(Fundamental and applied catalysis) geoffrey c bond metal catalysed reactions of hydrocarbons springer (2005)

Metal-Catalysed
Reactions
of Hydrocarbons


FUNDAMENTAL AND APPLIED CATALYSIS
Series Editors: M. V. Twigg
Johnson Matthey
Catalytic Systems Division
Royston, Hertfordshire, United Kingdom

M. S. Spencer
Department of Chemistry
Cardiff University
Cardiff, United Kingdom

CATALYST CHARACTERIZATION: Physical Techniques for Solid Materials
Edited by Boris Imelik and Jacques C. Vedrine
CATALYTIC AMMONIA SYNTHESIS: Fundamentals and Practice
Edited by J. R. Jennings
CHEMICAL KINETICS AND CATALYSIS
R. A. van Santen and J. W. Niemantsverdriet
DYNAMIC PROCESSES ON SOLID SURFACES
Edited by Kenzi Tamaru
ELEMENTARY PHYSICOCHEMICAL PROCESSES ON SOLID
SURFACES
V. P. Zhdanov
HANDBOOK OF INDUSTRIAL CATALYSTS
Lawrie Lloyd
METAL-CATALYSED REACTIONS OF HYDROCARBONS
Geoffrey C. Bond

METAL–OXYGEN CLUSTERS: The Surface and Catalytic Properties of
Heteropoly Oxometalates
John B. Moffat
SELECTIVE OXIDATION BY HETEROGENEOUS CATALYSIS
Gabriele Centi, Fabrizio Cavani, and Ferrucio Trifir`o
SURFACE CHEMISTRY AND CATALYSIS
Edited by Albert F. Carley, Philip R. Davies, Graham J. Hutchings,
and Michael S. Spencer

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Metal-Catalysed
Reactions
of Hydrocarbons
Geoffrey C. Bond
Emeritus Professor
Brunel University
Uxbridge, United Kingdom

With 172 illustrations


Geoffrey C. Bond
59 Nightingale Road
Rickmansworth, WD3 7BU
United Kingdom

Library of Congress Cataloging-in-Publication Data
Bond, G.C. (Geoffrey Colin)
Metal-catalysed reactions of hydrocarbond/Geoffrey C. Bond.
p. cm. — (Fundamental and applied catalysis)
Includes bibliographical references and index.
ISBN 0-387-24141-8 (acid-free paper)
1. Hydrocarbons. 2. Catalysis. 3. Metals—Surfaces. 4. Reaction mechanisms
(Chemistry) I. Title. II. Series.
QD305.H5B59 2005
547Ј.01—dc22
2004065818
ISBN-10: 0-387-24141-8
ISBN-13: 987-0387-24141-8

e-ISBN: 0-387-26111-7

Printed on acid-free paper.

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ACKNOWLEDGMENTS

No work such as this can be contemplated without the promise of advice and
assistance from one’s friends and colleagues, and I must first express my very
deep sense of gratitude to Dr Martyn Twigg, who more than anyone else has
been responsible for this book coming to completion. I am most grateful for his
unfailing support and help in a variety of ways. I am also indebted to a number of
my friends who have read and commented (sometimes extensively) on drafts of
all fourteen chapters: they are Dr Eric Short, Professor Vladimir Ponec, Dr Adrian
Taylor, Professor Norman Sheppard, Professor Zoltan Pa´al and Professor Peter
Wells (who read no fewer than six of the chapters). Their advice has saved me
from making a complete ass of myself on more than one occasion. As to the
remaining errors, I must excuse myself in the words of Dr Samuel Johnson, who

when accused by a lady of mis-defining a word in his dictionary gave as his reason:
Ignorance, Madam; pure ignorance.
One of the most pleasing aspects of my task has been the speed with which
colleagues world-wide, some of whom I have never met, have responded promptly
and fully to my queries about their work; Dr Andrzej Borodzi´nski and Professor
Francisco Zaera deserve particular thanks for their extensive advice on respectively
Chapters 9 and 4. Dr Eric Short has been especially helpful in teaching me some
of the tricks that have made the use of my pc easier, and Mrs Wendy Smith has
skillfully typed some of the more complex tables.
Finally, I could not have completed this work without the patient and loving
support of my wife Mary.

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There must be a beginning of any good matter . . .

SCOPE AND PURPOSE OF THE WORK
It is important at the start to have a clear conception of what this book is about:
I don’t want to raise false hopes or expectations. The science of heterogeneous
catalysis is now so extensive that one person can only hope to write about a small
part of it. I have tried to select a part of the field with which I am familiar, and which
while significant in size is reasonably self-contained. Metal-catalysed reactions of
hydrocarbons have been, and still are, central to my scientific work; they have
provided a lifetime’s interest. Age cannot wither nor custom stale their infinite
variety.
Experience now extending over more than half a century enables me to see
how the subject has developed, and how much more sophisticated is the language

we now use to pose the same questions as those we asked when I started research in
1948. I can also remember papers that are becoming lost in the mists of time, and
I shall refer to some of them, as they still have value. Age does not automatically
disqualify scientific work; the earliest paper I cite is dated 1858.
It is a complex field in which to work, and there are pitfalls for the unwary,
into some of which I have fallen with the best. I shall therefore want to pass
some value-judgements on published work, but in a general rather than a specific
way. While there is little in the literature that is actually wrong, although some
is, much is unsatisfactory, for reasons I shall try to explain later. I have always
tried to adopt, and to foster in my students, a healthy scepticism of the written
word, so that error may be recognised when met. Such error and confusion as
there is arises partly from the complexity of the systems being studied, and the
vii


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PROLOGUE

great number of variables, some uncontrolled and some even unrecognised,1 that
determine catalytic performance. Thus while in principle (as I have said before2 ) all
observations are valid within the context in which they are made, the degree of their
validity is circumscribed by the care taken to define and describe that context. In
this respect, heterogeneous catalysis differs from some other branches of physical
chemistry, where fewer variables imply better reproducibility, and therefore more
firmly grounded theory.
Nevertheless it will be helpful to try to identify what constitutes the solid,
permanent core of the subject, and to do this we need to think separately about
observations and how to interpret them. Interpretation is fluid, and liable to be
changed and improved as our knowledge and understanding of the relevant theory

grows. Another source of confusion in the literature is the attempt to assign only
a single cause to what is seen, whereas it is more likely that a number of factors
contribute. A prize example of this was the debate, now largely forgotten, as to
whether a metal’s ability in catalysis was located in geometric or in electronic
character, whereas in fact they are opposite sides of the same coin. It was akin to
asking whether one’s right leg is more important than one’s left. Similar misconceived thinking still appears in other areas of catalysis. So in our discussion we
must avoid the temptation to over-simplify; as Einstein said, We must make things
as simple as possible – but not simpler.

THE CATALYSED REACTIONS OF HYDROCARBONS
This book is concerned with the reactions of hydrocarbons on metal catalysts
under reducing conditions; many will involve hydrogen as co-reactant. This limitation spells the exclusion of such interesting subjects as the reactions of syngas,
the selective hydrogenation of α,β-unsaturated aldehydes, enantioselective hydrogenation, and reactions of molecules analogous to hydrocarbons but containing a
hetero-atom. For a recent survey of these areas, the reader is referred to another
source of information3 . There will be nothing about selective or non-selective
oxidation of hydrocarbons, nor about the reforming of alkanes with steam or
carbon dioxide. That still leaves us plenty to talk about; hydrogenation, hydrogenolysis, skeletal and positional isomerisation, and exchange reactions will keep us
busy. Reactions of hydrocarbons by themselves, being of lesser importance, will
receive only brief attention.
Most of the work to be presented will have used supported metal catalysts,
and a major theme is how their structure and composition determine the way in
which reactions of hydrocarbons proceed. Relevant work on single crystals and
polycrystalline materials will be covered, because of the impressive power of
the physical techniques that are applicable to them. There are however important


PROLOGUE

ix

differences as well as similarities between the macroscopic and microscopic forms
of metals.
This may be an appropriate time to review the metal-catalysed reactions of
hydrocarbons. The importance of several major industrial processes which depend
on these reactions – petroleum reforming, fat hardening, removal of polyunsaturated molecules from alkene-rich gas streams – has generated a great body of
applied and fundamental research, the intensity of which is declining as new challenges appear. This does not of course mean that we have a perfect understanding
of hydrocarbon reactions: this is not possible, but the decline in the publication
rate provides a window of opportunity to review past achievements and the present
status of the field.
I shall as far as possible use IUPAC-approved names, because although the
writ of IUPAC does not yet apply universally I am sure that one day it will. Trivial
names such as isoprene will however be used after proper definition; I shall try to
steer a middle course between political correctness and readability.
You must be warned of one other restriction; this book will not teach you to do
anything. There will be little about apparatus or experimental methods, or how to
process raw results; only when the method used bears strongly on the significance
of the results obtained, or where doubt or uncertainty creeps in, may procedures
be scrutinised.
Some prior knowledge has to be assumed. Elementary concepts concerning
chemisorption and the kinetics of catalysed reactions will not be described; only
where the literature reveals ignorance and misunderstanding of basic concepts will
discussion of them be included. Total linearity of presentation is impossible, but
in the main I have tried to follow a logical progression from start to finish.

UNDERSTANDING THE CAUSES OF THINGS
I mentioned the strong feeling I have that there is much in the literature on
catalysis that is unsatisfactory: let me try to explain what I mean. I should first
attempt a general statement of what seems to me to be the objectives of research
in this field.
The motivation for fundamental research in heterogeneous catalysis is to develop the understanding of surface chemistry to the point where the physicochemical characteristics of active centres for the reactions of interest can be

identified, to learn how they can be modified or manipulated to improve the
desired behaviour of the catalyst, and to recognise and control those aspects
of the catalyst’s structure that limit its overall performance.

If this statement is accepted, there is no need for a clear distinction to be made
between pure and applied work: the contrast lies only in the strategy adopted to


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PROLOGUE

reach the desired goals. In applied work, the required answer is often obtained
by empirical experimentation, now sometimes aided by combinatorial techniques;
in pure research, systematic studies may equally well lead to technically useful
advances, even where this was not the primary objective.
In the past, the work of academic scientists has concentrated on trying to
understand known phenomena, although there has been a progressive change of
emphasis, dictated directly or indirectly by funding agencies, towards the discovery
of new effects or better catalyst formulations. I have no wish to debate whether
or not this is a welcome move, so I will simply state my own view, which is that
it is the task of academic scientists to uncover scientific concepts and principles,
to rationalise and to unify, and generally to ensure that an adequate infrastructure
of methodologies (the so-called ‘enabling technologies’) is available to support
and sustain applied work. Industrial scientists must build on and use this corpus
of knowledge so as to achieve the practical ends. The cost of scaling-up and
developing promising processes is such that academic institutions can rarely afford
to undertake it; this sometimes means that useful ideas are stillborn because the
credibility gap between laboratory and factory cannot be bridged.
The objective of the true academic scientist is therefore to understand, and

the motivation is usually a strictly personal thing, sometimes amounting to a religious fervour. It is no consolation to such a person that someone else understands,
or thinks he understands: and although some scientists believe they are granted
uniquely clear and divinely guided insights, many of us are continually plagued
by doubts and uncertainties. In this respect the searches for religious and scientific
truths resemble one another. With heterogeneous catalysis, perhaps more than with
any other branch of physical chemistry, absolute certainty is hard to attain, and the
sudden flash of inspiration that brings order out of chaos is rare. It says much for
the subject that the last person to have heterogeneous catalysis mentioned in his
citation for a Nobel Prize was F.W. Ostwald in 1909.
For many of us, what we require is expressed as a reaction mechanism or
as a statement of how physicochemical factors determine activity and/or product
selectivity. What constitutes a reaction mechanism will be discussed later on. What
is however so unsatisfactory about some of what one reads in the literature is that
either no mechanistic analysis is attempted at all, or that the conclusions drawn
often rest on a very insubstantial base of experimental observation; magnificent
edifices of theoretical interpretation are sometimes supported by the flimsiest foundation of fact, and ignore either deliberately or accidentally much information from
elsewhere that is germane to the argument. I particularly dislike those papers that
devote an inordinate amount of space to the physical characterisation of catalysts
and only a little to their catalytic properties. Obtaining information in excess of
that required to answer the questions posed is a waste of time and effort: it is
a work of supererogation.4 Full characterisation should be reserved for catalysts


PROLOGUE

xi

that have interesting and worthwhile catalytic behaviour, and adequate time should
be devoted to this.
This book is not intended as an encyclopaedia, but I will try to cite as much

detail and as many examples as are needed to make the points I wish to make.
Three themes will pervade it.
(1) The dependence of the chemical identity and physical state of the metal
on its catalytic behaviour; integration of this behaviour for a given metal
over a series of reactions constitutes its catalytic profile.
(2) The effect of the structure of a hydrocarbon on its reactivity and the types
of product it can give; this is predicated on the forms of adsorbed species
it can give rise to.
(3) The observations on which these themes are based will wherever possible be expressed in quantitative form, and not merely as qualitative
statements.
Lord Kelvin said we know nothing about a scientific phenomenon until we can
put numbers to it. However, with due respect to his memory, numbers are the raw
material for understanding, and not the comprehension itself. We must chase the
origin and significance of the numbers as far into the depths of theoretical chemistry
as we can go without drowning. We shall want to see how far theoretical chemistry
has been helpful to catalysis by metals. For most chemists there are however strict
limits to the profundity of chemical theory that they can understand and usefully
deploy, and it is chemists I wish to address. If however you wish to become better acquainted with the theoretical infrastructure of the subject, please read the first four
chapters of a recently published book;3 for these my co-author can claim full credit.
The foregoing objectives do not require reference to all those studies that simply show how the rate varies with some variable under a single set of experimental
conditions, where the variable may for example be the addition of an inactive
element or one of lesser activity, the particle size or dispersion, the addition of
promoters, or an aspect of the preparation method. Such limited measurements
rarely provide useful information concerning the mechanism, and many of the
results and the derived conclusions have recently been reviewed elsewhere.3 We
look rather to the determination of kinetics and product distributions to show how
the variable affects the reaction mechanism.
To explore the catalytic chemistry of metal surfaces, and in particular of
small metal particles, we shall have to seek the help of adjacent areas of science.
These will include the study under UHV conditions of chemisorbed hydrocarbons, concerning which much is now known; homogeneous catalysis by metal

complexes, and catalysis by complexes adsorbed on surfaces (to a more limited
extent); organometallic chemistry in general; and of course theoretical chemistry.


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CONCERNING THE USEFULNESS OF MODELS AND MECHANISMS
The training of chemists inculcates a desire to interpret the phenomena of
chemistry through the properties of individual atoms and molecules. To this
end they have devised a variety of ways of symbolising and visualising their
composition, size and shape. The purely symbolic method of identifying elements, while successfully distinguishing a hundred or so by means of at most
two letters, requires subscripts and superscripts to define atomic mass, nuclear
charge and oxidation state, but there is no means of showing size or chemical
character.
Structural formulae of various degrees of sophistication may be used to show
how atoms are linked in a molecule, what the bond angles and lengths are, and
ultimately how orbitals are employed in bonding, but depictions of adsorbed
species and surfaces processes of a very elementary kind are still often used,
and all too frequently there is no diagram or sketch at all to show what is in the
writer’s mind. This is a pity, because most chemists have pictorial minds, and
a simple sketch can speak volumes. A flexible and informative symbolism for
surfaces states and events is urgently needed, because our ability to think innovatively and imaginatively is limited by the techniques we have to express our
thoughts. Words are very imperfect vehicles for ideas and emotions. Perception
of the third dimension is helped by molecular graphics, but such displays are
impermanent until printed, when the extra dimension is lost. Often there is no alternative to the use of some kind of atomic model to convey the structures of surface
phases.
Our belief that we can meaningfully describe the transformations of molecules
by a few squiggles on a sheet of paper is a major act of faith. Acts of faith have

their place in science as in religion, and our ability to create a conceptual model or
hypothesis is however no more than a set of statements, either formal or informal,
that increases the probability of successfully predicting an event or the outcome
from a given situation. Karl Popper asserted that no hypothesis can ever be proved
correct; it only remains plausible as long as no evidence is found to contradict it. A
few scientific ideas have graduated from speculation through theory to the status
of immutable and universal law; the Periodic Classification of the Elements and
General Theory of Relativity are two such, but unfortunately there is as yet little
in catalysis of which we can say ‘It will always be thus’.

1.
2.
3.
4.

D. Rumsfeld: There are things we do not know we do not know (2003).
Catalysis by Metals (Preface), Academic Press: London (1962).
V. Ponec and G. C. Bond, Catalysis by Metals and Alloys, Elsevier: Amsterdam (1996).
See Article XIV of the Articles of Religion in the 1662 English Prayer Book.


CONTENTS

CHAPTER 1. METALS AND ALLOYS
1.1. The Metallic State . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.1.1. Characteristic Properties . . . . . . . . . . . . . . . . . . . . . .
1.1.2. Theories of the Metallic State . . . . . . . . . . . . . . . . . . .
1.2. The Metallic Surface . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.2.1. Methods of Preparation . . . . . . . . . . . . . . . . . . . . . .
1.2.2. Structure of Metallic Surfaces . . . . . . . . . . . . . . . . . . .

1.2.3. Theoretical Descriptions of the Metal Surface . . . . . . . . . .
1.3. Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.3.1. The Formation of Alloys . . . . . . . . . . . . . . . . . . . . .
1.3.2. Electronic Properties of Alloys and Theoretical Models . . . .
1.3.3. The Composition of Alloy Surfaces . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2
2
9
14
14
16
22
24
24
27
29
31

CHAPTER 2. SMALL METAL PARTICLES AND SUPPORTED
METAL CATALYSTS
2.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1.1. Microscopic Metals . . . . . . . . . . . . . . . . . . . . . . . .
2.1.2. Instability of Small Metal Particles . . . . . . . . . . . . . . . .
2.2. Preparation of Unsupported Metal Particles . . . . . . . . . . . . . . .
2.3. Supported Metal Catalysts . . . . . . . . . . . . . . . . . . . . . . . .
2.3.1. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3.2. Methods of Preparation . . . . . . . . . . . . . . . . . . . . . .
2.4. Measurement of the Size and Shape of Small Metal Particles . . . . .

36
36
38
39
40
40
41
47
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CONTENTS

2.4.1. Introduction: Sites, Models, and Size Distributions . . . . . . .
2.4.2. Physical Methods for Characterising Small Metal Particles . . .
2.4.3. Measurement of Dispersion by Selective Gas-Chemisorption .
2.5. Properties of Small Metal Particles . . . . . . . . . . . . . . . . . . . .
2.5.1. Variation of Physical Properties with Size: Introduction . . . .
2.5.2. Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.5.3. Energetic Properties . . . . . . . . . . . . . . . . . . . . . . . .
2.5.4. Electronic Properties . . . . . . . . . . . . . . . . . . . . . . . .
2.5.5. Theoretical Methods . . . . . . . . . . . . . . . . . . . . . . . .
2.5.6. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.6. Metal-Support Interactions . . . . . . . . . . . . . . . . . . . . . . . .
2.6.1. Causes and Mechanisms . . . . . . . . . . . . . . . . . . . . . .
2.6.2. Particle Size Effects and Metal-Support
Interactions: Summary . . . . . . . . . . . . . . . . . . . . . . .

2.7. Promoters and Selective Poisons . . . . . . . . . . . . . . . . . . . . .
2.8. Sintering and Redispersion . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

47
52
58
60
60
63
65
66
67
68
69
69
74
75
77
78

CHAPTER 3. CHEMISORPTION AND REACTIONS
OF HYDROGEN
3.1. The Interaction of Hydrogen with Metals . . . . . . . . . . . . . . . .
3.2. Chemisorption of Hydrogen on Unsupported Metals and Alloys . . . .
3.2.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.2. The Process of Chemisorption . . . . . . . . . . . . . . . . . .
3.2.3. The Chemisorbed State: Geometric Aspects . . . . . . . . . . .
3.2.4. The Chemisorbed State: Energetic Aspects . . . . . . . . . . .
3.3. Chemisorption of Hydrogen on Supported Metals . . . . . . . . . . .

3.3.1. Introduction: Determination of Metal Dispersion . . . . . . . .
3.3.2. Characterisation of Chemisorbed Hydrogen . . . . . . . . . . .
3.3.3. Theoretical Approaches . . . . . . . . . . . . . . . . . . . . . .
3.3.4. Hydrogen Spillover . . . . . . . . . . . . . . . . . . . . . . . .
3.3.5. The “Strong Metal-Support Interaction” . . . . . . . . . . . . .
3.4. Reactions of Hydrogen . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

94
97
97
100
102
108
114
114
124
129
132
137
140
142

CHAPTER 4. THE CHEMISORPTION OF HYDROCARBONS
4.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154
4.1.1. Types of Alkane . . . . . . . . . . . . . . . . . . . . . . . . . . 154


CONTENTS

4.1.2. Types of Unsaturated Hydrocarbon . . . . . . . . . . . . . . . .
4.1.3. The Literature . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2. The Chemisorption of Hydrocarbons: An Overview . . . . . . . . . .
4.3. The Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.4. Identification of Adsorbed Hydrocarbon Species . . . . . . . . . . . .
4.4.1. The Catalogue - or ‘The Organometallic Zoo’ . . . . . . . . . .
4.4.2. The π and di-σ Forms of Chemisorbed Alkenes . . . . . . . . .
4.5. Structures and Properties of Chemisorbed Hydrocarbons . . . . . . . .
4.5.1. Detailed Structures of Chemisorbed Alkenes . . . . . . . . . .
4.5.2. Structures of Chemisorbed Ethyne . . . . . . . . . . . . . . . .
4.5.3. Structures of Chemisorbed Benzene . . . . . . . . . . . . . . .
4.5.4. Heats of Adsorption . . . . . . . . . . . . . . . . . . . . . . . .
4.5.5. Characterisation by Other Spectroscopic Methods . . . . . . . .
4.5.6. C6 Molecules . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.6. Thermal Decomposition of Chemisorbed Hydrocarbons . . . . . . . .
4.7. Theoretical Approaches . . . . . . . . . . . . . . . . . . . . . . . . . .
4.8. Chemisorption of Alkanes . . . . . . . . . . . . . . . . . . . . . . . .
4.9. The Final Stage: Carbonaceous Deposits . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

xv

154
155
156
158
161
161
169
176

176
178
178
180
186
186
186
190
196
197
198

CHAPTER 5. INTRODUCTION TO THE CATALYSIS OF
HYDROCARBON REACTIONS
5.1. The Essential Nature of Catalysis . . . . . . . . . . . . . . . . . . . .
5.1.1. A Brief History of Catalysis . . . . . . . . . . . . . . . . . . . .
5.1.2. How Catalysts Act . . . . . . . . . . . . . . . . . . . . . . . . .
5.1.3. The Catalytic Cycle . . . . . . . . . . . . . . . . . . . . . . . .
5.2. The Formulation of Kinetic Expressions . . . . . . . . . . . . . . . . .
5.2.1. Mass Transport versus Kinetic Control . . . . . . . . . . . . . .
5.2.2. The Purpose of Kinetic Measurements . . . . . . . . . . . . . .
5.2.3. Measurement and Expression of Rates of Reaction . . . . . . .
5.2.4. The Langmuir-Hinshelwood Formalism . . . . . . . . . . . . .
5.2.5. Effect of Temperature on Rate and Rate Constant . . . . . . . .
5.2.6. Selectivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2.7. Kinetic modelling . . . . . . . . . . . . . . . . . . . . . . . . .
5.3. The Concept of Reaction Mechanism . . . . . . . . . . . . . . . . . .
5.4. The Idea of the Active Centre . . . . . . . . . . . . . . . . . . . . . . .
5.5. The Use of Bimetallic Catalysts . . . . . . . . . . . . . . . . . . . . .
5.6. The Phenomenon of ‘Compensation’ . . . . . . . . . . . . . . . . . .

5.7. The Temkin Equation: Assumptions and Implications . . . . . . . . .
5.8. Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

210
210
211
213
214
214
215
216
218
221
223
225
227
229
234
239
246
247


xvi

CONTENTS

5.8.1. Reactors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247
5.8.2. Use of Stable and Radioactive Isotopes . . . . . . . . . . . . . 249
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251

CHAPTER 6. EXCHANGE OF ALKANES WITH DEUTERIUM
6.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.2. Equilibration of Linear and Branched Alkanes with Deuterium . . . .
6.2.1. Methane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.2.2. Ethane and Higher Linear Alkanes . . . . . . . . . . . . . . . .
6.2.3. Higher Linear Alkanes . . . . . . . . . . . . . . . . . . . . . . .
6.2.4. Branched Alkanes . . . . . . . . . . . . . . . . . . . . . . . . .
6.3. Equilibration of Cycloalkanes with Deuterium . . . . . . . . . . . . .
6.4. Interalkane Exchange . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.5. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

257
260
260
267
271
273
275
285
285
287

CHAPTER 7. HYDROGENATION OF ALKENES AND
RELATED PROCESSES
7.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.2. Hydrogenation of Ethene and Propene . . . . . . . . . . . . . . . . . .
7.2.1. Kinetics of Hydrogenation . . . . . . . . . . . . . . . . . . . .
7.2.2. Structure Sensitivity . . . . . . . . . . . . . . . . . . . . . . . .

7.2.3. Ethene Hydrogenation on Bimetallic Catalysts . . . . . . . . .
7.2.4. Reactions of Ethene and of Propene with Deuterium . . . . . .
7.2.5. Reactions on Single Crystal Surfaces . . . . . . . . . . . . . . .
7.2.6. The Reaction Mechanism: Microkinetic Analysis, Monte
Carlo Simulation, and Multiple Steady States . . . . . . . . . .
7.2.7. Catalysis by Hydrogen Spillover and the Reactivity of
Hydrogen Bronzes . . . . . . . . . . . . . . . . . . . . . . . . .
7.3. Reactions of the Butenes with Hydrogen and with Deuterium . . . . .
7.3.1. The n-Butenes . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.3.2. The Single Turnover Approach . . . . . . . . . . . . . . . . . .
7.3.3. Isobutene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.3.4. Exchange Reactions between Alkenes . . . . . . . . . . . . . .
7.4. Reactions of Higher Alkenes with Hydrogen and with Deuterium . . .
7.5. Hydrogenation of Cycloalkenes . . . . . . . . . . . . . . . . . . . . .
7.5.1. Cyclohexene . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.5.2. Other Cycloalkenes . . . . . . . . . . . . . . . . . . . . . . . .

292
297
297
303
306
307
319
321
325
328
328
333
334

335
336
338
338
339


CONTENTS

xvii

7.5.3. Substituted Cycloalkenes: Stereochemical Factors . . . . . . . 340
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 348

CHAPTER 8. HYDROGENATION OF ALKADIENES AND POLY-ENES
8.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.1.1. Types of Unsaturation . . . . . . . . . . . . . . . . . . . . . . .
8.1.2. Practical Applications of Selective Hydrogenation:
Outline of Mechanisms . . . . . . . . . . . . . . . . . . . . . .
8.2. Hydrogenation of 1, 2-Alkadienes (Allenes) . . . . . . . . . . . . . . .
8.2.1. Hydrogenation of Propadiene . . . . . . . . . . . . . . . . . . .
8.2.2. Hydrogenation of Substituted 1, 2-Alkadienes . . . . . . . . . .
8.2.3. Hydrogenation of Cumulenes . . . . . . . . . . . . . . . . . . .
8.3. Hydrogenation of 1,3-Butadiene . . . . . . . . . . . . . . . . . . . . .
8.3.1. General Characteristics of Butadiene Hydrogenation . . . . . .
8.3.2. Chemisorbed States of 1, 3-Butadiene . . . . . . . . . . . . . .
8.3.3. Hydrogenation of 1,3-Butadiene on Single Crystal Surfaces . . .
8.3.4. Hydrogenation of 1, 3-Butadiene on Supported
and Unsupported Metals . . . . . . . . . . . . . . . . . . . . . .
8.3.5. The Reaction of 1, 3-Butadiene with Deuterium:

Reaction Mechanisms . . . . . . . . . . . . . . . . . . . . . . .
8.3.6. Hydrogenation of 1, 3-Butadiene by
Bimetallic Catalysts . . . . . . . . . . . . . . . . . . . . . . . .
8.4. Hydrogenation of Higher Alkadienes . . . . . . . . . . . . . . . . . .
8.4.1. Linear Alkadienes . . . . . . . . . . . . . . . . . . . . . . . . .
8.4.2. Branched Alkadienes . . . . . . . . . . . . . . . . . . . . . . .
8.4.3. Cycloalkadienes . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

357
357
359
360
360
362
365
365
365
366
367
368
375
379
382
382
386
388
390

CHAPTER 9. HYDROGENATION OF ALKYNES

9.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9.1.1. The Scope of the Literature . . . . . . . . . . . . . . . . . . . .
9.1.2. Industrial Applications of Alkyne Hydrogenation . . . . . . . .
9.1.3. The Chemisorbed State of Alkynes . . . . . . . . . . . . . . . .
9.1.4. The Origin of Selectivity in Alkyne Hydrogenation . . . . . . .
9.1.5. Interpretation of Results: Some Preliminary Comments . . . . .
9.2. Hydrogenation of Ethyne: 1, In Static Systems . . . . . . . . . . . . .
9.2.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9.2.2. Kinetic Parameters . . . . . . . . . . . . . . . . . . . . . . . . .

395
395
396
397
398
399
400
400
401


xviii

CONTENTS

9.2.3. The Formation of Benzene from Ethyne . . . . . . . . . . . . .
9.2.4. The Reaction of Ethyne with Deuterium . . . . . . . . . . . . .
9.3. Hydrogenation of Ethyne: 2, in Dynamic System with Added Ethene .
9.3.1. Kinetics and Selectivity . . . . . . . . . . . . . . . . . . . . . .
9.3.2. Mechanisms and Modelling . . . . . . . . . . . . . . . . . . . .

9.3.3. Oligomerisation . . . . . . . . . . . . . . . . . . . . . . . . . .
9.3.4. Gaseous Promoters . . . . . . . . . . . . . . . . . . . . . . . .
9.4. Use of Bimetallic Catalysts for Ethyne Hydrogenation . . . . . . . . .
9.5. Hydrogenation of Higher Alkynes . . . . . . . . . . . . . . . . . . . .
9.5.1. Propyne . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9.5.2. The Butynes . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9.5.3. Alkyl-Substituted Alkynes Having More Than Four
Carbon Atoms . . . . . . . . . . . . . . . . . . . . . . . . . . .
9.5.4. Aryl-Substituted Alkynes . . . . . . . . . . . . . . . . . . . . .
9.5.5. Multiply-Unsaturated Molecules . . . . . . . . . . . . . . . . .
9.6. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

407
407
411
411
415
417
417
418
421
421
422
426
428
429
430
431

CHAPTER 10. HYDROGENATION OF THE AROMATIC RING
10.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.1.1. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.1.2. Industrial Applications of Benzene Hydrogenation . . . . . .
10.2. Kinetics and Mechanism of Aromatic Ring Hydrogenation . . . . . .
10.2.1. Introduction: Early Work . . . . . . . . . . . . . . . . . . . .
10.2.2. Kinetics of Aromatic Ring Hydrogenation . . . . . . . . . .
10.2.3. Rate Expressions and Reaction Mechanisms . . . . . . . . .
10.2.4. Temperature-Inversion of Rates . . . . . . . . . . . . . . . .
10.2.5. Hydrogenation of Benzene Over Bimetallic Catalysts . . . .
10.2.6. Exchange of Aromatic Hydrocarbons with Deuterium . . . .
10.2.7. Hydrogenation of Benzene to Cyclohexene . . . . . . . . . .
10.3. Hydrogenation of Alkyl-Substituted Benzenes . . . . . . . . . . . . .
10.3.1. Kinetic Parameters . . . . . . . . . . . . . . . . . . . . . . .
10.3.2. Stereochemistry of the Hydrogenation of
Alkyl-Substituted Benzenes . . . . . . . . . . . . . . . . . .
10.4. Hydrogenation of Multiple Aromatic Ring Systems . . . . . . . . . .
10.4.1. Polyphenyls . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.4.2. Fused Aromatic Rings: (1) Naphthalene . . . . . . . . . . . .
10.4.3. Fused Aromatic Rings: (2) Multiple Fused Rings . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

438
438
439
440
440
441
446
448

450
453
457
458
458
460
461
461
461
466
468


CONTENTS

xix

CHAPTER 11. HYDROGENATION OF SMALL ALICYCLIC RINGS
11.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
11.2. Hydrogenation and Hydrogenolysis of Cyclopropane . . . . . . . . .
11.2.1. Kinetics . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
11.2.2. The Reaction of Cyclopropane with Deuterium . . . . . . . .
11.2.3. Reaction Mechanisms . . . . . . . . . . . . . . . . . . . . . .
11.3. Hydrogenation of Alkylcyclopropanes . . . . . . . . . . . . . . . . .
11.3.1. Mono-alkylcyclopropanes . . . . . . . . . . . . . . . . . . .
11.3.2. Poly-alkylcyclopropanes . . . . . . . . . . . . . . . . . . . .
11.3.3. The Cyclopropane Ring in More Complex Hydrocarbons . .
11.4. Hydrogenation of Cyclopropanes Having Other
Unsaturated Groups . . . . . . . . . . . . . . . . . . . . . . . . . . .
11.5. Hydrogenation of Alkylcyclobutanes and Related Molecules . . . . .

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

473
477
477
481
482
484
484
488
490
491
494
499

CHAPTER 12. DEHYDROGENATION OF ALKANES
12.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
12.2. Dehydrogenation of Acyclic Alkanes . . . . . . . . . . . . . . . . . .
12.2.1. Introduction: Alkane Chemisorption . . . . . . . . . . . . . .
12.2.2. Supported Platinum and Platinum-Tin Catalysts . . . . . . .
12.2.3. Other Metals and Modifiers . . . . . . . . . . . . . . . . . .
12.2.4. Kinetics and Mechanism . . . . . . . . . . . . . . . . . . . .
12.3. Dehydrogenation of Cycloalkanes . . . . . . . . . . . . . . . . . . .
12.3.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . .
12.3.2. Reaction on Pure Metals . . . . . . . . . . . . . . . . . . . .
12.3.3. Reaction on Bimetallic Catalysts . . . . . . . . . . . . . . . .
12.4. The Chemisorption of Hydrogen on Platinum . . . . . . . . . . . . .
12.5. The Formation, Structure, and Function of Carbonaceous Deposits .
12.6. The Homologation of Methane . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

501
504
504
505
507
508
509
509
510
512
514
516
519
520

CHAPTER 13. REACTIONS OF THE LOWER ALKANES
WITH HYDROGEN
13.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 526
13.1.1. A Short Philosophical Digression . . . . . . . . . . . . . . . 526
13.1.2. Alkane Hydrogenolysis: General Characteristics . . . . . . . 527


xx

CONTENTS

13.1.3. Problems in Studying Reaction Kinetics . . . . . . . . . . . .
13.1.4. Ways of Expressing Product Composition . . . . . . . . . . .
13.2. Hydrogenolysis of the Lower Alkanes on Single Metal Catalysts:

Rates, Kinetics, and Mechanisms . . . . . . . . . . . . . . . . . . . .
13.2.1. The Beginning . . . . . . . . . . . . . . . . . . . . . . . . . .
13.2.2. Kinetic Parameters . . . . . . . . . . . . . . . . . . . . . . .
13.2.3. Mechanisms and Kinetic Formulations . . . . . . . . . . . .
13.2.4. A Generalised Model for Alkane Hydrogenolysis . . . . . .
13.2.5. Alkane Hydrogenolysis on Metals Other than Platinum . . .
13.3. Structure-Sensitivity of Rates of Alkane Hydrogenolysis . . . . . . .
13.4. Selectivity of Product Formation in Alkane Hydrogenolysis . . . . .
13.5. Mechanisms Based on Product Selectivities . . . . . . . . . . . . . .
13.6. Hydrogenolysis of Alkanes on Ruthenium Catalysts . . . . . . . . .
13.7. Effects of Additives and the Strong Metal-Support Interaction
on Alkane Hydrogenolysis . . . . . . . . . . . . . . . . . . . . . . .
13.8. Hydrogenolysis of Alkanes on Bimetallic Catalysts . . . . . . . . . .
13.8.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . .
13.8.2. Metals of Groups 8 to 10 plus Group 11 . . . . . . . . . . . .
13.8.3. Metals of Groups 8 to 10 plus Groups 13 or 14 . . . . . . . .
13.8.4. Platinum and Iridium plus Zirconium, Molybdenum,
and Rhenium . . . . . . . . . . . . . . . . . . . . . . . . . .
13.8.5. Bimetallic Catalysts of Metals of Groups 8 to 10 . . . . . . .
13.9. Apologia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

528
530
531
531
531
540
549
552

552
555
562
565
569
574
574
575
578
579
583
583
583

CHAPTER 14. REACTIONS OF HIGHER ALKANES WITH
HYDROGEN
14.1. Introduction: Petroleum Reforming and Reactions of Higher
Alkanes with Hydrogen . . . . . . . . . . . . . . . . . . . . . . . . .
14.1.1. The Scope of This Chapter . . . . . . . . . . . . . . . . . . .
14.1.2. Bifunctional Catalysis: Principles of Petroleum
Reforming . . . . . . . . . . . . . . . . . . . . . . . . . . . .
14.1.3. Reactions of the Higher Alkanes with Hydrogen . . . . . . .
14.1.4. The Scope and Limitations of the Literature . . . . . . . . .
14.1.5. The Principal Themes . . . . . . . . . . . . . . . . . . . . . .
14.2. Reactions of Higher Alkanes with Hydrogen: Rates and
Product Selectivities . . . . . . . . . . . . . . . . . . . . . . . . . . .
14.2.1. Activities of Pure Metals . . . . . . . . . . . . . . . . . . . .
14.2.2. Effect of Varying Conversion . . . . . . . . . . . . . . . . . .

592

592
592
596
597
598
599
599
601


CONTENTS

14.2.3. Reactions of Linear Alkanes with Hydrogen . . . . . . . . .
14.2.4. Reactions of Branched Alkanes with Hydrogen . . . . . . . .
14.2.5. Reactions of Cyclic Alkanes with Hydrogen . . . . . . . . .
14.2.6. The Environment of the Active Site: Effect of ‘Carbon’ . . .
14.3. Mechanisms of Alkane Transformations . . . . . . . . . . . . . . . .
14.3.1. A General Overview . . . . . . . . . . . . . . . . . . . . . .
14.3.2. Mechanisms of Skeletal Isomerisation . . . . . . . . . . . . .
14.3.3. Dehydrocyclisation . . . . . . . . . . . . . . . . . . . . . . .
14.4. Structure–Sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . .
14.4.1. Reactions on Single-Crystal Surfaces . . . . . . . . . . . . .
14.4.2. Particle-Size Effects with Supported Metals . . . . . . . . . .
14.5. Modification of the Active Centre . . . . . . . . . . . . . . . . . . . .
14.5.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . .
14.5.2. Metal Particles in Zeolites . . . . . . . . . . . . . . . . . . .
14.5.3. Platinum-Rhenium Catalysts . . . . . . . . . . . . . . . . . .
14.5.4. Modification by Elements of Groups 14 and 15 and
Some Others . . . . . . . . . . . . . . . . . . . . . . . . . . .
14.5.5. Other Bimetallic Catalysts . . . . . . . . . . . . . . . . . . .

14.5.6. The Role of Sulfur . . . . . . . . . . . . . . . . . . . . . . .
14.5.7. Metal-Support Interactions . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

xxi

602
609
616
621
624
624
625
628
629
629
630
634
634
634
635
637
639
643
644
647

INDEX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 657

1
METALS AND ALLOYS

PREFACE
This book in some ways resembles a detective story, but the criminal that we
seek is the answer to the question: which solid-state properties of metals determine
their behaviour as catalysts for the reactions of hydrocarbons? The search will lead
us from the bulk metallic state through the small supported metal particles whose
greater area makes them more fit to catalyse in a useful way; and from the reactions
of hydrogen and hydrocarbon molecules with both sorts of metal to their catalytic
interactions. The chain of cause and effect may not be straightforward.
In the metals of the Transition Series, where our attention will be focused, the
strength of interatomic bonding and all the parameters which reflect it vary greatly:
only six nuclear charges and their compensating electrons separate tungsten from
mercury. The clear physicochemical differences that separate the metals of the first
Transition Series from those in the second and third Series will be reflected in their
chemisorptive and catalytic properties, as will the subtler differences between the
second and third Series, for the understanding of which we are indebted to Albert
Einstein and Paul Dirac. Gold has always been seen as the ultimate in nobility
and iron as most liable to corrode; indeed this contrast was invoked by Geoffrey
Chaucer’s village priest, who in describing the high qualities needed in one of his
calling, asked rhetorically If gold rust, what shall iron do?
Metal surfaces are the place where chemical changes start and even on large
pieces of many metals the surface atoms are not quiescent but in a state of permanent
agitation; this has quite a lot to do with their reactivity. They are, as Flann O’Brien
remarked, livelier than twenty leprechauns dancing a jig on a tombstone.
While there are only some seventy-five metals, there are an infinite number
of binary alloys, and it is little wonder that some are better catalysts than the pure
metals that comprise them. In telling the story of catalysis by alloys we shall see
how suspects were wrongly identified, and how the real truth was discovered: but

1


2

CHAPTER 1

first we have to know something about the structure of metals and the metallic
surface.

1.1. THE METALLIC STATE
1.1.1. Characteristic Properties
Of the first one hundred elements in the Periodic Table (Figure 1.1), about
seventy-five are metals: in bulk form, most of them exhibit the characteristic physical properties of the metallic state, namely, strength, hardness, ductility, malleability and lustre, as well as high electrical and thermal conductivity. They owe
their chemical and physical properties to their having one or more easily removed
valence electrons: they are therefore electropositive, and most of their inorganic
chemistry is associated with their simple or complex cations.1,2 Metallic character
in certain Groups of the Periodic Table increases visibly with increasing atomic
number: while all the d-block elements in Groups 3 to 13 are obviously metals,

Figure 1.1. Periodic Classification of the elements.


METALS AND ALLOYS

3

of the Groups containing elements of the short Series, i.e. the sp-elements, this
is only true of Group 1 (Figure 1.1). In Groups 2, 3, 14 and 15, the early elements are either clearly non-metallic or are semi-metals (e.g. beryllium, boron).
The transition from non-metallic to semi-metallic to wholly metallic behaviour is

most evident in Group 14, in which silicon and germanium are semi-metals, tin
is ambivalent (the grey allotrope, α-Sn, is a semi-metal, while the much denser
white form (β-Sn) is metallic), and lead is of course a metal. In Group 15, arsenic
and antimony are semi-metals, but bismuth is a metal; in Group 16, tellurium and
polonium are semi-metals.3
It is however not always easy to decide what substances show metallic
behaviour.4−6 One criterion for distinguishing semi-metals from true metals under
normal conditions is that the co-ordination number of the former is never greater
than eight, while for metals it is usually twelve (or more, if for the body-centred
cubic structure one counts next-nearest neighbours as well). Other criteria have
been proposed. Which category an element falls into also depends upon the conditions employed; thus for example some metals lose their metallic character above
their critical temperature (e.g. mercury) or when in solution (e.g. sodium in liquid
ammonia). Interatomic separation is then large and valence orbitals cannot overlap, so electrical conduction is impossible. On the other hand, the application of
pressure causes some substances that are normally insulators or semiconductors
to behave like metals; thus for example α-Sn changes into β-Sn, in accordance
with Le Chatelier’s Principle.2 Similar changes also occur with other semi-metals
(e.g. silicon and germanium), and even hydrogen under extreme pressure shows
metallic character. Electrical conduction takes place when metal atoms are close
enough together for extensive overlap of valence orbitals to occur. All metals when
sufficiently subdivided fail to show the typical characteristics of the bulk state; the
question of the minimum number of atoms in a particle for metallic character to
be shown will be considered in Chapter 2.
The physical and structural attributes of the metals vary very widely: tungsten
for example melts only at about 3680 K, while mercury is a liquid at room temperature (m.p. 234 K), this change being produced by increasing the nuclear charge
only by six. The way in which the outermost electrons are employed in bonding
ultimately determines all aspects of the metallic state. This is a question which is
poorly treated if at all in text books of inorganic chemistry,1,2,7 so some further
description of the relevant facts and theories will be necessary. This information
bears closely on the chemisorptive and catalytic properties of metal surfaces, which
are our principal concern. When a metal surface is created by splitting a crystal,

bonds linking atoms are broken, and in the first instant the dangling bonds or free
valencies thus formed have some of the character of the unbroken bonds. We may
therefore expect to see some parallelism between the behaviour of metals as shown
by the chemical properties of their surfaces and the manner in which their valence
electrons are used in bonding.


4

CHAPTER 1

The metals of interest and use in catalysis are confined to a very small area
of the Periodic Table, so that most of our attention will be given to the nine metals
in Groups 8 to 10,8 with only occasional mention of neighbouring elements in
Groups 7 and 11, and of the earlier metals of the Transition Series (Figure 1.1). A
principal object of our enquiry will be to understand why catalysis is thus restricted
and our gaze will therefore be limited largely to the trends in metallic properties
that occur in and immediately after the three Transition Series.
The properties of metals that disclose how their valence electrons are used
may be put into four general classes: (i) mechanical, (ii) geometric, (iii) energetic,
and (iv) electronic. The mechanical class (hardness, strength, ductility and malleability) may be quickly dismissed, because in polycrystalline materials these are
mainly controlled by interactions at grain boundaries, and are influenced both by
adventitious impurities that lodge there, and by deliberate additions that result in
grain stabilisation, with consequent improvement in strength and hardness. With
single crystals, they are described by the plastic and elastic moduli, which in turn
are governed by the ease of formation and mobility of defects within the bulk under
conditions of stress. They bear some relation to the strength of metallic bonding,
but are of lesser interest than other properties. Metals having the body-centred
cubic structure are less ductile than those that have close-packed structures (see
below), because they lack the planes of hexagonal symmetry that slide easily past

each other.
Bulk geometric parameters are those that describe the arrangement of the
positive nuclei in space, and the distances separating them: the former is conveyed by the crystal structure and co-ordination number, and the latter by the
metallic radius. Most metals crystallise in either the face-centred cubic (fcc) or
the close-packed hexagonal (cph) or the body-centred cubic (bcc) structure; the
first two are alternative forms of closest packing (Figure 1.1). Four other structures are known: rhombohedral (distorted fcc: mercury, bismuth), body-centred
tetragonal (A4, e.g. grey tin9 ), face-centred tetragonal (indium, manganese), and
orthorhombic (distorted cph: gallium, uranium). Many metals exhibit allotropy, i.e.
they exhibit different structures in different regimes of temperature but in catalysis our only concern is with the form stable below about 770 K; of the metals
of catalytic interest, only cobalt suffers a phase transition below this temperature
(from cph to fcc at 690 K). Within the Transition Series there is a strikingly regular periodic variation in crystal structure; most metals in Groups 3 and 4 are cph
(aluminium is fcc), those in Groups 5 and 6 are bcc, those in Groups 7 and 8 are
again mainly cph (excepting iron, which is bcc, and manganese), while those in
Groups 9 to 11 (except cobalt) are all fcc at ordinary temperatures.3,10 Explanation
of this regularity will be a prime requirement for theories of the metallic state
(Section 1.12).
As the nuclear charge increases on moving across each Transition Series,
the number of valence electrons forming covalent bonds at first rises rapidly, then


METALS AND ALLOYS

5

Figure 1.2. Periodic variation of metallic radius and density in the Third Transition Series.

remains almost constant in Groups 5 to 10, and afterwards starts to fall. This
effect is clearly shown in metallic radius and density, values for which for the
third Transition Series are shown in Figure 1.2. To relate these two parameters
precisely, it is necessary to correct for changes in atomic mass. The plot of atomic

density (i.e. density/atomic mass) versus the reciprocal of the cube of the radius
(Figure 1.3) shows two good straight lines, one for the close-packed metals and
another of slightly lower slope for the more open bcc metals. Figure 1.4 shows the
periodic variation of the reciprocal cube of the radius for all three Transition Series:
in the First Series iron, cobalt and nickel have almost the same bond lengths, while
in the later Series the minimum bond length is shown at ruthenium and osmium.
The similarity between the bond lengths in the second and third Series is only
partly a consequence of the Lanthanide Contraction (see below).

Figure 1.3. Dependence of atomic density on the reciprocal of the cube of the radius for metals of the
Third Transition Series; open points, close-packed structures; half-filled points, bcc structure; filled
point, Hg.