Scanning Tunneling Microscopy in Surface Science, Nanoscience and Catalysis

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Scanning Tunneling Microscopy in Surface Science, Nanoscience and Catalysis Michael Bowker and Philip R. Davies, Editors

WILEY-VCH Verlag GmbH

Scanning Tunneling Microscopy in Surface Science, Nanoscience and Catalysis

Edited by Michael Bowker and Philip R. Davies

Scanning Tunneling Microscopy in Surface Science, Nanoscience and Catalysis Edited by Michael Bowker and Philip R. Davies

Further Reading K.W. Kolasinski

B.P. Jena, J.K.H. Hoerber (Eds.)

Surface Science

Force Microscopy

Foundations of Catalysis and Nanoscience

Applications in Biology and Medicine

2008

ISBN: 978-0-471-39628-4

2006

ISBN: 978-0-470-03304-3

M. Prutton, M. El Gomati (Eds.) D.G. Brandon, W.D. Kaplan

Microstructural Characterization of Materials 2008

Scanning Auger Electron Microscopy 2006 ISBN: 978-0-470-86677-1

ISBN: 978-0-470-02784-4

D.K. Schroder J.W. Niemantsverdriet

Spectroscopy in Catalysis An Introduction 2007 ISBN: 978-3-527-31651-9

Semiconductor Material and Device Characterization 2006 ISBN: 978-0-471-73906-7

Scanning Tunneling Microscopy in Surface Science, Nanoscience and Catalysis

Edited by Michael Bowker and Philip R. Davies

The Editors Prof. Michael Bowker Cardiff University Wolfson Nanoscience Lab and Cardiff Catalysis Institute School of Chemistry Cardiff, CF10 3AT United Kingdom Dr. Philip R. Davies Cardiff University Wolfson Nanoscience Lab and Cardiff Catalysis Institute School of Chemistry Cardiff, CF10 3AT United Kingdom

Cover illustration The STM images being past of the front cover picture have been kindly provided by the group of D. Wayne Goodman (authos of Chapter 3).

All books published by Wiley-VCH are carefully produced. Nevertheless, authors, editors, and publisher do not warrant the information contained in these books, including this book, to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate. Library of Congress Card No.: applied for British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library. Bibliographic information published by the Deutsche Nationalbibliothek The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at http://dnb.d-nb.de # 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim All rights reserved (including those of translation into other languages). No part of this book may be reproduced in any form – by photoprinting, microfilm, or any other means – nor transmitted or translated into a machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law. Printed in the Federal Republic of Germany Printed on acid-free paper Cover Design Formgeber, Eppelheim Typesetting Thomson Digital, Noida, India Printing Strauss GmbH, Mörlenbach Bookbinding Litges & Dopf Buchbinderei GmbH, Heppenheim ISBN: 978-3-527-31982-4

V

Contents Preface IX List of Contributors

1 1.1 1.1.1 1.1.2 1.2 1.2.1 1.2.2 1.2.3 1.2.4 1.2.5 1.3 1.3.1 1.3.2 1.3.3 1.3.4 1.4

2 2.1 2.2 2.3 2.4 2.5 2.5.1

XIII

Chirality at Metal Surfaces 1 Chris J. Baddeley and Neville V. Richardson Introduction 1 Definition of Chirality 1 Nomenclature of Chirality: The (R),(S) Convention 2 Surface Chirality Following Molecular Adsorption 4 Achiral Molecules on Achiral Surfaces 4 Lattice Matching 8 Chiral Molecules on Achiral Surfaces 12 Chiral Molecules on Chiral Surfaces 15 Chiral Etching 16 Chiral Amplification and Recognition 19 Chiral Amplification in Two Dimensions 19 Chiral Switching 20 Chiral Recognition 21 Prochiral Molecules Interacting with Chiral Surfaces 24 Conclusions 25 References 26 The Template Route to Nanostructured Model Catalysts Conrad Becker and Klaus Wandelt Introduction 29 Surfaces as Two-Dimensional Templates 31 STM Imaging of Oxide Films 34 STM Imaging of Metal Particles on Oxide Films 39 Template-Controlled Growth of Model Catalysts 44 Oxides as Templates 44

29

VI

Contents

2.5.2 2.6

Modified Templates Conclusions 51 References 52

3

In Situ STM Studies of Model Catalysts 55 Fan Yang and D. Wayne Goodman Introduction 55 Instrumentation 56 Visualizing the Pathway of Catalytic Reactions 59 Imaging of Adsorbates and Reaction Intermediates 59 Imaging Chemisorption on Metals 61 Determining the Sites for Chemisorption on Oxide Surfaces 64 Visualizing Reaction Intermediates and the Mechanism of Hydrogen Oxidation 71 Measuring the Reaction Kinetics of CO Oxidation 73 Metal Surfaces at High Pressures 81 In Situ Studies of Supported Model Catalysts 85 Monitoring the Growth Kinetics of Supported Metal Catalysts 85 Studies of the SMSI Effect 88 Sintering Kinetics of Supported Au Clusters 89 Outlook 91 References 92

3.1 3.2 3.3 3.3.1 3.3.2 3.3.3 3.3.4 3.3.5 3.4 3.5 3.5.1 3.5.2 3.5.3 3.6

4

4.1 4.2 4.3 4.4 4.5 4.5.1 4.6

5

5.1 5.2 5.2.1 5.2.2 5.2.2.1 5.2.2.2 5.2.2.3

50

Theory of Scanning Tunneling Microscopy and Applications in Catalysis 97 Gilberto Teobaldi, Haiping Lin, and Werner Hofer Catalysis and Scanning Tunneling Microscopy 97 Image Formation in an STM 98 Simulating Tunneling Currents 99 Simulating Chemical Reactivity 100 Catalytic Water Production 101 TiO2: A Catalytic Model System 106 Outlook 115 References 116 Characterization and Modification of Electrode Surfaces by In Situ STM 119 Dieter M. Kolb and Felice C. Simeone Introduction 119 In Situ STM: Principle, Technical Realization and Limitations Principle Considerations for In Situ Operation 120 Technical Realization 124 Tip Preparation and Isolation 124 Electrochemical Cell 126 Vibration Damping 127

120

Contents

5.2.3 5.3 5.3.1 5.3.2 5.4 5.4.1 5.4.2 5.4.3

6 6.1 6.2 6.3 6.3.1 6.3.2 6.3.3 6.3.4 6.3.5 6.3.6 6.4

7

7.1 7.2 7.3 7.4 7.5 7.5.1 7.5.2 7.5.3 7.6

Limitations 127 Imaging Single-Crystal Surfaces of Catalytically Relevant Systems 128 Preparation and Imaging of Metal Single-Crystal Surfaces 128 Bimetallic Surfaces 130 Strategies for Nanostructuring Surfaces 132 Oxidation–Reduction Cycles for Roughening and Faceting Surfaces 132 Surface Modification by an STM: An Overview 134 Metal Nanocluster Deposition via Jump-to-Contact 139 References 144 STM Imaging of Oxide Nanolayer Model Systems 147 Falko P. Netzer and Svetlozar Surnev Introduction 147 Experimental Aspects and Technical Developments 149 Case Studies: Selected Oxide–Metal Systems 152 Alumina Nanolayers on NiAl Alloys 152 Titanium Oxide Nanolayers 155 Vanadium Oxide Nanolayers 159 Iron Oxides on Pt(1 1 1) 169 Nickel Oxide Nanolayers 173 Ceria Nanolayers on Metal Surfaces 177 Synopsis and Outlook 182 References 183 Surface Mobility of Atoms and Molecules Studied with High-Pressure Scanning Tunneling Microscopy 189 Gabor A. Somorjai, Feng Tao, and Derek Butcher Introduction 189 Characterization of Surface Mobility of Molecules and Atoms High-Pressure STM Technique and Instrumentation 191 Mobility and Flexibility of Catalyst Surfaces at High-Pressure High-Temperature Reaction Conditions 197 Adsorbate Mobility During Catalytic Reactions 206 Ethylene Hydrogenation on Pt(1 1 1) 207 Hydrogenation of C6 Cyclic Hydrocarbons on Pt(1 1 1) 209 CO/NO Coadsorption on Rh(1 1 1) 213 Summary 216 References 216

189

VII

VIII

Contents

8

8.1 8.2 8.3 8.4 8.5 8.6 8.7 8.8

Point Defects on Rutile TiO2(1 1 0): Reactivity, Dynamics, and Tunability 219 Chi L. Pang and Geoff Thornton Introduction 219 Methods 220 Water Dissociation at Oxygen Vacancies and the Identification of Point Defects 221 O2 Dissociation at Oxygen Vacancies 229 Alcohol Dissociation at Oxygen Vacancies 229 Diffusion of Oxygen Vacancies and Surface Hydroxy 232 Tuning the Densities of Oxygen Vacancies and Surface Hydroxyl on TiO2(1 1 0) 234 Outlook 236 References 236 Index

239

IX

Preface The objective of this book is to highlight the important strides being made toward a molecular understanding of the processes that occur at surfaces through the unique information provided by the proximal scanning probe family of techniques: this principally involves scanning tunneling microscopy (STM) but some atomic force microscopy (AFM) experiments are also included. The chapters in this book describe several state-of-the-art examples where an atomic understanding of surface processes is developing out of atomically resolved information provided by STM and AFM. The focus of much of the work is on understanding the fundamentals of catalysis, a reflection of the huge significance of heterogeneous catalysis in society today, but the discoveries being made in this field will undoubtedly have a much wider significance in the field of nanoscience/ technology. Reaction equations are derived from the results of global reaction measurements and considerations of stoichiometry. Until recently, the intermediates (and in particular the surface species) involved in the mechanism and their spatial location have remained largely theoretical. The advent of STM has, for the first time, allowed us the possibility of getting direct insight into this area. An example is the molecular identification of the sequence of reaction steps and of the species involved in the reaction of gas-phase methanol with oxygen on a copper surface. This produces methoxy groups as the first step, in which the slightly acidic hydrogen from the alcohol is stripped by surface oxygen leading to water desorption, as shown below, where the subscript ‘‘a’’ refers to an adsorbed species. CH3OHg þ Oa ! CH3Oa þ OHa CH3OHg þ OHa ! CH3Oa þ H2Og In this case, as in many others, STM has enabled us to identify the active sites at which reaction between methanol and adsorbed oxygen takes place, and has also allowed us to verify, at the atomic and the molecular scale, that the reaction does indeed occur in the way the above stoichiometric equations describe. This is invaluable information in the quest to understand and improve catalytic processes.

X

Preface

The principal advantage of the probe methods is their extraordinary spatial resolution ( {1 1 0} is clockwise (R) or anticlockwise (S) about the kink atom. STM imaging of step–kink surfaces such as Cu{6 4 3} show a high degree of atom mobility at the step edges [39]. Sholl et al. used DFT simulations to show that naturally chiral metal surfaces retain their net chirality even after their local structure is disrupted by thermal step roughening [40]. More recently, Jenkins and Pratt showed that stepped bcc and hcp surfaces may be chiral in the absence of kinks [41]. It was realized at an early stage that the adsorption of two enantiomers at chiral step–kink sites was likely to occur with slightly different adsorption energies. In

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Figure 1.12 Schematic diagram showing the mirror equivalent step–kink arrangements of the fcc {6 4 3}R and fcc{6 4 3}S surfaces (Adapted with permission from Ref. [37]. Copyright 1996, American Chemical Society.)

catalysis, such small modifications to the reaction pathway can be amplified to make significant changes in selectivity. Attard, with an elegant series of cyclic voltammetry experiments, displayed not only a difference between the behavior of D- and L-glucose at the (R)-Pt{6 4 3} surface but also an equivalence between the behavior of D-glucose/ (R)-Pt{6 4 3} and L-glucose/(S)-Pt{6 4 3} [42]. Temperature-programmed desorption (TPD) has been used to identify subtle differences in adsorption energy for enantiomers at chiral surfaces. For example, Gellman and coworkers reported enantiospecific behavior of (R)- and (S)-propylene oxide on Cu{6 4 3} [43]. However, the adsorption of (R)- and (S)-butanol (the simplest chiral alcohols) produced TPD data that were indistinguishable on Ag{6 4 3} [38]. STM studies of the adsorption of chiral molecules on chiral surfaces are surprisingly sparse. Zhao and Perry showed that (R)-3-methylcyclohexanone forms ordered structures on Cu{6 4 3} with a molecular spacing consistent with the spacing of kinks on the ideal Cu{6 4 3} surface [39]. Kuhnle et al. were able to probe, with atomic resolution, the interaction of chiral molecules with kink sites in the case of cysteine on Au{1 1 0} [44]. Although the surface is achiral, it displays both (S)- and (R)-kinks in approximately equal numbers. Kuhnle et al. showed that dimers formed from (R)cysteine adopt different adsorption geometries at (S)-kinks from (S)-cysteine dimers, demonstrating enantiospecific adsorption at these chiral centers. Furthermore, dense, homochiral cysteine islands are found to preferentially grow from kink sites of a specific chirality [44]. 1.2.5 Chiral Etching

For an fcc crystal, the low-index faces (e.g., {1 1 1}, {1 0 0}, and {1 1 0}) are thermodynamically the most stable, having the lowest surface free energies.

1.2 Surface Chirality Following Molecular Adsorption

Chemisorption can lead to large changes in surface free energies. There are many examples where chemisorption of organic molecules on a low-index crystal face results in faceting of a metal surface. A number of factors influence the formation of facets including face-specific adsorption energies, the energy difference between kinks, steps, and terraces, substrate-mediated intermolecular interactions, and surface diffusion barriers. Recent studies of organic molecules adsorbed on lowindex surfaces have found that high-index facets can be formed with complex organic molecular adsorbates containing electronegative elements such as O and N atoms in their functional groups. In these systems, the energy gain, which drives the morphology change, could originate from the molecule–substrate interactions and substrate-mediated interadsorbate interactions, which stabilize the steps and kinks of the substrates. Organic molecules with carboxylic acid functionalities commonly exhibit faceting on metal surfaces. For example, STM investigations have revealed that formic acid [45], benzoic acid [46], and p-aminobenzoic acid [47] all exhibit faceting behavior on Cu{1 1 0}. It has been well established that at room temperature the carboxylic acid group is deprotonated to the carboxylate. A preferential alignment of step edges along the [1 1 2] directions can be easily identified for both formate and acetate. It seems likely that the driving force for the formation of this orientation of step edge is the ordering of the molecular species into c(2 2) arrangements. Surface structures formed by the adsorption of benzoic acid are much more complicated [46]. Benzoate species can adopt either flat-lying or upright geometries and may form several different periodic structures depending on coverage and annealing temperature. The related molecule p-aminobenzoic acid also displays extensive faceting on the Cu{1 1 0} surface as shown in Figure 1.13 [47]. In these cases, it is possible to identify two symmetrically equivalent (11 13 1) facets giving the characteristic sawtooth arrangement of facets. The fact that similar facets are observed for both benzoic and p-aminobenzoic acid leads to the conclusion that the formation of facets is directed by the flat-lying carboxylate units. In the case of formate and acetate, where vibrational spectroscopy reveals upright carboxylate units, step bunching is not observed leading to the proposal that the adsorbate-mediated step–step interaction required for step bunching is at best only weakly attractive when the carboxylate is perpendicular to the surface [48]. Pascual et al. [49] investigated the adsorption of the prochiral carboxylic acid 4[trans-2-(pyrid-4-yl-vinyl)]benzoic acid on Ag{1 1 0}. Following exposure to submonolayer coverages of PVBA and thermal processing, similar sawtooth facets were observed as for benzoic acid on Cu{1 1 0} (Figure 1.14). It was proposed that the formation of facets was driven by the interaction between the carboxylate and the {1 0 0} microfacets at step edges. The microfacets then act as chiral templates nucleating the growth of supramolecular PVBA structures. The chirality of the PVBA species at the microfacet determined the structure of the first four assembled rows of molecules. It is perhaps unsurprising that when a chiral adsorbate is used containing the carboxylate functionality, the distribution of facets produced becomes chiral. Zhao and coworkers carried out studies of the adsorption of a range of amino acids on

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Figure 1.13 High-resolution STM image (50 nm 50 nm, bias 1.14 V, tunneling current 6.1 nA) showing the faceted structure of p-aminobenzoic acid on Cu(1 1 0). (Reprinted with permission from Ref. [47]. Copyright 2003, Elsevier.)

Cu{0 0 1} [49–52]. In the case of the achiral glycine molecule, a tendency was found for the formation of (3 1 17) facets. Since there is neither rotational nor reflectional symmetry within individual facets, eight symmetry-related facets should be expected, that is, (1 3 17), (1 3 17), (3 1 17), (3 1 17), (3 1 17), (3 1 17), (1 3 17), and (1 3 17) The first four facets are rotationally equivalent to each other as are the final four. The two sets are related by reflectional symmetry to each other. When a chiral adsorbate, for example, S-lysine, is used, the reflectional symmetry is no longer valid and only rotationally equivalent facets should be formed. This was demonstrated elegantly by Zhao with STM [53]. The driving force for facet formation is proposed to be a three-point interaction involving the carboxylate group, the a-amino group, and the amino-terminated side chain. The simultaneous optimization of adsorbate–adsorbate and adsorbate–substrate interactions determines the stereochemistry of the facet. Surface faceting may be particularly significant in chiral heterogeneous catalysis, particularly in the Ni/b-ketoester system. The adsorption of tartaric acid and glutamic acid onto Ni is known to be corrosive and it is also established that modifiers are leached into solution during both the modification and the catalytic reaction [28]. The preferential formation of chiral step–kink arrangements by corrosive adsorption could lead to catalytically active and enantioselective sites at step–kinks with no requirement for the chiral modifier to be present on the surface.

1.3 Chiral Amplification and Recognition

Figure 1.14 (a) STM image (10 nm 10 nm, tip bias þ 0.52 V, tunneling current 0.5 nA) of a PVBA-induced sawtooth blade in a restructured Ag(1 1 0) surface terrace. (b) Structural model of the chiral kink arrangements induced by lateral interaction of molecular carboxylate end groups with Ag{1 0 0} microfacets. (Reprinted with permission from Ref. [48]. Copyright 2004, American Institute of Physics.)

1.3 Chiral Amplification and Recognition 1.3.1 Chiral Amplification in Two Dimensions

In Section 1.2.1, we discussed the phenomenon of adsorbate-induced chirality whereby the adsorption of achiral species (e.g., glycine) results in the formation of two mirror equivalent domains on the surface. It has recently been shown that the presence of relatively small mole fractions of chiral dopants can result in the exclusive

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formation of one of the two mirror equivalent domains of the achiral species. For example, succinic acid (HOOCCH2CH2COOH), an achiral molecule, forms two mirror equivalent domains h9 0 | 2 2i and h9 0| 2 2i on Cu{1 1 0} [54]. The doubly deprotonated succinate species are bound via both carboxylate groups to the Cu surface – the mirror relationship between the two domains is thought to arise from the twist of the carbon backbone of succinate with respect to the [0 0 1] surface direction. When as little as 2 mol% (R,R)-tartaric acid is coadsorbed with succinic acid, LEED beams associated with the h9 0 | 2 2i structure are extinguished. The opposite behavior is observed when the dopant is (S,S)-tartaric acid [55]. This behavior is analogous to the sergeants and soldiers principle observed for helical polyisocyanate copolymers [56]. The mechanism for this effect is proposed to be substratemediated. Succinate species are unable to form intermolecular H-bonds, so a chiral footprint imposed on the surface by a tartrate species is thought to control the adsorption geometry of the surrounding complex creating an effect that is amplified over 30–50 molecules in a given domain [55]. 1.3.2 Chiral Switching

In discussing adsorption-induced chirality, it is generally assumed that interconversion of enantiomers is highly unlikely since such an interconversion, for example, under thermal excitation, requires a reduced interaction with the surface and desorption is a more probable outcome. However, if the molecule is relatively large and the chiral center does not make a large contribution to the molecule–substrate binding, then there is the possibility that a low-frequency mode can be excited sufficiently on heating that a bond rotation is possible leading to a chiral switching even without the molecule fully leaving the surface. This effect has been observed by Linderoth and coworkers [57]. The molecule consists of a linear backbone formed out of three benzene rings connected by ethynylene spokes and is functionalized at each end with an aldehyde, a hydroxyl, and a tert-butyl group. The molecule is achiral in the gas phase but adsorption on Au{1 1 1}, with the main molecular backbone parallel to the surface, creates, by restricting rotation around the ethynylene spokes, two equivalent chiral centers such that molecules can be classified as LL, RR, or RL, the latter being internally racemic. The tert-butyl group can be readily identified in the STM image and its position relative to the molecular backbone determined, thereby permitting the chirality properties of the molecule to be determined. Thermal switching of conformations was noted and ascribed to a partial loss of binding at one end of the molecule. Detailed temperature-dependent studies allowed the barrier to switching to be determined as approximately 0.3 eV. More subtly, it is found that the internally racemic LR/RL conformation has a relatively low probability on the surface and that it is more likely to switch than the LL or RR enantiomers: the difference in barrier heights being 0.04 eV. This is related to the interadsorbate interactions, which favor the LL/RR molecules over LR/RL. The possibility for chiral switching of this type provides a new mechanism for the growth of large homochiral domains as an alternative to separation relying on interdiffusion.


1.3.3 Chiral Recognition

Perhaps the simplest form of chiral recognition is that in which one enantiomer, for example, A, of a chiral object displays a stronger interaction with a particular enantiomer of a second chiral object, for example, B, rather than its mirror image, B. Of the four possible diastereoisomeric interactions AB, AB; AB, AB, the first two form a mirror equivalent, enantiomeric pair as do the latter. However, the crossrelationships are inequivalent, nonmirror images, for example, AB and AB, and in a chemical system, there would be an energetic preference for one pair of enantiomers over the other. This is the key to the significance of chirality in biology and, therefore, in the need to develop chiral products in the pharmaceutical and agrochemical industries. Studying chiral recognition processes at surfaces is therefore relevant to a better understanding of the separation of enantiomers, for example, following their preparation in an insufficiently enantioselective reaction. It is also relevant to the development of biosensors and biocompatible materials. We have already covered the interaction of chiral molecules with chiral surfaces, which is an important example of chiral recognition and diastereoisomerism. In this section, attention is focused on chiral recognition between molecules adsorbed on surfaces and it is useful to distinguish between self-recognition processes and those involving different molecular species. The latter can be described by the AB system introduced above while extending the analogy to self-recognition; it is the energy differences between the species AA, AA; AA, AA that is of interest. Here, AA is the mirror image of AA, so these form a pair of enantiomers. Similarly, AA and AA are also enantiomers but somewhat trivially since they are also equivalent and might be described as internally racemic. We have already discussed examples of what is effectively chiral self-recognition, when we described the formation of chiral clusters, chains, and arrays following adsorption involving induced chirality in otherwise achiral species in Section 1.2.1. Now, we show examples of self-recognition between intrinsically chiral molecules adsorbed as a racemic mixture on achiral surfaces leading to segregation of enantiomers if the homochiral (AA/AA) pairing is preferred over the heterochiral interaction (AA). A nice example of this is revealed in the work of Besenbacher and coworkers [58] on the adsorption of a racemic mixture of D- and L-cysteine on Au{1 1 0}. At low coverages, STM shows the molecules are present in pairs and, on the basis of the alignment of any given pair with respect to the h1 1 0i direction, it can be identified as being either DD or LL. Notably, DL heterochiral pairs are not observed (Figure 1.15). The reason for the homochiral preference lies in the orientation of the cysteine molecules on the gold surface determined by AuS and AuN interactions. The carboxylic acid functionality is not involved in any significant interaction with the gold substrate but rather dominates the pairing interaction between enantiomers. This three-point bonding of each molecule, AuS, AuN, and OHO, drives the self-recognition preference for homochiral pairs [58]. A more subtle example of homochiral preference, which draws attention to the conformational changes in the molecules needed to achieve self-recognition, is that

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Figure 1.15 Adsorption of cysteine on Au{1 1 0}. Molecular model shows the deprotonated thiolate surface species. (a) Model of the reconstructed (1 2)-Au{1 1 0} surface; (b–d) show, respectively, dimers of L-cysteine, D-cysteine, and the two together characteristically rotated relative to the h1 1 0i azimuth. (Adapted with permission from Ref. [57]. Copyright 2002, Macmillan Publishers Ltd.)

of the dipeptide D-phenylalanine-D-phenylalanine (D-Phe-D-Phe) and its enantiomer L-Phe-L-Phe adsorption on Cu{1 1 0} [59]. Following adsorption of a racemic mixture at low coverage, isolated species are recognizable by the orientation of their principal axis with respect to the h1 1 0i azimuth of the substrate; the LL (DD) enantiomer is rotated 34 (counter) clockwise as shown in Figure 1.16. Density functional and molecular dynamics calculations support an interpretation that the molecule adopts a conformation similar to the gas phase, in which the amine and carboxylic acid functionalities lie on the same side of the principal molecular axis, the peptide backbone. In contrast, homochiral chains (D-Phe-D-Phe)n and (L-Phe-L-Phe)n are observed by STM to have the principal axis rotated by 74 and heterochiral chains are not observed. Calculations suggest that the conformation of each molecule in a chain is dramatically changed relative to the isolated molecules with, inter alia, the amine and carboxylic functionalities now lying on opposite sides of the backbone to optimize intermolecular zwitterion formation between the amine of one molecule and the carboxylic acid of its neighbor. The need to consider the dynamic nature rather than simple lock–key models of chiral recognition is thereby emphasized. Adenine as an isolated molecule has no symmetry elements and therefore might mathematically be considered chiral; however, as in the case of glycine (Section 1.2.1), this description is not useful in chemistry since the enantiomers differ only by inversion through the weakly pyramidal nitrogen atom of the amine functionality, the main body of the molecule being planar. The inversion corresponds to a low-frequency vibration and a low-energy barrier such that single enantiomers


Figure 1.16 Comparison of the structure of an isolated (L-Phe-L-Phe) on Cu{1 1 0} rotated 34 clockwise with respect to the h1 1 0i azimuth (a, c, e) and that of the molecules found in rows that are rotated by 74 (b, d, f) based on STM

images. The superimposed models indicate that the change in rotation is linked to a major change of conformation to enable strong intermolecular bonding. (Adapted with permission from Ref. [58].)

cannot be realized. However, adsorption of adenine on a Cu{1 1 0} surface gives rise to flat-lying molecules, which then have a high barrier to interchange of enantiomers; that is, chirality is induced by adsorption [60]. At coverages up to one monolayer, adenine forms homochiral dimers that link into homochiral chains, whose direction on the Cu{1 1 0} substrate is correlated with their chirality [60] as shown in the left panel of Figure 1.17. Subsequent adsorption of one enantiomer of phenylglycine leads to an intermolecular recognition process that favors the decoration of chains running in the (1, 2) direction by S-phenylglycine (right-hand panel of Figure 1.17) while R-phenylglycine decorates the mirror image (1, 2) adenine chains [61]. The origin of the strong interaction between the amino acid, which adsorbs on Cu{1 1 0} as the anion, and the nucleic acid base is electrostatic favoring the close approach of the carboxylate functionality of phenylglycine to the nitrogen of the adenines amine group, which

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Figure 1.17 The left-hand STM image shows homochiral adenine rows aligned in low symmetry but mirror image azimuths on a Cu(1 1 0) surface. On the right, adenine rows in the (1,2) direction are decorated with double rows of S-phenylglycine molecules, while no such interaction occurs with (1, 2) rows. (Adapted with permission from Ref. [60]. Copyright 2000, Macmillan Publishers Ltd.)

lies on the periphery of the chain [62]. Chiral recognition occurs because there is also a repulsive interaction between the amine groups of the two molecules and this is less for the favored enantiomer than for the other [62]. 1.3.4 Prochiral Molecules Interacting with Chiral Surfaces

In enantioselective catalysis, the key problem to overcome is the fact that the Gibbs energy change from gas-phase prochiral reagent to gas-phase product is identical for each enantiomeric product molecule. Hence, in the absence of any chiral influence on the reaction, a racemic mixture of products should always ensue. By providing a reaction pathway to one product that has a much lower activation barrier, the selectivity can be skewed to give an enantiomeric excess of one product. One of the most heavily researched examples of heterogeneous enantioselective catalysis is the hydrogenation of b-ketoesters over Ni catalysts [28]. The simplest b-ketoester is methylacetoacetate (MAA). This molecule is approximately planar and can adsorb via either molecular face with equal probability. The hydrogenation reaction is believed to occur via dissociative adsorption of H2 on the metal surface and attack by H from underneath the molecular plane of MAA. The stereochemistry of the chiral center thus produced would be determined by which face of the prochiral reagent lies down on the surface. To skew the reaction in an enantioselective direction, a clear requirement seems to be to restrict the adsorption geometry to exclusively one enantioface. In this respect, the coverage of chiral modifier is thought to be crucial. If the coverage is too low, the formation of 1 : 1 complexes between modifier and reactant could induce some enantioselectivity, but the adsorption of MAA on bare metal sites would be expected to occur racemically. If the coverage is too high, there

1.4 Conclusions

Figure 1.18 STM image (4 nm 4 nm) showing the 2D cocrystalline structure consisting of an ordered array of 1 : 1 H-bonded complexes of (R,R)-tartrate and methylacetoacetate species on Ni{1 1 1} giving a chiral h3 1 | 3 4i structure. (Adapted with permission from Ref. [62]. Copyright 2002, Elsevier.)

may be insufficient space for MAA to adsorb on the surface. Indeed, the sticking probability of MAA on Ni{1 1 1} covered by high coverages of (R,R)-tartaric acid [63] or (S)-glutamic acid [64] is essentially zero. At intermediate tartaric acid coverages, Jones and Baddeley showed that the adsorption of MAA caused a restructuring of the tartrate adlayer and the formation of an ordered array of 1 : 1 tartrate:MAA complexes (Figure 1.18). Interestingly, the geometry of each MAA molecule in the array appeared identical and corresponding to the geometry required for the formation of (R)-methyl-3-hydroxybutyrate – the product observed in excess in the catalytic reactions [63].

1.4 Conclusions

Chirality at surfaces can be manifested in a number of forms including the intrinsic chirality of the surface structure and even the induction of chirality via the adsorption of achiral molecules onto achiral surfaces. The ability of STM to probe surfaces on a local scale with atomic/molecular resolution has revolutionized the understanding of these phenomena. Surfaces that are globally chiral either due to their intrinsic structure or due to the adsorption of chiral molecules have been shown by STM to establish control over the adsorption behavior of prochiral species. This could have profound consequences for the understanding of the origin of homochirality in life on Earth and in the development of new generations of heterogeneous chiral catalysts that may, finally, make a substantial impact on the pharmaceutical industry.

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19 England, C.D., Collins, G.E., Schuerlein, T.J., and Armstrong, N.R. (1994) Interfac. Des. Chem. Sensing, 561, 202. 20 Richardson, N.V. (2007) New J. Phys., 9, 395. 21 Bennett, R.A., Pang, C.L., Perkins, N., Smith, R.D., Morrall, P., Kvon, R.I., and Bowker, M. (2002) J. Phys. Chem. B, 106, 4688. 22 Lackinger, M., Griessl, S., Heckl, W.M., and Hietschold, M. (2002) J. Phys. Chem. B, 106, 4482. 23 Gyarfas, B.J., Wiggins, B., Zosel, M., and Hipps, K.W. (2005) Langmuir, 21, 919. 24 Yoshimoto, S., Narita, R., and Itaya, K. (2002) Chem. Lett., 356. 25 Richardson, N.V. (2007) New J. Phys., 9, 395. 26 Schrock, M., Otero, R., Stojkovic, S., Hummelink, F., Gourdon, A., Laegsgaard, E., Stensgaard, I., Joachim, C., and Besenbacher, F. (2006) J. Phys. Chem. B, 110, 12835. 27 Webb, G. and Wells, P.B. (1992) Catal. Today, 12, 319. 28 Izumi, Y. (1983) Adv. Catal., 32, 215. 29 Lorenzo, M.O., Haq, S., Bertrams, T., Murray, P., Raval, R., and Baddeley, C.J. (1999) J. Phys. Chem. B, 103, 10661. 30 Lorenzo, M.O., Baddeley, C.J., Muryn, C., and Raval, R. (2000) Nature, 404, 376. 31 Barbosa, L. and Sautet, P. (2001) J. Am. Chem. Soc., 123, 6639. 32 Fasel, R., Wider, J., Quitmann, C., Ernst, K.H., and Greber, T. (2004) Angew. Chem. Int. Ed., 43, 2853. 33 Humblot, V., Haq, S., Muryn, C., Hofer, W.A., and Raval, R. (2002) J. Am. Chem. Soc., 124, 503. 34 Hofer, W.A., Humblot, V., and Raval, R. (2004) Surf. Sci., 554, 141. 35 Hermse, C.G.M., van Bavel, A.P., Jansen, A.P.J., Barbosa, L., Sautet, P., and van Santen, R.A. (2004) J. Phys. Chem. B, 108, 11035.

References 36 Hazen, R.M. and Sholl, D.S. (2003) Nat. Mater., 2, 367. 37 Halasyamani, P.S. and Poeppelmeier, K.R. (1998) Chem. Mater., 10, 2753. 38 McFadden, C.F., Cremer, P.S., and Gellman, A.J. (1996) Langmuir, 12, 2483. 39 Zhao, X.Y. and Perry, S.S. (2004) J. Mol. Catal. A – Chem., 216, 257. 40 Sholl, D.S., Asthagiri, A., and Power, T.D. (2001) J. Phys. Chem. B, 105, 4771. 41 Jenkins, S.J. and Pratt, S.J. (2007) Surf. Sci. Rep., 62, 373. 42 Attard, G.A. (2001) J. Phys. Chem. B, 105, 3158. 43 Horvath, J.D. and Gellman, A.J. (2001) J. Am. Chem. Soc., 123, 7953. 44 Kuhnle, A., Linderoth, T.R., and Besenbacher, F. (2006) J. Am. Chem. Soc., 128, 1076. 45 Bowker, M., Poulston, S., Bennett, R.A., and Stone, P. (1998) J. Phys: Condens. Matter, 10, 7713. 46 Chen, Q., Perry, C.C., Frederick, B.G., Murray, P.W., Haq, S., and Richardson, N.V. (2000) Surf. Sci., 446, 63. 47 Chen, Q., Frankel, D.J., and Richardson, N.V. (2001) Langmuir, 17, 8276. 48 Chen, Q. and Richardson, N.V. (2003) Prog. Surf. Sci., 73, 59. 49 Pascual, J.I., Barth, J.V., Ceballos, G., Trimarchi, G., De Vita, A., Kern, K., and Rust, H.P. (2004) J. Chem. Phys., 120, 11367. 50 Zhao, X.Y., Gai, Z., Zhao, R.G., Yang, W.S., and Sakurai, T. (1999) Surf. Sci., 424, L347.

51 Wang, H., Zhao, X.Y., and Yang, W.S. (2000) Acta Phys. Sin., 49, 1316. 52 Wang, H., Zhao, X.Y., Zhao, R.G., and Yang, W.S. (2001) Chin. Phys. Lett., 18, 445. 53 Zhao, X.Y. (2000) J. Am. Chem. Soc., 122, 12584. 54 Humblot, V., Lorenzo, M.O., Baddeley, C.J., Haq, S., and Raval, R. (2004) J. Am. Chem. Soc., 126, 6460. 55 Parschau, M., Romer, S., and Ernst, K.H. (2004) J. Am. Chem. Soc., 126, 15398. 56 Green, M.M., Reidy, M.P., Johnson, R.J., Darling, G., Oleary, D.J., and Willson, G. (1989) J. Am. Chem. Soc., 111, 6452. 57 Weigelt, S., Busse, C., Petersen, L., Rauls, E., Hammer, B., Gothelf, K.V., Besenbacher, F., and Linderoth, T.R. (2006) Nat. Mater., 5, 112. 58 Kuhnle, A., Linderoth, T.R., Hammer, B., and Besenbacher, F. (2002) Nature, 415, 891. 59 Lingenfelder, M., Tomba, G., Costantini, G., Ciacchi, L.C., De Vita, A., and Kern, K. (2007) Angew. Chem. Int. Ed., 46, 4492. 60 Chen, Q., Frankel, D.J., and Richardson, N.V. (2002) Langmuir, 18, 3219. 61 Chen, Q. and Richardson, N.V. (2003) Nat. Mater., 2, 324. 62 Blankenburg, S. and Schmidt, W.G. (2007) Phys. Rev. Lett., 99, 196107. 63 Jones, T.E. and Baddeley, C.J. (2002) Surf. Sci., 519, 237. 64 Jones, T.E., Urquhart, M.E., and Baddeley, C.J. (2005) Surf. Sci., 587, 69.

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2 The Template Route to Nanostructured Model Catalysts Conrad Becker and Klaus Wandelt

2.1 Introduction

Even though the majority of industrially produced chemicals are based on processes that include at least one heterogeneously catalyzed step, the basic physical and chemical properties that govern the action of a particular catalyst are far from being understood. Surface science can play a major role in the elucidation of the molecular principles that govern reactions on catalyst surfaces. This approach was brought to attention of a wider public when the Nobel Prize for Chemistry in 2007 was awarded to Gerhard Ertl for his studies of chemical processes on solid surfaces. The investigation of the basic physical and chemical properties of heterogeneous catalysts down to the molecular and atomic level is, indeed, one of the prime goals of surface science. On the route to ultimate understanding of heterogeneously catalyzed reactions, two major obstacles need to be overcome, and these are commonly referred to as the pressure gap and the materials gap. The pressure gap stems from the fact that industrial catalysts are usually run at rather high pressures, whereas surface science studies are generally undertaken in ultrahigh vacuum. In the recent past, however, considerable progress has been made to close this gap by making standard surface science techniques work at elevated pressures (see Refs [1–3]). Therefore, it seems likely that the pressure gap is not the major obstacle for understanding heterogeneously catalyzed reactions any more provided the structure of the catalytic surface is well known. Here the materials gap enters the scene. It is related to the fact that industrial catalysts possess a highly complex structure and topology, which makes them inaccessible for surface science tools. This problem can be overcome by the use of model catalysts of reduced complexity, that is, designed model catalysts that mimic the nanoscopic structure of the industrial catalyst. For many years, metal single-crystal surfaces have been – and are still – used for this purpose. However, since industrial catalysts are often composed of oxidesupported metal particles, extended metal surfaces are – even though of very low complexity – rather unrealistic models. More realistic model catalysts are provided by

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metal-on-oxide systems of low complexity. In this respect, it has turned out that thin oxide films on metal substrates can be used as support for the preparation of such systems. By depositing metal vapor on these films, model catalysts can be prepared, which possess the big advantage over bulk oxide materials that they can be investigated by surface science methods that rely on charged particles [4]. The big drawback of such systems is, however, that in general their complexity is still rather high. Owing to the nucleation process of metals on oxide surfaces, which is in general controlled by the defects of the surface, it is quite difficult to reach a uniform particle size and distribution since the defects are randomly distributed over the surface. This renders investigation of particle size effects difficult because only the ensemble average over a particular size distribution is experimentally accessible. The statistical arrangement of defects on the surface not only leads to different capture zones, and, as a consequence, to metal particles of different sizes (Figure 2.1a), but, in turn, also results in a different reverse spillover from the support to the individual particles during a catalytic reaction. This influences the reaction kinetics, which is closely connected to the arrival rate of reactants on the particle [5]. One way to overcome all these limitations is to use an ordered nanostructured model catalyst. The schematic representation of such a model catalyst is provided in Figure 2.1b. It will provide a uniform particle size and distribution, thus, enabling measurements on an ensemble of identical particles. The fabrication of such a system can be accomplished only by nanofabrication, and different routes can be imagined in this context. We will focus in the following section on the template-controlled growth of metal clusters on thin oxide films, which has proven to give excellent results in terms of low complexity. This approach has been successfully employed for metal-on-metal systems (for a comprehensive review see [6]) and has recently been extended to metal growth on oxide films. The key factor in this approach is the provision of suitable growth templates. Before we turn to this point, we will discuss in which way surfaces may be thought of as templates.

(a)

(b)

Figure 2.1 Schematic representation of a random model catalyst surface (a) and an idealized model catalyst (b). The black dots correspond to metal particles, dotted circles to their capture zones.

2.2 Surfaces as Two-Dimensional Templates


The concept of templates is well known in our contemporary life. Templates are generally used to shape a product. This can be, for example, the mold of a church bell or style sheet in office applications in computing. One common feature of templates is that the information, which is used to shape a product, is intrinsically encoded in the template. A good example in this context is a strand of DNA that sets the genetic sequence of new strands. If we want to use surfaces to act as templates, for example, in nanostructuring, we have to find a way to encode the information into the surface structure. This approach is commonly used in the fabrication of semiconducting devices, where lithography is employed to pattern the surface of a wafer. The pattern thus created is then used as a template for the following manufacturing steps. This approach, generally referred to as top-down approach, relies on the active patterning of the structure of the surface by external intervention. Even though this approach is frequently and successfully applied in a variety of areas, it is subject to a number of limitations in the context of model catalyst fabrication. First, if a lithographic process is used, spatial resolution is limited by either the physical properties of the radiation source employed for patterning (e.g., light, electron beams) or the chemical properties of the resist, which contains the pattern. In general, these limitations impose a resolution limited to a few tenths of nanometers. However, very well ordered model catalysts have been prepared in the past using this approach (see Figure 2.2). To fully exploit the nanoscopic properties of materials, for example, in catalysis, this structure size is much too large since it corresponds to a regime where the bulk properties of materials still dominate. An alternative approach can be the patterning of a surface by direct manipulation of atoms or molecules with the scanning tunneling microscopy (STM) [8], which has been successfully employed in the past

Figure 2.2 STM image of platinum particles on an oxidized silicon wafer. (Reproduced with permission from Ref. [7].)

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in many cases. In this case, only the size of the objects, which are manipulated, determines the resolution limit. However, it is still a top-down approach, which requires the intervention of the operator. Furthermore, the speed of the assembling process is rather slow and the patterning of surface areas on the micrometer scale may be tedious or even impossible. We, therefore, have to conceive an alternative pathway for creating nanopatterned templates. Fortunately, nature points the way for that providing us with nanopatterned surfaces that are derived from the intrinsic physical and chemical properties of the system itself. One possibility is to use the self-organization of colloidal particles to form regular nanostructured arrays [9, 10]. If applied to model catalyst design, this approach has, however, the disadvantage that the ligand shell, which stabilizes the particles, has to be removed without destroying the particles [11, 12]. Nanostructuring by self-organization is also found to occur in many metal surfaces. A particularly beautiful example is the herringbone or p chevron reconstruction of the Au(1 1 1) surface. This Au(1 1 1)-( 3 22) reconstruction, which has been observed and characterized by a large number of surface science techniques, provides us with a regular array of specific sites – the elbows of the reconstruction – that can act as nucleation sites [6]. The fabrication of a template using self-organization can be referred to as bottom-up approach since it relies on the inherent physical and chemical properties of the system. The beauty of this approach is that nature is doing the job; by choosing the components of the system, the self-organization process is controlled by the intrinsic properties of the system. The major obstacle that we are facing in this context is to set the controlling parameters of the self-assembly process in such a way that a desired nanopattern is generated. In the case of a pure metal surface such as Au(1 1 1), this of course cannot be done since it is a one-component system with no control parameters. However, other routes to nanostructured surfaces can be envisioned that provide an experimental control on the size and symmetry of the pattern. Among these, thin films provide the most promising route. This has been shown in a number of cases for metal films, oxide films, and molecular films. In the case of metal films, the difference in lattice parameters of substrate and film leads in many cases to the formation of strain relief pattern, to compensate for the lattice mismatch. This has been found for systems such as Ag/Pt(1 1 1) [13–15], Cu/Ru(0 0 0 1) [16], Ag/Cu(1 1 1) [17–19], and many others [20]. Likewise, oxide films can form regular nanopattern depending on the accommodation of the film to the substrate, and we will come back to this later. Finally, molecular films provide access to a whole new class of nanopatterned surfaces since in this case the intermolecular interactions of the molecules along with the molecule–substrate interaction play a decisive role in the pattern formation [21]. The latter case will thus provide us with interesting properties for the growth of nanostructures on surfaces but will have only a limited application for the fabrication of model catalysts because these should be, in general, metal-on-oxide systems. The nanopatterning of a surface is, however, only the first step toward a surface template. We will further need a specific property of the surface, which is encoded into the pattern. Only this will assure that the template will control the subsequent nucleation or growth processes. In many cases, this specific property will be


a particularly strong interaction (adsorption energy) at certain points of the surface [22]. This will provide us with a given density of nucleation sites that is determined by the nanostructure of the template. If we want to use these sites as a growth template, we have to control the kinetics of the process to reach nucleation on the template sites. Besides the adsorption energy of the particles on the traps, the flux of the impinging particles and the temperature of the substrate play a decisive role [23]. The latter has an important impact on the mobility of the particles and, thus, their ability to reach the traps. This has been illustrated in the work of Prevot et al. on the nucleation and growth of Au on the reconstructed N/Cu(1 0 0) surface [24]. This surface provides a square lattice of nucleation sites, which constitute a template for the ordered growth of Au clusters [25]. The kinetic Monte-Carlo (KMC) simulations shown in Figure 2.3 illustrate the importance of growth temperature for the action of the template. Only at intermediate temperatures (e.g., 240 K) is the growth perfectly template controlled. At this temperature, the diffusion of Au atoms on the surface is fast enough so that the nucleation sites are actually reached. At higher temperature also, the template is operative, all islands are found on template sites, but not all template sites are covered by islands, which is due to higher mobility of Au atoms. At lower temperature, the number density of islands exceeds that of the template sites indicating that besides the template-controlled (heterogeneous) nucleation, homogeneous nucleation also takes place. A more detailed picture of the temperature dependence of the growth is given in Figure 2.4, where the island density is plotted as a function of temperature. It can be seen that only in the temperature range from 207 to 288 K the growth is perfectly template controlled and the number of islands matches the number of available nucleation sites. This illustrates the importance of kinetic control for the creation of ordered model catalysts by a template-controlled process. Obviously, there has to be a subtle balance between the adatom mobility on the surface and the density of template sites (traps) to allow a template-controlled growth. We will show more examples of this phenomenon below. That not only an increased interaction energy at the traps can be responsible for a template-controlled growth but also an anisotropy of the surface diffusion

Figure 2.3 Kinetic Monte-Carlo simulations of Au growth on N/Cu(1 0 0) for three different temperatures. From left to right, T ¼ 180 K, T ¼ 240 K, and T ¼ 300 K. The coverage is 0.11 ML. (Reproduced with permission from Ref. [24].)

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Figure 2.4 Evolution of the island density with growth temperature for Au/N/Cu(1 0 0). (Reproduced with permission from Ref. [24].)

barrier may just be sufficient to control a growth process has been shown in several cases [26, 27]. Indeed, this leads to a template-controlled growth without specific interaction but by spatially confining the movement of the deposited particles. The nucleation is then homogeneous but controlled by the template. This approach is quite similar to the one used in lithography where the spatial confinement is provided by the lithographically produced pattern on the surface. Since surface diffusion in this case is the crucial parameter, the process will be subject to the same limitations in terms of temperature as in the case of template-controlled heterogeneous nucleation. Before we discuss the template-controlled growth of model catalysts in more detail, we will have to consider a few aspects of STM imaging of these systems. This will be crucial for the characterization of the model catalyst surfaces.

2.3 STM Imaging of Oxide Films

Even though scanning tunneling microscopy is nowadays a standard tool for surface characterization, special care has to be taken when using it for the investigation of thin oxide films. On clean metal surfaces, a simple metal–vacuum–metal tunneling gap is usually found [28]. In this case, the tunneling current can be to a first approximation described by the Tersoff–Hamann model [29], which postulates that the main contribution to the tunneling current is due to electrons coming from the Fermi edge. If we introduce a thin oxide film, the tunneling gap can be characterized by a metal–oxide–vacuum–metal junction (Figure 2.5). As a consequence, the band structure of the oxide film – in particular, the gap between the valence band (VB) and

2.3 STM Imaging of Oxide Films

(a) EVac φt

CB

φs

EF

EF

VB df

dg

(b)

CB

EF

EF

VB df

substrate

dg’

oxide

tip

Figure 2.5 Schematic representation of the gap of a metaloxide–vacuum–metal tunneling junction in the case of a low (a) and a high (b) sample bias voltage.

the conduction band (CB) – will play a crucial role in the tunneling process. If we disregard local effects – such as a local variation of the barrier height – and take only the average electronic structure of the film into account, we can distinguish two limiting cases. For small bias voltages, the oxide film will only weakly perturb the tunneling process. The current passes through the oxide film because no states for the tunneling electrons are available (Figure 2.5a). It will thus act much like an additional vacuum barrier. As a consequence, the distance from the tip to the sample will be the sum of the film thickness df and the distance between tip and film dg. This does not necessarily imply that the oxide film is invisible in STM. If the lateral structure of the oxide film is not homogeneous, the tunneling barrier will change as a function of the lateral position of the tip and consequently the z-position of the tip will also change, which will result in a contrast in the STM images. If, at small bias voltage, the tunneling current is increased, the tip will touch and eventually penetrate the film, leading to a direct overlap of the electronic states of the tip and the sample. This will cause variations in the tunneling current as a function of the lateral tip position, which can – in favorable cases – be used to image the surface of the oxide film with atomic resolution [30]. Since at moderate tunneling currents (2 ML oxide coverage [91, 113]. Ranke et al. [91] have attempted a qualitative interpretation of the different coincidence structures based on an ionic model of FeO and the decrease of the electrostatic

6.3 Case Studies: Selected Oxide–Metal Systems

energy of the polar FeO stack by reducing interlayer distances and increasing lateral lattice constants. While catching the essence of the overall lattice relaxations in the FeO(1 1 1) nanolayers, a purely ionic description of FeO is questionable, and the model is too simplistic to explain the sequence of coincidence structures observed as a function of coverage. High-level ab initio calculations of the various interfacial configurations will be necessary to shed light on the origin of the observed coincidence structures of FeO on Pt(1 1 1); this will be no easy task in view of the large unit cells involved. 6.3.5 Nickel Oxide Nanolayers

Thin nickel oxide (NiO) films are used in many oxidation catalysts and industrial chemical processes including steam reforming of methane to syngas [116], the oxidation of methane [117], hydrocarbons [118], methanol [119], CO [120], and other organic compounds [121, 122]. NiO is also employed in alkaline electrochemical systems, such as alkaline fuel cells [123, 124]. Owing to its defect structure, nonstoichiometric nickel oxide is a good p-type semiconductor and is therefore used as resistive sensors for reducing gases, such as H2 [125], CO [126], NH3 [127], and also NO2 [128], formaldehyde [129], and methanol [130]. Epitaxial growth of nickel oxide nanolayers and the characterization of their structure at the atomic level is thus of considerable fundamental and applied interest. Historically, ultrathin Ni-oxide layers have been first grown by oxidation of singlecrystal Ni(1 0 0) and Ni(1 1 1) surfaces (see Ref. [131] and references therein). The lattice parameter of NiO (4.176 Å) is by nearly 20% larger than that of metallic Ni (3.524 Å), which is not a favorable prerequisite for epitaxial NiO growth. B€aumer et al. [132] have reported a (1 0 0)-oriented NiO film growth by exposing a Ni(1 0 0) surface to 104 l O2 at 570 K. According to their STM and SPA-LEED data, the film consists of NiO(1 0 0) crystallites, which are tilted by approximately 8% with respect to Ni(1 0 0) surface normal. The authors concluded that by means of this tilt the surface strain, caused by the large misfit between Ni and NiO lattices, may be partially relieved [132]. On the other hand, a much more favorable epitaxial relationship for a NiO(1 0 0) layer growth has been established on the Ni(1 1 1) substrate, that is, NiO(1 0 0)||Ni(1 1 1) and NiO[0 1 0]||Ni[110] with a mismatch of only 1.4%, where locally ordered (2 2), (2 3), (3 2), and (3 3) superstructure cells of NiO(1 0 0) have been detected in LEED and STM [133, 134]. In particular, Hildebrandt et al. [134] used a variable-temperature STM to investigate in situ and in real time the oxidation of Ni(1 1 1) at elevated temperature. They found that the oxidation starts at the Ni step edges, which is followed by the transformation of the Ni(1 1 1) terraces into a threedomain NiO(0 0 1)-like layer, containing a mixture of (2 2), (2 3), and (3 3) NiO (0 0 1) unit cells, as shown in Figure 6.14. At higher oxygen exposures (>25 l), the formation of triangular bulk NiO(1 1 1) islands has been observed. More favorable lattice matching conditions have been achieved by growing epitaxial NiO layers on dissimilar metal surfaces, such as Au(1 1 1) [135, 136], Ag(1 0 0) [137–142], Cu(1 0 0) [143], Cu(1 1 1) [144, 145], and more recently

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Figure 6.14 Room temperature STM image (400 Å 270 Å, þ 1.0 V, 0.5 nA) after oxidation at 400 K. An enlarged image detail shows the structure of the (2 2), (2 3), and (3 3) cells in the NiO(0 0 1) regions. (Reproduced with permission from Ref. [134].)

Pd(1 0 0) [146–150] and Pt(1 1 1) [20]. The Neddermeyer group was first to demonstrate that owing to the small lattice mismatch (2%), well-ordered ultrathin NiO films can be prepared on a Au(1 1 1) surface by reactive evaporation of Ni metal in an oxygen atmosphere [135, 136]. Their STM images revealed tripoid-like ridge structures with a (2 2) periodicity, corresponding to a p(2 2) reconstructed NiO(1 1 1) surface, which has been interpreted in terms of Wolfs octopolar model [151]. The latter consists of {1 0 0} nanofacets, obtained by removing three-fourth of the surface and one-fourth of the subsurface ions, which results in a compensation of the polarity of the NiO(1 1 1) surface. More recent glancing incidence X-ray diffraction (GIXD) study has shown that the (2 2)-reconstructed NiO(1 1 1) films can be grown on the Au(1 1 1) surface up to 8 ML in a layer-by-layer fashion, and such films exhibit both possible terminations (O and Ni), separated by single-atom steps [152]. The Ag(1 0 0) surface was considered as an ideal candidate for the epitaxial growth of NiO films oriented along the (1 0 0) direction due to the low overlayer/substrate lattice mismatch ( þ 2.2%) and the low reactivity of silver toward oxygen. Interestingly, despite this good structural matching no pseudomorphic NiO(1 0 0) layer forms for submonolayer coverages. Instead, a 2D oxidic phase with a (2 1) periodicity grows in the form of two orthogonal domains on the Ag(1 0 0) surface up to a coverage close to 1 ML, as shown by STM images obtained by Bertrams and Neddermeyer [138]. On the basis of quantitative LEED analysis, Caffio et al. [142] have


proposed a structure model of the (2 1) phase similar to the (1 1 1) surface of NiO, which consists of a buckled NiO bilayer (O-terminated) with a distorted hexagonal structure. It has been argued that this polar NiO(1 1 1)-like nanolayer is stabilized by the electrostatic interaction with the metal substrate and is kinetically preferred over the NiO(1 0 0) structure in the early stages of oxide formation [142]. As the oxygen dose and/or the film thickness increases the (2 1) phase transforms into a pseudomorphic (1 1)-NiO(1 0 0) phase, which grows in a layer-by-layer mode up to 5 ML [139]. For higher coverages, as demonstrated by SPA-LEED analysis [139], the strain is relieved by the introduction of misfit dislocations with {1 1 0} glide planes, which leads to the formation of mosaics on the oxide film surface. Recently, we investigated in a series of joint studies [146–150] with the group of Granozzi in Padua the structure of NiO nanolayers supported on a Pd(1 0 0) surface. The Pd(1 0 0) surface differs from Ag(1 0 0) by a significantly larger lattice mismatch ( þ 7.3%) to NiO(1 0 0) and by the predominantly 4d character of the valence band near the Fermi edge, in contrast to the 5sp character of the silver top valence band. This difference in the electronic structure of Pd and Ag is considered to play an important role in the hybridization of states at the metal–oxide interface and in the substrate reactivity toward oxygen. The latter was found to strongly affect the morphology and degree of structural order of the Ni-oxide overlayer in the submonolayer coverage range via the oxide preparation procedure: reactive deposition (RD) versus postoxidation [148]. Irrespective of the oxide preparation route, an interface-stabilized c(4 2) phase forms in the early stages of Ni-oxide growth, as detected by LEED and STM [147, 148]. The PO procedure was shown to be beneficial for obtaining a well-ordered wetting 2D layer, which has been imaged with atomic resolution in STM (Figure 6.15a and inset). A structure model of this c(4 2) monolayer has been derived by quantitative LEED I–V analysis [147] and is presented in Figure 6.15b. It is based on a NiO(1 0 0) surface with O atoms sitting in on-top Pd positions and with one-fourth of the Ni atoms missing in a rhombic c(4 2) unit cell, which corresponds to a formal Ni3O4 stoichiometry. Hybrid-exchange density functional theory calculations of Pisani and coworkers [149] have confirmed the LEED I–V model, and it has been argued that the rhombic geometry of Ni vacancies is electrostatically preferred over a squared one due to the higher average distance between Ni vacancies in the former structure. We close this section with highlighting our recent results on the fabrication of quasi-one-dimensional (1D) Ni-oxide structures on vicinal Rh(1 1 1) surfaces via decorating their regular step arrays [153, 154]. Figure 6.16 shows STM images of the 0.3 ML Ni/Rh(15 15 13) surface after exposure to 15 l O2 at 300 C. Figure 6.16a and b show straight steps decorated by bright lines of protrusions, which display a linear 1D-(1) and 1D-(2) periodicity in terms of the Rh lattice parameter parallel to the step edges (Figure 6.16c and d). The oxide-free Rh terraces are covered by (2 1) domains of chemisorbed oxygen. The structure of Ni-oxide wires at the step edges has been rationalized in the DFTmodel of Mittendorfer and coworkers [153] for a 0.6 ML Ni-oxide decorated Rh(5 5 3) surface (Figure 6.16e). Accordingly, the first row of Ni atoms at the step edges is coordinated to four O atoms each, yielding the 1D-(1) periodicity at the upper and lower step edges. Additional O adatoms are adsorbed in

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Figure 6.15 (a) STM image of 1.25 ML Ni-oxide on Pd(1 0 0) (500 Å 500 Å, þ 2.0 V, 0.1 nA). The inset shows a highresolution STM image of the c(4 2) structure (35 Å 35 Å, þ 1.0 V, 1.0 nA). (b) Top and side views of the NiO(1 0 0)-based model for the c(4 2) overlayer. (Reproduced with permission from Refs [147, 148].)

a (2 1) lattice on the other Ni rows, resulting in the 1D-(2) lines of protrusions in the STM images and reproduced well in the simulated STM image in Figure 6.16f. Although the formal stoichiometry of the NiO stripes on the vicinal Rh(1 1 1) surfaces is NiO, it has no relation to the bulk NiO, since a fraction of the O atoms is shared between the Ni and the Rh surface atoms.


Figure 6.16 (a–d) STM images of 0.3 ML Ni-oxide layer on the Rh(15 15 13) surface: (a) 1000 Å 1000 Å, þ 1 V, 0.1 nA; (b) 150 Å 150 Å, þ 0.13 V, 1.0 nA; (c) 18 Å 18 Å, þ 0.13 V, 1.0 nA; (d) 45 Å 37 Å, þ 0.14 V, 1.0 nA; (e) DFT model of a 0.6 ML Ni-oxide on Rh(5 5 3); and (f) corresponding simulated STM image. (Reproduced with permission from Ref. [154].)

6.3.6 Ceria Nanolayers on Metal Surfaces

Cerium oxides are outstanding oxide materials for catalytic purposes, and they are used in many catalytic applications, for example, for the oxidation of CO, the removal of SOx from fluid catalytic cracking flue gases, the water gas shift reaction, or in the oxidative coupling reaction of methane [155, 156]. Ceria is also widely used as an active component in the three-way catalyst for automotive exhaust pollution control,

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where it is believed to play a crucial role as an oxygen storage-release agent to control the oxygen concentration on the catalyst surface during the different cycles of the operating engine. The latter involves oxygen transport from and to the surface, which is determined to a large extent by the formation, concentration, and mobility of oxygen vacancy defects at the surface and in the bulk. The ease of formation and annihilation of oxygen vacancy defects in cerium oxides is of course intimately connected to the propensity of Ce ions to undergo valence changes from þ 4 to þ 3 and vice versa. Therefore, it is not surprising that there has been and still is a strong interest in the atomic structure of cerium oxide surfaces and of the defects encountered thereon. There has been considerable effort to elucidate the surface structures of the low-index single-crystal surfaces of CeO2, which crystallizes in the cubic fluorite structure. According to theory, the most stable low-index surface is the (1 1 1) surface with an oxygen-terminating outer layer [157]. Note that in the (1 1 1) orientation, the CeO2 crystal may be regarded as a stack of hexagonal OCeO trilayer planes. N€ orenberg and Briggs [158] were the first to apply STM to the study of CeO2(1 1 1) surfaces, and they indeed observed images with atomically resolved features in a hexagonal arrangement. Because CeO2 is a wide-band insulator with a bandgap of approximately 6 eV, the experiments had to be performed on partially reduced CeO2x surfaces to enhance the electron conductivity for the STM experiments; the reduced CeO2x surfaces were obtained by annealing the samples in vacuum at high temperatures (1000 C). On the basis of their tunneling conditions (negative sample bias of several V, thus tunneling out of the filled states of the surface), N€ orenberg and Briggs concluded that they are imaging the oxygen atoms of the CeO2(1 1 1)-1 1 surface. The typical defect structures observed at the partially reduced CeO2x surfaces displayed triangular and linear dark contrast features, which were interpreted by N€orenberg and Briggs in terms of multiple oxygen vacancies [158]. The defect formation process on CeO2(1 1 1) single-crystal surfaces has recently been addressed in more detail using STM by Esch et al. [159], again on partially reduced samples. In combination with DFTcalculations, the local structure of surface defects has been ascribed to multiple surface and subsurface oxygen vacancies in various combinations. A better approach than using STM to study defects at CeO2 single-crystal surfaces, independent of the overall reduction state, is to use atomically resolved atomic force microscopy. Iwasawa and coworkers [156, 160, 161] have imaged the oxygen atoms on CeO2(1 1 1) surfaces with atomic resolution with noncontact AFM and have observed several kinds of defects, both point defects and multiple defects such as the line and triangular structures, which were also observed in STM before [158]. The creation of oxygen vacancies leads to the formation of reduced Ce 3 þ ions in the vicinity of the defect, which take up the two negative charges left behind per oxygen vacancy. This leads to local reconstructions of the atoms around the defect that can be visualized in the AFM images and that contribute to the stability of the defect. By means of successive AFM measurements of the same area on slightly reduced CeO2x surfaces, the dynamics of surface O atoms and the healing of defects by exposure to O2 from the gas phase has been observed directly by Namai et al. [161].


High-quality resolved images and atomic details of surface features on CeO2(1 1 1) in various oxidation states have also been obtained recently by dynamic AFM measurements by Gritschneder et al. [162]. Thin films of CeO2x with (1 1 1) surface orientation have been grown epitaxially on a variety of metal substrates. Mullins et al. [163] have prepared highly ordered CeO2x(1 1 1)-type films, up to 10 ML thick, by reactive evaporation of Ce in oxygen atmosphere on Ru(0 0 0 1) and Ni(1 1 1) surfaces. The oxide overlayers were aligned parallel to the principal azimuths of the substrates as judged from ion scattering spectroscopy and LEED measurements, and the degree of oxidation (i.e., the x in CeO2x) could be adjusted to some extent by the oxygen pressure during evaporation. No direct imaging information on the morphology and structure of these films was available in the early work of Mullins et al. [163]. However, Lu et al. [164] recently took up and modified the recipe of Mullins for the preparation of CeO2(1 1 1) films on Ru(0 0 0 1) and have shown that higher oxidation temperatures lead to flat ceria films with a low defect density. Schierbaum and Berner [165–167] have used a different approach to generate epitaxial ceria nanolayers on Pt(1 1 1), namely, the oxidation of Pt–Ce alloy surfaces; the latter were formed by depositing Ce metal onto Pt(1 1 1) surfaces followed by a high-temperature (1000 K) annealing treatment. The surface structures of the oxide nanolayers formed were consistent with CeO2(1 1 1) as revealed by STM, but heating of the oxide phase in vacuum at 900 K lead to a different structure, which was attributed to a Ce2O3 surface phase [167]. Epitaxial cerium oxide nanolayers have been deposited by reactive evaporation on Rh(1 1 1) by Eck et al. [168]. STM and LEED indicated that the ceria grows in the form of ordered CeO2 double-layer islands, with (1 1 1) faces parallel to the surface and orientationally aligned to the main azimuthal directions of the substrate. While the nanolayer films contained significant amounts of reduced Ce 3 þ species, for the thicker films (>6 ML) stoichiometric CeO2 was detected in XPS. Vacuum annealing of the CeO2/Rh(1 1 1) submonolayer nanostructures produced morphologically welldefined hexagonal islands (see Figure 6.17a), which displayed a characteristic Moire pattern in STM at their surfaces (Figure 6.17b and c) [169]; the latter is the result of the interfacial lattice mismatch and the interference effects between the CeO2 and the Rh lattices, which give rise to a (5 5) coincidence lattice of a compressed cerium oxide overlayer (5 aCeO2 7 aRh). When subjected to more reducing conditions, for example, by additional vacuum annealing at >600 C, an ordered array of surface defects has been observed on the ceria nanoislands as displayed in the STM images of Figure 6.18. In contrast to the STM images obtained from bulk-terminated CeO2 surfaces [158, 159], the STM images from the CeO2 nanolayers on Rh (Figures 6.17 and 6.18) have been recorded with positive bias voltages [169], where the empty states are imaged. Ab initio DFT calculations have revealed that the density of states within 1 eV above the Fermi level in the first CeO2 trilayer of the ceria–Rh system is made up by Ce 4f states, which are hybridized to a certain extent with O 2p states; according to the calculations, the bright maxima in the corresponding STM images correspond, therefore, to the cerium sites [169]. Using STM images with atomic resolution in combination with DFT calculations, the defects seen in Figure 6.18 have been

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Figure 6.17 Constant current topographic STM images of 0.5 monolayer (ML) of CeO2 on Rh(1 1 1). (a) 1850 Å 1850 Å, þ 1.99 V, 1 nA; (b) 200 Å 200 Å, þ 0.89 V, 0.83 nA; (c) 93 Å 57 Å, þ 0.78 V, 0.83 nA. A CeO2(1 1 1)1 1 unit cell (u) is indicated on the image (c). The inset to (a) shows a corresponding LEED pattern (electron energy ¼77 eV). (Reproduced with permission from Ref. [169].)


Figure 6.18 STM images of the 0.5 ML CeO2x–Rh(1 1 1) surface after annealing to 625 C. (a) 200 Å 200 Å, þ 0.93 V, 0.86 nA; (b) 100 Å 100 Å, þ 0.80 V, 1.05 nA. The grid of thin lines illustrates the superlattice of defects. (Reproduced with permission from Ref. [169].)

associated with oxygen vacancies. It has been suggested that this self-assembly of oxygen vacancies is facilitated by the lattice mismatch between the oxide overlayer and the metal substrate and that it may be ascribed to a strain-related reduction in the vacancy formation energy [169].

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An interesting point is the fact that the epitaxial growth of CeO2 overlayers has been observed on a number of noble metal surfaces [163, 165, 168], all of which display a large lattice mismatch of the order of at least 30% at the respective interfaces. It has been proposed that coincidence lattices with relatively low interfacial strain are formed in all these cases, in a manner similar to the one described above for the CeO2–Rh(1 1 1) interface, thus creating ordered low-energy interfaces that mediate the heteroepitaxial growth further [169].

6.4 Synopsis and Outlook

As in other fields of nanoscience, the application of STM techniques to the study of ultrathin oxide layers has opened up a new era of oxide materials research. New emergent phenomena of structure, stoichiometry, and associated physical and chemical properties have been observed and new oxide phases, hitherto unknown in the form of bulk material, have been detected in nanolayer form and have been elucidated with the help of the STM. Some of these oxide nanolayers are and will be of paramount interest to the field of advanced catalysis, as active and passive layers in catalytic model studies, on the one hand, and perhaps even as components in real nanocatalytic applications, on the other hand. We have illustrated with the help of prototypical examples the growth and the structural variety of oxide nanolayers on metal surfaces as seen from the perspective of the STM. The selection of the particular oxide systems presented here reflects in part their relevance in catalysis and is also related to our own scientific experience. We believe that the field of oxide nanolayer characterization is still in its early stages and that many novel oxide phases will be detected in the years to come. While the way to their structural characterization is essentially paved, the elucidation of other properties such as catalytic chemistry or physical properties including magnetic behavior is largely unexplored. The commercial availability of high-quality low-temperature STM instruments has opened the way to the observation of chemical reactions on a single-atom basis – keyword: single-atom catalysis – that is of fundamental scientific interest. The local spectroscopy capability of STS in probing the electronic structure at the single-atom level is central to this single-atom chemistry approach. Bond formations induced by the STM tip [170] or STM-induced catalysis [171] are directions of chemical nanostructure research, where increasing activities can be expected in the future. The reduction of the dimensionality of metal-supported oxide nanostructures from three- and two-dimensional to onedimensional (monoatomic oxide line structures [153]) or quasi-zero-dimensional (oxide quantum dots), thus creating low-dimensional oxide–metal nanoscale hybrid structures, is another area of scientific endeavor with promise of finding novel chemical and physical behavior. These latter systems will allow us to study, in a controlled way and at the atomic scale, the chemical reactivity of low-coordinated surface sites, which are abundantly present in practical catalyst systems but which escape direct scientific characterization due to the inherent lack of control at the

References

atomic level. In all these areas, STM techniques will play a central role, and the future of STM applications in the field of surface chemistry and catalysis, and oxide catalysis in particular, is bright and wide open.

Acknowledgments

This work has been supported by the Austrian Science Funds within the National Research Network Nanoscience on Surfaces. FPN acknowledges with gratitude the excellent hospitalities of Professor Wolf-Dieter Schneider, EPFL Lausanne, and Professor Charlie Campbell, University of Washington, during his sabbatical stays in Lausanne and Seattle in 2008.

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7 Surface Mobility of Atoms and Molecules Studied with High-Pressure Scanning Tunneling Microscopy Gabor A. Somorjai, Feng Tao, and Derek Butcher

7.1 Introduction

Atomic and molecular studies of solid surfaces have been an intensive research area over the past few decades. Most of these investigations were carried out in vacuum to take advantage of electron, ion, and molecular beam scattering techniques that provided surface sensitivity because of their high scattering cross sections. However, most surface phenomena occur at solid–high-pressure gas and solid–liquid interfaces that we call buried interfaces, including heterogeneous catalysis, electrochemistry, and corrosion, to mention a few. The lack of techniques for studies of buried interfaces was commonly called the pressure gap in surface science. There has been a great deal of effort in our laboratory and elsewhere to develop surface techniques that provide molecular information at buried interfaces. Our laboratory has focused on developing three of these techniques: high-pressure scanning tunneling microscopy (HPSTM), sum frequency generation (SFG) vibrational spectroscopy, and ambient pressure X-ray photoelectric spectroscopy (APXPS). This paper reviews the development of the HPSTM technique and our research results, including the determination of surface mobility, surface structure on the atomic and molecular scales, and reaction kinetics.

7.2 Characterization of Surface Mobility of Molecules and Atoms

Since the most active catalytic sites are usually steps, kinks, and surface defects, atomically resolved structural information including atomic distribution and surface structure at low pressure, possible surface restructuring, and the mobility of adsorbate molecules and of the atoms of the catalyst surface at high temperature and high pressure is crucial to understanding catalytic mechanisms on transition metal surfaces. The importance of studying the structural evolution of both adsorbates

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and catalyst surfaces at high pressure has driven the development of new surface analytical techniques to determine surface structure and mobility of molecules and atoms in the harsh conditions of industrial catalysis. Scanning tunneling microscopy (STM) has the capability to examine molecules and atoms of adsorbate and catalyst surface to address several crucial issues in heterogeneous catalysis, including the rearrangement of surface atoms, restructuring of the catalyst surface, and mass transfer. Since the advent of STM in the 1980s [1], it has been one of the dominant tools to study catalytic reactions with a vacuum surface science approach. This is because it has the unique ability to investigate adsorption and surface structure with atomic resolution and in some cases the electronic structure as well. This technique can be applied to a pressure range from ultrahigh vacuum (UHV) to atmospheric or high pressure since the tunneling process between the sample and tip occurs only in a very close range of 5–50 Å. We first applied STM to study adsorbate and surface structure at high pressure by filling reactant gases into an STM chamber and keeping them at the high-pressure during STM scanning [2, 3]. After the first demonstration, it has been applied to catalysis studies under a condition of a high pressure by a few groups [4–11]. Recently, a new high-pressure high-temperature STM reactor/UHV system was designed and homebuilt by our group in 2006 and 2007 [12]. In this design, the STM body, sample, and tip are placed in a small high-pressure reactor (19 cm3) installed in a UHV chamber. A sealable port on the wall of the reactor separates the highpressure environment of reactant gases in the reactor from the vacuum environment of the UHV chamber and permits one to change sample and tip in UHV. The combination of a sample transfer arm, wobble stick, and sample load-lock system allows convenient transfer of samples and tips between the preparation chamber, high-pressure reactor, and ambient environment. Experiments performed on inert samples such as HOPG and reactive samples such as hex-Pt(1 0 0) both in vacuum and in high-pressure conditions demonstrate the ability to perform in situ investigations of heterogeneous catalysis and surface chemistry at atomic resolution at a wide pressure range from UHV to a pressure higher than that of atmosphere and a temperature range from 300 to 700 K. Other than the STM techniques for catalysis studies at high-pressure conditions in our group, sum frequency generation vibrational spectroscopy and ambient pressure X-ray photoelectron spectroscopy were also developed for catalysis studies at high pressures. Early on we developed SFG vibrational spectroscopy to study surface chemistry at high-pressure conditions with an emphasis on catalysis [13, 14]. SFG is a surface-specific spectroscopy that can be used to measure vibrational spectra under realistic catalytic conditions to allow us to identify the reaction intermediates and products and therefore reveal the catalytic mechanism [15–17]. Recently, a synchrotron-based ambient pressure XPS was developed [18–20]. Since the XPS signal is measured through the collection of electrons ejected from the sample, the instrument performance is highly susceptible to scattering from gas molecules between the sample and the detector. The APXPS was developed with the idea of bringing a sample close enough to a differentially

7.3 High-Pressure STM Technique and Instrumentation

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