Poisoning and Promotion in Catalysis based on Surface Science Concepts and Experiments

Studies in Surface Science and Catalysis 70

POISONING AND PROMOTION IN CATALYSIS BASED ON SURFACE SCIENCE CONCEPTS AND EXPERIMENTS

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Studies in Surface Science and Catalysis AdvisoryEditors: B. Delmon and J.T. Yates Vol. 70

POISONING AND PROMOTION INCATALYSIS BASEDON SURFACE SCIENCE CONCEPTS AND EXPERIMENTS

M.P. Kiskinova Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia 1040, Bulgaria

and

Sincrotrone Trieste, Padriciano 99, 34072 Trieste, Italy

ELSEVIER

Amsterdam

- Oxford - NewYork - Tokyo

1992

ELSEVIER SCIENCE PUBLISHERS B.V. Sara Burgerhartstraat 25 P.O. Box 211,1000AE Amsterdam, The Netherlands Distributors for the United States and Canada:

ELSEVIER SCIENCE PUBLISHING COMPANY, INC. 655 Avenue of the Americas NewYork, NY 10010, USA

ISBN: 0-444-86947-6

0 Elsevier Science Publishers B.V., 1992. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the Publisher, Elsevier Science Publishers B.V./Academic Publishing Division, P.O. Box330,1000 AH Amsterdam, The Netherlands. Special regulations for readers in the USA - This publication has been registered with the Copyright Clearance Center Inc. (CCC), Salem, Massachusetts. Information can be obtained from the CCC about conditions under which photocopies of parts of this publication may be made in the USA. All other copyright questions, including photocopying outside of the USA, should be referred to the Publisher. No responsibility is assumed by the Publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein. Although all advertising material is expected to conform to ethical (medical) standards, inclusion in this publication does not constitute a guarantee or endorsement of the quality or value of such product or of the claims made of it by its manufacturer. This book is printed on acid-free paper. Printed in The Netherlands

PREFACE

Extensive work on surface research during the past two decades, occasioned by the development of various powerful surface-sensitive techniques, has resulted in a much-improved fundamental understanding of surface phenomena. Valuable information, concerning catalytic processes on metal surfaces, was derived from experimental data thus obtained. Such information concerns the characterization, on atomic as well as molecular scale, of the following surface factors: electronic structure, position and order of atoms and chemical composition of the catalysts. It also concerns the composition, structure and properties of the adsorbed layers, and lastly, the factors responsible for changes in catalytic activity and selectivity. In this book, an attempt is made to summarise current information which contributes t o the fundamental understanding of the effect of additives, some of which act as promotors, others as poisons, in a number of important catalytic reactions. A description of single- and double-component systems has been obtained by using surface-sensitive techniques, particularly suited for this purpose. For the benefit of the reader, a short summary of the main surface science techniques used in the studies considered in this book has been given in Chapter 2. Three general and interrelated topics are reviewed. The first concerns the interaction of electronegative (Cl, S, Se, C, N , 0, P) and electropositive (alkali metals) atoms with metal surfaces (Chapter 4). The second topic covers the chemisorptive properties of metal surfaces modified by varying amounts of additives with respect to different reactants ( CO, NO, N2, 02,Ha, C 0 2 , NH3, H 2 0 , and hydrocarbons) (Chapters 5 & 6). In particular, the adsorption kinetics and energetics and the electronic, structural and reactive properties of the coadsorbate systems are considered, whereby particular attention is given to recent surface science studies with well-characterized, single crystal metal surfaces. In these chapters, special attention is paid to showing the contribution of different factors (the nature and adsorption state of the modifier and the coadsorbed molecule, the structure of the adsorbed layer, the type of interactions in the mixed overlayers, etc.) t o the modifier effects. In the discussion of the third topic, model studies of several important catalytic reactions (Fischer - Tropsch synthesis, ammonia synthesis, GO oxidation, water-gas shift synthesis) on modified metal surfaces (Chapter 8) are considered. Emphasis is placed on the correlations between the chemisorptive properties of the modified surfaces and the observed changes in their catalytic activity and selectivity. Included in this discussion is the question to what V

Vi

Prefeace

extent various factors contribute t o the promoting or poisoning effect in the model experiments and how this affects actual catalytic operations. There is a chapter on the theoretical approaches to the definition of modifier effects also. I would like t o thank a number of people who worked with me on this subject. I am grateful to Prof. J.T. Yates, Prof. D.W. Goodman and Dr. A. Szabo with whom I had the pleasure to work on joint projects and thereby discuss many aspects of the poison phenomenon. I greatly appreciate the joint work, valuable discussions and constant exchange of scientific information and with Prof. H. Bonze1 and Dr. G. Pirug, that helped me to deepen my knowledge in the field of alkali-modified surfaces. Special thanks are due to my colleagues in the Institute of General and Inorganic Chemistry, Prof. L. Dr. M. Tikhov and Dr. G. Rangelov for their enthusiasm and Surnev, dedication to our joint research on alkali modification effects and for their helpful comments on the subject. I acknowledge the financial support of part of my research by the Alexander von Humboldt Foundation (Germany) and the Eastern Europe Program of NSF (USA). Maya P. Kiskinova

CONTENTS

PREFACE Chapter 1. INTRODUCTION Chapter 2. SURFACE SCIENCE METHODS 2.1. DYNAMICAL METHODS 5 2.1.1. Thermal Desorption 5 2.1.2. Molecular Beam Technique 6 2.1.3. Electron and Photon Stimulated Desorption 7 2.1.4. Secondary Ion Mass Spectrometry 8 2.2. STATIC METHODS 8 2.2.1. Low Energy Electron Diffraction 8 2.2.2. Work Function Measurements 9 2.2.3. Emission Spectroscopies 10 2.2.4. Absoption Spectroscopies 12 References 13

Chapter 3 EXPERIMENTAL APPROACH

15

References 18

Chapter 4 INTERACTION OF ATOMIC ADSORBATES, ACTING AS PROMOTERS OR POISONS, WITH SINGLE CRYSTAL SURFACES 4.1. ELECTROPOSITIVE ADDITIVES: ALKALI METALS 19 4.1.1. Surface Order and Surface Concentration 19 4.1.2. Nature of the Adsorption Bond 28 4.1.3. Summary for AlkalilMetal Adsorption Systems 39 4.1.4. Alkali Adsorption on Semiconductor Surfaces 40 4.2. ELECTRONEGATIVE ADDITIVES 41 4.2.1. Surface Structure, Site Occupation and Bond Lengths 41 4.2.2. Strength and Nature of the Surface Bonding 52 4.2.3. Conclusive Remarks 61 References 62

vii

19

...

Vlll

Con tents

Chapter 5 ADSORPTION O F GASES ON SURFACES MODIFIED BY ELECTRONEGATIVE ADDITIVES

5.1. CARBON MONOXIDE 69 5.1.1. General Remarks for CO Adsorption on Clean Metal Surfaces 69 5.1.2. Modifier Effect on the CO Adsorption Energy and on the Surface Adsorptive Capacity 72 5.1.3. Modifier Effect on the CO Adsorption Kinetics 80 5.1.4. Modifier Effect on the CO Adsorption Site Occupation 85 5.1.5. Surface Order in Mixed Overlayers 87 5.1.6. Modifier Effect on the CO Mobility and Bonding Orientation 92 5.1.7. Effect of the Substrate Surface Orientation on the Range and Strength of the Modifier Effect 94 5.1.8. Modifier Effect on the Electronic Structure of the Adsorbed CO Molecule 94 5.1.9. Modifier Effect on CO Dissociative Adsorption 96 5.1.10. Influence of the Chemical State of the'Modifier on the Strength of Poisoning 101 5.1.11. Conclusive Remarks 102 5.2. NITRIC OXIDE 104 5.2.1. General Remarks for NO Adsorption on Clean Metal Surfaces 104 5.2.2. Modifier Effect on the NO Molecular Adsorption 107 5.2.3. Surface Order in Mixed Overlayers 115 5.2.4. Modifier Effect on the NO Dissociative Adsorption 118 5.2.5. Differences in the Effect of Oxygen on the Adsorptive Properties and Reactivity of Pt(ll1) with Respect to CO and NO 120 5.2.6. Differences in the Effects of the NO Dissociation Products 0 and N on the Reactivity of Pt(ll1) and Rh(ll1) Surfaces with Respect to CO Oxidation and NO Reduction 123 5.3. NITROGEN AND OXYGEN 124 5.3.1. General Remarks for N2 Adsorption on Transition Metal Surfaces 124 5.3.2. Modifier Effect on the N 2 Molecular and Dissociative Adsorption 126 5.3.3. Modifier Effect on the 0 2 Adsorption 127 5.4. HYDROGEN 128 5.4.1. Hydrogen Dissociative Adsorption on Modified Surfaces: Adsorption Kinetics, Energetics and Capacity for Adsorption 129 5.4.2. Modifier Effect on the Surface Diffusion of Hydrogen Adatoms 133

69

Contents

ix

5.5. WATER 135 5.6. ORGANIC COMPOUNDS 139 5.6.1. Interaction of Hydrocarbons with Modified Metal Surfaces 140 5.6.2. Effect of S and C on the Interaction of Thiophene with Transition Metal Surfaces 147 5.6.3. Interaction of Alcohols, Aldehydes etc. with Modified Metal Surfaces 150 5.7. CONCLUSIVE REMARKS: THE POISONING EFFECT DESCRIBED WITHIN THE FRAMEWORK OF THE POSSIBLE INTERACTIONS IN THE COADSORBED LAYER 155 References 161 C h a p t e r 6 ADSORPTION OF GASES O N SURFACES MODIFIED BY ALKALI METALS 6.1. CARBON MONOXIDE 169 6.1.1. Alkali Effect on the Kinetics and Energetics of CO Molecular Adsorption 169 6.1.2. Alkali Effect on the Vibrational Properties of the Coadsorbed CO Molecules 178 6.1.3. Surface Order in Mixed Overlayers 183 6.1.4. Alkali Effect on the Electronic Structure of the Coadsorbed CO Molecules 186 6.1.5. Alkali Effect on the CO Bonding Orientation and the C0 Bond Length 198 6.1.6. Conclusive Remarks about the Behaviour of Mixed Alkali-CO Overlayers 200 6.1.7. Alkali Effect on the CO Dissociative Adsorption 203 6.2. CARBON DIOXIDE 208 6.2.1. Alkali Effect on the CO Adsorption Rate, the Surface Adsorptive Capacity and the Stability of the COa Adspecies 209 6.2.2. Alkali Induced Dissociation of COYand Secondarv Reactions in the Mixed Overlayer 212 6.3. NITRIC OXIDE 215 6.3.1. Alkali Effect on the NO Molecular Adsorption 215 6.3.2. Alkali Promoted Dissociation of NO and Secondary Reactions in the Mixed Overlayer 219 6.4. OXYGEN 227 6.4.1. Alkali Effect on the 0 2 Dissociative Adsorption: Adsorption Kinetics and Adsorptive Capacity of the Modified Surface 228

169

Con tents

X

6.4.2. Mutually Induced Stabilization of the Coadsorbed Oxygen and Alkali Species 233 6.4.3. 0 - Alkali Interactions in Mixed Overlayers 238 6.5. WATER 245 6.5.1. Alkali Effect on the H20 Molecular Adsorption 246 6.5.2. Alkali Induced Dissociation of H 2 0 and Stabilization of the Dissociation Products 247 6.6. HYDROGEN 251 6.6.1. Alkali Effect on the H2 Dissociative Adsorption and Saturation Hydrogen Coverage 252 6.6.2. Alkali Effect on the Adsorption State of the Hydrogen Adatoms 253 6.7. NITROGEN 256 6.7.1. Alkali Effect on the N2 Molecular Adsorption Kinetics and N2 Molecular Adsorption State 256 6.7.2. Alkali Promoted N2 Dissociative Adsorption 258 6.8. ORGANIC COMPOUNDS 261 6.8.1. Interaction of Unsaturated Hydrocarbons with Alkali Modified Metal Surfaces 261 6.8.2. Interaction of Alcohols with Alkali Modified Metal Surfaces 264 6.8.3. Conclusive Remarks 266 6.9. AMMONIA 267 6.10. ALKALI MODIFICATION EFFECTS AND FACTORS DETERMINING THE TYPE OF THE INTERACTIONS I N THE MIXED LAYERS 268 6.10.1. Mechanism of the Alkali Stabilization Effect on the Molecular Adsorption State of Acceptor-like Coadsorbates 269 6.10.2. Alkali Effect on the Adsorption Kinetics and Energetics 272 6.10.3. Alkali Effect on the Dissociation Propensity 274 6.10.4. Formation of Compound-like Species in the Mixed Overlayers 275 6.10.5. Conclusive Remarks 276 References 276 Chapter 7 THEORETICAL APPROACHES TO THE DESCRIPTION OF THE MODIFIER EFFECTS 7.1. PROMOTING EFFECT OF ALKALI METAL ADDITIVES 285 7.1.1. Theoretical Models for Alkali Adsorption on Metal Surfaces 285

285

Con ten t s

xi

7.1.2. Theoretical Models for the Alkali Effect on Coadsorbed Molecules 291 7.2. THEORETICAL MODELS FOR THE POISONING EFFECT OF THE ELECTRONEGATIVE ADDITIVES 298 References 306 Chapter 8 MODEL STUDIES O F SURFACE REACTIONS ON MODIFIED SURFACES

309

8.1. METHANATION AND FISCHER - TROPSCH SYNTHESES 310 8.1.1. Effect of Electronegative Additives on the Catalytic Activity and Reactivity 311 8.1.2. Alkali Promoters in Catalysts for CO Hydrogenation 317 8.1.3. Conclusive Remarks 320 8.2. CO OXIDATION: EFFECT OF ELECTRONEGATIVE ADDITIVES 321 8.3. REACTION OF NO AND CO: EFFECT OF S 324

8.4. WATER-GAS SHIFT REACTION 326 8.4.1. Effect of S on the Rate of the Water-Gas Shift Reaction 326 8.4.2. Effect of Cs on the Rate of the Water-Gas Shift Synthesis 327 8.5. AMMONIA SYNTHESIS: K PROMOTION EFFECT 329 8.6. CHEMICAL STATE OF THE ALKALI ADDITIVES UNDER THE REACTION CONDITIONS 331 8.7. CONCLUSIVE REMARKS: CORRELATIONS BETWEEN THE ADSORPTIVE AND THE CATALYTIC ACTIVITY AND SELECTIVITY 332 References 335 INDEX

337

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Chapter 1

INTRODUCTION

The importance of heterogeneous catalysis in modern industry motivated the great interest towards experimental and theoretical studies of gas - solid surface interactions. The phenomenon catalysis is a complex system of various processes. These concerns adsorption, desorption, surface and subsurface diffusion, interactions between chemisorbed atoms, molecules, and molecular fragments. Changes in the surface structure and properties induced by temperature, reactants, products, intermediates and desired or undesired additives are also concerned. Because of the complex nature of the catalytic reactions, many factors should be considered in order to explain the mechanism of each process and its relation t o the main characteristics describing the catalytic reactions: catalytic activity and catalytic selectivity. Catalytic activity is usually expressed by the rate of the desired catalytic reaction, e.g. the number of converted molecules per active site per second, or the number of product molecules per surface substrate atom per second (turnover number). The catalytic selectivity is a characteristic related to the fact that i n most cases, the catalytic reactions take place either successively ( A -kBl L Ck- D )

k3

and/or simultaneously (A

+)

in steps

where only one product of the i-th reaction step is the desired one. Consequently, the selectivity of the catalyst is defined as the fraction of reactant molecules converted to the desired product and is expressed by the ratio It is obvious that changes in selectivity are not always k,(desired)/ accompanied by changes in activity, because the total number of converted reactant molecules might not have changed. In the conventional methods for examining catalysis on an actual scale usually industrial processes are simulated, whereby the same form of supported or non-supported catalysts is used. They involve measurements of the reaction kinetics at various temperatures and partial pressures of the reagents, whereby identification of the rate-limiting step and its activation energy are aimed a t . In some cases the identity of the interacting species may be found by means of vibrational (infrared) spectroscopy. Information on the structure and properties of the catalyst, such as state of dispersion (size of crystallites or clusters), number of active sites and defeck, interatomic distances and coordination number of the catalyst clusters, is usually obtained by means of

CH,.

1

2

Chapter 1.

electron microscopy, X-ray scattering, X-ray absorption spectroscopy and adsorption studies. The usual correlations that come out of systematic studies of different reactions over a series of catalyst,s, concern the relationship between catalytic activity and selectivity for given types of reactions and some of the catalyst properties. By properties is meant the number of active sites, the heats of adsorption of the reactants or products, the position in the periodic table, and the prevailing structure of the micro - crystallites etc. However, the macroscopic reaction rate and product distribution, determined by conventional methods, does not provide a precise description of the single surface processes and the properties of the catalyst surface and surface overlayer on an atomic and molecular level. In order to obtain more accurate information, one should isolate single processes concerned with the catalytic reaction taking place on a well defined surface. This became possible after the development of precise physical methods for the characterization of the structure and chemical state of the solid surfaces and chemisorbed molecules, atoms and radicals. In order to simplify the systems and show the influence of the surface structure and composition on catalytic activity, model studies are usually carried out with single-crystal samples. This ensures investigation of the various steps of the catalytic reaction on well defined surfaces, the structure and composition of which can be changed in a controllable way. Since most of the surface science techniques work under ultra- high vacuum, the experimental conditions in the same chamber are most suitable for studies of chemisorbed, static layers. The sensitivity of most surface techniques enable studies of the interaction of isolated molecules or atoms with the surface, and the induced changes in the adsorbat,e and substrate surface structure and properties. Kinetic studies require a second reaction chamber where the chemical reaction can be carried out at elevated pressures with pure reactants. The connection with the surface analyses chamber ensures precise control of the catalyst surface before and after the reaction. New surface science techniques, applicable to cases inaccessible by the conventional methods, can give information on:

(1) the atomic and electronic structure of the surface, and the changes in this structure induced by the presence of adsorbates;

(2) the identity of the adsorbed molecules, atoms and molecular fragments and the perturbations in their electronic structure as a result of adsorption forces;

(3) the structure of the adsorbate layers, tqhecoordination of the adsorption sites, and the vibrational and energetic properties of the adsorbed species at different concentrations of the adsorbates; (4) the relation between the surface structure and composition and adsorp-

tive and catalytic properties by studying different single crystal surfaces and the introduction of defects (step, kinks, terraces) or foreign atoms with a known concentration, and

(5) the mechanism of the single processes involved in the catalytic reaction.

3 When trying t o describe the mechanism of surface reactions, one should take into account that, for reactions to proceed, including intermediate steps, there must be specific surface sites with specific properties. That is why any changes in the catalyst surface structure and composition might affect the catalytic activity and selectivity. By introducing foreign species, one can modify the properties of the catalyst surface reversibly or irreversibly. The additives may alter the geometrical structure and the chemical state of the surface. This may affect certain reactions. When these effects add to catalytic activity and/ or selectivity, the additivies are called promoters. When these effects detract from the catalytic act,ivity or cause undesired changes of the selectivity, the additives are called inhibitors (when the process is reversible) or poisons (when the process is irreversible). The promoters are generally introduced deliberately during the preparation of the catalyst in order to improve its properties. The catalytic poisons are different in origin. They can be (i) impurities, introduced during the catalyst preparation, which segregate on the surface and occupy certain active sites; (ii) impurities in the reactants or some of the reaction participants. Whether a species acts as a poison or as a promoter depends exclusively on the kind of the catalytic reaction concerned. Since most of the metal-catalyzed reactions, which will be considered in the present manuscript, require cheniisorption and/or dissociation of reactants with electron acceptor behaviour (CO, NO, 0 2 , N z , etc.), one should expect that elements which are electronegative in respect of the metal catalyst, should act as poisons and vice versa. Following this conventional definition, the terms electronegative and electropositive additives will be used hereafter. Over the past decade a great number of surface science studies have been dedicated t o coadsorption experiments on single crystal metal surfaces modified by foreign atoms. These model experiments play a significant role in understanding the contribution of various factors to the poisoning and promotion phenomena. The purpose of this monograph is to summarize what has recently been achieved by surface science model studies (on single crystal surfaces with controlled amounts of modifiers and coadsorbates) in developing a consistent picture of the mechanism of poisoning and promotion effects. The main scientific issues which will be considered are related to:

(1) the influence of additives on the chemisorptive properties of the substrate with respect to molecular and dissociative adsorption; (2) the contribution of different factors, such as atomic size, electronegativity, adsorption state, surface order, and surface concentratmionof the additive, on the direction and the strength of the modifying effects;

(3) the nature of the modifier - coadsorbate interactions, and (4) the relationship between the modified chemisorptive properties and the observed changes in the cat,alytic activity and selectivity of some catalytic systems.

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Chapter 2

SURFACE SCIENCE METHODS

Since a great number of books has already appeared, giving detailed descriptions of the surface science methods (e.g. refs. [I, 2]), a brief classification and a description of the most common techniques will suffice. 2.1

DYNAMIC METHODS

The techniques used, when applying these methods, cause the adsorbed molecules, atoms or radicals to be desorbed from the surface and analyzed in the gas phase. A mass-spectrometer is normally used as the gas analyzer. The desorption of the surface species can be achieved in various ways, e.g. by means of a programmed increase of the substrate surface temperature or by irradiation of the surface layer by electrons, photons, atoms or ions. 2.1.1

Thermal Desorption

This widely used method is based on measuring the flux of the desorbing species for a given adsorption system [3-81. The desorption is induced by heating the sample using an adequate temperature ramp (temperatureprogrammed desorption, TPD) [3-71, or pulse laser beam (laser-induced desorption, LID) [8]. The desorption flux is usually recorded and analyzed by a mass spectrometer. For a linear temperature ramp the desorption rate can be described by the relationship:

dN - V dP S _ dt - w[dt+l/p]l where N is the adsorbate surface concentration in molecules per cm ', P is the pressure change induced by the desorbing particles, S is the pumping rate of the system in 1 per sec, A is the adsorbent surface in cm 2 , V is the volume of the system and Tg is the temperature in the gas phase. With S >> V as the limit and a slow heating rate ( d T / d t ) the desorption rate ( d N / d t ) becomes proportional to the pressure changes induced by the desorbing particles. Taking into account that the modern experimental chambers are characterized by V 5 20 1, and S > 400 I.sec-', aiid d T / d t is usually less than 20' sec-l, 5

Chapter 2.

6

the mathematical description of the T P D spectra obtained is simplified considerably. Thus the traces of the T P D spectra usually reflect the desorption rate directly from the i-th adsorption state, as described by the Polanyi Wigner equation [9, 101:

where ni is the order of desorption, E, is the activation energy of desorption, v, is the frequency factor and N , is the surface concentration for the i-th adsorption state. Thus, in the case of E,, n, and v, independent of the adsorbate surface concentration the relation between the desorption parameters and the experimental thermal desorption curves for n, = 1 can be described by [4]:

Ei RT

- = In

(y) vi.T .

3.46,

(3)

where p = dT/dt, and Tpd is the temperature at the maximum of the corresponding T P D peak. For ni = 2, the relation can be given by [4]:

where Nio is the initial coverage. In actual fact, the desorption parameters can be independent of coverage only with the limits of very low coverages. This calls for full analyses of the complex T P D spectra obtained at moderate and high coverages [5, 9-11]. The T P D method is rather popular because it is relatively cheap and can be applied to many adsorption systems. The T P D data provide information about the adsorbate surface coverage (the area under the T P D spectra is directly proportional to the adsorbate surface concentration), the existence of different adsorption states (an appearance of several TPD peaks), the adsorption and desorption kinetic parameters, the existence of different adsorption states at different adsorbate coverages (from the changes in the shape of the T P D spectra), the interactions between the adsorbed species, the phase transitions in the adsorbate layer, etc. In order to obtain reliable information, it is necessary to avoid side effects, such as recording desorption from the sample supports, a temperature gradient across the crystal, possible laser-induced damage t o the surface, etc. [6,7]. When interpreting the data, especially the multi - peak T D spectra, care must be taken, because in some cases the temperature rise might cause interconversion between the adsorption states. Consequently, the different peaks in the T P D spectra do not necessarily reflect a coexistence of different adsorption st,at,es at the given adsorption temperature. 2.1.2

Molecular Beam Technique

Rapid progress has been made recently with molecular beam experiments because they represent a good approach to the heterogeneously catalyzed surface

2.1. Dynamic Methods

7

reactions [12-141. In these experiments, the sample or t h e detectors (or both) can be moved so that the molecular beams of the molecules, atoms or fragments under study can be directed and reflected elastically or inelastically by the substrate surface a t different angles. This ensures variation of the angle of incidence of the molecular beam, an independent change of the gas and surface temperature, and allows direct analysis of the reaction products by means of a mass spectrometer. The use of a mass spectrometer as an analyzer and lock-in phase sensitive detection restricts the measurements only to the primary reaction products of interest. Furthermore, by using a modulated molecular beam and by first analyzing the phase shift between the input and output signal, it is possible to obtain information about the residence time of the adsorbing species at different substrate temperatures and to deduce the adsorption/desorption parameters and the mechanism of the surface processes. 2.1.3

Electron and Photon Stimulated Desorption (ESD and PSD) and Electron Stimulated Desorption - Angular Distribution (ESDAD)

This method is based on analyses of the particles (ions and neutrals) desorbed as a result of electronic excitations of the adsorbed surface species, induced by electron or photon irradiation 11, 15-17]. A variety of molecular and fragment ions and neutrals ( either in ground or in electronically excited states) have been detected from the adsorbed overlayers as electron (photon)-stimulated products. Two mechanisms are proposed for explaining the electron- stimulated desorption. According to the first mechanism (known as Menzel, Gonier and Redhead model) [15, IS], the first step of electronic excitation is a FrankCondon transition from a ground to nonbonding (repulsive) state. This primary excitation can be followed by different events leading either to quenching of the excitation (by delocalization and trapping of the particles in an attractive potential well) or t o the removal of the excited particle from the surface (by transfer of the electronic energy to a nuclear motion as a result of the electronic rearrangements). On it,s way away, the excited particle may be transferred to another repulsive potential curve which determines the nature of the detected products (positive or negat>iveions or neutrals). In this model, the total desorption cross section is usually given by the relationship: u = u,.P,

where u, is the excitation cross section for the gas phase and P is the escape probability. P depends on the shape of the repulsive potential curve which determines the kinetic energy of the desorbing particle and the lifetime of the excited state a t the surface. According t o the second model suggested by Knotek and Feibelman [17], the primary excitation is a core excitation that decays by an interatomic Auger transition. As a result of this Auger decay, the ion which was negatively charged originally, loses electrons and becomes positively charged. This leads to the desorption of positive ions due to the inversion of the Madelung

8

Chapter 2.

potential. This model is primarily applicable to systems with an ionic bond at the highest valence state. As was mentioned above, while the desorbing particles are leaving the surface, they can undergo different transitions leading to the recapture or a change in the charged state. What happens very much depends on the environment of the adsorbed particles. Depending on the type of the excitation process involved, the cross section of the different ESD products can vary substantially with the changes of the adsorption state and the adsorbate coverage. These variations provide inherent information on the nature of surface - adsorbate interactions and bonding and the changes induced by changes in coverage or modifications of the surface. It is assumed that the directions of the desorbing beams are exclusively determined by the bond orientation of the initial state which allows the angular distribution measurements of the species desorbing by electron or photon stimulation, as developed by using the ESDAD method [18-21). A movable mass - spectrometer [20], a phosphorus screen [I91 or a resistive anode [21] can be used as detector for the ESDAD patterns of the excit,ed ESD products. The uniqueness of the ESDAD method is that i t is passing on information about single species on the surface. It does not require long-range ordering but azimuthal ordering for abnormal directions. This method turns out to be a sensitive probe, concerning the chemisorption bond angles, the site location, the amplitudes of the soft bending molecular vibrations parallel to the surface and their dependence on temperature, coverage and surface composition.

2.1.4

Secondary Ion Mass Spectrometry (SIMS)

This method is based on mass analysis of the species sputtered from the surface as a result of bombardment with highly energetic particles, usually Ar ions with energies in the keV range [I, 221. The emission of the surface particles is induced by energy transfer from the impact ions to the substrate lattice atoms or to the adsorbates. The mass analysis of the sputtered particles contains information about the surface composition. It also provides information on the local structure using the kind of the fragments monitored by the mass spectrometer and the angular dependence of the ion emission as a fingerprint. Depending on the density of the incident-beam current (ranging from 1 nA cm-2 t o 1 mA emd2), this technique can be used for accurate surface and bulk analysis, which provides information on a depth scale.

2.2

STATIC METHODS

These methods involve analyses of the stationary adsorbed phase by means of various spectroscopies based on the interaction of electroils and photons with the surface layer.

2.2.1

Low Energy Electron Diffraction (LEED)

This method is based on recording and doing spatial analysis of the elastically back scattered low energy (15-350 eV) primary electrons from the surface [l, 2, 23, 241. According t o the Lue de Broglie equation [l],which describes the

2.2. Static Methods

9

interference phenomena in electrons scattered by a crystal, the pronounced maxima in the angular distribution of the back scattered electrons displayed on the detector (phosphorus screen or resistive anode) reflect the periodicity of the surface and the possible variations as a result of reconstruction or formation of ordered adsorbate superstructures. In the case of a two - dimensional lattice consisting of parallel rows of atoms in the directions [k,h] and interatomic distances dk,h the Lue de Broglie equation becomes:

where A is the wave length, 90 the angle of the primary electron beam, andps the angle of the reflected electrons. Usually in the LEED systems, ‘po is 0’. Thus the diffraction pattern reflects the periodicity on the surface and the changes induced by the increase of coverage, introduction of coadsorbates, reconstruction of the substrate surface etc. The main driving force for obtaining ordered adsorbate structures on a single cryst,al substrate is determined by the type of adsorbate - substrate and adsorbate - adsorbate interactions. Complete structural analysis of the LEED patterns is possible applying the kinematic theory and measuring the intensity of the diffracted beam as a function of the direction and energy of the primary electron beam [23,24]. This ensures determination of the site location of surface species within the unit cell, and the corresponding adsorbate - adsorbate distances and adsorbate - substrate bond lengths. 2.2.2

W o r k Function Measurements

The change of the work function upon structural and/or composition changes on the surface provides valuable information on the electrostatic potential of the surface which affects its reactivity [25]. Work function is usually described by the relationship: 4 = A4 - m / e , (7) where A@ represents the electrostatic potential of the surface double layer and xel the chemical potential of the electron in the bulk. Since the chemical potential is a bulk property and is not much affected by modification of the surface, it is the change of the double layer that generally determines the work function changes that are observed. The surface potential in the adsorption systems arises because of the presence of dipoles. The potential difference on the two sides of the dipole layer is given by the equation:

Ad = 4 ~ , u N ,

(8)

where N and p are the concentration of the dipoles and the dipole moment of each species on the layer. When the concentration of the surface dipoles increases, the depolarization effects should be ascribed to dipole-dipole interactions. In this case, the surface potential can be expressed by the following equation: 4?rp0N A$ = (9) 1 +9a”

10

Chapter 2.

where LY is the polarizability of the adsorbed species and po is the initial dipole moment. The absolute values of the work function can be determined by three methods only: thermo-ionic, field emission methods and a He1 ultraviolet photoemission method [25, 271. Since usually the relative change of the surface potential, Ad, is the only important factor, many other methods can be used, such as a vibrating capacitor, a diode (retarding potential) method, a secondary electron method (cut-off of the low energy secondary electrons created by electron or photon irradiation on the surface), etc. [25]. It should be pointed out that, together with information about the sign and strength of the surface dipoles, some of the informat,ion on the uniformity of the surface layer is obtained by using these methods, e.g. by the diode method [26, 271. In mixed overlayers the coexistence of patches of different work functions, compositions and structures on the surface (which is difficult to detect with other methods) can be established. 2.2.3

Emission Spectroscopies

Depending on the kind of emission which is recorded and used for characterization of the overlayer, the emission spectroscopies can be classified into two general groups: A. Electron emission spectroscopies. These involve spectroscopies based on measurements of the intensities and the kinetic energies of the electrons emitted from various electron levels of the matter under investigation. Depending on the primary irradiation that causes the electron emission, these spectroscopies can be divided as follows: (a) Spectroscopies in which the primary irradiation is performed by electrons. The Auger Electron Spectroscopy (AES) falls in this category [1,2]. This is the most widely used technique for the quantitative and qualitative analysis of surface layers. The principle of the method is based on detection and analysis of the energy distribution of the Auger electrons ejected from the surface as a result of excitation induced by irradiation with electrons of primary energy (typically between 1500-5000 eV). The energies of the secondary Auger electrons is determined by the following relationship:

where E,, Ey and E , are the binding energies of the three electron levels involved in the Auger process, E, is the correction for relaxation of the levels as a result of changes in the amounts of the charges, caused by the creation of a core hole, and e4sp is the work function of the spectrometer. It is obvious from eq.(lO) that the kinetic energy of the emitted Auger electron is independent of the primary excitation energy. Information derived from the energy position, intensity and shape of the Auger spectra has been successfully used for the determination of surface concentration and changes in the chemical state of the adsorbate and substrake. The surface sensitivity of AES depends on the primary excitation energy and the energy of the given Auger transition. Usually, Auger electrons with a low kinetic energy are operative at valence electron levels and are much more sensitive to the interactions in the overlayer and changes in the chemical d a t e of the interacting species.

2.2. Static Methods

11

(b) Photoelectron spectroscopies, where the primary irradiation is electromagnetic (photons). In this class are: X-ray excited Auger electron spectroscopy (XAES), X-ray photo-electron spectroscopy (XPS), ultraviolet electron spectroscopy (UPS), extended X-ray absorption fine structure (EXAFS) etc. [1,2]. The XPS and UPS methods are based on detection of the kinetic energies, &n, of the photoelectrons ejected from discrete electron levels of the adsorbate or substrate as a result of irradiation by a monochromatic photon beam with an energy hv. When using the relationship:

one can determine the binding energy in electron level 12, E g , with respect to the vacuum level. XPS and UPS differ in the energy of the primary excitation beam. Usually, the irradiation sources for XPS give beams with energies by 150-8000 eV. XPS is most appropriate for the determination of the electron states in the core levels. The information originating from the XPS spectra includes data. on : the m t u r e of adsorbates a.nd suhstmtes, the surface concentration (which is directly proportional t.0 the int,ensity of the photoelectrons emitted from tlie deep core levels), the chmiges i n chemical state of the surface species (from the energy shifts), tlie coordination of the adsorbed molecules etc. The energy of tlie primary photon beam for UPS is less than 100 eV (usually 10-45 eV). The information based on UPS concerns mainly the valence electron states, binding energies of the molecular orbitals, and the changes induced by interactions on the surface. As has been mentioned in subsection 2.2.2., t8he UPS method can also be applied for work function measurements [I]. The EXAFS method is based 011 the fact, that the photoelectrons a,re emitted as a result of X-ray a.bsorption hack scattered off neighboring atoms. This results in interference between the outgoing and back - scattered photoelectron waves. This interference process produces a,n oscillatory modulation in the X-ray absorption spectrum within the energy range beyond the absorption threshold. Analyses of this oscilhtory fraction in the X-ray absorption spectrum provide information on the local structure around the absorbing species. The advantage of this met,Iiod is that, contrary to the diffraction methods (such as LEED and X-ray diffraction), it does not require a longrange periodicity. (c) Spectroscopies where the primary irra.diation is performed by noble gas ions and metastable atoms with low kinetic energies. To this class belong the metastable quenching spectroscopy (MQS) or penning ionization electron spectroscopy (PIES) [l]. This method is b a e d on t,he fact that the interaction of noble gas ions or excited neutrals with a surface layer forces neutralization accompanied by a.n electron emission. Depending on t,he deexitation mechanism, the resulting electron energy spectra. conbain information on t.he energy position of the molecu1a.r orbitals, the electron levels near t,he Fermi level etc. B. Photoemission spectroscopies. These a.re the spect,roscopies based on measurements of tlie intensities and the energies of the electroma.gnetic radiation emitted as a result of the interaction of slow electrons with the surface layer. In this class a.re the appearance potential spectroscopy (APS) [l]and the inverse photoemission spectroscopy (IPS) [1,28].

12

Chapter 2.

The APS method is based on measuring the X-ray intensity emitted from the surface irradiated with electrons, on which , in the background, which is steadily gaining in strength, a characteristic emission is superimposed. The latter appears when the primary beam energy equals the threshhold energy of excitation of an electron from a core level to an unfilled state above the Fermi level. APS spectra contain information about the core level binding energies and the density of unfilled states above the Fermi level. IPS is based on a process which is to photoemission, i.e. a radiative deexcitation of electrons. Thus, the energy distribution of the emitted photons reflects the electron density of the unoccupied stat,es of the adsorbate and substrate. It should be pointed out that the photoemission spectroscopies provide information about the empty electron density of states above the Fermi level and the unoccupied adsorbate electron orbitals, whereas the electron emission spectroscopies carry information exclusively on the occupied electronic states of the substrate and the occupied molecular or atomic orbitals of the adsorbates. 2.2.4

Absorption Spectroscopies

The absorption spectroscopies are based on monitoring of the energy spectra of the inelastically back-scattered primary electrons (electron loss spectroscopies) or reflected electromagnetic radiation (infrared spectroscopy - IR). A. Electron loss spectroscopy. Depending on the characteristic energy losses two categories of electron loss spectroscopies are distinguished on the basis of the energy of the primary electron beam: (a) Electron Energy Loss Spectroscopy (EELS), where the primary electron energies are of the order of 100 eV. The characteristic energy spectra are a result of energy losses caused by the induction of interband and intraband transitions, plasmon and core level electron excitations in the surface layers and one electron transitions in molecules, atoms or molecular fragments present on the surface. These losses are usually observed in the 1-50 eV range and the energies of the loss peaks are determined by the separation of the two electron levels involved in the induced electron transition or the excitation energy for the plasma oscillations. (b) High Resolution Electron Energy Loss spectroscopy (HREELS), where the primary energy is of the order of a few eV and the electron losses are caused by excitations of the vibrational modes (phonons) at the surface or in the adsorbed species. These losses are nearly of the order of 100 meV. HREELS has been successfully applied in studies of interfacial properties of thin films, and bonding configurations of adsorbed species. Information obtained by HREELS regarding the vibrations excited in the adsorbed molecular species is very similar to that offered by infrared spectroscopy, where the same excitations are induced by electromagnetic irradiation.

References

13

B. Infrared reflection absorption spectroscopy [29]. The advantage of this vibrational spectroscopy is the higher resolution than that of HREELS and the absence of possible electron beam effects. Recently, it is widely applied for the determination of the bonding mode orientation of adsorbed molecules and interactional effects between adsorbed species. It is worth mentioning here that the vibrational spectroscopies are directly related t o the fact that any adsorbate on the surface is vibrating. The possible vibrational modes are determined by the symmetry of the adsorbed species. The symmetry depends on the number of the substrate atoms participating in the formation of the adsorption bond. The possible adsorption sites on single crystal surfaces are determined by the crystallographic orientation of the surface plane. For example, for a fcc (111) surface there are one-, two- and three-fold adsorption sites, depending on the number of the nearest substrate surface atoms.

REFERENCES G . Ertl and J. Kiippers, Low Energy Electrons and Surface Chemistry, 2nd ed. (Verlag Cheniie, Weinheim, 1986) D. P. Woodruff and T. C. Delchar, Cambridge Solid State Sciences Series, eds. R. Cahn, E. Davies and I. Ward (Cambridge, 1986) G. Erlich, J. AppJ. Phys. 32 (1961) 4; Adv. Catalysis 14 (1963) 255 P. A. Redhead, Vacuum 12 (1962) 203 L. D. Schmidt, Catalysis Rev.-Sci. Eng. 9 (1974) 115 L. P. Levine, J. F. Ready and E. Bernalg, J . appl. Phys. 38 (1967) 531; 1EE J . Quantum Electron. QE-4 (1968) 18 [71 D. Menzel, in: Chemistry and Physics of Solid Surfaces, eds. R. Vanselow and R. Rowe (Springer Series in Chemical Physics, 1981) p.389 J. T. Yates, in: Experimental Methods of Experimental Physics vol.22, ed. R. L. Park (Academic Press, 1985) p.425 D. A . King, Surface Sci. 47 (1975) 384 E. G. Seebauer, A. C. F. Kong and L. D. Schmidt, Surface Sci. 193 (1988) 417

J. B. Miller, H. R. Siddiqui, S. M. Gates, J. N. Russel Jr., J . T. Yates Jr., J. C. Tully and M. J. Cardillo, J . Chem. Phys. 87 (1987) 6725 P. M. Merrill, Cat. Rev. 4 (1970) 115 M. P. D’Evelyn and R. J. Madix, Surface Sci.Reports 3 (1983) 413 J. A. Barker and D. J. Auerbach, Surface Sci. Reports 4 (1984) 1 D. Menzel and R. Gomer, J . Chem. Phys. 41 (1964) 3311 P. A. Redhead, Can. J . Phys. 42 (1964) 886 M. L. Knotec and P. J. Feibelman, Phys. Rev. Lett. 40 (1978) 904 J. J. Czyzewsky, T. E. Madey and 3. T. Yates Jr., Phys. rev. Lett. 32 (1974) 777

T. E. Madey, D. L. Doering, E. Bertel and R. Stockbauer, Ultramicroscopy I1 (1983) 187 and references therein D. Menzel, Nucl. Instr. and Methods in: Physics Research, Vol. B13 (1986) 50

M. Alvey, M. J. Dresser and J. T. Yates Jr., Phys. Rev. Lett. 56 (1986) 367 A. Benninghoven, J . Phys. 230 (1970) 403; Surface Sci. 57 (1975) 596

14

Chapter 2

[23]

J. B. Pendry, Low Energy Electron Diffraction eds. G. M. Conn and I<. R.

[24]

M. A. Van Hove and S. Y. Tong, Surface Crystallography by LEED,(Springer

[25]

J. Holzl, F. Schulte in: Solid Surface Physics , vo1.85 of Springer Tracts in

[26] [27] [28] [29]

Modern Physics (Springer, Berlin, 1979) p.85 A. G. Knapp, Surface Sci. 34 (1973) 289 I. I. Ionov, Soviet Phys. Tech. Phys. 43 (1973) 159 V. Dose, Surface Sci. Reports, 3 (1985) 337 and the references therein F. M. Hoffmann, Surface Sci. Reports 3 (1983) 107

Coleman (Academic Press, London, 1974) Berlin, 1979)

Chapter 3

EXPERIMENTAL APPROACH

A scheme of an experimental system for adsorption and reaction model studies is shown in figs. 3.1. and 3.2. The ultra-high vacuum (UHV) conditions with a base pressure less than P a can be achieved by using a combination of diffusion, turbo-molecular, ion-sorption and titanium sublimation pumps. The composition of the residual gases is checked by a mass-spectrometer and, depending on the type of the pumps used it consists of varying amounts of Ha, COz, CO, CH4, HzO and Ar. In addition to the mass-spectrometer the UHV analysis chamber is equipped with several other surface analytical facilities, the use of which depends on the objectives of the study.

7

C MA

Fig. 3.1. A scheme of a typical UHV experimental chamber equipped with selected surface sensitive techniques.

The usual experimental approa,ch of model surface science studies is by first obtaining substrate surfaces that a.re not contaminated by undesired atoms.

15

16

Chapter 3.

The cleaning procedure depends on the nature of the substrate. Several different kinds of treatment may be necessary, such as annealing, ion bombardment with high energy noble gas (Ar) ions, which remove the dirty surface layers; or chemical types of treatment, such as oxidation/reduction reactions. When a single crystal is used as a model catalyst, it should necessarily be accurately oriented and polished before insertion into the experimental chamber. After cleaning, the substrate surface is characterized with respect to structure and composition. The structure of the single crystal surface is usually determined by using the LEED method. The cleanliness is verified by means of XPS and AES. A well-ordered surface should show distinct reflections, which should characterize the corresponding crystallographic plane. A surface which does not show traces of contaminants, may be considered 'clean', i.e. free from foreign atoms. The criterion for accepting the cleanliness of the surface lies in the sensitivity limits of the AES or XPS method used, which will be of the order of 0.01 part of the monolayer (ML).

Analysis and Surface Preparrlioa Cbanber

Fig. 3.2. A combined system consisting of high pressure reaction and UHV analysis chambers, allowing translation of the crystal.

The amount of foreign particles on the surface is usually expressed by the ratio of the number of foreign particles per number of substrate surface atoms, taking the absolute number of the substrate surface atoms as one monolayer (ML). Another way of expressing this is the number of foreign particles per cm2. These two ways of expressing the foreign particles coverage differ by the factor of the number of the substrate atoms per cm2. In some cases, when the absolute coverage is unknown, the unit-fractional coverage is used.

17 I t is related to the maximum adsorbate coverage achieved under the actual experimental conditions. The calibration of the absolute adsorbate coverage is easiest when the adsorbate layer forms ordered superstructures. In the other cases relative methods can be applied, e.g. comparison of the intensities of TPD, XPS or AES spectra for a given adlayer with those corresponding to adsorbate layers with a known concentration. A controllable introduction of the modifiers can be performed in several ways. Alkali metals which are very frequently used as promoters in several important catalytic processes, are deposited in situ onto the clean substrate surface. Various alkali metal sources have been used, such as ion exchanged zeolites [l],alloys, consisting of a high percentage of a desired alkali metal [a], or compound dispensers (SAES getters: a mixture of alkali metal chromate and a Zr-A1 alloy which acts as a reducing agent) [3]. The principle which applies t o all sources is, after sufficient (several hours) outgassing under UHV conditions at appropriate temperatures, a flux consisting of pure alkali atoms and/or ions is obtained. The production of this flux requires relatively high temperatures (> 800 I<). The actual formation temperature of an alkali flux depends on the type of the source and the alkali metal present. The sources are designed in a way such, that the alkali flux can be directed and deposited onto the sample. Usually, after the outgassing procedure, the sources are calibrated for different working temperatures. This ensures a rough estimation of the conditions (evaporation at a given temperature) necessary to achieve the desired alkali coverage. Another procedure for the introduction of a modifier is to adsorb a gas which contains the modifier [4]. The adsorbed species, after appropriate heat treatments, dissociate on the surface and the undesired components are desorbed, whereas t,he modifier remains bonded to the surface. In this way, many modifiers, behaving as poisons in many catalytic reactions, are deposited on the surface of the model catalysts. For example, S is introduced by adsorbing H2S, P by PH3, Se by H2Se, Te by H2Te, C by CzH4 or CzH4, N by N2H4. In all these cases, after dissociation, H can be removed by desorption at elevated temperatures. Sometimes it is possible to introduce the modifier directly by dissociative adsorptioii, e.g.S by Sz, C1 by Cl2, 0 by 0 2 etc. The cleanliness and the concentration of the modifier overlayer can be determined by a variety of surface sensitive techniques, described in Chapter 2. Normally the following methods are applied: AES (where the intensity of the modifier’s most intensive Auger transition is measured), XPS (where the intensity of a chosen intensive core level peak of the modifier is measured), T P D (where the modifier can be desorbed at elevated temperatures, e.g. alkali metals). The calibration of the modifier AES or XPS signals to absolute coverages might be performed in several ways, e.g. by mass-spectrometric measurements of the undesired species resulting from dissociation which desorb upon heating (H in the case of H2S, HzSe, HzTe, PH3, NzH4, C2H2, and C2H4), direct thermal desorption of the modifier, when possible ( 0 2 , Cl2, N 2 , and alkali metals), radiotracer measurements, LEED, when the modifier forms ordered overlayer structures, etc. The uniformity and ordering of the modifier overlayer can be determined by scanning AES, LEED, and EXAFS. Furthermore, the modified surface can be characterized by the techniques available

Chapter 3.

18

in the chamber in order t o gain information about the nature of the surface UPS, TPD), the adsorption site (e.g. EXAFS, HREELS), the perturbations induced in the geometric structure of the substrate surface (e.g. LEED, EXAFS), the changes in electronic structures of the substrate and adsorbate as a result of the adsorption bond formation (e.g. XPS, UPS, MQS), etc. The next step, after modifying the substrate with a desired amount of an additive, is the adsorption studies. The modified surface is exposed to a gas (reactant or product of a certain catalytic reaction) whose interaction with the surface is of interest. In order to have a basis for comparison, these coadsorption experiments are usually preceded by the characterization of the gas - clean surface interactions. The adsorption experiments on modified surfaces include measurements a t various modifier and coadsorbate concentrations. This ensures a description of the coverage effects on the surface chemisorptive properties and on the strength of the modifier - coadsorbate interactions. The available experimental techniques are applied in order to show up all the aspects of the modifier effects on the chemisorptive properties of the surface. All experiments described above are designed so as to simplify the systems in order to obtain reliable information about each step involved in the complex catalytic processes. The conditions in these model adsorption experiments (specimens, pressures and environment) are far from the real technological conditions. More realistic conditions are created by using a second reactor chamber, in series with the first one. The model experiment is carried out in the first UHV chamber, kinetic stmudiesat elevated pressures in the second. Usually, this second chamber is equipped with a chromatograph or mass spectrometer for measuring the reaction rate [5-71. The advantage of these double chamber systems is that they ensure thorough characterization of the catalyst surface structure, composition, and chemisorptive properties before and after the reaction. As will be illustrated in the forthcoming sections, this combination enables precise studies of the effect of various modifiers with different surface concentrations and surface orders on the catalyst reactivity and selectivity. - modifier bonding (e.g. WF, LEED, SEXAFS, angle resolved

REFERENCES [l] [2] [3] [4] [5] [6] [7]

R. E. Weber and L. F. Cordes, Rev. Sci. I m t r . 37 (1966) 112 G. Broden and H . P. Bonzel, Surface Sci. 84 (1979) 106 SAES Getters S.p.A. Via Gallarate 215 Milano, Italy M. Kiskinova, Surface Sci. Reports 8 (1988) 359 G. A. Somorjai, Catalysis Rev.-Sci. Eng. 18 (1978) 173 H. P. Bonzel, G. Broden and H. J. Krebs, Appl. Surface Sci. 16 (1983) 373 D. W. Goodman, J . Vac. Sci. TechnoJ. 2 (1984) 873

Chapter 4

INTERACTION OF ATOMIC ADSORBATES, ACTING AS PROMOTERS O R POISONS WITH SINGLE CRYSTAL METAL SURFACES

4.1 E L E C T R O P O S I T I V E ADDITIVES: A L K A L I M E T A L S Among the most important additives used to promote a desired function of metal and metal oxide catalysts are alkali metals. They can be present at the surface in different manners ranging from sub monolayer quantities to surface or bulk compound. Originally, alkali metal salts were used in the manufacture of iron catalysts for ammonia synthesis [l]. Now alkali promoters are used in a number of catalytic reactions, e.g. Haber - Bosch (ammonia) synthesis, Fischer - Tropsch (hydrocarbon) synthesis, alcohol synthesis, water-gas shift reaction, etc. [2-41. Alkali metals are strongly electropositive elements with low ionization potentials (5.39, 5.14, 4.34, 4.18 and 3.89 eV for Li, Na, K, Rb and Cs, respectively). The first studies of alkali metals adsorption on transition metal surfaces were reported in 1920’s. A drastic lowering of the work fuiiction induced by the presence of alkali metals was established [5, 61. Since then a great number of studies have been dedicated to understanding the physics and chemistry of alkali metal adsorption on metal or semiconductor surfaces. 4.1.1

Surface Order and Surface Concentration of Alkali 0verlayers

A. Ordering and structural transformations of the alkali overlayers. When alkali metal is adsorbed on a single crystal metal surface, depending on the adlayer surface concentration and the adsorption temperature, two dimensionally commensurate and incommensurate ordered structures can be formed [7-161. At low temperatures a series of ordered structures are observed in the different alkali adsorpt>ionsystems [7]. Selected phase diagrams for N a on Ru(0001) [22] and Cs on Rh(100) [8, 121 are shown in figs. 4.1-4.3. It is obvious that, because of the significant changes in the adsorption state and

19

20

Chapter 4.

the surface diffusion rate of the alkali metals, the adsorption temperature at which ordered alkali structures exist, increases with increasing surface density of the adatoms. That is why in most cases where alkali adsorption is studied at room temperature, ordered overlayer structures are formed only a t coverages near saturation of the first overlayer. The formation of ordered structures is determined by the balance between the adatom - adatom interactions and the periodic surface potential contour. The relative magnitude of these forces determines whether the overlayer order will be commensurate (in-registry with substrate geometric structure) or incommensurate (out-of registry with the substrate geometric structure).

O

.

0.2

0.3

0.L

0.5

J

Na Coverage Fig. 4.1. A phase diagram for Na on Ru(0001). ‘Split ( f i x &)R30 epitaxy phase. ‘TR’ means transition region (from ref.[22]).

is a rotational

At low coverages the repulsive dipole-dipole interactions between the alkali adspecies are dominating. This leads to the stabilization of a structure with the largest possible distance between the adspecies. The adsorption sites are determined by the surface potential contour and in most cases these are the sites with the highest surface atoms coordination number (e.g. three-fold hollow sites on fcc (111) and hpc (0001) planes, or four-fold sites on fcc (loo), fcc (110) and bcc (100)).The mean mutual distance decreases with increasing surface density which also leads to occupation of less favourable adsorption sites. This subject will be discussed in greater detail in Subsection 4.1.2. Table 4.1. summarizes several selected data on the alkali induced superstructures observed on different substrate crystallographic planes at room temperature. Typical of the closed packed fcc(lll) , hpc(0001) and bcc(ll0)

4.1. Electropositive Additives: Alkali Metals

21

planes is the formation of hexagonal super-structures.

Fig. 4.2. Model structures for the different phases of Na on Ru(0001). Open circles represent Na atoms in symmetric sites. Hatched circles are Na atoms in asymmetric sites (from ref.[22])

Obeying the requirement about a minimum energy of the system, in some cases the highest coordinated sites are abandoned in order to form a structure where the repulsion between the species is the weakest. Examples of surface orders where various structures appear as a result of the phase tran-

22

Chapter 4.

i

CslRh (1001 LATTICE GAS

(333

t

5) K

- 300Y

0, L

3

c

e 200-

(155

OI

g :

e!

(145 t 5 ) . ..... ... .

loo-"

0

0.1

0.2

0.3

0.L

05

Cs Coverage

"r.1 o=f I3.21-ZCS

Fig. 4.3. Phase diagram, schematic LEED pattern and real space models for Cs on Rh(100) with increasing Cs coverage (from ref. [8])

4.1. Electropositive Additives: Alkali Metals

23

sitions with increasing alkali coverage are shown in figs. 4.2. and 4.3. Phase transitions leading to the conversion from commensurate to incommensurate superstructures are likely at high alkali coverages. Table 4.1. Ordered Superstructures and the Corresponding Alkali Coverages, @ A M , (in ML - adatoms per substrate surface atom) for Several Single Crystal Metal Planes a t T = 290 K. In brackets the surface atomic density in atoms.cm-' for each surface plane is given.

SURFACE fcc Ni(100) (1.6 x Na/Ni(100)

OAM

0 0.25 0.5 I
STRUCTURE 1x1 P(2 x 2 ) c(2 x 2 ) P(2 x 2 ) hexagonal-incommensurate hexagonal-incommensurate 1x1 disordered & x &)R30° 1 x 1 1 / 3 x 1/3 &)R30° (& x &)R30° 1x1 incommensurate in [1210] c(2 x 2) incommensurate in [310] c(2 x 2 ) incommensurate in mi01

(Ax

In their thorough studies on the structures of alkali metal overlayers and the driving forces of the surface phase transitions, Naumovets et a1 [9, 14, 161 have shown that even at temperatures as low as 100 I< the surface mobility of the alkali metals is very high so that the format>ionof ordered structures is not hindered kinetically. The general trend in the changes of the diffusion coefficient is a decrease with increasing coverages but, as is illustrated in fig. 4.4., this decrease is rather nonmonotonous and correlates with the structural changes occurring in the overlayer. An important conclusion from the alkali diffusion studies is that despite the alkali coverage and the crystallographic orientation, the diffusion coefficient and the type of the ordered structures are strongly dependent on the nature of the substrate. For example, for the same crystallographic planes with almost identical lattice constants, such as M o ( l l 0 ) and W(110), the phase diagrams and the surface diffusion rates for Li differ substantially [16]. This fact demonstrates the importance of the electronic surface structure in the formation of ordered overlayers.

24

Chapter 4.

Fig. 4.4. Plots of lg D versus Li coverage on W(Ol1) at different temperatures:(l) 130 K; (2) 175 K; (3) 200 K; (4) 225 I< (from ref. [9])

Consequently, the numerous structural data on the alkali metal-met,al systems provide evidence of the major factors contributing to the formation of ordered alkali metal structures are the adsorption temperature, the actual alkali coverage (which will be shown further on determines the changes in the strength of the adsorption bond and the dipole-dipole interactions) and the geometric and electronic structure of the surface. At low alkali coverages, the surface overlayer is built by positively charged species and the surface - adparticles bond is characterized by a large dipole moment, p. In these layers, the dipole-dipole repulsions are with an energy [17]:

These repulsions appear t o be the major interaction force. Because of the significance of these repulsive forces under the usual conditions of the catalytic reactions (room or elevated temperatures) alkali metals are uniformly distributed on the surface at low sub monolayer coverages and no ordered structures are observed. In addition to these dominating dipole-dipole interactions indirect interactions occurring via substrat,e exist as well. They are characterized by an energy term [18]:

u, s C O S ( 2 k F l . ) / d ,

(2)

where r l " ~is the magnitude of the Fermi momentum, and t varies from 1 to 5 depending on the shape of the Fermi surface in the considered direction. These

4 . 1 . Electropositive Additives: Alkali Metals

25

interactions are propagated through the electron and phonons and have an oscillating and anisotropic character. A recent opinion is that the substrate mediated interactions can superimpose with the dipole-dipole ones [15]. The resulting potential energy-oscillating contour seems to play an important role in the determination of the most favourable surface order at a given coverage. This explains, t o a certain extent, the dependence of the alkali metal surface order on the substrate nature and the crystallographic orientation. The influence of the crystallographic plane anisotropy on the rate of the surface diffusion of the adsorbate and the strength of the surface mediated interaction forces is the reason for the difference in stability of the ordered alkali superstructures on the different crystallographic planes. A typical example in this respect is the comparison of the hehaviour of alkali adsorbed on closedpacked ‘isotropic’ surfaces (e.g. fcc(ll1) and ( l o o ) , hpc(0001), bcc(ll(i), etc.) and open ‘anisotropic’ surfaces (e.g. fcc(llO), hpc(lOiO), bcc(ll2), etc.) [7]. The latter consist of closed-packed rows of atoms in one of the crystallographic directions spaced by troughs. Because the lateral interactions between the adsorbed species on these surfaces with anisotropic potential relief are also anisotropic, this usually leads to stabilization of ordered structures at higher temperatures and lower coverages. Because of the preference of the adsorption sites in the troughs, the saturated first layer on these surfaces tend to conform with the substrate by the formation of an incommensurate structure with alkali adspecies compressed in rows along the troughs (fig. 4 . 5 . ) .

Fig. 4.5. Model of surface structures of alkalis on fcc(llO), hpc(lOT0) and bcc(ll2)

A transition from commensurate alkali surface phases to incommensurate ones occurs at higher coverages where the dipole-dipole interactions are weakened as a result of depolarization effects due to the elect
26

Chapter 4.

transitions in the alkali layers is the necessity to balance the lateral interactions by shifting the adatoms to less favourable adsorption sites. The saturated first alkali layer consists of adspecies with negligible polarization. With a few exceptions, the saturated first alkali layer is characterized by the formation of hexagonal lattices. The shortest distance between the adjoining adatoms of the saturated first layer is often less than that corresponding to the alkali bulk lattice constant. This compression is generally higher for the 4 % for Na [8, 131. larger alkali atoms, e.g. up to 24 % for Cs but only This compression is observed for both commensurate and incommensurate structures. In recent theoretical considerations, the assumption is made that the contraction of the alkali layer is related to the formation of 2 D electron bands and polarization of alkali semi-core levels, i.e direct adatom-adatom interactions become important. These compression effects as well as other properties of the first alkali overlayer indicate that its properties are affected significantly by the substrate. The influence of the substrate nature on the alkali adatom surface order is clearly demonstrated when comparing the ordering tendencies in the alkali/transition metal and alkali/free electron metal systems [7-10, 19, 201. In the first case the electrostatic repulsion remains predominant throughout the monolayer growth, whereas in the second case 2D condensation readily occurs even at low alkali coverages above a certain adsorption temperature ( w 300 K) [19, 201. This tendency of clustering indicates t,hat the electrostatic interactions between the alkali adatoms on free electron metals can be easily overcome by cohesive interactions due to the overlaps of the wave functions between neighbouring adspecies. These results indicate that the horizontal corrugation of the surface potential relief is weaker in the case of the free electron metals. As will be discussed in the forthcoming subsections, besides the reduced barrier for surface diffusion, the enhanced tendency to clustering correlates with the measured smaller effective charge (dipole moment) and adsorption binding energy for alkali adspecies on electron free metals. Here, it is worth mentioning that the absence of a surface order and the strong trend to the formation of clusters characterize alkali-layer growth on alkali oxide (a-alumina) surfaces [19]. The tendency towards formation of ordered alkali overlayers at certain coverages is very helpful for the calibration of the absolute alkali coverage. According to the proposed structural models, the corresponding coverages at first layer saturation are determined exclusively by the size of the adatom and the crystallographic orientation of the surface.. For example, for fcc (111) planes of P t , Pd, Ni etc., the saturation coverage for ( f i x &)R30 structure of Cs and K is 0.33, whereas for the incoherent hexagonal structure of N a it is 0.5 [7]. Building alkali metal multilayers is possible when the substrate temperature is lower than the temperatures at which alkali metals sublimate. Condensation of the alkali metal in the second layer can still be accompanied by the formation of ordered structures, whereas further multilayer growth usually does not occur in an ordered epitaxial way [21-241. Since alkali multilayers already behave as pure alkali metals, they will not be considered in this review.

-

4.1. Electropositive Additives: Alkali Metals

27

."p.

0.1 0.2 UESIUM COVERAtE IHL)

0.3

Fig. 4.6. The variation of integrated intensity of selected half order beams from Cs induced (1 x 2)-MR structures of Ag (110) (full line) and Pd (110) (dashed line) (from ref. [61]).

B. Alkali induced surface reconstruction. Up to here only cases when alkali adspecies form various overlayer superstructures (in registry or not with the surface lattice) without affecting the original surface structure of the substrate have been considered. This is however not the case with all substrates. Depending on the nature of the substrate and the crystallographic plane, alkali metal adsorption also causes structural modification of the surface. These effects are very important with respect to the description of the induced changes in the properties of the substrate in the presence of modifiers. Recently a number of studies have shown that (110) faces of several fcc d-band metals (Ag, Pd, Cu, Ni) suffer a reconstruction in the presence of adsorbed alkali metals [7, 24-26]. This reconstruction requires some activation energy so that, at low adsorption temperatures (- 100 K), alkali adsorption leads t o the formation of different phases with increasing coverage without any modification of the (1 x 1) unreconstructed substrate. The onset of the alkali induced (1 x 1) to (1 x 2) reconstruction of the above-mentioned fcc(ll0) metals can be observed at different temperatures determined by the actual

28

Chapter 4.

alkali coverage. The missing-row model (every second (110) closed packed atomic row is missing) has been accepted for explaining the substrate surface atomic arrangement after the reconstruction [27-291. A rather interesting fact is that the reconstruction induced by the alkali metal is favoured only over a limited range of alkali surface concentrations, which are substrate dependent (see fig. 4.6). A restoration of the initial (1 x 1) substrate structure takes place at high alkali coverages. It has also been established that, for the same substrate, the critical coverages required to drive the reconstruction increases in the sequence Cs, K, N a [25, 301, i.e. the larger alkali atoms are more active. Another finding is that only fcc metals with partly-filled or filled d bands show a trend to reconstruction, whereas other similar two-dimensionally channeled surfaces such as hpc (1010) Ru or Co are stable in the presence of alkali metal. The ease of reconstruction of the above mentioned (110) faces can be ascribed to the small energy difference of the (1x 1) and (1x 2) unit cell, so that slight perturbations induced by the presence of an adsorbate can cause the reconstruction [24]. The transition from (1 x 1) to (1 x 2) surface structure is found to be an activated process, associated with a surface diffusion process of the surface atoms and adspecies. As a result of this surface diffusion, islands of (1 x 2) reconstruction are formed which grow in size within the corresponding alkali coverage range. Several explanations of the driving force of the alkali induced reconstruction are proposed. They account on effects due to the perturbations in the substrate surface electronic structure as a result of the the formation of the alkali - surface adsorption bond and/or variations of the dependence of the alkali - surface binding energy on the surface structure with increasing alkali coverage [25, 261. Another possible effect of the alkali adsorbates on the substrate surface structure is alkali induced stabilization of the reconstruction of a clean surface or lifting of the original reconstruction at certain alkali coverages. An example for the first case is the system Cs/W( 100) where the clean substrate exhibits a zig-zag chain reconstruction which is stabilized by the presence of Cs [8, 31, 321. An example for the second case is the K/Ag(llO) 2 x 1 system where at high K coverages the original 2 x 1 reconstruction is lifted [33].

4.1.2

Nature of the Alkali Adsorption Bond

A. Alkali induced work function changes. The first attempt to -1iaracterize the interaction of the adsorbed alkali atoms with the substrate surface and within the adsorbate overlayer are linked to measurements of the work function changes in alkali/transition metal systems [2G]. The characteristic work function changes during alkali metal adsorption on transition metal surfaces (A4 vs BAM) are illustrated in fig. 4.7. [34]. The A d ( 0 A M ) curves exhibit a strong initial decrease to a minimum followed by a rise to a near bulk alkali metal work function value at completion of the first overlayer. This shape of the A + ( e A M ) plots for transition metal surfaces is independent of temperature up to adsorption temperatures below the onset of desorption of the first alkali layer (- 300-350 I< depending on the type of the alkali metal). The initial slope of the &(eAM) plot can be used for the evaluation of the initial dipole moment due to the adsorbed alkali species.

4.1. Electropositive Additives: Alkali Metals

29

0 1 - K/Ru100011 2 - Cs/Ru(0001) 3 - K/Ruf IOiO) 4 - Cs/RullOiO)

-1 2

-

- -24

% I

8

a

-3 5

- 4 81

0

15

3 Alkali coverage

4.5 (X

6

75

lO''at/cm')

Fig. 4.7. W F changes, &#I, as a function of I< and Cs coverages for Ru(0001) (1 & 2) and for Ru(1070) (3 & 4) (from ref.[34])

Selected data of the calculated initial dipole moments, pol and the maximum work function changes, Aq5,,,, for several substrates and alkalis are given in Table 4.2. A full set of the available work function data for alkalifmetal adsorption systems can be found in refs. [26] and [36]. Assuming that in the case of alkali adatoms, (because of the large polarization) the measured work function changes are determined exclusively by the external electrostatic dipole moment, one can evaluate roughly the effective positive charge on the adsorbed alkali atom at a zero coverage using the simple relationship: cloy. d , (3) where d is the dipole length and y is the electronic charge. Usually d is determined on the basis of the substrate lattice constant, the ionic alkali radii and the assumption about occupation of the highest coordinated adsorption state at low alkali coverages. Thus, for N a , K and Cs on Ni(100) the following values were estimated q = 0.41,0.58,0.78 e- [35], respectively, for Na, I<, Cs on Ru(0001) q = 0.44,0.67 and 0.82 e- [44], respectively, and for I< on Pt(ll1) - q = 0.86 e-. The important piece of information carried by these approximate estimations is that even within the limits of very low coverages the alkali adatoms are not fully ionized. The general trend i n all work function data is that the induced dipole moment and the absolut,e values of A&,,, increase with decreasing ionization potential of the alkali metal. Besides, the po and q5,,

30

Chapter 4.

Table 4.2. Initial dipole moments, 2p0, maximal work function changes, A&,,, and initial heats of adsorptions, A H o , for adsorption of Na, K and Cs on different substrates.

SURFACE

2po(D)

AHo (kJ/mole)

6.8

242

3.2

13.5 21.0 7.8 14.2 20.4 5.8

295 325 258 297 322 255

3.4 3.6 3.5 3.9 4.1 3.15

10.2 14.6 10.5

275 305

3.4 3.6 3.7

A&,,,

(eV)

('#'abs)

Na/Ni(100) (5.2) K/Ni(100) Cs/Ni(100) Na/Ru( 000 1) I
18.8 3.2

-

-

180

4.6 2.0

(4.5) I
values depend on the substrate: they are larger for surface faces characterized by a higher clean surface work function. These large work function changes, observed upon alkali adsorption, are first explained assuming that the adsorption state of the alkali species is ionic [5]. According to the first theoretical considerations [45] it is assumed that at low alkali coverages, as a result of electron transfer t o the substrate, the valence ns-electron levels of the single alkali adatom are broadened in resonances and shifted above the Fermi level. T h e energy and width of this resonance varies with increasing alkali coverage because of the depolarization field of the surrounding ad-dipoles. As will be discussed in more details later, the experimental results exclude the existence of completely ionized alkali adatoms, i.e. within the framework of the valence resonance model one should take into account that even at the lowest coverages the low energy tail of the resonance extends below the Fermi level. That

4.1. Electropositive Additives: Alkali Metals

80

31

7

I . ,

b

1 No

2 K 3 cs

8

Fig. 4.8. (a) The changes of the alkali adatom dipole moments as a function of the alkali coverage; (b) The coverage dependence of the alkali desorption energies for Na, K and Cs adsorbed on Ru (0001) (ref.[37])

is why it is appropriate to introduce the term degree of ionicity of the alkali substrate adsorption bond. It accounts for changes in the contribution of the ionic component of bonding with increasing alkali coverage. The changes in the dipole moments due to the adsorbed alkalis estimated on the basis of the A4 vs OAM plots axe shown in fig. 4.8a. They can be satisfactorily fitted to eq. (9) in Chapter 2., which accounts for the induced decrease of the dipole moment with decreasing effective distances between the ad-dipoles as a result of strong depolarization effects. It is logical to expect that the strong repulsive interactions and the high mobility of the alkali adatoms will favour uniform distribution on the surface at room temperature at low coverages. For flat surface planes this assumption is confirmed by both LEED (see the phase digrams in figs. 4.1-4.3.). and WF studies; the shape of the I-V characteristics is uniform. For corrugated surface planes (e.g. Ru(1010)) both LEED and WF data (appearance of a shoulder in the I-V curves in fig. 4.9.) indicate a coexistence of islands of different surface superstructures (ordered and disordered alkali phase) [34, -

32

Chapter 4.

41, 431. The analysis of the evolution of the shape and the position of the retarding potential curves suggests development of small ordered domains and fairly inhomogeneous distribution of the alkali adatoms on the remaining surface. This result about a possible inhomogeneous distribution of the alkali adatoms is important because it explains why the absolute work function changes depend in some cases on the experimental method used for measuring the work function. This result for homogeneity of the alkali layers on flat and corrugated transition metal surfaces can be related t o the real catalysts which exhibit an anisotropic surface structure. It implies that, because of the possible uneven distribution of the promoter, the rate of the catalytic reaction might vary along the surface.

0

2

r,

2

6

Voltage lev1

Fig. 4.9. Normalized retarding potential curves for increasing coverages of (a) K on Ru(0001) with 8K: 1 - 0, 2 - 0.03, 3 - 0.07, 4 - 0.36; (b) K on Ru(lOT0) with OK: 1 - 0, 2 - 0.03, 3 - 0.13, 4 - 0.20, 5 - 0.35, 6 - 0.66. (from ref. [34])

For electron-free metal surfaces, the alkali induced work function changes are temperature dependent. They show the typical patterns observed on transition metals only for low adsorption temperatures [20] (see fig. 4.10.), whereas at room temperature above a critical (relatively low) alkali coverage the Aqj(0AM) dependence becomes linear. As is noted in Subsection 4.1.1., this temperature effect on the surface potential versus the alkali coverage dependence is due to the induced 2D condensation of the alkali adspecies above critical surface coverages. As is obvious from the data in Table 4.2., the initial dipole moment due to the same alkali is smaller for an electron free

4.1. Electropositive Additives: Alkali Metals

I

I

0

.

1

1

1

. 2 1 A Jodlum Cmvamge 0..

I

I

d

1

33

1 2

Fig. 4.10. K induced work function changes for adsorption at 100 K (open circles) and after annealing to 350 K or upon K adsorption at 350 K (open triangles) on A1 single crystals (from ref.[19])

substrate. This implies a lower degree of ionicity of the alkali - free electron metal bond compared t o the alkali - transition one. B. Spectroscopy data and their relevance to the charge redistribution as a result of alkali adsorption. The change in magnitude of the effective charge associated with the alkali species (assumed to be a measure of the contribution of the ionic component to the alkali - surface bonding) was confirmed by a couple of spectroscopic data. Photoelectron spectroscopy data demonstrate a shift of the core levels of the alkali metal adatoms to lower binding energies due to the depolarization effects with increasing alkali coverage. Depending on the type of the alkali and the substrate, the absolute value of this energy shift is in the range 0.3-0.8 eV [21, 26, 461. The direction of the alkali core level binding energy shift is in general agreement with the decreased charge transfer after the increase in alkali coverage. The increase in population of the alkali valence ns levels gives an intra atomic repulsion which pushes the core level electron energy to lower energies. It should be taken into account that the measured magnitude of this shift is affected by

34

Chapter 4.

initial and final state effects due t o the changes in relaxation. These many body effects are more likely the reason for the existing variations in the alkali core level shifts for some alkali/metal systems. For example, the Cs 5p energy for Cs/W(OOl) remains constant [47], whereas for Cs/Mo(001) even a higher energy shift of the Cs 5p core levels is observed with increasing Cs coverage

WI. More uniform are the

data on the core level shifts obtained by EELS, where the energy loss for excitations from ( n - 1)p to empty ns valence levels are measured because these intra atomic excitations should be less affected by the many-body effects. For all studied alkali/metal systems, the core level loss spectra exhibit an initial energy decrease up to coverages corresponding to the minimum of the A4(0,,) plot and levels off with further increase of the alkali coverage [26, 37, 48-51]. It should be pointed out that the absolute values of these energy shifts are much larger than the ones observed for the 1.5 eV for N a 2 p 3 s , core level shifts by photoelectron spectroscopy, e.g. 2.1 eV for K 4 p 5 s and 2.5 eV for Cs adlayers on Ru(0001) [36]. From the relation between these energy loss shifts and the charging state of the adatom, Anderson and Jostell [35]estimated that even a t very low coverages the positive charge on the adsorbed alkali atom is less than unity. The core level binding energies characteristic of the first alkali overlayer remain lower (- 1 eV) than that of a bulk like multilayer alkali film. Since this binding energy difference is mainly due to initial state effects, this indicates that the properties of the first alkali adlayer should be different from that of an alkali bulk film. Direct information about the position and the occupancy of the alkali adatoms valence resonances is obtained by metastable quenching spectroscopy (MQS) [52,53], UPS [54,55] and electron energy loss spectroscopy (EELS) [26]. MQS data showed the existence of alkali induced peaks associated with ns valence states even at the lowest alkali coverages. This result, together with the EELS data where one electron ns-np transitions are also detected at low alkali coverages, indicate that alkali metal adatoms should be considered as polarized rather than ionized. The coverage dependent change in electronic structure of the adsorbed alkali species involving a decrease of the polarization is evidenced by all spectroscopic data under consideration. At higher alkali coverages when a dense metal-like layer is built, the linear dependence between the MQS alkali 11s iiiteiisity and the alkali coverage is violated and the signal becomes narrow [52]. The UPS spectra s h o w a rise of narrow ns-derived peaks near the Fermi level, whereas the one electron ns-np loss peaks shifting initially to smaller energies change their character and transform into collective plasmon excitations in the metal-like overlayer

-

WI.

-

-

IPS is used as a probe of the behaviour of the unoccupied alkali adatom states just above the Fermi level, i.e. the n p states which participate as final states in one electron ns-np intra atomic transitions. The general trend in the energy position of these empty states up to moderate alkali coverages (shown in fig. 4.11) [26] is in quantitative agreement with the EELS data which predict a larger downward shift of the unoccupied valence states than is the case of the occupied states [26].

4 . 1 . Electropositive Additives: Alkali Metals

35

-HIAg(111)

(IPS) WCu(100) (EELS and A@ )

Fig. 4.11. The energy position of the I< 4p,-derived state for I
All spectroscopic data together with the observed work function changes are a strong confirmative indication that the adsorption state of the alkali adatoms changes significantly with increasing alkali coverage. On the basis of the data summarized above one should distinguish between the low coverage adsorption state when the alkali - surface bonding is strongly polarized (which is associated with a larger contribution of the ionic component t o the bonding) and to a much less polarized metal-like bonding at saturation of the first overlayer. The transformation (according to the data) from a ‘strongly polarized’ to a ‘metallic’ state is not abrupt, which means that the contribution of the ionic component gradually decreases in the intermediate coverage region.

36

Chapter 4

Since all transition metals exhibit the same type of Aq5(0,,) plots, it can be generalized that for the same substrate crystallographic plane the ionic contribution at low alkali coverages increases with decreasing ionization potential of the alkali adatom. Judging from the initial dipole moments and the tendency for 2D condensation, the contribution of the ionic component (degree of polarization) for the same substrate is smaller in the case of free electron substrates. C. Strength of the alkali adsorption bond at various alkali coverages. The coverage-induced changes in the character of the alkali - surface bonding affect strongly the adsorption binding energy and the alkali - surface bond lengths. This is very well illustrated by the thermal desorption alkali spectra for different alkali coverages shown in fig. 4.12.and the E d vs AM plot in fig. 4.8.(b), obtained on the basis of the TPD data. Since for an non activated adsorption process as is the case of alkali adsorption on metal surfaces, the desorption energy, Ed, is equal to the heats of adsorption, AH, the Ed data can be used as a measure of the adsorption bond strength.

Fig. 4.12. I< TPD spectra for various I< coverages on Ru(0001) (from ref. [44])

As can be judged from the data in fig. 4.8(b)., the variations in the alkali adsorption binding energy are rather large. The absolute changes with the built of the first alkali become larger with decreasing ionization potential of the alkali metal. The same trend is exhibited by the values of the initial heats of adsorption. As can be seen in fig. 4.8(b). and table 4.2. the absolute values

4.1. Electropositive Additives: Alkali Metals

37

of the initial heats of adsorption, A H A ~correlate , with that of the initial dipole moment, PO. Since PO relates to the contribution of the ionic component to the adsorption bond, this means that the latter plays an important role at low alkali coverages. Its contribution is dampened with increasing alkali coverage leading to the observed significant weakening of the adsorption bond. However, the adsorption energy measured on completion of the first alkali Layer remains higher than that corresponding to the sublimation energy of the bulk material. This supports the spectroscopic results that within the first layer the alkali ns-valence electrons remain involved in bond formation with the substrate surface. The variation of the nature of the alkali - surface bonding associated with the transition from a ‘polarized’ to a ‘metallic’ state affects also the bond lengths. As has been shown by recent SEXAFS studies of the system Cs/Ag(lll), the bond length increases from 3.2 to 3.5 A a s a result of weakening of the adsorption bond [58]. Although the trend in the coverage induced changes of the strength of the alkali adsorption bond is the same for all alkali - metal systems, there is a difference in the absolute values of the initial heats of adsorption for transition and free electron metals. The smaller alkali adsorption energies for free electron metals correlate well with the measured lower initial dipole moments. This result leads to different theoretical approaches to the description of the properties of alkali adlayers on metals. All recent realistic models exclude the existence of completely ionized adparticles whose valence electrons are delocalized into the substrate metal. An appropriate theoretical approach (using the simple jellium model) of alkali adsorption on free electron met,als ( A l , Cu) w a s first proposed by N.Lang et al. [59,60]. Further verifications, done by many other authors, can be found in refs. [61-661. Within the limit of low alkali coverages the electron-density contours corresponding to chemisorption of one adatom on a high-density jellium substrate indicate that the adatom electron density is displaced from the vacuum to the interface side. As a result, the corresponding ns-valence resonance of the alkali adatom becomes almost empty, which is interpreted as a charge transfer t o the substrate and the formation of an ionic type bonding at very low coverages. Recent theoretical studies of alkali adsorption on Al(001) [64] exclude charge transfer to the substrate. They describe the bonding and the observed coverage effects by assuming hybridization of adatom and substrate surface states and variation of polarization with increasing alkali coverage. For the case of a transition metal substrate which surface electronic structure is more corrugated because of the spilled out d-like states, the alkali adsorption bond is described by E. Wimmer et al.as a polarized covalent one [65, 661. The formation of this bond involves mainly alkali ns-valence states and s , d-like surface states of the substrate (Mo or W). This theoretical covalent bond model suggests a smooth variation of electronic features with increasing alkali coverage. Irrespective of the coverage, the alkali adatoms preserve partial occupation of the ns-derived states with a tendency towards increased polarization per adatom at lower coverages. It is worth pointing out that within the framework of both theoretical approaches one can explain

38

Chapter 4

satisfactorily most of the experimental results. D. Range of the alkali induced charge redistribution within the surface layer (local work function)”Obviously, the formation of an alkali adsorption bond and the induced changes in the electrostatic potential cause a charge redistribution within a certain surface region. Since these are the major alkali induced modification effects on the substrate surface, it is of great importance to know the lateral range of this effect. The theoretical models generally suggest a rather localized character of the alkali - surface bonding and the charge redistribution. The lateral range of perturbation of the substrate surface electronic structure due to the adsorption of an alkali atom or the so called ‘local work function’ has been recently probed by photoemission studies of noble gas adsorption on alkali modified metal single crystals [67-691. These studies are based on the simple principle that the noble gas (Xe) core level binding energies with respect to the vacuum level, EB(V)are independent of the substrate because of the weak Xe/metal interaction and the large size of the Xe adatom. Taking into account that EB(V)is a sum of two quantities: EB(v)= EB(F) 4,’ (4) (where EB(F)is the core binding energy with respect to the Fermi level and 4sis the local surface potential of the adsorption site) it becomes obvious that any changes in +s will cause changes in EB(F).This picture of a floating potential well of the adsorbed noble gases with the vacuum level has been also predicted theoretically in the calculations of Lang et a1 [70] for adsorption of a single Xe atom on a jellium surface. Thus. the difference in the,measured by photoelectron spectroscopy, E g ( F ) levels for Xe(5p,12) has been successfully used for the determination of the work function difference between the surface regions of affected and nonaffected by [67]. Fig. 4.13. presents the structure of a Xe-K coadsorbed layer for Ru(0001) covered with small amounts of K and a set of photoemission Xe(5p3/2,1/2) spectra. These results indicate that the state in which Xe next to I< is populated first, followed by occupation of the surface site next-nearest to an alkali adatom. According to the measured Xe 5p binding energies, one can draw the conclusion that the local electrostatic potential of the ‘promoted’ state next to K is much lower than that of the next nearest state. For K on Ru, the local work function decrease confined within -8 A around the K adatom is found t o be of the order of 0.5 eV, whereas beyond this sphere of 8 A, the so called long range work function is considerably smaller - 0.08 eV. This determination of the alkali-induced local work function changes on an almost atomic scale is an important finding for further coadsorption studies. Obviously, the introduction of an alkali modifier onto the metal-catalyst surface induces a significant non uniformity of the surface with respect to the spatial charge distribution. This implies that the strength of the promoting effect on the coadsorbed reagents or reaction products will differ significantly when changing the alkali - coadsorbate separation in the overlayer.

+

- -

4.1. Electropositive Additives: Alkali Metals

39

XelKI

Fig. 4.13. Structural model for Xe adsorbed on a 0.05 K/ Ru(0001) surface and the Xe ( 5 ~ ~ / ~ , , photoemission /2) spectra for increasing Xe coverages on O.OSK/ Ru(0001) (from re€. [67])

4.1.3

Summary for Alkali/Metal Adsorption Systems

The most important results contributing to the description of the behaviour of alkali atoms adsorbed on metals can be summarized as follows: 1. The interaction of alkali metals with both transition and free electron metals show the same general features: a strong initial work function decrease t o a minimum followed by a rise to a value close to the corresponding alkali bulk value on completion of the first overlayer;

coverage governed significant changes in the charge of the alkali adatoms due t o depolarizatioii taking place with decreasing separation between the alkali adatoin dipoles; a significant decrease of the alkali adsorption energy with increasing alkali coverage within the first layer; formation of ordered superstructures favoured at higher alkali coverages when the repulsive dipole-dipole forces are reduced; a gradual reduction of the contribution of the ionic component to the alkali adsorption bond, reflected by an increase of the occupancy in the alkali ns-valence levels with increasing alkali coverage

40

Chapter 4 and the formation of a 2D ns-band with a nearly free electron gas on completion of the first layer.

2. The ionicity and the strength of the alkali adatom - substrate bond depend on three major factors: the kind of alkali (its size and ionization potential), the nature of the substrate, and the actual alkali coverage. For the same alkali coverages they are larger for alkalis with a higher atomic number and for transition metal surfaces.

3. The kind of change of the adsorpt,ion state which is a function of coverage. It is characterised by ‘a strongly polarised’ bonding at low coverages and moves t o a ‘metallic’-like bonding at high coverages. This change in the bonding character is accompanied by an increase of the alkali surface bond length. 4. The lateral range of the alkali induced charge redistribution within the surface is rather short. The strong local changes are confined within the next nearest adsorption sites so that at sufficiently low alkali coverages one can distinguish between strongly ‘promoted’ and almost unaffected sites beyond a certain distance from the alkali adatom.

4.1.4 Alkali Adsorption on Semiconductor Surfaces Finally, some information is given on the behaviour of the alkali/semiconductor adsorption systems. The author believes that this will be useful t o the reader who wishes to evaluate the influence of the substrate surface electronic structure on the nature of the adsorption bond. Most of the semiconductor surfaces are characterized by active surface dangling bonds directed outwards and a bulk band gap. This suggests the important role of the hybridization between the adatom and the substrate surface states during the formation of the adsorption bond. It should be pointed out that besides the fundamental interest, the alkali adatoms can serve as promoters for the oxidising and nitriding semiconductor surfaces [71-741 . When describing the alkali - semiconductor systems a great deal effort has gone into the question whether the alkali adatom is bound in an ionic or in a covalent manner. The experimental data show similar work function changes as in the case of alkali - transition metal systems with exceptions of several systems where distinct minima in the work function curves are lacking [75]. Some differences were also observed in the EELS spectra where the 2D ns-band collective excitations did not appear on the completion of the first overlayer [76]. The photoelectron studies also support the views that there is a lack of metallization in some systems on completion of the first alkali overlayer [77]. On the other hand, the MQS data indicate that similarly to the case of the metal substrates, Cs 6s-derived levels exist at all Cs coverages with an abrupt increase in their charge density above a certain critical Cs coverage [78]. The alkali adsorption bond for most of the semiconductors is found t o be of the same strength as that measured for transition metals, but it depends t o a minor extent on alkali coverage [7G, 791. This seems unexpected because

4.2. Elect ronega t i ve Ad di t i ves

41

the depolarization effects with increasing alkali coverage should be larger for semiconductor substrates where the screening is much less effective than in the case of metals. A plausible explanation of the behaviour of alkali - semiconductor systems is that for substrates with active surface states projected outside the surface, a central role in the formation of the alkali - surface bond is played by the hybridization between these active surface states and the alkali valence states. The adatom dipole moment is interpreted as a result of polarization of the covalent bond [80]. This model of alkali - semiconductor bonding is very similar t o the strong polarized covalent bonding proposed by E.Wimmer et al.for alkali - W (Mo) [65, 661. Indeed, these transition metals also possess spill out d-like surface states. However, because of the more active dangling bonds of the semiconductor surfaces the coverage induced weakening of the alkali - semiconductor bond is smaller, which explains why in most cases no metallization is achieved on completion of the first layer. 4.2

ELECTRONEGATIVE ADDITIVES

The quantitative interpretation of the extent, strength and nature of the deactivation effects induced by the presence of electronegative additives, requires a fundamental knowledge of the structure and bonding of the additives on the catalyst surface. This knowledge is necessary in order to explain the contribution of the so called geometric (e.g. site blocking) and electronic (redistribution of the valence electron charge density of the adsorbate and substrate atoms) factors t o the observed changes in the reactivity of the surface. 4.2.1

Surface Structures, Site Occupation and Bond Lengths

Structural information regarding the geometry of the adsorbed layers of electronegative additives has been obtained already on various single crystal metal surfaces [7]. A variety of techniques, such as the LEED analysis, ion scattering, ARUPS, SEXAFS, SEELFS (surface estended energy loss structure) and HREELS, have provided accurate information on the coverage dependence of the adsorbate surface order, the adsorption sites, the adsorbate - substrate atom bond lengths and the adsorbate-induced reconstruction of the substrate surface. It is well known that the trend towards surface ordering depends on the types of interactions between the species on the surface. As will be discussed in the forthcoming subsection, in most cases the electronegative atomic adsorption is characterized by the formation of rather strong bonds of a covalent type. A. Ordering of the electronegative additives at various coverages. Since sulphur and other chalcogens are the most common poisons in real catalysis, recently many papers have appeared dealing with the crystallographic structure of the cheinisorbed states of these atoms on different transition metal single-crystal surfaces. Selected data concerning the observed S structures, site occupations and the measured bond lengths, L , on different crystallographic planes of several metals which are important in catalysis, are given in table 4.3. For most of the systems considered in table 4.3. the bond

42

Chapter 4

Fig. 4.14. LEED and TPD data from Pt (111) saturated with HzSe and subsequently annealed (from ref.[113])

lengths are for S coverages where S is in an adsorbed state residing in the highest coordination sites on the surface. The data in Table 4.3. [81-1111 show that for unreconstructed fcc ( l l l ) , (100) and (110), bcc(ll0) and (loo), the p(2 x 2) overlayer superstructure is formed a t an S coverage of 0.25. It should be pointed out that usually p(2 x 2) patterns appear at sulphur coverages far below 0.25. This indicates that S tends to form long-range ordered islands so that the substrate surface at 0s < 0.25 should be regarded as a mixture of p(2 x 2) S islands and S-free regions. As the coverage increases, these islands grow and form either larger domains if they are in-phase, or domain boundaries if they are out of-phase. As reported recently [112], in such a domain structure of a sulphur p(2 x 2) overlayer on Cu( loo), the local geometric structures of the S adsorption sites within the p(2 x 2) domains and at the domain boundaries are different. This is ascribed to different substrate surface reconstructions at the domain boundaries and the domains. Such nonuniformity of the modified surface might introduce certain difficulties in obtaining a clear picture of the modifier effects on the reagent gas adsorption at lower modifier coverages. With increasing S coverage beyond 0.25, a mixture of p(2 x 2) and c(2 x 2) or x &)R30° patterns exist till completion of the final c(2 x 2) order at Bs = 0.5 for the fcc(lOO), bcc(100) and fcc(ll0) planes, x 6 ) R 3 0 ° order at 0s = 0.33 for the f c G l l l ) or hpc(OOO1) and the (fi

(a

43

4.2. Electronegative Additives

Table 4.3. Sulfur Surface Structures, Adsorption Sites and Adsorption Bond Lengths ( L ) on Some Transition Metal Single Crystal Planes. In brackets the bond lengths of some bulk sulfides are given for the sake of comparison. ~

L

(A)

METAL

OVERLAYER ORDER

ADSORPTION SITE

Ni(ll1) [82-871 Pd(ll1) ~8,891 Rh( 111) [go, 911 Pt(ll1) [92-941 Ir(ll1) 1951 Ni( 100) [96, 971 Pd(100) ~981 Rh(100) [99, loo] P t ( 100) p o l , 1021 Fe( 100) ~091 Ni(ll0) [103,104] Rh( 110) [99, loo] Pt(ll0) [105-1071 Fe( 110) [lo81 NisFe(ll0) [110,111]

p(2 x 2) -+ ( A x &)R3Oo -+ 5& x 2 +8fix 2 p(2 x 2) x &)R30° -+ hexagonal ( & x &)R3Oo p(2 x 2) 4 hexagonal p(2 x 2) + x &)R3Oo --+ hexagonal p(2 x 2) -+ ( A x fi)R30°

three-fold four-fold three-fold

2.20 (NiS-2.39) 2.22

t hree-fold three-fold

2.18 (RhzS3-2.37) 2.28

t hree-fold

2.28

- (4 -

(a

p(2 x 2)

+

c(2 x 2)

four-fold

2.23

p(2 x 2)

-*

c(2 x 2)

four-fold

2.35

c(2 x 2)

four-fold

2.30

p(5 x 20) -* p(2 x 2) -* c(2 x 2) p(2 x 2) -t c(2 x 2)

four-fold four-fold

2.34 2.30

p(2 x 2)

c(2 x 2)

four-fold

2.31

c(2 x 2)

four-fold

2.39

c(2 x 2)

four-fold

-

p(2 x 2) c(2 x 2) c ( n x 1) p(2 x 2) -* c(2 x 2) P(2 x 3)

four-fold

-

four-fold

-

p(2 x 2)

p(2 x 2) p(2 x 2)

-

+

-

44

Chapter 4

planes. For the c(2 x 2) superstructure on fcc(100) and (110), bcc(100) etc. planes and the (4x &)R3O0 superstructure on f cc(lll) , hpc (OOOl), and other planes, all substrate surface atoms become directly coordinated to the modifier adatom. As will be shown in the forthcoming sections, the modifier structures at higher modifier coverages are usually associated with a complete deactivation of the surface. The same p(2 x 2), c(2 x 2) or ( Ax &)R30° ordered structures are formed in the case of the ot,her chalcogen atoms (Se and Te), C1, I, etc. [7]. In many cases, the maximum adatom coverages achieved exceed those corresponding to c(2 x 2) or ( f ix &)R3Oo orders, which are accompanied by the development of new LEED patterns. An example of the evolution of the LEED patterns upon increasing the Se coverage on P t ( l l 1 ) is given in fig. 4.14. [113]. Parallel LEED and T P D measurements have shown that the reverse conversion from the (4 x 4) to (& x &)R30° order occurs readily in the temperature range 800-900 K when part of Se desorbs. The data in fig. 4.14.,together with the AES calibration data indicate, that in the case of Se the maximum Se coverage achieved upon HzSe dissociative adsorption and the subsequent removal of hydrogen by desorption exceeds 0.5 (corresponding t o the (4 x 4) order). A hexagonal compact structure can be achieved at higher chalcogen coverages [loll. As will be discussed below, in most cases sulphur coverages exceeding 0.33 on fcc( 111) planes cause the development of complicated LEED patterns associated with an induced substrate surface reconstruction. Up t o coverages of 0.25, the same ordered p(2 x 2) structure is observed for the other smaller sized electronegative additives, such as C, N and 0 [7]. With these small adatoms the picture is more complicated at coverages exceeding 0.25 because of the induced substrate surface reconstruction. This will be discussed later. In the cases where no reconstruction or compound formation takes place, these adat,oms form the same ordered structures at higher coverages as summarized in table 4.3. For systems with a lack of reconstruction, the general trend to occupation of the highest coordination adsorpt.ion sites (C3,, and Cq, for single crystal planes considered in table 4.3.) is fulfilled for most of the systems under consideration [7, 1141. Exceptions to this rule occur usually in cases where the adatoms induce reconstruction of the substrate surface or the adatom surface interactions lead to the formation of a bulk compound. Fig.4.15. shows schemes of the location of the electronegative adatoms in the highest coordination (fourfold and threefold sites) on different crystallographic planes. There are some exceptions to this general trend usually with open crystallographic planes or originally reconstructed surfaces which exhibit rather large surface lattice constants. Such examples are the systems S/Re(1010) [115],and S/Ir(llO)-1 x 2 [llG]. Dynamical LEED studies of these systems have shown that the S adatom occupies three-fold sites between the surface and second substrate layer. This adatom location reduces the distance to the two closer ridge substrate atom supposing stronger interaction for the three-fold site. As can be seen in fig. 4.15., the number of the substrate atoms coordinated to the electronegative adatoms depends on the crystallographic

4.2. EIectronega t ive Addi tives

a

45

b

Fig. 4.15. Location of the adatom on different crystallographic single crystal planes: (a) three-fold site on fcc(ll1) and hpc(0001); (b) four-fold site on fcc(100); (c) fourfold, short-bridge and long bridge sites on fcc( 110); (d) four-fold and three-fold sites on bcc(ll0); (f) four-fold site on bcc(100).

orientation and the actual adsorption site. This determines the importance of the adatom surface structure in explaining the differences of the niodification effect on different substrate crystallographic planes. It is related to the fact that the substrate atoins directly coordinated to the modifier are most strongly affected. Depending on the actual substrate surface orientation, the lattice constant, and the atomic radii of the electronegative modifier, the adatoms located in the hollows of the surface are also more or less closely coordinated to the substrate atoms of the second layer . Thus, considering the local bonding geometry in determining of the total number of the substrate atoms coordinated to the adatom, the subsurface substrate atoms should also be involved in some cases. The actual location of the electronegative adatoms in the highest coordinated adsorption sites is not always centered in the hollow. Recently a

46

Chapter 4

thorough LEED analysis of the S/Mo(100) system [117] has shown a lateral displacement of the S adatoms towards the bridge sites in the 10 surface direction. This leads t o differences in the bond lengths between the S adatom and the top Mo atoms involved in the bonding: 2.33 and 2.58 A, respectively ( S - second layer Mo atom distance for the laterally displaced S atoms was found to be 2.57 A). Comparing these values for length with that corresponding to the centered hollow location of S (2.4681 to the top and 2.5781 t o the second Mo layers) it can be seen that, as a result of the lateral displacement, S becomes more closely attached to two Mo surface atoms. Undoubtedly, this will affect the strength of the S adsorption bond. The data in Table 4.3. indicate that the adatom - surface atom bond lengths tend to increase for higher adatom coordination numbers. Consequently, the dependence of the adsorption bond length on the coordination number is similar to that of the compound bulk bonds vs. the coordination number [118]. The measured adatom - substrate surface atom bond lengths are in most cases shorter than the corresponding Me - X bond lengths in the bulk compounds. As discussed in refs. [153] and [154], this implies a lower degree of ionicity between the substrate atoms and the modifier and stronger bonding compared to the corresponding bulk compound. In many cases the measured bond lengths for unreconstructed p(2 x 2) and c(2 x 2) structures show a negligible difference. This indicates that the adatom - adatom interactions are weak compared to the surface - modifier bonding. The location and the adatom - surface atoms bond lengths determine the distances between the modifier and the surface plane and between the modifier and the subsurface atoms. These characteristics are very important in describing the extent of the modifier effects, as will be discussed later. B. Electronegative adatom induced surface reconstruction.The changes in electronic structure of the substrate atoms involved in the strong interactions with the electronegative adatoms are expected to affect not only the nearest surface atoms but, in many cases, lead to a reconstruction of the substrate surface, extending typically over 2-3 atomic layers. The phenomenon of adsorbate induced surface reconstruction is very important especially with regard t o catalytic reactions which are sensitive to surface structure. The adsorbate induced reconstruction is coverage dependent and occurs most readily on more open crystallographic planes. At high adatom coverages, usually exceeding 0.33 for fcc (111) and hpc (0001) planes and 0.25-0.5 for fcc (100) and (110) planes, most of the transition metals used as catalysts (Ni, Fe, Mo, Cu, Pd, etc) undergo reconstruction and in some cases even a bulk compound is formed. Less severe cases of reconstruction are an adatom-induced expansion of the top substrate surface layer or a lateral displacement of the surface atoms as well as some changes of the position of the substrate atoms in the second layer. In the latter case the adatoms usually preserve the highest coordinated adsorption sites. As an example for such systems may be mentioned the 10 % expansion of the Ni(ll0) surface layer induced by c(2 x 2) S in a fourfold site [104], the buckling of the second layer of the Ni(100) crystal as a result of an interaction with oxygen in a fourfold site [121], the lateral displacement of

4 . 2 . Electronegati ve Ad& tives

47

I0011

Fig. 4.1G. Surface top and side views of oxygen adsorbed on Cu(llO)-2xl as deduced from SEXAFS data. Oxygen is adsorbed above the surface in the longbridge site with the surface reconstruction according to the missing row model (from ref.[126]).

the Ru top atoms coordinated with 0 and the buckling of the first and second layers for the p(2 x 2)O and three-domains p(2 x 1 ) 0 with an oxygen adatom in a threefold site on Ru(0001) [122], etc. It should be emphasised that the most recent LEED intensity analysis data show that strongly-bound atomic adsorbates can induce reconstruction even of the closed-packed surfaces at low adsorbate coverages as in the case of O/Ru(OOOl) [122]. More severe reconstruction of the single crystal surfaces is usually associated with missing of surface atomic rows or reorientation of t81iesubstrate surface atoms maintaining a new geometry. An example of the first case is the oxygen-induced (2 x 1) reconstruction of Cu(llO), where oxygen occupies lower symmetry long bridge sites lying 0.23 A above the top substrate layers (bond lengths top Cu-0 = 1.82 A, second Cu-0 = 1.98 A), as illustrated in fig. 4.16 [123-1261. In the second case the reconstruction is usually associated with the appearance of a variety of complicated LEED patterns. Examples

Chapter 4

48

of such systems are S / N i ( l l I ) [82, 83, 871, O/Cu(111) [161], S/Fe(llO) and (100) [108, 1091, C, N/Ni(100) [128-1321, O/Pd(100) [133], S/Pt(100) [134], etc.

Fig. 4.17. Structural pseudo - c(2 x 2)s - Ni(100) model corresponding to a x 2)s structure on a reconstructed N i ( l l 1 ) surface (from ref. [87])

(5a

Fig.4.17. presents the suggested new surface order induced on Ni(ll1) by S which satisfactorily explains the (5&x 2) and (8&x 2) LEED patterns[87] observed at 0s > 0.33. The reconstructed surface layer consists of domains of a distorted c(2 x 2)s - Ni(100) surface. The measured nearest top N i - S bond length (2.22 is the same as the one measured for the real c(2 x 2)s - Ni(100). It is smaller than that corresponding to the bulk compounds NiS (2.38 k) and Ni3S2 (2.28 A). This result excludes sulfide formation on the Ni(ll1) surface even for these high S coverages. A similar rearrangement, i.e. formation of rotational domains of distorted c(2 x 2)O - Cu( 100) has been suggested for explaining the complicated LEED patterns at high oxygen coverages on C u ( l l 1 ) [127]. Typical electronegative adatoms which usually induce a surface reconstruction are also C and N . C or N adsorbed on a Ni(100) surface at coverages exceeding 0.25 induce reconstruction leading to a ( 2 x 2)p4g structure [1281321. It has been supposed that this reconstruction involves rotation of the four nearest N i surface atoms, as is illustrated in fig. 4.18. Recent SEXAFS [129, 1311 and SEELFS [132] analyses favour a. fourfold hollow site in the rotated square as an adsorption site for the modifier adatom. Comparison of the behaviour of the 0, N and C/Ni(100) systems [131] have shown that the adsorbate induced (2 x 2)p4g reconstruction is not observed in the case of oxygen where 0 adsorbs in the fourfold site on the unreconstruct,ed surface. The measured modifier to the nearest neighbour top Ni distances are : 0 Ni = 1.97 N - Ni = 1.89 and C - Ni = 1.82 A. The short C - Ni bond indicates that the carbon adatom is located in the same plane or even

A)

A,

A,

4.2. Elect ronega t ive Additives

49

Fig. 4.18. Model proposed by LEED analysis for C(N) induced reconstruction of Ni(100) leading to ( 2 x 2)p4g structure. The arrows indicate t,he lateral distortion of the top Ni layer atoms (large circles) and the small circles represent the adatoms in the four-fold hollow sites (from ref.[132]).

slightly below the top Ni layer, whereas N and 0 are 0.11 and 0.88a above the top layer, respectively. On the basis of a similar cOmparison concerning the trend of the adsorbate-induced reconstruction for various systemsjt has been shown that, for the same substrate and crystallographic orientation, this trend depends on the adsorption bonding strength As has been shown in the theoretical treatment in ref.[135], in this case the electronic structure (filling of the valence levels) of the modifier is of main importance. Reconstruction of the substrate surface induced by the adsorption of electronegative modifiers appears to be a general phenomenon. It occurs usually above critical adatom coverages and occurs, in some cases, at elevated temperatures only. The tendency t o reconstruction is strongly dependent not only on the nature of the modifier and the substrate but also on the substrate surface orientation. The reconstruction can lead to the appearance of new active adsorption sites and/or elimination of others. Closer behaviour to real catalysis (where the catalyst is a polycrystalline sample or a highly-dispersed metal surface) is exhibited by the stepped metal surfaces. These surfaces reconstruct more readily a t lower adatom coverages. For example, no reconstruction has been observed with Pt(ll1) in the presence of S up to sulphur coverages of 0.33, while S adsorption of a stepped

50

Chapter 4

Pt(S) - [6(111)x(lOO) surface induces Pt(S) - [12(111) x2(100)] and Pt(S) - [6s(lll)xn(100)] reconstructions [136]. It has also been found that S occupies preferentially the step edges which are expected to be the most active adsorption sites. Here it is worth noting that the impurities such as 0, S, N , C , C1, etc., always occupy preferentially the defect sites, such as steps, grain boundaries etc., irrespective of whether they are introduced on the surface by adsorption or segregate from the bulk [137]. A similar stronger tendency towards reconstruction in the presence of S is observed for a stepped Cu(ll0) [138] surface compared to the behaviour of the flat Cu surface [139]. With increasing S coverage on stepped Cu(llO), a great number of complicated LEED structures are detected [137]. They indicate that the substrate surface undergoes a complex 2D and 3D faceting which is governed by the sulphur surface concentration. Stronger interactions between Ni and C or S have also been reported for stepped Ni surfaces, when faceting of the surface covered by C or S adsorbates is detected even at low modifier coverages [140]. More complicated is the characterization of the electronegative modifier structure on the single crystal alloy surfaces. This is due to the fact that the composition of the alloy surface is strongly dependent on the presence of impurities. Also the affinity of the different constituents of the alloys towards the adsorbate is different and one should expect preferential bonding to one of the elements. As an example, the surface composition and structure of S overlayers on two faces of a Ni3Fe alloy are shown in table 4.3 [110]. Comparison with the behaviour of the S/Ni and SfFe systems shows that the same surface order is observed only for the S/Ni( 100) and S/Fe( 100) systems. However, for the alloy surfaces, the highest coordinated adsorption site remains the most favoured one as an adsorption site. Another very important result for the S/NiFe alloy systems is the S-induced change in the surface composition at elevated temperatures as illustrated in fig 4.19. [110]. This temperature dependent surface composition of the sulphur covered alloy is explained by the preferential S - Fe interactions. The trend for the surface composition to be restored above a certain temperature is due to the tendency to disorder which is outweighing the tendency towards iron - sulphur bonding. C. Compound formation between the adatoin and the substrate or formation of a separate modifier phase.The most severe modification of the substrate surface occurs with the formation of two- or three-dimensional compounds as a result of interaction with the electronegative modifier. Usually this happens at elevated temperatures and high modifier coverages when compressed overlayers are formed. Typical examples of a compound formation are: (i) the formation of a surface oxide phase and growth of a bulk oxide above critical oxygen coverages on Ni [141, 1421, Cu [143], M o [144, 1451, Fe [146, 1471, and Ir [148, 1491, single crystal surfaces; (ii) the formation of a sulfide surface phase on Fe [lo$, 1501 and Mo [151, 1521 single crystals; (iii) the formation of metal carbides on W [153-1551, Mo [156, 1571, etc. In most cases the formation of a surface or bulk compound is also accompanied by the formation of ordered structures as monitored by LEED. A good example for such a system is the carbonization of W(100) [154]. For this system the LEED patterns show a sequential appearance of c(2 x 2)C, (1310)C, (6 x l)C and (5 x l)C structures [154, 1551. The final two structures which are a culmina-

4.2. Elec tronega ti ve A ddjti ves

51

Fig. 4.19. N i and Fe atomic fractions i n the first and the second planes versus temperature for S/NijFe(lll) (from ref.[llO])

tion of the substrate carbonization, appear only at adsorption temperatures N 1500 K when the surface reconstructs in such a manner that the carbon atoms are residing in interstitial positions between the first two layers of tungsten atoms. This atomic order of the substrate and the modifier is very similar to the layered structure of W2C. It is clear that in the case of surface or bulk compound formation the chemical properties of the substrate will match those of the new material, as for instance has been shown in refs. [158, 1591: the carbonization of some transition metals results in a material with catalytic behaviour similar to that of noble metals. Obviously, the formation of a n oxide phase will cause the properties of the metal catalyst to be converted t o those of the metal oxide, etc. The tendency towards compound formation with the same electronegative modifier under the same reaction conditions depends on the nature of the substrate metal. For example, when comparing the single crystal closely packed metal surfaces of Ni, P d and P t , which are from the same row of the periodic table, it transpires that the possibility of reconstruction and/or compound formation decreases in the sequence Ni, Pd, P t , i. e. going down the column. The Ni lattice can be disturbed by sulphur at elevated temperatures [107, 1611, it also forms a relatively stable surface carbide [162], and is rather easily oxidized [142, 143, 1611. The P t lattice is not penetrated by sulphur [91, 921, and no carbide formation or oxidation takes place under UHV conditions [158, 1601, whereas Pd exhibits intermediate behaviour [89, 134, 1581.

52

Chapter 4

Another possibility of a structural order of some electronegative additives on metal surfaces is that observed on phosphorus covered single crystal surfaces. The reports on the behaviour of the P/ Ni(100) [162], P/Rh(100) [163], and P/Pt(lll) [164] systems do not give unambiguous identification of the P structure and site occupation. It has been supposed that phosphorus either tends t o form istands of a P phase, or the isolated P adatoms may react to form a variety of compounds with the substrate surface atoms. A field emission study [165] has shown that, as a result of interaction with phosphorus, changes in the surface orientation of Pt take place and compound clusters are formed. The stoichiometry of the proposed Me,P, compounds depends on the P concentration and the substrate temperature. The lack of order in the phosphorus adsorption layers causes difficulties in determining the absolute phosphorus coverage and, as can be demonstrated in the forthcoming sections, it has a significant bearing on the strength of the phosphorus modification effects. It should also be mentioned here that, in the case of C, one should distinguish between the two possible surface states of carbon. The first state, i.e. the so called ‘carbidic’ carbon is the one considered above in describing the formation of differently ordered structures due to adsorption and/or carboninduced surface reconstruction. The ’carbidic’ carbon atoms are highly coordinated on the substrate surface and, in the cases of induced reconstruction, they may well occupy the interstices between the first and second layers as well. The second surface form of carbon is the so called ‘graphitic’ form. The formation of a basal layer of graphite is preceded by the growth of islands of a graphite phase in which the location of the carbon atoms cannot be specified [166, 1681. An important conclusion regarding the structural behaviour of the electronegative additives is that, except in some cases, they show a strong tendency towards the formation of ordered structures and towards the occupation of the highest coordinated surface states with submonolayer coverages. This assist an explaining the contribution of the geometric factor to the adatom modification effects, especially for low modifier coverages when, in most cases, the strongly bound additives do not cause substantial reconstruction of the substrate surface. 4.2.2

Strength and Nature of the Surface Bonding

A. Strength of the adsorption bond of the electronegative additives The strength of the substrate - adsorbate bonding is an important characteristic because it determines the stability of the ”poisons” on the surface. Various methods have been applied t o the evaluation of the metal-electronegative modifier bonding energy, EM-x. The choice of the method depends on the stability of the modifier adatom on the surface, i.e. on whether it can be removed from the surface by desorption a t elevated temperatures, or not. The determination of EM-x by direct thermal programmed associative desorption is possible in the case of less strongly bound additives, e.g. O?/P t, O?/Ru, Oz/Pd, NZ/Pt, S/Pt, etc. which have no strong tendency to compound formation. For all these cases the adsorption is a nonactivated process so that

4.2. Electronega tive Additives

53

the desorption energy measured from the T P D spectra can be directly related t o EM-Xaccording to the well known relationship :

where AH is the activation energy of desorption and D x , is the dissociation energy of the molecule X2 in the gas phase. Another method for evaluating of the E M - X values is based on the adsorption isobars or isosteres, for the case that the modifier is deposited on the surface by gas adsorption, e.g. HzS, H2Se, PH3, H2C2, etc. This method uses the Clausius - Clapeiron relationship for determining the heats of adsorption, A U.

d l-n P -

dl/T

AH --

R

The third method for direct evaluation of E M - X is the Born - Haber cycle, using adequate XPS data. Table 4.4. presents selected EM-^ values for the adsorption state of several 0.25). The covalent radii, of the most common poisons (for a coverage T , , the Pauling electronegativity, x, and the energies of the corresponding stoichiometric bulk compounds are given in brackets. The EM-^ values show that the affinity of most catalyst surfaces to these modifiers is very high and rather stable layers are formed. That is why it is impossible to avoid poisoning, because the temperatures at which the catalytic reactions usually take place are far below the temperatures required for the poison desorption in the cases when no compound formation or subsurface diffusion take place. However, as outlined in the previous subsection, a variety of secondary processes, such as temperature-driven adatom bulk diffusion/surface segregation, reconstruction and compound formation, are likely to happen at elevated temperatures. The selected data in Table 4.4. lead to the following two important coilclusions:

-

(i) comparison of the measured EM-Xvalues with the corresponding bond strengths of the bulk metal compounds shows that the adsorbed electronegative additives layers on the metal are more stable [118], as has already been predicted earlier by the measured shorter M - X bond lengths;

(ii) the measured EM-x values show a good correlat,ion with the observed tendency towards a substrate surface reconstruction and compound formation in the presence of the electronegative additives, e.g. for the same modifier reconstruction and compound formation occurs more readily on Fe than on Pt, while for the same substrate - C is the more active element.

B. Charge transfer involved in the formation of the adsorption bond (work f unc ti on changes).The forma.tion of the adatom - surfa.ce adsorption bond involves energetica,lly and symmetrically appropriate adsorbate

Chapter 4

54

Table 4.1. Adsorption bond strength, EM-^ in kJ/mole, for several adatom transition metal systems. For the sake of comparison the bond strength in NiS is given at the end of the table

METAL

Fe( 110) Ru(001) Rh( 111) Ir( 11 1) Pd( 111) Pt(ll1) Ni( 11 1) Ni(100) NiS

MODIFIER ( r c in

A; X )

S

C

N

(1.02; 2.5)

(0.77; 2.5)

(0.75; 3.0)

733 [169] 603 [169]

585 [175]

330 [go] -

290 [lo71 425 [lo"] 411 [157] 682 11741 360

-

635 489 569 633 555

[169] [169] [169] [169] [174]

-

530 545 530 564 490

[I711 [177] [178] [176] [179]

0 (0.73; 3.5) 530 445 725 389 367 352 543

[I691 [170] [173] [171] [171] [I721 [169]

and substrate electronic states. In order to obtain a more complete description of the properties of the modified surface, it is important to establish the perturbations of the surface electronic structure induced by the presence of adatoms on the surface. The first attempts to characterize the changes in surface electronic structure are the measurements of the adatom induced changes in the work function, A@, assuming a direct relationship between the dipole moment, m,estimated from the A@ value (see eq.(8) in Chapter 2.) and the amount of charge transfer between the adatom and the surface. According to the relationship rn = dq, where q is the charge transfer per atom and d is the dipole length (the component of the bond length perpendicular to the surface), one can estimate q for all adsorbate systems where the adsorption site and the adsorption bond lengths, 1, are known. Usually, for most of the common 'poisons' such as S, Se, P, and C, the measured work function changes upon adsorption are rather small (0.1-0.5 e V ) . This determines rather low q values. The sign of the work function changes is usually positive, which would indicate a charge transfer towards the electronegative additive adatom, but there are cases where the adsorption of the modifier induces a negative work function change. Some selected work function data for S adsorption on several transition metal single crystal surfaces may be considered. It so happens that on N i ( l l 1 ) [185], Ni(100) [180], Mo(100) [181] and Ru(0001) [182], S adsorption induces an increase of the work function in the range 0.14.5 eV for a monolayer coverage ( c(2 x 2)s for fcc (100) and bcc(100) surfaces and (6 x &)R30° for fcc(ll1) and hpc(OOO1) surfaces). On the contrary, S adsorption on Pt(100), P t ( l l 1 ) [134, 1831 and P d ( l l 1 ) [89] leads to negative work function changes. An example of A$vs. sulphur coverage dependence for positive and negative work function changes is shown

4.2. Elec tronega ti ve Additives

55

in fig. 4.20. [184]. From the data in fig. 4.20. one can evaluate that the charge transfer per S adatom can be evaluated within the limits of very low adsorbate coverages:- -0.04 e- for S/Ni(100) and +0.02 e- for S / P t ( l l l ) . Negative work function changes were measured also for other electronegative additives, e.g. C1 on W(110) I1851 and W(211) [36], N on W(100) [186], I on W(100) [36], Te on Ni(100) [84], etc.

-

0.4 0*3 3 0.2 QI u 3 0.1 “ 0

- 0.1

- O o2I

\9,

- O 3 Pt(l11)

Fig. 4.20. Work function changes induced by S adsorption on Ni(100) and Pt(l11) surfaces (from ref. [187])

The work function results indicate that the formation of a strong adsorption bond between most of the electronegative modifiers and the substrate involves a negligible charge movement in the Z direction to tthe adatom, and in some cases the charge movement seems to be even towards the substrate. Since all electronegative modifiers under consideration are expected to behave as electron accepter an explanation of the measured positive work function changes in some adsorption systems can be based on the fact that there are two contributions to the measured work function changes upon adsorption: the external electrostatic dipole moment, (determined by d and q ) and the internal polarization dipole moment, A@,,,, (determined by the interactions of the adspecies with the hybridized substrate surface d - p orbitals) [188, 1891. The adsorbate induced surface dipole A$’,nt is always negative in sign and its magnitude depends on the nature of the adsorbate and substrate and on the surface crystallographic orientat,ion. That is why, in the case of adsorbates where the adsorption bond formation involves a small charge trans-

56

Chapter 4

fer in the Z direction, the conventional arguments relating the sign and the magnitude of the experimentally measured work function to the electrostatic dipole moment only is not valid because the contribution of A@,nt(although small) should be also taken into account. This might lead to work function changes with a negative sign even for some acceptor adatoms. C . N a t u r e of the e le c t rone ga t ive adatom adsorption bond.The main conclusion from the relatively small and sometimes negligible charge transfer t o the electronegative adspecies, discussed above, is that the character of the Me - X adsorption bond is largely covalent. This is consistent with the theoretical predictions of Citrin [119] for the small percentage of ionicity of the bonding of the adatoms under consideration on the single metal surfaces, especially for additives such as S, Se, C, and Te, which are less electronegative. In order to qualify the degree of covalency or ionicity, Citrin used the simplest approach of comparison of the calculated covalent single bond lengths in a bulk-like system with measured adsorption bond lengths. The bond length of the covalent single bond was calculated using the simple relationship: RMex(calculated) = f M e -!- fx - C(&e

-xx),

(7)

where T M and ~ r x are the single bond covalent radii of the metal and the additive taken from ref. [118], X M and ~ X x are the corresponding Pauling electronegativities, and c is an empirical coefficient. The basic idea is that, comparing the calculated values of the single covalent bond, with the experimentally measured adsorption bond lengths, L , from the deviation one can judge the degree of the ionicity of the corresponding adsorption bond. L larger than RMex(calc.) suggests a relatively more pronounced ionic character, and vice versa. Although, as pointed o u t in ref. [llg], these simplified calculations are not strictly quantitative. This approach gives an idea about the trends in the character of the adsorption bonding. On the basis of the = D differences, the following trends are found. For adsorbate L systems where the modifiers have close covalent radii but different Pauling electronegativities the deviation D confirms the expected correlation of an increase of the ionicity of the bonding with increasing Pauling electronegativity. A convincing example is the comparison of the estimated D values for the following adsorption systems: (i) O/Ni(100) --Xo = 3.5, TO = 0.73, D = +0.12; N/Ni(100) -XN = 3.0, f N = 0.75, D = 0.00; C/Ni(100) - & = 2.5, f c = 0.77, D = -0.13; (ii) I/Cu(lOO) -XI = 2.5, q = 1.33, D = +0.08; Te/Cu(lOO) r ~ =, 1.37, D = -0.03;

-XTe

= 2.1,

(iii) I / C u ( l l l ) - D = +0.05, T e / C u ( l l l ) - D = +0.04. The results for modifier/Cu systems also show further that the degree of ionicity for the same adatom and substrate is dependent on the crystallographic orientation of the substrate. This reflects the dependence of L on the adsorption site first-neighbour coordination number, discussed in Subsection 4.2.1. An apparent trend is, that the ionicity increases with the coordination number (bond length). For the case of Te, it should be remembered that the Te

4.2. Elec tronega tive Additives

57

adsorption site on C u ( l l 1 ) is a six-fold one and the Te - nearest Cu bond lengths are 2.62 and 2.69 for Cu(100) and C u ( l l l ) , respectively [190]. The general trend of a larger coordination number (or L ) and a higher degree of ionicity is also evident in the case of S/Ni(100) ( D = -0.5), S/Ni(111) (D = -0.08), O/Ni(100) (D = +0.12), O / N i ( l l l ) ( D = +0.06), etc. [119]. This correlation between the coordination number and the ionicity of bonding is the same as that observed for bulk compounds [118].This is not surprising, taking into account the fact that the strength of the electronegative adatoms adsorption bonds is comparable with or even higher than the typical chemical bonds. When the trend for adatoms with t8hesame Pauling electronegativity but different adatom covalent radii is compared, it becomes obvious that the adatom dimensions should also be taken into account (the bond ionicity increases with adatom size). An example of the influence of the adatom dimensions of electronegative additives with the same Pauling electronegativity is the larger ‘covalency’ of the C adsorption bond (D = -0.13, PC = 0.77 [135]) on Ni(100), compared with that of S ( D = -0.05, T S = 1.04 [191]) and I (D= +0.21, 7-1 = 1.33 [192]) adsorbed on the same surface. A quantitative picture of the relative contribution of the Pauling electronegativity and the size of the adatoms to the nature (‘ionicity’ or ‘covalency’) of the adsorption bond is very difficult to make because as was stressed above, this simplified approach is rather empirical and outlines only the general trend, including any deviations. D. Bonding chemistry. For a thorough quantitative description of the nature of the adsorption bond of electronegat,ive additives, a detailed picture of the local electronic structure of the different adsorption sites as to directions, symmetries and energies of the hybridized occupied and unoccupied metal orbitals on the surface. The latter contribute to the formation of the adsorption bond. They determine the most favourable location where the appropriate modifier adatom electronic valence states will overlap with those of the substrate atoms. For the electronegative adatoms under consideration, it can be assumed that their highest lying p-electronic valence states will participate in the formation of the adsorption bond. The degree of coupling with the substrate electronic states depends on the energy, symmetry and occupied position of the modifier p-states: e.g. C has two partly filled p , and py atomic orbitals and one empty p r orbital; N has three partly-filled p orbitals, and 0 has one completely filled p , and two partly filled p , and p , orbitals. As discussed above, this results in significant differences in the nature of chemisorption on the same Ni( 100) surface. One of the most appropriate techniques used for determining the nature of the chemical bond of the adsorbed species, i.e. how the adatoms are bound t o the surface, is angle-resolved photoemission. It gives reliable information about the degree of involvement of the different modifier and substrate electronic valence states in the bonding when the adatoms form well ordered layers. As an example for a precise ARUPS characterization of chalcogen adsorption is the study of p(2 x 2), c(2 x 2) and (&x &)R30° sulphur overlayers on Ni(100) [193] and Ni(ll1) [194], respectively. As a result of the interaction of the S 3p,, 3py and 3p, levels with the appropriate symmetric Ni states

58

Chapter 4

(S being in a fourfold adsorption site), two types of molecular orbitals can be formed: a single orbital with a1 symmetry, involving S p , atomic orbital, and two doubly degenera.t.e orbitals with e symmetry involving the p , and p , atomic orbitals. In the real adsorbate syst,ems with ordered S overlayers, two dimensional band is formed due t o additional interactions, either direct or via the substrate between the S porbitals. Fig. 4.21. presents the experimental normal emission ARUPS spectra for ppolarized light at two angles of incidence. It is obvious that the energy positions of the 0.1 state built by the S 3 p , orbital and the e state composed by the S p , and p , orbitals depend on the S covera.ge. The binding energies of the al and e level are 4.2 and 4.9 eV, respectively, for p(2 x 2)S, whereas for c(2 x 2) s the a1 and e states reverse in binding energy, a,1 moving to a higher binding energy of 5.9 eV and e moving slightly to a lower binding energy of 4.4 eV. In addition, the changes in the and e binding energies wit11 increasing S coverage a.re a.ccompanied by changes in the width of the states: for the a.1 state the width increa,ses from 1.0 eV for p(2 x 2 ) s to 2.1 eV for c(2 x 2)S, whereas for the e st,at.e the width decreases from 2.1 to 1.4 eV, respectively. Although the deta.iled description of these observations can not be based on initial stmateeffects only, they contain important informa.tion on the relative contribution which the S 3p,, 3p, and 3 p , levels render towards the formatmionof the adsorption bond for two S structures formed a t va.rious S covemges. The data in fig. 4.21 indica.te that for higher S coverages, the contribution of the p , - Ni interaction becomes stronger, whereas a t p(2 x 2)s (qs = 0.25) the S p , and p , orbitals are more strongly involved in the formation of the a.dsorption bond. In addition, with increasing S coverage (from the p(2 x 2 ) s to the c(2 x 2)s overlayer), the S p bands dispersion increases. This suggests an enhmced interaction between the S p states within the c(2 x 2)s layer as a result of the decreased S - S separation at higher S covemges [193]. In the case of larger cha.lcogen a.dditives (Se and Te), the same trend in the p , and p,, derived states a,nd in the p band dispersion is observed with increasing adatom covera.ge on Ni(100), i.e energy inversion of the p , and p,, states and an increase in dispersion (see table 111. in ref. [195]). The case of 0 is the only one for which, the energy posit#ionof the p , level is alwa.ys below the p,, levels. Although it, goes from the p(2 x 2) to the c(2 x 2) coverage, the p z level is subjected to a stronger upwa.rd energy shift which approaches the p,, level. Obviously, these differences in the modifier plevels energy positions reflect their different overlap with the hybridized substra,te surface electronic states. It should be pointed out tha.t, 011 the basis of compa.rison between the p orbital energies of the free chalcogens and the values measured on the chalcogen adatom, one can get an idea of the ionicity of the adsorption bond. It has been found that, as predict,ed from t
-

-

-

4.2. Electronega ti ve Add; tives

59

INITIAL ENERGY ( e V )

Fig. 4.21. Normal emission spectra for S/Ni(100) for p-polarized light at two angles of incidence. The a1 state is built by the S p z orbital and the ‘e’ state is composed by the S p , and S p , levels (from ref.[193]) of oxygen adsorption on several inetal substrates, Doyen et alhave calculated that the charge transfer is of the order of 5 % [196].

The same tendency of an upward shift (a decrease of the binding energy) has been observed for the core levels of the modifier adatoms under consideration [197-1991. However, since the orbital binding energies derived from the photoelectron spectra depend not only on the initial state effects (chemical and electrostatic shifts) but also on the final state relaxation or screening effects (which are unique for each adsorption system), a quantitative conclusion regarding the variations in ionicity of the adsorption bond of different modifiers cannot be made on the basis of the UPS data alone. Up t o this point, the filled valence states of the adatoms which are located below the Fermi level were considered. For transition metals characterized by a high density of d-states at the Fermi level, the unoccupied antibonding modifier pderived states can not be detected by UPS because they lie above the Fermi level. Certain information on the enhancement of the local density of unfilled valence band states located above the Fermi level has been found for C, N , 0 and S adsorbed on Ni, Cr, and Ti, using the X-ray appearance method [199] and inverse photoemission [200]. Existence of antibonding p derived levels below the Fermi level has been detected by UPS for substrates (e.g. Cu) where the filled d-band lies below the Fermi level [196, 2011. In these cases the emission associated with the antibonding p levels has been detected a t energies below the Fermi level. The experimental evidence of the presence of bonding and antibonding electron states indicates that the formation of the

Chapter 4 adsorption bond should be described as a localized phenomenon in terms of a ‘surface molecule’. This means that the chemisorption bond energy depends on the efficiency of the overlap between the modifier and substrate surface atom orbitals. Thus, the measured energy level shifts of the orbitals involved in the formation of the chemisorption bond and the energy separation (gap) between the resulting bonding and antibonding states can be successfully correlated with the measured magnitudes of the modifier’s adsorption bond strengths. Thus far the only energy changes looked at were those electronic levels of the electronegative adatoms which, as a result of the adsorption bond formation, concerned both, occupied and unoccupied electronic states of the substrate surface. Undoubtedly, the substrate nature is another important factor in the strength and the nature of the surface - modifier bonding. Of interest are the atomic radii and the electronegativity of the substrate atoms as well as the surface geometric and electronic structure. The possible modifier induced perturbations in geometric structure of the surface have already been considered in the previous subsection. They are always accompanied by significant changes in the electronic structure of the substrate surface, due to the strong coupling of the additive adatoms with the surface atoms involved in the formation of the adsorption bond. These changes of the electron density distribution in the surface valence band are reflected by the energy shifts in both the occupied and unoccupied electronic states. In general, the local density of states near the Fermi level is supposed to be an important property of the catalyst surface because it determines the number of electronic states that can be excited from an occupied to an unoccupied state at the lowest energy cost. In most cases these are the states involved in the interaction with the reagents. Consequently the introduction of a modifier will affect the ability of the surface to respond to external perturbations [l89]. The magnitude of the adatom-induced alterations of the electron distribution at/or near the surface depends on the nature and strength of the chemical bond. The current ARUPS and IPE data have shown that all electronegative adatoms considered in the present review cause a suppression of the substrate surface emission from the valence d-band near the Fermi level and the appearance of new features in the near region below the Fermi level. Most often these features are attributed to an emission from chemically shifted d-states of the substrate atoms directly involved in the adsorption bond [191, 192 195, 197-199, 202-2051. Thus, the significant changes in the energy region of the Ni d band observed in the angle-resolved photoemission spectra when S is adsorbed, indicate a considerable involvement of the localized surface Ni 3d electron states in the adsorption bond [193]. However, as outlined in ref. [195], the intensity changes in the substrate d-band region are not related in any direct manner to the actual charge redistribution on the surface atoms. This means that the relative weight of the valence d-states and the delocalized s-valence states contributions to the interactions with the electronegative additives cannot be qualified on the basis of the presently available experimental data. The main conclusion that can be made on the basis of the available experimental and theoretical data is, that the strongest charge perturbations

4.2. Elec tronega t i ve Additives

61

as a result of the adsorption bond formation, are localized within the first substrate plane. It has been shown that the increased bonding charge in the adatom/surface interface is counteracted by a charge depletion below the surface top layer resulting in a change of the Me - Me spacing between the first and the second layer. Indeed, as was discussed in the previous Subsection, in many adsorption systems, an expansion in the substrate lattice has been observed. The changes in valence electron distribution of the substrate surface atoms also lead t o changes in the surface atom core level electron states. In the case of adsorbates with electron acceptor behaviour, an increase of the core level binding energy (downward shift) of the substrate atoms should be expected if the formation of the adsorption bond involves a substantial charge transfer to the adatom . Some authors [205] have tried to correlate the measured surface Mo 3 d 5 / , core level shifts, A E , induced by adsorption of different electronegative additives (O,C, and B) to the Pauling electronegativity difference, AX, between the modifier and the substrate metal. For the three chosen additives - 0, C and B. which are characterized by different electronegativities ( 3 . 5 , 2.5 and 2.0, resp.), but with closely covalent radii (0.73, 0.77 and 0.8, resp.), the authors have established a linear dependence between the measured DE values and the corresponding k values [205]. It has been shown that the adsorption of B, C and 0 causes the following increase of the surface 3d core level binding energy: AS, = +0.1 eV, A& = +0.2 eV and Eo = +0.4 eV. The trend in the A E values indicates that going from B to 0, the surface atoms become more electron deficient due to an increase of the adsorption bond ionicity. More significant surface core level shifts have been detected when, as a result of the surface - modifier interactions, a new chemical compound is formed. This is attributed to the fact that the true stoichiometric compounds usually have more ionic bonds [118]. 4.2.3

Conclusive Remarks

The most import,ant data reviewed in this subsection show that, in order to explain the poisoning action of the electronegative additives under consideration, the contribution of the following three main fact,ors should be taken into account for any particular system:

1. The number of substrate atoms involved in direct interactions with the modifier adatom. This depends on the structural order of the modifier overlayers on the surface and determines the blocking effect of the poisons. 2. The actual changes in electronic structure of the substrate atoms induced by the adsorption bond formation. The magnitude of these changes depend on the nature and strength of the adsorption bond, which is of a covalent type for most of the electronegative additives under consideration.

3. The Pauling electronegativity and the adatom size of the modifier which are the major factors determining the degree of ionicity of the adsorption

Chapter 4

62

bond. T h e s e factors a r e i m p o r t a n t for explaining the differences i n the severity (range) of the poisoning effect of different modifiers.

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Chapter 4

D. G. Kelly, A. J. Gellman, M. Salmeron, G. A. Somorjai, V. Maurice, M. Huber and J. Oudar, Surface Sci. 2 0 4 (1988) 1 [116] C.-M. Chan and M. A. Van Hove, Surface Sci. 183 (1987) 303 [117] D. G. Kelly, R. F. Lin, M. A. Van Hove and G . A. Somorjai, Surface Sci. [115]

224 (1989) 97 [118] L. Pauling, The Chemical Bond, 4th ed. Cornell University Press, Ithaca, N.Y., 1966 P. N. Citrin, Surface Sci. 184 (1987) 109 [119] C. H. Bartholomew, P. I(. Agrawal and J. R. Katzer, in: Advances in Catal[120] ysis, ~01.31,p.135 [121] W. Oed, H. Lindner, U. Starke, H. Heinz, K. Muller and J. B. Pendry, Surface Sci. 2 2 4 (1989) 179 [122] M. Lindvoos, H. Pfnur, G. Held and D. Menzel, Surface Sci. 2 2 2 (1989) 451 I1231 G. Ertl and J. Kuppers, Surface Sci. 24 (1975) 104 [124] H. Niehus and G. Comsa, Surface Sci. 140 (1984) 18 [125] M. Bader, A. Pushman, S. Ocal and J. Hasse, Phys. Rev. Lett. 57 (1986) 3273

A. W. Robinson, J. S. Somers, D. E. Ricken, A. M. Bradsliaw, A . L. Kilcoyne and D. P. Woodruff, Surface Sci. 227 (1990) 337 [127] R. W. Judd, P. Hollins and J . Pritchard, Surface Sci. 171 (1986) 643 [128] J. H. Onufrenko, D. P. Woodruff and B. H. Holland, Surface Sci. 81 (1979)

[126]

357 [lag]

M. Bader, C. Ocal, B. Hillert, J. Haase and A. M. Bradshaw, Phys. Rev. B

[130] [131]

35 (1987) 5900 W. Daum, S. Lehwald and H. Ibach, Surface Sci. 178 (1986) 528 D. Avranitis, K. Baberschke, L. Wenzel and N. Dobler, Phys. Rev. Lett. 57

(1986) 3175 [132] E. Atkei, U. Bardi, M. Maglietta, G. Rovida, M. Tokkini and E. Zamazzi, Surface Sci. 2 1 1 / 2 1 2 (1989) 93 [133] T. W. Orent and S. D. Bader, Surface Sci. 115 (1982) 323 [134] H. Gutleben and E. Bechtold, Surface Sci. 191 (1987) 157 [135] J. E. Muller, M. Wittig and H. Ibach, Phys. Rev. Lett. 5 6 (1986) 1583 [136] A.-M. Lanzilloto and S. L. Bernasek, J. Chem. Phys. 84 (1986) 3553 [137] H. J. Grabke, W. Paulitschke and S. R. Srinivasan, Reactivity of Solids, ed.

[138] [139] [140] [141] [142] [143] [144] [145] [146]

by J. Wood, 0. Lindquist, S. Helgesson and N. Vannerberg (Plenum N.Y. 1977), p.55 J. C. Bouliard and M. P. Sotto, Surface Sci. 1 8 2 (1987) 200; ibid. 195 (1988) 255; ibid. 2 1 7 (1989) 38 V. Maurice, J. Oudar and M. Huber, Surface Sci. 187 (1987) 312 H. V. Thapliyal and J . Blakeley, J. Vacuum Sci. TechnoJ. 15 (1978) 600 P. H. Holloway and J. B. Hudson, Surface Sci. 43 (1974) 123; ibid. 141 C. R. Brundle and A. F. Carley, Chem. Phys. Lett. 31 (1975) 425 C. Benndorf, A. Hohlfeld, C. Nobl and F. Thieme, in: Proc. ICSS-4 & ECOSS-3, Cannes, 1980 (Suppl. Le Vide No 201 (1980) p.363 C. R. Brundle, Surface Sci. 48 (1975) 99 E. Bauer and H. Poppa, Surface Sci. 88 (1979) 31 J. M. Van Zoest, J. M. Fluit, T. J. Vink and B. A. Van Hassel, Surface Sci. 182 (1987) 179

References

67

[147] K. Ueda and R. Shimizu, Surface Sci. 43 (1974) 77 [148] H. Conrad, J. Kuppers, F. Nitschke and A . Plagge, Surface Sci. 69 (1977) 668 [149] V. P. Ivanov, G. I<. Boreskov, V. I. Savchenko, W. F. Egelhoff Jr., and W. H. Winberg, J. Catalysis 48 (1977) 269 K. 0. Legg, F. Jona, D. W. Jepsen and P. M. Marcus, Surface Sci. 66 (1977) I1501 25 [151] T. Kikuchi and I<. Ishizuka, J. Res. Inst. Catal. Hokkaido Univ. 26 (1978) 7 11521 J. Oudar Conf. Catal. Deactivat. Poison., Lowrence Berkeley Lab., Berkeley May 24-26, 1978 M. Boudart and D. F. Ollis, Surface Sci. 23 (1970) 330 J. B. Benziger, E. KO and R. Madix, J. Cat. 54 (1978) 41 D.R. Mullins and S. H. Overburg, Surface Sci. 103 (1988) 455 C. Giullot, R. Riwan and J . Le Cant,e, Surface Sci. 50 (1976) 581 R. J . Madix, Catal. Rev.-Sci. Engrg. 15 (1977) 293 M. Boudart and R. Levy, Science 181 (1973) 593 M. Pendereau and J. Oudar, Surface Sci. 20 (1970) 80 R. Wandelt, Surface Sci. Rept. 2 (1982) 1 and references therein C. R. Brundle and A. F. Carley, Chem. Phys. Lett. 31 (1975) 425 M.Kiskinova and D. W. Goodman, Surface Sci. 108 (1981) 64 R. I. Hedge, J. Tobin and J . M. White, J. Vacuum Sci. Technol. A 3 (1985 339 C. E. Mitchell, M. A. Henderson and J. M. White, Surface Sci. 191 (1987 425 M. Mundshau and R. Vanselow, Surface Sci. 166 (1986) L131 N. M. Abbas and R. J. Madix, Appl. Surface Sci. 7 (1981) 241 J . L. Gland and G . A. Somorjai, Surface Sci. 41 (1974) 387 R. W. Joyner, G. R. Darling and J. B. Pendry, Surface Sci. 205 (1988) 573 K. W. Frese, Surface Sci. 182 (1987) 85 and references therein L. Surnev, G. Rangelov and G. Bliznakov, Surface Sci. 159 (1985) 299 G . Ertl, in: The Nature of the Surface Chemical Bond, Eds. T. N. Rhodin and G. Ertl (North-Holland, Amst,erdam, 1979) p.313 T. Matsushima, Surface Sci. 157 (1985) 397 G.B. Fischer and S. J . Schmieg, J . Vacuum Sci. Technol. A 5 (1987) 1064 W.F. Egelhoff, Jr., J . Vacuir~nSci. Technol. A5 (1987) 700 F. Boszo, G. Ertl and M. Weiss, J . Cata1,ysi.s 50 (1977) 519 H. Conrad, G. Ertl, J. Ktppers a i d E. E. Latta, Surface Sci. 50 (1975) ?96 H.Conrad, G. Ertl, J. Ktppers and E. E. Latta, Surface Sci. 65 (1977) 335 C.M. Comrie, W. H. Weinberg and R. M. Lambert, Surface Sci. 57 (1976) 619 W . F. Egelhoff, J. Vacuum Sci. Technol. A 3 (2985) 1305 E. L. Hardegree, P. Ho and J . M. Whit.e, Surface Sci. 165 (1986) 488 V. Maurice, L. Peralta, Y. Berthier and J. Oudar, Surface Sci. 148 (1984) 623 S. R. Kelemen and T . E. Fischer, Surface Sci. 87 (1979) 53 J. Billy and M. Abon, Surface Sci. 146 (1984) L525 M. Kiskinova, Habilitation Thesis, 1989 F. Bonchek, T. Engel and E. Bauer, Surface Sci. 97 (1980) 595

68

Chapter 4 D. L. Adams and L. H. Germer, Surface Sci. 27 (1971) 21

M.Kiskinova, Habilitation Thesis,

1989

E. Shustorovich and R. C. Baetzold, Appl. Surface Sci. 11/12 (1982) 693 P. J. Feibelman and D. R. Hamann, Surface Sci. 149 (1985) 48 F. Comin, P. H. Citrin, P. Eisenberger and J. E. Rowe, Phys. Rev. B 26 (1982) 7060

S. Brennan, J. Stohr and R. Jaeger, Phys. Rev. B 24 (1981) 4871 R. G. Jones, S. Ainsworth, M. D.Crapper, C. Somerton and D. P. Woodruff, Surface Sci. 179 (1987) 425 E. W. Plummer, B. Tonner, N. Holzwart and A. Liebsch, Phys. Rev. B 21 (1980) 4306

T. W.Capehart and T. N. Rhodin, Surface Sci. 83 (1979) 367 E. W. Plummer and W. Eberhardt, Adv. Chem. Phys. 20 (1982) 49 and references therein G. Doyen and G. Ertl, J. Chem. Phys. 68 (1978) 5417 R. Wandelt, Surface Sci. Reports 2 (1982) 1 and references therein A. Gelman, W. Tysoe, F. Zaera and G. A. Somorjai, Surface Sci. 191 (1987) 271

S. Anderson and C. Nyberg, Surface Sci. 52 (1975) 489 L. Klebanoff, R. I<. Jones, D. T. Pierce and R. Celotta, Phys. Rev. B 36 (1987) 7849

G. G. Tibbetts, J . M. Burkst.rand and J . C. Tracy, Phys. Rev. B 15 (1977) 3652

P. J. Jupiter, A. J. Viescas, C. Carbone, J . Lindau and W. E. Spicer, J. Vacuum Sci. Technol. A 3 (1985) 1517 G. B. Fischer, Surface Sci. 62 (1977) 31 D. Spanjaard, C. Guillot, M. C. Desjonqueres, G. Treglia and J. Lecante, Surface Sci. Rept. 5 (1985) 1 J. L. Grant, T. B. Fryberger and P. C. Stair, Surface Sci. 159 (1985) 333

Chapter 5

ADSORPTION OF GASES O N SURFACES MODIFIED BY ELECTRONEGATIVE ADATOMS

5.1

5.1.1

CARBON MONOXIDE General Remarks for CO Adsorption on Clean Metal Surfaces

The most exploited model for explaining the CO bonding on transition metal surfaces is the synergic mechanism proposed by Blyholder for the description of the metal - CO bond in transition metal carbonyls where CO is known as a ligand in the transition metal inorganic chemistry [l].According to the model proposed by Blyholder, the metal - CO bonding involves donation from the strongly directed CO - 5u highest occupied molecular orbital (HOMO) to the unoccupied or partly occupied s, p or d, metal orbitals, and back donation from the appropriate metal (most often & occupied states) to the CO 2x lowest unoccupied molecular orbital ( L U M O ) . As a result: (i) CO 50 HOMO forms a bonding orbital, which, according to the simple chemical bond model, should cause an increase of the 5u binding energy (BE);

(ii) the mixing of the unoccupied CO 27~levels with the metal dp electrons will form a bonding configuration with a large amount of d, character. The degree of backdonation depends on the availability of suitably oriented metal orbitals and is supposed to be the major contribution to the bonding on transition metal surfaces [2]. The most recent theoretical approaches [3, 41 have shown that electrons from the other orbitals (e.g. s p ) can be backdonated to a lesser extent or hybridization with resonant unoccupied metal states also might contribute to the bonding. Since the 5u HOMO is essentially nonbonding and the 2~ LUMO is strongly antibonding, the formation of the chemisorption bond results in weakening of the C-0 bond , an increase of the C-0 bond length and a reduction of the C-0 stretching frequency below the free molecule value. A schematic

69

70

Chapter 5.

of the CO molecular orbital perturbations during the formation of the metal - CO chemisorption bond is shown in fig.5.1.

2

co

co 2lT*

ZlT-,

gas phase

co on Metal (no interaction)

Strongly Chemisorbed co on Transition Metal

Fig. 5.1. Schematics of the CO molecular orbitals energy perturbations as a result of the CO interaction with a metal surface Adsorption of CO on transition metal surfaces has turned out to proceed via a precursor channel which ensures a non activated pathway to chemisorption [5]. The values of the reported CO adsorption binding energies for the molecular CO state on the Group VIII transition metals within the limits of low CO coverages (initial heats of adsorption, AH'(C0)) are ranging from 140 to 190 kJ/mole. On metals from Groups VIB and VIIB one should discriminate between molecular and dissociated CO coexisting on the surface, the molecular adsorption energies being within the same range [6, 71. Metals from Group IB exhibit a completely different behaviour, showing a rather weakly bound molecular CO state with adsorption energies of the order of 30-70 kJ/mole. Except for weakly-bound CO on Ag and Au, the CO overlayers form ordered structures on all other single crystal metal surfaces under consideration [5, 71. The orientation of the chemisorbed CO molecule with respect to the substrate surface is determined exclusively by the CO coverage. On most of the single crystal surfaces (up to moderate coverages), the CO molecule is bonded via the C atom with a C-0 axis normal to the surface. Existence of off-normal bonded CO molecules has been reported for compressed CO overlayers (tilt angles 6'-12') and stepped surfaces (tilt angles 18') [5]. Nonnormal bonding configurations (strongly inclined or 'lying down') with an involvement of l ? r and 4u molecular orbitals of CO are observed for some transition metal surfaces (Fe(100) [8], C r ( l l 0 ) [9] and Mo(ll0)) [ l o , 111 at N

5.1. Carbon Monoxide

71

low CO coverages. This abnormally bound state is assumed to be a precursor for CO dissociation. T h e dissociation propensity of CO molecules varies on the different single crystal metal surfaces. Fig. 5.2. illustrates the energetic situation for CO and its constituents - 0 and C on some transition metal surfaces. The energy level of molecular CO is determined on the basis of the measured initial heats of adsorption, AH’. T h e energy level of dissociated CO, Ec,o, is estimated using the relationship:

Ec,o = EM-c+ MM-o - Dco,

(1)

where EM-C and EM-^ are the metal-carbon and the metal oxygen binding energies on the metal surface and DCO is the dissociation energy of CO molecule in gas phase (256 kcal/mole).

250I

-W0

,

DCO

\ E

3 1500

-rc

d

-

F W

W

-

50 -

.0

c

Pd ( 1 1 1 ) P t (Ill) Rucoooi)

Ni ciio Feccio)

w (110)

C

al

c

0

a

50-

c+o

Fig. 5.2. Potential energy diagram for CO adsorbed on P d ( l l l ) , P t ( l l l ) , Ru(0001), Ni(lll), Fe(ll0) and W(110), presenting the molecular and dissociat,ed CO states and the activation energy, of dissociation, EI

The activation energy of dissociation of chemisorbed CO molecule, E:, is estimated using the relationship derived in ref. [12]:

where the AH’S are the heats of atomic chemisorpt,ion of the corresponding dissociation products.

72

Chapter 5.

Fig.5.2. illustrates quite well that the dissociation propensity of the chemisorbed CO cannot be simply relat.ed to the M-CO adsorption bond strength. Obviously, it depends on the affinity of the substrate to the atomic constituents C and 0. It can be expected that changes in the properties of the surface by introduction of modifiers will affect the surface behaviour with respect t o CO dissociation by influencing the energy position both of the molecule and the products of dissociation.

5.1.2

Modifier Effect on the CO Adsorption Energy and on the Surface Adsorptive Capacity

Information concerning the effect of different electronegative modifiers on the CO adsorption energy and on the capacity of the surface for CO adsorption has been obtained mainly by means of the T P D method. This technique is adequate for metal surfaces modified by C1, S, Se, C, N , P, etc., where the desorption of reagents, such as CO, NO, Hz and hydrocarbons takes place at much lower temperatures than that at which the adsorbed modifier can be removed. As will be illustrated below, the introduction of a modifier induces changes both in the location of the T P D maxima and in t.he shape of the T P D spectra, which reflect the different aspects of the modifier effect.

Fig. 5.3. Effect of different amounts of C1 and S on the CO TPD spectra from Ni(lO0) for saturation CO coverages at 100 I< (from ref. (131) Figs. 5.3 and 5.4. show the effect of increasing amounts of various additives on the CO T D spectra from Ni(100) [13, 141. All these modifiers, as outlined in subsection 4.2., occupy the highest coordinated sites tending t o form ordered structures. The results in figs. 5.3 and 5.4. indicate that the presence of a modifier causes a substantial reduction in the occupation of

73

5.1. Carbon Monoxide

the most tightly bound ,&-GO state with increasing modifier coverage. The lower temperature desorption peaks are depopulated less rapidly, which is accompanied by the appearance of new weakly-bound states, associated with desorption from modifier-induced surface states. It should be pointed out that up to certain modifier coverages the highest temperature &state (resulting of desorption from non affected surface states) is not completely removed. It remains preferentially occupied in cases of low CO coverages, followed by a sequential occupation of the less st
024

L 1 0 200 300 400 500 600 IEMPEAAlURE

(I()

TEMPERATURE (I’

Fig. 5.4. Effect of different amounts of C and N on the CO T P D spectra from Ni(lO0) for saturation CO coverages at 100 K (from ref. [14])

A very similar effect of the electronegative additives as that illustrated in figs. 5.3 and 5.4. has been reported for CO adsorption on other modified fcc(100) and fcc(ll0) surfaces, e.g. S/Ni(100) [15, 161, S/Ni(110) 1171, S/Pd(100) [Is-201, S/Pt(llO) [21], N/Rh( 100) [ 2 2 ] ,C/Ni( 110) [23], C/Ni( 100) & (110) [24], etc. Fig. 5.5. shows the CO TPD spectra from Ni(100) modified with different amounts of P (an additive which does not tend to form ordered surface structures). Obviously, the lack of ordering of the modifier and the tendency of P to form amorphous islands (see Subsection 4.2.) lead to a weaker effect on both the CO bonding strength and the CO coverage. When assessing the relative strength of the modifier effect induced by different electronegative additives, two factors may be considered: (i) the critical modifier coverage at which the most strongly-bonded CO adsorption state (associated with CO adsorption on a nonaffected site) is completely eliminated;

Chapter 5.

74

100

200

300

400 500 600 TEMPERATURE IKI

Fig. 5.5. Effect of different amounts of P adsorbed on Ni(100) on the CO TPD spectra for saturated CO coverages at 100 I< (from ref. [13])

(ii) the modifier induced reduction of the adsorptive capacity of the surface (saturation CO coverage at the given adsorption temperature).

Fig. 5.6. presents the induced changes in the relative population of the most-strongly bonded CO molecular /??-state and in the total CO saturation coverage a t 120 K as a function of the coverage of different modifiers, such as C1, S, N , C, and P on Ni(100). The data in fig. 5.6. demonstrate that the rapid initial decay of the total CO coverage is mainly due to the fast reduction of the &states. This eliminated completely for modifier coverages of 0.25 or less in cases when the additive adatoms form ordered p(2 x 2) structures (Cl, S, N, C). It is obvious that the strongest effect is observed in the presence of C1, when the complete &CO statmeelimination is observed at C1 coverages of 0.15. In the case of P, which does not form ordered overlayers, the modification effect is weaker and the elimination of &CO occurs at P coverages exceeding O.G. The same trend in the strength of the modification effect is preserved also for the total CO coverage. There the maximum CO coverage has turned out to drop from 0.62 for a clean Ni(100) surface to 0.15, 0.18, 0.25 and 0.45 for ordered p(2 x 2 ) overlayers of C1, S , N

-

-

5.1. Carbon Monoxide

75

and C, respectively, and to 0.55 for the disordered P overlayer. For the most severe 'poisons', such as C1 and S, the surface can be considered as completely deactivated at modifier coverages approaching 0.5 when the formation of the c(2 x 2) structure is fully completed.

U,

ec''d

00

1

0

01

02 03 04 05 06 A D D I l I V t C O V t R A G t IMLI

2

01

Fig. 5.6. Dependence of the occupation of the Pz-CO state and the total CO uptake on the additive coverage. (from ref. [13])

A very similar effect is exhibited by electronegative additives on the adsorptive capacity and the relative population of the various molecular adsorption CO states on modified fcc(ll1) and hpc(0001) surfaces. Typical examples are the data reported for CO adsorption on S / P t ( l l l ) [25-271, S/Ni(111) [28, 291, C/Ni(111) [24, 301, O / N i ( l l l ) [31], C/Rh(111) [32], S/Ru(OOOl) [33], etc. An illustration of the S-induced changes in the CO T P D spectra from fcc(ll1) and hpc(0001) metal surfaces is given in fig. 5.7., where with increasing S coverage on P t ( l l l ) , the following effects are observed : (i) the CO peak maximum at low CO coverages shifts from 440 K for a clean surface to lower temperatures (down to 360 K for p(2 x 2) 0.25 S / P t ( l l l ) ) , accompanied by an increase of its half width for the same CO coverages;

Chapter 5 .

76

-

(ii) the maximum CO coverage is reduced from 0.66 for a clean surface to 0.24 at 0.25 S / P t ( l l l ) and drops to 0 at ( Ax A ) R 3 0 ° 0.33 S/Pt( 111).

250

ilo

350

400

4.k

so0

TEMPERATURE (K)

Fig. 5.7. CO T P D spectra for clean and sulfided P t ( l l 1 ) at To= 90 K (from ref.

PI) Approximations based on the initial reduction in population of the most strongly bound CO-state induced by the introduction of increasing amounts of a modifier have shown that, for additives forming ordered overlayers, one modifier adatom affects 3 CO adsorption states on fcc( 111) and hpc( 0001) surfaces and 4 states on fcc(100) and (110) surfaces. Part of the possible adsorption sites is eliminated whereas another part is converted into new modified adsorption sites where the CO molecule is less strongly bound. It should be noted that at low additive coverages, modified and unmodified adsorption sites coexist. The critical coverages, at which the latter are eliminated, are determined by the effective radii of the modifier influence and by the stability of the initial additive adlayer order in the presence of the reagent. Both factors will be considered in detail in the forthcoming subsections. The influence of the modifier on the adsorption energy of the CO molecular form is very well reflected in the T P D spectra, shown in figs. 5.4, 5.5 and

-

-

5.1. Carbon Monoxide

77

5.7. The elimination of the most favourable adsorption sites, associated with the clean surface is accompanied by the removal of the highest temperature adsorption peak. The clearest picture of the effect of the modifier on t,he strength of the CO adsorption bond in the affected adsorption sites can be obtained by comparing the initial heats of CO adsorption for additive free substrate surfaces and for surfaces covered with an ordered modifier overlayer at which all clean surface sites have been eliminated.

p(2x2) S-Pt(111)

( f i ~ a ) R 3 0 S-Pt(111)

S adatom in three-told hollow site

0 nearest Pt alom blocked for CO adsorption 0 next nearest Pt atom with modified properties with respecl to CO adsorption 1 reduced CO sticking coefiicient 2 reduced P1-CO btnding energy 3 increased ESD efficiency 4 stiffened CO bending vibrations

Fig. 5.8. Structural models of S - modified Pt(ll1) by S coverages 0.25 and 0.33.

Table. 5.1. presents some selected data obtained for the desorption energies, E d , and the pre-exponential factors of desorption, v, of GO adsorbed on several clean and modified substrate surfaces. Some of the data in Table 5.1. are obtained without taking into account the possible variations of the pre-exponential factors in the presence of a modifier (e.g. for Ni(100)). This assumption is not accurate, because the changes in the adsorption bond strength and the presence of coadsorbat,es always affect the degree of localization of the adsorption stmate,which determines the pre-exponential factor. That is why, in most recent studies, both parameters were evaluat,ed. As is evident from the data summarized in table 5.1., there is a compensation effect of a concurrent decrease of both parameters. An accurate investigation on the relative contribution of Ed and v reduction has shown that Ed has a greater effect because the rate of CO desorption increases in the presence of a modifier [33]. Within the framework of the transition state theory [34], the

78

Chapter 5.

Fig. 5.9. A scheme of the possible CO adsorptioii sites for Ni(100) modified with p(2 x 2 ) 0.25 S and c(2 x 2 ) 0.5 S overlayers

compensatiiig effect can be quantified as a natural relatioilship between the activation energies and the initial and the transition state entropies. Summarizing the results in Table 5.1., the general conclusion is that the surface - CO binding energy in the adsorption sites affected by the electronegative adatoms is always reduced. The absolute value of this reduction, which reflects the strength of the modifier influence on the particular adsorption site, varies in different adsorption systems. Let the CO initial heats of adsorption be compared for modified and nonmodified fcc( 111) and fcc( 100) surface, e.g. p(2 x 2) 0.25 S - P t ( l l 1 ) with clean P t ( l l 1 ) and p(2 x 2) 0.25 S - Ni(100) with clean Ni(100). The C O / P t ( l l l ) systems are very appropriate for this comparison, because within the limits of low CO coverages, CO occupies ‘on top’ sites on both ( P t ( l l 1 ) and S - P t ( l l 1 ) ) surfaces. The Pt-CO binding energy for CO residing on the nest-nearest neighbour P t atom of 0.25 S-Pt(ll1) (the only possible adsorption site on this surface as shown in fig. 5.8.) is reduced by 48 kJ/mol compared with that of CO on S-free P t ( l l 1 ) . Almost the same reduction in the binding energy is observed when the CO coverage on clean P t ( l l 1 ) is increased to 0.4 (see the T P D spectra in fig. 5.7.). Consequently, the S effect on the CO adsorption energy for S-CO separation 3.12 A is of the same order of magnitude as that experienced by CO in the denser CO overlayers on S-free P t ( l l 1 ) where the closest CO-CO

-

79

5.1. Carbon Monoxide

-

distances are 3.8 k. The picture is more complicated in the case of modified fcc(100) surfaces where several distinct S affected sit,es are observed at p(2 x 2) 0.25 S. On Ni(100) the three S affected CO states a.re separated from S at 3.53 A, 2.8 k and 2.5 A, respectively (fig. 5.9.) when the CO binding energy decreases with decreasing separation between the coadsorhates (from 110 down t o 30 kJ/mole). However, the Ed results summarized in Table 5.1., do not show a simple correlation between the modifier electronegativity and the magnit.ude of CO adsorption energy reduction. As will he discussed in the next subsection, this is due t o the fact that, the type of CO a.dsorpt,ion sites on modified surfa.ces depends on the adsorption t>emperatureand the tendency towards reconstruction. The complete deactivation of the fcc(ll1) metal surface with respect to CO adsorption at S coverages of 0.33 (when the (fi x &)R30° S structure is formed) indicates that the surfa,ce sites (on top, bridge and threefold) involving nearest neighbour Pt atoms a.re unfavourable for CO adsorption and can be regarded as S blocked ones. The critical concent~ra~tion of the modifiers causing complete blockage of t.he surface is larger for the fcc( 100) planes. This is due to the fact that the substrat,e surfwe struct,ure det,erniines the ordering of the modifier and the effective distances between the coadsorbates on the surface. According to the da ta summarized in Table 5.1., a wea,kly-bound CO state can be detected even on c(2 x 2) 0.5 S-Ni(100) where t.he maximum SCO distance is 2.5 k (compared wit,li 2.8 k for CO in a. three-fold site on 0.33 S-Pt(ll1)). This iiidica,tes that, apa.rt from the modifier - reagent separation, the modifier surface configuration also plays an important role in the determination of the actual extent of the blocking effect. Thus far systems have been considered in which the coadsorption of CO does not disturb the initia.1 order of the additive overlayer, i.e 110 rearrangement in the modifier overhyer takes place upon int,roduction of CO. A substantial difference in the T P D specha (which reflects the energetic statmeof CO and the capacity of tlie surface) is observed i n some ca.ses when the coadsorption of CO can induce a reconstruction of the modifier overlayer. The CO T P D spectra from p(2 x 2 ) Se-Pt(ll1) i n fig. 5.10. show some peculiar changes in the shape of the CO T P D spectra with increming the CO coverage [35]. These T P D changes are usually associa,ted with a t.ransition from first order to fractiona.1 or quasi-zero order desorption kinetics [36, 371. It should be not,ed that, because of this effect, which indica.tes significant changes in the mechanism of CO desorption from selenided-Pt(ll1) at OCO > 0.06, the corresponding Ed and Vd data in Ta.ble 5.1. a,re calculated for OCO < 0.05, and Oco 0.2. As can be seen in fig. 5.10., at OCO 0.2 the CO desorption is described by a second lower temperature desorpt,ion pea.k which follows first order desorption kinetics. This pea.k a.ppea.rsonly at, adsorption beniperatures below 130 K. At higher adsorption t,enipera.t,ures,tlie CO coverage is saturated at 0.16 and the T P D features resemble a zero order desorption process. When comparing the kinetic data for a p(2 x 2) 0.25 S-Pt(ll1) surface, it can be seen tha,t the ca.pa.cityof p(2 x 2) 0.25 Se-Pt(ll1) for CO adsorpt,ion

-

-

-

80

Chapter 5.

Table 5.1. CO Desorpt,ion Energies, E d , and Pre-exponential Factors, n, for Some Transition Metals Modified by Electronegative Additives SURFACE

MODIFIER

E:

E,'

(kJ/mole)

u:

4

(sec-' )

clean 140 98 1015 P(2 x 2 ) s 110, 90 1013 1012 c(2 x 2 ) s 30 P(2 x 2)O 120 40 1013 [221 p(2 x 2)N -90 93 1013 (2 x 2)p4gC ~241 P d ( 100) clean 160 lox5 I301 p(2 x 2)s 86, GO Ni(111) clean 140 1O l 5 p(2 x 2)s 91 1381 p ( 2 x 2)O 105 10l2 1311 Pt(111) clean 154 96 10l5 1o3 106 82 1013 10" [261 p(2 x 2 ) s 110 82 1013 io1O p(2 x 2)Se [351 Ru(0001) clean 170 1Ol6 MI p(2 x 2 ) s 105 5.1010 E d and V d are the adsorption parameters at very low co coverages (the desorption energy is equal to the initial heat. of adsorption because CO adsorption is a non activated process). E,' and v,’ are the desorption parameters for the corresponding saturation CO coverages for clean and modified surfaces. Ni(100) ~411

-

-

is smaller (- 0.16 compared to 0.25 a t T, > 130'). As will be shown below, this reduced capacity is a result of the CO induced changes in t,he initial Se surface order. Reconstruction effects on the capacity ofsome p(2x 2) X-Ni( 100) surfaces have been reported in ref. [44].

5.1.3

Modifier Effect on the CO Adsorption Kinetics

The influence of the additives on the CO a.dsorption kinetics has usuaJly been characterized by: (i) the changes in the initial sticking coefficient, So,

(ii) the dependence of the sticking coefficient on the adsorption temperature, S(T),

(iii) the variations of the sticking coefficient with CO coverage, S(0).

5.1. Carbon Monoxide

81

es = 0 2 5

%o .22 .20 16

.06

!-

250

300

4-

:

350

400

450

5 0

Fig. 5.10. CO T P D spectrafrom clean P t ( l l 1 ) and Pt(1ll) modified with p ( 2 x 2 ) S - and p(2 x 2) Se overlayers. T, = 220 I<. Dashed TPD spectra in the upper panel are for T, = 90 I< (from ref. [35])

For evaluation of SO, S ( T ) and S(B) usually the CO coverage versus CO exposure plots are used. As a measure of the CO coverage one can also use the appropriate XPS (intensity of the 01s or C l s peaks), AES data (the intensity of the C(KLL) or O(KLL) Auger transitions), etc. Typical CO uptake curves for clean and modified surfaces, where no reordering is induced in the mixed overlayer, are shown in fig. 5.11. A similar effect of the electronegative overlayers on the CO adsorption curves is observed for the other modified single crystal surfaces [15, 16, 28, 311. At low adsorption temperatures (80-120 K.),for all systems under consideration a visible effect of tlie additives on tlie CO initial adsorption rate ( S o ) is observed a t modifier coverages close to or above that corresponding to a p(2 x 2) 0.25 X overlayer [15, 16, 211. The modifier coverages at which SO becomes zero depend 011 the adsorption temperature and the type and surface order of the additive adatoms. At, T, < 100 I<, SObecomes zero when

Chapter 5.

82

0.8

T, = 9 0 K

CI

J

2 0.6 w

99

0.4

0

u

0

0.2 0.0

0.0

3.0

6.0 9.0 12.0 CO EXPOSURE (mborsec x 106,

15.0

Fig. 5.11. CO coverage vs CO exposure for clean and siilfided P t ( l l 1 ) with different S coverages (from ref. [ 2 6 ] )

the formation of a (& x 4)R30 0.33 S overlayer is completed on fcc(ll1) and hpc(0001) surfaces , whereas on the sulfided fcc(lOO), hcc(100), fcc(ll0) surfaces, SO does not become zero even in the presence of a c(2 x 2) 0.5 S overlayer. The lower adsorption temperature ensures an increase in lifetime of the weakly-bound CO states. That is why the reduction of the adsorption rate is less severe than that observed at higher adsorption temperatures. It is worth pointing out that for clean transition metal surfaces, S " ( C 0 ) is close to unity and remains independent of the adsorption temperatures up to 400 I<, while for modified surfaces, So(C0) is a stronger function of the adsorption temperature. Since the introduction of a modifier affects wit*lia different strength the population of the CO surface states, it is important to find out how the local sticking coefficient of the different GO adsorption states changes with increasing modifier coverage. In real catalysis, the reaction temperatures are always above 300 K. That is why, it is of great interest to explain the variations of So of the most tightly-bound CO states as a function of the additive coverage. An illustration of SOvariations as a function of the additive coverage for CO adsorption at 300 I< is given in fig. 5.12. It is obvious that SOsuffers a pronounced, almost linear reduction with increasing additive coverage, which is most severe in the case of a chlorided surface. The influence of electronegative adatoins on the initial rate of molecular adsorption of reagents, such as CO and NO can be satisfactorily explained within the framework of the precursor state model for nonactivated adsorption

-

5.1. Carbon Monoxide

83

’0

ADDITIVE COVERAGE (ML)

Fig. 5.12. Initial sticking coefficient, So, of the &CO state as a function of the additive coverage, The dashed line represents the theoretical dependence according to the relationship S," = St(1 - 4Bx) (from ref. [?a])

[6, 391 where the sticking coefficient can be described by the relationship S=

Q(t, 0) 1 -k (vd/va) exP[-(Ed - E a ) / h T ]

'

Here, vd and va are the pre-exponentials for chemisorption and desorption from the precursor state, Q is the trapping probability, which is coverage and temperature dependent, and Ea and E d are the activation energies for chemisorption and desorptioii from the precursor state. As pointed out in ref. [5, 71, for CO adsorption on most of the transition metal surfaces under consideration ndexp(-Ed/RT) << v,exp(-E,/RT) and SOZ Q is close to unity. The reason for the reduction of So with the intaroduction of an electronegative additive can be understood if one considers the modifier surface order illustrated in figs. 5.8. and 5.9. and the possible adsorption sites for CO on these surfaces. Within the framework of the description given by eq. ( 3 ) the reduction of So can be ascribed t o a decrease in lifetime of the CO precursor on the modifier affected adsorption sites, wliich results in an increase of the second term in eq. (3), i.e the desorption rate of the precursor cannot be neglected for the modified surface sites. One reason for the reduced lifetime of the precursor can be its reduced migration mobility due to the changes of the potential energy surface for CO diffusion in the presence of the modifier adatoms. The absence of an effect on So at low adsorption temperatures (90-100 K) for S coverages much less than 0.25 on most of the substrates under con-

Chapter 5.

84

sideration [15, 16, 26, 311 can be satisfactorily explained by an non-uniform distribution of the modifiers because of the tendency of most of the electronegative adatoms to form ordered islands even at very low coverages (see subsection 4.2.1.). Thus, up to some critical additive coverages a presence of modifier-free islands is possible where the adsorptive properties of the clean surface are preserved.

I

0.3 h

J

I Pt(ll1) I I

%(S)

= 0.Z5

S/Pt(lll)

Fig. 5.13. CO coverage versus CO exposure on the sulfided and selenided P t ( l l 1 ) surface at T, = 90 and 220 K (from ref. [35])

For all modified surfaces, where the introduction of reagents does not disturb substantially the initial order of the modifier, the dependence of CO sticking coefficient on the CO coverage resembles that for the adsorption on a clean surface, despite the reduced So value. This S(0) behaviour is illustrated in fig. 5.13. for CO adsorption on p(2 x 2) S-Pt(ll1) and p(2 x 2) SePt(ll1) at 90 K . The initial constancy of the S(B) plots followed by a gradual decrease above certain CO coverages, reflects a precursor mediated process where the molecules occupy identical adsorption sites. A more complicated behaviour with respect to the CO adsorption kinetics is observed in the cases when the coadsorption of CO results in a substantial reordering of the modifier overlayer. The effect of the structural changes in the mixed overlayers on the CO adsorption kinetics is illustrated by the S ( 0 ) plot in fig. 5.14. obtained at an adsorption temperature of 220 K for selenided P t ( l l 1 ) . As indicated in this figure, the stepwise reduction of S at 0 ~ > 0 0.04 is accompanied by structural changes in the overlayer where the new surface order offers less favorable adsorption sites (see fig. 5.19.). Finally, one exception should be mentioned, when a typical electronegative adatom does not cause a reduction of the initial adsorption rate of CO. This

85

5.1. Carbon Monoxide

is the system p(2 x 2) 0.25 O / P t ( l l l ) , where CO preserves the same So as on a clean surface. As discussed in det,ail in ref. [40], the main reason for that is the great affinity of CO and 0 coadsorbed on P t ( l l 1 ) for COa formation. But on other substrates (e.g. Ni(ll1) [31]), 0 behaves as a typical 'poison' with respect t o CO bonding strength, adsorption rate and adsorptive capacity of the surface.

0 .6

0.5

90 K

ie---o,---o

A

\

1 ’

Q s e ( ~=) 0.25 0

\

i 1

A

0,

Fig. 5.14. CO sticking coefficient, vs. CO coverage on sulfided and selenided Pt(ll1) at T, = 90 and 220 I< (from ref. [35])

5.1.4 Modifier Effect on the CO Adsorption Site Occupation Vibrational spectroscopy data (HREELS and IRAS) as well as some selected ESDIAD data have shown that the presence of electronegative coadsorbates, occupying the highest coordinated sites, always causes changes in the nature of the molecular adsorption sites of the reagents. Comparison between the observed molecular stretching frequencies Me-C and C-0 and the GO TPD spectra gives a good basis for correlating the binding sites with the corresponding adsorption binding energies. Since CO adsorption on various bare metal surfaces exhibits a different sequence of the site occupation, it is impossible to generalize the observed effects induced by the modifier on the CO site occupat,ion. For example, fig. 5.15. shows that, as a result of increasing S coverages on S/Ni(100) [41], the original CO vibrational modes, associated with 'on-top' bonding (at Bco < 0.5) are replaced by: (i) three new modes ascribed to twofold (3 = 1910-1960 cm-l) or fourfold (hl = 1740 cm-') S affected sites in the next nearest position for the p(2 x 2) S-Ni( 100) surface and (ii) one mode ascribed

86

Chapter 5.

t o a fourfold (hz = 2115 cm-l) S affected site for the c(2 x 2) S-Ni(100) surface (see the structural model in fig. 5.9., where b, hl and h2 sites are indicated). The corresponding CO adsorption binding energies are shown in Table 5.1.(110, 90 and 30 kJ/mole, respect,ively).

Fig. 5.15. A set of TPD and HREELS spectra, illustrating the effect of preadsorbed S on CO adsorption states on Ni(100) for an adsorpt,ion temperahre of 90 I< (from ref. [41])

In the case of S/Pd( 100) [19] where on a clean surface CO occupies twofold (bridge) sites, the presence of S up to moderate sulphur coverages does not affect substantially the CO stretching modes, although the T P D spectra show a strong reduction in the population of the highest temperature state. This is not surprising because, as outlined above, the S-affect,ed b state has same bridge coordination (fig. 5.9.). Similar t,o the case of S/Ni(100,) a peak at 2110 cm-I can be distinguished 011 c ( 2 x 2) S/Pd(100) also [19]. The latter is associated with a h2 adsorption site. A very similar behaviour of the CO site occupation w a s observed for C/Pd (100) [19]. The coordination of the modifier-affected CO adsorption sites is different in the case of substrate surfaces with fcc( 111)-like crystallographic symmetry. Detailed studies of CO adsorption on S / N i ( l l l ) [as],O / N i ( l l l ) [31], S and S e / P t ( l l l ) [26, 35, 42 ] have shown that the CO* modifier-affected sites are more likely to be ‘on-top’ sites on the next nearest substrate atom. Thus, the IR studies [28, 311 have shown that the S induced CO adsorption state is characterized by a stretching frequency 2100 cm-’ and its relative concentration grows a t the expense of the original bridge and on top adsorption

-

5.1. Carboil hlonoxide

87

sites (fig. 5.8.). The difference between O / N i ( l l l ) and S/Ni(111) is that the S induced CO* band frequency at 2108 cm-l is invariant with 0s and 0 ~ 0 , whereas the 0 induced CO" band frequency suffers a strong coupling effect with increasing Bco (fig. 5.16.).

A

I3 : e4

a 0

2200

2100

Wavenumber

zoo0 lcm-')

Fig. 5.16. Effect of oxygen (right) and sulphur (left) precoverages on C-0 ing frequencies for CO adsorbed on modified N i ( l l 1 ) (from refs. [28, 311)

stretch-

Regardless of the assignment of the coordination of the most strongly affected CO adsorption site, comparison with the available vibrational data for the C-0 stretching modes shows the following common features: (i) the most strongly perturbed CO' state (the most weakly bound one) always exhibits the highest vibrational frequency mode (of the order of 2110 cm-') which is closer to that of a free CO molecule; (ii) up to certain critical modifier coverages, the original (clean surface) CO bands coexist with the modifier-induced ones, the frequencies of the former being only weakly perturbed by the presence of the additive ad atoms. 5.1.5

Surface Order in Mixed Overlayers

The effect of the electronegative adatoms on the surface order of the coadsorbed CO molecules is studied exclusively by LEED. On all single crystal surfaces, CO tends to form commensurately and incommensurately ordered structures at moderate and high coverages [ 5 , 431. This tendency is preserved

88

Chapter 5.

only in the presence of negligible amounts of additives (less than 0.05) but even then the CO induced spots are fainter and the background is larger than that for CO on a clean surface [15-19, 26, 281. At higher additive coverages CO adsorption does not give rise to ordered structures, which means that CO adsorbs in a disordered manner. The lack of order can be easily understood since the introduction of foreign adatoms prevents long range ordering of the CO molecule, because of hindered occupation of certain surface sites. At moderate and high modifier coverages, when the additive adatoms have formed ordered structures, the possible adsorption sites for CO are restricted and usually, the initial surface order determined by the modifier, is preserved. For an example, CO adsorption on p(2 x 2) S-fcc(ll1) substrates is possible only in tlie "on top" positions on the next nearest substrate surface atom (see fig. 5.8.), which means that at saturation the CO molecules are in the same p(2 x 2) order as the modifier adatoms. In the case of p(2 x 2) - fcc(100) surfaces, the picture is not very clear because, as pointed out in ref. [44], CO-induced reconstruction is possible in the case of some of the modifiers. Most recent studies of mixed electronegative adatoms - CO systems have shown that the CO coadsorption might disturb tcheorder of the electronegative adatoms and, in some cases, can even result in a rearrangement of the modifier. There is a tendency for GO to exert this effect, above certain CO coverages and adsorption temperatures. As illustrated by tlie LEED data in fig. 5.17., at Oco > 0.05 and T, > 240 K CO induces a certain disorder in the initially well ordered p(2 x 2) S overlayers on P t ( l l 1 ) . This disorder is temperature irreversible and the initial well-ordered p(2 x 2) S layers are restored only after CO desorption. More severe structural changes have been observed upon CO coadsorption on p(2 x 2) Se-Pt(ll1). As shown i n figs. 5.18. and 5. 19., the coadsorption of CO causes a phase transformation and an establishment of a new x 8 ) R 1 9 . l 0 order in the mixed CO + Se overiayer. In this new order CO molecule again is adsorbed on the next-nearest on top Pt site, but the capacity of the surface for CO adsorption is reduced at the expense of the increased CO-Se separation. Detailed studies have shown that this phase transition proceeds easily at 0.lG > Oco > 0.04 and T > 130 K. As will be discussed below, the observed CO induced disturbance in the initial surface order of the electronegative additives is accompanied by a reduction of the constraint imposed by the modifier on the CO frustrat,ed translational modes parallel to the surface. The observed relation between the structure in the mixed overlayer and the energetics of the CO adsorption site for selenided P t ( l l 1 ) indicate that one of the most important factors which determine the strength of the modifier effect, is the separation (effective distance) between the electronegative adatom and the coadsorbed GO molecule, whereas the coordination number of the former is of less importance. As shown above, the fi order with 6 Se atoms coordinated around CO with a CO-Se separation of 4.22 A, turns out to be energetically more favoured than the p(2 x 2) structure with 3 Se adatoms spaced around CO at a distance of 3.19 A. The rapid decay of the modifier - CO repulsive forces with the increasing

8

(a

5.1. Carbon Monoxide

co + SIPt(ll1) LEED AK, CO

-

ESOIAD DATA

e,, .

-

210 K '

89

0.15

T,=240

K

F-.

Fig. 5.17. Top: CO' ESDIAD patterns as CO is adsorbed at 90 I< and after annealing at 270 K or upon CO adsorption at 240 I<. Bottom: LEED pattern changes as C O + S / P t ( l l l ) layer is annealed after CO adsorption at 90 K (left); LEED pattern changes as CO adsorption takes place at 340 I< (right.) (from ref.

WI) coadsorbate effective distance explains satisfactorily the absence of a CO induced phase transition in CO/p(2 x 2) S-Pt(ll1). Because of the difference in the Se and S adatom sizes (covalent radii 1.16 A and 1.04 A, respectively) shorter, i.e. the GO-S rethe CO-S effective distance will be by 0.14 pulsive strain will be weaker. Consequently, even when assuming the same activation energy for the transformat,ion of the p(2 x 2) S and p(2 x 2) Se overlayers, thermodynamically the reconstruction will be less favourable in the case of sulfided Pt(ll1). It is quite obvious that the different geometrical arrangements of the additive adlayers are an important factor in determining the possible number of the modifier induced new adsorption states. A recent IR study of the temperature effects on the GO site occupation on Ni(100) - p(2 x 2) X (X = C1, S, 0, N , C) surfaces has indicated that the break in the correlation adatom electronegativity - strength of the poisoning effect in some systems is due

A

Chapter 5.

90

CO

+ 8.IpHtlll-

WED AND CO* ESDIAD DATA

Fig. 5.18. CO’ ESDAD patterns (left) and the corresponding LEED structures (right) as CO is adsorbed 011 p(2 x 3 ) Se-Pt(ll1) at 220 I< (from ref. [42])

t o the reconstruction of the additive adlayer induced by CO above certain temperatures [44]. Table 5.2. summarizes the temperature changes in the CO site occupation (as judged by the measured C-0 stretch frequencies) observed for Ni( 100) - p(2 x 2)X surfaces. C-0 frequencies above 2000 cm-’ are attributed to ‘on top’ CO (‘t’), whereas those below 2000 cm-’ are associated with bridge bonded (‘b’) CO [5]. In all mixed systems considered in table. 5.2., the increase of temperature causes partial CO desorption. It is obvious that in the case of C1, S and 0 the bridge bonded CO is removed, while in the case of N and C the ‘on top’ CO states are desorbed preferentially. Another important feature is that both the C-0 stretch frequencies and the adsorption energy of CO remaining on the S, 0 and C1 modified surfaces after heating to 300 K, are very similar to the ones measured for clean Ni(100). These results for a strong temperature dependence of the CO site occupation for Ni(100), modified with p(2 x 2) overlayers of S, 0, and CI have been explained by considering the possibility for CO induced p(2 x 2) to c(2 x 2) transformation of the modifier overlayer. As a result, the surface will consist of patches of

-

91

5.1. Carbon Monoxide

POSTULATED Se AND CO ARRANGEMENT I N THE TWO PHASES p(2x2kSe with disordered CO

($7x$7)R19.1°rnixed Se-GO ordered layer

Fig. 5.19. CO and Se arrangement for the p(2 x 2) and

(8 x fi)R19.1°

mixed

overlayers on Pt(ll1) (from ref. [42])

c(2 x 2) adatoms and patches of a clean surface where the CO adsorption sites are least disturbed. This is another example of the possible changes in the structural order of the adlayer when the adatoins and the reagent tend to separate in two phases, instead of forming one mixed surface phase as was the case of CO + Se on P t ( l l 1 ) [42] As outlined in ref. [44], therinodyiiam~callythe CO induced reconstruction of a p(2 x 2) adatom overlayer on Ni(100) will be favoured only in the case when the energy gain for CO moving from the modifier perturbed ‘bridge’ site t o an energetically more favourable ‘on top’ adsorption state is larger than the energy required for the p(2 x 2) to c(2 x 2) transition. Since the estimated energy gain is found to be 40 kJ/mole, the lack of reconstruction in the case of N fits in well with the estimated energy of 80 kJ/mole for the p(2 x 2)N t o c(2 x 2)N transformation (whereas for 0, S and C1 this energy is 40-50 kJ/mole) [44]. Although to date, few studies have been concerned with the temperature and the reordering effects on the C-0 staretchingfrequencies, CO bending vibrations and the overlayer surface order, the available data indicate that the possibility of CO-induced structural transformations cannot be excluded. These are similar to that observed for CO/Ni(100) - p(2 x 2)X [44], CO/Pt(lll) - p(2 x 2) S and C O / P t ( l l l ) - p(2 x 2) Se [35, 421 in the other modified systems under consideration. Such structural changes are more likely to be responsible for the lack of a straightforward correlation be-

-

-

92

Chapter 5.

Table 5.2. C-0 Stretch Frequencies, w , at Different Temperatures for p(2 x 2) Overlayers of Various Modifiers on Ni(100) (data from ref. [44]) ADDITIVE

w (cm-I) at

T = 170 I<

w (cm-I) at

bare Ni(100) p(2 x 2)Cl

1965 (‘b’), 2035 ( 1 ) 1945, 1955 (two ‘b’) and 2036 ( ‘ 2 ’ ) 1946 (‘b’) 1957 (‘b’) 1934 (‘b’) 2046 and 2067 (two 9’) weak 1937 (‘b’) 2081 (‘2’)

2038 (‘1’) 2028 ( ‘ t ’ )

p(2 x 2 ) s p(2 x 2)O p(2 x 2)N p(2 x 2)C

T, > 290 I<

2021 (9’) 2017 ( 1 ) weak at 1940 weak 1937 2060 ( ‘ t ’ ) with a reduced in t,ensity

-

(‘b’) means bridge CO bonding, and ( ‘ t ’ )means terminal CO bonding.

tween the strength of the poisoning effect a.nd the adatom electronegativity which applies in some cases to additives with close atomic sizes.

5.1.6

Modifier Effect on the CO Mobility and Bonding Orientation

Another important characteristic of the modified adsorption site is the effect on the molecule soft ‘bending’ vibrations parallel to the surface. These vibrations are sensitive to the modifier induced changes in the surface potential energy contour and are related to the adsorbate mobility and desorption rate. Information about the relative changes in the amplitudes of tlie translational (‘wagging’) vibrational modes by the introduction of a modifier ( S or Se) has been obtained recently from measuring the widths of the ESDIAD patterns of the excited neutral CO species from sulfided and selenided P t ( l l 1 ) . As described in refs. [45-471, the widths of t,he ESDIAD patterns of the neutral species are determined by the root-niean-square vibrational amplitudes of the Me-C-0 bending modes. The effect of p(2 x 2)S(Se) and x &)R19.l0 Se overlayers on tlie CO frustrated t,ranslational vibrational frequencies are summarized in table 5.3. Comparison of the wt data in table 5.3. and the wt values for a clean surface (48 c1n-l) shows that for both S and Se, the CO vibrational motions are constrained somewhat as compared to the unmodified surface. Considering the data on tlie reduced CO adsorption energies, which implies increased mobility and less hindered translational and rotational degrees of freedom, it is likely that the reason for the observed reverse effect (a hindrance of the CO surface motions) can be only the existence of substantial modifier - CO repulsive interactions. The trend in the changes of w t ( X ) going from p(2 x 2) to (fi x J?)R19.lo surface order shows that the new surface order established as a result of the CO induced phase transitions offers

(J?

5.1. Carbon hlonovide

93

CO adsorption sites with less hindrance to its vibrational freedom. Indeed, an inspection of the structural models for the arrangement of the coadsorx a ) 1 9 . l o orders shows that at the expense bates in the p(2 x 2) and of the reduced adsorption sites for CO (from 0.25 down to 0.16) the Se-CO separation in the J? order increases by 1.03 A (from 3.19 A for p(2 x 2) to 4.22 for the J? order). It should be noted that the order of Se is stabilized only by the presence of CO arranged in the same structural order. The sharp GO desorption peak froin the fi mixed overlayer reflects a drop in the CO desorption energy, because, as a result of the destruction of the J? configuration the less favourable CO p(2 x 2) adsorption configuration is restored.

(8

J?

Table 5.3. CO Frustrated Translation Vibrational Frequencies, wt,on S and Se Modified P t ( l l l ) , as Estimated on the Basis of the CO ESDIAD HWHM, 0,using the simple relationship w t ( 0 ) / w t ( X ) = O2(X)/0’(0),where: 0 ( 0 )= 6.3', w t ( 0 ) = 48 cm-' are the values for Bco

<

0.15 for clean P t ( l l 1 ) (data from refs. [16, 35,

421)

SU B STRATE p ( l x a ) S-Pt(ll1) p(2 x 2) Se-Pt(ll1) ( a x a ) 1 9 . 1 ' Se-Pt(ll1)

O( X ) (degrees) 4.5-5f 0.5 4.4 f 0.5 5.5 f 0.5

wt(X)

(cm-') 100-80 100 60

The ESDIAD results for CO adsorption on sulphided and selenided P t ( l l 1 ) with normally centered ESDIAD narrow patterns indicate that the CO molecule preserves its orientation with the C-0 axis normal to the surface both for the p(2 x 2) and (8x 8 ) R l g . l " surface orders. Certain slight 'offnormal' tilting of the coadsorbed CO, which causes extreme broadening of the ESDIAD patterns, were detected only in disordered CO Se overlayers [35]and at CO coverages larger than 0.25 on a p(2 x 2 ) O / P t ( l l l ) surface [40]. For transition metal surfaces where at low CO coverages the CO bonding orientation is not normal to the surface, the introduction of an electronegative modifier was found to inhibit the population of t,he "lying down'' adsorption state for CO. Such a modifier induced reorientation of molecularly adsorbed CO is observed for C r ( l l 0 ) [48,49] precovered with atomic oxygen. It should be pointed out that the CO state the intramolecular axis of which is nearly parallel to the substrate surface, is the most favourable adsorption state and serves as a precursor for CO dissociation. Thus, by removing this CO adsorption configuration, the electroiiegative adatoms are expect.ed to inhibit the CO dissociation as well.

+

94

5.1.7

Chapter 5. Effect of the Substrate Surface Orientation on the Range and Strength of the Modifier Effect

Summarizing all data concerning the various effects on the CO adsorption on modified surfaces, it is obvious that there is a significant difference in the effective range of the modifier effect for different substrate crystallographic planes. On the basis of available data, the best examples are the differences in the behaviour of the p(2 x 2) S-fcc(100) and fcc(ll1) planes (see figs. 5.8 and 5.9.), where S occupies fourfold hollow and threefold hollow sites, respectively. According to the assignments of the CO adsorption sites on the p(2 x 2) S modified surfaces, for the fcc(100) plane (fig. 5.9.), all S affected CO sites are bridge sites sharing a substrate surface atom with S, whereas for the fcc(ll1) plane all adsorption sites (bridge or on t.op) involving the nearest substrate surface atom are unfavourable for CO adsorption. Here it should be recalled that the separation of the threefold adsorption sites foq ( Ax fi)R30° S-Ni(ll1) from the S adsorption site is by 0.3 A larger than the separation of the four-fold adsorption site for c(2 x 2) S-Ni(100) (figs. 5.8 and 5.9). This result indicates that the strength and the extent of the effect of the same modifier and reagent cannot be simply correlated only with the separation between the adsorption sites because obviously the actual configuration of the modifier adsorption site is also of importance. Taking into account the present state of knowledge about the interaction of electronegative additives with the transition metal surfaces, described in the Section 4.2., the following explanation can be offered regarding the observed difference in the range of the S effect on the fcc(100) and fcc(ll1) planes. The interaction of S with the substrate atom from the second layer below the hollow is stronger for the fourfold S coordination on the fcc( 100) plane than for the threefold S coordination on the fcc(ll1) plane. As will be discussed in more detail in the forthcoming section 5.1.8., the stronger coupling with subsurface metal atoms will reduce the range of the blocking effect along the surface. This explanation fits in well with the data on the weaker poisoning effect in the case of additives such as C (which has the same electronegativity as S) which are deeply embedded in the substrate surface.

-

5.1.8

Modifier Effect on the Electronic Structure of the Adsorbed CO Molecule

Undoubtedly, the reduction of the adsorption energy of the CO molecule adsorbed on adsorption sites influenced by the additive adatoms will be accompanied by changes in the core and valence electron energies of the adsorbed CO. Recent ARUPS data on a CO-S/Ni(lOO) adsorption system [50] have revealed that the major effect of the coadsorbed S lies in the change in the energy position of the CO 517 level which moves by 0.7 eV to a lower binding energy, i.e. closer to its gas phase value. Since in the case of strong CO chemisorption (as outlined in subsection 4.1.1.) the stabilization of the 5s level is regarded as being due to 5u-metral bonding interactions, the S induced destabilization of the 5s levels indicates a reduction of the CO 5o-metal coupling in the S affected adsorption sites. This impeded 5a donation to the

-

5.1. Carbon Monoxide

95

metal can be ascribed to the repulsive interactions occurring bet,ween the CO 5a molecular orbitals and the energetically close 3 p orbitals of S . Another indication of the changes in the degree of coupling of the CO molecular orbitals with the metal in the modified adsorption sites is found in the work function results. Table 5.4. 0 1s and C 1s Core Level Binding Energies for CO Coadsorbed with Electronegative Adatoms SURFACE

Fe( 100)

[521 c ( 2 x 2) C-Fe(100) p(1 x 1) 0-Fe(100) c(2 x 2) S-Fe(100) Ni(100) [531 0.4 S/Ni(100) Rh(100)

P I

c0 (0 1s)

co ( C 1s)

(eV)

(eV)

530.6, 531.4, 532.2 (01) (02)

-

533.9 531.5 533.7 531.3

284.3

(03)

284.8 284.9 285.4 -

-

0.4 N/Rh(100) Mo( 110)

531.6 531.7

385.0

[541 (1 x 1 ) C-Mo(ll0)

533.1

285.5

~

Fig. 5.20. shows the typical changes induced in the work function data by the presence of electronegative ada.toms [31]. Similar work function results concerning CO adsorption on modified surfaces are reported for other systems where CO adsorption on a clean surfa.ce ca.uses positive changes in the work function (e.g. CO-S/Ni(lOO) [ l G ] , CO-O/Ir(llO) [15], etc.). The positive work function cha.nges induced by CO adsorpt,ion 011 a.11 c1ea.n transitmionmetal surfaces under considera.tion (with the exception of P t ) reflect the direction of the net charge transfer as a result, of the met,al/ CO 2 i ~bonding. It is obvious from fig. 5.20., tha.t on modified surfaces the total work function changes induced by the same amount of adsorbed CO tend to decrea,se with increa.sing modifier covera.ge. For very high modifier coverages, when the CO adsorption sites are strongly perturbed, negligible or even negative work function changes can be associated with the occupation of these sites. These work function results reflect the influence of the electronegative a.dditives on the contribution of the nietal/CO 277 backdona.tion to the CO adsorption bond. The smaller dipole moment of the perturbed CO molecules indicates a reduction of the metal/2i~coupling, i.e. weakening of the CO adsorption bond. Consequently, the ARUPS and the work function results indica.te that the

96

Chapter 5.

reduced metal surface - CO coupling in the affected adsorption sites is the result of modifier effects on both the donor and the acceptor component of the metal - CO bonding.

1.5

a

>,

1

2

1.0

al CI, e 0

z 0

._ 0 . 5 c

U E

a

A 0.10

OH@

LL

x

0

L

0.18 0.25

0.2

Co Coverage,

0.4

0.6

ecc0 (CO/Ni 1

Fig. 5.20. CO induced work function changes during adsorption on N i ( l l 1 ) modified with increasing amounts of oxygen (from ref. [31])

The changes in the metal surface - CO coupling also affect the core electron energies of the CO molecule. As can be expected, in the presence of a modifier both C 1s and 0 1s levels move to higher biiiding energies compared t o that measured for CO on a clean surface for the same adsorption site symmetry. Selected data concerning the effect of different modifiers on the 0 1s and C 1s binding energies for molecularly adsorbed CO are given in Table 5.4. This tendency of an increase of the 0 1s and C 1s binding energies of CO adsorbed in the modified sites is consistent with the general trend of reduced final state relaxation effects for adsorbates less strongly coupled with the surface.

5.1.9

Modifier Effect of the C O Dissociative A d s o r p t i o n

Thus far, the influence of the electronegative modifiers on the molecular adsorption state of CO was dealt with. As noted in subsection 5.1.1., the ten-

5.1. Carbon Monoxide

97

dency of the CO molecule to dissociate on clean transition metal surfaces increases going to the left in the corresponding row of the Periodic table. Under ultra high vacuum fractional CO dissociation at T > 300 can be observed on Fe, Mo, Re, W, Cr and Co surfaces [5, 71. The importance of CO dissociation as an intermediate step in the methanation and Fischer - Tropsch syntheses is well recognized [55]. This is due to the fact that the methane formation and the chain growth of higher hydrocarbons on the catalyst surface are preceded by rupture of the C-0 bond.

@,SO.

e. = .o 9.- .l

8.. .2

8. .a 9,. .I

Fig. 5.21. Changes in the CO TPD spectra on sulphur - covered Fe(100) with increasing sulphur coverage. CO exposure = 12 L (from ref. [59])

Recent HREELS and SEXAFS data [49,56-58] have defined the precursor state for CO dissociation as a tilted (flat-lying) molecular state characterized by an extensive occupation of t,he 27r CO molecular orbihls, compared to that for a normally bonded CO molecular adsorption state. As suggested in ref. [56], the energy positions and the intensities of the CO ?r and (I resonances in the SEXAFS spectra indicate an extreme elongation of the C-0 bond (1.47 A) and a lowering of the C-0 bond order of the precursor CO state. Consequently, in the adsorption state which serves as a precursor for CO dissociation, the CO occupied molecular orbitals are strongly rehybridized in comparison with the way CO is conventionally bonded. CO adsorption data

Chapter 5.

98

for systems where a fraction of the adsorbed molecules can dissociate have shown that the precursor state for dissociation is occupied first and the fraction of the dissociated molecules decreases with increasing CO coverage. The number of dissociated molecules is restricted for the lack of sufficient appropriate (highly coordinated) adsorption sites for the dissociation products. It is worth pointing out that even on a unmodified surface, the dissociation by itself introduces the electronegative adatoms C and 0 which can act as poisons if they do not participate in further reaction steps. Consequently, if the dissociation occurs at the adsorption temperature, the surface is gradually deactivated by the increasing amount of C and 0 adatoms with increasing CO exposure. Actually, under the real conditions of the catalytic reaction of CO hydrogenation when C and 0 do not exceed certain critical coverages, they can be removed from the catalyst surface by the reaction steps: (i) zC (ii) 0

+ yH - hydrocarbons(gas);

+ 2H = HzO(gas)

Since the precursor CO molecular state for dissociation is supposed to be more strongly coupled to the substrate, it should be expected that the presence of electronegative additives will significantly affect this state, leading to the inhibition of CO dissociation. The effect of the increasing sulfur coverages on the CO T P D spectra from Fe(100) is illustrated in fig. 5.21. The clean surface CO peaks designated as a1, Q , and a3 are due to desorption from molecular adsorption states, whereas the high temperature one - P-peak is the result of recombination of the dissociation products C and 0. Thorough studies have proved that only the CO as-state is related to CO dissociation. The dissociation is competing with the a3 molecular desorption and occurs at temperatures higher than 450 K [52, 59, 601. The introduction of S affects the desorption spectra from the CO molecular adsorption states in the usual way (as described above), i.e. by removing the original clean surface states and replacing them with new less-strongly bound states. The major S effect is a reduction of the dissociated fraction and complete inhibitsionof dissociation at the p(2 x 2) 0.25 S layer. A close inspection of the T P D spectra shows that the inhibition of dissociation cannot only be associated with the removal of the a3 ‘precursor’ state. This state with reduced population still exists at 0s = 0.25 but cannot undergo dissociation because S has blocked most of the fourfold sites which are the adsorption sites favoured by the dissociation products C and 0. At low Os, the presence of S also hinders, to a certain extent, the recombination of the dissociation products reflected by the increasing temperature of the /3 peak. The same effects consisting of the removal of the most strongly bound CO molecular states and reduction of the fraction of dissociated CO have been observed during CO interaction with sulfur predosed Re(0001) and Re(1010) surfaces [63]. Fig. 5.22. illustrates how, with increasing S coverage, the capacity of the surface for CO adsorption and the fraction of dissociated GO are reduced. The inhibition effects on CO dissociation of the other electronegative adatoms are quantitatively the same. An illustration of the differences in

5.1. Carbon Monoxide

99

Fig. 5.22. Total amount of adsorbed and the dissociated fraction (as evaluated from the total CO TPD area and the area under the p recombination peak) as a function of sulfur coverage on Re(1010) (from ref. [63])

strength of the effects of different modifiers are the CO TPD spectra show11 in fig. 5.23. [52]. It is obvious that the effect on the energetics and populat,ion of the molecular adsorption states, and on the dissociation propensity of GO becomes less deleterious in the sequence S, 0, C. The modifier induced reduction of the adsorption rate on the S, 0, and C modified Fe( 100) surfaces also decreases in the same direction. The effects of the three electronegative additives on the molecular adsorption bond, the dissociation propensity and the CO sticking coefficient are summarized in table 6.5. It should be noted that in the case of a carbided surface, the fact that the fraction of dissociated CO is small, might be due to the tendency to diffusion of carbon into the bulk at elevated temperatures and this would liberate some C-free areas. The same trend in the strengths of the C and 0 effects on CO dissociation is observed with modified Mo(100) [Gl]. It has been found that, with increasing carbon coverage, the dissociated fraction linearly decreases from 504% for one monolayer of C (Il40( 100)-(1 x 1)C). This observation iudicat.es that the suppression of CO dissociation is due esclusively to blocking of the available fourfold sites. In the presence of oxygen there is complete inhibition of the CO dissociative adsorpt,ion at half a monolayer of oxygen, accompanied by a considerable reduction in the desorption energy of the CO molecular state. Obviously, because of its higher electronegativity, oxygen behaves like a more severe poison than carbon. The oxygen-induced inhibition of CO dissociation cannot be related to blocking of the available fourfold adsorption sites

Chapter 5.

100

l

a

200

400 TEMPERATURE

80 0

600 (K)

Fig. 5.23. Effect of carbon, oxygen and sulfur adlayers on CO desorption from Fe(100): (a) clean surface, 10 L exposure; (b) c(2 x2)C, 10 L exposure; (c) p(1 x 1 ) 0 , 100 L exposure; (d) c(2 x 2)S, 1000 L exposure (from ref. [52])

alone. The main reason for the observed pronounced destabilization of the CO molecular state on a p( 1 x 1) O-Mo( 100) surface is supposed to be the substantially reduced backdonation response of the surface in the presence of oxygen. This enhances the efficiency of oxygen as an inhibitor to the CO dissociation process which surpasses the efficiency expected when considering only the effect of blocking of the fourfold sites. Since the additives C and 0 are also dissociation products, their simultaneous presence a t the surface is unavoidable at the catalytic reaction temperatures. Studies of the effect of increasing equal carbon and oxygen coverages on

Effect of Electronegative Modifiers on the Average CO Adsorption Binding Energies, E d , the CO Adsorption Rate, SO,and t,he Dissociated Fraction of CO, &,o, on Fe(100) for T, = 150 I< ( d a t a from ref. [52])

Table 5.5.

SURFACE Fe(100) c(2 x 2) C-Fe(100) (1 x 1) 0-Fe( 100) c(2 x 2) S-Fe(100)

Ed (kJ/mole)

so

95 75 75 45

0.2 lo-?

1 10-4

AC,O(%)

-- 525 0 0

101

5.1. Carbon Monoxide

Fe (100) [G2] and M o ( l l 0 ) [54] on the molecular and dissociative adsorption of CO have shown the following peculiarities:

+

(i) the same total amount of equal concentrations of C 0 on the metal surface causes a weaker reduction in the CO adsorption bond strength than the same amount of C (the measured initial heats of CO adsorption on c(2 x 2) 0.5 C-Fe(100) equals 77 kJ/mol, while 95 k.J/mol corresponds to (0.250 0.25C)-Fe( 100) [G2]);

+

(ii) the effect on the dissociation rate is average between that of a surfacemodified one with the same amount of C or 0 adatoms alone. The weaker effect of the mixed C + 0 overlayers than that of C alone is attributed to the possible immobilization of the effect if C and 0 are bound in close proximity on the surface. As regards the CO dissociation rate, it has turned out that the additive effects on the kinetic parameters ( activation energy of dissociation and frequency factor) tend to compensate each other, and the net effect on the dissociation rate is relatively small [54]. Consequently, the major reason for the inhibition of CO dissociation is more likely the enhanced probability of desorption of the molecular CO state, rather than the inhibition of the dissociation rat8e. 5.1.10

Influence of the Chemical State of the Modifier on the Strength of Poisoning

In this subsection the dependence of the modified properties of transition metal surfaces on the chemical state of the modifier will be considered. As outlined in chapter 4.2., one of the factors determining how strong the poisoning is, is the actual chemical state of the electronegative additive 011 the surface. A good example of the relat,ion between the chemical state of the modifier and its effect on the CO molecular adsorption state and dissociation propensity are the recent HREELS studies of CO adsorption on oxygen modified Cr(ll0). Three states of oxygen are studied: the chemisorption state, the subsurface oxide and the oxide phase. Table 5 . 6 . illustrates selected data about, CO site occupation and adsorption rate for oxygen-modified C r ( l l 0 ) [44, 641. From the data in table. 5.6. the following important information can be derived: (i) the chemisorbed atomic oxygen species has a stronger direct site-poisoning effect than that of the intermediate oxidation state of Cr( 110), where oxygen is located in the near-surface region; (ii) the oxidized surface is significantly passivated with regard to CO adsorption and the weak CO features can be at,trihuted to the existence of isolated metal defect sites because of imperfect areas in the oxide layer obtained under mild conditions. It is obvious that the removal of the ‘lying down’ CO-state which serves as a precursor for CO dissociation is favoured only when oxygen is present on the

Chapter ,5.

102

Table 5.6. CO Adsorption State, the Corresponding C-0 Stretching Frequencies, wco, and the Initial Sticking Coefficient, So, for C r ( l l 0 ) Modified with Oxygen (data from ref. [64])

SURFACE

ADSORPTION STATE

Cr(ll0)

‘lying down’ CO 1150, 1330 ‘top’ 1955-1 975 ‘bridge’ 1865 ‘top’ 1975 ‘bridge’ 1865 co3 ? 1500 ‘lying down’ 1150-1 330 ‘top’ 1840-1 955 new stat.e 3035 very weak feat.ures at 1170, and 1350, and 1915

C r ( l l 0 ) with chemisorbed eo 0.15 Cr(11O) sub- o xi de

5

Cr(ll0) ‘oxide’

wco (cm-’)

SO

-

1

3.10-*

surface or a surface oxide phase being formed.The effect of the sub-surface oxygen, despite the fact that its absolute concentration is higher than that of the adsorbed oxygen, exhibits significantfly weaker effects on both the occupation of the molecular adsorption stmatesand the dissociation of CO. The difference in the poisoning effects exhibited by the same modifier present on the surface in various chemical states is well demonstrated by comparing the adsorptive properties of substrates ‘carbide’ and ‘graphite’ overlayers. Thus, as shown in fig. 5.24., surface carbon in the carbide form on N i ( l l 0 ) has weaker effect on both the CO adsorption energy and the capacity of surface for CO adsorption than is the case of surface carbon forming a basal layer of graphite [23]. Since the graphite phase grows in islands, the presence of two CO adsorption states when the surface is not completely covered with a graphite layer, is associated with CO adsorbed at the periphery of the graphite islands (CO adsorption energy 50 kJ/mole) and CO molecules residing on C-free pat,ches (CO adsorption energy 96 kJ/mole, i.e. close to that for high CO coverages on a clean surface) [2G,661. A complete surface deactivation is observed on completion of a monolayer of graphitic carbon, because the CO heat of adsorption for a. graphite surface is as low as 15 kJ/mole [GG]. A similar effect of the growing graphite phase on CO adsorption is observed for carbon modified Pt( 111) [ G S ] .

- -

-

5.1.11

Concludiiig Remarks

As will be illustrated in Chapter 8., the adsorptive properties of modified surfaces determine to a large extent the changes in activity and selectivity of the metal catalysts. Since CO coadsorption with electronegative additives is characteristic, it serves t o explain the poisoning effect with respect to reagent

103

5.1. Carbon Monoxide

1

1

.

1

.

I

150200 250

1

I

I

I

.

I

.

I

.

I

.

300 350 400 450 500 TEMPERATURE ( K )

Fig. 5.24. CO TPD spectra for CO from N i ( l l 0 ) ( 2 x l ) C and N i ( l l 0 ) with carbon in graphite islands (from ref. [23]) molecules which exhibit electron acceptor behaviour. The major changes in the CO adsorption kinetics, energetics, site occupation, surface dynamics and dissociative propensity revealed by means of extensive surface science studies are now summarized. These changes are as follows:

(1) Reduction in the CO adsorption rate due to a decrease of the lifetime of the GO precursor for adsorption on the surface sites affected by the modifier. The magnitude of this reduction depends on the adsorption temperature and modifier coverage.

(2) Reduction in the total adsorptive capability of the surface with respect t o molecular CO adsorption due t,o blocking of the favourable adsorption sites by the modifier.

(3) Sequential elimination of the original CO molecular adsorption states starting with the most tightly bound one. The critical modifier coverages at which the original CO adsorption states are completely eliminated depend on the size, the electronegativity and the surface order of the additive adatoms. (4) Appearance of new less strongly bound CO molecular adsorption states. They are associated wit81ithe occupat,ion of different adsorption sites in the close vicinity of the additive adatoms. CO molecules adsorbed in

Chapter 5.

104

these affected sites exhibit different vibrational modes, electronic structure and reduced adsorption binding energies compared to CO molecules adsorbed on clean surfaces.

(5) Constraints of the CO frustrated translational vibrational motions parallel to the surface, which indicates that the modifier induced changes in the surface potential energy cont,our affects the CO mobility as well. As is well known, the adsorbate surface dynamic are an important factor in the surface reactions.

(6) Suppression of the CO dissociation propensity and complete inhibition of GO dissociation above certain additive coverages. This effect on the CO dissociation propensity is due to several factors: (i) a decrease in the lifetime of the molecular CO adsorption state which is a precursor for dissociation; (ii) blocking of the energetically most favourable adsorption sites for the transition state and dissociation product,s; (iii) a possible increase of the activation barrier for dissociation,provided that the binding energies of the dissociation products C and 0 are affected substantially by the presence of a modifier; (iv) impeded CO surface diffusion, etc. The first three factors determine the magnitude of the poisoning effect on the nearest and next-nearest substrate surface atoms. Similar effects of electronegntive additives on the adsorption of acceptorlike molecules, such as NO, Na, 0 2 , are presented in the forthcoming sections.

5.2 5.2.1

NITRIC OXIDE General Remarks for NO Adsorption

011

Clean Metal Sur-

faces

Extensive studies of NO interaction with single crystal metal surfaces during the last decade are directly related to the fact that NO is one of the most tedious components in the automobile exhaust gases. In looking for the most effective catalyst for NO reduction, fundamental knowledge about its chemisorptive behaviour on the catalyst surface is necessary. Of greatest interest as catalysts for NO reduction are the metals P t , Rh and Pd. The electron structure of the NO molecule is similar to that of CO, the essential difference being caused by an additional electron occupying the antibonding 2n level. The presence of this unpaired electron in the 27r molecular orbital of NO promotes dissociation probability for NO compared with CO (the dissociation energy, D N O , for the NO molecule in gas phase is 717 kJ/mole, compared to 1072 kJ/mole for CO [67]). That is why NO exhibits a more pronounced tendency to dissociative adsorption than CO.

5.2. Nitric Oxide

105

The description of the possible NO adsorption bonding on the metal surfaces is made on the basis of the NO bonding in transition metal - nytrosil complexes [68-701. Similarly to CO, the formation of the adsorption bond is via N , N-5a lone pair and partly filled 27r orbitals being involved in the bonding. Because of the presence of an extra unpaired 27r electron the following three types of NO bonding to the surface are possible: (i) ‘on top’ linear bonding, with donor 5a/metal d, and covalent NO27r/metal d , (Ir-like) components; (ii) ‘on top’ bent bonding, where only the NO 27r-orbital participates in the formation of a covalent s bond as a result of overlap between the unpaired 27r electron and a s electron of the metal. In this bent configuration the 50 orbital remains non bonding; (iii) two-fold bridge bonding, where one of the metal atoms is involved in the formation of a donor (a-like) bond with the 5a orbital and the other, in the formation of a covalent ( d i k e ) bond with the 27 NO orbital.

I

I

100

400

700

1oOo

1 m

TEMPERATURE IK)

Fig. 5.25. NO, N2 and (from ref. [SS])

0 2

TPD spectra for increasing NO coverages on Rh(ll1)

This bonding configuration also has a.n N-0 axis perpendicular to the surface plane. Evidently, the backbonding component of adsorption is largest in case (iii) and smallest in case (ii), i.e. the dissociation of NO will be easiest in the case of bridging NO adsorption configuration. The strongest backbonding component for the bridging configuration is also confirmed by the fact that

106

Chapter 5

the occupation of bridge sites is accompanied by a larger increase of the work function than those observed with adsorption in the ‘on top’ sites. Vibrational spectroscopy data (HREELS and IR) have shown that the twofold bridge bonding is the preferred adsorption configuration of NO especially at low NO coverages [71-801. Besides, a variety of other bonding modes, associated both ‘on top’ configurations [74, 76, 79, 81-83], threefold sites [84] and side-on bonded species [75], have been observed.

Fig. 5.26. Dissociated NO amount (as a fraction of saturated NO coverage at 100 K ) and the amount of molecularly desorbed NO as a function of t.he initial NO coverage for several single crystal metal surfaces (from ref. [86])

With the exception of P t ( l l l ) , P d ( l l 1 ) and Pd(100)’ NO adsorption on the transition metals (Nil Ru, Rh, Ir, Cu, Pt(100)) which were also studied is complex, with both molecular and dissociative adsorption states [85]. The relative amount of dissociated NO decreases with increasing NO coverage a t the expense of an increasing amount of undissociated NO [85-901. At low

5.2. Nitric Oxide

107

-

NO coverages the decomposition of NO usually occurs at temperatures 200300 K , the decomposition temperature increasing to 400 K for high NO coverages. The removal of dissociation products by associative desorption as N2 and 0 2 is possible for substrates which do not exhibit a strong tendency to nitride or oxide formation, e.g. on Ni surfaces the dissociation product 0 remains [89, 901, whereas it desorbs as 0 2 from Ru, Rh and P t [71, 86, 871. Figs. 5.25 and 5.26 illustrate the NO, N2 and O2 T P D spectra for increasing NO coverages on R h ( l l 1 ) and the dependence of the fraction of dissociated NO on the initial NO coverages for several substrates. From the T P D spectra in fig. 5.25 it is obvious that the 0 adsorption state on Rh, which is one of the most promising catalysts for NO reduction, is rather stable and can enrich the surface and alter the reaction pathways. Similarly t o CO, NO tends to form various ordered structures on single crystal surfaces determined by the NO coverage and substrate nature [43]. With most transition metals the NO initial sticking coefficient is close to unity and the initial heats of adsorption are in the range 100-120 kJ/mole [85911. At low NO coverages, the activation energy of dissociation for substrates such as Rh, Ru and Ni is of the order of 90 kJ/mole. If NO coverages are high, desorption of part of NO is necessary for the creation of vacant sites for NO dissociation [78]. 5.2.2

Modifier Effect on the N O Molecular A d s o r p t i o n

A. A d s o r p t i o n kinetics and energetics. As outlined above, from all NO/single crystal transition metal systems, studied to date, no NO dissociation occurs in the N O / P t ( l l l ) , N O / P d ( l l l ) and NO/Pd(100) systems [72, 73, 81, 82, 91-95]. That is why some of these systems will be used for illustrating the effect of electronegative modifiers, such as 0, S and Se, on the NO molecular adsorption kinetics and energetics. In contrast to CO [40, 96, 971, no oxidation to NO2 occurs in the NO 0 coadsorption system on P t ( l l 1 ) [82, 981, since thermodynamically the NO2 reduction is the favoured process [99]. Figs. 5.27. and 5.28. present the NO T P D spectra and the adsorption kinetics plot for p(2 x 2) 0.25 O-Pt(ll1). The higher temperature NO T P D peak from O / P t ( l l l ) , located a t 340 K saturates at NO coverage 0.25, whereas the maximum NO coverage on 0.25 O / P t ( l l l ) is 0.38. Compared t o the data for a clean Pt(ll1) surface, both the capacity of the surface to adsorb NO, and the initial heat of adsorption are reduced by the presence of oxygen. Similar results for NO adsorption on 0.25 O / P t ( l l l ) have been observed by M. Bartram et al. [98]. These authors have also worked at a higher oxygen coverage and have shown that in the presence of 0.75 0 the NO saturation coverage and adsorption energy are reduced further. The kinetic curves in fig. 5.28. show that the presence of 0.25 0 does not affect the initial adsorption rate of NO. Figs. 5.29 and 5.30. present the NO T P D spectra for p(2 x 2) 0.25 SPt(ll1) and p(2 x 2) 0.25 S-Pt(ll1) surfaces. In the case of a sulphided surface, NO desorbs in a single peak located a t 270 K . Together with the reduced adsorption capacity, the sulphided surface also exhibits a decrease in

+

-

-

-

Chapter 5

108

I

100

200

300

400

TEMPERATURE [K]

Fig. 5.27. NO TPD spectra for increasing NO coverages on p(2 x 2) 0.25 0P t ( l l 1 ) . Dashed curve shows the NO TPD spectra for saturated NO coverage on Pt'(111). To= 100 I< (from ref. [82])

0.5 a

3 NO FLUX [ 1O+’ 4M0LECULES/crn2]

Fig. 5.28. NO coverage vs.exposure plots for NO adsorption on clean (a) and modified with p(2 x 2) 0.25X overlayers P t ( l l 1 ) . X: (b) - 0; (c) S; (d) Se. To= 100 K (from ref. [82])

5.2. Nitric Oxide

109

-

3

n

1

1OL'

200

300

400

500

TEMPERATURE [K]

Fig. 5.29. NO T P D spectra for increasing NO coverage on ~ ( 2 x 2 0.25 ) S-Pt(ll1). T h e dashed curve shows the NO T P D spectrum for a saturated NO coverage on clean P t ( l l 1 ) . T, = 100 I< (ref. [82])

dT/dt= 1.5 K/sec

0.04 10

Fig. 5.30. NO T P D spectra for increasing NO coverage on p(2 x 2 ) 0.25 SeP t ( l l 1 ) . T h e dashed curve shows the NO TPD spectrum for a saturated NO coverage on clean P t ( l l 1 ) . T, = 100 I< (ref. [82])

Chapter 5

110

the NO adsorption rate (fig. 5.31.). The NO TPD spectra from a selenided surface are more complicated, consisting of three T P D peaks, located at 280, 245 and 225 K at saturation. The effect of Se on the adsorption capacity and adsorption rate at 100 K is similar to that of S. At elevated adsorption temperatures (- 200 K), the capacity t o NO adsorption on Se becomes much less which, as will be shown in the forthcoming sections, is due to structural changes in the overlayer. The effect of 0, S, and Se overlayers on the NO initial heats of adsorption (equal to the NO desorption energy in the limit of very low NO coverages, E z , ) , saturation coverage and initial sticking coefficient at adsorption temperatures 100 K and 200 (determined on the basis of the NO T P D data) is illustrated in Table 5.7. The reduced capacity of the 0.25 O / P t ( l l l ) at 200 K is obviously due t o the removal of the lower temperature state (see fig. 5.27.). In the case of Se, because of the NO induced phase transformation in the overlayer, the NO desorption spectra resemble a fractional order desorption process and this poses difficulties in accurately estimating Ez. The data summarized in Table 5.7. indicate that the molecular adsorption kinetics and energetics of NO are affected in the same way as for the molecular adsorption of CO. Comparing the electronegativities of the modifiers considered in Table 5.7. (3.5, 2.5 and 2,4) and the covalent radii (0.77, 1.04 and 1.16 A) for 0 , S and Se, respectively, the weakest poisoning effect is exerted by the modifier with the highest electronegativity: oxygen. This indicates that the of the modifier is probably a more important factor. Table 5.7. NO Desorption Energy, Ei (in kJ/mole), NO Saturation Coverage, 6sat for T, = 100 and 200 I< (in ML), and NO Initial Sticking Coefficient, SO,for a Clean and Modified P t ( l l 1 ) Surface. (from ref. [S2]) SURFACE

Ei

Pt(ll1) p(2 X 2) 0.25 0 - P t ( l l 1 ) 0.75 0 - P t ( l l 1 ) [98] p(2 X 2) 0.25 S-Pt(ll1) p(2 x 2) 0.25 Se-Pt(ll1)

99 91 86 79 76

N

6 (100 I<)

6 (200 I<)

So

0.55

0.4

1

0.40 0.15 0.17 0.16

0.25 0.17 0.08

-1

--

-

0.75 0.7

A more detailed study of the effect of different amounts of a modifier on the NO molecular adsorption rate has been proposed in ref. [102], where the S-induced reduction of the NO initial sticking coefficient, SO,is found to obey the relation &(S) = So(1 - 28s) (8s is S coverage in ML). Obviously, the influence of sulphur on the NO adsorption rate is weaker than that expected from the simple consideration of four blocked adsorption sites for adatom residing in a fourfold adsorption site. This is probably due to the fact that, for a precursor mediated adsorption process (as is the case of CO and NO molecular adsorption), the sites unfavourable for chemisorption can still serve as possible residence sites of the precursor state.

5.2. Nitric Oxide

111

I

Fig. 5.31. NO uptake versus NO exposure for clean (dashed line), and modified Pt(ll1): p(2 x 2 ) S (open circles) and p(2 x 2) Se (half filled circles) (from ref. 1821)

Studies of the effect of different modifier coverages on NO adsorption are helpful in establishing the sequence and the effectiveness i n the eliiiiinat 1011 of the NO adsorption states The rapid removal of the most strongly bound NO state is illustrated in Figs. 5.32. and 5.33., as well as the reduction in the total amount of NO adsorbed as a result of increasing S coverage on a Pd( 100) surface. It is obvious that for this system the greatest poisoning effect will 0.1. For this low S coverage range, be observed to a S coverage limit of the initial drop of 0 ~ indicates 0 that 1 S adatom blocks 3 adsorption sites of NO and all sites associated with the 535 I< T P D peaks are removed at 0s 0.1. According to the HREELS data, the effect of S on the adsorbed NO bond order, is rather small when S coverages are low, and the reason for that will be discussed in the next Subsections. Both the T P D and the vibrational data show that at 0s < 0.15, no new S-induced NO adsorption states appear At moderate and high S coverages, the reduction in the adsorptive capacity of the surface becomes less severe (1 NO per S) and new S induced weakly bound NO adsorption states arise. Similar effects, like those described above, on the amount and the desorption energy of molecularly adsorbed NO have also been reported for other modified transition metal surfaces, e.g. S/Rh(100) [loo], S/Pt(100) [ l o l l , S/Ni(100) [102], N , O / N i ( l l l ) [103], O/Rh(111) [78], and S/Pd [104]. B. Site occupation and electronic structure of the coadsorbed N O molecule. HREELS [78, 98, 1051, UPS [98] and ESDIAD [82] data have shown that in the presence of electronegative modifiers, the amount of NO in a two-fold bridge bonding configuration is greatly attenuated and completely

-

-

-

112

Chapter 5

166

2 M

NO/NO t S

42

.35

30

.a 15 .09

03S

31

Fig. 5.32. NO TPD spectra for saturated NO coverages from sulphur predosed Pd(100). T, = 80 I< (from ref. [95])

removed above certain modifier coverages. Instead, on surfaces modified by electronegative adatoms, NO occupies exclusively ‘on top’ sites. Compared with Me-NO and N-0 stretching frequencies for linear ‘on top’ NO on a clean surface the Me-NO stretching frequency decreases, and the N-0 stretching frequency increases with the introduction of the modifier and the increase of the modifier coverage. This effect is attributed to an increase of the NO bond order (i.e. a decrease of the degree of coupling of the NO molecule with the surface in the presence of the modifier). Table 5.8. illustrates the changes in the Me-NO and N-0 stretching frequencies and in the NO 2n, l n , 50 and 4a binding energies induced by the presence of 0.25ML and 0.75ML oxygen on P t ( l l 1 ) . It becomes clear from Table 5.8. that the main differences observed for the ‘on top’ NO species on clean and modified with 0.25 0 - P t ( l l l ) , and the ‘on top’ NO species on 0.75 0-Pt(ll1) are the appearance of a second low stretching frequency mode at 510 cm-’ and the increase of the binding energies of the NO molecular orbitals in the case of 0.75 0-Pt(ll1). Assuming that the presence of an electronegative additive should result in a reduction of the ability of the surface for r-backbonding, the observed changes in the NO vibrational and UPS can be satisfactorily explained with ‘on top’ bent NO bonding on 0.75

5.2. Nitric Oxide

NO/NO+S

Pd (100) Dosed at 8Ou

1-

0 2

113

.P.... '.--.. *...._----. ....._..., .........-........... ....

'1\

'.

-\--

I

.I

I

Sulfur

1 .4

.3

Covercqe (monoloyer)

Fig. 5.33. Effect of sulphur coverage on the total NO coverage (full line) aud the local coverages in the most strongly bound ( a t 535 K (dashed line)) and the less strongly bound ( a t N 440 K ) NO adsorption states on Pd(100) (from ref. [95])

0-Pt(ll1). As described in Section 5.2.1., this NO bonding configuration occurs with a negligible x-backbonding contribution. The same argument of a reduced backdonation ability of the modified surface explains the removal of the bridge bonding configuration, where the r-backbonding contribution is significant. Table 5.8. Pt-NO ( w l ) and N-0 ( w 2 )Stretching Frequencies (in cm-') and NO Z A , I T , 5u and 40 Binding Energies (in eV) for NO Adsorbed on a Clean and Oxygen-Covered P t ( l l 1 ) Surface (from ref. [98]) SURFACE Pt(ll1) Pt(ll1) 0.250/Pt( 111) 0.750/Pt(lll)

w1

w2

2*

lX+5fJ

two-fold bridge NO 1490 2.1 9.1 'on top' NO 390 1710 3.9 9.4 9.5 280 1740 2.9 Z 10 265 & 510 1775 3 . 3 450

4u 14.9 13.9 13.9 14.3

The same trend of a preferential 'on top' 1inea.rconfiguration on the modifier affected sites has been observed for NO/O-Rh(lll), where on a mod-

Chapter 5

114

ifier - free surface NO adsorbs in a two-fold bridge configuration for the whole coverage range at which T, = 100 K [78]. The presence of oxygen (0 < 00 < 0.8) causes a shift from bridge to an ‘on top’ linear NO configuration, and when oxygen coverages are very high (00 > 0.8), a backward shift to bridge NO takes place. Relative to the vibrational data, the NO desorption states distinguished in the NO T P D spectra, the following three NO adsorption configurations have been proposed:

-

(i) O-affected bridge NO, which appears when oxygen coverages are very high and desorption takes place at 280 K;

-

(ii) 0 affected ‘on top’ linear NO, which is the preferred adsorption state for moderate oxygen coverages and when desorption takes place at 380 K;

-

(iii) unperturbed bridge NO which exists up to moderate oxygen coverages and desorbs at 430 I<. The N-0 stretching frequency value measured for the oxygen affected ‘on top’ NO is slightly higher than that of the ‘on top’ linear (- 30 cm-’) NO frequency measured for the other P t group metals. Almost the same stretching frequency value was reported for NO on 0-Ru(0001) [log]. The effect of (O+N), produced by NO dissociation, on the NO site occupation on Ru(0001), i.e. moving from bridge to ‘on top’ adsorption site [log], is similar. Modifier-induced changes in the electronic structure of the NO molecule adsorbed on the affected sites are also evidenced by:

+

(i) the intensity and energy changes in the (Is 5a) emission peak in the MQS spectra [103]; (ii) the 0 1s or N 1s core level energy changes of the adsorbed NO molecule [l02], and (iii) the changes in sign of the dipole induced by NO adsorption on clean and modified surfaces [102, 1031.

+

N mixture Thus, for NO adsorbed on Ni(ll1) precovered with 0 or a 0 [103], MQS studies have shown that the (Is 5u) emission peak decreases and shifts t o higher binding energies with increasing amount of 0 or 0 + N present on the surface, whereas the intensity of the 2s emission hardly changes even after the complete disappearance of the ( 1 s 5a) peak (attributed to a change in the spatial extension of the Is and 5a orbitals). For the same systems parallel work function measurements have shown that the NO induced work function changes are in a direction opposite to those for molecular NO adsorption on a clean N i ( l l 1 ) surface, i.e. for a saturated NO coverage on 0.2 O / N i ( l l l ) , the work function drops down t,o -0.2 eV below the clean surface value, whereas a saturated NO coverage on a bare surface causes an increase of +1 eV. These work function results indicate opposite signs of the dipole moments for NO adsorbed on modified and unmodified fcc(ll1) surfaces, which agrees very well with the designation of the modifier-induced changes in the NO bonding configurations described above. The same trend

+

+

-

5.2. Nitric Oxide

115

in the NO induced work function changes reported for the NO/S-Ni(lOO) system, accompanied by a tendency towards an increase of the NO-0 1s binding energy (from 531.2 eV for a S-free surface to 532.2 eV for c(2 x 2) 0.5 S-Ni(100) [102]) is consistent with the decreased metal - NO coupling in the presence of electronegative adatoms. Modifier induced changes in the occupation of surface sites are more complicated in the case of fcc(100) single crystal planes. HREELS data for NO adsorption on S/Pd(100) [95] have shown that the appearance of the new Sinduced NO adsorption states is favoured above certain concentrations of the modifier on the surface. At low S coverages (6s < 0.15), the presence of S: (i) hardly affects the N-0 stretching frequencies of bridge-bonded (1510 cm-1) and ‘on top’ bonded (- 1700 cm-’) NO, and this indicates a negligible disturbance of the NO bond order; (ii) does not change the sequence which adsorption sites are filled (starting with the bridge bonding configuration); but (iii) prevents on top to bridge conversion, observed on S-free surfaces along with partial ‘on top’ NO desorption.

For higher S coverages (0.15 < 0s 5 0.5), three adsorption NO configurations are observed: one bridge, one ‘on top’ and a third very weakly bound NO (desorption temperature 165 K and stretching frequency 1750 cm-l). The latter was associated with the same h2 four-fold hollow site postulated for GO on c(2 x 2) S-Ni(100) [41, 981 (see fig. 5.9.). Finally, in order to illustrate the effect of the chemical state of the electronegative additives on the NO adsorption configuration, the vibrational data concerning NO adsorbed on (100) faces of NiO [lo61 are worth mentioning. The IR data have shown only linearly ‘on top’ bonded NO molecules with N-0 stretching frequencies 1800 cm-’ for all NO coverages, the adsorption sites being the Ni2+ ions in the oxide surface lattice. Comparison of these data with the vibrational data for NO adsorbed on a fcc(100) metal surface precovered with adsorbed oxygen, reveals that the oxidation of the surface results in a more severe reduction of the r-backbonding component as manifested by a higher N-0 stretching frequency (indicating a larger NO bond order) for linear ‘on top’ NO on a metal oxide surface.

-

5.2.3

Surface Order in Mixed Overlayers

The introduction of a modifier above certain critical coverages prevents the formation of the usual NO ordered structures as is the case for CO. LEED data 0.03 on the NO/S-Pd(lOO) system have shown that the presence of only ML sulphur prevents the formation of the p(4 x 2) NO ordered structure, with NO occupying two-fold bridge sites. The p(4 x 2) structure is readily observed on a S-free surface when by heating of a NO saturated surface to 410 K part of the ‘on top’ NO desorbs and another part converts into bridge bonded NO. The S induced inhibition of the long range order of the bridge bonded NO species agrees well with the observed impediment of the ‘on top’

-

Chapter 5

116

bridge conversion in the presence of S due to the reduced stability of the ‘on top’ NO species. However, the presence of small amounts of S (0s < O.l), does not prevent the formation of a c(2 x 2) NO ordered structure observed at NO coverages of 0.5 on a S-free surface. Since the appearance of c(2 x 2) patterns on a sulphided Pd( 100) surface is observed at NO coverages as low as 0.25 this indicates that up to certain value for S coverages, NO tends to adsorb in a separate phase where the local NO density reaches 0.5 ML. This tendency t o formation of a separate surface adsorption phase clearly explains the negligible influence of S on the N-0 stretching frequencies at 0s < 0.1 (because a few NO molecules are directly coordinated to the S adatoms), as discussed in Subsection 5.2.3. 4

N

b.

0.

c.

d.

I

Fig. 5.34. Changes in the LEED patterns induced by annealing of a p(3 x 2 ) 0.25 Se 0.16 NO mixed layer to different temperatures. For the sake of clarity, the NO TPD curve for a saturated NO coverage on p(2 x 2) Se-Pt(ll1) obtained at 90 K is given. The arrows connected with LEED panels indicate both the annealing temperature and the remaining part of the TPD spectrum (from ref. [82])

+

Studies of the effect of NO adsorption on the initial structural order of the modifier on the surface have been carried out for P t ( l l l ) , precovered with p(2 x 2) overlayers of oxygen, sulphur and selenium [82]. For p(2 x 2) 0.25 O-Pt(ll1) and p(2x 2) 0.25 S-Pt(ll1) no extra LEED spots or disturbance of the initial modifier surface order are detected upon NO adsorption in the temperature range 90-250 I< (at higher adsorption temperatures NO starts t o desorb). The picture is completely different in the case of a selenided P t surface where the initial p(2 x 2) Se order is preserved only when NO adsorption is carried out at temperatures lower than 170 I<. As shown in fig. 5.34., annealing of the mixed NO p(2 x 2) Se with saturated NO coverage

+

-

5.2. Nitric Oxide

117

of 0.16, obtained at 100 I<, to various temperatures causes streaking of the extra spots in the original p(2 x 2) patterns followed by the conversion of the p(2 x 2) pattern into a mixture of p(2 x 2) and (Ax &)R30° patterns at 230 K when part of NO desorbs. With a further increase in the annealing temperature and complete desorption of NO, the initial p(2 x 2) Se order is restored.

-

? 0

n 3

$

- 1

dT/dt=

1.5 K/sec

= 40 pA

v

0

.-x

--

C.

c n I

0

b. /

a.

100

200

300

temperature

400

[K]

Fig. 5.35. NO TPD spectrafrom: (a) asaturated NO coverage (- 0.07) on p(2 x2) NO+(& x &)R30 Se-Pt(ll1); (b) after additional 10 L NO exposure at 90 K on (a); (c) from saturated NO coverage on p(2 x 2) Se-Pt(ll1) obtained at 90 K (from ref. [SZ])

In fig. 5.35., the NO T P D spectra obtained after NO adsorption on p(2 x 2) S e / P t ( l l l ) at 220 K when the mixed p(2 x 2) + ( Ax fi)R3Oo structure is formed are compared with the NO T P D spectra obtained by direct desorption of NO from a p(2 x 2) S e / P t ( l l l ) surface, saturated with NO a t 90 I<. It is worth pointing out that the same NO T P D spectra as those observed after 220 K NO adsorption can be obtained if a ~ ( 2 x 2S) e / P t ( l l l ) surface saturated by NO at 90 K is annealed to 230 K prior desorption. The coverages of the Se and NO constituents in the overlayer with mixed p(2 x 2) + x &)R30 structures are 0.25 (initial) for Se and 0.07 for NO. The new structural order obviously offers less adsorption sites for NO, because the surface with an already formed mixed structure can not adsorb more NO even after a long additional NO exposure at 90 I< (curve b in fig. 5.35.). Considering the adsorptive capacity of the P t ( l l 1 ) surface, the best explanation of the new surface order is an NO-induced phase transition from &)R30° Se, accompanied by the formation of a separate p(2 x 2) Se to ( A X

-

(a

118

Chapter 5.

p(2 x 2) NO phase. This tendency of forming of two-phase, immisibly ordered islands, is a probable development where many coadsorbates with dominant repulsive interactions are concerned [43, 107l.The lack of NO induced restructuring on p 0 and p(2 x 2) S-Pt(ll1) and the decrease of the adsorptive capacity of the surface in the sequence 0, S, Se (see Table 5.7.) indicates the significant contribution of the size of the modifiers to the strength of the poisoning effect, i.e. the steric effect seems tooverweigh the electronic one. Indeed, comparison of the adsorptive capacities of P t(ll1 ) precovered with p(2 x 2) overlayers of oxygen, sulphur or selenium, shows that, in the case of oxygen the fraction of NO molecules beyond 0.25 (corresponding to the low temperature TPD peak in fig. 5.27.), resides on bridge sites, sharing a Pt atom with the adsorbed oxygen. ESDIAD data for NO adsorbed on a modified P t ( l l 1 ) surface [82] indicate that for p(2 x 2) S-Pt(ll1) and a selenided surface (in both structural orders) only ‘on top’ linearly-bonded NO species are present, whereas for p(2 x 2) O-Pt(lll), when the NO coverage exceeds 0.2 a bridge bonded configuration is also possible. The observed structural changes in the NO (C0)-electronegative adatom coadsorbate layers, when each adsorbate tends to form a separate adsorption phase complicate the description of the poisoning effect. However, the experimental data show definitely that, even when forming a separate phase, the molecular adsorption state of the reactants is destabilized (they desorb at a lower temperature than from a clean surface) and the local density that is reached in this separate phase, is far below that which is rea&ed bll a clean surface. Several factors should be considered when trying to explain these effects: (i) for the formation of dense compressed overlayers of CO or NO a longrange order is needed which is constrained by the presence of ordered modifier islands; (ii) the reduced mobility of the coadsorbed molecules which leads to an increase of the pre-exponential factor for desorption, i.e. an increase of the desorption rate; (iii) the possible propagation of the local modifier coadsorbed molecule repulsive interactions through the adlayer via molecule - molecule interactions; (iv) the existence of a long range electronic effect of the modifier which extends beyond the next-nearest neighbours. The relative contribution of each effect is likely to be different for the various coadsorbate systems, and no generally applicable definition can be proposed at this stage. 5.2.4

Modifier Effect on the NO Dissociative Adsorption

A significant fraction of NO readily dissociates on most of the transition metal surfaces at temperatures higher than 200 K. This described already in Section

5.2. Nitric Oxide

119

5.2.1. There are minor exceptions like some planes of P t and Pd. In these systems, where the NO dissociation process is competing with molecular NO desorption, the introduction of an electronegative additive always results in the inhibition of the dissociation, accompanied by the reduction of the surfacemolecular adsorption capacity. The fraction of NO, dissociated on S/Ni( 100) as a function of S coverage, and the corresponding NO and Nz T P D spectra is shown in fig. 5.36. It is obvious that NO dissociation is completely inhibited, for sulphur coverages of 0.25 when a p(2 x 2) S ordered structure is formed and all NO adsorption sites are influenced by the additive. The complex N2 T P D spectra are due to the influence of the other dissociative product (0) which remains on the Ni surface. The data in fig. 5.36. can be summarized as follows: (i) the relative fraction of dissociated NO molecules decreases with increasing S coverage; (ii) the inhibition of the dissociation is stronger than the reduction of the surface adsorption capacity for molecular adsorption; (iii) the sulphur induced reduction in saturated NO coverage is adsorption sites per S adatom, and (iv) the sulphur induced inhibition of NO dissociation is

per S adatom.

-

-

2 NO

4 NO molecules

For the NO-S/Ni( 100) system, what makes it complicated, is that, with the dissociation of the first NO doses 0 and N , are also introduced to the surface and they also inhibit, to a certain extent, further NO dissociation. This self poisoning effect accompanying the NO dissociation on a clean surface is weaker than that induced by S [89]. Comparison with the case of a N or 0 precovered surface has shown that the presence of sulphiir influences twice as many dissociation sites [102], i.e. this further proofs the relevance of the adatom size over the electronegativity with respect to the strength of the modifier. Here it is worth noting that, when the effect of 0 on the adsorptive properties of Ni is studied, the possible 0 induced reconstruction and formation of oxide islands will complicate the picture. As reported in ref.[l06], no NO dissociation occurs on NiO (100) surfaces. Generally, the same trend in the S ‘poisoning’ effect has been reported for NO/p(2 x 2) S-Pt( 100) [loll and NO/S-Rh( 100) [loo]. In the presence of a p(2 x 2) 0.25 S overlayer on Pt(100) the dissociation of NO is completely inhibited and the molecular adsorption state is destabilized (the NO T P D data suppose a reduction of the Pt-NO binding energy by more than 50 k.J/mol for NO adsorbed on p(2 x 2) S-Pt(100) [loll). As a result. of this inhibition of NO dissociation, the p(2 x 2) S-Pt(100) surface is completely deactivated in respect of the reaction CO+NO = COz+N2, which proceeds readily on S-free Pt(100). In the case of NO on sulphided Rh(100) it has been found that at 300 I< the presence of only 0.08 S inhibits Completely NO dissociat,ion which is accompanied by a significant decrease in the amount of molecularly-adsorbed NO.

120

Chapter 5.

05 04

03 02

ji 1

01

0 04

9

03 02 0 : 0

02 01

0

01

02

03

04

05

Fig. 6.36. (Left) NO and Nz TPD spectra from Ni(100) with different sulphur precoverages, 0s (in ML), exposed to various NO doses at 110 K. The NO exposures from bottom to top within each manifold are: 0.2, 0.2, 1.0, 1.5, 2.0 and 10 L. (Right) Dependence of the total NO coverage (a), dissociated NO fraction (b), and molecular NO fraction (c) on the S coverage, 0s. NO exposures from bottom to top in each panel is 0.2, 0.6, 1.0, 1.5, 2.0 and 10 L (from ref. [102])

Other electronegative adatoms, such as 0 and N , also cause a decrease of the NO dissociation propensity but they act as less severe deactivators than S [78, 103, 1091. In the case NO/O R h ( l l l ) , the presence of oxygen causes the following changes in the adsorption behaviour of NO : (i) destabilization of the NO molecular adsorption state and the adsorption state of the dissociative product N, both NO and Nz desorption occurring at lower temperatures in the presence of oxygen; (ii) inhibition of NO dissociation at 00 turbed bridge NO is removed.

> 0.8, i.e. when almost all undis-

Parallel T P D and HREELS studies indicate that no dissociation is likely for 0 affected ‘on top’ and bridge NO [78]. 5.2.5

Differences in the Effect of O x y g e n on the A d s o r p t i v e Properties and Reactivity of P t ( l l 1 ) with respect to CO and NO

In this Subsection is illustrated that, depending on the particular system, the same electronegative additive can serve as a ‘poison’ with respect to one

5.2. Nitric Oxide

121

reactant (NO) or can participate in a catalytic reaction with another (CO), although both molecules are behaving as electron acceptor coadsorbates. As has been outlined in Section 5.1., oxygen on P t ( l l 1 ) does not serve as a typical poisoning additive with respect to CO adsorption and participates as a reactant in the important (from the viewpoint of environmental protection) catalytic reaction of CO oxidation. It has been found that, contrary to the expectations concerning the behaviour of an usual ‘poison’, the increase of the oxygen-modified CO adsorption states resulting in a more facile reaction [96, 1081.

k---------A

-- 0.0

2.0

C02 product

CO rwnainlng (450K)

4.0 6.0 8.0 10.0 CO EXPOSURE [ r n b a r . ~ e c . l O - ~ ]

12.0

Fig. 5.37. CO coverage as a function of exposure o n clean P t ( l l 1 ) (dashed curve) and p(2 x 2) O-Pt(ll1). T h e amounts of CO and COz desorbing from CO/p(2 x 2) O-Pt(ll1) during heating are also shown (from ref. [40])

The effect of 0 on the adsorption rate and sequence in the adsorption site occupation of NO and CO on p(2 x 2) O-Pt( 111) will now be compared. In the case of NO, the presence of a p(2 x 2) 0 overlayer leads to: (i) a decrease of the NO initial sticking coefficient (see Table 5.7.); (ii) a reverse in the site occupation, with preferential occupation of linear ‘on top’ sites [98], and

-

(iii) an appearance of a weakly bound NO state with a desorption energy 36 kJ/mole at NO coverages exceeding 0.2, when NO starts to occupy bridge sites (see fig. 5.27.) [82, 981. In the case of CO, the presence of a p(2 x 2) 0 overlayer does not affect the initial rate of CO adsorption (see fig. 5.37.). This indicates that the CO molecules are trapped at 90 K with an equa.1 efficiency in the precursor

Chapter 5

122

OXYGEN COMRAGE CO C O V E W E (YL):

= 2 5 O/Pt 1 r.42 CO/W

2=.3* 3=.32 4z.24 5z.14

dT/dt= 1.4 K/D*c I

Fig. 5.38. COz, CO, and ref. [40])

0 2

TPD spectra from CO/p(2 x 2 ) O-Pt(ll1).

(from

state above each surface site, i.e. the presence of oxygen does not affect the lifetime of the adsorption precursor as has been supposed for the case when the additive acts as a ‘poison’. N o CO desorption occurs at temperatures below 280 K , even at a saturation CO coverage of 0 42 on p(2 x 2) O--Pt(lll), i.e. the oxygen induced destabilization effect (provided t,hat it exists) on the CO adsorption state is much weaker than in the case of NO. At temperatures above 280 K (as is illustrated in fig. 5.38.),the oxidation reaction takes place. However, comparison with the CO T P D spectra from a clean P t ( l l 1 ) surface (see the bottom panel in fig. 5.7.) for a CO coverage of the order of 0.40, where CO desorption starts at 320 I<, permits making the assumption that the effect of oxygen on the stability of the CO adsorption state (if there is any) is insignificant. The most interesting behaviour observed for CO adsorption on p(2 x 2) O-Pt( 111) is that there is no preferential occupation of the ‘nonmodified’ (‘on top’ next-nearest P t site in fig. 5.8.) and both ‘on t,op’ sites on the nearest (directly coordinated with 0) and nest nearest Pt are filled together [40]. As the bond orientation of the coadsorbed CO molecules is concerned, the ESDIAD data have shown that CO ‘on top’ bound on the nearest Pt atom is slightly tilted (- 5’). This bonding configuration of CO and 0 sharing the same P t atom is supposed to be a precursor for COz formation. The difference in behaviour of the NO + 0 and CO + 0 coadsorbed layers on Pt(ll1) has been discussed recently in ref. [98]. It has been shown that the ready CO oxidation is due not only to the lower activation barrier for CO oxidation (53 kJ/mol, compared to 110 kJ/mole for NO oxidation on Pt) , but

-

5.2. Nitric Oxide

123

also to the lower adsorption binding energy of NO. In the case of NO, sharing P t atoms with the oxygen adatoms (a bonding configuration which is likely to be a precursor for NO2 formation) the NO adsorption bind is strongly reduced. Thus, because of the reduced activation barrier for NO desorption, no NO2 formation is observed in NO 0 coadsorbate overlayers. Consequently, the absence of an oxygen-induced ‘poisoning’ effect with respect to CO adsorption on P t implies that the strong CO-0 affinity leading to the formation of C02 might compensate the repulsive forces between these two electron-acceptor coadsorbates.

+

5.2.6

Differences in the Effects of the N O Dissociation Products 0 and N and the Reactivity of Pt and Rh Surfaces with Respect to CO Oxidation and NO Reduction

Since, as outlined above, P t an Rh are presently the best catalysts used for the effective removal of the troublesome automobile exhaust gases NO and CO, it is of particular interest to know how the presence of the NO dissociative products will affect the adsorptive behaviour of the reagents. According to the T P D data summarized in ref. [86], the rate-limiting step in the CO(a) NO(a) = C02(g) N2(g) reaction on R h ( l l 1 ) is the reaction CO(a)+O(a) = CO,(g). On Rh(ll1) when allsites are affected by N adatoms both NO and CO adsorb in ‘on top’ sites, where the contribution of the 7rbackdonation to the adsorption bond is reduced. On the one hand, this leads to destabilization of the molecular adsorption state, and on the other, to inhibition of NO dissociation which is an intermediate step in the NO reduction process. Thus, because of its reduced adsorption binding energy (by 30-40 kJ/mol), in a close proximity of N , CO desorption occurs at temperatures close to and even lower than that at which CO oxidation on Rh takes place. This also leads to an increase in relative concentration of oxygen on the surface, which, above certain coverages, will start t o act as a deactivator of further NO dissociation. The inhibition of NO dissociation in the presence of additives is more severe in the case of Pt, where NO dissociation is more difficult. Because of the presence of at least four different species on the surface (NO, CO, 0 and N ) the real picture is much more complicated. Thus, one should take into account that, besides the electronegative adatoms (0,N and eventually C) there exists a mutual effect of NO and CO coadsorption. The latter is quite different for coadsorbed layers on Pt and Rh. In the case of R h ( l l l ) , NO acts t o a certain extent as a deactivator with respect t o CO adsorption. This causes (similarly to N) destabilization of the molecular CO adsorption state by inducing a discreet new CO adsorption state with a population increasing with the CO coverage. On the contrary, the NO adsorption behaviour on Rh(l1 l), including the dissociation propensity, are negligibly affected by the presence of CO. The only CO effect concerns the capacity for NO adsorption, which could be expected from a site exclusion view, because NO cannot remove CO from the surface. The absence of a measurable effect of CO on the NO dissociation can be attributed to the fact that CO occupies exclusively ‘on top’ sites. Thus the threefold sites favourable for 0 and N adsorption on fcc(ll1) surfaces are free.

+

-

+

Chapter 5

124

The picture is different for P t ( l l l ) , where the presence of CO causes a continuous reduction in stability and coverage of the molecular NO adsorption state with increasing CO coverage, whereas the effect of NO on the CO adsorption behaviour on P t ( l l 1 ) is much weaker. That is why, under the typical reaction conditions, where the concentratmionof CO in the gas phase 5-10 times larger than that of NO (both gases adsorb on Rh and Pt is with sticking coefficients close to unity), Rh will be a better catalyst for NO reduction, whereas Pt remaiiis the best one for CO oxidation. Finally, it should be noted that the above considerations merely express a very simplified picture. As has been shown in the previous subsections, when different species with the same surface dipole polarity (electron acceptors in this particular case) are coadsorbing, it is likely that they will tend to form separate islands. This tendency is frequently observed with coadsorbate systems, such as 0 CO, CO NO, etc.[43, 1071. Such a separation of the reactants in islands impedes the surface reaction to an additional extent.

-

+

5.3

+

NITROGEN AND OXYGEN

The interest in understanding the effect of electronegative modifiers on the interaction of N2 with transition metal surfaces has arisen because nitrogen is one of the reactants participating in the important catalytic reaction of ammonia synthesis, where iron is the most effective catalyst [110]. Since N2 dissociation is believed to be the rate limiting step in t>hehigh pressure ammonia synthesis from Nz and H z , it is important to find out to what extent the presence of additives such as S and 0 will affect the interaction of Nz with the catalyst surface. 5.3.1

General Reinarks for N? A d s o r p t i o n on Transition Metal Surfaces

The nitrogen molecule is isoelectronic with and structurally similar to the CO molecule. Generally, with both molecules the formation of the molecular adsorption bond is via the combination of a a / d , donor Component, where a orbitals are lone pair orbitals of the adsorbing molecules, and a En*/ dz acceptor component, where the E* are unoccupied antibonding orbitals in the adsorbing inolecules. The difference between the N2 and CO bonding is that the a valence orbitals 2uu and 3ug, shared equally between the two nitrogen atoms in a free molecule, mix upon interaction with a metal surface to form two new a orbitals which show a lone pair character to some extent. The resulting donor bond is weaker than that of CO (formed with the participation of the CO-5a lone pair only). By analogy with CO, the backdonation component in the Me-Nz bonding involves the lrgantibonding orbital [ l l l ] . The backdonation component is expected to be more important in CO than N2 bonding, since the 2r CO orbitals lie at a lower energy (of 0.6 eV) than the l?rg Nz ones [112]. As a result the adsorption binding energy of N2 is rather small rangkg from 20 t o 40 kJ/mol for transition metals, such as P d ( l l 0 ) [114], Re (1120) and Re(OOO1) [115], Fe(ll0) and Fe(100) [116], Ru(0001) [117], Ni(ll0) [118, 1191, etc. [113]. These weakly bound molecular

-

5.3. Nitrogen and Oxygen

125

states occupy usually ‘on top’ sites wit,li the molecular axis perpendicular to the surface plane [ill, 114, 117-1231. It is worth mentioning that studies of the influence of the nitrogen coverage on the Me-N:, and N-N stretching frequencies have shown that the Me-N2 stretching frequency increases (e.g. from 278 t o 291 cm-’ for N2 on Ru(0001) [117]), whereas the N-N frequency shifts to lower energies with increasing nitrogen coverage (e.g. from 2252 t o 2198 cm-’ for the same system [117]). This trend is opposite to the one observed in CO/transition metal adsorption systems. The reason for this behaviour has not been completely clarified, but one of the possibilities is the formation of a In, band with a significant dispersion as a result of overlapping between the spatially extended N2 17rg orbitals in denser nitrogen overlayers. It would be reasonable to suppose that, if the In, band broadens sufficiently, it might cross the Fermi level and induce the observed reduction in the N-N stretching frequency. Since the changes on the N-N stretching frequency with increasing Nz coverage are accompanied by a reduction in the NZ desorption energy, the above explanation will be adequate, supposing parallel changes in the donor bonding contribution. Because of the weak adsorption bond, the adsorption rate and the saturation nitrogen coverage for molecular Nz adsorption are strongly dependent on the adsorption temperature. Thus, at room temperature for most of the transition metals under consideration, the initial sticking coefficient is less than lo-’. Actually, at temperatures exceeding the desorption temperatures of the molecular nitrogen, the only stable state on the surface can be nitrogen atoms produced as a result of dissociative nitrogen adsorption [113]. It has been established that the dissociation propensity of N2 is structurally sensitive, e.g. Fe( 111) and polycrystalline Fe are more active with respect t o Nz dissociation and ammonia synthesis than the less open Fe(100) and Fe(ll0) planes [113]. Similarly t o F e ( l l l ) , the open plane of Re(1120) and polycrystalline Re is more active than the close-packed Re(0001) [115, 1241, etc. Recently, HEELS and ARUPS data [125-1281 have shown t,hat there is a second adsorption state of N:, on the open Fe(ll1) surface. The orientation of the N:, molecules in this state and the bonding to the surface differ from the ‘end-on’ linear configuration described above. The second Nz adsorption state det,ected on the open Fe(ll1) plane has an unusually low N-N stretching frequency and is described as a ‘side-on’ r-bonded complex with both N atoms interacting with the metal, with a stronger backdonation contribution compared to ‘end-on’ configuration. The ‘side-on’ Nz cqnfiguration is assumed to be an immediate precursor of Nz dissociation. As described i n ref. [127], the adsorption in this ‘precursor’ state can occur via the more weakly bonded Lend-on’linear state (by interstate conversion) and also by direct adsorption. The direct adsorption process has a rather low initial sticking coefficient (and is the only adsorption channel at higher temperatures and low pressures when the concentration of the ‘end-on’ linear state becomes negligible. The same ‘r-bonded’ N2 species have also been observed on a Cr( 111) surface [ 1291.

Chapter 5

126 5.3.2

Modifier Effect on the sorption

N2

Molecular and Dissociative Ad-

Oxygen, often introduced as an impurity in the reactant mixture is one of the most effective poisons in ammonia synthesis [130]. Studies of thg effect of increasing amounts of oxygen over polycrystalline Fe on the dissociative nitrogen adsorption have shown a linear decrease of the atomic N uptake with increasing oxygen coverage [131]. The effect of preadsorbed oxygen is assumed t o be due t o the fact that both nitrogen and oxygen prefer the same type of adsorption sites. This indicates that the two effects are additive because, as outlined in section 3.2.' both 0 and N cause reconstruction of the Fe surfaces with a strong tendency t o compound oxide and nitride formation. Similar effect on nitrogen dissociative adsorption on Fe is also induced by the presence of sulphur adatoms [113]. The effect of oxygen on the molecular adsorption process on N i ( 110), where dissociation under UHV conditions is negligible consists of a linear displacement of N2 with a negligible influence on the stability of t,he molecular state (slight downward shift of the N 2 T P D peak by 10 I<) and on the Me-N:! and N-N stretching frequencies [119]. The linear decrease of the saturation N2 coverage, e ( N z ) , is found to obey the following relationship:

-

where Omax(N2) is the saturation N:! coverage on a O-free surface, and e(0) and O,(O) are the actual oxygen coverage and the critical oxygen coverage at which O(N2) becomes 0. It has been established that 6,(0) is 0.5 and one oxygen adatom blocks one N2 adsorption site. This result agrees well with the simple blocking model of competitive adsorption. The study of the influence of ordered and disordered 0 overlayers on molecular nitrogen adsorption over Ru(0001) [117] hasbeendone in greater detail. I t has been found that the increase in oxygen coverage from 0 to 0.25 when the p(2 x 2) 0 surface structure is formed, causes a gradual attenuation of the original 94 I< and 117 I< N2 T P D states and an appearance of a new NZ TPD peak at 140 K. The original N2 T P D peaks completely disappear at p(2 x 2) 0.25 0 when only the new TPD feature at 140 K remains. An increase of oxygen coverage beyond 0.25 causes further reduction of the nitrogen coverage [the new O-induced N2 state) until the surface is completely deactivated for nitrogen adsorption at a 0.5 0 p(2 x 1) coverage. An interesting finding is that, irrespective of the reduction of the surface adsorptive capacity (following the same blocking mechanism a5 described by eq.(4)), the oxygen induced Nz molecular state is stabilized compared to a clean surface by 6 kJ/mole. This state is supposed to he 'on top' linear N 2 residing on the only next-nearest Pt atom on a p(2 x 2) 0.25 O / P t ( l l l ) surface (see fig. 5.8.). The Me-N2 stretchingfrequency of this O-affected site is 249 cm-' (by 30 cm-' lower than the lowest value on a clean surface), whereas the N-N stretching frequency is 2763 cm-' (by 15 cm-' higher than the highest 'on top' N 2 value on a clean surface). The vibrational data indicate that, as expected, the presence of oxygen inhibits the ability of the affected site for backdonation, i.e weakens the contribution of l r g / d T backbonding. In order

-

-

5.3. Nitrogen and Oxygen

127

t o explain the fact that nevertheless the N? adsorption state is stabilized, the authors suppose that the increased Lewis acidity of the next nearest Ru atoms enhances the ability of the surface to accept the Nz lone pair, i.e.enhances the contribution of the s donation bonding. Comparison with the effect of the same oxygen overlayers on the CO adsorption behaviour shows that together with the surface adsorptive capacity, the CO adsorption bond is also reduced (by 30 kJ/mole). The opposite effects of oxygen on the stability of the Nz and CO molecular states indicate that the backdonation contribution to the bonding is much weaker in the case of N?, whereas in the case of CO, the 27r backdonation is very significant.

-

5.3.3

Modifier Effect on the

0 2

Adsorption

Generally, the effect of the electronegative a.dditives on the oxygen dissociative adsorption is the same as that observed for CO, NO and N?. Oxygen adsorption on transition metal surfa.ces plays a. central role i n several important catalytic processes, such as the ca.talytic oxidation of CO and ammonia, where the oxidation processes are preceded by oxygen dissociative adsorption. Compared to Nz, NO and CO, oxygen adsorption proceeds dissociatively even at temperatures lower than room temperature on most metals so that the molecular adsorption state can be detected only at rather low adsorption temperatures. This is due t o the lower shbility of the oxygen molecular bond. Thus, the formation of a donor/acceptor adsorption bond causes weakening and rupture of the 0-0 bond as a result of the charge backdonation to the 1 7 molecular ~ ~ orbital. The introduction of electronegative adatoms causes the reduction of the rate of 0 2 dissociation because of blocking of the adsorption sites favourable for oxygen ada.t.oms. An example of the effect of an electronegative additive on the oxygen dissocktion rate is given in fig. 5.39. AS can be seen, a C1 covemge of N 0.25 suppresses the rate of oxygen dissociative adsorption more than ten times. The sa.turation oxygen coverage is observed t o decrease almost linearly with B c ~ falling to zero a.t C1 coverages close to 0.5 [186, 1871. Since the presence of CI does not affect significa.ntly the shape and position of the 0 2 T P D spectra, it has been suggested that the coa.dsorbed C1 and 0 atoms tend to form separate islands. This agrees well with the linear decline of the oxygen covera.ge with increasing Bcl “71. The electronegative additives which are electron acceptors a.ffect mainly the donation ability of the surface atoms, so that the reactants molecular adsorption state where the formation of the adsorption bond involves a significant backdonation contribution should be more severely destabilized. Since, as outlined above, the dissocia.tion of molecules with a donor/accept,or type of adsorption bond, such as CO, NO, Nz and 0 2 proceeds via a precursor with a dominating backbonding contrihut>ion,the presence of electronegative additives will always inhibit the dissociation of these molecules, irrespective of the effect on the sta.bility of the possible molecular adsorption states.

Chapter 5

128

Fig. 5.39. The effect of C1 coverage on the rate of oxygen dissociative adsorption on Ag(ll0) (from ref. [ISS]) 5.4

HYDROGEN

Hydrogen is a main reactant in a number of catalytic reactions, such ils Fischer

- Tropsch synthesis, ammonia synthesis, hydrogenation of hydrocarbons etc. The H:! molecule possesses a n occupied lug and an unoccupied la, orbital, Both orbitals are involved in the formation of bonding with transition metal surfaces. The existence of a high d-electron d a t e density at the Fermi level of the substrate is found to be the major parameter that governs the dissociation process. Thus, on most of the transition metals hydrogen readily dissociates, hydrogen adatoms forming a rather strong bond with the substrate (of the order of 240-300 kJ/mole [132]). Because of its small size, the hydrogen adatom is located close to the surface, occupying usually the highest coordinated sites. Pertinent to H adsorption is the smaller activation energy for surface diffusion compared t o other adsorbates. The high mobility of hydrogen on the surface is an important property explaining the fact that hydrogen diffusion is seldom rate-limiting in heterogeneous catalysis. The rate of the dissociative hydrogen adsorption on a particular surface depends on the height of the activation barrier which is small or negligible for most of the transition metal surfaces. The selected values of the measured initial sticking coefficients for hydrogen dissociative adsorption on clean single crystal surfaces show that with most of the transition metal surfaces, 5’0 is in the range from 0.5 to unity. Lower So values are likely to be due t o the existence of an activation barrier for adsorption.

5 . 4 . Hydrogen 5.4.1

129

Hydrogen Dissociative Adsorption on Modified Surfaces: Adsorption Kinetics, Energetics and Capacity for Adsorption

All studies up t o date have shown that the presence of electronegative additives results in a strong reduction in the hydrogen dissociative adsorption rate, a decrease of the hydrogen saturation coverage and significa.nt changes in the desorption parameters [13 - 15, 22, 23, 52, 61, 66, 133-1381.

Fig. 5.40. Effect of varying chlorine, sulphur and phosphorus precoverages 011 the Hz TPD spectra from Ni(100). Hz exposure 10 L; T, = 100 I< (from ref. [13])

Figs. 5.40.-5.42. illustrate the effect of various modifiers on the H2 T P D spectra, initial sticking coefficient and saturation coverage. Obviously, the introduction of electronegative adatoms causes a reduction in the hydrogen uptake, a shift of the H? T P D maxima to lower temperatures and a broadening of the T P D spectra. The effect of soiiie modifiers on the desorption parameters of Ha, as calculated from the T P D data, is presented in Table 5.9. It is obvious that the presence of electronegative additives causes a concomitant decrease of the desorption energy, Ed, and the preesponential factor, v. As will be discussed in more detail later on, the observed relatively small shift of the H:! T P D maxima accompanied by a marked broadening of the peaks, despite the relatively large E d and v changes, reflects the constraint on the recombination process due to the modifier induced changes i n the potential energy contour for hydrogen surface diffusion. It is interesting that, compared t o Ed, the effect of the electronegative additives on the Me-H bond is less severe (of the order of 10-30 kJ/mol). This indicates that the modifier influence on the H adsorption sites which are not blocked is, not significaiit In the case of other substrates, where the H? T P D spectra reveal the existence of more than one hydrogen adsorption state, e.g. H 2 / P t ( l l l ) [137], Hz/Mo(100) [61], etc. [132], the reduction in the ability of the surface to chemisorb hydrogen proceeds by a subsequent elimination of the high temperature adsorption states with increasing additive coverage. Thus, as illustrated

130

Chapter 5

Fig. 5.41. Hz TPD spectra for increasing hydrogen coverages on clean P t ( l l l ) , p(2 x 2) 0.25 S-Pt(lll) and ( Ax &)R30° 0.33 S-Pt(ll1). T, = 200 K (from ref. [137])

in fig. 5.41., the increase of the S coverage on P t ( 111) leads to elimination of the p3 peak at 400 K first, followed by a reduction of the less strongly bound pzstate. The saturation hydrogen coverages at 200 I< evaluated from the T P D data in fig. 5.40. are 0.3, 0.24 and 0.04 for clean P t ( l l l ) , p(2 x 2) S / P t ( l l l ) and (fi x fi)R30° S / P t ( l l l ) surfaces, respectively. The adsorptive ability of S-modified P t ( l l 1 ) is more severely affected a t higher adsorption temperatures, e.g. at 300 K it becomes 0 for the ( A X &)R30° S-Pt(ll1) surface and is of the order of 1/10 of the clean surface saturation coverage for p(2 x 2) S / P t ( l l l ) [13G]. As can be judged from the data presented in Table 5.10. and fig. 5.42., the effectiveness of modifiers with respect t o reduction of the adsorption capacity is more sensitive to the size of the additive adatoins, i.e. the larger chlorine and sulphur adatoms exhibit the most severe influence on the hydrogen uptake. The OH versus modifier coverage plots in the case of modifiers which tend t o adsorb in highly coordinated sites and form ordered structures, are very steep in the beginning. This indicates that an isolated additive adatom which competes with hydrogen for the same adsorption sites eliminates not only the site where it resides but also influences nearest-neighbour positions. The number of the latter is varying with the type of the modifier and the substrate crystallographic plane. Here it is worth noting that again in the case of additives which tend to form islands of a separate phase on the surface, the poisoning effect is less severe, because patches of unaffected areas persist until the surface is covered with a complet,e overlayer of the modifier. This is the case of phosphorus Ni(100) in fig. 5.42., where the reduction of hydrogen adsorption is linear. The same behaviour is exhibited by surfaces with a graphite deposit. For an example, in the presence of graphite islands on the Ni( 110) surface, hydrogen desorbing exactly like H 2 from clean Ni(ll0) has been det,ected [23].

5 . 4 . Hydrogen

131

ADOITIVI COVtllACt [ML)

0

'r

d

CI

i

05

Fig. 5.42. (Top) Dependence of the hydrogen saturation coverage, 6 , at 100 K on the additive coverage. (Bottom) T h e initial sticking for dissociative hydrogen adsorption, SO,as a function of the addit.ive coverage. T h e dashed line represents the theoretical dependence according to the relationship SO= So(clean)(l - 46%)' (from ref. [13])

Since graphite (phosphide) surfaces do not adsorb H?, the reduction in the adsorptive capacity depends simply on the fraction of the clean surface islands. The d a t a in Table 5.10. and figs. 5.39.-5.41. show that the introduction of electronegative adatoms induces a severe reduct,ion in the rate of hydrogen dissociative adsorption. The presence of additives affects the dissociative hydrogen adsorption in several ways: (i) by occupying and blocking highly coordinated surface sites favourable for hydrogen adsorption; (ii) by reducing the lifetime of the molecular precursor for dissociation, provided hydrogen dissociative adsorption proceeds via a precursor, and (iii) by creating an activation barrier for dissociation of the precursor The simple blocking effect of the modifier, (i), concerns the available adsorption sites for hydrogen adatoms. The extent of this effect can be established

Chapter 5

132

Table 5.9. Effect of Several Electronegative Adatom Overlayers on the Kinetic Parameters of Hydrogen Desorption, E d (in kJ/mol) and v (in cm2/s), and the MeH Bond Strength, E M ~ - (kJ/mol), H as Estimated from the Relationship E M ~ - H = (data from ref. [15, 52, 136, 137bl) 1/2(Ed

+

SURFACE ~~

Ni(100) p(2 x 2) S-Ni(100) c(2 x 2) S-Ni(100) Fe(lO0) c(2 x 3) 0-Fe(lO0) Pd( 100) 0.15 S-Pd(100) Pt( 111) p(2 x 2) S-Pt(ll1)

~

Ed

U

102 f 5 84 f 10 48 f 16 87 f 5 60 f 10 85 49 75

5x 4x 10-~

EM+H

~~

-

10-8 .5 x lo-*

10-2.5 10-6.8 10-~

266 f 5 256 f 10 239 f 16 258 f 5 245 f 10 256 240 253 244

studying the adsorptive capacity of the surface exposed directly to atomic hydrogen. As has been reported in ref. [52], there is a significant increase of the amount of adsorbed hydrogen (roughly by a factor of five) upon atomic hydrogen adsorption. Let us suppose that the blocked area around the additive adatom is determined by the repulsive interactions between the modifier and the hydrogen adatoms. Consequently, taking into account the small dimensions of the hydrogen adatom, it should be expected that the effective blocking radius will be of the order of the Van der Waals radius of the modifier atoms when the latter are located above the surface plane. As an example, for S on Ru(0001) the effective blocking radius is found to be 2 (the Van der Waals radius of S is 1.85 A). This means that for S located in a threefold site on Ru(0001), the three nearest-neighbour threefold sites should be excluded as sites for hydrogen adsorption because they are at a distance of 1.56 A. This prediction agrees with the finding that one S adatom blocks four sites for hydrogen adsorption on Ru(0001) [139]. However, the effective blocking radius becomes much smaller in the case of a smaller additive adatom, located deeply in the first layer of the substrate. This is the case of C/Mo( 100) where in the presence of t9c = 0.7 ML the adsorption capacity for atomic hydrogen is 0.3 ML, i.e. because of its small size one C (residing deep down in the fourfold surface sites) eliminates only one adsorption site. Following these considerations about the elimination of hydrogen adatom adsorption sites, it is obvious that a simple site blocking alone cannot explain the observed severe reduction of the initial sticking coefficient for dissociative adsorption, SO,(implying 6-9 sites blocked per one S or C1 adatom for fcc(ll1) [133, 135, 136) or fcc(100) [13] surfaces). Undoubtedly, the contribution of the kinetic factors (ii) and/or (iii) are also significant, especially in the case of additives with larger sizes. The differences in the efficiency of the poisoning effect for ordered additive overlayers and islands of a separate additive phase are very well demonstrated

-

A

133

5.4. Hydrogen

Table 5.10. Saturation Hydrogen Coverage, 8~ (in ML), and the Initial Sticking Coefficient for Hz Dissociative Adsorpt,ion, SO,for Several Clean and Modified with Electronegative Additives Single Crystal Surfaces (from ref.[l3, 15, 23, 52, 1371)

so

SURFACE ~~~~~~~~

~

Ni(100) p(2 x 2) S-Ni(100) c(2 x 2) S-Ni(100) p(2 x 2) C1-Ni(100) 0.5 P/Ni( 100) Ni( 110) (4 x 5) C-Ni(ll0) (2 x I) C-Ni(110) graphite/Ni(llO) Fe(100) c(2 x 2) C-Fe(100) p(1 x 1) 0-Fe(100) c(2 x 2) S-Fe(100) Pd(100) 0.08 S-Pd( 100) 0.15 S-Pd( 100) p(2 x 2) S-Pd(100) Pt(ll1) p(2 x 2) S-Pt(ll1) c(2 x 2) S-Pt(ll1)

~

~

T,

~

0.74 0.12 < 0.01 < 0.001 0.36 0.5

0.005 < 0.0015 < 0.001 0.5 0.1 0.4 0 1.0 0.84 0.42 0.11 0.3 0.2 0.04

120 K 120 I< 120 K < lod4 120 K 120 I< 2.5 x lo-’ -1 200 I< 5x 200 I< 5x 200 I< < 3 x 1 0 - ~ 200 I< 250 I< 3x 10-~ 250 I< 1 0 - ~ 250 I< 0 250 I< 100 I<

6 x lo-’ 7 x 10-2 < lod3

--

-

0.69So 0.31So

0.05So -

100 I<

100 I< 100 I< 200 I< 200 I< 200 1;

by comparing tlie adsorptive properties of a (4 x 5) C-Ni(ll0) surface and a N i ( l l 0 ) surface covered wit,h graphite islands, produced after a carbon phase transition. The latter came about as a result of heating of the (4 x 5) CNi(ll0) layer to 750 I< (the (4 x 5) C graphite transition which leads to N 40 % of the saturated graphite layer). It turns out that the 0.4 graphite surface adsorbs substantial amounts of H? on tlie remaining patches of unaffected areas, whereas the ( 4 x 5) carbide surface does not chemisorb hydrogen at room temperature [23]. For surfaces where the modifiers are arranged in islands, leaving completely clean patches, the changes in the initial rate of dissociative adsorption are less severe. As illustrated in fig. 5.41., 5’0 decreases linearly with the increase in the fraction of the surface occupied by modifier islands and falls t o zero on complet-ion of the modifier monolayer (since the latter does not adsorb hydrogen at all).

-

5.4.2

Modifier Effect on the Surface Diffusion of Hydrogen Adatoms

The surface mobility of tlie reactants is of great importance in catalytic processes [140]. As outlined a.bove, hydrogen adatom diffusion on clean metal

134

Chapter 5

-

surfaces occurs readily, the activation energy of diffusion being of the order of or less than 25 kJ/mol [132]. Since besides t,he effects on the adsorption kinetic parameters and Me-H bond strength, the introduction of foreign adatoms changes the surface potential contour for diffusion, it is necessary to find out how this is going to affect the mobility of H on the surface. Such fundamental knowledge will provide a valuable insight into the very important phenomena of surface diffusion directly related to catalysis. Fig. 5.43. presents the changes in the hydrogen surface diffusion coeficients induced by various sulphur coverages on Ru(O1). It is obvious that the presence of small amounts of S causes a severe reduction of the hydrogen diffusion coefficient approximately by a factor of 30 for a sulphur coverage 0.2. Since the ratio between the diffusion coefficients measured at 300 and 270 K remains constant, this indicates that the presence of S does not affect the activation barrier for diffusion (19 kJ/mol). Consequently, the presence of S should affect the pre-exponential factor in the hydrogen surface diffusion coefficient [142].

-

-

Sulfur Coverage

BS

(ML)

Fig. 5.43. Hydrogen surface diffusion coefficients on Ru(0001) at 300 and 370 I< versus sulphur coverage. The solid lines are Monte Carlo simulat,ion results for sulphur blocking ten three-fold adsorption sites (from ref. [143])

A similar strong reduction of the hydrogen surface diffusion coefficient has been observed in the presence of C which is also a common surface modifier in many catalytic reactions. The difference with a S additive is that the presence of a C deposit causes a change in the activation energy for hydrogen surface diffusion [141]. The observed differences have been associated with the existence of two possible mechanisms for hindrance of the coadsorbate surface mobility:

5 . 5 . Water

135

(1) A blocking mechanism in the case where the additive adatom residing in a highly coordinated site forces the coadsorbate adatom t o diffuse around this site, the precluded area being determined by the t,ype of the modifier. Obviously, in this case, the activation energy for diffusion is likely to remain constant and the restricted mobilit,y will be determined by the pre-exponential of the surfa.ce diffusion coefficient; (2) A trapping mechanism when the coadsorbate adatoms are permanently or temporarily trapped by the additive adatoms. In this case the formation of a H-additive bond should affect the activation energy for surface diffusion and might also cause a shift of the H? T P D spectra to a higher temperature due to the formation of a hydrogen-modifier bond. A deta.iled exa.mination of the dat8aconcerning the effect of S and C: additives on the hydrogen diffusion coefficient and the €12 TPD data. on Ru(0001) [141, 1421 has led to the conclusion that the site-blocking model is of importance in the case of S, w11erea.s the trapping mecha~nismexplains the effect of C. Actually, there is a great dea.1 of proof provided by HREELS and ESD of H attaching t o C atoms on met,al surfaces [143-1451, which supports the existence of a trapping mechanism. The Monte Carlo simulations performed in order to determine the effective radius of the S influence on hydrogen surface diffusion have shown that the experimental d a t a will fit a.ssuming that one S adatom residing i n a threefold site on the hpc (0001) surface perturbs ten threefold surface sites. They involve the S adsorption site, the three nea.rest neighbour and the six nextnearest neighbour sites. The restricted mobility of the hydrogen adat$omsin the presence of additives explains the observed drama.tic reduction of the 112 desorption preexponential factors (see Table 5.9.). This reduction reflecting a steric hindrance of hydrogen recombination on the surface can be directly related t o the observed severe restriction of the hydrogen surface diffusion in the presence of S adatoms. Undoubtedly, as will be illustrated in Section 8.1., the effect on the hydrogen surface mobility is one of the most import,ant,factors in explaining the S poisoning effect, on the methanat,ion reaction and the Fischer - Tropsch syntheses. 5.5

WATER

Water participates as a reactaiit or a product in numerous het,erogeneous catalytic reactions, such as water - gas shift synthesis, Fischer - Tropsch synthesis, etc. Isolated water molecules possess four doubly occupied orbitals, the two 0-H bonds being located i n t,he y r plane, and the two lone pairs, (3al and l b l ) , in the z z plane. Water adsorption on the single crystal metal surfaces under consideration is nondissociative. T h e bonding to the surface is formed via the oxygen atmomwith a 3u1 partly bonding lone pair and a Ibl non bonding lone pair being involved in bonding. T h e strength of the molecular adsorption bond with metal surfaces is typically of the order of 4065 kJ/mol, which indicates a relatively weak coupling with the metal surface.

136

Chapter 5

The formation of the molecular adsorption bond is accompanied by a decrease of the work function, i.e, with respect to its adsorption behaviour water should be considered as a n electron donor (opposite to CO, NO, Nz and 0 2 ) . The configuration of the water molecule chemisorbed on the surface depends on the actual adsorption site, because there is competition between 3al and I b l orbitals for optimal overlap with suitable surface &orbitals. Because of the non-bonding character of the water lone pairs participating in the formation of the adsorption bond and the relatively weak interactions with the surface, the internal molecule bonds are only slightly perturbed upon adsorption. The relatively weak coupling with the surface enables the formation of hydrogenbonded clusters even at low water coverages (the hydrogen bond strengths are of the order of 15-25 kJ/mol). More details about the water - metal surface interactions are given in the extended review of P. Thiel and T. Madey (14GI. Since, as outlined above, water acts as an electron donor in the bonding formation with the metal surface, the introduction of electronegative additives will be expected to influence the surface structure and reactivity of water in a way quite different from those observed for GO, NO, N?, 0 2 and H?. The influence of oxygen as an electronegat(ive additive is most widely studied. In order to understand the oxygen effect, it is necessary to consider separately the observed stabilization of the molecular adsorption state and the promotion of dissociation of the water molecule on some oxygen-modified single crystal metal surfaces. The best example for the oxygen effect on the molecular surface structure and bonding of water is the system H?O/ 0Ru(OOO1) [147-1491. It involves: (i) stabilization (increase of the adsorption binding energy) of the water molecular state;

(ii) destruction of the long range ordering due to the formation of H? 0H 2 0 hydrogen bonds on the O-free surface; (iii) changes in the H 2 0 orientation preferred on a clean surface due to direct water - oxygen interactions in the mixed overlayers, which overcome the formation of hydrogen bonds. The same stabilization and orientation effects on the molecular adsorption state is observed for the H20/O-Ni(lll) system [150, 1511. The experimental data [148, 150-1531 show that several water molecules can be influenced by each adsorbed oxygen atom, e.g. the number of affected water molecules reaches 6 to 8 for low oxygen coverages [152]. An excellent illustration of the oxygen-induced orientational changes of the coadsorbed water molecule are the ESDIAD results, shown in fig. 5.44. The change in the Ht ESDIAD pattern with the introduction of oxygen on Ni( 111) indicates that oxygen causes an azimuthal order in the OH bond orientation along [I121 azimuths. In addition, because of interaction of the water molecule with the coadsorbed oxygen via one of the hydrogen atoms (as illustrated by the structural model in fig. 5.44.) the molecule inclines towards the oxygen adatom, so that only the second hydrogen atom is seen in ESDIAD. The origin of the central beam (the only one that remains after heating) from the linear Ni-0-H group,

5.5. Water

137

118 Fig. 5.44. LEED (a), ESDIAD (b-g) patterns and a structural model for fractional monolayers of oxygen and water coadsorbed on N i ( l l 1 ) . Panel (b) shows the Ht ESDIAD pattern of water on clean N i ( l l l ) , and panel (c) shows the H+ ESDIAD pattern of water with coadsorbed oxygen. Panels (e-g) illustrate the heating induced changes in the H+ ESDIAD pattern when OH, species are formed. (from ref. [150])

formed as a result of oxygen promoted dissociation of water on the surface. Oxygen induced water dissociation and OH formation have been reported for Ag, C u , P t , P d a n d Ni single crystal surfaces [150-1571. It has been supposed t h a t the dissociation of water on the surface proceeds via a hydrogen abstraction reaction, which takes place in the temperature range 130-200 K: 0, HzO, = 20H,. T h e formation of adsorbed OH species on oxygen predosed metal surfaces is found to be very sensitive to t h e actual oxygen coverage, as illustrated in fig. 5.45. T h i s indicates that the dissociation of water via a hydrogen abstraction reaction is favoured only at relatively low oxygen coverages (maximum production of OH, in the 0.1-0.2 ML range), t h e oxygen saturated coverages becoming unreactive. Oxygen coverage being dependent on reactivity is supposed to be due to a geometrical site-blocking effect, because significant coverage-induced changes in the electronic properties of the adsorbed oxygen atoms are unlikely. However, if oxygen tends to form islands at high coverages, t h a n this adds to the reduction of the surface reactivity, because t h e reaction occurs only at the edges of the islands. An additional severe reduction of t h e reactivity towards hydrogen abstraction occurs when a n oxide phase is formed on the surface. As has been shown in ref. [158], both the

+

Chapter 5

138

' /

I

I

\ \

i

\ \

0

u

.4\

0 2 1

r,

01

02

03

0.

Oxygen Couerage (ML) Fig. 5.45. Dependence of the hydroxyl coverage on t,he initia1 oxygen coverage, 80, on C u ( l l 0 ) (from ref. [152])

clean Ni(ll0) and the heavily oxidized metal surface are completely inert to HzO adsorption at room temperature. As illustrated by the HzO T P D data in fig. 5.46. (where H 2 0 desorption results from OH, recombination), the dissociation of HzO takes place preferentially within a critical oxygen coverage span, where oxygen is in a n adsorption state. Summarizing the results for water coadsorption with electronegative adatoms, it becomes obvious that the effects of the electronegative additives with respect to the molecular adsorption state stability and the dissociative propensity of water are opposite to that observed in the case of CO, NO, N 2 , 0 2 and Hz. This is not surprising because in the case of water adsorption, the metal surface can be considered as an electron acceptor so that it is likely that the introduction of an electronegative modifier will increase the heat of water adsorption. Besides, the presence of additive adatoms with dipoles opposite to that of the coadsorbed water will also contribute change in the bonding energies and in the orientation of H20 via attractive electrostatic interactions. In the case of an oxygen modifier these interactions can lead to hydrogen abstraction and dissociation of the water molecules on the surface. The resulting OH, species are electron acceptors bonded via an 0 atom in an adsorption site which offers a n optimal overlap between OH x and the appropriate metal d orbitals.

139

5.6. Organic Compounds

-

-g;* -0.65

--.---. 0.6

0.1 1

,

1

300

-

0.06

A ’0.0 400 500 Temperature (K)

Fig. 5.46. Thermal desorption spectra for H 2 0 o n clean and oxygen covered N i ( l 1 ) ) . The formation of an oxide phase is observed at 00 > 0.5 (from ref. [158]) 5.6

ORGANIC COMPOUNDS

There is a great number of hydrocarbon reactions catalyzed by transition metals, e.g. hydrogenation of unsaturated hydrocarbons, dehydrogenation, dehydroisomerisation, dehydro- cyclisation, isomerisation, selective oxidation, etc. T h e complicated hydrocarbon syntheses involve a sequence of several distinct steps, t h e first one being always adsorption of the reactant - hydrocarbon molecule. W i t h the exception of some hydrogenation reactions, several alternative reaction paths are usually possible. They include hydrocarbon dissociation followed by fragmentation, rearrangement of the intermediates a n d secondary reactions within the adsorbed layer. Any single step may be favoured for a given structure and composition of the catalyst surface. Because of t h e complexity of the syntheses based on hydrocarbons, one cannot draw a definite line, whether additives are acting as poisons or promoters T h e reason for t h a t is t h e necessity to introduce a selective poison in order to inhibit the undesired reaction paths or to stabilize some surface chemical s t a t e or structure. There is tremendous variety of hydrocarbons and hydrocarbon interactions with clean a n d modified transition metal surfaces; it, is impossible to deal with these in the present review. T h a t is why several examples of electronegative additive effects on interactions of unsaturated hydrocarbons,

140

Chapter 5

alcohols and R I R ~ C Ocarbonyl compounds have been singled out. The main aspect thereby is the influence of some electronegative additives (0,S, C) on the molecular adsorption state, surface decomposition and secondary reactions in the mixed overlayers.

5.6.1

Interaction of Hydrocarbons w i t h Modified Metal Surfaces

Most of the catalytic reactions involving unsaturated hydrocarbons contain breaking the C-H and C-C bonds as a reaction step, followed by hydrocarbon adsorption on the surface. The formation of the molecular adsorption bond of olefins with the transition metal surface has been established exclusively on the basis of HREELS, ARUPS, IR and T P D studies of C2H4 adsorption on clean single crystal surface at low adsorption temperatures [163]. Two types of bonding with the metal surface have been identified. The first di-a type of bonding occurs via the two carbon atoms and involves a large amount of backdonation from the ds-hybrid metal orbitals to the antibonding r* orbitals of the adsorbed ethylene. As a result, the C-C bond order decreases, the C hybridization changing from sp2 to sp3. The second r type of bonding retains the sp2 hybridization of the C atoms involved in the double bond and the main contribution to the bonding is via x-donation from the molecule t o the metal ds-hybrid orbitals. The relative amount of the di-a and a-bonded ethylene varies with various transition metals and crystallographic planes, the di-a bonded state being the most favourable at low coverages and the more stable (with a higher adsorption binding energy) one. Since in both bonding configurations the ethylene C-C axis is parallel to the surface plane, one should expect the same steric blocking effect of the additives with respect t o di-a and a-bonded molecules. At elevated temperatures (2 300 I<), the di-a bonded species undergo subsequent dissociation starting with breakage of the C-H bonds and formation of ethylidyne species C-CH3, coordinated to three surface atoms. With a further increase of the adsorption temperature (> 400 K), decomposition to CCH and complete dehydrogenation occur. Oxygen is one of the most usual impurities but it also can participate as a reactant in some catalytic reactions, e.g. epoxidation of ethylene to ethylene oxide on silver catalysts [164]. This has given rise to a great number of recent studies dedicated to describing the different aspects of the influence of oxygen on the adsorption and the decomposition of ethylene and other olefins [165-1741 on single crystal metal surfaces. The introduction of oxygen (as well as other electronegative additives) to the surface generally leads to the reduction of the relative amount of di-a bonded molecular species in favour of dominating x-bonded species. On metal surfaces where no secondary reaction between the hydrocarbon radicals a i d coadsorbed oxygen atoms occurs (e.g. Ru and Fe [167, 1681) it has been found that the presence of oxygen reduces both - the total amount of adsorbed ethylene and the fraction of adsorbed ethylene molecules which decompose at elevat,ed temperatures. Table 5.11. presents the effect of oxygen overlayers on the adsorptive capacity and the decomposition efficiency of the surface. It is obvious that the decomposition of ethylene is completely hindered on Ru(0001) covered with a p(2 x 1) 0.5 0 overlayer. However, close inspection of the experimental data

5.6. Organic Compounds

141

has shown that no decomposition takes place at oxygen coverages exceeding 0.4 [167]. Table 5.11. Saturation Ethylene Coverage, OSat (in ML) at T, = 100 K , the Fraction Decomposed upon Heating, &ins (in ML), and the Type of Bonding, MeC2H4, of the Molecular Adsorption State for Clean and Oxygen Precovered Ru(0001) Surfaces (from ref. [167]) SURFACE

6,,,

edirs

Ru(0001) p(2 x 2) 0.25 0-Ru(0001) p(2 x 2) 0.5 0-Ru(0001)

0.3 0.12 0.10

0.08 0

0.24

Me-CzH4 di-a/

TI .I

HREELS da t a for a clean and oxygen modified Ru(0001) surface show t h a t p(2 x 2) 0.250 and p(2 x 1) 0.5 0 overlayers favour n-bonded molecules where C atoms remain sp2 hybridized, and remove the di-a (C sp3 hybridized) bonded molecules observed on an oxygen-free Ru (0001) surface. This effect of the oxygen adatoms has been ascribed mainly to significant oxygen-induced perturbations of the electronic properties of the surface (increasing of the Lewis acidity of the Ru surface atoms). The contribution of the simple steric blocking of the available adsorption sites is supposed to be less important, because both di-u and n-bonded molecules require almost the same surface space. As a result of this oxygen induced electron deficiency on the surface, the adsorption of x-bonded molecules which act as electron donors become more favourable. The dissociation of molecules in a x-bonding configuration wlll be less easy because the negligible backdonation involved in the formation of the n-bond is not capable of substantially weakening the double C-C bond. Another proof of the domination of oxygen induced electronic perturbations over steric blocking is the much weaker effect, of the p(2 x 2) 0.25 C overlayer on the ethylene bonding configuration, i.e. no removal of the di-a ethylene bonding is observed in the presence of a carbon coverage of 0.25. Since C and 0 have very close covalent radii, the more severe effect of 0 can be satisfactorily explained with the significant difference in the Pauling electronegativity of C and 0: 2.5 vs.3.5 (implying larger electronic perturbations on the surface induced by the adsorbed oxygen). Apart from the change in molecular bonding configuration and the inhibition of ethylene decomposition on the surface, the presence of oxygen on Ru(0001) has been found to cause stabilization of the intermediate products on the surface during the decomposition process. Thus, on an oxygen-free surface the only dissociation product detected is ethylidyne (CCH3) which decomposes readily to C and H at T > 400 I<, no stable intermediates were detected in the vibrational spectra. In the presence of oxygen, the ethylidyne decomposition proceeds via the following reaction steps, the intermediate products being detected in I

Chapter 5

142 the HREELS spectra [lG7]: ethylidene

’2’vinylidene 4 2 K C + methylidene

> 550K

C+H

With the exception of small amounts of GO formed at temperatures above 500 K (as a result of the reaction C,+O, = CO) no other secondary oxidation products (COz or HzO) are detected in ethylene + oxygen coadsorbate layers on Ru(0001). The lack of secondary oxidation products is also reported for oxygen modified Fe [168]. These results are not surprising since GO and hydrogen oxidation reactions do not take place on Ru and Fe surfaces under UHV conditions because of the high Me-0 bond strength. On the contrary, on oxygen-modified Pt, Pd and Ir surfaces, secondary reactions leading to the formation of COz and H 2 0 readily occur [165, 169172, 1751. Because of the activity of P t , Pd and Ir with respect to CO and hydrogen oxidation reactions, the effect of oxygen on the adsorptive capacity and the dissociation probability of CzH4 and other olefins is negligible at low and moderate oxygen coverages (i.e. the oxygen effect is restricted exclusively t o the removal of di-a in favour of r-bonded molecules). On Pd the presence of oxygen has been found even t o lower the temperature for dissociation of the unsaturated hydrocarbons from 500 down to 420 K [170], due to the fact that 0 efficiently withdraws hydrogen and inhibits any possible rehydrogenation. At T 2 400 K, the rate of ethylene dissociative adsorption on an oxygen precovered Pd surface is very high ( the initial sticking coefficient is close to unity) and complete dissociation is favoured because the reaction of water formation is faster than the rehydrogenation reaction and C can be readily removed by complete combustion to COz. At lower reaction temperatures, as a result of the reduced rate of COz formation, the amount of C increases on the surface. This leads to a decrease of the reaction rate of water formation because of the H lateral mobility thereby being reduced (by trapping to C as described the Subsection 5.4.2.) and further inhibition of the ethylene dissociation by blocking the appropriate adsorption sites [169-171]. Thus, a conclusion can be made that for transition metals which are good catalysts for CO oxidation, the effect of the oxygen additive is restricted only to the inhibition of the di-a molecular bonding of et,hylene. However, because of the oxygen-induced electron deficiency on the affected substrate surface atoms, the n-bonding (where the ethylene molecule acts as an electron donor) might be stabilized by the presence of an electronegative additive. More peculiar is the case of ethylene - silver systems, where the extensive HREELS and IR studies have shown that the molecular adsorption of C2H4 and higher olefins occurs only via a P-bonding without rehybridization of the molecule, the adsorption energy of ethylene being of the order of 40 kJ/mole [176, 1771. That is why it is not surprising that the introduction of an electronegative additive, such as oxygen leads to an increase in the amount of n-bonded ethylene with increasing additive coverage [176]. The adsorptive behaviour of ethylene on Ag surfaces is most likely the reason that Ag is the best catalyst for selective oxidation of ethylene to ethylene oxide. Ethylene oxide is a basic chemical for many important syntheses, e.g. production of polyesters, antifreeze etc. [164, 1781. There are several different views about the type of

-

-

5.6. Organic Compounds

143

the oxygen adsorption state participating in the formation of CzH4O. Some of the authors support the mechanism whereby diat,omic oxygen species are active for epoxidation by the following reaction [164, 1791:

CzH4(a) + @ ( a )

-+

CzH40

+ O(a),

whereas the atomic oxygen state exclusively combust.s ethylene by the surface reaction: CzH4(a) 6 0 ( a ) 2C02 2 H z 0 .

+

-

+

Other authors are of the opinion t,Iiat the direct pwticipation of 0 (a) in the formation of C2H4O cannot be excluded [180, 1811. Recently, it has been proposed that depending on the orientatmionof the Ag surface crystallographic planes, the oxygen ada.toms can be i n different adsorption sta.tes, which exhibit different reaction activities [182]. Thus, for the corrugated Ag( 110) surfa.ce, the low coverage fourfold coordinat,ed oxygen state is expected to have a diu type bonding and is supposed t,o lead to ethylene combustion. At high oxygen coverages threefold a.nd/or t.wofold states a.re occupied by species of oxy-radical character, which are a.ssumed t,o be the a.ctive epoxidation species [182]. As outlined above, besides ethylene oxide formation, the second rea.ction path leads to products of total combustion (CO? and HZO), so that much effort has gone into finding appropriate additives for a.chieving masimuni selectivity. It has turned out that typical electronega.t.ive additives, such as C1, S, Se, Br, etc.as well as typical electropositive (I<, Cs) additives are known t o increase the selectivit,y and activit,y of the Ag catalysk for the epoxidation reaction [183, 1841. In order to explain the effect of the electronegative additives on the ethylene oxide forinat,ion, coadsorption experiments involving electronegative adatoms and the reagents ethylene and oxygen have been performed in model systems. Figs. 5.47 and 5.48. present thermal desorption spectra of ethylene from Ag(ll0) modified by increasing a.mount,s of C1 or 0. Obviously, the int,roduction of elect.ronega.tive additives up t.0 covemges of 0.5 leads t o an increa.se in the adsorbed a.mount of ethylene and the appearance of a second higher temperature desorption pea.k due tro stabilization of the ethylene molecular adsorption state on the modifier affected sites. The measured heats of ethylene adsorption on C1 and 0 affected sites are 62 kJ/mol and 53 kJ/mol, respectively, compared to 40 kJ/mol for a clean surface. Irrespective of the increased stability of t.he ethylene molecular state no ethylene decomposition has been observed. At very high additive coverages the reverse trend is observed, which indimtes t1ia.t the adsorption of ethylene can ta.ke place on the affected sites, but not. over t.he sit,es a1rea.dy occupied by the additives. The more pronounced sta.biliza.tion effect and the la.rger relative amount of affected molecules i n the ca.se of CI suggests the prevalence of the size contribution over the electronegativity factor in determining the strength and the extent of the modification effect,. As has been a.lready discussed, the C1- a.nd O-induced electron deficiency in the affected surface Ag a.toms fa.vours the C2H4 a-electron donation which contributes mainly to the strength of C Z H a-bonding ~ on the surface. However, the enhanced affinity and capacity of the surface to a-bonded ethylene

Chapter 5

144

0 ’

AV I

I30

I90

TEMPERATURE (K)

Fig. 5.47. CzH4 TPD spectra from A g ( l l 0 ) surfaces containing various C1 coverages. T, = 134 I<. (from ref. [lSS])

molecular adsorption in the presence of electronegative additives cannot explain the enhanced selectivity for ethylene oxide formation. A satisfactory explanation of the observed inhibition of the complete ethylene combustion (GO:! formation) is the effect of electronegative modifiers on the rate of dissociative oxygen adsorption. As has been already discussed in the previous Subsection, the presence of electronegat
5.6. Organic Compounds

145

Ethylene TPD spectra from A g ( l l 0 ) containing various coverages of atomically adsorbed oxygen (from ref.[186]) Fig. 5.48.

is possible by favouring new reaction pathways [25, 188-1901. Studies of the S and C effect on the reactivity of Mo(100) with respect to several unsaturated hydrocarbons have shown that, while molecules, such as butadienes and butenes, undergo almost complete decomposition in several steps to C(af and H z , both C and S adatoms inhibit the complete decomposition of the adsorbed molecules under consideration. This S- and Cinduced inhibition of the complete hydrocarbon decomposition is accompanied by favouring new reaction paths leading to the formation of hydrocarbons with a higher hydrogen content. As illustrated in figs. 549. and 5.50., for 1, 3butadiene adsorption a hydrogenation reaction leading to the formation of a mixture of 30 % to 70 % 1- and 2-butenes is favoured at sulphur coverages ranging from 0.2-0.4 and C coverages of 0.2-0.8. Above certain S ( w 0.5) and C (- 0.8) coverages, the decomposition process is almost completely eliminated and only molecular butadiene desorption is observed. A similar effect of S and C on the decomposition reactivity of the surface is observed in the case of butenes. However, since even a clean Mo (100) surface is active towards l-butene hydrogenation, the S- or C-induced slight increase in hydrogenation activity at low additlive coverages (2 0.2) is followed by fast

Chapter 5

146

I

1,3-Butadlone/S/Mo( 100)

Sulfur C0v.r.g. (a)

1

8-

Fig. 5.49. 1,d-butadiene, 1,a-butene and Hz production as a function of S coverage (in ML) on Mo(100), as evaluated from the mass 54, mass 56 and mass 2 TPD peak areas (from ref. [188])

Fig. 5.50. 1,d-butadiene, 1,Z-butene and Hz production as a function of C coverage (in ML) on Mo(100), as evaluated from the mass 54, mass 56 and mass 2 TPD peak areas (from ref. [188])

5.6. Organic Compounds

147

-

reduction of the butane production at higher S or C coverages until complete inhibition a t S (C) coverages 0.4 [la81 has been reached. The TPD data of the molecular adsorption states of butadiene and butene indicate that in the case of S, no new molecular states are created (both from a. clean and from a S modified Mo( 100) surface the molecular desorption peak is at 150 K , i.e. the desorption energy is of the order of 30-40 kJ/inol). In the cases of a stable molecular adsorption state on a clean surface, e.g. benzene on P t ( l l 1 ) [25], the increasing amounts of S lead to continuous removal of the most tightly bound states accompanied by a strong reduction of the amount of benzene desorbing from the first overlayer. In the case of carbided Mo( loo), new higher temperature molecular desorption peaks associated with hydrocarbon adsorption on C illflueliced sites are observed with a desorption energy by 10-30 kJ/mol larger than the clesorptioii energy measured for a clean surface [188]. This stabilized molecular state is supposed to be bound to the nearest on top Mo sites which become more efficient electron acceptors in the presence of a C adatom residing i n a fourfold site. The absence of a similar stabilization effect in the case of S is attributed to the larger size of S which sterically hinders the adsorption 011 the most strongly perturbed site that is the nearest on top one. The enhancement of the molecular desorptioii fraction at the expense of the dissociated one and the promotion of the hydrogenation reaction within a cert,ain S- and C-coverage range indicates that the major effect of S and C is in blocking the favourable adsorption sites for the decomposition products H and C. Obviously, because of its larger size S blocks the sites for decomposition more efficiently and behaves like a more drastic poison than C. Generally, the same effect on the surface activity (blocking of the available adsorption sites and inhibition of the tota.1 dehydrogena.tion on the surface in the presence of S and C additives) is observed for adsorption systems involving saturated hydrocarbons [188, 1901.

-

-

5.6.2

Effect of S and C on the Interaction of Thiophene with Transition Metal Surfaces

The great interest in understanding t,he adsorption and decotiiposit,ion hehaviour of thiophene is directly related to the necessity to reinove the undesired sulphur compounds from the crude oil (generally present as part of a thiophene ring) T h e reason is that. these compounds iiitroduce S onto the catalyst surface which severely poisons catalytic reforming and other iinport,a,nt ca.talytic processes. Molybdenum, tungsten and ruthenium sulphide catalyst are found to be the most effective for the removal of S containing organic compounds by a hydro-desulphurization process. C2H4S adsorption and deconipositioii has been studied on several tra.nsition metal single crystal planes which exhibit different kinds of behaviour with respect to the thiophene molecular bonding configuration and varying efficiencies for decomposition of the adsorbed molecule. When the thiophene adsorption is performed at low temperatures, a molecular state is detected by means of vibrational spectroscopies. At low thiophene coverages on Mo( 100) and Mo(ll0) [188, 192,1931, Ru(0001) [194, 1951, Cu(100) [196], Ni [197,198],

Chapter 5

148

and Pt [199,200] single crystal surfaces, the thiophene ring is oriented parallel to the surface plane, bound through the aromatic ring via a ir-bonding. In this bonding configuration no selectivity in the C-H bonds breaking is observed. At high thiophene coverages in most cases a change in the bonding geometry takes place. The molecule stands up and a a-bonding via the lone electron pair on S is realized. In this bonding configuration, a selectivity in the C-H bond breakage is observed, the a-CH bond scission being more facile. The temperatures at which C-S, C-C and C-H bonds break, vary with the different substrate surfaces and the actual thiophene coverage. Cu(100) [196] is found to be completely inactive for thiophene decomposition under UHV conditions. On P t ( l l l ) , C-S bond breakage occurs at T > 290 K. The desulphurisation of thiophene, tetrahydrothiophene and 2,5-dimethylthiophene leads to H and a variety of hydrocarbon products preceded by metallacycle-like intermediates [199, 2001. On Ni(100) C-S scission takes place at 90 K, forming the C ~ H B metallacycle species which undergoes further dehydrogenation at T > 500 K [197]. On Ru(0001), C-S breakage occurs at 120 K, the formed metallocyclelike intermediates being further dehydrogenated stepwise with Hz desorption at 230, 305 and 450 K [194]. While from all systems described above, a certain fraction of the adsorbed thiophene desorbs molecularly, negligible molecular desorption occurs up t o monolayer thiophene coverages on Mo because Mo is one of the most effective substrates with respect to complete thiophene decomposition. The C-S cleavage is favoured at temperatures 100 K , whereas the break of the C-H bond and the complete decomposition to C, S, and H occurs:

-

(i) at low thiophene coverages (for the parallel ir-bonding configuration) at T < 500 K and (ii) at high thiophene coverages at T

-

650 K.

This coverage dependence on the thiophene decomposition temperature is due t o the change in the bonding configuration which introduces a high energy pathway for P-CH cleavage H2 is the only desorption gas product as a result of the complete thiophene decomposition on Mo. The modification of the transition metal surface with S adatoms causes the following identical changes in the surface activity with respect to thiophene adsorption and decomposition [lSS, 193-195, 198, 200, 2011:

(i) a decrease of the thiophene sticking probability and the saturation coverage; (ii) an inhibition of the total decomposition on the surface; (iii) an enhanced selectivity towards hydrocarbon formation after desulphurization at low up to moderate (0-0.25) sulphur coverages a t the expense of the reduced fraction of totally decomposed molecules, and (iv) a complete passivation of the surface for decomposition above critical S cover ages.

5.6. Organic Compounds

149

The main reason for the deactivation effects of the preadsorbed S is that sulphur blocks the multiatom ensembles and this prevents decomposition. The fact that thiophene hydrodesulphurisation can be observed up to rather high sulphur coverages, i.e. the amount of the decomposed fraction is decreasing almost linearly with increasing sulphur coverage [201], can be explained by the tendency of S to form islands of ordered structures leaving ensembles of bare Mo atoms. The enhanced selectivity towards hydrocarbon formation a t low and moderate S coverages supposes that the preferential orientation of thiophene in the presence of S should be a perpendicular one. After C-S scission, the hydrocarbon products desorb rather than decompose because, as described in the previous Section, S inhibits hydrocarbon dissociation as well.

SULFUR COVERAGE (ML)

Fig. 5.51. Effect of increasing S coverages on Mo(100) on the amount of desorbed HP and S residue (resulting from thiophene decomposition) (from ref. [20l])

The studied effect of adsorbed sulphur on the activity of Mo single crystal surfaces is a good example of the differences in activities of surfaces with adsorbed additive overlayers and the corresponding metal-additive compounds. It is well known that the layer compound MoSz is an excellent catalyst for the hydrodesulphurization reaction. The established deactivation effect of S on the decomposition of thiophene and related hydrocarbons on transition metal surfaces supports the mechanism according to which the active sites on MoS2 catalysts are the edge sites (anion vacancies) where Mo ions are present in

Chapter 5

150

a number of different oxidation states. These sites are likely to favour the formation of an electron donor s-bonding via the lone pair located at the S atom.

I

Thlophom/C/Yo( 100)

Fig. 5.52. Hz and C2H4S TPD spectra after thiophene adsorption on clean and carbon covered Mo(100) surfaces. (from ref. [188])

Similar but less severe is the effect of C adatoms on thiophene adsorption and decomposition [188, 1951. The only essential difference is that, in contrast to S, the presence of C on the surface creates a new higher temperature desorption state in the thiophene molecular desorption spectra [188, 1951. Fig. 5.52. presents HS and CzH4S T P D spectra from clean and C covered Mo(100) surfaces. It is obvious that C induced reduction of the amount of decomposed thiophene is compensated (to a certain extent) by an increase in the fraction of non dissociated thiophene and stabilization of the molecular adsorption state. That a similar stabilization of the hydrocarbon molecular state by C is not observed in the case of S, has already been discussed in the previous Subsection.

5.6.3

Interaction of Alcohols, Aldehydes, Carbonyl Compounds etc. with Modified Metal Suri’aces

Synthesis, decomposition and oxidation of alcohols are catalytic processes of great industrial importance. Since adsorption and decomposition of alcohols on catalyst surfaces is an important step in the aldehyde and carbonyl compound synthesis, a significant number of studies have been concerned with

5.6. Organic Compounds

151

the description of the influence of electronegative modifiers on the interaction of alcohols, aldehydes and carbonyl compounds with single crystal transition metal surfaces. The interaction of alcohols with transition metal surfaces can proceed via several reaction channels. As an example, consider the most extensively studied methanol (CH30H) adsorption and decomposition process. The following four major reaction channels of methanol interaction with metal surfaces are possible:

-

(1) Dissociative adsorption even a t temperatures as low as 100 K leading to the formation of methoxy (CH30,) and hydrogen (H,) species. This first step is observed on many clean surfaces, e.g. Fe(ll0) and (100) [204, 2051, Mo(100) [206], N i ( l l 0 ) [207], Pd(100y [208]', W(l00) [209]', Ru(0001) [210], etc;

(2) Decomposition of the methoxy intermediate to CO and H 2 , which takes place on Fe(100) [211], Mo(100) [206], Ni(ll0) [207], Pd(100) [208], and Ru(0001) [210] at temperatures varying with the different substrates (ranging from 200 to 400 I<); (3) Formation of aldehyde (CHzO) by dehydrogenation of the methoxy intermediate, which is exclusively favoured in the presence of electronegative additives ( 0 ,S, C) [66, 207,209, 211-2211; (4) Recombination of adsorbed methoxy species and H thereby forming CH30H. The effect of S on the adsorption and decomposition of alcohols leads to quite different kinds of behaviour than that observed on a clean surface. In some cases the influence of S is not, simply restricted to the reduction of the adsorption/dissociation rate and the surface adsorptive capacity but also leads to a significant alteration of the decomposition reaction path [66, 2121. Fig. 5.53. presents the sulphur induced changes in the distribution of CO and CHzO products from methanol adsorption layers on a Ni(100) surface [212]. For a S-free surface at T > 300 K CH30H undergoes complete decomposition to H and CO, whereas the molecular desorption takes place at T 200 K. The presence of S leads to the following effects:

-

(i) a decrease of the amount of molecularly adsorbed CHSOH, attributed to the usual induced reduction in the surface adsorptive capacity; (ii) a rapid decrease of the extent of H 2 and CO desorbing from the surface due t o inhibition which prevents complete methanol decomposition, and (iii) a change in the selectivity towards the production of CH20, which is favoured a t 0.2 < 6.5 < 0.5.

As can be seen in fig. 5.53., the decomposition is completely hindered on

-

p(2 x 2) 0.25 S--Ni(100), whereas the maximum in the formaldehyde amount occurs at 6s 0.3. Comparing the T P D spectra of the decomposition products arising from a clean and from a sulphided Ni(100) surface, leads to the conclusion that probably the selectivity changes induced by S are due to

152

Chapter 5

Fig. 5.53. Effect of preadsorbed S on the amount of HzCO and GO formed as a result of C H 3 0 H interaction with Ni(lO0) (from ref. [212])

(i) the increased stability of the intermediate methoxy-species because of the increase in the activation energy for dehydrogenation; (ii) the suppression of the C H 2 0 adsorption bonding configuration favouring further dehydrogenation, and (iii) the effective blocking of the hydrogen adsorption sites. The maximum of the C H z O production should be associated with competition between two S effects: stabilisation of the C H 3 0 , intermediate and blocking of the surface sites allowing C H 3 0 H dissociation to C H 3 0 and H . Thus, the surface becomes completely deactivated as the S coverage reaches 0.5 when no dissociation of C H 3 0 H is detected. An accurate study of the effect of S on the decomposition kinetics of methanol has been recently reported for S-covered Ru(0001) [220]. It has been found that the initial decomposition activation energy increases almost linearly with increasing S coverage from 33 kJ/mol for a S-free surface to 42 kJ/mol for a surface covered with f?s = 0.07. The increase of the activation energy is compensated for by an increase of the pre-exponential factor from 4.2 x 10' for a clean surface to 3.3 x lo7 for 0.07 S/Ru(OOOl). These changes in the kinetic parameters for methanol decomposition together with the dramatic reduction in the methanol initial decomposition rate indicate that the effect of the sulphur additive cannot only be explained by a simple site blocking mechanism. Obviously, perturbations of the surface electronic structure beyond the nearest neighbours should be also considered.

5.6. Organic Compounds

153

The effect of S on the formaldehyde interactions with P t ( l l 1 ) is simiIar [66, 1371. On a clean Pt(ll1) surface, CH2O decomposition causes mainly H2 and CO to be produced and very small amounts of C02, CH3OH and HCOOCH3. The presence of S reduces the amounts both of molecularly adsorbed and dissociated CH2O and changes the decomposition reaction path towards hydrocarbon formation (CH4) at the expense of HCOOCH3 and C02 formation. Carbon overlayers of C in the carbidic adsorption state affect the adsorptive capacity in the same way as S does. It also affects the activity of the surface with respect to the decomposition of alcohols, RlR2CO compounds and formates and in addition it alters the product distribution [66, 2261. However, as has been observed with the other hydrocarbons, the poisoning effect of C is weaker than that of S. A more severe deactivation effect is exhibited by graphitic overlayers, which are completely inert with respect to the decomposition of oxygen-containing hydrocarbons. S and C act exclusively as selective poisons, while the oxygen adatoin serves several different functions, i.e, oxygen can act as a reactant and/or poison. These actions involve the following steps: (i) hydrogen abstraction from the hydroxyle group of the alcohol and formation of adsorbed alkoxides;

(ii) stabilization of the adsorption state of the alkoxides (RCH20,) and prevention of their complete decomposition; (iii) alteration of the preferred adsorption geometry and the thermal stability of the carbonyl compounds obtained a result of partial dehydrogenation of the alkoxide species, and (iv) formation of carboxylate as a result of a nucleophilic attack of the adsorbed oxygen. The last step is not favoured on all 0-modified metals because it depends on the actual oxygen adsorption state, Figs. 5.54. and 5.55. illustrate the oxygen-induced inhibition of complete methanol decomposition at the expense of opening up a new reaction path for partial dehydrogenation resulting in the format,ion of formaldehyde. It is obvious that the maximum activity towards the formation of formaldehyde is achieved at oxygen coverages 0.2 -0 25. At high oxygen coverages (> 0.4), CH30H decomposition is completely inhibited and only molecular CH30H desorption is observed. This complete passivation of the oxygen precovered Fe(100) surface with respect to methanol dissociation is confirmed by the HREELS data which suggest that the molecular methanol is weakly bound via hydroxy hydrogen and surface oxygen [all]. In the case of 0-covered Ru(0001) [213], the presence of oxygen (up to 00 0.25) exhibits the following effects:

-

-

(i) promotion of the formation of methoxy species even at high methanol coverages where, on a clean surface, the methoxy-formation is favoured only at low methanol coverages;

Chapter 5

154

0.44

O.X!

+-0.20

II

f

I

Fig. 5.54. Effect of increasing oxygen coverages on Fe(100) on the H2 and GO TPD spectra. The oxygen induced changes in the CO and Hz TPD area reflect the reduction of the fraction of totally decomposed CH30H. (from ref. [211])

(ii) quenching of the second pathway of methoxy decomposition which leads to the formation of HzO, C and H on a clean surface. The facilitation of methoxy formation can be related to the hydrogen abstraction ability of the adsorbed oxygen. The inhibition of C-0 bond breaking, which occurs on a clean surface, can be related to the poisoning effect of oxygen with respect to the appropriate bonding configuration required for C-0 cleavage (as described below). Similar to the case of 0-covered Fe(lOO), high oxygen coverages on Ru(0001) act as poison with respect to methoxy formation because oxygen blocks the adsorption sites which would otherwise readily adsorb methoxy. Since the selective poisoning effect is very important with respect to selective oxidation of alcohols to desired products, it is worth discussing briefly the mechanism of the electronegative modifier action on the stability of the carbonyl compounds which is observed with all the Group VIII metals. The carbonyl compounds of the type RIRZCO, where R1 and RZ are H or hydrocarbon radicals, exhibit two different coordination geometries in the adsorption state associated with two types of bonding: (0)and ?72(c,o) [223-2251. The q l ( 0 ) bonding results from a donor component involving overlapping of the non-bonding oxygen lone pair orbital with a ds hybrid orbital of the metal surface, where the metal acts as an acceptor. The small metal to anti bonding s*-CO backdonation contributes negligibly to the q l ( 0 ) bonding. The role of the metal in this type of interaction is that of a weak Lewis acid and the

5.7. Conclusive Remarks

1

155

0

+l_jbb-%5

Fig. 5.55. Effect of increasing oxygen coverages on the HzO, HzCO and C H 3 0 H

TPD spectra from oxygen modified Fe(100). The appearance and increase of the intensity of the HzCO TPD spectra at 0.1 < 80 < 0.35 indicate the selective poisoning effect of oxygen (from ref. [211])

resulting q l ( 0 ) bond is rather weak (- 40 kJ/mol). The qz(C,O) bonding configuration results from overlapping of the a-CO bonding orbital with a ds hybrid acceptor metal orbital and an overlap of a metal ds-orbital with a 7r*CO orbital. Since the backdonation from the metal is the major contribution to the qz(C,O) bonding, it causes a decrease in the C-0 bond order. qz(C,O) species are more strongly bound than ql(C) species and exhibit a higher tendency t o further decomposition than the q I ( C ) bound ones. The formation and the preference of the qz(C ,0) bonding configuration depends crucially on the backdonation ability of the surface. That is why the introduction of electronegative additives, which enhances the Lewis acidity of the surface, will suppress the q2(C,O) bonding configuration in favour of the q,(C) one. Since q1 (C) bound carbonyl compounds exhibit a high preference for molecular desorption, the observed inhibition of the total alcohol decomposition in the presence of electronegative additives is explained satisfactorily. 5.7

CONCLUSIVE REMARKS: THE POISONING EFFECT DESCRIBED WITHIN THE FRAMEWORK OF THE POSSIBLE INTERACTIONS IN THE COADSORBED LAYER

Because of the complexity arising from the specificity of the modified systems and the variety of possible interactions in the coadsorbed layer, there is still no unified model for the mechanism of the poison action even for the idealized model systems described in the previous sections. Up to date the

156

Chapter 5

scientific efforts have succeeded in clarifying that the following main factors might contribute t o the poisoning effect, the weight of every one varying with the different modifiers: 1. Steric blocking. This is due to direct repulsive interactions between the modifier and the coadsorbed molecules which excludes the adsorption sites occupied by the modifier and hinders coadsorption over a certain area around the modifier. These interactions can involve direct repulsion between some spatially extended and/or energetically suited coadsorbate and modifier electron orbitals. The steric blocking is a short-range effect, and its effective radii depend exclusively on the modifier’s size and to a lesser extent, on the modifier’s Pauling electronegativity and adsorption site .

2. Substrate mediated electronic effects. These effects are associated with the changes in the surface local density of states near the Fermi level as a result of the formation of the modifier adsorption bond. As will be discussed in more detail in Section 7.2., the presence of electronegative adatoms perturbs substantially the substrate electronic states which can concern the formation of an adsorption bond with coadsorbates. Because of the significant screening effects in the metal substrates, the effective radii of the electronic effect usually do not exceed 5 A.

3. Electrostatic interactions between the modifier and the coadsorbate. The contribution of these interactions depends on the actual effective charge of the interacting species and the corresponding dipole lengths. The range of such interactions does not exceed the screening length of the metal (- 4 A). In the case of most of the electronegative adatoms with prevailing covalent type of bonding and small dipole lengths, this kind of interaction does not contribute substantially to the poisoning effect. Obviously, all factors summarized above, account for a rather localized modifier action. Considering the complexity due to the different kinds of interactions an attempted explanation will now be given as to the experimentally observed ‘poisoning’ effects on the adsorption properties of some single crystal planes modified by ordered adatom overlayers. Fig. 5.5G. presents schematics of fcc(100) and fcc(ll1) types of surfaces with the modifier adatom residing in the highest coordinated site. The substrate atoms directly bound to the modifier are assigned as nearest neighbours, and the substrate atoms next to the directly bound ones as next-nearest neighbours. Obviously, in the case of the fcc( 111) plane one should distinguish between the three close and six remote next-nearest neighbours. The separation of the various adsorption sites from the modifier site is determined by the lattice constant of the substrate. On the basis of the experimental data for larger electronegative adatoms, such as S, Se, Te and C1, the following may be considered:

(i) the adsorption sites which involve only substrate atoms directly coordinated t o the modifier as blocked and

5.7. Conclusive Remarks

157

0 QIIP

@ 0

Fig. 5.56. Schematics of the possible number of surface atoms which might be affected by the presence of an additive adatom located in the highest coordination site on fcc(ll1) and fcc(100) surfaces.

(ii) the adsorption sites sharing some substrate atoms with the modifier as subst an tially perturbed. This means that for the fcc (100) plane one modifier adatom blocks four on top, four bridge and one fourfold site and perturbs eight bridge, four close and four remote fourfold sites. For the fcc(ll1) plane, the modifier blocks three on top, three bridge and one threefold site and perturbs six close bridge, six remote bridge, three close and nine remote threefold sites. With increasing modifier coverage, because of overlap of the effect, the number of affected sites per modifier adatom decreases. Following this simple model, it is obvious that the coadsorbate's most tightly bound molecular states, associated with unaffected surface sites, should be rapidly removed with increasing modifier coverage. Indeed, as reported in refs. [28, 291 , at low S coverages, one S adatom was found t o eliminate 6 to 9 most tightly bound &CO bridge states on a Ni(ll1) surface, which is close to the sum of the blocked bridge and perturbed six close-bridge sites. In the case of Ni( 100) where CO favours on top sites, one S adatom is found to remove four on top &CO states, i.e. those blocked by the additive (see fig. 5.6.). In addition, as evidenced by HREELS and TPD data [41], CO is pushed to occupy the close-bridge and close fourfold sites, where the coupling with the substrate is reduced. The above simplified picture can describe satisfactorily the modifier effect on the adsorption rate of the coadsorbate and on the adsorptive capacity of the surface. Let molecular adsorption first be considered. It obeys a trapping-

158

Chapter 5

dominated precursor mechanism (see eq.(3) describing the sticking coefficient of non activated molecular adsorption). In the case of CO and NO, the initial sticking coefficient for molecular adsorption on most transition metals is close t o unity and is almost constant up to moderate coverages. This indicates that the elastic and nonelastic back scattering are negligible and all molecules hitting the surface are trapped and chemisorbed. On assuming invariance of the rates of adsorption and desorption of the precursor upon introducing the modifier, the observed strong decrease of So(P2) for the most strongly bound Pz-CO state on Ni(100) (fig. 5.12.) with increasing modifier coverage, Ox, might be described by introducing a factor (1 - aOx), i.e. SX = So(@z).(l fytjx). The coefficient accounts for the number of sites, where the lifetime of the precursor at room temperature is drastically reduced. Thus, assuming that the number of these sites is equal to the coordination number of the modifier fourfold adsorption site, CY can be substituted: a = 4. This means that, at p(2 x 2) adatom overlayers (Ox = 0.25), the Pz-CO adsorption state is completely eliminated. The experimental results in fig. 5.12. show that the effects of S and C1 are even stronger, i.e. a is larger than 4. This implies that the modifier perturbations are extended beyond the nearest neighbours, affecting also the lifetime of the precursor state on these sites. The situation with modified fcc( 111) surfaces (modifier residing in a threefold adsorption site) is similar, there the reduction of the local sticking coefficient for the most tightly bound &CO state in the case of S is stronger than that predicted by a = 3 [28, 291. In general, for most of the electronegative adatoms under consideration, which tend to form ordered overlayers, occupying the highest coordinated surface sites, the blocking efficiency with respect to the originally most tightly bound molecular state of NO and CO is ranging from 3 to 6 per modifier adatom (depending on the substrate and the type of the modifier). The effect of the electronegative modifiers on the total sticking coefficient, SX, and on the total adsorption capacity observed at low adsorption temperatures is much weaker. In most cases SX preserves the clean surface value a t low modifier coverages and is less affected by the presence of a modifier than predicted by a equal to the adatom coordination number. Our studies on CO and NO adsorption on sulphided and selenided P t ( l l 1 ) [26, 35, 821 have shown that, at an adsorption temperature of 90 I< for p(2 x 2) 0.25 S(Se) overlayers SX =- 0.5s0, while according to the factor (1 - 3Ox) it should be 0.25 So. This can be ascribed to: (i) new access enabling the occupation of energetically less favourable (affected) adsorption states, and/or (ii) a certain residence lifetime of the mobile precursor even on surface atoms directly coordinated by the modifier at sufficiently low temperatures. However, the elimination of the most-strongly-bound molecular state is independent of the adsorption temperatures and its removal is usually completed at BX p(2 x 2) 0.25 X on f c c(l ll ), hpc(0001), fcc(lOO), fcc(llO), bcc(llO), etc. single crystal planes. The reduction of the unaffected adsorption sites with increasing modifier coverage forces the trapped precursor to pass into the less favoured adsorption sites which are influenced to a different extent by the modifier until the surface is eompletely deactivated at certain critical modifier coverages. For the same coadsorbate, the critical modifier coverages vary with the type

5.7. Conclusive Remarks

159

of modifier, the crystallographic plane and the adsorption temperature. Now consider the effect of the electronegative modifiers on dissociative adsorption, where the molecular adsorption state plays the role of a precursor. As outlined above, the bonding strength of the molecular state and its concentration on the surface are reduced. Obviously, when the molecular desorption energy (which reflects the strength of the molecular adsorption bond) falls far enough below the activation energy of dissociation, the dissociative adsorption will be hindered. In addition, it is quite possible that the presence of electronegative adatoms can also cause an increase of the activation energy for dissociation and will affect the binding energy of the products. Besides these energetic factors, a third steric blocking effect also contributes to the inhibition of the dissociative process because the dissociation products compete with the modifier atoms for the same adsorption sites and two adjacent favourable sites are required by the dissociation products. That is why the reduction of the dissociative adsorption rate induced by the electronegative additives is very severe. For example, the (1 - a&) factor for dissociative Hz adsorption on sulphided Ni(100) [13, 141 and Pt(ll1) [35] is estimated to be i.e. within the limits of low sulphur coverages of the order of (1 - (8 f l)O,) (see fig. 5.42.). A similar reduction factor is also found for 0 2 adsorption on P t ( l l 0 ) [21]. It is worth discussing the possible reasons for the observed differences in the poisoning strength with various elect#ronegativeaddit.ives. For this purpose we several simpler cases are selected:

(1) Additive adatoms with close sizes (covalent radii), localized in adsorption sites with the same symmetry. For such systems it is likely that the adatoms will exhibit approximately the same steric blocking effect (covering exclusively the nearest neighbours). Consequently, the observed variations in the strength of the poisoning effect should be ascribed to differences in the modifier induced perturbations on the next-nearest neighbours. As an example of this case,t,lie deactivation effects of C1, S and P which have similar covalent radii (0.99, 1.02, 1.04 A) but different Pauling electronegativities (3.0, 2.5 and 2.1) can be compared. The increasing strength of the poisoning effect with respect to adsorption of acceptor-type reagents in the sequence P, S, C1, can be satisfactorily explained by the increased ionicity of the substrate - modifier bonding, causing an enhanced net charge movement in the 2 direction. This extends the range of the perturbations due to both the enhanced substrate mediated electronic effects and the increased direct electrostatic interactions, as will be considered in more detail in Chapter 7.2. (2) Additive adatoms with close electronegativities (implying negligible differences in the nature of bonding) but substantially different sizes, occupying identical adsorption sites (e.g. C and S, S and Se). In this case the observed differences in the strength of the poisoning effects can be attributed to the fact that the smaller adatoms are located deeper (and even embedded) in the substrate surface and can even interact with the subsurface substrate atoms. This leads to weakening of both the steric blocking effect (because the effective radii of strong repulsion between

160

Chapter 5 the modifier and the coadsorbate decreases with the modifier’size) and the electronic perturbation effects on the next-nearest neighbours. Thus, in the case of sufficiently small, additive adatoms (e.g. C, 0) which are imbedded in the surface layer, even the substrate atoms directly bound to the modifier can be considered as possible adsorption sites.

The picture becomes more complicated when one should compare the poisoning effect of additive adatoms possessing substantially different electronegativities and sizes. On such an example, the effects induced by 0 and S, where the electronegativity factor (3.5 and 2.5) and the size factor ( r c : 0.77 and 1.02 A) act in opposite directions. The available experimental data have indicated that the poisoning effect on the adsorption process is always weaker in the case of 0 (e.g. CO adsorption on S/Ni(111) [28] and O / N i ( l l l ) [31], NO adsorption on S(Se)/ P t ( l l 1 ) [82] and O / P t ( l l l ) [82], etc.). This supposes that the strength of the modifier induced perturbations and the range of the effect on the adsorptive properties of the surface are determined to a larger extent by the size rather than the electronegativity of the modifier. Most of the current experimental data presented in this Chapter support the above considerations which account for a rather localized action of the modifier, usually not extending beyond the next-nearest neighbours. Indeed, let it be assumed that the introduction of a foreign adatom changes the distribution of the electrons of the whole surface. This should lead to continuous variations of the coadsorbate adsorption energies, stretching frequencies, etc. with increasing modifier concentration and immediate removal of the unaffected adsorption sites, which contradicts the experimental data. However, some authors ascribe the certain influence on the shape and position of the coadsorbate T P D peaks and the stretching frequencies of the coadsorbates residing far beyond the affected sites (low modifier coverages), to possible long range effects. In the author’s opinion, no unambiguous conclusion about the long range effects can be made because, in the case of non uniform distribution of the additive adatoms a t low coverages, tending to form islands of ordered structures, the following factors might influence the position of the T P D peaks and the stretching frequencies of the coadsorbates residing beyond the affected area:

(i) overlap of the desorption peaks of the coadsorbate desorbing from unaffected a r e a and areas located close to the boundaries of the modifier ordered islands. This explains satisfactorily the observed small shift to lower temperatures and the increase of the CO T P D halfwidths for low CO coverages at 6 s =- 0.1, illustrated in fig. 5.10.; (ii) at the same coadsorbate coverage there is a more pronounced compression of the coadsorbate overlayer on the modified surface (compared to that on a clean surface) because of the modifier induced reduction in the unaffected surface area. It is well known that both the positions of the T P D maxima and the stretching frequencies are very sensitive to the actual density of the adsorbate overlayer. Thus, the observed small decrease of the temperature maximum ( 5 25 K) and the increase of the stretching frequencies ( 5 15 cm-’) associated with the unaffected

References

161

adsorption states of the coadsorbates agree well with the above consider a t ions. Up t o this point the description of the effects of the modifier has been restricted t o systems where the additive adatoms tend to form ordered overlayers. The experimental data have shown that the picture becomes more complicated in cases of:

(i) modifier-induced surface reconstruction; (ii) surface compound formation, and (iii) lack of a tendency to ordering and occupation of definite adsorption sites. Thus, as has been described in Chapter 4.2., rather often, 0, N or C adsorption on some transition metal surfaces can cause a surface reconstruction or formation of surface or bulk compounds (oxides, nitrides or carbides). When true transition metal carbides, oxides or nitrides are formed, then the activity of the surface is determined by both the changes due to the new type of modifier substrate interactions in the compound (which alters completely the electronic structure) and the substantial structural changes. This implies a completely new type of adsorption sites and substantially altered adsorptive properties of the surface. It is obvious that i n such cases the behaviour of the surface changes drastically. It will acquire the properties of the new phase, which does not necessarily mean that the poisoning effect will he more severe than that observed when the modifier is in an adsorption state. The lack of a surface order and the tendency to island formation usually lead to a substantial reduction in the relative number of the perturbed surface sites, because they are restricted to the neigbours of the modifier island boundaries. This is the reason for the substantially weaker poisoning effect of P and C forming phosphide or graphite islands [13, 651. It is worth mentioning that in the case of C one should distinguish between the two possible surface states of C. The first state is ‘carbidic’ carbon where C is in a typical adsorption state forming ordered overlayers and residing in the highest coordinated sites. In this state, C is strongly coupled with the substrate atoms and acts in the way described above for ordered electronegative adatoms. In the second ‘graphitic’ surface state the coupling between the C adatoms and the surface is reduced a t the expense of rather strong attractive C-C interactions being allowed to arise; the latter would lead to the growth of a graphitic phase. That is why, when C tends to form islands of a graphite phase, they coexist on the surface with fractions of a free surface until the first graphite overlayer has completely formed and the poisoning effect is thus much weaker.

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9679; ibid.

4687 1791

S. Lehwald, J. T. Yates Jr. and H. Ibach, in Proc. IVC-8, ICSS-4 & ECOSS3, No 201 of Le Vide (I 980) 221 D. E. Peebles, E. L. Hardegree and J. M. White, Surface Sci. 148 (1984) 635 M. Kiskinova, G. Pirug and H. Bonzel, Surface Sci. 13G (1984) 285 A. Szabo, M . Kiskinova and J. T. Yates, 1988 (unpublished data) F. H. Netzer and T. E. Madey, Surface Sci. 110 (1987) 251 H. Conrad, R. Scala, W. Stenzel and R. Unwin, Surface Sci. 145 (1984) 1 G. Broden, T. N. Rhodin, S. Brucker, R. Benbow and Z. Hurych, Surface Sci. 59 (1976) 593 and references therein T. W. Root, L. D. Shmidt and G. Fisher, Surface Sci. 134 (1983) 30 R. J. Gorte and L. D. Shmidt, Surface Sci. 111 (1981) 260 T. W. Root, L. D. Shmidt and G. Fisher, Surface Sci. 150 (1985) 173 H. Conrad, G. Ert,l, J. Kuppers and E. E. Latta, Surface Sci. 50 (1975) 295 H. Conrad, G. Ertl, J. Kuppers and E. E. Latta, Surface Sci. 65 (1977) 235 E. G. Seebauer, A. C. F. Kong and L. D. Schmidt, Surface Sci. 176 (1986) 134; ibid. 193 (1988) 417 R. J. Gorte, L. D. Schmidt and J. L. Gland, Surface Sci. 109 (1981) 367 C. T. Cambell, G. Ertl and J. Segner, Surface Sci. 115 (1982) 309 H. D. Schmick and H.-W. Wassmuth, Surface Sci. 123 (1982) 471 S. Jorgensen, N . D. S. Canning and R. J . Madix, Surface Sci. 179 (1987) 322

L. D. Gland and E. Kollin, J . Cbem. Phys. 78 (1983) 983; Surface Scr. 171 (1985) 260

T.Engel anf G. Ertl, in: Fundamental Studies of Heterogeneous Catalysis, ed. D. A. King and P. D. Woodruff (Elsevier, NY 1987) vo1.4, p.73 M. E. Bartram, B. Koel and E. Carter, Surface Sci. 219 (1989) 467 M. E. Bartram, P. G . Winham and B. E. Koel, Langrnuir 4 (1988) 240 H. Hochst, E. Covalita and G. B. Fusher, J . Vac. Sci. Tecbnol. A 3 (1985) 1554

T. F. Fischer and S. R. Keleman, J . Catalysis 53 (1978) 24 E. L. Hardegree and J. M. White, Surface Sci. 175 (1986) 78 F. Bozso, J. Arias, C. P. Hanrahan, J . T. Yates, Jr., R. M. Martin and H. Matiu, Surface Sci. 141 (1984) 591 Y. Matsumoto, T. Onishi amd I<. Tamaru, J. Cbem. SOC. Far. Trans. I 76 (1980) 1116

U. Schwalke, J. E. Parmeter and W. H. Weinberg, J . Cbem. Pbys. 84 (1986) 4036

E. Platero, B. Fubini and A. Zecchina, Surface Sci. 179 (1987) 404

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C. T. Kao, C . M. Mate, G. S. Blankman, B. E. Bent, M. A. Van Hove and G. A. Somorjai, J. Vac. Sci. Technol. A 6 (1988) 786 and references therein J. R. Engstrom and W. H. Weinberg, J. Vac. Sci. Technol. A 5 (1987) 640 B. H. Hayden, K. Kreutschman and A. M. Bradshaw, Surface Sci. 125 (1983) 366

G. Ertl, Catal. Rev. Sci. Eng. 21 (1980) 2101; J. Vac. Sci. Technol. A1 (1983) 1247

K. Horn, J. D. DiNardo, W. Eberhardt, H. J. Freund and E. W. Plummer, Surface Sci. 118 (1982) 465 G. J. Schultz, Rev. Mod. Phys. 45 (1973) 423 M. Grunze, in: Chemical Physics of Solid Surfaces and Heterogeneous Catalysis, vo1.4 (1982) p.143 Y. Kuwahara, M. Jo, T. Tsuda, M. Onchi and M. Nichijima, Surface Sci. 180 (1987) 421 G. Haase and M. Asscher, Surface Sci. 191 (1987) 75 M. Grunze, R. K. Driscoll, G . N. Burlamd, J. S. L. Cornish and J. Prichard, Surface Sci. 89 (1979) 381 A. B. Anton, N. R. Avery, T. E. Madey and W. H. Weinberg, J . Chem. Phys. 85 (1986) 507 M. Baubaker and M. Trenary, J . Chem. Phys. 85 (1986) 6100 A. Schenk, M. Hock and J. Ktppers, Surface Sci. 217 (1989) 367 B. J. Bandy, N. D. S. Canning, P. Hollins and J. Pritchard, J. Chem. SOC. Chem. Commun. (1982) 58 J. Stohr and R. Jaeger, Phys. Rev. B 26 (1982) 4111 R. A. De Paola and F. M. Hoffmann, Chem. Phys. Lett. 128 (1986) 343 R. A. Shigeishi and D. A. King, Surface Sci. G2 (1977) 379 M. Asscher and G. Sornorjai, Surface Sci. 143 (1984) L389 L. J. Whitrnan, C. E. Bartosh, W Ho, G. Strasser and M . Grunze, Phys. Rev. Lett. 56 (1986) 1984 M.-C. Tsai, U . Seip, I. S. Bassignara, J . Ktppers and G. E d , Surface Sci. 155 (1985) 387 M. Grunze, G. Strasser and M. Grunze, Appl. Phys. A 44 (1987) 19 and references therein N.-J. Freund, B. Bartos, R. P. Messmer, M. Grunze, H. Hulhenbeck and M. Neuman, Surface Sci. 185 (1987) 187 Y. Fuknda and M. Nagashi, Surface Sci. 203 (1988) L653 C. A. Vancini, Synthesis of Ammonia, Macmillan London 1971 G. Ertl and M. Hnber, Z. Phys. Chem. N. F. 119 (1980) 97 K. Christmann, Surface Sci. Rep. 9 (1988) 1 K. D. Rendulic and A. Winkler, Surface Sci. 74 (1978) 318 C. M. Pradier, Y. Berthier and J. Oudar, Surface Sci. 130 (1983) 229 E. Protopopoff and P. Marcus, Surface Sci. 169 (1986) L237 P. Marcus and E. Protopopoff, Surface Sci. 161 (1985) 533 N. Abbass and R. Madix, A p p l . Surface Sci. 7 (1981) 241 M. L. Burke and R. J. Madix, Surface Sci. 237 (1990) 1 L.-G. P e t e r s o n , H. Dannetun, J. Fogelberg and I. Lundstrom, A p p l . Surface Sci. 27 (1986) 275 J . A. Schwarz, Surface Sci. 87 (1979) 525

Chapter 5

166

G. Erlich and K. Stolt, Ann. Rev. Phys. Chem. 31 (1980) 603 C.-H. Mak, B. G. Koehler, J. L. Brand and S. M. George, J . Chem. Phys. 87 (1987) 2340

J . L. Brand, A. A. Deckert and S. M. George, Surface Sci. 194 (1988) 457 M. A. Barteau, J . Q . Broughton and D. Menzel, Surface Sci. 133 (1983) 443 M. A. Barteau, J. Q. Broughton and D, Menzel, Appl. Surface Sci. 19 (1984) 92

L. Surnev, G. Bliznakov and M. Kiskinova, Proc. IV. Sympos. Het. Cat., Varna 1979, p.91

P. Thiel and T. Madey, Surface Sci. Rep. 7 (1987) 211 and references therein D. Doering and T. E. Madey, Surface Sci. 123 (1982) 305 K. Kretzschmar, J. K . Sass, A. M. Bradshaw and S. Holloway, Surface Sci. 115 (1982) 183

P. A. Thiel, F. M. Hoffmann and W. H. Weinberg, Phys. Rev. Lett. 49 (1982) 501

T. E. Madey and F. Netzer, Surface Sci. 117 (1982) 549 F. P. Netzer and T. E. Madey, Phys. Rev. Lett. 47 (1981) 928 K. Bange, D. E. Grider, T. E. Madey and J. I<. Sass, Surface Sci. 137 (1984) 38

E. M. Stuve, R. J. Madix and B. A. Sexton, Surface Sci. 111 (1981) 11 G. B. Ficher and B. A. Sexton, Phys. Rev. Lett. 44 (1980) 683 J. R. Greighton and J. M. White, Surface Sci. 136 (1984) 499 E. Stuve, S, N. Jorgensen and R. J. Madix, Surface Sci. 146 (1984) 179 C.Nyberg and P. Uvdal, J. Chem. Phys. 84 (1986) 4631 C.Benndorf, C. Nobl and T. E. Madey, Surface Sci. 138 (1984) 292 K. Bange, R. Dohl, D. E. Grider and J . I<. Sass, Vacuum 33 (1983) 757 J. K. Sass, K. Bange, R. Dohl, E. Piltz and R. Unwin, Ber. Busenges. Phys. Chem. 88 (1984) 354 K. Bange, T. E. Madey and J. K. Sass, Surface Sci. 162 (1985) 252 T. E. Madey, Science 234 (1986) 316 N. Sheppard, J. EJectr. Spectr. Rel. Phenom. 38 (1986) 175 M. A. Barteau and R. Madix, in: The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis, vo1.4. eds. D. P. Woodruff and D. A King (Elsevier, Amsterdam 1984) p.139 E. M. Stuve and R. J. Madix, Surface Sci. 160 (1985) 293; J. Phys. Chem. 89 (1985) 105 M. A . Barteau, J. Q. Broughton and D. Menzei, AppJ. Surface Sci. 19 (1984) 92

M. M. Hills, J. E. Parmeter and W. H. Weinberg, J . Am. Chem. SOC. 109 (1987) 4224

U. Seip, M.-C. Tsai, J. Ktppers and G. Ertl, Surface Sci. 147 (1984) 65 L.-G. Peterson, H. Dannetun, J. Fogelberg and I. Lundstrom, Appl. Surface Sci. 27 (1986) 275 H. Dannetun, I. Lindstrom, L.-G. Peterson, Surface Sci. 173 (1986) 148; ibid. 193 (1988) 109

P. Berlowitz, C. Mergiris, J . B. Butt and H. H. Kung, Langrnuir 1 (1985) 206

H. Steininger, H. Ibacli and S. Lehwald, Surface Sci. 117 (1982) 685

References

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R. L. Palmer, J . Vac. Sci. Technol. 12 (1975) 1403 C. Backx and C. P. M. de Groot, Surface Sci. 115 (1982) 382 T. Marinova and I<. Kostov, Surface Sci. 181 (1987) 573 C. Backx, C. P. M. de Groot and P. Biloen, Appl. Surface Sci. G (1980) 256 E. M. Stuve, R. J. Madix and C. R. Brundle, Surface Sci. 152/153 (1985) 532

A. M. Brownstein, Trends in Petroleum Technology (Petroleum Publishing Company, Tulsa, 1976) C. T. Campbell, J. Catalysis 94 (1985) 436 R. B. Brant and R. M. Lambert, J . Catalysis 92 (1985) 364 R. A. van Santen and C. P. M. de Groot, J . Catalysis 98 (1986) 530 E. A. Carter and W. A. Goddard, Surface Sci. 209 (1989) 243 R. A. van Santen and H. C. Kuipers, Advan. Catalysis 35 (1987) 265 X.E. Verykios, F. P. Stein and R. W. Coughlin, Catalysis Rev.-Sci. Eng. 22 (1980) 197 C. T . Campbell and B. Koel, J. Catalysis 02 (1985) 272 C. T. Campbell and M. T. Paffet, Appl. Surface Sci. 19 (1984) 28 G. Rovida, E. Pratesi and E. Ferroni, J. Catalysis 41 (1976) 140 D. G. Kelly, M. Salineron and G. Sornorjai, Surface Sci. 175 (1986) 465 J. B. Benziger, E. I. KO and R. J . Madix, J. Catalysis 64 (1980) 132 I<. Kikowatz, I<. Flad and G. Horz, J. Vac. Sci. Technol. A5 (1987) 1009 C. H. Bartholomew, P. I<. Agraval and J. I<. Katzer, Advan. Catalysis 31 (1982) 135 E. Zaera, E. B. Kollin and J . L. Gland, Surface Sci. 184 (1987) 75 J. Roberts and C. M. Friend, Surface Sci. 186 (1987) 201 W.H. Heise and B. J . Tatarchuk, Surface Sci. 207 (1989) 297 R. A. Cocco and B. J. Tatarchul, Surface Sci. 218 (1989) 127 B. A. Sexton, Surface Sci. 163 (1985) 99 J. Stohr, E. B. Kollin, D. A. Fischer, J. B. Hastings, F. Zaera and F. Sette, Phys. Rev. Lett. 55 (1985) 1468 G. R. Schoofs, R. E. Preston and J . B. Benziger, Langmuir l ( 1 9 8 5 ) 313 J . Stohr, J. L. Gland, E. B. Kollin, R. J . Koestner, A. C. Johnson, E. L. Muetterties and F. Sette, Phys. Rev. Lett. 53 (1984) 2161 J. F. Lang and R. I. Masel, Surface Sci. 183 (1987) 44 A. G. Gellman, M. H. Farias, M. Salmeron and G. A. Somorjai, Surface Sci. 136 (1984) 217 M. Salmeron, G. A. Somorjai, A. Wold, R. R. Chianelli and K. S. Liang, Chem. Phys. Lett. 90 (1982) 105 H. Heinemann and G. A. Somorjai, Eds., Catalysis and Surface Science (Dekker, NY, 1989) P. H. McBreen, W. Erley and H. Ibach, Surface Sci. 133 (1983) L469 J. B. Benziger and R. J . Madix, J. Catalysis 65 (1980) 36 S. L. Miles, S. L. Bernasek, J . L. Gland, J. Phys. Cliem. 87 (1983) 1626 S. R. Bare, J . A . Stroscio atid W. Ho, Surface Sci. 150 (1985) 399 I<. Christman and J . E. Dernutli, J. Chein. Phys. 76 (1982) 6318 E. I. KO, J . B. Benziger and R. J . Madix, J. Catalysis 62 (1980) 264 J. Hrbek, R. DePaola and F. M. Hoffmann, J. Chem. Phys. 81 (1984) 2818

168

Chapter 5 J. P. Lu, M. Alberts, S. L. Bernasek and D. S. Dwyer, Surface Sci. 218 (1989) 1

R. J. Madix, S. B. Lee and M. Thornburg, J. Vac. Sci. Technol. A1 (1983) 1254

J. Hrbek, R.A. DePaola and F.M. Hoffmann, Surface Sci. 166 (1986) 361 B. A. Sexton, Surface Sci. 102 (1981) 271 S. Akhter and J. M. White, Surface Sci. 167 (1986) 101 S. Anderson and M. Person, Phys. Rev. 24 (1981) 3659 F. Solymosi, T. J. Tarnezi and A. Berko, J . Phys. Chem. 88 (1984) 6170 S. W . Jorgensen and J. M. Madix, Surface Sci. 183 (1987) 27 J. L. Davis and M. A. Barteau, Surface Sci. 197 (1988) 123 A. A. Deckert, M. V. Arena, J. L. Brant and S. M. George, Surface Sci. 226 (1990) 42

M. Bowker and R. J. Madix, Surface Sci. 116 (1982) 549 S. M. Gates, J. N. Russell and J. T. Yates Jr., Surface Sci. 171 (1986) 111 N. R. Avery, Surface Sci. 125 (1983) 71 A. B. Anton, N. R. Avery, B. H. Toby and W. H. Weinberg, J . Am. Chem. SOC.108 (1986) 684 A. B. Anton, J. E. Parmeter and W. H. Weinberg, J. Am. Chem. SOC. 108 (1986) 1823

R. Madix, Catalysis Rev.-Eng. 151 (1977) 293

Chapter 6

ADSORPTION OF GASES ON SURFACES MODIFIED BY ALKALI METALS

In this Chapter the experimental data revealing the effects of alkali metal additives on the adsorption and reaction properties of single crystal metal surfaces are considered and discussed The main emphasis will be on the following aspects:

(i) molecular adsorption kinetics and energetics;

(ii) mutually-induced changes i n the electronic structure of the coadsorbates; (iii) alkali effect on the dissociation propensity; (iv) interactions in the mixed coadsorbate layer, and (v) correlations between the alkali additive adsorption state and the strength of modification effect. 6.1

6.1.1

CARBON MONOXIDE Alkali Effect on the Kinetics and Energetics of CO Molecular Adsorption

As outlined in Subsection 5.1.1., the CO initial sticking coefficient for molecular adsorption on transition metal surfaces is close to unity. In the presence of alkali additives, usually above certain alkali coverages, a decrease in the CO adsorption rate is detected [l-201. The alkali coverages, above which such a reduction in the CO initial sticking coefficient takes place, are in the moderate coverage range (when the minimum in the work function versus alkali coverage plots is observed). The alkali effect on the CO adsorption rate is illustrated in figs. 6.1. and 6.2. It is obvious that the CO initial sticking coefficient drops significantly at high alkali coverages. Alkali metal introduction affects in a similar way, as that illustrated in figs. 6.1. and 6.2., the CO molecular adsorption rate on the other transition

169

Chapter 6 .

170

metal surfaces, such as Ni [3, 41, Ru [5-8, 14-16], Pd [lo], Pt [ I, 11, 121. In the case of Cu, where the initial sticking coefficient is less than unity, the introduction of alkali additive causes an increase of So almost linearly with alkali coverage up to moderate alkali coverages [20]. A further increase of the alkali coverage causes the usual So reduction, as discussed above.

Pt 11111 * K

O r }

CO

T=300K

s 05 Q

Y4

OL

&

03

a w V

8 02 01

n

0

1

2

3

co

~ EXPOSURE

5

6

7

e

9

0

(io-'m S )

Fig. 6.1. CO uptake curves for clean and K covered Pt(ll1) at 300 K (from ref.[l])

This reduction of the CO molecular adsorption rate at high alkali coverages does not lead to concomitant reduction in the capacity of the surface for CO adsorption. The decrease of the CO saturation coverage a t high alkali coverages reported by many investigators is usually obtained on the basis of comparison of the CO coverage achieved after a CO exposure (- 10 L), which will equal that necessary for saturating a n alkali-free surface. This is not accurate because these authors do not allow the fact that, due to the decrease in the CO sticking coefficient at high alkali coverages, CO exposures must be increased additionally in order to achieve a saturation coverage. Indeed, as illustrated in fig 6.3., the adsorptive capacity of alkali modified P t ( l l l ) , remains the same in the presence of high K coverages, but for achieving the CO saturation at high K coverages on P t ( l l 1 ) CO exposures of the order of 300-500 Langmuirs were used [l]. Similar results where the adsorptive capacity of the surface was found to be hardly affected at all were obtained for alkali modified Fe(ll0) [2], Ru(1010) [14-161, etc. These results concerning the influence of alkali additives on the CO molecular adsorption rate and the capacity of the surface for molecular adsorption can be explained satisfactorily within the framework of the precursor adsorption model. It is probable that,up to moderate alkali coverages, the alkali is in an adsorption state that does not visibly affect the lifetime of the CO I

171

6.1. Carbon Monoxide

t b

0

8

16 CO exposure / L

0

02

04

ek

(a) CO uptake curves for clean and modified by various K coverages Pd(100) surfaces; (b) Relative sticking coefficient for CO adsorption as a function of K coverage on Pd(100). Dashed line presents the CO coverage achieved after CO exposure of 16 Langmuir at 100 K (from ref. [lo]) Fig. 6.2.

precursor. Thus, the initial sticking coefficient a n d the kinetics of CO adsorption are preserved up to moderate alkali coverages, as illustrated in figs. 6.1 a n d 6.2. A t high alkali coverages when, as discussed in Section 4.1., the adsorption s t a t e of alkali adspecies undergoes considerable changes and becomes metallic-like on completion of t h e first overlayer, the reduced CO adsorption r a t e can b e attributed to a decrease of the lifetime of the CO precursor on the metal-like overlayer. As will be shown later on, t h e CO sticking coefficient on thick alkali overlayers is extremely low. This alkali-induced kinetic effect is not necessarily accompanied by a concomitant reduction in the adsorptive capacity of the surface, because, as will be shown below, t h e presence of alkalis does not eliminate the CO adsorption sites; om t h e contrary, it creates new, more favourable, ones. T h e uptake curve shown in fig. 6.3 (b) indicates nucleation-controlled CO adsorption kinetics which means t h a t a space for CO adsorption is created by a reaction between CO and the thick I( overlayer. In can be assumed t h a t t h e initial CO adsorption takes place on defects of the compressed 0.44 K overlayer.

Chapter 6.

172

Pt 11111 K t M

T-mK

O0.12 l

Fig. 6.3. (a) Saturation CO coverage versus I< coverage on Pt(ll1) at 300 I< (from ref. [l])(b) Uptake curve for CO adsorption on 0.44 K / P t ( l l l ) at 300 I< (from ref. [32])

Most often the information about the effect of the alkali additive on the stability of the molecular CO adsorption state on the modified surface has been obtained from the CO thermal desorption data. This kind of information is based on the alkali-induced changes in the peak temperatures of the CO TPD spectra. Figs. 6.4.-6.6. show series of CO TPD spectra from alkali modified single crystal surfaces. Cases where no CO dissociation occurs in the presence of alkali additives are chosen. A common feature in the CO TPD spectra from

173

6.I . Carbon Monoxide

0.7 0.4

0.2 0.1

0.0 1

1

1

300 5 0 0

I

I

700

1

I

900

T(K) Fig. 6.4. CO TPD spectra from a clean (dashed line) and N a covered Ru(1010) surfaces for saturated CO coverages. Hatched TPD curves are obtained at low CO coverages and are used for est,imation of the initial heats of CO adsorption (from ref ~41)

alkali modified surfaces is the appearance of a new CO desorption state located at higher temperatures. The peak maxima and the relative CO amount desorbing from the alkali-induced states increase with increasing alkali coverage at the expense of a decrease in the amount of CO desorbing in the temperature range associat5d with unaffected surface. In the case of corrugated surfaces, such as Ru(1010) [14-161 or Ni(ll0) [17], the CO TPD peaks due to desorption from alkali metal-influenced sites and unaffected sites are more distinct. This is due to the fact that the corrugated surfaces are anisotropic and possess a smaller number of adjacent promoted sites and a restricted mobility in one of the crystallographic directions. An important finding is that for most of the studied transition metal surface studied, the maximum CO peak temperature of the molecular adsorption state which w a s reached at high alkali coverages, is in the range 640-680 I< [16]. Another important finding is that above these critical alkali coverages, when the CO TPD peaks reach

Chapter 6.

174

Fig. 6.5. CO I'PD spectra from a clean (dashed line) K and Cs covered Ru(lOi0) surfaces for saturated CO coverages. Hatched TPD curves are obtained at low CO coverages and are used for estimating the initial heats of CO adsorption (from ref. [15])

their maximum temperature value, coincident alkali metal and CO desorption is observed, the peak shapes and temperatures being almost independent of the transition metal substrate (see fig. 6.7.). In general the three alkali additives (Na, K, Cs) have the same effect on the CO T P D spectra, but the amount of CO molecules affected by one alkali adatom seems to increase in the sequence from N a to Cs (compare e.g. figs. 6.4. and 6.5.). The observed alteration of the CO peak temperature for CO adsorbed on the promoted surface sites, CO*, can be ascribed to alkali induced stabilisation of the CO molecular state on the promoted sites, which, as will be shown later, does not necessarily mean an increase in the substrate - CO bond strength. Neglecting the possible influence on the pre-exponential (frequency) factor of desorption, it was estimated that the initial heats of adsorption of CO', AH;,, frori the CO T P D spectra for very low CO coverages (- 0.05, see e.g. the hatched T P D spectra in fig. 6.4.). This is possible because the alkali modified adsorption sites are occupied first. Fig. 6.8. presents the dependence of AH;, on the alkali coverage on Ru( 1010) estimated assuming a first-order frequency factor of s-l [24]. Obviously, AH,! increases linearly with alkali coverage from 31 for a bare surface up t o 47 kcal/mol at high K and Cs coverages and up to 45 kcal/mol at high N a coverages. It is worth pointing out that the maximum value of AH:, is achieved at alkali coverages exceeding the vaIues which correspond to the A q 5 ( 8 ~ ~minimum ) (see e.g.

- -

-

6.1. Carbon Monoxide

-

175

0 0

h

0 05

(b)

I

8,

, 300

.

,

400

.

,

500

.

, 600

Temperature (K)

G.G. CO TPD spectra from: ( a ) clean and (b), ( c ) and ( d ) I< precovered P t ( l l 1 ) at 100 I<. T h e CO coverages (in ML) are (a) 0.03, 0.05, 0.1, 0.13, 0.16, 0.33, 0.45, and 0.55; (b) 0.03, 0.05, 0.08,0.12, 0.20, 0.28, 0.40, and 0.5; ( c ) 0.03, 0.05, 0.09, 0.13, 0.19, 0.26, 0.35, 0.45, and 0.5; (d) 0.03, 0.05, 0.09, 0.12, 0.21, 0.29, 0.39, and 0.50 (from ref. [23])

Fig.

fig. 4.8.). At these critical alkali coverage values, coincident alkali and CO desorption also appears which is often associated with an autocatalytic desorption process. Assuming that CO desorption is independent of frequency factor desorption, almost the same maximum increase of AH:, for the promoted molecular CO state (of the order of 12-14 kcal/mol) can be found for the other transition metal surfaces [9, 161 Recently, more accurate estimations of the possible changes in the frequency factor for CO desorption have shown that the higher desorption temperature of promoted CO which increases monotonically with increasing I< coverage on P t ( l l 1 ) is a result of the combined effect of an increase in activation energy of desorption, Ed, (equal to AH,!?,) and a decrease of the frequency factor, vd. Fig. 6.9. presents the concomitant changes in Ed and vd measured at low CO coverages on P t ( l l 1 ) modified by I<, the K cover-

176

Chapter 6.

Fig. 6.7. (a) CO T P D spectra for increasing CO coverages from Ru(1010) doped with OK = 0.66 Coincident I< desorption for a CO coverage of 0.6 is presented by the dashed T P D curve. (b) Retarding potentia1 curves (RPCs) after varying CO exposures on 0.66I
ages being below the critical one at which coincident GO and K desorption occurs. Comparing the data in figs. 6.8. and 6.9., it is obvious that the trend in the alkali induced changes of the GO desorption energy (which reflects a stabilization of the CO molecular state) is the same. However, the absolute values of AH;, in fig .6.8. estimated, excluding the possible effect on vd, should be reduced ( by a factor of 2.5) provided that the opposite (anticompensating) effect of alkali additives on the frequency factor is a general phenomenon for all alkali-CO coadsorbate systems. At the present state of knowledge there is no unambiguous explanation for this alkali induced reduction in the frequency factor. This result definitely contradicts the expectation of a stronger CO-substrate bond in the promoted sites, because usually v becomes lower when the molecule-surface bond weakens [25]. The possible explanations therefore are:

-

177

6.1. Carbon Monoxide

b.

I

-. Q,

0 50

I

1

CO/ Alkali / R u (1070)

E

\

0

1: 4 0

m No

v

0

O K

o v

I

a

A Cs 30

I

1

I

0.2

0.4

0.6

0.8 Alkali Coverage ( M L )

Fig. 6.8. Dependence of the initial heats of CO adsorption, A H g o , on the alkali coverage, as estimated from the CO T P D spectra a t very low CO coverages assuming invariable frequency factor (from refs. [14, 151)

9

-.-

r)

c-.

c c

0.00

0.05 OK

0.10

0.15

(ML)

Fig. 6.9. T h e average values of E and v when llco coverage on P t j l l l ) (from ref. [23])

< 0.1 M L

as a function of I<

Chapter 6

178

(i) stabilization of the CO molecular state as a result of direct alkali-CO interactions which weakens the coupling of CO with the surface and softens some CO vibrational modes, and/or

(ii) changes in v d as a result of the K induced perturbations of the surface phonon spectra. The CO T P D spectra as well the other experimental data (e.g. XPS, HREELS, ESDIAD, W F , etc) show that the promoted adsorption states are occupied first. The coexistence of separated promoted and unpromoted states up to moderate alkali coverages indicate that the effect of the alkalis is probably short-ranged. The saturation coverage of the perturbed CO* states increases monotonically with alkali coverage. Subsequent population of CO states with T D spectra close to the clean surface ones has been observed for all CO alkali coadsorbate systems up to moderate alkali coverages: K/Ni(111) [3, 211, K/PtQ11) [ l a , 231, I<, Na, Cs/Ni (100) [4], I
+

6.1.2

Alkali Effect on the Vibrational Properties of the Coadsorbed CO Molecules

HREELS and IRAS data give important. information about the alkali-induced changes of the Me-CO and C-0 stretching frequencies of promoted CO' . The observed stretching frequencies of promoted CO' can be used as a local probe of the alkali-induced perturbation of the Me-CO and intlramolecular C-0 bonds. Figs. 6.10-6.14. illust,rate HREELS data of various CO coverages on clean and a,lkali-modified single crystal surfaces. Similar to the data presented in figs. 6.10-6.13. are the GO vibrational data for the alkali-CO coadsorba.te layers on other alkali-modified transition metal surfaces, where no CO dissociation occurs, such as I
6.1. Carbon hlonoxide

179

Fig. 6.10. CO HKEELS spectra for Pt( 111) covered with OK = 0.09 with increasing CO coverage (from ref. [32])

(iii) the alkali effect on the C-0 stretching frequencies a t low a.iid moderate alkali coverages becomes wea.ker (i.e. the C-0' stretching frequency) increases with increasing CO coverage; (iv) the promoted sites are preferentially occupied a n d several kinds of affected CO' species (with different stretching frequencies) can coexist with almost unaffected GO at low and moderate alkali coverages; (v) the C-O* stretching frequencies become independent, of CO covera.ge in t h e alkali monolayer regioii where they reach t,lieir lowest value (see fig. 6.12.); (vi) the C-0 stretching frequencies associated with adsorption on unpromoted sites (which a.re removed at moderate alkali coverages) are also slightly affected, suffering a small downward shift (of the order of 1020 cm-')[21, 341, and

Chapter 6

180

I

'

'

mini) .K 1

I,!. 0

. . 5

i

'

-----

I

.co

I-

l3JK

I , .

m

..

1 . .

m

o

_ _ . 1 1 _ . .

M

o

m

a

ENtRGY LOSS l i d 1

Fig. 6.11. CO HREELS spectra for P t ( l l 1 ) covered with @K = 0.16 with increasing CO coverage at 130 K (from ref. [32])

-

(vii) the Me-CO' stretching frequencies of the promoted CO" molecules are lower (by 100 cm-') than that of CO bonded on a clean surface, and in return CO coadsorption also causes a reduction in the Me-alkali adatom stretching frequency [32]. The observed significant reduction of the C-0 stretching frequency of promoted CO* species indicates a weakening of the C-0 bond. The first attempts to explain this effect relate the weakening of the C-0 bond to alkali induced strengthening of the substrate - CO adsorption bond as a result of an enhanced metal & / C O 2a backdonation. However, such an explanation contradicts the observed alkali induced reduction in the Me-CO stretching frequency. Thus, it seems more plausible to attribute the alkali induced changes in CO" vibrational bands to direct short range interactions between CO* molecules and alkali adspecies in the mixed layer (which is also in agreement with the observed CO-induced reduction in the Me-alkali adatom stretching frequency). It appea.rs that the alkali-induced changes in the local CO adsorption site coordination (e.g. from on-top to bridge, as proposed in

181

6.I . Carbon Monoxide

Fig. 6.12. CO HREELS spect,ra for P t ( l l 1 ) covered with OK = 0.44 for saturation CO coverage at 300 I< (from ref. [32])

ref. [12]) are not the only reason for the drastic changes in the CO stretching frequency of CO' , that have been observed. As will be discussed in more detail below, the main effect is due to substantial short-range attractive interactions between the coadsorbed species. The CO coverage dependence of the CO' stretching frequencies has been ascribed to a interdependence of the strength of the interactions between the alkali adspecies and CO on the CO/alkali stoichiometry which, as has been established by many authors, changes at low and moderate alkali coverages [8, 14-17, 31, 32, 351. Here, it is worth pointing out that CO/alkali stoichiometry changes are more pronounced on flat substrate surfaces (e.g. from 1:2 to 3:l on K / P t ( l l l ) [32]) than on corrugated surfaces (e.g. on Ni(ll0) and Ru(1010) the C0:AM ratio does not exceed 2:l). The suggested dependence of the C-O* stretching frequency on the CO*/AM stoichiometry in the mixed surface overlayer agrees extremely well with the observed trend in the C-0 stretching frequency changes in matrix isolated alkali metal-CO complexes (AM,CO) [36-381. These IR studies have shown that the C-0 stretching frequency increases with decreasing ’x’. As will be discussed in the next subsections, the changes in the C-O* stretching frequency with changing the CO*:AM stoichiometry also correlate with the observed continuous changes in the LEED patterns reflecting the tendency to form patches of ordered structures consisting of CO alkali, as well as with the shapes of the retarding potential curves used for work function

-

+

182

Chapter 6

Fig. 6.13. Vibrational spectra of CO on K-covered Ru(0001) at constant GO coverage and various K coverages (from ref. [8])

measurements. Generally, the CO vibrational data are consistent with the TPD data concerning the sequential filling of the alkali promoted sites and unaffected sites in the course of CO adsorption. Closer inspection of the available vibrational data have shown that the behaviour of the coadsorbate system changes considerably at high alkali coverages. It has turned out that the critical alkali coverages beyond which the C-O* stretching frequencies reach their lowest value and become independent of CO coverage are the same at which CO and alkali coincident desorption is detected in the T P D spectra. This finding indicates that the type of the interactions between the coadsorbates in the mixed overlayer changes drastically when the adsorption state of alkali adspecies becomes nearly metallic. That is why most authors prefer to treat the behaviour of the mixed CO-alkali coadsorbate systems at low and moderate coverages and at near monolayer region separately. As a critical point the alkali coverages, corresponding t o the minima of the Ad vs AM plots will be used further on.

6.1. Carbori Monoxide

183

Fig. 6.14. Relative loss intensities for the different C-0 stretching frequency bands on K-precovered Pt(ll1): (B) 1390-1420 cm-l; (0) 1510-1610 crn-'; (.) 16401730 cm-', @) 1800-1820 cm-l; (&) 2100 cm-'. The last two bands correspond to unpromoted bridge and on top CO (from ref [32])

Finally, it should be mentioned t h a t in t h e case of alkali doped noble ( C u [18, 191) a n d s p metals (Al(100)) [43]), t h e energy positions of the C0' stretching frequencies show negligible shifts with increasing CO coverage irrespective of t h e alkali coverage. As will be discussed in more detail later, this can be ascribed to t h e differences in t h e adsorption s t a t e of alkali adspecies a n d CO o n these surfaces.

6.1.3

Surface Order in Mixed Overlayers

As has been described in Chapter 4 . 1 . , a t room temperature alkali metals d o not form ordered structures in the submonolayer region on flat single crystal surfaces and alkali adspecies are supposed to be uniformly distributed up t o nearly monolayer coverages. On such alkali modified surfaces, e.g.

Chapter 6

184

K/Ru(0001) [5] Iioncauses reordering and the formation of new structures. A sequence of rather complex LEED patterns are found to appear sequentially with increasing CO coverage. Additional rearrangements in the mixed overlayers leading to new LEED patterns are observed upon annealing the mixed layer at temperatures up t o 500 I< [lo].

-

+

Table 6.1. LEED Patterns Observed for CO I< Mixed Layers on P t ( l l l ) , Pd(100) and N i ( l l 0 ) a t Different Temperatures, T (data taken from refs. [5, 10, 15, 17, 321)

SUBSTRATE

eI< (ML) eco

(ML)

Pt(ll1)

0.12 0.12 0.12 0.12 0.12 0.2-0.25 0.2-0.25 0.12 0.12 0.12 0.2-0.25 0.37 0.5 0.1 0.1 0.1 0.2 0.2 0.33 0.34 0.66 0.66

0

(fix

0.05 0.09

(6 x &)

Pd(100)

Ni(ll0)

Ru( 000 1)

Ru( l o l o )

0.23 0.32 0

< 0.1

---

0 0.5 0.4 N 0.4 0.35 0.1 0 0.2 0.7 0 0.25

-

-

0.34 0 > 0.5

LEED pat.tern

fi)R19.1"

R30°-diffuse mixed (2 x 2) ( 2 x 2) ( 2 6 x 2 6 ) R30"

+6

( 2 x 2)

(?A x 2fi)

R30"

-

(a x a) R arctan 2/3 (q y)R45" x

P(2 x 2) P(3 x 2) (fix &) R45" (1 x 3) ( 2 x 3) c ( 2 x 6) (1 x 2) c(2 x 4) x A) R30"

(a

(3 x 3) incommensurate (1 x 3)

T

(I0

110 110 110 110 110 110 110 250 250 500 500 500 500

300 300 300 300

300 130 130 300 300

Table 6.1. presents selected data for LEED patterns observed at various GO and alkali covera.ges and obtained at different temperatures for several coadsorption CO AM systems. Because of the restricted mobility of the adspecies across the troughs on corrugated single crystal metal surfaces, weak and diffuse fractional extra spots have indicated a certain degree of order at room temperature even at moderate alkali coverages. For these surfaces the introduction of CO leads to the appearance of new ordered structures.

+

6.1. Carbon Monoxide

185

A series of LEED patterns observed upon saturation of a Ru( 1070) surface precovered with different amounts of K or Cs are presented in fig. 6.15. They indicate that CO coadsorption causes a n appearance of new extra spots in both 1210 and 0001 crystallographic directions, which implies reordering of the overlayer along and across the troughs. Generally, the CO coadsorption causes removal of the alkali extra spots accompanied by the appearance of new spots.

Fig. 6.15. Schematics of the observed LEED patterns for K and Cs adsorption on Ru( 1070) and the reordering induced by subsequent saturation of the alkali modified surface with CO: (0) Ru spots, (0) I( or Cs induced extra spots, (4 extra spots induced aftep?CO coadsorption (from ref. [15])

The explanation of these rather complicated LEED patterns for mixed CO

+ alkali overlayers by real space models is difficult for the following reasons:

(i) it is not clear whether CO, alkali, or complexes containing hot,li species act as scattering centers; (ii) it is quite possible that islands with various CO*:alkali stoichiometries and also patches of unaffected CO can coexist on the surface, each possessing a specific order. However, LEED data indicate that the CO coadsorption reduces the thermal mobility of the alkali adspecies and buffers the repulsive electrostatic interaction between the alkali adspecies. This leads t o the formation of ordered patches of varying composition determined by the relative local coverages of the coadsorbates.Comparison of the structural results for mixed CO alkali overlayers with the observed CO coverage dependence of the CO' stretching frequency (see fig. 6.14.) and the evolution of the CO T P D spectra from alkali-modified surfaces (e.g. figs. 6.4.)

+

Chapter 6

186

shows that the CO coadsorption leads not only to the formation of islands of a mixed CO,AM, composition but also causes contraction of the alkali layer due to attractive interactions with GO'. Thus, after saturating the possible adsorption sites in the mixed patches, the contraction of the alkali adspecies in these patches leads to the formation of clean surface patches. Since the unaffected sites are the least favourable ones from energetic point of view, they are occupied last and behave very much like' that of CO adsorbed on an alkali-free surface. For example, an approximate estimation has shown that for O.lK/Ni(llO), where K is uniformly distributed on the surface, the COinduced contraction of K adspecies within the mixed ordered islands leaves 56 % of the surface unaffected [17]. The same trend is observed with the other CO alkali systems. The GO TPD spectra in j g . 6.4. that were observed spectra from unperturbed areas even for Ru( 1010) precovered with ON^ = 0.7 (close to completion of the first N a overlayer at 300 K), are in support of the contraction effect due to C0'-Na attractive interactions. Even at monolayer alkali coverages, CO coadsorption destroys the order of the alkali adspecies. This is in general agreement with the view that the CO' molecules are attached t o the substrate surface which is only possible if a considerable rearrangement and contraction of the alkali adlayers takes place. Studies of the temperature effect on the actual order of the mixed patches and the C-0 stretching frequency of CO adsorbed on unaffected surface patches have shown that at higher adsorption temperatures the average size of the mixed contracted islands is smaller because of the increased mobility of the coadsorbates. Hence, the CO molecules in the 'unperturbed' patches are, on the average, closer to the alkali adspecies at higher temperatures, which results in a slightly lower C-0 stretching frequency as compared to the C-0 stretching frequency measured at low temperatures. In the case of low alkali coverages the latter is almmt the same as that measured for GO adsorbed on a clean surface [17, 321.

-

+

6.1.4 Alkali Effect on the Electronic Structure of the Coadsorbed CO Molecules Information about the electronic properties of the coadsorbed species can be gained by measuring the work function changes, the binding energies of the valence and core electrons of the coadsorbates, the energy positions of the unoccupied CO states, etc. A. Work function measurements. Fairly common features in the CO induced work function changes are observed for all currently studied systems, regardless of the behaviour of the Aq5 vs. Oco plots for CO adsorption on a clean surface [l,5, 8-10, 14-16,27, 30, 31, 39-41]. The main features are best illustrated in figs. 6.16. and 6.17. They can be summarized as follows; (i) at low and moderate alkali coverages (up to those corresponding to the minima in the Aq5 vs. OAM plots), CO coadsorption always causes an increase of the work function; (ii) at higher alkali coverages (above those corresponding to the Aq5 vs. OAM minima), the behaviour of the work function plots becomes more

6.1. Carbon Monoxide

187

complex; (iii) the work function remains almost invariant or decreases slightly with the first GO doses and then increases with rising GO coverage. The striking similarity in the work function data that was observed for COalkali coadsorbate systems where GO preserves its molecular state, indicates the same type of the alkali-GO interactions irrespective of the nature of the transition metal. More detailed examination of the A$ vs. BCO plots in figs. 6.16. and 6.17. shows that in the alkali coverage region before the A$ vs. OAM minima, the absolute increase of the work function induced by CO coadsorption is proportional to the initial alkali coverage. In addition it should be pointed out that the major work function increase is observed with the initial GO dose, i.e. when CO occupies the alkali promoted sites. This means the the promoted GO* species should possess larger dipole moments. Thus for GO* on 0.09 K / P t ( l l l ) , an initial dipole moment of 1.8 D has been determined from the initial slope of the A$ vs. Oco plot in fig. 6.16.

0

1

2

LO EXPOSURE IlO-'Pa 5 3 L 5 6 7 8

)

9

10

=-.,,-.-.--T

11

0

I

eK=

LfO

*

002

/

PI (111) + K T = #)(IK

20

LO 60 80 CO EXWSURE (10.' Pa s \

100

Fig. 6.16. Work function changes versus CO exposure for clean and K-covered P t ( l l 1 ) at 300 K measured from the onset of the electron emission of He I UPS spectra (from ref. [I])

As outlined in Subsection 2.2.2., apart from the evaluation of the surface electrostatic potential changes, the shape of the retarding potential curves might also give information on the uniformity of the overlayer [42]. The evolution of the retarding potential curves (RPCs) presented in fig. 6.18. shows

Chapter 6

188

- 0.53 -3 1

0

.

1

.

I

0.2 OL! 0.6

.

,

0.8

Fig. 6.17. Work function changes versus Bco for clean and Na covered Ru(1010) at 300 I< measured from the onset of the retarding potential curves (RPCs). The arrows indicate the CO coverages above which a shoulder can be distinguished in t h e RPCs (from ref. [14])

that initially, the RPCs monotonically shift to higher voltages (indicating an increase of the work function) with negligible changes in their shapes. A t CO coverages which are close to that at which the work function (measured from the onset of the RPCs) saturates, the shape of the RPCs undergoes rather complex changes leading to the appearance of a well-distinguished shoulder. This shoulder separates the RPCs to low and high voltage sections and indicates the Coexistence of pat,ches of different work functions on the surface. Comparison with the CO T P D spectra corresponding to each RPC reveals that the work function changes measured from the onset of the RPCs, reflect the changes in the surface potential induced by CO adsorption on the alkalipromoted sites. After saturation of these sites, the work function changes induced by further CO adsorption on ‘unaffected’ areas are reflected by the appearance of a high voltage Section in the RPCs. The CO induced work function changes in the unperturbed surface patches are found to be similar t o that measured during CO adsorption on an alkali-free Ru(1010) surface [14, 15,421. This result confirms the LEED and vibrational spectroscopy data (discussed in the previous sections) suggesting coexistence of rather large ordered

6.1. Carbon Monoxide

189

patches of the contracted CO,AM, phase and alkali ‘free’ surface patches. The W F results have shown that these islands are large enough (> 0.08 pm) t o allow individual contribution to the retarding potential curves.

eco

Fig. 6.18. (a) CO TPD spectra from Ru(10i0) covered with OK = 0.35. The dashed line shows CO TPD curve for bare Ru(10i0) saturated with C O . (b) RPCs for increasing CO coverages on 0.35 K/Ru(lOlO). The insert presents Ad vs. Oco plot as measured from the onset of the RPCs (from ref. [15])

B. Changes in the core and valence electron spectra of CO and alkali coadsorbates-Thephotoelectron spectra have shown significant changes in the valence and core levels of the coadsorbed CO and alkali species in the mixed overlayers. XPS spectra of the C 1s and 0 1s regions of the CO molecules adsorbed on alkali promoted sites show a decrease by 1-2 eV in the 0 1s and C 1s binding energies [l,2, 5, 9, 16, 28, 41, 441. Figs. 6.19. and 6.20., show by way of example, 0 1s and C 1s spectra of CO on clean and K-covered P t ( l l 1 ) [l].Fig. 6.19(a) illustrates the sequential occupation of on top (0 1s = 532.6 eV) and bridge (0 1s = 531.0 eV) sites on a clean Pt(ll1) surface. Fig. 6.19(b) shows that at low K coverages they are preferentially occupied GO’ promoted sites characterized by a 0 1s binding energy of 531.3 eV followed by occupation of unaffected on top

-

Chapter 6

190

. l S . l O ' I% I M

111111111111 5

3

5

Y

5

.

m

Fig. 6.19. X-ray photoemission spectra of the 0 Is level for CO adsorption on clean (a) and K-covered (b) & ( L ) F t ( l l 1 ) . T, = 160 I< (from ref. [l]) sites at higher CO exposures. Obviously, as has been found by HREELS

-z W

* ex= 038

CllSl

1;

....

@"SO

'.\

?' :.

..

,

. . . ............ . . . .- ,F. . . . .'.. .. . . , . : .C

' - . I

I.

,

Fig. 6.20. X-ray photoemission spectra of the K(2p) and C(1s) levels for saturation CO coverage on clean and I<-covered P t ( l l 1 ) surface. T, = 300 I< (from ref. [l])

6.1. Carbon Monoxide

191

and T P D data,the promoted and unpromoted CO molecules can coexist up t o moderate alkali coverages. Figs. 6.19.(c) and 6.20. illustrate that above medium K coverages only promoted CO' characterized by 0 1s = 531.3 eV and C 1s = 285.8 eV can be distinguished in the XPS spectra. Table 6.2. presents some selected data of CO 0 1s and C 1s binding energies measured for CO adsorbed on clean and K modified surfaces. The observed reduction in the binding energies of CO 0 1s and C 1s levels of the CO' molecules residing in the promoted sites as compared with CO adsorbed on alkali-free surfaces is consistent with almost any model involving enhanced charge transfer to the promoted CO' (no matter whether it is a result of direct alkali-CO coupling or through substrate interactions). Along with the same trend in the alkali-induced changes of the CO 0 Is and C 1s binding energies for K promoted Cu(lOO), the introduction of alkali additive on Cu( 100) additionally reduces the intensity of the 0 1s and C 1s satellites by 20-25 % [44]. The existence of rather intense satellite features in the XPS spectra of the CO/Cu adsorption system is usually ascribed to the substantially smaller amount of metal d, to CO 27r level backdonation for CO bound to copper which leads to a rather weak bonding compared to CO bonding on typical transition metals. The weak coupling with copper, results in poorer screening via the CO 27r orbital which accounts for the appearance of satellites to the main lines of the core and valence CO regions. The reduction in the intensities of these satellites in the presence of K is interpreted in terms of an enhanced occupation of the CO 27r orbital and arising CO l?r-K 4s interactions [44, 451. As a result of these interactions in the mixed CO-K overlayer on Cu(100) the stability (as judged from the CO desorption energy [l8]) of the promoted CO' increases to the level of CO adsorbed on transition metal surfaces.

-

Table 6.2. CO 0 1s and C 1 s Binding Energies (in eV) of CO Adsorbed on Clean and K-covered Single Crystal Surfaces SURFACE Pt( 11 1)

Ru(0001)

Ni( 100) Fe(ll0) Cu(100)

OK

(ML)

0 0-0.15 > 0.15 0 0-0.2 > 0.2 0 > 0.2 0 0.3 0 0.3

0 1s

C 1s

Ref.

532.6 (on top) 532.6 & 531.3 531.3 532.2 (on top) 532.2 & 530.9 530.9 531.2 530.3 531.9 531.1 533 531

286.9 285.8 285.3 284.4 285.8 285.0 286.5 284.5

[l]

--

N

[5]

[41] [2] [44]

Chapter 6

192

The UPS data have shown that the presence of alkali additives also causes changes in the valence spectra of the promoted molecules [l, 2, 5, 12, 18, 39, 41, 44-52]. For most systems under consideration (e.g. K / P t ( l l l ) [l], K/Fe(llO) [2], K/Ru(0001) [46], K / R h ( l l l ) [47], K/Rh(100) [50], and K/Cu( 100) [44, 48,49]), the introduction of alkali additives is found to cause the following changes in the binding energies of the CO 4a,50 and l a IeveIs: (i) the 4 a and 5a-derived orbitals remain essentially unaffected or move slightly to higher binding energies, and (ii) the l7r-derived orbital experiences a visible shift to lower binding energies (of the order of 1 eV).

A clear picture of these changes is illustrated in fig. 6.21., obtained by ARUPS. The ARUPS spectra in fig. 6.21. indicate that the promoted CO* molecules preserve a straight bonding configuration (slight tilting has been supposed on the basis of the observed slight reduction in symmetry of the 50 and l x orbitals [46]). The CO* l x emission is shift,ed by 1.2 eV, while t8heother orbitals remain relatively unperturbed. This substantial shift of tlie 1x level implies that this orbital is involved i n a direct interaction with I<. This is reasonable because the l w orbital extends laterally from tlie CO molecule (whereas the a orbitals are localized at the ends of the CO molecules). This enables l 7 r to overlap with I< 4a orbitals. The negligible perturbations in the binding energies of the other CO induced levels indicate that there is no strong electrostatic effect. In the case of Cu(100) the introduction of K also leads to the removal of the satellite structure of the CO 4a-derived level due to the enhanced screening via the CO 27r level [44, 491. High resolution ARUPS data [49] have shown that the CO I n level splits in the presence of K on Cu( 100) due t o a direct bonding or/and to electrostatic interactions. Few systems (K/Ni( 111) and I
-

N

-

6.1. Carbon Monoxide

193

increases. This has been interpreted as an enhancement of the occupancy of the 27rb state [3].

-K

-lo

-5

0

E - E, leVl

Fig. 6.21. Angle resolved photoemission spectra of CO adsorbed on clean (a, b) and K-covered Ru(OOO1) (& = 0.33) surfaces. The insert presents a spectrum taken

for a clean Ru(0001) and a I<-covered Ru(0001) (from ref. [46]) The coadsorption of CO is found to affect the alkali np core levels causing a decrease in the binding energy (of the order of 0.3-04 eV) and an increase in the peak intensity up to 2 times [l,5, 30,41,44-471. The CO induced shift in the alkali np binding energies is in a direction opposite to that predicted by the simple charge transfer model on assuming a charge flow from alkali to CO which should result in an increase of the alkali np binding energies. This result has not been explained yet because both the possible initial state (due to direct CO-alkali interactions) and final state (extra-atomic relaxation effects) can contribute t o the observed alkali np binding energy shifts. It is more probable that the enhanced intensity of the alkali np peaks is due t o the removal of photoemission loss channels associated with one-electron or plasmon excitations of delocalized alkali ns valence states (or bands). It is worth pointing out that a similar slight binding energy reduction and intensification of the alkali core level lines have also been observed in the presence of other

Chapter 6

194

acceptor type coadsorbates, such as 0 2 , HzO, NO, etc. CO coadsorption is found to remove the K M23MM Auger transition resulting from the decay of the K 3 p hole (probably due to a crossed Auger process between the strongly coupled K and CO coadsorbates.) [46]. Another CO-induced effect is the observed changes in the K-LMM Auger line shape, illustrated in fig. 6.22. As shown in ref. [53], the changes in the h / H ratio (where h and H are the heights of the lower energy and the high energy sides of the K-LMM Auger line in the first derivative mode) can be used as an indication of the effective charge associated with K . This h / H ratio is found to be 0.45 for K in the ionic state. As can be seen in fig. 6.22., the h / H value monotonically decreases with increasing I< coverage due to the depolarization effects in the overlayer, described in Chapter 4.1. The coadsorption of CO always causes an increase of h / H , this indicating depletion of the electronic charge from the K adspecies.

f

K-L MM in 50

0.34 -

0.261

o

I

I

1

1

a2

O.L

0.6

0.8

@K

Fig. 6.22. Changes in the h / H ratio of the K-KLL Auger line as a result of increasing K coverage on Ru(lOi0); (x) CO coadsorption on I<-covered Ru(1010) for Bco = 0.6 (from ref. [15])

Important information on the mutually-induced changes in the core and valence electron levels of the coadsorbed CO and alkali species is also obtained from the electron energy loss data. fig. 6.23. presents the CO induced changes in the EEL spectra for K covered Ru(1010) surfaces. The main CO induced

6.1. Carbon Monoxide

195

changes in the K loss features can be summarized as follows: (i) a shift by energies;

-

0.5 eV of the K 3s core electron excitations to higher

(ii) removal of the K 4s one electron valence excitations existing at low and moderate I< coverages, and (iii) removal of the I< plasmon excitations characteristic for near to inonolayer K coverages.

'

K

I

3s

K3p

V

30

20

10

L O S S EhiERGYIeVI

Fig. 6.23. EEL spectra of I<-covered (1) and CO

+ K-covered (2) Ru(1010) surfaces

(from ref. [15])

Similar effects are observed with CO coadsorption on the Cs and N a core, valence and plasmon excitations [14-161. The major influence of the alkali additives on the CO loss features concerns the peak at 6.3 eV associated with electron transitions from the bonding 2nb to the anti bonding 2na state, whereas the peak at 14.5 eV, associated with transitions from the CO 5u level is unperturbed (fig. 6.23.). As can he seen in figs. 6.23 and 6.24., the energy

196

Chapter 6

-

of the co 2'ifb + 2n, transitions increases by 0.7 eV. This increase of the 27rb - 2 ~ splitting , accompanied by a depletion of the alkali valence ns-states indicates participation of the CO 2n* and alkali ns-states in extra coupling.

h(

lu

2 2 r\r

?

Fig. 6.24. EEL spectra of the low energy region for CO adsorbed on a clean and Cs-covered Ru( 101 0) surfaces (from ref. [15])

The influence of alkali additives on the empty anti-bonding 2ira energy level resulting from the formation of the CO adsorption bond has been measured by inverse photoemission (IPS) for CO adsorption on K promoted P t ( l l 1 ) [54]. The results have shown that the 2a, levels of promoted CO* are located a t lower energies (closer to the Fermi level) compared with CO on a clean surface where 2ira lies at 4.2 eV above the Fermi level. The CO* 2ira energy position is found to depend on the K covemge becoming more strongly shifted a t higher K coverages. Fig. 6.25. presents the IPS spectra for increasing CO exposures on K-covered P t ( l l 1 ) . Here, the CO* 2na related feature appears at 2.2 eV above the Fermi level. With increasing CO coverage, a second feature a t 4 eV develops which is associated with the unpromoted sites being filled up.

-

-

-

6.1. Carbon Monoxide

197

Pt (111) + K + CO

o IS

8,.

0

2 (E -E,I

L

6

8

lev

Fig. 6.25. IPE spectra of CO adsorbed on K-covered P t ( l l 1 ) for increasing CO coverages. = 0.15. T h e large peak riglit above the Fermi level is due t o empty Pt d-band states (from ref. [54])

Within the framework of the siinple bonding model the increased coupling of CO" in the presence of alkali should be expected to raise the absolute energy values of both the CO" Zx, and 2xb with respect to the Fernii level which is contrary t o the IPS data. This explicit discrepancy has been explained taking into account the drastic alkali induced modification of the surface potential barrier which pushes the spatially extended CO 2x’ orbitals closer to the Fermi level, thus providing a more effective overlap with appropriate surface or alkali electronic states located below the Fermi level. Obviously, the downward electrostatic shift might overcompensate for the alkali-induced enhancement of the 27fb - 27fasplitting action.

198

6.1.5

Chapter 6

Alkali Effect on the CO Bonding Orientation and the C-0 Bond Length

A couple of techniques have been used in order to establish whether the orientation of the promoted CO* species differs from that of the unpromoted ones. Some authors have proposed as a possible explanation of their ESDIAD [6] and MQS [59] data, the existence of flat lying or strongly tilted CO* species, but recently the existence of such species on alkali modified flat surfaces has been ruled out. It has been shown that the alkali additives have a profound effect on the excitation processes involved in the electron-stimulated desorption, causing complete conversion to ESD inactive promoted CO' species. Consequently, in the case of alkali-modified surfaces, the ESDIAD method can neither give information on the orientation of the promoted molecule on the surface [6, 22, 56-58]. ARUPS [26] and angle-resolved Auger lineshape [62, 631 measurements of CO adsorption on K/Ru(0001), SEXAFS measurements for CO adsorption on K/Cu(lOO) [48], K / N i ( l l l ) [51] and N a / P t ( l l l ) [64], and XPD measurements of CO adsorption on I < / P t ( l l l ) [GO, 811 have indicated that CO remains vertically oriented on alkali modified surfaces. Another important piece of information that becomes clear from the SEXAFS data is that the a resonance of the promoted CO* species is shifted by 4 eV towards the absorption threshold as compared with the a-resonance of unpromoted CO, while the energy position of the a-resonance remains almost unchanged. The enormous energy shift of the Is + r* absorption peak indicates an alkali induced lengthening of the C-O* bond distance by 0.12 [63, 641. Alkali-induced tilting of the promoted CO* molecules has been detected by means of XPD on I<-covered Ni(llO), where at 300 K , CO molecules are bonded normally to the surface plane on clean Ni(ll0) [60, 611. Fig. 6.26. presents normalized C Is XPD scans for CO adsorbed on M-covered P t ( l l 1 ) and Ni(ll0). It is quite obvious that the presence of I<-induces tilting of the CO* molecules only in the case of K/Ni(110) (tilting angle 32'). The tilted molecules are found to coexist with normally bound ones. Compared t o the normally bound species they are thermally more stable, i.e. the tilted CO molecules are the most strongly affected species. The reason for this Kinduced tilting of CO* on Ni( 110) has not been clarified yet. Two explanations are proposed:

-

-

A

-

(i) electrostatic interactions of the O-end of CO' residing on top the rows and the K6+ species in the troughs, and (ii) CO* adsorbed on the microfacet,s formed as a result of a K induced missing-row reconstruction [BO]. The current data about the structure of CO coadsorbed with alkali additives cannot give a straightforward description of the structure of the C0'alkali complex, because, as revealed by the vibrational data, this structure changes with the composition of the mixed overlayer. Thorough studies on the possible contribution of the alkali-induced reconstruction to the orientation of the CO molecule and the local stoichiometry of the CO-alkali complexes are lacking.

199

6. I. Carbon Alono.de

I

a

1.41 &=Y ,

10

-80

10

0

-40

80

polar anile 8 ( d e i )

" ' , T=300K

0.45

0.34

4

31.4 '

e

4

.d

.e

.s:1.0 4

- . ...b..

.

*'-.

0

g0.a. E

I

-80

.

-

.

'

"

.

.

0 LO -40 polar anglr 8 (dcg)

no

Fig. 6.26. (a) Normalized XPD C 1s scans of CO on K-covered P t ( l l 1 ) . (b) C 1s XPD s c a m in both emission azimuths for CO* coadsorbed with I< on Ni(l1O). The extra amount of CO is desorbed by briefly heating to 485 I{ (from ref. [GO])

Chapter 6

200

6.1.6

Conclusive Remarks about the Behaviour of Mixed AlkaliCO overlayers

A. Alkali modified transition metal surfaces. First mixed CO-alkali overlayers on transition metal surfaces (Ni, Ru, Rh, Pd, Pt) are considered. These exhibit almost the same behaviour with respect to CO and alkali adsorption alone, e.g. the same character and strength of the CO (AM) adsorption bonds and the same changes in the CO (AM) adsorption state with increasing adsorbate coverage. For these systems, the interactions in mixed overlayers, in absence of alkali induced CO dissociation, are expected to have the same effects. I t should be remembered that, within the limits of very low coverages, the Me-CO adsorption bond is much weaker than the Me-AM bonds (130-150 kJ/mol for CO compared to 240, 290 and 320 kJ/mol for N a , K and Cs, respectively). With increasing CO coverage, the Me-CO adsorption bond decreases by 50 kJ/mol at most, whereas the coverage induced reduction of the Me-AM bond strength can exceed 200 kJ/mole in the case of Cs. This substantial reduction in stability of the alkali adsorption state has been ascribed to a coverage governed change of the alkali adsorption state from a strongly polarized one at low coverages to a metal-like at high alkali coverage. In general, the alkali coverages beyond which the alkali overlayer starts to exhibit a metal-like character are those corresponding to the minima in the Aq5 vs. BAM plots. Further on the symbol B,,(AM) will be used for the critical coverages beyond which the layer becomes metal-like. The major changes in the alkali adsorption states can be associated with an increase in relative electron density of the ns-valence levels of the alkali adspecies going t o higher alkali coverages. The most important experimental results will be summarised thereby, emphasizing the common features observed for different substrates which are relevant to understanding the type of the CO-AM interactions in the mixed overlayer:

-

-

(1) Up to moderate alkali coverages promoted CO' species can coexist together with unpromoted ones, the relative coverage and the adsorption energy of the former increasing linearly with alkali coverage. The number of the promoted CO* molecules depends on the CO coverage, reaching three CO' and two CO* per alkah adatom for flat and corrugated substrate surfaces, respectively Up to moderate alkali coverages when the strength of the alkali-induced perturbations of the CO* adsorption state depends on the CO*:AM ratio, CO always desorbs before the alkali metal does.

-

-

(2) At alkali coverages exceeding B,,(AM), the CO' properties become independent of the CO coverage, and the CO*:AM ratio is 1. At these high alkali coverages coincident CO' and AM desorption is observed in the range of GGO-G80 K for the transition metals under consideration. The similarity in the CO and AM TPD peak shapes and their unusually small half widths are indicative of a n autocatalytic desorption process (decomposition of an AM-CO surface complex). It should be stressed that within the high alkali coverage range, CO causes stabilization of the alkali adsorption state as well. This is reflected by the observed high

6.1. Carbon Monoxide

201

temperature shift of the onset of alkali desorption

(3) The C-0' stretching frequency dramatically decreases with increasing alkali coverage, falling sometimes below 1400 cm-' at high alkali coverages, which indicates a substantial weakening of the C-0 bond.

(4) At alkali coverages below B,,(AM) the CO* adsorption always causes an increase in the work function proportional to the initial alkali coverage, whereas at high alkali coverages the CO* induced WF changes become weaker and more complex.

(5) The CO coadsorption results in contraction and reordering within the overlayer, which leads to the formation of patches of various CO/AM compositions and orders and unperturbed surface.

(6) The major perturbations i n the electronic structure of the promoted CO' molecules are: (i) a reduction in the CO 1~ binding energy and 0 1s and C 1s core level binding energies; (ii) delocalization of the 4u oxygen lone pair; (iii) a downward shift of bot,h the 2 ~ and b 2a, electronic levels accompanied by an increase in the 2Sb - 28, splitting due to the larger shift of the 2 i r b level; (iv) an increase of the C-0 bonding length indicating weakening of the C-0 bond.

(7) The interaction of CO with alkali adspecies leads to changes in the alkali adsorption ('charging') state, as confirmed by the following results. (i) removal of the AM losses due to the ns-valence one-electron transitions (at B(AM) < B,,(AM)), and removal of the collective ns electron (plasmon) excitations (at B(AM) > B,,(AM)), which indicates CO induced reduction in the alkali ns-valence state occupation; (ii) energy and intensity changes of the alkali np core levels, and (iii) removal of the alkali induced surface resonance state close to the CO l~ level from the energy point of view.

B. Alkali modified noble and sp- metals. For metal substrates, such as Cu, All Ag and Au, where the CO adsorption bond is rather weak (due to the lack of or a negligible d / 2 backhonding), ~ the introduction of an alkali metal does not always promote CO adsorption. It should be stressed that on these noble and sp -metal substrates, the adsorption state of alkalis is also characterized by a weaker bonding compared to the alkali bonding on the typical transition metal surfaces. A promotion effect reflected by the enhanced CO adsorption rate and substantial stabilization of the CO adsorption state is observed for K-covered Cu surfaces where the behaviour of the mixed overlayers is similar to that

202

Chapter 6

observed for the transition metal surfaces described above. There are certain deviations in behaviour in the observed weaker CO coverage dependence on the C-0' stretching frequency and the lower temperature (- 450-500 K) at which the coincident CO and I< (Cs) T P D peaks appear. Obviously, these deviations in the behaviour of the CO + K(Cs)/Cu systems should be associated with the difference in the adsorptive properties of Cu with respect to CO and K, compared to those of the typical transition metals under consideration. It is probable that the weaker coupling of the coadsorbates with the substrate interactions. surface would enhance the contribution of the C0'-AM For Al(100) [43], the alkali additives (Na or K) do not only create favourable adsorption sites and cause an increase of the CO sticking coefficient but go as far as promoting dissociation of CO. However, the CO-I<, Na/A1(100) system does not, behave i n the same way as the one mentioned above. There are several major differences which will be summarized below. The energy positions of the CO' induced bands are insensitive to the alkali and CO coverage. This coverage independence of the CO' vibrational bands can be related t o the differences not only in the electronic structure of the sp-metals, but also in the weaker AM-A1 interactions and the tendency to clustering of the alkali adspecies even at low coverages. Thus, it is probable that the stabilization of CO on alkali modified A1 is exclusively by CO-alkali coupling. Another interesting observation is the appearance of a CO' adsorption state at OK > 0.2, characterized by an extremely low stretching frequency (1060 cm-l), which cannot be explained by a normally bound CO molecule. As will be discussed in the next Section, this CO' (1060 cm-') molecular state readily dissociates at T < 200 I<. Actually, it should be stressed that all promoted CO* molecules undergo dissociation on alkali-modified Al( 100) and no CO molecular desorption is detected. In the case of Au and Ag [GG, G7], no CO promoted adsorption is observed in the presence of an alkali additive in the submonolayer alkali coverage region. This result indicates that the adsorption state of alkalis might be quite different on these substrates so that no favourable adsorption sites for CO can be created. C. CO Adsorption on thick alkali films grown on metal surfaces. Recent studies of the reactivity of I< films developed on A1 and Fe substrates, (film thickness of more than 50 A), have shown that CO can adsorb with a very low sticking coefficient (of at, 100 I< [MI. The adsorbed CO molecules are characterized by a relatively low C-0 stretching frequency (1750 cm-') and undergo dissociation upon annealing to 250 I<. These results confirm the suggestion about the existence of pure alkali-CO interactiom leading to substantial weakening of the CO intramolecular bond strength. The differences in the dissociation propensity of the CO molecules directly interacting with alkali adspecies in the mixed overlayer indicate that the reactivity properties of the adsorbed alkali additives are affected by the substrate, as will be discussed in detail in the next Section. The results presented in the previous sections and summarized above clearly indicate that the alkali-induced effects are rather complicated. Their explanation is not limited t o the framework of the simple Blyholder model as .alkali induced enhancement of backdonation to the CO 27r' orbital. It is quite

-

203

6.1. Carbon Monoxide

obvious that the type of interactions and the strength of the alkali effect depend substantially on the adsorption state of the alkali additive which varies with alkali coverage. This is not surprising because, in the case where shortrange coadsorbate-alkali interactions are of major importance, the strength of the promotion effect will depend on the relative occupancy of the alkali valence electronic states involved in these interactions. 6.1.7

Alkali Effect on the CO Dissociative Adsorption

The probability of CO dissociation on transition metal surfaces has already been considered in Section 5.1.1. As illustrated in fig. 5.2. , on most of the transition metals under consideration the dissociation of the GO molecule is kinetically hindered by a substantial activation barrier. In addition, for P t and P d also, it is thermodynamically unfavourable. Obviously, in order t o promote GO dissociation on the surface, the action of the alkali additive should lead to: (i) stabilization of the GO molecular adsorption state which plays a role of a precursor for dissociation, and

(ii) a decrease of the activation barrier for dissociation.

I

--

-1

Fig. G.27. Effect of various amounts of Na on the GO TPD spectra from N i ( 1 0 0 ) following adsorption at 300 I<. CO exposure 10 L (from ref. [4])

In the previous sections it has been illustrated tha.t indeed the alkali modifiers create promoted sites where the GO' molecular state is stabilized and the C0' bond strength is weakened, which should facilitate the CO dissociation.

Chapter 6

204

An overview of the current data has shown that, besides the same changes in the molecular adsorption state, the promotion of CO dissociation is substrate-specific. In particular, only for the following single crystal surfaces where 0 does not dissociate on a clean surface, evidence of alkali-induced CO* dissociation is reported: N a , K, Cs/Ni(100) [4, 31,741, Na/Rh(100) [50], and K , Na/A1(100) [43]. As is illustrated in fig. 6.27., for alkali-modified Ni(100) [4, 31,731 a second high temperature CO desorption peak appears at 800 K above certain alkali coverages. Comparison with the CO T P D data from transition metal surfaces where CO dissociation occurs (e.g. Fe [28, 29, 691) shows that a /3-CO T P D 800 K. In peak associated with C and 0 recombination also appears at addition, in the same 800 K temperature range a small CO2 T P D peak also desorbs from alkali modified Ni( 100) and traces of C contaminants remain on the surface after complete CO and alkali desorption. For CO on Na/Rh( 100) [50], a promotion of fractional CO dissociation is evident from the appearance of a 0 2 p related 5.5 eV peak in the UPS spectra when the N a coverage exceeds d,,.(AM) which corresponds to the minimum in the A 4 vs.d,,(AM) plots. More interesting is the case of albli-induced CO dissociation on Al( 100) [43] where on clean AI(100) the weakly bound CO desorbs at 125 K and the CO adsorption sticking coefficient is even at temperatures as low as 80 K. As discussed in the previous Section, the adsorptive capacity of the Al( 100) surface increases with increasing alkali coverage and all coadsorbed CO molecules undergo dissociation. Fig. 6.28. presents the sequence in CO' dissociation on alkali-promoted Al( 100). The range 1060-2060 cm-l represents the molecular CO state and the range 540-900 cm-l corresponds to CO dissociation products: A1 - 0, A1 - C and A1 oxide bands. The CO' molecules characterized by the band at 1060 cm-' decompose at 190 I<, the dissociation products creating a shoulder at 700 cm-l. A further increase of the temperature up to 390 I< leads to decomposition of the second type of CO' species accompanied by the creation of distinct bands due to the formation of A1203 and A14C3 phases. This facile CO dissociation on an alkali-covered Al( 100) surface supports the viewpoint that the direct alkaliCO coupling contributes at most to the alkali promotion action because in the case of the spmetals the CO coupling with the substrate is very weak. However, the extremely high promotion of CO dissociation on alkali-modified A1 can be explained by the fact that the heats of oxide and carbide formation on A1 greatly exceed those of the other substrates. This makes the dissociation thermodynamically very favourable. In addition, complete CO' dissociation is supposed to be also facilitated by the existing significant I<-C interactions. The latter stabilize the dissociation product C and prevent the formation of the inactive graphite phase 011 the surface. No CO dissociation is detected on alkali-promoted P t ( l l l ) , Ru(0001), Pd(100), N i ( l l l ) , Cu(100) and C u ( l l O ) , although all experimental data indicate the same alkali induced weakening of the C-0 bond. The CO isotope exchange observed at higher alkali coverages on K/Ni(111) [3], K/Rh (111) [47, 761, K/Ru(0001) [7,8] cannot be used as proof for alkali promoted CO dissociation. As has been proposed in refs. [3, 7, 8 47, 781 a non-dissociative

-

-

-

-

-

-

-

6.1. Carbon Monoxide

r

1

1

1

1

1

'

205

1

A1(100)IK(115K, no ann.)lCO(M)K."150L")

'

Ma'

KlSllAIU- 1Y

-+

Electron Energy Lor. (em - '1

Fig. 6.28. Changes in the vibrational band of GO and dissociation products as a result of of a mixed CO + I< layer on Al(100) to different temperatures (from ref. [43])

mechanism of isotope mixing due to an intermolecular exchange reaction between the promoted CO" molecules involved in the alkali-C0 surface complexes is most likely. This is confirmed by the fact that all the other T P D , vibrational and spectroscopic data do not give any evidence of CO induced dissociation on alkali modified Ni( l l l ) , Ru(0001) and Rh( 111) surfaces. Since the experimental data have shown that the alkali-induced weakenmolecular bond does not vary substantially with different ing of the C-0' transition metal surfaces, obviously the substantial differences with respect to promotion of CO dissociation should be associated exclusively with the stability of the dissociation products on various substrates. Indeed, a s has been discussed in ref. [77], the activation barrier for dissociation of a chemisorbed molecule is determined mainly by the heats of chemisorption of the atomic constituents. A certain contribution to this stability might be expected in the presence of alkali adatoms because the latter can form rather stable com-

Chapter 6

206

200

400

600

600

1000

Temperslure (K)

Fig. 6.29. CO TPD spectra from Fe(100) with increasing K coverages for the same CO exposure (from ref. [70])

pounds with oxygen. In the case of substrates where CO dissociation is thermodynamically and kinetically favourable on clean surfaces, such as Fe [2, 28, 29, 69, 701, Ta [71], Co [72], stepped Ni [74], etc. the presence of alkalis enhances the fraction of dissociated CO. Fig. 6.29. presents the effect of increasing amounts of K on the CO T P D spectra from Fe(100). A similar effect on the CO T P D spectra has been observed for K-covered Fe(ll1) [29]. The P-peak in the CO T P D spectra from Fe surfaces, located at 800 I<, reflects the relative amount of dissociated CO, whereas the highest temperature a3 peak is assumed to be the molecular precursor for dissociation. It has been established that the fraction of dissociated CO (P-peak) increases with increasing alkali coverage in a linear fashion at the expense of the gradual elimination of the weakly bound a1 and a2 molecular adsorption states. According to the XPS data [28], the number of dissociated CO molecules increases more rapidly than the number of I< adspecies, i.e. 1 K seems t o promote the dissociation of more than one CO molecule. Close inspection of the T P D data in fig. 6.29. shows that even at a high I< coverage no high temperature CO desorption peak associated with a stabilized CO' molecular state appears. On the contrary, the a3 state shifts to even lower temperatures. The absence of a high temperature CO" T P D state and the decrease of the desorption temperature of the a3-feature indicate that the activation energy barrier for CO dissociation is lowered. On the basis of the experimental data it has been established that the reduction in the activation energy for dissociation depends on alkali coverage. The lowest value of the dissociation barrier (96 kJ/mol compared to 122 kJ/mol for I<-free

-

6. I. Carbon Monoxide

207

surface) is achieved at alkali coverages around the A&',,(AM) minimum. K/Fe systems [28, 29, 69, TO] is the An interesting finding for CO observed simultaneous desorption of I< together with the recombination CO P-peak and the mutually-induced chemical shifts and changes in the shapes of the K 2p doublet and the 0 1s core peak associated with atomic oxygen produced by CO dissociation. These data are explained by the formation of a K,O surface complex (1 < 2 < a ) , which reacts with the surface carbon at temperatures 800 I<. This reaction leads to liberation of CO and I< in the gas phase leaving the surface free from C and 0 contaminants. Recently, the small amount of C 0 2 desorption at 590 K detected at high I< precoverages has been associated with CO' disproportionation leading to CO2 desorption and C deposit [4].

+

-

Fig. 6.30. CO TPD spectra from a Ni[6(111) x (IIO)] surface precovered with different amounts of I< (from ref. 1751)

A more complicated picture has been observed for CO adsorption on a I< promoted kinked Ni [G( 111) x (1lo)] surface. As can be seen from the T P D spectra in fig. G.30., the presence of I< leads to the typical strongly-stabilized CO* molecular state located at 660 I< a t high I< coverages and also enhances the fraction of dissociated CO molecules. It is obvious that the hehaviour of the I< modified stepped Ni surface can be regarded as a combination of the adsorptive properties of alkali modified surfaces (where CO dissociation is not favoured) and the reactive properties of surfaces where CO can dissociate even in the absence of an alkali promoter. A possible explanation is that only those CO molecules which are coadsorbed on I< promoted sites in the near vicinity of the kinks are more likely to dissociate, whereas the other molecules, adsorbed on promoted sites a t the terraces behave in the same

-

208

Chapter 6

way as CO on K/Ni(111) where CO dissociation is not promoted. Indeed, a close examination of the experimental data shows that the main increase in the number of dissociated molecules is observed at low alkali coverages when K adspecies adsorb preferentially at the kinks. At higher K coverages when the kink sites are saturated and K adsorbs on the terraces, the amount of dissociated CO levels off and the typical behaviour of promoted molecular CO adsorption is observed. The variations in the alkali effect on the CO dissociation propensity on the different substrate surfaces indicate that the reactivity of the actual substrate metal surface with respect to CO dissociation is of major importance. The reason for that can be well understood if one assumes that, in the case of the typical transition metal surfaces the presence of alkali adspecies would induce almost the same measure of stabilization of the CO molecular and dissociation product states and the resulting reduction in the activation barrier for dissociation. Evidently, as can be judged from the one-dimensional energy diagram for CO dissociation on various metals shown in fig. 5.2., it seems unlikely that the alkali induced increase in the CO adsorption energy (which does not exceed 60 kJ/mol [20]) and decrease of the activation barrier (of the order of 30 kJ/mol [70]) would be sufficient to favour CO dissociation on surfaces, such as Cu, P t, Pd, Ru, etc. Here, it should be pointed out that, in the case of Al, it is likely that the creation of molecular adsorption states (which are lacking on a clean surface) and can act as a precursor for dissociation, should be considered as the major alkali effect. There are a number of theoretical approaches where the authors attempt to explain the observations in the CO-alkali coadsorbate overlayers considering various possible interactions, such as direct electrostatic interaction, indirect (through metal) charge transfer, direct CO-AM coupling (involving appropriate electronic states of the coadsorbates) leading to ‘covalent’ or ‘ionic’ type bonding, etc. More detailed presentation of the current theoretical models will be given in Section 7.2.

-

6.2

CARBON DIOXIDE

COz plays a certain role in complex reaction systems such as Fischer - Tropsch synthesis where CO disproportionation (2CO -+ CO2+C) or the water-gas shift (CO +HzO+ CO2 Hz) are possible side reaction paths. In addition, the water-gas shift reaction is also an important catalytic process by itself. In these syntheses alkali additives are often used for changing the activity and selectivity of the catalytic reaction [80-841. Recently, a number of model adsorption studies have been conducted on the describing of the alkali effect on CO? adsorption and dissociation on alkali modified surfaces of Cu [20], Fe [79], Rh [85, 861, Pd [87-911, A1 [92] and Ag [93]. In some of these studies it has been attempted to shed more light on the possible types of interactions in the CO alkali adlayers which occur at high alkali coverages [20, 791 by comparison with the behaviour in mixed CO? alkali adlayers. Excepting Fe, CO? adsorption on the other metal surfaces considered above is rather weak and no COZ molecular or dissociative adsorption takes place at room temperatures f94-971. The highest molecular adsorption energy

+

+

+

6.2. Carbon Dioxide

209

(- 60 kJ/mol is reported for COz o n Rh(ll1) [85, 961. The most probable bonding configuration of this weakly-bound molecular COz state is supposed t o occur via the oxygen lone pair [98], although the most recent ARUPS studies of COZ adsorption on Ni(ll0) [99] and Fe(ll1) [loo] does not support vertical bonding configuration. As judged by the ARUPS spectra for CO:! adsorption on Fe(111) and N i ( l l 0 ) measured in the temperature range 85-300 I<, two CO2 adsorption states can be distinguished on the surface at 85 K . The main fraction consists of species with an undisturbed electronic structure, i.e. they exhibit the same molecular valence levels as gas phase COz or thick COz films. These undisturbed COz molecules are extremely weakly bound and lie flat on the surface. They desorb at temperatures below 140 I<. The electronic structure of the second adsorption state exhibits features which can be associated with COi- species. This state is described as a bent anionic COY, which serves as a precursor for further dissociation. The changes observed in the valence spectra with increasing temperature indicate that the C0;- species undergo the following decomposition on a Fe( 111) surface: CO,(g)

’3’C02(linear)+ CO,(bent)

-

180-3OOK

> ’>OK

COY (bent)

-

c o + o 300-39OIZ c - t o s o

In the case of COz adsorption on N i ( l l O ) , the ultimate step (CO dissociation) does not take place.

6.2.1

Alkali Effect on the CO2 Adsorption Rate, the Adsorptive Capacity of the Surface and the Stability of COZ Adspecies

The presence of alkali additives causes a significant influence on the surface adsorptive properties with respect to C 0 2 adsorption. This influence of alkalis can be summarized as follows: (i) an increase of the CO? sticking coefficient to unity which is reached at moderate alkali coverages;

(ii) an enhancement of the capacity of the surface for COz adsorption proportionally to the initial alkali coverage both at the expense of the weakly-bound species and the promoted ones; (iii) creation of new adsorption sites, where CO, is more strongly bound; (iv) an increase in reactivity for dissociation, and (v) conditions coming about which enable secondary reactions in the overlayer leading to the formation of oxalate-like and carbonate-like species. The latter two effects will be discussed in more detail in the next Subsection because each system exhibits its own reactivity peculiarities It should be pointed out that, because of the drastically enhanced reactivity of the modified surface with respect to CO, dissociation, it is almost impossible to cousider

Chapter 6

210

+

pure COz alkali overlayes, especially at higher alkali coverages and elevated temperatures. On K-covered Ag, HREELS and Raman spectra taken upon COz adsorption at 50 K show strong evidence of the formation of a binary surface compound of the type MetCO, [93]. The COT adspecies are characterized by 760 and 1260 and 1600 cm-' bending, symmetric and asymmetric stretching modes. They appear after the initial COz dose on I<-modified Ag. The intensities of the COT modes are proportional to the K coverage. With increasing GO2 coverage, the CO, related modes become obscure because of 670, 1320 and 2360 cm-' vibrational modes typical of physisorbed COz arising. A similar behaviour is also observed i n the presence of Cs and Li. The formation of binary surface compounds on alkali-modified Ag is found to compete with CO2 dissociation to CO and 0, which does not occur on alkali-free surfaces ~901. On K-covered Pd(100), the GO2 stability on t,he surface is reported to be dependent on alkali coverage, the CO? T P D peak of the promoted P-CO? state shifting gradually from 185 I< for 0.05 K/Pd(100) to 556 K for 0.4 K/Pd(100). No COz dissociation is observed up to moderate I< coverages (2 0.2) on 0.3, a new higher temperature COz Pd(100). At K coverages exceeding T P D peak at 672 K is detected which, as will be discussed later, is associated with alkali-induced COz dissociation [87, 881. Quite clear evidence about the nature of the stable alkali induced CO2 adsorption state on Pd is obtained from the ARUPS and HREELS data concerning CO:! adsorption on Na-covered P d ( l l 1 ) [90,91]. It should be pointed out that a difference in reactivityof this. system has been found whether alkali coverages are low or high. At low and moderate Na coverages at 90 I<, two kinds of species, namely physisorbed COZ and chemisorbed GO:- are identified by the vibrational spectra, similarly to the case of alkali modified Ag [93]. The COi- associated vibrational modes are at 282 cm-' (Me-CO2 stretch), 744 cm-' (bending mode) and 1210 cm-' (symmetric stretch). Upon mild annealing t o 120 I<, the physisorbed COz partly desorbs and partly transforms into CO;-. Further heating causes dissociation of C0;- to CO and 0 (7' > 135 I<), and only the GO stretching frequency (at 1530 cm-' at moderate N a coverages) remains at T > 190 I<. Taking into account that GO2 does not adsorb on Pd at 90 I<, the authors explain this Na induced enhancement of the surface adsorptive ability as being a result of the creation of promoted sites in the vicinity of the Pd - Na6+ dipoles where opposite COi- - Pd dipoles are formed. The bent anionic CO4- species serve as a precursor of dissociation as observed on N i ( l l 0 ) and F e(ll1) surfaces [99, 1001. The picture is more complicated in the case of high N a coverages when several losses appear in the 850-1800 cm-' region and secondary reactions occur readily, as will be discussed in the next Subsection. A similar effect of stabilization of a molecular adsorption state on the promoted sites coexisting with the unpromoted ones at low I< coverages has been reported for I<-covered Rh(l l1) [85]. In this system, the T P D spectra associated with the promoted state shift slightly (by only 30 K) to higher temperatures within the I< coverage region 0-0.2. Again, high temperature

-

-

-

-

6.2. Carboii Dioxide

211

desorption peaks, associated with CO2 dissociation and secondary reactions, develop a t moderate (0.14 ML) I< and high (2 0.36 ML) K coverages: at 500 and 720 K , respectively. In the case of Cs-modified C u ( l l 0 ) [20], Cs induced dissociation and CO desorption proceed below 200 I<, so that the C02 TYD peaks which appear at much higher temperatures (- 550, 620, 685), cannot be associated with an alkali-promoted molecular state. Since no C02 adsorption occurs on Cu( 110) at 110 K, a certain concentration of weakly bound COZ associated with the presence of Cs is evident from the C I s and 0 Is spectra upon high C02 doses at 110 I<. These weakly-bound species desorb at 130 I<.

-

-

Table 6.3. Peak temperatures (I<) of the COz, CO and alkali desorptioii states detected at different alkali coverages, HAM ( i n ML)

SURFACE

B r r n ~ ~DESORPTION STATES

coz

(TPD PEAK MAXIMA) GO AM

Cs/Cu( 110)

0

-

-

T, = 110 I< ref. [20]

< 0.2

550 550, 620 & 685 500 k 620-700 doublet 0-135 (Y & P-185 P-303 p & y-670 p-556 & 7-670 (Y-244-170 PI-31 3-342 PI Pz-505 P 1 , P z 8~ 7-714

210 210 210

0.20-,035 2 0.4

K/Pd(100) T, = 100 I<

N

0 0.05 ref.[87] 0.21 0.26 0.42 K/Rh(lll) 0 T a = 100 I< 0.03-0.19 or 300 I< > 0.14 ref. [85] > 0.36 0.01-0.46 > 0.18 K/Fe(100) 0 < 200 100 K 0.4 590 ref. [20] > 0.25' 590' Na/Al mu1tilayer 150 & 540 100 I< ref. [68] * - this concerns CO adsorption on I
2

-

doublet -

GOO & 684 -

-

250-900 > 700 680, > 700 500 & 620-700 250-1000 > 700 -

-

690 250-1100 > 740 > 740 720 & >

546-704 -

590 350-450' 470 & 540

720 & > 250-1100 590 590,sl 410, 470 & 540

Table 6.3. summarizes selected characteristic T P D data on CO, CO2 and alkali desorption from several alkali modified transition metal surfaces. The data in Table 6.3. show that the adsorptive and reactivity properties of alkali modified surfaces vary with the different substrates. As will be discussed in the

Chapter 6

212

next Subsection, this should be attributed to the influence of the substrates, which, as briefly summarized above, exhibit different reactivities with respect to the interaction with COz. In addition, the presence of alkalis will affect, to different degrees, the stability of the various species formed a t the surface as a result of the surface reaction. This might lead to different reaction paths on the different modified surfaces. However, it is evident that the reactivity enhances at high alkali coverages (usually beyond the values corresponding to the minimum in the A4 vs BAM curves when the alkali overlayer becomes metal-like). 6.2.2

Alkali Induced Dissociation of COz and Secondary Reactions in the Mixed Overlayer

As a basis for understanding the complex behaviour of mixed overlayers formed after COz adsorption on alkali modified metal surfaces, the observed interactions of bulk alkali layers with C 0 2 ca be used. HREELS data for COa adsorption on a thick N a overlayer deposited on Al(100) have shown that, depending on the surface temperature, the following reaction paths are possible [68]. At 100 I(, the vibrational spectra give evidence of the coexistence of

-

(i) a weakly bound molecular state characterized by 650 cm-' and 2350 cm-' bands (very close to the CO2 gas phase ones) which desorbs at 130 K and

-

(ii) a second state characterized by 920 cm-l, 1350 cm-' and 1650 cm-I bands which is stable up to 300 K .

-

These bands are the same as reported in ref. [98] for oxalate ions in alkali metal salts. Annealing of this alkali oxalate layer at 360 K leads to dramatic changes in the vibrational spectra where the oxalate bands are replaced by the 1100 cm-' and 1450 cn1-l bands, associated with the formation of carbonate GO, species. Further annealing to 420 I< leads to partial N a desorption, accompanied by decomposition of the carbonate species and the build-up of A1 oxide. As a result of the complex decomposition process, CO, C02 and 0 are released. As can be seen in Table 6.3., they desorb simultaneously with the remaining N a at 470 K and 540 K, respectively. Since the decomposition of the carbonate species occurs at temperatures at which the Na multilayer has been already desorbed, the influence of the substrate on this reaction step cannot be neglected, although the coincident Na, CO and CO2 desorption indicates that the species are mutually stabilized on the surface. However, for temperatures below 400 I<, the reaction steps of COz with the bulk alkali metal can be described as follows:

coz 1 2 K COz(weak)

Na, CO,

0

The oxalate species are likely to be formed as a result of C02 dimerisation or via the reaction AMzCOz COz + X2C204. Finally, annealing above 600 K leads merely to A1 carbide and oxide because, as described in the previous Section, CO readily dissociates on Al( 100) in the presence of alkalis PI.

+

6.2. Carbon Dioxide

213

Matrix-isolated studies of alkali metal-COz interactions [lo21 have shown that, depending on the relative concentration of the reactants, radical anion COT, dianion CO;-, radical oxalate CzO,, oxalate C20:- and carbonate Cog- species can be detected. The formation of oxalates is found t o be favoured in the case of N a and Li, whereas, in the case of Cs and K , carbonates are favoured. Oxalates decompose to metal carbonate and CO at elevated temperatures (- 300 K). The latter finding agrees extremely well with the above results on COZ interactions with thick N a films on A1(100), where the formation of carbonate species from oxalates is detected at 300 K. For the other COz-alkali modified metal systems, common features like that observed for thick alkali films are detected a t high alkali coverages if the alkali layers exhibit a metal-like character. For the COz-Cs/Cu(llO) system [20], the CO desorption peak appears at temperatures as low as 200 K and increases in intensity up to moderate Cs coverages. The relative amount of the released CO decreases drastically at high Cs coverage (> 0.3 ML), which is accompanied by the appearance of a high temperature shoulder in the CO T P D spectra. This Cs coverage effect indicates changes in the reaction path for CO production. Some of the following reactions can take place on the modified surface:

-

(i) dissociation of bent COi- species to CO and 0, similarly to the process observed for the COz-Na/Pd(lll) [89, 901, COz-alkali/Ag [93], COz/Ni( 110) [99] and COz/Fe( 111) [loo] systems; (ii) disproportionation of COa(a) to C03(u) and CO where C03(a) is stabilized as a surface Cs.zCO3 complex, or (iii) decomposition of a surface oxdate analogous to Cs.zCO3(a) and CO. The exact reaction paths and the type of the proposed Cs-zCO3(a) complexes are not very clear. The fact that up to moderate Cs coverages (- 0.2) the COZ desorption occurs before Cs desorption indicates that at low and moderate alkali coverages the adsorption state of alkalis does not permit the formation of a very stable Cs.zCO3(u) complex. One can only speculate that within the low alkali coverage range typical carbonate-like species are not formed and it is more likely t11a.t C0:- species are stabilized on the alkali modified sites. Stable carbonate-like species are formed and they are spectroscopically proved on Cs/Cu(llO) a t high alkali coverages when the alkali adlayers become metal-like [20]. These species participate in the formation of stable surface compounds which, as illustrated by the data in Table 6.3., decompose with simultaneous evolution of CO2 and Cs in the temperature range 620700 K. The formation of these stable carbonate-like surface compounds leads aIso to stabilization of the alkali adsorption state. The temperature range of decomposition of the alkali carbonate-like surface complexes is the same as that reported in refs. [loo, 1011 for decomposition of deposited K2C03 salt on Fe foil. Similar effects to that reported for Cs/Cu(llO) were reported for COZ interacting with I
Chapter 6

214

of the coexistence of a rather stable GO adsorption state at all I< coverages. The formation of a radical anion COY has been supposed as the first step of stabilization of CO2 on the surface followed by the carbonate-like species via either of the pathways described already above. Recent studies of GO2 adsorption on alkali modified P d(ll1) [89-911 have shown that the formation of carbonate-like species is favoured only at high alkali coverages as a result of interactions between CO2, and 0,. COZ, and 0, are formed as a result of preceding CO2 dissociation on the surface. It is likely that the lack of carbonate-like species at low and moderate alkali coverages is due to the fact that 0 bound to the substrate is less reactive than that bound to the alkali. As will be shown in Section 6.4., the formation of a l k a l i 4 complexes on the transition metal surface is favoured at high alkali coverages. On 0.4 K/Fe(lOO) [79], the stable species which remain above 100 K after desorption of the physically adsorbed GO:, are more likely analogous of the oxalates (with vibrational bands at 800, 1200, 1500 and 1650 cm-l). These species decompose above 400 I<. The new products are characterized by 1450, 500 and 200 cm-' vibrational bands. Since almost the same vibrational spectra are observed after annealing of mixed GO K overlayers on Fe(100) (500 and 200 cm-' are due to K-0 and Fe-0 stretching modes), the authors cannot say explicitly whether the band a t 1450 cm-' is due to carbonate-like or strongly promoted GO* species. The TPD spectra from C02--I
-

+

+

-

-

+

(1) The introduction of an alkali additive drastically enhances the adsorptive and reactivity properties of the surfaces. (2) The modification is not restricted t o an increase of the adsorption rate, the stability of the bent GO;- state and dissociation propensity, but also leads t o the formation of new intermediates and compound-like species as a result of secondary reactions in the overlayer. The type of these alkali stabilized intermediates (CO;-, CzHi-, GO:-) and their thermal stability are determined exclusively by the adsorption state of the alkali additives on the surface and the relative stability of oxygen and carbon on the specific surface. The formation of the most stable AM6+COtcarbonate-like surface compounds is favoured a t high alkali coverages and their thermal stability resembles that of the corresponding bulk compounds.

6.3. Nitric Oxide 6.3

215

NITRIC OXIDE

Compared to CO, the effect of alkali additives on NO adsorption is less extensively studied.The description of thealkali effect on the adsorptive ability and the reactivity of the surfaces with respect to CO and NO is equally important because the combination of this knowledge will help to assess the feasibility of alkali-promoted catalysts for pollution control.

6.3.1

Alkali Effect o n the N O Molecular Adsorption

T h e reported effect of alkali additives on the NO adsorption rate varies with different substrate surfaces [50, 107-1141 The general trend is that the initial sticking coefficient remains unchanged in the cases when it is close to unity o n an alkali-free surface (e.g. K / P t ( l l l ) [lo71 and K / R h ( l l l ) [log]) and increases with increasing alkali coverage when the clean surface sticking coefficient is less than unity (e.g. Na/Ag(llO) [ l l l , 1121 and I
-

(i) the orientation of the molecules approaching the surface; (ii) the surface phonon spectra which determine the trapping probability of the surface;

(iii) the density and distribution of the electron charge on the surface, which in the case of NO will create more favourable sites for cheinisorption As supposed in ref. [114], the main contribution to the enhanced adsorption rate on K-covered Rh(100) is the increased chemisorption probability which in the case of a I<-free surface is supposed to be limited by the available surface electron density (or more correctly by the available appropriate electronic states for the formation of an adsorption bond). It should be stressed that there is no correlation between the alkali induced changes in the molecular adsorption rate and the dissociation adsorption rate, because, as discussed in Subsection 6.1.7. the dissociation process is a multistage reaction not directly related t o the first step of molecular adsorption. The effect of the alkali additives on the adsorptive capacity of the surface is found depend on substrate and temperature. In the case of I
-

Chapter 6

216

0 n

ln

I v

d

-I

0 M

z 0

0

0

0.2 0.6 1.0 1.4 1.8 NO Exposure (L)

9 c-

2.2

n

A

I

9-

0

Fig. 6.31. (Top) NO uptake curves for clean (solid line) and 0.18 I<-covered (dashed line) Rh(100). (Bottom) The NO initial sticking coefficient, SO,normalized to the maximum observed Somax and the saturation NO coverage, @NO, as a functioii of @I<. (from ref. [114])

217

6.3. Nitric Oxide

a Olkl

i\

PtI1111.ra

’kI’

Rim1.K .N3 9,=017

NOE

~

I

.

.

.

.

l

.

.

.

.

r

. .

m

53s

1 . . . . 1 . . . . 535 m

E, I eV

b

1 " " " " ' l '

I

T=lZOK hw = 12536eV

A

Nlls)

Pt(lllI+K+NO

......... ., e,= 0.14 .. . .... . ... .

N(Q NIU

-:Ptllll) NO

. . l

Lo5

,

.

,

,

l

,

,

,

,

l

.

395

Loo €,lev

Fig. 6.32. (a) NO 0 1s spectra for clean and I<-covered P t ( l l 1 ) at 120 I< as a function of NO coverage (b) NO N 1s spectra of saturated NO coverages at 120 K on clean and I<-covered P t ( l l 1 ) (from ref. [107])

Chapter 6

218

The effect of alkali additives on the NO molecular adsorption state is similar to that observed for CO. The promoted adsorption sites are preferentially occupied during the initial adsorption process. Coexistence of promoted and unpromoted adsorption sites on P t ( l l 1 ) is observed at OK < 0.07 [lo71 and on Rh(100) - at BK < 0.1 (1141. Apparently, in the case of NO the elimination of the unpromoted states occurs at lower alkali coverages than the corresponding elimination for CO. As will be discussed below, this is likely t o be due t o the possibility of stabilization of two NO bonding configurations (,normally oriented and inclined), so that the alkali effect might be extended to the next-nearest neighbours. The observed alkali induced changes associated with the promoted NO molecular adsorption state can be summarized as follows: (i) a change of the 0 1s and N Is core level binding energies (see fig. 6.32.);

-

(ii) a stabilization of the molecular state by 15-30 kJ/mol, as measured for NO on I < / P t ( l l l ) [107], K/Rh(111) [lo91 and Na/Ag(llO) [ill, 1121; (iii) a decrease of the N-0' stretching frequency of normally bonded species by 160 cm-' at high I( coverages, whereas the N-0' stretching frequency of side-on bonded (inclined) species depends on Bit: it increases at low K coverages and decreases at BK > 0.1 [114];

-

(iv) stabilization of the side-on bonded dissociation precursor relative to the normally bonded desorption precursor (fig. 6.33.). In analogy to CO adsorption, on alkali-modified surfaces the occupation of the promoted sites leads to a substantial increase of the work function, indicating that the NO dipole is opposite to that of the coadsorbed alkali [107, 1091. For example, from the work function data shown in fig. G.34., a rather large initial dipole moment of 3.2 D (negative end outwards) for NO adsorbed on I< promoted sites on P t ( l l 1 ) was estimated. The work function data together with the observed decrease in the N-O* stretching frequency indicate enhanced occupation of the NO 2w' states and leads to altered adsorption energy and weakening of the N-0 bond in analogy to the case of CO. More information about the effect of the alkali and NO coverages on the bonding configuration of the promoted NO molecular species has been obtained for NO-K/Rh( 100) by means of the time resolved electron energy loss spectroscopy method [114]. It has been found that both the normally bound ( a q ) and the inclined ( a 1 )NO configurations which are detected on a K-free surface, preserve on the K-covered surface (although perturbed, as is evident from the changes in the N-0 stretching frequency). The stabilization of both adsorption configurations is explained by assuming that the K induced dipoles on the surface stabilize the az-NO-like configuration with respect to the a1-one, whereas the I( induced increase of the surface electron density stabilizes predominantly inclined ax-like NO. The major difference with the clean surface behaviour is that, as a result of the I( induced stabilization of the inclined a l-NO species, the total BNO necessary to induce a1 to a2-NO conversion increases with increasing BI< and at high al-NO remains on the

6.3. Nitric Oxide

219

surface even when the NO coverages are saturated (see fig. 6.33.) As will be shown in the next Subsection, the al-NO state serves a s a precursor for NO dissociation.

-

0.20

3

0.15

-

(b)

J

m

+

O.1°

0.05

--

f1

,.,,E/&-p

1

0.00 0.0

0.1

0.2

r.3

0.4

0.5

Fig. 6.33. A summary of the effect.s of K precoverage on the (YI-NO(inclined) and az-NO coverages. (a) solid line = unproinoted (YI-NO,dashed line = promoted ( ~ 1 NO'. (b) the minimum total 0 ~ necessary 0 to induce (YI-NOto ( Y ~ - N conversion. O (c) (YI-NOcoverage remaining at NO saturation coverage (from ref. [114])

6.3.2

Alkali Metal-Promoted Dissociation of NO and Secondary Reactions in the Mixed Coadsorbate Layers

A. Alkali promoted N O dissociation. The best system for illustrating the alkali promoted NO dissociation is NO-K/Pt(lll) where NO does not dissociate on a K-free surface. Evidence of decomposition of NO on I(-covered P t ( l l 1 ) at T > 300 K is given by the observed relative loss of nitrogen and by the increase of the O/N ratio beyond 1 corresponding to molecularly adsorbed NO at 120 I(. Similar to the case of CO, the amount of dissociated molecules is proportional to the I( coverage, as evidenced by the linear increase in the residual oxygen after annealing to 470 K in fig. 6.35. Further support for K promoted dissociation above 300 K is the appearance of an 0-related peak at 5 eV in the UPS spectra and the observed temperature-induced work function changes (see fig. 6.34.). Thus, a sudden drop in the work function occurs within the temperature range 320-360 K and the work function levels

-

Chapter 6

220

TEMPERATURE I K 500 300

100

.

3

-ea w

4 -2.0 9 u z

0

z

-30

3 LL Y

K

Pt (111l+K+NO

0

-10

T=120K E

I , , , , , , 0

01

02 03 OL COVERAGE ON(

05

06

Fig. 6.34. NO induced work function changes, Aq5 vs. @NO for clean and K-covered P t ( l l 1 ) at 120 K. The dashed line indicates the work function changes after heating the NO saturated K-covered P t ( l l 1 ) to various temperatures (from ref. [107])

-

off with further annealing to 500 K at a value 1.2 eV higher than the initial one (due t o the dipole of the coadsorbed 0). At T > 500 K, K starts to desorb simultaneously with some of the secondary products. The deviation of the O/N ratio from 1 indicates that the primary products of NO dissociation, 0 and N, are not equally stable on the surface. A thorough study of the alkali additive affect on the NO dissociation rate is performed for the system NO-K/Rh(lOO) [114] where fractional NO dissociation also takes place on a clean surface. Both TPD (fig. 6.36.) and vibrational data show evidence of the enhanced dissociative efficiency of the K modified Rh(100) [114]. A similar effect on the dissociative efficiency is also observed for K / R h ( l l l ) [log, 1101. The increased fraction of dissociated NO, which reaches 100 % at BI< > 0.25, correlates well with the observed Kinduced stabilization of the side-on bonded al-NO (which serves as a precursor for dissociation) relative to the desorption precursor CQ-NO. This favours the processes of al-NO* and cr2-NO dissociation (via conversion to al-NO). Note that the increased dissociation efficiency of the modified surface with increasing K coverage is accompanied with dramatic changes in the shapes and the temperature range of the N2 TPD spectra illustrated in fig. 6.36. (resulting

6.3. Nitric Oxide

22 1

from NO dissociation). Since there is no doubt that the presence of K would cause a reduction in the activation energy of the Nz desorption process which proceeds via recombination of N adatoms, the K-induced reduction of the N2 desorption temperature should be ascribed to secondary reactions in the mixed overlayer, as will be discussed below. 8

v)

c

z

Q

z 05 -

4

v)

* O

01s

A h Nls

0 W

500 TEMPERATURE I K

R 2

1

Pt(lll)+K+NO

0

POTASSIUM COVERAGE 8,

Fig. 6.35. (a) Relative 0 1s and N 1s intensities vs. the annealing temperature for clean and K-covered P t ( l l 1 ) (b) Relative intensity of the 0 1 s residual oxygen peak obtained after annealing to 470 I< as a function of I< coverage (from ref. [107])

In analogy with alkali-promoted GO dissociation, the same factors can explain the promotion of NO dissociation, namely: (i) alkali-induced reduction of the activation energy for dissociation (due to weakening of the intramolecular bond as a result of enhanced electron

222

Chapter 6

density in the anti bonding 27r' molecular orbital) and (ii) stabilization of the molecular state, serving as a precursor for dissociation. The established direct correlation between the alkali-induced increase in the relative population and the stability of the inclined al-NO species and the dissociation propensity confirm the suggestion made in the previous Section, that the lack of alkali-promoting effect on CO dissociation in some systems might be related to the ability of the modified surface to favour side-on bonded molecules which are precursors for dissociation. Obviously, this property is strongly dependent on the type of the substrate.

Fig. 6.36. Nz (solid lines) and NO (dashed lines) TPD spectra from Rh(100) as a function of NO exposure at 100 I< for various OK (ML): (a) 0.0; (b) 0.04; (c) 0.12, (d) 0.18; (e) 0.25, and (f) 0.41. The NO exposures are ranging from 0.1 to 2.1 L, the largest exposure leading to saturation (from ref. [114])

The most striking finding is that, besides the alkali-induced increase of the

NO dissociation propensity the NO dissociation rate on a K-covered Rh( 111) surface is lower than that on a K-free surface. As illustrated in Table 6.4., the reason for that is the drastically reduced pre-exponential factor of dissociation, v & s , which outweighs the effect of the alkali-induced reduction of the activation barrier for dissociation. At the current stage of knowledge, no unambiguous explanation of the alkali effect on the dissociation pre-exponential factor can be offered. Obviously, considering the effects of additives on the reactivity of the surface, possible perturbations on the dynamic properties of the surface should also be taken into account. Within the confines of the simple transition state theory, the smaller pre-exponential factor means that in

6.3. Nitric Oxide

223

the presence of an alkali modifier the transition complex for dissociation has a lower entropy, i.e. it is constrained entropically compared to the transition state on a K-free surface. Another possible explanation of the observed compensating decrease of vdiss might be that competing pathways with a lower activation energy unfold, which will account for the apparent low vd,ss value estimated from the constructed Arrhenius plots. As will be shown below, this alternative explanation cannot be rejected, not having the evidence of the promotion of secondary reactions in the mixed overlayers. Activation Energies, E, (kJ/mol) and Pre-exponential Factors, for NO Dissociation on Clean and K-covered Rh(100) Surfaces as Evaluated from Arrhenius Plots (from ref. [114])

Table 6.4. Vdiss (s-')

Ell

SURFACE

Rh( 100) 0.14 I
Vdiss

50.4 f 3 . 4 3 1 . 6 f 1.9 26.4 f 2 . 9

10"

'*' '*'

102 l o 4 3*0

B. Secondary reactions in mixed NO-AM overlayers. Evidence of secondary reactions in the mixed alkali NO overlayers involving the undissociated molecules and the dissociation products is obtained both from the spectroscopic data and the T P D data. Close inspection of the spectroscopic data for NO adsorption on I < / P t ( l l l ) [107, 1151 have shown that at low and moderate K coverages the only secondary reaction which is favoured after NO decomposition is:

+

Na

+ NO,

+

N2O

+

NzOy

+

Nzy

+ 0,.

-

This is evidenced by: (i) the complete removal of nitrogen after heating to 420 K and the substantial amount of oxygen remaining behind (see fig. 6.35.); (ii) the detection of N 2 and NzO desorption at 400 I< 11081. The other possible way in which nitrogen can get removed, i.e. associative N 2 desorption, is not likely because even from a clean P t ( l l 1 ) surface, N 2 desorbs at T > 450 K [116]. Secondary reactions, which lead to products stabilized on the surface by K , are observed at high K coverages (beyond the A 4 vs. B1t minima) and at elevated temperatures. Figs. 6.37. and 6.38 present the XPS spectra for the 0 1s and N 1s region obtained upon annealing of a mixed layer from 305 I< bo 570 K and after NO a.dsorption on the same I<-covered surface at 305 and 420 I(. Adsorption of NO at 305 I< gives rise to two new peaks i n the 0 I s and N Is region at 532.8 and 404 eV, respect,ively, in addition to the 531 and 400.3 eV ones characteristic of NO:. The intensity of the new peaks increases in the temperature range 300-450 at the expense of the NO:-related ones. Further annealing beyond 450 K leads to gradual removal of the new peaks

-

224

Chapter 6

T=S70K

T i 420K

& .. . . . . . . . Tr305K

...... .. .I

.... ...

E. I nV

-+

Fig. 6.37. Evolution of the 0 1s and N 1s XPS spectra from mixed NO K overlayersonPt(ll1) obtained a t 305 I< and annealed to 420 and 570 K. OK = 0.27 (from ref. [107]) Table 6.5. Binding Energies (in eV relative to the Fermi level) for Various NO2 Species (from ref. [lo"])

ORBITAL 6ai 1a2 4b2 3b 1b 5a 4a

CALCULATED NO, energies 3.5 4.1 5.0 10.4 10.4 11.4 15.7

NO,

EXPERIMENTAL ENERGIES NOz(a) N O ~ - ( K 6 + ) / P t ( l l l )

5.4

N

I1

N

14

-

3.1 5.2 9.6

3.0 4.0 5.4 9.6

11.3

11.6 13.7

-

accompanied by a drastic increase of the O/N ratio to 7 a.t 570 I<. Adsorption at temperatures which favour the formation of the new product leads to the predominance of the 532.8 and 404 eV peaks in the 0 Is and N 1s regions (as shown in fig. 6.36.), which are originating from NO:- species. The identification of the new K stabilized product as NO:- is also supported by the

225

6.3. Nitric Oxide

h w = 12536

eV

. ..

. . 535

530

105

#J

395

E, I eV

Fig. 6.38. 0 Is and N Is XPS spectra from mixed NO + I< overlayer on Pt(ll1) obtained at 305 K and 420 I<. 8K = 0.26 (from ref. [115])

UPS spectra measured for this mixed layer (Table G.5.).

Comparison with

a valence band spectrum of LiN02 [117] and with theoretical orbital energies of NO, [118] yields a rather good agreement between the principal features. This supports the above definition of the stabilized product to NO;-. Another proof is that a similar N 1s binding energy of 404.5 eV and 0 1s binding energy of 533.3 eV are measured for a K N 0 2 salt deposited on Fe and

-

Pt [103]. The dissociation of NO and the formation of NO:- are interrelated. The absence of spectroscopic evidence of atomic nitrogen ( N 1s peak at 397 eV) a t 305 I( (see fig. 6.37.) iiidicat.es that this dissociation product is not stable under these conditions. However, the elimination of N by a simple associative desorption of Nz is not likely because, as outlined above, atomic nitrogen is bound rather strongly on P t [llG]. It has turned out that N, can be easily removed by a secondary reaction leading to NzO formation, followed by rapid NzO desorption. This reaction readily occurs at high K coverages. The appearance of a N,-related peak at 397.3 eV when the NO adsorption is performed at 420 I< (fig. 6.38.) has been explained by the fact that at higher adsorption temperatures the steady state coverage of NO, is drastically reduced and the formation of NO;- predominates. Thus, all NO molecules striking the surface adsorb dissociatively or react directly with 0, t o form NO6,(u). Hence, a certain N, amount can accumulate because of the induced NO, deficiency. The mechanism of NO;- formation via the reaction NO, 0, = NO:- (K stabilized) has been proved by subsequent 0 2 and NO

-

+

Chapter 6

226

adsorption on K-covered P t ( l l 1 ) [115]. This K stabilized secondary product decomposes at T > 500 leading t o subsequent desorption of N2, 0 2 and K. It should be pointed out that the dissociation products 0 and N and the secondary product NO;- cause stabilization of the K overlayer. The stabilization effect of K with respect to the secondary product NO:can be explained as follows. The NO2 molecule has a singly occupied 6al orbital. This molecule is generally quite unstable in contact with transition metal surfaces, and usually undergoes dissociation into atomic 0 and NO [119]. K promotion can be associated with an increase of the charge donated to the 6al NO2 orbital so that the molecule becomes negatively charged. This has a stabilization effect because the NO, anion is isoelectronic with the SO2 molecule which is much more stable than the NO2 molecule. Furthermore, the formation of NO:- species is expected to cause a decrease of the bond angle (134' for NO2 and 113' for NO,), which makes the newly formed secondary product thermodynamically more stable. Thus, in the presence of a sufficient amount of K the reaction between the coadsorbed 0 and NO to form NO2 is facile in the temperature range of 300-450 I<. The NO;- species is stable up t o 500 K. Summarizing the effects of alkalis on the adsorptive properties and the reactivity of the surface with respect to NO, the following conclusions can be drawn:

-

(1) The presence of alkalis stabilizes the molecular adsorption state and promotes NO dissociation, the fraction of dissociated NO increasing linearly with the alkali coverage.

(2) Following NO dissociation, secondary reactions between the undissociated molecules and the dissociation products N, and 0, are possible. They lead to: (a) unstable products which immediately desorb or decompose, releas-

ing a gaseous product according to the scheme:

These secondary reactions are favoured at all K coverages and are an efficient pathway for removing the dissociative product N, from the surface. The rate and the efficiency of the reaction depends on the NO, surface concentration, which, as described above, depends on temperature and alkali coverage. (b) alkali stabilized secondary products formed as a result of the following reaction:

NO,

+ 0,

4

N06,-(K6').

This reaction is favoured only at alkali coverages exceeding the A4 vs. OAM minimum. This indicates that the formation of the NO:- anionlike product requires a substantial electron density in the alkali nsvalence states. The stabilization of this product is most likely a result

6.4. Oxygen

227

of direct alkali-N02 coupling leading to the formation of a nitrite-like salt. The last assumption is confirmed by the fact that the stability of the secondary product is comparable with that of the adsorbed ICN02 salts [103, 1041. 6.4

OXYGEN

Oxygen adsorption on alkali modified metal surfaces has received great interest due to its relevance in two important fields: (i) heterogeneous catalysis where alkali additives serve as promoters with oxygen participating as a reagent (selective oxidation of ethylene to ethylene epoxide, partial oxidation of methanol to formaldehyde, etc.), a product of an intermediate step, such as dissociation of NO or CO, an impurity in the reaction gas mixture, etc.; (ii) electronics where the alkali metals are used for enhancement of the electron emission of photocathodes and thermoionic emitters. A great number of model studies have already described different aspects of oxygen interaction with alkali modified single crystal planes of Ag [120-1271, Cu [128-1321, P t [127, 133, 1341, Pd [91], Ni [135-1381, Ru [139-1431, Fe [144], M o [145, 1461. W [147-1491, and Au [150]. Oxygen adsorption on clean metal surfaces proceeds according to following main stages : (1) Molecular adsorption where peroxo- and superoxo-like bonded species are detected. This state is usually observed only at low temperatures and its stability is strongly dependent on the nature of the substrate surfaces. The reason for the observed facile dissociation of 0 2 a (compared t o CO, NO and N 2 ) is the weaker 0-0 intramolecular bond due to the pair of electrons in the anti bonding 1 ~ orbital. ; A relatively stable molecular state is detected on noble metal surfaces (Ag, Cu) [151], where the backdonation to the l$ is weaker. The molecular state partly desorbs and partly dissociates at temperatures above 250 K on Ag surfaces and above 150 I< on Cu surfaces. Comparison of the reactivity of different crystallographic planes shows that the molecular state is less stable on the more open planes [151]. A molecular peroxo-like adsorption state is also detected at 100 K on P t and Pd single crystal planes [152-1541. On the other transition metals under consideration, the molecular adsorption state is not stable at T 2 100 K , because of the enhanced contribution of the metal to l$ backdonation, so that only dissociative oxygen adsorption takes place above these temperatures.

(2) Dissociation of the molecular stsate,which occurs above a certain temperature depending on the type of the substrate. When oxygen adsorption is carried out a t room or elevated temperatures, the lifetime of the molecular state is very short and only the atomic adsorption state is favoured. With some metal surfaces (e.g. Ag, P t and Pd), the sticking coefficient related to dissociative oxygen adsorption is lower than that measured at low temperatures. It varies from, e.g for one of the least reactive metals (Ag) t,o N 1 for the most reactive ones (e.g. N i ,

228

Chapter 6 W and Mo). The capacity of different surfaces for atomic oxygen adsorption and the atomic adsorption binding energy also vary with the different substrates. With metals which exhibit a great affinity to oxygen, the adsorption energy is rather high and heating of the adsorbed layer to high temperatures leads t o diffusion of oxygen into the bulk rather than to associative desorption of 0 2 .

(3) Depending on the actual reaction conditions and the kind of the substrate, further reaction steps (when the oxygen coverage exceeds certain critical values) can lead to surface and bulk oxide growth. With the substrates under consideration, evidence of build-up of an oxide phase under UHV conditions is obtained in respect of Ni, Fe, W and Mo single crystal surfaces. As outlined above, oxygen adsorption on metal surfaces is exclusively dissociative under real catalytic conditions (room or elevated temperatures). That is why in this chapter the effect of alkalis on the dissociative oxygen adsorption and the possible interactions between the oxygen adatoms and the alkali adspecies in the mixed overlayer are the main factors to be considered. 6.4.1

Alkali Effect on the Oxygen Dissociative Adsorption: Adsorption Kinetics and Adsorptive Capacity of the Modified Surface

A. Oxygen sticking coefficient and saturation coverage on alkalimodified surfaces. The presence of alkali additives always leads to an increase of both the dissociative adsorption rate and the adsorptive capacity of the surface in cases where the initial sticking coefficient for dissociative adsorption and the saturation atomic oxygen coverage are less than unity on an alkali-free surface. Figs, 6.39. and 6.40. present typical oxygen uptake curves, obtained for alkali covered surfaces and the dependence of the initial sticking coefficient for dissociative oxygen adsorption, SO,on the alkali coverage. I t is obvious that the initial sticking coefficient and the saturation oxygen coverage increase with the alkali coverage. So is a linear function of the alkali coverages in accordance with the model for the local effect of the alkali modifiers, described in Section 4.1. So reaches the maximum value of unity at various alkali coverages with the different alkali metals. Comparison with the corresponding A4 vs. eAM plots shows that SO attains unity at alkali coverages beyond the work function minima when the overlayer acquires a metal-like character. In order t o illustrate the relation between the magnitude of the alkali promoting effect and the cross sections of the actual alkali modifier,the insert in fig 6.40. presents So values plotted against the normalized alkali coverages BAM/BXM, where 6 i M is the alkali coverage corresponding to the first alkali overlayer (0.5, 0.33 and 0.29 for Na, K and Cs on Ru(0001), respectively). The fact that the SO values in the insert of Fig. 6.40. fit on a straight line confirms the view that the equilibrium charge transfer distance depends on the size of the corresponding alkali valence shell [155]. The higher electron density localized in the vicinity of the alkali adspecies is most probably one of the reasons

6.4. Oxygen

0

T

229

2.0 u) 6.0 O2 EXPOSURE “Tambar s)

Fig. 6.39. Oxygen uptake curves for I< (left) and C s (right)-dosed Ru(0001) T = 300 K (from refs. [139, 1401)

at

for the enhanced So and saturation oxygen coverage. From the traces of the oxygen uptake curves in Fig. 6.39. It is obvious that, parallel to the increase of the initial sticking coefficient, the presence of alkali additives also leads to independence of the adsorption rate in a rather wide oxygen coverage range. Obviously, the critical oxygen coverages up to which the initial adsorption rate is preserved increases with increasing alkali coverage. Generally speaking, the same trend of a linear increase of S O ,and saturation oxygen coverage with increasing alkali coverage and constancy of the 3xygen adsorption rate up to a certain alkali coverage determined 00 has been observed for Kcovered Fe(ll0) [144], Na, I<, Rh-covered Ag [121, 125, 1261, Kcovered Pd(100) [91], Na-covered Cu [130], I<-covered Au [150], N a , I<, C‘s-covered Ru(0001) [16, 139-1411, and I<-covered P t ( l l 1 ) [133]. It is worth pointing out that, compared to Ru(0001), where the initial sticking coefficient 3n a clean surface is relatively high (- 0.35) for substrates, such as Au, Ag and P t , where So on a I<-free surface is 5 So reaches also unity at high alkali coverages near completion of the first layer. As will be discussed below, depending on the relative affinity of the alkali adlayer and the substrate to oxygen, they will compete for an oxygen bonding and in some cases (e.g.

Chapter 6

230

'*

-

cs

/boo -'' Na

I . -

I

0

v)

0

.P("O'-

gem! .3 .4 .5 %.m. ,

0

.1

.2

1

.6

Fig. 6.40. Initial sticking coefficient, SO,as a function of alkali coverage, @ A M . T h e insert shows SOversus normalized 8AM/@;M coverage (from ref. [140])

alkali covered Au or Ag) both associative and dissociative adsorption can be detected even in the presence of alkali additives. For substrates which do not tend to form an oxide phase, the maximum oxygen coverage achieved in the presence of alkali additives usually does not exceed 1 ML (with respect to the substrate surface atom density) on completion of the first alkali overlayer. Higher oxygen coverages can be achieved when the alkali coverage exceeds one layer, and then the behaviour of the system becomes strongly dependent on the reactive properties of the actual alkali metal. In the case of alkali-modified substrates, where the clean substrate surface exhibits a high reactivity with respect to oxygen dissociative adsorption (e.g. So 1 for an alkali free-Ni surface) and where substrate oxidation occurs above critical oxygen coverages, the alkali induced effect consist of N

(1) Suppression of the self-poisoning action of the adsorbed oxygen leading to a rapid drop of the sticking coefficieiit on a n alkali-free surface (see the dashed line in fig. 6.41.). As illustrated in fig. 6.41., the initial oxygen adsorption rate retains its initial value up to a certain critical 00, which increases linearly with increasing alkali coverage. The highest efficiency of the modifying effect is observed in the presence of the largest alkali additive (Cs) for alkali modified Ni(100) (135, 1371. (2) Enhancement of the oxidation rate because the preserved high adsorption rate produces the critical oxygen coverages needed for the formation

6.4. Oxygen

231

Fig. 6.41. Dependence of the oxygen sticking coefficient, SO,on 00 for various Na, K and Cs coverages on Ni(100). T h e dotted curve presents the S vs. Bo plot for a clean Ni(100) surface. T, = 300 I< (from ref. [137])

of surface (NiO), nuclei and a developing oxide phase more rapidly As can be seen in Fig. 6.42., introduction of alkali additives leads to shortening and disappearance (above certain alkali coverages) of the long plateau. This plateau is due to the very slow growth of the (NiO) islands because of the strong self-inhibition effect on the oxygen sticking coefficient on an alkali-free Ni surface. The alkali effect on the oxygen adsorption kinetics and rate of the oxide growth, described above, indicates that the mechanism of the promoting action is essentially the same. The effects on the SO values and on the S(B0) dependence can be explained within the framework of the precursor mediated adsorption process [156]. The increase of So (when it is less than unity on a clean surface) can be due to an increase of the trapping probability and/or a decrease of the k d / k , ratio, where k d and k , are the rates of desorption of the molecular precursor and k , is the rate of dissociation. Supposing there are small changes in the pre-exponential factors, then this means that the alkali species create promoted sites characterized by a stabilized molecular precursor and a reduced activation barrier for dissociation. It is more probable that this is a result of the enhanced charge transfer to the oxygen la;. As will be discussed below, in the case of substrates, such as Au and Ag, which cannot compete for oxygen bonding with the alkali additives, a contribution t o reduction of the activation barrier for dissociation is the substantial stabilization of the atomic adsorption state via direct interactions with the alkali species. Thus, pushing down the atomic oxygen potential well will lower the intersection of the atomic and precursor state potential curves. The same

Chapter 6

232

I

0

10

20 OXYGEN

30

40

50

EXPOSURE(L)

Fig. 6.42. Oxygen uptake curves as a function of oxygen exposure for increasing alkali modifier on Ni(100). B ~ M= 0.15. (from ref. [137])

arguments for the creation of more favourable promoted adsorption sites explain the observed S(B0) independence up to certain critical oxygen coverages, Bo(cr.), satisfactorily. The dependence of Bo(cr.) on alkali coverage is due t o the exhaustion of the promoted sites which are proportional to the surface concentration of the alkali species. Because of the enhanced electron charge density within the surface and the stabilization effect of the alkali additives on the atomic adsorption state, the adsorptive capacity of the surface is also enhanced. Thus, in the case of easily oxidized substrates where the initiation of the oxidation process requires a critical oxygen coverage, the presence of alkalis also facilitates the oxidation process by providing more oxygen on the surface. It should be stressed that irrespective of the presence of alkali additives, the onset of the oxidation process requires the same critical oxygen coverage. This indicates that t,he presence of alkali additives does not influence the activation barrier for oxide formation,merely only speeds up the oxygen uptake. Apparently, substrate oxide formation cannot be promoted by the presence of alkali additives. This agrees extremely well with the experimental results where no alkali induced oxidation is observed in substrates, such as Pt, Cu, Ru and Ag which cannot be oxidized at 300 I< under UHV conditions. In summary, the current experimental results have shown that, depending on the relative affinity of the alkali adspecies and the substrate surface to oxygen, the dissociative oxygen adsorption on an alkali modified surface can

6.4. Oxygen

233

proceed via the following main channels: (i) adsorption on alkali modified sites with prevailing stabilization via alkali-oxygen interactions; (ii) adsorption via a modified state followed by the formation of a substrate0 bonding, or (iii) simultaneous direct adsorption on promoted and alkali free sites which is likely in cases of low alkali coverages on a substrate exhibiting a high activity with respect to 0 2 adsorption. The first and the second channels are responsible for the observed promotion effect on the adsorption rate and the enhanced saturation coverage. 6.4.2

Mutually Induced Stabilization of the Coadsorbed Oxygen and Alkali Species

On most of the metal surfaces under consideration, dissociative oxygen adsorption is the only or the predominant process on alkali modified surfaces. That is why the effect of the alkali additives on the stability (adsorption binding energy) of the oxygen adatoms will mainly be considered.

7/10' I

Fig. 6.43. ref. [9 11)

0 2

TPD spectra from clean and K-covered Pd(ll1). T, = 100 I{ (from

For substrate surfaces, such as Ag, Pd or P t , where the molecular oxygen adsorption state is relatively stable a t lower adsorption temperatures, the

234

Chapter 6

introduction of alkali additives eliminates the molecular adsorption state, because of promotion of the dissociative oxygen adsorption. Fig. 6.43. presents the effect of increasing K coverage on the 0 2 T P D spectra from Pd( ll1) . As can be seen, K removes the low temperature 0 2 T P D peaks associated with the molecular adsorption state at the expense of a substantial increase of the capacity for atomic oxygen adsorption reaching satmurationcoverage of one at high K coverages (the saturation atomic oxygen coverage on K-free surface is 0.25). An interesting feature in the 0 2 T P D spectra in fig. 6.43., which, as will be shown below, appears in the 0 2 TPD spectra from other alkali modified surfaces, is the sharp T P D peak at 660 K . This peak arises at K coverages beyond the WF minima, accompanied by coincident K desorption. The stabilization effect of alkali a.dditives on the oxygen atomic adsorption state depends on several factors. The most important are:

-

(i) the relative affinity of the substrate and the alkali adspecies to oxygen; (ii) the relative adsorption bonding strength of the alkali and oxygen adspecies, which is determined by the type of the substrate surface and the actual adsorbate coverages, and (iii) the adsorption temperature

a

I 1000

1200

l4OO

1600

T( K1

Fig. 6.44. (a) TPD spectra of 02-saturated Ru(0001) at, 300 K followed by Na, K and C s deposition (@AM = 0.05). (b) Na, K and Cs TPD spectra from the mixed overlayer. (from ref. [137])

The best example of the case when the substrate cannot compete with the alkali additive for oxygen bonding, is K-covered Au [150]. In this case, oxygen

6.4. Oxygen

235

is stabilized on the surface exclusively via the formation of a bonding with the alkali adspecies and desorbs simultaneously with the alkali additive. Evidence of alkali-oxygen interactions leading to stabilization of the atomic oxygen adsorption state is observed in the mixed alkali-oxygen overlayers on Ag and Pt at all alkali coverages. For these substrates, the atomic oxygen .initial heat of adsorption, AH:, is smaller than the alkali initial heat of adsorption, AH:, (e.g. 213 kJ/mol for 0 against 290 kJ/mol for I< on P t ( l l 1 ) [31]). Thus, the presence of Cs on a Ag(ll0) surface leads to higher temperature 0 2 T P D peaks at 595 and 630 K compared to the associative oxygen peak at 580 K from a Cs-free surface [122]. Because of the relatively weak O-Ag bonding strength, oxygen is adsorbed always in association with the alkali adspecies on an a.lkali modified Ag surface, the O/AM stoichiometry being independent of the alkali coverage up to completion of the first overlayer. In the case of mixed 0-I< overlayers on P t ( l l l ) , t,he presence of I<, regardless of its initial coverage, ca,uses always a high temperature shift of the 0 2 T P D spectra (indicating a sta.bilization of the atomic oxygen adsorption state), accompanied by simultaneous I< desorption [134]. With substrates where AH: is larger than AH:, (e.g.- 400 kJ/mol for oxygen and 238, 297 and 322 kJ/mol for Na, K and Cs on Ru(0001) [lG]), alkali-induced stabilization of the atomic oxygen adsorption state can be observed above a certain oxygen coverage when the oxygen-substrate bonding strength becomes of the sa.me magnitude or less than tha.t of the 0.5, the O-Ru binding energy drops to alkali-substrate one (e.g. at 00 312 kJ/mol). Fig. 6.44. presents the alkali-induced stabilization effect of a saturated oxygen layer on Ru(0001) at 300 I< (60 = 0.5). Obviously, the post deposition of the same small amount of N a , K or Cs leads to a high temperature shift of the 0 2 T P D spectra, the effect becoming stronger in the sequence Na, K , Cs. Fig. 6.45.presents a set of 0 2 , I< and I<20 T P D spectra. obtained at various coadsorbate coverages. The appearance of new T P D peaks and the oxygen induced higher temperature shift of t,he I< T P D peaks illustrate the mutual I< and 0 stabilization in the coadsorhate overlayer. Since more discussions on the alkali-oxygen interactions will he offered in the next, Subsection, it may be pointed out here h a t the observed alka.li-induced st,abilization of a higher oxygen coverage a,nd the appearance of coincident oxygen, I< a.nd K 2 0 desorption at 900 I< (fig. 6.45.). Such coincident alkali, oxygen and alkali oxide desorption froin Ru(0001) is t8ypicalof high alkali coverages (beyond the work function minima) a i d is a.lso observed with Na- and Cs-covered Ru(0001) [16, 139-1411. The ma.in rea.sons for the visible stabilization effect of alkali additives in systems where the oxygen-substrate bond is weaker or comparable with the alkali-substrate can be rela.ted to the creation of more favourable adsorption sites as a result of the enhanced electron charge density on the surface and the occurrence of substantial attractive interactions between the coadsorbates. These interactions become substantial in cases when the alkali competes successfully with the substrate for oxygen bonding. The stabilization of the alkali adsorption sta.te observed upon oxygen coadsorption, is also determined by the relative strength of the coadsorbatesubstrate bodding. In the case of substrates (e.g. P t) where AH: is smaller

-

-

-

-

-

-

Chapter 6

236

.n 3

Fig. 6.45. (a) 0 2 TPD spectra following different oxygen exposures on Ru(0001) covered with 0.33 ML K. T h e dashed line presents the 02 TPD curve from clean Ru(OOO1) saturated with oxygen. (b) I< TPD from mixed K 0 layer for different K coverages after saturation with oxygen. The dashed line presents K T P D spectra for 0.05 K from oxygen-free Ru(0001) (c) KzO TPD spectra for K and oxygen saturated mixed layer. T, = 300 I< (from ref. [140])

+

than AH:,, stabilization of the alkali overlayer is observed at higher alkali coverages. In the case of substrates when AH: is larger than AH:,, oxygen coadsorption always causes strengthening of the alkali surface bond irrespective of the initial alkali coverage (see fig. 6.46.). From the data shown in Fig. 6.46. the author has estimated that the increase of the relative oxygen concentration leads t o an increase of the Na initial heat of adsorption (adsorption binding energy) from 274 kJ/mol for clean Ru(0001) to 393 kJ/mol for a mixed overlayer with do = 0.5. In the case of Cs, which is characterized by a larger AH& on a clean surface, the magnitude of the oxygen induced stabilization at 00 = 0.5 is less than t,hat in the case of N a (- 84 kJ/mol). A similar stabilization of the alkali adsorption state induced by coadsorption of accepter like molecular and atomic coadsorbates is a common phenomenon.

237

6.4. Oxygen

, 0

\

500 700

900

*

1100 Tempera t we ( K )

1300

Fig. 6.46. Chaiiges in the Na T P D curves for Ru(0001) covered with S N = ~ 0.15 caused by increasing oxygen coverages. Dashed line presents the Na TPD curve from 0-free Ru(0001) (from ref. [139]).

There are two main contributions responsible for this mutually-induced stabilization of the coadsorbates. The first is that the coadsorbed species, which have opposite charges, play the role of a buffer, reducing the repulsive dipole-dipole interactions. The second one mainly accounts for the occurrence of direct alkali-coadsorbate interactions. The observed oxygen coverage dependence of the magnitude of the stabilization effect illustrated in fig. 6.46. is more likely due t o both the increase in number of oxygen atoms coordinated in the vicinity of the alkali and the possible change in the adsorption state of the oxygen adspecies at higher oxygen coverages. Indeed, for the case of the 02-Ru(0001) system at 00 > 0.25 the oxygen-Ru distance becomes larger, which leads to an increase of the electrostatic potential induced by the oxygen adatom [157]. Undoubtedly, this will enhance the strength of the 0-alkali interactions and the stabilization effect related to them. In the case of substrates characterized by an extremely strong bonding with oxygen and a prevailing tendency to oxide formation (e.g. 0-Ni bonding is 543 kJ/mol compared to 322 kJ/mol for the most strongly bound alkali adatom - Cs), it is difficult to evaluate the magnitude of the alkali induced stabilization effect because oxygen solely bonds to the substrate forming an oxide phase beyond certain oxygen coverages. On these substrates, evidence of direct alkali-oxygen interactions has been observed only in cases of niultilayer alkali overalyers, which, as will be discussed below, behave similarly t o bulk alkalis. Regardless of the lack of direct evidence of substantial alkali

-

-

238

Chapter 6

oxygen interactions (due to the fact that 0 adspecies on these substrates are located very near by and sometimes even embedded in the surface layer), an enhancement of the alkali adlayer thermal stability is always observed on these oxygenated surfaces [159]. According to recent reports, the initial heats of alkali adsorption for metal oxide surfaces do not differ substantially from that measured for a clean surface [158]. However, because of the presence of oxygen on the MeO, surface, the adsorbed alkali species (preferentially bound to the oxygen centers) are characterized by a larger initial dipole and the coverage related depolarization effects are much weaker than that observed for alkalimetal surfaces. This results in a lack of metallization up to the completion of the first alkali layer which is niuch more stable than on a clean surface. A common feature for all systems considered above is that always (regardless of the relative affinity of the substrate to oxygen and alkalis) the oxygen induced stabilization of the alkali overlayers is accompanied by a substantial increase in the amount of alkali that can be deposited on the surface. 6.4.3

0-Alkali

Interactions in Mixed Overlayers

As outlined above, the various substrates under consideration exhibit large variations of the oxygen-metal bonding, which determines whether the coadsorbed oxygen will be bonded to the alkali modifier preferentially or t o the substrate. Due to the fact that the strength of the oxygen-substrate and alkali-substrate bonding also depends on coverage, the competition for oxygen bonding varies with the relative alkali and oxygen coverages. Three main cases are considered now. Table 6.6. 0 1s Binding Energies (in eV) of Different Oxygen Species (Oa, 0: and 0::) Adsorbed on Alkali Covered P t ( l l 1 ) [133], Ag [127] and Cu [132]. (&,t corresponds to the completion of the first alkali layer)

SURFACE (T,) P t ( l l 1 ) (300 K )

K/Pt Ag (300 K ) K/Ag K/Ag

Cs/Ag CslAg C U ( 11 0)

0 1.0 0 0.4 -1

0.25 -2 0 < 0.5

-

530 530.5 -

-

528.9

-+

529.5

-

-

528.2 8L 530.2 528.0 8L 530.7 528.5 8L 530.3 530.2 N 528.5

533.2 533.0 -

-

529.5-530.0 Cs/Cu(llO) 529.5 0, and 0: indicate oxygen adatoms on unpromoted and promoted sites.

Firstly, substrates with a relatively low metal-oxygen adsorption bond (= < 250 kJ/mol) will be considered. On these substrates, the occurrence of

6.4. Oxygen

239

direct a l k a l i 4 interactions involving the oxygen adatoms located in the alkali promoted sites is favoured at all oxygen coverages. Typical examples of such systems are O-alkali promoted Au, Ag, P t , and Cu. In these systems, because of the lower affinity of the substrate to oxygen, it is possible t o detect different types of oxygen species on the surface, depending on the actual alkali coverage. At alkali coverages before reaching the minima in the Ad vs. BAM curves, XPS and UPS spectra show coexistence of promoted and unpromoted oxygen adatoms, with preferential occupation of the alkali-promoted sites. Selected XPS data of the 0 1s region for clean and alkali-covered surfaces are given in Table 6.6. A survey of the available experimental data and those summarized in Table 6.6. has shown that oxygen adatoms on an alkali free surface are characterized by an 0 Is binding energy of 530 eV in the case of substrates where no oxidation occurs. The oxygen species adsorbed on the alkali promoted sites (which are preferentially occupied) are characterized by 1 eV lower 0 1s binding energy suggesting an increase of the negative charge on these oxygen species. However, because of the complexity of the system and the absence of evidence of the formation of a substrate oxide phase, the enhanced negative charge on the promoted oxygen adatoms is more likely associated with direct alkali-oxygen interactions. The larger negative charge associated with oxygen occupying the promoted sites is consistent with the substantial work function increase observed with the first oxygen doses for low and moderate alkali coverages [123, 1241. This increase in the work function also indicates that the coadsorbed promoted oxygen is located on top or in the same plane as the alkali adspecies. Currently, it is accepted that, from an energy point of view,it is more favourable for both atoms to be in the same plane [160, 1611. The initial 0-xygen-induced decrease in the work function at alkali coverages exceeding the work function minima is more likely due to an oxygen-induced increase of the effective positive charge on the alkali adspecies which, up t o a certain oxygen coverage, can compensate for the dipole of the negatively charged oxygen. However, some of the authors ascribe this initial work function decrease t o underlying oxygen species [123, IZS]. Since usually the same work function changes are also observed a t high alkali coverages upon molecular adsorption (e.g. CO), the formation of a double layer seems less probable. Another support for the coplanar adsorption of oxygen and alkali atoms is the oxygen induced development of distinct new LEED patterns associated with the formation of alkali-oxygen complexes with varying stoichiometries [126, 1341. The thermal decomposition of these complexes leads to simultaneous alkali and oxygen desorption. The preferential bonding of oxygen from the promoted sites to the alkali is a common phenomenon i n the case of the noble metals However, in the submonolayer region and even 011 completion of the first alkali layei these alkali-oxygen interactions do not lead to the formation of typical stoichiometric alkali oxides [133]. The appearance of a third 0 1s peak a t 533 eV at high alkali and oxygen coverages on I< or Cs-covered Ag is ascribed to a weakly-bound molecularly adsorbed oxygen. According to the XPS data it is a very small fraction of the adsorbed oxygen and desorbs at T < 400 I< [ 120, 1271. It is suggested that these weakly-bound molecular oxygen species on alkali modified Ag are more

-

-

-

Chapter 6

240

likely bound to the substrate and stabilized by the presence of alkalis on sites released as a result of the oxygen-induced contraction of the overlayer. The rejection of the possibility of forming compound-like alkali-dioxygen species is based on the fact that the latter are rather stable and require a metal-like alkali overlayer, as will be discussed below. Secondly, substrates with a relatively large metal-oxygen adsorption bond strength will be considered, which are expected to compete with alkali adspecies for the bond formation with oxygen. A typical example for that is the behaviour of the alkali-modified Ru(0001). The oxygen-Ru adsorption bond decreases from 400 kJ/mol at low oxygen coverages down to 312 kJ/mol at 00 = 0.5 [162], whereas the alkali-Ru adsorption bond in the first layer varies within a wide range with increasing alkali coverage (from 320 kJ/mol down to 120 kJ/mol - see Fig. 4.8.). The large Ru-0 bonding strength at low oxygen coverages implies that the coadsorbed alkali adspecies are not expected t o compete with the substrate for oxygen bonding. This is demonstrated by the T P D and the work function data in Figs. 6.44. and 6.47. where a t low alkali coverages no evidence of a substantial alkali influence on the Ru-0 bonding is observed. The 0 2 T P D traces remain the same up to 00
-

-

-

-

-

6.4. Oxygen

24 1

oxygen-substrate bonding. Evidence of competition between the alkali additives and Ru for oxygen bonding and the formation of relatively stable alkali-oxygen surface complexes is observed at 00 > 0.6, i.e when less strongly-bound oxygen species are present. These oxygen coverages can be achieved above critical alkali coverages, the latter being within the range of the Df vs.0AM work function minima. The occurrence of direct alkali- oxygen interactions at 00 > 0.6 is evidenced by the following main experimental data: (i) the steep increase of the work function (see fig. 6.47.), which indicates the appearance of dipoles with a large effective negat,ive charge, accompanied by the appearance of a shoulder i n the retarding potential curves, indicating the formation of patches of various work functions on the surface [42, 139, 1401;

-

(ii) the appearance of coincident AM, 0 2 and A M 2 0 desorption at 850900 I<, supposing decomposition of a rather stable surface complex (see fig. 6.44.) [139-1411; (iii) the occurrence of new structures in the ELS spectra related to transitions in the alkali-oxide phase (e.g. the loss features at, 3.4 and 6.2 eV in fig. 6.48. [139, 140, 1421; (iv) the appearance of a variety of LEED structures, which indicates existence of ordered domains of varying alkali-oxygen composition; (v) the difference between the 0 1s binding energy of the oxygen adatoms involved in the formation of the surface complex (531.1 eV) and the oxygen adatoms bound to Ru (- 530 eV) [143]; (vi) the appearance of a n intense I<-0

stretch at

-

240 cm-l [141], and

(vii) the negative shift i n the binding energy of the alkali core levels of the order of 0.5 eV [127, 133, 1441. The exact stoichiometry of the alkaii-oxygen surface complexes has not been clarified yet. For saturated I< monolayer coverages, a stoichiometry 1:2 [141] or 1:l [143] has been proposed. Features in the HREELS, UPS and XPS spectra, supposing the existence of peroxo- or superoxo-like species are detected only at T < 200 I< for monolayer I< coverages [141, 1431, but they are not stable at 300 K. Parallel analysis of the available data on Na, I<, and Cs modified Ru(0001) shows that, irrespective of the type of the alkali, the oxygen adsorption at high alkali coverages proceeds via the following steps at 300 I<:

(1) Oxygen molecules dissociate with a high probability on the promoted surface, which is followed by a ‘spill over’ of the atomic oxygen forming an adsorption bond with Ru. These oxygen adatoms remove the alkalisurface interface plasmon (the 24 eV feature in Fig. 6.48.) and cause the initial invariance or reduction of the work function (see fig. 6.47.).

Chapter 6

242

S sk 0

a bare OK b 0.05 c 0.21 d 0.33

3

2

I

.

.

0

.2

.4

I

0

1

.a

.6

I

0

1.0

I

I

I

I

0.2 0.4 0.6 08 1.0

Fig. 6.47. Work function changes,Aa),as a function of 80 on Ru(0001) with different amounts of (a) K and (b) Cs. T, = 300 K.The insert in (a) presents the normalized RPCs for selected points of curve c (from ref. [139, 1401)

6.4. Oxygen

243

The absence of a substantial alkali oxygen coupling at low 60 when the oxygen-Ru bonding is predominating, is evidenced by the fact that the removal of the alkali ns-plasmon excitations (the feature at 2.5 eV in fig. 6.48.) does occur at low oxygen coverages but is observed at 0.3 < eo < 0.8.

-

(2) Alkali-surface complexes are formed. This requires critical oxygen and alkali coverages above which alkali-oxygen interactions will dominate over the interaction with the substrate. This process leads to the removal of the ns-plasmon excitations because these alkali states are involved in direct coupling with the oxygen adatoms. The formation of AM-0 complexes is accompanied by the appearance of a shoulder in the retarding potential curves and leads to compression of the alkali overlayer. Analysis of the behaviour of the RPCs during the increase of 80 and BAM or upon annealing of alkali-modified surfaces saturated with oxygen shows that the patches of AM-0 complexes are characterized by a lower work function than the remaining surface. Since the A+(Oo) data are taken from the low-current parts of the RPCs, the A+(do) plots in fig. G.47. reflect the work function changes of the AM-0 complexes due to an increasing O/AM ratio. The final WF value a n d the stoichometry of the AR4-0 complexes always reach those of a surface saturated with A M and 0 at 300 I< and uniforinly covered with complexes. As will be discussed below, typical alkali oxide compounds can be formed a t alkali coverages exceeding one monolayer because at B A M less than a monolayer the influelice of the substrate cannot be neglected. This influence is well demonstrated by the varieties in the behaviour of the oxygen treated surfaces precovered by an alkali monolayer. In the case of Ni [137, 1641, the Ni-0 bond formation is predominant (features indicating the presence of 02- and adsorbed species appear first). In the case of Ru [139-1431, because of its relatively large affinity to oxygen, a substantial fraction of oxygen participating in the formation of relatively stable surface complexes prefers to remain on Ru after decomposition of the surface complex a t 900 I<, whereas a large amount of alkali desorbs. In the case of Cu [163], the 0-Cs complex formed is distinctly different from that of oxides identified for thick Cs overlayers and high oxygen coverages. At Cs coverages up to one overlayer oxygen also interact with the Cu substrate and a large fraction remains on the surface after removal of Cs. In the case of Pt and Ag, the main fraction of the oxygen is stabilized by interactions with the alkali additives and desorbs together with the alkali during decomposition of tthe surface complexes. The influence of the substrate is also noticed by the decomposition temperature of the surface 650 I< for Ag [125], 800 K complexes which, on K modified surfaces is: for P t [134] and 900 I< for Ru [140, 1411. The third case concerns substrates which tend to form oxides and for these oxygen-alkali systems, at OAM up to a monolayer, the substrate-oxygen bonds remain dominating and an oxide phase is built above critical oxygen coverages Typical examples of such systems are alkali modified N i [135-1371, Fe [144,

-

-

-

-

Chapter 6

244

CI

W

40

36

32

28 24 20 16 ENERGY LOSS(&)

12

8

L

Fig. 6.48. Second derivative ELS spectra for increasing oxygen coverage, 00, on 0.33 K/Ru(0001) T, = 300 K (from ref. [140])

1501 and W [147-1491. Evidence of substantial alkali-oxygen interactions is found only on cases of inultilayer alkali coverages when an alkali oxide phase can be formed. The experimental data concerning oxidation of thick alkali layers deposited on metals show that the final oxidation stage is the formation of superoxides, the presence of the 0; ions being proved by means of photoelectron and metastable quenching spectroscopies [164-1671. The continuous oxidation of alkali films proceeds in three stages, as is clear from the UPS, MQS and WF data [164]. They involve: (i) dissociation and incorporation of 02-ions below the surface, while the surface preserves its alkali metallic character; (ii) transformation of the subsurface suboxide continuously to peroxide accompanied by adsorption of some oxygen atoms on the alkali surface; (iii) complete oxidation to superoxide which is accompanied by a substantial increase of the work function due t o 0, species on the surface.

6.5. Water

245

This final superoxide product is observed for Na, K, and Cs thick layers. More facile formation of alkali superoxides is observed during simultaneous alkali and oxygen deposition. Because of the high reactivity of the oxide ions, depending on the nature of the substrate, part of oxygen can be transferred and bound to the surface after annealing and decomposition of the bulk-like alkali oxides formed. Thus, thick alkali overlayers can be used as a reservoir for active oxygen species which facilitates the oxidation of substrates important to industry. A typical example is the recent interest in alkali promoted oxidation of semiconductors (e.g. Si) where the thick alkali oxide films transfer the active oxygen to the substrate upon annealing which leads to a drastic enhancement of the oxidation rate [168, 1691. Summarizing the behaviour of the alkali promoted metal surface with respect to dissociative oxygen adsorption, the following important conclusions can be drawn:

(1) With all substrates the introduction of alkali additives leads to an increase of the oxygen adsorption rate and the adsorptive capacity of the surface but do not affect the activation barrier for oxidation of the substrate, i.e. alkalis do not induce oxidation of the substrate; (2) The competition between the substrate and the alkali adspecies for oxygen bonding determines the preferred adsorption state of oxygen on the modified surface. (3) The strength of the attractive interactions between the coadsorbed alkalis and oxygen adatoms depends on the relative strength of the substrate0 and substrate-alkali bonding. Formation of stable alkali-oxygen surface complexes is observed when oxygen-alkali interactions dominate over that with the substrate. (4) The properties of the alkali-oxygen surface complexes are influenced by the substrate and are quite different than that of bulk alkali oxides. 6.5

WATER

The rising interest in the effect of alkali additives on water adsorption and interaction on metal surfaces is not restricted to catalytic processes only. Such studies also provide a basis for describing the structure and properties of double layers formed on the electrodes during electrochemical reactions. Surface science studies on the interaction of water with alkali-modified metal surfaces have been reported for several systems: Li/Ag(llO) [170], Li/Ru(OOOl) [171], Na/Ru(0001) [172, 175b], I
246

6.5.1

Chapter 6

Alkali Effect on the Molecular Adsorption State of Water

The reactivity of the alkali modified surfaces with respect to water varies with different substrates and shows a definite dependence on the actual alkali coverage and temperature. Thus, no water dissociation is observed for Li and Cs-covered Ag(ll0) [170, 177, 1781. On the other metals under consideration up t o certain alkali coverages also only adsorption of molecular water is favoured. The effect of alkali additives on the molecular adsorption state is best illustrated in ref.[l80] where TPD, WF, HREELS and ESDIAD studies reveal that the presence of Cs (by analogy with the other coadsorbate systems) leads to the creation of locally-promoted sites where the adsorption molecular state exhibits a completely different behaviour compared with that on an alkali-free surface. The major characteristics of the water molecules adsorbed on the promoted sites can be summarized as follows. Promoted sites are oc50 K cupied first and the desorption temperature from these sites is by higher than that from the first water adsorption layer on a clean surface. The occupation of the promoted sites causes an increase in the work function (opposite t o the decrease induced by water adsorption on a clean surface) , which indicates that the promoted and non-promoted water molecules have opposite polarizations. The vibrational bands of the promoted molecules are characterized by a narrow 0-H stretching mode at 3180 cm-' corresponding to the low frequency component of the triplet (associated with the effect of hydrogen bonding on the 0-H band) observed as a wide peak at 3370 cm-' for water on an alkali free surface. In addition, the libration peak at 745 cin-' (associated also with hydrogen bonded water clusters) is not observed when water adsorbs on the promoted sites. ESDIAD patterns indicate a difference in the orientation of the water molecule adsorbed on the promoted sites. After saturation of the promoted sites which are estimated to be less than 3 per Cs adatom, further occupation of non affected sites leads to features typical of water adsorption on an alkali-free Cu(ll0): a T P D peak at 180 K,a decrease of the work function, an appearance of the dominant HzO libration mode at 745 cm-' and the wide 0-H triplet mode at 3360 cm-'. Similar stabilization of negatively-charged promoted water molecules has been evidenced by the H 2 0 T P D and WF data for Cs/Ag [178], Na, I< and Cs coverages on Ru(0001) [172, 175b], and N a and K on P t ( l l 1 ) [174, 175aI. The only plausible explanation of the obtained WF, HREELS and ESDIAD data [172-175, 1801 obtained, is an alkali-induced reorientation of the water molecule due t o direct interactions between the coadsorbates. The experimental results support the theoretical calculations where promoted water molecules are bound to the alkali metal with a molecular plane tilted to the surface normal and hydrogen atoms pointing towards the surface [182]. Thus, the observed change in sign of the dipole moment associated with promoted water molecules is dominated by the change in orientation of the molecule rather than a charge transfer. The absence of the libration 745 cm-' mode and the triplet 0-H structure indicates that the reoriented promoted molecules exist as monomers because of the hindrance to the formation of hydrogen bonds. Recently, an attempt has been made to describe the strength of the ef-

-

-

-

6 . 5 . Water

247

fect of different alkali metals on the H 2 0 reorientation and the role of the substrate [175]. In these studies the dipole moments of the HzO molecules, affected by N a and K on P t ( l l 1 ) [175a] and by Na, K and Cs on Ru(0001) [175b], are compared and discussed. It has been shown that the HzO dipole moment which is indicative of the degree if tilting of the H 2 0 molecules increases with increasing the ionicity of alkali metal, i.e. in the sequence: N a , K , Cs. Comparison of the degree of HzO tilting for the same alkali species but different substrates was also made for Ru(0001) and P t ( l l 1 ) modified by Na or K [175a]. It has been found that the alkali-induced tilt is stronger, the weaker the H20-substrate interaction is, i.e for the particular two cases the reorientation of HzO on alkali-modified P t ( l l 1 ) is larger. A higher degree of alkali-induced reorientation generally correlates with more difficult dissociation of HZO. 6.5.2

Alkali Induced Dissociation of Water and Stabilization of the Dissociation Products

Excepting Li and Cs-covered Ag (110) [170, 1781, all other alkali modified surfaces under consideration become reactive for HzO dissociation above a certain critical submonolayer alkali coverage. The latter depends on the type of the alkali and substrate and the adsorption temperature (e.g. at 100 K for N a and I( on Ru(0001) the critical coverages are 0.06 [175b], for Cs on Ru(0001) - 0.11 and for Na and I< on P t ( l l 1 ) - 0.1 [173-175a]). Recently, a model has been proposed where t8hedifference in the critical alkali coverages, required for dissociation to occur, is correlated to the alkali-induced degree of H 2 0 reorientation. According to this model the stronger tilting of H z 0 leads to weakening of the HzO-substrate interactions and impedes the dissociation of H 2 0 assisted by the substrate [175]. The XPS, UPS [174, 175, 1811, HREELS [173] and ESDIAD [172, 1731 studies show convincingly that the alkali induced dissociation of H 2 0 leads to the formation of OH species. They are identified by:

-

(i) an 0 1s peak at 530.9 eV (the 0 I s binding energies of molecularly adsorbed water and ice layer are 532.2 and 532.9 eV, respectively) [174];

-

5 and 9 eV due to OH l x and 3a orbitals ( H 2 0 (ii) two UPS peaks at adsorbed on a clean surface is identified by peaks at 6.0, 8.1 and 12.0 eV corresponding to l b l , 3Ul and l b z molecular orbitals) [174-176, 1811; (iii) vibrational bands in the 1340-1500 cm-' and 3570-3650 cm-' regions associated with AM-OH bending and O-H stretching modes in an AM-OH complex similar to that observed for alkali hydroxyls. At low adsorption temperatures the dissociative adsorption is accompanied by molecular adsorption, followed by the build-up of an ice layer at high H20 exposures. The molecular adsorption state is found to be stabilized regardless of the presence of OH species. This state is characterized by a negative dipole and molecular orbital binding energies slightly higher than those of HzO on clean P t ( l l 1 ) . For K / P t ( l l l ) , complete desorption of the molecular

248

Chapter 6

0.L

I

I

1

I

7

,

Pf (1111 + K T Hfl T==K 30 x1U' h s H20

MONOLAYER

L A -26

O O

a1

I

I

a2 POTASSIUM COVERAGE

0.3

OL

&

Fig. 6.49. Saturated OH coverage versus the K coverage on P t ( l l l ) , as a result of HzO adsorption on I<-covered P t ( l l 1 ) . T, = 305 I< (from ref. [174])

-

state occurs at 270 K, i.e. at a temperature of 70 K , which is higher than that for K-free P t ( l l 1 ) . This indicates stabilization of the molecular state coadsorbed with I< and OH. Consequently, at elevated adsorption temperatures the tendency is towards dissociative adsorption. Thus, studying water adsorption at elevated temperatures enables getting information on the alkali additive effect on the dissociative adsorption alone. In analogy to the other systems described in this Chapter, the reactivity of the surface for dissociative adsorption is determined by the alkali coverage. Fig. 6.50. shows that the OH coverage is linearly related to the K coverage on P t ( l l 1 ) with a stoichiometry close to one OH group per K adatom.This agrees with the view that compound-like surface complexes are formed locally. The critical coverage necessary for the promotion of HzO dissociation indicates that the formation of these complexes requires a critical population of the alkali ns-valence states which are involved in the bonding with OH. Fig. 6.50. shows the expected increase of the work function induced both by the adsorbed electronegative OH groups and the coadsorbed water molecules a t low temperatures. This result- indicat,es that the molecular adsorption state has a negative dipole even in the presence of OH, species. This has been also proved by following the work function changes during desorption of the molecular state after annealing to 270 K. The cross-over of the two Ad vs. water exposure curves and the steeper initial slope at 300 K in fig. 6.50. indicates that the formation of OH is faster at 300 K, i.e. the dissociation process is thermally activated. Compared with the OH, species formed in the presence of coadsorbed oxygen where the disproportionation reaction occurs

6.5. Water

0

2

L

249

6

10

8

12

Exposure ( lo-' Pa. s 1 2.0

1

1

I

I

8

10

I

1.5

8

a

1.o

0.5 0' 0

1

2

I

I

4

6

Exposure

I lo-'

J

12

Pa s )

Fig. 6.50. Changes in the work function, A$, induced by (top) H2O adsorption on Pt(ll1) at 105 I< and (down) OH (T, = 305 I<) and (OH + H 2 0 ) (T, = 100 I<) adsorption on K / P t ( l l l ) (from ref. [174])

-

at 200 I< (see Section 5.5.), the OH, species on alkali modified surfaces are remarkably stable. They decompose at temperatures above 500 I< so that their stability is similar to that of bulk I
250

Chapter 6

Fig. 6.51. (a) Relative i1~tensit.yof t,he Pt 4f levels of the oxide layer and the metallic Pt substrate as a function of the anneal temperature. (b) Pt 4f level binding energy shift with respect t o the Pt metal ( E B = 70.9 eV) as a function of the annealing temperature (from ref. [183])

HzO is easier on substrates which are more active towards H 2 0 dissocia.tion by themselves. Comparing P t ( l l 1 ) and Ru(0001) [175a], it has been shown that alkali-induced dissociation of HzO on Pt( 111)leads only to OH, whereas alkalienhanced dissociation of H 2 0 on Ru(0001) results in an oxygen secondary product as well. The studies on HzO interaction with thick K layers on P t ( l l 1 ) have definitely shown that bulk-like K also dissociates water at low temperatures, e.g. 125 K, so that the major contribution t o dissociation is supposed to be due t o direct alkali-water interactions. These studies have also helped to clarify the type of the stabilized water molecules in the presence of OH, and K, at low temperatures. The water molecules on the surface covered by OH and If are identified as hydration water stoichiometrically linked to KOH. This hydration water has a higher adsorption energy than either H 2 0 on transition metals or on alkali-promoted surfaces where no water dissociation occurs (described in the previous Subsection) [173, 1761. Recently, it has been reported that KOH formed as a result of a reaction between a thick I< adlayer and water on P t ( l l 1 ) is very active with respect to Pt oxidation at 400-480 I< under UHV conditions. This has been evidenced by the 2.7 eV shift of P t 4f levels t,o higher binding energies, as illustrated in fig. 6.51. This is really surprising because no Pt oxide formation has been detected in the presence of mixed oxygen-alkali overlayers (see Section 6.4.). Thermodynamic considerations favour the formation of a product of the type

6.6. Hydrogen

251

of KzPt(OH)G, which is stable up to 500 K [183]. Summarizing the results on the interaction of water with alkali modified metal surfaces, the following conclusions can be drawn: (1) The alkali induced stabilizat,ion of the molecular adsorption state when dissociation is still not favoured, is apparently due to electrostatic interactions between the H 2 0 dipole and the dipole of the alkali adspecies. These interactions cause a reorientation of the water molecules and favour hydration of the alkali species at the expense of hindering the formation of hydrogen bonded clusters. The local interaction between HzO* and alkali adspecies is indicated by the sequential filling of the promoted and non-promoted sites at low alkali coverages.

(2) Excepting Li and Cs-covered Ag, for all other metals under consideration, the presence of alkali additives induces the dissociation of H 2 0 above certain critical alkali coverages. The stoichiometry alka1i:OH is close t o unity and the stability of this surface complex is simCar to the stability of the (AM OH) compounds. This indicates that the OH stabilization is predominantly via direct AM6+ - OH6- interactions. This is in good agreement with the observation that the H2O dissociation is favoured above critical alkali coverages. In analogy to the behaviour observed in the other alkali coadsorbate systems, the stabilization of the dissociation product OH via direct interactions requires a certain occupancy of the alkali valence ns-states which depend on alkali coverage. That is why a governing factor for OH formation becomes the alkali coverage.

(3) The detailed mechanism of OH formation on alkali-modified surfaces is different from that of OH formation on oxygen-covered surfaces described in Section 5.5. The coadsorhed oxygen does not affect the HzO bonding (it remains via 0 towards the surface) hut it initiates weakening of the one of the O-H bonds by interaction with H eventually leading to dissociation at elevated temperatures. On contrary, the H20-substrate bonding is substantially altered in the presence of alkali adspecies, the degree of alkali-induced reorientation being determined by the ionicity of the alkali metal and the nature of the substrate. The principal difference between the OH species formed in the presence of alkalis and oxygen additives, lies in their stability: the AM6+OH6- complexes are stable up t o 500 I<, whereas the OH species formed on an O-covered surface desorb below 350 I<.

(4) The stabilization of a molecular adsorption state on AM6+0H6--covered surfaces is associated with hydration water. The structural configuration of hydration water has not been clarified yet. 6.6

HYDROGEN

Interaction of Hz with alkali-modified surfaces has been much less studied compared t o CO, although H2 is the second reactant participating in the important Fischer - Tropsch and Ha.ber - Bosch syntheses. The observed

Chapter 6

252

effect of the alkali additives on the product distribution in Fischer - Tropsch synthesis suggests that the presence of alkalis leads either to the reduction of the hydrogen concentration on the surface or to the creation of new hydrogen adsorption states which makes hydrogen less accessible to C-H bond formation [184]. As has already been described in Section 5.4., excepting Al, H2 adsorption on the metal surfaces, which will be considered below, is dissociative. Thus, in all current studies of €I2 adsorption on alkali-modified Fe [28, 1851, Ni [186, 1871, Mo [188], Pt [189, W [190, 1911 and Ru [192] the effect of alkali additives on the dissociative adsorption of hydrogen is considered. 6.6.1

Alkali Effect on the H2 Dissociative Adsorption and Saturation Hydrogen Coverage

The presence of alkali additives always leads to a reduction of the dissociative adsorption rate and the saturatioii coverage achieved upon adsorption. The only exception is the earliest study on hydrogen adsorption on I<-covered Fe(100) in which an increase of the hydrogen sticking coefficient and saturation coverage with increasing potassium coverage [28] is reported.

I '

Temperature ( K 1

Fig. 6.52. Hz TPD spectra from K-covered N i ( l l 1 ) for various K coverages. The insert illustrates the reduction in saturation hydrogen coverage as a function of OK (from ref. [187])

The strength of the inhibition effect, observed in the other studies varies with the different substrates. This difference is well illustrated in figs. 6.52. and 6.53. for K-covered Ni(ll1) and K-covered W(100). In the case of alkalimodified Ni(100) and Ni(ll1) [186, 1871 and P t ( l l 1 ) [189], the initial sticking

6.6. Hydrogen

253

coefficient for dissociative adsorption, So, and the saturation hydrogen coverage decay exponentially with increasing alkali coverage. As calculated from the experimental OH vs. OK plots in ref.[189], the rapid decrease of hydrogen coverage obeys the relationship: OH = exp(-168~). A similar relationship is valid for I<-covered Ni(ll1). This means that hydrogen dissociative adsorption is almost completely inhibited at t9K 5 0.1 on these surfaces, implying 10 H sites blocked by one K adspecies. The inhibition effect of K on the H2 dissociative adsorption on modified W surfaces, illustrated in fig. 6.53., is less severe. The K-covered stepped W(100) surface has a reduced reactivity but preserves 20 % of its adsorptive capacity a t 300 K even at a monolayer K coverage (fig. 6.53). It should be clarified that in fig. 6.53. the K coverage in the first layer is accepted as unity, whereas in fig. 6.52. the K coverage is given by the usual monolayer definition, used in this book, which is K adatoms per substrate surface atom. One of the reasons for the observed difference in the strength of the inhibition effect should probably be associated with the different affinity of the substrate surfaces to hydrogen. Indeed, the W-H adsorption energy is larger than the Pt-H and Ni-H ones. Another factor that might contribute to the inhibition effect is the different accommodation ability of the substrates. Thus, it is possible that on Pt and Ni surfaces, the diffusion of the H2 precursor, i n order to lose energy and accommodate before dissociation, is more important than on the W surface. Consequently, the introduction of an additive which constrains H 2 surface diffusion should inhibit the dissociative hydrogen adsorption on P t and Ni more efficiently. In the case of alkali additives, the least probable explanation of the reduced dissociation rate is the blocking effect leading to a reduction in the number of the sites favourable towards atomic hydrogen adsorption. The suggestion that the alkali additive acts in a way t o eliminate the dissociation channels rather than the atomic hydrogen adsorption sites has recently been proved t o be true by studying the effect of Li, N a or I< on atomic hydrogen adsorption on Al(100) [193, 1941. It has been shown that the modified A1 surface preserves its adsorptive capacity with respect to atomic hydrogen adsorption and, as will be discussed below, both the hydrogen adatoms and the alkali species are mutually stabilized on the Al( 100) surface.

-

-

6.6.2

Alkali Effect on the Adsorption State of the Hydrogen Adatoms

The effect of alkali additives on the stability of the coadsorbed hydrogen atoms is best depicted by the Hz T P D spectra which always shift to higher temperatures with increasing alkali coverage (see fig. 6.52.). Table 6.7. presents selected activation energies and pre-exponential factors for hydrogen desorption, as evaluated from available H? T P D data. The desorption kinetic results in Table 6.7. show that there is a compensation effect, i.e. the pre-exponential factor increases with the activation energy for desorption. An inspection of the current H 2 T P D data on clean and alkali-modified metals have shown that tche magnit,ude of the alkali-induced higher temperature shift depends on the desorption temperature of hydrogen from a clean surface. For substrates where the H desorption from a clean surface occurs at

Chapter 6

254

I2 10

08

8

\

0.6 03

02 00

Fig. 6.53. The effect of increasing I< coverages on the Hz initial sticking coefficient (solid line) and saturation coverage (dashed line) relative to a clean stepped W( 100) surface (from ref. [191])

T < 500 K (Al, Pt, Ni, Fe), the alkali-induced shift does not exceed 500 I(. It should be pointed out that this temperature represents approximately the temperature range of decomposition of dry alkali hydrides. The fact that it happens at alkali coverages when the alka.li overlayer becomes metal-like, indicates the possibility of the formation of alkaIi hydride-like species. The magnitude of the alkali-induced temperature shift is largest for Al( 100) from 350 K to 500 I<, whereas for the other metals where Hz desorbs at higher temperatures from a clean surface, the total shift is smaller. Table 6.7. Activation Energy, E,, (in kJ/mol) and Pre-exponential Factor, u, (in cm2/s) for a Second Order Hz Desorption Process SURFACE

E L 3

Pt(ll1) 0.08 K / P t ( l l l ) 0.16 I < / P t ( l l l ) 0.23 I < / P t ( l l l ) Ni( 111) O.l5I
42 62 121 168 89 144

v 1.4 x -

1.7 1.7 x 7.5 x

Ref. [189] ~ 9 1 ~391 ~391 [187] [I871

6.6. Hydrogen

255

In the case of substrates, e.g. stepped W(100) [191], where H2 desorption from a clean surface completes at T > 540 K , the alkali-induced shift to higher temperatures is less than 50 K. These results indicate that hydrogen stabilization via direct interactions with the alkali additives will be favoured on substrates which bind hydrogen more weakly so that alkali can compete for hydrogen bonding. The best example of such a substrate is A1 where the formation of a K6+H6- surface compound is evidenced by the characteristic for alkali hydrates vibrational bands: Na(K)-H stretching frequencies at 1850 (1650) cm-' and 1715 (1500) cm-' and a Na(K)-H deformation band at 800 (775) cm-' [193]. These bands are clearly observed at high alkali coverages when the decomposition of the alkali hydrate-species leads to simultaneous AM and H2 desorption. The formation of K6+H6- species is also supposed t o take place on the basis of the ESDIAD data for K-covered Ni(ll1) [187] and Ru(0001) [192]. However, a straightforward identification of the alkali hydrate-like species in the other studied systems is more difficult because of the very low hydrogen coverage that is obtained upon H2 dissociative adsorption. In the case of H coadsorbed with I< on W(100) [191], where K cannot compete successfully for hydrogen bonding with the substrate, there is no evidence of substantial I<-H interactions leading to alkali hydrate formation. An interesting finding is that the presence of K on W leads to a decrease of the H effective negative dipole. This behaviour is opposite to the usual one when electron acceptors coadsorb on promoted sites (for alkali coverages before the work function minimum). An explanation of this effect is that H has a negligible polarizability, so that the measured effective dipole moment is mainly associated with the increase in the K dipole due to the buffering effect of the H adatoms. This buffering effect is responsible for a certain stabilization of the I< overlayer at high I< coverages (appearance of a I< peak at 500 I<) but even then no coincident I< and Hz desorption is observed (Hz desorption from these surfaces occurs at 600 K ) . In this case, one cannot decide whether the slight stabilization effect of I< is due to direct interaction with H or the higher temperature shift is a result of the hindrance to the recombination process by the presence of an additive. The major findings about the effect of alkali additives on the reactivity of metal surfaces with respect to hydrogen adsorption can be summarized as follows:

-

-

(1) The introduction of alkali additives always reduces the dissociation adsorption rate of Ha substantially. Since the strength of the inhibition effect varies with the different substrates. This effect on the dissociation rate is most likely due t o the reduction of the lifetime of the precursor for dissociation. (2) The quenched Hz dissociation on alkali covered surfaces leads to the reduction of H atomic concentration on the modified surfaces. (3) The hydrogen atomic adsorption rate is hardly affected by the presence of alkalis, so that the same hydrogen coverage can be achieved on a modified surface upon direct adsorption of atomic hydrogen.

Chapter 6

256

(4) The hydrogen atomic adsorption state is stabilized by the presence of

alkalis. The magnitude of the stabilization effect and the tendency to form alkali hydrate-like species on the surface is determined by the competition between the substrate and alkalis for the hydrogen bonding. 6.7

NITROGEN

The effect of alkali additives on Nz adsorption is of relevance for real catalysis where alkali promoters are found to increase the activity of the iron catalysts used for ammonia synthesis considerably [195]. As outlined in Section 5.3., the typical Nz adsorption rate is very low and Nz dissociation is the ratelimiting step in ammonia synthesis. Experimental data for Fe and Re (which are good catalysts for ammonia synthesis), as well as for other metal surfaces, have shown that the molecular Nz adsorption state is weakly bound, whereas the atomic N adsorption state is strongly bound. The height and width of the crossing region between the potential energy surfaces of these two states is a dominating factor regarding the reaction rate of dissociation. Two molecular adsorption states, assigned as Y - N ~and a - N z are observed, the latter being distinguished only on Fe(ll1) surfaces which are the most active in respect of dissociative nitrogen adsorption. As described in Section 5.3., the y-Nz state is bonded normally to the surface with a predominant contribution of the Sa/metal donation component to the bonding. a - N z detected on Fe(ll1) is described as a side-bond configuration with a major contribution of the backdonation metal/lr* component to the bonding. The a-state is supposed to be a precursor for dissociation. Recent molecular beam studies have shown a rapid increase of the N2 dissociative sticking coefficient with the increase of the kinetic energy of the incident N 2 beam [196, 1971. This has been used as an indication that the NZ dissociation can proceed via a tunneling mechanism, i.e. Nz tunnels from the molecular potential well through the barrier to the atomic well. This mechanism can explain the possibility for NO dissociation on surfaces where the side-bonded configuration is not observed. In the forthcoming Section the effect of alkali additives on both the molecular states and on the dissociative nitrogen adsorption will be considered.

6.7.1

Alkali Effect on the Nz Molecular A d s o r p t i o n Kinetics and Nz Molecular A d s o r p t i o n State

The alkali effect on the adsorption kinetics and energetics of the 7-Nz state is best illustrated on substrates where no other molecular adsorption form is detected, e.g. K/Ru(0001) [198] and K / N i ( l l l ) [199]. For both cases K-preadsorption inhibits Nz adsorption and Nz adsorption is found to be completely suppressed a t OK 2 0.1. This suppression is accompanied by a decrease in the desorption energy by 4 kJ/mol (the Nz-clean surface bonding on the substrates under consideration is rather weak 20 kJ/mol). The UPS and HREELS data [198] have shown that the Nz-induced features on clean and K-covered Ru(0001) are qualitatively the same, which indicates a lack of substantial interactions between the coadsorbates. This is also confirmed by

-

-

6.7. Nitrogen

257

the lack of substantial changes in the binding energy, width and int,ensity of the K 3p core level. The observed alkali-induced reduction of the y-Nz adsorption rate until complete inhibition of adsorption at relatively low alkali coverages is reached and accompanied by destabilization of the adsorption bond is ascribed to the repulsive alkali-Nz interactions (both I< and y-Nz dipole moments are with the positive end outwards) and the reduced u donation because of the enhanced electron density on the surface in the presence of coadsorbed potassiumb The repulsive radius around the K species is derived to be approximately 5 A , i.e. they extend to the next-nearest neighbours. The accommodation chance of the surface being able to provide is probably affected because of a possible unfavourable reorientation of the approaching molecules by the K dipoles which perturbs the Nz 5a-surface coupling. The recent results on K adsorption over Nz-covered Ni(ll1) [199] have shown t h at in the reverse adsorption mode, the K adsorption process is more effective in overcoming the repulsive barrier and in activating short-range attractive interactions with the coadsorbed Nz molecules. In this adsorption mode, K induces a substantial stabilization of the N2 molecular adsorption state at alkali coverages beyond the Ad vs.81c minimum (the Nz T P D peak shifts from 80 I< on a I<-free surface to 100 I< at moderate K coverages (OK 0.2), and a N z peak at 250 I< is observed for 0.36 I
-

-

-

Chapter 6

258

-

the N-N bond order. The annealing experiments have shown that the y*-Nz state desorbs at 120 K, i.e. at temperatures by 30 I< higher than that of Y-Nz. The removal of the y*-state leads t o an increase in a-Nz band intensities implying that a fraction of the y-state can be converted into an a-state. Probably, because of the reduced space for side-on bonded Nz at d K = 0.43, the y*-related 1670 cm-' band predominates. N

Table 6.8. Vibrational Bands (in cm-') of Molecular Adsorption States of Nz in the Presence of Different Amount,s of Adsorbed I<, OK, (in ML) o n Fe(ll1) [203]

0 0.05 0.11

2100 21001

-

21001

-

-

-

1415 1410 -1405

1 t

1165 11651

(obscure) 1600-1800 1400 1 1160 (very weak) 0.3 1720 13851 11501 0.43 1670t 1340 1 t and 1 indicate an increase and a decrease of the band intensity, respectively 0.16

-

N

An interesting finding is that at OK 5 0.16 the alkali induced increase in the amount of the n-bonded N2 is distributed between the a - N 2 and a f - N 2 (promoted state in close proximity of K) in a way that three a states are created for every a* promoted state. This is attributed to the longer range effect leading to a more effective y to a conversion. This is explained by a Kinduced reduction of the activation barrier for this conversion process, which leads to a substantial increase of the equilibrium coverage of the &-state that serves as a precursor for dissociative adsorption at higher temperature. The stabilization of the large amount of n-bonded Nz in the presence of I< is due to the enhanced charge density on the surfacc which reduces the self-poisoning effect observed on a I<-free surface. 6.7.2

Alkali Promoted Nz Dissociative Adsorption

By analogy with CO, alkali promoted N z dissociative adsorption has been reported for substrates where dissociative NZ adsorption occurs also on alkalifree surfaces. This fact implies that the substrate determines the ability of the system to adsorb Nz in the n-bonded a-Nz state which is a precursor for dissociation. Thus, the introduction of K on Fe(ll1) and Fe(100) [201, 2041 leads to an increase of the initial sticking coefficient for dissociative Nz adsorption, U O ,a t T, = 430 I<. As shown in fig. 6.54., 6 0 increases linearly with increasing I< coverage, similarly to the case of dissociative 0 2 adsorption discussed in Section 6.4.

6.7. Nitrogen

259

Fig. 6.54. Changes in the initial sticking coefficient for Nz dissociative adsorption on I<-covered F e ( l l 1 ) as a function of I< coverage. T, = 430 K. (from ref. [201])

However, the enhanced rate of dissociative N2 adsorption, does not lead t o a substantial increase of the saturated atomic N coverage on the modified Fe surface. This indicates that K does not affect the stability of the N atomic state substantially.Here, it should be pointed out that at the reaction temperature of these studies (430 K ) l OK cannot exceed N 0.2 ML, i.e. it is less than the K coverage corresponding to the minimum of the DF vs.O~ 220 K the adsorption in the a - N 2 state cannot be explained by a y - N z (Y - N2 precursor mechanism, the K promotion should operate via the direct channel for a-Nz adsorption [205]. 4

260

Chapter 6

Since the direct channel for a-Nz adsorption is probably an activated process, the presence of alkali additives is expected to reduce the activation barrier for Q-N2 adsorption. This can be explained taking into account that the presence of alkali additives perturbs the potential energy surface in a way which enhances the ability of the surface towards coupling with the 1 ~ N2 * orbital. The increased equilibrium molecular a-Na coverage and its stabilization in the presence of K account for the increased dissociation rate at elevated temperatures, because the ratio dissociation rate/desorption rate becomes larger. However, the weakening of the N-N bond of the a*-N2 might contribute t o a decrease in the activation barrier towards dissociation, but this effect is negligible because no substantial changes in the apparent activation energy of ammonia synthesis are observed when an alkali promoter is introduced. The negligible perturbations in the activation energy for dissociation, EC;,,, , correlate with the view that E:,,, is determined exclusively by the potential energy surface of the dissociation product [77]. In the case of nitrogen on Fe, nitrogen adatoms are strongly bound to Fe and their adsorption state is not affected by the presence of K up t o moderate OK. Summarizing the data concerning the effect of alkali additives on the adsorptive properties of the modified surfaces with respect to nitrogen the following main conclusions can be drawn: (1) The presence of alkali additives inhibits N2 adsorption in a u-bonded y-N2 state which behaves like an electron donor. This is due to the repulsive barrier introduced by the alkali adspecies which acts at a radius 5 8, and is mediated through the substrate by the charge density of redistribution.

-

(2) The presence of alkali additives enhances the adsorption rate, stability and saturation coverage of the .Ir-bonded a-N2 state which behaves like an electron acceptor. This is a result of the alkali induced enhancement of the charge density on the surface which leads to perturbations in the potential energy contour that are favourable towards the a-N2 state.

(3) Since the a-N2 state serves as a precursor for N2 dissociation, a consistent increase of the Nz dissociation adsorption rate is observed at high temperatures in the presence of I<. The major contribution to the increased N2 dissociation rate is supposed to be due to: (i) the enhanced equilibrium coverage of the Q-N2 state due to a Kinduced concomitant reduction in the activation barrier to direct a-Nz adsorption and (ii) an increase of the a-N2 desorption energy.

6.8. Organic Compound

6.8 6.8.1

26 1

ORGANIC COMPOUNDS Interaction of Unsaturated Hydrocarbons with Alkali Modified Surfaces

T h e major interest in understanding the effect of alkali promoters on the interaction of hydrocarbons with single crystal metal surfaces is due to the fact that they are products of the important Fischer-Tropsch synthesis, where alkali promoters are widely used. Unfortunately, only several studies have appeared recently with specific reference to the CzH4 interaction with alkali covered P t ( l l 1 ) [20&210] and C6H6 and C4Hs interaction with K-covered P t ( 111) [211, 2121. Generally, since the hydrocarbon molecules considered above serve as net R or a- donors during the adsorption bond formation (see Subsection 5.G.l.), it can be expected that the electropositive additives would act as inhibitors with respect to the surface reactivity towards these molecules.

< w

a 4

c2%

\

O

n I-

A

0.07

A 100

300

300

500

TemmKahn I K

0.05

0.96 700

900

3

300

500

700

Temperature

( K

eoo 1

Fig. 6.55. C2H4 (a), H2 (b) and C2Hs (c) TPD spectra recorded after ethylene adsorption on clean and I<-covered Pt(ll1). T, = 100 I<. OK values are relative to the saturation I< coverage in the first layer taken as unity. The real coverage in M L (I< adatoms per surface atom) is 3.03 times smaller (from ref. [206])

The molecular adsorption state of C2H4 on P t ( l l 1 ) a t T 2 100 K consists only of a di-a bonded species with a significantly reduced C-C bond order. Very weakly R-bonded CzH4 species which retain their initial sp2 hybridization are found a t T < 50 I< [209]. Progressive annealing of a C2H4 layer adsorbed at 100 K leads to:

(i) fractional (- 54 %) molecular desorption at 280 K ,

262

Chapter 6

-

(ii) decomposition viaseveral steps ofCCH3 (Bl-Hz T P D at 310 K), C,H(@ZH2 at 510 K) and C formation (a broad p3- H~peak at 600 K) (- 44 %) and

(iii) hydrogenation to C ~ H G (CZHG T P D peak at

-

300 K) (2 %).

Fig. 6.55. presents the effect of increasing amounts of K on the CzH4, C~HG and Hz T P D spectra. It is obvious that, at the expense of the decreased amount of decomposed and hydrogenated CzH4, a new weakly-bound molecular state appears a t 150 K.

-

a

Fig. 6.56. T h e effect of increasing I< coverages on the amount of decomposed CzH4. BK(sat.) = 0.33 M L (from ref. [208])

Fig. 6.56. illustrates the relative amount of decomposed ethylene as a function of I< coverage on P t ( l l 1 ) . Comparison of the data in figs. 6.55. and 6.56. show that the dramatic decrease in the amount of decomposed C2H4,is accompanied by substantial changes in the relative amount of H2 desorbing under the three peaks. In addition, a new peak, denoted as p', in fig. 6.55. and Table 6.9. arises. The relative changes in the P-H2 peak intensities are summarized in Table 6.9. It should be stressed that, in order to evaluate the absolute Hz desorbing amounts under the T P D peaks, one should consider the dramatic changes in the Hz evolution which, as illustrated in fig. 6.56., drops by 80% OK 0.033 (corresponding t o OK/O,(sat) = O . l ) , changes negligibly at 0.033 < OK < 0.18 and drops to 0 at OK 0.24. The HREELS and UPS [207-2101 data identified the alkali-induced weaklybound molecular adsorption state as a ?r-bonded CzH4 species, the C-C

-

-

263

6.8. Organic Compound

Table 6.9. The Amount of Hz Desorbing Under each TPD Peak (in %) Relative to the Total Desorbing Amount at a Given K coverage, OK, (in ML) o n a Pt(ll1)

Surface. (from ref. [206, 2081) OK

P1

0

27 (310 I<) 0 0 0 0 0 0

0.017 0.04 0.10 0.13 0.16-0.23 > 0.24

18’ 0 27 (350 I<) 27 (380 K ) 27 (420 I<) 10 0 0

P2

46 (510 K) 46 73 73 (520 I<) 90 100 (570 I<)

183

27

(w

600 I<)

27 0 0 0 0 0

0

bonding being almost undistorted compared to a gas phase molecule. As outlined above, the major contribution to the x-bonding belongs to the donor component. Since the donor bonding will be inhibited i n the presence of electropositive additives, it might be compensated to a certain extent by enhancement of the x* acceptor interactions. However, the fact that the C-C bonding remains almost unperturbed, indicates that such compensation is negligible. Most probably, the alkali induced appearance of the n-bonded C2H4 is due to the hindrance of the x- to di-a C2H4 interconversion which occurs readily on an alkali-free surface [210]. This is confirmed by the fact that the total ethylene coverage remains unchanged up to OK 0.13, although a t this alkali coverage, the fraction of the di-a species is negligible. The alkali inhibition of the adsorption in a di-a configuration (completely removed at OK 0.12) fits well in a steric blocking model where the modifier's nearest neighbours (- 8 per K adspecies) are supposed to become unfavourable adsorption sites. Since the di-a bonding configuration requires two unperturbed P t atoms, it has turned out that they are no longer available at OK 2 0.12 (for randomly-distributed I< adspecies) [206]. The inhibition of adsorption in the di-a state leads to the observed drastic reduction in the decomposed fraction because this molecular state serves as a precursor for dissociation. The I(-induced dramatic decrease of ethylene decomposition is also accompanied by changes in the stable intermediate and the decomposition pathway -Ha), HREELS as is evidenced by TPD (appearance of 8' at the expense of and SIMS data [206, 2081). It turns o u t that the presence of alkali stabilizes an ethylidene (CHCH3) intermediate on the surface, which converts to ethylidyne CCH3 at T > 350 accompanied by evolution of P * - H 2 . The higher temperature of P*-H2 desorption as compared to PI-H? indicates that K increases the activation energy towards CCH3 formation. The formation of CCH3 is completely inhibited at OK > 0.15. The small fraction that decomposes above this coverages is supposed to involve sequential breaking of the C-H bonds in the K stabilized CHCH3 with nearly the same activation energy. The inhibition of the formation of CCH3 and the stabilization of CHCHs can be attributed

-

-

264

Chapter 6

to the blocking effect of I< because CCH3 is bonded via three s-bonds to the surface (against two bonds for CHCH3). At 0.13 < OK < 0.25 the total capacity of the surface linearly decreases and the surface is completely deactivat,ed in regard to C2H4 adsorption at t91< > 0.25 [207]. This agrees extremely well with the steric blocking effect because the metal-like K overlayer is completely inactive with respect to adsorption of C2H4. Generally speaking, the same inhibition effect on the decomposition reaction and destabilization of the molecular adsorption state has been reported for C s H ~ a n dC4H8 adsorption on K-covered Pt(ll1) [211, 2121. The alkali-induced destabilization effect on the molecular adsorption state of olefin has also been reported for ethene and phenylethene adsorption on Cs-covered Ag [213, 2141. Combined with the promotion effect on oxygen adsorption and dissociation on Ag surfaces (discussed in Section 6.4.), this results in the activity of the coadsorbed oxygen participating in the epoxidation reaction having changed. In the presence of alkalis, the selectivity of the Ag catalysts is initially changed towards combustion to COz and H2O. This is due to the alkali-induced enhancement of the oxygen adatom concentration which favours the stripping of the weakly-acidic hydrogen atoms rather than the electrophilic attack on the double C-C bond. Thus, for understanding the mechanism of the alkali action in changing the selectivity of the Ag catalyst towards epoxidation it is necessary to account for the secondary reactions which happen on the surface after COZ formation. As has been already shown in Section 6.2., the COi- and C0;- species are stabilized on the surface by the alkali additives. Obviously, the formation and stabilization of COi- will deplete adsorbed oxygen from the surface and will exert a poisoning effect on further total oxidation, i.e. the epoxidation will be favoured. 6.8.2

Interaction of Alcohols with Alkali Modified Metal Surfaces

As has been briefly described in Section 5.6., the chemistry of alcohols on transition and noble metal surfaces is Characterized by a variety of reaction products which are determined by the actual properties of the surface. Because of the industrial importance of the reactions involving alcohols, it is of interest t o know the effect of electropositive additives on the reactivity and selectivity of the surface with respect to alcohol decomposition. Here results on methanol interaction with I<-covered Ru(0001) [215, 2161, and Rh(ll1) [217] and Na-covered Cu(l l1) [181, 2181 will be considered. It should be pointed out that the methanol molecules readily decompose on Ru and Rh surfaces, while the Cu( 111) surface is inert to CHSOH dissociation. Another difference between Ru, Rh and Cu is that the adsorbed alkali species are characterized by a smaller adsorption binding energy on Cu surfaces. These differences in the substrates reactivities with respect to the coadsorbates should be recogiiised in the behaviour of the mixed overlayers. As with most metal substrates, the formation of methoxy species mediates CH3OH decomposition, which on Ru(0001) proceeds via the following two

265

6.8. Organic Compound parallel pathways: 22K

CHsOH,

CH30Hg

> 253'

+

CO,

H20,

'z CH30, +Ha +

+ Ha 3?K

+ C, + Ha

HZg CO,

+

'ZKCO,

HzO, (210K)

A common feature of the effects of K independently of OK is that the second pathway is quenched which leads to HzO formation via a C-0 bond breakage. Besides, there are substantial differences in the surface decomposition chemistry at low K coverages (ahead of the minima of the A 4 v s . 6 ~plots) and at high alkali coverages when the alkali layer exhibits more pronounced O . l ) , when the I< adspecies metal-like properties. At low I< coverages (BIG are strongly polarized, substantial changes in the dissociation steps, leading to CO and H products, are observed. The reaction scheme for 0.1 I
-

CH30Hg (27010 CH30Hg +

CO,

+ H,(at

85K +

CHBOH,

T > 270 I<) 42'H2g

585IC -+

CO,

Obvioudy, at low K coverages the presence of K inhibits the 0-H bond breakage and the formation of methoxy species. The coadsorbed methanol is stabilized by the presence of K up to 240 K. Both desorption and decomposition occur within a very narrow temperature range (240-270 K) and no metlioxy species are identified by the vibrational spectroscopy. The higher Hz and CO desorption temperatures are consistent with the alkali promotion effect on hydrogen and CO adsorption reported in Sections 6.1. and 6.6. The mechanism of the I< induced hindrance of methoxy formation cannot be explained satisfactorily at the present stage of knowledge. As evidenced by bond is weakened on 0.1 K/Ru(0001) compared to a HREELS, the 0-H CH30H multilayer on a clean surface. A possible explanation is that, because of the enhanced surface charge density, the weaker CH 30 q1( 0) donor bonding configuration is suppressed. However, if t,his is the case, the alkali additives would be expected to favour the p ( C , O ) bonding configurat,ion where the backdonation component is of main importance. However the quenching of the C-0 bond breakage and the absence of any spectroscopic evidence of 172(C10) species reject this supposition. Since HREELS spectra show no evidence of direct K-methanol interactions, it is more likely that the modifying effect may be related to a steric hindrance and/or substrate-mediated interactions due to the I< induced charge density perturbations. Completely different is the behaviour of the K modified Ru(0001) a t high alkali coverages when the metal-like character of the overlayer is predominant. Thus, in analogy to HzO and 0, at OrmK 0.33 (which corresponds to completion of the first layer) potassium methoxide species are formed via a direct K-OCHS bonding. The thermal stability (> 450 K) of these species is larger than that of the methoxy species on clean Ru(0001) and methanol on

-

-

266

Chapter 6

0.1 K/Ru(0001). A similar K-OCH3 surface compound is identified on 0.33 K / R h ( l l l ) [217]. The influence of the adsorption state of alkali on the promotion action is also reflected by the enhanced reactivity of the Cu(ll1) surface in the presence of Na. For Na-covered Cu( lll),formation of Na6+OCHi- species is evidenced by UPS even at low N a coverages [181, 2181. Since the Cu( ll1) surface is inert towards CH30H dissociation, obviously the N a adspecies serve as local reactive centers. The local stabilization of methoxy-species by Na even at low N a coverages can be ascribed to the relatively weaker coupling of N a with Cu compared to the K-Ru (Rh) bonding at low K coverages. One can speculate that this means that the effective electron density in the N a 3s-valence states is larger, which favours direct interactions with the coadsorbates over coadsorbate - substrate interactions. The thermal stability of the sodium methoxide is within the same temperature range (- 450-500 K) as that of K-OCH3. 6.8.3

Conclusive Remarks

Summarizing the results in this Chapter about the alkali modification effect on the reactivity of single crystal metals surfaces with respect to selected unsaturated hydrocarbons and alcohols, the following conclusions can be made: A. Unsaturated hydrocarbons.

(1) The presence of alkali additives prevents the adsorption of unsaturated hydrocarbons in a di-a molecular bonding configuration at the expense of an increasing amount of the weakly ?r-bonded state. (2) The decomposition of unsaturated hydrocarbons is drastically reduced by the presence of alkali additives within the low alkali coverage region and is completely quenched above certain alkali coverages. (3) The presence of alkali additives also inhibits the hydrogenation of the unsaturated hydrocarbons and stabilizes a new C,H, intermediate that participates in the hydrocarbon decomposition. The action of the alkali additives can be satisfactorily described considering both: (i) the steric blocking effect leading to a rapid reduction of the unaffected surface atoms which participate in the adsorption of the di-a bonded molecules and (ii) the perturbation of the surface electronic structure which makes the surface less reactive towards donor-like adsorbates. Considering the utilization of alkali additives in the synthesis of Fischer - Tropsch, one can conclude that, besides the promoting effect of the alkalis with respect to dissociation of one of the reactants (CO) the presence of alkalis will also facilitate the removal of unsaturated hydrocarbon products from the surface (by decreasing their adsorption energy). An additional effect on the selectivity can be related to the alkali induced inhibition of the hydrogenation of the unsaturated hydrocarbons.

6.9. Ammonia

267

B. Alcohols. (1) For substrates which exhibit a high reactivity in the decomposition of alcohol, the behaviour of the modified surface depends on the actual alkali coverage: (a) in the presence of small amounts of an alkali additive (low alkali

coverages) the major effect is constrained to inhibition of the first step of methanol decomposition (methoxy formation) at the expense of stabilization of the methanol molecular state; (b) at high (near to a monolayer) alkali coverages, direct interactions between the metal-like alkali overlayer and methanol leads t o the formation of rather stable alkali methoxides.

(2) For substrates inactive with respect to CHZOH decomposition, and also less strongly coupled t o the alkali adatoms, alkali methoxide-like species are formed by direct local alkali-OCH3 interactions within a larger submolayer alkali coverage range. 6.9

AMMONIA

The effect of alkali promoters on ammonia adsorption is directly related to the important ammonia synthesis. Ammonia molecules produced as a result of the surface stepwise reaction between N, and Ha have a certain residence time on the surface and under the reaction conditions surface NH3 species can either diffuse along the surface before desorption or can decompose to their constituents. Thus, NH3 can perturb the dissociative adsorption of the reactants by blocking the surface sites, and reduce the effective rate of ammonia production. NH3 adsorption on alkali free transition metal surfaces occurs via the N lone orbital where the molecule acts as an electron donor with respect to the metal (similarly to HzO). As can be expected, coadsorption of alkali additives leads to weakening of the ammonia adsorption bond [219-2211. This can be associated with the enhanced charge density in the presence of alkali which prevents the formation of a donor-like adsorption bond. The decrease of the desorption energy of ammonia, E d , on Na-covered Ni(ll0) and Ru(0001) [220, 2211 is found to depend on the alkali coverage according to the relationship: Ed = E: - UO,, 312 . Besides, the presence of alkalis causes a perturbation of the NH3 orientation on the surface due to the occurrence of attractive interactions between t h e positively charged alkali species and the negative N end of the ammonia molecule. However, in contrast to H 2 0 (where also alkalis induce reorientation of the adsorbed molecule), the dissociation propensity of ammonia is affected negligibly by the presence of alkali additives. Similar destabilization of the molecular adsorption state of NH3 is observed for I
-

Chapter 6

268

time of the NH3, molecule relates exponentially to the adsorption energy, this means that ammonia will be more easily removed from the surface in the presence of alkali promoters. Thus, similar to the case of Fischer - Tropsch synthesis, the alkali additives exercise several different actions. On the one hand they promote the dissociative adsorption of the reactants and, on the other, they facilitate.the:removalof the reaction product NH3 from the surface. 6.10

ALKALI MODIFICATION EFFECTS A N D FACTORS DETERMINING THE TYPE OF INTERACTIONS IN THE MIXED LAYERS

The interactions of various molecules and atoms with alkali-modified surfaces described in Chapter 6. show definitely that the possible effects of alkali additives in the catalytic chemistry are rather complex. In some respect they are more complicated than the effects of the typical poisons (described in Chapter 5.), because the alkali influence cannot be simply due to effects on: (i) the adsorption rate and adsorptive capacity of the surface; (ii) the stability of the adsorption state; (iii) the strength of a certain intramolecular bond, and (iv) the dissociation propensity of the coadsorbed molecules. In most of the cases additional reactions with the active participation of the alkali additives are favoured leading to the formation of new intermediates and compound-like species. (e.g. AM6+COi-, AM6+C20:-, AM6+NO;-, AM6+OH6-, AM:+06-, AM6+C06- and AM6+0CH:-, formed as a result of COz, NO, H20, CO and HOCH3 interactions with alkali modified substrates). Many of these compound-like surface species exhibit a stability similar to that of the corresponding bulk chemical compounds. The conditions favouring the formation of such compound-like species on the surface vary with the different substrates. In general, they are determined by the competition between the substrate and the alkalis for binding the coadsorbate. Since the reactivity properties of the alkali additives on the surface depend on their adsorption state (the relative occupancy of alkali valence ns-states), direct interactions with the coadsorbates and stabilization of secondary products on the surface are usually observed above certain alkali coverages. In this Section the major factors which contribute to the strength of the alkali modification effects will be outlined. These account for the extensive amount of chemistry of the alkali-coadsorbate stable compound-like species formed as a result of direct attractive interactions in the mixed overlayers. Particular attention will be paid to the correlations between the alkali induced stabilization and reactivity effects and the alkali adsorption state. On the basis of the current experimental results one can definitely state that there is a negligible difference in the action of Na, K and Cs additives, whereas the strength of the promoting effect changes substantially with increasing alkali coverage. As has been already described in Section 6.1., the

d,,

6.10. Alkali Modification Effects and . . .

269

alkali coverage determines, to a great extent, the adsorption state of the alkali species on the surface. Let us remember that: (i) the rather strong adsorption bond at low alkali coverages associated with a large dipole can be described by the dominant contribution of the 'ionic' component to the bonding and (ii) due t o depolarization effects the ionic contribution is strongly reduced at high alkali coverages, so that the overlayer becomes metal-like and the coupling with the surface becomes rather weak. As revealed by the spectroscopic data, the alkali coverage changes induce significant perturbations in the electron density of the alkali valence ns-states which are converted into a two dimensional band at alkali coverages exceeding the critical ones. It is quite obvious that the alkali valence ns-states are energetically and sterically the most appropriate ones t o participate in direct coupling with the coadsorbates. T h at is why the adsorption state of the alkali metals, which determines the degree of occupancy of the ns-valence states, is of significant importance.

6.10.1

Mechanism of the Alkali Stabilization Effect on the Molecular Adsorption State of Acceptor-like Coadsorbates

Since the system that has been studied most extensively and thoroughly is the one on the molecular adsorption of CO on various alkali-modified surfaces, the author will base his views exclusively on the experimental evidence obtained for this system. Moreover, CO adsorption on most of the alkali-modified surfaces under consideration, is non dissociative, which facilitates the description of the alkali-induced effect on the molecular adsorption state. Generally, the trends observed in CO-alkali overlayers can be extended to other acceptorlike molecules which are bonded to the substrate in a way similar to CO. In most recent reviews [9, 16, 1021 and papers, e.g. refs. [8, 15, 19, 28, 461, the major contribution t o the promoting effect is believed to be the direct coupling between the promoted CO molecular orbitals ( 2 ~ 'and/or IT) and the alkali valence ns-states. The consistency of this idea is confirmed by the fact that the alkali induced increase in the stability of the promoted CO* state correlates with the increase in the relative occupation of the alkali ns-valence states, going from low to high alkali coverages. This means an enhancement of the degree of coupling between CO* and AM, followed by the formation of the same type of rather stable CO*-AM surface complexes a t high alkali coverages when the alkali overlayer becomes metal-like. As has been outlined above, the experimental data indicate that in the case of the alkali modified transition surfaces which behave very similarly with respect t o CO and AM adsorption alone, the critical alkali coverages above which the stable compound-like surface complex is formed are on the right side of the A+ vs. BAM plots (i.e. when the plasmon losses indicating the formation of a two dimensional ns-band arise). It will be remembered that above these critical alkali coverages: (i) the CO* vibrational bands reach the lowest stretching frequency and become independent of CO coverage; (ii) coincident CO and AM desorption peaks are detected, and their positions are independent of CO coverage and of a further increase in the

Chapter 6

270

alkali coverage up t o completion of the first alkali layer; (iii) the highest values of the initial heats of CO' adsorption which are a measure of the stabilization effect of alkalis is achieved, and (iv) the CO coadsorption leads to stabilization of the alkali overlayer. It might be argued that the strength of the promotion effect, governed by alkali coverage is due to the increased number of alkali adspecies which interact with a single GO' molecule. This argument has been used frequently when explaining the observed differences in the strength of the effect at low GO coverages, e.g. the linear increase of the initial heat of adsorption, AH:,, or the trend of reduction in the CO' stretching frequency, WCO., with increasing initial alkali coverage. The following experimental facts are in opposition to the strength of the above argument: (1) If the strength of alkali-induced promotion effect is determined by the average number of alkali adspecies affecting a single GO' molecule, then substantial differences in the A H g o and WCO- should be observed for modified flat and corrugated transition metal surfaces, e.g. Ru(0001) and Ru(1010) or Ni(ll1) and Ni(ll0). Comparison of the available T P D and vibrational data of these surfaces [5, 6 , 8, 15, 3, 17, 211 definitely shows very similar kinds of behaviour of the modified flat and corrugated surfaces, especially above the critical alkali coverages. Coincident CO' and I( T P D peaks are observed at die same temperature of 680 K from both K/Ru(0001) and I
(2) If the strength of the promoting effect is exclusively determined by the number of alkali adspecies, affecting a single CO' molecule, then the

temperature (reflecting the alkali-induced stabilization effect) of the coincident CO' and AM peaks and the corresponding W C O - should also depend on the actual CO coverage. On the contrary, in addition to the observed CO coverage independence the areas under the CO" and AM coincident T P D peaks indicate that the most favourable CO' : AM = 1 stoichiometry is preserved even when the number of the coadsorbed GO molecules is less than that of the AM adspecies. As will be discussed below, at alkali coverages lower than the critical ones, when the alkali adspecies desorb after CO, the direct CO-AM coupling is weaker. Then the preferred 1:l stoichiometry can be violated, which leads t o the observed relationship of the CO coverage and the GO' T P D peaks and W C O - . This can be associated with the coexistence of patches of various compositions on the surface.

(3) The Me-AM and Me-CO' vibrational bands are at lower energies than those measured for monoadsorbate systems. This means that the

6.10. Alkali Modification Effects and . . .

271

coupling of CO* and AM with the substrate is reduced at the expense of the direct CO-AM coupling which is a general trend in the quantum chemistry. The existence of direct CO' 27 (and/or l.lr)-ns alkali interactions, which might result in the formation of a covalent or ionic type of bonding with preferred 1:l stoichiometry has already been proposed in several theoretical models (see Chapter 7.) in order to explain the observed alkali influence on the GO I n and 2x-derived bonding and antibonding levels, as well as the CO induced changes in the alkali core levels and the depletion of the valence ns-induced excitations which indicate a reduction of the electron density in the ns-states. It should be stressed that the theoretical models based on the predominant direct coupling between the coadsorbates are able to explain satisfactorily the experimental results obtained a t high alkali coverages, when the overlayer is metal-like and the part played by the alkali ns-states in coupling with the substrate is drastically reduced. These conditions favour strong direct interactions between the coadsorbates involving a substantial redistribution of the electron densities in the alkali ns and CO l a and 2n-derived states, which can be considered as chemical type interactions. Some authors believe that the strong CO-AM coupling might even lead to "salt" formation where the anion can be of the kind of C,Oi- (almost identical to bulk oxocarbon compounds [18, 191). In order to extend the direct coupliiig model to explaining the behaviour of coadsorbate systems at low and moderate alkali coverage, It should be taken into account that the adsorption state of the alkali adspecies within the coverage range before reaching the work function minima is Characterized by a rather strong polarized bonding with the substrate where the valence ns-states are involved. This implies a reduced contribution of the ns/2a ( l a ) coupling to the alkali associated stabilization effect on CO*. Obviously, within the lower alkali coverage range the AM-substrate interactions are dominating over the AM-CO' ones and CO* desorption precedes that of the alkali metal. However, the correlation between the net occupation of the ns-states (determined by the actual alkali coverage) and the strength of the promoting effect is reflected by the gradual increase of the stabilization effect with increasing alkali coverage. Within the limits of very low alkali coverages when the alkali species are strongly polarized, the stabilization effect is the weakest. One might think that at these very low alkali coverages the electrostatic interactions would be of major importance but it is obvious that they induce negligible perturbations in the CO bonding. Within the framework of the model for a dominating contribution of the direct coupling t o the mutually-induced stabilization of the coadsorbates on the surface at high alkali coverages and the alkali-induced stabilization of GO* a t low and moderate alkali coverages, there now is a satisfactorily explanation for the fact that the mixed coadsorbate layers on transition metals, which have similar adsorptive properties with respect to CO and AM, behave in a very similar way. In support of this assumption are the results on CO adsorption on alkali-modified Cu and A1 [17, 18,431. CO and alkali adsorption on these substrates is characterized by a substantially weaker chemisorption

272

Chapter 6

bond compared to the transition metal surfaces considered above. In the case of All as a result of the weaker AM-A1 coupling, clustering of the AM species is detected even at low alkali metal coverages. Therefore, in these systems the major contribution to the stabilization of the CO' state is via direct coupling and the formation of CO*-AM compound-like complexes even at low alkali coverages, as is evidenced by the TPD and the vibrational data. Finally, a brief discussion of the observed differences in the magnitude of the promoting effect with various alkali additives, e.g. Na, K, Cs will be given. Considering only the electrostatic interactions, one could expect that the magnitude of the promoting effect should increase with increasing size and decreasing ionization potential of the alkali adspecies. Furthermore, with the larger K and Cs adatoms, the depolarization effects and the metallization of the overlayer take place at lower coverages compared to the smaller Na adatoms. Within the framework of the direct coupling model this means that the maximum magnitude of alkali-GO* coupling will be achieved at higher absolute N a coverages, as is in fact established experimentally. The observed slightly weaker stabilization effect of N a (asjudged by the coincident desorption peak maxima, (e.g see fig. 6.8.) might be attributed to:

(i) differences in the 2 x electrostatic shift induced by N a and I< or Cs (which determines the separat,ion between the 2a and 12s-levels), (ii) steric reasons due t o the smaller size of Na, etc. The same mechanism of alkali-induced stabilization effect on the molecular adsorption state can be applied for explaining the more stable molecular adsorption state in the promoted sites of other molecules, such as NO, 0 2 , and n-bonded (.Y-N~,where the acceptor metalla* component contributes mainly to the adsorption bond. However, because of their weaker intramolecular bond, these molecules dissociate more easily. In the case of COz , the presence of alkali favours the adsorption and stabilization of bent anionic C0;- which serves as a precursor of further dissociation. For all molecules described above, the alkali stabilization effect has a bearing on the existing bonding configurations on an alkali-free surface. Usually, the trend in the alkali promotion effect with respect to the molecular adsorption is t o favour and stabilize molecular bonding configurations where the adsorbate behaves like an eiectron acceptor with respect to the surface. Alkali-induced changes in the molecular bonding configurations are observed in the case of H20 and NH3 adsorption. For these systems reorientation of the promoted molecular adsorption state occurs due to direct interactions with the coadsorbed alkalis.

6.10.2

Alkali effect on the A d s o r p t i o n Kinetics and Energetics

Here, it is proposed that the fact be considered that it is not always possible t o find a correlation between the observed stabilization effect on the molecular adsorption state and the molecular adsorption rate. An alkali-induced increase in the molecular adsorption rate is observed in the case of 0 2 , GO2 and in the case of the a-bonded CY-N~ state on Fe( 100). In

6.10. Alkali Modification Effects and

. ..

273

the first two cases this can be associated simply with the high affinity of alkalis to oxygen and COz and the suppression of the self-poisoning effect of the coadsorbates. In the third case the increase of the adsorption rate is attributed t o an alkali-induced reduction of the activation barrier for a-NZ adsorption. The promotion effect on the molecular adsorption rate is more likely t o be related to the charge-transfer process operating on the molecules approaching the surface from the gas phase. A possible mechanism of the alkali influence on the adsorption rate can thus be the fact that the charge exchange between the approaching molecules and the surface becomes facilitated and this can occur further away than the equilibrium distance of the non-promoted surface (due t o the alkali induced increase in the surface electron density and decrease in the work function). Such an explanation fits satisfactorily with the observed invariance of the CO sticking coefficient (which is unity for a clean transition metal surface) up to moderate alkali coverages. However, when the alkali overlayer becomes metal-like (at alkali coverages beyond the A4 vs. OAM work function minima), the CO sticking coefficient drops significantly. The effect of high alkali coverages on the sticking coefficient is explained fully by the reactivity of thick alkali layers. While in the case of 0 2 and CO?, the thick alkali layers exhibit a high reactivity with respect to O2 and CO? molecular adsorption, in the case of CO the adsorption rate on a thick alkali overlayer is very low. This is probably due to the appearance of an activation barrier for CO adsorption and/or to the drastic reduction of the lifetime of the CO precursor state. However, despite the drastically-reduced sticking coefficient, rather strong pure alkali-CO interactions are favoured after CO adsorption. Finally, the cases where alkali additives completely inhibit the adsorption process within the whole alkali coverage range should be considered. These are the cases of molecular adsorption where the adsorbate plays the role of an electron donor. Typical examples are the results on the alkali effect on the a-bonded (donor-like) ethene and y-Nz molecules. The alkali-induced reduction in the sticking coefficient can be explained by mechanism similar t o that proposed above for the promotion of the adsorption rate of acceptorlike molecules. In the case of donor-like adsorbates, the presence of alkalis will set up a repulsive barrier to the approaching molecules. However, in the case of Y - N ~ ,as soon as this barrier can be overcome, short-range attractive interactions with K can ensure a rather stable N2 adsorption state (more likely of an a-type). The absence of a similar acceptor-like bonding configuration ,in the case of ethene leads to complete suppression of ethene adsorption in the molecular state serving as a dissociation precursor and the action of the alkali additives in this case is similar to the action of the typical poisons. In general, the alkali effects 011 the molecular adsorption kinetics are determined by the type of the adsorption bond of the adsorbate, the reactivity of the substrate and the thick alkali films. In the alkali coverage region before the work function ininimuin, ttlie effect is governed by the following two factors: (i) the alkali-induced changes of the equilibrium distance for charge exchange between the approaching molecule and the surface and (ii) the relative stabilization of the molecular adsorption state on the pro-

Chapter 6

274 moted sites.

At high alkali coverages, when the properties of the surface resemble those of thick alkali overlayers, the adsorption rate is determined exclusively by the actual reactivity of the alkalis towards the coadsorbate, irrespective of the fact that in many cases after adsorption the adsorption state is more stable than on an unpromoted surface.

6.10.3

Alkali Effect on the Dissociation Propensity

The observed alkali-induced enhancement of the dissociation propensity of acceptor-like molecules follows from the induced stabilization of the molecular adsorption state (via an increase of the charge density in the a'-antibonding molecular orbitals) and enhancement of the equilibrium concentrations of the acceptor-like molecular state which serves as a precursor of dissociation. Thus, considering the potential energy surfaces for adsorption and dissociation (see e.g. fig. 5.1.), it can be concluded that the presence of alkali additives leads t o an increase in the ratio; 'dissociation rate/desorption rate' at the expense of (i) an increased concentration of the molecular precursor state and (ii) a possible reduction in the activation barrier for dissociation, as judged by the weakening of the intramolecular bonds due the enhanced electron density in the antibonding orbitals. This general correlation: stabilization of the adsorption state - enhancement in the dissociated fraction is fulfilled always in the cases when a fraction of the coadsorbate dissociates on an alkali-free surface (e.g. N 2 and CO on Fe, NO on Ru, Rh, Fe, 0 2 on transition metals, etc). In the other cases where the clean surface is inactive in respect of adsorbate dissociation alkali-induced dissociation is favoured usually when: (i) the dissociation process is thermodynamically favourable and the activation energy barrier for dissociation is of the order of the activation energy for desorption of the molecular state (e.g NO on P t ( l l 1 ) ) ; (ii) the alkali additives cause substantial stabilization of the dissociation products which leads to sufficient lowering of the activation barrier for dissociation (e.g. AM stabilization of OH6- species in the case of water dissociation). Since the strength of both stabilization effects (on the molecular precursor and on the dissociation products) is determined by relatively strong direct attractive interact ions arising between the alkali adspecies and the coadsorbed adspecies, the onset of dissociation usually requires a critical alkali concentration for systems where the alkali-free substrate is inactive in respect of dissociation. This agrees well with the views that the relative occupancy of the alkali ns-valence states (dependent on the alkali coverage) is of major importance for the observed promoting effects on alkali modified surfaces.

6.10. Alkali Modification Effects and . . .

275

6.10.4 Formation of Compound-like Species in the Mixed Overlayer The formation of compound-like surface species is related to the cases when the alkali-adsorbate direct attractive interactions dominate over that with the metal substrate. The tendency to formation of such compound-like complexes on the surface is determined by two main factors:

(1) The ability of the alkali adspecies to participate in direct interactions with the coadsorbed species. This ability gains strength with increasing alkali coverage because, due to the depolarization effects leading to weakening the alkali-substrate coupling, the effective charge density in the alkali valence ns-states arises; (2) The competition between the alkali and the substrate for binding the coadsorbed species which is best illustrated in the case of oxygen interaction with various substrates modified with alkalis. Current experimental data have shown that above certain critical alkali coverages (determined by the specific system) the alkali modifiers can participate in the formation of compound-like species with the following types of coadsorbates: (1) Molecular coadsorbates. A typical example is the formation of AM6+C0*complexes at high alkali coverages, as discussed i n Subsection 6.1.6.

(2) Atonic coadsorbates or niolecular fragments which are products of dissociative adsorption. Typical examples are (i) the formation of AMi+Oi- surface complexes which, depending on the affinity of the substrate t o oxygen, are observed above critical oxygen and alkali coverages; (ii) the formation of hydride-like species (AM6+H6-); (iii) the formation of hydroxide-like species (AM6+OH6-) following alkali promoted water dissociation; (iv) the formation of methoxide species (AM6+OCH!-) following dissociative adsorption of CH30H, etc.

(3) Products of secondary reactions. These reactions are favoured only in mixed alkali-coadsorbate overlayers. Typical examples are the coadsorption systems CO2 + alkali and NO + alkali. In both cases the most stable secondary products (t,hecarbonate-like AM6+CO;- and the nitrite-like AM6+NOi-) are formed at high alkali coverages when the alkali overlayer exhibits metal-like properties. That is why these surface complexes have a thermal stability close t o that of the corresponding bulk compounds. Current d a t a concerning the stability of the compound-like alkali surface complexes reveal that many new intermediates can be stabilized even at catalytic reaction temperatures. As described in the sections of this Chapter,

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the conditions of stabilization and decomposition of these surface complexes vary with the different substrates. As outlined in Subsection 6.1.6., a similar behaviour of the modified surfaces, governed exclusively by the adsorption state of the alkalis, can be observed in the cases where the adsorptive ability and the reactivity of the substrates with respect to the alkali additive and the coadsorbate are similar.

6.10.5 Conclusive Remarks In summary, the following general conclusions can be drawn: 1. The effect of the alkali promoters on the molecular state of acceptor-like molecules and on the dissociation propensity of these molecules is largely dependent on the alkali-coadsorbed molecule interactions involving coupling between the AM ns-states and the molecule antibonding 7r-derived states. Since the relative occupancy of the alkali ns-valence states is determined by the actual alkali coverage, one should expect that optimal alkali concentrations should be used in real catalysis. Obviously, it is of importance also establish whether the alkali additives are localized on the metal clusters or on the support because, as outlined above, the alkali adsorption state depends strongly on the substrate nature.

2. The possibility of formation of rather stable compound-like surface complexes with the active participation of the alkali additives plays an important role in the catalytic reactions. It might lead to changes in the reaction pathways or to suppression of certain surface reactions. For example, the stabilization of hydrogen atomic state and the inhibition of the Ha diffusion on the surface in the presence of alkali modifiers turns out to be one of the reasons for the preferential production of unsaturated hydrocarbons in Fischer - Tropsch synthesis.

3. In the case of reagents, intermediates or products which behave as donors during the formation of the adsorption bond with the surface the alkali additives act as poisons with respect to both the molecular and the dissociative adsorption. Such ‘poisoning’ action towards unsaturated hydrocarbons and ammonia will undoubtedly have a positive effect on the catalytic reactions (e.g. CO hydrogenation and ammonia synthesis), because the presence of alkalis will facilitate the removal of the reaction products from the surface.

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W.D. Clendening, J.A. Rodriques, J.M. Campbell and C.T. Campbell, Surface Sci. 216 (1989) 429 G. Pirug, H. Bonzel and G. Broden, Surface Sci. 122 (1981) 1 E. Garfunkel anf G.A. Somotjai, Surface Sci. 115 (1982) 441 G. Bliznakov, M. Kiskinova and L. Surnev, Proc.IV Conf. Solid Surfaces, Cannes, 1980 p.505 C.A. Papageorgopoulos and J.M. Chen, Surface Sci. 52 (1975) 53 M. Kiskinova, L. Surnev and G. Bliznakov, Surface Sci. 104 (1981) 240 P. Dolle, M. Tommasini and J. Jupille, Surface Sci. 211/212 (1989) 904

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M. Kiskinova, G. Rangelov and L. Surnev, Surface Sci. 150 (1985) 339; &id. 172 (1986) 57 L. Surnev, G. Rangelov and M. Kiskinova, Surface Sci. 179 (1987) 283 R.A. DePaola, F.M. Hoffrnann, D. Heskett and E.W. Plummer, J . Chem. Phys. 87 (1987) 1361 L. Surnev, G. Rangelov, E. Bertel and F.P. Netzer, Surface Sci. 184 (1987) 90 J. Hrbek, Surface Sci. 205 (1988) 408 G. Pirug, G. Broden and H.P. Bonzel, Surface Sci. 94 (1980) 323 R. Riwan, P. Soukiassian, E. Guillot, J. Lecante, S.S. Zuber and J. Cousty, Surface Sci. 146 (1984) 382 P. Soukiassian, R. Riwan and J. Lecante, Surface Sci. 152/153 (1985) 522 C.A. Papageorgopoulos and J.L. Desplat, Surface Sci. 92 (1980) 119 J.M. Chen and C.A. Papageorgopoulos, Surface Sci. 26 (1971) 499 C.A. Papageorgopoulos and J.M. Chen, Surface Sci. 39 (1973) 313 P.H. McBreen, S. Mooke, A. Adnot and D. Roy, Surface Sci. 194 (1988) L112 M.R. Rajuman, I<. Prabhakaran and L.N.R. Rao, Surface Sci. 233 (1990) L237 H. Steiniger, S. Lehwald and H. Ibach, Surface Sci. 123 (1982) 1 N. Freyer, M. Kiskinova, G. Pirug and H. Bonzel, Surface Sci. 166 (1986) 206 T. Matsushima, Surface Sci. 157 (1985) 297 N.D. Lang, S. Holloway and J.K. Norskov, Surface Sci. 150 (1985) 24 D.A. King in: Chemistry and Physics of Solid Surfaces, v01.2 (1977) p.87 T. Rahman, A.B. Anton, N.R. Avery and W.H. Weinberg, Phys. Rev. Letts. 51 (1983) 1979 S. Kennou, M. Kamaratos and C.A. Papageorgopoulos, Vaccum 41,(1990) 22 C.A. Papageorgopoulos, Surface Sci. 104 (1981) 643 J.W. May and C.E. Carrol1,Surface Sci.29(1972) 85 E.V. Albano, Appl. Surface Sci. 14 (1982) 183 L. Surnev, G. Rangelov and G. Bliznakov, Surface Sci. 159 (1985) 441 P. Dolle, P. Louis and J. Jupille, Vacuum 41 (1990) 174 B. Woratschek, W . Sesselrnann, J. Ktppers and G. Ertl, J. Chern. Phys. 86 (1987) 2411 E. Bertel, F.P. Netzer, G. Rosina and H. Saalfeld, Phys. Rev. B 39 (1989) 6082 E. Bertel, N . Memmel, W. Jacob, V. Dose, F.P. Netzer, G. Rosina, G. Rangelov, G. Astl, N. Rosch, P. Kanappe, B.I. Dunlap and H. Saalfeld, Phys. Rev. B 39 (1989) 6087 J . Hrbek, G.O. Xu, T.K. Sham and M.-L. Shek, Vacuum 41 (1990) 153 R. Miranda, in: Physics and Chemistry of Alkali Metal Adsorption, eds. H.P. Bonzel, A.M. Bradshaw and G. Ertl, Materials Science Monographs, 57 (Elsevier, 1989) p.425 L. Surnev, in: Physics and Cltemistry of Alkali Metal Adsorption, eds. H.P. Bonzel, A.M. Bradshaw and G. Ertl, Materials Science Monographs, 57 (Elsevier, 1989) p.173

Chapter 6

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J.L. Sass, I<. Bange, R. Dohl, E. Piltz and R. Unwin, Ber. Bunsenges. Phys. Chem. 88 (1984) 354 S. Semancik, D.L. Doering and T.E. Madey, J . Vac. Sci. Technol. A 3 (1985) 1571; Surface Sci. 176 (1986) 165 D.L. Doering, S. Semancik and T.E. Madey, Surface Sci. 133 (1983) 49 P.A. Thiel, J . Hrbek, R.A. dePaola and F.M. Hoffmann, Chem. Phys. Letts. 108 (1984) 25

M. Kiskinova, G. Pirug and H. Bonzel, Surface Sci. 150 (1985) 319 (a) H. Bonzel, G. Pirug and C. Ritke, Langmuir (in press); (b) G. Pirug, C. Ritke and H.P. Bonzel, Surf.Sci. (submitted) H.P. Bonzel, G. Pirug and A. Winkler, Chem. Phys. Letts. 116 (1985) 133; Surface Sci. 175 (1986) 287 E.M. Stuve, R. Dohl, I<. Bange and J.I<. Sass, J. Vac. Sci. Technol. A 3 (1985) 1571

E.M. Stuve, K. Bange, J.I<. Sass, in: Trends in In terf. Electr., ed. A.F. Silva, NATO ASF Series, Vol.cl79 (Reidel, 1986) p.255 I(. Bange, D. Grider and J.K. Sass, Surface Sci. 126 (1983) 437 D. Lackey, J. Schott, B. Straehler and J.K. Sass, J. Chem. Phys. 91 (1989) 1365

J. Paul, Surface Sci. 160 (1985) 599 H.P. Bonzel, G. Pirug and J.E. Mtller, Phys. Rev. Letts. 58 (1987) 2138 G. Pirug, R. Dziembaj and H.P. Bonzel, Surface Sci. 221 (1990) 553 P.H. Emmett ed., Catalysis vo1.4 (Reinhold, New York 1956) p.46, 330 G. Ertl, S.B. Lee and M. Weiss, Surface Sci. 111 (1981) L711 Y.M. Sun, H.S. Luftman, J.M. White, Surface Sci. 139 (1984) 379 A.M. Lanzillotto, M.L. Dresser, M.D. Alvey and J.T. Yates Jr., J. Chem. Phys. 89 (1988) 570 M.-L. Ernst-Vidalis, M. Kamaratos and C.A. Papageorgopoulos, Surface Sci. 189/190 (1987) 276 H.L. Zhou and J.M. Whit,e, Surface Sci. 185 (1987) 450 C.A. Papagergopoulos and J.M. Chen, Surface Sci. 39 (1973) 283 Z.Y. Li, R.N. Lamb, M. Allison and R.F. Willis, Surface Sci. 211/212 (1989) 931

G.H. Rocker, C.L. Cobb, H. Metiu and R.M. Martin, Surface Sci. 208 (1989) 205

J. Paul and F.M. Hoffmann, Surface Sci. 194 (1988) 419 J. Paul, R.A. dePaola, F.M. Hoffmann, in: Physics and Chemistry of Alkali Metal Adsorption, eds. H.P. Bonzel, A.M. Bradshaw and G. Ertl, Materials Science Monographs, 57 (Elsevier, 1989) p.213 S. Bosch, A.M. Hasch, G. Stern and H. Wolf, D R P 249.447, 1910, BASF, Ludwigshafen C.T. Rethner and D. Auerbach, Comments A t . Mol. Phys. 20 (1987) 153 C . T . Rethner and H. Stein, Phys. Rev. Letts. 59 (1987) 2768 R.A. dePaola, F.M. Hoffmann, D. Heskett and E.W. Plummer, Phys. Rev. B 35 (1987) 4336 E. Umbach, Appl. Phys. A 47 (1988) 25; in: Physics and Chernastry of Alkalr Metal Adsorption, eds. H.P. Bonzel, A.M. Bradshaw and G. Ertl, Materials Science Monographs, 57 (Elsevier, 1 9 8 9 ) ~ 241 .

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E.L. Garfunkel, J.J. Maj, J.C. Frost, M.H. Farias and G.A. Somorjai, J . Phys. Chem. 87 (1983) 3629 E.L. Garfunkel, M.H. Farias and G.A. Somorjai, J . Am. Chem. SOC.107 (1985) 349

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Chapter 7

THEORETICAL APPROACHES T O THE DESCRIPTION OF THE MODIFIER EFFECTS

As described in Chapter 4., electronegative additives and alkali metals exhibit substantial differences with respect to the type of bonding with the substrate metal surface. This determines the essential differences in the induced charge perturbations within the surface, which are most important to the reactivity properties of these systems. In this Chapter the main theoretical approaches will be presented, which aim a t establishing the main factors responsible for the modification effects and which offer a coherent explanation of the behaviour observed in real systems. 7.1

7.1.1

PROMOTING EFFECT OF T H E ALKALI METAL ADDITIVES

Theoretical Models for Alkali Adsorption on Metal Surfaces

Adsorption phenomenon of alkali metals have attained a long standing interest by the theoreticians during the last few decades [l-lo]. Early attempts were aimed at explaining the extraordinary large work function decrease induced by alkali adsorption, which presupposes significant charge perturbations within the surface. Langmuir has proposed a rather simplified explanation which has been used as a basis in the initial theoretical treatments of the properties of the alkali-metal systems [I, 111. The Langmuir model considers the adsorbed alkali species as positive ions (with a fractional net charge) which donate their valence ns-electron to the substrate. These ions and their negative image charge form a dipole layer which is responsible for the observed reduction in the work function. The decrease in the work function and in the alkalisubstrate adsorption energy with increasing alkali coverage are ascribed to depolarization effects leading t o a reduction in the effective dipole moment per alkali adspecies. This oversimplified picture was abandoned later because the adsorption process, as a localized phenomenon,should be treated quant u m mechanically [2, 12, 131. However, a constraint to a complete theoretical

285

286

Chapter 7.

quantum-mechanical treatment of the alkali-metal surface interaction is introduced by the fact that a t the present stage it is very difficult to give an accurate description of the real electronic structure and the energetics of a surface covered with adsorbed species. That is why several theoretical approaches have been used in an attempt to attain a more realistic picture.

(a) 0.60 1

I,

0.40 0.20 0.00

-0.20 -10

(b) 0.8 h

2 E 0.6 a

-8 -6 -4 -2 ENERGY RELATIVE TO VACUUM (eV)

I

K ..

1

'I

0

1

0.4

E

v

c

i

0.2 0.0

E -0.2 -10

-8 -6 -4 -2 ENERGY RELATIVE TO VACUUM (eV)

0

Fig. 7.1. (a) T h e s and p, components of the N a state on a high density jellium surface. (b) The s, p z and d z 2 components of the K state density on a high density jellium substrate ( r S = 2) (from ref. [ 5 ] )

7.1. Promoting Effect of the Alkali Metal Additives

287

In this Chapter, the localized nature of alkali adsorption and t.he following two related subjects will be summarised briefly:

(i) spatial redistribution of a charge density due to the formation of an adsorption bond and (ii) changes in the electrostatic potential in the vicinity of the adsorbed alkali adspecies. One of the first and relatively successful models has been proposed by Lang et al,[5, 141. The properties of the alkali species within the limits of a single adatom are described by self-consistent calculations supposing that the alkali adatom interacts with a jellium surface. In this model, the substrate is presented as consisting of a uniform positive background (substituting the substrate ion cores) and valence electrons that have lost their real identification with a particular surface atmom.Within this approximation the substrate has a nearly free electron delocalized character. Fig. 7.1. illustrat,es that, the interaction of an alkali atom with this jellium surface causes a broadening of its valence state into a resonance centered well above the Fermi level. This indicates that the alkali ns-valence resonance is less occupied than its free atom counterpart due to a net flow of charge from the alkali to the alkali-substrate interface region. The induced electrostatic potential calculated for the single adatom limit is presented in fig. 7.2. [15]. The results i n fig. 7.2. show that the perturbations induced by the electrostatic do not, exceed distances of 3.5 d; from the center of the adatom, i.e. one should expect, that the nearest-neighbour adsorption sites will be affected most efficiently. Within the framework of this treatment, the increase in magnitude of the electrostatic potential with increasing size of the adatom (which agrees extremely well with the adsorption dipole moments of the corresponding alkali adspecies) is due t o the increase in the spatial extension of the valence shells of the heavier adatoms. This model describes satisfactorily the initial part (before the minimum) of the characteristic work function vs. alkali coverage curves. In general, as outlined in Section 4.2.2., the minimum of the work function plots corresponds to the breakdown of the metallic-behaviour of the alkali overlayer. Although the simple concept of a substrate substituted by a jellium is most appropriate for free electron sp metals, the theoretical results obtained by this approximation are applicable t o some basic properties of the alkali adsorption systems. Moreover, when the substrate and the adlayer are not commensurate, a jellium model for either the adsorbate layer or the substrate should be used if one wants to account for the alkali coverage cha.nges [G, 10, IG-191. Recently, using the method of Newns Anderson, the local densities of states and the effective charges of alkali a.dspecies forming two dimensional islands have been calculated taking into account the direct intera.ction between the adspecies [17]. These calculations ha.ve revealed that the effective charge on the adspecies decreases the coordination number, the total atom number and the interaction parameter increase. Parallel alterations in the shape of the LDOS curves leading to broadening or splitting are also observed.

-

~

288

Chapter 7.

Fig. 7.2. (Ieft) Contour maps of the induced electrostatic potentid for Li, Na and K at their calculated equilibrium distances outside a jellium (rS = 2) surface. The contour values are i n eV. (right) The electrostatic potentials from shown as a function of the distance outside the surface at a lateral distance of 5 bohr from the adatoms (from ref. [15])

Obviously, in the usual case of initial random alkali adsorption, the changes in the effective charge should be expected at moderate and high coverages. However, in the case of adlayer growth in islands (e.g. alkali adsorption on Al(100)) one can expect substantial changes in the effective charge of the adspecies at low coverages. The point which is being disputed most, is the character (covalent or ionic) of alkali-substrate bonding. Lang et a1.[5, 141, as well as the theoretical treatments based on the Anderson Hamiltonian [6], presuppose a drastic change of the adatom electronic structure from basically ionic within the limits of low coverages (in the so called charge transfer region) to a metallic neutral state at high coverages. These changes are ascribed to the depolarization field at the adatom sites. Other theoretical works concerning adsorption of alkalis on jellium [16, 171, where the alkali layers are described by a discrete lattice,

7.1. Promoting Effect of the Alkali Metal Additives

289

consider the dipole moment as a result of polarization of the alkali adsorbate due t o hybridization between the adatom and the surface states excluding any charge donation to the substrate. In this model the adatom region is essentially neutral if the charge density is averaged within an alkali adatom sphere. The adatom induced dipole moment is attributed to the adatom polarization arising as a result of the alkali-substrate orbital mixing. The decrease in the polarization (dipole moment) and in the alkali adsorption bond strength with increasing alkali coverage is interpreted as being due to the strengthening effect of the alkali-alkali bonding a t the expense of the weakening effect of the a l k al i s u b s t r a te bonding.

Q!

1,

-20

vr

-3.0

ao

4.0

bp

I

1

. 2 k.u . ) .

12.0-

+

Fig. 7.3. Total binding energy, VT = Vcov KOnof a single Li atom chemisorbing on Cu(100) as a function of the perpendicular distance 2. VT is obtained by solving the model Hamiltonian for Li in a four fold position. V,," is calculated from VT - Ken. (from ref. [lo])

A covalent type of alkali substrate bonding has also been proposed on the basis of the all-electron self-consistent local-density-functional results of Wimmer et a1.[7, 8, 201. For an ordered dense c(2 x 2) Cs overlayer on W(100) and Mo(100) the chemisorption bond is described as a result of hybridization between Cs(6s) and W(OO1) surface states of dz2 character, projected far

290

Chapter 7.

outside the geometric surface. These authors suggest that even at Cs low coverages, the chemisorption bond is polarized covalent rather than ionic. However, at the present stage, calculations with a discrete-lattice treatment of both the substrate and the adsorbate are performed only in the case of a dense alkali overlayer. A significant covalent contribution to the chemisorption bond due predominantly t o coupling to metal sp states has been reported for the adsorption of alkalis on Cu(lOO), described by a Hamiltonian model approach [lo]. Theoretically predicted changes in the binding energy of a Li atom in a fourfold position above a Cu(100) face upon approaching the surface are shown in fig. 7.3. It is obvious that the covalent contribution dominates the binding in the adsorption well. In this study, the alkali adsorption bond is treated in the common quantum chemical way as resulting from ionic and covalent components. The hybridization energy, the changes in electron-electron repulsion energy and the dipole layer repulsion all make covalent contributions. The static image energy before hybridization (V,,, = I&, - (4 - IPO)),makes an ionic contribution, where Km is the interaction energy of the singly, positively charged alkali core with its image inside the surface, 9 is the work function of the substrate surface, and IPo is the gas phase ionization energy of the alkali metal). The ionic component increases when the atomic number (IPO) increases from Li to Na; this was proved experimentally. In our opinion, the most accurate treatment of the alkali chemisorption bond is the quantum chemical approach whereby variations i n the contribution of the ionic and the covalent components are believed to occur. These variations are determined by the following main factors :

(i) the atomic number of the alkali additive: within the limits of a single adatom the ionic component increases as the atomic number, i.e. with decreasing of alkali ionization energy (ii) the alkali coverage (the ionic part of the bonding predominates at low coverages) , and (iii) the work function and discrete electronic structure of the substrate surface (they determine the degree of hybridization of the alkali ns-valence states with appropriate substrate surface states and the magnitude of

Ken). A common feature of all the theoret,ical models summarized above is, that they suggest a highly-localized character of the alkalisurface bonding at low alkali coverages. As a result the charge redistribution in the interface and electrostatic potential are induced in the vicinity of the adsorbed alkalis. The significant changes in magnitude of the alkali-alkali and alkalisubstrate coupling with increasing alkali coverage is expected to affect the alkali-coadsorbate interact ions in mixed overlayers significantly From band-structure calculations, which exclude a charge donation to the substrate surface conduction band, it can be predicted that the alkali adsorbates will not affect the substrate surface layer significantly. This has been confirmed recently by high resolution surface core level photoemmision measurements of the changes in the W 4f levels induced by alkali adsorption [21].

7.1. Promoting Effect of the Alkali Metal Additives

291

The W 4f levels are extremely sensitive t o the interactions with adsorbates. Table 7.1. presents the adsorbate induced shifts of W(110) surface core level binding energy. The data in Table 7.1. support the view that the major contribution to the alkali modification effect is due to direct coupling with the alkali species rather than to substrate mediated stabilization effects. Table 7.1. Na, K and Cs Induced Changes of the W(110) Surface 4f7p Core Level Binding Energy, A E B , (in meV) for Low and Saturation Alkali Coverages. The Oxygen Induced Changes are given for the Sake of Comparison (from ref. [21]) SURFACE

AEB

SURFACE

0.12 Cs/W(llO) 0.37 Cs/W(110) 0.14 K/W(l10) 0.42 K/W(IIO)

+3f2 -20f3 +5f2 -12f3

0.18 Na/W(IIO) 0.57 Na/W(110) 0.12 O/W(110)

+170f10

0.37 O/W(llO)

+520f30

AEB Of2 -28f3

Before discussing the theories considering alkali-coadsorbate interactions it should be mentioned that there is one more study, in which long-range perturbations in the surface local density of states (LDOS) near the Fermi level have been predicted [9]. Since more details of this study will be presented in the next Section, it is mentioned here that a 1/4 monolayer of Li adsorbed on a double layer of Rh(001) film has been found capable of an increase in the electron charge density over the larger part of the surface, along with an increase in the LDOS near the Fermi level (the magnitude of which is supposed t o be a measure of the ability of the surface to interact with the adsorbents). The perturbations in the LDOS near the Fermi level remain evident even remote from the Li adsorption site (beyond the next-nearest neighbours). In some papers this longer-range effect is ascribed to be responsible for the weak shift in the coadsorbate vibrational spectra observed in the presence of small amounts of alkalis. 7.1.2

Theoretical Models for the Alkali Metal Effect on Coadsorbed Molecules

Most recent theoretical models for the localized nature of the alkali-substrate bonding favour theoretical treatment of the alkali modification effect where the contribution of the alkali-coadsorbate direct interactions dominate over the possible substrate mediated indirect interactions. However, the reduced work function is a general phenomenon for all alkali-metal systems. That is why the starting point of the description of the direct adsorbate-adsorbate interactions should be the common effect of alkali additives concerning the modification of the electrostatic potential in the surface region. All theoretical treatments have shown that the alkali induced modification of the surface electrostatic potential causes a significant shift of the CO-related

Chapter 7.

292

molecular orbitals to larger binding energies (as referred to the Fermi level of the substrate). The theoretical treatments concerned here are: (i) the generated perturbation theory, applied to CO-K/Ni( 100) [22]; (ii) All-electron local density functional calculations of an ordered c(2 x 2) CO and K arrangement on Ni( 100) [23]; (iii) Multiple Scattering - X a and Green's function density of states calculations of CO coadsorbed with K or Li on Ni clusters [24]; (iv) Molecular Orbital Method calculations for CO on K-covered P t [25] etc.

(aJ

I

h

-12

-10

l

-a

l

-6

I

-4

-2

4

I I

2

4

Fig. 7.4. The positions and widths of the 4u, 5 0 , l x and 2x' CO bands for (a) a I< layer on c(2 x 2) CO layer on a Ni(100) three layer film, and (b) a c(2 x 2) CO Ni(100) (from ref.[23]). Long and short vertical bars indicate positions of MS-Xa energy levels with a weight w > 10 % and 1 % < w < 10 %, projected onto C spheres. (from ref. [24])

+

Fig.7.4. illustrates the calculated densities and positions of the CO 4a, 5a, In and 2n' levels for clean and K-covered Ni(100) [23, 241. From the same theoretical calculations one can also predict a shift towards higher binding energies of the core C 1s (by 0.34 eV) and 0 1s (by 1.25 eV) levels. There is no doubt that the electrostatically-induced changes in the energy positions and the densities of the electronic states of the coadsorbed molecules are expected t o alter the way of mixing with the substrate and the alkali

-

-

7.1. Promoting Effect of the Alkali Metal Additives

293

electronic states. This uniform electrostatic shift of all levels is contrary to the experimental data where the C 1s and 0 1s binding energies decrease in the presence of alkalis and the 5a shift is negligible (see Section 6.1.). Obviously, the discrepancy with respect t o the experiment should be attributed to the chemical effects due to the predominant direct alkali-adsorbate interactions (provided that the final state screening effect for strong CO chemisorption is not substantially changed by the presence of alkalis). A strong interaction between the 21r* and the Li s-p-like states due to their proximity in energy is evidenced by the calculated contour plots of wave functions in ref. [23]. The rearrangement of the charge in the 27r* levels leads to a pronounced compensating effect on the CO and Li occupied orbitals due to screening. In addition, some mixing of Li p and CO I n is also considered. The theoretical results, shown in fig. 7.4., also give some evidence of a direct interaction between the K 3 p electrons and the CO I T states elicited by a pronounced broadening of the CO lT-derived band. This result supports the experimental data for CO adsorption on Cu(100) and Ru(0001) covered with one K layer "261. Several theoretical models have been proposed for the direct alkalicoadsorbate interactions. Taking into account the wealth of experimental results which show a great variety of behaviour of the mixed overlayers, the systems are divided into two main categories: (i) systems where the alkali-adsorbate interactions are weaker than the adsorbate (alkali)-substrate interactions and (ii) systems where the alkali-adsorbate

interactions are predominant.

For weak interactions, the interaction energy (within the framework of the effective medium theory) consists of two terms. The first accounts for a pure electrostatic (dipole-dipole) interaction between the adsorbates, and the second, for direct or indirect hybridization between the adsorbates [15, 271. The change in adsorption energy of the molecular adsorbate (2) induced by the presence of an additive adatom (1) is given by the relationship:

6E =

J (1

6$Ta(r)n2(r)dr

+6

J

nz(E)EdE.

--w

In the first term, 6$1(r) denotes the difference arising in the electrostatic potential after introducing an alkali adatom to the molecule/ substrate system, n2 is the charge density induced by the coadsorbed molecule. The integral is over the near-adsorbate 2 region denoted by 'u' and the superscript ( - u ) indicates that only charges outside region 'a' are to be included in calculations of the electrostatic potential. The second term is the difference in one-electron energies with and without an alkali additive, integrated over the molecule induced one electron density of states nz(E). Assuming that for alkali adatoms with large dipole moments the electrostatic dipole-dipole interactions will prevail, then the first term is considered in more detail in ref. [15, 271.

Chapter 7

294

Following the self-consistent calculations of electrostatic potentials, 641, for alkali adspecies on a jellium surface (see fig. 7.1) described in the previous Section, it becomes obvious that, as 641 is negative for alkali adatoms, one should expect that ~ E < B 0, i.e. the coadsorbate adsorption energy becomes larger. This approximation gives a quantitative description of the direction of the promotion effect. The predicted stabilization of the weakly bound Nz molecular adsorption state on Fe( 111) estimated by combining the calculations of the electrostatic potential ( d d l d r ) and the dipole induced by the molecule pl is 9 kJ/mol. This agrees well with the experimental data for weakly bound species and low alkali coverages. Obviously the model can give a satisfactory explanation in the case of very low alkali coverages when the magnitude of the electrostatic potential is largest and the alkali-coadsorbate interactions are weak compared to the strong alkali-substrate coupling. Another useful quantitative conclusion which is obtained from the electrostatic model is the dependence of the magnitude of the electrostatic potential on the distance outside the surface (see fig. 7.1). Close to the surface, the electrostatic potential is rapidly screened and this explains why the coadsorbed molecules are more strongly affected than the coadsorbed atoms. This dependence of the electrostatic potential on distance is also supposed t o be one of the reasons for the difference between the stabilization effect and the effect on the activation barrier for dissociation. The electrostatic contributions from eq. (1) can account for a Stark shift in the molecular stretching frequency due to the electric field being induced by the alkali adspecies. Model calculations for CO on a Cu cluster in the presence of a n external electric field of the same magnitude as that induced by alkali additives at low alkali coverages, have predicted a C-0 frequency shift of the order of 50-100 cm-'. This value is close to the values measured in the limits of very low alkali coverages [as]. Obviously, additional interactions should be considered in order to rationalize the dramatic reduction i n C-0 stretching frequency at high alkali coverages. On the basis of the experimental results it can be postulated that the electrostatic interactions and the shifts in the antibonding molecular levels induced by the electrostatic interactions are important at low alkali coverages and can explain the relatively weak perturbations in the adsorption state of the coadsorbed molecules (as summarized in Section 6.8.). However, the observed substantial changes at high alkali coverages, when the coadsorbate-alkali distances are shortened and the alkali-substrate coupling is substantially reduced, require theoretical models which account for direct coupling between the alkali valence electrons and the coadsorhate molecular orbitals. One of the pioneering works where the direct interactions due to an overlap between CO 27r* and K 4s orbitals are considered as predominating at short distances, is the simple LCAO calculations for CO adsorption on p(2 x 2)K/Fe(100) [29]. In these calculations, only the Fe 3d and K 4s states are considered as important for coupling with the CO 5 a and 27r* states. The calculations have shown that the coupling of I< 4s and CO 27r' causes:

-

(i) an energy shift, to higher binding energies of the bonding molecular orbitals formed between these states, i.e an increase of the adsorption

7.1. Promoting Eflect of the Alkali Metal Additives

295

energy of CO; (ii) splitting and energy shifts of the 2 ~ derived ' orbitals above the Fermi level (in good agreement with the IPS results), and (iii) no change in the 5a energy position which is also in agreement with the experiment. The recent total energy calculations for coadsorbed K and CO using a Pt15-K2-C0 cluster where the C-K distance is 3.0 A, consider K as an ion with effective charge q* = 0.6 [30]. As a result of the charge transfer, a local filling of the P t 5d metal orbitals over the region where they overlap with the K 4s is found. The most dramatic effect induced by the coadsorbed K is the reduction of the internal CO bonding charge, Qc-0 from 1.25e for CO on a clean surface t o 1.17e in the presence of K. This effect is ascribed to an indirect charge transfer from K to GO through the surface, increasing the occupation of the CO 27r* orbital. The calculations also allow the prediction that the K-induced stabilization of the GO adsorption state is accompanied by a change of the CO adsorption place from an on-top t o a bridge position. In this model, the effect of the electrostatic potential of the K ion has been found to be rather weak and the calculated energy shifts of the 3a, 4a and l 7 r CO states are negligibly small. This is explained assuming that the electrostatic potential of the K ion is compensated for by the electron charge flow into the CO molecule. Thus, the CO molecule is considered as negatively charged and its local potential reduces the effect of the external field due to the K ion. The stabilization factor in this model for the formation of a K-CO complex at the surface is supposed t o be the electrostatic attraction arising between the negatively charged CO and the K ion. The contradiction of this model with the recent views for the nature of alkali-substrate bonding, lies in the suggestion about a charge transfer from the alkali to the substrate. Another piece of theoretical evidence saying that the interactions between the alkali metal and CO are predominantly ionic, is based on simple energetic considerations (using a Born - Haber cycle) and the generalized valence bond method [31]. The Born - Haber cycle consisting of the steps, schematically given below, demonstrates that the ionic K-CO bonding is energetically favourable:

+ e; + 2.0 eV; (b) CO, + e g +. CO, + 1.5 eV; (c) K$ + CO, (K+CO-) - 5.3 eV. (a) K,

+ K:

+

It is obvious that, with respect to the separately bound CO, and K,, the ionic complex formed is bound by an extra 1.8 eV due to the Coulombic stabilization. In this ionic K+CO- model (in contrast to the previous one [30]), the electron charge to the CO 27r* orbital has been transferred directly from K rather than through the surface. In this model the stabilization of CO is through K, and the direct substrate-GO bond may be weakened due to the

296

Chapter 7.

additional electron-electron repulsion between the negatively charged CO and the substrate. The calculated C s O bond length and stretch frequency for CO- in the ionic complex (1.27 A and 1505 cm-l) and the predicted downward shift of the substrate-CO vibrational frequency agree well with the experimental data presented in Section 6.1. This model can also explain satisfactorily the observed CO-induced removal of the alkali ns-valence excitations and the disappearance of the Auger peak associated with the decay of the K 3 p hole as being due t o the complete transfer of the alkali valence electron. The shifts of the C 1s and 0 1s levels t o lower binding energies in the presence of alkalis are also consistent with a CO- species. In terms of the proposed model one can also explain the observed coincident alkali and CO desorption because, as outlined above, the CO stabilization is governed by direct Coulombic interactions with the alkali adspecies. Here, it should be stressed that the behaviour, predicted in this model fits in well with the relatively strong alkali effect observed at high alkali coverages where the K adspecies attain a metal-like character, i.e. when the alkali-adsorbate interactions become predominant. This is not surprising because the application of the model implies rather weak coupling of the alkali to the substrate, which is observed at high alkali coverages. It is also obvious that actually one cannot expect the existence of completely ionized species on a metal substrate and therefore the more accurate presentation of the surface complex is K6+C06- . The effects produced by the strong electric field in the K6+CO-' complex (formed at saturation (0.33) K coverages on Ru(0001)) have also been studied by cluster calculations [32] in order to explain the following effects which were observed experimentally by UPS, ARAPS and vibrational spectroscopy: (i) the splitting of the CO 1~ level; (ii) the rehybridization of the CO 4a orbital and (iii) the drastic decrease in the C-0

stretching frequency.

The calculations of clusters representing: (i) normal chemisorbed CO (sp3 carbon), (ii) rehybridized CO (sp' carbon) and (iii) a EC-0salt complex (where the C-0 bond is parallel to the surface plane (in contrast to CO;) considered in the previous models) have led to the conclusion that (ii) and (iii) are not favourable, i.e. the alkali effect cannot be explained by simple variations in the covalent bonding. A satisfactory explanation of the experimental data ( 1 splitting, ~ 40. hybridization, C-0 stretching frequencies and the CO induced work function changes observed at alkali coverages beyond the work function minimum) is given when the calculations are performed by placing a NiCO cluster in a strong electric field which can be experienced next to a K + ion (at a distance which is the sum of the C covalent and the I< ionic radii). It has been found that, as a result of an extra charge flow from the metal to the CO 2~ orbital, CO becomes negative and a #*+CO-' complex can be formed. The theoretical models dedicated to alkali metal-CO coadsorption on transition metals can be applied generally to another similar coadsorbate, such as NO, N2 or 0 2 , which behaves as electron acceptors in the formation of the adsorption bond.

7.1. Promoting Effect of the Alkali Metal Additives

297

-

>

t!

Bc S

w

0.5

Fig. 7.5. HzO adsorption energy and HzO induced change of the cluster dipole as a function of the tilt angle (Y for zo = 2.7 A. The curve labelled XKO = w is calculated for zo = 2.5 A without K. The vertical arrows show the equilibrium values of A p with and without K (from ref. [33])

A completely different behaviour and action can be expected when molecules, such as HzO and NH3 coadsorb with alkali adspecies. Since these admolecules have the same sign of their dipole moment as that of the alkali adspecies, attractive interactions can be expected only if they reorient in the presence of alkalis attaining a dipole moment opposite to that of the alkali adatoms. Ab initio cluster calculations of a Ptlo-K-H20 cluster have shown that, in the presence of K , the minimum of the H20 potential shifts from a tilt angle of 90' for a clean surface to a tilt angle of 160", i.e. the Hatoms point towards the surface [33]. AS is shown in fig. 7.5., the tilted geometry is also more stable leading t o an increase in the adsorption energy by N 35 kJ/mol. The calculated dipole moments for H20 adsorbed with and without K are -2.0 D and +1.2 D, i.e. they fit in sign but are double the size than the actual values found experimentally. These calculations confirm the predictions of Norskov et al. based on the 'effective-medium theory' [15, 271 for a reorientation of the H 2 0 molecule due to an electrostatic influence. The

298

Chapter 7.

calculations have shown that the interactions between K and the reoriented H2O are characterized by a small charge transfer (- 0.06 e) from H2O to K which is consistent with the upward shift of the K 3p level detected by UPS. The selected theoretical approaches presented in this Section confirm the general views about the complexity in behaviour of mixed alkali-coadsorbate overlayers. It is obvious that theoretical models which account only for the effect of the electrostatic potential perturbations explain satisfactorily the relatively weak perturbations in the molecular bonding of acceptor-like molecules (e.g. CO) within the limits of low alkali coverages. This has been also demonstrated by simulation of the contribution of the electrostatic interactions replacing the alkali by NH3 (a molecule which also induces substantial work function changes) [34]. Dipole-dipole interactions are also responsible, to a large extent, for the modifier-induced reorientation of molecules such as H 2 0 and NHB. The theoretical models by which explanations are sought t o the drastic changes in the molecular bonding and electronic structure account for predominating alkali--coadsorbate interactions involving an electron charge transfer and strong coupling (Coulomb interactions) between oppositely charged alkali and coadsorbate species. 7.2

THEORETICAL TREATMENTS OF THE POISONING EFFECT OF THE ELECTRONEGATIVE ADDITIVES

A number of theoretical studies have already been done on the factors contributing t o the poisoning action. As is the case with alkali additives, several approaches have been made by the theoreticians. In many cases, calculations of the modification effects induced by alkalis and electronegative additives were done in parallel by different workers. They found that the general difference in the typical electronegative additives is their smaller size and the small change in the electrostatic potential induced by the formation of an adsorption bond. Because of the relatively weak perturbations in the electrostatic potential, induced by the electronegative additives, the attempts to explain the effect of the electronegative additives by the sign and the magnitude of the electrostatic potential around the adatom on a jellium edge are less successful, especially in the case of small adatoms. Generally, considering the first term in eq. (l), it can be predicted that the introduction of additives leading to positive 64 changes will cause a reduction of the adsorption energy of acceptor-like adsorbates. By using self-consistent calculations of the electrostatic potential could predict that the poisoning around PI S, and C1, Lang et a1.[15] strength will increase in the sequence P, S , C1 due t o an increase in the electrostatic potential, associated with the adatom, in the same direction. The range of these interactions is short (up t o 3-4 A), i.e. of the order of the screening length of the metal. The weaker effect of 0 (electronegativity 3.5) compared to C1 (electronegativity 3.0) is attributed to the smaller size of 0. Being embedded in the jellium, the 0 electrostatic potential becomes more efficiently screened, which rapidly diminishes the effect. Since in actual catalysis oxygen atoms are often present in connection with the alkali promoters, a question arose as to what extent the presence of 0 will destroy the alkali effect. As

7.2. Theoretical Treatments of the Poisoning Effect o f . . .

299

has been shown in ref. [15], considering the location of both adatoms above the jellium surface it turns out that the range and the absolute magnitude of the K electrostatic potential is much larger. Consequently, the negative effect of 0 is expected t o be rather small. Obviously, the electrostatic potential model can explain qualitatively the modifier effect of highly polarized adatoms located well above the jellium plane. Usually, this is not the case with electronegative additives, because, as shown in Section 4.2., most of the electronegative adatom bonds are of a prevailing covalent character with a negligible charge movement in the z direction. In these systems, the adatom induced perturbations of the discrete nature of the real surface (see the substantial shifts in the W surface electron levels induced by 0 shown in Table 7.1.) and the possible direct (orbital overlap) or through metal interactions gain more importance. The effect of the electronegative additives on the substrate electronic structures has been treated in a group of calculations using self-consistent models where the substrate description is closer to the actual one. These methods require a high degree of symmetry of the adsorbates and, in most cases, they are performed a t higher modifier coverages, the modifier being located in a nonrealistic site.

5 a

a

1

lb-

1

A

Fig. 7.6. Partial densities of states, DOS, projected onto C spheres for and (b) CO-S/Ni (from ref. [23])

(a)

CO/Ni

Wimmer et a1.[23], using the same method applied to alkali adsorption on Ni(100) (FLAPW calculations), have confirmed the expectations that the weak perturbations in the electrostatic potential induced by adsorption of electronegative additives (S on Ni(100)) would induce a very small shift (less than 0.5 eV) in the molecular levels of the coadsorbate. As can be seen

300

Chapter 7.

in fig. 7.6. there is a substantial broadening in the lw/5a band from 1 eV for CO/Ni( 100) t o 2.5 eV for CO-S/Ni( loo), which indicates direct S-CO interactions. The most important results in this study are that:

(9 the

formation of a Ni-S bonding leads to a substantial depopulation of the Ni 3dza-like states which, together with the repulsive short range S-CO interactions accounts for the site blocking effect

(ii) the charge redistribution leads to a reduced Ni 3dz2 density near the C atom located in a next t o S top-site, which explains the destabilization effect of S on the CO adsorption state.

c L.-

.-..

1

.I

Fig. 7.7. (a) Valence charge densities (in a.u.) and (b) Fermi level LDOS's, in (eV x a3)-' for a C1 x &)R45°-covered (top) and a clean Rh(100) 2 layer film. Hatched regions correspond to the same range of densities as those observed on a

(a

clean surface. (from ref. 191) One of the well-known studies is the self-consistent linearized augmented plane wave (SLAPW) calculations of Feibelman and Hamann who examined the influence of several poisons (Cl, S, P) on the valence charge densities and on the Fermi level local density of states of Rh(001) 2-layer slabs. As illustrated in fig. 7.7., the additive induced charge density perturbations decrease exponentially with the distance and are screened to zero in the immediate vicinity ( at nearest neighbour distances 5 2.5 A). In contrast to this, similar plots for the Ef-LDOS reveal poor screening extended at distances as large as that to the next-nearest neighbour. It is predicted that this relatively longrange effect might play a role in surface chemistry supposing that Ef-LDOS are a measure of the number of electronic states available for interacting with

7.2. Theoretical Treatments of the Poisoning Effect of. . .

301

the reagent. The strength of the effect is calculated to increase in the sequence P, S, C1, i.e. with increasing additive electronegativity. By using the extended Htckel tight-binding model, Zonnevylle et al. [35] have traced the correlations between the interadsorbate separation and the poisoning effect for p(2 x 2)CO p(2 x 2)s on a Ni(100) three layer slab. The calculations indicate for short separations (CO on-top site shared with S in a fourfold site), the S-GO interactions are strongly repulsive and mainly a site blockage effect is operative. At intermediate separations, where CO is moved in a bridge position, sharing one substrate atom with S, the resulting weakening of the Ni-GO bonding is due to a reduced CO 27r population. This effect is explained by the reduced ability of the specific (zzand yz) Ni surface orbitals for back donation into the CO 2%' orbitals. In this intermediate position, the calculations also predict a reduction in the CO 5 a contribution to the bonding due to the locally enhanced density associated with S p levels. The main conclusion in this study is that at short distances (S-CO 5 1.8 A) sulphur induces a site blockage by means of a direct repulsive mechanism. The modification effect observed at S-CO distances of the order of 2.7 %I is a form of through-bond coupling, i.e. S acts indirectly by perturbations of the electronic structure of the substrate. In the case of larger separations (S-CO 2 3.6 A) the predicted small reverse effect is difficult to explain. The effect of electronegative modifiers on the substrate valence states has also been predicted by Benziger and Madix who applied simple LCAO-type calculations to describe GO adsorption on Fe(lOO), modified by S, C or 0 [29]. They have taken into account only the interactions between the Fe 3d valence states, modifiers npstates and CO 5 a and 2n* molecular orbitals. The calculations have revealed that the modifier's npstates interact with the same Fe 3d states which are involved in the Fe-GO bonding, resulting in the reduction of the Fe 3d-CO 2n* coupling. In addition, it is found that in the case of S , direct 3 ~ 2 interactions ~ ' are also possible, pushing the GO 2n* level to higher energies above the Fermi level, i.e. further reducing the coupling with the substrate. The observed differences in strength of the poisoning effect between S, C and 0 is ascribed to a different form of n p 2 n * coupling which is determined exclusively by the size of the modifier and its distance above the surface. This simple LCAO approach allows the prediction that GO 5alsubstrate coupling remains unaffected, which contradicts the experimentally observed destabilization of the 5a-derived orbital in the presence of S. Profound theoretical studies, considering the extent of the effect of several electronegative adatoms (C, S, P, C1) on the LDOS near the Fermi level, have been performed recently by MacLaren et al. [24, 36-38] who applied the muffin-tin approximation to clusters on modified Ni(100), N i ( l l 1 ) and Rh( l l 1) surfaces. They have shown that the effective range even of C1, being the strongest poison, does not extend beyond the next-nearest neighbours, i.e. it is less than 5 A. These calculations indicate that the modifier effect contains two contributions:

+

-

(i) a decrease of the LDOS near the Fermi level which causes a decrease of the matrix elements for both the 5a/metal and the metal/27r* coupling

Chapter 7

302

Fig. 7.8. (top) Plan view of a cluster used t o investigate the poison/Ni( 100) system showing cluster 1 which includes three layers of Ni atoms. The other two clusters add poisons either round the central site (near) or as a remote ring of poisons around the central site (far). (bottom) Calculations for the clusters. T h e LDOS’s are calculated above the central Ni atom in a top layer a t a distance of 1.23 A: (-) with only three layers of Ni atoms; (- -) with three layers of N i atoms and four near poisons; (-.-.) with three layers and eight far poisons. (a) S with vertical spacing between the first Ni plane and S, Z = 1.3 A; (b) C, the first plane is relaxed by 0.2 A and Z = 0.1 A; (c) C, except there is no relaxation and 2 is the same as for S , i.e. 1.3 A. (from ref. [36, 371).

and (ii) a direct interaction between the additive p levels and GO 5a levels which

7.2. Theoretical Tkeatments of the Poisoning Effect o f . . .

303

are close in energy, so that the modifier-induced increased charge in this region hinders the Sa/metal donation. An important issue in these theoretical studies is the correlations established between the local geometry of the poison adatom, its electronegativity and the strength of the poisoning effect. It has been shown that in some cases the difference in magnitude and range of the poisoning effect is determined by the size (distance of the adatom to the surface) rather than by the electronegativity of the adatom. Fig. 7.8. shows the cluster and the LDOS's calculated above the central Ni atom at a distance of 1.23 A for S and C. These atoms possess thc same electronegativity (2.5) but different covalent radii (1.02 A (S) and 0.77 A (C)), which determines the height of the poison above the surface. It is obvious that C exhibits a much weaker effect on the nearest neighbour sites and almost no effect on the next-nearest ones. This indicates that the closer the position t o the second substrate layer, the weaker the poisoning effect. The authors stressed that the effect is not due to simple electrostatic screening (because they consider neutral species) but rather t o the ability of the poison induced perturbations to propagate across the surface.

0

'02

.04

.06

-08

.lO

.12

.f4

-16

adatom coverage (NL)

Fig. 7.9. Comparison of the variation of 8co with an adatom coverage (normalized to 1 when the poison coverage is 0) to catalyst activity, as calculated by the numerical simulations. The full lines represent the calculation: A-A - nearest neighbour poisoning only (e.g. C); u-e - nearest and next-nearest poisoning (e.g. S). The dashed lines represent the experimental results taken from ref.[39] for Ni(100) modified by: A-A - P, 0-0 - C, V-V - S. (from ref. [36])

The authors in ref. [36] have gone further. They have tried to relate the

304

Chapter 7.

observed induced changes in the density of states to the adsorptive properties of the modified Ni(100) surface and its catalytic activity in the methanation rGaction. They have used a simple approach, assuming that the poison reduces the number of the adsorption sites and that the poison sites have lost their methanation activity. Fig. 7.9. presents the trends in S and C poisoning effects on the adsorption of the most strongly bound Pz-CO, calculated for a 100 x 100 Ni(100) matrix; it is assumed on the basis of the LDOS results, that C poisons only the nearest neighbours whereas the S action extends over the next-nearest neighbours. The calculations agree quite well with the experimental data. This confirms the importance of the location of the poison above the surface predicted by the theoretical results in fig. 7.8.

0

42

44

46

-10

adatom coverage

-12

014

*46

(ML)

Fig. 7.10. Comparison of the rat+eof methanat,ion with an adatom coverage to the catalyst ‘activity, for various ranges of t,he interaction. Range: 0-0 1.8 8, (nearest neighbour poisoning); 0-0 - 3.9 8, (nearest & next-nearest poisoning); X-x 5.3 8, +--+ - 6.1 8.The dashed line presents the experimental dat,a (from ref. [ 3 6 ] ) The application of these simplified model calculations for explaining the drastic reduction in the methanation rate by the presence of S have not been very successful. As illustrated in fig. 7.10., comparison of the experimental and the theoretical plots shows that the predicted reduction assuming deactivation extended to the next-nearest neighbours is much weaker than the actual one. As discussed in the next Chapter, this discrepancy is due to the fact that the influence of S in actual catalysis involves also a substantial reduction of the dissociative adsorption rate and the diffusion rate along the surface, which are not taken into account in this study. Since present theoretical approaches cannot offer a complete quantitative explanation of the mechanism of the poisoning effect, statistical mechanical

7.2. Theoretical Treatments of the Poisoning Effect o f . . .

305

E le VI 3 2 1

0

Fig. 7.11. Total energy changes versus reaction coordinate along the preferential dissociation path, illustrating the influence of C1 and K on CO dissociation. E’ is the activation energy for CO dissociation (from ref. [44])

models (using Monte Carlo simulation methods) have also been applied in order to describe experimental results that were observed [40-431. In these studies the term ‘effective’ lateral interactions is introduced which accounts both for the direct interactions and those mediated through the substrate perturbations. These lateral forces are limited to pairwise interactions the magnitude and sign of which are varied in order to model the poisoning effect. The general observation on different pairwise interactions between the poison adatoms (of magnitudes -5 (attractive) t o +10 kJ/mol (repulsive)) is that, in the case of attractive interactions between the poison species, island formation is favoured [40]. This leads to a weaker poisoning action, as observed experimentally. For repulsive forces the effect is more severe and can be fitted to the severe poisoning effect observed e.g. by the presence of S. Calculations for a three-state lattice-gas with nearest-neighbour interactions between the poison (X) and the coadsorbate (A) with varying chemical potentials of X and A have given an explicit set of criteria that can describe the strong poisoning effect [41, 421. It has also been shown that in addition

Chapter 7

306

the next-nearest neighbour perturbations may be chosen in such a way as to enhance the observed effects of the nearest neighbour interactions [43]. Finally, the results of cluster calculations are mentioned where the influence of electronegative and electropositive additives on the dissociation propensity of acceptor-like molecules, such as CO, have been considered [40]. The authors have used the molecular orbital method and clusters of Ni4, Ni&l and Ni3K with a flat lying CO molecule (believed to be the precursor state for dissociation). Fig. 7.11. shows the calculated total energy changes as a function of the reaction coordinate for the three clusters. Obviously, the calculations predict that the barrier for CO dissociation increases slightly (by 0.3 eV) in the presence of C1 and substantially drops in the presence of K. However, as has been already discussed in chapters 5. and 6., the experimental data have not given unambiguous proof of the effect of additives on the dissociation barrier, although it is believed to exist. The difficulties come from the fact that the activation energy for dissociation is more strongly dependent on changes in the potential energy surface of the dissociation products. When summarizing the theoretical results selected in this Section it can be concluded that in the case of electronegative additives the poisoning effect on acceptor-like molecules is mainly due to perturbations of the electronic $ructure of the substrate in the vicinity of the adatom and strong repulsive short range interactions which are responsible for the site blocking effect.

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[38]

D.R. Jennison, J . Vac. Sci. Technol. A5 (1987) 684 H.P. Bonzel, G. Pirug and J.E. Mtller, Phys. Rev. Letts. 58 (1987) 2138 D. Lackey and D. King, J. Cliem. SOC.Far. Trans. I83 (1987) 2001 M.Zonnevylle and R. Hoffmann, Langmuir 3 (1987) 452 J.M. MacLaren, J.B. Pendry, D.D. Vvedensky and R.W. Joyner, Surface Sci. 162 (1985) 1322 J.M. MacLaren, J.B. Pendry and R.W. Joyner, Surface Sci. 165 (1986) L80; ibid. 178 (1986) 856 J.M. MacLaren, J.B. Pendry, R.W. Joyner and P. Meehan, Surface Sci. 175

[39] [40] [41]

M. Kiskinova and D.W. Goodman, Surface Sci. 108 (1981) 61 A.E. Reynolds, D.J. Tildesley and J.F. Foord, Surface Sci. 191 (1987) 239 P.A. Rikvold, J.B. Collins, G.D. Hansen and J,D. Gunton, Surface Sci. 203

[42] [43]

J.B. Collins, P.A. Rikvold and E.T. Gawlinski, Phys. Rev. B 38 (1988) 6741 J.B. Collins, P. Sacramento, P.A. Rikvold and J.D. Gunton, Surface Sci. 221

[44]

D. Tornanek and K.H. Bennemann, Surface Sci. 154 (1985) L261

[32] [33] [34] [35] 1361 [37]

(1986) 263

(1988) 500

(1989) 277

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Chapter 8

MODEL STUDIES OF SURFACE REACTIONS O N MODIFIED SURFACES

Systematic studies of the adsorptive properties of clean and additive-modified single crystal metal surfaces have provided a foundation on which to build with further applications of this knowledge and this will ensure progress in future fundamental catalytic research. While the low-pressure studies with well-defined single crystal surfaces are eminently suited for the description of the elementary adsorption/ desorption processes by means of the powerful surface science techniques, the correlation between these results and the catalytic surface reactions calls for kinetic measurements maintained at high pressures. As has already been described in Chapter 3 . , dual-chamber systems have been developed in order to bridge the adsorption and reaction studies (see fig. 3.2). They ensure a characterization of the surface structure and composition before and after the high-pressure study by means of selected surface sensitive techniques in an UHV analysis chamber. The latter can be also used for parallel UHV adsorption studies necessary for accumulating the basic information on: (i) the electronic and geometric structure; (ii) the composition, and (iii) the adsorptive properties of the catalyst surfaces. These studies are rather promising in that they may well result in a better understanding of surface modifier action which alters catalytic activity and selectivity. In catalysis the additive action includes both the ensemble (site blocking) and the ligand (electronic) effect. Usually, both effects act simultaneously, so that it is difficult to determine the relative contribution of each one That is why detailed knowledge about the surface composition and ordering of the various additives on well-defined surfaces and a comparison of the adsorptive and catalytic properties of these surfaces can shed light on the relative importance of the ligand and the ensemble effects in a particular cata!ytic reaction.

309

Chapter 8.

310

This Chapter is limited to several examples of model surface reaction studies. They illustrate the success in understanding the additive influence at molecular and atomic level achieved by parallel kinetic measurements, surface analyses and surface science adsorption studies. 8.1

METHANATION A N D FISCHER - TROPSCH SYNTHESES

CO hydrogenation concerns catalytic problems of great importance for the production of synthetic fuel and other important for the chemical industry hydrocarbons. This reaction is very appropriate for model single crystal studies because it is structurally insensitive. As has been shown by a number of data collected using single crystal catalysts, the rates and the activation energies agree well with the values found for real catalysts [l-41. This facilitates the extension of the information gathered in model studies with well-defined single crystal metal surfaces to real high area supported catalysts. The mechanism of hydrocarbon synthesis from CO and H2 involves the following main steps: (i) adsorption and dissociation of CO; (ii) dissociative adsorption of

H2

;

(iii) hydrogenation of the CO dissociation products to a variety of adsorbed hydrocarbon intermediates (involving formation of adsorbed monoiiiers and polymerization to form carbon chains) and water (0 removal is also possible via COL,formation), and (iv) formation and desorption of the final hydrocarbon products.

A number of side reactions, such as hydrocracking, formation of oxygenate products, coke deposition, etc. are also possible, which complicates the description of the process under the conventional catalytic conditions. The activity and selectivity of the different metal catalyst. surfaces varies significantly. As could be seen from the model surface science studies [I, 51, two carbon types can be distinguished on the surface: (i) an active carbidic form which participates in further hydrocarbon formation and (ii) a graphitic inactive phase which is formed after carbidic carbon has reached a certain level (at low H2 pressures and high temperatures). The growth of a graphite phase leads to deactivation of the catalyst surface. Since the surface concentration of hydrogen is controlling the steady state level of the carbide, dramatic changes can be expected in the presence of additives which affect: (i) the adsorptive ability of the surface with respect to the reactants hydrogen and CO; (ii) the CO dissociation propensity which determines the carbon deposition on the surface, and (iii) the lifetime of the reaction products (hydrocarbons, water and COz).

8.1. Methanation and Fisher 8.1.1

-

Tropsh Syntheses

311

Effect of Electronegative A d d i t i v e s on the Catalytic A c t i v i t y and Reactivity

Model catalytic studies of the C O methanation reaction on a Ni( 100) surface have shown a severe reduction in the reaction rate on the introduction of small amounts of S [6-81. These model experiments are performed in the apparatus described in fig. 3.2. It has been found that CH4 formation on a clean surface is independent of partial pressure of CO, PCO,and is first order with respect to the Hz partial pressure, PH,. T h e rate-limiting step is supposed to be the hydrogenation of the surface ‘carbide’ carbon leading t o the formation of a methyne group [ll]. The CO hydrogenation under the reaction conditions PH,fPc0 = 4 and T, = 500-700 K proceeds with a low level carbide-like carbon on the surface (less than 0.1 ML), which increases up to 0.4 ML at reaction temperatures above 700 K. At T > 700 K, the formation of the inactive graphite phase becomes favoured wl~ichdeactivates the catalyst surface [l]. As outlined above, a similar increase of the surface carbon level and build-up of a graphite phase is observed when the PH,/Pco ratio decreases. Fig. 8.1. presents the Arrhenius plots for CH4 synthesis on a clean and on a S-modified Ni( 100) surface. From the Arrhenius plots it is obvious that the reaction rate drops significantly. Furthermore in the presence of S, the deviation of the Arrhenius plots from linearity starts at lower temperatures, similarly to the case of reduced PH, on a clean surface. Comparison of the slopes of the linear segments of the Arrhenius plots obtained for a clean and S-modified surface shows that they are the same. This indicates that, within the accuracy of the experiment, the introduction of S does not cause a change in the activation energy of the methanation reaction. The studies of the reaction rate at various partial pressures of the reactants have shown that the reaction order with respect to P H ~ and PCO depends on the actual S coverage. A t ‘low’ S coverages (0 < 0s < 0.1) the reaction rate remains first order with respect to PH2 and zero order with respect to Pco. A characterization of the catalyst surface after the reaction indicates that, compared to a clean surface, the steady state concentration of the ‘carbide’ carbon level increases with 0s (for S coverages ranging from 0 to 0.1). This increase in the surface carbon level is proportional to the sulphur coverage a t 0s < 0.1 and is substantial (compared to a clean surface level) a t reaction temperatures when deviation of the Arrhenius plots from linearity occurs. The distribution of the reaction products is also affected by the presence of S. The relative amount of higher unsaturated hydrocarbons is found to increase at the expense of CH4 formation. This indicates h a t the S induced deactivation of the catalyst is also accompanied by a n alteration of its selectivity. At high S coverages (beyond 0.15) the reaction rate is found to become first order with respect to PCO and the Auger characterization of the catalyst surface after the reaction shows a severe decrease of the steady state carbon level below that for a S-free catalyst surface. Before trying to explain the effect of S on the Ni(100) activity and selectivity, i t should be mentioned that almost the same effect on the CO hydrogenation has been reported for a S-covered Ru(0001) catalyst [3]. The S effect

Chapter 8.

312

1\

lo

1

i

CLEAN N1(100)

I \

N

3-

10

1

1.2

1

1.4

1

1

1.6

1

1

18

1

2.0

1 / 1 . 1 0 ~ ( ~ ~ )

Fig. 8.1. Arrhenius plots of the methanation rate over a clean and sulphided catalyst at total pressure 120 Torr and PH,/Pco= 4 (from ref. [S])

at low and high S coverage regions can be explained satisfactorily, taking into account the behaviour of the Hz/S/Ni(lOO) (Ru(0001)) and CO/S/Ni(100) (Ru(0001)) adsorption systems, described in sections 5.1 and 5.4. As evidenced by the CO T P D da ta in figs. 5.3. and 5.6., at low S coverages, there are still enough more-tightly bound (&-state) CO molecules which are providing active carbon on the surface, albeit with a reduced efficiency compared to the clean surface. That is why the fact that the steady-state C level increases at Bs < 0.1 means that the rate of methyne formation is strongly inhibited. This should be associated with the following effects of S (described in details in Section 5.4.): (i) an S induced deficiency of Ha due to a drastically reduced adsorptive ability of the surface with respect to dissociative hydrogen adsorption and (ii) S induced changes of the hydrogen surface mobility reflected by the drastic reduction of the surface diffusion coefficient in the presence of sulphur.

8.1. Methanation and Fisher

-

313

Tropsh Syntheses

1.OI

0-0

0.8 0.6

0.4

0.2 0.0

1

0.0

0.1

0.2

0.3

0.4

0.5

0s

Fig. 8.2. Relative changes in the methanation rate, NcH,, as a function of sulphur coverage on Ni(100), compared to the S induced relative changes of the H a ( B H ) and (data Pz-CO,(Bco) coverages and Ha surface diffusion coefficient at 300 K (DH). taken from refs. [8, 131)

Fig. 8.2. presents the relative methanation rate, the corresponding relative Ha and &CO, coverages (121 and the hydrogen diffusion coefficient [13] as a function of 8s. It is mentioned here that the hydrogen diffusion data are obtained for S-covered Ru which exhibits almost the same decline in the methanation rate in the presence of S as does Ni(100) [3]. Comparison between the reaction rate reduction and the reactant coverage reduction reveals that the poisoning effect on the methanation rate is more severe (one S atom deactivates more than 10 surface sites). A better fit exists between the decline in the methanation rate versus sulphur coverage and the corresponding hydrogen diffusion data at T = 300 K [13]. Consequently, summing up the effects leading to a hydrogen deficiency on the surface and t o a restricted hydrogen surface mobility, both the severe reduction of the catalytic activity and the alteration of the selectivity to production of heavier and unsaturated hydrocarbons can be explained satisfactorily. It is obvious that while the CH4 formation requires four hydrogen atoms the relative H/C ratio is smaller for the heavier and unsaturated hydrocarbons. At high S coverages, the most tightly bound CO state which is the main channel for a ‘carbide’ carbon production is eliminated. This leads to a deficiency of ‘carbide’ carbon as confirmed by the absence of surface carbon after the reaction. Consequently, a t high S coverages, in addition to the reduced ability of the surface to hydrogenate, the carbon atom, the C formation step (CO dissociation) is also inhibited due to the decrease in the life of the CO molecular state and the

314

Chapter 8.

eventual increase in the activation barrier for CO dissociation. As has been shown in ref. [ll], the activation energies for carbide formation and carbide hydrogenation are rather close. Thus, one cannot expect a change in slope of the linear segment of the Arrhenius plots at high S coverages, if the CO dissociation becomes rate-limiting (provided that the S effect on the activation barrier for dissociation is negligible). The model studies of CO hydrogenation on Ni and Ru single crystals agree very well with the observations for a S induced deactivation of supported catalysts [ll-131. The correlations between the reaction and adsorption data for the S induced effects on the methanation reaction rate and adsorptive properties of the surface reveal the mechanism of the S poisoning action. They provide a profound explanation of the prediction of Dalla Betta et al.about the S deactivation action on supported Ni/A1203 catalysts. They related the S effect mainly to the reduced activity of the surface with respect to the hydrogenation steps, while the formation of C-C bonds is supposed to be less severely hindered [111. The correlation between the drastic reduction in methanation rate in the presence of small amounts of S on the surface and the inhibition of both the hydrogen dissociative adsorption and the hydrogen adatom diffusion on the surface cannot be explained by a local steric effect which predicts much weaker perturbations. Obviously, the possible perturbations in the LDOS near the Fermi level should be considered. The effective range in the case of S has been predicted t o extend t o the next-nearest neighbours. As outlined in ref.[13], this longer range electronic effect is probably contributing to the hindrance of hydrogen diffusion on the surface which explains the severe S-induced reduction of the methanation rate. Obviously, the disagreement between the theoretical calculations of the methanation rate on S-covered Ni(100) [17] and the experimental results is due to the fact that in the theoretical study only the S-poisoning effect on CO adsorption is considered. The contribution of the electronic (ligand) factor to the S poisoning effect on the methanation reaction has been confirmed by comparative studies of the Cu and S influence on the methanation reaction on Ru(0001) [3] and the P and S influence on the methanation reaction on Ni(100) [7, 81. Cu and P have smaller Pauling electronegativities (1.9 and 2.1 compared to 2.5 of S). Cu is found t o exhibit only a simple blocking effect and at 0cu = 0.1 the methanation rate is decreased only by 10 % compared to a more than 80 % decrease in the presence of the same S coverage. The substantially smaller magnitude of poisoning in the case of P, illustrated in fig. 8.3., is consistent with the adsorption results reviewed in sections 5.1 and 5.5. These comparative studies illustrate the importance of the ‘electronic’ effect in the poisoning phenomena. The tendency of P to form two-dimensional islands on the surface contribute to its lower poison effect. This tendency is enhanced in the case where the electronegativity difference between the modifier and the substrate is smaller. As outlined in Chapter 5. and ref. [18], when two-dimensional poison islands are formed on the surface, almost unaffected surface patches coexist on the surface up to rather high modifier coverages. In this case the reduction of the adsorptive capacity and the decrease in the reaction rate are less severe than in the case of uniformly-distributed poison

8.1. Methanation and Fisher - Tropsh Syntheses

315

adatoms.

0.2 0.3 0.4 Additive Coverage (ML)

0.1

0.5

0.6

Fig. 8.3. Methanation rate, N C H as ~ a function of S or P coverage on a Ni(100) catalyst at 120 Torr, Pk,/Pco = 4 and reaction temperature 600 K (from ref. [8])

0.0

0.1

0.2

0.3

0-0

Ni(lO0)

4-4

w(110)

0-0

YO

0.4

0.5

0.6

0s

Fig. 8.4. Relative changes in the methanation rate over Ni(100), Mo and W(110) as a function of S coverage. Reaction temperature 600 K (from refs. [6, 9, lo])

316

Chapter 8.

Both catalysts (Ru and Ni) considered above exhibit very similar behaviours in respect of the methanation reaction, which can be related to similar adsorptive properties of these metals with respect to S, GO and Hz. Certain differences in the strength of the S poisoning effect towards the CO methanation reaction is observed for W(110) [9] and Mo(100) [lo]. These two metals differ substantially in their adsorptive properties from Ni( 100) and Ru( 000 1): (i) Mo and W are less stable with respect to a S induced reconstruction which becomes facile at 8s > 0.25; (ii) CO readily dissociates on Mo and W under UHV conditions; (iii) Mo and W exhibit a stronger tendency to metal carbide formation These differences in the adsorptive properties are reflected by the reactivities of these surfaces. Model reaction kinetic studies have shown that under the same reaction conditions the activation energy of the methanation reaction is the same for Ni(100) and Mo((100) (- 103 kJ/mol) [lo] and half that value (- 56 kJ/mol) for W(110) [19]. The typical reaction conditions involve a much higher ‘carbide’ carbon level in the case Mo (100) (compared to Ni(100)) and a stable tungsten carbide phase in the case of W(110). Consequently, one could expect that the comparison of these three catalysts (Ni, Mo and W) would reveal to what extent the poison effect depends on the substrate properties. Fig. 8.4. shows the plots of the relative reaction rates for methanation on S-covered Ni(100), Mo(100) and W(110). Obviously, the S induced reduction in the reaction rate is less severe for Mo(100) and W(110). The correlation between the reaction rates and the coadsorption data for GO and H2 on S/W(llO) and S/Mo(100) has turned out t o be less satisfactory than that in the case of Ni(100). This is due t o complications arising from the additional effects of the CO dissociative products which interact strongly with the Mo and W substrates. This leads t o a substantial disturbance of the surface properties, especially when reconstruction and formation of a new surface phase are favoured. That is why it is difficult to propose a thorough explanation of the observed differences in the strength of the S poisoning effect. A possible reason is that the dissociation ability of the Mo and W surfaces, although strongly reduced, is not completely inhibited up to relatively high S coverages. Indeed, as evidenced by the U H V coadsorption studies, the CO dissociation is completely inhibited on S/W(110) at 8s as high as 0.4. As far as hydrogen adatom diffusion is concerned, no data are available, but it is possible that the hydrogen mobility might be less severely affected by the presence of S on W or Mo. Another important argument is that the Mo and W surfaces tend t o form a rather stable carbide phase, i.e. the action of S on a metal carbide surface, rather than on a clean surface should be considered. It is likely that the carbide surface would be more resistant t o S poisoning and/or would favour the formation of two-dimensional S islands. At this point thorough surface science adsorption studies are needed in order to throw more light on the actual adsorption state of S on Mo and W under the reaction conditions.

-

8.1. Methanation and Fisher 8.1.2

-

Tropsh Syntheses

317

Alkali Promoters in Catalysts for CO Hydrogenation

In contrast to the exclusively deactivating effect of electronegative additives (e.g. S) with respect to the CO hydrogenation reaction, the action of alkali additives is more complex. Although alkalis are considered as promoters, in practice they usually do not cause an increase of the total catalytic activity. The alkali additives action is mainly related to changes in the selectivity of the catalysts, compensation of the S poisoning effect and suppression of the conversion of the active 'carbidic' carbon into an inactive graphite phase [20, 211. The effect of K in model studies of the methanation reaction on Ni( 100) [22] is the same as that observed for supported catalysts [23]. Gas chromatography product analyses and subsequent surface analyses have indicated that the introduction of K causes: (i) a decrease in the rate of methane formation in an almost linear fashion for 0 < OK < 0.15; (ii) an increase of the rate of heavier hydrocarbon production, and

(iii) an increase of the steady-state carbon level compared to a K-free surface.

L

Fig. 8.5. Comparison of product distribution (weight percent) observed for clean and K-covered Ni(100) at reaction temperature of 500 K, pil,/Pco = 4, a total pressure of 120 Torr and @K = 0.1 ML. (from ref. [21])

Chapter 8.

318

Fig. 8.5. presents the K-induced redistribution of the reaction products, which reveals that K acts as a promoter with respect to the formation of unsaturated hydrocarbons and higher molecular weight products. Basically the same alkali effect is observed on K-promoted Fe [23-261 which is the main catalyst used in the Fischer-Tropsch synthesis. It has been found that a relatively larger amount of higher molecular weight hydrocarbons is produced but the total reaction rate is reduced. A series of model studies with Fe foils and moderate surface area catalysts and parallel surface analyses have already succeeded t o clarify several aspects of the mechanism of alkali action in Fischer - Tropsch reactions. A certain measure of success has also been achieved in the attempts to identify the nature of the C-containing species on the surface and to reveal the chemical state of the alkali promoter under the reaction conditions.

50 40

K

y 30 0 z 20 10 0

CARBON NUMBER

CARBON NUMBER

Fig. 8.6. A distribution of products obtained over K promoted and unpromoted iron powders. The products were evaluated under steady state conditions (Hz : CO = 3 : 1, Total pressure 7 bar, T = 540 K. (from ref. [26])

Fig. 8.6. presents the distribution of the reaction products obtained under identical reaction conditions on a clean and on a K promoted Fe catalyst. Together with the increase in percentage of heavier hydrocarbons at the expense of methane production, the oxygenated hydrocarbons production also shifts from alcohols to aldehydes, i.e. the general tendency is to produce hydrocarbons with a lower hydrogen content. In contrast to the case of CO hydrogenation on a K-covered Ni( 100) surface, the presence of K on Fe leads to a reduction in rate of formation of all hydrocarbons, i.e. in the case of Fe K acts as a deactivator with respect to the total reaction rate. The K-induced change in selectivity towards the formation of heavier and unsaturated hydrocarbons and the decrease in the reaction rate are accompanied by a sharp increase of the surface carbon. A detailed XPS study of the C 1s region has revealed that a variety of C-containing surface species are present on the

8.1. Methanation and Fisher - Tropsh Syntheses

319

surface after different reaction times. Fig. 8.7. presents a series of C 1s spectra for a clean and a K-covered Fe surface recorded after different reaction times. It is obvious that on Kcovered Fe in addition t o the 'carbidic carbon' at 283.6 eV and the 'graphitic' carbon at 284.7 eV, there are t w o more peaks at 289 eV and 285.8 eV, associated with 'carbonate' like species and polymerized carbon or polymethylene, respectively [24, 251. The formation of the latter is directly related to the greater probability of chain growth in the presence of alkalis. It is apparent from fig. 8.7. tha t K favours the increase in C surface concentration. This leads t o surface deactivation when a substantial amount of a graphite phase is formed. Under actual catalytic conditions, X-ray diffraction studies have shown that an active carbide phase (Fe5C3) is formed during the reaction.

I " " l " " I " " I ~

Q

~

'

~

'

"

"

'

C l k l AFTER REACTION AT SLBK

"

"

'

'

,

"

"

,

~

9

A

n

Fig. 8.7. X-ray photoemission spectra of Fe foil after CO hydrogenation at 548 K in CO/H2 = 1:20 at 1 bar total pressure and varying reaction times. (a) C Is spectra from K-free Fe. (b) K 2 p and C 1s spectra from K-covered Fe, 8~ 0.3 (from ref.

-

~31)

Model studies of CO hydrogenation reaction on K-promoted Co surfaces have shown that the presence of K favours the formation of 'carbide' carbon but its reactivity is rather low and at high K coverages no change in product distribution is detected [27]. Although numerous processes are involved in the CO hydrogenation reaction on alkali promoted surfaces, a reliable explanation of the alkali action can be proposed by considering the following critical reaction steps:

(1) CO dissociation. It is enhanced by the presence of K, as evidenced by

Chapter 8.

320

the adsorption data in Section 6.1. This leads to the observed increase in the C concentration on the surface, as evidenced by the surface analysis of the catalysts after the reaction. The higher C concentration favours the polymerization reaction but it also has a negative effect on the total reaction rate because it facilitates graphite precipitation and this deactivates the surface. (2) Hydrogen dissociative adsorption. It is inhibited by the presence of K (see Section 6.5) which leads t o a reduction in the H atomic concentra-

tion. This contributes to the increased C/H ratio on the surface and favours the formation of hydrocarbons with a lower hydrogen content.

(3) Hydrogenation reactions. Here, the fact that the stability of the adsorbed hydrogen atoms is enhanced in the presence of K should be considered. This might reduce the mobility and reactivity of H atoms and in addition, it contributes to the formation of hydrocarbons with a lower hydrogen content. (4) Hydrocarbons desorption. This process is favoured by the presence of alkali additives because they cause a destabilization of the adsorption

state of the unsaturated hydrocarbons (see Section 6.7.). This facilitates the removal of the reaction products from the surface. 8.1.3

Conclusive Remarks

The results concerning additive effects on the CO hydrogenation reaction, obtained by model reaction studies on well defined catalysts, and comparison with the available surface science data on the chemisorptive properties of these modified reaction products (hydrocarbons and HzO), provide a fundamental insight into the mechanism of the additive action. The main factors contributing t o the modifier-induced changes in the activity and selectivity of the metal catalysts used in the CO hydrogenation reactions can be summarized, as follows:

I. ElECTRONEGATIVE ADDITIVES ACTION AS CATALYST POISONS (e.g. S, P, C) CAN BE SUMMARIZED AS FOLLOWS: (1) A reduction in the overall catalyst activity. This effect in the case of the most common and severe poison S is exclusively due to the S induced inhibition of the H2 dissociative adsorption rate and the mobility (diffusion coefficient) of hydrogen on the catalyst surface. As a result of the reduced H/C ratio on the surface in the presence of S the deactivation of the catalyst is accompanied by an increase in the fraction of the hydrocarbons with lower H content (heavier and unsaturated hydrocarbons). (2) A much larger magnitude of the poisoning effect than that expected by assuming a simple blocking effect. This shows the importance of the additive induced perturbations in the LDOS near the Fermi level (ligand effect) in the case of additives with a larger size and

8.2. CO Oxidation: Effect of Electronegative Additives

321

electronegativity. The contribution of the ligand effect becomes larger with increasing electronegativity difference between the poison and the catalyst. (3) A dependence of the strength of the poisoning effect on the distribution of the poison adatoms. It is weaker in the cases when the poison adatoms tend to form two-dimensional islands leaving unaffected patches of rather high modifier coverages. (4) A difference in the magnitude of the poisoning effect on the various elementary reaction steps involved in CO hydrogenation. It depends on the actual surface concentration of the poison and determines the extent to which the reaction rate depends on the partial pressure of the reactants. Thus, the inhibition of the CO dissociation rate above certain critical poison coverages (e.g. 0.1 in the case of S/Ni(100)) leads to a first-order dependence of the reaction rate on the CO partial pressure (at 0 5 0s 5 0.1 the reaction is zero order with respect to Pco).

-

11. ALKALI ADDITIVES ACTION AS PROMOTERS CAN BE SUMMARIZED AS FOLLOWS:

(1) A change in selectivity of the catalysts towards the formation of heavier and unsaturated hydrocarbons. This alkali effect is related to: (i) the enhanced CO dissociation propensity (ii) the inhibition of the Hz dissociation rate and (iii) the suppression of hydrogenation and destabilization of the adsorption state of unsaturated hydrocarbons. As a result, the formation of hydrocarbons with longer chains and a lower hydrogen content is favoured. A decrease in the overall reaction activity in the case of alkali promoted Fe. This has a bearing on a lower hydrogenation activity of the surface which can be attributed to stabilization of the atomic hydrogen adsorption state in the vicinity of alkali additives, as observed in the adsorption studies. Undoubtedly, this stabilization effect will perturb the mobility of hydrogen on the surface and this, in turqdetermines the extent to which the carbide carbon intermediate in the form of hydrocarbon products is removed. 8.2

CO OXIDATION: EFFECT OF ELECTRONEGATIVE ADDITIVES

The CO oxidation reaction is directly related to the solution of the pollution problem. P t , Rh and Pd are in the catalyst category most often used for that purpose. Recent model studies on different single crystal surfaces of the same metal have shown that, similar to the CO hydrogenation reaction, CO oxidation is structurally insensitive [31-341. This facilitates bridging the model reaction studies to the real catalyst systems because the possible structural changes induced by the additives will not alter the properties of the modified surfaces substantially.

Chapter 8.

322

As described in ref. [34], the reaction proceeds along the LangmuirHinshelwood mechanism and involves the following elementary steps: (i) CO molecular adsorption; (ii) oxygen dissociative adsorption; (iii) interaction between CO, and 0, and formation of CO:! which desorbs immediately. The net rate of Con formation is governed by the individual steps and is a function of the reaction temperature and the CO and 0 2 partial pressures. Under actual catalysis conditions, where the relatively high CO pressures favour a build-up of appreciable CO coverages which partly inhibit 0 2 adsorption, the rate is usually a positive first-order with respect to PO,, and a negative first-order with respect t o CO. Model studies of CO oxidation on P t ( l l 0 ) are carried out at different S coverages under steady state conditions at constant CO and 0 2 partial pressures. COZ production is monitored by a mass spectrometer. It has been found that at 540 K the oxidation rate, Rco,, falls to zero with increasing sulphur coverage, Bs, according t o the relationship: &oZ = 1 - k&, where k = 3.5.

:

Adsorbed CO Concentrotion

Rote of COz Formation

Sulfur Coveroge 8

Fig. 8.8. Composite plot of the normalized quantities of adsorbed CO, sulphuroxygen reaction time and rate of COz formation as a function of calibrated sulphur coverages (from ref. [35])

Fig. 8.8. presents the dependence of the relative CO2 reaction rate and CO coverage on the S surface concentration. It is obvious that the reaction

8.2. CO Oxidation: Effect of Electronegative Additives

323

rate is inhibited at O S , when still sufficient GO can be adsorbed on the surface.

CO+l 20, .COz pc0= 8 TQf; I = 5 0 0 1 m : nh (100) 0 :nh (1111

1

0.14

1

0.1

0.6

1.0

6.0 10.0

w.0 t00.0

m.0

+

Fig. 8.9. (a) Rates of C02 formation from CO 0 2 on R h ( l l 1 ) and Rh(100) as a function of 0 2 partial pressure. (b) Amount of oxygen remaining on the surface of R h ( l l 1 ) and Rh(100) as a function of 0 2 partial pressure. PCO= 8 Torr, T = 500 K . (from ref. [31]

This strong sulphur poisoning effect correlates well with the observed strong S-induced inhibition of the oxygen dissociative adsorption rate. Since the latter determines the surface concentration of the oxygen adatoms involved in

Chapter 8.

324

the CO,+O, = C02 reaction step, for constant CO and 0 2 gas pressures, the above relationship between Rcoa and 0s fits in satisfactorily with the factor (1 - 70s) describing the S-induced reduction of the 0 2 dissociation at low

-

4s *

-

With the exception of Pt, where the surface is stable with respect to oxidation up to a PCO : Po3 ratio of 1:150 (a total partial pressure 150 Torr) and T = 500 K, other metal catalysts (Rh, Ir and Pd) can be deactivated at high oxygen pressures due to the formation of an oxide phase. Fig. 8.9a. illustrates the rates of COn formation from CO and 0 2 on Rh(l1l) and Rh(100) as a function of the 0 2 partial pressure at 500 K and PCO = 8 Torr. The change in the reaction order at high oxygen pressures from a positive first order to a negative order in oxygen pressure is coincident with the increase in oxygen level measured after the reaction and presented in fig. 8.9b. The vehement change in the pressure dependence of oxygen is ascribed to the growth of a RhzO3 oxide phase which leads to deactivation of the catalyst surface. As is evident from the data in fig. 8.9., the deactivation of Ru(100) occurs at slightly lower oxygen partial pressures, which agrees well with the lower resistance (discussed in Section 4.2.) of the more open crystallographic planes with respect to the adsorbate-induced reconstruction and compound formation. Similar oxygen-induced suppression of the CO oxidation reaction at high oxygen coverages is reported for Ir and Pd surfaces [32]. Within the framework of the surface science adsorption studies, the observed deactivation effect of the oxide phase can be ascribed to (i) the reduced adsorptive ability of the oxidized surface to adsorb CO and (ii) the change in reactivity of the oxygen species which participate in the formation of an oxide phase. These model studies, which describe the effect of the partial pressure of the reagents on the reaction rate have revealed the possible negative effects that might be induced if itheoptimalreaction conditions are not met. It has been demonstrated how the reagents (above a certain surface concentration) can act as deactivators by building a new, less active, phase on the catalyst surface. This effect is similar to the effect of forming of an inactive graphite phase in the case of CO hydrogenation where the active participant is the intermediate carbidic carbon. 8.3

REACTION OF NO AND CO: EFFECT OF S

The catalytic reaction of NO and CO to produce mainly COz and N2 is also of great importance in pollution control. The reaction steps involved in this reaction are : (i) CO and NO adsorption; (ii) NO, dissociation to 0, and N,, (iii) COz formation via a CO,

+ 0, surface reaction, and

8.3. Reaction of NO and CO: Effect of S (iv) N2 formation via N,

+ N,

or N,

325

+ NO reactions.

The rate-limiting step under the actual catalytic conditions is supposed to be N2 formation which causes the rather strongly bound nitrogen atoms (which block surface adsorption sites for the reagents) to be removed from the surface. In contrast to CO oxidation and hydrogenation, this reaction is structurally sensitive as is evidenced from the comparison of the reaction rates in model studies on R h ( l l 1 ) and Rh(100) surfaces [32]. The R h ( l l 1 ) surface has turned out to be N 5 times less active than the Rh(100) one. Different rate limiting steps with different activation energies are believed to be responsible for the observed structural sensitivity.

1.0

r 7 \

+

Fig. 8.10. Effect of increasing S coverages on the NO CO reaction rate on Pt(100) expressed by the relative amount of the COz formed (from ref. [36])

The introduction of S leads to suppression and complete inhibition of the CO reaction above certain S coverages. Model low pressure massspectrometric studies of the NO CO reaction on S covered Pt(100) have shown that the reaction is completely inhibited by a p(2 x 2) 0.25 S overlayer [33]. As illustrated in fig. 8.10., the reaction rate decreases drastically a t 0s exceeding 0.1. Comparison with the surface science adsorption studies of the S effect on the CO and NO adsorption, presented in sections 5.1 and 5.2., shows that the presence of S reduces the adsorptive capacity of the surface with respect to both reagents. Thus, on fcc(100) surfaces the NO dissociation is completely inhibited a t 0s = 0.25, the inhibition of the dissociation propensity being stronger than the reduction of the adsorptive capacity. Besides, the presence of S perturbs substantially the potential energy contours for surface

NO

+

+

Chapter 8.

326

diffusion and this prevents the adsorbed molecules and atoms from coming in contact with each other and react. 8.4

WATER-GAS SHIFT REACTION

+

+

The water-gas shift reaction CO H2O = H2 C02 on catalysts based on a Cu/ZnO mixture is an important step in numerous industrial processes to produce hydrogen [37].The main reaction steps involve: (i) H 2 0 dissociation to OH, and Ha; (ii) CO2 formation via two possible ways: CO, +OH, Ha or OH, 0, H a , CO, 0, -+ ( 3 0 2 , and --.f

+

+

+.

HCOO,

--.f

CO2

+

(iii) H2 formation by a recombination of H a . The reaction is independent of the CO partial pressure and 0.5-1 order with respect t o H 2 0 partial pressure, the reaction order is within the range 0.5 to 1. The rate-limiting step is supposed t o be HzO dissociation. Since in real catalysis metallic Cu is believed to provide the active sites for this reaction, recent model studies have shown that C u ( l l 1 ) can serve as good model catalyst. Parallel-medium pressure kinetic measurements and UHV surface analysis and adsorption studies have provided a good basis for a fundamental understanding of the reaction mechanism and the influence of additives, such as S and Cs on the reaction rate [38-431.

8.4.1

Effect of S on the Rate of the Water Gas-Shift Reaction

Cu-based catalysts are rather easily deactivated by the presence of H2S impurities, which has given rise to the model medium-pressure studies on the influence of S overlayers on the reactivity of a Cu(ll1) single crystal surface [39]. As shown in fig. 8.11., the rate of the water-gas shift reaction decreases linearly with increasing sulphur coverage and drops to zero at saturation point 0s = 0.36 which corresponds to a S(LVV)/Cu(LVV) Auger peak-to-peak ratio of 0.5. The rate of reaction decay is found t o fit to the factor (1- 2.60s) which is much less than the rate decay factor of (1 - 100s) found for the S effect on CO hydrogenation reaction, described in Subsection 8.1.1. A simple statistical analysis [45] indicates that the linear dependence of the reaction rate on 0s presupposes that 3 surface Cu atoms are blocked by one S adatom. This agrees with the location of the S adatom in a threefold surface site on the C u ( l l 1 ) plane and the tendency t o form ordered structures. This means that the S-poisoning action can be restricted only to a simple site blocking (ensemble) effect. Since H2O dissociation is the rate-limiting step of the water-gas shift reaction, the S blocking effect is due to the prevention of the H 2 0 dissociative adsorption which requires no more than two unaffected Cu surface atoms. Special attention in the reaction studies with the model system S/Cu( 111) has been paid to the possibility of removing S from the surface by high temperature treatment with 500 Torr H2 (which appears to be one of the reaction

-

-

8.4. Water-Gas

Shift Reaction

327

products). It has turned out that the probability of removing S is very low (- lo-’), which correlates with the observed irreversible deactivation of actual catalysts by the presence of S.

0.0

0.3

0.4

1

CU(lt1) t o TORR 2 0 TORR

*

W

=

HZ

.

.

OlZK

nzo co 0 co2

5

\

0

1 0.0

0.1

0.2

0.3

S/CU

AES

RAT!O

0.4

\I 0.5

Fig. 8.11. Rate of the water-gas shift reaction over C u ( l l 1 ) as a function of sulphur coverage at 612 K , 26 Torr CO and 10 Torr HzO (from ref. [39])

8.4.2

Effect of Cs on the Rate of the Water-Gas Shift Synthesis

In actual catalysis it has been found t ha t the introduction of Cs salts increases the rate of the water-gas shift synthesis on copper-based catalysts [37]. The model kinetic studies of the CO HzO reaction on Cu(ll1) dosed with CsOH solution and dried in air prior to the reaction have revealed that the presence of a Cs promoter influences the reaction in the following way:

+

-

(1) The reaction rate increases linearly with increasing Cs coverage a t the 6ca range 0-0.13, achieving an enhancement by a factor of 15 at Bcs = 0.13 (corresponding to Cs/Cu AES ratio 0.075 in fig. 8.12.). (2) The apparent activation energy of the reaction increases from 8 3 . 0 f 8.6 kJ/mol for a clean surface to 98.4 f 12 kJ/mol for a Cs-covered C u ( l l 1 ) surface with 6s = 0.13. (3) T h e reaction orders with respect to PCOand P H ~ remain O unchanged.

Chapter 8.

328

CI

u1

3 0.4

0.01 0

' 2

1

4

Cs/Cu

1

6

’

8

' 10

' 12

' 14

I

16

AES RATIO / ( l / l O O )

Fig. 8.12. Variation of the water-gas shift rate with Cs coverages on C u ( l l 1 ) at 564 K. PCO = 26 Torr, &,o = 10 Torr. The data represent the average reaction rates (between 0.67 and 10.67 min reaction rate (from ref. 1401)

The observed increase in the apparent activation energy of the reaction Eapp= E(H2O)diss - E ( H 2 0 ) d e s indicates that the Cs promoting action cannot be related to reduction in the activation energy for H z O dissociation, because the adsorption experiments have shown that E ( H 2 0 ) d e S increases even slightly in the presence of alkali additives. That is why the authors suggest that the enhanced reactivity of the surface is more likely due to a large increase in the frequency factor for dissociation of the adsorbed H 2 O from 8 x 10l1 s-l to 1 x 1014 s - l , which compensates for the increased activation energy for dissociation. Within the confines of the activated complex statistical theory formalism for the rate processes, the increase in the frequency factor for H 2 0 , dissociation indicates that the entropy difference between the adsorbed species and the activated complex is larger (- 5 times) in the case of Cs-covered Cu(ll1). Provided that Cs does not constrain the transition complex entropically, the enhanced frequency factor can be related t o the reduced mobility (entropy) of H 2 0 , attached to Cs, which agrees well with the observations in the coadsorption experiments (see Section 6. 4.). The findings in the model study of the Cs promoted water-gas shift synthesis on C u (ll 1) indicate that cs participates directly in the rate limiting step and the authors propose a mechanism where water dissociation occurs at the surface Cs complex. Further on, in analogy with the observed behaviour in mixed coadsorbate systems of alkali adspecies with C02, 0 and O H , the authors have suggested a 'surface redox' mechanism involving also a carbonate intermediate (as a result of the intermediate reaction C02 0, Cs = Cs.C03,(+CO) + 2C02$ Cs,) [40, 431. The post re-

-

+

-

+

+

8.5. Ammonia Synthesis: K Promotion Effect

329

action Auger and XPS analysis have revealed that several different oxygen and/or C containing species, such as Cs,Oy(,), CsOH, and Cs.nCO,(,) can be stabilized on the surface in the presence of Cs. The presence of a Cs promoter on Cu(ll1) was found to compensate for the poisoning effect of S to some extent.This is simply due to the fact that because of the Cs induced enhancement of the reaction rate, a higher S-content is required for a complete deactivation of the catalyst. The model adsorption and reaction studies have shown that Cs does not affect the H2S adsorption rate and S influences the reaction rate in the same fashion as on a Cs-free surface. 8.5

AMMONIA SYNTHESIS: K PROMOTION EFFECT

The industrial importance of ammonia synthesis has gained great interest in surface science studies. As has been discussed in refs. [46-511, the adsorption results obtained under UHV conditions, combined with model reaction studies, offer a detailed insight into the mechanism of ammonia formation and the action of alkali promoters. The overall catalytic reaction over Fe catalysts proceeds according t o a Langmuir-Hinshelwood mechanism and involves the following main steps: (i) dissociative N2 and Hz adsorption ; (ii) hydrogenation of the adsorbed atomic nitrogen to ammonia via intermediate imine and amine surface species; (iii) desorption of the ammonia product [46, 471. The rate-limiting step in the reaction is associated with the dissociative adsorption of nitrogen molecules. High concentrations of N, and NH3, act as deactivators of the reaction because they block the active sites for Nz dissociation. The rate limiting step of ammonia synthesis is described by the equation: R N= ~ k ~ h ~-( ON1 - ONH3)’, where k~ is the rate constant for equilibrium reactions:

N,

N2

3 K + -HZ =’ NH3, 2

dissociation. ON and O N H ~ depend on the

and

NH3,

5 NH3g,

where Ii‘l and Eiz are the corresponding equilibrium constants. Obviously the role of hydrogen adatoms is to remove N, and create available sites for Nz dissociation. From the above basic relationship the reaction rate of ammonia synthesis is given by the following equation [50]:

330

Chapter 8.

Recent model studies of the structural sensitivity of ammonia synthesis performed with several single Fe crystals have shown that the catalyst activity decreases in the sequence Fe(ll1) >Fe(211) >> Fe(100) > Fe(210) >> Fe(ll0) [48, 491. The authors have suggested that C7 sites are responsible for the high activity of the open Fe(ll1) and Fe(211). Comparison with the adsorptive properties of the different Fe planes have shown that Nz dissociative adsorption is most facile on the open Fe (111) surface (see Section 5.3.). Treatment of the inactive phases of Fe(100) and Fe(ll0) with water vapour in the presence of aluminum oxide is found to produce a new active surface phase due t o the induced surface reconstruction [49]. The main results obtained by model reaction studies of the effect of K additive deposited (using SAES getter) on well-defined and atomically-clean Fe(100) and Fe( 111) surfaces prepared and characterized by LEED and AES under UHV conditions are presented in Table 8.1. and can be summarized as follows [50]: Table 8.1. Apparent Ammonia (.A) and Hydrogen ( Z H ) Reaction Rate Orders, Activation Energy, Ea (in kJ/mol) and Equilibrium Constant Kz for Clean and Kcovered Fe(100) and Fe(ll1) Surfaces. Reaction conditions: K coverage: 0.15 of the saturated first layer; reaction temperature: 673 K; P N = ~ 5 atm, P H = ~ 5-15 atm, PNH =~ 0 Torr. Total pressure 20 atm. (data from ref. [50])

5

SURFACE

ZA

tH

Ea

h-2

Fe( 100) K/Fe(100) Fe(ll1) K/Fe(lll)

-0.6f0.07

-

-

-

-0.35f0.07 -0.49f0.07 -0.34f0.07

-

-

-

0.76 0.44

75.3 f 3 76.3 f 5

0.19 0.37

(1) The introduction of a K modifier does not change the activation energy of the reaction as judged by the Arrhenius plots obtained for a clean and a K-covered surface;

(2) The apparent reaction orders of ammonia and hydrogen are markedly changed;

-

(3) Initially, at zero ammonia partial pressure, the reaction rate is increased by 30 % in the presence of K and the magnitude of the promotion effect increases further on as conversion progresses. (4) The surface science (AES) analysis of the catalyst surface before and

after the reaction has shown that, irrespective of the initial amount of K deposited on the surface, the steady-state K coverage under the working conditions is 0.15 of the saturated first K overlayer.

-

8.6. Chemical State of the Alkali . . .

331

(5) Besides K , which is the only adsorbed species on the surface before the reaction starts, Auger analysis after the reaction has shown the presence of 0 an N on the surface. 0 originates from water and oxygen impurities and serves as a stabilizer of K under the reaction temperature. Auger analysis has not shown a K-induced change in the N level. This result should be taken with caution because the surface science measurements are performed under UHV conditions, i.e. some weaklybound N-containing species present on the surface at high pressures could be removed by transferring the catalyst into the UHV analysis chamber. The observed promotion effects on the ammonia synthesis correlate well with the main K induced effects in the corresponding coadsorption systems as is verified by the surface science studies described in Chapter 6. They are:

(1) An increase of the equilibrium coverage and the stability of the molecular a - N z state which serves as a precursor for N2 dissociation. This causes an increase of the Nz dissociation rate, which is consistent with the observed K induced increase of the reaction rate by 30 % at zero ammonia partial pressure. The invariance of the activation energy indicates that the K effect on the activation barrier for dissociation is negligible.

-

(2) A decrease of the activation energy for NH3, desorption.This reduces the lifetime of ammonia on the surface and opens more active sites for Nz adsorption. The significant increase in promotion effect as conversion progresses, the decrease in the ammonia reaction order and the increase of the equilibrium constant Kz indicate that the promotion effect of K is predominantly due to the suppression of the ammonia inhibition effect by favouring the ammonia desorption and hindrance of ammonia readsorp t ion. 8.6

CHEMICAL STATE O F THE ALKALI ADDITIVES UNDER THE REACTION CONDITIONS

The high reactivity of the alkali additives towards some of the reactants in the catalytic reactions or towards the usual impurities, present in the reaction gas mixture (oxygen, water vapour, etc.), requires clarification of the chemical state of the alkali promoters under catalytic conditions. Undoubtedly, the presence of a variety of reactants, intermediates and products will affect the adsorption state of alkali additives even if they are deposited on the surface under UHV conditions as metals. In actual catalysis, alkali promoters are introduced as alkali salts which also suffer substantial transformation during conditioning of the catalyst and under the reaction conditions. Thus, by varying the initial deposit (KNOZ, K N 0 3 , K 2 C 0 3 salts or elementary K), it has been found that, after completing heat treatment and reduction in the reaction chamber, the resulting layer always appears as a strongly adsorbed ‘KO’ or ‘KOH’ phase with unknown stoichiometry. As has been discussed in

332

Chapter 8.

the previous Chapter, the presence of oxygen in connection with an alkali promoter is not expected to destroy the entire promotion effect, especially when 0 is bound to the surface. The reason for that is the substantial difference in the location of the two adatoms above the surface, which implies a small negative effect on the K action. Furthermore, the chemical state of the alkali can change with the surface environment and temperature and the resulting states may contain elemental K, K oxides, K hydroxyls, K carbonates etc. [29, 301. T h e formation of this great variety of alkali surface complexes has been detected also in the model coadsorption systems discussed in Chapter 6. Considering the chemical nature of the alkali additives under the real catalytic conditions, their promotion action should probably be correlated with the effects observed at low alkali coverages in the coadsorption systems, when the alkali adspecies are strongly polarized. Consequently, the promotion effect can be related to the action of alkali cationic species. The fact that the different anionic species can modify the effect in a specific way, should be considered as well. In some cases, the alkali complexes can exhibit a particular activity with respect to certain reactions and can be involved directly in the formation of intermediate surface complexes. For an example: (i) the action of alkali additives in enhancing the reactivity with respect t o oxygen adsorption and the oxidizing properties of some of the alkali oxides are widely used in catalytic oxidation of carbon and they facilitate coke removal from the catalyst surfaces; (ii) Cs is believed to participate in the formation of a Cs.C03(,) intermediate in water-gas shift synthesis, etc. Finally, it is mentioned that, on some metal faces (see Chapter 4.1), alkali adsorption can cause a reconstruction which will alter the surface properties further. Such alkali-induced structural changes are expected to have some impact on structurally-sensitive catalytic reactions. In such cases, whether the ‘structural’ effect will lead to enhancement or suppression of the typical ‘electronic’ effect of the alkali additives should be considered. Structural effects of alkali additives are most common in the case of alkali-promoted metal oxides where restructuring of the metal oxide films and/or the formation of complex loosely-packed structures is possible in which alkali ions participate [52]. Obviously, in the case of oxide catalysts, alkali additives modify the bulk structure of the catalyst and create a new active phase, i.e. the promotion effect is due t o the restructuring of the surface to a large extent. 8.7

CONCLUSIVE REMARKS: CORRELATIONS BETWEEN THE ADSORPTIVE AND THE CATALYTIC ACTIVITY AND SELECTIVITY

The examples in this Chapter present combined model kinetic measurements on well-defined metal catalyst surfaces a t near t o the actual catalysis conditions. UHV surface analysis demonstrate that in most cases there is an excellent correlation between the additive effect on the catalyst activity and

8.7. Conclusive Remarks: . . .

333

reactivity and the corresponding chemisorptive properties of single crystal metal surfaces modified by the same additives with respect to the reactants and products involved in the catalytic reaction. A survey of the coadsorption and model reaction studies have shown that the mechanism of the additive action is multilateral. The same additive can serve as a poisonor as apromoter in the different reactions. The changes in the activity are often accompanied by changes in selectivity by altering a particular reaction channel. Several representative examples are given:

(1) K acts as a typical promoter in ammonia synthesis which leads to enhancement of the Fe catalyst activity. In this case, K action is a proper combination of promotion and deactivation effects. The promotion effect is observed in respect of the rate limiting step - Nz dissociation, while the deactivation effect concerns ammonia adsorption by blocking of the active surface sites. (2) K does not act as a promoter in Fischer - Tropsch synthesis with

respect t o the overall surface reactivity of the Fe catalyst but it substantially modifies the catalyst selectivity towards the production of the desired heavier and unsaturated hydrocarbons. This change in selectivity combines three main effects of K: (i) the enhanced dissociation propensity of CO; (ii) the reduced dissociation rate of Hz, and (iii) the reduced lifetime of the hydrocarbon products. (i) and (ii) favour the production of hydrocarbons with a lower hydrogen content and the formation of longer C-C chains. (iii) facilitates the removal of products which block the surface sites.

(3) T h e severe S deactivation effect on the CO hydrogenation reaction is accompanied by a desired change in the selectivity towards production of heavier hydrocarbons. This correlates well with the stronger poisoning effect on the adsorption and dissociation of hydrogen and restricted mobility of the hydrogen species on the surface revealed by the coadsorption studies. The mechanism of additive action has been revealed on the basis of accumulated surface science information regarding the perturbations of the surface adsorptive properties by the introduction of modifier adspecies. The following main factors contribute to the poisoning effects:

1. The reduced adsorptive capacity of the surface. This is usually limited to a short range site blocking effect which is extended to the nearestneighbours; 2. The reduced stability (lifetime) of the adsorbed species on the surface; 3. The inhibition of the dissociation adsorption rate of the reactants;

334

Chapter 8.

4. Hindrance of the surface mobility of the reactants. In the case of typical severe poisons (e.g.S), the effects (2), (3) and (4) can be extended to the next-nearest neighbours. This indicates the existence of a longer range influence due to induced perturbations in the electronic structure and adsorptive properties (ligand effects)of the surface atoms which are not directly coordinated to the poison adatom. The relative contribution of each of these four effects to the poisoning effect is unique for the particular catalytic reaction and poison. Comparison of the poisoning effect on a particular catalytic reaction rate with the corresponding adsorption data provides information on the factors that contribute predominantly to the poisoning effect. In the CO hydrogenation reaction S exhibits a longer range ligand effect related to the inhibition of the rate limiting step (methyne formation) which depends on the hydrogen surface mobility and dissociative H2 adsorption rate. Both processes, as presented in Section 5., are very sensitive to S-induced perturbations which extend beyond the nearest neighbours. In another system, such as the watergas shift reaction, S exhibits merely a site-blocking effect due to the fact that the rate limiting step ( H 2 0 dissociation)is less sensitive to the perturbations on the next-nearest neighbours. Comparison of the poisoning effect induced by the same modifier in the same reaction for different substrates with the corresponding data on the adsorption behaviour of these substrates (e.g. CO hydrogenation on Nil W and Mo) have demonstrated the alterations in the strength of the poisoning effect due t o the tendency of the substrate to reconstruct or form compounds with the reactants. The surface science coadsorption studies have shown that the reconstruction and compound formation substantially change the properties of the catalyst surface including the resistance to the additive influence. The model reaction and surface science studies also clarified the role of the additive distribution (ordering) on the surface. In has been proved that the poisoning effect is less severe when the additive adspecies tend to form two-dimensional islands on the surface. In general, this tendency is exhibited by poisons with smaller Pauling electronegativity. The factors contributing to the promotion effect of the alkali additives are more complex and they are not common to all coadsorbate systems. The only common effects observed with most of the acceptor-like coadsorbates (reactants or products in the catalytic reactions) are:

1. An increase of the dissociation propensity of the coadsorbing molecules. 2. Stabilization (increase of the lifetime) of the coadsorbed species via direct attractive interactions and even formation of stable alkali-coadsorbate surface complexes.

The influence on the adsorptive capacity varies with the different systems, In some systems secondary reactions in the mixed layers are also possible. The high reactivity of the alkali additives complicates defining the part which each factor plays in the modifying effects observed, because usually, alkali additives are directly involved in intermediate reactions which form relatively

References

335

stable new intermediates and surface compounds. As has been described in Section 8.6., in contrast t o most of the typical poisons the actual chemical state of the alkali additives varies with the reaction conditions. From the surface analysis it can be seen that the alkali active state is stabilized by anionic-like species, most often 06or OH6-. That is why it is more probable that the alkali action should be correlated with the perturbations observed in the adsorption studies at lower alkali coverages where the alkali adspecies are strongly polarized. Consequently the main contribution to the promoting effect in actual systems should be attributed t o the electrostatic interactions described in the previous Chapter. Finally, it is mentioned that another important action of the promoters in actual catalytic reactions is that they compensate somewhat for the deactivation induced by the poisons, i.e. the catalyst resistance to poisons is enhanced.

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D.R. Strongin and G.A. Somorjai, J. Catalysis 109 (1988) 51 S.R. Bare, D.R. Strongin and G.A. Somorjai, J. Phys. Chem. 90 (1986) 4726 E. Mtckenhausen, ‘Bodenkunde’ DLG Verlag, Frankfurt (1985) 232

INDEX

Absorption spectroscopy, 12 AES, 10 Alcohols, adsorption and decomposition - alkali modified surfaces, 264-266 - clean surfaces, 150 - surfaces modified by C, 0 or S, 151-154 Alkali metals, adsorption, 19, 39, 40, 285 - adsorption bond, 36, 285-290 chemical state under reaction conditions, 331-332 - CO induced changes in electron levels, 189 - core level shifts, 33 - formation of compound like surface species, 275 - induced reconstruction, 27-29 - 0 induced stabilization, 237 - ordered overlayers, 19-25 - phase transitions, 25-27 - valence resonances, 286 - work function changes, 28-34, 38, 287 Aluminium, alkali adsorption, 33 - CO adsorption, 204 Ammonia, adsorption and desorption, 267 - K promotion effect, 330 - synthesis, 329 Butadiene, adsorption and decomposition, 146 Butene, adsorption & decomposition, 146, 147 Carbon, carbide phase, 50 - effect on butadiene adsorption, 146 _ _ on CO adsorption, 73-75, 80, 92 - _ on hydrogen adsorption, 132-134 - _ on thiophene adsorption, 150 - graphite phase, 52 - surface bonding, 52, 54 - surface ordering, 44 Carbonate, secondary product, 212 Carbon dioxide, adsorption clean surface, 209 - adsorption alkali modified surface, 210-212 - dissociation & secondary reactions, 212-214 - from CO oxidation, 321-324 - from water gas-shift reaction, 326-328 Carbon monoxide, adsorption & dissociation, 69-71 Carbon monoxide on deactivated surfaces, - adsorption kinetics, 8&85 - adsorption sites, 85-87, 92 - desorption kinetics, 72-76 - dissociative propensity, 96-98 - electron level binding energies, 95, 292 - hydrogenation 310-316 ~

337

- oxidation

321-323 with NO, 324 - surface coverage, 75, 82, 84 - surface mobility, 92-94 - surface ordering, 87-91 - water gas-shift reaction, 326 - work function, 96 Carbon monoxide on alkali promoted surfaces - adsorption kinetics, 169 - core level binding energies, 189-191 - desorption kinetics 173-176 - dissociation propensity, 199-203 - filled and empty valence levels, 192, 193 - hydrogenation, 317-320 - initial heats of adsorption, 177 - interactions in mixed layers, 275, 291-295 - vibrational spectra, 178-183, 205 - surface ordering and bond orientation, 184, 195 - water gas-shift reaction, 326 -work function, 186189 Catalysts, catalytic activity and selectivity, 1 - Ag for ethylene epoxidation, 143 - Go for CO hydrogenation, 319 - Cu for water gas-shift synthesis, 326-328 - Fe for ammonia synthesis, 330 _ - for CO hydrogenation, 318 - Mo for CO hydrogenation, 315 - Ni for CO hydrogenation, 311-315, 317 - P t for CO oxidation, 322 - _ for NO reduction, 325 - Rh for CO oxidation, 323 - _ for NO reduction, 325 - Ru for CO hydrogenation, 311 - W for CO hydrogenation, 315 Chlorine, effect on CzH4 adsorption, 144 _ _ on CO adsorption, 72, 75 - _ on Hz adsorption, 129 - _ on 0 2 adsorption, 128 Cobalt catalyst, CO hydrogenation, 319 Copper, K adsorption, 35 - oxygen adsorption, 47 -water adsorption, 138 - water gas-shift synthesis, 326-328 Chromium, CO adsorption, 101, 102 Cesium, adsorption, 19 - dipole moments, 30 - in water gas-shift synthesis, 327 - surface ordering, 22, 23 - work function, 29, 30 Decomposition, alcohols, 150-152, 264-266 - reaction

338 - butadiene, 145-147 - butene, 145-147 - ethylene, 141, 261-263 - formaldehyde, 152 - olefins, 261 - thiophene, 147-149 Dipole length, 9, 29 Dipole moment, 9, 55 - adsorbed alkalis, 30, 31 - adsorbed CO, 187 - adsorbed NO, 220 Dissociative adsorption, CO, 71, 97, 203 - COz, 209 - Hz, 131-133, 252 - H20, 135-138, 287-291 - 0 2 , 127, 228-231 - Nz, 126, 258 - NO, 118-120, 219-221 EELS, 12 Electronegative additives, 4, 41 Electropositive additives, 4, 19 Electronic structure, alkali adspecies, 33-35, 285-290 - CO molecule, 69-71, 94, 186, 292, 299 - NO molecule, 104, 217 - S adatoms, 59 - substrate surface, 60, 291, 300 Electrostatic interactions, 286-289 Electrostatic potential, 9, 38, 288, 293 Emission spectroscopies, 10-12 Ethylene, adsorption and decomposition 141-143, 261-263 - epoxidation and combustion, 143, 144 ESDAD, 7 EXAFS, 11 Folmaldehyde from methanol, 151, 165 HREELS, 13 Hydrogen, adatom surface diffusion, 133, 134 - adsorption on clean surfaces, 128 -- on modified surfaces, 129-134, 251-255 - decomposition product, 146, 149, 154, 265 - ESDIAD patterns, 137 - in CO hydrogenation, 311-319 - in ammonia synthesis, 330 Hydrate, 256 Hydrogenation of CO, 317-320 Hydroxyl formation from water, 135, 247-252 Ionicity of bonding, 56 IPE, 12 IRAS, 12 Ionization potentials, alkali metals, 19 Iron, adsorption of electronegative additives, 43, 50,51 - adsorption of CO, 71, 95, 97, 100, 191, 206, 301

_ _ of c 0 2 , 2 1 1 - _ of 112 133 - _ of Nz, 125, 257-260, 294 - catalyst

in ammonia synthesis, 329-331

- _ in Fischer-Tropsch synthesis 318, 319 - NH3

adsorption, 267 LEED, 8 Lithium, surface diffusion, 24 -- adsorption energy, 289 - electrastatic potential, 288 Methane, production, 317 Methanol, adsorption k decomposition, 151, 267 - K stabilized methoxy, 268 Methanation reaction, 317-320 MQS, 12 Molecular beam technique, 6 Molybdenum, adsorption of butadiene, 146-148 - adsorption of thiophene, 148150 - carbide formation, 50 - catalyst in CO hydrogenation, 315 - CO adsorption and decomposition, 95, 99 - core level shifts, 61 - Cs adsorption, 34 - sulfur adsorption, 46 Nickel, alkali adsorption 23, 30 _ _ induced reconstruction, 27 .- electronegative adatoms, 43, 4850, 54-59 - catalyst in CO hydrogenation, 311-313, 316 - CO adsorption, 71-75, 78, 80, 86, 87, 103, 184, 191, 199, 203, 292 - H2 adsorption, 129, 131, 133, 252-254 - HzO adsorption, 136 - methanol adsorption, 152 - Nz adsorption, 257 - 02 adsorption, 231, 232 - oxidation, 232, 243 NiFe alloy, S adsorption, 51 Nitric oxide, adsorption, 104 - - on deactivated surfaces, 107-115 - _ on alkali modified surfaces, 21S219 - dissociative propensity, 105, 118, 219-221 - dissociation products, 106 - reaction with CO, 324 - secondary reactions, 223 Nitrogen, adsorption, 124 - - on deactivated surfaces, 126 - - on alkali modified surfaces, 256 - dissociative propensity, 127, 258 - in ammonia synthesis, 329 - adatoms as modifiers, 41, 48, 49, 54, 56, 124 Organic compounds, adsorption and decomposition, 139-154, 261-266 - products of Fischer-Tropsch synthesis, 317, 318 Oxidation, A1(100), 205 - alkali multilayers, 243

339 143 144, 120, 321 - Ni, 271, 243, 232 - P t ( I I I ) , 250 Oxygen, adatoms bonding, 44, 47, 54, 56 - adatoms as modifiers, 80, 87, 92, 95, 96, 100, 102, 108, 110, 113, 121, 123, 133, 130-138, 141, 145 - adsorption on alkali modified surfaces, 227-237 - - on deactivated surfaces, 127 - dissociation product, 119, 204, 209 - impurity in ammonia synthesis, 331 - in CO oxidation, 121, 321 - in C2H4 epoxidation, 143 - induced electrostatic potential, 298 - induced surface reconstruction, 47, 48 - interactions in mixed layers, 238 Phosphorus, adsorption on surfaces, 52 - induced electrostatic potential, 294 - effect on CO adsorption, 74, 75 - - H2 adsorption, 129 - poison in CO hydrogenation reaction, 303, 315 pi-bonding, olefins, 141 Palladium, C2H4 adsorption and decomposition, I42 - CO adsorption, 71, 171, 184 - C02 adsorption, 210, 211 - NO adsorption, 112, 113, 115 - 0 2 adsorption , 229, 233 - S adsorption, 43 PIES, 11 Phase diagram, Cs on Rh( loo), 22 - N a on Ru(0001), 20 Photoelectron Spectroscopy, 11 Photoemission Spectroscopy, 11 Poisons, 3 - in CO hydrogenation, 304, 311-315 - in CO oxidation, 321 - in NO reduction, 324 - in water gas-shift synthesis, 326 Potassium, adsorption, 23, 30-33 - chemical state under reaction conditions, 332 - desorption spectra, 36 - dipole moments, 30 - electrostatic potential, 288 - surface ordering, 23 - work function, 29, 30, 33 - state density, 286 Potassium salts, promoters in ammonia synthesis, 330 - - in Fischer-’Ikopsch synthesis 317 Promoters, 3 - in ammonia synthesis, 330 - in CO hydrogenation, 317 - in Fischer-Tkopsch synthesis, 318, 319 - in water gas-shift synthesis, 327 - c2114, - CO,

Pt, C2114adsorption and decomposition, 142, 261-263 - GO adsorption, 71, 76, 80-85, 88, 93, 172, 175, 177-184, 187, 190, 197, 199 - CO oxidation, 20-125, 321 - 112 adsorption, 254 -- K adsorption, 23, 30 - N adsorption bonding, 54 - Na adsorption, 23, 30 - NO adsorption, 112, 113, 219 - NO dissociation, 221 - NO secondary reactions, 223 - 0 2 adsorption, 230, 233 - oxidation, 250 - S adsorption, 43, 50, 54, 55 - Se adsorption, 42 - water adsorption, 248 Reconstruction of surfaces, induced by alkali adatoms, 27 - induced by electronegative adatoms (C, N , S, 0),4650 Reduction, NO, 324 Rhenium, CO adsorption, 99 Rhodium, CO adsorption, 95, 123-125 - C02 adsorption and decomposition, 208, 211 - CO oxidation, 121, 321 - Cs adsorption, 22 - NO adsorption, 105, 106, 123, 216, 222, 223 - NO reduction, 324 - 0 bonding, 54 - S bonding, 43, 54 - valence charge density, 291, 300 Ruthenium, alkali local work function, 39 - C bonding, 54 - C2H4 adsorption and decomposition, 140, 141 - CO adsorption, 71, 80, 84, 188191, 193-196 - CO hydrogenation, 314 - Cs adsorption 29-31 - H2 adsorption, 134, 132 - H2O adsorption, 246 - K adsorption, 29, 30-32, 36, 39 - methanol adsorption, 153, 264 - Na adsorption, 21-23, 31 - NO adsorption, 114 - 0 bonding, 54 - 0 2 adsorption, 230, 231, 234-236 Selenium, adsorption bond, 42 - CO induced reordering, 91 - effect on CO adsorption, 80, 81, 84, 85, 92, 93 - _ on NO adsorption, 109-111, 117 - NO induced reordering, 116 - surface ordering, 42 Selective oxidation, ethylene, 144 Semiconductors, alkali adsorption, 40 Silver, alkali adsorption, 27, 35

340 - alkali

induced reconstruction, 26, 27 for epoxidation, 144 - CzH4 adsorption, 143-145 - H 2 0 adsorption, 137 - 0 2 adsorption, 128 SIMS, 8 Sodium, adsorption, 20, 21 - dipole moments, 30, 31 - electrmtatic potential, 288 - state density, 286 - surface ordering, 20-23 - work function, 30 Steric blocking, 156 Sticking coefficient, effect of alkali modifiers, 273 _ _ of electronegative modifiers, 157-159 Substrate mediated effects, 156 Sulphur, adsorption bond, 43, 54, 55, 58-60 - adsorption sites, 43, 48 - effect on CO adsorption, 72, 75-87, 89, 92, 93, 95,97-100, 299, 301 - _ on alcohol adsorption, 152 _ _ on butadiene adsorption, 146 - _ on Hz adsorption, 129-133 - _ on H surface diffusion, 134 - _ on NO adsorption, 107-113, 116, 119 - - on thiophene adsorption, 147-150 - in CO hydrogenation, 304, 311-316 - in CO oxidation, 322 - in NO reduction, 325 - in water-gas synthesis, 327 - induced electrostatic potential, 298 - _ surface reconstruction, 48 - surface ordering, 43 Surface diffusion, H adatoms, 133-135 - Li adatoms, 24 - effect in CO hydrogenation, 313 Surface ordering, alkali overlayers, 20-23, 25 - CO in mixed layers, 87-92, 184-186 - electronegative additives, 41-44 - NO in mixed overlayers, 115-117 - water in mixed overlayers, 137 Tellurium, adsorption bond, 56 Theoretical models, alkali adsorption, 286-291 - alkali effect on CO adsorption, 291-296 - alkali effect on H 2 0 adsorption, 297 - catalytic activity, 304 - LDOS near the Fermi level, 300-303 - effect of CI on CO adsorption, 305 _ _ of S on CO adsorption, 299, 301-303 - valence charge density, 355 TPD, 5 Tungsten, alkali induced core level shifts, 291 - carbide formation, 51 - catalyst in CO hydrogenation, 315 - H2 adsorption, 254 - catalyst

- Li

diffusion, 24 UHV, 15 - analysis chamber’s, 15-17 UPS, 11 Valence band, 58-60 Valence levels (resonances), alkali metals, 30, 286 - electronegative adatoms, 58-60 - CO molecule, 70, 94, 192, 193-195 - HzO molecule, 135 - Nz,molecule, 124, 125 - NO molecule, 113, 114 - 0 2 molecule, 227 XAES, 10 XPS, 11 Water, adsorption, 135 - adsorption on alkali modified surfaces, 246 - alkali induced dissociation, 247-250 - alkali induced reorientation, 248, 297 - Br and 0 induced reorientation, 138 - ESDIAD and LEED patterns, 137 - 0 induced dissociation, 137 Water gas-shift synthesis, 326-328 - effect of Cs, 327 - effect of S, 326 Work function changes, 9 - induced by alkali adspecies, 28-32 -- CO, 96, 187-189 - - electronegative adatoms, 55 - - H20,249 _ _ NO, 220 - _ 0 adatoms, 242

34 1

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Methane Conversion. Proceedings of a Symposium on the Production of Fuels and Chemicals from Natural Gas, Auckland, April 27-30, 1987 edited by D.M. Bibby, C.D. Chang, R.F. Howe and S. Yurchak Innovation in Zeolite Materials Science. Proceedings of an International Symposium, Nieuwpoort, September 13-1 7, 1987 edited by P.J. Grobet, W.J. Mortier, E.F. Vansant and G. Schulz-Ekloff Catalysis 1987. Proceedings of the 10th North American Meeting of the Catalysis Society, San Diego, CA, May 17-22, 1987 edited by J.W. Ward Characterization of Porous Solids. Proceedings of the IUPAC Symposium (COPS I), Bad Soden a. Ts., April 26-29, 1987 edited by K.K. Unger, J. Rouquerol, K.S.W. Sing and H. Kral Physics of Solid Surfaces 1987. Proceedings of the Fourth Symposium on Surface Physics, Bechyne Castle, September 7-1 1, 1987 edited by J. Koukal Heterogeneous Catalysis and Fine Chemicals. Proceedings of an International Symposium, Poitiers, March 15-1 7, 1988 edited by M. Guisnet. J. Barrault. C. Bouchoule, D. Duprez, C. Montassier and G. Perot Laboratory Studies of Heterogeneous Catalytic Processes by E.G. Christoffel, revised and edited by 2. Pael Catalytic Processes under Unsteady-State Conditions by Yu. Sh. Matros Successful Design of Catalysts. Future Requirements and Development. Proceedings of the Worldwide Catalysis Seminars, July, 1988, on the Occasion of the 30th Anniversary of the Catalysis Society of Japan edited by T. lnui Transition Metal Oxides. Surface Chemistry and Catalysis by H.H. Kung Zeolites as Catalysts, Sorbents and Detergent Builders. Applications and Innovations. Proceedings of an International Symposium, Wurzburg, September 48, 1988 edited by H.G. Karge and J. Weitkamp Photochemistry on Solid Surfaces edited by M. Anpo and T. Matsuura Structure and Reactivity of Surfaces. Proceedings of a European Conference, Trieste, September 13- 16, 1988 edited by C. Morterra, A. Zecchina and G. Costa Zeolites: Facts, Figures, Future. Proceedingsof the 8th International Zeolite Conference, Amsterdam, July 10-14, 1989. Parts A and B edited by P.A. Jacobs and R.A. van Santen Hydrotreating Catalysts. Preparation, Characterization and Performance. Proceedings of the Annual International AlChE Meeting, Washington, DC, November 27-December 2, 1988 edited by M.L. Occelli and R.G. Anthony New Solid Acids and Bases. Their Catalytic Properties by K.Tanabe, M.Misono, Y. Ono and H. Hattori Recent Advances in Zeolite Science. Proceedings of the 1989 Meeting of the British Zeolite Association, Cambridge, April 17-1 9, 1989 edited by J. Klinowski and P.J. Barrie Catalyst in Petroleum Refining 1989. Proceedings of the First International Conference on Catalysts in Petroleum Refining, Kuwait, March 5-8, 1989 edited by D.L. Trirnrn, S. Akashah, M. Absi-Halabi and A. Bishara

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