Table Of ContentChemisorption and Reactivity on
Supported Clusters and Thin Films
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Series E: Applied Sciences· Vol. 331
Chemisorption and Reactivity on
Supported Clusters and
Thin Films
Towards an Understanding of
Microscopic Processes in Catalysis
edited by
Richard M. Lambert
Department of Chemistry,
University of Cambridge,
Cambridge, U.K.
and
Gianfranco Pacchioni
Department of Inorganic, Organometallic and Analytical Chemistry,
University of Milan,
Milan, Italy
Springer-Science+Business Media, B.V.
Proceedings of the NATO Advanced Study Institute on
Chemisorption and Reactivity on Supported Clusters and Thin Films:
Towards an Understanding of Microscopic Processes in Catalysis
Erice, Trapani, Sicily
July 15-26, 1996
A C.I.P. Catalogue record for this book is available from the Library of Congress.
ISBN 978-90-481-4807-3 ISBN 978-94-015-8911-6 (eBook)
DOI 10.1007/978-94-015-8911-6
Printed on acid-free paper
AII Rights Reserved
© 1997 Springer Science+Business Media Dordrecht
Originally published by Kluwer Academic Publishers in 1997
Softcover reprint of the hardcover 1 st edition 1997
No part of the material protected by this copyright notice may be reproduced or
utilized in any form or by any means, electronic or mechanical, including photo
copying, record ing or by any information storage and retrieval system, without written
permission from the copyright owner.
CONTENTS
Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. vii
Introduction to Heterogeneous Catalysis
R.M. Lambert .............................................. .
Thin Films as Model Catalysts
D.R. Rainer and D. W Goodman . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 27
Metal Deposits on Thin Well Ordered Oxide Films: Morphology, Adsorption and
Reactivity
M. Biiumer, J. Libuda and H. -J. Freund ........................... 61
The Growth and Stability of Ultrathin Films on Metal and Oxide Surfaces
T.E. Madey ............................................... 105
Size Effects in Heterogeneous Catalysis: A Surface Science Approach
C. Henry, C. Chapon, S. Giorgio and C. Goyhenex . . . . . . . . . . . . . . . . . . . . 117
Supported Clusters, Structure, Reactivity and Microscopic Processes in Catalysis
M. Ichikawa ............................................... 153
Quantitative Determination of Molecular Adsorbate Structures
D.P. Woodruff . ............................................ 193
The Structure and Reactivity of TiOi11O) Supported Palladium and Rhodium
B.E. Hayden ............................................... 215
Angle-Scanned Photoelectron Diffraction: A Structural Probe for Near-Surface
Atomic Layers
G. Granozzi and M. Sambi ..................................... 237
Co-Adsorption on Metal-Oxide Crystal Surfaces: Cases of CO/Cu/ZnO(OOOl) and
CO/NalTiOi11O)
P.J. M~ller ................................................ 267
Theory of Adsorption and Surface Reactions
B. Hammer and J.K. N~rskov ................................... 285
Density Functional Cluster Calculations on Metal Deposition at Oxide Surfaces
N. Rosch and G. Pacchioni .................................... 353
vi
Theory of Heterogeneous Catalytic Reactivity using the Cluster Approximation
R.A. van Sanlen ............................................ 371
Cluster Modelling of Oxide Surfaces: Structure, Adsorption and Reactivity
G. Pacchioni .............................................. 395
Theoretical Modelling of Chemisorption and Reactions on Metal-Oxide Surfaces
L.G.M. Pettersson, M. Nyberg, J.-L. Pascual and M.A. Nygren ............ 425
Stability of Polar Oxide Surfaces: Oxygen Vacancies and Non-Stoichiometric
Reconstructions
e.
Noguera, A. Pojani, F. Finocchi and J. Goniakowski ................ 455
Computer Simulation of Structural, Defect and Surface Properties of Solids
e.R.A. Callow, D.H. Gay, MA Nygren and D.e. Sayle ................. 479
Index ...................................................... 523
PREFACE
Heterogeneous catalysis is of immense technological and economic importance on a global
scale because it provides the backbone of the world's chemical and oil industries. For this
reason, among others, the subject is intensively researched in both academic and industrial
laboratories. The innate complexity of practical catalytic systems suggests that useful
progress should be achievable by investigating key aspects of catalysis by experimental
studies on idealised model systems. Thin ftlms and supported clusters are two promising
types of model systems that can be used for this purpose because they mimic important
aspects of the properties of practical dispersed catalysts. Similarly, appropriate theoretical
studies of chemisorption and surface reactions using cluster models or extended slab
systems can provide valuable information about the factors that underlie bonding and
catalytic activity. This book is based on the lectures given at the 32nd course of the
International School of Material Science at the Ettore Majorana Centre, Erice Sicily
between July the 15th and 26th 1996. The title of the course was "Chemisorption and
reactivity on supported Clusters and thin ftlms: towards an understanding of microscopic
processes in catalysis". The Institute dealt with the fundamentals of heterogeneous
catalysis from phenomenological, experimental and theoretical points of view. An
important theme of the course was its emphasis on the design and use of suitable model
systems whose properties could be expected to mimic key aspects of the behaviour of
practical catalysts which generally are much more complex and poorly defmed materials.
This volume describes experimental and theoretical approaches to the surface chemistry and
catalytic behaviour of metals, metal oxides and metaIlmetal oxide systems. It should be of
value to advanced undergraduates taking courses in chemisorption and catalysis and to
graduate students undertaking research in this field. An introduction to the principles and
main themes of heterogeneous catalysis is followed by detailed accounts of the application
of modem experimental· and theoretical techniques to fundamental problems in catalytic
science. The description of a variety of advanced experimental methods to problems of
structure, bonding and reactivity at well defmed surfaces is complemented by an account of
theoretical procedures including Hartree-Fock, density functional and computer simulation
~hniques.
The editors gratefully acknowledge the support provided by NATO, the sole sponsor, to
whom thanks are also due. Without this support, neither the course nor the production of
this book would have been possible. We are indebted to Professor A. Zichichi, Director of
the Majorana Centre, whose help was essential for the planning of the school. Finally, it a
pleasure to acknowledge the hospitality and generous help provided by all the staff at the
Ettore Majorana Centre, whose efficiency, attention to detail, and unfailing good humour
made the school the successful and highly enjoyable occasion that it was.
Richard Lambert
Cambridge, England
Gianfrnnco Pacchioni
Milan, Italy
October 1996
vii
INTRODUCTION TO HE1EROGENEOUS CATALYSIS
R. M. LAMBERT
University of Cambridge
Department of Chemistry
Lensfield Road
Cambridge
CB21EW
INTRODUCTION
Chemisorption is a necessary precursor to heterogeneous catalysis. Thin ftlms and clusters
provide a means of studying chemisorption experimentally, and there are theoretical
approaches related to each of these avenues. Our purpose is to achieve a fundamental
understanding of heterogeneous catalysis, therefore let us begin by asking why this is
considered to be an important and worthwhile goal. Few would disagree that most research
on catalysis is at least partly driven by the desire to improve existing technology or to
invent new technology. A principal purpose of this introductory chapter is to put the
subject in context, to indicate what is known and what remains to be done, and to give an
indication the most important avenues for future development. Another is to introduce
basic ideas about the nature of solid catalysts, to show how they may be approximated by
idealised thin film systems and to indicate what may be learned by this approach. A third
objective is to give a simple overview of where we stand with respect to theories of
catalysis. Finally, these themes are illustrated with some specific examples. The reader
will be aware that many of the themes introduced here are developed in much greater detail
in the chapters that follow this one.
In terms of the world economy and with respect environmental protection, heterogeneous
catalysis, especially by metals, and especially by the platinum metals, is of global
strategic importance. To some degree, this is reflected in the fact that the most academic
research is focussed on adsorption and catalysis by metal surfaces -although it must be
admitted that the relative ease of studying metals plays a very significant part here. To
appreciate the scope of our subject, consider first some urgent environmental issues.
Reactions occurring on the surfaces of polar stratospheric cloudsl or volcanic inorganic
aerosols[2] are implicated in ozone destruction. Much research in electrocatalysis[3] and
photocatalysis[4] is aimed at the development of fuel cells, solar energy conversion or
CO2 conversion -issues that are of importance with respect to minimising the greenhouse
effect. Control of toxic emissions from automotive and stationary combustion engines is
achieved entirely by the application of catalytic systems. The economic impact of
heterogeneous catalysis is no less striking. More than 90% of the output of world
chemical industry depends on the operation of heterogeneously catalysed reactions. More
than 25% of global industrial output (not just the production of chemicals) depends on
heterogeneous catalysis by the Pt metals alone. 20% of world GNP is derived through the
operation of heterogeneously catalysed processes, equivalent to about $5,000 billion/year.
Governments and industry constantly demand more efficient and cleaner processes, cheaper
R.M. lAmbert and G. Pacchioni (eds.).
Chemisorption and Reactivity on Supported Clusters and Thin Films. 1-26.
© 1997 Kluwer Academic Publishers.
2
feedstocks and cheaper chemical plants -and better performing products. Catalysis enables
all of these. This is exemplified by the huge sales of catalysts themselves (as opposed to
the products made by employing them) amounting to $9 billion/year and growing at
5%/year. The principal industrial applications of catalytic technology lie in the in
following areas.
Oil refining (cracking, isomerisation, reforming, hydrocracking). The industry is
undergoing m~or consolidation and developments in the Far East are likely to be crucial.
Petrochemicals production (polymers, syngas, selective oxidation and reduction). Most
research is driven by the desire to reduce costs by discovering new routes or by improving
the selectivity of existing routes.
Environmental protection (80% autocatalysts; 20% stationary sources). Suppression of
diesel exhaust and volatile organic compounds emission probably hold the key to future
growth in this area.
The m~ority of solid catalysts are broadly divisible into two classes: metals (including
alloys) and oxides (including binary or ternary mixed oxides). We shall consider only these
classes and neglect all others - for example sulphides of the transition metals which are
also industrially important. Many catalytic processes are operated at the solid gas interface,
and it is this interface that we shall concentrate on here. However, it should be borne in
mind that many important catalytic processes, including electrocatalysis, are operated in
the liquid phase. Such systems are difficult to address experimentally: the presence of the
solvent introduces great complexity, both in the acquisition and in the interpretation of
data. Therefore we shall not consider reactions at the solid liquid interface.
Experimental studies of catalytically relevant phenomena have advanced enormously in the
last two decades or so, thanks to the use of highly characterised surfaces and the
development of a range of sophisticated techniques. These so-called "surface science"
methods are to a large extent based on electron spectroscopic techniques which exploit the
energy dependence of the inelastic mean free path of electrons in solids[5] . By working
with ejected electrons whose kinetic energies lie in the range - 50 to 500 eV, one may
achieve the required surface sensitivity. Such methods enable us to study the atomic
arrangement and electronic properties of surfaces, especially extended surfaces, in great
detail. With well defined surfaces, working under ultra high vacuum conditions, very
detailed information about the dynamics of chemisorption and the structure and bonding of
adsorbed species can be obtained. To extrapolate from this information to an understanding
of the observed activity and selectivity at elevated pressures of highly dispersed working
catalysts is a formidable task, because of the complexity of the latter. Typically, practical
heterogeneous catalysts are relatively poorly characterised. The catalytically active surface
phase may include a variety of different crystal planes of unknown chemical composition
interacting to an unknown degree with the support pbase. It is in this connection that
experiments on well characterised thin films and clusters can be especially valuable. Most
such work has tended to focus on metals as catalysts, albeit in some cases addreSSing their
interaction with oxide supports. However, corresponding work on oxides in their own
right, though much more scarce, is gaining momentum. Although experiments on oxide
surfaces involve a higher degree of experimental difficulty, this is now an important
growth area[6] and is likely to remain so -many industrially important catalysts are oxides
and most industrially important catalyst supports are oxides.
3
Advances in theory have provided vital fundamental understanding, and the most important
ab initio theoretical techniques are discussed in detail elsewhere in this volume. Suffice to
say here that in recent years density functional calculations have come to play an
increasingly important role in chemisorption calculations, in regard to both energetics and
dynamics[7], especially with respect to metal surfaces. At the simplest level of
approximation (jellium model) density functional theory has been used to address a variety
of chemisorption problems, in eluding promoter effects, and a number of important
conclusions have emerged[8,9] Such calculations are now starting to make an important
contribution to our understanding of adsorption on both perfect and defective oxide
surfaces [1 0]. Standard density functional calculations treat the exchange-correlation energy
in the local density approximation, i.e. the electron density is treated as locally uniform.
However, this gives rise to systematic "overbinding" which can introduce significant errors
in the results. The accuracy of such calculations can be improved by application of
generalised gradient approximations in which the exchange-correlation energy is represented
in a form that explicitly contains gradients of the electron density[11].
Despite impressive advances in both experiment and theory, it remains true that valid
generalisations are few and it is difficult to make specific predictions that are of direct
practical utility. This reflects the sheer chemical and structural complexity of real catalytic
systems. A great proportion of current research on catalysis concentrates on finding the
link between structure and function. This link is one of the great unifying themes of
chemical theory which has found triumphant expression in the field of organic chemistry
where it has culminated in the ability to predict accurately the outcome of chemical
reactions taking place between very complicated molecules in the solution phase. Many
of these homogeneous reactions are operated catalytically. Let us take a step closer to our
present subject. The structure, spectra and catalytic behaviour of metal cluster compounds
in solution are known and understood with a high degree of certainty. Related to this is the
fact that homogeneous catalysts are far more selective than their heterogeneous
counterparts. Why? Because in solution reactions take place between well defined
molecular entities that can be studied in very great detail by methods that on the whole
yield unambiguous information. Usually, we need consider only one type of reactive
encounter in each elementary step: A collides with B; we need not worry about A', A" ...
B',Bft and so on. Not only do we know the molecular and electronic structure of the
reactants and products very well, in many cases it is possible to isolate and study the
intermediates that are formed in some overall process.
With a heterogeneous catalyst the corresponding problem is much more difficult. A whole
range of very different adsorption sites and surface species may be present and one cannot
be certain that the measurement technique provides information about the chemically
significant fraction of the adsorbed layer -a point that we shall return to below. Because
the heterogeneous system is characterised by a spread of adsorption sites and adsorbed
species, chemical specificity, usually termed selectivity, is often rather poor. Many
reaction channels may be accessible and a distribution of products is produced. Why then
bother with heterogeneous catalysis at all? The answer is simple. Compared with their
homogeneous counterparts, heterogenous catalysts are capable of delivering ve~y high
productivity because intrinsic turnover rates are high and because they can be cpera.ed in a
continuous mode (as opposed to batch mode). Also, again in contrast with homogeneous
catalysts, one is not faced with the problem of separating the catalyst from the products.
