Catalysis: Vol 17 (Specialist Periodical Reports)

Catalyis Volume 17 A Specialist Periodical Report Catalysis Volume 17 A Review of Recent Literature Senior Reporters...

Author: James J Spivey (Editor) | George W Roberts (Editor)

34 downloads 1164 Views 27MB Size Report

This content was uploaded by our users and we assume good faith they have the permission to share this book. If you own the copyright to this book and it is wrongfully on our website, we offer a simple DMCA procedure to remove your content from our site. Start by pressing the button below!
Report copyright / DMCA form

DOWNLOAD PDF

Catalyis Volume 17

A Specialist Periodical Report

Catalysis Volume 17 A Review of Recent Literature Senior Reporters J.J. Spivey, Louisiana State University, Baton Rouge, Louisiana, USA G.W. Roberts, NC State University, Raleigh, North Carolina, USA Reporters

0. Augustsson, Perstorp Formox, Perstorp, Sweden K. Badii, Vaxjo University, Vaxjo, Sweden M. Boutonnet, KTH, Stockholm, Sweden C.K. Costello, Northwestern University, Evanston, Illinois, USA N. Cruise, Perstorp Formox, Perstorp, Sweden K.M. Dooley, Louisiana State University, Baton Rouge, Louisiana, USA S. Eriksson, KTH, Stockholm, Sweden M. Faghihi, Vaxjii University, Vaxjo, Sweden S. Go"boIos, Hungarian Academy of Sciences, Budapest, Hungary J.G. Goodwin Jr, Clemson University, Clemson, South Carolina, USA C.S. Heneghan, Cardiff University, Cardiff, UK G.J. Hutchings, Cardiff University, Cardiff, UK S. Kim, University of Pittsburgh, Pittsburgh, Pennsylvania H.H. Kung, Northwestern University, Evanston, Illinois, USA M.C. Kung, Northwestern University, Evanston, Illinois, USA J.L. Margitfalvi, Hungarian Academy of Sciences, Budapest, Hungary C.A. Querini, INCAPE, Santa Fe, Argentina M . Rahmani, Vaxjo University, Vzxjo, Sweden W.D. Rhodes, University of Pittsburgh, Pittsburgh, Pennsylvania S. Rojas, KTH, Stockholm, Sweden M . Sanati, Vaxjo University, Vaxjo, Sweden S.H. Taylor, Cardiff University, Cardiff,UK

RS.( advancing the chemical sciences

If you buy this title on standing order, you will be given FREE access to the chapters online. Please contact [email protected] proof of purchase to arrange access to be set up.

I

Thank you.

ISBN 0-85404-229-6 ISSN 0140-0568 A catalogue record for this book is available from British Library

8 The Royal Society of Chemistry 2004 All rights reserved Apartfjom anyfair dealingfor the purposes of research or private study, or criticism or review as permitted under the terms of the U K Copyright, Designs and Patents Act, 1988, this publication may not be reproduced, stored or transmitted, in any form or by any means, without the prior permission in writing of The Royal Society of Chemistry, or in the case of reprographic reproduction only in accordance with the terms of the licences issued by the Copyright Licensing Agency in the U K , or in accordance with the terms of the licences issued by the appropriate Reproduction Rights Organization outside the U K . Enquiries concerning reproduction outside the terms stated here should be sent to The Royal Society of Chemistry at the address printed on this page. Published by The Royal Society of Chemistry, Thomas Graham House, Science Park, Milton Road, Cambridge CB4 OWF, UK Registered Charity Number 207890 For further information see our web site at www.rsc.org Typeset by Vision Typesetting, Manchester, UK Printed by Athenaeum Press Ltd, Gateshead, Tyne & Wear

Preface

It is my pleasure to welcome Prof. George Roberts of NC State University as my co-editor for Volume 17 of this book series. We have worked together to provide reviews of current topics in catalysis, and we trust that the subjects presented here are of interest. Catalysis continues to be applied to a wide range of chemical reactions. New applications of catalysis, new synthesis methods, and new research into the molecular level mechanisms have provided insight into the applied and fundamental processes occurring on the working catalyst. For example, Kerry Dooley (Louisiana State University, Baton Rouge, LA) reviews the catalysis of condensation reactions leading to ketones. There are a number of such reactions: aldol condensation and decarboxylation/condensation of acid and aldehydes, for instance. Despite a great deal of industrial and academic interest in these reactions, the mechanisms are not entirely clear. This is because of the change in mechanism (and product distribution) with temperature, and the complexity of the reaction. Prof. Dooley provides a detailed review of the most important reactions leading to ketones on a range of catalysts. Jozef Margitfalvi and Sandor G8bolos (Hungarian Academy of Sciences, Budapest, Hungary) provide a comprehensive review of the interaction of metal and metal ions in nanoscale clusters. They show that there are unique catalytic properties derived from the molecular interaction of these types of clusters. Their review summarizes the literature on five case studies that exemplify this type of interaction: Sn-Pt, supported Au, Sn-Ru, Re-Pt, and several Cu-containing catalysts. They discuss both oxidation and hydrogenation reactions on these types of catalysts, and provide detailed summaries of the literature, as well as examples from research in their own labs. S. Rojas, S. Eriksson, and M. Boutonnet (KTH, Stockholm, Sweden)focus on the use of microemulsion techniques for catalyst synthesis. They discuss this as an alternative to traditional methods such as impregnation, ion exchange, and use of organometalliccomplexes. One specific advantage of the microemulsion method is that it results in a typically narrow particle size distribution. This is true because the metal particle is formed without being influenced by the support. They describe the specific processes used to prepare catalysts with this technique. Jim Goodwin, So0 Kim, and William Rhodes (Clemson Univeristy, Clemson, SC) review the concept of turnover frequency, a widely used measure of catalytic reaction rates. They review various methods of measuring this property: chemisorption and isotopic tracing, for example. Their analysis also compares TOF values for structure-insensitive reactions like methanation and structure-

vi

Preface

sensitive reactions like ethane hydrogenolysis. Isotopic tracing is shown to be a more accurate measure of true catalytic turnover frequency. Catherine Heneghan, Stuart Taylor, and Graham Hutchings (Univ. Cardiff, Cardiff, UK) discuss the oxidation of volatile organic compounds using heterogeneous catalysts. This review supplements considerable work done by this research group in the past. Their review deals with both the more widely used noble metal catalysts as well as metal oxides. The different mechanisms on these two classes of materials are presented and analyzed. As the authors state, this may lead to the development of a catalyst with applicability to the wide range of VOCs that must be dealt with in industry. Mehri Sanati, Mohammad Rahmani, Khashayar Badii, and Mostafa Faghihi (Univ. Vaxjo, Vaxjo, Sweden), Neil Cruise and Ola Augustsson (Perstorp Formax, Perstorp, Sweden), and Jerry Spivey (Louisiana State University, Baton Rouge, LA) also review VOC oxidation catalysts, but focus on the deactivation processes that take place in industrial practice. Specifically, they focus on the deactivation mechanisms associated with silica and phosphorous poisoning. General mathematical models of the deactivation process are presented, and applied specifically to deactivation of VOC oxidation catalysts. Mayfair and Harold Kung, along with Colleen Costello (Northwestern University, Evanston, IL) review catalysts for CO oxidation over Au catalysts. This is an important reaction in the development of fuel processors to produce hydrogen for fuel cells. The authors discuss the unusual behavior of nanoparticles of Au, and point out that there is no consensus on the nature of the active site and the mechanism. Their review focuses on the preparation and effect of the support, the nature of the active site, the mechanism, and deactivation of these catalysts. Finally, Carlos Querini (INCAPE, Santiago, Argentina) reviews the literature dealing with the characterization of coke. The difficulty in identifying the chemical and physical properties of coke on the working catalyst are well known. The author describes temperature programmed methods, spectroscopy, and extraction methods as alternatives to characterize the structure of coke. He provides specific examples of these methods in a way that will helpful to those working in the field. The editors wish to thank the authors for the effort they have put into these chapters, and the Royal Society of Chemistry for their support. Comments and suggestions are welcome. James J . Spivey Department of Chemical Engineering Louisiana State University Baton Rouge, LA 70803 jjspivey/lsu.edu

George W. Roberts Department of Chemical Engineering NC State University Raleigh, NC 27695 groberts/eos.ncsu.edu

Contents

Chapter 1 Role of Metal Ion-Metal Nanocluster Ensemble Sites in Activity and Selectivity Control b y J . Margitfalvi and S. Gb'bolos 1 Introduction 1.1 Historical Background 1.2 Type of Active Sites 1.3 Mono- and Bimetallic Supported Catalysts 1.4 Promotion of Supported Metal Nanoclusters 1.5 Characterization of Supported Metal Catalysts 1.6 Subject of Contribution 2 Case Studies 2.1 Supported Sn-Pt Catalysts 2.2 CO Oxidation on Supported Gold Catalysts 2.3 Supported Sn-Ru Catalysts 2.4 Re-Pt/A1203Catalysts 2.5 Copper-Containing Catalysts 2.6 Other Types of Supported Catalysts 3 Conclusions References Chapter 2 The Destruction of Volatile Compounds by Heterogeneous Catalytic Oxidation By C.S. Heneghan, G.J. Hutchings and S.H. Taylor

1

1 1 1 2 5 5 6 8 8 47 55 67 77 91 94 96 105

1 Introduction 105 2 VOC Abatement 106 3 Operational Parameters Affecting the Catalytic Combustion of v o c s 106 3.1 Tempertature 106 3.2 System Preheating 107 3.3 Space Velocity 108 3.4 Type of VOC 108 3.5 VOC Mixtures 109 Catalysis, Volume 17 0 The Royal Society of Chemistry, 2004 vii

...

Contents

Vlll

3.6 VOC Concentration 3.7 Deactivation 4 Catalysts used for VOC Abatement 4.1 Noble Metal Catalysts 4.2 Design of Catalyst Supports 4.3 Gold as a VOC Destruction Catalyst 4.4 Metal Oxide Catalysts 4.5 Mixed Catalyst/Sorbent Systems 4.6 Comparison of Noble Metal and Oxide Catalysts 5 Conclusions References Chapter 3 CO Oxidation Over Supported Au Catalysts B y M.C. Kung, C . K . Costello and H . H . Kung

1 2 3 4 5 6

Introduction Preparation of Supported Au Catalyst Nature of Au Active Site Reaction Mechanism Catalyst Deactivation Conclusion References

Chapter 4 Coke Characterization B y C.A. Querini 1 Introduction 2 Temperature-Programmed Techniques 2.1 Temperature-Programmed Oxidation 2.2 TPO Studies of Different Catalytic Systems 2.3 Temperature-Programmed Hydrogenation 2.4 Temperature-Programmed Gasification 3 Electron Microscopy 3.1 Naphtha Reforming 3.2 Coke on Nickel Catalysts 3.3 1-Butene Isomerization 4 Electron Energy Loss Spectrocopy (EELS) 4.1 Naphtha Reforming 4.2 1-Butene Isomerization 4.3 Other Reactions on Zeolites 5 Infrared techniques (FTIR, DRIFTS) 5.1 Cracking 5.2 Isobutane Alkylation 5.3 1-Butene Skeletal Isomerization 5.4 Butene Dehydrogenation 5.5 Other Reactions on Zeolites

111 112 113 113 125 127 128 143 144 148 148 152

152 152 154 158 161 163 163

166 166 167 167 171 175 176 177 177 177 178 178 178 179 179 180 180 180 182 183 183

ix

Contents

6 Laser Raman Spectroscopy 6.1 Classic Laser Raman Spectroscopy (LRS) 6.2 UV-Raman Spectrometry (UV-RS) 7 Dissolution of Support and Solvent Extraction 7.1 Naphtha Reforming 7.2 Coke on Zeolites 7.3 Paraffins Dehydrogenation 7.4 Propene Oligomerization on Heteropoly-Acids 7.5 n-Butane Isomerization 8 Neutron Scattering and Attenuation 9 Nuclear Magnetic Resonance (NMR) 9.1 13CCP/MAS-NMR 9.2 'H NMR 9.3 '29XeNMR 9.4 129SiMAS NMR 10 Auger Electron Spectroscopy (AES) 11 X-Ray Diffraction (XRD) 12 Secondary Ion Mass Spectrometry (SIMS) 13 Sorption Capacity: Surface Area and Pore Volume 13.1 Coke on Zeolites 13.2 Residue Hydrotreating 13.3 Isobutane Dehydrogenation 14 X-Ray Photo-electron Spectroscopy (XPS) 14.1 Coke on Zeolites 14.2 Residue Hydrotreating 14.3 Isobutane Dehydrogenation 15 Ultraviolet-Visible Spectroscopy (UV-VIS) 15.1 Isobutane Alkylation 15.2 n-Butane Isomerization 16 Electron Paramagnetic Resonance (EPR) 17 Coke Formation Rate 18 Concluding Remarks References

183 183 184 186 187 187 188 188 188 189 189 190 193 193 194 194 195 196 197 197 199 199 200 200 200 200 200 20 1 201 202 203 203 206

Chapter 5 Deactivation of Oxidation Catalysts for VOC Abatement by Si and P Compounds 210 By M . Rahmani, K . Badii, M . Faghihi, M . Sanati, N . Cruise, 0.Augustsson and J.J. Spivey 1 Introduction 210 21 1 2 Effect of Organo-silica Compounds 2.1 Chemical Properties of Hexamethyldisiloxane (HMDS) 212 2.2 Deactivation Effect of Hexamethyldisiloxane (HMDS) on Oxidation Catalysts 212

X

Contents

2.3 Effect of Hexamethyldisiloxane (HMDS) Concentration 218 2.4 Effect of Catalysts and Supports 218 2.5 Effect of Deactivation Temperature 220 2.6 Effect of Reactor Design 22 1 2.7 Mechanism of Deactivation 223 3 Deactivation by Phosphorus Compounds 226 3.1 Introduction 226 3.2 The Influence of Phosphorus Poisoning 228 3.3 Support Effects 238 3.4 Mechanism and Kinetics 239 4 Mathematical Modeling of Deactivation by Si and P-Based Compounds 24 1 4.1 Mathematical Approaches 241 4.2 Analytical and Numerical Methods 242 4.3 Modeling of Catalyst Poisoning by Organosilicon Compounds 243 4.4 Modeling of Poisoning by Organophosphorous Compounds 243 4.5 Optimization of Active Phase Distribution For Deactivating Systems 250 4.6 Summary 252 References 254 Chapter 6 Microemulsion:An Alternative Route to Preparing Supported Catalysts B y S. Rojas, S. Eriksson and M . Boutonnet

258

1 Introduction 258 2 Formation of Nanoparticles in Microemulsions 259 2.1 What is a Microemulsion? 259 2.2 Structure of Microemulsions 260 2.3 Microemulsions as Synthesis Medium 261 2.4 Some Relevant Aspects of Microemulsions for Particle Preparation 26 1 3 Metal Oxides by Microemulsion 265 3.1 Introduction 265 3.2 Catalytic Oxide Materials 265 3.3 Oxide Materials 267 4 Metal-based Catalysts Prepared by Microemulsion 272 4.1 Introduction 272 4.2 Unsupported Catalysts 272 4.3 Supported Catalysts 275 4.4 Microemulsion vs Traditional Techniques 283 5 Concluding Remarks 288 References 289

Contents

xi

Chapter 7 Catalysis of Acid/Aldehyde/Alcohol Condensations to Ketones By K . M . Dooley 1 Introduction 2 Decarboxylative Condensation, Acids 3 Decarboxylative Condensation, Aldehydes and Alcohols 4 ‘One-step’.4:do1 Condensations to Ketones 5 Lower Temperature Condensations to Ketones 6 Catalyst Properties - Decarboxylative Condensations 7 Catalyst Properties - ‘One-step’ Aldol Condensations References

Chapter 8 Turnover Frequencies in Metal Catalysis: Meanings, Functionalitiesand Relationships By J.G. Goodwin Jr, S. Kim and W.D. Rhodes 1 2 3 4 5

Introduction Determination of TOF Based on Chemisorption Determination of TOF Based on SSITKA Relationship of TOFChem and TOFITK to Site Activity Comparison of TOFChem and TOFITK for Actual Reactions 5.1 Methanation: a Classic Structure-insensitive Reaction 5.2 Methanol Synthesis 5.3 Ethane Hydrogenolysis: a Classic Structure-sensitive Reaction 5.4 Ammonia Synthesis 6 Conclusions References

293 293 294 298 303 306 308 312 314

320 320 32 1 32 1 322 325 325 336 341 343 344 345

1 Role of ’Metal Ion-Metal Nanocluster’ Ensemble Sites in Activity and Selectivity Control BY JOZSEF L. MARGITFALVI AND SANDOR GOBOLOS

1

Introduction

1.1 Historical Background. - In heterogeneous catalysis, the entity involved in the catalytic cycle is an active site or active center located at the surface of a solid material. This idea goes back to the second half of the nineteenth century. For example, Loew suggested’ that when a molecule interacts with the catalyst the ‘sharp corners’ of the catalysts are involved in the break up of the molecule into atoms, i.e., these sites are more reactive than others are. More precise definition of the active sites was first given with respect to metal catalysts. Langmuir has described active sites as an array of sites that can chemisorb an atom or molecule in a localized mode.2 In his model Langmuir suggested that all available active sites are identical. Taylor was the first who proposed that a solid surface with catalytic properties may contain not one, but many types of active site^.^'^,^ He focused on the heterogeneity of the surface of catalysts, ascribing special activity to surface atoms whose coordination to other surface atoms is low. The other very important prediction made by Taylor is related to the ‘reaction induced’ formation of active sites. He stated ‘the amount of surface which is catalytically active is determined by the reaction catalysed’. This principle has been evidenced in several catalytic reactions. It will also be shown in this review that surface species formed in situ play an important role in the generation of a new type of active site containing ‘metal ion-metal nanocluster’ ensembles.

1.2 Type of Active Sites. - In heterogeneous catalysis the following type of actives sites can be distinguished: (i) metallic, (ii) acid-base, (iii) red-ox type, and (iv) anchored metal-complex. The catalytic sites may contain one of the above types of active sites or can include several types of sites. In case of different type of sites the catalysts are bifunctional6 or multifunctional. For instance, Pt/A12037 and Pt/mordenite* are typical bifunctional catalysts containing both metallic and acidic types of active sites. On the other hand, Pt or Pd supported on silicon carbide: nitride,1° or Pt/L-zeolite” are mono-functional catalysts. There are important industrial reactions, such as isomerization and aromatization of linear hydrocarbons, which requires bifunctional catalysts, such as chlorinated Catalysis, Volume 17 0 The Royal Society of Chemistry, 2004

1

2

Catalysis

Pt/A1203.12In these catalysts the two types of sites have to be located sufficiently close to each other so that transport between the sites would not be rate limiting in the overall process. Metal catalysed reactions are differentiated introducing the concept of facile and demanding rea~ti0ns.l~ In principle a single atom should be adequate for a facile (structure insensitive) reaction, while an ensemble of surface atoms is required to form a catalytic site adequate for demanding (structure sensitive) reactions. Consequently, there are reactions, which requires more than one species to form m~ltiplets'~ or ensemble^.'^^^^^^' In other words, some reactions depend on the surface geometry (e.9. hydrogenolysis of hydrocarbons), while other may not (e.9. hydrogenation of olefinic double bond). Red-ox type catalysts are mostly used in oxidation or related types of reactions.I8 For instance, vanadium catalysts containing ions of different valence state are used in the oxidation of benzene to maleic anhydride.19 Bismuth molybdate catalyst can be used both for the oxidation or ammoxidation of propene.20Anchored metal-complex catalysts combine the advantage of both homogeneous and heterogeneous catalysts, however in these catalysts the molecular character of the active sites is maintained.2l In the last generation of this type of catalysts, heteropolyacids are fixed first to the support and in the second step different metal-complexes are anchored to the heteropolyacid. In this way highly active and stable catalyst have been prepared for different reaction^.^^^^^

1.3 Mono- and Bimetallic Supported Catalysts. - The key factor in designing supported metal catalysts is the knowledge about the reaction mechanisms and information about the role of different types of active sites in a given step of the catalytic reaction. The performance of supported mono-functional monometallic catalysts is governed by the metal particle size, metal dispersion, overall morphology of the metal nanocluster, the character of metal-support interaction, and the electronic properties of the In bifunctional supported metal catalysts in addition to the above listed factors the metal/acid balance," and the type and strength of the acid function26play a key role in the overall performance. In case of bimetallic catalysts, other properties, such as surface composition and the potential stabilization of one of the metal components in ionic form, are the most crucial determining the performance of the catalyst. It is noteworthy that combination of modern methods enables the chemist to characterize both active sites of supported metals and the reaction intermediates formed. Additionally, quantum chemical calculations become more and more powerful tools in understanding chemical interaction controlling and governing both the catalyst structure and the catalytic perf0rmance.2~ In the last decade much attention has been paid to metal nano-clusters including supported nanoparticles as one of the promising advanced nanoscopic materials.27Elements easily forming supported metal nanoclusters are Group VIII and IB transition metals as follows: Pt, Ir, Pd, Rh, Ru, Ni, Co, and Au, Ag, Cu. It is interesting to note that the heat of formation of the oxides of these metals is low (usually below -AHf = 40 kcal/mol at 25 "C referred to one oxygen atom28).

3

1: Role of ‘Metal Ion-Metal Nanocluster’ Ensemble Sites

Therefore, the oxides of these metals can easily be reduced to zero valence. The reduction of metal oxides with high heat of formation (above 100 kcal/mol) (e.g. SO2,Ti02,Zr02,A1203,Ce02,Nb205,MgO, La2O3) is rather difficult, therefore they are usually applied as catalyst supports. Other transition metals, such as V, Cr, Mo, W, Mn, Re, Fe, Zn, and sp-metals such as Ga, In, Ge and Sn with an intermediate value for the heat of formation of their oxide (ca. 60-90 kcal/mol) are frequently used as promoters in supported metal catalysts. The metals with an intermediate heat of formation of oxide are usually present in the heterogeneous metal catalysts in the form of isolated ions or nano-sized oxide crystallites even under reducing or reaction c ~ n d i t i o n s . ~These ~ , ~ ~metal , ~ ~ ions behave as Lewis acid sites. These sites can be involved in the polarization of multiple bonds or electron donor groups of substrate molecules, and they can activate carbonyl compounds, nitriles, nitro-compounds, and CO molecule chemisorbed on the surface of heterogeneous ~ a t a l y s t s . ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ’ The Lewis acid strength of metal ions is said to be proportional to the generalized electronegativity of metals in their oxide form (Xi = (1 2Z)XO, where Xi and Xo are electronegativity of metal in the oxide and elemental form, respectively, and Z = valence state of metal in the oxide).36The Lewis acid strength, expressed as the electronegativity of metals in their highest oxides, is significantly higher for Cr(20.8), Mo( 16.9), W( 18.2), Mn(11.2), Re(22.5), Ge(18.0) and Sn(15.3), than for other metals. Therefore, these metals can be used as promoters in different reduction or oxidation catalysts requiring activation of the substrate molecule by Lewis acid sites. It is also known that large number of catalytic reactions, such as catalytic naphtha reforming, hydrogenation of unsaturated carbonyl compounds, oxidation of CO or methanol, require both metallic sites and Lewis acid sites for activating hydrogen or oxygen and substrate molecules, respectively. Metal nanoclusters of Group VIII or Group IB catalysts supported and stabilized on irreducible oxides and promoted by a metal ion can fulfill this requirement. In these catalysts metal ions or metal oxide species of the promoter interact with metal nanoclusters at the cluster-support interface, or can be stabilized on the top of the metal nanocluster. Due to the intimate contact between the metal ion of the promoter and the metal nanocluster bimetallic ensemble sites are formed.37.38 These types of sites can also be formed by high temperature reduction of metal catalysts supported on a slightly reducible oxide support, such as Ti02.39740 Based on a careful literature survey and recent results published by the authors of this review, it can be assumed that in a number of heterogeneous catalytic reactions ‘metal ion - metal nanocluster’ ensemble sites are operative. In these catalysts the metal ions have to be located in atomic closeness to the metal nanocluster. As far as the action of supported bimetallic catalysts is concerned, the main theories suggest either geometric and/or electronic effects to account for the improved catalytic properties. For instance, in platinum based naphtha reforming catalysts, the electronic modification of platinum particles may be induced by an interaction with an oxide layer of the promoter41or by alloy formation!2 The electronic modification results in a change in the Pt-C bond strength of adsorp-

+

4

Catalysis

tion of hydrocarbons and hence alters the activity and selectivity of the reforming type catalysts.43 Geometric or ensemble effects arise due to the dilution of the surface of the given active metal by an inactive one. For instance, this is the case for A,B, type binary alloy catalysts containing Pt, Pd or Ni as active metals and Au, Cu, Sn, etc. as diluting elemenfs.l6 The ensemble effect can induce different structure sensitivities of the reactions. For instance, the dilution of active metal (Pt) surface into smaller ensembles by addition of inactive species, such as Sn or Ge, selectively poisons demanding reactions (e.g. hydrogenolysis and coke formation) that requires relatively large clusters or ensembles of adjacent metal atoms. While structure insensitive reactions (double bond hydrogenation, aromatization or isomerization) can occur on single isolated atom^.^,^^ In bimetallic reforming-type catalysts the presence of separate oxidized promoter species, e.g. Sn(I1)and Ge(IV),46results in a change of the acidity, affecting both the activity and selectivity of the cata1yst:l Bond and co-workers have classified bimetallic or modified supported catalysts as follows: 47 i. Formation of bimetallic and/or alloy type particles from a pair of elements showing substantial or complete miscibility (for example, alloy type supported Sn-Pt, Sn-Pd catalysts, see section 2.1.2); ii. Formation of bimetallic clusters from pairs of elements showing limited solubility; (for example, supported Sn-Ru catalysts, see section 2.3); ... 111. Incorporation of a third component that cannot be reduced to the zerovalence state but coming into contact with the metal particle (PtMo03/Si02,R U - T ~ O ~ / S ~ItOwas ~ ) .suggested ~ ~ > ~ ~ that in these catalysts the character of interactions is similar to catalysts with Strong Metal Support Interaction (SM SI); iv. Addition of a third component that mainly interacts with the support. In this way either the character of metal-support interaction is altered or the electron density of supported metal nanocluster is changed (for example, alumina supported Sn-Pt catalysts prepared by conventional methods, see section 2.1.1); v. Addition of other species (for example species electronegative in character, which act as selective or non-selective temporary poisons).50 The above classification suggests that under properly chosen condition the subject of this chapter, i.e. ‘metal ion-metal nanocluster’ ensemble sites (MIMNES)” can be formed in most of the above types of catalysts. For instance, from bimetallic clusters of type (i) and (ii) MIMNES can be formed under conditions of mild oxidation. In catalysts type (iii) MIMNES should exist both under oxidative and reductive environment. In catalysts type (iv) any metalsupport interaction with the involvement of non-reducible oxide can also be considered as MIMNES. The only requirement for the formation of MIMNES is the atomic closeness of the two types of sites.

I : Role of 'Metal Ion-Metal Nanocluster' Ensemble Sites

5

1.4 Promotion of Supported Metal Nanoclusters. - In the last decade a growing body of data provided evidence for the presence of specific active sites on the periphery of metal particles composed of metal site and specific sites on the support surface (adlineation sites). For example the enhanced activity of transition metal catalysts supported on/or promoted with reducible oxides TiO2, wo3, Nb2O5, etc. in carbonyl bond hydrogenation was attributed to the formation of specific s i t e ~ . ~Boffa, * , ~ ~Bell and Somorjai have proposed that the hydrogenation of carbonyl bonds on the surface of rhodium promoted by oxide species such as TiO,, ZrO,, TaO,, WO,, etc. proceeds via the activation of C=O bond through simultaneous adsorption of the carbon end to the metal site and oxygen end to the Lewis acid site on the oxide.53 A similar model was proposed by Vannice et ~ 1 to explain . ~ ~ the extremely high activity of 0.95%Pt/Ti02 reduced at 500 "C in acetophenone hydrogenation, and the enhanced selectivity toward crotyl alcohol in crotonaldehyde hydrogenat i ~ n . ~The ' model also implies the creation of special sites at the metal-support interface that can coordinate the oxygen end of the C=O bond and thereby specifically activate the carbonyl bond. The enhanced selectivity of Ru/Zr02 toward cinnamyl alcohol in cinnamaldehyde hydrogenation was also ascribed to the formation of Ru-Zr"+ sites at the periphery of the nanoparticles. The presence of mixed Ru-Zr"+ sites appeared to decrease the strength of the C=O bond, thus facilitating the hydr~genation.~~ Similar interfacial active sites created in Pt/Mo03 and Pt/W03 upon high temperature reduction were suggested to favor the isomerization of ally1 alcohol to propanal at the expense of hydrogenation to propan01.~~ Bell and S ~ m o r j aproposed i~~ the concept of the interfacial active site involving the coupling of a metal center and a Lewis acid/base site to form adjacent centers. The latter sites are formed either in the oxide support or the added promoter. It was suggested that these active sites might be crucial in the conversion of the molecules with polar functional groups (such as CN, CS and NH). Close analysis of data presented in the above references37.39.40.52~53~54.55 shows that in all cases the character of interactions strongly resembles the presence of 'metal ion-metal nanocluster' ensemble sites.

1.5 Characterizationof Supported Metal Catalysts. - Chemisorption of different probe molecules and Temperature Programmed Reduction (TPR) studies are frequently used to study the metal dispersion, surface composition and oxidation state of metals in mono- and bimetallic supported catalysts. Combined use of CO, hydrogen and oxygen chemisorption as well as oxygen-hydrogen titration can provide information about the dispersion and surface composition of metal n a n o c l ~ s t e r s TPR . ~ ~ ~studies ~ ~ of bimetallic catalysts can give information about the type, the reducibility, and the oxidation state of metal components. In addition, the position of TPR peaks can be used to characterize the type of interactions of the metal species in the c a t a l y s t ~ . ~ * ~ ~ ~ Traditionally, IR spectroscopy of adsorbed CO serves as a tool to gain knowledge about the electronic state and dispersion of supported m e t a 1 ~ . ~ ~ * ~ ~ . ~ The spectra of adsorbed CO are known to be the result of the interplay of the

Catalysis

6

interaction between metal d-orbitals and a-bonding and a-antibonding orbitals of adsorbed COeaX-ray photoelectron spectroscopy (XPS),65966 X-ray absorption fine structure (EXAFS)67and X-ray absorption near edge structure (XANES)68 are also used to determine both the electronic state and the environment of metal species in supported catalysts. There are indications in the literature suggesting the formation of electron deficient metal particles in e.g. A1203-basedand halogenated solid catalyst^.^^ However, the mechanism of this process and the nature of anchoring sites are not quite clear. Broensted acid sites, as well as strong Lewis acid sites may be considered as surface centers70 stabilizing small metal particles (Pt, Pd, Ir, Ni) and causing their positive charging.24

1.6 Subject of Contribution.- The aim of this contribution is to give an overview of catalytic systems consisting of very specific type of active sites, the so-called ‘metal ion - metal nanocluster’ ensemble sites. In this respect we shall differentiate two main types of catalysts containing ‘metal ion - metal nanoparticles’ ensemble sites as shown in Figure 1. In catalysts type I the ‘metal ion - metal nanocluster’ ensemble sites are formed at the perimeter of nanoparticles stabilized on reducible oxide supports. Typical representatives of this type of catalysts are Pt/Sn02” or R u - S ~ / A ~ ~ O ~ ~ ~ and Pt/Ti0273catalysts. In catalyst type I1 the above sites are created and stabilized at the surface of the first metal. The stabilization of highly active metal ions at the atomic closeness of metal nano-cluster can be achieved by one of the following methods:

A

B

- Pt’ensemblesites

“Snn?Pt” ensemble sites

i4

support Figure 1

Two forms of ‘metal ion-metal nanocluster’ ensemble sites. A - Type I . ‘Metal ion-metal nanocluster’ ensemble sites formed at the perimeter of nanoparticles. B - Type I I . ‘Metal ion-metal nanocluster’ ensemble sitesformed at the surface of mono- and bimetallic nanoparticles (Reproduced from ref. 107,139 with permission)

7


(i) adsorption of metal ions at the surface of the metal n a n o ~ l u s t e r ~ ~ ~ ~ ~ (ii) selective oxidation of one of the components of supported bimetallic nanoparti~le;7~-~~ (iii) incomplete reduction of the active phase in monometallic catalysts (see supported Cu or Cu-Zn catalyst^):^.^^ (iv) incomplete reduction of one of the active phases in bimetallic catalysts (see Re-Pt/AlzOJ catalysts).” The ‘metal ion - metal nanocluster’ ensemble sites can contain either one or two metal components. In different forms of copper and gold catalysts, the metal can exist in both forms, i.e., as metal ion and the metal nanocluster. These systems will be considered as a mono-element ‘metal ion - metal nanocluster’ ensemble sites. However, as it will be demonstrated later, systems containing two elements are more common. In most of these systems the metal ions are formed from elements of well known red-ox metals, such as tin, rhenium, iron, tungsten, molybdenum, etc. while the metal nanoparticles are noble metals, such as platinum, ruthenium, etc. In this work we shall focus on preparation, stabilization, and use of catalysts containing ‘metal ion - metal nanocluster’ ensemble sites. We shall discuss different case studies summarized in Table 1. With regard to the bimetallic systems involving a platinum-group metal and a second metal, usually having lower or no catalytic activity, many studies have been carried out with the aim to investigate the chemical state of both metals. Thus, Pt, Ir, Os, Pd, Rh, and Ru catalysts among others have been studied, using Fe, Co, Ni, Ge, and Sn as second metal. Most of the studies have shown that the second metal after reduction remains as a cationic species associated with the platinum-group metal, this sites being responsible for the selectivity improvement, e.g. in the hydrogenation of unsaturated aldehydes to unsaturated alcoho1.8WJ3

Table 1

List of case studies related to the involvement of ‘metal ion - metal nanoparticle ensemble sites’ in activity or selectivity improvement

Catalysts

Reaction

Substrate

Product

Sn-Pt/SiO,

Hydrogenation Hydrogenation Low temperature CO oxidation Low temperature CO oxidation Hydrogenolysis Hydrogenolysis Hydrogenation Hydrogenation Hydrogenolysis Reductive alkylation

crotonaldehyde benzonitrile

crotylalcohol secondary amine

Au/MgO Sn-Ru/A1203 Re-Pt/A1203 Cu-chromite CuO-ZnO

*Me = CH,, Bu = C,H,.

co co

co2 co2

ethyl laurate dodecanol butylacetate butanol carbonyl compounds alcohol, amine lauric acid dodecanol ethyl laurate dodecanol butylamine MeNHBu*

8

Catalysis

In the present review the role of supported 'metal ion - metal nanocluster' ensemble sites in activity and selectivity control of different catalytic reactions will be discussed. Evaluation of literature data and interpretation of author's recent results obtained in the activation of different organic carbonyl compounds, nitriles, and the CO molecule will be given. The catalytic performance of metallic nanoclusters promoted by metal ions or reducible metal oxides will be discussed in separate chapters according to the type of metal forming the nanocluster.

2

Casestudies

2.1 Supported Sn-Pt catalysts. - 2.1.I General Approaches Used for the Preparation of Supported Sn-Pt Catalysts. Supported bimetallic Sn-Pt catalysts can be prepared in different ways. Co- or subsequent impregnation techniques are often used, followed by high-temperature decomposition of pre-adsorbed precursors of both metals in reductive or oxidative atmospheres. For example, alumina supported Sn-Pt catalyst were prepared by (i)impregnation of commercial reforming Pt catalyst (0.3 wt. YOPt and about 0.6 wt. YOCl) with an acetone solution of SnC14 x 5H20;g4(ii) co-impregnation with appropriate amount of SnC12 x 2 H 2 0 and H2PtC16in dilute HCl;g5(iii) co-impregnation using an acetone solution containing H2PtC16and snc12;86(iv) co-impregnation with nonacidic precursors Pt(NH&(OH), x XH20 and tin (11)tartrate (SnC4H406).g7 The latter method resulted in a catalyst containing no chlorine. Sn-Pt/Si02catalysts were prepared, for example, by successive impregnation with (i)an acetone solution containing H2PtC16and SnC12;(ii) by coimpregnation with appropriate amounts of H2PtC16x 5H20and SnC14 x 5H20in acetoneggor with (iii) methylene chloride solution of ci~-[PtCl~(PPh~)~] followed by impregnation with an acetone solution of SnC12.g9The main drawback of these approaches is that during high temperature treatment, the formation of bimetallic surface species takes place only by chance. The above disadvantage can be avoided by using different anchoring techniques. The major advantage of anchoring techniques is that the chemistry of the system (parent catalyst and modifier compound) controls the formation of desired surface species, an advantage not often found upon using conventional modification methods. To obtain tin-platinum supported catalysts by anchoring, the following two approaches have been applied: i) anchoring platinum complexes (e.g. PtCl, Pt(CqH7)?' into tin ions bonded to the support and ii) anchoring of individual platinum-tin complexes or clusters. In the latter approach the composition of Sn-Pt bimetallic particles can be controlled effectively by selection of the starting inorganic complexes with different Sn:Pt ratios (e.g. [PtC13SnC13l2-, [PtC12(SnCl3)2)I2-, [PtC12Sn(PPh3)2], [Pt( SnC13)5]3-, [Pt3SngC120]~-:~,~~,~~ or bimetallic ionic compound with Pt:Sn ratio 1:1 [Pt(NH3)4] [SnC16]? Anchoring of tin ions onto the surface of the support is based on the reactivity of surface OH groups of Si02 or A1203. This has been accomplished by using

1: Role of 'Metal Ion-Metal Nanocluster' Ensemble Sites

9

either vapors of Sn(OOCCH3)2or an alcohol solution of SnC4.%Upon using this approach different organometallic and alkoxide compounds, metal halogenides, salts of organic acids, etc. were also attached to the surface of different support materials containing OH groups? It is necessary to emphasize that the surface composition of supported nanoclusters strongly depends on the method of preparation. If the formation of platinum-tin alloy phases should be avoided, tin must be introduced first onto the support by (i) exchange, (ii) coprecipitation of tin and aluminum oxides and (iii)by sol-gel synthesis of the Sn/A1203system. For example, Sn-Pt catalysts with high platinum dispersion (up to 900/) were prepared by the sol-gel method by adding tetrabutyltin to a homogeneous solution containing aluminum tri-secbutoxide (TBA), followed by impregnation of dried and calcined solids with an aqueous solution of H2PtClb.95 In solvated metal atom dispersion (SMAD) method solvated atoms prepared at very low-temperature are used as transient, highly reactive organometallic reagent for the deposition of Sn-Pt bimetallic particles onto different supports.96 In another approach chemical vapor deposition (CVD) using tin organometallic compounds was applied.97For example, the selective reaction of Sn(CH3)4vapour with Pt nanoparticles supported on Si02appears to be very promising preparation method. Methods of Controlled Surface Reactions ( C S R S ) ~ *and * ~ ~Surface Organometallic Chemistry (SOMC)'00~'0'~'02 were developed with the aim to obtain surface species with Sn-Pt interaction. In CSRs two approaches have been used: (i) electrochemical, and (ii) organometallic?' Characteristic feature of the organometallic approach is that both CSR and SOMC results in almost exclusively supported alloy type bimetallic nanoclusters. Studies on the reactivity of tin organic compounds towards hydrogen adsorbed on different transition and noble metals have revealed new aspects for the preparation of supported bimetallic catalysts. The formation of surface alloys, phase segregation at the surface, site sensitive chemisorption, changes in electronic properties, surface reconstruction, etc. have been investigated by a range of surface science methods.lo3 2.1.2 Preparation of Alloy Type Sn-Pt/SiOz Catalysts. Supported bimetallic Sn-Pt catalysts can be prepared using different methods and approaches. However, exclusive formation of alloy type nanoclusters can be achieved by using methods of surface organometallic chemistry, namely by applying Controlled Surface Reactions (CSRs) between hydrogen adsorbed on platinum and tin tetraalkyls. CSRs between Sn(C2H5)4 and hydrogen adsorbed on supported platinum (see reaction (1) below) has been first described in 1984.9' Under properly chosen experimental condition the reaction between Sn(C2H5)4and surface OH groups of the support has been completely suppressed. Consequently, reaction (1) provided direct tin-platinum interaction that was maintained upon decomposition of the primary surface complex (PSC)in a hydrogen a t m ~ s p h e r e (see ~ ~ ?reaction '~ (2) below). The net result is the formation of alloy type bimetallic surface entities

10

Catalysis

as it has been evidenced by Mossbauer spectros~opy.'~~ This approach led to the monolayer coverage of platinum by tin organometallic specieslo6as shown in Figure 2. The monolayer coverage resulted in Sn,,,,h/Pt,ratios around 0.4 (Snanchamount of tin atoms anchored, Pt, - number of surface Pt atoms). The surface chemistry involved in the formation of monolayer coverage can be written as follows:98999

Pt-Sn&,,,

+ (4 -x)/2 H2 -.Pt-Sn + (4 -x) RH

(2)

The material balance of tin anchoring indicated that under monolayer coverage of platinum by PSC the average value of x, is around 1.5. This fact pointed out that Pt nanoclusters are covered by -SnR3and -SnR2moieties formed in 1:l ratio. It has been suggested that all anchored surface organometallic species with general formula of -Snq4,)and x> 1 can be considered as Coordinatively Unsaturated Primary Surface Complex (CUPSC). The increase of the Snanch/Pts ratios required the formation of new anchoring sites for tin tetraalkyl. The CUPSC is believed to be one of these new sites, which can be used to anchor additional amount of tin tetraalkyls, provided the concentration of tin tetraalkyls is high enough. It has also been proposed that in excess hydrogen the extent of coordinative unsaturation can further be increased, i.e.,in this way higher Snanch/Ptsratios can be obtained. The validity of above suggestions has been proved and the Snanch/Pts ratio increased up to 2: 1.'06 Surface reactions involved in this new approach can be written as follow^:'^^'^^ PtHads. Pt-SnR3

A Figure 2

+

+ SnR4+Pt-SnR3 + RH x Had,. + Pt-SnR(3-,)

+

x RH

(3) (4)

B

Computer modeling of organometallic moities anchored onto the platinum nanocluster, monolayer coverage. A - anchoring of SnR, (top view); B anchoring of SnR2 (side view), R = -C2H5 (Reproduced from ref. 106 with permission)


11

Reaction (3) and (4) describe the formation of PSC and CUPSC, respectively. The coordinatively unsaturated surface species interacts with Sn& used in large excess (reaction (5)) resulting in surface complex in the second layer (SCSL). Similar surface species have been suggested when supported rhodium was modified with tin tetrabutyl.lo1In the presence of excess hydrogen the SCSL is partially hydrogenolysed resulting in coordinatively unsaturated species in the second layer (reaction (6)),which also interacts with Sn& (reaction (7)). The net result is the formation of slab-like Multilayered Surface Complex (MLSC) on the Pt surface. Reaction (3) - Reaction (7), which take place in the presence of a solvent, are referred as tin anchoring reactions in the presence of hydrogen. This anchoring process is shown in Scheme 1. Scheme 1 shows two routes for tin anchoring. Route 1 takes place in excess hydrogen, while route 2 in excess tin tetraalkyls. Route 1 is more preferable than route 2, as it provides high tin coverages and strongly decreases the amount of tin introduced into the support either by adsorption or a side reaction with the involvement of the surface OH groups of the support. New types of tin anchoring sites were created when both PSCs and CUPSCs were mildly oxidized to Oxidized Surface Organometallic Complex (OSOC), with general formula of -Sn,Rb0,.’06The addition of trace of oxygen led to the immediate formation of ethylene, i.e. surface chemistry involved in tin anchoring was altered. In this case as shown in Scheme 2 the lone pair of electrons of the

II

IL SCSL

Scheme1

Scheme of tin anchoring in the presence of excess hydrogen or excess tin tetraalkyl (Reproduced from ref. 107 with permission)

12

Catalysis

oxygen atom in -Sn,RbO, moieties are involved in tin anchoring. As the number of anchoring sites increased further, the amount of tin anchored significantly increased.lo6 It has been proposed that in the presence of oxygen the build-up of the second and subsequent layers can be written as follows:'o6

In reaction (8) oxygen containing multilayer species (OMLSC)) are formed, which instantaneously react, forming of ethylene and MLSC (11). The approach using trace amount of oxygen during tin anchoring will be denoted as route 3. The decomposition of surface organometallic complexes formed in tin anchoring steps (see reactions (3) - (9))was accomplished as a gas-solid reaction in the temperature range between 25-300 "C. The decomposition in a hydrogen atmosphere led to the formation of alloy-type bimetallic surface entities. More details on the decomposition of different surface organometallic complexes can be found e1sewhere.lo6These Sn-Pt catalysts will be referred as (H) type catalysts. However, the decomposition of surface organometallic complexes can also be

I

support

support

Scheme 2

Tin anchoring in the presence of trace amount of oxygen (Reproducedfrom ref. 107 with permission)

I

13

I : Role of 'Metal Ion-Metal Nanocluster' Ensemble Sites

carried out in the presence of oxygen.lo8These Sn-Pt catalysts will be referred as (0)type catalysts. In this case the tin organometallic species were transformed into tin-oxide like surface species on the top of supported platinum nanocluster. Upon subsequent reduction this tin-oxide type species could be partially or fully reduced resulting in alloy type bimetallic nanoparticles. In the case of a silica support, the reduction in a hydrogen atmosphere above 350°C was complete, while on alumina supported platinum part of tin-oxide migrated onto the support.lo8 Supported Sn-Pt catalysts prepared by CSRs will be designated as follows. The reaction route used will be indicated by numbers 1,2,3, corresponding to tin anchoring in the presence of excess hydrogen, excess tin tetraethyl, and trace amount of oxygen, respectively. The atmosphere (hydrogen or oxygen) used for the decomposition of surface organometallic moieties will be denoted as (H) or (0).Thus, designation, such as (H-3) indicates that the catalyst has been prepared in the presence of trace amount of oxygen and hydrogen atmosphere was used in the decomposition. 2.1.3 Characterization of Alloy Type Sn-PtlSiOz Catalysts. The alloy type SnPt/SiOz catalysts were characterized by chemisorption and Mossbauer spectroscopy. Results of CO and hydrogen chemisorption are summarized in Table 2.'08 From data presented in Table 2 the following conclusions can be drawn:

Upon increasing tin content the number of accessible Pt sites decreases; The H/Pt ratio decreases much faster than the CO/Pt ration. This is an indication that tin strongly blocks kink and corner sites involved in hydrogen activation; The hydrogen chemisorption on the supported bimetallic nanoclusters can completely be suppressed; The site blocking effect of tin is slightly higher in (0)type catalysts;

Table 2

Chemisorption properties of silica and alumina supported Sn-Pt catalysts prepared by decomposition of surface organometallic species (Reproduced from ref. 108 with permission)

Catalysts Pt/SiO; Sn-Pt/Si02 Sn-Pt/Si02 Sn-Pt/Si02 Pt/A120, Sn-Pt/A1203 Sn-Pt/A1203 Sn-Pt/A1203

Sn/Pt"

H/Ptb( A )

CO/Ptb( B )

AIB

-

0.337 n.a. (0.225) 0.162 (0.129) 0.105 (O.OO0) 0.341 0.212 (0.174) n.a. (0.122) 0.169 (0.086)

0.361 n.a. (0.278) 0.245 (0.210) 0.168 (0.157) 0.703 0.509 (0.439) n.a. (0.354) 0.486 (0.273)

1.07 n.a. (1.24)

0.6 0.8 1.7 -

1.4 2.3 4.3

1.60(00) 2.06 2.40 (2.52) n.a. (2.90) 2.88 (3.17)

in at/at, calculated from AAS analysis and the overall material balance of tin anchoring. first number catalysts type (H), in parenthesis catalysts type (0). ' Pt content: 3 wt. %. Pt content: 0.3 wt. %. a

Catalysis

14

(v) In the case of the alumina support, the decrease of the amount of chemisorbed hydrogen and C O is less pronounced. This indicates that at high Sn/Pt, > 1.4 a definite part of tin was introduced into the support and not to the p l a t i n ~ m . ~ ~ ~ ' ~ * Typical Mossbauer spectra of (H-3) type Sn-Pt/Si02 catalysts with different Sn/Pt, ratios are shown in Figure 3, the corresponding computer evaluation of the spectra is given in Table 3.1°9 These results indicate that in these (H-3) type catalysts after reduction at 300 "C there are only three tin containing species and the ratios of these species is strongly depends on the Sn/Pt, ratios. These catalysts contain two alloy phases: a platinum-rich Sn-Pt(a), which corresponds to Pt3Sn alloy phase, and a tin-rich Sn-Pt(b) phase, which can be related to Pt2Sn3or PtSn2 phases.ll0 Upon increasing the tin content the ratio of the tin rich phase increases from 14 to 54 %. Based on this finding it has been suggested that the surface of the bimetallic Pt-Sn nanocluster is enriched by tin. A characteristic feature of these catalysts is the

-6

-4

-2

0

2

4

6

v / mm s' Figure 3

Mossbauer spectra of Sn-PtlSiO, catalysts with diflerent SnlPt (atlat) ratios. SnlPt ratios: N" 1 - 0.6, N" 2 - 1 .O, N" 3 - 1.3. Catalysts type (H-3) (Reproducedfrom ref. 109 with permission)

15

1 : Role of 'Metal Ion-Metal Nanocluster' Ensemble Sites

Table 3

Results of the computer evaluation of Mossbauer spectra (Reproduced from ref. 109 with permission)

Sample No SnlPt, ~

~

1

2

Species

F WHM, mms-'

mms-l

-

-

-

-

1.56 2.31 3.33

-

1.23 0.75 0.75

70 14 16

0.97 1.03 0.87

38 47 15

1.03 1.13 0.76

32 54 14

RI,

%

~~

0.6

1.o

Sn4+ Sn-Pt(a) Sn-Pt(b) Sn2+ Sn4 Sn-Pt(a) Sn-Pt(b) Sn2 Sn4 Sn-Pt(a) Sn-Pt(b) Sn2 +

+

3

QS,

IS, mms-'

1.3

+

+

-

1.56 2.20 3.69 -

1.49 2.15 3.82

-

0.71 -

0.92 -

-

1.19

-

-

-

-

Sn-Pt/SiO, catalysts, type: (H-3). IS: isomer shift, mm s-'; QS: quadrupole splitting,mm s-'; FWHM: full-width at half-maximum, mm s-'; RI: relative spectral area (%). Errors on IS, QS and RI values are 0.03mm s-' and +/10 re]. %, respectively.

presence of Sn2+species. This behaviour has been observed only in catalysts prepared in the presence of trace amounts of oxygen. Because the relative amount of this species is almost constant (see Table 3), it has been suggested that these ionic forms of tin are stabilized at the perimeter of the Pt nanocluster. Further results of Mossbauer spectroscopy of Sn-Pt/Si02catalysts are given in Table 4, where characteristic features of (H-1) and (0-1) type catalysts are ~ompared.~' From results of Mossbauer spectroscopy presented in Table 4 the following information can be extracted: In (H) type Sn-Pt/Si02 catalyst upon its contact with air the tin is strongly oxidized; The (H) type catalyst after reduction at 300 "Ccontains two alloy phases: a platinum-rich Sn-Pt(a), which corresponds to Pt3Sn alloy phase, and a tinrich Sn-Pt(b) phase, which can be related to Pt2Sn3or PtSnz phases."' The ratio of these to alloy phases is roughly 1:l; Because no oxygen has been used in the tin anchoring step in (H) type catalyst the reduction of tin at 300 "Cis complete, i.e.,this catalyst contains no ionic forms of tin; However, the results show also that if the reduction has been performed at 200 "Ctin is not fully reduced; In the (H) type catalyst the tin rich phase oxidizes faster than the Pt rich phase (see the as received sample); The (0)type catalyst as prepared contains only tin (IV) oxide, this form of tin partially or fully covers the Pt nanocluster;

16

Catalysis

Table 4

Catalyst samples

Comparison of the Mcssbauer parameters of ( H ) and ( 0 )type of Sn-Pt/Si02catalysts (Reproduced from ref. 3 5 with permission) Species

IS, mm s-I

Sn4+ Sn-Pt(a) Sn-Pt(b)

0.09 1.25 1.98

Sn4+ Sn-Pt(a) Sn-Pt(b)

0.43 1.30 2.17

Sn-Pt(a) Sn-Pt(b) Sn4+

1.32 2.21 0.08

Sn4 Sn-Pt(a) Sn-Pt(b) Sn2+ Sn4 Sn-Pt(a) Sn-Pt(b)

0.48 1.24 2.35 2.60 0.62 1.20 2.17

+

+

QS,

FWHM,

mm s-'

Mms-'

RI, % 55 27 18 8 38 54 49 51 100

0.86 1.45 1.03 0.80 1.17 1.31 1.44

1.46 0.99 1.57 1.30 1.56

16 33 44 7 7 23 70

1-44

0.83 1.20 2.17

Catalysts with Sn/Pt (at/at) = 1.0. I S isomer shift, mm s-'; QS quadruple splitting, mm s-'; FWHM:full-width at half-maximum, mm s- *;RI: relative spectral area (%). Errors on IS, QS and RI values are 0.03 mm s- and / 10 rel. YO,respectively.

'

+

The reduction of tin(1V) oxide formed at the top of platinum nanocluster is incomplete at 200 "C; The full reduction of ionic forms of tin in (0)type catalyst requires temperatures higher than 300 "C; After reduction at 300°C the (0) type catalyst contains also alloy phases similar to the (H) type, however the ratio of these two phases is different; (0)type catalysts reduced below 300 "Ccontain also both tin (IV) and tin (11)oxides.

2.1.4 Use of Alloy Type Sn-PtlSiOl Catalysts in Selective Hydrogenation of a,/%Unsaturated Aldehydes and Ketones. 2.1.4.1 Literature Background. The hydrogenation of a,p-unsaturated aldehydes onto unsaturated alcohols is a very important reaction in the field of selective hydrogenation. In the presence of heterogeneous catalysts both the aldehyde and the olefinic double bond of unsaturated aldehydes can be hydr~genated."'*'~~.~'~ The reaction network involved in the hydrogenation of unsaturated aldehydes is given in Scheme 3.83 This scheme contains both parallel and consecutive reactions. The key issue is how to tune the selectivity of a given supported catalyst towards the formation of unsaturated alcohols. In other words the main question is how to change from

17


R-CH-CH-C,

Scbeme 3

qo

acrolein motonaldehyde

Reaction network invdved in the hydrogenation of a# unsaturated aldehydes

thermodynamic selectivity control to kinetic selectivity. This field has been recently reviewed by different Therefore, in this section no attempt is made for a detailed overview. We shall only refer to experimental data related to the possible formation of 'metal ion -metal nanocluster' ensemble sites and their involvement in activity and selectivity control. The first evidence related to the enhanced formation of unsaturated alcohols was mentioned by Adams et In their studies it had been demonstrated that both the activity and the selectivity of Pt black or platinum oxide catalysts to cinnamyl alcohol formation increased in the presence of iron chloride or zinc acetate. The effectiveness of the above additives had been further proved by Rylander who used carbon supported platinum ~ata1ysts.l'~Gallezot and Richard82carried out detailed kinetic studies on the hydrogenation of cinnamic aldehyde over Pt/C carbon catalysts varying the amount of FeC12 added to the reaction mixture. Figure 4 shows the influence of FeC12 both on the initial rate and the initial selectivity. These figures provided valuable information about the role of iron as a beneficial additive. The addition of iron resulted in not only pronounced increase of the selectivity for unsaturated alcohols, but considerable increase in the reaction rate. Figure 4 shows also that above the optimum iron concentration both the rate and the selectivity decreases.82 In additional experiments it has been shown that iron is interacting with platinum, i.e., it is located in atomic closeness to Pt. In the bimetallic nanocluster, due to the high electropositivity of iron, there is an electron transfer from iron to platinum. The net result is the formation of electron deficient iron species at the Pt surface. The authors suggested that these electron-deficient or low-valent iron species on the Pt surface might act as Lewis acid adsorption sites. These sites, due uZ.11491*59116

18

Catalysis A Initial selectivity (%) 50. 4

FdPt 1

.

1

1

1

0,s

Figure 4

1

1

1

1

Felpt d

1

1

1

1

03

1

1

1

.

1

1

1

InJZuenceof the FelPt ratio on the performance of Pt/C catalyst in cinnamaldehyde hydrogenation. A - initial rate of cinnamaldehyde (I 0 - 3 rnollmin gc,,J; B - initial selectivities (Reproduced from ref. 82 with permission)

to the high electron affinity of Mn+sites, can polarize the carbonyl bond, i.e., in this way they are involved in the activation of the C=O double bond as shown in Figure 5. Consequently, the key issue in this activation step is the Mn+- carbonyl interaction, taking place in the atomic closeness to the platinum sites.82 The selective reduction of the carbonyl group requires the creation of polar sites that interact with the C=O bond and thus lead to its preferential activation. This may be achieved by using bimetallic catalysts or supported noble metals on partially reducible oxides.82 Gallezot and Richard82have classified this effect into three groups: (i)Catalysts where metallic promoters are added in ionic form. This process may result in the formation of the cationic species deposited on the metal nano-particles or, due to the hydrogenation conditions, these cationic species can be reduced producing bimetallic particles. (ii) Catalysts involving bimetallic particles, where electropositive metal atoms are associated in the same particle with metal atoms of higher red-ox potential (usually, platinum-group metals). (iii) Catalysts involving oxidized metal species at the metal support interface, usually produced by migration of partially reduced support species to metal particles, resulting in decoration of metallic species. Among the transition metals, platinum and rhodium are the most studied and

1 H !

r t Figure5

Pt

P t - Pt

Scheme of C=O bond activation by electropositive iron atoms on platinum surface (Reproduced from ref. 82 with permission)

19


among the selectivity promoters, iron, Ge, Ga, tin etc. are widely employed in the hydrogenation of unsaturated aldehydes to unsaturated a l c ~ h o l s . ' ' ~Figure ~~'~6 and Figure 7 show the influence of different additives on the selectivity of various unsaturated alcohols over modified platinum catalysts. These results unambiguously show that tin is one of the most effective modifiers for selectivity improvement. Figure 8 shows the influence of tin in the gas phase hydrogenation of crotonaldehyde over tin-modified Rh/Si02catalysts prepared by using CSR.83$' l8 Figure 8 clearly shows the suppression of double bond hydrogenation and enhancement 100

0 E

20 0

No

Ti

V

L L d Ga

F

Gc

S

Promotor X in PtX catalyst acrolein

Figure 6

B crotonaidehydc

-

o me crotonaldehyde

Selectivity of unsaturated alcohols in hydrogenations of diflerent a,punsaturated aldehydes. Promoters used are shown. Selectivity is higher when the C = C bond bears substituents (Reproduced from ref. 8 1 with permission) 30

0

1

Na

Ti

V

Fe

Ga

Ge

' Sn

PromotorX in PtX c a t a l y s t 8 methacrolein

Figure 7

acrolein

methyl vinyl

ketone

Selectivity of unsaturated alcohols in hydrogenations of different a , p u n saturated aldehydes and ketone. Notice the zero selectivity with unsaturated ketone (Reproduced from ref. 8 1 with permission)

20

Catalysis

1

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Sn/(Rh+Sn) Figure 8

Selectioity to crotyl alcohol and n-butyraldehyde us. atomic ratio Sn/(Rh + Sn) in the gas phase hydrogenation of crotonaldehyde ( C A ) over Rh-Sn/SiO,-CSR catalysts at constant conversion of crotonaldehyde = 15 %. ( T = 40 O C , Proto[ = 2 MPa, H 2 / C A = 20) (Reproduced from ref. 83 and 11 8 with permission)

of carbonyl hydrogenation upon increasing the overall tin content. The methodology of the addition of tin into supported platinum or rhodium seems to play an important role in the behaviour of the active phase obtained. Controlled surface reactions of organometallic compounds with metal surfaces result in the formation bimetallic systems with specific properties in the hydrogenation of different unsaturated ~ o m p o ~ n d sHowever, . ~ ~ the ~ nature ~ ~ ~ ~ ~ ~ of the Sn-Pt or Sn-Rh bimetallic phase formed, and its influence on the final properties of the catalyst, have not been yet well determined and this is still a subject to be investigated. Santori et al. have studied the hydrogenation of several compounds (crotonaldehyde, cinnamaldehyde, butyraldehyde, 2-butanone, benzaldehyde and cyclohexene) over Pt/SiOz catalyst modified with tin CSRs.li9In the hydrogenation of butyraldehyde and butanone, the adsorption of the ql-(0) type appears as highly favorable, both from a geometric and electronic point of view. In the benzaldehyde hydrogenation, the increase in the catalytic activity for bimetallic catalysts associated with electronic effects, i.e. the presence of ionic tin and of phenyl group. In the case of the cyclohexene, geometric and electronic, as well as steric effects lead to a strong reduction of the hydrogenation rate of C=C bond. These results can be extrapolated to explain the behaviour of the unsaturated a,P-aldehydes. The hydrogenation of the C=O group is promoted and the adsorption modes favorable to the C=C hydrogenation are inhibited by tin. XPS analysis of silica-supported Sn-Pt catalysts shows that the binding energy of platinum shifted towards lower values of approximately 1 eV with respect to

1 : Role of ‘Metal Ion-Metal Nanocluster’ Ensemble Sites

21

Pt/Si02, this shift can be interpreted as an electronic transfer from tin to platinum. These modifications strengthen the hypothesis of the electronic effects induced by tin as it was previously discussed to explain changes in the H2 and CO chemisorption (see Table 2). It is important to note that the increase in the electronic density of platinum is observed not only in the systems where tin is ionic state, but also in the case of the catalyst containing tin only in zero valent state. For this reason, it was proposed that even in the case of metallic tin forming superficial alloys, polarized states of Pt*- and Sn*+ sites are generated, which would be important in the chemisorption of reagents containing double C=O and C=C bonds.’22The existence of ‘Lewis acid sites’ (electronic modifications), due to the presence of Sn*+ and ionic tin, tends to promote the attack of hydrogen on the C=O group. In further studies it has been demonstrated that a supported platinum catalyst can also be modified either by addition of cobalt, germanium, iron or tin halides. In the hydrogenation of cinnamaldehyde the modification resulted in much higher activities and selectivities for the hydrogenation of the aldehyde group than on the parent platinum catalysts as shown in Table 5.82,123 Ponec et al. has recently demonstrated that in the selective hydrogenation of crotonaldehyde over Sn-Pt catalysts the selectivity of crotylalcohol increased with time on The observed phenomenon had been ascribed to ‘reaction induced selectivity changes’, however the exact origin of these changes was not clarified. It should also be mentioned that in addition to the presence of Lewis-acid type surface entities there is another requirement for the preparation of selective catalysts used in the hydrogenation of a,P-unsaturated aldehydes. In this reaction both the adsorption of the substrate via its olefinic double bond and the re-adsorption of the formed crotylalcohol should be suppressed in order to increase the selectivity for the formation of the unsaturated alcohol. 2.1.4.2 Gas Phase Hydrogenation of Crotonaldehyde Over Alloy Type Sn-Pt/Si02 catalysts. In these studies different Sn-Pt/Si02catalysts prepared by using CSRs were tested. On the parent Pt/Si02 catalyst no crotylalcohol formation was observed. The introduction of tin into platinum resulted significant selectivity improvement and the selectivity for the formation of crotylalcohol (Sc=o) strongly depended on the Snanch/Ptsratio. A characteristic feature of this type of

Table 5

Eflect of Metal Chlorides on Cinnarnaldehyde Hydrogenation (Reproduced from ref. 82 with permission) ~~

~

k x lo3

Selectivity

Additives

(s-l g p t - 9

(%I

none COCI, FeCl, SnC1, GeCl,

0.08 0.8 2.4 10.0 17.8

10 25 64 75 94

22

Catalysis

Sn-Pt/Si02catalysts is the increase of the yield of unsaturated alcohol with time on stream as shown in Figure 9 . l l 1 * l 2 l While the yield of a saturated alcohol increases, that of saturated alcohol, and hydrocarbons decreases. The net result is the pronounced increase of the ScG selectivity. Consequently, the behaviour of alloy type Sn-Pt/Si02 catalysts prior to reaching the steady-state activity strongly resembles that observed by Ponec et ~ 2 Z . l ~ ~ However, the selectivity of Sn-Pt/Si02catalysts prepared by using CSRs, as it will be shown latter, was much higher than that prepared by conventional methods. Steady-state selectivity data obtained in gas phase hydrogenation of crotonaldehyde over different Sn-Pt/SiO, catalysts are summarized in Table 6 and Table 7.lo7 Sn-Pt/Si02 catalysts below monolayer coverage show only a slight increase in the selectivity of unsaturated alcohol, however the activity of these catalysts is much higher than that of the parent Pt/Si02catalyst. These results already show the positive effect of tin in carbonyl activation. However, due to the relatively low tin content of these catalysts, the adsorption of the substrate molecule by its olefinic double bond is still possible, consequently the formation of butiraldehyde is very pronounced. The use of Sn-Pt/SiOz catalysts with multilayer tin coverage resulted in pronounced increase in the SCe selectivity as shown in Table 7. The highest SCa selectivity was around 90 %. It should also be emphasized that the introduction of tin increased the overall activity of all alloy type Sn-Pt/SiOz bimetallic catalysts compared to the parent Pt/Si02. In this respect the behaviour of Sn-Pt/SiO, bimetallic catalysts strongly resembled platinum catalysts modified with iron (see Figure 4). 0.10

To

0 0

5

I0

15

20

Pulse number Figure 9

Time on stream pattern of the gas phase hydrogenation ofcrotonaldehyde over Sn-Pt/Si02catalyst (catalyst type (H-3), Snanch/Pts= 2.9). Reaction temperature: 80 "C,amount of catalyst: 100 mg, flow rate: 90-mllmin. Reaction products: 0 - crotylalcohol, - butyraldehyde, 0 - hydrocarbons, x - butylalcohol. (Reproduced from ref. 11 1 and 121 with permission)

23


Table 6

Gas phase hydrogenation of crotonaldehyde to crotylalcohol over Pt/Si02 and Sn-PtlSi02 catalysts with monolayer tin coverage (Reproduced from ref. 107 with permission) ~~

Selectivity (%)

Catalysts, SnanchlPts

Wini a

SAL

SOL

Pt/SiO,

1.68

95

0

0

5

Sn-Pt/SiO,, 0.22

6.05

82

5

13

0

Sn-Pt/Si02,0.39

4.17

71

4

20

5

UOL

HC

initial rate, w, in pmol &atp1s-' determined from the conversion - contact time dependencies extrapolating to zero conversion, measured at 5 % conversion. Temperature of re-reduction = 300 "C, reaction temperature = 60 "C, [C], = 0.64mmol/dm3, catalysts = 40-80g. Abbreviations: SAL - butiraldehyde, SOL - butanol, UOL - crotylalcohol, CH - hydrocarbons. a

Table 7

Gas phase hydrogenation of crotonaldehyde over Sn-Pt/Si02catalyst with multilayer tin coverage (Reproduced from ref. 107 with permission)

Catalysts, SnancdPts

' '

&lw?lol got - s -

Pt/SiO,

-

0.50

0

Sn-Pt/SiO,, 0.38

2.3 1

4.3 1

40

Sn-Pt/SiO,, 1.40

2.21

1.13

65

Sn-Pt/SiO,, 2.50

0.77

0.09

90

a initial rates, determined from the conversion - contact time dependencies extrapolating to zero conversion, measured at 5 % conversion. Temperature of re-reduction = 300 "C, reaction temperature = 60 "C, [C], = 0.64mmol/dm3, catalyst: 40-80g. Abbreviations see Table 6.

Figure 10 shows the dependence of the crotylalcohol selectivities of the conversion over different Sn-Pt/Si02 catalyst~.''~The higher the Sn/Pt ratio the higher the crotylalcohol selectivity. Over catalysts with high Snanch/Pts ratios the selectivity drop between 8 and 50 YOconversion was less than 10 YO.These results show that in Sn-Pt/SiOz catalysts with high Snanch/Pts ratio both requirements of the selective formation of unsaturated alcohols, i.e., the activation of the carbonyl group and the suppression of the readsorption of formed unsaturated aldehyde, are fulfilled. As shown in Figure 9 in the initial part of TOS curve the formation of hydrocarbons was very pronounced. The analysis of hydrocarbons formed in the initial part of time on stream showed marked difference between Pt/SiO, and Sn-Pt/SiO, catalyst as given in Table 8.lo9 Over the monometallic catalyst, formation of C3 hydrocarbons, while on Sn-Pt/SiO, catalyst formation of butadiene was observed. The formation of C3 hydrocarbons indicates that on the parent catalyst decarbonylation of the sub-

24

Catalysis

0.6

0.1 O

c m j w

-

0.2

0

i

0.4

0.6

Conversion Figure 10

Gas phase hydrogenation of crotonaldehyde on diferent Sn-PtlSiO, catalysts. raDependence of the crotylalcohol selectivities on the conversion. Snonch/Pts tios: 0 - 0.00, - 0.44, X - 2.1, - 2.90. Preactivation temperature: 300 "C, reaction temperature: 80 "C,[ C ] , 0.64 mmol/dm3,Catalysts type (H-3), 80 mg (Reproduced from ref. 107 with permission)

+

Table 8

Catalysts

The distribution of C& hydrocarbons formed during the pretreatment of Pt/Si02and Sn-Pt/Si02catalysts with crotonaldehyde-hydrogen mixture (Reproduced from ref. 109 with permission) Propane ~~

Pt/SiO, Sn-Pt/SiO,

Propene

1,3-butadiene

2 0

91

2-butene

butane

13 9

68 0

~

17

0

0

Snanch/Pts = 1.4,pretreatment 80 "C with crotonaldehyde- hydrogen mixture (lltorr/750 torr). Samples accumulated in the first hour of pretreatment. The overall yield of hydrocarbonsis around 4 Yo.

strate molecule takes place. Contrary to the Sn-Pt/Si02catalyst, the formation of C3hydrocarbons was not observed at all. The formation of butadiene indicates that over Sn-Pt/SiO, catalyst the abstraction of oxygen from crotonaldehyde takes place. It had been suggested that the oxygen abstracted from crotonaldehyde was involved in the oxidation of surface tin atoms. This may be the crucial step in the formation of 'metal ion - metal nanocluster' ensemble sites, i.e., in the formation of tin-containing Lewis-acid sites in atomic closeness to platinum, as shown in Figure ll.'07 Based on the above figure it has been suggested that the activation of the carbonyl group can take place on the 'Snn+ - Pt' ensemble sites as depicted in Figure 11. On the other hand the enrichment of tin at the surface of the bimetallic nanocluster strongly decreases both the adsorption of crotonaldehyde and readsorption of the formed crotyl-alcohol by their olefinic double bond. In-situ FTIR measurements provided further proof for the decarbonylation of crotonaldehyde over Pt/SiO2 catalysts.'21Under experimental conditions used in gas phase hydrogenation an intensive band of linear and a weak band of bridged


25

'Snm-Rensemb@site /

Figure 11

Activation of the carbonyl group in a,P unsaturated aldehydes over 'Sn"+-Pt' ensemble sites (Reproduced from ref. 107 with permission)

10.2

I

I

I

I

I

2250 2150 2050 1950 1850 1750 Wavenumber / cm-' Figure 12

Carbonyl region of FTIR spectra measured over Pt/Si02 and Sn-Pt/Si02 catalysts treated with crotonaldehydelhydrogen mixture at 80 "C.1 - PtISiO,, 2 - Sn-Pt/Si02 (Snanch/PtS = 0.5), 3 - SnPt/Si02 (Snanch/Pts = 2.3). (Reproduced from ref. 121 with permission)

26

Catalysis

carbon monoxide was observed on the parent Pt/Si02 catalysts as shown in Figure 12. Upon increasing tin content the intensity of the band of linear CO strongly diminished. This observation is in full agreement with the proposed decarbonylation of crotonaldehyde. These results can explain the low activity of the parent platinum catalyst, i.e. the low activity of Pt/Si02 catalyst can be attributed to the poisoning of Pt sites with adsorbed C0.'219125 The in situ formation of ionic forms of tin was proved by Mossbauer spectroscopy. These results are summarized in Figure 13 and Table 9.'@ As shown in data given in Figure 13 and Table 9, the freshly prepared Sn-Pt/Si02catalysts (H-3 type) have two forms of tin. The majority of tin is alloyed with platinum (isomer shift in the range of 1.40-2.31mm s-') resulting in a Pt rich (Sn-Pt(a))and a tin rich (Sn-Pt(b))phases., while the minority is in Sn2+ form (isomer shift in the range of 3.33-3.82 mm s-' Pt alloy phases). Detailed assignment of these samples has been given in section 2.1.1. In samples treated with crotonaldehyde/hydrogen mixture (samples 1-cr, 2-cr,

Table 9

~~

Results of Mossbauer spectroscopy on diflerent Sn-PtlSiOz catalysts. Comparison of catalyst freshly reduced and used in crotonaldehyde hydrogenation (Reproduced from ref. 109 with permission) ~

Samples, No

SnlPt (at/at)

Species

1

0.6

Sn4 Sn-Pt(a) Sn-Pt(b) Sn+* Sn4+ Sn-Pt(a) Sn-Pt(b) Sn+* Sn4+ Sn-Pt(a) Sn-Pt(b) Snf2 Sn+4 Sn-Pt(a) Sn-Pt(b) Sn+2 Sn+4 Sn-Pt(a) Sn-Pt(b) Sn+* Snf4 Sn-Pt(a) Sn-Pt(b) Sn+2

2

3

1-cr

1.o

1.3

0.6

2-cr

1.o

3-cr

1.3

+

IS, mms-'

QS, mms-'

F WHM, mms-'

RI,

%

-

-

1.56 2.31 3.33

1.23 0.75 0.75

70 14 16

0.97 1.03 0.87

38 47 15

1.03 1.13 0.76 0.77 1.26 1.01 0.78 1.09 1.07 1.12 1.19 0.78 0.79 1.31 0.78

32 54 14 7 62 19 12 13 31 46 10 15 21 54 10

-

-

0.71

1.56 2.20 3.69

-

-

1.49 2.15 3.86 0.69 1.56 2.3 1 3.64 0.7 1 1.42 2.28 3.57 0.85 1.54 2.23 3.64

1.19 0.96 -

1.20 0.80 -

1.18 0.44 -

0.83

-

-

Note. IS, isomer shift, mm s-I, Qs, Quadrupole splitting, mm s-I, FWHM, full width at half maximum, mm s-', RI, relative spectral areas (%). Errors on IS, QS and RI values are 0.03 mm/s, and +/- 10 re1 YO,respectively.Catalyst type (H-3).


-6

-4

-

2

0

v

Figure 13

2

4

6

27

8

I mm s '

Mossbauer spectra of Sn-PtlSiO, catalyst samples with diflerent SnlPt ratios. 1,2,3 - original samples, I- cr, 2-cr, 3-cr - samples treated with crotonaldehydelhydrogen mixture at 80°C (Reproduced from ref. 109 with permission)

and 3-cr, respectively) both Sn-Pt alloy phases and the Sn2+form are maintained, however in these samples due to the treatment with crotonaldehyde a new form of tin appeared with isomer shift in the range of 0.69-0.85 mm s-l. This form of tin was assigned to Sn4+.The formation of Sn4+ species in samples treated with crotonaldehyde has been considered as an indirect evidence for the 'reaction induced activation' of supported Sn-Pt nanoclusters by the substrate. This kind of activation of Sn-Pt/SiO, catalyst has been suggested in earlier studies.126 It has to be emphasized, as it emerges from data given in Table 9, that the Sn4+species are formed mostly from the platinum rich Sn-Pt(a) alloy phase. The high IS value of the Sn4+ species was attributed to the presence of delocalized diene-type ligands in the coordination sphere of Sn4+species.127Consequently, it is the first experimental evidence indicating that in the working alloy type Sn-Pt/Si02 catalyst, SnO,L, type surface organometallic species containing either butadiene or crotonaldehyde as stabilizing ligands are formed. These species should be formed in the atomic closeness of the active Pt-Sn phase. As a result, this evidence can be considered as the first proof of the formation of 'metal ion metal nanocluster' ensemble sites involved in the activation of unsaturated aldehydes as shown in Figure 11. 2.1.4.3 Selective Hydrogenation of Cinnamaldehyde. The hydrogenation of cinnamaldehyde was investigated over alloy type Sn-Pt/Si02catalysts under pressure. The corresponding experimental data are summarized in Table 10.1°7 As shown in Table 10, in this liquid phase hydrogenation reaction a similar

28

Catalysis

Table 10 Liquid phase hydrogenation of cinnamaldehyde over diflerent alloy type Sn-PtlSiOz catalysts. (Reproduced from ref. 107 with permission) Initial reaction ratea E x p . Catalysts, Pressure, No Snanch/Pts bar Overall W,,, W,,,

Selectivities,

SOL

SAL ~

1 2 3 4 5

Pt/SiO, 4 Sn-Pt,0.4 4 Sn-Pt, 1.6 4 Sn-Pt, 1.6 40 Sn-Pt, 2.4 40

2.2 17.7 1.8 3.7 2.2

1.1 13.0 1.7 3.4 1.6

1.1 4.8 0.06 0.2 0.5

40 17 5 4 9

%*

~

2 1 0 0 3

UOL

~~

58 82 95 96 88

*Selectivity measured at 10 % conversion.a in pmol GC,,-' s-' , selectivity measured at 10 Yo conversion. Reaction temperature = 60 "C,[Cl0 = 120 mmol/&,,, catalysts type (11), 50 g. Abbreviations: see Table 6.

trend has been observed as in the gas phase hydrogenation of crotonaldehyde, i.e., the introduction of tin into platinum strongly increased the reaction rate and leads to a very pronounced selectivity increase. However, the highest selectivity was obtained at relatively low Sn,,,h/Pt, ratio. It is interesting to note also that the parent Pt/Si02catalyst showed relatively high Sco selectivity in this reaction. This selectivity value was increased further by introduction of tin. The highest selectivity measured in this reaction was 96 YO at 10 % conversion. Data presented in Table 10 show also that the hydrogen pressure had a positive effect on the rates, however, it did not affect the high Sc4selectivity. In this reaction the conversion - Sc=oselectivity dependence showed also an increase, as shown in Figure 14.'07However, in this case the extent of the increase is relatively small. This type of kinetic pattern can also be attributed to the in situ formation of 'metal ion - metal nanocluster ensemble sites'. 2.1.4.4 Hydrogenation of Methyl Vinyl Ketone. The selective hydrogenation of the keto group in methyl vinyl ketone is considered a great challenge. The lack of positive results in this reaction has been attributed to (i) the steric hindrance of the alkyl groups, and (ii) the decreased reactivity of the keto carbonyl group compared to the aldehyde group in molecules with conjugated double bonds. Preliminary results obtained in the liquid phase hydrogenation of methyl vinyl ketone over a Sn-Pt/Si02 catalyst (Snanch/Pts= 1.43) are shown in Figure 15.'07 No unsaturated alcohol formation was observed on the parent catalyst. As emerges from these results, the SC+ selectivity is very low and decreases upon increasing the conversion. However, the use of catalyst treated previously with crotonaldehyde/hydrogen mixture in the gas phase resulted in a slight increase in the selectivity values. This increase was more pronounced at conversion below 10 YO, and it was maintained in the whole conversion range. This behavior was reproducible, however experimental conditions of the above treatment (i.e., duration, crotonaldehyde concentration, temperature) had no measurable influence on the selectivity improvement. Despite the modest results obtained in this reaction, the slight increase of the

29

1: Role of 'Metal Ion-Metal Nanocluster' Ensemble Sites 0.97

..->

c,

0.91

Y

I

-I

+

0 0)

v)

0.85

0.83 0

0.05

I

I

0.1

0.1 5

0.2

Conversion Figure 14

Selectivity-conversion dependencies in the hydrogenation of cinnamic aldehyde in batch reactor on Sn-PtlSiO, catalyst at H 2 pressure 3.5 bar - 0 and 40 bar Snonch/Pts = 1.61. Catalysts type (H-3). Temperature of the pretreatment in hydrogen = 300 " C ,reaction temperature = 40 "C, initial concentration of the substrate = 120 mmol/g,.,,, (Reproduced from ref. 107 with permission)

+

0

0.2

0.4

0.6

0.8

1

1.2

Conversion

Figure 15

Liquid phase hydrogenation of methylvinil ketone on Si02 supported Sn-Pt = 1.43. Eflect of the pretreatment with crotonaldehyde on catalyst, Snanch/Pts the selectivity. Selectivity - conversion dependencies without and with pretreatrespectively. Temperature of the pretreatment in crotonaldehyde, 0 and ment in hydrogen = 300 "C, temperature of the pretreatment with crotonaldehyde = 80 "C, concentration of crotonaldehyde = 30 mmol/dm3, reaction temperature = 40 "C, initial concentration of the substrate = 120 mmol/g,,,. (Reproduced from ref. 107 with permission)

+,

30

Catalysis

Sc4 selectivity strongly indicates on the involvement of ionic forms of tin formed during the treatment with crotonaldehyde. Consequently, in this reaction the 'metal ion-metal nanocluster' ensemble sites are involved in the activation of the keto group and the formation of unsaturated alcohol. However, these ionic forms of tin do not have enough strength to activate the keto group. 2.1.5 Use of Alloy Type Sn-Pt/Si02 Catalysts in Benzonitrile Hydrogenation. Earlier studies on liquid-phase hydrogenation of benzonitrile have shown that the addition of small amount of tin chloride to Pt/nylon catalyst increases also the rate of reduction of the -CN A similar trend has been previously reported for the hydrogenation of the carbonyl group in cinnamald e h ~ d e and , ~ ~ hydrocinnamaldehyde,12*or the -NO2 group in nitroben~ene.3~ Physico-chemical characterization of the Pt-Sn/nylon catalysts showed that tin was always present mainly as tin ions.'29 Therefore, it was suggested that the presence of tin ions enhanced the reactivity of the -CN group. The electrophillic nature of tin ions increased the polarization of the nitrile group present in the organic substrate, facilitating the attack of the chemisorbed hydrogen.33 The liquid phase hydrogenation of benzonitrile had also been investigated over alloy type Sn-Pt/Si02 catalysts prepared by CSRs.13' Prior to reaction the Sn-Pt/Si02catalysts (0.25 g) were re-reduced in H2at 300 "C. The hydrogenation of benzonitrile was carried out in ethanol at 60°C and 4 bar H2 pressure. Tin content of the catalysts ranged from 0.05 to 0.63 wt. YO,whereas Sn/Pt surface atomic ratios determined by chemisorption measurements were between 0.1 to 3.5. In the hydrogenation of benzonitrile the following reactions take place: (Ph = Phenyl group) PhCN 2 H2 PhCH2NH2 (10)

+

2 PhCH2NH2 PhCH2NH2

+

+

+ NH3 PhCH3 + NH3

(PhCH&NH

+ H2

+

(11)

(12)

The main product of the reaction was dibenzyalmine. As seen in Figure 16 the selectivity of dibenzylamine was around 75 YOand was not influenced by the level of the surface Sn/Pt atomic ratio.*30Upon increasing the surface Sn/Pt atomic ratio the selectivity of toluene formation decreased and that of the primary amine increased. The addition of tin to Pt led to an increase in the turnover frequency (TOF) by a factor of 2. TOF showed maximum at surface atomic ratio of Sn/Pt = 1 as shown in Figure 17.13' These results are in good agreement with earlier findings obtained in the liquid-phase hydrogenation of benzonitrile on catalysts prepared by the addition of tin chloride to Pt/nylon catalyst.33 Because the hydrogenation of the nitrile group takes place on the Pt sites, the rate enhancement observed upon addition of tin can only be explained by the cooperation of Pt and tin sites on the surface of alloy-type nanocluster and/or the metal support interface. The results obtained on alloy type Sn-Pt/Si02 catalysts strongly resemble results attained on silica supported Ni-Fe catalysts with 75 Yo

31


.

$ 80-,

.

Dibenzylamine

Benzylamine n 0.0

0.4

0.2

0.6

1.o

0.8

Sn/Pt surface atomic ratio Figure 16

Relationship between product selectivity and SnlPt surface atomic ratio in the hydrogenation of benzonitrile over Sn-PtlSiO, catalysts (Reproduced from ref. 130 with permission)

*O

1

15-

F

v,

u:

10-

?

5-

0

Figure 17

1

.

1

-

1

-

1

-

1

-

1

-

1

'

1

Correlation between turnover frequency and SnlPt surface atomic ratio in the hydrogenation of benzonitrile over Sn-PtlSiO, catalysts (Reproduced from ref. 130 with permission)

Ni and 25 % Fe ~ 0 n t e n t . In l ~ this ~ study the improved activity was attributed to the formation of either Ni-Fe"+ or Ni- Feo alloyed sites. It has been suggested that in both types of sites, due to the charge transfer form Fe, the charge on the nickel site increased. Based on the above analogy the activity increase over Sn-Pt/SiOz was explained by the formation of 'Sns+-Pt' ensemble sites on the surface of bimetallic

32

Catalysis

nanoclusters. It was suggested that highly dispersed positively charged tin species, by polarizing the triple bond, enhanced the reactivity of the -CN 2.1.6 Use of Alloy Type Sn-Pt/Si02Catalysts in Low Temperature CO Oxidation. 2.1.6.1 Literature Background. Pt/Sn02'32J33and Pd/Sn02134J35 catalysts are widely used as low temperature CO oxidation catalysts. With respect to these catalysts a synergism between the oxide and the metal phases has been suggested. Bond et al. proposed a bifunctional mechanism based on the spillover of both CO and oxygen from the noble metal to tin Sheintuch et al. also considered a spillover, but it was related only to C0.'35In another explanation the formation SnPt alloy phase has been ~uggested.'~~ Local temperature increase of Pt sites and the promotion of the reaction on adjacent SnO2 sites had also been propo~ed.'~' Recently an alternative reaction mechanism'38has been proposed suggesting that the reaction takes place at the Pt-Sn02 interface and the Lewis acid sites of the interface are involved in the activation of CO molecule as shown in Figure 18.1°7 The analysis of the above scheme and the scheme describing the activation of the carbonyl group in unsaturated aldehydes shows obvious similarities (compare Figures 5 and 11 with Figure 18). The S C e selectivity of alloy type Sn-Pt/Si02catalysts in the hydrogenation of crotylaldehyde strongly depended on the Sn/Pt (at/at) ratio and increased with the time on stream (TOS).'" This fact has been considered as direct evidence for the involvement of tin in aldehyde activation. Results obtained by in situ Mossbauer spectroscopy provided further prooflogthat in the beginning of the TOS experiment the Sn-Pt alloy phase poor in tin was oxidized by crotonaldehyde, and a correlation between the SC=O selectivity and the formation of Sn4+ sites was found. The Sco selectivity of Sn-Pt/Si02catalysts has been attributed to the in situ formed Sn4+species. For this reason it has been suggested that alloy type Sn-Pt/Si02catalysts can effectively be used in CO oxidation as the reaction atmosphere is favourable for the in situ formation of ionic forms of tin in the atomic closeness of platinum. Consequently, it has been expected that these Sn-Pt/SiO, catalysts should have higher activity than the parent Pt/Si02 and the commonly used Pt/SnO2 catalysts. 2.1.6.2 Low Temperature CO Oxidation Over Alloy Type Sn-Pt/SiO, Catalysts. Figure 19 shows selected Temperature Programmed Reaction (TPRe) results obtained in the oxidation of CO over Sn-Pt/Si02catalysts with different

I platinum I Sn02

Figure 18

Pt-Sn02 interface inuolued in the activation of CO in PtlSnO, catalysts (Accordingto ref. 135)

33

1 : Role of 'Metal Zon-Metal Nanocluster' Ensemble Sites

T

0

50

100

150

200

T PC

Figure 19

The injluence of tin content on the activity of Sn-Pt catalysts in the oxidation of CO. Temperature Programmed Reaction (TPRe) results. Catalysts: 0 Pt/SiO,; X - Sn/Pt = 0.22, (H-2) - type catalyst, A - Sn/Pt = 0.41, ( H - I ) type catalyst 0- Sn/Pt = 0.81 ( 0 - 3 ) - type catalyst (Reproduced from ref. 35 with permission)

. Figure 20

Activity of Sn-PtlSiO, catalysts in CO oxidation. Dependence of TSovalues of the Sn/Pt (atla0 ratio. (0)= 79 torr, (m) = 16 torr (Reproduced from ref. 107 with permission)

Sn/Pt The temperature (T50), at which 50 % CO conversion has been achieved was used to compare the activity of catalysts. The T5ovalues measured at two CO pressure levels are shown in Figure 20."' Results given in Figure 19 show that the parent Pt/SiOz catalyst is quite inactive. However, the introduction of tin significantly increases the activity of the catalysts, resulting in a pronounced decrease of the T50 values. Figure 20 shows that catalysts with Sn/Pt (at/at) ratio around 0.2-0.5 have high activity around room temperature at Pco = 16 torr. Further results given in Table 11 show that the TSovalues strongly depend on

34

Table 11

Catalysis

InJEuenceof the temperature of reduction on the activity of Sn-Pt/Si02 catalysts (Reproduced from ref. 35 with permission)

Catalytic runs

Catalysts, and pretreatment

7-50,

1 2 3 4*

(H) type, T H 2 = 340 "C (H) type, T H2 = 200 "C (0)type, T H=~340 "c (0)type, TH2= 200 "C (0)type, after run No4 followed regeneration at TH2= 340 "C

69 69 63 101 68

5*

"C

* TPRe experiment with temperature ramp up to 200 "C.

the temperature of reduction of the alloy type Sn-Pt/Si02catalysts in hydrogenP' There was only a minor difference between (H) and (0)type catalysts reduced at 340°C. However, the (0)type catalysts were sensitive for the temperature of reduction in hydrogen. These results are in a good agreement with results of Mossbauer spectroscopy discussed earlier (see Table 4). After reduction at 200 "C the (0)type Sn-Pt/Si02 catalysts are not fully reduced, they still contain both four- and bivalent forms of tin. Consequently, in highly active alloy type SnPt/Si02 catalysts the high activity has to be attributed to the stabilization of supported bimetallic nanoclusters and not to the formation of stabile ionic forms of tin. It should be emphasized that after TPRe run up to 350"C, all alloy-type Sn-Pt/SiOz catalysts without re-reduction had very low activity. Thus, on platinum nanoclusters covered by bulk type tin-oxide layer the number of required 'metal ion -metal nanocluster' ensemble sites is very low. The experimental data given in Table 11 strongly indicated that the activity of the alloy type Sn-Pt/Si02 catalysts was controlled by the surface composition of the bimetallic nanoclusters and the reduced form of the Sn-Pt nanoclusters is more active than a fully oxidized form. Additional experiments have proven that the activity of catalysts used in TPRe experiments can be completely restored after reduction in hydrogen at 340°C. 2.1.6.3 In situ Mossbauer Measurements. In situ Mossbauer spectra were taken during the room temperature oxidation of carbon monoxide on Sn-Pt/SiOz catalyst ((0-3) type (sample 11-3 in ref. 139) both at 27°C and -196°C. These spectra are presented in Figure 21 A and B, respectively and the corresponding data are summarized in Table 12.'39 The as received Sn-Pt/Si02catalyst contained Sn4+oxide as the dominating phase (see spectra a in Figures 21A and 21B). This finding is obvious because after preparation the sample was kept in air. The high temperature (300°C) activation in hydrogen results in the formation of bimetallic phases in overwhelming dominance (see spectra b in Figures 21A and 21B).

35


4

Figure 21

4

-

2

0 2 mmls

4

6

8 - 4 - 2 0 2 mmls

4

6

Mossbauer spectra of Sn-Pt/Si02catalysts ((H-3) type, SnlPt (atlat) = 0.68) under diferent experimental conditions. ( A ) spectra obtained at 27 "C,( B ) spectra obtained at -1 93 "C.Catalyst samples: (a) as received, (b) afer reduction at 300 "C, (c) in situ measurement in CO oxidation at 27 "C, (d) in situ reduction in hydrogen at 27 "C,and (f)in situ treatment in CO at 27 "C (Reproducedfrom ref. 139 with permission)

The reduced Sn-Pt/Si02catalyst had two alloy type species with isomer shifts between 1.20-1.56 mm s-'and 2.23 - 2.35 mm s-', respectively."' These species were the main components in this catalyst accounting for around 85 % of the overall amount of tin. These phases were attributed to Pt-rich (PtSn(a)) and tin-rich one (PtSn(b)) (see explanation given in section 2.1.2."' It is worth mentioning that after reduction at 300°C the Sn4+(ox)species were almost completely reduced. The Sn2+and Sn4+species found in reduced sample may have originated from organometallic species anchored at the periphery of the nano-cluster and subsequently become incorporated into the upp port.^^'^^^^ Another possible explanation for the presence of ionic forms of tin is that their full reduction requires slightly higher temperature and prolonged reduction time. Figures 21A and 21B show that the character of in situ Mossbauer spectra taken in the presence of CO + O2 at room temperature has been completely changed compared to the reduced form of the catalyst (compare spectra b and c). The oxidative atmosphere in the presence of CO resulted in (i)oxidation of both PtSn(a) and PtSn(b) alloy phases to Sn4+ in a high proportion and (ii) the

in CO at 25 "C

f

+

+

+

-

-

0.65(1)

-

0.65(2)

-

-

0.70(1)

-

-

0.65(2)

QS

0.97(3) 1.60(19) 1.40(8)

1.45(7) 0.96(7) 1.79(23) 1.53(12)

47 28 25

18 30 43 27

53 46 75 7

82 18

1.07(4) 1.53(17) 1.44(5) 1.87(7) 1.07(2) 0.90(14)

RI

FWHM 0.00(1) 1.39(2) 0.55(3) 2.76" 1.37(3) 2.4 1(6) O.Ol(1) 0.94(2) 3.4 1(2) 1.81(2) O.Ol(2) 1.24(2) 2.44(2) -0.04(2) 0.79(3) 3.31(2) 1.68(2) - 0.05(1) 0.88" 1.93(1)

IS

1.29(1) 1.66(4) 0.79( 14) 1.38(14) 1.66(16) 1.96(14) 1.22(1) 1.07(8) 0.81(6) 1.44(6) 1.03(3) 1.74(10) 1.74(5) 1.16(2) 1.03(8) 0.51(6) 1.57(4) 1.10(2) 1.46(10) 1.52(3)

0.72(1)

-

-

0.65

-

2.1 1

0.66

-

0.7 1

-

2.03

-

0.73

-

2.32"

-

FWHM

QS

77 K

3.5 4.7

77 23 4 12 41 42 62 11 4 23 31 34 35 58 10 2 29 50 21 30

4.7 (3.0) 5.1

4.9 5.9 4.6 6.6

4.1 4.8 3.1 6.1

OrfA

RI

(IS: isomer shift, relative to SnO,, mm s-'; QS: quadrupole splitting,mm s-', FWHM: line width, mm s-', RI: relative intensity %, b: fA = - d In (A3W/A,,) / dT x 10 -3) RI is a derived parameter with an error summarized from those of the components (baseline, intensity, FWHM - is estimated at ca. 10% relative YO) ("I constrained parameters

inCO O2 at 25 "C

+

in H2 at 25 "C

inCO + O2 at 25 "C

e

d

C

b

O.OO(1) Sn4+(ox) 1.37(7) PtSn(a) Sn4+(sf) Sn2 Pt Sn(a) 1.34(2) 2.24(5) PtSn(b) Sn4+(ox) O.OO(1) Sn4+(sf) 0.80'"' Sn2+ PtSn 1.58(3) Sn4+(ox) -0.01(2) Pt Sn(a) 1.23(4) 2.34(4) PtSn(b) Sn4+(ox) Sn4+ (sf) Sn2 PtSn Sn4 (ox) - 0.06( 1) 0.80" Sn 4 + ( ~ f ) PtSn 1.91(2)

As received Treated in H,at 300 "C

a

IS

Camp.

300 K

Results of in situ Mossbauer spectroscopy on (0-1)type Sn-PtlSiOz catalyst used in room temperature oxidation of CO (Reproduced from ref. 139 with permission)

Samples Treatment

Table 12

o\

w

37

1 : Role of 'Metal Ion-Metal NanocEuster' Ensemble Sites

appearance of a new alloy phase with IS value around 1.58-1.81 mm s-'(see spectra c in Figures 21A and 21B). This isomer shift is close to that of the PtSn (1:1) thus this new third alloy component has been denoted as PtSn (1:l) alloy phase. In the presence of CO + O2 at room temperature the main component is the bulk tin-oxide like Sn4+ species with IS value around zero. Furthermore, an additional new component appeared with IS between 0.80 - 0.94 mm s-' (and after a repeated CO + O2treatment at 0.79 mm s-', as well (see spectra e)). This value is beyond the lowest IS assigned for bimetallic tin alloys. Based on literature analogies'43this component has been assigned to a highly mobile and reactive surface species, Sn4+(sf),since its IS value is significantly different both from that of oxidic Sn4+and Pt rich alloy phases. The room temperature treatment of catalysts used in CO oxidation in a hydrogen atmosphere resulted in very pronounced changes in the composition of the catalyst. Interestingly, not only the mobile and highly reactive surface species, Sn4+(sf),but part of Sn4+(ox)is reversibly transformed to the original alloy phases. The reduction of the Sn4+(sf)phase is complete and the proportion of Sn4+(ox)drops from 62 to 31 YOarea in the spectrum and simultaneously the platinum-rich PtSn(a) and the tin-rich PtSn(b) components reappear in 34 and 33 YOrelative intensity, respectively. This result indicates that 82 YOof the original alloy content has been restored by room temperature hydrogen treatment (compare samples b and d). The re-reduction completely eliminated the newly formed PtSn (1:1) alloy phase, as well. Consequently, the intimate contact between Sn and Pt, both in the reduced and the working catalyst containing supported alloy-type nanoclusters, is convincingly demonstrated by the room temperature regeneration in hydrogen. This regeneration clearly proved the Sn4+ Snotransformation at room temperature both in PtSn(a) and PtSn(b) bimetallic phases. In addition, it is worth comparing the effects of reduction at 300 "C and room temperature re-activation in hydrogen: the corresponding IS values of PtSn(a) and PtSn(b) are very close and even their proportion is the same. In a subsequent room temperature CO + O2treatment partial re-oxidation of metallic tin to Sn4+(ox)and Sn4+(sf)was evidenced again with simultaneous disappearance of both PtSn(a) and PtSn(b) components in the catalyst (see spectrum e in Figure 21B). This experiment provided additional proof for the reversibility of nanocluster reconstruction. However, the treatment with pure CO (see spectrum f) resulted in only slight alteration in the ratio of the Sn4+(ox)and Sn4+(sf)phases without reconstruction of the original alloy phase. From the -d ln(A300/A77)/dT values the following information was obtained. The low values are characteristic of strong ionic bonds (for instance, for bulk-like SnOz phase a value of 1.0 x is ~btained.'~'However, in large organic complex molecules with looser bonds -d lnA/dT values close to have been obtained.l4 Strong interaction of tin with oxygen is reflected in the -d In A/dT = 3 x value estimated for Sn4+(ox)after contacting the catalyst with C O 0 2 mixture. In contrast, after room temperature re-activation in hydrogen (see

-

+

38

Catalysis

sample d), Sn4+(ox)exhibits a significantly larger -d lnA/dT value (5.87 x than in samples c or a. Further, the surface character of Sn4+(sf)is reflected in large -d In A/dT values (6.08 x after CO O2treatments. The comparison of PtSn(a) and PtSn(b) component -d In A/dT values provide further insight. For the Pt-rich PtSn(a) component, a relatively low value (4.1- 4.6 x is obtained, indicating the incorporation of tin into the core of bimetallic particles. Whereas, tin in PtSn(b) is more loosely bound as -d In A / dT = 4.8 - 6.5 x lod3indicates. Based on the -d In A/dT values the strongest Pt-Sn interaction is in the PtSn (1:1) component formed after treatment with CO (see sample f). In summary, results of Mossbauer spectroscopy studies indicate that the primary interaction with oxygen leads to a strong enrichment of tin in the surface layer in the bimetallic particles and tin, both in the surface layer and in the bulk of the nanocluster, is oxidized to Sm4+In the presence of CO O2mixture two forms of Sn4+oxide have been found, i.e., (i) a more stable one, Sn4+(ox)with isomer shift around zero, and (ii) a mobile one, Sn4+(sf)with IS = 0.79-0.94 mm s-l. During the room temperature CO oxidation no separation of the oxidized Sn4+forms was observed, as both the Sn4+(ox)and Sn4+(sf)components were easily reconverted in hydrogen at room temperature to PtSn(a) and PtSn(b) phases. During the CO + O2reaction, the surface of the nanoparticle containing of both mono and bimetallic metallic sites are probably covered mostly by Sn4+ species, while providing simultaneous access to CO on the surface of the formed PtSn (1:l) or Pt phases. In this way indirect experimental proof for the formation of 'metal ion - metal nanocluster' ensemble sites has been obtained.

+

+

2.1.6.4 In situ FTIR Measurements. Three different catalysts were investigated by in situ FTIR measurements: Pt/Si02, Sn-Pt/Si02(Sn/Pt (at/at) =0.19 sample A) and Sn-Pt/Si02(Sn/Pt (at/at) = 0.68, sample B). The characteristic frequencies of both the linear and the bridged CO bands are summarized in Table 13.'39 The addition of tin into the Pt/SiO2 catalyst decreased slightly both the frequency and the intensity of linear CO band. On the parent Pt/Si02 and the Sn-Pt/Si02, sample (A) the bridged CO band appeared around 1855 cm-'. However, on Sn-Pt/Si02, sample (B), due to the relatively high tin loading, no bridged CO band was observed. Similar results were obtained both on Pt/Si02145J46 and Sn-Pt/Si02147J48J49 catalysts. The observed behaviour of alloy type Sn-Pt/Si02 catalyst is characteristic for tin modified supported platinum catalysts and has been attributed to the dilution of the platinum surface with tin. Data in Table 13 show that in the parent platinum catalyst the addition of oxygen to CO had no detectable changes in the spectrum. However, in both Sn-Pt/Si02catalysts the addition of oxygen resulted in noticeable shift in the CO band frequencies. This shift indicates that in the presence of oxygen the surface composition of the supported tin-platinum nanocluster has been altered and the extent of these changes depends on the Sn/Pt ratio (compare samples (A) and (B)). After addition of oxygen catalyst sample (A) strongly resembles the parent platinum catalyst. On this sample the YCO frequency of linear CO band was

-

-

2083 (9.12) 2082 (9.30) -

-

2078 (8.90) 2082 (8.46)

-

1865 (0.88) 1865 (1.78) 1865 (0.65) 1860 (1.58)

a

Frequency and normalized intensity data (the latter are given in parenthesis). addition of CO first, introductionof oxygen first. Spectra measured after 40 min equilibration; n.d., not determined; n.m. not measurable. A - Sn/Pt = 0.19, B - Sn/Pt = 0.68.

4

+

In the presence of CO O2 mixtureb In the presence of CO 02mixtureC After room temperature reduction of sample No 2 in H2

2

3

Adsorption of CO

1

+

Experimental condition

N"

Catalysts

2070 (8.18) 2076 (6.66) 207 1 6.15 2083 (n.d.)

(n.m.) (n.m.) 1860 0.57 1860 (n.m.) 1860 (n.m.)

Table 13 Summary of in situ FTIR experiments: Frequency and Intensity Changes of the Linear and Bridged C O Adsorption Band under Condition of C O Oxidation at Room Temperature (Reproduced from ref. 139 with permission)

40

Catalysis

shifted towards the high frequency region (AVco= 4 cm-') and the intensity of the bridged CO band increased significantly. In sample (B) after exposure to oxygen the band position of linear CO is also shifted (Avco = 6 cm-') and there is a noticeable half width broadening. The analysis of band intensities indicates also that substantial replacement of chemisorbed CO by oxygen can only be seen in sample (B). The treatment of the catalyst used in CO oxidation for one hour at room temperature resulted in a decrease in the v C 0 band frequency from 2076 to 2071 cm-' (see Table 13 exp. No3). This new value is very close to the frequency of the v C 0 band measured on the fully reduced (B) sample (see Table 13 exp. N"1). Consequently, partial reduction of the formed oxide phase at room temperature has also been shown by in situ FTIR measurements. Results presented in Table 13 indicate also that the addition of oxygen to the Pt-Sn/Si02 (B) catalyst followed by introduction of CO has resulted in substantial differences in the FTIR spectra of chemisorbed CO (see experiment NO4 in Table 13).In this sample the low frequency v C 0 band at 2070 cm-' disappeared and the spectrum strongly resembled that of the parent platinum ((vCO)lin = 2083 cm-', (vCO)br= 1860 cm-l). This result indicates that upon contacting the supported Sn-Pt nanocluster with pure oxygen fast oxidation of the supported alloy phases takes place. The oxidation leads to rapid segregation of phases and the subsequent addition of CO cannot restore the surface composition formed in the presence of CO + O2 mixture (compare exps. No2 and No 4). This segregation leads to the reappearance of pure Pt sites ((vCO)lin = 2083 cm-', (vCO)br = 1860 cm-'). Catalytic experiments showed also that the activity of Sn-Pt/Si02 catalyst was very low when oxygen was introduced first. Difference spectra obtained on sample (B) in the presence of CO + 0 2 mixture and in pure CO are shown in Figure 22A and Difference spectra were taken at 5, 15,30 and 60 minutes after exposure of oxygen.lo7 Results obtained at 10 torr (see Figure 22A) imply that upon exposure to oxygen the intensity of the band at 2070 cm -', characteristic for the adsorption of CO on bimetallic nanoparticles, decreases, while that of the band at 2086 cm- ', characteristic for the adsorption of CO on pure platinum, increases. Parallel to the above changes, a very pronounced increase was also observed in the intensities of the (vCO)b,band at 1840 cm-'. However, the difference spectra indicate that the oxygen induced surface reconstruction is relatively fast as there is no measurable difference between these spectra. The increase of the concentration of CO up to 50 torr resulted in slightly slower, but pronounced changes induced by oxygen (see Figure 22B). The character of these changes is similar to that measured at Pco = 10 torr. However, after eight minutes only minor alteration can be seen, although after 25 minutes there are no further measurable changes in the band intensities both around 2070 cm-' and 2086 cm-'. Parallel to the above changes, a notable increase was also observed in the intensities of the (vCO)b, band at 1840 cm-'. This change appeared to be continuous in the whole time interval. These results might indicate that the bridged CO band is more sensitive to reflect minor surface reconstruction than the linear one. The comparison of results obtained at two

41


c

2200

I

I

2100

2000

1900

Wavenurnbers, cm.'

Figure 22

1800

1700

Wavenumbers, cm"

Diflerence FTIR spectra of adsorbed CO in the presence and absence of oxygen on Sn-Pt Si02 catalysts ( H - 1 type, SnlPt (atlat) = 0.68). A - Pco = 10 torr, Poz = 5 torr; B - Pco = 50 torr, PO2 = 25 torr, in situ room temperature spectra: (a) 5 min,, (b) 15 min, (c) 30 min and (d) 60 min of reaction (Reproducedfrom ref. 107 and 139 with permission)

CO partial pressure levels indicates a pressure dependence of surface reconstruction, i.e., the lower the CO pressure the higher the rate of oxygen induced reconstruction of supported Sn-Pt nanoclusters. The above results strongly indicate that upon introduction of the CO oxygen mixture the supported Pt-Sn alloy phases are transformed and segregated. This is a time dependent process, however the rate of these surface transformations is relatively fast. The net results of changes induced by oxygen are as follows: (i) the number of bimetallic tin-rich alloy sites around 2070 cm-' diminishes, and (ii) and number of pure Pt sites what adsorb CO around 2086 cm-' increases. The formation of pure Pt sites have been evidenced in Sample (A), as well, i.e., the site separation is characteristic both for platinum-rich and tin-rich alloy phases. These results are in a good agreement with the results of Mossbauer spectroscopy, where in the presence of oxygen the transformation of both PtSn(a) and PtSn(b) alloy phases has been shown and the formation of tin oxide like phases has been demonstrated (see Table 12). Consequently, results of in situ FTIR provided further proof for the PtSn + Sn4++ Pt conversion taking place during the low temperature CO oxidation.

+

2.1.6.5 Structure of Alloy Type Sn-Pt/Si02 Catalysts used in Low Temperature CO Oxidation. Both Mossbauer and FTIR spectroscopy provided sufficient proof of surface reconstruction during the low temperature CO oxidation. However, the above reconstruction appeared to be reversible as the reversible interconversion of PtSn c* Sn4+ + Pt was demonstrated by both spectroscopic techniques. This reversibility can only be achieved if the segregation described above is within the supported nanoparticle, i.e., when surface reactions involved in CO oxidation do not result in formation of separate Pt and tin-oxide phases on the silica support. In in situ experiments it was demonstrated first time that both metallic and ionic species can co-exist in the same supported particle (nanocluster) and both

42

Catalysis

the parent alloy phases and the newly formed ionic and metallic phases appeared to be highly mobile and reactive even at room temperature. Based on these results it had been suggested that in room temperature CO oxidation over Sn-Pt/Si02 catalysts the supported bimetallic nanocluster is oxidized and the oxidation of the alloy phases leads to strong reconstruction. The net result of these transformations is the formation of the following phases or sites: Tin oxide phase (IS = 0.0 mm s-', QS = 0.6-0.7mm s-') Appearance of a highly mobile Sn4+(sf) phase (IS = 0.79-0.94mm s-') Formation of free platinum sites (vCOlin= 2086 cm-', vCObr= 1840 cm-') Appearance of a third alloy phase, PtSn (1:l)(IS = 1.6-1.9mm s-'). The schematic view of the supported nanocluster prior and after its reconstruction in the presence of CO and O2mixture is shown in Figure 23 and 24. Based on the above results it had been suggested that the oxidation of CO takes place at the 'Sn 4+ - Pt' ensemble sites formed in situ. These sites are considered to be 'metal ion - metal nanocluster' ensemble sites. The possible

SnO, \

SiO,

Figure 23

Schematic view of silica supported Sn-Pt nanocluster after reduction in hydrogen at 300 "C.Catalyst type (H-3)

sio, fl hTI

well dispmried Pt or Ptsn (1 :l)p)use

mobite Snf++f> Sni(0x) segregated Pt

Figure 24

Schematic view of silica supported Sn-Pt nanocluster after its reconstruction in CO O2mixture at room temperature. Catalyst type (H-3) (Reproduced from ref. 139 with permission)

+

43

1 :Role of 'Metal Ion-Metal Nanocluster' Ensemble Sites

mode of the activation of the CO molecule on the above sites was also modeled and calculated. The results of these calculations are discussed in the next paragraph. 2.1.6.6 Computer Modeling of CO Activation. Results presented so far indicate that the chemical nature of heterogeneous catalytic activation of both the carbonyl group in aldehydes and the CO molecule should have a common basis. The high activity of catalysts prepared by using CSRs can be related to the activation of the carbonyl moiety by Sn4+species formed during the reaction. The ionic forms of tin stabilized on the platinum surface attract electrons either from the oxygen atom of the CO molecule or the carbonyl group. The flow of electrons from the CO (or carbonyl moiety) to Sn4+strongly weakens the carbon-oxygen triple (or double) bond. Consequently, in CO oxidation over the Sn-Pt/Si02 catalysts prepared by using CSRs the C O molecule is strongly perturbed and reacts at low temperature. The above perturbation of the CO molecule has been modeled and cal~u1ated.l~~ In the first approach ab initio Hartree-Fock calculations were made for hypothetical (CO + Sn'"))clusters, where n = 0, + 2 and +4. Results for the single CO molecule and for cases n = 0 and +4, are summarized in Table 14. These data undoubtedly demonstrate that the carbon-oxygen bond is weakened in the proximity of the charged tin atom and the weakening effect is proportional to the charge of the tin atom. The calculation of the Mulliken charges and the bond order of the contributing atoms provide further information (see data given in Table 15). These data show that the effect of the neutral tin atom in the proximity of the CO molecule on the atomic charges and bond orders is insignificant. However, when the tin atom is charged, electron transfer occurs from the CO molecule to the tin cation. The charge of the carbon atom in the C O molecule becomes more positive, and this makes the oxygen atom more negative and reduces the positive charge of the tin cation. It has been shown that the increase in the nucleophilic nature of the oxygen atom makes it possible to form a strong interaction between the C O molecule and the tin cation as shown in Table 16. As emerges from these data the Mulliken bond order increases and the distance decreases between the 0 and Sn'")atoms if the tin atom is charged. However, the increase of the positive charge on the

Table 14

Bond lengths and Mulliken bond orders of the carbon-oxygen bond in the carbon monoxide molecule and in CO + Sn'") (Reproduced from ref. 35 with permission) bond length (A)

co CO CO

+ Sno + Sn4+

Mulliken bond order

STO-3G

3-21 G ( * )

STO-3G

3-21 G( *)

1.15 1.15 1.28

1.13 1.13 1.28

2.51 2.46 1.61

2.22 2.10 1.25

44

Catalysis

Table 15

Mulliken atomic charges in the carbon monoxide molecule and in CO Mn) complexes (Reproduced from ref. 35 with permission)

+

C

co CO CO

Sn

0

STO-3G

3-21 G ( * )

STO-3G

3-21G(*)

STO-3G

3-21G(*)

0.20

0.44 0.53 1.48

- 0.20

-0.44 -0.47 -0.64

-0.03

-0.06

3.28

3.16

+ Sno 0.24 + Sn4+1.03

-0.21 -0.31

-

carbon atom reduces the electronic charge density in the proximity of the carbon nucleus, and thus reduces its ability to form a covalent bond. Hence the result of these effects is the weakening of the carbon-oxygen bond in the carbon-monoxide molecule. Consequently, the results of the ab initio Haertee-Fock calculations strongly support the involvement of the electron transfer from the CO molecule to the Sn4+site in the activation of the CO molecule. According to the results of in situ Mossbauer spectroscopy the formation and stabilization of the PtSn (1:l) alloy phase has been shown under condition of room temperature CO oxidation (see results presented in Table 12). One of the most active surfaces of the PtSn (1:l) alloy phase, the (110)phase, was chosen to model the interaction of the CO molecule with the metal surface.'39 The computer modeling and the related calculations were made on density functional level. In this model a small cluster of the (110) surface of the PtSn phase94as shown in Figure 25a, was selected in order to calculate and investigate the interaction of the CO molecule with the metal surface. Two alignments of the CO molecule relative to the metal cluster were examined, for total charges N = 0, 4. For the linear alignments, the CO molecule is perpendicular with the surface and no S n ( N ) . . 0 . . interaction is possible. In contrast, for the bent structures, the oxygen atom of the CO molecule chemisorbed on the Pt is near to a tin atom, allowing interaction between them. The structures of the above alignments are shown in Figures 25b and 25c, for neutral surface. The results of the above calculations for the CO molecule and for its interaction with N-fold charged metal clusters are presented in Table 17, for N = 0 and +4.13' These results show that with respect to the Sn(N).... 0 interaction, and the parameters of the CO bond the alignment of the CO molecule relative to the

+

Table 16

Distances and Mulliken bond orders between the oxygen and tin atoms in CO + Sn(*) (Reproduced from ref. 35 with permission) distance

CO CO

+ Sno + Sn4+

(A)

Mulliken bond order

STO-3G

3-21 G ( * )

STO-3G

3-21 G ( * )

2.66 1.93

2.72 2.0 1

0.08 0.85

0.12 0.76

45

I: Role of ‘Metal Ion-Metal Nanocluster’ Ensemble Sites

B

W

C

A Figure 25

Alignment of the CO molecule chemisorbed at the cluster of the (I 10) surface of the PtSn (1:l) alloy phase. ( A ) the (I 10) surface of PtSn (1:l) alloy phase (the PtSn cluster used in D F T calculation is shown in black): ( B ) linear coordination of CO on the PtSn cluster; ( C ) bent coordination of CO on the PtSn cluster. The bent structure is required for the activation of the CO molecule (Reproduced from ref. 139 with permission)

metal cluster is more important than the total charge of the system. The calculations reveal that the alignment strongly affects the CEO bond, as the bent structures have more significant effect on the bond lengths and bond orders than the linear structures. On the other hand, it has also been demonstrated that for the neutral system the linear structure is energetically more stable than the bent alignment, while the opposite relationship has been found for the charged alignment. These results support the hypothesis that on the uncharged cluster the chemisorbed CO molecule is perpendicular to the metal surface with no Sn(N).... 0 interaction. However, if the surface is charged, the alignment of the molecule is bent and the CEO bond is weakened because of the involvement of ‘Sn4+-Pt’ ensemble site in this interaction. Consequently, these calculations strongly support the relevance of our hypothesis with respect to the involvement of ‘metal ion - metal nanocluster’ ensemble sites and the C - 0 - Sn4+interaction in the increased activity of alloy type Sn-Pt/SiOz catalysts in low temperature CO oxidation.

46

Catalysis

Table 17 Bond lengths and Mayer bond orders of the C = 0 bond in the (cluster)N complexes and the carbon-monoxide molecule and in CO ienergy diflerences between the bent and linear structures (AE = Ebent- Elinear)

(Reproduced from ref. 139 with permission)

co CO CO

+ (cluster)’ + (cluster)4+

bond length (A)

Mayer bond order

linear

linear

bent

1.317 1.338 1.324

E

bent

(kcaljmol)

1.42 1.42

- 34.89

2.46 1.415 1.406

2.02 2.18

205.41

With respect to the involvement of the ‘Sn4+-Pt’ensemble sites in the activation of both the carbonyl group in unsaturated aldehydes and the CO molecule, the following difference has to be discussed. Upon using alloy type Sn-Pt/Si02 catalysts high selectivity for the formation of unsaturated alcohols was obtained at Sn/Pt (at/at) > 0.8. Contrary to that in CO oxidation, the best results were attained at Sn/Pt (at/at) = 0.4 - 0.7. The above difference was explained by a specific need for surface sites to suppress the readsorption of formed unsaturated alcohol. The suppression of the re-adsorption of unsaturated alcohol can only be achieved if platinum is strongly diluted by metallic tin, i.e., the surface concentration of the tin-rich alloy phase is relatively high. Therefore this reaction requires alloy type Sn-Pt/SiOz catalysts with high Sn/Pt (at/at) ratio. In CO oxidation there is no need for tin-rich alloy sites. The in situ formation of Pt or SnPt (1:l)sites enhances the chemisorption of both reactants. This is the reason that the segregation process (see Figure 24) is not harmful for the reaction. Contrary to that, it is very beneficial for the formation of ‘Sn4+-Pt’ensemble sites. However, at high Sn/Pt (at/at) ratio (Sn/Pt >1)the formed Pt (or SnPt (1:l))sites might be covered by the inactive Sn4+(ox) phase formed in situ. In this way the number of sites required for CO chemisorption decreases and the rate of CO oxidation diminishes, as shown in Figure 20. 2.1.7 Summary on Alloy Type Sn-Pt/Si02Catalysts. Results shown in these case studies provided unambiguous evidence that in alloy type Sn-Pt/Si02catalysts Lewis type ionic Sn”+or polarized metallic Sns+ sites can be formed. These sites due to the formation of alloy type supported nanoclusters are in atomic closeness. In this way ‘Sn4+-Pt’or MIMNES can be formed. In most of the cases these sites are formed in situ. Characteristic feature of these new types of active sites is the activation of different polarizable functional groups, such as carbonyl or nitrile, and the triple bond of carbon monoxide. Due this polarization the activity of these groups increases strongly resulting in high selectivity and high reaction rate in the hydrogenation of the carbonyl or nitril group and high rate of the oxidation of the CO molecule.


47

CO Oxidation on Supported Gold Catalysts. - 2.2.1 Literature Background. The first papers on the preparation and use of highly active supported gold catalysts were published in the late 1 9 7 0 ~ . ' ~ ~In3 ' ~ the ~ last 10-15 years supported gold catalysts have received great academic interest, due to their high activity in low temperature CO oxidation. Supported gold catalysts have also been used in combustion of saturated hydrocarbons, reduction of nitrogen oxides, hydrogenation of unsaturated carbonyl compounds, e t ~ . The ' ~ ~activity of supported gold catalysts appear to be a function of the gold dispersion and the concentration of interfacial sites. Supported gold catalysts have recently been reviewed by different a u f h o r ~ . ' ~ ~ ~ ' ~ ~ In studies on supported gold catalysts different gold precursor compounds and miscellaneous support materials, such as Ti02,'569'579'587'59 Ni0,16' Fe2o316071 56 c0304156"60 CuO,'@Zr02,158,161 Mg(OH)2'59and various transition metal hydroxides'62have been investigated. The support materials were classified as active (Fe304, TiOz, NiO,, COO,)and inert materials (A1203, MgO).'63 From results presented so far on low temperature CO oxidation the following main conclusions can be drawn:

2.2

(i) supported gold catalysts are highly active even at -70 "C, (ii) only nanoclusters with particle sizes in the range 1-6 nm show high activity, (iii) not only the size, but the shape of the nanoclusters has a significant influence on the activity of supported gold catalysts, (iv) supports with redox properties strongly enhance the activity of supported gold catalysts, (v) a number of different reaction mechanisms have been proposed in low temperature CO oxidation. Several models have been suggested to explain the high activity of gold catalysts. The proposed models of low temperature CO oxidation are summarized in Figure 26.'63 The first model assumes that oxygen adsorption takes place directly on the gold nanoparticles and the role of support is to stabilize very small gold particles with highly reactive gold sites or crystallite faces (reaction pathway 1 in the

Figure 26

Possible reaction schemes for the CO oxidation over supported Au catalysts (AulFe304) (Reproduced from ref. 163 with permission)

48

Catalysis

above f i g ~ r e ) . ' ~In. ' the ~ ~ second model it is suggested that the oxygen adsorption takes place on the support or at the metal-support i n t e r f a ~ e . ' ~ ~Oxygen ~'~~~'~*~ vacancies on the semiconductor type support materials, such as Fe304,T i 0 2 or ZnO may be considered as sites involved in oxygen adsorption (see reaction pathways 2a and 2b in the above figure). However, it was not discussed whether this oxygen molecule dissociates into Oads. or reacts directly with adsorbed CO (pathways 2a or 2b, respectively). In the third pathway it is suggested that oxygen after its adsorption dissociates, producing lattice oxygen, what can subsequently react at the metal-support interface with CO adsorbed on gold (see pathway 3 in the above figure).'66*'68 Reaction pathway 2 strongly resembles that of proposed for CO activation over Pt/Sn02 catalysts at the Pt-Sn02interface (see Figure 18). Costello et al. studied the deactivation and regeneration phenomenon during room temperature CO oxidation over Au/y-A1203catalyst, which was as active as the most active supported Au catalysts reported in the l i t e r a t ~ r e .The ' ~ ~ initial rapid loss of activity could be avoided if either hydrogen or water vapor was present in the reaction mixture. Thermal treatment above 100°C in a dry atmosphere also deactivated the catalyst. The original activity could be recovered by exposure of the deactivated catalyst to either hydrogen or water vapor at room temperature. These results suggested that hydroxyl group, most likely associated with a Au(1) cation located at the gold nanocluster - support interface is involved in CO activation as shown in Figure 27 and Scheme 4.17*It has been proposed that the active site is an ensemble of Au+-OH together with Au(0) atoms. As shown in Scheme 4 the CO oxidation was proposed to proceed via the insertion of CO into the Au+-OH bond to form a hydroxycarbonyl, which is oxidized to bicarbonate. Decarboxylation of bicarbonate completes the reaction cycle.171 The above results and some recent literature data related to the involvement of ionic form of gold in CO a c t i ~ a t i o n , 'encouraged ~ ~ ~ ' ~ ~ the authors of this review to demonstrate that MIMNES can also be formed in supported gold catalysts. If a non-active support, such as MgO is used in this case (AU)'+,-(AU)~, type ensemble sites can be formed provided the reduction of the gold precursor compound is controlled. In the literature there has been no investigation of the role of additives or

Au

Figure27

O H

Schematic drawing of an active site for CO oxidation over supported gold catalysts (Reproducedfrom ref. 171 with permission)


Scheme 4

49

Oxidation of CO over supported gold catalyst. Involvement of 'metal ion - metal nanocluster' ensemble sites in CO activation (Reproduced from ref. 171 with permission)

modifiers on the activity of supported gold catalysts in low temperature CO oxidation. The aim of this work is to demonstrate that ascorbic acid can modify Au/MgO catalysts to improve their activity in low temperature CO Oxidation. 2.2.2 Preparation and Characterization of AulMgO Catalysts. 2.2.2.1 Preparation. Three different Au/MgO catalysts were prepared using HA&& as a precursor compound:'74(i)base, (ii)re-suspended and (iii)modified with ascorbic acid. The modification of the base catalyst with ascorbic acid is associated with a slight color change. The color of the re-suspended catalyst after drying at 100 "C is beige with a light gray tone. The modified catalyst samples after modification with ascorbic acid and subsequent drying at 100°C were light bluish-violet or light pinkish-red. Ascorbic acid is considered to be a mild reducing agent, consequently it is suggested that in its presence the spontaneous auto-reduction of the gold precursor compound observed both in the impregnation and drying period and will be more controlled. It has been assumed that (i) by changing the amount of ascorbic acid added the ratio of ionic/metal gold can be altered and (ii) the controlled reduction will increase the ratio of smaller gold particles required for low temperature CO oxidation. 2.2.2.2 Characterization by U V-VIS Spectroscopy. Figures 28A and B show the diffuse reflection UV-VIS spectra of the different gold catalysts prior to and after reduction in hydrogen at 350 "C,re~pective1y.l~~ Three adsorption bands around 240,390 and 565 nm have been observed in all samples. Based on literature data these bands correspond to (i) Au+ cations, (ii) (Au),b+,and (iii) (Au),, respectiveIY.''~Figure 28A definitely shows that during the preparation of these samples spontaneous autoreduction of the gold precursor compound takes place. The autoreduction leads to the formation of both ionic and metallic forms of gold.

50

Catalysis

0.4

0.1

0.0

1

I

200

-

I

300

.

I

400

.

.

500

~

600

I

-

700

I

.

800

,

.

I

-

900

Wavelength, nm

A

0.74 0.6 0.5

1: 0.2 0.1

0.0 200300400500600700800900 Waveknght, nm

B

Figure 28

Difluse reflectance U V-VIS spectra of diflerent gold catalysts. A ) Catalysts prior to reduction. 1 - Au/Mg(OH)?P, 2 - O . ~ - A U / M ~ ( O H )3~-~2.4O~, after reduction in hydrogen at 350 C. 1 A U / M ~ ( O H ) ~B~)OCatalysts ~. Au/MgO"""",2 - 0.7-Au/MgOmod, 3 - 2.4-Au/MgOmod (Reproducedfrom ref. 174 with permission) O

The spectra reveal also that (i) autoreduction begins under conditions of the resuspension (see sample (1)in Fig. 28A, (ii) there is a certain amount of ascorbic acid which is not involved in the autoreduction (compare samples (1) and (2) in Fig. 28A), (iii) above a minimum level the addition of ascorbic acid increases the amount of both metallic gold and the positively charged gold nanoclusters, (Au), and ((AU)~'+, respectively, while amount of Au+ cations is practically constant. The reduction of samples in hydrogen at 300 "Cresulted in further increases in the intensity of bands at around 390 and 560 nm, while the band at around 240 nm completely disappeared. The disappearance of the band at 240 nm indicates

51

1: Role of 'Metal Zon-Metal Nanocluster' Ensemble Sites

the complete transformation of the gold precursor compound. After reduction in hydrogen the character of the UV-VIS spectra in the range between 300 and 600 nm is similar to that stabilized prior to the reduction. However, as shown in Figure 28B for samples 1 and 2, the band at around 560 nm is broader than in the corresponding samples shown in Figure 28A. In sample 3 the plasma-resonance peak is shiftec from 560 to 545 nm and the peak is narrower compared with the corresponding peak in the other two samples. These findings indicate that the addition of ascorbic acid enhances the stabilization of supported gold nanoparticles in the sizes below 5.0 nm required for low temperature CO oxidation. It should be noted that the reduction in hydrogen does not lead to the disappearance of (Au),"+ species. It has been suggested that the positively charged gold nanoclusters play a crucial role in the activation of CO. 2.2.2.3 Characterization by CO Chemisorption. Figure 29 shows the adsorption isotherms of CO over re-suspended Au/MgO ~ata1yst.l'~ It has to be emphasized that it was the first attempt to demonstrate that the amount of CO chemisorbed on gold could be measured. The calculated amount of chemisorbed CO shows a saturation level around 800-1000 torr of CO. Selected results of CO chemisorption are summarized in Table 18.'74These data indicate that both Au/MgO catalysts and the MgO support chemisorb CO. However, the latter has much less chemisorption capacity than the gold containing samples. The amounts of CO chemisorbed on resuspended Au/MgO catalyst (catalysts (I) and (11))roughly correspond to CO/Au = 0.10 - 0.12, i.e., the dispersion of gold in these Au/MgOcatalysts is about 10 - 12 %. Results presented in Table 18 show that the modification with ascorbic acid resulted in a substantial increase in 0.25 0.20

T

E 0.15

0.05 0.00

I

I

I

I

I

I

0

200

400

600

800

1000

I 1200

Pressure, torr

Figure 29

Adsorption isotherms of CO on Au/MgOresusp catalyst (Sample N o 3 in Table 18.). W - total volume of adsorbed CO, A - volume of physisorbed CO, 0volume of chemisorbed CO. Amount of catalyst: 0.13 g (Reproduced from ref. 174 with permission)

52

Catalysis

Table 18

CO chemisorption on MgO and diferent Au/MgOresusp catalysts (Reproduced from ref. 174 with permission) ~

VCO,

No

Catalyst

1 2 3 4 5 6 7

MgO Air and H2at 350 "C Au/MgO""usP(I)b Air and H2 at 350 "C Au/MgOesusP(II) Air and H2 at 350 "C Au/MgOesuS~(II)H2at 350 "C O . ~ - A U / M ~ O ~H2 " ~at 350 "C 0.7-Au/MgOmod Air and H2 at 350 "C 2.2-Au/MgOmod H2 at 350 "C

Treatment

mils

~~

CO/Au (atlat)

measured

Correcteda

0.095 0.554 0.547 0.569 0.540 0.525 0.813

-

-

0.642 0.632 0.675 0.630 0.600

0.113 0.111 0.120 0.110 0.103 0.176

1.005

Corrected by the amount of CO chemisorbed on MgO and for the amount of water lost due to transformation of Mg(OH), to MgO, prepared in amount of 1.3 g, (11) - reproduction of (I) in amount of 1.3g. Thermal treatment: heating in air to 350 "Cand keeping 1.5 h at this temperature followed by reduction in hydrogen for 1 h. a

the amount of CO chemisorbed and the dispersion value increased to 17.6 %. This dispersion value might correspond to a particle size below 5 nm. 2.2.2.4 FTIR Results on Adsorbed CO Molecule. C O chemisorption was performed on modified Au/MgO catalyst after two treatment procedures: (a) treatment in air followed by reduction in hydrogen, and (b) treatment in hydrogen. The corresponding spectra are presented in Figure 30A and B.'74 Over the catalyst pretreated in air and reduced in hydrogen the exposure of CO resulted in one relatively broad carbonyl band at around 2115 cm-' (see Figure 30A). Upon increasing the duration of CO exposure, the position of this peak shifted slightly to the low frequency region, to 2106 cm-'. Parallel to this shift a very broad band appeared in the low frequency region between 1800 and 2200 cm-I. The switch of the CO flow to pure helium resulted in complete removal of the CO band around 2106 cm-', but this had no influence on low

A

iIo.005

' '-D

3% CO (10 min)

-

i";"--

2

.-.

3%CO (30min)

2106

1:

/ ,

-

~

,.3,"/. CO (60min)

,

6

2125

100025

c-

g; 48 -- -J' ~'.~----.JhCO(3Ornm) 1

9

.3% CO (60 mn)

'

?

.

,

Figure 30

.

,

.

,

.

,

.

I

7

.

1

.

I

.

I

.

I

.

I

FTZR spectra of chemisorbed CO on 3.4-Au/MgOmodcatalyst, carbonyl region. A - catalyst pretreated in air followed by reduction in hydrogen; B - catalyst pretreated in hydrogen (Reproduced from ref. 174 with permission)


53

frequency the broad band. Complete removal of this broad band was observed only after thermal treatment in helium above 250 "C after 1 h. The sample treated only in hydrogen showed two overlapping CO bands at 2125 and 2105 cm-' (see Figure 30B). Almost similar overlapping CO bands have been obtained on Au/A1203 catalyst after its treatment with oxygen.'76The increase of the CO exposure time resulted in slight changes in the ratio of these two bands in favour of the lower frequency band. It should be mentioned that on this sample the appearance of the broad carbonyl band between 1900 and 2200 cm-I is negligible. However, the broad band between 1800 and 2200 cm-' was not observed in this sample. According to Boccuzzi et ~ 1 . the l ~ linear ~ CO band on small gold particles appears at 2106-2116 cm-'. Grunwald et a1.16' ascribed the bands around 21 112123 cm-' to CO chemisorbed on step and kink sites, while the bands around 2128-2135 cm-' were assigned to CO chemisorbed on positively polarized gold sites. Consequently, the appearance of a CO band at 2125 cm-' and above, obtained on modified Au/MgO sample pretreated only in hydrogen indicates that this catalyst might contain more ionic forms of gold than the one treated both in oxygen and hydrogen. This form of gold seemed to be quite stable as the duration of CO exposure did not result in a notable intensity change (see Fig. 30B). The asymmetric character of the CO band on a sample pretreated both in air and hydrogen indicates that the Au/MgO catalyst might also have ionic gold species, but in this sample the proportion of the ionic gold is relatively low. Based on literature data discussed above the following assignment was done for the carbonyl bans presented in Figures 30A and B: (i) the carbonyl bands at 2105 cm-': CO chemisorbed on metallic gold nanoclusters (Au)',; (ii) the carbonyl bands at 2125 cm-': CO chemisorbed on positively charged gold species (Au)'+,; (iii) the broad band between 1800 and 2200 cm-': chemisorbed CO, or spilled over to MgO. 2.2.3 Use of Au/MgO Catalysts in Low Temperature CO Oxidation, Figure 31 and Figure 32 show results in the temperature range of -30-250 "C obtained by Temperature Programmed Reaction (TPRe) technique using different types of Au/MgO cata1y~ts.l~~ These results clearly show that the base Au/MgO catalyst was less active than the re-suspended one, while the catalysts modified with ascorbic acid have the highest activity. The TPRe curves presented in Figure 31 and 32 show also unusual TPRe pattern of all Au/MgO catalysts. Upon increasing the reaction temperature from -30°C to 100 - 120°C the conversion of CO decreases. This decrease is very substantial in all catalysts. Upon further increase of the temperature, the conversion of CO increases and almost full conversion is reached around 250°C. A similar effect, i.e. a decrease in activity on increasing the temperature has recently been observed on Au/Mg(OH), catalyst under dry condition^.'^^ The authors attributed this behavior to 'negative activation energy'. It should be emphasized

54

Catalysis 1.o

0.8

--5t? 0.6 > 0

0.4 0.2 0.0

-50

0

50

100

150

200

250

300

Temperature, O C

Figure 31

Oxidation of carbon monoxide on digerent unmodijied AuIMgO catalysts using TPO techniques. Catalysts: 0 - A u / M g P s p (fresh, 0.150 9); A Au/MgOesusp(used, 0.150 g), x - Au/MgOresuSp, (fresh, 0.075 9); 0 Au/MgObase(fresh, 0.150 9); + - Au/MgOb"' (used, 0.150 9); 0Au/MgObase (fresh, 0.075 g). Catalyst pretreatment: calcination in air at 350 C for 1.5 h followed by reduction in hydrogen at 350 "Cfor 1 h (Reproduced from ref. 174 with permission) O

c

1.0

-

0.8

--

'E B

s

0

-50

0

50

100

150

Temperature, O C

Figure32

200

.-

250

300

The inJluence of the amount of ascorbic acid on the activity of Au/MgOmod catalysts. Results of TPO experiments. Catalysts: Au/MgOreSUSP, 0O.7-Au/MgOmod,0 1 . ~ - A u / M ~ O ", A" ~2.3-Au/MgOmod,x - 3.4-Au/MgOmd, + - 5.3-Au/MgOmod(the corresponding numbers indicate the amount of ascorbic acid in mg used to modify 1 g Au/Mg(OH), catalyst). Amount of catalyst: 0.075 g. Catalysts pretreated in hydrogen at 350 "Cfor 1 h (Reproduced from ref. 174 with permission)

that no other catalyst systems, such as Au/Ti02, Au/Fe203or Au/ZrO2 showed similar behavior. With respect to the activity decrease observed upon increasing the temperature, the deactivation of Au/MgO catalysts by surface species formed in situ, such


55

as carbonates, has been suggested. This has been confirmed by the formation of different carbonate IR bands observed under condition of CO chemisorption. The catalyst poisoning hypothesis has been further supported by TOS results obtained at -50, -30 and 0°C on different Au/MgO catalysts. In two hours on stream the modified catalysts showed no deactivation, while both the re-suspended and the base catalysts on he lowest deactivated relatively quickly. The strong deactivation of pure Au/MgO catalysts during TOS has recently also been reported by other a ~ t h o r ~ . ~ ~ ~ * ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ Summing up the results of catalytic experiments the following conclusions can be drawn:

(0

the parent activity of Au/MgO catalysts increases in the following order: base < Pd > Rh >> Au This can be correlated reasonably well to the heats of adsorption of oxygen on evaporated metal films, as determined by Brennan and co-workers; these were 275 kJ mol-' for platinum and palladium and 312 kJ mol-', for rhodium [22].

2: The Destruction Of Volatile Organic Compounds

115

The discrepancy between activity and heat of adsorption for platinum and palladium is not discussed. However, the authors assert that the differences in the activities of platinum and palladium are not due to a fundamental difference in mechanism, but rather are caused by a greater strength of adsorption of ethylene in the presence of oxygen on platinum than on palladium, as indicated by a zero order dependence of ethylene on platinum compared to a first order dependence of palladium. The highest catalytic activity is observed for metals which have weakly bound oxygen species. The low activity of gold cannot be explained by strongly bound oxygen species, but is probably due to the fact that the chemisorption of oxygen does not occur readily on gold, although gold/oxide interfaces do provide sites for oxidant adsorption. From the early work by Patterson and Kemball[20], it is clear that platinum tended to have the highest activity for complete oxidation of noble metals, and had a relatively low tendency to catalyse partial oxidation. For these reasons, platinum is the most commonly used and widely-studied of the noble metals. The destruction of chloroform has also been studied by Lou and Lee using a Pt/A1203 alloy catalyst [23]. The authors have concentrated on the fact that many catalysts do produce undesirable products such as CC14, C2Ch and CO and they have considered the nature of the adsorbed species by interpreting kinetic data. The catalyst was prepared using a wash coating method to produce a catalysts with a bulk density of 0.6-0.7 g ml-' with a BET surface area of 1.267 m'g-', an active metal surface area of 65-75% and porosity of 80.03%. Catalysts were tested at atmospheric pressure and temperatures ranging from 200 to 475"C, with space velocities of 22,500 h-' and 57,000 h-'. The study shows that at a constant temperature, reaction rate increases almost linearly with CHC13 content. The effect of an oxygen rich environment on the reaction was studied, with the conclusion that there is no noticeable difference for temperatures between 290 and 350°C, as the rate was zero order with respect to O2concentration. The main products of this reaction were C02, Cl2 and HCl, with trace amounts of CO and CC14produced across the temperature range. Between 275 and 400°C a linear relationship between the destruction of CHC13 and the formation of C02, C12and HCl was reported. Above this temperature, conversion of CHCl3 did not continue to increase at the same rate with temperature and neither did the production of HCl. In contrast, relative C12 concentration increased and this observation is attributed to the Deacon reaction. 2HCl

+

'/202-----*

H20

+ Cl2

The paper concludes that the Pt catalysts produced by the wash coat method are highly active for the oxidation of CHC13,and produces no significant amounts of products that contain C-Cl bonds. HCl and C12are the only chlorine containing compounds formed at temperatures above 450°C. It must be noted that all experiments were conducted using dry air. They have suggested that water vapour may have an effect and indeed the authors highlight that it requires further study. Rhodium and palladium may also be effective catalysts, as illustrated by

116

Catalysis

Nagata and co-workers [24]. In the oxidation of a mixture of CFC115 (CF3CF2Cl)and n-butane in air at 500°C and a gas hourly space velocity of 15,366 h-' rhodium and palladium catalysts supported on alumina showed activity for combustion comparable to that of platinum at the same temperature. 100% conversion of n-butane and 40% conversion of CFC115 was obtained over all three catalysts under these conditions. This observation is not unconditionally observed, since the activity of the noble metal may be adversely affected by certain reaction conditions. One cause of this is the differing susceptibilities of noble metals to deactivation by VOCs and the products of complete and incomplete oxidation. For example, palladium supported on alumina catalysts are unsuitable for the combustion of CFC113 (C2C13F3),as the noble metal component sacrificially reacts with product fluorine and chlorine species and is thus lost from the catalyst by volatilisation [25]. In contrast, platinum, rhodium and iridium show high activity for combustion under the same reaction conditions, although all are greatly affected by poisoning, causing deactivation. Platinum catalysts are not always appropriate for the combustion of volatile organics. At high temperatures platinum catalysts are susceptible to sintering, causing deactivation [26]. However, it has been stated that temperatures in excess of 300°C are essential in the combustion of chlorinated hydrocarbons in order to prevent strong interaction of HCl with the catalyst surface, leading to extensive deactivation of the catalyst [27]. This gives a limited range of temperatures in which the noble metal catalyst may be used, which is not ideal given the widely varying susceptibilities of different VOCs to catalytic combustion. Structural changes, similar to those seen for platinum, are observed for palladium supported catalysts at temperatures in excess of 450°C in the presence of oxygen [28]. In general, however, noble metals other than platinum and palladium have few commercial applications in catalytic combustion, due to their relative instability and lower activity. The use of rhodium as the component of automotive exhaust catalysts responsible for hydrocarbon oxidation is widely reported, though this has been extensively reviewed by Shelef and Graham [29] and will not be dealt with here. An interesting recent development in the use of gold supported catalysts in combustion has been reported by Haruta and co-workers [30]. The activity of gold for the low-temperature oxidation of carbon monoxide has previously been widely reported [31]. In this study, Haruta determined that a catalyst consisting of 1 wt% gold supported on a-Fe203is active for the combustion of methanol and its decomposition derivatives (HCOOH, HCHO) at temperatures below lOO"C, at a gas hourly space velocity of 2,OOOh-'. Comparison of T50, the temperature at which the catalyst combusts 50% of the initial amount of hydrocarbon, for Au/Fe203and the conventional combustion catalysts Pd/A1203 and Pt/A1203 gave the order of reactivity:

which is in contrast to the order determined by Patterson and Kemball [20], in which gold was found to have low activity. In all cases, the only carbon-contain-


117

ing product observed was COZ. In addition to the obvious advantages of a catalyst with comparable activity to platinum and palladium for low temperature oxidation, it was proposed that the Au/Fe203catalyst may be beneficial as its activity was not suppressed but enhanced by the presence of moisture. The reason for this is not clear, although FT-IR studies have indicated that the adsorption of C O on the catalyst surface is enhanced by the presence of moisture ~321. One class of possible alternative catalysts that show high activity and selectivity for combustion are bimetallic noble metal catalysts, which may have significant advantages over the single component catalyst. Activity is not always enhanced, as illustrated by a study into the combustion of chlorobenzene and xylene over platinum catalyst both in the pure form and with palladium or manganese added as active ingredients [33]. The catalysts considered were 0.15% Pt, 0.1% Pt,O.l% Pt/0.02% Pd,O.l% Pt/O.l% Mn, 0.05% Pt and0.05% Pt/0.02% Pd, all supported on Ba-modifed y-alumina. At a constant space velocity of 10,000 h-I, temperature was varied in the range 177-467"C, to determine the temperature required to give 50% and 90% conversion of a variety of concentrations of xylene and chlorobenzene, both separately and in a two-component mixture. Pure chlorobenzene could not be oxidised with 90% efficiency at any temperature over any catalyst other than 0.15% Pt/alumina , which gave 90% conversion at 440°C. This catalyst was also found to be more active than all others for combustion of chlorobenzene-xylene mixtures, achieving 90% conversion of a 1 mg dm-3 xylene and 2 mg dm-3 chlorobenzene feed at 300°C. Addition of palladium to the platinum catalyst was not seen to increase activity, with 90% conversion of the above mixture at 327°C for the 0.1% Pt/0.02% Pd catalyst. Addition of manganese decreased the temperature at which 90% conversion of the mixture was obtained to 290°C. This catalyst underwent significant deactivation as a result of poisoning by chlorinated compounds, with xylene conversion falling by 47% following exposure to the chlorinated compound. The catalyst could not be completely reactivated by heating in air at 347"C, and the deactivation observed is similar that seen for the pure platinum catalyst. Due to their low combustion activities, platinum-manganese and platinum-palladium were not found to be acceptable alternatives to the single component catalyst. Platinum-palladium bi-metallic catalysts, however, have shown high activity in the combustion of certain volatile organics. This was illustrated by Skoglundh et al. [34], who proposed the use of a platinum-palladium bi-metallic catalyst, with a Pd/Pt ratio of 4/1, supported on hydrothermally pre-treated alumina, for the oxidation of xylene. The hydrothermal support was prepared by treating alumina at 814°C for 2 hours in 100% steam; its merits and preparation are discussed later in this review. This catalyst had a lower light off temperature (235°C)for the oxidation of 220ppm xylene at a space velocity of 144,000h-' and a total noble metal concentration of 5 pg mol-', than the corresponding pure platinum catalyst. Conversely, a 1/4 ratio of platinum to palladium does not promote the combustion activity in the same manner. The behaviour of bi-metallic catalysts may differ markedly from those of the

118

Catalysis

pure noble metal catalyst. A bi-metallic Pt-Pd catalyst supported on hydrophobic fluorinated carbon studied by Sharma and co-workers [161 demonstrates this, The bi-metallic catalyst showed higher combustion activity in the range 200-4OO0C, at a space velocity of 3,000-15,000 h-1, for the combustion of methylene chloride, with a maximum 60% conversion observed at 400"C, significantly higher than for the pure metal supported on the same material. Cordonna et al. made similar observations for the oxidation of a number of hydrocarbons over bi-metallic platinum-palladium catalysts. Catalytic activity was found to increase markedly with increased platinum content, the most active catalyst had a Pt/Pd ratio of 4/1, in accordance with previous work by Skoglundh [34]. Gonzalez-Velasco et al. have further studied the use of platinum-palladium catalysts [35] and have addressed the problems associated with the destruction of hydrogen deficient chlorinated VOCs in an attempt to minimise the production of COC4 and C12. This has been approached by the addition of toluene, water and hexane to the reactant and feed. Many studies have previously added water to the feed, and although studies have investigated mixtures of the VOCs, not many have investigated the importance of hydrogen transfer from other reactants during VOC destruction. The catalysts employed were 0.5 wt% Pt and Pd on y-A1203.They were calcined in air at 550°C for 4 h before being reduced at the same temperature for a further 2 h. Characterisation of the catalysts showed loadings of 0.44 wt% Pt and 0.42 wt% Pd with dispersions from hydrogen hemisorption of 53% for Pt and 43% for Pd. Trichloroethylene was fed into the reactor after mixing with various concentrations of water (1000 ppm, 7500 ppm, 15000 ppm) and it was reported that over Pd/A1203,adding water had no effect on the reaction. Over Pt/A1203 the catalyst activity was enhanced by water up to 400"C, but was inhibited above this temperature. Increasing the concentration of the water caused this enhancement or inhibition to be more pronounced. The activity of both catalysts was effected by the addition of hexane of toluene. Over Pd/A1203 the light off temperature for the reaction was reduced from 400 to 325°C in the presence of both toluene and hexane. Over Pt/A1203the decrease in light-off temperature was from 425 to 325°C for toluene and hexane. The authors suggest that this enhancement in activity is due to the exothermic oxidation of the added hydrocarbons increasing the surface temperature of the catalyst. Without added water the amount of C2Cl4 formed reached a maximmum at 450°C for both Pt/A1203 and Pd/A1203, with approximately twice the yield formed over Pd/A1203compared to Pt/A1203.With the addition of water (1000 ppm) C2C14 was still produced but the concentration was reduced considerably, and by the addition of 1500 ppm water it was reduced further. As expected similar improvements in HCl selectivity over C12were observed when water was added. The addition of water also decreased CO selectivity, over Pd/A1203 the amount of CO decreased by a factor of 4 between 350 and 400°C. With higher H20 concentrations (7500,15000 ppm) no CO was observed at all. The authors suggest that the formation of OH- species on the surface of the metal decreases


119

selectivity towards CO by promoting the water gas shift reaction and the observed reactivity is consistent with previous work [36]. The addition of hexane and toluene also had a beneficial effect. Over Pd/A1203 the amount of C2Ch formed was 40 times lower in the presence of hexane whilst it was 20 times lower with toluene added. C2C14formation was also inhibited and it was apparent that the production of C12was suppressed. It was also noted that the chlorine balance was often low, suggesting that chlorine was retained by the catalyst and this eventually could lead to deactivation. These results indicate that in the bi-metallic catalyst the active surface is not a simple mechanical mixture of platinum and palladium, and a new structure responsible for the high activity is formed on the surface, which has not as yet been satisfactorily determined, although alloy effects are thought to play an important role. Platinum catalysts promoted with base metals have been studied by Bo-Hyuk Jang et aE. [37]. The catalysts contained vanadium, chromium, manganese, cobalt, copper and barium and were prepared by impregnation with the various promoters added to a slurry of Pt/A1203. Three groups of catalyst were prepared containing different base metal loadings of 2, 6 and 18 wt%. All catalysts were dried for 20 min at 120"C, and then calcined at 500°C for 1 h. Catalysts were tested in a fixed bed microreactor with a gas stream consisting of 10% C02,4% O2and 1000 ppm of chlorinated hydrocarbon with a gas hourly space velocity of 20,000 h-'. The system temperature was then raised from ambient to 450°C at a ramp rate of 5" min-'. The chromium-promoted catalyst was the most effective for destroying the chlorinated hydrocarbons, with 85% conversion at 450°C. It was evident that the chromium catalysts were most active, with vanadium the next active showing a maximum conversion of 53%. The other catalysts did not achieve 50% conversion. Further studies with the most active chromium and vanadium promoters showed that in general the activity increased as the loading of chromium and vanadium increased. Accelerated ageing of the catalysts on line was performed by heating to 800°C for 120 h. This treatment caused the catalyst activities to drastically decrease. The best catalyst was still the chromium-promoted system but conversion decreased to 40% at 450°C. The performance of the vanadium catalyst after ageing was poor and the authors attribute this to the melting point of vanadium (690"C), which caused loss of the promoter from the catalyst. It is also evident that the BET surface area of the vanadium aged catalyst decreased by 84% from the initial value. However, these studies indicate that the incorporation of other metals with platinum may have a beneficial effect for complete oxidation activity. A number of catalysts consisting of mixed noble metal - metal oxide have been proposed for combustion processes and these are discussed later in this review. Most noble metal catalysts are supported, the most common support material being y-alumina. This role of the support is to increase the dispersion and thus enhance activity, economically this also results in a decrease of the quantity of the costly noble metal component required for the catalyst. The dispersion of the noble metal on the catalyst support is therefore of prime importance. In a highly

120

Catalysis

dispersed catalyst, the interactions between the noble metal and the support can have considerable effects on catalytic behaviour. A poorly dispersed catalyst may have less interaction with the support, resulting in properties similar to that of the bulk metal. As metal concentration is increased particle size may increase, so support interactions will decrease. Variations in particle size are also associated with changes in the density of active sites, with associated effects on the catalysis. The concentration of the noble metal can affect both catalytic activity and resistance to deactivation, hence the amount of noble metal on a supported catalyst is of prime importance. The former is illustrated by Skoglundh [34], who noted that the overall noble metal content of a Pt-Pd catalyst (ratio 4/1) is of vital importance in determining the light off temperature for xylene oxidation, as the light off temperature markedly decreased as noble metal content increased in the range 5-20 pg mol-'. These results are shown in figure 4. Regarding susceptibility to deactivation, Mendyka [33] determined that a

I

Figure 4

I

I

I

I

Efect of noble metal content on T,, for the combustion of 220 ppm xylene by Pt-Pd bimetallic catalyst supported on a hydrothermally treated washcoats at GHSV= 144,000 h-' [34]: v 5 pmol per gram, o 1 Opmol per gram, h 20 pmol per gram


121

catalyst with high noble metal content (0.6-1.5% Pt) is relatively poison-resistant in the oxidation of xylene and chlororbenzene, under the conditions previously described. However, the use of additional noble metal will significantly increase the cost of the catalyst, a highly undesirable situation in commercial operations, and will not completely prevent poisoning. As such a completely different type of catalyst with higher poison resistance might be preferable for effluents in which the levels of chlorinated compounds and other poisons are such that deactivation of a noble metal catalyst is likely. The effects of metal concentration on activity are discussed in greater depth by Spivey and Butt [l5]. Their review deals extensively with catalyst deactivation, hence the causes of deactivation are not considered in this review. However, the effects of water and chlorinated compounds in total oxidation have led to the development of a variety of novel combustion catalysts, which will be discussed briefly. The presence of moisture in the feed gas is a common cause of catalyst deactivation. Water vapour may be present as a contaminant in the effluent stream, and is also produced by combustion. This poses a particular problem for alumina supported catalysts. Adsorption of water vapour on the surface of the catalyst is unavoidable at the low temperatures necessary for catalytic oxidation to be economically viable. It has been suggested that the use of a catalyst which is hydrophobic in nature and will thus prevent adsorption of water on its surface will have major advantages over the conventional hydrophilic catalysts. In addition, when the hydrophobicity of a catalyst to water increases, hydrocarbons are often more readily adsorbed on the catalyst surface where they can subsequently react. This will result in an increase in the number of active reduced sites on the catalyst surface, which have previously been shown to be essential in the mechanism of combustion reactions over noble metal catalysts. Hydrophobic systems have been extensively studied by Chuang and co-workers [38], and have comparable activity to conventional hydrophilic catalysts, such as Pt/alumina, at significantly reduced temperatures. The high catalytic activity appears to be due to the hydrophobicity of the catalyst to water and to the high ratio of the surface reduction rate constant (k,) over surface re-oxidation rate constant (k3), obtained from the Mars-van Krevelan mechanism previously detailed. The hydrophobic catalyst employed consisted of 6 mm ceramic Raschig rings coated with a mixture of hydrophobic fluorinated carbon (containing 60% fluorine) and Teflon, impregnated to a platinum loading of 0.2 wt.%. The contact angle of the catalyst with water was 109",indicating its hydrophobic surface properties. The catalyst was tested for the combustion of a feed stream consisting of 250 vppm of benzene, toluene or xylene, or a bi- or tri-component mixture of these, diluted in air to 10-60 vppm at a gas hourly space velocity of 3,200 h-' in the temperature range 90-150"'. The experimental procedure consisted of measuring the reactor inlet and outlet concentrations at a series of different feed compositions and constant temperatures. The oxidation of benzene, toluene and xylene, as the sole component of the effluent, was complete in the temperature range 9O-15O0C,with no partial oxidation products or CO detected. At 130°C and inlet concentration of 45 vppm over 90% of the VOC stream was converted. This was significantly higher activity than that observed for a conventional hydrophilic catalyst (250-

122

Catalysis

300°C required for comparable conversion). It can be seen that there is a significant decrease in the temperature required for combustion to occur with the hydrophobic catalysts. A detailed kinetic study of the combustion process indicated that the surface reduction rate constant, kl, in the Mars-van Krevelan mechanism increases with temperature faster than the surface re-oxidation rate constant, k3. That is, the surface concentration of adsorbed oxygen (02) is higher than that of (C,H,-0) at higher temperatures. This occurs in spite of the fact that kl is more sensitive to variations in temperature than k3.The ratio kl/k3 for a Pt/hydrophobic catalyst in the combustion of benzene, toluene and xylene at 90-130°C was calculated to be in the range 466-3880, which can be compared to 24-2490 for a conventional hydrophilic catalyst (Pt-Ni/A1203)in the oxidation of benzene and n-hexane at 140-221°C C341The higher values of this ratio for the hydrophobic catalyst suggested that the hydrophobicity of the catalyst may accelerate the desorption of water from the catalyst surface and thus accelerate the forward reaction (step 4 in the Mars-van Krevelan mechanism). This may result in a reduced catalyst surface, which has previously been determined to favour high activity. A similar situation exists in the oxidation of CO on pure platinum metal [18], such that the surface concentration of CO decreases as the reaction temperature increases. Because the carbon atom of CO is known to adsorb to the platinum surface, a similar adsorption mechanism may exist for structurally similar hydrocarbons. This postulate is consistent with the mechanism employed. The addition of water to the feed gas does not affect the conversion of benzene, toluene and xylene on the hydrophobic catalyst, as there was no significant loss in conversion as the proportion of water vapour increased. A further advantage of the hydrophobic catalyst is that the combustion of VOC mixtures containing two and three components did not depress conversion when compared to the single components. This was in clear contrast to results previously described, in which interactions between the components of mixtures often resulted in the inhibition of oxidation of the mixture’s weakly adsorbed components, with the result that increased temperature was required to achieve high levels of destruction compared to the components alone. A further benefit of using a hydrophobic catalyst is that there is lower sensitivity of the reaction rate to the oxygen to hydrocarbon ratio. Chuang demonstrated this by comparing the combustion of formaldehyde over Pt/hydrophobic support to that of conventional hydrophilic Pt/alumina [39]. It is generally accepted [17] that the activation energy for oxygen desorption is much lower than that for combustion, and this was confirmed by detailed kinetic studies of the hydrophobic catalyst. The rate limiting step for both catalysts was the reaction between formaldehyde and surface oxygen and maintaining a high oxygen to formaldehyde ratio was of vital importance in achieving high C 0 2 selectivity. For the Pt/alumina hydrophilic catalyst, C 0 2 selectivity falls as the ratio decreased, particularly at low temperature. However, for the hydrophobic catalyst selectivity only decreased if the ratio was below 500, a situation which is rarely encountered in practical applications. Because the selectivity to C 0 2 was


123

low and the dependence on the ratio was high for the hydrophilic catalyst, the hydrophobic catalyst might therefore offer significant advantages. It was observed that whilst conversion of formaldehyde took place at 63"C, activity was low and selectivity to C 0 2 negligible, with partial oxidation to HCOOH occurring at a significant rate. Selectivity to C02increases with increasing temperature, reading 100% at 125°C and 100% conversion was observed at temperatures in excess of 150°C. This suggests two reaction pathways between formaldehyde and adsorbed oxygen, one producing complete oxidation products at high temperatures, the other forming the partial oxidation product HCOOH at lower temperatures. The authors suggest that this indicates that at least two types of active sites are involved in the reaction. With increased temperature selectivity to C02 increased, indicating that sites active for complete oxidation become more active. Detailed kinetic studies showed that the rate of consecutive oxidation of HCOOH becomes increasingly greater than the rate of desorption of surface HCOOH. As a result, less of the unwanted partial oxidation product HCOOH was produced and selectivity to C02 increased. The presence of water vapour in the feed gas may have a beneficial effect on the combustion of chlorinated VOCs. For ease of removal from the effluent, HCl is the preferred chlorinated product of combustion. As HCl generally inhibits the rate of combustion, it might have been expected that any increase in its formation would result in a decrease in reaction rate. However, this does not occur, since the inhibition effects of C12are similar to those of HCl. It has been widely reported that water in the feed gas acts to enhance the production of HCl during the oxidation of a chlorinated VOC. This is sometimes attributed to the use of water as a source of hydrogen atoms [40]. However, other papers have proposed the existence of alternative reaction mechanisms in the presence of water, which give increased selectivity to the production of HCl and may also increase the conversion of the organic material. One such mechanism has been detailed by Rossin and co-workers [41], who state that the presence of water appears to play an important role in the overall reaction sequence, minimising the production of Clz in favour of HCl without significantly affecting the catalytic activity. During the oxidation of chloroform over 2% Pt/alumina, HCl was the major chlorinated reaction product. The oxidation of chloroform in humid air might be expected to proceed as follows: CHC4

+ 0 2 e C O 2 + HCl + C12

This reaction is not consistent with the observed results, as negligible C12 was produced, suggesting that the majority of C12was converted to HCl. As there is water in the effluent, it might be supposed that the Deacon reaction: 4HCl

+ + 2C12 + H20 0 2

is responsible for the production of HCl. Studies of the kinetics of the process indicate that this is not the case. Equilibrium concentrations of HCl and Cl2, calculated using the values determined in this study, indicate that the Deacon reaction, if it occurred, would be shifted towards the formation of C12,hence this cannot be responsible for the preference to HCl.

124

Catalysis

Bond and Sadeghi [42] previously suggested that the hydrogen required to make HCl may be produced by the combustion of the hydrocarbon. Lester further speculated that the formation of HCl can be favoured by adding hydrogen, in the form of water or organics, to the feed gas [43]. It was further proposed that the conversion of halogenated organics in humid air streams involves hydrolysis, and the presence of water is essential to limit the formation of Cl2. Thus, a reaction mechanism consistent with Rossin and Farris' products is:

+

+

CHC13 [ O l d [COC12] HCl C02 + 3HC1 [COCl2] + H20The first step involved an interaction between chloroform and adsorbed oxygen, whereby HCl is abstracted to yield a phosgene intermediate, which is consistent with the results obtained form a kinetic study of the reaction. Phosgene then undergoes rapid hydrolysis to yield C 0 2and HCl. A slight inhibition of the rate is observed in the presence of water. This is attributed to the rate limiting step for the reaction being the reaction between chloroform and oxygen to form phosgene. If the reaction between phosgene and water were the rate limiting step, the addition of water would be expected to increase reaction rate. This mechanism was found to be consistent with the products and kinetics of the reaction. Further evidence for the existence of a hydrolysis mechanism is provided by Papenmeier and Rossin [12] and by Agarwal and co-workers [40]. Agarwal detailed a hydrolysis reaction mechanism which is said to predominate in the presence of water vapour, providing both increased activity and increased selectivity to HCl compared to the oxidation mechanism operating in 'dry' conditions. This mechanism was consistent with the observed reaction products and with the kinetics of the oxidation. The initial observation was that, in the oxidation of cyanogen chloride in air over a 2.15% Pt/alumina catalyst, 78,000 ppm water vapour in the inlet gas acts to enhance conversion, giving 98% conversion at 375°C and a space velocity of 170,000 cm3h-'g-' compared to a maximum 20% conversion for the corresponding dry feed stream at 440°C and 46,000 cm3h-'g-'.. The conversion in the presence of water was mainly to HCl and C02, with decreased selectivity to partial oxidation products and C12.The dry feed stream produced negligible HC1 and C02, with CO the only carboncontaining product detected. It was proposed that the enhancement in conversion was due to the existence of an alternative hydrolysis pathway in the presence of water. Such a mechanism has been reported previously by Lester and Marinangeli [44] for the oxidation of cyanogen chloride over a platinum supported on titania catalyst. Agarwal [40] found that the presence of water vapour acted to reduce the apparent activation energy in their system, from 96 kJ mol-' at 390-440°C for the dry feed gas to 54 kJ mol-' at 185-215°C for the humid feed gas. Experimental conditions were such that diffusion resistance was negligible, and therefore, the lower activation energy for the humid feed gas suggests the presence of an alternative reaction mechanism. This was confirmed by further experiments in which the catalyst particle size was reduced to approximately half its original size, consequently no change in apparent activation energy was


125

detected. By varying the oxygen concentration of the feed stream in the range 0-21%, it was determined that cyanogen chloride conversion is independent of the concentration of oxygen, hence the rate of its destruction is zero order over this range. This suggests that hydrolysis was the major pathway for cyanogen chloride destruction. Two reaction mechanisms are proposed, one for the dry feed stream: 2CNCl

+

02-4

Cl2

+ N2 + 2/X C02 (where 1<X Pd/A1203> Pd/A1203-Si02 However, the reverse order was observed for platinum catalysts also for the oxidation of methane [51]. The acidic properties of the support are of prime importance, with solid superacids showing high activity at relatively low temperatures [49]. Ishikawa and co-workers have found that in the oxidation of

126

Catalysis

propane by supported platinum catalyst, the activity of the supported catalysts follows the order:

-

-

Pt/Si02 > Pt/A1203 Pt/Ti02 Pt/Ce02 > Pt/ Zr02 > Pt/La203 With the exception of silica, this correlates well to the order of acid strength of the catalyst: Ti02 > Si02 > Z r 0 2> La203 The type of support influences the metal dispersion also, which may account for the high activity of silica, and the very low activity of lanthana. As acidic properties appear to influence activity, the doping of the catalyst with SO:' has been proposed as a means of increasing the acidity of the catalyst, and thus increasing its activity [46,48]. Zirconia has been proposed by Hubbard et al. [47] to be a possible superior support material to alumina. For highly dispersed platinum, the oxidation of propane occurs at a rate as high as two orders of magnitude higher over Pt/zirconia than over Pt/alumina. At higher platinum concentrations, the influence of support is negligible and the rate constants for both catalysts are the same. The change in activity was proposed to be due to a difference in interaction between noble metal and support. The differences in activity between supports might be expected due to the great differences in structure for alumina (poorly crystalline, high BET surface area) and zirconia (highly crystalline, low surface area). Analysis of infrared CO adsorption data indicates that there is little interaction between platinum and zirconia [52]. It can therefore be proposed that highly dispersed platinum is active for propane oxidation provided that it is supported on an inert material, such as zirconia. It is also stated that interaction with alumina deactivates platinum, with only one third of sites active for propane oxidation compared to the zirconia supported catalyst [53]. Hence, a zirconia support may have significant advantages over an alumina support in combustion of noble metals. The effects of hydrothermal pre-treatment of an alumina support have been investigated by Skoglundh [34]. Alumina was treated either thermally at 500°C in air for 2 hours, or hydrothermally by treating at 814°C for 2 hours in 100% steam. The noble metal component of both catalysts consisted of Pt-Pd in the ratio 80:20. Relative to the thermally treated catalyst, the hydrothermally treated catalyst had a lower light-off temperature in the combustion of xylene than both the thermally treated catalyst and the pure, unsupported platinum particles. It has been determined that hydrothermal pre-treatment decreased surface area and shifted the pore size distribution of the catalyst to larger pores [54]. It is suggested that larger molecules diffuse more rapidly in a hydrothermally treated support and can thus reach catalytically active metal more rapidly, hence the increased activity. Hydrothermal pre-treatment may also alter the metal-support interaction. Measurements of platinum particle size on this support have indicated that the noble metal is present as larger crystallites than in thermally treated catalysts, hence there is less interaction with the support material. It is


127

clear that these thermal treatments affected both the structure of the support and the morphology of the platinum crystallites. Other possible pre-treatment have been considered, for example, the exposure of Pt/zirconia and Pt/alumina to hydrogen at 500°C increase the oxidation rate of both catalysts [53]. However, pre-treatment of Pt/zirconia in oxygen at 500°C has negligible effect on its activity [48]. This is said to be due to a lack of interaction between noble metal and support, preventing re-dispersion of Pt at low concentrations.

4.3 Gold as a VOC Destruction Catalyst. - Continued research into the use of noble metal catalysts for complete oxidation reactions is required to determine the composition of catalysts most active for the process and the mechanism by which these operate. In spite of considerable research into alternative supports, varied noble metal loadings, etc., the susceptibility to deactivation of these catalysts remains a problem, particularly in the oxidation of chlorinated compounds. For this reason, alternative classes of catalysts active for VOC combustion are required. Recently, there has been considerable interest in gold as a heterogeneous oxidation catalyst. Gold based catalysts on a variety of supports have been employed by Baoshu Chen et al. for the complete oxidation of dichloromethane [55]. The use of alumina has been shown to degrade at high temperatures with HCl. Supporting the metal on Ti02/V205or Cr203/A1203has lessened this tendency. It has also been shown [56] that a small amount of gold on a cobalt oxide can enhance chlorinated hydrocarbon destruction. The gold catalyst, Au/C0304 was prepared by co-precipitation, by ageing the catalyst at 60-70°C for an hour before it was washed with distilled water. After drying, the catalyst was prepared by calcination for 8 h at 350°C. Water was co-fed into the reaction feed stream to suppress the Deacon reaction that forms chlorine gas. The effect of gold loading on activity was investigated, and the results compared with conventional noble metal catalysts. Reaction conditions were 500 ppm dichloromethane and 0.5 wt% water in air, giving a gas hourly space velocity of 15,000 h-'. Catalysts with a gold loading of 0.2,1,5 and 10Wt% were prepared and all demonstrated an increase in activity compared to the Co304 support alone. For example the temperature for 50% conversion was 3 10°C without gold but decreased to 210°C when it was present. The reaction rate was also increased by 25 times at 300°C when in gold was added to the support. It was also reported that the activity was independent of gold loading. In some cases CHC13 and CCl, were produced as by-products at temperatures below 250"C, however, above this temperature only HCl and C02 were produced. Comparison with other noble metal catalysts showed that the gold catalysts were active at lower temperatures. A conversion of 50% dichloromethane was achieved at 210°C for Au/Co304 compared to Cr203/A1203(260"C), 0.5% Pt/A1203 (345°C) and 0.5% Pd/A1203 (395°C). These observations were explained in part by poisoning of the noble metal catalysts by chlorine at low temperatures. Whilst it has been reported that that over a test run of 130 h the 5% Au/Co304catalyst produced a consistent conversion of 95%. Conversely the

128

Catalysis

0.5% Pt/A1203catalyst was stable for around 70 h with a lower conversion of 40% whilst the 0.5% Pd/A1203catalyst showed a decrease in conversion from 25% conversion to zero after 25 h. The catalytic combustion of n-hexane, benzene and 2-propanol was investigated using Au/Ce02/A1203and Au/A1203by Centeno et aZ. [57]. The catalysts were prepared by the method of deposition-precipitation. They showed that ceria enhances the fixation and final dispersion of the Au nanoparticles and stabilises them at lower crystallite sizes. The addition of ceria therefore improves the activity of the gold particles for the oxidation of VOCs. They propose that the enhanced activity may be caused by an increase in the mobility of lattice oxygen and controlling and maintaining the required oxidation state of the active Au nanoparticles. Minico et.aZ. [58] have studied the oxidation of alcohols, acetone and toluene using Au/Fe203prepared by coprecipitation in the presence of excess oxygen. The high activity of the catalysts was ascribed to the increase in the mobility of lattice oxygen.

4.4 Metal Oxide Catalysts. - The use of metal oxide catalysts for oxidation reactions has been well documented, and general reviews concerning catalytic activity [171 and mechanistic principles [59] have been published. Metal oxide based catalysts have been specifically applied to the combustion of VOCs [l5] although their use is not as widespread as catalysts based on noble metal systems have been most extensively studied. It is generally accepted that oxide catalysts show greater resistance to poisons when compared to noble metal catalysts, they also have the advantage that the catalyst tends to be less expensive. However, metal oxides frequently show lower catalytic activities which may require the use of lower space velocities and higher temperatures to give comparable performance. A class of oxide catalysts which have been employed for combustion reactions, particularly hydrocarbon combustion are oxides with the perovskite structure, possessing the general formula AB03[60]. The activities of several unsubstituted component B oxides (B03)have been compared with perovskite oxides for the catalytic oxidation of propylene [61], this is shown in figure 5. Catalytic activity is expressed in terms of the temperature at which the combustion of propylene occurred at a given rate. Catalysts below the straight line showed enhanced activity over the component oxide due to the formation of a perovskite structure. Conversely for catalysts above the line activity was reduced. Although there is a degree of scatter most catalyst are generally distributed close to the line indicating that the activity of the unsubstituted perovskite oxides are primarily determined by the nature of the B component oxide. The most active catalysts being based on the oxides of Co and Mn. The catalytic combustion of methane over perovskite type catalysts has been investigated by Arai et al. [62]. Methane is one of the most stable alkanes and is relatively difficult to combust by virtue of the high strength of the C-H bond which must be activated. Studies were performed using relatively high space velocities in the range 45,000-50,000 h-' with a 2% methane feed in air. The catalytic activity, expressed as the temperature required for 50% conversion, is

2: The Destruction Of Volatile Organic Compounds SrCe0,-CeO 650

-

o2

129 SrTi0,-Ti0

o2

CaTi0,-TiO, 0

Figure 5

Efect of perovskite substitution on the activity for propylene oxidation 1611; T is the temperature at which propylene oxidation rate = 1 O-' mol m-2 s-I

shown in table 2 for a series of unsubstituted perovskite type oxides. Carbon dioxide was the sole reaction product over all the catalysts tested. Comparison of the activity was made with a Pt/alumina catalysts, and although the perovskite oxides were less active, this was only marginal in the cases of LaCo03, LaMn03 and LaFe03which showed much lower surface areas. The activity of the lanthanum perovskite oxides were enhanced by substitution of Sr2+,this was particularly prevalent for the lanthanum manganate variable, La&Sro.4Mn03,which showed a T50%of 482"C, 36°C lower than Pt/alumina. The substitution of Sr2+ for La3+ leads to the formation of positive holes and/or oxygen vacancies, and it is the formation of these defective structures which are considered to impart the high catalytic activities. It has been reported that on cooling in oxygen and heating in vacuum these oxides absorb or liberate large amounts of oxygen, this phenomenon is clearly important in catalytic reactions and the behaviour of these oxides and the nature of the oxygen species have been previously reviewed [60]. A more detailed study of the combustion of methane over a series of Mg doped LaCr03 perovskites has been reported by Saracco et al. [63]. The catalysts prepared were LaCrl.xMgx03 , where 0 < X< 0.5, and were synthesised by the method denoted as the 'citrate method', which briefly consisted dissolving the constituent metal nitrates in citric acid solution. After heating the solution the catalyst precursor was obtained and subsequently calcined at 1100°Cto produce the final catalyst. Catalysts were screened for activity using 1.5% methane, 18%

130

Table 2

Catalysis

Activity of perovskite type oxides for methane combustion expressed as temperature required for 50% conversion [62]

Catalyst

G O / OC

Surface area /m2g-' ~

~~~

LaCo0,

3.0

535

LaMn0,

4.0

579

LaFe0,

571

LaCuO,

3.1 0.6

LaNi0,

4.8

702

LaCr0,

1.9

780

146.5

518 834

Pt/alumina thermal oxidation

-

672

oxygen with the balance helium. Substitution of Mg into the perovskite structure enhanced the combustion activity, as LaCr03 showed a TSo%= 692°C which was reduced to 553°C for LaCro.sMgo.s03. Catalyst activity was improved further by supporting the perovskites on MgO, although more importantly the support stabilised small perovskite crystallites reducing sintering. Kinetic analysis of the supported catalyst showed that the reaction was first order with respect to methane, and that dissociative adsorption of oxygen exerted a significant role on the reaction mechanism. The experimental observations were consistent with the operation of an Eley-Rideal type mechanism for methane combustion over these cat a1y st s. The combustion of other VOCs by perovskites, besides alkanes and alkenes, has also been investigated. Ling et al. [64] have studied LaNi03catalysts for the combustion of ethanol and acetaldehyde, comparing activity of that for methane combustion. Oxidation of 1 vol.% VOC in air (total flow-100 ml min-', 0.lg catalyst) followed the order for ease of combustion; ethanol z acetaldehyde >> methane The temperature required to attain 90% methane conversion was ca 600°C whilst equivalent conversion of ethanol and acetaldehyde were reached around 400°C The preparation of monolith perovskites by extrusion of plastic pastes, comprising perovskite powder, binder, acid peptizers and some surfactant, suitable for use in high temperature incineration processes have been reported by Isupova et al. [65]. Preliminary catalytic activity results showed that the monoliths were active for the combustion of butane, gasoline, methane and chloroform in air at GHSV = 12,000 h-', although the required reaction temperature was high, in the region of 1000°C.It was reported that catalysts were used continually for over one month at 900°C without loss of monolith integrity, mechanical strength and catalytic activity. The preparation of such active monoliths is an interesting concept and one which may be applied to other oxide catalysts,


131

especially for use in VOC destruction which requires high volumetric throughput with a low pressure drop. The destruction of halogenated VOCs, particularly those of short chain length, are of great industrial and environmental importance, and a considerably number of studies using oxide catalyst have investigated this area. The oxidation of 0.74 vol.% dichloroethylene by a wide range of oxides has been studied by Imamura at 3,600 h-' space velocity [66]. Catalytic activity was defined in terms of C02 yield and at 650°C the activity was ranked in the order: Cr203 > Mn203 > Co304 > CuO > La203> Ce02 > NiO > MgO-CaO > MgO > CaO > ZnO > Si02-Al203 > V205 > Si02-Ti02 Although transitions metal oxides demonstrated the highest activity they all showed high yields of C12 whilst acidic catalysts such as Ti02-Si02and Si02A1203 produced HCl almost exclusively. C12 is highly reactive and can readily react with carbon containing substrates forming harmful by-products. The product distribution over the metal oxides changed quickly with time as CO selectivity increased at the expense of C02, indicating that the combustion activity decreased with time on line. This behaviour was not observed over the acidic catalysts which were concluded to be more suitable for dichloroethylene destruction. A range of solid acid catalysts, including Ti02/Si02,zeolite Y, various mordenites and A1203/Si02,were tested for the destruction of 1% 1,2-dichloroethane in air at 3,600 h-' space velocity [67]. The most active catalyst was TiOz/SiOz which showed 100% conversion at 400°C, CO was a major product showing a CO/CO2 ratio of ca. 2.5. Zeolite Y also showed a high conversion of 98.5% at the same temperature, however, the carbon balance was poor (33.5%0),highlighting that the zeolite Y catalyst tended to have a short lifetime, being rapidly deactivated by deposition of carbonaceous organic residue in the zeolite pore structure and the catalyst surface. All of the zeolite-based catalysts were rapidly deactivated and had inferior activity to TiO2/SiO2, which showed stable activity, these other zeolites were also deactivated by carbon deposition. Imamura et al. [68] have also investigated the destruction of dichlorodifluoromethane (CC12F2)over a similar range of solid acid catalysts used previously [67] and single and mixed oxides. Again Ti02/Si02exhibited the best performance, initially showing 95.7% conversion at 5,900 h-' space velocity, other acidic catalysts such as mordenite and A1203/Si02showed higher conversions, however, these catalysts deactivated quickly. The TiOz/SiO2 catalyst also deactivated, conversion decreased markedly after 150 minutes reaching a steady state of ca. 10% after approximately 300 minutes use. This deactivation was due to the attack of corrosive fluorine on the silicon component of the catalyst, as during use silicon was removed from the catalysts and deposited at the reactor outlet. The addition of CaO prolonged catalyst lifetime by reacting with fluorine, although the catalyst still showed significant deactivation as conversion decreased to 50% after 600 minutes time on line. Comparison with transition metal oxides (Ti02,Cr203,Mn203,Co304and Fe203),ZrP207,Zr02/Mo03,Ti02/Zr02 and CaO showed that all, with the exception of Cr2O3 and CaO, gave appreci-

132

Catalysis

ably CC12F2conversion although activity was generally lower than Ti02/Si02 and deactivation was a very rapid process. These studies highlight problems associated with the corrosive nature fluorine from the destruction of fluorinated VOCs which often leads to catalyst deactivation of both metal oxides and noble metals [25]. This is illustrated by the work of Imamura and co-workers in comparing the destruction of 1,2 trichloroethylene and dichlorodifluoromethane over the same group of catalysts. The oxidation of CC12F2by a Ti02 (anatase) catalyst has also been investigated by Karmakar and Greene [69]. Catalytic activity was determined using 1500-2000 ppm CC12F2in air with 10,500 h-' space velocity in the presence and absence of 5000-6000 ppm co-fed water. The addition of water had no significant effect on initial catalytic activity, but dramatically increased the HCl/Cl ratio and increased the selectivity to C02, which was 100% at all reaction temperatures. Time on line studies at 300°C with co-fed water showed that less than 5% decrease in conversion was observed after 4 days continuous operation. Over the same time period the Ti02surface area was reduced from 170 m2g-' to 40 m2g-', 50% of the reduction took place in the first 1-1.5 hours, simultaneously the catalyst activity increased. This increase was due to the increase in catalyst acidity which was a consequence of surface fluorination. It is interesting to note the differences in Ti02 deactivation characteristics between the two studies investigating CCl2F2 decomposition [67,68]. The differences can be related to the concentration of the fluorinated VOC and fluorine containing products and residence time within the catalyst bed. The former catalysts [68], which deactivated quickly, used a CC12F2concentration 3-4 times greater and a space velocity approximately 0.5 that of the latter [69]. The influence of water in the feed also had an important effect as the extent of deactivation was significantly reduced by the presence of water. The decomposition of HCFC-22 (CHClF2) by a series of acidic single and dual component metal oxides has been studied by Li et al. [70]. Initial studies over single metal oxides showed the order of reactivity; Ti02> Z r 0 2> Cr203 > W03 Ti02 was the most active, in agreement with other catalyst systems for the decomposition of fluorinated VOCs [67,68], producing 97.3YOconversion at 400°C with 84.1YOselectivity towards COX. ZrOz was marginally less active but only showed 36.2% COX selectivity at the same temperature. The range of dual component oxides tested included Zr02/Ti02, Cr203/Ti02, Cr203/Zr02, Co304/Zr02,V205/Co304,Zr02/V205, Zr02/MnO2, V205/Mn02,Co304/Mn02 and Zr02/W03.The highest activity was shown by Zr02/Ti02,Cr203/Ti02and Cr203/Zr02producing at least 85% CHClF2 conversion at 350°C and >99% at 400°C. The addition of water vapour increased the conversion and selectivity to COX whilst decreasing the selectivity to CHF3. It was proposed that water promoted the removal of fluoride ions from the catalyst surface which was then more active for the decomposition reaction. Time on line studies of Cr203/Zr02 in the presence of water indicated that conversion decreased marginally over 50 hours operation, whilst initially C 0 2selectivity decreased but levelled at ca. 70%


133

after 20 hours. In the absence of water, CHClF2 conversion also decreased slightly with time, however, a dramatic increase in C 0 2selectivity accompanied by an increase in CHF3 selectivity was observed, such that after 50 hours operation CHF3was the predominant product with ca, 60% selectivity. Powder X-ray diffraction studies of the Cr203/Zr02fresh catalyst identified a Cr203 phase, whilst after use in the absence of water the diffraction intensity of Cr2O3 was reduced and intense peaks from ZrF4 were identified. For the catalyst tested in the presence of co-fed water Cr2O3 was unaffected and only relatively small ZrF4diffraction peaks were present showing that water in the feed suppressed the transformation of the oxide to a fluoride phase maintaining catalyst activity. Nagata et al. [24] have studied the decomposition of a range of CFCs including CFC-113 (CF2C1CFCl2), CFC-114 (CF2ClCF2Cl) and CFC-115 (CF3CF2Cl)in the presence of hydrocarbons, which were also oxidised, by a series of acidic metal oxides and supported metal oxides. The relative order for CFCs destruction was; CFC-113 > CFC-114 > CFC-115 and was related to the number of chlorine atoms in the substrate as destruction was successively more difficult as the number decreased. The most active catalyst was y - A l 2 0 3 showing higher conversions than zeolites Y, L, mordenite, ferrierite and ZSM-5 and the mixed oxide Si02/A1203.100% conversion of CFC-113 and CFC-114 over y-A1203 in the presence of n-butane were reported at 450°C and 500°C respectively, whilst CFC-115 conversion was 45.1% at 600°C. The only carbon products were CO and C02. CFC-115 conversion over y-A1203increased as the partial pressure of ethane, propane and n-butane increased but no increase was observed when the partial pressure of methane was altered. Supporting chromium, cobalt, zinc, molybdenum, cerium, vanadium and tungsten oxides on y-Al203 enhanced the activity for CFC-115 destruction, vanadium and tungsten were the most effective but only increased conversion to 59.0% and 60.0% respectively at 600°C. The y-A1203 catalysts did show some deactivation during the first two hours on stream ,however, after 4 hours they were reported to show stable activity which is promising but a considerably quantity of CFC-115 was not destroyed and activity must therefore be improved further. The catalytic oxidation of dichloromethane was investigated by Van den Brink et al. [71], using y-A1203, which is commonly employed as a support of noble metals for catalytic oxidation. Studies used a combination of flow and infrared spectroscopy experiments over a range of reaction temperatures. This paper is interesting as it provides a comparison with many of the studies using alumina supported catalysts, and it demonstrates that alumina is not a passive component in many chlorinated the VOC oxidation reactions. Studies were performed using Teflon coated tubing to prevent reactions between the substrate and vessel walls. 0.3 g of catalyst was used with a gas flow of 50 ml min-' with a composition of 1000 ppm CH2C12,89% He, 10% 0 2 and 1% H20. from a baseline temperature of 200°C a heating rate of 5" min-' was applied up to the desired reaction temperature and the system was allowed to stabilise for 20 min before effluent gases were analysed.

134

Catalysis

Initially dichloromethane was introduced to the reactor at 300°C and after 10 min on line no dichlormethane was eluted from the catalyst bed and CO was the only reaction product detected. CO remained the only product for a further 25 min on line. After this time the CO MS signal was reduced and CH3Cl was also detected in the reactor effluent along with unreacted CH2C12. The conversion of CH2C12was 92%. HCl was only detected after several hours on stream once the y-A1203surface was saturated with chorine. Above 350°C C02 was produced, but CO still remained the dominant product. The carbon balance for these experiments decreased from 95% at 325°C to 60% at 500°C over a period of 30 h. A Darkening of the catalyst was also observed and was attributed to the deposition of carbonaceous material on the surface. Water was also important in the reaction, with 1YOin the feed the temperature required for 50% conversion of CH2C12was increased from 270 to 320°C when compared with no co-fed water. Through FTIR studies it was suggested that there is a strong interaction between CH2C12 and adsorbed O H groups on the catalyst surface, and dichloromethane formed hydrogen bonds with the OH groups. The authors identified two types of species present on the surface when CH2C12 reacted with the y-A1203 at 250°C. They propose a reaction scheme (figure 6) in which the first step was the displacement of a chlorine atom from dichloromethane at the alumina surface to form a chloromethoxy species (1). Further interaction resulted in the loss of another chlorine to form species (2) which is in equilibrium with the chemisorbed formaldehyde analogue (3).

c1

I

A1

/ \

Figure 6

c1

/A99.5% conversions respectively. Time-on-line studies for the destruction of 0.12% chlorobenzene at 450°C showed that the catalyst was not deactivated as 99.9 YO conversion was maintained during 400 hours continuous operation. These catalysts were also active for the oxidative abatement of other VOCs and it has been demonstrated that toluene, butylacetate and cyclohexanone can also be destroyed at relatively low temperatures. Considering the high space velocities employed in these studies, uranium based catalysts are amongst some of the most active oxide catalysts investigated for VOC destruction. The combustion of acetaldehyde and trimethylamine, which are common VOC odour pollutants, have been investigated over a series of mainly metal oxide based catalysts [78]. Catalysts were supported on y-A1203 and were all prepared by impregnation of the nitrate solution with the exception of the Pt system which was prepared using H2PtCls.Catalyst activity was screened using a GHSV = 30,000 h-' with 50 ppm VOC the balance being air. The order of activity for acetaldehyde combustion was:

-

Ag > Mn203 CuO > PdO > Pt > Fe2O3 > NiO > Co304 The activity of Ag was significantly better than all other catalysts showing 90% conversion at 200"C, whilst the best oxides were Mn2O3 and CuO both showing 87% conversion at 300°C. Trimethylamine was oxidised at slightly lower temperatures and followed the order of activity:

-

Ag = Mn2O3 = Pt > PdO CuO > Fe2O3 > Co304> NiO Based on these results a series of Ag/Mn203/y-A1203catalysts with varying Ag content were prepared by co-impregnation to a total loading of 2 wt.%. The addition of Ag improved the activity of both supported and bulk Mn2O3, although after 2 hours calcination Ag/y-A1203 was still more active than Ag/Mn203/y-A1203.However, when the catalysts were calcined for 22 hours the activity was approximately equal and after 42 hours calcination Ag/Mn203/yA1203was more active. Surface area determination showed that these effects were not dependent on the catalyst surface area. The combustion activity of Ag/Mn203/y-A1203 was not significantly effected by the presence of water vapour

138

Catalysis

in the feed. Oxygen TPD studies indicated that surface oxygen species were ca. 2.9 greater on the Ag/Mn203based system compared to those on Mn2O3, and it was these surface oxygen species which were important for the enhancement of combustion activity. The incorporation of metals with oxide based catalysts have also been investigated, Vassileva and co-workers have studies the addition of Pd [lo] and Ag [79] to 30 wt. % V2O5 supported on y-A1203. Both catalysts were tested for the destruction of benzene at 300 h-' space velocity, the addition of Pd and Ag both promoted the catalyst activity. The promotional effect was due to the activation of oxygen by the metal component and modification of the vanadium redox properties. Results from ESR spectroscopy and X-ray diffraction studies were consistent with the proposal that V4+ species within the V2O5 lattice were responsible for the catalyst activity. The most active catalysts were those with high metal dispersion, particularly in the case of Ag, as the activity was reduced when Ag was chemically bound to vanadium oxide and metallic Ag phases were present. Additional studies of the Pd/V205/y-A1203 [SO] system have identified the phases during use and it has been proposed that the efficient delocalisation of electrons in V4+ion clusters in these phases facilitated the redox cycle important for oxidation. The addition of metals to oxide catalysts, which are already active for combustion processes, is an interesting approach which attempts to combine the beneficial aspects of both types of catalyst system. The incorporation of the metal component generally increases the activity, however, the destruction of halogenated VOCs have not been investigated and problems associated with deactivation may take place. The oxidative destruction of methylene chloride has been studies by Jiang et al. [S 13 over sulfated oxide catalysts, consisting Ti02/S04, ZrOz/S04 and Ce02/S04,prepared by sulfuric acid impregnation of the oxides. Experimental conditions for catalyst testing used a reactant stream of 959 ppm methylene chloride in air at GHSV = 2,210 h-'. Ti02/S04was the most active catalysts showing complete CH2C12conversion at 275"C, at equivalent temperature ZrOz/S04and Ce02/S04showed conversions of 90.6% and 82.9% respectively. HCl was the sole chlorine containing product, although CO was a major reaction product showing selectivities in the range 86-89%, the balance was C02. Determination of the oxygen adsorption capacity and acidity, by desorption of NH3, showed a direct relationship with catalyst activity suggesting that both factors may be important for catalytic activity. The addition of 2 vol.% water to the reaction feed suppressed catalyst activity markedly from 100% conversion to 50.1YOfor Ti02/S04at 275°C. In an attempt to improve the low C 0 2selectivity shown by the sulfated materials a bifunctional catalysts consisting Ti02/S04 with 5% CuO was developed. The addition of CuO did produce a beneficial effect, increasing the C02 selectivity from 12% to 60% at 275°C. The authors consider that one possible mechanism for the increase in C 0 2 selectivity was sequential oxidation of CO by the CuO component which is a similar conclusion shared by the present authors over uranium oxide catalysts for the oxidation of butane and benzene [76].


139

The destruction of benzene by Cu/Cr and Co/Cr mixes oxide systems supported on y-A1203 and y-A1203/Si02have also studies by Vass and Georgescu [82]. The catalyst precursors were prepared by co-precipitation of 1:1 molar ratios of the metal nitrates in aqueous solutions containing tartaric acid. The supported catalyst was prepared by two methods, the first involved solubilisation of the precursor followed by impregnation of the support with the precursor solution. The second involved formation of the precursor on the support by successiveimpregnation with tartaric acid and then metal nitrate solutions. After drying the precursors were calcined at 700°C for 6 hours prior to use. The catalysts were tested for the combustion of 5% benzene at 4,000 h-' space velocity and 1% benzene at 10,000 h-'. Under both conditions the catalysts prepared by successive impregnations were more active, and irrespective of the preparation method the y-Al203 catalyst was more active than that supported on y-A1203/Si02.The Cu/Cr system was more active than Co/Cr, the best performance was shown by the Cu/Cr/y-A1203catalysts which showed 100% conversion of 1% benzene at 320°C and 10,000h-'. The authors report that in the region of 70-80% conversion traces of maleic anhydride were detected and concluded that it was sequentially combusted as at higher conversions C 0 2was the sole reaction product. Kang and Wan [83] have also studied the combustion activity of y-A1203 supported chromium and cobalt oxide catalysts, with particular reference to the effects of acid and base additives. These catalysts were found to be active for the combustion of ethane and activity was increased by the preparation of a mixed chromium/cobalt supported oxide phase. This enhancement was attributed to the release of lattice oxygen from the binary oxide being a more facile process than the singular oxides. The addition of a base additive, potassium, to Cr/Co/yA1203 reduced the ethane conversion whilst increasing the C 0 2 selectivity to 100%. Conversely the addition of Si02,an acidic component, enhanced ethane conversion and reduced C 0 2selectivity. It was therefore considered that initial ethane activation was via C-H bond breaking on Br-nsted acid sites of the Si02/A1203.Whilst the base additive enhanced the adsorption of oxygen which is involved in the formation of COz, the reverse effect on C02 production is observed for the acid additive. Agarwal et al. [SS] have investigated the long term activity of a commercial chromia-alumina catalyst (ARI. Technologies Ltd.) for the destruction of a mixture of C1-C2chlorinated VOCs and a mixture of Cs-C9hydrocarbons with 50 ppm trichloroethylene. Catalyst deactivation was studied at constant conversion, increasing the temperature when necessary to compensate for loss of intrinsic catalyst activity. Over 153 days in a fixed bed reactor at 23,970 h-' space velocity no increase in temperature was required for the oxidation of the chlorinated feed stream although CO selectivity gradually increased indicating that deactivating to place to some extent. Activity in a fluidised bed showed no deactivation under the same conditions, it was concluded that physical attrition and loss of chromium, via the oxychloride, were beneficial by continually exposing fresh chromium catalysts. Although beneficial to catalyst activity the loss of chromium by volatile product formation has further environmental conse-

Catalysis

140

quences and release to the environment must be avoided. Destruction of the mixed chlorinated/hydrocarbon stream indicated that benzene and trichloroethylene were the most difficult to destroy and the temperature had to be increased from 385°C to 418°C over 210 days to maintain >99YOconversion. This decrease in the activity of the catalyst was due to a decrease in the catalyst surface area which decreased by approximately 20% for the mixed stream whilst remaining unchanged for the solely chlorinated stream. Manganese oxide catalysts promoted with transition metal oxides, in particular CuO, have long been established as effective oxidation catalysts for carbon monoxide [85]. The same type of catalyst has also been applied to the oxidation of VOCs. A detailed kinetic study of the combustion of acetone over a supported hopcalite (CuMn204)catalyst has been reported by Linz and Wittstock [86]. Studies monitored the reaction products along the length of a fixed catalyst bed showed that partially oxidised products, particularly acetaldehyde, were significant products within the bed, although at the bed exit only ca. 2% of the carbon products were partially oxidated the balance was COZ. The same type of behaviour was exhibited during the oxidation of isopropyl alcohol and butyl acetate and the oxidation process of the VOCs can be described by a simple parallelconsecutive reaction network (figure 8). A series of rate equations for the reaction network were expressed and rate constant calculated from curve fitting the experimental data. The derived rate constants obeyed Arrhenius behaviour and by using the kinetic data the reactor can be designed specifically so that the outlet concentration of the pollutant VOC and partially oxidised by-products can be reduced below legislative limits. Klissurski et al. [87] have examined the combustion of acetone, toluene and styrene by zinc-cobalt spinel oxides supported on alumina. Catalysts were prepared by co-precipitation with sodium carbonate from a mixed zinc/cobalt nitrate solution at pH 9. The supported catalyst was prepared by deposition of the precursor on y-A1203 from a suspension in dimethylformamide and water. The supported precursor was dried at 150°C and calcined at 300°C to produce the catalyst. The bulk and supported catalysts both showed the formation of zinc cobaltite spinel structures which were thermally stable. Microreactor studies at 15,000 h-' space velocity showed that the components of a mix of acetone, toluene and styrene were destroyed at 225"C, 280°C and 350°C respectively. The VOC concentrations were not specifically expressed but it is assumed that they

acetaldehyde acetone

'14

partially oxidied products (PO) Figure 8

Simple parallel-consecutive reaction network describing the oxidation of VOCs by a supported hopcalite catalyst [86]


141

are within the range 0.5-3.0 gm-3. The catalyst showed no deactivation or partial oxidation products and remained active when the space velocity was varied in the range 7,500-30,000 h-'. Activity determination in a semi-large scale pilot plant installation at 10,OOO h-' showed similar trends, showing 99.9% acetone conversion at 180°C and conversions of 99.8% at 280°C for toluene and 320°C for styrene. A mix of VOCs containing acetone, ethyl acetate, benzene, toluene and styrene were also efficientlycombusted at 320°C in the pilot plant tests and it is evident from these studies that oxide catalysts based on cobalt spinels demonstrate high combustion activity. An interesting development in the destruction of VOCs, in particular chlorinated compounds, has been the development of catalysts based on copper and chlorine [88,89]. Catalysts were prepared by incipient wetness impregnation of a silica support by metal chloride and KCl solutions. A wide range of metal chlorides, including CuCl, ZnC12, FeC12, MnC12, CoC12, SnC12, NiC12, SbC13, MoClS, and CdC12 were prepared and tested for the destruction of methylene chloride (10.000 ppm) at 300 h-' space velocity. CuC1/KCl/SiO2 was the most active of the chloride catalysts, showing 100% conversion at 350"C, this compared to 38.0% for MnCl2/KC1/SiO2which was the best performance after the CuCl based catalyst. Comparison with the oxides Cr2O3, CuO, Co304and M n 0 2 supported on Si02showed that CuC1/KCl/SiO2 was more active and although CrzOJSi02 activity was only slightly lower the selectivity to COX from CuC1/KCl/SiO2was significantly better than the oxide catalysts which produced appreciable quantities of non-combustion products. After use the Cr203/Si02 catalyst showed an 18% decrease in the Cr content, probably due to the formation of volatile Cr02C12,whilst analysis of CuC1/KCl/SiO2 showed no decrease in Cu content or loss of activity. Kinetic investigations of methylene chloride oxidation indicated that the reaction was first order with respect to CH2C12and zero order with respect to O2 above 0.15 atm partial pressure, pulsing experiments with CHzC12and O2 indicated that significant amounts of active oxygen were stored by the catalyst during 0 2 pulses. It has also been shown that the catalyst can function in a redox cycle mode and evidence from TPR studies suggests that the copper (11)oxychloride species, CuO. CuC12, was the active component for CH2C12destruction. The oxidation of deuterated methylene chloride showed a kinetic isotope effect, kH/kD = 1.48, suggesting that C-H bond breaking was an important procedure in the rate determining step. The CuC1/KC1/SiO2and CuC1/Si02 catalysts have also been used to destroy a range of other VOCs including 1,2 dichlorobenzene, carbon tetrachloride and ethylene oxide which were all completely destroyed within the temperature range 300-500°C.These catalysts based on copper and chlorine show high activity for the destruction of a range of VOCs, the results presented here were all obtained at a relatively low gas hourly space velocity of 300 h-', it would therefore be interesting to investigate these catalyst at higher flow rates which would more applicable for industrial applications. The use of zeolite catalysts in the total oxidation of VOCs has been previously highlighted [58-591, zeolite based catalysts have particularly been used for the destruction of a range of chlorinated organics. Early studies by Chatterjee and

142

Catalysis

Greene [90] investigated the oxidative destruction of methylene chloride by zeolite Y in the H form and ion exchanged with cerium and chromium. The zeolite was supported on a low surface area cordierite honeycomb monolith by application as a wash coat. Below 425°C the catalytic activity decreased in the order; Cr-Y > H-Y > Ce-Y however, above 425°C the activity was similar, with all catalysts showing >90% conversion, major reaction products were CO and HCl. The addition of 2.7 vol.% water into the reaction stream suppressed the conversion, particularly below 400"C, and was most marked with the H-Y and Ce-Y catalysts, this effect was reversible and it was considered due to active site blocking by the water molecule. The oxygen uptake capacity and acidity for the zeolites correlated directly with activity and it was considered that the exchanged metal cation determined these parameters and ultimately the activity. A dual site oxidation mechanism involving the adsorption of methylene chloride at Br-nstedacid sites and oxygen adsorption on the metal cation was feasibly proposed. The use of metal exchanged zeolites has been investigated further by Chatterjee et al. [91] to include other transition metals such as cobalt and manganese in addition to chromium. The catalysts were prepared as previously described [90] and were tested for the destruction of methylene chloride, trichloroethylene and carbon tetrachloride in the presence of 1.3 vol.% water. Co-Y showed superior performance producing complete conversion of methylene chloride and carbon tetrachloride at 350°C and 200°C respectively, whilst conversion of trichloroethylene was not observed below 400°C. Impregnation of the transition metal exchanged zeolites with chromium solution resulted in the formation of Cr2O3 in the matrix, significantly improving the destruction of trichloroethylene to >90% at 325°C. Partially oxygenated products were not detected and the major reaction products were CO, C 0 2 and HCl, 100% C02 selectivity was observed for carbon tetrachloride oxidation over all catalysts. However, for the oxidation of methylene chloride and trichloroethylene, CO selectivities for the ion exchanged catalysts were high, CO2/CO ratios were in the range 0.02-0.05, the added Cr2O3 improved C 0 2 selectivity but the C 0 2 / C 0 ratio was still only 0.2-0.3. A more detailed study investigating the effects of catalyst composition on cobalt exchanged chromium impregnated zeolite Y has been reported [92]. Increasing the degree of cobalt exchange increased the acidity and oxygen uptake of the catalysts without effecting the surface area. Increasing the Cr2O3 impregnation caused a decrease in acidity by dealumination and higher loadings reduced the surface area by structural loss and pore/channel blocking. The destruction of methylene chloride was improved by increased Co exchange, whilst increasing Cr203loading was detrimental, on the other hand trichloro-ethylene destruction increased with increasing Cr203and to some extent Co exchange. The authors concluded that methylene chloride destruction was essentially a function of the acid sites, forming a carbonium ion which was subsequently oxidised by oxygen adsorbed at the cobalt cation sites. The presence of Cr203was detrimental as a consequence of the reduced surface area and acidity. Conversely, trichloroethy-


143

H\c/H

CI

' v I 'Cl

0-

I

H+

co2+

Al /o\Si

Al /*\Si

I t

HCI

Figure 9

co3+

CO

H

I

Al /'\Si

"7'

c---

Reaction mechanism proposed for methylene chloride oxidation by Cr203/Co-Y [931

lene conversion was strongly dependent on the chromium sites although the acidic and cationic sites were also influential in the destructive sequence. Surface reaction mechanisms based on evidence from in situ transmission FTIR studies, have been proposed for the destruction of methylene chloride and carbon tetrachloride over Co-Y loaded with Cr203[93]. The proposed reaction mechanism for methylene chloride destruction is shown in figure 9. A similar type of mechanism was also proposed for carbon tetrachloride oxidation, important steps in both mechanisms were the adsorption of the chlorinated hydrocarbon on the Br-nsted acid sites and dissociative adsorption of molecular oxygen on Co cation sites. The formation of CO and C02 products were via parallel reaction pathways, with little or no COz formed by sequential CO oxidation. The formation of phosgene as a reaction intermediate has been identified during CCl, oxidation at space velocities above 15,000 h-' in a wet feed stream and as low as 2,400 h-' in a dry stream. The selectivity of phosgene also increased with time on line as the catalysts deactivated from 99% conversion to ca. 10% conversion after 22.5 h use. The use of zeolite-based catalysts for VOC oxidation, in particular chlorinated VOCs, has been demonstrated at low temperatures, however, they deactivate relatively quickly and will therefore require regular regeneration, which may prove impractical for commercial operation. The very low selectivity towards C 0 2and the formation of potentially hazard by-products such as phosgene also needs to be addressed. 4.5 Mixed Catalyst/Sorbent Systems. - The process efficiency of catalytic oxidation may be enhanced by incorporating a sorbent component in the system. In conventional catalytic oxidation acid products such as HCl and C12 produced

144

Catalysis

during chlorinated hydrocarbon destruction must be scrubbed from the effluent stream before emission to the atmosphere. The use of a catalyst-sorbent system simultaneously destroys the VOC substrate and captures the halogenated oxidation products negating the need for downstream scrubbers. Stenger et al. [94] have described the use of such catalysts for the destruction of trichloroethylene, trichlorofluoromethane and toluene in the concentration range 30-350 ppm at 6,700 h- space velocity. The catalyst comprised copper and manganese oxides, which were the catalytic components, supported on the sorbent component sodium carbonate. All the VOCs were completely destroyed in the temperature range 250-400°C. Powder X-ray diffraction showed that sodium carbonate captured the chlorine products and was converted to sodium chloride with subsequent release of COz,this was confirmed by X-ray photoelectron studies. A chlorine balance determined during trichloroethylene destruction indicated that 98% of the chlorine was retained by the sorbent. Catalysts lifetime testing showed that there was no trichloroethylene in the effluent, however, dichloromethane was a by-product and increased slightly in concentration from 1 ppm to 5 ppm over 5 days operation. Theoretically the lifetimes of such catalyst-sorbent systems are limited and lifetime will be a function of the flow rate, VOC type and concentration. A different type of catalyst-sorbent system based on transition metal exchanged zeolites for the destruction of chlorinated VOCs has been investigated by Greene et al. [95]. Chromium exchanged Y and ZSM-5 based zeolites were used to physisorb VOCs from a humid air stream at ambient temperatures, the upper portion of the bed was then heated to catalytically active temperatures, typically 300"C, whilst the lower portion was slowly temperature ramped to desorb the VOCs which were catalytically destroyed. A gas hourly space velocity of 2,400 h-' was used and Cr-ZSM-5 was found to have the highest activity for simple VOC oxidation, 9 5 YO conversion of trichloroethane and dichloromethane at 300"C, and also had a relatively high sorption capacity. A process which alternatively trapped and catalytically destroyed 110 ppm trichloroethylene using a Cr-ZSM-5 sorbent/catalyst has been described. The cycle time was determined to be ca. 24 hours and comparison of the relative energy use showed a 93% reduction compared to a conventional reactor as the sorbentcatalyst process only required heating for 7% of the cycle time. 4.6 Comparison of Noble Metal and Oxide Catalysts. - A few studies have directly compared the activity of noble metal and oxide based catalysts. The combustion of a range of Cs-C9hydrocarbon VOCs in humidified air by 0.1% Pt/3 YONi/A1203and ceria promoted hopcalite commercial catalysts has been compared [96]. The Pt based catalyst showed no deactivation during 253 continuous operation, whilst over 297 days the temperature of hopcalite catalyst required an 85°C increase to maintain > 99% conversion. However, the final operating temperature of the hopcalite catalyst was 4OO0C, 30°C lower than the isothermal operating temperature of the Pt system. A first order concentration deactivation model was developed and predicted a 362 day lifetime for the

GHSV/h-'

70000 70000 70000 15000 10500 10000 10000 9500 6700 5900 5900 5900 5900 5900 5000 5000 3600 3600 3600 3600 3200 2400 2400 2400 2400 2361

Temp. / "C

400 350 450 400 400 325 390 430 270 500 500 500 500 500 500 500 400 650 400 400 130 350 350 350 350 325

voc(s)

benzene chloro benzene butane mixture of Cs-C9hydrocarbons CFzClz benzene benzene mixture of C5-C9hydrocarbons trichloroet h ylene dichlorofluoro methane dichlorofluoro methane dichlorofluoro methane dichlorofluoro methane dichlorofluoro methane C,CI,F, C2C13F3 1,2- dichloroethane trichloroethane 1,2- dichloroethane 1,2- dichloroethane benzene, toluene and xylene

methylene chloride methylene chloride trichloroethylene trichloroethylene trichloroethane

Comparison catalyst activties for combustion of VOCs

Cu/U/Si02 Ce/Cu/Mn02 Ti02 Cu-Cr/y-A120, Co-Cr/y-A1203 0.1%Pt/3 %Ni/A1203 Cu/MnO/Na2C03 mordenite A Si02/A1203 Zr02/Mo03 Ti02-SiOz ZrP207 Pt/Zr02 Pt/ZrOz-(PO,) Ti02/Si02 Cr203 zeolite Y mordenite B Pt/ hydrophobic fluorinated carbon (60% fluorine) co-Y H-Y Cr-ZSM-5 Cr-Y co-Y

U308

U308

Catalyst

Table 3

1406 996 1098 1023 1500

10000 10000 10000 500 2000 10000 10000 500 31 6000 6000 6000 6000 6000 11000 11000 10000 10000 10000 10000 45

Inlet

99.9 99.7 95.0 99 98.0 100 100 99 99 100 99.2 95.7 95.7 95.3 99 98 100 99.7 98.5 95.3 95 100 100 99.4 98 100

673 K) conditions is found in the behavior of catalysts such as Ce02or Ce02/Mg0. For these materials, little primary aldol product is formed from the reaction of 1-propanol regardless of the conversion, while the more basic MgO gives almost entirely primary aldol product (3-hydroxy-2-methylpentanal), again regardless of c o n v e r ~ i o nUnless . ~ ~ the Ce02is absolutely necessary to catalyze the final decarboxylation (in reaction (4) or Scheme 3), this evidence suggests a parallel, non-aldol route at these conditions. In other words, it is suggested that the problem with the strong bases of high M - 0 bond strength (e.g., MgO) is not that they cannot form carboxylates (MgO forms bulk acetates at these conditions,)," it is that they cannot undergo the reduction needed to generate a ketene-type intermediate. However, further studies with 13C-labeled

302

Catalysis

feeds are probably necessary to settle this question. Limited studies have been made to determine if an added oxidant could replenish the surface oxygen removed in aldehyde oxidations, when they occur. Thermodynamic calculations show the oxidation of aldehyde using water as an oxygen source would be favorable. 2 RCHO

+ H20

+-*

R2CO

+ C02 + 2H2

(9)

AH298 (acetaldehyde) = -27.5 kJ mol-' Experiments have shown that both ketone and COZ production increase dramatically when water is added. Claridge et al. performed preliminary studies using water in decarboxylative condensation, and found that a 1/1 propanal/water feed led to an increase in pentan-3-one and C 0 2 formation, over Fukui et al. have shown that adding water to an isobutyraldehyde feed (0.6-0.75 mol water/mol aldehyde) increased conversion and the selectivity to diisopropyl ketone over Zr02.23Plint et al. found that adding 20% water to their butanol/02 feed gave a high yield to 4-heptanone; however, with time on stream the selectivity to 4-heptanone d e c r e a ~ e d . ~ ~ When cyclopropanecarboxaldehyde(CCAld) was condensed with acetic acid in pulse reactor experiments, the only primary product arising from CCald was the acid CCA.39-40 The ketone (MCPK) production rate was -4 times slower with acid/aldehyde than acid/acid feeds. An aldol route to MCPK could proceed through either of the following reactions: CH3COCH3 + C3HSCHO CH3CHO

+ C3HsCHO

+

C3HSCH(OH)CH2COCH3

(10)

+

C3HSCH(OH)CH2CHO

(11)

Both intermediates could conceivably decompose to MCPK by oxidative decarboxylation to give COXand water or by a concerted decarboxylation reaction to acetaldehyde starting from the first intermediate, or to formaldehyde starting from the second intermediate. However, neither intermediate, nor their dehydration products, nor acetaldehyde, formaldehyde, or CO were found even in trace quantities. Therefore, it appears that in this case as well the ketone is not being produced by an aldol reaction, but rather by a decarboxylative condensation reaction of the aldehyde and acetic acid, using oxygen from the surface as needed. When water was added, an increase in ketone formation was observed when comparing runs performed at the same CCald partial pressures. The reaction order for water was estimated to be 0.2. With oxygen present a much lower selectivity was attained in acetic acid/CCAld condensations than with either water present or with no additive. Although CCA production was greater, this did not enhance the production of ketones. It was impossible to say whether the inhibition resulted from more total oxidation or from deactivation of active sites for condensation. Plint et al. found that the yield to 4-heptanone from butanol was maximized at a butanol/oxygen ratio of 6/1, using a 40% Ce02/Mg0 catalyst, at one particular flow rate.67At other flow rates the optimum ratio varied from 3/1 to 1/1. They also found that butanol conversion was positive order with respect to oxygen. But they con-

-

7: Catalysis of AcidlAZdehydelAlcohol Condensations to Ketones

303

cluded that the product distributions differed to such an extent that no correlation between selectivity to 4-heptanone and oxygen content was possible. Claridge et al. studied the reaction of propanal to 3-pentanone using O2or N 2 0 as They observed that at 773 K total oxidation occurred. The data from our work show that an increase in CCald/02ratio increases the production of ketone, suggesting total oxidation is the more probable cause of the lower ketone yields. Experiments with acid/aldehyde feeds were also conducted using deuterium oxide and acetic-d3-acid-hin place of water and acetic acid, r e s p e c t i ~ e l y .The ~~-~ addition of D 2 0had a significant effect on the reaction rate, so that a reaction involving water is most likely a slow step. The addition of acetic-d3-acid-halso decreased the ketone turnover ratio slightly, so the breaking of a C-H bond in acetic acid could also be a slow step in the mechanism, or there could be two parallel overall reactions, one using and one not using water. The kinetic isotope effects for these reactions were 1.5 for CH3COOH/CD3COOHand 6.7 for H20/D20. As with the acid/acid reactions, these kinetic isotope effects are in the expected range of carboxylate decomposition, based on data for formic acid.4l For aldehyde/aldehyde feeds (CH3CHO/CCald), the side products from CCald were 2-cyclopropyl-1-butene, and 3-cyclopropyl-1-butene, both formed from the aldol condensation of CH3CH0with CCald, but also some CCA. With such feeds the production of ketone was greatly decreased, indicating the necessity of at least one easily formed carboxylate for decarboxylative condensation. The addition of water increased the rate of production of ketone, again suggesting that water is supplying oxygen to the surface to produce carboxylate intermediates. There were also a number of side products produced from CH3CH0, by aldol self-condensation. The aldol condensations involving either CH3CH0 or CCald are in this case faster than their decarboxylative condensation reactions. From thermodynamic calculations it is obvious that while all the reactions discussed above are thermodynamically possible, certain reactions are more probable. For example, thermodynamic calculations suggest that the condensation of acetic acid/CCald is more likely to proceed to C 0 2 as a byproduct, and this is in fact what was observed. When looked for, more C02 is found than CO in the products of aldehyde/aldehyde c o n d e n s a t i o n ~ .It~ ~ is*also ~ ~ important to note that the I& values for the acid/aldehyde and aldehyde/aldehyde formation of the ketone are both typically larger than for the acid/acid reaction. From this one might expect the turnover rate for ketone production to be larger for these two feeds. However, this is not what is observed. Again, this indicates some involvement of an adsorbed carboxylate, which cannot be formed directly from an aldehyde. 4

‘One-Step’ Aldol Condensations to Ketones

Lower temperature processes and catalysts exist for so-called ‘one-step’ aldol condensations to produce ketones by condensation of two ketones or of a ketone

304

Catalysis

and an aldehyde to give initially the aldol product, followed by dehydration to the a,P-unsaturated ketone (enone), and then by hydrogenation to the saturated ketone; all of these can occur using the same catalyst. In some cases it even appears possible to start with the corresponding alcohols which are first dehydrogenated to the ketone^;'^'^^-^^ this presumably obviates the need for H2 addition. When H2is used, a typical ratio to ketone or aldehyde is between 0.25 and 1. When an alcohol is used to supply the hydrogen, higher molar ratios to the ketone can be used. Higher selectivities to the product of a single condensation (e.g., methyl isobutyl ketone from acetone) are obtained by using H2 pressures well above atmo~pheric,7~ or by the addition of 5 wt% recycled product of the single aldol conden~ation.~~ The alcoholic dehydrogenation reaction requires a transition metal component unless it is possible to operate above -470 K, where moderately basic catalysts also become active for this r e a ~ t i o n . ~An ~ - ~example ' catalyst would be a MgA10, hydrotalcite of low A1 content. These catalysts can activate Hz/D2 exchange at these temperatures. The alcohol adsorbs dissociatively to surface alkoxide and hydroxy, a step requiring proximity of a basic and a Lewis acid There follows a hydride abstraction, mediated by another Lewis acid group, and heterolytic association to give H2. When a ketone and an aldehyde are condensed in a cross-aldol reaction, excess ketone must be used to avoid aldehyde self-condensation, and even so the surface is essentially saturated with aldehyde at low temperature. Exceptions are reactions of easily enolizable ketones at ambient and lower temperatures.8l Therefore increasing the temperature often increases selectivity as well, a situation reminiscent of decarboxylative condensation, by allowing for greater ketone adsorption. An enolate formed from an unsymmetric ketone can be of two types; the less electronegative (usually more substituted) side is deprotonated preferentially by many homogeneous transition metal complexes with N-donor or halide ligand~,8~ in- ~agreement ~ with local hard-soft acid-base But steric hindrance plays a greater role with heterogeneous catalysts, to such an extent that the less substituted side is often deprotonated preferentially.s8*85 Some examples of 'one-step' processes for methylketone production are as follows.

-

-

(1) Methylpentylketone from the acetone/isopropanol/n-butanol reaction at -520 K, low pressure, LHSV 1, with major side reactions to acetone and heavier condensation The primary reaction is:

CHCOCH3

+ CH3(CH2)30H -> CH3CO(CH2)4CH3 + H20

(12)

(2) Methylketones or methylvinylketones from a ketone/aldehyde/H2 reaction at -430-470 K, 1-2 MPa H2,86or at higher temperatures with an alcohol as hydrogen source,59~74-76~s7 or at 95% selectivity at 70% conversion with a secondary amine hydrochloride ( 40/1 aldehyde/amine)/Nb205catalyst mixture at 390-420 K,lo3 or at lower selectivity (- 40-50%) but almost complete conversion using typical H-form zeolites at 670-730 K.'04 Most commercial enone synthesis methods are not applicable to all desired enones. The acetylene condensation/hydration method can only make enones with even numbers of carbon^,'^^ while formaldehyde condensation preferentially gives branched products at higher molecular weights.lMOther methods use expensive oxidation reagents and start with the allylic alcohols.'07Yet other methods use even more expensive reagents,lo8-lwor electrochemical methods

-

306

Catalysis

with relatively low yields and complex product distributions."' Converting the enone to the dihydroxyenone is possible by nucleophilic epoxidation followed by selective hydrolysis. The epoxidation reaction is well known."' A peroxide dissolved in a strongly basic medium must be used (Weitz-Scheffer reaction). The selective hydrolysis of the epoxide to the dihydroxy compound proves more difficult. The trick is to avoid conversion to the trio1 or to a diketone, especially under the basic conditions.

5

Lower Temperature Condensationsto Ketones

The high temperatures and/or byproducts often associated with the reactions discussed above have led to attempts to produce ketones by condensations at near ambient conditions. Most of the catalysts for these reactions are soluble transition metal complexes, and there have been few accompanying studies of the stability and reusability of the complexes. For example, the condensation of boronic acids (mostly phenylboronic acids) with anhydrides takes place at 50%. While not strictly a condensation, aromatic ketones can also be produced from fluorenes, indenes, xanthenes, etc. by first chlorinating them at the 9-position, then oxidizing at the same position with a strong acid at near ambient phase transfer conditions, with a tetraalkylammonium salt as ~ata1yst.l~~

-

-

-

6

Catalyst Properties- Decarboxylative Condensations

While several studies have compared various metal oxides for decarboxylative condensation reactions of acids and aldehydes, most were limited in scope due to the low conversions and/or partial pressures employed. In particular, the detrimental effects of water, often present in the feeds and always a product of the reactions, have usually been ignored. Regenerability of spent catalysts has also been given scant attention. In previous work using Ce02 and other rare-earth oxide (REO) catalysts, ketonization selectivity was found to be low compared to that of aldolization, cracking or reductive coupling reaction^.'^^^^^^^*'^^ However, in recent work it has been determined that many supported REOs are selective, long-lived decarboxylative condensation catalysts at high conversions, especially when acetic acid is one reactant.'0~12*27-28~40 Among the more common oxides outside the transition metal oxides, it was found that Ce02- or ZrOl-containing catalysts demonstrate the best overall

309

7: Catalysis of AcidlAldehydelAlcohol Condensations to Ketones

behavior for ketonization of acids/aldehydes at high c o n v e r ~ i o n Hi . ~gh~ ~ ~ ~ ~ ~ loading Ce02/A1203materials prepared by incipient wetness impregnation give STYs (space-time yields) in excess of 4 wt ketone/(wt cat-h) for over 14 h continuous operation without regeneration for methylcyclopropylketone and 2-undecanone s y n t h e s i ~ , 2 ~and - ~ ~for 9 ~acetone." ~ STYs in the range 1.8-2.0 have been reported for 3-pentanone, 6-undecanone and 7-tridecanone synthesis.1° With periodic regeneration these materials show no loss in STY over a few weeks of use, and in fact the selectivity improves through the first few regenerations, suggesting that interaction between the R E 0 and the support takes place. Deactivation seems to be associated with coking rather than excessive reduction, because the oxidation state of the pre-reduced (more active) catalyst and the used catalyst are similar. REOs such as Pr203 and Sm203,which are less reducible than Ce02at typical reaction conditions, have been reported to be more selective for the ketonization of but we have found them to be less active and less stable at high conversions. We and others have also tried to optimize the support and preparation method for Ce02-supported catalyst^,'^*'^^^^^^^ both unpromoted and those promoted with alkali or alkaline earths. A1203 appears to be the best support in terms of overall behavior, although Ti02is close. In contrast to suggestions from other work: we and others have found that alkali and alkaline earth promotion does not improve high loading Ce02-based catalysts, neither by eliminating undesired reactions (e.g., lactonization or cracking of acids) nor by decreasing rates of coke formation.65While some enhancement of Z r 0 2 activity with Nadoping has been this appears to be the result of better stabilization of the active tetragonal phase in Zr02. Mixing Ce02 with a d9 or dl0 transition metal (with the possible exception of Co), or with a metal oxide of wider base strength distribution (e.g., CaO), is usually detrimental unless the reactants are resistant to dehydrogenation, isomerization and cracking reactions, e.g., acetic acid. Therefore desirable catalysts (e.g., supported CeO2 or Zr02)appear to be characterized by a relatively amphoteric or weak base nature, a narrow distribution of surface basi~ity,'~~'~-'~ and relatively poor dehydrogenation activities. There is often little relationship between acid/base site distributions in supported MO, and classical measures of cation Lewis acidity/basicity such as electronegativity or the isoelectric point of charge (IEP). However, in general it is true that a cation of high electronegativity is likely to have more acid sites at the 'strong' end of the scale (as measured by heat of adsorption for an amine or NH3) than one of low electronegativity. The reverse is true for basic sites. In terms of relative base strengths, it is generally agreed that, e.g., ZnO >> Ce02 > ZrO2. Relative to typical supports such as A1203 (IEP 7-9) and Ti02(anatase, IEP 6-6.5), supported Z r 0 2 (IEP -6-6.7), Nd203, Ce02 (IEP 6.7-6.8) and ZnO (IEP 8.7-9.7) all have more basic sites per unit area. But these oxides are also characterized by a relative absence of strongly basic sites, especially when compared to M0,'s such as Li/A1203,Ca/A1203or Sr/La203.'6~18*22~'34 Z r 0 2 actually shows some Lewis acid sites by pyridine adsorption at 773 K, and 4-7 p,mol/m2 NH3 a d s ~ r p t i o n , 'while ~ ~ in comparison Ti02 normally gives many more acidic than basic sites upon titrations.20Ce02is similar to Zr02.

-

-

-

-

-

3 10

Catalysis

For a typical catalyst such as supported Ce02, the optimal loading is g Ce02/m2.The results of calculations for the Ce02 loading -0.7-0.9 x corresponding to a monolayer leads to a range of results. For example, assuming equal percentages of (11l), (110) and (100) planes exposed, with half of the Cexf coordinatively unsaturated, gives a surface density of 1.0 x g Ce02/m2, which corresponds to the transition density from mostly two-dimensional rafts to three-dimensional crystallites as determined by TEM and EPR.'36For 75% of the Cex+coordinative unsaturated, a surface density calculation gives a result similar to the density (- 1.7 x calculated assuming that the low-temperature reduction peak in TPR measurements corresponds to removal of 25% of surface oxygens to create a surface similar to Ce203.'37Taking into account the inability of incipient wetness impregnation to fill the smallest pores of the support, and the inaccuracy of the TPR calculations (it is difficult to deconvolute the surface reduction from the bulk reduction peak), it is probable that the optimum Ce02 loading corresponds to near one monolayer of C e 0 2 0 n the supports. Ce02crystallizes in the fluorite (fcc) structure with octahedral coordination of the cations. This structure is maintained upon reduction to at least CeOl.7and sometimes 10wer.'"O-'~~ Metastable phases of varying compositions, all with defective fluorite structures, have been observed, suggesting that the diffusion of oxygen vacancies into the bulk is rapid at reduction temperatures; this is a reason why the surface and bulk reduction peaks in the TPR spectrum overlap. The (defective) fluorite structure is retained even to high degrees of reduction in CeOX/Zr0,.'"O The gradual increase in ketonization activity of these catalysts is probably the result of reduction and spreading of Ce02 from 'islands' to a more two dimensional layer structure, and some intercalation in the support to ultimately form, e.g., CeA103or CeTi03.141-'43 Redox activity in the temperature range of interest is important because the mechanisms of ketonization all require reduction/oxidation of the surface. Reduction of high surface area Ce02 itself (to Ce203)typically begins at 600 K with a 'surface' peak maximum of 750-770 1020-1 100 K, and final stoichiometry at >1100 K, a 'bulk' peak maximum of K of Ce01,75-1.8.1401144-146 The amount of reduction at 723-773 K can vary from 0.01-0.13 O/CeOz, depending on factors such as surface area and heating rate.'39,145-146,147 Reoxidation at 450 K is rapid.'39Partly reduced Ce02surfaces are very strongly dehydr~genating,'~~ and generate alkyl moieties as sometimes hypothesized in the 'ketene' mechanism. This is especially true at lower pressure (of acid or aldehyde) conditions, as present, e.g., in TPR experiments or with alcohol feed^.^^'^' Surface reduction of >1 monolayer of Ce02/A1203with H2 takes place at 560-900 K (peak max. 730-860 K),'38,'433'469'49 and therefore within the range of acid condensation. This is a lower temperature range than for bulk Ce02,and a higher-temperature bulk peak is also sometimes seen. Rogemond et al. report reductions of as high as 0.24 mols O/mol Ce02associated with this peak.'49The total amount of reduction is also higher than for Ce02alone, to CeO1.54-1.58/A1203 at >1100 K.'& Rapid redox cycling ('oxygen storage') experiments to mimic redox

--

-

-

-

311

7: Catalysis of AcidjAldehydejAlcohoI Condensations to Ketones

reactions showed that the supported CeO, would exist at steady-state as Ce01,98-1.99 at 773 K.'433'46 Mixed Ce/REO's (e.g., La, Y, Hf) and Ce/Zr mixed oxides are often more easily reduced than Ce02 itself. These ions are either similar in size to Ce3+ or smaller, providing for increased oxygen mobility. However, the (defective) fluorite structure is retained during the reduction unless the doping exceeds several For degree of reduction vs. temperature, some of the differences in the literature are undoubtedly due to differences in the intimacy of mixing between the oxides. Better reducibility, however, does not necessarily translate into enhanced hydrogenation or ketonization ability. With the exception of La203,most of the other REO's are poorer hydrogenation catalysts than Ce02itself,'51and Ce02is much better than 1/1 Ce02/Zr02for typical ketonizations.28Fornasiero et a1 found that upon repeated redox processes the 1/1 mixture sintered;lMthis may be why Ce/Zr oxides made poorer acid condensation catalysts. The TPR behavior of the mixed oxides supported on A1203is unclear. XRD observations suggest that while reduction takes place, any formation of CeA103 is inhibited by the Zr02.152 If further increases in dehydrogenation activity are desirable in the ketonization process, a d6-d8transition metal can be supported on the active oxide l a ~ e r . ~There ~ . ' ~can ~ be lost yield to saturated h y d r o c a r b o n ~ . ' ~Most ~ ' ~ ~of the commonly used transition metals exhibit limited miscibility with REOs. An exception is C U . ' At ~ ~ambient conditions, the Cu is mostly dispersed as Cu+; some Cu+ can be maintained under reducing conditions even at 723 K.'55When supported on A1203,CeO, suppresses formation of inactive CuA1204up to very high temperature. Of the transition metal oxides, Fe203and Mn02/Mn304appear to be the best single metal ketonization catalyst^.'^-'^^^^ They are also quite effective when used in combination with Ce02in the alcohol to ketone dehydrogenation/condensations, more so than either the d6 or d9 metals.65Both oxides are easily dispersed on A1203.There is some disagreement over the active state of the Mn-containing catalysts. Pestman et al. reported that irreversible reduction to MnO takes place, while most others consider the active form to be a mixture of the IV to I1 formal oxidation states." The interconvertibility of the Mn-oxides at high temperature is probably important in their ability to fulfill the varied requirements of the dehydrogenation/condensation reactions. In contrast to the REOs, Zr02, etc., Nb2O5 and related group VB oxides are highly acidic (IEP O S ) , especially in the presence of water. The dispersion of Nb2O5 is particularly good on A1203,forming a distorted NbO6 octahedral 2-D 1 a ~ e r .This l ~ ~ structure is not dehydrated even upon drying until 770 K, and the acidity is thought to arise from a hydroxyl adjacent to the surface Nb = 0 bond. This and related mixed-metal oxides are poor ketonization catalysts, resulting in products more characteristic of aldolization, and excessive isomerization of the reactant acids and product ketone^.^^-^^ Strong basicity and low M - 0 bond strengths result in bulk acetate formation; pure oxide catalysts of this type are relatively inactive for ketoni~ation.6~"~~~

-

%.138,1409150

-

-

312

Catalysis

Oxides with a wide distribution of acid and base sites (e.g., La203/La(OH)3, Nd2O3, or mixed oxides of the alkaline earths with Ce02) tend to catalyze cracking of aldehydes and ketones:* or at 700 K followed by rehydration is a suitable activation procedure; the optimal Mg) ratio is usually -0.25-0.33. More Lewis acid or more weakly Al/(Al basic Bronsted sites (-OH) typically enhance catalytic activities for both the

+

7: Catalysis of AcidlAldehydelAlcohol Condensations to Ketones

313

condensation and dehydration reactions, but too many Lewis acid sites can reduce selectivity due to multiple aldol condensations and/or Michael additions, e.g., isophorone from acetone. Both site density increases can be effected by increasing the Al/(Al + Mg) r a t i ~ . ~ Layered ~ - ~ ' double hydroxides derived from anions other than the carbonates can exhibit greatly different acid-base properties, e.g., those derived from exchange of some borate or silicate anions are primarily weak Lewis acids.80 Considering single metals rather than alloys, copper functions effectively for the selective hydrogenation of the a$-unsaturated product, in combination with moderately basic mixed oxides such as Ce02,MgO/A1203(high Al/Mg ratio) or K/Mg/CeO,, as long as H2 partial pressures are relatively low. One way to accomplish this is to use partly or wholly alcoholic feeds, which generate their own surface hydrogen. Copper appears to have little effect on acid/base strength distributions, and in particular does not inhibit neighboring basic ~ i t e s . ~ ~ ? ' ~ ~ However, it is difficult to control further aldol condensations with these materials, even at

Catalysis: Vol 17 (Specialist Periodical Reports)

Catalysis: Vol. 19 (Specialist Periodical Reports)

Catalysis: v. 18 (Specialist Periodical Reports)

Catalysis, Volume 22 (RSC Specialist Periodical Reports)

Photochemistry (Specialist Periodical Reports) (Vol 13)

Chemical Modelling: Vol. 2: Applications and Theory (Specialist Periodical Reports)

Organosphophorus Chemistry (Specialist Periodical Reports) (v. 4)

Photochemistry, Volume 37 (Specialist Periodical Reports)

Heterocyclic Chemistry: v.3 (Specialist Periodical Reports) (Vol 3)

Heterocyclic Chemistry: v.4 (Specialist Periodical Reports)

Organometallic Chemistry, Volume 36 (Specialist Periodical Reports)

Organometallic Chemistry, Volume 31 (Specialist Periodical Reports)

Theoretical Chemistry,vol 2 (Specialist Periodical Reports) (v. 2)

Catalysis: A Review of Chemical Literature: Vol 15 (Specialist Periodical Reports)

Catalysis: A Review of Chemical Literature: Vol 16 (Specialist Periodical Reports)

Alkaloids Volume 4: Specialist Periodical Rep (Specialist Periodical Reports) (v. 4)

Nuclear Magnetic Resonance: Volume 37 (Specialist Periodical Reports)

Terpenoids and Steroids: Volume 12 (Specialist Periodical Reports)

Terpenoids and Steroids: Volume 3 (Specialist Periodical Reports)

Dielectric and Related Molecular Processes: Volume 1 (Specialist Periodical Reports)

Electrochemistry: v.8: A Review of Chemical Literature (Specialist Periodical Reports) (Vol 8)

Electrochemistry Vol 6: The Periodic Table of the Elements Wall Chart (Specialist Periodical Reports) (v. 6)

General and Synthetic Methods (A Specialist Periodical Report, Vol 11)

Dielectric and Related Molecular Processes, Vol. 1: A Review of Chemical Literature (Specialist Periodical Reports)

Organophosphorus Chemistry (Volume 18) Specialist Periodical Report

Gas Kinetics and Energy Transfer: v.4: A Review of Chemical Literature (Specialist Periodical Reports)

The Alkaloids: A Review of Chemical Literature: v. 8 (Specialist Periodical Reports)

Inorganic Chemistry of the Transition Elements: v. 4: A Review of Chemical Literature (Specialist Periodical Reports)

Electrochemistry: v.10: A Review of Chemical Literature (Specialist Periodical Reports)

Inorganic Chemistry of the Transition Elements, Volume 2 (Specialist Periodical Reports) (v. 2)

The Alkaloids: A Review of Chemical Literature: v. 6 (Specialist Periodical Reports)

Catalysis: Vol 17 (Specialist Periodical Reports)

Catalysis: Vol. 19 (Specialist Periodical Reports)

Catalysis: v. 18 (Specialist Periodical Reports)

Catalysis, Volume 22 (RSC Specialist Periodical Reports)

Photochemistry (Specialist Periodical Reports) (Vol 13)

Chemical Modelling: Vol. 2: Applications and Theory (Specialist Periodical Reports)

Organosphophorus Chemistry (Specialist Periodical Reports) (v. 4)

Photochemistry, Volume 37 (Specialist Periodical Reports)

Heterocyclic Chemistry: v.3 (Specialist Periodical Reports) (Vol 3)

Heterocyclic Chemistry: v.4 (Specialist Periodical Reports)

Organometallic Chemistry, Volume 36 (Specialist Periodical Reports)

Organometallic Chemistry, Volume 31 (Specialist Periodical Reports)

Theoretical Chemistry,vol 2 (Specialist Periodical Reports) (v. 2)

Catalysis: A Review of Chemical Literature: Vol 15 (Specialist Periodical Reports)

Catalysis: A Review of Chemical Literature: Vol 16 (Specialist Periodical Reports)

Alkaloids Volume 4: Specialist Periodical Rep (Specialist Periodical Reports) (v. 4)

Nuclear Magnetic Resonance: Volume 37 (Specialist Periodical Reports)

Terpenoids and Steroids: Volume 12 (Specialist Periodical Reports)

Terpenoids and Steroids: Volume 3 (Specialist Periodical Reports)

Dielectric and Related Molecular Processes: Volume 1 (Specialist Periodical Reports)

Electrochemistry: v.8: A Review of Chemical Literature (Specialist Periodical Reports) (Vol 8)

Electrochemistry Vol 6: The Periodic Table of the Elements Wall Chart (Specialist Periodical Reports) (v. 6)

General and Synthetic Methods (A Specialist Periodical Report, Vol 11)

Dielectric and Related Molecular Processes, Vol. 1: A Review of Chemical Literature (Specialist Periodical Reports)

Organophosphorus Chemistry (Volume 18) Specialist Periodical Report

Gas Kinetics and Energy Transfer: v.4: A Review of Chemical Literature (Specialist Periodical Reports)

The Alkaloids: A Review of Chemical Literature: v. 8 (Specialist Periodical Reports)

Inorganic Chemistry of the Transition Elements: v. 4: A Review of Chemical Literature (Specialist Periodical Reports)

Electrochemistry: v.10: A Review of Chemical Literature (Specialist Periodical Reports)

Inorganic Chemistry of the Transition Elements, Volume 2 (Specialist Periodical Reports) (v. 2)

The Alkaloids: A Review of Chemical Literature: v. 6 (Specialist Periodical Reports)

Recommend Documents