Catalysis by Metals and Alloys (Studies in Surface Science and Catalysis)

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Studies in Surface Science and Catalysis 95

CATALYSIS BY METALS AND ALLOYS

This Page Intentionally Left Blank

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.

No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the publisher, Elsevier Science B.V., Copyright & Permissions Department, P.O. Box 521, 1000 AM Amsterdam, The Netherlands. Special regulations for readers in the U.S.A.-This publication has been registered with the Copyright Clearance Center Inc. (CCC), 222 Rosewood Drive Danvers, MA 01923. Information can be obtained from the CCC about conditions under which photocopies of parts of this publication may be made in the U.S.A. All other copyright questions, including photocopying outside of the U.S.A., should be referred to the copyright owner, Elsevier Science B.V., unless otherwise specified. No responsibility is assumed by the publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein. This book is printed on acid-flee paper Printed in The Netherlands

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


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.


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


~,

= 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)


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


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.


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].


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


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


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,


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)


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].


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

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


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.


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~" "\ ~"

\, \\. \\ \ \\\ \

\ \ \\\

"-~,/\ \ 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/


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


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


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


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


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 ferromagnetism 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


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.


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.


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


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.


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

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137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152

chapter 1

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.


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


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.


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


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 (0B.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].


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


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


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.


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


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-


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:


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)


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].


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


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].


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].


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


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


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.


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


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


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


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.

References

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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

140

chapter 2

I32

"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

134

135

136

137 138 139 140 141 142 143

144


145

141

L.D.Landau, A.I.Khiezer, E.M.Lifschitz, "General Physics, Mechanics", Pergamon Press, Oxford (1967)


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,


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


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.


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


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.

Ni

6-

4-

-1 87o/,cu-13./o.i

//

in "o

2-

>, c "13

/./

~ - ~

1...IEI~

//

0

r

I

-6

I

i

-4 energy

i

-

~

(eV)

I

+2

0 EF

a) I f~

!

4 4~176176 ~ %F

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].

Ag ;

20 >"

g

I

2

*

lJAk> .

.

.

:

o

~ 8

~

;

t

~

~

o

~

l

/

,P~,i

12 |

i

~ 5~

,~

T25~

i

o

;

'

i

6] ; ^~ g

o

~

2

O r

o

~

o

~

:

~

i

2 5%t

,

i:~ o

I

I

i

I'I

0 8

6

. z,

I

*l

,:,~,~/

T

, !,,I

T [

,

I

energy

:

"I

' :i[

:

8

0-1

binding

t,

t..J /; J

01,.-

It 2

t

tr~

0 .

:i

~T,o~ :i Ag [ ,

8

.

,

~ i

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/',

of" ~

~/60Olo~

.

_L-

~

Ia

{eV)

6

4

2

0-1

154

chapter 3

/

5-

1

/

4-

Ag

/ ,

-

3

.,..~

,,----,

oCu ~Ni

[]

~ Cu

3.0-

:>

o

,.._,,

Itl

2.0

2Pd

Ni

/ Pd

1-

I -

d BAND

d BAND POSITION

0---

0

I

0.5

1.0

O~

0.5 X

Pd

1.0-

J

I

0

X

Agl-x

WIDTH

x

0-

0

I

i

Cu 1-x Ni

x

figure 10 figure 10 figure 11

I

i

0.2 0.L. 0.6 0.8 1.0 X

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].


), (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-

c~

tems: 9 - AgCd; 0 - Agln; (9 t_

(9 f(9

"D rnn

f,

100

'

!

80

Atomic

'

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

9-~

0

)i

0)

~-~///

C

2-

o ,~",4~,1., 0

.

u C 121 ,

,

,

'~

6

2

Distance

~

0

0

.

.

~

.

.

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].


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

Total ..'.

9 .

I Ni-Nid

t

,...[.

!/~Nid ::1~.-, 9

""

I ".

I

i ;.,

|"

TN

i

.."

9""'" tl

Ni

"|:

"~.':.

":'.\..

0

N .i . ALLOY . .

I

tNi-Noble I Metal

"'".Total

I

9

I ..I

1:3

... -..

~

g

I

~! "

I

,

~

(eV)

-

....

, .oto,-.-

I

"

.~.

t

l

'.

." :

-!.."

'

~ Ni d

"'.

.. 9

E F

E F

BE

,, "

9

~

-~

~

-

=

BE

leVI

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


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].

!!~e A Ee x p -1

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

UJ

,, ..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

I l i ~ - -

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.


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].


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


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].


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


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25 26 27 28 29 30

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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)


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)

KA1

4) KA>>I

K>> 1

K-1 K=f(Xg) K.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_
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

360

chapter 7

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.


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

chapter 7

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


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

chapter 7

{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].


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

chapter 7

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.


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).

368

chapter 7

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


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

370

chapter 7

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


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

372

chapter 7

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.


"lD (ll r'~ I..

0

Vrn
~ "-_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


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

chapter 8

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


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)(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.

482

chapter 11

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).


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.

484

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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)


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.

486

chapter 11

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


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


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

chapter 11

[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


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

chapter 11

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


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.

494

chapter 11

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


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


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

chapter 11

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].


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

chapter 11

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).


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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chapter 11

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-


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


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

506

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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
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.


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


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

518

chapter 11

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-


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.

520

chapter 11

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


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


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.


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


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].

References

E.K.Rideal, J.Chem.Soc. 121 (1922)309 E.K.Rideal, Adv.Catal. 9 (1957) 8 S.A.Goddard, R.D.Cartwright, J.A.Dumesic, J.Catal. 137 (! 992) 186 J.A.Dumesic, D.F.Rudd, L.M.Aparicio, J.E.Rekoske, A.A.Trevino, "The Microkinetics of Heterogeneous Catalysis", ACS Washington, 1993 G.C.Bond, P.B.Wells, Adv.Catal. 15 (1964) 92 S.Siegel, M.Dunkel, Adv.Catal. 9 (157) 15

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"The Catalytic Process from Labotarory to Industrial Plant", Italian Chem.Soc., 1994 C.Hoang Van, P.Pichat, M.N.Mozzanega, J.Molec.Catal. 92 (1994) 187 198 199 E.J.Grootendorst, R.Pestman, R.M.Koster, V.Ponec, J.Catal. 148 (1994) 261 V.Ponec, Int.Quant.Chem. 12(2)(1977) 1 200 K.Hotta, T.Kubomatsu, Bull.Chem.Soc.Japan 45 (1972) 3118 201 G.C.Bond, Chem.Ind. (1967) 2018 202 P.N.Rylander, A.G.Cohen, Proc.2nd Int.Congr.on Catal., Paris, 1960, Editions 203 Technip.Paris, 1961, vol. 1 p.977 P.N.Rylander, L.Hasbrouck, S.G.Hindin, I.Karpenko, G.Pond, S.Starrick, Engelhard 204 Ind.Tech.Bull. 8 (1967) 25 G.C.Bond, D.E.Webster, Ann.New York Acad.Sci. 158 (1969) 540 205 G.C.Bond, D.E.Webster, Plat.Met.Rev. 9 (1965) 12; 10 (1966) 10; 13 (1969)57; 206 Chem.Ind. (1967) 878 D.V.Sokol'skii, K.K.Dzhardamalieva, A.G.Sarmuzina, T.Tonmanov, Dokl. 207 Akad.Nauk.SSSR 176 (1967) 1093 S.Galvagno, A.Donato, G.Neri, R.Pietropaoli, Z.Poltarzewski, J.Molec.Catal. 42 208 (1987) 379 209a B.Coq, A.Tijani, F.Figueras, J.Molec.Catal. 71 (1992) 317 P.Beccat, J.C.Bertolini, Y.Gauthier, J.Massardier, P.Ruiz, J.Catal. 126 (1990) 451 b M.Nakamura, H.Wise, Proc.6th Int.Congr.on Catal., London, 1976, Chem.Soc.Lon210 don, 1977, vol.2, p.881 P.B.Babkova, A.K.Avetisev, G.D.Lyubarskii, A.I.Gel'bshtein, Kinet.Katal. 13 211 (1972) 345 A.van den Burg, J.Doornbos, N.K.Kos, W.J.Ultee, V.Ponec, J.Catal. 54 (1978) 243 212 S.Galvagno, Z.Poltarzewski, A.Donato, G.Neri, R.Pietropaoli, J.Molec.Catal. 35 213 (1986) 365 Z.Poltarzewski, S.Galvagno, R.Pietropaoli, P.Staiti, J.Catal. 102 (1986) 190 214 S.Galvagno, Z.Poltarzewski, A.Donato, G.Neri, R.Pietropaoli, J.Chem.Soc.Chem.215 London (1986) 1729 216 C.G.Raab, J.A.Lercher, Catal.Lett. 18 (1993) 99 T.B.L.W.Marinelli, J.H.Vleeming, V.Ponec, Proc.10th Int.Congr.on Catal., Budapest, 1992, Elsevier, 1993, vol.B, p. 1211 S.Nishimura, H.Taguchi, Bull.Chem.Soc.Japan 36 (1963) 873 217 R.G.Davlesupova, D.V.Sokol' skii, Kinet.Katal. 8 (1967) 1378 218 S.Galvagno, A.Donato, G.Neri, R.Pietropaoli, J.Molec.Catal. 58 (1990) 215 219 J.H.Sinfelt, H.Hurwitz, R.A.Shulman, J.Phys.Chem. 64 (1960) 1559 220 J.W.A.Sachtler, M.A.van Hove, J.P.Biberian, G.A.Somorjai, Phys.Rev.Lett. 45 221 (1980) 1601 197

Catalytic hydrogenation and dehydrogenation 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253

537

Chen Xu, B.E.Koel, Surf.Sci. 304 (1994) 249 J.H.Sinfelt, J.L.Carter, D.J.C.Yates, J.Catal. 24 (1972) 283 H.E.Swift, J.E.Bozik, J.Catal. 12 (1968) 5 M.Masai, K.Mori, H.Muramoto, T.Fujiwara, S.Ohnaka, J.Catal. 38 (1975) 128 M.Masai, K.Honda, A.Kubota, S.Ohnaka, Y.Nishikawa, K.Nakahara, K.Kishi, S.Ikeda, J.Catal. 50 (1977) 419 D.A.Aranda, F.B.Noronha, M.Schmal, F.B.Passos, Appl.Catal.A Gen. 100 (1993) 77 J.Volter, G.Lietz, M.Uhlemann, M.Hermann, J.Catal. 68 (1981) 42 Liwu Lin, Tao Zhang, Jingling Zang, Zhusheng Xu, Appl.Catal. 67 (1990) 11 G.del Angel, F.Tzompantzi, R.Gomez, G.Baronetti, S.de Miguel, O.Scelza, A.Castro, React.Kinet.Catal.Lett. 42 (1990) 67 Hoang Dang Lanh, Ng.Khoai, Ho Si Thoang, J.Volter, J.Catal. 129 (1991) 58 S.Engels, M.Wilde, Z.Phys.Chem.,Leipzig 264 (1983) 432 J.H.Sinfelt, Catal.Rev.-Sci.Eng. 9 (1974) 147 J.H.Sinfelt, J.Catal. 29 (1973) 308 C.H.F.Peden, D.W.Goodman, J.Catal. 100 (1986) 520 C.H.F.Peden, D.W.Goodman, J.Catal. 104 (1987) 347 V.M.Gryaznov, V.S.Smirnov, M.Slinko, Proc.6th Int.Congr.on Catal., London, 1976, Chem.Soc.London, 1977, vol.2, p.894 M.E.Ruiz-Vizcaya, O.Novaro, J.M.Ferreira, R.Gomez, J.Catal. 51 (1978) 108 P.Biloen, F.M.Dautzenberg, W.M.H.Sachtler, J.Catal. 50 (1977) 77 Hoang Dang Lanh, Ho Si Thoang, H.Lieske, J.Volter, Appl.Catal. 11 (1984) 195 Changmin Kim, G.A.Somorjai, J.Catal. 134 (1992) 179 G.N.Pirogova, N.N.Rymar, T.A.Lagutina, Russ.J.Phys.Chem. 66 (1992) 1715 K.Tamaru in "Dynamic Heterogeneous Catalysis", Academic Press, London, 1978, chapter 4 J.Fahrenfort, L.L.van Reijen, W.M.H.Sachtler in "Mechanism of Heterogeneous Catalysis", Elsevier, Amsterdam, 1960 p.23 G.M.Schwab, Disc.Faraday Soc. 8 (1950) 166 H.C.Tingey, C.N.Hinshelwood, J.Chem.Soc. 121 (1922) 1668 E.Iglesia, M.Boudart, J.Phys.Chem. 95 (1991) 7013; J.Catal. 88 (1984) 325 L.H.Dubois, T.H.Ellis, B.R.Zegarski, S.D.Kevan, Surf.Sci. 172 (1986) 385 F.Solymosi, J.Kiss, I.Kovacs, Surf.Sci. 192 (1987) 47 R.J.Madix, Catal.Rev. 15 (1977) 293 N.D.S.Canning, R.J.Madix, J.Phys.Chem. 88 (1984) 2437 F.Fukuda, S.Nagashima, Y.Noto, T.Onishi, K.Tamaru, Trans.Faraday Soc. 64 (1968) 522 K.Tamaru, Trans.Faraday Soc. 55 (1959) 824

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254 255 256 257 258 259

K.Fukada, T.Onishi, K.Tamaru, Bull.Chem.Soc.Japan 42 (1969) 1192 H.M.C.Sosnovsky, J.Chem.Phys. 23 (1955) 1486 W.J.M.Rootsaert, W.M.H.Sachtler, Z.Phys.Chem. NF 26 (1960) 16 M.J.Duell, A.J.B.Robertson, Trans Faraday Soc. 57 (1961) 1416 E.M.A.Willhoft, A.J.B.Robertson, J.Catal. 9 (1967) 358 J.Farhrenfort, L.L.van Reijen, W.M.H.Sachtler, Ber.Bunsenges. Phys.Chem. 64 (1960) 216 P.Sabatier, Ber. 44 (1911) 2001 A.A.Balandin, Adv.Catal. 19 (1969) 1 G.C.A.Schuit, L.L.van Reijen, W.M.H.Sachtler, Proc.2nd Int. Congr. on Catal., Paris, 1960, Editions Technip.Paris, 1961, vol. 1, p.893 K-I.Tanaka, K.Tamaru, J.Catal. 2 (1963) 366; Kinet.Katal. 7 (1966) 272 P.Fuderer-Luetic, I.Brihta, Croat.Chem.Acta, 31 (1959) 75 M.A.Newton, S.M.Francis, Yongxue Li, D.Law, M.Bowker, Surf.Sci. 259 (1991) 45 G.Rien~icker, H.Muller, Z.Anorg.Allg.Chem. 357 (1968) 255 D.D.Eley, D.M.McMahon, J.Coll.Interface Sci. 38 (1972) 502 J.K.A.Clarke, E.A.Rafter, Z.Phys.Chem.N.F. 67 (1969) 169 D.D.Eley, P.Luetic, Trans Faraday Soc. 53 (1957) 1483 K.Gossner, H.Bischof, J.Catal. 32 (1974) 175 R.F.Howe, A.Metcalfe, J.Catal. 14 (1969) 55 G.Rien~icker, J.Volter, Zeit.Anorg.Allg.Chem. 296 (1958) 210 C.Suzuki, I.Matsuura, Bull.Chem.Soc.Japan 39 (1966) 1104 D.W.McKee, J.Catal. 14 (1969) 355 D.D.Eley, D.M.McMahon, J.Coll.Interface Sci. 38 (1972) 502 F.Haber, S.Grinberg, Z.Anorg.Chem. 18 (1898) 37 D.A.Dowden, P.W.Reynolds, Disc.Faraday Soc. 8 (1950) 184 D.W.McKee, Trans Faraday Soc. 64 (1968) 2200 Y.L.Lam, J.Criado, M.Boudart, Nouv.J.Chem. 1 (1977) 461 G.Tammann, Z.Anorg.Allg.Chem. 111 (1920) 90 J.M.Thomas, W.J.Thomas, 'Introduction to the Principles of Heterogeneous Catalysis' Academic Press, London, 1967 K.J.Laidler in "Chemical Kinetics" 3rd ed. Harper and Row, New York, 1987 P.Stoltze, J.K.Norskov, Phys.Rev.Lett. 55 (1985) 2502 P.Stoltze, Physica Scripta 36 (1984) 824 D.W.Taylor, P.J.Smith, D.A.Dowden, C.Kemball, D.A.Whan, Appl.Catal. 3 (1982) 161 P.J.Smith, D.W.Taylor, D.A.Dowden, C.Kemball, D.Taylor, Appl.Catal. 3 (1983) 303

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

539

R.J.Kalenczuk, J.Chem.Tech.Biotechnol. 59 (1994) 73 A.Ozaki, V.Aika in "Catalysis" (editors" J.R.Anderson, M.Boudart) Springer Verlag (1981) vol. 1 p.87


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chapter 12

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

chapter 12

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].

544

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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 _~

220iii 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

D.O.Hayward, B.M.W.Trapnell in "Chemisorption", Butterworths, London, 1964 B.E.Nieuwenhuys, Surf.Sci. 126 (1983) 307 Idem in "Elementary Reaction Steps in Heterogeneous Catalysis" (editors: R.W.Joyner, R.A.van Santen) Kluwer Acad.Publ. (1983) p.155 B.Harrison, M.Wyatt, K.G.Gough in "Specialist Periodical Reports: Catalysis", Chem.Soc.London, vol.5 (1982) p. 127 J.L.Gland, B.A.Sexton, G.B.Fisher, Surf.Sci. 95 (1980) 587 M.B.Ward, M.J.Lin, J.H.Lunsford, J.Catal. 50 (1977) 306 M.M.Khaw, G.A.Somorjai, J.Catal. 91 (1985) 263 C.Nyberg, P.Uvdal, Surf.Sci. 204 (1988) 517 A.Sandell, A.Nilsson, N.Martensson, Surf.Sci. 241 (1991) L 1 H.D.Schmick, H.W.Wassmuth, Surf.Sci. 123 (1982) 471 H.Conrad, G.Ertl, J.Kuppers, E.E.Latta, Surf.Sci. 65 (1977) 235 W.Ranke, Surf.Sci. 209 (1989) 57 J.L.Gland, B.A.Sexton, Surf.Sci. 94 (1980) 355 M.Kiskinova, Y.Pirug, H.P.Bonzel, Surf.Sci. 136 (1984) 285 N.Kruse, G.Abend, J.H.Block, J.Chem.Phys. 88 (1988) 1307 C.Comrie, W.H.Weinberg, R.M.Lambert, Surf.Sci. 57 (1976) 619 R.J.Gorte, L.D.Schmidt, J.L.Gland, Surf.Sci. 109 (1981) 267 G.Pirug, H.P.Bonzel, H.Hopster, H.Ibach, J.Chem.Phys. 71 (1979) 593 V.K.Agrawal, M.Trenary, Surf.Sci. 259 (1991) 116 J.H.Gohndrone, R.I.Masel, Surf.Sci. 209 (1989) 44 J.Kanski, T.N.Rhodin, Surf.Sci. 65 (1977) 63 J.C.L.Gomish, N.R.Avery, Surf.Sci. 235 (1990) 209 P.D.Schulze, D.L.Utley, R.L.Hance, Surf.Sci. 102 (1981) L9 H.A.C.M.Hendrickx, A.M.E.Winkelman, B.E.Nieuwenshuys, Surf.Sci. 175 (1986) 185 G.Camtero, C.Astaldi, P.Rudolf, M.Kiskinova, R.Rosei, Surf.Sci. 258 (1991) 44 V.Schmatloch, N.Kruse, Surf.Sci. 269/270 (1992) 488 T.W.Root, G.B.Fisher, L.D.Schmidt, J.Chem.Phys. 85 (1986) 4679 J.S.Villarubia, L.J.Richler, B.A.Gurney, W.Ho, J.Vac.Sci.Technol. A4 (1986) 1487 D.L.Vuckovic, S.A.Jansen, R.Hoffmann, Langmuir 6 (1990) 741 H.P.Bonzel, T.E.Fischer, Surf.Sci. 51 (1975) 213 E.Umbach, S.Kulkarni, P.Feulner, D.Menzel, Surf.Sci. 88 (1979) 65 H.Conrad, R.Scala, W.Stenzel, R.Unwin, Surf.Sci. 145 (1984) 471 A.Sandell, A.Nilsson, N.Martensson, Surf.Sci. 251/252 (1991) 971 A.F.Carley, G.Rassias, M.W.Roberts, W-T.Han, Surf.Sci. 84 (1979) L227

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15 16 17 18 19 20 21 22 23 24 25

26

27 28

29

30 31

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68a b 69 70

71 72 73 74 75 76 77 78

79 80 81

577

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93a b 94a b C

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Oxidation reactions

579

A.Ayame, T.Kimura, M.Yamaguschi, H.Miura, N.Tkeno, H.Kanoh, I.Toyoshima, J.Catal. 79 (1983) 233 96 G.Papin, M.Christman, N.Sadeghi, C.R.Acad.Sci.Paris 284 (1977) 791 97 C.Backx, C.P.M.de Groot, Surf.Sci. 115 (1982) 382 P.J.van den Hoek, E.Barends, R.A.van Santen, J.Phys.Chem. 93 (1989) 6469 98 R.W.Clayton, S.V.Norval in "Specialist Periodical Reports: Catalysis", Chem.Soc. 99 London, vol.3 (1980) 70 100 G.Rovida, F.Pratesi, E.Ferroni, J.Catal. 41 (1976) 140 101 N.W.Cant. W.K.Hall, J.Catal. 52 (1978) 81 102 W.F.Richey, J.Phys.Chem. 76 (1972) 213 103 N.W.Cant, W.K.Hall, J.Catal. 27 (1972) 70 104 A.B.Ernin, J.A.Rabo, P.H.Kasai, J.Catal. 30 (1973) 109 105 C.F.Cullis, T.G.Nevell, Proc.Roy.Soc.London A 349 (1976) 523 P.A.J.M.Angevaare, E.J.Grootendorst, A.P.Zuur, V.Ponec, Stud.Surf.Sci.&Catal. 55 (1990) 861 106 J.R.Anderson, C.Kemball, Trans Faraday Soc. 51 (1955) 966 G.H.Chuch, N.Kruse, W.A.Schmidt, J.H.Block, G.Abend, J.Catal. 119 (1989) 342 107 108 V.S.Bagotzky, Yu.B.Vassiliev, Elektrochim.Acta 11 (1966) 1439 P.Davies, R.T.Donald, N.H.Harbond in "Catalyst Handbook" 2nd ed. (editor: M.V.Twigg) Wolfe Publ.Ltd. (1989)490 109 L.N.Kurina, V.P.Morozov, Zhur.Fiz.Khim. 50 (1976) 904; 51 (1977) 2257 110 I.E.Wachs, R.J.Madix, J.Catal. 53 (1978) 208 111 N.J.Jaeger, R.Ottensmeyer, P.J.Plath, Ber.Bunsenges.Phys.Chem. 90 (1986) 1075 A.Th.Haberditzl, N.I.Jaeger, P.J.Plath, Z.Phys.Chem.Leipzig 265 (1984) 449 112 G.K.Boreskov in 'Geterogennyi Kataliz v Khimicheskoi Promishlenosti Goskhimizdat, Moscow (1955) p.5 G.K.Boreskov, V.S.Chesalova, Zhur.Fiz.Khim. 30 (1956) 2560 113 O.M.Poltorak, V.S.Boronin, A.N.Mitrofanova, Proc.4th Int.Congr.on Catal. Moscow, 1968, Akademiai Kiado, Budapest, vol.II (1971) p.276 114 W.Manogue, J.R.Katzer, J.Catal. 32 (1974) 166 J.J.Osterman, J.R.Katzer, W.H.Manogue, J.Catal. 33 (1974) 457 115 I.Langmuir, Trans Faraday Soc. 17 (1922) 621 l16a A.G.Daglish, D.D.Eley, Proc. 2nd Int.Congr.on Catal., Paris, 1960, Edit.Technip. Paris, vol.2 (1960) p. 1615 b R.L.Moss, L.Whalley, Adv.Catal. 22 (1972) 115 C G.M.Schwab, K.Gossner, Z.Phys.Chem. N.F. 16 (1958) 39 117 F.C.M.J.M.van Delft, G.H.Vurens, M.C.Angevaare-Gruter, B.E.Nieuwenhuys, Stud.Surf.Sci.& Catal. 30 (1987) 229 118 S.H.Oh, J.E.Carpenter, J.Catal. 98 (1986) 178

95

580 119

chapter 12

A.G.van de Bosch-Driebergen, M.N.H.Kieboom, A.Dreumel, F.C.M.J.M.van Delft, R.M.Wolf, B.E.Nieuwenhuys, Catal.Lett. 2 (1989) 235 120 J.Siera, Ph.D.thesis, Leiden University, The Netherlands, 1992 121 A.D.Logan, M.T.Paffet, J.Catal. 133 (1992) 179 122 P.C.Liao, J.J.Carberry, T.H.Fleisch, E.E.Wolf, J.Catal. 74 (1982) 307 123 K.I.Choi, M.A.Vannice, J.Catal. 131 (1991) 36 124 M.Skoglundh, L.O.L6wendahl, J.E.Ottersted, Appl.Catal. 77 (1991) 9 125 G.Tammann, Z.Anorg.Chem. 111 (1920) 90 126 M.Kowaka, J.Jap.Inst.Metals 23 (1959) 659, according to G.C.Bond in "Catalysis by Metals", Academic Press, London, (1962) 450 127 A.B.van Cleave, E.K.Rideal, Trans Faraday Soc. 33 (1937) 635 128a H.Hirano, T.Yamada, K.Tanaka, J.Siera, B.E.Nieuwenhuys, Proc.10th Int.Congr.on Catal., Budapest, 1991, Elsevier, part A (1992) p.345 b A.Sasahara, H.Tamura, K-I.Tanaka, Catal.Lett. 28 (1994) 161 129 L.Ya Margolis, S.Z.Roginskii in 'Problemy Kin.i Kataliza' (editor: S.Z.Roginskii) Acad.Sci.Press, Moscow, 9 (1957) 107 130 H.R.Gerberich, N.W.Cant, W.K.Hall, J.Catal. 16 (1970) 204 131 P.V.Geenen, H.J.Boss, G.T.Pot, J.Catal. 77 (1982) 499 132 W.H.Flank, H.C.Beachell, J.Catal. 8 (1967) 316 133 G.C.Nelson, Surf.Sci. 59 (1976) 310 134 R.L.Moss, D.H.Thomas, J.Catal. 8 (1967) 162 135 D.Cormack, D.H.Thomas, R.L.Moss, J.Catal.32 (1974) 492 136 R.L.Moss in "Specialist Periodic Reports: Catalysis", Chem.Soc.London, vol. 1 (1977) 37 137 M.Masai, A.Kubota, K.Honda, Memoires of the Faculty of Eng., Kobe University 26 (1980) 195 138a K.C.Taylor in 'Catalysis, Science and Technology' (editors: J.R.Anderson, M.Boudart) Springer Verlag, Berlin, N.Y. 5 (1984) 119 b K.C.Taylor, Catal.Revs.Sci.Eng. 35 (1993) 457 139 K.C.Taylor, Stud.Surf.Sci.& Catal. 30 (1987) 97 140 L.L.Hegedus, J.J.Gumbleton, Chem.Tech. 10 (1980) 630 141 F.G.Dwyer, Catal.Revs. 6 (1972) 261 J.Wei, Adv.Catal. 24 (1975) 57 M.Shelef, K.Otto, N.C.Otto, Adv.Catal. 27 (1978) 311 B.Harrison, B.J.Cooper, A.J.J.Wilkins, Platinum Met.Rev. 25 (1981) 14 B.J.Cooper, Platinum Met.Rev. 38(1) (1994) 2 T.J.Truex, R.A.Searles, D.C.Sun, Platinum Met.Rev. 36(1) (1992) 2 142 P.Phillips Jr., British Patent 6096 (1831) (see A.J.B.Robertson, Platinum Met.Rev. 19 (1975) 64)

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


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.


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


/ 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_ 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,. ..,=,

~

>

&

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(,.)

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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9

10 11

12 13 14 15 16 17 18 19a b C

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Syngas reactions 106 107

713

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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


729

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

Volume 2

Volume 3

Volume 4

Volume 5

Volume 6 Volume 7 Volume 8 Volume 9 Volume 10 Volume 11

Volume 12 Volume 13 Volume 14

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

731 Volume 35 Volume 36 Volume 37 Volume 38 Volume 39 Volume 40 Volume 41


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Volume 47 Volume 48 Volume 49 Volume 50


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

732 Volume 54 Volume 55 Volume 56

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

734 Volume86 Volume 87 Volume 88 Volume 89

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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

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