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MOLECULARSPECTROSCOPYOF OXIDE CATALYSTSURFACES
Anatoli DavydovUniversity of Alberta, Edmonton, Canada
Syntroleum Corporation, Tulsa, Oklahoma, USA
Edited by N. T. Sheppard
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Copyright 2003 John Wiley & Sons Ltd, The Atrium, Southern Gate, Chichester,
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Library of Congress Cataloging-in-Publication Data
Davydov A. A. (Anatoli Aleksandrovich)Molecular spectroscopy of oxide catalyst surfaces / Anatoli Davydov ; edited by N.T. Sheppard.
p. cm.Includes bibliographical references and index.ISBN 0-471-98731-X (cloth : alk.paper)
1. Metallic oxides Surfaces Analysis. 2. Molecular spectroscopy. I. Sheppard, N. T. II.Title.
QD509.M46 .D38 2003
541.33 dc212002191080
British Library Cataloguing in Publication Data
A catalogue record for this book is available from the British Library
ISBN 0-471-98731-X
Typeset in 9.5/11.5pt Times by Laserwords Private Limited, Chennai, IndiaPrinted and bound in Great Britain by TJ International, Padstow, Cornwall
This book is printed on acid-free paper responsibly manufactured from sustainable forestryin which at least two trees are planted for each one used for paper production.
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Dedicated to my wife Marina
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CONTENTS
Preface xi
Symbols and Abbreviations xiii
Introduction xv
1 Theoretical fundamentals and experimental considerations of thespectroscopic methods used in surface chemistry 1
1.1 Electronic spectroscopy 1
1.1.1 Transmission spectra 4
1.1.2 Diffuse reflection spectra 5
1.2 Vibrational spectroscopy 5
1.2.1 Infrared spectroscopy 11
1.2.2 Photoacoustic spectroscopy 18
1.2.3 Raman spectroscopy 19
1.3 Electron energy loss spectroscopy 21
1.4 Inelastic electron tunneling spectroscopy 221.5 Inelastic neutron scattering spectroscopy 23
1.6 Other vibrational spectroscopies 23
1.6.1 Infrared ellipsometric spectroscopy 23
1.6.2 Surface electromagnetic wave spectroscopy 23
1.7 In situ measurements 24
1.8 Quantitative measurements 25
2 The nature of oxide surface centers 27
2.1 Systems investigated 27
2.1.1 Solid structures 27
2.1.2 Surfaces 28
2.1.3 Active sites 29
2.2 Spectra of oxide surfaces 31
2.2.1 Vibrations of metal oxygen bonds on oxide surfaces 32
2.2.2 Molecular forms of adsorbed oxygen 44
2.2.3 Surface hydroxyl groups 56
2.3 Determination of the nature of surface sites and their chemical properties
using the adsorption of simple molecules 77
2.3.1 Adsorption of ammonia and pyridine 78
2.3.2 Adsorption of carbon monoxide 95
2.3.3 Adsorption of hydrogen and nitrogen 114
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viii CONTENTS
2.3.4 Adsorption of water 120
2.3.5 Adsorption of nitrogen oxide and nitrogen dioxide 123
2.3.6 Adsorption of carbon dioxide 133
2.3.7 Adsorption of hydrogen sulfide 139
2.3.8 Adsorption of sulfur dioxide 146
2.3.9 Surface isocyanate complexes 157
2.4 Determination of acidic surface properties 161
2.4.1 Protic acid sites 162
2.4.2 Lewis acid sites 166
2.5 Determination of basic surface properties 171
2.6 Surface defects 177
3 Study of cation states by DRES and FTIR spectroscopies of the probe
molecules 181
3.1 Copper-containing systems 1823.1.1 Zeolites 182
3.1.2 Oxides 200
3.2 Nickel-containing systems 207
3.2.1 Zeolites 207
3.2.2 Oxides 215
3.3 Co-containing systems 217
3.3.1 Zeolites 217
3.3.2 Oxides 218
3.4 Iron-containing systems 220
3.4.1 Zeolites 2203.4.2 Oxides 222
3.5 Silver-containing systems 223
3.6 Palladium-containing systems 228
3.6.1 Zeolites 228
3.6.2 Oxides 235
3.7 Rhenium-, ruthenium-, and rhodium-containing systems 237
3.8 Platinum-containing systems 238
3.8.1 IR-Spectra of CO adsorbed on supported metals 238
3.8.2 Cationic states of platinum 248
3.9 Molybdenum-containing systems 252
3.9.1 Molybdenum aluminum oxide compounds 252
3.9.2 Molybdenum silicon oxide compounds 253
3.9.3 Molybdenum titanium oxide compounds 255
3.10 Vanadium-containing systems 257
3.10.1 Vanadium titanium oxide compounds 259
3.10.2 Vanadium silicon oxide compounds 263
3.10.3 Vanadium aluminum oxide compounds 266
3.11 Chromium-containing systems 269
3.12 Effects of the states of adsorption sites on the stretching frequencies of
adsorbed carbon monoxide and nitrous oxide and the problem of detecting
the states of cations in oxide catalyst surfaces 2713.12.1 M2+CO, Mn+CO (n >2) 272
3.12.2 M+CO and M0 CO 274
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CONTENTS ix
4 Interactions of inorganic compounds with oxide surface
active sites 277
4.1 Organometallic complexes 281
4.2 Metal carbonyls and nitrosyls 282
4.3 Interactions with simple acids and bases 284
4.3.1 F- and Cl-modified oxide systems 285
4.3.2 SO42-modified oxide systems 286
4.3.3 BO32-modified oxide systems 291
4.4 Heteropoly compound systems 294
4.4.1 Effects of the supports 295
4.4.2 Acidic properties of molybdenum heteropoly compounds 300
4.5 Thermal stabilities of molybdenum compounds, decomposition mechanisms
and the role of modifiers 303
4.5.1 Bulk and supported heteropoly acids 303
4.5.2 Modified molybdates 305
4.6 Cationic modification 308
5 Formation of surface complexes of organic molecules 309
5.1 Complexation of alkenes 310
5.1.1 Complexation with OH groups 310
5.1.2 Carbenium ions and alkoxy compounds 313
5.1.3 Interaction with cations 327
5.1.4 Interaction with cation anion pairs 342
5.1.5 The complexation of alkenes with surface oxygen 351
5.2 Complexation of aryls and aryl halides 355
5.2.1 Hydrogen-bonding 3555.2.2 Alkylaromatic carbenium ions 358
5.2.3 -complexes 366
5.2.4 Interaction with ionic pairs 373
5.2.5 Complexation with surface oxygen 376
5.2.6 Formation of aryl halide complexes 378
5.3 Complexation of alkynes 381
5.3.1 Silicon dioxide zeolites 381
5.3.2 Aluminum oxide 385
5.3.3 Zinc oxide 386
5.3.4 Titanium oxide 387
5.4 Complexation of alkanes 389
5.4.1 Interactions with OH groups, carbenium-like ions 389
5.4.2 Interaction with cations 392
5.4.3 The activation of C H bonds in alkane molecules 395
5.5 Complexation of chlorofluorocarbons 407
5.6 Complexation of nitriles 411
5.6.1 Acetonitrile 411
5.6.2 Benzonitrile 415
5.7 Complexation of alcohols 416
5.7.1 Saturated alcohols 416
5.7.2 Phenol 4275.8 Complexation of aldehydes and ketones 430
5.8.1 Formaldehyde and acetaldehyde 430
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x CONTENTS
5.8.2 Acrolein 435
5.8.3 Benzaldehyde 439
5.8.4 Maleic anhydride 440
5.8.5 Acetone 442
5.9 Complexation of acids 445
5.9.1 Formic acid 445
5.9.2 Acetic acid 453
5.9.3 Acrylic acid 453
5.9.4 Benzoic acid 455
5.10 Deactivation catalysts due to carbonaceous depositions as a result of
catalyst interactions with hydrocarbons and their derivatives 456
6 The mechanisms of heterogeneous catalytic reactions 459
6.1 Reactions involving carbon monoxide 461
6.1.1 The oxidation of carbon monoxide 461
6.1.2 The water-gas shift reaction 4666.1.3 Carbonization and hydroformylation 473
6.1.4 The synthesis and decomposition of alcohols 475
6.2 Reactions with the participation of hydrocarbons 479
6.2.1 Complete oxidation of hydrocarbons and their derivatives 479
6.2.2 Selective transformations of alkenes 483
6.2.3 Partial oxidation 499
6.2.4 Ammoxidation of hydrocarbons and their derivatives 518
6.3 Transformations of aldehydes and ketones 526
6.3.1 Oxidation of acrolein 526
6.3.2 Oxidation of formaldehyde 5316.3.3 Transformation of acetone 531
6.3.4 Hydrogenation of aldehydes and ketones 532
6.4 Transformations of alcohols 532
6.4.1 Dehydration of alcohols 532
6.4.2 Dehydrogenation of alcohols 536
6.4.3 Methanol oxidation to formaldehyde 538
6.5 Transformations of nitrogen-containing compounds 545
6.5.1 Decomposition of nitric oxide 545
6.5.2 The reduction of nitrogen oxides 552
6.5.3 Reactions of NOx and NH3 Mixtures 556
References 559
Index 643
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PREFACE
Molecular spectroscopic methods, together with X-ray diffraction, have played key roles in estab-
lishing the concepts of coordination chemistry, as originally developed in the study of individual
transition-metal complexes in aqueous solutions or the solid state. This present book is concerned
with the even greater importance of molecular spectroscopic methods in developing similar under-
standings of the coordination chemistry of oxide surfaces where application of diffraction methods
is much more difficult. The adsorption of molecules on the surfaces gives rise to ligands attached
to free sites on the surface cations.The book commences with an account of the basic theoretical principles and experimental
techniques of the various molecular spectroscopic methods as applied to surfaces, namely the
electronic (UV Vis), vibrational (transmission IR, diffuse reflection, reflection absorption IR
and Raman), electron energy loss, inelastic electron tunneling, and inelastic neutron scattering
spectroscopies. Special attention is devoted to in situ measurements where the oxide or catalyst
sample is in contact with the adsorbate or reactant. The local approach has been chosen as the
basis of the spectroscopic analysis of adsorption on the active sites of the oxide surfaces, while
the collective properties of the solid adsorbents, based on analysis of their crystal structures, is
used to describe the sites themselves. This approach is applied to pure oxides and also to oxide
systems such as cation-substituted zeolites, heteropoly compounds of molybdenum, or supported
catalysts prepared by ionic exchange or the interaction of the support with various complexes.
In some cases, the crystallographic positions of both cations and anions can be unambiguously
determined by means of molecular spectroscopic (ESR, UVVis, Mossbauer, etc.) or diffraction
(for zeolites, etc.) methods.
An attempt has been made to cover all of the spectroscopic literature on oxide adsorption
studies, covering many different oxide adsorbate systems in a comparative manner. Because
the number of such publications is now very large (numbered in thousands), it is impossible to
analyze all of them individually in one single book. A particular goal is to provide a critical
analysis of the literature on the interpretation of the spectra of surface compounds on oxides
going back to the earliest days of the 1950s. A comparative analysis of the changes in the IR
spectra of adsorbed molecules, based on an improved knowledge of the bonding between the
adsorbed molecule and the surface site, has allowed this present author to improve the reliability
of interpretation of many of the spectra. Special emphasis is placed on the spectral characteristics
of active sites on oxide surfaceshydroxyl groups, or coordinatively unsaturated surface cations
and oxygen anions. The concept of the decisive role played by surface sites in surfacemolecule
adsorption is used to systematize and classify the spectral data relating to the interaction of
numerous organic and inorganic molecules, and their transformation products, with the types of
surfaces referred to above. The structures of many surface species have been identified from the
spectroscopic data.
A detailed account is presented of methods for spectroscopically characterizing the oxida-
tion state and degree of coordination of surface cations and oxygen anions by the adsorption ofprobe molecules such as NH3, pyridine (Py), CO, CO2, H2, N2, H2O, NO, NO2, H2S and SO2(Chapter 2). Special attention is paid to the critical investigation of protic and aprotic acidic and
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xii PREFACE
basic surface centers, including specific correlations for comparing the strengths and concentra-
tions of surface centers on different oxides, zeolites, supported oxides, etc. by using the UVVis,
ESR and IR spectral characteristics of the adsorbed probe molecules, particularly CO and NO.
This includes the testing of cation states during the process of stationary-state heterogeneous
catalytic reactions. Systems containing Cu, Ni, Co, Fe, Ag, Pd, Re, Ru, Rh, Pt, Mo, V and Cr
are examined in detail. The vibrational frequency ranges of the CO and NO probes characteristicof different surface states are presented.
Attention is also paid to the interactions of organometallic (allylic and other types) and inorganic
compounds (such as metal carbonyls), simple acids and heteropoly compounds with various
supports (Al2O3, SiO2, TiO2 and MgO), i.e. to the problems that occur during the preparation or
modification of supported catalysts. The dependence of the structure and properties of the surface
complexes formed and the properties of the catalytic systems are also shown.
The complexation of many organic molecules alkenes, alkene halides, alkynes, aryls, aryl
halides, alkanes, nitriles, alcohols, aldehydes, ketones and acids (saturated and unsaturated, aro-
matic and non-aromatic) with different oxide systems are critically examined. The surface
compounds formed are classified in relation to the nature and properties of the available surfacecenters (H+, OH, O2, Mn+, Mn+O2, etc.).
The final chapter is devoted to discussions of possible mechanisms of catalytic reactions as
deduced from spectroscopic identification of the reaction intermediates. The latter identifica-
tions are based on the comparison of the rates of reaction with those of the transformations
of surface compounds. The catalytic reactions discussed include carbon oxide oxidation, the
water gas shift (WGS) reaction, the synthesis and decomposition of alcohols, carbonization,
hydroformylation, full and partial transformations of alkenes (including isomerization, hydro-
genation, oligomerization, polymerization, cracking and metathesis), partial oxidation of alkanes,
alkenes and aryls, ammoxidation of hydrocarbons, alcohols and aldehydes, conversion of alco-
hols, transformations of aldehydes and ketones, NO decomposition, NO+ CO, NO/hydrocarbons,
and reactions between NO and NH3.
Taking into account common understandings and the results of the analysis of detailed schemes,
the mechanisms of heterogeneous catalytic reactions can be classified as stepwise (when sequen-
tial interactions of the reaction components occur) or associative (where the stages of product
separation and interaction of the reaction mixture components with the catalyst occur in parallel)
with the help of spectroscopic analyses.
This book is intended for specialists working in the fields of surface physical chemistry, surface
science, adsorption phenomena and heterogeneous catalysts.
Special thanks are due to Professor N.T. Sheppard for his attention, interest, valuable correc-
tions and useful advice, to Professor J.T. Yates Jr for important comments, and also to my wife,
Dr M. Shepotko, and son, Davydov, A.A. Jr, for their help in the preparation and design ofthis book.
Anatoli Davydov
Tulsa, OK, USA
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SYMBOLS AND ABBREVIATIONS
AES Auger electron spectroscopy
AFS atomic fluorescence spectroscopy
AO atomic orbital
BAS Brnsted acid site
BS basic site
CTB charge-transfer band
DRES diffuse reflection electron (UV Vis) spectroscopyDRIRS diffuse reflection infrared spectroscopy
EELS electron energy loss spectroscopy
EHM extended Huckel Method (quantum-chemical calculations)
EPR electron proton resonance (spectroscopy)
ES electron spectroscopy
ESR electron spin resonance (spectroscopy)
FTIR Fourier-transform infrared (spectroscopy)
GC gas chromatography
HFB high-frequency band
HM Huckel Method (quantum-chemical calculations)
HOMO highest-occupied molecular orbital
HPA heteropoly acid
HPC heteropoly compound
HREELS high-resolution electron energy loss spectroscopy
IETS inelastic electron tunneling spectroscopy
INSS inelastic neutron scattering spectroscopy
IP ionization potential
IRAS infrared absorption spectroscopy
IRES infrared ellipsometric spectroscopy
IRS infrared spectroscopy
LAS Lewis acid siteLFB low-frequency band
LOMO lowest-occupied molecular orbital
M metal
Mn+cuo coordinatively unsaturated octahedral site (on a metal)
Mn+cus coordinatively unsaturated site (on a metal)
Mn+cut coordinatively unsaturated tetrahedral site (on a metal)
MO molecular orbital
MS mass spectrometry
MY-A faujasite-type zeolite containing preferably strong
associated cationsMY-I faujasite-type zeolite containing preferably isolated cations
NMR nuclear magnetic resonance (spectroscopy)
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xiv SYMBOLS AND ABBREVIATIONS
NRS nuclear-resonance spectroscopy (Mossbauer spectroscopy)
PAS photoacoustic spectroscopy
R hydrocarbon fragment
RAIRS reflection absorption infrared spectroscopy
RS Raman spectroscopy
SAPO silicoaluminophosphateSCR selective catalytic reduction
SERS surface-enhanced Raman spectroscopy
SEWS surface electromagnetic wave spectroscopy
SIC surface isocyanate complex
SIMS selected-ion mass spectrometry
SMSI strong metalsupport interaction
TPD temperature-programmed desorption
UEP unshared electron pair
UHV ultra-high vacuum
UV Vis spectroscopy in the ultraviolet and visible regionVAPO vanadiumsilicophosphate
WGS water gas shift
XPS X-ray photoelectron spectroscopy
XRD X-ray diffraction
A absorbance
A0 absorption coefficiente electron charge
E activation energy
g parameter of level split in ESR spectrumK reaction rate constant
N number of active sites
PA proton affinity
q charge (on a dipole)
Q heat of absorption
r interatomic distance (A)
deformation vibration
extinction coefficient(cm2 molecule1)
surface coverage
vibration wavenumber (reciprocal wavelength, 1/) (cm1)
out-of-plane deformation vibration
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INTRODUCTION
Numerous technological processes are dependent on the nature of the molecular and chemical
interactions which occur on contact of various media with solid surfaces. These include, for
example, the separation of mixtures by adsorption, heterogeneous catalysis, the chromatographic
separation of pure substances, the production of polymer and lubricant fillers, materials for micro-
electronics and the manufacture of controlled-property semiconductors, pigments and catalysts.
It is therefore an important objective to gain a better understanding of the nature of the processes
which occur at the surfaces of solids.This book is concerned with the development of the principles of the coordination chemistry
of oxide surfaces as brought about by the use of analyses of experimental data obtained by
means of molecular spectroscopy methods which are the most widely used in chemistry [135a].
The concepts of coordination chemistry, originally established for transition-metal complexes
in aqueous solutions [35b e], can frequently be usefully extended to heterogeneous systems
consisting of transition-metal ions dispersed on the surfaces (or incorporated within a solid matrix)
of oxides despite the differences in behavior of transition-metal ions at gassolid or liquidsolid
interfaces [30, 35f h]. The phenomena occurring at the latter interfaces can be described in
terms of the coordination chemistry concept of coordination number of the transition-metal ion
(the number of atoms donated by ligands) which can vary over a wide range. At the solid surface,
a transition-metal ion has necessarily a lower coordination number than in the bulk of the solid
and so it can complete its coordination sphere by bond formation through adsorption from the
gas or liquid phase. The first coordination sphere predominantly determines the reactivity and
properties of the central transition-metal ion in both homogeneous and heterogeneous systems,
although the influence of the addition of ligands to the coordination spheres is much greater in
heterogeneous systems. This can lead in the latter case to the existence of a number of transition-
metal ion complexes and, moreover, this is the cause of the creation and stabilization of species
with unusual oxidation states or coordination numbers at solid surfaces which are different from
those in the bulk of the oxide. Compared with homogeneous coordination chemistry, such species
show new types of reactivity. Thus, these peculiar features of heterogeneous coordination systems
containing transition-metal ions open up new, special and unique potentialities in adsorption and
catalysis [30, 35fh].
The principles of coordination chemistry, established essentially by Grinberg, Jorgensen and
Werner, were based on results obtained for solution complexes with the single transition-metal
ion surrounded by ligands [35be]. The properties of these complexes were the main subjects
of study, for example, reactivity, structure, the nature of the different chemical bonds involved,
the presence of optical and/or geometric isomers, the number of isomers, optical and magnetic
properties, chemical reactivities, etc. Different theoretical treatments, such as crystal field, molec-
ular orbital, valence bond, etc., improved the understanding of the transition-metal complexes in
solution or in the solid state.
Attempts to apply these theories to catalytic processes have been more successful for homoge-neous than for heterogeneous systems. The main results are that (i) the properties of the partially
filled d-orbitals of oxide surfaces can be studied at a molecular level by using probes with
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xvi INTRODUCTION
characteristic optical properties, (ii) oxide surfaces may act as either s-donor or p-donor ligands
and hence be classified within the spectrochemical series of ligands, (iii) an oxide support can
play a role in reactivity similar to that of a solvent, and (iv) the distinctions and similarities
between interfacial coordination chemistry and surface organometallic chemistry can be made in
a similar fashion as for complexes in solution [30, 35fh].
Molecular interactions at solid surfaces are very complicated because of a number of factors.The most important of these are the nature and properties of the adsorption centers, which deter-
mine the types of surface complexes formed. Hence, the main tasks in surface chemistry are the
identification of these surface centers, the determination of their concentrations and characteristic
chemical properties (for instance, via investigations of the interaction between a given center and
adsorbed molecules with different chemical properties) and the establishment of the relationships
between bulk crystal structures and the nature of the surface centers.
Morrisons term surface center [35i] will be applied in this book to describe the microscopic
group of atoms which exhibits a particular chemical activity on the surface. This term applies
to a surface atom of the lattice with a free bond, the free bonding orbital with a low ionization
potential, etc.; such centers can be situated on a uniform surface or can occur in nonhomogeneousareas, where their activity is often highest. It has been shown that surface heterogeneity has a
great effect on the chemical properties of surfaces as it leads to a wide range of different types
of adsorption centers. These centers may be related to defects, e.g. sites where a crystal defect
meets the surface. It should be noted that the surface itself, in both the microscopic and the
electronic senses, is a major defect of a three-dimensional crystal structure. Hence, any real
surface is in principle nonhomogeneous. Additional nonhomogeneity is caused by a variety of
possible microscopic defects (steps, cracks, dislocations, corners, etc.) or point defects (vacancies,
interstitial atoms, substitution or insertion atom sites). Defect concentrations on real surfaces
increase with diminishing crystal size. In highly dispersed systems, they may reach, or at times
exceed, the concentrations of normal surface sites.In addition, any real surface may have chemical nonhomogeneities formed during its prepa-
ration or from adsorption in the form of surface chemical compounds which modify the surface
properties. It is therefore evident that all properties of a surface must be taken into account when
considering its further interactions. Cause and effect can be difficult to distinguish here. Although
the surface itself determines the nature of the adsorption of molecules from its surroundings, those
adsorbed molecules often modify the surface properties.
Classifications of surface centers, plus a knowledge of the chemistry of surface complexes (the
chemical properties governing the interaction of molecules with each type of center), make it
possible to characterize the various possible types of surface compounds formed after adsorption.
The facility for surface-compound formation is more diverse than, and qualitatively different
from, that of individual molecular complexes since the surface may incorporate cationic states of
various coordinations and valencies, sometimes unusual ones. Location on a surface can change
the oxidative reductive properties of a ligand site; adsorption is dependent on the collective
properties of the solid and multi-centered adsorption can occur [30].
The study of the nature and properties of a surface entails great experimental difficulties. As
the traditional methods of defect analysis (electro-physical methods and radioactive labeling)
practically cannot be applied to polydispersed materials, it is clear that the development of
concepts of the mechanisms of molecular processes on solids (which require information about the
nature of bonds formed, surface structure, molecular mobility, etc.) is impossible without spectral
analysis, which yields direct data on interactions at the molecular level. A particularly versatile
role is played by vibrational spectroscopy. ESR (EPR) and NMR spectroscopies are limited toselected elements; the former requires paramagnetic ions or radical forms of adsorption, while
the latter requires nuclei with a magnetic moment. In addition, changes resulting from adsorption
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INTRODUCTION xvii
can be detected by UV or visible spectroscopy only for the limited proportion of species that
give well-defined electronic absorption spectra. The vibrational spectroscopic methods (especially
Fourier-transform infrared (FTIR), Raman and high-resolution electron energy loss spectroscopy
(HREELS)) are by far the most versatile techniques for the analysis of surface layers on solids.
Vibrational spectroscopy provides data on the composition and structure of surface compounds,
the nature of the bonds formed between adsorbed molecules and the surface, and the existenceof different types of surface compounds and active surface centers. As the vibrational spectrum
reflects both the properties of the molecule as a whole and the characteristic features of separate
chemical bonds, this method offers the fullest possible information on the perturbation experienced
by a molecule on contact with the solid surface, and often determines the structure and properties
of adsorption complexes and of surface compounds.
Terenin [17, 35j] first pioneered the successful study of adsorption using molecular spec-
troscopy in the near-infrared region. Today, the literature contains much generalized experimental
material on the character of interactions of various molecules with surfaces of silica gels, alumina
gels, aluminosilica gels and zeolites as determined by infrared spectroscopy in the fundamental
region. The principles of the study of surface compounds and adsorbed molecules by molecularspectroscopy have been described in several books [1835a]. New experimental developments
have been reviewed pertaining to studies (for example, by IR spectroscopy) of solids at high
pressures and/or temperatures in various chemical media, to the determination of kinetic param-
eters for individual stages of surface reactions, and to the use of computers to process spectral
data in order to improve the volume of information.
This book does not repeat a consideration of these problems but instead concentrates on the
analysis of surface properties, the interaction of relevant simple molecules with the surface of the
solid, and the reactions which occur on the surface of oxides of the transition-metals. This latter
choice has been made for two reasons: first, transition-metal oxide systems were not extensively
studied by molecular spectroscopy in the earlier days and the more recent results have not beensubjected to detailed analysis and comparison, and secondly, such oxides and their surfaces are
important because of their wide use as heterogeneous catalysts. The main objective of this work
is to systemize scientific approaches to spectral studies in this area of surface chemistry. The
entire volume of data obtained for each individual system cannot be examined in detail. Bearing
in mind that catalysis is of central importance in a conceptual chemistry of the surface, and
that it provides a means for achieving chemical transformations in the laboratory and in nature,
this author considers it important to discuss the currently considered concepts by using catalytic
systems and processes as examples.
As the main objects of early molecular spectroscopy studies were systems involving silica
gels, alumino gels and aluminosilica gels, and as it proved fairly easy to obtain their spectra in
the OH-stretching region, the greatest attention was at first focused on establishing the existence
and significance of surface hydroxyl groups in adsorptive interactions. At that time, there were
practically no methods accurate enough for the qualitative, let alone quantitative, differentiation of
coordinatively unsaturated cations and anions on the surface of oxides. The difficulties involved in
the analysis of the nature and surface properties of the latter oxides also proved a major obstacle
to determining the character of the interactions between such centers and adsorbed molecules. In
fact, until recently no data have become available on some aspects of the interaction between,
say, alkenes and Lewis centers, even in such well-studied systems as alumina and crystalline or
amorphous aluminosilicates, which are widely used as catalysts in hydrocarbon transformations.
The specificity of the material discussed here required novel approaches to the properties of
the surface. It is known that in the important stage of initial complex formation, mechanisms ofcatalytic reactions involve interactions with transition-metal ions and/or oxygen on the surface
of the catalyst. Evidently, both the nature and valency/coordination state of the cations, together
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xviii INTRODUCTION
with the properties of the surface oxygen ions, exert a considerable influence on the character of
activation and, possibly, on the directions of transformation of the adsorbed molecules. Hence,
there is a need to find methods for the identification of such centers on the surface and to establish
their concentrations and their differences in chemical properties.
The use of probe molecules, based on the analysis of their spectral changes as a result of
donor or acceptor molecules interacting with their opposites on the surface, have been widelyused for investigating these centers. This book contains a preliminary description of the authors
approach to the analysis of surface properties, which includes the isolation of specific interactions
of elementary simple molecules so-called probe molecules with every possible type of center
on the solid.
This authors views on the main goals of molecular spectroscopy in surface chemistry can be
described as the analysis of the important relationships between the nature of the surface centers
on oxides, and the forms and directions of transformation of molecules adsorbed on them; these
considerations account for the manner in which the material is presented. Information about the
surface states of solids (particularly oxides), about their active sites, and also about the structures
of surface compounds formed upon adsorption of the various types of molecules, is very important.Such information can be obtained by means of different physico-chemical methods, among which
those of molecular spectroscopy [116] are the most widely used in chemical applications. In
the past few decades, these have been very effectively used to investigate the surface chemistry
of oxide systems [1735].
Taking into account the large amount of literature covering these fields, the methods of IR and
UVVis spectroscopies have been the principal means of investigating the surface chemistry of
such solids. A short introduction to the development and analysis of the data obtained by means
of these techniques, and the theory of such methods, is given in Chapter 1 of this book. For
the EPR method, the strong reasons for its use as a tool for the investigation of transition-metal
chemistry on oxide surface have been described in reference [35h], while the theoretical basis ofsuch applications has been reviewed in various references [25, 31, 33]. Therefore, in this present
book I will only use the results of this informative method.
The various molecular spectroscopic methods play even more of a key role in the development
of the concepts of the surface coordination chemistry originally established for transition-metal
complexes in aqueous solutions. The local approach has been chosen as the basis of the inter-
pretation of the absorption bands which characterize the active sites on oxide surfaces and the
interaction of these sites with adsorbing molecules. The collective properties of the solid adsor-
bents are also used, based on the analysis of the crystal structures of the oxides or of systems
such as cation-substituted zeolites, heteropoly compounds of molybdenum, precisely prepared
(for example, by ionic exchange or interactions of supports with complexes) supported systems
in which the crystallographic positions of both cations and anions can be well and unambiguously
determined by means of ESR, UVVis or Mossbauer spectroscopic methods. In the middle of
the 1980s, a first attempt was made to develop the principles of surface coordination chemistry of
transition-metal oxides based on the analysis of principally my own spectroscopic investigations
of oxides surfaces and the compounds formed upon them through adsorption (of a limited number
of molecules) [30]. In this present book, practically all of the international literature has been
analyzed for the developing concepts and experimental data over a very wide range of molecular
reactants and oxide systems. A correct comparative approach limits the number of published
works that have to be considered individually, such as those which take into account results for
only one molecule or another. The total number of such publications is so large (in thousands)
that it is impossible to analyze all of them in detail in one single book.A dramatic increase in the number of recent studies concerning applications of molecular spec-
troscopic methods to studies of the surface states, and to the structures of adsorbed species, has
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INTRODUCTION xix
led to the writing of this present book. More sensitive methods, such as FTIR, laser Raman spec-
troscopy and Surface-enhanced Raman Scattering (SERS), the sensitivities of which are order of
magnitude greater then those of earlier vibrational spectroscopic methods, and especially the cre-
ation of new methods for obtaining vibrational spectra (diffuse reflection, high-resolution electron
energy loss spectroscopy, electron tunneling spectroscopy and the spectroscopy of electromagnetic
waves), as well as the wide use of computers to process the spectral information (for instance,to subtract spectra and to separate complex spectral contours into those of separate components),
have stimulated progress in the study of the physico-chemical properties of heterogeneous sys-
tems and processes proceeding at the gassolid, gasliquid and solidliquid interfaces, at
the molecular level [1766]. Due to these advances, it is now possible to obtain practically any
spectra which reflect the interaction of a surface with molecules in the gaseous or liquid states.
It is reasonable to affirm that these developments in spectral methods have caused a renaissance
in interest in the surface chemistry and catalytic applications [67108] of oxides and metals.
The appearance of more sensitive methods and new techniques has become a reason for a
reassessment of a significant part of the spectral information obtained ten and more years ago,
based on the use of more modern techniques. New spectral effects and characteristics, which couldnot be observed in earlier studies because of very low concentrations of the corresponding surface
species, or because of low values of extinction coefficients, can now be detected. Unfortunately,
authors of such new studies do not always cite the related previous work, although they use
the main ideas and conclusions as a basis for interpretation. This is why, in this book, the
data obtained in the 1970s and 1980s or earlier are reexamined, along with those from studies
published during the past decade.
A significant contribution to the development of spectroscopic investigations of surface com-
pounds has been made by the evolution of cryoscopic methods [109, 110], which allow the
registration of the spectra of individual molecules, their fragments, ions and radicals, both in
free states and in interaction with clusters of metal particles of different sizes (from monatomicupwards) or with cations (anions) frozen in a matrix. Since these interactions have similari-
ties with such structures formed on solid surfaces, the use of this cryoscopic data helps in the
interpretation of the spectra of reactive surface compounds.
The development of the catalytic systems based on the organometallic compounds supported
on different supports (so-called precise catalysts) [111115] became another way in catalytic
chemistry that contributed a lot to the identification of the surface compounds.
It should be particularly pointed out that, in spite of the now relative ease of obtaining spectral
information (including spectra registration), the interpretation of the spectra obtained and asso-
ciated phenomena (which are investigated by means of spectral methods) is extremely difficult.
In order to solve these problems, it is necessary to have knowledge in at least three areas: spec-
troscopy itself, the surface chemistry and heterogeneous catalysis. It cannot be assumed that the
presence of an absorption band, close in position to that expected in the spectrum of a possi-
ble fragment, provides a sufficient reason to draw such a conclusion about the structure of the
complex (and all the more about its subsequent participation in different interactions). Such a
simplified approach to the use of spectral information to explain, for example, catalytic phenom-
ena, has often led to wrong conclusions which in turn have caused further mistaken studies. As
will be shown below, doubtful surface compounds are often postulated as intermediates in cer-
tain reaction mechanistic schemes. This is why this author, who has devoted more than 30 years
to the application of spectroscopic methods in surface chemistry, and particularly heterogeneous
catalysis, has undertaken the difficult task of summarizing and analyzing the voluminous related
literature and data on the investigation of surface centers and the interactions of these centerswith a large number of molecules of both inorganic and organic types. Principal attention is paid
to the spectral identification of surface compounds (especially to evidence for their formation)
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xx INTRODUCTION
formed upon the interaction of different molecules with solid surfaces. The development of spec-
tral criteria to identify such compounds is the second subject of this book, since at present the
main problem in using physical methods in surface chemistry is to obtain reliable proof that the
suggested surface compounds are really present.
Chapter 1 summarizes the fundamentals of the different vibrational spectroscopy methods, and
their main strengths and weaknesses with respect to each other, as well as their different fieldsof application in studies of surface chemical problems. Several substantial books and reviews
devoted to such subjects, and analysis of the applications of different spectroscopic methods in
adsorption and catalysis, have already been published [27, 43, 44, 52, 6366]. Among them, the
books devoted to particular spectroscopic methods [24, 25, 31, 32, 34, 34a] have been written
by leading scientists in their fields. Readers who are interested in the possibilities of some
particular spectral method can find the answers in the appropriate text. The main question of in
situ studies, including kinetic and spectral measurements, have also been described in reasonable
detail [35a, 6365].
It should be pointed out that such numerous data have been obtained by different spectral
methods during investigations of surface phenomena and that it is impossible to describe andanalyze all such data in a single book. In general, studies can be divided into two groups,
(i) those devoted to spectroscopic investigations, and (ii) works in which spectroscopic methods
are used to check on the information obtained by nonspectroscopic methods. The second group
is widely represented in the literature, and is used for the study of adsorption and catalysis.
However, the spectral information obtained in a large number of such works is not sufficiently
reliable and the formation of one or another type of a postulated surface compound is not always
proven. For these reasons, the conclusions of such studies are not always discussed in detail in
this book, although appropriate ones will be mentioned. Studies of the other group, which form
the basis of this book, are as a rule carried out by highly qualified spectroscopists and are based
on a detailed analysis of the spectral information obtained. These studies contain very importantdata about solids and the surface compounds formed during interaction with different molecules.
Chapter 2 is devoted to the examination of spectral images of both simple oxides and binary
oxide surface systems in the various regions where they occur, and includes discussions of
the following: (i) the fundamental frequencies of the lattices, (ii) surface cationanion bonds,
(iii) molecular states of adsorbed oxygen, and (iv) surface hydroxyl group vibrations. Particular
attention is given to the study of electron-donating centers, oxygen ions, electron-accepting centers
and coordinatively unsaturated cations. The principles of crystal structures of oxides are used to
interpret the spectra, because it has been shown earlier that the homogeneity or otherwise of
an oxide surface, as well as the properties of different surface centers, depends on the degree
of dispersion and is related to the exposure of different crystal faces. The reactivity of surface
centers to different adsorbed molecules is analyzed for numerous oxide systems. The different
types of surface centers observed on oxide systems are classified, and the procedures used for
their investigation by means of molecular spectroscopy are analyzed.
Probe molecules with spectral parameters sensitive to the state of different adsorption centers
are described. Particular attention is paid to data on the identification of both Brnsted and Lewis
acidic sites on the surfaces of different oxide systems through the IR spectra of adsorbed ammonia
and pyridine. Practically all of the data in the literature on spectral patterns from ammonia bonded
to coordinatively unsaturated surface cations are considered in this chapter. Correlations revealed
between the changes in the frequencies of the symmetric deformational vibration of the ammonia
molecule due to adsorption ammonia and the acceptor abilities of coordinatively unsaturated
surface cations show the relative strengths of different cation acceptor centers on the oxidesurfaces (the established influence of the nature of the cations on their acceptor ability). It is
shown that ammonia adsorption opens the way to the identification of surface acidbase pairs
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INTRODUCTION xxi
(both surface cations and anions) which lead to ammonia dissociation and the formation of NH 2and OH species. For a number of systems involving acid-forming oxides, the presence of protic
acid centers is demonstrated, in which a proton compensates for a charge on several surface
anions through mobility.
Analysis of the different spectral methods of determination of both the acidic and basic proper-
ties of an oxide surface is represented in this chapter. Sections where these questions are examinedcontain (i) equations to calculate the strengths of both protic and aprotic surface centers in simple
and complex oxide systems, (ii) quantitative data on the different nature of active surface centers,
and (iii) extinction coefficients and values of integral intensities for a number of the most com-
monly used probes (NH3, Py and CO). In several cases an analysis of the correlations between
the quantitative characteristics of different types of surface centers and the pre-treatment and/or
preparative conditions of the surfaces is carried out. Spectral characteristics of both regular and
defective surface centers are also examined in this chapter.
In Chapter 3, the states of different cations in zeolites, oxides, supported oxides and heteropoly
acid systems, which depend on preparation methods and other experimental conditions are ana-
lyzed on the basis of detailed examinations of diffuse reflection electronic and ESR spectra.Special attention is paid to the cations of copper, nickel, cobalt, palladium, silver and platinum,
and to their changes upon modification by supports or other active components. Spectral char-
acteristics of metal particles are dependent on their sizes, types of support (including strong
metalsupport interaction (SMSIs)) and upon lateral interactions between adsorbed molecules.
Analysis of the spectra of adsorbed species on a number of metals, as well as the spectral
manifestations of the complexes formed at the active-component support interface, are also
considered.
The data covered in Chapter 3 illustrate the special roles of CO and NO molecules as probes.
Use of these molecules, coupled with the direct study of cationic states (by diffuse reflectance
electron spectroscopy (DRES), ESR, etc.), enables the establishment of correlations betweenCO (NO) and both the charge and coordination states of the cations. A number of model
examples of real oxide surfaces, created on the basis of oxide crystal structures, are examined.
These models describe sufficiently the experimental data obtained about interactions of the probes
with both active and inactive surface centers and surface structures. Sections are devoted to the
detailed examination of the spectral data in this field available from the literature concerning the
interactions of different types of oxide surfaces with other simple molecules such as H 2, N2, NO,
H2S, SO2, NO2 and CO2. Such data establish the direct interrelationships between the nature
of the activation and properties of the surface sites and the activation of these molecules. For
each molecule, the analysis of its electronic structure is carried out and changes observed upon
complexation are explained in terms of the changes in the vibrational spectra. It is shown that the
N2 molecule is a good probe for investigating electron-acceptor centers, whereas the H2 molecule
is a unique one for describing the properties of acidbase pairs. This chapter concludes with data
about the limits of spectral ranges characteristic for the complexes of CO (NO) with the variable
valence and coordination states of different cations, and also with metal clusters of different sizes.
The potentialities of spectral methods to investigate the interactions of inorganic or organometal-
lic compounds with oxide surfaces are briefly summarized in Chapter 4. Principal attention is
given to data representing the analysis of spectral changes resulting from the interactions between
molybdenum heteropoly acids (HPAs) and the surfaces of different supports, and also the changes
in properties of such unsupported systems themselves. It is shown that the examined spectra enable
us to determine whether or not such interactions occur, and can also detect the interactions of
different types of organometallic systems such as metal- allylic and carbonyl compounds of dif-ferent cations or metals. The results of the modification of oxide surfaces by anions, in particular
SO42, occupy a significant part of the data discussed in this chapter.
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xxii INTRODUCTION
Interactions between oxide surfaces and organic molecules of different types, such as alkanes,
alkenes, alkynes, aryls (including their halide derivatives), nitriles, alcohols, aldehydes, ketones
and acids, are presented in Chapter 5. Wherever possible, the complexation with each type of
surface center is differentiated and analyzed separately. Spectral images of the complexes formed,
their features and spectral parameters, which depend on the type and particularities of the surface
center, are examined in detail.Using the spectral data, general correlations are demonstrated between the character of the
nature and reactivity of the complex being formed from a particular adsorbate and its dependence
upon the nature of the surface center. The formation of new types of surface species (such as
unsaturated compounds interacting with coordinatively unsaturated surface cations), first revealed
by spectral features, is described for a number of systems. This chapter also contains numerous
energy and spectral characteristics of the surface compounds, such as the - and -allylic com-
plexes of unsaturated hydrocarbons, formed upon dissociative interactions of organic molecules
with oxide surfaces.
Special attention is paid to the estimation of the character of CH bond activation in alkanes.
Analysis of recently obtained data on methane activation allows the representation of a schemewhich establishes and provides a correlation between the active surface centers and the particular
intermediate species formed upon them. The latter provide a possibility for understanding the
pathway by which hydrocarbons are converted to other products on the different surfaces. As
a result of the spectral classification of the different adsorption species, new types of surface
compounds have been shown to be formed on oxide surfaces, which are otherwise absent or
extremely unstable under normal conditions.
Finally, in Chapter 6 a discussion is given of the mechanisms of a number of catalytic reactions
which have been investigated by means of vibrational spectroscopy.
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1 THEORETICAL FUNDAMENTALSAND EXPERIMENTALCONSIDERATIONS OF THESPECTROSCOPIC METHODSUSED IN SURFACE CHEMISTRY
Three principal types of problems may be distinguished in the application of molecular spec-
troscopic techniques in surface chemistry, namely (i) the characterization of the surface, (ii) the
estimation of the type and structures of surface compounds, and (iii) the obtaining of information
required to understand the mechanisms of the processes proceeding on the surface of a solid. The
first problem requires the determination of the types and properties of surface centers, which are
dependent on the structure and morphology of the solid. The second and third problems concernadsorption processes and involve the study of the following: (i) the structures and properties of the
surface compounds forming at different surface centers, (ii) reaction intermediates, and (iii) the
directions of their transformations to the products. During recent years, the situation in this areahas greatly improved, and different spectroscopic methods are now available for these studies.
The general principles of all of the techniques is the interaction between the incident radiation
or particle beam and the specimen and the following analysis of both the nature and energy of
the beam after such interaction. The energy regions involved in the different spectral methods
involving electromagnetic radiation are represented in Figure 1.1, while Table 1.1 summarizes
some of the essential characteristics of the various techniques.It is clear today that any progress in the field of surface chemistry is impossible without
the application of such modern methods of molecular spectroscopy as UVVis spectroscopy,
Fourier-transform infrared (FTIR) spectroscopy, Raman spectroscopy, electron energy loss spec-
troscopy, (EELS) including high-resolution electron energy loss spectroscopy (HREELS), and
neutron spectroscopy.
The main feature of these methods is that they are nondestructive analytical methods, because
the electromagnetic (radiation) or particle beams which are used, disturb the investigated systeminsignificantly. This is why such vibration techniques in different electromagnetic variants, such
as transmission, reflectance and emission, or the low-energy electron loss spectroscopies, are very
widely used nowadays.
1.1 Electronic spectroscopy
Electronic spectroscopy (ES) is normally concerned with the valence electronic transitions
between molecular orbitals. The transmissions between the electronic levels (Figure 1.2) are
Molecular Spectroscopy of Oxide Catalyst Surfaces. Anatoli Davydov 2003 John Wiley & Sons, Ltd ISBN: 0-471-98731-X
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2 SPECTROSCOPIC METHODS IN SURFACE CHEMISTRY
103
105 107 109 1011 1013 1015 1017 1019
101 101 103 105 107 109 1011
(m)
(Hz)
Region
Process NMR ESR
Molecular
rotations
Molecular
vibrations
Electronic
transitions
Imer-electronic
transitions
Nuclear
excitations
Radiofrequency Microwave IR UV X-Rays
Large Medium Short VHF Far ConventionalNear Near Far
Visible
Figure 1.1. The regions of the electromagnetic spectrum, classified according to the experimental techniquesemployed and the molecular information that can be obtained.
E2
E1
E0
~1 eV
~0.1 eV~0.01 eV
vn
v1 rnr1
Figure 1.2. Scheme of energy levels: E, electronic; v, vibrational; r, rotational.
located in the range of the electromagnetic spectrum (50 000 3000 cm1) this is the basis ofUVVis spectroscopy. The energies associated with the electronic jumps are large enough to
provokevibrations of the molecule, and the transitions are thereby broadened [6, 810, 33, 116].
Light in the UV Vis region of the electromagnetic spectrum can be used to study the
electronic transitions of the substrates. According to the nature of the electronic jumps,
the electronic transitions found in organic and inorganic chemistry can be classified into
several groups: (i) d d transitions (Figure 1.3(a)), (ii) charge transfers, (iii) transitions(Figure 1.3(b)), and (iv) n (Figure 1.3(b)). In the far-UV range are found other transitions,e.g. (n ) and ( ) (Figure 1.3(b)). Charge transfers occur due to electron transfersfrom an occupied orbital localized on a donor to an unoccupied orbital of an acceptor. In organic
systems, these transitions are between electron acceptors and electron donors and produce theabsorption bands in the UV and visible regions of the spectra with 103 106 (see below).In inorganic systems, the charge-transfer phenomena are of two types, involving an electron
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3
Table1.1.C
omparativecharacteristicsofthe
differentspectralmethodsusedinsurfacechemistry.
Characteristic
Technique
Molecularspectroscopy
R
esonancespectroscopy
Surfacestudies
IR,PAS
Raman
UVVis
EPR
NMR
Mossbauer
Neutron
EELS
XPS
AFS
SIMS
spectroscopy
Thicknessa
nalyzed
mm
mm
mm
mm
mm
100m
0.1mm
m
2050A
1020A
23A
Areaanalyzed
cm2
m2
cm2
cm2
cm2
cm2
mm2
cm2
cm2
cm2
cm2
Sampledeg
radation
No
Possible
No
No
No
No
Possible
Verysmall
Possible
Possible
No
Samplepreparation
Easy
Easy
Easy
Easy
Easy
Easy
Difficult
Difficult
Easy
Easy
Easy
Quantitative
measurem
ents
Possible
difficult
Possible
Yes
Yes
Yes
Yes
Yes
Possible
Possible
Gaseous
atmosphe
re
Yes
Yes
Yes
Yes
Yes
Yes
Yes
No
Difficult
Difficult
No
Temperaturerange
(C)
196to500
196to500
196to500
269to1000
196to200
269to400
269to800
Ambient
180to600
180to6
00
Ambient
Information
obtained
Functional
groups;
adsorbing
species
Functional
groups;
adsorbing
species
Degreeof
oxidation;ion
symmetry;
adsorbing
species
Paramagnetic
species;
degreeof
oxidation;
symmetry
Func
tional
groups
Degreeof
oxidation;
symmetryof
environment
Adsorbing
species;
atomic
structure
Metalligand
bonds
Degreeof
oxidation;
surface
composition
Surface
composition
Surface
composition
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4 SPECTROSCOPIC METHODS IN SURFACE CHEMISTRY
eg
6Dq
4Dq
10Dq
l2g
Octahedralfield, Voh
Sphericallysymmetrical
repulsiond
Free ion
(a)
(b)
s*
s
p*
p
p p* n p n s* s s*
n
Figure 1.3. Schemes of electron transfers: (a) d d electron transitions in an octahedral field; (b) relativeenergies of electronic transitions between different types of orbitals typical of organic molecules (,
and n).
transfer in different directions: (i) from an orbital mainly localized on the metal to that mainly
localized on the ligand (M L), and (ii) in the opposite direction (L M). The energies ofthese transitions is higher than that for d d transitions, and accordingly the absorption bandsare in the UV region of the spectra ( 103 106). Optical spectra can be directly obtained byeither internal or external reflectionabsorption techniques (reflectance spectroscopy).
1.1.1 TRANSMISSION SPECTRA
The transmitted light of intensity I is related to the incident light intensity I0 by the transmit-
tance, T, given by I /I0 (0< T < 1). For thin samples, the transmittance can be related to the
concentration of the absorber (c) and the thickness of the sample (l) by the LambertBeer law,
as follows:
T () = exp(lc) (1.1)
where is known as the molar absorption coefficient (cm2 mol1). The optical density orabsorbance, A(= log(I/I0) is also used frequently. It is often preferable to use this parameter inthe integral form (A), as follows:
A = 2
1
A() d= 2
1
ln[I0()/I()] d (1.2)
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VIBRATIONAL SPECTROSCOPY 5
UVVis spectroscopy is distinguished by a fairly high sensitivity. In particular, the intensity of
the absorption for allowed one-electron d d transitions is characterized by a molar absorptioncoefficient of the order of1 100. The value is significantly lower for complexes with a highsymmetry and is much greater in the case of the bands characteristic of the charge transfer in
the complex.
1.1.2 DIFFUSE REFLECTION SPECTRA
In surface chemistry, the UV Vis spectroscopic method is usually used in its diffuse reflec-
tion modification. The radiation reflected from a powdered crystalline surface consists of two
components, i.e. (i) that reflected from the surface without any transmission (mirror or specular
reflection), and (ii) that absorbed into the material and which then reappears at the surface after
multiple scattering. Modern spectrometers minimize the first component, and the term reflectance
is thus used for diffusely reflected radiation [25, 117].
Since only a part of the diffuse radiation is returned to the detector, measurement of the diffused
intensity is difficult. For this purpose, a special integrative sphere (Table 1.2), coated inside witha highly reflecting layer, such as MgO or BaSO4, is used. Such a sphere increases the part of
the diffused intensity that reaches the detector (3050 %). Spectra are recorded in ratio with a
sample which has similar diffusion characteristics to the sample under investigation, but without
any absorption losses.
The evaluation of the intensities of diffuse reflectance spectra is based on the theory of Kubelka
and Munk. The reflectance is given by R= I /I0 (0< R
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6
Table1.2.Basicexperime
ntalprinciplesandapplicationsofthedifferentspectralmethodsusedforsurfacechemistryanalysis.
Method
Commonscheme
Thickness
analyzed
Quantitative
measurements
Information
obtained
UVVis
Source
Integratingsphere
Detector
Standard
Sample
mm
Possible
Ionsymmetry;
ad
sorbing
sp
ecies;degree
ofoxidation
Transmission
IR
Source
Scatteredlight
Scatteredlight
Transmittedlight
Detector
KBr
KBr
Sample
disk
mm
Possible
Adsorbedspecies;
su
rfaceactive
sites;functional
groups
Reflectio
nIR
(RAIR
S)
Source
Detector
Reflectingsam
ple
mm
Possible
Adsorbedspecies;
su
rfaceactive
sites;functional
groups
ATR
Sample
Crystal
IRradiation
mm
Possible
Adsorbedspecies;
su
rfaceactive
sites;functional
groups
Diffuse
reflection
(DRIR
S)
Source
Collectingmirror
Samplepowd
er
Detector
Diffusively
reflectedlight
mm
Possible
Adsorbedspecies;
su
rfaceactive
sites;functional
groups
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7
.
EmissionIR
Furnace
Detector
KBr
Sample
mm
Possible
Adsorbedspecies;
su
rfaceactive
sites;functional
groups
Raman
UVVISorNIRbeam
Detector
90
back-scatteredbeam
mm
Possible,butwith
difficulties
Functionalgroups;
ad
sorbedsites;
su
rfacestructure;
bulkstructure;
structureof
ad
sorbedspecies
EELS
Ultra-high-
vacuumchamber
Electronbeam
Impact-scatteredbeam
Dipole-scatteredbeam
Sample
m
Metalligand
bonds;phase
transitions;
ch
emical
structure
IETS
Metal
Metal
Elec
tronflow
Adsorbate
Oxidelayer
m
Possible,butwith
difficulties
Vibr
ationalspectra
ofminute
quantitiesof
m
aterials
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8 SPECTROSCOPIC METHODS IN SURFACE CHEMISTRY
frequencies of atoms within the molecules, but allows us to explain the existence of rotational
frequencies only if the electronic motions occur at the same frequencies.
The vibration of a diatomic molecule can be reduced to the motion of a single particle of
reduced mass m. In this model, the problems are simplified by considering that the diatomic
molecule can be analogous to the harmonic oscillator (Figure 1.4), in which two masses ( m1 and
m2) are joined by a perfect spring of length r0. A restoring force fis directly proportional to thedistance r , as follows:
f= kr= m(d2r/dt2) (1.4)
where k is known as the harmonic force constant, and is a function of the potential energy U in
accordance with Hookes law:
f= dU/dr= kr (1.5)
Integration of this equation leads to the following parabolic relationship see (Figure 1.4(a)):
U= 1/2kr 2 (1.6)
For diatomic molecules AB, r represents the displacement of the atoms from the equilibrium
separation r0. A small displacement of one of the masses relative to the other will cause the
system to vibrate as a simple harmonic oscillator with a frequency given by the following:
0= (1/2 )
k/m (1.7)
where m is the reduced mass of the system. At the assignment of the frequencies observed
in the infrared spectra, this relationship is often used in conjunction with isotopic exchange, in
particular deutero exchange, as follows:
X H/X D=
2 = 2mx/(2 + mx) (1.8)
where mx is the mass of the X atom. This ratio is 1.37 for OHOD and NHND, and 1.36 for
CHCD (i.e. ca.
2).
According to quantum theory, the energy of the molecule is given in terms of a series of discrete
energy levels, E0, Ev1 , E
v2 , etc. (see Figure 1.2), and each discrete molecule must exist at one
or other of these levels. The frequency of absorption or emission of radiation for a transmission
between the levels with energies E0 and E1 is given by the following:
= (E1 E0)/ h (1.9)where h is the Planck constant.
From the Schrodinger wave equation, the total energy of vibration is as follows:
Evib= h(v + 1/2) (1.10)
where is the frequency of vibration of the oscillator and v is the vibrational quantum number.
For any transition between quantized levels in which v v= 1.
E= h (1.11)
The differences between two levels arise directly as a result of the quantum-mechanical deriva-tion of Equation (1.10). For the simple quantum-mechanical model, the presence of combination
bands and overtones in the spectrum is forbidden, because such bands involve jumps between
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VIBRATIONAL SPECTROSCOPY 9
Potentialenergy
(b)
DeD
r0
V=0V=1V =2
Internuclear separation
r
f= kr
U= 1 kr22
Potentialenergy
Equilibriumposition
(a)
Separation
Figure 1.4. Potential-energy functions for (a) a mass and spring system obeying Hookes law, and (b) a
real diatomic molecule with a dissociation energy De and equilibrium bond length re (where r0 represents
the first energy level).
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10 SPECTROSCOPIC METHODS IN SURFACE CHEMISTRY
several different quantum levels. There are no such stringent rules in the case of an anharmonic
oscillator, where overtone and combination bands can appear, often weakly in the spectra, accord-
ing to the following:
Evib
=h(v
+1/2)
xe(v
+1/2)2 (1.12)
wherexe is the anharmonicity constant. Introduction of this parameter leads to the potential curve
shown in Figure 1.4(b).
The movements of the atoms in a molecule during vibration can be approximately classified
into two groups, i.e. (i) bond stretching, and (ii) angle deformations. For an N-atomic molecule,
the number of fundamental vibrations is 3N 6 for a nonlinear and 3N 5 for a linear molecule.There are four types of vibrations, i.e. , , , and (Figure 1.5). Generally, the frequencies of
these vibrations decrease in the order > > > .
All molecules can be classified into a limited number of symmetry groups, which obey the rules
of group theory. A knowledge of the symmetry group of a molecule allows the determination
of the symmetry classes of the 3N 6 normal modes of vibration and their activities in IR andRaman spectroscopies.Assignment of the bands in the spectrum to particular types of vibrations is an important stage
and is based, as a rule, on group-characteristic (of limited dependence on the nearest molec-
ular environment) modes. Calculations based on vibrational theory are used for more accurate
assignment [4, 1416]. These calculations show that there are no strictly characteristic vibrational
modes; frequencies of many group vibrations are coupled and make certain contributions to each
other. A vibration with a minimum contribution from other vibrations is known as a group char-
acteristic. Usually, interpretations of the spectra of adsorbateadsorbent systems are made by the
stretching
symmetricstretching
symmetricstretching
asymmetricstretching
asymmetricstretching
bending
bendingin-plane
bendingout-of-plane
bending
bending
+ +
rocking twisting wagging
+
++
+
(a) Linear molecules (b) Non-linear molecules
Figure 1.5. Fundamental modes of vibration of (a) linear, and (b) nonlinear molecules.
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VIBRATIONAL SPECTROSCOPY 11
comparison with the spectra of bulk compounds or fragments isolated in a matrix. However, in
this way only certain types of compound can be identified. Calculations of the vibrational spectra
of individual surface species based on the modeling of the potential function of the molecule
(vibrations in the force constants of the potential function) are useful for interpreting changes in
vibrational frequencies and their reactions to changes in the force field of molecules subjected to
the influence of a highly nonuniform field at an adsorbent surface [19, 118].Several forms of vibrational spectroscopy are now in routine use, i.e. (i) transmission infrared,
(ii) Raman, (iii) diffuse reflection, (iv) reflectionabsorption infrared, and (v) electron energy
loss, but in the study of surfaces none has found wider application than infrared spectroscopy.
1.2.1 INFRARED SPECTROSCOPY
The infrared (IR) region corresponds to the energies of the vibrations and rotations of molecules.
If a molecule is subjected to IR radiation whose frequency is equal to that of one of its oscillators,
this oscillator will resonate and absorb part of the radiation. The absorption (emission) intensity
is given by the transition probability between the ground and excited states. Not all vibrations are
observedonly those transitions corresponding to vibrations with variation of the dipole momentare active in IRS. The intensity of the infrared band is proportional to the square of the change
in dipole moment. The principles of this method have been presented in numerous books and
reviews [15, 1116, 119123] and are summarized in Table 1.2.
In the study of processes occurring on surfaces, transmission, reflectance, emission and diffuse
reflection infrared spectroscopies are used.
Transmission spectroscopy
A common infrared transmission spectrum is obtained as a result of the direct transmission of an
infrared beam through a sample when the following conditions apply: = +1 (/Q)0 = 0,where is the dipole moment, and Q is a normal coordinate. As in UVVis spectroscopy,the spectrometer records the transmission, T (= I /I0= exp(kl)), the intensity of which can befound from the LambertBeer law (Equation (1.1)). The sensitivity of this method is determined
by both the characteristics of the radiation detector and by the absorption coefficient of the
medium. Approximately a 10-fold gain in sensitivity can be achieved by the use of the Fourier
transform (FT) technique [1, 120].
To study the spectra of bulk oxides, dilution in either KBr (down to 400 cm1) or CsI (downto 200 cm1), or polyethylene disks are used. If the sample is stable in air and does not reactwith KBr or CsI, these are the methods most often used. However, if any interactions take place,
then the techniques of attenuated total (internal) reflection (ATR) or emission spectroscopies have
to be used.
For both cases, in order to obtain a typical transmission IR spectrum from oxides in the
region of surface vibrations (in this case, the thickness of the corresponding sample is ten times
more, as a minimum, than that of the corresponding samples prepared in an immersion media),
the sample has to allow at least a partial transmission, preferably 10 % or more, of the IR
beam. Scattering of the radiation by the particles can be another factor which leads to low
transmission. The transparency is improved when the oxide particle sizes are small relative to
the radiation wavelength. Scattering may be significantly reduced by the use of pressed disks
or highly dispersed samples (particles size less than 1 m). The latter can be prepared in two
different ways, i.e. (i) sedimentation of the dissolved sample from an inert solvent or from air
onto a transparent window, or (ii) by using an electrical field [42]. When a material is pressed into
a thin flat self-supported disk, the scattering, which takes place during the transmission of the IRradiation through the sample, can be substantially reduced. In general, such a disk should be from
one to a few tenths of a millimeter in thickness and have a density between 10 and 100 mg cm2
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12 SPECTROSCOPIC METHODS IN SURFACE CHEMISTRY
Figure 1.6. A typical mold used for compressing self-supported pellets of adsorbent in infrared studies.
in order to give good transmission in the infrared and also to have good mechanical properties.
It is better to press the samples in a demountable mold (Figure 1.6) [19, 42], which then allows
adjustment of the lateral pressure on the compressed sample by means of a number of screws
and plates, in order to prevent cracking of the thin disk and to facilitate its removal from the
mold. Preparation of such thin samples, especially from transition-metal oxides or zeolites, istruly an art. If the solids are not transparent or cannot be molded as disks and hence have
to be handled as powders, the following methods can be useful: (i) the use of a grating; (ii) a
(powder) technique in which the powder is finely pulverized, sifted, and then spread out on an
IR-transparent disk (such as KBr, CaF2 or a silicon single crystal) and, if this covering is stable,
then covered by a second disk; (iii) dilution of the sample in a compound that is transparent
and inert to the reaction being studied (e.g. SiO2 and Al2O3); (iv) the use of special cells. The
problem of sample preparation is minimized in the case of diffuse reflection spectroscopy [25,
61, 122124].
To record the infrared spectra of adsorbed molecules, special vacuum cells are used these
are available in many different and widely variable types, depending on the system being studied[1821, 2326, 2835, 42, 4446, 63, 64]. Ideally, these cells should be designed so that the
sample can be heated up to 1273 K, and cooled to liquid nitrogen temperature, to adsorb/desorb
both gases and vapors at different temperatures, be able to maintain a high vacuum, and to record
the spectra of adsorbed molecules without exposing the pellet (disk) to the air. The main problem
here is achieving an hermetic sealing of the windows (plates) which are transparent in the
IR (Table 1.3 [42, 119, 125127]). Frequently, such a sealing is made by the use of different
cements and glues with low vapor pressures, or alternatively by using O-rings and flanges.
There are two types of cells used for recording infrared spectra under high vacuum over a wide
temperature range: (i) a cell, large in length, which gives the possibility of separating a heater
from the region of the sealed windows, i.e. where the recording of spectra and the heating of the
sample are carried out in different parts of the cell or if a static sample is heated, a cooling of
the windows is required (see, for example, a cell design resulting from the work of this present
author (Figure 1.7) [42, 128]); (ii) a cell with a very short optical pass (
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VIBRATIONAL SPECTROSCOPY 13
Table 1.3. Various optical materials used for lenses and windows in infrared studies.
Material Low-energy cutoff
(cm1)Comments
Sapphire >1600 High mechanical strength; inert;
connected with metal; hard; expensiveQuartz >2500 Good for high-temperature work and in
the overtone region; insoluble; easy to
work in fused form
LiF >1200 Good dispersion in the near-IR region;
easily scratches
MgF2 1400 Strong; chemically durable
CaF2 1200 Inert to most chemicals; tends to be
costly; slightly soluble; good from
73373 K
MgO >1200 Hard and costly; can be sealed to a
high-expansion glass
Silicon 1100 Inert; insoluble; connected with glass; not
transparent at high temperatures
NaF 1000 Slightly hydroscopic
BaF2 900 Hard; expensive
ZnS 714 Good up to 1073 K; strong; chemically
durable
NaCl 600 Slightly hydroscopic; cheap; easily
worked
KCl 550 Slightly hydroscopic; cheap
AgCl 500 Photosensitive; can corrode metals
ZnSe 500 Good up to 573 K; soluble in acids
KBr 350 Hydroscopic; easily scratched; used aspowder for pressed-disk technique
CsBr 250 Very hard; expensive
KRS-5 250 Very soluble; expensive; toxic; deforms
under pressure
CsI 180 Very hard; expensive
been reviewed by Little [18], Kiselev and Lygin [19], Delgass et al. [25], Shchekochikhin and
Davydov [42], Bell [119, 127], and Basu, Ballinger, Yates et al. [128a], as well as in numerous
original studies.
The best materials for windows directly connected to glass are silicon, MgO and AgCl, while
sapphire is the best material for connection to metals (see Table 1.3). Quartz cells, which are
usually routinely used in the visible and UV regions of the spectrum, can also be employed in
the infrared region, but only above 2000 cm1. A very convenient cell is one with silicon singlecrystal windows (Figure 1.8(a)) but, unfortunately, this cannot be used at temperature higher than
473 K because of the reduced transparency of the silicon single crystal as a result of internal
electron transfer mechanisms.
There are several types of cells of minimized volume, used in order to record spectra under
dynamic conditions (e.g. at high temperatures during catalytic transformations so-called in
situ conditions see, for example, Figures 1.7(b) and 1.10). A simple flow-cell-reactor made
from metal [42] or quartz [64, 65] practically without any free volume, has been proposed(Figures 1.7(b) and 1.11). The main part of such a cell is a reactor made from metal or quartz.
Both the reactor and windows (CaF2, BaF2, ZnS or ZnSe) are polished and clamped to each
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14 SPECTROSCOPIC METHODS IN SURFACE CHEMISTRY
Furnace
Thermocouplepocket
Pyrex
Catalyst disk
NaCl window
Quartz sampleholder
Quartz retainingring
10
10/1
19
11
13
2
4
56 7
812
(a) (b)
9
Figure 1.7. Schematics of cell-reactors used for studying the spectra of adsorbed molecules at (a) room,and (b) high temperatures: 1, cell body; 2, sample holder; 3, evacuation port; 4, case for thermocouple;
5, container for heater; 6, 10, cooling channels; 7, 11, flanged connectors; 8, windows; 9, connectors; 12,
sample pocket.
Tovacuum
10 mmSi window
NaClwindow
Liquid N2
Siliconwindow
(a) (b)
Figure 1.8. Schematics of vacuum cells with silicon windows: (a) for low-temperature investigations;
(b) with a short optical path.
other [64, 65]. Connections between the cell body and the windows is achieved by using iridium
or gold foils, or with Teflon [42]. The Graseby Specac company now produces standard in situ
high-temperature cells for temperature ranges up to 773 K, as well as high temperature and
pressure cells.
Emission spectroscopy
Methodological difficulties in the study of surface species caused by strong scattering or absorp-
tion of infrared light by the adsorbing sample can be eliminated if emission spectra are recorded
[1, 25, 35]. This method, however, is less frequently used since the intensities of the emissionbands are quite low, except at higher temperatures. Emission spectra are usually produced by
heating the sample above 473 K (Figure 1.12) [129] and are the most appropriate in cases where
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VIBRATIONAL SPECTROSCOPY 15
Figure 1.9. Schematics of types of sample holder.
reactions proceed on surfaces at high temperatures (see Table 1.2). The spectrum is obtained bymeasuring the ratio of the emitted radiation at any wavelength to that emitted by a perfect black
body at the same wavelength and temperature. According to the Kirchoff law, = a , wherea is the absorbance (fra