bertel, A.Goldmann, Surf.Sci. 199 (1989) 87 L.Hanley, Xingcia Guo, J.Y.Yates Jr., J.Phys.Chem. 93 (1989) 6754 M.D.Alvey, J.T.Yates Jr., J.Am.Chem.Soc. 110 (1988) 1782 Xueping Xu, C.M.Friend, J.Phys.Chem. 93 (1989) 8072 A.K.Meyers, J.B. Benzinger, Langmuir 5 (1989) 1270 J.F.M.Aarts. K.G.Phelan, Surf.Sci. 222 (1989) L853 J.R.Anderson, C.Kemball, Trans Faraday Soc. 51 (1955) 1782 A.Farkas, L.Farkas, J.Am.Chem.Soc. 61 (1939) 1336 J.L.Davis, M.A.Barteau, J.Am.Chem.Soc. 111 (1989) 1782 H.Luth, G.W.Rubloff, W.D,Grobman, Surf.Sci. 63 (1977) 325 N.R.Avery, Surf.Sci. 125 (1983) 771 N.R.Avery, A.B.Anton, B.H.Toby, W.H.Weinberg, J.Electr.Spectr.Rel.Phenom. 29 (1983) 233 M.A.Henderson, Y.Zhon, J.M.While, J.Am.Chem.Soc. 111 (1989) 1185 C.J.Houtman, M.A.Barteau, J.Catal. 130 (1991) 528 J.G.Chen, J.E.Crowell, J.T.Yates Jr., Surf.Sci. 172 (1986) 733 B.A.Sexton, Chem.Phys.Lett. 65 (1979)469 G.R.Schoofs, J.B.Benziger, Surf.Sci. 143 (1984) 359 R.J.Madix, J.L.Gland, G.E.Mitchell, Surf.Sci. 125 (1983) 481 R.P.Eischens, W.A.Pliskin, Proc. 2nd Int.Congr.on Catal., Paris, 1960, Technip. Paris (1961) Vol. 1, p.789 K.I.Tanaka, K.Tamaru, J.Catal. 2 (1963) 366 J.C.Slater "Introduction to Chemical Physics", Dover Publ.N.Y. (1970) McGraw Hill, 1939 D.F.Ollis, J.Catal. 23 (1971) 131 D.W.Hoffman, J.Catal. 27 (1971) 374 J.J.Burton, E.Hyman, D.G.Fedak, J.Catal. 37 (1975) 106 W.M.H.Sachtler, G.J.H.Gorgelo, J.Jongepier in "Basic Problems in Thin Films Physics", Proc.Int.Symp.Clausthal, 1965 (editors: R.Niedermayer, H.Mayer) v/dHoeckx Ruprecht, G6ttingen (1966) 218 J.L.Meijering, Acta Metallurgica 5 (1957) 257 P.E.C.Franken, V.Ponec, J.Catal. 42 (1976) 398 G.K.Boreskov, Kinet.Katal. 10 (1969) 1 C.G.Robbins, H.Claus, P.A.Beck, J.Appl.Phys. 40 (1969) 2269 T.J.Hicks, B.Rainford, J.S.Kouvel, G.G.Low, Phys.Rev.Lett. 22 (1969) 531
72
137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152
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P.A.Beck, Met.Trans. 2 (1971) 2015 J.S.Kouvel, J.B.Comley, Phys.Rev.Lett. 24 (1970) 598 Y.Ito, J.Akimitsu, J.Phys.Soc.Japan 35 (1973) 1000 F.Acker, R.Huquenin, Phys.Lett. 38A (1972) 343 E.Vogt, Phys.State Sol.(a) 28 (1975) 11 J.Vrijen, Ph.D.Thesis, Utrecht University, The Netherlands (1977) J.Smithels, Metals Reference Book, Vol.II (1967) Butterworths (a collection of phase diagrams) J.B.Darby, K.M.Myles, Metall.Trans. 3 (1972) 653 W.B.Pearson "A Handbook of Lattice Spacings and Structures of Metals and Alloys", Pergamon Press, Oxford (1958) A.Schneider, U.Esch, Z.Elektrochem. 50 (1944) 290 W.Trzebiatowski, J.Berak, Bull.Acad.Polon.Sci. C1, III, II (1954) 37 A.S.Darling, R.A. Mintern, J.C.Chaston, J.Inst.Metals 81 (1952/53) 125 L.J.van der Toorn, Ph.D.Thesis, Delft Technical University, The Netherlands (1960) H.de Jongste, F.J.Kuijers, V.Ponec in "Preparation of Catalysts", (editors: B.Delmon, P.A.Jacobs) I (1975) 207 R.Delhez, E.J.Mittemeijer, Chem.Weekblad (1977) 105 F.M.Mittemeijer, R.Delhez, J.Appl.Phys. 49 (1978) 3875 K.Schubert, E.Jahn, Z.Metalkunde 40 (1949) 399 M.Hansen "Constitution of Binary Alloys", 2nd ed.McGraw Hill, N.Y. (1958) J.Hafner "From Hamiltonians to Phase Diagrams" (the electronic and stat.mech.theory of s,p-bonded metals and alloys), Springer, Berlin (1987) H.Kleykamp, J.Nuclear Materials, 201 (1993) 193 J.T.Taylor, Platinum Metals Rev. 29 (1985) 74 F.R.de Boer, R.Boom, W.C.M.Mattens, A.R.Mieding, A.K.Niessen, "Cohesion in Metals" (Trans.Metal.Alloys) North Holland Publ.Amsterdam (1988) H.Brodowsky, H-J.Schaller, "Thermochemistry of Alloys", Kluwer Acad.Publ. (1989)
73
Chapter 2
EXPERIMENTAL TECHNIQUES OF SOLID STATE PHYSICS RELEVANT TO RESEARCH ON ALLOYS The great merit of the work of Dowden [1] is that it caused those working in catalysis to think about the subject in terms of solid state physics, and the contribution of the solid state physics to the understanding of heterogeneous catalysis has been growing ever since. The techniques of solid state physics supply us with information on solids used as catalysts, on the role of supports and promoters, on structure, energy spectra and orientation of various chemisorption complexes. Catalytic research without these techniques is now unthinkable. When Dowden published his first paper [1] the range of available techniques was somewhat limited. Magnetic moment and electrical conductivity measurements were mainly used. At the periphery of general interests there were also electro-optical measurements (not very well understood at that time), determination of electronic contribution to heat capacities, Hall effects (not well understood in transition metals) and some others of minor importance. None of these methods was suited to the study of parameters most interesting for catalysis by metals and alloys, such as the full
density-of-statescurves
and
other details of electronic structure. The situation in surface chemistry and catalysis on metals and alloys changed dramatically when in the late sixties various spectroscopic techniques became available. These techniques also made possible the analysis of surfaces and chemisorbed layers, and the maturation of catalysis from the black-box empirical approach to the science was accelerated. The techniques most relevant for catalysis are reviewed below. Photons, electrons, ions, neutral molecules and electromagnetic fields can interact with solids and one or more of these particles (or the field quanta) can be followed after their release from the solid. For example, irradiation of the sample by photons can lead to emission of electrons (photo-electron emission spectroscopy, PES) or emission of another photon (X-ray fluoresence). Irradiation by electrons is often used to perform Auger Electron Spectroscopy (AES), etc. Scattering of slow ions (ions in - ions out) is a very powerful technique in surface chemical analysis and is referred to as low energy ion surface scattering, LEISS; scattering of neutrals such as helium atoms is also a very good tool to analyse how perfectly flat are the surfaces of single crystals.
74
chapter 2
2.1 Photo-electron Spectroscopy (PES) In the early days of PES, two types of source were available to excite electrons in the sample: the use of X-ray quanta led to X-ray photo-electron spectroscopy, XPS, and of UV quanta to ultraviolet photo-electron spectroscopy, UPS. XPS is also called Electron Spectroscopy for Chemcial Analysis (ESCA). The spectral yield can be intergrated over a broad range of emission angles, or determined at one fixed angle or by varying the emission angle (angle resolved, AR). The advent of synchrotron radiation sources has made the transition from one to another of these spectroscopies easy and the subdivision less sharp. 2.1.1 Instrumentation A typical PES apparatus consists of a source (X-ray tube or a resonance source), an analyzer tube permitting only electrons of one energy to reach the detector and a detector, as in figure 1 [2].
Electron
Filament
~~
detector
analyzer
X-roy source Sample
figure 1 Schema of the experimental arrangement used in X-ray photoelectron spectroscopy: an incandescent filament emits electrons which produce X-ray emission from for example a magnesium anode. The X-rays impinge on a sample, producing photoelectrons which are selected by their energy and finally collected by the detection system, which always comprises an electron multiplier.
In this figure the use of an X-ray tube is indicated. In UPS some type of resonance source of light is usually used; a gas at a defined pressure is excited by a discharge and fluoresence leads to de-excitation and emission of monochromatic photons (HeI, 21.21; HelI, 40,83; NelI, 16,85, 16.67; NelI, 26,9, 27,8 and 30,5, all in eV) [2-4].
Experimental techniques of solid state physics
75
A powerful modem source of photons of variable energy, pulsed and polarized, is the synchrotron (an electron accelerator), now available for surface chemistry research at many places (see figure 2).
e-
~ 11oops
F,~
TIME / l ' , , I C H~R0 TRON wr RADIATION
-ANALYZER
~FOCUSS_ING MIRRORS
__ _ ~ - ' ~ ,~'ENTRANCES~LIT DE~EC~RTO~RJJ .._~'GRATING ----" MONOCHROMATOR ..... ~ E XiT-SLIT SAMPLE
I0I~
!
v
!
vvvv,
1
|
v
-
v vvvvv
I
v
i
,
Tv,,,
!
T
i
!
vvvv
I
h '~c
1013
u LU ~n 1012
figure 2 Synchrotron Radiation: (Top) Scheme of tangential Synchrotron Radiation beam and monochromator used in photoelectron spectroscopy experiments. (Bottom) Photon flux calculated for two bending magnets as a function of photon energy. The photon flux is defined as the number of photons per unit time emitted with a certain band width and into an angle element in the plane of the electron orbit: more details in the text.
Z 0 k-OT 1011 G.
1010 0.1
, , ,,,,,,
I
, ,
PHOTON h,v
I0 ENERGY --
..... I00
i ,,,
1000
(KEY)
Analyzers are of different shapes and work by different principles: spherical, as in figure 1, 127 ~ deflection analyzer, retarding field analyzer or cylindrical mirror analyzer. Also the use of a magnetic field has been described in the literature [2-4]. A s detector a multiplier of the channeltron type is most frequently used, but more sophisticated detection systems have also been designed [2-4]. A typical spectrum obtained by a commercial XPS spectrometer is shown in figure 3.
76
chapter 2
I 150kc"
lOOk-
o 50k
OIs
]
OKLL
'
8;o
'
6;o
~-~ 500 c
700 ,
,
,
C
'
9 () 0
'
AI2s
surface of aluminium.
~oo
Binding
Kinetic
Is
figure 3 A spectrum as obtained with commercial XPS equipment, showing elements detected in the
200
0
Energy ( e V ) 1 1'00
Energy
'
13'0 0
(eV)
'
1500 '
>
2.1.2 Basic principles and phenomena in PES In photo-emission an electron is ejected from the sample by a photon. The energy hv of the photon is known and the kinetic energy Eki n o f the emitted electron is determined by the spectrometer. The difference between the two energies is defined as the binding
energy of the electron, BE. When the electron comes from level Ej, notation is BEj. The level denoted by j is one either of the atomic core-levels or of the molecular or valence band levels. The important question is, how are Ej and BEj related? By definition BEj is the difference in the energy of the initial neutral state and of the final ionized state, and with N electrons involved in the initial state, it is BEj
=
h v - Ekin
(l)
BEj
=
E(N-1)f- E(N)i
(2)
The final state cannot be described as a system with all energy levels in the same place on the energy scale as in the initial state, with one orbital corresponding to Ej ionized. All electrons feel that one electron from their neighbourhood is missing and when we want to make the energy balance in terms of orbital energies this can be accounted for by introducing the so-called energy of relaxation E rel o f the other electrons, so that the equation becomes: BEj = -Ej- Ejr~ + other corrections
(3)
If equation 2 were used, the term of E tel would not need be introduced, but when one wants to refer the results to the ground state levels Ej, the use of E rel is a necessity. However, the use of Ej makes discussion on BE values easier. Besides E re~ other small corrections are needed, but discussion of them would extend this simple introduction too far.
Experimental techniques of solid state physics
77
The BE values of core levels are only slightly dependent on the molecular environment, so that the XPS technique, concerned as it is with these levels, is a good analytical tool for elemental analysis. For example the Is levels of carbon, nitrogen and oxygen can always be found at about 284, 399 and 530 eV respectively. Small deviations of these values of the order of a few % depend on the chemical environment of the ionized atoms and are called chemical shifts. The analytical use of XPS was responsible for the technique being originally designated Electron Spectroscopy for Chemical Analysis (ESCA). The value of the BE is not the only information which can be extracted from XPS core level results. The highest intensity peak is often flanked by satellites. When ionization of an electron with a given BE is accompanied by excitation of another electron, a satellite appears at a higher BE; it is called shake-up peak. When simultaneously two electrons are emitted, one speaks of a shake-off peak. The appearance of such peaks is the most obvious manifestation of relaxation processes following ionization. Metals do not show separate satellite peaks because most of the shake-up and shake-off processes involve the conductivity band, which has a semi-continuous spectrum of energies. These relaxation processes reveal themselves with metals as peak broadening and a high background level. This short introduction cannot be exhaustive but interested readers will find several excellent monographs and reviews on this subject when looking for additional information [3,4]. The mere appearance of BE's in the region of energies related to molecular orbitals can supply information on whether or not a molecule such as carbon monoxide or nitrogen oxide dissociates under given conditions. However, more details of the characterization of adsorbed molecules can be gained by XPS, and in particular by UPS, and some of these will be discussed below. Since the use of XPS and UPS in catalysis by alloys is very much dependent on the proper understanding of the final state effects of for example relaxation and screening, we shall pay more attention to these phenomena.
2.1.2.1 Ionization and relaxation effects on the binding energy of electrons in atoms and molecules Let us start with the question of relaxation in atoms and follow in this the discussion presented by Hedin and Johansson [5]. Ej rel can be treated conveniently as the sum of three contributions" one due to the inner shell, the second to intra-shell effects and the third to an outer-shell contribution:
Ej rel= E tel (n' < n)
+ E rel
(n' = n) + E re~ (n' > n)
(4)
where n is the principal quantum number of orbital j which is being ionized, and n' is that
78
chapter 2
for other, passive electrons. The inner term E tel (n' < n) is negligible, since these electrons are only a very little influenced by the presence or absence of an electron in the higher energy level (mostly electrons which are outside the space where the emitted electron comes from). The intra-shell term is intermediate in size; usually it is less than 5 eV. The largest is the outer-shell relaxation term (n' > n), because a removal of an inner shell electron causes an outer-shell electron to feel that the positive nuclear charge is effectively increased by almost one unit. This picture has led to an equivalent core model for calculation of Ejtel values (see below). Martin and Shirley present [6] in the compendium edited by Brundle and Baker [4] the values which are shown in table 1. table 1 Calculated atomic relaxation energies for the orbitals of light atoms (eV) Atom
ls
2s
2p
3s
3p
3d
He Li Be B C N O F Ne Na Mg A1 Si P S C1 Ar K Ti Mn Cu
1.5 3.9 7.0 10.6 13.7 16.6 19.3 22.1 24.8 24.0 24.6 26.1 27.1 28.3 29.5 30.7 31.8 32.8 35.4 40.1 48.2
0.0 0.7 1.6 2.4 3.0 3.6 4.1 4.8 4.1 5.2 6.1 7.0 7.8 8.5 9.3 9.9 10.8 13.0 17.2 23.7
0.7 1.6 2.4 3.2 3.9 4.7 4.4 6.0 7.1 8.0 8.8 9.6 10.4 11.1 12.2 14.4 18.8 25.7
0.7 1.0 1.2 1.3 1.4 1.6 1.8 2.2 3.6 5.1 7.7
0.2 0.4 0.6 0.9 1.1 1.4 2.0 3.4 4.9 7.2
2.0 3.6 5.3
4s
0.3 0.4 0.3
These values were calculated by various theories in a high, many-electron approximation. The values give a good impression of the size of the effect we are speaking about and which plays an important role in XPS on small particles and alloys. Table 1 and the above explanation make it clear that, in atoms, the lower the level being ionized, the higher will be the relaxation energy. This holds equally well for
Experimental techniques of solid state physics
79
molecules and solids such as alloys, but with these systems other relaxation processes are also possible. The extra-atomic, viz. those of the neighbouring-atom, electrons can contribute to the relaxation energy. Experimental values for small molecules show that for the second row elements the ls BE's are lower by 2-3 eV than those calculated for the same atom in the free state. This deviation from the free-atom behaviour increases with molecular size but the effect levels out and large molecules show limiting BE's which are at maximum 5-6 eV lower than the corresponding atomic ones. Martin and Shirley [6] in the compendium by Brundle and Baker [4] point to an observation which is very interesting for chemists, concerning the ionization potential of alcohols. The potentials corresponding to the ionization of the oxygen lone pair, i.e. the molecular orbital, localized on the oxygen atom, vary by about 4 eV and this is almost exactly the variation in the extra-atomic relaxation energy E re1 as calculated theoretically [6]. E tel increases and ionization potential decreases in the order: H20 > CH3OH > C2HsOH > (CH3)2CHOH > (CH3)3COH. A nice example illustrating various aspects of measured BE values is the case of the n-alkanes, CnH2n+2 , from n = 1 to n = 16. When measured in the gas phase, alkanes show C ls BE shifts which decrease from zero for CH4 to about - 0.6 eV for CllH24 [7,8]. A careful theoretical analysis [9] has revealed the following. First, an estimate of the shift is made by initially neglecting the final state effects and considering only the initial state differences. It is obvious that the core level binding energy of an electron in an atom should be related to the effective charge on that atom. However, it is not sufficient to consider only the nuclear charge of the atom being ionized. For example, with an organic molecule, quantum mechanics allows us to calculate the effective charge on a given carbon atom quite exactly. From simple electrostatic considerations one would lead us to expect that the BE of a core-level electron should be inversely related to the calculated charge density of the valence electrons on the atom being ionized. Let us call that charge density Q and the electrostatic potential energy V originating from the neighbouring atoms in a molecule or in a lattice. Then, with C1 and C 2 being constants, BE = C1. Q + V + (C 2 E rel)
(5)
If the relaxation effects are also considered, the term in brackets should be added. In most cases E re~ is neglected and the method is then called the Ground-Potential Method (GPM). With the best possible choice of constants the GPM predicts a +0.37 eV shift for ethane and +0.57 for n-butane. The absolute value would not be a matter of concern, but the sign
and trend are the opposite of the experimental ones! A better agreement can only be obtained when E rel is included, and this is done in the so-called Transition State Method, by using the equation
80
chapter 2
Erel= C 3 AQ* + AV*
(6)
where AQ* and AV* are respectively the changes in Q and V accompanying the transition from the intial to the final states [12]. One has to take the Q and V values somewhere at the half-point of the transition from the initial to the final state. The exact procedure is not relevant for discussion concerning alloys, but we shall remember that the correct sign and trend of BE values can only be obtained when the relaxation energy is properly accounted for [9]; that point is very important for the discussion in chapter 3.
2.1.2.2 Relaxation effects in solids, particularly metals With insulators, all the effects mentioned so far have to be considered, and one also must not forget the polarization of the lattice when discussing the extra-atomic relaxation. The largest relaxation effects are to be found in metals. Upon photo-emission of a core electron from an atom in a metal, the mobile valence band electrons are attracted toward the positive hole created by the ionization. In contrast to molecules and insulators, the existence of a pseudo-continuous valence band in metals allows the screening charge to be transferred on the atom being ionized. This screening by the valence electrons is easier, the higher the density of states near the Fermi energy (see chapter 1), and this is the situation with the incomplete valence band of transition metals, which has predominantly d-character. This mechanism of screening, shown schematically in figure 4, is however seriously disturbed when the metal is in the form of very small metal particles. Diminishing the size causes the energy levels to lie further apart and to bring an extra electron to the lowest unoccupied level (just above the Fermi Energy EF) costs more energy than with bulk metals which have well developed bands with almost continuously changing energy levels. Less screening due to small particle size means a lower relaxation energy and thus a higher BE. This is one of the effects behind the correlation repeatedly found with metals: higher BE for a smaller average particle size. This effect will be discussed again later (chapter 5). Another contribution to this shift is due to the different cohesion energy in small particles (see section 2.1.2.4; the theory of Johansson and Martensson). Metals, with their mobile electrons, responding fast to any change in external fields or to the charge created by irradiation, can also bring about remarkable changes in the relaxation energy of adsorbed molecules. Electrons to screen a hole on the adsorbed molecule can be attracted from the metal, as we shall see below.
Experimental techniques of solid state physics
81
E VQC
g
EF hv
ore
L_I
L_/
figure 4 Energy levels (schematically, the distances between the levels are not on the scale) in a metal. Core levels and valence band are indicated. Photoionization causes electrons in the valence band to move towards the electron hole; this is called extra-atomic screening.
2.1.2.3 Relaxation effects in adsorbed atoms and molecules
In the region of energy where ionization of valence electrons in molecular orbitals of chemisorption bonds occurs, the formation of new chemisorption states has been observed. It has been concluded that, for example, hydrogen atoms act as a positive potential which creates new states [13]. The same observation of chemisorption states has been also made with nitrogen [14] and several chalcogenides [15]. Obviously, the initial state phenomena dominate these particular results concerning valence electrons. Core-level PES displays a great variety of phenomena: shifts in band-positions, and changes in the band shape and width [15-18]. Results to illustrate this are presented in figure 5. The change in BE as a function of coverage is explained [18] by variation of the relaxation energy. While the 4ds/2BE's are assumed to be different for different sites, and the hollow sites to be occupied first and a-top sites later, the explanation offered is in terms of initial states. However, this explanation cannot be accepted as a definitive one, since the different sites also undoubtedly differ in the extra atomic relaxation, i.e. screening. Similar difficulties have been encountered with discriminating amongst the various contributions such as bonding effects, electrostatic effects of the surface dipoles and relaxation and screening effects to the BE of adsorbed oxygen (see e.g. [19]). We shall return to this particular point when discussing metal-on-metal systems.
82
chapter 2
I / Pt
0.1" ~ m
(111)
FI
A 03 t_ >,, O
It
0.2-
0 0
E
I
0.3-
1
0.4-
I
49.4
I
I
I
|
49.0
I
I Z.d 512 Binding
i
I
Energy
l
48.6
I
I
!
I
48.2
(eV)
figure 5 XPS of iodine adsorbed on Pt(111). 4d5/2 binding energy vs coverage for the two doublets in the iodine spectrum. The height of each vertical bar is proportional to the doublet's intensity [16].
With adsorbed molecules the spectra are even more complicated than with pure metals and adsorbed atoms. With UPS and synchrotron radiation, one probes the valenceelectron levels, corresponding to the MO's in adsorbed molecules or new MO's formed upon chemisorption. As a rule of thumb, one can expect that in this region of energy the information gained will mainly concern initial states. However, in the BE shifts of the core-levels, the final state effects are very important and sometimes even dominating. Let us demonstrate all this with carbon monoxide, the most studied of all molecules. The C transition in the ls XPS spectrum of carbon monoxide chemisorbed on various metals exhibits a multiplet structure which was first suspected to be due to there being several different adsorbed states; that is, the multiplicity was ascribed solely to initial state phenomena. However,
later and more thorough experimental and theoretical
analyses revealed that it arises from differently-screened final states [20-23]. At this
Experimental techniques of solid state physics
83
moment only some details of these final state effects are still a matter of discussion [22], not the principle of explanation. A symmetrical nitrogen molecule also shows a multiplet in the region of the N l s transition. This molecule being oriented perpendicularly to the surface experiences different potential environments on the lower and the upper N-atoms, and this has been suggested as the main reason for the doublet structure [23]. However, the two atoms also experience different screening due to the interaction with the metal electrons, and this seems to be at the moment an even more likely explanation for the presence of a doublet. A free carbon monoxide molecule shows three bands in the region of MO energies, at about 14, 17 and 20 eV. The two latter bands show vibrational structure. They are ascribed respectively to the 5o, 1~ and 4o molecular orbitals, having well-known energies [25]. A comparison with the measured BE's reveals that E re~ is 1-2 eV, being as expected the largest for the 4o electrons. Upon adsorption the vibrational structure disappears due to shortening of the final state life-time; fast screening manifests itself here, and instead of a three-peak structure one having two-peaks appears. This is a consequence of the large shift in BE of the 5o-electrons due to the bonding. The appearance of a UPS spectrum of carbon monoxide on a metal is shown schematically in figure 6.
=
__
-"
N( E) "r A ------.. B - - - - C . , - - -
oO~
" EF: 0 2 L..to
~
Secondaries of
[~
/i:~
~ Kinetic energy
16i~i21o8 g Z 2 b ev ~,t,,'~s~ctto Evac.sample
covered
, 2 # jFc,,,~
spectrometer
Energy below EF 8 10 {2 12 16 eV : ,n,t,at state e~ergy .,. 9
"zero"
Secoo~ / of sample
d-band f
~
ANIE)
hv-r
O-EF-
z.
8
12 16 eV Energy below EF
figure 6 Left: principal shape of an UP spectrum excited by He I radiation. The relative intensities of primary and secondary electrons are very dependent on the particular experimental arrangement, e.g. angle resolved or integrated field free or field applied conditions, and may differ from this illustration considerably. Right: Schematic representation of the changes observed in UP spectra upon adsorption. The top curve gives the difference N(E)covered-N(E)clean. Refer to the text for details [3].
84
chapter 2
After a certain amount of discussion and wrong assignment of peaks in earlier papers, it has been finally established that the lower energy peak corresponds to the 4or level, while the higher energy emission is from the lrt and 5or orbitals, together. This assignment has finally been made definitive by studying the dependence of the intensities on the angle of incidence of a polarized light and by predicting and verifying the dependence of the intensity on the energy of the photons used [23, 26-28]. As mentioned above, PES does not determine the orbital energies, but the defined binding energies BE. For an adsorbed molecule we can write BEads = BEgas - O _+ (f.AO) - E rel + E B~176
(7)
The term in brackets reflects the fact that the work function 9 changes on adsorption and this change too is to some extent reflected in the BE shifts (0
is related to
various initial state bonding effects. None of these terms is negligible in the PES of MO's, but the exact contributions of individual terms can be estimated only roughly [29-33]. Whatever the origin of the BE shifts is, the spectra are always a source of valuable information. Surprisingly little of the potentially obtainable information has been collected with adsorption on alloys, although one very interesting application of Xe XPS to the analysis of surfaces, including those of alloys, will be discussed below.
2.1.2.4 Theoretical and semi-empirical calculation of binding energies in atoms, molecules, metals and alloys A full theoretical calculation of BE's by equations 1 and 2 comprises the relaxation effects implicitly, while semi-empirical calculations mostly aim to estimate
E rel
explicitly.
Examples which follow illustrate this. Several calculations of BE's of model compounds and of some real systems using various approximations have already been mentioned [20-22,30-32]. Let us now turn to the very interesting cases of metals and alloys. Calculating the BE's is here an extremely difficult task and making approximations is unavoidable. Usually the binding and relaxation effects are not explicitly separated in a comparison of free atoms with atoms in a metallic state [20-22,30-32,34]. A very interesting and instructive paper by Weimert et al. however does so [35]. The results in this paper give us a feeling about the size and importance of individual effects: the values are as follows. For the Au 4f7/2 level the BE found experimentally is 83.90eV, the theoretically predicted value being 83.7eV. The contributions to it are: 17,7eV for the final state relaxation and 4-4.6eV for the extraatomic relaxation-screening energy. Further, the measurable difference between atom and metal is
Experimental techniques of solid state physics
BEatom - BEsoli d = EVaCatom - EEFsolid - (I~solid = 2 . 3 e V
85
(8)
in good agreement with semi-empirical theories. The local potential around the core electron involved in ionization is influenced by the participation of the valence electrons in the metallic bonding. This initial state effect is estimated to be about -2.3eV and is thus directed against the extra-atomic relaxation. The values presented above are not absolutely exact but the overall picture offered [35] is probably correct. Binding energies can be sometimes predicted with a surprisingly high degree of accuracy using semi-empirical procedures. These are based on the equivalent core model and on thermodynamic data. Let us now see how it works for atoms, molecules and metals. Consider the BE of one of the ls electrons of lithium, which has only three electrons and the configuration (ls)2(2s) 1. Following the literature [36,37] we perform the following hypothetical steps: we shall ionize the 2s orbital (we spend 5.39eV), then we remove one of the ls electrons (75,62eV), and finally put one electron back (screening simulated) in the 2s orbital (unknown AE is released). This three-step process creates the hole in the ls orbital (such as the XPS ionization step which we analyse) and to calculate the energy needed to do this, we must know the energy change AE involved in the third step. According to the assumption of equivalent cores, we put this AlE equal to the energy gained by adding an electron to Mg +. The latter value is - 18.21eV, and the BE for Li ls is thus estimated as 62.8eV (figure 7). The experimental value is 64,85eV. The equivalent core approximation is based on the assumption (see above) that removing one of the inner electrons is felt by the outer electrons as an increase in the nuclear charge by one unit. So far the relaxation in atoms. Since a proper acknowledgement of the relaxation and screening effects is so important for metals and alloys, let us consider one more example, now concerning molecules. We shall calculate the difference between the BE of the ls electron in dinitrogen and say in nitrogen dioxide, taking the former as a standard zero for predicting BE shifts of other nitrogen-containing molecules [37]. The difference in BE is related to the following charge transfer reaction: N O 2 + N2 +(is) ~
NO2 +(is) + N2
(9)
where +(Is) indicates that the ionization took place from the ls orbital. In the equivalent core level approximation NO2 +r is put equivalent to O3+(valence) and N2+r to NO +(valence), where the index in brackets again indicates which level is ionized. Thus, one can also write: N O 2 +(is) + NO+(valence)= O3+(valence) + N2 +(is)
(10)
86
chapter 2
to which reaction we ascribe an energy effect: ABE = O. By addition of equations 9 and 10, the following is obtained: N O 2 + N O +(valence) ~
O3 +(valence) + N 2
(11)
and the accompanying enthalpy change equates to the BE difference looked for. It may thus be calculated from known ionization data. In this and other similar calculations the contributions of the initial and the final states are intermixed, but from the whole structure of the reasoning and from equation 11 one can expect that in reality it is the difference in E 'el that is being calculated. This is a point to remember for the explanation of results obtained with alloys.
Li 5.39eV
atom:
5,62 eV
(?)
--
I I I
18,21 eV ut
epual t o
I
I (?)
Mg+ figure 7 Equivalent core model. A lithium atom ionized twice and thereafter the screening electron is put into the 2s orbital. This state describes the situation in lithium after the core level ionization and full screening, and is put equivalent to the ionized Mg +.
The literature also offers many examples of calculations of BE's of series of related molecules or solids, especially oxides, with complete neglect of relaxation phenomena. From simple electrostatic considerations one concludes that there should be a relation between the ionization energy of a core-level electron and the local valenceelectron density on the atom being ionized. When two compounds are compared, ABE = kl.A Q
+ k2
(12)
Experimental techniques of solid state physics
87
In a better approximation one adds the differences in coulombic potential as caused by charges other than just valence electrons, for example, the charges of anions, or effectively charged fragments of the same molecules. Since in the long run E rel can never be completely neglected, for a series of related molecules or solids the difference in BE from a chosen standard is ABE = kl'(AQ) + AV
+ L~Erel +
K 2'
(13)
With this expression we return to the original question: what is the importance of the individual terms in equation (13) and what does a change in BE as determined experimentally actually mean? The transition state model (see above) represents one attempt to resolve the problem and with its most sophisticated description (see equation 13) this model too explicitly recognizes the importance of E rel. The problem of predicting the BE's of various metals was succesfully solved by Johansson and Martensson in 1980 [38]. Their semi-empirical calculation was based on two assumptions: 1) the final state is completely screened by a valence electron attracted from the pool of conducting electrons, 2) the equivalent core approximation can be used. These assumptions are built into a Born-Haber cycle as shown on the left side of figure 8. The cycle comprises the following steps. 1) Evaporation of 1 mol of Z atoms from a large amount of the metal (Ecoh (Z), is consumed); 2) ionization of the evaporated atoms (I(Z)); 3) addition of valence, i.e. screening, electrons to the Z ions, making excited Z" atoms; 4) condensation of Z" atoms into a Z" metal (Ecoh (Z") is relaxed); 5) dissolution or implantation of the Z" metal into a large piece of Z metal (Eimp). The remaining difference in the energy diagram is the BE in Z metal of an electron in the orbital in question. According to the equivalent core approximation, a excited Z" atom is equivalent to a valence-ionized (Z+I) atom (see the example in figure 7 as an analogy). With this assumption, the righthand side of figure 8 is obtained, and for the difference in BE of a free and a metal atom the following holds: A(BE)=EVB (gas)-EFB (met)=I(Z+ 1)+Ecoh(Z+ 1)-Ecoh(Z)-Eim p
(14)
the 'V' and 'F' upper indexes indicate the different standard states. In figure 9 the symbol A stands for the difference in BE between an atom and a solid of the same electron. A comparison of the values predicted in this way and those measured is excellent, as can be seen in this figure.
88
chapter 2
v a l e n c e ionized (Z+I) - atom (Z.1) §
core ionized Z-atom Z " - (. 1 ion)
// ~
(Z) Z
-
(Z+ll-atom
atom
Z'"
_ ~Ecoh r
(
Z-atom Z
E
I Z+I
17 ,~
"
me t a l
Z
imp .. (Z)
Z
metal
I Z- metal
I Z- metal J ..with .
coh
I Z -
Z+1)-metol~zim~p(Z) with
F --- [(Z§
-impurity
E B (met.)
I|
EB(met.)
figure 8 First, we remove one atom from metal Z, a process which defines the cohesive EZcoh. Ionization of the core electron of the free atom involves energy EV(gas). This Z"-ion is neutralized by acquisition of an electron into the lowest possible valence orbital of the Z" atom, in the presence of core hole, whereby Iz'' is released. After this we bring together a macroscopic number of Z" atoms to form a solid, yielding
EZ"coh. One
of these metallic
sites is then dissolved in the host metal Z, releasing the indicated energy.
Let us now return to analyse in a qualitative manner what has actually been done in this calculation of the BE. A difference between the BE's of a free metal atom and an atom in the metal (ABE) arises due to the following initial and final state effects. 1) When a metal atom is placed in a metal lattice, its electrons feel the positive charges from other nuclei in the neighbourhood. This is an initial, binding effect, let us call it coordination or environmental energy, E env, which increases BE. 2) Placing an atom in a lattice can lead to a redistribution of its electrons: for example, the free state of palladium is (4d)l~ whereas that of the palladium metal is (4d)9'7(5s) ~
Ec~
~
(let us call it configuration energy,
3) Valence electrons, being differently distributed in a free atom and in an atom in
metal lattice, create a different field at the location of the electron where it is being removed (EBOND). Last, but by no means least, the relaxation energy is different
(mF-rel). TO
a first approximation one can say that the I(Z+I) term is present in equation 14 mainly because of the relaxation effects, while the thermodynamic terms cover both relaxation
Experimental techniques of solid state physics
89
effects and initial state effects; the first term is however larger than the rest of terms put together. A similar scheme to that for pure metals can be constructed for alloys and the results of calculations performed by Verbeek [38], according to this scheme are shown in figure 10.
figure 9 Evaluation of the free atom-metal core-level shifts calculated by the theory (solid line). The filled bars represent experimental shifts. The open squares and rings denote quasiexperimental shifts between experimental solid-phase binding energies and semiempirically calculated atomic binding energies [381.
A
leVI
0
1Q
s
Imp - E z . ~ (Z) o
Rb
L
Sr
.
i
Y
,
-Zr 1
N'b
.....
1
No
,
1
Tc
t
Ru
I
h
.............
Pd
Ag
Id
C
~
In
L__
Sn
Cu 50Pt5o
Cu -shifts
CU5oPd5o AE (eV)
figure 10 Core level shifts of some 50-50 copper alloys. Open bars - calculated, shaded- measured. Calculation according [38].
90
chapter 2
The application of this theory to alloys is very important. Not only does it help us to understand the chemical shifts due to alloying, but the X-ray photoemission spectra of core levels offer a method for estimating enthalpies of alloying. However, let us now attend to our first task: can the BE of elements in alloys or the change in BE due to alloying be successfully predicted by thermodynamic (Born-Haber) cycles? The answer is affirmative. Where the enthalpy of alloy formation is not known, values obtained from the semi-empirical Miedema theory [39] can be used with success. Let us follow the very instructive discussion [40] concerning results on the shift in palladium and silver core levels [41]. Recalling the equivalent core approximation and the relevant theory [38], the ultimate value for BE of silver means that it is easier by about l eV to reproduce the electronic structure of cadmium, hypothetically formed in the equivalent core-level model, from silver in palladium than in an silver matrix. This is not surprising, since the formation of an Cd-Pd alloy is quite exothermic [39] and one can say that the matrix helps the formation of the final state. The two initial states (Ag in Ag and Ag in Pd) do not differ very much, since the enthalpy of formation of Ag-Pd alloys is low, palladium and silver forming an almost ideal solid solution. This picture obviously stresses the role of final states, although the calculations are based on the total energy estimates, which does not explicitly split the contributions from the initial and final states. What would now be the prediction based on initial states and in particular on charge transfer amongst alloy components? One would expect transfer of electrons from silver to palladium, and this should lead in the ground state model to a decrease in the BE of Pd 3d5/2. However, we see an increase. The charge transfer idea clearly fails here. Egelhoff [40] also showed that there are many similar changes opposite to those expected from bulk electronegativity differences, for example with Pd-Cu and Pd-Ni alloys. There are also other phenomena which indicate that charge transfer cannot offer a right explanation: with Au-Sn, Nb-Ni, Pd-Ti, Pd-Zr and Cu-Zr, the BE's of both components increase by alloying, which would of course be impossible if charge transfer were the main reason of the BE shifts [42]. The theory of Johansson and Martensson [38,39] also predicts correctly the socalled surface core-level shifts (SCLS) and those induced by adsorbed molecules. Let us start with the former ones. High resolution angle-resolved PES allows us to identify the core-level differences between the bulk and the surface atoms. When the d-band is less than half filled, the SCLS's are to higher BE's, but with metals having more than five d-electrons, they are to the lower BE's. This can be seen in figure 11, where the results and various predictions are compared.
91
Experimental techniques of solid state physics
0 75
't,,;,..~.,.
Rosengren
......
Tomanek . . . .
050
".
"__~'Mt
Jo~,~o, u m r t ~ Experiments
0.25
......
--.0-
~'.. 0.0 -0.25
~
'
.
~
-050
'~
~.
figure 11 The difference in BE's of core levels of atoms in the bulk and in the surface, the surface core level shift, for different metals of the third long period of ele ments. Different theories are compared with experiments. Only values for closed-packed surfaces are considered [431.
-0.75 Yb
l_u
Hf
Ta
W
Re
Os
Ir
Pt
Au
If the Born-Haber cycle is performed according to the example shown above [38,43], the SCLS are found to be for an element Z: A B E = ~r
- ~z
(15)
where 7 is the surface free energy of the elements as indicated. The Johansson-Martensson theory [38] is a total energy approximation and the initial (i.e. bonding) and final (i.e. relaxation, screening) effects are both implicitly involved in equation 15. However, there is another theory which ascribes SCLS solely to initial effects and this theory [44] is based on the fact that the local densities of states (DOS) on surface atoms are narrower than that on bulk atoms. This narrowing causes the DOS's in the surface and in the bulk to be occupied to a different degree. Equalisation of E F between bulk and surface atoms is achieved by transfer of electrons, but the effect is different for elements having more or less than half-filled d-bands. The effect of narrowing, viz. a higher occupancy and an equivalence of E F, is shown in figure 12 [40]. This model also predicts correctly the change in the sign of SCLS. The transfer of electrons is of a modest size; only 0.004 electrons per atom cause a shift of 0,1eV in the BE of a core level. However, the testing point is tungsten, where the
4f7/2
surface core-level shift is
about 0.31eV. This is of the same size as that caused by adsorbed oxygen. However "it would be very unreasonable to say that the ionicity of tungsten surface atoms is the same as that of the adsorbed oxygen" [45]. A detailed theoretical analysis [45] revealed the following. The SCLS can be split into the following hypothetical contributions: SCLS = A E env + A E c~ + z~Echtr" + A E rel
(16)
92
chapter 2
where /~env is due to the effect of other nuclei in the neighbourhood or of their absence on the surface, AEc~ is due to changes in the occupation of atomic levels, /~ch.tr. due to a charge transfer between the bulk and the surface (if there is any) and AE rel is the difference in the relaxation energies between the surface and bulk atoms. It was deduced theoretically for tungsten that AEenv is 0.26eV and that the rest of the effect is due to /~ch.tr. and AE re~. However, for a half-full band the AEchtr should be zero anyway, so that one can estimate/~E rel
to
BULK METAL
be about 0.05eV and here/~kE c ~
SURFACE: NARROWED
~ D.O.S.
I
I
t ~
I
i
CORE LEVEL
-~r-I
be approximately zero [45].
figure 12 SCLS described by the assumed changes in the initial state. The surface narrowed density of state distribution is
_-It- -
BAND
I
SHIFTS UPWARDS
tO
occupied to a different E, causing redistribution of electrons between the bulk and the surface. The bulk and the surface must have the same Fermi Energy E F. As a consequence, the whole
picture for surface states is shifted upwards by the electron transfer causing a shift in the core-level energy. The shift is predicted to be to smaller BE's (corelevel shifts up in the diagram)for metals with a more than half-filled d-band and to larger BE's for less than half-filled d-band [401.
Adsorbate-induced shifts have also been the subject of experimental and theoretical studies. The results obtained with metal-on-metal layers are especially interesting, since they concern systems which are very closely related to alloys. Let us consider [45] a single crystal plane nickel and a monolayer on it. The BE of the bulk Ni 2p~/2 core-level is 852.77eV and that of a nickel surface atom is 852.34eV. When this atom resides below a copper cover, its BE increases to 852.45eV. It is interesting to see what theories can account for this effect, which is also observed with many other systems. The first idea which probably comes to any chemist's mind is that of the old rigid band theory, which would predict a charge transfer from copper to nickel. Such transfer would be expected to decrease and not to increase the BE of Ni 2p3/2 level. A way of avoiding this difficulty would be to say that the surface electronegatives are different from those of the bulk and that the charge transfer is indeed from nickel to copper. Those who like this type of interpretation would go so far as to suggest this, but it is probably better to look for another explanation. Let us therefore consider the explanati-
Experimental techniques of solid state physics
93
on based on the narrowing of the surface density of states distributions. Nickel stays at the right-hand side of its period which means that its SCLS is towards lower BE, corresponding to more electrons on the surface atoms. A layer of copper on the top of nickel surface atoms increases the coordination of the surface nickel atoms. An increase in coordination leads to an increase in the BE of nickel surface atoms, by which atoms due to copper on their top become more like bulk nickel atoms. Indeed, an increase in BE is found experimentally, and thus at first glance this theory is superior to the rigid band model and the idea of charge transfer between nickel and copper. Nevertheless, in the theory based on narrowing of the surface band, a small charge transfer also takes place. The final test of this idea is provided by tungsten [45], for which the band narrowing effect can be excluded, because tungsten has a half-filled d-band. However, with tungsten the effect of coordination (effect of environment, leading to E env) can be estimated theoretically and we know in advance it has to increase the BE. For example, with a caesium layer on tungsten the charge transfer in the expected sense, i.e. from caesium to tungsten, and the relaxation, screening effect both work in the same direction (to decrease the BE of tungsten atoms) and it is difficult to disantangle these two effects [45]. It is very interesting to note that a layer of potassium on platinum causes an increase in the BE of electrons of platinum. To explain it solely by charge transfer would
mean to assume a charge transfer from platinum to potassium. This example shows again the importance of environmental (coordination) effect on BE's. The total energy consideration can also explain very well the adsorption-induced SCLS. This can be demonstrated by the shift in the BE upon adsorption of carbon monoxide on Ni(100). An ionized and screened nickel atom is according to theequivalent core-approximation (ECA) "equal" to copper. The chemisorption bond strength for Cu-CO is lower than for Ni-CO and this difference in the total energy is the main contribution to the adsorption induced SCLS of 0,67eV found experimentally [40,46]. The last phenomenon to be discussed is another BE shift in adsorbed layers, that in metal-on-metal layers. Representative results are shown in figure 13 [47]. The Au 4f BE value for a monolayer of gold on platinum was taken here as an arbitrary zero. The negative BE shift is then explained by the appearance of gold atoms in a coordinated state on the perimeter of gold islands and the positive BE shift by the growing number of gold atoms in a high coordination state. The position of an atom determines its E env and E tel. The difference between the value for a surface layer of gold, for which the BE is zero, and the extrapolated bulk value agrees with the results for the SCLS of gold on gold, viz. 0.38eV. This stresses the importance of the E env term: it does not matter too much whether a monolayer of gold is on platinum or on gold as long as only the coordination is increased. The equivalent core model approach can also predict BE shifts in adsorbed molecules reasonably well. For example, by this theory the non-equivalence of two
94
chapter 2
nitrogen atoms in adsorbed nitrogen molecules, e.g. on nickel, can be very well explained [40,48]. Following the ideas of the ECA theories, one would predict interesting core level shifts for the surface atoms as well as for atoms in adsorbed layers on alloys. However, this potentially useful information is still missing from the literature.
EPITAXIAL Au LAYERS ON Pt(100) AB.E.(eV)
.
.
.
.
-0.6
o~
0.4
0.2 0.0
figure 13 The BE shift ABE(eV) for gold epitaxially grown layer on Pt(lO0).
~BULK-
URFACE
0.2 0.4 EDGES--
0.6 0
1 Au
!
2
1
3
i
4
COVERAGE
5 8
2.1.2.5 Integrated and angle-resolved spectra of valence band electrons in metals and alloys If all other parameters are kept constant, the intensity of photo-emitted electrons depends on the following factors: (i) the probability M of the transition between the initial and final state, (ii) the probability of finding an electron in the initial state which in its turn is proportional to the density of occupied states (usually described by the symbols DOS or N(E)), (iii) the probability of finding a final state fitting the resonance condition, this being proportional to the density of unoccupied final states of the appropriate energy. It is a good approximation to assume that the probability M is constant for a given valence band. Further, by using primary photons of higher energies, higher final energies Efina I can be reached and since then the N(E)final is approximately proportional to moderately varying
function N]Efinal, one can then neglect the modulation of the N(E)initial by N(E)final. I(E) = C. M. N(E)final.N(E)initial
= k.N(E)initial.
(17)
Recognition of these simple conclusions was very important for the theory of alloys and the first results obtained by photoemission studies on valence band electrons immediately
Experimental techniques of solid state physics
95
revolutionized this field of science [50]. Let us compare the predictions of the following theories: (i) the rigid band theory and (ii) the various theories assuming a strong local perturbation in alloys, using the scheme in figure 14. I I
ICEI
I
,,'1"
comman
/ T
~
"d"
occupation as in Ni- Cu alloy as in Ni
~.~.~~p
J I ~
" band
experim.- Ni
"li /\ i
band
~
~
band
experim. -
Ni-Cu alloy I I
0 (=E F )
I I
figure 14 Intensity of photoemission current as a function of energy, as predicted by the rigid band theory (A) and as measured (B,C,D). A) Rigid d-band, broader in alloy than in pure nickel B, C,D) Experimental distributions, as indicated. It is obvious from this figure that the rigid band theory and its inherent electron transfer from copper to nickel are not confirmed by experiment. The shape in D) is predicted by various theories assuming a strong but local perturbation potential in alloys and also experimentally found.
>B.E.
If nickel and copper formed a common rigid band, the effect of alloying would be a higher occupation of the common d-band due to s electrons coming from copper. The dband would grow broader in alloys. If nickel in copper (and vice versa) represent a strong localized perturbation, localizing the d electrons around the respective atoms, alloying of nickel (see B) with copper (see C) would lead to a spectrum like in D of figure 14, also found experimentally. If there were a transfer of electrons from copper to nickel accompanying the formation of two separate bands, the band of nickel should shift up, and that of copper down. Figure 14 illustrates the fact commonly found experimentally, namely, that
96
chapter 2
the bands in alloys do not move in the energy scale, and thus nothing points to an extended charge transfer. We will return to this point in chapter 3. The simple picture that the distribution of photo-emitted electrons I(E) is only a slightly modulated density-of-states c u r v e N(E)initial is based on spectra obtained by integrating the current over all angles. Further details over the electronic structure of metals and alloys can be obtained by using the angle-resolved spectra, i.e. using a series of I(E) curves determined at different angles of photo-emission. This can be illustrated (not described in full detail) using examples taken from the literature [51]. Determination of the band structure [51-53] from valence band PES results is possible, but only under certain conditions. The reason is that, upon photo-emission, the vector
k(ll ) (electron momentum vector parallel to the surface of emission) is conserved,
but the perpendicular component k(_L) is not. However, energy is always conserved (Efina 1 is the sum of Einitia I .4- hv) and this is the second condition helping the analysis. There are several techniques for overcoming the inherent difficulty of the non-conservation of one component [52]. More recently, a method has been applied which is conceptually not very difficult and yet very powerful [53]. One assumes that the final state band is a free-electron-like band, that is, that Efinal = (h2k2tot/2m) + V o
(18)
The corresponding initial state energy is calculated from the energy conservation law: Einitial(ktot) 4- hv = (h 2k2tot]2m ) + V o
(19)
The constant V o, the so-called inner potential, has to be chosen by fitting procedure. The c o n s e r v e d k2tot is given by: k2tot - 2 m ~ 2. [Einitia I (ktot) + hv - @]. sin 2 0
(20)
where O is the emission angle taken from the normal to the plane studied. The primary results have the form shown in figure 15. From these results Einitia ! is calculated, and this together with O fixes the ktot 2. E(k) or E(| is then plotted in the same graphs as the theoretically calculated E(k) or E(| this is shown in figure 16. Deriving band structures from the experimental results is easier when the body to be analyzed is two-dimensional, since then the non-conservation of k(_l_) plays no role. This is exactly so for layer compounds, such as for example graphite [54], for ordered adsorbed layers of atoms or molecules [55], and for surface layers of metals [56].
Experimental techniques of solid state physics
I
I
I t I ! I i Ni(100), r'XWK hv : 2 1 . 2 2 eV \ 1~/=45"
I
97
I
e=o ~
_
~30" ~~~---~
_ 40 ~
--
56r
_
_j
eoL
-
~
T~-"
J ~=0
I
1
Energy
J
J
(eV)
l -5
l
I
I
=
/
figure 15 Angle-resolved photoemission from the Ni(lO0), F XWK (see chapter 1 for this notation) ,% mirror plane; n is normal to the plane.
A full description or prediction of angle-resolved photo-emission spectra of valence bands requires not only the analysis of the angular behaviour, i.e. the position of peaks as a function of E(O), but also the calculation of correct intensities. Several difficulties attend such calculations [57], but the results are the most valuable pieces of information on the electronic structures of alloys [58]. Yet further details of the structure of solids can be obtained by monitoring spin polarised angle-resolved spectra. In particular, catalytically interesting alloys would be a good object for study, but these difficult measurements remain a task for the future [59].
98
chapter 2
Ni ( 11 0 ), h v -
2 1 . 22
eV
EF=0 .~.
9
,,,-
-
5
L
I
I
I
RLUX
-
RXWK
EF=O
.
.
.
.
.
.
.
-
._ UJ
bl 90
I
45
0 Emission
45
90
angle
figure 16 Comparison of measured and calculated dispersion curves. Triangles -measured peak positions, shaded areas- calculated peak positions assuming a 4eV final state broadening. a) Calculated from the band structures by Moruzzi et al. [511. b) Calculated from the semi-empirical band-structure [53].
2.1.2.6 Quantitative analysis by XPS So far we have only described the application of XPS to the study of electronic structures of alloys. However, it is also possible to gain some information by XPS on the composition of alloy surface layers [3]. Since the ionization probability of a core-level is almost independent of the valency and of the environment of the element to be determined, the intensity of photoemitted electrons, i.e. integrated area of a peak, is closely related to the number of atoms in the analyzed area [3]. The peak area determination is
Experimental techniques of solid state physics
99
performed after subtracting the background for which good procedures have already been suggested [3,4,60]. The signal I A corresponding to certain electron transitions on an element A is composed of contributions from several near-to-surface-layers. The signal is weaker, the larger the distance of the layer z from the surface. The signal decreases by a factor f with the distance z from the surface: (21)
f = e x p [ - z / Z ( E a) cos 0]
where 19 is the angle between the normal and the emission directions and ~(EA) is the mean free path of electrons. If the density of atoms A at x, y, z is p(x,y,z), to obtain the total intensity I A, we must integrate (see eq.22) over x,y,z and also over the angle between the directions of incidence and collection 7, and the azimuthal angle in the x,y plane ~. Then 2
I,
f f= L,(v)f f Jo(x,y) T(x,y,v,O,Ea)f y=o
r
xI
y
pafdzdxdydrddpdy
(22)
z
where (YA is the photoionization cross-section, L a describes the angular asymmetry of the
photoemission, Jo the flux of primary photons and T the transmission of the electron analyzer. Often the so-called spectrometer transmission is introduced which is defined as Gi =
T(x,y,EA) dxdy and its value is taken approximately to be inversely proportional to
EA, the kinetic energy of electrons leaving atom A. If the flux of incoming electrons Jo(x,y) is homogeneous over the whole illuminated spot, it can simply be taken as a constant. Exact integration over the z coordinate, i.e. ~ (z).f. dz, is often replaced by a density-analyzed volume term, p. )~ (EA).CosO. A', where A' is the irradiated area.
IA = B.A' LA. Jo. PA. 6,(E2.X(E2.c~
0
(23)
The cross sections CYAhave been tabulated by Scofield [61] and LA(7) has been calculated too [62]. One often uses the following simplified expression: ia = B / oa" La"
pA.~.(Ea).E]~'
(24)
and determines the densities DA with respect to an element for which the constant B' is taken arbitrarily as unity. For an element in a pure state, the symbol IA~ is used. It is then convenient to take
IA]I~
ratios, whereby some of the unknown factors can be eliminated.
Following Ertl and Kuppers [3], and Seah [61], we shall look now in more detail at the two more interesting cases of bimetallics: (i)
A and B forming a homogeneous solution; and
100
(ii)
chapter 2
A covering B up to a coverage ~)a"
For case (i), the ratio of normalized XPS intensities taken from equation 23 is
x,/q
(O.x,(E,)
o,
(25)
Here ~'AB stands for the mean free path of electrons from A in the matrix A + B. For a metal M, one can use the semi-empirical relation: _312
(26)
171/2
)tM = 0.41. a M .,.,M
In this estimate the atomic size a M is calculated from the average density PM, the Avogadro number N AV and the atomic mass MM:
(27)
10-3"MM aM = PM NAV
and knowing that pA ~ equals aA-3 and OA in an AB alloy equals X A. aAB"3, where X g is mole fraction of A, we may write:
(28)
P~ PB
Xn ~,aB,]
By substituting equation 26 into 25 and rearranging the terms, we arrive at the following equation for the ratio of concentrations, or their molar ratios in the alloy A-B"
CA _ Xa _(aa] 312 IA/lff
(29)
When a not too high accuracy is needed, one can put the term in brackets equal to unity and use tabulated sensitivities for IA~ and IB~ [3], to calculate the required ratio CA/CB. In case (ii), the signal is split into two contributions: one from the bare surface and one from the covered surface. These are respectively (1-OA)IB~ and OA.IB~ where f is the attenuation factor (see equation 21). The total signal is: IB =IB~ ((1-0'1) + 0 A e x P [ -
aA COS0]/~A
(30)
The signal from the adsorbed layer IA equals OAIA~ SO that the ratio to be used for
Experimental techniques of solid state physics
101
determination of IDA is ~
o,,
(31) 1-OA(1-exp[ -aA cosO])
When clustering of one of the alloy components takes place or when surface segregation occurs, the above used procedures are less adequate, since they give us only a rough idea of the average composition within a few nm of the surface rather than an exact value. Problems also arise when supported metals rather than unsupported ones are to be measured. We shall now turn our attention to this problem. XPS signals can be also used to estimate the dispersion of monometallic supported catalysts (for determining the dispersion see chapter 7). As a first approximation one can consider a model in which a fraction | of the support is covered by a metallic layer of a thickness tM. By analogy with the equations 27 and 28 we can write [63,64]:
I M/I~
OM(1-exp(- Z-~u~))
(32) $
o
~vvlls~vv
1-Ou(1-exp(-
tu )) (M)
~'supp
whereby the M-index stands for metal and ~supp(M) for the mean free path in the metal of electrons of an element of the support used for the analysis, etc. The second equation to be used to detemine both tM and |
by an XPS technique alone is: (33)
pu.S.Ou.t M = XMIX~.pp
where S stands for the surface area of lg of the support and X M and
Xsuppfor weight
ratios
of the metal and support in one gram of catalyst. This model has been named the stratified
layer model [63]; more sophisticated models have also been developed. The model developed by Kerkhof and Moulijn [64] has also been extended to supported layers in which segregation of one component occurs [65], following the ideas of Defosse et a1.[64]. A common problem with all measurements on supported metals is that the determination is usually performed on pellets. Upon compression, the material of the support tends to surround the hard metal particles, and this partially covers them; this suppresses the signal from the metal and an apparently lower dispersion of a metal is observed. The error is less serious when various but similar catalysts based on the same support are compared and when the analysis is performed on pellets broken in vacuo.
102
chapter 2
2.2 A u g e r S p e c t r o s c o p y
2.2.1 Basic principles We may use figure 17 to describe an Auger process and to show how it is related to other electronic processes.
E kin (X PS )
Evac Valence
I X- ray I fluorescence I I I
E
band
&
UJ
L3
L2
L1
= = t. e
~
2
: -_ 9
"•L3
P3/2
~2Pl/ 2S 2
g: I I._
.& E K
=
9
IS
o3 ._J
r~ I
T Ekin (Auger)
I
Q; t_
I
c
I
.9
t
2
I I I
~ ~, "~ :~" .c
I
t'
I I I I I
I I
It. Auger
Process
I L electron I takes away I I deexcitation
energy
figure 17 Relation between various processes used in the indicated spectroscopies left: scheme of energy levels (not on scale) middle: de-excitation by hv-excitation, after reoccupation of the primarily ionised level K. right: de-excitation by electron emission (Auger process).
Excitation of an electron from the K-shell into vacuum requires the energy Eprim. The energy carrier can be either an X-ray photon as in XPS, or an electron, as in Auger Electron Spectroscopy, AES. As described in section 2.1, in XPS the kinetic energy Ekin of the escaping electrons is determined. In the solid, various electron rearrangements follow the ionization. First, the hole in the K shell is filled by one of the electrons from a higher level (L 3 level in fig.17) and then the energy is released by one of two subsequent processes: (1) emission of a photon giving rise to X-ray fluorescence; (2) emission of another electron, in fig.17 an electron of the L 1 shell, into vacuum with the release of energy E(L3)-E(K); this gives rise to Auger Spectroscopy.
Experimental techniques of solid state physics
103
As indicated in fig.17, the second process is a radiation less transition and is called an Auger process [66]. When going along the periods of the Periodic Table from left to right, the contribution of an Auger process to deexcitation decreases and that of X-ray emission increases. To excite Auger electrons and to monitor them by measuring their number and Ekin, the equipment for XPS (see section 2.1) can be used. However, to increase the sensitivity of the measurements a powerful source of electrons is often used for ionization instead of an X-ray source, and furthermore the spectrum is recorded in derivative form. This arrangement is what is usually implied by the acronym AES. Examples of both integral and derivative spectra are shown in figures I8 and 19.
8
LzM~.sM~.s
L3M~.sM:.s
7000
Z
Krypton
i
I 6000 c~ co
"t
I~M2~M~s I7 "
5000
N
IzMz,jM~s
_
N ~000 3000
2000
#/",,,~m~o 12 0
1300
I(metJc energy leVI
1~00
1500
figure 18 Photon excited Auger spectrum of krypton in integral form, measured by an XPS equipment [67].
The first important characteristic feature of an Auger emission peak is its position on the energy scale. For example, an Auger process as shown schematically in figure 17 would produce an emission peak at the kinetic energy of the monitored electrons
Eu.(K L3L1), where
104
chapter 2
(34)
E~an(KL3L1) = E(K) - E(L3) - E(L1) - AE
The correction term AlE collects together all deviations from the tabulated values of E's. These are caused by the fact that after ionisation, electrons in the L 3 and L 1 levels are in the field of a higher positive charge than in a neutral atom. The electron in the L1 level is even influenced by two holes, which also interact with each other, and this all is represented by the term AE, an estimate for which can be obtained by using the equivalent core model (see section 2.1).
~00
,
150
,
Z00
,
,
Z50 3 0 350 Electron energy [eV]
~.00
,
~.50
5 0
,
550
figure 19 Auger Electron Spectra of a beryllium sample [68]. a) Energy distribution I(E); b)first derivative of I(E); dI/dE. (the commercial AES equipments record b)).
Equation 34 shows two important aspects of the Auger process. The measured kinetic energy Eki n is independent of the primary ionising energy, and therefore less monochromatic but more intense sources of ionisation such as electrons can be used. Further, the first three terms in equation 34 can be found in tables and Auger spectra can thus serve as a very valuable tool in qualitative element analysis. For analysis of solid samples the natural energy zero level is the Fermi level of the system comprising the sample and the spectrometer and then, as with XPS (see section 2.1), the work function of
Experimental techniques of solid state physics
105
the spectrometer should be added to Ekin(K L L) before making comparison with tabulated data. When observing the same element in different compounds and environments we can often see a chemical shift in the position of the Auger peaks, the reasons for which are essentially the same as those causing the analogous shifts in XPS and can be related to differences in either the ground state or the final state which is here a two-hole-state. AES equipment has a relatively lower resolution than XPS machines usually have and thus the use of chemical shifts as a source of information on the solid is not as frequent as with XPS. However, a combination of measurements of XPS and Auger shifts is very useful. Auger transitions in solids sometimes involve electrons from the valence band, for example E(K V~V2) indicates a transition involving two levels of the valence band. The Auger signal is then broadened and its shape and intensity reflect two convoluted density of states function N(E) of the valence band. Analysis of such spectra and a comparison of predicted and measured peak shapes have been reported [69]. For more detailed information on the principles and application of Auger spectra the reader is referred to several monographs on this subject [3,70]. Very good introductions can be found in the monographs by Ertl and Ktippers [3] and by Woodruff and Delchar [3]. Auger spectroscopy reveals interesting differences between various forms of carbon on metallic surfaces [71]. This is illustrated by figures 20 and 21, which show Auger spectra in the usual derivative mode. Figure 20 shows Auger spectra of carbon monoxide adsorbed on platinum and of methane adsorbed dissociatively on rhodium. Spectrum c of figure 20 shows the pure carbon spectrum, obtained from b by subtraction of the rhodium spectrum. Such carbon signals are typical for carbon in molecules or in carbides. Figure 21 shows an Auger spectrum obtained after chemisorption of ethene on platinum at different temperatures. This form of signal is typical of graphitic carbon. In this way it has been shown that graphite layers are formed much more easily on platinum than on e.g. rhodium or nickel [72]. Deposition of carbon and formation of graphite layers can be greatly suppressed by alloying platinum with for example copper [72]. Deposition of carbon is accompanied by transport of platinum to the surface, thus counteracting the segregation established in vacuum. Simultaneous determination of XPS and Auger chemical shifts can supply us with information on screening and relaxation processes in solids. This aspect of Auger spectroscopy has been already briefly mentioned above, but further details are available in the literature [73].
106
chapter 2
i 1" rl ~ I v ~ ! 1 I !
9
I
100
I
-T-
300
!
I
I
500
I
I
l
I
I
I
rlll]]l
I
E---~
-v -> E (eV)
figure 20 Typical Auger spectra (dN/dE versus E) of Pt (a) and Rh (b) covered with a submonolayer of molecular carbon stemming from CO and C H 4, respectively: c) A typical 'molecular' carbon KVV Auger spectrum of CO chemisorbed on Rh resulting from the spectrum subtraction technique. The energy scaling is indicated in steps of 5eV; the position of 275eV is indicated [72]. figure 21 (right side) The carbon KVV Auger spectra after chemisorption of 0.5 mbar ethene on Pt during 5 min. The sensitivity of the vertical axis has been decreased by a factor 2.5 for practical reasons. (a) 30OK; (b) 40OK; (c) 520K; * indicates 275eV [721.
2.2.2 Quantitative analysis by Auger Electrons Spectroscopy. Many valuable results on the composition of alloy surfaces have been obtained by this technique. Analyses have been performed with single crystal planes, foils, evaporated films and unsupported metals and alloys. Working with supported metals is more difficult, because pressing of a powder into a pellet, which is necessary for measurements in UHV-
Experimental techniques of solid state physics
107
AES apparatus, leads to burial of the hard metal particles in the support material. One has to use for analysis pellets broken in vacuo [75a,b]. The success of AES in analysing catalytically important materials is because Auger transitions of most metals release electrons with kinetic energy between 100 and 1000 eV. Since the surface sensitivity of the analysis is higher, the shorter the inelastic mean free path of electrons X, the escape depth should be as short as possible, and kinetic energies between 100 - 1000 eV are thus particularly suitable, as can be seen in figure 22. The universal
~(E)kin function
for various materials is shown in figure 22 [74] and we can
easily see that at about 100eV the X-function has a minimum. However, even at the optimum kinetic energy of about 100 eV, when X is at its minimum, an Auger signal detected outside the sample comes from at least two uppermost layers; AES is not sensitive to the outermost layer only. This is a very important point to remember for analysis of surface segregation in alloys.
figure 22 Inelastic mean free path of electrons in solids, as a function of the kinetic energy of electron E. A universal curve for
100 -
solids, as derived from the literature data on various elements and compounds.
10m
1I
1
I
10
I
100
I
1000
E,eV
Auger peaks modulate the background signal and the subtraction or suppression of the latter is very important when accurate results are required. The simplest way to do it is to measure in the derivative mode, as a standard AES apparatus does, and to use the peak to peak height measurements. This technique has met with much success and most of the results on alloys and chemisorbed layers have been obtained in this way. The problem of the background can arise when it changes too rapidly, as it does at E less than 160 eV, or when it is impossible to measure the standard and the sample in the same apparatus. There are several useful prescriptions on how to solve the background problems [75c,d,76]. When X-ray-excited Auger spectra (XAS) are the basis of analysis, one has to follow one of the procedures developed in the literature [77]. Sometimes the ratio of the Auger signal to the background can be used to gain additional or more accurate information [78].
108
chapter 2
For quantitative analysis based on AES, it is necessary to have information on the attenuation of the signal by the layers of the material analysed and on the so-called backscattering factors. Attenuation of a signal by a layer of thickness z is given approximately by (see equation 21) a factor
exp[-z/X], where ~, is a parameter of attenuation. In the literature
there are three names of ~, that are used interchangeably, although there is in fact some small difference in their meaning: (i) inelastic mean free path of electrons, (ii) attenuation length and (iii) escape depth. The first is what is obtained by calibration with overlayer films, and the term escape depth is better reserved for the ~zos O term. The inelastic mean free path is obtained by theoretical calculation, and some special experiments, and can differ from attenuation length by as much as 15% [79]. However, bearing in mind the accuracy of the calculations as well as that of experimental determinations, the difference between the two can probably be neglected. Nevertheless, for accurate analysis it is better to use the experimentally determined value of ~, measured in the apparatus used for analysis, than values from the universal ~, vs.E curve. Accurate analysis requires taking into account the fact that some of the Auger electrons are a consequence of back-scattering processes in the solid [81]. The electron beam giving the primary ionisation also produces electrons which have high enough energy to produce further Auger electrons. The back-scattering factors needed in proper data evaluation can be obtained by essentially the same type experiments as those used to determine ~, [82]. If ~Ax is the ionisation cross-section of level X in atom A,
Eprim the
energy of primary electrons and h(E) the electron spectrum of backscattered electrons, then the total ionisation cross section is expressed as a sum of the two terms: one corresponding to the primary ionisation and the other to the total effect of secondary ionisations:
o
-
o
1~pr~rn f o 4x(E)h(E).dE
(35)
Eax
Equation 35 is often approximated by:
OAx~ (Ep,,m) [1 +rm~t:(Eax,Eo) l = o lx~ /
(36)
Here r' stands for [l+rmatr ]. The effect of backscattering can either be estimated by calibration [82] or it can be roughly calculated by using one or another of the emprirical or semi-emprirical equations [81,83]. The principal equation for the Auger signal intensity for element A, I A, based on a continuous exponential attenuation, reads as follows:
Experimental techniques of solid state physics
109
/
I a = IoY o a sectz[i +rmatr(a,EA) 1. T(EA)D(Ea). f
~
I
~'matr cosO
.dz
(37)
In equation 37 all effects caused by backscattering are expressed by [l+rmatr ], I o stands for the intensity of the primary beam, qt is the probability that de-excitation occurs by an Auger transition, c~ the angle of primary beam incidence to the normal, |
the take-off
angle of Auger electrons and ~matr the attenuation length in which a signal is attenuated by a factor I/e, T(E) the tranmission coefficient of the signal through the spectrometer, and D(E) the detector efficiency. For a homogeneous sample, p is not a function of distance z and the integral leads to pA.~matrix COS O. It is further convenient to work with QA, the normalized IA/IA ~ ratio, where the intensities I A and IA~ ~ is the signal intensity for pure element) are determined in the same apparatus: Ia QA -
IA ~
[1 +rmatr(Ea)lpa.~.an(Ea)
(38)
[1 +rA(EA)lPA~
Further approximations can proceed along the lines explained above for r and in paragraph 2.1.2.6. for ~,, where the step from 9A, and pA~ to XA, the molar ratio of A in a binary alloy, is also outlined. The problem with alloys is that they are not homogeneous in the z-direction, because in general they show surface segregation of one of the components. There are several ways of handling this problem. It has usually been assumed that the normalized peak ratio (IAI~176 is simply proportional to the surface atomic ratio XAsurf/xBsurf. By surface, one means then either the outermost layer only, as was the case in the earliest literature, or more properly the surface layer with a thickness of about equal to ~. How incorrect the first procedure was will be shown below. The second approximation is less incorrect, but, when the segregation is limited to one layer and one works with a concentration which is not properly averaged over several layers, one loses a lot of potentially available information which is crucial for catalysis. An even better approximation can be achieved by evaluating the layer-by-layer contributions of several outer layers to the total Auger signal, and by comparing the calculated and measured ratios of signals. Let us now turn our attention to that procedure. A very useful model for calculating the Auger signals has been suggested by Gallon [84]. In this model, solids are represented by planes (1,2 .... n) parallel to a surface, the incident electron beam has a probability P of being scattered by the first layer and probability (I-P) of penetrating deeper. For the second layer a fraction P (I-P) of the original beam is scattered and (l-P) 2 penetrates, etc. The probability that an Auger electron is released towards the surface is W and the probability that this electron reaches vacuum
110
chapter 2
is E s. With N o primary electrons per second, No(l-P) n-1 electrons are scattered by the n-th layer, thereby producing No(l-P) "-l. PW electrons, of which No(l-P) n-~. PWEs nq reach vacuum. The total Auger current from the n-layers is then: I(n) = N oP W [ 1 +(1 - P ) ~ +..... (1 -P)"
(39)
or after performing the summation:
I(n) = N oP W 1 -(1 _p)n E~ 1 -(1 -P) E s
(40)
For a monolayer I(1), the result is given by NoPW; for the signal I(oo) one obtains: I(oo) = N o P W
1 1 -(1 - P ) E
(41)
With these expressions; the final equation reads I(n) - I(~o) [ 1 - [ 1 - / -I(1) ~ ) ] "1 = I(oo) [1 -[1 -N1]" ]
(42)
It is very easy to determine I(~), which is the I ~ in equation 38, and the only parameter to be determined is I(1). The fraction I(1)/I(oo) is abbreviated as Nl. Let us now analyse the relation of the Gallon equation 42 to the equation which results from the model of exponential attenuation. When a current of electrons originates at z = ;L, it loses 63% of its intensity before reaching the surface (see equation 21). If n~ layers are necesaary to suppress 63% of the signal, these nx atomic layers form the layer of thickness ;L:
l(n)
I(~) =
I(1) "x
1 -[1-/--~) ]
- 0.63
and nx.ln [1 I(1)] __
(43)
With ~ = n~ d(hkl), where d(hkl) is the distance between the crystallographic planes, I(1)H(~) = 1 -exp(d/~.)
(44)
and the equivalence of the equations is obvious. Equation 44 can be used to calculate I(1)
Experimental techniques of solid state physics
111
when this is not known, from the crystallographic parameter d and the value of ~ either tabulated or calculated from one of the empirical equations (see above). Vice versa, an experimental determination of I(1) can supply an accurate ~, value [82,85]. It has already been mentioned above that using Auger peak heights as a direct measure of outermost layer composition can lead to very incorrect conclusions. To support this statement we shall now analyse a hypothetical alloy A-B in different approximations. To assume that Auger signals reflect the composition in only the outermost layer is equivalent to the assumption that N1 of equation 42 is equal to unity. However, in reality N1 is always 0.5 or lower! We shall see immediately how this fact influences the conclusions. Experimentally the Auger peak intensities I A and IB are determined and are converted by elemental sensitivity factors into magnitudes called here PA and PB" Assuming that N1 is equal to unity leads to the conclusion that the atomic fraction X A of A in the outermost surface layer is equal to P defined as PA/PA+PB or (I+(PB/PA)) -1. We know however that we have to build up the signal from contributions of several layers (1,2 ....i) whereby the fraction of the total signal coming from the i- th layer is Ni, A (and Ni,B). Thus the xi's must be calculated from an equation such as 45 which in its turn is derived from equation 42. Nj,a Xi,~ r,,a i=l
(45)
Ni~ (1 -X/,a)r,,B i--1
Figure 23 compares three cases. On the extreme left of the figure P is equal to experimentally determined PA/(PA+PB). It means that N1 is put equal to 1 and ri,A equal to ri,B. The straight line in the extreme left corresponds to the case of no segregation. Thus, comparing the experimentally determined P with the straight line leads to a conclusion that there is only a marginal surface segregation. Such was the evaluation of results in earlier literature on palladium/silver and nickel/copper alloys. However, we know now that the evaluation could not be correct because the maximum possible value of N1 is not unity but only 0.5. Let us take the experimental results represented by the curve for P on the extreme left and re-evaluate them under the following assumptions" (i) N1 = 0.5, (ii) rA = rB, (iii) segregation is limited to the outermost layer. We obtain the curve shown in the indicated (N~ = 0.5) part of the figure. Finally, we keep assumptions (ii) and (iii) and assume that N1 = 0.25. These last mentioned assumptions correspond very well to the situation with palladium-silver alloys. The results of the last evaluation are shown in figure 23, too. The conclusion is straightforward. The experimental results indicate by the ratio P, at first glance a negligible segregation, but a proper evaluation of the same results, leads to a very different conclusion, namely that there is a pronounced segregation.
112
chapter 2
X$
- - - N~ =0 . 2 5 ,---,-.
=
/ ~
:
05~
05"
OS
Xbulk
! 05
05
Xbulk
figure 23 Analysis by AES of hypothetical alloy AB [86] left: The curve is representing the sensitivity-corrected ratio of signals PA/PA+PB. In the crudest approximation this ratio is taken as the molar ratio x~ of the component A in the outermost layer. middle: The experimental results represented by the curve for P are here analysed with the assumption that NI = 0.5 (as it is with Ni-Cu alloys) right: Curve P is evaluated by using indicated values of N~. The experimental ratio P and the 'N I = 0.2' case describe approximately the situation with the Pd-Ag alloys.
It is easy to predict the ratio of normalized Auger signals P A/PB when values of
XA,i
are known. However, usually we want to determine XA,i from the measured PA/PB values; to do that further approximations are necessary. We can choose one from the following procedures. (i) Values of X i are predicted by one or another theory of surface segregation and the parameters of the segregation equations and the X i values are fitted to the experimental results. (ii) We assume that segregation is limited to only the outermost layer, so that we merely have to calculate Xsu~f,A. In this case Xbu~k,Acan be put equal to the average composition of the sample. (iii) A relation is assumed connecting X1,A with X2,A and X3A and further. (iv) The latter relation can also be determined experimentally by peeling off the surface layers by sputtering. This procedure requires caution and the knowledge of the possible preferential sputtering of one of the components [82,85]. The more an alloy approaches the state of an ideal solution, the better the assumption (ii) would be fulfilled. A very instructive review on the problems of a proper quantitative evaluation of the AES results has been published by Lejcek [86].
Experimental techniques of solid state physics
113
2.3 Other methods
2.3.1 Ion scattering techniques: Low Energy Ion Scattering (LEIS) This is a very powerful technique for analysing the surface composition of solids with the highest possible surface sensitivity [87-92]. When an ion beam of energy say 0,20,5 keV approaches a metal surface, most of the ions are neutralised and this holds especially for all ions which penetrate below the surface to the deeper layers or are multiply scattered by two of the surface atoms. The small fraction of ions detected after their scattering by the first layer appears with an energy E~ at the ion detector. If the primary beam of ions of molecular weight M~ has energy E ~ the scattered beam will have energy E 1 after having been scattered by an atom of the molecular weight M 2.
_
M1
M 1 ' EO
'
EI
surface M2
\
\
/
\ \ \ \ \ figure 24 Ion scattering process. An ion with a mass M 1 and Energy E o collides with an atom (ion) in the surface (mass M 2) and is scattered back to the gas phase, with energy El.
With the geometry as shown in figure 24, classical mechanics using the energy and momentum conservation laws [145] derives for inelastic scattering:
114
chapter 2
El
1
Eo
(1 + ( M 2 / M 1 ) )
{ )''212
cosOI_+ M22
_
(46)
sinZOl
Equation 45 is particularly simple for |
= 90, when
m-1 M1
E1
(47)
~+1 M1 By measuring El and knowing E o and M1, M2 is determined and the intensity of the scattered beam thereby brings information about the surface composition. An interested reader is refered to the literature for further details of this technique [87-92]. Early instruments worked with an electrostatic 127 ~ energy analyser [87], which is still used in some modified and improved versions of the early LEIS apparatus [88]. The commercial instruments use the cylindrical mirror analyser (CMA), which is also frequently used in AES. The most advanced version of the mechanism built by Brongersma et al. [89] also makes use of this analyser. Those already having a CMA in their surface science facility will probably be interested in the apparatus designed by Niehaus and Bauer [90]. Advanced machines measure by time-of-flight mass spectrometer both neutrals and ions [91]. Many interesting aspects of using the ion beam technique have already been reviewed [92]. An example [93] of a LEIS apparatus is shown in figure 25.
I t
,oN
V
~'
\
_1 9 QUADRUPOLE T LENSES
STATION
ELECT,OSTAT,C
MAGNET
i-,,j~r~',,,~.~ 7
~
- Y'\ I"~" "- L ~
U Hv
PUMPING STATION . . . . . . .
~
t A~I-~.
S,STEM A.UJ~4".t5~
I _ IAIPLiFiERI 9 J
/
I
figure 25 A version of a Low Energy Ion Surface Scattering apparatus (schematically).
TARGET ? _Ba_S_---i _ 1 _ SUPPLY T
t ITIM R [ SCALER ~ I s c A E L E R l ~ CUF~EAI~Tk J [INTEGRATORi"
While the main aim of LEIS is the elemental analysis of surfaces, the very small fraction of the double-scattered ions can be used when measurable to gain information on
115
Experimental techniques of solid state physics
surface structure, e.g. lattice spacing [94]. It is a great advantage of this technique that it is so surface sensitive. It is one of the very few that are sensitive to only the outermost layer of a solid. However, proper application of this technique requires much skill and experience: even when only a very small primary ion current is used, some damage to the surface cannot be completely excluded. One therefore has to work at slightly elevated temperatures to ensure some annealing of the damage caused by selective sputtering. 2.3.2 Medium and high energy ion scattering (0.2-2 MeV) When primary ions are accelerated to the energy of one or more MeV (e.g. by a van der Graaf accelerator), they penetrate deep into the solid and are scattered by nuclei by a coulombic interaction, as was first recognized b y Rutherford. With a primary beam (Eo, M1) and symbols as used above, upon an impact perpendicular to the surface:
El_ mlo:O M212 "~o- ( MI+M2 )
(48)
When the layer to be analysed is thin, there will be a sharp maximum for a given O for each mass M2 of elements in the thin layer at energy E~ according to equation 48. However, when the layer is thick, many subsequent losses of energy of primary as well as of scattered ions occur, and this produces a step-like signal with a tail towards low energies. When a thin layer of a heavy element is supported by a thick layer of a lighter element, the intensity of the backscattered ions shows a pattern like that shown schematically in figure 26.
figure 26 A schematic
Int
1 Ep
presentation
of
Rutherford backscattering spectra for a thick substrate with a thin (monolayer) layer of a heavier element on the surface. Ions of primary Ep are used.
116
chapter 2
A medium energy ion beam of, for example, protons appears to be an extremely powerful tool for surface crystallography [95,96]. The technique has been applied to single crystals of pure metals, with or without simple adsorbates (Pt, Ni, O, S), to some semiconductors (e.g. Si) and to the problems of surface melting. An application to the adsorption on alloys or to metal-on-metal films can be expected in the near future. The experimental set-up for such measurements is shown in figure 27 [95,96].
..~'Faraday cup
sputter ion gun
-••
~~""'-~~'~l~~Ph
mline. ~ bea n e ~ ~ . ~ ' "'1~-'~~ '''~ :.r ~ _.
target J "
~ \ diaphragm
' / " ~ " ] ~ ~'~'~'
~~.
~
dia;hragms + Faraday cup
ragms s,it v l"~o.~~~.~,J ~ ~
~ ~ ~~
.lL'nultigliers X . I ~
~. ~~"
ion beam
figure 27 Ion beam scattering on a target; a scheme of experimental set-up [95]. ESA - electrostatic analyzer.
Figure 28 can be used to explain what is being measured. Imagine a beam of protons
coming from the left in the [314] direction and let it strike, for example, a
Ni(110) plane. The ions are scattered in a cone indicated by shadowing in figure 28, or they continue to move through the channels between the atoms (channeling of the beam). It is also so with the atom indicated in the same figure by o); this is called a shadowing
effect. The primary beam as indicated in figure 28 is scattered through an angle O and the scattered ions are hindered in their movement by atoms of the higher lying layer. This is called blocking. The cone of the leaving ions is at slightly different value of O when the outermost atoms, instead of being at their bulk positions, are in their relaxed surface positions. Determination of the scattering angle | at which the scattered ion beam shows a minimum in intensity allows an exact assessment of the surface relaxation [96]. There are already several examples of very successful use of this technique in surface science [97].
Experimental techniques of solid state physics
117
bulk f
[3i4]
[0 i] 0
~--surface
A
figure 28 Scattering configuration in the (111) plane of Ni(llO). A proton beam is incident in the [314] direction. Surface- and bulk-blocking is observed in the [011] direction. A displacement x of the first layer relative to the second layer produces an angular shift A| of the surface blocking cone [951.
2.3.3 Ion neutralisation spectroscopy (INS) by slow ions When an ion having a high ionisation potential, such as He*, collides with low energy (- 5eV) at a metal surface, it can be neutralised by an Auger process: an electron of a higher-lying valence band jumps into the low-lying ionised He atomic level. The energy of neutralisation is partially released by emitting into vacuum an Auger electron, the energy distribution I(E) of which reflects the density of states N(E) of the metal. In fact N(E) is self-convoluted, since it plays a role both in neutralisation and in emission. Quantitative deconvolution may sometimes be difficult, but the experimentally obtained I(E)-distribution is at least a good fingerprint, which can be understood in a qualitative way quite easily. Figure 29 shows results for several clean surfaces and for the adsorption of selenium, sulfur and oxygen on Ni(100) [98]. Most of the results obtained by this technique originate in the Bell laboratories [99]. The technique is as elegant as it is difficult, but since very similar information is also supplied by photoelectron spectroscopy, a method succesfully introduced later, not much has been published recently on the use of INS. However, its potential, in particular in combination with other techniques described in this chapter, remains.
118
chapter 2
2/..'
He* ions 5 e', Ge{111}
Ni {100}
c(2x2)-Se
v U.I
Y
Clean
x
m
__
16
M X
aO
,:; c o i_
•
-/
,|c
.o
-
8-
,g
.,...
g
....,
20
0 O4
!
!
0 /.. 8 112 16 la)Ejected electron energy. EKIeV)
(b)Ejected
electron energy. E K ( e v )
figure 29 ION Neutralization Spectroscopy (INS). Results for several clean surfaces (left) and for selenium, sulfur and oxygen adsorbed on Ni(lO0) [98].
2.3.4 Secondary Ion Mass Spectrometry (SIMS) When ions of sufficient energy collide with a solid surface, they either penetrate into the bulk, becoming implanted there, or they also knock atoms off the surface. The process of sputtering can be used to remove the surface layers in a controlled way and in combination with other tools such as AES or XPS to produce information on the composition of the surface layers down to a desired depth: this is known as depth profiling. When sputtering by primary ions is combined with mass spectrometric analysis of the released secondary ions, one speaks of Secondary Ion Mass Spectrometry (SIMS). Several reviews provide good information on the development of this very useful technique [100]. It is unique in the sense that one can perform surface and bulk analysis in one apparatus and, moreover, gain information on the variations in the composition normal to the surface. The technique has a rather high sensitivity. A typical energy of ions bombarding the surface is 2-4 KeV, and most frequently used gas is argon. According to the current density of primary ions one distinguishes static (SSIMS) and dynamic SIMS. In the former case the current density is about 10-l~ A cm -2 and this ensures that it takes several hours to remove by ion sputtering a monolayer. When the current density of 10-2 A cm -2 is used, dynamic conditions are achieved and a monolayer is removed in about 10 ms. The latter conditions are used in depth profiling, the former in analysis of adsorbed layers. Sputtering yields can be predicted for simple homogeneous materials [100].
Experimental techniques of solid state physics
119
However, an analysis of surfaces of alloys covered by other elements such as oxygen, carbon, sulfur is a purely empirical procedure. Although the technique has been applied to catalysts from its very first development [100], its main application nowadays is in the materials science and electronics industries, where it belongs to the most powerful techniques used. When the surface of aluminium is bombarded, A1§ A12§ A13§ and A14+ ions can be detected in the gas phase. Similarly, when carbon monoxide is adsorbed on a metal surface at which it is bonded to two or more atoms. M2CO + and M3CO § are observed in the gas phase. This qualitative information can be very useful but it is still difficult to derive very desirable quantitative information on such matters as ordering and homogeneity of alloys, exact coordination of adsorbed species, multiplicity of adsorption bonds etc., only from SIMS results and analysis based on cluster ions observed upon sputtering. The main problem is that one does not know exactly how many of the clusters detected in the gas phase were formed in the process of sputtering and how much of the cluster structure was already there before sputtering [101]. Since we do not know exactly to what extent the distribution of sputtered ions of varying composition (e.g. M-CO, M2-CO, M3-CO) reflects the distribution of the various adsorbed species, SIMS is in this respect a qualitative rather than a quantitative tool for analysis of adsorbed layers. 2.3.5 High-Field Emission Techniques Emission of electrons from a solid can be achieved by heating, i.e. thermoemission, or by bombarding with carriers of a sufficient energy, i.e. ions, electrons or photons. However, it is also possible to achieve a cold emission from metals by applying a sufficiently high electric field and causing tunnelling of electrons from the metal. The process is shown schematically in figure 30.
vacuum
tunnelling
figure 30
Metal
Applied field
A metal with its valence band and work function 9 indicated. Tunneling occurs when the potential decreases steeply enough.
120
chapter 2
Electrons can tunnel into vacuum when the field F is of the order of 1-5 V nm -~ and when the barrier outside the metal is very narrow; this requires use of the metal in a form of a very sharp tip. The current I measured at the anode is given approximately by: I/V 2 = a exp [ - b
(I)3/2/cV]
(49)
where a,b,c are constants and ~ is the metal work function [102-105]. A field-emission tube is shown in figure 31 [105].
f
I
Pumps
Anode ring
figure 31 Field emission electron tube.
Phosphor screen
Cu ball
Shield
Field emission of electrons does not have atomic resolution. One sees on the screen bright and dark spots, which are imaging the various crystallographic planes which form the almost spherical surface of the microscopic single-crystal at the very top of the metal tip. Atomic resolution can be achieved when the polarity is reversed, metal then being positive, and when helium atoms are used for the imaging. They are much heavier than electrons and at low temperature they have much less tangential energy than electrons tunnelling at the Fermi level (from which the electron emission comes). As a consequence, after their ionisation at the surface, when an electron of helium tunnels into the metal, He+ ions are shot against the screen along a straight line trajectory [106-109]. In this way He+ ions image the spots where they were created from atoms. A picture such as in figure 32 appears on the screen. When the positive voltage on the tip is sufficiently high, field desorption, i.e. evaporation, of surface atoms can occur. The inventor of the high-field techniques, E.W.M~iller, suggested using this phenomenon to analyse the surface composi-
121
Experimental techniques of solid state physics
tion of the tip [106-109]. This is possible when the mass of the desorbing atoms can be determined by a time-of-flight mass spectrometer. Such atom-probe equipment is shown in figure 33.
i:::::::~t(1 O) (]xl;
Mixed
(ix2)
(I x ~,
figure 32 (a) (lxl)Pt(llO) surface prepared by low temperature field evaporation. Between two photos from (a) to (e) is the application of one pulsed-laser heating of the surface to about 400K for 5 ns with the applied field turned off. Note a gradual (lxl) to (lx2) reconstruction of the surface. At (e) the surface is completely reconstructed. After (e) this surface is then gradually field evaporated to reveal the structure of the underneath layer, which is found to have the (lxl) structure. This surface reconstruction is therefore consistent with the simple missing row model [110].
The atom-probe technique has been very useful in studying diffusion of metals on metals, the ordering of alloys, surface segregation in alloys, defects in alloys as compared with pure metals and some other metallic and surface science problems.
122
chapter 2
~' ~II1~~ " t ~ Trigger ~ L S ~ ~ Coldfinger~.....,l ~ Bellows ] . J ~ [ t ~--.~ / Lens+0"6Vc ! ViewingflTip_~,.'_-"- / _ ~ ~ _ _. _~ D -_
Image---~-~~ I. , gas
1 1 Pump
beh~
~-~'LPump
figure 33 Atom-probe field emission microscope. The emitting tip, time-of-flight tube (with a probe-hole, lens and detected) and the electronics are schematically shown.
2.3.6 Microscopes with atomic resolution By making use of diffraction phenomena [111 ], a classical electron microscopy can also achieve atomic resolution in solids. This high-resolution electron microscopy (HREM) depends on the use of very high voltage, which is used to accelerate electrons already emitted. Some applications of HREM to alloys have already been reported [112]. The future applications will be more in metallurgy and materials science in general than in surface science and catalysis, but there is a potential of this technique for checking alloy formation in small supported metal particles. In 1982 B innig and R6hrer disclosed [113] that they had constructed an apparatus which permitted the observation of solid surfaces at atomic resolution. The principle is as follows. A sharp metallic tip can emit or absorb electrons to or from another solid by the tunnelling mechanism described in paragraph 2.3.4. The condition is that the tip is sharp and the voltage of the correct value. Tunnelling depends exponentially on the distance between the atoms of the tip and those of the surface under study. Strictly speaking, the current depends on the form and extension of the electron wave functions, of both the tip and the surface and on their mutual overlap. These functions decay exponentially outside the solid. The tip scans the surface to be studied and this movement is achieved by using a piezoelectric transducer. The usual arrangement is that the tunnelling current is kept constant and one assumes thereby that the distance from tip to surface is also kept constant. However the reader can certainly feel that this is not always exactly true. The surface is repeatedly scanned over (in figure 34 - horizontally). A feedback circuit applies a correction voltage to the lift of the transducer normal to the surface. The magnitude of this additional voltage as a function of the position of the tip in the x-y plane is converted
Experimental techniques of solid state physics
123
by computer into a map of the surface corrugation. The equipment for Scanning Tunnelling Microscopy (STM) is shown in figure 34 [114].
figure 34 A schematic description of the scanning tunnelling microscope. The tip (shown enlarged) scans the surface being moved in x,y and z direction by piezoelectric transducer [123].
Xpiezo TIP
Iref SAMPLE
4~
MONITOR
The equipment in figure 34 can also be placed in an ultra-high-vacuum chamber [115]. Studies have already been published on subjects such as epitaxial growth of metals [116], adsorption of simple adsorbates on various single crystal planes of metals and semiconductors, and morphological changes of catalyst surfaces by adsorption and reaction [117]. Complicated structures of some clean surfaces (e.g. silicon) have been also succesfully analyzed by this method [ 113,117]. Although the method has the power of fascination and undoubtedly holds great promise for the future, it is not free from difficulties and problems. An example to illustrate these is the adsorption of silver on silicon [119]. The pictures obtained showed the presence of two components but it is not easy to show immediately what is what [119]. However, this is not the only problem. The STM essentially maps the convolution of the wave functions of the tip and the wave function of the surface, each showing a different degree of the (de)localization and extension outside the solid. Thus, with one and the same system, various maps can be obtained as a function of the shape of the tip, the atomic-scale shape of the surface and the distance between them. Also reversing the polarity very often leads to a change in the observed picture. The STM technique is usually applied to simple crystal planes, although attempts to study materials with rough surfaces, e.g. powders, have also been made [120]. Binnig and coworkers have developed another apparatus which is also applicable to ceramics, to a broad spectrum of polymers and to biological objects [121]. The principle on which it works is as follows. A very sharp tip is brought close to the surface; at a very
124
chapter 2
short distance the tip starts to feel attraction to the surface, but upon a further approach to the surface, repulsion by the surface prevails. The tip is mounted on a kind of balance, and a very small deflection of the lever can be measured together with the resonance frequency of the lever: the deflection of the lever shows the magnitude and direction of the attraction-repulsion force, while the change in the resonance frequency is proportional to the gradient of the force. On scanning, the tip-to-sample distance is controlled by a feedback circuit maintaining either a constant deflection or a constant resonance frequency of the lever. There are several techniques for measuring the deflection extremely accurately; amongst others one can use a second tip operating in the STM mode. This Atomic Force Microscopy (AFM) will certainly find many applications in surface chemistry and catalysis in the near future. When an alloy is being studied it can appear difficult to identify the components of the alloy. It can be helpful if a gas is available which is adsorbed strongly and selectively by only one of the alloy components. However, it is not easy to find, since adsorption of carbon monoxide on many metals is too weak for this purpose and it would be pushed over the surface by the tip. Another option for identifying the components of alloys or the objects sitting on the surface is to measure the I(V) or dI/dV characteristics of the spots which have to be identified. This is actually what can be called a surface tunnelling
spectroscopy [ 122]. 2.3.7 Work function measurements From the beginning of studies on the thermo-emission of electrodes [hot cathode devices], people realized the necessity of defining a parameter which would characterize the work needed to bring an electron out of a metal into vacuum. This parameter has been called work function 9 [124,125] and is usually expressed in eV. Work function can be defined by means of the Fermi energy, the electrostatic potential energy difference from bulk to vacuum due to surface dipoles D, the electrostatic energy put externally on the metal ~ and/or by the effective potential energy of electron inside, Veff (in), or outside the metal,
Veff(out ). While 9 is independent of the choice of
the zero potential energy and standard levels, the magnitudes of the quantities that constitute it are not. Most important is the choice of zero with respect to which EF is expressed. Two conventions are convenient: in theoretical solid state physics, the zero to which E F is expressed is often the bottom of the conductivity band; for spectroscopic and chemisorption studies, a more convenient zero is the vacuum level, Evac which is the energy of an electron at rest infinitely distant from the surface. The definition of 9 is immediately seen from figure 35.
Experimental techniques of solid state physics
125
( 1 0 -7 ) m V eff
f
(out)
EF Veff
(in)
rdist =
Vef f
{out)
-
Veff(in)
-
EF
( 1 0 -7 ) m
t
Eva c ( o o )
i /
/
qJ e l s t
oT
EF
dist @ = ( _E F)
t
~elst
=
( -la )
+_ D
figure 35 Parameters used to define the work function ~ of a metal (its valence band is shown). EF
- Fermi energy (electrochemical potential),
p
- chemical potential o f electrons,
D
- surface dipole layer
9 ~t~t
- outer-potential.
The choice of definition as shown has been made on the following grounds. When an electron is leaving a metal, it feels the attraction of the positive charge left behind, viz. the image potential. When measuring ~, we are not interested in that, so the minimum distance rdist from the surface (see figure 35, upper part) to be used in the definition of is about 1 Jam. The bulk of a metal is separated from the vacuum by a dipole layer which creates a potential energy step of D (figure 35, lower part). When very far from the surface the electron no longer feels the potential difference due to the dipole layer and
126
chapter 2
only at short distance (rdist --- 1 ~tm) does it feel the dipole layer to be infinitely large. Therefore ~ is defined as the work necessary to extract an electron from the metal and bring it into vacuum to the distance of 1 lum from the surface.
The work function is defined in such a way that it is independent of the electrostatic potential which one puts on the metal during the measurement. It is therefore incorrect to speculate, for example, that the work function of a small metal particle can be changed by putting it in the electrostatic field of an ionic metal oxide support (see chapter 5). While 9 is independent of the electrostatic potential on the metal, except for the effect of the surface double layer, the total energy parameter, viz. the Fermi energy E F, is dependent on the potential. Thus while the contact potential created upon a contact of two metals changes the position of EF with regard to the vacuum, it does not influence the work function. It is thus easy to determine the work function O, but difficult to determine EF. The literature (e.g. on XPS) reveals many misunderstandings of these simple facts. The potential energy difference D due to the surface dipole layer (figure 35) is an important feature of a metal, and is a feature that is characteristic of the crystallographic structure of the surface. Its main component arises from the fact that, on the vacuum side, electrons can move further from the nuclei, escaping the electron-electron repulsion and being less strongly bound to the metal than is the case in the bulk. When the surface is rough and open, the electrons can also fill the spaces between the surface atoms and then they can move less in the direction of the vacuum. As a consequence, rough surfaces have a lower work function than flat ones having a high density of atoms. This lowering of by surface roughness is called the Smoluchowski effect [126]. Thus various crystal faces of a metal have different work functions as seen in table 2, although they have all the same Fermi energy E F (again often misunderstood). According to figure 35 (the equation at the bottom), when two metals with ~1 and 9 2 respectively are in electric contact (i.e. they have the same E F then), they create a contact potential difference among them. O 1 - 0 2 = A~l/elstat
(50)
In other words an electron just outside the surface of a metal acquires an electrostatic potential difference m~Jelstat , SO that it does not leave the metal having the energy
Evac(~176
On this point also one can find many mistakes in the literature. Measurements of the work function have played a very important role in studies on alloys. By this technique the problem of surface segregation in vacuum, as well as that of gas-induced surface segregation, has been attacked experimentally for the first time [ 128]. The work functions of alloys are difficult to interpret because they reflect the varying surface dipole layer D, which dipole layer changes due to the differences in composition and differences in roughness, and varying electronic structure of the solid (E F, in figure 32). It is at the moment impossible to disentangle the individual contributions
Experimental techniques of solid state physics
127
to work function changes by alloying, but following these changes can, nevertheless, supply important finger-print information on the surface of alloys [127,128]. There are several techniques which can be used to determine work function or its changes. The most important ones are: 1) diode characteristic, including the technique based on scanning by electron beams; 2) contact potential change measurements (vibrating condenser and condenser charging techniques); 3) field emission measurements; 4) photo-electric yield measurement which gives in absolute value of ~. The interested reader is referred to the literature [ 124,125]. 2.3.8 Extended X-ray Absorption Fine Structure (EXAFS) Results obtained by this technique are discussed on several places in this book (chapters 3,5,7). Here we shall introduce some basic terms. When X-rays pass through a material of a thickness x, they are absorbed according to the Beer-Lambert Law: I = Ioe-~x
(51)
where Io is the intensity of the incoming and I of the emergent b e a m and ~ is the absorption coefficient. When px is plotted as a function of the photon energy, a characteristic difference can be noticed between the absorption of the X-ray in atomic gases on one side, and a solid state material on the other side, as shown in figure 36.
ABS
ABS
(,ux)
XANES
-" l,
J
|
...._EXAFS
"...
figure 36 Absorption of X-rays (x = thickness of the Left: absorption in Right: absorption in
E
E
characterized by btx, as a function of energy of the X-ray photons absorbing layer). a medium having a short or a long range order solids having a short range order.
128
chapter 2
Solid materials show namely a fine structure in absorption near the ionisation edge, whereas atomic gases show only the usual smooth form reflecting the dependence of the ionisation cross-section on the energy of the ionising particles or photons. The appearance of this fine structure has been known for a long time and the basis of the explanation, which is still valid, was first suggested by Kronig [130]. A quantum mechanical derivation of the equation describing the fine structure of absorption has been offered [131], and several books and reviews illustrate and instruct how the method of EXAFS can be applied to problems of catalyst surfaces and materials science [132]. An example of EXAFS of nickel is shown in fugure 37.
--
0.03
.
.
.
.
.
.
.
.
.
(b) O.4
0.02
"2
,at
o.oi
-0.2
o.8
i
....
~
....
~(I-')
~'s - ~
~176
~
4
s
,.(I)
s
lo
figure 37 Left: A typical function (oscillations in absorption intensity) with wave vector
12
k -1
(k
momentum photons) as the coordinate. Right: Fourier-transformed function often having maxima just at the distances of the atomic shells around the atom absorbing the photon. Results for nickel are shown. Notice; only an exact mathematical analysis can show whether the maxima as on the right are really the distances to the shells.
The probability of absorption of a photon is given by:
Pi,f = const. [Mif [Pie(E)
(52)
where i and f stand for initial and final states, Pif is the convolution of the densities of states and M is the matrix element for the transition between the initial and final states. The initial state wave function ~i is in principle known and the final state function
Experimental techniques of solid state physics
129
can be constructed as follows. The outcoming function for the emitted electron is linearly combined with the functions describing the back-scattering, i.e. the emergent electrons from neighbouring atoms. In this way the delocalized character of the function describing the emitted electron is accounted for. The interference (see figure 30, right) between the outcoming and the backscattered functions can be constructive or destructive, according to the distance between the atoms, the energy of the emitted electrons, that is, its wave length, and the field causing the back-scattering. With some simplifying assumptions, the function g(k) is the sum of terms taken over i-coordination shells around the atom monitored. The result is then: ~(k) = 1/k
(53)
•i [Ai.sin {2kRi + ~i( k)}]
where k is the photoelectron wave vector, R i the distance to the scattering shell and (I)i(k) the phase shift caused by scattering. The amplitude term is: A i = (Ni/Ri2). Fi(k ). exp[-2 (I)ik2]
(54)
where N i is the number of atoms at the distance R i, F is the back-scattering factor and the exponential term reflects the thermal movement of atoms, i.e. Debye-Waller-like term. Before the experimental results can be analysed using equations 53 and 54, several steps are necessary. First, the background, viz. the extrapolated curve of the pre-edge absorption (figure 36), has to be subtracted. Then the oscillations have to be separated from the background by relating them to the smooth line corresponding to the absorption without the fine structure (the middle-line); the picture thus obtained is converted into a ~(k) function (see figure 37). To make a correction for decreasing absorption with increasing k, a function ~(k).k 3 or ~(k)k is taken and it is Fourier-transformed (~) into a ~trans()c(k)k") function which peaks at distances of individual shells. A backscattering function F(k) must be used in this procedure, as well as the correct phase shift ~ i ( k ) . There are several possible ways for doing this, but the most reliable is by calibration with known molecules or solid compounds. The EXAFS technique has already produced much extremely valuable information on alloys, both crystalline and amorphous, including those present in small particles. The use of this technique, which is one of the very few which can be applied in situ during a catalytic experiment, will therefore continue. Determination of alloy structures from the Ri's c a n be complicated, because with an increasing number of components the difficulties of data evaluation increase correspondingly, but in principle it is quite reliable. However estimation of the number of nearest neighbours N i based on intensities of peaks is accompanied by more serious problems. We shall turn to these problems in the chapter on small particles.
~'
130
chapter 2
If high resolution results on X-ray absorption are available, with a resolution approximately ten times better than is necessary for structural EXAFS, one can analyse, aided by a large body of theory, the X-ray Absorption Near the Edge Structure, i.e. the socalled XANES region of the overall absorption (see figure 36). This type of analysis has been already applied to alloys and used for analysis of the apparent valence in intermetallic compounds [133]. The analysis of pure metals has also brought interesting information on the electronic structure of materials [134]. With many metals, the absorption easily accessible to measurement probes the unoccupied local partial density of states, i.e. that corresponding to a certain type of orbital. Therefore this region has attracted the attention of those who wanted to see if there was any change in the occupation of the d-band of transition metal alloys due to alloying. Attempts have been made to determine the d-band occupation in various supported metals and to trace a possible charge transfer between small metal particles and the acceptor/donor centres on the support, by comparing the intensity of the very edge absorptions with those of various known compounds [135-139]. An analysis using model catalysts with a known content of platinum ions revealed that: a) a simple approach [135,138] using only one edge profile is very crude and in fact inapplicable to catalysts; and b) when a better approach [136] was used, factors other than occupancy (e.g. particle size) determines the height of the edge, i.e. the intensity of the white line. In principle this method is only applicable when the absorption edge concerns transition to states near the Fermi energy and not into the free (i.e. vacuum) state [137]. Errors concerning this point can also be found in the literature. EXAFS and XANES as described above are bulk methods, but can be applied to surfaces, if they are made surface sensitive. Surface sensitivity can be achieved by using a different method of detection or by monitoring those species which are only on the surface [140-142]. In the first case one can use the secondary electron yield as a measure of absorption of X-rays; this proposition has been tested and found satisfactory. In the second, EXAFS analysis is performed on the absorption edges of atoms in the adsorbed layer (oxygen, carbon, sulfur etc.). The surface XANES is also called NEXAFS (near edge...) and for surface-sensitive EXAFS the acronym SEXAFS is used frequently. SEXAFS and NEXAFS have been used with success to determine structural features of chemisorbed layers; the use of light, polarised and oriented in desired way with respect to the surface, is of great help [ 143,144]. While XPS makes use of single monochromatic photons with an X-ray tube serving as a source, and the number and kinetic energy of photo-electrons is monitored, with X-ray absorption techniques the attenuation of the photon beam is monitored as a function of the variable photon energy. For the latter, a very intensive source (10 7 times the X-ray tube intensity) of X-ray photons of variable energy is necessary; this is at present only available in a synchrotron storage ring. In SEXAFS and NEXAFS modes of
Experimental techniques of solid state physics
131
measurement, the total yield of secondary electrons is monitored, rather than the attenuation of the primary beam. Earlier in this chapter the final state effects in core level XPS were discussed extensively. XANES often shows a much lower sensitivity to final state effects, and thus performing XANES and XPS on the same materials can be very useful when the effects of changes in the final and initial states have to be disentangled [132]. However, this is not an easy task experimentally.
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chapter 2 Verlag, Berlin, Vol.21 (1956) 176 W.P.Dyke, J.K.Trolan, Phys.Rev. 89 (1953) 799 M.E.Alferieff, C.B.Duke, J.Chem.Phys. 46 (1967) 923 A.J.Melmed, J.Appl.Phys. 36 (1965) 3585 E.W.Mtiller, "Chemistry and Physics of Solid Surfaces", (editors: R.Vanselow, S.Y.Tsong) C.R.C.Press (1977) p.1 E.W.Mtiller, Adv.Electr.& Electron Physics 13 (1960) 83 E.W.Mtiller, "Methods of Surface Analaysis" (editor: A.W.Czanderna) Elsevier, Amsterdam (1975) E.W.Mtiller, S.V.Krishnawamy, S.B.McLane, Surf.Sci. 23 (1970) 112 T.T.Tsong, Surf.Sci.Repts. 8 (1988) 127 J.M.Cowley, Progr.Surf.Sci. 21 (1986) 209 A.Howie, U.Valdre, "Surface and Interface Characterization by Electron Optical Methods", NATO ASI series, Plenum Press (1988) W.O.Saxton, D.J.Smith, Ultramicroscopy 18 (1985) 39 R.Sinclair, G.Thomas, J.Appl.Crystallogr. 8 (1975) 206 J.C.H.Spence in "Experimental high-resolution Electron Microscopy", Clarendon Press, Oxford (1981) G.Binnig, H.R6hrer, Helv.Acta 55 (1982) 726 G.Binnig, H.Rhrer, G.H.Gerber, E.Weibel, Phys.Rev.Lett. 49 (1982) 57; 50 (1983) 120 V.Kuk, P.J.Silverman, Rev.Sci.Instrum. 60 (1989) 165 G.E.Poirier, J.M.White, Rev.Sci.Instrum. 60 (1989) 3113 F.Besenbacher, F.Jensen, E.Laesgaard, K.Mortensen, I.Stensgaard, J.Vac.Sci.Technol.B 9 (1991) 874 N.Shimizu, T.Kimura, T.Nakamura, I.Umehn, J.Vac.Sci.Technol.A 8 (1990) 333 R.Wiesendanger, G.Tarrach, D.Btirgler, T.Jung, L.Eng, H.J.Guntherordt, Vaccim 41(1-3) (199) 386 D.R.Peale, B.H.Cooper, J.Vac.Sci.Technol.A 8 (1990) 345 M.Bott, T.Michely, G.Comsa, Surf.Sci. 272 (1992) 161 R.Schuster, J.V.Barth, G.Ertl, R.J.B6hm, Phys.Rev.B 44 (1991) 13689 F.Besenbacher, I.Stensgaard, L.Ruan, J.K.Norskov, K.W.Jacobson, Surf.Sci. 272 (1992) 334 S.Chiang, R.J.Wilson, C.M.Mate, H.Ohtani, Vacuum 41(1-3) (1990) 118 R.Koch, O.Haase, M.Borbonus, K.H.Rieder, Surf.Sci. 272 (1992) 17 B.A.Cowans, K.A.Jurman, W.N.Delgas, Yz.Li, R.Reifenberger, T.A.Koch, J.Catal. 125 (1990) 501 D.F.Ogletree, C.Ocal, M.Marchon, G.A.Somorjai, T.Beeke, W.Siekhaus, J.Vac.Sci.Technol.A 8 (1990) 297
Experimental techniques of solid state physics 118
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R.M.Tromp, R.J.Hamers, J.E.Demuth, Phys.Rev.B 34 (1986) 1388 H.Neddermeyer, St.Tosch, J.Microscopy 152 (1988) 149 R.M.Tromp, R.J.Hamers, J.E.Demuth, Science 234 (1986) 304 R.J.Hamers, R.M.Tromp, J.E.Demuth, Surf.Sci. 181 (1987) 346 R.M.Feenstra, J.A.Stroscio, J.Tersoff, A.P.Fein, Phys.Rev.Lett. 58 (1987) 1192 H.J.W.Zandvliet, H.B.Elswijk, E.J.van Loenen, Surf.Sci. 272 (1992) 264 E.J.van Loenen, J.E.Demuth, R.M.Tromp, R.J.Hamers, Phys.Rev.Lett. 58 (1987) 373 R.J.Wilson, S.Chiang, Phys.Rev.Lett. 58 (1987) 369 T.Yokotsuka, S.Kono, S.Suzuki, T.Sagawa, Surf.Sci. 127 (1983) 35 G.Kreysa, J.Gomez, A.Baro, A.J.Arvia, J.Electr.Anal.Chem. 265 (1989) 67 Z.Zhang, M.M.Lerner, V.J.Marty, P.R.Watson, Langmuir 8 (1992) 369 D.R.Denley, J.Vac.Sci.Technol.A 8 (1990) 603 D.M.Komiyama, J.Kobayashi, S.Murita, J.Vac.Sci.Technol.A 8 (1990) 608 G.Binnig, C.F.Quate, Ch.Gerber, Phys.Rev.Lett. 56 (1986) 930 M.D.Kirk, T.R.Albrecht, C.F.Quate, Rev.Sci.Instr. 59 (1989) 833 Y.Kuk, P.J.Silverman, J.Vac.Sci.Technol.A 8(1) (1990) 289 J.Chen, J.Vac.Sci.Technol.A 6 (1988) 319 H.Neddermeyer, S.Tosch, Festk6rperprobleme 29 (1989) 133 R.Tromp, "Chemistry & Physics of Solid Surfaces", (editors: R.Vanselow, R.Howe) Springer Verlag series on Surf.Sci. 7 (1988) 547 N.Garcia in "Surface and Interface Characterization by Electron Optical Methods", (editors: A.Howie, U.Valdre) Plenum Press, N.Y (1988) p.235 Y.Kuk, P.J.Silverman, Rev.Sci.Instr. 60 (1989) 165 E.Feuchtwang, P.H.Cutler, Physica Scripta 35 (1987) 132 C.Herring, M.H.Nichols, Revs.Modern Phys. 21 (1949) 185 R.V.Culver, F.C.Tompkins, Adv.Catal. 11 (1959) 67 R.Smoluchowski, Phys.Rev. 60 (1941) 661 W.M.H.Sachtler, G.Dorgelo, W.van der Knaap, J.Chim.Phys. 51 (1954) 491 R.Bouwman, W.M.H.Sachtler, J.Catal. 19 (1970) 127 D.P.Woodruff, T.A.Delchas, "Modem Techniques of Surface Sciences", Cambridge Press (1986) p.356 R.de L.Kronig, Z.Phys. 70 (1931) 317; 75 (1932)468 D.E.Sayers, E.A.Stern, F.W.Lytle, Phys.Rev.Lett. 27 (1971) 1204 D.E.Sayers, F.W.Lytle, E.A.Stern, Adv.X-ray Anab. 13 (1970) 248 T.M.Hayes, J.B.Boyce, Solid State Phys. 37 (1982) 173 E.A.Stern, Phys.Rev.B 10 (1974) 3027 P.A.Lee, G.Beni, Phys.Rev.B 15 (1977) 2862 P.A.Lee, B.J.Pendry, Phys.Rev.B 11 (1977) 2795
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"X-ray absorption" (editors: D.C.Koningsberger, R.Prins) Wiley & Sons, N.Y. (1988) (Chem.Anal.Series, Vol.92) "Determination of Structural Features of Crystalline and Amorphous Solids" (editors: B.E.Rossiter, J.F.Hamilton) Wiley & Sons, N.Y. (1990) B.K.Teo, Acc.Chem.res. 13 (1980) 412 C.Vlaic, J.C.J.Bart, Recl.Trav.Chim. Pays-Bas 101 (1982) 171 J.Wong, Mat.Sci.Engin. 80 (1986) 107 A.Bianconi, M.Campagna, S.Stizza, Phys.Rev.B 25 (1982) 2477 A.Bianconi, S.Modesti, M.Campagna, K.Fisher, S.Stizza, J.Phys.C 14 (1981) 4737 A.Balzarotti, M.DeCrescenzi, L.Incoccia, Phys.Rev.B 25 (1982) 6349 M.Benfatto, M.DeCrezcenzi, L.Incoccia, Solid State Commun. 46 (1983) 367 G.N.Greaves, P.J.Durham, G.Diakeim, P.Quinn, Nature 294 (1981) 139 L.Grunes, Phys.Rev.B 27 (1983) 2111 F.Szmulowicz, D.M.Peax, Phys.Rev.B 17 (1978) 3341 J.E.Mtiller, J.W.Wilkins, Phys.Rev.B 29 (1984) 4331 M.Brown, R.E.Peierls, E.A.Stern, Phys.Rev.B 15 (1977) 738 F.W.Lytle, J.Catal. 43 (1976) 376 P.H.Lewis, J.Catal. 69 (1981) 511 F.W.Lytle, P.S.P.Wei, R.B.Gregor, G.H.Via, J.H.Sinfelt, J.Chem.Phys. 70 (1979) 4849 A.N.Mansour, J.W.Cook, D.E.Sayers, J.Phys.Chem. 88 (1984) 2330 A.N.Mansour, J.W.Cook, D.E.Sayers, R.J.Emrich, J.R.Katzer, J.Catal. 89 (1984) 462 J.A.Horsley, J.Chem.Phys. 76 (1982) 1451 P.Gallezot, R.Weber, R.A.Dalla Betta, M.Boudart, Z.Naturforsch. 34(a) (1979) 40 M.J.P.Botman, A.J.den Hartog, V.Ponec, Stud.Surf.Sci.& Catal. 48 (1989) 179 J.St6hr, D.Denley, P.Perfetti, Phys.Rev.B 18 (1978) 4132 J.St6hr, C.Niguerra, T.Kendelewicz, Phys.Rev.B 30 (1984) 5571 A.Bianconi, Appl.Surf.Sci. 6 (1980) 392 J.St6hr, Z.Phys.B Cond.Matter 61 (1985) 439 D.P.Woodruff, Surf.Interface Anal. 11 (1988) 25 D.A.King, Chemistry in Britain, (1986) 819 A.L.Johnson, E.L.Muetterties, J.St6hr, F.Sette, J.Phys.Chem. 89 (1985) 407 R.J.Koestner, J.St6hr, J.L.Gland, J.A.Horsley, Chem.Phys.Lett. 105 (1984) 332 A.L.Johnson, E.L.Muetterties, J.St6hr, J.Am.Chem.Soc. 105 (1983) 7183 M.D.Crapper, D.P.Woodruff, J.Vac.Sci.Technol.A 5 (1987) 914 D.A.Oetka, J.St6hr, "Chemistry and Physics of Solid Surfaces" (editors: R.Vanselow, R.Howe) Springer Verlag series on Surf.Sci. 7 (188) 183 J.L.Gland, ibid, p.221
133
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136
137 138 139 140 141 142 143
144
Experimental techniques of solid state physics
145
141
L.D.Landau, A.I.Khiezer, E.M.Lifschitz, "General Physics, Mechanics", Pergamon Press, Oxford (1967)
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143
Chapter 3
THE E L E C T R O N I C STRUCTURE OF ALLOYS; E X P E R I M E N T A L RESULTS The metals that are most important as catalysts are those in Groups 8-11 of the Periodic Table, i.e. the iron triad, the group of platinum metals, and copper with silver in the 1 l th group. The metals of Groups 8-10 belong to the transition series and distinguish themselves from other metals by having a partially occupied (n-l) d-band, n being the principal quantum number of the valence sp electrons. Unpaired d-electrons of these elements are responsible for the paramagnetism or ferromagnetism observed. From the very beginnning of the modem theory of alloys [1], magnetic measurements were the first, and for a long time almost the only, source of information on the electronic structure of transition metal alloys. Dowden's famous papers [2] strengthened the interest of chemists working on catalysis in the presence and properties of the d-band holes, and magnetic measurements become very popular in the world of catalysis [3]. After the discovery of the M6ssbauer effect, measurements of the chemical shift in M6ssbauer spectra became another very important source of information, although a sophisticated theoretical background is required to understand these results in detail [4]. However, the most important part of the available information has been supplied by the photoelectron spectroscopies (UPS, XPS) and the analysis of the shape and the position of various emission bands in these spectra. Here also some problems in the interpretation of spectra persist and render some of the conclusions debatable, including those we make below.
3.1 Magnetic measurements With a system of N uniform particles, such as ions, domains of parallel-oriented magnetic moments, or individual unpaired electrons, each with a magnetic moment pp, the total magnetic moment M of all N particles would at temperature T be given by the Langevin equation: M]lVIsa t =
coth (~/kT)- (kT/ppH)
(1)
where Msa t is the total saturation magnetic moment of the volume unit, i.e. magnetisation, at very high magnetic field strength H and at low T. When M is very much less than the saturation moment
Msa t
(equal to N~), equation 2 holds:
144
chapter 3
M =
N
]Ll2pH/3kT
(2)
Plotting the measured moment against H T l supplies us with the value of N).12p. Now the value of Mp for an electron is well known, so it is possible to determine from the magnetic susceptibility which is the magnetic moment per unit volume, of the material studied, a most interesting quantity, i.e. the number N a of unpaired electrons per atom of a metal. Figure 1 shows the susceptibility results collected by Taniguchi et al. [5]; in the early literature on alloys (PdAu, PdAg, PdCu alloys) [6-8] the number of unpaired electrons per atom in palladium metal was estimated to be about 0.6.
600
Pd Y
-i o 121
400
E ~D
Pt
-9 2 0 0
T i
o
- - --"-
x
r
r
r
f
3 number
'
5 of
Rh
.. -,.
-0 s ' ~ ~ , 7
outer
electrons,
__, 9
lq
q
figure 1 Magnetic susceptibility of close-packed transition elements, at room temperature [ref 5]
Ferromagnetic materials can attain and keep the saturation magnetisation in a broad range of temperatures above the absolute zero (see figure 2) and the value Msa t is again used to determine the magnetic moment per atom and from this to calculate the number Na of d-holes per atom. Measurements made in this way and reported in the early literature included the following: Nd(Fe ) = 2.2, Na(Co) = 1.6 and Na(Ni) = 0.6 [8]. The very early literature ascribed the magnetic moment found as described above to the spin-moment; however, the experimental evidence that this was correct came much later. For example, Mook and Shull [9] determined for nickel by neutron scattering, a technique that supplies spin-density maps, that (a) the magnetic moments are indeed localized on individual atoms, (b) 80% of the total magnetic moment is due to electrons in tzg-orbitals, (c) apart from 0.65 M~ (P~ = Bohr magneton) coming from the 3d spin, there is only 0.055 PB related to the 3d-orbital, so that a strong quenching is obviously occurring, and (d) - 0.105 laB originates from negatively exchange-polarized 4s electrons. The results are shown in figure 3.
145
The electronic structure of alloys; experimental results
T < Tc
T > Tc
ferromagnetic
paramagnetic
M/Msat
X ( T ) -1
1 -
f
0.5
1
~,
T/T c
figure 2 Typical magnetisation curves, metals of iron group left: ferro-magnetic behaviour under the critical (Curie-)temperature Tc right: para-magnetic behaviour above Tc magnetic susceptibility # M-magnetisation Ms,,,- saturated magnetisation. Nickel nucleus
[100]---~
_
(3
2
o.oo85 "B .&-3
figure 3 The distribution of the magnetic moment (spin density) in the (100) plane of a nickel crystal, as revealed by the diffraction of polarized neutrons [9].
o o
-0 O 0 8 5 P B J ~ - 3
Cl
2
Nickel
nucleus
146
chapter 3
When an alloy is formed from a para- or ferromagnetic metal such as iron, cobalt, nickel, palladium or platinum, all metals with an uncomplete d-band, and a metal with one or more valence s electrons, a decrease of the atomic magnetic moment la is observed. For example, results have been reported for nickel [8] as shown schematically in figure 4.
0.5
figure 4 Experimental data on magnetic moments (per
t~ / atom
atom) of Ni in Ni-Cu and Ni-Zn alloys, in Bohr magnetons. \
\ \ \
I
I
'~
i
I
5O
!
I
1
%
of element added
Results such as these led to the formulation of the so-called Rigid Band Theory (R.B.T.) (see chapter 1) and almost all results published between about 1936 and 1968 were interpreted by this theory. The basic assumption was that a charge transfer occurs between the alloy components. In this model copper should supply one, and zinc two, selectrons , into the holes in the d-band of nickel, thus eliminating one or two unpaired electrons on the nickel and so decreasing the average magnetic moment per nickel atom. The same assumption was used to explain magnetic results on alloys of palladium with copper, silver and gold and also on the palladium-hydrogen system. However, as we shall see below this later appeared to be an incorrect explanation. According to the R.B.T., the nickel-copper alloys with more than 60% copper should have been diamagnetic; instead, a strong paramagnetism was observed. This was at first explained as being due to nickel clusters in the copper matrix [8], while others assumed that some ferromagnetic impurities were disturbing the results. It was only later, when strong paramagnetism in extremely pure diluted nickel in copper alloys had been definitely established [10], that many papers appeared which showed that nickel atoms indeed keep their unpaired electrons, and that no signs of a charge transfer from copper to nickel could be seen. The observation of giant paramagnetic clusters of nickel in copper concentrations less than 40% has led to a new theory explaining ferromagnetic behaviour,
The electronic structure of alloys; experimental results
147
such as that shown in figure 4, by an overlap and mutual interaction of giant moments, but not by any charge transfer from copper to nickel [ 11-16]. A new interpretation was also given later to the magnetic behaviour of the palladium alloys. While in pure palladium the broad 4d band is cut by the Fermi energy E F (see chapter 1, and this chapter below), and thus has unpaired (i.e. magnetizable) electrons, dilution of palladium with a Group 1B metal (Group 11 metal) leads to an narrowing of the 4d band, so that with alloys of silver and gold containing less than about 40% palladium it lies completely below E F, becoming fully occupied and causing the alloys to be diamagnetic. The Group 11 metal thus causes redistribution of 4d and 5s electrons, although this does not amount to a charge transfer from the Group 11 metal to palladium. The results for the Pd-H system were explained similarly [17]; dissolved hydrogen diminishes the overlap between metal atoms and the 4d band is in consequence narrowed and completely under the Fermi energy [17]. X-ray diffraction results are a possible source of information on the spatial distribution of d-electron density in metals and alloys; similarly neutron diffraction reveals spin density contours. However, neither of these techniques indicates a marked difference in the density monitored due to alloying [18]. Theoretical investigation [19] has shown the distribution of spin densities in a monolayer of iron atoms to be uninfluenced by placing it on a noble metal, but in a relaxed state on tungsten it was indeed influenced. The atomic moment was lowered from 2.55 laB, for the unrelaxed layer to 2.18 lab in the relaxed layer. This change in spin density is due to the formation of a chemical bond between tungsten and iron atoms [19]. Experimental results obtained with such layers by spin- and angleresolved photoemission and M6ssbauer spectroscopy confirm this conclusion [20]. For a number of alloys of interest as catalysts such as palladium-copper and -silver, platinum-silver and platinum-gold, parallel results exist on (i) magnetic susceptibility, (ii) Knight shift in NMR of platinum and iii) Knight shift of silver. Further, theoretical calculations are available on the electronic structure or these alloys [21]. The totality of this information confirms consistently the picture developed above: a dilution of palladium or platinum in the matrix of silver or gold causes a narrowing of the d-band, so that below about 40% of the Group 10 metal it's whole d-band falls below the Fermi energy, becoming fully occupied, the magnetic susceptibility changes from paramagnetic to diamagnetic. This does not, however mean that it is only the electrons of silver or gold which fill the holes in the platinum or palladium d-shells. Summarizing, we can conclude the following. Magnetic measurements on alloys reveal various kinds of interaction between the components: a very weak one (in the sense of consequences for the atomic magnetic moments), as in nickel-copper, or stronger ones, as in palladium or platinum alloys mentioned above. In the latter case the observed change in the average atomic moment is caused either by spatial and orbital redistribution of electrons or by formation of new hybridized orbitals, which also leads to a spatial
148
chapter 3
redistribution of electrons. However, this is not a charge transfer in the usual sense, since we also do not speak of a charge transfer upon formation of an hydrogen molecule from two hydrogen atoms.
3.2 The M6ssbauer effect In many cases this is just another probe into the electronic structure of alloys, and the broad spectrum of results already available offers very valuable information [4,22]. Without attempting to make a complete survey and full analysis of published work, we shall concentrate our attention on a few of the most general papers. We present only a simple introduction to the principles of M6ssbauer spectroscopy (see also chapter 7). M6ssbauer spectra are observed upon recoilless emission and resonant absorption related to the decay of excited nuclear states. A M6ssbauer spectrometer consists typically of a source containing an element undergoing radioactive decay, an absorber with the same element but in a compound (alloy) of interest, and the detector. The energy of emitted quanta is fine-tuned by moving the source, which causes a Doppler effect. If the difference between two energy levels of a nucleus is E o, the energy of the emitted y radiation Ev equals (E o - ER), where ER is the energy spent by recoil. Since the atoms of the same element to be excited in the absorber need Eo for a resonant excitation, none can occur with emitted quanta of energy Ev (Ev < Eo). However, under suitable conditions of vibrational excitation of the lattice (ER < hv, where hv is the phonon energy), a fraction of emission and absorption events takes place without exchange of any recoil energy ER. This recoilless fraction gives rise to Mtissbauer spectrum. Due to the so-called hyperfine interactions, one observes not only differences in the nuclear energy levels but also the very small but easily measurable influence of the electromagnetic field round the nuclei. The field strength varies due to (i) isomer (chemical) shift phenomena, (ii) electric quadrupole splitting and (iii) magnetic hyperfine splitting. The isomer shift AS reflects the difference in the electron density at the nucleus of an atom in the absorber and in the source. Only electrons for which the wave functions have an amplitude on the nucleus, mainly s-electrons, are felt by the nucleus, and manifest themselves by an isomer shift in the M6ssbauer spectra. It thus reflects the difference in different compounds or alloys in the electron density Apo of s-electrons. A nucleus can possess an electric quadrupole in one of its states, and this can interact with the electric field around the nucleus. Two possible orientations of the dipole with respect to the field are possible and this causes a splitting in the energy levels of the nucleus. T h e surrounding of the nucleus, viz. the outer electrons, can also be a source of a magnetic field which in its turn causes a Zeeman splitting of the nuclear energy levels. Thus, for elements suited for observing the M6ssbauer effect very valuable information
The electronic structure of alloys; experimental results
149
can be obtained from the spectra. It goes much beyond the purpose of this book to explain further how M6ssbauer spectra may be used to study alloy catalysts. For this detailed information the reader is referred to the literature [22]. Alloying of many metals leads to an isomer shift AS. The first impression created by the results was that a relation existed between the isomer shifts and the electronegativities W of the host and impurity atom: AS = K(~IJhost-
Such relation is apparently
kI/imp).
(!) supported by results such as those shown in figure 5. Ao
12.5 (volume
Q.I Ul
E E I--U_
9
10.0 -
o < o., b,-
corrected 3d
7.5-
HOST S
o 5d
x~.
)
HOSTS
9 Ld
T t~ or" m
x
HOSTS
5.02.50.0
I
2
I
1
L
I
1
6
HOST
I
8
1
~o
I
I0
NUMBER
figure 5 Isomer shifts of 197Alg as a dilute impurity in various host-elements. The shifts are corrected for volume changes due to alloying [23].
However, when the results were fully analyzed, doubts appeared about such a simple conclusion and about charge transfer as the reason for changes in Apo. It sometimes looked as if one s electron was being transferred, where one would consider 0.1 electron a more likely figure [23]. The picture was therefore completed by making the reasonable assumption that the charge transfer due to s electrons is compensated by a charge transfer of d electrons in the opposite direction. When theoreticians helped to analyze the relation between isomer shifts and electronic structure, new important conclusions were formulated [24]. The calculations revealed that in some cases the charge transfer found is inconsistent with simple electronegativity arguments and with models which assume hybridization of obitals forming the d-band. Further, the theoretical analysis pointed to another feature of an apparent charge transfer, which was called "trivially obvious" [24], but surprisingly had been ignored earlier. If an impurity is placed in a matrix formed by atoms of which the
150
chapter 3
valence orbitals are of a very different 1-components, (say, in other words, orbitals extending far from the neighbouring site on the impurity atom site), the tailing of the orbitals brings a charge on the site probed, which charge, however, has nothing to do with a charge transfer in the usual sense or with hybridization of orbitals, i.e. formation of delocalized chemical bonds, or with screening effects. Yet this is the most important component of the electron density changes which in their turn can be monitored by the isomer shift in M0ssbauer spectroscopy [24].
3.3 Photoemission Spectroscopy (PES) The most important information on the electronic structure of metals and on the changes in it due to alloying has been already obtained (see chapter 2) by various types of PES, which relate respectively to the valence bands and the core levels. The source of the information is the position of emission bands in valence band spectra and binding energies BE's for core levels; the shape, width and the spin-orbital splitting of emission bands; and the satellite structure and spin-orbit splitting of the core level emissions. We shall begin with a discussion on the BE or band position shifts. With free gas molecules the problem of BE determination is easy. All molecules being compared have the same zero level
Evac(oo),
correponding to an electron at rest, very
far from the molecule which has been ionized. However, the problem of how to compare BE's or band positions becomes more difficult when solids such as metals are involved. Figure 6 shows what happens when two metals A and B with two different work functions 9 are (a) separated or (b) in contact. Being in contact, the metals build up a contact potential difference (CPD), which arises from the fact that electrons flow to the extent of about 1 or 2 electrons per 100 atoms of the surface involved, from the metal of smaller work function to that with the higher one. The first metal becomes positively and the second negatively charged, and OcpD arises which is equal to (OB-OA). For our purpose A is the sample under study, and B is the spectrometer. The kinetic energy of an electron in the spectrometer E K~N equals hv-O B, irrespective the value of 9 A or the presence or absence of the CPD. As long as one can identify the common E F exactly and use it as a zero reference level, there are no problems. However, if one wants to know the binding energy with respect to the vacuum level, the problems are serious. Experimentally, the work function is easily and accurately measurable; however the position of the EF with respect to
E vac
is not. The difference E vac - E F is certainly formed from 9 A plus a portion
of the CPD which is unknown, because it is unclear how much CPD there is, when the metals are permanently connected and whether there is an electron current through the gas phase. The OcpD is usually ignored, when shifts in the BE's are discussed, but it must not be forgotten.
The electronic structure of alloys; experimental results
Metals
disconnected
151
9
V//A upon c o n t a c t e Metals
connected -I
9
I
E Rin ( m e a s u r e d at s p e c t r o m e t e r )
hv ,
Evac
.
figure 6 Scheme for definition of Binding Energies, related to different zero-levels (EF, Evac).
E F
(BE)j
EVQC I
I ( B E l j EF
i
EKin tb
work
= hv-
| j - level ( B E ) EF - t B
function
expressed
as
energy
per
electron
Taking EF as the reference level has two other consequences. (i) Changing of ~A by adsorption (A(I)ads) does not exert any influence on the position of the aligned EF, observed by the spectrometer, although the A~ad s changes the position of the EF with respect to E vac, of the metal taken alone, ii) The change by adsorption A(/)ad s does not produce a shift in the core level binding energy of bulk atoms with respect to EF of the metal. Surface atoms can however exhibit a different BE due to A~ads, as a consequence of electrostatic effects and different coordination (see chapter 2). The matter of the reference level is sometimes complicated by the fact that spectroscopists and chemists automatically relate EF to the vacuum level, while theoreticians relate the highest occupied metal level E F to the bottom of the valence band. From this Babylonian use of words, problems arise leading to statements such as "adsorption does not change the E F postion" and "adsorption does change the EF position" made about the same system. Both statements can be true, but the first holds only when E F is taken from the bottom of the valence band, and the second when EF is measured from E vac (see e.g. discussion in [25]). Moreover, there is no complete uniformity in the literature on the signs of individual terms in the various equations in which BE and 9 appear. Moreover, the work function is sometimes taken as a potential energy, sometimes as potential, the energy then being e~. However, being warned and knowing the existence of these problems, the reader will easily recognize which of the possible alternatives is being applied in any particular case. Finally, a serious problem of the reference level is also inherent in the BE's of
152
chapter 3
alloys, where (I)alloy is not equal to either O a or OB, and is then particularly difficult when components of the alloy form clusters. A very good discussion on those problems can be found in the short monograph by Egelhoff [26]. Being aware of potential difficulties we can now discuss the main results obtained on alloys by PES. The most important milestone in developing of the application of PES to alloys is the work of Seib and Spicer [27]. They recognized the important potential that UPS has for studying the electronic structure of alloys, and suggested the way in which UPS could be used to provide a check of the validity of different theories: this is shown in figure 7 [28]. At the time this was published the only theory which correctly predicted the shape of the valence band spectra was the theory of virtual bound states. Later, predictions of qualitatively the same character were also made by more sophisticated theories, for example, the Coherent Potential Approximation (see chapter 1).
25 % Ni
pure
in
Cu
mE F
--E F
2eV
2eV
Cu
rigid-bond
model
virtual -bound - state model
figure 7 Prediction by Seib and Spicer of the photoemission patterns of Cu and Ni-Cu alloys, according to two, at that time (1970) available theories. Also other more sophisticated theories lead to the same appearance of spectra as the virtual-band models [28] (see also chapter 2).
Systematic studies made by many workers [28-30] revealed very similar features in the valence band spectra of other alloys. Their main characteristics can be seen in the figures which follow. Figure 8 shows the UPS-valence band spectra of nickel-copper alloys of the indicated compositions [28], and figure 9 shows the XPS valence band spectra of palladium-silver alloys [29]. Figure 10 [31] shows the evaluation of the two most important parameters of the spectra: the d-band centroid position and width. Similar results for nickel-copper are shown in figure 11 [29]. Conclusions from all these results are fairly straightforward. Diluting a metal A in another metal B makes the behaviour of
The electronic structure of alloys; experimental results
153
A more atomic. Overlap integrals dictating the band width are smaller and diluting can change the electronic configuration, as happens with palladium where the metal-like configuration 4d 9,6 4s0,4 becomes atomic-like (4d 1~ 4s~
but there are no signs of a
pronounced or even a barely measurable charge transfer between the components.
c-
~-
w~.~77 % Cu - 2 3 %
,'/'/ / x _ / ~
dD .._..
figure 8 Experimental photoemission results, by which Seib and Spicer [28] demonstrated the invalidity by the Rigid Band (charge transfer) Theory (compare with fig. 7). Due to experimental limitation of the equipment at the time of publication (1970) the low energy side of the spectra is not reliable.
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figure 9 Valence-band spectra of Ag-Pd alloys obtained with monochromatiled X-rays. The inelastic component as well as the background has been substracted. (a) Silver-rich alloys and (b) palladium-rich alloys; the alloy compositions refer to atomic fractions [29].
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154
chapter 3
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I
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Ni
figure 11 Photoemission parameters for a series of Pd-Ag alloys [29]. Ni-Cu alloys, centroid position of the particular d-bands [29].
An interesting comparison is offered by several papers dealing with silver and gold alloys [31]. They compare UPS band positions for silver alloyed with cadmium, indium, palladium or tin and the most essential information from these studies is shown in figure 12. We observe that the silver 4d emission band shows maxima at BE's of about 4 and 6eV. On dilution in other elements, each band splits in two. The essential feature is that all points lie on common curves, irrespective of which the other alloy component is. The width of the silver 4d band decreases on alloying, but again in the same way for all the alloys studied. Obviously, the bands shift due to varying d-d electron interactions among the silver atoms in the alloys, and not because of charge transfer between the components [32]. The same conclusion can be drawn about alloys of gold with cadmium, indium, gallium and palladium, bands of which overlap with those of gold; one would therefore expect some mutual perturbation. However, the interaction between the alike atoms dominates the spectral behaviour, a phenomenon actually predicted earlier by Friedel [32].
The electronic structure of alloys; experimental results
), (9
155
figure 12
..._..
tn
].experimentel
c~ (9 rl
UPS
error
binding
energies
of the
peaks in the A g 4d valence band spectra
"D
as
a function
of Ag
concentration for the alloy sys-
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tems: 9 - AgCd; 0 - Agln; (9 t_
(9 f(9
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f,
100
'
!
80
Atomic
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A _ AgPd and x - AgSn. .....
The full curves show variation of the Ag 4d peak nearer to 4 J
e V while the broken curves show
9
variation I
60
'
percent
I
z.0
i
I
20
silver
i
0
of the Ag
4d peak
nearer to 6 e V. The arrows at 0% Ag indicate the Ag free ion 4d splitting of 0.6 eV [311.
Alloys such as nickel-copper, formation of which is endothermic, and palladiumsilver, formation of which is weakly exothermic, are examples of systems in which the components do not tend to form stronger bonds between different atoms than between alike atoms. However, some combination of metals (Pt-Sn, Pt-Ti, Pt-Zr, etc.) do so and one then speaks of intermetallic compounds. This is indeed a name which fits very well the observed phenomena, such as changes in the valence band spectra (see below); electric conductivity, where some of the intermetallic compounds are semiconductors rather than metals; and mechanical properties, where alloying leads to a loss of ductility. It does not matter too much whether the compound is crystalline or amorphous, e.g. a metallic glass; in both cases short range effects prevail [33-39]. The paper by Fuggle et al. [40] is particularly illustrative and it reviews in an excellent way results on sixty different nickel and palladium alloys and intermetallics, documenting the difference between these two systems. We shall analyze the main points of this work below. First, let us look at some of the ideas needed to form the physical picture of the alloys discussed. The right-hand part of figure 13 shows charge density contour maps obtained theoretically [33] for superimposed non-interacting nickel and silicon atoms and the lefthand part the contours for NiSi 2, an intermetallic compound. The most important consequence of compound formation is the increase of electron density along the nickel-silicon line, reminiscent of the formation of covalent bonds in free molecules. :i'his is an important point to keep in mind. Figure 14 shows how the formation of an intermetallic compound is reflected in the valence band spectra of Ni3Ti obtained by UPS [34].
156
chapter 3
5
2 ~
00(!
0.~
0
_s 121
2 U c
s
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)i
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0
0
.
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~
.
.
i
.
6
along [110] (a.u.)
figure 13 Left: self-consistent crystalline charge density for NiSi 2 plotted in a (110) plane. The contours are given in units of electrons per cubic Bohr. Contours are logarithmically spaced, five to the decade (0.1, 0.16, 0.25, 0.40, 0.63, 1.0). Right: superimposed neutral atomic charge densities for NiSi 2. Caption as in fig.4 [33].
t
Ni
figure 14 Left: comparison of the UPS energy distribution at hv = 21.2
I !
-4
!
-2
E-E F (eV)
!
EF
-4
k.
I
-2
E-E F (eV)
EF
for Ni with the SCF-x ~-SW calculation for Ni4. The height of the bars are proportional to the number of occupied electrons. Levels within 0.2 e V are combined. Right: comparison of the UPS energy distribution of hv = 21.2 eV for Ni3Ti with the SCF-% e~SW calculation [34].
The electronic structure of alloys; experimental results
157
The energy spectra of a Ni 4 and a Ni3Ti cluster were calculated by combining levels separated by less than 0.2eV into one, shown by the bar, and putting the height of the bar proportional to the number of electrons in these orbitals [34]. These discrete spectra were then compared with the valence band spectra of nickel and of Ni3Ti. We can see a far-reaching similarity and we can gain even more detailed information from the tables in the paper. The spectra show the following typical features observable with intermetallic compounds of transition metal elements. i) The levels near the maxima correspond to orbitals which have in all cases a very high contribution from the nickel d-orbitals. ii)
The d-band is narrower in alloys.
iii) iv)
The whole d-band is shifted to higher BE's. Because the d-band in the intermetallic compound is further below the E F and has
fewer holes, the average atomic magnetic moment is lower. With some other intermetallic compounds, effects (iii) and (iv) are even more pronounced than here. The decrease in BE is achieved by admixing orbitals of s-character with the dorbitals, although the band is still formed by levels corresponding to orbitals of the highest occupied levels of the metals. A reorganization of the charge distribution occurs in consequence, so that the d-band, which is now below the EF, becomes more occupied by electrons from the s-band. Detailed and extended studies performed with zirconium intermetallics have revealed that the decrease in BE of the d-band is most probably correlated with the enthalpy of formation of the intermetallic compounds [40,41]. This again points to the formation of strong, less delocalized chemical bonds between the unlike elements; loss of ductility is observed. Electrons of the transition metal become less reactive by these changes and some intermetallic compounds resemble noble metals, e.g. in chemisorption and overall low reactivity. Figure 15 [40] summarizes a broad comparison of UP spectra of various alloys and intermetallic compounds. Changes in the valence band spectra due to the formation of intermetallic compounds recall the changes observed upon formation of chemical compounds. This suggests the direction in which the chemisorption and catalytic properties of the intermetallics may differ from those of pure metals. Angle-resolved valence band spectra (ARPES) are already available, for example, for some copper alloys [42,43]. These may be compared with theoretical predictions and the comparison offers a deeper insight in the properties of these alloys. As far as the strength of interaction between unlike atoms is concerned, palladium-copper alloys lie between palladium-silver and nickel-copper on one hand and palladium- or nickel-zirconium intermetallics on the other. We can observe changes in the spectra due to alloying, and these bear witness to the influence of palladium on the band which can be ascribed to copper. However, in these spectra the palladium band is split, lying on the flanks of the copper band; the position of these flanking bands is a function of palladium concentration, indicating d-d electron interaction, and therefore bonding between palladium atoms.
158
chapter 3
~
nsities of .S_t.gwtesin Ni Alia 1
of States in Ni Alloys]
Densities
I I
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figure 15 Photoemission patterns of alloys of different types [40]. Left: schematic diagram of Ni d-state and total state densities in alloys with elements of similar electronegativity, such as CrNi 2. A similar diagram could be applied to Pd or Pt alloys, with low heats of formation. Right: schematic model of the Ni d-state density in Ni and Ni alloys such as ScNi and other intermetallics with a high heat of formation. A similar model could be applied to Pd and Pd alloys. For further discussion see text.
Palladium-copper alloys are also interesting from another aspect. On alloying, the BE of 3d palladium core levels increases by 0.5eV and that of copper 2p levels decreases by 0.1eV [44]. If the BE changes due to alloying were caused solely by a charge transfer, one would expect them to be of the opposite sign and to be more nearly equal. However, such a discrepancy is found frequently and not only with the palladium-copper system, as we shall below. An early but in several respects a pioneering paper [45] considered core level and valence band spectra for the palladium-antimony, platinum-bismuth and gold-tin combinations. The so-called electron configurational changes, indicated by narrowing of the d-band and decrease in its BE, should also be used as the main basis for explaining the core level shifts, rather than speculating about charge transfer between the components. Several theories of metals and alloys, such as those due to Matt and Jones, HumeRothery, Miedema, and Engel and Brewers, and the rigid band theory, assume a charge transfer between the components. This idea is very popular amongst chemists in general. When valence band spectra [27,28] did not bring clear evidence for charge transfer, the
The electronic structure of alloys; experimental results
159
attention of chemists was refocussed on core level spectroscopy, and in particular on the easily observable BE shifts caused by alloying. The above mentioned theories , all based on the idea of a charge transfer, correctly predicted some thermochemical quantities and this was in its turn often seen as evidence of charge transfer. In spite of this, with the platinum-hafnium system, for example, Miedema's theory predicted a charge transfer from hafnium to platinum, while the Engel-Brewer theory (see chapter 1) predicted it in the opposite direction (see chapter 1). With many intermetallics, the BE's of both components increase, and in other cases the changes are in opposite directions from those which electronegativities would predict. Nevertheless the belief that core level BE shifts constitute direct evidence of charge transfer between components of an alloy is in some people unshakable, and, when bulk electronegativities do not predict the experimental shift correctly, the surface electonegativities are postulated to be different (see below). The core level BE shifts in figs.16 and 17, coded as AEc are very well predicted by the theory by Johansson and Martensson [46] (see chapter 2). The application of this theory requires a knowledge of the heat of formation of the alloys in question, and of alloys of those metals which are effectively formed according to the equivalent core model (see chapter 2, PES). Where these values are not known, they are usually calculated by Miedema's theory [47], which is known to make good predictions. An example showing the degree of agreement between the theory [46,48] and various experimental results [49] is shown in figures 16 and 17 [48].
AEC I
eV
1
C w
~
~" ~ +~,~*
~ §
core-
level shifts for
com-
metallic
pounds and solid solutions [48].
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figure 16 Calculated vs measured
~0 + v J 4 0
I
§ --
1 I
i
eV
9 compounds .
solid
solutions
160
chapter 3
] "r Ti t
i
Cu 0 ~
r
.Pt
,Cu
-I
x-
tt
1
figure 17 Core-level shifts AE as a function of the alloy composition. Various literature data (points) are compared with calculations according to the JohanssonMartensson theory [48].
>
Cu 0
Pd
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<~ -I
I
I
0
0.5
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Systems which are in many respects similar to alloys are metal-on-metal layers. Theoretical band structure calculations and ARPES results already exist for several interesting combinations. For example, silver monolayers on the (111) and (001) planes of nickel and copper and on (111) planes of gold and silicon have been studied [49]. In all six systems, the silver overlayer structure is very close to the hexagonal close packed Ag(111) structure. The band dispersions, E(k) (see chapter 1), are to a first-order approximation determined by interactions within the silver monolayer itself, and are surprisingly independent of the electronegativity of the substrate. The dispersions are mainly shifted - as a whole - by a certain AE, due to an electrostatic potential jump at the surface, having different values for different substrates. There are therefore no signs of an electron transfer in or out of any substrate.
particular orbitals of the silver layer, due to the contact with the
The palladium-silver system is of great interest, theoretically, spectroscopically and catalytically. Therefore, it is useful to inspect the experimental results in more detail. Figure 18 [50] compares the palladium 4d band as seen in the spectrum obtained with a fractional (0.1) monolayer of palladium on gold(100) with that observed in very dilute
The electronic structure of alloys; experimental results
161
bulk alloys of palladium with silver. The partial monolayer shows many more features of bulk palladium than do the dilute palladium atoms which recall of the behaviour of free atoms (viz., band narrowing).
(a) figure 18 The Pd 4d-photoemission feature for (a) a 0.1 monolayer palladium coverage on
>,, ..i.-, .m C" .4-, r"
s
silver (100) and (b) for palladium dissolved into the silver matrix. The back-
J
ground has been subtracted in each case. The full curves are experimental. I
The broken curves are the best fits to the measured spectra using two lorent-
!
(b)
zians of FWHM Asa = 0.3 e V (a) and 0.5 eV (b) to represent the hybridized, spin-orbit-split Pd 4ds/2 and 4d3/2 levels [501.
I
!
-3
-2 Energy
of
initial
I
-1 s t a t e (eV)
Comparison of observed and calculated ARPE spectra of a copper film on silver(100) gave good agreement when zero charge transfer was assumed [50]. Also other details of the behaviour of the system have been published [51 ]. Whether or not a submonolayer of a metal shows a BE in the valence-band spectra, different from that in a dilute solution, depends very much on the atom-atom (d-d electron) interactions. The change in Ni 2p3/2 BE which occurs with a submonolayer of nickel on gold is almost identical to that observed in dilute nickel in gold alloys, but the
4f7/2 BE in a submonolayer of gold on nickel is not the same as that for gold dissolved in nickel. This indicates that the electron configuration as defined by the occupation of the
Au
various orbitals depends on coordination and the sensitivity of the given atom to it. It is very interesting to see that the BE shifts show same dependence on growing coverage of nickel on gold as for gold on nickel (see figure 19). This emphasises that shifts are mainly governed by mean coordination number and by density of atoms in the monolayer. Let us suppose for a moment that there is a charge transfer between nickel and gold; according to the RBT this would be expected to be from gold to nickel, while
162
chapter 3
according to the work functions it should be in the opposite direction. The first difficulty would be that the experimental shifts in BE are to lower values for both components. The second difficulty would be the similarity between the curves in fig.19. One would expect a continuous decrease for nickel and a mirror image increase for gold, but instead we observe a single universal curve. Further speculation based on an assumed charge transfer is obviously pointless.
XPS Core
Line Shifts
Ni / A u bO
~_ _J
a E i-O
z
Surface
Layers
1.2-
0.8-
I ~ /
o
(~
and
in Au / N i
I
Au
Ni
I -
0.0I lllll|
i
I'1
I III1|
0.0 1 Monolayer
I
0.1
I
I
Illli
I
1
I
I
I
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19
Coverage
figure 19 Normalized core-level binding energy shifts for Au on Ni and Ni on Au. The full line is the normalized change of the mean Au-Au and Ni-Ni nearest neighbour coordination number as function of the overlayer coverage. The coordination rather than a charge transfer dominates the behaviour of the layers [52].
Goodman and his coworkers have performed a very extensive and systematic study on the spectra and chemisorptive properties of various metal-on-metal layers; it forms a very solid basis for further discussion [53,54]. The first clear-cut conclusion is that a monolayer of one metal on another has different properties in chemisorption than thicker layers, the shifts in BE indicating that the layers also have different electronic properties in either their initial or final states, or in both. Figure 20 shows the results for palladium and for copper monolayers on a number of metals; similar results for nickel are also available.
The electronic structure of alloys; experimental results
163
&BE (eV)
figure 20 Tdes o
Photoemission parameter- the Binding Energy shift ABE- and the temperature of desorption
K
0.6
-1500
of Pd (above) and Cu (below) full layers on
O.4 - 1460
indicated metals [53].
0.2 -1420 Ta
W
Re
Ru
eV)
&BE _
Tdes o
K
-1260
0.20
-1200
-0.2 -11&0 To
Mo
Re
Ru
Rh
One has however to keep in mind the following points. First, the extra-atomic screening causing a decrease in BE is stronger, the higher the density of levels at the Fermi surface for the substrate. In figure 20, this takes place on going from left to right. Second, when a monolayer of a metal is put in an average electrostatic potential energy jump on the metal surface, the electrostatic potential will further influence the values of BE's and this jump is likely to be different for copper and palladium. These are probably the two dominant effects, since the ARPES showed that the effect of the structure mis-match is of smaller influence [49-52]. These two effects taken together would also explain the results in figure 20. However, an explanation based on an assumed charge transfer is preferred [53]. The authors say: "For supported monolayers of Pd (electron-rich admetal), the magnitude of the perturbations induced by the loss of electron density increases as the fraction of empty levels in the valence band of the metal substrate increases: Ru< Re < W < Ta. Cu has a 4s valence band that is half empty. Therefore, supported Cu can act as an electron donor or electron acceptor depending on the relative fraction of empty states in the valence band of the metal substrate. For Re, the 5d valence band is also half empty, and as a consequence only a minor perturbation is observed for the CUl,o/Re(O001) system. Adsorption of Cu on metals to the left of Re (substrates with valence band more than half empty) produces a reduction in the electron density of the adatoms. On the other hand, when Cu is deposited on metals in the right side of the periodic table (elements with valence bands more than half occupied), electrons flow from the substrate into the 4s band of the admetal ".
164
chapter 3
The conclusion is drawn [53,54] that the chemisorption of carbon monoxide and its temperature-programmed desorption (TPD) confirms the picture of an electron transfer: with carbon monoxide adsorption, the electron shift into the 2r~ orbitals is crucial and this shift is easier, the further to the right the metal lies: the further to the right, the higher are the Tmax values of the TPD spectra. The picture developed in [53] and represented by the last two paragraphs is certainly nicely self-consistent and has a certain attraction. Nevertheless, it is necessary to stress the danger of relying blindly on a picture based on initial state effects only, and including moreover the concept of charge transfer. It seems suspicious that this picture [53,54], has to assume frequently an electron transfer in unexpected directions: ruthenium---)copper, palladium---)copper, palladium---~tantalum, palladium---~ruthenium, etc. The claim that surface electronegativities are very different from bulk ones, and therefore allow such surprising charge transfers, such as palladium to copper, to occur has been so far not confirmed by any other independent experiment. Layers of alkali metals and alkaline earths on transition metals are very important in hot-cathode devices, and layers of transition metals on refractory or on non-transition metals are potentially very interesting for nanotechnology in the electronic industries. These are sufficient reasons to justify their having been extensively studied by theoretical methods [55-57]. A discussion on the chemisorptive, catalytic and spectroscopic properties of these layers is now very much helped by the availability of electron-density contour maps, which these theories [55-57] have produced. Without attempting to achieve a complete presentation of the available results, we shall just discuss two extreme examples. The first example [55] concerns adsorption of sodium on aluminum, and contour maps for this system are shown in figure 21. At all degrees of coverage by sodium, the charge on the sodium atom is spatially redistributed: its vacuum side is deprived of electrons, which concentrate in the space between it and the outer aluminum atoms. The changes in the electron density around atoms deeper within the aluminum are small. Further, the charge density changes become more continuous and more delocalized (i.e. more metal-like) when the sodium coverage approaches unity. This picture explains the usual drop in the work function ~ observed upon adsorption of alkali metals on metals. With this picture in mind it is not surprising that 9 tends to approach the value for pure bulk sodium as its coverage of the surface approaches unity. The value of 9 for several transition metals is higher than the first ionisation potential of sodium (5.13V) and this has led to the idea that adsorption of sodium therefore has a fully ionic character (Na§ Figure 21 shows a shift of electrons in the correct direction and A~ as predicted is also what one would expect; it is then very much a question of definition, whether to call sodium at low coverages an Na § cation, or to visualize the situation as a strongly polarized Na(+)-AI(-) bond. Some workers, all with good reasons, prefer the former [58], others the latter [55].
The electronic structure of alloys; experimental results
20
,.--.,
5 O
---
f
a)
165
(b) .,s
.-'_-2"-,
'~
1C
N
\
-6
0
6
-6
0
6
-6
0
6
X (a.u.)
figure 21 Contour maps of the difference charge 8p(r) in the vertical-cut plane containing the Na and nearest two A1 atoms. (a) |
= 1/2, (b) 6) = 88 and (c) 6) = 1/8. The Na and AI atoms
are shown by filled circles. The shaded and hatched areas indicate the regions where 6p(r) = .001 a.u. and 6p(r) = 0.0005 a.u. respectively. The dot-dashed lines correspond to 6p(r) = 0 [551.
Alkali metal adlayers on transition metals represent an extreme in the polarity to be expected for the metal layer-metal bond. Much more subtle shifts in electron density arise when, for example, iron is deposited on tungsten. This case has also been analyzed theoretically and the results [56] are shown in figure 22. The work functions and ionisation potentials of the two metals do not differ greatly and there is thus no reason to expect a charge transfer in either direction. When a layer of silver is placed on the iron layer, atoms in the latter assume characteristics of the
bulk, the electron density around them
being spherically symmetrical. The first layer of tungsten atoms show the same electron contours, irrespective of the presence of the silver outermost layer. There is therefore, little evidence for an iron-to-tungsten charge transfer, and the nature of the surface layer has obviously no influence on underlying layers. Further evidence against the charge transfer concept comes from measurements of the Yb/Mo(110) system [59], where the BE shift for Yb is +0.88eV, compared to +l.7eV for Yb outermost layer on Yb metal. Although there are contributions to the observed shift from both initial and final states, the effects of the latter predominate.
166
chapter 3
I
W (I-I) w(c)
W(I-1) ~"
, (o)
w(c) (b)
figure 22 Valence charge density contour map for a) Fe/W(110) and b) Ag on Fe/W(110) layer. Maps are on the (001) plane, perpendicular to the surface; contours in units of lO%/(at unit) 3. Each contour line differs by a factor of ~12. The main features to be noted: charges of the Fe and W(I) layer do not interpenetrate, a top layer of Ag makes the Fe surface layer more bulk. Domination of coordination above the charge transfer effects is clearly visible.
Anyone who very strongly believes in charge transfer between alloy components, or between metal and metallic ad-layers, will be not convinced by the arguments presented above. One of the objections would be to say that electronegativities in the adlayers are very different from those in the bulk solid [53,54] and in the absence of any information on surface layer electronegativities, this seems to be a safe statement. However, most readers would agree that the case of potassium on platinum is quite straightforward. It would need unlimited courage to suggest a charge transfer in the sense of platinum to potassium due to different surface and bulk electronegativities. Yet the Pt 4f BE shifts to a higher BE [60], so that it obviously cannot be dominated by a charge transfer effect; the alternative hypothesis (based on the effect of coordination or the initial state) [58] quoted above offers a good explanation for the observed values. With the pioneering theoretical and the experimental papers [61,62] as a basis, angular resolved photo-emission from adsorbed layers of metals, metalloids or molecules could in principle be used to analyze surfaces of alloys and to establish the role in
The electronic structure of alloys; experimental results
167
chemisorption of the geometrical and electronic structures of alloy surfaces. However, very little use has so far been made of this potential. Studies of the adsorption of sulfur on nickel by Angle Resolved Auger Spectra show that a potential indeed exists in this direction [61,62].
3.4 Soft X-ray emission and absorption Even early publications on the electronic structure of metals described X-ray emission and absorption as a source of information on band structures. Their importance has not diminished with the passage of time, as some recent papers show; they are continuously being improved [63-65] and made more sensitive to the effects of alloying. The emission of X-rays can involve either the valence band electrons, or transitions between two core levels (i.e. valence band-core, core-core). In valence band-core level spectra, X-radiation occurs following the emission of electrons from the solid and creation of a vacancy in an inner atomic core level; usually K or L core-levels are involved. The intensity of emission I(E) is approximately given by: I(E) = const.M(E).N(E)
(3)
where M(E) stands for the probability of the electron transition and N(E) is the electronic density of states in the solid, at the energy E corresponding to the measured energy. Papers published in about 1970 reported results on the behaviour of alloys, incompatible with the rigid band theory [66]. This behaviour which could later be described by newly arising theories (for example, the CPA, see chapter 1) has been confirmed recently [67]. It has also been confirmed that the number of d-holes in nickel-copper alloys is not changed by alloying in any pronounced way, leaving open the possibility that perhaps this number does not change at all. For example, Durham et al. [67] showed that the 4s3d---~2p3/2 transitions in nickel-copper alloys produce two emission bands, which are separated by 2eV at all concentrations, a behaviour which cannot be harmonised with the assumption that electrons are transferred from copper to nickel. The same information is available for the platinum-gold system, which is also catalytically interesting [68]. Wenger and Steinemann [69] studied the L emission bands of a large number of alloys and in particular those of various intermetallic compounds. They took a courageous step and used the intensity of the bands to calculate changes in the number of d electrons. The results for various aluminum compounds are shown in figure 23. We observe an increase and then a decrease in the d-electron count (i.e. And is not zero), nothing like that seen with nickel-copper, where And -- 0. This finding should be understood in the same sense as that in which the results in section 3.3 have already been discussed: s- electrons from the conductivity band and d electrons are located in new hybridized orbitals, thus
168
chapter 3
effectively increasing for example the number of d-electrons associated with nickel in the nickel-aluminium system. This hybridization has however an opposite effect with, for example, vanadium.
An d
figure 23 d-electron count changes (due to hybridization of d-orbitals with other ones), formally expressed as "d-charge transfer" An d. Alloys of 60% of A1 with indicated metal [691.
+1.0-
0,5
-
-O.l.
-
T=
I
V
I
Cr
I
Mn
I
Fe
I
Co
I
Ni
Cu
Some results on alloys of titanium with iron, cobalt and nickel were discussed in section 3.3. It was shown there that PES indicates the formation of new hybridized orbitals between unlike atoms, the energy of which is shifted to lower values. The number of dholes in the common band is decreased as a consequence of the fact that the part of the band corresponding to the states with a high contribution of d-orbitals is in the case of alloys completely under Fermi level. It is interesting to see that this picture is equally and completely confirmed by an X-ray emission study on these alloys [70]. The results concerning the shift in the emission band cannot be explained by a transfer of 3d-electrons in any direction, since the shift is negative for both components. However, theoretical analysis [70] has revealed that the shift would be compatible with intra-atomic configurational changes, with nickel changing from 3d 9 4s I to 3d 8 4s 2 and titanium from 3d 3 4s 1 to 3d 2 4s 2. X-ray absorption results are also an important source of information (albeit not easy to obtain) on the presence or absence of the d-band holes. Gudat and Kunz [71] have measured with nickel-copper the M absorption (3p---~3d) edge intensity with the nickelcopper system, while, Cordts et al. [72] have measured the K and L absorption edges concerning transitions related to the d-band holes. In none of these studies has a change in a number of d-band holes on nickel been detected. Most recently, this conclusion has been confirmed by a detailed study of a large number of various nickel-copper compositions; the results of this last study are shown in figure 24 [73].
The electronic structure of alloys; experimental results
i
,.,=..
m CD
,4.,=1
>
1.0
'HI--.
0 In
'l
i
0.8
T
i
""~ll
0.6
r >
i
I
Pd-Au
Pd-Ag
"', 0.4
4-.
i
",,,,,,,,,
m
r:3 2 ::1 2Q.
'i
169
I L
O9 r r
"6
>,
-o.go 'HI-.,
:)r -
', Rigid Band Theory
0.2
0.0
,L
h,,
0
I
I
"~2 0 Atom
I
I
.'i
40 Percent
i
60
~i..
80
i
I O0
IB M e t a l
figure 24 Electronic structure parameter of indicated alloys as a function of alloy composition. The ordinate represents the number of unfilled d-states per atom relative to the number for pure transition metal. The solid line through the data points was derived from X-ray absorption data. For comparison, a prediction based on the rigid-band model of the Ni-Cu electronic structure is shown as the broken line [73].
3.5 Conclusions Recalling the results presented in sections 3.1-3.4, we draw the following conclusions. Alloys can be subdivided in three categories characterized by (a) typical PES features in the valence band spectra and, (b) the magnitude of their enthalpies of formation, AHf. Group 1. Enthalpy of formation is positive and the components form bi- or multiphasic systems. Alike atoms tend to cluster and the PES features resemble a combination of those of the individual components (e.g. nickel-copper, Group 2.
platinum-gold). Almost ideal solutions having small negative enthalpies of formation. The PES features are not very different from those of the pure metals, yet we observe subtle changes in the spectra caused by dilution of one element in a matrix of the other. These changes include peak shape and width variations, spin orbital splitting, and position of the band when it reflects the strength of d-d electron interactions. (see fig.12)
170
chapter 3
Group 3.
Intermetallic compounds having a large negative formation enthalpy. Here changes are seen by PES and X-ray emission/absorption. They simulate formation of bonds between unlike atoms. In extreme cases the compounds are semiconductors and are not ductile. Their surface reactivity is very low. Examples include alloys of hafnium, zirconium and tin with nickel, palladium and platinum.
Metal-on-metal layers differ from both the analogous alloys and from bulk metals (see also chapter 7). However, they are more reminiscent of bulk metals than of dilute alloys (see fig.18) and charge transfer of any appreciable size is highly improbable.
References
10 11 12 13 14 15 16 17 18
N.F.Mott, Proc.Phys.Soc., London 7 (1935) 571 idem, Adv.Phys. 13 (1964) 325 D.A.Dowden, J.Chem.Soc. (1950) 242 P.W.Selwood, "Adsorption and Collective Paramagnetism", Academic Press, London (1962) V.I.Goldanskii, R.H.Herber, "Chemical Applications of M6ssbauer Spectroscopy", Academic Press, N.Y.(1968) "Applications of M6ssbauer Spectroscopy" (editor R.L.Cohen) Academic Press, N.Y. (1980) S.Taniguchi, R.S.Tebble, D.E.G.Williams, Proc.Roy.Soc. London A 265 (1962) 502 E.Vogt, Ann.der Physik 14 (1932) 1 B.Svensson, Ann.der Physik 14 (1932) 699 N.F.Mott, H.Jones, "The Theory of the Properties of Metals and Alloys", Clarendon Press, Oxford (1936), reprinted from corrected sheets in 1956 by Dover Publ.USA M.A.Mook, C.G.ShuI1, J.Appl.Phys. 37 (1966) 1034 R.B.Coles, Proc.Phys.Soc., London B 65 (1952) 221 C.G.Robbins, H.Claus, P.A.Beck, Phys.Rev.Lett. 22 (1969) 1307 E.Vogt, Phys.State Solids (b) 50 (1972) 653 J.P.Perier, B.Tissier, R.Tournier, Phys.Rev.Lett. 24 (1970) 313 A.Kidron, Phys. Lett. 30A (1969) 304 T.J.Hicks, B.Rainford, J.S.Kouvel, G.G.Louw, J.B.Comley, Phys.Rev.Lett. 22 (1969) 531 B.Mozer, D.T.Keating, S.C.Moss, Phys.Rev. 175 (1968) 868 T.R.P.Gibb Jr. J.McMillan, R.J.Roy, J.Phys.Chem. 70 (1966) 3024 C.Herring, J.Appl.Phys. 31 (1959) 1
The electronic structure of alloys; experimental results
19 20
21
22
23 24
25 26 27 28 29 30
171
J.Weiss, J.J.DeMarco, Rev.Modern Phys. 30 (1958) 59 R.Nathans, C.G.Shull, B.Shirane, A.Anderson, J.Phys.Chem.Solids 10 (1959) 138 S.C.Hong, A.J.Freeman, C.L.Fu, Phys.Rev.B 38 (1988) 12156 R.Kurzawa, K.P.K~imper, W.Schmitt, G.Gtintherodt, Solid State Commun. 60 (1986) 777 M.Przybylski, U.Gradmann, Phys.Rev.Lett. 59 (1987) 1152 H.Ebert, J.Abart, J.Voitl~inder, J.Phys.F (Metal Phys.) 14 (1984) 749; Z.Phys.Chem.NF 144 (1985) 223 W.S~inger, J.Voitl~inder, Z.Phys.B 44 (1981) 283 H.Ebert, H.Winter, J.Voitl~inder, J.Phys.F (Metal Phys.) 14 (1984) 2433 P.Weinberger, J.Staunton, B.L.Gyorffy, J.Phys.F (Metal Phys.) 12 (1982) 2229 H.Winter, G.M.Stocks, Phys.Rev.B 27 (1983) 882 J.A.Dumesic, H.Topsoe, Adv.Catal. 26 (1977) 121 N.N.Greenwood, T.C.Gibb in "M6ssbauer Spectroscopy", Chapman & Hall (1971) G.K.Wertheim, "M6ssbauer Effect: Principles and Applications", Academic Press (1964) F.J.Berry, "M6ssbauer Spectroscopy" in "Determination of Structural Features of Crystalline and Amorphous Solids", Vol.5, p.273 (editors: B.W.Rossiter, J.F.Hamilton) Wiley & Sons, N.Y. (1986) J.W.Niemantsverdriet, "Spectroscopy in Catalysis", VCH, Weinheim (1993) R.E.Watson, L.J.Swartzendruber, L.H.Bennett, Phys.Rev.B 24 (1981) 6211 R.E.Watson, J.W.Davenport, M.Weinert, Phys.Rev.B 35 (1987) 508; 36 (1987) 6396 T.E.Cranshaw, J.Phys.F (Metal Phys.) 10 (1980) 1323 J.E.Tansil, F.E.Obenshain, G.Czejzek, Phys.Rev.B 6 (1972) 2796 J.Chem.Soc.Faraday Trans I, 83 (1987) Discussion on p.1963/1964 W.F.Egelhoff jr. in "Core Level Binding Energy Shifts at Surfaces and in Solids" Surf.Sci.Repts. 6 (1986) 253 D.H.Seib, W.E.Spicer, Phys.Rev.Lett. 20 (1968) 1441; 22 (1969) 711 and idem Phys.Rev. 187 (1969) 1176 D.H.Seib, W.E.Spicer, Phys.Rev.B 2 (1970) 1676 S.Htffner, G.K.Wertheim, J.A.Wernick, Phys.Rev.B 8 (1973) 4511 P.O.Nilsson, Physik Kondens Materie 11 (1970) 1 H.S.Reehal, P.T.Andrews, J.Phys.F (Metal Phys.) 10 (1980) 1631 C.Norris, H.P.Myers, J.Phys.F (Metal Phys.) 1 (1971) 62 A.D.M.McLachlan, J.G.Jenkin, R.C.G.Leckey, J.Liesgang, J.Phys.F (Metal Phys.) 5 (1975) 2415 P.T.Andrews, L.A.Hissott, J.Elect.Spectr.& Related Phenom. 5 (1974) 627 P.Steiner, H.Hochot W.Steffen, S.Htffner, Z.Physik B 38 (1980) 191
172
31
32 33 34 35 36
chapter 3 A.Bansil, Phys.Rev.B 20 (1979) 4025, 4035 A.J.Pindor, W.M.Timmerman, B.L.Gyorffy, G.M.Stocks, J.Phys.F (Metal Phys.) 10 (1980) 2617 B.E.A.Gordon, W.E.Timmerman, B.L.Gyorffy, J.Phys.F (Metal Phys.) 11 (1981) 821 S.Htffner, G.K.Wertheim, J.H.Wernick, A.Melera, Solid State Commun. 11 (1972) 259 J.A.Nicholson, J.D.Riley, R.C.G.Leckey, J.Liesgang, J.G.Jenkin, J.Phys.F (Metal Phys.) 7 (1977) 351 Idem, J.Elect.Spectr.& Related Phenom. 15 (1979) 95 J.Friedel, J.Phys.F (Metal Phys.) 3 (1973) 785 Y.J.Chabal, D.R.Hamman, J.E.Rowe, M.Schltitter, Phys.Rev.B 25 (1982) 7598 T.E.Fischer, S.R.Keleman, K.P.Wang, K.H.Johnson, Phys.Rev.B 20 (1979) 3124 B.J.Waclawski, D.S.Bondreaux, Solid State Commun. 33 (1980) 589 O.Oelhafen, E.Hauser, H.J.Guntherodt, K.H.Bennemann, Phys.Rev.Lett. 43 (1979) 1134
37 38
39 40 41 42
43 44 45 46 47 48
O.Oelhafen, E.Hauser, H.J.Guntherodt, Solid State Commun. 35 (1980) 1017 A.Amamou, Solid State Commun. 37 (1980) 7 G.N.Derry, P.N.Ross, Solid State Commun. 52 (1984) 151 S.D.Cameron, D.J.Dwyer, Surf.Sci. 176 (1986) L857, the last two quotations should be compared with ref.35, where charge transfer is also assumed. R.M. Friedman, J.Hudis, M.L.Perlman, R.E.Watson, Phys.Rev.B 8 (1973) 2433 J.Fuggle, F.U.Hillebrecht, R.Zeller, Z.Zolnierek, P.A.Bennett, Ch.Freiburg, Phys.Rev.B 27 (1983) 2145, 2179 L.Brewer, P.R.Wengert, Metallurg.Trans. 4 (1973) 83 H.Asonen, C.J.Barnes, M.Pessa, R.S.Rao, A.Bansil, Phys.Rev.B 31 (1985) 3245 H.Asonen, M.Lindroos, M.Pessa, R.Prasad, R.S.Rao, A.Bansil, Phys.Rev.B 25 (1982) 7075 R.S.Rao, A.Bansil, H.Asonen, M.Pessa, Phys.Rev.B 29 (1984) 1713 G.S.Sohal, R.G.Jordan, P.J.Durham, Surf.Sci. 152/153 (1985) 205 N.Martensson, R.Nyholm, H.Calen, J.Hedman, B.Johansson, Phys.Rev. B 24 (1981) 1725 P.M.Th.M.van Attekum, J.M.Trooster, J.Phys.F.(Metal Phys.) 9 (1979) 2887 B.Johansson, N.Martensson, Phys.Rev.B 21 (1980) 4427 N.Martensson, B.Johansson, Solid State Commun. 32 (1979) 791 A.R.Miedema, P.F.de Chatel, F.R.de Boer, Physica B 100 (1980) 1 A.R.Miedema, Z.Metallkunde, 70 (1979) 345 B.H.Verbeek, Solid State Commun. 44 (1982) 951 G.G.Kleiman, V.S.Sundaram, J.D.Rogers, M.B.de Moraes, Phys.Rev.B 23 (1981)
The electronic structure of alloys; experimental results
173
3177 V.S.Sundaram, M.B.de Moraes, J.D.Rogers, G.G.Kleiman, J.Phys.F (metal Phys.) 11 (1981) 1151
49 50
51 52 53 54
55 56 57 58
59 60 61
62
63 64 65 66
N.J.Shevchik, D.Bloch, J.Phys.F (Metal Phys.) 7 (1977) 543 A.P.Shapiro, T.C.Hsieh, A.L.Wachs, T.Miller, F.C.Chiang, Phys.Rev.B 38 (1988) 7394 G.C.Smith, C.Norris, C.Binns, H.A.Pandmore, J.Phys.C (Solid State Phys.) 15 (1982) 6481 G.C.Smith, C.Norris, C.Binns, J.Phys.C (Solid State Phys.) 17 (1984) 4389 D.L.Weissman-Nenocur, P.M.Stefan, B.B.Pare, M.L.Shek, I.Lindau, W.E.Spicer, Phys.Rev.B 27 (1983) 3308 N.G.Stoffel, S.D.Kevan, N.V.Smith, Phys.Rev.B 32 (1985) 5038 P.Steiner, S.Htifner, Solid State Commun. 37 (1981) 279 J.A.Rodriguez, R.A.Campbell, D.W.Goodman, J.Chem.Phys. 95 (1991) 5716 J.A.Rodriquez, D.W.Goodman, Surf.Sci.Rept. 14 (1991) 1 R.A.Campbell, J.A.Rodriquez, D.W.Goodman, Surf.Sci. 240 (1990) 71 J.A.Rodriquez, R.A.Campbell, D.W.Goodman, J.Phys.Chem. 93 (1991) 2477 H.Ishida, K.Terakura, Phys.Rev.B 38 (1988) 5752 S.C.Hong, A.J.Freeman, C.L.Fu, Phys.Rev.B 38 (1988) 12156 R.Richter, J.G.Gay, J.R.Smith, Phys.Rev.Lett. 54 (1985) 2704 C.L.Fu, A.J.Freeman, Phys.Rev.B 33 (1986) 1611; 35 (1987) 925 G.A.Benesh, D.A.King, Chem.Phys.Lett. 191 (1992) 315 M.Scheffer, Ch.Droste, A.Fleszar, F.Maca. G.Wachutka, G.Barzel, Physica B 172 (1991) 143 A.Stenborg, O.Bj6rneholm, A.Nilsson, N.Martensson, J.N.Andersson, C.Wigren, Surf.Sci. 211/212 (1989) 470 G.Apai, R.C.Baetzold, P.J.Jupiter, A.J.Viescas, I.Lindau, Surf.Sci. 134 (1983) 122 J.B.Pendry, J.Phys.C 8 (1975) 2413 B.W.Holland, J.Phys.C 8 (1975) 2679 H.Freud, M.Neumann, Appl.Phys.A 47 (1988) 3, see also chapter 2 for this subject R.Baudoing, C.Gaubert, E.Blanc, D.Aberdam in "Physics of Solid Surfaces" (editor: M.Laznicka) Elsevier (1982) Stud.Surf.Sci.& Catal. Vol.9, p.87 R.Baudoing, E.Blanc, C.Gaubert, Y.Gauthier, N.Gnucher, Surf.Sci. 128 (1983) 22 M.L.L~ihdeniemi, E.Ojala, I.Tterakura, K.Terakura, J.Phys.F (Metal Phys.) 13 (1983) 521 M.L.L~ihdeniemi, E.Ojala, M.Okochi, J.Phys.F (Metal Phys) 13 (1983) 513 V.V.Nemoshkalenko, in "X-ray Emission Spectroscopy of Metals and Alloys" (in Russian) (publ.house: Naukova Duma, Kiev) (1972) D.Fabian, J.de Physique 32 (1971) C4-17 (suppl. 10)
174 67
68 69 70 71 72 73
chapter 3 P.J.Durham, D.G.Raleby, B.L.Gyorffy, C.F.Hague, J.M.Mariot, G.M.Stocks, W.M.Temmermans, J.Phys.F (Metal Phys.) 9 (1979) 1719 M.C.Munoz, P.J.Durham, B.L.Gyorffy, J.Phys.F (Metal Phys.) 12 (1982) 1497 P.Weinberger, J.Staunto, B.L.Gyorffy, J.Phys.F (Metal Phys.) 12 (1982) L199 A.Wenger, S.Steinemann, Helv.Phys.Acta 47 (1974) 321 E.K~illne, J.Phys.F (Metal Phys.) 4 (1974) 167 W.Gudat, C.Kunz, Phys.State Sol. 52 (1972) 433 B.Cordts, D.M.Peax, V.Azaroff, Phys.Rev.B 22 (1980) 462 G.Meitzner, D.A.Fischer, J.H.Sinfelt, Catal.Lett. 15 (1992) 219
175
Chapter 4
S U R F A C E C O M P O S I T I O N OF ALLOYS
4.1 General remarks on surfaces of metals
When a chemical bond is formed between two or more atoms, the total energy of the whole system decreases. This also holds true for a macromolecule, such as a metallic crystal. Inversely, when the bonds are broken, by, for example, cutting a crystal into two pieces and forming two new surfaces, the total energy increases. The energy associated with the formation of surface is called surface energy AE, and is given by: A
,xE=fdE= f
(1)
o
where dA is the differential increase in surface area and 7 energy needed to form a unit surface area. Cutting the crystal causes the atoms in the surface to feel a different crystal potential V, which manifests itself by the appearence of new energy levels in the band structures, i.e. in the E(k) diagrams (chapter 1) or in the one dimensional band schemes. The wave functions of these states are highly localized on the surface atoms, since the perturbation in the crystal potential is also localized. The surface state functions decay exponentially in the direction of vacuum. Nevertheless, due to their form, electrons appear in the vacuum at a distance greater than the half a lattice plane distance. This leads to a formation of a surface dipole layer, which then contributes substantially to the work function (see chapter 3). A simple picture of a dipole layer formed by a spillover of an isotropic electron cloud would suffice for simple s,p-metals, but for the transition metals the picture is more complicated. These have electrons in spatially structured and clearly identifiable d-orbitals, which when they emerge from the surface are occupied to a different degree from their bulk analogues. Examples of calculations on the surface states of transition metals, for example of palladium, are already available [1]. Desjonqu~res and Cyrot-Lackman [2] used a simple model to calculate the surface energy of transition metals by quantum mechanical techniques. They applied the equation due to Allan [3]:
176
chapter 4
EF
?s =
1 0 fj o
oo
EE
(2)
AN, ( e , U o ) d e - Z M U~ -
i=1
The sum is taken over individual layers of the metal and A N i is the deviation in the local density of states on the i-th layer. Z M and Z s are the numbers of d-electrons in the bulk of the metal M and in its surface respectively. Further, Uo is the change in the potential due to the existence of the surface. The last two terms are added to prevent double counting of Coulomb interactions. By this procedure a correlation is predicted as shown schematically in figure 1. figure 1 Dependence
(schematically)
of
the surface energy of metals on the occupancy by the electrons of the d-bands [21 Z - number of d-electrons, from 1 to 10. The maximum is at Z equal to five.
Zmetal When we compare this figure with figure 28 of chapter 1, we see that the surface energy shows a similar dependence on the band occupation as does the cohesive energy: there is a maximum near the half-filled band. Indeed, a relation between Ts and the cohesive energy Ecoh should and does exist, as we shall see below. The relation between Ys and Eco. was already mentioned in the very early literature [4] and later several authors analyzed this relation in more detail [5,6]. The basic idea behind these calculations is as follows. In the bulk of a given metal each atom has n neighbours. It is assumed that the cohesion in the metal can then be described as arising from n electron-pair bonds. In the surface An of these bonds are broken and the correponding dissociation energy is taken as the surface energy. If the surface layer is of thickness d, the surface energy per unit volume of the surface layer is defined as Tls = %/d (the average d is calculated from the average density 19, as p = d-3). If the molar volume is VM, the cohesion energy per unit volume is Ecoh.VM-~ = above lead to a self-evident relation:
8cob. The considerations presented
Surface composition of alloys
177
1
~'s _ A n Ecoh
(3)
rt
Actually, this equation does not always predict the experimental values of qt~1 very well, but it can still be useful for rough estimates. As we shall see below, there is yet another approach for using the idea of "broken bonds". If a potential energy curve is known for pair-wise interactions, a better approximation than equation 3 can be achieved. In this respect, the simplest potential is the Morse curve, and the most elaborate is the Mie potential, used for example by Machlin [7]. Wynblatt [8] used a potential due to Baskes and Melius [9] and calculated the surface energy as "/, = ]~ (~k - ~bulk)
(4)
k
Here ~k is the energy of an atom lying in the 1,2 . . . .
k th
layer, given as the sum of
interaction potentials, over all other atoms in the crystal, at distances rj 1 1
The potential used [9] is rather short range, so that it is predicted that not more than the four outermost layers contribute to qts; they are thus of crucial importance for predictions of surface segregation (see section 4.2.3 below). Table 1 shows the values of ~'s at 0 K, as derived by Miedema [10] from various experimental values spread over the literature. Useful information on experimental values of qt's, their temperature dependence etc. can be found in a paper by Overbury et al. [10]. One of the probable reasons why various empirical or semi-empirical equations predicting qts-values fail is the phenomenon of surface reconstruction, not accounted for in earlier theories. Let us now turn our attention to that. A mere look at the surfaces formed by different sections through crystals shows immediately that the resulting unsaturation varies from surface to surface (see figure 2). Surface energy can be lowered when atoms in the surface layer(s) move out of their positions and form new bonds among themselves. For example, the (100) face of the fcc metals can be stabilized if atoms of the outmost layer rearrange themselves to form a hexagonally closed packed structure. The (110) plane of the fcc structure can reconstruct by losing alternate rows of atoms, which allows those underneath to form new, stronger bonds, etc. This is schematically shown in figure 3 [see e.g. 11-13].
178
chapter 4
[o
1~]11 10011 (o)
--101~1
; 1oi'11 (c)
(b)
(d)
10111
[----------~
[e~l
L~
101~1
(e)
10011 (g)
(f)
[o011 11101
10101
------~--10101
(h)
figure 2 The lowest-indexes planes of the three basic structures: bcc [a)-d)], fcc [e)-g)], hcp [h,i]. The crystallographic orientation of the plane is indicated [12b].
A
B
C
[11iEO~]~~, 1 x
x
.R
figure 3 Different forms of the reconstructed (110) surface. Perspective view of a surface. A) Unreconstructed surface; B) missing-row (MR) reconstructed; C) pairing-row (PR) reconstructed [12e].
A further complication in the considerations concerning surfaces is the presence of adsorbing gases. Adsorption means formation of new bonds and this leads to a decrease in the total energy, through the decrease in the surface energy. This is shown schematically in figure 4 for the metals A and B.
Surface c o m p o s i t i o n o f alloys
table
179
1
Surface energies ]t(mJ/m 2 at OK [10].
(A)
li s
M
y
1100
La
900
Zr
1950
Hf
2200
2600
Nb
2700
Ta
3050
Cr
2400
Mo
2950
W
3300
Mn
1600
Tc
3050
Re
3650
Fe
2550
Ru
3050
Os
3500
Co
2550
Rh
2750
Ir
3100
Ni
2450
Pd
2100
Pt
2550
Cu
1850
Ag
1250
Au
1550
M
y
Sc
1200
Ti
2050
M
with
~cidsorbclte
ofree
r
2'
dsorbate
I I I I
I I I I I
with
free
2 Nads
0 Nads
figure 4 Diagrams explaining the gas-induced segregation. Surface energy of two metals A and B. Upper-lines, adsorbate free surfaces, y(A) > y(B). Other lines, surface energy as a function of the number of adsorbed molecules. At the amount adsorbed 1' and 2', y(B) > y(A): Then adsorption reverses the direction of segregation and we observe gas induced (re-)segregation.
180
chapter 4
We observe that in the case shown Y*s exceeds ~s when the surface is free of adsorbates. Thus in vacuum, the surface of an AB alloy will be enriched in B (see below). However, in the presence of gas which is adsorbed more strongly on A than on B, ~ is less than "~s and the sign of segregation is reversed. This phenomenon of gas-induced segregation was first observed by Bouwman et al. with carbon monoxide on palladium-silver [14] and was later seen with many other alloys and gases. Reconstruction of the surface can be abolished [15] or created [16] by adsorption of gases. It naturally influences the strength of chemisorption bonds, so that enthalpy of adsorption of oxygen and carbon monoxide on single crystal planes vary according to the occurrence or absence of reconstruction [17]. The process of gas-induced reconstruction can lead to oscillation in the rates of surface reactions [15]. The differences in surface energies between various crystal planes determined the equilibrium shape of unsupported small metal particles in vacuum, which can be derived from the so-called Wulff construction [18], schematically shown for a section of a crystal in figure 5.
\
figure 5 Shape of a crystal, determined by
N
I
thermodynamic factors - surface
energies "~(hkl)"Planes are set in such a way that Yh~tforms a
\
(ool)
normal to the (hkl) plane. "~ (lOO)
By such a procedure, the surface energies of various planes are represented by vectors, which are drawn as normals to the planes derived by their indices. When these planes are set at the tops of the corresponding vectors they define altogether the shape of the metal particle. When 7ak~is changed by adsorption of gases and the gas-induced change AyaR~is a function of the indices (h,k,1) of the plane, adsorption can reshape the particles, providing the temperature allows it. This phenomenon is most probably behind the various effects of pretreatment (oxidation/reduction at different temperatures) on the specific catalytic activity of metal particles [19,20]. It should also not be forgotten that contact with a strongly interacting support will influence the surface/interface energy, and that this can cause the shape of metal particles to differ from these expected in the free state [21].
Surface composition of alloys
4.2
Binary systems with surface segregation
4.2.1
Chemical approach, kinetic and thermodynamic description of equilibrium
181
All chemists are acquainted with the arguments which had led Guldberg and Waage to their formulation of the reaction equilibrium constant. An analogy of such reasoning can be used to predict the equilibrium for surface segregation [22]. We consider exchanges of positions of the alloy components A and B between the surface (index s) and the bulk (without index). If there are nA moles of A and n B moles of B in unit volume, the stoichiometric equation of the exchanges reads: 1
--A~+
na
1 =__1 __1a B Bs + nn nn nA
(6)
This exchange proceeds until the equilibrium of the segregation is achieved. With activities aB and aA, the equilibrium constant is:
K A = ~
(an)t'a~
; p-
nn
(7)
Replacing activities by the corresponding mole fractions x in the bulk and y in the surface and activity coefficients f, we arrive at
:/a-yA/P /
(8)
K,,ol = f al (fnY ; K -
(9)
Ka ~l_xa) "--~a 9K since
To simplify, we consider only the case of p=l, and by substituting: K
-
K~~
-
K/
in equation 8, we arrive at:
(10)
182
YA--
chapter 4
Kx a
(11)
I +(K/-1)XA
The surface composition YA as a function of the bulk composition XA is shown schematically in figure 6.
YA figure 6 Surface
~j/J~
composition
(atomic ratio YA) as a function of the bulk composition (XA) [22]
/;/
1)
KA<<1
2) KA=I 3) KA>>1
4) KA>>I
K>> 1
K-1 K=f(Xg) K<
1
•
A
If the exchange described by equation 6 is not accompanied by a change in the thermal, i.e. non-configurational, entropy, then:
~,:oxp/ ~~xc~I K ! = exp
( -AHe~c'ss] RT }
~1~) (13)
where AHexchis the enthalpy change accompanying the exchange reaction (equation 6) and AHexcess is related to the non-ideal behaviour of the mixture. This is a simple approximation for the so-called regular solutions, and is often referred to as the Fowler-Gugenheim or Bragg-Williams approximation. The parameters AHexch and AHexcess can be calculated from, for example, a broken bond model, as we shall see below.
Surface composition of alloys
4.2.2
183
Simple thermodynamics of segregation Following the paper by Overbury and Somorjai [10] on this topic, we shall first
define the generalized total Gibbs free energies (enthalpies) G for the surface and for the bulk, of an alloy A B: s s , G s =E s + p v s - T S S - T A =P.AnA + [.tBn B,
(14)
Gb=Eb+PVb
(15)
-
TS b = l . t Ab n Ab + P,BnB b b
In these equations the lower index indicates the component and the upper one the location and A is the surface area. For an ideal solution, the extreme right-hand side of the equation can be written as: +
n tix a
b
t,,o
nffRTlnxff
= Gt'
(16)
There is, of course, also an analogous equation for GS; the chemical potentials are defined by equations 14 and 15 as
lXA=~ ; ~n = ~,8 na ) T,P,,,~ t 8 nn ) r,e,,, ~
(17)
where again the lower index of n indicates the component, the upper index the location. At equilibrium the chemical potentials of component A in the surface and in the bulk are the same: s
b
(18)
~A = ~a
~ta
+ RTlnx~-~,
~
t 8,,,,J
= IXa
+ RT
For the pure metal A: p.]o_ I'ta
(20)
ts,,,) The last three equations can easily be written by analogy for component B. In what follows, we shall assume that:
184
chapter 4
6A
6A
-a
(21)
8n;
With this, we shall now formulate the equation for the equilibrium between the components where both are present in the bulk and in the surface: $
y,a
+ R T l n x ~ - R T lnxba = y2a + R T l n x B - R T l n x ~
(22)
Knowing that x A + x B = 1, we rewrite this as: s
b
xa
xa
.exp (1-x~)
(~'n-va)a
(23)
RT
1-x~
In the approximation of ideal and regular solutions, the change in the vibrational entropy caused by exchange according to equation 6 is neglected and equation 24 is used: (~'n - ~'a)a = - A H ~ exch
(24)
With this the thermodynamic derivation supplies exactly the same equation as the kinetic approach in 4.2.1. The approximation of ideal solution predicts the highest segregation of any theoretical model; introduction of non-ideality can decrease the segregation effect and introduction of size effects can even reverse its direction, as we shall see below. This has to be kept in mind, when seeking to use simple equations such as 8, 11 or 23 to interpret adsorption and catalytic phenomena in terms of surface segregation.
4.2.3
Broken-bond model of alloy surfaces. In this section the ideal solution model [24,25] will be treated in detail and it will
be briefly indicated how the regular solution model and some higher approximations can be derived from it. A very comprehensive and illuminating analysis of the broken-bond model was published in 1974 by Williams and Nason [23]. By equations shown below, Williams and Nason calculated the enthalpy H per bond, whether A-A, B-B or A-B, from the heats of sublimation AH~
of metals A and B and the number of nearest neighbours of each atom
in the solid Z, which is for example twelve for the bulk of the fcc structure, eight for the bulk of the bcc structure, nine for the (111) surface of fcc solids, etc. Bond enthalpies are then expressed as:
Surface composition of alloys
AH ~
185
(A)
2AH ~
Z
(B)
Z
1 (Haa + HBB) + Q Hart - ~-
(25)
Parameter f~ stands for the excess enthalpy and represents the deviation from the ideal solutions and its explicit form will be shown below. Williams and Nason used in their analysis the model shown in figure 7, in which four surface layers
are explicitly treated
as potentially deviating from the bulk and are placed on the homogeneous bulk alloy. To work with this model we have to distinguish the number of vertical and lateral nearest neighbours of each atom, respectively Z v and Z1; and to add an index indicating whether the layer considered is one of the four surface layers or whether it lies within the bulk. For a binary solution of two metals of the same crystallographic structure equation 25 is used in further description. Broken Pair Bond Pair Bond \,, N \\
8
//
Vacuum
//
Interface
o
5 a 2'X2
Surface Gs' SS' Hs
0
O
~, 3,x3 t,"
z
i I
0
l,X!
~
i
>
O
4,X 4
O O xb O
O0
O
O O O O
figure 7 A model of binary alloy surface layers, used for calculations [23]
Bulk IAxbBl.Xb) Gb Hb sb
186
chapter 4
Ideal solutions We shall first discuss the ideal solution model ( ~ - 0) and use the following symbols and expressions. x 1 = fraction of A atoms in layer 1 (and analogous symbols for other layers) 1
-
x 1 = fraction of B atoms in layer 1.
Zll = the total number of lateral bonds. V2Z~ x~x 1 = number of lateral bonds AA
V2Z~1 X~Xl HAA = enthalpy associated with these bonds Zll x 1 (1-Xl)HAB = enthalpy associated with the A-B bonds. By analogy similar equations can be written for vertical contributions to the enthalpy and for the layers 2,3 and 4 and for the bulk. The factor Y2 in some of the expressions is introduced to avoid counting of the same bond twice. Analogous expressions are used for the vertical bonds; for example, the enthalpy associated with vertical A B bonds, i.e. bonds out of the first layer is Z~vX2(1-Xl)HAB, etc. With these terms the enthalpy difference AH s has been calculated as AH s = (enthalpy of all bonds in the four surface layers) (enthalpy of all bonds in four layers of the bulk) By "all" is meant the total number of lateral and vertical AA, BB and AB bonds. The entropy of mixing per atom in the bulk and in the first layer of the alloy is then [24,25]:
ASmix, b : S b - X b S ~
)S~
= - k [ x b l n x b +(1-xt,)ln(1-xt,)]
(26)
[XllI1x 1 +(1-xl)ln(1-xl) ]
(27)
ASmix, 1 = Sl -Xl S~ - ( l - x 1 ) S ~ B = - k
The index ~ indicates the state of a pure metal. With AS s , calculated as the difference between the enthalpy of the four surface layers and that of four layers in the bulk, the change in surface free enthalpy is given by: AG s = AHS_TAS
s
(28)
The total free enthalpy Gv is the sum of G s + G b, so that the equilibrium conditions for the first four layers are: 5G r 5x n
--
5G 9 5x
+
5G a 5x
(29)
Surface composition of alloys
6G s
o
6%,
187
(30)
For ideal solutions A
~
(31)
X1 -
1+,4" where an abbreviation is used: A *= -
xb
exp
1 -x b
-AH~ap
Zlv .kT
(32)
and AHva p = AH~
(a) - A H~
(33)
By performing the calculations one arrives at the conclusion that X2 =X 3 =X 4 =X b
(34)
or in other words, for an ideal solution, the surface enrichment is limited to the outermost layer. Equations 31 and 32 should be compared with equations 11, 23 and 24 given earlier in this chapter. First layer compositions for the (111) plane of ideal binary alloys with an fcc structure are shown in figure 8. When different planes are cut through an AB alloy having a cubic structure, different numbers of bonds are split. This too has its influence on the surface enrichment as we can see immediately in figure 9. Figures 8 and 9 present a graphical illustration of two important tendencies: the element with the higher sublimation energy disappears from the surface since it costs less energy to cut the bonds of lower energy at the surface, i.e. the bonds associated with the metal with the lower sublimation energy. Furthermore, for analogous reasons the first layer enrichment is the strongest with the most densely packed surfaces (figure 9). However, it must not be forgotten that this is only true for an ideal solution, and when both pure metals have the same crystallographic structure and atomic size.
188
chapter 4 1.0
1.0
x
0.8
X
0.8-
~o,,,
d
c-
"0,.
0
O .m 4-,
4-,
t) CJ t_
u_
t3 C} t_
i1
0.6
0.6-
E
E
0
0
<
<
I_.
I._
~
O.l.
O.L
C~ ...3
0 _J
ul
EL
.t_ I.t.
0.2
0.0
0.0
Bulk
0.5
Atom
0.2
0.0 0.2.5
0.3
fcc111 f c c l O 0 bcc 100
1.0
Froction.X
O.L
fcc110
0.5
bcc100 bcc 111
Surface Roughness Z l v / Z 1
figure 8
figure 9
figure 8. First layer composition xl for ideal solution binary alloys, as a function of bulk composition x b, for the fcc(111) surface. full circles zkH,, b = 2kT; empty circles All,, b = 5kT; full squares zkH,, b = l OkT; empty squares zkH,ub = 20kT [23] figure
9. First layer composition,
ideal solution approximation,
binary alloys.
The
influence of the surface roughness. empty circles AHs, b = 5kT; full squares AH, ub = lOkT; (- - -) x b = 0.96; (-) x b = 0.80 [23]
Regular solutions In the regular solution theory of solid solutions, also called Bragg-Williams or FowlerGuggenheim approximation, HaB = 112 (Haa + Hnn) + f~
(per bond)
(35)
where E2 is related, as an excess enthalpy term, to the value of the activity coefficient f, as follows:
Surface composition of alloys
189
lnfa ~ lnfB
f~
ln f a m
kT
(36)
ln f n
(37)
m
(1-xb) 2 Z
x~ Z
This symmetrical form for the activity coefficients fA and fB, with a single value of f2, is a consequence of the approximation used. In this approximation, non-ideal behaviour influences only the HAB values, but not the ASs values, i.e. ideal mixing is retained. In the influence on H no asymetrical effects, such as an asymmetric size effect, are considered. In the next steps of the derivation, the whole procedure of calculating G s (equations 25-28) is repeated but now with HAB containing the term f2. After applying the equilibrium conditions for GT and G s, four equations for four variables x1,x2,x3,x4 result. Of these, only the two first equations are presented here as examples: ~ - 2 O ( Z x b - Z, x ,
-~1 L,)
Z Al-I ap + k T l n [ X l ( 1 - x b )
x2(1-x b)
0 : 2t~ ( Z x b - ZlX 2 - ZvX 1 - Z~x3) + k T l n - - - -
(38)
(39)
[Xo(1-x 3) As can be seen in figure 10, negative values of the regular solution parameter f2 suppress the surface enrichment, the positive values do the opposite. The sign of f2 has also an important role in establishing the concentration profile over the four surface layers (see figure 11). It demonstrates that an endothermic excess enthalpy, i.e. negative f2, causes a lower enrichment in B in the first layer and a reversed enrichment (in A, now) in the second layer. In endothermically formed alloys the components tend to form clusters. We have mentioned this effect in Chapter 1. The thorough analysis by Williams and Nason [23] not only demonstrated clearly the effects of a number of principal factors such as differences of cohesive sublimation energy, differences between crystallographic planes, the effects of non-ideality etc., but in the original paper the analysis was also extended to cases of surfaces with surface relaxation, i.e. where different bonding strengths were assumed in the surface and bulk layers, and to the effects of the particle size.
190
chapter 4
1.0
l=Xb x
figure 10 First layer composition, regular solution ap-
0.8-
proximation, binary alloy, % = 0.9. The influence of the regular solution parameter, for
C
.s U O i_
u_
fcc(111) surface. Symbols as in figure 8 [231
0.6-
fi 0
O.Z.0 J
0.2-
0.O
- 0.5
Regular
I
1
-0.3
I
I
1
0.0
Solution
m
.L I
0.3
I
0.5
Parameter. Q/RT
A
0.1
L_
o
3-
...J
Bulk
50.0
l
I
figure l l Regular solution composition profile. f c c ( l l l ) surface: zkHs,b = lOkT, % = 0.8, f~ = +_ O.lkT
I
I
I
0.5
I
Atom F r a c t i o n
I
I
I
1.0
Surface composition of alloys
191
There are several ways in which the particle size can influence the surface composition of alloys; (i) there are different planes exposed to vacuum in small and large particles; (ii) due to different electronic structures of small and large particles, the surface energy per atom is different, also for planes of the same index; (iii) with small particles there is an insufficiency of B atoms in the bulk to achieve an enrichment as predicted by the theory derived for semi-infinite crystals, for example, such as equations 11 or 31. Figure 11 from the paper by Williams and Nason [23] shows the region for allowed enrichments expressed as x 1 values for different bulk compositions x b and for
different
dispersions D of the metal. Dispersion is defined as Ns/N T, where Ns is the number of atoms the first surface layer and N T the total number of atoms. If the mean composition of the particle, i.e. composition in a non-segregated system, is (Xb)o, an obvious balance equation must hold:
(xb) o =x 1D +x b (1 -D)
(40)
This leads to the results as shown in figure 12
1.0--[
:~
IA
A
A A
.
6
~
0'81A A A ~O [A A A/ Eo 0
o tv iT
o4:1
/vv
0.0 0.0
A ~
(Xb) o -0.9
~
v
(Xb) o -0,5
(Xb) o -0.2
0.5 Dispersion. D
1.0
figure 12 Regions of feasible values for first layer compositions: small particle mass balance: (A) for (%)0 = 0.5, (<) for (%)0 = 0.2.
192
chapter 4
4.2.4
Regular solution model for systems with components of different molar volume.
McLean [26] has derived an equation by which the energy Esize accompanying the transfer of an odd-sized atom from the bulk into the strain-free surface of an alloy AB is calculated: E~u,, =
GK(rA-rB)2.r3 2 (3K + 4G) r a
-24n
(41)
In this equation r g and r B are the radii of the pure metals, r the radius of the minor dissolved metal atom in the alloy, K the bulk modulus of the solute, G the sheer modulus of the solvent. Friedel [27] suggested eliminating the unknown r by applying the condition of equilibrium. The main problem with the application of equation 41, which is used to calculate an additional term in the energy H s of the surface formation, is its symmetry: the overand under-sized solute atoms should cause the same effect. However, due to the asymmetry of the potential curves for pairwise bonding such as the Lennard-Jones 6-12 potential curve, the compression of a lattice should require more energy than an expansion. Moreover, strain energy is never completely released when a solute atom appears in the surface. The literature offers several examples of calculations predicting and describing surface segregation in alloys with strain [28-30]. Following the approach developed therein, Van Langeveld [31] calculated the surface segregation using the interatomic distance in the alloy R b which was either taken from experimental results or calculated from Vegard's law:
Rb=XbR~ +(1-Xb)R~
(42)
The potential energy for AB, AA or BB pairs (i,j) is taken as Lennard-Jones 6-12 potential curve: e0(R b)=e~ ~J 2( -~bij)6 -(
)~2
(43)
which expression written in shorthand notation reads as
eq(Rb) =e~ Oq With this
(44)
Surface composition of alloys
193
1 Hb=Z [-~xb e ~ , , u +x~(1-xb)e ~*aB + 2
+_
1
(l_Xb)2g
2
o
~
BB
o
IilBB]
(45)
and Z has the usual meaning of total number of nearest neighbours. This and analogous equations are used to calculate G s, as in the foregoing paragraph, with substitutions such as e~
= HAg, etc. Van Langeveld [31] illustrated the effect of varying the R~176
ratio by the curves shown in figure 13.
1.0
--
0.5
~
-
/
/
~
.~/<-~ /
/ / f : .............. .....
-
/
/"
...'"
........
." //
" "-'- 9-._ . . . . .
/'
/
i
.I
9
/ / "
-
//Z"
!
0
I
~
i
I
I
0.5
~
i
l
i
10
----~ X b
figure 13 The surface composition of a hypothetical alloy system with a varying ratio R~176 The binding energies e~ of all pairs are assumed to be equal the enthalpy of sublimation of both components being 418 kJ/mol. The calculations were performed for a temperature of 750K and a (111) crystallographic plane of an fcc lattice, in assuming the surface relaxation parameter 5 to be zero. Zero-line: R~176 ( - - -)R~176 (-.-.-)R~
= 1.15R~
[31].
In this figure the enthalpies of sublimation of the two components were taken to be 418 kJ mol -~. Thus, the results in figure 13 show the segregation purely due to differences in the
194
chapter 4
radii. In his work, van Langeveld also showed how the activity coefficients are related to asymmetric size effects. He applied his theory mainly to platinum-copper alloys [32] and limited himself to a two-layer model. Later Gijzeman [33] improved the theory in several details and suggested further how to use it for other alloys, such as Ag-Pd, Cu-Ni, Cu-Pd, Ni-Pt, Ag-Au, Fe-Ni and Cu-Pt. Strohl and King [34b] have developed several of the theories published earlier in a very general multicomponent multilayer thermodynamic theory in a high approximation; three or more components and four or more layers can be considered. They showed how to use it, for example, for Ag-Au, Cu-Ni and Cu-Ni-Pt semi-infinite alloys. Also the paper by Lee and Aaronson [34a] is important for such cases. 4.2.5
Monte-Carlo calculations of surface segregation Monte Carlo techniques were first applied [34-39] to the problem of surface
segregation in semi-infinite alloys, that is, to systems which can also be described by other more conventional thermodynamic methods. However, this technique can be used with greatest advantage when applied to small alloy particles, and small bimetallic particles including those with components of zero or limited mutual solubility, etc. For example, in an early paper, Sundaram and Wynblatt [37] performed calculations for cubo-octahedra containing 38 and 201 atoms, and simulated in their model surface segregation, and the ordering and clustering of components. In the Monte Carlo technique, one generates different configurations of the systems, e.g. in the case of an AB alloys, different distributions of the atoms A and B, and then macroscopic thermodynamic functions, such as average energy, entropy or free energy, are calculated by averaging over series of configurations. Strohl and King described [34,39] their algorithm for Monte Carlo calculations as follows: 1) An atom is selected and allowed to exchange positions in the lattice with either 2)
another atom or a vacancy. The change in configurational energy AE, resulting from the switch described in
3)
step 1, is calculated. If AE is negative, the new configuration is accepted. If AE is positive, a random number is selected from a uniform distribution. If the quantity exp(-AE/kT) is greater than this random number, the new configuration is accepted, otherwise the
4)
old configuration is retained. Steps 1-3 are repeated until "equilibrium" is reached that is no significant changes
in total configurational energy occur with continued exchanges. With this calculation it is very important how the parameters defining the potentials of interactions are chosen. The pairwise bond-energies are taken to be dependent on the coordination of each atom and the cohesion energy, the energy to create a vacancy and the
Surface composition of alloys
195
surface energy are used to calculate the dependence of ~i3 on the coordination number n. Figure 14 shows an illustrative example of results of calculations performed in this way.
b m
ATOM Cu Pt
~
~
,.~~~
~
NUMBER 962 1444
ATOM
DISPERSION
Aq Pt
0.5766 0.1255
~~'%
TOTAL DISPERSION = 0.3059
NUMBER
576 1830
DISPERSION 0.9583 0.1005
TOTAL DISPERSION = 0.]05q
..~,~,~,~5.v
ATOM NUMBER Au Pt
569 1837
DISPERSION 0.9649 0.1018
TOTAL DISPERSION = 0.]059
.,Is
,g..4kQ~k~:S
_
~,~.~ figure 14 Illustration of platinum ensemble sizes in Pt-Group 11 cubo-octahedral particles. Light atoms represent platinum and dark atoms represent the Group 11 element (T=550K): (a) Cu-Pt; (b) Ag-Pt; (c) Au-Pt [40].
Analogous calculations for the rhodium-copper system which is one of low miscibility revealed that even at low copper contents the low coordination number sites on edges are completely occupied by copper atoms [40]. The Monte-Carlo technique has recently been applied to calculate surface segregation in interstitial alloys [99]. It should be obvious from the above description of various treatments of segregation problems how important it is to use the approximation or model most appropriate for the problem to be solved. Similarly, it is very important to choose the values of empirical parameters carefully. Analysis of the problem of correct parameters can also be found in the literature [ 10,41 ].
196
4.2.6
chapter 4
Metal-on-metal layers There are several reasons why we should be interested in these systems. First, it
might sometimes be convenient to study problems of metal-metal interaction with this approach, which can help analysis of interactions between metals in a quite well-defined way. Second, there is a possibility of creating materials which are new and have properties which otherwise cannot be observed with bulk metals. Thin metal-on-metal layers can be considered as a specific state of aggregation of matter and knowledge of their properties is important far beyond the field of catalysis. In discussing the problem of metal-on-metal layers we shall follow excellent reviews by Bauer and Rhead [42,43]. Additional information, in particular on catalytically interesting systems, can be found elsewhere [44]. At low coverages of a metal adsorbed on a substrate metal, the interaction of individual atoms with the metal surface can be analysed. The first crucial problem is that of the adsorption site. Table 2 [44] summarizes results on systems studied up to 1990. The expectation (see Chapter 1) that adsorbed atoms should prefer the highest coordination sites, i.e. those at hollows or ledges, is confirmed by the experiments. However, this does not mean that the conclusion is problem-free, since theoretical predictions are still sometimes at variance with experimental results, which in their turn also have some problems of their own. For example, the determination of the site position should be performed at such low coverages that effects of lateral interactions are excluded. The disturbing effects of the measuring technique should also be excluded, e.g. the effect of a high electric field in FIM (see chapter 2). However, by and large, the rule of highest coordination sites as preferred locations for adsorption is certainly fulfilled for most cases. Atoms of the 5d transition metals adsorbed on tungsten show the usual picture of the chemisorption binding strengths. There is a maximum in the binding energy for metals having about one half of the d-levels occupied; this is shown in figure 15 from the compilation made by Bauer [42]. We see again the picture which we saw in some other places in this book (figure 1 this chapter and figure 28 from chapter 1). From a certain coverage up, individual atoms feel not only the interaction with the substrate, but also the mutual lateral interaction in the layer. The latter is of complicated character, being composed of several components. The most important ones are the dipoledipole repulsive interactions which occur through-the-vacuum and the weaker are repulsive and attractive interactions which occur through-the-metal. The latter are not so strong that it would change very much of the character of the adsorbed metal (see the chapters 1-3), but they play a role in establishing of the structure of the adsorbed layer. Finally, when the adsorbed metal layer is dense, the metal-metal interaction, leading to the formation of two-dimensional or three-dimensional metal islands, becomes most important. Due to the interplay of the various interactions, dimers, trimers or islands of metal atoms can be observed on the substrate surface. When watching the migration of atoms on the surface
Surface composition of alloys
197
one can observe correlated movement of atoms at various distances from each other [45] and also the reverse process of dissociation of dimers. There is a correlation between the atomic chemisorption bond strength and the dissociation energy of dimers [42]; the higher the former, the lower the latter. table 2. Site geometries for adsorbed metal atoms at low coverage Adatom
Substrate
Site location
W
W(111)
lattice hollow
W
W(110)
3-fold hollow
Pd
W(110)
3-fold hollow
Re
W(110)
lattice hollow
p(lxl)Cd
Ti(0001)
3-fold fcc hollow
p(lxl)Co
Cu(lll)
3-fold symmetric hollow
Co
Cu(11 o)
In through, in 4fold hollow
p(lxl)Cu
W(100)
4-fold hollow
p(lxl)Cu
Ni(100)
4-fold hollow
p(lxl)Ni
Cu(lll)
3-fold fcc hollow
p(lxl)Fe
Cu(ll0)
fcc hollow
c(2x2)Hg
Ni(100)
bridge or 4-fold
p(2x2) and
Ni(100)
4-fold hollow
p(2x2)Te
Cu(lO0)
4-fold hollow
c(2x2)Pb
Cu(100)
4-fold hollow
c(2x2)Se
Ni(110)
hollow site
c(2x2)Na
Ni(100)
4-fold hollow
c(2x2)Na
AI(IO0)
4-fold hollow
c(4x2)Cs
Rh(100)
4-fold symmetric hollow
c(2x2)K
Au(110)
substitutional site
c(2x2)Te or Se
198
chapter 4
10-
8
-
9
,x
\\ ,,
\
i
6-
/..-
\\
2 -
0
Hf
Ta
W
Re
Os
Ir
Pt
Au
2
3
/-.
5
6
7
8
9
t
I
!
I
I
!
I
I
n 5 d ~
figure 15 Binding energies of 5d transition-metal atoms on various tungsten surfaces derived from critical voltages for field desorption: crosses, {110}; squares {100}, circles {112}; triangles {111}; open and closed symbols indicate different evaluation [12]. These curves can be compared with the results of the theory explained in chapter 1 and more specifically in figure 1 of this chapter.
When the coverage by the adsorbed metal is further increased, various structures develop in the adsorbed layers. When the adsorbate/adsorbent interaction is very strong, which is the case with two metals differing sufficiently in their electronegativities, i.e. sufficiently differing ionisation potentials of atoms or work functions of metals, ordered structures can arise. Ordering often occurs due to the combination of strong adsorption forces with strong lateral repulsion forces. The phase diagram then shows regions of commensurate and incommensurate structures, next to a disordered gas-like structure, etc. An example of such a phase diagram, expressed in terms of LEED patterns, is shown in figure 16 [46].
Surface composition of alloys
1000
199
Gd/W(110)
T'
/
1000K
800-
.
2[
/ !
600.
800-
400
0
0.2
0.4
0.6
0.8
2D -gas condensate
1.0
COVERAGE 8 (MONOLAYERS)-~
ads
figure 16 Left: limits of existence (phase diagram) of various LEED diffraction patterns observed upon adsorption of Gd on W(110) plane [46]. Right: phase diagram (schematically) as usually observed with noble metals adsorbed on refractory metals [12,43,46]
The other extreme is formed by systems with a relatively weak chemisorption bond strength, but sufficiently strong lateral interactions. The phase diagram in the region of temperatures between say 700 and 1300K and at low coverages (O = 0.1-0.2) is then very simple and just indicates the limits of existence of the two-dimensional gas and its mixture with two-dimensional condensate. This is schematically shown in figure 16, representing the situation with Group l l(Ib) metals on tungsten [43]. The reader can notice that the region of the disordered gas-like layer is limited at all practical temperatures of metal condensation to a very narrow region of surface coverage (O < 0.1). For catalysis, the most interesting layers are those of transition and noble metals. Bauer [42] compiled the results for W(ll0), the most dense plane, and for W(100) as substrates, and his conclusions are that platinum, palladium and nickel and the Group 11 metals tend first to form one-dimensional chains on the W(ll0) surface. At higher coverages, incommensurate close-packed structures of palladium, silver or gold are formed after a certain activation heating. On W(100) one has to distinguish two temperature regions. Below 800K either no superstructure at all or only a poorly developed p(2xl) structure is formed. The monolayer then appears always to be pseudomorphous. At temperatures higher than 800K, all adsorbates have a well ordered c(2x2) structure, which is best developed round | = 0.5. When the temperature is about 0.3-0.5 of the melting point of the adsorbate, the ordering which is observed is sometimes accompanied by other parallel processes: reconstruction of the surface layer and/or a vertical mixing at temperatures above 0.3 of the melting pont of the substrate, leading to the formation of alloy layers. The latter
200
chapter 4
phenomenon is annoying when properties of pure elemental layers are the objective of the study, but in catalytic work one often requires formation of surface alloy layers. They are very useful in catalytic studies on clean single-crystal-plane surfaces [47]. A review of the results obtained with these alloys has been published [44] and table 3 shows some results for catalytically interesting alloys. Valuable information on the electronic structure of metal-on-metal layers has been obtained by measuring the work function [42,44]; the form of the plots of work function as a function of the adsorbate metal coverage often resembles that in figure 17.
(
clean substrate
figure 17 Work function as a function of the surface coverage. Typical form of a curve found with adsorption of alkali metals on refractory or noble metals
~final I
0.5
I
I
1.5
(~ ads
This curve is typical of alkali metal adsorption on transition or noble metals, and its form has been explained as follows. First, the atoms are adsorbed with a pronounced shift of electrons from the original electron density spheres around alkali metal nuclei to the space between them and the nuclei of the surface atoms. This creates a dipole oriented with its positive charge to vacuum and a large decrease in work function is a consequence. When the adsorbed layer becomes more dense, more and more interactions occur laterally in the layer and the dipole-layer gets depolarized; the original shift is partially removed. Finally a continuous layer of the alkali metal is formed, and this has a similar dipole, and thus work function, to thicker alkali films. It is interesting to note that, in some other cases even when the electronegativity difference between the interesting metals is much less, the curve still has the form shown in figure 17, albeit with a less pronounced minimum. Such a curve has even been observed with tungsten atoms adsorbed on a W(110) surface, i.e. upon interaction of atoms with the same electronegativity. However, even in such cases some redistribution of electrons in the space around adsorbed atoms takes place, and the work function is reduced in consequence.
Surface composition of alloys
201
tabel 3. Well-ordered thin film alloys by interdiffusion Dosed metal
Substrate
LEED pattern
Comments on surface structure
Au
Cu(100)
c(2x2)
Like Cu3Au(100) surface with half of atoms in topmost layer Cu; all atoms in second layer are Cu. Atomic positions were determined by LEED
Pd
Cu(100)
c(2x2)
Same as above, like Cu3Pd (100). Atomic positions determined by LEED (111)
Sn
Pt(lll)
p(2x2)
Like Pt3Sn(111) surface with 25% of topmost layer Sn
Sn
Pt(lll)
(~/3x~/3) R30
1/3 of topmost layer is Sn
A1
Ru(O001)
(~/3x~/3) R30 ~
Looks like quasi-hexagonal plane of bulk A12Ru with 2/3 of topmost layer A1
Au or Cu
Pt(lll)
p(lxl)
Probably a quasi-random substitutional alloy (solid solution) with variable composition in topmost layer
Au
Pd(lll)
p(lxl)
Same as Au/Pt(111) above
Te
Cu(111)
(2~/3x~/3) R30 ~
Te substitutionally replaces every third atom in Cu(111) surface plane
Zr
Pt(lO0)
c(2x2)
Cu, Ag or Au
w(loo)
c(2x2)
Cu substitutionally replaces W in topmost W(100) plane
202
chapter 4
Smoluchowski [48] predicted many years ago that a rough surface should have a lower work function than a flat one of the same material. According to him, on rough surfaces electrons spillover into the ledges or troughs, making other more exposed atoms on the surface positively charged. Smoluchowski's model of rough surfaces is shown in figure 18. The same effect occurs when isolated atoms of tungsten are put on a single crystal plane of the same metal.
figure 18 A schematic illustration of the Smoluchowski effect [48] on rough surfaces. The signs + and- indicate the accumulation of the respective charges. The problems of metal-on-metal layers already discussed are closely related to the more general problem of growing a three dimensional metal or alloy layer on another substrate. Three mechanisms of growth can be distinguished: 1) Frank-van der Merwe mechanism: this is a layer by layer growth and it is typical for metal-on-metals. 2) Volmer-Weber mechanism: metals on a substrate form nuclei, which further grow into three dimensional crystals. This mechanism is typical for a metal-on-insulator system. 3) Stranski-Khrastanov mechanism: the substrate is first covered by a monolayer and further growth proceeds in the form of three dimensional crystals. This is typical for metal-onmetal growth, in particular for systems with limited solubility. This knowledge on growth mechanism is important for catalyst preparations (see chapter 7). Growth mechanism is influenced by the condition of growth: temperature, flux of atoms of the growing metal and the presence of additives which can influence the surface energy.
4.3
Surface segregation in catalytically interesting alloys
4.3.1. Nickel-copper alloys Nickel-copper alloys have the longest history of investigation, a history marked by re-appearing controversies and by development of better and better techniques for the study of this system. Available results cover almost all conceivable forms: evaporated alloy films, powders, foils, epitaxial layers and single crystal planes. Enthalpies of sublimation and surface energies of the components in question differ
Surface composition of alloys
203
so much that there should never have been much doubt about the occurence of segregation. However, it was indeed doubted. The first information on surface composition was obtained by a chemical probe: H 2 adsorption [49-51]. In these papers the conclusions of the rigid-band theory (see chapter 1) were ignored and it was believed (correctly!) that nickel and copper atoms retain their individual and characteristic catalytic and chemical properties in alloys. In other words, the number of hydrogen atoms adsorbed per unit surface area of alloys and under standard conditions should be equal or at least proportional to the number of nickel atoms per unit area, that is - should indicate very accurately the surface molar concentration of nickel. At the beginning, there were some problems and for example it had to be decided at which temperature and pressures one can expect the hydrogen just to cover all the nickel atoms. However, very similar results were obtained with both evaporated films and with alloy powders [49-51], indicating that in a broad range of nickel bulk composition (90-30%) its surface concentration should be between 20-10% and confirming the consistency of results. An obvious conclusion followed, in compliance with the theory as we now know it: there is a substantial surface segregation. However, up until about 1970, the rigid band theory (RBT) had still been considered by many to be valid and the sceptic doubted the results obtained by chemisorption probes. They said that the small extent of chemisorption on nickel-copper alloys was actually due to the fact that the electronic structure of nickel in nickel-copper alloys was different from that of pure nickel and not to there being less nickel in the surface. A completely filled d-band was predicted for alloys with XNi greater than 0.6 and alloys with XNi lower than that should also have had a different d-band occupancy from that of nickel, with all the resulting consequences for chemisorption which followed from the RBT and Dowden's theory of chemisorption. It was therefore extremely important to check the surface composition by an independent, physical method. About 1970 apparatus for Auger Electron Spectroscopy became available and was applied to the problem of segregation in the nickel-copper system. The first conclusion was surprising; no segregation was detected [52,53]. However, Helms [54] has shown that this was an artefact of the method used. Auger peaks in the region of 700-900 eV were used for analysis [52,53] and at these energies only about 10% of electrons are emitted from the first layer. The rest come from the second and much deeper layers: the mean free path )~ is about five layers and 64% of the emitted electrons come from this region. This makes it hardly possible to detect any segregation. However, if one uses, as Helms did, the Auger peaks round 100 eV, the first layer contributes about 50% to the total emitted electron current and surface sensitive analysis is indeed possible (see chapter 2). Several papers described the latter choice of surface sensitive AES peaks and they all confirmed the existence of a considerable copper segregation to the surface [55-57]. Results obtained by Kuijers [57] are shown in figure 19. Results obtained by Low Energy Ion Scattering (LEIS) which is a method sensitive to only the first layer are also shown in figure 19.
204
chapter 4
100
%Ni
80-
60
L0
20
first .
.
-
I
I
I
20
z.0
I
"
I
I
60 bulk
l~ye : "
I
I
80
I
I
100
% Ni
figure 19 Concentration of nickel in the first and second layer as a function of the bulk concentration [57]. With the concentration as shown in this figure the AES experimental results can be exactly reproduced. The three points are the concentrations determined experimentally at 500~ by Brongersma and Buck [55] by ion scattering spectroscopy.
These results have been confirmed qualitatively by XPS/UPS and quantitatively by several other methods such as time-of-flight mass spectrometry of field desorbed atoms (see chapter 3) or combined XPS/AES measurements [58]. In a situation that seemed to be well established, a paper by Sakurai et al. [59] appeared which again doubted the earlier conclusions and claimed that at the highest copper contents there is an opposite segregation: more nickel in the surface layer than in the bulk layers. However, when this region was subjected to new very carefully performed LEIS measurements [60], it appeared that the claim [59] was an artefact of the method. It was probably caused by preferential copper evaporation, the consequences of which were not completely removed by annealing. There are some further problems with nickel-copper alloys which have to be mentioned and commented on. First, carbon monoxide adsorption shows a higher surface nickel concentration than all other techniques; sometimes it looks as if there were no copper segregation at all [61]. However, this is most likely explained by gas-induced segregation of nickel. This is very easily observable by IR spectroscopy, as figure 20 shows [62].
Surface composition of alloys
CO-odsorption
009
- Ni-Cu(alloy/SiO
2 /
t = 5 min
205
I
I
figure 20 Infrared spectra of carbon monoxide adsorbed on Ni-Cu 50/50 alloy.
i
t = 120 m i n
007
-
005
-
003
-
Pco = 6.2 Torr; (---) after 5 min.; (- - -) after 120 min. [62].
:'/ I, 1
001
I
!
1
I
i
2200 2160 2120 2080 20/.0 2000 wavenumber
~ (cm -1)
We observe in figure 20 that the IR absorption at lower wave numbers, ascribed to carbon monoxide on nickel, slowly increased, and that ascribed to Cu-CO decreased, when the sample of a copper-nickel alloy powder remained longer time in the IR beam at ambient temperature. However, nickel can only appear in the uppermost layer after it comes into contact with carbon monoxide, while still in its subsurface position. This interaction with the subsurface layer and the subsequent nickel extraction is possibly responsible for the higher extent of adsorption of carbon monoxide than that of hydrogen. Another possibility is that atoms in the surface and subsurface layer dynamically interchange their positions and when it happens in the presence of carbon monoxide or another gas, nickel atoms can become trapped in the surface layer by adsorbing gas. Close inspection of all available results (compare [49-51] with [55-57]) reveals that hydrogen adsorption systematically indicates slightly higher concentrations of nickel in the surface than do the physical techniques. This can be caused by one or by combination of the following effects. a) There is hydrogen induced segregation of nickel to the surface [63]. b) There is a spillover of adsorbed hydrogen atoms from nickel to copper, causing a steady state coverage of copper by Hads [64]. c) Nickel does not adsorb hydrogen atoms alone but each isolated atom of nickel creates in its immediate neighbourhood mixed nickel-copper sites which can do it [65].
206
4.3.2
chapter 4
Palladium alloys Palladium is a very good catalyst for various hydrogenations and it continues to
attract much attention from catalytic chemists. At the time when the rigid band theory dominated the science, palladium seemed to be an ideal metal to confirm Dowden's theory of catalysis by metals. When alloyed with silver or gold, it changes from being paramagnetic to diamagnetic and this has been explained by the RBT as a consequence of electron transfer from silver or gold to palladium. Alloys with more than 60-70% silver or gold should be inactive, according to these theories. Other theories then RBT as well as the experiments show that the electronic structure of pure palladium and palladium in alloys differ (see chapters 1 and 3). However, when the bulk content of silver or gold rises above 70%, there should not be much palladium in the surface, as both Group 11 metals have much lower melting points than palladium. The question is then, when the alloys with more than 70% silver or gold are found to be inactive, is it because they have practically no palladium in alloys in the surface or is an alloy with only 30% palladium in the bulk inactive because its electronic structure is different from that of pure palladium? The developments in this field were almost as controversial as with nickel-copper alloys. Christman and Ertl [66] studied palladium-silver alloys by AES and concluded that there was
no
surface enrichment whatsoever. Wood and Wise observed an enrichment in
silver, but a somewhat negligible one [67]; however, this conclusion was premature. Research [68] performed later revealed that they had used calibration factors which were not correct and with correct values the experimental ratio of signals indicated actually a much higher segregation [68]. It is essential to realize that even the ratio of well-corrected signals still does not reflect surface composition correctly. As we showed in chapter 3, this is due to the fact that the largest contribution to the measured Auger signals of palladium and silver (about 80%) comes from the bulk and this part of the signal is not affected by segregation. When all of this is taken into account, we observe that silver segregation is actually very pronounced and not negligible as claimed [67,68]. See figure 21. Figure 21 shows also the results of the evaluation of the IR bands of adsorbed carbon monoxide [69], which is adsorbed on palladium in a multiple-coordinated or in a single-coordinated structure, in which the molecule is bound respectively either to two to four palladium atoms (bridged form) or to a single atom (linear form). Sachtler [69] evaluated from the band intensities the quantity q = Is/(I s + Im). (S = single coordinated, m = multiply). If one makes several very well acceptable assumptions such as, for example, that carbon monoxide is adsorbed linearly on isolated atoms and linearly + bridged on palladium clusters, and calculates the number of clusters from the random statistics, one can derive the surface composition of alloys [68,69]. Sachtler's calculations contained an unknown parameter ~,, which is the ratio of specific IR absorbances of the linear and the bridged (multicoordinated) forms of adsorbed carbon monoxide.
Surface composition of alloys
207
100 %Pd surf
60-
oO~
I
20, f.'~ o .,..,--,,
J
o.O.~
I
1
20
/~0 %
Pd
1
60
1
80
100
bulk
figure 21 Comparison of the surface composition of Pd-Ag alloys sintered in vacuo (- - -) and after interaction with CO (...) with the calculated surface composition according to infrared CO adsorption data ( - ) and the theoretical model of Sachtler (see text for details).
Figure 21 shows that with two reasonable extreme values of ~, = 1/5 and ~, = 1, curves are obtained between those corresponding to carbon monoxide induced resegregation (uppermost curve) and to vacuum segregation (lower curve). Since IR results must reflect some gas-induced segregation, they should lie somewhere between the extreme cases seen by AES. One can conclude that the IR results for carbon monoxide adsorption are, in principle, fully explained by the surface composition of palladium without any recourse to electronic structure effects. Experimental results on the palladium-gold system are also available [70], but they should be re-evaluated in a higher approximation. The system palladium-copper has also been studied and a modest segregation of copper to the surface found [67b,71]. For some applications as well as for fundamental studies, palladium-nickel alloys [72] are also interesting: nickel changes the electronic structure of palladium differently from silver. Surface enrichment is not pronounced and it seems that palladium accumulates in the surface.
208
4.3.3
chapter 4
Platinum alloys
Because of its application in three-way catalysts for treating exhaust from internal combustion engines, in naphtha reforming and in various hydrogenations, platinum is probably the catalytically most important transition metal. Many attempts have been made to improve the behaviour of platinum by adding a second metallic element, and papers on platinum alloys are therefore numerous.
Palladium-platinum alloys form a very suitable system for studying fundamental problems in determining surface composition. Many of the parameters necessary for a proper quantitative analysis and for theoretical predictions are known or can be calculated from quite reliable empirical expressions (surface energies, back-scattering factors, escape depths, etc.). Analysis of this system has been published [73]; from this paper figure 22 is reproduced showing the surface composition, compared here with results on benzene hydrogenation [74]. 100
/'
%Pt surf
/
/
/
/
/
/
i/ill'l / i/I/ /
60-
20
t'tl 10
I
0
/
I r
I
2o
I
z.o % Pt
o
1
60
t
I
80
I
figure 22 Comparison made by Kuijers [681 of catalytic data in relative units of benzene hydrogenation (full points) with calculated surface compositions by AES. -- Surface composition of alloy sintered in vacuo. - - Alloys treated with carbon monoxide or propane.
lO0
bulk
Platinum-gold alloys form at about 770K a system with a somewhat large region of coexistence of platinum rich and gold rich phases [75]. Platinum has the higher melting point and higher sublimation and surface energies, while the gold atom has a larger size. This is all favourable for segregation of gold to the surface, and indeed this has been reported [76] to occur. Platinum-silver alloys show a more pronounced miscibility gap, but silver enrichment can be expected at any bulk silver concentration. Platinum-copper alloys
Surface composition of alloys
209
have been discussed earlier in this chapter. It is found that copper is enriched in the surface layers, but to a lesser extent than the bond-breaking effects would predict. The effect of broken bonds is here counteracted by size effects which push the larger platinum atoms into the surface.
Platinum-nickel alloys appear to be a very interesting system to study [77-79]. If only bond breaking (enthalpy-) effects were considered, a pronounced nickel surface segregation would be predicted. However, size effects should certainly be more important here than with platinum-copper and atomic size effects, taken alone, would lead to platinum segregation. Experimental results reported in early papers indicated a pronounced platinum segregation, and this has led to the speculation that some additional effects such as ordering, surface relaxation or surface electronic structure effects would play some role [79]. Analysis by LEED supplied the following picture. An alloy 50-50 bulk composition, exposing the (111) plane as the surface, has almost 100% platinum in its outermost layer. Weigand et al. [80] established by ion scattering spectroscopy that, while on the (111) plane segregation of platinum occurs, on the (100) plane there is almost no segregation, while on the (110) plane the segregation of nickel is observed. This clearly demonstrates the subtle balance of various effects (enthalpy, size, and ordering effects) with one or another of them prevailing on each plane. It is important to note that, as van de Riel et al. [80] have established, nickel atoms do not cluster in the uppermost layer of the alloy, so that ordering effects are possible. Thanks to its attractiveness, the platinum-nickel system now belongs to the best studied group of alloys and it is very satisfying that various methods (AES, XPS, LEED, XRD) supply mutually consistent results [80], which moreover can be rationalized by existing theories [77,80]. With this system one can also learn what pitfalls lie in the way of an investigator on this field. Single crystals of nickel contain always contaminants (oxygen, sulfur, carbon) which are difficult to remove. To do that, a long chemical treatment in oxygen and hydrogen, and ion bombardment, is necessary. While oxygen induces nickel segregation to the surface, ion bombardment sputters nickel out of the crystal preferentially. Thus, a long and tedious cleaning must be followed by an equally long and tedious equilibration. However, with the sample in vacuum, contaminations from the deep layer can again appear on the surface, and this makes repeating of the cleaning unavoidable [77-80] and it is very difficult to find the point in the procedure at which the surface is clean and perfectly equilibrated.
Platinum-rhodium alloys are not only interesting from the scientific point of view, but they are also important practically, being the active component of the automotive three- way catalysts. Early investigations [81] showed that this is not an easy system to study. Enthalpy-based calculations would predict rhodium segregation, while surface energy differences would indicate platinum segregation. It has been found that at 8001400K segregation of platinum occurs, while below 800K the effect is suppressed or even
210
chapter 4
reversed. Since Rh203 is stable below 800K, a possible role of contamination-induced segregation was immediately suggested, and has been found experimentally [81]. However, claims that at low temperature the segregation of rhodium is real continued to appear [82]. The temperature-dependence of the segregation, as well as its unusual size and sign, led to speculation [83] that there must be also an entropy term involved in the driving force for surface segregation. The idea of entropy-driven segregation can easily be explained if we turn back to equation 6, describing the exchange of atoms between the bulk and the surface. In the steps which followed in the derivation, we assumed that the exchange changes only the enthalpy of the system, whereby the configurational entropy is assumed to be ideal. This leads to equation 7, or in another treatment of the problem, to the equations 25 and 27. Tacitly one assumes that there is a difference between the thermal entropy of the inner and of the outermost layers, but this difference is taken to be the same for all metals. Doubts have been expressed about that and it has been pointed out that the surface Debye temperatures, such as were known at that time, indicated that the thermal (i.e. vibrational) entropy of the surface platinum atoms should be higher than that of rhodium. That would explain segregation of platinum and also its temperature dependence [83]. A review summarising the earlier results appeared [84], formulating the problems, and interest in these alloys, both theoretical and experimental, has continued [85,86]. The surface enrichment of platinum in well-equilibrated alloys seems to be now firmly established, as well as the oscillatory character of the depth profile. A recent theoretical paper, using the so called tight-binding Ising model (i.e. a model comprising ordering effects), explained satisfactorily the sign and size of segregation and the oscillatory profile [87a]. Another theoretical paper applied the effective medium theory (density functional based method, see chapter 1) to calculate the atom-atom interactions in alloy clusters, from which calculations the equilibrium surface composition results. This paper too predicted, in good agreement with the available results, platinum enrichment in the surface [87b]. The platinum-rhodium system has also been investigated by the coherent potential theory (see chapter 1) and platinum enrichment was predicted [79]. An estimate was also made of the absolute size of the vibrational entropy term and it was concluded that effects arising from it can be neglected [87a]. While platinum-rhodium alloys are so important for automotive catalysts, platinumiridium alloys are one of the most important combinations, next to platinum-rhenium, for naphtha reforming catalysts. Their surface composition has therefore been also studied, and results are available for well defined and for practical supported catalysts [88-92]; as expected, the surface is enriched in platinum. Figure 23 shows [89] what composition of the first and second layer would fit the AES results obtained with equilibrated evaporated films.
Surface composition of alloys
211
100 %Ir
-
_----
_
figure 23 Calculated composition of the first (- - - ) and second (-.-.-) atomic layer for equilibrated PtIr alloys as a function of the bulk percentage of iridium. Such composition of two upmost layers would reproduce the experimentally found AES results [89].
6O
20
/ )
l
I
20
'
I
/.0 %
Ir
i
I
60
~
I
80
'
100
bulk
There is not enough known about the segregation of alloy components in the case of intermetallic compounds, such as the Pt-Sn, Ni-Sn or Ni-A1, Cu-A1 systems [93,94]. An interesting comparison has been made [94] of results of various methods applied to the alloys of the nominal composition of Pt0.75Sn0.25 and Ni0.75Sn0.25. These well-equilibrated alloys were prepared by classical metallurgical methods, broken under vacuum and then investigated by XPS, AES, LEIS and by carbon monoxide chemisorption. Index (a) in table 4 indicates results obtained from an unsophisticated analysis based on peak intensity ratios; (b) is based on calculations which take into account a correction for the bulk contribution; (c) refers to experiments with 2 KeV Ne + ions in LEIS. However the tin enrichment seen here with powders is likely to be a consequence of a preferential termination of small particles (prepared by grinding) by planes relatively rich in Sn and not by additional enrichments of individual planes [94b]. It is very likely from the literature available that this also holds true for other intermetallic components of platinum and nickel. Some authors speculate that platinum alloys can show such a surface segregation of that metal that the other component of the alloy would hardly be present in the outermost layer. Such an idea might explain the behaviour of alloys such as platinum with chronium, rhenium or iridium, etc. in catalytic reactions [95]. It is obvious that, in platinum-tin alloys, tin is easily visible (by AES and chemisorption) in the surface. (N.B. a close inspection of all catalytic results would reveal to the reader that the catalytic behaviour of platinum-rhenium or platinum-iridium also clearly demonstrates the presence of rhenium or iridium in the surface [96]).
2 12
chapter 4
table 4 Surface composition of Ni3Sn and Pt3Sn Method
Pt__q~Sn
XPS
Pto.68Sno.32
AES ALES, XPS
Pto.60Sno.no Pto.sSno.5
Ni0.22S%.78 Nio.lSno.9
LEIS CO chemisorption
Pto.43Sn0.57 Pto.n2Sno.58
Ni0.27S%.73 Nio.~sSno.85
Ruthenium, rhodium, osmium, iridium and platinum with silver or copper form systems of a low or a very limited solubility. It is then even more obvious that the low melting silver or copper accumulates on the surface. In these cases clustering of the adsorbed non-transition metals occurs [67b,97] and catalytic results indicate a preferential occupation of the sites which would otherwise be most active in hydrogenation of hydrocarbons. Conclusions. Investigations performed with single crystal planes or sintered films, representing respectively well or reasonably-well defined surfaces, have confirmed that the basic ideas expressed in the theoretical introduction to this chapter are correct. Various effects influencing surface segregation have been established. The majority of them are correlated with the enthalpy of sublimation or surface energy of the pure components, with atomic sizes and with gas-induced segregation. One has also to pay attention to the effects of ordering and surface relaxation or surface entropy. Theoretical considerations (section 4.2.3) predict, and experience confirms, that surface segregation can be absent in very small particles of supported alloys, which are the most important systems from the point of view of practical catalysis. Of course, the presence of a support brings about additional complications, such as the possibility of a lower reducibility of one of the metal precursors, the influence of the support on the morphology of the metal particles and, by that, on the surface segregation, and a trivial but tedious problem of metal separation (see chapter 7). If one wants to take advantage of using a support, i.e. to prepare small alloy particles, the heat treatment must be limited to the temperatures where no serious sintering occurs. Even then, homogeneity is by no means secured and inhomogeneous alloys can be formed with small clusters of one metal component in the other. For example, larger than equilibrium clusters of nickel are found on nickel-copper alloys, and for all these reasons many prefer to speak about metallic or
Surface composition of alloys
213
multimetallic catalysts, rather than about alloy catalysts. With practical catalysts having high dispersion, it is more important to check whether alloys are formed at all on the support than to worry too much about the surface segregation. In very small particles the segregation is not pronounced anyway, and often to neglect it would cause less error in the conclusions than neglecting the separation of metals and inhomogeneity in alloy composition in particles on supports. Supported metals and alloys are not easy to study by AES or XPS. When the samples are in a form of fine powders, pressing into a pellet can easily accumulate the support onto the external surface of the sample. Obviously, one should preferably work with pellets broken in situ. Samples containing insulators as supports are also difficult to investigate because of charging. One has to get rid of it by using a flux of opposite charges. Yet such measurements are still much desired, both for fundamental and for practical reasons. Pioneering work on this subject does exist [98] and there is probably more information still being kept confidential.
References
8
9 10 11 12a b
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Surface composition of alloys 30 31 32 33 34a b C
35 36 37 38 39 40 41 42
43 44 45
46 47 48 49 50 51 52 53
215
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54 55 56 57 58
59 60 61 62 63 64 65
66 67a b 68 69 70 71
72
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Surface composition of alloys 73 74 75
76
77
78 79 80
217
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81
82 83
E.van der Riel, S.Deckers, F.H.P.M.Habraken, A.Niehaus, Surf.Sci. 243 (1991) 49 Y.Gauthier, Y.Joly, R.Baudoing, J.Rundgren, Phys.Rev.B 31 (1985) 6216 R.Baudoing, Y.Gauthier, M.Lundberg, J.Rundgren, J.Phys.C (Solid State Phys.) 19 (1986) 2825 Y.Gauthier, R.Baudoing-Savois, J.Bugnard, V.Bardi, A.Atrei, Surf.Sci. 276 (1992) 1 F.W.Williams, G.C.Nelson, Appl.Surf.Sci. 3 (1979) 409 W.B.Williamson, H.S.Gandhi, P.Wynblatt, T.J.Truex, R.C.Ku, AICHE Symp. series 76 (201) (1980) 212 T.T.Tsong, D.M.Ren, M.Ahmat, Phys.Rev.B 38 (1988) 7428 M.Ahmad, T.T.Tsong, Surf.Sci. 149 (1985) L7 N.Sano, T.Sakurai, J.Phys.Coll. (36th Int.Field Emiss.Symp.) C8-321/C8-325 A.D.van Langeveld, J.W.Niemantsverdriet, J.Vac.Sci.Technol.A 5(4) (1987) 558;
218
84 85 86 87a b 88 89 90 91 92 93 94a b 95 96 97 98 99
chapter 4
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219
Chapter 5
P H Y S I C A L P R O P E R T I E S AND STRUCTURES OF S M A L L M E T A L AND A L L O Y PARTICLES
Theoretical background to size effects. In chapter 1 we have discussed how two atoms form a molecule (figure 13), several atoms a cluster, and finally, how many atoms form a metal. When the number of atoms is large, the energy levels form a quasicontinuous band of levels. Molecules and small clusters on the other hand form a discontinuous system of discrete levels. The work function of a metal differs from the ionisation potentials of atoms, molecules and small clusters made of atoms of the same element. Electronic and crystallographic structure and thermal properties of small clusters differ from those of large metal particles and single crystals. One can expect that all these differences should be somewhat reflected in catalytic behaviour. An extensive literature exists on this subject and recent reviews contain collections of almost all known results bearing on the problem [1,2,3]. Let us go now through the individual aspects of this problem systematically and consider in sequence: (a) electronic structure of small particles and its investigation by theory and experiment, (b) thermal behaviour of small particles, (c) the role of epitaxy, facetting and of exposure of special sites, and later, at the proper place, (d) the side effects accompanying the catalytic reactions. All these aspects contribute to the final sometimes complicated picture of how the activity of a metal varies with particle size. Particle size effects are of great interest for those who investigate chemisorptive and catalytic properties of alloys, first because alloy catalysts used in practice always consist of small particles, and second because a metal A in alloy AB can form small clusters and one therefore needs to know what effects, in principle, can be related to the cluster size. Of course, in the sense of electronic structure a naked cluster of A differs from a cluster of A in an alloy AB, but the similarities are still not negligible. Very often the presence of B in supported catalysts diminishes the particle size of the supported metal A, and if the metal particle size is important for the reaction of interest, diminution of particles can simulate effects of alloying. We shall turn to these problems below [5,7].
5.1
The electronic structure of small free metal particles
220
chapter 5
The following are the principal features which we need to consider: energy band structure, surface dipole and electronic structures of atoms in different positions on the surface. At what particle size can we expect measurable differences in the electronic structure of small particles from that of bulk metals? We can suspect that effects should be observed when the spacing between energy levels is larger than kT at the temperature of observation. At room temperature kT is 2.5x10 -2 eV, which means that with an energy band width of 10 eV the effects can be expected with particles of approximately 400 atoms, corresponding for nickel to a diameter of about 2 nm [1,2]. As we shall see below, a more exact approach leads to about the same conclusion. However, notice that we are speaking above only of effects related to the discreteness of the energy band, and there are many more effects related to particle size. The most direct approach to the problem mentioned above would be to take a metal atom M and consider the formation and properties of M 2, M 3, M4 ... MN and examine how the properties such as binding energy per atom, ionisation potential, density of states, etc. behave as a function of N. However, it is not an easy task. First, one observes that the energy spectrum of an M cluster depends not only on the size (N) but also on the method used to perform the calculations. Unfortunately, a strong correlation exists: the larger the size, the more crude is the approximation that has to be used.
0-
Cu13
Cu 13
Cu 19
HARTREE-
Xa-SW
EXTENDED
FOCK
HUCKEL
.,.,,
.c_ t/3 LIJ (.9 n," LIJ Z W
-5-
~
~
.._1
rr
O
s -15-
s ~
s
s.d
eg ~st2g
-10 -
I--rn
s
~ s
figure 1 Comparison of orbital energies for copper clusters as determined by the Hartree-Fock [41, X~-SW and extended Hiickel methods [5].
s -d
~ s
5 e g
t2g -20
These points are illustrated in figure 1 [3], where we see the energy spectrum of Cu13 and CUl9 particles, as calculated by different quantum chemical methods: HartreeFock [4], X~-scattered wave techniques [5] and Extended-Htickel-Theory (EHT) [6]. The difference between the results is obvious, and is quite representative. Bartel et al. [7] made calculations by X~-SW and EHT techniques for Ir6 and Pt 6 clusters with similar results.
Physical properties and structures of small metal and alloy particles
221
However, they showed that, with an acceptable adjustment of parameters in the EHT, a good coincidence between theoretical spectra could be achieved. Important features to note from theoretical calculations are: the ionisation potential of small copper particles, 5-6 eV, is smaller than that of the free atom (7.24 eV), but higher than the work function of copper metal (4.5-5 eV, according to the crystallographic plane). The spectrum is clearly very discrete, but two methods (X~-SW, EHT) predict correctly that the narrow band of d-levels is inside a broad band of s-levels. Ionisation potentials (I.P.) of small naked (i.e. without ligands) and unsupported clusters of iron have been determined experimentally and figure 2 shows the results obtained [8]. We can see that the trend predicted by theory is experimentally observed; the theory also predicts small oscillations around a smooth background function. These have also been observed also for other metals [9]. 8 -
9A t o m
.'I
mlmm mlmmmml~ml Work
i
i
i
figure 2 The ionization potentials of iron clusters determined from the photoionization spectra plotted vs cluster size. The work function of bulk iron is also indicated [8].
i
j
i
i
i
I
i
I
I
I
I
"'"Ul
Function i
I
!
i
I
o
Iron
I
'
Cluster
Size
(Atoms)
The convergence of the properties with increasing number of atoms in the cluster has been theoretically analyzed albeit by a theory in a lower approximation. For example, for clusters with bcc structure the binding energy E varies [10] as: Eb(N) = Eb(~) ( 1 - 1,079N -1/3)
(1)
while the band width BW varies as
BW(N) =BW(oo) ( 1 - 5,875 N -2/3)
(2)
A cubo-octahedron with 13 atoms already shows some of the features of bulk metals. For example, figure 3 shows the density of state curves for a Nil3 cluster and for bulk nickel; for chemisorption the most important part of the density of state functions is that close to Fermi Energy (E - 0 in fig.3), and here the similarity is quite marked; Pt13 and Pdl3 clusters offer a very similar picture.
222
chapter 5
N(E)
figure 3 Comparison of the majority-spin electronic density of states, N(E) for bulk Ni(points) and an Nil3 cluster (full line) [51.
a.Ll.
oO
~176
o,
~ I
0.4
I
0.2
I
0.0
E (Ry)
The behaviour of small copper particles has been analyzed in greater detail [5]. For example, the form of the highest occupied and the lowest unoccupied orbitals has been identified and an estimate made of the charge distribution within the clusters 9 According to this estimate, the surface bears a positive charge of 0.08-0.04 electrons/atom. It is thus possible that the effective surface dipole of small particles is different in its sign and size from that of bulk metals 9 The problems of the spatial form of the 3d nickel orbitals in small particles and in bulk metals, and of emission properties, have been addressed [11]. It was concluded that the differences are somewhat limited and manifest themselves mainly in a slight narrowing of the d-band and small changes in satellite structure. With 5d metals the band narrowing accompanying particle size decrease may be more pronounced than with the 3d and 4d metals 9 An important probe into the electronic structure of metals is photo-electron spectroscopy (XPS, UPS, see chapters 2 and 3). Small particles show an imperfect screening (lower by about 10% than bulk metals) which can lead to binding energy shifts mainly of core level electrons by 0.5-1.5 eV [12]. Let us now take a brief look at other electronic properties which depend on metal particle size [13]; a particular good review on various metals has been published by Halperin [ 1]. The first effect to be mentioned is the colour of small metal particles. It is known that metal colloids are coloured and this phenomenon is explained nowadays by the theory of plasma oscillations. Imagine a small particle embedded in a medium of different dielectric constant (a liquid, a zeolite, etc.). When placed in oscillating electric field, such as visible light, such a particle develops an oscillating surface charge which, in its turn, imparts oscillations to the electron density in the particle. We then observe collective plasma-oscillations. The simplest theory relates the width of the absorption band to the restriction of the mean free path of electrons in small particles [ 1,13,14].
Physical properties and structures of small metal and alloy particles
223
The magnetic susceptibility of conduction electrons can be monitored by NMR. While the chemical shift (i.e. a shift in the position of an NMR peak with regard to tetramethylsilane, or other standard compound) is of the order of 10 ppm in molecules, the shift from a diamagnetic platinum H2Pt(OH)6 to bulk platinum is 4.9x103 ppm. This large Knight shift is with Pt, Sn, Cu, A1, as Halperin [1] concludes particle size dependent, being smaller the smaller the diameter of the particle [15]. With platinum, as Rhodes et al. claim [15], the changes are more complicated. Different electronic specific heats and Electron Spin Resonance (ESR) spectra should be expected for the smallest particles. Differences are predicted for particles having odd and even numbers of atoms and these are found experimentally [1,13]. For example, an extensive literature already exists for lithium clusters, both on theoretical predictions of the most stable form of clusters [16] and on the ESR spectra [17,18]. The latter provide implicit information on the form of clusters and hybridisation of various orbitals, as shown for Li 7 clusters [18]. We shall turn to some other features determined by ESR below. Direct measurements of magnetic properties and determination of magnetic susceptibilities reveal interesting effects. For example, with platinum particles, the Curie constant was found to be inversely proportional to the average diameter of the Pt particles [19]. This is ascribed by Halperin [1] to the surface atoms having different magnetic moments to those of the bulk. Palladium has been studied in detail by Ladas, Dalla Betta and Boudart [20], who used to advantage the fact that dissolving hydrogen in palladium, causes an expansion of the lattice spacing and changes the d-electron population from d 9'7 to d 1~ Subtracting the susceptibilities measured before and after saturation by hydrogen allows determination of the d-electron susceptibility. Again, the susceptibility was a function of the metal dispersion, as with platinum [19]. Vanadium particles varying in size from 9 to 30 nm also show magnetic susceptibilities varying inversely with diameter [21]. In an ideal situation, the ESR method would represent the most unambiguous approach to the determination of magnetic susceptibility, which is proportional to the integral of the ESR signal. The method should be also helpful in establishing quantum size-effects, because it can in principle easily distinguish between conduction electrons and those localized on atoms. This statement by Halperin [1] is certainly true, but the results reviewed by him showed that for the catalytically most interesting metals, such as platinum and silver, definitive conclusions cannot yet me drawn. The reason seems to be that when small particles are stablized on a support, two effects immediately influence the measurements: (i) the inherent non-uniformity of metal particle size in these materials, (ii) minute quantities of impurities such as Fe 3§ in the supports of practical catalysts.
224
5.2
chapter 5
Equilibrium shape and thermal properties of small metal particles The geometry of clusters with one, two or three atoms seems to be undisputable.
An Mn-cluster has more possibilities: rhombic, square and tetrahedron structure. Theoretical calculations [22] in quite high approximations (ab initio, configurational interaction, CI calculations) have produced relevant results with interesting differences: Li 4 and Na4 have the lowest singlet state energy in a rhombic form, but the optimal geometry for the lowest triplet state is a square. However, this square is bent with Li 4. A tetrahedron, which one would intuitively favour, exhibits a biradical character and does not yield a minimum on the singlet energy surface. M 5, M 6 and M 7 clusters offer even a greater variety of possibilities. Again, by the ab initio CI calculation it was discovered that clusters which can be considered as sections of a fcc crystal have a relatively high binding energy per atom. However, with Li 6 a pentagonal pyramid (see figure 4, below) and with Li 7 a pentagonal bi-pyramid appeared to be even more stable [ 16]. The calculations [ 16] thus confirmed the stability of the often postulated pentagonal arrangement (see below), but at the same time also showed that predictions made by more simple theory, or on only intuitive grounds, must be used with caution. One may consider a cubooctahedron as the most likely structure for the M13 clusters. Here a so-called full shell structure is created round the central atom; it always contains 10n 2 + 2 atoms. The cubo-octahedron, with some other possible structures of small crystals [23], is shown in figure 4. Structures 2 and 3 in figure 4 can be described as a truncated octahedron. Structure 4 is a cubooctahedron with (lll)-triangular faces. Structures 5 (decahedron) and 6 (icosahedron) show elements of five-fold symmetry, and have claimed to be seen by various experimentalists (see below). According to the fullshell formula, the octahedra contain 13, 55, 147, 309, 561 ... atoms and the full shell icosahedra contain exactly the same numbers of atoms. For more details on the geometrical form of small supported metal particles the reader is referred to the literature [23,24]. The shapes of metal clusters stabilized by ligands show even a greater variety than the free and unsupported naked clusters [25]; this is also true for compound clusters [26]. Bonacic-Koutecky et al. have summarized in a very exhaustive review all the problems of the theoretical description of small Group Ia, Ib and IIa metal clusters [1] and have made predictions as to which structures are the most stable ones. The form of supported small metal particles can change when the particles are brought into contact with chemisorbing gases (see also chapter 4) which can change the surface energy of different geometrical arrangements in different ways [27]. When the adsorbates can produce species such as atomic carbon, oxygen or sulfur, a penetration of these atoms into the structure can also induce changes in the habit of the metal crystallites or can even change the size of particles by causing their disintegration.
Physical properties and structures of small metal and alloy particles
!
M6
>.jw M[}- cubooctahedron (truncated octahedron)
(~oo)
225
figure 4 Various possible forms of small metal particles 1) M 6, pentagonal pyramid 2) M 13-cubooctahedron, with indicated position of atoms in a cube (see also 4) 3) cubooctahedron (Mx) with (111) hexagonal faces 4) cubooctahedron (Mx) with (111) trianular faces 5) decahedron 6) icosahedron
(tit)
5
6
However, even in vacuum small metal particles cannot be considered as really rigid; available information on the thermal behaviour of small particles supports this statement. Halperin [1] has pointed out the main variations to be expected when the size of particles is diminished: the surface vibration modes must shift to lower frequency and the restricted size truncates the phonon spectrum at the large wave vector (~,-1)side. As a consequence, an enhanced heat capacity is observed with small particles (see e.g. [1,2831], and a lower Debye temperature. This indicates a weaker binding and increased vibrational amplitudes of metal atoms forming the surface. The possibilities of enhanced movement are very important from the point of view of catalysis; they make gas-induced segregation in alloys easily possible (see chapter 4) as well as partial shape-reconstruction [27]; they make the sintering process by particle coalescence easier, and last but not least they intervene in an important way in several measurements used for catalyst characterization, such as EXAFS ( see chapter 2). Clausen et al. [32] have performed a very extended study by molecular dynamics simulation of copper clusters with sizes between 17 nm and 7.0 nm, i.e. with 256 up to 17.000 atoms. It appeared that the atomic motion is not harmonic, even at low temperatures, and this causes asymmetric distributions of distances in pairs of atoms. In other words, many atoms at or near the surface have much greater vibrational amplitudes than those inside the particles. If a model EXAFS calculation is performed on such vibrating systems, the number of nearest neighbours N c derived from the EXAFS intensities (see chapter 2) will be surprisingly low. This when found experimentally is usually explained
226
chapter 5
by the existence of small particles, because small particles have a low coordination number N c. Indeed, metal particle sizes derived by EXAFS were always suspiciously small [32,33]. On the basis of this analysis [32] one would also tend to doubt the earlier statements claiming that small particles in for example zeolites show a contraction of some interatomic distances. The error involved in neglecting the thermal movement is by no means negligible [32]. For catalysts for which classical EXAFS analysis gave sizes of 1,01,1 nm, the new approach [32] supplies values 2,4-3,4 nm, in a much better agreement with the XRD values (3,0-3,9 nm). Marcus and Tsai showed in an earlier paper [34] that the mean-square-relative-displacement,
appearing in the Debye-Waller factor of the
equations used for EXAFS analysis, is a simple function of temperature and they established that the factor f, in equation 3 below, is different for atoms in the nearest and the next nearest shells: o2(/) _ o2(0) =f . T312
(3)
The analysis [34] has been made for massive metals (diluted copper in titanium), but it has consequences for analysis of small particles, too. Speaking about thermal properties of small particles, we have to add a few words about sintering of metals dispersed on supports. There are several mechanisms of sintering and there are several ways of diminishing its effect on the catalysts. Sintering is usually thought to proceed by one or a combination of the two of the following mechanisms [35,36]: 1) migration of small crystals and their coalescence upon collision; 2) Oswald ripening: small particles release atoms more easily than large ones and migrating atoms are captured by large particles. A mathematical description and a complete analysis of various ways of sintering can be found in the literature [37]. Migration of small particles (mechanism 1) can be prevented by anchoring metal particles to the support by a chemical glue. This is usually achieved by ions which adhere firmly to or are built in the structure of the support. Weyl [38] has pointed to the effects of nucleation catalysis, for example, to the effect which Pb 2+ ions have on precipitation of BaSO 4. If lead ion is a part of a support lattice, BaSO 4 crystallizes on it at lower supersaturation and adheres better to it. A similar effect is involved in the old practice of making well-adhering silver mirrors: the glass surface is flushed first by a solution containing tin or other higher valency ions and then silver is deposited by reduction of silver nitrate. The effect of anchoring is mentioned also in some recent papers [39]. Our reader has to realize that, with some alloy catalysts, one of the components can play the role of an anchor for the other component particle. For example, in the case of platinum-rhenium naphtha reforming catalyst (see chapter 13), it is very likely that
Physical properties and structures of small metal and alloy particles
227
some of the rhenium ions are built in the A1203 surface probably as Re(IV) and play a role in thermal stabilization of the metal particles. A similar situation could also exist with platinum-iridium alloys. Components which are not reduced easily, and are moreover stabilized against reduction by forming mixed oxides with the support, quite frequently function as anchors (nickel, cobalt, iron, copper) [38,39].
5.3
Adsorption sites on small metal particles The obvious effect of preparing metals as particles of small size is the important
increase in the metal surface area. Model calculations on a homogeneous system of particles Of a uniform size produce the results shown in Table 1. The fraction of atoms at the surface is frequently termed the dispersion D, but in the more recent literature the percentage or fraction exposed FE is also used [1,41-43]. For larger particles, simple mathematical expressions suggested by Bond can be used to obtain quick information [42]. Cubo-octahedral particles expose valley sites of trigonal symmetry on the (111) faces and of tetragonal symmetry on the (100) faces. Next to it, there are also sites on corners a total of N c and edges (Ne). Simple calculations by Bond [42] illustrate how the different types of site are represented in the surface (total number Ns) and how the fraction exposed Ns/Nto t varies with the particle size r s. A number of atoms in the planes, Np,
defined as Np = Ns - (N c + Ne) increase with the particle size; however, the fractions Ns/Nto t (equal to D), Ne/N s and Nc/N ~ decrease with the particle size of the complete cubo-
octahedra. It is likely that small particles try to avoid formation of sharp edges with highly unsaturated atoms and tend to become more sphere-like, as in incomplete octahedra. This aspect should lead to the appearence of special sites, absent on both very large and very small particles. For example, it has been argued [44] that small particles should expose amongst others the so-called Bs-sites, where (100) and (111) planes cross each other. One of such sites ((100)-step), on which an adsorbed species is coordinated by five atoms, is shown in figure 5. [3:.
figure 5 fcc structure (001) surface plane, the fivecoordinated position (called B s site) is indicated.
228
chapter 5
table 1 Model calculations based on spheres of uniform size [40] metal Ni
Pd
Pt
d(nm)
number of atoms p. part.
area (m 2 g-l)
dispersion*
2
381
336
0.554
4
3045
168
0.276
6
10273
112
0.184
8
24364
84
0.138
10
47636
67
0.110
2
285
253
0.611
4
2279
125
0.304
6
7687
83
0.203
8
18231
62
0.152
10
35646
50
0.121
2
227
139
0.617
4
2219
70
0.308
6
7483
47
0.205
8
17748
35
0.154
10
34702
28
0.123
(* fraction exposed of all atoms)
The IR spectra of physically adsorbed nitrogen can probably be used to count Bs-sites [44];
they may also be important for carbon monoxide adsorption on nickel [45], since
the population of the multiply-coordinated CO molecules could be correlated with the presence of the Bs-sites. It seems on the other hand that multiply-coordinated CO was much less represented on palladium in the form of very small particles, although it is abundant on the Pd(111) surface planes [46]. This last paper pointed to another fact which may be of some relevance to catalytic reactions: while on smooth surfaces CO molecules shift between various sites very easily, for example, under influence of increasing/decreasing surface coverage, on small particles is this type of mobility much more restricted. The movement of molecules between various sites or positions on the surface is quite common [47] and its presence on large particles and absence on small particles could play a role also in catalytic reactions. Moreover, the presence of the other component in the alloy surface could influence these mutations, too.
Physical properties and structures of small metal and alloy particles
5.4
229
Reactivity of small metal particles As we have seen above, a platinum particle with a diameter of 2 nm has 220-230
atoms. Platinum catalysts with silica as a support can be quite easily prepared with this dispersion; with A1203 as a support the particles can be even smaller, by a factor of two or three. It is difficult to go beyond this, and supported metal particles of guaranteed still smaller size are prepared mainly in cages of zeolites. The smallest metal particles are prepared as naked, i.e. ligand-free, clusters in special UHV/molecular beam equipment (see below) or as condensed on the field emission tip (see chapter 7, section 3). Let us start the discussion with the reactivity of the smallest metal particles, containing between 10-30 atoms, according to the metal. Obviously, these sizes of particle overlap with sizes common with metal-in-zeolite systems. An example of the experimental set up for the production of adsorbate-free or adsorbate-covered clusters is shown in figure 6 [48].
J
Ion Detector
'D
I
Time-Of-Flight[ I I,,---Time-Of-FligMoss ht Spectrometer Tube~ :, l ctor L !I~ ~iilSVociumChcl~be r [ Rei
UVLoser for / G r e e n Loser for Photoionizotion Voporizotion
figure 6 Left: pulsed cluster beam apparatus:
ReogentInjection
Metal Torget Rod
~Voporizotion
Loser
Right: details of cluster source and reactor.
When the reactivity of clusters is being studied, gases are introduced at the 'reagent injection' point. Mass spectrometric analysis detecting the particles with attached adsorbate species reveals how the adsorption activity varies with particle size. One can determine the rate of adsorption and the maximum adsorption capacity, as well as the stability of various clusters. Very interesting results have been obtained by this technique. Abundant information exists concerning the interaction of hydrogen [48,49] with various metals: the measurements have been made with V, Fe, Co, Ni, Nb, Rh, Pd, Ta and
230
chapter 5
Pt. There are differences in detail and also between the results of various workers, but we observe one common feature [48]: the reactivity is size-dependent, although it varies nonmonotonically with particle size. Reactivity follows the variation of ionisation potential of clusters, being higher for clusters of smaller ionisation potential (see in figure 7).
>
5.0
100 o
>,
~
10
tn ._ c
u_ c
5.5
~oo
w
o ~ I
o rn
oi~
1.0
I~[oo
0.1
,I
6.0
0.01
_
I
6.5t.tJ
0
r~
L_ <
--
.-
5 o
,I
rr
,
1'0
Fe A t o m s
,
per
i
20
,
figure 7 Ionization potential of iron clusters (vertical lines) and relative reaction rate measurement of iron clusters with dihydrogen (open circles), both as a function of particle size [48].
30
Cluster
An attractive idea is that this could be related to the activation of hydrogen by a shift of electrons into its antibonding orbitals (see chapter 1, on the mechanism of dissociation). When the particle contains more than about 25 atoms, the reactivity becomes more or less independent on size [48]. Metals differ in their sensitivity to the particle size effect on the reactivity towards the first hydrogen molecule [48,49]. For example, rhodium and platinum are quite insensitive, while iron, cobalt, niobium and vanadium are quite sensitive. The total amount of hydrogen which can be attached to a metal particle varies with the metal. Iron seems to adsorb at most only one hydrogen per surface atom, while palladium in the form of clusters with four to ten atoms can bind two to three atoms of hydrogen per palladium atom, with the smallest particles showing the largest relative capacity. Although we have to be careful in transferring these conclusions to the field of small supported metal and alloy particles, the observation that the very small particles can show unexpected stoichiometry is very relevant for experimentalists who characterize metal particle size by chemisorption of hydrogen or carbon monoxide (see chapter 7, section 4). The interaction of small naked clusters with methane is very interesting, because in several aspects it is surprisingly different from that of small supported metal particles. For example, methane is found to interact dissociatively with neutral and with positivelycharged platinum clusters and with palladium, but it does not do so with iron, niobium or rhodium. Positively charging a platinum cluster changes some details of the curve of variations of reactivity with cluster size [48]. Although the strengths of C-H and H-H bonds do not differ much, the activity pattern of clusters of various sizes differs. While the
Physical properties and structures of small metal and alloy particles
231
smallest clusters of iron seem to be unreactive towards hydrogen, the smallest clusters of platinum are the most reactive ones. However, dehydrogenation of benzene on niobium or vanadium clusters seems to require four to five metal atoms to occur. Obviously further experimental and theoretical research will be needed to elucidate these details. In the meantime the fascination with this newly arising field will continue [49,50]. Early research concerning chemisorption and catalysis on metals relied much on the information obtained by one of the first surface science methods available to chemists, namely, work function measurement (see chapter 3). This revealed that, on most metals with guaranteed clean surfaces, hydrogen when adsorbed extracts electrons from the vacuum electronic surface. Ammonia did the opposite. It was important to learn that this also holds for very small metal clusters [48,51]; their ionisation potentials vary in consequence of adsorption on them in the same directions as the work functions of bulk metals do. Boudart and Hwang [52] have observed that formation of hydrides of palladium can be suppressed by diminishing the particle size. An attractive explanation of these results was that dissolution of hydrogen requires that palladium atoms surround dissolved hydrogen atom from all directions, since alloying with silver also suppresses hydride formation too. This could be a very satisfying explanation, if there were no contradictions with the results of the study with naked palladium clusters [48], a contradiction which has not yet been explained. By using EXAFS, it is in principle possible to determine the average number of nearest neighbour atoms and the information can be converted, by using calculations on models, into information on the average particle size and t h e dispersion ('fraction exposed'). Small particles of iridium and rhodium can adsorb more than one hydrogen atom per metal atom (H/M > 1) [53]. When we consider the results of a very recent analysis of model calculations [32], we can see that the H/M > 1 values are actually found for quite large particles. Such stoichiometry is, according to the results with naked clusters, perhaps possible, but chemisorption measurements on supported metal catalysts are bedevilled by the possibility that some of the hydrogen taken up may be used for purposes other than chemisorption on the metal (e.g. reduction of cations, hydrogen spillover). Information on the distribution of adsorption sites according to the activation energy of desorption, possibly reflecting the heats of adsorption, can be obtained by temperature-programmed desorption (TPD). Schats [54] has prepared Ni/SiO2 catalysts with widely varying particle sizes and has monitored TPD of hydrogen from these catalysts; the results are shown in figure 8 [54,55]. When normalized to the highest rates of hydrogen desorption, the results for very different catalysts do not differ in a pronounced way; actually, they are surprisingly similar. This implies that the large and small particles expose on their surfaces more or less the same sites for hydrogen adsorption.
232
chapter 5
0.80
0.40
0.00
-
300
~bo
fi~ure 8 J-o
sbo
6bo
|
600
K TEMP.
Thermal programmed desorption profiles of hydrogen obtained with different Ni catalysts with particle sizes varying from 1 to 4 nm. Curves are normalized at their maximum values and plotted in arbitrary units [54,55].
An alternative but a most convenient way of obtaining information on adsorption bond strength is adsorption calorimetry. Such measurements have also been made [56] and it was established that the integral heat of adsorption of hydrogen on palladium was sizeindependent for sizes of 3-1000 nm, but was twice as large for still smaller Pd particles. The same has been found for carbon monoxide [56]. For rhodium a comparison has been made [57] of Rh(111) single crystal plane, and for rhodium in the form of FEM tip filament, evaporated film and carefully cleaned powder. All samples showed very similar heats of hydrogen adsorption in the surface coverage region from | = 0 up to 0.3. A great similarity was still there even at O = 0.6. This implies that also for rhodium different sites do not differ much in their bonding strength with regard to hydrogen. A detailed study has been undertaken by Altman and Gorte [57]. They found that (i) the difference between single crystals and polycrystalline macro- and microcrystals is much less pronounced for hydrogen than for carbon monoxide; (ii) the difference between samples of various size was more important with platinum than with rhodium and; (iii) polycrystalline materials most probably expose sites corresponding to low index planes next to sites on highly stepped surfaces. Another measure of reactivity is the activity of various particles in carbon monoxide dissociation. However, we shall postpone that discussion to a later section where the closely-related phenomenon of methanation and higher hydrocarbon synthesis will be discussed. Small metal particles (platinum, palladium) on supports seem to be oxidized much more easily than metal particles of larger size, confirming the higher reactivity of atoms in
233
Physical properties and structures of small metal and alloy particles
small particles. EXAFS and XRD results are quite clear on this point [58]. The differences in the reactivity of small supported metal particles towards hydrocarbons can manifest themselves not only in the global way, by for example differences in the total number of molecules adsorbed, but also in a more subtle way. We have seen in chapter 1 that hydrocarbons can be bound to the surface of metals by single (M-C) or by double and multiple bonds (M=C). Exchange of hydrogen atoms in methane for deuterium can supply information on the relative abundance on surfaces of single and multiple bonds [55]; we shall discuss it in more detail in chapter 10. Results of such experiments are summarized in figure 9.
multiple t bonding CD/.,
/
s
/
/ / /
CH3D 2.5-
/ / I //
////
1.5-
0.5-
.IlllI'.......1 1
2
5
~Ni
.
.
.
.
<-Ir
.
10
.
.
20
d. n m
figure 9 Propensity of metals (characterized by the CD4/CH3D ratio measured at standard conditions) to form metal-to-carbon multiple bonds, as a function of particle size for platinum, iridium and nickel.
The results in figure 9 show that, with three metals studied, platinum, iridium and nickel chemisorption behaviour is determined by the particle size; the smaller particles tending to form fewer of the multiple metal-carbon bonds than the flat surfaces. If the multiple bond were in one or another way related to the simultaneous engagement of several metal surface atoms, in other words, if an ensemble of active metal atoms of a certain size were required for multiple bonding to occur, we would conclude that due to the geometry of small particles and possibly also due their different thermal vibrational behaviour, ensembles of the required size are less abundant on small particles. This would be an acceptable conclusion, in principle. Thus, the particle size effect on the chemisorption bond has an obvious effect on the catalytic activity and selectivity [55,57,58].
234
chapter 5
A very interesting example of reactivity being dependent on particle size is found with the photographic process. Photons strike finely dispersed silver halide grains in the photographic film and create a few silver atoms per grain. This so-called latent image centre triggers a fast reduction of the whole grain in the developer. It now appears that a minimum cluster size of at least four silver atoms is required to act as a catalyst for the grain reduction. The reduction of metal halides in the so-called electrode-less plating seems to be similar [59]. Small supported metal particles, in particular of sizes which are common with metal-in-zeolite systems, are also very reactive in carbonyl formation. Somewhat unusual stoichiometric carbonyls and subcarbonyls can be reversibly formed at relatively low pressures of carbon monoxide [60,61]. This too confirms that the reactivity of very small particles is high; however this need not mean that in all cases very small particles bind everything more strongly.
Another aspect of particle size effects to consider is that
particle size can influence the chemisorption modes leading to side reactions such as carbon deposition in hydrocarbon reactions [55,56,62,63].
5.5
Polarization and charging of small metal particles by a support A physicist, coming new to the field of catalysis, would certainly be surprised that
so many chemists ascribe almost mystical properties to metal particles, just because they are very small and placed on a support. We have already discussed some electronic properties of small support-free particles, and their experimentally established reactivities. Let us now turn our attention to the
possible effects
of support on the properties of metals.
Van Santen et al. have performed calculations on Ir4 clusters [64]. In these papers a tetrahedron is ascribed to Ir4 (about this choice; see section 5.2), and it is concluded that a metal particle of this size can be polarized by the electric field of, for example, Mg 2+ ions in the support. The same effect can be expected from co-adsorbed ions. This polarization modifies the chemisorption bond strength of hydrogen atoms by about 10%. However, when the metal particles grow, the screening properties develop, due to the increase of the density of states at the Fermi energy, N(EF). A high density of states allows accomodation of an additional screening electron at the expense of only small increase in the total energy (chapter 2, section 1). It is difficult to set an exact limit to the particle size at which the screening becomes normal, but the literature suggests that we should put that limit at about 200 atoms per particle. For platinum is this (see table 1) a diameter of about 2 nm. However, model calculations show that even particles of a much smaller size show density of states (N(E) functions which already have many of the features of bulk metals. This makes the region between, say, 4 atoms and 200 atoms a grey zone, where speculation can florish. However, some indication of the size at which an efficient screening starts
Physical properties and structures of small metal and alloy particles
235
can be obtained on basis of the estimates by Mukherjee and Bennemann [64], who were able to explain the literature results on the stability of clusters by assuming that it is limited by Coulombic repulsions and imperfect screening. They assumed for lead particles that the screening constant L varies as: k ---k B (n - 300) 270
(4)
where ~'a represents the bulk value. The use of equation 4 led to the conclusion that doubly-charged spherical lead particles, Pbn m§ (m=2) are stable when n exceeds 30, triplycharged particles when n exceeds 45 and that full screening sets in at n=300, all in concordance with the experimental results. However, transition metals, having a high density of d-states, show
a much better screening; while lead needs 30 atoms to be able
to bear two positive charges without a Coulombic explosion, Ni32+ clusters have been observed experimentally. The screening in small particles, such as metals in zeolites, should therefore not be underestimated. We shall return to this point below, in the discussion of the so-called electron deficiency of small particles and its effect on chemisorption and catalysis. Screening of an external field, which causes this field to be felt around but not through the metal, is very efficient in bulk metals and large metal particles. Two model calculations show this in an explicit way. Smith et al. [65] have made the following accurate estimate of screening for Cu(001). If a positive charge appears at a certain atom, it immediately attracts electrons from the neighbourhood, and these screen off the effect of the positive charge on other electrons. A small shift of the electron density is enough to do the job, the whole screening (excess) density being only 2% of the ground state density. The whole screening charge accumulates very near to the positive charge, 76% of it being within about a quarter of an interatomic distance. As a consequence, even the next neighbour atom then feels very little of the positive charge. There is a different screening and relaxation on the surface and on the bulk atoms, leading to about 0.3 eV higher binding energy of electrons on surface atoms [65]. Aers and Inglesfield [66] have made very similar calculations for a Ag(001) surface. They inposed an electric field on this plane, of strength E = ___ 0.02 atomic units, which is equivalent to 1.03x101~ Vm -1 (or 1V /~-1) and calculated the distribution of the screening charge. Their result is in figure 10, where the first arrow marks the position of the nuclei of the surface layer, and the second the position of the first subsurface layer. We observe immediately that the electric field which would be created by an ion of a promoter, support or by any polarized or charged adsorbed particle does not penetrate through the metal. It spreads according to the classical law of electrostatics through the vacuum outside the metal, but does not spread either horizontally in or
236
chapter 5
perpendicularly to the surface (see figure 11).
t.O r--X
~
g
figure 10 Planar-averaged screening char-
ii,
,'/
+' " ~ - 2
'7
ge at Ag(O01) surface for fields E = +0.02 a.u. (solid line) and
~
2
E
= -0.02 a.u. (dashed line)
[661. 2
/. ~ , 6
Z~a .u.)
metal figure 11 Effect of a charge on a metal, possible interactions discussed by various authors are indicated here. Left: through-the-vacuum (coulombic) interaction. Right: through-the-metal interaction, effective on a limited distance.
Another idea as to how small metal particles could be modified comes from the theory of the metal-semiconductor interface and the theory of p-n semiconductor junctions [67]. It is known that the high work function of a metal, that is, its high electron affinity, causes an attraction of electrons from a semiconductor into the metal in contact with it; former then becomes depleted of electrons for some distance below its surface. The removal of negative charge from a semiconductor causes a decrease in the potential energy of electrons and the bands and donor levels then bend in the direction from the metal into the semiconductor. The situation is illustrated by figure 12.
237
Physical properties and structures of small metal and alloy particles
figure 12 Bending of bands in the n-type semiconductor as a consequence of electron transfer from the semiconductor to a metal with a high ~. The depth to which the donor levels are exhausted in the semi-conductor (L) is indicated.
I] ~ r"
Valence
band
L
"-i
A barrier is created and, when the donor levels are at the level of the common Fermi energy, further transfer of electrons to the metal stops. Early text books on solid state physics [68] made a quite reliable estimate of the number of electrons that can be transferred before the movement is arrested by the electrostatic energy barrier due to bending. With the use of realistic parameters, the maximum transfer of electrons per l cm 2 interface has been calculated to be 3.5x1012 electrons [69]. With transition metals in contact with a semiconductor, it is about 0.003 electron per metal atom of the interface. Imagine now a cube of 6x6x6 (216) atoms, having 36 atoms in contact with the semiconductor: if the whole approximation does not fail at these dimensions, one can expect a transfer of 0.1 electron per particle. Even when we are suspicious of a conclusion based on the calculation, we would not expect more then 1 electron per particle to be transferred. This figure should be compared to the (minimum) 216 valence electrons which such metal particle has (for platinum it would be rather 2160 electrons). Yet very much has been expected from this relatively small electron transfer [70]. For example, concerning Pt/zeolite catalysts it has been said [71]: "everything happens as if our small platinum clusters are electron deficient, maybe as a result of partial electron transfer from the platinum cluster to the zeolites ..... Thus, the clusters behave more like iridium, the element to the left of platinum in the periodic table". This picture of electron-deficient small particles does not explain how very limited electron transfer, such as that estimated at various places above, or even a partial transfer [71], should cause such a fundamental change in catalytic properties (from platinum to iridium-like). Furthermore, the sign of the electron transfer should have been a matter of more concern than it was. The thermionic work function of alumina is 4.7 eV [72] and, assuming that the band gap is 2 eV, we estimate the lowest acceptor level to be at 3.7 eV. This has to be compared with 5.5-6.0 eV of the work function of platinum. Small particles have even a larger ionisation potentials, so that the transfer in the suggested direction should be more difficult than with large particles. Of course, the formation of a non-ionic bond between platinum and alumina is still possible, but how should that have the effect of making the platinum particle behave as if all of its surface atoms were more like those of iridium?
238
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It is however still possible that theoretical considerations and estimates expressed above are incorrect and that one should rely more on experimental information: therefore, let us see what is available. In Chapter 3, a simple description of the principles of EXAFS was presented, and figure 36 showed directly the form of the spectra obtained. Let us consider platinum on a support. The intensity of the first peak of the spectrum, the so called white line, is proportional to a) the number of potentially responding atoms in the system, b) to the number of the final state energy levels and c) to M, the parameter characterizing the probability of the transition from the initial into the final state. If, for example, platinum ions are added to the Pt~
on the support, the white line intensity would increase
[73-75], because there are more acceptor levels in the system and the intensity should probably also increase if the platinum particles really became electron-deficient. Small particles indeed show a higher white line intensity after a proper normalization, but at the moment it is impossible to say which of the following factors is prevailing, i) Small particles have a different electronic structure and thus a different transition moment M. ii) The small particles prepared in systems which stabilize some ions, for example, those of platinum against reduction and these ions are responsible for the higher white line intensity [74]. iii) They really are electron-deficient, having relatively more unoccupied energy levels than large particles. The attempts to disantagle these effects still continue [76], since a definitive solution has not been reached yet. It is a well established phenomenon that small metal particles bind their coreelectrons more tightly, and this has been accepted by many as evidence of the electron deficiency of small particles. Examples of experimental results can be found in [77] or in the references in the review by Che and Bennett [1]. The great majority of the existing results can be summarized in the form of figure 13.
BE
(XPS)
figure 13 Binding energy of small metal particles (schematically).
4 S IZE,nm
I'0
bulk
)
Physical properties and structures of small metal and alloy particles
239
The size at which the switch from the high to the bulk values occurs is not very different for different supports, as is shown with palladium on ZnO/SiO2, A1203 and active carbon. The phenomenon of higher binding energies may be due to various effects other than electron deficiency (for example, imperfect extra-atomic screening [65,78,79]), which are not dependent on the support. Moretti et al. [80] have performed a very nice analysis of the so-called Auger parameter, t~' for small palladium particles. In this case
igAuger a / = BE
+ "kin
(Ms N45N45)
where BE stands for the Pd3ds/2 binding energy, and
(5)
EkinAuger is the kinetic energy of
electrons emitted by the indicated Auger transition induced by X-ray excitation (see chapter 2). By using the earlier electrostatic theory of the parameter t~' [81,82], the conclusion was reached that a difference in t~' between two samples of the same or similar material is essentially due to the extra-atomic screening: A t t / = 2 A R extra - a t o m k
(6)
According to this analysis the electron polarisibility of small palladium particles of only 13 atoms is already very high, approximating to that of the bulk metal! This is very relevant for the earlier discussion in this book, since polarisibility is so closely related to screening. This varies with particle size but remains close to that of the bulk even for quite small particles; the effect of particle size on the binding energies is therefore o n l y 01 eV. This is the order of the particle size effects on the binding energy of core-level electrons. On the other hand the existence of screening means that there is a very little effect of charges through-the-metal. The results [80] can be explained purely by changes in the final state and it is thus not necessary to invoke to the electron deficiency of small supported metal particles [80]. H +, NH4§ or Na § in the vicinity of the metal particles also hardly change the electron polarisibility of a palladium cluster exposed to the field of the cations [80]. The electrostatic theory of the a ' parameter as used here is a serious simplification but nevertheless the conclusions seem to be very well supported by all the results and the basis of the simple theory is sound. Very valuable information has been obtained by M6ssbauer spectroscopy on small ligand-stabilized clusters. They contained one, two or three shells and the M6ssbauer spectroscopy revealed how deep the field of ligands penetrated in the metal. In agreement with the results on the metal-to-metal layers, the field was found to be fully screened off by about two shells of metal atoms [83]. Some were ready to go a step further and in some earlier papers they claim on the basis of some other arguments that a cation, e.g. H § from an OH group of for example zeolite, attached to a metallic particle can make the whole particle electron deficient and
240
chapter 5
hence change its electronic and catalytic properties [84,85]. The problem with this model is that the endothermicity of splitting off H § from OH could hardly be compensated by its attachment to the metal particle and it is also not explained how the H § charge influences the adsorption through the metal particle of say 10-20 atoms. However, the authors [84,85] nicely explain a great number of observations by this idea. Some other ideas have been proposed, which however are apparently at variance with the theory of solid state physics. For example, there are speculations that the work function of a metal such as platinum can be decreased by the electric field of a support. That is of course impossible, since the work function (see chapter 2) is defined in such a way that it does not depend on the external potential; in contrast to the Fermi energy, the position of which with regard to the vacuum level does depend on the external potential. Another incorrect idea is that with particles as large as 5-10 nm the position of the Fermi level can be changed by filling the metal valence band by transfer of electrons from an electron donor (e.g. reduced TiO2 [86]). Attention has been concentrated above on the problem of a possible charge transfer from or to [87] small particles. These conclusions are mostly based on the observed variation in the binding energy shifts with a varying particle size. However, altering particle size also changes the photo-emission band width and the spin-orbital splitting, wherever the latter occurs [88]. A small metal particle looks less like a bulk metal and acquires spectroscopic features which remind of the free atoms [89]. The chemical reactivity of small particles can also follow this trend which is theoretically explained by papers quoted in section 5.1 and [89].
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247 Chapter 6
T H E C A T A L Y T I C CYCLE
6.1
Prelude
The basic requirements for efficient catalysis can be simply expressed by a diagram (figure 1) of the catalytic cycle. This represents a bimolecular reaction A + B ---~ C and it identifies the five steps which must proceed unhindered if a catalysed reaction is to be sustained. These are (i) transport of the reactants to the surface, (ii) their chemisorption at the active centre, in the right form and not too strongly, (iii) their reaction to form the chemisorbed product, (iv) the desorption of the product and (v) its transport away from the surface. If all steps proceed smoothly, the active centre is regenerated and is able to act in a large number of cycles. By the aid of this simple diagram, we can therefore identify the places at which trouble can arise: mass-transport limitation of the rate (i), chemisorption in an inappropriate way for obtaining the desired product (ii), or chemisorption of the reactants so strongly that they are loath to react (iii). Failure of the product to desorb, either because it is too strongly bound, or because it is converted into some other species which cannot desorb (iv), and inability of the product to diffuse away from the surface (v), are additional sources of potential difficulty. In what follows we will need to have frequent recourse to this diagram to identify the particular trouble-spot we are considering, or to indicate why alloys behave differently from pure metals. Using alloys instead of pure metals can, in principle, influence any of the chemical steps, (ii) to (iv).
figure 1 The catalytic cycle
CATALYTIC CYCLE FOR REACTION : A + B ~ C
|
248
6.2
chapter 6
The role of reaction mechanism in catalytic research At the heart of the study of heterogeneous catalysis lies the question of how
catalytic activity depends on chemical and physical structure of the catalyst. This simple statement embraces much if not all of both the fundamental and the applied research which is conducted in this field. For those motivated by insatiable curiosity, the manner in which a catalyst's chemical nature and structure determine its catalytic behaviour is a matter of supreme interest: for those whose objectives take a more practical tum, this issue is of no less importance, for the design of better or longer-lived catalysts necessitates an understanding of the relevant fundamental principles. Academic scientists are concemed with - one might almost say obsessed by - the question of the mechanism by which a catalytic reaction occurs. Of great relevance to such basic studies, as well as to the design of large-scale industrial plant, is the response of the reaction rate
to the operating variables, especially temperature and concentration of
reactants and products. Investigation of reaction kinetics in the laboratory is not so popular now as it once was. At the time your authors started their research it was almost the only tool available for studying catalytic mechanism. Kinetics gradually fell from favour for two reasons: first, more exciting (and expensive!) methods became available for characterising catalysts, so that more intimate pictures of their architecture could be obtained, and although the resulting discoveries did not directly address the issue of mechanism they did provide more logical means of defining catalytic activity. Secondly, it came to be realised that kinetic measurements alone could rarely if ever lead to a satisfactory and unambiguous statement of mechanism. It often turns out that significantly different mechanisms can generate rate expressions that are quite similar and that the quality of the measurements is too poor to discriminate between such similar rate equations. This problem is particularly acute with multistep processes, such as reactions of hydrocarbons. It is however an important criterion of any proposed mechanism that it should be consistent with the observed kinetics.
The present statement of our understanding of how catalysed reactions proceed is due to the development and use of techniques other than the simple measurements of rate. The use of the stable isotope of hydrogen, i.e. deuterium, revolutionised our appreciation of the complexity of hydrocarbon interactions with hydrogen on metal surfaces [1-5]. For some purposes the use of radioactive isotopes (3H, 35S, 14C, etc.) is advantageous [6]. Analysis of reaction products, first done by tedious chemical methods [7], later aided by mass spectrometry and infrared spectroscopy, and finally most precisely and rapidly by gas chromatography, has provided another window into reaction mechanism. Paradoxically the more complex the mixture of products that a reaction affords, whether isotopically or chemically different, the more immediate is the insight we can gain into the nature of the surface processes that are responsible, because the more are the constraints applied to the
The catalytic cycle
249
conceivable possibilities. The wealth of information produced by the study of a multipath reaction far outweights the additional trouble that has to be taken in product analysis. Reactions exhibiting some kind of selectivity are always to be preferred to those yielding only a single product if fundamental understanding of surface phenomena is the goal of the research. The development of improved procedures for the study of reaction kinetics has not however been ignored. The introduction of microprocessor-control allows the collection of results without the presence of an operator, and techniques are available for very frequent analysis during transient changes to reaction conditions; indeed it is believed by some that transient kinetics are more informative than those obtained under steady state conditions (see section 6.3.3). Implicit in the specification of a reaction mechanism is some statement concerning the involvement of the surface. The concept of the active centre [8] in heterogeneous catalysis (figure 1) is an old and well-tried one, and one which we shall need to explore in detail at a later stage. For the moment, however, we will simply note that the systematic alteration of the composition of the active surface, especially by diluting the active component with an inactive one, is richly informative concerning the likely size of the active centre. The contribution which the study of alloys and bimetallic systems has made to our knowledge of catalytic mechanisms is a significant one, and it will demand our careful attention in the pages that follow. Because of the participation of the surface, the definition of "reaction mechanism" for a heterogeneously-catalysed reaction is incomparably more difficult than for a homogeneous process. Since the first rule of exploration is to be able to recognize the object of your search when you have found it, it may be helpful to offer some suggestions as to what constitutes the minimum information which is needed. The problem is undoubtedly most acute in reactions of hydrocarbons (hydrogenation of alkenes, alkynes, alkadienes; hydrogenolysis, isomerisation and cyclisation of alkanes), and it is to this area, as an example, that the following remarks are addressed; they are based on an earlier attempt which is now 30 years old [5]. Minimal knowledge of "reaction mechanism" demands answers to the following questions. (1) What are the main products of the reaction, and what are the principal routes by which they are formed? (2) What are the relative concentrations of the reactants in the adsorbed phase? How do these change with reactant concentrations in the fluid phase, and with temperature? (3) Are by-products formed, e.g. "carbonaceous residues'? If so, how are they formed and what influence do they have on the principal reactions? (4) What intermediate adsorbed species are kinetically significant? What is the "mostabundant surface intermediate" (MASI) [9]? (5) What is the rate-determining step for the principal reaction? With this information we might say we have reached the end of the beginning, if not the beginning of the end. We do not seek, for example, to explore the precise architecture of the active centre, not the concept that the reactants themselves may
250
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in some sense be able to create active centres for themselves [10]. Study of mechanism in the field of heterogeneous catalysis is more akin to skinning an onion than peeling an orange. At no point does the whole truth stand revealed in all its glory.
6.3
Methods of investigating catalysed reactions The goal of the research having been expressed, at least in general terms, we now
examine ways of achieving it. The experimental procedures for the measurement of the rates of catalysed reactions have been developed into a fine art, and apparatus has been constructed to handle all conceivable types of catalyst and combinations of reactant phases. However, before giving a brief description of some of these systems, it is important to understand how the influences of heat transfer and mass transport are to be recognised and eliminated, for many features in the design of apparatus are intended to address these issues. 6.3.1
Heat transfer and mass-transport limitation Most catalysed reactions are exothermic, and especially with oxidation reactions the
heat evolved can be large. The problem is then to secure the efficient removal of this heat to the surroundings in order to limit the increase in temperature experienced by the catalyst. This is chiefly a problem only where the reactants are gaseous; heat transfer from the catalyst is much easier if it is in contact with a liquid phase. In the laboratory it is usually possible to work at such low conversions or with such a diluted reactant mixture that the rate of heat production is small and it can therefore be easily transferred by the fluid medium and then removed from the reactor. If however, the rate of heat production becomes greater than the rate at which it can be removed by conduction, convection or radiation then the temperature of the catalyst starts to rise and the reaction rate and rate of heat production escalate uncontrollably. In a temperature-programmed experiment, a temperature (measured outside the reactor) will be reached at which the catalyst temperature starts to exceed that measured, and a further small increase in the external heating will cause the observed rate to rise dramatically: this point is called the light-off tempera-
ture. On lowering the external temperature, hysteresis may be observed, as the heat of reaction served to maintain the catalyst at a higher temperature than that measured. This effect may be used diagnostically in the absence of other complicating factors to detect inefficient heat transfer. The main complication is that some reactions can also show hysteresis due to the mechanism; these reactions can often be brought into oscillations. Means of controlling temperature rise in industrial reactors will be considered in the next section. In the catalytic cycle depicted in figure 1, the overall rate will be controlled by
The catalytic cycle
251
whichever step is the slowest [11,12]. In order to make best use of the catalyst, it is generally desirable that the transport of reactants to the surface should not be rate-limiting, otherwise, the catalyst will be idle while waiting for reactants to arrive. There are rare examples (fat-hardening is one) where in order to isolate an intermediate product it is necessary to work under conditions of mass transport limitation. More usually however steps are taken to minimise the risk of this occurring, the chance of which is greatest when the catalyst is porous and working at high temperature and high conversion. Problems of heat dissipation and of mass-transport control often occur together [11,12]. In gas-solid systems, mass-transport limitation of the rate can occur either because the transport outside the catalyst particle is slow or, if the catalyst particle is porous, because transport within the pores governs the rate. In the former case, which is characterised by a low temperature-coefficient and a less than first-power dependence of rate on catalyst weight, there is little to be done except to lower the rate of the catalytic process, e.g. by using a less active catalyst, lowering the pressure of reactants, or the temperature, or increasing the flow-rate. Pore-diffusion limitation can be controlled by changing the texture of the catalyst particles (e.g. removing microporosity) or diminishing the particle size. If such limitation is absent, the rate should be independent of particle size. Lowering temperature or activity will also be effective, but increasing the flow-rate has little effect on diffusion within pores. Mass-transport limitation is frequently encountered where a liquid phase is present, for example, in catalytic hydrogenation of organic liquids or solutions [13-15]. The chief reason for this is that the solubility of H i in most liquids is very low, and hence the transport of H 2 from the gas phase to the active centre on the catalyst surface is often ratelimiting. The criteria by which this situation is recognised include those listed above for gas-solid systems (low temperature coefficient, rate not proportional to catalyst weight), but in addition the rate will be a positive function of the efficiency of agitation (shaking or stirring) (figure 2), since good mixing decreases the distance that H2 molecules have to migrate through the liquid and thus increases their flux to the catalyst surface.
{Q) figure 2 Dependence of reaction rate on speed of agitation using different Rote quantities of catalyst (left) and on catalyst weight (right). The /5 diffusion-free zone lies below ~ the dotted line.
-~ .
,~,
--
...'"
Agitation efficiency
Catalyst weight
252
chapter 6
The rate of mass-transport dnA/dt is proportional to the concentration gradient dc/dx between the catalyst surface and the bulk (fluid liquid) or gas phase according to Fick's First Law: -dnA/dt = AD dc/dx
(1)
where A is the area across which the flux occurs and D the diffusion coefficient. The effect of slow hydrogen transport is shown in figure 3. A slow transport creates a gradient between the liquid and the catalyst particle and it can influence the observed rate as shown in figure 3, right-hand side.
\A' \ \ \
I
.. ~ !
4
Diffusionlaver
x
Bulk solution
j
1 I i
B'
\ \\
~z _c B
A
' Distance from surface
I/T
figure 3 Left: concentration gradients in the neighbourhood of a surface. I; no diffusion limitation: H partial diffusion limitation; III complete diffusion limitation. Right: Arrhenius plot showing the onset of diffusion limitation. AA'; surface reaction is rate-limiting; BB' reaction is diffusion-limited. Observations follow AXB' since the slower of the two processes controls the rate.
For liquid reactions design of the reactor is, of course, important; quality of mixing is improved by having baffles set into the reactor wall, but the easiest way to improve mass-transport of dissolved H2 is to increase its concentration in solution by using superatmospheric pressure. Solubility of H2 is usually roughly proportional to applied pressure, and much faster rates then result; means of achieving this will be indicated later (section 6.3.4). As with gas-solid systems, mass-transport within catalyst pores can also occur, but this cannot be eliminated by improved agitation. By making particles smaller, the length of the pores, i.e. the maximal distance for transport, can be shortened. Special preparation techniques can produce materials with wider pores. The following section gives a brief outline of the various types of reactor that can be used either in the laboratory or on an industrial scale for conducting catalysed reactions in gas-solid or gas-liquid-solid systems. Then some further detail will be provided
The catalytic cycle
253
concerning various reactor designs. 6.3.2
Methods for conducting reactions in gas-solid systems We shall be much concerned with the study of catalysed reactions in the gas-phase
and there are a number of different ways in which this can be done. The basic distinction in terms of reactor design is whether the reactants remain more or less fixed in space or whether they are continually on the move. Basic studies are often made in a small static
batch reactor, typically a glass vessel holding the catalyst in some form, and capable of operating at up to 1-2 atm. pressure. Movement of the reactant/product mix depends entirely on convection, so that very rapid reactions cannot be followed. Conversely however such a reactor is ideal for looking at very slow reactions, since they may be followed for hours or even days on end. This form of reactor is also suitable where the reactants are expensive, e.g. isotopically labelled molecules. An intermediate type of reactor is the recirculatory or race-track reactor, in which a fixed volume of reactants is forced by a magnetically-operated pump to move around a closed loop through a small bed of catalyst. This motion helps to avoid mass-transport limitation, and is often a convenient compromise between the purely static reactor and the dynamic mode. A simple system suitable for studies on metal films, metal or alloy wires, etc. is shown in figure 4.
3 6
13
figure 4 A simplified scheme of equipment for studies with metal and alloy films at low pressures (0.1-50 Torr) [16a], IM-ionisation manometer, C-reactor, with evaporated film, Pcirculation pump, K-cold finger, MS-mass-spectrometer, VP-Vac-lon getter pump, CMmembrane manometer, P-mercury diffusion pump, 1-6,18-uhv valves, 7,13,17-metallic, high vacum valves, 14-16-glass valves.
254
chapter 6
In these systems the rate may be directly derived from the time course of the reaction as indicated by pressure change or other analytical means, and so it is straightforward to obtain orders of reaction and apparent activation energy by altering the initial conditions. Their main disadvantage is however that a steady state is at no time established; this disability is overcome by the use of a continuous-flow reactor, where reactants are caused to flow through a bed of catalyst which is more or less fixed in space. Concentrations vary with distance through the bed, but not with time, unless deactivation is occurring; kinetic information therefore has to be obtained by changing the flow-rate of the reactants.
o
FA ~, A
--~I FA
FA-d FA
d•
figure 5
x
t>
Scheme f o r mass balance analysis in a continuous flow reactor.
The easiest derivation of the appropriate formula to use is based on a mass-balance analysis of an element of catalyst bed, the volume of which is Adx, where A is the crosssection area (figure 5). If the rate is expressed per unit volume (V) of the bed, the massbalance statement is FA - (FA - dFA) = rA Adx = r, dV
(2)
where F A is the flow-rate of the reactant A into the reactor, expressed in the same units (e.g. mol s-1) as those used for the rate r A. This simple equation, representing the conversion of A into a single product, neglects all effects of longitudinal or transverse masstransport within the bed, as well as the influence of conversion on flow (which happens when the reaction changes the number of molecules; in other words the reaction is assumed to take place without change in volume and is also taken to be thermoneutral. Expressing F A as F A = F~ (1-OtA) (Z A
(3)
being the conversion and F~ the flow-rate of A at the entrance to the bed, and t~A as 0~A
=
(C~
- CA)/C~
(4)
The catalytic cycle
255
where CA is the corresponding concentration of A and C~ its initial value, the rate is given by rA = dodd(V/F~
(5)
If the rate is expressed per unit weight W of catalyst, the denominator becomes d(W/F~ and if per unit surface area S it is d(S/F~ Since the time a molecule of A spends in the catalyst bed is inversely related to the flow-rate F A, the denominator can be replaced by d1: where "~ is the apparent contact time. It is then straightforward to write down expressions for reactions of various orders in A; for example if the reaction is first-order rA = k C A = k C ~
(6)
(1-0~A)
and a A
(7)
r, = f d~a[kC~ (1 - o~a) 0
A simple continuous flow apparatus is illustrated in figure 6 (BTS catalyst removes oxygen). A
r 1
r
/
,.\\
t ,
~
r
/.,
BTS
"~
,. -, -,
mol sieve
H 2 in valves: 9operated automatically O operated manually
S.V.
s
c
reactor
to G.L.C.
s- saturator with HC c- c o n d e n s o r T- temperature read out sr- second reactor sv- sampling valve
figure 6 Continuous flow apparatus.
It is possible to conduct fundamental studies using only very small amounts of catalyst in a microreactor [16b]. An apparatus permitting in situ examination of the catalyst by IR spectroscopy simultaneously with product analysis by gas-chromatography has been described [17] and is shown in figure 7.
256
chapter 6
1
COIH2 co
6
3 <
9
3
'
16
figure 7 flow system: I-dry ice trap; 2-1iquid N 2 trap; 3-molecular sieve; 4-De-oxo unit; 5-flow controller gas mixer; 6-6-port valve; 7-4-port valves; 8-10-port valve; 9-infrared cell; lO-infrared spectrometer; l 1-data station; 12-column; 13-hot wire detector; 14-sarnple loop (100 pl or 500 pl); 15-flowmeter; 16-Perkin-Elmer Sigma-3B gas chromatograph.
Infrared cell: I-carrier gas (He or H 2) inlet; 2-outlet; 3-resistance heater; 4-asbestos insulator; 5-CaF2 optical window; 6-Viton O-ring; 7-catalyst disc; 8-sample holder; 9- spring; lO-cooling water.
./
L
-------~ 10
The catalytic cycle
257
Due to the rapid loss of activity which catalysts often suffer in use, another dynamic configuration often used in the laboratory involves pulsing small amounts of the reactant into a continuous flow of another by means of a gas-sampling valve or similar device; in this pulse-flow reactor the catalyst may be kept in a reasonably clean state, since the reaction time is short and if the continuously-flowing gas is H i in hydrogenation reactions or 02 in oxidations, it may regenerate the surface between pulses. Accurate kinetic measurements cannot however be made in this way, as the concentrations of reactants continually change as the pulse passes over the catalyst, although valid product analysis can be performed. In order to avoid this problem, short reaction-pulse techniques have been developed [18,19]; the use of short (1-2 min) pulses of reactants in a known ratio, with a single analysis made at the end of the pulse, followed by a cleansing stream of H 2 (or 02) before the next reaction period, allows kinetic results of high quality to be obtained. When continuous-flow reactors are used in the laboratory, the catalyst is usually in the form of small particles which, under the relatively slow flow-rates generally used, remain fixed in space. This arrangement constitutes what is known as a fixed-bed reactor. If however, the catalyst grains are very fine, a continuous fast reactant flow will agitate them and the bed will appear to 'boil': we then have a fluidised-bed reactor. Because the particles of catalyst are in constant motion and collide with walls of the reactor, both lateral and longitudinal temperature gradients are minimised and flow of heat to the reactor walls is encouraged. This manner of operation is widely used in industry and in a recent development, applicable to catalysts which deactivate rapidly (e.g. fluid cracking catalysts, FCC), the gas (or liquid) flow causes the catalyst particles to move continuously from the reactor zone to a reactivating zone and back again: this is the rising-solids reactor. An important variant of the fixed-bed reactor involves the use of ceramic monoliths or honeycombs as supports for the active phase. They have been used particularly for treatment of vehicle exhaust and they comprise blocks of a ceramic material such as alundum or mullite permeated by a large number of small regular channels. They are in a sense magnified zeolites; they offer minimal obstruction to gas flow, they are strong and rigid, and therefore apt to suffer from thermal and mechanical shock. Their use is however justified in situations where very short contact times have to be used [20]. With large-scale industrial reactors, to work at low conversion is uneconomic, and means have to be found of achieving high conversion in a single pass while at the same time limiting the rise in temperature of the catalyst [21]. Excessive temperature rise might harm the catalyst and lead to a decrease in the amount of desired product formed. Several options are available [11,21]. With fixed-bed reactors the catalyst can be placed in small tubes (-- 1 cm diameter) around which a heat-transfer liquid flows: up to 1000 such tubes can be filled into a reactor system (figure 8), giving good control over maximum catalyst
258
chapter 6
temperature. This is a multi-tubular reactor.
Reactants in
Reactants in
f eac,or
Catalyst beds
~' t u b a s
.oO.o;-oo:;i~_o
u~O.Ooe.~ d
C
Coolant
ant
out
\
j
Coolant in
P r o d u c t s out figure 8 Multi-tubular flow reactor.
LJ g P r o d u c t s out figure 9 Split-bed reactor
Alternatively, the fixed-bed can be divided into narrow portions interspaced with zones having coils through which a heat-transfer liquid is circulated and in which the gas temperature is lowered: the temperature rise is limited by the thickness of each portion of the bed, and control over the maximum temperature reached is thereby achieved (figure 9). This is the split-bed reactor. 6.3.3
Transient kinetics and temporal analysis of products (TAP)
These two related but distinct techniques provide information on the nature and reactivity of intermediates involved in catalytic processes. The term transient kinetics refers nowadays mainly to observations made in a continuous-flow system under nonsteady state conditions brought about by a sudden step-change in composition of the reactant gas stream entering the catalyst bed. It may be that the concentration of one reactant is changed or even reduced to zero; consequential changes in product concentrations are then monitored at frequent intervals (ca. a few seconds) either by using a very rapid analytical procedure (e.g. FTIR, quadrupole mass-spectrometer) or by taking samples in quick succession for later analysis by GC.
The catalytic cycle
259
The principle of the method can be illustrated by reference to the reaction scheme
kl A
k2
--+
B
--+
(8)
C
Now in the steady-state it is the rate of formation r c of product C which is measured and this equates to rc
[B]
= k2
(9)
It is impossible to evaluate k 2 because it is always linked with the concentration of the surface-bound intermediate B. If however the flow of A to the catalyst is stopped, r c falls because B continues to react but is not replenished. The time-constant of the decay of r c is the reciprocal of k 2, so the value of k 2, and hence also the concentration of B in the steady-state, can be separately estimated (see figure 10a). In the case of the more complex scheme A
kl --+
B
k2 --+
k3 ---+
C
D
(10)
measurement of the rate of formation of D follows a different course, because on stopping the flow of A, the intermediate C initially continues to be replaced from B and it is only as the latter decays that the concentration of C falls. The rate r D therefore follows the curve shown in figure 10b, the shape of the course being determined by the reciprocals of both k 2 and k 3. b
CI
c
d
Rate of formation of product
t-----~
t -----~
t
=
~
t ----~
figure 10 Transient kinetics: changes in rate of product formation following alterations in reactant flow conditions at the times indicated by the broken lines (see text for descriptions).
There is an implicit assumption in the foregoing that stopping the reactant flow does not affect the values of the rate constants for the removal of the intermediates. However, problems arise when the flow of one ot two reactants is stopped, because the
260
chapter 6
surface concentration of the remaining reactant may increase and so affect the rate of product formation. An example, of this is provided by CO methanation: (11)
CO + 3H 2 ---9 CH 4 + H 2 0
If the flow of CO is stopped, the rate of production of methane initially increases because in the absence of CO the H 2 can occupy more of the surface, thus accelerating the rate at which intermediate species are transformed into methane [23] (figure 10c). The removal of accumulated carbonaceous residues (section 6.6) as methane may also contribute to this effect. Problems of this sort are overcome by the use of isotopic variants of the transient kinetic measurements. If for example, the flow of normal (i.e. 12C) CO is abruptly replaced by 13CO, there is no disturbance in the chemistry of the steady state, but the formation of ]2CH4 will gradually be replaced by ~3CH4 (see figure 10d). From the time-constant of the change, the rate constant for converting the Cl intermediate into methane can be estimated [24]. These techniques are not new; they have been practised for a number of years [2527] and are assuming a growing importance in the analysis of kinetics and mechanism. It is claimed that the response of rate to changes in reactant concentration can be more reliably determined and information derived from transient measurements than from conventional steady-state measurements [28]. In temporal analysis of products (TAP) we are concerned with changes occurring at the catalyst surface on a much shorter time-scale and much more sophisticated apparatus is needed than for following transient kinetics. The essence of the method is as follows [29] (see figure 11). A
Cryo - shield ~, 77K glass I beads :.i]catalyst
.....
! ./ B
10
-7
-6 10 Torr
Tort
-9
10 Torr
figure 11 Simplified scheme of a TAP reactor [30]. A,B - gas inlets; QMS - quadrupole mass spectrometer. The high-speed valves are magnetically operated: as they are shown, a pulse of A has been admitted and a pulse of B is ready for admission.
The catalytic cycle
261
The reactor, valve assembly and quadrupole mass-spectrometer (QMS) are contained within a differentially-pumped vacuum system; the valve assembly is capable of allowing either a continuous flow of a gas to the catalyst or very small pulses (-- 1018 molecules) of short duration: on leaving the catalyst, molecules move towards the QMS, their flux being trimmed into a molecular beam. Without the reactor the mean residence time of the pulse is about 1 ms with a half-width of 200 Its. In the pulse mode, emergence of a single pulse is detected by the QMS which records either the masses of products formed or the timedependence of a single species (reactant or product) by fixing the value of m/e. A sequence of pulses of the same molecule can also be used (e.g. 1 s-1) or most informatively pulses of two different molecules in the so-called pump-probe format. Here the two pulse-valves are activated in sequence, the species formed on the surface by the first pulse being detected as products formed by interaction with the second ('probe') pulse. Attenuation of the time interval between the pulses allows quantitative information to be obtained concerning the manner in which the first molecule reacts with the surface. Almost infinite variation in the operating procedure is possible, and it has been particularly applied to the study of selective oxidations ([30] and references therein). While undoubtedly providing new and interesting information, the conditions used are far removed from the steady-state of operation and proper caution has to be used in transferring information from one system to another. 6.3.4
Ways of performing reactions in gas-liquid-solid systems [13-15, 31-33] When the catalyst is a fine powder and is easily dispersed in the liquid medium,
reactions are readily conducted in a small batch reactor in the laboratory. The required three-phase contact is achieved by vigorous shaking (typically 800-1000 min -1) or by a magnetically-operated stirrer. Consumption of the gas is measured volumetrically at constant pressure and temperature, a procedure which easily lends itself to automation (figure 12). Most hydrogenation reactions on an industrial scale are performed at superatmospheric pressure, for the reason explained above: the reactor is then called an autoclave. The simplest type, which will work safely to about 3 atm. pressure, is just a stout glass vessel, suitably guarded. It is however possible to work at very much higher pressures using stainless steel equipment with glass or Teflon liners (figure 13). In these cases, progress of the reaction is followed by the fall in pressure or by analysis of extracted samples. Very great care needs to be exercised when handling H 2 with a catalyst and an organic medium, especially if high pressure is involved. The risk of accident is non-negligible and due attention must be paid to the locally-applying health and safety regulations. Such equipment is usually housed in a dedicated blast-proof laboratory and is operated by remote control. For many reactions the steel walls of the reactor have to be made inert by covering them by a gold or Teflon film.
262
chapter 6
solash trSpS
[
z\ r--- ~ Iu%Xinbge
[
J
{
.
'~= ~176
'
]
~ra:~atr ee~
/%
'~//
.
.
'.
.
.
(
~"~
I
I
resetvol
v
ume
figure 12 An automated apparatus for recording change in volume of 1t2 at constant atmospheric pressure as a function of time. Consumption of gas by reaction causes pressure to fall: this makes photocell start motor, which winds reservoir up until pressure equalized: photocell then cuts out. Paper is on drum which is turned by small constant-speed motor to give time base, operation of reservoir motor gives (with suitable calibration) the volume uptake.
? stirring
cooling
figure 13 A simple high and medium pressure autoclave.
H2
f
l
0
J
0
s - ~ . l
J
f
l
J
r
,,t
i
f
l
J
, E-~
f.,,fy,,j ,,,,'jj
heating
The catalytic cycle
263
Other ways of conducting gas-liquid reactions are available. In a trickle-column reactor the liquid phase is allowed to percolate under gravity through a bed packed with granular catalyst, while the gas is passed concurrently: countercurrent operation tends to lead to flooding of the bed [11] (figure 14). Liquid reactant
Gaseous reactant
in
figure 14 Trickle column reactor.
in ~] 1t~'~
Gas
--
Catalyst bed
Liquid reactant
in
Excess gas out
in
---.-
l
Excess gas out
,
wire mesh basket containing catalyst
0 .~~~~---"~
Liquid
product
out
Liquid product
figure
15 "~
Continuous Stirred-Tank Reactor
(CSTR) (schematically).
Conditions of temperature pressure and flow-rates can be adjusted to secure the desired conversion in a single pass. In the continuous stirred-tank reactor (CSTR), the granular catalyst can be held in a wire-mesh basket which is rotated rapidly in the liquid, which flows into and out of the vessel at a controlled rate. The gas is injected under the catalyst basket, and the vessel may be pressurised. The mean residence time of the liquid is adjusted to achieve the desired conversion [11,21] (figure 15). Finely-divided powder catalysts may also be investigated using a bubble-column reactor in which the catalyst is kept suspended in a column of the liquid by means of fine bubbles of the gas rising through it. These last devices are not widely used in the fundamental study of liquid-gas reactions, although certain of these reactions have great industrial importance; the phenomena of heat and mass transfer in the three-phase systems are however an interesting field for chemical engineers.
264
6.3.5
chapter 6
Catalysis under UHV conditions It is now possible to follow catalytic reactions proceeding on the surfaces of single
crystals of metals and alloys pretreated under ultra-high vacuum (UHV) conditions. The area available for catalysis is small, so that sensitive analytical methods may have to be used. Single crystals can be cut at an angle to the low Miller index places, thus producing
stepped surfaces which are thought to mimic the small particles present in supported metal catalysts. Metal particles deposited by evaporation/condensation onto well-defined oxide surfaces such as those of quartz, A1203 and TiO 2 constitute good model catalysts; these and continuous metal films can also be prepared and studied under UHV conditions. Surfaces that have not been prepared in situ need to be cleaned, typically by a combination of heat treatment and ion bombardement, which removes the outermost layers from the surface. Adsorbed O atoms (on metals) and other rubbish, e.g. carbonaceous residues, can also be removed by alternating treatments with 02 and H2 at elevated temperatures. UHV systems are usually provided with a number of spectroscopic techniques, such as XPS, AES, LEED and HREELS, which amongst others can monitor the effectiveness of the cleaning treatment, as well as of course rapid pumping systems capable of lowering the pressure inside to at least 10-1~ Torr. The first studies made by this technique were confined to regions of very low reactant pressures (less than 10 -6 T o r r ) because no means were available to isolate the place where catalysis occurred from the pumping system. Catalytic investigations in the laboratory are conducted at pressures 10 6 tO 10 9 times higher than this, and in industry it is not uncommon to use pressures above 100 atm. People therefore spoke of a pressure gap between the fundamental and the technical work, and attention was directed towards bridging this gap. Pioneering developments in technique took place in the laboratory of G.A.Somorjai: in his apparatus the single crystal or metal surface prepared in situ is enclosed in a small reactor volume which formed part of a flow or recirculation system. Gas analysis is preformed either by mass spectrometry or by gas chromatography. An example of the design of equipment used is illustrated in figure 16 [34,35]. 6.3.6
Scaling-up [36] Experimental work in the laboratory, whether industrial or academic, is carried out
on a scale which is obviously much smaller than that of industrial processes; nevertheless if the work is exploratory in nature there is at least the possibility that there may one day be the need to use the result to design either a pilot plant or even a full-scale reactor system. Apart from the scale of operation, the other principal difference is that laboratory work is normally confined to low conversion so as to minimise heat generation and sequential reactions, whereas industrial reactors will preferably operate so as to achieve an
The catalytic cycle
265
economically-useful conversion in a single pass. The scaling-up problem therefore falls into two main parts: (i) changes of reactor size, and (ii) increased difficulty of heat and mass transfer. The latter problem is related to the fact that to achieve a required flow through the reactor, the catalyst must be in form of large pieces and not as fine powder.
Gas
Chromo tograph
Manipulat~
,, <
Potassium Gun ~-~ ~ ~ J
Argon Ion
Cylindrical r---..] Mirror -----"
Analyzer rl~-J X-roy .--'~'11 I---, Source -'_I
~ Gun "7
-'~ Moss
._~Spectrometer
~,
\
Leak Vok,es
S.S/H~~
~ Diffusion
Pump
lonizolion Gouge
Bellows
~,
)
l
Hydrolic
l[-~
[ V l~ch-1 -
~)
Pislon
(~)
Sampling Valve and Loop
Pressure Gouge
~pMeCh.
ump
1
> I~ICirculalion Pump
P~ILv_W_J
SAMPLE
CMA
~;~
SIDEVIEWWITHHIGHPRESSURECELLOPEN
figure 16 Schematic representation of one type of apparatus capable of carying out catalytic reaction rate studies on single-crystal surfaces of low surface area at high pressures and also to perform surface characterization in ultrahigh vacuum [16c,34].
A typical large-scale reactor might contain at least 105 or
even
10 6
times more
catalyst than a laboratory microreactor. One consequence of this for fixed bed reactors is that edge-effects are very much less on the large scale. Furthermore it is difficult with a laboratory reactor to replicate high-conversion conditions if this means using the long contact-times which automatically result with large catalyst beds: to do so would entail having flow-rates so low that they are difficult to control and measure.
266
chapter 6
Methods for limiting temperature rises in industrial reactors have already been mentioned (section 6.3.2). The occurrence of temperature-gradients in fixed bed reactors, even of the split bed and multitubular type, are difficult to reproduce on a small scale, but these can be important; for example, catalyst in the latter zone may deactivate more quickly. Although much progress has been made in understanding the problems of scaleup, a great deal of it is still based on empirical knowledge and experience and the proof of the pudding remains in the eating.
6.4
Kinetics of heterogeneously catalysed reactions
6.4.1
Ways of expressing the rate of a catalysed reaction It is far more difficult to find a satisfactory way in which to express the rate of a
catalysed reaction than is the case with a homogeneous reaction. It is of course straightforward in a given system with a known amount of catalyst, and with all other relevant variables defined, to observe that a certain number of moles of reactant are being converted to product in unit time. It is equally simple to define the rate at which each particular product is formed, relative to the total rate, and hence the selectivity of its formation, provided the stoichiometry of the reaction is known. The trouble starts when one seeks to relate the observed rate to the quantity of catalyst. There is no problem in expressing the rate per unit weight of catalyst taken e.g. mol S -1 g-lcat.
(12)
In the case of supported metals, the rate may be better defined in terms of the amount of metal present, e.g. mol s -1 gMl,
(13)
but a more meaningful definition would be in terms of the area of the active component, sometimes called the areal or specific rate [37] (the IUPAC recommendation is to use the former term, but many authors consider the terms as synonymous) and even better in terms of the number of surface atoms in the active phase i.e. the turnover frequency, TOF [38], also sometimes called turnover rate or number. The rate per total metal atom has been termed the atomic rate AR [39], and if the fraction of metal atoms at the surface is called degree of dispersion D (or the fraction exposed, FE [37]), then TOF = AR/D
(14)
The catalytic cycle
267
However, ambiguities arise from several directions. (1) Even in a monometallic catalyst, not all the surface atoms (as counted by H2 chemisorption or by electron microscopy) are necessarily active or capable of participating in an active centre: some may be eliminated by poisons in the reactants or formed during the reaction. (2) The number of active centres (which cannot be directly measured) will frequently be less than the total number of surface atoms, because each active centre may comprise a number of surface atoms (NB, the Balandin number or ensemble size). The ratio of the number of centres to the total number of surface atoms is the Taylor ratio RT: thus NB x RT = 1
(15)
(3) In bimetallic catalysts, the problem is compounded by our likely ignorance of the surface composition under reaction conditions, of the composition of the active centres and of their number. Thus while with unsupported metals (wires, films, powders, etc.) some of these problems become insignificant, with supported metals, expecially the bimetallics, the optimal form of a valid expression of the rate is fraught with difficulties. All authors should be required to state explicitly what assumptions underlie their rate statements; unfortunately many do not. An example of an attempt to evaluate the rates per active ensemble is presented in chapter 13. We will focus in what follows on reactions in the gas phase in an open flow reactor. The first piece of information that product analysis provides is the fractional conversion ~ based on one of the reactants, which with a knowledge of the reactants' rate of flow into the reactor F~ reactor:
gives for low conversion the rate per unit volume of the
r A = - dnA/dt = ~F~
(16)
Thus, if the rate is constant with time and if mass-transport control is absent, conversion will be inversely proportional to reactant flow rate, and this indeed constitutes one of the useful tests for the intrusion of diffusional or other mass-transfer limitation (section 6.3.1). It is commonly found, especially in reactions of hydrocarbons, that when a flowsystem is used the conversion will decrease with time-on-stream because one or more deactivation processes are at work. Often a steady-state is attained, but the conversion may then be much lower than the initial value. It is far from routine, and indeed experimentally difficult, to measure the catalyst's free surface area in the steady-state: this as noted above is a major source of ambiguity in estimating the "real" turnover frequency. While it is accepted that the steady-state rate of the catalysed reaction will depend in some way on temperature and reactant concentrations (perhaps on product concentrati-
268
chapter 6
ons too), it is sometimes forgotten that the turnover frequency is only a rate and is therefore susceptible to the same variables. It has the units of frequency only because the units of amount of reactant removed and amount of surface atoms cancel. Authors sometimes regard TOF as a kind of fundamental constant, quoting its value without reference to the conditions used: editors and referees should be alert to this possibility. 6.4.2
Introduction to Langmuir-Hinshelwood kinetics [4,11,38,40,41] The manner in which rate varies with experimental conditions is often represented
by an equation of the form:
dt
(17)
- kP;P
where PA, PB--- are the pressures of the reactants (and products if they affect the rate) and a,b .... are the "orders of reaction". This form of rate expression is called a Power Rate Law, but it is an empirical expression of only limited value. Since the orders are frequently non-integral, it is not always easy to define the dimensions of the rate constant k. At the next level of sophistication, the rate is taken to be proportional to a function of the surface concentrations of the chemisorbed reactants Og, Oa .... With the assumption that the interaction of adsorbed A with adsorbed B is the rate-limiting step, the rate becomes:
dt
-
(18)
klOAOB....
and one then seeks to relate the O terms to the corresponding gas-phase concentrations, by means of the Langmuir equation. This equation [42] is simply derived by equating the rates of adsorption and of desorption at equilibrium; in the most straightforward case, i.e. the adsorption of a single species A without dissociation. ka PA (1-Og) = k d O A
(19)
|
(20)
= bg PA/(1+b A PA)
where k a and k d are respectively the rate constants for adsorption and desorption,
I~) A
is the
fraction of surface covered by adsorbed A molecules, P A is the equilibrium pressure of A above the surface and b g (= ka/kd) is the adsorption coefficient for A. Og can also be expressed as ng/ng,m, where n g is the number of molecules adsorbed at pressure PA and ng, m is the number adsorbed when the surface is saturated. Adsorption from dilute solution
can be similarly formulated. It is however necessary to stress the assumptions which
The catalytic cycle
269
underlie the simple derivation, shown above, since they are frequently and conveniently forgotten; they are as follows. (1) Each adsorption site can be occupied by only one molecule of A. (2) It is adsorbed without dissociation or decomposition and can therefore desorb unchanged. (3) The strength of the bond formed by A to the surface, or the enthalpy of adsorption, is independent of the coverage | i.e. there are no mutually attractive or repulsive interactions between adsorbed molecules, either directly, or indirectly through the solid, and all adsorption sites are of equal quality. It is then a small step to set down equations defining the surface coverages by two species A and B, both present in the gas phase at equilibium pressures PA and PB; thus for example Og = bg PA/(1 + bg PA + bB PB)
(21)
It then follows that OA/OB = bA PA/bB PB
(22)
If the molecule A dissociates into n fragments on adsorption and therefore occupies n sites = (bA PA)l/n/[1 + (bA PA)TM]
|
(23)
Obedience of a given set of experimental results for adsorption of a single species to the Langmuir equation can be tested (see figure 17) by plotting for example PA/| versus PA. 80
i
70-
figure 17 Hydrogen chemisorption on EUROPT-1 (6.3%Pt/Si02): Langmuir equation plot for dissociative chemisorption.
60-
%
50-
x
8 t,O
-a_*' 30 20-
O. " ~ ! 0
.... 1
i
i'
2
3
'
|
~
9
i
|
5
6
'
'
|
7
'! . . . . . . . 8 9
Pg/~//Torr'/2
Figure 17 shows that the chemisorption of H 2 on a Pt/SiO2 catalyst (EUROPT-1) obeys in the indicated region of pressure the Langmuir equation for dissociative chemisorption. By plotting PH2~ versus V H20.5 , a very good linear plot is obtained, the reciprocal of the
270
chapter 6
slope giving nil,m; hence, assuming a H/Pt surface atom ratio of unity, the degree of dispersion of the metal is estimated as 1.17. Possible reasons for finding values greater than unity will be considered later. If then the rate-determining step is the catalytic unimolecular decomposition of adsorbed A and if the product X is not adsorbed, -dnA/dt = kl OA
(24)
Here k 1 is the first-order rate constant for the slow step. Thus this unimolecular reaction will be first-order in A at low values of |
(i.e. low PA or low b A) and zero-order in A
when OA is unity (i.e. high PA or high bA). At intermediate values the order will appear to be fractional and positive (figure 18), but over the whole range the rate is expressible as (25)
-dnA/dt = kl bA PA/(I+bA PA)
First order region
/
iI /
Rate
I
/
Froctionol-order region
Izero-order I region
I I I
I
figure 18 Dependence of the rate of the unimolecular decomposition of A on its gas-phase pressure.
In the case of a bimolecular surface reaction for which the rate-determining step is A+B X, and X is not adsorbed, the rate expression becomes: -dng/dt = kl OA O13 = kl bA PA bB PB/( 1 + bg PA + bB PB)2
(26)
The form of the observed variation of rate as PA is varied with PB held constant, and vice versa, depends both on the values of the adsorption coefficients and on the level at which the constant pressure is held. Results for a typical case, where bA is comparable with bB,
The catalytic cycle
271
are shown in figure 19.
1
7: 0.5
figure 19 Dependence of the rate of a bimolecular surface reaction between A and B on the partial pressure
~2 0.0
of A (PA) at constant pressure of B (Ps). The top section shows how the fraction of surface covered by each rectant changes with PA, the rate being proportional to the product 0 A, 0 8.
PA
At a constant temperature, the rate passes through a maximum a s P A is increased, the position of the maximum denoting the point at which | and | are equal. Above this point, further increase in P A inhibits the reaction because A is then occupying too much of the surface. The rate can be approximated by power rate law, for low P A, with a positive fractional order. On raising the temperature (not shown), the rate naturally goes up, but the maximum as PA is varied becomes broader and moves to higher pressures: the inhibiting effect of A at higher pressures is less marked. The reasons for these changes are as follows. Because both bA and bB are equilibrium constants, their values will decrease as temperature rises, due to adsorption being always exothermic. So the values of | and | at fixed pressures will also decrease, but not necessarily to the same extent; the ratio |174
will in principle be temperature
dependent OA/ Oa = (ba Pa/ba PB) = b~176
(27)
Coverage by the component having the greater enthalpy of adsorption will change then more rapidly with temperature. At very high temperatures, where | and | are both low, and the denominator in (26) is small and nearing unity, the rate will be proportional to the first power of the pressure of each reactant. Both Irving Langmuir and Sir Cyril Hinshelwood [43] were instrumental in developing this methodology in the period 1915-1935; it is now universally known by the term Langmuir-Hinshelwood kinetics. It is not necessary to go into much further detail here concerning the extension of
272
chapter 6
the above concepts to more complex situations. Equations describing dissociation of reactants in a bimolecular reaction, or allowing for strong adsorption of the product X are readily written down. Even more complex scenarios are described in standard texts [4,9,12,40], the classic work of Hougen [44,45] covering almost every conceivable eventuality. The work of J.C.Jungers [46] was an inspiration to early workers on heterogeneous kinetics, and is notable for the great care with which measurements were made and interpreted. Systems of extreme complexity (e.g. reactions of alkanes with H2) have also been addressed [19,28], but the chief practioners of kinetic modelling are now chemical engineers [47]: they have for example been much occupied with the reactions of petroleum reforming [50]. It is surprising that, given the constraints in the derivation of the Langmuir equation, the application of Langmuir-Hinshelwood kinetics has been so successful [44,45]. In particular the enthalpy of adsorption very frequently decreases from high to very low values with increasing coverage. Other equations relating coverage to equilibrium pressure, allowing for this variation of adsorption enthalpy, have been given; they are associated with the names of Freundlich and Temkin [4,a,b], but they have not been much used as a universal basis for rate expressions. The kinetics of NH 3 synthesis have however been thoroughly modelled, originally by Temkin and Pyzhev [4,12,48]; this reaction has attracted recent attention as attempts have been made to estimate rates given by practical catalysts from the results obtained with single crystal faces [49]. Kinetic modelling is not an easy thing to do. By "kinetic modelling" is meant identifying a rate expression which adequately describes the variation of rate with reactant concentrations, temperature etc. [46,50]. Even when the system is under kinetic control, difficulties can arise. First, experimental results of high quality, covering a wide range of the relevant variable are needed, but can only be obtained when the catalyst is in a stable condition. Secondly, it is found that quite different rate expressions can give almost equally good fits to the experimental results [19]. When this situation arises, subsidiary criteria have to be introduced. Which rate expression is based on the most sensible and logical mechanism? Do the constants of the equation vary with temperature as expected and are their temperature coefficients of a reasonable order of magnitude? In spite of these difficulties, kinetic analysis is of real value in unravelling reaction mechanisms. Although by itself it can never lead to an unambiguous mechanistic conclusion, the proposed mechanism can not be accepted as valid unless and until the kinetic consequences have been evaluated and tested. It has to be emphasized that the Langmuir-Hinshelwood kinetic formalism described above is applicable only to simple reactions, whereas most reactions of practical interest proceed through complex mechanisms in which there are various intermediates and alternative reaction pathways. In such cases, analysis of results obtained over a broad range of temperatures and pressures is likely to show that the nature of the slow step
The catalytic cycle
273
varies with conditions, a possibility not covered by the treatment given above. One further limitation implied in the above formulation of Langmuir-Hinshelwood kinetics is that it is the interaction of two adsorbed species which constitutes the ratedetermining step. Occasionally it is evident that adsorption of one of the reactants or the desorption of one of the products is rate-limiting, and this situation is readily recognised by modelling. The possibility that the slow step might be the reaction of for example gaseous A with adsorbed B was visualised by Eley and Rideal [51]. The Rideal-Eley (or Really-Ideal) mechanism does not however seem to operate frequently [53a]. Reactions in flow systems do not always attain to the steady-state, for reasons other than deactivation. Under some circumstances the rate can oscillate; this effect is particularly found with oxidation reactions, such as oxidation of carbon monoxide, and its cause has been much debated [52]. The phenomenon has been closely examined using single crystals of metals [53b,c], and in these cases it is certain that the predominant surface phase changes in a cyclical manner. However, while some models relate the oscillations in rate to phase changes, others explain them purely on the basis of the mechanism. This is important because the latter model [53d,e] can explain oscillations on those metals the surface of which does not undergo phase changes. It is possible that the same effect somehow operates with highly dispersed metals on insulating supports, but the extremely exothermic nature of oxidation reactions means that in these cases the temperature of the particles may differ considerably from that measured externally. Oscillation of the true temperature of the catalyst may also contribute to the effects observed with supported catalysts. 6.4.3
Activation energy
On increasing the temperature, the rate constant k~ for the catalysed reaction will rise in accordance with the Arrhenius equation k 1 = A exp (-Et/RT)
(28)
where A is the pre-exponential factor and E t the true activation energy. However, the rate of a unimolecular process is given by equation 24, so that its temperature coefficient is the product of that of kl and that of |
Since the adsorption coefficient bA is an equilibrium
constant, its temperature dependence is given by the Van't Hoff isochore d In bA/dT = AHa~
2
(29)
where -AHa~ is the standard enthalpy of adsorption of A. It means that AHa~ is generally negative: thus |
decreases with rising temperature. The rate does not increase as quickly
274
chapter 6
as it might, although the Arrhenius law often seems to be obeyed, at least over some limited range of temperature; the slope of the Arrhenius plot then gives what is called the apparent activation energy Ea, which is, in the simplest case, related t o E t by [54] Et =
E~- A H a ~
(30)
When there are two different reactants A and B, the apparent activation will be a function of the pressure PA of the more strongly-adsorbed molecule, because when PA is lOW and hence both | and | are low E a = E t + AHa,A ~ + AHa,B O
(31)
However, when PA is high and OA is occupying much of the surface, E a = E t - AHa,A O +/~LIa,B O
(32)
A number of examples are known in which Ea is a function of the pressure of one of the reactants; an example is shown in figure 20 [55], although in this case the form of the rate equation is somewhat more complex than that of a simple bimolecular process. In extreme cases, the rate at which the surface concentration of one of the reactants decreases with rising temperature can exceed the effect on the rate constant; the rate then passes through a maximum (figure 21 [56]), and the apparent activation energy assumes a negative value at high temperatures. 150
1
,r
u.l
50
0
i
I
figure 20 n-Butane hydrogenolysis on 0.3%Pt/A1203 (EUROPT-3). Apparent activation energy as a function of hydrogen pressure [551.
S
~.._ 100O E \
I
o
O'.2
0'.6
Oi8
Hz pressure/otrn
This is a situation encountered, for example, with oxidation of carbon monoxide on metals. Since as noted above | will decrease less quickly with increasing temperature than OB, the Arrhenius plot obtained by using fixed reactant pressures may be non-linear.
The catalytic cycle
275
i
figure 21
!
l
i
I
O-
Benzene hydrogenation over EUROPT-1
-a-
(6.3%Pt/Si02). TOF as a function of temperature. PH2 = 80 KPa, Psz = 9.3Pa
"r- 2-
E a (low temp.region) = 4.5 kJ tool -1
u.\ 3-
[56].
~" t."~
0
-5-6-7-
1'.8
z'.z
zi6
1000/T(K
3'.0
!
"1)
There is an additional explanation for non-linear Arrhenius plots. We saw earlier (section 6.3.1) that mass transport limitation of the rate occurs when the rate of arrival of the reactants at the surface is slower than the reaction on the surface (see figure 1). Now the diffusion process has a relatively low temperature coefficient, so that it is possible on raising the temperature to move from a regime of kinetic control to one of mass-transport control, when the reactants' rate of reaching the surface no longer keeps pace with the catalytic reaction. Various possible scenarios were analysed in detail many years ago by Ahlborn Wheeler [57], and others. 35 '7, o ;-,9 r
30
E
o o o 25 a
o E 2o < o
15
0
I
I
40
80
I
120
160
E (kJ/mol) figure 22 Compensation effect in the exchange of methane with deuterium over metal films [58]. Open points: stepwise exchange; filled points: multiple exchange, a, Ni; b, Pd; c, Pt; d, Rh; e, W.
276
chapter 6
Measurement of the rate as a function of reactant concentrations at several temperatures in principle permits determination of model to be acceptable in the Popperian sense,
Et
and AH Oa,A,B. For the chosen reaction
E t must
be positive and the enthalpy terms
negative and of reasonable magnitude [9,47,50]. Such comprehensive kinetic studies are only rarely performed, at least by chemists, and there is correpondingly little information available on the magnitudes of true activation energies of heterogeneously catalysed reactions. 6.4.4
The compensation phenomenon This last point has to be kept in mind when considering what is perhaps the most
ubiquitous and puzzling phenomenon in the whole heterogeneous catalysis, namely the socalled compensation effect. There is frequently found to be, for a series of related catalysts, an excellent linear correlation between the apparent activation energy of a reaction and the logarithm of its pre-exponential factor E = m In A + x
(33)
An example is shown in figure 22 [58]. Similarly striking correlations are found when (i) the same catalyst is exposed to a variety of pre-treatments and (ii) when a single catalyst is used for a number of related reactions. Sometimes the experimental points lie on two or more lines (figure 23 [59]). Such a linear relation requires the individual Arrhenius plots to intersect at a single temperature, sometimes called the isokinetic temperature [60,61]. There has grown up a very considerable literature on the compensation effect, and there have been many and varied attempts at an explanation, none of which is thoroughly convincing [60-63]. The effect has obvious similarities to the linear free-energy relationships found in organic reaction systems [61], and may therefore find its origin in a sympathetic relation between the entropy and enthalpy of activation. What is perhaps not so widely realised is that exactly parallel relationships are observed in almost all activated processes: examples which have been cited include homogeneously catalysed reactions, thermionic emission (the Richardson equation), solid-state diffusion, the life-time of semiconducting devices and the death-rate of bacterial cells. It would be a brave scientist who advocated a common origin for such diverse phenomena. This is clearly not the place to enter into a prolonged discussion of compensation, but one or two short comments are in order. (i) The effect can be used in a purely empirical or finger-printing way, to identify groups of catalysts sharing some common feature: thus perhaps reaction systems (i.e, reaction and catalyst) which have the same transition state in the rate-limiting step may show Arrhenius parameters which give a compensation effect [64]. (ii) It is by no means clear that the same effect would be found with true activation energies. What is quite
The catalytic cycle
277
certain is that compensation effects can be artificially generated by using an incorrect method of data analysis [65], and there are well-established cases in which apparent activation energy can be varied at least two-fold, simply by changing the concentration of one of the reactants, through the varying effect that this has on its surface coverage and hence on the rate (see equation 26, and figure 20) [55,66,67]. This variation has been shown to lead, inevitably, to a compensation effect [55]; at least for reactions for which the rate expression is complex, it is unwise to say the least to place reliance on an apparent activation energy or to try to give it a physical interpretation. Rooney [68] has recently provided a very clear formulation of the compensation effect in terms of variable heats of adsorption of reactants. Pd
figure 23 Compensation effect in the hydrogenolysis of ethane catalysed by silica-supported metals (circles [59]; triangles, G.C.Bond, G.Hierl, J. Catal.61 (1980) 348).
80-
Rh
Ni 'E
Pd
70-
U
,R
r
O
E
60-
e
e-
50-
160
1{0
2{0
E / k J tool "1
6.4.5
Selectivity [4] The term selectivity is used (sometimes very loosely) to describe the relative
amounts of alternative products formed in a catalysed process. We should strictly speak of the degree of selectivity with which a particular product is made and which can be anything from 0 to 100%, but many authors use the word 'selective' to represent a situation in which the desired product occurs in much less than 100% yield. It is preferable to express the degree of selectivity Sx with which a particular product is formed as the fraction or percentage it constitutes of the reactant removed or of all products. Thus for the reaction
278
chapter 6
A+B
~X+Y+Z
(34)
S x = Wx/(Wx + We + Wz)
where w may be expressed on a weight, volume or molar basis. Because, there are possible sources of confusion, e.g. in the hydrogenolysis of alkanes, it is always desirable to give precise definitions of one's terms. Selectivity can arise in a number of different ways. (1) A single reactant (A) or combination of reactants (A + B) can form alternative products (X and Y) in parallel processes, thus: A + B ---) X
(35)
A + B ----)Y Here both products will be seen from the earliest stages of the reaction, although their ratio (or the selectivity of either) will not necessarily be independent of conversion, since the dependence of their rates on the concentrations of A and B may not be the same, and their activation energies will almost certainly differ. (2) The same reactants may form products X and Y sequentially: A+B
~X~Y
(36)
In such a case, the selectivity for X (which is often the desired product) depends on the relative values of the rate constants for the two steps. If for example, X readily vacates the surface, perhaps because it is less strongly adsorbed than one of the reactants and so is efficiently displaced by it, then Sx can approach unity. If however X reacts quickly with either A or B, then S x will be low. (3) In the most general case, both parallel and sequential routes will be available, and except at very low conversion there may be competition of the products with the reactants for the available surface; reactions of the type A + X ---) Z may also occur in parallel. These considerations are, for example, of great interest and importance in the reactions of H 2 with hydrocarbons on metal catalysts [5]: some examples of systems exhibiting the phenomenon of catalyst selectivity are shown in the following scheme. I
C2H 2
~
ethyne II
C4H 6
1-C4H 8
1-butene
~
ethene ~
1,3-butadiene III
C2H 4
C4H 8
----)
n-butenes ---)
C4H 8
cis-+ trans-2 butene
C2H 6
ethane C4Hlo
n-butane ~
C4Hlo
n-butane
279
The catalytic cycle
IV
n-C4Hlo
C H 4 + C2H 6 + C3H 8
V
n-C6H:4
branched
C6
isomers
4- C 6 cyclic products
+ hydrogenolysis products Selectivity also occurs in an important way in metal-catalysed oxidations for example: C H 3 O H + 02 -.---)
HCHO + H20 C O 4- C O 2 q-
H20
(37)
or in the syngas reactions: CO + H 2
---->
CH3OH higher alcohols hydrocarbons
(38)
We shall encounter numerous examples of reactions where improved selectivity to the desired product occurs when an alloy catalyst is used in place of a monometallic one. 6.4.6
Epilogue
One final word of caution concerning the kinetics of heterogeneously-catalysed reactions is in order. There are significant but poorly comprehended differences between kinetics determined in static and in dynamic modes, at least for some reaction systems. In the dynamic mode, the catalyst is operating in a quasi-stationary way, that is, conversion will not vary with time unless deactivation is taking place: the response of the rate to variation of reactant concentration is established by making a step-change, after which a new steady condition results. In the static mode, however, reactant concentrations are established at the start, after which they fall only gradually as the reactants are consumed, Orders of reaction should therefore be derivable from the conversion-dependence of the rate. Under certain conditions the rates of hydrogenation of multiply-unsaturated hydrocarbons do :not respond to changes in reactant concentrations when they occur in this way, although introduction of a step-change does have the expected effect [69,70]. These observations, made long ago, are not widely known and are not readily explained; perhaps a re-exarnination of the effect would be in order.
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6.5
Structure sensitivity and particle size effects
6.5.1
Principles and concepts It may be helpful now to develop in somewhat greater detail some of the ideas
which have been touched on above, the understanding of which is fundamental to the ensuing presentation of the catalytic properties of metals and alloys. The concept of the active centre (figure 1) was first mooted by Hugh Stott Taylor in 1925 [8]; it arose from the observation that catalysts could be poisoned by a number of molecules quite insufficient to cover the surface, and hence that there must be a few locations on the surface which possessed all the activity shown. He speculated
that these active centres might
comprise surface atoms of low coordination number and he generalised this concept in a most far-sighted way by stating "that the fraction of the surface that is active depends on
the reaction being catalysed". Thus some reactions might be able to utilise all surface atoms for catalytic purposes; others might have highly specific requirements and thus only be able to use a small fraction. Much later Michel Boudart [71] introduced the terms facile and demanding to describe these two situations and later they were replaced [72] by the terms structure-insensitive and structure-sensitive: these are the terms now in general use. Taylor's original idea was expanded and applied by the Russian scientists Kobozev [73] and Balandin [74,75] in the period between the two World Wars; we shall see below what further refinements of those concepts are desirable. The belief has gradually arisen, based on evidence from a variety of sources, that the active centre for a reaction deemed to be structure-sensitive contains a number (possibly quite a large number) of the catalytically active atoms. Sometimes the numbers suggested are so large as almost to defy common sense [76]; these may however arise from the use of an over-simplified model of surface structure and composition. For the moment, to be brief, a reaction is said to be structure-sensitive in the broad sense of the word if its rate (or better its TOF) is highly dependent on (i) metal particle size in the case of monometallic catalysts; or (ii) surface composition in the case of alloy catalysts; or iii) crystallographic structure of the exposed surface(s), whether of small metal particles or of single crystals. Since the experimentally observed effects may in fact have somewhat different physical causes, it will be prudent to assign these three situations respectively to (i) particle-size sensitivity, (ii) ensemble-size sensitivity and (iii) plane-structure sensitivity. Even this further stage of classifying activity-structure relationships may ultimately prove inadequate. The experiments that led Boudart to propose the simple dualistic classification involved the annealing of platinum particles without changing their size [77]; alternative explanations may be possible, but it is simplest to suppose that the heat treatment just produced a smoother surface. The observation that the TOF of a reaction is particle-size sensitive can be taken to mean that some specific geometry of active centre is
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281
needed, or that tight constraints apply to the kind of site that can be effective, and that it is unlikely that such sites will be found in either large or small particles, depending on the sense of the change of TOF with size. The argument is based on the use of geometric models, such as those described by Hartog and Van Hardeveld [78,79] and by Poltorak [80]. The geometric basis for particle-size sensitivity has been questioned [81] and its status needs re-examination. First, there is the possibility that surface mobility smoothes out geometric and structural features on small parrticles under reaction conditions, especially if the temperature is high or the reaction very exothermic (see chapter 5). Surface melting temperatures can be much lower than bulk values for small particles, and very small particles are nearly all surface. Second, size effects may be a consequence of changes in electronic structure: for very small particles the band-width decreases and the properties of surface atoms approaches those of free atoms (see chapter 5). Third, the theoretica]t calculations of Hartog and Van Hardeveld [78,79] apply only to assemblies of particles having the same number of atoms, and in the main to particles formed from just such a number of atoms as will make perfectly regular shapes, with complete outer layers. Particles of this type are statistically improbable, and bearing in mind the breadth of size distributions commonly encountered their relation to the real world is not strong. It is readily shown that the fraction of surface atoms having a specified coordination number fluctuates wildly as the outer layer of a perfect form is built up to make the next-larger form. The only parameter of surface structure that changes smoothly with the number of atoms in a particle is the average coordination number, which increases towards a limiting value, dependent on the particle shape, as size increases. Particle-size sensitivity therefore does not necessarily have a geometric origin. One further recent refinement of these theoretical concepts is the idea, implicit in Taylor's original formulation [8], that there can be degrees of structure sensitivity. David Avnir [82,83] has defined a reaction dimension D R log(r[t-1]) = log k + D-2R log (R[nm])
(39)
where r is the rate, R the mean particle radius and k is a constant, and has tested the equation against a number of experimental results. The concept has however been criticised [39], although the basic idea that there is no black or white but only shades of grey, when considering structure sensitivity, must be correct. We may develop this point in another way. A reaction whose degree of structure-sensitivity is zero is structureinsensitive: there are no constraints on the type of site that will suffice for an active centre, so this must mean that a single atom of any coordination number will do. At the other extre, me, a reaction whose degree of structure-sensitivity is unity will require a very specific, indeed unique, active centre, for which the number of atoms and the coordination
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number of each will be defined. Between these limits, which is where most reactions lie, there will be various extents of stringency in the specification of the active centres. We must suppose that many reactions can proceed, with different efficiencies, on various types of centre: this is what much of the work on single crystal surfaces teaches [84]. The central problem of catalysis is to relate the kinetic parameters of the reaction to the architecture of the active centre; analysis of the arguments involved in deducing the size of an active centre will be considered at a later stage. Thus far our discussion has emphasized the probable role of the size and structure of the active centre in determining its effectiveness, with a passing mention of other possibilities: we have yet to consider the ways in which rates per unit, that
is to say
TOF's, may and do change with particle size. In principle they may either increase or decrease with size, or show some more complex form, e.g. a maximum (see figure 25). We may distinguish between intrinsic and induced effects. Intrinsic effects are caused by the properties of the particles themselves; examples of this would be (i) the large fraction of atoms of low coordination number in small particles at which it is difficult to form multiple M-C bonds [85,86], and (ii) the greater probability of finding large ensembles, where these are required, in large particles. Induced effects are secondary consequences of size, and may include for example (i) a smaller chance of deactivation of small particles by carbon deposits [85,86] and (ii) a greater tendency to become oxidised. It is not always easy to separate all these effects. Thus it will appear that the shape of the curve relating TOF to size (figure 24) will provide some clue as to the factors determining catalytic activity in a particular system. A TOF increasing with size (curve 3) will assign importance to low coordination number atoms and atomically rough surfaces. When attempting to draw such conclusions it is necessary to remember that particle size distributions are never as narrow as we might wish. The simultaneous operation of opposing factors in different size ranges can explain the occurrence of a maximum in the curve (figure 24, curve 4). In general, size effects due to whatever cause are most likely to be found with particles less than 5 nm in size; this is what Poltorak defined as the mitohedrical region [80].
figure 24 Possible forms of dependence of specific activity on particle radius r. Case 1: specific activity independent of r. Case 2: specific activity decreases with increasing r, i.e. smaller crystallites more active than larger ones. Case 3: specific activity increases with increasing r, i.e. larger crystallites more active than small ones.
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It should hardly be necessary to add that knowledge of the structure-sensitivity of a given reaction on one metal is no sure basis for predicting what will happen on another metal; in the terms of Taylor's hypothesis, a change in the effective valencies of surface atoms is likely to alter the active fraction. Per contra, reactions of the same class are likely to show similar structure-sensitivities on a given metal. It has to be said that the state of our comprehension of site requirements of catalysed reactions remains at an elementary level. It is trite but true to say that in catalysis theoretical developments usually lag behind practical applications. There is a great need for more thoughtful experimentation to define the state of a catalytic surface during use, and for more refined models to account for what is seen. In any case, the reasons of particle size dependence of catalysis activity can be very different with different reactions. Let us now survey the progress made so far in elucidating the sensitivity of some characteristic catalysed reactions to the structure of the catalytic surface. 6.5.2
Hydrogenation of multiple carbon-carbon bonds This is a subject that will be reviewed more fully in chapter 11: here we cite only
illustrative examples, obtained with single-metal catalysts, to establish the degree of structure-sensitivity that these reactions have shown. Re:suits are available for a number of alkenes (ethene [87-89], propene [90], cyclopente, ne [91] and cyclohexene [92]), and for cyclopropane and methylcyclopropane on metals that do not cause low-temperature hydrogenolysis (palladium, platinum) [90]. Hydrogenation of all these molecules show little dependence of rate on particle size in the range 1-100 nm, i.e. for dispersions between about unity and 10-2; if anything, TOF's decrease slightly with decreasing size [89]. Changes are however less than ten-fold, so these reactions are generally regarded as being particle-size insensitive. Metals differ in their rates of ethene hydrogenation, but activation energies remain remarkably constant [93], so the variable term is clearly the pre-exponential factor; differences in the extent to which carbonaceous deposits deactivate the surfaces may therefore be responsible. It is possible that there may be a minimum effective size for a metal particle, as there is an indication that very small particles show a decreased activity [89]. Alkynes seem to behave somewhat differently; the TOF for ethyne hydrogenation on Pd/A1203 catalysts decreases with decreasing size of particle [94-96], this being the opposite trend to that noted as possible with alkenes. 1,3 Butadiene behaves in the same way as ethyne [96-98]. Di-tert-butylethyne was selected [91] as a molecule that might show marked structure-sensitivity, because the tert-butyl groups protect the triple bond from easy access to the surface: somewhat surprisingly only a very small particle-size effect was seen. It appears that alkynes and alkadienes coordinate very strongly to exposed
284
chapter 6
low-coordination atoms, forming unreactive complexes [95-97]; if the triple-bond is sterically protected, this may not be possible, but a weaker form of attachment can lead to apparent structure-insensitivity. The hydrogenation of benzene, which has been studied on nickel [99], rhodium [100] and platinum [101,102], is not strongly particle-size sensitive; once again, TOF's vary less than a factor of ten, and very small nickel particles (< 1 nm) appear to be less active than larger ones. The exchange of benzene with deuterium does however show a more marked particle-size-sensitivity (see chapter 11). 6.5.3
Hydrogenolysis of the C-C bond The hydrogenolysis of the C-C bond, as in alkanes (e.g. ethane [103-105], propane
and n-butane [ 106-109] and cycloalkanes (methylcyclopentane [ 110,111]), is commonly regarded as the archetypal structure-sensitive reaction; TOF's do indeed often vary with particle size, but the trouble is that experimental results, when surveyed in their totality [112], show no conformity to any single mode of behaviour. All the four forms shown in figure 24 are to be found, and others besides. Results for ruthenium catalysts [109a] that appear to be trustworthy show a clear and marked increase in TOF with increasing particle size (figure 25) and this is readily interpreted in terms of a greater probability of finding the needed large ensembles of atoms in large particles. Hydrogenolysis of cyclopropane, that is, its conversion to methane + ethane, as opposed to its hydrogenation to propane is also structure-sensitive on ruthenium catalysts; formation of excess methane (i.e. methane/ethane > 1) clearly requires large particles [113]. The reasons for the lack of conformity of the results in the literature on hydrogenolysis are [112] not easily discerned. It may be that we expect too much of studies which, while in the main have been carefully conducted, explore only limited facets of each system. We may try to list some of the parameters needing closer attention; these would include (i) the breadth of the particle size distributions (ii) the possible presence of toxins from the precursor (e.g. C1-) or the support (e.g. $2-), (iii) self-poisoning by carbonaceous residues, and (iv) differences in the manner in which reactions are conducted (e.g. temperature, contact time, reactant concentrations). Recent detailed measurements on the kinetics of hydrogenolysis of the lower alkanes [114] has begun to resolve some of the discrepancies: thus for example the form of the dependence of rate on hydrogen pressure is particle-size-sensitive, and relative rates for large and small particles depend on the hydrogen pressure [114]. The pressure at which the rate is maximum is higher for large particles and the 'order' in hydrogen above the maximum is less negative. By using a rate expression based on Langmuir-Hinshelwood formalism (section 6.4.2), these effects can be traced to differences in the heat of adsorption of hydrogen, and the enthalpy change of the dehydrogenation that precedes the activation of the alkane. In view of the complexity of
The catalytic cycle
285
the proce,;s and the many variables that may affect its rate, it is perhaps little wonder that the literature is so discordant. ( H / Ru ) tot 0
0
J
0.5 I
i
1.0 i
1.5
nC 4
A
I0 0 ~2
-
-
3
~
figure 25 Dependence of TOF on dispersion for hydrogenolysis of of n-butane on Ru/A1203 catalysts [114]. Open points -first HTR; full points - after oxidation and LTR; half-filled points second HTR.
In some respects, cycloalkanes behave differently from linear alkanes" the hydrogenolysis sometimes seems to be structure-insensitive and the reaction more akin therefore to hydrogenation. 6.5.4
Hydrogenation of carbon monoxide
The particle-size-sensitivity of this reaction has been investigated on each of the metals that have practical importance for the synthesis of hydrocarbons by the FischerTropsch process (nickel [115,116], cobalt [117], iron [118], ruthenium [119-122]), and a certain generalization seems possible. There is a strong and general tendency for the TOF to decrease with decreasing particle size, the more rapidly when this becomes less than 3 nm. On these metals the principal route to hydrocarbons is by dissociation of carbon monoxide (see chapter 14), and we know from chapter 1 that this requires an ensemble of several atoms. We also know [85,86] that small particles are less well able than large particles to bind carbon atoms by multiple carbon-metal bonds. These considerations taken together would serve to explain the observed particle-size dependence.
286
chapter 6
Dissociation of carbon monoxide precedes its disproportionation: (i)
CO
~
C + O
(ii)
CO + O
,~
CO2
(iii)
2CO
~
C + CO2
(40)
On nickel [123,124], iron [125] and palladium [126], small particles are more active than large ones; however there may be different reasons for this seemingly uniform behaviour. With iron, and probably also with nickel, the first reaction is probably the faster, so it must be the second reaction that shows the particle-size-sensitivity, at least, when the reaction steps differ indeed in their rates. Palladium on the other hand is reluctant to dissociate carbon monoxide, but is very active for its oxidation: so the particle-size effect must be connected with the first reaction. This suspicion is confirmed by the observation that defects on the surface of a single crystal of palladium can initiate dissociation of carbon monoxide [127]. 6.5.5
Ammonia synthesis The industrial importance of the Haber process for the synthesis of ammonia has
been responsible for much fundamental research; the chemisorption of the reactants and the interaction of the atoms and radicals formed thereby are reactions that are particularly suited to study both on supported metals and on single crystals. Various catalysts have been fully characterized by adsorption and by physical techniques, and the rate of synthesis appeared to be lower on small particles than on large ones [128,129] (figure 26). It was noticed, however, in confirmation of earlier observations [130], that nitrogen adsorption causes reconstruction of iron surfaces, creating more of the atomically rough (111) planes of the bcc structure of iron. This surface contains deep pits in which an atom or molecule may become coordinated to no less than seven iron atoms. Such a site is shown in figure 27. The conclusion drawn [128,129] from this work on supported iron particles have been confirmed in all essentials by work on single crystals of iron [131,132] and of rhenium [133]. The usual promoters in ammonia synthesis catalysts, namely potassium and alumina, appear to influence not only the chemisorption of nitrogen and ammonia, but also the process of reconstructing the surface during operation [134]. Of course, the deep holes in the Fe(111) plane cannot exist in very small iron particles, but the size of the iron particles in industrial catalysts, which consist predominantly of iron, is quite large.
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287
figure 26 TOF for ammonia synthesis on magnesia-supported and unsupported iron catalysts, using hydrogen, carbon monoxide or nitrogen adsorption to count active sites [137].
1000 o
100
NH ~
o
o
10-
301~n"
o9
t/9 c,") o
1 A
x Z
0.1
o/:
"t7:
0.01
0.001
6
-
"'r'l ' I' 2
4
i'1 6
o
' 1 ' 1 ~i'r 8 10 12 14. 6 0 6 2 d Fe' n m
F~(~il)
W(210) 9
Fr
figure 27 Low index surface planes of bcc iron and the active sites C7 [16c, 133]. Schematic representations of the idealized surface structures of the (111), (211), (100), (210) and (110) orientation of iron single crystals. The coordination of each surface atom is indicated.
The nitrogen molecule does not dissociate readily, and on most metals of Groups 10-12, including those most active for other hydrogenation, it does not dissociate at all. Even on iron, the deep sites on the (111) plane are needed to provide positions where the nitrogen molecule can be accommodated under the influence of surrounding iron atoms in a strongly-held molecular precursor state [135,136]. The promoters can also help to stabilize it. The vital importance of this precursor state is nicely confirmed by reference to figure 28, where it is shown that structure-sensitivity is only shown when the rate is related to the total surface area as defined by chemisorption of hydrogen or carbon monoxide [137]: when the number of active sites is counted by nitrogen chemisorption
288
chapter 6
and the rate is expressed relative to this number, the apparent structure-sensitivity disappears. The study of this system therefore teaches us another important lesson, one that may have wider significance. In hydrogenation and hydrogenolysis studies, and quite generally, the number of active sites is tacitly assumed to equate to the number of hydrogen atoms or carbon monoxide molecules adsorbed: sometimes but rarely, the
number of surface metal atoms is derived by other means, e.g. TEM. The fact that the surface stoichiometry H/M S can also be particle-size dependent is also frequently ignored. In these reactions we have no means of counting the true number of active centres: it is
no wonder the literature is in such a mess. 6.5.6
Oxidation reactions Results on this class of reaction have been reviewed [112]; molecules studied
include hydrogen, carbon monoxide and ethene. In the case of hydrogen, the reaction is particle-size-sensitive, but only when hydrogen is in excess [138]. In other cases it seems that larger particles are more effective than small ones. Probably because the latter more readily suffer complete oxidation, or at the very least inactivation through very strong chemisorption of oxygen. It is necessary to be clear in our thinking when we pose the question: is this reaction indeed particle-size-sensitive? There will be more about these reactions in chapter 12.
6.6.
Catalytic consequence of metal-support interactions
We conclude this introductory chapter on the principles of catalysis with a short description of effects which may arise when the catalytically-active phase is dispersed on the surface of a material which can stabilize it as small particles: this material is usually oxidic and of high surface area, so that the active particles may be well separated from each other, and their coalescence minimised. A full description of the supports and the ways in which supported metal catalysts are prepared is given in the following chapter. Having discussed at some length the effect that may follow from using metal or alloy particles of extremely small size, it is only right to note that they cannot be used in the total absence of another phase. True, aerosols of metals can be made, and have been studied to a very limited extent; colloidal dispersions have received more attention. Of infinitely greater interest and importance are supported metal particles, and before getting down to practicalities we must examine in what ways the metal particles and their support may mutually interact. When the catalytic properties of a supported metal are being studied, the first question requiring an answer is: where is the reaction taking place? The obvious answer -
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289
"on the metal" - is not always true, or at least wholly true. There are two other possibilities: (i) bifunctional catalysis in which part of a complex sequence of reactions takes place on acidic centres on the support; and (ii) spillover catalysis, in which species migrating from metal to support (or vice versa) can participate in the reaction. Bifunctional catalysis is well recognized as the mode by which petroleum reforming occurs under industrial conditions: a fuller account will be given in chapter 13, but the essential features are (a) dehydrogenation of alkane on the metal (normally platinum); (b) gas-phase migration of alkene to the acidic centre (usually on 7-A1203) where it isomerizes; (c) return migration again via the gas-phase to the metal where it is re-hydrogenated. There are numerous and growing instances of spillover catalysis. Here reactive intermediates formed on the metal (especially hydrogen atoms) migrate over the surface from metal to support, where much of the reaction may occur [139]. As an example we may cite the hydrogenation of carbon dioxide on rhodium catalysts: carbon monoxide was detected on the metal and formate ion on the support; carbon dioxide chemisorbed by insertion in surface hydroxyl groups and methane was formed via formate ion by reaction with spillover hydrogen atoms. Other reactions where good evidence for spillover catalysis exists include carbon monoxide oxidation, oxidation of carbon monoxide by nitric oxide and the water-gas shift reaction (CO + H20 , - CO 2 -I- H2) [139]. It has recently been invoked to explain differences in TOF with different types of support used for catalysts to hydrogenate benzene (see chapter 11). The possible occurrence of spillover catalysis is yet another complication in the study of particle-size effects. There have been a number of theoretical studies on the equilibrium shape of small metal particles, both in vacuum and gas atmospheres (see chapter 5)" it is an interesting question whether the same principles apply to particles sitting on a support [140]. In theory, for solids as for liquids, the shape of such particles should depend on the balance of forces between the three interfaces, the stable configuration being that which minimises the free energy of the whole system. It is however extremely difficult to obtain reliable information on the shape of supported particles in the size range of catalytic interest. High resolution transmission electron microscopy is starting to produce evidence of their morphology, particularly in the case of microcrystalline supports such as titania [141a] or magnesia [141b]. Obviously differences in particle shape can lead to differences in catalytic behaviour, through the operation of a metal-support interaction of this kind. We shall see in the next chapter how with some supports it is possible for some metal ions to become firmly fixed to the support, in a way that makes their reduction very difficult. This can lead to a bimodal particle size distribution, with the relative proportions of the two types of particle changing with metal loading: usually the larger particles become more dominant as the loading increases. Moreover, the stabilized ions can show some catalytic activity of their own. Here is yet another factor to complicate the study of particle-size effects, and to generate phenomena attributable to metal-support interactions.
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chapter 6
With the noble metals of Groups 10-12 they are seen with alumina and titania, but to a lesser extent with silica. Unreduced ions may help to form a kind of chemical glue between the metal particle and the support, thus reducing its mobility and its tendency to sinter; they may also have the effect of making the particle seem electron-deficient. Indirect evidence for Pt-O-Si bonds in a Pt/SiO2 catalyst has come from an interpretation of the thermal desorption spectrum of hydrogen in conjunction with the volumetric uptake and the size estimated by electron microscopy [142]. A long-standing subject for discussion concerns the ability of a metal particle's electronic structure to be modified by the support on which it resides. Inventive ways of examining this question were conceived many years ago by Schwab [143] and Solymosi [144], who put metals on various semi-conductors and semi-conductors on various metals. Positive results were obtained for systematic changes in the activation energy of simple reactions such as formic acid decomposition (see chapter 11), but the work fell short of modern standards in terms of eliminating other possible explanations. It is now generally held that the transfer of electrons from the support to the metal is improbable, since the first electron moved would set up an image charge which would hinder further movement (see also chapter 5). In what are amongst the most cited papers of the literature of catalysis, S.J.Tauster and his colleagues reported [145-147] that when the noble metals of Groups 10-12 were supported on titania, or a number of other reducible oxides, they lost much of their capacity to chemisorb hydrogen or carbon monoxide. These surprising observations have generated an enormous literature in the years since 1978 when the first papers appeared, and they have been a stimulus to research on supported metal catalysts in general. They have been frequently cited and reviewed [139,147-150], and only a short resume is possible here. The earliest explanations involved partial reduction of the support by spillover hydrogen, forming Ti 3§ ions which then reduced the active metal: Ti 4+ + H
--~
Ti 3+
+ H+ (41)
Ti 3+ + Pt ~
~
Pt-
+ Ti 4§
There was also early evidence to suggest a change in the morphology of platinum particles, due to a strong metal-support interaction (SMSI) involving the Ti 3+ ions. There then appeared a great many results to show that reduced titania species (TiO2_x) were able to decorate, or partially cover, or even completely mask, the metal particles. There is no doubt some truth in all of these ideas. The effect of SMSI on catalytic activity for structure-sensitive reactions is frequently disastrous, but the results are not always in agreement. Structure-insensitive reactions are obviously less affected and syngas reactions
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291
and hydrogenation of aldehydes and ketones can even be promoted. The effect is reversed by oxidation and even by water, so that with reaction of syngas in which water is a product it is hard to maintain the original SMSI state. It can also be reversed by oxygen atoms liberated by the dissociation of carbon monoxide, e.g. on Rh/V203_x. Similar effects can be produced in a more controlled fashion by adding titania as a structure-modifier to an active metal on a support such as silica. Model catalysts (see chapter 7) have also been made to elicit the mechanism of the effect. As well as titania, much work has also been performed with vanadium oxide (V203), niobia, zirconia and others. Loss of chemisorptive power is also experienced when heating metals on silica, magnesia and alumina to high temperatures, but in these cases the effect are due either to toxicity from within the support or the formation of intermetallic compounds. As mentioned above the outstanding beneficial effect of the SMSI is seen in the hydrogenation of carbon monoxide and of aldehydes and ketones, where unusually fast rates are observed without keeping the SMSI effects intact. These are thought to be a consequence of the formation of more active sites at the periphery of the metal particles; sites of this kind have also been implicated in certain types of hydrocarbon transformation. The SMSI concept has been invoked to explain a wide variety of other phenomena not obviously having the same cause. Systems such as the modification of iron catalysts by manganese for the production of alkenes and even petroleum reforming by Pt-Re/A1203 catalysts have been stated to exhibit the symptoms of SMSI. It is likely too that some of the lack of reproducibility of results in the literature on quite straightforward catalysts (Pt/A1203 etc.) may arise from the adventitious or unwitting occurrence of an SMSI. A number of oxides used as supports (e.g. A1203, TiO 2, V203) are soluble in strong acid and from the use of hydrochloric acid there may be formed chloro-species in solution (A1C14-, TiOC13- etc.) which during drying may precipitate onto the metal, giving after reduction the appearance of SMSI. Although the flood of literature on this subject has now somewhat subsided, there is a continuing interest in the practical potentialities of the effect, and there are still many unresolved questions concerning the finer details of hydrocarbon transformations. These considerations will serve to keep SMSI as an active area of research.
References 1
A.Farkas, L.Farkas, E.K.Rideal, Proc.Roy.Soc.A. 146 (1934) 630
2 3
G.C.Bond, J.Sheridan, D.H.Whiffen, Trans Faraday Soc. 48 (1952) 715 C.Kemball, Bull.Soc.Chim.Belg. 67 (1958) 373
4a
G.C.Bond, "Catalysis by Metals", Academic Press, London/New York, 1962
292
7 8
9 10a b
11 12 13 14 15 16a b C
17 18 19 20a b C
21
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b 50 51a b C
d 52
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d e
f 54 55 56 57a b C
d 58 59 60 61 62 63 64 65 66 67 68 69 70 71
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72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101
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The catalytic cycle 135 136 137
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299 Chapter 7
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P R E P A R A T I O N AND C H A R A C T E R I Z A T I O N OF M E T A L AND A L L O Y CATALYSTS
There are two kinds of catalysis research, each with its own distinct objectives: (i) that mainly conducted by workers in industry, having the purpose of devising either a more active, more selective, stabler or longer-lived catalyst for an already operating process, or of designing a suitable catalyst for a new process; and (ii) that mainly performed by academics, designed to achieve some degree of understanding of how catalysts work [1]. Of course, in pursuit of the practical objectives it may well prove necessary to make some fundamental studies, but these will be targeted and rarely will be simply curiosity-driven; and similarly some academic will pursue studies having a distinct application in view, either because they enjoy a challenge or because industry pays them to do it. This broadly valid distinction serves as a convenient introduction to our classification of methods of preparing and studying metal and alloy catalysts. Fundamental work is directed to explore how the surface of an alloy differs in its catalytic behaviour from that of a single metal, and to explore such effects as modification of ensemble size, changes in electronic structure and the spillover of reacting species from one alloy component to another. For such work there is the need to have surfaces whose structure and composition can be defined by direct observational methods, before, during and after reaction.
Macroscopic materials are preferred for these studies. Metal or alloy films are used much less than formerly, and other polycrystalline forms (wires, foils, etc.) have now largely be superceded by single crystals and "surface alloys" (section 7.1). These forms all have low surface areas and except in a very few specific cases have little practical application: ammonia oxidation and the Andrussow process are however the important exceptions. Small unsupported particles having a higher surface area can be prepared for use in either the gas phase or the liquid phase (or sometimes both). For the latter, "blacks" formed by chemical reduction, Adams oxides, Raney catalysts and colloidal dispersions are of interest and use; mixed oxides formed by thermal decomposition of precursors lead on reduction to metal powders which were formerly often used (section 7.2). However a great many industrial processes require catalysts suitable for prolonged use in the gas phase, and here attention is usually directed to supported alloys because of their stability and (sometimes) ease of regeneration. A number of different methods have been tried and found effective (section 7.3); however while it is possible to use a catalyst
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in an important process without knowing intimate details of its structure, the same cannot be said of basic research, and there are a number of difficulties to be described in section 7.3 in the way of making supported alloy catalysts which meet all the stringent criteria that should be applied to a material for use in a fundamental study. There is however a considerable body of knowledge concerning the choice of support, the best precursors and pretreatment conditions to guide the aspiring worker in this field. Finally having made catalysts, one will need to know whether ones' aspirations have been fulfilled. This requires the most thorough possible physical characterization, through quantities as simple as the BET surface area to the use of refined techniques such as EXAFS (section 7.4). It is however important to realize that information obtained in excess of that required to answer the questions posed represents a waste of time and money: it is what the old English Prayer Book used to describe, in Article XIV of the Thirty-Nine Articles of Religion, as "a work of supererogation". In particular one often wonders at the thinking, if any, behind papers which describe the most penetrating characterization of catalysts which appear to have low or generally uninteresting catalytic activity. Effort should be reserved for materials that merit it.
7.1
Macroscopic materials
7.1.1
Polycrystalline materials Metals and alloys in forms categorized as macroscopic are those in which the
assembly of the microcrystals of which the material is composed are readily visible to the naked eye: such forms have been primarily but not exclusively employed in the study of catalyzed reactions in the gas phase. The principal physical forms to be considered are
single crystals, wires, films, granules and coarse powders; these forms comprise the metal(s) chiefly in the zero-valent state or are produced therefrom by some type of comminution. They are therefore distinguished from the small unsupported particles to be discussed in section 7.2, which in the main are formed by chemical reduction of precursor compounds, and having a generally smaller particle size are more suited for use in liquid or three-phase systems. The surfaces of those macroscopic forms which have been exposed to the atmosphere will be to some degree contaminated, and need to be cleaned before use: dissolved impurities can also cause problems. In this section attention is reserved to polycrystalline materials, excluding metal films, which will be considered in the following section. Metals and alloys as commonly experienced are comprised of innumerable small crystallites of irregular size, shape and orientation, welded together under the influence of heat to provide materials processing the traditional attributes of the metallic state: strength,
Preparation and characterization of metal and alloy catalysts
301
hardness, malleability, high thermal and electrical conductivity, etc. These crystallites, which are typically some ~tm in size, are separated from each other by grain boundaries, along which cracks occur when the material is under stress. Many of the mechanical properties such as hardness, strength and elastic modulus are more determined by the microcrystalline structure than by the chemical composition of the crystallites, since deformation is governed by the ease with which they can be parted from each other, rather than by the strength of the bonds between the atoms. It is for this reason that the physical properties of metals and alloys depend so much on their thermal history, and upon their purity, for impurities often tend to congregate at grain boundaries, and thus influence physical properties disproportionately to their concentration. These effects, which lie at the heart of the science of metallurgy, are nowhere more clearly seen than with the element iron, the many manifestations of which, from pig-iron to stainless steel, bear witness to the changes that can be wrought by impurities and additives. Deliberate additions of compounds of the more refractory transition metals can greatly improve the high-temperature stability of the noble 8-10 Group metals, the lifetime of which, in applications such as furnace windings, is limited by the rate of which the crystallites grow: mechanical failure follows from excessive crystal growth, and the addition of grain-stabilisers diminishes the rate of crystal growth, with corresponding practical benefits. However this branch of science only rarely impinges on catalytic practice. Much of the early work on the use of alloys as catalysts was performed using wires and foils: this work is particularly associated with the names of Schwab [2], Eley [3], Dowden [4] and Rien~icker [5]. The principal reason for them being used was no doubt their ready availability, since they were manufactured for other purposes: the palladium-gold alloy wires used by Eley and Couper [6] in their epic experiments on parahydrogen conversion were for example available because they served as heat fuses. Wires were a particularly convenient form to use, because they could be heated electrically and their surfaces could be well cleaned by hydrogen reduction and by heating in vacuo. Accurate estimation of their temperature during use was however difficult as end-losses through the connections were difficult to assess. Much of Rien/icker's pioneering work [5] and also that of Dowden and Reynolds [7,8], employed foils, which were prepared by metallurgical methods. When the required alloys were not already available in either of these
forms, it
was necessary to prepare them from the pure components. The following statement, taken from the paper [2] in which G.M.Schwab summarized his research carried out during and immediately after the Second World War, gives a succinct account of procedures used at the time. "The alloys were prepared from chemically pure metals in porcelain crucibles under suitable molten salts in a gas blast-furnace, and the phases present were found by Xray analysis. Ductile metals were used as foils, brittle alloys as pieces, and the total geometrical area of the pieces estimated by measuring their dimensions".
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Although much of this early work, carried out in the 1940-1960 period, would fail to meet today's standards, and although the interpretations were based in the main on over-simplified models that were ultimately shown to be incorrect, it nevertheless put alloys on the map, and the principal features of the catalytic properties of alloys, especially those involving pairs of elements from Groups 10 and 11, were clearly delineated for the first time. Subsequent more careful work with other types of surface has served only to confirm and extend the original observations, but it is strange that Schwab's work on Hume-Rothery alloys, comprising pairs of sp-metals, has attracted so little later attention. The one and almost the only industrial application for an alloy in a macroscopic form is the platinum-rhodium gauze used for ammonia oxidation [9-11 ]: 4NH3 + 502 ~ 4NO + 6H20 The nitric oxide (nitrogen monoxide) is then oxidized to the dioxide, which is absorbed in water to give nitric acid (see chapter 12). The oxidation reaction requires high operating temperatures (1170-1220K) and very short contact times ( 1 0 -3 - 10-4S): the form of catalyst used almost without change since the inception of this process is a bed of finely woven gauzes, made from 0.06 mm diameter wire with 1020 apertures cm -2. The loom needed to weave a gauze from such wire is itself a masterpiece of technology, as for low-pressure reactors gauzes of up to 4m diameter are employed. A principal feature of this reaction is the restructuring of the wire that occurs during use: in the course of a few days the surface of a new gauze becomes roughened, and the rearrangement continues inexorably to the point where small fragments are blown off the surface by the gas which is flowing through the bed of gauzes with a very high linear velocity. The alloy crystallites also grow in size, and the gauze eventually disintegrates. With pure platinum these changes take place with unacceptable rapidity; the ideal gauze material would therefore be one with which the initial surface roughening developed quickly, because only when this has happened does conversion attain its maximum value (close to 100%); however, it is also desirable to arrest the subsequent changes which lead ultimately to the gauze's destruction. A large number of alloy formulations were prepared and tested during the 1920's and 30's, and in the course of this work [12] it was found that addition of 5 or 10% of rhodium to platinum increases conversion efficiency and reduces catalyst loss; higher rhodium concentrations lead to material which is difficult to draw into fine wire. There have been several studies of the restructuring, for which scanning electron microscopy is an ideal tool [10,11]; however the mechanism by which it occurs is complex and obscure, the one certain fact being that the changes do not occur when the gauze is simply heated in air at the normal reaction temperature. The literature contains several detailed accounts of the process, the equipment and conditions; a recent review [9] has described the types of gauze used in the Commonwealth of Independent States (formerly the Soviet Union),
Preparation and characterization of metal and alloy catalysts
where alloys composed of 81% platinum-35%rhodium-15%
303
palladium-0.5%ruthenium
were favoured as being just as effective and somewhat cheaper than those used in the West [9]. In a closely-related process, named after the Russian scientist Andrussow, hydrogen cyanide is synthesized by the reaction CH 4 + NH 3 + 3/202 ~ HCN + 3H20 The same type of catalyst is used and reaction conditions are similar, save that the temperature is somewhat higher (- 1400K). Disintegration of the gauze also occurs during use [10]. The only other industrial process for which a macroscopic form of metal is used is the oxidation of methanol to methanal (formaldehyde) catalyzed by granular silver: this however, seems to be used as a pure metal rather than as an alloy. 7.1.2
Metal films [13-20] The use of foils, wires and granules for fundamental research had severe limitati-
ons; their areas were low and their surfaces sometimes difficult to clean. There was a perceived need for a higher area form, the surface of which would be formed in clean conditions, such as a vacuum. The use of evaporated metal films (perhaps more properly called condensed metal films) was developed and popularized by Otto Beeck and his associates at the Shell Development Laboratory at Emeryville, California; although not the true begetter [21,22], this group described procedures for forming, characterizing and using such films, and reported extensive measurements of heats of chemisorption and catalytic activity for films of a number of different metals [23-27]. The idea was rapidly taken up by a number of others, notably by Kemball and his co-workers [28] for his studies of exchange of alkanes with deuterium, by Mignolet [29] for the measurements of surface potential and by Rideal, Trapnell and their students for work on chemisorption of hydrogen and other molecules [30-32]. The basic idea of preparing a metal film is simplicity itself. A wire of the element which it is desired to cast into a film is mounted in a glass vessel through which it is connected to electrical leads; the wire is degassed by being heated electrically, and the vessel is pumped while being heated externally. The system resembles nothing so much as an electric light bulb with a tungsten filament. When a sufficiently good vacuum is obtained, the vessel is cooled and the current through the wire is increased, so that atoms evaporate from the wire and condense on the walls of the vessel. For reasonably refractory metals, and provided the vessel walls are cold, it is possible to obtain quite porous films with areas up to 60 m 2 g-1. Many modifications and refinements of the procedure are
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possible. Metals that cannot be drawn into wires may be physically attached to a more refractory metal (tungsten is often used for this purpose) and the film is thrown by electrical heating to a temperature at which the desired metal vaporizes but the refractory one does not. With the use of very thin glass walls, the reactor may be deployed as a sensitive calorimeter [27,33-35], and the properties of films and the way they change following chemisorption of gases can also be characterized by electrical conductivity, surface potential measurements [29], magnetic properties [36] and many other techniques. Apparatus of considerable complexity is required for such studies: an example of that used for measuring photoelectrical work function is shown in figure 1 [37]. ---Ig
18-
1
9
'o
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figure 1 Upper part. Photoemission work function cell I. For adsorption on films at low temperature. The main parts: 1- and 18- Dewar; 2-film support; 3- thermocouple; 4- external glass tube; 5- quartz windows; 6- and 7- evaporation sources; 8- platinum electrode; 9tungsten anode lead; 10- tungsten cathode lead; 11- and 12- leads; 16- shutter; 22-24leads for shutter movement (continued on the next page).
Preparation and characterization of metal and alloy catalysts
305
lower part: Photoemission work function cell II. For preparation of well-sintered films. The main parts: 1- Pyrex tubing; 2- tungsten rails; 3- photo cathode, below left expanded, showing the film substrate (8) quartz oven (9) and movable carrier underneath. Cathode can be placed under a heat shield (17). 14- thermocouple connected to leads in a constant temperature bath (21). 23-24- evaporation sources.
(
Photoemission work function celI II, the total view.
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Procedures for forming films and for studying species adsorbed on them have been described a number of times [15-19,38-40]. The two volumes edited by J.R.Anderson, although now more than 20 years old, give a full and detailed account of work performed up to about 1970 [13]: they were written at the time when interest in metal films was at its apogee, and are an impressive memorial to a technique which has all but passed into history. The more recent book edited by Wissman [19] is also a mine of information on this subject. While the procedure for preparing a films of a single metal is comparatively straightforward, the preparation of homogeneous alloy films is more problematical. There are three ways in which this can be attempted [14]: (i) by sequential evaporation of the components, (ii) by simultaneous evaporation of the components and (iii) by evaporation from an alloy wire. In each case it is necessary to heat the film in order to homogenise it, unless the wall is maintained at high temperature (--670K). Method (iii) works well where the two components do not differ too greatly in volatility, although the more volatile naturally evaporates preferentially and the film will not have the same composition as the wire: this will not matter however providing it is analyzed. Method (i) was found suitable for preparing nickel-copper films [41-45], while both methods (i) and (ii) have been used for making palladium-silver films [46-50]. The composition of the film can be controlled either by adjusting the currents through the wires or leads (method ii) or by the times and
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the currents (method i). It is necessary to watch out for reaction between the metal of interest and its refractory support (e.g. tungsten), since this may result in co-evaporation of both when it is not expected. The composition and homogeneity of the films can be assessed by X-ray diffraction (giving a mean lattice parameter), X-ray fluorescence, electron microscopy and work function measurement, as well as of course by catalytic activity. Alloy films (platinum + copper, iron or nickel) have also been prepared by RF sputtering [51 ]. In addition to the work on nickel-copper and palladium-silver alloys cited above, a number of other alloy systems have been prepared and used; these include palladium with platinum [52,53], rhodium [54,55], gold [56-58], tin [57,59] and nickel [60,61]; gold with platinum [62,63], nickel [64], iridium [65,66] and rhenium [67]; and rhodium with platinum [59] and copper [68]. Although they represented a vast improvement on the prior art, and permitted the development of new ideas concerning metal and alloy catalysis, they were not without their limitations and drawbacks. Films of the lower-melting metals were prone to sinter; they seemed more liable to deactivation by carbon deposition than were supported metal catalysts (see section 7.3); and some totally spurious effects were observed when attempts were made to measure heats of chemisorption calorimetrically, due to inadequate distribution of gas doses over the films. They were of course polycrystalline, exposing various crystal faces at their surfaces. Films are now little used in fundamental work, but the technology has been put to good use in preparing (i) metal-on-metal catalysts (see section 7.5), (ii) small metal particles by evaporating metal atoms onto an oxide substrate, preferably a single-crystal oxide surface, giving what is sometimes called a model catalyst (section 7.3.3) and (iii) very thin single crystals suitable for use in the measurement of heats of adsorption [69,70]. To provide a better understanding of how metal-on-metal catalysts are made, we must first discuss in further detail the mechanisms by which films are formed. The manner in which a film of condensed metal atoms grows depends principally on the chemical nature of the substrate to which they adhere, and its temperature [71-73]; these factors together determine the movement of an atom after its first sticking collision. At high temperature, or generally when the interaction between the atom and the substrate is weak, the atom will be mobile after collision, and will migrate around the surface until it meets another of its kind, with which it may form a stable bond: this pair may then act as a nucleus to which other atoms can adhere. There will then develop islands of monoatomic thickness, and as the deposition continues some atoms will arrive on these islands as well as on the substrate. At low temperatures, or generally when the cohesive energy of the deposited metal is large, the former atoms will remain where they collide and so a second layer starts to be formed before the first is complete (see figure 2). In addition, if adhesion to the substrate is weak, atoms arriving there may migrate to the growing metal
Preparation and characterization of metal and alloy catalysts
307
particles rather than forming new nuclei. This gives the so-called Volmer-Weber mode of film growth.
figure 2 Volmer-Weber film growth mechanism "s
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3
Number of monolayer equivalents
If however metal atoms in the second layer are mobile and if they bond more strongly to the substrate than to themselves, they will travel to the edge of the island and fall over, thus extending the island until a complete first monolayer is formed. If then the same procedure operates with islands formed on top of the first layer, a second coherent layer will be formed, and this process may continue with the formation of a number of flat coherent layers (see figure 3): this is the so-called Frank-van der Merwe growth mode. An intermediate situation arises when after the formation of the first one or two monolayers that the growth of three-dimensional particles becomes preferred (see figure 4): this gives the Stranski-Krastanov mode of growth. The ideal Frank-van der Merwe mode is not the usual one, either when the substrate is another metal or when it is a semiconductor or when it is an oxide such as
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glass. It is however observed in a few cases of metal-on-metal film growth (e.g. Sn/InSb(100) and Sn/CdTe(100) [71 ]). Following changes in the strength of Auger signals during deposition can reveal the type of growth mode (see figure 5): deposition of molybdenum on Cu(100) follows the Stranski-Krastanov mode, since after formation of two coherent layers the results depart from those calculated for layer-by-layer growth, more markedly at higher substrate temperature [74]. Sometimes of course direct observation by electron microscopy will reveal the growth mode; for example, deposition of lead on graphite leads to the formation of solid droplets that do not wet the graphite surface [71 ].
1
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300K
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Metal films may also be prepared by thermal decomposition of a volatile and unstable compound of the metal. This technique is of course much used in the preparation of semiconductor devices, but seems to have been little used for metals or alloys of catalytic interest. As an example of the potential usefulness of the method, it has been reported that quite clean films of sodium and of potassium can be formed by decomposing their azides [40]. 7.1.3
Other methods of comminution A number of other methods have been employed to produce highly dispersed metal
Preparation and characterization of metal and alloy catalysts
309
particles from massive metal, and some have led to observations of catalytic activity. Colloidal dispersions of pure metals have been made by striking an arc between fine wires in a liquid medium, a technique associated with the name of Bredig. Organosols of several metals have also been made in this way [39]. On heating a metal above its melting temperature in an inert atmosphere, a metallic "smoke" is formed, from which small perfectly formed crystals form and may be trapped on a cold probe [39,75], and discharging a bank of condensers through a wire causes it to explode into atoms which aggregate into fine particles [76]. Klabunde's technique [77] for forming highly dispersed metal involves evaporating metal from a heated wire onto the surface of a frozen organic substance. After warming and evaporation of the solvent the fine metal powder remains. Before this method was reported, Blyholder had studied by IR spectroscopy the adsorption of carbon monoxide and other molecules on metals, formed by evaporation in a low pressure of helium and deposited on a heavy oil such as Nujol [78-80]. Colloidal dispersions can also be formed by abrasion, an effect which is observable every time one cleans an aluminium saucepan with a Nylon scrubber. Most of these methods could be adapted to form well dispersed alloy particles, but none seems yet to have done so, and indeed none of these methods have received widespread approbation as applied to pure metals. Sputtered films have also been used, the first report [81] being as long ago as 1933. More recently, films of iron and nickel, and of platinum-copper, have been prepared by RF sputtering [51]. 7.1.4
Single crystals "The old order changeth, giving place to new", and the growing availability during
the 1960's of large single crystals of a number of metals, coupled with improvements in access to ultrahigh vacuum (uhv) conditions, began a new era in the study of interfacial phenomena at the gas-solid interface. Metallic single crystals could be cut to expose chosen low Miller index plane, which could then be polished and cleansed in situ by bombardment to yield after prolonged outgassing a surface which was as near perfect, perfectly clean, as could be wished. In a significant development, G.A.Somorjai and
any ion and co-
workers [82] realized that cutting a single crystal at a slight angle to a low index plane would produce a surface having a defined concentration of steps and kinks, thus simulating the rougher surfaces exhibited by small metal particles: some examples are shown in figure 6.
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fcc
fcc
(977)
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(b)
figure 6 Schematic representation of the surface structures of several stepped (a) and kinked (b) crystal faces deduced from the bulk unit cell. Contraction of interlayer spacing and other modes of restructuring that are commonly observed are not shown [82].
Preparation and characterization of metal and alloy catalysts
311
The major attraction of the use of the single crystal surfaces, next to the well defined structures, was the range of applicable methods available to characterize its cleanliness and structure before and after use, and to identify the nature of adsorbed species residing thereon. Outstanding among these techniques were Auger Electron Spectroscopy (AES) for estimating impurity levels, low energy electron diffraction (LEED) for determining surface structures, XPS and UPS for atomic and molecular analysis, and IR spectroscopy and EELS (electron-energy loss spectroscopy) for studying interatomic motions in adsorbed species. The list of methods used is of course very much longer: almost every acronym seems to have been applied to the study of single crystal surfaces and some further consideration to the type of information afforded will be given in section 7.4 (see also chapter 2). When working with single-crystal planes, whether as catalysts in their own right, or as supports for other layers, or as a basis for epitaxial or two-dimensional alloys, it is first necessary to cut the single crystal accurately in order to expose the desired plane: this is a task demanding considerable skill. The crystal has to be accurately aligned and this is done with the aid of the Laue diffraction technique [83-85]. An adjustable goniometer combined with a surface-polishing device has been described [86]. It is possible to exercise in situ control of the plane cut by means of LEED. Helium-beam scattering at thermal energies is a very sensitive detector of small deviations from the desired angle, these manifesting themselves as steps [87]. It is of course now possible to examine the perfection of single-crystal surfaces by means of scanning-tunnelling microscopy (STM) and atomic force microscopy (AFM) [88]. Having produced the wished-for plane with the appropriate degree of accuracy, it is then necessary to clean it of all impurities by a combination of reduction, heating, ion bombardment and annealing. Single crystals of some metals, particularly palladium, iron and nickel, contain dissolved impurities (carbon, oxygen, sulfur) which continue to diffuse to the surface after it has been cleaned. Prolonged treatment in vacuo may therefore be necessary before obtaining a crystal whose surface can be maintained in a pristine condition. An alternative could be to grow in UHV a thick epitaxial film of, say, nickel on nickel. These problems have also been mentioned in section 4.3.3. Reference has already been made to the development of techniques permitting the measurement of heats of adsorption of very thin (200 nm) single crystals grown epitaxially on cleaved rock salt which is dissolved after the film is formed [69,70]. While an enormous amount of work has been performed and published concerning the surfaces of single crystals of pure metals, much less has been reported on alloys in the form of bulk random solid solutions or ordered phases. Not only is there surface segregation to contend with (see chapter 4): the common occurrence of restructuring to minimize surface free energy has only recently been recognized and its implications for alloy systems has not yet begun to be explored. Work, including catalytic measurements [89],
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has however been done with PdlsCU85 [92], Pd3Ce [90], Pt80X20 (X = Fe, Co, Ni) [91] and other platinum-nickel [92] single crystals. There is however now more use of and interest in two-dimensional alloys formed by evaporating atoms of one component onto the welldefined surface of the second. This technique is briefly described in the following section. 7.1.5
Two-dimensional alloys
This term is employed to describe surfaces prepared by the evaporation and deposition of atoms of one metal onto a single crystal of another; theoretical considerations of the nature of the bonds formed between unlike atoms have already been described (chapter 3). These systems are also described as "metal-on-metal" model catalysts. They possess certain advantages over bulk alloy single crystals: (i) the composition of the surface is readily altered by increasing the coverage by the deposited metal; (ii) the problem of surface segregation can be avoided, and (iii) one can gain access to systems such as silver/ruthenium where the solubility of the former in the latter is zero, so that bulk alloys cannot be prepared. A very great deal of work has been carried out with the (0001) surface of ruthenium, which is isotropic, modified by copper [93-101], silver [ 102-104] and gold [96,105108]. The large difference in chemisorptive and catalytic properties and in physical characteristics between ruthenium and the Group 11 elements makes this a most attractive set of systems for fundamental study. A short description of the various possible modes that can be adopted in the growth of a deposit on a substrate has already been presented (section 7.1.2, figures 2-4); the form adopted depends not only on the chemical interaction between the components, but also on the temperature. This is well illustrated by the copper-ruthenium system, where two conditions for the deposition have been used [109]: deposition of copper onto ruthenium at 540K, afterwards cooled to room temperature, and at 1080K, followed by rapid quenching to 300K. Under the former conditions, the Auger line-intensity plot shows no breaks corresponding to layer-by-layer growth, and evidence for strong lateral interaction between copper atoms giving island formation even at submonolayer coverages is provided by the development of a two-dimensional band structure [110]. Under the latter conditions, very distinct breaks in the copper Auger line-intensity plot are seen, strongly suggesting that the growth mode is of Frank-van der Merwe type. The behaviour of the surfaces in chemisorbing hydrogen and carbon monoxide, and in respect of physical properties, confirms this conclusion. When silver is deposited on the non-isotopic Ru(10i0) surface, three-dimensional growth of silver crystallites begins at a coverage of about 0.6, i.e. the Stranski-Krastanov mode is adopted; although this implies strong interaction between silver atoms, some ruthenium atoms are still exposed when the equivalent of five monolayers has been laid down [ 104]. The number of two-dimensional alloys being formed in this way is increasing
Preparation and characterization of metal and alloy catalysts
313
rapidly. Fe/Cu(100,110,111) surfaces have been comprehensively studied by Geus et al. [111-113], Sn/Pt(111) and Sn/Ni(111) surfaces have been used for n-butane hydrogenolysis [114] and Cu/Rh(100) for oxidation of carbon monoxide [115]. An EXAFS study [116] has revealed structural and dynamical information on the Co/Cu(ll0,111) surfaces. Work has also been reported on Au/Pt(111) [ 117], and extensive studies have been performed on surfaces of transition metals modified by partial monolayers of alkali metal atoms, with the object of modelling promoter effects [118-120]. 7.1.6
Intermetallic compounds [121-123]
Intermetallic compounds are prepared by argon arc melting of a mixture of the appropriate metals in the desired amounts; where necessary, they may be afterwards homogenized by vacuum melting. Compounds whose catalytic properties have been investigated include Ce2M (M = A1, Co, Ni [124] and Ru, Co, Fe [125]), other rare earth compounds containing iron, cobalt, nickel, ruthenium [126-128] and copper [129,130], CeRh3_xPdx and ZrRh3_xPdx [ 131 ] and Zr-Ni-Co and Zr-Ni-Cu compounds [ 132]. 7.1.7
Interstitial alloys [121,133]
This term is applied to compounds formed between metals of the transition series and small atoms having only sp electrons, namely, boron, carbon, silicon, nitrogen and phosphorous: the latter fit into interstices or gaps in the metal lattice, which often does not suffer a radical change, although it may expand. Hydrides are not regarded as interstitial alloys, although there are some resemblances; with aluminium, we are in the realm of the Raney alloys (see section 7.2). The compounds may be stoichiometric or non-stoichiometric; they may be formed either through a gas-solid interaction, using for example metal or oxide plus methane or ammonia (in preference to nitrogen), or by precipitation from a solution of a metal salt with a reducing agent containing one of the atoms in question (section 7.2.1). There have been instances where interstitial alloys have been produced unwittingly, i.e. where it had been thought that a pure metal "black" was being formed, it only being realized later that a part of the reductant had ended up in the product. They may also be formed during use, as for example the nitrides of iron in ammonia synthesis. Metal borides continue to attract interest [134]. Interstitial alloys are distinguished from other compounds (oxides, sulfides, etc.) by having bonds that are substantially covalent. They are formed most readily by and are most stable with metals in the centre of the transition series (Groups 3 through 10; the Group 2 carbides have a different structure) and especially the base metals of Group 8-10: the noble metals of Groups 8-10 do not in general form them, a fact which is in no small way related to their high catalytic activity. They are hard, high-melting materials,
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possessing electrical conductivity; special efforts are required to reproduce them in higharea form. Their abilities as catalysts have attracted attention in a number of ways. There are some quite old studies of the iron nitrides in connection with ammonia synthesis and of iron carbides and nitrides in relation to Fischer-Tropsch synthesis; Lotz and Sebba [135] as long ago as 1957 evaluated the activity of various transition metal nitrides for the former reaction. It was however the recognition that the carbides of tungsten and of molybdenum are in some ways similar to the noble metals of Groups 8-10 that has caused considerable research effort to be devoted to their study. It is not an easy field in which to work as the surface composition is never quite known with certainty: for example small amounts of oxygen often have an activating effect [ 136]. We do not propose to include a detailed coverage of the physical and catalytic properties of interstitial alloys, and we therefore cite only a few references which the interested reader may use as a basis for further enquiry. It is however worth noting recent claims that (i) molybdenum nitrides show better hydrodesulfurisation activity than the conventional Co-Mo/A1203 [137] and (ii) that Mo2C in high area form is superior to normal platinum catalysts for n-hexane isomerization, both in selectivity and in activity [138]. 7-Mo2N has also been synthesised in high area form [139]. The long-term stability on stream of such materials has not yet been evaluated, nor have the problems of largescale manufacture been explored. It will be some time before they can replace the welltried and tested traditional catalysts. The formation of the palladium silicide Pd2Si has been observed after hightemperature reduction of Pd/SiO 2 catalysts [140] and it is possible that this and analogous processes which may occur with AI203 [141] and with MgO [142] may go some way towards explaining phenomena referred to as 'metal-support interaction' (see chapter 6). 7.1.8
Amorphous alloys [143]
Amorphous metals and alloys (also called metallic glasses) are made by the extremely rapid cooling (105-106Ks -l) of molten metal. Pure metals however easily recrystallize and the stable formation of an amorphous or glassy material, which is in fact a supercooled liquid, requires the ordering process to be hindered during cooling, and this is readily achieved by the use of additives. The products, which possess only short-range order, are of considerable interest as catalysts; they can be produced in a wider range of composition than is the case with their crystalline counterparts, they exhibit only a single phase without surface segregation of either component, they are non-porous and appear to have a high concentration of coordinatively unsaturated sites on their surface. Their availability offers a ready means of testing hypotheses of catalytic action (see chapter 9) which are conditional on there being an array of surface atoms in a regular local geometri-
Preparation and characterization of metal and alloy catalysts
315
cal arrangement [143]. The necessary fast rate of cooling is achieved by melt-quenching methods, in which the molten alloy is brought into contact with a rapidly-moving heat sink, such as a rotating wheel. Numerous variations of the basic concept have been used and have been described [144]. A great many alloy compositions have been studied: zirconium is a ready glassformer, and the catalytic activity is often contributed by added iron, nickel, palladium or copper. Combinations familiar in the context of interstitial alloys, and close relatives, are also particularly convenient to prepare in glassy form; thus nickel and/or iron with boron and/or phosphorus, and palladium with silicon or germanium have often been used. Alloys of the noble metals of Groups 8-10 with zirconium, M25Zr75 (M = Rh, Os, Ir and Pt), have also been made and tested, as have three-component alloys such as (Pd25Cu75)33Zr67 [143]. Either because of their low surface area or because of impurities accidentally acquired during manufacture, or due to their chemical composition, amorphous metals often show only low activity before "activation". Treatment by nitric, hydrochloric or hydrofluoric acid, or by sodium hydroxide, followed by a high temperature oxidationreduction treatment has the desired effect [143]. However, oxidation of alloys containing the less noble elements (boron, phosphorus, zirconium, etc.) leads to a segregation of the components, not generally restorable by reduction. Glassy materials are of course metastable with respect to crystalline forms, and during use may undergo recrystallization at temperatures as much as 200K below the normal (vacuum) transition value, T c. However, some glassy materials have quite high T c values; for Run6Zr54 it is greater than 1000K [145]. The reactants and products may however cause the components to segregate and oxides to be formed. Amorphous alloys have been tested in a great range of catalytic applications and some quite useful levels of activity have been observed for Fischer-Tropsch synthesis and methanol synthesis [143]. Further information will be provided in later chapters. Some of their most outstanding properties are however shown in electrocatalysis, e.g. electrolysis of water and of brine; they have also been considered as electrode materials for fuel cells.
7.2
Small unsupported metal particles
7.2.1
Metal "blacks" by chemical reduction
It has been known since the earliest days of the chemistry of metal sait solutions that a number of metals could be formed and precipitated by the action of liquid-phase reducing agents or solutions thereof, including formaldehyde (methanal), formic (methanoic) acid, sodium formate (methanoate), hydrazine hydrochloride, hydroxylamine,
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hypophosphorous acid (H3PO2) and hypophosphite salts. The metals reducible in this way under ambient conditions include those of Groups 8-10, 11 and 12, but the base metals of Groups 8-10 require stronger reducing agents (e.g. LiA1H4, NaBH4 or their close relatives [146]) or more forcing conditions, e.g. higher temperature, than the noble metals of these Groups. It is best to regard these products as coagulated colloids, for these reducing agents or ones very similar can with careful control of experimental procedures form stable colloidal suspensions (see section 7.2.2). The method was formerly used quite extensively to prepare binary alloys for catalytic use. Systems investigated include platinum-gold, platinum-iridium [147], palladium-gold [ 148-150], palladium-silver [ 151 ] and palladium-boron [ 152] prepared by reducing appropriate salts with NaBH4: the adventitious formation of borides was noted above, but here the use of the tetrahydridoborate ion was deliberate. Rhodium-iron powders have been prepared by reducing (RhCODC1) 2 (COD = cyclo-octadiene) and FeC13 solutions with LiBEt3H [153]. Kulifay [154] has prepared and characterized a number of alloy blacks and intermetallics, particularly those involving mercury as one component, by using mild reducing agents (H3PO 2, hydrazine, hydroxylamine), but compounds such as PtAu 3, PdCu 3 and PtBi 2 were also made. 7.2.2
Colloidal metals [20,155]
The term colloidal metal or metal colloid is applied to a suspension, usually aqueous, of very small metal particles, typically a few nm in size. They are formed by careful and controlled reduction of a dilute solution of a metal salt, and in the absence of impurities, especially highly charged ions of opposite charge to that carried by the colloid, they are stable for long periods of time. In some cases, their preparation is easy and never fails: a brown colloidal suspension of platinum particles is readily prepared by boiling a dilute solution of chloroplatinic acid (H2PtC16) with a freshly prepared solution of trisodium citrate. The colloid forms and the reduction is complete in about 30 min; no stringent hygienic precautions are needed, and the colloid, containing platinum particles about 2 nm in size, will be stable for some weeks, but a precipitate of platinum black will gradually be deposited. The use of protecting agents (carbohydrates, synthetic polymers, etc.) is often recommended; they adsorb on the surface of the colloidal particles, thus guarding them against coagulation, but their presence is bound to interfere with their catalytic activity. The reader is encouraged to study the very thorough paper by Turkevich and his associates conceming the preparation and characterization of colloidal gold [156]. It is a model of its kind and is one of the early (and very dramatic) applications of electron microscopy to finely divided materials. Later work from this group dealing with other metals is also noteworthy [157], as is the Australian work on colloidal metals [158]. Very small gold particles (1-2 nm) are formed by using P(CH2OH)4C1 as reductant [159].
Preparation and characterization of metal and alloy catalysts
317
The method is therefore capable of producing metal particles in the size range of catalytic interest; size distributions can be quite narrow [156], and the mean size and the breadth of the distribution depends on the relative rates of nucleation and growth, and are thus to some extent potentially under experimental control. However, the use of this method as a weapon to tackle the problem of particle size effects has never been comprehensively developed. While for certain reactions, such as liquid-phase hydrogenations, the colloid may be used directly, it is possible to adsorb the particles onto a supporting material (section 7.3.1) and thus to produce a supported metal catalyst (section 7.3). The first use of this procedure in the early 1950's was to prepare a Pt/A1203 catalyst [ 160], and it has since been applied to a number of other cases (see for example [161]). The way is therefore open to making a series of supported metal catalysts containing narrow and controlled size distributions by operating on the solution chemistry that determines the development of the particles, rather than relying on the chemistry of the interaction of the metal precursor with the support surface as a means of control; this is a notoriously difficult and dangerous thing to do, as it is almost impossible to effect a change in particle size without introducing other differences. However, the work with colloids has not yet been performed. Two other quite recent developments merit attention. (1) Techniques have been developed for preparing stable metal colloids in non-aqueous media. Colloids of platinum, palladium, rhodium and ruthenium, about 2 nm in size, have been prepared by reducing the chlorides with silanes such as (EtO)3SiH and Mez(EtO)SiH [162]: these are liquids, and their oxidation products act as the disperse medium. The reduction of Pd(DBA) 2 (DBA = dibenzylideneacetone) in
CHzC12by hydrogen leads to 5 nm palladium particles
which are stabilized by polyvinylpyrrolidone [163]. (2) Use has been made of reverse micelles, i.e. of very small droplets of an aqueous solution dispersed in an organic phase (a microemulsion) as a route to colloidal metals. A detailed description of the method has been given [164]; the isolation of a very small quantity of the metal ion in the droplets ensures that on reduction with hydrazine particles of 2-5nm in size will be formed. Such colloids of platinum, palladium and rhodium have been described [164], and a method for depositing them on a support (pumice) and some aspects of their catalytic behaviour have also been published [165]. Less attention has been paid by catalytic chemists to what might be termed the rational synthesis of particles of colloidal dimensions, starting from organometallic complexes containing one or a few metal atoms. The chance of obtaining a truly monodisperse colloid is highly attractive, and the way is opened to a much more systematic analysis of particle size effects. An impressive example of this approach is the preparation of the Au55 cubo-octahedron from AuC1P~ 3 (~ = phenyl) by reduction with diborane; a thorough investigation of its optical and structural properties has been reported [166]. Although covered by chlorine and triphenylphosphine ligands, the particle has many of the
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characteristics of metallic gold. Relatively little attention has been given to the preparation of alloy colloids. Two methods can be envisaged: (i) sequential reduction, using a colloid of one metal as a nucleus for the reduction of a salt of the second metal, and (ii) simultaneous reduction of two salts. It might however be expected that much luck would be needed for this latter methods to produce a homogeneous product. However, contrary to expectation, homogeneous platinum-gold particles in narrow size distributions have been prepared [167,168] by the trisodium citrate reduction of mixed chloride solutions; mean sizes increased with gold concentration, and the particles were supported on graphite for a study of their behaviour in n-butane
hydrogenolysis. Palladium-gold colloids prepared
similarly have been
supported on carbon [161] and palladium-copper colloids have been prepared either by heating a solution of the two acetates in 2-ethoxyethanol at 408K or by reducing their solution in ethanol by hydrogen at room temperature. In a further application of the nonaqueous procedure, platinum-silver colloids have been prepared by reducing the complex Ag2(Ptox 2) (ox = oxalate) in ethylene glycol with NaBH 4 [169]. The structure of palladium-rhodium colloids prepared by reducing salts with aqueous ethanol has been examined by EXAFS [ 170]. 7.2.3
Reduction of binary oxides
For some years the only way to prepare alloys of the base metals of Groups 8-10 in a condition of reasonably high surface area was by reducing binary oxides formed by thermal decomposition of mixed hydroxides or basic carbonates [20,38,171]. This procedure was used to prepare nickel alloys with iron, cobalt and copper, and of iron with cobalt, but their specific surface areas were quite low, and the method was little used when confidence was gained in the ability to make supported alloys. Some metal carboxylates are believed to decompose directly to the metal in vacuo, e.g. M(HCOO)2 ~ M ~ + H 2 + 2CO 2, but the procedure does not seem to have been much applied to make alloys. While thorough and complete reduction of Groups 8-10 base metal oxides by hydrogen requires quite high temperatures [172] and in the case of iron the use of very pure and dry hydrogen, reduction of the noble Groups 8-10 metal oxides proceeds swiftly at room temperature, and so for liquid phase hydrogenation of organic compounds it is frequently acceptable to introduce the catalytic element as its oxide. Only occasionally does the reactant seriously inhibit the reduction. Many years ago, Adams [173] devised a simple procedure for converting a platinum salt into an oxide, a procedure which, unlike other methods he had tried, gave fairly reproducible materials. His product is logically
Preparation and characterization of metal and alloy catalysts
319
known as Adams platinum oxide. What is done is simply to heat the salt in molten sodium nitrate in a stainless steel beaker for some hours; the oxide is then recovered by leaching the sodium nitrate, and is thoroughly washed with hot water. There can be a vigorous release of nitrous fumes and other pyrotechnics as the reaction gets under way, so proper care has to be taken. The product is usually formulated as PtO2, but some sodium is strongly retained, and it may be better regarded as a sodium platinum bronze. The method is readily adapted to the preparation of binary oxides, a number of which have been prepared and tested for liquid-phase hydrogenations [174-178]. In almost every case there was a definite synergetic effect, being particularly marked with the platinum-ruthenium combination [179]. Palladium-silver alloys have also been prepared from oxides formed by thermal decomposition of the nitrates [ 180]. Alloys of platinum with ruthenium, rhodium and palladium have been formed by reduction of mixed sulfides at about 1000K [ 181 ]. 7.2.4
Raney alloys
A Raney alloy, named after the inventor, Murray Raney [182,183], is an alloy of a metal having catalytic properties with aluminium; the latter is then substantially removed by leaching with strong alkali, although some aluminium and alumina may remain [20]. The active metal, in the form of a fine powder, is highly porous, and is a suitable form for use in liquid-phase hydrogenations. Because of its open structure, a Raney metal is sometimes referred to as a skeletal metal, particularly in the Russian literature. Raney nickel is probably the most widely used catalyst for hydrogenations in the liquid-phase. A great deal of research has been performed to determine the best nickel to aluminium ratio and the most favourable conditions for leaching etc. [184-186]. We do not propose to review this material in any detail, because the final product is essentially a pure metal, although the other small components (A1, A1203, NaOH) noted above, may modify its properties slightly. Many other metals, including the noble Groups 8-10 metals, can be used in place of nickel; Raney platinum and palladium have been reported [187-189] and Raney ruthenium is effective for methanol synthesis [190]. Indeed, combinations of active metals have been used, presumably giving a bimetallic Raney alloy (e.g. platinum with rhodium or silver [191] and iron-nickel [192]). Fas'man has recently introduced the interesting concept of supported Raney alloys, using Pt-Cu/A1203 as a example [193,194]. A comprehensive review of structure and bonding in transition metal aluminides has recently been published [195].
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7.3
Supported metal catalysts
7.3.0
Introduction "Supported metal catalysts comprise 0.1-20 weight % of a metal of Groups 8-11
dispersed over the surface of a support, which is typically a high-surface-area oxide. They are widely used on an industrial scale and in research laboratories. Principal large-scale uses include hydrogenation of animal and vegetable oils (fat hardening), petroleum reforming to make high octane gasoline, and treatment of vehicle exhaust to minimize environmental pollution. These materials are effective as catalysts because the active metallic phase is present as extremely small particles, having a degree of dispersion D of 10-100%. They are firmly anchored to the surface and are widely separated from each other, and hence do not readily coalesce or sinter" [1]. Like all attempts at generalisations, the above statements, while reasonably accurate, reveal only a very small fraction of the wealth of information available in the scientific and patent literatures on these materials. Their very great practical importance has generated an enormous number a publications, the flow of which shows no sign of diminishing. The advantages to be reaped in terms of greater utilisation of atoms of the active metal by progressive reduction of particle size and increase in surface-to-volume ratio were described in chapter 5, and the use of degree of dispersion as a basis of expressing rates was discussed in section 6.4.1. It hardly matters whether the particles are supposed to be spheres, hemispheres, cubes, square pyramids, or any other shape: the rate at which exposed surface area increases with lowering of size is much the same [196]. While there are numerous ways of forming small crystallites having dispersions greater than 10% (see section 7.2), the major practical challenge is to find means of rendering them usable in the often strenuous conditions in which major industrial and environmental control processes operate. The problem is of course that small metal particles are thermodynamically unstable with respect to the bulk material: the driving force towards coalescence is equal and opposite to the work performed in subdividing the bulk material, i.e. in breaking numerous metal-metal bonds. The smaller the particle, the less stable it is: atoms are least stable of all. The first approach to a solution of the problem was essayed by D~Sbereiner [197] who admixed platinum black with clay in order to dilute its catalytic power. The concept has been refined, and in the present century the strategy has been chiefly to form the metal particles and attach them firmly to the surface of a support, which is often a higharea oxide and which is usually but by no means always catalytically inert. In fact very many different materials have found use as supports (section 7.3.1) and they are used in a variety of physical shapes, that chosen in any particular use being determined by the
Preparation and characterization of metal and alloy catalysts
321
reactor configuration and considerations of mass-transport control. Numerous procedures have also been developed for forming the metal particles; these may be classified as follows. (1) Introduction of a simple or complex ion to the support, followed by reduction: this embraces the methods of impregnation, adsorption from solution and ion-exchange, and is by far the most widely practised method (section 7.3.2). (2) Deposition from solution onto the support of a compound of the active metal, typically the hydroxide, followed by calcination and reduction. (3) Introduction to the support of a compound of the metal in its zero valent state, e.g. a carbonyl or rc-allyl: this is a method of growing interest (section 7.3.3). (4) Introduction to the support of preformed metal particles, i.e. colloids or aerosols, a procedure but little used (see section 7.2.2). (5) Introduction to the support of single atoms of the metal produced by vacuum evaporation: this is a technique of increasing use to form model catalysts for basic studies, especially of sintering (section 7.3.3). (6) Simultaneous formation of precursors to both support and active metal by
coprecipitation, a method little used with metals other than the base metals of Groups 811. In the following sections we shall first consider supports (section 7.3.1), then methods 1,2 and 6 as applied to both single metals and alloys (section 7.3.2); then methods based on zero-valent compounds, and atoms (section 7.3.3). Next we deal with techniques designed to modify a monometallic catalyst by addition of a second element (section 7.3.4), and then other less used methods, e.g. those classified under method 6 in the last paragraph. Finally we shall describe reduction procedures and the use of temperature-programmed reduction (TPR) to follow them. 7.3.1
Supports [20,198-201] The requirements that have to be met by a material that is to act as a support for
metal particles in an industrial or environmental control process are very stringent. Whether in fixed or fluidized bed, the catalyst is expected to demonstrate acceptable activity over prolonged periods, and in the case of catalysts used for vehicle exhaust control the operating conditions may change drastically on a very short time-scale. For three-phase batch reactions proceeding under milder conditions, the catalyst should be reusable on a number of occasions. These criteria imply very careful attention to the nature and composition of the chosen material. One elementary consideration is that the material should be chemically stable under the conditions of use. Clearly substances having basic character will not last long in acidic media, and vice versa. A particular problem arises in systems containing sulfur trioxide; alumina is not suitable because of its tendency to form the sulfate, so silica [202] and titania are preferred. Neither graphite nor activated carbon can be used in strongly oxidizing conditions and silica cannot be used in the presence of steam for fear of forming
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volatile ortho-silicic acid. Small particles used in fluidized beds, and in three-phase systems, need to be hard in order to resist attrition, by which, in consequence of continued rubbing together, the particles will be slowly reduced to fine dust which is either blown from the reactor or in three-phase systems will cause filtration problems. For fixed bed systems, a great variety of macroscopic shapes have been used: rough granules, pellets, extrudates, tablets and rings (figure 7) [203].
figure 7 Shapes of catalyst particle for use in a fixed bed reactor [203].
Pellet
Ring
@
q)
Tablet
Granule
Sphere
% Extrudate
Much ingenuity has been exercised in designing and forming shapes which will possess the greatest possible external area for a given volume (e.g. a cylindrical tablet penetrated by a hole, or having a serrated edge (figure 8).
figure 8 Forms of catalyst particle having increased external surface area.
For such forms to be stable, the fundamental particles must not be too hard, since a certain degree of plasticity is required for them to adhere firmly to each other under pressure. However the body once made must be hard and strong, resistant to thermal shock, and cheap. These considerations frequently lead to the choice of alumina in one of its many manifestations. For many applications the support must have a high surface area in order to enable crystallites of the active phase to be well separated from one another. Values up to about 50m2g-~ can be achieved simply by lowering the size of the fundamental units: thus for
Preparation and characterization of metal and alloy catalysts
323
example Degussa P-25 titania is reported [204] to have an area o f - 55 m2g-1 and a mean particle size o f - 5 - 1 0 nm. To obtain areas much above 50 mZg-1 it is necessary for the fundamental particles to be microporous. Pores are conventionally classified in an arbitrary way as (i) micropores, less than 1 nm in size, and thus being channels of not much greater than atomic dimensions within the primary particles; (ii) mesopores which are 1-50 nm i n size, and are due to larger cracks within primary particles; and (iii) macropores which are voids and crevices between primary particles, and are greater than 50 nm in size. Techniques for the structural investigation of porous solids will be treated in section 7.4.1. The ability to form and to maintain microporosity, and the associated high surface area, is limited to a comparatively few substances: essentially it is the sp-oxides alumina and silica, and their mixtures, and carbon, which have this property. It is no accident that the ability to form zeolites having molecular sieve behaviour is also limited to the same classes of substance, and their close congeners. Other oxides and chalogenides cannot sustain microporosity, so that for examples the oxides of the transition series elements and those of post-transition groups can at best be prepared as very small non-porous particles, having areas typically not more than 50mZg-1 or at the very highest approaching 200mZg -1 [205]. Both silica and alumina are available with areas up to 500-600mZg -1 [202] while activated carbon typically weighs in at 1000-1200mZg -1, almost the theoretical maximum. When larger surface areas are reported, they result from a misinterpretation of the experimental results: the adsorbate condenses in the micropores, filling them completely at a pressure at which only the formation of a monolayer on a flat surface is possible. A matter of very great importance is the acid-base character of the support surface [206,207]. Except in the case of petroleum reforming and a few other types of reaction which demand the use of a bifunctional catalyst (see below), it is generally thought desirable that the support should be catalytically inert. While it may well be true that, in some systems not conventionally regarded as bifunctional, the support plays an important role, perhaps through the spillover of reactive species [208-210], it is equally so that in many cases the support is responsible for unwanted or parasitic reactions. This is particularly the case in reaction of hydrocarbons such as steam-reforming or dehydrogenation, where acidic groups at the support's surface can initiate carbocationic polymerization of unsaturated intermediates, a process which terminates in carbon deposition. Since a bifunctional petroleum reforming catalyst must comprise an acidic support, together with a metallic or alloy component, one might say that it contains the seeds of its own destruction, and indeed the problem of keeping the formation of the carbon deposit under control has taxed the ingenuity of catalytic chemists for almost half a century. In monofunctional systems, it is usual to kill off the acidity by neutralizing acidic groups with an alkali: both sodium and potassium bases are widely used for this purpose. The same procedure is used to fine tune the acidity of bifunctional catalysts. The surface chemistry of oxides of interest as catalyst supports has been widely
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studied. Although there is much that is complex, because of the different environments that are available to surface ions, it may be presented simply in the following way. A freshly-cleaned surface of an oxide will on exposure to the atmosphere rapidly interact with water vapour as shown schematically in figure 9. OH=
O
2+
M 2+
M
=
O
=
O
2+
M 2+
M
H20
OH 2+
=
M
O
bulk figure 9 Schematic representation of how water reacts with an oxide surface.
-
OH-
2+
M =
O
2+
OH- M 2+
M
=
O
bulk
For example with magnesium oxide this reaction occurs very easily. Most oxide surfaces are hydrated, and indeed there may be multilayers of adsorbed water molecules: the first layer is very tenaciously held, especially on alumina and silica, and often cannot be removed entirely without heating to the point where the pore structure collapses. The above simple representation of water chemisorption illustrates how there may come to be different types of hydroxyl ion, differing in their basicity. The preferred mode of dissociation of the M-OH bond determines whether the surface will have (BrCnsted) acidic or basic character. M § OH-
~"
M-OH ~"
M-O-H +
Thus when M is an electropositive element (Mg, Ca, Ba...) the surface will be basic, but when it is more electronegative (e.g. Si) it will be acidic. Both types may coexist, giving an amphoteric oxide (e.g. ot-A1203, ThO 2, ZrO 2 .... ). Acidity caused by protons is termed BrCnsted acidity, while that due to coordinatively-unsaturated surface ions, e.g. A13+, which can react with a base or can liberate protons from water, is termed Lewis acidity. Application of these concepts in the context of adsorption and ion exchange for catalyst preparation will be considered in the following section. Non-stoichiometric compounds formed from oxides whose cations differ in valency will exhibit acidity (of the BrCnsted type) which is greater both in terms of the number of acid centres and of the acid strength
than that shown by either oxide separately. The
substitution of an A13+ ion in the framework of silica will generate a charge imbalance which in the presence of water is neutralized by a proton (see figure 10). Amorphous silica containing about 13% alumina has been much used, for example, as a cracking catalyst, until largely replaced by zeolites whose acidity is exceptional; it is still used as a diluent for the more active zeolites. The same principle applies to many other combinati-
Preparation and characterization of metal and alloy catalysts
325
ons of oxides whose ions are of different valency.
B r o n s t c d acid s i t e
Lev,is acid site
L --Si--
I
/ i
0 H
1 Si--
l/ ~
--Si---
i i-
0
I --
'\
i
()
x\ H "
O
AI
c~'' --
I 0
I \\
0
--
t
-k--- S i - -
/
,
1
/ /
Si--
figure 10 Representation of an acidic site in silica-alumina.
macropore
rnesopore figure 11 Formation of silica gel (schematically)
Micro-, meso-, macropores and aggregates of particles are shown [214].
The chemistry involved in the preparation of oxides intended as catalyst supports has also been intensively studied. The first and often critical step is the formation of a sol from a solution of a suitable salt [211]: on heating, this polymerizes to form a gel which after washing and calcination gives the high-area material. In figure 11 the primary particles are those of the sol, and their aggregates are what are formed on ageing. Traditionally, aqueous solutions were used, but for some years there has been growing interest in products made by what is termed the sol-gel process, in which an organometallic compound, typically an alkoxide, in an organic solvent such as ethanol is hydrolyzed to
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form a sol, which polymerizes to a gel [212]. Removal of the solvent under supercritical conditions leads to aerogels having very high areas and very low bulk densities [213]. Gels formed in aqueous media can also be solvent-exchanged before being dried in this way. The chemistry associated with alumina is particularly complex [20,202,203,214216]. There are several different hydroxides (gibbsite, bayerite, nordstrandite), an oxyhydroxide (boehmite, A10(OH)) and at least four polymorphs of alumina itself. The products formed at each stage depend critically on the conditions used: a simplified summary of the known transitions is shown in scheme I.
Scheme
[
Gibbsite
air
l~lli~,,,,
air
l~li~ '
-
~,
-o.,%
'~72<~~~//D iir,t..~BOehmite
(~'~,~~'1020K
i Bayeriet
__
~
r air
l
~li~,,r
)
air~
Transitions in an aluminum hydroxide-oxide system [214].
All the alumina phases shown in this scheme are metastable relative to o~-alumina, to which they all convert at high temperature: the 0~-phase has only a low surface area. y-Alumina is probably the most widely used oxide support and its crystal structure is well known [199,214-217], It has been regarded as a spinel, the general formula for which is AB204: in the y-alumina spinel structure there is a face-centred cubic lattice of oxide ions and 1/8 th of the tetrahedral and 1/2 of the octahedral interstices are filled with cations. In this structure there are chains of edge-sharing octahedra linked through edgesharing to orthogonal chains in planes above and below, and through corner-sharing to bridging tetrahedra. Thus, since the true cation: anion ratio in a spinel structure is 3:4, the
Preparation and characterization of metal and alloy catalysts
327
structure of pure y-alumina might be represented as A18/3 [11/304 [217]. However, the presence of residual water in the structure has to be counterbalanced, and protons may occupy certain cation vacancies. The surface chemistry of y-alumina has been thoroughly investigated [207] and surface acidity constants and concentrations of charged groups such as A10- and A1OH2§ determined as a function of pH in the range 3-11. A quantum mechanical description of the alumina structure has also been given [218]. The comparatively open structure of 7-A1203 and the other high area forms (11, g, etc.) make it easy to understand how the introduction of defects can lead to the development of micro- and mesopores. The process of sintering, which starts above 1000K, occurs by migration of defects and of ions, to form more compact structures. For high-temperature applications, including steam-reforming and vehicle exhaust treatment, this process is very undesirable. It can be arrested by introducing other cations into the surface, where defect and ion migration starts; the La 3+ ion is particularly suitable for this purpose [219]. It is not uncommon to find that commercially available supports contain substances which can act as, or can generate, poisons for catalytically active metals. Both alumina and titania if prepared from sulfate solutions can incorporate sulfate ions in their structures, and these are exceedingly difficult to remove by washing or even by calcination. Hydrogen atoms migration to the support by spillover during reduction of a catalyst precursor can lead to formation of hydrogen sulfide and this can (as one of the authors knows to his cost) produce very misleading results in activity tests. Formation of hydrogen sulfide is easily detected by placing lead acetate paper downstream of the sample in a TPR experiment. Technical grade alumina also contains other cations, expecially Fe 3+ which create strong ESR signals: it is difficult to perform good ESR measurements on commercial alumina-supported catalysts. One of the earliest indications of the occurence of hydrogen spillover was the observation of reduced impurity species in alumina [220]. Chloride ion is of course strongly held by alumina, titania and many other oxides (less so by silica), but can be removed by treatment with hot water, steam or ammonia. Unit operations involved in the large-scale manufacture of catalyst supports have been described by Stiles [221]. It is impossible to start to do justice to the wealth of knowledge available on zeolites in the space of a short paragraph. They now occupy a central position in catalyst technology [203] and also find use as dehumidifiers and water softeners. Societies have been formed for their study and international conferences on them regularly held: they have a very rich literature [222,223]. Essentially they are crystalline aluminosilicates formed by corner-sharing of
SiO 4
and
A10 4
tetrahedra; they are characterized by having
pores of uniform size which connect larger cavities known as supercages. They occur in a range of Si: A1 ratios, and the excess negative charge induced on the framework, as shown above, has to be balanced by a cation M, typically sodium, according to the formula Mv(A102)x(SiO2)yzH20 which characterizes an aluminosilicate zeolite. If the cation is M +, v
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equals x; if it is M 2+, v equals x/2 and so on. They are also observed to form a great variety of crystal structures. Perhaps the simplest are the so-called faujasite series, in which the basic sodalite unit, comprising 24 A104 or SiO 4 tetrahedra forming a cubooctahedron, is linked either via its six square faces or via four of its hexagonal faces to other sodalite units to form a three-dimensional network. The former mode affords zeolite A and the latter zeolites X and Y. The effective size of the pores can be altered by changing the size of the cation: zeolites can be decationated by first exchanging the alkali metal cation by NH4+ which is then thermally decomposed to give the protonic form. Zeolites are excellent catalysts in their own right for reactions initiated by protons, e.g. those having carbocations as intermediates: their effectiveness is due partly to the number and strength of the acidic hydroxyl groups on the framework and partly to the very high electric field gradients which exist in the neigbourhood of uncomplexed cations. However for our purposes, namely, to consider zeolites as supports for metals and alloys, their two most relevant properties are (i) their ion exchange capabilities, whereby ions such as Na § or protons can be exchanged for cations of the catalytic metal, and (ii) their molecular sieve characteristics which mean that only molecules of a certain small size are able to enter and diffuse through the pores. Some information concerning the process of ion exchange by complex metal cations and the mechanism of their conversion into small metal or alloy clusters [224] will be presented in the next section. Aluminosilicate zeolites are not particularly stable, being metastable with respect to low-area forms analogous to quartz or t~-alumina, e.g. mullite. They are therefore unsuited to use at very high temperatures, or in damp atmospheres where ortho-silicic acid may be formed. Many types of zeolite occur naturally, but many have also been synthesized. This usually requires treating solutions or dispersions of aluminium and silicon compounds under hydrothermal
conditions, often in the presence of a species such as a tetra-alkyl-
ammonium which seems to act as a template around which the structural units form. However, despite their great practical importance, the mechanism of their synthesis is still not completely clear. Other combinations form analogous structures having molecular sieve behaviour; the term zeotype has been coined to cover any microporous crystalline solid showing molecular sieve characteristics [222]. Aluminium phosphate is one such combination; however it lacks the acid character of aluminosilicate zeolites. A zeolitic form of essentially pure silica is also known; this is called silicalite. Many other ions can be substituted in either aluminosilicate or aluminophosphate or silicalite frameworks, imparting additional catalytic character; zeolites containing iron, vanadium or titanium have attracted much attention, but they have not yet found use as supports for metals or alloys. A kind of poor man's zeolite is formed by expanding the two-dimensional sheets in naturally occurring clays and propping them apart with pillars: but the products have likewise not been much used as catalyst supports [225].
Preparation and characterization of metal and alloy catalysts
329
Activated carbon [226,227] is widely used as a support, especially for noble metals, when the catalyst is to be used in a three-phase system; its use for gas phase processes is more restricted. It is prepared by carbonization at high temperature in vacuum or inert atmosphere of some naturally-occuring cellulosic or carbonaceous material, such as peat (already partially-decomposed vegetable matter), coconut-shell, wood or coal. Activation is usually performed by a high-temperature steam-treatment, catalyzed by some added alkali; the resulting partial oxidation generates the microporosity to which the high surface area of the product is attributable. A major problem with activated carbon (or activated charcoal as it is sometimes called) is that the raw material is of variable quality and purity, and it is difficult to obtain exactly reproducible products. Separation into fractions using a fluidised bed produces more homogeneous materials, the surface of which can be rendered more uniform (and more reactive) by mild oxidation, for example, by impregnating with nitric acid and drying. Numerous different oxygenated groups are formed at the surface, e.g. -CH2OH, -COOH, CHO, etc. and some of these act to assist the binding of the metal salt which is the precursor to the catalytic metal, either by ion exchange or adsorption (see section 7.3.2). However attention has been turned to artificial sources such as polymers and plastics. Carbonization of certain types of polymer affords products having some molecular sieving ability, and carbonization of fabric woven from fibreforming polymers leads to charcoal cloth which finds use in air purification and as a wound-dressing. Heat treatment of high-area carbon in an inert atmosphere leads ultimately to graphite, which has a low surface area, but finds some application, e.g. in fuel cell electrodes, where electrical conductivity contributes to its usefulness. An attempt has even been made to use buckminsterfullerene as a support of noble metals [228]. So far we have spoken only of pieces of material that are homogeneous in their composition, and the porosity of which is caused by natural channels lying between the basic units of which it is composed. There is one radically different type of support, known as a monolith, which is widely used in systems for treating vehicle exhaust and which acts as a support for an alloy catalyst. The principle is as follows. A structure resembling the wax honeycomb of a beehive is first fabricated, using a non-porous ceramic material such as c~-alumina or mullite (a silica-alumina): this provides the high temperature strength and stability, but because of its low porosity is not itself a suitable support for metal particles. It is therefore covered or wash-coated with a thin layer, -- 0.1 mm thick, of a porous material such as y-alumina, in which metal particles can comfortably exist. The parallel channels in the simplest design are 2-3 mm diameter and thus there is little obstruction to gas flow (figure 12): high space velocities can be achieved at little cost in pressure drop, so such material is especially convenient to place in the exhaust system of a vehicle, where any interference to unrestricted gas flow is unwelcome. Much more complex structures can be made, in which for example the channels suffer frequent changes in direction. Monoliths (the word means "one stone") are available in quite large
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blocks, but have not yet found use in chemical process technology: possible reasons for this are the comparatively great cost of the material and its liability to fracture if a thermal shock is applied.
k.
kfk]kJkJ
p-
(b)
figure 12 Typical cross-sections of monolithic supports: (a) honeycomb; (b) corrugated.
In addition to the chemical types that have featured so largely in the foregoing discussion, many others have found limited use in small scale operations, particularly those involving three-phase systems. Palladium as a hydrogenation catalyst is sometimes supported on either calcium carbonate or barium sulfate; for laboratory use in organic synthesis, kieselguhr ('diatomaceous earth') has been popular. Polymers such as Nylon and silk have been tried [229], both being easily fished out of the reaction medium. There is much current interest in the use of magnesia [202] as a support for platinum and palladium in reactions related to petroleum reforming, although the raison d'etre has not yet been revealed. Numerous other oxides (ZnO, SnO 2, M o O 3, V 2 0 5 ..... ) have been used in fundamental research on spillover catalysis and metal-support interactions. Finally a word needs to be said about model catalysts. They will be referred to again in section 7.3.3, but briefly they are formed in vacuo by condensing metal vapour onto the surface of single crystal oxides: various oxides and related compounds have been used in this way (alumina, mica, sapphire ...) as well as graphite crystals exposing their basal plane. Alternatively the face of a single crystal of metal can be superficially oxidized, and this can simulate an oxide support on which metal particles can be deposited. 7.3.2
Use of precursors in positive oxidation states [20,199,206,230] The science of catalyst preparation has as its objective the preparation of a material
that will meet its design criteria expressed as activity, selectivity and stability. It is advancing to the point where it is, or will shortly become, possible to specify a desired composition and structure in the certain knowledge that practical means can be predicted
Preparation and characterization of metal and alloy catalysts
331
for achieving the required formulation. It is rarely possible to foretell with any precision what the activity or the selectivity of a new catalyst will be, but forecasting the structure that a given preparative route will result in is a safer bet. Very many factors need consideration at the design stage [199]. Experience shows that the final structure of a supported metal catalyst depends on the chemical nature of the support, its surface area and porous character, its purity and previous pretreatment, the metal precursor, the preparation method, the solvent, the age of the solution, the methods of drying, whether calcination is performed before reduction, and the reduction conditions (nature and concentration reductant, temperature, time) [1]: this list is not necessarily exhaustive. Any or all of these factors may influence the mean size of the metal particles, their size distribution, their morphology and their location within the support granule, and hence their catalytic performance. Some elements seem to be more temperamental than others: the activity of ruthenium catalysts is more difficult to control than that of platinum catalysts [1 ]. Indeed the commonly-experienced lack of reproducibility of catalytic activity on a sample-to-sample basis using portions drawn from the same bottle, to say nothing of batch-to-batch reproducibility of preparations, is a humiliating reminder of the lack of control which we have over the complex chemistry involved; but in view of the large numbers of factors requiring control, as listed above, it is perhaps not all together surprising that precise replication eludes us. Fortunately, product selectivities are generally much less variable and this indeed is of itself valuable information. There is however a strong argument [1] for using a standard or reference catalyst [231,232] wherever possible. The reader who wishes to be informed of the extensive literature on the preparation of supported monometallic catalysts is referred to the excellent books and review articles available [20,199,215,216,233]. Here we can only give the briefest summary of the salient considerations, with emphasis on those features whicl] are relevant to the preparation of supported alloy catalysts. In section 7.3.0 the principal modes of incorporating a metal precursor, where the element in on a positive oxidation state, were listed. The term impregnation is used to describe the process whereby a solution of the metal precursor is drawn into the empty pores of the material that is to act as the support. This is exactly what happens when a dry sponge is put in water. Providing the surface of the support is hydrophilic, capillary forces pull the solution into the pores, which in most cases are filled within a few minutes: only the smallest micropores may be immune, since some complex ions with their hydration spheres may be comparable in size. With hydrophobic supports such as graphite, it may be better to use a non-aqueous solvent having a lower surface tension than water, or to add a surface-active agent. Silica which has been heated to high temperature and has been completely dehydroxylated (e.g. quartz, glass) cannot be re-hydroxylated, and is hydrophobic, as one sees by looking at the window-pane when it rains. Only if there is a large proportion of pores that are only open at one end is it necessary to remove air by
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evacuation before the solution is introduced. We must now enquire what chemical processes may occur between the precursor species in solution and the surface of the support. In the first place there may be no interaction at all. In this case, on removal of the solvent, there will be formed innumerable small crystallites of the precursor salt, each independent pore perhaps containing one or two such crystallites [234], the size of which will depend on the concentration of salt in the impregnating solution. Thus if the number of crystallites is constant, and if on reduction each forms a metal particle, the metal area will be proportional to the two-thirds power of the metal loading [234,235]. However, the particle size distribution will be broad, reflecting the range of pore sizes present. Where there is no adsorption of the precursor solute, there will be movement of solution through the pore structure as the solvent evaporates. Solvent loss naturally starts at the external surface of the support granule, and occurs preferentially from the wider pores where due to the Kelvin effect the vapour pressure is higher than in the narrow pores. The solution therefore gradually retreats into the smaller pores, becoming more concentrated, so that at the end quite large particles are formed, but only in the interior. Better dispersions are therefore obtained by rapid drying, because then solvent removal is faster than solution movement. Interesting results have recently been obtained by using microwave radiation to remove solvent [236]. It is however unusual to find a system in which the metal-containing ion under no circumstances interacts with the support, and although an operation recognized as impregnation may have been performed it is more than possible that some chemical interaction takes place. However before considering what form this might take, we will deal with some experimental aspects. When preparing catalysts in the laboratory on a scale of a few grams or tens of grams, impregnation to incipient wetness is often used. Here the volume of solution taken is such as to fill the pores of the support, pore volumes generally lying between 0.1 and 1 cm3g-~; aerogels may have much higher pore volumes (up to 5 cm3g-1). The solution is added quickly to the support and is absorbed completely. Alternatively an excess of solution is used, the solvent being removed by evaporation, e.g. in a rotary evaporator, and all or most of the salt ends up on the support. This method may have to be used when the precursor salt has limited solubility or where a high loading of the metal is desired. For large-scale manufacture the support may be placed in wire baskets which are dipped into the solution for a certain time; after raising and draining, they pass automatically to a hotair oven where the solvent evaporates. The location of the active metal within the support granule is of immense importance. We will consider the possibilities by reference to a 2 mm rectangular pellet, which is a commonly-used form of support; however very similar considerations apply to microscopic particles as found, for example, in activated charcoal. There are four basic scenarios (figure 13) [199,230]. (i) The metal is concentrated in a very thin layer (-- 0.1
Preparation and characterization of metal and alloy catalysts
333
nm thick) close to the external surface: this is the egg-shell mode. (ii) It is in a thin layer below the surface, but not penetrating to the centre: this is the egg-white mode. (iii) It is entirely located in a small zone near the centre of the particle: this is egg-yolk model. (iv) The metal may of course be uniformly distributed throughout the granule. Which mode is preferred depends on the conditions of use; prime considerations are the ease of diffusion of reactants and of products through the pore structure, and the likelihood that due to mechanical vibration the external surface will be gradually lost through attrition or abrasion. If this latter eventuality is probable, then mode (i) is discounted; if pore-diffusion limitation is not expected, the mode (iii) is selected. If both are likely, the mode (ii) or perhaps mode (iv) are the best compromises. We will now outline some of the factors that enable these modes to be realised in practice. Pt/AI 203
(o) SHELL ADSORPTION OFCHLOROPLATINIC ACID.
(b)
(c)
{d)
INCREASING ACID STRENGTH
figure 13 Variation of metal concentration with distance from the external surface of alumina pellet: a) egg-shell mode; b) egg-white mode; c)d) egg-yolk mode. Coadsorption of an acid added to chloroplatinic acids can cause shifts from a) to d). [2431.
As discussed in the last section, the surfaces of oxidic supports are usually hydroxylated, although the stability of the hydroxyl layer is very variable. In the case of titania, for example, dehydroxylation begins (at--423K) almost before removal of physically adsorbed water is complete. The acid-base character of the hydroxyls is represented in terms of the following equilibria:
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chapter 7
MOH2 +
,~
MOH + H+
...K1
MOH
,-
MO-+ H§
...K2
We then define the zero-point charge (ZPC) or the isoelectric point (IEP) as the pH at which a particle of the support in aqueous suspension carries no net charge. This pH is defined by pH = (pK~ + pK2)/2 and at this point the concentrations of MOH2 § and M O are equal, pH has therefore been termed, somewhat grandly, a "surface charge selection switch" [237]. At pH's above the IEP, cations will be adsorbed, because the surface carries a negative charge; below the IEP, only anions will be adsorbed, because it will have a positive charge. These simple chemical considerations underlie the phenomena of adsorption from solution and ion
exchange, both of which are used in catalyst preparation [206]. The literature reveals some lack of precision concerning terms such as exchange, adsorption, neutralization, precipitation, complexation, etc., used to describe the association of metal containing ions with supports. Brunelle [206] has proposed the following definitions: in an acidic medium, polarisation of surface hydroxyl may proceed as M - OH + H§ -
~,
M - OH2+A-
(a)
M - OH + H+A -
,~
M+A-+ H20
(b)
In a basic medium the processes are M - O H + B*OH -
,~
M(OH)2-B §
(c)
M - OH + B+OH -
,-
MO-B § + H 2 0
(d)
He regards a and c as adsorption, and b and d as neutralization. Exchange is properly represented as M+A( + A 2-
,~
M+A2- + A (
(e)
MO-B1 + + B2 +
,~
MO-B2 + + B1+
(f)
For large-scale manufacture, methods based on these principles are sometimes preferred to the impregnation procedures used on a small scale in the laboratory: complete extraction of a metal ion from solution onto the support in suspension is however desired, for then after filtration the liquid can be discarded. Application of the principles therefore demands first an understanding of the ionic composition of the solution and of the nature
Preparation and characterization of metal and alloy catalysts
335
and concentration of the surface groups on the support at the relevant pH. Such an understanding is not always regarded as a prerequisite. "It would seen trite and unnecessary to observe that one would expect poor results from trying to cation exchange an anionic support and vice versa, if this were not in fact what is so frequently attempted. Similarly one would not normally attempt cation exchange of protons at low p H or anion exchange at high pH, and yet this again appears to be what is frequently attempted" [ 199]. Caveat operator!
1:
The commonly-used catalyst supports can be classified as follows [199]. zeolites - strong cation exchangers
2:
silica
- a weak cation exchanger
3:
alumina
- a weak anion and cation exchanger
4:
magnesia
- a stronger anion exchanger
5:
carbon
- can form charge-transfer complexes as an electron-donor, but can
also act as a weak cation exchanger. The solution chemistry of the elements of Groups 8-11, from which most metallic catalysts are made, can be extremely complex: it is not easily summarized. The base metals (Fe, Co, Ni, Cu) form aquated cations in the absence of a ligating anion, so nitrate, perchlorate and sulfate solutions will contain chiefly these uncomplexed ions. With the halogens however anionic neutral and cationic complexes can form, depending on the metal:halogen ratio (e.g. FeC12+, FeC12+, FeC13, FeC14-). Ammine and related complexes (e.g. with ethylenediamine, 1,2 diamino-ethane) are also formed except with iron. The chief difference with the noble metals of these groups is that uncomplexed cations are less easily found (except in the case of silver), and only exist in strongly acidic media containing non-complexing ions. In dilute solution the cations often hydrolyse with ease, forming oxo- or hydroxo species: this is especially so with ruthenium, rhodium and palladium. The noble metals (again excepting silver) are most cheaply available as their simple or complex chlorides (H2PtC16, Na2PtC16, PdC12, RhC13, etc.), although some are themselves mixtures, containing the metal in different oxidation states (e.g. RuC13 may contain some RuC14). Ammine complexes are also readily available and are suitable for cation exchange [206,238]. Care has to be taken, because the local pH in the pores of the support may affect the nature or properties of the ions in solution. Acidic groups present at the surface of activated carbon or of ]t-A1203 can hydrolyse the normally very stable PtC162- ion, as has been shown by visible-UV spectroscopy, the hydrolysis being stimulated by visible light; use of different platinum salts as precursors leads to catalysts having quite different properties [239]. Kennedy [240] has summarized the solution chemistry of the Groups 8-11 elements. The practice of ion exchange or adsorption from solution is very simple. The support is agitated in the solution in question usually at room temperature, until equilibration has been achieved; this usually takes a few hours. After filtration, the support is
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washed to remove occluded solution; exchanged ions will not be washed out by neutral water. As noted above, it is convenient in large-scale manufacture to achieve complete extraction of the metal-containing ions from solution, but this may be undesirable for other reasons. If only a limited supply of ions is available, they will adsorb or exchange at the first site they encounter, and the consequence may be a high metal concentration on the periphery of the particle and nothing in the middle. The metal dispersion in an egg-shell mode may therefore be poor. In order to overcome this difficulty, there is added to the solution a competing or sacrificial cation, e.g. NH4§ in the case of ammine complexes. The ammonium ion competes for the available exchange sites, so in this way a more even distribution of the metal through the support particle can be achieved. Similarly stronglyadsorbing anions (NO3-, F-, acetate, citrate and other carboxylate anions) may be used to obtain a more even spread of for example platinum in alumina when PtC162- is used [199]. Che has given [237] a detailed and systematic treatment of the various processes that can occur when a metal-containing ion encounters the surface of an oxidic support. We return for a moment to the imgregnation method. It is perfectly possible for there to be some ion exchange or adsorption of ions starting when the support and solution come into contact: this will lead on drying and/or calcination to metal ions closely engaged with the support. This engagement may take the form of an ion firmly anchored in an octahedral hole in the surface of 7-alumina; the ion may enter the support lattice by isomorphous substitution (Ni 2§ in MgO); or in the limit a new compound may be formed (as with Ni 2§ and SiO 2 [237,241-243]). Ions in solution occluded in the pores will after drying form microcrystals of the original salt, or some related compound, which after calcination will form microcrystals of an oxide. In general metal ions which have interacted strongly with the support will be more difficult to reduce than those in noninteracting oxide particles. There are a number of clear examples of this in the literature, where temperature-programmed reduction (see section 7.3.6) reveals two separate parts to the process of reduction. This is likely to yield a bimodal distribution of metal particle sizes, so that any study of particle size effect based on average size estimates for catalysts made by impregnation is likely to be without much significance. Catalysts made by ion exchange or adsorption from solution are less likely to suffer this drawback; the consequences for the preparation of alloy catalysts will be considered in due course. The distinctions between the various techniques for preparing supported metals are more apparent than real; the terms used apply to the procedures employed rather than to the chemistry of the surface processes. Thus in impregnation there is often an ion exchange step, or a neutralization of acidic groups, and the ionic character of this interaction may survive the further steps in the preparation, and be responsible for the
chemical glue that has often been thought to hold metal particles in place on the support. It is somtimes advantageous to anchor the precursor to ions of another element, which will in turn help to fix the metal particle [241-243]. This is exactly the same technology as is
Preparation and characterization of metal and alloy catalysts
337
used to form silver mirrors, e.g. in Dewar flasks: here tin ions are the anchoring agent, and other examples lie in the area of film preparation known as electroless plating. Before returning briefly to talk again about calcination, we will mention two other procedures which are in practice limited to the base metals of Groups 8-11; these are
deposition-precipitation and co-precipitation. The former term applies when the metal ion in a solution in which the support is suspended is caused to deposit as a solid on the surface of the support; typically what is deposited is the hydroxide, but precipitants other than hydroxyl ion (e.g. dimethylglyoxime for nickel) have been described. Occasionally the support surface is sufficiently basic to cause hydrolysis by itself; thus the PdCI42- ion is hydrolysed by a suspension of calcium carbonate. It is usually unsatisfactory to add a basic solution to a suspension of the support in a solution containing a non-interacting ion, because much of the hydroxide will be precipitated away from the support surface. The slow homogeneous generation of hydroxyl ions by slow decomposition of urea: CO(NH2) 2 + H20 NH3 + H 2 0
~
=
CO 2 + 2NH 3 NH4 + OH-
overcomes this problem, because the concentration of hydroxyl ions remains low and no local high concentration can arise. Geus, who pioneered this technique, has applied it to nickel and copper catalysts [243]; it does not however seem to have been used with noble metals forming insoluble hydroxides (Ru, Rh and Pd) although there is no reason why it should not be. Co-precipitation differs from other methods in that it does not start with a preformed support, but precursors to the support and the metal are precipitated simultaneously: thus from a solution of Ni(NO3) 2 and AI(NO3) 3 a binary hydroxide can be obtained, and after calcination and reduction a nickel/alumina catalyst is obtained. It matters whether the material after drying is calcined or not. Consider the case of RuC13/TiO2 [244]. This may be reduced directly to give a quite high ruthenium dispersion, but some chlorine ion will be strongly retained; after calcination of RuC13/TiO2 at 623K, RuO 2 is formed and much of the chloride is lost by hydrolysis. Reduction then affords a significantly different catalyst, e.g. one which is much more active for alkane hydrogenolysis [244]. One or two other tips may be offered. The stability of the support needs to be considered; thus T-alumina is soluble in acidic media below about pH3 and the surface vacancies thus created have been implicated in the adsorption of the PtC162- ion [245]. Moreover the A13§ ions may re-deposit on the precursor during drying, as AI(OH)3, A1C13 or oxychlorides: these may perhaps encapsulate or at least decorate the metal particles after low-temperature reduction, although above 550K they will probably migrate back to the support. This type of corrosive impregnation is very important for the thermal stability
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of catalysts made in this way [245,246]. The process of dissolution and re-deposition may also occur during the preparation of metals supported on transition metal oxides such as VzO 3 or TiO2, and the resulting species may contribute to the so called strong metal-
support interaction (SMSI). Both silica and activated carbon are unstable in strong alkali, forming colloidal suspensions, and below pH6 the three-dimensional framework of silica starts to disintegrate, with the formation of new hydroxyl groups which can serve to bind the precursor [247]. It is vitally important to analyse the metal content of the catalyst before use; one cannot rely on the nominal value, as the actual value can be substantially less (quite often by at least 10%). Analysis is most conveniently performed after drying, when the metalcontaining species may be readily taken into solution. The metal content is required for estimation of dispersion and hence of TOF: its value cannot be safely assumed, and the accuracy with which it is known limits the accuracy of all other derived quantities. A novel method for preparing small and reasonably uniform particles on a silicon wafer is to place a drop of a metal salt solution on it, and then spin it at high speed [248]: copper (II) oxide particles of 4_+ 1 nm in size have been made and the effect of rotation frequency examined. The reduction step has not yet been performed, however, but should be easy and the method should be generally applicable. A number of reports have recently appeared describing the preparation of supported metal catalysts by the sol-gel technique [249,250]; they appear to have some interesting properties. It was stated above (section 7.3.0) that ceramic monoliths needed to be coated with a thin layer of a high-area oxide which would acts as the support for the active metal. In fact the catalytic species in ionic form can be introduced before the monolith is made, but this is not a recommended or generally used procedure. Efforts have been made to impregnate the monolith without the wash-coat, but these have not been very succesful. Most commonly either the support is applied, and then the catalytic species are introduced by impregnation or ion exchange, or a catalyst is prepared and applied as a wash-coat [199]. There is much unrevealed technology involved in these operations. Metallic monoliths have also been made, and superficial oxidation gives a material to which catalytic metals can adhere. If from the foregoing remarks the reader should conclude that the preparation of a supported metal catalyst in a controlled manner is difficult, then he might suspect that to make a proper supported alloy catalyst might be well-high impossible. Indeed some years elapsed before research workers possessed sufficient courage to try. The path is strewn with difficulties and dangers. The full structural evaluation of a supported alloy catalyst is a difficult and daunting task, rarely undertaken: but at least we can pose a number of questions that require answers if we are to know whether a particular procedure has achieved its objectives.
Preparation and characterization of metal and alloy catalysts
339
1
Are the two components fully alloyed, i.e. are particles containing only one
2 3 4 5 6
component absent? Do all particles have the same average composition? What is the average size, and what the size distribution of the alloy particles? Are the particles homogeneously distributed throughout the support granules? Is there a preferential segregation of one component to the surface? Irrespective of whether such segregation occurs, does one of the components prefer
7
to occupy a particular class of surface site? Is the surface concentration of either component altered by the presence of
chemisorbed molecules or by the pretreatment applied? The thoughtful reader may be able to add further questions to this list. In a number of published studies, the amount of characterization reported is quite inadequate to provide definite answers even to the first five of these questions. Reliance is placed on changes in activity and selectivity, perhaps aided by results on chemisorption of hydrogen and/or other molecules. XPS is of limited value as it only assesses the external surface of the catalyst granule and signals emanating from more than a few nm below the surface are not received. Scanning and transmission electron microscopies with their linked techniques of electron diffraction, energy analysis of secondary X-rays etc. are essential for obtaining information on the local composition and distribution of metal/alloy particles. Some examples of the use of these methods will be presented below. X-ray diffraction gives useful information provided the particles are of a size to give coherent diffraction (> 5 nm), but inhomogeneity of composition will lead to excessive line broadening, with consequence loss of accuracy in estimating lattice parameters. Frequent use is made of temperature-programmed reduction (TPR; see section 7.3.6) as an aid to deciding whether or not alloying has occurred. It is impossible to give a detailed survey of the results of all attempts to prepared supported alloys starting with the catalytic elements in positive oxidation states. All that can be offered is a broad-brush review, concentrating on general trends. One or two generalisations emerge clearly from the literature. (1) In the great majority of cases where alloy catalysts have been prepared in research laboratories, the ions are introduced by impregnation, either simultaneously (the more usual) or sequentially, possibly with an intervening drying or calcination. Ion exchange is practised much less often [251-253] and deposition-precipitation hardly ever [254]. (2) Of the possible supports, silica is the most commonly used, and in general alloying is most easily accomplished on this support [255,256]. Much work has also been performed with alumina [257-261], and a little with titania [262,263] and magnesia [264,265]. (3) Of the possible metal salts, chlorides of the noble metals of Groups 8-11 are most usually used for impregnation and nitrates in the case of the base metals. The preference for using simple impregnation rather than ion exchange is not easily
340
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understood: the latter method would require more preliminary research before a suitable procedure was found, but most of us just want to prepare a catalyst in the quickest possible way, and then get on with the more interesting work involved in using it. The great problem with the impregnation method is the possible chromatographic separation of ions that may occur as the mixed solution makes its way through the pore structure. It is believed with some supporting evidence that, following reduction, a high temperature annealing will allow interdiffusion to occur, and a more homogeneous product to be formed. There is of course a risk that excessive sintering may take place. One further generalisation might have been added to the above list. It appears that genuine alloys are more likely to arise by direct reduction of the adsorbed chloroprecursors than by reduction of the oxides that result from calcination [266,267]. It is here that the outstanding difference between alumina and silica emerges: bare cations coordinate a more strongly to the former, and as noted above complete reduction can be difficult. In the case of silica, however, calcination usually leads to microcrystals of oxide that do not interact strongly with the support: their reduction may lead to separate metallic particles, requiring high-temperature treatment to homogenise them. There is clear evidence in some systems that oxidation of an alloy phase gives separated oxide particles, which do not re-form an alloy on mild reduction [266]. It may be helpful to indicate how a typical alloy catalyst is prepared by impregnation. A platinum-copper on silica catalyst has been made in the following way [268]. "The metals were dissolved in aqua regia and the solution was added to a suspension of silica in amounts required to obtain metal loading of 9%. The slurry was vigorously stirred and the water slowly evaporated at 100~
then it was dried and before use stored in a vacuum
desiccator. The last stage of catalyst preparation was reduction in situ, by hydrogen at 450~ at 1 atm. pressure, for at least 15 h". Quite homogeneous alloys were prepared in this way, as XRD analysis showed. Two alloy systems have been more intensively investigated than all others put together: the platinum-rhenium [269] and ruthenium-copper systems [270,271]. The first, widely used in petroleum reforming, has proved a major challenge to scientists. The purpose of many studies has been to establish to what extent, if any, an alloy or bimetallic cluster is formed between the two elements, and thus to what extent the rhenium becomes reduced to the zero-valent state. Early studies by TPR led to contradictory conclusions and this was traced to the important role of water vapour in mobilising adsorbed species during calcination, which is always recommended [272,273], and reduction [274]. It is now generally agreed that some of the rhenium is associated with platinum particles: rhenium having the higher latent heat of sublimation would in vacuum occupy the centre of the particle, and platinum the periphery [275], provided the particles are large enough to have a recognisable middle and outside. Rhenium atoms at the exterior would preferentially occupy positions of high co-ordination number, and models showing this behaviour
Preparation and characterization of metal and alloy catalysts
341
have been used to account for the behaviour of the mixed metal catalyst compared to that of platinum/alumina [276]. However, it is likely that some of the rhenium will remain as Re 4§ ions residing in octahedral holes in the "/-alumina surface, indeed early efforts to recover rhenium (as well as platinum) from spent catalyst showed this to be the case. Catalysts for academic use are usually made by impregnating the support with a solution containing hexachloroplatinic acid (H2PtC16) and ammonium perrhenate (NHaReO4); alumina is normally used as support, in imitation of industrial practice. There are two further complications to note. (i) In use the catalyst is deliberately sulfided and there are supposed to be ReS x "molecules" at the surface, fragmenting the surface platinum atoms into small ensembles [277]: studies on unsulfided catalysts therefore related only distantly to performance under industrial conditions. (ii) Chloride ion on the support, stemming from the H2PtC16, performs essential functions not only in providing the acidic surface required for a bifunctional catalyst, but also in generating mobile chloro- or oxychlorospecies of the metals, the mobility of which during calcination and reduction helps to form alloy particles. This area has been well explored in the context of the regeneration of spent reforming catalysts [278], and the special role played by chloride ions constitutes some justification for the generally successful use of chloride salts in preparing supported alloys. We shall return to these problems in chapter 13. The ruthenium-copper system, and related systems involving osmium, silver and gold, have commanded much interest for different reasons [ 270,271,279]. Miscibility in these systems is very slight, and alloy particles, which are quite easily prepared on silica and magnesia [256,264,265], comprise ruthenium cores which are partly or completely covered by a layer of chemisorbed copper atoms to an extent depending on the particle size and the atomic ratio of the metals. There have been thorough investigations of the importance of the type of silica used [280] and of preparation conditions [281]; the desirability of omitting calcination has already been noted [266]. The accompanying table 1 serves to indicate the range of bimetallic alloys that have been prepared using ionic species containing the active metals. It also highlights gaps remaining to be filled: many binary systems have not yet received attention. There is for example apparently no systematic study of the platinum-lead system. What also is crystal clear is that platinum is the favourite component from Groups 8-10 for alloy studies. Very many physical and chemical techniques have been deployed in order to characterize supported alloys: these include selective chemisorption of hydrogen, carbon monoxide and other molecules and spectroscopic studies of their adsorbed states (especially IR spectroscopy in its several manifestations, also 1H NMR) (see section 7.4 and chapter 8), thermal analytical methods (see section 7.3.6), EXAFS and XANES, XRD, XPS, M6ssbauer spectroscopy and many others. A general overview of the scope and power of these techniques in elucidating the finished product will be presented in section 7.4. Some of these techniques and others not listed above, including laser-Raman
342
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spectroscopy [282], visible-UV spectroscopy [239,283] and differential scanning calorimetry [284], have been used to unravel processes occuring during catalyst preparation. Each method has its strengths and its limitations; the more independent methods that are used, the more reliable is the resulting picture. A single method rarely does more than scratch the surface. As an indication of the problems that can arise and of the relevance of physical characterization, we cite results obtained with a series of palladium-gold/silica catalysts prepared by hydrazine reduction of the dried impregnated chlorides [285,286]. Grains of silica--- 60 vm in size were embedded in resin, and sectioned: figures 14 and 15 show how the metal concentration and the mean size (which was quite large) changes with distance from the surface. The composition in each of several zones at increasing distances from the surface was then assessed by electron-probe micro-analysis. Total metal concentration and particle size is greatest near the outer edge of the grain and there is a marked tendency for palladium to be found close to the outer surface, its concentration decreasing sometimes markedly with depth of penetration. Results of this kind, rarely published, show just how important it is to have a full physical characterization before conclusions concerning composition and catalytic activity are drawn.
18 (D
4
E
16
o> (D
3-
.c_
2
E o~
1-'
14 9 0 Au40
0
10 0
6o
12
/._.x
" ~ Pd40 Au60 ( ~ , , ~ , . ~
1
5
i
10
1
15
i
20
Distance from surface / pm
figure 14 Quantitative electron microscopy of Pd-Au/SiO 2 catalysts: dependence of total metal concentration on distance from the surface of the support grain for Pd6oAu4o and Pd4oAu6o [285,286].
o
~
1'o
&
,6,
l'S
2'0
Distance from surface / #m
figure 15 Quantitative electron microscopy of Pd-Au/Si02 catalysts: variation of mean particle size with distance from the support surface for PdeoAu4o and Pd4oau6o [285,2861.
__ ___ Rh
41 I
408-410
258 -
437-439
388. 441
443
354, 444
Preparation and characterization of metal and alloy catalysts
table 1 A guide to the preparation of supported alloy catalysts by conventional methods (Note: the numbers refer to references given at the end of the chapter)
a4_~
344
chapter 7
This table is only intended to give selective and illustrative references to the literature: each box contains no more than three references: where more are needed, the code letter directs the reader to the following lists. A"
Ni-Cu: 254,260,399,424,425,458
B:
Ru-Cu: 252,256,265,266,280,281,370,444
C:
Pd-Au: 251,253,462-463
D:
lr-Pt: 257,259,262,269,364
E:
Pt-Re: 269,272,273,277,353,391-393,464,465
F:
Pt-Sn: 269,333,351,441,448
Occasionally a combination of methods is used. Thus for example, the components may be added separately by ion exchange and impregnation [287] or one may be incorporated by impregnation and the second as a zero-valent compound [288,289]. One last generalisation could be noted. For catalysts prepared in academic laboratories, it is almost always hydrogen that is used to reduce a precursor in the dry state. Sometimes liquid-phase reductants such as hydrazine or hydroxylamine are used [285]; these generally work well with the noble metals at or near ambient temperature. Very rarely some more esoteric methods are used. Ammonia ligands can act as reductants; thus heating ion-exchanged ammine complexes in an inert atmosphere produces elemental metal [244], while reducing groups on the surface of activated carbon [199] or even the carbon itself [290] can cause reduction to occur. The prize for making the smallest supported alloy catalyst must go to two Czech research workers who recently described [291] the preparation of a palladium-molybdenum alloy on an alumina-coated tungsten field-emission tip. 7.3.3
Use of zero-valent compounds and atoms The use of zero-valent compounds for the preparation of supported metal catalysts,
and more recently for supported alloy catalysts, has attracted enormous attention. The philosophy is readily appreciated. One starts with the element of interest in an already highly-dispersed state, which it is hoped with careful handling may be maintained; the need for a reduction step, with its attendant risks of particle growth, is avoidable; and a number of spectroscopic techniques, especially FTIR and EXAFS, can be applied to study the interaction of the precursor compound with the support surface. The major problem involved in this approach arises from the fact that single atoms or small groups are only stable when surrounded by ligands such as carbon monoxide, which have to be removed before catalytic activity is obtained. It is in the process of removing the ligands that the problems arise, and some growth in particle size is almost inevitable.
Preparation and characterization of metal and alloy catalysts
345
Following the pioneering work of Webb and Thomson [292] many groups began to study the interaction of carbonyl compounds with oxidic supports. Carbonyl compounds dominate the organometallic chemistry of the transition metals, and all inorganic chemistry texts devote important sections to them. A number of metals form simple mono-, bi- or tri-nuclear carbonyls (see table 2); of the metals of catalytic interest, only palladium, platinum and the Group 11 metals do not, and the first two of these form carbonyl halides. Many metals form hydridocarbonyls, and the carbonyl group may co-exist with other ligands such as n-allyl, cyclopentadienyl, etc.: if one or more of the ligands is not zerovalent, the complex will carry a charge that has to be balanced by a cation or anion. Carbonyl compounds containing two or more metal atoms may accommodate different elements provided they came from the same group of the Periodic Classification (see below). table 2 Simple* neutral carbonyls of transition metals Group
5
6
7
8
9
10
CO2(CO)8 CO4(CO)12
Ni(CO)4
First row
V(CO)6 Cr(CO)6
M2(CO)~0
Fe(CO)5 Fe(CO)9 Fe3(CO)12
CO6(CO)16
Second row
-
Tc2(CO)lo
Ru(CO) 3
Tc3(CO)12
Ru2(CO) 9 Ru3(CO)12 Os(CO)5
Rh2(CO)8 Rh4(CO)12 Rh6(CO)16 Ir2(CO)8
Os2(CO) 9
Ir4(CO)12
Os3(CO)12
Ir6(CO)16
Third row
-
Mo(CO)6
W(CO)6
Rh2(CO)I0
* In the case of other elements, the carbonyls are either very unstable or are anionic or require other ligands to impact stability. Osmium also forms carbonyls containing 5,6,7 and 8 osmium atoms.
A great deal of detailed information is available on the manner of the interaction of carbonyl compounds with hydroxyl groups on oxidic supports [293-296]. Very often the reaction can be represented as oxidative addition of an acidic or amphoteric hydroxyl to the complex:
-S-OH + Mx(CO)y ----) HMx(fO)y_2(O-S) + 2CO where S represents the support cation: Carbonyl hydrides can become attached to the
346
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support by deprotonation: -S-OH + HnMx(CO)y ~ S*[Hn_lMx(fO)y]- + H20 The interested reader is referred to the cited articles for further information on this fascinating area of chemistry The course of decarbonylation depends on the nature of the oxide support. On magnesia and alumina, Ru3(CO)l 2 decomposes to give, after a hydrogen reduction at 723K, islands of ruthenium atoms bonded to the support through oxygen atoms, whereas on vanadia, silica and titania these oxygens are absent, and clusters of respectively greater than 12 and 6 ruthenium atoms are directly attached to the support [295]. Although it cannot be assumed that other preparative routes will lead to a similar distinction, the work with carbonyls has stimulated new thinking on the possible origins of metal-support interaction effects. rt-Allyl complexes were championed by Yermakov as providing a route to highlydispersed supported metals. They are readily prepared from a Grignard reagent, e.g. PdBr 2 + 2C3HsMgBr ~ Pd(C3Hs) 2 + 2MgBr 2 Bis-(rl3-allyl) complexes are known for nickel, palladium and platinum, tris-(rl3-allyl complexes for chromium, iron, cobalt, rhodium and iridium, and tetrakis complexes for mobyldenum and tungsten. They react with surface hydroxyls and propene is liberated: 2-S-OH + Pd(C3Hs) 2 ~ Pd(-O-S)2 + 2C3H 6 This chemistry does not however by itself lead to the preparation of alloy catalysts. The availability of carbonyl compounds containing two different metal atoms has given rise to attempts to use them as precursors to small alloy particles. The first experiments were reported by J.R.Anderson some twenty years ago, and the concept has been energetically pursued by other workers, notably Guczi in Budapest [297]. Chemisorbed forms and derivatives thereof, still retaining many or most of their carbon monoxide ligands, show interesting catalytic properties, especially in syngas reactions; but these bimetallic clusters cannot be regarded as alloys, so this work is not reviewed here. There have however been a number of apparently succesful efforts to produce small alloy particles by decomposition and reduction of chemisorbed carbonyls, although it seems probable that in many cases the naked cluster is unstable, and some particle growth occurs. Certainly an Ir/A1203 catalyst derived from Ir4(CO)l 2 shows the characteristic catalytic properties of iridium [298]. Among the binary carbonyls that have been used are Co2Rh2(CO)12 [299,300] and Co3Rh(CO)I 2 [299], H[Co3Ru(CO)12] [301] and Na[Co3Ru-
Preparation and characterization of metal and alloy catalysts
347
(CO)~2] [302], Fe3_xRux(CO)~2 [303], and other iron-containing multinuclear species [304]. Binary complexes containing the cyclopentadienyl ligand (cp) in addition to carbon monoxide have also been employed (cpMolr3(CO)~ ~ and cp2Mo2Ir2(CO)~0 [298], and the corresponding tungsten compounds [305]). The coexistence of two metals in the same complex is however no guarantee that an alloy particle will result from its decomposition, especially where they are incompatible (i.e. of low mutual solubility) in the metallic state. The complex (~t-H)AuOs3(CO)~0PPh 3 and (Ia2-C1)AuOs3(CO12PPh3 decompose on silica to give gold particles and OSx(CO)y species [306]. It may not always be necessary that the two different atoms should be in the same complex. Effective platinum-rhenium catalysts have been made by the interaction of (a Chini complex) with Re2(CO)I 2 [307]. Other organic complexes containing different metal atoms have been tried: those whose use has been reported include RhzCu2(p-totyl)4 [308], PdCu(OAc)4 [309], and a binary rc-allyl complex containing palladium and molybdenum [310]. Phosphine complexes [311] and trimethylplatinum chloride have also be employed [312], the latter in conjunction with an iron salt. [NEt4] 2 [Pt3(CO)615
A number of preparations have also been reported in which a combination of a conventional impregnation, leading to a monometallic catalyst, followed by a treatment with an organometallic complex, has formed a supported alloy. Examples of this appraoch include Pt-Mo/SiO 2 using a rt-allyl complex [313], Fe-Ru o n S i O 2 [288] and on A1203 [314] using the carbonyl, Ru-Pt/SiO 2 formed by treating Pt/SiO 2 with cpRuC1 [289], and Fe-Ru/SiO 2 and Ni-Cu/SiO 2 made by reacting Cu/SiO 2 with the respective carbonyls [315]. An Fe-Pt/7-AI203 catalyst has been prepared analogously [314]. Important extensions of this concept designed to ensure that the second metal interacts specificially with the first metal, and not with the support, will be considered in the following section. Considerable interest has been shown in producing artificial supported metal catalysts by alloying metal atoms formed at heated sources under UHV conditions to condense on the surface of an oxide or other material that is suitable as a support for metal particles, e.g. graphite [120,316,317]. The procedure is thus equivalent to that described above for forming condensed metal films, except that the number of atoms deposited per c m 2 is smaller. Because the condensed atoms do not bond strongly to the support, they are mobile and coalesce to form small particles, the shape of which, through the operation of epitaxial factors, depends on the chemical nature and crystallographic structure of the surface [317]. Such model catalysts, as they are called, are very suitable for studying changes in shape with gas atmosphere, the kinetics of sintering, a field of study pursued especially by Ruckenstein and his associates [316,318], and redispersion. Oxide supports may be either polycrystalline powders or single crystals such as mica, sapphire, magnesium oxide, quartz or diamond. Many of the features exhibited by practical catalysts can be incorporated in model
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analogues; for example, promoters such as alkali ions can be added, also by condensation, and partial overlayers of another oxide such as titania can be introduced to simulate the effects of the strong metal-support interaction [319]. It is also possible to produce very small particles by these methods, and basic studies on particle size effects have been reported [320]. Ion-implantation has also been employed as a route to model supported metals, but none of these procedures has been much used for preparing alloys. 7.3.4
Methods specific to alloys The methods to be noted in this section begin with an already-prepared supported
metal from Groups 8-10, to which a second metal is added by a procedure which is predicted to lead to all or most of it becoming associated with the first metal. Such a method will lead to a partial, or in the limit a complete, coverage of the first metal by the second, so that what is formed is a kind of two-dimensional alloy, somewhat similar to those produced by evaporating metal atoms of one kind and condensing them on the single crystal surface of another (see section 7.1.5). The added metals that have been studied include gold [321-326], silver [327], silicon, germanium, tin and lead [282,328-333], and the chemistry is ordered so that a defined reaction takes place between a compound of the additive and the surface of a reactive metal rather than with the support. One feature of this general procedure that has attracted particular interest is the possibility that the added metal may prefer one type of surface site to another, so that in principle the opportunity arises to observe structure-selectivity through the controlled elimination of one class of surface site. A convenient method for introducing the second metal is to use hydrogen chemisorbed on the first as reductant, for example: 3PtH + AuC14 --+ Pt3Au + 3HC1 + Cl The coverage by gold that is achieved thus depends on the amount of hydrogen chemisorbed and on the concentration of gold in solution. At low gold coverages the gold atoms are deposited randomly over the whole platinum surface, but at higher coverages they are set down preferentially on high-index planes through direct redox reactions that can be represented as: 3Pt + 2AuC14-+ 4C1- --~ 3PtC142- + 2Au ~ 3Pt + 4AuC14- + 2C1- --~ 3PtU62- + 4Au ~ The work by Barbier and his colleagues on the platinum-gold system has been described in a number of papers [321-326]. A similar method has been used to place silver on the
Preparation and characterization of metal and alloy catalysts
349
platinum particles in the Pt/SiO 2 EUROPT-1 [327] and Margitfalvi and his associates have used very similar chemistry to prepare supported platinum-tin and platinum-palladium catalysts [334-335]. Deposition of the Groups 4, 14 and 15 elements onto preformed particles of the Groups 8-10 metals has been successfully achieved by the reaction of chemisorbed hydrogen with a tetra-alkyl compound of the second metal; alkane is liberated and the adsorbed species are decomposed in inert atmosphere. After reduction, the second metal resides on the surface of the first. Once again it is possible to control the coverage by the additive, and to produce combinations with pairs of metals (or a metal + semi-metal) where low mutual solubility would otherwise prevent study. This procedure, which was originally developed at the French Petroleum Institute (I.F.P.) [282], has been applied in particular by Figueras, Coq and their associates. Germanium, tin, lead and antimony have been deposited on rhodium [329-331], palladium [328,329], ruthenium [331,332], platinum [333] and nickel [329]. While germanium appears to be deposited randomly, the heavier elements of Group 14 seem to prefer sites comprising atoms of low coordination number; this can be concluded from the study of reactions such as the hydrogenolysis of 2,2',3,3'tetramethylbutane, where central-bond fission to give isobutane occurs most readily on free sites containing low coordination-number atoms. For this technique of changing the composition of active centres to be successful, the particles of the active metal must of course be large enough to expose ensembles of atoms in low coordination number; thus sizes in the range 2-4 nm are desirable. The combination is unstable under oxidising conditions, so that a calcination step is to be avoided: the separated oxides cannot be recombined by reduction. The literature contains numerous reports of other approaches to the modification of a supported metal of Groups 8-10 by other species, but in these cases the additive is deliberately intended to remain in a positive oxidation state or is likely to do so despite the best endeavours to the contrary of those doing the work. There is indeed a kind of penumbra between true alloys, where both elements are indisputably in the metallic state, and promoted metals where the additive is not meant or expected to be reduced at all. The uncertainty lies in (i) whether the additive is totally or even partially reduced (e.g. as in Pt-Re/AI203), (ii) whether the use of a zero-valent precursor ensures that the element in question remains reduced [313], (iii) whether atomic hydrogen or spiltover hydrogen is capable of effecting reductions that molecular hydrogen at low temperatures cannot (e.g. M o 6+ ~
Mo ~
[336], and (iv) whether even if the added element starts by being zero-
valent it remains syngas reactions, These interesting research for some
so under reaction conditions. This last concern applies especially to where oxidising species such as water may be present on the surface. questions will no doubt ensure that catalysis remains a lively field of years to come.
350
7.3.5
chapter 7
Reduction to the active form Except in those cases where the active components are clearly already in a reduced
state, the final stage in preparation of an effective catalyst is the reduction of a precursor (oxide, salt or complex) to zero-valent metal. Especially with alloy catalysts, success in achieving the desired configuration and the necessary intimacy of contact between the components depends critically on the choice of reduction conditions. Among the variables that must be specified and controlled in the case of a gas-phase reduction are: the nature of the reductant, its concentration, the temperature ramp rate, the length and temperature of isothermal periods, the purity (especially the dryness) of the gas stream, and the size and shape of the bed of precursor. It is common
practice in academic laboratories to
reduce a separate aliquot of precursor before each experiment or series of experiments: while this has certain advantages (e.g. pre-reduced catalyst may become poisoned in storage), the number and sensitivity of the variables makes precise replication of activity measurements by this procedure almost impossible. Thankfully, product selectivities often seem less variable than rate measurements. Precursors that are readily reduced may in fact be brought to the active form at the start of the reaction, e.g. in a catalytic hydrogenation, by one reactant, i.e. hydrogen acting as reductant. It is for example common industrial practice to supply palladium/charcoal catalyst as unreduced wet filter-cake having the appearance and general character of old boot polish. Provided it is to be used in a hydrophilic solvent, the presence of the 25-30% w/w of water is of no significance. For catalysts of this type, that is those intended for use in liquid phase hydrogenations relevent to fine chemicals manufacture, where prereduction is necessary it is best effected by reagents such as alkaline formaldehyde, sodium formate, hydroxylamine or hydrazine salts or hypophosphites, in liquid medium. However, these operations have not so far been much applied to the preparation of alloys, and will therefore not be described in detail. Catalysts that are to be used in the gas phase are in general reduced in that phase, most commonly with hydrogen as the reductant. Carbon monoxide has certain attractions: it is a more effective reductant in the thermodynamic sense than hydrogen, i.e.carbon dioxide is a less strong oxidant than water, but the risk of carbon deposition through decomposition usually roles it out of consideration. Hydrogen is a kinetically active reductant, and reduction of some of the more easily reducible precursors (e.g. Pd 2§ may start at temperatures as low as 200K. In general, because of the exothermic nature of some precursor reductions, it is preferable if small metal particles are desired, to reduce as slowly as possible and at as low a temperature as possible: thus reductions may be beneficially conducted with a dilute mixture of hydrogen in nitrogen or other inert gas, using low ramping rates. The chemistry involved in the reduction process naturally depends on the type of
Preparation and characterization of metal and alloy catalysts
351
precursor. If it is an oxide or hydroxide, or other precursor subjected to calcination, the product of reduction by hydrogen is simply water, but reduction of a chloride will form hydrogen chloride, which may not immediately vacate the surface. Indeed with oxide supports such as alumina and titania, chloride ion is very strongly retained, and of course the success of alumina as a support for petroleum reforming catalysts is predicated on this fact. It is worth noting too that the water formed by reduction of an oxide may hydrate the surface of the support, or just become physically adsorbed on it: the successful application of the hydrogen-oxygen titration for measuring metal dispersion hinges on this. Reduction of nitrates without calcination can also lead to some complex chemistry, since the firstreduced metal can catalyse reactions of the gases liberated: thus the products may contain not only nitrogen monoxide (nitric oxide) and dioxide, but also ammonia and nitrogen. It is often desirable to know the precise conditions under which a precursor will become reduced to an active form, and whether the process is a simple or a complex one, that is to say, whether the reduction takes place in one stage or in more than one. In the latter case one would infer the existence of two or more different precursor species of different reducibility. A widely-used (and sometimes mis-used) technique for studying these questions is temperature-programmed reduction (TPR) [337-341]. This is one of an extensive suite of thermal-analytical techniques, in which changes in some property of the solid or the total system are followed as the temperature is raised linearly. The measurable properties include sample weight (as thermogravimetric analysis, TGA) and temperature (as in differential thermal analysis, DTA) [342,343], change in heat content (as in differential scanning calorimetry, DSC) [284] or composition of a gas stream passing over the sample (TPR). In these procedures, the observed signal may be either differential in character, as with DTA, DSC or TPR, or integral, as with DTA, where the total weight of the sample is recorded. The connection between the two is shown in figure 16; however, where necessary differential and integral signals are readily interconverted electronically. It is however a well-established fact that small signals are more easily detected when in the differential form. Much use is made of this principle in spectroscopic techniques (e.g. Auger spectroscopy, see chapters 3 and 4) and electron paramagnetic resonance (EPR) spectroscopy. The apparatus usually employed for TPR is sketched in fig 17. The principle of the method is as follows. A dilute mixture of hydrogen in an inert gas (nitrogen, or preferably argon) flows through one arm of a thermal conductivity cell (katharometer) and then over the sample; water or other condensible products are held back in a cold trap, and the gas proceeds through the second arm of the katharometer and thence via a flow-meter to exhaust. Consumption of hydrogen due to reduction of the sample is revealed by the detector wires differing in temperature: they form two arms of a Wheatstone bridge and the out-of-balance signal is captured by a chart-recorder or now most usually is put straight on to a VDU through a microprocessor.
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TGA
TPR, DTA, DSC
signal
G weight
integral effect
(-)dG dT
I
\
measured
time
T ~
time
figure 16 Recording of different analyses: TPR, DTA, DSC where the derivative is measured experimentally (such as the difference in the signal from Kz and K 2 in figure 17) and the total change obtained by integration. (~
|
1
cleaning
Reactor
figure 17 A simplified scheme of equipment for TPR; K 1 and K 2 katharometers.
Signal stability both on long and short time scales is important for obtaining results of good quality: control over gas flow rate, ambient temperature around the detector and mains voltage need particular attention. Signal calibration is achieved by injecting a known volume of pure hydrogen. Typical TPR profiles are shown in figure 18. For most common purposes the profile is characterised by the Tmax values and by the integrated volume of hydrogen consumed, although many workers inexplicably omit to measure this latter quantity: this is unfortunate because it often permits the composition of the precursor to be deduced, and deconvolution of complex profiles may reveal intermediate states of reduction. For example, it is quite clear that reduction of MoO 3 to Mo ~ proceeds via MoO 2, etc.
Preparation and characterization of metal and alloy catalysts
~B e-
.9
353
figure 18 TPR profiles of Pt, Re and Pt/Re on "~Al203 catalysts calcined at 798K (A) 0.375%w Pt 0.2%w Re/~[-A1203, (B) 0.375%w Pt/y-A1203, (C) 0.2%w Re/~[Al203 [3411.
E c o o
Lo "0 tO 0
-C
I
300
I
500
I
700 T(K}
I
900
I
1100
The basic method is capable of extensive refinement. More sophisticated methods of gas analysis can be used (e.g. mass spectrometry) to identify reduction products; evolution of hydrogen chloride has been detected by the simple expedient of passing the effluent gas through water and monitoring the change in conductivity [344]. Re-oxidation can be studied by temperature-programmed oxidation (TPO) using an oxygen-helium mixture, but for reasons that are not entirely clear the resulting profiles are usually broad and without significant features. TPO can also be used to estimate types and amounts of carbonaceous deposits. TPR has been applied to estimate the amount of atomic oxygen deposited on copper catalysts by decomposing dinitrogen oxide (nitrous oxide) [345]. A rapid and approximate method to assess catalytic activity is to pass a reactant mixture over a catalyst as the temperature is raised and to monitor conversion in some convenient way (temperature-programmed reaction). Reverting now to the basic TPR procedure, numerous experimental variables can affect the quantitative aspects of the results. In an epic study [346], Monti and Baiker examined the effect on the Tmax for nickel oxide of varying gas composition, flow rate and heating rate: Tmax is depressed by increasing the hydrogen concentration and by decreasing the heating rate. Maximum sensitivity is achieved by using a small hydrogen content (e.g.
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1%), for then the fractional change in composition is greatest: but the corresponding disadvantages are that (i) the reduction rate is continuously changing as the concentration changes, and (ii) all or nearly all the hydrogen may be consumed in a period of rapid reaction, leading to flat-topped peaks. Some workers have therefore used concentrations of hydrogen as high as 60%, with a corresponding loss of sensitivity. The normally used 5 or 6% concentration represents a suitable compromise. Kinetic parameters are usually determined isothermically in a static system, but theory is available to permit useful numbers to be derived from TPR: it is closely analogous to that used to understand thermal desorption spectra. The detector signal is proportional to the rate of reduction r, which can be expressed as: r = -dC/dt = ko C p (1-~) q exp (-E/RT) where C is the hydrogen concentration, ot the fraction of oxide already reduced, p and q are respectively orders of reaction in hydrogen and in unreduced oxide, and E is the activation energy of the process. The linear heating rate dT/dt is usually denoted by 13. This form of equation clearly replicates the principal feature of the results, namely, the rate increases, passes through a maximum and then falls to zero as the process is completed. At the temperature at which the rate is a maximum, the following equation holds: E]RT2max = k CPmax q(1-o~)q-l 13-1exp (-E/RTmax) whence by taking logarithms ln(T2max CPmax/~) q- ln[q(1-~) q-l] = (E/RTmax) + ln(E/Rk) If p and q are both unity ln(T2max CPmax/13) -- (E/RTmax) + ln(E/Rk) Since the logarithmic terms change more slowly with temperature than the linear term, it follows that the activation energy is proportional t o Tmax. However the literature records only comparatively few detailed studies employing this kinetic formalism. The kinetics of reduction of large oxide particles has been formulated [347] in terms of the nucleation of small metal centres following heteropolar adsorption of hydrogen molecules on the oxide surface and reduction by dehydration. Thereafter the reaction accelerates as hydrogen atoms are formed by dissociation on the metal centres, the maximum rate occurring when the interface between the metal and oxide is greatest. It is however doubtful whether this is a useful model to describe the reduction of very small
Preparation and characterization of metal and alloy catalysts
355
oxide particles or of the highly dispersed species that may be present after drying of precursors prepared by impregnation or ion exchange. This is an area in which a comprehensive study is still awaited. By far the greatest use made of TPR is therefore in a finger-printing sense, to recognise the number and types of precursor species and their reducibility. It is for example clear that simple impregnation and drying, using alumina support, often generates a binodal TPR profile (see figure 19), the sizes of the two peaks varying systematically with total metal loading [348]. The lower temperature peak corresponds to small oxide particles that are not chemically interacting with the support, whereas the higher temperature peak is due to metal ions that are firmly entrenched in interstices in the alumina surface. A TPR study of the reduction and re-oxidation of the Pt/SiO2 EUROPT-1 [349] reveals the complexity that an apparently simple system can show.
0.5% M 1%M 2%M C 0
I
I
I @
I @ I
/
E "-s
/
//
C 0 0
cy
,,|
"T" 0
t~ n"
I I
I 1 0
T/K
1
2
[M]/%
figure 19 (a) TPR profiles representing two species of different reducibility at various metal loadings, and (b) corresponding peak areas as a function of loading.
Extensive use has been made of TPR in seeking evidence for the formation of alloys in bimetallic catalysts [255,258-260,265,349-355]; the logic employed is simple, but fallible. One presumes that if one observes two peaks, each of which is clearly associated with the reduction of one of the components, then there has been the creation of two separate metallic phase and no alloy formation has occurred (see figure 18) [341]. Per contra if one sees only a single peak when reducing a bicomponent precursor, it is supposed that the more easily reduced species has catalysed the reduction of the less easily reduced, perhaps by the intervention of hydrogen spillover, and that there is then every
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chance of an alloy having been formed. Unfortunately, these assumptions are not necessarily true. There is good evidence [266] however that the occurrence of a single peak, in reducing mixed copper and ruthenium oxides on silica, does not signify to alloy formation, as only separate metal particles result: this is perhaps due to very effective and rapid hydrogen spillover giving action at a distance. Reduction of chloride/nitrate precursors affords two peaks, not well resolved, but continuing the hydrogen treatment at higher temperatures initiates surface migration, with consequent alloy formation. Extreme caution is needed in the interpretation of the results of TPR measurements, but the relatively low cost of the equipment and the ease of its operation will ensure its continuing and well deserved place in catalysis research.
7.4
Characterization of alloy catalysts
7.4.0
Introduction When the preparation and pretreatment of a catalyst is completed, and it is ready to
go to work, the question arises: does it have the desired configuration? In this section we are chiefly concerned with supported alloy catalysts, since the theory and practice of techniques such as XPS, AES, LEED and SIMS, which are important for characterizing single crystals, films and the like have already been covered. Some of these techniques, particularly XPS and AES, are of course also applicable to practical catalysts, but their theoretical basis will not be described again. The answer to the above question comes in two stages. The first, a comparatively trivial one, concerns the physical structure of the whole catalyst as expressed by its surface area, porosity and pore size distribution, The term "trivial" is used, not to suggest that these factors are unimportant to the successful operation of the catalyst in a practical context, but to imply that the techniques are well-established and the basis for interpreting the observations is well-understood. For this reason, this part of the question will be treated only briefly. The more weighty second part to the question addresses those issues set forth in section 7.3.0 concerning the extent of alloying, the homogeneity of the metal particles, their surface composition etc. To these and cognate questions concerning the method of their formation a great variety of powerful investigational techniques has been deployed, and section 7.4.2 describes the more important of these with brief introductions to the relevant theory where necessary, and some illustrative examples of their application to characterizing alloy catalysts.
Preparation and characterization of metal and alloy catalysts
7.4.1
357
Characterisation of the physical structure of supported metal catalysts.
As noted in section 7.3.1, the materials used as supports for small metal and alloy particles are frequently microporous, for only in this way can a sufficiently high surface area be generated to keep the particles well separated from each other. It is useful to know the surface areas of the support and indeed that of the finished catalyst, which is sometimes smaller [356], and also to know the porosity (i.e. the total pore volume Vp) and the distribution of pore sizes. From the surface area and pore volume, a mean pore radius r is readily estimated if one assumes the pores to be cylindrical and non-intersecting: r = 2 Vp/S Thus if r is 2nm and Vp is 1 cm3g-1, the surface area is 1000 m2g-1. At temperatures less than the critical temperature, but not more than about 100K above their normal boiling points, the vapours of liquids and gases, are physically
adsorbed on the surfaces of solids. The forces responsible for this interaction depend on the molecule in question and on the chemical nature of the surface, but they are of the same kind as those that hold molecules together in liquids, and are collectively described as Van der Waals forces, comprising dipole-dipole interactions, induced dipole charges, London forces due to mutually-induced fluctuating dipoles, and nuclear quadrupole forces. They are non-specific and the bonding lacks directional character: they are usually weak, and the heats of physical adsorption are only a little greater than the corresponding heats of liquefaction. It is sometimes possible to estimate a surface area roughly by applying the Langmuir equation O
= N/Nma x =
bp/(1 +bp)
where | is the fractional coverage and N the number of molecules adsorbed at the equilibrium pressure p, N m a x is the number adsorbed at saturation, i.e. the monolayer capacity and b is the adsorption coefficient. By using the equation in the linearized form p/N
=
p/Nmax + 1/bNmax
and plotting p/N versus p, the monolayer capacity is obtained from the slope: whence by assuming a value for the cross-sectional area of a single molecule, the area of the material is obtained. The Langmuir equation holds when only a single monolayer is formed; it is however more informative to measure a full adsorption-desorption isotherm at the temperature of the normal boiling point of the adsorbate (see below). For substances of
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moderate or high surface area (> 10m2g-~), the most suitable and widely used adsorbate is nitrogen; for materials of lower surface area it is necessary to use krypton or xenon [357]. If the adsorption isotherm is measured close to atmospheric equilibrium pressure at the boiling point of the adsorate, i.e. the saturation pressure, multilayer formation will occur. The isotherm will adopt one of the five forms, depending on the type of porosity and on the adsorbate; the relevant theory was first addressed by Brunauer, Emmett and Teller [358]. The standard classification of isotherms embraces all the types known at the time Brunauer wrote his book [359] (see figure 20). The type I isotherm is commonly found with zeolites and with activated carbons: the limiting uptake is due, not to monolayer formation, but to micropore filling. Information concerning the adsorption potential and its changes due to the presence of metal particles can be obtained when results are plotted in coordinates of the potential theory [360]. With isotherms of types II and IV, the monolayer capacity can be estimated using the BET theory [357-360] (see below): most of the supports usually used for metals and alloys show isotherms of this type. Isotherms of types III and V are characteristic of cases where the adsorption potential is weak and interaction between adsorbed molecules strong, e.g. adsorption of xenon or of pentane on alkali halide crystals or on carbon blacks.
Type
Type II
I
Type Ill
Type 19"
Type
Nods
P/Po figure 20 Types of adsorption isotherm (drawn schematically) as classified by Brunauer [359]
(note - types H and IV isotherms often show hysteresis as in figure 21 below), p~ saturation vapour pressure.
In the low pressure region of types II and IV we may apply the well-known BET equation
p Na~(p o-p)
--
1 NmaxC
+
C-1
p
NwaxC Po
where Po is the saturation vapour pressure, Nad s is the amount adsorbed, Nma x the monolayer capacity and C is given by
Preparation and characterization of metal and alloy catalysts
359
C = exp[(Ha-H1)/RT] Ha
being the heat of adsorption for the first layer, and H1 that for the second and subse-
quent layers, this being equated to the heat of liquefaction of the adsorbate. So by plotting the left hand side of the equation against P/Po, both
Nma x
and C can be obtained from the
slope and intercept. The type IV isotherm is of interest as it usually shows a hysteresis loop because the desorption branch of the isotherm does not follow that of the adsorption branch (see figure 21 ).
Type A
Type B
Type E
Nods
P/Po figure 21 Three types of hysteresis loop according to De Boer's classification.
This is because the evaporation of liquid from fine pores does not occur as readily as its condensation: the vapour pressure at a highly concave meniscus is less than that at a flat surface, because molecules at the surface are held more strongly. The effect of surface curvature on saturation vapour pressure was recognized many years ago by Lord Kelvin in the context of fine liquid droplets [359-361]: the effect on a liquid in a fine pore is described by the Kelvin equation: ln(p/po) =-(2VT/rRT)cos where V is the molar volume of the liquid, y its surface tension, r is the pore radius and the contact angle, usually taken to be zero. The relative pressure P/Po at which condensation will occur in a pore of given size can thus be determined and by reversing the procedure the isotherm may be analyzed to give a pore size distribution. A more exact procedure takes account of the presence of the physisorbed layer in forming the curved surface on which condensation occurs [362,363]. Hysteresis loops vary widely in shape, and these have also been classified: types A, B and E are shown in figure 21. The type A loop is given by open-ended cylindrical
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pores, type B by slit-shaped capillaries and type E by open-ended or closed pores of variable radius. Very detailed information about the pore structure of a solid can therefore be revealed by study of the nitrogen physisorption isotherm. Commercial apparatus has been available for some years for the automated measurements of physisorption isotherms, so the work may be performed painlessly. The only important decision that needs to be taken concerns the outgassing conditions; if not stringent enough, small microspores may retain gas, and particularly physisorbed water; if too stringent, sintering may occur. There is one point which should be brought to the attention of the reader. When, at a certain point the measurement of adsorption at increasing pressure (adding new doses of gas admitted to the sample) is interrupted and the pressure is decreased, the resulting isotherm cuts the hysteresis loop as shown in figure 22 and the lower isotherm is not obtained reversibly. Without a barostatic facility, the pressure at the beginning of the dosing is higher than at the steady state ('equilibrium') reached with the same dose. The point of the isotherm then appears within the loop and consequently a wrong distribution of pores according to radius is obtained.
figure 22 Adsorption isotherm showing a capillary condensation and scanning of hysteresis.
~
While the interpretation
---------~
x
of physisorption isotherms provides
information
on
micropores and mesopores, it is unable to say anything concerning macropores and larger voids. Fortunately these can be sensed by mercury porosimetry, which covers the mesopore and macropore region: the pressure p required to force liquid mercury
of
surface tension 7 into a pore of radius r is given by the equation p = 27 cos 0/r where ~ is the contact angle. Thus by monitoring the rate of change of the liquid volume in which the solid is immersed with applied pressure, a pore-size distribution can be obtained. A pressure of 70MPa is needed to force mercury into pores of 10nm radius, and this is readily attainable; in fact, pores as small as 1.5nm and as large as 104nm (10 lam) can be detected and estimated.
Preparation and characterization of metal and alloy catalysts
361
Other specialised techniques for studying the porosity of solids include small-angle X-ray scattering (SAXS), neutron diffraction and 129Xe NMR [364-366]. The latter is particularly relevant to zeolites, but the characterization of their internal structure is beyond the scope of this work. 7.4.2
Physical methods of characterizing small alloy particles The methods applicable to the characterization of small supported alloy particles
are the same as those used for single metals, where the identical question is posed. Indeed almost all techniques of which we shall have also found use in the context of single metals, and in some cases the interpretation is complicated by the presence of the second metal to the point where no meaningful conclusion is possible. A first and central question concerns the mean size of the particles and their size distribution, and for more than four decades the use of transmission electron microscopy (TEM) has provided the most direct and unambiguous information. The relevant theory and pratice has been described on numerous occasions [367]; the very much shorter equivalent wavelength of electrons in comparison with visible radiation permits a much finer resolution and modem instruments employing accelerating voltages of 100keV have point-to-point resolutions of about 0.3 nm under ideal conditions. With a little imagination it is possible to see the images of single atoms in small metal clusters [368]. The great assest of TEM is that it allows the construction of a size distribution histograph from counting the individual sizes of 500-1000 particles. For alloys it is important that the size distribution be as narrow as possible, otherwise there will inevitably be a range of surface composition which alter with size. The use of electron microscopy to follow sintering and re-dispersion in model catalysts has already been noted [316]. Much of the skill in the successful use of TEM as applied to supported metals lies in the sample preparation since it is a transmission technique, the sample must be thin, and the simplest method is to scatter fine particles of the catalyst into a support made from a material of low atomic mass that does not absorb or scatter electrons. A partial film of evaporated carbon on a copper grid (a so-called holey carbon film) is particularly convenient. In order to obtain a good dispersion of the catalyst particles they may first be immersed in a volatile liquid and disaggregated by ultrasound. More elaborated techniques can be used. Catalyst particles may be held in a thermosetting resin from which when solidified very thin sections may be cut with a microtome having a glass or diamond knife. Shadowed carbon replicas also reveal details of particle size and shape and if the support offers too much interference (for example, because it contains ions of high atomic mass) it can be eliminated by dissolution, after which the metal particles are recaptured and looked at in isolation. Although TEM has often be used to obtain size distributions of supported alloy
362
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particles (see for example [343,369-371], the great limitation of the technique is that is not analytical in the sense of immediately revealing the composition of the electron-adsorbing phase. In the high resolution mode (HRTEM), using electron energies of 0.5 to 1 MeV, lattice images can be observed and these may serve to identify the phase in question. Otherwise it is necessary to operate in the diffraction mode and obtain an electron diffractogram which may also be interpreted to pin-point the major phase present, although such patterns can only be formed by collecting electrons from a specified area of the sample (selected area electron diffraction) or by scanning the electron beam over the sample (STEM) [367]. Atomic composition is of course also accessible through energy analysis of back-scattered secondary electrons and secondary X-rays. We may turn now to methods of utility both for metals and alloys. When X-ray quanta impinge on a crystalline material, at certain angles of incidence 19 the reflected quanta reinforce one another, while at other angles they interfere and cancel: this gives rise to the well-known phenomenon of X-ray diffraction, described by the Bragg law: nK = 2d sin 19 where ~, is the wavelength of the X-radiation, d the distance between adjacent layers of atoms or ions, and n is the order of the reflection. Thus d is easily measured if ~ and 19 are known. It is a powerful method for bulk structure determination and estimation of lattice constants by one of the standard methods [372] serves to identify the diffracting phase. For particles below about 100 nm in size, however, the diffraction lines begin to broaden, and this effect can be used to give mean particle sizes in the range 5-50 nm by using the Scherrer equation which gives the volume-averaged particle diameter a as: a = 0.9x/B cos | where g is the peak width at half-height in radians. This is an approximate relation, to which corrections have to be made for instrumental broadening and for the fact that Xradiation is not exactly monochromatic, but for example a doublet [373]. The method is nevertheless straightforward to use and is quite satisfactory for comparing a series related catalysts; professional crystallographers however regard it as a very crude method. It is possible to generate a particle size distribution by analysis of the peak profile, and in its most elaborate form consideration is given to the concentration of defects, which manifest themselves as local deviations of the lattice constant, and to lattice strain. The theoretical basis is provided by the Warren-Averbach procedure [374] which has been applied to a number of systems [375,376]. It might seem doubtful whether these methods should be applicable to alloys, since inhomogeneity of composition from particle to particle would contribute an additional factor to line-broadening; nevertheless a very
Preparation and characterization of metal and alloy catalysts
363
detailed version of peak profile analysis for alloys has been advanced [377], making allowance for intraparticle concentration gradients. X-radiation incident upon a surface can be either diffracted, scattered or absorbed: the last gives rise to EXAFS (extended X-ray absorption fine structure) which will be considered below, while observation of scattered radiation is the basis of small-angle X-
ray scattering (SAXS) [378]. Particle sizes may be deduced [371,379], but there may be interference from pores, which should first be eliminated by filling or compression. An obvious requirement for obtaining a diffraction pattern is that the particles should be crystalline and of sufficient size to give coherent diffraction. This need does not apply to EXAFS, where local structure can be probed in systems not exhibiting extended order. The basis of the method is as follows [339,380,381]. The absorption of X-rays by a solid leads to the ejection of a photoelectron having energy Ek such that: E k = hv - E b where E b is its binding energy and hv the energy of the incidentphoton. The absorption spectrum therefore consists of a number of edges, each corresponding to the binding energy of an electron orbital in a component atoms. In the case of a free atom, the photoelectron escapes without hindrance, but in a solid it will be scattered by interaction with neighbouring atoms or ions, and constructive interference between outgoing and backscattered waves leads to a diffraction effect which manifests itself as the fine structure above the absorption edge (see chapter 2). This contains information on interatomic distances from and coordination number of the scattering centre (see also chapters 2 and 3 for other applications). Specifically, the periodicity of the modulation of the absorption is a function of interatomic distance r between the absorbing and backscattering atoms, and the phase shifts 5ij caused by the potentials at these centres on the photoelectron, while the intensity is controlled by the number of back-scattering centres Nj and their scattering amplitudes Fj(k). The amplitude is dampened by thermal and static disorder in the solid. The K edges of most of the 3d and 4d metals are accessible, as are the L(III) edges of the 5d metals. The realisation that structural information could be extracted from the EXAFS was most clearly demonstrated by the work of Sayers, Lytle and Stern [382]. The procedure is complex, but may be summarized as follows. The parabolic backgrounds and the freeatom spectrum are first subtracted; the signal is then expressed as a function of the wave number k of the photoelectron where:
k : 2~/2meEk[h = 27r,~/2me(hv-Eb)]h where m e is the electron mass. Here hk/2rt is the momentum of the wave quantum and
364
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{2meE k is the equivalent particle momentum. The EXAFS function g(k) is the sum of the scattering contributions of all neighbouring atoms" X(k) = ~[~ Fj (k) sin (2krj + 5~j(k)) J where the subscript j defines the coordination shells around the emitting atom. The essence of the analysis is to recognise all the sine contributions in z(k); the most suitable mathematical procedure to achieve this is Fourier analysis. The major problem lies with quantifying the phase shifts, this is best done through the use of analogous model compounds. It is unnecessary to rehearse in detail the operation by which the raw results are interpreted; this has been described on many occasions [339,380,381], and instead we concentrate on illustrating the application of the method to small metal particles and alloys. Analysis of the EXAFS of single metal catalysts yields a mean coordination number from which an average particle size can be deduced [383]" comparison of the values obtained for nickel, platinum, rhodium and iridium catalysts with hydrogen monolayer capacities has helped to establish that H/Ms ratios greater than unity are possible, and may approach a value of three in the case of iridium. However the proper manner of data analysis is still being debated [384] and particle sizes for lighter elements may be underestimated because the anharmonicity of vibrational motion at the surface has not be properly evaluated. Alloys of copper with ruthenium [270,271,385,386], osmium [387], gold [371], rhodium [388], silver [389], rhenium, iridium and platinum [390] have provided clear structural information, helped by the fact that the large difference in atomic number enables them to be differentiated as components of a coordination shell. The work of Sinfelt and co-workers on ruthenium-copper catalysts [385] has confirmed the model of chemisorbed copper atoms on a ruthenium core, arrived at on other grounds: the ruthenium EXAFS profile resembles that of Ru/SiO2, while the copper profile is unlike that of Cu/SiO 2 because of the smaller number of Cu-Cu interactions. Analysis of results is more difficult when atomic numbers are similar, as with platinum plus rhenium or iridium. For obvious reasons these systems have nevertheless attracted considerable attention [271,272,386,390-394]; with the Pt-Ir/SiO2 system it was however necessary to use artificially high metal loadings (10% of each). Other systems successfully examined include rhodiumiridium [395] and platinum-chromium [396]. It needs to be appreciated that with the observed EXAFS, in an average over all oxidation and dispersion states of the element in question, and for materials that are not completely homogeneous, the information derived will only have a limited significance. The structure of the X-ray absorption spectrum in the region of the edge (XANES = X-ray absorption near-edge structure) also contains potentially useful information [381].
Preparation and characterization of metal and alloy catalysts
365
On the low-energy side of the continuum, there are transitions to unoccupied valence states, giving rise to a 'white-line' feature which can be particularly intense when there is a high density of dipole-allowed transitions. The white-line intensity may therefore be correlated with the occupancy of d-levels, and in the osmium-copper system [387] (see chapter 3) XANES analysis has suggested that there are fewer unfilled d-states than in pure Os/SiO 2. However other factors than the mere presence of the copper may operate and the theoretical basis for interpreting XANES is not yet well established. The application of X-ray photoelectron spectroscopy (XPS) to estimating the surface composition of alloys has been treated in detail in chapter 2. It is often used to examine practical catalysts [339,397], as the instrumentation is now widely available and very reliable, however, it is necessary to enter a number of caveats concerning the use of the techniques and the interpretation of the results. The first concerns pretreatment of the sample. Clearly if information relevant to the practical use of a catalyst is to be obtained, it must have the appropriate activation. The use of a sample pretreatment facility in which a precursor can be calcined and reduced and from which it can be transported into the spectrometer without exposure to air is essential. Then it must be remembered that the volume of sample from which the signal comes is exceedingly small, and replicate samples should be tested to ensure the absence of heterogeneity. Even so the photoelectrons emerge from only a very thin layer close to the surface (typically 1-2 nm), so that the fraction of a sample that is sensed is minute. With porous catalysts, where metal or alloy particles may be dispersed throughout the support grain, and where there may concentration gradients normal to the surface, misleading results can be obtained: misleading that is in the sense of being unrepresentative of the whole mass of sample. Interpretation of results, except in the most superficial manner, can also present difficulties. The presence of carbon on the surface is often indicated by a strong and broad C ls signal, which is sometimes (unwisely) used as a calibration of the binding energy scale, i.e. to estimate the charging correction. The problem is that a number of different types of carbon atom (aliphatic, graphitic, etc.) contribute each having slightly different binding energies because of chemical shifts. For example, the C ls signal interferes seriously with the study of ruthenium catalysts, since the Ru3d transitions occur at almost the same binding energies. Complex signals arising from several different components, or even broad or asymmetric signals from a simple material, may be deconvoluted to show what individual components are present, but so easy is it to do this with the help of a computer that the imagination can readily run riot: some published deconvolutions could be classified as an art-form rather than a scientific exercise. Nevertheless, used with caution and common sense, it is able to reveal minor amounts of unexpected oxidation states or trace impurities. It is possible to estimate surface concentrations from signal intensities modified by published [398] sensitivity coefficients (see chapter 2), but once
366
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again their quantitative value has to be treated cautiously. So commonly is the technique employed that it would be pointless to try to list all published work on XPS as applied to practical alloy catalysts. However the interested reader might consult the following references to form an impression of the scope and limitations of the technique: platinum-rhenium [391], iron-manganese [350], nickel-copper [399], platinum-molybdenum [400], nickel-rhodium [401], palladium-lead [343,402]. Another very popular technique, although one more limited in its availability and scope than XPS, is M6ssbauer spectroscopy (see also chapter 3) [339,403]. This is based on the observation that nuclei held rigidly in a solid matrix can undergo recoil-free emission and absorption of X-radiation; the separation of the nuclear energy levels can be estimated to an accuracy of 1 in
l014,
which is sufficient to detect weak interactions
between a nucleus and its electronic environment. As an example we may consider the 57Co nucleus which transforms to the excited state of 57Fe, which in turn decays to its ground state by emitting either an internal conversion electron or X-rays with a characteristic energy of 14.4 keV; these may then be absorbed in iron nuclei in the sample under study. In the case of free atoms, the energy of the X-rays Er is less than 14.4 keV (i.e. Eo) because some is imparted to the nucleus as recoil energy ER: thus
E+ = E o - E R = Eo(1-
1 2)
2mc
where m is the nuclear mass and c the velocity of light. Emitted photons cannot then be re-absorbed, but if the absorbing nucleus is within a solid lattice, the recoil energy is taken up by the collective phonons. If however E R is smaller than the phonon energy, a number of emission or absorption events can occur in a recoil-free manner, provided the average value of E R is maintained over a large number of events. Thus a fraction of the emitted Xrays can be re-absorbed: thus is the M6ssbauer effect. The nuclear levels in the absorber will not however exactly equal those in the emitter, because the electronic environments will not be the same: it is therefore necessary to fine-tune the energy Er by moving either the source or the absorber relative to the other, and utilising the Doppler effect. Then
E+ = Eo(l + ~) C
where v is the relative velocity. In order to observe hyperfine interactions in iron, we need to use Eo_+ 500 ~eV which corresponds to Doppler velocities of _+ 10 mm.s -1. The transmittance of X-rays by the sample as a function of Doppler velocity for a moving single-line source and a stationary sample is therefore as shown in figure 23A.
Preparation and characterization of metal and alloy catalysts
367
%
|
Q r .g
E 09 r
I--
V1
V1
V2
V1
V2 V3 V4 V5
V6
Dopier velocity
figure 23 Principle types of M6ssbauer spectrum. A: singlet, B: quadrupole doublet, C: magnetic sextuplet (Zeeman effect).
The hyperfine interactions between a nucleus and its surroundings are a sensitive indicator of the chemical state of the atom or ion; they are of three sorts. (i) The isomer shift 5 arises from the Coulombic interaction between the nucleus and the s-electrons; since excitation of the nucleus changes its size, the Coulombic interaction also changes. The isomer shift therefore reflects the density of s-electrons at the nucleus and hence the oxidation state of the iron. It also depends on the thermal motion of the atoms or ions within the lattice and from the way in which 5 varies with temperature the Debye temperature may be estimated. (ii) Electric quadrupole splitting AE (figure 23B) arises from the fact that the excited nucleus is ellipsoidal rather than spherical, and therefore possesses a nuclear quadrupole moment. The nucleus can therefore orient itself in two ways of slightly different energy in an electric field gradient, so that two transitions from the ground state become possible. (iii) Finally, magnetic hyperfine splitting (the Zeeman effect) (figure 23C) arises through interaction of the nuclear magnetic dipole moment and the magnetic field at the nucleus; this interaction permits six transitions to occur, the separation of which is proportional to the strength of the magnetic field. For further information on the theory of M6ssbauer spectroscopy, the reader is referred to the cited references [339,403]. There are only a limited number if isotopes of elements of catalytic interest that are suitable for examination by M6ssbauer spectroscopy (see table 3).
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table 3 Isotopes suitable for MOssbauer spectroscopy Isotope
Source
Half-life
Energy/keV
57 Fe ll9Sn 121Sb 197Au 99 Ru
57 Co 119rnSn 121mSn 197 Pt 99 Rh 193 Os 195 Au
270d 245d 75y 20h 16d 32h 192d
14.4 23.9 37.2 77.3 90.0 73.0 98.8
193ir
195pt
Almost all the published work refers to catalysts containing either iron or tin, although limited work has been performed with ruthenium [404]. Amongst the alloys investigated are those of iron with platinum [287,405-407], palladium [407-410], ruthenium [406,411414], rhodium [411] and nickel [415] and those of tin with iridium [342] and especially platinum [416]. The technique is not structural in the sense that structures can be deduced; interpretation depends chiefly on comparisons with known compounds or phases. Further useful techniques for characterizing small metal and alloy particles are based on magnetic properties associated with nuclei and atoms. A nucleus that possesses spin angular momentum exhibits the phenomenon of nuclear magnetic resonance (NMR): it may take 21 + 1 orientations, so that for hydrogen (~H), for which I equals g2, there are two possible orientations, designated o~ and 13, corresponding to quantum numbers mi of _ g2. In a magnetic field B, these have different energies, given by E = - gtzB = - 'flaBm I where gtz is the component of the magnetic moment on the Z-axis, 3~is the magnetogyric (or gyromagnetic) ratio of the nucleus and h is h/2rr. The product yB is termed the Larmor frequency co; thus E - - - mihco The term 7 is usually positive, so that the 13 state has the slightly greater energy; if radiation of frequency v impinges on the sample, the energy states resonate with the radiation provided hv = y h B
Preparation and characterization of metal and alloy catalysts
369
Under this resonance condition, there is strong coupling between the nuclear spins and the radiation, so that strong absorption of the radiation occurs as the spins make the ot to 13 transition. This constitutes nuclear magnetic resonance. Its usefulness in determining chemical structures arises chiefly from the phenomenon of the chemical shift (r due to the perturbation of the Larmor frequency by the effect on the nucleus in question of the diamagnetic field surrounding it: 2rc~ = 7(1 + (y)B Its value is about 10 -6 of the Larmor frequency, and the chemical shift is conventionally expressed relative to some reference compound. Concerning the application of this marvellous technique to liquids, organic molecules and indeed to living systems, we need not speak: the theory, practice and application are well described in numerous text books, Its utility in the study of metals and alloys has been limited [403,417,418]. Considerable work has been done on NMR of adsorbed molecules using the 1H [419,420] and 13C nuclei, but here we will focus on the use of the 195pt and 129Xe nuclei. In the case of metals, an additional perturbation to the Larmor frequency arises through the polarisation of the conduction electrons by the magnetic field at the nucleus, resulting in a further displacement of the Larmor frequency, called the Knight-shift, which can be much larger than the chemical shift [421]. Marked changes in NMR line shape for Pt/A1203 catalysts of various dispersions have been reported [422], but the technique is difficult and has not yet found wide application (see also chapter 5). Chemical shifts on 129Xe NMR through interaction with hydrogen atoms has been more widely used, Particularly in the context of zeolites and estimation of the size of metal particles therein, or on conventional supports [365,366]. Again, the technique does not yet seem to have been much used in the context of alloys, although an examination of platinum-iridium clusters in NaY zeolite has recently been reported [364]. There is however no reason why it should not been quite informative concerning the structure of alloy catalysts. Studies of the magnetizability of metals and alloys contributed greatly to the development of early (incorrect) theoretical concepts (see chapter 3) [423]. The intensity of magnetization M is proportional to the magnetic field strength H, i.e. M=)cH where Z is the magnetic susceptibility: when this is positive, the material is said to be paramagnetic and when negative it is diamagnetic. A characteristic feature of many metals is their weak temperature-independent paramagnetism. Iron, cobalt and nickel and their
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alloys are of course ferromagnetic, that is, they can be permanently magnetized below the Curie temperature T c (see table 4); above this temperature they show a normal temperature-dependent paramagnetism. The saturation moment PM, sometimes termed the atomic magnetic moment, when expressed in Bohr magnetons, gives the average number of unpaired electrons per atoms at the absolute zero (see table 4). table 4 Curie temperature T c and atomic magnetic moments ~M of ferromagnetic metals Metal
T~(K)
E~ (Bohr magnetons)
iron
1053
2.22
cobalt
1148
1.71
nickel
631
0.606
The early work mentioned above was concerned very largely with the nickelcopper system, and measurements had been made before 1960 on the way in which saturation moment, Curie temperature and electronic specific heat coefficient varied with composition [38,423]. During this period the first results on systems appropriate to catalytic use were described [38,254]. It is curious that in more recent times relatively little use has been made of magnetic methods in the study of alloys: such reports as we have noted have been very largely confined to the nickel-copper combination [399,424427], although carbon-supported platinum-iron alloys have also been investigated in this way [428]. Magnetic susceptibility studies of diamagnetic systems have been almost entirely neglected. Techniques based on magnetism suffer from the same limitation as many other namely, they integrate over the whole sample, and any heterogeneity of structure or composition is not revealed. 7.4.3
Characterization of supported metals and alloys by selective gas chemisorption
It would not be appropriate to leave this review of methods for characterizing supported metal and alloy catalysts without touching on the procedure which is probably the most widely used of all, namely, selective gas chemisorption. The principle upon which the method operates is very simple: one chooses a gas which will chemisorb only on the metal or alloy, and not on the support, and which will form a complete monolayer under ambient conditions, or close to them, and then measures -for example- the volumetric adsorption isotherm. From the derived monolayer volume and an assumed stoichiometry for the chemisorption process, one estimates the number of surface metal atoms
Preparation and characterization of metal and alloy catalysts
371
participating; this allows estimation of turnover frequencies, and in the case of pure metals with some further assumption concerning particle shape one may derive a mean particle size. The apparatus needed is relatively cheap, and the method although tedious is deservedly popular. Instrumentation for the automated determination of adsorption isotherms is commercially available, but expensive. However, as with all methods of analysis to obtain consistent, accurate and meaningful results requires attention to be paid to numerous factors, both theoretical and practical. The first relates to the initial cleaning of the surface, which clearly must be complete before the isotherm is measured. Metallic particles are normally covered by a layer of chemisorbed oxygen and/or sulfur and/or organic rubbish; this will need to be removed by treatment in hydrogen at the lowest possible temperature, to avoid sintering; oxidative treatments must be used with great caution, as in some cases either particle growth or disaggregation may occur. Adsorbed hydrogen is then removed by pumping at elevated temperature, typically overnight; hydrogen being more weakly chemisorbed than the initial contaminants is the more easily got rid of. The catalysts should then be ready to have its adsorption isotherm measured. The selection of the adsorbate has then to be made. Hydrogen is most commonly used, for it chemisorbs dissociatively on almost all metals of catalytic interest, although weakly on copper and silver, and not at all on gold. There are two main problems with the use of hydrogen: (1) the surface stoichiometry, i.e. the number of hydrogen atoms per surface metal atom H/M s, is uncertain, although very widely assumed to be unity. Certainly in the case of small particles of groups 8-10 metals it may be very much higher [429]. (2) The occurrence of hydrogen spillover, i.e. migration of hydrogen atoms from metal to support, is a well attested phenomenon, but it is often claimed that this does not occur under the conditions under which adsorption isotherms are normally measured. While hydrogen chemisorption is often almost instantaneous, at least at low coverages, there are cases (ruthenium powder is one) where the process is notoriously slow, and long equilibration times are needed for each dose; alternatively superambient temperature is used, with the attendant risk of spillover and interactions with contaminants occurring. While in the simplest theoretical visualization of the process, viz. H 2 -4-
2M~
--+
2 H - M~
there is only one type of chemisorbed hydrogen atom, there is much evidence to show that adsorption energies decrease with increasing coverage, and this effect is often expresses simplistically by designating some adsorption as 'strong' and some as 'weak', the latter being that which can be removed by a few minutes pumping. Measurement of a second isotherm therefore records only the 'weak' component, and subtracting this from the first isotherm therefore gives the 'strong' component, for which the H/M s ratio is perhaps more
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sensibly taken as unity (see figure 24).
Q
>
figure 24 Hydrogen chemisorption: 1, first isotherm; 2, second isotherm (weak form only) A, difference isotherm. .-
PH2(kP ) While the literature records instances of impressive agreement being obtained between particle size estimates made by hydrogen chemisorption, electron microscopy and other methods [430], it is also true that there are well-established cases of lack of agreement: with EuroPt-1 for example [431] the amount of hydrogen chemisorbed exceeds that expected on the basis of a mean particle size of 1.8 nm as derived by TEM. It may also be helpful to plot the results according to the Langmuir equation to obtain a more realistic value for the monolayer volume [432,433]. With EUROPT-1 careful experimentation can provide quite consistent values for monolayer volume [231,356,432,433], but the H/Ms ratio exceeds unity by some 20%. For the greatest accuracy and in general with microporous supports, due attention has to be paid to the pumping speed at the sample if a really clean surface is to be obtained [433]. An alternative procedure to the volumetric one is the pulse-flow method. Here the sample is cleansed after reduction by purging with an inert gas, then small pulses of hydrogen are injected at regular intervals; several may be wholly adsorbed, then some partially and finally when the surface is saturated they pass unchanged (see figure 25). Knowing the volume of each pulse, the total volume retained can be deduced. Alternatively, a single large pulse can be used and from the fraction reaching the detector the amount retained can be found. With the small pulse methods, desorption begins as soon as the pulse has moved through the sample, so that the amount recorded as adsorbed actually increases with the interval between the pulses. Variation of the interval provides a method for studying desorption kinetics.
Preparation and characterization of metal and alloy catalysts
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0
Vrn<
.... j /
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E
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.X I
~X"
/ X.
.s
•
•
/ .... t
!
tA t
373
t
t
t
A !
r
Cl
a.
t 1 1 1 Pulse number
figure 25 Pulse flow methods to determine the monolayer chemisorption capacity Vm.
Returning to the volumetric methods, difficulties can arise if chloride ion is present and if the support retains it strongly (e.g. TiO2, A1203) [434]. Apparently chloride partitions itself between metal and support, and its presence on the metal or its direct neighbourhood interferes with the chemisorption of hydrogen, causing also long equilibration times. In such a case the isotherm does not show the characteristic sharp 'knee' of a 'good' isotherm (figure 24). It is a source of concern that so many papers cite H/Mto t values without describing how these were derived from the experimental measurements, and without showing examples of the isotherms obtained. The eternal watchword for the careful scientist is : caveat lector.t Another common fault is to omit chemical analysis of the active component. Catalysts made by impregnation having typically only 90% of the nominal metal loading (less if this is high); nevertheless results are often quoted to three significant figures, based on nominal metal content! Analysis of the exact metal content of a catalyst containing -- 1% metal, e.g. to an accuracy of 1% is no mean feat: but it has to be done if the implied worth of the result is to be justified. Chemisorption of hydrogen on alloys will be treated fully in the following chapter. It suffices now the remark that the principal question that arises in using it as a means of characterizing supported alloys is whether the hydrogen atoms remain wholly on the active component (Ni, Pt ...) or whether they can spillover onto the inactive one (Cu, Ag .... ). Depending on the answer to this question one may have a means of estimating either the number of active atoms on the surface or the total surface area. Frequent use has also been made of chemisorption of carbon monoxide for titrating surface metal atoms. Its sole advantage over hydrogen is that the possibility of spillover to
374
chapter 7
the support is virtually eliminated, but there are many disadvantages too. Chief, among these are the questions of whether the chemisorption is dissociative or non-dissociative and if the latter how much is 'bridged' and how much 'linear'. Diluting the active metal in matrix of inactive metal can cause a change from bridged to linear bonding, so that the use of an ancillary technique (e.g. IR spectroscopy) is needed before certain results can be obtained. A further concern with very small particles especially of rhodium and of ruthenium is that they may disintegrate to form adsorbed carbonyl groups such as M(CO)2. The pulse-flow method can be applied with carbon monoxide, using hydrogen as carrier gas, which is easily displaced by the more strongly adsorbing carbon monoxide and which also keeps the surface in a fully reduced state. Chemisorption of oxygen has also been used, but has been less popular. The main problem here is that of sub-surface adsorption, leading in the case of small particles to almost complete oxidation. The process needs to be performed at sub-ambient temperatures if these risks are to be avoided. In the case of alloys, oxygen chemisorption being very exothermic, complete disrupting to two separate oxide phases, not reversible by reduction, is a distinct possibility. Finally the use of chemisorption titrations needs to be mentioned. The basic idea will be conveyed by the following equations: M20 + 2H 2
----) 2MH + H20
2MH
~
+ 0 2
M20 4- H20
Causing hydrogen to react with a layer of chemisorbed oxygen atoms, here assigned the stoichiometry O/M s = 0.5, doubles the amount of hydrogen taken up, thus increasing the accuracy of the measurement. Titrating the chemisorbed hydrogen with oxygen completes the cycle and offers another route to estimating the number of surface metal atoms. The cycle can of course be repeated any number of times, and it has: slow changes in amounts consumed have been detected. There has been considerable discussion on the stoichiometry of the two processes, and the O/M s ratio seems to be particle size dependent. The water formed is assumed to be taken up by the support. Titrations with carbon monoxide have also been tried: M20
+ 3CO
2M-CO + 3/202
~
2M-CO
----) M20
+ CO 2
+ 2CO 2
Chemisorbed hydrogen has also been titrated with alkenes (1-pentene and ethene). These titration methods have found only limited use in the study of alloys [434], although they have been applied to petroleum reforming catalysts containing platinum and a second element (rhenium, tin .... ) [435].
Preparation and characterization of metal and alloy catalysts
375
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S.-H.Wu, W.-P.Huang, W.-J.Zhao, W.K.Wang, J.-X.Lin, Cuihua Xuebao 13 (1992) 181 G.Leclercq, T.Romero, S.Pietrzyk, J.Grimblot, L.Leclercq, J.Molec.Catal. 25 (1984) 67 G.Leclercq, S.Pietrzyk, L.Gengembre, L.Leclercq, Appl.Catal. 27 (1986) 299 W.Palczewska, A.Jabl6nski, Z.Kaskur, G.Zuba, J.Wernich, J.Molec.Catal. 25 (1984) 307 W.N.Delgass, G.L.Haller, R.Kellerman, J.H.Lunsford, "Spectroscopy in Heterogeneous Catalysis" Academic Press, New York, 1979 C.A.Clausen, M.L.Good, J.Catal. 38 (1975) 92 C.H.Bartholomew, M.Boudart, J.Catal. 25 (1972) 173; 29 (1973) 278 J.H.A.Martens, R.Prins, J.W.Niemantsverdriet, J.Catal. 108 (1987) 259 J.J.Burton, R.L.Garten in "Advanced Materials in Catalysis" Academic Press, chapter 2 (1977) 33 R.L.Garten, D.F.Ollis, J.Catal. 35 (1974) 232 R.L.Garten, J.Catal. 43 (1976) 18 R.L.Garten, E.B.Prestridge, J.Catal. 35 (1974) 232 J.W.Niemantsverdriet, D.P.Aschenbeck, F.A.Fortunato, W.N.Delgass, J.Molec.Catal. 25 (1984) 285 A.M.van der Kraan, A.Nonnekens, F.Stoop J.W.Niemantsverdriet, Appl.Catal. 27 (1986) 285 F.J.Berry, L.-W.Lin, C.-G.Wang, R.-Y.Tang, S.Zhang, D.-B.Liang, J.Chem.Soc.Faraday Trans.I 81 (1985) 2293 F.J.Berry, L.-W.Lin, H.-Z.Du, D.-B.Liang, R.-Y.Tang, C.-Y.Wang, S.Zhang, J.Chem.Soc.Faraday Trans.I 83 (1987) 2573 G.del Angel, C.Medina, R.Gomez, B.Rejai, R.D.Gonzalez, Catal.Today 5 (1989) 395 G.B.Raupp, W.N.Delgass, J.Catal. 58 (1979) 337, 348 M.C.Hobson Jr., S.L.Goresh, G.P.Khare, J.Catal. 142 (1993) 641 "NMR Techniques in Catalysis", (editors: A.T.Bell, A.Pines) Dekker, New York, 1994 A.D.H.Clague, "Specialist Periodical Reports (Catalysis)" (editors: G.C.Bond, G.Webb) Royal Society of Chemistry, London, 7 (1985) 75 X.Wu, B.C.Gerstein, T.S.King, J.Catal. 121 (1990) 271 X.Wu, B.C.Gerstein, T.S.King, J.Catal. 123 (1990) 43 C.P.Slichter, "Principles of Magnetic Resonance", 2nd edn., Springer, New York, 1982 H.E.Rhodes, P.-K.Wang, H.T.Stokes, C.P.Slichter, J.H.Sinfelt, Phys.Rev.B 26 (1982( 3599
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N.F.Mott, H.Jones, "The Theory of the Properties of Metals and Alloys", Oxford U.P., Oxford, 1936 J.A.Dalmon, J.Catal. 60 (1979) 325 S.D.Robertson, S.C.Kloet, W.M.H.Sachtler, J.Catal. 39 (1975) 234 W.Romanowski, React.Kinet.Catal.Lett. 4 (1976) 129 W.G.Reman A.H.Ali, G.C.A.Schuit, J.Catal. 20 (1971) 374 C.H.Bartholomew, J.H.Anderson Jr., M.Boudart, J.Chem.Soc.Faraday Trans.I 75 (1979) 257 B.J.Kip, F.B.M.Duivenvoorden, D.C.Koningsberger, R.Prins, J.Catal. 105 (1987) 26 J.W.E.Coenen, Appl.Catal. 75 (1991) 193 A.Frennet, P.B.Wells, Appl.Catal. 18 (1985) 243 G.C.Bond, Lou Hui, J.Catal. 147 (1994) 346 C.Hubert, A.Frennet, Catal.Today 17 (1993) 469 L.Guerrero, G.Diaz, S.Fuentes. React.Kinet.Catal.Lett. 36 (1988) 217 G.Leclercq, H.Charcosset, R.Maurel, C.Bertizeau, C.Bolivar, R.Frety, D.Jaunay, H.Mendez. L.Tournayan, Bull.Soc.Chim.Belg. 88 (1979) 577 M.Hoang, A.E.Hughes, T.W.Tumey, Appl.Surf.Sci. 72 (1993) 55 T.Mallat, S.Szabo, J.Petro, Acta Chim.Hung. 124 (1987) 147 T.Mallat, J.Petro, S.Szabo, J.Sztatisz, React.Kinet.Catal.Lett. 29 (1985) 353 Zhong-ru Guo, Hong-xin Shi, Hui-zhen Xu, J.Catal.(Cuihua Xuebao) 14 (1993) 12 H.E.Swift, J.E.Bozik, J.Catal. 12 (1968) 5 J.K.A.Clarke, I.Manniger, T.Baird, J.Catal. 54 (1978) 230 S.Leviness, V.Nair, A.H.Weiss, Z.Schay, L.Guczi, J.Mol.Catal. 25 (1984) 131 A.Ramachandran, D.K.Chakrabarty, Appl.Catal. 42 (1988) 229 M.W.Smale, T.S.King, J.Catal. 116 (1989) 540; 125 (1990) 235 J.M.Cowley, R.J.Plano, J.Catal. 108 (1981) 199 D.E.Resasco, G.L.Haller, Appl.Catal. 8 (1983) 99 J.H.Anderson Jr., P.J.Conn, S.G.Brandenberger, J.Catal. 16 (1970) 326, 404 T.Mallat, Z.Bodnar, A.Baiker, O.Greis, H.StiJbig, A.Keller, J.Catal. 142 (1993) 237 J.H.A.Martens, R.Prins, Appl.Catal. 46 (1989) 31 M.Chen, L.D.Schmidt, J.Catal. 56 (1979) 198 W.Palczewska, A.Jablonski, Z.Kaszkur, C.Zuba, J.Wernisch, J.Mol.Catal. 25 (1984) 307 T.Mallat, S.Szabo, J.Petro, Appl.Catal. 29 (1987) 117 H.R.Aduriz, C.E.Gigola, A.M.Sica, M.A.Volpe, R.Touroude, Catal.Today 15 (1992) 459 R.Frety, M.Guenin, P.Bussiere, Y.L.Lam, J.Mol.Catal. 25 (1984) 173 T.M.Tri, J.Massardier, P.Gallezot, B.Imelik, J.Mol.Catal. 25 (1984) 151 J.F.Faudon, F.Senocq, G.Bergeret, B.Moraweck, C.Clugnet, C.Nicot, J.Catal. 144
424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456
Preparation and characterization of metal and alloy catalysts
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393
Chapter 8
ADSORPTION ON ALLOYS A solid adsorbent placed in a gas or liquid phase causes an increase in density of that phase in its immediate neighbourhood. The increase can be so pronounced that it is sensible to speak about an adsorbed layer of the adsorbate, or an adsorption phase. The forces of interaction causing this phenomenon can be divided into two classes: physical and chemical. The subdivision is meaningful and useful, but in some cases it might be difficult to establish which of the two types of adsorption is actually taking place. However, to make a difference between living and inorganic nature is always useful too, and is accompanied with the same difficulties. The physical adsorption layer is analogous to the liquid phase and can be due to one or more of the following interactions: (i) interaction between fluctuating dipoles (London, or dispersion force), (ii) interaction between the surface dipoles and dipoles induced in the molecules, (iii) interaction between permanent dipoles. The totality of the physical forces is often called the "van der Waals interaction". Chemisorption is mediated by the same forces as the formation of bonds in molecules. Electrons of the adsorbed molecules are delocalized in the direction of the solid and in terms of classical bond theory we can speak of a covalent sharing of electrons or, in the extreme case, of electron transfer and ion bond formation. Chemisorption also comprises weakly bound forms, such as hydrogen bonds or the so-called charge transferno bond adsorption (for example, xenon on metals). Chemisorption which is taking place under the critical temperature of the adsorbate is always accompanied by interaction due to physical forces. Sometimes the chemisorption bond is so strong that it pays to split the bond(s) in the molecule and adsorb the fragments separately. In what follows we shall mainly be concerned with chemisorption. When metals A and B differ sufficiently in their electronic structure, the surface containing A and B atoms will show different adsorption characteristics from surfaces of the individual metals A and B. In the simplest case, when B is inactive and the effect of B on the electronic structure of A is marginal, the addition of B to A will simply cause a suppression of adsorption capacity when the same surface areas are compared, and the lateral interaction in the layer adsorbed on the A-B alloy may be weakened by the dilution of A in B. Interesting effects can occur when the adsorption of the molecule requires that each adsorbed molecule or its fragment is to be attached to several contiguous surface A atoms. Dilution of A in metal B makes finding larger surface ensembles of A atoms less probable, and the adsorption capacity decreases with decreasing mole fraction of A in the
394
chapter 8
surface faster then linearly. The decrease is even sharper when the B atoms for whatever reason inactivate the surrounding A atoms. This can certainly happen when the B atoms change substantially the electronic structure of the A atoms. In the first case we speak of a
geometric or an ensemble size effect; in the other case, of an electronic structure or ligand-effect [1,2]. When not only A but also B is active we have always to consider the possibility that ensembles AraBn may behave differently from Am+n or Bin+n. If this happens we speak of an ensemble composition effect. As we shall see below, there are good reasons to believe that all of these scenarios have been observed at some time, although they are not of equal importance. Usually one considers first ensemble size and ensemble composition effects, and only when they seem to be insufficient to explain the results are electronic effects evoked. Very often the indications for electronic effects are seen in changes in the activation energy of desorption, i.e. shifts in TPD maxima, in an unexpected change in adsorption capacity induced by alloying, or in some changes in the solid state or the spectroscopic parameters characterising the adsorbed layer, e.g. peak positions and peak widths in the spectra. Let us analyze these problems step by step, and thereafter take a closer look at the available adsorption/desorption results for alloys.
8.1
Ensemble size and composition effects The term ensemble for a small number of contiguous surface atoms necessary for a
certain act of adsorption originates in the work by Kobozev [3]. Balandin [4] used the term multiplet to describe the same concept. When it started to emerge and be experimentally established that some adsorbed species are coordinated to single metal atoms, while others are multiply coordinated to several surface atoms, and that many molecules can be bonded to metal surfaces through more than one of their atoms (see chapter 1 and for hydrocarbons chapter 10), it required only a small step to postulate the necessity of ensembles of a certain size for certain reactions [5-8]. This idea appeared later to be very helpful in rationalizing the results obtained with alloys of an active with an inactive metal.
8.2
Ensemble statistics and the extent of adsorption
Before World War II, Kobozev and his associates in Moscow studied the activity of supported metal catalysts of very low loadings. They noticed that the total (A) and the specific (A/o~) activities showed a maximum in the A vs. ~ or A/or vs. o~ curves. Here ot stands for metal loading, expressed as the coverage of the support surface by metal atoms. The first (obviously incorrect) idea was that metal atoms can exist on the surface of the
Adsorption on alloys
395
support as single atoms, pairs, triplets or even as larger flat ensembles. They used [3] the Poisson statistical distribution to calculate how many ensembles of a certain size can be formed at given a and from the experimental determined position of a maximum in the A vs. ot and A/o~ vs. a curves determined the size of ensemble required for the reaction in question. The idea that supports bear such small ensembles on their surface was incorrect (their catalysts were black, and single atoms or, say, triplets cannot be black), but Kobozev's idea of ensembles of metal surface atoms as adsorption sites can be considered as a further elaboration of Taylor's idea of the active site (see chapter 6). Also their use of statistics had aspects that can be applied to catalysis by alloys, and this application can be found in later papers by Dowden [7] and Martin [9]. We shall follow the latter work below and the reader should notice that the distributions of binomial statistics used [7,9] transform into Poisson distributions for large systems. Let us consider qB indistinguishable single atoms of an inactive metal B in the matrix of active atoms A forming together a surface of an alloy catalyst having N lattice sites. The number of ways W by which B can be placed on N sites is given by equation 1.
W(qB,N ) =
N! qn(N-qB)!
(1)
As an example let us take a square lattice and assume that to adsorb a certain molecule we need an ensemble consisting of four contiguous A atoms. In this case, to make one ensemble of four atoms of A, we have to distribute qB over N-4 positions. This can be done Wn-times, where W4 =
(N-4) !
(2)
qn !(N-4-qn) ! The probability of forming from a random distribution of A and B an ensemble of four A atoms, that is a B-free ensemble of a required size, is
p=
W4
W(qB,N)
: (N-q-3)(N-q-2)(N-q-1)(N-q) (N-3)(N-2)(N-1)N
(3)
By substituting
On=qa]N
(4)
we obtain for very large values of N and not too large qB: p=(l_0n)4 For a more general case
(5)
396
chapter 8
(6)
P = ( 1 - O n ) 'n for a site comprising m atoms of A (see figure 1)
0.8
figure 1 Probability P of finding one A atom (straight line) or two (line in the middle) or three (circles) contiguous A atoms (P = (1-0) m) as a function of the bulk AB alloy composition (xs).
0.6
0.4
o,2\
0.2
'o:2'o:,
'o:6'
o'.8'
'
xB
When an atom of an inactive metal B modifies the activity of surrounding A atoms (due to electronic structure effects or due to the size of B), the value of the exponent can be effectively higher [9]. An ensemble can be inactivated not only by alloying but also by poisoning, for example, by sulfur, which inactivates several surrounding atoms due to its size. The statistical treatment is complicated but actually it is of the same kind as that shown above; a general and mathematically exact analysis of it has appeared [10]. Some [ 11 ] prefer to use the equations
P=(1-bOB)"
(7)
P=(1-bOB)
(8)
when the molecule being adsorbed requires m sites and the presence of B, either as an inactive metal atom in the surface or as a poison on the surface, inactivates b sites. It can be seen that these equations are just alternative approximations of equation 6 and other more complicated ones [10]. When the components A and B are both active, the ensembles can have a varying composition and activity. For example, for the four atom ensemble we have to consider the mixed ensembles: AB3, A2B2, A3B , the number of which can be again calculated by binomial statistics [7]. If from the m atoms in the ensemble of a required size, n A atoms should be of the sort A, the probability of finding such an ensemble is
Adsorption on alloys
P.,,n =
m
w
397
?1a
_
" X A (1-XA) mnA na ! ( m - n ~ ) T
(9)
when the surface contains a mole fraction XA of atoms A. From Pn,m the number of ensembles suitable for a given adsorption can be calculated; (this question will be developed more fully in chapter 13). The statistics become more complicated when the distribution of alloy components is not random. When the alloying is accompanied by an endothermic change in heat content, the components tend to form clusters, and when the alloys are formed with an
exothermic change they tend to be ordered. Both effects disturb the random distribution. A theory describing simple non-ideal cases is also now available [12,13]. Calculations meant to represent the situation with the non-ideal platinum-gold alloys have been performed [13] where gold is considered as inactive, but this characteristics does not further influence the statistics. Results of such calculations are shown in figure 2 and illustrate very well what kind of results can be expected. 1.0
curve1 o~
\\
2 o~
o
0.8
etc
-
/111 /
3 a ~176 e t c
,
0.6
~--
,\ ,. r\%..
=o
--~-
/ /
5o.o o
~ \\~.-- -~
0.4
ce ensembles in (100) platinumgold surface. Full lines, using co
3 b o~o e t c " -% . . . . .
,\ . . . . .
figure 2 Calculated distribution of surfa-
_
/
= 1.25 kJ mol 1. Broken lines,
/./
using co = 0 (random mixing). The probabilities are defined so
I
that, given a surface platinum atom (o), the value is the chance
k;
of that atom having zero to four
0.2
nearest 0
neighbour
platinum
atoms, co used here is identical 0
0.2
0 .L. X(s)Pt
0.6
0.8
1.0
with parameter Z, introduced in chapters 1 and 4.
In principle one would predict similar statistical distributions for systems such as platinum with rhenium or iridium, where both atoms are active. For these systems it is quite conceivable that a mixed ensemble such as that labelled 2 or 3 in figure 2 has better properties than the extremes labelled 1 or 5. Since the numbers of these ensembles should show a maximum as a function of Xpt and since all ensembles of the same composition are of the same quality, the activity as a function of Xpt in the unmodified, i.e. unsulfided, platinum-rhenium or other similar alloys can show a pronounced maximum. Then, an
398
chapter 8
assumed activity should be ascribed to each kind of ensemble and the total activity in chemisorption or catalysis calculated by summation of individual contributions. An example of such calculation will be treated in detail in chapter 13. The numbers of ensembles 2 and 3 in figure 2 show a maximum as a function of Xr~. As we shall see in chapter 13 such a maximum is often observed with alloy catalysts containing two active metals (platinum-rhenium, platinum-iridium, iridium-osmium, etc.). It should be noted that the above-mentioned statistical expressions hold only for large planes of alloy particles and when edge-effects can be ignored. Predictions as made above can be checked by measuring, for example, the extent of adsorption of a certain gas on A and on B by thermal desorption, and by integrating the peaks corresponding to the identifiable A and B sites. When making that or other similar measurements, one must be aware of some pitfalls such as gas-induced segregation (see chapter 4) and side reactions such as carbon deposition from carbon monoxide, whereby the deposition can be stimulated or suppressed by the second metal.
8.3
Adsorption as a probe of active sites on alloys
Probably the simplest way to characterize alloys by adsorption/desorption phenomena is to perform Temperature Programmed Desorption (TPD) from their surfaces. Under well-defined experimental conditions and after careful calibration, the integrated TPD peaks supply information about the extent of adsorption. Moreover, the position of the desorption peaks can be sometimes related to individual alloy components and sometimes the shifts in the peak positions, when they really occur, can represent whatever their origin is - very valuable additional information. However, some caution is necessary when explaining the latter phenomenon, as will be demonstrated by the following example (figure 3). We consider first hypothetical TPD spectra shown by a pure metal A obtained using different initial coverages. This is shown on the left side in the figure. Now, we replace some atoms A by virtually inactive atoms B which suppress the overall adsorption; thus where N is the number of adsorbed molecules Nmax(alloy) < Nmax (A). When desorption of a few adsorbed molecules from the alloy surface takes place, several situations can arise, of which two are shown in figure 3. Case i) Adsorption is so much weaker on the alloy that the desorbing peak due to the small adsorption followed appears outside the envelope formed by Nmax o n the pure metal A (upper right side). Case ii) The peak is shifted but stays within the envelope (lower right side). Before we proceed any further we should discuss the origin of the shifts on homogeneous well-defined pure metal surfaces showing TPD spectra such as those in figure 3 on the left. Such shifts also occur with ideal adsorption without lateral interaction, when the desorption is of high order, for example, of second order in N. At low N all adsorbed species have an opportu-
Adsorption on alloys
399
nity to be adsorbed on as large as possible an ensemble, but at higher coverages molecules push each other out from their first positions, for example, from the valleys between the surface atoms on to the tops of metal atoms. Desorption from such a layer starts at lower temperatures and the desorption peak shifts to lower temperatures than for ideal layers. This holds also for a desorption of first order, whose Tmax does not shift with 6) in the ideal case (one speaks thus of an induced heterogeneity). This latter effect can be aggravated by mutual repulsive interaction, which shifts the desorption peak to even lower temperature. Alloying can suppress such interactions, which can be either repulsive or attractive, and cause further shifts. However, the alloying of an active with an inactive metal can from the very beginning of the adsorption cause adsorbed molecules to be on the tops of single atoms instead of covering ensembles of A atoms by residing in the valleys between them. In this case the small desorption peak from the incipient adsorption will be at lower temperatures than with pure A, but obviously will not leave the region covered by the pure A envelope. Finally, it is in principle possible that the shifts in TPD peaks are due to electronic structure effects, in other words, due to the fact that atoms of A are different when they are placed in B, instead of in their own matrix.
Alloy A - B
/f Pure metal A
"----~I ~iln
II
clplent
----->T ~//f
incipient,
)jy,,olXy
figure 3 Left: TPD from metal A, from incipient, full and intermediate surface coverage. The shift in the position of maximum occurs due to second order desorption (rate -- 6)2) or due to lateral repulsive interactions. Right side, above. TPD from an alloy AB, from incipient coverage (curve i); for comparison TPD from full coverage on metal A is shown in the same graph (curve f). Obviously, alloying creates sites with a weak chemisorption bond not present on pure metal A. Right side, below. TPD from an alloy AB, from incipient coverage (curve i, alloy). The TPD is compared with desorption from sparsely covered metal A (incipient metal A) and from fully covered A (curve f). It is obvious that in this case alloying does not create any site adsorbing more weakly than the sites on pure metal A.
400
chapter 8
Keeping all this in mind, we can draw a cautious conclusion. Except for those cases where the TPD peaks are shifted clearly and far outside the envelope corresponding to Nmax on pure A, one cannot use TPD peak shifts as evidence of an electronic structure effect due to alloying. No case is known to us where the shift is like that in figure 3, upper right part. Poisoning of the surfaces by sulfur or by other elements (phosphorus, arsenic, antimony, bismuth ...) has, as far as adsorption/desorption is concerned, much in common with alloying of an active metal A with an inactive metal B. The simple rules for qualitative analysis of TPD spectra, described above, also hold in these cases. In several known and practically interesting cases, shifts are indeed observed, but they are within the envelopes established with pure A (see below and in [14]). Other probes for the surface of alloys are combinations of adsorption and spectroscopic measurements; both physisorption as well as chemisorption have been used to this end. Molecules of xenon appear to be a very good probe for sites, when the xenon corelevel binding energies (BE) are monitored by XPS, the principles of which are described in chapter 2. This can be illustrated by figure 4.
J Ru-Cu I 0-7 Torr 1/,.0 L
IIV /,.72
I I , l w , 6.72 8.72 Binding Energy
11.0 8.0 5.0 3.0 2.0 1.0 0.5
t,.52
6.52
8.52
Binding Energy (eV} XelCu (~Xe/Cu-Ru Xe/Ru (bl
Ru(001)
r/~O
0
Side View
Top View
figure 4 Left, XeJp3/2,1/2 photoemission spectra for xenon atoms adsorbed on a surface prepared by vapor deposition of 0.4 monolayer of copper onto a smooth Ru(O001) surface maintained at lOOK. The surface was annealed at 520K before xenon adsorption. The spectra were obtained after various exposures, except for one spectrum obtained with xenon present at a pressure of 10 z Torr. Right (a), deconvolution of the 11 L(angmuir) spectrum from the figure on the left. (b) Schematic representation of xenon adsorption sites [15].
Adsorption on alloys
Xenon shows a doublet of emissions from the
401
5P3/2 and
5pv2 orbitals. The binding energy is
a complex quantity and reflects effects of the local electric field and changes in the relaxation screening energy due to adsorption at different places on the surface [15], but an exact disentangling of the individual contributions is not necessary when the method is simply used as a fingerprint for the surface. In the case illustrated by figure 4, copper had been evaporated on flat, annealed ruthenium single crystal plane and the various sites (Ru, Cu and Ru next to Cu) could be identified by xenon adsorption, as indicated in the figure. In another experiment, not shown, xenon was adsorbed on rough (i.e. stepped or bombarded) single plane surfaces with and without copper and analogous measurements were also performed with powders. Powders looked very much like ion-bombarded single crystal planes, it also appeared that with powders more of copper can be introduced into the surface of ruthenium particles than into single crystal planes. Xenon adsorption as probe can be also used to advantage in combination with NMR techniques; this has been particularly used in studies on metal-in-zeolites systems [16,17]. Since the pioneering work of Eischens [18] and Terenin [19], vibrational spectroscopy in its different modes (IR) [18], Raman, Electron Energy Loss Spectroscopy (EELS) [20], or tunneling mode [21] firmly holds first place in the affections of catalytic people. It is true that in most cases we are watching with this spectroscopy the spectators of the catalyzed reaction, rather than the most reactive intermediates, but nevertheless this technique has supplied us by with the most valuable information. This is also true for research on alloys, as we shall see below. Carbon monoxide seems to be a very suitable probe for testing the binding on metal and alloy surfaces (see chapter 1). Its 56 electrons are delocalized in the metal as they disappear partially from the metal-carbon region, and the d-electrons of the metal are shifted in the direction of the antibonding 2rt-orbitals. In comparison with hydrogen or oxygen atoms it is a more complicated mechanism of binding and this gives more hope for seeing some subtle effects reflecting the expected electronic structure changes due to alloying. Indeed, the very early papers on alloying claimed that ligand effects had been seen by IR spectroscopy. In these measurements, IR spectra only at full coverage by carbon monoxide have been considered and the frequency decrease, Av, has been interpreted as a consequence of shifts in electron densities from atom B to atom A and from A atom into the 2rt-orbitals of carbon monoxide adsorbed on A. However, the correct explanation of the Av shifts is as follows [22,23]. The carbon monoxide molecule constitutes a quite large vibrating dipole and, when the dipoles interact, their vibration frequency increases. Actually, two collective vibrations take place in the layer of interacting dipoles; the dipoles vibrating symmetrically show a higher frequency of absorption which is IR visible, while the anti-symmetrical vibration occurs at a lower frequency but this vibration is IR invisible because the effective dynamic dipole is zero. Only when the
402
chapter 8
dipoles are not the same as in 12CO and 13CO or C O 16 and C O 18 can we also see the antisymmetrical vibration in IR spectra [22,23]. The increase in the symmetrical vibration frequency of the carbon monoxide layer can be suppressed in three ways; (i) by adsorbing a mixture of 13CO and 12CO molecules, so that at high dilution the frequency of isolated ~2CO molecules (singleton frequency) can be determined (figure 5);
figure 5 Stretching vibration frequency v(CO) as a function of the surface composition
v (CO)
y/ J A
x
o
X = p(12CO)/[p(12CO)+p(13CO)] alloy
x
(schematically) Left: alloying has a purely diluting effect. Right: alloying has simultaneously a diluting effect (=f(X)) and a ligand effect, the latter manifests itself by the value Av ~ at X-->O.
(ii) by diluting one metal (A) in another metal (B) on which carbon monoxide is either not adsorbed at all or is adsorbed but shows a different C-O stretching vibration frequency (VA~VB); (iii) by creating only a small coverage of carbon monoxide and making sure that the molecules are adsorbed far from each other: equilibration at elevated temperatures can help to achieve that. It is obvious that, when one wants to see the ligand effect of alloying, one must establish the effect on the singleton frequency and not the shift in v(CO) at full coverage. When using 13CO/~2CO mixtures at full coverage as in figure 5, the interaction of dynamic dipoles, which is a resonance phenomenon, is eliminated by the fact that the dipoles are of different magnitude. Elimination of the interaction is most complete at the lowest molar fraction X(12CO) = p(12CO)/[p(12CO) 4- p(13CO)]. Extrapolation to zero mole fraction shows the maximum possible size of the ligand effect on the v(CO) frequency. We shall see below that with all but one (Pt-Pb) of the systems already studied (viz. Pt-Au, Pt-Cu, Pd-Ag, Pt-Re and Pt-Sn), the maximum possible ligand effect of alloying is very small (see table 1). The largest Av shift seen was with the Pt-Pb system [24]; it cannot be excluded that in this case it was a through-the-vacuum effect of lead (or Pb n+) on the neighbouring carbon monoxide adsorbed on platinum. It is not the only benefit from studying IR spectra that shifts can be seen in the positions of absorption bands. In favourable cases, IR spectra can reveal changes due to alloying in the binding of molecules to the surface, for example, to show the presence or absence of binding through hetero-atoms in adsorbed molecules, or the change upon alloying from multiple coordinated to single coordinated species, caused by an ensemble size effect. The last effect was first clearly demonstrated by the paper of Soma-Noto and Sachtler [2], which presented results obtained with the Pd-Ag-CO system (see figure 6).
Adsorption on alloys
403
table 1 An estimate of the maximum possible size of the ligand effect in carbon monoxide adsorption (Av, in cm-~), for various alloys Alloys
Av(cm-1)
Pt-Cu Pt-Au Pd-Cu Pd-Ag
3 5 -0 10
Pt-Re
10
c m -1
2~00 II
Pt-Pb
2000
I
9
30
~800
I
~
~700
I
~1
s
,
'
~900
I
s
5/
_ Pd/
g
1.0 h/h*l
~ --- . . . .
o.s-t ,,o/
0u
100
,
80
,
60
,
40
210
Pd %
3.5/6.5/ ,,t
figure 6 Left: absorbance ratio of high frequency (h) bands above 2000 r 1 to total absorbance of adsorbed CO as a function of Pd composition in Pd-Ag alloys: (0-) Pco = 0.01 Torr (- -) maximum value of h(h+l). Right: spectra of CO adsorbed on Pd-Ag alloys: (-) Pco = 0.01 Torr (- -) Pco = 0.5.
404
chapter 8
Later, similar results were obtained with the Ni-Cu-CO system [25]. Suppression of the presence of multicoordinated carbon monoxide on palladium-silver alloys seemed at first to be too strong to be explained by an ensemble size effect only. However, when the surface concentration of palladium in the alloy had become known, it appeared that the IR spectrum could be very well explained just by considering the surface concentration of palladium (see chapter 4) and simple ensemble statistics such as explained above. EELS has made the region of metal-carbon and metal-nitrogen stretching frequencies accessible and in principle, these should reveal ensemble effects much more easily. However, this possibility has been only sparsely explored up to now; moreover, the low resolving power in this region of the spectrum might prohibit the obtaining of useful results. We have seen that shifts in the position of the TPD peaks due to alloying can actually be caused by the influence which alloying has on intermolecular interactions, either attractive or repulsive, in the adsorbed layer. The same holds for shifts seen by IR spectroscopy and, of course, this has also to be considered when analyzing results obtained by calorimetry on alloys or by various electrochemical methods (see below).
8.4
Adsorption of simple gases on alloys
8.4.1
Adsorption of hydrogen Due to its apparent simplicity and to ideas originating from the rigid band theory
the nickel-copper system has for many years enjoyed great popularity among those engaged in catalysis research. Indeed, much is already known about the electronic structure (chapters 1 and 3) and the surface composition of these alloys (chapter 4). When unsupported alloys are used, one can most easily determine the sticking probabilities and the maximum extent of hydrogen adsorption. The magnitude of the latter compared with the surface area as determined by physical adsorption of krypton or xenon was the first indication that the surface nickel content is different from the bulk [26]. Later, when more became known from independent methods such as Auger Electron Spectroscopy or Low Energy Ion Scattering about the exact surface composition (chapters 3 and 4), one could draw further conclusions on the influence of alloying on hydrogen adsorption. It appeared that it could either be responsible for a slight adsorption-induced attraction of nickel to the surface or lead to a marginal coverage of copper by hydrogen. The e x t e n t of hydrogen adsorption has not led [26] to a suspicion of a ligand effect occurring with this system. The next step in studies concerning adsorption on alloys is usually the comparison of TPD curves obtained on pure metals and series of alloys. An example of such studies
Adsorption on alloys
405
on nickel-copper alloys and hydrogen is represented by figure 7, from the paper by Silverman et a1.[27], who used a 90% Ni-10%Cu surface formed by the (110) plane of a single crystal. H2/H2(190K) = 15x
b3 E~
r}2. /~ a
10-8A
O s_
=260'
'
'36o'
'
'~6o'
figure 7 Thermal desorption spectra of H 2from 90%Ni/lO%Cu(llO) bulk alloy with different surface compositions after maximum exposure of the surface to H2 at 19OK. The surface compositions were: (a) lO0%Ni; (b) 87%Ni/13%Cu; (c) 77%Ni/23%Cu; (d) 61%Ni/39%Cu; (e) 35%Ni/65%Cu.
'
Temperature (K)
By ion bombardment, which preferentially removed copper, they prepared surfaces with a lower copper content. The results in figure 7 show that removing of copper from the surface changes the population of individual states (o~, 131, 132) but not so much the
character of the states, i.e. their position in the TPD profile. This conclusion could be very well harmonized with the finding that the sticking probability dropped in a pronounced way on diminishing the average nickel ensemble size on the exposed surface: one would conclude from the results that different states are associated with different ensemble sizes. Chebab et al. [28] studied the same system using the same way of preparing various Ni-Cu (110) surfaces from single crystal alloy. The results were similar to those in figure 7, only it seems that in this paper there was a slight shift in the position of TPD peak maxima. This shift was explained by recalling the fact that on the Ni(110) surface individual hydrogen atoms are attached to parallel rows of nickel atoms and have an attractive interaction among themselves. When copper is introduced in the rows there are fewer pairs of mutually attracting atoms and the heat of adsorption decreases, so a shift of TPD peaks to lower temperature occurs. Such a shift, however, has apparently not been observed by Yu et al., who were the first to use this technique for research on nickelcopper alloys [29]. In favourable cases adsorption calorimetry can be a very direct method to establish whether or not a ligand effect due to alloying does occur. Prinsloo et al. [30] determined the heats of hydrogen adsorption on various Ni-Cu/SiO 2 powders as a function of
406
chapter 8
coverage, by using very accurate volumetric and calorimetric measurements (Calvetcalorimeter). Results are shown in figure 8.
adsorbed 0 ,
hydrogen
pmol
~
6
2 0,
,
2.
,
. 0
~.
rE 2 m e t a l 8
,
2
.
6
I
996 :
,
/..
~ ( o )
8 8 8:5~ ( . ) 566
le) ----n
100
o
..o. . o , . . . .
._
g o
75
o
N
50
c
hydrogen adsorbed
on s i l i c a - s u p p o r t e d nickel Fmol m-2metal(e)
figure 8 Differential heats of adsorption of hydrogen at 296K as a function of the coverage of silica-supported nickel-copper catalysts, o: Ni; o. Ni98/Cu2; +" Ni88/Cu12; 13: Ni80/Cu20; 69: Ni73/Cu27; o. Ni54/Cu46, according to [30].
The calorimetric heats of adsorption were measured [30] as functions of the extent of hydrogen adsorption, for alloys of various compositions x (in their case the bulk composition was almost equal to the surface composition x). They saw that all calorimetric results for all compositions x would form one universal curve for all alloys, when they formally considered an alloy with, for example, x% copper as already having 1.66 x preadsorbed hydrogen (see figure 8). For such a universal curve there are two possible explanations in terms used above: (i) copper changes the population of ensembles of various sizes and the larger ensembles are those which adsorb more strongly and their presence in the surface is suppressed by alloying; (ii) copper disturbs the H-H attractive interactions by which the average heat of adsorption is decreased. A third explanation based on electronic structure effect was however preferred [30]: copper fills by its electrons the d-holes on nickel atoms, a phenomenon which we already know (chapter 3) does not occur. Two other papers [31,32] concerning heats of adsorption of hydrogen report results which are apparently at variance with these findings. The heat of adsorption was observed [31,32] to fall at quite low bulk concentrations of copper, from 109 kJ/mol" to 59 kJ/mo1-1
Adsorption on alloys
407
[31]. However, if we consider the information on the extent of hydrogen adsorption in these papers then we quickly arrive at the conclusion that the drop in the surface concentration of nickel with increasing copper content is much more pronounced in this latter work [31] than in the former [30], obviously due to different methods of preparation. As a consequence, large nickel ensembles have disappeared probably faster in the latter [31] than in the former case [30]. This is the most likely explanation of the difference between the results. The reader who prefers explanations in terms of electronic structure effects would find in [31,32] some verbal support for such ideas; however, then it is more difficult to explain the difference between the two sets of results only on this basis. The platinum-gold alloy system recalls the nickel-copper system: there is a miscibility gap and thus a tendency to clustering of components. The effect of gold on the extent of hydrogen adsorption, as well as on the TPD spectra, allow similar conclusions to be made as with nickel-copper. Figure 9 compares the phase diagram with the extent of variation of work function and hydrogen adsorption [33-35]. The results suggest that the surfaces of films tend to keep a composition near to that of the phase boundaries in the phase diagrams over a large range of platinum concentrations. Figure 10 [35] shows the TPD profiles of hydrogen desorbing from platinum and one platinum-gold alloy.
T oK
r
7x
ev
1500-
5.7
1100-
1.6
5.5
700-
5.3 2'0
'
6'0
'
0.2
2'0
6'0
0 OU 0
20
0
60
%Pt
%Pt
%Pt
(o)
(b)
(c)
figure 9 Results on hydrogen adsorption and data concerning the phase (a) and surface compositions (b,c) of Pt-Au evaporated and equilibrated films. (a) Phase composition diagram showing that at the equilibration temperature applied, two phases coexist with about 3% Au in Pt and 17% Pt in Au. In this region the exposed surface should show a quite constant composition. (b) Photoelectric work function as a function of bulk composition indicating a constant surface composition in broad range of alloy bulk compositions. (c) Hydrogen~xenon adsorption ratio, indicating the presence of Pt in the alloy surface.
408
chapter 8
,~12 k col /mole
12 IM=2
figure 10 Desorption spectra of hydrogen for pure Pt and an Au-rich (60% Au) alloy, obtained with heating rate + 57 deg/min, using evaporated films.
10
CI,U,
e
Pt
0./. IM=2 OU
0.2 Pt/Au
J |
,
2 t. 200 ~
|
i
|
6 g 0 /.O0; K i
The results (peak maxima in the same position) suggest that the same hydrogen adsorption modes exist on platinum and on alloys, and this has been interpreted [35] as an indication that hydrogen can be held by ensembles of various sizes, the distribution of which changes with composition: there are fewer large, strongly binding ensembles on the alloy than on pure platinum. Thermal desorption studies with powders show usually a lower resolving power than those performed with films and single crystals, and the peak position can even be influenced by slow transport phenomena in powders. It is nevertheless interesting to note that the results obtained with films [35] lead to the same conclusion as those obtained with powders [ 13]. Paffett et al. [36] have studied hydrogen adsorption on the Pt(111) surface and on the same surface partially covered by copper evaporated onto it; they come to the conclusion that the effect of copper can be described as simply a site blocking. Copper has a very small solubility in ruthenium and there is no physical reason to suspect that copper should exert an exceptionally strong influence on its electronic structure. It was therefore so surprising when a report appeared [37] claiming a very strong suppression of hydrogen adsorption by copper on Ru(0001). It looked as if each copper atom deactivated many ruthenium atoms or, in other words, that hydrogen required an ensemble of 5-9 atoms. This is quite unlikely and therefore the idea of a strong electronic structure effect was invoked to explain the result. However, the method of determining the amounts of deposited copper has been criticized [38-40] and indeed, later papers described an improved technique to determine it. It then appeared that the necessary ensemble size for hydrogen may be only about 3-4 atoms [39] or even just one ruthenium atom [40]. Copper forms alloys with palladium, which show a weak endothermic effect of formation. The components thus perturb each other slightly more than the components of Cu-Ni, Cu-Ru or Au-Pt alloys. Nevertheless, results obtained with Pd-Cu/SiO 2 powders
Adsorption on alloys
409
indicate a simple blocking effect of alloying [41 ] with no signs of electronic effects. Godbey and Somorjai [42] have studied platinum-rhenium alloys prepared by evaporation of rhenium onto a Pt(111) surface. Pure Re(0001) showed with hydrogen a TPD peak at higher temperature than does Pt(111). However, when rhenium was dosed onto (and into) platinum, the TPD peak maximum shifted to lower temperatures. The shift was of a size which would correspond to a decrease by 11 kJ/mole -1 in the activation energy of hydrogen desorption, which is 80 kJ/mole 1 on rhenium free surface. Changes of this size can easily be due to a suppression by rhenium of the attractive H-H interactions (see above). An explanation of the decrease by an assumed electronic structure change in platinum due to rhenium was however preferred [42]. The intermetallic compounds are very interesting materials but are very difficult to study [43]. With platinum-tin and nickel-aluminium, it is very difficult to be sure that they do not contain small clusters of the active metal which would overshadow the at best low activity of the compound. It is therefore important that Plummer [43] has undertaken the very difficult task of studying single crystal faces of nickel-aluminium intermetallic compounds. These appeared to be inactive towards hydrogen at moderate temperatures, but they adsorbed carbon monoxide and water, both of which decompose and oxidize the alloy. Verbeek and Sachtler [44] studied well equilibrated, metallurgically prepared platinum-tin intermetallics Pt3Sn and PtSn. Information obtained by TPD (see figure 11) indicates that the same ensembles might be present on the surface of all three materials. The unexpected suppression of the low temperature state on Pt3Sn might either be due to the different crystallography of the surfaces of various alloys or, as the authors preferred to say, to a ligand effect. f
figure 11 Desorption (TPD) of carbon monoxide from platinum (full line), Pt3Sn compound (short dashes) and PtSn (long dashes). !
200
400 Desorption
8.4.2
600 Temp.,K
Carbon monoxide adsorption as a probe. The mechanism of formation of the bond between carbon monoxide and a metal is
more complicated than that involved in hydrogen chemisorption. Carbon monoxide can
410
chapter 8
participate in bonding to several metal atoms at once and the relative contributions of donation and back-donation to the bonding should, in principle, depend sensitively on the electronic structure of the adsorbing surface. It was with carbon monoxide that for the first time the ensemble size effect was so clearly demonstrated by IR spectra of the molecule adsorbed on palladium-silver alloys supported on Aerosil [2]. The most relevant result of this study has been already shown in figure 6. We observe in figure 6 that alloying of palladium with silver strongly suppresses the presence of multicoordinated species that give rise to absorption bands at lower frequencies than those due linear species, but causes only marginal shifts in the positions of the absorption maxima. These small shifts were later explained by the suppression of the intermolecular repulsive dipole-dipole interactions on surfaces in which palladium is diluted by silver atoms [22,23,46]. The suppression of multicoordinated species can be fully described by the ensemble statistics and by the experimentally determined surface composition (see figure 21 in chapter 4). These conclusions have been later confirmed by IR spectra of ~3CO/'2CO mixtures on the same alloys [45]. The closely related palladium-copper system has been also studied by IR spectroscopy on powders [45] and by TPD on the (111) face of a PdvsCu25 single crystal. In both cases it appeared that electronic structure effects were small or might be even negligible.
A
B
C
C
.~ r
C
ZI i_
L. 0
~
o.
<1
t...
o. <1
,,
!
,
300
!
,
!
500
,
!
,
b c o
700
300
500
700
300
500
700 TIK)
figure 12 TPD profiles of CO adsorbed at 250K on the indicated surfaces [46]. A: Pd(lll). Exposures before desorption (a) 0.3; (b) 0.6; (c) 1.5; (d) 3.0; (e) 6.0 nbar B: C.
"
Pdol-Ago9(lll). Exposures before desorption (a) 0.15; (b) 0.3; (c) 0.6; (d) 1.2; (e) 3.0; (f) 6.0; (g) 12.0 nbar s. Pdo.7-Cuo.3(lll). Exposures before desorption 0 0.15; (b) 0.3; (c) 0.6; (d) 1.2; (e) 3.0; (f) 6.0 n bar s.
Figure 12 shows a comparison on which this conclusion, based on TPD, is derived. We observe that on the (111) face of Pd~0Agg0 single crystal the same state (i.e. the same TPD
Adsorption on alloys
411
peak) exists as on pure Pd(ll 1) at a high coverage by carbon monoxide. The latter is known to correspond to molecules on the tops of surface atoms. Carbon monoxide is pushed (at high coverages) into these less favourable sites by intermolecular interactions. The same effect can obviously be achieved when a silver atom is present and makes adsorption on the site between this silver atom and surrounding palladium atoms (a site formed by the valley between the atoms) less strongly binding than the tops of the palladium atoms (for comparison see figure 6). Suppression of the multicoordinated adsorption and a relative enhancement of the single-coordinated (linear) carbon monoxide can be also achieved by diminishing the size of palladium particles [45]. The palladiumcopper alloys show practically the same kind of sites as pure palladium (compare C with A in figure 12) only their relative population in the surface is different. Very similar results are known for palladium-copper supported alloys [47]. It has been reported that, in compliance with the conclusions above, the main effect of copper is to suppress the presence of multicoordinated carbon monoxide. Larger ensembles of palladium in palladium-copper alloys can be created by oxidation and mild re-reduction [41 ]. Sufficiently large smooth planes of palladium tend to adsorb carbon monoxide in the valley position as multicoordinated species. When the size of palladium particles is very small [45], or when the metal is diluted by an inactive metal (copper, silver or gold), the population of multicoordinated carbon monoxide is lower. Platinum is in many respects similar to palladium: it too has a high density of states at the Fermi level, and a very similar occupation of the d-states in the valence band. Nevertheless, for reasons not yet well understood, carbon monoxide prefers the single-coordinated on-top position on platinum. Thus, there is one effect less to be watched for with platinum alloys, but on the other hand we should see the ligand effects of alloying better here because the pure metal and the alloys show the same prevailing species, namely, the singly-coordinated carbon monoxide. This has been used to advantage in the study [23] which was the first to attempt to quantify the size of the ligand effect by using information obtained with PtCu/A1203 alloys. With the strategy explained above (see figure 5), the molar ratio 13CO12CO was varied in full monolayers adsorbed on different alloys. Extrapolation of x(12CO) to zero supplied a value of the carbon monoxide singleton frequency. We can observe in figure 13 (compare with figure 5) that the maximum size of the ligand effect on the stretching frequency is of the order of uncertainity of extrapolation, something like 0-2 cm-l! The same result has been found with Pd-Cu/Aerosil alloys [45a]. However others (45b) who studied carbon monoxide adsorption on Cu3Pt(111) single crystal plane saw a shift in the TPD profile maximum from 300 to 330K, where the surface of alloy with random distribution was annealed into a surface with ordered structure. They say that this suggests a possible ligand effect from copper on platinum. Yates and Somorjai [45c] studied adsorption of carbon monoxide on epitaxially grown Cu-Pt(111) and Cu-Pt(553) surfaces and compared them with alloys prepared by annealing of these layers. They
412
chapter 8
concluded that epitaxial copper atoms just block the carbon monoxide adsorption sites without affecting the energy of desorption. However, the energy of desorption from platinum sites is reduced by 20kJ/mole, when copper is alloyed, i.e. damped in the platinum surface. An electronic through-the-metal effect of copper on neighbouring platinum atoms would manifest itself in both cases, so that the reduction can most likely be ascribed to the polarization component of the chemisorption forces whose modification is probably also responsible for the weakening of the chemisorption bond strength upon adsorption on metal-on-metal monolayers (see below). 2080-
CO
( c m -1 )
figure 13 Wavenumbers of the frequency "12C0" IR absorption band maxima of CO adsorbed on Pt as a function of isotopic composition. All samples supported on Al203, at constant high coverage by CO. (o) Pt; ( a) Pt42Cu58" (11) et31Cu69" Notice, there is no influence of alloying on the singleton frequency [23].
2060 -
20/,0
2020
-
o
!
!
50
lOO % 12 CO
Platinum-copper alloys are formed exothermically and, when the particles are large enough, the alloys show tendency to ordering. In contrast with it, platinum-gold alloys are formed endothermically and the components show a tendency to cluster. In any case, one can expect less mutual perturbation in the electronic structure of individual components with platinum-gold than with platinum-copper alloys. In conformity with this expectation are results [48] describing IR spectra of carbon monoxide adsorbed on Pt-Au/SiO 2 alloys: the results are thought to be best explained by the conclusions presented above for PtCu/A1203 alloys: gold is just a solvent of platinum. Very important information conceming carbon monoxide adsorption on the (111)Pt-Au single crystal plane has been reported [49]. This work showed again how important it is to consider properly the repulsive dipole-dipole interaction forces when analyzing results on carbon monoxide adsorption. As shown in figure 14, on the left side, the TPD peak maximum shifts downwards by about 75K with increasing extent of carbon monoxide adsorption on pure platinum. The reader should notice that it is a first-order desorption, so that in the ideal case of no interactions the maximum should not move at
Adsorption on alloys
413
all. The shift is due to repulsive interactions and the size of the shift corresponds well with their expected strength. When gold is evaporated onto the platinum(111) surface, it diminishes adsorption of carbon monoxide by a simple blocking mechanism (i.e. there is a linear decrease of Nco with gold coverage) and the TPD peak maximum is not shifted (see figure 14, on the right, the lower line). Obviously, gold forms islands on platinum. However, when the Pt-Au(lll)
system was annealed and gold islands dissolved in
platinum, the TPD peak maxima show a shift now. At high content of gold in the surface the TPD peak maximum of carbon monoxide desorption from a coverage nearing the maximum is at the same temperature as with scarcely covered pure platinum surface. Obviously, gold atoms dispersed on or in the surface of platinum prevent crowding of carbon monoxide and eliminate the intermolecular repulsive interactions.
35 L CO Clean
t_
o
Pt(lll)
Tads = 268K
550
~ 1]
F-
g 0.1 c3
550
CO
1 Exposure
10 (L)
-or ~ 5 o o-4> ~ "-_t~
350
alloys
o
Fractional
0.5 Gold
Surface
_
.
~.o Coverage
figure 14 Left: variation of the desorption peak temperature with CO exposure (1 L = 1 Langmuir = 1.3 x 1 0 -6 mbar.sec (1 x 1 0 -6 Torr.sec)) Right: variation with surface gold content of the desorption peak temperature, after saturation exposure of 36 L, for epitaxial and alloy surfaces. The more diluted is Pt in PtAu alloy, the lower is the lateral repulsion and the higher is the desorption temperature. [491.
Copper dissolves in ruthenium much less than in platinum. There is no reason to expect upon contact of copper with ruthenium any strong mutual perturbation of electronic structures of the components will occur, but nevertheless very interesting effects have been observed. First, when a copper film is evaporated on ruthenium single crystal, new states appear which have been spectroscopically identified as interface states, localized between the two types of atoms [50]. It is not likely, and experiments seem to confirm, that they are involved in chemisorption on copper, but their appearence is, nevertheless, a very interesting phenomenon. We may compare various metal-on-metal systems and look at the position that the ruthenium-copper system adopts. Thanks to very systematic and intensive research by
414
chapter 8
Goodman et al., reviewed recently [51], there are sufficient results to make a comparison sensible. The published results [51] are schematically shown in figure 15.
A T.K T
0.8
& B.E.
-200 -100 -0.4
+100
C___u N__ .~i N~ Re
Ru
Mo
N._~i W
Pd Ru
Pd Re
Pd W
Pd Tq
&T,K ,,,,,t,,, ' ~ T
-0.6
& B.E. (-)
80 "[3,
40 --'I~
C._.~u Re
"--~ (B.E.)
0.2
. . . . . . . . . . . . . . . . . . . . . .
C._~u Ru
C_ .9_u Rh
C__9u . Pt
-0.2
(+)
figure 15 Correlation between the shift z ~ E in the Binding Energy of the core level and the shift in the desorption maximum in the desorption profile AT, for metalon-metal layers (as indicated) AT indicates weakening of adsorption; A(BE) binding energy shift in the adsorbate. If the final state effects were responsible for the A(BE), a plus value of A(BE) would mean suppression of screening, and vice versa. [511.
When looking for an explanation of the results summarized in figure 15, it is temptating to speculate along the following lines. Screening of an XPS-hole depends on the contact with the surrounding atoms (see chapter 2), therefore it is expected to be poorer when the metal particle with the atom ionized is smaller. Similar effects can be expected when a monolayer or a submonolayer amount of a metal is condensed on an unlike metal. Therefore, a high binding energy BE of atoms in a condensed monolayer (i.e. the positive ABE value in fig.15) can be an indication that the screening is poorer by electrons from unlike atoms in the substrate than from alike atoms. This poorer screening probably has its consequences for carbon monoxide adsorption, which on a metal is mainly
but not solely an interaction with the outermost layer. The sublayer contributes not only to the screening of the ionization holes on atoms on the outermost layer, but it also contributes to binding of carbon monoxide by a contribution which can be called a deformation or polarization effect (see e.g. [52]). It seems to be a very attractive idea to relate the effect of hole screening (ABE) and the polarization contribution to the chemisorption bond strength, they depend namely on the same factors. One can expect that poorer screening parallels smaller polarization effects in chemisorption bond formation and this would explain the trend seen in figure 15. Contrary to the case of layers of one transition metal on another, a transition metal sublayer beneath copper strengthens the adsorption of carbon monoxide on copper and
Adsorption on alloys
415
most likely also improves the screening effects in the copper overlayer. The d-electrons of transition metals are very efficient in screening. These factors would explain the trend in the lower part of figure 15. An alternative explanation would be that the orbitals of atoms in the transition metal substrate participate in bonding through the hollow sites in the first layer. However, an explanation based on charge transfer between the overlayer and the sublayers (see also chapters 1 and 3) was preferred [51]. Whatever explanation we accept, we come invariably to the conclusion that in the ruthenium-copper system a minimum (see fig.15) of mutual disturbance in the electronic structure of the components can be expected, and that is important for what follows. Adsorption of carbon monoxide on annealed Ru(0001)-Cu surfaces has been studied by FTIR [53a]. The IR spectra allow us to distinguish clearly in the systems with submonolayer copper films between the ruthenium sites [v(CO) = 1980-2058cm -~] and the copper sites [v(CO) = 2068-2080 cm-~]. There is not much change seen in the properties of ruthenium sites due to the presence of copper covering or replacing some ruthenium atoms. However, a very interesting influence of ruthenium on the adsorption properties of the copper films is seen, but before starting a discussion on this point we must mention one detail of the IR spectra of carbon monoxide on copper. When carbon monoxide is adsorbed on any transition metal, its stretching vibration frequency v(CO) increases with coverage due to dipole-dipole interaction. This can be suppressed or even eliminated, e.g. by adsorbing 12CO from mixtures with 13CO. When such suppression is applied to carbon monoxide adsorbed on copper, one observes that there are actually two effects accompanying an increasing coverage, almost precisely cancelling each other: an increase in frequency due to dipole-dipole interaction and a decrease due to an interaction which has been called a 'chemical shift'. The exact nature of the latter is still a matter of discussion, but as possible explanations a variation in the contributions to the binding of the sp and d- copper orbitals, or formation of an intralayer CO-n-orbital-bond have been suggested [53b]. Whatever the nature of the chemical shift is, it is obvious that the copper metal orbitals of the sublayers are involved. Figure 16 shows that a monolayer of copper on ruthenium shows a shift in v(CO) like those observed with transition metals and not like that observed on bulk copper, which is very surprising at first glance, but it can be explained by considering the mutual cancellation of the two opposing effects. Only with very thick layers (8 or more monolayers) does the behaviour of a copper film recall that of bulk copper; probably, as discussed in relation to figure 15, we see here again an indication of interaction of carbon monoxide with lower layers [51], possibly through deformation or polarization effects [52]. However, the phenomenon in figure 16 has been taken as evidence that the electronic structure of copper is
modified by the ruthenium underlayer [51,53]. IR bands which can be interpreted as CO adsorbed on some scarce ruthenium-
copper mixed sites have been reported [53a]. Their existence is also supported by the
416
chapter 8
reference to (Ru Cu CO) + ions seen by SIMS [54], and by TPD results [51]. OCu 2080- ~) 2070 7
E U
U C 13" 0 t_ LL .C U 0
0
I
o
o
2080 -0:p 2070
8ML
o
0
o
0
3ML
figure 16 Vibrational frequency shifts vs exposure of the C-O stretch as a
CO
function of Cu coverage. 2080 2070 -r 2080 2070
IML
o
o
-
0
0.60
0 0 0
2080-
0.35
2 0 7 0 - o~176
2080 2070
_
00
o
o
o
0.25
Exposure ILl
Most recently, the ruthenium-copper system has been also studied in the form of the Ru(1010) face covered to various extents by copper [55]. EELS results for electronic interorbital transitions as well as the work function changes do not indicate any ligand effect of copper on Ru-CO bonds. The TPD spectra for copper coverages less than unity show only a small decrease in the temperature of the peak maxima. Instead of stressing the ligand effect of copper, it is pointed out that the sticking coefficient for carbon monoxide near full coverage decreases with copper coverage and, because of the microscopic reversibility, the preexponential factor in the rate of desorption must decrease correspondingly: this should lead to a decrease in temperature for the peak maxima in the TPD profiles. In terms of statistical mechanics this means a copper-induced delocalization of atoms in the adsorption layer and a decrease in the life-time and mobility of the precursor state for carbon monoxide desorption [56]. The great popularity of the ruthenium-copper system, to which popularity the fact that it had been studied first by a leading industrial laboratory made a contribution, also led to an interest for the related ruthenium-gold system [57]. Gold has a clearly higher work function than ruthenium and it is more inert than copper, so that some differences in the behaviour of the two systems could be expected. Two series of the bimetallic goldruthenium system were studied, one equilibrated at 540K and one at 1110K. With the former a sympathetic variation was observed between the apparent activation energies of
Adsorption on alloys
417
gold desorption from ruthenium and the preexponential factors. This behaviour probably reflects the formation of two-dimensional islands of gold leading to an increase in the activation energy of its desorption, and formation of a low melting metal layer after formation of full monolayer (decrease in Edes). The normalized saturated coverage by carbon monoxide decreases with gold coverage in such a way that on surfaces annealed at 540K one gold atom replaces one molecule of carbon monoxide. However, when the surface has been annealed at l ll0K, the decrease in saturation coverage by carbon monoxide is sharper, as if one gold atom destroyed two sites for carbon monoxide adsorption. In any case, the positions of the TPD peak maxima for the 1110K-equilibrated surfaces, as well for Cu/Ru(0001) and for Au/Ru(0001), are not shifted by the presence of either gold or copper (see figure 17). The conditions under which clustering occurs with copper and gold have been fully analyzed: even with systems of a very low mutual solubility, surface alloying can occur, also with single crystal planes. It goes without saying that this holds even more for highly dispersed metals. However, in contrast to ruthenium-copper, no indication for mixed states is obtained with the ruthenium-gold system.
figure 17 Comparison of TD spectra after a saturation coverage of CO at 150K from clean Ru(O001). Cu/Ru(O001) and Au/Ru(O001) bimetallic surfaces prepared at high temperature (= l lOOK). Coverage by l b metal is indicated [571. 200
300
z.O0
500
T [K] - - ~ "
In commercial terms the most important alloy of platinum is the Pt-Re/A1203 alloy used in reforming of naphtha. A comprehensive report on the extended and well coordinated French research on these catalysts was presented in 1979 by Leclercq [58]. Temperature-programmed reduction of their catalysts, prepared by co-impregnation of H2PtC16 and HReO4, showed one peak. With a rhenium mole fraction less than 0.8, reduction occurred below 453K; pure perrhenic acid was reduced above 573K. It was thought that at lower rhenium contents a part of rhenium remained unreduced. By IR spectroscopy essentially only Pt-CO species were seen and their concentration, as measured by extinction, decreased linearly with rhenium content. The technique explained in the text of figure 5 above has also been applied to
418
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platinum-rhenium alloys [58]. It appeared [24] possible to speculate that some ligand effect might occur; however, this would cause not more than about an 11 cm -1 decrease in the singleton frequency (see figure 18). An increase of the same size would be caused on raising the fractional coverage from that at which there were only isolated molecules to one of 0.45; the definition of coverages is that as used before [24]. ~ 2100 1 Vco {cm-1) 2050 2 1 0 0 ] Pt
0
Re
'
100/0
, % 12CO
, 100
1950
0
50
r
% 12 CO
100
figure 18 Left: wavenumbers of high-frequency bands as a function of the isotopic composition, o: PtsoRe5o; A: PtRe (PRD 62.5); o: Pt5oSn5e Right: wavenumbers of IR absorption band maxima of adsorbed 12C0/13C0 mixtures as a function of isotopic composition of the adsorbed layer for Pt-Pb/Al203 alloys. ~ Pt96Pb4; I1: PtssPb12; A: Ptz3Pb2z; A: Pt41Pb59; El: PtlsPb82. The dotted lined indicates the analogous curve for Pt/Al203 (1). For reasons of clarity the lowfrequency band points are omitted with the exception of those for the PtssPb12 sample, to indicate the comparable behaviour (compare with figure 5).
Practically relevant research is usually performed with powders, i.e. catalysts which simulate as much as possible the behaviour of industrial catalysts. However, to solve some fundamental problems, it is much better to use well-defined surfaces. Different TPD profiles are shown by pure platinum and by pure rhenium, but surfaces with 0.3 and 0.55 rhenium monolayer showed profiles which looked very much like that for pure platinum [42]. This is actually not very surprising since those who studied carbon monoxide adsorption by IR spectroscopy also saw only species which looked like Pt-CO [24,58]. However, the maximum extent of carbon monoxide adsorption slightly increased when rhenium was present, the saturation capacities being 7.5x1014 molecules cm 2 and 4x10 Inmolecules cm 2 on Pt(111) and Re(0001) respectively [42]. With 0.3 monolayer of rhenium on platinum the adsorption capacity rose by 40%. This synergetic increase was ascribed to an electronic structure effect; the results [42] with hydrogen were explained in the same way, see above. However, one must not forget that owing to the different crystallographic
Adsorption on alloys
419
structures (platinum is fcc, rhenium hcp) a restructuring of surfaces accompanied by changes in adsorption capacity is not excluded. Most recently, Shpiro and Joyner [59] have suggested that in these catalysts the outermost surface is practically only platinum, the electronic properties of which
are modified by a rhenium-rich underlayer; this could
be an alternative explanation of these results [42]. There is however one problem with the last suggestion put forward [59]. The zeolite cages, for which the model was suggested, cannot accomodate such large particles: and yet a particle with a full shell of rhenium atoms surrounded by a full shell of platinum atoms has a total of 55 atoms, and thus cannot fit in the cage. By the criteria mentioned above (see figure 5 and the corresponding text), a surprisingly large effect of alloying has been seen with Pt-Pb/A1203 alloys, namely a decrease of 37cm -1 in the singleton wavenumber (see figure 18). That could be a consequence of a ligand effect, which however would be then much larger than with several more strongly exothermically-formed alloys such as platinum with rhenium or tin (see fig.18). Another explanation could be that lead atoms or Pb "+ ions
on
the surface of
platinum or of the alloy cause this frequency shift [24]. The nickel-copper system has been studied by many of the techniques of experimental and theoretical physics and by those of surface chemistry, and the adsorptive properties have also been repeatedly studied. Probably this is the alloy system we know most about, but it does not mean that everything is understood in every detail. An example of where our knowledge is incomplete is just the adsorption of carbon monoxide on these alloys; IR spectra of adsorbed carbon monoxide are shown in figure 19. While with evaporated nickel-copper alloy films, the amounts of hydrogen adsorbed at saturation were quite reproducible, the results for adsorption of carbon monoxide on the same films were much more scattered [51]. This is most likely due to the fact that the mere presence of carbon monoxide in the system extracts nickel out of the alloy and a reverse, gas-induced surface segregation occurs [61,62]. It is difficult to keep this phenomenon under strict control and to achieve the same reproducibility as with hydrogen adsorption. However, there are yet more sources of discrepancies. When alloys are prepared from oxides via carbonates, by a reduction at temperatures higher than a critical one of about 470-520K, an alloy is prepared with less copper enrichment in the surface than when films are condensed and equilibrated at low temperatures, where separation of phases is possible, whereby a copper-rich phase can form the outer surface. Similarly, as with other systems, very valuable results have been obtained by IR and these were moreover combined with magnetic studies [25]. The ratio of absorption intensities of single and multiply coordinated carbon monoxide has been quantitatively evaluated from the IR spectra and it clearly shows that alloys have fewer ensembles allowing multiply-coordinated species adsorption to form [25]. This can be seen in figure 20.
420
chapter 8
-1
cm 2100 _ I
2000 I
1900 1
I
f I
i
9
I
1800 I
s ---
~-- figure 19 Infrared spectra of CO adsorbed on Ni-Cu alloys at about 0.02 Torr pressure: (-) the alloys were evacuated 18 hr at 623K before adsorption; (--) the alloys were evacuated 2 hr at 623K before adsorption [21. Notice the suppression of multicoordinated CO. (v(CO)<2000 l cm).
AA
1.0 -~
AA § AB
figure 20 Ratio AA(A a arAB) as a function of the alloy composition. This ratio indicates the population of the surface by single (A) and multiply (B) coordinated CO molecules [251.
0.8
0.6
0.4 0.2
~
"
3b
s'o
'
7b
'
9'o
-
Cu*/,
Magnetic measurements were evaluated [25] by a parameter c~, defined by equation 10, a -
4AM~a, ~
(10)
where L~Msa t is a change in the saturation magnetic moment caused by adsorption of amount q. This parameter clearly decreases with increasing copper content as if, indeed, fewer
nickel atoms were involved in binding carbon monoxide on nickel diluted in
copper. The vibration frequency decreases with increasing copper content and this has been ascribed in this paper to a ligand effect of alloying [25]. However, the shift is of the order of the dilution effects of copper on the carbon monoxide layer (see the text above and around figure 5). The electron energy loss spectroscopy (EELS) can easily be applied to well-defined single crystal surfaces. This mode of vibration spectroscopy confirmed [63] what had been found with Ni-Cu/SiO 2 powders and, moreover, possibly indicated the presence of a small
Adsorption on alloys
421
number of mixed nickel-copper ensembles. Mixed ensembles were also claimed to have been observed by others [64,65]. Shifts found in the position of maxima in TPD profiles obtained with nickel-copper alloys have been explained by many workers as an indication of electronic structure effects of alloying. However, when thinking about the shifts due to electronic structure changes and desorption from mixed ensembles, one has also to consider the following trivial but important detail concerning the experimental technique used. Those working with single crystal faces have usually only an alloy of o n e bulk composition at their disposal and alloys of varying compositions are then prepared by sputtering copper out of the surfaces first and then, by a subsequent heat treatment, atoms of copper are partially restored to the surface. Prepared in this manner the alloys have two characterization parameters, which are mutually related: the more nickel in the surface, the more defects and, vice versa, the more copper in the surface the more ideal the crystal is. The presence of and the variation in the number of defects can also cause new IR bands and TPD maxima to appear, as well as shifts in their positions to occur. Defects are known to have profound effects on carbon monoxide adsorption [66]. A very peculiar case are the intermetallic compounds. It is very difficult to prepare single crystals of them, it is difficult to cut well-defined planes and it is also difficult to secure that after cleaning (by reduction-oxidation cycles or by ion bombardment) annealing will bring the planes into thermodynamic equilibrium, without appreciable clustering of the transition metal component. The latter is more difficult to achieve than seems at first glance (compare various papers on this subject [43]). Detailed studies exist on Pt3Ti and Pt3Co [67,68]. While the Pt3Ti(lll) surface exposes both platinum and titanium atoms, it seems that with Pt3Co the (100) and (111) surfaces are formed almost completely from platinum atoms. Powders of cobalt-platinum supported alloys show IR spectra of adsorbed carbon monoxide which could be interpreted in the same way [69]. Thus, there could be large differences in surface compositions among these compounds, which otherwise show many similar features. With (100) and (111) Pt 3 Co planes, compound formation decreases the temperatures of the maxima in the TPD profiles. This is explained by a ligand effect on the behaviour of the platinum atoms. However, it can also be that the deformation or polarization contributions to the heat of carbon monoxide adsorption from atoms surrounding the platinum atom bearing carbon monoxide are different when these atoms are again platinum or when they are cobalt atoms. Since in the latter picture cobalt atoms and platinum atoms stay virtually unmodified, the effect can perhaps be better described as an ensemble-composition effect than a ligand effect. Adsorption on titanium or titanium-platinum sites is dissociative and on platinum sites molecular. Appearance of loose oxygen atoms complicates the observations, since oxygen can lead to decomposition of the alloy and to segregation of components. The study of carbon monoxide adsorption on Pt3Ti [67] demonstrates the difficulties one meets when working with intermetallic compounds. The compounds can be quite
422
chapter 8
inert towards one gas (e.g. hydrogen), but with another molecule (e.g. carbon monoxide) they decompose. When the latter is adsorbed on Ni3Ti it very easily dissociates at 310K, carbon atoms diffuse into the bulk and remaining oxygen atoms cause clustering and segregation of TiO 2. Such chemisorption, combined with a reaction, takes the elements very far from the state which they have in an endothermically or weakly exothermicallyformed alloys such as nickel-copper alloys. The conclusions of this paragraph also apply to many of the amorphous glasses in which the amorphization is induced by elements such as boron, phosphorus, zirconium, rare earths and other non-noble metals (see section 7.1) which interact with oxidising agents very vigorously. The practically most important intermetallic compounds are those of platinum and tin. They can be prepared in a well-defined form metallurgically but for practical catalysis they are prepared supported on alumina. Infrared spectra and TPD of adsorbed carbon monoxide, when properly evaluated, do not indicate that platinum atoms in these catalysts have adsorption properties very different from those in pure platinum (see figure 18 above). This earlier conclusion has been fully confirmed by recent papers [70]. While the experimental results seem to be well established, the explanation stands less firmly. The results could mean an absence of any ligand effect. However, this behaviour could also be a consequence of a structure in which the more or less inactive platinum-tin matrix contains ensembles of pure platinum in the surface. Results obtained with ethene lead to the same conclusions, as we shall see below. Rather surprisingly, not much progress can be reported on adsorption taking place on alloys of two active metals. We have already touched the alloys of platinum with rhenium and with cobalt, but there are some more which can be mentioned. The most difficult to study are obviously those where the alloy components have very similar adsorption properties. Nevertheless, those topics have also been studied and will be briefly reviewed. Nickel with cobalt alloys on silica [71] have been prepared by co-impregnation of nitrates and a long reduction, and the IR spectra of adsorbed carbon monoxide were measured and analyzed. The results are shown in figure 21; one can observe that both components of the adsorbent are visible by the stretching frequencies of adsorbed carbon monoxide. There is very little shift in the position of the bands. Iron-nickel powders gave a shift in v(CO) with composition [71], as is usual with adsorption on alloys, but adsorption on individual components can be recognized as well. Platinum-ruthenium alloys have also been studied by IR spectra of adsorbed carbon monoxide [72]. With this system, too, carbon monoixde adsorbed on individual components could be recognized. This provides a possible way of estimating surface composition. A later paper from the same laboratory reported that samples with as little as 10% ruthenium already show dissociation of carbon monoxide at room temperature [73]. From what is known [74] of dissociation of carbon monoxide (see chapter 14), we can conclude
Adsorption on alloys
423
that occurrence of dissociation demonstrates the presence of ruthenium ensembles even at this low content of ruthenium. It was very interesting to see that when both carbon monoxide and nitric oxide are present in the gas and in the adsorbed phase, the former is adsorbed preferentially on platinum and the latter on ruthenium. The presence of ruthenium is also essential for the formation of isocyanate species.
Co-Ni
Co z
E
80
X3
-o 60 "1o 121
N
40
E
20
n
9
m
9
nn
-
9
mmn n n
_
9
l i B _
9
o <
9
II I1_ 9
m
Ni 2200
._
9
2100
- -
,
I1_
,
!
2000
,
,
1900
1800
Frequencies (cm -1)
figure 21 Infrared absorption by CO on Co-Ni alloy particles. The bars indicate the main absorption bands and the height of bars represent the relative intensity of absorption [71].
Platinum and palladium have a very similar electronic structures (e.g. hardly distinguishable Fermi surfaces, see chapter 1) and they form alloys which behave almost ideally. Infrared spectra of adsorbed carbon monoxide show features of singly coordinated molecules, the IR bands for Pd-CO and Pt-CO coinciding, and, next to that, of multiply coordinated molecules [75]. The latter is assumed to be present only on ensembles of several atoms of palladium. Alloying of platinum (which alone shows very little or no multiple coordinated species) with palladium, strongly suppresses the occurrence of multiple coordinates species, which is marked by bands at 1880-1980 cm -~. This is what one would expect if an ensemble of several palladium atoms is required for such adsorption. It was suggested [75] that the suppression of multiply coordinated species is so strong that one should assume the operation of ligand effects. However, a variable population of palladium-rich and platinum-rich particles as the composition is changed would produce the same result and it is known that oxidation/reduction treatments of the catalysts stimulates such a segregation into two kinds of particles. Platinum and rhodium alloys showed in the IR spectra of carbon monoxide both Rh-CO and Pt-CO species, which moreover vibrated in mutually coupled modes [76]. The presence of platinum suppressed also the formation of rhodium carbonyls.
424
8.4.3
chapter 8
Adsorption of hydrocarbons and of some other gases A very comprehensive and instructive review [77], in which is given an extended
list of the observed band frequencies, has demonstrated how rich is the information obtained by vibrational spectroscopies on the chemisorption of hydrocarbons by single crystal planes of metals. In great contrast to it is the very scarce information concerning the influence of alloying on the spectra. Reactions of hydrocarbons on alloys will be discussed later in this book (chapter 13). In relation to this it is important to note how the chemisorption of hydrocarbons is influenced by the presence on or in the surface of inert atoms. For example, it has been established [78,79] that the dissociative adsorption of cyclohexene, benzene, cylohexane or cyclopentane on Pt(111) is suppressed by bismuth evaporated onto platinum. Since these somewhat large molecules can also bind weakly to bismuth, the initial sticking probability is barely influenced by it. Ethene is a very suitable model molecule and it has been used in studies concerning the copper and gold overlayers on Ru(0001) [80], and on Cu-Rh(ll 1) alloys [81]. The spectra shown by these systems can be described as the sum of those shown by each metal acting independently. The prevailing low temperature adsorption mode of ethene on copper, silver, gold and palladium is a 7t-complex (see chapter 1), but on ruthenium and rhodium it is a di-~-bonded form [80]. Also on platinum the di-~-bonded form seems to prevail [77]. These forms of adsorption can be seen in figure 22 and in principle they should also exist on some alloys.
0 H
figure 22 Reactive modes of ethene adsorption: 1 and 3, di-o-adsorption; 2, rc-complexed ethene.
The rt-complexed ethene does not easily dissociate upon heating but the di-~bonded one does [77,82-87]. Gold and silver overlayers on a ruthenium surface have interesting properties [80]. As with carbon monoxide, a monolayer of gold and copper on ruthenium behaves differently from a layer of 2-5 atomic layers, and it shows a clearly stronger adsorption of ethene. Either the d-orbitals of the ruthenium surface participate in bonding due to a overlap with the carbon orbitals in hollow sites on the surface, or the deformation-polarization component of the adsorption bond is strengthened by the presence of these underlying orbitals in the underlayer.
Adsorption on alloys
425
By heating the substrate-overlayer system to higher temperatures (500-1100K), some dissolution of the Group 11 metals in ruthenium can be achieved. The results indicate that their presence and in particular their dissolution also induces n-complex-like bonding on ruthenium atoms. Preadsorption of oxygen induces the same change in bonding. Although the view is strongly expressed [80] that these results indicate an operation of ligand effects, nothing really excludes the possibility that they are consequence of simple blocking effects. Sassen et al. [88] studied ethyne and ethene adsorption on palladium and PdCu(100) single crystal planes, and the influence of co-adsorbed oxygen or co-deposited carbonaceous layers. In their interpretation, dilution of palladium causes a switch of adsorption mode from ethylidyne (three bonds) to ethylidene (two bonds), and slows down the transition from rt-complexed alkene into more dissociated modes. The results obtained with palladium are therefore very similar to those obtained on ruthenium [80]. However, it is believed [88] that the results are best explained by assuming mere blocking effects of copper or of the carbonaceous hydrogen-lean species, or of preadsorbed oxygen. In the study [83] of bismuth overlayers and of the use of bismuth to create a blocking effect, the following very interesting experiments were performed. Bismuth was evaporated onto a layer of a hydrocarbon (benzene, cyclohexene, etc.) adsorbed on platinum and the displacement of preadsorbed molecules was then followed. In this way surface reaction intermediates otherwise not observable were displaced into the gas phase and could be detected by a mass spectrometer [83]. This approach to detect intermediates by displacement had been used earlier [84,85] and some other workers had used scavanging by C H 3 radicals to detect intermediates on surfaces [86]. It is an obviously very powerful technique, but in the time of fascination by spectroscopic techniques it is not widely employed. Mueterties has at different occasions repeatedly pointed out that the fathers of the coordination chemistry, i.e. Werner and his contemporaries, used just the displacement technique as their main research tool in the study of coordination complexes
[87]. The platinum-nickel system is apparently simple, but is actually very difficult to understand in detail. Massardier et al. [89] studied the Pt78Ni22(111) plane and the PtsoNis0(111) plane by EELS. Adsorption of benzene and ethene on the former plane is similar to that on pure Pt(lll); however, the latter plane is very different. Platinum segregates strongly to the surface and forms a quasi-pure platinum layer (see chapter 4) which, however, does not adsorb these molecules even down to 180K. Interestingly the Ph0Ni90(lll) plane adsorbs benzene in the same way as all other metals [90]. The surprising behaviour of the 50-50 alloy is ascribed to lattice strain in the outermost layer induced by the high content of nickel in the layer beneath [89]. The lattice parameter of platinum is 0.3924 nm, but that of 50-50 alloy is 0.3755 nm. If this explanation is correct, and it seems to be so, it would be an interesting case of an electronic structure effect due
426
chapter 8
to overlayer formation. It is a quite realistic idea, since the lattice contraction would shift the energy of platinum d-orbitals upwards, due to the stronger d-d electron repulsion in the compressed layer, and the observed consequences namely, a worse rt-complexing, could logically follow. Without the precursor n-complex-like state, there would be no adsorption at higher temperature, either. Further research by spectroscopic and surface crystallographic techniques on the adsorption characteristics 50-50 alloys is thus very desirable. When pure platinum is compared with platinum-tin compounds, differences are first observed at the macroscopic level: admixture of tin causes a clear increase in brittleness. Of course, there are also changes on the spectroscopic level; photoemission from the energy levels in the region of the platinum 5d-orbitals is shifted to lower energies and taken all together one would expect large differences between the adsorption behaviours of platinum on one side and of platinum-tin alloys on the other. Even a complete loss of platinum-like properties in chemisorption would not be surprising. In comparison with the expectation, the differences are very modest: only the extent of C-H bond dissociation is very substantially different, being lower on alloys. An important start in the study of platinum-tin alloys has been made quite early [44]. Figure 23 shows the TPD profiles of ethene from two metallurgically prepared intermetallic compounds Pt3Sn and PtSn.
figure 23 Desorption of ethene from PtaSn (-) and PtSn (- -)
I
1
200
I
300
Desorption
I
~00
!
500
Temp. K
It is obvious that an increase in tin content suppresses the high temperature peak, which most likely is related to the dissociatively adsorbed species. Parffet et al. [91] performed a very detailed study by LEED, TPD, UPS and EELS on alloys which they prepared by evaporation of tin onto the (111) face of platinum and a subsequent anneal at 1000K. Two different platinum-tin surfaces, were prepared which could be described by models shown in figure 24.
Adsorption on alloys
427
p(2 X 2)
Pt3Sn(111) ~)Sn= 0.25
(43 X ,,/3) R30~ Pt2Sn
esn= 0.33
figure 24 Structural models for Sn/Pt surface alloys obtained after annealing (to IO00K) Sn adlayers on Pt(111) as outlined in the text.
The results obtained with these surfaces are of great importance for catalysis. First, there was no formation observed of ethylidene or ethylidyne or vinyl (CHCH2) or ethyne on either alloy surface, in sharp contrast with pure P t ( l l l ) where all these species are observed at room or slightly higher temperatures [92]. The results are compatible with a model according to which non-dissociative ethene adsorption requires four platinum atoms, but adsorption with dissociation needs six platinum atoms. The results obtained with bismuth-platinum surfaces, mentioned above, allow the same conclusions. However, there are also some small changes induced by the presence of tin in the molecular adsorption of ethene.
88
~3.0 9.9i 6.8
C2HAHe-,I UPS
"
I
I
I
%
I
I
/
l l/
~ H2Ii'CH
I
/
/-Oo', :; ''
/I,
ETHYLENE I
1
]
I I I I( I
~
I
";
....
BINDING ENERGY
;:EF (eV)
figure 25 Ultraviolet photoelectron spectra (using the Hell, 40.8 eV, line)for ethene chemisorbed on P t ( l l l ) (bottom curve), the p(2x2) Sn/Pt(l l l) surface alloy (middle curve) and the (~13x~13)R30~ Sn/Pt(lll) surface alloy (top curve). The energy distribution curves shown are different spectra obtained by subtracting, without scaling the corresponding clean surface spectrum from the ethene-covered surface spectrum. The spectra were obtained for saturation ethene coverage at 12OK.
428
chapter 8
Figure 25 shows the UPS spectra with identification of emitting orbitals, for ethene on pure Pt(111) and on the two indicated alloys. The differential spectra shown in figure 25 clearly demonstrate (see the large negative peaks), that molecularly adsorbed ethene, which is identified by UPS and EELS as di-~-bonded ethene, suppresses emission from the d-orbitals, at about l eV binding energy. While on platinum the desorption occurs at 285K, for the p(2x2) alloy surface it occurs at lower temperatures. The temperature of adsorption varies with the ethene coverage the heat of ethene molecular adsorption was estimated 71 kJ/mo1-1 on Pt(111), 65-60 kJ/mo1-1 on the p(2x2) alloy and 49-45 kJ/mo1-1 on the (~/3x~/3) R 30~ Alloy formation obviously induces changes in the binding strength, the origin of which is a matter of discussion. Two possible reasons for the decrease in the molecular adsorption bonding strength have been suggested: (i) the electronic structure of the alloy components is not very much changed by alloying, but alloying causes changes in the coordination of di-cr-bound ethene; instead of the carbon atoms bonding to an ensemble of platinum atoms as on pure Pt(lll), with platinum-tin alloys a Pt2Sn ensemble might be used for adsorption, or adsorption might take place on two platinum atoms in an 'on-top' mode as shown in figure 22. (ii) The bonding is identical, e.g. an 'on-top' adsorption on both alloys and on pure platinum, but platinum atoms are importantly modified by tin. The authors [91] say that they "simply cannot rule out the possibility (i)", but they give preference to explanation (ii). Recently, Cartright and Dumesic [100] performed an important study on platinumtin alloys by microcalorimetry and M6ssbauer Spectroscopy, in order to explain why PtSn/SiO2 is superior to Pt/SiO2 in selective dehydrogenation of methylpropane. This complex study revealed that alloying did not remove all sites which could strongly adsorb hydrogen or carbon monoxide, but suppressed strongly their appearance in the alloy surface, compared with pure platinum surface. The remaining strongly adsorbing sites are put in relation with the high dehydrogenation action. Suppression of hydrogenolysis causing high dehydrogenation selectivity is explained by elimination on alloys of highly dehydrogenated species multiply bonded to the surface. It was concluded that 'ensemble size effect is primarily responsible for the higher dehydrogenation activity of catalysts containing tin' [ 100]. Environmental problems caused by electricity production and by automotive transport generated much interest in the adsorption of gases which we have been already considering, but also in some other gases such as nitrogen, nitrous oxide and nitric oxide. We shall turn now our attention to them. Adsorption of oxygen and nitrous oxide has also been studied [92] with the popular system Cu-Ru(0001), and several interesting effects were observed. A very small amount of copper, only 0.02 monolayer, dramatically retards dissociative adsorption of nitrous
Adsorption on alloys
429
oxide, but has little effect on the total uptake of the oxygen, and this results in nitrogen being continuously released into the gas phase. Suppression of the sticking probability is attributed to the influence of copper on the precursor state. The adsorption of oxygen from di-oxygen is not influenced by small amounts of copper. The possibility cannot be completely excluded that 0.02 of a copper monolayer is just the amount needed to block defects in Ru(0001), which defects can be suspected to be most active in dissociative adsorption of nitrous oxide [92]. Adsorptions of nitric oxide and of oxygen are of great importance with platinumrhodium automotive exhaust catalysts; they have therefore been intensively studied with single crystal planes of these alloys. It has been established that oxygen induces a very strong surface segregation of rhodium [93], just as sulfur does [94]. Since nitric oxide dissociates on rhodium and on some stepped surfaces of platinum, its adsorption induces rhodium segregation as well. Adsorption of nitric oxide has been studied by various surface science techniques [95] and it has been established that not only the composition, but also the formation, of various surface structures depends very much on the composition of ambient gases. The fraction of molecularly adsorbed nitric oxide is high on Pt(100), much lower on Rh(100) and with Pt25Rh75(100) plane it lies in between [96] (see chapter 12). Adsorption of carbon monoxide on the same alloy surface causes a clustering of platinum, but no particularly pronounced segregation [97]. A review [97] is available where other details of the studies already performed are summarized. Alloys of platinum with either ruthenium [72,73] or with palladium [75] on silica have been studied as adsorbents for nitric oxide, and as catalysts for the nitric oxide and carbon monoxide reaction, by transmission IR spectroscopy. As already mentioned above, formation of alloys can be confirmed in this way and the IR bands corresponding to individual alloy components can be distinguished; the latter feature is very useful for following the surface reactions. This approach has already been successfully applied [85]: carbon monoxide tends to bind to platinum sites, while nitric oxide prefers rhodium sites, and also mixed platinum-rhodium sites are probably revealed by IR spectra of carbon monoxide adsorbed on PtsoRhs0/SiO2 powders. With nitric oxide, which we have seen above to be a more 'corrosive' gas than carbon monoxide, the alloys show the same IR bands as pure rhodium. Alloys mentioned in this paragraph show a composition-dependent formation of isocyanate at slightly elevated temperatures.
0
0 Scheme I
N
N
I
I
430
chapter 8
Adsorption of nitric oxide on metals can occur in one or two modes: the linear mode, where the molecule is normal to the surface or the bent mode, in which the oxygen is bent towards the surface. The adsorption of nitric oxide has already been studied on iron, cobalt, nickel and their alloys [99]. The population of the bent mode is always higher on alloys than on pure metals and shows a remarkable parallel with reactivity in the reaction with carbon monoxide. 8.4.4
Adsorption on incomplete layers of alkali metals on transition metals Closely related to the phenomena occurring on alloys or on metal-on-metal layers,
is the adsorption of gases on transition metal surfaces doped by alkali metals. Alkali metals influence very strongly the adsorption of gases such as carbon monoxide and after the pionering papers [101-103] a long list of papers appeared and the results have been critically evaluated and reviewed [104,105]. The reason for this interest was, next to the fact that relevant experiments could be relatively easy performed, the potential application of alkalis as promoters of many catalysts [106]. With simple gases pronounced effects have been seen on the sticking coefficient, which can decrease as with hydrogen [103,109], or increase as with oxygen [103,104] or with nitrogen [108]. Further, the chemisorption bond strength is markedly increased as repeatedly observed for carbon monoxide by TPD measurements [104] or by calorimetry [110]. Carbon monoxide coadsorbed with alkali metals, forms compounds
Mm(CO)n
[111,112], which, when carbon monoxide is being adsorbed on a weakly adsorbing metal (copper, silver) or layer on another metal, recall the well-defined compounds such as e.g. squarates [ 113-116]. These compounds can be considered as having ionic character. Next to compound formation on shortest distance, adsorbed alkali metal atoms which appear to the gas phase as alkali ions, also influence the co-adsorbed layer by the electric field around the positive charges [117-118]. Thus they influence most strongly the dipoles in other coadsorbed molecules [ 119-122].
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66 67 68 69 70 71
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434 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88
89
90 91 92 93 94 95
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96 97 98 99 100 101 102 103
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Chapter 9
CATALYSIS BY ALLOYS - GENERAL FEATURES
9.1
Basic problems
The final goal of catalytic studies with alloys is easy to formulate: (i) by using alloy catalysts one should learn more about the functioning of metals as catalysts; this is the fundamental aspect: and (ii) one should be able to find empirically or to design rationally catalysts having better activity, selectivity and stability. Matters relating to the second point are already documented in the patent and scientific literature. So, for example, the platinum-iridium alloy catalyst used in naphtha reforming is several times more active per unit volume of reactor than the platinum catalysts originally used. These catalysts have been rationally designed on the basis of knowledge concerning the reactions of hydrocarbons on the individual metals. The most succesful combination, namely platinum-rhenium, was found empirically and shows superior selectivity and stability in the sulfided state. The same holds for platinum alloys used in the pharmaceutical industry for reactions such as transalkylation and hydrogenation of molecules with hetero-atoms, or for palladium alloys used in selective hydrogenation of ethyne, etc. However, the more intriguing for a scientist is point (i) above. One keeps asking questions concerning the relation of the activity and the selectivity, the latter being more and more important, to the chemisorption and thus to the surface structure and composition. We then wonder whether it is possible by alloying to change the chemisorption bond strength, a factor which certainly influences the activity of the catalysts. We shall tum to some reforming and hydrogenation reactions later in this chapter, and we shall touch the general problems. First, we have to make an inventory of the arguments available to support the idea that some adsorbates can be bound simultaneously to several surface atoms while other adsorbates are activated sufficiently by a single atom. Chapter 1 presents several examples of binding to ensembles of active atoms (see chapter 8 for a definition of this term). Carbon monoxide is, for example, most easily activated for dissociation in the hollow site or valley position on nickel surfaces [1]. Hydrocarbons form multiple metal-carbon bonds, as the vibrational spectroscopy and the exchange reactions with deuterium show (chapter 10). It is not certain in all cases, but it is quite probable for many that the multiple bonds actually involve several surface metal atoms. This idea is strongly supported by results obtained by LEED on the adsorbed ethylidyne radical (see chapter 1), and by quantum
438
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chemistry calculations in high approximation, which showed that even such fragment as the methyl radical prefers to be adsorbed at a hollow site. It is then a quite straightforward and safe extrapolation to expect that fission of a C-C bond upon hydrogenolysis of hydrocarbons would require two such hollow sites, or so to speak an ensemble of active sites. When we have advanced so far that we feel convinced some chemisorption modes require ensembles of atoms and not just single atoms, we have to worry whether such modes are not only spectators of a reaction but its reactive intermediates. If in the hydrogenolysis of ethane or of higher hydrocarbons the fragments are all bound by hollow sites, or on at least two different surface atoms (chapter 8, figure 22), we already have an example of reactive species bound to ensembles of atoms. Another example is carbon monoxide dissociation and the subsequent hydrogenation of the carbon and oxygen fragments in the Fischer-Tropsch reaction. Carbon from dissociation remains in the hollow site [1], and the preferential site for oxygen is also the hollow site, and both atoms react with hydrogen in the synthesis gas reactions (see chapter 14). When we accept the idea that in some reactions the strongly bound species held on ensembles of surface atoms are reactive, we can look around to see whether we can find some arguments in the catalytic results alone (the arguments based on chemisorption are presented in chapter 8) to justify speculations on the role of ensembles. In relation to the last problem we shall consider figure 1.
ACTre I
\
\ \
figure 1 Areal activity, relative to the activity of the pure metal activity as a function of the alloy surface composition. pure A
\
\ X
surf B
When the areal activity of an alloy (activity per unit total alloy surface area) of an active metal A with an inactive metal B follows closely the straight line 1 in figure 1, we would probably not hesitate to conclude that each single atoms of A can act as an active site. When the curve 2 is the result, the reasons for such a form can be as follows. 1) The reaction needs ensembles of active A atoms in order to occur. 2) The electronic structure of atoms of A is so much modified by B that their activity is much lower than that of atoms A in the pure metal. 3) Introduction of B leads to so much of a side reaction,
Catalysis by alloys - general features
439
detrimental for the activity, that the total activity per single atom is much lower on alloys. 4) With small particle size alloys, exposing sites of very high activity on edges, corners, defects, etc. metal B-atoms segregate to those most active sites. To eliminate this complication one has to work with larger particle alloys or with single crystal planes. The possible role of side reactions was much underestimated in earlier papers. However, it is very well possible that for instance a hydrocarbon reaction deposits in the steady state much more carbon on the surface of a certain AB alloy than on pure A metal. Another example could be a deeper oxidation of, and thus also stronger deactivation of, AB in comparison with pure A. A combination of 1) to 4) is also possible. Obviously, when we are looking for results which might possibly support the idea of ensembles acting as active centres, we have to make sure that effects 2), 3) and 4) are absent or are of small importance. It seems that nickel-copper alloys are very well suited for this purpose. Studies of their solid state physics have provided abundant evidence that the electronic structure changes to nickel atoms induced by copper are marginal (see chapter 3). The reason for various changes in chemisorption behaviour (see chapter 8) due to alloying are still a matter of discussion, but we can nevertheless conclude that all these results allow us to speculate that the changes are due to varying concentration of nickel atoms in the surface, and to consequential ensemble size effects, but not a result of changes in electronic structure or ligand effects on nickel sites. Finally, it has been established that, for example, selfpoisoning by carbon in hydrocarbon reactions is suppressed by addition of small amounts of copper to nickel (see below) and not enhanced; with Fischer-Tropsch synthesis the selfpoisoning might be, however, slightly higher on alloys than on pure nickel. Having all this in mind we can compare the activity patterns shown by nickel-copper alloys in various catalytic reactions. This has already been done [2] and the result is shown in fig. 2; the bulk and not the surface concentration is used in this graph taken from the original paper, and therefore it should be realized that the surface nickel concentration follows approximately the lower edge of the band comprising the rates of reactions of group I. The group I of alloying-insensitive reactions comprises hydrogenation of unsaturated C-C, C-O and (most probably) C-N bonds and the reverse reactions, as well as the related hydrocarbon-deuterium exchange reactions. The group of alloying-sensitive reactions comprises hydrogenolysis of C-C, C-N, C-O bonds and the Fischer-Tropsch synthesis [2]. The conclusion [2] has been made that the alloying-sensitive reactions are actually ensemble size sensitive reactions. Of course, such a conclusion requires an additional support, and hydrocarbon reactions can serve as a model for this investigation. However, with them we have first to answer the question why with some reactions the activity increases after addition of small amounts of copper. It has been shown [3] that when hydrogenation of benzene is monitored at low temperatures, i.e. when selfpoisoning by carbonaceous deposits is of limited
440
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extent, nickel-copper alloys have a lower areal activity than pure nickel catalysts. However, at high temperatures, when the formation of deposits is more extensive and the deposits can become highly deactivated by graphitization, nickel-copper alloys are more active than pure nickel [3]. Benzene hydrogenation [3] has been frequently used as a test reaction and this reaction shows, with alloys in which a real atomic dispersion of the active metal in the surface is possible, such as the diluted platinum-gold alloys, a certain ensemble size sensitivity, too [4,5a].
Activity I standard, N i lo -1 _
/ /
~0-3 _
/
i0-s _
Ni
figure 2 Catalytic activity if Ni-Cu alloys. Activity patterns as a function of bulk composition. Phenomenologically, all reactions can be subdivided into two groups according to the limits indicated in the figure (schematically).
Cu
Further, one has to investigate whether some changes in the electronic structure of nickel are not too small to be detected by spectroscopic techniques or by TPD and other chemisorption studies, but yet are important enough to manifest themselves by a sharp drop in activity of nickel by alloying as seen with reactions of the group II (in figure 2). This problem has been addressed by a study on palladium-gold alloys [5b]. Fig.3 shows that the selectivity S for non-destructive reactions of hexane increases with gold content, in the same way as with nickel-copper alloys (see below). This is equivalent to saying that hydrogenolysis is suppressed by alloying of nickel with copper as well as by alloying of palladium with gold. These alloys have one aspect in common: in both cases the active metal is diluted in the matrix of a virtually inactive metal, i.e. the average ensemble size is diminished by alloying. On the other hand, they differ in the changes in electronic structure induced by alloying: nickel remains paramagnetic (ferromagnetic) in all alloys with copper and the 3d holes thus stay unoccupied even in very diluted nickel alloys, while palladium in alloys with gold changes its electronic structure from about 4d9"75s ~ to 4d~~ ~ and thus becomes diamagnetic (see chapter 8). Since the effect of alloying on the catalytic reaction is in both cases the same, one can conclude that the main effect by which the hydrogenolysis is suppressed is the ensemble size effect, that is to say the dilution of the active metal by the inactive one. The effect of d-orbital occupation on the suppression of hydrogenolysis is in this specific case a second rate effect, if it exists at all.
Catalysis by alloys- general features
441
figure 3 9 S
%
n-Hexane reactions. Selectivity parameter S and the MCP content among products, at comparable conditions.
50-
Pd: conversion ~ = 1.02%; 656K. 39% Au: o~ = 0.7%; 633K. MCP
30
J
10
J
f
48% Au: o~ = 0.6%; 646K. Alloying is accompanied by a sharp drop in the activity. The extent of all reactions extent
of
except
hydrogenolysis
non-destructive
(i.e.
reactions
[5b]), is plotted as a function of alloy
0
'
' 20
'
' 40
'
6'0
bulk composition.
At % A u
Results obtained and the conclusions arrived at with nickel-copper alloys cannot however be automatically extrapolated to other alloy systems. Therefore, we can find in the literature many papers which continue to discuss the catalytic behaviour
of alloys at
the level of the two following fundamental questions. (i)
Are the catalytic effects of alloying a consequence of an ensemble size effect or
(ii)
Does the alloying of two active metals lead to the formation of mixed ensembles with special catalytic properties?
are electronic structure changes equally or even more important?
In particular, the first question attracted much attention in the literature and we shall analyse below several attempts to find an answer. Actually, electronic structure changes induced by alloying should always be proven first by physical methods when they are intended to form a basis of explanation of catalytic results, but since a sceptic can always say that all such methods are less sensitive to small changes in the electronic structure of active metal atoms than a catalytic reaction itself, we have to consider in detail which results are usually used to document the importance of electronic structure effects in catalysis.
9.2
Investigation of electronic structure effects by means of catalytic reactions
We believe that the comparison mentioned above of palladium-gold alloys with nickel-copper alloys with regard to one and the same reaction (i.e. hydrogenolysis) is an example of a possible approach. However, its applicability seems to be limited, and
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attempts have been thus made to use other results as indications of the existence of the ligand effects in catalysis. We shall now discuss some of them.
'Inexplicable'changes Electronic structure effects are often introduced when phenomena are observed which seem to be inexplicable by ensemble size sensitivity of a reaction. Let us mention some examples. A system shows a so-called synergism. It means the activity of an alloy catalyst is higher than that of either component in the state of a pure metal. Indeed, an electronic structure effect could potentially manifest itself by such behaviour; however, alternatives should be checked first before one accepts this explanation as definite. The possible alternatives are the following (i) mixed ensembles show a higher catalytic activity than any of the pure metals, although the electronic structure on each individual metal atom in the alloy is not much different from the structure in a pure metal state (see chapter 13). (ii)
Selfpoisoning is less severe with alloys than with pure metals. We have seen an example of it with nickel-copper alloys in hydrocarbon reactions [3]. This explanation could also be considered in cases such as hydrogenation by palladium alloys, since hydride formation, which can lead to selfpoisoning, can be suppressed by alloying. Still another potential application of the idea that alloying suppresses selfpoisoning can be oxidations with alloys of which one component is more easily oxidized, and thereby deactivated, than the other. (iii) Some reactions show a direct or, via selfpoisoning, indirect particle size effect. Therefore, one must not forget that alloying or just the presence in the catalyst of a second metal can lead to a higher dispersion and (iv) can also have morphological effects, i.e. can lead to exposure of different crystallographic planes. Yermakov considered the anchoring effect of ions of one of the component as one of the most important effects in alloy catalysts [6]. Certainly the particle size and morphological effects should be always considered and this is also the reason why these effects were discussed in detail in chapter 5. Another factor which is very often brought into discussions as evidence of electronic structure effects is the change in the apparent activation energy. When changes in the activation energy due to alloying have been reliably established, which is not easy and is not done frequently (see chapter 6), one has still to consider that mixed ensembles (A 2, B, AB 2, etc.), if operating, may show different chemisorption bond strengths from pure metal ensembles, and as a consequence show different activation energies. When admixture of B in A causes the reacting molecule or its fragments to be no more adsorbed in hollow sites, where it would be bound to the ensemble of surrounding atoms, but are pushed by the presence of B into the 'on-top' position, then, if reaction is still possible, the activity per site may be different in pure metals and in alloys and the activation energy may be different too.
Catalysis by alloys - general features
443
Changes in the temperature dependence of rates or rate constants, i.e. changes in the apparent activation energy, can also be introduced by a temperature-dependent selfpoisoning, which in its turn can be different on metals and on alloys. The number of working (poison-free) sites can be a function of temperature and partial pressures and this function can in it's turn be different for metals and alloys. Sometimes the drop in areal activity caused by alloying can seem to be inexplicably large. If simple statistics of ensemble size distribution is applied (chapter 8), and all the effects on the activity are ascribed only to the presence or absence of ensembles of required size, the size so determined can sometimes be unreasonably large, being something like 15-30 atoms. It could then be tempting to attribute the very large decrease to electronic structure changes. However, it is also possible that certain sites such as defects in single crystal planes or special sites such as edges on small metal particles are the places of the highest catalytic activity (e.g. B5 sites, discussed in chapter 5). If the added inactive metal congregates to these places, a small amount of the added metal can have very pronounced effects; notice, that the results on nickel-copper alloys were mostly obtained with large particle catalysts!. If one blindly applies simple ensemble statistics to the case of small particles, the resulting ensemble size would seem to be unreasonably large. The list of alternatives is not yet exhausted. Once again one has to consider selfpoisoning effects and the role of several undersurface layers in the alloy before one makes a definitive conclusion that catalytic data alone supply sufficient evidence for electronic structure effects. For example, sulfur can be considered as a poison which blocks the site it is sitting on and, because of its size, several nearest sites; one has to consider the Van der Waals radius of sulfur. However, the drop in the methanation activity of nickel caused by sulfur is still much larger than expected if the simple blocking mechanism were operating [7], and speculations on electronic structure effects are very tempting. Nevertheless, one should not forget results that show sulfur strengthens the selfpoisoning by carbon deposition, inducing graphitization of the layer [8]. In this way, the drop in the activity is a simultaneous effect of sulfur and of the deactivating carbon deposit. Indeed, higher carbon AES signals were seen [9] with sulfur- poisoned nickel catalysts than with unpoisoned ones [7].
The technique of competitive and parallel hydrogenations This method was first suggested as a means of characterizing pure metals and to demonstrate the role of the electronic structure of individual pure metals in hydrogenation [ 10], but in principle it should also be applicable to detecting electronic structure effects of alloying or promotion. The method has indeed been applied to such problems [11-14]. It is claimed [10] that this method is much more sensitive to ligand effects than are physical methods and that it can also detect small crystal-face specific differences [ 13].
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It is claimed [11-14] that the constant Kr/B, the ratio of the kinetically determined adsorption coefficients of T__oluene and B__enzene respectively, is raised when the environement makes an adsorption site more electron defficient and/or when the density of states at the Fermi energy on the adsorption site is lowered. This explains why KX/B increases in the sequence P d < Pt < Rh < Ir < Os < Ru. Value of Kr/B greater than unity implies that benzene is hydrogenated faster when alone than when mixed with toluene. The stronger effect of charge deficiency or of lower density of states at the Fermi energy N(EF) on the adsorption of toluene is explained by saying that it is a stronger donor of its rt-electrons, and therefore reacts more sensitively to charge deficiencies or low N(EF) than benzene. At first glance, this seem to be a very attractive method to detect possible changes in the electronic structure of alloys, since hydrogenations are not at all sensitive or are only moderately sensitive [2-5] to ensemble size effects. However, a closer inspection of the available information on toluene and benzene adsorptions reveals some problems. It is known [15] that, when the benzene ring is substituted by alkyl-substituents, the exchange reaction with deuterium always starts on the side chains. It means that at temperatures at which KTra is being determined [11-14] the -CH 3 group forms an additional centre at which adsorption by dissociation of C-H bonds can occur. Such adsorption makes exchange on the ring, and also addition of hydrogen to the ring, more difficult, and when the bond from the carbon of the CH 3 group to the metal is a multiple one [15], it may even be impossible. It is interesting to see that the sequence of metals shown above, from palladium to ruthenium, corresponds very well to the propensity of metals to break C-H bonds and to form multiple metal-carbon bonds" (viz. Pd < Pt < Rh < Ir < Ru)
[16,17].
So if the latter parallel is of physical importance, the competitive hydrogenation cannot be regarded as a reliable method to determine the local charge deficiency, or low density of states. This pessimism is strengthened by a report that sintering and poisoning both change the ratio of the reactivities of hydrogen on the ring and on the side chain [18]. Perhaps the competitive hydrogenation method is better suited to detect how alloying influences the reactivity of the C-H groups than to detect ligand effects, and this would be of considerable value too.
9.3
Important side effects of alloying We have already mentioned that introduction of a second metal or metal ions can
change the particle size distribution, which in its turn can influence the activity and the selectivity directly, or indirectly via selfpoisoning. Practical catalysts, except of those used as wires, gauzes, or Raney metals, are used as metal or alloy on a support (chapter 7). The support has to stabilize small particles, and to prevent sintering, and sometimes it can also work as a poison-scavenger, or a storing-
Catalysis by alloys - general features
445
place for carbon deposits. However, in several practically very important catalytic reactions, the support is not inert; it bears active sites and the activity of these sites can be influenced very much by ions of the precursors from which the alloy will be formed. A typical example is the presence of the acidic sites on the ](-A1203 surface, which sites make the catalyst behave bifunctionally: a part of the reaction takes place on the metallic phase and a part on the support (see chapter 13). There are also many cases of spillover of adsorbed species. Hydrogen atoms or alkenes can migrate from the metal to the support and for example, propene in cyclopropane reactions can migrate from sites on the support to metallic sites. The presence of ions of Ir, Re, Sn or Ge next to platinum ions in the precursor state of the reforming catalysts not only determines the properties of the alloy phase, but it can also influence the number and quality of the acidic sites on the support. Moreover, the transition metal ions which survive the reduction can themselves be active. When the activity of a support is so essential for the total activity of a catalyst as it is with petroleum reforming catalysts, the influence of Re n+, Sn n+, etc. ions on the poisoning of the sites on the support has also to be considered [19]. Temperature-programmed oxidation of used catalysts has revealed that the largest volume of deposit is on the support [20], and the presence there of transition metal ions can possibly help the continuous in situ regeneration of the acid sites during operation. Some reforming catalysts are used after modification by sulfurization. Of course, different alloy components have different propensities f o r holding sulfur [21]. For example, rhenium alone or rhenium in platinum holds sulfur much more firmly in a hydrogen atmosphere than pure platinum; iridium is more comparable with platinum in this respect. In any case, this is also an aspect which has to be kept in mind when discussing results obtained with industrial catalysts; they often work after having been intentionally or adventitiously modified by various poisons or modifiers in the feed. In the chapters which follow, several important groups of reactions catalyzed by alloys will be discussed. While the agreement between experimental results obtained by different workers is quite good, and the results as such are very selfconsistent, the explanations put forward by various authors are, as could be expected, very different. However, we hope that this review of various effects discussed on a very general level will be helpful as a guide through the following chapters. If a reaction of A with B produces the products C and D, the selectivity to product C can in this reaction be defined for example as:
Sc=rc[(rc+ro )
=
1 (1 + ro/rc)
The ratio of rates can in its turn be written as:
(1)
446
chapter 9
EO ro
k~
e
rc
xrf(papBpcpo)
No
(2)
_ e__sc k ~
c .e
Rr.f(papnpcPo)
Nc
where next to the obvious terms, N o and Nc stand for the numbers of sites (individual atoms or ensembles of atoms) for the particular reaction. It is interesting to note that almost all the "classical" literature focuses attention on the exponential terms and parameters E o, E c, while the work on alloys (and on promoted metals, too, see chapter 14) revealed that the term No/Nc is the most important.
References 1
2 3 4 5a b 6 7 8
9 10 11 12 13 14
15
W.L.van Dijk, J.A.Groenewegen, V.Ponec, J.Catal. 45 (1976) 277 V.Ponec, Int.J.Quant.Chem. 12(suppl 2) (1977) 1 W.A.van Barneveld, V.Ponec, Recl.Trav.Chim. (English) 93 (1974) 243 S.Puddu, V.Ponec, Recl.Trav.Chim. (English) 95 (1976) 255 V.Ponec, Proc.6th Int.Congr.on Catal., London, 1976, Chem.Soc.London, Vol.2 (1977) p.851 C.Visser, J.G.P.Zuidwijk, V.Ponec, J.Catal. 35 (1974) 1407 Yu.I.Yermakov, Proc.7th Int.Congr.on Catal., Tokyo, 1980, Kodansha-Elsevier, Vol.A (1981) p.57 M.Kiskinova, D.W.Goodman, Surf.Sci. 105 (1981) L265; 108 (1981) 64 P.W.Wentrczek, J.G.McCarty, C.M.Ablouw, H.Wise, J.Catal. 61 (1980) 232 M.Kiskinova, private communication T.T.Phoung, J.Massardier, P.Gallezot, J.Catal. 102 (1986) 456 R.Szymanski, H.Charcosset, P.Gallezot, J.Massardier, L.Tournayan, J.Catal. 97 (1986) 366 T.M.Tri, J.Massardier, P.Gallezot, B.Imelik, Stud.Surf.Sci.& Catal., Vol.11 (1982) 141 J.Massardier, J.C.Bertolini, T.M.Tri, P.Gallezot, B.Imelik, Bull.Soc.Chim.France (1985) 333 J.Barrault, A.Alouche, A.Chaffik, V.Paul-Boncour, S.Probst, Proc.9th Int. Congr.on Catal., Calgary, 1988, Chem.Inst.Canada, Vol.2 (1988) p.642 D.G.Blackmond, A.Waghray, R.Oukaci, B.Blanc, P.Gallezot, Stud.Surf.Sci.& Catal. Vol. 59 (1991) 145 R.J.Harper, C.Kemball, Proc.3th Int.Congr.on Catal., Amsterdam, 1964, North
Catalysis by alloys - general features
16 17 18 19
20 21
447
Holland Publ.Co., Vol.2 (1965) p.1145 J.W.Hightower, C.Kemball, J.Catal. 4 (1965) 363 E.H.van Broekhoven, V.Ponec, J.Molec.Catal. 25 (1984) 109 C.Kemball, Catal.Revs. 5 (1971) 33 M.J.Philips, E.Crawford, C.Kemball, Nature 197 (1963) 487 R.Burch, A.J.Mitchell, Appl.Catal. 6 (1983) 121 L.H.Ludlum, R.P.Eischens, ACS Div. Petrol.Chem.abstracts, Seattle, 1976, p.375 R.J.Bertolacini, R.J.Pellet in "Catalyst Deactivation" (editors: B.Delmon, G.F.Froment) Elsevier (1980) p.73 V.K.Shum, J.B.Butt, W.M.H.Sachtler, J.Catal. 96 (1985) 371; 99 (1986) 126 J.Biswas, G.M.Bickle, P.G.Gray, D.D.Do, J.Barbier, Catal.Revs.Sci.Eng. 30(2) (1988) 161 K.C.Mills, "Thermodynamic Data for Inorganic Sulphides, Selenides and Tellurides", London, Butterworths (1974) A.B.Anderson, S.Y.Hong, J.L.Smialek, J.Phys.Chem. 91 (1987) 4250
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Chapter 10
R E A C T I O N S OF H Y D R O G E N AND A L K A N E - D E U T E R I U M E X C H A N G E
10.1
Reactions involving only hydrogen and its analogues
Introduction The concern of this section is with catalysed reactions in which hydrogen and its analogues can participate. The term 'analogue' is used as an omnium gatherum for the isotopes deuterium (D or 2H) and tritium (T or 3H), for the molecules as well as for the corresponding atoms, and for the spin isomers, of which the ortho- and para- forms of molecular hydrogen are most important. The terms dihydrogen, dideuterium etc. are sometimes used to describe the molecular state where it is important to distinguish it. The dissociation energy of dihydrogen in the ground state is 436 kJ/mo1-1, which means that it is not until very high temperatures are reached that there is any appreciable concentration of hydrogen atoms in equilibrium with the molecular form: at 2000 K the concentration of atoms is only 0.081% [1]. At ambient temperature the fractions of molecules in the ortho-form, i.e. having the nuclear spins parallel, are for hydrogen 75%; for deuterium 66.7%; and for tritium also 75%. The consequence of the extremely strong covalent bond in these diatomic molecules is that any reaction between them, or indeed any reaction between any one of them and any other type of molecule, requires the intervention of a catalyst to dissociate them into atoms, or failing this some high energy source such as an electrical discharge or an intense form of radiation (e.g. as in mercurysensitised photolysis). The only exception to this generalisation is the interconversion of the spin isomers, which can be effected by a 'flip' of nuclear spin in the presence of a strong magnetic field, such as exists closely to paramagnetic ions, without dissociation of the molecule. Table 1 lists the various possible reactions of hydrogen and its analogues, and the terminology used to describe them. Where isotopic variants are possible only the most commonly used are listed. The reactions listed in Table 1 have played a leading role in the development of concepts and theories concerning catalysis by metals and alloys [2]. In the period up to about 1960, they were the reactions chiefly responsible for the early clarification of the principles governing metal catalysis, and it is not difficult to see why this should be so. The chief reason is that all of these reactions proceed easily and can be followed with relatively cheap but accurate apparatus. The formation of hydrogen atoms by desorption
450
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from an electrically-heated wire can be quantitatively determined by
W O 3 o r M o O 3,
which
are turned blue by the atoms, due to production of the non-stoichiometric 'hydrogen bronzes', or by a thermistor which senses the temperature rise produced by the surfacecatalysed recombination. The recombination of atoms, produced for example in an electric discharge, can also be followed by the temperature increase of the catalysing surface, e.g. by the change in its electrical resistance. table 1 Catalysed reactions of hydrogen and its analogues name
reaction*
Dissociation
H2
+ 2*
---) 2H*
---) 2H. + 2*
Atom recombination
2H.
+ 2*
---) 2H*
---) H2 + 2*
Isotopic equilibration
H2 + D2 + 4* ---) 2H* + 2D* --~ 2HD + 4*
Spin-isomer equilibration
pH 2 + 2*
---) 2H*
---) oH2 + 2*
Isotopic exchange
H* + D 2 + 2* ~ D* + HD + 2*
Hydrogen ion discharge
H § + * + e-
--) H
---) V2H2 + *
* A possible number of active atoms involved in the step is indicated since none of these numbers has been definitely established
Isotopic exchange and equilibration, and para-hydrogen conversion, can all be followed by observing the change in the thermal conductivity of the gas, which is significantly different for the various reaction participants: this technique was indeed generally and effectively employed until at least 1950, when isotopic analyses began to be performed mass-spectrometrically. This latter method is however far from fool-proof, due to significant memory effects arising from adsorption of the components on the inner metal surfaces, and their exchange with species remaining from previous analyses. The old method had much to commend it. A second powerful motive for studying these reactions was their formal simplicity, and the uniquely inevitable nature of the products formed. However, even with unsupported metals (wires, films, single crystals, etc.), variations in surface topography due to defects impurities different crystal planes, and in adsorption energetics with coverage, have limited the fundamental significance of the results obtained. It is now clear that adsorbed hydrogen atoms can exist in a number of different adsorbed states, in some of which the atoms may lie within or beneath the surface. Interconversion of these forms is usually rapid, and their differentiation by kinetic techniques is not always possible; sometimes however it is only the more weakly bound that are reactive, while the more strongly held
Reactions of hydrogen and alkane-deuterium exchange
451
forms have only 'observer status'. These uncertainties are compounded in the case of supported metals. We then have to consider not only the effects of a distribution of particle sizes and the dependence of adsorption energetics on particle size and shape [3], but the propensity of hydrogen atoms to migrate from the metal to the support by a process known as spillover (French, 6pandage). Many details of this occurrence still remain obscure and are the subject of continuing research [4-6], but the process is responsible for (i) the exchange by deueterium of hydrogen atoms in hydroxyl groups on the support, (ii) formation of hydrogen bronzes when oxides such as V205, WO 3 o r MoO 3 are used as supports, (iii) reduction of cations in the support to lower oxidation states and (iv) reduction of carbon deposits, oxygenated molecules adsorbed on the support etc. Thus although by studying the reactions of hydrogen and its analogues one avoids the confusion that always to some degree attends the reactions of hydrocarbons, the small size of the hydrogen atom gives it a kind of chameleon quality that leads to other equally frustrating sources of uncertainties. One or two further remarks about the reactions listed in table 1 are in order. It may not be obvious why a catalyst is needed to effect the recombination of gaseous hydrogen atoms. The reason is that two atoms, on colliding in the gas phase, have so much energy that the incipient bond is immediately broken again, unless the excess vibrational energy can be removed by a third body, which is conveniently provided by a surface. The positions of equilibria in reactions involving isotopes are not simply determined on statistical grounds, because the effects of isotopic variations on physical properties such as mass, vibration frequencies, moments of inertia etc. are such as to necessitate the use of statistical thermodynamics to account for the observed equilibrium constants. Thus for example the equilibrium constant for H 2 + D2 ~ 2HD has a value of about 3.27 at 298 K, but this rises towards the statistical value of 4 as the temperature is raised [2]. The earliest work on catalysis by alloys of the reactions of hydrogen was motivated by a desire to test the consequences of the collective electron model on catalytic activity. We shall shortly be reviewing some of the information obtained, although by this stage it must be abundantly clear to the reader that this approach has now been consigned to the dustbin (trash can) of science. Nevertheless, subject only to limitations connected with the primitive apparatus used, and attendant questions of surface purity, the results still have validity and require explanation. The collective electron model supposed that the available electrons (and the 'holes' resulting from their absence) were equally shared by all the atoms present, and that the electron concentration (electron/atom ratio) or the density of states at the Fermi surface or some other related quantity would determine catalytic activity, which was thought to be governed by electron-transfer processes. Many of the early results appeared to confirm this expectation, so that the discovery (described and explained in chapters 1 and 3) that atoms in an alloy retain their chemical identity more or less completely, while more in tune with chemists' intuition, created certain problems in
452
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the re-interpretation of the catalytic results according to the new ideas. Some additional questions have also had to be posed: for example, if in an AB alloy the element B is normally unable to chemisorb hydrogen, while element A can, are atoms of B in the surface capable of retaining hydrogen atoms, and what role if any is played by twocomponent sites such as AB, A2B, ABa, etc.? Something has already been said on this problem in chapter 9 and we shall attend to some of these questions in the next section and chapter 13. One last remark is needed. It will be quite apparent to the reader of the catalytic literature that the reactions that are the subject of this section are no longer an active field of research: indeed our search of the literature has revealed very few relevant publications in the last 25 years, and in the otherwise comprehensive volume on the role of hydrogen in catalysis [6] they receive only incidental mention. Perhaps the science has moved on, but if it has it has left behind if more questions than were ever answered. Perhaps it has been tacitly concluded that the questions which arose were "too difficult" or not even very important, and indeed some of the early discussion of mechanism now seems somewhat recondite and irrelevant. Perhaps also the equipment needed was not sufficiently complex or expensive, so that scientists' experimental virility and fund-raising powers were not adequately tested: or perhaps it became possible to study other more fascinating subjects. Most probably it was thought by those who controlled the purse-strings that there were no important applications for the reactions of hydrogen, and that the fundamental but unanswered questions could wait. Whatever the explanation - and it is is likely that all these factors contributed - it is certain that the study of reactions of hydrogen went out of fashion. Social historians of science (if they exist) have an interesting task in explaining why we do certain types of work at certain times, but we cannot allow ourselves to speculate further at the present time. It is one of the paradoxes of science that what seems to be, and might be, most simple turns out to be most complex, while what is more complicated turns out to be more informative. What we report in the next sections is work mainly performed more than a quarter of a century ago: when the fashion changed, about 1970, attention could be turned to more complex and 'relevant' subjects. However, we must record what was done, as it represents an important step along the road. It would be wrong to ignore it because people no longer work in this area. Perhaps the time will come to return to the study of these most basic of catalysed reactions. 10.1.1 Reactions involving hydrogen atoms The next more complicated effect that occurs when dihydrogen interacts with a metal, after chemisorption, is dissolution. Within the metals of Groups 8-10, it is chiefly palladium and its alloys that exhibit the well-known tendency to dissolve hydrogen
Reactions of hydrogen and alkane-deuterium exchange
453
exothermically [7]; the easy migration of atomic hydrogen across a concentration gradient within the metal provides a ready means of generating extremely pure dihydrogen, although in practice a palladium-silver alloy is used [8] because regular changes in lattice parameters caused by irregular use result with pure palladium in "hydrogen embrittlement", which limits its utility. Palladium membranes have also been extensively investigated as catalysts [9,10]; very simply, dihydrogen is brought to one side of a palladium foil, and a hydrogen acceptor to the other, where a controlled hydrogenation takes place. The stoichiometric hydrides formed by elements particularly of Groups 3-5 and related intermetallic compounds are also well known, and some of them have been examined as stoichiometric hydrogenation catalysts not needing the presence of hydrogen molecules (see also chapter 7): however they appear to have few advantages over conventional catalysts, and some disadvantages. What is perhaps somewhat less well known is that nickel and nickel-copper alloys can also form B-hydride phases [11], but they are usually prepared electrolytically. The other noble metals of Groups 8-10 absorb small amounts of hydrogen endothermically and it has been suggested that occluded hydrogen (the physical nature of which is not absolutely clear) is responsible for the low selectivity usually shown by iridium in reactions of alkenes and alkynes [12]. There have been numerous investigations of the effects of alloying on the interaction of dihydrogen and dideuterium with palladium: the alloys with gold [13-15] and silver [13,16,17] have received particularly attention, but those formed with many other metals [18-20] and semi-metals such as boron [21] and ternary alloys [22] have also been studied. It is unnecessary to describe the results in any detail, as the relevance of dissolved hydrogen to the catalytic properties of the element is generally unproven. There have been suggestions that it acts as a poison both for palladium [23] and for nickel [11] in the same sense that metals of Group 11 will lower their activity, but this concept was in vogue when observations were being interpreted by the collective electron model, and dissolved hydrogen atoms were supposed to be partially ionised, donating electrons to palladium's vacant d-band: today we know that hydrogen in palladium keeps its electron, and its effect is due to dilation of the metal lattice. Equilibrium solubilities of hydrogen decrease as the concentration of the added element rises [14,18]; ironically, in the cases of silver and gold, they become zero at about the point where the d-band of palladium would be filled if the simple collective electron model were valid [18]. An empirical relation between the expansion of the lattice at the B-phase boundary and the valence electron concentration of the hydrogen-free alloy has been proposed [14]. The rate of increase of lattice parameter with dissolved hydrogen concentration is independent of gold content [18]. Alloys of palladium with gold and with silver differ in respect of their interaction with hydrogen in small but significantly different ways [13]. There are several particular reasons for questioning whether dissolved hydrogen affects the catalytic properties of palladium in any meaningful way. (1) The ct-PdH phase
454
chapter 10
is -at about one bar- only stable below about 350K: its formation and decomposition is readily observable during TPR experiments. (2) Strongly adsorbed molecules such as ethene and carbon monoxide poison dissolution, since the hydrogen molecule must first be dissociatively chemisorbed before the atoms can dissolve. (3) Solubility decreases with decreasing particle size [24]. It is nevertheless recommended that anyone using palladium or a palladium alloy for a reaction in which hydrogen is a reactant or product should be alert to the possible effects that hydride formation might have [13,25]. Very little work has been performed on the atomisation of dihydrogen by heated metal filaments since the subject was reviewed in 1964 [26] (see also [2]). This is presumably because little information is thereby obtained on the chemisorbed state of the atoms, the lifetime of which must be exceedingly short at the necessarily high temperatures employed (1000-1800K). The reverse process of hydrogen atom recombination [2,27,28] was however intensively studied for a period during the 1960's and a short review of this work is in order. In the method used by Linnett and co-workers [29,30], hydrogen atoms were generated by a radio-frequency discharge and were allowed to diffuse into a cylindrical tube closed at the other end. The catalyst in the form of a foil was placed along the length of the tube, and the concentration of atoms at any point was measured by a movable silver-coated thermocouple. The effectiveness of the surface in catalysing the recombination is measured by the recombination coefficient 7, which is derived from the equation relating atom concentration n at any point along the tube to distance x from the source: n .: n o e x p [ ( - Y c [ 2 R D ) l / 2 x ]
(1)
where n o is the concentration at the source, ~ the mean atomic velocity and D the binary diffusion coefficient for the mixture [29]. Measurements were made with the foils at ambient temperature. What is most significant about the results obtained with all three alloy series (palladium-gold [29] and -silver [30] and nickel-copper [11]) was that higher values of the recombination coefficient were found towards the centre of the alloy series than with the more active of the two components, which was the Group 10 metal (Pd-Au, -- 30-40% Au; Pd-Ag, -- 40% Ag; Ni-Cu, --60% Cu). The increased efficiency was observed [29] with both hydrogen-free and hydrogen-saturated foils, but the value of 7 was less with the latter. The toxic effect of dissolved hydrogen was also noted in the nickelcopper series [11]. Less extensive results were obtained for the palladium-gold system at higher temperature, using a different technique [31]. The increased effectiveness produced by alloying palladium or nickel with a Group 11 element was attributed to a decrease in the M-H bond strength, but once again this was discussed in terms of the filling of d-band vacancies. The favoured mechanism was that of collision of a gaseous atom with an adsorbed one"
Reactions of hydrogen and alkane-deuterium exchange
Had s +
455
H. ~ H 2
A re-examination and re-interpretation of the results that this simple reaction affords is long overdue. When an electric current is passed through an aqueous solution of an electrolyte, the net reaction is the discharge of protons at the cathode, from which dihydrogen is evolved, and of hydroxide ions at the anode at which di-oxygen appears: 2H30 + 2e- --> H 2 + 2H20 4OH-- 4e-
--> 0 2 .q.- 2H20
With most metallic cathodes operating at low current density I, the potential becomes much more negative when the current flows,and the difference between the actual and the reversible (open-circuit) potential is the overpotential 1"!. This is related to the current density by the Tafel equation: 1"1 = a + b log I a and b being constants. The current density is a measure of the rate of conversion of hydrated protons H30 § into dihydrogen. There has been much discussion in the literature concerning the rate-limiting step in this process and two possibilities have been canvassed [2]. Following the discharge of the ion as H3 O+ +
e- ---> Hads + H20
there may ensue either the recombination of the atoms or an ion-atom reaction: Had s + H 3 0 + +
e- ~ H 2 + H 2 0
Attempts to resolve this question have been made [2,32-34] by trying to correlate current densities with work function, activity for para-hydrogen conversion or hydrogen-deuterium exchange, heat of hydrogen chemisorption and other physical properties of the metals concerned. Most probably the slow step is the atom recombination on metals such as platinum, on which the atoms are weakly held and mobile. There have been a number of studies of the electrochemical efficiencies of alloys. With the nickel-copper system [33,35,36] current density decreases with increasing copper content, while with palladium-gold there is an activity maximum at about 40% gold [37]. Operated in reverse, these reactions are central to the successful operation of fuel cells, and this consideration has motivated much of the work in this area.
456
chapter 10
It is however unfortunate that the electrochemical, electrocatalytic and catalytic fraternities rarely mingle, because they have much to learn from each other. An interesting example of the success that may attend "technology transfer" is provided by the application of cyclic voltametry to the study of single crystals. The current-voltage plot (voltammogram) gives peaks at particular voltages corresponding to the adsorption or desorption of species in solution. The area under a peak represents a charge and this may be used to estimate the absolute coverage of the species in question. Thus for example on Pt(111) the total hydrogen charge is 240x10 -6 C cm -2, which on dividing by the electronic charge gives the number of hydrogen atoms adsorbed as 1.5x10 ~5 cm 2, i.e. exactly the number of platinum atoms on this surface. Detailed studies have been reported of stepped platinum surfaces [38], and the procedure has also been used to study hydrogen adsorption on Pt(111) partially covered with bismuth atoms [39]. This seems to be a technique of very considerable promise.
10.2
The equilibration of hydrogen + deuterium and para-hydrogen conversion [2,40-42].
An introduction to these reactions and motivation for their study has already been sketched at the beginning of this chapter: here we shall only be concerned with reactions and mechanisms in which dissociation of the reactant molecule(s) (Ha, D2, p-H2, o-D2) takes place. Of the many attractions in the study of these reactions not the least is the fact that reactants and products are to all intents and purposes chemically equivalent, so that for example the free energies of chemisorption of dihydrogen and dideuterium are so little different that one may assume equal coverages at equal pressures. Ortho- and parahydrogen of course lose their distinction after dissociative chemisorption. This family of reactions (Table 1) proceeds with extreme facility: activation energies are low, being often in the range 10-20 kJ mo1-1 [2], and reactions can therefore be followed over a wide range of temperature, and certainly down to 80K. The earliest mechanistic proposal for the hydrogen-deuterium reaction was advanced by Bonhoeffer and Farkas: it was the fairly obvious sequence of dissociative chemisorption, followed by random recombination: H2 +
2* ~
2Hads ---) 2HD + 4 *
D2 + 2 .
--~
2Dad~
The first doubts concerning this simple mechanism arose when it was realised that these equilibrations could proceed under conditions of low temperature where hydrogen and
457
Reactions of hydrogen and alkane-deuterium exchange
deuterium atoms were apparently strongly bonded [2]. Rideal therefore suggested an altemative mechanism, namely a radical chain between molecules and atoms:
~D, H ,
+
9
+
D2
~ ss
,,'" ,"
"'-,",
~
D,
+
HD
+
x
H
D
Evidence for the existence of the necessary vacant sites was provided by statistical calculations on the pairwise occupation of adjacent sites, assuming immobile adsorption. This mechanism was further modified by Couper and Eley [23], who proposed that the triatomic transition state might be accommodated on a single atom, thus: S
D
s
9 %
H%
s
D
9 f
This removes the necessity for vacant sites, which are in any event improbable except at the lowest temperatures because of the ready mobility of hydrogen atoms. Very determined efforts were made to resolve these mechanistic dilemmas through the measurement of orders of reaction; these efforts were reviewed in 1962 [2] and finally by Rideal himself in 1968 [41 ]. The first principle of scientific logic may be stated as follows: when two or more intelligent persons inspect the same facts and draw two or more different conclusions, it is most probable that all are partly right, and partly wrong. This goes for politicians too. One of the faults of mechanistic discussion in the field of heterogeneous catalysis has been the desire to force all observations to support one or other proposed simple scheme. As R.W.Emerson remarked "Foolish consistency is the hobgoblin of little minds". Thus it seems most likely that all of the proposed mechanisms may operate under some circumstances of temperature, pressure or catalyst type. Indeed several of the most recent papers concerned with pure metals [43-47] (all more than 20 years old!) support the view that different mechanisms operate in different ranges of temperature. On nickel [44,46,47] and on platinum [45] it seems that at low temperature (< 300K for nickel and < 200K for platinum) a mechanism of the Rideal-Eley type operates, whereas at higher temperatures the Bonhoeffer-Farkas mechanism adequately accounts for the results. Other types of experiment have been performed to adduce evidence for the sorts of mechanisms engaged in by hydrogen species at metal surfaces. Hydrogen atoms placed on a gold surface by decomposition of formic (methanoic) acid do not exchange with gaseous
458
chapter 10
dideuterium [48], although atoms deposited on gold by gas plasma do [ 49]. Hydrogen atoms deposited on copper and on gold by dissociation of molecules at a heated tungsten wire also exchange with dideuterium [50]. Although hydrogen deuteride is formed in the thermal decomposition of mixtures of SnH 4 and SnD 4, none appears when SnH4 is decomposed in the presence of dideuterium [51 ]. This seems to indicate the absence of the Rideal-Eley mechanism. However, it is probable that on these low activity catalysts the rate of exchange is greatly influenced by the concentration of surface defects; it would in any event be dangerous to extrapolate or generalise these observations. Throughout the foregoing discussion on mechanism there has been an unspoken implication that all adsorbed hydrogen or deuterium atoms are energetically equal: it may however be, as with George Orwell's animals, that some are more equal than others. Experiments have been performed to compare the fraction of adsorbed hydrogen desorbable with that exchangeable with dideuterium, as a function of coverage [2]. Thus for example on a nickel film at room temperature only 30% can be desorbed, while the whole can be exchanged. This might seem to support the idea of the Eley-Rideal mechanism, but the observation is also explicable by decreasing in the strength of the hydrogen-metal bond as the surface coverage increases, due to repulsive interactions between adsorbed species [2]. This means that, provided the adsorbed atoms are exchanged amongst themselves, and are energetically equivalent at high coverages, all may be sufficiently weakly adsorbed to exchange readily by any mechanism. There is however considerable evidence to show that in some circumstances a part of the adsorbed hydrogen is unexchangeable, or only exchangeable with great difficulty, and such species may constitute a poison for reactions such as atom recombination [11,52] or isotopic equilibration [2]. Where such species may reside is unclear, but they are perhaps deeply embedded in the surface, or (in the case of nickel) even dissolved. It is said that there is no such thing as a perfect world, and it is always the good that suffer. Similarly there is no such thing as a perfect catalysed reaction, that is, one that is perfectly suited to the evaluation of catalytic activity. Due to its small size and the spherical symmetry of its ls orbital, the hydrogen atom can happily occupy a variety of types of site on a metal surface, it can diffuse, migrate, dissolve, spill over and generally behave in a most undisciplined manner. For this reason it is difficult, perhaps impossible, to connect the measured rate of reaction and the associated kinetic parameters (order of reaction, activation energy, etc.) with any one specific elementary process. Work in more recent times has revealed how detailed aspects of surface structure (e.g. roughness) can affect rates of equilibration processes [53,54]. Steps on single crystal platinum surfaces were found to be many times more active for hydrogen-deuterium equilibration than the compact f c c ( l l l ) surface, although whether the very large factor claimed [53] is actually valid is uncertain in view of later work, which has been reviewed by Thomson [54]. Wherever the precise truth lies, the fact remains that this simplest of all reactions, which is
Reactions of hydrogen and alkane-deuterium exchange
459
facile in terms of its ready occurrence and small energy requirement, is indeed structuresensitive. Because of the convenience and imagined simplicity of the isotopic equilibration and para-hydrogen conversion it has been the natural choice of many workers for assessing the effects of alloying on catalytic activity. Curiously enough, only quite few systems have been examined: the nickel-copper system on several occasions [33,52,55,56], and alloys of palladium [23,55,57,58] represent most of the work reported. Somewhat discordant results have been reported for the nickel-copper system. With foils, Rien~cker and Vormum [2,55] found rates of para-hydrogen conversion that decreased only slowly with increasing copper content, and activation energies of about 2025 kJmo1-1. With nickel-copper plates [33], rates of hydrogen-deuterium equilibration declined smoothly with increasing copper concentration, but with films [56] at 233K there was an initial sharp drop, followed at more than 20% copper by a slower one, such that in this latter range the rate per nickel atom was independent of copper content. Rates of parahydrogen conversion on nickel-copper powders at 77K were independent of copper content between 5 and 92%, these rate being 103 times less than for pure nickel, and ten times greater than for pure copper [52]. This last work demonstrated very clearly the important role of strongly retained hydrogen as a poison for this reaction. The experiments performed by Couper and Eley [23] on para-hydrogen conversion on palladium-gold wires appeared to support the collective electron model as the basis for understanding "the electronic factor in catalysis" (figure 1). 8
!
6 o
o o
4
5-5
(
A
2
E
,,,,.
E -2 0
6"01
.
-6 -8
S-0
o
m
"
4.5
(a) 9 ~
0 50 ~00 Y, Au in Pd- Au alloy
4.0
-
0
"~
(b)
60 9.
.
1
5
.
.
.
I
1
I0 IS E (kcal. mole"~)
20
Figure 1 Para-hydrogen conversion on palladium-gold alloy wires: a) dependence of activity on composition; b) compensation effect: the composition (% Au) to which each point refers is indicated.
460
chapter 10
These workers [23] also observed a sudden two-fold increase in activation energy when the gold content rose above 40%, which they connected with the filling of the palladium 4d band with 6s electron supplied by the gold. This work has often been cited in support of this theoretical approach: it is unfortunate that it can no longer be sustained (see chapters 1, 3 and 9). Alloys containing about 30% gold are somewhat more active than pure palladium and this observation [2] has been used to explain the similar maximum found with hydrogen atom recombination [29]. It is surprising that this important piece of work has never been repeated. The reaction has also been studied using palladium-silver wires [57] and foils [58]. The results obtained with palladium-silver wires (figure 2) are not dissimilar from those obtained with the palladium-gold alloys, although the activation energy does not exhibit such an abrupt jump and there is no activity maximum.
r
0
figure 2 Comparison between heights of Fermi surfaces (o: arbitrary zero) and activation energies for para-hydrogen conversion (o) as functions of composition in the palladium-silver system.
// 50
100
atomic % silver
The position of the Fermi energy with regard to the top of the d-band, a parameter referred to as 'height of Fermi surface' was estimated [57]. It is very interesting to see that this parameter is closely correlated to the activation energy of the para-hydrogen conversion (see figure 2). The palladium-silver system is discussed in chapter 3 (see the magnetic and EXAFS results) and thus we know that in this system the occupation of the palladium d-band changes by alloying, although the electrons which increase the occupancy of the d-band are not taken from silver. Alloying suppresses the d-d electron interaction, and thus makes the band narrower and more fully occupied when its top level shifts from above the Fermi energy to a position under it. The results in figure 2 indicate strongly that there could be a correlation between the activation energy and either (i) the
Reactions of hydrogen and alkane-deuterium exchange
461
occupancy of the d-band, or (ii) the d-d electron interaction, the latter influencing the metal-hydrogen bond strength. There is a strong correlation between the rate of hydrogen atom recombination (see figure 3) and the metal-hydrogen bond strength, and owing to the similarity of the two reactions the likely, common rate-determining step (rate of desorption) we can expect such correlations also for para-hydrogen conversion. 50--
WI
i
....
I -"
i
'~-
?
figure 3 Relative efficiencies of metals in hydrogenatom recombination as a function of their initial heat of adsorption of hydrogen; various results from the literature [21.
T O
E 40
8 i
:~ 30
Ni
_
I
20
_
!
!
I
.
I
_!_
!
5O 60 70 80 90 Relative efficiency in H atom recombination
100
One of the curious features of both the wire systems is that the Arrhenius parameters show a negative compensation effect [2] as can be seen in figure 1, although the points there are too scattered to make a definitve conclusion. With foils [58], the rate is initially constant but falls at about 40% silver to another plateau, before decreasing yet again (see figure 4). The first step is accompanied by a change in compensation line. 19
9 ,c-
'
L~O
O
%
17-
oo'o
i
E u
u
.o
15-
0
\\
\
13-
L.
O
8\
\0
o
E
figure 4 Dependence of rate of parahydrogen conversion on composition of palladium-silver alloys o: [571 9: [581.
9
11-
En O
() I
0
25
I
I
75
50 At. %
Ag
100
462
chapter 10
Work with platinum-gold alloys [42,59] is complicated by the existence of the miscibility gap, and the cleansing and surface roughening produced by an oxygen pretreatment; according to [60], the platinum-copper system also shows a miscibility gap. Palladium-copper foils behave similarly to nickel-copper foils [55]; it is interesting to note that ordered alloys of copper with platinum [60] and palladium [55] show almost exactly the same rates as their disordered counterparts, thus indicating the structure-insensitivity of the reaction (see figures 5-7). i
*
i.
,
8
I
+0/
O
-r,., ~
0
6
rJ-%9'
"a
E
67-Sfo 83-$
E ""
~" >~ - - I "~
!
!
~o~
4
m't
-~
o
u
a-2-
/ 9 83-5
2
0
(b)
(a) -3 0
50
I00
,
,~
0
i
a
E f, kcaL mole 1)
% Cu in P t - C u alloy
figure 5 Para-hydrogen conversion on platinum-copper alloy foils a) activity as a function of composition b) compensation effect: the composition (% Cu) to which each point corresponds is indicated: o: disordered; o: ordered alloys !
2-0,
T
%
4-5
9
i i
"
~U
u 0 I
7.
3~ .>_
I-5
'd
I-0
u o 0
0"5
4.0
E
(a) 0
s 5O
3-5
I0
3.0
eTo
100
% Cu in Ni-Cu alloy
figure 6 The same as in figure 5 as for copper-nickel alloy foils
4"
(b) I
6
,
l
8
,
I
10
(kcaL mole"1)
I
12
Reactions of hydrogen and alkane-deuterium exchange
.-.
T
E
z.5,
I
2-0
o O m T.
.E.
E~
463
5-0 i'0
I
g,,,
_ ds~
p~3
0
._~ u O
.-~ 3.0 -I-0
(a)
O
!
0 figure 7
50
100
% Cu in Cu-Pd alloy
2
0
.
2
0
~
46 8 I012 E (kcal. mole-1)
The same as in figures 5 and 6 as for palladium-copper alloy foils.
It is not easy to evaluate this work, most of which was performed before 1970, i.e. before surface analytical techniques became routinely available. What is certain is that many of the surfaces used must, by today's standards, have been extremely dirty and only in one or two instances is information on surface composition available. It is also clear that, with powders at least, strongly retained hydrogen acts as a poison. Added to these uncertainties, the results were discussed in terms of the inappropriate collective electron model. There has been no systematic attempt (see our attempt in the comments to figure 2) to re-interpret this work in terms of current theory, and because of the uncertain quality of the experimental work the effort may not be worthwhile. Modem theory would however suggest that explanations should be sought in terms of surface composition, i.e. taking account of surface segregation and of the size of the ensemble of the active atom, for although these reactions are traditionally classified as structure-insensitive there is evidence (see above [53,54]) that under certain conditions steps and defects can exhibit higher activity and there is no general proof that single atoms can constitute an active centre. Indeed with the ruthenium-copper system [61] it has been claimed (although denied by others, see chapter 8) that an ensemble of several ruthenium atoms is needed for the dissociative adsorption of a hydrogen molecule. The statistics of occurrence of ensembles of various numbers of active atoms have been calculated by several authors ([62]; see also chapters 8, 9 and 13), and these may be applied in the re-interpretation of the results summarized above. A further complication is the possibility, for which there is some evidence (see chapters 9 and 13), that sites comprising two or more atoms of a different kind may also demonstrate activity sometimes higher than that of either component. All the old work ought actually to be repeated; the topic is promising enough. For example, a comparison of nickel-copper alloys with palladium-silver or palladium-copper alloys would
464
chapter 10
be very desirable for catalytic as for the heat of adsorption results. Extensive measurements of hydrogen equilibration have been performed with rare earth metals and their alloys and intermetallic compounds with copper by Zhavoronkova and her colleagues [63]. The compounds have higher catalytic activity than the individual components.
10.3
Exchange of alkanes with deuterium [2,64-66]. We move now from reactions where both reactants and products are to all intents
and purposes chemically identical to the next stage of complication in which the two reactants are chemically different but the products are the same as the reactants, except for their isotopic composition. Of course the small differences in energy between X-H, X-D and X-T bonds lead to tiny effects on physical properties and chemical reactivity of molecules that contain them, and useful conclusions can sometimes be drawn from kinetic
isotope effects, that is, the change in reactivity produced by isotopic substitution [67]. In this section however we shall be chiefly concerned with the catalysis of the exchange of alkanes with deuterium over metal and alloy catalysts. Unsaturated hydrocarbons (alkenes, alkynes, aromatics, etc.) that can undergo hydrogenation at the same time as exchange, although sometimes under somewhat milder conditions, will be treated in the next chapter. The exchange of other saturated molecules (alcohols, amines, etc.) with deuterium has also been studied and there have been a few investigations of the exchange of organic molecules with heavy water (D20) or tritiated water (T20/HTO in H20 ), catalysed either by heterogeneous catalysts or by metal complexes in solution [68,69]. The specific labelling of complex organic molecules by tritium is of growing importance in biological and medical research, enabling the fate of such molecules in living systems to be followed. Consideration of these interesting matters is however beyond the scope of this book. Investigation of the exchange of alkanes with deuterium opens the way to revealing how the electronic and geometric structures of elements and their alloys determine reactivity, in a manner not clearly shown by the reactions of hydrogen described above. These proceed too readily to provide a diagnostic test for reactivity [70]. The activation of alkanes by metal surfaces is much more difficult than that of hydrogen, so that reactions such as isotopic equilibration that require their dissociative chemisorption discriminate more precisely between metals on the basis of their activity. It is however not only the activity of metals that is exposed: we can now differentiate species that the alkane can form by its chemisorption [71]. This kind of information will be of immense help in understanding a wide range of phenomena involved in reactions of hydrocarbons and other compounds on metals and alloys. To great advantage this information can be compared
465
Reactions of hydrogen and alkane-deuterium exchange
with that obtained by various spectroscopies [64]. It has been known since the time of the earliest work on alkane exchange [72] that these molecules can be activated for adsorption by dissociation of a C-H bond and the gaseous molecule can be re-formed by recombination of the fragments. If this occurs in the presence of deuterium, isotopic substitution can occur, e.g. CH 4
----) CH3* + H* CH3D + HD
D2
~ 2D*
Subject to the influence of zero-point energy differences, the forward and reverse processes will proceed randomly, and through reiteration of these reactions an equilibrium will ultimately be attained. The position of equilibrium will be governed by simple statistics, modulated by effects of isotopic substitution: equilibrium constants can be calculated by methods of statistical thermodynamics where all the required basic information is available, sometimes with great accuracy, and it is a useful check on the accuracy of the analytical method to measure concentrations at equilibrium and then to compare them with theoretical values. To a first approximation, that is, ignoring isotopic effects, the composition of an equilibrium equiatomic mixture of methane and deuterium will be CH 4 1
6.25%
CH3D 4
25%
CH2D: 6
37.5%
CHD 3 4
25%
CD 4 1
6.25%
The relative proportions correspond of course to the coefficients of binomial distribution, and the general expression for the binomial expansion (a + b) n can be used to calculate approximate equilibrium concentrations for any number of exchangeable hydrogens and any hydrogen:deuterium ratio. There are several surprising aspects about the dissociative chemisorption of alkanes. (1) C-H bonds break more easily than C-C bonds although they are stronger: thus for example in ethane the C-H bond strength is 401kJ mol -I while that of the C-C bond is only 347kj mo1-1. (2) The C-H bond breaks much less readily than the H-H bond, although the strengths are about the same. (3) Methane is much less reactive than ethane, although the C-H bond strengths differ only a little (in methane it is 423kj mol-1). To understand fully the causes of these observations would require a deep knowledge of bonding theory, but two simple concepts allow some appreciation of the situation. First, in methane and higher alkanes the carbon atom and the C-C bond is effectively shielded by the hydrogen atoms, so that the molecule cannot approach the surface closely enough for C-C activation to occur. The C-H bond is less shielded, so that this is the bond that breaks, even although
466
chapter 10
it is the stronger (see chapter 13). Secondly, ethane and higher alkanes show a stronger interaction with the surface through weak
H...M bonding, such as is responsible for
physical adsorption and hydrogen-bridge bonding. With methane only the bond to be broken can so interact, so its residence time is shorter and its chance of activation is thereby lowered. Figure 8 shows in histographic form the minimum temperatures at which methane, ethane, propane and cyclohexane react with deuterium on a number of metals. Methane is always the least reactive, and cyclohexane the most. Too much should not be read into the differences in activity shown by the metals, e.g. in Groups 4-6, as it is likely that the more active have become the least active in consequence of self-poisoning by carbon deposition and carbide formation. "The last shall be first, and the first last". Zr
V
Ta Cr Mo W
Rh Ni
Pd Pt
300 A CHL + D2 ,.--..
%_
200
100
x
C2H 6 + D2
0
C3H 8 + D 2
O C6H12 § D 2
0
-100
figure 8 Activity of metals in various hydrocarbon-deuterium exchange reactions. Temperature range in which a particular reaction has observed is shown for individual metals (indicating the reactivity of the system).
The exact energetic modes needed for dissociative chemisorption (e.g. vibrational, rotational or translational in the molecule; vibrational in the solid, i.e. lattice phonons) is now the subject of research by the techniques of molecular dynamics. In studying isotopic exchange, the composition of the initial products is of immense interest, because here the reaction is under kinetic control, and in fact very often the relative yields of products do not correspond to the equilibrium distribution. Reactions are conducted with a considerable excess of deuterium, so that the change of the products hydrogen deuteride and dihydrogen reacting again are minimised [73]. In particular it has been observed [2,66] that highly or completely deuterated molecules arise in the initial products, simultaneously with those formed by repetitive stepwise exchange of a single hydrogen atom at a time. The former process is termed multiple exchange (see below).
Reactions of hydrogen and alkane-deuterium exchange
467
The mechanism by which this occurs (with higher hydrocarbons than methane) is most easily understood by reference to ethane [74]. Whereas for stepwise exchange it is only necessary to postulate an ethyl radical as an intermediate: C2X6
~
C2H5 +
X
(single exchange)
(where X = H or D), for multiple exchange a more dissociated form is proposed: -X
Sx6
+•
-X
(multiple exchange)
x c*
+•
*
,
Repeated interconversion of the ethyl radical and the 1,2-(or o~13-) diadsorbed ethane can lead to complete substitution of all the hydrogens before the final act of desorption occurs. This mechanism, which is reminiscent of the old Horiuti-Polanyi mechanism for the hydrogenation of alkenes [2], has general applicability to linear alkanes and similar molecules. Clearly it cannot work for methane, so instead a methylene (CH2=*) intermediate was suggested, schematically: -• CX4 ~
CX 3 ~
CX 2 +X ~-~
The possibility of even more dehydrogenated intermediates (methyne, and carbon atoms) has been canvassed, but the thermochemistry is apparently against their extended formation. The tendency to form multiply-exchanged products is signaled by the initial value of the mean number of deuterium atoms M entering the products. Published results [2,74] need to be interpreted with care, however, first, because the temperature may not always be the same: multiple exchange has the higher activation energy, so that less active metals such as palladium and platinum may show high values of M in exchange of ethane and higher alkanes with deuterium because of the necessarily high temperature that is used [74,75]. On the other hand, the occurence of multiple exchange has also been attributed to the use of unsintered films having highly defective surfaces [76]. Secondly, of course effects due to carbon deposition and incorporation into the surface must always be expected. In general the base metals iron, cobalt and nickel, and the first triad of noble metals (ruthenium, rhodium [77], palladium) give extensive multiple exchange of higher hydrocarbons, whereas iridium usually, and platinum sometimes [71,76] give chiefly stepwise exchange. This is also the main reaction with tantalum, molybdenum and tungsten, perhaps because of their high activity: all are active below 273K [74].
468
chapter 10
There is however no comprehensive and satisfactory model for interpreting all these observations. Clearly in most cases there are two parallel processes operating, in one of which the adsorbed alkyl radical has a high tendency to revert to alkane and in the other of which it prefers to lose a further hydrogen or deuterium, thus entering the maelstrom of multiple exchange. The behaviour of methane does not however always mimic that of the higher alkanes, because ability to form ao~-diadsorbed species does not run parallel to that for otl3-species. Other bases for understanding the results have been advanced [78,79], but the fact remains that the corpus of information is very incomplete and the results available cannot be with confidence attributed to any particular feature of the catalytic metal. Recent studies of the exchange of neohexane (2,2-dimethylbutane) [77] and of other alkanes and cycloalkanes [77,80] with deuterium on various rhodium catalysts, supplementing earlier work on palladium and platinum catalysts [81], demonstrates how complex and indeed structure-sensitive are these reactions. Variation of particle size affects turnover frequencies by quite large factors, and exchange mechanisms as well: the type of pretreatment and presence of residual hydrogen also produce major effects. It is postulated that no less than eight distinguishable active centres have to be operating in order to understand the results. This seems rather a lot and the passage of time may bring some simpler (and therefore better) model. We may usefully recall the pre-Keplerian model for the motion of the planets, with its cycles, and epicycles: but we must remember too the words of Einstein: " We must make the truth as simple as possible, but not simpler". Thus although alkane-deuterium exchange has great potential as a reaction for characterising metal catalysts at the present time, its sensitivity exceeds our ability to control and interpret the factors responsible for deciding their physical structure. Although multiple exchange in methane is diagnostic for formation of carbon-metal multiple bonds, it is of interest to see whether other alkanes can demonstrate the same ability. Clearly ethane may but does not have too. A molecule which shows the necessary character and on which much work has been done, is cyclopentane: on many metals it readily exchanges up to five hydrogen atoms, these clearly being those on one side of the ring, and exchangeable by repeated interconversion of mono- and ~fS-diadsorbed species. However, unlike cyclohexane, which is sufficiently flexible to form a staggered a13 species by which exchange may be propagated from one side of the ring to the other, the ring in cyclopentane is not large enough to permit this. Nevertheless on certain metals, particularly palladium, multiple exchange giving up to cyclopentane -d~0 is observed, the activation energy being higher than for stepwise exchange. It is believed that one of the intermediates that allows exchange to move to the other side of the ring is an ota-diadsorbed species (schematically):
C
C
.
i
I
C,
/C C
II a~
Reactions of hydrogen and alkane-deuterium exchange
469
and indeed there are similarities between multiple exchange in methane and in cyclopentane [64,82]. Exchange of cyclopentane has also been much used by Sachtler et al. to characterise the behaviour of metal particles in zeolites [83,84]. It was Kemball who originally noticed [66] a correspondence between activity in multiple exchange and in hydrogenolysis. This led to the idea that multiple carbon-metal bond formation was responsible for hydrogenolysis, a concept confirmed by the work of Anderson [64], Van Broekhoven and Ponec [85] and many others. The utility of the idea will be seen in chapter 13. The c~o~-and c~B-diadsorbed intermediates so far discussed are not adequate to describe exchange in alkanes containing quaternary carbon atoms. Thus in neopentane, exchange in one residence of more than one hydrogen necessitates postulating an c~7species:
\ / ci-h
/ C
cI%
\ h
In a molecule such as 2,2-dimethylbutane, up to five hydrogens can exchange in one residence, but the requirement to form the my-species, which is more difficult than forming the o~ot-species, inhibits propagation of the exchange past the quarternary carbon atom [77]. Likewise a hetero-atom, such as oxygen (e.g. in diethylether), confines multiple exchange to one end of the molecule. Much midnight oil has been burnt in discussing the fine detail of these intermediates and their interconversion, and much ingenuity has been exercised in selecting and designing molecules the exchange character of which will delineate more clearly what intermediates can be formed under what conditions. Thus for example on a palladium film cis-l,2,3,4-tetramethylcyclopentane exchanges twelve hydrogens, but the 1,1,3,3-isomer no more than two [2]. In bicyclo [2.2.1] heptane only two hydrogens are exchangeable at a single residence. The interested reader may wish to employ his favorite molecular graphics package to understand the reasons for these findings. Although mass-spectrometry is the traditional and indeed inescapable analytical method, the use of microwave spectroscopy and of nuclear magnetic resonance (NMR) to identify positions of substitution in complex molecules such as 3,3'-dimethylpentane has added an extra dimension to the work. It is still quite unclear why adjacent metals can behave so differently as catalysts for the exchange of complex molecules: each metal shows a characteristic 'fingerprint' through a preference for forming a particular intermediate [71]. Unfortunately the chemical rules governing metals' behaviour have not yet been elucidated, nor has the enormous body of knowledge of organometallic chemistry yet been of assistance. Much work remains to be
470
chapter 10
performed in this field. Alkane-deuterium exchange has been used to assess diffusional processes occurring within microporous catalysts, and to assist the understanding of hydrogen transfer steps under petroleum reforming conditions [86]; in this latter work, the exchange of atoms between n-octane-d~8 and methylcyclohexane-d0 has also been followed. We may move on now to discuss work on alkane exchange with alloy systems. Once more it is necessary to emphasize that much of it is now quite old, and questions arise over surface composition and cleanliness, as well as the proper theoretical framework for interpreting the results. One can detect several distinct motives for the performance of this work, as indeed for that on other catalytic systems. Initially, confirmation was sought for the results of the type described in the last section which appeared to support the collective electron model for alloy catalysis. Later, interest moved to the kinds of adsorbed hydrocarbon species formed on metals, so that work on alloys was enterprised in the expectation, or at least the hope, that through the operation of ensemble size effects or ligand effects (see chapter 8) it might become clear what structures acted as intermediates in particular reactions. The aspirations have only been partially fulfilled, but the work so generated has enriched the catalytic literature and broadened the base for the understanding of reaction mechanisms. What has been reported in the literature may be divided into two classes: (i) studies of alloys of a Group 10 metal with a Group 11 metal which by itself has minimal activity (e.g. copper-nickel [73,87-90], palladium-gold [67,91,92], platinum-gold [93] and rhodiumsilver [94]) and (ii) examination of systems comprising two metals from Groups 8-10 showing different activities or reaction characteristics (for example, palladium-platinum [76,95], palladium-nickel [82] etc.). In the former class, the main effect to be expected is the lowering of the mean size of the ensemble of active atoms, but in the second class the formation of active sites of bimetallic ensembles is a distinct possibility. The reaction of cyclopentane with deuterium on nickel-copper films [88] is accompanied by self-poisoning, which decreases in importance as the copper content rises: hydrogenolysis is only shown by nickel above 340K, and the effect of copper content on the exchange is quite small. It is therefore clear that exchange can manage with a smaller group of nickel atoms than the destructive reactions. With silica-supported nickel-copper [73,96] rates of both stepwise and multiple exchange were suppressed by increasing the copper content, the latter somewhat more than the former. Orders of reaction and apparent activation energies were recorded and true activation energies were derived by the Ternkin equation: E a = E t - n Qi where n is the order of reaction in deuterium and
Qi its isosteric heat of adsorption.
Reactions of hydrogen and alkane-deuterium exchange
471
Values of E t were about 47 kJ mo1-1 for stepwise exchange and 63 kJ mo1-1 for multiple exchange. The results were interpreted according to a model proposed by Frennet and coworkers [97] in which the initiating step is the collision of a methane molecule with an adsorbed deuterium atom surrounded by a specified number of vacant sites, thus:
CH4
+
D
+ 3"
~
[CH4D]
~
H
+
3"
****
Multiple exchange is supposed to require complete dissociation of the molecule into carbon and hydrogen atoms, this needing a cluster of five nickel atoms: the numbers of nickel atoms composing the active centres for these processes was deduced from the variation of their rates with copper content. The mechanism was not affected by addition of copper. The validity of this interpretation depends on the propriety of using random statistics, i.e. on assuming that nickel atoms are randomly dispersed on the surface, and that the surface composition is the same as that of the bulk. Neither of these assumptions is likely to be true in practice, nor indeed it is clear how the above mechanism might explain the ~13 mechanism in higher alkanes, or the behaviour of even more complex molecules [78]. Studies have been reported of the exchange of methane with deuterium on palladium-gold films on mica [91] and on powders formed by reduction with sodium borohydride (tetrahydridoborate) [67]. The exchange was substantially [91] or exclusively [67] stepwise. With the latter catalysts the rate showed a slight maximum at 90% palladium, but fell precipitately between 50 and 40% palladium. Other alkanes (propane, n-butane and n-pentane) showed similar behaviour on the films [91], as did neopentane, the exchange of which was also stepwise at lower temperatures. Two groups have examined cyclopentane-deuterium exchange on films [91,92]; both detect a maximum in multiple exchange activity at some point in the series, but a minimum in roll-over selectivity was found at 12% gold [91]. It was speculated that the species responsible for roll-over [64,82] may also occur during alkene hydrogenation. Information on this reaction has also been reported for platinum-gold [93] and palladium-tin [92] films, and the methane-deuterium exchange has been studied on Rh-Ag/SiO2 catalysts by a transient response method [94]. Exchange of methane and of cyclopentane with deuterium has been described for a number of alloys formed from metals within Groups 8-10 [67,76,82,95,98-100]. The observations can be summarized as follows. (1) An activity maximum is sometimes shown, usually near the centre of the composition range (CH4, Pd-Ru [67]: CH4, Pt-Rh [95]). This is a phenomenon often encountered with alloys of this type, and other examples will be met in the following chapter.
472
chapter 10
(2) Where one metal has a much greater propensity for one type of reaction (e.g. roll-over over cyclopentane, multiple exchange of methane) this is rapidly suppressed by addition of the other element, the conclusion being that these processes require larger ensembles of the active element than alternative less demanding processes (i.e. one-side and stepwise exchange) (CH4, Pd-Rh [95]; cCsH12,Ni-Pd [82]). (3) Compensation effects are met with when the Arrhenius parameters are reported [67,76]. Surface structures in alloys of platinum with iridium [99,100] and with rhenium [101,102] have been assessed by means the exchange of cyclopentane and of neopentane with deuterium.
References
9 10 11 12
13 14 15 16 17
N.N.Greenwood, A.Earnshaw, "Chemistry of the Elements", Pergamon, Oxford, 1984. chapter 3 G.C.Bond, "Catalysis by Metals", Academic Press, London, 1962 A.Guerrero, M.Reading, Y.Grillet. J.Rouquerol, J.P.Boitiaux, J.Gosyns, Z.Phys.D. 12 (1989) 583 P.A.Sermon, G.C.Bond, Catal.Rev. 8 (1974) 211 "New Aspects of Spillover Effect in Catalysis" (editors: T.Inui, K.Fujimoto, T.Uchijima, M.Masai), Stud.Surf.Sci.& Catal. 77, Elsevier, Amsterdam 1993 "Hydrogen Effects in Catalysis" (editors: Z.Paal, P.G.Menon) Dekker, New York, 1988 F.A.Lewis, Plat.Met.Rev. 38 (1994) 112 J.Philpott, D.R.Coupland in "Hydrogen Effects in Catalysis" (editors: Z.Paal, P.G.Menon), Dekker, New York, 1988, chapter 26 B.J.Wood, H.Wise, J.Catal. 5 (1966) 135 V.M.Gryaznov, M.G.Slinko, Faraday Disc.Chem.Soc. 72 (1981) 73 W.Palczewska, A.Frakiewicz, Z.Karpinski, Bull.Acad.Pol.Sci. 17 (1969) 687 P.B.Wells in "Proc.Symp.Electrocatalysis" (editor: M.W.Breiter), The Electrochem.Soc.Princeton, N.J. 1974, p. 1 A.G.Burdon, J.Grant, J.Martos, R.B.Moyes, P.B.Wells, Faraday Disc.Chem.Soc. 72 (1981) 95; see discussion on this paper, pp.173-179 M.Shamsuddin, O.J.Kleppa, J.Chem.Phys. 71 (1979) 5154 A.Maeland, T.B.Flanagan, J.Phys.Chem. 69 (1965) 3575 K.Allard, A.Maeland, J.W.Simons, T.B.Flanagan, J.Phys.Chem. 72 (1968) 136 S.Kishimoto, N.Yoshida, T.Tanaka, T.B.Flanagan, J.Chem.Soc.Faraday Trans.I 85 (1989) 1787 F.A.Lewis, W.H.Schurter, Naturwiss. 8 (1960) 177
Reactions of hydrogen and alkane-deiaterium exchange
18 19 20 21 22 23a b C
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76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97
98 99 100 101 102
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477
Chapter 11
CATALYTIC H Y D R O G E N A T I O N AND D E H Y D R O G E N A T I O N
11.1 11.1.1
Hydrogenation of alkenes General principles The addition of hydrogen to the carbon-carbon double bond is one of the simplest
catalysed reactions in which the products are plainly different from the reactants, and the hydrogenation of ethene, the smallest alkene, is therefore the archtype of a great family of reactions, some of which have great industrial importance. For this reason it has been much used (perhaps too much?) by those wishing to demonstrate the effects of some variable of the type of catalysts under study on an easy yet significant reaction. Like many other catalysed reactions, it was first observed by the grandfather of catalysis, Paul Sabatier, and it has been the continued object of systematic and quantitative study for three-quarters of this century [1-4]. There is no scientist of any stature, working in the field of catalysis by metals, who has not at some time hydrogenated ethene. Part of its attractiveness lies in its apparent but deceptive simplicity. It appears capable of yielding only a single product - ethane - and thus of being studied for example by change of pressure in a constant volume reactor: and to a certain level of sophistication this is true. But as with so many other catalytic systems, the more one delves into it, and the more refined and revealing the analytical methods employed, the more one comes to realise that this simplicity is in fact, like beauty, only skin-deep, and that there are countless complications and ramifications to this nominally straightforward reaction. Let us now try to set down in a systematic and logical manner what some of these are. We are concerned in this section only with molecules that contain a single, nonconjugated carbon-carbon double bond, with its • and rt components: molecules containing two or more such bonds conjugated with each other or with some other unsaturated function will be dealt with later. We do however include here linear, branched and cyclic alkenes of any size. The first and major complication, to which we shall return, is the question of 'carbon deposition', i.e. the formation on the surface of 'hydrocarbonaceous residues' that, in the case of gas-phase reactions at least, may rapidly cover a substantial fraction of the active surface. Of such importance is this effect that it will be treated in some detail in a later section (11.1.2). Secondly, in addition to this parasitic reaction, there are two other
478
chapter 11
processes observable with ethene: these are (a)
homologation and (b) isotopic exchange.
These may be described formally as
and
2C2H 4 + H 2
---)
C4H 1o
C2H 4
----)
C2H3D + HD,
+ D2
the latter reaction being in part responsible for the formation of a range of deuterated ethanes [3,4]. Thirdly, when four or more carbon atoms are present in the molecule, positional and geometric isomers of the monoalkene become possible, and these can interconvert during hydrogenation (see scheme I, below). Of course, the processes of double-bond migration and of cis-trans isomerization can be followed as the alkene reacts with deuterium, and there is considerable mechanistic information in comparing, for example, the deuterium distribution in the three n-butenes formed when 1-butene reacts with deuterium [5]. Fourthly, a further stereochemical facet of the reaction class becomes visible with cyclic alkenes and other appropriately substituted molecules: for example, hydrogenation of 1,2-dimethylcyclohexene [6] may in principle give either cis- or transdimethylcyclohexane depending on whether the hydrogen atoms are added to the same or to different sides of the molecule (see also scheme I). In the following paragraphs these additional processes will be considered further. The observation of the ethene-deuterium exchange reaction [7] was one of the early mechanistic triumphs of the use of deuterium as an isotopic label: hitherto it had been assumed that hydrogenation by the simultaneous addition of two hydrogen atoms was all that was entailed. Horiuti and Polanyi [8] therefore proposed the mechanism (scheme II, below), which bears their name, in which the atoms added consecutively, an adsorbed ethyl radical being the intermediate; for the addition-exchange reaction: ---)
2D
C2H 4 + 2*
---)
C2H4
C2H 4 + D
~
C2H4D ----)
C2H4D + D
---)
C2H4D 2
D2 +
2*
C2H3D + H
In this early work, the exchange was detected by HD in the gas phase formed by desorption of the hydrogen atom released in the exchange, together with a deuterium atom: this was followed by the change in thermal conductivity of the non-condensible gaseous fraction, this being the only analytical technique available at the time. Incidentally, this work was carried out with 'hydrogen' that was only about 30% enriched in deuterium. The
Catalytic hydrogenation and dehydrogenation
479
role of ethyl radicals as the half-hydrogenated state in the mercury-photosensitised ethenehydrogen reaction had in fact been established by H.S.Taylor some years before. Hydrogenation of higher alkenes is accompanied by other reactions which are presented by scheme I. Scheme I Reactions accompanying the hydrogenation of. higher linear and cyclic alkenes CH3
H
CH3 /
C=
C
~
CH3 ~.m,.-'~
H
H
cis- 2- butene CH~-CH2---CH =
H ~
C=
C
~
cis- trans isomerization
CH3
trans - 2- butene CH 2
~
CH~--CH--CH--CH 3
1 - butene
(>(oH3
double-band migration
2- butene H _
CH3
CH 3
H
.
CH3
+
~
N
CH3
CH 3 1,2 - dimethylcyclohexene
cis - 1,2 - dimethyl cyclohexane
trans- 1,2 - dimethyl cyclohexane
The Horiuti-Polanyi mechanism, of which the above steps are only an incomplete statement for the ethene-deuterium reaction, is capable of considerable extension and refinement [9], but is still regarded as containing the essential truth [3,4]. What remain under discussion are questions such as the precise form of the reactive adsorbed ethene molecule and the source of the hydrogen (deuterium) atoms involved in the addition steps. It required the advent of mass-spectroscopy to reveal the true splendour of the ethene-deuterium reaction. It emerged that, with nickel wire as catalyst, all possible deuterated ethenes and ethanes (including C2H6, i.e. ethane-do) were formed by reiteration of the basic steps, as well as dihydrogen and hydrogen deuteride [10]. Indeed, ethane-d o was the major initial product, due to the high concentration of hydrogen atoms that accumulated on the surface through the reactions shown above. Only one hydrogen atom in ethene was substituted for deuterium in a single residence. Similar studies using a series of platinum catalysts [11,12] revealed an early example of a support effect, and showed that alkene exchange was less prominent with platinum than with nickel. Later systematic work by Kemball [13] was rationalised in terms of a quantified description of the HoriutiPolanyi mechanism in which numerical probabilities were assigned to four basic steps (scheme II).
480
chapter 11
Scheme 1-I Quantitative description of the Horiuti- Polanyi mechanism [13] C2x 4
' q 1-2-+H
C2X 5H ~ ~ 1 C2X 4
-X
~ ~r
+H
C2X4H ' ~ ~ 1
P
C2x 4
~~"
C2X5
~m,.-s
C2X 5
~ ~1, - -
C2X 4
~m,.-q
+D
+X
+D
C2x 5 C2X5D C2X 6 C2X4D
It is then possible to write down equations for eighteen simultaneous reactions, since there are six possible ethenes and twelve ethyls, including positional isomers. Calculated amounts of the product ethanes and ethenes are then obtained for any set of probability parameters p,q,r and s, by solving the equations (most easily by matrix inversion). It is interesting to note that more information is available by calculation than by experimentation (e.g. the isomers CH3-CD 3 and CHD-CHD 2 are calculated individually, but cannot be separated by mass-spectrometry). Mechanistic work performed before 1965 has been reveiwed [5,9] ; the reader is referred to the outstanding work of Dumesic and his associates [3,4] for the most recent statements on this reaction. With this last exception, it is remarkable how little further research has been performed on alkene-deuterium reactions since that date; despite their great potential for sensing the surface of metallic (or alloy) catalysts, and the availability of analytical methods even more refined than mass-spectrometry [14-16], the focus of interest has shifted to other matters; and your authors too, like so many others, have succumbed to the temptation to move on to more timely projects. The work of Francois Gault and his colleagues [17,18] revealed new and unsuspected mechanistic richness in the exchange of alkenes over iron and nickel catalysts. Another development of major importance was the application of microwave spectroscopy by Hirota and his associates [15,16,19,20] (see also [21]); this has been subsequently continued by Naito and Nanimoto [22,23]. The technique permits analysis of positional isomers of deuterated molecules, and its scope and power are illustrated by the following examples. (1) Over nickel, palladium and rhodium catalysts, propene-3d 1 (CH2D-CH=CH2) undergoes self-exchange, forming propene-d o and propene-d2; use of microwave spectroscopy established that on all metals deuterium atoms were liberated as 7t-allyl complexes
Catalytic hydrogenation and dehydrogenation
481
were formed (see scheme III), and that these subsequently reacted with n-adsorbed propene to give cy-propyl radicals, whence the exchanged propene molecule was reconstructed [22,23].
Scheme Mechanism of self-exchange of propene--3d 1
CH 2 = CH -CH2D
.--~-I,,--
CH2~ CH-CH2D
+ D
H2C-CH
- CH 2
+ D
..)(.
--x-
f
CH - c H D -CH2D
-.x-
CH2D-CH - C H 2 D
CH2_CH D _ CH2D
-----D--
C H 2 = CH-CH2D
+ D
CH2D-CH -CH2D
~
CHD =CH - CH2D + D
(2) Over rhodium, nickel and iridium catalysts, isobutene (2-methylpropene) reacts with deuterium to form the monodeutero-products (CH3)3CD and (CH3)2C=CHD; this requires different butyl radicals to be involved in exchange and in hydrogenation [20]. 1,2Dideutero-2-methylpropane is the major product obtained by reacting isobutene with deuterium over nickel [24]. (3) Analysis of monodeutero-products from the reaction of propene with deuterium on nickel, platinum, palladium and copper catalysts is given in table 1 [22]. These results counteract the impression that may have been created by the earlier examples that all metals (at least those of Groups 8-10) behave similarly, and form related if not identical intermediates. The analyses shown in Table 1 are initial ones, obtained when the mean number of deuterium atoms in the propene was only 0.01-0.03: in the cases of palladium and platinum, relative amounts of the four products changed as the reaction proceeded due to 'intramolecular isomerization' [23]. These observations are not readily explicable in mechanistic terms, although it is clear that over nickel exchange proceeds by dissociative chemisorption of the C2-H bond, rather than via a n-allyl radical as in self-exchange.
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table 1 Relative amounts of propene-dl, isomers formed initially by the reaction of propene with deuterium [22].
d
Ha
H3CN C=C /
if/
\H
Catalyst
T/K
a/%
b/%
c/%
d/%
Ni
200
2
3
95
0
Pd
210
35
33
16
16
Pt
235
9
57
28
6
Cu
263
14
14
72
0
Ni-Cu
180
11
12
78
0
Pd-Cu
225
20
17
58
5
Pt-Cu
248
13
9
68
10
Note: the alloys were prepared from mixed oxides and contained 60% copper (surface concentrations 70-80% copper)
The latter work [22] provides one of the very few examples of a study of an alkene-deuterium reaction on alloys. With this system also the self-exchange -scheme IIIcan take place [22]. Alloys of copper with nickel, palladium and platinum showed product distributions having distinct similarities to those of copper, although rates were faster than that of copper as seen by the temperatures used in table 1. It may be that, while copper as expected occupies most of the surface due to its smaller surface energy, a few atoms of the Group 10 metal provide a channel by which more hydrogen atoms attain the copper surface than would otherwise be possible. A study of this reaction on Pt-Au/SiO2 has recently been published [251. One other observation merits a brief mention. The interaction of ethene and deuterium on nickel-copper films containing low concentrations of nickel results only in exchange and not addition (i.e. hydrogenation) [26]. It was suggested that isolated nickel could effect exchange. We return now to the matter of alkene isomerization (see scheme I). A moment's thought shows that both cis-trans isomerization and double-bond migration may proceed by addition of a hydrogen atom, followed by abstraction of a different atom (scheme IV).
Catalytic hydrogenation and dehydrogenation
483
Scheme IV Mechanisms of alkene isomerization
Alkyl reversal Cis- trans isomerization" CH3
CH3
CH3 +H
C=C H
CH3 ~
f~ / CH ~ C m
H/
H
H
CH3
H -H ~
.I
~
CH3
/
C=C .I
H
Double - band migration
CH3- CH2
_
CH - -
I
CH2
~
CH3- CH2- CH -CH3
/
-H
~
CH3- CH - -
I
CH - CH 3
Via ~'- allyl intermeditate
1 - Butene
-H ~
H C CH3 ./.5".-- ."N, / H2C ' ' CH
+H ~.m,.-
Cis - and trans - 2 - butene
Alternatively a rt-allylic intermediate could also be effective (see also scheme IV). A detailed study of the interaction of the butene isomers with deuterium over palladium, platinum and iridium catalysts [5,21] suggested that the addition-abstraction route was operative, but there is clear evidence of isomerization of 1-butene to 2-butenes over iron [17] and nickel [18] films without deuterium incorporation, which is only explicable by an intra-molecular hydrogen atom transfer, involving hydrogen abstraction at a certain stage. On the base metals iron, cobalt and nickel, and on palladium and rhodium the formation of rc-allylic species is always a strong possibility. Unfortunately, and inexplicably, there are almost no studies of alkene isomerization on alloy catalysts to report [27,28]. While the processes involved in isotopic exchange of alkenes are of purely academic interest, the corresponding isomerizations do have practical significance, most importantly in fat hardening. The selective hydrogenation of multiply-unsaturated natural oils to a stable product containing on average about one carbon-carbon double bond per hydrocarbon chain requires first the isomerization of non-conjugated double-bonds into conjugation [29]: that is why nickel is an appropriate catalyst, and why both palladium [30] and copper have been considered. The former has failed on cost grounds and the latter because traces of copper ion dissolved from the catalyst catalyse autoxidation of the product. Unfortunately nickel contains the seeds of its own destruction, so to speak, because it also catalyses cis-trans isomerization of the exclusively cis-conformers originally present to the less-desirable trans-isomers (e.g. oleic to elaidic acid groups). What is needed is a catalyst that will hydrogenate and give double-bond migration but not cis-trans isomerization: it has not yet been discovered, although palladium-based catalysts hold out some promise.
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It is important to understand that a metal active in the reactions of alkenes with hydrogen shows essentially the same characteristics irrespective of its physical form, type of support and method of preparation. There may be minor differences [12], but in broad outline the principal metals exhibit a characteristic 'reaction fingerprint' in processes occurring in both gas and liquid phases, with a variety of solvents [5] and in a great range of related molecules. The difference in character between palladium and platinum is particularly clearly marked: with the former, isomerised and/or exchanged alkenes appear abundantly as products, their rates of formation being typically comparable with or even exceeding that of hydrogen addition. With platinum on the other hand, the appearance of altered alkenes is usually slow and subsidiary to hydrogenation. The consistency of the available evidence is quite remarkable and we shall encounter another manifestation of what is probably the same effect when we come to consider the selective hydrogenation of alkynes and alkadienes. It appears that iron, cobalt and nickel, and probably rhodium, share the character of palladium, and iridium resembles platinum. Not much work has been carried out with ruthenium and osmium as hydrogenation catalysts [31]. The difference can be understood in terms of two factors, which may themselves be related: (i) a strong tendency (already mentioned) of metals in the first two rows to form rc-allylic intermediates, and (ii) ready desorption of the adsorbed altered alkene into the fluid phase in the case of these metals. Analogies have been drawn with the stabilities of organometallic complexes contained rt-coordinated alkenes as ligands [5]. We suggest a useful definition of a structure-insensitive reaction or family of reactions is that it shows the same broad reaction characteristics under a variety of circumstances. By such a definition the reactions of alkenes with hydrogen or deuterium fall into this category. 11.1.2 Carbon deposition It is a well-known fact that as soon as any active metal catalyst is exposed to an alkene, or indeed any other unsaturated hydrocarbon or carbon-containing molecule, its surface becomes at least partially covered by 'acetylenic residues' [32] or 'carbonaceous species' or, as we shall call it for the sake of brevity, carbon. The effect has been widely noted and studied [33-35] and its general occurrence is not in dispute: what is uncertain however is what role if any it plays in the mechanisms of alkene reactions. It is not easy to summarise what is known, but certain trends emerge clearly from the literature. (1) At low to moderate temperatures, the 'carbon' takes the form of a somewhat dehydrogenated derivative of the reacting alkene: for example the (111) surfaces of platinum and rhodium are on exposure to ethene quickly covered by ethylidyne (CH 3C-) radicals which have low reactivity [36]. (2) Formation of ethylidyne is suppressed by the presence of hydrogen. At higher temperatures by further dehydrogenation these species polymerize into a two-dimensional layer [37] which may ultimately graphitise [35]. (3)
Catalytic hydrogenation and dehydrogenation
485
They are bonded to the surface by one or more carbon-metal multiple bonds. (4) They are formed more easily and are held more tenaciously by the base metals of Groups 8-10 than by the noble metals, and are more prevalent on metals to the left of Group 8 in the Periodic Classification: there is thus a qualitative correlation between carbon deposition and stability of bulk carbide phases. Note however that carbon can dissolve into palladium in some circumstances [38]. (5) On active catalysts their formation begins well below ambient temperature: Pt/A1203 catalyst suffers rapid deactivation for ethene hydrogenation even at -- 210K [39]. (6) Such deposits are not readily or completely removed by hydrogen treatment even at high temperature: carbon so removed appears only as methane. Oxidation is needed for complete cleansing. (7) Carbon deposition is a damned nuisance. It is as certain as anything can be that the great majority of studies of the reactions of alkenes (and other hydrocarbons) with hydrogen have been performed with surfaces at least partly contaminated by carbon, and it seems possible that in some cases with reactions in the gas phase only a very small fraction of the virgin surface actually participates in the continuing reaction [21]. Even a recent sophisticated study of ethene hydrogenation uses what is admittedly an 'equilibrated' surface [3]. In fact it is difficult to know what else can be done, as there seems to be no way in which carbon formation can be completely avoided. One therefore might as well live with it, because a clean surface is like the proverbial free lunch - there is no such thing - but the consequences of doing so have to be appreciated. For example, it is meaningless to derive a turnover frequency based on any estimate of the total surface and it is rare to find the presumed available area being estimated after
the reaction, which may in any event be difficult. Attempts to
determine structure-sensitivity, i.e. rate dependence upon particle size, are therefore also doomed to failure. Much of what is contradictory in the literature on hydrocarbon transformations is due to carbon deposition. It is known that, with carbon deposited by alkanes, larger metal particles are selectively deactivated [40] and flat planes or terraces in preference to edges and corners [35]. Thus in all probability the small fraction of surface that typicallly remains active is constituted of tiny particles and the edges of larger ones. Certainly a carbon deposit acts as any other selective poison or modifying species or alloying element [41]. The active fraction of surface may thus be a function of (i) the metal, (ii) its particle size distribution, (iii) the temperature and (iv) the reactant ratio. It is not clear whether carbon deposition is such a problem for reactions conducted in the liquid phase, or in the presence of a solvent. A suggestion [42] which has attracted much attention is that many of the characteristics of alkene hydrogenation, including its seeming structure-insensitivity, may be explained if the reaction occurs on top of a primary carbonaceous overlayer, the hydrogen content of which is a major or perhaps the only source of atomic hydrogen. We will revert to this idea in the following section.
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11.1.3 Kinetics and mechanism of hydrogenation It was necessary to suffer the discursion of the last section, so that the possible role of the carbon deposit in the reaction mechanism could receive due attention. The popularity of ethene hydrogenation as a means of evaluating catalytic activity of metals and alloys is attested by a cursory glance at the literature, and its attractiveness does not fade with time: indeed one might say of this reaction, as did Shakespeare of Cleopatra - 'Age cannot wither nor custom stale her infinity variety'. However, as was said in the opening paragraphs of this chapter, its formal simplicity is a snare and a delusion, for small molecules can sometimes act in ways denied to higher homologues. It is worth recalling in this context the words of Pierce W.Selwood [43] written in 1962: 'No problems in surface chemistry have been more hotly debated than the adsorption and hydrogenation mechanism for ethylene; and few debates have resulted in such meagre conclusions'. The passage of more than three decades has not invalidated these views. As noted elsewhere in this book, chemists have an interest amounting to an obsession in finding a mechanism for the reaction under study. To a certain extent this is of course fitting and proper, but it only becomes a valid exercise when sufficient reliable information is avialable. Would that we all had the humility of Sir Eric Rideal, who speaking of ethene hydrogenation said in his plenary lecture to the First International Congress on Catalysis [2]: 'A great number of workers in the field of catalysis from Sabatier onward have given explanations of the mechanism of the reaction; I myself have advanced three. At least two must be erroneous and judging by the fact that no less than three communications are to be made on this subject during this week, it is quite likely that all three of them are wrong'. The formal kinetics of the ethene hydrogenation are quite straightforward, although showing some variation with catalytic metal, its physical form and temperature [3]. Broadly speaking the reaction has a positive order in hydrogen, which at least over platinum increases with temperature from 0.5 to about 1.0, due to a change in ratedetermining step: the order in ethene is either zero or negative. Similar values are recorded for higher alkenes [5,9]. Activation energies are remarkably consistent at about 40-50 kJ tool -1, although work performed before 1960, which has been reviewed [9,44], led to some very low values: diffusion limitation is often encountered and is not easily avoided. The problem is that a number of formal mechanisms based on the Horiuti-Polanyi proposal (section 11.1.1 and scheme II) can generate the same reaction orders, and it is well understood that kinetic measurements can never of themselves lead to the establishment of a sure mechanism: equally however any mechanistic proposal needs to be compatible with the observed kinetics. Mention was made in the last section of the suggestion [42] that a carbonaceous overlayer or carbon deposit was the vehicle whereby hydrogen was added to the alkene
Catalytic hydrogenation and dehydrogenation
487
molecule [36,45]. One of the principal bases for the idea was the similarity in kinetic parameters shown by a variety of different metals; what was not made clear however was how this model might account quantitatively for the products of alkene-deuterium reactions, or more generally for the systematic differences in reaction character that different metals show. The idea remains sub judice: many obvious and straightforward experiments (e.g. deuterium exchange with the carbon deposit) that would test the concept have not yet been performed. It was also at one time believed that the ethylidyne radical CH3-C-, for which much evidence accrued from studies of ethene chemisorption on single crystal surfaces [36,45], was the reactive intermediate in hydrogenation. Once again the proponents of the concept omitted to explore whether or how the idea might explain the broader sweep of experimental observations, and it was no longer defensible after it had been shown that the species was quite unreactive [36]. It has now been relegated to the role of a spectator. Quite the most profound investigation of ethene hydrogenation has been that conducted by Dumesic and his co-workers using a Pt/SiO 2 catalyst; it has already been referred to several times [3,4]. Their conclusions rest on a quite complete analysis of the products of the reaction with deuterium at various temperatures and reactant ratios, of orders of reaction at various temperatures and on TPD measurements made with platinum single crystals. A most impressive body of information is available, from which they conclude that there are two modes of deuterium chemisorption, and that atoms formed in either way need to be 'activated' before they can react: one of these routes is competitive with ethene and the other is not. While this analysis might be taken to provide support for a role for the carbon deposit, we must remember Popper's dictum that 'It is only possible to disprove a hypothesis, not to prove it'. Other mechanistic formulations may be devised and may be more economical in reaction steps. The following thoughts are offered. (1) It is assumed that the fraction of free surface remains constant, and unaffected by reactant ratio and temperature. (2) Reaction orders are only expressed in power rate law language and not in Langmuir-Hinshelwood terms. (3) No allowance is made for a reaction involving dideuterium with adsorbed ethene or ethyl: if it has a higher activation energy than reaction of deuterium atoms, this might explain why the order in deuterium increases with temperature from 0.5 to unity. (4) Although there is a prima facie case for accepting their mechanism as at least a basis for discussion, one cannot be certain that it would apply to other alkenes and other metals. Nevertheless the interested reader is strongly recommended to study these papers [3,4], and those cited therein, with the greatest care. Two more techniques need to be noted before we proceed to see what is known about alkene hydrogenation on alloys. A most fascinating and novel technique has been developed by Robert L.Augustine and his students, termed the single turnover (STO) method [46,47], wherein a hydrogen-covered surface is subjected to a pulse of an alkene such as 1-butene and the products of the one-step interaction analysed. These would
488
consist of (i) affixed to the this way, the assessed. The
chapter 11
n-butane, (ii) isomerised 2-butenes, and (iii) butyl radicals which remain surface and which can be removed only by a further pulse of hydrogen. In number of sites responsible for each type of reaction can be quantitatively technique has the advantage (or disadvantage ?) of describing the condition
of the virgin surface, and should provide an invaluable source of information on surface structure. Unfortunately (i) it does not appear to assist understanding of selectivity in alkane reactions at higher temperatures [48] nor (ii) it does reflect the situation on the surface during continuous hydrogenation. It is however an interesting development of the earlier alkene titration techniques devised by Sermon [49] and Leclercq [50]. It does not yet appear to have been applied to alloys. Finally it is worth mentioning the role of theoretical analysis in the discussion of reaction mechanisms. Following early fumbling and amateurish attempts to draw analogies between chemisorption and coordination complexes, and between mechanisms in homogeneous and heterogeneous systems [51], there have been many more skilled applications of theoretical constructs to the still unresolved questions of hydrogenation mechanisms [46]. The types of methodology used have been helpfully described by Romanowski [52]. It would be an exaggeration to say that this work has revolutionised our perception of the situation but it is right and proper that theory and practice should walk hand in hand (if possible). 11.1.4 Hydrogenation of alkenes by alloys Once again it is convenient to classify the available information by the type of alloy system used, and to consider first the effect of adding an inactive Group 11 or other metal to an active metal of Groups 8-10. The prize for the most popular system is won by nickel-copper: there have been a number of studies of ethene hydrogenation using powders [52-56], foils [44,57,58] and films [59-63], particularly during the period 1950-1970. Some very early work on this system was performed by Schwab and Brennecke [64]. The results are not unexpectedly somewhat discordant, and with benefit of hindsight we can recognize the following factors as contributory. (1) The surface concentrations are not in general the same as the bulk concentrations, due to the occurrence of surface segregation of copper and of a miscibility gap, as explained in chapter 4. This accounts for the different results obtained with films sintered at different temperatures [62] and when films and foils are compared with powders [57]. (2) The almost inevitable rapid formation of carbon deposits, to an extent that will depend on surface composition, defect concentration particle size etc. In those cases where addition of copper to nickel leads to an increase in activity [53,54,59] the cause is very likely a lowering in the extent of self-poisoning. (3) There is however another possible explanation for the scatter of results, and this is the promotional role played by strongly retained hydrogen, a phenomenon investigated by Keith Hall, Paul
Catalytic hydrogenation and dehydrogenation
489
Emmett and their associates [55,59,62]. This will clearly be absent initially from films prepared in high vacuum, but it is a feature of copper-nickel powders (not the pure metals [55]), and is attributed to the presence of dissolved oxygen atoms to which the hydrogen atoms become attached. Its modus operandi remains something of a mystery, but it can exert a powerful influence on catalytic activity. The paper by Tuul and Farnsworth [44] deserves a special commendation: H.E.Famsworth was one of the earliest, but forgotten, pioneers of the study of adsorption and catalysis on metal surfaces in UHV conditions, and his work [44,65] (and other references cited therein) represented a quantum jump in surface cleanliness achieved. It required great strength of character and experimental skill to construct and operate a UHV system in the early 1950's. If one attempts to discern a pattern in the way that the rate of ethene hydrogenation varies with copper content, one can only say that nickel is much more active than copper, and that the manner of variations is influenced by the factors mentioned above: sometimes there is a smooth decrease [53], often a maximum [44,54,59,62] and on occasion a period of almost constant activity before a catastrophic fall [57,60]. Activation energies are very variable, and are at times suspiciously low, especially on contaminated surfaces (compare [52,53] with [44]). Early studies were conducted on hydrogenations in solution (styrene [65], benzene [66], cinnamic acid [67]), and there are a few reports of the behaviour of other nickel alloys (with gold [62] and tin [68]) in ethene hydrogenation. It is very surprising that there are so few and such superficial investigations of alkene hydrogenation and related reactions over alloys of the noble metals of Groups 8-10 with an inactive metal. What there is to report includes some quite old work by Rien/icker and his colleagues on foils (Pt-Cu and Pd-Cu [69]) where activity for ethene hydrogenation at first falls slowly as the copper concentration is increased, and then very quickly at a composition close to that of pure copper [9]. In these cases the low activity points lie on a separate compensation line [9], which may betoken the operation of a different type of active centre in this concentration range [70]. With palladium-gold microspheres [71], and less obviously with palladium-silver alloys [9,72], rates increase as the Group 11 element's concentration is raised, in the former case by as much as 60%. Some information is available on supported palladium- and platinum-tin catalysts [68,73] and there is recent brief report on the ruthenium-copper system [74]. Apart from what is said in the last section concerning the propene-deuterium reaction (see table 1), there is little more to tell. It seems incredible that no one has thought to look at, for example, the ethene-deuterium reaction on palladium-gold alloys. There are one or two studies only of alloys formed by elements within Groups 810. Nickel-palladium films show a maximum rate of ethene hydrogenation towards the centre of the composition range [75], this being assigned to a diminution of the toxic effect of hydrogen dissolved in the palladium; Ni-Pd/SiO2 catalysts have also been studied
490
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[76]. Addition of iron decreases the activity of platinum catalysts for propene hydrogenation [77], while iron-rhodium catalysts supported on graphite after reduction at high temperature give selective isomerization of 1-butene [28]. This behaviour, ascribed to a true alloy, is reminiscent of the behaviour of dilute copper-nickel catalysts in ethene deuteration [26]. There have also been reports of alkene (and alkyne) hydrogenation on intermetallic compounds [78-80], amorphous metals [81] and colloidal alloys [82]. 11.1.5 Hydrogenation of the cyclopropane ring Cyclopropane itself is a fascinating molecule, partaking as it does of the properties of both alkenes and alkanes; it is, as one says, 'Neither fish, flesh, fowl nor good red herring.' Theoretical chemists have had a field day seeking the best way to describe bonding in this molecule [83-90]. The strain induced by the 60 ~ angle between the carbon atoms means that addition reactions can proceed with ease and the ring is readily hydrogenated at ambient temperature or even below [9,91,92]. From some points of view the molecule appears to react as if it comprised a methylene radical (CH2:) interacting with the rt-orbitals of ethene. Its homologues are even more useful: methylcyclopropane reacts [93,94] to give either n- or iso-butane (scheme V), and this reaction has sometimes been used as a means of characterising the surface of small metal particles ([94], and references therein).
Scheme V Products of hydrogenation of methylcyclopropane CH
CH
3
\ /
CH 3
CH CH
CH
2
methylcyclopropane
3
~
+H 2
>
CH2~ CH / n - butane
CH3
\
+
CH
....... CH3
CH / 3 isobutane
Early kinetic work revealed that hydrogenation of cyclopropane on supported Group 10 metals was first order in the hydrocarbon and zero in hydrogen [9], suggesting that the surface was mainly covered by the latter and that the concentration of hydrocarbon radicals was low. Later it was shown that the reaction with deuterium gave deuterated propanes, the distributions of which resembled those obtained in propane exchange, with propane-d 8 being a principal product; there was small formation of exchanged cyclopropane. More detailed analysis, especially on the products of the reaction with hydrogen over nickel [92,95-97] and ruthenium [98-100] showed that hydrogenolysis of C-C bonds could
491
Catalytic hydrogenation and dehydrogenation
occur, giving methane and ethane (scheme VI). Sometimes the methane: ethane ratio barely exceeds unity, showing that only two C-C bonds are broken, but on large ruthenium particles at high temperature there is considerable excess formation of methane [98]. It seems likely that these further bond-breaking steps occur through a propyl radical formed by adding a hydrogen atom to cyclopropane: because they occur at temperatures below that at which desorbed propane could react further (scheme VI).
Scheme VI Hydrogenation and hydrogenolysis of cyclopropane
CH / CH 2
+H
2 ~
CH 2
+
H .,
t~>
C3H8
CH 2 - CH 2 - CH .~ + 3H
C2H6 + C H 4
In the expectation that the process of hydrogenolysis might require a larger ensemble of active atoms than hydrogenation, there have been several studies of the effects of adding copper or gold to a metal of Groups 8-10 (Ni-Cu [95-97]; Fe, Co-Cu [97]; Ru-Au [101]). Addition of copper to nickel leads to a selective suppression of hydrogenolysis, and when the results are corrected for the true surface concentration of copper it appears that, quite precisely, hydrogenation requires two nickel atoms and hydrogenolysis three [96]. Addition of gold to ruthenium inhibits excess methane formation, but has little effect on the hydrogenation/hydrogenolysis ratio, so that the two reactions probably involve the same intermediate. With Rh-Ir/SiO2 catalysts, rhodium is much more active than iridium, but maxima in rates both of hydrogenation and of hydrogenolysis are seen at about 40% iridium [102] (see also chapter 13).
11.2
Hydrogenation of alkynes and alkadienes
11.2.1 General principles In the hydrogenation of multiply-unsaturated hydrocarbons we encounter a new phenomenon, or at least one which has only features tangentially in what has gone before. This is the concept of
degree of selectivity, or selectivity for short. Molecules that contain
two or more unsaturated functions can usually be hydrogenated in such a way that intermediate products can be obtained in some degree of selectivity, which sometimes approaches 100%. Scheme VII lists some of the more important reactions that show this
492
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Scheme Vll .Examples of selectivity in hydrogenation of multiply unsaturated hydrocarbons Reactant
Intermediate product ( s )
Ethyne
Ethane
Ethene
HC --- CH
CH
-CH 3 3 Propane
H2C = CH 2
Propyne
Propene
CH3-C=CH Propadiene
Final product
CH3-CH=CH 2
CH3- CH2- CH3
(ailene)
CH 2 = C H = C H 2 2 - Butyne CH3-C=C-CH
3
1 - Butyne CH3-CH2-C-CH 1,3 - Butadiene H2C= CH - C H = C H
m
1 - Butene CH3-CH2-CH=
CH 2
2 - Butenes CH3-CH=CH-CH
n - Butane
3
1,2 - Butadiene H2C=C = C H - C H 3
But -1- yne - 3 - ene
1,3- Butadiene
Butenes
(vinylacetylene) CH = C - CH =CH 2
characteristic: each reactant is transformed into the product shown in the next column to the right by addition of one mole of hydrogen. Carbon-carbon unsaturation may accompany a variety of other unsaturated functions (e.g. the aromatic ring, as in styrene (phenylethene) or phenylethyne, etc., or the carbonyl group as in crotonaldehyde). Selective reduction of some of these combinations will be considered later. Processes based on some of these reactions have considerable industrial importance and this has occasioned much research. Perhaps the most important family of reactions is that employed by the petrochemical industry to remove small concentrations of multiplyunsaturated molecules from streams comprising chiefly alkenes, either C 2, C 3 or C4. These streams arise by fractionation of steam-cracked naphtha, and virtually complete removal of alkynes and alkadienes is essential before the alkenes can be further processed by polymerisation, selective oxidation etc. [103]. In a succesful process to achieve this, the catalyst must (i) not hydrogenate any of the alkene, and (ii) desirably reduce the alkyne/alkadiene to alkene rather than alkane. The first of these requirements introduces a second facet of selectivity: in a mixture of A + B, it may be possible to obtain a selective reaction
Catalytic hydrogenation and dehydrogenation
493
of component A without affecting B. This will happen if for example A is much more strongly adsorbed on the surface of the catalyst than B: we then speak of thermodynami-
cally controlled selectivity [104]. When the intermediate product is desired, it is necessary for it to be able to desorb more quickly than it is further hydrogenated; this is mechanistically-controlled selectivity: X
A-----P--
X
Y
as
~
as
Y
The two factors come together because to obtain X in high selectivity it must not readsorb once it has vacated the surface. Fortunately, alkynes and alkadienes are both much more strongly adsorbed than the corresponding alkenes, so there is a very favourable thermodynamic factor in this system. Thus in a batch reactor no alkene will react until nearly all the alkyne or alkadiene has been removed. Equally fortunately there is one metal -palladium- that is outstandingly active and selective for their hydrogenation; indeed, selectivities to alkene approaching 100% can often be obtained. The other metals of Groups 8-10 are active in some degree but are normally much less selective. Alkadienes are almost as strongly chemisorbed as alkynes, and their selective reduction to alkenes is therefore also possible [5,103]. Before proceeding to detail, there are two other features of these reactions to note: first, stereochemistry. Reduction of a disubstituted alkyne can give, as primary products, either the cis or the trans-alkene: palladium catalysts often show a high degree of stereochemical specificity to the cis-isomer, showing that both the hydrogen atoms are added to the same side of the molecule (scheme VIII). Much research has been directed to obtaining the highest possible yields of cis-isomers, as these are normally the desired product (e.g. in synthesis of vitamin A); high stereospecifity usually accompanies high selectivity. Secondly, during its hydrogenation, ethyne can unfortunately undergo a
hydropolymerisation to oligomers containing C4, C6, etc. molecules. This aspect of the reaction has also been investigated, and some partial solutions have been obtained [9]. In the following sections we shall briefly review the kinetics and mechanism of alkyne hydrogenation with emphasis on ethyne, and its reaction with deuterium and its polymerisation. We shall consider stereochemical aspects, and the effects of alloying and of other modifiers on the reaction characteristics. We shall then give some attention to the corresponding reactions of alkadienes.
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Scheme VIII
Hydrogenation of 2-butyne
CH - C - C- CH 3
H
H
L
~>
CH 3
H
CH 3 J
C -'--~. C
H
Cis- 2- butene
11.2.2 Kinetics and mechanism of alkyne hydrogenation When ethyne is hydrogenated in a batch reactor over a catalyst containing a metal of Groups 8-10, the product ratio remains constant until almost all the ethyne has reacted: then a much faster hydrogenation of the ethene ensues [5,9,21,103,105,106]. This demonstrates clearly the absence of hydrogenation sites that will chemisorb ethene noncompetitively, at least at the partial pressures involved. This is the case even when ethyne hydrogenation proceeds with somewhat low selectivity (this being defined as the percentage of ethene in the C 2 products) as it does with platinum [107,108]. On Pd/A1203 catalysts, the thermodynamic factor favouring ethyne is greater than 2000 [21,105], but when ethene is added in sufficient excess, in order to simulate industrial conditions [103,105,109] it can adsorb and be hydrogenated on sites not blocked by ethyne. By sophisticated application of isotopic labelling, using for example 13C [105] or 14C [21] labelled ethene and ethyne, and double-labelling techniques [109] it has proved possible to identify three types of site [21,108]. (i) A type I (or type X) site at which ethyne is hydrogenated only to ethane. (ii) A type II site at which ethyne is reduced to ethene. (iii) A type III (or type Y) site at which added ethene may be reduced by ethane (scheme IX). There have been no serious attempts to allocate these functions to any particular geometric features of the metal particles, for example by systematic variation of particle sizes, although as we shall see shortly this approach has provided very useful insights into the behaviour of C4 hydrocarbons. A possible reason for this omission is the fact that the work performed with palladium has often employed a commercial catalyst which has a very low metal content (e.g. 0.04%). There has been a hint [109] that the Type III site may in fact be on the support and be fed with hydrogen by spillover from the metal. Another very plausible suggestion is that non-selective hydrogenation occurs only on the 8-PdH phase and not on the t~-PdH phase, which because of the effect of particle size on the thermodynamics of hydride formation [110] is probably absent from catalysts in which the palladium is highly
Catalytic hydrogenation and dehydrogenation
495
dispersed [21]. Tendency to form the 13-PdH phase is also suppressed by alloying with platinum [111] and presumably other metals also. It is curious that no one has thought to apply the single turnover method to alkyne hydrogenation. Scheme IX r-- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
'
Type
I
!
I
1 ,
/tt
H
9 * H
H
H\
+H
H
;
H-C=C--H -+ I
9 H{a
__\ ....
.
C2H2 (g) ,Z
|!
I Sites
/H
C=C---* /i- -\ +H ** H
r-
!\ H\ /H H --*I C2H6(g) II, C=C~---* / C--C--H \ +21"I', / \ + H 9I H ', ,, * H
H\
'
-
i C2H4(ads)
,,
{
,
i
',
/
T y p e II S i t e s
I
Type Sites
llI
i
''1, i ,
. . . . . . . . . . .
..i
A reaction scheme portraying possible routes is shown in scheme IX [108]. It is interesting to note that ethylidyne or vinyl and ethylidene species feature as intermediates; these were recognised in the last section as being unreactive, and perhaps the principal components of 'carbon deposits' formed from alkenes. This suggests a possible general principle: species formed parasitically in an easy reaction may be essential intermediates in a more difficult reaction. We noted above that alkyne (and alkadienes) are usually less
reactive than alkenes. Orders of reaction are typically first or greater in hydrogen and zero or somewhat negative in alkyne [9,103]; the latter betokens competitive adsorption of the two reactants, but orders in hydrogen greater than unity are only explained with difficulty without invoking the involvement of dihydrogen in reactions with adsorbed hydrocarbon species. An alternative suggestion [112] that the area of reactive surface is a positive function of hydrogen pressure, through its removal of over-strongly adsorbed hydrocarbon moities, depends for its credibility on the ease with which their formation may be reversed when hydrogen pressure is raised. Selectivities on all metals not unexpectedly decrease with increasing hydrogen pressure [5]. Activation energies are variable, but frequently in the region of 60 kJmo1-1, higher than for alkenes [9]. Selectivities increase with increasing temperature, except for palladium [5]. Interesting correlations have been developed by workers at the Institut Francais du P6trole between the inhibiting effect of alkyne concentration on the rate (i.e. negative order of reaction) and the extent to which the rate is a function of particle size (structure sensitivity) [107]. Over rhodium [107], palladium [113], and platinum [107] catalysts, rates of hydrogenation of 1-butyne in the liquid phase decrease with decreasing particle size;
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1,3-butadiene behaves similarly over palladium and rhodium, but over platinum there is no variation. There is then a good correlation between negative orders and the appearance of particle size sensitivity; only for butadiene over palladium is the expected negative order not observed. The explanation advanced [107] is that alkynes and alkadienes chemisorb extremely strongly on surface atoms of low coordination number (e.g. edge atoms), with perhaps two molecules forming a kind of organometallic complex, which is unreactive. In the limit, the metal may dissolve into the liquid phase when a strongly complexing molecule such as vinylacetylene (but-l-yne-3-ene) is present [103]. Thus activity per unit area diminishes with decreasing particle size, i.e. as the fraction of surface atoms with low coordination number increases, and the occurrence of negative orders is neatly explained. It would be interesting to study the properties of alkynes chemisorbed on stepped single crystal surfaces. In the references quoted, small metal particles are also thought to be electron-deficient because their activity is improved by addition of an electron donor such as piperidine [107]. Particle size effects having an electronic origin have also been claimed in 1,3-butadiene hydrogenation catalysed by palladium particles formed by an atomic beam method [ 114]. Hydrogenation of 1-butyne leads exclusively to 1-butene [107,113] and of 2-butyne to predominantly cis-2-butene [5,115]. Product selectivities from hydrogenation of 1,3butadiene and of 1-butene are independent of dispersion over palladium [113], suggesting that with the former it is just the number of active sites, and not their composition, that changes with particle size. Clearly in these cases there is no facility for the adsorbed alkene to isomerise before it is forced from the surface. A strange phenomenon was observed many years ago concerning the gas-phase hydrogenation of ethyne over nickel in a constant volume batch reactor [ 116,117]. When the hydrogen; ethyne ratio exceeds unity, the pressure-time curve is strictly linear (i.e. the rate remains constant) until all the ethyne has reacted, after which the rate accelerates. This by itself is not easily explained, because the rate is in fact proportional to the initial hydrogen pressure, but it does not respond to changes in hydrogen pressure as the reaction proceeds. Then if the hydrogen pressure is suddenly changed by adding more, the rate increases in proportion to the quantity added. It would appear that some kind of surface chain reaction is set up in the initial instants and that it is self-sustaining until a step change in pressure is made. These observations have never been repeated or followed up. The reaction of ethyne with deuterium has been studied on seven of the nine metals of Groups 8-10 (not iron or cobalt) [5]. Deuterated ethynes are not returned to the gas phase over rhodium, iridium, platinum and palladium, neither is hydrogen deuteride over nickel and platinum. Novel information comes from the analysis of the dideuteroethene (C2H2D2), because this exists as three isomers (cis, trans and asymmetric), the proportions of which can be estimated by infrared spectroscopy. The cis-isomer usually predominates but 30-40% of the trans-isomer is formed over the metals of Groups 8 and 9; the
Catalytic hydrogenation and dehydrogenation
497
asym-isomer is always a minor component. Scheme X illustrates possible routes to the formation of these products: the ethylidyne radical postulated as the intermediate for forming the asymmetric isomer is also implicated in the non-selective hydrogenation route (see scheme X).
Scheme X Routes to the formation of dideuteroethenes in the reaction of ethyne with deuterium H
\
H--C--
I
C--
H
H
+D ~
~
// C
C~
D
I
+D ---,--
H
D
\
/
C -- H
/
\
H
D
H
\
H
~
// C
C--" D
H ~
\
C --
C
I
I\
I H
H--C~C--H
I
/
\
/ C
II
C
I
H
H
+D ~
c i s - or
trans-
dideuteroethene
D
+D ~
H
\
/ C
H
+D ~
asym- dideuteroethene
II
C --D
I
The last feature of alkyne hydrogenation needing attention is hydropolymerisation. On all metals the hydrogenation of ethyne leads to the formation of considerable amounts of oligomers, mainly straight-chain hydrocarbons containing even numbers of carbon atoms, in parallel with the C2 products: with nickel, the oligomers can be the major product [5]. They are also formed by alkadienes, but not by alkenes. In the industrial processes for removing traces of multiply-unsaturated molecules from alkene streams [103], the Pd/A1203 catalyst over a period of time produces a 'Green Oil', which fouls the plant and deactivates the catalyst: this is a complex mixture of hydrocarbons formed by hydropolymerisation of ethyne. It is an unmitigated nuisance, and many expedients have been examined to decrease or eliminate its formation. Sheridan [118] proposed that the free-radical form of the vinyl radical (scheme X) initiated a surface chain reaction (scheme XI): this seems to be an explanation that satisfactorily explains the observations. Higher homologues (propyne [119,120], butynes [115,121], etc.) give progressively smaller amounts of oligomers, due to steric interference with the C-C bond forming step. The increase in ethene selectivity with conversion in a static reactor, and with increasing particle size using Pd/SiO 2 catalysts, has been correlated with the reluctance of small
498
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particles to form carbonaceous residues, the formation of which may be important in achieving high selectivity. Scheme XI Hydropolymerization of ethyne [ 118]
H--C----C--H
I
**
\C--
C
.
.
I
/
Ix
H
9
C H
I
H
C
I
H
C
I
C
/
I\
There does not appear to have been any comprehensive attempt to model the hydrogenation of alkynes in a quantitative manner, incorporating all the available information on temperature and pressure effects, the results of isotopic labelling with deuterium, ~3C and ~4C etc. Certain underlying causes of the observed behaviour therefore remain obscure: the outstanding activity and selectivity shown by palladium seems to be related to its ability to absorb hydrogen, and indeed its excellent selectivity may be a consequence of there being an abnormally low concentration of hydrogen atoms at the surface: reaction may occur as dissolved hydrogen atoms emerge from beneath the surface, attacking adsorbed hydrocarbon species from below. Another possibility is that dissolved hydrogen changes the electronic structure of palladium and its propensity to attract the r~electrons of ethene is thereby diminished. 11.2.3 Hydrogenation of alkynes by alloys The main purpose of the extensive work that has been carried out with alloys, and with addition of surface modifiers (apart from the satisfaction of curiosity), has been to improve even further the selectivity and stereospecificity shown by palladium. One might think with Shakespeare that 'To gild refined gold, to paint the lily, to add another hue unto the rainbow, were wasteful and ridiculous excess'; but palladium is not quite perfect and the polymeric 'Green Oil' problem is a major one. Modifiers (or selectivity promoters or selective poisons, as they are sometimes called) such as carbon monoxide or nitrogen bases or metal cations achieve the same end by different means, and because their modus
operandi can sometimes be described as the formation of a two-dimensional surface alloy they must receive some attention. Only a little work has been done with alloys of metals that normally show only low selectivity [122,123]. The addition of gold to palladium in the form of powder results in an increase in activity [124], and of silver to Pd/o:-AleO 3 in some improvement in selectivity [125].
Catalytic hydrogenation and dehydrogenation
499
Following this early work there have numerous patents granted that claim amelioration of palladium's shortcomings by addition of small amounts of other elements (see [126] for an example): the trouble with patents is that they do not invariably achieve the standard of scientific rigour that publications in refereed journals have to meet, and it is therefore not easy to assess the merits of the claims. Moreover, of course, in patents one does not have to explain one's discovery. In an informative study [115] it was shown that the variation of rate of 2-butyne hydrogenation brought about by changing the composition of palladium-gold alloy wires was chiefly attributable to alterations in the pre-exponential factor, which reflects the number of active sites, thus categorizing alkyne hydrogenation as an ensemble-size insensitive reaction which might require only a single palladium atom as its active centre. Addition of copper to dilute Pd/AI203 leads to increased stability and lower ethane formation [109]. A study of nickel-copper and cobalt-nickel alloy powders [127] revealed the interesting fact that the relative activities of the former series were temperature dependent because activation energies were not all the same. While at 323K nickel was much more active than any alloy, at 423 and 473K the maximum activity was in the centre of the series because here the activation energies were higher. It is a sobering thought that much ingenuity has been exercised to explain a manner of variation of activity with alloy composition that may be an artefact of the particular temperature chosen for the comparison. Ranveer S.Mann and his associates have extended this work to cover propyne [120] and 2-butyne [ 121 ]. The best known and longest established improved version of a palladium catalyst for liquid-phase hydrogenation is Pd/CaCO3 selectively poisoned by Pb ions, usually in the form of Pb(OAc)4, and the nitrogen base quinoline [128]: it is generally referred to as the Lindlar catalyst. Its success has stimulated considerable research into the role of lead [129131] which may be present either as an oxide or as the ordered alloy Pd3Pb [131]. The effect of vacuum-deposited lead on the surface of a palladium single crystal has also been examined [130], as has the palladium-boron system [132]. One cannot totally ignore the possibility that one of the factors responsible for the egregious properties of palladium is the formation of an interstitial palladium-carbon alloy during ethyne hydrogenation [38,133]. The addition of either gold or copper to iridium [122] or of platinum to iridium or rhenium to platinum [123] all lead to increased ethene selectivity in ethyne hydrogenation, although usually at the sacrifice of some activity. It seems that in these cases, ensemble size determine the adsorption modes and by that the resulting selectivity. The activities of a number of intermetallic compounds for the hydrogenation of 1butyne (Pd-RE [134]) and of 1-octyne (Zr-Rh-Pd and Zr-Rh-Ru [135], MRh3_xPdx where M=Ce or Zr [136]) have been investigated.
500
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11.2.4 Hydrogenation of alkadienes As noted in the introduction to this section, alkadienes (both conjugated, and nonconjugated, provided there is not more than one methylene group between the double bonds) are adsorbed with strengths comparable to those of alkynes and therefore also show the phenomenon of selective reduction to intermediate alkenes. By and large the selectivities that various metals show in the hydrogenation of allene (propadiene), 1,3-butadiene and 1,4-pentadiene [5] are similar to those given by alkynes: palladium is again outstanding in this respect. The process as exemplified by butadiene is however in some respects more complex than that say of 2-butyne, in that the relative amounts of the three isomeric butenes vary considerably from one metal to another, and with reaction conditions [5,21], due to various amounts of 1,2- and 1,4- addition, and great ingenuity has been exercised in converting the observations, including those arising from the use of deuterium, into convincing mechanisms [21]. A selection of the results obtained is given in table 2. It is unnecessary to enter in great detail into the somewhat complex mechanistic schemes that have been devised. The principal indicator of mechanism is the trans:cis ratio of the 2-butene: on palladium and sometimes on cobalt this is very high (8-14) and this betokens a mechanism (called B) based on syn- and anti-~-allyl intermediates which cannot interconvert on the surface, the proportions which simply reflect the amount of the two conformers in the gas phase. In mechanism A, which describes the behaviour of other metals, showing a trans:cis ratio of about unity, intermediates are either ~-alkenes or ~alkyls that may interconvert more freely, although the intervention of ~-allylic species at some points in the mechanism is tolerated. Abbreviated representations of these two mechanisms, both of which may operate side-by-side, are shown in scheme XII. The selective reduction of 1,2- and 1,3-dienes is also important in the treatment of steam-cracked naphtha fractions to prepare them for further petrochemical processing [103], and recent studies of butadiene hydrogenation in the liquid phase have shown the same kind of particle size sensitivity as given by 1-butyne on palladium [113] and rhodium [107] catalysts, but a lack of particle size dependence in the case of platinum [107]. Once again the notion of strong complexation of diene to atoms of low coordination number is advanced to explain this type of structure sensitivity where it occurs. This effect is also at work in the study of palladium particles of various sizes formed by atomic-beam vacuum deposition onto carbon or graphite [137]. Large particles (> 2.8 nm) behaved like bulk palladium, but small ones (< 1.4 nm) were quickly deactivated probably by complexation; a marked size dependence of rate was shown in between. Butene selectivities were 100% but their distribution was not given. Related work [114] has already been noted. In the liquid phase [113] palladium shows the expected high trans:cis ratio (12), independent of particle size. Supported gold catalysts are also active and very selective for butadiene hydrogenation ([21]; see table 2).
Catalytic hydrogenation and dehydrogenation
501
Scheme XII Possible mechanisms for the hydrogenation of 1,3-butadiene [21] C4H 6
mechanism A H2C
-'--
I
*
H2C
---
I
*
CH3 ~
CH
H2C
\
CH ~
H2C
CH
I
~
CH ~
I
CH ~
--
\
*
H2C CH 3 ~ CH ~
CH ~
CH - -
I
CH 3
I
~
CH
\ CH /\
CH 2
CH 3 "*
CH
CH3 ~
~'CH
CH
H2CJ/\
CH
\
CH
I
CH
~
CH3~
CH
CH2~CH
CH
~'cH
--- CH2
I
*/
CHa trans - 2 - butene
cis - 2 - butene
1 - butene
C4H 6
mechanism B H2C ~
CH
I
H 2C _._---" --,C%
I ..~
cH -- cH I
,'','
cH
CH 2
I
1L H2C - -
CH
I
H2C ~
CH - -
CH
CH 3
CH3
CH 3
\
H2C ~
CH --- CH
I \
I
CH ~
CH2~
CH
CH 3
\
/ CH --- CH
I
CH 3
trans - 2 - butene
CH 3
1 - butene
cis - 2 - butene
Note. The hydrogen atoms added and removed are not shown, and some of the possible intermediates are omitted for the sake of clarity.
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table 2 Selectivities and butene isomer distributions for the hydrogenation of 1,3-butadiene on alumina-supported metals [5] Metal
T/K
S
1-b
t-2-b
c-2-b
Fe
471
0.980
23
45
32
Co
348
1.000
28
64
8
Ni
373
>0.99
27
63
10
Cu
333
1.000
87
6
7
Ru Rh Pd
273 289 273
0.736 0.943 1.000
69 51 65
19 32 33
12 17 2
Os
297
0.431
65
19
16
Ir Pt Au
297 273 443
0.251 0.501 1.000
59 72 58
19 18 13
22 10 28
Notes: Selectivity = (total butenes)/(total butenes + butane) The distributions shown for nickel and cobalt are of the 'Type B' kind, probably representative of sulfur-contaminated surfaces.
There have been comparatively few systematic studies of alkadiene hydrogenation on alloys, although there are several claims of improved alloy catalysts in the patent literature. Rates shown by 1,3-butadiene on nickel-copper films at 328K are independent of composition in the range 3-97% copper, as expected for the 'cherry' model (see chapter 4), and this rate is a hundred-fold less than that shown by nickel but ten times larger than that given by copper [138]. The alloys show high trans:cis ratios, as does nickel, but the 1butene:2-butene ratio decreases with increasing copper content because copper is loath to form rt-allylic intermediates. Pumice-supported palladium-gold alloys show similar product distributions throughout the composition range, and the effect of the transition from the ~to the 6-PdH phase at 353-403K on products was noted [139]. Studies of butadiene hydrogenation on Pd-Cu/Nb205 [140] have been reported, and there have been several studies on simple crystal alloys [141a-c]. With PtsoNis0(111), which has a platinum outer layer, the rate at 300K is a little faster than on Pt(111), but the selectivity to butenes is much higher (>80%) [141a]. In the case of PtsoNis0(110) [141b], equilibration at 800-900K leads to platinum enrichment in the surface, while higher temperatures (l100-1200K) result in a higher nickel surface concentration (see chapter 4). In the latter state the surface is both active and selective for butadiene hydrogenation. Studies of Pt80Fe20(111), Pt75Ni25-
Catalytic hydrogenation and dehydrogenation
503
(111) have also been reported [141c]. It is however unfortunate that so much effort is spent in characterization and so little in conducting a full and proper catalytic investigation: no detailed interpretation of the observation has been advanced. Amorphous interstitial alloys of phosphorous and boron with nickel have also been employed as catalysts for butadiene hydrogenation [81 ]. Isoprene (2-methyl-l,3-butadiene) is an interesting molecule, since the two doublebonds are rendered non-equivalent by the substitution of the methyl group, and the three isopentenes (2-methyl-l-butene, 3-methyl-l-butene and 2-methyl-2-butene) are all formed [9,142]. On palladium-gold and -silver alloys, however, their relative amounts are only slightly affected by composition as expected for an ensemble size-insensitive reaction [143]. As noted above, the hydrogenation of animal, fish and vegetable oils to stable products fit for human consumption involves reduction of non-conjugated double bonds to a product containing principally a single double-bond. Nickel catalysts are universally employed, but a beneficial effect of alloying with copper has been reported [ 144]. We conclude this section with a review of some of the principal unanswered questions surrounding the hydrogenation of alkynes and alkadienes. A central question is whether in these systems alloying simulates the behaviour of small metal particles, i.e. whether the properties of active atoms in small ensembles on an alloy surface are the same as or similar to those of small ensembles on small particles of a pure metal, where the mean coordination number is low. Do reactants bind as strongly in the former case as in the latter? There is an almost complete dearth of quantitative information (e.g. on heats of adsorption) to answer this question, but the provisional answer, based on the above survey of the literature has to be 'no'. The factors determining activity and selectivity in the hydrogenation of alkynes and alkadienes include the following. (1) Particle size, through the effect of complexation of reactants to sites containing low coordination number atoms, although there is no clear evidence that particle size p e r se affects selectivities or product distributions. (2) Particle size, through self-poisoning by carbonaceous residues, most readily visible by the use of fresh catalysts in static or recirculating systems, and not easily seen in continuous flow systems. (3) With palladium, a particle size effect through the solubility of hydrogen and the formation of the unselective fS-PdH. (4) With alloys of palladium with a Group 11 element, effects due to changes in the solubility and diffusibility of hydrogen in the alloy: the effect of particle size on solubility of hydrogen in alloys has not been investigated. (5) In the case of palladium, selective formation of intermediate products may be assisted by a low surface concentration of hydrogen atoms in consequence of their propensity to
504
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dissolve. (6) Low selectivity, as often shown for example by iridium [5], may be a consequence of the presence of occluded hydrogen, as suggested by a comparison of the behaviour of supported catalysts and wires [145].
11.3
Hydrogenation of aromatic compounds
11.3.1 General principles Hydrogenation of the homocyclic aromatic ring, as in benzene and its homologues, and in fused ring systems (e.g. naphthalene), is achieved by the metals of Group 8-10 both in the liquid and gas phases, but with some difficulty. The six-membered ring is very stable, and more forcing conditions are needed than for example with alkenes or alkadienes. It has been suggested [107] that there is a kind of 'volcano' relationship in the hydrogenation of unsaturated hydrocarbons (see figure 1), with aromatics having low reactivity because they do not interact strongly enough with metal surfaces to give an optimum ratio of adsorbed molecules to adsorbed hydrogen atoms.
Reactivity figure 1 Volcano relationship for the reactivity of unsaturated hydrocarbons in hydrogenation [107].
I oromo i sl ---i
~
:r
l
..s alkadienes]
Q. 0
0 D
Ikenes
f
I
While it is true that the interaction is not strong so long as the ring remains intact and no C-H bonds are broken, measured orders of reaction [9,146,147] indicate it is sufficient to ensure almost complete surface coverage. One clearly discernible facet of the hydrogena tion of benzene and its homologues is that in general no intermediates such as cyclohexene or cyclohexadienes appear in the fluid phase: once the resonating system of electrons is disturbed, the rest of the reaction proceeds apace, and indeed the possible intermediates when tested by themselves in fact react much faster than the parent molecule. There are
Catalytic hydrogenation and dehydrogenation
505
exceptions to this generalisation: the use of extremely high space velocities can cause some cyclohexene to appear in the products and benzene hydrogenation in the liquid phase with ruthenium catalysts, especially in the presence of water, in which they work particularly well, also gives useful yields of cyclohexene [148,149]. Phenol can also be reduced to cyclohexanone in a bifunctional system [150] (scheme XIII). With these few important exceptions, the reduction of simple aromatic rings may safely be assumed to proceed in a single adsorption step, so that progress of the reaction may be followed in such a simple way as barometrically in a constant-volume reactor or by analysis of condensed products (e.g. by refractive index) using a continuous flow system. The reaction is a favoured one for use in laboratories not well provided with modern analytical equipment, and partly for this reason there are numerous reported studies of the reaction over alloy catalysts. Scheme XIII Hydrogenation of phenol to cyclohexanol
metal OH
+2H 2
phenol
~-OH cyclohexenol
,'
acid
>
0,==0 cyclohexanone
Because of the great stability of the aromatic ring, the resonance energy of which is about 150 kJ mol -~, the reverse reaction, namely the dehydrogenation of cyclohexane, is particularly favoured and can be studied at much more moderate temperatures than the corresponding dehydrogenation of linear alkanes. The constant Kp for the equilibrium C6H 6 + 3 H 2 ~ C6H12
has a value of unity at 560K, at which temperature the opposing processes are exactly in balance. Dehydrogenation will be considered in a later section (chapter 13), but since by the principle of microscopic reversibility the same mechanism must operate in both directions what is said concerning hydrogenation has clear implications for dehydrogenation. If one reads somewhere that, for example, nickel is a better dehydrogenation catalyst than platinum, while for hydrogenation the reversed order is observed, this must be understood as a statement concerning the side reactions: dehydrogenation occurring at higher temperature than hydrogenation is more plagued by side reactions (hydrogenolysis, carbon deposition) and these side reactions are of larger extent on nickel than on platinum. Hydrogen atoms on the aromatic ring can be exchanged for deuterium atoms using either dideuterium or heavy water (D20) in the presence of a metal catalyst. Intermediates
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and mechanisms will be discussed below. Much enjoyment and indeed profit can be had by studying the hydrogenation of fused ring systems: interesting and quite complex stereochemical aspects arise [9,151,152]. Similarly there is much interesting stereochemistry in the hydrogenation of heterocyclic rings [9], but its detailed consideration is beyond the scope of this book [153-155]. 11.3.2 Hydrogenation and exchange of aromatics on pure metals The principal kinetic features of benzene hydrogenation are not in dispute. Orders of reaction in hydrogen are about 0.5 while those in benzene are slightly positive; activation energies are usually in the range 42-63 kJ mol -~, tending to be somewhat higher for nickel [56,146] than for platinum [147] or palladium [156]. However although the immediate deduction is of a surface almost saturated with benzene, the molecules probably lying flat on the surface, and of a rate-determining step involving a single hydrogen atom, there is sufficient space between the adsorbed molecules to allow para-hydrogen conversion and dihydrogen-dideuterium equilibration [9], or chemisorption of carbon monoxide [157], to proceed more or less unhindered. Scheme XIV Possible mechanisms of exchange and hydrogenation of benzene
[~
+D2 H
-~
[~
~,
I
*
+HD D
Dissociatively adsorbed benzene +H
,,
4_;-X
x - adsorbed benzene
H
J~
+3H
Y cyclohexane
+2H <
H
H t
H i
H
, H
/
H ~ \H
H
~- adsorbedcyclohexene
Catalytic hydrogenation and dehydrogenation
507
The substitution of the hydrogen atoms for deuterium by isotopic exchange proceeds in essentially stepwise fashion [158-160], with a higher activation energy and a lower order in deuterium [160] than for addition of hydrogen. It would appear that different intermediates and mechanisms are involved: exchange must proceed by dissociative adsorption to give a phenyl radical (scheme XIV), with limited further dissociation to a benzyne radical to explain the small amount of multiple exchange that is sometimes observed. The need for extra free sites to accomodate the released hydrogen atoms accounts for the negative order in deuterium, so that dissociative adsorption must be the rate-limiting step. The first-formed adsorbed state of benzene probably involves an interaction of the delocalised re-electrons with a number of metal atoms (see chapter 1). On nickel however magnetic studies [ 161] have indicated the formation of a species with a 'magnetic bond order' (based on a number of unpaired electrons in the metal that become paired) of 8, interpreted as a benzyne radical. The expectations of Balandin's Multiplet Hypothesis [162] that benzene would chemisorb flat and in register with atoms in planes of trigonal symmetry (e.g. fcc(ll 1)) are only partially fulfilled by modern LEED studies [36]. However the slow step in hydrogenation is probably the addition of the first hydrogen atom to the ring (scheme XIV), after which a series of ~-allylic (and analogous) species may be formed. These will be identical to those involved in the dehydrogenation of cyclohexane and cyclohexene. It is possible that in the reaction with deuterium these intermediates undergo further exchange, because there are regularly more heavily exchanged cyclohexane molecules observed than can be accounted for by adding six deuterium atoms to the only slightly exchanged benzene [159,160]. The interesting observation [163] that with platinum catalysts hydrogenation activity correlates with the amount of weakly chemisorbed hydrogen seems never to have been followed up. The extensive work by Vannice and his colleagues on palladium [156] and platinum [147] catalysts using a variety of supports has led to the speculation that spillover hydrogen on more acidic supports reacts with benzene adsorbed thereon and accounts for the higher turnover frequencies observed. Although within limited ranges of nickel particle size the turnover frequency appears to pass through a maximum [146,164-166] or to decrease with increasing size [146], when the results are viewed together [146] it is clear that this hydrogenation is essentially particle size insensitive, while exchange proceeds better on large particles than small [159 ]. There is no indication as to whether exchange can also proceed on the support, so the origin of its apparent particle size-sensitivity remains unresolved. With platinum [147] and palladium [156] catalysts, variations in TOF attributed to the support and to spillover of hydrogen onto it obscure any particle size effect there may be, but very high values are reported for platinum black [147]. Using 7-A1203 as support, however, TOF values were independent of particle size both for platinum [167] and palladium [168] (see [167] and [169] for reviews of early work).
508
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The substitution of one methyl group for a hydrogen has little effect on kinetic parameters for hydrogenation except the pre-exponential factor. With increasing degree of substitution by methyl groups the rate decreases progressively, but to an extent that varies both with the position of substitution and with the catalyst. To a first approximation the rate for a compound having n methyl substituents is given by rn = ro 2 -n where r o is the rate for benzene. The penta- and hexamethyl compounds are hydrogenated only with difficulty. The products obtained by hydrogenating the xylene isomers and more heavily substituted molecules consist of mixtures of the corresponding isomeric cyclohexanes; in the former case, the thermodynamically favoured cis-isomer predominates [9,151]. A comment on the hydrogenation of fused ring and heterocyclic systems was ventured at the end of the previous section. Of the metals of Groups 8-10, the base metals are least active, perhaps because of their propensity to induce hydrogenolysis and to form a carbonaceous overlayer, followed by palladium, which is distinctly less active than platinum [147,156]. However films of tungsten and iron are active at or below 273K, specific rates (mol h-lm2 xl02) for benzene hydrogenation at 358K and exchange at 373K have been given as respectively: platinum 3.0, 4.7; iridium 5.1, 0.089; osmium 0.9, 0.36; values for rhenium were too low to measure [ 160]. 11.3.3 Hydrogenation and exchange of aromatics on alloys There has been a number of studies of benzene hydrogenation on nickel-copper alloys [62,56,146,170-174]. Before going into further detail we must remember that in many cases the surface compositions were not measured and their relation to overall composition can only be guessed. A further feature for which there is evidence, and which can affect or even determine the activity pattern, is the tendency of alloys containing Group 11 metals to be less susceptible to poisoning than the pure active metal. In view of these uncertainties it is pleasing to note that three studies, using powders and covering the whole composition range, agree nicely: at higher temperatures (about 430K) rates are maximal for alloys containing 30-40% copper, but this maximum becomes progressively less evident as the temperature is lowered, because the apparent activation energies are higher for the alloys than for pure nickel. The results of other workers, although less complete, are not in disagreement [146,170]. The reaction has been estimated to need an ensemble of about three atoms, but hydrogenolysis to methane which is observed at high temperature [146,170] is quickly suppressed by adding copper and therefore needs a larger ensemble [ 146].
Catalytic hydrogenation and dehydrogenation
509
An equally clear but somewhat different picture emerges from a study of thermally equilibrated nickel-copper films, the surfaces of which contain a constant 23% nickel [171]. Rates of hydrogenation and exchange are constant in this region, the latter being much faster: the activation energy for the alloys is about twice that for pure nickel, and once again this could be due to the absence of deactivation in the case of the alloys. Not surprisingly there have been several reports of benzene hydrogenation and related processes on alloys of palladium [71,172,175-178] and of platinum [179-181] with metals of Group 11. On palladium-gold films [175], rates of hydrogenation of benzene and of p-xylene decreased rapidly with increasing gold content, although on alumina-supported catalysts the now-familiar increase in rate was seen [71]. There is a useful discussion [175] of this effect following the paper by these authors, concerning another reaction, in the proceedings of the Sixth International Congress on Catalysis. What was of particular interest with the films [175] was the persistence of exchange, albeit slow, at gold contents so high that hydrogenation had ceased. Similar findings have been reported for toluene [177]. These observations recall that made with the ethene-deuterium exchange reaction on nickel-copper films [26]. It is however difficult to reconcile results that point to the exchange reaction proceeding on a smaller ensemble than that needed for hydrogenation with the variation of specific exchange rate with particle size, noted above. The addition of copper to platinum in NaY zeolite leads to homogeneous alloys, the activity of which for hydrogenation of benzene [179] and toluene [181] decreases with copper content. Similar results have been found with Pd-Cu/SiO2 catalysts [178] and with powders [172], both of which show activation energies (-- 45 kJ mo1-1) that are independent for copper concentration. It has also been noted [178,179] that one effect of adding copper is to displace the temperature of the reversible maximum in the rate, sometimes attributed to desorption of hydrogen, to higher temperatures. Once again from the form of the rate variation it was concluded that an ensemble of three active atoms was required for hydrogenation [178]. Palladium-gold [172] and platinum-gold [180] catalysts show similar behaviour. The effects of adding copper to rhodium [179], iridium [179,182] and ruthenium [179,183] on benzene hydrogenation have been reported: in every case there is a rapid loss of activity as the copper concentration is increased. Adding tin to platinum also leads to a substantial loss in activity [73]. The well-known beneficial effects of adding rhenium to platinum have prompted the use of the reactions of benzene with hydrogen and with deuterium as a means of assessing surface composition, the nature of which is still the subject of debate [184]. In the Pt-Re/a-A1203 system [185], rates of both hydrogenation and exchange exhibit two maxima, one at 15-20% rhenium and the second at 75% rhenium: however the shapes of the curves are such that the ratio of the two rates rc/rh has a distinct minimum at about 60% rhenium. No detailed model has been developed to account for this behaviour and
510
chapter 11
indeed it is doubtful whether sufficient information is available to warrant the effort. We shall return to these alloys in chapter 13, where we show that mixed ensembles may be responsible for the maxima observed. The effects of adding rhenium to palladium [186] and to iridium [182] have been reported for benzene hydrogenation; the latter system also shows a rate maximum. The interesting behaviour shown by zeolite-encapsulated platinum + molybdenum alloys in nbutane hydrogenolysis is not reproduced in benzene hydrogenation [187], where the rate decreases monotonically with increasing molybdenum content. A similar lesser effect on benzene hydrogenation has been noted with Pt-Mo/SiO2 catalysts [ 188]. A Pt75-Zr25/ graphite catalyst shows a TOF one-third that of the corresponding platinum catalyst [189]. In a rare example of the use of technetium in catalysis, its effect on platinum for benzene hydrogenation has been described [190]; platinum-chromium alloys have also been examined [ 191 ]. Finally we must refer briefly to work on alloy catalysts comprising pairs of elements selected from within Groups 8-10: (Pt-Ir [185], Pt-Pd [174,192,193], Pt-Ru [ 174,194,195], Pt-Rh [ 174], Pd-Rh [ 196], Pd-Ru [ 174] and Ir-Os [ 185]). Most usually there are no dramatic variations in activity, but in the platinum-ruthenium system two independent studies [194,195] have shown the occurence of a substantial activity maximum towards the centre of the composition range. The same behaviour is seen in the platinum-iridium and iridium-osmium combinations [185], and the effect has been attributed either to bimetallic ensembles that are more active than those of either metal alone (see also chapter 13), or to a change in the strength of hydrogen chemisorption [194]. Work concerned with the liquid-phase hydrogenation of aromatic compounds bearing unsaturated substituents will be considered in the following section. To conclude this section on hydrogenation of aromatic compounds, it may be useful to draw some conclusions and to offer a few comments. (1) Benzene hydrogenation is well esetablished as a reaction the TOF of which is somewhat insensitive to particle size. (2) Exchange of benzene with deuterium is on the contrary particle-size-sensitive, TOF's being greater on large particles. (3) Hydrogen spillover may contribute significantly to the hydrogenation rate where the support is acidic. (4) Activities of pure metals vary considerably, for reasons that are not entirely clear. (5) Alloy systems sometimes show rate maxima, but only for the nickel-copper system is it yet possible to speculate on likely reasons. It is evident that in many reported studies the activity pattern (that is, the effect of the variable under investigation on the rate) has only been established under a limited range of experimental conditions, e.g. at a single temperature or at a single set of reactant
Catalytic hydrogenation and dehydrogenation
511
concentrations, and only rarely is the selectivity parameter represented by the exchange/hydrogenation ratio brought into play. We are therefore placed in the impossible position of having to try to interpret results based on a single section through the problem: we do indeed 'now see as in a mirror, imperfectly'. We may therefore make the following recommendation for acquiring fuller understanding of the correlation between catalyst structure and composition, and behaviour in reactions of aromatic compounds. (A). With an alloy system, the composition should be altered in increments of not more than 10% so that a complete picture of the effect of alloy composition can be obtained. (B) The temperature should be varied, an apparent activation energy derived, and the point of the rate maximum identified. (C) Orders of reaction for both reactants should be obtained for at least one temperature,
and the kinetic equation defined. (D) Wherever possible,
deuterium should be used in place of hydrogen, so that corresponding information may be collected on the exchange reaction at the same time: the additional insights obtained will more than compensate for the added cost and effort needed. This is all true for other reactions also.
11.4
Hydrogenation of other unsaturated groups
11.4.1 The problem of diffusion limitation We now move to a somewhat different area of catalysis, because although we are still concerned with the addition of hydrogen to multiple bonds (other than those between carbon atoms) the field of application is chiefly that of the fine chemicals industry and of the research and development activities that underpin it. The molecules to be treated are in the main of considerable molar mass and the functional groups interact strongly, e.g. by hydrogen bonding. Thus many of them are liquids or solids in the normal state, and their hydrogenation must therefore be conducted in the liquid state or in solution. This simple fact creates numerous difficulties for the scientists wishing to conduct fundamental research in this field; there are more variables to consider, and much greater difficulty in obtaining meaningful kinetic information, than is the case with gas-phase reactions. So while the relevant literature (some to be found in journals of organic chemistry) may seem to the catalytic chemist even more vague and imprecise than that which he is used to reading, the processes are of immense importance to the pharmaceutical and fine chemicals industry, and for this reason they command our attention. One of the principal difficulties encountered in working with three-phase systems (hydrogen, gas; catalyst, solid; organic molecule, liquid or in solution) is to secure conditions under which the rate of reaction will not be limited by the diffusion of hydrogen through the liquid to the surface of the catalyst. For this to occur, the hydrogen
512
chapter 11
must first dissolve in the liquid: relevant considerations are as follows. (i) The solubility at normal temperature and pressure: alcohols for example are good solvents for hydrogen (one liter of ethanol dissolves about 20cm 3 hydrogen), so the choice of solvent is important. (ii) The pressure: the solubility is approximately proportional to the applied pressure in the range of interest, so reactions are frequently conducted in autoclaves, i.e. pressurized reactors, working at 3-100 atm pressure. (iii) The temperature: solubility decreases with rising temperature and this counteracts the normal effect on the rate constant. (iv) The conditions of agitation" it is vitally important to secure intimate contact between the three phases, so that the distance that the dissolved dihydrogen molecule has to diffuse is minimised. Much ingenuity has been deployed in the design of reactors to achieve this end, but a detailed consideration of this subject is outside the scope of this work: the interested reader should consult the cited references [29,153-155] . The importance of these factors is as follows. If the rate of reaction is not. limited by the flux of dihydrogen to the catalyst surface, the reaction is under chemical control; but if it is so limited, it is said to be diffusion-controlled (or mass-transport controlled) and the catalyst is then being used at less than its full potential, because for part of the time it is idle while awaiting the arrival of a dihydrogen molecule. Reference to figure 1 of chapter 6 may aid appreciation of this problem. The generally desirable situation of chemical control is thus most probably encountered at low temperature, high pressure and with efficient agitation. The occurrence of diffusion-limitation is recognised by (i) a low temperature coefficient (activation energy less than 20kJ moll), (ii) a rate which does not increase in proportion to the weight of catalyst used, and (iii) a rate which rises when conditions of agitation are made more vigorous. The relation between these last factors is illustrated in figure 2. There are rare instances (fat-hardening is one of them) where it is advantageous or even essential to work under diffusion-limited conditions, e.g. where an intermediate reduction product is what is desired. The situation near the solid surface can also be discussed in terms of a Nernst diffusion layer, the thickness of which is inversely proportional to the flow of liquid parallel to the surface (see figure 3).
t~
figure 2 Rates as a function of catalyst weight and agitation conditions.
- o ,
rpm
7/'3
units
/
l / i
!
r
.o _ t.O
0
l ! ! h,," !000 2000 0 Speed of agitation (r.p.m.)
! l I ! 2 3 Catalyst weight (units)
Catalytic hydrogenation and dehydrogenation
513
The choice of solvent is important, not only because of the extent to which it dissolves hydrogen, but also because it may adsorb competitively with the reactant and thus in effect be a catalyst posion. The physical form of the catalyst will depend on the type of reactor used [29,197]" microporous supports of high surface area (e.g. activated charcoal) are not infrequently used, but then the metal needs to be located close to the external surface of the catalyst granule, otherwise the rate may become limited by diffusion within the pore structure, and this is altogether more difficult to overcome than diffusion limitation without the granule. Two other ploys for avoiding these difficulties may be noted. (i) It has been mentioned that with the use of ultrasound better mixing at the molecular level can be achieved. (2) The use of dihydrogen can be avoided altogether by substituting it by a hydrogen atom donor such as cyclohexene, formic acid or an alcohol; the catalyst then mediates the transfer of the atoms from the donor to the acceptor [198]. ',Ner nst l a y e r
figure 3 Variation of concentration at the surface with speed of rotation of catalyst in the solution (i.e. with rate of movement of liquid past the surface) The thickness of the Nernst layer, where the concentration is less than in the bulk, is inversely proportional to this rate.
r0
"-.:
-~
~ i
,,1-
I I
I I
I i
Bulk solution i
?
I
/J/I
/-/~
,
I
I
,
,
t
Distance
,,
from
surface
From the foregoing it will appear that it is extraordinarily difficult to obtain kinetic information such as the reliable Arrhenius parameters and orders of reactions for reactions proceeding in a three-phase system, and indeed much of the information available is qualitative (e.g. amount of product formed in the given time) or semi-quantitative (rate or conversion per unit weight of catalyst): selectivities where appropriate are of course informative. In spite of these limitations there is extensive information available to delineate the capabilities of metals and alloys in the hydrogenation of complex molecules.
514
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11.4.2 Reactions and catalysts The purpose of this section is to provide an overview of the principal types of reaction to be catalysed and of the chief problems encountered, and to enumerate the classes of catalyst that have found greatest use. The next section will briefly describe how these reactions and their attendant problems have been assisted by the use of alloy catalysts. The hydrogenation of compounds containing only a single functional group is generally uncomplicated; so it is when there are two reducible groups having quite different reactivities, as for example with nitrobenzene, which is easily reduced to aniline without touching the aromatic ring. The nitro(-NO2), nitroso-(-NO), keto-(> C=O) and aldehyde (-CHO) groups can be reduced to respectively the amino-group or secondary or primary alcohols with the right catalyst under the right conditions. Even in these cases there is the formal possibility of simultaneous or sequential hydrogenolysis of C-N or C-O bonds to form for example an alkane, although it is rarely difficult to avoid this occurrence. The homocyclic and heterocyclic aromatic rings may also be hydrogenated, but more forcing conditions are needed than for the groups listed above. Electron-rich substituents, such as -OH and -NH2, appear to help anchor the molecule to the surface, since phenol and aniline are more easily hydrogenated than benzene. The nitrile group (- C-N) is harder to reduce, and the carboxylic acid (-COOH) group is even more difficult. However, oxide - promoted metals, which sometimes can be prepared from alloys, are suitable catalysts for this reaction [199]. A review on nickel-copper alloys as catalysts for hydrogen addition to various groups is available [200]; it can be seen how scarce is the information on this subject. The fun really starts when the reactant molecule contains two functional groups of comparable reactivity: these may be either (i) two unsaturated groups, or (ii) one unsaturated group and another susceptible to hydrogenolysis. These may be illustrated by two of the classic problems of catalytic hydrogenation in the liquid (sometimes also in the gaseous) phase: the hydrogenation of unsaturated aldehydes and of substituted nitrobenzenes.
The first of these long-standing problems concerns the hydrogenation of molecules containing the conjugated C=C and C=O groups, as in acrolein, crotonaldehyde (2-butenal) or cinnamaldehyde (3-phenylpropenal) (see scheme XV). In these cases the desired products are the unsaturated alcohols and not the saturated aldehydes or the fully reduced products. Unfortunately the C-C bond is the more reactive, and much effort has gone into finding catalysts that will achieve the desired goals. The success attending the use of alloys will be noted in the next section.
Catalytic hydrogenation and dehydrogenation
515
Scheme XV
Hydrogenation of unsaturated aldehydes
CH3-CH = CH - CHO
CH3-CH = CH- CH2OH crotyl alcohol '~,
/
CH3- CH2- CH2 - CH2OH /n-butanol
CH3-CH 2-CH 2-CHO ' n - but y raldehyde
crotonaldehyde
/ C6 H5-CH = CH - CHO 'N~ cinnamaldehyde
C6 H5- CH = CH - CH2OH cinnamylalcohol "~k / C6H 5-CH 2-CH 2- CHO " 3- phenylpropanal
C6 H5- CH2- CH2 - CH2OH 3 - phenylpropanal
The second old problem, still attracting interest, is that of selectivity reducing an aromatic nitro-group in the presence of a halogen substituent (see scheme XVI). The aromatic ring activates the C-C1 bond, rendering it easy to hydrogenolyse, so that it is difficult to avoid the formation of some aniline. Once again we shall see that alloys have been able to provide some solutions to these problems. Scheme XVI
Hydrogenation of halonitrobenzenes (+ hydrogenolysis) NH2
X X Y
u NH2
z
E.g. X=Br, Y=Z=H X=Y= H,Z=CI
The metals used for catalytic hydrogenation in the liquid phase are largely confined to the noble metals of Groups 8-10, and nickel: the other base metals, and copper, do not in general possess the required activity, and of the noble metals it is palladium and platinum that enjoy the greatest use. A considerable mythology or (to put it more kindly) body of experience, has grown up concerning the suitability of particular metals to particular reactions [153,155], but the information is widely disseminated through the
516
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chemical literature, and it is only the occasional conference or collection of papers [155] that brings some of it together. It is nevertheless difficult to codify and impossible to rationalise. In the absence of a guiding theory, progress is haphazard and slow. As to the forms of catalyst favoured, some trends are evident. Unsupported metals (colloids, blacks: see chapter 7) are suitable only for small-scale laboratory work, but Raney nickel (usually promoted by chromium or other metals) has been and continues to be extremely popular as a general purpose and quite cheap hydrogenation catalyst. Other metals, especially the noble metals, can be prepared in the Raney (i.e. an alloy-) form (see also chapter 7), and can be modified by addition of other metal salts [201]. Much interest was shown a number of years ago in alloy catalysts prepared in the same way as Adams platinum oxide (chapter 7), and some of their interesting properties will also be described later. Most usually and especially for large-scale operations, the metals are used in a supported form, alumina and activated carbon being the most popular supports. The latter can also act to remove toxic impurities and metal is simply recovered from the exhausted catalyst by burning it. 11.4.3 Alloy catalysts in liquid-phase hydrogenation [153,202] There have been a number of studies of the hydrogenation of nitrobenzene using alloys as catalysts; of the pure metals, palladium shows the best activity (the reaction proceeds readily at room temperature), but addition of silver seems to suppress the rate through an increase in activation energy [5]. Quite remarkably higher rates are shown by combinations made by the Adams method, of platinum-rhodium [203,204] and platinumruthenium [205-207], than are possible with either metal individually. Even small additions of base metals (which are possible converted in situ into oxidic promoters) can effect considerable rate enhancements [206]. In the case of platinum in nitrobenzene hydrogenation, phenylhydroxylamine is an important intermediate product and although inclusion of tin initially gives an increase in rate the selectivity to the intermediate is not affected [208]. The reasons for these effects are somewhat obscure; they were originally (and inevitably) discussed in terms of electron/atom ratios, using the collective electron model, but more recent discussions have concerned the creation of bimetallic sites or, in the case of tin [208], the activation of the molecule through the oxygen atom by stannous ions. Alloy systems in the unsupported state sometimes show higher active areas than the single metals [204], and there is the possibility that, where as with platinum one or more [208,209a] intermediates are formed (this is not so with palladium or rhodium), the two components may be effective because each accelerates a different step in the total process [204,208]. This idea finds support in the observation [203] that physical mixtures of two supported metals can be more effective than either separately. One also has to remember that a component of an alloy that is easily oxidised by the reactant or a product may be
Catalytic hydrogenation and dehydrogenation
517
effective as a promoter (i.e. as ions) rather than by conventional alloying. The problem of hydrogenating a nitrobenzene on which a halogen atom is also substituted, without causing hydrogenolysis of the carbon-halogen bond, was noted above as a classic problem in liquid-phase hydrogenation catalysis. Highly dispersed Pt/A1203 catalysts give quite high yields of the desired intermediate p-chloraniline in the reduction of p-chloronitrobenzene, but its yield can be further increased by modification with an element of Group 12 (Zn) or 14 (Ge, Sn, Pb) [209]. A careful kinetic study [209a] leads to the conclusion that the selectivity improvement arises from a weakening of the adsorption of the required product, so that its chances of desorption before suffering dechlorination are improved. The literature contains few references to the use of alloy catalysts for the hydrogenation of ketones [202,210-212] and aldehydes [207,212,213]. Most attention has centred on the problem of the selective hydrogenation of unsaturated aldehydes containing the C=C-C=O group, as in acrolein crotonaldehyde or cinnamaldehyde (see scheme XV). On most metals the C=C bond is much the more reactive, so that reduction to the desired and useful unsaturated alcohols has proved difficult; however some significant progress has been made through the use of alloy or modified catalysts [201,214-216]. The addition of tin to a Pt~ylon catalyst raises the selectivity to unsaturated alcohol from about zero to 75% when the fraction of tin exceeds about 0.15 [214]; germanium is even more effective, selectivities of 90-95% being reported [215]. As remarked above, however, it is more than probable that these additives will be in a positive oxidation state when they act, and that they do so by engaging the carbonyl group in a way that activates it. An example of this mode of action may be provided by the Pt80Fe20(111) single crystal, which is both more active and more selective to unsaturated alcohol than P t ( l l l ) in the hydrogenation of crotonaldehyde [209b]. Other catalyst systems examined include Pt-Ni/TiO2 [216] and modified Raney cobalt [201]. There are many other reports, especially in the literature of the 1960's of the beneficial effects (chiefly in terms of rate) obtained by using alloys for hydrogenating organic compounds. The part of this work which is associated with the names of S.Nishimura in Japan [217], P.N.Rylander (Engelhard Industries [153,203,204]), Bond and Webster [202,205,206] (Johnson Matthey), and D.V.Sokol'skii (Alma Ata [207,218]) employed alloys prepared by the Adams route (see chapter 7). Molecules examined included benzonitrile [203,219], aniline [217], pyridine [203], 1,4-butyndiol [203,218], methylbutynol [206], cyclohexanone [206] and cyclohexanone oxime [204,219]. As noted above, because this work was performed at a time when measurement of surface area and surface composition was not routine, it is impossible to be certain of the cause of the generally observed higher activity shown by alloys. Probably a number of effects operate simultaneously; the likely complexity of the situation is high-lighted by the observation that the degree of rate enhancement depends on the solvent used [204]. It is also unclear
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to what extent these undoubted advantages of faster rates or lower catalyst loadings, have benefitted manufacturers in the fine chemicals sector. There is a strong preference to use supported metals rather than an unsupported form wherever possible, and the benefit derived by use of Adams alloys may not always be transferable to the supported state. The recovery of spent catalyst containing two metals may also prove more expensive than when only a single metal is present. Nevertheless it seems inevitable that the clear merits of modified platinum catalysts in making of unsaturated alcohols [214-216] will ultimately find commercial application in this and analogous reactions.
11.5
Dehydrogenation
11.5.1 Dehydrogenation of alkanes and cycloalkanes Dehydrogenation of the lower alkanes to the corresponding alkenes is an industrially attractive reaction, for the products are more useful and therefore more valuable than the reactants. Unfortunately because it is an endothermic process, significant conversions are only obtained at quite high temperatures, at which, when metal catalysts are used, other destructuve reactions such as 'carbon deposition' occur freely (section 11.1.2). This problem may be evaded by changing to a mixed oxide catalyst, or by attempting oxidative dehydrogenation using a metal or alloy [27]. Indeed the free energy change is in this case far more favourable due to the large heat of formation of water, although there has been little research on the role that metals and alloys might play in this direction. The dehydrogenation of cycloalkanes ('naphthenes') to aromatics is a desired process in petroleum reforming, and for this reason it has been widely studied, usually with cyclohexane as the typifying reactant; few studies have been reported in which methylcyclohexane [220] or cyclohexene [221,222] have been used. The free energy changes are more favourable for the cyclic alkanes than for linear molecules, because of the resonance energy of the product aromatic ring, and the equilibrium constant has a value of unity (i.e. there are equal concentration of C6 reactant and product at equilibrium) at about 560K. It is therefore possible to follow both the hydrogenation of benzene and the dehydrogenation of cyclohexane at the same temperature, although it is usual practice to employ a higher temperature range for the latter: a typical range would be 600-800K. Since pure metals, which are initially very active, are quickly deactivated by the formation of strongly-held 'carbonaceous residues' (see section 11.1.2), much effort has been applied to finding alloys or modified metals that will not suffer this drawnback. From a more academic standpoint interest has centred on the apparent lack of structure-sensitivity of the reaction, and thus on the number of atoms comprising the active centre. Since the early recognation that dehydrogenation of cyclohexane is a structure-
Catalytic hydrogenation and dehydrogenation
519
insensitive (strictly, ensemble-size insensitive) reaction as shown by the lack of dependence of TOF on copper content in the nickel-copper alloy series [223], and following the accumulation of evidence from a variety of sources [33,34] that the further dehydrogenation which presages formation of 'carbonaceous residues' needs larger ensembles, a great many papers have appeared, describing the capacity of alloy catalysts to sustain this reaction continuously. Unfortunately not all these papers address the fundamental questions noted above, and the surface composition
is not always estimated: the tendency of this
reaction to selfpoison militates against systematic basic investigation. Nevertheless the thesis that dehydrogenation can manage with a smaller ernsemble of active atoms than can 'carbon' formation is amply borne out of the observations. In almost every study it is found that it is the rate or extent of the latter process that the more rapidly suppressed as the concentration of the inert element is increased. Tin appears to be a particularly effective modifier for nickel [68,224-226], palladium [68,226] and platinum [227,230]; with this last system it has been suggested [229], that the effect of tin in weakening the adsorption of the hydrocarbon species [222] permits a readier migration of carbon precursors to the support, as it is found that a larger fraction of the carbon residues there than in the case of unmodified Pt/AI203 [229]. (N.B. Acid centres created by stannic ions in alumina would have the same effect). Modifiers such as antimony, tin and copper have a negative effect on dehydrogenation activity at low temperature, but a beneficial effect at higher temperature, at which self-poisoning is more of a problem [231]. Table 3 provides an overview of the alloy systems studied: while it makes no pretence to be comprehensive or complete, it does give some impression of the range of systems examined, and will provide an entry to the literature for those desiring to pursue the matter further. To conclude this section, we refer to a recent paper [222] describing a 'surface science' study of the dehydrogenation of cyclohexene on a Pt(111) surface modified by deposition of tin, in amounts such that the surface exhibited tin atoms in a p(2x2) lattice, corresponding to the Pt3Sn phase, a(~/3x~/3)R30 ~ lattice which implies the Pt2Sn phase. The binding energy of the cyclohexene molecule decreases with increasing tin concentration, which was taken to result from 'a substantial electronic effect of tin on the Pt(111) surface'. However, in the face of all the other evidence presented in this book, it is at least necessary to consider whether a steric or geometric obstruction of the platinum atoms might not have the same effect. The structure of the adsorbed cyclohexene changes from the di-c~ form on Pt(111) to a hydrogen-bonded form on Pt2Sn: this suggests the interesting possibility that its further dehydrogenation may occur without the need for it to be strongly chemisorbed.
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table 3 Tabulation of references to the dehydrogenation of cyclohexane by alloy catalysts. Ni
Pd
Pt
Ru,Os
Cu
223,231,233
-
-
234-236
Ag
232
237
-
Au
-
-
221,238,239
Sn
66,224-226
66,226
222,227-230
Sb
231
-
240
Pb
231
-
228
Active metal/modifier
Re
-
186
241
Cr
-
-
191
Mo
-
-
74
Ti
-
242
-
11.5.2 Decomposition of formic (methanoic) acid [9,243,244] Despite the fact that this reaction has hardly any practical importance, it has been studied by a great many workers, especially in the early days of the development of the basic ideas concerning catalysis by metals and alloys [9,244,245]; the earliest report dates from 1922 [246]. The reasons for selecting this reaction were (i) it is slightly closer to systems of real interest than, say, para-hydrogen conversion; (ii) the products are simple molecules; (iii) for which reason the reaction may be followed with very simple apparatus, e.g. by some barometric means, since reaction is necessarily accompanied by an increase in the number of molecules. The reaction can be performed in a static or flow-system; it is readily possible to identify the products and to determine the conversion by classical gasanalysis, since the only two modes of decomposition are HCOOH
--~
H20
+ CO
--~
H2
+ CO2
The first of these tends to be found on oxide surfaces, because of their ability to chemisorb the water molecule as hydroxyl groups. The second mode is favoured by clean (noble) metal surfaces, although in the case of base metals (e.g. nickel, copper) the watergas equilibrium H20 4- CO ~ H 2 4-
CO 2
521
Catalytic hydrogenation and dehydrogenation
may subsequently come into play, and confuse the picture. However, carbon monoxide appears to be a primary product over nickel, but not over copper [247]. The simplicity of the molecule and of the adsorbed species to which it may give rise have ensured that its chemisorption and decomposition have attracted the interest of physical chemists and surface scientists to this day: various types of vibration spectroscopy (e.g. EELS on copper [248], R h ( l l l ) [249]), XPS, XANES and other procedures have been deployed to assist the understanding of what happens [250,251]. The initial step in chemisorption involves the loss of the more acidic hydrogen atom, with formation of a carboxylate group that may occupy either one or two adsorption sites (see scheme XVIII). The released atoms desorb as dihydrogen and after longer exposures the other products also appear. Scheme XVII Forms of the chemisorbed formate group H
0
0
I
monodentate
H
H
C
C
I
I
C ,'
O' ~xx
',0 s s SS
bidentate
O'
I
',0
I
bidendate, bridging
The early pioneers of catalytic research lacked many, indeed most, of the tools of investigation now commonly available. Kinetic measurements, that is, of reaction rates and product selectivities, were the only method whereby reaction mechanisms could be probed, and the decomposition of formic acid is yet another example of a system for which it is difficult to gain a reliable picture by kinetic information alone. For example, in the cases of copper and silver, which show almost exclusively dehydrogenation, the rate is first order in formic acid pressure at low pressure and/or high temperature, but of zero order at high pressure or low temperature. This would suggest the operation of a Langmuir-Hinshelwood mechanism, with the rate being proportional to the concentration of adsorbed acid: r=kOF =kaPF/(1 +apF+...) where F represents formic acid and a its adsorption equilibrium constant. Depending upon the metal and the experimental conditions, further terms describing the inhibiting effect of adsorbed products can be added to the denominator. However, Tamaru and his associates [243,252-254] have shown by direct gravimetric measurement of the extent of adsorption
522
chapter 11
that the reality can be more complicated than this. With silver [253,254] the rate was indeed proportional to |
but with copper |
continued to increase in a broad range of
pressure, including the region where the rate was apparently zero order. Complications also occur with nickel [252,253]; although the rate is approximately first order the contribution of the fast first order reaction decreaseswith increasing coverage by formate, while a slow decomposition also contributes to the overall process as the infrared spectra clearly show. Formic acid decomposition appears to be structure-sensitive, since its rate depends on the geometry of the single crystal surface employed. On Ag(111), the activation energy is 67kJ mol-', whereas on the (110) face it is 127kJ mol -I [255]. Face-specificity of the reaction is also conveniently studied by field-emission microscopy [256]. There are numerous mentions in the literature of the effect that the type of pretreatment has on activity and kinetic parameters for this reaction; the role of defects has also been stressed [257,258], and it is significant that these effects are greatest for the least active metals, such as those of Group 11. This is because electron-deficient sites may possess enhanced activity and may be present in greater concentration in defective surfaces. There was much confusion in the early literature on hydrogen chemisorption on copper, for this reason [9]. Sufficient information has long been available to permit speculation as to how the solid state properties of a catalyst determine its efficiency for his reaction, whether expressed as the rate at some chosen temperature, or as the inverse of the temperature at which a selected rate is obtained. Since it is the decomposition of the formate group, in one its manifestations shown in scheme XVIII, that is likely to be rate-determining, it is reasonable to examine whether activity correlates with the heat of formation of the corresponding metal formate, expressed per metal ion equivalent. For the metals of Groups 8-10 the rate decreases as the heat of formation rises, platinum and iridium showing the highest activity and the base metals the lowest, see figure 4:
~3"0 I
Pt
" 2.8[
i
I
:
'qu ~u
~.
3.0[
u
eIr
I
,
,
,
figure 4
j
='~,o 2.4
, ,do 2-4lu
~ .~- 2 . 2 -
0 N~
~" "~- 2 2
go 7b
9b ,60 I10
Heat of formation of metal formate
(kcal..e~uiv.-1)
9Ag
~
~-~
...~ 2.o o_.~ 1.8
,
28 f ~ . 2-6
2"6
69
,
2.0
J
Heat of formation of metal
formate (kcal..r
-1)
il0
continued
Catalytic hydrogenation and dehydrogenation
523
oPt olr" Tp
400
o
~
6oor[ 7'o
~iF.e~
log r. -0.8
~o ,& Heat8b of formation, kcal/equiv.
,;o
figure 4 Activity in formic acid decomposition, whereby the temperature of reaction or it's reciprocal is taken as a measure of activity, plotted as a function of the heat of formate formation per metal equivalent.
For the metals of Group 11, however, the reverse is true (figure 4, page 522, right), so that when all the results are plotted together a kind of 'volcano curve' is obtained (figure 4, this page) [244,256,259]. Such curves imply that there is an optinum strength of adsorption for the reactive intermediate; if less, the surface is not fully utilised, if more, the intermediate becomes too stable and unreactive and the reaction in effect is self-poisoned [29]. Correlations of this type were predicted by Sabatier [162,260] and by Balandin [261] and more recently by Schuit et al. [262,263]. Naturally, values of parameters such as the heat of formation of the key intermediate are not always available. In such cases one may make use of the observation [262] that the heats of formation of many types of compound are linearly related, so that for example the heat of formationm of the highest oxide (i.e. where the element is in its highest possible oxidation state), expressed per metal ion equivalent, may be universally used as the basis for correlations. Extensive and accurate data are available for heats of formation of oxides. Subsequently Tanaka and Tamaru [263] proposed the use of the heat of formation per oxygen atom, i.e. the strength of the average metal-oxygen bond in the relevant oxide, as the basis of comparison. However, in the case of formic acid decompo-
524
chapter 11
sition, it does not matter greatly which parameter is used. Numerous studies of formic acid decomposition of alloys have been conducted and work performed before 1962 has been reviewed [9]. Inevitably much of this early work is of semi-quantitative value only, as the surface concentrations and cleanliness of the catalytic surfaces were not adequately defined or controlled. Nevertheless, some systematic regularities have been observed. Not surprisingly attention has been focussed on the Group 10-11 systems, and the nickel-copper [9,247,264], nickel-silver [264] and palladium-copper [9,265],-silver [9,266] or-gold [9,267-269] combinations have proved especially attractive. However in view of the uncertain quality of the early studies [9] it is equally unsurprising that no fully consistent picture emerges with regard to the dependence of activity on composition. All that is certain is that the reaction on Group 11 metals proceeds more slowly and with a higher activation energy than on Group 10 metals. In the palladiumsilver series, the rate is almost constant in the range 100-70% palladium [266]; the activation energy can also be constant over a wide range of composition [9]; and in the case of the nickel-copper system [247] this is so over the entire range. This last study is one of the most recent and thorough studies of formic acid decomposition and it employed both powders and silica-supported catalysts; it was concluded that the TOF per nickel atom was constant, and that
the active centre for this structure-insensitive reaction
comprised just one or two nickel atoms. This conclusion does not necessarily conflict with the observations mentioned above concerning pretreatment and defects which relate particularly to sp-metals. Other alloy systems have been investigated although none recently. Schwab described his results, obtained over a long period of time, for alloys involving only spmetals [245]; although some systematic variations of activation energy on electron/atom ratio were found, one may think with benefit of hindsight that activation energy is an all too capricious measure of activity, and too likely to be influenced by the pretreatment and pre-conditioning of the surface [266]. Other systems which have received attention include silver-gold [270], copper-gold (ordered and disordered) [271], iron-nickel [272] and alloys of iron, cobalt and nickel with chromium, vanadium and molybdenum [273]. In this last case, activation energies correlated with a decrease in heat of hydrogen chemisorption at 30% coverage, and with an increase in the ratios of the numbers of electrons in the s- and d-bands, although it was unclear how these were estimated. As with all formally simple catalysed reactions, close investigations using the techniques of surface science [247,265] reveal unsuspected complexities and wealth of information of much interest to the theory of heterogeneous catalysis, and formic acid is likely to remain an object of study for some time to come.
Catalytic hydrogenation and dehydrogenation
525
11.5.3 The decomposition of hydrogen peroxide The necessity to study this reaction in the liquid phase has advantages and disadavantages; the former include the simplicity of the apparatus needed, and the ability to examine electrocatalytic aspects; the latter include the sensitivity of the rate of release of gas bubbles to the gross topography of the surface, and the sensitivity of the rate to pH [274,275]. The overall reaction is of course 2H202 ----) 2 H 2 0 + 0 2
and its mechanism has been a subject of speculation since the end of the last century [276]. The Haber-Weiss formulation of the homogeneous reaction [9,275] defines the initial step as H202 4- e-
~ HO. + OH-
but the increase in activity observed at alkaline pH [274,275] suggests [275] that it is the anion HO2- rather than the neutral molecule that may be the reactive species, because chemisorbed oxygen appears to be a reactive intermediate, at least on palladium-gold alloy wires: H202
~
H+
+
H O 2-
~
0 2
4-
OH- +
O
+
OH-
H O 2-
4-
O
9
4-
HO 2- --~
*
By this last reaction the surface is re-oxidised, so that the cycle may continue and the hydroxyl ions combine with protons to form water. McKee [274] has measured the specific rates given by the noble metals of Groups 8-11, and his results are shown in table 4. Alloy powders of platinum with palladium, rhodium or ruthenium prepared by borohydride reduction of salt solutions, were also examined, the rates being monotonic functions of composition. With palladium-gold alloys prepared in the same way, rates at various pH's showed maxima at about 20 wt% gold, although the rates given by wires [275] were constant across the entire composition range. Although, based on results for the nickel-copper system, it was originally claimed [277] that electron-rich sp-metals were more effective than those having d-band vacancies, in accordance with expectations based on the initiation of the dissociation by electronacceptance as shown above, it now appears [275] that the position of the Fermi level has little or no influence on the catalysis, and that the strength of the metal-oxygen bond may be the determining factor. This idea receives some qualitative support from the data in
526
chapter 11
table 4. For further discussion of this subject, the reader is referred to the cited appers [274,275], which although of some antiquity represent the most recent discussions of the mechanism of hydrogen peroxide decomposition. The reverse reaction of hydrogen peroxide formation, from di-oxygen and dihydrogen is briefly discussed also in section 11.6 and in chapter 12. table 4 Activities of metals for hydrogen peroxide decomposition [274] metal Ru Rh Pd
rate(cm-2min~)x 106, 300K 0.91 0.66 2.2
Os
16.2
Ir Pt Au
6.2 19.4 0.079
11.5.4 Decomposition of alcohols The decomposition of methanol is another reaction which attracted early attention in the development of theories of metal and alloy catalysis [9,278]: it achieved practical interest at the time when methanol was being considered as an energy source for fuel cells (see chapter 12), which however sometimes operated more easily with its decomposition products: CH3OH
---) CO + 2H 2
This time is perhaps now past, but there is some continuous attention to this and related reactions in connection with the stability of oxygenated molecules on catalysts that can synthesise them from syngas. In addition to the straightforward dehydrogenation shown above, other simultaneous or sequential reactions may occur, yielding methane, water, carbon dioxide, formaldehyde (methanal) and methyl formate (methyl methanoate) as products.
527
Catalytic hydrogenation and dehydrogenation
11.6
Hydrogenation of diatomic molecules: oxygen and nitrogen This chapter concludes with brief treatments of the reduction by hydrogen of two
contrasting diatomic molecules. The reaction of hydrogen with oxygen is considered by some as the archetypal structure-insensitive reaction (more about this in chapter 12), proceeding with facility on many metal and alloys, indeed with explosive violence in the case of the most active: an impressive lecture demonstration can be provided by this system. The synthesis of ammonia on the other hand is certainly very structure-sensitive and only a few catalysts can catalyse this reaction. However in both cases the number of possible adsorbed intermediates is very limited, but this does not stop the derived kinetic expresssions having some degree of complexity. There have been relatively few detailed kinetic studies of the hydrogen-oxygen reaction. Over palladium the rate is first order in hydrogen concentration, and there is no equilibrium between dihydrogen and dideuterium when the reaction is performed with their mixture [279]. Thus in agreement with expectation based on the relative strengths of adsorption the surface must be almost completely covered with oxygen atoms, to the virtual exclusion of hydrogen. The reaction may therefore proceed either by a kind of Eley-Rideal mechanism (scheme XIX): Scheme XVIII H"
or
"
" n s
H2
+
O
~
"O~
~
H20
+
H2
+ 20
~
2OH
~
H20
+
O
or by a normal (Langmuir-Hinshelwood) bimolecular process, utilising a very low concentration of hydrogen atoms: Hads +
Oad s
----ff OHaas
-------~H20
Kinetic measurements do not allow discrimination between these possibilities. The study [279] of the palladium-gold alloy system does however contain a major surprise. We have already noted a number of instances where the addition of gold to palladium leads to an increase in activity: a similar increase is noted here, but it is significantly larger than any of the other instances quoted. It occurs, as a broad peak, at about 60% at gold, at which composition the TOF is some 50 times greater than for pure palladium. The effect is ascribed, inevitably, to a ligand effect, but no direct evidence has been produced to substantiate the idea. It is interesting to note that Tammann's work [280]
528
chapter 11
on this reaction on palladium-gold and -silver alloy wires is probably the first scientific study of catalysis by alloys. The industrial importance of ammonia synthesis has ensured that it has been a subject for fundamental research for many decades. The main outlines of the mechanism were established by the Russian scientists Temkin and Pyzhev in the late 1930's, and their work was supplemented by distinguished contributions from the Americans (Kokes and Emmett; Boudart and Taylor) and the Dutch workers at Dutch State Mines (Scholten, Zwietering). Their work has been extensively described and reviewed [9,28 l] and it is not intended to provide a lengthy recapitulation here. Briefly, on promoted iron catalysts, dinitrogen chemisorbs with difficulty and there is a considerable coverage of the surface by hydrogen atoms. Nitrogen chemisorption may well be the slow step, at least in some circumstances, and be related to the presence of 'deep' active sites and defects in the surface (see chapters 5 and 6, discussion on particle size effects). Argument has centred on questions such as the following. (i) Does dinitrogen chemisorb directly onto bare iron surface, or does it react with an adsorbed hydrogen atom? (ii) What precisely is the role of the basic promoters (K +, Ca 2§ etc.) and do they accelerate nitrogen chemisorption or strengthen the chemisorption in molecular form? (iii) What is the most abundant surface intermediate (nitrogen atoms or imino (NH) groups)? To answer the second question, the full panoply of surface science methodology has been applied, and results of very high quality have been obtained. Two groups of workers have endeavoured with some success to use information obtained from single crystal studies to estimate rates under industrial conditions [282]. It is probable that research on the mechanism of ammonia synthesis has now reached the point of diminishing returns, there being only nuances and shades of meaning left to be resolved. While metals to the right of iron are inactive for ammonia synthesis, iron's activity can be improved by inclusion of 5% cobalt [283-285]. Much attention has also be given in recent years to the development of an effective ruthenium-based catalyst for this process [286].
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532
92
93 94
95 96 97 98 99 100 101 102 103 104
105 106 107 108 109 110
111 112 113 114 115 116 117 118 119 120
chapter 11
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260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281
282 283 284
Catalytic hydrogenation and dehydrogenation 285 286
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OXIDATION REACTIONS In this chapter two groups of oxidations will be discussed: (i) oxidation by dioxygen and (ii) oxidations by nitrogen oxides. In both cases alloy catalysts have already been applied.
12.1
F u n d a m e n t a l s - c h e m i s o r p t i o n of the reactants J
Dissociative adsorption of dioxygen occurs on all the metals of Groups 1-13, except on gold, even at quite low temperatures [1,2]. At slightly elevated temperatures, this develops into bulk oxide formation, and thin foils of platinum and silver become even permeable to oxygen at high temperatures. Only at very low temperatures does molecular adsorption of dioxygen prevail. Although the heat of molecular adsorption is about 40 kJ mo1-1, the sticking probability (i.e. fraction of colliding molecules that remain adsorbed) is near to unity. It is interesting that in the molecularly adsorbed state the molecule lies parallel to the surface. At slightly elevated temperatures dissociation occurs [3]. At higher temperatures the sticking probability decreases, because the molecular precursor to dissociation is no longer stable and it desorbs, while the slightly slower dissociative adsorption cannot catch all impinging molecules. Some other general aspects of oxygen adsorption, such as adsorption bond strength and adsorption sites, have already been mentioned in chapter 1 of this book. The adsorption of molecules to be oxidized is briefly discussed elsewhere in this book: hydrogen in chapter 1; alkenes and saturated alkanes in chapters 1, 8-11 and 13; carbon monoxide in chapters 8 and 14. Let us now focus our attention on those molecules the adsorption of which is not discussed elsewhere in this book.
Nitrogen monoxide (nitric oxide) It is mostly because of the necessity to remove this gas from various waste gas mixtures that people are interested in the adsorption and reactions of this gas. However, it is also (together with nitrous oxide) a potentially interesting source of oxygen for selective oxidations [4]. When nitric oxide is adsorbed on a rhodium filament at 80K and its thermal desorption is followed, desorption peaks due to nitric oxide, nitrogen and dioxygen are observed (see figure 1). The heat of adsorption of molecularly adsorbed nitric oxide is
542
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about 110 kJ mo1-1 [2a,b]. In many respects these results are also representative of the other Group 8-10 metals. Some information is available on all of them but we now know most about platinum, palladium and rhodium, the components of the automotive catalysts [2c]. TDS
NO / Rh
'
2;0
'
figure 1 Temperature Programmed Desorption of NO, N2 and 02, following adsorption of NO on a rhodium filament at 8OK.
~;o
'
6;o
'
8;o
'
,doo'
,;'oo
TtK)
The electronic structure of nitric oxide is similar to that of carbon monoxide (see chapter 1). It has however one more electron, which occupies the 2rt antibonding orbital and is responsible for its higher reactivity and its higher tendency to dissociate, when compared with carbon monoxide. The preferred orientation of nitric oxide is perpendicular to the surface, with the nitrogen down, coordinating so that it sits either in a hollow or 'on top'; in many cases it has been observed to be tilted, more frequently than is the case with carbon monoxide. Tilted molecules are supposed to be precursors of dissociation [2]. As already mentioned, dissociation of nitric oxide is easier than that of carbon monoxide. Metals of the Groups 8-10 show important similarities, but also a great variety in details of their behaviour. Palladium is a metal that is very reluctant to dissociate nitric oxide; it clearly prefers molecular adsorption and at low temperatures dimers of nitric oxide have been reported on Pd(100) [5]. Other planes of palladium are similar [2,6,7], but other metals show a higher activity in dissociation [2a,b] as the following examples demonstrate. Platinum has planes of different activity: the (111) plane is quite inactive [8] but defects increase its activity [9], and the more open and stepped surfaces are most active [10]. With carbon monoxide, iridium is just as inactive in dissociation as platinum, but with nitric oxide Ir(111) and Ir(100)-stepped show some dissociation even at 300K [11]. Not surprisingly, the element still further to the left in the Periodic Table, i.e. rhenium, readily dissociates nitric oxide at ambient temperature [12]. The same trend holds likely in other periods of the table, too. Rhodium dissociates nitric oxide [6,13] and this is one of the reasons why this element is used for the automotive catalysts (the other reason being that nitrogen is not converted on rhodium into undesired ammonia). Ruthenium seems to be even more active in dissociation than rhodium [14], and in the first long
Oxidation reactions
543
period, appreciable activity in dissociation starts with nickel [15]. It seems that dissociation is a reaction requiring an ensemble of at least two active atoms [16]. A high surface coverage by molecular adsorption can block the dissociation, which seems to require free sites.
Adsorption of nitrogen, ammonia and some related molecules In comparison with carbon monoxide or nitric oxide, dinitrogen is quite inert, although there is again some similarity in the electronic structure [17,18]. The heat of its molecular adsorption is only about one-third of those of the other molecules. The early transition metals adsorb dinitrogen dissociatively and allow formation of nitrides, but of the Groups 8-10 metals only iron, osmium and ruthenium exhibit dissociation at not too high temperatures. The dissociation occurs through a weakly bound state of molecular adsorption and is promoted by defects in the structure [19]. This suggests a pronounced crystal face specificity for this adsorption, which is also established by other experiments [2,20], already discussed in chapter 6. Unlike dioxygen, dinitrogen is adsorbed preferentially with its molecular axis perpendicular to the surface [21,22]. Thanks to its electrons in the non-bonding molecular orbital, ammonia is easily adsorbed by many metals with nitrogen down. On some metal planes it is adsorbed in a multicoordinated form in hollow sites, while on some others the on-top adsorption is preferred [23]. Pre-adsorption or co-adsorption of oxygen promotes the adsorption (see chapter 1). The heat of molecular adsorption is comparable to that of nitric oxide. A comparison of the activity of different metals for ammonia decomposition has been made by Logan and Kemball [15]; a comparison between single crystal planes of various metals, or metals in form of filaments, will emerge from papers on ammonia decomposition (Pt [25], Rh [26], Ru [27]). Platinum is a very active metal in forming or splitting the Na~s-H bonds [28], but it can adsorb dinitrogen dissociatively only at such high temperatures that the formation of ammonia is very much disfavoured thermodynamically. This is in compliance with the conclusion [15] that the recombination-desorption step of adsorbed nitrogen is rate determining in ammonia decomposition on metals [15]. When adsorbed nitrogen atoms can be formed from another molecule, for example from nitric oxide, then ammonia is formed easily on platinum, which aspect is a matter of concern with automotive catalysts. There, the formation of ammonia has to be avoided. Hydrazine, NzH4, is a molecule for which a fast decomposition/oxidation is most desirable, since it is often used as a booster for jet motors. As with ammonia, pre- or coadsorption of oxygen promotes adsorption of hydrazine. Iridium and rhodium are very active metals for these reactions [29]. Amines are adsorbed in a similar way to ammonia, the nitrogen-bonded hydrogens being more reactive than those attached to carbon atoms [30].
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Adsorption of dinitrogen oxide (nitrous oxide) On s,p-metals with low work function, dissociative adsorption of nitrous oxide prevails down to low temperatures. On noble metals, such as platinum, and in particular on its high work function planes, the prevailing form is a weak molecular adsorption. This means that when nitrous oxide is formed on those planes, it desorbs very easily. When molecular adsorption is observed, the UV-photo-emission spectrum can be fully explained by theoretical calculations [31]. Easily occurring dissociation, e.g. on rhodium, is documented by a number of papers dealing with polycrystalline [32,33] and monocrystalline surfaces Adsorption is in all cases most likely terminal, with
[34,35,36].
nitrogen down, often with the
molecular axis tilted. The N-N bond strength is higher (476 kJ mol -~) than the N-O bond strength (180 kJ mol-~), so that with all metals dissociation into dinitrogen and an adsorbed oxygen is the most likely pathway [38]. The higher the affinity of the metal for oxygen, the higher the tendency to dissociate the N-O bond in the nitrous oxide molecule. Ruthenium, tungsten [35] and iron [37] are examples of it. The activity order therefore follows the sequence which we meet also with other reactions: Ru > Rh > Ir > Pt.
Adsorption of products of oxidation reactions - water, carbon dioxide. These are two very stable molecules (thermodynamic sinks) and, once they have been formed, they desorb easily [2] under the reaction conditions used in most practical applications. As far as water is concerned, most of the available information is again on platinum, rhodium and ruthenium. Adsorption on metals is mediated by the electrons on the oxygen atoms [39-43]. Coadsorbed or preadsorbed oxygen can strengthen the interaction of water with the metal surface. On Pt(111) adsorbed water makes hydroxy groups from preadsorbed oxygen [43,44], but on Rh(111) this reaction occurs to a lesser extent under otherwise comparable conditions [43]. Carbon dioxide can be adsorbed in various modes, as one can learn from the extensive review by Solymosi [45]. With co- or preadsorbed oxygen, carbonates can be formed [45]. Adsorption is crystal-face and thus structure sensitive, as results obtained by FEM [46] and by experiments with macrocrystal planes [47] revealed. Dissociation of the C-O bond occurs very easily on s,p-metals as well as on rhenium [45]. On rhodium, molecular and dissociative adsorption coexist, dissociation occurring at temperatures above 165K [48]. This contrasts strongly with the behaviour of palladium, iridium and platinum where molecular adsorption persists up to the temperature of desorption [48-50]. Adsorption on the Group 11 metals is weak and molecular [45]. Van Tol [48] summarizes the adsorption modes possible on platinum and rhodium in the way shown in figure 2.
Oxidation reactions
545
Pt
0 0 C
;
"q:) 8 (3
I
Rh
0
0
O.
0
CI
M
figure 2 Possible adsorption geometries of carbon dioxide on platinum and rhodium. The bending of carbon dioxide on rhodium is at low temperature probably not so pronounced as indicated in the figure, although in the limit, near to dissociation, the M-C-O angle could be quite large.
Effect of alloying on adsorption The Pt0.25-Rh0.75(ll1) single crystal plane has been studied by means of carbon monoxide adsorption. It appeared that platinum sites (i.e. sites characterized by the corresponding stretching vibration frequency of adsorbed carbon monoxide) adsorbed more strongly than rhodium sites. Platinum, which segregates to the surface, tends to form clusters there, but by repeated desorption and readsorption of carbon monoxide the clusters can be disrupted and the platinum atoms dispersed in rhodium [51]. Adsorption of carbon monoxide on other alloys is discussed in extenso in chapters 8 and 14 (see also [51]). Adsorption of oxygen and sulfur, even in minute amounts, stimulates very strongly the segregation of rhodium to the surface of platinum-rhodium alloys [52]. On the other hand, carbon monoxide, whose affinity for rhodium and platinum also differs, does not cause measurable changes in the surface composition [53] although, under certain conditions, it changes the distribution of components. Much is known about adsorption of nitric oxide on the platinum-rhodium alloys. For example, on the Pt-Rh(100) surfaces its adsorption is accompanied by other parallel and consecutive processes (notice the difference from the (111) plane [53]): dissociation induces extraction of rhodium from the bulk of the alloy and reconstruction of the outermost layer into a stable ordered oxygen layer. On rhodium-rich alloys these steps are faster than on rhodium-lean alloys [54]. In this and also in some other papers [53,55,56] the extent of dissociative adsorption of nitric oxide was compared on Rh(100), Pt0.zs-Rh0.75(100) and Pt(100) surfaces. Its extent decreased in the indicated order. The alloy surface was interesting in that at low nitric oxide coverages it behaved like a rhodium surface but at high coverages as a platinum surface. The behaviour of other potentially interesting alloys is not known in such detail,
546
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but something can of course be derived by analogy with the above information and from information on nitric oxide adsorption on individual metals (see above). Surprisingly, there is no information on fundamentally interesting, but practically rarely used alloys as for example platinum-palladium or palladium-gold, etc.
12.2
Selected information on simple oxidation reactions on metals
Oxidation of di-hydrogen The practical application here is not the production of water, but the removal of oxygen from hydrogen. Oxygen-free hydrogen is required not only for catalytic processes but also in the nuclear energy industry in the production of deuterium. The long-used Pd/A1203 and Pt/SiO2 catalysts have been replaced in many applications by a very efficient, stable and high surface area Ni/SiO 2 catalyst, promoted by various additives such as chromium oxide. Results of experiments with single crystal planes, with the micro single crystal planes of the field emission tips, and with evaporated films and powder catalysts, allow us to put the metals in the sequence of decreasing activity, as follows [2,57]: Pd,Pt > Ir > Rh,Ru,Os > Ni > Fe,Co .... + other metals It is known that interaction of preadsorbed oxygen with atomic hydrogen takes place even at 80K [57]. The more difficult step is the recombination of hydroxyl groups OHads + OHads
~ Oad s +
H20
(1)
which is obviously a reversible reaction (see section 12.1). The existence of such a reaction as the rate-determining step was postulated some time ago on basis of indirect evidence [58]. The XPS and UPS experiments failed to show evidence for hydroxyl groups as intermediates [59], but EELS and laser-induced fluorescence have proven their existence as intermediates on rhodium and platinum [60]. It would be now extremely interesting to know how various alloys behave in the formation and recombination of hydroxyls, but this information is not yet available. Early experiments in which the interaction of hydrogen atomized in the gas phase, and of molecular hydrogen, with oxygen preadsorbed on different metals revealed that hydrogen reacts from the adsorbed state. This makes it very likely that the whole reaction is of the Langmuir-Hinshelwood type. The field emission observation has nicely visualized [2] how patches of adsorbed oxygen are, starting from their outskirts, successively removed by mobile hydrogen atoms. These choose selectively the sites with the weakest
Oxidation reactions
547
oxygen-metal bonds at which to react with oxygen atoms. This fits well the picture that the sequence of Group 8-10 metals according to their activity as shown above follows the reversed order of the metal-oxygen bond strength [2].
Oxidation of carbon monoxide by dioxygen This reaction is not performed in order to prepare carbon dioxide. One needs however to remove carbon monoxide from other gases, as from waste gases of combustion installations or of motors, stationary or moving. Its removal by oxidation is also the purpose of the catalytically active filters (in gas-masks, or upon refreshing respiratory gases in closed spaces such as spaceships, submarines, etc.). Low temperature oxidation of carbon monoxide has also to be achieved in long-working CO2 lasers [61]. The relation of the observed catalytic activity to the parameters characterizing the metals or their oxides is less straightforward here than with the hydrogen-oxygen reaction. For example, it appeared that at low temperature (T < 500K) the Rh(111) surface shows a higher activity than the Rh(100) surface, but above 500K, the order of activity is reversed! [62]. Since both reactants are strongly adsorbed and on most metals the heat of carbon monoxide adsorption is higher than the heat of molecular oxygen adsorption (the precursor of dissociation), carbon monoxide makes oxygen adsorption more difficult and the order of metals' activities must depend on the Pco]Po2 ratio [63]. According to everything that is known, the reaction on Pd(111), and most likely on all metals, is of the Langmuir-Hinshelwood (L-H) type, as experiments with molecular beams and research by a combination of all relevant surface science techniques very convincingly has shown [64]. When the parameters obtained with these techniques are substituted into the corresponding L-H kinetic equations a very good fit is obtained to experimental results with model and also with practical catalysts [65]. However, this is only true for the steady state reaction in the region of high temperatures where the surfaces are spasely covered by reactants. The rate in the steady state under standardized conditions with platinum metals shows frequently a very typical dependence on temperature as shown in figure 3. The increase reflects the increasing extent of the surface free of carbon monoxide with increasing temperature. Hence the activation energy is practically equal here to the heat of adsorption of carbon monoxide. The decrease in rate with increasing temperature is due to two facts: the extent of carbon monoxide adsorption dramatically decreases and so also does the sticking probability of dioxygen [2]. The reaction seems here to be quite structure insensitive and can be brought into oscillation. For platinum this is very well explained by surface reconstructions [66], but an alternatively explanation based on a feedback reaction step of free-site generation can possibly explain the results too (see, nitric oxide reactions below).
548
chapter 12
ei oxid rate
figure 3 Surface coverage of a metal (e.g. plati-
8 (CO)
num) by CO and oxygen and the rate of
,, I [
oxidation, all as a function of temperature (schematically); to the left of the line the surface is CO rich. If the rate
oxidation .
"
.
constant is not a function of the O's el0)
and if the rate is exactly first order with respect of both O's, then the maximum of the oxidation rate is at T, where both O's curve intersect. T
Oxidation of carbon monoxide by nitric oxide This is a reaction of great importance for automotive catalysts. There is good evidence that the reaction comprises the following steps: nitric oxide dissociation, the Oads + CO,d s reaction and recombination/desorption of Nads as dinitrogen [67]. It thus has many features in common with the other reactions discussed above. The rate-determining step is, according to the metal and reaction conditions, either the nitric oxide dissociation or the COads + Oads reaction. The activity of platinum metals increases in the order Pt < P d < Rh < Ru [68a] and the kinetics have been already established for various metals [68b]. The activity order reflects the propensity for nitric oxide dissociation and (inversely) the strength of the inhibiting effect of carbon monoxide, the latter being more pronounced at lower temperatures. However, at high pressures of nitric oxide, its dissociation can be suppressed by the lack of the necessary vacant sites [69]. Nitrogen atoms produced by nitric oxide dissociation can either recombine and desorb, or react with carbon monoxide to form an isocyanate group (-NCO) [70]. This is unstable on metals, but when it can migrate to the support it can survive there and be detected in the IR spectra [70]. This reaction also can be brought into oscillation [71]. It is important to note that these oscillations have been explained, for surfaces which are not being reconstructed under operating conditions, by the so called 'vacancy' model. The vacancy must be present in the adsorbed layer to enable the nitric oxide to dissociate [72]. Since oscillations in carbon monoxide oxidation on palladium cannot be explained by reconstructions because most of the palladium single crystal planes do not reconstruct, it could be that the vacancy model is actually a more general explanation of oscillations [73] than any other. It is very likely that this reaction is of the Langmuir-Hinshelwood type [74].
Oxidation of hydrogen by nitric oxide It can be expected that, with the possible exception of palladium, dissociation of
Oxidation reactions
549
nitric oxide plays an important role in this reaction. There should then be some similarities observed between this reaction and oxidation of carbon monoxide. However, there are also remarkable differences. While for oxidation of carbon monoxide by nitric oxide the activity order is Ru > Rh > Pd > Pt, for oxidation of hydrogen by nitric oxide it is: Pd > Pt > Rh > Ru [68]. Obviously, in the absence of carbon monoxide and its strong adsorption, in particular suppressing nitric oxide adsorption on platinum and palladium, the order is more like that for the hydrogen-oxygen reaction. This shows a structure sensitivity which parallels that for nitric oxide adsorption and its dissociation. When the surface coverage by nitric oxide is too high, dissociation is inhibited and this also plays a role here [75-77]. With most of the metals studied, the sequence of reaction steps starts with nitric oxide dissociation, only with palladium the first step could be hydrogenation in the direction of hydroxylamine, followed by dissociation.
Oxidation of ammonia by dioxygen The stoichiometric equation 4NH 3 + 5 0 2 ---) 4NO + 6H20
(2)
indicates the start of the way by which nitric acid is produced from ammonia. In excess oxygen and at suitable temperatures (800-900K) the higher oxide N204 is also formed, which on being dissolved in water produces nitric acid [78]. While this and analogous reactions to higher oxides are desirable in nitric acid production, they are a nuissance when they occur as a consecutive side reaction in automotive catalysis. On the other hand, the reaction: 4NH 3 + 302 :=~ 2N 2 + 6H20
(3)
which should be avoided in nitric acid production, is welcome in the catalysis taking place in car exhausts. The literature [78] indicates that, of the several metals tested, platinum appeared to be the only one satisfying, at least to some extent, the technological requirements. The problem was (and remains, until now) the losses of catalyst under severe oxidation conditions (800-900K, 1-5 bar total pressure in the feed). To suppress these technologists turned to platinum-rhodium alloys which have been and still are the most frequently used catalysts (see also chapter 7). We shall turn to these catalysts below in the discussion of oxidations on alloys.
Oxidation of ammonia by nitric oxide The reaction
4NH 3 + 4NO + 02 --, 4N 2 + 6H20 is of great importance for the
technology of cleaning waste gases from power stations. Ammonia is the only reductant
550
chapter 12
which can selectively and efficiently extent remove nitric oxide from mixtures containing oxygen [79]. In principle, platinum group metals are good catalysts for this reaction [80], but the technology prefers the use of oxides, e.g. VzO5/YiO 2 [81]. The technological problem is an exact dosing of ammonia, since release in the atmosphere of unreacted ammonia is undesirable. When metals are used as catalysts, the temperature must be kept below a certain limit since at high temperatures ammonia would be oxidized to nitric oxide by dioxygen present in the mixture.
Oxidation of saturated hydrocarbons on metals Partial oxidation of methane [82,83] 2CH 4 + 02 ----) 2CO + 4H 2
(4)
is an alternative for the presently prevailing technology for the production of syngas by the classical steam-reforming [84]: CH 4 + H20 ~ CO + 3H 2
(5)
or for the modem carbon dioxide reforming: CH 4 + C O 2 ~
2CO + 2H 2
However it is very likely, as van Looy [83] has shown, that this is actually a two-step reaction: water and carbon monoxide are produced first and water reacts further by reaction (5). Platinum group metals, introduced into the reactor as mixed oxides such as Pr2Ru207, Eu2Ir207 or RhVO4, appeared to be good catalysts [82]. When a RhVO 4 catalyst was examined by XRD before and after the reaction, it appeared that even after a mere heat treatment at temperatures higher than 500K, the catalyst decomposed into metallic rhodium and vanadium oxides [83], so that one can speak of promoted metal(s) as catalysts for reaction (4). When the reaction on RhVO 4 was monitored at about 800K, only carbon dioxide and water were produced; at 1000K, carbon monoxide and hydrogen formation predominated. Platinum has also been reported to catalyse oxidative methane coupling to ethane and ethene [85], but here the oxidic catalysts show a much superior selectivity. Full oxidation of methane to carbon dioxide and water is desirable in car-exhaust gas purification and in the removal of traces of methane from waste gases after combustion of natural gas in power stations. Some fundamental information on the kinetics of this reaction on supported palladium and on platinum ribbons is available [86]. Oxidative coupling of ammonia and methane into hydrogen cyanide [88],
Oxidation reactions
NH 3 + 1.4 CH4+ 1.82
551
0 2 ---'> 0.86
HCN + 0.5 CO + 0.036
CO 2
+ 0.78 H 2 + 0.07 N 2 + 3.07 H20,
(6)
the so called Andrussow process [87], is another oxidation reaction which is industrially performed on large scale with platinum as a catalyst. Full oxidation of methane or the oxidative coupling reactions of methane are accompanied by two side processes: (i) at high temperature (T > 1400K) reactions of a radical character take place in the gas phase; (ii) in spite of the presence of oxygen, the catalyst surface can be covered by unreactive forms of carbon [88]. Both types of side reactions can in principle be influenced by using alloys instead of pure metals, but to our knowledge this potential has not been explored yet. Oxidation of methane on e.g. palladium can also be influenced by gaseous additives [89]. There is the very interesting finding that total oxidation to carbon monoxide and carbon dioxide can be turned into formaldehyde formation when a chlorinated surface is formed by the additives. One can speculate that palladium chloride temporarily formed is responsible for this pathway of reaction. A study [90] showed that on platinum and palladium filaments, the activation energy for alkane oxidation decreases from ethane to butane, being constant for higher hydrocarbons. Oxidation of alkenes on metals Industrially, the most important process is the epoxidation of ethene on silver
catalysts [91], formally described as: CH2 = CH 2 +
1/2 02
=
CH2_.. CH 2 \ / O
(7)
The mechanism of this reaction has been a matter of extremely interesting and long-lasting discussion. Early kinetic studies lead to the conclusion that it is atomic oxygen which is active in epoxidation of ethene [92]. However, others pointed to the fact that atomic adsorption of oxygen exists on all Group 8-11 metals (except gold) but including silver, but one can only speculate on the existence of appreciable amounts of molecular oxygen in the case of silver and this would explain its exceptional position in this reaction. It has also been proven by IR spectra that dioxygen participates in complexes with ethene [93]. A very often used argument concerned the apparent stoichiometric limit which seemed strongly to confirm the idea that atomic oxygen burns ethene into water and carbon dioxide, while molecular oxygen leads to oxirane (epoxide) [93]. The three following reactions describe
the stoichiometry when carbon dioxide is the product of deep oxidation:
552
chapter 12
* 0 2 + *C2H 4 4 * 0 + *C2H 4 2CO
+ *C 2
---->C2H40 + *O + * --~ 2CO + 2H20 + 4* 2CO 2 + *
these equations lead to a selectivity for oxirane of 4/5; i.e. 80% [93]. At the moment when the evidence for molecular oxygen being the crucial species in epoxidation seemed to be almost absolutely conclusive, new results appeared which returned everything to 'square one'. First, by sensitive tools such as EELS it has been shown that under reaction conditions there is no detectable molecular oxygen present, while atomic oxygen can be stripped off the surface by ethene, forming epoxide, albeit at low selectivity [92,95a], Second, when atomic oxygen labelled as 180 has been prepared on a silver surface, this oxygen appeared in the initially formed epoxide and not 160 from the ethene-oxygen mixture admitted thereafter [91,94]. The idea of molecular oxygen as the crucial species could still be saved by saying that it is just that very small hardly detectable fraction of molecular oxygen from the equilibrium 2Oad s = O2ads, which leads to epoxidation. The occurrence of molecular adsorption of dioxygen under real reaction conditions, i.e. at higher temperatures but also at a high oxygen pressure, still has support, some of which is recent [95bc,]. It has to be noted that silver is strongly promoted for epoxidation by chlorination and by the presence of alkali promoters. The best laboratory catalysts even surpass the magic pseudo-stoichiometric limit of the above-mentioned maximum selectivity [90,91]. In the earlier literature one read about chlorine as an element preventing dissociation of the required molecular oxygen [93], but the following aspect must not be forgotten. Silver does not adsorb ethene, but preadsorption of oxygen and formation of subsurface oxygen, in whatever stoichiometry, promotes ethene adsorption [99]. Ethene can be bound, albeit weakly, to silver ions [93,97,98], and chlorine or subsurface oxygen in the catalyst certainly increase the concentration of silver ions. Silver and alkali elements form mixed carbonates [96] in which silver, or other Group 11 metal ions, can be stabilized against reduction. Of course, one can also expect weakening of the Ag-Oads bond by promoters, and this can also be beneficial for epoxidation [91,98]. In any case, the amount of oxygen adsorbed is suppressed by chlorination of silver [100]. Higher alkenes are not epoxidized with any practically useful selectivity. This is most likely caused by the presence of the labile, reactive allylic hydrogen in propene and higher alkenes. Once dissociation of C-H bonds sets in, it is difficult to stop and carbon oxides are produced. When, by a substitution on propene, the hydrogen abstraction is made slightly more difficult, epoxidation selectivity increases, albeit marginally (from several % to 20-30%) [100,102] as comparison of propene and trans-2-butene shows [101]. Gold obviously catalyses only the full combustion [lO1] of ethene, but from propene some acrolein (propanal) can be produced. Copper is itself oxidized in the
Oxidation reactions
553
presence of oxygen, and copper (I) oxide oxidizes the higher alkenes to the corresponding aldehydes and ketones whereby it is mostly in the allyl position that oxygen appears. This is however sometimes accompanied by a double bond shift before oxidation. Cant and Hall [103] made a very extended comparison of oxidation reactions with ethene, propene, 1-butene, cis-2-butene, trans-2-butene, isobutene and two 2-pentenes on supported iridium catalysts. They established that the reactivity of those alkenes decreased in the indicated order. About 40% of each alkene (at various temperatures close to 370K) could be converted into products of partial oxidation. The dominant products were acetic acid with ethene and acetone with isobutene. Results from several earlier papers were summarized [103] and a very useful generalization was made: (i) palladium and iridium are selective in oxidation of ethene, but they cut one carbon away from the molecule when propene or higher alkenes are oxidized. (ii) Platinum, rhodium, ruthenium and gold are unselective with ethene, (mainly carbon dioxide is produced), but they form acrolein from propene. A very interesting system is palladium-doped vanadium pentoxide [104], which under mild conditions (380-450K) oxidized 70% of ethene into acetaldehyde. Palladium ions were believed to be the active sites [104]. Unfortunately this possibility had not been checked in earlier papers on alkene oxidations. An exception is one [105] in which saturated alkanes (C~-C4) were compared with cyclohexane and with cyclopropane, which due to its electronic structure behaves very much like propene. It is also possible that cyclopropane isomerizes upon adsorption to propene (see chapter 13). It was concluded that at 590-870K it is palladium oxide and not palladium metal which is active. It is important with respect to the mechanism of oxidation reactions that which has been established for iridium with regard to acetic acid formation by using 14CH2 = C H = CH 3. One carbon atom is cut away when acetic acid is formed (such reaction takes place on Ru, Rh, Ir, Pt and Au), and it is always the labelled one [105].
Oxidation of alcohols It is known from alcohol-deuterium exchange reactions that an interaction of a metal with an alcohol starts at the hydroxyl group. Thereafter, CH bonds are activated, alcohol is dehydrogenated and at a certain stage the C-O bond is or can be broken. It is still a question for discussion exactly at which stage of dehydrogenation of the fragment it happens [107]. Only when the hydroxyl group is kept away from the surface by, for example, interaction with a hydrophilic solvent, do the C-H bonds react with the metal first [ 108]. The industrially most important oxidation of an alcohol is the oxidative dehydroge, nation of methanol to formaldehyde. The name itself indicates the mechanism and the most frequently used catalyst is silver [108]. It is a metal which is, without oxygen, not very active in (de-)hydrogenations and which does not dissociate carbon monoxide at the high temperatures necessary for dehydrogenation. Oxidative dehydrogenation of alcohols
554
chapter 12
into various aldehydes or ketones is also of practical interest [99]. The conclusion from a comparison of C1-C4 alcohols was that the higher the molecular weight of the alcohol, the larger extent of side reactions [109]. With copper, the reactions can be considered as running on an oxidized surface or even oxide, the oxygen of which can be active in C-H bond fission [ 110]. As with many other oxidative reactions, deep alcohol oxidations can be brought into oscillation [ 111 ]. The full oxidation of methanol is not yet interesting for the chemical industry, but because of the use of methanol as a jet-propellant, such as in famous V-1 missiles, the interest might be hidden within military secrets.
Particle size effects in oxidations The general problems of particle size effects in chemisorption are discussed in chapter 5, while the catalytic effects are discussed in chapter 6. The effects related more specifically to oxidation reactions are discussed below. table 1 Activity of various platinum catalysts per unit surface area for oxidation reactions: Form
Area(cm 2R-1)
k/cm2
rate/cm 2
SO~ oxidation
H2+O 2
0.37
14
0.2%/SIO2
3x105
0.5%/SIO2
7x105
0.40
-
sponge
1.7x103
0.23
-
filament
20.6
0.26
5.5
foil
6.9
1.74
9.0
In 1955 Boreskov et al. [112] published a comparison which is presented in table 1. The results led Boreskov to the conclusion that there is no dependence of specific activity for sulfur dioxide oxidation on particle size in the case of platinum. Later, Poltorak [113] stressed that, when the measurements are extended down to still smaller particle size, then some oxidation and related reactions show a very pronounced sensitivity to particle size, among them hydrogen peroxide decomposition and oxidation of alcohols. The areal activity of the smallest particles studied was considerably lower, sometimes a hundred times lower, than that of large particles. The same conclusion was also mentioned in papers by Manogue, Katzer et al. [114], who suggested that the reason for the observed phenomenon is that small metal particles are more easily converted during reaction into metal oxide, which in the reaction they studied was less active than the metal. This seems to be a plausible explanation, although one should not forget another aspect, discovered
Oxidation reactions
555
later, that small particles are also less active in formation of multiply coordinated adsorbed species such as CHads,
Cads, Oads, NHads. Nads, etc.,
which are the necessary intermediates for
the corresponding oxidation reactions (see also chapter 13).
12.3
Oxidations on alloys
12.3.1 Oxidation of carbon monoxide There are good reasons to expect that the behaviour of metal catalysts in the title reaction can be improved by alloying. However, not much of this potentially interesting field has yet been explored, although valuable ideas on this subject are available [116]. Even in the early literature the kinetics of the reaction was established and these can be rationalized as follows. The rate of carbon monoxide oxidation is proportional to the number of oxygen molecules impinging per second on the surface free of strongly adsorbed carbon monoxide in unit time: when for Oco the Langmuir expression is substituted, with aco standing for the adsorption constant, one obtains equation 8: qco
_
-1
(8)
RT _ -1
r =APo2.(1-Oco) =APo2.aco.e.
['co
the validity of which has been confirmed by the most recent papers. Alloying can create, for example, either more active mixed ensembles, i.e. adsorption sites with a lower qco, or sites which can adsorb oxygen atoms but not carbon monoxide 9 Daglish and Eley [116a] have shown in their classical paper that there is a distinct difference in the activation energy of oxidation between the palladium-rich and gold-rich alloys, used in the form of wires. This is shown in figure 4. /.0 i
a; o
E E~
figure 4 The apparent ac:ivation energy of carbon monoxide oxidation as a function of alloy composition.
20
LLi
8
I0
I
Atomic
i
40
% Pd
I
556
chapter 12
The reaction shows a compensation effect (see also chapter 6). This behaviour (figure 4) was discussed in relation to the electronic structure of palladium. As we have seen in chapter 1, palladium-rich alloys expose palladium atoms with partially occupied d-orbitals, but those are fully occupied when the bulk concentration of gold exceeds 60-70%. One can speculate that gold-rich surfaces expose only very few palladium atoms, so that the pre-exponential factor of the rate is low on them. On the other hand, these palladium atoms bind carbon monoxide only weakly, because single atom sites are available, with a lower qco, so that the exponential term is large, thanks to the low activation energy (the latter is about equal to qco, see equation 8). Gold-rich alloy surfaces therefore show different kinetics, and the rate is proportional to |
|
and the adsorption of carbon
monoxide is less inhibiting than on palladium or palladium-rich alloys. Two other papers dealing with the very closely related palladium-silver alloys (films and foils) [116b,c] report that the activation energy decreases when going from silver to silver-rich palladium alloys. However, there is an even larger drop in activation energy when a small amount of silver is added to palladium. The order of reaction with respect to carbon monoxide varies in agreement with the behaviour expected according to Langmuir-Hinshelwood mechanism: it is -1 on the palladium-rich side, and +1 on the silver-rich side. Oxidation of carbon monoxide by oxygen is a very important function of automotive catalysts. These catalysts contain in most cases platinum and rhodium, being at least partially involved in alloy formation. Pure Pt/SiO2 and Rh/SiO2 catalysts differ in the sense that rhodium is a better catalyst when the gas mixture has a reducing character but platinum is better when the atmosphere is oxidizing. This has to do with the inhibition of the reaction by carbon monoxide, which is more pronounced on platinum than on rhodium and more at low temperatures than high. With alloys, in the form of powders or single crystal faces, oxygen causes segregation of rhodium to the surface, which is platinum-rich in vacuum (see chapter 4). As a consequence of all these effects acting together, the
Pto.vsRh0.25 and Pt0.sRho.5 single crystal planes behave very much like pure platinum in stoichiometric carbon monoxide-oxygen mixtures. The Pt0.zsRh0.75 alloy has a behaviour [117] intermediate between those of platinum and of rhodium. Figure 5 shows the hysteresis phenomena arising from gas-induced effects on the surface composition and/or the structure of the alloy surfaces. The curves show the effect of increasing and decreasing temperature on the rate of carbon dioxide production under standard steady flow conditions. Effects such as gas-induced segregation and even phase separation are more difficult to control with powder catalysts and this could be the reason for controversy in the literature as to whether [118] or not [119] there is any synergetic effect of alloying on the activity of Pt-Rh/AI20 3 and Pt-Rh/SiO2 catalysts.
Oxidation reactions
557
875K _
T equi
/,
o
m
1400K
---~
A
_= ._ c-
2
,4 c~ o c_)
J
"
i
400
i
III
i
600
i
l
i
800
v
,.oo
1000
TIK
600
J
8;o' ~o'oo
TIK
c ._
2
g
2
a3
u3 uJ
~
1
d
0
C
x
< -
0
400
600
800
1000
v--ki
"
400
TIK
i
~1
--i
600
i
800
i
|
1000
TIK
figure 5 Temperature Programmed Reaction, followed by mass spectrometry, with a stoichiometric flow of carbon monoxide and dioxygen, a) Rh-rich surface of Pt-Rh(410). b) Pt-rich surface of Pt-Rh(410). The temperature was first increased, then decreased, and finally increased again, as the arrows indicate. The AES signals of carbon and oxygen are shown in c) and d). These signals reflect the changes in the surface composition caused by the reaction taking place in the gases of the feed. c) Rh-rich surface of Pt-Rh(410) d) Pt-rich surface of Pt-Rh(410). The relation of both hystereses is clearly demonstrated by these results.
Pc
T
CO . 0 2 (111)
0 2
(a.u.)
(410)
il
,
300
s6o
temperature
760
= (K )
960
figure 6 Oxidation of carbon monoxide stoichiometric mixture with oxygen, on several alloy surfaces: Pt-Rh(l l l), Pt-Rh(lO0), Pt-Rh(410) and Pt-Rh(210). All prepared from the Pto.25Rho.75alloy.
558
chapter 12
Figure 6 shows the comparison of the steady state rates of carbon monoxide oxidation on different single crystal planes of pure platinum and platinum-rhodium alloys [2,120]. These crystal planes were all fabricated from a Pt0.25Rh0.75 single crystal and they exposed surfaces of the compositions shown in table 2. table 2 Composition as determined by AES of various surfaces of Pto.25Rh0.75 following annealing at 1300K Surface
single crystal
first layer platinum concentration (at %)
(lll)
32
(100)
40
(210) (410)
55 40
Alloying a noble metal with an sp metal seemed to be attractive for several reasons. There was a hope that the additive would create sites which would adsorb oxygen, but bind carbon monoxide much more weakly, leading to a rate enhancement. Further, in automotive catalysts one tries to remove carbon monoxide and nitric oxide simultaneously, and the presence of an element which could promote nitric oxide dissociation seemed to be a good way to improve the catalytic behaviour of noble metals such as palladium or platinum. Success in this direction would moreover help to decrease the dependence of automotive catalysis on very expensive rhodium. Important information on the Pd-Sn alloys has been obtained by comparison of the adsorptive and catalytic behaviour of Pd(100) and cx(2x2)-Sn-Pd(100) crystal plane. It appeared that under steady state conditions of carbon monoxide oxidation the alloy surface consists of an SnO x layer on a top of the Pd(100) substrate [133]. The alloy catalyst is more active than the monometallic surface, but the rate enhancement is of the same magnitude as with other metal oxide systems, in which the promoting oxide is introduced into the catalyst in a different way [133 and references therein]. Palladium forms a continuous series of solutions with copper (chapter 1) and these alloys have also been studied. As expected, the reaction causes a surface enrichment in copper and formation of copper oxide [122,123] (chapter 4). The reaction patterns corresponding to individual components are actually additive and 'none of the catalytic behaviour appeared to be due to a ligand effect in this bimetallic system' [123]. The automotive catalysts also often contain palladium. This does not surprise us,
Oxidation reactions
559
since Skoglundh et al. [124] showed that a small amount of platinum (20% of the metal content) in Pd-Pt/AI203 (washcoat) leads to the best catalyst for complete oxidation of xylene and of carbon monoxide. Results [124] are presented in figure 7.
260 250
(a
270 ,
230q o9 5
"~
E _~
220<,
j
250 J
2",0, /
,,'~-~~_~~w '
240
4y'
I
1230!
:o':eo 6o~.o looo Os.CO z.O. 50 80-20
0"100
2o8o
t
w.-
o
2~.0~
--~
220
80|
c
,
0"100
6oi~.o !ooo ~.0.60 8020
9
,
.1
20':80 50':/.. 0 100-0 Z.O-60 80 "20
Pd Pt. tool - % rQtio
figure 7 The effect of composition of noble metals on I'5ofor three concentrations of noble metals (Pt+Pd); 5 (V'l), 10 (A) and 20 ( 0 ) pmol per gram catalyst over hydrothermally treated washcoats, for the catalytic oxidation (SV -- 144 000 h l) of (a) 220 ppm xylene; (b) 900 ppm xylene and (c) 4600 ppm carbon monoxide (the lower the temperature, the higher the activity).
A very important reaction in car-exhaust catalysis is the oxidation of carbon monoxide by nitric oxide. Since, as we have seen earlier in this chapter, during this reaction nitric oxide dissociates, the reaction
has very much in common with carbon
monoxide oxidation by dioxygen, which also dissociates before it enters the reaction with carbon monoxide. However, there are also some minor differences, as detailed studies [2,120] have revealed. Several single crystal planes, which are described in the text adjacent to figure 6 above, have been fabricated. It was established that (i) carbon monoxide inhibition is still observable with the alloy (figure 8), (ii) the maximum on the rate-temperature curve is sharper for oxidation of carbon monoxide by nitric oxide than for
560
chapter 12
oxidation by dioxygen (figure 9) and (iii) the activity patterns as characterized by the temperature-dependence of carbon dioxide formation are quite similar for both oxidations (figure 10).
figure 8 Steady state rate of C02 production versus temperature for three different NO~CO ratios over the Pt-Rh(11 I) surface. The NO~CO ratios are 1/5, 1 and 5 for (a),(b) and (c), respectively.
(111) NO CO
APco 2
5
(a.u.)
/ (, -!tw
300
,
",',,b -
500
temperoture ~
figure 9 A comparison of the steady state rates of C02 production for the (a) CO+NO and (b) C0+02 reactions on the Pt-Rh(111) surface under stoichiometric reaction conditions.
l
(K)
AP
1
1111)
CO2
(a.u.)
~
/t
CO+NO 300
500
temperature
~ (K)
Further, it was concluded that at low temperatures nitric oxide dissociation is in no case the rate-determining step. However, at high temperature and on the Pt-Rh(111) surface the dissociation is suppressed by the short life-time of molecular nitric oxide on the surface and can even become the rate-determining step. By variations in the pre-treatment, surfaces could be prepared which were either platinum-rich or rhodium-rich in comparison with standard surfaces (table 2). The platinum-lean surfaces were then more active in nitric oxide dissociation than the platinum-rich ones, this effect being most pronounced
Oxidation reactions
561
with the Pt-Rh(111) surface. The reader will remember that the Pt(111) surface is the one which is very ineffective in nitric oxide dissociation. A comparison reveals further (figures 6-10) that on Pt-Rh(111) the onset of carbon monoxide with nitric oxide reaction lies at slightly lower temperature than that for the oxidation of carbon monoxide by di-oxygen.
t
CO+ NO
(210) ./f"'X
{111]
A Pco 2 (a.u.)
/".7--/(LI0 )-.
/,'
"~\X
f-5'X__ 4t
,
300
s6o
'
temperature ~
760
'
(K)
960
figure 10 A comparison of the steady state rates of C02 production in stoichiometric mixtures versus temperatures for the CO+NO reaction over the Pt-Rh(lll), (100), (410) and (210) under stoichiometric reaction conditions.
12.3.2 Oxidation of hydrogen This reaction has been found to be either of zero or of a negative order in oxygen for most of the platinum group metals. It means that suppression of strong oxygen adsorption or promotion of hydrogen adsorption is desirable in order to achieve high catalytic activity. Therefore, there is some hope that by using alloys one can attain this goal. However, the results already obtained in this direction are certainly not abundant. In a very early paper, Tammann [125] noticed that adding silver to palladium decreased the activity of palladium and the drop in the activity was in particular sharp at about 80% silver in the wire. Palladium-gold alloys behaved in a very similar way. The same conclusion can be drawn from the paper by Kowaka [126], the results of which are plotted in figure 11. The drop in activity is similar to that observed with carbon monoxide oxidation. It may be explained by supposing that palladium greatly diluted in silver has an electronic structure such as 4d1~176 i.e. different from the electronic structure of pure
palladium d975s~ (see chapter 3). However, it must not be forgotten that alloys with more than 70% silver have also practically no palladium in the surface. The fact that the activity falls so abrubtly supports the first explanation, since the palladium surface concentration varies more smoothly. There is also some information available on copper-gold alloys [127]. However, it is very likely that, being similar to platinum-tin alloys, they are actually converted under the running oxidation into gold covered by copper(I) oxide. Hydrogen can also be oxidized by nitric oxide, which reaction is again interesting because of the nitric oxide removal from waste gases. Studies have been published on
562
chapter 12
platinum, rhodium and platinum-rhodium alloys [128]. The most relevant results concerning their activity and selectivity are shown in figure 12 [128a]. We observe here that pure platinum is very active but the selectivity to nitrogen is poor. The selectivity of rhodium is excellent, but its activity is low. By the use of the alloy one may obtain a convenient compromise of high activity and reasonable selectivity. In the (100) and (110) planes of platinum-rhodium alloys have been studied [128b].
-0.5
1
-0
3.0-
-O.5 -1.0
2.0-
-2.5
figure 11 The activity of Pd-Ag alloys in the hydrogenoxygen reaction [126]. (o) The variation of 103Ti-1 (left vertical axis) with alloy composition (T i being the temperature at which the rate constant is 1 min-l). (o) The variation of log k ~ at 373K with composition (vertical axis on the right side).
1.0--
0
l
/-,10 Atom
1
810
% Ag
The reaction of nitric oxide with hydrogen can also lead to useful products. One can, for example, produce in this way hydroxylamine which is an intermediate used in the production of Nylon. For this purpose various alloys of palladium, platinum and some sp elements, such as germanium or tin, have been reported as good catalysts. A product of hydrogen oxidation which seems to have a bright future in industrial organic syntheses is hydrogen peroxide since it minimizes waste. Presently, the largest volume of hydrogen peroxide is produced by a homogeneous reaction using alkylanthraquinones. However, the interest for heterogeneous catalysts, which are generally easier to use in continuous processes and permit easy separation of the catalyst from the reaction mixture, is increasing all the time. It has been suggested to use transition metals, mainly platinum and palladium, supported by solid acids such as WO 3 or ZrO 2, etc. [148]. However, it seems that hydrogen oxidation to peroxide can in future be another major industrial application of alloy catalysts. It has been reported [149] that platinum-palladium alloys show a behaviour superior to that of individual components. In this patent, alloys were prepared as colloids mixed with supports, such as SiO2 or active carbon. The results are schematically represented for all supports by the curve shown in figure 13. One can speculate on reasons for the superiority of alloys: mixed ensembles (see chapters 9 and 13), suppression of hydride formation, elimination or creation of certain sites on the surface, etc. Only future research can show what the exact reason is.
Oxidation
reactions
563
l~176 t
3
._ >
i__i
I']
100 50
Pt-Rh(111) Pt-Rh(lO0) Pt(lO0) Pt-Rh(/-,lO) Rh[lO0)
I00~ _J
., ;
I I
I
50-
.,.-.
u
<
0
,
Pt-Rh (111)Pt-Rh[100) Pt ii00) Pt-Rh(Z.10)Rhl100)
figure 12 Above, on the left: the selectivity of the NO-H 2 reaction to dintrogen determined at a total conversion of 10% at 520K (a) and 575K (b). Below, on the right: the NO conversion after 3 minutes of reaction time at 520K.
0
I'M 1"
10
0
9
0
|
,
,
,
1
0.5
,
!
,
!
I
1.0 Pt
Pt
+ Pd
figure 13 Oxidation of hydrogen by oxygen, Pt-Pd alloy catalysts [149]; selectivity to hydroperoxide as a function of alloy composition.
564
chapter 12
12.3.3 Epoxidation and other oxidation reactions of alkenes The reaction of ethene with dioxygen (see also section 12.2) on silver-containing catalysts can be considered as a system of two parallel reactions +1//202 CzH 4
~
C2H40
(EO)
(9)
+302 CzH 4
~
2H20 + 2CO 2
(10)
It has been shown by using isotopic labelling that most of the carbon dioxide is formed directly from ethene and only a small part arises from the consecutive oxidation of EO(oxirane) [129,130]. For propene oxidation to various products the same conclusion holds [131]. This probably indicates that there is a link between the way in which ethene is adsorbed and the reaction which follows; according to the ethene adsorption mode either (9) or (10) follows. Epoxidation of propene on silver is almost impossibly difficult, since the selectivity is very low and only when silver is modified by many various additives (Te,S,Au,Cd)
some
PO is produced. It is agreed that this is because the reactive allylic
hydrogen induces deep oxidation through dissociative adsorption of propene. Therefore, with ethene also one may speculate that the molecular non-dissociative adsorption induces epoxidation according to 9, while the dissociative adsorption of both oxygen and ethene opens the way leading to deep oxidation (10). However, there is some room for different roles to be played by various forms of oxygen (atomic, single or multiple coordinated, subsurface oxygen, molecularly adsorbed oxygen, etc.). Gold does not adsorb ethene dissociatively and the adsorption of oxygen on gold is certainly of a very low extent. Yet there is on gold some reaction of ethene to carbon dioxide and water, but not to oxirane, and of propene to acrolein [131]. Perhaps the presence of very small amounts of oxygen on gold in the form of collision complexes induces dissociative adsorption of the alkene, in a similar way to that proven for ammonia adsorption (see chapter 1) and this dissociative adsorption leads to these products. On the other hand, the absence of gold ions which would adsorb ethene molecularly and activate it in the direction of epoxidation (as the silver ions possibly do, see section 12.2) prevents epoxidation. According to some earlier papers e.g. [93], the main role of gold in silver should be to suppress the atomic and enhance the molecular adsorption of oxygen. As we know, molecular oxygen was the postulated intermediate of epoxidation [93]; however, the more recent papers cast some doubts on this idea [91,94d,e]. The influence of gold on silver was always a matter of interest in mechanistic studies. It was expected that gold could beneficially weaken the bond between the metal and atomic oxygen or increase concentration of molecular oxygen. Whatever was considered as most important for oxirane formation, these two ideas seemed to be confirmed by
Oxidation reactions
565
the results [132]. The selectivity to oxirane shows a somewhat pronounced maximum at about 20% gold, but its yield, thanks to the lower activity of the alloys, shows only a very flat maximum, as can be seen in figure 14. The alloys were prepared by a co-reduction of nitrates, using methanal (formaldehyde). It was suggested that the metal-oxygen bond strength varies due to the varying lattice constants of the alloys. Geenen et al. [131] used Ag-Au/~-A1203 catalysts and they reported that the selectivity to oxirane decreased
monotonically with increasing concentration of gold. They reported a surface enrichment in silver, which has been confirmed by other work [133]. o
o -20/-, C x -232%
>iii L3
o -28
x O iii
Z iii
C
figure 14 Mole percent yield of ethylene oxide (oxirane) versus alloy composition [132].
15
>I
o
W
o
~-Z W ~_~ n~ LLJ EL LLJ 0
!
0
!
20 ATOM
I
i
40 PERCENT
1
i
60
i
i
80
GOLD
When palladium is admixed with silver, selectivity to epoxidation smoothly decreases. This has been seen for both the alloy films and for silica-supported alloys [143,135]. Comparison of these catalysts as presented in the literature [136] is shown in figure 15. The palladium-rich alloys with silver oxidize ethene to mainly carbon dioxide and water. However some ethanal is also seen [134]. Palladium-gold alloys produce ethanal (acetaldehyde) too, but selectivity is not very high on palladium-rich alloys. This can be seen in figure 16 [130]. Palladium and its gold alloys do not produce more than traces of oxirane. However, there is an interesting report that Pd-Sn/SiO 2 catalysts show a modest transient activity for oxirane formation. However, in repeated pulses of the etheneoxygen mixture this activity disappears, most likely due to the formation of a tin oxide phase [ 137]. Silver forms solution alloys with 0-42% cadmium, the oxide of which is not very stable at high temperatures, but it can be expected that oxygen will induce surface segregation of cadmium rather than of silver. Experiments [132] revealed that surfaces kept in vacuum (instead of in contact with oxygen) show a pronounced silver enrichment.
566
chapter 12
The presence of cadmium in silver (or cadmium oxide in silver) increases the selectivity to oxirane, as can be seen in figure 17.
z. 0
supported a l l o y s (&88K
figure 15 Selective oxidation of ethene to ethylene oxide (oxirane) over palladium-silver alloys; (lefthand plot), 1% (o) and 4% (A) loading on silica; (right-hand plot) evaporated films [136].
)
1:3 .w
X
o c"
&
20
A
t-
>,, \
>
.o
1,9
0
100
U3
I
!
80
\
I~
% IL
100
I
I
80
I
60
Composition (atom % Ag)
60
figure 16 Ethene oxidation selectivity to ethanal at 373K as a function of alloy composition.
c
_
/.0
Q.
> "" u
20
&l u3
0
0 Atom
&O per Cent
8O Pd in Au
Oxidation reactions
567
T.~
o
A 217
9O
l-
a 224 0
eo
~
.x
~o
Epoxidation
~ Combustion
i
2o
6o 50
24
|
g
o
Surface
;o
8'o
Composition %
Cd
~6 12
U
i
,
f'
~
|
I rl ,
8'0
l
Surface Composition % Cd
figure 17 Left: effects of surface composition on the selectivity of alloys. Right: effects of surface composition on activation energies of epoxidation and combustion reactions.
Moss et al. [135,136] studied palladium-rhodium films as catalysts for the total oxidation of ethene at 423K. Rhodium was clearly the least active catalyst, since it is oxidized under reaction conditions, the activity of palladium being comparable with the most active alloys. The activity-composition pattern was, however, complicated and could indicate simultaneous blocking and promoting effects of rhodium oxide(s) on palladium. Propene can be selectively oxidized to acrolein by silver-gold alloys with o~-A1203 as support. This can be seen in figure 18, in which the results concerning oxirane formation are also seen [ 131 ].
o
80
Z
40
bO
o-f.~ 0
Gold
figure 18 Selectivities to propene oxide (PO), ethylene oxide (EO) and acrolein over Ag-Au/o~-Al203 catalysts (except for the catalyst with 76% gold at the surface, which is a pure alloy). Propene oxidation at 473K, ethene oxidation at 533K. 0.4 fraction
0.8 at
surface
568
12.4
chapter 12
Practical applications of oxidation reactions on metals and alloys If we assume that all cars now having catalytic converters fitted have the standard
three-way catalyst, then the total amount of noble metals in them exceeds by a factor of two to three the amount of platinum in the reforming catalysts that produce the petrol they use, notwithstanding the lower concentration of metal in the former. This indicates how large is the business potential of these catalysts. 12.4.1 Three-way catalysts Catalytic converters in the car exhaust system were first time used in the USA in the new models introduced in the autumn of 1974. Their main function was to burn carbon monoxide and unburned hydrocarbons (HC) to carbon dioxide. The legal norms later became more and more stringent and included also the removal from the exhaust gases of the various nitrogen oxides, NOx. Thus, three types of reactions, two oxidations (carbon monoxide, hydrocarbon) and one reduction (NOx) have to reach a high conversion simultaneously, using a single reactor. The demand to catalyze three reactions also gave the name 'a three-way catalyst' to the system. This catalyst can presently satisfy all legal norms when it works at a sharply controlled air/fuel ratio (A/F). The reason of this can be seen in figure 19. conv.%
,~co
100 -
figure 19 Conversion of gases indicated in the figure, as a function of the air-to-fuel (A/F) ratio. Only in a narrow range of
50
A/F values can a high enough conversion of all gases be achieved.
Ox A O13
/
' 14
"
'-"
15
1
'6
F
The 'window' in the A/F ratio for an optimal operation of the catalyst in all its main functions is fairly narrow and the correct value has to be controlled and maintained by an oxygen sensor and special electronics. The whole equipment comprises an air pump, and the control of exact fuel supply requires the use of a fuel injector, as can be seen in figure 20. Several good reviews are available describing the construction and function of the converters and these reviews also offer to a reader an historical look at the development of the automobile catalytic converters [ 138-141 ].
Oxidation reactions
569
ELECTRONIC CONTROL MODULE MANIFOLD ABSOLUTE ~PRESSURE SENSOR INJECTOR SYSTEM
AIR CLEANER ~
~
" ~ ~
MASS AIR
.~,;~'
~. ,
~|)7~~
//~/t~" ~-
EXH
OXYGEN SENSOR--/
v,~(~'? /TF FUEL
~
~/ .,
'
"
3 wAY CATALYST ~]--
VAPOR CANISTER
TORQUE CONVERTER CLUTCH CONTROL
/~~'~~
COOLANT SENSORJ
FE TURN
~'~
Closed-loop emission control catalyst equipped vehicle.
system
on a three-way
figure 20 Configuration of the motor, catalyst and control system in a car [139].
The main reactions occurring in catalytic converters can be described concisely by the following equations (hydrocarbons are represented by (-CH2-)n)2CO
II
~
2CO 2
(11)
2CO + 2NO
~
N 2 + 2CO 2
(12)
(-CH2-) n + 1.5NO 2
~
NCO 2 + NH20
(13)
(-CH2-) n + 3nNO
~
1.5N 2 + NCO 2 + nH20
(14)
2NO + 5 H 2
----)
2NH 3 + 2H20
(15)
2NO + H 2
----)
N20 -t- 2H20
(16)
CO 2 -'[- H 2
~
CO + H20
(17)
+ 0 2
Reactions of group I decrease the emission of toxic gases, but reactions of group II should be suppressed as much as possible, since they produce environmentally undesired gases. For example, the formation and release of NH 3 would finally lead in nature to the formation of NOx and therefore it should be definitely prevented. On the other hand a part of the gases that are reactants in group II would be removed in the converter by reactions as"
570
chapter 12
6NO + 4NH 3
~
4N 2 + 6H20
(18)
2N20
----)
2N 2 +
0 2
(19)
2NH3
--~
N 2 + 3H 2
(20)
Also undesired is steam reforming of hydrocarbons, since this would release carbon monoxide. In principle one cannot completely exclude the occurrence of reactions such as the reverse water-gas-shift reaction (17), oxidation of hydrogen, methanation, etc. which are also for various reasons undesirable. However, their extent will probably be very limited with platinum-rhodium catalysts. Basically, one of two forms of catalyst is used in the converters, either alumina pellets or a ceramic monolith covered by a thin alumina wash-coat (see chapter 7). The metal or alloy component is mounted on these supports from solution. Most of the catalysts contain platinum and rhodium, which are, at least partially, alloyed; the total metal loadings do not exceed 0.5 wt%. The catalysts further contain about 5% of
CeO 2
which additive works as an oxygen reservoir, as a promoter of catalytic reactions and a stabiliser of the alumina [ 138-141 ]. During their operation, the catalysts are subjected to severe thermal stress and poisoning by sulfur, phosphorus, lead, and antimony. The self-poisoning can be in some cases partially removed by self-regeneration under other driving conditions. The problem with the catalysts is the cost and availability of platinum and even more of rhodium. Therefore, much research is being carried out on a new class of catalyst, in which these metals would be replaced; it seems that promoted palladium catalysts are very promising. It can be expected that research will continue along these lines, being focussed in the future on the effects of alloying and promotion. A special and tedious problem is the removal of carbon-rich particulates from the exhaust gases of Diesel motors. This problem is being studied on several places in the world. A new topic will probably be the use of surface-modified and/or alloy materials in the combustion chamber, bringing about new impulses for catalysis research. The reader who wants to follow these developments will profit from the series of 'Studies in Surface Science & Catalysis', in which proceedings of specialized symposia on automotive catalysis regularly appear. It is interesting to note that the reactor technique developed for catalytic converters will possibly have some impact on reactor technology in the classical chemical industry, in particular for those reactions for which a fast removal of products from the reactor is desirable, or when the gas flow must be high for some other reason.
Oxidation reactions
571
12.4.2 Oxidation of ammonia Heterogeneous oxidation was the subject of the first patent in the field of catalysis [142]; it concerned sulfur dioxide oxidation on platinum. Since that time much has changed and the expensive and poison-sensitive platinum has been completely replaced by alkali-promoted vanadium pentoxide catalysts. However, in ammonia oxidation, platinum still retains its dominant position (see also chapter 7). The discovery of the oxidation of ammonia on platinum is attributed to Kuhlmann (see Davis et al. [78]), and its conversion into an industrial process to W.Ostwald. It was never an easy process, as the problems of corrosion, poisoning, catalyst losses, etc. were always there. The following reactions are involved, and all highly exothermic [78]: 4NH3 + 502
------)
4NO + 6H20 AH - -899,9 kJ/mol
(21)
2NO + 02
~
N204
AH - -171,6 kJ/mol
(22)
4HNO3
AH = -73,6 kJ/mol
(23)
2N204 + 2H20 + O2 ~
The last reaction describes an absorption reaction, which is fast in comparison with another reaction: 4N204 + 4H20
---)
4HNO3 + 4HNO 2
(24)
which is endothermic (AH = +83kJ/mol). It is important to notice that N204 dissolution in water is easy in the presence of oxygen and at elevated pressures. Reactions with water of nitrogen dioxide monomer formed at high temperatures and low pressures are slow. At high temperatures, ammonia can be lost by another very exothermic reaction: 4NH3 + 302
----)
2N 2 + 6H20
(25)
When unreacted ammonia leaks into the reactor section rich in nitric oxide, an exothermic reaction takes place: 4NH 3 + 6NO
~
5N 2 + 6H20
(26)
which also causes loss of ammonia. A simplified scheme of the industrial plant for nitric acid production is shown in figure 21. As mentioned in chapter 7 the catalyst used in the process is gauze, woven from platinum-rhodium alloy wires. The content of rhodium is 5-15%. The severe process conditions cause changes in the state of the catalyst and these have been the subject of several studies by electron microscopy and surface science techniques [144].
572
chapter 12
Compressor
-D
Oxidation/ absorption
F i i t er t,..,...i I~
Ammonia
Secondary air
~1 ilr~;e Nitric acid
Ammonia E~o porat or
Vent
Expander
Filter==
~
section
Waste~ heat boiler
Condensate
Bleacher Nitric aci
figure 21 Simplified flow diagram for a single pressure operation in a pressure nitric acid plant.
Environmental legislation is becoming more and more stringent and companies are being pressed to decrease to zero any exhalation of nitrogen oxides. This will likely renew interest in fundamental research for this process, with possible new catalysts on the horizon. 12.4.30xirane (ethylene oxide, EO) production Oxidation of ethene to oxirane oxide is a large industrial operation. Oxirane is used as an intermediate in numerous synthetic organic reactions, as a feedstock for the production of ethylene glycol (1,2-dihydroxy-ethane used as antifreeze liquid) and as a liquid rocket propellant. The first practical procedure for air-oxidation of ethene was designed by Lefort [145], and since then the technology has been improved in many aspects. The modem technology makes use of oxygen, as done for the first time by Shell in 1958. The catalyst is exclusively silver, but it is always promoted by various additives such as chlorine and alkalis. Thermally stable catalysts have been designed; they contain small nuclei of a transition metal (e.g. platinum) with a thick silver shell around it [146]. In most practical applications alumina is used as the support. The installations comprise units for careful purification of the feedstock and allow an in situ regeneration of the catalyst. The most serious problem is the large exothermicity of the reaction which requires special multitubular reactors with a very well controlled temperature regime. Oxirane is usually absorbed in water, concentrated thereafter and purified. The main by-product is acetaldehyde and
Oxidation reactions
573
some of the by-products are not produced in the reactor but in the heated pipe lines or even during the separation in solution. Other aspects of the technology are described in standard texts [ 147]. 12.4.4 Electrocatalytic oxidations Basic textbooks on physical chemistry teach us how the second law of thermodynamic restricts the thermal efficiency of a heat machine. Calculations with the Carnot cycle formulate this restriction quantitatively. When the chemical energy is not used to produce heat (and from heat, electricity), but the oxidation reactions are taking place in an electrochemical fuel cell, producing electricity directly, restrictions imposed by Carnot cycle are removed. As fuel hydrogen, methanol or hydrocarbons can be used, at least in principle. In practice, the hydrogen-oxygen fuel cells seem to have reached a high level of technological development and futurologists speak sometimes about 'hydrogen economy'. The known problem is still a fabrication of an optimal oxygen electrode. For some small applications methanol as a fuel is a promising option [151]. Figure 22 shows essential features of all. In these cases platinum-tin electrodes appeared to be superior to pure platinum electrodes. This applications next to naphtha reforming were the main incentive for some research done with platinum-tin catalysts. Some other alloys such as platinumtitanium have also been explored as catalysts for various electro-oxidations [152].
EXTERNAL LOAO------.~
-CARBON DIOXI DE
METHAN'OL, oULPHURIC
" ~ A NODE
AIR
~
CATHODE
6 H*, 3/20,~, 6e -.-~. 3H20
MEMBRANE
CH3OH H20 9 -,,-CO 2
OVERALL CH3OH ,3/202-~CO 2
figure 22 Methanol fuel cell.
METHANOL
*
2H20
96 H ' , 6e"
574
chapter 12
References 1
2a b
9
10 11 12 13
14
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68a b 69 70
71 72 73 74 75 76 77 78
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579
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Oxidation reactions 143 144
145 146 147
148 149 150 151
152
581
W.Ostwald, US Patent 858,904 (1902) J.M.Hess, J.Phillips, J.Catal. 136 (1992) 149 T.P.Chojnacki, L.D.Schmidt, J.Catal. 115 (1989) 473 Yuantan Ning, Zhengfen Yang, Huazhi Zhao, Platinum Met.Rev. 39(1) (1995) 19 R.T.Horner, Platinum Met.Rev. 35(2) (1991) 58; 37(2) (1993) 76 A.R.McCabe, G.D.W.Smith, A.S.Pratt, Platinum Met.Rev. 30(2) (1986) 54; 32(1) (1986) 11 T.E.Lefort, Fr.Patent 729 952 (1931); US Patent 1,998.878 (1935) J.W.Geus, private communications Kirk-Othmer Encyclopedia of Chem.Technol., 3rd edition, Wiley N.Y. vol.9 (1980) p.440 J.Berty in 'Applied Industrial Chemistry' (editor: B.E.Leach) Academic Press, vol.1 (1983) p.207 H.Nagashima, M.Y.Ischinki, Y.Hiramatsu, US patent 5,236 692, Mitsubishi Co. (1993) L.W.Gasser, J.A.T.Schwartz, US patent 4,832 938, E.I.Dupont (1989) C.G.M. van de Moesdijk, Ph.D.thesis, Utrecht University, The Netherlands, 1979 D.G.Lowering, Platinum Met.Rev. 33(4) (1989) 169 D.G.Cameron, G.A.Hards, B.Harrison, R.J.Potter, Platinum Met.Rev. 31(4) (1987) 173 B.J.Piersma, W.Greatbach, Platinum Met.Rev. 30(3) (1986) 120
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583
Chapter 13
REACTIONS OF ALKANES AND REFORMING OF NAPHTHA
13.1
Fundamentals
13.1.1 Adsorption of hydrocarbons under reaction conditions Some of the spectroscopic and other surface science techniques, such as UV-VIS or FT-IR spectroscopies, are well suited to monitor adsorption in situ, while catalytic reactions are running. However, many other techniques such as EELS, LEED, XPS and UPS, requiring high vacuum, are not applicable. Moreover almost all spectroscopic techniques which can be used with metals and alloys suffer from the inherent drawback that the most observable and easily visible species are usually those which are only of indirect importance for the reaction studies, being for example poisons arising in situ from the reaction itself. It is extremely difficult to detect species which can be considered with confidence as reactive intermediates. Even the name indicates that their steady state concentration on the catalyst surface will be low. Thus, regardless of the ever continuing accumulation of the most valuable information provided by various spectroscopic techniques, new attempts are always being made to identify the reactive intermediates by chemical methods; some catalytic reactions indicate quite clearly what these intermediates must be. Very important information on the reactivity and adsorption modes of hydrocarbons on metals and alloys has been obtained by the hydrocarbon-deuterium equilibration reactions. Kemball and his associates were responsible for much of progress in this field, by developing the theoretical basis of these reactions [1-3]. The literature offers several reviews on this subject [3-5], the matter is also discussed in chapter 10.2. Exchange reactions Exchange reactions have revealed several important aspects of the behaviour of alkanes on catalysts, as we have already seen in chapter 10. Let us briefly summarize the points (i)-(v) which we have to keep in mind when discussing reforming reactions on metals and alloys. (i) Although the bond strength (dissociation-energy) of C-H bonds is higher in many molecules than the dissociation energy of the C-C bond, the C-H groups react first [1-4] (see also chapter 10). All alkanes can be bonded to the active site through a multiple
584
chapter 13
bond on one carbon-atom [4,5] (ii) Formation of metal-carbon bonds (see the estimates in [6-10]) can make the dissociations in alkanes thermodynamically feasible. (iii) Methane is considerably less active than higher alkanes and needs higher temperatures for its exchange [4] (see also chapter 10). Recent molecular beam and vibrational spectroscopy investigations revealed why just the adsorption of methane is so difficult. To initiate the C-H dissociation, methane molecule must be pressed against the surface [11 ] to achieve a better contact of both C and H with the surface atoms, as shown schematically by scheme I: Scheme I
Methane is a rather small molecule, its Van der Waals interaction with the surface is weak and therefore, either a high temperature of the whole system or at least a high kinetic energy of
CH 4
such as in supersonic beams, is necessary to press it against the surface for
it to be dissociatively adsorbed. Higher alkanes interact physically with the surface in a way which is strong enough to allow good sticking and subsequent dissociation, without any activation energy or with only a small one. Figure 1 shows an appropriate LennardJones diagram, describing the dissociative adsorption of alkanes and alkenes. One can expect: the higher the molecular weight, the stronger the physical interaction, the deeper the minimum on the curves 1 and 2 and the lower the activation energy of dissociative adsorption.
Epot
If,\
I 2~~~----~olko nes / ~lkenes3 v~-. diss. adsorbed
Io'c-H' distclnce from the
surface
figure 1 Lennard-Jones potential energy diagram showing schematically the energy changes along the transitions: alkanes ~ dissociative adsorption (1,3) alkenes ---) dissociative adsorption (2,3).
As mentioned on other places in this book, this diagram does not show exactly how a molecule reaches its final state of adsorption. A molecule can bounce against the walls of potential wells, the exact movement depending on details of energy transfer between the molecule and the metal or alloy surface, and on other things which cannot be shown by such a simple diagram. However, by using such diagrams we can easily indicate
Reactions of alkanes and reforming of naphtha
585
the energy of the system at different distances of a molecule from the surface, regardless of how it gets in the given state. The extent of multiple exchange on different metals can be quantitatively characterized by parameter M (see chapter 10), defined as m
M - E i=l
where di stands for concentrations (in %) of the product containing i deuterium atoms. For methane, m equals 4. While palladium and platinum show a low extent of multiple exchange with methane, ruthenium, cobalt, nickel and rhodium give much more. One can conclude that the first two metals form multiple bonds such as
/~
CH
CH2
/1\
II
Scheme II
reluctantly, but the latter metals do it easily. When we consider a C-C bond splitting in its simplest possible form, viz.
I
C
c-
-~
*
-~
c
c
I \ / -X-
/ C
+ \
\
/ ~ ~
Scheme III
II
/ C
\ "4
We have to assume that it can occur either by radical-forming, with an activation energy of 340 kJ/mol or higher, or when metals help (as indicated in scheme III) by forming multiple bonds dissociation can presumably occur with a lower activation energy. Experimentally found activation energies of C-C hydrogenolysis are always lower than about 200 kJ/mol, so that a mechanism using metal-carbon multiple bonds is very likely. Therefore, Kemball suggested and verified [3c] a sympathetic correlation between the propensities for multiple exchange on the one hand and hydrogenolysis on the other. Methane is a very good diagnostic molecule for establishing the multiple bond formation, but it is not ideal in all respects. The multiple bonds between the molecule and the metal surface can be formed
only to one and the same carbon atom. For example a
higher alkane such as ethane can form multiple bonds and induce multiple exchange by forming offS-bound species (see chapter 10). Therefore, it was necessary to check whether
586
chapter 13
Kemball's correlation between multiple methane exchange and hydrogenolysis of a higher alkane also holds when multiple exchange running through the ac~ species is established with the same molecule for which the hydrogenolytic results are considered. A molecule which would allow such correlation to be studied appeared to be cyclopentane. Before we discuss the results, a few words must be said about the exchange reaction of this molecule. Cyclopentane exchange gives as initial products CsH9D by reversal of its adsorption as a cyclopentyl radical, and molecules having from two to five deuterium atoms formed by multiple 0tl3...c~...0~13 exchange on one side of the molecule. Somehow or other [4,5] the molecule may 'roll over' so that hydrogen atoms on the other side can exchange; CsD10 will then be the major product. A temperature may be chosen such that the a13 mechanism gives mainly CsHsD5 while the ~o~-adsorbed species are still present, giving CsH8D 2. Thus the low ratio ds/d 2 under these conditions indicates whether a metal readily forms the ota species: the results of this study [5b] are shown in figure 2, which compares this ratio with activity at a high temperature for cyclopentane hydrogenolysis.
figure 2 The relation of the activity in cyclopentane hydrogenolysis with the propensity to form multiple bonds. Activity is characterized by the temperature region (bars) at which the conversion increases from the first traces to 3% while the ds/d2 ratio is measured at the conversion in exchange of o~=15%.
573
-
PV
T(K}
/ /
/
/..73
-
./I/RI h /
~_ _. __ ~ 373 -
"~ -'Ni
Co
Ir
Ru
273
0
1
i
dS/d 2
Obviously there is a sympathetic correlation between the hydrogenolytic activity and the propensity to form metal-carbon multiple bonds. Hydrogenolytic activity has also been plotted as a function of the multiple exchange parameter M, established with methanedeuterium exchange on the same metals. The results are shown in figure 3.
Reactions of alkanes and reforming of naphtha
673 Pd
T(K) 573 -
.~.... . . . .
-.\ \
473 -
\ \ ~Rh
373 273
0
|
1
I
2
N 1523 K)
587
figure 3 The relation of the activity in cyclopentane hydrogenolysis with the parameter M, characterizing the formation of multiply bound species of methane. Multiplicity M of the methane/D e exchange reaction is determined at 523K and 10% overall exchange of methane. Activity in cyclopentane hydrogenolysis, as in figure 2. [5b1.
We can now quite safely conclude that the hydrogenolytic activity and the propensity in mutiple bond formation (c~a) follow - as suggested by Kemball [3] - the same sequence: Ni, Co, Ru > Ir > Rh >> Pt, Pd
(2)
It is quite unfortunate that, up to now, there is no theoretical explanation available of this order in activities. The smaller particles of all metals are less efficient in forming metalcarbon multiple bonds, and are also (except platinum) less active in hydrogenolysis (5a,b,c]. However they can survive selfpoisoning and keep their activity longer. This has to be kept in mind in the discussion of the results obtained with e.g. ruthenium-copper alloys (see below). (iv) Higher alkanes can induce multiple exchange by c~g-bound species. There was some doubt [1-3] whether with these molecules c~-bound species are of any importance at all. However, there seems to be no reason for such doubts. Figure 4 shows at its left side the product distribution of ethane/D 2 exchange on Pt(II) complexes. The high contribution by d 3- and d6-products indicates that with the mono-nuclear Pt(II) complexes and ac~ multiple bond is indeed possible. On the right side of figure 4 is the product distribution obtained with a platinum film [2,3]. We can immediately see that the distribution also shows a high d 3 peak. It is impossible to explain it by one single exchange mechanism yielding dl-, d 2- and d3-products as well as the d6-product. Thus two or more different multiple exchange c~,13 mechanisms had to be postulated. Alternatively, one can assume that, next to the ag multiple exchange leading to d6-product, there is a parallel multiple c~a exchange. The second possibility seems to be more likely. Kemball et al. [1-5] have established that ag adsorption of alkanes is not only easier than the c~a adsorption, but also easier than the ct7 and other similar adsorptions (see also chapter 10).
588
chapter 13
Pt 2§
%d i
Pt-metol
100~
134~
40
20-
I 2
3
I
4
I
5
6
I 2
3
Z. 5 6
D-exchonged
figure 4 Initial product distribution of the exchange of ethane with deuterium (platinum) and with
D20 (Pte+) [2,3,5c,d].
(v) When interpreting the results obtained by exchange reactions at higher
temperatures or when planning such experiments one has to be aware of the following complication. The large difference in the reactivities of the C-H and the C-C bonds means that the results of exchange reactions can be analysed in a straightforward manner usually only in the region of temperature where C-C bonds do not react. When the temperature is raised to induce skeletal reactions, the molecule undergoes many consecutive reactions on the C-H bonds before a reaction on one of the C-C bond is accomplished. By this circumstance, it is generally impossible to derive from the results on exchange reactions information on the binding to the surface of intermediates in simultaneous reactions. In some favourable cases this is easily possible and very valuable information can be then obtained. For example, in the exchange reaction of adamantane there is a simultaneous isomerizarion [12] and since the only product of exchange was that with one deuterium atom it was concluded that skeletal isomerization must be in principle possible from the state in which the intermediate species is bound by a single M-C bond to the surface. They suggest, for example, for isomerization of neopentane (2,2-dimethylpropane) an intermediate shown in figure 5; the reactive intermediate here has the form of a pseudo metal-bound carbenium ion. This approach [12] is actually a combination of the exchange reaction with the use of 'archetype' or 'diagnostic' molecules, an approach which is treated in the next section.
589
Reactions of alkanes and reforming of naphtha
/ CH
i 2
CH
CH ," ,3 ,," ' , , / C H
3
C'---CH ~
\
~M~
CH
3
C"
3
/CH
~
,i, ~M~
"'"D CH CH
3
CH 3
2
C
I\ c
~M~
H3
figure 5 Pseudo-carbenium-ion mechanism of 3C-skeletal isomerization [12] schematically.
Diagnostic or archetype molecules Another possible approach in the identification of reactive intermediates is to use the so-called diagnostic or archetype molecules. J.R.Anderson called them 'archetype' molecules, because their structure makes them suited to form just one reactive intermediate. This approach can be most easily explained with the molecule neohexane (2,2dimethylbutane). i
ct5'
f C C
C
I C--- C C 4 ~C ~" I
I
C
I
C4 I
C--_C
~C
I
\ c--dc~c c~c '
I
[somerization : 2.3-dimethylbutane 2- methylpentane
3-methylpentane
Hydrogenolys~s: C1 . 2-methyl butane
CI .2-methylbutane
C1. neopentane
C2.2-methylpropane
figure 6 Adsorption modes of neohexane and the products arising from these modes. C1 = methane; C2 = ethane.
Figure 6 shows two o~7-bonded and one o~6-bonded species, with a list of products underneath which can be associated with each of these intermediates. When the reaction is monitored at a low conversion and the system does not show too much consecutive
590
chapter 13
reaction, one can derive from the product distribution which intermediates are the most favoured on the surface of the metal or alloy studied. It appeared from studies using neohexane that only platinum and palladium favour the aT modes at low temperature, and they also show appreciable isomerization. All other metals show a strong preference for the o~g-adsorption mode, and hydrogenolysis [13,14]. The propensity to o~7 adsorption and isomerization follows the order: Pt>__Pd>Ir>Ni > other metals. The order is somewhat similar to (but the reverse) of that seen in the foregoing section, concerning multiple bond formation. The role of consecutive reactions increases as Pt_
%
% CONC.
CONC. 5O
615K
639K
662K
30-
615K
IT
-2
% CONC.
662 K
639K
_-6
10-
50-
-10
%
CONC. 622 K
C1C2iB 2MP N P 3MP
639K
661K
-10
622K
639K
661K
Prop iP nHX CHX Bu nP McP
figure 7 Product distributions obtained with reactions of neohexane on iridium catalysts; small (upper part) and large (lower part) particles are compared Cl-methane, C2-ethane, iBisobutane, NP-neopentane, 2MP-2methylpentane, 3MP-3methylpentane, prop-propane, iPisopentane, nHX-n-hexane, CHx-cyclohexane.
Small particles (the upper part of figure 7) which are known to be less influenced than large particles (lower part) by carbonaceous layers formed during the reaction (chapter 6), show even at the lowest temperature a lower molar concentration of neopentane than of methane. It means that the ~B-mode, which induces the production of the same amounts of
Reactions of alkanes and reforming of naphtha
591
neopentane and methane, leads simultaneously to consecutive reactions of the still adsorbed isopentyl species. This and other similar consecutive reactions cannot be suppressed by shortening the contact time, since the consecutive reactions occur during one sojourn of the reactant on the surface. Large particles, covered by carbonaceous layers to a higher extent, show more of isomerization and adsorption by the ~y-modes. Obviously, selfpoisoning favours a small extent of consecutive reactions and stimulates formation of o~y'-mode [15]. It seems that three phenomena are sympathetically correlated: (i) the activity in the formation of the multiple metal-carbon bonds (see chapter 5) (ii) the activity in hydrogenolysis of alkanes, and (iii) the extent of consecutive catalytic reactions in the adsorbed state. The influence of several consecutive steps occuring upon one adsorption sojourn on the surface on the product distribution must not be underestimated. We meet this problem not only with exchange reactions and with reactions of diagnostic molecules discussed here, but also in reactions with 13C or 14C labelled molecules. Another example of the use of diagnostic molecules concerns molecules which can form carbon-metal multiple bonds but which due to branching in the molecule cannot form a system of two or three conjugated C=C bonds. An example of such a molecule is 2,2dimethylpentane: this molecule does not undergo cyclization at moderate hydrogen pressures although it should if forming of metal-C multiple bonds were a sufficient condition for it [16,17]. The conclusion therefore is that formation of a conjugated multiply-unsaturated structure (which cannot be formed here) is a pre-requisite for cyclization. Zimmer et al. [18] reported that, when the hydrogen pressure is decreased and the surface coverage by hydrogen is thereby suppressed, new routes (probably through multiple consecutive reactions in the adsorbed state) become available which lead to an extended cyclization of 2,2-dimethylpentane and similar molecules. It seems that, when | is low, the metal-carbon multiple bonds are formed more easily and they are also more favoured at equilibrium. At higher pressures of hydrogen, i.e. higher OH' s, the temperature must be increased if such multiple bonds are to be formed, and the prevailing intermediates contain carbon-carbon double bonds, i.e. polyenic species. While neohexane can be used to establish whether a given metal or alloy is able to form t~B-, t~y- or t~y'-bonds, the 2,2,3,3-tetramethylbutane molecule can be used to test the reactivity of adsorbed species for internal C-C bond fission. With Pt/AI203 the propensity for this splitting is high as witnessed by high concentration of isobutane in the products [ 19]. This could be an indication of the occurence of t~8- or t~t~88- or similar bonding. The use of the following molecules belongs to the same group of experiments. For example, from 2,2,4-trimethylpentane, various xylenes can be formed. However, this is only possible after 1,1,3-trimethyl-cyclopentane has been formed. Since the mechanism of Finnlayson et al. [16,17] cannot operate because of the quartenary carbon-atom, we can
592
chapter 13
say that the intermediate for aromatization is a 5C-ring, which must be 'standing' and not 'lying'. It is most likely that the intermediate on the way from 1,1,3-trimethylcyclopentane to xylenes is a 3C-ring comprising the carbon bearing the two methyl groups. Obviously, by using a proper iso-octane molecule, the existence of two intermediates can be proven [20]. The fastest way to make benzene from an alkane such as hexane is by a sequence of dehydrogenations, viz. hexane-hexene-hexadiene-hexatriene-benzene. This has been proven by using ~4C labelled alkanes and alkadiene [21-26]. This way is so much preferred at high temperatures that some aromatics are formed from substituted cyclopentanes by the sequence: Cs-ring opening - dehydrogenation - C6-ring closure. When
platinum
and palladium
alloys
are investigated
as y-A1203-supported
catalysts, it is sometimes more difficult to gain clear information on intermediates adsorbed on the metal, because of the occurrence of simultaneous reactions on acidic centres [20,21,27], leading to the same products. 13.1.2 Kinetics of skeletal reactions It is mainly hydrogenolysis which has been studied by analyzing kinetics and for which investigators have tried to relate the parameters of the kinetic equations to the composition and the structure of the intermediates participating in the rate-determining step of the reaction. However, the same principles can be applied to derive equations for e.g. isomerization, and in some cases this has been done, too [28]. The first kinetic equation rationally derived for hydrogenolysis of ethane is probably that by Cimino et al. [29]. It was assumed that ethane is adsorbed dissociatively and this adsorption is in equilibrium with the gas phase ethane when the state of C2Hx has been achieved. This is expressed by the equations: C2H 6 = C2H x + a H 2
(3)
a- 6-x
(4)
The following and rate-determining step is then the C-C bond rupture involving gaseous or weakly adsorbed di-hydrogen: The rest of the reactions leading to C H 4 formation are fast. The equation: k reac
C2Hx + H 2
. . . . . . ) CHy + CH z
(5)
Reactions of alkanes and reforming of naphtha
593
(6)
k~Pc~n6.( 1 -O q n ) = kr,,~, "Oc~vl,P ~ can be rewritten as
oc: m =
=A
with n_
n
(1-via)
(8)
rate = kreacOCzHx.PH2 = g Pc2n~.PiI~ which is easily verified [28,29]. It appeared that the exponent m which is given by:
m = 1 -na = 1 - 6 n + 3x 2
(9)
increases with increasing temperature. However n is almost independent of temperature and that would mean that x should increase with temperature: x indicates the content of hydrogen in the adsorbed species, but this should probably rather decrease than increase as the temperature is raised. There are more logical contradictions in this treatment, and therefore, Sinfelt and Taylor [30] suggested a different sequence of steps: k1 C2H 6 ~
C2H 5 + Haas (equil)
(10)
k2
K2 CzH 5 + Had s
-----> CzH x + ~ H 2
CzH x + H 2
------) CH 2 + CH x
(11)
k
A steady state is then assumed for |
(12) and, with a = 2 and b = ki/k K 2, the final
equation was found in the form:
rate= klPc2a~ ~ pc2n6.p~ 1 +bpH 2
(13)
594
chapter 13
The above treatment is not without problems and modifications of it followed later [31]. These equations could explain the negative order in hydrogen pressure found in a certain temperature region
(m < 0), but failed to explain why with some metals (e.g. iron and
rhenium) m exceeded unity. Further it remained unexplained why, in the formal power rate law r = k pH2m pHcn, the parameter m is larger than unity, for
CnH2n+2with n greater
than 2. Some workers therefore proposed that reactive desorption, in which hydrogen participates, is the rate-determining step [32]. However, there are also suggestions that the rate of adsorption is the rate-determining step [33]. Let us take now two examples to show how the variations with temperature of the hydrocarbon and hydrogen formal orders can be rationalized by full kinetic equations derived with the assumption that the C-C bond rupture is the rate-determining step. (i) It is assumed that the hydrocarbon fragment involved in the C-C bond rupture retains at different temperatures and pressures, or with different hydrocarbons, a different number of H atoms: a low number at low hydrogen pressure and high temperature and vice versa [34]. In what follows, two situations are analyzed: the C-C bond rupture occurs from (a) CzH 5
and (b) from C2H2. The adsorption site is indicated by Z, all species are assumed to
be bound to a single site. a) C2H 5 Z + Z - C H 3 Z + C H 2 Z; b) CzH 2 Z + Z - 2CH Z It is assumed that, with an excess of adsorbed hydrogen on the surface, the rate r of ethane hydrogenolysis is in case a):
r= klPc:~ Pv~ (Pe2H6+k2PH2)2
(14)
For case b), with C2H 2 undergoing the C-C bond rupture:
r=
k3Pc2HeP~2 --
(Pc2H~+lr
(15)
2.5.2
)
At a constant hydrogen pressure, the rate can show a maximum as a function of the hydrocarbon pressure (figure 8). At a constant hydrocarbon pressure and in the higher range of hydrogen pressures a negative order (in the approximation of the power rate law) in hydrogen is predicted. Moreover, for case b) a region of hydrogen pressure is predicted to exist, in which the rate of reaction increases with increasing hydrogen pressure, that is, hydrogen has here in the approximate power rate law a positive order. In principle the idea of shifting the rate-determining step amongst the various elementary steps of the surface reactions, i.e. going for example, from C2H5 to C2H2 as the key intermediate, can explain the change in the hydrogen order when going from, for instance, ethane to heptane. One has just to assume then that the H/C ratio on the C-C group undergoing fission is lower with heptane than with ethane, which is not unreasona-
Reactions of alkanes and reforming of naphtha
595
ble. ~Qtel
3.010f
figure 8 Hydrogenolysis of ethane at constant hydrogen pressure (1.2 kPa); the rate as a function of ethane pressure. Catalyst: 1.1% wt Pt, 0.7% wt Fe, on silica. Curve 1)is plotted according to
;,00~.
eq. 14, curve 2 according to eq. 15. [34].
0,002
ii) The second approach represented at the time of its publication [35] a substantial innovation in the routine of writing down the kinetic equations according the procedure worked out by Hougen and Watson and others (see chapter 6) a long time ago. In this approach [35] it is properly acknowledged that there are always side reactions accompanying the target hydrocarbon reaction, viz. reactions of deposition and removal of the carbonaceous layer. The latter reaction leads to a steady state coverage of the surface by carbonaceous deposits and this is a function of the hydrogen pressure and temperature. In other words, the approximate power rate law should be written as: r =kNwork P~cP~
(16)
where Nwork stands for the number of sites on the working surface uncovered by the carbonaceous layer. It can reasonably be assumed that: r
N~o~k = const.pn 2
(17)
When the mechanism of the reaction leads to equations such as 14 or 15, which can be approximated by 16, with m less than zero, the total order in hydrogen pressure, r + m, can be positive. This short treatment probably suffices to illustrate the problems one has with using and determining kinetics. For any practical application, knowledge of the kinetics is essential, since it forms the basis of reactor and process design. Further, mechanisms suggested on other than a kinetic basis should be checked by formulating and verifying the kinetic equation. However conclusions from kinetics on mechanism and key intermediates should always be made cautiously. Since very different mechanisms can lead to very similar equations, kinetics does not supply in this respect more than a hint, which nevertheless, can be very valuable (see also chapter 6).
596
chapter 13
13.1.3 Model reactions of alkanes on metal catalysts With regard to skeletal reactions of hydrocarbons, let us now consider the segment of the periodic table of elements which is most relevant for metal catalysis. We can conclude on basis of the literature, with some extrapolation, that the metals of the Groups 3-6 show strong tendencies to be converted into stable hydrides and carbides upon contact with hydrogen and hydrocarbons respectively. The non-stoichiometric carbides can show sometimes interesting catalytic properties [36-38], but we do not intend to discuss this matter in detail. Formation of hydrides and carbides is suppressed in many alloys. Not much is known about Group 7, but the properties of elements in Groups 8-12 are known quite well. Since Groups 11 and 12 show a very low or zero activity in skeletal reactions of hydrocarbons, we focus just on the iron triad and platinum group metals (table 1). table 1
8
9
10
Fe
Co
Ni
Ru
Rh
Pd
Os
Ir
Pt (VIII)
All these metals are active in hydrogenolysis, this reaction always being strongly prevailing, except with palladium and platinum. However, as we shall see below, the manner of hydrogenolysis is not the same on all metals. Platinum in a pure state as a film [39] or mounted on an inert carrier such as carbon or silica [40] can induce isomerization and
dehydrocyclization and at higher temperatures also aromatization providing the molecule contains sufficient carbon atoms for these processes to occur. Palladium deactivates very quickly but it shows a high selectivity to dehydrocyclization [41]. A comparison of metals is illustrated by two tables. Selectivity parameter S(i~ indicates how much of the reactant is converted in each of the three groups of reactions: isomerisation, cyclisation and hydrogenolysis (cracking). table 2
Selectivity of supported metals in n-hexane reactions [42,43] Metal T(K)
&so._~__% S_Scycl_~__S,:rack
type of cracking
Pd
678
17.5
42.7
39.8
terminal
Pt Fe
568 523
Ni
523
0 0 0
0 0 0
100 100 100
multiple multiple multiple
Reactions of alkanes and reforming of naphtha
table 3
597
Selectivity of supported metals in heptane reactions [43,44] metal T(K)
SScracl~%
Pd
573
Pt
548
Rh
386
Ru
361
Ir
398
table 4
Sis~
S_Scycl
90.5
6.2
3.1
37.4
46.8
15.8
93.0
7.0
0
92.5
7.5
0
87
13
0
Product distribution in n-pentane reactions with
H2
%
% molar Catalyst
T,K
]~.I-t2
~-pent C1
Pt/SiO 2 (16wt%)
585
0.9
0.1
619
0.9
0.1
Ni/SiO 2
623
2.5
( 19wt %)
623 623
c_c_.2
G
G
5
17
15
3
6
20
18
4
0.5
85.9
6.6
4.7
8.1
0
5.0
0.5
77.0
8.5
8.3
5.6
5.0
2.0
52.0
0.5
3.0
4.4
i-Cs
c-Cs
S_aso
52
6
67
43
7
59
0
0
0
0
0
0
0
0
~ycl
iso-C 5 = isopentane; c-C5 = cyclopentane;
Siso + Scycl-~- Scracking = 1 (cracking is hydrogenolysis) Experiments performed in an open flow apparatus under the conditions indicated.
Hydrogenolysis
For several reactions the order of activities in hydrogenolysis of the Group 8-10 metals has been already established. For hydrogenolysis of cyclopentane under mild conditions and minimal selfpoisoning [45]: Ru > Ni,Co > Ir > Rh >> Pt,Pd For ethane hydrogenolysis according to Sinfelt [46]: Os>Ru>Ni>Rh>Ir>Re>Co>Fe>Cu>Pt, or according to Sarkany and Tetenyi [47], also for ethane, Ru > Rh > Ir > Co > Ni > P d > Pt
Pd
(T=478K) (T = 525K)
The position of some metals is quite clear: ruthenium always stands among the metals of greatest activity and platinum and palladium are always amongst the least active. The
598
chapter 13
metals nickel, cobalt, rhodium and iridium do not differ very much in their activity, which is always quite high, and a small variation in the measuring conditions, leading to variations in selfpoisoning, can easily change the order of activity and cause discrepancies between various groups of workers. Hydrogenolysis can proceed in several manners,
as we shall schematically
demonstrate by the example of n-hexane: a) terminal fission CCCCCC ~ CCCCC + C ~ CCCC + 2C ~ CCC + 3C etc. b) internal fission
CCCCCC ~ 2CCC CCCCCC ~ CCCC + CC
c) random fission, when the probabilities of splitting at any position are equal d) multiple fission
CCCCCC ~ 6C; CCCCCC ---->2CC + 2C, etc.
To characterize the fission quantitatively, several parameters have been suggested. The multiple fission parameter M e appeared to be very useful and sensitive [48]" n-1
(n-i)c,
Mr---
(18)
i=2
C1
wherein n is the number of carbon atoms in the initial molecule, and
Ci
the concentration
of a molecule with i carbon atoms. Upon terminal fission each step releases just one molecule of methane (C1) so that Mf should be one. If the fission is multiple, M e is less than 1 and if internal splitting prevails, M e exceeds 1. Hungarian workers who have produced a great wealth of information on hydrocarbon reactions prefer to use the socalled depth of hydrogenolysis, characterized by the fragmentation parameter ~ [49]: n-1
~=
t
(19)
n-1
~ i Cin1
Montarnal and Martino have characterized terminal splitting in relation to internal and multiple fissure, by using the parameter Pf [50]" C1
c_:
(20)
which increases when multiple splitting is more important and decreases when the internal fission becomes prevailing. Whatever parameter is used, the conclusions are usually similar. The application of M e leads to the conclusion that for example nickel, cobalt and
Reactions of alkanes and reforming of naphtha
599
ruthenium show a very high propensity for multiple splitting and platinum the lowest. Similarly, those who use the parameter ~ derived from their results that ruthenium, osmium and nickel prefer deep hydrogenolysis, while the opposite is true for platinum, palladium, iridium and rhodium. Exactly the same is found for multiple fission when using parameter Pf: it increases in the order platinum, ruthenium, nickel, cobalt. It has already been indicated elsewhere that the o~13 adsorption mode can be considered as a typical intermediate of fast hydrogenolysis. However, the reactions of neohexane reveal that ~7 adsorption can also induce hydrogenolysis. Where possible, this reaction will probably use several surface atoms as the active site(s), but with platinum it is also likely that metathesis-like intermediates (metallo-cyclobutane) operate. Metallocyclobutane is a well-established species in organometallic chemistry; the situation is summarized in figure 9.
\c_c /
ot[3.
/I
\/
I x.
N
=
N
c Jl ~
\/
+
figure 9 Hydrogenolysis induced by o~fi or c~7 adsorption modes, respec-
c II N
tively.
\
ct'y
/
C \/(~ / \ / ~, N
The
~-bound
species
can be adsorbed on sites with
N /\ /CiC
N
\/ + C
more than one metal atom (see scheme 3, above).
II
I~
N
\/
/ \ N
--
-~
-~
The reactivity in hydrogenolytic splitting of different positions in hydrocarbon molecules has been systematically studied by Leclercq et al. [51], with Pt-A1203 catalysts. They defined a reactivity parameter: 60
=
observed rate of reaction of a particular bond rate expected, statistical random rupture
The statistical rate is obtained as the total rate of hydrogenolysis divided by the number of bonds which can be broken (4 in pentane, 5 in hexane, 6 in methylcyclopentane, etc.) The results of such evaluation are shown in table 5.
600
chapter 13
table 5 Rates of hydrogenolysis of C-C bonds at 573K (PH2 = 0.9 atm.; P
Hydrocarbons 1 2
C-C-C-C
Broken bonds 1
2
Reactivity factor: co 0.9 1.2
Hydrocarbons 123 C-C-C-C-C-C-C
hydrocarbon ----
Broken bonds
0.1 atm.)
Reactivity factor: co 1.1 0.6 1.3
123 C-C-C-C
1
I
2
C
3
08 0.5 1.9
C 1123
C-C-C-C I
0.8 2.2 0.35
C 1 2 3 4
1
0.85
I
2
C
3 4
0.9 1.2 1.15
C-C-C-C-C
1 2
C-C-C-C-C
1
I
2
C
3
3
1 2
C-C-C-C I
I
1
2
1.8 0.4 0.6
0.7 1.65
C 112 3 4 C-C-C-C-C I
1.45
C C 112 3 4 C-C-C-C-C I
I
C
C
C 112 3
C-C-C-C
CC
0.25 3.7 0.09
I I
0.35 3.8 0.09 1.05
0.7 2.7 0.6
CC 1 2
C-C-C-C-C I
I
C
C
1
2
1.15 0.45
CC 112 I
0.45 4.3
C-C-C-C I I
CC 1234
C-C-C-C-C I I
CC
5
1 2 3 4 5
0.95 0.8 0.85 1.75 0.7
1 2
C-C-C-C-C III
CCC 3
1.1 0.5 1.6
Reactions of alkanes and reforming of naphtha
601
We can see from the results obtained with 2,2,3,3-tetramethylbutane that with platinum the ~x8 adsorption mode can also induce hydrogenolysis. When the aB, o~y, ~8 modes of binding are all possible, we probably cannot exclude formation of the 1,5 or 1,6 species also. The latter ones could be some of those intermediates which can lead to dehydrocyclization and aromatization. Establishing similar reactivity tables, such as table 3, is more difficult with other metals because of the larger extent of multiple (in the adsorbed state) and consecutive (via the gas phase) reactions. We have learned above several very well-established facts. (i) There is a correlation of activity in hydrogenolysis with the propensity to form multiple bonds. (ii) With all species so far postulated as necessary intermediates of hydrogenolysis we expect formation of multiple bonds. Therefore, one can expect that metals which, under the reaction conditions, form species with a lower H/C ratio would be more active in hydrogenolysis than the metals which give a higher H/C ratio. This can be documented by two references from the literature [52,53]; table 6 shows the H/C ratio of species formed from ethene [53]. The lower index indicates the temperature at which the result shown is obtained. H/C ratio of species formed from ethene
table 6 cat.
(I--I/C)a28
(I--I/C)483
Co
0.82
0.33
Ni
1.12
0.25
Rh
1.2
1.34
Pd
2.1
1.83
Pt
2.5
2.05
The higher H/C ratios correlate with the lower hydrogenolytic activity (see above). The other very similar example is from cyclopropane hydrogenation. Cyclopropane reacts at quite low temperatures according to two reactions: c - C 3 H 6 -I- H 2 ----) C3H 8 (hydrogenation)
(21)
c-C3H 6 -!- 2 H 2 ---> C2H 6 + C H 4 (hydrogenolysis)
(22)
Metals have been found to be more selective for propane (reaction 22), the higher the H/C ratio in the adsorbed layer formed by adsorption of cyclopropane at low pressures on metal films at room temperature [52]. Hydrogenolysis of the C-C bond requires vice versa a low H/C ratio. Notice that due to the electronic structure of cyclopropane, reaction (22)
602
chapter 13
reminds us of the hydrogenation of alkenes more than the C-C bond splitting in saturated molecules (see also chapter 11).
Dehydrocyclization There is a large variety of intermediates to be considered here: di
5C-intermediates
/
/
C
C
c/C--/c
~
c/Cc/c
I_cS
/ /c /
N
N
(o) Carbenic
and
5C-intermediates
I
I
C
C
I
C--C
Olefinic
C--C
I
I
C--C
C
N
N
N
N
l
II
II (d)
c<-.. c
/C~ C C
I
I
C--C
ill
Nm~
C
111
~mN
(e}
(schematically)
/C----,C\
C
II
(c}
\
/C
I
C
5C-intermediates
I
.
/Cx C C
I
C --C
(b)
(o)
(schematically)
/Cx C C
/C~ c c
/Cx C C C~C
carbynic
(b)
C--C
/C
--,\
=
C\c /
=
,,
s
C
~" " ~ c c~C~ /,- .,\ C'--
C
figure 10 Possible cyclic intermediates of isomerization. The same intermediates operate in dehydrocyclization and aromatization.
None of these species has been ever directly seen by any spectroscopy under reaction conditions, but for each of them there are some, more or less strong, reasons to assume
Reactions of alkanes and reforming of naphtha
603
their existence. With metals in their pure state unmodified by e.g. sulfur or other modifier, operating as monofunctional catalyst, the formation of intermediates such as these depicted in figure 10 can to an appreciable extent be expected only with platinum and palladium. However, small amounts of cyclic and aromatic hydrocarbons are formed from alkanes with other metals of Groups 8-10. We have already mentioned in this chapter that 1,6-aromatization can also occur by successive dehydrogenation accompanied probably by rt-complexing of intermediates; this holds in particular at higher temperatures. It seems that diminishing of the particle size of platinum stimulates this way of aromatization or the reverse way of ring opening.
Isomerization This group of reactions can proceed through two classes of intermediates. (i) Intermediates involving three carbon atoms (3C-intermediates) seen clearly in isomerization via (xT'-intermediates with neohexane, and in isomerization of butane. These two molecules cannot form any other intermediate leading to isomerization. (ii) 5C-intermediates involving five carbon atoms of hydrocarbons with six or more atoms, elegantly proven by 13C labelling by Gault et al. (see chapter 1) With these 5C-intermediates, isomerization actually takes the form of a two- step reaction: ring closure, followed by ring opening; the latter occurring at a different place on the ring from that at which the closure took place. The 3C-intermediates suggested in the literature are collected in figure 11 [5,39,55]. The 5C-intermediates are shown above in figure 10.
3C-Intermediates
\
/c2 C1
II ~ " 1o)
of Isomerizotion
/c\ J'~C 3
I
C~C
I
(b)
\ ,'cH3 /
C
"C"
\
(c)
/c C
\
/
C
(d)
figure 11 The 3C-intermediates of isomerization (notice, not all bonds but only the structure is indicated). With the first three intermediates in figure 11, isomerization is again a 'ring closure - ring opening' reaction but only three carbon atoms are involved. The last of the intermediates shown (on the right side) has been suggested by Gault et al. [54d,56]; for isomerization it should split into a carbene species and a rt-complexed alkene, which should then rotate, and after the rotation the molecule should be reconstructed again. However, some doubts arise about the last two steps [57].
604
chapter 13
13.1.4 Reactions on supported metal catalysts Introduction of platinum on alumina catalysts in naphtha reforming revolutionized the production of high quality gasoline. These catalysts dominated the scene for about twenty years after their introduction [58] and, because of their enormous commercial success, the amount of fundamental knowledge concerning their functioning is respectable. Even in the early stages of research and use of these catalysts, it was suggested [58-60] that the prevailing mechanism is as follows. An alkane is dehydrogenated on the metal (this is called the metallic function of the catalyst) and alkenes migrate or are transferred via the gas phase to the acidic centres on alumina (see chapters 6 and 7). There carbenium ions are formed which induce skeletal rearrangements. Rearranged alkenes are then backhydrogenated on the metal. It appeared later that some isomerization and aromatization also occurs on the metallic particles [60,61], but the prevailing mechanism with industrial catalysts is still assumed to be the bifunctional catalysis as described above. The basic facts concerning the acidic mechanism are the following. Alkenes are converted on Br6nsted acid sites (OH groups) into secondary carbenium ions, for example: CH2 = C H - C H 2 - C H 2 - C H 2 - C H 3 + H + =~ CH
3 -
+CH - CH 2 - CH 2 - CH 2 -
CH
3 ~
CH
3 -
(23)
CH2 - +CH - CH 2 - CH
- CH
3
The bond in the 13 position to the positive charge is activated and can be broken, whereby an alkene and a shorter carbenium ion are formed; this constitutes acidic cracking. Carbenium ions can also accept the arising fragment (alkene formed) and this leads to a more stable tertiary carbenium ion. By hydride transfer from another molecule or by splitting off of a proton, an isomerized or a shortened alkene is formed. The overall scheme showing schematically reactions of n-hexane on a bifunctional catalyst is presented in figure 12.
13.2
Model
reactions
of alkanes
on alloys
13.2.1 Nickel, cobalt and iron alloys with Group 11 elements. Nickel-copper alloys, although of limited practical importance, are very useful for fundamental research. Their electronic structures and surface compositions are quite well known, and alloys well defined by XRD can be easily prepared without any support. The latter aspect was important for hydrocarbon reactions with which one had always consider the reactions on the support, i.e. the possibility of bifunctional catalysis. When, for example, alloying of palladium with gold (0.6%wt Pd + 0.1; 0.6; 1.2%wt Au) causes in
Reactions of alkanes and reforming of naphtha
605
reactions of heptane (Hz/heptane ratio of 5.1; 4.6 at 454~ a shift from hydrogenolysis to isomerization [61], one may ask whether this is because of alloying alone, all reactions observed occurring on the metallic phase, or whether alloying suppressed hydrogenolysis by palladium, which could still produce enough alkenes to sustain isomerization on the support. Such problems were however not present when the study was made with unsupported nickel-copper alloys such as evaporated films [62,63].
n-HEXANE
CYCLOHEXANE
1L
CYCLOHEXENE
METHYLCYCLOPENTANE
~
CYCLOHEXADIENE
..,
n-HEXENE
ISOHEXANES
~
ISOHEXENES
1L
METHYLCYCLOPENTENE
ME T H Y L C Y C L O P E N T A D I E N E
BENZENE
figure 12
REACTIONS
ON
ACIDIC
CENTERS.
AFTER
MIGRATION
Reactions of n-hexane on a bifunctional catalyst (schematically, the stoichiometry is not indicated). Horizontally: reactions on acidic centers. Vertically: reactions on the metal.
Cyclopentane-deuterium exchange has been studied [62] on nickel and on well equilibrated nickel-copper alloys. It appeared that the exchange reaction was only modestly influenced by alloying, but the side reaction of hydrogenolysis, yielding, amongst other products deuterated methane, was dramatically suppressed. This selectivity effect produced by alloying was seen as important and therefore a study with other molecules followed soon. Roberti et al. [63] studied the exchange of methylcyclopentane and obtained results which fully confirmed those obtained before [62]. Beelen et al. studied cyclopropane reactions [64]. Hydrogenation to propane can be considered as simple addition of H 2 to an unsaturated bond [52], but production of ethane and methane clearly requires a hydrogenolytic splitting of a C-C bond. Figure 13 shows that the effect of alloying on the two types of reactions is very different. The general character of these results was further confirmed by a study on hexane reactions, with silica-supported nickel-copper alloys [65]. Results are summarized in figure 14. In the same period of time, the group of Sinfelt in the EXXON laboratory worked on exactly the same problems. Their first paper [66] on nickel-copper alloys even appeared
606
chapter 13
in the same issue of J.Catal. as the work described above [62]. The figure which summarized their findings on the selectivity effects of alloying was so convincing that research on alloys was thereby greatly stimulated throughout the world. The figure in question (15) compared the variations in the rates of cyclohexane dehydrogenation with those of ethane hydrogenolysis. o
H2/xe
Aso[
A.
H21q.d l.
ct
%
:b
~'o
6b
-1.0
-20
-0.I~
,12
.0.2
,4
8"OO/oCu
20
4o
60
80 % Cu
figure 13 Nickel-copper alloys (unsupported). a) On the left. ct, the ratio of extents of hydrogen to xenon adsorption, established with evaporated alloy films, prepared in uhv [62]. fl, the relative occupation of alloy surface, by hydrogen adsorbed irreversibly at 293K; fl is taken unity for nickel [66]; for nickelcopper powder alloys. b) On the right. The reactions of cyclopropane and hydrogen; activity in arbitrary units A H - activity in hydrogenolysis to methane and A s - the total activity in all reactions, including hydrogenation to propane. The latter is the highly prevailing reaction on alloys. Support-free powder alloys are used.
While the experimental results obtained with different hydrocarbons and alloy catalysts of different formulation gave a self-consistent and actually a very well reproducible picture, the explanations suggested by groups concerned were quite varied. One group [66] believed first that their results could be explained just by considering the expected variations in the d-character of bonds (see chapter 1 on this parameter) and the effect of them on the chemisorption bond strength of hydrogen. However, when it later appeared [67] that various metal pairs, including those which show hardly any miscibility and thus perturb each other's electronic structure to the least possible extent, behave catalytically in a very similar way, the original theoretical basis for the explanation was abandoned [68].
Reactions of alkanes and reforming of naphtha
607
On the other hand, another group suggested [62] that the selectivity effects are mainly due to dilution of the active sites, which are nickel atoms or nickel ensembles, in the virtually inactive matrix. However it was still believed at that time that there should be some room for operation of electronic structure effects [65]. Later in the seventies a great volume of information appeared (see chapters 1, 3 and 8) about the electronic structure of nickelcopper alloys, and it became clear that those features which were suspected [65] as being a consequence of some electronic structure effect could be explained by dilution effects.
SIMIEL
x----'
%1 /<'~'1
17~ 004 ,.4 k-x ~ x
~
.
-I,
/____ _
%
x
_.,.
M
\
'. s
1
-"
~I.
l
I'
'
I
+
-
t
2'o
-
I
- -
I '
,b
z '
A
"
z
6B
,
80 1
A2
100%Cu 1
figure 14 Reaction parameters o f n-hexane conversion by nickel and nickel-copper alloys. Rate per g c a t a l y s t - r w, rate per cm 2 surface - r~; A 1 - log r w at 603K, A 2 = log r~ at 603K, activation energy o f the overall reaction Eacp fission parameter M, selectivity parameter S; Characterizing the extent o f non-destructive reactions (mainly isomerization); all as a function o f alloy bulk composition (in atomic % copper). Support-free powder alloys used as catalysts.
Results obtained with n-hexane have also been confirmed by those obtained in reactions of smaller hydrocarbons such as ethane, propane and butane, studied on NiCu/SiO 2 alloy catalysts [69a]. A simple statistical treatment of ensemble size distribution and its application to these reactions leads to a required ensemble size for hydrogenolysis of 12-20 atoms, depending on the reaction (see also chapters 8 and 9). Silver is not miscible with nickel but it also strongly suppresses hydrogenolysis of n-butane [69b].
608
chapter 13
I
1
I
1
I
9
1
10 6
.
Cyclohexane dehydrogenation . . ~ -=----=
.
-
o
x
I
i Ethane hydrogenolys*s 10-
1-
L 0
20
40
60
80
100
Copper content (at. %)
figure 15 Activities of nickel-copper catalysts for the hydrogenolysis of ethane to methane and the dehydrogenation of cyclohexane to benzene [66].
Reactions of cyclopropane have been studied on iron-copper and cobalt-copper alloys, the results being comparable with those obtained with nickel-copper alloys [70a]. Unlike nickel-copper, the other pairs show only a limited miscibility but, nevertheless the conclusions for all three systems are similar. (i) At low temperatures nickel can be more active than alloys, but at higher temperatures the alloys are better since they are obviously less poisoned by carbonaceous deposits. (ii) Addition of copper to iron, cobalt and nickel always suppresses the selectivity to hydrogenolysis to methane and ethane. The selectivity effect of copper on nickel has recently been again confirmed by Cole and Robertson [70b]. We have seen above (section 13.1.2) how kinetic equations can be derived and related to the mechanism of dissociative adsorption of alkanes. Ollis and Taheri [71] analyzed in this way the kinetics of ethane and pentane hydrogenolysis on nickel and nickel-copper alloys, and concluded that on alloys the intermediates participating in the rate-determining step, which is here assumed to be C-C bond splitting, always contain one
Reactions of alkanes and reforming of naphtha
609
hydrogen atom more than the analogous intermediates on pure nickel. It is as if the presence of copper in the surface suppressed C-H dissociation and the lower hydrogenolysis activity were the consequence of it [52,53] (see section 13.1.3 on hydrogenolysis). Langenbeck et al. [72] studied cyclohexane dehydrogenation to benzene on nickel catalysts. Dehydrogenation is accompanied here by hydrogenolysis, but the latter reaction could be strongly and selectively suppressed when oxides such as zinc oxide, titanium or thorium oxides or zinc powder, were added to the nickel supported by silica, alumina or other oxides. This was possibly the first observation of pronounced selectivity effects of alloying on reforming reactions. A possible role of alloying was admitted, but finally an explanation in terms of the theory of Schwab et al. [73] and Solymosi [74] was preferred. They believed [73,74] that n-type semiconductors when used as supports inject their electrons into the nickel particles and consequently the selectivity is modified. Having in mind what has been said in chapters 5 and 8, the explanation in terms of alloying is more likely ( a parallel SMSI effect, discussed in chapter 6, can also be expected with for example titanium oxide). In any case, it is surprising how this important paper has been ignored by all other authors in this field. Perhaps the reason is in the title ('Die Wirkung von Metalloxyden mit Elektronentiberschussleitung auf die Katalytische Eigenschaften von Nickel bei der Umwandlung von Cyclohexan'), which did not attract the attention of people working on selectivity problems and on alloy catalysts. The same topic was studied again [75] with the knowledge on catalysis by alloys gathered in the intervening time. According to this study, nickel-copper alloys on alumina show in the reaction of cyclohexane an increase in selectivity to benzene, which remains constant over a broad composition range. However with nickel-lead and nickel-antimony catalysts, the selectivity to benzene, measured after two hours on stream, increases as a function of increasing content of the metal added. However, even the maximum selectivity established in this way is lower than the initial time-on-stream selectivity of pure nickel. Lowering of the selectivity to benzene on bimetallic catalysts as compared with nickel is even more pronounced at 573K. It was suggested [75] that selectivity is positively modified by the second metal only when the self-poisoning by carbonaceous deposits plays an important role. This is the case at high temperatures and low hydrogen pressures. Suppression of hydrogenolysis leading to a higher aromatization selectivity in nhexane reactions at 773K has also been reported for nickel-tin on alumina catalysts [76], probably due to a combined effect of alloying and of modification of the acidic sites on alumina surfaces. When alumina-supported catalysts are used at high temperature, the potential bi-functionality must not be forgotten. Unsupported nickel-copper powder alloys have been used in a study of neohexane (2,2 dimethylbutane) reactions. Alloys with about 50% nickel showed a slightly higher isomerization selectivity than pure nickel (compare with figure 7, for iridium), but this was not the main effect observed. The most pronounced effect of alloying was on the extent of
610
chapter 13
consecutive reactions (multiple breaking of bonds) occurring during one sojourn of neohexane on the surface. The primary reaction produces methane and neopentane via the 2C-orB-adsorption mode. At higher temperatures more methane and less neopentane is observed, which indicates that isopentyl species can react at those temperatures without leaving the surface, as there is no influence of contact time. The point where consecutive reactions suppressed neopentane production is at about 500K for pure nickel but about 550K for a Ni65Cu35 alloy. Obviously, copper suppresses the consecutive reactions of the adsorbed isopentyl species. A very interesting combination of two active metals appeared to be nickel with palladium. Moss et al. [77], who worked with well-defined alloy films, took first a reaction which should not be very sensitive to alloying, namely hydrogenation of ethene. Then they went over to hydrogenolysis of ethane [78] in which reaction palladium and nickel differ very much in their activities, even more than nickel and copper (see paragraph 13.2). In the first case the variation of activity with composition was modest, but in the second case it was more pronounced. The most interesting result was that the rate of product formation correlated very well with the percentage of nickel atoms in mixed nickel-palladium ensembles comprising six atoms. With surface composition determined by AES and by performing the statistical calculation of ensemble distribution by a MonteCarlo technique, the result shown in figure 16 was arrived at. The cobalt-palladium system is in many respects similar to nickel-palladium. Continuous solutions are formed of a very active metal with another which is almost inactive in hydrocarbon reactions. Both metals of these systems are present in the surface of alloys. Results for various reactions are shown in figure 17 for palladium-cobalt/silica catalysts [79]. In another section below, results obtained with platinum-cobalt catalysts are discussed and the reader will notice similarity between the systems. Here we can consider platinum as being the far less active component of the couple. Variations in the aereal (specific) activities are of several orders of magnitude, showing a minimum at certain concentrations of cobalt. However, selectivities show the picture one would expect: cobalt is active mainly for hydrogenolysis, but palladium also in isomerization and dehydrocyclization. When comparing these results [79] with those for the nickel-palladium system [78] one could come to the conclusion that mixed ensembles of cobalt and palladium, unlike those of nickel and palladium, are of a
very low activity. The 'activity-composition' function does not show any kind of behaviour (a plateau or a curve with a maximum) which could be ascribed to the operation of mixed ensembles in the palladium-cobalt system.
Reactions of alkanes and reforming of naphtha
I
.E E
611
I
I
I
3
L. L_ O
-
\
z O
9
~ rr O
100
X
2
x
m u.l .__i rn
9
\
ixl
\
11
\
~t
-r (,3 LU
z u.l cl
\A
\
-
50
OC
z \
if) A\
A
~_.
l
l
20
40
SURFACE
l
w X
\
_
J.
COMPOSITION,
)
_
60
1.,
80 atom
~
O
F--
\
~ ,~."~_ o Pd
Pd
figure 16 Above: rate of methane formation at 573K over nickel-palladium alloys replotted against apparent surface composition (symbols); percentage total surface atoms participating in mbced ensembles is shown. On the left: mixed nickel-palladium ensemble on (111) surface, n = 6 with central nickel doublet (nickel atoms shaded).
612
chapter 13
~,,.lO -5
k
100
k '~
Bo
U
g
o0_6 L
;9
|
E L,.
2 1 0 -7
0
.~
,
50 otorn */, cobalt
"]100
100
"~
B
60
4'.-
o c
40
0
g
-6
20
0
0
50 otom % cobott
100
0 e
~"
figure 17 Catalytic performance of lOwt% Pd-Co/Si02. (A) Catalytic activity for the reactions of hydrogen with neopentane at 515K (v) n-hexane at 553K (o) and MCP at 553K (+). (B) Selectivity changes - neopentane isomerization (13) and in n-hexane reactions: hydrogenolysis (o), isomerization (o), MCP formation (+), and fragmentation factor ~ (A), defined in the same way as 9 in section 13.1.3.
13.2.2 Palladium alloys We have already discussed some alloys of palladium in combination with nickel or cobalt, where palladium played the role of the component of lower activity. Now we shall turn our attention to alloys of palladium with a metal which, for the reaction in question, shows a much lower activity than palladium. Visser et al. [80] used unsupported palladium-gold alloys for skeletal rearrangements of n-hexane and found that alloying suppresses hydrogenolysis, thereby increasing selectivity mainly to dehydrocyclization. Karpinski [81] studied butane reactions on alloys and we can draw the same conclusion from both papers [80,81]: alloying palladium with an inactive metal selectively suppresses the destructive reaction of hydrogenolysis. These results are shown in the figures 18 and 19. Isomerization of n-butane and exchange reactions were studied on palladium-gold films [81]. It is worth noting that isomerization of n-butane is claimed to occur from adsorbed species which have lost only one hydrogen upon chemisorption. We know already that such intermediates have been suggested for isomerization (see chapter 10 and section 13.2). The large decrease observed [81] in rate of methane exchange is ascribed to changes in the electronic structure of palladium. This is of course in principle possible in a reaction involving hydrogen isotopes, and it would not be very surprising, but one also must not forget that surface segregation of gold could be very pronounced, and responsible for the strong suppression of exchange activity. Clarke and Taylor [82a] have studied cyclopentane exchange reactions on palladium-gold
Reactions of alkanes and reforming of naphtha
613
alloys, with simultaneous deuterolysis of the cyclopentane ring as a side reaction; they saw the same strong suppression of the ring fission as seen with nickel-copper alloy films [62].
,....> s 50-
MCP
30-
I 0 --~
J I I0
0
I 20
I 30
I 40
I 50
1 60
At%
Au
figure 18 Selectivity S in non-destructive reactions and the MCP content among products, under comparable conditions. Pd, conversion ~ = 1.02%, 656K: 39% Au, ~ = 0.7%, 633K." 48% Au, ~ = 0.6%, 646K.
20 0
l,(
x q-
IU
@
20
=3 i
x
/s
ii '
,-
~o 10
et-
Z
,u
~
x O
i
~
2 z i
X i
2o
30
i
40
9"o~ d
.... J
~so
lo
2o
3o %
z~o 9okt
figure 19 Left: catalytic activity of Pd-Au(lll) alloys in hydrogenolysis of n-butane at 590K (o) and of propane at 624K (x). Right: catalytic activity of Pd-Au(111) alloys in isomerization of n-butane at 590K.
614
chapter 13
Alumina-supported palladium-gold alloys have also been studied by O Cinneide and Gault [82b]. The results on selectivity in n-hexane reactions indicate the same tendency as in the case of cyclopentane [82a]. By using carbon-labelled molecules, it was possible to study the contribution to isomerization by the 5C-mechanism (chapter 1 and section 13.1); this was high (more than 80%) with pure palladium and remained high in the alloys. Henriques et al. [83] studied the reactions of n-heptane using palladium-tin containing HY-zeolites as catalysts; they observed a suppression of isomerization and of coke deposition by tin. It is likely that in these
bifunctional catalysts
the addition of tin
mainly influenced the behaviour of the zeolite. The possibility that the added metal influences substantially the catalytic behaviour of the zeolite must be always considered with other combinations than this. Palladium-tin films [82a] showed a suppression of those mechanisms of cyclopentane-deuterium exchange which could be suspected to occur on ensembles of several palladium atoms. Thus, the inert component performs the same role as others (Au, Cu in Ni or Pd) do: it suppresses the availability of ensembles of several palladium atoms, with predictable consequences. 13.2.3 Ruthenium alloys Ruthenium and copper have different crystallographic structures (hcp and fcc, respectively) and are almost immiscible; yet addition of copper to ruthenium sometimes causes very dramatic changes in chemisorption and catalytic behaviour. The reason is that, although copper does not dissolve, it adheres to the surface of ruthenium crystallites. Adsorption of copper on ruthenium is expected to be strongest on defects in the ruthenium structure and places such as those around the edges of ruthenium microcrystals and other similar places. Ruthenium-copper alloys were introduced by the EXXON laboratory and since then have enjoyed great popularity amongst catalyst chemists. Sinfelt et al. [84] prepared these alloys in the form of macroscopic aggregates by co-reduction or by sequential reduction in aqueous solutions using hydrazine, and subsequent reduction and sintering in hydrogen. These aggregates showed a very different behaviour in cyclohexane dehydrogenation and hydrogenolysis as can be seen in figure 20. Ethane hydrogenolysis was also suppressed strongly by addition of copper. The amount of copper which would be necessary to cover the whole surface of the small aggregates - in the case studied - would be about 1.5wt%. It is therefore obvious from figure 20 that copper forms small microcrystals on the surface of ruthenium, because at 4wt% copper there is activity observed which is still higher than that of pure copper. This picture by Sinfelt et al. [84], developed from indirect catalytic information, has been perfectly confirmed by analysis using XPS [85], EXAFS [86], and by adsorption of hydrogen [84]. The general qualitative conclusions made from this work on aggregates were later fully
Reactions of alkanes and reforming of naphtha
615
confirmed by studies using single crystal planes [87].
10 4 Q o i 0
I...
Deilydrogenation
___n
X
E s
. dro 'en
(.9 (l)
10 2 z3 u o
E
.._... 4.-.' t~
l
figure 20
1 0
1 4
I 8
1 12
Copper content (at. %)
The rates of dehydrogenation and hydrogenolysis of cyclohexane on ruthenium-copper aggregates as a function of composition [84].
Ruthenium-copper alloys prepared in the form of very small particles on supports [67,86] were subjected to EXAFS studies. These reconfirmed the picture suggested on basis of catalytic results of copper accumulating on the surface of small ruthenium crystals [86]. The catalytic measurements with these catalysts showed extensive agreement with the results obtained on various other supported alloys (called in [84-86] bimetallic clusters). This attractive ruthenium-copper system has kept chemists interested and new papers are still appearing. In spite of the general agreement on some points, some apparent controversies appeared, too. Let us turn our attention to them. A series of papers has appeared from the laboratory of D.W.Goodman on the behaviour of copper/ruthenium(0001) catalysts in hydrogenolysis of ethane [88,89] and figure 21 is representative of the results and the conclusions drawn [89]. Copper suppresses hydrogenolysis almost proportionally to its surface coverage. Such behaviour could be expected from a system in which copper forms islands on ruthenium, but does not diminish the size of the ensembles of ruthenium on the remaining uncovered ruthenium surface. The effect of sulfur which is known to block several sites simultaneously, and also spreads over the surface, thus diminishing the availability of large ensembles, is more pronounced [89]. Sinfelt analyzed [91] these results and pointed to the fact that, while
616
chapter 13
single crystal planes of ruthenium with copper on top do not exhibit a strong suppression of ethane hydrogenolysis, supported alloy particles do, probably because of their heterogeneity. Sinfelt speculated that in powders copper blocks the active defect sites which would survive selfpoisoning in the steady state, while copper islands formation occurs on the single crystal plane. This was a very good guess by Sinfelt, as later appeared. Smale and King [92] have probably solved these problems satisfactorily. They used 1H-NMR to determine the ruthenium surface exposed and evaluated the rates of ethane hydrogenolysis per surface ruthenium atom. Then they plotted these rates as a function of copper content and obtained the picture shown in figure 22, below. 1.0
0.8 o
o COPPER
LLI ,< n~ LU
9SULFUR
>__
-
I,<
..J
LU rr
0.2
0.0 0.0
figure 21
0 1 ADATOM
t . 0.2
0.3
0.4
0 5
0.6
,SURFACE C O V E R A G E ( M O N O L A Y E R S )
Relative rate of ethane hydrogenolysis on a Ru(O001) catalyst as a function of copper coverage (reaction temperature 550K). Sulfur poisoning results from another paper have been replotted for comparison in the same figure [89].
Monte-Carlo calculations (see chapter 4) performed in King's laboratory [92] showed that the first portion of copper added to a system of ruthenium crystallites preferentially covers the defects, edges and comers (specific decoration), the sites which are here assumed to have a higher activity in hydrogenolysis. When the amount of copper added is increased, it starts to form islands and finally microcrystals on the flat planes of ruthenium particles. Such island formation has no selectivity effect and almost no effect on the activity expressed per ruthenium atom. With increasing temperature self-poisoning occurs and then the presence of copper can be beneficial by suppressing it (see chapter 9 and [93]). Therefore, addition of copper can at higher temperatures cause an increase in the rate per
617
Reactions of alkanes and reforming of naphtha
ruthenium atom of the steady state reaction rate (see figure 22). This effect of copper on ruthenium is thus very similar to that observed with nickel-copper alloys. Self-poisoning can be influenced to a very important extent by alloying. 100
10
9493
9528
a 508
o 548
figure 22 Rate of ethane hydrogenolysis at different temperatures (+ 1 standard deviation) per surface ruthenium as measured by proton NMR. Dependence on copper surface concentration [92].
r...- { {
rc D ET eg
c
0.1
0.01 0.0
1
I
1
02
0.4
06
0.8
Cu Atomic Fraction
Peden and Goodman have studied dehydrogenation and hydrogenolysis of cyclohexane on copper/ruthenium(0001) and have observed an enhancement of cyclohexane dehydrogenation when ruthenium was covered by copper [88,89,93]. They suggest that this could be either (a) an electronic structure or ligand effect on the adsorption bond strengths or (b) a mutual synergism in the activities of the metals, for example, with hydrogen supplied to copper by spill-over from ruthenium causing copper to become active. Others [88,89,91,93] also suggest that the strained copper layer could itself become active. Finally, one also cannot exclude a role for mixed ensembles in these reactions. The conclusions drawn by Smale and King [92]
and their model of specific
decoration were confirmed by Sprocket al. on n-butane hydrogenolysis on rutheniumcopper/SiO2 catalysts [94]. A slight increase in internal fission was observed when the fraction of copper in the catalysts was increased. They point out that surface coverage of ruthenium by hydrogen, OH, depends on the copper content, and OH in its turn is known to influence the selectivities. The values of selectivities in different pathways are correlated with the order in hydrogen pressure and both should depend on the composition of the catalysts. Bond and Xu Yide [95] compared the behaviour of propane, n-butane and isobutane on two types of catalyst: ruthenium on silica and ruthenium-copper on silica.
618
chapter 13
Evaluation of the results was performed by using the comprehensive scheme developed by Kempling and Anderson [96]. The RusoCus0/SiO2 catalyst showed in its reduced state a much lower activity at 463K than the pure ruthenium on silica catalyst; however the steady state activity did not show any selectivity effects of copper on ruthenium. A model of specific decoration described above [92,94] is compatible with all these results. It was also demonstrated clearly [95] how minute details of the preparation procedure can influence catalytic behaviour. At the level of a general statement, probably all investigators in the field of catalysis by alloys would admit that a support can influence alloy formation, the crystal texture, surface composition, etc., but there are few detailed studies of the problems. However, important information in this respect is slowly appearing. It has been shown [97a,b] by TPR that the formation and stability of ruthenium-copper catalysts as well as their activity depend very much on the support. Ruthenium-gold catalysts do the same [97c,d]. The test reaction here was hydrogenolysis of propane. It was concluded that suppression of hydrogenolysis acts preferentially on the multiple hydrogenolytic fission to methane [97b]. ~oo[
figure 23 Hydrogenation of cyclopropane on Ru-Au alloys. Influence of temperature on selectivity S. PH = 0.20 atm; Pcp = 0.03 atm. Alloys of different Ru(-R) content: rq, R 100; o, R 089; A, R 064; o, R 010; Pure Ru catalysts: | 0.6%wt Ru/Si02; I"1, Ru sponge. The region of higher temperatures shows a pronounced effect
s(%)j ;
|
8
6
40-
of the support.
1
50
1
t00 T (*C)
I
~50
The ruthenium-gold system is expected to be similar to ruthenium-copper. Galvagno et al. used cyclopropane reactions to examine ruthenium-gold/SiO e catalysts, as discussed before in comparison with nickel-copper. They reported [97d] that the selectivity changes produced by gold are most pronounced at higher temperatures, where secondary reactions occur freely. The selectivity S to propane formation is high for all catalysts at temperatures lower than 373K, but at 423K the ruthenium-rich catalysts show lower values, while the copper-rich catalysts do not (see figure 23). This confirms that also with this reaction hydrogenolysis requires larger active centres.
Reactions of alkanes and reforming of naphtha
619
13.2.4 Platinum in combination with Group 11 elements Platinum is an universal, versatile catalyst, used in many reactions, and therefore the literature concerning this metal is very extensive: a large part of it deals with reactions of saturated hydrocarbons. No wonder, since before the advent of the catalysts for control of vehicle exhaust, the largest application of platinum-containing catalysts was in naphtha reforming. Besides investigations on these industrial catalysts containing rhodium, iridium, gold or lead and modifiers, many studies were devoted to model systems such as platinum -Group 11 alloys. With regard to platinum-gold alloys and hydrocarbon reactions, one can speak of a system comprising one active and one inactive metal. Comparison with the other similar systems discussed above was therefore a great challenge. Experiments [98] with platinumgold alloys prepared by hydrazine co-reduction mixed with inert silica and finally annealed at temperatures ensuring that the surfaces at least were equilibrated [99], revealed a pattern already known from studies on other systems such as nickel-copper: platinum very diluted (x~ < 17%) in gold shows a higher selectivity although at much lower total activity, for non-destructive reactions. This can be seen in figure 24. 1.0-
figure 24 Reactions of n-hexane and hydrogen on pure Pt/SiO 2 and on Pt-Au/Si02 equilibrated dilute alloys (bulk xpt < 17%). Content of platinum in %, average (bulk) composition shown. Selectivities to hydrogenolytic cracking, total of cyclization reactions and isomerziation.
0.8-~ 0.60.4-
0.2-
0
/ ___ .~,/ ol o
0
/
,
,
2
,
,
4
,
,
6
,
o,"
n ,
8
/
_,
S c y ,io / / /
Scr ,
10
,
to"' ,
12
,
i / [ .
12
80
,-~
100
%Pt
At the higher pressures of n-hexane and hydrogen as used in experiments with supported alloys [99], it was mainly isomerization which was favoured in this way. At low pressures in experiments with evaporated and annealed films, it was mainly dehydrocyclization [100]. However, the results collected in the early stage of studies with alloys, already
620
chapter 13
signaled that the results depend on the hydrocarbon used [98-101] and the details of preparation conditions. The latter aspect was explicitly addressed by a comprehensive investigation [102]; in this paper several platinum-gold catalysts prepared as before [98] by hydrazine reduction showed a higher selectivity to destructive reactions and not lower as with earlier catalysts [98]. The only difference between the methods used [98,102] was that in the latter study platinum and gold were reduced in an adsorbed state on a support, and this probably caused a complete reversal of the catalytic effects of gold on platinum [102]. Substantial progress in understanding this apparent dichotomy has been made, through the use of single crystal alloys made in situ by evaporating gold onto platinum single crystal planes. Reactions with these catalysts have been monitored [103] in UHVapparatus equipped with an isolation valve, allowing work with mixtures of 200 Torr of hydrogen and 20 Torr of n-hexane. The results have been summarized [103] by a schematic graph shown in figure 25: Au - Pt (111) § H2 , 573K
/VV
H2/HC= 1 0 , P t o t = 2 2 0 T o r r 1.0
Isomerization
>,
C-5Cyclization
Hydrogenolysis
~Epitaxial systems
/
"5
-.-Alloys
/
aJ 0.5
~,r omatization
/" 9
" "--..-.
"~
~ . "~.
"\ 9 --..
0
I
'
i
I
gold
"
.
.
.
.
!
0
coverage
figure 25 Schematic summary of various results obtained with Au-Pt(lll) alloy catalysts in nhexane reactions, at the indicated conditions as a function of fractional Au-coverage. Epitaxial grown gold layer, results shown by full line. Alloys were obtained by equilibration of that layer [1031.
We see very clearly that suppression of hydrogenolysis is only observed with annealed surfaces in which gold is dissolved. Gold on the platinum surface (in a non annealed system) has different effects from those of gold which is in the surface. This throws new light on earlier results [98-102] and on the apparent contradictions. The single crystal results obviously indicate to us what the origin could be of the poor reproducibility sometimes observed in results obtained with powders. This is also the message which can
Reactions of alkanes and reforming of naphtha
621
be drawn from a later paper [104], from which it appeared that gold has a very different effect when evaporated on Pt(100) plane rather than on the (111) plane and when it is annealed or not. While alloying with gold increased the total activity of the P t ( l l l ) surface up to a surface coverage by gold of 0.4, the selectivity was influenced as shown in figure 25 above. However, with Pt(100) surface the total activity decreased in proportion to the gold coverage. Suppression of selectivity to hydrogenolysis was observed again, but in contrast to the situation with P t ( l l l ) there was no enhancement in isomerization selectivity. It was suggested that perhaps an ensemble in a form of a mixed triplet PtzAu is particularly well suited for isomerization. Whatever the explanation, these results provide yet another reason why platinum-gold alloys are so sensitive to the preparation procedure. The differences in activities in various reactions between platinum and platinumcopper alloys are much smaller than with the platinum-gold couple. There are two possible explanations: a possible spill-over of hydrogen from platinum sites to copper sites, thereby making copper sites active is the first possibility, and further, one must consider seriously the possibility of the operation in catalysis of mixed ensembles. It has been reported [105] that, with platinum-copper/silica alloys, decrease in overall activity of n-hexane-hydrogen reaction occurs, but that selectivity to hydrogenolysis increases as the platinum concentration decreases [105]. This is shown in figure 26: T = 290~
0,8---
0.4
~ ~
/
--C~
o _
~
~
u.. -" I~i
/
o%
o :t~ 0
1~ l"3
2;
0 A --- w - 0
~
6'0
a'o
~)
100
%Pt figure 26 Reactions of n-hexane and hydrogen on Pt-Cu/SiO 2 alloy catalysts. The selectivity to isomerization (squares), hydrogenolytic cracking (triangles) and total cyclization (circles) all as a function of average platinum content in the alloys.
The cracking patterns looked like that of platinum or of nickel diluted in copper [65]. Cyclization selectivity was increased by addition of copper as did also the contribution of the cyclic 5C-mechanism to isomerization [106]. This indicates that in dilute alloys the active sites are platinum atoms in very small ensembles of platinum atoms and that these
622
chapter 13
behave differently from those in pure platinum surfaces. There was no speculation [105106] on ligand effects influencing the properties of platinum atoms, since the temperature dependence of all groups of reactions did not indicate variations in the corresponding apparent activation energies. Results with n-hexane [105] were confirmed by those obtained with neo-hexane (2,2-dimethylbutane) [107] as can be seen in figure 27: 1.0
hydrogenolysis ---o-isomerisotion
0.8
0 A
S
A
0.6 _
0.40
0.2-
0
'
2'0
'
-~'0
'
6'o
'
8'0
% Pt
'
100
figure 27 Neohexane reactions with hydrogen on Pt-Cu/Si02 catalysts, selectivities for hydrogenolysis and isomerization, both as a function of alloy bulk composition. These results were obtained in the temperature region 553-593K, but for each catalyst in a region in which the selectivities were almost independent of the temperature.
As mentioned in section 13.1, the product pattern of neo-hexane reactions also offers information on the adsorption modes. On the basis of this information it could be concluded [107] that the proportions of cz7 and cz~/'modes stay constant for all alloys. The czg-adsorption mode is almost negligible over the whole range of composition [107]. This means that the same species are always formed, but they participate in isomerization on pure platinum and in hydrogenolytic cracking on platinum diluted in copper. Several possible reasons were considered. First, this result could be an effect of a carbonaceous layer present on platinum surfaces but missing to some extent on platinum-copper surfaces. Den Hartog et al.[ 108] addressed this problem using a pulse-micro reactor. Under these conditions the cracking pattern was in detail somewhat different from that seen earlier [106], but the increase of hydrogenolytic selectivity in dilute alloys was confirmed even for virtually carbon-free surfaces. Selectivity in n-hexane reactions did not depend on the number of pulses, and the fraction of surface atoms covered by carbon atoms retained from the first pulse of n-hexane/H 2 mixture was estimated to be less than 5%. Thus the idea of selectivity being mainly determined by deposited carbon could be discarded and
623
Reactions of alkanes and reforming of naphtha
the idea of mixed ensembles proposed in earlier papers [106,107] was re-examined [108].
C
\
C/ \
C Pt
/
C
\C / /
Ca)
C C
\ / / C\
C l
P~
C C t
C / C
Pt
(b)
\ / / C\
C i
Cu
C C i
/C
Pt
figure 28 Possible c~7' adsorption modes of di-adsorbed neohexane.
(c)
Upon comparison of the different adsorption modes shown in figure 28, one possible explanation which emerges is that complexes (a) and (b) lead to isomerization while complex (c) leads to hydrogenolysis. Later [109], it was also admitted that complex (a) might lead to hydrogenolysis. Originally, this possibility was disregarded [107] because dilute platinum-gold alloys did not show enhanced hydrogenolysis. However, one has to realize that platinum and gold form alloys endothermically, and that this stimulates formation of platinum three-dimensional clusters in the bulk or two-dimensional ensembles, in the surface, and thus also in dilute platinum-gold alloys there could be present platinum ensembles suited for isomerization by complexes such as (b). On the other hand the smallest platinum particles show [109,5a-c] enhanced activity in hydrogenolysis but not an enhanced activity in multiple-bond formation, a correlation which otherwise holds for other metals. This points to metallocyclobutane complexes, such as (a) above. For the time being, both these possibilities for explaining the enhanced hydrogenolysis should be kept in mind. 13.2.5 Alloys containing rhodium, iridium and Group 11 metals Rhodium and copper show a very limited miscibility, so that even at low contents of copper the surface of rhodium can to a very large extent be covered. A very interesting comparison has been made of reactions between ethane, pentane and neohexane and hydrogen occuring on rhodium and rhodium-copper/SiO2 catalysts [110]. The reaction of ethane was most sensitive to surface alloying. Adding 10 at% copper to rhodium suppressed the rate of ethane hydrogenolysis by two to three orders of magnitude. Ethane cannot undergo reactions other than hydrogenolysis and dehydrogenation via 2C-~13 complexes. With pentane, the main reaction was again hydrogenolysis, although there was a significant increase in isomerization selectivity, from 1-2% for pure rhodium to 5-15% for alloys. Suppression of hydrogenolysis by copper was however much smaller with pentane than with ethane. This was ascribed [110] to the existence of an one-site, 3C-ay adsorption mode, inducing hydrogenolysis as suggested by Rooney earlier [111]. The most interesting with regard to the possible mechanism are the results obtained with neo-hexane
624
chapter 13
(2,2-dimethylbutane). Here we observe considerable ~B-hydrogenolysis to neopentane even on those catalysts which show an almost zero activity in ethane hydrogenolysis. The same situation exists with nickel-copper alloys [64]. There are several possible explanations to be considered. (i) With neohexane, but not with ethane, one site 2C~B hydrogenolysis is possible, perhaps due to the presence of the secondary carbon atom. (ii) Neohexane forms not only a 3Cc~? intermediate (a metallacyclobutane-like complex), but also a 4Cc~8 intermediate (a metallacyclopentane-like complex). Both cycles are assumed to split upon simultaneous formation of metal-carbenic species. Coq et al. [116] have examined rhodium-copper/SiO 2 catalysts in reactions of nhexane, 2-methylpentane, 2,2-dimethylbutane, methylcyclopentane and 2,2,3,3-tetramethylbutane (TMB) with hydrogen. The presence of copper suppressed all reactions, but the extent of suppression of hydrogenolysis, which is the principal reaction, depends on the structure of the molecule. Particle size and alloying effects were compared for all these reactants. For all reactants except the last, the effect of added copper was much more pronounced than the particle size effect; however, with TMB it was the other way round. An explanation was suggested which assumed that, to break the middle bond in the TMB molecule, the molecule must be bound to the metal surface by carbenic bonds which require large ensembles of rhodium atoms to be available. Such large ensembles can be found only on flat faces of large rhodium particles and that is responsible for the particle size effect. The other molecules can react on the same planes too, but they have also a fast alternative, which is to react on the edge-, comer- and defect- sites. These sites are however first occupied by copper when this metal is added to rhodium, and as a consequence
a much more pronounced effect of copper is observed with this group of
molecules, and vice-versa the particle size effect is less important with them. Iridium-gold films were studied with n-butane and n-hexane by Plunkett and Clarke [112]. The presence of gold mainly suppressed terminal hydrogenolysis, so that dehydrocyclization had a better chance to occur, and finally, at the highest temperature used and at the highest content of gold in the films, it was a pronounced effect. Suppression of hydrogenolysis of n-heptane was also observed at low pressures of heptane on iridium
foils covered by gold [113]. Results obtained with films [112] were confirmed by experiments with iridium-gold/SiO2 catalysts [114], and the usual selectivity effects were observed here, too. Den Hartog compared iridium/SiO 2 (5%wt loading) with another iridium/SiO 2 catalyst in which 20% of the iridium had been replaced by copper [115]. The two metals are practically immiscible so that a close similarity is to be expected with the ruthenium/copper system.
Reactions of alkanes and reforming of naphtha
100-
625
figure 29 Reactions of n-hexane and hydrogen on
I "
-z,.0
S(%)
Ir-Cu/Si02 alloy catalysts. Selectivities for hydrogenolysis (-.-.-), dehydrocyclization ( - - -) and isomerization (full
50-
-2.0
line) are shown as functions of the average iridium content. Also shown is the total conversion per gram catalyst (+) in a logarithmic scale. Temperature
/u7 ' s
-0
oYolr
of the reaction is 600K for all catalysts.
-01
The results shown in figure 29 show that upon addition of copper the total activity is much decreased (right side scale), but the selectivity in hydrogenolysis is slightly
increased (left side scale). It is also important to see that the character of the fission changes from internal (see sections 13.1 and 13.2) to terminal. This change is shown in figure 30.
16-
figure 30 Multifission parameter My (see section
Mf
13.1.3 - f o r definition) as a function of temperature, for 3 catalysts; = 100% Ir
12-
9
8-
9
=
lr89CUll
= IrsoCu2o [115]. l.-
~o
s6o
s~o
060 rcKI
Relative participation of the internal (typical for iridium) and terminal (typical for nickel) fission varies and is reflected by the parameter of multiple fission Mf. It is to be preferred to use the more exact treatment suggested [96] and applied [95], but it is not easy to analyse fully the kinetics of reactions on many alloys. It is worthwhile noticing that addition of copper to either iridium or nickel causes with regard to M e opposite effects on the character of fission. It decreases Mf in the case of iridium but increases it with nickel.
626
chapter 13
Let us stress that the exact reason is not known but a possible role of mixed ensembles of iridium-copper, but not of nickel-copper, cannot be excluded. Another explanation could be that edges of iridium particles are most active in internal fission, but they are the first to be blocked by copper [116]. With nickel, the shift in selectivity could be due to an ensemble size effect. The reader should notice that these are all speculative explanations. Foger and Anderson [117] compared low-loaded (0.8-1.5wt% of metal) iridium/Si02 and iridium-gold/SiO2 catalysts having from zero up to 86% gold, and they also observed a very pronounced dependence of the suppression of hydrogenolysis by gold on the molecular structure of the reactant. Activation energies did not vary by addition of gold in the range of temperature 440-550K, but the frequency factor did change: to only a small extent over a broad range of composition when neopentane was the reactant, but it varied much more strongly when either n-butane or neohexane (reacting here exclusively by the 2C-o~B adsorption mode), was used. It was concluded [117] that neopentane hydrogenolysis takes place solely via a 3C-~y intermediate (since there is no other possibility) while neohexane and n-butane react preferentially via a 2C-c~B fission. It would appear that the first reaction needs larger iridium ensembles. When we combine the results of these investigations [115,117] we can consider that iridium-copper mixed ensembles, on which we speculated above, are indeed operating in hydrogenolytic fission on flat parts of the particle surface. Brunelle et al. [118] prepared various rhenium-iridium and copper-iridium catalysts with alumina as a support, and compared the selectivities with those of osmium, platinum and palladium. The most essential results of this study are shown in figure 31. It is a somewhat complex figure, containing much information. However, if we focus on the most essential points, we conclude the following. Adding copper to iridium causes an increase of selectivity for the non-destructive reactions (F), this selectivity increase being accompanied by activity decrease (E). The suppression is most pronounced
with hydrogenolysis (E), the reaction which we already know as a large ensemble reaction (see chapter 9). With iridium-rhenium alloys we observe synergetic effects which as we shall see below can be ascribed to the mixed-ensembles (B,A). The window C shows that the character of splitting is different on rhenium and iridium, but varies smoothly over the alloys.
627
Reactions of alkanes and reforming of naphtha
00' 0
0.5
1 ,,t % O,
~176 ~.,&
o 1~
or./" ,.~ ~ I
"
.! i
I
1
C
~
0.6 ........... 04
~
' o
3.2 II
_ Ne
F
13
\t N
D
05
Jr" k-Os
0
Ir
O.S
~
-
I~
figure 31 Left: variations in properties of Re-Ir/AleO 2 catalysts with atomic fraction of iridium. Comparison with Os-alumina catalysts. A. Hydrogen chemisorption in cm~/g catalyst (o) B. Turn-over measured at 303K for benzene hydrogenation (a) and 458K for n-pentane hydrogenolysis (o) in mole/h.site. C. Depth of n-pentane hydrogenolysis at 458K expressed by equation(1)(D). Right: variations in properties of Ir-Cu/Al203 catalysts with the atomic fraction of copper. Comparison with Pt-Pd/Si02 catalysts. D. Hydrogen chemisorption in cm3/g catalyst (o). Carbon monoxide chemisorption variation with wt% copper (*). E. Turn-over measured at 373K for benzene hydrogenation (,) and 523K for pentane hydrogenolysis (.), isomerization (V) and cyclization (n). F. Isomerization (v) and cyclization (u) selectivities at 523K.
628
chapter 13
13.2.6 Platinum-rhenium model catalysts We shall see below that in their working state in industrial reactors these catalysts are always used with an alumina support (often 7-alumina) and modified by sulfur. However, to understand the processes on these catalysts better, many have studied them in a sulfur-free state and also supported by inert supports such as silica, or not supported at all. It was from the beginning very important to establish the particular role of sulfur and of the acidic alumina support which is needed for bifunctional catalysis. The most logical step to take is to study first well-defined support-free single crystal planes. These are experiments which, ironically enough, have been performed last and only very recently [119,120]. However, the results of these experiments form the best point at which to start a general discussion on earlier results. With ethane [120], cyclohexane and n-hexane [119] in the feed, platinum-rhenium single crystal catalysts show a synergism: alloys are more active than the individual components. Rhenium is very active in hydrogenolysis. Sulfur enhances the dehydrogenation activity of pure platinum as well as of platinum covered by rhenium. Amongst the skeletal reactions occurring on platinum, C6- and Cs-dehydrocyclizations and isomerization are suppressed most by rhenium, while hydrogenolysis is promoted. The presence of sulfur promotes on all catalysts (pure platinum as well as platinum with rhenium), the C5- and C6 dehydrocyclizations, but isomerization remains low. The latter fact is very important for the later discussion of results obtained with commercial platinum-rhenium-sulfur/alumina catalysts working as bifunctional catalysts (see below). The main results obtained [119] are summarized in figure 32. 1.0 0 [~.
O.Z.
.-
,o Hydrogenolysis
121 9- -
~ r-
"--
~
o Cyclization
o Cyclization
|
c
0.5
9Isomerization
9Hydrogenolysis 02
o Isomerization
o Dehydrocyclization
~
9Dehydrocyclizalion
o E
0.0
lr
0.0
~-
-,-
~
~
~- r
1.0
.
.
.
.
Re Coverage (ML)
2.0
0.0
.....
0.0
Re
- ..... 3?"
1.0
~ ....
2.0
Coverage
v-
3.0
(M L )
figure 32 Reaction conditions: T = 573K, P,_hexa,e= 20 Torr and PH2= 200 Torr. Left: rate of n-hexane conversion on P t ( l l l ) as a function of Re coverage. Right: rate of n-hexane conversion on sulfided P t ( l l l ) as a function of Re coverage. The catalysts were sulfided by depositing the saturation amount of sulfur.
Reactions of alkanes and reforming of naphtha
629
Let us now return to several other points of fundamental information that are important for the understanding of industrial catalysts. Under the conditions used (UHV apparatus, equipped with an isolation valve, reaction conditions: 100 Torr H 2, 10 Torr ethane, T=573K), the rate of hydrogenolysis is very low on pure platinum(l 1 l) surface [119,120]: (5x10 -3 tool methane site-is -1) and is considerably higher on rhenium(0001) (0.55 mol methane site -1 s-1 on an annealed surface and 1.8 mol methane site -~ s-1 on a surface roughened by argon bombardement). When two or more monolayers of rhenium had been laid down on platinum(111), the activity was again 0.5 mol methane site -1 s -1. At coverages of rhenium on platinum between 0.6 and 1.0 monolayer, a synergistic effect was seen and the activities reached values of 2-3 in the above units. Since, at the temperature of reaction, the penetration of rhenium into platinum is negligible, we can ascribe the higher hydrogenolytic rates to the presence of rhenium
on
the surface of platinum. When,
vice versa, platinum is deposited on rhenium, again the rate of hydrogenolysis is higher than that of either of the metals in the pure state. The most active platinum-on-rhenium catalyst was comparable with a clean platinum(l 1 l) plane, when this plane had been sputtered and there was
no
subsequent anneal. In another experiment the platinum-on-
rhenium surface was annealed at 923K, at which temperature platinum penetrates into rhenium, which in its turn appears on the surface. Such alloy surfaces showed the highest hydrogenolytic activity. The fact that platinum-on-rhenium catalysts show a lower activity than the platinum-rhenium alloy should be kept in mind for further discussion, because there are
theories which ascribe to platinum on rhenium a higher activity than that of
platinum itself. Also the fact that rhenium on platinum has an even higher hydrogenolytic activity is worth remembering. It shows that rhenium manifests itself by its reaction fingerprint in the product distribution, with hydrogenolysis prevailing. Three monolayers of rhenium on platinum at 573K in a sulfur-free state also showed a low but easily measurable activity for benzene formation [119]. This can be compared with the results of a study of reactions with hydrogen and n-hexane, methylcyclopentane,
3-methylpentane,
n-pentane
and isobutane
on
evaporated
rhenium
and
rhenium-gold [121]. The films were inhomogeneous mixtures of various rhenium-gold phases but the message was clear: adding of gold to rhenium suppresses hydrogenolysis, so that it is possible to work at the higher temperatures (600-700K) which favour dehydrogenation and benzene formation. It is not excluded that this dehydrocyclization runs partially on a carbonaceous layer extending over the alloy surface, by a stepwise dehydrogenation. The so-called 1-6 dehydrocyclization was indeed a prevailing pathway to benzene [122]. Results are presented in table 7 [121].
[able 7 Iilrn coinpo\ition .itorii%Re
temp./
CH, 34.8 43.6 72.0 507 574
100
hintcred
100
carburized
1 ~
78.5 sintered
sintered
*
21.3 1.6 23.7* 34.4* 71.7*
594 654 694
47.5 55.3 32.3
613 629
20.0 24.5
* I
CZH,
646
30.6 21.8
C,H, 11.3 2.2 0.8 20.0 21.8 16.8
initial product distribution 1nin-
3.7 2.3 traces
k0!J
2methylpentane
min-’
S,#
%
C4H,0
‘SH12
11.2 17.7 12.6
0.4 0.3 0.2
19.8 28.7 11.6
1.o 2.2 0.5
0.32 0.32 0.29
20.2 16.2 5.3
-
36.1 19.2
traces
0.047 0.040 0.049
0.99 0.32 0.64 0.97 0.03 1.0 0.92 0.94
35.2
0.003 0.005 0.012
1.0 1.0 0.65
36.5 41.6 100
0.038 0.019 0.009
0.63 0.58 -
0.038 46.0 0.045 39.3 -
0.54 0.61
traces 5.9
-
630
Reaction of n-hexane and hydrogen on rhenium and rhenium-gold films
traces
9.2 19.1
traces
5.5 16.5
-
-
5.3
I-
T
-
-
2.5 14.7
; :1
0.08 0.06
0.35
0.39
~
Percentage methane and ethane combined
# S,, S, and S , are selectivities for hydrogenolysis, isomerization and benzene production expressed as the fraction of reactant consumed in the designated
process divided by total consumption.
chapter 13
Reactions of alkanes and reforming of naphtha
631
Results very similar to those with rhenium-gold films are also obtained with tungsten-gold and tantalum-gold films; admixture of gold and the presence of carbonaceous deposits convert the hydrogenolytic catalysts into aromatization catalysts [121]. 13.2.7 Platinum-rhenium on alumina (sulfur-free catalysts) The presence of a support, so beneficial in industrial use, introduces many complications which must all be considered in understanding the function of the industrial catalysts (see chapter 7). Alumina can cause separation of metal precursors during preparation of catalysts by the impregnation technique and it was at first not selfevident that the components, platinum and rhenium, operate by being alloyed or at least in close proximity to each other. Some workers [122-124] deny that and think that the two elements act separately. It was in the beginning not evident at all that all, or a considerable part, of rhenium in the catalysts really becomes reduced under conditions of industrial operation. At first glance, the literature offers a controversial picture, with conclusions such as 'almost all rhenium is quantitatively reduced' up to 'under operating conditions rhenium oxide is substantially unreduced and it is present as Re(IV)', which is thought by many people to be beneficial for the performance of the catalyst [124-134]. However, a closer inspection of the results allows us to make apparently consistent and reliable conclusions. Such a consistent picture is built up from several ingredients which will now be discussed in a logical but not a chronological sequence. When a small amount of an oxide is mounted on alumina, which is a spinel with many defects, both stoichiometric and non-stoichiometric, the first cations added occupy the defect sites (see chapter 7). Only when these defect sites are filled and a complete monolayer has been formed do crystallites of the active supported oxide start to appear. This is illustrated in figure 33 by the MoO3-A1203 system, the results for which are being taken here as representative of extended systematic studies performed at Beijing University [135]. One can expect that rhenium oxides behave in a similar way to that of molybdenum trioxide, the onset of whose crystal formation is clearly seen. c,')
9o 0,40121
>, 0
figure 33 Molybdenum
i
in
crystalline
form, as a function of its content in the MoO/Al20 ~ powders. Similar behaviour
0.20 "
trioxide
can also be expected for the formation O-O---C
0
~
0.20
0.40
0.60
Total Mo03,g/g ~ - A I 2 0 3
of a rhenium oxide layer on alumina.
632
chapter 13
Industrial catalysts usually contain only about 0.6%wt of total metal, while model catalysts prepared for laboratory research contain up to 2%wt of metals. Rhenium, or other cations on the defect sites, are difficult to reduce, and thus when rhenium loadings are low, a number of rhenium cations can coincide with the expected (because the exact number is not known) content of defect sites on the surface. Knowing this, one is not surprised that in the literature quoted [125-134] those who studied catalysts with a very low loading did not observe full or even appreciable reduction of rhenium, while those who studied catalysts with a high metal loadings have seen that most of the rhenium was reduced. These conclusions are mainly based on TPR and TPD measurements, but there were also attempts to determine the content of ionic rhenium in a more direct chemical way. Its extraction by acetylacetone is reliable, but since alumina becomes dissolved it is difficult to avoid some reduced rhenium from alloy particles appearing in the gel-like solution used subsequently for analysis by X-ray fluorescence. By this method up to 25% of rhenium present was found to be ionic in reduced catalysts. However, this high figure is, because of these difficulties, probably an overestimate [136]. When the extraction is performed by another method, which appeared to be successful for ionic platinum, i.e. by diethylamine/acetic acid (4%) mixture, the content of ionic rhenium was estimated as less then 5% [137]. The concentration of Re(IV) has also been determined by calibrated ESR signals [138]. Rhenium dioxide is the most stable oxide, so it can be expected that most of the unreduced rhenium would be in the 4+ state. By using ESR, it was concluded that in platinum-rhenium catalysts the fraction of unreduced rhenium was less than about 10%; for pure rhenium or alumina catalysts less than about 20%. Rhenium could also have been present in another valency, so that only a rough estimate could be made from all these measurements. However, if we rely on the techniques used, the conclusion is that the platinum-rhenium/alumina catalysts contain of 10_+5% ionic unreduced rhenium. This amount is of such a small size that it allows us to speculate that platinum-rhenium alloys are indeed formed, and operate also under standard industrial conditions, but on the other hand it is large enough that it would be premature to deny the possible beneficial effect of rhenium(IV) on reactions occuring on the support. The proven presence of metallic rhenium still does not prove that alloys are formed and operate in naphtha reforming; however a strong indirect evidence exists for alloy formation. First, it has been shown that the presence of water, even in small amounts, increases substantially the migration of rhenium oxide species on alumina [131,139-141]. When such mobile species collide upon reduction in progress with a platinum particle, and platinum metal particles are certainly formed earlier than rhenium particles, the oxide will be reduced by hydrogen delivered by spill-over from the neighbouring platinum. Once rhenium metal appears, reduction can continue by hydrogen which is also dissociated on rhenium. A very strong indication for the easy migration of rhenium has been presented [141]. Using cyclopentane hydrogenolysis as a fingerprint reaction for alloy formation it
Reactions of alkanes and reforming of naphtha
633
was found that, when a physical mixture of Pt-Na-Y and Re-Na-Y zeolites was coreduced, the product patterns of exchange reactions clearly indicated that extensive alloy formation indeed took place. This means that rhenium species can migrate from one crystal to another and join platinum in other zeolite cages. It has also been established that the presence of carbon in the system makes the reduction of rhenium easier [134]. This is important in relation to the following. When reforming catalysts are deactivated they must be regularly regenerated by burning off the carbon deposits. The platinum-rhenium alloy is then re-converted into segregated oxides, but a repeat reduction in the presence of small amounts of water obviously can restore the alloy. It is the water which is always present in the system after the oxidation and re-reduction steps which very much helps alloy formation. Margitfalvi et al. [142] have studied the influence of an oxidation-reduction cycle on the product distribution in the n-hexane-hydrogen reaction. They prepared the catalysts shown in table 8 by depositing rhenium onto platinum covered by hydrogen (see chapter 7). These catalysts (no.l-4) showed only a small suppression of the total conversion and of hydrogenolysis, the latter change being achieved by carbonaceous deposits. When the system was treated by an oxidation-reduction cycle, that is, when the formation of alloys was stimulated, the catalysts started to show the usual selectivities of Pt-Re/A1203 catalysts. It can be seen in the table that a part of the Cl-units produced by hydrogenolysis appears in toluene, a CT-hydrocarbon formed from n-hexane; the presence of rhenium on the platinum promotes this step. The effects of oxidation-reduction cycles are complex. Botman et al. [137] saw that upon calcination the rhenium oxide does not leave the support, as some authors earlier suspected, but it penetrates into the interior of alumina. When the catalyst is being rereduced, then, depending on the duration of reduction, rhenium returns more or less completely to the surface of alumina, where rhenium ions are reduced. This is probably the background for the newest technology in platinum-rhenium catalyst preparation, which makes use of a more crystalline 0~-A1203 and makes the catalysts with more rhenium than platinum (typically 0.3wt% platinum; 0.6%wt rhenium), so that enough rhenium is all times at the disposal on the outer surface of alumina. Botman [137] also established that the presence of rhenium in the catalysts increases the fraction of platinum which is in the ionic state after reduction, in particular, when chlorine is also present in the system. Industrial catalysts are prepared with the use of hexachloroplatinic acid and when it is not removed by extra steps, chlorine is always partially retained by the carrier. Its presence is beneficial for the bifunctional operation of the catalysts since chlorine creates more of strong acid sites on the alumina. This however influences the reducibility of the platinum and the rhenium, irrespective of the fact that, upon reduction of the catalysts with hydrogen, a part of the chlorine is removed as hydrogen chloride.
m w
table 8 Comparison of the catalytic properties of Pt/AI,O, and Re-Pt/Al,O, catalysts in hexane dehydrocyclization (reaction temperature: 753K, H,; n-hexane = 5 : 1)
I temperature of pretreatment (K)
It
1
catalyst
1
0,
1
H,
1
convefsion
C,-C,
I
selectivity data
benzene
I
I
isohexane
I
P
I
methylcyclo-
1 pentane
I 1
l
toluene
0.190
0.60
0.06 1
0.069
0.062
0.092
0.525
0.120
0.096
0.120
0.190
0.560
0.089
0.093
0.035
Re-Pt/Al,O,
*
848
0.25
0.160
0.473
0.100
0.140
0.094
Pt/AI,O,
673
773
0.61
0.077
0.690
0.084
0.027
0.098
Re-Pt/Al,O,
673
773
0.75
0.070
0.725
0.1 10
0.026
0.042
Re-Pt/Al,O,
773
673
0.72
0.6 1
0.740
0.092
0.024
0.056
Pt/AI,O,
773
773
0.78
0.1 10
1 0.700
I 0.096
1 0.024
1 0.044
Re-Pt/Al,O,
773
773
0.80
0.05 1
0.740
0.1 10
0.023
0.044
Re-Pt/Al,O,
773
848
0.73
0.060
0.690
0.150
0.029
0.036
* Calcination omitted; catalyst reduced
after contact with moisture in air.
i1
I/
1
Reactions of alkanes and reforming of naphtha
635
All this is very well illustrated by a detailed study performed by Munuera et al. [143,144] which also revealed that by oxidation-reduction cycles one can produce catalysts with different amounts of rhenium in the surface and that these catalysts differ in their cracking patterns. Rhenium has a typical multiple'cracking character which appears when more rhenium is in the surface. Such a state can be created by oxidation and mild reduction. Platinum shows internal cracking and this appears only after a more thorough reduction. Bond and Gelsthorpe studied hydrogenolysis of propane, n-butane and isobutane and compared Pt/A1203 with Pt-Re/AI203 catalysts (EUROPT-3 and EUROPT-4). They established that adding rhenium enhances the propensity to multiple hydrogenolysis [145a]. A comparison of results obtained with these catalysts by various laboratories has been compiled [145b], this shows again how important to the performance of the catalysts are the fine details of the preparation method. A simple molecule such as n-butane probes mainly the hydrogenolytic properties of these catalysts. The activity per unit metal surface area shows a very pronounced maximum when about 80% of metal content is rhenium [ 146]. We shall see more of this type of activity pattern below. A detailed kinetic study has been performed on the reactions of ethane and n-butane on Pt-Re-A1203; the rate varied according to the equation proposed by Cimino et al. [29] in section 13.1. Sharp maxima in the activity vs. catalyst composition curves have been confirmed by several other groups [148-150] and have also been found for molecules other than those already mentioned.
lO
figure 34 Specific activity (mole hlm-2), per unit alloy surface area of Pt-Re/Al203 alloy catalysts, in cyclopentane hydrogenolysis. Activity as a function of at% rhenium in the alloys on different carriers:
-2 10
Pt/~-Al203 catalyst (stars); Pt/Al203 Degussa (full circles);
1 0 3_ .]
10
-4
-
0
Pcyclopent = 0.1 bar, Pm = 0.9 bar, T = 513K [1461. i 20
t.lO
610
810 %Re
10 0
636
chapter 13
Figure 34 shows the results for cyclopentane hydrogenolysis [146] and figure 35 shows the selectivity pattern for heptane hydrogenolysis and isomerization [150] and similar results are known for n-butane [151 ]. Explanations for this behaviour differ. Many authors of the original and reviewing papers were inclined to see in the maxima, and in the assumed underlying synergism of platinum and rhenium, a clear indication for ligand effects of alloying. However, two other explanations seem to be more attractive. (i) Mixed
ensembles can be formed which allow a higher rate of catalytic reaction than ensembles of any of the components taken alone [ 107,148,151 ]. Notice that this behaviour has been seen also with support-free single crystal planes, so it can be ascribed to the metallic function of the catalysts. (ii) There are different rate-determining steps on platinum and rhenium, being C-C bond rupture on the former and removal of methane or other fragments on the latter metal. This explanation seems to be well supported by some results for platinumrhenium catalysts [149], but similar maxima in activity vs composition curves are also shown by the platinum-iridium and by iridium-osmium combinations [151]. In particular, in the case of iridium-containing catalysts it does not seem likely that this active metal would retain hydrocarbon fragments strongly, as does rhenium. Therefore, the first explanation seems to be the most universal one.
@
(a)
figure 35
OJ.-
Conversion
of
n-heptane
on
(Pt+Re)/Al203 catalysts. Selectivities in isomerization (a) and
0.2
~ 1 7 6
O-
|
w
n
~
~
i
hydrogenolysis (b).A = 773K, [3
t
= 973K, 9 = 1073K. Selectivities are plotted as functions of the
2'0
'
Z.'O
'
6'0
'
8'0
'
100
%
(at%) [150].
(b}
0.~
-
j
i
9
9
0.2 o
2'0
'
~'0
'
6'0
average rhenium content
'
8'0
'
~60
Reactions of alkanes and reforming of naphtha
637
Using neohexane (2,2-dimethylbutane) as a test molecule allows us to analyze the prevailing surface complexes (see section 13.1). The overall activity pattern obtained with this molecule again shows a maximum at about 75mo1% rhenium. This is shown in figure 36 [152], where the activity is expressed per mg of total metal, but the surface areas of the metallic components do not vary much. The catalysts with 20% and 80% rhenium have an average particle size of 1.3 nm and 1.0-1.7 nm respectively, but the activity per unit weight differs more. The different catalyst compositions carry a fingerprint of either platinum or rhenium in the product idstribution. On platinum-rich alloys, t~T-complexes are observed and isomerization of neohexane, as well as its hydrogenolysis, takes place. On rhenium-rich surfaces t~6-complexes and, of course, hydrogenolysis prevail: this is seen in figures 37 and 38. 1,5
!
\
I
E iS
1
\
%
I I
>,. ..,=,
~
>
<
\
I
0,5
[] ii
Series
W , O
7-
1
-T-
40
20
1
9 Series 2 Ketlen
|
60
80
100
% Re / R e + P t
figure 36 Activity for the reaction of neohexane with hydrogen (expressed as conversion per mg catalyst) as a function of alloy composition at 548K. Series 1 catalysts are prepared from dissolved commercially available salts, series 2 catalysts are prepared by dissolution of metals in aqua regia and impregnation on A1203 and the 'Ketjen' are the commercial catalysts. The two commercial catalysts are CK 455 (0.47wt% Pt, 0.475 wt% Re) and CK 433 (0.296 wt% Pt and 0.31 wt% Re).
The selectivity to neopentane is used as a diagnostic tool to establish the contribution of the txl3-mechanism. However, in particular on pure rhenium, neopentane reacts further, while still in the adsorbed state, to yield butane and propane. This leads to the apparent decrease of the contribution of the t~6-mechanism in figure 38.
638
chapter 13 4o
~
3o
9
}
~o
0 9
1
o~'r. s e r a e s
2
0,3'. K e t i e n
F
10
@
0
o~
c~'Y. s e r i e s
9
0
n
.
,
20
.
40
-
~
r
60
,
80
.,5,, loo
B
a'y".
seT,es
1
9 e3".
ser,es
2
~h",
Ketjen
% Re / R e + P t
figure 37
Neohexane - hydrogen reaction: selectivity in the 3C ~"~ and ot~[' mechanisms (isomerization and hydrogenolysis together) as a function of catalyst composition (catalysts as in figure 36). 80
6O
\
.,..., > .,.., Q.) V)
40
O
2O
/.
\
\
C;' % c ~ . series 1 9 % r~h'. series 2 .1~ % n i l , c o m m e r c i a l
2O
> (o
~5
) % P + B. series 1
'::,
9 % P + B. ser,es 2 ~.
0
20
40
60
80
% P + B. c o m m e r c i a l
100
% Re / R e + P t
figure 38 Neohexane - hydrogen reaction: the selectivity S (2C~fl) versus catalyst composition. Below: the selectivity of propane and butane formation (both characteristic of multiple splitting) versus catalyst composition (catalysts as in figure 36).
Reactions of alkanes and reforming of naphtha
639
Multiple fission of adsorbed intermediates formed from neohexane or neopentane points likely to adsorption through three primary carbon atoms, viz. a 4C-complex, with a tripodic structure. With sulfur-free catalysts, the role of acidic sites is somewhat marginal: the nonacidic silica-supported platinum catalysts show approximately the same picture as the alumina-supported catalysts. We have therefore neglected the role of acidity (bifunctionality) in the whole discussion above on the platinum-rhenium/alumina catalysts in reactions at low temperatures. However, as we shall see below and also in figure 39, the situation concerning hexane is very different with bifunctional catalysts modified by sulfur or sulfur (see section 13.4). 100
200--Y
50
100
0 o
50
"I, Re
1oo
!
0
50
"/.Re
I~0
figure 39 Reaction of n-hexane with hydrogen: yields Y, defined as a product of conversion and the corresponding selectivity, are proportional to the rates. Left: Y as a function of rhenium content in Pt-Re/Cl-Al203, chlorine containing catalysts. Hydrogenolysis (squares), isomerization (circles), cyclization (triangles). Right: the same, for chlorine-free Pt-Re/Al203 catalysts.
13.3
Fundamental studies on reforming catalysts.
Even before the advent of platinum-rhenium/alumina catalysts, it was common industrial practice to modify platinum/alumina catalysts before and during catalytic reforming of naphtha by small amounts of sulfur, deposited in situ from molecules such as, for example, thiophene. With platinum-rhenium/alumina the amount used is smaller and it is added in a controlled way, before the reforming process has been started. Sulfur treatment was found to be beneficial even with pure platinum catalysts, leading to a lower hydrogenolysis and a higher yields of aromatic, but it is absolutely essential with platinum-rhenium/alumina [153-165]. Without sulfur these catalysts show a too high
640
chapter 13
hydrogenolytic activity, until this starts to decay slowly due to the carbonaceous deposits formed at high temperatures [163-165]. Under reforming conditions alumina is active through its acid sites, the high number of which is due to the presence of chlorine (see above). Obviously we have to investigate what are the roles of the individual components in a catalyst containing platinum, rhenium, sulfur, chlorine and alumina, all together. To have an idea which aspects of the results presented below should be inspected most closely, let us start by listing the various suggestions formulated in the literature to explain the superiority in reforming of the Pt-Re-S, C1/A1203 catalysts. 1) Rhenium ions can be built in the structure of alumina and once there are quite stable to reduction. It is known that metal cations in alumina can serve as an anchor for platinum or platinum-rhenium particles, making them more stable against sintering. Thus this could be a positive role for rhenium in the catalyst [166]. 2) Rhenium ions modify the surface of alumina in such a way that coke is formed either in smaller amounts or it is more easily removable by hydrogen, or during regeneration by air and hydrogen [ 124,159-165, 169-171 ]. 3) Rhenium modifies the availability and the reactivity of hydrogen on the surface, by influencing such phenomena as ' spill-over' of hydrogen [ 165,172]. 4) Sulfur forms during sulfurization of the catalyst very strong Re-S and less strong Pt-S bonds. Therefore, and Pt~
the working surface of alloy particles would be formed by Re-S, Pt-S
The presence of sulfur partitions the surface into very small platinum
ensembles or even single platinum atoms, which are sufficiently active in dehydrogenation of the feed for the bifunctional mechanism to operate freely, but which cannot be covered by continuous carbon layers. The metallic function is mostly limited to aromatization via dehydrogenation. Some isomerization accompanies it [155,156]. 5) Partitioning of the platinum surface into small ensembles is indeed known to suppress the formation of graphitic layers [173]. This effect too, has been suggested to operate on platinum-rhenium/alumina catalysts and has been assumed to contribute to the superior properties of these catalysts [156,174]. It has been shown [174] that most of the carbon is deposited on the support and chlorine plays a role in anchoring the coke precursors. However, the smaller amount of carbonaceous species which is on the metallic surface is at the end of the day much more detrimental to the activity of the catalysts. When rhenium is present, the residues have a higher H/C ratio [174]. Formation of graphitic coke has been proven to be a structure-sensitive reaction [175,176] so that there is a quite firm basis of evidence supporting this effect of rhenium. 6) It is still very popular to speculate that rhenium and sulfur both exert a ligand effect on the adsorption of hydrogen atoms or hydrocarbon fragments on platinum [92,150]. For example, it is said that rhenium donates its electrons to platinum, but it is partially counteracted by electron shifts from platinum or rhenium towards sulfur [164]. One can find remarks in the open literature and patents in which it is clearly assumed that shifts of
Reactions of alkanes and reforming of naphtha
641
electrons are mediated by the support. Such shifts have to explain the effect of other additives to the Pt-Re/AI203 catalysts. However with regard to these ideas some scepticism is not misplaced. Concluding this inventory, we would like to direct the attention of our reader to some review papers, where a similar attempt has been made to list the possible modes by which rhenium acts [177,178]. All the first five points have a sound basis in experimental results; one can trust them all to play some role in reforming reactions. However, the picture is not yet complete, because all these theories did not sufficiently acknowledge the bifunctionality of Pt-Re-S,C1/AI203 catalysts. To complete the picture we shall present some results of a study [179] on the specific role of bifunctionality. Figure 40 compares the variation in yields, which are proportional to rates, in the n-hexane reactions on sulfurized Pt-Re/AI203 and Pt-Re,C1/AI203 catalysts as the rhenium content is varied [115,179]. ,
, =
_
~oo I
S*II
I0~\
50-~
!/~
2.5-
o
5'O
"hRe
100
0
///'cr'''" ~
"-~9 .-~ .......~.
50
"h Re
100
figure 40 Left: yields (proportional to rates)for the various groups of reactions for sulfided chlorine-free catalysts versus the rhenium content of Pt-Re/A1203. Indicated with arrows; the physical mixture Pt/S/AI203 + Re/S/AI203. Yields in hydrogenolysis and cyclization with the physical mixture catalyst have been omitted for reasons of clarity. They are similar to those found for 50%Pt-50%Re/A1203. Right: group selectivities for the sulfided chlorine-free catalyst as a function of the rhenium content of Pt-Re/S/AI203. Indicated with arrows are the results obtained for the physical mixture of Pt/S/AI203 + Re/S/AI203. (Symbols as in figure 39)
All catalysts contained l%wt of metals and were prepared by chemical impregnation of alumina. Platinum nitrate, made from H2Pt(OH)6, and H3ReO5 were the metal precursors in the preparation of chlorine-free catalysts. Figure 39 demonstrates that there is an effect of
642
chapter 13
chlorine on the yields but this does not influence the conclusions we are going to make. The question to be answered now is whether in the chlorine-free catalysts the acid centres of the alumina have some role. The answer can be obtained by selective poisoning experiments using pyridine. Pyridine is irreversibly bound, under the conditions applied [179], to the acid centres on the alumina, but it can be removed from the metallic part almost completely. Therefore, by using pyridine one can make visible which part of the reforming reactions is related to the acid centres, manifesting itself by irreversible disappearance of activity, and which part can take place on metal sites, without any assistance of the support; this is the slowly regenerating part of the reactions. When pyridine is administered to the sulfur and chlorine-free Pt-Re/A1203, yields of all products of the n-hexane/H 2 reactions fall, but two groups of reactions recover to a great extent when admission of pyridine is stopped: dehydrocyclization and hydrogenolysis [179,180]. In contrast to these reactions, isomerization reactions are hit most by pyridine, and their suppression is irreversible. This is reflected by activity patterns as shown in figure 41. 1.0
-3.0
I
to--O-o- I 20 Y
0.5 O
o
t(h )
i
2'o
rrt. I::~'R1DINE
-10
O
t (h )
}.
-0
!
o
t(h)
figure 41 Yields for a presulfided chlorine-free 25%Pt-75%Re/A1203 catalyst for the various groups of reactions. Treac t .= 623K. First part: after reduction (in hours), 2nd part: an increasing amount of pyridine added to the gas flow (ml), 3rd part: after the addition of pyridine has been stopped (t in hours), 4th part." after a reduction for 5 hours at 673K (t in hours). Symbols as in figure 39.
When the chlorine-free Pt-Re/AlzO 3 catalysts were sulfurized by thiophene (3 hrs. on stream with 8% thiophene in H2 stream at 1 atm at 570K, followed by
15hrs. on stream
with pure hydrogen at 670K to remove weakly bound sulfur and hydrogenate carbon species), the selectivity pattern changed from that shown on the left side of figure 42 to that on the right side of the same figure.
Reactions of alkanes and reforming of naphtha
643
100 S'I,
loo 1
S('/,) /
t
t
f
/ /
50 ~
^
/
50 L
_
ar
, 9 " ".~],
0
0
5~3
"/,Re
-r
100
0
0
.~.1~
% Re
figure 42 Left: group selectivities as functions of the rhenium content for the chlorine-free catalysts Pt-Re/Al203. Right: Group selectivities for the presulfided chlorine-free catalysts as functions of the rhenium content in Pt-Re/S/AI203. Indicated with arrows are the results obtained for the physical mixture of Pt/S/AI203 + Re/S/AI203. Symbols: squares: hydrogenolysis; circles: isomerization; triangles: cyclization.
Sulfur brings about a very dramatic change in the selectivity pattern. While in the sulfurfree catalysts the selectivity to isomerization decreases monotonically when going from pure platinum to rhenium, it increases in the series of sulfurized catalysts. We observe in figure 42 that rhenium-rich sulfurized catalysts show an important suppression of hydrogenolysis and dehydrocyclization, the two reactions which we identified (fig.41) as taking place on the metallic surface. The reader will notice that without sulfur these are very active catalysts for hydrogenolysis. The picture of yields in figure 40 leads us to conclude that sulfur suppresses all reactions, both metallic and bifunctional, but suppression is less pronounced for isomerization. At 525K isomerization is largely an acidcatalyzed reaction on catalysts for which alumina is used as a support. It is possible that suppression of the skeletal reactions on the metallic surface is accompanied by a slight enhancement of the dehydrogenation reactions on metallic sites, from which effect the selectivity to isomerization on acidic sites can also profit. The same approach as used earlier in studies on platinum-rhenium catalysts has been applied to platinum-iridium catalysts [180-181] and the same conclusions concerning the role of sulfur and bifunctionality have been arrived at. The overall picture of bifunctionality and of sulfur effects applies also with other hydrocarbons; however, it is interesting to see some differences in points of detail. For example, much of the aromatization of methylcyclopentane to benzene occurs on the acidic centres of alumina [182]. This is true for platinum, platinum-iridium, platinumcobalt and platinum-copper catalysts [182]. With neohexane (2,2 dimethylbutane), sulfur
644
chapter 13
causes a switch from a strongly prevailing metal function to bifunctional behaviour [115]. With sulfurized catalysts, 2,3-dimethylbutane, almost absent when metal activity prevails, becomes the most prominent product. This switch is most dramatic with chlorine-containing Pt-Re-S/A1203 catalysts. Let us return to the basic information on the platinum-iridium catalyst. This very important naphtha reforming catalyst is one of the few industrial catalysts where one can speak about a rational design, in contrast to the more usual 'trial and arror' approach. Sinfelt et al. [46,183] at the EXXON laboratory, New Jersey, systematically studied the hydrogenolytic properties of metals and observed that iridium is very active, its deactivation slow and regeneration by burning off carbonaceous residues easy. On the other hand, it was known that carbonaceous deposits are responsible for a great part of the deactivation of the Pt/A1203 catalysts. Therefore, by putting some iridium into platinum it was intended to decrease deactivation. Since iridium would be diluted in these catalysts by the much less active platinum, the hydrogenolytic activity of iridium was expected to be suppressed and should not harm the high selectivity to aromatization. The expectation was perfectly fullfilled and the catalysts appeared to be very useful. The activity of the new platinumiridium catalysts on a volume basis was about three times as high as that of standard platinum/alumina or platinum-rhenium/alumina catalysts, and moreover there was a very high yield of aromatic, ensuring production of a high octane-number gasoline. Note that these catalysts too are always used in a sulfurized state. Results of the very comprehensive research performed are described in Sinfelt' s monograph [ 183] and key papers [ 185-188]. Sinfelt et al. studied platinum-iridium catalysts with a maximum of 1%wt platinum and 1%wt iridium, in a high metal dispersion, i.e. with metal clusters in which practically all atoms were exposed to the gas phase. This study also revealed the limits of the present science in characterization of such catalysts. Some of the mono- and bimetallic catalysts showed H/M (M = Ir or Pt) and CO/M ratios higher than one. With CO/M it is most probably caused by the formation of stoichiometric or substoichiometric carbonyls, but with hydrogen the reason is less obvious [184-186]. EXAFS studies showed that bimetallic catalysts contain mixed clusters and a very thorough analysis of EXAFS spectra revealed that the metals mix, but the solution is not homogeneous. Catalysts with 10wt% iridium and 10%wt platinum could be also studied by XRD and they appeared to be solution alloys [183,185,186]. It has been also seen that a pronounced gas-induced segregation of components is caused by oxygen or other corrosive gases. In particular, with catalysts consisting of large bimetallic particles, the restoration of the state before segregation can be a lengthy process. M6ssbauer spectroscopy has also been used, with 57Fe as a probe. This technique showed too that clusters consist of both platinum and iridium [187]. Using hydrogen (0.97 atm) and ethane (0.03 atm) mixtures, it was shown that catalysts containing 0.3wt% iridium and 2.7wt% platinum gave a hydrogenolysis rate that was about seven times lower than that for pure iridium catalysts, although the total amount of metals
Reactions of alkanes and reforming of naphtha
645
was ten times larger. This indicates that platinum suppresses the activity of iridium. Those who pioneered research on platinum-iridium catalysts paid a great deal of attention to whether or not the two metals were completely mixed; they even expressed surprise when that happened to be the case. The reason for their doubts was that the older metallurgical literature reported phase segregation and immiscibility at certain platinumiridium ratios.. However, one must not trust the older literature in all details; at the time the work was done, vacuum technique was much below the present standards and moreover a sealed quartz tube was used for long annealing procedures at very high temperatures, since both platinum and iridium are high-melting metals. However, it is known that a) quartz is permeable for oxygen at temperatures of 1300K and above; b) traces of oxygen in sealed tubes stimulate segregation of iridium in the form of an oxide. On the other hand the preparation of platinum-iridium bimetallic catalysts usually comprises some steps which stimulate alloy formation, such as mixing of metal precursors and reduction by wet hydrogen, so that alloys can be formed and are indeed observed. Conditions suitable for alloy formation are already created in process of drying the precursors [188]. As a study by Raman spectroscopy showed, the presence of chlorine in the system also stimulates mixing upon reduction [189]. This knowledge can now be used to an advantage in choosing proper precursors. Regeneration of catalysts used in reforming leads, due to the oxidation step, to the separation of components, and introduction of chlorine-containing gases into the process of regeneration can help to restore good mixing of the metals on the surface of alumina. Growth of knowledge concerning platinum-iridium alloys is in many respects very similar to the case of platinum-rhenium alloys [153]. Ramaswamy et al. [190] reported that adding iridium to platinum only makes the catalysts worse from the point of view of reforming. The sulfur-free alloy catalysts are too active in hydrogenolysis and only when modified by sulfur do the alloys show properties superior to those of pure platinum on alumina catalysts: this statement has been confirmed [151,191-193]. The similarity between platinum-rhenium and other catalysts will be discussed below. Figure 43 shows the activity of (Pt+Ir)/~-A1203 catalysts for hydrogenolysis of cyclopentane as a function of iridium content; the reader will have already noticed how general is this picture. We shall see it again for the iridium-osmium system, and what is found for cyclopentane is also found for butane hydrogenolysis. An explanation based on assumed electronic structure changes was favoured [151], but the possibility that the maxima in the activity curves just mean an optimal composition and number of mixed ensembles was not explicitly excluded: this seems to be at the moment the most attractive explanation.
646
chapter 13
Ar 15
figure 43 Activity of (Pt+Ir)/o~-Al203 (lO3.mol h-lm -2 metal) as a function of % iridium. Hydrogenolysis of cyclopentane. T = 463K, Pc = 0.1 attn., PH2 = 0.9 atm [1511.
"o
2~
so
7~s
~6o% I r
Let us turn to the problem of selectivities and the influence thereon of the sulfurization of alumina-supported catalysts. This can be seen in figure 44 [194a]. 100 ~ Pt-Re/AI203
100
Pt-Re/AI203 .S
S%
S%
t
50-~
50
figure 44 Selectivities in the reacti-
I
100
Pt-Ir/AI203
~
so R~% 1do
1 0 0 /-rP t _ i r / A i z 0 3
§
/
5%
S%~
50
50
00 50 Ir% 100 100S%Pt-Co/ , . . ~ ~ 50
0
o
5'0 Re% 100
50 Ir% 100 S%10
50 ~ ~ 1 AI203 §
0
,
50 Co% 100 'I,"
0
0
,'~ 50 Co% 100
ons of n-hexane with hydrogen at 620K, standard reaction
conditions
[194], as functions of catalyst composition (at%). On the left: indicated catalysts, sulfur free. On the right: the same catalysts, after standard sulfurization (see the text). Symbols as in figure 39. Notice the crucial difference induced by sulfur.
Reactions of alkanes and reforming of naphtha
647
Conclusions derived from figure 44 are straightforward. Addition to platinum of a metal active in hydrogenolysis always leads to an increase in selectivity to hydrogenolysis (left side of figure 45). In this respect rhenium, iridium and cobalt behave exactly the same. When sulfur is added to the catalyst the same effect again occurs with all systems: isomerization selectivity increases and cyclization selectivity decreases. With rhenium-rich and cobalt-rich sulfurized catalysts, the selectivity to hydrogenolysis is lower than that of a sulfurized platinum/alumina catalyst. Only with a sulfurized Pt-Ir/A1203 catalyst is there a slight increase in the selectivity to hydrogenolysis. To understand these similarities and differences, one has to examine the thermochemistry of sulfide formation [155]. Thermochemical data (see table 9) show clearly [194b] that rhenium, iridium or cobalt placed in a platinum surface capture sulfur preferentially and hold it more firmly than does platinum alone. With iridium, as with platinum, deactivation by sulfur is less complete and therefore the contribution to hydrogenolysis by iridium in iridium-platinum sulfurized catalysts is not completely removed by sulfur. One must not forget that the activity of pure iridium in hydrogenolytic cracking is several orders of magnitude higher than that of platinum, so that even a small fraction of iridium atoms not covered by sulfur can induce a fast hydrogenolysis. table 9 Enthalpy of formation - AHf of some sulfides
-AHf, 298K in kJmo1-1 Pt S Pt S 2
82 110
Re S 2 Re S 3 Re 2 S 7
178 208 450
Ir2 S 3 Ir S 2
105 133
Co S
358 119 153
Co 3 S4
Co $2
Important additional information on the working of these catalysts can be obtained when, instead of only selectivities, rates also are compared [194]. This comparison has been performed with a series of home-made catalysts and with two commercial catalysts (EUROPT-3 and a Ketjen platinum-rhenium catalyst). If the measured activity at 620K of
648
chapter 13
each sulfur-free catalyst is put equal to 100, a picture is obtained with the same but now sulfided catalysts, as shown in figure 45. In all cases, the rate of hydrogenolysis is suppressed very strongly. The rate of isomerization is suppressed in the most pronounced manner by sulfur with the catalyst which already shows a high selectivity to isomerization before sulfurization. On the other hand, with catalysts which show the lowest selectivity to isomerization before sulfurization (Ir, Pt-Re), the rate of isomerization, and thus not only the selectivity, is higher on sulfided catalysts than on those that are sulfur-free. These facts, and the knowledge [152] that with neohexane only 2,3-dimethylbutane is produced by sulfided catalysts, have of course, led to the suspicion mentioned above that, in spite of the relatively low temperature in these model reactions (620K), acid-catalyzed reactions
1.
must play an important role with sulfided catalysts.
120
[ s-
100
EE t
80-
80-
60
60-
~,0
t.0
20-
20 Pt
r"
~
Ir Pt-Ir 1WT%
.=_ Ru
[ PYRIDINE-FREE
100 %
Pt
==L
Pt-Re
0.3 IWT% Euro Pt 3
bifunct. mech.
-
Pt
Pt-Ir IWT%
Ir
Ru
Pt
Pt-Re
0.3 IWT % Euro Pt 3
figure 45 Left: relative rates (100 = given catalyst in the sulfur free state) after sulfurization Right: relative rates (100 = given catalyst after sulfurization) after poisoning by pyridine: the irreversible loss of particular activity indicates the extent of the bifunctional mechanism. In both parts: the left bar: hydrogenolysis, the right bar: isomerization.
To establish definitely the role of acid centres in the overall conversion of hydrocarbons on various catalysts, poisoning by pyridine has been used [179-182]. When the steady state of the catalysts was achieved after about 20 hours (let us call it state 'A'), pyridine was administrated in pulses, until a new steady state was achieved (state 'B'). The reaction was then continued without pyridine for about 4 hours (state 'C' achieved), a period which was followed by reduction in pure H 2 at 620K for about 5 hours (state 'D'). When the reaction was restarted thereafter with standard reaction mixtures and at a
Reactions of alkanes and reforming of naphtha
649
standard temperature, a new steady state was finally achieved (designed as state 'E'). The results obtained in this way with n-hexane as reactant are evaluated in the following manner. An idea already mentioned above is followed, based on the paper by Robschlager et al. [195], that pyridine poisons both metallic as well as acidic sites of a bifunctional catalyst, but at suitably chosen temperatures the former is reversible and the latter irreversible. By using this criterion the following picture of the importance of reactions catalyzed by acid centres can be established. With sulfur-free catalysts, the activity in the final state 'E' is always 90% or more of the steady state at 'A'. The poisoning can thus be considered as essentially reversible and the activity of any of the sulfur-free catalysts is almost purely due to the metallic sites. With sulfur-containing catalysts, the situation is quite different. In some cases hydrogenolysis is still poisoned reversibly, see figure 45, left bars, but with the two commercial catalysts the poisoning of hydrogenolysis is to a large extent irreversible. Isomerization (right side of the bars) is in all cases more influenced by irreversible pyridine poisoning than hydrogenolysis and with the two commercial catalysts (0.3%Pt/A1203 the so-called EUROPT-3 and the platinum-rhenium catalyst EUROPT-4), which are specially acidified during their production, the suppression of isomerization by pyridine is most dramatic. Comparing of sulfur-free with the sulfided catalysts thus reveals a clear shift in selectivities, from reactions catalyzed by metallic sites to reactions catalyzed by acidic sites. Speculations on the existence of the shift demonstrated by figure 45 can already be found in some earlier literature [157]. The shift in selectivities caused by sulfur deserves some more comment. Reading the literature, one finds no indications that irreversibly bound sulfur (the form which played a role in the above experiments) would increase the number or acid strength of acid sites on the alumina. Our preliminary experiments with Pt-Re/A1203 confirmed this conclusion [196-197]. However, sulfur certainly decreases the number of sites active in 'metallic' reactions, ((de-)hydrogenation, most of hydrogenolysis and cyclization, and a part of isomerization). Dehydrogenation is necessary for acidic isomerization and yet the activity in isomerization increases in some cases after sulfurization (figure 45). It means that these catalysts have possibly a lower total dehydrogenation activity, but nevertheless after sulfurization produce more of those intermediates which induce isomerization, obviously on account of the intermediates of other elementary reactions. The speculation is that this shift in selectivity for the formation of various intermediates should be due to something like that shown by the very schematic picture in figure 46. Intermediates multiply bound to metallic sites are replaced on sulfided catalysts by unsaturated intermediates which are more loosely bound, migrate more easily and can form carbenium ions on acidic sites; these in their turn induce isomerization and other reactions catalyzed by acid centres. Recently, hydrogenolysis of ethane has been compared with hydrogenolysis of n-
650
chapter 13
hexane on Ir/SiO2 and Ir + S / S i O 2 catalysts. It appeared that the activity of iridium in nhexane reactions is suppressed by sulfur, but it is not eliminated completely. However, the activity in ethane hydrogenolysis is suppressed totally by the same standard sulfurization. It has been concluded from this observation [197] that the multiply-bound surface intermediates derived from the o~B binding, which is the only possibility for ethane, require a larger ensemble of metal surface atoms in order to be formed than the complexes of the metallocyclobutane structure, or n-complexed species.
\/ c/ ii
*
c
\/
~c / i -x--
~c -
II *
c/
/ c
I\
--c
-
*
c
I
\c /
I\
II .)(_
\ / \ /
--c--c
c c / \ / \
~I
*
/c C~c/
t c/c -)(-
c____ c ~
-~ c
L -)(- l C~C
figure 46 A shift from multiply bound to weakly bound intermediates represents a transition from 'externally' to 'internally' unsaturated molecules.
The role of acidic centres in maintaining the isomerization activity was proven in another way by experiments in which platinum-rhenium and platinum-copper catalysts were prepared on inert silica. After a standard sulfurization procedure these catalysts having no acid centres completely lost their activity, while catalysts prepared on alumina as we have seen above- did not.
13.4
Various combinations containing two transition metals
13.4.1 Combinations containing platinum A very extended collection of results accompanied by a thorough analysis has been published by Oliver et al. [198], who studied platinum-rhodium alloys on inert silica as
Reactions of alkanes and reforming of naphtha
651
support. By using propane, butane, 2-methylpropane (isobutane), 2-methylbutane and 2,2dimethylpropane (neopentane) they were able to establish the following reactivity sequence of bonds between primary, secondary and tertiary carbon atoms on the series of alloy catalysts: (24)
CII -Cii > C I - CII > Cii - Cii I > C I - Cii I > C I - C i v
Since rhodium is much more active for hydrogenolysis than platinum (2x102 - 103 times, according to the molecule used) an attempt was made first to correlate the activity with the presence of certain rhodium ensembles. If their necessary size is m, the probability of finding such ensembles is -- xmp.h, where x is the atomic fraction. However, this approach appeared not to be successful. Then they considered m i x e d ensembles. For example, when m is taken as 3 and the activity of the various mixed ensembles is assumed to vary as indicated in table 10, the number of various ensembles can be calculated by using the binominal statistics. Results are shown in table 10: table 10 Characteristics of triatomic rhodium-platinum ensembles Type of ensemble Rel.no ensembles
Rh 3 X3Rh
Rh2Pt 3X2Rh(1-XRh)
RhPt2 3X~(1-XRh)2
Activity of ens.
aRh
a~'3~ aVSpt
aV3Rh
a~'Pt
Pt3 (1-XRh) 3
apt
The total activity of all ensembles taken together, is then given by a sum A (see below) where R is a constant, m is the required ensemble size (number of atoms) and i shows how many of these atoms are Pt-(i-atoms) and how many Rh-(m-i atoms)-atoms. Table 10 shows how the triatomic (m=3) ensembles are described, but in a full analogy description can be extended to any value of m. Results obtained by using equation 25 are compared with experimental results in figure 47. A
=
i -m E
m -i
m-i
R,,m.X ~
_ i]3 (1-xm) i aRh3 apt
(25)
i=0
In an earlier paper from the same laboratory, it was pointed out that platinum is much better in forming 3CffT complexes (iso-units) than rhodium, which excells by its propensity to form 2Cc~7 complxes, the ideal intermediates of hydrogenolysis [199]. Therefore, an exact quantitative analysis might be very complicated. Moreover, pure platinum and pure rhodium tend to split molecules in different places; for example, with
652
chapter 13
n-alkanes platinum prefers internal splitting (see section 13.1) while rhodium is active in multiple or terminal splitting.
-2
0 ,
platinum (%) 40
!
!
!
80 v
!
a
-2
-3
m=6 .,.a
7
-3
-4
-4
0
E -4
.-"
i
E
"--4
"..4
o
-5
-5
m--8 ~
O
2 9
-6
() figure 47
'
40
'
8'0
'
platinum (%)
Comparisons of the activities of the bimetallic catalysts with values calculated for ensembles of m atom (indicated) assuming that the activity of an ensemble varies geometrically with its composition: lines are calculated, points are experimental results: (a), 2-methylpropane at 463K; (b), 2,2-dimethylpropane at 463K; (c), butane at 398K.
On the platinum-rhodium system information is also available concerning evaporated films and reactions of unbranched C4,C5 and C6-alkanes [200]. A faster than linear decrease in hydrogenolysis rate was observed when platinum was added to rhodium (see above). Only on platinum-rich films there was some isomerization, but traces of benzene were found even on pure rhodium. A closely related combination
is platinum-ruthenium. The difference is that the
latter system does not form a continuous series of homogeneous alloys, but the formation of several phases can be accompanied by surface segregation of elements and this all can lead to a pronounced heterogeneity of alloy particles. In spite of these potential difficulties, monotonically varying and mutually selfconsistent selectivity patterns have been observed with Pt-Ru/SiO 2 and Pt-Ru/A1203 catalysts in reactions of n-heptane. The results on the selectivity [201] for silica-supported alloys are shown in figure 48, the total tumover number decreasing with increasing ruthenium content. This could indicate that at
653
Reactions of alkanes and reforming of naphtha
653K the catalyst worked under conditions of severe selfpoisoning, since ruthenium alone is much more active in hydrogenolysis of ethane than platinum. Other workers [202] have studied their catalysts most probably under conditions of a more limited selfpoisoning, as can be expected for reactions of cyclopentane/H 2 at 573K, they observed what others have observed with platinum-rhenium and platinum-iridium also: the total turnover number has a maximum in the middle of the composition range. S
x
1 X
0.75
SH 05 (
SC
x
SA
o 2~ N
Ru/(Pt + Ru) 25
50
15
IO0
figure 48 Conversion of n-heptane on (Pt-Ru)/Si02 catalyst. Selectivities: A aromatization (toluene); C cyclization (ethyl- and dimethylcyclopentane); 1.isomerization; H hydrogenolysis (T = 653K).
Diaz et al. [203] have studied platinum-ruthenium alloys in order to establish how the different mechanisms and intermediates (3C or 5C, see chapter 1) contribute to the overall isomerization of hexanes and how these contributions can be influenced by mixing these metals. Also the methylcyclopentane ring splitting is found to be influenced by alloy formation. The most interesting point is that small amounts of ruthenium (strongly diluted in platinum, thus not forming large ruthenium ensembles) strongly activate platinum without inducing pronounced hydrogenolysis, so typical for pure ruthenium. When discussing above the results concerning platinum-iridium, we have already presented some information on the platinum-cobalt alloys, which in some respects recall the platinum-iridium combination. Platinum-cobalt on alumina catalysts reduced at 973K, were also studied by Zyade et al. [204]; some of their results obtained with 2-methylpentane at 573K are shown in table 11. We can see the frequently observed synergism in total activities and suppression of isomerization on platinum by cobalt, and an enhancement of terminal splitting and suppression of formation of 3C~7 complexes (iso-units) when cobalt
654
chapter 13
is added. This information also confirms the picture which we have already constructed on the basis of results obtained with Pt-Re, Pt-Rh, Pt-Ru and Pt-Ir catalysts. Let us now turn our attention to another closely related alloy, the platinum-nickel system. As we have seen in chapter 4, there is extensive high quality information available on this system and this is important for good understanding of the catalytic results. Most of this information on surface composition as well as on catalysis by these alloys has been produced by various French laboratories. Platinum-nickel alloys have been studied with alumina [205] and active carbon [206,207] as supports. An extended discussion of catalytic effects of alloying platinum with nickel can be found [207]. For example, it was suggested that the frequently observed synergetic activity maximum found here for isomerization and hydrogenolysis of neopentane (2,2-dimethylpropane) by alloys with about 10-20% platinum in nickel should be ascribed to the electronic structure effects [207]. Very fine details of differences in catalytic behaviour which are induced by alloying can be established by monitoring the conversion of labelled molecules, for example, of 2-methyl(213C)pentane. The presence of a molecule of 3-methyl(313C)pentane in products witnesses (see chapter 1) the operation of a 5C(cyclic-) mechanism; the so-called bond-shift mechanism operating through 3C-~7complexes is related to the occurence of 3-methylpentanes labelled at another position in the molecule. When the 13C label appears at a place which cannot be reached by either of these mechanisms, the reason for it is most likely a repetitive conversion during one sojourn of a molecule on the surface. Some information does exist conceming the comparison of the kinetics of hydrogenolysis of ethane [207] measured on 1 . 1 % Pt/SiO 2 and on a ( 1 . 1 % Pt + 0.7 % Fe)/SiO2 catalyst. By a detailed analysis based of the models currently used (see section 13.1.2), it was concluded that ethane is adsorbed in a more dehydrogenated state at a low
PH2/Pc2H6ratio on Pt-Fe/SiO2 than on pure platinum catalysts. An interesting subject from the point of view of fundamental research is the platinum-palladium system, with which we can see again how the individual alloy components impose their fingerprint on the product distribution and the overall selectivity pattern. Palladium prefers, as do nickel, cobalt and ruthenium, terminal splitting (demethylation), while on pure platinum at low temperatures 3C0~7-complexes and internal fission are favoured. By using hexane isomers, it has been shown [209] that the contribution of terminal cracking is decreased by platinum in a clearly non-linear way. This could be explained by stating that, as with the other metals mentioned above, terminal splitting requires large ensembles of palladium (or of nickel, cobalt or ruthenium in other alloys), although it was believed [209] that the results actually reveal the operation of electronic structure effects in alloying.
Reaction of 2-methylpentane at 573K on Pt-Co/Al,O, reduced at 973K S: selectivity in isomers Activity expressed in pl(s.g)-'
I
I
Pt At.%
Act. %S x lo3
I
6C,
3C2
2C,
C,+C,
C,+C,
3-MP
n-C,
MCP I
&
nC,
nC,
I
modes iso/C2
16
0.5
1.5
10.5
2.7
0.9
1.2
21.5
20
9.1
1
0.8
12
33
12.1
1.2
0.5
100
12
84
0
0
2.5
4
10
25.5
20
38
80
285
81
0
0
4.5
4.5
10
40.5
29.5
60
16
60
0
0
7.5
13
19.5
19
50
20
63
1
1
4.5
15.5
18.5
0
195
0
96.5
2.5
1
14.5
&
I
Reactions of alkanes and reforming of naphtha
table 1 I
655
656
chapter 13
Platinum-palladium alloy catalysts have also been studied by another group of workers [210,211]. With alloys in the form of evaporated films and supported on silica, reactions of n-pentane, n-hexane and 2,2-dimethylpropane (neopentane) with hydrogen were monitored. Several interesting but not yet fully explicable effects were observed. For example, in n-pentane conversion the selectivity to n-pentane cyclization was higher for alloys than for either of the pure metals. The ratio of the selectivity to 1,6-closure to that of the 1,5-closure was also higher on alloys than on pure metals. In the reaction of hydrogen with n-hexane, 3-methylpentane and methylcyclopentane, saturated C6-products prevailed as expected at higher hydrogen pressures, while alkenes and mainly benzene prevailed in hydrogen-lean mixtures. The overall activity exhibited a minimum at about the equimolar composition, where isomerization selectivity also showed a maximum. The selectivities appeared to be influenced by i) hydrogen pressure, ii) the extent of selfpoisoning by carbonaceous deposits iii) alloy composition, so that all three factors are interrelated. This makes the picture very complex. With platinum-palladium alloy catalysts the isomerization of 13C-labelled molecule has also been studied on A1203-supported catalysts [209], and the variation between the external splitting on palladium-rich and-amongst others- internal splitting on platinum-rich alloys was again reported. Catalysts containing platinum and a sixth group metal (molybdenum, tungsten) have been studied by several groups [212-215]; explanation of the effects found is not easy. It is usually uncertain whether complete reduction of molybdenum or tungsten oxides occurs under conditions used, and this leads to additional problems, since unreduced oxides can create or demolish acid centres on silica or alumina supports, respectively. Yermakov et al. [212] studied ethane hydrogenolysis and found that Pt-Mo/SiO2 catalysts showed a high activity which they ascribed to electronic structure effects. The same type of catalyst (Pt-Mo/SiO2) was studied by Leclercq et al. [213] using n-butane hydrogenolysis; they ascribed the observed behaviour either to ligand effects and/or formation of mixed ensembles. Y-zeolites which are active catalysts in their own right have been used as supports for platinum and molybdenum containing catalysts [214]. Hydrogenolysis of nbutane was also faster on mixed metal catalysts than with monometallic catalysts with this support. Kuznetsov et al. [215] studied reactions of neopentane (2,2-dimethylpropane) and found a synergetic enhancement of the hydrogenolytic activity with bimetallic Pt-Mo/SiO2 catalysts. Results obtained with tungsten containing platinum catalysts show the same general picture [215a]: addition of tungsten suppresses isomerization and promotes the hydrogenolysis. However, for platinum-chromium catalysts a suppression of hydrogenolysis have been reported [215b]. With respect to the various catalysts discussed above (Ni-Pt; Pt-W, Pt-Mo) the following results could be of interest. When nickel was evaporated on W(100) or W(110) planes, effects were observed on the hydrogenolysis of butane, which were ascribed to the
Reactions of alkanes and reforming of naphtha
657
strained structure of nickel on W(100) [216]. Such effects of strain can in principle also exist in some dispersed catalysts, when the components differ in all respects so much as for example nickel and tungsten. In the same laboratory [217] nickel evaporated onto platinum was observed to give a non-linear increase the activity in ethane hydrogenolysis with surface coverage by nickel. Such non-linearity is often considered as an indication of cluster formation. Garin and Maire who studied the reaction of C6-alkanes on a long list of platinumbased alloys, published a summarizing review on this subject [218]. They concluded that there are two main surface complexes inducing hydrogenolysis, the 2Ct~7- and 3Co~7complexes. The sequence of bonds in the order of decreasing reactivity is thought to be [2181:
2CtxT: C I - Cii , 3Cr C I - CiIi,
(26) (27)
C I - CI, C i i - Cii C i i - CN, C i i - Cii I
This sequence of reactivities differs from that which we saw above for platinum-rhodium catalysts, but the reason for the discrepancy is not known. A summary of results in graphic form is in figure 49.
iso mode C2mode
Ni 2-
Pt -W
Pt-Pd 8
I~
I -
Pd
%, ~, / / P t-N i ~%1,,/ II
iC o
Pt -Co 0.3
Pt-lr
0.2-
Ir
Pt-Ru
0.10.06
0.1
012 0.3 0.z.
2
I
3 45
~0
figure 49 2-Methylpentane (2MP) hydrogenolysis reactions [218]. The ratio on the horizontal axis used to display the results shows which bond splitting is easier: terminal splitting leading to 2methylbutane (supposed to be induced by C2, oV3-adsorption mode) or splitting at the isounit, leading to n-pentane. The value of the parameter called 'iso-mode', used on the vertical axis, is derived from propane plus pentane concentrations; that of the parameter called 'C2unit' from isopentane plus isobutane concentrations.
658
chapter 13
13.4.2 Transition metal alloys without platinum Iridium-cobalt catalysts have been studied [219] using the reactions of alkanes, for example, of 2-methylpentane. By using isotopic labelling the contribution of 3Cc~Tcomplexes to isomerization increased, as did hydrogenolysis activity, when cobalt was added to iridium. This is the same effect as seen with platinum-cobalt catalysts. An explanation assuming changes in the electronic structure of the components, induced by alloying was also favoured here. Iridium-ruthenium catalysts have been studied with n-butane hydrogenolysis. Ruthenium is a metal with a strong preference for terminal and multiple splitting (section 13.1.1), while iridium at low temperatures prefers internal splitting. Alloy catalysts show a transition from one characteristic pattern to the other [220]. Iridium-osmium catalysts with alumina as support show the same activity-composition dependence as several other platinum alloys (see above). This can be seen in figure 50 [151], where the results for benzene hydrogenation are shown. In these cases, the possibility of mixed ensembles operating should not be forgotten. In any case this idea seems to be the most universally applicable explanation.
r3~l
figure 50 Activity of (Ir+Os)/ct-Al203 (mol.h-l.g 1 of metal) as a
20
function of at% osmium. Hydrogenation of benzene. T = 373K, 0
P8 = 0.05 atm., Pm = 0.95 attn.
10
[1511.
O
0
0
I
/
i
50
I '1~
100
%Os
13.4.3 Multimetallic cocktails The patent literature contains several suggestions to use more than two transition metals in industrial catalysts. Your authors are not aware of an industrial application of these multimetallic catalysts, but it might be nevertheless useful to know which combinati-
Reactions of alkanes and reforming of naphtha
659
ons have already been suggested. Without trying to be complete, we mention here that, for example there is a proposal to use the almost obvious combination Pt-Re-Ir/S/A1203. In these catalysts there is 0.1-0.2 wt% of each transition metal and 0.05-0.15 wt% of sulfur [221]. EXXON laboratory owns patents [222] which suggest a combination such as: 2%wt Rh, 0.01-2.0 Ir, 0-2wt Re, 0-2wt% Sn, 0-3.5wt% of halides, 0.01-2wt% Pt, with A1203 as support. The support has been moreover modified by adding silica and alkaline earth metals. The UOP company has another idea [223], namely, 0.05-1wt% Pt, 0.1-2wt% Ge, Pd (as oxides) 0.05-1wt% Rh, Ir, Ru, Co or Ni 0.5-1.5wt% C1, the rest being A1203. We have seen in the foregoing section what impact the presence of particular components has, so that the reader can make for himself a picture of the complicated function of these multimetallic catalysts. Owing to low loadings and the presence of elements which increase the dispersion of metals on alumina, the fraction of metal atoms exposed would approach unity in most of such catalysts.
13.5
Platinum-tin and other related catalysts
13.5.1 Platinum-tin catalysts These are very important reforming catalysts and their popularity seems to be ever growing [224]. The whole available arsenal of physical methods (chapter 2) has been applied here, but the definitive picture of the working catalyst, including the mechanism of reforming with them, is not yet available. However, the volume of knowledge gathered up to now is very respectable and the albeit slowly emerging picture is very interesting. In some respects the situation with these alloys is very similar to that with platinum-rhenium catalysts. We can find papers which try to explain the total behaviour of these catalysts by just considering the platinum-tin alloy formation [224,225], while others prefer to assume the presence of unreduced tin and speculate on its effects on anchoring metal particles, acidity of the support or coking and other side reactions [226,227]. With the available knowledge one can conclude that most likely all these effects are operating under reforming conditions. However, before presenting the catalytic results we shall turn to the question of the state of the catalyst. A very powerful tool to investigate the state of platinum-tin catalysts is MOssbauer Spectroscopy. This is applicable also under in situ conditions which makes it particularly attractive. Moreover, it is rather a short-range-order method than a long-range-order-XRDmethod and this is also an important factor for catalysts with very small particle size. Analysis is performed by standard procedures of curve-fitting [226,228,233] and it is not a trivial procedure. However, the results are in many respects rewarding. The picture emerging from the most recent literature [231-233] is that one or more
660
chapter 13
alloy phases containing Sn ~ are present. In addition tin is present in bi- and tetravalent oxides, and in chlorides, when chlorine containing precursors are used. The fraction of platinum present in the alloy phase increases with increasing tin concentration. The first portions of tin added are probably firmly bound to defects of alumina, dissolved in the structure, etc. and are not easily reduced (if at all). The presence of chlorine in the precursor facilitates alloy formation [232-234], as we saw also with iridium- or rheniumcontaining catalysts. M6ssbauer spectroscopy also revealed that reduction of tin is easier and can even approach completness when silica is used as a support [233]. Another bulk technique, the common X-ray diffraction (XRD), can be used only with model catalysts containing a higher platinum and tin content than the commercial reforming catalysts [235]. The dominant phase seen is the stable intermetallic compound PtSn, but not all tin is present as an alloy. Layers of tin aluminate may also be formed. EXAFS can be easily performed with both the platinum and tin edges (L and K, respectively) [234]. Again, the Pt-Sn alloy phase and Sn-O bonds have been observed. The use of XPS should have been most informative, but it was not. Some papers claimed the presence of exclusively ionic tin [236-238] and only under the pressure of evidence coming from other techniques and due to the development of the XPS technique and data evolution, did opinions started to change. Some later papers report the presence of zero-valent tin, most of it forming an alloy with platinum [239-243]. Having in mind the summary of the results, namely that catalysts contain platinum and tin in various phases, most of platinum being alloyed, we can now review the catalytic behaviour. In advance the following can be said. In principle the presence of tin compounds can modify acid-catalyzed reactions and the presence of tin ions on platinum can influence aromatization/cracking selectivity ratio. Moreover, it is not known whether the Pt-Sn alloy is active in adsorption of hydrogen and hydrocarbons at all. It is quite possible that only clusters of platinum in platinum-tin alloy particles or small platinum particles (unalloyed platinum) are active, but not the alloy in the form of an intermetallic compound. Early patents [224] as well as the early open literature point out what is the advantage of using platinum-tin catalysts instead of pure platinum catalysts: at higher temperatures and under more severe conditions, i.e. a higher hydrocarbon: hydrogen ratio, the hydrogenolytic splitting is on alloy catalysts lower than on Pt/A1203 and the aromatization higher [244-246]. There are indications that the harmful coke precursors created on the metallic surface, these precursors being dehydrogenated fragments of reacting molecules, are in the case of platinum-tin catalysts more mobile and less firmly attached to the metallic surface. Further, it is claimed that coke is mostly deposited on the support and that the metallic function of the catalyst stays preserved for a longer period of time [247]. Notice that a similar picture has also been suggested above for Pt-Re-S/A1203 catalysts. Suppression of coke formation under more severe conditions is reported by several other
Reactions of alkanes and reforming of naphtha
661
workers and can be considered as very well established. The effect is more pronounced with, say, n-heptane than with lower hydrocarbons [248-251], again in agreement with the idea that the presence of tin suppresses the detrimental side reactions. While the facts concerning platinum-tin catalysts seem to be clear, explanations have been suggested based on a broad variety of ideas. While Bacaud et al. [226] stress the effect on acidity, Beltramini et al. [251] deny the existence of this effect. Those who concentrate their attention on the metallic part of the catalysts can be subdivided into two groups. The first group offers an explanation based on the ensemble size effect: smaller platinum ensembles should show a much lower hydrogenolysis rate, while dehydrogenation together with dehydrocyclization should be influenced to a much lesser extent [225,250,251]. The second group believes that the beneficial effect of tin on the platinum comes either from electronic interactions of zero-valent tin on platinum, or from the interaction of platinum particles with tin ions [227,244,249]. An important paper for all future attempts to explain the behaviour of platinum-tin alloys has been published by Coq et al. [253a], who studied Pt-M/AI203 catalysts, with M being Sn, Pb, Ge, A1 or Zn. They used a series of hydrocarbons - n-hexane, methylcyclopentane and 2,2,3,3-tetramethylbutane. In particular, the last mentioned molecule was interesting: with pure platinum catalysts, small particle catalysts show a 3Cc~y-dimethylation, leading to 2,3,3-trimethylbutane, while large particles split the molecule into two isobutanes (2-methylpropanes). When a small amount of metal-M-precursor was adsorbed on the metal (an organometallic compound was decomposed by reaction with hydrogen preadsorbed on the platinum), then with M/Pt lower than 0.2 in atomic contents, small particles started to behave as large ones. It means that the first dose of added M-metal, occupied special sites abundantly present on small particles, likely at edges and defects. Of the metals mentioned, tin seemed to be the most efficient. Essentially the same results were obtained with ruthenium and rhodium as the transition metal component [253b,c]. Several groups of workers have compared tin as an additive to platinum, with some other elements used as additives: Pb, Ge, Mo, W. Re. Tin seems to be the best additive for enhancement of aromatization [250,254,255]. The problem with all platinum-containing alloy catalysts is regeneration. An oxidative treatment clearly separates the components, but since it is possible to mix the components as oxides or chlorides, regeneration is in principle possible. The fact that platinum-tin catalysts are used in practice shows that the proper way of regeneration has been already found by industrial laboratories. Tin and platinum form in an ionized state several very stable coordination complexes and thus the anchoring of one component on the 'template' formed by ions of the other component can operate here to our benefit
[2581. The beneficial effect which tin has on platinum catalysts can also be achieved with germanium. Germanium similar, to tin, exists in several valencies: Ge(0), Ge(II) and
662
chapter 13
Ge(IV), also forming platinum-germanium intermetallics [259-263]. With germaniumcontaining catalysts an enhanced isomerization is sometimes reported, but otherwise the effects on selectivities and the suggested explanation of effects are mostly analogous to those for platinum-tin systems. Platinum on alumina catalysts can be also modified by elements such as tellurium or antimony [256]. In some refineries, compounds containing antimony are indeed added to the feedstock and they have a beneficial effect on reforming and fluidized catalytic cracking, but the traces of these components appearing then in gasoline are a problem for car exhaust catalysts. The propane to benzene reaction can be performed on zeolites loaded with various sp metals, semi-metals or their oxides (Ga, Te, Bi). It has been reported that platinum-tin intermetallics are very good catalysts for this aromatization in combination with ZSMzeolites [264]. 13.5.2 Other catalysts containing tin and related elements Results of a detailed study of the iridium-tin system have been reported [265]. Conclusions
are very similar to those derived from the platinum-germanium and plati-
num-tin systems: C-C bond fission is suppressed and aromatization of higher hydrocarbons is enhanced. The first point is of particular interest since iridium clearly prefers the 2Cc~Bsplitting at all temperatures, while platinum only at a very high temperature, being at low temperatures mostly active through the formation of 3Cot"f-complexes. Nevertheless a great similarity is observed. Cyclohexane can be aromatized or the ring can be opened and further hydrogenolyzed. Addition of tin to iridium has an effect on these reactions as illustrated by table 12 [265].
table 12 Cyclohexane conversion at 526K; rates of formation (a.u.) Benzene
Methane+Ethane Prooane
Butane/Pentane/Hexane
_
Ir/SiO 2
8.7
1.7
0.5
0.65
0.7
4.2
Ir+Sn/SiO 2
1.3
0
0
0
0
0
Reactions of alkanes and reforming of naphtha
13.6
663
Reforming of naphtha Catalytic reforming of naphtha [266-269] forms a part of the oil refining network
and it stands in this network next to other units such as distillation, dehydrosulfurization, cracking, etc. A scheme of a reforming unit is shown in figure 50. COMP
-'~ Gas
R1
NAPHTH
H-E
R3
PI Gasoline
figure 51 A simplified scheme of a reforming unit in a three-reactor configuration (R1-R3). The position of the compressor (COMP), separation subumit (SEP), pump (P), heat exchange (H-E) and re-heaters (H) are indicated.
It has been mentioned above (13.1) that reforming is a complex of dehydrogenation processes accompanied by hydrogenolysis, isomerization and dehydrocyclization to the C 5 and the C 6 substituted alkyl rings and aromatics. These reactions occur partially on the acidic support and partially on the metal, therefore we speak of bifunctionality of the catalysts. Octane numbers of some hydrocarbons are given in table 13. Obviously, isomerizations, leading to branched hydrocarbons and aromatization, are the most desired reactions. Let us mention some pieces of information concerning chemical and technological aspects of naphtha reforming; more details can be found in the literature [266-270]. As far as the chemistry is concerned, many isomerizations can reach equilibrium and the higher the temperature the more branched molecules are present at equilibrium. This is seen in table 14.
664
chapter 13
table 13 Octane numbers of pure hydrocarbons research octane number (clear)
Hydrocarbon Paraffins: n-butane
113
n-pentane
62
n-hexane
19
n-heptane
0
n-octane
- 19
2-methylhexane
41
2,2-dimethylpentane
89
2,2,3-trimethylbutane
113 Aromatics:
Naphthenes(alkanes)" n-butane
113
methylcyclpentane
107
benzene
99
n-pentane
62
1,1-dimethylcyclopentane
96
toluene
124
n-hexane
19
cyclohexane
110
1,3-dimethylbenzene 145
n-heptane
0
methylcyclohexane
104
isopropylbenzene
n-octane
- 19
ethylcyclohexane
43
1,3,5-trimethylbenzene 171
table 14 Equilibrium distribution of iso-alkanes at 750K. number of branches [269] hydrocarbon
0
1
2
3
C6
25
C7
15
45
30
-
45
40
-
C8
15
45
25
5
132
Reactions of alkanes and reforming of naphtha
665
Further, thermodynamic calculations reveal that for temperatures above 600K and hydrogen pressures between 1 and 3 MPa the content of cycloalkanes is below 5%. However, at 800K, aromatics are almost the only components favoured, except with C6 hydrocarbons. The rate of all reactions is favourably enhanced by an increased temperature, too, so that reforming is performed at somewhat high temperatures. However, the upper limit of temperature is mainly dictated (apart from thermodynamic limitations) by the extent of side reactions. These are induced by a too deep dehydrogenation of the feed, leading to coke formation. A compromise is to work at about 750-800K. Due to coking, deactivation of the catalysts slowly progresses and the temperature of reforming must be increased to keep the conversion high. After a certain period of such operation, the catalyst has to be regenerated semicontinuously or swing-reactors have to be used (i.e. two parallel reactor units of which one is always being regenerated). After several regenerations, the catalyst has to be replaced, but this is a very costly operation [270-272]. Alloy catalysts such as Pt-Re-S/A1203 have the life span 5-10 times as long as the originally used Pt-S/A1203 catalysts. In all respects the new catalysts appear to be superior [273]. A typical three reactor configuration is shown in figure 51. It indicates that reactors of unequal size are used and the units comprise external heaters (H), heat exchanger (HE), pumps (P) and compressors, the latter being necessary in recycling of feedstocks. The reactors have an operating temperature of about 770K, and the pressure of about 30 atm. hydrogen, needed at the inlet of the process, is produced by the process itself. The process also supplies hydrogen for other processes of oil refining. The inventors of new, robust and stable catalysts based on platinum and rhenium also caused development in the engineering of the process: improvements which have been introduced have been described [270-273]. The units presently used also contain provisions for re-activation by oxygen and re-dispersion of the metallic components, using oxygen and chlorine compounds. During these steps the support is also cleaned and recharged with chlorine [274].
References 1 2 3a b C
4
C.Kemball, Proc.Roy.Soc., A 207 (1951)541; A 217 (1953) 376 J.R.Anderson, C.Kemball, Proc.Roy.Soc., A 223 (1954) 361 C.Kemball, Adv.Catal. 11 (1959)223 C.Kemball, Bull.Soc.Chim.Belg., 67 (1958) 373 C.Kemball, Catal.Rev., 5 (1971) 93 G.C.Bond in "Catalysis by Metals", Academic Press, London, (1962) G.C.Bond in "Heterogeneous Catalysis", Oxford Sci.Publ.,Clarendon Press, Oxford, 2nd ed. (1987)
666
5a
b C
d e
f g 6
9 10 11 12 13 14
15 16 17 18 19 20 21a
chapter 13 E.H.van Broekhoven, V.Ponec, Progr.Surf.Sci., 19(2) (1985) 351 E.H.van Broekhoven, V.Ponec, J.Molec.Catal., 25 (1984) 109 V.Ponec in "The Chemical Physics of Solid and Heterogeneous Catalysis" (editors: D.A.King, D.P.Woodruff) Elsevier, Vol.4 (1982) 365 A.E.Shilov, Pure.Appl.Chem., 50 (1978) 725 R.L.Burwell Jr., Annu.Rev.Phys.Chem., 15 (1964) 131 R.L.Burwell Jr., Catal.Rev., 7 (1972) 25 L.Guczi, K.Ujszaszi, React.Kin.Catal.Lett., 8 (1978) 489 T.L.Cottrell in "The Strength of the Chemical Bond", Butterworths, London, (1958) H.A.Skinner, Adv.Organomet.Chem., 2 (1964) 49 "Handbook of Chemistry and Physics", C.R.C.Press, Cleveland "Comprehensive Organometallic Chemistry" (editors: G.Wilkinson, F.G.A.Store, E.W.Abel) Pergamon Press, N.Y., Vol.6 (1982) 42 C.T.Mortimer in "Reaction Heats and Bond Strengths", Pergamon Press, Oxford (1962) R.T.Anderson in "Chemical Bond and Bond Energy", Academic Press, N.Y. (1971) S.Cerny, M.Smutek, F.Buzek, J.Catal., 47 (1977) 166 M.Smutek, S.Cerny, J.Catal., 47 (1977) 178 P.Tetenyi, Proc.6th Int.Congr.on Catal.London, 1976, Chem.Soc.London, Vol.1 (1977) p.456 R.Evans, M.Polanyi, J.Chem.Soc., (1947) 252 M.E.Winfield, Australian J.Sci.Rev., A4 (1951) 385 S.T.Ceyers, Science, 249 (1990) 113 M.A.McKervey, J.J.Rooney, N.G.Samman, J.Catal., 30 (1963) 330 M.Vogelzang, M.J.P.Botman, V.Ponec, Faraday Disc.Chem.Soc., 72 (1981) 44 Z.Paal, P.Tetenyi, React.Kin.Catal.Lett., 12 (1979) 131; 7 (1977) 39 Z.Paal, P.Tetenyi, Nature, 267 (1977) 234 Z.Paal, P.Tetenyi, Appl.Catal. 1 (1981) 9 M.W.Vogelzang, V.Ponec, J.Catal., 111 (1988) 77 O.E.Finnlayson, J.K.A.Clarke, J.J.Rooney, J.Chem.Soc.Faraday Trans.I, 80 (1984) 191 C.O.Donohoe, J.K.A.Clarke, J.J.Rooney, J.Chem.Soc.Faraday Trans.I, 76 (1980) 345 H.Zimmer, P.Tetenyi, Z.Paal, J.Chem.Soc.Faraday Trans.I, 78 (1982) 3573 H.Zimmer, M.Dobrovolszky, P.Tetenyi, Z.Paal, J.Phys.Chem., 90 (1986) 4758 G.Leclercq, L.Leclercq, R.Maurel, J.Catal., 50 (1979) 87 J.E.Germain, 'Catalytic Conversion of Hydrocarbons', Academic Press, N.Y. (1969) H.Pines in "The Chemistry of Catalytic Hydrocarbon Conversions", Academic
Reactions of alkanes and reforming of naphtha
b C
d 22
23 24 25 26 27
28 29 30 31
32
33 34 35
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P.Esteban Puges, F.Garin, F.Weisang, P.Bernhardt, P.Girard, G.Maire, L.Guczi, Z.Schay, J.Catal., 114 (1988) 153 H.Hamada, Appl.Catal., 27 (1986) 265 US Patent no. 5,066.632 US Patents nos. 4,966 682; 4,966 878; 4,968 408 US Patents nos. 4,791 087; 4,737 483 F.M.Dautzenberg, German Offenlegunsschrift, 2,121 763 (1971) F.M.Dautzenberg, H.W.Kouwenhoven, German Offenlegungsschrift, 2,153 891 (1972) F.M.Dautzenberg, J.N.Helle, P.Biloen, W.M.H.Sachtler, J.Catal., 63 (1980) 119 J.Volter, G.Lietz, M.Uhlemann, M.Hermann, J.Catal., 68 (1981) 42 Yang Weishen, Lin Litru, Fan Yineng, Zhang Jingling, Catal.Lett. 12 (1992) 267 R.Bacaud, P.Bussi6re, F.Figueras, J.M.Mathieu in "Preparation of Catalysts" (editors: B.Delmon, P.A.Jacobs, G.Poncelet), Elsevier, (1976) 509 H.Lieske, J.Volter, J.Catal., 90 (1984) 96 R.Burch, L.C.Garla, J.Catal., 71 (1981) 360 R.Burch, J.Catal., 71 (1981) 318 R.Bacaud, P.Bussi6re, F.Figueras, J.P.Mathieu, C.R.Acad.Sci.Paris, ser.C, 281 (1975) 159 V.H.Berndt, H.Mehner, J.Volter, W.Meise, Z.Anorg.Allg.Chem., 429 (1977) 47 K.J.Klabunde,Y-X.Li, K.F.Purcell, Hyperfine Interact., 41 (1988) 649 Y-X.Li, Y-F.Zhang, K.J.Klabunde, Langmuir, 4 (1988) 385 Y-X.Li, K.J.Klabunde, B.H.Davis, J.Catal., 128 (1991) 1 V.I.Kuznetsov, A.S.Belyi, E.N.Yarchenko, M.D.Smolikov, M.T.Protasova, E.V.Zatoloking, V.K.Duplyakin, J.Catal., 99 (1986) 159 N.A.Pakhonov, R.A.Buyanov, E.M.Moroz, E.N.Y.Yurchenko, A.P.Chernychev, N.A.Zaitseva, G.R.Kotelnikov, React.Kinet.Catal.Lett., 14 (1980) 329 G.Meitzner, G.H.Via, F.W.Lytle, S.C.Fung, J.Phys.Chem., 92 (1988) 2925 R.Srinavasan, R.J.DeAngelis, B.H.Davis, J.Catal., 106, (1987) 449 B.A.Sexton, A.E.Highes, K.Foger, J.Catal., 88 (1989) 466 S.R.Adkins, B.H.Davis, J.Catal., 89 (1984) 371 J.M.Stencel, J.Goodman, B.H.Davis, Proc.9th Int.Congr.on Catal.,Calgary, 1988, Canadian Chem.Inst., Ottawa, vol.3 (1988)p.1291 Y-X.Li, J.M.Stencel, B.H.Davis, React.Kinet.Catal.Lett., 37 (1988) 273 Y-X.Li, J.M.Stencel, Appl.Catal., 64 (1990) 71 R.Srinivasan, B.H.Davis, Platinum Metal Rev., 36 (1992) 151 H.A.Laitenen, J.R.Waggoner, C.Y.Chan, P.Kirzenztejn, D.A.Asbury, G.B.Hoflund, J.Electrochem.Soc., 133 (1986) 1586 S.D.Gardner, G.B.Hoflund, D.R.Schryer, B.T.Upchurch, J.Phys.Chem., 95 (1991)
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679
Chapter 14
SYNGAS REACTIONS
14.1
Fundamentals
14.1.1 Historical introduction The observation that carbon monoxide can be hydrogenated to methane with nickel as catalyst was first reported by Sabatier and Senderens [1]; they made this observation when they systematically studied all kinds of hydrogenations on nickel catalysts. Orlov was probably the first to detect formation of higher hydrocarbons [2], a reaction which later received the name of two outstanding German chemists, who developed the technology of liquid-fuel production from coal: Fischer and Tropsch [3]. The process also attracted the attention of fundamental research workers and reviews describing the development of our knowledge of this reaction between 1923 and 1950 can be found in the literature [4-8]. Later it appeared that with alkali-promoted and nitrided iron catalysts various oxygencontaining molecules can also be produced [9,10]. Finally, the important bulk chemical, also suitable as fuel -methanol- was prepared from syngas (CO/I--I2/CO2) on oxidic [11] as well as on metallic catalysts [12]. Obviously, syngas (CO]H 2 or C0/I--I2/C02) is a very versatile mixture and various catalysts induce reactions in very different directions (scheme I).
CH/"'H20 %
~~4 CH30H
Scheme I Reactions o f syngas on metal containing
/
oxygenotes
CO/H2
catalysts.
Reactions
1,2 and 5,6 can be catalysed also by pure metals; reaction 7 to
CnH2n, CnH2n+2, H20.
alcohols
reactions
and
most
likely
3 and 4 require
a
promoted metal as a catalyst.
The key-word for fundamental research is thus SELECTIVITY. How to identify the factors determining selectivity, and how to use this knowledge in practice, are the questions which have kept scientists interested in this reaction network for more than 70
680
chapter 14
years. Selectivity to any particular product or group of products is determined by the reaction mechanism and it can be manipulated by (i) catalyst composition, including the presence of promoters and gaseous modifiers, and (ii) by composition of the reaction mixture. We shall return to this problem below. The first idea concerning the mechanism of hydrocarbon synthesis was formulated at the time when almost nothing was known about polymerisation reactions and the transition from C 1 to C n (n = 2,3 ...) seemed difficult conceptually. Fischer and Tropsch knew that iron carbide was formed from iron and carbon monoxide and suggested that higher hydrocarbons are formed from bulk carbides [3-8]. This theory has even obtained some very recent supporters [13]. However, there were many objections raised against this theory or its two-dimensional version (surface carbides instead of the bulk ones) [14]. The most important one suffices as an example. The product distribution shows very little of branched-chain hydrocarbons, while one would expect that branching would occur freely when carbon chains present on the surface of carbides or in the bulk were hydrogenated directly to hydrocarbons; the distance between two carbon atoms in these chains is approximately the same as the C-C bond length in alkanes. Therefore the original carbidic theory was replaced by the idea of the chain growth by dehydrocondensation of hydroxycarbene groups [5], the latter being formed by recombination of carbon monoxide and two hydrogen atoms.
HO
I
IH HOI
\
I
H HO
/- . . . . \ / C
C
!1
rl
x
•
H
\/ C
Scheme H Dehydrocondensation mechanism, using hydroxy carbene.
•
Later a theory appeared which assumed that hydroxycarbenes are first hydrogenolysed to methylene groups which polymerise by step-by-step addition of
CH 2
groups [4].
Speaking generally we can divide the mechanisms shown in the literature into two groups, differing in the order of elementary steps: (i)
dissociation of carbon monoxide, hydrogenation of carbon into CH x and of oxygen into water, CHx polymerisation into hydrocarbons;
(ii)
hydrogenation of adsorbed carbon monoxide to species such as hydroxycarbene, followed by hydrogenolytic splitting or splitting accompanying the condensation reaction.
Syngas reactions
681
For a long time pathway (ii) had the most supporters. Its fans pointed out how strong the carbon monoxide bond is (more than 1 MJ/mol) and the kinetics also seemed to support the idea [15]. However, as we shall see below, transient kinetic experiments and experiments with labelled molecules have finally revealed that for the most active FischerTropsch synthesis catalysts, namely iron and cobalt, and also for nickel and ruthenium, the main pathway is that described by (i). In the past there was very little doubt expressed about the possible mechanism of the methanol synthesis. Everybody believed that metallic copper hydrogenates carbon monoxide into methanol and the other components of the industrial catalyst Cu/ZnO/AI203 are present just to stabilize the dispersion of copper. However, doubts about this mechanism have appeared and they remain (see below). One hypothesis is that copper(I) sites are the loci of the hydrogenation of carbon monoxide by hydrogen which is supplied from other parts of the catalyst surface e.g. ZnO or copper metal [16]. Higher oxygenates such as aldehydes and alcohols are believed to originate from a carbon monoxide insertion reaction [17]. When a methyl group (or in general, an alkyl group) recombines with CHx, the hydrocarbon chain grows. When it recombines with adsorbed CO, an acyl or similar group arises, and this was and still is believed to be the main pathway to higher oxygenates. It has also been suggested [17] that hydrocarbons are formed by this addition and by a subsequent hydrogenolysis of the C-O bond, but it does not seem at the moment that this is the universal and prevailing mechanism of hydrocarbon formation. 14.1.2 Present ideas concerning the mechanisms
Synthesis of hydrocarbons The simplest product of synthesis is methane. When methane is the desired product, we speak of methanation. Araki et al. [18], in an attempt to elucidate the mechanism of methanation, performed experiments with evaporated nickel films, to avoid potential complications due to a support. They covered the nickel surface by labelled carbon (13C) using the disproportionation reaction:
213C0 ~13C+13C02 (T)
(1)
and left only a part of the surface free for adsorption of carbon monoxide and possible formation of oxygen-containing intermediates. Then they admitted a 12CO/H2 mixture to the catalyst and watched to see which methane appeared first - 13CH4 from 13C laid down by reaction (1), or 12CH4 which would be formed, if such pathway existed, from e.g. hydroxycarbene. It appeared that 13C reacted first and formation of unlabelled 12CH4 started only when practically all the 13C was removed from the surface. This is schematically
682
chapter 14
shown by figure 1. The same conclusion was arrived at by others, who worked independently on this topic at almost the same time [ 19-21 ].
~
~
:
Me
,'*C
. . . . . . . H ~- .~/ s/
to I
figure 1 Two possible pathways on nickel pre-
25 o
covered by u C (=*C). On the left is the
"\\I \
H
\
result expected when C from CO dissoCH L
ciation must be formed first. On the right; the result when the reaction runs via
oxygen-containing
intermediates.
Observed behaviour resembles the lefthand scenario. >t
>t
Biloen et al. [19a] applied the same procedure to study the formation of higher hydrocarbons on supported nickel catalysts. They saw that, for example, some butane molecules contained two or even more ~3C atoms from the carbon predeposited on the surface. It means that not only methanation but also formation of higher hydrocarbons is possible without oxygen-containing intermediates. In the case of methane, the experiments revealed that the pathway (i) (see above) via dissociation of carbon monoxide is much faster than any other proposed pathway. The conclusion that oxygen-containing intermediates are not a necessary prerequisite for hydrocarbon chain growth has also been arrived at by others [19,b,c]. Important support also came from transient state experiments [20,21]. However, not all metals behaved in the same way; with palladium the evidence was not unambiguously in favour of the carbon monoxide dissociation pathway, and the results could have been equally well explained by assuming that some oxygen-containing species (formyl group, hydroxycarbene, methoxy group, adsorbed methanol) are intermediates in formation of methane [21]. Experiments [22] with palladium and palladium-silver alloys seem to confirm this conclusion for this particular metal. Another important detail known about the mechanism is the following. When an active metal such as nickel or cobalt is diluted in copper, which is almost inactive, the activity per unit area of surface for methane and higher hydrocarbon formation falls much more than the number of active atoms in the surface. In other words, the most likely explanation (see chapter 9) is that an ensemble of several atoms is required for methanation or higher hydrocarbon formation [18,23-26]. Discussion continues as to whether one needs only three or four atoms, or even more, for instance, as many as ten or fifteen (see chapter 9). However, irrespective of this uncertainty it seems now to be well established that not one but several atoms form an active site. Araki [18] has proven that the
Syngas reactions
683
ensemble is necessary for dissociation of carbon monoxide, the hydrogenation of surface carbon being influenced by alloying to a much lesser extent since it does not require such large ensembles. The next question is how alloying influences the formation of higher hydrocarbons. There was some controversy on this point in the literature (compare [23] with [24] and [25]), but this problem now seems to be solved. At low temperatures alloying of nickel or cobalt with copper leads to suppression of higher hydrocarbon formation, but at higher temperatures alloys produce more higher hydrocarbons. Those responsible for this discrepancy [23-25] worked at different temperatures and consequently their results differ. The reason for the behaviour described is less certain but the likely explanation is that copper in the nickel or the cobalt surface decreases the concentration of the CH x monomers on the surface and suppresses the formation of higher hydrocarbons at low temperatures. At higher temperatures a secondary reaction of reversed splitting of alkyl chains becomes possible, but this reaction is less probable on alloys than on pure active metals. Another possible role of copper could be related to the following point. When carbon monoxide dissociates on the surface, not all carbon is immediately taken away by hydrogenation. A part of it is converted into an inactive carbon of which again a part can be slowly hydrogenated but a certain part always forms an almost totally inactive carbonaceous, probably graphitic, layer [20,27,28]. The presence of carbon of different reactivities has indeed been proven by various chemical and physical methods, e.g. NMR. Alloying can influence these transitions and by that the availability of active CHx groups on the surface. We have seen in chapter 13 that alloying can influence the reaction under study by changing the direction or extent of a side reaction. Here we see such an effect again and it is something that has to be always kept in mind with any reaction studied on alloys. There was a period of time when kinetics and some spectroscopies seemed to indicate the participation of oxygen-containing intermediates in the synthesis of hydrocarbons on pure metals. However, these results now have another explanation. It appeared that oxygen-containing species are only seen in IR spectra when supported metals are used as catalysts or when metal oxide promoters are placed on the surface of the active metal [29]. No oxygen-containing intermediates were observed in experiments with metal singlecrystal planes [30]. With kinetics it was a different story. In the semi-empirical approximation having the form of a power rate law, the rate r of methanation is: "
"
r = k P c o Pt~2
(2)
Experiments showed that in a certain region of pressure, n should be near to unity or even be larger. In a higher approximation, equation (2) should be rationalised by LangmuirHinshelwood kinetics, in terms of On, Oc, Oo, etc. Let us now consider that the rate r is
684
chapter 14
proportional to OH (r = const.OH). In its turn, OH is proportional to the square root of hydrogen pressure, so that n should be approximately equal to V2. When n > 0.5 or even n > 1.0 is found experimentally, we have a problem. To solve this one has to make one of the following assumptions. a)
The rate-determining step is a reaction between surface carbon and at least two hydrogen atoms (three-atom-collision) [31 ];
b)
The rate-determining step is the reaction between CH and Haas, or even CH2.aos + Hads, but not the most likely one Cads + Haas [33,34];
c)
The rate-determining step is the formation or conversion on the surface of an oxygen-containing intermediate [ 15,32,33].
The first two suggestions did not seem likely, so that those who believed in kinetics as a reliable tool to establish mechanism were all in favour of suggestion c). However, as we have seen above, this suggestion
disagrees with all other more convincing information
supplied by transient state techniques, isotopic labelling, etc. The explanation of the apparent controversy seems to be offered by the results in a paper published by Goodman et al. [35]. They established that the
working
part of the surface |
free of carbon, is a
function of hydrogen pressure. For example, when the fraction of the surface not covered by 'carbon' is given by one of the following equations:
0 ~ =k/p~ 2
0~
k~P~~
(3)
-
the rate equation, taking the reaction C + H ~ CH as the rate determining step, becomes (4)
r = c o n s t . 0 ~ . O H. 0 c
where const, comprises various elementary constants. We then have a solid basis for explaining why the order in hydrogen may be greater than one-half, as, assuming steady state concentration of single carbon atoms on the working surface, |
is approximately
constant. Both the terms O . and O ~ depend on hydrogen pressure to the half-power, so that the rate is then first order to hydrogen. One is therefore not obliged to claim the existence of oxygen-containing intermediates to explain how n can be unity. Earlier studies on Fischer-Tropsch Synthesis (FTS) were solely concentrated on the two metals, iron and cobalt. However, renewed interest in FTS after the oil crisis in 1973 extended the information on the activity of metals to almost the whole Periodic Table. Summarizing this information in brief, we can state the following. 1)
Metals of the Groups 3-7 of the periodic table are active in carbon monoxide adsorption and dissociation. However, oxides and some of the carbides formed upon carbon monoxide dissociation are very stable, being irreducible at temperatu-
Syngas reactions
685
res at which the hydrocarbon chain growth of the FTS is thermodynamically still possible 9 This makes these metals inactive in the FTS. However some of their carbides or oxycarbides seem to show some activity [36].
2)
The metals iron, cobalt, nickel, ruthenium and osmium are the most active ones. They adsorb carbon monoxide strongly and dissociate it easily at moderate temperatures and they do not form oxides that are irreducible at synthesis conditions.
3)
The metals palladium, iridium and platinum are of very low activity 9 They all adsorb carbon monoxide, palladium even rather strongly, but they are all very reluctant to carbon monoxide dissociate it. Both the thermodynamic and kinetic factors are responsible for this.
4)
Rhodium takes in all respects a middle position between the metals under 2) and
5)
Metals of Groups 11-13 of the Periodic Table are virtually inactive.
3).
The active metals listed under 2) and 3) provide the activity pattern presented by figure 2, where the results shown are replotted from the literature [ 15,32].
figure 2 Activity patterns in methanation; (TON)
-9
as a function of the position of a metal in the periodic system.
0
,,.-,..
1000100-
z
i Rh
10-
0 I--
1-
0.1
Pd
-
0.01 Cu I
8
9
10
11
12
Group number The distribution of hydrocarbons according to the number of carbon atoms n c is given by the so-called Schulz-Flory-Herrington-Anderson distribution function [37], log[C,] = nc.loga + log[ C1 ]
where
Cn
(5)
stands for the mol fraction of hydrocarbons with n c carbon atoms 9 The parameter
686
chapter 14
o~, representing the probability of chain growth, is highest for the most active metals (iron, cobalt and ruthenium); c~ > 0.9 can be achieved at low temperatures. Oxygenate formation sometimes follows the same law [38]. The parameter o~ decreases with increasing temperature and with increasing hydrogen pressure. Factors increasing the rate of carbon monoxide dissociation, as the presence of suitable promoters or an increased carbon monoxide pressure, can increase it.
Synthesis of higher oxygenates Even long before World War II, chemists noticed that iron catalysts can be modified by additives in such a way that they produce appreciable amounts of aldehydes, alcohols and even acids [9,10]. Higher alcohols and hydrocarbons produced by the same catalyst often exhibit parallel Schulz-Flory-Herrington-Anderson distributions (eq.1) [38]. This by itself indicates that chain growth by recombining CHx and CmH x units can be interrupted by recombining CmHx with COads instead of with the CHxunit. We know now that the recombination of CmH x 4- CO can also take place on a pure metal surface [39,40]. However, further hydrogenation of the intermediates in the presence of carbon monoxide in the gas phase is only possible in the presence of a promoter (see below). With rhodium, carbon monoxide dissociation on the unpromoted surface is much slower than that near to promoter patch on the metal surface, where the hydrogenation of the acyl group, formed by a recombination of CH x with CO, is also fast. Thus, of all metals rhodium produces most of ethanol. There will be more about this in the section on promoters.
Synthesis of methanol The industrial catalyst is Cu/ZnO/A1203. However, palladium catalysts have been found to be of a comparable activity [41,42]; at the time, it was a surprise for everybody. Also rhodium [43-45] can be prepared in a form producing methanol, also with a selectivity nearing that of copper [45a]. There were attempts to explain the activity in methanol synthesis of platinum, rhodium or palladium catalysts by special particle sizes, by a particular particle shape or by activation of the support [46-48], which in the latter case was seen as the locus of methanol formation; also a particular metal-support interaction was suggested as the reason [45]. Driessen et al. [49] prepared Pd/SiO2 catalysts with varying amounts of magnesium promoter and established that the activity was strictly proportional to the percentage of palladium (up to 4% of the total metal) present in an ionic form (see figure 3). Hindermann et al. [50] showed, with the same catalysts, that the activity in methanol synthesis was proportional to the concentration, measured under standard conditions, of the formyl group on the surface of the catalyst. Theoretical papers bound these two papers [49,50] together, by showing that the formation of a formyl group is easier and the stability of the formyl is higher, when the palladium site bears a positive charge [51,52].
Syngas reactions
687
o
0.4CH30H act. (%)
~o
0.3-
I
0.2 ,,1
c o n v e~/r s i~o
n
-
to CH30HversiOn o
0
I
1
%Pd
I
!
2
3
figure 3 Activity (yield) in CH30H formation at 488K (defined as total conversion x selectivity x 10 2) as a function of the relative (to the total Pd contenO content of Pd extractable by acetylacetone under standard conditions (most likely Pd"+); (~) Mg promoted, chlorine containing Pd/Si02 catalysts; (m) promoted, chlorine free; (o) La promoted, yield of all products (o) La promoted, methanol yield [49]
n §
There is abundant evidence that with many catalysts there are also formate groups present on the surface, sometimes, as with copper catalysts, as the only detectable intermediates and sometimes accompanying formyl groups [50]. If an equilibrium such as that indicated by scheme III is possible, then it would be extremely difficult to establish which of the two groups is the primary intermediate formed directly from syngas and which is the reactive intermediate. In any case, the suggestion that Pd n+ (probably Pd(I)) is the site at which methanol is built up from adsorbed carbon monoxide with atomic hydrogen produced on palladium metal seems to be very well supported by various pieces of evidence. Additional indirect support comes from the fact that a similar relation as in figure 3, has been established for several other metals [43,44,53], as for example figure 4 shows for ruthenium.
H
H
\ C=O
Mm"O
I
pdn+OM m§
\ C=O
Mm'' 0
I
Pd n+ OMm+O
Scheme III HCO-group adsorbed in a form of formyl (left) or formate (right) on a catalyst with Pct"+ stabilized by a promoter (for the sake of clearity, charges on oxygen sites are not shown).
688
chapter 14
figure 4 Relationship between extracted Ru "+ and methanol formation rate at 353K [53]
9
3
--0---
-
0 i
E
i
s
i
'~
2-
".2.
1
t
!
-
!
I
i
i
cO
-r 0
it 0
0 ~ 0 Extracted
I
0.5
I
1.0
I
1.5
Ru n+ / 10-/~ m o l - g -1
Koningsberger et al. [54] prepared catalysts from alloys of iron with other metals, e.g. iridium, which do not dissociate carbon monoxide well. It is an attractive explanation that in this case Fe n+ is the site on which methanol is constructed, the hydrogen atoms being produced by iridium. Finally, it should be mentioned that oxides of various metals (Fe, Co, Zn) can be active in methanol synthesis without a metal in zero valency state, but then the temperature of synthesis must be high and thus the pressure of syngas must be high also. All results now available can be self-consistently explained by the model just described: M n+ is the active site on which the methanol molecule is built up while the metallic component (M ~
in the nearest neighbourhood supplies atomic hydrogen.
However, it has to be mentioned that an equally self-consistent picture can be suggested based on formate intermediates, the latter being adsorbed on the carrier or promoter. The most interesting and discussed catalysts are the copper-containing combinations, including the industrial Cu/ZnO/A1203 catalysts. There are claims that either the active site is on the copper metal (i.e. a Cu~
[12,55] or that the site is Cu(I) [12],
stabilized by the support or promoter. The first alternative is based on the fact that in CO-CO2-H 2
mixtures the activity was proportional to the metallic copper surface, as
measured before the reaction, while others point to the equally good correlation between activity and Cu(I) concentration [12]. The argument that Cu ~ atoms constitute active sites has been countered by observations [56,57], that pure Cu ~ on pure silica is totally inactive, and that only when oxides stabilising the copper ions are present in the surface does the correlation between activity and metal area [55] hold. It is important to know that supportfree but alkali-promoted copper is active in methanol synthesis and the activity is very well correlated with the concentration of Cu(I) species [53b]. This correlation is shown in figure 5. It was expected that the question as to whether Cu ~ or Cu + is the active site would be resolved by experiments with pure metal single crystal planes. However, such experi-
Syngas reactions
689
ments are only conclusive when (i) the metals do not contain contaminants, e.g. silicon, which in the form of silica can stabilize Cu(I) or palladium ion, and (ii) in the direct neighbourhood of the single crystal planes copper or palladium, there are no ions which could serve as active sites, such as those of tantalum, iron or chromium. These ions can be present in the single crystal holders or be brought incidentally as a carbonyl onto the surface of the metal. So far it has been found by one group that single crystals of palladium were active [58] but copper crystals were not [59]. Another group claims, however, an activity of pure single crystal planes of copper [60], so that the problem is not yet solved to everybody's satisfaction, neither is it known what the source of the discrepancy is.
">
.~
0
u T
0 Q;
o
10
E
:3_
u
i
Cs
Lo ~ N0 0.00 Cu §
I
0.02 Surface
figure 5 Rate of methanol synthesis as a function of Cu + concentration in catalysts promoted by the alkalis indicated. This correlation
I
0.04 Mole
I
0.06 Fraction
0.08
should be compared with that for Pd and Ru and qualitatively the same result has also been obtained for Rh [43,44,53b].
14.1.3 The role of promoters Many additives have been found to promote the performance of catalysts in syngas reactions, among them the alkali oxides [61] and transition metal oxides [62]. The role of these additives seems to be different in the different reactions of Scheme I (see section 14.1.1). Introduction of additives can be effected by impregnation (co-impregnation or secondary impregnation), by deposition from the gas phase or by using alloys as precursors. We shall return to the last point below (section 14.3.4).
Methanation The oxides [61,62] mentioned above are all suited as promoters in this reaction. There is some unity of opinion only on the question of what is promoted: most of the authors agree that this is the rate of carbon monoxide dissociation, the acceleration of which is achieved by stronger back-donation of electrons from the metal into the carbon monoxide antibonding orbitals. Controversies exist concerning the answer to the question,
how is the carbon monoxide dissociation promoted? The list of ideas concerning promotion effects comprises at the moment the following points, illustrated by examples of
690
chapter 14
relevant references. (1) The promoter donates its electrons to the metal which is then more easily able to donate then into the antibonding orbitals of carbon monoxide [63]. (2) A promoter or a support receives or gives away electrons into the metal, thus changing by the orbital occupation of the surface metal atoms. This makes the metal atoms behave differently [64]. (3) Promoters change the reactivity of carbon monoxide by changing its dipole [66] or occupation of carbon monoxide antibonding orbitals [67,68], in both cases by an electrostatic field interaction through the vacuum (gas) side of the catalysts. (4) The promoter forms an intermediate with carbon monoxide, the latter being weakly bound through its oxygen to the cation of the promoter [69,70]. Solid states physics does not seem to support the ideas presented under (1) to (2): this has been repeatedly shown in several reviews [71-73], but there is good theoretical and experimental support for the ideas under (3) and (4).
Higher hydrocarbonformation Promoters stimulate dissociation of carbon monoxide (see above) and enhance thereby the supply of carbon atoms to the metal surface. This means that the surface concentration of CHx-monomers is then higher and a greater rate of formation of higher hydrocarbons ensues. At the same time, alkalis suppress the presence on the surface of hydrogen atoms and reactive desorption by hydrogenation is suppressed [74]. When the concentration of promoters is too high, formation of waxes or even of unreactive graphitic carbon can prevail. Thus, there is always an optimum concentration of promoters, different for different reactions and metals as catalysts. Promoters can positively influence the lifetime and reactivity of carbon atoms on the surface [75] and in this respect one has to think mainly of transition metal oxides (V, Nb, Ta, Th, Ce, etc.).
Higher oxygenate formation Promoters have been proven to facilitate the hydrogenation of the R-CH=O groups [76]. To be able to perform this function, they have to be closely associated with the metal (being for example on the metal) [40] and activate the oxygen atom of the C=O group [77]. A very small amount of a promoter is enough to exert this function; sometimes the contaminants present in commercial silica or alumina supports are sufficient to make rhodium a good catalyst in the title reaction [39,40] without addition of other promoter.
Methanol synthesis Presence of promoters (alkalis, ZnO, transition metal oxides) in catalysts containing palladium, platinum, rhodium or copper is extremely beneficial [56]. Those who are convinced that positive ions (Cu(I), Pd(I) etc.) are essential for methanol synthesis would say that promoters primarily function by stabilizing Cu(I), Pd(I), etc. against reduction by hydrogen. Scientists who prefer to regard metal atoms as active sites would look for the
Syngas reactions
691
explanation in one or other electronic effect (through-the-metal or through-the-support influence) or would recall the function that the promoter can have in activating carbon monoixde on the vacuum side of the catalyst [57]. With the attention which this problem still enjoys in the literature one can hope that we shall learn more about it in the near future. In a paper on the nature of the active sites [80], the ideas expressed above have been put together in a tentative picture of the working surface of a promoted metal. This schematic picture is shown in figure 6. It is indicated here that, for FTS, the site adsorbing and activating carbon monoxide strongly must comprise atoms of the two outermost layers (see chapters 8 and 9). For methanol synthesis this scheme assumes that unreduced ionic species are the active sites. We may add a few words about the way in which a promoter appears on the metallic surface. First, a metal can be co-impregnated or post-impregnated (secondary impregnation) by a promoting compound or its precursor. However, the promoting compound on a metal can also be created from the support. For example nickel on titania, when reduc, ed at elevated temperature, becomes covered by T i e x species (strong metal support interaction discussed in chapter 6), etc.
/ /
'/' 'x§
0
CH x CmHn I *I -" A Chain growth
...d "'.22" l "I
Dissociation
Mete I M
x
/u
(C § H----~ ( / ~ y d
)
/1
figure 6 Schematic form of active sites, upper part: a model of a surface active in synthesis of C1 and C2+-oxygenates. Base - an active metal particle M (Rh, Ru, Co...) attached to support (not shown). Blocks of promoting oxides (transition metal oxides, as
optimal for C2+-oxygenate formation, basic metal oxides, as optimal .for CH30H synthesis) are shown, the black spots indicate the position of the active metal ions (M n+) stabilized by the promoter. A vertical cut through the active sites (black)for carbon monoxide dissociation, embedded in an inactive metal. Carbon monoxide in the steady state stays perpendicular to the surface and only when activated towards dissociation is its oxygen tilted towards the surface. It should be remembered that CO is mobile and can migrate over various sites; therefore a site-adsorption is only observed by the fast vibrational or electronic spectroscopies but not by slow scanning tunnelling microscopy.
692
chapter 14
14.1.4 Some of the ideas behind the work with alloys Some of the alloys used in laboratory experiments in the recent past have been tested in the hope of finding a better catalyst by a 'trial and error' method. However, in many cases, there was some additional idea behind the combination used. Let us illustrate this with some examples. Figure 7 shows on the left values for methanation turnover frequency obtained by Vannice [15,32] plotted as a function of heat of adsorption of carbon monoxide. On the right are the same results, but now replotted [78] as a function of heat of formation of the highest oxide, expressed per oxygen atom. Volcano-shaped correlations when observed give always a hope that the right mixture of catalyst components would be the optimal catalyst, and the combinations of metals close to the maximum are therefore the most
! co
attractive ones. co
~oo~
N"
3Fe
Ni
10
to
1
Pt
0.1
~,Pd
o r
Z
0
10
h Ic~r P d
if)
T
7
~ Fel
RuI:~/
0
.
X
...-~----~c o
i
....,
cJ
m
100
co
x
0.01
I
Z
Cu 0.001
m~
60
I
| I 100
l I 140
CO heat of a d s o r p t i o n { k 3 / M o l )
Pd
0.1 ~
0.01 I 0.001
,
20 -AHf
,
4w0 O
I
6'0
(k3 /Mol )
figure 7 Volcano-shaped activity patterns. Left: activity results by Vannice as a function of carbon monoxide heat of adsorption; Right: the same results plotted [78] as a function of the Tanaka-Tamaru parameter (heat of formation of highest oxide of the given metal per oxygen atom).
For example, suppose one wanted to achieve a catalyst that would produce shorter hydrocarbons, through an enhanced hydrogenation activity of an alloy. This improvement was expected from addition of platinum or palladium into cobalt or iron. The same improvement could also be expected from, for example, copper in cobalt or iron, but of course the price in this case would be a much lower total activity. The combinations ruthenium-iron or ruthenium-cobalt could be expected to be free of the last shortcoming and, moreover, one could expect a suppressed deactivation because of carbidization.
Syngas reactions
693
Further, attempts have been made by alloying to decrease the sensitivity of the common FTS or methanol synthesis catalysts to sulfur poisoning. The use of molybdenum, tungsten, nickel or palladium have been suggested to this end. We shall see some of the results of these attempts in the coming sections.
14.2
Alloys in Fischer-Tropsch synthesis" combinations of active and inactive metals. Even the early literature mentions combinations such as iron-copper or cobalt-
copper, but then one mainly speculated on the influence that the added copper had on the reducibility of the active metal [79]. Later, the title alloys were used to establish the role of ensembles in methanation and in FTS. Alloys with components known not to perturb each others' electronic structure were the most suitable systems for the study of this question. As already mentioned, results obtained with nickel-copper alloys lead most straightforwardly to the conclusion that the activity is suppressed by copper more quickly than the concentration of nickel in the surface falls [18,23-25] and thus large ensembles are obviously necessary. The results showing this can be seen in figure 8. Discussion in chapters 8 and 9 shows that the most active ensembles probably comprise atoms from two outermost layers of the metal.
A
0 b
0.2-"
"b-4> - o- 3
$2+
a___o_
-2
_ 0.1-
3
_
-4~1
o
l
i
I
Cu(at
I
!
%)
8'o
|
O-
2 I
s2o s;o T,K
figure 8 Right: activity (rate in arbitrary units, logarithmic scale) as a function of alloy composition, T=593K, I arm., standard conditions; powdered alloys prepared from carbonates. (1) formation of C2 and C3 hydrocarbons; (2)formation of methane; (3) results obtained with evaporated films (r = 573K, total pressure 0.6 Torr) [23]. Left: comparison of selectivities in Cshydrocarbon formation over three unsupported powder catalysts (ex.carbonates). (1) pure Ni; (2) 3% Cu-Ni alloy: (3) 10% Cu-Ni alloy. Notice the different influence of alloying on the selectivity at low and high temperatures
[251.
694
chapter 14
It is interesting from the practical point of view that, when the formation of hydrocarbons (that is essentially carbon monoxide dissociation) is suppressed, catalysts containing nickel-copper or copper-cobalt can be further modified by the presence of promoters to become catalysts for higher alcohol formation! These alloy catalysts, containing added alkalis and/or TiO 2 or another similar oxide, have already been successfully tested in a pilot plant for a process producing the so-called fuel-alcohols [26]. The systems ruthenium-copper and ruthenium-gold have also attracted the attention of a considerable number of investigators. By their main features the results confirmed what has been seen with nickel-copper alloy catalysts [24,81-88]. The necessity for the presence of large ruthenium ensembles for fast FTS was confirmed by some papers [24a,81-86], but a closer inspection of details revealed that we meet the following problems with these alloy systems, as with hydrocarbon reactions (see chapter 13). When the ruthenium particles are very small, there is not much difference in the observed phenomena in comparison with nickel-copper catalysts. In small ruthenium particles, copper placed anywhere partitions the surface into even smaller ensembles. However, when the ruthenium particles are larger and flat planes develop, one observes a very different behaviour. The first atoms of copper or gold added to ruthenium appear on the defect sites, edges, etc, causing a dramatic change in activity and selectivity. However, when these particular sites have been occupied, copper or gold form islands on the flat planes of the particles, causing almost a linear decrease in activity and probably no change in selectivity [88]. The use of supports such as magnesia can cause further complications. Such a support can be transformed into another compound by the precursors (e.g. into MgC12) and, upon drying during the preparation, the oxide formed can capture Run+-ions and prevent their reduction. Stabilization of ions against reduction can have consequences for the catalytic behaviour tested subsequently. According to some workers [53,80] stabilization of Ru n+ should open the pathway to methanol synthesis and induce in this way the formation of other products (including methane) from methanol or its precursors. Cobalt-copper and iron-copper alloy catalysts also behave very similarly to the other catalysts discussed above [23,26,79,89]: copper suppresses the activity but in some cases (higher temperatures) there might be a slight enhancement of the higher hydrocarbon formation [89]. We shall turn to other properties of these combinations below in the discussion of copper-based catalysts. It has already been mentioned in chapter 5 that the effect of lowering the size of metallic particles sometimes has a similar effect to that of adding an inactive metal to an active one. Indeed, in agreement with this general idea, it is reported that for ruthenium [90,91,93] and nickel [92,93] diminishing of the particle size leads to a decrease in the rate of FTS. The link between alloying and particle size effects could be carbon monoxide dissociation, which like that of the C-C bond, needs large ensembles. Small ensembles, as
Syngas reactions
695
we discussed elsewhere, are less suited to form metal-carbon multiple bonds, which formation is necessary for fast dissociation of the C-O or the C-C bond.
14.3
Alloys in Fischer-Tropsch synthesis: combinations of two active metals
14.3.1 Iron-ruthenium alloys After having looked at figure 7 we understand immediately why this combination is so attractive. Moreover, it can be remarked that ruthenium deactivates more slowly than iron, and that this should allow the catalyst to work at higher temperatures where the formation of the desired lower alkenes would become more favourable. A particularly large volume of valuable information on preparation, characterization and catalytic properties of Fe-Ru/SiO2 and Fe-Ru/A1203 catalysts has been produced by the group of Guczi [94-99] and by Niemantsverdriet et al. [100]. Iron-ruthenium is not an easy system to work with. It can be expected [101] that a mixture of particles of different compositions will arise whatever preparation technique is used. Iron oxide is more difficult to reduce than ruthenium compounds, so that its presence in the operating catalysts is very probable. Nevertheless, by application of various techniques a quite detailed picture has been obtained of this system. There is certainly some mixing of the two metallic components in the zero valence state [97-100] and the admixture of iron to ruthenium increases the dispersion of the alloy particles. [100,102,103]. It has been reported that Fe-Ru catalysts indeed show a lower selfpoisoning and a somewhat higher yield of low alkenes [99,104]. These catalysts can be in principle further modified by addition of promoters. These and similar alloys can be conveniently prepared from carbonyls with the use of good variety of supports. Some preparations are described in chapter 6. 14.3.2 Iron-cobalt, iron-nickel and other iron-containing allloys. Another combination of two metals of very high activity is that of iron with nickel or cobalt [89,105-109]. With iron-cobalt, solid solutions can be prepared with zero up to about 75% cobalt in iron and from zero up to about 10% iron in cobalt. Mixing on the surface of silica has been proven to occur [109]. It has been reported that alloy catalysts show less carbide formation [109] and more alkene formation [89], as compared with pure metals. In the iron-nickel system solutions exist in regions of up to about 15% nickel in iron and up to 15% iron in nickel. With these alloys, the same can be expected with regard to carbide suppression as with iron-cobalt alloys. In syngas reactions cobalt differs greatly from nickel. While the first metal shows,
696
chapter 14
at almost all reaction conditions, a tendency to form the highest fraction of higher hyd:ocarbons (a high value of ~ in the Flory-Schulz-Herrington-Anderson equation), nickel is vice versa mainly a methanation catalyst. It is then interesting to see that the formation of alloy particles, proven to be possible [ 110], does not lead to such dramatically different catalytic results as one would expect. This can be seen from Table 1. table 1 Product distributions in the syngas reaction over iron, cobalt, nickel and their alloys [89].
cat.
% total
Mole percent of hydrocarbons
con-
version
El
C2
C2=
C3
C3=
C4
C5
Fe-K-1
2.6
62.5
7.3
5.9
2.5
11.8
6.4
3.6
Fe-2
3.3
63.1
8.0
5.1
3.1
10.4
6.8
3.6
Fe-1
3.3
63.5
8.3
4.6
3.0
10.6
6.4
3.7
Fe-Co-
2.0
67.5
4.3
7.7
1.2
10.5
5.7
3.1
3.8
67.5
8.6
2.9
3.8
8.8
5.9
2.5
Co-1
3.1
79.3
4.7
1.8
1.5
7.1
3.9
1.8
Ni-1
2.3
82.8
6.9
trace
4.3
2.5
2.6
1.1
2.3
84.3
7.1
2.0
3.4
.,
1 Fe-Co1
0.11
All catalysts were prepared by impregnation of Davison 62 silica with loadings as indicated. The table demonstrates some potential (a limited one) for these alloys to enhance alkene yields and figure 9, which summarizes the effects of alloying on the constant c~ of the Schulz-Flory-Herrington-Anderson equation, shows the effect of alloy composition on the production of higher hydrocarbons [105]. Interesting are also combinations in which another metal from the platinum group is added to iron or cobalt. Such a metal will be a good hydrogenation catalyst, but will not dissociate carbon monoxide easily. These combinations have been tested with the aim of
Syngas reactions
697
preparing catalysts giving a high yield of oxygenates. Indeed, catalysts with a quite high selectivity for methanol synthesis have been prepared in this way [54,111]. Combinations of iron-palladium and iron-iridium were thus successful in this striving, but iron-platinum catalysts did not seem to produce methanol at all [113]. On the other hand rutheniumplatinum alloys do so [114]. With iron-palladium alloys, formation of alloys was confirmed by M6ssbauer spectroscopy [111,112] and the activity in methanol synthesis is ascribed to these alloys [111] It was believed that the change in the electronic structure induced by alloying made these alloys good methanol synthesis catalysts. However, lanthana had a similar effect on palladium as iron had and in this case [112] the activity for methanol synthesis was attributed to the presence of species such as PdLaOx perhaps containing Pd +. With palladium-iron in zeolite catalysts [111] the presence of iron ions was clearly seen, but since their concentration did not correlate with methanol synthesis activity, the activity was ascribed to the alloys and not to the ions. Very close to these systems are the Pd-Ru/SiO2 catalysts [115], the results for which seem to show a smooth transition from activity for hydrocarbon synthesis to activity for methanol synthesis [116a], (figure 10). "to
figure 9 Chain growth probability as a function of alloy composition [105].
0.8
13.
o
0.4
.c_ tO As. (.9
o
0 Fe
50 mol%Co
100 Co
50 100 50 100 m o l % N i Ni mol% Fe Fe
(
JE
figure 10 Activity of Ru-Pd catalysts in CO hydrogenation as a function of their composition at 523K. (1) in CHsOH synthesis; (2) in synthesis of CI-C6 hydrocarbons.
O
E
1 >
&
0
0
50
A t o m i c % Pd
100
698
chapter 14
Palladium in NaY zeolite, either by itself or in combination with nickel, is active in methanol synthesis [ 116b]. 14.3.3 Cobalt-containing alloy catalysts Cobalt-containing catalysts have already been mentioned above, mainly in relation to the classical FTS of hydrocarbons. Let us now look at some combinations not yet mentioned which have been studied with the aim of preparing catalysts for the production of higher oxygenates. In this respect the cobalt-iridium alloys have been found to offer some advantage, but the production of oxygenates obviously depends very much on the way the catalysts are prepared; the course of the induction period [117,118] is important too, as well as the presence or absence of additional promoters, such as alkalis. Similarly combinations such as cobalt-rhenium and cobalt-ruthenium have been studied with and without additional promoters [119,121]. An increased selectivity to oxygenate formation has been reported for some of these catalysts. Cobalt alone, provided it is promoted, can similarly to iron produce higher oxygenates without any other transition metal being present. This has been known for a long time [122]. It has been mentioned above (section 14.1.3) that the role of promoters in higher oxygenate formation is to promote formation of acyl and similar higher groups and their hydrogenation. With a metal reluctantly dissociating carbon monoxide, promoters can also stimulate its dissociation. On the other hand a promoter can tune down the dissociation power of metals such as iron, cobalt, nickel or ruthenium by dividing their surface in smaller ensembles. Such promoter species can also be produced from the combinations mentioned above, e.g. with binary alloys, such as cobalt-ruthenium, cobalt-rhenium etc. The promoters' role can be played then by compounds such as NaCoOx, Na(RuCo)Ox, etc. Thus it is not clear whether with alkali-promoted cobalt catalysts or with (Co+Re)A1203 catalysts we are really describing them well by calling them alloy catalysts. In any case they are fully alloyed and in the metallic state only before the reaction.
14.3.4 Other alloys and pseudo-alloys A great stimulus to research on rhodium-iron alloy catalysts was given by the paper by Bhasin et al. [123a], of the Union Carbide Laboratory. It is now known that promotion of the reaction into the direction of oxygenate formation is not due to Fe ~ in alloys but due to iron oxides present under reaction conditions [123b]. It is sure, that in rhodiummanganese or iron-manganese catalysts the manganese component is in an oxidic form [124]. For rhodium it is also known for sure [44] that rhodium ions, so beneficial for methanol synthesis, are not required for higher oxygenate formation, provided there are
Syngas reactions
699
oxides of another transition metal to take the role of promoter [125] in hydrogenation of aldehydic intermediates. Perhaps, in the absence of the second metal, rhodium silicates, rhodium oxides or other rhodium compounds can play the role of promoter to the rhodium metal. The chance to produce in situ transition metal oxides is particularly important when carbonyls or other similar clusters are used as precursors in catalyst preparation [ 1261127]. When alkalis are used as promoters, compounds of rhodium or iridium can also be formed, as has been observed with e.g. promoted alloys such as Rh-Ir (Mn, Li)/SiO 2 [128] and with other combinations (e.g. Rh-Ag) [129]. Active carbon can be, with its quite inert surface, an interesting support under reducing conditions, and indeed papers have appeared which indicate it [130,131]. table 2
Alcohol production with various alloy catalysts, carbon monoxide" hydrogen = 1" 1, P = 404 kPa, GHSV = 9420h -1 [132] Catalyst
Ru-W Co-Mo Co-W Rh-Mo Rh-W
mole % CH3OH 523K 573K
CH3CH2OH 523K 573K
3.5 2.5 5.3
2.3 2.8 1.2 28 28.4
9.1 5.4
16 1.0 0.4 31.0 18.7
We have already discussed above the situation of an alloy catalyst being active in oxygenate formation, thanks to the presence of a promoter, the second metal being there just to suppress some undesired activity, such as too fast dissociation of carbon monoxide [133]. There are more catalysts that can be added to this group, but typical examples are catalysts containing molybdenum or tungsten. Two tables from a paper by Foley et al. [132] are representative of the results obtained. To explain them the two-site model schematically shown in figure 11 was suggested. This scheme acknowledges properly that unreduced molybdenum compounds are present, but experience with other systems suggests that Rh(8 +) MoO x should also be o___n_nthe rhodium surface, not only on the support. Results contained in other papers [134a,b] would also be in agreement with such modified picture.
Obviously with rhodium-molybdenum, rhodium-manganese
[133,134d,e]
and
perhaps also with rhodium-iron, the catalysts for oxygenate formation under operating conditions are actually
oxide-promoted metals rather than just zero valent alloys. The
same holds for the intermetallic compounds to be discussed in the next section. For some time iron-manganese catalysts were considered as very promising for alkene production [ 134c,d,e].
700
Ruthenium and alkali-promoted ruthenium catalysts [ 1321
table 3 Catalysts
T (K)
P GHSV @Pa) (h.’)
Ru/A120,
523
101 404 101 404 101 404
573 Ru-W-Nd A1203
523 548 573
Ru-W-W
523
A1203
548 573 Ru-W-CS/ A1201
523
573 rI = moles carbon
2400 0.6 9420 4.1 2400 1.1 9420 2400 0.7 9420 0.7 101 2400 2.2 404 9420 1.9 101 2400 5.9 404 9420 2.5 101 2400 1 404 9420 0.9 101 2400 3.3 404 9420 3.7 101 2400 7.9 404 9420 11.5 101 2400 1.5 404 9420 I .8 101 2400 5.7 404 9420 3.3 101 2400 14.7 404 9420 7.9 monoxide converted x (gc0p).’
g cat.
rl
0.33 0.33 0.33
8.04. 5.494.10.’ I .474.I 0.’
1.55 1.58
0.38 0.44 0.29
0.37 0.37 0.37 0.37 0.37 0.37 0.37 0.37 0.37 0.37 0.37 0.37 0.36 0.36 0.36 0.36 0.36 0.36
2.5605.10-’ 1.0242. 8.0472. lo-’ 278. 21581 36578 3.6578.10.’ 13168 12071 54136 28897 16826 1 5.6392.10.’ 2.7068.10-4 21429 49625 55264 11.8798
7.53 4.35 7.01 5.91 2.88 1.17 6.8 4.17 7.13 5.9 3.42 3.08 53.8 4.08 2.22 3.57 1.74 1.52
1.58 1.08 0.97 0.64 0.58 2.64 I .73 0.9 0.92 0.59 0.59 0.52 0.03 0.67 0.58 0.5 1 0.46 0.55
chapter 14
548
%carbon monoxide conversion
Syngas reactions
figure 11 Two sites model for reduced 3% Rh-Mo/TAl203 catalysts [132].
701
CO.H 2 CO.H 2
Hydrocarbons
Oxygenates
14.3.5 Intermetallic compounds as precursors of catalysts for syngas reactions Nickel and cobalt form with rare earth elements a series of intermetallic compounds; they decompose in the presence of syngas into nickel and cobalt metals and a mixture of compounds in which oxides dominate [135,136]. The same happens with various intermetallic compounds containing zirconium, such as nickel-zirconium compounds [137] and some others [137-144, see also chapter 7]. Probably most studies have been performed with intermetallic compounds of copper, because these lead to very active catalysts of high density for methanol synthesis. For this reaction compounds of cerium have been used [138] as well as those of zirconium [137,141,142], thorium [143] and titanium [142] and various rare earth and Group 5 metals [139,140]. Compounds of rhenium with silver and gold have been investigated too [138]. Table 4 below illustrates the results obtained with three-component catalysts: zirconium-rare earth- copper. Figure 12 shows typical results illustrating the development of activity with time; this course by itself indicates that changes in phase composition are caused by interaction of the intermetallic compound with the reaction mixture. Figure 13 shows the XRD analysis of the changing composition with the niobium-copper intermetallic compound. We observe here that with increasing exposure the intensity of diffraction peaks for oxides and hydrides increases. The considerable activity that is sometimes observed with catalysts prepared from intermetallic compounds of gold is surprising. This has been found both for hydrocarbon synthesis and for methanol synthesis from carbon dioxide [145]. Copper supported on zirconia by classical methods is also active in methanol synthesis. However, this is not true for gold or for a catalyst prepared from the goldcerium compound, when carbon monoxide is the reactant [146].
702
chapter 14
table 4 Methanol activity of Zr/RE/Cu alloy derived catalysts activated at 50 bar and 513K RE component
composition
RE mole fraction
methanol activity (mol/kg.h) a Rm R20
Zr:
RE:
Cu
Nd Nd Nd Nd Nd
0.8 0.8 0.7 0.6 0.6
0.2 0.2 0.3 0.4 0.4
2.0 1.5 2.0 2.0 1.5
0.067 0.084 0.1 0.13 0.152
15.2 7.8 11.8 15.5 9.7
9.2 3.6 8.8 8.3 6.0
Dy Dy
0.7 0.7
0.3 0.3
2.0 1.5
0.1 0.12
16.7 12.2
9.5
Y Y
0.95 0.85
0.05 0.15
1.5 2.5
0.02 0.05
11.8 14.4
11.0 10.3
La
0.7
0.3
2.0
0.1
10.0
6.8
Sm
0.7
0.3
2.0
0.1
11.5
6.2
Yb
0.7
0.3
2.0
0.1
10.5
9.8
activation energy b (kJ/mol)
51.5
8.0
57.3
48.1
a) measured at a space velocity of 36 000 h1 b) apparent activation energy for methanol synthesis R m = maximum rate observed, R~o = rate after 20 hrs. on stream
2O t-
.at
.~
~ .....
'6 E
v
~= >_ ~o i(,.) <
_J o z <: -1i--
D ...
'
... ...
5 /
z
&"
0
,
,200
1 400
TIME ON L I N E ( M I N )
figure 12 Comparison of the activation profiles of three alloys of general composition with binary allow ZrCu 2 950 bar, 513K) (*) x=l" (o) x=l.4; (El) x=2; (A) ZrCu (2) [140].
Zro.7Gdo.3Cu
x
Syngas reactions
703
9 Ox tcle
-
Hyclr" :i d . . . . . . .
figure 13 Synthesis gas activation of NdCu precursor. Increasing vertical offset of XRD patterns corresponds to increasing exposure (O-18h) at 15 bar/423K. (Asterisks indicate the aluminium sample holder [139a].
"1 t'-]
Cu
, t ILIIi
40 TNO-THETA
'
60
'
80
(DEGREES)
14.3.6 Promoted and alloyed copper catalysts Whatever the mechanism of methanol synthesis, we believe in, we know for sure that Cu/ZnO/AI203 is an excellent catalyst for this reaction. When required to be used for synthesis of higher oxygenates (i.e. production of fuel alcohols) it has to be promoted and C-C bond formation has to be stimulated. This has been achieved by either a) promotion by alkalis, of which caesium salts are the best [147] or b) by adding to copper metals which can stimulate dissociation of carbon monoxide, such as cobalt or nickel [26,148]. 14.3.7 Interstitial compounds of iron and cobalt It has been known for some time that iron modified by the presence in it of nitrogen, that is, iron nitride, shows an enhanced selectivity to higher oxygenates [9,10]. Their surface layers are converted in use into carbides and, perhaps, iron and carbonitrides [49,150]. Cobalt, iron and nickel all form borides which can also be used as catalysts for FTS [151-154]. Borides are easily prepared by, for example, reduction of iron acetate in tetrahydrofuran by diborane, and the stoichiometric composition of the boride can be reached. These catalysts sometimes show only a temporarily higher activity than the pure metals and a clearly higher resistance to sulfur poisoning. Catalysts containing cobalt and boron show a respectably high selectivity to alkenes. The beneficial effect of boron is thought [151-154] to be due to electron donation from boron to the transition metal atom, by which electron shift the metal-to-carbon monoxide back-donation and hence its dissociation should be facilitated. Sulfur is not only a serious poison of cobalt and iron catalysts, but it can also show some beneficial effects [155]. Sulfur on the surface is beneficial for alkene formation. When a suphide-containing catalyst (e.g. MoS2) is alkali-promoted, it can show very interesting activity and selectivity in methanol and higher oxygenate formation [156-158].
704
14.4
chapter 14
Industrial processes with syngas
The reader is referred to specialised reviews and monographs for all details of the technologies based on syngas as the raw material [159-162]. Below, just as an example to illustrate the potential for using of bimetallic catalysts, two existing industrial processes will be briefly described: synthesis of hydrocarbon (Diesel fuels) and synthesis of methanol. Syngas is however also used in reactions with alkenes for hydroformylation (or processes based on hydroformylation) and numerous other homogeneously catalysed reactions and, prospectively, also in various polymer productions. Nevertheless, we shall confine ourselves to the first two processes. Since the first design of the industrial process [3,4,163], the catalyst for liquid fuel production from syngas has undergone a considerable development (actually from cobalt [163] to iron [3,4] and back to cobalt [164] as we shall see below), and the reactor was also improved. With the latter, the great problem is the exothermicity of the reaction (about 145 kJ/mol per carbon atom built in). This necessitates the fixed bed reactor to be of a multitubular form, and a typical reactor in industrial use has 1500-2200 tubes. Other ways of handling the problem is to use a fluidised-bed reactor or a slurry reactor. In the last case, the catalyst is suspended in molten high molecular weight waxes, which can also be produced by FTS under different conditions. A reactor for FTS forms a part of a complex network which is shown in figure 14. The scheme shown is that of Sasol I, a plant in South Africa, which uses local coal for the production of syngas. The fixed bed reactor filled with alkali-promoted catalyst forms only a very small part of the whole investment; the most expensive part of the process is always the production of syngas itself. More recently designed productions of liquid fuels are based on natural gas (CH4) instead of on coal. Natural gas is first converted by partial oxidation or steam reforming into syngas. Whatever the way of syngas production, it is always the most expensive unit of the whole plant. The more modem Sasol II plant combines syngas from coal with that from methane since the two routes provide different CO/H 2 ratios. Sasol II operates with a fluidised-bed reactor, shown in figure 15. While fixed-bed reactors can make use of the cheapest form of the catalyst, namely alkalised fused iron, the fluidised-bed reactor requires a catalyst in form of light particles for example, as produced in situ from co-precipated iron and other oxides.
Syngas reactions
705
Air
Coal
Coal Water
plant
FT
t
I
[ >
Rectisol
~__~C02. H2S
lw~ -uP i---~BTX
I Alcohols Ketones I
I1_ . ] Reformer
I-
I
L work - upj
-" 1 H JOryogenic L ~ , m m o n , ~ ~ - I L syn. /
CH, '
l-Tar 8 oil [---~Creosote
[
L Naphtha
rcl og. ark-up 9 _ I F[I ~o,~,~t
Town gas
plant I
Separation
( NH4)2SO 4 Phenols
t
I
I -'1
Phenosolv
figure 14 Flow scheme of liquid fuel production Sasol I [161,162].
~
i
- I
lw~
0il
~
Oligo-
. Gasoline
i
Fuet Gasoline
o,,
N2 NH3
TAI L GAS ....----
CYCLONES S E T T L I N G HOPPER
HEAT EXCHANGERS
CATALYST
STAND PIPE
SLIDE V A L V E
- -
FEED GAS
figure 15 Reactor with transported fluidized bed as used in Sasol II [162].
GAS AND C A T A L Y S T
706
chapter 14
The most modem version of the FTS industrial process is the Shell Middle Distillate Synthesis. By this synthesis, aromatic-free high quality Diesel oil is produced from natural gas occurring in very remote locations (e.g. Malaysia) [164]. It is a two-step production combining two well-developed steps . A cobalt catalyst, promoted by oxide(s) from Group 4, is used at such low temperatures and at a suitable pressure/composition of syngas that heavy waxes are produced without selfpoisoning of the catalyst. As figure 16 shows, a suitable catalyst should have a chain growth probability o~ higher than 0.9. Under such conditions one makes the maximum possible use of available carbon and minimizes methane production. The melted waxes are then very selectively cracked down - by zeolites - to the Diesel oil. 100-
80
figure 16 Product distribution for Fischer-
C~ +C 2 fuel gas
~
/ .
C3+C 4 LPG
~ 6o
Tropsch synthesis as a function oJ the parameter alpha, the chain growth probability (Shell -
-
commercial information). o.7~ L., I-"
0.80
o.ss
classical catalyst new catalyst
o.~o
o.~
probability of chain growth, a ~~1 -i Shell -~
development
catalyst
C~H, -
gas
Synthesis"- HPS L__._.___J gas [
H,~
Distillation facilities
HPS J l SGP
9Shell Gasification Process
HPS:
9Heavy Paraffin Process
HMU
9Hydrogen Manufacturing Unit
HGU
9Wax Production Unit
--"--
Naphta Kerosene ---- Gasoil ,,..--
" ~
~'~ Paraffine
WPU-~
Wax Wax Wax
figure 17 Shell middle distillate synthesis process - a simplified flow scheme (Shell- commercial information).
Syngas reactions
707
The flow-sheet of the process is shown in figure 17. It seems that at the moment no industrial plants use zero valent alloy catalysts. However, a small selection of patents [165-168] illustrates that there is some industrial interest in the future use of such catalysts. In particular, when one wants to optimize short chain alkene production [165] or Diesel oil production [166,168], alloying can help. To conclude this section, we will add just a few words on the methanol process [169]. The overall scheme of this relatively simple process is shown in figure 18. The most expensive unit of the whole plant, the production of syngas by steam reforming, is not shown in this scheme. As with FTS, this process also requires expert engineering to design reactors with good regulation and control of the highly exothermic reactions. Figure 19 shows several reactors for methanol synthesis [169]. BFW
BFW /.
5
8
,.Synthesis ,.-
6
9
7
1 ~ l "~
ii -~~ i
, ~ ,
Purge Gas *
j
{Raw Methanol
figure 18 Haldor Tops6e low-pressure methanol process. (1) Synthesis gas compressor; (2) recycle compressor; (3) heat exchanger; (4-6) methanol reactors; (7,8) BFW preheaters; (9) purge gas preheater; (10) purge gas expander; (11) cooler; (12) separator; (13)flash drum. Gas Inlet I Catalyst Charging Manhole ~ Manhole
-''~
"'
"~
Lozenges
Catalyst Gas inlet
Loading rmocouples
III IIIIITCatalyst ~
Catalyst
Catalyst ',,,.,/ \ , . / c a t a l y s t \ j . /
D i scha rge ~ : , ~ ~ " ~ D i s c harg e Manhole " y ~' M a n h o l e _ ~ : ~ Oas Outlet Oas Outlet (a)
(b)
figure 19 Various reactors used in methanol synthesis [169].
~ Oa~slnlet Gas Outlet (c)
708
chapter 14
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10 11
12 13 14 15 16 17 18 19a b C
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chapter 14
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126
Syngas reactions
141 142 143 144 145 146 147 148
149 150 151 152 153 154 155
156 157 158 159 160 161 162
715
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717
EPILOGUE It is customary to finish a scientific publication with a short statement under the title of "Summary" or "Conclusions", which epitomises the intention of the work and the messages it intends to convey. Often we read this part first, as it may with luck save us the trouble of reading the whole paper. With a book the situation is somewhat different: the ground covered is so broad and the concepts and principles described are so varied, that any attempt to summarise what has been said must be doomed to failure. Nevertheless a few general issues have arisen, some of which we must confess are personal "bees in the bonnet", which we should underscore: we do so without apology, knowing that the reader will expect to see the imprint of the authors' minds on what is written. It has not been our intention to impress on the reader our own sense of the importance of things, or so to emphasise our beliefs concerning the proper interpretation of experimental observations, or the manner in which decent research should be conducted, that the value of published work is obscured or lost. True, both of us have strongly-held convictions, and where these shine through, as they inevitably must form time to time, the reader must understand that these are our honest and sincerely held beliefs, and we advance them in the confidence that their acceptance and adoption will help the progress of the science. Where however our interpretation differs from that of the authors, we are usually at pains to draw attention to this, perhaps even too often. What then are the major issues that we wish to focus on? Catalysis, and especially catalysis by alloys, is a subject of enormous complexity: but at the simplest level it is quite easy to prepare an alloy catalyst and to study its behaviour in some novel reaction. To do so is not complicated: the difficulties arise in the interpretation. What is the cause of the effects observed? We have stressed that so numerous are the factors at work that it is usually difficult and sometimes impossible to distinguish them or to evaluate them separately. A good example of this kind of problem is the old argument about whether it is "geometric" or "electronic" factors that determine the catalytic activity of metals. In fact, of course, the two are so closely interwoven that it is meaningless to try to split them. It is the electronic structure that controls atomic radius, but changing the number of valence electrons will alter a great many physical properties besides radius: and who is to say which determines the catalytic activity? One of the major constraints to the advancement of the science of catalysis (and perhaps to some other sciences as well) arises from our necessity to describe concepts by words rather than symbols. Except in a very limited way, one cannot "speak" symbols, or discuss scientific issues without recourse the words - which by their essence lack a
precise
718
Epilogue
meaning or significance. How do we know whether "water" means exactly the same to you as it does to us? More appositely, how do we know that the concept of "electron transfer" means exactly the same to you as to us? Much of the heat that has been generated, at scientific meetings and in the literature, as to whether electron transfer between alloy componentsdoes or does not occur, derives from the use of inadequately defined words. Thus a movement of electrons from one class of orbital to another, under the influence of a changed environment, a change which can certainly happen, may to some be "electron transfer", but not to others. Our conclusion, which can surely not have escaped the reader's notice, is that, for pairs of metals that are not too dissimilar in electronic structure, the concept of "electron transfer" as it is generally understood is not a proper or useful way to account for the observed changes in catalytic behaviour that take place when the proportions of the two are altered in an alloy. However, as their electronic structures become more different, the chance of "electron transfer" increases, so that in e.g. Pt3Ti and PtSn intermetallic compounds, it is clear that some "transfer" (or better- a shift) has taken place (see the Prologue). One therefore has to guard against making ex cathedra statements such as: "electron transfer never takes place": we have to be more circumspect and specific, referring our statements to named systems and defining the criteria by which the judgement is made. As the old proverb says: "In argument there is much heat, but little light". One further example of Humpty Dumptys' precept (see the Prologue) concerns the meaning of the term "alloy". Somewhat to our surprise we found that it was widely defined and used to signify any mixture of two or more metals, irrespective of the number of phases present or whether indeed there was any mixing at the atomic level. We would have liked to have seen and to have used a more exact implication of the word, but we felt that this would only exacerbate confusion: it has the merit of brevity, and that excuses a lot. The recent literature also affords other examples of imprecise or incorrect terminology: sometimes the tragic lack of an education involving either the Greek or Latin languages has led to the use of words that have no valid etymology, and in other instances there may be alternative terms having equal merit (e.g. sulfurisation, sulfidation). It is not our task to pontificate on matters of language or to act as arbiters of good taste in written English: we have however endeavoured to present on material in clear and unambiguous language. Where we ourselves have been guilty of inconsistency, we simply cry "Emerson!", for it was the American essayist Ralph W. Emerson who wrote: "Unnecessary consistency is the hobgoblin of little minds". While on the subject of language and terminology, we must mention our anxiety to employ names of chemical compounds, and units, that are both acceptable and comprehensible: acceptable that is to those who decree what ought to be used, and comprehensible to those older scientists and those reading the older literature, containing as it does information which is still of great usefulness. We believe that some of the names for compounds approved by IUPAC (e.g. ethene, ethyne, ....) are steadily gaining global acceptance, but in
Epilogue
719
a number of cases where it is still difficult to speak the official name, e.g. methanal (formaldehyde) or methanoic acid (formic acid) etc., we have used both names at least once, to provide a kind of Rosetta stone for those who still find the translation difficult. We are however confident that IUPAC systematic terminology and units (K, not ~
J, not
cal .... ) will eventually gain universal acceptance. While on this theme, we have to ask forgiveness for using only the simple alphabet of the English language, which does not use diacritical signs, such as accents, the cedilla or umlaut. We apologise to those whose names are consequently mis-spelt, and ask for their understanding. A second general question which compiling this book has raised in our minds is the danger of half-truth, by which we mean attempting to draw conclusions about mechanisms or the role of the solid state in catalysis on the basis of an incomplete study of the reaction. It is very noticeable how many studies of catalytic systems, expecially those involving alloys, (i) prepare the catalysts by haphazard methods, (ii) characterize them at inordinate length (and cost), and then (iii) spend what appeared to be little more than a morning in assessing their catalytic quality. This is not good science, or a good use of resources, for often it is cheaper and quicker to make catalytic measurements than to perform TEM or XPS. On the basis of such a few measurements, made often at only one temperature and one set of reactant concentrations, a great theoretical edifice of interpretation is constructed. It is of course impossible ever to make a complete study of a reaction, but some attempt has to be made to determine activation energies and orders of reaction before sufficient information is to hand to justify much discussion. The simplest example to illustrate the pitfalls that are possible is shown in the accompanying figure.
In(rate)
D~~
Ol
rate
: \\---.2 103/T(K)
~1
,
2
,
3
4/1
2
I
3
composition
4/1
I
2
3
4
Arrhenius plots for an alloy system: 1 and 4 represent the pure components, 2, 3 are alloys containing 1/3 or % of each component.
720
Epilogue
What is the effect on the activity of adding the second component? The question has no answer, because it is without meaning - unless the temperature is specified. If the reaction is only examined in a limited range of conditions, the derived conclusions have likewise to be delimited. Failure to acknowledge this simple concept is partly responsible for the many disagreements to be found in the literature. For all good chemists, the need to account for what they observe in terms of a "model", or to "interpret" their results within the framework of some currently accepted theoretical paradigm, is over-riding. Nevertheless it is regrettable that some authors appear to lack any profound curiosity concerning cause-and-effect, and are content simply to report what they see, without speculation as the underlying reasons. In making criticisms of this kind we must accept the difficulty mentioned above of identifying any single "cause" in a system of great complexity, however clear the effect may be; and we must accept the late Sir Karl Popper's dictum that it is possible to disprove a hypothesis, never to prove that it is uniquely true. Sensible speculation is however always justified and almost always necessary: pictorial sketches of likely or possible mechanisms are especially useful when they provide some sense of the scale of events at the atomic level, e.g. of the relative sizes of atoms or relative distances involved in reaction steps. Many authors eschew such aids, although whether this is due to lack of artistic ability or to the abovementioned lack of curiosity is not always clear. The word that should be for ever on the chemist's lips is: why? Why does this catalyst work better than another? Why cannot I repeat the preparation? And sometimes even: why did I ever start to study catalysis? Science advances by the iteration between experiment and theory: between stepwise refinement of the model and ever more sophisticated experimentation. The strictest test of a model is its ability to predict: one that make an incorrect prediction has to be rejected or refined, and the new one is then subjected to the tests which will prove or disprove it. This cyclical nature of theory and experiment, which lies at the heart of the scientific method, justifies the need for interpretative models, for without them the next round cannot be played. We build them on a foundation of scientific knowledge and experience that stems from two centuries of endeavour; and this confidence in our basic knowledge and in the validity of the methodology gives the lie to those who believe science is a social construct: scientific knowledge is true, because science works. Although we started with the intention to write a book about catalysis by alloys, and although this is reflected by the emphasis given to alloys in the early theoretical chapters, it soon became clear to us that one cannot build a house starting at the second floor, but rather that one must lay foundations and construct the first floor before seeking to go higher. Complex through catalysis by single metals undoubtedly is, it is simpler than the corresponding phenomena shown by alloys: we have therefore included some quite brief treatments of the theory of the structure of metals and of their catalytic properties as introductions to the text concerning alloys. The size of the corpus of knowledge even on
Epilogue
721
ness that was possible 35 years ago [1] is now out of the question: but we hope that there is sufficient information, and references, included to guide the reader, should he or she wish to pursue any topic further. The bias towards alloys rather than single metals, which is the chief feature of this book, reflects not only the literature of the recent past, but also by extrapolation their likely importance in the future. A great many of the major applications of the metallic state in catalysis involve alloys: we have only to think of petroleum reforming, ammonia oxidation and the treatment of vehicle exhaust to appreciate their very great practical importance. The regions in which alloys have not yet risen to prominence are those that relate to the "fine chemicals" industry, for example, catalytic hydrogenation involving regiospecificity or enantio-selectivity. There are however good indications that, in reactions such as the hydrogenation of unsaturated aldehydes to unsaturated alcohols, catalysts that may be classified as alloys within the broad definition adopted in this book, show distinct promise. Their application to other polyfunctional molecules and to the isolation of intermediate products remain targets for future research. This book is a joint effort: readers may wish to amuse themselves by trying to guess which of us wrote which chapters, but the task should not be hard, knowing the interests and proclivities of each of us. We have read, and commented liberally on, each others drafts and an effort has been made to homogenise the text, in terms both of the use of the English language and of scientific propriety. We hope therefore that it will not be necessity to append the type of comment made in the preface to the book [2] written about 30 years ago by D.O.Hayward and B.M.W.Trapnell. Paraphrased to current circumstances it would read: "The chapters in which the science is sound but the English was poor were written by V.P.; those in which the science is poor but the English is good were written by G.C.B." We end with an apology: here is yet another book on catalysis, and "in the making of many books there is no end, and much study is a weariness to the flesh". Amen to that: but we hope that our efforts will help our readers to chart their ways through the jungle of catalytic literature, or at least that part we have tried to map. If we have made that task easier, our efforts will have been well rewarded.
References
[1] [2]
G.C.Bond, "Catalysis by Metals", Academic Press, London, 1962 D.O.Hayward, B.M.W.Trapnell, preface to "Chemisorption", 2nd ed., Butterworths, London, 1964
722
Acknowledgement The authors express their most sincere thanks to all publishers who gave their permission to reproduce in this book figures and tables published already elsewhere. The authors acknowledge with pleasure the help they received from their respective Universities, Leiden and Brunel (London). The authors feel much obliged to mrs.H.Knegtel, who performed all typeand lay-out work.
723
Subject index
A
absorption, X-rays (EXAFS), 127,167,363 acidity, surfaces of oxides, 324 activation energy, 4,273 activation, by electron transfer, 2 active carbon, as support, 329 Adams oxides, 299, 319,516 adsorption site, 227, 231, 280 adsorption, alloys, 393 adsorption isotherms, classification, 358 adsorption, the role in catalysis, 2,247 aerogel, formation of 326 alkene titration, metal surface area, 488 alkene, exchange, 481 alkene, oxidation, 551, 564 alkyne hydrogenation, particle size effect, 495 alloying (in)sensitive reactions, 439 alloys of" Ni-Co, 423 Ni-Cu, 59,202,405,406,419,439,462,482,489, 491,502,508, 524, 605, 693 Ni-Pd, 489, 610 Pd-Ag, Au, Cu, 60,206,342,402,408,410,459,463,489,498,502,509,561,612 Pd-Cu, 411,425 Pt-Ag, 195 Pt-Au, 62,195,402,407,412,462,619 Pt-Co, 421,646,653,655;AgAu,565,567 Pt-Cu, 60,195,402,340,411,414,462,489,509,524,621 Pt-Ir, 210,499,644,646 Pt-Ni, 209,425,502,517,654 Pt-Pb, 402 Pt-Pd, 525,559,563,566,656 Pt-Re, 60,340,402,409,417,509,628,634 Pt-Rh, 209,556,571 Pt-Ru, 422,429,652 Pt-Sn, 62,212,402,409,426,489,519,659 Pt-Ti, 421 Ru-Cu, Au, 341,408,413,414,417,489,491,614,694,
alloys, list of preparations, 343 alloys, amorphous, 314 alloys, rigid band theory, 2, 28 alloys, mono-, bi-phasic, 4,56 alloys, twodimensional, preparation of 312 alloys, interstitial, 4,313,703 alloys, substitutional, 4 alloys, definition of 4 alumina, formation of 326 alumina, structure of 327 alumino-silicates, 325,328 ammonia synthesis, particle s&e effect, 286 Andrussov process, 303 angle-resolved photoemission, 94 apparent activation energy, 274 Auger electron spectroscopy, 73, 102 autoclave, 262 B
Ballandin theory, 507,394 band broadening, chemisorption, 36,37 band, width, metals, 10 band-bending, 237 bands, stucture of, 97 bands, formation of 10 bifunctional mechanism, 289,605,643,650 binding energy (spectroscopy), 88
binding energy shifts, 160,163,238,414 Block, theorem, function, 8 bond order conservation, 49 bond order, Pauling, 20 Bonhoeffer-Farkas mechanism, 457 Born-Karman, conditions, 8,14 Bragg-Williams approximation, 182 Brillouin zone, 12,15 Brillouin zone, Hume-Rothery theory, 17 Bronsted acidity, 324 Brunauer-Emmett-Teller (BET) isotherm, 358 C calorimetry, alloys, 406 capillary condensation, 359 carbon monoxide, chemisorption of 41'45 carbon deposition, 609
724
carbon, active, 329 carbon monoxide, oxidation of 547,555 carbon particulates, car exhaust, 570 carbonyls, preparation metal catalysts, 345 catalysis, heterogeneous, 1,2 catalysis, elementary steps of 247 catalysis, definition, 2 catalysis, alkanes, 602 catalytic cycle, 247 charge density map, 41,156,165 charge transfer, MOssbauer, 149 charge transfer, metal-support, 234,290 charge transfer, photoemission, 158 charge transfer, calculation of 33 charge transfer, EXAFS, 169 chemical shift, chemisorption, XPS, 85 chemical shift (photoemission), 77 chemisorption titration, 374 chemisorption, definition of 393 chemisorption, alloys, 393 chemisorption, theory of 36,41-50 clusters (metals), ionization potential of 236 clusters (metals), properties of 220 clusters, theory of alloys, 34 Coherent potential theory (CPA), 28 cohesion (binding energy, 176,198 colloids, metallic, preparation of 309 compensation effect, 275,277 complexes, on surfaces (see also intermediates), 51 concentration gradients, 252,511 contact time, apparent, 255 contact potential 150 Continuous Stirred-Tank Reactor (CSTR), 263 conversion, definition of 267 coordination number, 20,281 coprecipitation, preparation of catalysts, 337 core level shifts, surfaces, 91 core levels, shifts of 84,89,160 correlation, hydrogenolysis and exchange, 585 corrosive chemisorption, 545 crystal shape theory, 180 Curie temperature, 370 cyclic intermediates, 52,602 D
d-band, position of 39 d-character, Pauling, 21,22 de Haas-van Alphen effect, 10
Subject index
Debye-Waller factor, effects of 226 decoration model alloys, 616 dehydrocondensation, 680 density of states, calculations, 29,222 density of states, definition, 11 density of state, relation to photoemission, 16,94 depth, hydrogenolysis, 598 diffusion limitations, 252,511 dispersion, metals, 266 dispersion function, 12 double-bond migration, 483 E
egg-shelL egg-white, egg-yolk catalysts, 333 electrocatalysis, 455 electrode-less deposition, 349 electron spin resonance, metal clusters, 223 electron microscopy, transmission, 361 electron density map, 41,156,161 electron deficiency, metal particles, 234 electronegativity (Miedema), 26 electronic structure effect, adsorption, 394, 438,441 Eley, heat of adsorption theory, 48 Eley-Rideal mechanism, 273 emission, X-rays, 167 energy, dispersion function of 12 energy bands, 10,237 energy, activation, 273 energy, cohesion, 176 energy, surface formation, 175 Engel-Brewer theory, 23 ensemble composition effect, 394 ensemble size effect, 394,438,441,607 ensembles, mixed, 442 ensembles, active sites, 438 enthalpy, excess, 182,188 entropy, configurational (mixing), 186 entropy, thermal, 182 equivalent core model 78,86 evaporated metal films, 303 exchange, alkenes, 481 exchange, homomolecular, 449,450,464,583 F fat hardening, 483 Fermi surface, 12,15 Fermi energy, 12,15,16,37,39 Fermi-Dirac function, 16 ferromagnetism, 144,370 field emission techniques, 119
Subject index
films, as catalysts, 303 final state, effects of 77 Fischer-Tropsch, mechanism of 680 foils, as catalysts, 301 Fowler-Guggenheim approximation, 182 fragmentation parameter, 598 fragments, molecular, adsorption of 46 Frank-van der Merwe mechanism, 307 free-electron approximation, 8 fuel cell, 573 G Gallon model, Auger analysis, 109 gas-liquid-solid systems, 261 gauzes, as catalysts, 302,571 glass, metals, alloys, 314 gradients, concentration, 252,511 grain-stabilizers, 301 Green, operator, 28 ground-potential method (photoemission), 79 group orbitals, 38 growth, metal-on-metal layers, 307 H
half-hydrogenated state, 479 Hamilton, operator, 9,28 heat transfer, 250 homogeneity, alloys, 339 Horiuti-Polanyi mechanism, 478,480 Hume-Rothery theory, 17,23 hydrides, alloys, 231 hydrocarbons, exchange reactions, 466 hydrogen atoms, recombination of 453 hydrogenation, particle size effects, 283 hydrogenolysis, particle size effects, 284 hydrogenolysis, hydrocarbons, modes of 597 hydropolymerization, 493 hydroxylation, surfaces of oxides, 324 hysteresis, capillary condensation, 359 I
ideal solutions (alloys), 182,187 tmpregnation, 331 mcipient wettness impregnation, 332 inhibition, CO reactions, 556 inhomogeneity, alloys, 339 Initial-state, effects of 77,92 intermediates, 51-53,585,589,599, 602 intermetallics, 4,158,313,703 intermetallics, hydrogenation by, 499 interstitial alloys, 4,313,703 ion scattering (LEIS), 113 ion neutralization spectroscopy, 117
725 ion-exchange, catalyst preparation, 328,334 ionization potential clusters, size dependence, 236 ions, the role of in syngas reactions, 684 IR spectra, adsorption on alloys, 402 isochore, 273 isoelectric point, 334 isomer-shift (M6ssbauer), 149 isomer-shift isomerization, cis-trans, 483 isomerization, alkanes, 603 J jellium, 19.41 K
Kelvin equation, 359 kinetics, transient, 259 kinetics, Langmuir-Hinshelwood,268 kinetics, relation to mechanism, 248 kinetics, skeletal reactions, hydrocarbons, 592 kinetics, principles of 266 kinetics, methanation, 683 kinetics, artefacts of 683 Knight shift, 147,369 Kobozev, theory, 394 L Langmuir isotherm, 268,357 Langmuir-Hinshelwood kinetics, 268 Lennard-Jones, potential energy, 47,584 Lewis acidity, 325 ligand effect, definition of 394 ligand effect, hydrocarbon reactions, 617,628,636 Lindlar catalyst, 499 Low Energy Ion Scattering, (LEIS), 113 M
magnetization, 143, 369 mass-transport, 248,250,511 mechanism, bifunctional, 289 mechanism, relation to kinetics, 248 mercury porosimetry, 361 metal -blacks, 299,315 metal-insulator, interface, 237 metal-on-metal layers, 196 metal-support interaction, 289 methane, activation of 584 microreactor, 253 Miedema, theory, 25 mixed ensembles, hydrocarbons, 623,645,651,658
726 moment, magnetic, distribution of 145 monoliths, 330 Monte Carlo, theory of segregation, 194 morphology effects, alloys, 442 Morse, potential, 49 MOssbauer spectroscopy, 147,239,366 multiple bonding, chemisorption, 437,585 multiple exchange, 466,587 multiple fission parameter, 598 multiplets (Ballandin), 394 N nearly free electron approximation, 8,17 nuclear magnetic resonance, 368 0 octane numbers, 664 operator, kinetic energy, 7 operator, Green, 28 operator, potential energy, 7 operator, Hamilton, 7 orbital, metallic (quant.theory), 7 orbital, atomic, 8 orbital, metallic (Pauling), 21 orbital, crystal, 7 orbital, valence (Pauling) 21 orbital, atomic (Pauling) 201 orbital, overlap of 10 orbitals, bonding, antibonding, 35,36 order, reactions, 270 ortho-hydrogen, reactions of 449 oscillations, reaction rate, 273 oscillations, vacancy model, 548 overlap, integral,, 10 oxidation, temp~erature-programmed~PO), 353 oxidation, alkenes, 551,564 oxides, reduction of, 318 oxygenates, from syngas, 686,690 P para-hydrogen, reactions of, 449 paramagnetism, 144 particle size, effects of, 219,280,495,507,554,590 Pauling, theory of metals, 19 Pauling, theory of chemisorption, 48 phase composition, monolayers, 199,201 phase diagrams, 57-63,199 photoemission, angle resolved, 94,157 photoemission, small particles, 238 photoemission, spectroscopy, 73, 74 pi-allyl complexes, prep.cats., 360,346
Subject index
polarization, small particles, 234 porosimetry, 360 porosity, catalysts, supports, 357 precipitation, prep.cats., 337 precursors, metal catalysts, 330 pressure gap, closing of 264 promotor, effect of 287,550,552,687,689,691 pseudomolecules, chemisorption, 38 pulse flow reactor, 257
Q R
radius, single bond, 20 Raney metals, 299, 319 rate, areal, defintion of 266 reaction order, 270 reaction, dimension of 281 reaction rates, definition, 254,266 reactions, structure sensitivity, 281 reactions, particle size effects, 281 reactions, demanding, facile, 280 reactivity, small particles, 229 reactor, continuous stirred tank (CSTR), 263 reactor, split-bed, 258 reactor, pulse flow, 257 reactor, continuous flow, 254 reactor, recirculatory, 253 reactor, batch, 253 reactor, monolith, honeycomb, 257 reactor, fluidized bed, 257 reactor, multitubular, 258 reciprocal space, 12 recombination reaction, hydrogen, 453 reconstruction, surfaces, 177 reduction, preparing of cats. 318,350 reduction, temperature-programmed (TPR), 351 reforming, unit, 663 regular solutions, 182,184.188,192 relaxation effect (spectroscopy), 77,80,239 residues, carbonaceous, 477,609 resonance (Pauling), 21 Rideal mechanism, 273,457 Rigid Band Theory, 2,27,95,146,152,169 S Sasol unit, 705 satellites, photoemission, 77 scattering, multiple theory of 28 Schulz-Flory product distribution, 685 segregation, gas induced, 179 segregation, theory of 181
Subject index
selectivity, effect of alloying, 445 selectivity hydrocarbons on metals, 596 selectivity, definitions, 277,445 selectivity consecutive hydrogenations, 492 self-exchange, 482 shake-off shake-up, peaks, 77 shape, crystals, theory of 180,224 shapes, catalyst particle, 322 Shell Middle Destillate Process, 706 silicagel, formation of 325 silicalite, 328 silicides, formation of 314 single exchange, 466 single crystal, preparation of 309 single turnover method (STO), 487 Smoluchowski effect, 202 sol-gel process, 325 solutions, ideal (alloys), 182,187 solutions, regular (alloys), 182,184,188,192 space, real, reciprocal, 12 spillover, 289 spin density, 144 sputtered films, preparation of 309 states, initial, final, 16 statistics, ensemble theory, 394 Stranski-Krastanov mechanism, 307 structure sensitivity, catalysis, 280,484 sulfurization, reforming catalysts, 640,647 supports, metal catalysts, 321 supports, classification of 335 surface analysis, Auger, 102 surface analysis, XPS, 99 surface analysis, SIMS, 118 surface, structure, 310 surface energy, 175 surface reconstruction, 177 surface composition (alloys), 182 surface segregation, 111 surfaces, contraction, relaxation of 310 surfaces, core level shifts of 91 synchrotron, radiation of 75 synergism, alloying, 609,628,636 syngas reactions, particle size effects, 285 T Taylor ratio (active sites), 267 Temperature-Programmed Reduction (TPR), 351 Temperature-Programmed Oxidation (TPO), 353
727 Temperature-Programmed Desorption (TPD)alloys, 231,399,405,410,414,417 Temporal Analysis of Products (TAP), 258 theory, Nyrop, 2 theory, Dowden, 2 thermodynamics, alloys, 54 transition state model (XPS) 87 Tunnelling Electron Microscopy (TEM), 120,123 Tunnelling Field Emission (FEM), 119 turnover ferquency, determination, 338 turnover frequency, definition (TOF), 258 U V vacancy model, oscillations, 548 valence band, photoemission, 94, 153 Vegard law, 192 volcano shape correlation, 523,692 Volmer-Weber mechanism, 307 W
wash-coat, 329 Wigner-Seitz cell, 31 wires, as catalysts, 301, 459 work function, metal-on-metal, 200 work function, masurements of 120, 125,304 work function, definition, 124 work function, small particles, 221,240 Wulff construction, crystal shape, 180 X
X-ray absorption (EXAFS), 363 X-ray diffraction, 362 Xenon, XPS, alloys, 400 Xenon, NMR, 369 Y Z
zeolites, structure, formation, 327
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S T U D I E S IN SURFACE SCIENCE A N D CATALYSIS
Advisory Editors: B. Delmon, Universite Catholique de Louvain, Louvain-la-Neuve, Belgium J.T. Yates, University of Pittsburgh, Pittsburgh, PA, U.S.A. Volume 1
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Preparation of Catalysts I.Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the First International Symposium, Brussels, October 14-17,1975 edited by B. Delmon, P.A.Jacobs and G. Poncelet The Control of the Reactivity of Solids. A Critical Survey of the Factors that Influence the Reactivity of Solids, with Special Emphasis on the Control of the Chemical Processes in Relation to Practical Applications by V.V. Boldyrev, M. Bulens and B. Delmon Preparation of Catalysts I1. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Second International Symposium, Louvain-la-Neuve, September 4-7, 1978 edited by B. Delmon, P.Grange, P.Jacobs and G. Poncelet Growth and Properties of Metal Clusters. Applications to Catalysis and the Photographic Process. Proceedings ofthe 32nd International Meeting ofthe Societe de Chimie Physique, Villeurbanne, September 24-28, 1979 edited by J. Bourdon Catalysis by Zeolites. Proceedings of an International Symposium, Ecully (Lyon), September 9-11, 1980 edited by B. Imelik, C. Naccache, Y. Ben Taarit, J.C. Vedrine, G. Coudurier and H. Praliaud Catalyst Deactivation. Proceedings of an International Symposium, Antwerp, October 13-15,1980 edited by B. Delmon and G.E Froment New Horizons in Catalysis. Proceedings of the 7th International Congress on Catalysis, Tokyo, June 30-July4, 1980. Parts A and B edited by T. Seiyama and K. Tanabe Catalysis by Supported Complexes by Yu.l. Yermakov, B.N. Kuznetsov and V.A. Zakharov Physics of Solid Surfaces. Proceedings of a Symposium, Bechyhe, September 29-October 3,1980 edited by M. L~iznieka Adsorption at the Gas-Solid and Liquid-Solid Interface. Proceedings of an International Symposium, Aix-en-Provence, September 21-23, 1981 edited by J. Rouquerol and K.S.W. Sing Metal-Support and Metal-Additive Effects in Catalysis. Proceedings of an International Symposium, Ecully (Lyon), September 14-16, 1982 edited by B. Imelik, C. Naccache, G. Coudurier, H. Praliaud, P. Meriaudeau, P. Gallezot, G.A. Martin and J.C. Vedrine Metal Microstructures in Zeolites. Preparation - Properties- Applications. Proceedings of a Workshop, Bremen, September 22-24, 1982 edited by P.A. Jacobs, N.I. Jaeger, P.Jin3 and G. Schulz-Ekloff Adsorption on Metal Surfaces. An Integrated Approach edited by J. Benard Vibrations at Surfaces. Proceedings of the Third International Conference, Asilomar, CA, September 1-4, 1982
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edited by C.R. Brundle and H. Morawitz Heterogeneous Catalytic Reactions Involving Molecular Oxygen by G.I. Golodets Preparation of Catalysts II1. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings ofthe Third International Symposium, Louvain-la-Neuve, September 6-9, 1982 edited by G. Poncelet, P. Grange and P.A. Jacobs Spillover of Adsorbed Species. Proceedings of an International Symposium, Lyon-Villeurbanne, September 12-16, 1983 edited by G.M. Pajonk, S.J. Teichner and J.E. Germain Structure and Reactivity of Modified Zeolites. Proceedings of an International Conference, Prague, July 9-13, 1984 edited by P.A. Jacobs, N.I. Jaeger, P. Jia3, V.B. Kazansky and G. Schulz-Ekloff Catalysis on the Energy Scene. Proceedings of the 9th Canadian Symposium on Catalysis, Quebec, P.Q., September 30-October 3, 1984 edited by S. Kaliaguine and A. Mahay Catalysis by Acids and Bases. Proceedings of an International Symposium, Villeurbanne (Lyon), September 25-27, 1984 edited by B. Imelik, C. Naccache, G. Coudurier, Y. Ben Taarit and J.C. Vedrine Adsorption and Catalysis on Oxide Surfaces. Proceedings of a Symposium, Uxbriclge, June 28-29, 1984 edited by M. Che and G.C. Bond Unsteady Processes in Catalytic Reactors by Yu.Sh. Matros Physics of Solid Surfaces 1984 edited by J. Koukal Zeolites: Synthesis, Structure, Technology and Application. Proceedings of an International Symposium, Portoro~-Portorose, September 3-8, 1984 edited by B. Dr~aj, S. Ho~,evar and S. Pejovnik Catalytic Polymerization of Olefins. Proceedings of the International Symposium on Future Aspects of Olefin Polymerization, Tokyo, July 4-6, 1985 edited by T. Keii and K. Soga Vibrations at Surfaces 1985. Proceedings of the Fourth International Conference, Bowness-on-Windermere, September 15-19, 1985 edited by D.A. King, N.V. Richardson and S. Holloway Catalytic Hydrogenation edited by L. Cerveny New Developments in Zeolite Science and Technology. Proceedings of the 7th International Zeolite Conference, Tokyo, August 17-22, 1986 edited by Y. Murakami, A. lijima and J.W. Ward Metal Clusters in Catalysis edited by B.C. Gates, L. Guczi and H. Kn6zinger Catalysis and Automotive Pollution Control. Proceedingsof the First International Symposium, Brussels, September 8-11, 1986 edited by A. Crucq and A. Frennet Preparation of Catalysts IV. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Fourth International Symposium, Louvain-la-Neuve, September 1-4, 1986 edited by B. Delmon, P. Grange, P.A. Jacobs and G. Poncelet Thin Metal Films and Gas Chemisorption edited by P. Wissmann Synthesis of High-silica Aluminosilicate Zeolites edited by P.A. Jacobs and J.A. Martens Catalyst Deactivation 1987. Proceedingsof the 4th International Symposium, Antwerp, September 29-October 1, 1987 edited by B. Delmon and G.F. Froment
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Keynotes in Energy-Related Catalysis edited by S. Kaliaguine Methane Conversion. Proceedings of a Symposium on the Production of Fuels and Chemicals from Natural Gas, Auckland, April 27-30, 1987 edited by D.M. Bibby, C.D. Chang, R.F. Howe and S. Yurchak Innovation in Zeolite Materials Science. Proceedings of an International Symposium, Nieuwpoort, September 13-17, 1987 edited by P.J. Grobet, W.J. Mortier, E.F.Vansant and G. Schulz-Ekloff Catalysis 1987. Proceedings ofthe 10th North American Meeting ofthe Catalysis Society, San Diego, CA, May 17-22, 1987 edited by J.W. Ward Characterization of Porous Solids. Proceedings of the IUPAC Symposium (COPS I), Bad Soden a. Ts., April 26-29,1987 edited by K.K. Unger, J. Rouquerol, K.S.W. Sing and H. Kral Physics of Solid Surfaces 1987. Proceedings of the Fourth Symposium on ....... Surface Physics, Bechyne Castle, September 7-11, 1987 edited byJ. Koukal Heterogeneous Catalysis and Fine Chemicals. Proceedings of an International Symposium, Poitiers, March 15-17, 1988 edited by M. Guisnet, J. Barrault, C. Bouchoule, D. Duprez, C. Montassier and G. Perot Laboratory Studies of Heterogeneous Catalytic Processes by E.G. Christoffel, revised and edited by Z. Paal Catalytic Processes under Unsteady-State Conditions by Yu. Sh. Matros Successful Design of Catalysts. Future Requirements and Development. Proceedings ofthe Worldwide Catalysis Seminars, July, 1988, on the Occasion of the 30th Anniversary of the Catalysis Society of Japan edited by T. Inui Transition Metal Oxides. Surface Chemistry and Catalysis byH.H. Kung Zeolites as Catalysts, Sorbents and Detergent Builders. Applications and Innovations. Proceedings of an International Symposium, WiJrzburg, September 4-8,1988 edited by H.G. Karge and J. Weitkamp Photochemistry on Solid Surfaces edited by M. Anpo and T. Matsuura Structure and Reactivity of Surfaces. Proceedingsof a European Conference, Trieste, September 13-16, 1988 edited by C. Morterra, A. Zecchina and G. Costa Zeolites: Facts, Figures, Future. Proceedings of the 8th International Zeolite Conference, Amsterdam, July 10-14, 1989. Parts A and B edited by P.A. Jacobs and R.A. van Santen Hydrotreating Catalysts. Preparation, Characterization and Performance. Proceedings of the Annual International AIChE Meeting, Washington, DC, November 27-December 2, 1988 edited by M.L. Occelli and R.G. Anthony New Solid Acids and Bases. Their Catalytic Properties by K. Tanabe, M. Misono, Y. Ono and H. Hattori Recent Advances in Zeolite Science. Proceedings of the 1989 Meeting of the British Zeolite Association, Cambridge, April 17-19, 1989 edited by J. Klinowsky and P.J. Barrie Catalyst in Petroleum Refining 1989. Proceedings of the First International Conference on Catalysts in Petroleum Refining, Kuwait, March 5-8, 1989 edited by D.L. Trimm, S. Akashah, M. Absi-Halabi and A. Bishara
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Future Opportunities in Catalytic and Separation Technology edited by M. Misono, Y. Moro-oka and S. Kimura New Developments in Selective Oxidation. Proceedings of an International Symposium, Rimini, Italy, September 18-22, 1989 edited by G. Centi and E Trifiro Olefin Polymerization Catalysts. Proceedings of the International Symposium on Recent Developments in Olefin Polymerization Catalysts, Tokyo, October 23-25, 1989 edited by T. Keii and K. Soga
Volume 57A Spectroscopic Analysis of Heterogeneous Catalysts. Part A: Methods of Surface Analysis edited by J.L.G. Fierro Volume 57B Spectroscopic Analysis of Heterogeneous Catalysts. Part B: Chemisorption of Probe Molecules edited by J.L.G. Fierro Volume 58 Introduction to Zeolite Science and Practice edited by H. van Bekkum, E.M. Flanigen and J.C. Jansen Heterogeneous Catalysis and Fine Chemicals I1. Proceedings of the 2nd Volume 59 International Symposium, Poitiers, October 2-6, 1990 edited by M. Guisnet, J. Barrault, C. Bouchoule, D. Duprez, G. Perot, R. Maurel and C. Montassier Chemistry of Microporous Crystals. Proceedings of the International Symposium Volume 60 on Chemistry of Microporous Crystals, Tokyo, June 26-29, 1990 edited by T. Inui, S. Namba and T. Tatsumi Natural Gas Conversion. Proceedings of the Symposium on Natural Gas Volume 61 Conversion, Oslo, August 12-17, 1990 edited by A. Holrnen, K.-J. Jens and S. Kolboe Characterization of Porous Solids I1. Proceedings of the IUPAC Symposium Volume 62 (COPS II), Alicante, May 6-9, 1990 edited by F. Rodriguez-Reinoso, J. Rouquerol, K.S.W. Sing and K.K. Unger Preparation of Catalysts V. Scientific Bases for the Preparation of Heterogeneous Volume 63 Catalysts. Proceedings of the Fifth International Symposium, Louvain-la-Neuve, September 3-6, 1990 edited by G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon Volume 64 New Trends in CO Activation edited by L. Guczi Catalysis and Adsorption by Zeolites. Proceedings of ZEOCAT 90, Leipzig, Volume 65 August 20-23, 1990 edited by G. ()hlmann, H. Pfeifer and R. Fricke Dioxygen Activation and Homogeneous Catalytic Oxidation. Proceedings of the Volume 66 Fourth International Symposium on Dioxygen Activation and Homogeneous Catalytic Oxidation, BalatonfQred, September 10-14, 1990 edited by L.I. Simandi Structure-Activity and Selectivity Relationships in Heterogeneous Catalysis. Volume 67 Proceedings of the ACS Symposium on Structure-Activity Relationships in Heterogeneous Catalysis, Boston, MA, April 22-27, 1990 edited by R.K. Grasselli and A.W. Sleight Catalyst Deactivation 1991. Proceedings of the Fifth International Symposium, Volume 68 Evanston, IL, June 24-26, 1991 edited by C.H. Bartholomew and J.B. Butt Zeolite Chemistry and Catalysis. Proceedings of an International Symposium, Volume 69 Prague, Czechoslovakia, September 8-13, 1991 edited by P.A. Jacobs, N.I. Jaeger, L. Kubelkova and B. Wichterlova
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Poisoning and Promotion in Catalysis based on Surface Science Concepts and Experiments by M. Kiskinova Catalysis and Automotive Pollution Control I1. Proceedings of the 2nd International Symposium (CAPoC 2), Brussels, Belgium, September 10-13, 1990 edited by A. Crucq New Developments in Selective Oxidation by Heterogeneous Catalysis. Proceedings of the 3rd European Workshop Meeting on New Developments in Selective Oxidation by Heterogeneous Catalysis, Louvain-la-Neuve, Belgium, April 8-10, 1991 edited by P. Ruiz and B. Delmon Progress in Catalysis. Proceedings ofthe 12th Canadian Symposium on Catalysis, Banff, Alberta, Canada, May 25-28, 1992 edited by K.J. Smith and E.C. Sanford Angle-Resolved Photoemission. Theory and Current Applications edited by S.D. Kevan New Frontiers in Catalysis, Parts A-C. Proceedings of the 10th International Congress on Catalysis, Budapest, Hungary, 19-24 July, 1992 edited by L. Guczi, F. Solymosi and P.Tetenyi Fluid Catalytic Cracking: Science and Technology edited by J.S. Magee and M.M. Mitchell, Jr. New Aspects of Spillover Effect in Catalysis. For Development of Highly Active Catalysts. Proceedings of the Third International Conference on Spillover, Kyoto, Japan, August 17-20, 1993 edited by T. Inui, K. Fujimoto, T. Uchijima and M. Masai Heterogeneous Catalysis and Fine Chemicals II1. Proceedings ofthe 3rd International Symposium, Poitiers, April 5-8, 1993 edited by M. Guisnet, J. Barbier, J. Barrault, C. Bouchoule, D. Duprez, G. Perot and C. Montassier Catalysis: An Integrated Approach to Homogeneous, Heterogeneous and Industrial Catalysis edited by J.A. Moulijn, P.W.N.M. van Leeuwen and R.A. van Santen Fundamentals of Adsorption. Proceedings of the Fourth International Conference on Fundamentals of Adsorption, Kyoto, Japan, May 17-22, 1992 edited by M. Suzuki Natural Gas Conversion I1. Proceedings ofthe Third Natural Gas Conversion Symposium, Sydney, July 4-9, 1993 edited by H.E. Curry-Hyde and R.F. Howe New Developments in Selective Oxidation I1. Proceedings of the Second World Congress and Fourth European Workshop Meeting, Benalmadena, Spain, September 20-24, 1993 edited by V. Cortes Corberan and S. Vic Bellon Zeolites and Microporous Crystals. Proceedings of the International Symposium on Zeolites and Microporous Crystals, Nagoya, Japan, August 22-25, 1993 edited by T. Hattori and T. Yashima Zeolites and Related Microporous Materials: State of the Art 1994. Proceedings ofthe 10th International Zeolite Conference, Garmisch-Partenkirchen, Germany, July 17-22, 1994 edited by J. Weitkamp, H.G. Karge, H. Pfeifer and W. H61derich Advanced Zeolite Science and Applications edited by J.C. Jansen, M. St6cker, H.G. Karge and J.Weitkamp
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Oscillating Heterogeneous Catalytic Systems by M.M. Slin'ko and N.I. Jaeger Characterization of Porous Solids II1. Proceedings of the IUPAC Symposium (COPS III), Marseille, France, May 9-12, 1993 edited by J.Rouquerol, F. Rodriguez-Reinoso, K.S.W. Sing and K.K. Unger Catalyst Deactivation 1994. Proceedings of the 6th International Symposium, Ostend, Belgium, October 3-5, 1994 edited by B. Delmon and G.F. Froment Catalyst Design for Tailor-made Polyolefins. Proceedings of the International Symposium on Catalyst Design for Tailor-made Polyolefins, Kanazawa, Japan, March 10-12, 1994 edited by K. Soga and M. Terano Acid-Base Catalysis I1. Proceedings of the International Symposium on Acid-Base Catalysis II, Sapporo, Japan, December 2-4, 1993 edited by H. Hattori, M. Misono and u Ono Preparation of Catalysts VI. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Sixth International Symposium, Louvain-La-Neuve, September 5-8, 1994 edited by G. Poncelet, J. Martens, B. Delmon, P.A.Jacobs and P. Grange Science and Technology in Catalysis 1994. Proceedings of the Second Tokyo Conference on Advanced Catalytic Science and Technology, Tokyo, August 21-26, 1994 edited by Y. Izumi, H. Arai and M. Iwamoto Characterization and Chemical Modification of the Silica Surface by E.F.Vansant, P. Van Der Voort and K.C. Vrancken Catalysis by Microporous Materials. Proceedings of ZEOCAT'95, Szombathely, Hungary, July 9-13, 1995 edited by H.K. Beyer, H.G.Karge, I. Kiricsi and J.B. Nagy Catalysis by Metals and Alloys by V. Ponec and G.C. Bond
