Series Editor: Graham J. Hatchings
Catalysis by Ceria and Related Materials edited by
A. Trovarelli
Imperial College Press
Catalysis by Ceria and Related Materials
CATALYTIC SCIENCE SERIES Series Editor: Graham J. Hutchings (Cardiff University)
Vol. 1
Environmental Catalysis edited by F. }. }. G. Janssen and R. A. van Santen
Vol. 2
Catalysis by Ceria and Related Materials edited by A. Trovarelli
Vol. 3
Zeolites for Cleaner Technologies edited by M. Guisnet and J.-P. Gilson
Forthcoming: Supported Metals in Catalysis edited by J. A. Anderson and M. F. Garcia
Series Editor: Graham J. Hutchings
Catalysis by Ceria and Related Materials edited by
Alessandro Trovarelli Universita di Udine, Italy
ICP
Imperial College Presi
Published by Imperial College Press 57 Shelton Street Covent Garden London WC2H 9HE Distributed by World Scientific Publishing Co. Pte. Ltd. 5 Toh Tuck Link, Singapore 596224 USA office: 27 Warren Street, Suite 401-402, Hackensack, NJ 07601 UK office: 57 Shelton Street, Covent Garden, London WC2H 9HE
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CATALYSIS BY CERIA AND RELATED MATERIALS Copyright © 2002 by Imperial College Press All rights reserved. This book, or parts thereof, may not be reproduced in any form or by any means, electronic or mechanical, including photocopying, recording or any information storage and retrieval system now known or to be invented, without written permission from the Publisher.
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a mio padre 22 settembre 2001
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PREFACE Rare-earth oxides have been widely investigated as structural and electronic promoters to improve the activity, selectivity and thermal stability of catalysts. The most significant of the oxides of rare-earth elements in industrial catalysis is certainly Ce0 2 . Its use in catalysis has attracted considerable attention in recent years, especially for those applications, such as treatment of emissions, where ceria has shown great potential. This is documented by the increasing number of scientific articles and patents that have appeared on this topic in the last few years. There are also several emerging applications or processes for which cerium oxide is currently being actively investigated. Specifically, Ce0 2 has potential uses for the removal of soot from diesel engine exhaust, for the removal of organics from wastewaters (catalytic wet oxidation), as an additive for combustion catalysts and processes, and in redox and electrochemical reactions. In addition to these applications, much effort has been dedicated recently to studying the role of ceria in well-established industrial processes such as Fluid Catalytic Cracking and ThreeWay-Catalysts, where Ce0 2 is a key component in catalyst formulation. With this book, I have tried to collect reviews of several aspect of the chemistry and catalytic properties of ceria and related materials which in my opinion are relevant in the future development of the field. Catalysis by ceria and related materials enjoyed contributions from industrial, academic and government laboratories from around the world (Austria, Denmark, England, France, Italy, Japan, Spain, the Netherland, U.S.A.) involved in the study of characterization and catalytic properties of ceria and Ce02-containing materials. The first part of the book deals with fundamental characteristics of ceria from the point of view of catalytic applications. After an introductory chapter on production, mining and safety issues, a series of chapters (chp. 2-7) cover structural properties, preparation chemistry and fundamentals of characterization and redox/oxygen storage properties. A specific section (chp. 6) has been dedicated to ceria-zirconia due to the importance this material has in current three-way catalysis for auto-exhaust treatment. To complete the first part there is a chapter on computational studies (chp. 8) and one on the properties of ceria surfaces and films as model for catalytic studies (chp. 9). The second part of the book centers around a few catalytic applications each dealing with important commercial processes involving Ce0 2 in some stages. The use of ceria in auto-exhaust treatment and the role of sulfur in catalyst deactivation is specifically addressed in chapters 10 and 11 for spark ignited engines and chapter 12 for diesel engines. The relevance of ceria in total oxidation catalysis is the main vn
Vlll
Preface
topic of chapter 13 while chapter 14 deals with the use of ceria in removal of organics from wastewaters. A specific chapter (chp. 15) has been dedicated to applications of ceria in electrocatalysis. Other relevant catalytic applications including FCC are reviewed in chapter 16. I hope that although not exhaustive the book will give the reader a glance on the state of the art in this field with the point of view of scientists that have been involved in this field for years. I also think it would be a valuable contribution for those who want to enter this field. I would also like to thank all contributors to the volume and I hope you enjoy reading it. Alessandro Trovarelli University of Udine
CONTENTS Preface
vii
1. Mining, Production, Application and Safety Issues of Ceriumbased Materials
2
Karl Schermanz 1.1. Mining 1.1.1. Rare-Earth Sources and Ceria based Rare-Earth Minerals 1.1.2. Mining and Ore Refining 1.2. Production and Application 1.2.1. Production Processes for Cerium and Cerium Derivatives (Overview) 1.2.2. Cerium Containing Materials and their Commercial Application 1.3. Safety Issues 1.3.1. Toxicological and Ecological Behaviour of Cerium Compounds 1.3.2. Occupational Health 1.4. References 2. Structural Properties and Nonstoichiometric Behavior of C e 0 2 Alessandro
\ \ 2 3 3 6 9 9 \Q 1\ 15
Trovarelli
2.1. Structural Properties
15
2.1.1. The Higher Oxides of Cerium (CeOI714-Ce02) 2.7.2. The Composition Range CeOl714-Ce0ls 2.2. Defect Structure Analysis 2.2.1. Defect Types 2.2.2. The Structure of Defects in Oxygen-deficient Ceria 2.3. Transport Properties 2.3.1. Electrical Conductivity 2.3.2. Oxygen Diffusion in Ceria 2.4. References
17 22 24 24 28 32 33 44 46
3. Synthesis and Modification of Ceria-based Materials Gin-ya Adachi and Toshiyuki Masui IX
51
X
Catalysis by ceria and related materials
3.1. Introduction 3.2. Solid to Solid Synthesis 3.2.1. Ceramic Method 3.2.2. Mechanical Milling 3.3. Liquid to Solid Synthesis 3.3.1. Precursor Method 3.3.2. Precipitation and Coprecipitation Method 3.3.3. Hydrothermal and Solvothermal Synthesis 3.3.4. Sol-gel Method 3.3.5. Surfactant-assisted Method 3.3.6. Emulsion and Microemulsion Method 3.3.7. Flux Method 3.3.8. Electrochemical Methods 3.3.9. Spray Pyrolysis 3.3.10. Impregnation Method 3.4. Gas to Solid Synthesis 3.4.1. Gas Condensation or Sputtering 3.4.2. Chemical Vapor Deposition 3.5. Modification of Bulk and Surface 3.5.1. Effects of Dopants 3.5.2. Structural Modification by Redox Aging 3.5.3. Surface Modification 3.6. References 4. Chemical and Nanostructural Aspects of the Preparation and Characterisation of Ceria and Ceria-Based Mixed Oxide Supported Metal Catalysts
51 52 52 53 54 54 55 59 61 63 65 66 67 68 69 69 69 71 72 72 74 75 76
85
Serafin Bernal, Jose J. Calvino, Jose M. Gatica, Carlos Lopez Cartes and Jose M. Pintado 4.1. Introduction 4.2. Preparation of M/Ce0 2 and Closely Related Catalysts 4.3. Characterisation of M/Ce0 2 and Closely Related Catalysts 4.3.1. Some Challenging Aspects of These Characterisation Studies 4.3.2. Chemical Characterisation ofM/Ce02 and Closely Related Catalysts 4.3.3. Nanostructural Characterisation Studies 4.3.4. The Nature of the Strong Metal/Support Interaction Effects in NM/Ce(M)02.x Catalysts
85 89 95 95 96 123 149
Contents 4.4. References
5. Studies of Ceria-containing Catalysts Using Magnetic Resonance and X-ray Based Spectroscopies
XI
153
169
Jose C. Conesa, Marcos Ferndndez-Garcia and Arturo MartinezArias 5.1. Introduction 5.2. EPR 5.2.1. Ceria-related Spectral Characteristic 5.2.2. Surface Studies Using Oxygen as Probe Molecule 5.2.3. Surface Analysis Using Neutral Radicals as Probe Molecules 5.2.4. Metal-related Signals in Supported Catalysts: Cation Dimers and Redox Studies 5.3. NMR 5.3.1. NMR-active Species in the Solid Catalyst 5.3.2. Adsorbed Species 5.4. XPS 5.4.1. Ceria-specific Spectral Features 5.4.2. Quantitation of Redox States 5.4.3. Studies ofCe Redox Behavior in Catalytic Oxide Materials 5.4.4. Catalysts with Supported Platinum Group Metals 5.4.5. Catalysts with Other Supported Metals 5.4.6. XPS Studies on the Adsorption of Diverse Molecules 5.5. XAFS 5.5.7. Ceria-specific Spectral Features 5.5.2. Methods of Analysis 5.5.3. XAFS Studies of the Cerium Oxide Phase in Catalytic Materials 5.5.4. Studies of the Metal Component in Ceria-containing Catalysts 5.6. References 6. Structural Properties and Thermal Stability of Ceria-Zirconia and Related Materials
169 169 169 173 180 181 183 183 184 186 186 187 190 193 196 198 199 199 201 202 205 207
217
Jan Kaspar and Paolo Fornasiero 6.1. The Ce0 2 -Zr0 2 Phase Diagram
217
Xll
Catalysis by ceria and related materials 6.2. Effects of High Temperature Reducing and Oxidising Treatments 6.2.7. Effects of Oxidising Atmosphere 6.2.2. Effects of Reducing Atmosphere 6.3. Effects of Aliovalent Doping on Thermal and Phase Stability 6.3.1. Doped Ce02 Materials 6.3.2. Doped Ce02-Zr02 Materials 6.4. Effects of Addition of A1203 to Ce0 2 -Zr0 2 Mixed Oxides 6.5. References
7. Oxygen Storage/Redox Capacity and Related Phenomena on Ceria-Based Catalysts
224 224 228 230 230 232 234 236
243
Daniel Duprez and Claude Descorme 7.1. Introduction 7.2. Oxygen Storage Capacity Measurements 7.2.1. OSC Measurements at Low Frequency 7.2.2. OSC Measurements at High Frequency 7.2.3. Oxygen Buffering Capacity 7.3. Elementary Steps Involved in OSC Processes 7.3.1. Inventory of Elementary Steps 7.3.2. Oxygen Activation and Equilibration 7.3.3. Oxygen Species Involved in OSC 7.3.4. Oxygen Diffusion 7.4. OSC and Catalysis 7.4.1. Effect of Additives and Poisons on OSC 7.4.2. Role of OSC in Catalytic Reactions 7.4.3. On-Board Diagnostic 7.5. References
243 243 244 252 256 257 257 262 264 266 267 267 268 273 276
8. Computer Simulation Studies of Ceria-based Oxides
281
M. Saiful Islam and Gabriele Balducci 8.1. Introduction 8.2. Computational Techniques 8.3. Bulk Defect Chemistry 8.3.1. Ce4+/Ce3* Reduction Energetics 8.3.2. Defect Clustering 8.4. Oxygen Ion Migration
281 282 285 285 287 290
Contents 8.5. Surface Properties 8.5.1. Surface Structures ofCe02 8.5.2. Surface Structures of Ce02-Zr02 8.5.3. Surface Redox Behavior ofCe02 and Cel.^,rJD2 8.5.4. Surface Segregation of Oxygen Vacancies and Metal Ions 8.6. Conclusions 8.7. References
xiu 295 295 297 300 303 305 306
9. Ceria Surfaces and Films for Model Catalytic Studies Using Surface Analysis Techniques
311
Steven H. Overbury and David R. Mullins 9.1. Introduction and Scope 9.2. Techniques for Preparation of Ceria Films and Model Catalysts 9.2.1. Preparation of Model Ceria Supports 9.2.2. Preparation of Model Ceria Supported Catalysts 9.3. Structure of Ceria Surfaces 9.3.1. Theoretical Studies of Structure and Defects on Clean Ceria Surfaces 9.3.2. Experimental Studies of Surface Structure 9.4. Chemisorption Studies on Clean Ceria Surfaces 9.4.1. CO and C02 9.4.2. NO, N20 and N02 9.4.3. 02 9.4.4. H20, H2and-OH 9.4.5. S02 9.4.6. Alcohols and Carboxylic Acids 9.4.7. Hydrocarbons 9.5. Reducibililty of Ceria Surfaces 9.6. Studies of Chemisorption on Metal Loaded Ceria Surfaces 9.6.1. CO 9.6.2. NO, N02 9.6.3. C2H4 9.7. Coadsorption and Reaction Studies on Ceria Model Catalysts 9.7.1. Coadsorption of CO and NO 9.7.2. Coadsorption of C2H4 and NO 9.7.3. Coadsorption of CO and Water 9.7.4. Reactor Studies 9.8. Overview
311 313 313 317 318 318 320 322 323 324 325 325 326 327 328 328 330 331 332 334 334 334 336 336 336 337
Catalysis by ceria and related materials
XIV
9.9. References
10.
Ceria and Other Oxygen Storage Components in Automotive Catalysts
338
343
Mordecai Shelef, George W. Graham and Robert W. McCabe 10.1. Origin and Evolution of "Oxygen Storage" in Automotive Catalysts 10.2. Interaction of Ceria with the Active Noble Metals 10.3. Deactivation of Oxygen Storage 10.3.1. Thermal Deactivation 10.3.2. Chemical Deactivation 10.4. Other Materials Providing Oxygen Storage 10.5. Special Uses of Oxygen Storage 10.5.1. Role of Ceria in On-board Catalyst Diagnostics 10.5.2. Light-off Enhancement ofTWCs and Hydrocarbon Traps 10.6. Oxygen Storage and NOx Traps 10.7. Outlook 10.8. References 11. S0 2 Poisoning of Ceria-Supported, Metal Catalysts
343 348 350 350 357 363 365 365 368 371 372 374 377
Raymond J. Gorte and Tian Luo 11.1. Introduction 11.2. Effect of S0 2 on Catalytic Activity 11.2.1. Oxygen Storage Capacity 11.2.2. Steady-State Reactions 11.3. Chemistry of S0 2 Poisoning of Ceria 11.3.1. Thermodynamic Considerations 11.3.2. Surface Investigations 11.4. Lessons from Catalysts for S0 2 Reduction 11.5. Future Directions 11.6. Subject Index 11.7. References
377 378 378 380 381 381 382 386 386 387 387
12. Cerium and Platinum Based Diesel Fuel Additives in the Diesel Soot Abatement Technology 391
Contents
xv
Michiel Makkee, Sytse J. Jelles and Jacob A. Moulijn 12.1. Introduction 12.2. Experimental 12.2.1. Engine Experiments 12.2.2. Flow-reactor Experiments 12.3. Results 12.3.1. Engine Experiments 12.3.2. Flow-reactor Experiments 12.4. Discussion 12.4.1. Function of Platinum 12.4.2. Performance of the Base Metal 12.4.3. Reaction Network 12.4.4. NOx Reduction 12.5. Conclusions 12.6. References 13. Fundamentals and Applications of Ceria in Combustion Reactions
391 394 394 397 397 397 399 401 401 402 403 403 405 405 407
Michel Primet and Edouard Garbowski 13.1. Introduction 13.1.1. Catalytic Combustion 13.1.2. Ceria Structure 13.1.3. Ceria in Catalysis 13.1.4. Ceria and Total Oxidation 13.2. Catalytic combustion on Ceria 13.2.1. CO Oxidation 13.2.2. Oxidation of Hydrocarbons 13.2.3. H2 Oxidation 13.3. Catalytic Combustion on Ceria Containing Oxides 13.3.1. Ceria Associated with Zirconia 13.3.2. Ceria Associated with a Support 13.3.3. Ceria Associated with Transition Metal Oxides 13.4. Ceria Associated with Noble Metals 13.4.1. Silver Associated with Ceria 13.4.2. Rhodium Associated with Ceria 13.4.3. Platinum Associated with Ceria 13.4.4. Palladium Associated with Ceria 13.4.5. Conclusion on Noble Metals Associated with Ceria 13.5. The Future of Ceria in Catalytic Combustion
407 407 408 409 409 410 410 410 412 412 All 414 415 416 417 All All 418 420 421
Catalysis by ceria and related materials
XVI
13.6. Conclusions 13.7. References 14. Ceria-based Wet-Oxidation Catalysts
423 425 431
Seiichiro Imamura 14.1. Introduction — Background of Wet-Oxidation 14.2. Catalysts 14.2.1. Mn/Ce Composite Oxide 14.2.2. Modification of Mn/Ce Composite Catalyst 14.2.3. Ceria-promoted Precious Metal Catalysts 14.2.4. Other Ceria Based Catalysts 14.3. Summary — Role of the Catalysts 14.4. References 15. Ceria-based Electrodes
431 432 432 437 439 445 446 449 453
Mogens Mogensen 15.1. Background 15.2. The Chemistry of Ceria 15.2.1. Types of Defects and Reactions 15.2.2. Thermodynamic Properties 15.2.3. Lattice Parameters of Pure, Doped and Reduced Ceria 15.3. Electrical Conductivity 15.3.1. Electronic Conductivity 15.3.2. Ionic Conductivity 15.4. Ceria Based Fuel Electrodes for SOFC 15.4.1. Hydrogen/Ceria Electrodes 15.4.2. Oxidation of Hydrocarbons on Ceria Based Electrodes 15.5. References
453 455 455 457 462 466 466 468 471 472 413 476
16. The Use of Ceria in FCC, Dehydrogenation and Other Catalytic Applications
483
Marta Boaro, Alessandro Trovarelli, Carla de Leitenburg and Giuliano Dolcetti 16.1. Introduction 16.2. Treatment of SOx
483 484
Contents
xvn
16.2.1. Fluid Catalytic Cracking (FCC) 16.2.2. de-SOx de-NOx Processes 16.3. Ethylbenzene Dehydrogenation 16.4. Other Catalytic Reactions 6.4.1. Environmental Applications 6.4.2. Syn-gas Production 16.5. Conclusions 16.6. References
485 487 490 493 493 493 497 497
Index
501
CHAPTER 1 MINING, PRODUCTION, APPLICATION AND SAFETY ISSUES OF CERIUM-BASED MATERIALS
KARL SCHERMANZ R&D Rare Earths, Treibacher Auermet Produktionsges. m.b.H., A-9330 Althofen, Austria; e-mail: karlschermanz©treibacher.at
1.1.
Mining
1.1.1. Rare-Earth Sources and Ceria based Rare-Earth Minerals Under the classification of rare-earths, there are 15 lanthanide elements and the 2 elements appearing above lanthanum in the periodic table, scandium and yttrium [1]. The lanthanides are divided into two groups: the first four elements are referred to as the eerie or light rare-earths, while the remaining are called the yttric or heavy rareearths [2]. Rare-earth minerals occur in a variety of geologic environments. Concentrations exist in igenous, sedimentary and metamorphic rocks. The rare-earths are constituents in over 160 of minerals [3], but only a few are recovered for commercial production. Bastnasite, Monazite, Loparite, Xenotime and 'Rare-earth bearing Clay' are the major sources of the world's rare-earth supply. Bastnasite, Monazite and Loparite are considered to be the principle cerium ores (Table 1.1). Bastnasite, a rare-earth fluorcarbonate mineral, forms as an igenous or hydrothermal mineral and occurs as an accessory mineral in several large deposits. The most important deposits containing bastnasite as a high grade accessory mineral are at Mountain Pass, California, United States and Baiyunebo, Nei Mongolia, Autonomous Region, China. While the Mountain Pass bastnasite is believed to have formed by a coprecipitation process the Baiyunebo's bastnasite is considered to be hydrothermal in origin and formed by alteration of dolomite in the presence of large amounts of flourine [5]. Monazite, a rare-earth phosphate, is one of the most abundant rare-earth minerals. It occurs as an accessory mineral in granitic and metamorphic rocks, pegmatites, vein deposits, as a dendrital mineral in placer deposits and as a 1
2
Catalysis by ceria and related
materials
hydrothermal and supergene mineral in carbonatites. It is classified as a lightlanthanide mineral and is usually enriched in cerium [6]. It contains also significant amounts of Thorium which together with other trace elements causes radioactivity of the mineral. Monazite is very often associated with other heavy minerals such as ilmenite, zircon and rutile. Those other minerals are usually the economioc driving force for exploiting the deposits and hence monazite is almost always derived as a by-product of the production of titanium- and zirconium-containing minerals. Several countries supply monazite, or monazite derivatives, onto the world market. Extensive deposits along the coast of Western Australia are processed for ilmenite and are the major source of world monazite. Other regions of Australia, along with India, China and Brazil also supply the mineral [7]. Table 1.1. Distribution of rare-earth elements in commercial used rare-earth minerals in % [4] Bastnasite
Bastnasite
Monazite
Loparite
Xenotime
Clay Y-low
Clay Y-rich
(China)
(USA)
(Australia)
(Russia)
(Malaysia)
(China)
(China)
La20,
27,2
32,3
23,9
25,0
1,3
29,8
2,2
Ce0 2
48,7
49,2
46,0
50,5
3,2
7,2
1,1
Pr 6 O u
5,1
4,5
5,1
5,0
0,5
7,1
Nd 2 0,
16,6
12,0
17,4
15
1,6
30,2
1,1 3,4
Y203
0,3
0,1
2,4
1,3
61,9
10,1
64,1
Others
rest
rest
rest
rest
rest
rest
rest
Phosphate containing rock in certain areas contains a few-percent of lanthanides, e. g. the apatite deposits in the Kola peninsula in the Commonwealth of Independent States (C.I.S.). Loparite, a Nb-mineral containing rare-earths is also present and is the leading source of rare-earths for the C I S . [8].
1.1.2. Mining and Ore Refining Bastnasite is mined from hard rock deposits. Production in China is a by-product of iron ore mining while U.S. production is solely for rare-earths. Ore is recovered by drilling and blasting. The ore is crushed, ground and subjected to flotation. The bastnasite fraction is floated off and thereby seperated from other minerals to produce a concentrate. Bastnasite can be converted directly, without separating individual rareearths, to other derivatives such as sulphate or chloride by dissolution in acid. The following step to crack the concentrate for further processing used in the U.S. is to roast in air and then to leach with HCl. This produces an insoluble cerium rich
Mining, production and safety issues of Ce-based materials
3
fraction (cerium concentrate) and a soluble cerium depleted (lanthanum rich) fraction (lanthanum concentrate). An alternative process for cracking bastnasite concentrate is used in China. The concentrate is roasted with sulfuric acid followed by an aquous leach to produce a solution containing the full natural ratio of the rare-earth elements. The rare-earths are then precipitated as sulfates or hydroxides which are converted into chlorides by hydrochloric acid treatment. After removing valuable heavies (Sm and beyond) the initial cerium-containing product will be a lightlanthanide (La, Ce, Pr and Nd) rare-earth chloride. Monazite concentrate is processed either with sulfuric acid, like bastnasite, to produce a mixture of sulfates but the usual process is an alkaline treatment. The alkali process is preferred since it removes the phosphates more readily [9]. Whichever method is chosen the radioactive thorium must be completely removed. After benefication the monazite concentrate is finely ground and reacted with a hot concentrated sodium hydroxide at 140° to 150°C. Insoluble hydroxides of the rareearths and thorium are formed while trisodium phosphate and excess sodium hydroxide remain in solution. The next step is hydrochloric acid attack on the solids portion. The thorium remains insoluble and a crude thorium hydroxide can be filtered off. Trace contaminants that do carry through into solution, such as uranium and lead, as well as some thorium, are removed by coprecipitation with barium sulphate in a deactivation step. The cerium-containing product will be a rare-earth chloride differing only marginally in the proportions of the various rare- earths present, to the analogous rare-earth chloride produced from bastnasite. Loparite is decomposed in hot concentrated sulfuric acid and addition of ammonium sulfate. The rare-earths and thorium separate as double sulfates and are removed by filtration. The remaining solution of sulfates contains titanium, niobium and tantalum and is removed for separate processing. The double sulfates of rare-earths and thorium are converted to carbonates followed by dissolution in acid. Thorium is seperated by precipitation when the alkalinity of the solution is raised by the addition of sodium- or ammonium hydroxide.
1.2. Production and Application 1.2.1. Production Processes for Cerium and Cerium Derivatives (Overview) Cerium is characterized chemically by having two stable valence states, Ce(IV) and Ce(ffl). This property is used in several production processes for the recovery of
4
Catalysis by ceria and related materials
cerium. Commercial production of cerium is by solvent (liquid-liquid) extraction (SX), selelective precipitation and ion exchange (IX). Whereas the fractional precipitation process will yield solid cerium-compounds the liquid-liquid extraction and ion exchange process will produce solutions of cerium. The cerium will be isolated usually by precipitation as an oxalate, carbonate or hydroxide. Cerium-oxalate, -carbonate and -hydroxide are considered to be the most important precursors for cerium-derivatives on a commercial scale. The cerium derivatives are yielded from these compounds by additional chemical and/or physical treatment. For example, cerium oxide may be formed easily by calcining cerium carbonate or/and cerium oxalate respectively.
1.2.1.1. Liquid-Liquid (Solvent) Extraction SX Liquid-liquid (Solvent) extraction (SX) is the most widely used commercial process for the separation of rare-earths. Once the starting precursor has been prepared, this separation technology is independent of starting mineral and different feedstocks can ultimately be processed by the same separation routines and equipment. Solvent extraction has the advantages of continous multiple-stage separation and it is easy to automate. One of the disadvantages of the SX technique is that it is not easy to modify the separation schemes for the production of higher purity products. A further disadvantage is that operating costs for small capacity units are still high [10]. The extraction procedure rely on the differential partitioning of metal soluble complexes between immiscible aqueous and organic phases. Mixing the two solutions and separation of the aqueous and organic phase leads to an equilibrium of distribution of the rare-earths between the two phases. The elements with the highest affinity for the chelating agent are enriched in the solvent. The degree of separation is maximized by optimization of operating conditions by linking of many SX cells. The aqueous feed flows one way while the organic flows the other. In commercial practice up to 100 cells or even more will form a circuit. In industrial processes organic phosphates e.g. tri-butyl-phosphate (TBP), carboxylic compounds like versatic acid, and phosphoric acids, e.g. di-2-ethyl-hexylphosphoric acid (DEHPA), are used as extractants or complexing agents. For the aqueous phase strongly acidic solutions, usually nitric- or hydrochloric acid are used, and for the organic phase commercial aliphatic or aromatic solvents are applied. Passing a mixed rare-earthand Y-feedstock through an SX circuit will result in a cut into two fractions. A
Mining, production and safety issues of Ce-based materials
5
single element could be cut off from one end of the group or the mixed feedstock can be split into two fractions, each containing several elements. The cut will depend on economics and on the demand for the mixture or the single element. A fraction of Ce, La, Nd and Pr derived from bastnasite or monazite is a typical feedstock in the recovery process of cerium on a commercial scale. Separation of the rare-earth elements may be achieved by splitting the mixed rare-earth elements into a cerium/lanthanum and didymium (Nd/Pr) fraction first. The cerium/lanthanum fraction may be used as a further feedstock in a second extraction stage and will yield high pure cerium and lanthanum solution respectively. Cerium can then be precipitated as, for example, an oxalate or a carbonate which may be used as precursor for cerium derivatives.
1.2.1.2. Selective Precipitation Separations by selective precipitation depend primarily upon basicity differences. These differences can only operate when equilibrium between the solid phase and the solution is complete. Cerium is seperated commercially based on its reduced basic property in the tetravalent state. By adjusting the pH of a mixed rare-earth solution the cerium may be selectively precipitated out as a cerium(IV). In mixed rare-earth solutions the rare-earths are present in the trivalent state. To precipitate the cerium, cerium(III) must be converted into cerium(IV) by an oxidizing agent, e.g. hydrogenperoxide. The more soluble trivalent rare-earths are dissolved causing concentration of the less soluble cerium(IV). Selective hydrolysis is another way of concentrating cerium(IV). Hydrolysis to a basic nitrate or sulfate is effected by diluting and boiling a concentrated solution containing the trivalent rare-earths and cerium(IV). A high purity cerium is obtained, but a complete separation cannot be achieved by this method.
1.2.1.3. Ion Exchange (IX) Ion exchange techniques are used for the separation of relatively small amounts of high pure rare-earths. In this process, a rare-earth ion, R3+ in solution, exchanges with ions on a solid ion exchanger, a natural zeolite or a synthetic resin, which is normally called the resin [11]. Cerium is separated from the other rare earths based on differences in adsorption and selective elution. In a typical process, thorium free rare-earth solutions are run
6
Catalysis by ceria and related materials
through multiple columns of ion exchange resins. The resins are designed to have an affinity for rare-earth ions. Typical resins used are sulfonic polymer beads containing monovalent hydrogen or ammonium cations on the resins. After loading, the resins contain a mixed distribution of rare-earths and unabsorbed ions are flushed from the column. Removal of cerium and other rare-earths from the loading resins (elution) is effectuated by an organic eluting agent, such as hydrogen EDTA, ammonium EDTA, or other hydrogen-bearing eluting agents [12]. EDTA agents form complexes with the rare-earths to form a mixed rare-earth EDTA solution which is feeded to a separation column. Partial separation already occurs in the loading column as the heavy rare-earths have a greater affinity for EDTA than the light rare-earths. In the separation column the rare-earth EDTA solution interacts with copper ion resins. The rare-earth ions travel along the column and emerge in the order of their association with the eluent. This method can produce 99,999% to 99,9999% pure rare-earths but suffers from the length of time it takes to purify a given amount.
1.2.2. Cerium Containing Materials and their Commercial Application The applications of ceria based materials are related to a potential redox chemistry involving Cerium(III) and Cerium(IV) , high affinity of the element for oxygen and sulfur, and absorption / excitation energy bands associated with its electronic structure. Important areas for application of cerium based materials are catalysis and chemicals, glass and ceramics, phosphors and metallurgy.
1.2.2.1. Application of Cerium in Catalysis and Chemicals Huge amounts of catalyst are consumed for refinery operations to convert crude oil into lower molecular-weight fractions (fluid catalytic cracking). Many of the catalyst compositions available contain lanthanides including cerium [13]. A major technological application of steadily growing importance for cerium is the vehicle emission control to remove pollutants from vehicle (auto-exhaust) emissions [14]. This market currently consumes a significant portion of the annually production of cerium derivatives. The ability of cerium oxide to act as an oxidizing agent underlies the potential use of various cerium derivatives as additives to aid combustion. In order to reduce the particle emissions in 'Diesel' cerium containing compounds are used as additives in this application [15].
Mining, production and safety issues of Ce-based materials The control of sulfur oxide emissions is becoming more important. Several catalyst additives containing cerium and/or lanthanides can act as the SOX control agent [16]. Cerium also has minor uses in other commercial catalysts [17]. The dominant catalyst for the production of styrene from ethylbenzene is an alkali-promoted ironoxide based material. The addition of a few percent of cerium oxide to this system improves activity for styrene formation. The ammoxidation of propylene to produce acrylonitrile is carried out over catalytically active complex molybdates. Cerium, a component of several patented compositions [18], supports the chemical reaction. Cerium Fluoride can be used as an additive to lubricant formulations to improve extreme-pressure[19]. In addition, this fluoride has been proposed as a high temperature lubricant. Metal catalysts, included in paint formulations promote drying. Cerium carboxylates are used as 'through' driers, i.e. to promote drying in the body of the paint film [20]. Cerium carboxylates[21], such as cerium octanoate, are needed to improve properties of silicone polymers. Comparable beneficial effects can be seen by incorporating into polymeric silicones the cerium as oxide. Cerium(IV) compounds with suitable reducing agents, readily initiate the redox polymerization of, for example, vinyl monomers [22]. This property is used to initiate graft polymerization of vinyl monomers onto cellulose, wool, starch, cotton, etc. in order to, e.g. improve mechanical strength, resist moisture penetration and reduce micro-organism attack.
1.2.2.2. Application of Cerium in Glass and Ceramics Cerium oxide is the most efficient polishing agent for most glass compositions [23]. This application consumes a significant portion of the cerium products produced annually. A major use for cerium compounds is the decolorization of glass. Glass can be decolorized by addition of Ce(IV) to the glass melt. Economical additions of cerium, as cerium concentrate or pure cerium oxide convert iron to the low-absorption Fe(H) form [24]. Most damage caused by light to exposed materials is due to u.v. radiation. Cerium(IV) in particular makes glass opaque to near u.v. radiation but shows no absorption in the visible and cerium (111) also shows u.v. absorbing behavior but somewhat less marked than for cerium(IV)[25]. The photostability of pigments can be enhanced by addition of cerium. It provides pigments with light
7
8
Catalysis by ceria and related materials
fastness and prevents clear polymers from darkening in sunlight. Television glass faceplates are subjected to electron bombardment which tends to cause browning of the glass due to the creation of color centers. This effect is suppressed by the addition of cerium oxide. When used in glass compositions (at a low weight percentages) along with comparable amounts of titanium oxide, cerium oxide produces a deep yellow coloration [26]. Rare-earth sulfides, among them also cerium are used in glass and ceramics as colorants to replace toxic CdS [27]. Cerium oxide has a high refractive index, and is an opacifying agent in enamel compositions [28] used as protective coatings on metals. The addition of cerium oxide, for example, to zirconia produces a material with exceptional toughness and good strength [29]. Cerium oxide-doped zirconia is used also in thermal barrier spray coatings on metal surfaces[30].
1.2.2.3. Application of Cerium as Phosphors Cerium is an essential component in several of the new generation of phosphors in tricolor lamps that have made possible more efficient and more compact fluorescent lighting [31]. The cerium atom, upon excitation by energetic cathode-ray electrons, produces a characteristic emission (luminescence) [32]. This property underlies the use of some cerium containing phosphors in specialized CRT applications [33].
1.2.2.4. Application of Cerium in Metallurgy Traditionally the item most widely associated with cerium has probably been the pyrophoric iron-mischmetal alloy for lighter flints, still in use. Mischmetal is to be termed the mixture of metals of the light lanthanides La, Ce, Pr and Nd. The high affinity of cerium for oxygen and sulfur underlies the use of ceriumcontaining ferro-alloys to improve the physical properties of highstrength low-alloy (HSLA) steels [34]. In the iron casting process cerium is considered to remove free oxygen and sulfur from the melt [35]. Several commercial alloys use micro-additions of (pure) cerium to the alloy to significantly improve this oxidation resistance, provide creep resistance and confer a longer operating life [36].
Mining, production and safety issues of Ce-based materials
9
In electrolysis self-forming anode technology is used whereby cerium oxide coatings are deposited onto conducting ceramic substrates [37]. Cerium oxide, provides an alternative to thorium oxide, a common additive in welding electrodes that is now being phased out for environmental reasons [38].
1.3. Safety Issues 1,3.1. Toxicological and Ecological Behaviour of Cerium Compounds 1.3.1.1. Toxicological Behaviour of Cerium Compounds Compounds of cerium and the other rare-earth elements are basically of low toxicity [39], see Table 1.2. Table 1.2. Acute Toxicity data of Cerium compounds Compound
type of test
Cerium Oxide
Lethal dose, 50% killed Lethal dose, 50% killed Lethal dose, 50% killed Lethal dose, 50% killed Lethal dose, 50% killed Lethal dose, 50% killed Lethal dose, 50% killed Lethal dose, 50% killed Lethal dose, 50% killed Lethal dose, 50% killed
Bastnasite Cerium Fluoride Cerium carbonate (aq) Cerium Nitrate
Cerium Chloride
route of exposure oral
species observed Rodent-rat
dose
ref.
> 5000 mg / kg
[40]
oral
Rodent-rat
> 5000 mg / kg
[40]
oral
Rodent-rat
> 5000 mg / kg
[40]
oral
Rodent-rat
> 5000 mg / kg
[40]
oral
Rodent-rat
3154 mg / kg
[41]
Intraperiton.
Rodent-rat
216mg/kg
[41]
intravenous
Rodent-rat
37 mg / kg
[41]
oral
Rodent-rat
2111mg/kg
[41]
subcutan
Rodent-rat
4000 mg / kg
[42]
intravenous
Rodent-rat
6,3 mg / kg
[41]
The water soluble salts are more likely to cause systemic effects when ingested. If the rare-earths are administered orally, only a small fraction of the rare-earths are absorbed by the intestines. This probably accounts for the low toxicity of the rareearths when taken orally. Solubility is a critical aspect of the oral bioavailability of
10
Catalysis by ceria and related materials
the material and the nature of the anion is often the important determinant of a material's toxicity. When vapors or dust are inhaled, the rare-earths are considered to be more toxic but tend to remain in the lungs and are only slowly absorbed into the body. If injected subcutanously, most of the injected material remains in place. The amount that is absorbed tends to collect in the liver, spleen and kidneys. By far the most toxic reactions are obtained if the rare-earths are introduced by means of intraperitoneal or intravenous injections. Large doses to experimental animals have caused writhing, loss of muscle coordination, laboured respiration, sedation, hypotension, and dead by cardiovascular collapse [39].
1.3.1.2. Ecological Behaviour of Cerium Compounds The lanthanides can enter the environment by leaching of lanthanide containing minerals into the ground water as well as by the release through crustal weathering into the atmosphere. In addition industrial operations, particularly refineries and automobiles, can also be sources.[43] In the soil the lanthanides are immobile under a wide variety of pH conditions, due to the low solubility of salts such as carbonates and phosphates. Concentrations in ground water are much lower than those of the soil through which the water percolates. In most natural waters, because the lanthanides sorb strongly to silicates and humic material, the bulk of the Ln content including cerium is associated with such colloidal particulates [44]. In the marine environment a depletion of cerium relative to the other lanthanides is found that is attributed to the oxidation of cerium (III) to highly insoluble Ce(IV) (OH)4-type species.
1.3.2. Occupational Health Some of the literature on the toxicological behavior of lanthanides has arisen because of the concern about exposure to radioactivity. Radiation health scientists generally believe that any dose of radiation, however small, carries with it an increased risk of some adverse health effect, such as cancer. This does not mean that everyone who receives an exposure will suffer an effect. It means the risk of a radiation-induced health problem is increased. Even if a particular effect does occur in an individual, it is not possible, to determine with current scientific methods that it was caused by radiation exposure [45].
Mining, production and safety issues of Ce-based materials In the past inadequate separation had produced Th-contaminated rare-earth products. Progressive lung retention was observed after inhalation of dust containing rare-earth oxides and derivatives. The damage of the lung did not seem to be attributed to the rare-earths, but to thorium and its disintegration products [46]. Continual progress in rare-earth processing has reduced the radioactive impurities in rare-earth products substantially, so they are practically free of radioactivity today. Current processing technology for mineral recovery and for the subsequent lanthanide separation results in products that meet all regulatory requirements.
1.4. 1. 2. 3.
4. 5.
6.
7. 8. 9. 10. 11. 12.
References Hedrick, J. B., The American Ceramic Society Bulletin, 74, August 2000. Bounds O. Ch., "The Rare Earths: Enablers of Modern Living", JOM, October 1998. Gschneider, Jr. K. A., Fine Chemical for the Electronics Industry II: Chemical Applications for the 1990's; Ando, D. J. and Pellatt, M. G. eds., Royal Society of Chemistry (1991). Yan, J., Rare Earth Market - Challenge and Opportunity, China Rare Earth Information, Vol. 6, Dec 2000. Drew L., Quingrun M., Weijun S., The Geology of the Byan Obo Iron-Rare Earth-Niobium Deposits, Inner Mongolia, China; U. S. Geol. Surv., Reston, VA, 1990. Mariano A. N., Economic Geology of Rare Earth Elements. Chap. 11 in Geochemistry and Mineralogy of Rare Earth Elements, Mineralogical Soc. of America, Review in Mineralogy, 21 (1990) 309 - 327. Hedrick, J. B., Templeton D. A., Rare Earth Minerals and Metals 1989, BuMines Minerals Yearbook, May 1991, 14 -15. Habashi, F., "The Discovery and Industrialization of the Rare Earths", CIM Bulletin, Jan - Feb. 1994. Narayanan N. S. et al, Processing of Monazite at the Rare Earths Division, Udyogamandal (India), Mater. Sci. Forum, 30 (1988), 45. The Economics of Rare Earths & Yttrium, Roskill Information Services, Tenth Edition 1998, p. 19. Gschneider, Jr. K. A., Speciality Inorganic Chemicals; Thomson, R. ed., Royal Society of Chemistry (1981). Sinha S. P., Complexes of the Rare Earths, Pergamon Press, 1966, pp. 66 - 79
\\
12
Catalysis by ceria and related
materials
13. Wachtere W., Nguyen V., U. S. Patent 6,022,471, 8 February 2000. 14. Funabiki M. et al, Catal. Today, 10 (1991), 33; Wu J. et al, Pat. WO 98/13139. 15. Mouraoand A. M„ Falst C. H, U. S. Patent 4,522,631, 11 June 1985. 16. Kim, G., U. S. Patent 5,627,123, 6 May 1995. 17. Kilbourn B. T., J. Less Common Metals., 126 (1986), 101. 18. Brazdil J. F and Graselli R. K., /. Catal, 79 (1983), 104. 19. Dumdum J. M. et al.,A New Solid Lubricant Additive for Greases, Pastes and Suspensions, paper from Annual Meeting Nat. Lubric. Grease Inst., 1983, Oct. 23 - 26, Kansas City, Mo. 20. Ducros P., /. Less Common Metals, 11 (1985) 37. 21. Heidingsfeldova et al, Kautsch. Gummi Kunstst., 37(8) (1984), 694. 22. McDowall D. J. et al, Prog. Polym. Sci., 10 (1984), 1. 23. Khaladji J., Peltier M., Rare Earth Polishing Compositions, U. S. Patent 4,942,691, 24 July 1990. 24. SchuttT.C, Ceram. Bull, 51(2) (1972), 155. 25. Arbuzov V. I. and Belyankina N. B, Phys. Chem. Glasses (Eng. transl. of Fiz. Khim. Stekla), 16(4) (1990), 317. 26. Xu S. et al, J. Non Cryst. Solids, 112 (1989), 186. 27. Chopin T., Dupuis D., Rare Earth Metal Sulfide Pigment Compositions, U. S. Patent 5,401,309, 28 March 1995. 28. Nedeljkovic A. I. and Cook R. L., The Vitreous Enameller, 26(1-2) (1975), 2. 29. Tsukuma K., Am. Ceram. Soc. Bull, 65(10) (1986), 1386. 30. Holmes J. W. and Pilsner B. H, Proc. Natl. Therm. Spray Conf. 1987, (publ. 1988 ASM), 259. 31. Smets B. M. J., Mat. Chem. Ph s., 16 (1987), 283. 32. Blasse G. and Bril A., /. Chem. Phys., 47 (1967), 5139. 33. Bril A. et al, Philips Tech. Rev., 332 (1971), 125. 34. Waudby P. E., Int. Metals Rev., 2 (1978), 74. 35. Linebarger H. F. et al, The Role of the Rare Earth Elements in the Production of Nodular Iron, in Am.Chem.Soc. Symposium series 164, "Industrial Applications of the Rare Earths" ed. Gschneidner K. A., publ. 1981, 20 36. Cosandey F., Met. Trans., 14A (1983), 611. 37. Walker J. K. et al, J. Appl. Electrochem., 19 (1989), 225; Duruz J-J. et al., U. S. Patents 4,948,676; 4,960,494; 4,614,569. 38. Sadek A. A. et al, Met. Trans. A, 21A (1990), 3221. 39. Sax N. I. and Lewis R. J., Dangerous Properties of Industrial Materials, 7 th ed., Vol. II, 743.
Mining, production and safety issues of Ce-based materials
13
40. Liebert M. A., Journal of the American College of Toxicology, 12 (1993) 617. 41. Environmental Quality and safety, Supplement (Stuttgart, Fed. Rep. Ger.), Vol. 1, 1975. 42. Archiv fiir Experimentelle Pathologie und Pharmakologie (Leipzig, Ger. Dem. Rep.), 100 (1923), 230. 43. Gomez I. and Gordon G. G., Science 229 (1985), 966; Kitto M .E.et al., Environ. Sci. TechnoL, (1990) August. 44. Choppin G. R., Eur. J. Solid State Inorg. Chem., 28 (1991), 319. 45. State Government Information and Services of Washington, US http://www.doh.wa.gov/hanford/publications/health/monlO.htm 46. Haley P. J., Health Physics, 61(6) (1991), 809.
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CHAPTER 2 STRUCTURAL PROPERTIES AND NONSTOICHIOMETRIC BEHAVIOR OF C e 0 2
ALESSANDRO TROVARELLI Dipartimento di Scienze e Tecnologie Chimiche, Universita di Udine, via Cotonificio 108, 33100 Udine, Italy; e-mail:
[email protected]
2.1. Structural Properties Cerium with a 4f25d°6s2 electron configuration can exhibit both the +3 and +4 oxidation states, and intermediate oxides whose composition is in the range Ce203Ce0 2 can be formed. Thermodynamic data indicate that cerium metal is unstable in the presence of oxygen and that Ce 2 0 3 and Ce0 2 are easily formed1 (Table 2.1). The final stoichiometry is strongly dependent on temperature and oxygen pressure. For example, cerium metal reacts easily with oxygen to form the sesquioxide at about 10"93 atm of oxygen at 573K. Ce 2 0 3 is also unstable toward oxidation and is oxidised as pressure increases to the homologous series Cen02n.2m up to 10"40 atm of oxygen, when Ce0 2 starts to form. Similar behaviour is observed in the corresponding TbOx0 2 , PrO x -0 2 systems, although the pressure at which these dioxides are formed is considerably higher (103 atm of oxygen for Tb0 2 and 0.17 for Pr0 2 at 573K) and the stable forms of Pr and Tb under ambient conditions are, respectively, PrO, 83 and Tb0 179 2 . The dioxide Ce0 2 crystallises in the fluorite structure, which is named after the mineral form of calcium fluoride. It has a face-centred cubic unit cell (f.c.c.) with space group ¥m3m, (a=0.541134(12)nm, JCPDS 34-394). In this structure, each cerium cation is coordinated by eight equivalent nearest-neighbour oxygen anions at the corner of a cube, each anion being tetrahedrally coordinated by four cations. The structure, which is illustrated in Fig. 2.1, can be thought of as a ccp array of cerium ions with oxygens occupying all the tetrahedral holes. Extending this structure by drawing cubes of oxygen ions at each corner reveals the eightfold cubic coordination of each cerium, which alternately occupy the centre of the cube. It is therefore also possible to move the origin and redraw the elementary cell as a primitive cubic array of oxygen ions, Fig. 2.1 (b,c), in which the eight coordination sites are alternately empty and occupied by a cation. This clearly shows that there are large vacant 15
Catalysis by ceria and related materials
16
octahedral holes in the structure, a feature which will be significant when we consider the movement of ions through the defect structure in Sec. 2.3, Table 2.1. Some thermodynamic properties of Ce, Pr and Tb oxides1.
M Ce Pr Tb
AH°2<)!i(kJ/mol) -1089 -958 -972
2M + 1.502 = M 2 0 3
Cea Prb Tbb
-1796 -1810 -1865
-1708 -1735 -1776
152 159 157
MO L5 + 0.25O2 = M0 2
Ce Pr Tb
-191 -44 -39
-172 -31.5 -
-
reaction M + 02 = M02
AG°,qs (kJ/mol) S°?qs (J/Kmol) -1025 61.5 79.9 -899 82.8 -
a: A-type sesquioxide; b: C-type sesquioxide.
(b)^
@ cerium ® oxygen
Figure 2.1. The crystal structure of CeOz: (a) unit cell as a ccp array of cerium atoms. The ccp layers are parallel to the [111] planes of the f.c.c. unit cell, (b) and (c) the same structure redrawn as a primitive cubic array of oxygens.
Structural properties and nonstoichiometric behavior
17
Historically, the oxides of cerium in the range Ce 2 0 3 -Ce0 2 were treated using the classical point defect model of non-stoichiometry, in which oxygen-vacant sites were considered to be present in the lattice in a randomised fashion, in conformity with the law of statistical thermodynamics. Later on, particularly from the 60s, increasing evidence was accumulated to indicate the formation at low temperature of stoichiometric phases originating from the fluorite lattice by removal of oxygen ions and ordering of the vacancies formed. It is from the pioneering work of Bevan3,4, Brauer5"7 and co-workers that a first reliable picture of the structural behaviour of reduced ceria was developed. This was later refined by the structural studies of Eyring's group8, and Ray et al.910, and has been recently been rationalised by the development of geometric models for defect ordering1112, which allow the description of all known structures and also the prediction of possible phases which might have eluded detection during experimentation.
2.1.1. The Higher Oxides of Cerium (CeO!7I4-Ce02) Reduced ceria results from the removal of O2" ions from the Ce0 2 lattice, which generates an anion vacant site according to the following scheme: 4Ce4+ + O2- -» 4Ce4+ + 2eVTJ +0.5O2 -» 2Ce4+ + 2Ce3+ + • + 0.5O2
(2.1)
where • represents an empty position (anion-vacant site) originating from the removal of O2" from the lattice, here represented as an oxygen tetrahedral site (Ce40). Electrostatic balance is maintained by the reduction of two cerium cations from +4 to +3. Ce0 2 is reduced at elevated temperatures and low oxygen pressures to form a seeming continuum of oxygen-deficient non-stoichiometric oxides, which upon cooling organise into highly ordered fluorite-related superstructures, often with complex stoichiometrics. Several studies have indicated that this continuum exists above 685°C in a range of CeOx composition from 1.714<x<231314. This represents the so-called a phase, a disordered non-stoichiometric fluorite-related phase which is stable at high temperatures. The high-temperature X-ray diffraction pattern of this phase does not show superstructures and the lattice parameter a of the cubic phase increases as x decreases515. Fig. 2.2 compares lattice parameter variation (calculated from the high-temperature X-ray diffraction pattern and extrapolated at 298K) with increasing deviation from stoichiometry. Since the contribution of thermal dilation has been subtracted (for Ce0 2 and Ce02.x this value is about 6.34xl0' 5 A/K5,15),
Catalysis by ceria and related
18
materials
lattice expansion is a consequence of the reduction of Ce4+ions to Ce3+, the radius of the Ce3+ ion being larger than that of Ce4+ (1.14A vs 0.97 A, according to the data of Shannon and Prewitt16). At lower temperatures, the a phase transforms through a disorder-order process into a series of ordered, fluorite-related phases which can be described with the generic formula Cen02n_2m. These mixed-valent compounds have compositions between Ce0 1714 (Ce7012, n=7, m=l) and Ce0 2 . The seemingly continuous variation of x at low temperatures is due to the co-existence of two phases with different stoichiometrics (n, n+1). Pure Cen02n.2m, in contrast, exists only in a narrow composition range. 5.7-
5.65-
^5.6-
"5 A-" A
55o C 5.5-
5.45
W* o; £3'
5.4-
1.95
1.9
1.85
1.8
1.75
1.7
1.65
1.6
1.55
1.5
composition (x in CeO ) Figure 2.2. Lattice expansion at different degrees of reduction: values determined at high-temperature and extrapolated at 293K 515 (•,<>), values measured at room temperature4,9 ( A , 0 ) , ( ) values calculated from ref. 17.
Table 2.2 summarises the main phases observed in the region Ce02-CeO, 714. It will be seen that there is some conflict in the literature concerning both the composition and the structure of intermediate phases, only some of which have been fully characterised by structural refinement. Unfortunately, several anion-deficient structures remain unexplained. There are two major problems involved. First, it is difficult to obtain a single crystal. Second, the utility of X-ray powder diffraction
Structural properties
and nonstoichiometric
behavior
19
Table 2.2. Some components of Ce0 2 . x phases
phase
x in CeOx
Comp.a
ref.
Source
notes
X-ray diffraction at 1128K
Space group P2,/n, a=6.781A,&=11.893 A, c=15.823 A, P=125.04°
9,19
X-ray/neutron
a =6.757A, fe=10.260A c=6.732,a=90.04° P=99.80°,-)t96.22o
20 21,22
Specific heat Electron microscopy
4,5
X-ray
20
Specific heat
1.833
1.817-1.818
(P) b
CeuO20
1.812-1.805
P. 8'
1.806
Ce 6 2 0n2
8
Electron microscopy
E
1.8
^- e lO^M8
9
X-ray/neutron
20
Specific heat
20 21
Specific heat Electron microscopy
P7 M39 M29 M19
1.79 1.795 1.793 1.789
Ce 39 O 70 Ce 29 U 52 Ce| 9 0 3 4
M13
1.769
*-ei3^23
(DC
1.775-1.785
Ce 9 0 lfi
Rhombohedral cell, (pseudohexagonal) a=3.810A, c=9.538 A
" a=c
4
X-ray
9
X-ray/neutron
10,19
X-ray/neutron
20 8 4
Specific heat Electron microscopy X-ray
=6.750A, /?=8.400A a=99.4op=99.21c, "^75.00°
Rhombohedral cell, a=3.910 A, c=9.502 A
(5)1
1.714
Rhombohedral cell (pseudohexagonal) a=6.785A, a=99.42°
Rhombohedral cell, (pseudohexagonal) a=3.912 A, c=9.657 A a: stoichiometric composition, if available; b: original names of phases, assigned by Bevan4, are indicated in parenthesis. 1.72-1.710
20
Catalysis by ceria and related materials
techniques is limited by the modest x-ray scattering factor of oxygen, which means that details of the oxygen sublattice structure may be inaccessible. Both problems may be overcome in principle by neutron diffraction, a more effective approach since the coherent neutron scattering amplitudes of cerium and oxygen are comparable and profile refinement technique enables structural refinement of powder diffraction profiles. All the structures reported in Table 2.2 are fluorite-related. Although the f e e . symmetry of the unit cell is lost, both metal and oxygen remain essentially facecentered cubic close-packed and the diffraction signals (whether from X-ray, neutron or electron diffraction) corresponding to the fluorite structure are the strongest in the pattern. More complex patterns consisting of superstructures are therefore formed. Xray diffraction patterns typically show pseudocubic symmetry consisting of a number of peaks which are related to those obtained from the f.c.c. fluorite structure and which are often split owing to their lower symmetry. Significantly, the value of the pseudocubic lattice parameter calculated at low temperature is close to the value reported for the a phase, even though the a phase is disordered and the low temperature phases involve structural transitions (compare Fig. 2.2). This indicates that the cerium cations do not change their relative positions by varying oxygen pressure and temperature, and that the change of symmetry upon reduction or heating is mainly due to formation of vacant oxygen positions and oxygen migration. The phase diagram of the Ce-0 system, especially in the region of the higher oxide phases, was first constructed thanks to the work of Be van4, Brauer 5 and Bevan and Kordis3. Their results highlighted the formation of at least three CeOx phases in this range: the so-called [3 phase, with composition in the range 1.805<x<1.812, a y phase (1.775<x<1.785), and the 8 phase (1.710<x<1.72). No precise stoichiometrics were formulated but it was clear that formation of discrete phases in a very narrow stoichiometric range took place at low temperature. The dynamics of the formation of these phases was further clarified by the work of Ray et al.9,10. These authors found evidence for the formation of Ce 7 0 12 (similar to the 5 phase reported by Bevan4), Ce9016, and Ce n 0 2 o (the composition of which is close to that reported for the y and p phases). In addition, a Ce10O18 composition phase was reported and the crystal structure of Ce 7 0 12 was determined10. The structure is regarded as a rhombohedral distorsion of the fluorite lattice (space group R 3 with o= 6.79 and oc=99.4°) caused by ordering of oxygen vacancies along one direction. The relevant structural representation is shown in Figure 2.3(a). The idealised structure may be derived from Ce0 2 by placing strings of oxygen vacancies along the [ l l l ] F direction of the fluorite lattice. In this case, cerium ions situated on this axis (one seventh of total) are six-coordinate, their near-neighbours (six sevenths) being seven-
Structural properties
and nonstoichiometric
behavior
21
coordinate 23. Fig. 2.3(b) shows the structure of Ce 7 0 12 along the [211]F plane of the parent fluorite lattice. The projection of the rhombohedral unit cell and of the fluorite sublattice are evidenced.
Figure 2.3. (a) linear string characteristic of Ce 7 0 1 2 structure showing vacancies accommodated along the line. A cluster of primitive cubic array of oxygen is also evidenced, showing six and seven metal coordination, (b) structure of Ce 7 0 12 in projection along plane [21 l] F of parent fluorite lattice.
Some complications were introduced into the system in this composition region by Ricken et al.20 and Riess et al.24, who confirmed the previously suggested stoichiometries at n=7,10 and 11 but also found stable phases at an oxygen-cerium ratios of 1.808 (P^ and 1.79 (P2). At the same time, Knappe and Eyring8 confirmed the existence of phases at n=7 and 11 (not 9 and 10) by electron microscopy and determined two new phases at n=19 (Ce19034) and n=62 (Ce620112) as well as a series of other less clearly defined phases in the range CeOx 1.76<x<1.79. These compositions closely correspond to the ones reported independently by Ricken et al.19. Recently, the crystal structures of Ce 7 0 12 and Ce n O 20 have been confirmed by refinement of syngle crystal neutron diffraction data18. The composition range of x=1.8 to 2 was investigated in greater detail by Sorensen18. This author reported a phase corresponding to Ce 6 O u at high temperature, in addition to a series of higher stoichiometries at oxygen-to-cerium ratios of 1.875, 1.970 and 1.989 deduced from thermodynamic data25. Indexing of the XRD pattern showed Ce 6 O u to be isostructural with Pr 6 O n and to have a monoclinic unit cell, (a=0.6781, 6=11.893, c=15.823nm, (3=125.04°; JCPDS 32-196). Structural information on the remaining phases was not found and other investigators have not indicated phase transitions within this region at high temperatures 8'26. Fig. 2.4 illustrates the relevant phase diagram. Transformation temperatures in the CeOx (x>1.714) region are based on the work of Ricken et al.20. Although a
22
Catalysis by ceria and related materials
considerable amount of work has been carried out, the detail of the diagram is still incomplete and some data are missing as a result of experimental difficulties arising from annealing temperatures, fast heating and inadequate quenching, which can give rise to non-equilibrium state with possible variations in transformation temperature. For example, the maximum miscibility gap temperature has been observed at x=1.93 and T=910K20; x=1.93, T=958K3; x=1.95 T=903K27'28; and x=1.918, T=958K'.
1.5
1.6
1.7
1.8
1.9
2.0
1.7 1.8 L9 composition x in CeOx
2.0
1200
1000 T(°C) 800
600
400
1.5
1.6
Figure 2.4. Phase diagram of Ce02 adapted from refs. 3,20,22,29.
2.7.2. The Composition Range
Ce01J14-Ce015
At an O/Ce ratio lower than 1.714, the Ce0 2 phase diagram is dominated at high temperature by the presence of the o phase, a non-stoichiometric (Ce203+5) phase
Structural properties and nonstoichiometric behavior
23
which crystallises with the body-centered cubic type-C rare-earth oxide structure. There is a narrow a - a miscibility gap between the c and the a phase413. The Ctype sesquioxide Ce 2 0 3 , (the composition is uncertain and could be better described as Ce(\50_L53) is the compositional end member of the non-stoichiometric c phase series and crystallises in the bixbyite structure (space group la3), which can be considered as a double-edge fluorite structure with one fourth of the oxygen sites vacant and regularly ordered. The two structures are closely related, Fig. 2.5, for their cation arrays are almost identical and the anions occupy tetrahedral sites in both cases. In the fluorite structure, anions occupy all available sites whereas in the bixbyite they fill only three quarters in a perfectly ordered array. The b.c.c. unit cell of the latter has an edge length about twice that of the f.c.c. and a value for the unit cell dimension (a=11.21A) may be calculated by interpolation from the curve of the unit cell parameters of the cubic C structure for other lanthanides against the ionic radius for cations30"32. This agrees well with the value calculated by extrapolating at x=1.5 the value of the fluorite lattice parameter measured in the composition range 1.65<x<2, assuming Vegard-type behaviour for Ce0 2 -Ce 2 0 3 in the entire composition range7'31. Calculation of the lattice parameter of cubic Ce 2 0 3 using empirical relationships developed for fluorite-type Ce0 2 -M 2 0 3 solid solutions17 indicates a value of a=\ 1.24 A (ap=5.63 A, compare Fig. 2.2). Although sesquioxide formation is very common for other lanthanides, especially when synthesised at low temperatures222, it has not been unambiguously reported for cerium. The value calculated above for the cell parameter is reasonably close to the value of 11.26A measured by Courtel and Loriers using electronic diffraction on a sample of Ce 2 0 3 formed by rapid oxidation of cerium metal exposed to oxygen at room temperature30. The identification of Ce 2 0 3 has also been attempted in a few other studies31,32. The analysis of x-ray diffraction patterns led to the suggestion that cubic Ce 2 0 3 formed following reduction of high surface area ceria32. Unfortunately, the author does not report the precise stoichiometry. This makes it very difficult to distinguish the Ctype sesquioxide from the non-stoichiometric C-type a phase, which in the region Ce 2 0 3+5 with 8=0.2-0.33 presents very similar structural features4,5. For example, the value of the unit cell parameter of 11.13 A calculated by Perrichon for "cubic Ce 2 0 3 " is much closer to the value of 11.126 A reported for Ce 2 0 333 4 than to the values of 11.21 or 11.26 A calculated and found respectively for C-type Ce 2 0 3 . In another study, the stabilisation of nanosize particles of ceria as cubic Ce 2 0 3 was suggested on the basis of calculation of the lattice parameter of ceria sols having diameter of a few nanometers31. The decrease of particle size strongly stabilised Ce3+ ion, and the extrapolation of lattice parameter values suggested that the C-type sesquioxide exists when constituted by nanoparticles of diameter less than 1.5nm, with most of Ce
24
Catalysis by ceria and related materials
ions located near the surface. This is consistent with the enhanced stability of oxygen vacancies in the surface of ceria in comparison with oxygen vacancies in the bulk33, and with the enhanced reducibility of small ceria clusters34 compared to bulk ceria. Recently, the crystal structure of Ce0 1 6 8 (Ce20336) has been determined by refinement of single crystal neutron diffraction data19. A value of the unit cell parameter of 11.111(2) A has been found (space group lai), in agreement with the value of 11.126 A reported by Bevan4. At high degrees of reduction, only the hexagonal A-type sesquioxide Ce 2 0 3 (0 phase, in Fig. 2.4), which crystallises with an hexagonal unit cell (space group P32/m, a=0.389, c=0.607nm; JCPDS 23-1048), has been prepared35. Preferred formation of A-type sesquioxide does not seem to be related to energetic factors alone. Calculations reported for the lattice energy of a hypothetical cubic structure are similar to those obtained for the hexagonal cell36, suggesting a similar probability of formation for the two oxides. Preferred formation of the A-type Ce 2 0 3 is probably related instead to its lower reactivity toward oxygen in air at room temperature32 and possibly to the nature of the starting ceria since high surface area and small crystallite dimensions should favour C-type Ce 2 0 3 formation. It seems, however, that it is easier to form C-type Ce 2 0 3 by controlled oxidation of cerium metal at room temperature30,37 than by reduction of the dioxide. The diagram in this region is reported in Fig. 2.4 and was constructed largely thanks to the contributions of Height and Bevan22 and Kitayama et al.29. The latter study confirmed the existence at high temperature of a non-stoichiometric o phase with C-type structure in the region CtOlfl5A1(t (identified as Ce 3 0 5 -Ce 3 0 5A ), the coexistence in this region of the miscibility gap (between the C-type and the fluorite), and a region where the A-type sesquioxide co-exists with C-type a phase.
2.2. Defect Structure Analysis
2.2.7 Defect Types Defects in ceria can be intrinsic or extrinsic. Intrinsic defects may be present because of thermal disorder or can be created by reaction between the solid and the surrounding atmosphere (i.e. redox processes) whereas extrinsic defects are formed by impurities or by the introduction of aliovalent dopants. We shall draw attention here
Structural properties and nonstoichiometric behavior
25
only to processes of the first kind (i.e. intrinsic defects) and come back to extrinsic defects in section 2.3.
e „ - - '
1
,,
III „ "r'
III
II
v"-"""
III
_^^
^
^ ^ III ^ II ^ * 1 •
^-^
-
^
'
,_-
III o
Figure 2.5. Relations between the structure of Ce0 2 and that of the C-type sesquioxide. The unit cell of Ce 2 0 3 is obtained by packing the three octants as indicated. The octants are derived from the fluorite structure by ordered elimination of two oxygen per unit cell31.
There are three possible thermally generated intrinsic disorder reactions in ceria that do not involve exchange with the gas phase. These defects, which are of the Schottky (Eq. 2.2) and Frenkel (Eqs. 2.3 and 2.4) types, can be represented using the Kroger and Vink defect notation, which will be used throughout this chapter: CeCe + 2 0 0 < Ce r „ «-» C e " " +
o 0 « - o", + v0
V""Ce + 2Vd + CeO: r ,Ce
AE=3.53eV
(2.2)
AE=ll.lleV
(2.3)
AE=3.2 eV
(2.4)
In the above defect reaction, 0 0 and CeCe represent oxygen and cerium at their respective lattice sites, V0 and V'"Ce indicate respectively an oxygen and a cerium vacancy, and Cej**" and O"| a cerium and oxygen ion in interstitial position. The effective charge (i.e. the charge expressed in terms of the charge normally present in the same position in the host lattice) is indicated by a dot (•) for each positive charge and a prime (') for each negative charge. Thus, for example, V0 indicates a
26
Catalysis by ceria and related materials
doubly positive-charged oxygen vacancy and O j is a doubly negative oxygen interstitial. From variation in AE38, it is evident that the predominant defect category is the anion Frenkel-type, (Eq. 2.4), which leads to the formation of pairs of oxygen vacancies and oxygens in interstitial positions. Generally, these defects are present in low concentration and do not produce any deviation from stoichiometric composition. In ceria, however, a high concentration of defects can be formed by exposure to reducing gaseous atmospheres. Upon reduction, ceria has excess metal compared to its anion content, that is to say its cation/anion ratio is greater than 0.5. There are, in principle, two ways in which Ce0 2 can accommodate variation in composition. In the first case, oxygen vacancies are assumed to compensate the holes formed on reduction. If we remove oxygen, the crystal will end up with an overall positive charge and we need to introduce two electrons for each oxygen ion moved in order to keep the crystal neutral. These electrons are associated with two cerium atoms that will change charge from +4 to +3. The effective charge of the anion vacancies is positive, thus neutralising the negatively charged holes. The process illustrated in Fig. 2.6(a) is generally represented as: Ce0 2 <^ Ce02.x + x/202(g)
(x<0.5)
(2.5)
or the following defect reaction can be written: Ce0 2 <-> 2xCe'Ce + (l-2x)CeCe + xV0 + (2-x)0 0 + 0.5xO2(g)
(2.6)
the reaction implies that as x moles of atomic oxygen are removed from the lattice, the corresponding quantity of O2' sites are occupied by oxygen vacancies, leaving 2-x moles of O2' anions in their original positions. On the cation side, 2x moles of Ce3+ are formed (Ce'Co), leaving l-2x moles of Ce4+. Cation interstitials provide an alternative way to create positively charged defects. These interstitials may be formed by transfer of cerium cations located on the surface to an interstitial position and by the removal of two anions to the gas phase for each cerium interstitial formed. The process can be simply represented in the following equation: (l+x)Ce0 2 <-> Cei +x 0 2 + x02(g)
(x<0.33)
(2.7)
which does not however show where electrons are localised. Several possibilities exist and these are highlighted more satisfactorily in the following defect reactions,
Structural properties and nonstoichiometric behavior
27
which show formation of either triply, Ce,"' (Eq. 2.8), or quadruply, Cej"" (Eq. 2.9), ionised cerium interstitials: (l+x)Ce0 2 «-> xCe"* + 3xCe'Ce + (l-3x)CeCe + 2 0 0 + x0 2 (g)
(2.8)
(l+x)Ce0 2 ^ x C e j " " + 4xCe'Ce + (l-4x)CeCe + 2 0 0 + xQ2(g)
(2.9)
This situation is illustrated in Fig. 2.6(b), where one cerium(IV) cation is present in an interstitial position with four neighbouring Ce3+ ions.
Figure 2.6. Schematic representation of (a) an oxygen vacancy (V0) and (b) a quadruply ionised cerium interstitial (Cej) in an idealised reduced Ce0 2 surface. Ce cations are shaded.
Identification of the nature of defects in Ce0 2 is a goal that has eluded researchers for several years. The dependence of conductivity data on oxygen pressure was interpreted using models that involved both oxygen vacancies39 and cerium interstitials40 as predominant defects. This was a result of the difficulty of measuring the expected Po2"1/6 dependency of conductivity over the full range of oxygen pressure available39 as predicted theoretically if double ionised vacancies/electrons are the predominant defects. Other studies which depended more directly on the type of defect present in Ce02.x also pointed to an oxygen vacancy model. Steele and Floyd41 concluded, on the basis of oxygen diffusion data in ceria and yttria-doped ceria, that
28
Catalysis by ceria and related materials
the predominant defects in oxygen-deficient ceria are anion vacancies and similar conclusions were reached after studies of the variation of thermodynamic parameters AH(02) and AS(02) with temperature14. Support for this view also comes from the results obtained by x-ray42, neutron diffraction43 and combined dilatometric and x-ray lattice parameter measurements 15. These studies concluded that the lattice expands as a function of increasing defects. The cerium sublattice is not strongly perturbed by the defects, which instead cause the formation of a defective oxygen sublattice.
2.2.2. The Structure of Defects in Oxygen-deficient Ceria As we have already seen, redox processes perturb the charge balance in Ce0 2 . Electrical neutrality is restored by the creation of a charge-compensating defect (oxygen vacancy) which can be thought as a point defect, i.e. a species confined to one site, or to a small group of sites. Even when the departure from stoichiometry is small, there is reason to believe that the defect structure of non-stoichiometric phases are more complex than a picture of isolated point defects would suggest. It is well-known that the oxygen lattice is the seat of non-stoichiometric variation and that the generic formula of reduced phases is thus Ce0 2 . x . However at low temperature, these phases show ordering of oxygen vacancies in a regular fashion, generating a homologous series of phases which have compositions expressed by the formula Cen02n.2m. Any possible representation requires that point defects should be ordered to give superstructures of the Ce0 2 fluorite lattice which can be derived either by incorporating oxygen atoms into vacant sites in the type-C sesquioxide structure or by creating anion vacancies in the perfect fluorite lattice. There has been a strong effort to rationalise and elucidate a structural principle which will account for all the anion-deficient, fluorite-related, mixed-valent binary oxides of cerium, praseodymium and terbium. This is a key step not only for the solid-state chemistry of these materials but also for a large class of fluorite-related materials involved in applications such as fast oxygen conductors and as catalysts. The two main theoretical approaches to the problem were developed by Martin11,21 and by Kang and Eyring44, and will be illustrated in the following sections.
2.2.2.1. The Coordination Defect Model11,44 A convenient way to represent the structure of Ce0 2 is shown in Fig. 2.7(a). Ce4+ ions are arranged in a face-centred cubic fashion and a cubic lattice of O2" anions is
Structural properties and nonstoichiometric behavior
29
located inside the metal cage. In this way, it is possible to think of the unit cell as comprising eight octants, Fig. 2.7(b), each having the composition Ce05O (i.e. one eighth of each of the four cerium ions shared by the other octants and one oxygen ion). The removal of oxygen from the lattice generates an octant with a hole whose composition is C e 0 5 O This implies that the resulting vacancy will carry a virtual positive charge which may cause a perturbation in the position of ions in the vicinity of the vacant site. The perturbation may affect the four tetrahedrally coordinated nearest-neighbour cerium ions and also the six next-neighbour oxygen
(a)
(b)
(c)
(d)
Figure 2.7. (a) Huorite unit cell of composition Ce4Og; (b) single octant of composition Ce 0 s O; (c) the coordination environment of a vacant site (i.e. the coordination defect) showing the six nearest neighbours octants and their assembly; (d) view of the coordination defect along [001] plane. Small black circles are cerium cations, larger gray circles are oxygen anions.
ions. This has been observed and reported for Ce, Pr and Tb systems10,12"43'46. For example, expansion by about 0.15 A of Ce atoms around an oxygen-vacant site is accompanied by a contraction of the six octahedrally disposed oxygen ions around the vacancy by about 0.3 A in Ce 7 0, 2 ' 0 . The values are summarised in Table 2.3. The defect is therefore not localised in one single oxygen site but instead extends tosix additional surrounding oxygen positions and comprises the anion vacancy site Ce 0 5 n plus the six octants which octahedrally coordinate the site, Fig. 2.7(c). The composition of this defect cluster is CessOfQ equivalent to CeO^ 714, which is also the reduced phase of greatest thermal stability (8-phase reported by Bevan4). This means that CeO, 7)4 can be derived from close packing of coordination defect units in
30
Catalysis by ceria and related materials
Table 23. Average bond distance in M 7 0 ] 2 (M=Tb,Pr,Ce).
Cation Tb Pr Ce
00(A) 2.375 2.493 2.510
0-Oa(A) 2.659 2.764 2.782
M-CKA) 2.473 2.538 2.553
M-Oa(A) 2.303 2.394 2.409
Volume" (A3) Ref. 263.32 45 46 295.68 301.68 10
a; average O-O and M-O distance estimated assuming an ideal fiuorite lattice, b: volume of rhombohedral unit cell of M 7 0 12 .
three dimensions; distribution of coordination defects in a [100]F layer of Ce^O^ is represented in Fig. 2.8, where cerium ions belonging to the same plane are outlined in black. In each [100]F layer, the closest packing of coordination defects is obtained by aligning them in parallel rows separated by holes, which are needed to accommodate the axial octant of the layer above and below the initial layer. As a result of this packing, coordination defects are aligned along certain preferential directions. The crystallographic translational unit of the phase is outlined in black and has the composition C e O O i 2 (i.e.CeO,714). The close relation between this superstructure and the structure of the fiuorite cell can be clearly seen.
Figure 2.8. Distribution of coordination defects in a [001 ] F layer of Ce 7 0 1 2 with the unit of composition Ce 7 O n outlined. White rectangles can accommodate oxygen belonging respectively to the axial octant of each coordination defect situated above or below this layer.
The composition of other known Cen02n.2m phases can be obtained by appropriate packing of octants containing the defect (Ce 3 .5OO and octants with no
Structural properties and nonstoichiometric behavior
31
vacancy (Ce 35 0 7 ). It emerges that oxygen vacancies in reduced phases are wholly contained in families of [213] planes separated by oxide-intact planes. This approach makes it possible to describe several structural features of aniondeficient fluorite-related binary oxides of Ce, Pr and Tb but it is fundamentally descriptive rather than predictive. This limit was pointed out by Khang and Eyring44, who recently built a model that is also capable of predicting the stractural features of unknown phases in the series Cefi^m12'41'44'41 • The Rang and Eyring model too, features a structural defect unit upon which the entire 3-D lattice is constructed. On the basis of structural analysis carried out on the lower oxides of Ce, Pr and Tb, the following underlying principles governing
Figure 2.9. Representation of units containing coordination defect used to model Ce„02„_2m oxides, with their orientations along [112]F plane. Module F has no vacancies; module D, has one vacancy in lower octant (there are four possible bottom vacancies whose D b modules have b= 1,2,3 and 4); module U 1 has one top vacancy (there are four possible U1 modules with t=l,2,3 and 4); W' 3 is one of four possible modules with two vacancies, a top vacancy and a vacancy in the lower octant (W b with t,b=3,l;4,2;l,3;2,4)44.
these structures can be evidenced: (i) the covalent character of bonding is much less significant compared to ionic interaction, which is quite reasonable considering that the fluorite structure is held to be favoured by strongly ionic compounds; (ii) all the structures of the lower oxides of Ce, Pr and Tb are fluorite-related and derived from the parent fluorite lattice, although the vacant oxygen sites break the cubic symmetry; (iii) the structural element which can be most conveniently utilised is the coordination defect as introduced by Martin11; (iv) overlapping of oxygen ions in coordination defects is not allowed so none of the six oxygens surrounding the
Catalysis by ceria and related materials
32
vacancy can be associated to more than one coordination defect; (v) relaxation of the cation sublattice is a stabilising factor for the intermediate structures. Table 2.4. Unit cell content of members of series CenO;
Phas e t
c 5
P M19 M29 M39
e 8'
n
m
Modular content
7 9 11 12 19 29 39 40 62
1 1 1 1 2 3 4 4 6
W, 3U,3D F, 4U, 4D 3F, 4U, 4D 4F, 4U, 4D 3F, 8U, 8D 5F, 12U, 12D 7F, 16U, 16D 8F, 16U, 16D 14F, 24U, 24U
Unit cell content Ce7012 Ce9016 Ce n 0 2 o V_-C|9^-'22
Ce 1 9 0 3 4 Ce 2 90 52 Ce 39 O 70 (-e4o0 7 2
Cefi20|17
x in CeO, 1.714 1.778 1.818 1.833 1.789 1.793 1.795 1.800 1.806
Keeping in mind these basic features, all the members of the homologous series can be constructed by appropriate packing of the fluorite-type modules derived from four basic units. Fig. 2.9 shows these four units, which contain a total of 13 modules (four U D and W modules and one F module). In the generic formula Cen02n.2m, n is the number of modules required to construct the supercell and m is an integer number correlated to the number of vacancies in the unit cell. There are always two oxygen-vacant sites in a unit cell or an integral number, m, of these vacancies. Table 2.4 lists the values of n and m and the modular content of established higher oxides of cerium. For example, the first member of the series Ce 7 0 12 can be constructed by packing n=7 modules (three of type D, D2, D3 and D4 three of type U, U lt U2 and U4, and one of type W, W3,). Fig. 2.10 shows the configuration of the modules that make up the structure of Ce 7 0 12 displayed in the [112] plane on the [100] and [010] directions.
2.3. Transport Properties Electrical conduction and other transport properties of oxides, such as oxygen diffusion, are mainly determined by the presence, concentration and mobility of
Structural properties and nonstoichiometric behavior
33
010
Figure 2.10. Packing of modules used to build Ce 7 0 12 showing the unit cell in projection. Modular sequences along [100] and [010] are, respectively, D, U2 D, Wr, U1 D, U4 and U2 U1 D, W 3 , U4 D,
DI4' f
lattice defects. It is obvious that the application of materials as solid oxide electrolytes relies on these transport properties 48'49, which are also believed to play a key role in catalysis. Oxygen transport materials have long been used as oxidation and ammoxidation catalysts, for which purpose the material must be susceptible to rapid reduction by the reagent and reoxidation by the incorporation of gaseous oxygen in the lattice50'51. Recently, with the introduction of ceria and ceria-zirconia as oxygen storage/release promoters in three-way catalyst formulations, it has become evident that transport properties could play a key role in processes where the availability of oxidant from the gas phase is not constant. Provided diffusion of anions is sufficiently fast, a continuous supply of oxygen from the bulk to the surface guarantees a constant concentration of active surface oxidation sites, thus enabling a fast surface catalytic reaction in the absence of other kinetic limitations. This section will attempt to summarise the fundamental features of the transport behaviour of ceria which will help the understanding of some catalytic properties of ceria-based materials illustrated in subsequent chapters.
2.3.1. Electrical Conductivity Total electrical conductivity in a solid (ot) is defined as the sum of conductivity contributions from each of the charge carriers present in that solid:
't=5>j
(2.10)
34
Catalysis by ceria and related materials
The charge carrier may be electronic (either electrons, e, or holes, h) or atomic (cation or anion defects). Each of the partial conductivities is given by the expression: C7j=CjZjeiuj
(2.11)
where o, is the partial conductivity measured in Scm"1 (lSiemens=lohm"'), and Cj, Z|e, and |ij are the carrier concentration (per cm3), charge (Coulombs) and mobility (cmYVs) respectively. In oxides, there is generally more than one charge carrier and it is important to identify for each the transport (or transference) number which defines the fraction of total conductivity contributed by that charge carrier: K=^~
(2.12) j
In cases where ionic and elctronic conduction occur together, (case of mixed conduction): o.ot=Oi+oei-tiai+telael
(2.13)
it may be noted that tj+tel=l and tel=te+th.
2.3.1.1. Electrical Behaviour of Ceria Ceria can be classified as a mixed conductor showing both electronic and ionic conduction. Its electrical properties are strongly dependent upon temperature, oxygen partial pressures and the presence of impurities or dopants. All these variables affect charge carrier concentration, which ultimately, together with charge carrier mobility, determines electrical conductivity. For the general case where electrons, holes and oxygen vacancies are the primary charge carriers in Ce02.x, total conductivity is given by: a t = [CeQ jeAie + [h* Je^h + [V6 ]2e//V(i
(2-14)
At high temperatures and low oxygen partial pressures, ceria behaves as an n-type semiconductor and electrons liberated following reduction are the primary charge carriers. The reaction which leads to non-stoichiometry is:
Structural properties
0
0
~
and nonstoichiometric
behavior
Vfl + 2e" + 0.5O2(g)
35
(2.15)
where the electronic defect e" can be regarded as equivalent to the presence of a Ce3+ ion, or in defect notation Ce'Ce. These electronic defects are accompanied by the formation of ionic defects (oxygen vacancies) of the same order of magnitude. For example, Ce0 2 may easily form Ce0 1 8 at 1000°C and reduced oxygen pressure (P(O2)=10"16)39,52, resulting in an oxygen-vacancy concentration of ca. 5.0xl021 cm"3, with a similar level of electron concentration (with ref. to Eq. 2.14 [Ce'Ce]=[V0]»[h']). The concentration of oxygen vacancies is related to the deviation from stoichiometry, x in Ce02_x, by the expression nv=4x/ao3, where ao is the lattice parameter of Ce02.x and nv is the number of oxygen vacancies. According to the electroneutrality relationship the number of electrons (localised on Ce3+ ions) is ne=[4x/a03]53. Since electron mobilities are generally orders of magnitude greater than ionic mobilities (|j.e»|j,Vi)), this deviation from stoichiometry does not usually lead to mixed conduction. In cerium oxide, however, electronic conduction does not take place through a band model but occurs instead through the formation of small polarons27,54, where the electron is self-trapped at a given lattice site (Ce3+) and can move only to an adjacent site by an activated hopping process similar to that shown by ionic diffusion. This strongly reduces electron mobilities, which for Ce0 2 are in the order of 10"4-10"2 cmYVsec52"54. Ionic and electronic mobilities for Ce0 2 containing samples are compared in Table 2.5. Table 2.5. Electronic and oxygen defect mobilities of ceria-based materials at 1273K.
Material
Ref.
[I; (cirrVVsec)
C e U | 992
27 27 54 55 56 a 56 57b 57
2.4x10'5 2.5xl0' 6 1.3xl0-4 7.5xlQ-4
Ce0 177 (-e(J 1975 LeO> 1997 Ce0 192 Ce0 1 8 ^- e 0.9^-' a 0.1*-'l.9
u.e (cmVVsec) 8.1xlO-3 2.2xl0"3 2.0xl0 2 2.0xl0"4
LengiliiffiUiois a: calculated from ionic conductivity data in ref. 56 using Nernst-Einstein relation; b: calculated from C: and E, data in ref. 27 using the following relations: a—C/TexpC-E/kT) and (Tpniiifli
36
Catalysis by ceria and related materials
The closeness of the values for the two parameters allows a regime of mixed conduction to be operative in non-stoichiometric ceria. Nevertheless, the greater the departures from stoichiometry, the more evident the electronic contribution becomes. Interestingly, a decrease in the number of charge carriers has been observed27,54 in these regions. The phenomenon is related to some degree of local order at high temperature, which immobilises charge carriers. This indicates that the hightemperature fluorite phase, where vacancies are believed to be randomly distributed within the lattice, may be constituted by regions of local short range order. Transition from n-type to p-type conduction is observed at lower temperatures and higher oxygen partial pressures near stoichiometric composition, where electronic conductivity arises from holes introduced by impurities56'58. Residual impurities are inherently present in all ceria samples and generally comprise other rare-earth elements1439. Several investigators have also reported significant levels of calcium impurity in samples of polycrystalline powders as well as in Ce0 2 single crystals (between 200 and 1000 ppm)52,56,5861. The presence of these impurities strongly affects ceria's electronic transport properties. For example, if a divalent cation impurity (I2+) is present in Ce0 2 lattice, the formation of holes is controlled by the two following equilibrium reactions: IO «-
1"^ + V0 + 0 0
Va + 0.5O2 <-> 0 0 + 2h*
(2.16) (2.17)
where h* indicates an electron hole. At high oxygen partial pressures, the equilibrium (Eq. 2.17) shifts to the right and oxygen fills the impurity-created vacancies. The consequent production of electronic hole carriers increases the a h contribution to conductivity ([h*]>[Ce'Ce] inEq. 2.14). Fig. 2.11 shows the partial conductivity of two samples of ceria with different purities at 1 atm of oxygen pressure, as measured by Panhans and Blumenthal58. The contribution of a h to conductivity is also evident at lower temperatures, where it competes with the ionic component. On increasing sample purity, the ionic and hole contribution to conductivity decreases as fewer extrinsic vacancies are present in the sample and the o t is the result of equal contributions by vacancies, electrons and holes. In the same investigation Panhans and Blumenthal found that the ionic conductivity of Ce0 2 x single crystal is more than two orders of magnitude higher than that of the polycrystalline material at 600°C, whereas it is similar at 1000°C. This difference was attributed to a grain boundary effect62 which results in a higher boundary
Structural properties and nonstoichiometric behavior
37
resistance to ionic conduction at low temperature (higher activation energy). At high temperatures, the grain itself is the limiting factor.
1000 800 500 temperature (°C)
1000 800 . 500 temperature (°C)
Figure 2.11. Partial conductivities of Ce02.x at 1 atm. of oxygen partial pressure: (—) CeO^ 99.9% purity; ( ) Ce02.x 99.99% purity; ( ) Ce02., single crystal 99.9% purity.
2.3.1.2. Effect of Dopants Ionic conductivity in pure ceria may result from intrinsic disorder generated by three possible defect mechanisms (Eqs. 2.2-2.4)38. Energetic considerations indicate that the anion Frenkel-type disorder is the predominant route to intrinsic defect formation but it is not responsible for high ionic conductivities in ceria. Ionic conductivity in pure ceria is in fact believed to be negligible and the observed values strongly depend on the level of impurities. At a temperature of 973K, values of the order of 10"5 Scm"1 are typically reported63,64. These values are found with a level of impurities below lOOOppm and they increase significantly when ceria is doped with aliovalent oxides such as CaO, Y 2 0 3 and various rare earths or when these ions are present as high-level impurities, which is quite common in the case of low-grade ceria.
38
Catalysis by ceria and related materials
The extremely open structure of the fluorite assembly tolerates a high level of atomic disorder, which may be introduced either by reduction or by doping. On reduction, both vacancies and electrons are present, thus giving rise to a large electronic contribution to conductivity. When an aliovalent solute (M2+/M3+) is dissolved in Ce0 2 , the crystal lattice must compensate for the excess negative charge. It can do so, in principle, by three mechanisms, referred to as vacancy compensation (Eq. 2.18), dopant interstitial compensation (Eq. 2.19), and cerium interstitial compensation (Eq. 2.20). In the case of the M3+ dopant cation, these mechanisms can be represented as: xMO, 5 + (l-x)Ce0 2 <-> xM'Ce + 0.5xV6 + (l-x)CeCe + (2-0.5x)Oo
(2.18)
xMO, 5 + (l-x)Ce0 2 <- 0.25xM"' + 0.75xM'Ce + (l-x)CeCe + (2-0.5x)Oo
(2.19)
xMO,.5 + (l-x)Ce0 2 <- xM'Ce + 0.25xCe"" + (l-1.25x)CeCe + (2-0.5x)Oo
(2.20)
Empirical calculations carried out for M3+ cations show that vacancy compensation is clearly the preferred route38, at least for large dopant cations (radius >0.8A). Formation of interstitials is also ruled out by measurements of true density and comparison with calculated values61,65. For the smaller cations (i.e. Al3+), some compensation via dopant interstitial may occur. The reactions described in Eq. 2.18 and 2.21 (for a divalent cation) therefore summarise the main route to defect formation in solid solutions of the type Ce,.xMxO2.05x and Ce!.xMx02.x respectively. xMO + (l-x)Ce0 2 *-* xM"Ce + xV0 + (l-x)CeCe + (2-x)0 o
(2.21)
These reactions imply that when x moles of dopant oxide are added, Ce4+ sites of Ce0 2 are filled with x moles of dopant cation (M2+/M3+) and (l-x) moles of host cation Ce4+. Similarly, O2" sites are occupied by vacancies and host anions. Conductivity data exist for a large number of doped cerias57,61,64,66"72 and, at high 0 2 pressure, the presence of extrinsic vacancies is responsible for large values of ionic conduction. The results are generally reported as logarithmic plots of o~ (or oT) vs the reciprocal temperature (Fig. 2.12). Using Eq. 2.11 and bearing in mind the expression for mobility73, we may obtain the dependence of electrical conductivity with temperature for oxygen ion conductors: —AH cT = A e x p — ^ 2 -
(2.22)
Structural properties and nonstoichiometric behavior
39
where A is a constant (that also depends on the number of charge carriers), k is the Boltzmann constant, and AHra is the migration enthalpy of oxygen ions. This equation is valid if vacancies are free and randomly distributed throughout the sample. If the vacancies are not free but linked to dopant cation instead, a so-called defect association between vacancies and cations is formed, which decreases conductivity73,74. This association is mainly due to the coulombic attraction of the defects caused by their effective charges in the lattice. Thus, for a divalent cation, one simple defect associate is possible: M"Ce +V e ~{M" C e V 0 }
(2.23)
while two situations are likely with a trivalent dopant: 2M'Ce + V0 ~ {M'CeVaM'Ce}
(2.24)
M'Ce +V 8 ~{M' C e V 8 }-
(2.25)
This may be taken into account by modifying the activation energy of Eq. 2.21 and adding another enthalpy contribution AHa as the enthalpy of formation of associated defects74. The conductivities of some ceria-containing compounds obtained from the literature have been compiled and shown as solid lines in Fig. 2.12. The main dopants for ceria belong to the alkaline earth or rare-earth metal series and the majority of doped samples exhibit conductivity values which fall into a rather limited band (gray band in Fig. 2.12), which points to a similar behaviour for all doped ceria samples. Exceptions are pure ceria and ceria doped with redox elements like Pr and Tb which give rise to electronic contribution to conductivity. An important requirement is that a homogeneous solid solution forms between the two oxides, which maintains the fluorite structure since the presence of a second phase or phase inhomogeneity due to insufficient solubility can affect ionic conductivity. The very low values of conductivity found for BaO and MgO-doped ceria were in fact attributed to the low solubility of these oxides into the lattice of Ce0 2 67 . When the dopant element exists in one oxidation state, conduction is mainly ionic at atmospheric pressure of oxygen (for example t~l for Bi71,72, Ca, Sm and other rareearth metals69'70-75).
40
Catalysis by ceria and related materials
1.00x10"
1000
800
T(K)
500
l.OOxlO" 0 -
1.00x10''!
l.OOxlO"5 0.0007
1 0.0011
0.0015
0.0019
0.0023
1/T(K4)
Figure 2.12. Electrical conductivity data of various ceria-containing materials: 1 Ce0 2 70 ; 2 Ce0 2 M ; 3 >m Ce 07 Tb 03 O 2 . 8 ; 4 Ce 0 5 Pr 0 5 0 2 . g ; 5 Ce 0 ,Bi 0 2 O 2 ^ ; 6 Ce 0 g Bi 0 2 O M ; 7 Ce°0.83' 0 0.17 , -'l.915 • Ce 0 g Sm 0 2 O 1 9 ; 9 Ce 0 8 Gd 0 2 O 1 9 ; 10 Ce(, 8 Y 02 O 19 ; 11 Ce091Ca00t>O191 .
However, when more than one oxidation state is possible (i.e. Ce,.xTb(Pr)x02.s), a contribution to conductivity arising to electrons associated with Tb3/4 and Pr3M transitions (0.4
Structural properties
and nonstoichiometric
behavior
41
introduction of di- and trivalent dopants. However, a few exceptions exist. The introduction of Bi3+ and Nd3+ (ionic radii 1.11A and 1.12A respectively) gives solid solutions with high ionic conductivities69'71'79. This can be explained by the fact that BiO, 5 itself has a high electrical conductivity, while the conductivity of Nd-doped samples can be associated with a favorable low binding energy 79.
2x10
1x10
1x10
0.95
1
1.05 1.1 1.15 Radius of dopant cation (A)
Figure 2.13. Relation between ionic conductivity at 1073K and radius of dopant cation. Data relative to Ce,.xMxO2.05x with x= 0.3 (D) 69 and x=0.2 (0,+) M - 68 . On the right the binding energy against the radius is shown (•, 79 ).
The behaviour of ionic conduction with dopant concentration depends on several factors. In principle, for aliovalent dopants, ionic conduction should increase with the degree of substitution as a result of the increase in the number of oxygen vacancies. This is the consequence of a low value for AHra, which depends both on the oxygen binding energy in the lattice and the free volume through which the oxide ions migrate. The first term is the average binding energy (ABE) of the Ce(M)-0 bond in the oxide which is correlated to the energy needed to break the cation-oxygen bond. This value can be estimated using available thermodynamic data80, and for solid solutions of the type Ce,.xMxO2.05x (with M=rare-earth cation) the following relation can be used: ABE = ^ [ B E C e 0 2 ] + f[BE M 2 0 3 ]
(2.26)
42
Catalysis by ceria and related materials
where BE is the binding energy and 8 is the coordination number of cations in a fluorite assemby. By doping ceria with other rare-earth oxides, the ABE decreases with increasing dopant content. The free volume, defined as the difference between the fluorite unit cell volume and the volume occupied by all ions present within the unit cell, is also a function of the dopant and it generally increases with the degree of substitution. Therefore it is expected that conductivity increases with the free volume and with the decrease of ABE. However experimental observations imply that the conductivity is not simply proportional to the concentration of oxygen vacancies and it is widely observed that it goes through a maximum70,71'73'79'81'82. Values of ionic conduction for some samples of doped cerias are shown in Fig. 2.14 as a function of dopant content. It is possible to distinguish two regions: at low dopant concentration (generally <3-5 mol% M 2 0 3 or <1.5-2.5 mol% anion vacancies, the so-called dilute range78'79) conductivity increases with concentration, while the opposite is observed in the high-concentration range. Maximum values of conductivity are strongly dependent on the dopant and result from minimum activation energy and/or maximum solubility of the two oxides. The explanations which are given to understand the decrease of oxygen-ion conductivity are usually qualitative and involve increasing defect interactions with increasing dopant concentration, which eventually cause a sharp reduction in mobility. Generally the dopant content corresponding to the maximum of conductivity depends on temperature, since the activation energy varies with composition. A quantitative evaluation of these parameters takes into account attractive interactions between the dopant cations and mobile vacancies. The composition corresponding to the minimum activation energy is determined by the range of these interactions which can involve also third or fourth nearest neighbours79. The activation energy of conduction against the concentration of dopant cation in ceria was calculated by Faber et al. from conductivity measurements79. For all samples investigated they found a minimum in the activation energy for conduction (Fig. 2.15) and this was attributed to the type of interactions between the dopant cation and the mobile oxygen vacancies. In another study, Yu Wang et al81 attributed the decrease of activation energy in the dilute range to coulombic interactions while in the highconcentration range the decrease of c and increase of Ea were attributed to the presence of vacancy traps that limit vacancy mobility, thus making the concept of free vacancy no longer valid at high dopant concentrations. Other explanations for the drop in conductivity were related to the distribution of dopants, which may be no longer random. For certain compositions and choice of dopants, long-range ordered structures, or structures containing ordered microdomains can be obtained. These have been reported to affect negatively conductivity82.
Structural properties
and nonstoichiometric
1x10"
behavior
43
1x10 1x10 1x10"
1x10" 1x10
e a
1x10 1x10" 1x10 1x10 1x10 0.02
0.1 1 10 20 dopant oxide M^O-, (mol%)
1x10"
0.02
0.1 1 10 20 dopant oxide N^Oo (mol%)
Figure 2.14. Ionic conductivity of Ce0 2 -M 2 0 3 solid solutions versus composition (mol% of dopant cation M) for M =Yb(«), Y(B), Gd(*), La(A), and Nd(D)) measured at 600K (right), and at 300K left79.
1.15-
1.05
>
0.95
0.85
0.75-
0.65 0.02
0.1 1 10 20 dopant oxide M2O3 (mol%)
Figure 2.15. Activation energy against composition for Ce0 2 -M 2 0 3 solid solutions: M=Yb(*), Y(B), Gd(*), La(A), and Nd(D)79.
44
Catalysis by ceria and related materials
2.3.2. Oxygen Diffusion in Ceria Oxygen diffusion in ceria and related systems is of interest for several reduction and oxidation reactions, where it may be the rate-controlling step, thus determining the kinetics of the process. The rate of several catalytic oxidation reactions which occur anaerobically, i.e. without oxygen from the gas phase, but using oxygen from the solid, may depend on oxygen diffusion parameters. In addition, reduction of ceria, which is believed to involve two distinct steps (surface and bulk reduction), may be dependent on the availability of bulk oxygen. Diffusion data are generally reported in terms of a diffusion coefficient D (cm2/s), and its temperature dependence is best described by an Arrhenius equation of the form: D=D0exp^j
(2.27)
where D0 is the pre-exponential factor which contains terms related to the concentration of ionic carriers, and Ea is the overall activation energy, which includes terms related to the energy needed to move the defect, the energy for defect formation and the defect association energy. Mass transport in ceria as well as in other fluorite-related materials is several orders of magnitude faster for anions than for the metal83,84. Measurements of bulk oxygen diffusion are relatively scarce in the literature and they are summarised in Table 2.6. Alternatively, values can be estimated from ionic conductivity data using the Nernst-Einstein relation: D =
_O i kT Ci(Ze)2
where a{ is the ionic conductivity, k is the Boltzmann constant, T the temperature, Cj the concentration of ionic carriers (N/cm3) and Ze their charge (Coulomb). In the case of ceria or doped ceria, the equation becomes: D =
a i kTa !L 16xe2
where ag is the lattice parameter and x the number of vacancies per unit formula.
Structural properties and nonstoichiometric behavior
45
Values of oxygen diffusion calculated using Eq. 2.29 are reported in Fig.2.16
along with oxygen self-diffusion data measured experimentally. It will be seen that oxygen diffusion data span several orders of magnitude and are dependent on the temperature and the degree of stoichiometry. Temperature is the most important parameter. It has been noted that the activation energy of stoichiometric ceria is higher than that of Ce02.x and doped ceria, see Table 2.656. This can be explained Table 2.6. Oxygen diffusion data of Ce02-containing oxides.
3.6
1123-1423
1.5X10"4
19.2
1123-1423
C 0.8*0.2Oi.90
1.7xl0 4
18.3
1123-1423
^0.6^0.4^1.8
5.1xl0"3
21.3
1123-1423
Da cm2/s -
Ce0 2
-
-
623
14.3
923-1773
Sample
T(K)
Do cm2/s
Kcal/mol
Ce0 2
5.3 xlO2
73.2
1423-1829
0.3%Gd-CeO2
9.5xl0' 5
21.7
1373-1573
Ce0 2
1.9xl0"4
24.8
1123-1423
Ce0 2
1.4xl0"4
21.3
1123-1423
v-CU] 92
CeO, 8 C e o j i Q ^ O , 95 e
Ce,. x Ca x 0 2 . x
5
1.5xl0" 6.2xl0'
3.7xl0
6
3
11.9
1123-1423
Notes"
Ref.
-
85
sc
56,86
pes
56,86
sc
56,86
85
sc
56,86
pes
56,86
pes
56,86
pes
56,86
5xl0' 18
Rh/Ce0 2 catalyst
87
-
pes
61
0.01<x<0.08 Ce0 2 C e O J 9837 C e O [ 8 i35
CeO, 7<jn4
3640
77
1350-1600
-
_ -
_ -
1244
6.2xlO"5
pes
5
1244
11.7xl0'
1244
5
8.6xl0"
D,pcs
88 89
D,pcs
89
D,pcs
89
a: diffusion coefficient (D), and chemical diffusion coefficient ( D ) determined at the temperature indicated; sc: single crystal, pes: polycrystalline sample.
by a decrease in the energy needed to create a defect in Ce02.x compared to Ce0 2 . In the latter case, almost all the anions in the sublattice are occupied, while in Ce02.x or solid solutions anion vacancies are already present. The value of D is also dependent on the degree of non-stoichiometry x. Because of the high concentration of oxygen defects in nonstoichiometric fluorite-type oxides, diffusion in Ce02.x is faster than diffusion in Ce0 2 ; empirically, at a first approximation, D/x =cost.83.
46
Catalysis by ceria and related
materials
Dependence on x, however, is limited to 1.5-2 orders of magnitude and mainly correlated with variations at low x. Together with the level of impurities present inherently in all ceria samples, this makes measurements of oxygen diffusion in ceria and Ce02.x a very delicate task, which may explain the rather scattered data in Fig. 2.16. 1250
1000
800
T(K)
6E-6
1250
1000 T(K) 800
700
1E-6
1E-7
1E-8
1E-91E-10 3E-10 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 0.7 0.8 0.9 1.0 1.1 1.2 lOOO/TCK"1) 1000/T (K"1) Figure 2.16. Left: oxygen diffusion data of various ceria-containing materials: 1 Ce0 2 , single crystal; 2 CeQ 2 polycrystalline sample; 3 Ce 0 g Y O 2 O 1 9 ; 4 Ce 0 6 Y 0 4 O l g ; 5 CeO !S 34 ; 6 C e ^ C a ^ O , / ' ; 7 (0.3%)Gd-doped Ce0 2 85 ; 8 Ce0 2 88 . The shaded area corresponds to oxygen diffusion values calculated from ionic conductivity of rare-earth doped ceria Ce,.,M x O 2 . 05 , with x=0.2 and 0.3 6 8 6 9 using Eq. 2.29. Right: details of oxygen diffusion coefficients of rare-earth doped ceria C e ^ M j O ^ j , with x=0.3; from top to the bottom (M=Eu,Nd,Ho,Gd,Er,Sm>Y,La,Yb,Dy).
2.4.
References Morss, L. R. Handbook on the Physics and Chemistry of Rare Earths Elsevier Science: New York, 1994; Vol. 18, pp 239-291. Adachi, G. and Imanaka, N. Chem. Rev. 98 (1998), 1479-1514. Bevan, D. J. M. and Kordis, J. /. Inorg. Nucl. Chem. 26 (1964), 1509-1523.
Structural properties and nonstoichiometric behavior
4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31.
47
Bevan, D. J. M. J. Inorg. Nucl. Chem. 1 (1955), 49-59. Brauer, G. and Gingerich, K. A. /. Inorg. Nucl. Chem. 16 (1960), 87-99. Brauer, G. and Holtschmidt, U. Z. Anorg. Allg. Chem. 265 (1951), 105-116. Brauer, G. and Gradinger, H. Z. Anorg. Allg. Chem. 277 (1954), 89-95. Knappe, P. and Eyring, L. J. Solid State Chem. 58 (1985), 312-324. Ray, S. P.; Nowick, A. S. and Cox, D. E. J. Solid State Chem. 15 (1975), 344-351. Ray, S. P. and Cox, D. E. /. Solid State Chem. 15 (1975), 333-343. Hoskins, B. F. and Martin, L. R. Aust. J. Chem. 48 (1995), 709-739. Kang, Z. C ; Zhang, J. and Eyring, L. Z Anorg. Allg. Chem. 622 (1996), 465-472. Campserveux, J. and Gerdanian, P. J. Solid State Chem. 23 (1978), 73-92. Panlener, R. J.; Blumenthal, R. N. and Gamier, J. E. J. Phys. Chem. Solids 36 (1975), 1213-1222. Chiang, H. W.; Blumenthal, R. N. and Fournelle, R. A. Solid State Ionics 66 (1993), 85-95. Shannon, R. D. and Prewitt, C. T. Acta Cryst. B 25 (1969), 925-1048 Kim, D. J. Am. Ceram. Soc. 72 (1989), 1415-1421. Sorensen, O. T. J. Solid State Chem. 18 (1976), 217-233. Kummerle, E. A. and Heger, G. J. Solid State Chem. 147 (1999), 485-500. Ricken, M.; Nolting, J. and Riess, I. /. Solid State Chem. 54 (1984), 89-99. Martin, R. L. /. Chem. Soc. Dalton (1974), 1335-1350. Haire, R. G. and Eyring, L. Handbook on the Physics and Chemistry of Rare EarthsElsevicr Science: New York, 1994; Vol. 18, pp 413-505. Sawyer, J. O., Hyde, B. G. and Eyring.L. Bull. Soc. Chim. France. (1965), 1190-1199. Riess, I.; Ricken, M. and Nolting, J. /. Solid State Chem. 57 (1985), 314322. Sorensen, O. T. in Nonstoichiometric Oxides, Ed. Sorensen, O. T., Academic Press: New York, 1981; pp 1-59. Korner, R.; Ricken, M. and Nolting, J. /. Solid State Chem. 78 (1989), 136147. Tuller, H. L. and Nowick, A. S. /. Phys. Chem. Solids 38 (1977), 859-867. Blumenthal, R. N. and Hofmaier, R. L. J. Electrochem. Soc. 121 (1974), 126-131. Kitayama, K.; Nojiri, K.; Sugihara, T. and Katsura, T. /. Solid State Chem. 56 (1985), 1-11. Courtel, R. and Loriers, J. Comptes Rendus Acad. Sci. (Paris) 230 (1950), 735-737. Tsunekawa, S.; Sivamohan, R.; Ito, S.; Kasuya, A. and Fukuda, T. NanoStructured Materials 11 (1999), 141-147.
48
Catalysis by ceria and related
materials
32. Perrichon, V.; Laachir, A.; Bergeret, G.; Frety, R.; Tournayan, L.; Touret, O. /. Chem. Soc. Faraday Trans. 90 (1994), 773-781. 33. Sayle, T. X. T.; Parker, S. C ; Catlow, C. R. A. Surf. Sci. 316 (1994), 329-336. 34. Cordatos, H.; Ford, D.; Gorte, R. J. J. Phys. Chem. 100 (1996), 1812818132. 35. Barnighhausen, H. and Schiller, G. /. Less Common Met. 110 (1985), 385390. 36. Conesa, J. C. Surf. Sci. 339 (1995), 337-352. 37. Loriers, J. Comptes Rendus Acad. Sci. (Paris) 231 (1950), 522-524. 38. Minervini, L.; Zacate, M. O. and Grimes, R. W. Solid State Ionics 116 (1999), 339-349. 39. Tuller, H. L. and Nowich, A. S. /. Electrochem. Soc. 126 (1979), 209-217. 40. Blumenthal, R. N.; Lee, P. W. and Panlener, R. J. J. Electrochem. Soc. 118 (1971), 123-129. 41. Kang, Z. C. and Eyring, L. J. Alloys Comp. 249 (1997), 206-212. 42. Faber, J.; Seitz, M. A. and Mueller, M. H. J. Phys. Chem. Solids 37 (1976), 909-915. 43. Faber, J.; Seitz, M. A. and Mueller, M. H. /. Phys. Chem. Solids 37 (1976), 903-907. 44. Kang, Z. C. and Eyring, L. Aust. J. Chem. 49 (1997), 981-996. 45. Zhang, J.; Von Dreele, R. B. and Eyring, L. J. Solid State Chem. 104 (1993), 21-32. 46. Von Dreele, R. B.; Eyring, L.; Bowman, A. L. and Yarnell, J. L. Acta. Cryst. B (1975), 971-974. 47. Kang, Z. C. and Eyring, L. /. Alloys Comp. 275-277 (1998), 30-36. 48. Boivin, J. C. and Mairesse, G. Chem. Mater. 10 (1998), 2870-2888. 49. Tuller, H. L. Solid State Ionics 52 (1992), 135-146. 50. Mazanec, T. J. Solid State Ionics 70/71 (1994), 11-19. 51. Gellings, P. J. Catal. Today 12 (1992), 1-92. 52. Riess, I.; Janczikowski, H. and Nolting, J. J. Appl. Phys. 61 (1987), 49314933. 53. Blumenthal, R. N. and Sharma, R. K. /. Solid State Chem. 13 (1975), 360364. 54. Naik, I. K. and Tien, T. Y. J. Phys. Chem. Solids 39 (1978), 311-315. 55. Dawicke, J. W. and Blumenthal, R. N. /. Electrochem. Soc. 13 (1986), 904909. 56. Steele, B. C. H. and Floyd, J. M. Pwc. British. Ceram. Trans. 72 (1971), 5576. 57. Tuller, H. L. and Nowich, A. S. J. Electrochem. Soc. 122 (1975), 255-259. 58. Panhans, M. A. and Blumenthal, R. N. Solid State Ionics 60 (1993), 279-298.
Structural properties and nonstoichiometric behavior
49
59. Chiodelli, G.; Flor, G. and Scagliotti, M. Solid State Ionics 91 (1996), 109121. 60. Chang, E. K. and Blumenthal, R. N. J. Solid State Chem, 72 (1988), 330337. 61. Blumenthal, R. N.; Brugner, F. S. and Gamier, J. E. J. Electrochem. Soc. 120 (1973), 1230-1237. 62. Gerhardt, R. and Nowick, A. S. J. Am. Ceram. Soc. 69 (1986), 641-646. 63. Shuk, P.; Greenblatt, M.; Croft, M. Chem. Mater. 11 (1999), 473-479. 64. Eguchi, T.; Setoguchi, T.; Inoue, T. and Arai, H. Solid State Ionics 52 (1992), 165-172. 65. Virkar,A.V. and Hong,S.J. J. Am. Ceram. Soc. 78 (1995), 433-439. 66. Inaba, H. and Tagawa, H. Solid State Ionics 83 (1996), 1-16. 67. Yahiro, H.; Ohuchi, T.; Eguchi, K. and Arai, H. /. Mater. Sci. 23 (1988), 1036-1041. 68. Dirstine, R. S.; Blumenthal, R. N. and Kuech, T. F. /. Electrochem. Soc. 126 (1979), 264-269. 69. Kudo, T. and Obayashi, H. /. Electrochem. Soc. 122 (1975), 142-147. 70. Huang, W.; Shuk, P.; Greenblatt, M. Chem. Mater. 9 (1997), 2240-2245. 71. Dikmen, S.; Shuk, P.; Greenblatt, M. Solid State Ionics 112 (1998), 229307. 72. Li, G.; Mao, Y.; Li, L.; Feng, S.; Wang, M.; Yao, X. Chem. Mater. 11 (1999), 1259-1266. 73. Kilner, J. A. and Steele, B. C. H. Nonstoichiometric Oxides, Ed. Sorensen, O. T., Academic Press: New York, 1981; pp 233-269. 74. Kilner, J. A. and Waters, C. D. Solid State Ionics 6 (1982), 253-259. 75. Yahiro, H.; Eguchi, K. and Arai, H. Solid State Ionics 36 (1989), 71-75. 76. Shuk, P.; Greenblatt, M. Solid State Ionics 116 (1999), 217-223. 77. Butler, V.; Catlow, C. R. A.; Fender, B. E. F. and Harding, J. H. Solid State Ionics 8 (1983), 109-113. 78. Gerhardt, R. and Nowick, A. S. Solid State Ionics 5 (1981), 547-552. 79. Faber, J.; Geoffrey, C ; Roux, A.; Sylvestre, A. and Abelard, P. Appl. Phys. A 49 (1989), 225-232. 80. Cook, R. L. and Sammells, A. F. Solid State Ionics 45 (1991), 311-321. 81. Wang, D. Y.; Park, D. S.; Griffith, J. and Nowick, A. S. Solid State Ionics 2 (1981), 95-105. 82. Nowick, A. S. Diffusion in Crystalline Oxides, Ed. Nowick, A. S.; Murch, G. E., Academic Press: New York, 1984; pp 143-188. 83. Matzke, Hj. Nonstoichiometric Oxides, Ed. Sorensen, O. T., Academic Press: New York, 1981; pp 155-232. 84. Etsell,T. H. and Flengas, S. N. Chem. Rev. 70 (1970), 339-376. 85. Freer, R. J. Mater. Sci. 15 (1980) 803-824.
50
Catalysis by ceria and related materials
86. Floyd, J. M. Indian J. Technol 11 (1973), 589-594. 87. Martin, D.; Duprez, D. J. Phys. Chem. 100 (1996), 9429-9438. 88. Kamiya, M.; Shimada, E. and Ikuma, Y. J. Ceram. Soc. Japan 106 (1998), 1023-1026. 89. Millot, F. and De Mierry, P. /. Phys. Chem. Solids 46 (1985), 797-801.
CHAPTER 3
SYNTHESIS AND MODIFICATION OF CERIA-BASED MATERIALS GIN-YA ADACHI and TOSHIYUKIMASUI Department of Applied Chemistry, Faculty of Engineering, Osaka University, Osaka, Japan; e-mail:
[email protected]
3.1. Introduction Ceria has been recognized as a key material of the three-way catalysts in automobiles since it can release and uptake oxygen owing to the following reversible reaction:1"8 Ce0 2 = Ce02.x + (x/2)02
(3.1)
This oxygen storage capacity (OSC) is useful to adjust the air/fuel ratio achieving a high conversion efficiency of the pollutants such as CO, NOx, and hydrocarbons. In accordance with the recent enforce of restrictions, it is essential to eliminate the pollutants at low temperatures when an automobile engine has just started.2 Generally, oxidation of Ce02_x occurs at room temperature, while the reduction of Ce0 2 starts at 473 K. However, pure Ce0 2 deactivates in its OSC when the exhaust temperatures exceed 1123 K due to sintering of the Ce0 2 particles and decrease in surface area.9 Since the reduction of Ce0 2 is essentially limited to the surface,10 it has been believed that a high surface area is essential for obtaining high OSC. However, recently it has been elucidated that the OSC of the ceria-based materials depends on the formed phase rather than the surface area.11"20 Synthesis and preparation process affects some properties of ceria-based materials such as formed phase, particle size, surface area, catalytic activities, and OSC etc. Therefore, many studies on synthesis, preparation, and modification have been carried out to develop the ceria-based materials of high catalytic activities, OSC, and thermal durability. In this chapter, conventional processes and recent advances in the synthesis and modification of the ceria-based materials are reviewed with the dependence of these methods on the characteristics of the materials.
51
Catalysis by ceria and related materials
52
3.2. Solid to Solid Synthesis 3.2.1. Ceramic Method Many inorganic solid materials have been synthesized by reacting a solid with another solid. The oldest and the most common method of preparing multicomponent materials is a direct reaction of corresponding solid compounds at high temperatures, where reacting atoms can diffuse through solid phases to the reaction front more easily. Since solid-state reactions are often very slow, high temperatures are necessary to complete the reaction. This method has prevailed and been convenient to prepare solid materials, particularly in a form of polycrystalline powder, with low cost for production on the industrial scale. Let us consider the thermal reaction of two single crystals of compounds A and B in contact along one face (See Figure 3.1). The first stage of the reaction is the formation of nuclei of the product phase. Nucleation is generally difficult when the structure of the product is different from the one of the reactant phases, because considerable structural reorganization of the lattice is required to form the product. This rearrangement costs high energy, hence the nucleation will occur at elevated temperatures. However, if the structures of the products and reactants are similar, like in the ceria-based materials system where Ce0 2 is usually used as one of the reactants, the nucleation occurs very easily.
A B Figure 3.1. Schematic illustration of a solid-state reaction.
After the nuclei of product C have formed, the reaction to form the product layer is limited at two reaction interfaces: one at the A-C interface and the other at the B-C interface. At the growth stage of the product layer, diffusion of ions from reactants A and B occurs through the product layer C. The initial reaction is prompt because of short diffusion path, but a further reaction becomes slower as ions must diffuse longer distances with growth of the product layer. As a result, the reaction rate gradually becomes slower with the time and a number of undesired phases may
Synthesis and modification of ceria-based materials
53
form in many cases. Therefore, various modifications are employed to increase reaction rates in solid-state synthesis. Using small particles, cooling and regrinding of the samples, and ball-milling treatment often facilitate the solid-state reactions. These processes enhance blending and increase surface area of the reactant particles. A high surface area of the reactants is an important factor, because it increases the contact area of the reacting solids. The contact area is multiplied by pressing the powder into a pellet. In the ceria-based materials system, the starting reactants usually used are oxides. The solid-state method are employed for the determination of phase diagrams especially in Ce0 2 -Zr0 2 systems.21"32 Typically, the starting materials are mixed in an agate mortar or in an agate ball mills for several hours, as dry powders or as wet slurries using solvents (methanol). The mixed powders are isostatically pressed into pellets at 200 MPa and fired in air at 1000-1400 °C for several hours. After firing, they are crushed, ground, pressed again at 200 MPa and refired at 1600-1800 °C. Finally, quenched products are crushed and ground again into powders. Using this method, several Ce0 2 -Zr0 2 mixed oxides are synthesized in a whole range of this system, and a detailed structural phase transition has been characterized.21"32
3.2.2. Mechanical Milling The use of mechanical milling is also a suitable method for powder preparation. The feature of this procedure is to obtain powders of small crystallite size of a few nanometers with a high concentration of lattice defects. There are many mechanochemical studies on the synthesis of alloys, solid solutions, nanophase materials, and intermetallic compounds. Of course, the mechanical alloying also has been applied to prepare mixed oxides containing cerium oxide to enhance catalysis, such as Ce02-TbOx,33 Ce02-Hf02,33 Ce02-Zr02,33"37 CeO r ZrO r MnO x> 38 and CeO r Zr0 2 -Cu0. 38 The basic process of mechanical alloying is illustrated in Figure 3.2. The starting materials usually used are oxides. Powders with typical surface area in the range of 10-50 m 2 g -1 are placed in a high-energy vibratory ball mill in the stoichiometric amount to contain Ce[.xMx02 (0 < x < 0.9). This mill is equipped with zirconia balls and vials made of stabilized-zirconia. During the ball-milling, the particles are subjected to local high pressure and mechanical deformation during collisions with the hard zirconia balls. Long duration of ball milling results in formation of nanosized grains having dislocations with high density. This process is a simple and effective method for preparing Ce02-based
54
Catalysis by ceria and related materials
materials and the presence of structural defects in the mixed oxides promotes the increase in the oxygen storage capacity.33"37
ee
Figure 3.2. Schematic illustration of the mechanical milling.
3.3. Liquid to Solid Synthesis The solid-state method requires high temperature heating or high mechanical energy for diffusion of reactants. Synthesis in liquid phase is useful to prepare solid compounds that contain the different cations in an ideally atomic dispersion. There are two methods concerning this concept: one is a precursor method and the other is a co-precipitation method. 3.3.1. Precursor Method Ceria-based oxides can be obtained by the decomposition of some compound precursor, such as hydroxide, nitrate, halides, sulfates, carbonates, formates, oxalates, acetates, and citrates.39,40 For example, nanosize or porous cerium oxide particles have been prepared at low temperatures by pyrolysis of amorphous citrate,4142 which is prepared by the evaporation of the solvent from the aqueous solution containing cerium nitrate (or oxalate) and citric acid. In the case of mixed oxides, the precursor containing some cations in the same solid salts is prepared. In the same manner of ceria particles, the precursors complexing some cations with citrates are useful to synthsize ceria-zirconia mixed oxides and their derivatives.43 Also, Ce0 2 -Ln 2 0 3 solid solutions, where Ln = La, Pr, Sm, Gd, and Tb, have been synthesized from the precursors obtained by the evaporation of nitrate solutions at 353 K in air from an intimate mixture of their respective metal nitrates.44 The precursors are dried and then heated at 673 K to remove nitrates, followed by calcination at 1073 K for 12h.
Synthesis and modification of ceria-based materials
55
A variation of the precursor method is that the mixed precursors are polymer complexes. A powder of composition Ce0J2Zr088O2 has been synthesized at mild temperatures (873 - 1073 K) by a polymer complex solution method using polyvinylalcohol (PVA) and by a polymerizable complex method using ethylene glycol.45"47 In these methods, a solution of polyvinylalcohol (PVA) (or ethylene glycol), citric acid, and metal ions are polymerized to form a gel precursor with randomly distributed cations. Heating of these precursors at a mild temperature produces mixed oxides that are compositionally homogenous at an atomic level. In addition, the combustion process that modifies the precursor method is also attractive. It requires only a short duration of few minutes to produce metal oxides having fine size and large surface area. The required quantities of metal nitride are mixed with urea, and the mixture is dissolved in a minimum amount of water. The content is transferred to a Pyrex dish and then is introduced into a preheated muffle furnace maintained at 773 K. The water evaporated in a few minutes to produce a fluffy powder. Ceria and Ce[.xPrx02.y fine powders have been synthesized by this method.48"51
3.3.2. Precipitation and Coprecipitation Method Chemical precipitation is a widely used method for synthesizing solid materials from solution. This method utilizes a liquid-phase reaction to prepare insoluble compounds that are crystalline or amorphous precipitates. The precipitate usually is composed of fine particles, and, of course, ceria-based fine particles can be synthesized by this method. Usually, ceria preparation is carried out by calcination of the hydroxide or oxalate gel precipitated using the reaction of aqueous solution of inorganic cerium salt (Ce(N03)3, CeCl3, CeS0 4 , and (NH4)2Ce(N03)6) with alkali solution (NaOH, NH4OH, and (NH2)2-H20) or oxalic acid.3952"54 In a typical precipitation process, oxide powders or their precursors are obtained by adding a solution containing metal cations directly to a precipitant. However, simply adding the solution to the precipitant has little control on particle size and morphology because of the rapid change of solution concentration and the discontinuous nature of precipitates formation. Th pH of precipitation may be different for each metal ion component, that is to say, the composition at beginning of the precipitation is different with that at an end. To improve this disadvantage, a homogenous precipitation method has been developed. In this process, precipitants are generated simultaneously and uniformly throughout the solution using the controlled release of the reaction-participating ligands by another chemical source in
56
Catalysis by ceria and related materials
the solution. For example, urea and hexamethylentetramine slowly decompose to yield ammonia by heating at 343 - 353 K. Applying this method, ceria55"57 and ceria-yttria58 particles of spherical shape with a narrow size distribution have been prepared. Another process to obtain uniform fine ceria particles is the forced hydrolysis method that is useful for preparation of metal oxides and hydroxides.
Figure 3.3. Transmission electron micrograph of Ce0 2 particles synthesized by the forced hydrolysis method. (Reproduced with permission from ref. 59. Copyright 1988 American Chemical Society.)
The particles shown in Figure 3.3 can be prepared from tetravalent cerium salt solution (CeS0 4 -4H 2 0, (NH4)4Ce(S04)4-2H2Q, and (NH^CeCNO^) in low concentrations by low temperature aging in a sealed vessel.59 The metal ions are solvated by water molecules which can be deprotonated to give hydroxide or oxide particles. This method is very sensitive to the concentration, temperature, and pH value of the solution. In the synthesis of mixed oxides, the co-precipitation method is the most commonly used wet-chemical process. Salts of the several metals are dissolved in the same solvent (water is the most popular one). Ideally, a quantitative and simultaneous precipitation of all the cations occurs without segregation of any particular constituents in the precipitates. This ideal situation is very rare in most cases, especially more than two metal cations are involved. Differences in solubility between several precipitating phases affect the precipitation kinetics of each metal
Synthesis and modification of ceria-based materials
57
ion component. Therefore, it is reasonable to think that homogeneous coprecipitation at an atomic level is very difficult and the most of resulting precipitate are considered as a homogenous mixture of fine particles.45 However, this method is a very popular technique, and plenty of ceria-based mixed oxides have been prepared (see Table 3.1) such as Ce02-MnOx,6061 Ce0 2 -La 2 0 3 , 62 Ce02-TbOx,6364 Table 3.1.
Synthesis condition, average particle ;size, and BET surface :area of ceria-based
materials synthesized by the precipitation method. Sample Ce0 2
Calcination
Average
temp. / K
size / nm
/mV
723
65
60,61
80
72
53
69
22
62
873
-
43
69
873
35
63
75
923
-
18
69
41
71
49
72
6
69
14
72
6
68
1
69
49
62
1173
-
NH4OH
873
36
(COOH),
923
-
Starting materials
Precipitant
Ce(N03),-6H20
NH4OH
773 823 873
973 973 1173 1173 1200 1373
Ce 0 , 5 Mn 02s O 25
Ce(NO,),-6H20
Ce
Mn(N03),-6H,0
o.5oMno.5cA-8
NH4OH + H 2 0 2
873
(COOH)2
923
NH4OH
723 723 723
.....?.?tA5¥.?flJi?if. ^o/zs^^-zs^z-s
Ce(NO,),-6H20
NH4OH
873
La(NOj),-5H,0
NH4OH + H,0 2
873
NH4OH
873
NH4OH + H 2 0 2
873
NH4OH
873
NH4OH + H2Qj
873
NH4OH
873
Ce^LaoyOjg
^0.15^^.25^2-5
^-e0.80
"0.20^2-5
Ce(NO,),6H 2 0 Tb(NOj)3-5H20
Ce090Nd010O2g
CefNO,), aq.
Ref..
10
66
80
60,61
64
60,61
55
60,61
23
62
53
62
22
62
40
62
25
62
41
62
45
63
8
64
60
75
19.5
66,67
Nd(N02)j aq. Ce
Pr
0
Ce(NO,), aq.
Ce
Pr O
Pr(NO,), aq.
Ce Ce
Pr 0 Pr O
Table 3.1. (Continued).
7.75 13.3 2.44
Catalysis by ceria and related materials
58
Sample
Starting materials
Precipitant
^ e 0 . 8 0 " 0.20^2
Ce(N03),-6H20
NH4OH
Calcination
Ave. size
temp. / K
/ nm
1200
SBET
/mV
Ref..
26
68 70
HfCl4 Ce 3 ZrO g
Ce(N03)3-6H20
(NH2)2H20
ZrO(N03)2-2H,0
e
*- 0.90^0.10^2-6
CeCNO,), aq.
373
12
573
13
-
773
13
58
70
873
-
57
70
70
973
15
56
70
1073
-
54
70
1173
17
41
70
1273
20
22
70
1373
34
3
70
1473
74
1
70
1573
103
0.1
70
NH4OH
873
31
70.2
75
NH4OH
773
-
85
72
ZrO(N03)2 aq. Ce 0 8 3 Zr 0 1 7 O 2
Ce(N03)3-6H20 ZrO(N03)2-7R,0
973 1173
^ e 0.80
0.20 2
Ce(N03)3-6H20
NH4OH
ZrO(N0 3 ) 2 xH 2 0
823 973 1073 1173 1200 1373
^- e 0.75^ r 0.25^*2
Ce(N03)3-6H20
NH4OH
973
NH4OH
773
58
72
27
72
87
69
73
69
47
69
29
69
29
68
4.5
69
36
71
104
72
70
72
ZrOCl2-8H20 ^ e 0.67^ r 0.33^2
Ce(N03)3-6H,0
NH4OH
973
-
(COOH)2
1273
-
4
73
NH4OH
773
-
97
72
ZrO(N03)2-7H20
973 1173
e
r
^- 0.50^ 0.50^2
Ce(N03)3-6H,0
25
72
61
71
ZrOCl2-8H20 ^- e 0.5O^ r 0.50^2
Ce(N0 3 ), aq. ZrO(N03)2 aq.
^e0.47^r053^2
Ce(N0 3 ) 3 6H 2 0 ZrOCNOjJj^HjO
973 1173
^O^S^O.TS^
Ce(N03),-6H20
^ e O.20^ r O.80^2
ZrOC!2-8H20
NH4OH
973 973
^• e o.l5^ r 0.85^2
973
Ce
973
0.l0Zr0.90O2
Table 3.1. (Continued).
62
72
19
72
58
71
46
71
43
71
44
71
Synthesis and modification of ceria-based materials
Sample Ce
Starting materials
7r
^ ^ 0 . 5 6 1 " 0.374
Y 1
0
0.065W 2-5
Ce(N03)3-6H20
Precipitant NH4OH
59
Calcination
Ave. size
temp. / K
/nm
/mV
S
BET
Ref..
873
9.0
68
74
ZrO(N0 3 ) 2 xH J 0
1173
-
15
74
Y(NO,),6H 2 0
1273
14.0
1.3
74
(NH4)2Ce(N03)6
(COOH),
873
5.3
54
74
Y(N03),-6H20
1173
74
Zr(C 2 0 4 ) 4 nH 2 0
1273
-
16 10
74
(NH4),Ce(N03)6
(COONH4),
873
6.4
39
74
Y(N03)3-6H20
1173
11
74
Zr(IV) citrate
1273
-
8
74
(NH4)2Ce(N03)6
NH4HC03
873
8.2
27
74
Y(N03)3-6H20
1173
-
14
74
ZrCCjO^-nHjO
1273
30
11
74
873
37
61.0
75
(COONH4)2
*-'eo.90*0.1(r-'2-S
Oxides
Ce 0 ,x»Bao. 1 o02- S
dissolved in
32
68.0
75
*-e0.90^r0-10^2-8
HN0 3 or HCl
46
48.0
75
*-' e 0.90^ a 0.10^2-5
49
47.7
75
Ce
o.90 0.10 2-5
41
53.6
75
" 0.90 t - U 0.10*^2-5
39
58.8
75
^-^0.90 "®0. 10^2-8
40
52.4
75
Ce
0.10 2-6
37
62.6
75
^ e 0.90^°0.10^2-5
40
57.7
75
^nonNdninOj-S
36
60
75
Zn
O
t e
0.»
Mn
O
Ce02-Nd203,65 Ce02-PrOx,6567 Ce02-Hf02,68 Ce02-Zr02,68-73, Ce02-Zr02-Y203,74 and Ce0 2 -MO x (M = Nd, Y, Ba, Sr, Ca, Pb, Mn, Co)75 as shown in Table 3.1.
3.3.3. Hydrothermal and Solvothermal Synthesis Hydrothermal synthesis is well known in mineralogy and geology fields for growth of minerals and ores.76 High temperature and high pressure water can be used as a transfer medium of heat, pressure, and mechanical energy, an adsorbate that works as a catalyst, a solvent which dissolves or reprecipitates the solid materials, and a reagent which acts as a mineralizer. These works have been used in processing of inorganic materials in the preparation of single crystals and particularly of fine powders with nanosized to submicron particles.77 Besides water (hydrothermal
60
Catalysis by ceria and related
materials
synthesis) ammonia water or some organic solvents are also important reaction media. Generally these processes are known as "solvothermal methods". Table 3.2. Synthesis condition and average crystalline size of hydrothermally crystallized ceria-based materials.
Sample
Starting materias
Solvent
17 K
Ce0 2
Ce(N0 3 ) 3 -6H 2 0 Ce(N0 3 ) 3 -6H 2 0
NH4OH NH4OH
573 453
14 17
79 80
Ce(S0 4 ) 2 -6H 2 0 Ce(S0 4 ) 2 -6H 2 0
NH4OH Urea, H 2 0
453 453
3 12.5
80 81
i- x Eu x 0 2 . 5
Ce(N0 3 ) 3 -6H 2 0 Eu(N0 3 )-5H 2 0
NH4OH
533
40-50
82
Ce!. x Pr x 0 2 . 8
Ce(N0 3 ) 3 -6H 2 0 Pr(N0 3 ) 3 -6H 2 0
NH4OH
533
35-49
83
Ce,. x La x 0 2 . 5
Ce(N0 3 ) 3 6H 2 0 La(N0 3 ) 3 -6H 2 0
NH4OH
533
28-44
84
Ce,. x Ca x 0 2 . x
Ce(N0 3 ) 3 6H 2 0 Ca(N0 3 ) 2 -6H 2 0
NH4OH
533
40-50
85
Ce,. x Sm x 0 2 . x , 2
Ce(N0 3 ) 3 -6H 2 0 Sm(N03)_3_-6H20
NH4OH
533
40-68
85
(Ceo^Smoi^.^Tb/Pr^O,,,,^
Ce(N0 3 ) 3 -6H 2 0 Sm(N0 3 ) 3 -6H 2 0 Pr(N0 3 ) 3 -6H 2 0 Tb(N0 3 ) 3 -5H 2 0
NH4OH
533
7-14
86
CeL^BiPj.s
Ce(N0 3 ),-6H 2 0 Bi(NO,) 3 6H 2 0
NH4OH
533
25-40
87
Ce(N0 3 ),-6H 2 0 Bi(NO,) 3 -5H 2 0
NaOH, H 2 0
513
13-19
88
Ce
Ce 012 Zr 088 O 2
Ce,. x Zr x 0 2
Ce(N0 3 ) 3 -6H 2 0 ZrO(N0 3 ) 2 -2H 2 0
Ce(N0 3 ) 3 -6H 2 0 ZrOCl2-8H20
size / nm
Ref..
H20
523
7.5
89
CH3OH
523
89
i-C3H7OH n-C,H„OH
523 523
6.5 7 6.5
89
NH4OH
453
6-20
90
89
Synthesis and modification of ceria-based materials
61
In preparing fine particles of inorganic metal oxides, the hydrothermal method consists of three types of processes: hydrothermal synthesis, hydrothermal oxidation, and hydrothermal crystallization. Hydrothermal synthesis is used to synthesize mixed oxides from their component oxides or hydroxides. The particles obtained are small, uniform crystallites of 0.3-200 |am in size and dispersed each other. Pressures, temperatures, and mineralizer concentrations control the size and morphology of the particles. In the hydrothermal oxidation method, fine oxide particles can be prepared from metals, alloys, and intermetallic compounds by oxidation with high temperature and pressure solvent, that is, the starting metals are changed into fine oxide powders directly. For example, the solvothermal oxidation of cerium metal in 2-methoxyethanol at 473-523 K yields ultrafine ceria particles (ca. 2 nm). 78 The hydrothermal crystallization is the most popular technique in preparing ceria-based nanoparticles. Precipitation from aqueous solutions under elevated temperature and high pressure are involved in the process. Usually the hydrothermal crystallization is carried out as follows. An excess amount of precipitates is added to the cerium salt solutions. The precipitated gels are sealed in Teflon-lined autoclaves and hydrothermally treated at 423-573 K for several hours. The autoclaves are quenched and the crystalline powder products are washed and dried. Using the hydrothermal crystallization method, a number of ceria-based nanoparticles have been prepared as summarized in Table 3.2. The particle size clearly depends on the reaction temperature and the starting materials used. It is shown that by heating at low temperature and by using tetravalent cerium salt solutions smaller particles can be obtained.
3.3.4. Sol-gel Method A sol-gel process is an important technique that can be used to synthesize many materials in a variety of shapes and forms. This method is especially suited for the synthesis and preparation of ultrafine oxide materials at relatively low temperatures. A sol is a stable colloidal dispersion of small particles suspended in a liquid. The particles are amorphous or crystalline and particle aggregation is prevented by electrostatic repulsion. The particles in some sols interact to form a continuous network of connected particles called a gel, instead of aggregating to form larger particles. Drying a gel simply by evaporating the interstitial liquid gives rise to capillary forces causing the gel to shrink and causing the formation of cracks as a result of the differential stresses generated in the drying gel. The resulting dried gel is known as a xerogel. When the wet gel is dried under supercritical conditions, the
62
Catalysis by ceria and related materials
pore and network structure of the gel is maintained even after drying. The resulting gel in this case is called aerogel. These sol-gel materials are frequently applied to catalysts and catalyst supports because they have high surface area. Table 3.3. Synthesis condition, average particle size, and BET surface area of ceria-based materials synthesized by the sol-gel method.
Sample
Starting materials
T/K
Ce0 2
CeCl3-7H20
r. t.
Ce(CH3COCHCOCH3)3 Ce(N0 3 ) 3 -6H 2 0
Ref.
size / nm
/ m2 g'1
4.0-5.5
33-75
92
775
5.9
110
94
923
44.3
-
98
Ceo.98Zro.n2O2
Ce(N03)3-6H2C>, Zr(OC3H7)4
873
-
62
96
Ce oy Zr 0 jU2
Ce(CH3COCHCOCH3)3, Zr(OC4H9)4
775
4.7
94
94
Ce(N0 3 ) 3 -6H 2 0, Zr(OC3H7)4
873
4.4
75
96
Ce(N0 3 )„ Zr(OC3H7)4
1053
56
95
35
95
55
95
44
95
Ce 0 8 /.r 0 2 O 2
Ce(CH3COCHCOCH,)3, Zr(OC4H9)4
775
-
109
94
C e o. 75^02502
Ce(N0 3 ) 3 -6H 2 0, Zr(OC3H7)4
873
4.0
63
96
Ce 07 Zr 03 O 2
Ce(CH3COCHCOCH3)3 Zr(OC4H9)4
775
4.2
187
94
Ce(CH3COCHCOCH3)3, Zr(OC4H9)4
775
-
138
94
Ce(N0 3 ) 3 -6H 2 0, Zr(OC3H7)4
873
4.0
71
96
Ce(CH3COCHCOCH3)3, Zr(OC4H9)4
775
154
94
Ce(N0 3 ) 3 -6H 2 0, Zr(OC3H7)4
873
56
96
Ce0.16^r0.84O 2
Ce(N0 3 ) 3 -6H 2 0, Zr(OC3H7)4
873
-
46
96
Ce 09 Ti 01 O 2
Ce(N0 3 ) 3 -6H 2 0, Ti(OC4H9)4
923
29.4
Ceo.gT'o^^ Ceo.7Tioj02
25.5
923
24.6
^-e0.6 1*0.4^2
923
21.6
Ceo.5Tio.5O2
923
20.5
Ceo.4Tio.602
923
19.5
-
98
923
1173 Ce(N0 3 ) 3 , Zr(OC4H9)4
1053 1173
e
r
^ 0.6^ 0.4^2
Ce 05 Zr 05 O 2
98 98 98 98 98
A method of producing the sol is to hydrolyse reactive metal compounds, for example alkoxides, M(OR)„, where M is a metal (e.g. Ce and other rare earths, Al, Ti, Zr, etc.) and R is an alkyl group (e.g. methyl, CH 3 , ethyl, C 2 H 5 , or propyl,
Synthesis and modification of ceria-based materials
63
C3H7). In the sol-gel method, metal alkoxides are generally dissolved in an alcohol (methanol, ethanol, or iso-propanol are usually used) and addition of water causes hydrolysis of matal alkoxides. M(OR)n + n H 2 0 -> M(OH)n + n ROH
(3.2)
This is followed by a series of condensation reactions between hydroxide groups and the overall reaction is represented by the following chemical equation: M(OH)n -> MO„n+ n/2 H 2 0
(3.3)
It can be seen that this method allows mixed oxide gels to be produced readily by mixing of their alkoxides solutions prior to hydrolysis. The sol-gel synthesis of some rare earths oxides has been carried out for the first time in 1971.91 For the preparation of ceria-based oxides, cerium isopropoxide, cerium acetylacetonate, cerium nitrate are used as the precursors. The water necessary for the hydrolysis reactions is brought in by adding directly or by the hydrated cerium nitrate. Using this method, Ce0 2 , 92 Ce02-PrOx,93 Ce02-Zr02,93"96 and Ce02Ti0297'98 particles have been synthesized (Table 3.3.). Minor modified sol-gel methods have been applied to the synthesis of Ce0 2 -Zr0 2 solid solutions.17,99100 In this case, the precursors obtained by the evaporation of solvent are digested to be hydrolyzed at 363 K for two days. These processes are good methods to obtain fine powders, but have a drawback in the high cost of metal alkoxides.
3.3.5. Surfactant-assisted Method The use of templating techniques for the synthesis of mesoporous solids has recently opened up new opportunities in the design of novel high-surface area materials for catalytic applications.101 This consists of using surfactants as templating agents for the creation of mesopores with regular structure. This approach has been applied to the synthesis of transition metal oxides using different organic molecules as templating agents.102"104 Anionic, cationic, and amphoteric surfactants can be employed with success in the preparation of high-surface area materials. In a few of these compounds, ordered pore structure was obtained even after calcination, but in the majority cases the regular pore structure collapse by the calcination. Mesoporous ceria and ceria-zirconia powders with high surface area have also been prepared using a surfactant-assisted method to prepare catalysts containing Ce0 2
64
Catalysis by ceria and related
materials
with improved textural and redox/catalytic properties.105"108 The ceria and ceriazirconia solid solutions have been prepared by a reaction of a cationic surfactant with the hydrous mixed oxide produced by co-precipitation under basic conditions. In a typical synthesis, the materials are prepared by slowly adding an aqueous ammonia water (25%) to an aqueous solution containing CeCl3-7H20, ZrOCl2-8H20, and a cationic surfactant (e.g. cetyltrimethylammonium bromide) until the pH reaches 11.4-11.5. After precipitation, the mixture is stirred for 60 min and then sealed and placed in a thermostatic bath maintained at 363 K for 90-120 hours. The mixture is then cooled and the resulting precipitate was filtered and washed repeatedly with water and acetone to remove the free surfactant. The obtained powder is dried at 333 K for 24 h and then calcined at temperatures of 623 - 1173 K for at least 2 h under an air flow. The elimination of surfactants on calcinations gives high surface area samples, although the regular pore structure is not obtained. The mean crystalline sizes of the particles distributed in the region of 2 - 5 nm for ceria and 4-18 nm for ceria-zirconia, respectively. Surface areas in an excess of 200 m^g"1 are obtained after calcinations at 773 K, which drop to ca. 40 m2-g"' after calcination over 1173 K as summarized in Table 3.4. Table 3.4. Mean particle size, BET surface area of ceria and ceria-zirconia solid solutions synthesized by the surfactant-assisted method107,108. Sample CeO z
Cacination temp. / K
Average size / nm
Surface area / rn^g'1
723
2-5
231 126
1073 1273 ^ e 0.80^ r 0.20^2
723 923
30 - 100
208 8-10
1173
15-18
723 923
4-6
56 235 170 115
1073 1173
163 124
1073
*-"6o.68^r0.12^2
36
13-15
40
The enhancement of the surface area of ceria-based materials related to the surfactant effect that reduces the surface tension inside the pores by decreasing capillary stress during drying and calcination processes. Better thermal stability is related to the structural arrangement and the morphology of the inorganic-organic
Synthesis and modification of ceria-based materials
65
composites which is produced by an exchange between the deprotonated hydroxy group of the oxides and the alkyl ammonium cation upon drying and calcination. These features could also contribute to the enhanced textural stability of these materials in comparison with those prepared by the conventional precipitation methods.
3.3.6. Emulsion and Microemulsion Method An emulsion liquid membrane (ELM) system has been studied for the selective separation of metals. This system is a multiple phase emulsion, water-in-oil-inwater (W/O/W) emulsion. In this system, the metal ions in the external water are moved into the internal water phase, as shown in Fig. 3.4. The property of the ELM system is useful to prepare size-controlled and morphology controlled fine particles such as metals,109 carbonates,"0111 and oxalates.112"116 Rare earth oxalate particles have been prepared using this system, consisting of Span83 (sorbitan sesquioleate) as a surfactant and EHPNA (2-ethyl-hexylphospholic acid mono-2-ethylhexyl ester) as an extractant.114""6 In the case of cerium, well-defined and spherical oxalate particles, 20 - 60 nm in size, are obtained. The control of the particle size is feasible by the control of the feed rare earth metal concentration and the size of the internal droplets. Formation of ceria particles are attained by calcination of the oxalate particles at 1073 K, though it brings about some construction of the particles probably caused by carbon dioxide elimination.
Figure 3.4. Schematic illustration for the formation of particles in the ELM method.
The ELM method is convenient to synthesize fine particles in |0.m scale but not suitable for the preparation of nanoparticles. The production of single nanometer particles is nowadays one of the most important and attractive technology. Applying
66
Catalysis by ceria and related
materials
chemical reactions in microemulsions is one of the powerful methods for obtaining ultrafine nanoparticles.117 This method is based on the use of reversed micelles as small reactors. The microemulsion is composed of two immiscible liquids and a surfactant. In water-inoil microemulsions, nanodroplets of aqueous phase within the reversed micelles are dispersed in oil phase.
Figure 3.5. Schematic mechanism for the formation of particles in the microemulsion method. (Reproduced with permission from ref. 117. Copyright 1993 Academic Press Inc.)
Figure 3.5 shows a schematic picture of this process. After mixing two microemulsons containing the reactants, the interchange of reactants is carried out during the collisions of the nanodroplets in the microemulsions. The interchanging process is very fast and the final size of the particles is controlled by the droplet size. Once the particles attain the final size, the surfactant molecules protect against further growth of the particle. The size of the droplets can be controlled in the range of 5 - 50 nm by varying the microemulsion system itself. Using this method, ceria and ceria-zirconia mixed oxides have been synthesized.118"120 The average particle sizes of the particles are around 2 - 4 nm, and depend on the synthesis conditions.
3.3.7. Flux Method The flux method is a well-known method used for single crystal growth. It has not been applied to the synthesis of fine powders because usually high temperature heating is necessary to obtain molten salts. However, the modified flux method has been reported for the preparation of fine particles of Ce,.APrx02 solid solutions.121 In the preparation of the powders by the flux method, molten salts of alkali metal hydroxides, nitrates, and chlorides are used as solvents. The use of molten salts,
Synthesis and modification of ceria-based materials
67
especially in NaOH-KOH eutectic mixtures, accelerates the kinetics of formation of the desired compounds by enhancing diffusion coefficients. The precursors, cerium(IV) ammonium nitrate (NH4)2Ce(N03)3 and praseodymium(III) nitrate Pr(N03)3-6H20, are added in a 1:1 weight ratio to the molten salts at 673 - 873 K, and the melt is maintained for 15 - 120 min. After the melt is quenched to room temperature, the reaction products are washed with water and then dried at 393 K. Well-crystallized Ce^P^Oj (x = 0 - 10) powders with very fine size (10 - 20 nm), narrow size distribution, and a clearly spherical shape are obtained.
3.3.8. Electrochemical Methods Electrochemical synthesis is an attractive method for preparing oxide ceramic films and powders because it offers the advantages of low-temperature synthesis, low cost, high purity, and controlled microstructure. In the electrochemical synthesis of oxide particles, both anodic (redox change) and cathodic methods (base generation) can be employed. In the redox change method, a metal ion or complex is oxidized at the electrode surface. The pH value of the solution is adjusted so that the initial oxidation state is stable, and then the electrogenerated higher oxidation state experiences hydrolysis to a metal hydroxide or oxide. In the cathodic method, cathodic currents are used to generate a base at an electrode surface, and the electrogenerated base then hydrolyses metal ions or complexes. The pH value at the electrode surface is considerably higher than that of the bulk solution. Nanocrystalline cerium (IV) oxide powders with an average particle size of 10 14 nm have been prepared by the cathodic base electrogeneration method.122123 The nanocrystalline Ce0 2 powders are prepared in the cathode compartment of a divided electrochemical cell. The cathode is a platinum wire and the anode is a platinum mesh electrode. The cathode compartment in the divided cell contained 0.5 moM"1 cerium (III) nitrate and 0.5 mol-1"1 ammonium nitrate, and the anode compartment contained 0.5 mol l"1 ammonium nitrate. The two compartments are separated with a medium porosity glass frit. The electrochemical synthesis is run in the galvanostatic mode at a current density of 1 A c m 2 and the particle size is controlled by adjusting the solution temperature.
Catalysis by ceria and related materials
68 3.3.9.
Spray
Pyrolysis
Spray pyrolysis is one of the effective methods to prepare homogenous and nonagglomerated sphere particles. The particles are generated by spraying a liquid precursor and by the subsequent reaction of the aerosol droplets in a furnace. Precursors usually employed are aqueous solutions of metal salts. Figure 3.6 is a schematic picture of this process. The first step on spray pyrolysis is the atomization process. This can be carried out using variety of atomizers (pressure, ultrasonic, vibration, disk rotation, electrostatic force, etc.). Heating the aerosols in air or nitrogen converts the salts to oxides after evaporation of the solvent. The size of the product particle is proportional to that of the aerosol droplets. Therefore, the particle size and the size distribution are determined by the employed atomizer. In this process, porous particles are easily formed. The porosity is controlled by changing the precursor concentration in the droplets or by adjusting temperature profile in the furnace. Hollow particles can also be prepared when the solute concentration gradient is created during evaporation of solvent. Easy scaling up is a major advantage of this method, too. Instead of an electronic furnace, an r.f. inductively coupled plasma (ICP) is also used to generate high temperatures. This method is named a spray-ICP technique and has been applied for a wide variety of oxides.'24 Another variation of the spray pyrolysis is the spray- drying method that uses slurry or sol of metal
\/
II •
furnace •
I I IT—I Particle deposition Figure 3.6. Schematic illustration of the spray pyrolysis.
Synthesis and modification of ceria-based materials
69
oxides and hydroxides instead of aqueous solutions. Well-crystallized ceria nanoparticles have been synthesized by this sol-spray technique at low temperatures.125
3.3.10. Impregnation Method Impregnation method is used for preparing catalysts that ultrafine particles are deposited on high-surface area supports. A solution containing the catalyst component (single or plural) is impregnated into the pore of the support and adhered by the subsequent drying and calcinations. The metal compounds are held by adsorption of metal cations on the basic site on the support (O2" or basic OH") or by ion exchange between matal cations and H+ of acidic OH" groups. Usually the former is called an impregnation method and the latter is called an ion exchange method. The impregnation method is convenient to disperse a small amount of wellfine particles on the surface of the support. An amount of supported particles depends on the concentration of the solution and pore volume of the support. Incipient wetness impregnation method is also used for precious control of the amount of the deposition. After evacuation of the support, the solution corresponded to the pore volume is added little by little and the surface of the support uniformly get wet. Ceria powders that are doped various cations such as Ca2+, Mg2+, Al3+, Y3+, 3+ Sc , Al3+, Th4+, Zr4+, and Si4+, have been prepared according to the wetness impregnation method to investigate their effects on thermal stability of ceria.7126 These impregnation methods are, of course, useful to synthesize ceria-based oxides supported on another oxides such as silica and alumina.127"129 The most advantage of the method is that highly dispersion is obtained.
3.4. Gas to Solid Synthesis 3.4.1. Gas Condensation or Sputtering The most commonly used technique of gas to solid synthesis involves the condensation of a vapour produced by the heating of a solid or a liquid starting material. This technique is very popular in preparation of thin films and nanoparticles. A model of an apparatus is shown in Figure 3.7. This comprises an
70
Catalysis by ceria and related materials
ultrahigh vacuum chamber, equipped with a liquid nitrogen-cooled finger, scraper and collector. The vacuum chamber is first pumped to a vacuum < 10"5 Pa by an oildiffusion pump equipped with a liquid-nitrogen trap or by a turbomolecular pump. In the case of powder synthesis, the chamber is then filled with a few hundred Pa of a high-purity inert gas. The starting material, usually in the form of a solid powder (mostly a metal) is vaporised by resistive heating in a boat or crucible made from a refractory metal (e.g. Mo, W or Ta). Alternative evaporation energy can be employed such as high-power lasers, ion bombardment (sputtering), or electron beam. If the chamber is in vacuum, the ensuing vapour strikes a substrate positioned over the boat or the crucible, where it condenses and forms a thin solid film. Liquid N^
Z3£$_4— Inert ; Vacuum 1—©—I Figure 3.7. Schematic mechanism for the formation of particles in the gas condensation method.
In the particle synthesis, metal atoms produced by the heating collide with the inert gas atoms to decrease the diffusion rate of the atoms from the source region. The collisions also cool the atoms to induce the formation of small clusters of fairly homogeneous size. The clusters grow mainly by cluster-cluster condensation to give nanoparticles with a broader size distribution. A convective flow of the inert gas between the warm region near the vapour source and the cold surface carries the nanoparticles to the cooled finger, where they are let to deposit. The inert gas pressure, the evaporation rate, and the gas composition can control the characteristics
Synthesis and modification of ceria-based materials
71
of the particles. For example, decreasing either the gas pressure in the chamber or the rate of evaporation of the metal decreases the particle size. The formed particles are then scraped from the cold finger and collected. Using two or more evaporation sources produces alloy particles. Mixing or replacing the inert gas with a reactive gas can produce oxide and other materials. Metal oxide nanoparticles such as ceria-based oxides have also been produced by the controlled oxidation of metal or alloy nanoparticles synthesized by evaporation of the metal. Nonstoichiometric cerium oxide-based catalysts, CeOz.x and Cu or La-doped Ce02.x nanoparticles, have been prepared by this method.130"132 The average particle sizes are about 8 nm. The nonstoichiometric Ce02.x based materials exhibit greater catalytic activities than precipitated ultrafine particles. For example, the light off temperatures for S0 2 reduction by CO, CO oxidation, and CH4 oxidation are 373 - 453 K lower for the pure and La-doped Ce02_x nanoparticles than for the respective precipitated samples.132 Nanostructured ceria particles have also been synthesized by direct thermal evaporation of cerium oxide from a tungsten crucible in an He-atmosphere of 1000 Pa.133 The crystalline size distributions are narrow with maxima between 3 and 3.5 nm diameter.
3.4.2. Chemical Vapor Deposition In chemical vapor deposition (CVD) reactive vapor precursors react to produce solid materials in the gas phase or at the solid-gas interface on the substrate surface at appropriate temperatures. Typical precursors used in the CVD process are metal hydrides, metal chlorides, and metal organic compounds. In the case that the precursor species are metal organic compounds, the process is called metal-organic chemical vapor deposition (MOCVD). The precursor molecules are introduced into a reactor sometimes with a carrier gas and decompose by means of heat, irradiation of UV light, or electrical plasma formed in the gas. Thermal CVD is the most commonly used method. This technique has an advantage that refractory materials can be vapour-deposited at relatively low temperatures. The CVD method is usually used to produce a thin film material which is formed on a heated substrate. However, nanostructured particles of ceria and ceriayttria have been synthesized by some arrangements of the apparatus. Figure 3.8 shows the schematic CVD reactors for synthesizing ceria-based nanoparticles.134 Two types of rector has been presented. The nanoparticles are collected either on a cooled quartz susceptor (A) that is in a furnace, or in a cold wall container outside the furnace (B). The precursor cerium chloride set on the container is evaporated and
Catalysis by ceria and related materials
72
introduced into the reactor with an argon carrier gas, where the reaction gas oxygen is fed separately into the reactor. The gas velocity of the nozzle is adjusted such that oxygen will not diffuse into the precursor area. The powders collected using apparatus B (20 - 30 nm) are smaller than those collected using apparatus A (30 80 nm).
(A)
Nozzle
Evaporator
Furnace Air in — » • = &
ll-rrr— Airoutfc Reactor outlet Susceptor (B)
Nozzle
Evaporator
Exhaust gas
Cold wall Figure 3.8. Schematic diagrams of the CVD reactors. (Reproduced with permission from ref. 134. Copyright 1999 Elsevier Science B.V.)
3.5. Modification of Bulk and Surface
5.5.7. Effects of Dopants Usually, the sintering of the catalytic components causes deactivation of catalysts. For example, the automotive three-way catalysts deactivate by the sintering of fine particles of precious metals and alumina supports as well as Ce0 2 when the catalysts are subjected at high temperatures for a long time.9 Sintering of Ce0 2 facilitates the
Synthesis and modification of ceria-based materials
73
sintering of the precious metals and the decrease in oxygen storage capacity. Therefore, it is very important to stabilize Ce0 2 and to avoid sintering when it is used at high temperatures. As a work on this line, thermal stability of ceria and doped ceria was investigated in detail.126 Generally, doping of different cations significantly stabilize the cerium oxide against sintering. Among many different cations, the addition of zirconium, especially the formation of ceria-zirconia solid solutions, is very effective in the inhibition of the sintering.9 Another important effect of the cation doping is increase in the number of defects (oxygen vacancies) that produce a material with a higher oxygen storage capacity. The incorporation of aliovalent elements (divalent or tetravalent cations) into Ce0 2 lattice produces lattice oxygen anion vacancies by a charge-compensating effect of foreign cations: (1-x) Ce0 2 + 0.5 x M 2 0 3 -> Ce1.xMxO2.05x + y Vo (1-x) Ce0 2 + x M O - > Ce^MA-x + zVo
(3.4) (3.5)
where M is a divalent or trivalent cation and Vo is an oxygen anion vacancy. For example, the introduction of Ca2+75, La3+,128 and Gd3+ 135 increases the OSC chiefly through the creation of defects. In other cases, doping isovalent elements like Zr4+ and Hf4+ into the Ce0 2 lattice strongly affects the redox properties of ceria7,16,68 by increasing both total17 and kinetic69 oxygen storage. In this system, bulk properties of the material play a key role in the extent of reduction rather than the surface. The excellent redox behavior of these materials is appears when the creation of structural defects by the variation of cell parameter takes place in a cubic system. Indeed, for ceria-zirconia, it has been reported that the optimum composition is around Ce!.xZrx02 with 0.2 < x< 0.5,l6,35,17,136,137 and in this range bulk diffusion of O2" ion is approximately two orders of magnitude higher for ceria-zirconia than for pure ceria.137 The mixture of these effects described above is obtained by the doping of rare earths elements with variable oxidation state. For example, incorporation of terbium or praseodymium increases both oxygen desorption at lower temperatures and the creation of oxygen vacancies than those of pure ceria. The former is due to the lower binding energy of a lattice oxygen in the mixed oxides and the latter is to the existence of trivalent terbium and praseodymium ions. In addition, a similar effect is also provided by the ternary oxides Ce0.6Zr0.4.xMxO2.^2 (M = Y3+, La3+, and Ga3+).43 The formation of solid solutions is not the only way to modify the redox property of ceria. The incorporation of small amounts of silica in a form of ceria-
74
Catalysis by ceria and related materials
silica composite is very effective to increase the redox activities.138 In this case, silica does not form solid solutions with ceria, and thus the material consists of domains of amorphous silica and nanocrystalline ceria. The separated silica aggregates help to remain the ceria particles small enough to have a lower energy of reduction and oxidation.
3.5.2. Structural Modification by Redox Aging Another interesting feature observed in ceria-zirconia solid solutions is the reduction temperature decrease after a cycling of high-temperature reduction (1000 - 1523 K) and subsequent reoxidation at middle temperatures (700 - 873 K) in CexZr!_x02 with x = 0.4 - 0.6.,117'139 This redox aging strongly affects the redox behavior and dramatically increases oxygen storage capacity. The reason for the changes has been explained by a slight rearrangement of atoms from their original positions, the formation of a cubic phase, and the increase in bulk anion mobility into a modified ceria. In the Ce0 2 -Zr0 2 system, a monoclinic structure is stable for the Ce0 2 molar contents of less than 20%, while a cubic phase is formed for the Ce0 2 contents higher than 80%.24,25'30'3' In the intermediate region, the true nature of the Ce0 2 -Zr0 2 phase diagram is still unidentified due to the presence of stable and metastable tetragonal phases, but three different tetragonal phases t, t', and t" has been distinguished on the basis of XRD and Raman characterization.22"32 (see Chapter 6 for details). With composition Ce05Zr05O2, a cubic pyrochlore Ce2Zr207 phase is obtained by hydrogen reduction of the stable tetragonal t' phase above 1323 K. When the reduction temperature is lower, a Calyrelated cubic phase is obtained. The pyrochlore phase has a regular arrangement of cerium and zirconium ions, where the CaF2-related phase has random cation arrangement. These phases are easily reoxidized at 700 - 873 K to form metastable cubic k and tetragonal t'mem phase, respectively, maintaining their cation arrangements.14 The oxygen atoms in these metastable phases are relatively unstable compared with those in the starting tetragonal t' phase and, therefore, these materials can release oxygen at lower temperatures than the t'tetragonal phase. The effects on the low temperature reduction are larger for the cubic k phase than that for the tetragonal t'meta phase. The pyrocholre-based cubic metastable phases can be obtained by the hydrogen reduction and subsequent reoxidation of tetragonal ceria-zirconia mixed oxides. In addition, these solid solutions can be synthesized by means of the thermal
Synthesis and modification of ceria-based materials
75
decomposition of cerium zirconyl oxalate in an argon flow at 1273 K and subsequent oxidation at 673 - 873 K in air.140 In this case, carbon produced during the thermal decomposition of the cerium zirconyl oxalate works as a reducing agent. Since the Gibbs free energy at 1273 K for the reduction of Ce0 2 is lower than for the reduction by hydrogen,141 the reduced phase is produced effectively by the reduction with carbon particles homogeneously dispersed in an atomic level.
3.5.3. Surface Modification The cubic pyrochlore-based ceria-zirconia solid solution is a good material. However, it has been reported that the reduction temperature has become higher after the hightemperature oxidation.""13139 This is due to the transformation from pyrochlorebased-cubic to r*-tetragonal phase which is more stable man k, t'mm, and f phases.12,13 As a result, the oxygen release temperature increases. The increase in oxygen release temperature is a serious problem, since deactivation of the subcatalysts affects overall catalytic activities. Recently, one of the solutions to overcome this problem has been proposed.142143 This does concern surface modification of the pyrochlore-based oxides. It is known that cerium and zirconium chlorides provide vapor phase complexes with aluminum chloride at elevated temperatures.144"146 The new surface modification technique utilizes the formation of these vapor complexes to remove and modify the top surface of the pyrochlore ceria-zirconia solid solution. This method is named "chemical filing". Application of the above complexes formation has already been demonstrated for the vapor phase extraction and mutual separation of rare earths based on the so-called chemical vapor transport (CVT).147,148
Metal chloride
MC^
Vapor comp|ex
NH4CI
Oxim
— * Chlori nation™"'
"*•"
Vaporize
Modification of surface
Figure 3.9. Schematic illustration of the chemical filing process. (Reproduced with permission from ref. 150. Copyright 2000 Elsevier Science S.A.)
Catalysis by ceria and related materials
76
Figure 3.9 shows a schematic representation of the chemical filing process. The first step of the process involves the chlorination of the surface of the pyrochlore-based ceria-zirconia sample. The extent of the chlorination can be controlled by the concentration of the chlorine gas and/or chlorination time and the cerium and zirconium chlorides partially formed on the surface are vaporized and transported by the formation of gaseous complexes with aluminum chloride. This chemical filing process is carried out at 1273 K to stabilize the surface modification effects at high temperatures. A similar effect can also be achieved by chlorination with ammonium chloride followed by dominant vaporization of formed zirconium chloride.149 The chemical filing technique is very effective in modifying the redox property in the low temperature regions. The reduction temperatures of the chemically filed samples become lower than those of the non-filed ones without decreasing in the amount of the released oxygen. The redox activities of the chemically filed samples are maintained even after several reduction and reoxidation aging at 1273 K. The reasons for these better redox activities have been attributed to the formation of trace amounts of Ce0 2 ultrafine particles with the evolution of zirconium and subsequent stabilization of the metastable k and t'meta phases.143149
3.6. 1. 2. 3.
References
Trovarelli A., Comments Inorg. Chem., 20 (1999), 263-284. Kaspar J., Fornasiero P., and Graziani M. Catal. Today, 50 (1999), 285-298. Kaspar J., Graziani M., Fornasiero P., Ceria-Containing Three-Way Catalysts, in Handbook on the Physics and Chemistry of Rare Earths, Vol. 29, eds. Gshneidner, Jr. K.A. and Eyring L. (Elsevier, Ireland, 2000), 159-267. 4. Yao H. C. and Yao Y. F., J. Catal., 86 (1984), 254-265. 5. Trovarelli A., Catal. Rev. Sci. Eng., 38 (1996), 439-520. 6. Trovarelli A., de Leitenburg C , and Dolcetti G., CHEMTECH, 27 (1997), 32-37. 7. Ozawa M., Kimura M., and Isogai A., /. Alloys Compds., 193 (1993), 7375. 8. Murota T., Hasegawa T., Aozasa S., Matsui H.. and Motoyama M., /. Alloys Compds, 193 (1993), 298-299. 9. Ozawa M., Kimura M., Sobukawa H., and Yokota K., Toyota Tech. Rev., 27 (1992), 43-53. 10. Laachir A., et al., /. Chem. Soc. Faraday Trans., 87 (1991), 1601-1609.
Synthesis and modification of ceria-based materials
11
11. Otsuka-Yao S., Morikawa H., Izu N., and Okuda K., /. Japan. Inst. Metals, 59 (1995), 1237-1246. 12. Otsuka-Yao-Matsuo S., Omata T., Izu N., and Kishimoto H., J. Solid State Chem., 138 (1998), 47-54. 13. Izu N., Omata T., and Otsuka-Yao-Matsuo S., J. Alloys. Compds., 2 7 0 (1998), 107-114. 14. Omata T., Kishimoto H., Otsuka-Yao-Matsuo S., Ohtori N., and Umesaki N., /. Solid State Chem., 147 (1999), 573-583. 15. Kishimoto H., Omata T., Otsuka-Yao-Matsuo S., Ueda K., Hosono H., and Kawazoe H„ J. Alloys. Compds., 312 (2000), 94-103. 16. Fornasiero P., Di Monte R., Ranga Rao G., Kaspar J., Meriani S., Trovarelli A., and Graziani M., J. Catal., 151 (1995), 168-177. 17. Fornasiero P., Balducci G., Di Monte R., Kaspar J., Sergo V., Gubitosa G., Ferrero A., and Graziani M., J. Catal., 164 (1996), 173-183. 18. Vlaic G., Fornasiero P., Geremia S., Kaspar J., and Graziani M., /. Catal., 168 (1997), 386-392. 19. Vlaic G., Di Monte R., Fornasiero P., Fonda E., Kaspar J., and Graziani M., J. Catal., 182 (1999), 378-389. 20. Fornasiero P., Di Monte R., Vlaic G., Kaspar J., and Graziani M., /. Catal., 187 (1999), 177-185. 21. Yoshimura M., Tani E., and Somiya S., Solid State Ionics, 3/4 (1981), 477481. 22. Tani E., Yoshimura M., and Somiya S., /. Am. Ceram. Soc, 77 (1983), 506-510. 23. Yashima M., Morimoto K., Ishizawa N., and Yoshimura M., /. Am. Ceram. Soc, 76 (1993), 1745-1750. 24. Yashima M., Morimoto K., Ishizawa N., and Yoshimura M., J. Am. Ceram. Soc, 76 (1993), 2865-2868. 25. Yashima M., Arashi H., Kakihana M., and Yoshimura M., /. Am. Ceram. Soc, 11 (1994), 1067-1071. 26. Yashima M., Takashita H., Kakihana M., and Yoshimura M., /. Am. Ceram. Soc, 11 (1994), 1869-1874. 27. Yashima M., Sasaki S., Yamaguchi Y., Kakihana M., Yoshimura M., and Mori T., Appl. Phys Lett, 72 (1998), 182-184. 28. Meriani S., Mater. Sci, Eng., 71 (1985), 369-373. 29. Meriani S., /. Phys. C Suppl. 2, 47 (1986), C1-485-C1-489. 30. Meriani S. and Spinolo G., Powder Dijf., 2 (1987), 255-256. 31. Meriani S., Mater. Sci. Eng., A109 (1989) 121-130.
78
Catalysis by ceria and related materials
32. Duran P., Gonzalez M., Moure C , Jurado J.R., and Pascal C , J. Mater. Sci., 25 (1990), 5001-5006. 33. Zamar F., Trovarelli A., de Leitenburg C., and Dolcetti G., Stud. Surf. Sci. Catal., 101 (1996), 1283-1292. 34. de Leitenburg C , Trovarelli A, Zamar F., Maschio S., Dolcetti G., and Llorca J., /. Chem. Soc. Chem. Commun., (1995), 2181-2182. 35. Trovarelli A., Zamar F., Llorca J., de Leitenburg C , Dolcetti G., and Kiss T., J. Catal., 169 (1997), 490-502. 36. Suda A., Kandori T., Terao N., Ukyo Y., Sobukawa H., and Sugiura M., J. Mater. Sci. Lett., 17 (1998), 89-90. 37. Enzo S., Delogu F., Frattini R., Primavera A., and Trovarelli A., J. Mater. Res., 15 (2000), 1538-1545. 38. Primavera A., Trovarelli A., Llorca J., Terribile D., de Leitenburg C., and Dolcetti G., Mater. Res. Soc. Symp. Proc, 454 (1997), 247-252. 39. Eyring L., The Binary Lanthanide Oxides: Synthesis and Identification, in Synthesis of Lanthanide and Actinide Compounds, eds. Morss L.R. and Meyer G. (Kluwer, Dordrecht, 1991), 187-224. 40. Adachi G. and Imanaka N., Chem. Rev., 98 (1998), 1479-1514. 41. Powell B.R., BloinkR.L., and Eickel C.C., J. Am. Ceram. Soc, 71 (1988), C104-C106. 42. Dong X., Hong G., Yu D., and Yu D., J. Mater. Sci. TechnoL, 13 (1997), 113-116. 43. Vidmar P., Fornasiero P., Kaspar J., Gubitosa G., and Graziani M., /. Catal., 171 (1997), 160-168. 44. O'Connel M. and Morris M.A., Catal. Today, 59 (2000), 387-393. 45. Kakihana M., /. Sol-Gel Sci. TechnoL, 6 (1996), 7-55. 46. Kakihana M., Kato S., Yashima M., and Yoshimura M., /. Alloys Compds., 280 (1998), 125-130. 47. Yamamoto S., Kakihana M., and Kato S., J. Alloys Compds., 297 (2000), 81-86. 48. Sekar M.M.A., Manoharan S.S., and Patil K.C., J. Mater. Sci. Lett., 9 (1990), 1205-1206. 49. Varma H.K., Mukundan P., Warrier K.G.K., and Damodaran A.D., J. Mater. Sci. Lett., 9 (1990), 377-379. 50. Varma H.K., Mukundan P., Warrier K.G.K., and Damodaran A.D., J. Mater. Sci. Lett., 10 (1991), 666-667. 51. RajendranM., Mallick K.K., Bhattacharya A.K., J. Mater. Sci., 33 (1998), 5001-5006.
Synthesis and modification of ceria-based materials
79
52. de Leitenburg C , Trovarelli A., and Kaspar J., J. Catal., 166 (1997), 98-107. 53. Audebrand N., Auffredic J.-P., and Louer D., Chem. Mater., 12 (2000), 17911799. 54. Nakane S., Tachi T., Yoshinaka M., Hirota K., and Yamaguchi O., /. Am. Ceram. Soc, 80 (1999), 3221-3224. 55. Matijevic E. and Hsu W.P., J. Colloid Interface Sci., 118 (1987), 506-523. 56. Chen P.I. and Chen I.W., J. Am. Ceram. Soc, 76 (1993), 1577-1583. 57. Chu X., Chung W., and Shumidt L.D., J. Am. Ceram. Soc, 76 (1993) 21152118. 58. Aiken B., Hsu W.P., and Matijevic E., J. Am. Ceram. Soc, 71 (1988), 845853. 59. Hsu W.P., Ronnquist L., and Matijevic E., Langmuir, 4 (1988), 31-37. 60. MachidaM., Uto M., Kurogi D., and Kijima T., Chem. Mater., 12 (2000), 3158-3164. 61. Machida M., Uto M., Kurogi D., and Kijima T., J. Mater. Chem., 11 (2001), 900-904. 62. Colon G., Navio J.A., Monaci R., and Ferino I., Phys. Chem. Chem. Phys., 2 (2000), 4453-4459. 63. Bernal S., Blanco G., Cauqui M.A., Corchado P., Pintodo J.M., and Rodoriguez-Izuqierdo J.M., Chem. Comm., (1997), 1545-1546. 64. Blanco G., Calvino J.J., Cauqui M.A., Corchado P., Lopez-Cartes C. Collex C , Perez-Omil J.A., and Stephan O., Chem. Mater., 11 (1999), 3610-3619. 65. McCullough J.D., J. Am. Chem. Soc, 72 (1950), 1386-1390. 66. Logan A.D. and Shelef M., J. Mater. Res., 9 (1994), 468-475. 67. Graham G.W., Haack L.P., and Shelef M., J. Mater. Res., 11 (1996), 19601971. 68. Zamar F., Trovarelli A., de Leitenburg C , and Dolcetti G., J. Chem. Soc. Chem. Comm., (1995), 965-966. 69. de Leitenburg C , Trovarelli A., Lorca J., Cavani F., and Bini G., Appl. Catal. A., 139 (1996), 161-173. 70. Kawabata A., Hirano S., Yoshinaka M., Hirota K., and Yamaguchi O., J. Mater. Sci, 31 (1996), 4945-4949. 71. Sergent N., Lamonier J.F., and Aboukais A., Chem. Mater., 12 (2000), 38303835. 72. Bozo C , Guihaume N., Garbowski E., Primet M., Catal. Today, 59 (2000), 33-45. 73. Masui T., Peng Y., Machida K., and Adachi G., Chem. Mater., 10 (1998), 4005-4009.
80
Catalysis by ceria and related materials
74. Kirichenko O.A., Graham G.W., Chun W., and McCabe R.W., Stud. Surf. Sci. Catal, 118 (1998), 411-420. 75. Zhang Y., Andersson S., and Muhammed M., Appl. Catal. B. Env., 6 (1995), 325-337. 76. Barnes H.L. (ed.), Geochemistry of Hydrothermal Ore Deposits, 3rd Ed., (Weily, NY, 1997). 77. Yoshimura M. and Somiya S., Mater. Chem. Phys., 61 (1999) 1-8. 78. Inoue M., Kimura M., and Inui T., Chem. Commun., (1999), 957-958. 79. Zhou Y.C. and Rahaman M.N., J. Mater. Res., 8 (1993), 1680-1686. 80. Hirano M. and Kato E., J. Am. Ceram. Soc, 79 (1996), 777-780. 81. Hirano M. and Kato E., /. Am. Ceram. Soc, 82 (1999), 786-788. 82. Shuk P., Greenblatt M., and Croft M., J. Alloys Compds., 303-304 (2000), 465-471. 83. Shuk P. and Greenblatt M., Solid State Ionics, 116 (1998), 217-223. 84. Dikmen S., Shuk P., and Greenblatt M., Solid State Ionics, 126 (1999), 8995. 85. Huang W., Shuk P., and Greenblatt M., Chem. Mater., 9 (1997) 2240-2245. 86. Huang W., Shuk P., and Greenblatt M., Solid State Ionics, 113-115 (1998), 305-310. 87. Dikmen S., Shuk P., and Greenblatt M., Solid State Ionics, 112 (1998), 299307. 88. Li G., Mao Y., Li L., Feng S., Wang M., and Yao X., Chem. Mater., 11 (1999), 1259-1266. 89. Sato T., Dosaka K., Yoshioka T., and Okumaki A., J. Am. Ceram. Soc, 7 5 (1992), 552-556. 90. Hirano M. and Kato E., J. Ceram. Soc. Jpn., 104 (1996), 958-962. 91. Madzdiyasni K.S. and Brown L.M., /. Am. Ceram. Soc, 54 (1971) 479-483. 92. Imoto F., Nanataki T., and Kaneko S., Ceram. Trans., 1 (1988) 204-210. 93. Narula C.K., Haack L.P., Chun W., Jen H.W., and Graham G.W., J. Phys. Chem., 103 (1999), 3634-3639. 94. Overbury S.H., Huntley D.R., Mullins D.R., and Glavee G.N., Catal. Lett., 51 (1998), 133-138. 95. Rossgnol S., Gerard F„ and Dupez D., /. Mater Chem., 9 (1999), 1615-1620. 96. Rossgnol S., Madier Y., and Dupez D., Catal. Today, 50 (1999), 261-270. 97. Rynkowski J., Farbotko J., Touroude R., and Hilaire L., Appl. Catal. A., 203 (2000), 335-348. 98. Luo M., Chen J., Chen L., Lu J., Feng Z., and Li C, Chem. Mater., 13 (2001), 197-202.
Synthesis and modification of ceria-based materials
81
99. Meriani S. and Soraru G., in Ceramic Powders, (ed) Vicenzini P., (1983), 547554, Elsevier, Amsterdam. 100. Fornasiero P., Kaspar J., and Graziani M., J. CataL, 167 (1997), 576-580. 101. Kresge C.T., Leonowicz M.E., Roth W.J., Vartuli J.C., and Beck J.S., Nature, 359 (1992), 710-712. 102. Antonelli D.M. and Ying J.Y., Angew. Chem. Int. Ed., 34 (1995), 20142017. 103. Sun T. and Ying J.Y., Nature, 389 (1997) 704-706. 104. Yada M., Kitamura H., Machida M., and Kijima T., Inorg. Chem., 37 (1998), 6470-6475. 105. Terribile D., Trovarelli A., de Leitenburg C , and Dolcetti G., Chem. Mater., 9 (1997), 2676-2678. 106. Terribile D., Llorca J., Boaro M., de Leitenburg C., Dolcetti G., and Trovarelli A., Chem. Commun., (1998), 1897-1898. 107. Terribile D., Trovarelli A., Llorca J., de Leitenburg C , and Dolcetti G., /. CataL, 178 (1998), 299-308. 108. Terribile D., Trovarelli A., Llorca J., de Leitenburg C , and Dolcetti G., Catal. Today, 43 (1998), 79-88. 109. Majima H., Hirato T., Awakura Y., and Hibi T., Metall. Trans. B, 22B (1991), 397-404. 110. Davey R.J. and Hirai T., /. Cryst. Growth, 171 (1997), 318-320. l l l . H i r a i T . , Okamoto N., Komasawa I., and Davey R.J., Langmuir, 13 (1997) 6650-6653. 112. Yang M., Davies G.A., and Garside J., Powder Technol, 65 (1991), 235-242. 113. Hirai T., Nagaoka K., Okamoto N., and Komasawa I., /. Chem. Eng. Jpn., 29 (1996), 842-850. 114. Hirai T., Okamoto N., and Komasawa I., AlChEJ., 44 (1998), 197-206. 115. Hirai T., Okamoto N., and Komasawa I., /. Chem. Eng. Jpn., 31 (1998), 474-477. 116. Hirai T., Okamoto N., and Komasawa I., Langmuir, 14 (1998) 6648-6653. 117. Lopez-Quintela M.A. and Rivas H., J. Colloid Interface Sci., 158 (1993), 446-451. 118. Masui T., Fujiwara K., Machida K., Adachi G., Sakata T., and Mori H., Chem. Mater., 9 (1997), 2197-2204. 119. Masui T., Fujiwara K., Peng Y., Sakata T., Machida K., Mori H., and Adachi G., J. Alloys Compds., 269 (1998), 116-122. 120. Fernandez-Garcia M., Maetinez-Arias A., Iglesias-Jues A., Belver C , Hungrfa A.B., Conesa J.C., and Soria J., J. Catal, 194 (2000), 385-392.
82
Catalysis by ceria and related materials
121.BondioliF.,CorradiA.B., and Manfredini T., Chem. Mater., 12 (2000), 324330. 122. Switzer J.A., Am. Ceram. Soc. Bull, 66 (1987), 1521-1524. 123. Zhou Y., Phillips R.J., and Switzer J.A., J. Am. Ceram. Soc, 78 (1995) 981-985. 124. Suzuki M., Kagawa M., Syono Y., and Hirai T., /. Mater. Sci., 17 (1992) 679-684. 125. Mani T.V., Varma H.K., Damodaran A.D., and Warrier K.G.K., Ceram. Int., 19 (1993), 125-128. 126. Pijolat M., Prin M., Soustelle M., Touret O., and Nortier P., J. Chem. Soc. Faraday Trans., 91 (1995), 3941-3948. 127. Bensalem A., Bozon-Verduraz F., Delamar M., and Bugli G., Appl. Catal. A., 111 (1995), 81-93. 128. Miki T., Ogawa T., Haneda M., Kakuta N., Ueno A., Tateishi S., Matsuura S., and Sato M., /. Phys. Chem., 94 (1990), 6464-6467. 129. Yao M.H., Baird R.J., Kunz F.W., and Hoost T.E., J. Catal., 166 (1997), 6774. 130. Tschope A. and Ying J.Y., Nanostr. Mater., 4 (1994), 617-619. 131. Tschope A., Liu W., Flytzani-Stepahanopoulos M., and Ying J.Y., /. Catal., 157 (1995), 42-50. 132. Tschope A., Schaadt D., Birringer R., and Ying J.Y., Nanostr. Mater., 9 (1997), 423-432. 133. Guillou N., Nistor L.C., Fuess H., and Hahn H., Nanostr. Mater., 8 (1997), 545-557. 134. Bui W., Choy K.L., Stelzer N.H.J., and Scoonman J., Solid State Ionics, 116 (1999), 225-228. 135. Cho B.K., J. Catal., 131 (1991), 74-83. 136. Cuiff J.P., Blanchard G., Touret O., Seigneurin A., Marczi M., and Quemere E.,SAE paper970463 (1997), 115-127. 137. Boaro M., de Leitenburg C , Dolcetti G., and Trovarelli A., J. Catal., 193 (2000), 338-347. 138. Rocchini E., Trovarelli A., Llorca J., Graham G.W., Weber W.H., Maciejewski M. and Baiker A., J. Catal., 194 (2000), 461-478. 139. Baker R.T., Bernal S., Blamco G., Cordon A.M., Pintado J.M., Rodorigez Izquierdo J. M., Fally F., and Perrichon V., Chem. Commun., (1999), 149150. 140. Masui T., Peng Y., Machida K., and Adachi G., Chem. Mater., 10 (1998), 4005-4009.
Synthesis and modification of ceria-based materials
83
141. Gmelin Handbuch der Anorganischen Chemie, System Nr. 39, Sc, Y, La und Lanthanide - Seltenerdelemente, Teil CI, Hydride; Oxide, Springer-Verlag, Berlin, (1974), p.256. 142. Masui T., Peng Y., Machida K., and Adachi G., /. Alloys Compds., 2 9 2 (1999), L8-L10. 143. Ozaki T., Masui T., Machida K., Adachi G., Sakata T., and Mori H., Chem. Mater., 12 (2000), 643-649. 144. Gruen D. M. and 0ye H. A., Inorg. Chem. Lett., 3 (1967), 453-455. 145. 0ye H. A. and Gruen D. M., /. Am. Chem. Soc, 91 (1969), 2229-2236. 146. Boghosian S. and Papatheodorou G. N., J. Phys. Chem., 93 (1989), 415-421. 147. Adachi G., Shinozaki K., Hirashima Y., Machida K., J. Less-Common Met., 169 (1991), L1-L4. 148. Jiang J., Ozaki T., Machida K., and Adachi G., J. Alloys and Compds., 2 6 0 (1997), 222-235. 149. Masui T., Nakano K., Ozaki T., Adachi G., Kang Z.C., and Eyring L., Chem. Mater., 13 (2001), 1834-1840. 150. Masui T., Ozaki T., Machida K., and Adachi G., /. Alloys Compds., 3 0 3 304 (2000), 49-55.
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CHAPTER4 CHEMICAL AND NANOSTRUCTURAL ASPECTS OF THE PREPARATION AND CHARACTERISATION OF CERIA AND CERIA-BASED MIXED OXIDE-SUPPORTED METAL CATALYSTS
SERAFIN BERNAL, JOSE J. CALVINO, JOSE M. GATICA, CARLOS LOPEZ CARTES and JOSE M. PINTADO Departamento de Ciencia de los Materiales, Ingenieria Metalurgicay Quimica Inorgdnica, Facultad de Ciencias, Universidad de Cadiz, Apartado 40, 11510 Puerto Real (Cadiz), Spain; e-mail:
[email protected] 4.1. Introduction For the last twenty years, the research effort on ceria-based catalytic materials has steeply increased (1). An important part of this effort has been devoted to M/Ce02 and closely related systems, i.e.: catalysts consisting of transition metals supported on the higher rare earth oxides (Ce02, Pr02-x, Tb02-x), and ceria-containing mixed oxides, all of them with fluorite-related structure. Table 4.1 summarises the open literature dealing with their preparation, characterisation and catalytic behaviour. Though not exhaustive, Table 4.1 is aimed at presenting an overview of these studies. Data included in this table show that the number of papers specifically dealing with M/Ce02 and related systems follows an evolution with time rather parallel to the statistical analysis reported in ref (1), in which the role of ceria-based materials as active phases and promoters was also considered. In accordance with this evolution, the past decade has been particularly active in the investigation of these catalysts. As deduced from Table 4.1, the investigations on M/Ce02 and related systems include a variety of supported transition metal catalysts. However, the studies devoted to Pd, Pt and Rh (NM) are particularly numerous. This specific interest on NM/Ce02 is certainly due to the close relationship existing between these model systems and the TWCs (Three Way Catalysts), nowadays used in the control of the exhaust emissions from spark-ignited motor vehicles (61,117,246-248). Also remarkable is that the research effort, which was initially focused on NM/Ce02 systems, is in recent years progressively shifting towards mixed oxide-supported catalysts: NM/CeM02.x. This evolution clearly reflects the influence of new developments in redox materials for TWC applications. In effect, as recently reported (246), in the latest 1980's, the poor textural stability of ceria was considered to be a major deactivation cause of TWCs; 85
86
Catalysis by ceria and related
materials
accordingly, new oxygen storage materials, with improved resistance against sintering ought to be developed. Alternative ceria-based mixed oxides (61,246,247), started to be investigated at the beginning of the 1990's. Nowadays, ceria-zirconia materials are used in the latest generation of commercial TWC's.
Table 4.1. Overview of the open literature dealing with M/Ce02 and closely related supported metal catalysts. Catalyst
References
Ag/Ce02
(2-4)
Au/Ce02
(5-9)
Co/Ce02
(10-14)
Cu/Ce02
(6,7,15-26)
Fe/Ce02
(10)
Ir/Ce02
(27-30)
Ni/Ce02
(10,17,24,26,31-43)
Pd/Ce02
(10,27,44-85)
Pd/Pr02-x
(86-91)
Pd/Tb02.x
(82,92)
Pd/CexZri_x02
(48,55,93-104)
Pd/PrxZn.x02.y
(100)
Pd/Ce,.x.yPrxZryOM
(48)
Pt/Ce02
(26,27,51,57,61,65,67,105-153)
Pt/Pr02.x
(127,154)
Pt/Tb02.x
(127)
Pt/CexTbi.x02.y
(109,115,155)
Pt/CexZr|.x02
(50,93,98,97,101,156-161)
Rh/Ce0 2
(5,27,57,61,65-67,109,110,117,126,128,143,162-230)
Rh/Pr02.x
(231)
Rh/Tb02.x
(232)
Rh/Ce.Pri^O^
(233)
Rh/Ce„Tb1.x02.y
(234)
Rh/C^ZriA
(98,101,104,161,235-241)
Ru/Ce02
(13,27,80,242-245)
The outstanding role of the TWC technology in orienting the research activity on M/Ce02 and related systems is also suggested by Table 4.2. This table summarises the catalytic studies performed on them. Though a variety of reactions, including CO and
Chemical and nanostructural
charaterization
ofmetal/ceria
systems
87
C0 2 hydrogenation, ammonia synthesis, hydrogenation of unsaturated organics, methane reforming, hydrogenolysis of saturated hydrocarbons, and wet oxidation of pollutants in waste waters have been tested, the main reactions taking place in the autoexhaust converters have received special attention. Such is the case of CO and hydrocarbon oxidations, NO reduction, steam reforming, and water-gas shift reaction. As deduced from Tables 4.1 and 4.2, the studies specifically dealing with the other pure higher rare earth oxide supports (praseodymia and terbia) are much scarce. However, a number of recent investigations have shown that the incorporation of praseodymium (69,100,274-278) and terbium (115,187,279-281) ions into the ceria lattice may improve its redox behaviour very significantly, thus becoming materials with potential interest in TWC technology and several other catalytic applications. This Chapter will be specifically devoted to the preparation and characterisation of the M/Ce02 and closely related systems. We shall discuss first the general procedures reported in the literature for dispersing the metal phases onto the ceriabased supports. Special attention will be paid to some specific preparative aspects, which may have important chemical and nano-structural effects on the resulting catalysts. Regarding the characterisation studies, after reviewing some of the most peculiar and challenging problems to be faced, we shall analyse the chemical and nanostructural information presently available. Finally, some concluding remarks summarising our current knowledge of the metal/support interaction effects occurring in M/Ce02 and related systems will be presented. Table 4.2. Some reactions investigated on ceria-supported metal catalysts and related systems. Reaction CO + Oz
Hydrocarbon Oxidation
Water Gas Shift Reaction (CO + H2O)
Steam Reforming of light hydrocarbons (C„H2„+2 + H2 0)
Catalyst (References) Pd/Ce02 (55,57,58,65,102,153) Pd/Ce,.xZrx02 (55,94,99) Pd/Pr02.x (55) Pt/Ce02 (57,65,122,123,140,153) Pt/CexTbi.x02.y (115) Rh/Ce02 (57,65,153,178,180,202,200,215,249) Cu/CeO; (19,25,250) Ag/Ce02 (200) Au/Ce02 (8) Ni/Ce02 (32) Pd/Ce02 (83) Pd/Ce,.xZrx02 (99) Pt/Ce02 (251) Pt/Ce,.xTbx02.y (115) Rh/Ce02 (200) Rh/Ce02 (57,180,208,211) Pd/Ce02 (57) Pt/Ce02 (57,139,252) Ni/Ce02 (57) Fe/Ce02 (57) Co/Ce02 (57) Pd(Pt)(Rh)/Ce02 (59)
Catalysis by ceria and related
materials
Table 4.2. (Cont.) Some reactions investigated on ceria-supported metal catalysts and related systems. Reaction Partial Oxidation of CHt CO2 reforming of CH4 Removal of pollutants from waste waters (Catalytic wet oxidation)
NO Reduction
N 2 0 Decomposition CH3OH Decomposition Ethanol Decomposition Dehydrogenation of Cyclohexane H2 + CO
H2 + COz H2 + N2 Hydrogenation of unsaturated organics
Hydrogenolysis of Alkanes
Catalyst (References) Ni/Ce02(Cel.xLax02.o.5,1) (31) Pt/Ce,.xZrx02 (158) Pt/Ce,.xZrx02 (156,253) Ag/Ce02 (4) Cu/Ce02 (21) Pt/Ce02 (134) Rh/Ce02 (134) Ru/CeQ2 (242,243,254) Cu/Ce02 (15) Pd/Ce,.xZrx02 (51,95,255) Pd/Ce02 (51,104,256) Pt/Ce02 (128) Rh/Ce02 (128,257) Rh/Cei.xZrxQ2 (128,238) Rh/CexPr,.x02.y (233) Rh/Ce02 (128,174,185) Pd/Ce02(PiO2.x) (54) Rh/Ce02 (182) Pt/Ce02 (47,111) Ir/Ce02 (258) Ag/Ce02 (259) Au/Ce02 (9) Co/Ce02 (14) Cu/Ce02 (259-261) Ni/Ce02 (38,40,41,262-264) Pd7Ce02 (82,84,85,265-268) Pd/Pr02.x (82,84,85) Pa7Tb02.,, (82,92) Pt/Ce02 (125-127,131,139,252,) Pt/Pr02.x(Tb02.x) (127) Rh/CeQ2 (126,197,221,225,228,269) Ni/Ce02 (40) Pd/Ce02 (74,270) Rh/Ce02 (27,196,201,221,225) Fe(Co)/Ce02 (271) Ru/Ce02 (244,272,271) Ir/Ce02 (30) Ni/Ce02 (24,36,39,41) Pd7Ce02 (72,73) Pt/Ce02 (108,118,131,142) ^h/CeQ 2 (171,198,221,227) Co/Ce02 (11) Ni/Ce02 (42,43,264) Pd/Ce02(PrOx)(TbOx) (82) Pt/Ce02 (125,131,146) Rh/CeQ2 (221,273)
Chemical and nanostructural charaterization ofmetal/ceria systems 4.2. Preparation of M/Ce0 2 and Closely Related Catalysts Impregnation techniques constitute the most usual procedure for depositing metal phases onto ceria and ceria-based supports. Aqueous solutions of a variety of transition metal precursors are commonly used. The impregnation step is usually followed by drying at 373 K-383 K, calcination at temperatures typically ranging between 673 K (5,110,118,163,179,189,205) and 773 K (16,27,73,98,115,138,139,201,282), and finally, reduction at temperatures varying within a wide range of values between 473 K and 1173 K. With almost no exception, hydrogen is used as the reducing agent. Sometimes, the uncalcined metal precursor/support system is directly reduced (70,124,166,193). The procedure above is particularly useful for preparing supported noble metal (NM: Pd, Pt, Rh) catalysts. Though obviously sensitive to the support surface area, metal loading, and the specific experimental protocol, this procedure, at the laboratory scale, often leads to well dispersed metal systems with relatively narrow metal particle size distributions (97,117,183,235). The interaction of ceria with atmospheric CO2 and H 2 0 does not induce any significant hydration and/or carbonation phenomena affecting the bulk of the oxide (283). Also relevant, ceria exhibits in aqueous solutions a good chemical stability against leaching. Therefore, in contrast to that reported for the rare earth sesquioxides, impregnating solutions varying over a wide range of pH values, from acidic (5,27,58,99,110,111,113) to base (117,124,135) character, may be used without inducing, during the impregnation step, any significant textural or structural change on the support. By contrast, very important textural, nanostructural and chemical effects may occur during the reduction step. They will be discussed later on in this chapter. The nature of the metal precursors also deserves some comments. If analysed their chemical constitution, particularly that of the most investigated noble metals, two major precursor categories may be distinguished, the chlorine-containing ones: PdCl2 (27,44,47,51,52,54,72,73,74,101,284), H2[PdCL,] (58,99), [Pd(NH3)4]Cl2 (82), [Pt(NH3)4]Cl2 (108,138,285), PtCL, (111,158) H2[PtCl6] (27,50,101,108,112-114,118, 139), and RhCl3 (5,27,101,163,165,166,170,171,181,186,193,196,236,238,282,286, 287); and the chlorine-free precursors: Pd(N03)2 (48,70), [Pd(NH3)2(N02)2] (84) [Pt(NH3)2(N02)2] (134,180), [Pt(NH3)4](N03)2 (108,110), [Pt(NH3)4](0H)2 (124,135), and Rh(N03)3 (27,98,134,163,171,179,180,185,189,193,233). The distinction is relevant because of the profound chemical and structural differences observed between the catalysts prepared from these two groups of metal precursors. It is presently well known that NM/Cei.xZrx02 (282,288), and particularly NM/Ce02 (5,52,72,79,108,110,163,165,166,181,193,195,205) catalysts, prepared from chlorine-containing metal precursors may incorporate large amounts of chloride ions into the supports. Direct experimental proofs of such an incorporation have been obtained from chemical analysis (72,108,170, 205,282), TPO-MS (193) XPS (5,110,166) and structural characterisation studies (52,72,79,210,289). Regarding the latter studies, both XRD (52,72,79,289) and high resolution electron
89
90
Catalysis by ceria and related materials
microscopy/selected area electron diffraction (HREM/SAED) (52,72,210) data have clearly shown the presence of the tetragonal CeOCl phase in different ceria-supported Pd and Rh catalysts. Figure 4.1 accounts for the HREM study performed on a Rh/Ce02 catalyst, exRhCla, reduced at 973 K. The micrograph has been interpreted as due to a CeOCl microcrystal in [010] orientation growing parallel to the ceria matrix crystal. This interpretation has also been confirmed by computer simulation techniques (210). Very recently (52), Kepinski et al have studied by means of XRD and HREM the mechanism of formation of the CeOCl phase in a Pd/Ce02 catalyst prepared from PdCl2. In agreement with the proposals made by several authors (163,170,195,193), it is suggested in ref. (52) that, under flowing H2, at the lowest reduction temperatures, 423 K, the CI" ions are strongly chemisorbed on ceria, a progressive incorporation into the oxygen vacancies at the support taking place as the reduction temperature is increased.
^
$• 3fo»*a8iMfl»si$aafra8gs ^
s
s
#fc \
Figure 4.1. HREM image and SAED pattern corresponding to a Rh/Ce02 catalyst (ex-RhCl3) reduced at 973 K. The lattice spacings at 0.68 run are interpreted as due to a tetragonal CeOCl micro-crystal seen along the [010] zone axis. The interpretation is confirmed by matching the experimental and computer simulated HREM images (bottom part of the figure). Both the experimental HREM image and the inset SAED pattern show the structural relationship existing between the ceria matrix and the CeOCl microcrystal grown from it (210).
According to their HREM study (52), between 423 K and 573 K, the very first crystallites of CeOCl could be identified. As T ^ is further increased, they would grow by coalescence; in two dimensions first, and finally, above 673 K, as confirmed by XRD, in a three-dimensional way. Inherent to the growth of the CeOCl cystallites, a heavy loss of the catalyst surface area is observed, thus indicating that the incorporation of the chloride ions induces profound changes in the textural, structural, and, as will be
Chemical and nanostructural charaterization ofmetal/ceria systems discussed below in the chemical properties of the ceria support. As revealed by TPR-MS (Temperature programmed reduction/Mass spectrometry), under flowing H2, the cerium oxychloride phase shows a remarkable thermal stability, being hardly decomposed at temperatures as high as 1200 K (110,170,193). By contrast, its stability is much lower under oxidising conditions (52,170,193,195). In ref. (52) the decomposition in air is suggested to occur even at 298 K, though at such a low temperature the resulting Ce02 phase would protect the oxychloride against the reaction progress. Several other re-oxidation studies, like those reported in refs. (193,195) clearly indicate that much higher temperatures are actually required to thoroughly decompose the CeOCl phase. Thus, the TPO-MS diagram reported in Figure 4.2 shows that upon heating in a flow of pure oxygen a Rh/Ce02(Cl) catalyst, Cl2 evolution starts to be observed at 600 K. The trace consists of two peaks occurring at about 773 K and 1050 K. Though some magnetic balance results reported in (193) suggest that the high temperature feature may actually correspond to chlorine species trapped by the oxidized rhodium phase, its origin is uncertain. The TPO-MS trace in Figure 4.2 also suggests that a calcination of the precursor/support system, prior to reduction, may favour the elimination of a significant part of the chlorine originally present in the metal precursor. If so, the specific calcination conditions would become an important variable in determining the chlorine content of the final catalyst. Though no detailed studies are to our knowledge available, there are some rough indications of the effects induced by some specific calcination treatments. Thus, a Rh/Ce02 catalyst prepared from RhCl3, and calcined in dry air at 673 K, has been reported to retain, after reduction, about 75% of the total chlorine deposited onto the support as RhCl3 (205). Regarding the remaining higher rare earth oxides (Pr02.x, Tb02.x), the XRD data reported in ref. (82) suggest that PrOCl and TbOCl are present in Pd/Ln02-x (Ln: Pr, Tb) catalysts prepared from [Pd(NH3)4]Cl2, and further calcined at 673 K, prior to reduction. As in the case of ceria-supported catalysts, the calcination treatment at 673 K seems to be too mild as to ensure the elimination of the chlorine deposited during the impregnation step. Deposition techniques from non-aqueous solutions of the metal precursors have also been reported. Thus, some Pd/Ce02 samples have been prepared by grafting the palladium acetylacetonate from a benzene solution (64,71,290). High purity methanol and tetrahydrofuran have also been used as solvents for depositing Rh acetylacetonate onto ceria (273). In the case of Pt and Rh acetylacetonates, toluene was the selected solvent (291). Likewise, a ceria-supported iridium catalyst was prepared by impregnation from a n-hexane solution of Ir4(C0)12 (28). In all these cases, the procedure was completed with the corresponding solvent elimination, calcination and reduction steps. Very recently (50), an acetone solution of H2[PtCl6] (2 mg of Pt/cm3) was used in the preparation of Pt (5 wt %)/Ce02(Ceo.75Zr0.2502) catalysts. The procedure consisted of five successive cycles of adsorption, filtration, acetone elimination, calcination at 773 K, and reduction at 673 K. The metal dispersion remained constant (40 %) after each of the cycles, thus indicating that the preparation
91
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method allows to increase the number of supported Pt particles without modifying their size distribution (50). Several other less conventional methods have also been reported. Thus, a series of Pd/Ce02 and Pt/Ce02 catalysts with metal loadings ranging from I to 10 wt% have been prepared by a combustion method from an aqueous solution containing (NH4)2Ce(N03)6, PdCl2 or H2[PtCl6] and oxalyldihydrazide (ODH: C2H6N402), which was used as the fuel (51). Temperatures as high as 1273 K are estimated to be reached during the combustion. The resulting catalysts were used without any further reduction treatment. Though some characterisation studies are reported (51), the actual size distribution of the metal particles and their availability at the surface of the catalysts are uncertain.
a
I
273
473
673
873
1073
1273
Temperature (K)
Figure 4.2. TPO-MS trace accounting for the Cl2 (m/c: 70) evolution from a Rh/CeCh catalyst pre-reduced at 773 K. Catalyst prepared from RI1CI3 (193). Experimental conditions: Pure O2 flow rate: 60 cm'.min; Heating rate: lOK.min"1.
An electroless method has been applied to the preparation of a Pd/Ce02 catalyst (77). The procedure consists of irradiating the ceria support with a high power UV laser beam; then, the photo-activated oxide is soaked in an aqueous solution of PdCl2 in excess of concentrated ammonia. The suspension is finally treated with a solution of hydrazine hydrate while stirring. In this way, a catalyst sample with high metal loading, 8.9%, and very large metal particles (average size: 39 nm) could be obtained. In some other cases, the noble metals were deposited onto ceria from colloidal suspensions. Thus, a stabilized rhodium hydrosol with an average particles size of 5 nm was used in the preparation of a Rh(l%)/Ce02 catalyst (182). Likewise, a series of Pd/Ce02 catalysts with 0.5, 2.5 and 5 wt.% have been prepared from microemulsion of metallic palladium, further destabilized by addition of tetrahydrofuran (78). The Pd/Ce02 catalyst samples investigated in refs. (70,72) were also prepared by following a rather unusual method. In this particular case, a colloidal dispersion of the support was suspended in an aqueous solutions of either Pd(N03)2 (70,72) or PdCl2
Chemical and nanostructural charaterization ofmetal/ceria systems (72). After drying and reduction, the resulting high loaded (9%) metal catalysts exhibit a very peculiar nanostructure. Thus, for the Pd(N)/Ce02 catalyst reduced at 573 K (20h), the Pd and Ce0 2 mean crystal sizes, as determined by XRD, were 14 nm and 11 nm, respectively (70). The evolution of the crystal size data with the reduction temperature: 15.4 nm (Pd) and 17.4 nm (Ce02), after reduction at 773 K (20h); and 28.5 nm (Pd) and 27.6 nm (Ce02), after reduction at 973 K (20h); also indicated a low microstructural stability (70). A rather similar evolution was reported to occur on the sample prepared from PdCl2. In the case of the ceria supported non-noble metal catalysts, both conventional impregnation techniques (16,32,264,292) and precipitation of the metal precursor onto the ceria support have been used (43). Some Pa7Ce02 catalysts have also been prepared in the latter way (284,293). Co-precipitation from a mixed solutions containing both Ce3+ and the corresponding transition metal cation is a rather common preparation procedure (23,36,39,294). In some cases, bulk Ni/Cei.xLaxO2.0.5X catalysts were obtained by the urea co-precipitation/gelation method (31). Ceria-supported gold (8) and high-loaded (15%) palladium (44,54) catalysts have also been prepared by coprecipitation techniques. A good deal of fundamental information about NM/Ce02 catalysts has also been obtained from experimental studies carried out on model systems consisting of noble metals deposited on Ce0 2 single crystals (177,192), or more often, on oriented thin films in which ceria or ceria-related oxides were grown on different single crystals: aAl2O3(0001) (63,141,191), Si(lll) (159,295), Pt(lll) (179), Ru(0001) (162,164), or thin films: A1203 (65), Zr0 2 (191). The ceria thin films, typically 10-20 (am thick (57,59,63,65,69,141), were grown onto the supports either by spray pyrolysis techniques (63,65,141), by evaporating Ce metal in the presence of 0 2 (10 7 Torr) (191), or by electron beam evaporation of Ce0 2 (179). In most of cases, the noble metals were supported onto the different ceria substrates by vapour deposition techniques. The metal particle size resulting from this preparation procedure typically ranges from 3 nm to 20 nm, or even larger (295) depending on the metal coverage and annealing conditions (65,177). Some interesting fundamental studies have also been performed on model systems prepared by depositing ceria onto Pt(l 11) (136,140), and Rh(l 11) (213) single crystals. Finally, the structural nature and catalytic behavior of Pt/Ce02 systems generated by the oxidation with different chemical agents of some Pt-Ce (CePt2 and Ce7Pt3) crystalline alloys (123) have also been investigated. Regarding the preparation of praseodymia and terbia supported metal catalysts, the information available is rather scarce. All the reported studies have dealt with dispersed noble metal samples. Though metal vapor deposition has been applied in some cases (231), the impregnation techniques have constituted the most usual preparation procedure. Chlorine-containing (53,82,85,127,175,278), and chlorine-free (53,84,232,278) metal precursors have been used. As already reported, PrOCl and TbOCl have been identified in praseodymia and terbia supported catalysts prepared from chlorinated precursors (82). Water (82,85,127,175), and non-aqueous solvents,
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like acetone (232), or N,N-dimethyl formamide (53,84), have been used in the preparation of the impregnating solutions. There are some chemical properties of the praseodymium and terbium oxides which, in our view, may be relevant in designing specific preparation methods for M/Ln02-X and, may be more interesting for Pr- and Tb-containing mixed oxide supported metal catalysts (48,100,109,115,155,233). As is known, both praseodymia and terbia may undergo disproportionation reactions. In the case of praseodymia, the process may even take place by prolonged exposure to atmospheric air, at 298 K. As a result, Pr(OH)3 and P1O2 are formed (296,297). Consequently, the impregnation, in acidic media, of an aged-in-air praseodymia sample may induce dramatic nanostructural effects. In effect, during the drying treatment following the impregnation step, the leached Pr(III) should be expected to co-precipitate with the metal precursor, thus leading, after reduction, to a highly dispersed, but partly non-accessible, metal phase. If so, the praseodymia-supported metal catalysts may show a rather anomalous chemisorptive behaviour. As in the case of lanthana-supported metal catalysts (298,299), this effect, may wrongly be interpreted as due to a SMSI-like phenomenon similar to the one exhibited by the M/Ti02 catalysts (300-302). Praseodymia and terbia may also undergo solvolytic disproportionation in diluted acidic aqueous solutions (303-307). This means that, even when starting from a true, non-aged-in-air, oxide phase, the conventional impregnation treatments in acidic media, may also induce some leaching, the nanostructural consequence of which has been described above. Since the solvolytic disproportionation is known to be much slower on terbia (306), this side effect should be expected less significant in the preparation of terbia-containing catalysts. It would be noted, however, that the leaching rate is strongly enhanced by ultrasonic irradiation (306,308,309). The use of ultrasounds in this kind of preparative procedures (278) should therefore be avoided. The very limited characterisation studies performed on NM/Ln02-x systems hardly allow to be conclusive in relation to the influence of the preparation procedure on the chemical and nano-estructural properties of these catalysts. Nevertheless, the comments above suggest that, in contrast to ceria, the impregnation and drying steps may play an important role in determining such properties. In particular, the above mentioned steps of the preparation procedure should be carefully controlled in fundamental studies aimed at investigating the strong metal support interaction phenomena in praseodymia and terbia supported systems. The use of non-aqueous solvents may be helpful in preventing the side effects commented on above. The chemisorptive behaviour exhibited by a Rh/TbOx catalyst prepared by impregnation in dry acetone media seems to confirm the interest of this alternative (232). However, the absence of parallel studies on catalysts prepared by impregnation from aqueous solutions does not allow to be conclusive in this respect.
Chemical and nanostructural charaterization ofmetal/ceria systems 4.3. Characterisation of M/Ce0 2 and Closely Related Catalysts
4.3.1. Some Challenging Aspects of These Characterisation Studies The characterisation of metal phases dispersed on ceria and related oxide supports was soon recognized as a very challenging problem (81,130). A number of reasons justify the difficulties found in this sort of studies. Ceria and related mixed oxides are known to chemisorb large amounts of H2 (310-314) and CO (64,75,76,120,227, 230,311,315-317), two classic probe molecules for characterising supported metal phases. In addition, the chemistry involved in these chemisorption processes is acknowledged to be rather complex (117). Thus, as a function of the ceria BET surface area and redox state, it may adsorb variable amounts of hydrogen (314). Moreover, the presence of highly dispersed noble metals strongly enhances this process, it taking place to a very significant extent even at room temperature (97, 98,209,217,218,235). The spillover rate, however, is sensitive to variables like the reduction/evacuation treatment applied to the catalyst prior to chemisorption (204, 235), or to the presence of chloride ions in the support (195). Accordingly, as will be discussed below in further detail, the interpretation of the chemisorption data should be made carefully; otherwise, very misleading conclusions may be obtained. On M/Ti02 catalysts, the increase of the reduction temperature leads to drastic changes in their chemical and nanostructural properties. At sufficiently high reduction temperature, typically 773 K, a strong inhibition of the metal chemisortion capability (300,302,318-320) with simultaneous partial covering of the metal particles by the reduced support (Decoration) (137,171,321-323) is known to occur. This peculiar phenomenon, which is referred to as SMSI (Strong Metal Support Interaction) effect, is acknowledged to be associated with reducible supports (300,302). Accordingly, on M/Ceria and related catalysts, similar effects might be expected to occur. In fact, many authors (28,41,64,70,73,74,78,82, 131,133,137,138,179,196,219,221,224,227,258) have speculated on the existence of some kind of strong metal/support interaction effect in ceria-supported metal catalysts. This phenomenon is relevant by itself, but also because of its obvious implications in the characterisation of metals supported on ceria and related oxides. In M/Ce(M)02-x catalysts, partial rather than complete inhibition of the metal chemisorption capability is usually observed (See Table 4.5 below). In the earlier studies, most of which have dealt with catalysts reduced at temperatures not higher than 773 K, electronic metal/support interaction effects (133), metal decoration (64,78,219,221,227), and NM-Ce alloying phenomena (324,325) have been suggested to play a role. More recently, the characteristics of the metal/support interaction effects occurring in these catalysts have been reviewed, and a model aimed at interpreting the chemical and nano-structural changes occurred in NM/Ce0 2 on increasing the reduction temperature has been proposed (117). In ref. (117), the analogies and differences observed between NM/Ti0 2 and NM/Ce0 2
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systems are also discussed. Section 4.3.2 will be devoted to the chemical characterisation studies. Because of the relationship existing between the support reduction degree and the occurrence of the deactivation phenomena mentioned above, we shall review first some of the major problems to be faced in relation to the redox characterisation of ceria and related oxide supports, sub-section 4.3.2.1. Then, we shall discuss the chemisorptive properties of these catalysts. In particular, section 4.3.2.2, will be devoted to the adsorption of H2 and CO, by far the two most commonly used probe molecules. Special attention will be paid to the relationship existing between chemisorptive behaviour and reduction temperature. We shall also report on some recent hydrogen chemisorption studies, in accordance with which, the sensitivity to the deactivation phenomena may vary from one noble metal to the other (97,117,235), being also influenced by the presence of chlorine in the support (163). The nano-structural characterisation studies will be reviewed in section 4.3.3. We shall review the High Resolution Electron Microscopy (HREM) data at present available. In the first part, section 4.3.3.1, we shall discuss how HREM technique may provide reliable particle size distributions, and therefore dispersion data, for the metal phase in NM/Ce(M)02_x catalysts. This information has been crucial for unequivocally establishing the existence of metal deactivation effects, and, consequently, for characterising the metal/support interaction phenomena occurring in these catalysts. Also very important are the studies aimed at gaining information about the nano-structural evolution undergone by these catalysts when reduced at increasing temperatures. This topic will be discussed in section 4.3.3.2. In particular, we shall review the HREM studies showing the occurrence of both metal decoration and NM-Ce alloying phenomena, as well as the experimental reduction conditions leading to the onset of these effects. The reversibility is a major characteristic feature of the SMSI effect as defined in the original Tauster's works (300). Accordingly, section 4.3.3.3, will be specifically devoted to the HREM studies aimed at better defining the re-oxidation conditions ensuring the recovery of these catalysts from the decorated or alloyed states. Finally, some concluding remarks summarising the progress made in the understanding of the strong metal/support interaction phenomena exhibited by the NM/Ce(M)02-x catalysts will be presented in section 4.3.4.
4.3.2. Chemical Characterisation ofM/Ce02 and Closely Related Catalysts
4.3.2.1. Redox Characterisation Studies Many different techniques have been used to probe the redox state of ceria and related oxide systems both in the presence and in the absence of a supported metal phase: EPR (5,46,94,95,189,227,326,327), XPS (132,166,177,179,193,199,224,289,328-330),
Chemical and nanostructural charaterization ofmetal/ceria systems electronic conductivity measurements (204,263), V-UV spectroscopy (331,332), FTIR spectroscopy (76,311,333,334), X-ray absorption spectroscopy (XAS) (93,205,289, 328,335,336), and magnetic balance (71,93,193,195,204,217,218,331,337-340). Chemical techniques, like the temperature programmed reduction (TPR) (27-29,58,61, 73,110,186,201,219,248,337,338,341-346), and, re-oxidation studies by oxygen pulses (98,115,160,186,337). The combination of 0 2 Pulses/TPO (Temperature programmed oxidation) (203) have also been applied in these redox characterisation studies. All these techniques have provided interesting pieces of information, however, most of them show some intrinsic experimental limitations, which do not allow their use in reliable quantitative determinations of the widely varying reduction degrees found in M/Ce(M)02_x catalysts. XAS, and the Faraday magnetic balance seem to be particularly interesting options. Probably due to the requirement of synchrotron radiation, the XAS studies, though very interesting (205), are presently rather scarce. Consequently, the magnetic balance studies have played an essential role in the understanding of the redox chemistry of these catalysts, and particularly, in the investigation of the H2-NM/Ce02 interaction. Faraday Magnetic Balance Studies In accordance with their electronic configurations, Ce4+ is a diamagnetic cation, whereas Ce3+ shows a paramagnetic behaviour, with a magnetic moment of 2.5 BM. Because of this specific property of ceria, the magnetic susceptibility data may be correlated with the concentration of Ce3+ ions, and therefore with the reduction degree of ceria, in a straightforward manner (331). The same is true for ceria-based mixed oxides in which the alio-cation shows a stable diamagnetic behaviour, i.e. not modified by the different reduction treatments. This is the case of the ceria-zirconia system, for which a number of magnetic balance studies have already been reported (93,310,333,337,338,347-349). Also very important, the Faraday microbalance allows the magnetic measurements to be performed in a wide range of temperatures, under varying chemical environments (71,193,195,204,340). Table 4.3 summarizes a magnetic balance study carried out on two Rh/Ce02 catalysts prepared from Rh(N03)3 (N) and RhCl3 (Cl) precursors (193,195). By analysing the results reported in this table, some major contributions of this technique to our current understanding of the redox chemistry of these catalytic systems will be highlighted. Regarding the Rh/Ce02 (N) catalyst, Table 4.3 shows that the overall concentration of Ce3+ species is actually determined by the addition of two different components. They have been referred to as reversible and irreversible contributions (195). The first one, is associated with the hydrogen chemisorbed on the ceria support. This hydrogen form may be eliminated, with inherent reoxidation of the catalyst, by simple evacuation, it being restored by a further treatment with H2. In accordance with Table 4.3, for a mean surface area catalyst (49 m2.g"'), reduced at a moderate temperature (623 K), the reversible contribution represents about two thirds of the total concentration of Ce3+ species. This observation is in full agreement with the magnetic
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Catalysis by ceria and related
98
materials
studies reported for several other chlorine-free Rh/Ce02 (204,217,218) and Pd/Ce02 (71) catalysts. The so-called irreversible contribution actually measures the concentration of oxygen vacancies created in the ceria lattice by the reduction treatment. It may be determined from the residual concentration of Ce3+ species in the catalyst outgassed under conditions ensuring the complete elimination of the chemisorbed hydrogen. In accordance with the TPD (temperature programmed desorption) studies of hydrogen chemisorbed on some Rh/Ce02 (166,204,219), and Pt/Ce02 (l 17,135) catalysts, 773 K is an evacuation temperature high enough as to achieve this objective. Table 4.3. Faraday magnetic balance study of the redox behaviour of two ceria-supported rhodium catalysts prepared from Rh(N03)3 (N) and RhClj (CI) metal precursors. Metal loading and BET surface area of the catalysts were 3 wt% and 49 mlg"1 respectively. Data taken from (195). Run
% Ce3+
Treatment *
1 H2 623 K(lh), cooled to 295 K under H2 2 Run 1 + Evacuation at 623 K (lh) 3 Run2 + H 2 295K(20h) 4 Run 3 + H2 523 K (lh); cooled to 295 K under H2 5 Run4 + Evacuation 773 K (lh) 6 H 2 773K(lh);cooledto295KunderH 2 7 Run 6 + Evacuation 773 K (lh) 8 Run7 + O 2 295K(20h) 9 Run 8 + 0 2 623 K (lh); cooled to 295 K under 0 2 10 Run 9 + 0 2 773 K (lh); cooled to 295 K under 0 2 11 Run 10 + Evacuation 773 K(lh) (*) Hydrogen and Oxygen pressures: 300 Torr.
N 11.4 5.0 10.1 11.7 4.1 14.3 6.5 1.6 1.7 1.9 1.5
CI 22.1 20.7 20.6 21.7 20.9 22.9 20.4 12.8 5.2 0.7 1.7
Table 4.3 also shows that the irreversible contribution increases with the reduction temperature. However, even for the sample reduced at 773 K, it represents less than 50% of the overall concentration of Ce3+ species. Some conclusion should be outlined from the above results. In the Rh/Ce02-(N) catalyst, the actual redox state of ceria, as determined by the concentration of Ce3+ species, is very sensitive not only to the reduction temperature, but also to variables like the evacuation temperature, or the cooling conditions, following the reduction treatment. Ceria will show a completely different redox state when the catalyst is cooled under hydrogen, or under high vacuum/inert gas flow. A distinction should therefore be made between the concentration of Ce3+ species, i.e. the redox state of ceria, and that of the oxygen vacancies. Otherwise, highly overestimated values may determined for the latter. The relevance of this point would become obvious when interpreting the temperature programmed reduction diagrams, to be discussed bellow. Likewise, it is worth recalling that the oxygen vacancies are considered to play an important role in determining the onset of the strong metal/support interaction effects
Chemical and nanostructural charaterization of metal/ceria systems (179), or some peculiar catalytic properties (196) of the NM/Ce02 catalysts. The role played by the dispersed metal in the H2-Ce02 interaction also deserves some comments. The mechanism of this interaction is strongly modified by the metal. Both through-the-metal adsorption (spillover) and desorption (back-spillover) processes are much faster than the corresponding reactions on the bare oxide. As shown by Table 4.3 and several other magnetic balance studies (71,217,218), on M/Ce02 catalysts, hydrogen spillover, with concomitant reduction of ceria, may occur even at room temperature. On bare ceria, hydrogen chemisorption is negligible below 473 K (218,331). Similar differences may be noted for the hydrogen desorption, also occurring at much lower temperature on M/Ce02 catalysts (204). Moreover, because of the strong influence of the metal on the kinetics of the hydrogen chemisorption, the relative weight of the reversible and irreversible contributions to the overall ceria redox state might well be significantly different on the bare oxide and the M/Ce02 catalysts reduced at the same temperature. Unfortunately, no specific studies have been addressed in this direction. If the results reported in Table 4.3 for the (N) and (CI) catalysts are compared, some dramatic differences may be noted. Thus, the ex-chloride sample reduced at 623 K shows an overall reduction degree, 22.1%, much larger than that exhibited by the (N) catalyst, 11.4%. Likewise, the effect of the evacuation treatment at 773 K is very different on the (CI) and (N) samples. In the (CI) catalyst, the irreversible contribution represents as much as the 90% of the total reduction degree. For the (N) sample, this contribution is much smaller, 36% for the sample reduced at 623 K, and 45% after reduction at 773 K. The response to the 0 2 treatment at 298 K is also very different. The reoxidation is almost complete in the (N) catalyst, whereas more than 50% of the Ce3+ species are still present in the ex-chloride sample. Assuming that most of the oxygen vacancies are titrated, the existence in the (CI) sample of a third contribution to the overall concentration of Ce3+ species becomes obvious. As deduced from the magnetic balance results commented on above, also confirmed by XAS data (205), the use of chlorine-containing metal precursors may deeply modify the redox properties of ceria. The reversible contribution plays a minor role, thus suggesting that the presence of chlorine heavily disturbs the H2-Ce02 interaction (209). This has also been confirmed by some volumetric (209) and TPD studies (166). Likewise, the above results show the existence of a new very important contribution to the total reduction degree reached by ceria. It is specifically related to the CI" ions incorporated into the ceria lattice. Though it does not revert on outgassing, no oxygen vacancies would be associated with this third contribution (52,163,193,195). The relative weight of the redox effects due to the presence of chlorine in M/Ce02 catalysts should be expected to vary from one sample to the other. Factors like, the metal loading, the chlorine content of the metal precursor, the calcination treatment prior to reduction, or the BET surface area of the support, would probably be relevant.
99
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Catalysis by ceria and related
materials
Temperature Programmed Reduction (TPR) Studies TPR is a routinary technique in the redox characterization of M/CeC>2 and related catalysts. It has been very extensively used in comparative studies aimed at establishing the influence of variables like the chemical composition (281,341,343,344,350-353), or the high temperature ageing treatments (187,288,337,347,354), on the reducibility of ceria-based mixed oxides, both in the presence and in the absence of a supported noble metal. Particularly noticeable are the studies on ceria-zirconia catalytic systems. As an example, the TPR diagrams have provided key information about the profound differences existing between the Ce0 2 -Zr0 2 mixed oxides and pure Ce0 2 in relation to their high temperature ageing behaviour. This technique has clearly shown that the redox properties of ceria become deteriorated on ageing, which is consistent with the relationship existing between its low-temperature reducibility and BET surface area (283,355,356). The ceria-zirconia response is much more complex, the changes induced by the ageing treatments on their TPR traces showing that a significant enhancement of the low-temperature reducibility may occur (186,241,338,354,357).
W
5(Xt
7'X»
WK)
1UM1
I.HK>
innptT.iiuri' iKl
Figure 4.3. TPR-(TCD) study of a series of NM(1 wt%)/Ce02 catalysts. Traces corresponding to Rh/Ce02, (b), Ir/Ce02 (c), Ru/Ce02 (d), Pt/Ce02 (e) and Pd/Ce02 (fj The diagram for the bare ceria support (trace a) has also been included for comparison. Reducing gas mixture: 5.22% H2/Ar. Results taken from (27).
Most of the reported results were obtained in experimental devices coupled to a thermal conductivity detector (TCD) (27-29,58,61,73,102,110,86,201,219,248,342,345, 358,359). In some cases, the analysis of the evolved gases was performed by means of mass spectrometry (MS) (97,98,160,232,283,338,356,360). Figure 4.3 depicts the TPR-(TCD) diagrams corresponding to a series of NM/Ce02 (NM: Rh, Ir, Ru, Pt, Pd) catalysts (27).
Chemical and nanostructural charaterization of metal/ceria systems
101
If the TPR profiles for the NM/Ce02 catalysts and the bare support, also included in Figure 4.3, are compared, a common high temperature feature centred at 1090 K may be noted. This peak is generally interpreted as due to the bulk reduction of ceria (61, and references there in). In agreement with several earlier studies (73,110,283), the position of this peak does not seem to be modified by the presence of any supported metal. This observation is typically interpreted in terms of a kinetic model (205) which assumes that the high temperature reduction process is controlled by the slow bulk diffusion of the oxygen vacancies created at the surface of the oxide. As deduced from Figure 4.3, the peak observed on the bare ceria at around 835 K, usually assigned to a surface reduction process, is almost completely lacking in the M/Ce02 catalysts. By contrast, they show some new features at much lower temperatures (300 K-550 K). The hydrogen consumption associated with these peaks largely exceeds the amount needed for fully reducing the supported metal phase (27). From these two observations, it is generally concluded that, in the presence of the metals, the peak appearing at 835 K on bare ceria is strongly shifted downwards in temperature, and, therefore, that the surface reduction of ceria is very much enhanced. Upon integration of the corresponding TPR traces in the low temperature (300 K-500 K) region, the stoichiometry of the resulting Ce02.x phases were determined (27): CeOi.95, for the Rh and Ru catalysts, and Ce0 197 , for the Ir sample. The interpretation suggested above is the usual one in TPR-TCD studies (61,73,219,248). Nevertheless, a recent study has critically revised this interpretation (355). By taking into account the available information about oxygen diffusion coefficients in ceria, Trovarelli et al (355) conclude that, in contrast with the classic interpretation, the TPR trace for ceria is not controlled by the rate of diffusion of the oxygen vacancies. On the contrary, they suggest that the surface reduction process and the difference of both thermodynamic and kinetic properties existing in the ceria microcrystals as a function of their size are critical factors rather. Based on these principles, the authors have developed a computer model, which allows them to interpret the main characteristic features of the ceria TPR trace. In particular, they justify the well known difference existing between the diagrams shown by the low (about 4 m .g"1) and high surface area (> 50 m2.g"') samples. Regarding the interpretation of the TPR-TCD diagrams, there are some additional aspects worth of commenting on. First of all, it should be recalled that the source of chemical information is the hydrogen consumption. However, as shown by the magnetic balance studies, at moderate reduction temperatures, hydrogen consumption and oxygen vacancies cannot be correlated in a simple way. As expected from the magnetic data reported in Table 4.3, in the 300K-500K range, the amount of hydrogen chemisorbed on ceria may be much larger than that of oxygen vacancies. To avoid misleading interpretations, the simultaneous recording of both H2 and H 2 0 signals is highly recommended. The TPR-TCD diagrams may be affected by the nature of the metal precursor. For catalysts prepared from a chlorine-containing metal precursor, there is an additional source of hydrogen consumption, that associated with the generation of Ce3+ species
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Catalysis by ceria and related
materials
with concomitant incorporation of CI" into the ceria lattice (52,163,193). In accordance with the scheme proposed in ref. (163), this contribution, which, as deduced from Table 4.3, may be very important, would not imply the creation of oxygen vacancies, but the replacement of the lattice oxygen by chloride ions. The interpretation of the TPR-TCD experiments may also be disturbed by the occurrence of side reactions due to the presence of contaminating species. Surface and bulk carbonates are often present in ceria-containing samples (331). Under flowing pure (283) or diluted (339,361) hydrogen, the carbonates are known to be reduced to CO and CH4 species, not retained by the trap located at the outlet of the reactor. Accordingly, the application of adequate cleaning pretreatments prior to running the TPR experiment should also be considered as a relevant experimental precaution. The application of standardised oxidising pre-treatment is particularly important in mixed oxides whose actual redox state may vary from one treatment to the other. Finally, in a recent study on Pt/Ce02 catalysts (138), the TPR-TCD experiments were run from 203 K. This allowed the authors to detect a significant hydrogen consumption below 298 K. This information is obviously lost in experiments started at room temperature.
400
600
000
1000
1200
Temperotor. (K)
Figure 4.4. TPR-MS study of a 4% RhATbOx-(N) catalyst (b) and the corresponding terbia support (a). The diagrams were recorded in a flow of pure H2 (60 cm'.min"'); heating rate 10 K.min'.
In the case of the remaining M/LnOx (Ln: Pr, Tb) catalysts, very few TPR studies are to our knowledge available (232). Figure 4.4 shows the TPR-MS traces recorded for a Rh/TbOx catalyst and the corresponding bare oxide. The experiments were run on samples pretreated/cleaned in 5%02/He flow at 973 K, in a flow of pure H2. Under these recording conditions, the H 2 0 (m/c: 18) evolution is the only source of experimental information. As deduced form Figure 4.4 (b), reduction of TbOx to Tb203 is achieved well below 600 K. If compared with the behaviour exhibited by the Rh/Ce02 catalysts reduced either in a flow of pure (203) or diluted (327) hydrogen, the reducibility of the terbia supported catalyst is much higher. In effect, as deduced from the results reported
Chemical and nanostructural charaterization ofmetal/ceria systems in (203,327), full ceria reduction to Ce 2 0 3 is not observed even for reduction temperatures as high as 1173 K. The comparison of TPR-MS diagrams reported in Figure 4.4 (a) and (b) also allows us to conclude that, the dispersed rhodium phase, enhances the reducibility of terbia, thus suggesting that, under the investigated conditions the reduction process is controlled by the activation of the hydrogen molecule. Quantitative Determination of Oxygen Consumption by Reduced NM/Ce02 and related Catalysts. Oxygen Storage Capacity. Since OSC (Oxygen Storage Capacity) was introduced as a way of determining on quantitative basis the capability of ceria-based supports to release oxygen under reducing conditions, and to uptake it under oxidising conditions (359), numerous studies (35,98,160,180,186,203,249,337,341,354,362-367) have included OSC measurements, as a routine way of characterising the redox properties of these catalytic systems. From the experimental point of view, the OSC is determined by measuring the 0 2 consumed by the catalyst after reduction under isothermal conditions. This consumption is mostly determined by using the oxygen pulse technique. Re-oxidation temperatures ranging from 700 K (354) up to 773 K (359) are typically applied. In accordance with the results reported in Table 4.3, the above quoted range, and even lower temperatures ensure the complete re-oxidation of ceria. The same is true for ceria-containing mixed oxides in which the alio-cation shows a single redox state (Zr4+, La3+, Y3+). By contrast, for the mixed oxides including a second reducible cation, like Tb, the full re-oxidation leading to the dioxide (CexTb1.x02) cannot be achieved (335). Moreover, the final state, CexTbi.x02-y, critically depends on the selected re-oxidation temperature and oxygen pressure (368-371). In the case of chlorine-containing catalysts, the re-oxidation conditions should also be carefully established. In effect, as deduced from Table 4.3 and Figure 4.2, also confirmed by some XPS (110) and XAS (205) studies, the complete removal of the chloride ions trapped in the ceria lattice may require harder re-oxidation conditions. In these latter catalysts, there is an additional experimental problem to be considered. If, as usual, the experimental device includes a TCD detector, the chlorine evolved on pulsing may disturb the oxygen signal, and, therefore, the estimate of its consumption. Two types of OSC measurements are generally distinguished (61,359,372): the ultimate OSC, which accounts for the redox behaviour of catalysts submitted to prolonged reduction treatments at temperatures well above 773 K (186,350,354), and the dynamic OSC (48,98,160,359). In the latter case, reducing and oxidizing pulses are alternatively injected on the sample at a predetermined temperature. Dynamic OSC measurements are thus aimed at simulating the oscillations occurring in the redox nature of the exhaust gases emitted by the motor vehicles. Therefore, they are particularly relevant in characterizing TWC's. The so-called ultimate OSC measurements actually constitute an alternative way of determining the redox state reached by support reduced at different temperatures.
\ 03
104
Catalysis by ceria and related materials
This approach represents a simple and low cost procedure. It has, however, a number of experimental limitations, which should be taken into account when interpreting the recorded data. Since no standardized procedures are at present well established, a variety of experimental protocols have been followed in the determination of the ultimate OSC. This obliges to be careful when comparing data obtained from different sources. Some of the variables, like the temperature and time of reduction, the nature of the reducing agent, typically H2 and to a lesser extent CO, or its partial pressure, may influence the final OSC results. Likewise, depending on the selected reduction temperature, the BET surface area of the support may also play a role. When using oxygen uptake data for characterising the actual redox state of support in NM/Ce02(CeM02-x) catalysts, there are some other experimental details to be considered. Such is the case of the evacuation conditions following the reduction treatment (98,160), the oxygen consumption due to the supported metal phase, or, when using CO as reducing agent, the likely occurrence of side reactions leading to the formation of carbonaceous deposits (113,117,164,192). Likewise, the presence of chlorine in the catalyst to be reoxidized may also complicate the interpretation of the reported results. As already discussed, the use of H2 as reducing agent implies that, in addition to the creation of oxygen vacancies, hydrogen chemisorbed species are formed. Accordingly, prior to reoxidation, an evacuation treatment ensuring the elimination of these species should be applied. Otherwise, the chemisorbed hydrogen would also be titrated with the oxygen pulses. The relative contribution of the chemisorbed hydrogen to the total oxygen consumption may be particularly important in the case of high surface area supports and moderate reduction temperatures ( T ^ < 773 K) (98,160). Regarding the contribution of the supported metal phase to the OSC values, the nature of the metal, its loading and dispersion, or the selected re-oxidation temperature may be relevant. In cases like ref. (359), where low loading (< 0.2%) Rh, Pd and Pt catalysts have been studied, a common O/M: 1 value was used for correcting the metal contribution to the OSC values. They were determined by pulsing oxygen at 773 K. However, this should be considered as a rough estimate. As deduced from a number of studies on supported rhodium catalysts (194,203,273,373,374), Rh 2 0 3 seems to be the more likely oxidation product under the experimental conditions usually applied in ultimate OSC determinations. In the case of Pt and Pd, however, not so well defined O/M ratios may be observed depending on the metal dispersion, and the specific oxidation conditions (375,376). As shown by the results reported in Table 4.4, to be commented on below, this contribution may become very significant as the metal loading is increased. A slightly different way of determining the ceria redox state in M/Ce02 and related catalysts, also based on oxygen consumption measurements, consists of the successive application of oxygen pulses at 298 K, and temperature programmed oxidation (TPO). This procedure has been fruitfully used for characterizing a series of Rh/Ce02 catalysts reduced over a wide range of temperatures (203). Table 4.4 reports
Chemical and nanostructural
charaterization
ofmetal/ceria
systems
1 05
on the reduction degrees determined for ceria in catalysts reduced at 773 K, 873 K and 973 K. Oxygen uptakes corresponding to both the pulse experiments, at 298 K, and the subsequent TPO-TCD runs are also reported. The TPO traces are depicted in Figure 4.5. The oxygen consumption data in Table 4.4 are expressed as apparent O/Rh ratios. The ceria reduction degree was determined as the difference between the total oxygen uptake (Pulses + TPO) and the one assigned to Rh. The O/Rh value assigned to the metal re-oxidation was assumed to be: 1.5. As deduced from Table 4.4, this O2 uptake is equivalent to a 12% reduction of ceria, being therefore far from negligible.
800 Temperature (K)
Figure 4.5. TPO-TCD diagrams corresponding to a Rh(2.4%)/Ce02-(N) catalyst reduced with flowing pure H2 at 773 K (A), 873 K (B), and 973 K (C), for lh, then evacuated for lh at T^m, and finally treated with 5%02/He pulses at 298 K until no oxygen consumption was observed. Also included is the TPO trace (D) for the bare support reduced/evacuated at 973 K (lh) and further submitted to the same 0 2 pulse treatment. Experimental TPO conditions: amount of catalyst: 200 mg; 5%02/He Flow rate. 60 cm3.min"'; Heating rate: 10 K.min"1. Diagrams taken from (203).
Table 4.4. Ceria redox state in a series Rh(2.4%)/Ce02 catalysts reduced at different temperatures. Oxygen consumption data as determined by combining Oxygen Pulses at 298 K and TPO-TCD studies. Data taken from (203). Redn./Evac. Conditions
0 2 consumption Pulses at 298 K (Apparent O/Rh)
0 2 consumption TPO-TCD (*) (Apparent O/Rh)
773 K(lh)/773 K(lh) 0.8 873 K(lh)/873 K(lh) 3.4 973 K(lh)/973 K(lh) 5.1 (*) Values in parenthesis correspond to the integration of the TPO-TCD diagrams reported in Figure 4.5.
Ceria Redn. Degree (% of Ce203)
1.8(0.1) 8% 1.9(0.2) 30% 2.2 (0.5) 46% first peak (the one centered at 373 K) in the
106
Catalysis by ceria and related materials
The TPO traces in Figure 4.5 are characterized by two main peaks. The first one, at 363 K, the intensity of which grows with the reduction temperature, has been assigned, in agreement with the diagram recorded for the bare support reduced at 973 K (trace D), to the ceria re-oxidation. Therefore, the support oxidation, though occurring to a large extent during the pulse experiment, cannot be completed at 298 K. Approximately, a residual 10% of the total re-oxidation can only be achieved on heating at about 373 K. This contrasts with the observation above in the sense of the fast ceria re-oxidation often observed at room temperature. It would be considered, however, that the pulse technique used in (203) implies the injection of small doses of O2, thus preventing the overheating of the sample, unavoidable in the case of treatments with a large excess of oxygen. Likewise, the low ceria surface area, and the relatively high reduction temperatures investigated in (203) may also justify the observed differences. The second oxygen consumption effect in the TPO traces reported in Figure 4.5 consists of a broader rather asymmetric feature peaking at 773 K. It has been interpreted as due to the full oxidation of the rhodium particles.
4.3.2.2. Study of the H2 and CO Interaction with M/Ce02 and Related Catalysts. The chemisorption of some selected probe molecules, typically H2 and CO, is a routine procedure for characterizing supported metal catalysts. In addition to providing basic information about the chemical properties of the dispersed metal phase, these studies are commonly applied to the estimate of metal dispersion data. For catalysts containing reducible oxide supports, as is the case of M/Ce(M)02.x systems, the chemisorption studies may also be used for detecting the metal deactivation effects due to the occurrence of a SMSI effect (300,301). On M/Ti02 catalysts, the classic SMSI systems, it is now well established that reduction at about 773 K strongly inhibits the metal chemisorptive capability (171,302,318-320). The chemisorption data reported for M/Ce(M)02_x catalysts have also suggested the occurrence of such an effect. It is certainly an interesting question which deserves some further discussion in this chapter. Upon reviewing the chemical characterization studies dealing with M/Ce02 and related catalysts, we may notice that a good deal of H2 and, to a lesser extent, CO chemisorption data are presently available. Table 4.5 summarizes some of these data. Most of them were obtained from volumetric measurements carried out at 298 K. As acknowledged in the Trovarelli's review work (61), data obtained from the conventional volumetric studies cannot, in general, be interpreted in a straightforward manner. Even if excluded the occurrence of a SMSI-like phenomenon, there are a number of side effects, which may very much complicate the interpretation of these chemisorption data. Accordingly, the H(CO)/M ratios thus determined should be considered, in principle, as apparent values rather than a true estimate of the metal dispersion. Thus, it is known that ceria shows a poor textural stability, particularly
Chemical and nanostructural
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ofmetal/ceria
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107
Table 4.5. Hydrogen and CO chemisorption data corresponding to a number of M/Ce02 and related catalysts. Except otherwise indicated, the reported values were determined from volumetric chemisorption studies at 298 K. SBET m 2 .g' 24 24 24 24 19 a 19" 20 20
Taedn. (K) 473 573 673 773 473 773 523 673
128 85 42 82 75 42 80
673 623 923 923 923 923 923 923
Pd(0.73%)/CeO2-(Cl)
15.9 15.9
448 773
Pd (4.89%)/Ce02-(Cl) Pd(4.74%)/Ce02-(C1) Pd(1.58%)/Ce02-(N)
-
673 773
115 115 19" 19" 20.4
473 673 473 773 423 773 973 1173
Catalyst (*) Ir(0.6%)/CeO 2 -(N)
Ir(1.0%)/CeO2-(Cl) Ni (7.5%)/Ce02-(N) Ni (9.5%)/Ce02-(N) Ni (2.3%)/Ce02-(N) Ni(1.5%)/Ce02-(N) Ni(1.8%)/Ce02-(N) Ni(15.7%)/Ce02-(N) Ni(3.9%)/Ce02-(N) Ni(1.3%)/Ce02-(N) Ni(1.5%)/Ce02-(N)
Pd(1.0%)/CeO2-(Cl) Pd(0.53%)/Ceo.6gZro.3202-(N)
20.3 Pd (0.64%)/Ce„.68Zro.3202-(N)
85
500
Pd(0.7%)/PrOx-(Cl)
10 10 5.3 5.3
448 773 573 773
30" 9"
673 673
27 27 30 30
473 773 673 773
Pd(1.6%)/PrOx-(N) Pd(7.1%)/PrOx-(Cl) Pd(7.8%)/TbOx-(Cl) Pt(1.77%)/Ce02-(C1) Pt(0.6%)/CeO2-(Cl)
Apparent H/M CO/M 0.16 0.11 0.10 0.07 2.97 0.79 0.01 0.01 0.00 0.03 0.10 0.07 0.26 1.70 0.40 0.09 0.41 0.39 1.70 0.08 0.08 0.05 0.02 0.003 0.000 3.40 2.70 0.63 0.27 2.15" 0.17" 0.07" 0.05" 11.3 0.08 0.06 0.02 0.01 0.12 0.18 0.02 0.0 0.01 0.0 0.01 0.59 1.00° 0.40 0.70' 0.24 0.79 0.12 0.38
Ref. (30)
(27) (41) (262) (263) (38) (38) (43) (43) (43) (43) (85) (377) (377) (71) (27) (97)
(98) (85) (53) (82) (82) (131) (125)
108
Catalysis by ceria and related
materials
Table 4.5 (Cont). Hydrogen and CO chemisorption data corresponding to a number of M/Ce02 and related catalysts. Except otherwise indicated, the reported values were determined from volumetric chemisorption studies at 298 K. SBET
Catalyst (*) Pt(1.0%)/CeO2-(Cl) Pt(7.0%)/CeO2-(N)
19" 19" 34 34
Pt (0.55%)/Ceo.68Zr0.3202-(N)
21.2 21.2
Pt^^SyoVCeo^Zro^OrOT
100 100 15 15 30 30 21 21 99 62" 62" 62" 62' 62" 62" 130 130 62 62 11 12 7 7 49 49 49 49 85 18
Rh(1.6%)/Ce02-(N) Rh(1.0%)/CeO2-(Cl) Rh(0.25%)/CeO2-(Cl) Rh(3.0%)/CeO2-(Cl) Rh (2.9%)/Ce02-(N)
Rh(2.4%)/Ce02-(N)
Rh(1.9%)/Ce02-(C1) Rh (3.0%)/CeO2-(Cl)
Rh(1.0%)/CeO2-(N)
TRedn.
(K) 473 773 473 773 423 423 773 773 973 1173 500 500 500 500 500 500 500 500 473 500 773 500 773 500 773 623 623 773 773 523 623 773 773 623 623 773 773 623 773
Apparent H/M CO/M 1.26 0.23 0.61 0.19 5.20 0.85" 0.42 0.42d 0.26 0.10 20.3 0.94d 3.15 0.10" 4.20 0.26d 5.20 0.85d 3.21 2.68 1.06 0.84 0.86 0.55 1.76 0.66 0.17 0.15 3.97 1.41d 0.75 0.82d 0.36 0.36 0.27 0.21 0.71 0.99d 0.71 0.88d 2.13 0.97 0.35 0.23
Ref. (27) (117) (97)
(98)
(269) (221) (221) (221) (209)
(209)
(209) (209)
(230)
Chemical and nanostructural
charaterization
ofmetal/ceria
systems
\ 09
Table 4.5 (Cont). Hydrogen and CO chemisorption data corresponding to a number of M/Ce02 and related catalysts. Except otherwise indicated, the reported values were determined from volumetric chemisorption studies at 298 K. Apparent H/M CO/M (K) Ref. 92 623 1.27 0.32 (230) 13 773 0.22 0.06 Rh(1.0%)/CeO2-(N) 20 623 0.84 0.62 (230) 623 18 0.65 0.31 Rh(2.1%)/Ce02-(C1) 55 a 400 (227) 0.5 55" 653 0.2 19" 473 (27) Rh(1.0%)/CeO2 0.93 19a 773 0.61 Rh(0.5%)/CeO2-(Cl) 154 473 1.80e (186) 154 473 0.21f 38 1000 1.14" 38 1000 0.15r 423 2.38 (235) 23.3 Rh (0.78%)/Ceo.6gZro.3202-(N) 423 0.52d 23.3 773 0.47 773 0.49d 20.8 973 0.41 : Rh (0.5%)/Ceo.6gZr0.3202-(N) 20.1 1173 0.23 (235) : Rh (0.5%)/Ceo.5Zro.502-(N) 29 473 0.53e (186) 29 473 0.27f 18 1000 0.21° 18 1000 0.20f Rh (0.69%)/Ceo.6gZr0.3202-(N) 94 500 (98) 12.0 500 94 0.69d Rh (4.0%)/TbOx-(N) 623 (232) 39 1.02 773 36 0.82 32 973 0.54 26 623E 0.60 Ru(1.0%)/CeO2-(Cl) 473 (27) 19a 0.98 19" 773 0.57 Ru (2.0%)/CeO2-(Cl) 140 623 0.57 (242) 129 773 0.55 115 898 0.35 87 973 0.02 -_ (*) Metal loading :(wt%). (CI) and (N) stand respectively for catalysts prepared from chlorine-containing and chlorine-free metal precursors. a BET surface area corresponding to the starting support sample. b As determined from volumetric adsorption data. Maximum pressure: 12 Torr. c Determined from IR spectroscopy of chemisorbed CO (Integrated absoprtion data for the catalyst reduced at 473 K normalized to 1.00). SBET
Catalyst (*) Rh (2.5%)/Ce02-(N)
mV
TRcdn.
110
Catalysis by ceria and related materials
d
As determined from volumetric adsorption data recorded at 191 K. As determined from volumetric adsorption data recorded at 308 K. f As determined from volumetric adsorption data recorded at 233 K. B Sample previously reduced at 973 K and further reoxidised at 973 K. e
under reducing conditions (339,378,379). Consequently, reduction treatments at T ^ > 773 K may induce strong sintering effects on the ceria support (209). Associated with it, an, in principle, uncontrolled fraction of the metal particles may become encapsulated. If so, the decrease of the H(CO)/M values cannot be unequivocally interpreted as due to metal sintering or to the existence of a true metal/support interaction effect. In spite of its likely relevance in mean/high surface area catalysts, information about their textural evolution with T ^ is often lacking (64,72,138,179,221,264). H2 Chemisorption Studies In the particular case of the hydrogen chemisorption studies, the interpretation of the experimental data may be heavily disturbed by the occurrence of spillover phenomena. A number of magnetic balance studies carried out on Rh/Ce02 (193,195,217,218), Pd/Ce02 (71), and Pd/Ceo.esZro^Oj (93) catalysts, an example of which is presented in Table 4.3, have clearly shown that large amounts of hydrogen can be transferred to support, even at room temperature. Consequently, as reported in Table 4.5, conventional chemisorption studies at 298 K. may lead to H/M » 1. It is presently well established that the contribution of the hydrogen spillover to the apparent H/M values is highly sensitive to a number of variables (117). Among them, the BET surface area of the support (98,209), and the reduction/evacuation conditions applied during the catalyst preparation. Regarding the latter variable, it is generally acknowledged that the spillover rate, at room temperature, decreases as the reduction/evacuation temperature is increased (204,209,235). Accordingly, the comparison of the H/M values determined after low and high temperature reduction should be made carefully because of the different weight of the spillover contribution. A third relevant factor influencing the spillover is the presence of chlorine in the catalyst. The results reported in (110,166,209) strongly suggest that chlorine very much disturbs the hydrogen chemisorption onto the support, thus having a negative influence on both the quantity and rate of the spillover processes. Because of the generalized use of the chorine-containing metal precursors, the influence of this factor should always be considered. Some experimental procedures aimed at minimising the contribution of the spillover to the H/M ratios have been developed. Low temperature (191 K) adsorption studies have shown to be an interesting alternative to the conventional measurements at 298 K (209). This technique has been fruitfully applied both to the characterisation of the metal phase in NM/Ce02 (117,171,209,222) and NM/CexZr,.x02 (97,186,235). Figure 4.6, shows an example of these studies. It deals with Pt supported on a texturally pre-stabilized Ceo.6sZro.32O2 sample (SBET: 23 m2.g"'). For all the applied reduction treatments, the catalyst surface area remained constant (97). Accordingly, metal
Chemical and nanostructural charaterization ofmetal/ceria systems encapsulation effects could be excluded. In Figure 4.6, the ratio of the metal dispersion data as determined from H2 adsorption at 191 K (DH = H/Pt,) and HREM (DHREM = Pt/Pt,); i.e.: DH/DHREM = (H/PtO/tPt/Pt,) = H/Pt, is plotted against Tred. In the absence of deactivation effects, H/Ptj should remain constant and equal to 1. As T ^ is increased and the metal deactivation starts to be noticeable H/Pts would become < 1, its deviation from unity indicating the intensity of the phenomenon. In accordance with the results shown in Figure 4.6, Pt deactivation starts to be noticeable at reduction temperatures as low as 573 K, and progressively increases with Tredn. It may also be deduced that for the catalyst reduced at/below 773 K, its chemisorptive behaviour may be fully recovered upon reoxidation at 700 K and further reduction at 423 K. For the highest reoxidation temperatures regeneration is only partial. A rather similar study has been reported for Rh(0.78 wt%)/Ce0.68Zr0.32O2 (235). From their comparison, it could be concluded that Pt exhibits higher sensitivity than Rh to the deactivation effects. To summarise, the combination of HREM and lowtemperature chemisorption studies has shown to be a powerful characterisation tool. From its application, critically important information about the upper limit Tredn values not inducing the catalyst deactivation, the regeneration effects associated with specific re-oxidation conditions, or the role played by the nature of the supported metal can be assessed. 1.20
1.00
0.80
Pt
0.60
0.40
D„ = H/PtT
0.20
DH'DHREM = H / P t s •
DHREM = P'S'P'T J
0.00 273
473
I 673
I
1. , J 873
L 1073
1273
Reduction Temperature (°C)
Figure 4.6. Metal deactivation effects in a Pt(0.55 wt%)/Ceo.68Zro.3202 as revealed by the combination chemisorption at 191 K (DH = H/Pt,) and HREM metal dispersion (DHREM = IVPtt) data. Plot of H/Pt, (DH/I against 1^, Fresh catalyst (O). Catalyst reduced at the indicated temperature, then re-oxidised at 700 K, and reduced at 423 K(*).
Very recently, 'H-NMR technique has been applied to the investigation of the hydrogen chemisorption on Rh/Ce02 catalysts prepared from both Rh(N03)3 and RhCl3 precursors (163). Because of the different chemical shift shown by the hydrogen species adsorbed on the metal and the ceria support, the evolution of the rhodium
HI
112
Catalysis by ceria and related materials
chemisorption capability as a function of TrsA could be studied. Within the investigated range of T ^ values (373K - 773K), a progressive metal deactivation starting to be noticeable at 573 K is observed on both (N) and (CI) catalysts. The deactivation effect is larger in the case of the (N) sample, thus allowing the authors to conclude that the presence of chlorine in the ceria support diminish the intensity of the strong metal/support interaction effects. Likewise, it is shown in ref. (163) that the reoxidation at 673 K of the catalysts reduced at 773 K fully recovers them from the deactivated state. Among the chemical effects associated to the onset of the SMSI state in titania supported metal catalysts, the practical suppression of their chemisorptive properties is often observed (171,302). By contrast, numerous hydrogen volumetric chemisorption data for M/Ce02 and related catalysts, some examples of which are reported in Table 4.5, suggest that, though the apparent H/M ratio generally decreases on increasing the reduction temperature, no complete inhibition of the H2 chemisorption capability occurs upon reduction at 773 K, the typical temperature at which the SMSI state is induced in M/Ti02 catalysts. Since changes in the relative weight of the hydrogen spillover or support sintering effects may also contribute to a lowering of the H/M values, the interpretation of the results reported in Table 4.5 as a proof of a true metal deactivation effect is far from obvious. Moreover, significant hydrogen chemisorption have been observed on Rh/Ce02 (186), Rh/Ceo.68Zr0.3202 (235) and Pt/Ce02 (135) catalysts reduced at 1000 K and 973 K, respectively. CO Chemisorption Studies The interpretation of the CO chemisorption data is not free from difficulties. In addition to the metal-dependent stoichiometric problems, it would be recalled that ceria may chemisorb large amounts of CO (64,75,76,120,227, 230,315-317). This side process is sensitive to the support redox state, being generally acknowledged that the amount of chemisorbed CO grows as the ceria reduction degree is increased (64,316). It is also known that the presence of chlorine in the ceria support may significantly diminish its CO chemisorption capability (75,166). On Pt/Ce02, a method allowing to substract the support contribution from the total amount of chemisorbed CO has been reported (120). The authors claim its usefulness for determining true metal dispersion data. In accordance with the CO/M data summarized in Table 4.5, the apparent CO/M values, typically decrease with the reduction temperature. However, as already noted for the hydrogen adsorption, no drastic inhibition effects are generally observed. In good agreement with the volumetric data, most of the FTIR studies of CO chemisorbed on Pl/Ce02 (130,131,133), Pd/Ce02 (64,78), and Rh/Ce02 (219) also show partial deactivation effects. An exception to this rather general observation is the case of a Pd/Ce02 catalyst, for which a complete suppression of the CO chemisorption capability (78) has been reported to occur. The FTIR spectroscopy of chemisorbed CO has also been fruitfully used for characterizing these catalysts (62,64,78,81,105,113,130,132,133,163,165). Because of
Chemical and nanostructural charaterization ofmetal/ceria systems the specificity of the bands due to CO interacting with the metal, their integrated absorption may be correlated with the number of exposed metal atoms. This method, which, in contrast to conventional volumetric procedures, is free from the perturbations inherent to the support contribution, has been applied to estimate the metal dispersion in Pt/Ce02 (130) and Pd/Ce02 (81) catalysts. The FTIR technique has also been used to probe the occurrence of metal deactivation effects associated with the onset of strong metal/support interaction phenomena in Pd/Ce02 (62,64,78), and Pt/Ce02 (105,133). In the particular case of ref. (105), a chlorine-free Pt/Ce02 catalyst showing stable textural properties and metal dispersion in the whole range of investigated reduction temperatures is studied. As deduced from the evolution of the integrated absorption values for the vco.pt bands, the metal chemisorption capability progressively decreases on increasing Tred from 473 K to 773 K. It is also shown that the catalyst may partly recover from the deactivated state (Tred = 773 K) by a very mild re-oxidation treatment consisting of heating it at 473 K, under 300 Torr of C0 2 . These observations, as well as the conclusions drawn from a parallel HREM study (105), allow the authors to conclude, in good agreement with the model proposed in ref. (117), that the deactivation phenomena occurring in the catalyst reduced at 773 K are of electronic nature. In addition to the metal deactivation effects which obviously would make useless this procedure in metal dispersion studies, the likely occurrence of CO dissociation should also be considered (113,164,168,192). Temperature Programmed Desorption Studies (TPD) Temperature Programmed Desorption (TPD) is a technique widely used for characterizing the thermal evolution of different probe molecules chemisorbed on M/Ce02 and related catalysts. From these studies, information about the chemical interaction of these molecules with both the dispersed metal phase and the support could be gained. This section will mainly deal with the TPD studies on chemisorbed H2 and CO, two of the most extensively investigated probe molecules. Temperature Programmed Desorption of Chemisorbed H2 In a recent review work (117) on the chemical and nano-structural characterization of NM/Ce02 catalysts, a detailed study of the H2 interaction with a Pt/Ce02 catalyst reduced at temperatures ranging from 473 K to 773 K is reported. The experimental techniques used in this work were TPD-MS and Isotopic Transient Kinetics (ITK) of the H2/D2 exchange at 298 K. The catalyst sample was carefully selected in order to minimise the Pt and support sintering effects in the investigated range of reduction temperatures. Likewise, a chlorine-free metal precursor, [Pt(NH3)4](OH)2, was used in the preparation of the catalyst. Figure 4.7 shows the TPD-H2 diagrams recorded for the above catalyst reduced at either 473 K (Traces A) or 773 K (Traces B), further evacuated at 773 K, and finally treated with flowing H2 at 298 K (Traces A,, B,), 473 K (Traces A2, B2), or 773 K (Trace B3). After lh treatment with H2 at the temperatures mentioned above, the
113
114
Catalysis by ceria and related materials
catalyst was cooled to 298 K, first, and then to 191 K (Liquid/Solid Acetone cold trap), always in a flow of H2. Finally, the corresponding TPD experiment was run in two steps. From 191 K to 298 K, low temperature TPD, the sample was heated freely, by removing the cold trap, whereas from 298 K upwards, a heating rate of 10 K.min" was applied. Diagrams in Figure 4.7 correspond to the latter step, high temperature TPD. This experimental protocol is close to that applied in a number of TPD-H2 studies on different Rh/Ce02 (124,166,204,222,380) and Pt/Ce02 (135) catalysts. The hydrogen treatment at 298 K is similar to that used in conventional chemisorption studies, whereas the treatments at temperatures above 298 K, are aimed at inducing the hydrogen transfer to the support. In this way, information about the spillover phenomena could also be gained. The integration of the traces in Figure 4.7 allowed the amount of hydrogen chemisorbed after the different treatments to be determined. Table 4.6 acounts for these quantitative results. Volumetric data obtained under similar conditions to those applied in the TPD experiments are also included in Table 4.6 for comparison. A good agreement may be noted between the amounts of chemisorbed hydrogen as determined from the TPD experiments (Low and high temperature steps of the thermodesorption) and volumetric studies. The quantitative TPD data reported in Table 4.6 also provide information about the role played by the spillover phenomena. Thus, for the catalyst reduced/evacuated at 773 K, and further treated with H2 at 773 K, the total amount of desorbed hydrogen (H/Pt = 1.02) is much larger than that determined after adsorption at 298 K (H/Pt = 0.21). In ref. (117) the latter value is assumed to be due to the metal, the difference, A(H/Pt) = 1.02 - 0.21 =0.81, being therefore assigned to spillover. If this amount is referred to nm2 of the ceria surface area, 6 H atoms.nm"2 is obatined. This value is close to the highest amount reported for the hydrogen chemisorbed on bare ceria (314), thus indicating that the hydrogen treatment (773 K) applied to the Pt/Ce02 catalyst leads to a saturated support. As discussed in ref. (117), even in the case of the catalyst reduced at 473K, the amount of hydrogen chemisorbed at 298 K (H/Pt = 0.61) is mainly assigned to the metal, thus concluding that, upon increasing the reduction temperature from 473 K to 773 K, a significant deactivation of the metal does occur. The same conclusion was drawn from the ITK study of the H2/D2 exchange (117). In conclusion, the quantitative analysis of the TPD-H2 results obtained by following the experimental protocol applied in (117) allows to gain very useful information about two major aspects of the hydrogen chemisorption studies, the metal chemisorption capability and the characteristics of the spillover phenomena occurring in these catalysts. The morphology of the traces reported in Figure 4.7 also deserves some comments. If the TPD-H2 diagram corresponding to bare ceria, trace C, is compared with that recorded for the Pt/Ce02 catalyst treated in a similar way, trace B3, a remarkable shift of the highest desorption peak from 773 K (C) to 470 K (B3) may be observed. This effect is interpreted as due to a change in the desorption mechanism.
Chemical and nanostructural charaterization ofmetal/ceria systems Associated with the presence of the dispersed platinum phase, a through-the-metal, back-spillover, mechanism, much faster than the direct recombination of the hydrogen species chemisorbed on the support, would mainly govern the desorption reaction. Also remarkable is the close analogy between trace B3 in Figure 4.7, and diagram reported in ref. (204) for a Rh/Ce02 catalyst reduced at 773 K and further treated with H2 in the same way as indicated above
i
1
r
300
500
700
900
Temperature (K)
Figure 4.7. TPD-MS study of the H2 desorption from a Pt(7%)/Ce02 catalyst reduced at: A) 473 K; B) 773 K. After reduction, the samples were evacuated at 773 K (lh) in a flow of He, and treated with flowing H2 (lh) at 298 K (Al and Bl); 473 K (A2 and B2); and 773 K (B3). Then, they were cooled to 191 K (solid/liquid acetone cold trap), and finally the TPD-MS diagrams were recorded in two steps: from 191 K298 K (free heating of the sample), and from 298 K upwards The reported diagrams correspond to the latter step. Trace C corresponds to the bare support reduced at 773 K (lh) and further cooled to 191 K, always in a flow of H2. Catalyst prepared by impregnation from an aqueous solution of [Pt(NHj)4](OH)2; Support surface area: 34 m2.g"' .Experimental TPD conditions: amount of catalyst 200 mg; He flow rate: 60 cm3.min''; Heating rate: 10 K.min"'. Diagrams taken from (117).
The reduction temperature also has a significant influence on the shape of the TPD-H2 diagrams. Regarding traces A2, B2, and B3, they consist of rather broad poorly resolved features. Since the hydrogen is initially chemisorbed on both, the
115
Catalysis by ceria and related materials
116
diagrams suggest that the corresponding desorption processes are not easily resolved. This observation, which certainly complicates the interpretation of the diagrams, is not, however, unexpected in view of the effect of the temperature of evacuation on the magnetic susceptibility of different M/Ce02 catalysts on which hydrogen has been preadsorbed (71,204,217,218). All these results clearly show that, in the presence of a dispersed noble metal, the hydrogen desorption from ceria starts at fairly low temperatures, being significant at 373 K. Since the TPD peaks due to H2 desorption from supported noble metal phases typically occur below 473 K (63,141,381), some overlapping between both, the direct desorption reaction from the metal and the backspillover process, seems to be unavoidable. Table 4.6. Study of the H2 chemisorption on a Pt/Ce02 catalyst reduced (lh) at 473 K. or 773 K evacuated (lh) at 773 K, and further treated with H2 as indicated. Comparison between the quantitative data obtained from TPD-MS and volumetric chemisorption experiments. Data taken from (117).
TR«1H
(K)
473
Further H2 treatment
TPD (191-298 K) 0.15
Evac.773K/H 2 298K/ Cooling H2191 K 0.13 473 Evac. 773K/H 2 473K/ Cooling H2191 K Evac. 773 K/H2 298 K/ 0.06 773 Cooling H2191 K 0.04 773 Evac. 773 K/H2 473 K/ Cooling H2191 K 0.05 773 Evac. 773 K/H2 773 KV Cooling H2191 K a Prior adsorption, the reduced catalysts were evacuated at 773 K.
Apparent H/Pt TPD (>298K) 0.46
Vol. Ads. (298K) a 0.61
0.92
1.03
0.15
0.19
0.68
0.77
0.97
A remarkable difference between the trace A2 (T^j,,: 473 K) and traces B2 and B3 (Tram: 773 K) in Figure 4.7 is the position of the main peak. In the latter case, it is clearly shifted towards higher temperatures. This suggests that the reduction temperature significantly affects the chemical properties of the Pt/Ce02 catalyst. Moreover, the displacement of the highest peak from 390 K (trace A2) to 470 K (traces B2 and B3) indicates that the hydrogen species desorbing at the lowest temperatures are particularly disturbed by the reduction treatment. Though not exclusively, these desorption forms would mainly be associated with the metal, thus indicating a certain deactivation of the platinum. Some other results are consistent with this proposal. Figure 4.8, shows the TPD diagrams recorded for a Pt/Ce02 catalyst reduced at several increasing temperatures, cooled under H2 flow, and then exchanged with flowing D2 at 298 K (lh) (135). The traces for H2, HD, and D2 show that both the fraction of chemisorbed hydrogen being exchanged with deuterium, and the intensity of the desorption peak at about 390 K steadily decrease with T ^ . Also worth outlining is the evolution of the overall TPD traces (H2+HD+D2) with T^h, which is similar to that
Chemical and nanostructural charaterization ofmetal/ceria systems deduced from Figure 4.7. Since the supported Pt ought to play a major role in the H2/D2 exchange at 298 K, the parallel evolution of the desorption peak at 390 K and the exchange intensity has been interpreted as due to deactivation of the metal phase as the temperature of reduction is increased from 473 K to 773 K. This observation is consistent with the quantitative data reported in Table 4.6.
300
500
700
Temperature (K)
Figure 4.8. TPD-MS study of a Pt(2.5%)/Ce02 catalyst reduced at 623 K (A), 773 K (B) and 973 K (C), cooled under H2 to 298 K, and then treated with flowing D2 at 298 K (lh). In addition to the traces corresponding to D2 (m/c: 4), HD (m/c: 3) and H2, the overall desorption diagrams (H2+HD+D2) are plotted. Diagrams taken from (135). Catalyst prepared by impregnation from an aqueous solution of [Pt(NH3)4](OH)2; Support surface area: 3.5 m2.g"' . Experimental TPD conditions: amount of catalyst 200 mg; He flow rate: 60 cm'.min"'; Heating rate: lOK.min" .
The comparative analysis of traces Ai ( T ^ : 473 K) and B! 0 ^ : 773 K) in Figure 4.7, is also interesting. Both traces show a peak at about 350 K, which may be assigned to the direct desorption from the metal, and a second broader feature. In the case of trace A2 this latter desorption effect is centered at 670 K. For the catalyst reduced at 473 K (trace Al), it appears as two ill resolved peaks shifted towards lower temperatures. The intensity ratio between the feature at 350 K and that corresponding to the forms desorbing at higher temperatures, which is significantly smaller for the catalyst reduced at 473 K, is also remarkable. As discussed above, the hydrogen treatment at 298 K would mainly lead to hydrogen chemisorption on the metal, with a smaller contribution of the spillover. If so, the higher intensity of the peak at 350 K, the one typically associated with metal, in trace Bi is unexpected. A rather similar observation on a Rh/Ce02 catalyst, also prepared from a chlorine free metal precursor, has been interpreted as due to the activation of the spillover
117
118
Catalysis by ceria and related
materials
process by the TPD heating programme (204). Since the ceria support would be practically free from chemisorbed hydrogen at the beginning of the TPD ramp, this interpretation suggests that as the temperature is increased, part of the hydrogen initially chemisorbed on the metal is transferred to the support, being desorbed later on the TPD run via a backspillover. The differences observed between traces Aj and Bj would thus be explained in terms of the faster spillover/backspillover processes on the catalyst reduced at 473 K. A second relevant conclusion from this observations is that, because of the interconversion of chemisorbed species ocurring during the TPD experiment, the recorded traces do not account for the actual nature of the species initially chemisorbed on the catalysts. The influence of the chlorine present in the ceria supported metal catalysts has also been recently investigated (166). This study was performed on two (0.5%)Rh/CeO2 catalysts prepared respectively from Rh(N03)3 and RhCl3. After impregnation and drying at 383 K, the uncalcined samples were reduced at 573 K (2h). Before running the TPD experiments, they were further heated in flowing He at 873 K (0.5 h), then in a flow of 3%02/He at 573 K (0.5 h), reduced again at 573 K (0.5 h), and heated up to 873 K in a flow of He. Finally, following a procedure similar to the one described above, the catalysts were treated with H2 at several different temperatures ranging from 298 K to 573 K, and cooled to 298 K under flowing H2. Figure 4.9 depicts the corresponding series of TPD diagrams. On varying the hydrogen treatment conditions, trends similar to those reported above for the Pt/Ce02 catalyst are obtained. Table 4.7 reports on the quantitative desorption data determined by integrating the TPD traces in Figure 4.9. As expected, the amounts of desorbed H2 increases with the temperature of treatment. Also remarkable is the difference observed between the ex.
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0 373
473
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773
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673
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Figure 4.9. TPD-MS study of the H2 desorption from two 0.5% Rh/Ce02 catalysts prepared by impregnation of a low surface area ceria sample (3.5 m2.g"') from an aqueous solution of either Rh(NOs)3 (N) or RhCl3 (CI). The catalysts were reduced with flowing H2 at 573 K, heated in a flow of He at 873 K, and finally treated with H2 at 298 K (a), 373 K (b), 473 K (c), or 573 K (d). Diagrams taken from (166). Experimental TPD conditions: amount of sample: 350 mg; He flow rate: 40 cm'.min"'; Heating rate: 30 K.min"1.
Chemical and nanostructural charaterization ofmetal/ceria systems chloride and ex-nitrate catalysts, the chemisorption capability of the latter being significantly larger. Though no metal dispersion data are reported in (166), the difference found after the hydrogen treatment at 298 K suggests that the ex-chloride sample is much poorly dispersed. On increasing the temperature of treatment, however, the difference also increases, thus indicating that both the spillover rate and the support chemisorption capability are disturbed by the presence of chlorine. Regarding the remaining higher oxide (terbia and praseodymia) supported metal catalysts, much less information is available. Figure 4.10 shows the TPD-MS diagram corresponding to the H2 desorption from a 4% Rh/TbOx catalyst reduced at 623 K and further cooled to 298 K under flowing H2 (232). The trace consists of a broad rather asymmetric feature peaking at about 373 K. Table 4.7. Amount of hydrogen desorbed from Rh(0.5%)/CeO2 catalysts following hydrogen adsorption in the temperature range: 298-573 K. Data takenfrom(382).
Adsorption Temperature (K) 298 373 473 573
Amount of desorbed H2 (umol.g'1 cat.) Rh/CeQ2 (N) Rh/CeQ2 (CI) 8.4 4.5 13.8 7.2 24.0 13.8 26.8 15.7
Figure 4.10. TPD-MS study of the H2 desorption from a (4%)Rh/TbOx catalyst reduced at: 623 K and further cooled to 298 K under folwing H2. Diagrams taken from (232). The catalyst, with a BET surface area of 39 m2.g"' was prepared by impregnation from a dry acetone solution of Rh(N03)j. Experimental TPD conditions: amount of catalyst 200 mg; He flow rate: 60 cm3.min"'; Heating rate: lOK.min"1.
Though no quantitative TPD data are reported, a parallel volumetric study shows that, after activating the adsorption at 623 K, i.e. under conditions similar to those applied in the TPD experiment, the amount of chemisorbed H2 expressed as apparent H/Rh ratio is: 1.19 (232). Since slightly smaller value, H/Rh: 1.02, was determined upon adsorption at 298 K, it is concluded in (232) that the dispersed metal phase exhibits a high chemisorption capability, which is consistent with the shape of the TPD
119
Catalysis by ceria and related
120
materials
trace in Figure 4.10. This observation is remarkable because the reduction treatment at 623 K leads to a fully reduced support, Tb203, thus suggesting that no metal deactivation is associated with this heavily reduced state. Temperature Programmed Desorption ofChemisorbed CO Some of the very first studies dealing with the CO interaction with ceria supported noble metal catalysts consisted of TPD-MS experiments carried out on powdered Pt/Ce02 catalysts (149,330). By combining TPD studies of isotopically labelled 0 2 and CO species with XPS and IR spectroscopy, the authors were able to show that, as a function of the pre-reduction conditions, CO and C0 2 might be interconverted on their catalysts. On a sample pre-calcined at 673 K, further flash-heated under vacuum at 800 K, and finally treated with CO at 200 K, the subsequent TPD run showed that 63% of the preadsorbed CO was actually desorbed as C0 2 (149). This fraction was much larger than that observed in a parallel study on the bare support. On repeating the experiment in a cyclic manner, successive but decreasing amounts of C0 2 were observed. Conversely, on the catalyst reduced with H2 (Pm: W4 Torr) at 773 K (5 min), prior CO adsorption, CO was practically the only desorption product. The initial behaviour of the catalyst could be restored by subsequent oxygen treatment at 373 K (149). Figure 4.11 summarizes the TPD-MS diagrams recorded after the two latter experiments. a
/' \
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«,0
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Figure 4.11. TPD-MS study of the CO interaction with a (2%)Pt/CeOj catalyst, a) Sample reduced with H2 (10 4 Torr, at 773 K for 5 min) prior CO adsorption (10" Torr, at 200 K. for 1 min). b) The catalyst pre-reduced at 773 K was further treated with 0 2 (10 4 Torr, at 373 K for 5 min), flash heated to 800 K, and finally treated with CO as indicated above. (—) CO and (—) C0 2 traces. Diagrams taken from (149). Catalyst prepared by impregnating ceria with an aqueous solution of H2[PtCl6], drying at 373 K, and further calcination at 673 K (12 h). The mean microcrystal Pt size was estimated to be 10 nm.
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In the case of the C0 2 adsorption/desorption studies, the opposite behaviour was found. When starting from the catalyst reduced at 773 K, about 50% of the preadsorbed C0 2 was desorbed as CO, the process being completely suppressed on the oxidized catalyst. In accordance with their findings, the authors propose as the most likely a
Chemical and nanostructural charaterization ofmetal/ceria systems reversible mechanism consisting of the oxidation of the platinum chemisorbed CO with ceria lattice oxygen species, or the replenishment of the ceria oxygen vacancies by C0 2 with inherent chemisorption of the resulting CO on the metal crystallites located at the close vicinity of the vacancies. Similar findings have been reported in a series of interesting TPD-MS studies of CO chemisorbed on model NM/Ce02 (NM: Rh, Pd, Pt) catalysts prepared by metal vapour deposition on ceria thin films and single crystals (63,141,178,214). In accordance with (63) the three noble metals above are active in enhancing the ceria reduction by CO. Some earlier studies on a Pt/Ce02 model catalyst had suggested a lower activity of this metal; in (63), however, the discrepancy found between the two platinum catalysts is interpreted as due to differences in the microstructure of the supports. A rather similar effect has been observed on a series of Rh/Ce02 catalysts prepared by vapour metal deposition onto Ce0 2 (lll) and CeO2(100) single crystals, and polycrystalline substrates (168). The fraction of chemisorbed CO oxidized to C0 2 during the TPD run was found to be substantially larger on the catalysts supported on polycrystalline ceria. The conclusion reached from all these observations is that the oxidation of the CO chemisorbed on NM/Ce02 catalysts is more sensitive to the support microstructure than to the nature of the noble metal (63,168). In a recent work, the interaction of CO with rhodium supported on both oxidized and pre-reduced Ce0 2 (lll) single crystal faces has been investigated with the help of HREELS (High resolution electron energy loss spectroscopy) and TPD-MS (192). As shown in Figure 4.12, the TPD-MS spectra corresponding to the Rh/Reduced Ce0 2 (l 11) treated at 300 K with a saturation dose of 13C180 shows two main features. A broad 13C180 peak centered at 480 K, similar to that reported on the Rh/Oxidized Ce0 2 (lll), which is interpreted as due to the CO desorption from the metal. The second, more intense, feature is observed at 600 K. The relevant observation is that the peak actually consists of 13C160. Though still observed on the oxidized catalyst, the peak shows a much smaller intensity. The results are interpreted as a proof of the occurrence of 13C180 dissociation on the Rh/Reduced Ce0 2 (l 11). The surface oxygen vacancies in the vicinity of the metal would play a direct role in the process, which would be followed by a rapid exchange with the 160 of the ceria lattice, and finally, by the re-oxidation of the adsorbed 13C atoms, with these oxygen species (192). Similar findings have also been interpreted as due to CO dissociation occurring on Rh supported on a partially reduced ceria ceria thin film grown on yttria-stabilized cubic zirconia (YSZ) (168). In this latter work, the occurrence of CO dissociation is considered to be metal specific, not being observed for either Pd or Pt supported on reduced ceria/YSZ. This proposal contrasts with a recent HREM study in which carbon deposits have been shown to be formed upon reduction of a Pt/Ce02 powdered sample with CO(5%)/He, at 773 K (117). In (168), even for Rh/Ce02, the CO dissociation activity was reported to be sensitive to the support microstructure, not being observed on polycrystalline ceria thin film supported catalysts.
121
122
Catalysis by ceria and related materials
13C180 12C160
12C180
300
400 500 600 Temperature (K)
700
Figure 4.12. TPD-MS study of the l3C180 interaction with a model catalyst consisting of Rh supported on Ce02(l 11) either oxidized or pre-reduced. Diagrams taken from (192).
A Faraday magnetic balance has also been applied to the investigation of the CO interaction with a Rh/Ce02 catalyst (206). In this way, quantitative data about the redox evolution undergone by the ceria support could be obtained. After impregnation with Rh(N03)3, the catalyst was outgassed at 773 K, treated with 0 2 at 298 K and evacuated again at 773 K, being further treated with CO (P co : 300 Torr), at 298 K. In (206), the results are compared with those obtained on the same catalyst treated with H2 instead of CO (218). After 24 h in contact with CO, at room temperature, some ceria reduction occurs (206). However, the percentage of reduced ceria, 5.5%, is much lower than that reported for the same treatment with H2, 21.2%. If, in a process which might be compared with the TPD experiments commented on above, the CO-pretreated catalyst is evacuated at 773 K, the reduction degree increases up to 7.4%. By contrast, the evacuation of the H2-pretreated catalyst leads to a strong ceria re-oxidation, the final reduction degree being 3.4%. Therefore, in the presence of highly dispersed Rh, the chemistry of the ceria interaction with H2 and CO is completely different. In the first case, the high reduction level must be interpreted as due to reversibly chemisorbed hydrogen on the support. Upon evacuation, a relevant fraction of this hydrogen form would be desorbed as H2 via a back-spillover mechanism. As a result, a ceria sample with a low concentration of oxygen vacancies is finally obtained. In the case of CO, on the contrary, the increase of the reduction level observed on evacuation suggests, in accordance with the TPD results commented on above, that a fraction of the CO probably chemisorbed on the metal is desorbed as C0 2 .
Chemical and nanostructural charaterization of metal/ceria systems 4.3.3. Nanostructural Characterisation Studies. High Resolution Electron Microscopy (HREM) has proven as a very useful technique in the structural characterisation of supported metal catalysts (383-386) in general and, in particular, of noble metal catalysts supported on ceria-based oxides (52,70,72,97,105,109,117,124,135,137,139,144,147,155,171,182-184,194,203,209, 210,218,226,234,235,387). The extraordinary possibility to reveal fine details of the local structure of both the supported particles and the supports make of HREM a rich source of information. Through such a window, the influence upon the nanostructure of the catalyst synthesis method, the procedures for the activation of the metal precursors or the effects of the working conditions can be monitored and the results eventually correlated with the observed macroscopic chemical and catalytic properties. Systematic studies are presently available for Pd/CeO2(49,52,72,70,97), Rh/Ce(M)02-x (109,117,171,182-184,194,203,209,210,218,226,234,235,387) and Pt/Ce(M)02.x (97,105109,117,124,135,137,139,144,147,155,184,387) catalysts. In this section the following points will be addressed: 1) the application of HREM in the determination of metal dispersion; 2) the structural features of metal-support interaction effects and their evolution under reducing environments in the temperature range 473 K-1173 K and, finally, 3) the reversibility of the interaction effects with oxidation treatments. Before going into the results it is important to bring the reader familiar with some concepts that will be frequently used henceforth. Figure 4.13 shows a typical HREM image of a supported metal catalyst, in this case of a Pt(5%)/Ceo.8Tbo.202-x catalyst reduced at 623 K. A large number of nanometer-sized Pt particles sitting on the surface of a larger Ce0.8Tbo.202.x crystallite can be readily recognised. Note however that the image of the small particles can be of two different types: 1) profile view, or edge-on, images. They correspond to the metal particles which are sitting in the projection along the perimeter of the support crystallite, like those marked with black arrows; 2) plan or top view images. This is the case of particles, like those marked with white arrows, whose image is formed on a support background. In the first case the electron beam goes through the sample along a direction which is contained in the interface plane between the supported particle and the support in such a way that the images of these two components do not overlap with each other. As reported in (184), to a large extent the interaction of the electron beam with the catalyst in this imaging mode takes place separately on the particle and the support. In other words there is no interference between the information generated in the interaction of the electron beam with the particle with that coming from the support. In the case of plan view images the information due to the particle and the support overlap in the space. From the point of view of the interaction of the electron beam with the sample, the most relevant aspect in this case is the occurrence of a double diffraction process. The electron beams, which aregenerated in a first diffraction process in the particles, are further diffracted by the support crystallites. Doubly
123
Catalysis by ceria and related materials
124
diffracted, or Moire-type, beams are generated which do contribute to the synthesis of the HREM image that now contains, superimposed in the same regions, information coming from both the particle and the support. It can be easily understood that the interpretation of plan view images is more complex than those recorded in profiles. Nevertheless, it is extremely important to emphasise the absolute need to cope with plan view images. A simple visual inspection of Figure 4.13 rapidly reveals the large amount of particles which are imaged in top view conditions in
5 nm
Figure 4.13. HREM image of a Pt(5%yCe».gTbo.2Q2.s catalyst reduced at 623 K in flow of hydrogen. Particles observed in profile view mode are marked wMi black arrows and those metal particles marked with white arrows correspond to plan view mode.
comparison with those which are imaged edge-on. Thus, if the conclusions from a HREM study have to be statistically sounded, the interpretation of plan view images is imperative. Concerning image processing, the operation most frequently shown here will be Fourier analysis of the HREM images. This type of analysis allows to extract the values of the spatial frequencies which are contributing to the synthesis of the image. In the following we will use the term Digital Diffraction Pattern or DDP to make reference to the log-scaled plot of the power spectrum of the Fourier Transform of the intensity distribution of digitised images. From these DDP information regarding both the spacing of lattice planes and the angles between them can be obtained, i.e. information useful to detect a particular crystalline phase.
4.3.3.1. Determination of Metal Dispersion in NM/Ce(M)02.x Catalysts by HREM. Evolution of Dispersion under Reducing Conditions. As depicted in Figure 4.13, HREM images provide a direct visualisation of the particles present in a supported catalyst Diverse parameters of these particles can be
Chemical and nanostructural charaterization ofmetal/ceria systems
125
individually determined and, if a sufficiently high number of particles are analysed, an statistical treatment of the information can be performed. One of these parameters which can be evaluated by simple measurements on the images is the size of the particles. Given that HREM is a projection technique, such an estimation would certainly correspond to the projected size. Although in the case of non-spherical particles different parameters could be used for the estimation of particle diameter (388), once a precise protocol is chosen for such a measurement this can be applied to every particle detected in an image. Thus, in (183) the largest distance that can be measured from the image of the particle is considered as estimate of the particle diameter. According to image simulation studies (184) the visibility limit for small f.c.c. type metal particles supported on ceria in profile or plan view images could be about 1 nm. By measuring directly on the image the diameter of a large number of supported particles, as previously precised, particle size distributions or histograms, like those shown in Figure 4.14 can be obtained. From these histograms statistical parameters like the average particle diameter (dm) or the volume-surface diameter (dvs) can be quantitatively estimated according to the following expressions, dm= X n r d i / X n ;
(4.1)
d vs = I n r d i 3 / I n r d i 2
(4.2)
100
80
-1
"I 20
0
0
5
10
15
Diameter (nm)
20
5
10
15
20
Diameter (nm)
Figure 4.14. Particle Size Distributions corresponding to a Rh(2.4%)/Ce02 catalysts submitted to reduction treatments at increasing temperatures in the 623 - 1173 K range; and cumulative curves showing percentage of particles versus diameter obtained for each reduction temperature (183).
Catalysis by ceria and related
126
materials
Following the change of these distributions, or the corresponding parameters, the influence of treatments on the particle size can be monitored. Thus metal particle sintering phenomena can be followed by doing these measurements. To illustrate these ideas, Figure 4.14 shows the results of a study of sintering under pure hydrogen at increasing temperatures, of a Rh(2.4%)/Ce02 catalyst. According to these data, metal sintering starts to be significant at T ^ > 973 K, being much more intense upon reduction at 1173 K. The sintering effects of reduction treatments at 973 K and 1173 K can be easily appreciated on the cumulative plot of the right side of the Figure 4.14. This approach has been followed in our lab to study the sintering of the metal phase in NM/Ce(M)02-x catalysts. Figure 4.15 shows an overview of the results obtained for a variety of rhodium and platinum catalysts for reduction treatments, under flowing pure hydrogen, in a wide temperature range (623 K - 1173 K). Values are compared in terms of average diameters (dm) . To facilitate the comparison, these values have been normalised by the average diameter determined at the lowest reduction temperature (0623). The following conclusions are worth being highlighted from this figure: 1) For T ^ < 773K minor sintering effects are observed on all the investigated catalysts; 2) Platinum is much more prone to sintering than rhodium. Differences in the sintering behaviour are enhanced with temperature. Thus, note the steep increase in average diameter when going from 973 K up to 1173 K in Pt, in comparison with the change observed in the same T,^ range in the case of Rh; 3) The nature of the support, either pure ceria or Ce/M mixed oxide, does not play an important role on the sintering behaviour of the metals. Note that all the Rh or the Pt curves are very close to each other.
- * - Pt/Ce02 -A^-
§ 2
573
Rh/Ce02 - O - Pt/Ce08Tb02Ox -A-- Rh/Ce08Tb02Ox ••- RWCeo^ZrojjO, -•- Pt/Ce068Zr0J2O2
673
773
873
973
1073
1173
1273
T r e d (K) Figure 4.15. Evolution of the particle average diameter for different rhodium and platinum supported catalysts with the reduction temperature in the 623 - 1173 K range.
Chemical and nanostructural charaterization ofmetal/ceria systems
127
From the volume-surface diameter (dvs) the dispersion of the supported phase (D) can be evaluated. For this purpose a relationship between this parameter and D is required. Different authors have suggested an influence of the particle shape on this relationship (389,390). To check this point, a series of models of metal particles with shapes more or less close to those observed in the experimental HREM images (octahedron, cube-octahedrons, cube, sphere) and of increasing size were built using the Rhodius program (184). For each model the total number of atoms (NT) and the number of surface atoms (Ns) were counted up, and from their ratio (Ns/NT) the dispersion (D) was calculated. A plot of the D values obtained for all the particles, with different shapes, vs. the relative volume-surface particle diameter (d^), as defined by equation [3], has been plotted in Figure 4.16. The term dat in this equation stands for the atomic diameter of the metal. dns = dvs/dat = G>rdi3 / £nrdi2) / dat
(4.3)
From the analysis of Figure 4.16 the following conclusions can be obtained: 1) For all the investigated shapes, dispersion values decrease steeply in the the range of relative diameter 0-20. This would correspond approximately to sizes in the range 0-8 nm for metals like Rh or Pt. For larger d„s values a slow and steady decrease of dispersion is observed; 2) Within the range of shapes investigated, it does not seem to exists a significant influence of the shape on dispersion. The larger deviations between different shapes are observed, as expected, for the smaller particles but in the worst of cases these differences are smaller than 0.1; 3) A shape-independent relationship
1,00 -
0 A
0,90 0,80 -
•
0,70 -
Cube-octahedron with square and hexagonal faces Cube-octahedron with square and Ida ngular aces Cube Octahedron Sphere Power Eight Polynomial Regression Para nctcrs; b[0] - 1.28 b[l] = -0.15 b[2] = 9.54C-3 b[3] = -3.49e-4 br4] = 7.58e-6 b(5]«-9.66c-S b[6]-6.69e-10 b[7] = -I.98e-12 b[8] = 4.060-16 r* = 0.995
a
2 0,60 -1 1 0,50 0,40 -
m
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0,30 -
^1" ^ w ^ .
0,20 0,10 0 00 • )
i
10
1 20
1 30
I 40 ''i-vs "" < W
1 50 d
1
60
{ 70
! 80
at
Figure 4.16. Curve showing the dependence of the dispersion values with the morphology of the metal particle.
Catalysis by ceria and related
128
materials
between D and d™ can be established by calculating the best-fitting curve for the whole set of data included in the figure. The coefficients corresponding to a power of eight polynomial regression, the value of the correlation coefficient as well as the plot of the best-fitting curve have been included in Figure 4.16. Thus, by estimating the d„s values from the experimental histograms and using Figure 4.16, the dispersion of the metal phase in a supported catalyst can be evaluated. Data included in Table 4.8 illustrate the evolution of this parameter with reduction temperature for some ceria supported catalysts. A more detailed consideration of this topic has been reported in (183). Concerning the role of chlorine on the sintering behaviour of ceria supported catalysts, Table 4.8 shows some dispersion data that allows a comparison of rhodium catalysts prepared from ex-nitrate and ex-chloride precursors. A similar trend is observed in the two catalysts. Taking into account the lower loading of the catalyst prepared from the chloride precursor and the very small changes of surface area of the two catalysts, these data would suggest that the ex-chloride catalyst is slightly more affected by sintering, though the differences are rather small. Assuming a visibility limit of the small particles of 1 nm and placing this value in the drvs scale in Figure 4.16, we come to the conclusion that the maximum dispersion detectable using HREM images should be around 85% for Rh or Pt. In samples in which a significant fraction of the small particles were less than 1 nm in size, the histograms would show only the tail of the real distribution and the dispersion calculated from them would result underestimated. For such samples alternative techniques like EXAFS would be more convenient. In any case, the range of dispersion reliably covered by HREM is rather wide. It should be also emphasised that HREM is unique in the sense that it provides the complete particle size distribution, whereas other physical techniques, like EXAFS or XRD, can only estimate an average particle size value. For samples in which the real distribution is skewed, large differences can exist between the actual dispersion, as estimated from the d^, and that obtained from dm. This makes HREM a more reliable and complete source of information about dispersion in samples in which the size of the particles lie within the experimental limits of this technique. Table 4.8. Evolution of average diameter (dm) and dispersion (D) with reduction temperature of rhodium catalysts prepared from nitrate (N) and chloride (CI) precursors. Tre(in(K)
Rh(N)(2.4%)/Ce02
Rh(Cl)(1.9%)/Ce02
dm(nm)
D(%)
dm (nm)
D (%)
773
3.3
28
3.4
27
973
3.7
26
4.8
21
1173
6.6
16
8.0
14
Chemical and nanostructural charaterization of metal/ceria systems 4.3.3.2. Nanostructural Features of Metal-Support Interaction Effects in NM/Ce(M)02.x Catalysts. Their Evolution with Reduction Temperature. In this section we will focus in the detailed description of the most relevant features of the nanostructure of the catalysts upon treatments under reducing atmospheres at increasing temperatures, covering the range 473 K - 1173 K. Figure 4.17(a) shows a HREM profile view image recorded on a Rh(0.5%)/Ceo.8Tbo.202-x catalyst reduced at 773 K, which exhibits the major characteristics of the catalysts treated at temperatures lower than 973 K, with independence that they are Rh or Pt catalysts (117,135, 155,184,194). Note in this case the bidimensional resolution of the image both on the particle and on the support. The presence of 0.220 run spots at 71° in the DDP of the particle, Figure 4.17(b), allows to assign this image to a [110] zone axis orientation of metallic, f.c.c, Rh. Likewise the DDP from the support, Figure 4.17(c), shows (1-11)Ce0.8Tbo.202-x (0.312 run) and (002)-Ceo.8Tbo.202-x (0.277 ran) diffraction spots characteristic also of a [110] zone axis orientation of fluorite. Two major conclusions can be drawn from these results: 1) after the reduction treatments, even at 473 K, the transformation of the precursors into the metal takes place with formation of the f.c.c. crystalline phases. In all the Rh and Pt catalysts studied, no other phase has been identified for reduction temperatures lower than 1073 K either on pure ceria or mixed oxide type (cerium-terbium, cerium-zirconium) supported catalysts (97,117,135,155, 194,235). 2) Metal particles do not grow with random orientations on the surface of the support but, instead, under well-defined crystallographic relationships. Note for example the alignment of the diffraction spots of ceria and Rh in the DDPs shown in Figures 4.17(b) and 4.17(c). As demonstrated further on, this is a consequence of an epitaxial relationship between the fluorite-type supports and the f.c.c. metal particles. A close look at the surface structure of the particles, like that shown in Figure 4.17(a), indicates that the particles are well-faceted. Likewise the surfaces are clean. No evidence of decoration by support overlayers has been obtained for reduction treatments at temperatures lower than 973 K. The same is observed in all the experimental studies at present available for NM/Ce(M)02.x catalysts (97,155,235). The shape of the particles is in general close to that of cube-octahedrons truncated along (111) or (001) planes. Rounding and stepping of the surfaces is usually observed, which makes the particles closer to hemispheres, but much more complex morphologies are not frequent. Thus, twinned particles do appear, though in a low percentage, in the case of the catalysts reduced at the higher temperatures, for which sintering, as we have already mentioned, starts to be significant. These twinned particles are very likely formed by coalescence of smaller f.c.c. units as the reduction temperature is raised. These observations are in good agreement with those reported by Cochrane et al. (147) and by Datye et al. (137,144,139), also for Pt/Ce02 catalysts. As mentioned above, one of the most remarkable features of these images is the appearance of metal-support orientation relationships. Such phenomenon indicates the
129
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Catalysis by ceria and related materials
existence of at least a structural interaction between the metal and the support. These peculiar growth schemes of the metal particles on top of the support surface could explain the metal dispersion values (around 30%) observed even in the case of catalysts prepared from low surface area ceria supports (4 m2/g) and appreciable metal loading (2.5% Rh or 5% Pt). Profile view HREM images have been particularly interesting to elucidate the specific crystallographic features of these orientation relationships (155,184,194). These images allow an identification of the metal and support planes which lie in contact at the interface and, at the same time, the determination of the zone axis orientation of the two components of the interface. Table 4.9 summarises our findings for the growth of metal particles on the two major type of ceria surfaces, (111) and (001). Results are identical for Rh and Pt catalysts. Moreover, the orientation relationships described in this table do hold for reduction temperatures in the range 473 K - 1173 K, whenever the supported particles remain metallic type and monocrystalline. As we will describe further, in the case of platinum catalysts, a transformation of the metallic particles into an intermetallic phase takes place at 1173 K. Though in this case specific orientation relationships have also been observed with respect to the support, their characteristics differ from those related in Table 4.9. Table 4.9. Crystallographic features of the metal/support orientation relationships observed in NM/Ce(M)02., catalysts. Planes at the interface (hkl)
Zones Axis [uvw]
Support
Metal Particle
Support
1-11
110
110
1-11
011
110
002
110
110
002
110
010
Figures 4.17 and 4.18 illustrate the two types of orientation relationships found for (lll)-metal//(lll)-support contacts at the interface. In Figure 4.17 a parallel alignment of the metal and support zone axis takes place, in such a way that planes with the same (hkl) indices in the metal and the support run parallel. Note for example the perfect alignment across the interface of the (111) planes of Rh and Ce0.8Tbo.202-x in Figure 4.17, also evident from the comparison of the DDPs, Figure 4.17(b) and 4.17(c). Simulated images, Figure 4.17(e), obtained for a supercell modelling this parallel orientation relationship, Figure 4.17(d), show contrast features matching those observed in the experimental ones. A twin or 60° rotated relationship has also been
Chemical and nanostruetural charaterization ofmetal/ceria systems
Kh
4
131
_iUU-_
Figure 4.17. Experimental contrast of the supported particle b) and the support c), Model showing a rhodium particle grown on the mixed image of a Rh(0.5%)/Ceo.gTbo^02.x catalyst reduced at 773 K a). DDPs obtained from the oxide crystal with a parallel orientation relationship d). Simulated image obtained from the model e) (109).
observed for this type of interface, Figure 4.18. In this case the (hkl) planes of metal and support are related by a mirror operation, a (111) type support interface plane acting as the mirror plane. Note in Figure 4.18(a) how the (1-1-1) planes of the support are reflected into the (1-1-1) planes of Pt, the topmost (1-11) plane of ceria being the mirror. This mirror relationship is also observed in the DDPs, Figure 4.18(b) and 4.18(c). If a line is traced through the central spot and the (1-11) spots, the metal and support (hkl) spots do lie at opposite sites of the line and at the same distance. Structure modelling, Figure 4.18(d), and simulation, Figure 4.18(e), do confirm this interpretation. Figure 4.19 contains an idealised model of a metal//support interface involving (111) planes of both components. Note than in this case a six-fold symmetry does exist. A 60° rotation of the metal particle does not make any change at the interface level, this being very likely the origin of the two orientation relationships observed experimentally. In other words, the two orientation relationships correspond to the same interface structure.
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Catalysis by ceria and related materials
In the case of (lll)-metal//(001)-support contacts, a parallel and a rotated orientation relationships have also been observed. Figure 4.20 shows examples recorded on a Pt/CeagTboA-x catalyst. Figure 4.20(a) illustrates the rotated relationship, for which the [110]-metal zone axis is perfectly aligned with the [010] zone axis of the support. The parallel orientation relationship involves an alignment of the [110] zones axis of metal and support, Figure 4.20(c). Note mat for this type of interface the only planes which lie strictly parallel are, in both cases, the (002)-support planes and the (1-1 l)-metal planes, see DDPs in Figures 4.20(b) and 4.20(d). Growth of the metal particles on its {001} planes has not so frequently been observed. In any case, a nearly parallel orientation relationship between the metal and the support has also been found for particles grown on {001} ceria surfaces. All the examples shown try to emphasise that the phenomenon of orientation between the structure of the metal and the support, though varied in characteristics, is generalised. It is observed for all the metals and supports here considered, and with
^
v
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w
Figure 4.18. Experimental image of the Pt(4%)/Ce02 catalyst reduced at 773 K a). DDPs obtained from the contrast of the supported particle b) and the support c). Model showing a platinum particle grown on the mixed oxide crystal with a 60° rotated orientation relationship d). Simulated image obtained from the model e)(109).
Chemical and nanostructural charaterization of metal/ceria systems independence that the catalysts have been prepared from nitrate or chlorine containing metal precursors, or the temperature at which the precursor is activated in hydrogen. iB''«%'«^e^®^®:'»T
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Figure 4.19. Model showing the structure of a metal/support interface involving (111) planes of both crystalline phases (194).
Figure 4.20. Experimental images of a Pt(5%)/CeojTboA catalyst reduced at 773 K a) and c). DDPs obtained from the images b) and d).
133
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Catalysis by ceria and related materials
Plan view HREM images have been extremely useful in the examination of metal-support orientation relationships (194,70), from two points of view: 1) to improve the statistics of the profile view observations; 2) to detect and quantify small misalignments of metal and support structures (109,387). Concerning the first point we should state that HREM plan view images have confirmed the different orientation relationships listed in Table 4.9 (194). As an example, Figure 4.21(a) shows a HREM image of a Pt/Ceo.sTbo.202-x catalyst in which the low frequency Moire contrasts of a Pt particle are clearly observed. In the DDP of this image, Figure 4.21(b), the Pt (111) and (002) spots are observed exactly aligned with those of the support, and so are observed the different Moire spots (marked as Haym). These Moire spots can be interpreted as due to a parallel orientation relationship between Pt and the mixed oxide support, according to the following crystallographic equations: (l-ll)-Pt || (l-ll)-CeTbO x [110]-Pt || [110]-CeTbOx Hi = gPt ("111) - gCeTbOx ("111) (J-2,1 = 2gp, (-111) - gceTbO* ("111) = gCeTbOx ( " H I ) - M-1.I M-1,2 = gPt (1-11) - gCeTbOx (1-11) fe = 2gp, (1-11) - gCeTbOx (1-11) = g&TbOx (1-11) " Hl.l
Figure 4.21. Experimental images of a Pt(5%)/Ceo,8Tboj02.x catalyst reduced at 623 K a). DDP obtained from the image b).
The g(hkl) notation used in these equations refers to the reciprocal (hkl) vector of the metal or the support (subindex). Note how the Moire spots result from linear combinations of the metal and support reciprocal vectors.
Chemical and nanostructural charaterization of metal/ceria systems A detailed treatment of the plan view images of the twin orientation relationship as well as those involving contacts with {001} support planes can be found in (194,391). As previously mentioned, plan view HREM images are particularly well suited to detect small misalignments between metal and support (387). The calculated images included in Figure 4.22 clarify this idea. This figure shows simulated images in profile and plan views for two situations: a) a Pt cubeoctahedron particle grown on ceria under a perfect parallel orientation relationship (images marked with a 0° label), b) the same Pt particle but rotated 2 degrees around the (1-11) axis which is normal to the interface (images marked with a 2° label). The DDPs of the plan view images have also been included at the right column. According to these simulations, the 2° rotation of the Pt metal particle on top of the ceria (1-11) surface does not produce any significant change in the profile-view image of the particle. Only very subtle changes take place that would be masked by the usual noise of the experimental images. No measurable change is neither observed when comparing the DDPs of these two profile view images. So we could say that this imaging mode is not sensitive to this small rotation.
Figure 4.22. Simulated images obtained using a model of a platinum particle supported on a (111) surface of CeOz in a parallel orientationrelationshipboth in profile view a) and planar view b). The same particle rotated 2° from the perfect alignment between both structures c) and d). The DDPs obtained from the planar view simulated images are also shown (387).
In contrast, plan view images are significantly affected. A simple naked eye comparison of these images does reveal a clear rotation of the dark Moire" bands when the particle is rotated. A more detailed analysis of the contrasts of these two images
135
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Catalysis by ceria and related materials
indicates that the rotation of the particle is amplified by the double diffraction process. Thus, in the perfectly aligned case, the metal, support and the two Moire spots (n, and H2) are all aligned, in such a way that a straight line connecting all these reflection can be drawn on the DDP. In the rotated case, the line connecting the two Moire spots lies at an angle 25° from that connecting the centre of the diagram and the metal or the support spots. A gain factor about 12 is working in this case. These calculations prove that plan views are extremely sensitive to misorientation. The interest of stressing this point is simply to make clear that situations that do not imply perfect orientation relationships between the metal particles and the support can be distinguished and precisely characterised, even in the case that very subtle rotations are involved. Figure 4.23 present one experimental case in which a slightly rotated rhodium particle has been detected on ceria. In this case the experimental plan view image, Figure 4.23(a), shows zig-zagged, low spatial frequency, Moire bands which suggest a rotated situation. The DDP of this region, Figure 4.23(b), shows Rh and Ce0 2 (111) spots which, within the experimental error, are parallel to each other, see enlargement in Figure 4.23(c). Nevertheless the position of the |ii and n2 Moire spots is clearly out of alignment with the spots of the metal and the support, Figure 4.23(d). By measuring the angle between the lines which have been traced on the DDP and using appropriate equations (387), a 2.1° rotation of the particle
Figure 423. Experimental image of a Rh(2.4%yCeC>2 catalyst reduced at 773 K a). DDP calculated from the experimental image b). Enlargements of the DDP c) and d) (387).
Chemical and nanostructural charaterization of metal/ceria systems
137
has been estimated. After long exposure of particles like the one shown in this Figure to the electron beam no change to a perfectly oriented situation could be observed. This allows also to disregard the idea that the orientation relationships are beam induced artefacts. If this was the case, situations so close to a perfect alignment as this mentioned here would be expected to change quite readily. According to our statistics most of the particles are usually under one of the orientation relationships already mentioned. The number of missoriented particles is usually very small. HREM studies have also revealed several other nanostructural phenomena, the nature and reduction conditions leading to their onset will be discussed below. For T ^ >. 973 K, mobilisation of the fiuorite type ceria based supports and their migration on top of the small metal particle surfaces take place. Figure 4.24 shows an interesting example of this decoration process. This HREM image was recorded on a Pt/Ceo,gTbo.202.x catalyst reduced at 973 K. A Pt particle sitting on a large, flat support surface is observed. The lower half of the particle seems to be embraced by a mixed oxide support layer. Likewise on the uppermost (111) plane of this particle a row of black dots, with a separation about 0.34 nm from each other, is also evident. This distance is larger than that corresponding to the closest Pt atoms in the f.c.c. structrure,
lh$4 am
Bt
Figure 4.24. Experimental image of a Pt(5%yCteo.!iTbo.202.» catalyst reduced at 973 K a). Model considering a platinum particle covered by a small monolayer of mixed oxide b). Simulated image calculated using the model c) (155).
138
Catalysis by ceria and related materials
but matches fairly well with the distance of 0.33 nm between neighbouring lanthanide cations along the [1-12] direction of the CeojTbojA-x oxide. These considerations suggest that the contrasts at the surface can not be due to metallic platinum atoms but should instead be ascribed to a coverage by a (111) support layer. Image calculations, Figure 4.24(c) have confirmed this interpretation. According to the structural model employed for this calculation, Figure 4.24(b), the black dot contrasts at the surface of the particle are due to a (111) CeasTbo^O^x cap covering the surface of the metal. Note than in this case the covering layer is extremely thin and with lateral dimensions about just 1 nm. In spite of its small size, it can be perfectly detected in the HREM image. Therefore, this example does not only confirm the occurrence of decoration effects in these catalyste after reduction treatments at 973 K or higher temperatures but also allows one to appreciate the detection limit of these decoration effects by HREM. According to these results, even a monolayer coverage can be detected in experimental images recorded in profile view conditions. Conversely, if decoration layers are not observed in the HREM images of a particular sample, we should admit that decoration is not taking place. In other words, the presence of "not visible" layers can be disregarded. In contrast to that reported for NM/Ti02 catalysts (322,323), the decoration layers are usually crystalline. Moreover it is clear that the decoration process involves the migration of the support over the surface of the particles instead of the particle sinking inside the support.
JLEOL
*"
4 run
Figure 4.25. Experimental image of a Rh(2.4%yCeQ2 catalyst reduced at 1173 K a). Image corresponding to the catalyst Rh(0.5%yCe&,Thi,jOji,reducedat 1173 K b) (194).
Chemical and nanostructural charaterization of metal/ceria systems
139
In Pd/Ce02 catalysts, Kepinski et al. (70) have also reported the ocurrence of decoration by thin ceria overlayers upon long (20 h) reduction treatments at 873K. When the reduction temperature is increased up to 1173 K differences between the behaviour of rhodium and platinum catalysts are observed. In the case of the Rhodium supported catalysts, no other interaction phenomenon appears. At this temperature HREM images show, Figure 4.25, metallic f.c.c. decorated rhodium particles. Though, as suggested by the calculations made in (194), on models taking into account this decoration effects, the estimation of the extension of the decorated surface is not an easy task, it can be at least qualitatively stated that the fraction of decorated surfaces increases with respect to that observed upon reduction at 973 K. In some cases the particles are only partially covered, like that shown in Figure 4.25(a), whereas some others appear completely encapsulated within a support shell, as it is the case of the particle in Figure 4.25(b). Also interesting, a large number of the metal particles seem to be grown on a support pedestal. Note how in this case the metal particle has been raised up on a column of a height 5 times the dm (Ceo.8Tbo.202.x) spacing. This feature is clearly observed in Figure 4.25(b). Though we do not have at this moment a clear explanation, this result suggests a lower surface energy state of the support during the crystallisation of these exotic nanostructures.
Figure 4.26. Experimental image of a Pt(4%)/Ce02 catalyst a) and a Pt(5%)/Ceo,gTbo202.x catalyst c) reduced at 1173 K. DDPs from the experimental images b) and d) (124,155).
The interaction of the support with Pt upon high temperature reduction treatments, T ^ > 1173 K, is much stronger than that observed for rhodium catalysts. HREM images recorded on Pt/Ce02 catalysts, prepared either from nitrate or chloridecontaining metal precursors, have revealed the transformation of metallic platinum into
Catalysis by ceria and related materials
140
an intermetallic CePt5 phase. Particles of this intermetallic have been identified in HREM images recorded along a number of different zone axis orientations, as well as on the basis of selected area electron diffraction patterns (124). Figure 4.26(a) shows a HREM image of this catalyst. Note the dramatic transformation of the contrasts in the supported particle with respect to those observed in catalysts reduced at lower temperatures. In this case the image is characterised, see DDP in Figure 4.26(b), by a rectangular pattern of fringes at 0.466 nm and 0.422 nm that can be assigned to a [010] zone axis orientation of the CePts phase. From the five different intermetallic phases described in the literature for the CePt system (392-396) (CePt, CePt2, CePt5, Ce3Pt2 and Ce7Pt3), the only one which has been detected is the Pt-rich CePts. No evidence of the formation of particles of compositions with higher cerium content has been obtained. According to the available literature (397) this is the thermodynamically most stable phase, thus suggesting that equilibrium conditions are reached during the reduction treatment at 1173 K. In the case of the Pt/CeogTbo^.x catalyst, the formation of particles of a LnPt5 (Ln = Ce, Tb) phase, isostructural with CePts, has been confirmed (155). Figure 4.26(c) shows a HREM in which a particle of this intermetallic is present. The details of the DDP, Figure 4.26(d), can be interpreted as due to a [011] orientation of the alloy phase. HREM thus provides evidence about the incorporation of the lanthanides present in the support to the metal particles but, in the case of the catalysts based on the mixed Ce/Tb oxide, it fails to reveal the extent to which each of them come into the alloyed state. From the analysis of the contrasts in the HREM images of the intermetallic particles it is not possible to precise this point.
900
1000
1100
1200
1300
Energy Loss (eV) Figure 427. EELS spectrum obtained on a (5%)Pt/Ceo.8Tbo.202., catalyst reduced at 1173 K. Signal corresponding to the analysis of a supported particle a) and that associated to the support b) (155).
In order to get further information about the composition in lanthanide elements of the alloy particles the Pt/CeogTboA-x catalyst reduced at 1173 K was examined in
Chemical and nanostructural charaterization ofmetal/ceria systems (155) using nanoanalysis by Electron Energy Loss Spectroscopy (EELS). By locating a nanometer-sized electron probe directly on the supported particles, their content in lanthanide elements could be estimated. Figure 4.27 shows EELS spectra representative of the results obtained in this study. The energy loss region shown corresponds to the Ce and Tb M4, M5 white lines. As depicted in the inset image, spectrum a was recorded on the support whereas spectrum b was obtained from a location in a supported particle far from the interface, to avoid in the latter case interference with the support.
880
895 910 Energy LossteV)
Figure 4.28. EELS spectrum obtained on a Pt(5%)/Ceo.gTbo.202.x catalyst reduced at 1173 K in the energy range corresponding to Ce-My lines (155). The EELS spectrum recorded on the support shows, as expected, the Ce M4, M5 peaks at about 900 eV as well as the M4, M5 lines of Tb. The relative intensities of these two sets of peaks are in good agreement with the Ce/Tb ratio in the mixed oxide. The spectrum recorded in the particle also contains the cerium lines, whereas only traces of the Tb features could be observed. This suggests a selective incorporation of Ce into the Pt lattice (155). The comparison of the fine structure of the Ce M4, M5 peaks of the support and the particle, Figure 4.28, also reveals some interesting information. Note the 1.8 eV shift to lower energies, the increase of the M4/M5 intensity ratio and the attenuation of the right side lobes in the spectrum recorded in the particles. All these changes can be interpreted as due to a decrease in the oxidation state of the cerium atoms which have incorporated into the supported particles. In fact these fine structure features are in good agreement with those observed for cerium in intermetallic compounds like CePd3, CeAl2 or in y-Ce. In all these compounds the formal oxidation state of cerium is zero. In (70) Kepinski et al. have reported, on the basis of XRD diagrams as well as from the precise analysis of HREM lattice fringe spacings, a 2.1% increase in the lattice parameter of the palladium particles in Pd/Ce02 catalysts after prolonged
141
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Catalysis by ceria and related materials
reduction treatment above 973 K. They have interpreted this result as due to the incorporation of Ce into the Pd particles. In contrast with the Pt catalysts, no intermetallic, with a definite composition and structure, is formed in the case of Pd. Regarding the LnPt5 phases, there are some other nanostructural observations worth of being outlined. The particle shapes are close to that corresponding to a beryltype facetting. This is the equilibrium shape corresponding to crystals with P6/mmm spatial group, as is the case of the CePts phase. This shape involves a preferential growth of the crystal along the c axis of the hexagonal unit cell. Figure 4.29 contains the results of an image simulation study that confirms this point. In the lower part of the figure the structural model built to calculate the simulated profile (b) and plan view (d) HREM images is depicted. A good match of both the outline of the particle and the details of the contrast of the experimental images (a and c) is observed.
Figure 4.29. Experimental images of a (5%)Pt/Ceo.gTbo.202.« catalyst reduced at 1173 K registered in profile view a) and planar view c). Simulated images obtained using models considering well faceted (beryl type morphology) CePt3 particles supported on a mixed oxide crystal b) and d). Model of a supported intermetallic particle used to obtain the simulated images e) (155).
Chemical and nanostructural charaterization of metal/ceria systems Secondly, according to both HREM images and diffraction patterns, the intermetallic particles grow under particular orientation relationships with respect to the support. The following equations describe the relationships we have found: (001)CeK5 II ( 0 0 2 ) ^ , [010]cem5 II [ 0 1 0 ] ^ (OOl)cePtf II ( 0 0 2 ) s u p p o r t ,
[OlOJcePtS || [ 1 1 0 ] s u p p o r t
A detailed comparison of the structure of the intermetallic and of the metal allows to establish a correlation between them (391). Taking into account this correlation it can be proved that the orientation relationships observed with the intermetallic particles are directly derived from those observed in the metal/support systems (391). Finally we will mention that in the case of a Pt/Ceo.7Zr0.302.x catalyst no evidence of the formation of this intermetallic has been obtained after treatments similar to those applied on supports containing exclusively reducible lanthanide elements.HREM images recorded on this catalyst upon a reduction treatment in (5%)H2/Ar at 1173K only showed Pt particles (97). Additional data obtained after more severe treatments, 1173K in pure hydrogen, lead also to the same conclusion. Thus, the presence of zirconia in the lattice seems to play an important role on the migration capability of ceria moieties. 4.3.3.3. Reversibility of Metal-Support Interaction Effects: Decoration and Alloying When considering metal-support interaction effects, the whole set of Electron Microscopy data presented in the previous section point out some important differences between the behaviour of noble metal catalysts supported on ceria and that of titania-supported catalysts. Much higher reduction temperatures are required in the case of ceria-type supports to observe nanostructural features similar to those described for the so called SMSI effect. The reversibility is a major characteristic feature of the SMSI effect (300-302). In the case of NM/Ti0 2 , reoxidation at about 773 K, followed by a reduction at low temperature, 473 K, is known to be effective for recovering the catalysts from the SMSI state (300-302,323). Probably by analogy with these earlier studies on titaniasupported noble metal systems, similar reoxidation temperatures (773 K) have also been applied to NM/Ce0 2 catalysts for recovering their chemisorptive and/or catalytic properties from the deactivated state (133,144,221). Data commented below, in which the nanostructural changes of Rh and Pt catalysts in a redox cycle have been followed, prove, nevertheless, that drastic differences are also observed in the reversibility behaviour of ceria based systems, and also that more severe treatments are required to recover this family of catalysts from their corresponding interaction states.
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Figure 430. Experimental images of a Rh(2.4%)/Ce02 catalyst reduced at 1173 K and further oxidised at 773 K a), and re-reduced at 623 K b). Image of die same catalyst reduced at 1173 K and re-oxidised at 1173 K c).
Figures 4.30 and 4.31 show sequences of HREM images taken at the steps of the reoxidation / reduction protocol usually applied to revert metal-support interaction effects, for a Rh(2.4%)/Ce02 and a Pt(5%)/Ce02 catalysts respectively. Prior to these treatments the two catalysts were reduced in a flow of pure hydrogen at 1173 K to induce the decorated and alloyed states. The re-oxidation treatment at 773 K of the heavily reduced Rh/Ce02 sample, Figure 4.30(a), leads to the formation of big, rounded shaped particles. Fringe analysis of these images using Fourier transform techniques, inset in the same figure, reveals the presence of lattice spacings in the 0.24 - 0.28 nm range. Such spacings can not be due to a metallic rhodium phase but instead, as shown in Table 4.10, they could be assigned to any of the rhodium sesquioxide phases. The complexity of the HREM contrasts, as revealed for example in the waving appearance of the fringes, and the small differences in the lattice spacing of the different oxide phases involved, preclude nevertheless an unequivocal assignment of this type of particles to a particular oxide. Thus, the 0.27 nm value observed in the
Chemical and nanostructural charaterization oftnetal/ceria systems
145
DDP could be related, within the experimental error, to any of the following family of planes: (110)-Rh2O3 I, (104)-Rh2O3 I; (002)- Rh203 II, (211)-Rh203 II; (200)Rh203 III, (114)-Rh203 III, (020)-Rh2O3 III. Likewise the 0.24 nm spot could be assigned to the following possibilities: (006)-Rh2O31, (012)-Rh2O3 II, (021)-Rh2O3 II, (115)-Rh203 III, (210)-Rh2O3 III or (120)-Rh2O3 III.
Figure 431. Experimental images of a Pt(4%)/Ce02 catalyst reduced at 1173 K and further oxidised at 773 K a). Image of the catalyst oxidised at 773 K and further reduced at 623 K b). Image of the same catalyst reduced at 1173 K and re-oxidised at 1173 K c).
Figure 4.32 shows an EELS spectrum which provides complementary information about the chemical composition of these particles. The spectrum was recorded in spot mode inside a particle like the one shown in Figure 4.30(a), i.e. imaged in profile. To avoid interference from the support, a region a few times the electron spot size (about 1 nm) far from the particle/support interface was analysed. As deduced from Figure 4.32, the EELS spectrum contains the Rh-M3 and O-K
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peaks, which confirms that the particle corresponds to an oxidised form of rhodium. Also important, the presence of Ce-M4,M5 white lines in the spectrum clearly indicates that these particles also contain cerium atoms. This result may be interpreted as due to the coexistence of ceria and rhodium in the large particles resulting from the reoxidation treatment. Accordingly, heating in a flow of pure 0 2 , at 773 K does not induce a net segregation of the support phase from the decorated metal particles. Table 4.10. Some lattice spacings characteristic of rhodium, ceria and rhodium sesquioxide phases. d-spacing (nm)
flikl)
0.219
111 111 002 113 006 110 104 210 002 211 020 102 012 021 104 113 020 114 200 120 210 115
0.312 0.270 0.224 0.231 0.257 0.273 0.297 0.269 0.260 0.258 0.252 0.239 0.233 0.299 0.297 0.272 0.262 0.257 0.241 0.233 0.231
Figure 4.30(b) shows the Rh/Ce0 2 catalysts after the final reduction treatment at 623K. We can still see some small patches on top of the metal particles which can be identified by fringe analysis as corresponding to a fluorite-like material. In particular, in the surface patch observed on the particle depicted in Figure 4.30(b), 0.27 nm (002)-CeO2 and 0.31 nm (lll)-Ce0 2 lattice planes are identified. Hence, oxidation at 773 K, followed by a mild reduction, does not recover the catalyst from the decorated state induced by the reduction treatment at 1173 K. Likewise, the metal dispersion (16%) does not change with respect to the one determined for the catalyst reduced at 1173 K (see Table 4.8). In the case of the Pt/Ce0 2 catalyst, the oxidation at 773 K with pure 0 2 , Figure
Chemical and nanostructural charaterization of metal/ceria systems 4.31(a), transforms the monocrystalline intermetallic particles into complex aggregates. Lattice fringes running in a variety of directions are clearly visible in these aggregates and suggest that these particles consist of a mosaic of small, missoriented, nanocrystals. Both (lll)-Pt (0.23 ran) and (lll)-Ce0 2 (0.31 nm) dspacings can be identified. The square insets at the right of Figure 4.31(a) show two enlargements where these spacing can be more clearly noticed. In the DDPs of these aggregates reflections due to metallic platinum and fluorite Ce0 2 are also present. The spots coming from f.c.c. platinum are only a few whereas those related to ceria are numerous and form a ring at 0.31 nm. These features in the HREM images and in the DDPs suggest the coexistence of metallic platinum and Ce0 2 in the polycristalline aggreagates resulting from the reoxidation at 773 K of the intermetallic particles. Moreover, these aggregates seem to consist of a core of platinum, comprised of one or at least only a few f.c.c. units, covered by a large number of nanometer-sized, randomly oriented, ceria surface patches. A final reduction of the Pt/CeQ2 catalyst at 623 K, does not change the nanostructure of the catalyst and, therefore, no effective recovering of the catalyst is achieved. In Figure 4.31(b) we can observe the same complex aggregates detected after the oxidation at 773 K. They remain even with a re-reduction at higher temperature (773 K).
Figure 4.32. EELS spectrum obtained from a supported particle on a Rh(2.4%)/Ce02 catalyst reduced at 1173 K and oxidised at 773 K.
These observations confirm that a treatment in 0 2 at 773 K is severe enough to destroy the intermetallic but does not regenerate the situation corresponding to a Pt/Ce0 2 catalyst directly reduced at 773 K or lower temperatures, i.e. a system consisting of small metal particles dispersed on the support. To summarise, the
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application of the standard regeneration treatment does not recover the Pt/Ce0 2 catalyst from the alloyed state. Figures 4.30(c) and 4.31(c) show HREM images representative of the catalysts reduced at 1173 K and further oxidised in pure O2 at 1173 K. The structure of both catalysts is clearly different from that observed after re-oxidation at 773 K. Notice that in this case both materials seem to be formed by small, crystalline, metal particles dispersed over the ceria surface. Fringe analysis confirms that these crystallites consist of metallic rhodium and platinum, respectively. Thus, the DDPs of the larger particles observed in the image of the Pt catalyst show 0.8 nm Moiretype fringes aligned with the (111)-Ce02 reflections. These spots arise from double diffraction in the (lll)-Pt and (Ill)-Ce02 planes under a parallel orientation relationship. Therefore this result, in addition to confirm the presence of metallic Pt particles in the sample oxidised at 1173 K, suggest that these particles are epitaxially grown on the support. A detailed inspection also reveals that the exposed surfaces of these particles are clean, i.e. free from support overlayers. Also worth of noting is that the metal dispersion estimated from the histogram corresponding to the Rh/Ce0 2 catalyst reduced at 1173 K and further reoxidised at the same temperature is 32%. This value is double than that determined for the sample directly reduced at 1173 K. Therefore, the high temperature reoxidation treatment not only induces the recovery of the catalyst from the decorated state; it also seems to induce the rhodium redispersion. The average diameter of the platinum particles estimated from the micrographs of the Pt/Ce0 2 sample reduced at 1173 K and further oxidised at 1173 K is also smaller than that of the catalyst directly reduced at 1173 K. This observation might well be interpreted as due to platinum redispersion induced by the high-temperature reoxidation treatment. Nevertheless, additional scanning electron microscopy and EDS analytical studies carried out on the regenerated Pt/Ce0 2 sample have shown (117) the simultaneous occurrence of very large platinum particles in the micron range size. This means that, in the case of the Pt/Ceria catalyst, the high temperature reoxidation treatment, in addition to inducing the reversion from the alloyed state, leads to a severe metal sintering. To summarise the results concerning the study of reversibility of metal-support interaction states, we could first state that the classic reoxidation treatment at 773 K does not allow the recovery of the NM/Ce0 2 catalysts from the decorated or alloyed states. The noble metal/ceria phase separation may only be achieved upon reoxidation at temperatures well above 773 K. This observation represents an additional major difference between titania and ceria supported noble metal catalysts. Moreover, the likely regeneration of NM/Ce0 2 catalysts reduced at 773 K by reoxidation at 773 K would actually prove, in good agreement with earlier HREM studies on the reduced catalysts (117,194), that the observed deactivation effects are not due to decoration or alloying phenomena, rather consisting of purely electronic effects (105). Finally, the reoxidation treatment at high temperature, 1173 K, appears to be
Chemical and nanostructural charaterization of metal/ceria systems suitable for recovering the metal particles from the decorated or alloyed states. The analysis of the evolution undergone by metal particle size distributions suggests some differences in the behaviour of rhodium and platinum catalysts. Thus, the microanalytical studies performed on the platinum catalyst indicate the coexistence of a fraction of highly dispersed metal crystallites with another one consisting of very large particles, over 100 nm. In the case of the rhodium catalyst no similar evidence could be obtained, thus suggesting an effective metal redispersion.
4.3.4. The Nature of the Strong Metal/Support Interaction Effects in NM/Ce(M)02-x catalysts. As deduced from the literature reviewed in previous sections of this chapter, a good deal of both chemical and nano-structural information about M/Ce0 2 and closely related catalysts is presently available. As a result, a significant progress has been made in the understanding of some of the most puzzling aspects of their behaviour. Such is the case of the strong metal/support interaction effects. The specificities of this phenomenon in NM/CeC>2 catalysts have recently been reviewed (117), a model accounting for its most relevant chemical and nano-structural features being proposed. As we shall briefly discuss below, this model is consistent with the latest results appeared in the literature. These very recent studies deal with catalysts consisting of noble metals supported on both ceria- and ceria-based mixed oxides, in particular Ce/Zr and Ce/Tb. Regarding the nano-structural aspects, there are a number of general common characteristics worth of being outlined. With a few exceptions, which will be commented on below, the available HREM data, including those recently published on Pt(Rh)/Ce0.8oTbo.2o02-x (109,115,155) and Pt(Rh)/Ce0.68Zro.3202 (97,235), are in good agreement with the description advanced in ref. (117). In accordance with this proposal, for catalyst reduced at T < 773 K, the only nano-structural peculiarity is the existence of well defined crystallographic relationships between the supports and the metal crystallites grown on them. Details of these structural relationships, as revealed from the analysis of both profile and plan view HREM images, are given in section 4.3.3.2. The phenomenon is completely general; it has been observed for all the investigated metals (Rh, Pd and Pd), and supports (Ce0 2 , Ce0.8oTbo.2o02-x, and Ceo.68Zro.32O2). Also remarkable is that it does not seem to depend on the reduction temperature, it being observed from the lowest Tred values. Metal decoration is also a general characteristic feature. As shown in section 4.3.3.2, it has been observed in Rh, Pd and Pt catalysts supported on ceria and all the investigated mixed oxides (Ce/Tb and Ce/Zr). It is important to stress, however, that the HREM studies have only provided unequivocal proofs of covering phenomena on catalysts reduced at temperatures well above 773 K, typically 973 K. In accordance with the results discussed in Section 4.3.3.2, a well characterised inter-metallic phase could only be observed on ceria- (124) and ceria/terbia-
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supported (155) Pt catalysts, its formation requiring the highest reduction temperatures, typically 1173 K. No HREM evidence of the CePt5 intermetallic phase could be obtained in the case of the Pt/Ceo.68Zro.3202 catalyst, thus suggesting that the presence of zirconium ions in the ceria lattice prevents the incorporation of the heavily reduced cerium into the metallic phase. Some XRD and SAED indications of alloying phenomena have also been reported on Pd/Ce0 2 catalysts (70,72). In this particular case, however, no well defined inter-metallic phase could be identified. On rhodium catalysts no evidence of alloying effects have been reported as yet. The behaviour against re-oxidation of heavily reduced, i.e. decorated or alloyed, catalysts is an additional, very important, aspect of their nano-structural characterisation. The HREM studies carried out on ceria-supported Rh and Pt catalysts reduced at 1173 K, section 4.3.3.3, have shown that the usual re-oxidation treatment at 773 K does not allow their regeneration. It leads to the formation of big polycrystalline aggregates consisting of a mixture of noble metal and ceria phase&r The effective separation of metal and support is only possible at significantly higher re-oxidation temperatures. These observations suggest that the experimental protocols for recovering the ceria-containing catalysts from the SMSI state should be revised, reoxidation temperatures higher than 773 K being required. A wealth of experimental chemisorption data are presently available for NM/Ce(M)02.x catalysts. As reported in section 4.3.2.2, the increase of Tred generally induces significant modifications on their chemical behaviour. In most of cases, partial rather than complete inhibition of their chemisorption capability is reported. In many cases, however, the techniques and/or experimental routines do not allow an unequivocal interpretation of the reported H(CO)/NM data. As noted in section 4.3.2.2, quite often, the role played by a number of very important side effects, like the metal or support sintering, the adsorption of the probe molecules (H2 and CO) onto the supports, the presence of chlorine in them, or the reversibility of the deactivation phenomena, has not been established. By contrast, there are a number of recent studies (97,117,163,235) from which meaningful conclusions may be drawn. Moreover, some of them (97,163) have provided some additional fine details about the nature of the metal/support interaction effects occurring in ceriabased catalysts. Very recently, HREM and H2 volumetric chemisorption studies at 191 K have been fruitfully combined in the investigation of the metal deactivation effects occurring in Rh/Ceo.68Zr0.3202 (235) and Pt/Ceo.68Zr0.3202 (97) catalysts reduced at increasing temperatures from 423 K to 1173 K. The mixed oxide support, which was the same in both cases, consisted of a texturally pre-stabilized sample, the BET surface area of which remained unmodified throughout the whole series of reduction treatments. Accordingly, metal encapsulation effects could be disregarded. The catalysts were prepared from chlorine free metal precursors. Finally, the chemisorption experiments were run at low temperature, 191 K, in order to minimise the spillover contribution (117,209). By taking these experimental precautions, the authors propose that, in the
Chemical and nanostructural charaterization ofmetal/ceria systems absence of metal deactivation effects, the ratio (DH/DHREM) = 1; where DH and DHREM stand for metal dispersion as determined from H2 adsorption and HREM, respectively. If some deactivation occurs, DH/DHREM should become < 1, the deviation from unity measuring the intensity of the effect. For the Pt/Ceo.68Zr0.3202 catalyst (97), the plot of DH/DHREM against T ^ (Fig. 4.6) shows that the loss of chemisorption capability for H2 starts to be noticeable at Tred = 573 K, and progressively increases with T^. Two relevant conclusions may be drawn from these results: a) In good agreement with a previous study on Rh/Ti02 (318), and a very recent 'H-NMR investigation of the hydrogen chemisorption on Rh/Ce02 catalysts (163), the metal deactivation is a progressive effect; and b) Also in agreement with ref. (105,117,163), the chemical perturbations are observed at T^j values well below those at which metal decoration or, eventually, alloying phenomena occur. Consequently, as proposed in refs. (117,105), for Tred < 773 K, the electronic metal/support interaction effects are responsible for the observed deactivation. This proposal is further supported by the regeneration experiments carried out in refs. (97,105,117). Thus, in ref. (97), the Pt/Ceo.68Zro.3202 catalysts reduced at temperatures ranging from 423 K to 1173 K were further re-oxidised at 700 K, and finally reduced at 423 K. A full recovery of the hydrogen chemisorption capability is reported for catalysts reduced at T ^ < 773 K, the regeneration being only partial for samples reduced at 973 K or 1173 K. Since, as discussed in section 4.3.3.3, re-oxidation treatments at or below 773 K do not allow to recover the ceria-based catalysts from the decorated or alloyed states, the observations above provide an additional evidence of the role played by the electronic effects in these deactivation phenomena. Rather similar conclusions may be drawn from the chemical effects produced by re-oxidation treatments with 0 2 at 673 K (163), or the even milder with C0 2 , at 473 K, applied in ref. (105). In the latter case, the CO chemisorption capability of a Pt/Ce02 catalyst reduced at 773 K could be partly restored by heating it at 473 K under 300 Torr of C0 2 . This regeneration effect is interpreted as due to the C0 2 dissociation with inherent support re-oxidation and CO chemisorption on the Pt particles (105). The different behaviour exhibited by Pt/Ce068Zr0.32O2 and Rh/Ce0.68Zro.3202 catalysts also deserve some comment. If compared the (DH/DHREM) - T^j plots reported in ref. (97) for these two catalysts, we may note that, in spite of being prepared from the same mixed oxide support, and having rather similar metal atom loadings and particle size distributions, the deviation of the (DH/DHREM) from unity starts to be observed at much lower Tred on the Pt catalyst. Moreover, even at the highest reduction temperatures, 973 K or 1173 K, (DH/DHRENOPI « (DH/DHREMV This suggests that Pt and Rh show remarkable differences of sensitivity against deactivation. In conclusion, the intensity of the metal/support interaction effects occurring in ceria-based catalytic systems depends on the nature of the metal, being higher in the case of Pt. Also worth of noting is the conclusion drawn in ref. (163) about the influence of the metal precursor on the intensity of the deactivation effects occurring in Rh/Ce02 catalysts. By using 'H-NMR, the authors investigate the influence of T^j on the H2 chemisorption capability of two catalysts prepared respectively from Rh(N03)3 and
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RhCl3 precursors. As T ^ is increased from 373 K to 773 K, two major effects are observed. First, the intensity of the so-called line B (IB), the one associated with the HRh interaction, remains constant up to T ^ = 573 K, decreasing progressively in the Tred = 573 - 773 K range. Second, the evolution with T^i of the chemical shift characterising line B runs parallel to the one observed for IB. These effects are interpreted as due to changes occurred in the chemical properties of the Rh particles, i.e. to metal deactivation effects. Qualitatively, both CI- and N- catalysts evolve with T ^ similarly. However, there are significant differences in quantitative terms, the relative deactivation effects being stronger in the case of the chlorine-free sample. The authors (163) conclude that the substitution of the O2" ions in the ceria lattice by CI" species modifies the metal/support interface, thus disturbing the electronic interactions taking place between the Rh particles and the reduced ceria in the N catalyst. Finally, it is interesting to compare the characteristics of the metal/support interaction phenomena occurring in ceria-related and titania systems. Analogies and remarkable differences may be observed between them. Among the analogies, we may note that in both cases the metal deactivation is a rather progressive effect. Electronic perturbations are observed first, i.e. at the lowest T ^ values, typically well below 773 K, then, at higher reduction temperatures metal decoration effects occur. There are, however, two major differences. The reduction temperatures required to induce the migration of the support on top of the metal crystallites are far higher in the case of the ceria-containing catalysts. More specifically, on NM/Ce(M)02.x catalysts reduced at 773 K, the classic reference temperature for inducing this effect in titania-supported catalysts, no HREM proof of metal decoration has been reported as yet. The same is true for the Pt-Ce alloying phenomena. These are remarkable observations because most of the studies on ceria-based catalysts at present available have applied reduction temperatures not higher than 773 K. Therefore, the deactivation effects reported in these studies, though sometimes interpreted in terms of the occurrence of decoration or alloying phenomena, are more likely to be due to purely electronic effects. The second major difference between ceria- and titania-supported systems is related to the re-oxidation conditions allowing to recover them from decorated or alloyed states. By analogy with the well established conditions for regenerating M/Ti02 catalysts from the SMSI state, re-oxidation treatments at 773 K, and even lower temperatures, have often been applied to ceria-based systems. However, as shown by the HREM studies reported in section 4.3.3.2, this reference temperature does not allow the reversion of the above effects. Consequently, if the catalyst is recovered from a deactivated state, it should be interpreted as a proof of the absence of significant decoration or alloying in the catalyst. Acknowledgements: Financial support from the CICYT (Project: MAT-99-0570) and the Junta de Andalucia (Group: FQM-110) are gratefully acknowledged.
Chemical and nanostructural charaterization of metal/ceria systems 4.4. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.
Trovarelli A., de Leitenburg C , Boaro M. and Dolcetti G., Catal. Today 50(2) (1999), 353-367. Imamura S., Yamada H. and Utani K., Appl. Catal. A 192 (2000), 221 -226. Murrell L. L. and Carlin R. T., J. Catal. 159 (1996), 479-490. Imamura S., Uchihori D., Utani K. and Ito T., Catal. Lett. 24 (1994), 377 Soria J., Conesa J. C. and Martinez-Arias A., Colloids Surfaces A: Physicochem. Eng. Aspects 158 (1999), 67-74. Liu W. and Flytzani-Stephanopoulos M., J. Catal. 153 (1995), 304-316. Liu W. and Flytzani-Stephanopoulos M., J. Catal. 153 (1995), 317-332. Waters R. D., Weimer J. J. and Smith J. E., Catal. Lett. 30 (1995), 181-188. Shaw E. A., Walker A. P., Rayment T. and Lambert R. M., J. Catal. 134 (1992), 747-750. Hilaire S., Wang X., Luo T., Gorte R. J. and Wagner J., Appl. Catal. A 215 (2001), 271-278. Mouaddib N., Perrichon V. and Martin G. A., Appl. Catal. A 118 (1994), 6372. Mouaddib N., Perrichon V. and Primet M., J. Chem. Soc, Faraday Trans. 185 (1989), 3413-3424. Bruce L. A., Hoang M., Hughes A. E. and Turney T. W., Appl. Catal. A 100 (1993), 51-67. Wang Z. L., Colliex C , Paul-Boncour V., Percheron-Guegan A., Achard J. C. and Barrault J., J. Catal. 105(1987), 120-143. Bera P., Aruna S. T., Patil K. C. and Hedge M. S., J. Catal. 186 (1999), 36-44. Martinez-Arias A., Fernandez-Garcia M., Soria J. and Conesa J. C., J. Catal. 182 (1999), 367-377. Lamonier C , Ponchel A., D'Huysser A. and Jalowiecki-Duhamel L., Catal. Today 50 (1999), 247-259. Luo M. F., Zhong Y. J., Yuan X. X. and Zheng X. M., Appl. Catal. A 162 (1997), 121-131. Daniell W., Lloyd N. C , Bailey C. and Harrison P. G., Journal de Physique IV-7 (1997), 963-964. Abou Kai's A., Bennani A., Ai'ssi C. F., Wrobel G. and Guelton M., J. Chem. Soc, Faraday Trans. 88 (1992), 1321-1325. de Leitenburg C , Goi D., Primavera A., Trovarelli A. and Dolcetti G., Appl. Catal. B 11(1996),L29-L35. Lamonier C., Bennami A., D'Huysse A., Abou Kai's A. and Wrobel G., J. Chem. Soc, Faraday Trans. 92(1) (1996), 131-136. Aboukais A., Bennani A., Damonier-Dulongpont C., Abi-aad E. and Wrobel G., Colloids Surfaces A: Physicochem. Eng. Aspects 115 (1996), 171-177. Wrobel G., Lamonier C., Bennani A., D'Huysser A. and Aboukais A., J. Chem. Soc, Faraday Trans. 92 (1996), 2001-2009.
153
154
Catalysis by ceria and related materials
25. Tschope A., Ying J. Y. and Chiang Y. M., Mater. Sci. Eng. A -Struct. Mater. 204(1995), 267-271. 26. Liu W., Sarofim A. F. and Flytzani-Stephanopoulos M., Appl. Catal. B 4 (1994), 167-186. 27. de Leitenburg C , Trovarelli A. and Kaspar J., J. Catal. 166 (1997), 98-107. 28. Tournayan L., Marcilio N. R. and Frety R., Appl. Catal. 78 (1991), 31-43. 29. Frety R., Da Silva P. N. and Guenin M., Appl. Catal. 58 (1990), 175-188. 30. Da Silva P. N., Guenin M., Lecrercq C. and Frety R., Appl. Catal. 54 (1989), 203-215. 31. Zhu T. and Flytzani-Stephanopoulos M., Appl. Catal. A 208 (2001), 403-417. 32. Tang S., Lin J. and Tan K. L., Catal. Lett. 51 (1998), 169-175. 33. Belyaev V. D., Politova T. I., Marina O. A. and Sobyanin V. A., Appl. Catal. A 133 (1995), 47-57. 34. Colliex C , Lefevre E. and Tence M., Inst. Phys. Conf. Ser. 138 (1993), 25-30. 35. Kacimi S., Barbier Jr. J., Taha R. and Duprez D., Catal. Lett. 22 (1993), 343 36. Wrobel G., Sohier M. P., D'Huysse A., Bonnelle J. P. and Marcq J. P., Appl. Catal. A 101 (1993), 73-93. 37. Inui T., J. Alloys Comp. 193 (1993), 47-51. 38. Ramaroson E., Tempere J. F., Guilleux M. F., Vergand F., Roulet H. and Dufour G., J. Chem. Soc, Faraday Trans. 88 (1992), 1211-1218. 39. Sohier M. P., Wrobel G., Bonnelle J. P. and Marcq J. P., Appl. Catal. A 84 (1992), 169-186. 40. Barrault J. and Alouche A., Appl. Catal. 58 (1990), 255-267. 41. Barrault J., Alouche A., Paul-Boncour V., Hilaire L. and Percheron-Guegan A., Appl. Catal. 46 (1989), 269-279. 42. Maubert A., Martin G. A., Praliaud H. and Turlier P., React. Kinet. Catal. Lett. 24 (1984), 183-186. 43. Akalay I., Guilleux M. F., Tempere J. F. and Delafosse D., J. Chem. Soc, Faraday Trans. 783 (1987), 1137-1148. 44. Matsamura Y., Shen W. J., Ichihashi Y. and Okumura M., J. Catal. 197 (2001), 267-272. 45. Bigey C , Hilaire L. and Maire G., J. Catal. 198 (2001), 208-222. 46. Fernandez-Garcia M., Martinez-Arias A., Salamanca L. N., Coronado J. M., Anderson J. A., Conesa J. C. and Soria J., J. Catal. 187 (1999), 474-485. 47. Yee A., Morrison S. J. and Idriss H., J. Catal. 186 (1999), 279-295. 48. Jen H. W., Graham G. W., Chun W., McCabe R. W., Cuif J.-P., Deutsch S. E. and Touret O., Catal. Today 50 (1999), 309-328. 49. Kepinski L., Catal. Today 50 (1999), 237-245. 50. Hori C. E., Brenner A., Simon Ng K. Y., Rahmoeller K. M. and Belton D., Catal. Today 50 (1999), 299-308. 51. Bera P., Patil K. C , Jayaram V., Subbanna G. N. and Hegde M. S., J. Catal. 196(2000), 293-301. 52. Kepinski L. and Okal J., J. Catal. 192 (2000), 48-53.
Chemical and nanostructural charaterization of metal/ceria systems
155
53. Sudhakar C. and Vannice M. A., Appl. Catal. 14 (1985), 47-63. 54. Usami Y., Kagawa K., Kawazoe M., Matsumura Y., Sakurai H. and Haruta M.,Appl. Catal. A 171 (1998), 123-130. 55. Bunluesin T., Graham G. W. and Gorte R. J., Appl. Catal. B 14 (1997), 105115. 56. Bensalem A. and Bozon-Verduraz F., Reaction Kinetics and Catal. Lett. 60 (1997), 71-77. 57. Bunluesin T., Gorte R. J. and Graham G. W., Appl. Catal. B 15 (1998), 107114. 58. Luo M. F., Hou Z. Y., Yuan X. X. and Zheng X. M., Catal. Lett. 50 (1998), 205-209. 59. Craciun R., Shereck B. and Gorte R. J., Catal. Lett. 51 (1998), 149-153. 60. Kepinski L. and Wolcyrz M , Appl. Catal. A 150 (1997), 197-220. 61. Trovarelli A., Catal. Rev. - Sci. Eng. 38 (1996), 439-520. 62. Badri A., Binet C. and Lavalley J. C , J. Chem. Soc, Faraday Trans. 92(9) (1996), 1603-1608. 63. Cordatos H. and Gorte R. J., J. Catal. 159 (1996), 112-118. 64. Bensalem A., Muller J. C , Tessier D. and Bozon-Verduraz F., J. Chem. Soc, Faraday Trans. 92(17) (1996), 3233-3237. 65. Bunluesin T., Putna E. S. and Gorte R. J., Catal. Lett. 41 (1996), 1-5. 66. Sayle T. X. T., Parker S. C. and Catlow C. R. A., J. Phys. Chem. 98 (1994), 13625-13630. 67. Murrell L. L., Tauster S. J. and Anderson D. R., Catalysis and Automotive Pollution Control II (1991), 275-289. 68. Filotti L., Bensalem A., Bozon-Verduraz F., Shafeev G. A. and Voronov V. V., Appl. Surf. Sci. 109 (1997), 249. 69. Bunluesin T., Gorte R. J. and Graham G. W., Appl. Catal. B 14 (1997), 105115. 70. Kepinski L. and Wolcyrz M., Appl. Catal. A 150 (1997), 197-220. 71. Bensalem A., Bozon-Verduraz F. and Perrichon V.,J. Chem. Soc, Faraday Trans. 91 (1995), 2185-2189. 72. Kepinski L., Wolcyrz M. and Okal J., J. Chem. Soc, Faraday Trans. 91(3) (1995), 507-515. 73. Souza Monteiro R., Bellot Noronha F., Chaloub Dieguez L. and Schmal M., Appl. Catal. A 131 (1995), 89-106. 74. Fan L. and Fujimoto K., J. Catal. 150 (1994), 217-220. 75. Badri A., Binet C. and Lavalley J. C , J. Phys. Chem. 100(20) (1996), 83638368. 76. Binet C , Badri A., Boutonnet-Kizling M. and Lavalley J. C , J. Chem. Soc, Faraday Trans. 90 (1994), 1023-1028. 77. Bensalem A., Shafeev G. and Bozon-Verduraz F., Catal. Lett. 18 (1993), 165171. 78. Binet C., Jadi A., Lavalley J. C. and Boutonnet-Kizling M., J. Chem. Soc,
156
Catalysis by ceria and related materials
Faraday Trans. 88 (1992), 2079-2084. 79. Wolcyrz M. and Kepinski L., J. Solid State Chemistry 99 (1992), 409-413. 80. Trovarelli A., Dolcetti G., de Leitenburg C. and Kaspar J., Stud. Surf. Sci. Catal. 75 (1993), 2781-2784. 81. Duplan J. L. and Praliaud H., Appl. Catal. 67 (1991), 325-335. 82. Le Normand F., Barrault J., Breault R., Hilaire L. and Kiennemann A., J. Phys. Chem. 95 (1991), 257-269. 83. Shyu Z., Otto K., Watkins L. H., Graham G. W., Belitz R. K. and Gandhi H. S., J. Catal. 114 (1988), 23-33. 84. Sudhakar C. and Vannice M. A., J. Catal. 95 (1985), 227-243. 85. Mitchell M. D. and Vannice M. A., Ind. Eng. Chem. Fundam. 23 (1984), 8896. 86. Usami Y., Kagawa K., Kawazoe M., Matsumura Y., Sakurai H. and Haruta M.,Appl. Catal. A 171 (1998), 123-130. 87. Bunluesin T., Graham G. W. and Gorte R. J., Appl. Catal. B 14 (1997), 105115. 88. Fuchs S. and Hahn T., Chemical Engineering and Processing 32 (1993), 225231. 89. Le Normand F., Barrault J., Breault R., Hilaire L. and Kiennemann A., J. Phys. Chem. 95(1991), 257-269. 90. Sudhakar C. and Vannice M. A., J. Catal. 95 (1985), 227-243. 91. Mitchell M. D. and Vannice M. A., Ind. Eng. Chem. Fundam. 23 (1984), 8896. 92. Vannice M. A., Sudhakar C. and Freeman M., J. Catal. 108 (1987), 97-111. 93. Norman A., Perrichon V., Bensaddik A., Lemaux S., Bitter J. H. and Koningsberger D. C , Topics in Catalysis In Press (2001) 94. Fernandez-Garcia M., Martinez-Arias A., Iglesias-Juez A., Hungria A. B., Anderson J. A., Conesa J. C. and Soria J., Appl. Catal. B 31 (2001), 39-50. 95. Martinez-Arias A., Fernandez-Garcia M., Iglesias-Juez A., Hungria A. B., Anderson J. A., Conesa J. C. and Soria J., Appl. Catal. B 31 (2001), 51-60. 96. Gonzalez-Velasco J. R., Gutierrez-Ortiz M. A., Marc J. L., Botas J. A., Gonzalez-Marcos M. P. and Blanchard G., Appl. Catal. B 25 (2000), 19-29. 97. Gatica J. M., Baker R. T., Fornasiero P., Bernal S. and Kaspar J., J. Phys. Chem. B 105 (2001), 1191-1199. 98. Hickey N., Fornasiero P., Kaspar J., Gatica J. M. and Bernal S., J. Catal. 200 (2001), 181-193. 99. Luo M. and Zheng X., Appl. Catal. A 189 (1999), 15-21. 100. Narula C. K., Haack L. P., Chun W., Jen H. W. and Graham G. W., J. Phys. Chem. B 103 (1999), 3634-3639. 101. Fornasiero P., Kaspar J., Sergo V. and Graziani M., J. Catal. 181 (1999), 5669. 102. Bekyarova E., Fornasiero P., Kaspar J. and Graziani M., Catal. Today 45 (1998), 179-183.
Chemical and nanostructural charaterization ofmetal/ceria systems
157
103. Graham G. W., Jen H. W., Chun W. and McCabe R. W., Catal. Lett. 44 (1997), 185-187. 104. Fornasiero P., Balducci G., Kaspar J., Meriani S., Di Monte R. and Graziani M., Catal. Today 29(1-4) (1996), 47-52. 105. Bernal S., Blanco G., Gatica J. M., Larese C. and Vidal H., J. Catal. 200 (2001), 411-415. 106. Bitter J. H., Cauqui M. A., Gatica J. M., Bernal S., Ramaker D. E. and Koningsberger D. C , Stud. Surf. Sci. Catal. 130 (2000), 3183-3188. 107. Rupprechter G., Calvino J. J., Lopez-Cartes C , Fuchs S., Gatica J. M., PerezOmil J. A., Hayek K. and Bernal S., Stud. Surf. Sci. Catal. 130 (2000), 20212026. 108. Abid M. and Touroude R., Catal. Lett. 69 (2000), 139-144. 109. Bernal S., Baker R. T., Burrows A., Calvino J. J., Kiely C. J., Lopez-Cartes C , Perez-Omil J. A. and Rodriguez-Izquierdo J. M., Surface and Interface Analysis 29 (2000), 411-421. 110. Holgado J. P. and Munuera G., Stud. Surf. Sci. Catal. (1995), 109. 111. Anna Y., Morrison S. J. and Idriss H., J. Catal. 191 (2000), 30-45. 112. Holmgren A., Azarnoush F. and Fridell E., Appl. Catal. B 22 (1999), 49-61. 113. Holmgren A., Andersson B. and Duprez D., Appl. Catal. B 22 (1999), 215230. 114. Holmgren A., Duprez D. and Andersson B., J. Catal. 182 (1999), 441-448. 115. Bernal S., Blanco G., Cauqui M. A., Corchado M. P., Larese C , Pintado J. M. and Rodriguez-Izquierdo J. M., Catal. Today 53 (1999), 607-612. 116. Imamura S., Higashihara T., Saito Y., Aritani H., Kanai H., Matsumura Y. and Tsuda N„ Catal. Today 50 (1999), 369-380. 117. Bernal S., Calvino J. J., Cauqui M. A., Gatica J. M., Larese C , Perez-Omil J. A. and Pintado J. M., Catal. Today 50 (1999), 175-206. 118. Sepulveda-Escribano A., Coloma F. and Rodriguez-Reinoso F., J. Catal. 178 (1998), 649-657. 119. Imamura S., Fukuda I. and Ishida S., Ind. Eng. Chem. Prod. Res. 27 (1998), 718. 120. Holmgren A. and Andersson B., J. Catal. 178 (1998), 14-25. 121. Lavalley J. C , Waqif M., Bazin P., Saur O., Blanchard G. and Touret O., Appl. Catal. B 11 (1997), 193-205. 122. Shalabi M. A., Harji B. H. and Kenney C. N., J. Chemical Technology and Biotechnology 65(4) (1996), 317-324. 123. Hardacre C , Rayment T. and Lambert R. M., J. Catal. 158 (1996), 102-108. 124. Bernal S., Calvino J. J., Gatica J. M., Larese C , Lopez-Cartes C. and PerezOmil J. A., J. Catal. 169 (1997), 510-515. 125. Kalakkad D., Datye A. K. and Robota H., Appl. Catal. B 1 (1992), 191-219. 126. Chojnacki T., Krause K. and Schmidt L. D., J. Catal. 128 (1991), 161-185. 127. El Fallah J., Le Normand F., Boujana S. and Kiennemann A., European Journal of Solid State Inorganic Chemistry 28 (1991), 433-436.
158
Catalysis by ceria and related materials
128. LoofP., Kasemo B., Andersson S. and Frestad A., J. Catal. 130 (1991), 181191. 129. Dauscher A., Hilaire L., Le Normand L., Miiller W., Maire G. and Vasquez A., Surface and Interface Analysis 16 (1990), 341-346. 130. Primet M., El Azhar M., Frety R. and Guenin M., Appl. Catal. 59 (1990), 153163. 131. Meriaudeau P., Dutel J. F., Dufaux M. and Naccache C , Stud. Surf. Sci. Catal. 11(1982), 95-104. 132. Zhou Y., Nakashima M. and White J. M., J. Phys. Chem. 92 (1988), 812-818. 133. Daniel D. W., J. Phys. Chem. 92 (1988), 3891-3899. 134. Duprez D., Delanoe F., Barbier Jr. J., Isnard P. and Blanchard G., Catal. Today 29(1996), 317-322. 135. Bernal S., Cauqui M. A., Cifredo G. A., Gatica J. M., Larese C. and PerezOmil J. A., Catal. Today 29 (1996), 77-81. 136. Hardacre C , Roe G. M. and Lambert R. M., Surf. Sci. 326 (1995), 1-10. 137. Datye A. K., Kalakkad D., Yao M. H. and Smith D. J., J. Catal. 155 (1995), 148-153. 138. Golunski S. E., Hatcher H. A., Rajaram R. R. and Truex T. J., Appl. Catal. B 5 (1995), 367-376. 139. Kalakkad D., Datye A. K. and Robota H. J., J. Catal. 148 (1994), 729-736. 140. Hardacre C , Ormerod R. M. and Lambert R. M., J. Phys. Chem. 98 (1994), 10901-10905. 141. Zafiris G. S. and Gorte R. J., Surf. Sci. 276 (1992), 86-94. 142. Yoshitake H. and Iwasawa Y., J. Chem. Soc, Faraday Trans. 88 (1992), 503510. 143. Nunan J. G., Cohn M. J. and Donner J. T., Catal. Today 14 (1992), 277-291. 144. Kalakkad D., Datye A. K. and Robota H., Appl. Catal. B 1 (1992), 191-219. 145. Levy P. J. and Primet M., Appl. Catal. 70 (1991), 263-276. 146. Dauscher A. and Maire G., J. Mol. Catal. 69 (1991), 259-270. 147. Cochrane H. D., Hutchinson J. L., White D., Parkinson G. M., Dupas C. and Scott A. J., Ultramicroscopy 34 (1990), 10-16. 148. Boujana S., Demri D., Cresseley J., Kiennemann A. and Hindermann J. P., Catal. Lett. 7 (1990), 359-366. 149. Jin T., Okuhara T., Mains G. J. and White J. M., J. Phys. Chem. 91 (1987), 3310-3315. 150. Otsuka K., Hatano M. and Morikawa A., Reactivity of Solids 1 (1985), 87-93. 151. Mendelovici L., Tzehoval H. and Steinberg M., Appl. Surf. Sci. 17 (1983), 175188. 152. Mendelovici L. and Steinberg M., Appl. Catal. 4 (1982), 237. 153. Summers J. C. and Ausen S. A., J. Catal. 58 (1979), 131-143. 154. Fuchs S. and Hahn T., Chemical Engineering and Processing 32 (1993), 225231. 155. Blanco G., Calvino J. J., Cauqui M. A., Corchado M. P., Lopez-Cartes C ,
Chemical and nanostructural charaterization ofmetal/ceria systems Colliex C , Perez-Omil J. A. and Stephan 0., Chem. Mat. 11 (1999), 36103619. 156. Noronha F. B., Fendley E. C , Soares R. R., Alvarez W. E. and Resasco D. E., Chemical Engineering Journal 82 (2001), 21 -31. 157. Kenevey K., Valdivieso F., Soustelle M. and Pijolat M., Appl. Catal. B 29 (2001), 93-101. 158. Pantu P., Kim K. and Gavalas G. R., Appl. Catal. A 193 (2000), 203-214. 159. Norman A., Sporken R., Galtayries A., Mirabella F., Keveney K., Pijolat M., Baker R. T. and Bernal S., Mat. Res. Soc. Symp. Proc. 581 (2000), 345-350. 160. Hickey N., Fornasiero P., Kaspar J., Graziani M., Blanco G. and Bernal S., Chem. Commun. (2000), 357-358. 161. Rao G. R., Fornasiero P., di Monte R., Kaspar J., Vlaic G., Balducci G., Meriani S., Gubitosa G., Cremona A. and Graziani M., J. Catal. 162 (1996), 19. 162. Mullins D. R., Kundakovic Lj. and Overbury S. H., J. Catal. 195 (2000), 169179. 163. Force C , Belzunegui J. P., Sanz J., Martinez-Arias A. and Soria J., J. Catal. 197 (2001), 192-199. 164. Mullins D. R. and Overbury S. H., J. Catal. 188 (1999), 340-345. 165. Kondarides D. I., Zhang Z. and Verykios X. E., J. Catal. 176 (1998), 536-544. 166. Kondarides D. I. and Verykios X. E., J. Catal. 174 (1998), 52-64. 167. Cunningham J., Farrell F., Gibson C. and McCarthy J., Catal. Today 50(2) (1999), 429-443. 168. Putna E. S., Gorte R. J., Vohs J. M. and Graham G. W., J. Catal. 178 (1998), 598-603. 169. Nelson A. E., Yip J. and Schulz K. H., Appl. Catal. B 30 (2001), 375-387. 170. Fajardie F., Tempere J. F., Manoli J. M., Djega-Mariadassou G. and Blanchard G., J. Chem. Soc, Faraday Trans. 94 (1998), 3727-3735. 171. Bernal S., Calvino J. J., Cauqui M. A., Cifredo G. A., Jobacho A. and Rodriguez-Izquierdo J. M.,Appl. Catal. 99 (1993), 1-8. 172. Pfau A., Sanz J., Schiervaum K. D., Gopel W., Belzunegui J. P. and Rojo J. M., Stud. Surf. Sci. Catal. 101 (1996), 931-940. 173. Putna E. S., Vohs J. M. and Gorte R. J., J. Phys. Chem. 100 (1996), 1786217865. 174. Cunningham J., Hickey J. N., Cataluna R., Conesa J. C , Soria J. and MartinezArias A., Stud. Surf. Sci. Catal. 101 (1996), 681-690. 175. Demri D., Chateau L., Hindermann J. P., Kiennemann A. and Bettahar M. M., J. Mol. Catal. 104 (1996), 237-249. 176. Martin D. and Duprez D., J. Phys. Chem. 100 (1996), 9429-9438. 177. Stubenrauch J. and Vohs J. M., J. Catal. 159 (1996), 50-57. 178. Cordatos H., Bunluesin T., Stubenrauch J., Vohs J. M. and Gorte R. J., J. Phys. Chem. 100 (1996), 785-789. 179. Pfau A., Sanz J., Schierbaum K. D., Gopel W., Belzunegui J. P. and Rojo J.
\ 59
160
Catalysis by ceria and related materials
M , 11th. Int. Congr. Catal. Abstracts. Eds. J. W. Hightower, W. N. Delgass, E. Iglesia and A. T. Bell. (1996), 931-940. 180. Taha R., Martin D., Kacimi S. and Duprez D., Catal. Today 29 (1996), 89-92. 181. Martin D. and Duprez D., J. Phys. Chem. B 101 (1997), 4428-4436. 182. Imamura S., Yamashita T., Hamada R., Saito Y., Nakao Y., Tsuda N. and Kaito C , J. Mol. Catal. A 129 (1998), 249-256. 183. Bernal S., Calvino J. J., Cauqui M. A., Perez-Omil J. A., Pintado J. M. and Rodriguez-Izquierdo J. M., Appl. Catal. B 16 (1998), 127-138. 184. Bernal S., Botana F. J., Calvino J. J., Lopez-Cartes C , Perez-Omil J. A. and Rodriguez-Izquierdo J. M., Ultramicroscopy 72 (1998), 135-164. 185. Oi J., Obuchi A., Bamwenda G. R., Ogata A., Yagita H., Kushiyama S. and Mizuno K., Appl. Catal. B 12 (1997), 277-286. 186. Fornasiero P., Kaspar J. and Graziani M., J. Catal. 167 (1997), 576-580. 187. Bernal S., Blanco G., Cauqui M. A., Corchado M. P., Pintado J. M. and Rodriguez-Izquierdo J. M., Chem. Commun. (1997), 1545-1546. 188. Martinez-Arias A., Soria J. and Conesa J. C , J. Catal. 168 (1997), 364-373. 189. Martinez Arias A., Soria J. and Conesa J. C , J. Catal. 168 (1997), 364-373. 190. Overbury S. H., Huntley D. R., Mullins D. R., Ailey K. S. and Radulovic P. V., J. Vacuum Science & Technology A 15(1997), 1647-1652. 191. Putna E. S., Vohs J. M. and Gorte R. J., Catal. Lett. 45 (1997), 143-147. 192. Stubenrauch J. and Vohs J. M., Catal. Lett. 47 (1997), 21-25. 193. Bernal S., Calvino J. J., Cifredo G. A., Gatica J. M., Perez-Omil J. A., Laachir A. and Perrichon V., Stud. Surf. Sci. Catal. 96 (1995), 419-429. 194. Bernal S., Botana F. J., Calvino J. J., Cifredo G. A., Perez-Omil J. A. and Pintado J. M., Catal. Today 28 (1995), 219-250. 195. Bernal S., Calvino J. J., Cifredo G. A. and Rodriguez-Izquierdo J. M., J. Phys. Chem. 99(1995), 11794-11796. 196. de Leitenburg C. and Trovarelli A., J. Catal. 156 (1995), 171-174. 197. Demri D., Hindermann J. P., Kiennemann A. and Mazzocchia C , Catal. Lett. 23 (1994), 227-236. 198. Fajardie F., Tempere J. F., Djega-Mariadassou G. and Blanchard G., J. Catal. 163 (1996), 77-86. 199. Pfau A., Schierbaum K. D. and Gopel W., Surf. Sci. 331-333 (1995), 14791485. 200. Putna E. S., Stubenrauch J., Vohs J. M. and Gorte R. J., Langmuir 11 (1995), 4832-4837. 201. Trovarelli A., de Leitenburg C , Dolcetti G. and Llorca J., J. Catal. 151 (1995), 111-124. 202. Bunluesin T., Cordatos H. and Gorte R. J., J. Catal. 157 (1995), 222-226. 203. Bernal S., Blanco G., Calvino J. J., Cifredo G. A., Perez-Omil J. A., Pintado J. M. and Varo A., Stud. Surf. Sci. Catal. 82 (1994), 507-514. 204. Bernal S., Calvino J. J., Cifredo G. A., Laachir A., Perrichon V. and Herrmann J. M., Langmuir 10 (1994), 717-722.
Chemical and nanostructwal charaterization ofmetal/ceria systems 205. El Fallah J., Boujana S., Dexpert H., Kiennemann A., Majerus J., Touret 0., Villain F. and Le Normand F., J. Phys. Chem. 98 (1994), 5522-5533. 206. Laachir A., Perrichon V., Bernal S., Calvino J. J. and Cifredo G. A., J. Mol. Catal. 89 (1994), 391-396. 207. Padeste C , Cant N. W. and Trimm D. L., Catal. Lett. 28 (1994), 301-311. 208. Padeste C , Cant N. W. and Trimm D. L., Catal. Lett. 24 (1994), 95-105. 209. Bernal S., Botana F. J., Calvino J. J., Cauqui M. A., Cifredo G. A., Jobacho A., Pintado J. M. and Rodriguez-Izquierdo J. M., J. Phys. Chem. 97 (1993), 41184123. 210. Bernal S., Botana F. J., Calvino J. J., Cifredo G. A. and Perez-Omil J. A., Inst. Phys. Conf. Ser. 138 (Section 10) (1993), 485-488. 211. Shido T. and Iwasawa Y., J. Catal. 141 (1993), 71-81. 212. Soria J., Martinez-Arias A., Coronado J. M. and Conesa J. C , Appl. Surf. Sci. 70/71 (1993), 245-249. 213. Warren J. P., Zhang X., Andersen J. E. T. and Lambert R. M., Surf. Sci. 287/288(1993), 222-227. 214. Zafiris G. S. and Gorte R. J., J. Catal. 139 (1993), 561-567. 215. Zafiris G. S. and Gorte R. J., J. Catal. 143 (1993), 86-91. 216. Barbier Jr. J., Marsollier F. and Duprez D., Appl. Catal. A 90 (1992), 11-23. 217. Bernal S., Calvino J. J., Cifredo G. A., Rodriguez-Izquierdo J. M , Perrichon V. and Laachir A., Chem. Commun. (1992), 460-462. 218. Bernal S., Calvino J. J., Cifredo G. A., Rodriguez-Izquierdo J. M., Perrichon V. and Laachir A., J. Catal. 137 (1992), 1-11. 219. Cunningham J., Cullinane D., Sanz J., Rojo J. M., Soria J. and Fierro J. L. G., J. Chem. Soc, Faraday Trans. 88(21) (1992), 3233-3240. 220. Soria J., Martinez-Arias A. and Conesa J. C , Vacuum 43 (1992), 437-440. 221. Trovarelli A., Dolcetti G., de Leitenburg C , Kaspar J., Finetti P. and Santoni A., J. Chem. Soc, Faraday Trans. 88(1992), 1311-1319. 222. Bernal S., Calvino J. J., Cifredo G. A., Jobacho A. and Rodriguez-Izquierdo J. M.,J. Rare Earths (Special Issue) 2 (1991), 838-841. 223. Gonzalez-Elipe A. R., Holgado J. P., Alvarez R., Espinos J. P., Fernandez A. and Munuera G., in Fundamental Aspects of Heterogeneous Catalysis Studied by Particle Beams, Eds. Brongersma,H.H.; van Santen,R.A. (Plenum Press, New York, 1991)227-235. 224. Munuera G., Fernandez A. and Gonzalez Elipe A. R., Stud. Surf. Sci. Catal. 71 (1991), 207-219. 225. Trovarelli A., de Leitenburg C. and Dolcetti G., Chem. Commun. (1991), 472473. 226. Bernal S., Botana F. J., Calvino J. J., Cifredo G. A., Garcia R. and Rodriguezlzquierdo J. M., Ultramicroscopy 34 (1990), 60-65. 227. Cunningham J., O'Brien S., Sanz J., Rojo J. M., Soria J. and Fierro J. L. G., J. Mol. Catal. 57 (1990), 379-396. 228. Lavalley J. C , Saussey J., Lamotte J., Breault R., Hindermann J. P. and
161
162
Catalysis by ceria and related materials
Kiennemann A., J. Phys. Chem. 94 (1990), 5941-5947. 229. Pan M., Cowley J. M. and Garcia R., Micron Microscopica Acta 18 (1987), 165-169. 230. Bernal S., Botana F. J., Garcia R., Kang Z. C , Lopez M. L., Pan M., Ramirez F. and Rodriguez-Izquierdo J. M., Catal. Today 2 (1988), 653-662. 231. Putna E. S., Vohs J. M., Gorte R. J. and Graham G. W., Catal. Lett. 54 (1998), 17-21. 232. Bernal S., Calvino J. J., Cifredo G. A., Gatica J. M., Larese C. and Pintado J. M., J. Alloys Comp. 225 (1995), 633-637. 233. Imamura S., Tadani J., Saito Y., Okamoto Y., Jindai H. and Kaito C , Appl. Catal. A 201 (2000), 121-127. 234. Blanco G., Calvino J. J., Cauqui M. A., Corchado M. P., Lopez-Cartes C. and Perez-Omil J. A., Inst. Phys. Conf. Ser. 161 (Section 10) (1999), 541-544. 235. Gatica J. M., Baker R. T., Fomasiero P., Bernal S., Blanco G. and Kaspar J., J. Phys. Chem. B 104 (2000), 4667-4672. 236. Fomasiero P., Rao G. R., Kaspar J., L'Erario F. and Graziani M., J. Catal. 175 (1998), 269-279. 237. Vlaic G., di Monte R., Fomasiero P., Fonda E., Kaspar J. and Graziani M., J. Catal. 182 (1999), 378-389. 238. Fajardie F., Tempere J. F., Manoli J. M., Touret O., Blanchard G. and DjegaMariadassou G., J. Catal. 119 (1998), 469-476. 239. Vlaic G., Fomasiero P., Geremia S., Kaspar J. and Graziani M., J. Catal. 168 (1997), 386-392. 240. Fomasiero P., Kaspar J. and Graziani M., J. Catal. 167 (1997), 576-580. 241. di Monte R., Fomasiero P., Graziani M. and Kaspar J., J.'Alloys Comp. 275277(1998), 877-885. 242. Barbier Jr. J., Delanoe F., Jabouille F., Duprez D., Blanchard G. and Isnard P., J. Catal. Ill (1998), 378-385. 243. Imamura S., Uematsu Y., Utani K. and Ito T., Ind. Eng. Chem. Res. 30 (1991), 18. 244. Niwa Y. and Aika K., J. Catal. 162 (1996), 138-142. 245. Saeki M. J., Uchida H. and Watanabe M., Catal. Lett. 26 (1994), 149-157. 246. Shelef M. and McCabe R. W., Catal. Today 62 (2000), 35-50. 247. Kaspar J., Fomasiero P. and Graziani M., Catal. Today 50 (1999), 285-298. 248. Harrison B., Diwell A. F. and Hallett C , Platinum Metals Review 32(2) (1988), 73-83. 249. Engler B., Koberstein E. and Schubert P., Appl. Catal. 48 (1989), 71-92. 250. Liu W. and Flytzani-Stephanopoulos M., J. Catal. 153 (1995), 317-332. 251. Mendelovici L. and Steinberg M., J. Catal. 93 (1985), 353-359. 252. Mendelovici L. and Steinberg M., J. Catal. 96 (1985), 285-287. 253. Stagg-Williams S. M., Noronha F. B., Fendley G. and Resasco D. E., J. Catal. 194 (2000), 240-249. 254. Imamura S., Fukuda I. and Ishida S., Ind. Eng. Chem. Res. 11 (1988), 718-721.
Chemical and nanostructural charaterization ofmetal/ceria systems 255. Di Monte R., Fornasiero P., Kaspar J., Rumori P., Gubitosa G. and Graziani M.,Appl. Catal. B 24(2000), 157-167. 256. McCabe R. W. and Usmen R. K., Stud. Surf. Sci. Catal. 101 (1998), 355-368. 257. Cho B. K., Shanks B. M. and Bailey J. E., J. Catal. 115 (1989), 486. 258. Guenin M., Da Silva P. N. and Frety R., Appl. Catal. 27 (1986), 313-323. 259. Shaw E. A., Rayment T., Walker A. P., Jennings J. R. and Lambert R. M., J. Catal. 126 (1990), 219-227. 260. Shaw E. A., Rayment T., Walker A. P., Lambert R. M., Gauntlett T., Oldman R. J. and Dent A., Catal. Today 9 (1991), 197-202. 261. Owen G., Hawkes C. M., Lloyd D., Jennings J. R., Lambert R. M. and Nix R. M., Appl. Catal. 33 (1987), 405. 262. Barrault J., Alouche A., Paul-Boncour V., Hilaire L. and Percheron-Guegan A., Appl. Catal. 46 (1989), 269-279. 263. Herrmann J. M., Ramaroson E., Tempere J. F. and Guilleux M. F., Appl. Catal. 53(1989), 117-134. 264. Turlier P., Praliaud H„ Moral P., Martin G. A. and Dalmon J. A., Appl. Catal. 19 (1985), 287-300. 265. Idriss H., Diagne C., Hindermann J. P., Kiennemann A. and Barteau M. A., Stud. Surf. Sci. Catal. 75 (1993), 2119. 266. Diagne C , Idriss H., Pepin I., Hindermann J. P. and Kiennemann A., Appl. Catal. 50 (1989), 43-53. 267. Diagne C , Idriss H., Hindermann J. P. and Kiennemann A., Appl. Catal. 51 (1989), 165-180. 268. Diagne C , Idriss H., Hindermann J. P. and Kiennemann A., Appl. Catal. 51 (1989), 165-180. 269. Katzer J. R., Sleight A. W., Gajardo P., Michel J. B., Gleason E. F. and McMillan S., Disc. Faraday Soc. 72 (1981), 121-133. 270. Fan L. and Fujimoto K., Appl. Catal. A 106 (1993), L1-L7. 271. Walker A. P., Rayment T. and Lambert R. M., J. Catal. 117 (1989), 102-120. 272. Walker A. P., Rayment T., Lambert R. M. and Oldman R. J., J. Catal. 125 (1990), 67-76. 273. Datye A. K., Kalakkad D., Cunningham J., Cullinane D., Farrell F. and Morris M. A., Stud. Surf. Sci. Catal. 96 (1995), 237-248. 274. Bernal S., Blanco G., Cauqui M. A., Martin A. I., Pintado J. M., Galtayries A. and Sporken R., Surface and Interface Analysis 30 (2000), 85-89. 275. Knauth P. and Tuller H. L., J. European Ceramic Society 19 (1999), 831-836. 276. Trovarelli A., de Leitenburg C. and Dolcetti G., CHEMTECH 27 (1997), 3237. 277. Sinev M. Y., Graham G. W., Haack L. P. and Shelef M., J. Mater. Res. 11 (1996), 1960-1971. 278. Logan A. D. and Shelef M., J. Mater. Res. 9 (1994), 468-475. 279. Bernal S., Blanco G., Cauqui M. A., Corchado M. P., Pintado J. M., Rodriguez-Izquierdo J. M. and Vidal H., Stud. Surf. Sci. Catal. 116 (1998),
163
164
Catalysis by ceria and related materials
611-618. 280. Zamar F., Trovarelli A., de Leitenburg C. and Dolcetti G., Stud. Surf. Sci. Catal. 101 (1996), 1283-1292. 281. Bernal S., Blanco G., Botana F. J., Gatica J. M., Perez-Omil J. A., Pintado J. M. and Rodriguez-Izquierdo J. M., J. Alloys Comp. 207/208 (1994), 196-200. 282. Fomasiero P., Hickey N., Kaspar J., Dossi C , Gava D. and Graziani M., J. Catal. 189 (2000), 326-338. 283. Bernal S., Botana F. J., Garcia R., Ramirez F. and Rodriguez-Izquierdo J. M., Mater. Chem. Phys. 18 (1987), 119-127. 284. Shen W. J., Ichihashi Y., Okumura M. and Matsumura Y., Catal. Lett. 64 (2000), 23-25. 285. Hori C. E., Permana H., Ng K. Y. S., Brenner A., More K., Rahmoeller K. M. and Belton D., Appl. Catal. B 16 (1998), 105-117. 286. Vlaic G., Fornasiero P., Martra G., Fonda E., Kaspar J., Marchese L., Tomat E., Coluccia S. and Graziani M., J. Catal. 190 (2000), 182-190. 287. Fornasiero P., Fonda E., Di Monte R., Vlaic G., Kaspar J. and Graziani M., J. Catal. 187(1999), 177-185. 288. Fornasiero P., Hickey N., Kaspar J., Montini T. and Graziani M., J. Catal. 189 (2000), 339-348. 289. Le Normand F., Hilaire L., Kili K., Krill G. and Maire G., J. Phys. Chem. 92 (1988), 2561-2568. 290. Tessier D., Rakai A. and Bozon-Verduraz F., J. Chem. Soc, Faraday Trans. 88(1992), 741. 291. Salasc S., Perrichon V., Primet M., Chevrier M. and Mouaddib N., J. Catal. 189 (2000), 401-409. 292. Mauger A., Escorne M., Paul-Boncour V., Percheron-Guegan A., Achard J. C. and Barrault J., J. Phys. Chem. 92 (1988), 6004-6009. 293. Shen W. J. and Matsumura Y., Phys. Chem. Chem. Phys. 2 (2000), 1519-1522. 294. Hoang M., Hughes A. E. and Turney T. W., Appl. Surf. Sci. 11 (1993), 55-65. 295. Guillaume C., Ph. D. Thesis, Facultes Universitaries Notre-Dame de la Paix (1995) 296. Eyring L. in HandBook of the Physics and Chemistry of Rare Earths, eds. Gschneider, K. A. Jr. and Eyring L. (North-Holland Publishing Company, 1979), 337-399. 297. Bernal S., Botana F. J., Cifredo G. A., Calvino J. J., Jobacho A. and Rodriguez-Izquierdo J. M., J. Alloys Comp. 180 (1992), 271-279. 298. Bernal S., Calvino J. J., Lopez-Cartes C , Pintado J. M., Perez-Omil J. A., Rodriguez-Izquierdo J. M., Hayek K. and Rupprechter G., Catal. Today 52 (1999), 29-43. 299. Bernal S., Botana F. J., Garcia R. and Rodriguez-Izquierdo J. M., Stud. Surf. Sci. Catal. 48(1989), 123-132. 300. Tauster S. J., Fung S. C. and Garten R. L., J. Am. Chem. Soc. 100 (1978), 170175.
Chemical and nanostructural charaterization ofmetal/ceria systems
\ 65
301. Tauster S. J., Accounts of Chemical Research 20(11) (1987), 389-394. 302. Haller G. L. and Resasco D. E., Advances in Catalysis 36 (1989), 173-235. 303. Kang Z. C. and Eyring L., J. Solid State Chemistry 75 (1988), 52-59. 304. Kang Z. C. and Eyring L., J. Solid State Chemistry 75 (1988), 60-72. 305. Eyring L., J. Alloys Comp. 207/208 (1994), 1-13. 306. Gasgnier M., Albert L., Derouet J., Beaury L., Maestro P., Chopin T. and Caro P., J. Solid State Chemistry 112 (1994), 367-375. 307. Kang Z. C. and Eyring L., Metal. Trans. 22A (1991), 1223. 308. Gasgnier M., Derouet J., Albert L., Beaury L., Caro P. and Deschamps M., J. Solid State Chemistry 107 (1993), 179-185. 309. Gasgnier M., Albert L., Derouet J., Beaury L. and Caro P., C. R. Acad. Sci. Paris 318, Serie II (1994), 915-920. 310. Daturi M., Finocchio E., Binet C , Lavalley J. C , Fally F. and Perrichon V., J. Phys. Chem. B 103 (1999), 4884-4891. 311. Binet C , Daturi M. and Lavalley J. C , Catal. Today 50 (1999), 207-225. 312. Fierro J. L. G., Soria J., Sanz J. and Rojo J. M., J. Solid State Chemistry 66 (1987), 154-162. 313. Rojo J. M., Sanz J., Soria J. and Fierro J. L. G., Zeitschriftfur Physikalische Chemie Neue Folge 152 (1987), 149-158. 314. Bernal S., Calvino J. J., Cifredo G. A., Gatica J. M., Perez-Omil J. A. and Pintado J. M., J. Chem. Soc, Faraday Trans. 89 (1993), 3499-3505. 315. Li C , Sakata Y., Arai T., Domen K., Maruya K. and Onishi T., J. Chem. Soc, Faraday Trans. 85 (1989), 929-943. 316. Li C , Sakata Y., Arai T., Domen K., Maruya K. and Onishi T., J. Chem. Soc, Faraday Trans. 85 (1989), 1451-1461. 317. Bozon-Verduraz F. and Bensalem A., J. Chem. Soc, Faraday Trans. 90 (1994), 653-657. 318. Belzunegui J. P., Sanz J. and Rojo J. M., J. Am. Chem. Soc. 112 (1990), 4066 319. Belzunegui J. P., Rojo J. M. and Sanz J., J. Phys. Chem. 95 (1991), 3464-3466. 320. Sanz J., Belzunegui J. P. and Rojo J. M., J. Am. Chem. Soc. (1992), 6749-6754. 321. Logan A. D., Braunschweig E. J., Datye A. K. and Smith D. J., Langmuir 4 (1988), 827-830. 322. Braunschweig E. J., Logan A. D., Datye A. K. and Smith D. J., J. Catal. 118 (1989), 227-237. 323. Bernal S., Botana F. J., Calvino J. J., Lopez C , Perez-Omil J. A. and Rodriguez-Izquierdo J. M., J. Chem. Soc, Faraday Trans. 92 (1996), 27992809. 324. Meriaudeau P., Dufaux M. andNaccache C.,ACSSymp. Ser. (1986), 118-122. 325. Meriaudeau P., Dutel J. F., Dufaux M. and Naccache C , Stud. Surf. Sci. Catal. 11 (1982), 95-104. 326. Fernandez-Garcia M., Martinez-Arias A., Iglesias-Juez A., Belver C , Hungria A. B., Conesa J. C. and Soria J., J. Catal. 194 (2000), 385-392. 327. Kai's A. A., Bennami A., Ai'ssi C. F., Wrobel G. and Guelton M., J. Chem.
166
Catalysis by ceria and related materials
Soc, Faraday Trans. 88 (1992), 1321328. Overbury S. H., Mullins D. R., Huntley D. R. and Kundakovic Lj„ J. Catal. 186 (1999), 296-309. 329. Galtayries A., Sporken R., Blanchard G. and Caudano R., Journal of Electron Spectroscopy and Related Phenomena 88-91 (1998), 951-956. 330. Jin T., Zhou Y., Mains G. J. and White J. M., J. Phys. Chem. 91 (1987), 59315937. 331. Laachir A., Perrichon V., Badri A., Lamotte J., Catherine E., Lavalley J. C , El Fallah J., Hilaire L., Le Normand F., Quemere E., Sauvion G. N. and Touret O., J. Chem. Soc, Faraday Trans. 87 (1991), 1601-1609. 332. Binet C , Badri A. and Lavalley J. C , J. Phys. Chem. 98 (1994), 6392-6398. 333. Daturi M., Finocchio E., Binet C , Lavalley J. C., Fally F., Perrichon V., Vidal H., Hickey J. N. and Kaspar J., J. Phys. Chem. B 104 (2000), 9186-9194. 334. Binet C., Jadi A. and Lavalley J. C., J. Chimie Physique 88 (1991), 499-508. 335. Arashi H., Naitoh H. and Ishigame M., Solid State Ionics 40/41 (1990), 539543. 336. Li P. and Chen I. W., J. Am. Chem. Soc. 11 (1994), 118-128. 337. Fally F., Perrichon V., Vidal H., Kaspar J., Daturi M., Lavalley J. C , Blanco G., Pintado J. M. and Bernal S., Catal. Today 59 (2000), 387-393. 338. Baker R. T., Bernal S., Blanco G., Cordon A. M., Pintado J. M., RodriguezIzquierdo J. M., Fally F. and Perrichon V., Chem. Commun. (1999), 149-150. 339. Perrichon V., Laachir A., Bergeret G., Frety R., Tournayan L. and Touret O., J. Chem. Soc, Faraday Trans. 90(5) (1994), 773-781. 340. Badri A., Lamotte J., Lavalley J. C , Laachir A., Perrichon V., Touret O., Sauvion G. N. and Quemere E., European Journal of Solid State Inorganic Chemistry 28 (1991), 445-448. 341. Fornasiero P., di Monte R., Rao G. R., Kaspar J., Meriani S., Trovarelli A. and Graziani M., J. Catal. 151 (1995), 168-177. 342. Fornasiero P., Kaspar J. and Graziani M., J. Catal. 167 (1997), 576-580. 343. Bernal S., Blanco G., Cauqui M. A., Cifredo G. A., Pintado J. M. and Rodriguez-Izquierdo J. M., Catal. Lett. 53 (1998), 51-57. 344. Bernal S., Blanco G., Cifredo G. A., Perez-Omil J. A., Pintado J. M. and Rodriguez-Izquierdo J. M., J. Alloys Comp. 250 (1997), 449-454. 345. Golunski S. E., Hatcher H. A., Rajaram R. R. and Truex T. J., Appl. Catal B 5 (1995), 367-376. 346. Hu Z., Kunimori K. and Uchijima T., Appl. Catal. 69 (1991), 253-268. 347. Vidal H., Kaspar J., Pijolat M., Colon G., Bernal S., Cordon A. M., Perrichon V. and Fally F., Appl. Catal. B 30 (2001), 75-85. 348. Vidal H., Kaspar J., Pijolat M., Colon G., Bernal S., Cordon A. M., Perrichon V. and Fally F., Appl. Catal. B 27 (2000), 49-63. 349. Vidal H., Bernal S., Kaspar J., Pijolat M., Perrichon V., Blanco G., Pintado J. M., Baker R. T., Colon G. and Fally F., Catal. Today 54 (1999), 93-100. 350. Vidmar P., Fornasiero P., Kaspar J. and Graziani M., J. Catal. Ill (1997), 160-
Chemical and nanostructural charaterization ofmetal/ceria systems 168. 351. Zamar F., Trovarelli A., de Leitenburg C. and Dolcetti G., Chem. Commun. (1995), 965-966. 352. de Leitenburg C , Trovarelli A., Llorca J., Cavani F. and Bini G., Appl. Catal. i4 139 (1996), 161-173. 353. de Leitenburg C , Trovarelli A., Zamar F., Maschio S., Dolcetti G. and Llorca J., Chem. Commun. (1995), 2181-2182. 354. Fornasiero P., Balducci G., di Monte R., Kaspar J., Sergo V., Gubitosa G., Ferrero A. and Graziani M., J. Catal. 164 (1996), 173-183. 355. Giordano F., Trovarelli A., de Leitenburg C. and Giona M., J. Catal. 193 (2000), 273-282. 356. Fornasiero P., Kaspar J. and Graziani M., Appl. Catal. B 22 (1999), LI 1-L14. 357. Rao G. R., Kaspar J., Meriani S., di Monte R. and Graziani M., Catal. Lett. 24 (1994), 107-112. 358. Fornasiero P., Balducci G., Kaspar J., Meriani S., di Monte R. and Graziani M., Catal. Today 29 (1996), 47-52. 359. Yao H. C. and Yao Y. F. Y., J. Catal. 86 (1984), 254-265. 360. Fornasiero P., Kaspar J. and Graziani M., Appl. Catal. B 22 (1999), LI 1-L14. 361. Zotin F. M. Z., Tournayan L., Varloud J., Perrichon V. and Frety R., Appl. Catal. 98(1993), 99-114. 362. Hickey N., Fornasiero P., Di Monte R., Kaspar J., Graziani M. and Dolcetti G., Catal. Lett. 72(1-2) (2001), 45-50. 363. Hori C. E., Permana H., Ng K. Y. S., Brenner A., More K., Rahmoeller K. M. and Belton D., Appl. Catal. B 16 (1998), 105-117. 364. Miki T., Ogawa T., Haneda M., Kakuta N., Ueno A., Tateishi S., Matsuura S. and Sato M., J. Phys. Chem. 94 (1990), 6464-6467. 365. Loof P., Kasemo B. and Keck K. E., J. Catal. 118 (1989), 339-348. 366. Herz R. K. and Shinouskis E. J., Ind. Eng. Chem. Prod. Res. Dev. 24 (1985), 385-390. 367. Su E. C , Montreuil C. N. and Rothschild W. G., Appl. Catal. 17 (1985), 75-86. 368. Kordis J. and Eyring L., J. Phys. Chem. 72 (1968), 2030-2044. 369. Burnham D. A., Eyring L. and Kordis J., J. Phys. Chem. 72 (1968), 4424-4431. 370. Eyring L., J. Solid State Chemistry 1 (1970), 376-385. 371. Esteva J. M., Karnatak R. C , Dexpert H., Gasgnier M., Caro P. E. and Albert L., J. Physique 41 (Sup. 12) (1986), C8-955-C8-959. 372. Kaspar J., Graziani M. and Fornasiero P., in Handbook on the Physics and Chemistry of Rare Earths, eds. Gschneider,K.A.,Jr.; Eyring,L. (Elsevier Science B.V., Amsterdam, 2000), 159-267. 373. Beck D. D., Capehart T. W., Wong C. and Belton D. N., J. Catal. 144 (1993), 311-324. 374. Logan A. D., Braunschweig E. J., Datye A. K. and Smith D. J., Ultramicroscopy 31 (1989), 132. 375. Wang C. B. and Yeh C. T., J. Catal. 178 (1998), 450-456.
\ 67
168
Catalysis by ceria and related materials
376. Borgna A., Le Normand F., Garetto T., Apesteguia C. R. and Morawcck B., Catal. Lett. 13 (1992), 175-188. 377. Fan L. and Fujimoto K., J. Catal. 150 (1994), 217-220. 378. Pijolat M., Prin M., Soustelle M., Touret O. and Nortier P., J. Chem. Soc, Faraday Trans. 91 (1995), 3941-3948. 379. Perrichon V., Laachir A., Abouamadasse S., Touret O. and Blanchard G., Appl. Catal. A 129 (1995), 69-82. 380. Datye A. K., Hegarty B. F. and Goodman D. W., Faraday Discuss. Chem. Soc. 87 (1989), 216(1-8). 381. Chin A. A. and Bell A. T., J. Phys. Chem. 87 (1983), 3482. 382. Kondarides D. I. and Verykios X. E., J. Catal. 174 (1998), 52-64. 383. Yacaman M. J., Appl. Catal. 13 (1984), 1-25. 384. Heinemann H., Catalysis Review - Sci. Eng., Special Issue on Electron Microscopy 34 (1992). 385. Catal. Today, SpecialIssue-HRTEMfor Catalysts, ed. Ledoux M. J., 23 (1995). 386. Ultramicroscopy, Special Issue-Characterization of Catalysts, ed. Smith D. J., 34 (1990). 387. Baker R. T., Calvino J. J., Lopez-Cartes C , Mira C , Perez-Omil J. A. and Rodriguez-Izquierdo J. M., Inst. Phys. Conf. Ser. 161 (Section 10) (1999), 537540. 388. Matyi R. J., Schwartz L. H. and Butt J. B., Catal. Rev - Sci. Eng. 29 (1987), 4199. 389. Hardeveld R. V. and Hartog F., Surf. Sci. 15 (1969), 189-230. 390. Borodzinski A. and Bonarowska M., Langmuir 13 (1997), 5613-5620. 391. Lopez-Cartes C , Ph.D. Thesis, Universidad de Cadiz (2000). 392. Bronger W., J. Less Common Metals 12 (1967), 63-68. 393. Le Roy J., Moreau J. M. and Paccard D., Acta Crystallografica B34 (1978), 913. 394. Compton V. B. and Matthias B. T., Acta Crystallografica 12 (1959), 651-654. 395. Dwight A. E., Conner R. A. and Downey J. W., Acta Crystallografica 18 (1965), 837-839. 396. Le Roy J., Moreau J. M., Paccard D. and Parthe E., Acta Crystallografica B33 (1977), 2414-2417. 397. Jacob K. T. and Waseda Y., Bull. Mater. Sci. 13 (1990), 235-244.
CHAPTER 5 STUDIES OF CERIA-CONTAINING CATALYSTS USING MAGNETIC RESONANCE AND X-RAY SPECTROSCOPIES
JOSE C. CONESA*, MARCOS FERNANDEZ-GARCIA and ARTURO MARTINEZ-ARIAS Institute de Catdlisis y Petroleoquimica, CSIC Campus de Cantoblanco, 28049 Madrid, Spain 5.1. Introduction Spectroscopies provide powerful tools for probing at molecular levels catalysts and catalytic reactions. Some of them, as e.g. IR spectroscopy, give detailed information especially on reacting species, while others specialize in exploring active sites at a more localized atomic level; in some cases these are quite element-specific, which facilitates to probe those sites selectively. This chapter deals with several spectroscopies of this highly specific kind, which happen to imply the two opposite extremes of the electromagnetic spectrum, viz. radio- and microwaves (magnetic resonances: EPR and NMR) and X-rays (XPS and X-ray absorption spectroscopy, XAS). Besides, these tools have in common, except perhaps in the NMR case, the property of being sensitive to redox states of catalytically important elements, and are thus quite useful in studying ceria-based catalysts since the effectivity of these relies largely on the active redox behavior of Ce0 2 . Here a (non-exhaustive) review is given of the informations that can be (and have been) gained through them about ceria catalysts, with due previous indications of the particularities of each tool as applied to Ce-containing systems. The EPR case, where relevant ceria-specific methods have been developed and used in the authors' laboratory, will be presented in more detail.
5.2. EPR 5.2.1. Ceria-related Spectral Characteristics 5.2.1.1. Signals Due to Ce3+ The action of many Ce-based catalysts rely on the operation of the Ce4+-Ce3+ redox couple, as shown by X-ray or magnetic susceptibility techniques; these latter evidence 169
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the paramagnetic character, at least from room temperature (RT), of Ce + ions formed upon reduction in various conditions [1, 2, 3], One would expect thus that EPR will give useful data on the formation and action of those reduced cations in catalysis. The EPR features of Ce3+ are governed by their 4f* configuration, in which the spin-orbit LS coupling dominates over the chemical environment effects and a 2Fi/2 ground state results. The sixfold multiplicity of the latter is (weakly) split by environments of symmetry lower than cubic in three so-called Kramers doublets; EPR transitions may then appear within the lowest lying doublet, which can be treated as having an effective spin S=V2. The theory predicts (in first approximation) the values of the main EPR parameters g; for each doublet type; thus for a (Jz=± Vi) doublet (the ground state in Cih symmetry) gj= 18/7 and g||=6/7 [4]; for a (Jz=±3/2) doublet, gn=18/7 and gj=0, and for the (Jz=±5/2) case g||=30/7 and gi=0 [5, 6]. Depending on the crystal fields present, doublets which are mixtures of these may occur, leading to intermediate g values and eventually splitting gj_ in two to give an orthorhombic g tensor [5, 6]. Examples approaching those values can be found in the literature [5,6]. Unfortunately, the detection of Ce3+ species by EPR is normally affected by fast spin-lattice relaxations which broaden the EPR lines beyond detection at all but very low temperatures (near liquid He or below) [4]; detection of Ce3+ in all the cases said above required temperatures below 20 K [5, 6]. As far as we know, in systems as those of interest here EPR features ascribable to Ce3+ within these theoretical schemes have been detected at 77 K only in special cases, like some MoOx/Ce02 and VOx/Ce02 catalysts [7] showing a broad signal with g±=2.46 and g p 0.85, thus close to the mentioned C#, case [4, 8], Such detection of Ce3+ at 77 K implies a smaller coupling with the lattice vibrations; this might be due to an effect of nearby Mn+ cations, leading to a relatively long spin-lattice relaxation time. Apart from this, it is worth noting that Cu-containing ceria has ben seen in some cases to display, after reduction or high temperature calcination, very broad signals in the 77 K EPR spectrum [9, 10], which were tentatively ascribed to Ce3+ ions undergoing spin interactions somehow influenced by copper, although a clear theoretical justification of this is still lacking. In the literature, a narrow (quasi-)axial signal around g=1.96 (typically g±= 1.9681.967 and gf= 1.947-1.936) has been observed frequently in EPR spectra of CeCvbased materials [11, 12, 13, 14,15, 16]. There is controversy concerning its assignment; some works [11, followed in ref. 15] ascribed it to Ce3+ located in specific symmetry sites. Although these g values could in principle be compatible, within the mentioned theory, with a mixed (J2=±l/2|±5/2) ground state, other authors [12] prefer, in view of these g values (much closer to the free-electron value ge= 2.0023, and differing from those normally observed for Ce3+) and of the small width and easy detection of the signal at relatively high temperature (up to at least 370 K [17]), to assign it to quasi-free (or
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conduction) electrons or to electrons trapped at particular oxygen vacancies of the ceria lattice; in the latter case the moderate deviation of its g values from g,. would be due to some orbital mixing with empty states of nearby Ce4+ ions. A bottlenecked system where both Ce3+ and conduction electrons participate forming a strongly coupled resonance system has been proposed more recently to explain the characteristics of that signal and solve this issue [17]. In any case, the magnitude of this signal is correlated with the level of impurities in the samples, suggesting that both facts are intimately related; thus in most pure, partially reduced Ce0 2 samples this signal has very low intensity [16] or even is absent [18]. Besides, its intensity does not correlate with the reduction degree of the samples, showing that it cannot be used as measure of the amount of Ce3+ nor for evidencing the presence of Ce3+ in a given sample, as erroneously stated in the literature many times. Also, 0 2 adsorption experiments on reduced Ce0 2 samples, usually performed at 77 K, produce large signals (due to Ce4+-02", see below) whose intensity does not correlate with the signal at gx« 1.968 (if any) present before adsorption [11, 16]; the centers producing this latter interact only weakly with 0 2 , which shows its marginal importance in redox processes occurring on these samples [16]. It can be concluded that most or all of the Ce3+ formed upon reduction of Ce02-based systems is EPR-silent at 77 K. For Ce02-containing samples reduced in vacuum at T>673 K, a narrow (AHPP * 5 G) symmetric signal can as well appear at g«2.003, being assigned to electrons trapped at surface oxygen vacancies or to carbonaceous impurities [16, 14].
5.2.1.2. Signals Due to Adsorbed Species Direct EPR detection of Ce3+ is difficult, but EPR allows the investigation of ceria materials through the use of probe molecules. Advantage is here taken of the high sensitivity and particular structural information afforded by the EPR technique; besides, such experiments are easier to handle (experiments at 77 K normally suffice) and yield surface-selective information. Indeed EPR monitoring of 0 2 adsorption at low temperature on reduced Ce-containing samples, giving structural information on the state of the surface before adsorption, has proven to be very useful in its study [16, 18, 19, 20]; it allows in addition to examine the redox and/or oxygen handling properties of the surface, very important for the performance of practical catalysts [21]. The method is based on the formation of paramagnetic superoxide species (02") upon 0 2 adsorption on reduced cerium centers, following a process that can be summarized in the equation (using formal charges for the sake of simplicity):
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Ce3+-V0 + 0 2 -* Ce4+-02"
(5.1)
Such transfer will occur, in principle, if the energy of the adsorbed anion formed lies below the Fermi level of the solid [22], the electrostatic stabilization of the Mn+-02" complex playing a fundamental role in the energetics of the process [23]. From this point of view, a classification of pure or mixed oxides into two types is proposed [22]. To the first type, including relatively few systems, belong those capable of transferring electrons to 0 2 without any prior reductive treatment; the second one includes semiconducting oxides which must be reduced or otherwise activated before 0 2 adsorption. Ceria-related materials are generally included in the second class, although generation of levels in the gap by doping can produce materials of the first type. Insulating oxides which are themselves unable to transfer electrons to 0 2 are at the limit of the second case; their study by this method requires prior modification of their electronic properties, e.g. by introduction of transition ions in their framework (thus modifying their band structure) or by irradiation preconditioning [22]. It was noted long ago that the g parameters of superoxide (02"-Mn+) complexes are sensitive to the nature of the cation M involved, allowing 02" to be used as probe of oxide surfaces. A simple ionic model [24], which which most 02"-Mn+ species comply, predicts that the deviation from ge of one of the g values (gz in the usual convention for these systems) varies inversely with the cation charge [25]. However, this model also predicts that another of the g values (gx) should be very close to g.. That this is not the case for 02" adsorbed on Ce4+ was remarked since the first reports on these centers [11, 26], and has been noted in many later studies [11, 13, 14, 16-20, 27, 28, 29]. The spectrum feature corresponding to gx can be identified using 170-enriched 0 2 , as it is known that the resulting hyperfine splitting is centered around gx [25]. This deviation of gx is ascribed to a mixing of the 02" orbitals with the 4f orbitals of Ce4+, i.e. to a sizeable covalent (less ionic) character in the cation-02" bond; it may be that the special hybridization of the Ce4+ state in oxidic environment (see section 5.4.1 below) plays an important role in this. The large value of the spin-orbit coupling of the Ce 4f orbital is then responsible for the significant magnitude of this deviation. In this sense, higher gx shifts would indicate a higher covalent character in the Ce-02" bond [13, 29, 30]. Although this needs still theoretical confirmation, a joint analysis of XANES and EPR spectra in Ce02/y-Al203 samples points to the validity of that correlation [30]. Other probe molecules used for the EPR study of Ce02-based systems are NO and nitroxyl radicals [31, 32, 33]; in this case, the neutral molecules themselves are paramagnetic, and give information without electron transfer. In the case of adsorbed NO, gz values are sensitive to the crystal field strength at adsorption sites of oxides in exactly the same way as in the case of 02"; NO has been thus used to study the
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characteristics of coordinatively unsaturated (cus) Ce + sites at ceria surfaces. On the other hand, the use of nitroxyl radicals as probe molecules provides information on the characteristics of Lewis acid-type centers at the surface, on the basis of the changes in the g and A values of the complex produced upon adsorption on those centers [33].
5.2.2. Surface Studies Using Oxygen as Probe Molecule 5.2.2.1. Pure, Unsupported Ceria Samples With the term "pure" we mean here samples with a very low level (presumably below ppm) of the impurities that can give signals in the EPR spectra. Most frequent in ceria are Mn ions, which give sets of six lines [18, 19] (but see Section 5.2.1.1 above about the sharp signal with g« 1.968). The need of severe impurity control is very important in EPR works, given the high sensitivity of this technique; otherwise, interference of impurity signals with the relevant features may difficult the spectrum analysis, and even may lead to errors (for example, signals ascribed to oxygen radicals in some cases [34] may well be due to Cu2+ impurities attending to the spectrum shape). Note that, due to the basicity of ceria [35], the presence of carbonate-type species in as-prepared samples is almost unavoidable; severe thermal treatments are needed if one wants clean sample surfaces. In any case, 0 2 adsorption at low temperature (between 77 and ca. 300 K) on thermally reduced samples produces in the spectra new signals which can be classified in two groups, summarized in Table 5.1: i) OI-type signals, presenting the lowest g value at g « 2.011 and the highest one at g = 2.037-2.031. They show a (quasi)axial spectral shape with g| > gj.. ii) Oil-type signals, presenting the lowest g value at g < 2.008 and the highest one Table 5.1. Parameters (principal values of g and " 0 hyperfine structure tensors) of EPR signals assigned to O2" species and formed upon O2 adsorption on CeC>2 previously submitted to outgassing treatments. Axis assignment is based on experiments using "O-enriched oxygen (see ref. 16 for more details).
signal
g tensor
A tensor (Gauss)
ref
OI type
gr2.032gx=2.011 g,=2.031g2=2.018g3=2.011 gl=2.037g2=2.014g3=2.011 g,=2.031g2=2.016g3=2.011 g,p2.037/2.034 g±=2.011
A=73
Ay,Az<2 Ax=74-76
14 18 18 16 16
Ay,Az<2; Ax=76 Ay,Az<2; Ax=70, 83
18 16 16
Oil type
gl=2.047g2=2.013g3=2.0078
gz=2.038 gy=2.010 gx=2.008 gz=2.052-2.048 gy=2.008 gx=2.006
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2.037 (a)
(b)
3260
x 3
x 1
3280
3300
3320
3340
3360
3380
3400
3420
3440
M a g n e tic Field (G ) Figure 5.1. EPR spectra at 77 K obtained upon 0 2 adsorption at RT on high surface area Ce0 2 previously outgassed at (a) 473 K and (b) 773 K.
at g=2.055-2.038. With an orthorhombic lineshape, they are broader than OI-type. Typical examples are shown in Fig. 5.1: atype OI signal (gj = 2.037, gj_ = 2.011) appears for a sample outgassed at 473 K, while for the sample outgassed at 773 K a more complex spectrum is observed, formed by the overlapping of one type OI signal at gn = 2.034 and gi = 2.011 and two type Oil signals, having gi = 2.038, g2 = 2.010 and g3 = 2.008, and g, = 2.052, g2 = 2.008 and g3 = 2.006, respectively. The assignment of these signals to 02~ radicals was confirmed using 0 2 enriched in the 170 isotope [16]. The data showed that the radical giving signal OI has both O atoms equivalent, at least in the EPR time scale (note that these radicals may be not "frozen" at 77 K; the possibility that motional effects lead to apparent equivalencies in an unsymmetric case must be considered in a thorough analysis [16]), and is thought to lie parallel to the surface. O-atom inequivalency appears for at least some of the OIItype species; this does not necessarily imply an end-on geometry of 02" on the vacancy, as proposed in [18], but can reflect steric effects in a non-smooth surface [36], or changes in the directionality of the Ce orbitals involved in bonding [37]. A detailed study led to an assignment of these signals based on several observations, as the different reduction degree needed for obtaining each signal; the higher ease (or rate) of reoxidation shown by the centers producing Oil-type signals; and the easier desorption of the species producing signals OI, which implies a stronger interaction of the species giving signal Oil with the surface. This indicated that these latter are formed on sites with a higher reduction level, i.e. with some degree of clustering or accumulation of anion vacancies, while OI signals would be formed on isolated vacancies [16, 18]. Experimental proof of such vacancy associations has been
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Table 5.2. Parameters (principal values of g and " 0 hfs tensors) of EPR signals due to 0{ species formed upon 0 2 adsorption on HC1- or NO-modified Ce0 2 . Axis assignment as in Table 5.1 (see ref. 16 for details).
signal
g tensor
A tensor (Gauss)
ref.
(HCl-modified sample) OC11 OC12
g,=2.027 g2=2.020 g3=2.012 gz=2.026gx=2.023gy=2.012
Ax=74
40,41 40,41
(NO-modified sample) ON
gz=2.028gx=2.019gy=2.012
Ax=73
32
given by STM studies of Ce0 2 (111) surfaces [38]; the model is supported by computer modelings predicting also such association for the same Ce02 surface [39].
5.2.2.2. Chemical Modification of Active Sites at the Ce02 Surface Also surface changes induced by adsorption of other molecules can be analyzed in this way; indeed 02" signals (summarized in Table 5.2) different from those described above were observed on ceria previously contacted with HC1 or NO. In the case of Ce0 2 impregnated with HC1, dried and air-calcined, 0 2 adsorption after thermal outgassing produced signals with shifted gx values (OC11 and OC12, Fig. 5.2), which were observed also on Au/Ce02 prepared from gold chloride and could be assigned to 02", rather than to other species as O" or 03~, using 170-enriched 0 2 [40]. The shifted g values mean that CI is affecting the 02"-stabilizing Ce ions. XPS data had shown in that work and others [41] the strong retention of CI in Ce0 2 (Cl 2p3/2 peak at 198 eV) even after thermal treatments, in contrast with other anions as e.g. nitrates, a fact related to the stabilization of Cl by formation of CeOCl-type microphases [42, 43]. These 02" radicals quickly vanish upon RT outgassing, in contrast with those in Cl-free ceria, indicating an easier reversal of reaction (1) above, i.e. a stabilization of Ce3+ by Cl. This implies electron orbital levels of lower energy, and thus fits with the higher gx shift observed, which implies a higher covalent character in the Ce4+-02" bond, i.e. a higher degree of electron density transfer from the radical to the Ce cation [44]. In a similar study on the effects of NO [32], the centers at reduced Ce0 2 surfaces able to form signal Oil with 0 2 were shown to disappear upon NO adsorption at RT, while those forming signal 01 were affected so that this signal changed into a new, orthorhombic one ON (parameters in Table 5.2) which also corresponds to a 02" species as revealed by adsorption of 170-enriched 0 2 . The parameter changes are in the same direction as those produced by Cl; furthermore, the corresponding species was easily
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desorbed by outgassing at RT, as observed for the Cl-modified signals. Thus it is suggested that monoanionic species produced by NO adsorption (N02", N03") and detectable by FTIR spectroscopy [32] replace O2" ions in the environment of the Ce ions, lowering the electron energy levels of the latter and increasing the Ce4+-02~ bond covalency. On the other hand, hyponitrite species {cis and trans- N 2 0 2 '), also detected by FTIR, were the result of the elimination of associated vacancies, and correlated with the generation of N 2 0 at RT upon NO adsorption on outgassed ceria.
5.2.2.3. Supported Ceria Samples The same methods were applied to alumina-dispersed ceria, the key support in classical TWCs. Table 5.3 summarizes the parameters of 02-derived signals obtained on different preactivated supported ceria samples, including in the comparison others reported in the literature for related systems. In some cases 01 or Oil type signals (Table 5.1) are found, indicating the apparition of well formed Ce0 2 particles for low ceria dispersion or high cerium load. However, new signals not found in pure ceria (those found on ceria-alumina being labeled as OCA type, see Fig. 5.3) dominate in all these cases; their assignment to 02" species was again confirmed by experiments with 17 0enriched 0 2 [29]. With the 0C1 and ON type signals described above they have in common large gx shifts, revealing support-induced modifications of the cerium environment which lead to higher covalency in the Ce4+-02" bond; and also an easier elimination by outgassing at RT, interpreted as well as due to higher stability of the Ce3+ state (i.e. higher reducibility of alumina-dispersed ceria, as already reported [45]). Note that this higher lability observed for increasing gx (i.e. for higher covalency in the complex) is in line with the lability observed for 02~-Co3+ complexes with high covalent
signal 0CI1 (a)
signal 0CI2
TBI
3360 33S0 3400 Magnetic Field (G)
Figure 5.2. EPR spectra at 77 K of HCl-modified Figure 5.3. EPR spectra at 77 K obtained upon 0 2 Ce0 2 . (a) After 0 2 adsorption at 77 K on a sample adsorption at 77 K on samples pre-outgassed at outgassed at 473 K. (b) After 0 2 adsorption at RT 773 K. (a) l%Ce0 2 /Al 2 0 3 ; (b) Ce0 2 . on a sample outgassed at 773 K.
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Table 5.3. Parameters of EPR signals assigned to 02" species formed on supported ceria samples. Axis assignment made as in Table 5.1 (see ref. 16 for more details).
Sample
g tensor
A tensor (Gauss)
ref
Ce02/Si02
gz=2.028gx=2.0158gy=2.011
Az<10Ax=75Ay<5
13
Ce-X Zeolite
glf=2.034 g±=2.010 (01 type signal) gl=2.024g2=2.021g3=2.011
A=78 A,=24A2=66A3=12
27
Ce02/Y-Al203: Signal OCAl gx=2.026g,r2.012 Signal OCA2 gz=2.028-2.027 gx=2.022 gy=2.012 Signal OCA3 gz=2.028-2.027 gx=2.017 gy=2.011
29,30 Az<5 Ax=78 Ay<3 Az<4 Ax=73 Ay<3
gz=2.031gx=2.016gy=2.012 |(OItype gr2.037gx=2.011 | signals) gz=2.038 gx=2.008 gy=2.010 (Oil type) character [37]. This lability implies that the same outgassing conditions must be kept when comparing spectra of adsorbed oxygen species on ceria-alumina systems in different situations; the lack of constancy in such conditions might be the cause of some spectral differences observed on Pt,Rh-containing catalysts [46]. Bidimensional ceria patches ("2D-Ce") or even isolated Ce species, with cerium in direct contact with alumina and thus having aluminate-type ligands in place of Ce-bound oxide ions (the latter being more basic and electron-destabilizing since Ce is less electronegative than Al [29]), were proposed to explain the detection of such OCA species; this agrees with works reporting a decrease in ceria basicity upon dispersion on alumina [47]. As seen in Table 5.3, 02" species with shifted g values have been detected also for cerium on other less basic supports. In one case [27] the g values (gi=2.024, g2=2.021) were interpreted, based on the variations of Ai and A2 values, in terms of partial averaging due to rotational motion at 77 K; however, the estimation of those parameters from the spectra given there is difficult [25], and this interpretation remains unproven. The study, completed with results from other techniques, established a model [29, 30,48] with ceria being present as 2-D and 3-D Ce species (Fig. 5.4); the proportion of the latter would increase with increasing Ce load (for a given preparation method), and the Ce sites characteristic of each degree of dispersion would be distinguished and characterized by the g values and linewidths of the 02" species spectra. Note that, due to the higher reducibility of cerium in 2D-Ce in comparison with 3D-Ce, the 02" species generated on the former appear in much larger amounts (per unit area) than on the latter, and thus dominate the spectra of Ce02/Al203 [30].
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species Ce02-2D sites type OCA
species Ce02-3D sites type OI and Oil
-~\
r
T-
>
AI2O3 Figure 5.4. Scheme of alumina-supported ceria species generating different types of 02" radicals.
5.2.2.4. Mixed Oxide Samples High surface area ceria-zirconia (at. ratio Ce:Zr»1.0) prepared by a microemulsion method has been studied in the same way [49], The presence of both Zr4+-02" (features at gz=2.037-2.032, gy=2.0Q9 and gx=2.002) and Ce4+-02" species (similar to those observed on ceria, Table 5.1), Fig. 5.5, showed the presence of both cations at the surface. The large intensity of the Zr4+-02" signal in comparison with that found under similar conditions in Zr0 2 evidenced a true mixed oxide nature of the sample surface, in which 02" species produced by electron transfer from Ce3+ become stabilized on a nearby Zr4+ ion, of smaller radius than Ce4+. This has been supported recently by joint analysis of EPR and FTIR spectra (with CH3OH as probe) [50]. Similar results were found on Al203-supported (Ce,Zr)02 samples made via microemulsions, evidencing that the dispersed phase had also homogeneous mixed oxide character [51]. It is noteworthy that coprecipitated Ce-doped Zr0 2 samples (with and without additional Y doping), after outgassing at 720 K and 0 2 adsorption, displayed signals formed mainly by 02~~Zr4+ species, with thermal stability depending on the level of Y-doping [52]; the capacity of Ce to facilitate surface reduction near Zr sites is thus high even for low Ce:Zr ratios. On the other hand, only Ce4+-Q2" species appeared in a Ce-Zr (1:1 at ratio) mixed oxide subjected to redox cycling at 1273 K [53]; this fact, and the FTIR analysis of species formed upon CH3OH adsorption, led to propose that severe redox treatments modify the surfaces of these samples, producing, apart from some Ce enrichment, patches or islands with structures similar to those of pure Zr0 2 and Ce0 2 . In the same context, studies using this technique performed on microemulsionprepared Ce-Ca mixed oxides allowed to conclude that significant enrichment in Ca is produced at the surface of the samples by increasing their calcium content (up to at. ratio Ca/Ce =1/1), affecting largely their oxygen handling property [54]. Indeed almost exclusively Ca2+-02" signals, and no Ce4+-02", appeared for a Ce-Ca mixed oxide with
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at. ratio Ca/Ce=l/1, while a sample with at. ratio Ca/Ce=l/9 displayed a mixture of OIand Oil-type signals plus a small amount of Ca2+-02" species, Fig. 5.6.
5.2.2.5. Supported Metal Catalysts Application of the same technique in a series of works on ceria-based metal catalysts has verified the role of anion vacancies at the support surface in reactions of interest for TWCs [45, 55, 56, 57, 58, 59, 60, 61, 62], the comparison of the type and amount of 02" species formed in metal-free and metal-containing systems (or in bimetallic and reference monometallic systems) being fruitful in this respect [55-62]. Thus, 02" species with somewhat shifted g values (gz=2.030-2.029, gx=2.017-6, gy=2.011) and broader than usual were formed by 0 2 at 77 K on Pd/Ce02 after CO treatment at RT; no signal appeared on the Pd-free support in the same conditions, suggesting that those radicals formed at Pd/ceria interfaces [57]. This is supported by their strong decrease upon warming from 77 K to RT, attributed to Pd-favored further evolution of the radical to complete the oxidation of vacancies; other species observed, with features and behavior similar to those found for pure ceria, were then ascribed to sites far from the metalsupport interface [57]. Those interface species appear also on Pd catalysts using as supports Ce0 2 /Al 2 0 3 or (Ce,Zr)02 [57, 60, 61], Thus CO reduces also here easily sites at the metal/ceria (or rnetal/ceria-zirconia) contact, this process constituting probably the initial step in CO oxidation reactions; results in the same line have been obtained for copper/ceria catalysts [56, 59]. Similar studies have shown also the oxidative interaction of NO with oxygen vacancies at these same interfaces, giving support to ideas on the beneficial role of PdCu alloys or of contacts between Pd and two-dimensional (Ce,Zr) oxide entities for NO reduction reactions [58,61,62]. Time-resolved EPR experiments have been used to monitor the evolution of 02"
Figure 5.5. EPR spectrum at 77 K after 0 2 adsorption at 298 K on a ZrCe0 4 sample preoutgassed at 773 K.
Figure 5.6. EPR spectra at 77 K after 0 2 adsorption at 298 K on Ce-Ca mixed oxides with C e / C a a t o m j c r a t i o s 0f; (a) 9/1; and (b) 1/1.
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radicals in oxidation reactions over TWC-type systems [28, 63, 64]. Thus, in studies on CO oxidation 02"-CO reaction rates were measured [63, 64]; a significant increase in them was observed in presence of precious metals, suggesting that 02" formed in sites at (or close to) ceria-metal interfaces are active for the reaction. More recent results suggested however that O2* species at that interface, rather than 02~, are the main active agents for CO oxidation on a well oxidized system [45], implying that the 02~-CO reaction rate over Pt/Ce02/Al203 may depend strongly on the ceria reduction degree. Similar proposals have been done for CH4 oxidation over Ce0 2 /Al 2 0 3 [28].
5.2.3. Surface Analysis Using Neutral Radicals as Probe Molecules NO radicals adsorbed at 77 K on outgassed Ce0 2 give a typical spectrum [31-33] of quasiaxial shape, with a 3-peak set due to hyperfine coupling of the unpaired electron with nitrogen (1=1) and parameters (from computer simulation) gx= 1.993, gy= 1.996, gz=1.900, AX(I4N)=33 G and unresolved Az and Ay (with estimated upper limits of 5 and 20 G, respectively). Increasing the vacuum pretreatment temperature (Tv) led to a higher amount of NO radicals formed upon adsorption at 77 K, indicating formation of increasing amounts of coordinatively unsaturated (cus) Ce4+ [32]. A general decrease in the gz (or g||) value of these radicals was observed as well, being interpreted as due the higher average reduction degree of the ceria surface [32]. This agrees with the decrease subsequently observed in gz upon increasing the amount of NO adsorbed at RT, since as mentioned before such adsorption leads to surface reoxidation [32,65]. NO adsorption has been used also to study Ce0 2 /Si0 2 catalysts [33]. On samples outgassed at 720 K two different adsorbed NO species appeared, with gz values of 1.9216 and 1.9528 (A(14NO) = 31.9 G for both), and were ascribed to the presence of two types of cus Ce4+ with different electron acceptor properties; this correlated with the observation of two different Ce3+ species in fluorescence spectra of reduced samples. The gz values observed in that study, higher than those measured on Ce0 2 , were then attributed to the lower coordination of the Ce4+ cations on the silica. Results in the same line were found by the same group using the nitroxyl radical TEMPO (2,2,6,6-tetramethylpiperidine-l-oxyl) as probe molecule [33]. The species adsorbed on Ce0 2 /Si0 2 showed gy and A| values significantly different from those of the species adsorbed on pure Si0 2 or (to a lower extent) from that found on pure Ce0 2 ; this was attributed to the stronger electron acceptor properties (higher Lewis acidity) of the cus Ce4+ cations present at the silica-supported sample. Determination of the maximum amount of TEMPO molecules adsorbed on those sites and experiments of pyridine-TEMPO coadsorption allowed to estimate the surface concentration of cus
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Ce + cations; the latter measurements revealed also the presence of two types of cus Ce4+, in agreement with the results obtained with NO adsorption [33].
5.2,4. Metal-related Signals in Supported Catalysts: Cation Dimers and Redox Studies Information on the redox state and coordination of metal ions in ceria catalysts can be obtained by ESR as in many other systems. It is peculiar of Ce0 2 , however, that it seems to facilitate especially the formation of transition metal ion pairs in its lattice [56, 66, 67, 10, 68], EPR being the technique able to detect them. A typical example is that of Cu2+ dimers found by different groups in Cu/Ce02 samples [56, 66-68], their spectrum being best resolved after high temperature calcination (Fig. 5.7). It shows signals (both with axial symmetry character) due to Cu2+ monomers, recognizable by the 4-line hyperfine splitting due to the magnetic Cu nuclei (features at g1=2.029, Ai=28 G and g||=2.237, Ay=170 G), overlapping other signals due to Cu2+ dimers, the complex structure of which has been explained [68] on the basis of the established theoretical description of exchange- and dipole-coupled spin pairs [69, 70]. These dimer signals are characterized by axial g values (gm = 2.207, gi = 2.040) and a main twofold fine structure splitting (with a value of the anisotropic spin-spin coupling D=0.066 cm"1, computed from the da and d± peak separations) combined with 7-fold hyperfine splittings (Aj = 84 G and A± = 15 G) which have a 1:2:3:4:3:2:1 intensity pattern (due to interaction with two equivalent ions); together with these features, weaker ones appear at half field field values which arise from Ams=± 2 transitions, allowed in second order approximation. With application of the mentioned theoretical treatment of spin-spin coupling an interion distance of 3.4 A has been estimated for these dimer species [68], which can be compared with the 3.82 A distance between nearest Ce ions in ceria. The intensity ratio between single ion and pair species in coprecipitated (Cu,Ce)Ox strongly increased with Cu/Ce ratio [71]; the amount of pairs was maximum for ratios»0.6, and agglomerated species dominated for higher Cu contents. Such pairs were detected as well for Cu2+ in TI1O2, which has also fluorite structure [72]. Similar dimers arising from, respectively, Mo5+ and V4+ were observed also for Mo/Ce02 and V/Ce02 materials; in both cases the computed interion distances are similar to those found for the Cu2+ dimers [68]. On the other hand, pairs of Rh ions in a mixed valence state (with total spin S=1/2), probably corresponding to Rh2+-Rh3+ configurations, have been detected for Rh/Ce02 [73], presenting a 3-fold hyperfine splitting in a 1:2:1 pattern (evidencing the equivalency of the Rh ions, having 1= Vi) and parameters g± = 2.297, gB = 1.980, A x = 11 G and Affl = 16 G. All these data show the particular ability of Ce0 2 for stabilizing dimeric structures in its lattice. For calcined
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Catalysis by ceria and related systems
9=4052 H = 1631 G
r
d„=1240G 1
UM g ±= 2.029 -4i=28G Figure 5.7. EPR spectrum at 77 K of a CuOx/Ce02 calcined in dry air at 1073 K, showing well-resolved features of a Cu2+ dimer species (from ref. [68])
NiOx/Ce02 catalysts no cation pairs were detected [74]; only well-resolved signals of Ni3+ ions in the bulk (g||=2.018, g±=2.358) and broad signals (g=2.47-2.20, Tdependent; AHPP>1000 Gauss) due to antiferromagnetic NiO domains were found. EPR has allowed to study in several cases the evolution of ceria-based systems and of active species on them during surface and catalytic reactions [55, 56, 59, 75, 76, 9, 77, 78]. A typical example of these are studies on the mentioned CuOx/Ce02 system, highly active in CO oxidation [56, 59]. Thus, for impregnated catalysts of this type calcined in air at 773 K the EPR spectra showed various Cu2+ species differing in their degree of dispersion, i.e. isolated species (with varying coordinations), dimeric species and CuO-type clusters; well formed CuO particles also exist, but are EPR silent due to antiferromagnetic interactions in this phase. Correlation between the redox properties of these species under CO/0 2 and the catalytic activity for CO oxidation allowed to identify CuO-type clusters as most active in this reaction, in view of the easy reductionoxidation observed for the sites located at the interface between such species and the ceria support; in turn, joint analysis of EPR and in szfa-DRIFTS data identified redox processes occurring upon interaction of the catalysts with CO/O2, from which a reaction mechanism was proposed [59]. The CuOx:Ce02 system showed also a peculiar response to reduction: in a coprecipitated sample with at. ratio Cu/Ce=0.5 calcined at 673 K, H2 treatments at 373 K and above led to progressive disappearance of the Cu2+ signals, but in addition a very broad line (AHPP»1000 G, g=2.41-2.25 depending on reduction temperature) with large integrated intensity appeared starting for reduction temperatures T»400 K [9]. Such line, tentatively assigned to Ce3+ interacting with Cu, was reported to appear also in impregnated CuOx/ceria after calcination at 1173 K [10]. On the other hand, those same coprecipitated samples showed the Cu2+ dimers to be more resistant to
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reduction by SH2 than isolated or clustered Cu [77]. In another study, Cu-doped cerialanthana solid solutions were studied before and after work in CO+NO reaction [79]. Reduction of Cu + was verified by EPR, with changes indicating coordination modification in some cases, but the most striking result was the observation at RT of a very broad, asymmetric spectrum (extrema located between 1000 and 2000 Gauss) after prolonged exposure to reaction; this was ascribed to the formation of "spin bags" of interacting O" species in the solid, generating strong local fields which shifted the EPR lines [80].
5.3. NMR NMR studies devoted to ceria-containing systems are scarce, since there is no Ce isotope with magnetic nucleus and the only suitable oxygen isotope, 1 7 0, has very low natural abondance. Therefore, except for a few 170-based results the existing studies concentrate either on other metal components incorporated into the catalyst or on adsorbed species containing NMR-active nuclei.
5.3.7. NMR-active Species in the Solid Catalyst 17
0-based studies in Ce02 have dealt mainly with ceramics for non-catalytic uses (e.g. ion conductors) [81, 82]. Recently the defect chemistry of a (La,Ce) oxide was studied with 139La and 17 0 MAS NMR measurements (the latter being viable without isotopic enrichment when the sample had high crystalline perfection) [83]. 17 0 signals at 875 and 375 ppm were found after calcination at 1273 K; the first one appeared in Ce02 while the second one increased with the amount of La and was ascribed to the anions around La3+ and/or Ce3+. The 139La spectra indicated dopantvacancy association (giving complex anisotropic spectra) and dissociation (single line spectra) for calcination at low and high temperature respectively. Possible effects of such associations on the material surface chemistry were indicated. 119 Sn MAS NMR data were taken for a SnCe0 4 sample obtained by oxidation of a SnCe0 35 pyrochlore and showing good properties of oxygen uptake and release at low temperatures, similar to those of (Ce,Zr)0 2 used in TWCs [84]. The data revealed that upon full oxidation 30% of Sn went into 7-fold coordination (giving rise to a -872 ppm line besides the -670 ppm line ascribed to 6-coordinated Sn), confirming neutron diffraction data indicating occupation of interstitial sites by
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Catalysis by ceria and related systems
O2" in similar way as reported for an oxidized pyrochlore-type CeZr03.5+x sample [85]. 51 V MAS NMR data were obtained on VO x /Ce0 2 samples of potential interest for NO SCR [86]. Even with low V contents (1 at%) two V5+ signals appeared. A sharp one, due to CeV0 4 (detectable at higher V loads in XRD data), had a -427 ppm shift; another anisotropic one with -565 ppm shift was ascribed to polymeric (V-O-V) entities and vanished for higher V load while a new signal at -609 ppm due to V2O5 appeared. The shift of the two first signals changed with temperature, indicating interaction with paramagnetic Ce3+ even in the highly dispersed species. 27 Al MAS NMR data complemented XRD and IR studies of the stabilization of Y-A1 2 0 3 by La or Ce in sol-gel preparations [87]. The detection of tetrahedral Al in the spectra was correlated with the persistence of the Y-AI2O3 phase; La was proven to be a better stabilizer than Ce, which segregated to give a separated Ce0 2 phase after calcinations at 1100 °C. Unfortunately, no 139La data were given. 27A1 spectra were obtained also for fresh and aged TWCs with and without Ce [88]. For fresh catalysts the presence of Ce led to the observation of a signal with a chemical shift halfway between 4- and 6-fold coordination values, which was ascribed to interaction with Ce3+ in an unspecified mixed phase; after air aging at 900 °C the signal vanished, implying breakdown of that phase upon sintering.
5.3.2. Adsorbed Species NMR data about hydrogen species on ceria supports and Rh/Ce0 2 catalysts have been reported by Sanz et al. The decrease of protonic species during decomposition of ceria precursor gels was followed [89]; the increase in the spectrum second moment for outgassing above 300 °C indicated the increasing presence of paramagnetic species interacting with the proton, i.e. Ce3+ centers. Reduction with H2 (in a closed cell) of a pre-outgassed sample led to growth of the 'H NMR signal, which increased in linewidth and shifted its position especially for reduction at 673 K and above, although the second moment (indicating formation of paramagnetic sites) increased already at lower temperatures [15, 19]. This was interpreted, together with gravimetric, IR and EPR data, as indicating formation of a bronze-like phase containing both OH" (surface and bulk) and hydride groups. A Rh/Ce0 2 catalyst prepared from a chloride precursor was also studied [90]. After H2 adsorption a 'H NMR line appeared, upfield shifted from the main signal due to ceria-held species. It was ascribed to H atoms adsorbed on Rh° particles, the shift arising from Fermi contact with the metal conduction electrons; thus specific
Studies by magnetic resonance and x-ray spectroscopies
Ih/CdVO)
185
U/CtO,(N)
TV-773K
Ti-STJK
Tr-3BK
300 400 500 600 700 800
T reduction (K)
300 400 500 600 700
T reduction (K)
Figure 5.8. NMR data of H2 adsorbed on Rh/CeC>2 catalysts. Left: spectra with H2 adsorbed at RT for ex-chloride and ex-nitrate samples after reduction at increasing temperatures. Middle: plot of the intensity of the shifted line B in the first and second reduction run (filled and open symbols, resp. Circles: ex-Cl sample; diamonds: ex-N03). Right: ibid, for the shift of line B (from ref. [93]).
monitoring of metal-adsorbed H species was allowed by this technique. This lateral line vanished upon reduction in H2 at or above 573 K, and was recovered on a sample reoxidized and newly reduced at low temperature; the SMSI effect could be thus directly followed. The lateral line shift (reflecting an average of two adsorbed forms, strong and weak) decreased also with increasing reduction temperature [91]; This was ascribed to electronic interaction with the reduced support and/or with hydride species located at the interface of the latter with the metal. A similar effect was observed with Rh/Ce0 2 prepared from Rh nitrate and pure ceria [92], showing thus that the SMSI behavior was not due to a chloride effect. A more detailed comparison of ex-chloride and ex-nitrate samples reported recently [93] shows that with the chloride precursor the reduction of Rh is delayed and gives lower metal particle size (which produces higher initial NMR line shifts), but otherwise once the metal is generated the SMSI-type behavior proceeds similarly (except that it is more marked for the ex-nitrate sample), even after redox cycling (Fig. 5.8). 13 C MAS NMR allowed to follow the interaction of hexane, marked at one methyl, with supported Pt catalysts [94]. For Pt/Ce0 2 a large signal, broadened and shifted to lower 5 values, was found at low temperatures, indicating interaction with (paramagnetic) surface Ce3+ centers; the same effect, more modest but still visible, was observed for Pt/Ce0 2 /Al 2 0 3 . By heating the sealed ampoule in the spectrometer above 623 K the hydrocarbon conversion was followed in situ; 18 products were identified. Pt appeared more active supported on Ce0 2 /Al 2 0 3 than on the single oxides, and selectivities also differed: ceria led to lower isomerisation selectivities, and was supposed to act as hydrogen atom buffer in the reaction process.
Catalysis by ceria and related systems
186
5.4. XPS 5.4.1. Ceria-specific Spectral Features For cerium, XPS studies normally use the excitation of the 3d level. Complex multipeaked spectra are obtained (Fig. 5.9); the peaks are labeled following the de facto standard nomenclature given by Burroughs [95], who was the first to describe this spectrum in detail although its interpretation was only partially correct. The more complex shape of the Ce4+ component, displayed also by other compounds with Ce in (formally) the Ce4+ state [95, 96, 97], is usually explained as due to a multipleconfiguration character of the initial ground state; i.e. the Ce ion should not be considered as a pure Ce4+ state, with configuration describable as 4f°Ln (L" designating the fully occupied valence band, formed in this case basically by p orbitals of the ligand oxygen atoms), but as a mixed Ce4+-Ce3+ state, i.e. mixed with a 4f'Ln"' configuration due to the very similar energies of the 4f and orbital ligand valence levels [98, 99, 100, 101]. Some authors [96, 102; see also an ensuing discussion in ref. 103], based of the shape of the 5s XPS spectrum and the pressure dependence of XANES spectra, have claimed that a mixed configuration is not needed to explain these data, which would be well understood assuming a single configuration with strong derealization of the involved valence band orbitals into
910 M O . a MS.at E„/eV
900
M0
M0
Binding Energy («V)
Figure 5.9. XPS spectra of the Ce 3d level in ceria. Left: spectrum of a partially reduced ceria, with fitted individual components (from ref. 2). Right: spectra for pure Ce4+ and Ce3+ (from ref. 194).
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the Ce 4f states (i.e. covalency). The same picture had been proposed in earlier electronic band structure calculations [104], being supported also by works based on brehmsstrahlung isochromat spectroscopy [105] which led to exclude a mixed valence scheme. On the basis of the rather good simulation of the experimental XPS features achieved within the impurity Anderson model (IAM) [106, 107] the mixed valence model is normally preferred (in the following, however, the state of Ce in Ce0 2 will continue to be designated Ce4+). All discussions and calculations do agree that an important amount of electron density, estimated to amount to ca. 0.6 electrons, is delocalized from the ligand atoms over the Ce 4f orbitals. It is worth noting that this more complex character of the spectra is a characteristic common to the M 4 T oxidized state in all the lanthanides which present it (Ce, Pr and Tb). Whichever may be the exact theoretical picture, there is agreement that the Ce02 spectrum shape is due to the convolution of the expected spin-orbit splitting with a three component splitting, the latter appearing because after ejection of the 3d photoelectron the Ce ions may be left in three different electronic states. Thus the v -u spin-orbit doublet results from the Ce ion being left in a [*]4f°Ln configuration (here [*] represents the hole left in the core level), and doublets v -u and v-u from final states which are different mixings of the [*]4f1Ln-1 and [*]4f2Ln_2 configurations [100]. On the other hand, the ground state of the (formally) Ce3+ species present in reduced materials has not hybridized character, but is described as a single 4f'Ln configuration. Here two final configurations appear, both mixtures of final states [*]4f'Ln and [*]4f2Lnl, which give rise to two spin-orbit doublets (v0-u0 and v -u). This spectrum shape is common to all lanthanide Ln + species.
5.4.2. Quantitation of Redox States In ceria-based catalysts, one main aspect studied with XPS is the cerium redox state (i.e. the proportion of Ce4+ and Ce3+) for different preparations or treatments. Peak u , located at EB«916.5 eV, which appears to be typical of Ce4+ and absent for Ce3+, and consequently decreases upon reduction, has been used frequently with this aim. The percentage %(u ) of the total integrated spectrum intensity (after baseline subtraction) lying within this peak in pure, nonreduced Ce0 2 has been computed to amount to 13.7% [108]. The %(u ) value of an analyzed material is frequently used in the literature to quantify its redox state, assuming e.g. that the the quotient betwen it and the value for Ce0 2 gives the fraction of Ce4+ in the material. It has been however pointed out in early works [109], that this value is not proportional to the amount of Ce4+ present; specifically Romeo et al. [110] claimed that peaks v
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Catalysis by ceria and related systems
and u (characteristic of Ce3+) increase in reduction processes long before the u peak decreases. Accordingly, %(u ) values should be used only for comparative or semi-quantitative purposes, although this limitation has been frequently neglected (but see below comments arising from the results of other quantitation methods). Another method proposed is the decomposition of the spectrum in individual peaks via least-squares fitting [2, 110]. The intensity ratio between (u°-v° + u - v ) and total spectrum would give then the fraction of Ce3+. Note that the situation may be a bit more complicated than that: a careful analysis of this kind [111], using spectra with rather good resolution (thanks to the use of a flood gun to minimize broadening by charging effects), concluded that the peaks in the (u-v) doublet of Ce4+ are asymmetric (this could arise from the atomic multiplet structure in view of recent theoretical simulations [106]; such asimmetry, however, has been questioned [117]), and that two small satellites at EB«895 and 913.5 eV, interpreted as "shakeup" peaks, are additionally present in the Ce3+ spectrum. With such complex spectra (ten peaks must be taken into account, or more if the mentioned satellites and asymmetries are considered), the fitting process may be slow and require using constraints, e.g. fixing peak positions or spin-orbit splittings and peak ratios. A third possibility lies in the use of statistical methods, for example factor analysis (FA) [112, 113, 114]. Here a set of spectra are handled with matrix algebra and statistical criteria to infer how many independent components (i.e. individual chemical species) contribute to the data; additional criteria may allow to get the actual component shapes. This has been applied to Ce0 2 and related materials subjected to reduction [111, 115, 116, 117]. In these works it was found that in each set only two components of fixed shape, formally arising from Ce3+ and Ce4+ (Fig. 5.10), suffice to reproduce by linear combinations all the spectra; other components reflecting intermediate redox states or shape modifications during the treatments did not occur. It was noted however that good separation of Ce3+ and Ce4+ components is sometimes not well achieved with this method [117]. This procedure can be very powerful to improve the accuracy of the cerium reduction degree quantitation in Cebased systems; however, it cannot be applied when the number of independent spectra is very small, and needs that the shapes of the individual components do not change within the spectrum set. This may be difficult in some cases, as it has been found that sample charging during data acquisition may produce spectrum broadening to different degrees depending on the reduction level (which influences the electrical conductivity of the solid) [118]; this problem is minimized using a flood electron gun to counteract charging [111] or when the sample is in form of a thin film on a conducting substrate. A different problem may appear if an important modification in the Ce ions environment, e.g. a change in the electronic energy
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Studies by magnetic resonance and x-ray spectroscopies
levels or the polarizability of the ligand oxygens (as could happen by substitution of oxide by sulphate, aluminate, etc.), changes significantly the the position of the peaks or the degree of hybridization of the Ce 4f orbitals (and consequently the relative intensity and separation of the spectrum components). In such case, of course, even the %u " value could be misleading, and only the method based in a decomposition in single peaks would be valid. Detailed studies on this issue do not seem to have been conducted to date in ceria catalytic materials. If only two components of fixed shape exist, corresponding to the two redox states (as indicated by the FA studies), then the analysis can be simplified as done by Appel et a. [119]: the Ce4+ component (obtained from a pure Ce0 2 spectrum) can be subtracted from a given spectrum in amounts appropriate to cancel the u peak; the Ce3+ fraction is thus obtained, which allows to quantify both. It should be noted that this conclusion of the FA contradicts the above mentioned claims of non-linear relationship between the u peak area and total reduction degree. One may note here that the FA results [111, 117] reproduce the small shake-up satellites described above as part of the Ce3+ component; the latter of these overlaps the u peak of Ce4+, and if not taken into account may contribute to the non-proportionality between %(u ") and the amount of Ce4+. The presence of a small component near the u peak position is visible in other spectra of nominally pure trivalent Ce [120]. Also, earlier works [121] suggest a nearly linear dependence between the v peak intensity and the reduction degree which is not a pure proportionality, as it includes some (u - v ) peaks intensity in the pure Ce3+ state (Fig. 5.11); this might correspond to the mentioned satellite located near the u peak. In summary, there are still issues requiring further work if a well-stablished quantification method is desired. In any case, the accuracy of any method is affected -i
1
1
1
1
1
'
r
•
'
20 40 «0 BO 100 % < * 0 , IN THE COMPOSITE SPECTRA 0
50
100
1SO
200
4+
2SO
Figure 5.10. XPS shape of Ce and Ce3+ components obtained with Factor Analysis from a series of Ce02.x spectra (ref. 117)
Figure 5.11. Correlation between fraction of XPS intensity within the u line and fraction of Ce4+ in the spectrum of samples with partially reduced ceria (ref. 156).
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Catalysis by ceria and related systems
by the difficulty in obtaining a good baseline in such complicated spectra; note, for example, the imperfect reproduction of the spectrum in the region around 893 eV observed in Fig. 5.9. It must be recalled also that, as reported initially by Paparazzo [122], Ce0 2 can suffer spontaneous reduction at RT during XPS measurements by action of the X-ray irradiation combined with the UHV environment. Even initially observed Ce3+ amounts may correspond to in-spectrometer reduction as shown by Laachir et al. [2], who found in the XPS spectra of calcined ceria Ce3+ amounts higher than compatible with the magnetic susceptibility data and showed that C0 2 adsorption leads to the absence of those Ce3+ species. This reduction is strongly influenced by the composition and type of material: thus ceria samples with low crystallinity are reduced by X-rays more easily than well-crystallized ones [123]. Possible mechanisms of oxygen loss during XPS experiments (a photon-stimulated desorption) as well as charging effects in this system have been examined in detail [124]. It must be noted that reduction under the XPS experiment conditions can occur also for ceria-supported metals [125, 126].
5.4.3. Studies ofCe Redox Behavior in Catalytic Oxide Materials 5.4.3.1. Studies on Unsupported Ceria Powders The reduction of Ce0 2 has been studied in a number of works within a catalytic perspective. Thus Le Normand et al. [106] reduced high surface area Ce02 with H2 in a spectrometer-attached chamber; Ce3+ was clearly detected only at or above 973 K, but full reduction was not attained. The same authors gave later quantitative data on the reduction degree based on multi-peak fitting of the spectra [2]. Both highand low surface area ceria reached intermediate reduction plateaus at ca. 723 K; for the former material the amount of Ce3+ found corresponded to a (surface) stoichiometry of CeOi.85, equal to that measured by magnetic susceptibility at similar intermediate plateaus, implying that no stoichiometry gradient existed in this material after such reduction treatments. Surface conditions influenced strongly the ease of reduction: a strongly sintered sample always presented in the XPS spectra, for any treatment temperature, lower reduction degrees than the high surface area material, which showed some Ce3+ formation already upon H2 treatments at 573 K. Ce3+ species formed upon reduction of Ce0 2 are known to be reoxidized quickly by exposure to even small 0 2 pressures [127]; this easy Ce3+ oxidation was observed even for the (highly amorphous) hydroxides which appear in the first stages of preparation via precipitation [128]. It is thus striking that nanocrystalline
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191
ceria formed by controlled oxidation of nanocrystalline Ce metal (obtained by magnetron sputtering methods) showed in XPS a partially reduced state, higher than explainable by in-spectrometer reduction, even after exposure to air [129]; those samples were fully oxidized only at elevated temperatures. Effects of chemical treatments can appear also on the Ols peak(s) in the XPS spectrum. For the O2" ions in the bulk, the position has been reported to be lower (by ca. 0.7 eV) in Ce 2 0 3 than in Ce0 2 [130], but in partially reduced samples the changes are small and are frequently not analyzed. More relevant is the relatively frequent observation of a peak with significant amplitude appearing upon reduction (and also in other conditions) with binding energy ca. 2 eV higher than that of the main O2" peak (this difference has been observed in some case to slowly vary with time [131]; this was ascribed to charging effects, assuming that the additional peak was due to a surface species). There is debate concerning the exact origin of such peak in each case; it has been ascribed, depending on the case, to species of type peroxide/superoxide [132], carbonate [133, 134 120], hydroxide [135, 110, 128] or O2" ions influenced by accumulated nearby anion vacancies [116]. Carbonate species are sometimes confirmed observing the corresponding contribution in the C Is region [134] or by the coincidence in Ols binding energy with that of a pure carbonate reference [133]; OH groups are not easy to exclude, especially if H2 is used for reduction. In another case in which reduction was achieved via sputtering, and furthermore the corresponding species were shown (using angle-resolved measurements) to occur in subsurface regions and to disappear by reoxidation with dry 0 2 , only the assignment to vacancy-influenced oxide ions seemed sensible [116]. It is noteworthy that in the latter case the data showed that upon reoxidation the additional oxygen species in the bulk decreased to higher extent than those at the surface, evidencing a preference of those reduced centers for surface positions.
5.4.3.2. Ceria in Supported and Mixed Oxide Forms Ce0 2 /Al 2 0 3 supports were studied in detail by Shyu et al. [108], who examined specimens with different ceria loadings prepared by impregnating Al203 with Ce3+ nitrate and calcination at 1073 K. From XPS-measured Ce/Al ratios they concluded that transition from highly dispersed ceria species to bulk phases occurred for loads of ca. 2.7 |amol/nm2. The percentage of Ce4+ (evaluated with the u peak) increased with the Ce load, suggesting that the dispersed phase, described by them as a CeA103 precursor (this perovskite phase was detected by XRD after H2 reduction at 1273 K), contained Ce as Ce3+, in amounts up to 90% for the lower load samples.
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Catalysis by ceria and related systems
However, TPR data in the same work displayed reduction peaks even for these latter samples; it might be that the highly dispersed phase contained initially Ce4+ (as proposed elsewhere on the basis of XANES data [30], see Sect. 5.5.3.1 below) and suffered spontaneous reduction in the spectrometer. Note that easy photoreduction of highly dispersed or ill-crystallized alumina-supported ceria by Xrays in the XP spectrometer has been claimed in the literature [123]. In any case, the dispersed phase was reduced completely by H2 around 773 K, while for higher Ce loads important amounts of Ce4+ remained after reduction at 1173 K; however, such influence of Ce load on reducibihty is not always observed [136]. The highly reduced state (supported CeA103?) is reoxidized partially by calcination at 873 K, more completely at 1200 K [108]. An increase of the amount of XPS-detected Ce3+ with decreasing Ce load in alumina-supported Ce02 has been found also in samples containing also supported barium oxide [137], which supposedly leads to the formation of a surface Ba aluminate which hinders the formation of Ce aluminate. A higher reducibihty of ceria when combined in mixed oxides has been reported in a number of cases. Already a 20% of Ti leads to an increase in reduction (compared with pure Ce0 2 ) under the X-ray beam [138]; however, these samples were amorphous, which may have influenced this behavior. A well-crystallized material with a 1:1 Ce:Ti ratio and only Ce0 2 peaks in the XRD pattern (but with a highly Ce-enriched surface: the presence of part of Ti in amorphous form could not be excluded) did suffer an important photo-reduction under the X-ray beam [139]; other specimens (crystalline CeTi0 3 , amorphous solids and unidentified crystallized phases) studied in the same work displayed complex reducibihty patterns. In a related work Leinen et al. [140] observed, starting from Ce02-coated Ti0 2 powders, that after sputtering with either Ar+ or 0 2 + the resulting highly reduced Ce3+ state, presumably present as an intimately mixed amorphous (Ti,Ce) oxide, had much higher resistance to reoxidation by 0 2 at RT than a sample in which Ce was reduced (almost completely: again a dispersion influence) not by sputtering but with H2 at 773 K. The same authors also reported that, both in reductions with H2 and via Ar+ ion sputtering, an increase in reducibihty was observed for supported ceria in the order (pure Ce0 2 )
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under UHV was observed as well for Ceo.12Zro.s2O2 calcined at 1273 K [144]; after heavy Ar+ ion sputtering reduced Zr species (assigned as Zr3+ or even Zr°) were then also found coexisting with a fraction of Ce4+, suggesting segregation of Ce and Zr in separate regions (otherwise electron transfer from Zr3+ to Ce4+ should occur). On the contrary, in the cited work on aerogels [143] no Zr reduction by sputtering occurred, while Ce was fully reduced to Ce3+. It is worth mentioning here the work by Graham et al. [145] on a Ceo.7Zro.3O2 solid solution: as expected, the XPSdetected Ce:Zr ratio did not change upon H2 reduction at 873 K (no cation migration should occur), but did increase (up to. 50%) upon the same reduction after precalcining the sample up to 1425 K, which led to segregation into Zr-rich and Zr-poor phases (XRD data). It was interpreted that after reduction of the calcined sample the electron mean-free-path (and thus the cation "visibility") increased in the significantly reduced Ce-rich regions due to the lowered oxygen content, but not in the nearly pure zirconia regions which suffered almost no reduction. Those authors thus proposed using such changes in XPS-measured cation ratios upon moderate reduction to ascertain spatial phase separation. For Ce:Pr mixed oxides similar result and interpretation were presented. Mixed oxides of Ce and other lanthanides (La, Nd) were studied by Kubsch et al. [133], who found segregation of these trivalent ions to the surface in samples calcined at 1253 K; a high EB peak in the Ols feature was observed, being ascribed to carbonates formed in the trivalent ion-rich surface. A tendency to formation of such mixed oxides was detected in (Ce,Tb)Ox supported on lanthana-promoted alumina: after calcination the La XP spectrum displayed two components, ascribed to La species on the A1203 surface and dissolved into the Ce,Tb oxide respectively [146]. A high EB Ols peak observed there was ascribed to both alumina O2" ions and carbonate species on La-rich zones while the (Ce,Tb) oxide gave lower EB values.
5.4.4. Catalysts with Supported Platinum Group Metals Many groups have reported XPS data on this type of systems. Before any further discussion it is important to note that, although a number of effects may be induced by metal-ceria interactions, they can be also provoked, or at least enhanced, by the presence of chlorine introduced with the metal. The strong retention at the surface (or/and in the bulk) of ceria and related oxides, even after redox treatments, of chlorine coming from the transition metal salt used in preparing the catalyst is well documented [109, 138, 147, 148, 40, 149, 150, 151]. It has been verified that CI
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Catalysis by ceria and related systems
stabilizes the Ce state, for example comparing XPS, magnetic susceptibility or XAFS data of catalysts prepared from chloride and nitrate [148, 149, 152] (although in some case XPS data have been reported indicating a higher Ce reduction level for ex-nitrate than for ex-chloride catalyst [150]) or examining the results of the interaction of ceria with Cl-containing organic molecules, e.g. CCU in which Ce 3d binding energies 1.5 eV below that of Ce02 are found [153]. This is interpreted as due to the stability of Ce3+ oxychloride-type species, and indeed in some cases the formation of a CeOCl phase is verified by XRD or Raman techniques [109, 154, 153]. The retained chlorine may affect also the redox state of the supported metal; so in RhO x /Ce0 2 it leads to the presence of Rh3+ after reduction in conditions which would leave only Rh+ in the absence of CI [148]. A higher redox state has been also observed for Pd on ceria-alumina when CI is present [149]. Thus when evaluating published results it must be always remembered that, if a metal chloride was used for preparation of a ceria-containing catalyst, CI is likely to be retained and may affect XPS results and physico-chemical properties. This is frequently overlooked. XPS data indicating that supported metals catalyze the reduction of ceria (supported or not) by H2 were reported already in the late eighties for specimens prepared from chlorides (Pd: [109]; Pt: [155, 156]) or nitrates (Pd: [121]). Examination by XPS of such facile ceria reduction has been reported from then on by several groups for Pt group metal catalysts prepared without CI (e.g. Rh/ceria [127, 157], Pd/ceria [150] and other systems [158]) or with CI (e.g. Rh/ceria [159], Pt/ceria [160] and Pd/ceria [150]). It is worth noting that in some case outgassing at high temperature after reduction led to the observation of only a small fraction of Ce3+, which was interpreted as due to catalysis of the reverse process (back spillover of hydrogen) [157]. Just outgassing can lead to ceria reduction a Pt/Ce0 2 (ex-Cl) catalyst [155]. In some cases the metal is found to catalyze also the oxidation of (pre-reduced) ceria: actually, Pd and Pt on Ce0 2 /Al 2 0 3 favor the reoxidation of a previously formed CeA103 surface phase (stable in air at RT) giving back Ce0 2 [121, 156]. A similar effect was observed in [147]. Support reoxidation may happen even for interaction with a H 2 -C0 2 mixture in Rh/Ce0 2 [159]. Reverse influences (effect of Ce on the redox state of the supported metal) have been also observed. In some cases the reports claim that ceria specifically favors the retention of the metal in an oxidized state [88], although less than thoria in the case of Rh/M0 2 /Si0 2 [126]. In any case, XPS is used regularly to check the metal redox state after catalyst preparation or diverse treatments. Oxidized states have been reported for Pd and Pt on Ce0 2 catalysts prepared by a combustion method leading mainly to (+2) and mixed (+2/+4) states respectively [161]; on the
Studies by magnetic resonance and x-ray spectroscopies
195
basis of XPS intensity ratios it was then concluded that Pd cations were located inside the ceria phase, while most of Pt lied highly dispersed at the surface due to cationic radio mismatch. Positively charged Pd was reported for (ex-chloride) Pd/Ce0 2 even after H2 reduction at 573-773 K, while EXAFS indicated metal-like clustering [162]. Cationic Rh and Pt was observed on ceria(/alumina) catalysts both before and after reaction in a car-exaust-type lean gas mixture [163]; zerovalent states were then found after operation in rich mixture. A particularly intriguing XPS result was obtained on reduced Rh/CeC^: Rh° in it was reoxidized more effectively with a CO+0 2 mixture than with 0 2 alone [127]. This was ascribed to an assistance of Rh-adsorbed CO for breaking the 0 - 0 bond in 0 2 adsorbed (maybe as 02~) at anion vacancies of the support surface located near the Rh particle. In other cases, however, an enhancement in the reduced state of the metal has been claimed; binding energies even lower than those of the pure metal have been observed in reduced systems, being ascribed to support-to-metal electron transfer. This has been reported e.g. for Rh/Ce0 2 /Si0 2 [126], Pd/Ce0 2 [164] and Ce0 2 -Pd/Si0 2 [165], being proposed as cause of the higher catalytic activities found in the latter cases. No such electron transfer from ceria is however observed for an (ex-chloride) supported Pt system after H2 reduction at 773 K [160], nor for Rh° species formed after outgassing in UHV an (ex-nitrate) Rh/Ce0 2 calcined sample [166]. XPS is particularly useful to assess metal dispersion characteristics and mutual distribution of catalyst components through the study of spectrum intensity ratios. So, in Pd/ceria systems with Zr or Si added to the support, after aging in CO+H2 or S0 2 +0 2 mixtures XPS peak rations verified that Si segregates to the surface but Zr does not [167]. Also, in Pd/ceria/zeolite systms, the higher (Pd/Si)surf ratios observed in samples with higher (Ce/Si)surf ratios (within the same bulk Ce content) evidenced a specific association-interaction of Pd with surface-segregated ceria clusters [168]. A particular behavior was reported for Pt deposited on ceria by reduction with formaldehyde and then calcined at 773 K in air [169]: the Pt/Ce ratios obtained by XPS, quite similar to the bulk values and remaining nearly constant after Ar+ sputtering, indicated penetration of Pt into the oxide support in a highly oxidized state (Pt2+/Pt4+) according to the Pt 4f peak positions. Rh/Ce ratio profiles obtained during Ar+ sputtering of Rh/Ce0 2 catalysts [170] were analyzed in detail with a mathematical method developed by the same authors [171] allowing to ascertain particle size distributions of the metal together with the presence of part of it diffused into the support as solid solution (Fig. 5.12). The results indicated that, while an ex-nitrate sample presented increasing particle sizes upon calcination and reduction, an ex-chloride specimen having initially very small particles developed a bimodal particle size distribution upon calcination and led after subsequent
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Catalysis by ceria and related systems
reduction to high amounts of nearly atomically dispersed Rh in the bulk coexisting with metal particles in the range 1-5 nm. A Rh/CeOjICII-O a
Rh/C«02(NI-0
•
Rh/CtOjlCII-H
^-a
1
SPUTTERED
THICKNESS/nm .
o Rh/C«0,(N)-H "•*•"•-,«_
-f
-
—VA-«-
-t-
Figure 5.12. Particle size analysis in supported catalysts through XPS/ion sputtering experiments. Left: predicted profiles for several supported particle sizes in monodisperse particle model. Right: experimental data for oxidized and reduced ex-chloride and ex-nitrate Rh/CeO> samples (from ref. 170).
5.4.5. Catalysts with Other Supported Metals XPS has evidenced mutual metal-ceria effects on the respective redox states also for nickel catalysts. The degree of reduction of Ni in Ni/ZrC>2 prepared by sol-gel methods was seen to increase by doping the support with Ce [172]; this could be due to a dispersion improvement effect, also evidenced by XPS. The reciprocal effect (increase in Ce reduction by Ni) only occurred however for high Ce doping levels. NiOx-ceria mixed oxides prepared by coprecipitation, precursors of hydrogenation catalysts, were shown to form solid solutions for at. ratios Ni/Ce < 2.0, according to the evolution of XPS-measured surface ratios and the particular XPS spectra observed for Ni2+, different from those of NiO or NiCr 2 0 4 [173]; a model for the structure of these materials was derived, containing ceria with a surface layer of (Ce,Ni)Ox solid solution and NiO particles of specific sizes. In that case, nickel in the solid solution remained partly unreduced under H2 at 573 K, in contrast with NiO present in samples with Ni/Ce > 2 which was fully reduced; Ni also catalyzed partial reduction of ceria in those conditions. Ni-Ce-Al mixed oxide catalysts were also studied [174], analyzing the atomic ratio profiles obtained upon Ar+ sputtering; structure models of different complexity (NiO particles plus pure ceria or mixed oxide) as well as the above mentioned method of sputtering data analysis by Gonzalez-Elipe at al. [171] were used to analyze the data. It was concluded that Ni associated to the Al component distribution in the mixed material.
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Copper-ceria systems have been studied with XPS by several groups. It must be noted that in this case complications for the interpretation of Cu 2p peak positions may arise, especially when clear Auger lineshapes are not available, due to the significant dependence of EB values on Cu-oxide coordination and particle size reported in the literature [75 and references therein]. A trend to stabilize the Cu+ state when in low concentrations was reported for (La-doped) coprecipitated CuOx-ceria catalysts, CuO being formed for higher Cu loads [125]; the XPSdetected Cu:Ce ratio increased with sintering for both species, so it was deduced that the Cu solubility in ceria was rather low, and that the Cu+ species, insoluble in diluted HNO3, belonged to surface CuOx clusters displaying high catalitic activity, which had a maximum concentration around 10 Cu/nm2. An unusual XPS feature at EB~930 eV was reported also in this work, being ascribed to isolated Cu+2 species. The same peak was also observed in coprecipitated copper-ceria catalysts by Lamonier et al. [71], who however ascribed it to charging effects. These latter authors also disagreed with those of ref. [125] in claiming that only Cu2+ was present in the calcined catalysts, and that copper oxide did form solid solutions with ceria, on the basis of XRD studies; they proposed a model with isolated Cu2+ and small Cu2+ clusters on the surface of the solid solution besides CuO crystallites. In any case, Cu2+ species with XPS features different from those of CuO (EB ca. 1.5 eV higher) were also observed on impregnated CuOx/Ce02, being ascribed to isolated species similar to those present in zeolites [56]. Solid solution of Cu in ceria was also confirmed by XRD, at least up to ca. 15 at%, for coprecipitated catalysts used in catalytic wet oxidation (CWO) of phenol [175]. Here XPS showed an increase in the amount of Ce3+ with increasing amount of Cu, which was present in part as Cu+. A special stability of Cu+ even after H2 reduction at 773 K was reported for Cu/Ce02/Al203 prepared by impregnation [75], and was tentatively ascribed to insertion of Cu in a perovskite-type Al(Ce,Cu)Ox phase (with Ce3+ stable even after 0 2 adsorption) on the basis of electron diffraction data; the increased activity for CO+0 2 and NO+CO reactions was related to these Cu+ species. The XPS atomic ratios found in this latter work, compared with those of a Cu/Al 2 0 3 sample, indicated that cerium decreased the sintering of copper during reduction, while their profiles during subsequent Ar+ sputtering, showing initially an increase in the Cu:Ce peak ratio, suggested a coverage of the sintered Cu particles by ceria (a SMSI-type effect) and/or a surface enrichment with Ce of the mentioned (Al,Ce,Cu) oxide. Significant Cu+/Cu° ratios (higher than in pure microcrystalline CuOx) after intermediate temperature H2 reduction were reported also for (Cu,Ce) oxides resulting from oxidation of (Cu,Ce) alloys prepared by magnetron sputtering [176];
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here Cu+ could be oxidized to Cu2+ by C0 2 at 473-673 K. On the other hand, studying impregnated CuO x /Ce0 2 /Ti0 2 systems [177] it was found that the introduction of Ce increased the proportion of Cu2+ (as evaluated from the proportion of its shake-up satellite) in comparison with the CuO x /Ti0 2 case, while Cu did not influence the degree of reduction of Ce in the support, which was higher than in a pure Ce0 2 sample. Cu+ stabilization favored by ceria was reported also by Hu et al:. after acting in the CO+NO reaction the fraction of Cu2+ in (impregnated) CuO/ceria decreased while it did not so in CuO/alumina [178]. A few works in the literature report XPS results for other ceria-supported metals. In the late eighties data for Mn catalysts [179] showed higher proportion of Mn2+ (as deduced from EB values and shake-up satellite intensity) on ceria (which led to unselective methanol oxidation) than on titania or alumina; the XPS data also noted that with Mn ceria was reduced by H2 already at 573 K. In the case of coprecipitated (Mn,Ce) oxides (used for CWO of phenol) presenting surface enrichment in Ce (added as BET area promoter), significant amounts of Ce3+ appeared only for atomic ratio Ce:Mn >3 (for which a Ce02-type solid solution existed), otherwise mainly Ce4+ was found [180]. A comparative study of Co/ceria and Co/ceria/silica [181] reported that Co catalyzes the reduction of ceria by H2 (presumably via spillover) much more effectively in the second case; also the reduction of Co to Co0 was more effective in the presence of ceria. Dong et al. reported XPS data on Ce02-supported Mo0 3 and W0 3 which indicated monolayer formation of M6+ states up to ca. 5 at/nm2, also upon calcination of physical mixtures (thus evidencing wetting-spreading phenomena); this was correlated with a model of surface site occupation (implying M/Ce~2/3) on (111) ceria surfaces [182, 183]. For (ex-chloride) Au/Ce0 2 no stabilization of cationic Au species was seen after calcination at 923 K [125]. In chromia-ceria catalysts XPS showed that Cr6+ was the main species in the samples with low Cr load which had the best activity for ODH of isobutane, while higher Cr loads led to inactive Cr 2 0 3 [184]. In VO x /Ce0 2 /Al 2 0 3 samples displaying V-Ce synergistic effects for NO SCR, the formation of a surface CeV04 phase (presumably enhancing catalytic activity) was confirmed by the change of the Ce 3d XPS spectrum upon addition of V into one identical with that of CeV0 4 [185].
5.4.6. XPS Studies on the Adsorption of Diverse Molecules The use of Ce0 2 in catalytic S0 2 removal, and the possible effects of this gas on the operation of TWCs, has led to studies on S0 2 -Ce0 2 interaction. Early XPS data on
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S0 2 adsorption on a typical TWC showed formation of a Ce + sulphite at RT and a redox process giving at 773 K larger amounts of a Ce3+ sulphate, which decomposed in N2 at 1173 K giving back S0 2 and Ce4+ [186]. The metal might have a relevant role in the course of reaction, since on well oxidized Ce0 2 only SC«4~ was detected in the S 2p spectrum after S0 2 interaction at RT (without formation of Ce3+, so that SO molecules or some other undetected product must be formed), while SC>3~ species appeared for reduced surfaces [187]. Surface S-containing species were detected also on CuOx/Ce02 catalyst after catalyzing the S0 2 +CO reaction [188]; partially sulfided Cu and partially sulfated ceria were observed on an active catalyst, while one inactivated by working at 773 K or below had a completely sulfated ceria surface. Pt-Ce0 2 catalysts (supported on A1203 or not) gave Ce3+ (oxi-)sulphate even in S0 2 +0 2 mixtures (while Pt remained reduced) [160]. This sulfate resisted H2 treatment at 773 K (while the A1203 surface sulphate was reduced) but not at 973 K; higher amounts of Ce3+ remained in the A1203supported case. Cu+ sulfide species appeared after contact of H2S at RT with a coprecipitated, preoxidized (Cu,Ce)Ox catalyst, and remained stable after subsequent contact with 0 2 also at RT [77]. Accumulation of organic species on ceria-based catalysts has been studied in a few instances. Thus catalysts based on coprecipitated (Mn,Ce)02, deactivated after use in CWO of phenol, displayed in the XPS C Is region a superposition of several carbonaceous species, with significantly higher fraction of aromatics in Ptcontaning samples explaining their smaller efficiency [189]; the decrease in the Ce:Mn ratio measured in the specimens with more C accumulated implied that the latter deposited mainly on the ceria component, which appeared thus to be less active in oxidation. Coprecipitated Nixeria catalysts (in fact, mixed oxyhydrides) were studied after being used in enantioselective hydrogenation of carbonyl compounds using tartaric acid as chiral agent [190]. After reaction, the XPSdetected nickel remained in oxidized state (Ni2+) in spite of the reducing conditions, which was ascribed to the complexing effect of tartrate at the surface; on the contrary, reduced cerium (Ce3+) was observed even after exposure to air, presumably due to protection by the adsorbed organic layer. 5.5. XAFS 5.5.1. Ceria-specific Spectral Features X-ray Absorption Fine Structure (XAFS) spectroscopic studies of ceria-based catalysts concentrate frequently on the Ce Lm-edge, in which electrons from the Ce
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2p level are excited with photon energies of ca. 5720 eV or higher. Fig. 5.13 shows the Ce Lm-edge absorption spectrum of Ce0 2 . As usual in XAFS, two different regions are considered: the first one (left part of Fig. 5.13), usually called XANES region (X-Ray Absorption Near Edge Structure), reflects electronic transitions to empty bound states located below the vacuum level (pre-edge features) and to quasi-bound or continuum states at or above that value (resonances). The first ones are absent in Ce0 2 , and the second ones produce peaks and shoulders superimposed on the absorption edge jump. For ceria the main peaks are due to quasi-atomic Ce 2p3/2 "^ 5d5/2,3/2 electronic transitions [191], as determined by the dipole selection rules. XANES probes mainly low-lying discrete and extended electronic states; as these depend on the bonding characteristics and electronic structure, the technique can give information about the local geometry and electronic state. The EXAFS region (right part of Fig. 5.13) follows the XANES one; it probes also continuum states above the vacuum limit, but with higher energy. In the case of the Ce Lmedge, the close presence of the L ir edge (due to the Ce 2pi/2 excitation) limits to ca. 440 eV the utilizable energy range of structure-dependent oscillations (analyzable via Fourier transform to give information on Ce-neighbor distance). For Ce0 2 , the XANES is dominated by four resolved resonances, named in Fig. 5.13 A, Bi, B2 and C following literature labeling [109]; such large peaks at an edge jump are known as "white lines" for historical reasons. The double-peak white line structure of Ce0 2 arises from two possible configurations in the final state of the 2p3/2 -> 5d5/2,3/2 transition; the usual theory explains that this arises from the initial ground state hybridization described in section 5.4.1 above. As explained there, some debate exists in the literature concerning whether this hybridized
Energy (eV) Figure 5.13. XAFS spectrum of Ce0 2 at the L m edge; both XANES and EXAFS regions are highlighted. The inset presents the (k2-weighted) Fourier transform of the EXAFS oscillations.
Studies by magnetic resonance and x-ray spectroscopies character has to be interpreted as truly multiconfigurational [191] or can be described also as a simple Ce-O covalency [102]. In any case, two final state configurations, describable as [*]4f1Ln"15d1 and [*]4f°Ln5d1, appear as possible in the excitation. The first state is split by the influence of the cubic ligand field on the 5d excited electron, giving rise to the B] and B 2 peaks. Some authors have assigned the B2 peak to a Ce3+ impurity on the basis of its intensity invariance with pressure [102, 192]; this last assignment seems however dubious as the crystal field splitting of the B-peak was not fully accounted for in those works. The second state gives peak C in Fig. 5.13; the absence in the latter of a discernible splitting, in spite of the presence of a 5d' excited electron which should lead to similar crystal field effects as in the B feature, can be due to a higher linewidth. Peak A arises from transitions to states of adequate symmetry at the bottom of the conduction band [191]. Ce3+ species, with no hybridization in the ground state, display a single white line Bo located around 5726 eV (near the B t peak) and ascribed to a [*]4f'L"5d' final state [191, 193]; crystal field splitting effects are not discerned in it. Such splitting effects are, on the other hand, visible in the oxygen K-edge features of the spectra of these materials [194]; however, this edge is not frequently scanned on catalyst samples due to its low energy (aprox. 530 eV) and the need to be recorded under vacuum.
5.5.2. Methods of Analysis Ce L lir edge XANES spectra have been analyzed using different approaches. The simplest one involves fitting the whole XANES spectrum (after baseline subtraction and normalization) to linear combinations of reference spectra. For the case of Ce, a variant has been developed which fits the spectrum to a sum of individual contributions representing the different resonances present. Each contribution is computed as a convolution of lineshapes representing the unoccupied density of states having appropriate symmetry (usually a gaussian/parabolic function with adjustable center and width), the experimental resolution (typically a gaussian) and the core-hole lifetime (a lorentzian with width taken from tabulated data); the resulting convoluted functions are added to a sigmoidal step function representing the edge. Note that the parameters defining the edge have critical importance; the position of its inflection point, if badly chosen, can alter the fitting results, particularly for the features located close to the edge. This approach has been used to examine the reduction of the ceria component in M/Ce02 specimens under H2 [109, 42]. Changes in the Ce Lm-edge shape are then evaluated as a function of the Ce4+/Ce3+ ratio, computed using the four main resonances (three for Ce4+ and one
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for Ce + ). Calibration of the intensity of the feature B 0 gives the percentage of Ce in each spectrum and consequently its dependence with the reduction temperature. Another method of analysis makes use of a subtraction procedure and has been applied to investigate the oxidation state of a series of Ce-Zr mixed oxides during a temperature-programmed process [195]. A difference spectrum between the sample at a defined state and the calcined, fully oxidized material displays a positive peak at the position of feature B0, characteristic of Ce3+, and a negative peak at the position of feature C, characteristic of Ce4+. The overall peak-to-peak amplitude is then proportional to the average reduction degree of Ce in the sample. A statistical approach of the Factor Analysis class, especially adapted for XANES spectroscopy [196, 197], has been used to analyze spectra of Ce-containing catalysts, not for the cerium edge but for others corresponding to the active metals [61]. This method is particularly fruitful when applied to temperature-programmed experiments, which provide a relatively large number of spectra in a homogeneous series so that the advantages of a statistical technique are utilized at maximum. Ce Lin-edge EXAFS spectra are analyzed in the usual ways (background subtraction, normalization, fitting in the k and R spaces of respectively the EXAFS function and its Fourier transform FT, etc.) [198]. However, the close presence of the Ce L ir edge limits the power of the technique to describe local coordination. For bulk Ce0 2 , the k and R ranges utilizable for fitting (see Fig. 5.13) would allow to get information of the Ce environment up to the fourth shell, but in practice only the oxygen coordination shell is confidently described in supported catalyst materials; analysis of the next shell(s) containing metal or cationic species can be tried, but it must be noted that any neighbor different from Ce (either from the underlying carrier or from a Ce-containing mixed oxide), if formed by a heavy atom, may be practically indistinguishable form that of Ce, as its specific contribution would dominate the EXAFS spectrum only in the range k>8 A'1, which is not utilizable due to the close presence of the Ce L Ir edge [198, 199],
5.5.3. XAFS Studies of the Cerium Oxide Phase in Catalytic Materials 5.5.3.1. Structural and Electronic Details of the Cerium Oxide Materials The possibility of obtaining data from both XANES and EXAFS regions allows to get information on both geometric and electronic characteristics of a material. Thus, XANES data were taken for ceria samples with different particle sizes in the nanometer scale [200], showing a decrease in B:C peak intensity ratio and in B r B 2
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Figure 5.14. Ce Lni XANES spectra for Ce02 samples of different particle sizes (from ref. 200).
peak energy separation for particle sizes decreasing below the 10 nm range (Fig. 5.14); note that a different peak labeling was used in that work. The first fact indicated a decreasing participation of the 4f1Ln"1 component in the ground state (lower covalency degree and higher ionicity in the Ce-0 bond), the second one a decrease of the crystal field on the Ce ion. On the other hand, the EXAFS analysis in the same work showed a lower Ce-0 distance in the smaller particles (e.g. lower by ca. 3% for 2 nm particles than for bulk Ce0 2 ). A single picture explaining these facts remains difficult; one would expect that shorter Ce-0 distances would lead to higher orbital mixing (higher covalency) and crystal field, and that higher bond ionicity would lead to a crystal field increase; on the other hand, the Madelung field may be incompletely developed in very small particles, which would lead to a lower crystal field. The data do indicate that the electronic properties of the Ce site are changed in very small particles; this might be due to quantum size effects, i.e. confinement of the extended electronic states (possibly increasing the Pauli repulsion within the Ce-0 bond), like those known to occur in semiconductors. Electronic state modifications also appeared in the XANES spectra of Ce0 2 /Al 2 0 3 samples with low Ce load where according to EPR 2-D patches with strong ceria-alumina interaction were almost exclusively present [30]. The spectrum shape, basically similar to that of Ce0 2 and not to that of the CeA103 perovskite, indicated clearly a Ce4+ state, without admixture of Ce3+ as proposed in the literature for a "CeA103 surface precursor" [108]. Although the analysis was only qualitative, a decrease observed in the B r B 2 peak splitting indicated a lower ligand field, attributed to both a small particle size and increased ceria interionic distances induced by epitaxial effects (deduced from the position of first continuum resonance peak beyond the white lines); a stabilization of the Ce 4f orbitals in comparison
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Photon E l W B y ( e V )
Temperature (K)
Figure 5.15. Ce Lm XANES data of (Ce,Zr) oxides. Left: sample reduced at increasing temperatures. Right: degree of Ce reduction for different samples and temperatures (from ref. 195)
with bulk ceria was also concluded, explaining the higher stability of Ce3+ states and the higher Ce4+-02" bond covalency in the EPR spectra detected. In the field of ceria-based mixed oxides of catalytic interest, XAFS techniques seem to have been applied to date only to (Ce,Zr)02 materials [199, 201]. In one case, the symmetry decrease of the O2" anion sublattice originated when substituting Ce with Zr in the oxide lattice was investigated by EXAFS at the Zr K edge. It was concluded that the Zr-0 coordination changes from a 4+2 model in samples of large particle size (>90 nm), with tetragonal lattice symmetry, to a 5+2 model in samples with particle sizes below 20 nm and a structure of cubic symmetry; a relationship between local properties around Zr and oxygen handling properties was proposed [199]. There is however some difficulty with the data analysis in this system; recent work [202] claims that if different E0 parameter values are admitted for different oxygen shells in the EXAFS analysis, as seems to be needed to obtain correct results for pure tetragonal Zr0 2 (with well known crystal structure), a 4+4 model rather than a 4+2 model might result for the bulk (Ce,Zr)02 material. On the other hand, current work in this laboratory examining XANES at the Ce Lni-edge [201] shows white line features nearly identical to those in ceria, suggesting that only moderate changes are induced by Zr in the Ce electronic state.
5.5.3.2. Behavior of Cerium Oxides under Reactive Atmospheres In the field of catalysis, the reduction of ceria was the subject of the first XAFS studies (and of most of the subsequent ones) on this material, due to the importance of the redox properties of ceria for its action in TWCs and other catalysts; a great advantage of XAFS in this arises from the possibility of obtaining the spectra while the sample is under the action of reacting gases and high temperatures. Ce Lin-edge XANES spectra were thus able to monitor the reduction of ceria under H2, showing
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that Ce is fully reduced to Ce above 1100 K [42]. A similar study on Cei_xZrx02 specimens of different Zr content (Fig. 5.15) revealed that compositions with x>0.3 dramatically increase the rate of reduction [195]. In all cases, the presence of a metal, typically Rh, strongly enhances the reduction rates, allowing co-reduction of both support surface oxygen and the metal at low temperatures (above 400-500 K) [109, 194]. The reoxidation process is however more difficult to analyze with conventional XAFS, as its rate is quite high and complete oxidation is reached in times shorter than the characteristic XAFS spectrum acquisition time (1 min). The effect of SO2 gas has been also examined in relation with the behavior of ceria as sorbent/catalyst for the removal/destruction of this pollutant [187]. An O K edge XANES study complemented XPS data (see above) showing that contact with SO2 produces sulfates which thermally decompose in the 390-670 K range, and that the presence of anion vacancies favors the formation of S0 3 instead of S0 4 . The promotion of ceria with copper was shown to enhance the SO2 removal activity in all cases (reduced/oxidized state). The behavior of cerium in metal-loaded TWCs under reaction atmospheres has been also examined. The Ce LIn-edge XANES region is very sensitive to reductive or oxidative atmospheres, the Ce3+/Ce4+ ratio increasing/decreasing with the net reduced/oxidizing nature of the gas mixture [194, 109, 203]. However, in presence of stoichiometic gas mixtures ceria-based catalysts show only marginal modifications in their XANES spectrum shape (and, consequently, in their average oxidation state) with respect to calcined materials, even in conditions giving high conversion of NO, which is the pollutant most difficult to eliminate and needs the highest reaction temperatures [61, 203]. In fact, EXAFS analysis points out the presence of a Ce-0 distance characteristic of ceria (2.34 A), without evidence of the somewhat higher distances expected for Ce3+ in fluorite Ce02-X phases nor of the shorter ones (2.12 A) present in bulk hexagonal Ce 2 0 3 [203]. All these data would thus suggest that in typical reaction conditions, TWCs contain fully oxidized Ce, except at the surface where some Ce3+ centers may also exist. Similar studies carried out on Pd TWC-type catalysts using (Ce,Zr)02 as promoter gave essentially the same result, in spite of the higher reducibility of that mixed oxide [61].
5.5.4. Studies of the Metal Component in Ceria-containing Catalysts Most of the XAFS studies on these catalysts, particularly the EXAFS ones (which provide little information in the Ce case due to the short spectral range utilizable), are devoted to the metal active phase of the catalysts. Such studies have appeared
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for catalysts in which ceria forms at least partly the support of noble metals in use as TWCs [193, 194, 109, 61, 203, 204, 149, 205], as well as for methanol synthesis or decomposition [206] and NH3 synthesis [207]. In the first type of use, in addition to the above mentioned studies devoted to the Ce component particular attention has been paid to examine the effect of some preparation variables. For classical Pt-Rh specimens, it has been shown that coimpregnated catalysts tend to form highly homogeneous Pt-Rh alloys, which depress hydrocarbon oxidation activity with respect to other samples (prepared by stepwise impregnation) which contain a higher degree of heterogeneity in the binary particles and some surface ensembles of Rh atoms (responsible for hydrocarbon oxidation) [205]. For Pd, CI" ions from the precursor salt, remaining after calcination located at the metal-support interface, seemed responsible for maintaining some oxidized Pd atoms (strongly attached to those CI" ions), which conferred to the noble metal higher resistance to sintering under net reducing atmosphere [149]; a similar mechanism was postulated for Rh supported on pure Ce-Zr mixed oxides [204]. In general, under stoichiometric mixtures the noble metals appear to be partially reduced, the zero-valent contribution growing with the reaction temperature; however, the presence in the EXAFS spectra of metal-O contributions at distances characteristic of oxides indicate the existence of oxidized atoms at the surface, which probably correspond to stabilized centers at the metalsupport interface [61, 203, 204]. A similar situation is found for Pd/Ce0 2 systems used in methanol synthesis. Post-reaction samples displayed in the EXAFS pattern an increased contribution of Pd-0 oxide distances while the Pd-Pd distances decreased, showing that the (near) surface regions of the metal are mostly oxidized in reaction conditions; furthermore, a feature in the EXAFS FT at 3.19 A was proposed to arise from a Pd-O-Ce structure indicating significant Pd-support interaction [206]. For Ru systems used in NH3 production, the obtention of high dispersions, critical for maximizing activity, was favored by the use of ceria supports, an effect which according to XAFS data occurred via establishment of metal-support interactions [207]. Acknowledgements. Thanks are given to Profs. J. Soria, G. Munuera and A. R. Gonzalez-Elipe for useful comments and discussions. Support from CICYT (Project Nr. MAT2000-1467) is also acknowledged.
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5.6. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.
Perrichon V., Laachir A., Bergeret G., Frety R. and Tournayan L., J. Chem. Soc. Faraday Trans. 90 (1994), 773-781. Laachir A. et al, J. Chem. Soc. Faraday Trans. 87 (1991), 1601-1609. Badri A. et al, Eur. J. Solid State Inorg. Chem. 28 (1991), 445-448. Wertz J. E. and Bolton, J. R., Electron Spin Resonance (McGraw-Hill, 1972), p. 339. McLaughlan S. D. and Forrester, P. A., Phys. Rev. 151 (1966), 311-314. Barrie J. D., Momoda L. A., Dunn B., Gourier D., Aka G. and Vivien D., J. Sol. St. Chem. 86 (1990), 94-100. Abi Aad E., Zhilinskaya E. A. and Aboukais A., J. Chim. Phys. 96 (1999), 1519-1526 Abragam A. and Bleaney B., Electron Paramagnetic Resonance of Transition Ions, (London/New York, Oxford Univ. Press (Clarendon), 1970), p. 308 ff. Wrobel G., Lamonier C , Bennani A., D'Huysser A. and Aboukais A., J. Chem. Soc. Faraday Trans. 92 (1996), 2001-2009. Soria J., Conesa J. C , Martinez-Arias A. and Coronado J. M., Sol. St. Ion. 6365 (1993), 755-761. Dufaux, M., Che, M. and Naccache, C , Comptes Rendus Acad. Sci. Paris C268 (1969), 2255-2257. Gideoni M. and Steinberg, M., J. Sol. St. Chem. 4, (1972) 370-373. Che M., Kibblewhite J. F. J., Tench A. J., Dufaux M. and Naccache C, J. Chem. Soc. Faraday Trans. 1 69 (1973), 857-863. Mendelovici L., Tzehoval H. and Steinberg M., Appl. Surf. Sci. 17 (1983), 175-188. Fierro J. L. G., Soria J., Sanz J. and Rojo J. M., J. Sol. St. Chem. 66 (1987), 154-162. Soria J., Martinez-Arias A. and Conesa J. C , J. Chem. Soc. Faraday Trans. 91 (1995), 1669-1678. Oliva C , Termignone G., Vatti F. P., Forni L. and Vishniakov A.V., J. Mater. Sci. 31 (1996), 6333-6338. Zhang X. and Klabunde K. J., Inorg. Chem. 31 (1992), 1706-1709. Rojo J. M., Sanz J., Soria J. and Fierro J. L. G., Z. Phys. Chem. N.F. 152 (1987), 149-158. Martinez-Arias A., Coronado J. M., Conesa J. C. and Soria J., Rare Earths, eds. Saez Puche R. and Caro P. (Madrid, Ed. Complutense, 1997), p. 299-315. Yao, H. C. and Yu Yao, Y. F., J. Catal. 86 (1984), 254-265.
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22. Anpo M., Che M., Fubini B., Garrone E., Giamello E. and Paganini M. C , Topics Catal. 8 (1999), 189-198. 23. Pacchioni G., Ferrari A. M. and Giamello E., Chem. Phys. Lett. 255 (1996), 58-64. 24. Kanzig W. and Cohen M. H., Phys. Rev. Lett. 3 (1959), 509-510. 25. CheM. and Tench A. J., Adv. Catal. 32(1983), 1-147. 26. Setaka, M. and Kwan, T. Bull. Chem. Soc. Japan 43 (1970), 2121-2131. 27. Vedrine J. C , Wicker G. and Krzyzanowski S., Chem. Phys. Lett. 45 (1977), 543-547. 28. Haneda M., Mizushima T. and Kakuta N., J. Chem Soc. Faraday Trans. 91 (1995), 4459-4465. 29. Soria J., Coronado J. M. and Conesa J. C. J. Chem. Soc. Faraday Trans. 92 (1996), 1619-1626. 30. Martinez-Arias A., Fernandez-Garcia, M., Salamanca L. N., Valenzuela R. X., Conesa J. C. and Soria J., J. Phys. Chem. B104 (2000) 4038. 31. Niwa M., Minami T. and Murakami Y., Bull. Chem. Soc. Japan, 49 (1976), 565-566. 32. Martinez-Arias A., Soria J., Conesa J. C , Seoane X. L., Arcoya A. and Cataluna R., J. Chem. Soc. Faraday Trans. 91 (1995), 1679-1687. 33. Markaryan G. L. and Lunina E. V., Kin. Catal. 37 (1996), 133-139. 34. Craciun, R., Solid St. Ionics 110 (1998), 83-93. 35. Li, C, Sakata Y., Arai T., Domen K., Maruya K.-i. and Onishi T., J. Chem. Soc. Faraday Trans. 1 85 (1989), 929-943. 36. Che M., Tench A. J. and Naccache C, J. Chem. Soc. Faraday Trans. 1 70 (1974), 263-272. 37. Giamello E., Sojka Z., Che M. and Zecchina A., J. Phys. Chem. 90 (1986), 60846091. 38. NQrenberg H. and Briggs G. A. D., Phys. Rev. Lett. 79 (1997), 4222-4225. 39. Conesa J. C , Surf. Sci. 339 (1995), 337-352. 40. Soria J., Conesa J. C. and Martinez-Arias A., Coll. Surf. A158 (1999), 67-74. 41. Soria J., Martinez-Arias A., Conesa J. C , Munuera G. and Gonzalez-Elipe, A. R., Surf. Sci. 251/252 (1991), 990-994. 42. El Fallah J. et al.,J. Phys. Chem. 98 (1994), 5522-5528. 43. Kepinski L., Wolcyrz M. and Okal O., J. Chem. Soc. Faraday Trans. 91 (1995), 507-515.
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44. Soria J., Martinez-Arias A., Coronado J. M. and Conesa J. C , Topics Catal. 11/12 (2000), 205-212. 45. Martinez-Arias A., Coronado J. M., Cataluna R., Conesa J. C. and Soria J., J. Phys. Chem. B102 (1998), 4357-4365. 46. Lecomte J. J., Grange P., Leclercq L., Lamonier J. F., Aboukais A. and Leclercq G. Coll. Surf. A158 (1999), 241-247. 47. Morterra C , Bolis V. and Magnacca, G., J. Chem. Soc. Faraday Trans. 92 (1996), 1991-1999. 48. Conesa J. C , Martinez-Arias A., Fernandez-Garcia M. and Soria J., Res. Chem. Interm. 26 (2000), 103-111. 49. Martinez-Arias A., Fernandez-Garcia M., Belver C , Conesa J. C. and Soria J., Catal. Lett. 65 (2000), 197-204. 50. Martinez-Arias A., Fernandez-Garcia M., Hungria A. B., Conesa J. C. and Soria J., J. Alloys Comp. 323-324 (2001), 605-609. 51. Fernandez-Garcia M. et al., J. Catal. 194 (2000), 385-392. 52. Markaryan G. L. et al., Coll. Surf. A151 (1999), 435-447. 53. Fornasiero P., Montini T., Graziani M., Kaspar J., Hungria A. B., MartinezArias A. and Conesa J. C. (submitted). 54. Iglesias-Juez A., Hungria A. B., Galvez O., Fernandez-Garcia M., MartinezArias A., Guerrero-Ruiz A., Conesa J. C. and Soria J., Stud. Surf. Sci. Catal. (in press). 55. Martinez-Arias A., Soria J. and Conesa J. C , J. Catal. 168 (1997), 364-373. 56. Martinez-Arias A., Fernandez-Garcia M., Soria J. and Conesa J. C , J. Catal. 182 (1999), 367-377. 57. Fernandez-Garcia M. et al., J. Catal. 187 (1999), 474-485 58. Fernandez-Garcia M., Martinez-Arias A., Belver C , Anderson J. A., Conesa J. C. and Soria J., J. Catal. 190 (2000), 387-395. 59. Martinez-Arias A. et al., J. Catal. 195 (2000), 207-216. 60. Fernandez-Garcia M. et al.,Appl. Catal. B: Environmental 31 (2001), 39-50. 61. Martinez-Arias A. etal.,Appl. Catal. B: Environmental 7>\ (2001), 51-60. 62. Fernandez-Garcia M., Martinez-Arias A., Anderson J. A., Conesa J. C. and Soria J., Proc. 12xh Intl. Cong. Catalysis {Stud. Surf. Sci. Catal. 130), eds. Corma A., Melo F. V., Mendioroz S. and Fierro J. L. G. (Elsevier, Amsterdam, 2000), 1325-1330. 63. Sass A. S., Shvets V. A., Saveleva G. A., Popova N. M. and Kazanskii V. B., Kinet. Catal. 27 (1987), 777-784.
210
Catalysis by ceria and related systems
64. Tarasov A. L., Przhevalskaya L. K., Shvets V. A. and Kazanskii V. B., Kinet. Catal. 29 (1989), 1020-1027. 65. Niwa N., Furukawa Y. and Murakami Y., J. Coll. Interf. Sci. 86 (1982), 260265. 66. Aboukai's A., Bennani A., Ai'ssi C.F., Wrobel G., Guelton M. and Vedrine J. C , J. Chem. Soc. Faraday Trans. 88 (1992), 615-620. 67. Aboukai's A., Bennani A., Ai'ssi C.F., Wrobel G. and Guelton J., J. Chem. Soc. Faraday Trans. 88 (1992), 1321-1325. 68. Abi-Aad E., Bennani A., Bonnelle J.-P. and Aboukai's A., J. Chem. Soc. Faraday Trans. 91 (1995), 99-104. 69. Smith, T. D. and Pilbrow, J. R.; Coord. Chem. Rev. 13 (1974), 173-278. 70. Eaton, S. S., More, K. M., Sawant, B. M. and Eaton, G. E., J. Am. Chem. Soc. 105(1983), 6560-6567. 71. Lamonier C , Bennani A., D'Huysser A., Aboukai's A. and Wrobel G., J. Chem. Soc. Faraday Trans. 92 (1996), 131-136. 72. Aboukai's A., Bechara, R., Ai'ssi, C. F. and Guelton, M. Chem. Mater. 3 (1991), 557-559. 73. Soria J., Martinez-Arias A., Coronado J. M. and Conesa J. C , Appl. Surf. Sci. 70/71 (1993), 245-249. 74. Davidson, A., Tempere, J. F., Che, M., Roulet, H. and Dufour, G, J. Phys. Chem. 100 (1996), 4919-4929. 75. Fernandez-Garcia M., Gomez-Rebollo E., Guerrero Ruiz A., Conesa J. C. and Soria J., J. Catal. 172(1997), 146-159. 76. Martinez-Arias A., Cataluna R., Conesa J. C. and Soria J., J. Phys. Chem. B102 (1998), 809-817. 77. Aboukai's A., Galtayries A., Abi-Aad E., Courcot D. and Grimblot J., Coll. Swr/A154 (1999), 335-342. 78. Bera P., Aruna S. T., Patil K. C. and Hedge M. S., J. Catal. 186 (1999), 36-44. 79. Oliva, C , Forni, L., Ezerets, A. M., Mukovozov, I. E. and Vishniakov, A. V., J. Chem. Soc. Faraday Trans. 94 (1998), 587-592. 80. Oliva, C , Forni, L. and Vishniakov, A. V., Appl. Magn. Reson. 14 (1998), 283-292. 81. Fuda K., Kishio K., Yamauchi S. and Fueki K, J. Phys. Chem. Solids 46 (1985), 1141-1146. 82. Adler S. B., Smith J. W. and Reimer J. A., J. Chem. Phys. 98 (1993) 76137620.
Studies by magnetic resonance and x-ray spectroscopies
211
83. O'Neill W. M. and Morris M. A., Chem. Phys. Lett. 305 (1999), 389-394. 84. TollaB. Etal.,J. Mater. Chem. 9 (1999), 3131-3136. 85. Thomson J. B, Armstrong A. R. and Bruce P. G., J. Am. Chem. Soc. 118 (1996), 11129-11133. 86. Cousin R., et al, Coll. Surf. A158 (1999), 43-49. 87. Vazquez A., Lopez T., Gomez R., Bokhimi, Morales A. and Novaro O., J. Solid St. Chem. 128(1997), 161-168. 88. Engler B., Koberstein E. and Schubert P., Appl. Catal. 48 (1989), 71-82. 89. Fierro J. L. G., Rojo J. M. and Sanz, J., Coll. Surf. 15 (1985), 75-87. 90. Cunningham J., O'Brien S., Sanz J., Rojo J. M., Soria J. and Fierro J. L. G., J. Molec. Catal. 57 (1990), 379-396. 91. Cunningham J., Cullinane D., Sanz J., Rojo J. M., Soria J. and Fierro J. L. G., J. Chem. Soc. Faraday Trans. 88 (1992), 3233-3240. 92. Pfau A., Sanz J., Schierbaum K. D., Gopel W., Belzunegui J. P. and Rojo J. M., Proc. 11th Intl. Congress on Catalysis (Stud. Surf. Sci. Catal. 101), eds. Hightower J. W., Delgass W. N., Iglesia E. and Bell A. T. (Elsevier, Amsterdam, 1996), 931-940. 93. Force C , Belzunegui J. P., Sanz J., Martinez-Arias A. and Soria J., J. Catal. 197 (2001), 192-199. 94. Ivanova 1.1, et al, Coll. Surf. A158 (1999), 3-11. 95. Burroughs P., Hamnett A., Orchard A. F. and Thornton G., J. Chem. Soc. Dalton Trans. (1976), 1686-1698. 96. Kaindl G., Wertheim G. K., Schmiester G. and Sampathkumaran E. V., Phys. Rev. Lett. 58 (1987), 606-609. 97. Matsumoto M. et al, J. Electr. Spectrosc. Rel Phen. 78 (1996), 179-182. 98. Fujimori A., Phys. Rev. B28 (1983), 2281-2283. 99. Kotani A. and Parlebas J. C , J. Physique 46 (1985), 77-82. 100. Kotani A., Mizuta H., Jo T. and Parlebas J. C , Solid St. Commun. 53 (1985), 805-810. 101. Kotani A., Jo T. and Parlebas J. C , Adv. Phys. 37 (1988), 37-85. 102. Kaindl G., Schmiester G., Sampathkumaran E. V. and Wachter P., Phys. Rev. B38 (1988), 10174-10177. 103. a) Hiifner S., Steiner P. and Kinsinger V., Phys. Rev. Lett. 61 (1988), 2628; b) Kaindl G. and Wertheim G. K., ibid. 61 (1988), 2629. 104. Koelling D. D., Boring A. M. and Wood J. H., Solid St. Commun. 47 (1983), 227-232.
212
Catalysis by ceria and related systems
105. Wuilloud E., Delley B., Schneider W.-D. and Baer Y., Phys. Rev. Lett. 53 (1984), 202-205. 106. Le Normand F., El Fallah J., Hilaire L., Legare P., Kotani A. and Parlebas J. C , Solid St. Commun. 71 (1989), 885-889. 107. Kotani A. and Ogasawara H., J. Electr. Spectrosc. Rel. Phen. 86 (1997), 6572. 108. Shyu J. Z., Weber W. H. and Gandhi H. S., J. Phys. Chem. 92 (1988) 49644970. 109. Le Normand F., Hilaire L., Kili K., Krill G. and Maire G., J. Phys. Chem. 92 (1988), 2561-2568. 110. Romeo M., Bak K., El Fallah J., Le Normand F. and Hilaire L., Surf. Interf. Anal. 20(1993), 508-512. 111. Trudeau M. L., Tschope A. and Ying J. Y., Surf. Interf. Anal. 23 (1995), 219226. 112. Malinovski E. R. and Howery D. G., Factor Analysis in Chemistry (Wiley, New York, 1980). 113. Malinovski E. R., Analytical Chem. 49 (1977), 606-617. 114. Gaarenstroom S. W., Appl. Surf. Sci. 26 (1986), 561-574. 115. Gonzalez-Elipe A. R., Fernandez A., Holgado J. P., Caballero A. and Munuera G., J. Vac. Sci. Technol. A l l (1993), 58-65. 116. Holgado J. P., Munuera G., Espinos J. P. and Gonzalez-Elipe A. R., Appl. Surf. Sci. 158(2000), 164-171. 117. Holgado J. P., Alvarez R. and Munuera G., Appl. Surf. Sci. 161 (2000), 301315. 118. Padeste C , Cant N. W. and Trimm D. L., Catal. Lett. 28 (1994), 301-311. 119. Appel L. G. et al, Phys. Sta. Sol. B192 (1995), 477-91. 120. Creaser D. A., Harrison P. G., Morris M. A. and Wolfindale B. A., Catal. Lett. 23 (1994), 13-24. 121. Shyu J. Z., Otto K., Watkins W. L. H., Graham G. W., Belitz R. K. and Gandhi H. S. J. Catal. 114 (1988), 23-33. 122. Paparazzo E., Surf. Sci. Lett. 234 (1990), L253-L258. 123. Park P. W. and Ledford J. S., Langmuir 12 (1996), 1794-1799. 124. El Fallah J., Hilaire L., Romeo M. and Le Normand F., J. Electr. Spectrosc. Rel. Phen. 73 (1995), 89-103. 125. Liu W. and Flytzani-Stephanopoulos M., J. Catal. 153 (1995), 304-316; ibid, 317-332.
Studies by magnetic resonance and x-ray spectroscopies
213
126. Lindner U. and Papp H., Fresenius J. Anal. Chem. 341 (1991), 387-394. 127. Munuera G., Fernandez A. and Gonzalez-Elipe A. R., Catalysis and Automotive Pollution Control II (Stud. Surf. Sci. Catal. 30), eds. Crucq A., Frennet A. (Elsevier, Amsterdam, 1991), p. 207-219. 128. Abi-Aad E., Bechara R., Gimblot J. and Abouka'fs A., Chem. Mater. 5 (1993), 793-797. 129. Tschope A. and Ying J. Y., Nanophase Materials, ed. Hadjipanayis C. and Siegel R. W. (Kluwer Acad. Publ., 1994), p. 781-784. 130. Praline G., Koel B. E., Hance R. L., Lee H.-I. and White J. M., J. Electr. Spectrosc. Rel. Phen. 21 (1980), 17-30. 131. Hughes A. E., Gorman J. D., Patterson P. J. K. and Carter R., Surf. Interf Anal. 24(1996), 634-640. 132. Dolle P., Drissi S., Besancon M. and Joupille J., Surf Sci. 269/270 (1992), 687-690. 133. Kubsch J. E., Rieck J. S. and Spencer N. D., Catalysis and Automotive Pollution Control II (Stud. Surf. Sci. Catal. 30), eds. Crucq A., Frennet A. (Elsevier, Amsterdam, 1991), p. 125-138. 134. Wolf D., Heber M., Griinert W. and Muhler M., J. Catal. 199 (2001), 92-106. 135. Koel B. E., Praline G., Lee H.-I., White J. M. and Hance R. L., J. Electr. Spectrosc. Rel. Phen. 21 (1980), 31-46. 136. Schmitz P. J., Usmen R. K., Peters C. R., Graham G. W. and McCabe R. W., Appl. Surf Sci. 72 (1993), 181-187. 137. Masuda K., Kawai M., Kuno K., Kachi N. and Mizukami F., Preparation of Catalysts V (Stud. Surf. Sci. Catal. 63), eds. Poncelet G., Jacobs P. A., Grange P. and Delmon B. (Elsevier, Amsterdam, 1990), p. 355-364. 138. Dauscher A., Hilaire L., Le Normand F., Muller W., Maire G. and Vasquez A., Surf Interf. Anal. 16 (1990), 341-346. 139. Dauscher A., Wehrer P. and Hilaire L., Catal. Lett. 14 (1992), 171-183. 140. Leinen D., Fernandez A., Espinos J. P., Holgado J. P. and Gonzalez-Elipe A. R.,Appl. Surf. Sci. 68 (1993), 453-459. 141. Galtayries A., Sporken R., Riga J., Blanchard G. and Caudano R., J. Electr. Spectrosc. Rel. Phen. 88-91 (1998), 951-956. 142. Martinez-Arias A. et ah, Langmuir 15 (1999), 4796-4802. 143. Sun Y. and Sermon P. A., J. Mater. Chem. 6 (1996), 1025-1029. 144. Zhu H. Y. and Hirata T., J. Mater. Sci. Lett. 12 (1993), 749-751.
214
Catalysis by ceria and related systems
145. Graham G. W., Roe C. L., Haack L. P. and Straccia A. M., J. Vac. Sci. Technol. 18(2000), 1093-1095. 146. Galtayries A. et al, Surf. Inter/. Anal. 27 (1999), 941-949. 147. Bak K. and Hilaire L., Appl. Surf. Sci. 70/71 (1993), 191-195. 148. Kondarides D. I. and Verykios X. E., J. Catal. 174 (1998), 52-64. 149. Kili K., Hilaire L. and Le Normand F., Phys. Chem. Chem. Phys. 1 (1999), 1623-1631. 150. Shen W.-J. and Matsumura Y., Phys. Chem. Chem. Phys. 2 (2000), 15191522. 151. Dai H. X., Ng C. F. and Au C. T., J. Catal. 199 (2001), 177-192. 152. Bernal S. et al., Catalysis and Automotive Pollution Control III (Stud. Surf. Sci. Catal. 96), eds. Frennet A. and Bastin J. M.(Elsevier, Amsterdam, 1995), 419-429. 153. Weckhuysen B. M., Rosynek M. P. and Lunsford J. H., Phys. Chem. Chem. Phys. 1(1999), 3157-3162. 154. Wolcyrz M. and Kepinski L., J. Solid St. Chem. 99 (1992), 409-413. 155. Jin T., Zhou Y., Mains G. J. and White J. M., J. Phys. Chem. 91 (1987), 59315937. 156. Shyu J. Z. and Otto K. J. Catal. 115 (1989), 16-23. 157. Bernal S., Calvino J. J., Cifredo G. A., Rodriguez-Izquierdo J. M., Perrichon V. and Laachir A., J. Chem. Soc. Chem. Commun. (1992), 460-461. 158. Hoang M„ Hughes A. E. and Turney T. W., Appl. Surf. Sci. 72 (1993), 55-65. 159. Trovarelli A., Dolcetti G., de Leitenburg C , Kaspar J., Finetti P. and Santoni A., J. Chem. Soc. Faraday Trans. 88 (1992), 1311-1319. 160. Loof P., Kasemo B., Bjornkvist L., Andersson S. and Frestad A., Catalysis and Automotive Pollution Control II (Stud. Surf. Sci. Catal. 30), eds. Crucq A., Frennet A. (Elsevier, Amsterdam, 1991), p. 253-274. 161. Bera P., Patil K. C , Jayaram V., Subbanna G. N. and Hedge M. S., J. Catal. 196(2000), 293-301. 162. Matsumura Y., Shen W.-J., Ichihasi Y. and Okumura M., J. Catal. 197 (201), 267-272. 163. Nunan J. G., Robota H. J., Cohn M. J. and Bradley S. A., Catalysis and Automotive Pollution Control II (Stud. Surf. Sci. Catal. 30), eds. Crucq A., Frennet A. (Elsevier, Amsterdam, 1991), 221-237. 164. Usami Y., Kagawa K., Kawazoe M., Matsumura Y., Sakurai H. and Haruta M.,Appl. Catal. A: General 171 (1998), 123-130.
Studies by magnetic resonance and x-ray spectroscopies
215
165. Kang J. H., Shin E. W., Kim W. J., Park J. D. and Moon S. H., Catal. Today 63(2000), 183-188. 166. Soria J., Martinez-Arias A., Fierro J. L. G. and Conesa J. C , Vacuum 46 (1995), 1201-1204. 167. Jen H.-W. etal, Catal. Today 50 (1999), 309-328. 168. Weyrich P. A., Trevino H., Holderich W. F. and Sachtler W. M. H., Appl. Catal. A: General 163 (1997), 31-44. 169. Imamura S. etal, Catal. Today 50 (1999), 369-380. 170. Gonzalez-Elipe A. R., Holgado J. P., Alvarez R., Espinos J. P., Fernandez A. and Munuera G., in Fundamental Aspects of Heterogeneous Catalysis Studied by Particle Beams (NATO ASI Series C 227) eds. Brongersma H. H. and Van Santen R. A. (Plenum, New York, 1991). 171. Gonzalez-Elipe A. R., Espinos J. P., Fernandez A. and Munuera G., J. Catal. 130(1991), 627-641. 172. Montoya J. A., Romero-Pascual E., Gimon C , Del Angel P. and Monzon A., Catal. Today 63 (2000), 71-85. 173. Wrobel G., Sohier M. P., D'Huysser A., Bonnelle J. P. and Marcq J. P., Appl. Catal. A: General 101 (1993), 73-93. 174. Ponchel A., D'Huysser A., Lamonier C. and Jalowiecki-Duhamel L., Phys. Chem. Chem. Phys. 2 (2000), 303-312. 175. Hocevar S., Batista J. and Levee J., J. Catal. 184 (1999), 39-48. 176. Tschope A., Trudeau M. L. and Ying J. K., J. Phys. Chem. B103 (1999), 8858-8863. 177. Larsson P.-O. and Andersson A., J. Catal. 179 (1998), 72-89. 178. Hu Y. et ah, Appl. Catal. B: Environmental 31 (2001), 61-69. 179. Baltanas M. A., Katzer J. R. and Stiles A. B., Acta Chimica Hungarica 124 (1987), 341-356; ibid, 357-369. 180. Chen H., Sayari A., Adnot A. and Larachi F., Appl. Catal. B: Environmental 32(2001), 195-204. 181. Ernst B., Hilaire L. and Kiennemann A., Catal. Today 50 (1999), 413-427. 182. Dong L. and Chen Y., /. Chem. Soc. Faraday Trans. 92 (1996), 4589-4593. 183. Dong L. etal, J. Phys. Chem. B104 (2000), 78-85. 184. Moriceau P., Grzybowska B., Gengembre L. and Barbaux L., Appl. Catal. A: General 199 (2000), 73-82. 185. Centeno M. A., Malet P., Carrizosa I. and Odriozola J. A., J. Phys. Chem. B104 (2000), 3310-3319.
216
Catalysis by ceria and related systems
186. Harrison B., Diwell A. F. and Hallett C , Plat. Met. Rev. 32 (1988), 73-83. 187. Rodriguez J. A., Jirsak T., Freitag A., Hanson J. C , Larese J. Z. and Chaturvedi S., Catal. Lett. 62 (1999), 113-119. 188. Liu W., Wadia C. and Flytzani-Stephanopoulos M., Catal. Today 28 (1996), 391-403. 189. Hamoudi S., Larachi F., Cerrella G. and Cassanello M. Ind. Eng. Chem. Res. 37(1998), 3561-3566. 190. Leclercq E., Rives A., Payen E. and Hubaut R., Appl. Catal. A: General 168 (1998), 279-288. 191. Soldatov A. V., Ivanchenko T. S., Delia Longa S., Kotani A., Iwamoto Y. and Bianconi A., Phys. Rev. B. 50 (1994), 5074-5080. 192. Hu Z., Bertram S. and Kaindl G., Phys. Rev. B 49 (1994), 39-43. 193. Karnatak R. C , J. Alloys Comp. 192 (1993), 64-68. 194. Mullins D. R., Overbury S. H. and Huntley D. R., Surf. Set. 409 (1998), 307-319. 195. Overbury S. H., Huntley D. R., Mullins R. F. and Gleaves G. N., Catal. Lett. 51(1998), 133-138. 196. Fernandez-Garcia M., Marquez-Alvarez C. and Haller G. L. J. Phys. Chem. 99 (1995), 12565-12569. 197. Marquez-Alvarez C , Rodriguez-Ramos I., Guerrero-Ruiz A., FernandezGarcia M. and Haller G. L., J. Am. Chem. Soc. 119 (1997), 2905-2913. 198. Sayers D. E. and Bunker B. A., X-Ray Absorption, eds. Koningsberger D. C. and Prins R. (New York, Whiley, 1988), Chp. 6. 199. Fornasiero P., Fonda E., Di Monte R., Vlaic G., Kaspar J. and Graziani M., J. Catal. 187(1999), 177-185. 200. Nachimuthu P., Shih W.-C, Lin R.-S., Jang L.-Y. and Chem J. M., J. Solid. St. Chem. 149 (2000), 408-413. 201. Fernandez-Garcia M. et al., unpublished results. 202. Lemaux S., Bensaddik A., van der Eerden A. M. J., Bitter J. H. and Koningsberger D. C , J. Phys. Chem. B105 (2001), 4810-4815. 203. Holies J. H. and Davis R., J. Phys. Chem. B104 (2000), 9653-9660. 204. Vlaic G. etal.,J. Catal. 190 (2000), 182-190. 205. Pirault L., Guerin M., Maire F., Marecot P. and Barbier J., Appl. Catal. A: General 111 (1998), 249-258; ibid. 199 (2000), 109-122. 206. Matsumura Y., Shen W.-J., Ichihashi Y. and Ando H., Catal. Lett. 68 (2000), 181-183. 207. Izumi Y., Iwata Y. and Aita K.-L, J. Phys. Chem. 100 (1996), 9421-9428.
CHAPTER 6 STRUCTURAL PROPERTIES AND THERMAL STABILITY OF CERIA-ZIRCONIA AND RELATED MATERIALS
JAN KASPAR and PAOLO FORNASIERO Dipartimento di Scienze Chimiche, University of Trieste, via L. Giorgieri 1,134127 Trieste, Italy; E-mail:
[email protected]
6.1. The Ce0 2 -Zr0 2 Phase Diagram Due to the importance of stabilised zirconias in the field of ceramics, investigation of the features of the Ce02-Zr02 phase diagram attracted interest of investigators since 1950s [1-14]. For fabrication of ceramic materials and also for solid-oxide fuel cells, where doped-Zr02 are extensively employed, the comprehension of the phase diagram is of critical importance to prevent undesirable mechanical or chemical transformations of the material under working conditions [15]. For example, the martensitic tetragonal-to-monoclinic (t->m) transformation is responsible for the high fracture toughness of zirconia ceramics [16]. At ambient pressure pure Zr0 2 exhibits the occurrence of the following transformations, depending on the temperature [15]:
Monoclinic
H00K
< P2xlc
Tetragonal > P42/nmc
264ox
<
Cubic > =— Fm3m
(6.1)
A general structural relationship between these various Zr0 2 phases is illustrated in Fig. 6.1. The phase transformations mechanism depicted in reaction (6.1) is a general behaviour for the Zr02-containing ceramics in that by doping the monoclinic (rn) phase, tetragonal (t) and cubic (c) phases can be stabilised, also at room temperature (RT). Typically di-, tri- and tetra-valent cations are employed as dopants and the reactions by which these cation may dissolve into the Zr0 2 lattice is described in reactions (6.2-6.4), using the Kroger-Vink notation: 217
Catalysis by ceria and related materials
218
xMO + (1 - x)Zr02 -» xM"Zr + xV" + (1 - x)Zrxr + (2 - x)Ox0
(6.2)
xMOl 5 + (1 - x)Zr02 -> xMZr + 0.5xF" + (1 - x)ZrxZr + (2 - 0.5X)OQ (6.3)
xM0 2 + (1 - x)Zr02 -+ xMxZr + (1 - x)Zrxr + 20^
Idealized c-ZrQ (CaF: structure)
(6.4)
t-Zr02
O Oxygen anion
* •
»
Zr cation
D Oxygen vacancy Stabilized c-ZrQ
Figure 6.1. Structural relationships among the monoclinic, tetragonal and cubic phases in the Zr0 2 system [17,18]
The most significant differences between monoclinic, tetragonal and cubic structures is the change in the coordination of the Zr atoms which is seven in themonoclinic phase and eight in the other two {vide infra). The presence of sevenfold coordination in the m-Zx02 is consistent with the strong covalent character of
Structural properties and thermal stability of ceria-zirconia
219
the Zr-0 bonding [19] and the relatively small ionic radius of the tetravalent zirconium (0.084 nm) [20]. This makes the eight-fold fluorite-type of coordination unfavourable. A detailed description of the mechanism by which the doping leads to formation of the so-called PSZ (?-phase, partially stabilised zirconia) or FSZ (cphase, fully stabilised zirconia) is out of the scope of the present chapter, however, it is important to notice that doping with low-valent dopants leads to generation of oxygen vacancies in the lattice (Eqs. 6.2-6.3). Presence of oxygen vacancies in the solid then provides a way of minimising the stress generated in the Zr0 2 lattice by adopting the unfavourable eight-fold coordination. Generally speaking, over-sized, with respect to Zr, tri-valent cations provide more effective stabilisation compared to the under-sized ones due to the fact that the oxygen vacancies are preferentially located close to Zr in the former case, thus releasing the lattice stress due to the eight-fold coordination [21]. Conversely, the undersized cations compete with Zr for the oxygen vacancy leading to poorer stabilisation. As for the Ce0 2 -Zr0 2 is concerned, presence of three Ce0 2 -Zr0 2 phases is also observed at RT (Fig.6.2.). Despite the intense work on the definition of this phase diagram [1-14], its exact appearance is often matter of discussion. We adopt the phase classification suggested by Yashima et al. [22-24], which further distinguishes the tetragonal phases into t, f and t" phases, according to the type of the tetragonal distortion and its nature, i.e. /-phase which is stable, and f, t" which are metastable. 3073
2273 CU
t
ZD
1473CI)
•V* / !m
Q.
F a> 673-
273 0
20
40
60
80
100
Ce02 content / mol % Figure 6.2. Phase diagram of the Ce02-Zr02 binary system. Adapted from [25]. As shown in Fig.6.2., below 1273 K the phase diagram shows a monophase region of monoclinic (m) symmetry for Ce0 2 molar contents less than =10%, while for Ce0 2 contents higher than 80% cubic (c) phase was reported [8,14]. In the
Catalysis by ceria and related materials
220
intermediate region, the true nature of Ce0 2 -Zr0 2 phase diagram is still unclear. In this region indeed a number of stable and metastable phases of tetragonal symmetry are observed [10,11,13]. The three tetragonal phases designated t, t' and t" can be distinguished on the basis of XRD and Raman characterisation [22-25]. These phases can be prepared at high temperatures by solid state synthesis and upon cooling the /-form, which is a stable one, can be formed through a diffusional phase decomposition, while the /'-form is obtained through a diffusionless transition and it is metastable. The /"-form is also a metastable phase and it is intermediate between /' and c. The /" phase shows no tetragonality of the cation sublattice and it exhibits an oxygen displacement from ideal fluorite sites. It is often referred to as a cubic phase because its XRD pattern is indexed in the cubic Fm3m space group [26]. This is due to the fact that the cation sublattice prevalently generates the XRD pattern. For sake of clarity the characteristics of all the phases are summarised in Table 6.1. Table 6.1 Classification of the phases in the Ce0 2 -Zr0 2 binary system.
Phase Monoclinic (m) Tetragonal (/) Tetragonal (t1) Tetragonal (/") Cubic (c)
Composition range
Tetragonality
Space
(% mol Ce)
(da)
group
0-10 10-30 30-65 65-80 80-100
>1 >1 1 1
P2i/c P42/nmc P42/nmc P42/nmc Fm3m
The phase boundaries as indicated in Fig.6.2. and Table 6.1. should be considered very approximate due to the fact that in the case of the metastable tetragonal phases, the kind of distortion from the fluorite type structure is highly sensitive to the particle size. Thus Yashima et al. observed that the t" phase is formed for Ce0 2 contents above 65 mol%, while we reported the formation of the same phase for a Ceo.5Zro.5O2 sample [26]. The reasons for such apparent discrepancies can be rationalised by analogy with pure Zr0 2 . In fact, at RT the mZr0 2 is the thermodynamically stable phase, however, either tetragonal and cubic Zr0 2 have been stabilised at RT provided that fine particles are formed in the synthesis. Different explanations have been advanced to account for the stabilisation of /-Zr0 2 : surface and strain energy effects [27-29]; strain energy effects generated at domain boundary [30]; structural similarity between the amorphous Zr0 2 and /-Zr0 2 [31] and/or topotactic crystallisation of/-Zr0 2 from the amorphous precursor [8]. The latter two explanations are rather kinetic than
Structural properties and thermal stability of ceria-zirconia
221
thermodynamic and they rely on the fact that bands attributed to the t phase were detected by Raman spectroscopy in the amorphous Zr0 2 , indicating that a partial ordering of the structure is present already in the precursor [32]. Nevertheless, all these investigations point out that below a critical crystallite size, the tetragonal phase is favoured over the monoclinic one. Consistently, by using extremely fine particles, even the c-Zr0 2 was stabilised at RT [33-35]. In addition to the above considerations, one should note that specific compounds have also been proposed to exist in the Ce0 2 -Zr0 2 system: tetragonal Ce2Zr3Oio [36] and cubic Ce 3 Zr0 8 [37,38,38]. The existence of the former compound, however, has not been confirmed [7,8]. Finally, in the discussion of the features of the Ce0 2 -Zr0 2 phase diagrams, it must be considered that upon reduction of the Ce0 2 -Zr0 2 mixed oxides a still different situation is observed [4] and the presence of different phases was inferred. For example, Otsuka-YaoMatsuo et al. suggested that under reducing conditions at high temperatures (> 1273 K), the pyrocholore Ce2Zr207 may originate [39]. By a re-oxidation around 873 K, the pyrocholore compound produces a so-called K phase, which is structurally related to the cubic phase with a double pseudofluorite cell [39]. This phase may originate another /* tetragonal phase upon high temperature oxidation (1323-1423 K) [40,41], The appearance of the different phases is clearly related to the metastable nature of the Ce0 2 -Ce 2 03-Zr0 2 system and is discussed below in some detail. The variation of the structural properties of the ceramic type of the Ce0 2 -Zr0 2 mixed oxides was investigated in detail by different researchers [22-25,42-44]. As indicated in Table 6.1., the structural properties of the mixed oxides can be related to the parent simple oxides, in that at the cerium rich side of the phase diagram c, and t" or /' phases are present, which all appears to be either fluorite type, i.e. Ce0 2 , structure or its a slight tetragonal distortion. In fact, a ratio of cla = 1.00-1.01 is detected in the t -phase, while all the cations occupy perfect fluorite cation positions in the /"-phase, the tetragonal distortion being due only to the oxygen displacement. On the other hand at the Zr02-rich side of the Ce0 2 -Zr0 2 phase diagram, the structural properties appear dominated by Zr0 2 , hence typical structural features of this oxide are observed for Zr0 2 contents typically higher then 70 mol%. The XRD method can be successfully apply to detect the /'-/" phase boundary by analysing the (112) and (400)/(004) reflections respectively at about 42° and 6164° 29 (Fig. 6.3.) [22,23], even though, due to the low intensity of these peaks, confirmation by using a synchrotron radiation or even neutron scattering source may be desirable [45,46]. Due to the low intensity of the conventional XRD technique to the oxygen atoms, the t"-c phase transition is best detected by using
Catalysis by ceria and related materials
222
Raman spectroscopy [24,25]. Six Raman-active modes of A lg + 3 Eg + 2 B l g symmetry are observed for tetragonal Zr0 2 (space group P42/nmc), while for the cubic fluorite structure (space group Fm3m) only one F2g mode centred at around 490 cm"' is observed for c-Zr0 2 [22,24,25,32]. An example of the variation of the Raman patterns with composition is reported in Fig.6.4. X=70
400,„
X=65
3
X=60
i
r
X=50 004,,|1
61
62
/I400,'
63
20(deg.)
64
200
400
600
800
1000
1
Raman Shift (cm" )
Figure 6.3. Effect of Ce02-Zr02 composition Figure 6.4. Effect of Ce02-ZrO2 composition on the (X = mol% of Ce02) on t'-t" phase boundary as Raman spectra. detected by XRD. Consistent with the prediction, a single sharp band is observed at about 465 cm" for c-phases obtained for Ce0 2 contents higher then 80 mol%. 18 out of the 19 predicted modes are observed at the Zr02-rich side where /w-phase is formed. For 1
Structural properties and thermal stability of ceria-zirconia
223
the intermediate compositions, the situation is more complex, in fact the six Raman modes predicted for the tetragonal phases are not always observed as more and more Ce02-is added to Zr0 2 . For example, only three bands (two very weak) are observed for the Ceo.6Zro.4O2 sample. The related structural data obtained from the EXAFS measurements are reported in Table 6.2. Table 6.2. Structural parameters for Zr—O bonding in CexZr|.x02 solid solutions as detected from EXAFS measurements.
Sample /-Ce0.2Zr0.8C>2 r-Ceo.5Zro.5O2 r-Ceo.5Zro.5O2 r-Ceo.6Zro.4O2 c-Ceo.gZro.2O2
CN° Zr—0 4 4 3 2 4 2 4 2 4 2
ub (A x lfj2) 6.2±0.5 6.3±0.6 6.5±0.4 6.5±0.4 8.0±0.3 8.0±0.3 7.6±0.3 7.6±0.3 6.9±0.3 6.9±0.3
Rc
(A) 2.089±0.006 2.301±0.010 2.117±0.005 2.352±0.005 2.124±0.006 2.340±0.010 2.134±0.003 2.328±0.008 2.164±0.003 2.329±0.006
AE0 (eV) 0.4±0.2 0.4±0.2 2.1±0.2 2.1±0.2 0.9±0.4 0.9±0.4 1.1±0.2 1.1 ±0.2 1.0±0.2 1.0±0.2
° Co-ordination number.b Debye-Waller factor,c Bond distance.
A perusal of the data reported in Table 6.2. and Fig.6.4. suggests some cautions about the real nature of the lattice distortion in the Ce0 2 -Zr0 2 mixed oxides. In fact, the true tetragonal distortion, characterised by four short and four long Zr—O bonds (Fig.6.1.) was detected by EXAFS only in the /-phase. In the t' and t" the oxygen shell nearest to zirconium is still divided into two subshells, however, the external subshell appears relaxed and only two out of four oxygen atoms were detected by EXAFS. On the contrary, the Ce—O coordination appears relatively unaffected by the presence of zirconia and conserves the typical local structure of the fluorite. Lack of detection of oxygen atoms in the first Zr coordination shell in the EXAFS spectrum is associated with a high structural disorder generated by insertion of Zr0 2 into the lattice, which partially erases the EXAFS signal [42]. In summary, the Ce0 2 -Zr0 2 phase diagram features some still unresolved problems due to the presence of the metastable phases and the type of the distortion of the oxygen sublattice with sample origin. As shown in the following section,
224
Catalysis by ceria and related materials
these aspects are further made complex when high temperature treatment are applied to these mixed oxides.
6.2. Effects of High Temperature Reducing and Oxidising Treatments
6.2.1. Effects of Oxidising Atmosphere The effects of redox treatments on the chemical/structural behaviour of Ce02-Zr02 mixed oxides has attracted the interest of a number of authors, principally due to their use as OSC promoters, even though investigation of effects of reducing atmosphere on this system dates back to 60s [3,4], Due to the metastable nature of the system it is difficult to fully rationalise the effects of either reducing or oxidising atmosphere due to the variety of different phases reported in the literature, particularly for the Ceo.5Zro.5O2 composition. Any attempt to correlate the structural properties with the per-treatment (reducing or oxidising) should bear in mind that under oxidising conditions at 1273-1473 K phase separation into Ce02-rich and Zr02-rich phases, typically occurs to give approximately Ceo.2Zro.sO2 and Ceo.gZro.2O2 compositions [47,48]. This is in agreement with the phase diagram. On the contrary, formation of a solid solution occurs when the calcination temperature is increased above 1573 K. Consistently, such high temperatures are routinely employed to prepare ceramic type of Ce02-Zr02 mixed oxides over the whole range of compositions [5]. The high reaction temperature and the partial reduction of Ce4+ sites [22] both contribute to enhance the cation mobility in the lattice, due to the presence of oxygen vacancies and reduction of Ce4+ to Ce3+ which expands the lattice [49], thus allowing solid solutions to be attained. The critical importance of the calcination temperature and time upon the structural behaviour must be underlined. The assessment of formation of a solid solution in the synthesis is generally based on the measurement of the XRD pattern, by comparing the calculated cell parameter with some of the reported models describing the variation of cell parameter with the composition of the Zr0 2 or Ce0 2 doped material [50-53]. According to Vegard's rule, due to the smaller Zr4+ ionic radius (0.084 nm) compared to that of Ce4+ (0.097 nm) [20,54], a linear decrease of lattice parameter, or cell volume when the tetragonal phases are also included, is expected to occur upon insertion of increasing amounts of Zr0 2 into the Ce0 2 lattice. This is what actually happens (Fig.6.5.) even though there are some subtle but appreciable differences in the lattice parameter according to the origin of the
Structural properties and thermal stability of ceria-zirconia
225
sample, i.e. sintered ceramic type (LSA) or a high surface area (HSA) sample sintered below 1273 K.
0.544
c_ 0.539 L_
(0 0.534 i— as a.
% 0-529 _J
0.524 40
50
60
70
80
90
100
Ce0 2 Content (% mol) Figure 6.5. Variation of the lattice parameter in the LSA (calcination temperature T > 1273 K) and HSA (calcination temperature T < 1273 K) Ce02-Zr02 mixed oxides with increasing amount of Zr0 2 inserted in the Ce0 2 lattice.
A rationale for this observation must take into account that the assessment of the true homogeneity of high surface area solid solutions is not an easy task when conventional characterisation techniques such as powder XRD are used. Due to the nanometer size of the crystallites of the as-prepared HSA products, the XRD patterns generally feature severely broadened peaks which can easily mask compositional inhomogeneities. Presence of compositional inhomogeneities facilitates the phase segregation upon calcination. This treatment is often employed in thermal stability studies of the Ce0 2 -Zr0 2 mixed oxides [55,56]. For such studies calcination temperatures of or above 1273 K are typically employed, leading to phase segregation [55,56]. By using pulsed neutron scattering technique, Egami et al. [57] were able to show that local inhomogeneities may be present in the Ce0 2 Zr0 2 mixed oxides, leading to domain type of structure at a local level, which is undetectable by conventional XRD techniques. The presence of such local Ce02-rich and Zr02-rich dominions then favours phase segregation even after calcination at relatively mild temperatures. Such phenomena are obviously favoured
226
Catalysis by ceria and related
materials
\ \ \ l!
(4)
i
JlA
/'
A
/I . 65
85
f Figure 6.6. Effects of calcination at 1273 K on two Ceo.5Zro.5O2 samples (A. commercial, B: ex-citrate) and Rietveld profile refinement of the XRD pattern of Ceo.5Zro.5O2 (sample B calciend for 5 h at 1273 K): (1) fresh samples, (2) calcined at 1273 K. for 5 h, (3) calcined at 1273 K for 100 h and (4) Rietveld analysis (Courtesy of Dr. Di Monte University of Trieste).
at intermediate Ce02-Zr0 2 compositions, e.g. for Ceo.5Zro.5O2, where the highest tendency for phase segregation is suggested by the thermodynamics. Recently, we have shown that calcination at 1273 K for 5 h may provide a single phase Ceo.5Zro.5O2 product as assessed by a Rietveld profile fitting of the XRD pattern [58]. The comparison of the effects of calcination on a commercial type of Ce0.5Zr05O2 sample with those obtained on the ex-citrate sample are remarkable (Fig.6.6.). Both samples appear as a single phase product when the XRD patterns of the fresh samples are considered, however, the commercial sample showed clear indication of phase segregation into Ce0 2 - and Zr02-rich phases after calcination at 1273 K for 5 h. Notably, this phase separation is present also in the ex-citrate sample but it occurs slowly so that after 5 h of calcination presence of single phase Ceo.5Zro.5O2 could be assessed by Rietveld analysis of the XRD spectrum. Noticeably, some residual Ceo.5Zro.5O2 was present even after 100 h of calcination at 1273 K (Fig.6.6.). In our view, this difference in the kinetics of phase segregation should be associated with a more homogeneously - randomly - distributed cation
Structural properties and thermal stability of ceria-zirconia
227
distribution being attained by the citrate synthesis. There is a further point of interest in this result: it provides a quick and easy criterion for the detection - to the level of sensitivity of this technique - of compositional inhomogeneities induced by an inadequate synthesis method. Provided that a solid solution with randomly distributed Ce and Zr was formed by the synthesis method, the different lattice parameters between the ceramic and HSA type of samples are still observed (Fig.6.5.) and need to be commented on. A careful XRD and Raman investigation of a Ceo.6Zro.4O2 solid solution prepared by different synthesis routes, i.e. high temperature (1873 K) solid state synthesis and a citrate polymeric resin route [59], disclosed different local Zr—O local structure according to the origin of the sample. In particular, the EXAFS signal due to the oxygen shell around zirconium could be conveniently fitted a two subshell model with four oxygen atoms at a short distance (0.2134 nm) and two at a distance of 0.2328 nm in the ceramic type of mixed oxide, while a 5+2 model reasonably accounted for the signal in the ex-citrate sample. The different symmetry of the M—O bonding was also confirmed by Raman spectroscopy. Since both the sample were in an oxidised form, lack of detection of the eight nearest oxygen atom neighbours, which would be expected for a tetragonal phase, cannot be associated with presence of oxygen vacancies. The indication arising from EXAFS is again that the oxygen atoms are in a highly disordered form around the zirconium atom, thus partially erasing the average EXAFS signal. The structural disorder appears to be the intrinsic property of the mixed oxide, rather then a thermal disorder, as confirmed measurements carried out at liquid He temperature [42,43]. The susceptibility of the distortion of the oxygen sublattice to the sample origin is also highlighted by the HREM data reported by Torng et al, who showed that cubic zirconia in arc-melted Zr0 2 -Ce0 2 exhibit extra reflections of l/2-(odd odd odd) type in X-ray or electron diffraction patterns, which are forbidden for the ideal fluorite structure. The extra reflections can be explained in terms of a regular displacement of oxygen ions in the [111] directions rather then in the [100] as predicted by the tetragonal P42/nmc space group [60]. Recently, Mamontov et al, by using pulsed neutron diffraction technique, suggested that Ce0 2 -Zr0 2 mixed oxides contain a significant concentration, of vacancy-interstitial oxygen defects, which are generated by insertion of Zr0 2 into the Ce0 2 lattice [61]. Unfortunately, the commercial sample employed by these authors contained significant amounts of lanthana as impurity, which could easily perturb the properties of their mixed oxides [62]. No evidence for presence of significant amounts of Ce3+ sites was indeed detected in earlier studies of Ce0 2 -Zr0 2 mixed oxides by XANES and
228
Catalysis by ceria and related materials
magnetic susceptibility measurements [47,63-65]. An important point, however, arises from all these studies: there is an extreme variability of the fine structural details in these mixed oxides which, apparently, depends on sample origin and even the pre-treatment. The type of phase separation which occurs under oxidising condition also depend on the sample composition. Thus, as shown in Fig.6.6., the phase segregation occurs via a decomposition of the Ceo.5Zro.5O2 phase, while for higher Ce0 2 contents segregation of the Zr02-rich phase is at the core of the process leaving a phase which is enriched in Ce0 2 with time of calcination [48]. As suggested by Pijolat and co-workers [48], the driving force for phase separation is the presence of a critical particle size above which the Ce02-Zr0 2 mixed oxides tend to decompose, in a similar fashion as Zr0 2 phase transformations do occur. This critical size is around 15 nm for Ceo.5Zro.5O2, but it changes with sample composition.
6.2.2. Effects of Reducing Atmosphere Generally speaking, phase separation is unfavoured under reducing conditions, even at very high temperatures. The effects of reduction followed by an oxidation were investigated in detail by a number of investigators, particularly for the Ceo.5Zro.5O2 composition [22-24,26,39-41,66-75]. Generally speaking, in these studies the sample are subjected to a high temperature reduction, followed by an oxidation treatment, which can be carried out either at high of mild temperature. The results obtained on the Ceo.5Zro.5O2 composition are summarised in Fig.6.7. It appears that both reduction and oxidation temperature critically affect the nature of the phase formed. It is indeed observed that when the reducing treatment is carried out above 1373 K, the pyrochlore Ce2Zr207 is formed by the reduction. The pyrochlore structure may be described as an ordered cubic close-packed array of cations with Ce3+ and Zr4+ located respectively at the 16c and 16d sites in the space group Fd3 m [71]. Oxide ions occupy 7/8 of the tetrahedral sites between the cations. As exemplified in Fig.6.7., the pyrochlore structure is related to that of fluorite, but with ordered cations and ordered oxygen vacancies. This structural ordering is achieved under high temperature reducing conditions and can be conserved by reoxidising under mild conditions. Thus, Thomson et al. reported formation of an intercalated Ce2Zr207.36 compound which was obtained by re-oxidation of the pyrochlore under very mild conditions [71,72], the degree of oxygen intercalation
Structural properties and thermal stability of ceria-zirconia
229
increased up to Ce2Zr207.97 when the oxidation temperature was increased to 773 K [70]. By increasing the oxidation temperature above 973 K, a decreases of intensity of the XRD peaks associated with cation ordering in the pyrochlore structure was observed, suggesting that the statistically disordered more stable fluorite structure was formed. These observation appear to be fairly in line with those previously reported by Otsuka-Yao-Matsuo et al. [39-41,68,69,76], who showed the critical role of the temperature of the reducing and oxidising atmosphere in generating the different phases. f-Ceo.sZro.502-x
f-ceo.5zro.502
©z^j§T~p£ •x*
@s
H2
c
v^"jp~"~^£:
T<1373K ^
1MX$J 3
H2 IA 02 \ T > 1 3 7 3 K ^ | T>873K
O2
t
02 \T>873K
Ce2Zr207,36
©
Zr or Ce
Ce
Zr
O O O
O vacancy
Figure 6.7. Scheme of Ceo.5Zro.5O2 transformations induced by reductive and oxidative treatments.
Relatively few investigation addressed the stability under reducing (and oxidising) conditions of Ce02-Zr02 with compositions different from Ceo.5Zro.5O;, [40,62,76]. Otsuka-Yao-Matsuo et al. investigated effects of reduction at 1323 K, and oxidation at 873, 1323 or 1423 K, on various CexZr,.x02 composition with x = 1. 0.80, 0.6, 0.5, 0.4 by XRD and temperature programmed reduction techniques [40]. They suggested that the high temperature reduction process can be depicted as
230
Catalysis by ceria and related materials
a formation of a pyrochlore type of compound in which at x = 0.5, Ce and Zr + are ordered, generating the pyrochlore, while for other compositions, the cations with 3+ and 4+ are in an ordered arrangement, much like in the pyrochlore, while unreduced Ce4+ is randomly mixed with Zr4+. Thus formation of a pyrochlorerelated of structure would be the driving force leading to structural transformation of these mixed oxides. Notice that these studies were carried out on single phase ceramic type of Ce0 2 -Zr0 2 solid solutions, however, it should be noted that indication of possible cation ordering was detected also in a HSA Ceo.6Zro.4O2 mixed oxide subjected to redox-treatments up to 1273 K [62], suggesting a more general validity of this proposal. Finally, the fact that formation of pyrochlore plays a crucial role in determining the chemistry under high temperature reducing conditions was recently confirmed by Ikryannikova et al. who observed separation of pyrochlore when Ceo.3Zro.7O2 was reduced at 1470 K [77]. In line with the above comments, only a <-Ceo.3Zro.7O2 was observed for reduction temperatures below 1220 K. Also earlier studies on Zr02-rich compositions (Ceo.12Zro.8sO2) reported triggering of phase separation upon reduction at very high reduction temperatures [78,79], however, in this case m-Zx02 may segregate which could provide a driving force for such a process.
6.3. Effects of Aliovalent Doping on Thermal and Phase Stability8
6.3.1. Doped Ce02 Materials As above outlined, the purpose of this chapter is to illustrate the aspects related to catalytic materials, ceramic or other type of materials being out of scope. The idea of using aliovalent dopants to modify the catalytic properties, and particularly lowvalent cations dates back to early 90s [80,81]; it was sought that creation of oxygen vacancies could improve the mobility of oxygen in the bulk, increasing the efficiency of Ce0 2 as oxygen storage material in the automotive applications. Since then a number of different system has been investigated, and the aspects related to the oxygen storage property in the doped Ce02-containing three-way catalysts have been discussed recently [82]. From a structural point of view, there is only a limited interest in the binary Ce02-MxOy (M= low - less then IV - valent cation), systems in " Also other elements, which can formally assume also a tetravalent state, such as Si, Tb or Pr will be included in this section, even if not properly aliovalent.
Structural properties and thermal stability of ceria-zirconia
231
that the fluorite lattice of Ce0 2 is conserved, the major modifications being related to the amount and ordering of the oxygen vacancies that are created by dissolving low-valent cations in the lattice. Strictly speaking, this does not mean that the chemistry of non stoichiometric of Ce0 2 phases does not represent interesting features; on the contrary there has been many studies in the literature aimed at elucidating how the non stoichiometric defects can order in the fluorite lattice and these systems feature a rich chemistry (see for example [83-88]). Also the presence of oxygen vacancies, which may be created by doping of Ce0 2 , has long been recognised as an important factor in determining both the conductivity and catalytic properties of these materials [89-94]. Clearly, the first and major structural change in the doped-Ce02 with respect to the undoped oxide is the lattice parameter, provided that the dopant cation radius is different from that of Ce4+. As above indicated, several authors reported empirical relationships which can be usefully employed for confirmation of the presence of doped Ce0 2 solid solutions and even for predicting the solubility limits [50-53,95]. In addition, binary compounds such as, for example, perovskite-related SrCe0 3 [96], can be formed, description of which is clearly outside the present scope. It should be noted that also formally pentavalent dopants such as Ta(V) have been inserted into the Ce0 2 lattice [97,98]. Generally speaking, stoichiometric oxides such as CeTa0 4 or CeNb0 4 , where a trivalent cerium cation is present, can be prepared under different reaction conditions In principle, there are two possible mechanism by which the electroneutrality of the solid can be achieved when the pentavalent dopant is added, i.e. by generation of interstitial oxygens or by reducing Ce4+ to Ce3+. The latter mechanism is clearly operating in both CeTa0 4 or CeNb0 4 , however, it should be noted that by annealing these compounds in the presence of oxygen, Ce3+ sites can be at least partially oxidised to Ce4+ leading to different phases which are generally related to the parent compound rather then to the Ce0 2 fluorite lattice [97,98]. To our knowledge, the catalytic relevance of these phases is still to be established. There are a few studies concerning doping of Ce0 2 with formally tetravalent dopants other then zirconium, Si, Ti and Hf being those reported [99-102]. In the case of Si, formation of a Ce9.33(Si04)602 phase upon reduction of a mixed Ce0 2 Si0 2 phase was observed [101]. Upon re-oxidation at 873 K this phase decompose into amorphous Si0 2 and Ce0 2 . Using Ti(IV) as a dopant, CeTi0 3 may be formed where cerium is stabilised in the trivalent state [100]. Clearly, both Ti and Si (ionic radii in the Vl-fold co-ordination of 0.0605 and 0.040 nm, respectively) appear to be too small to effectively stabilise the Ce0 2 fluorite lattice, as these cations prefer a six fold co-ordination. In fact, Ce(IV) is a strong oxidant in solution and the
232
Catalysis by ceria and related materials
stabilisation of the tetravalent state occurs in the solid state due to the formation of the fluorite lattice [82]. Use of variable valence dopant has also attracted some interest. Generally speaking, the aim of these studies was to improve the OSC property, accordingly the structural information is relatively scarce. Thus, for example, Bernal and coworkers investigated a La203/Ce02/Tb02 mixed oxide and the effects of reduction and re-oxidation [103]. They found that the starting oxide can be fully reduced to a Ce(III) and Tb(III) containing phase which consists of a hexagonal, A-type, mixed sesquioxide. Quantitative TPO then showed that this reduced phase can be fully oxidised :to a La203/Ce02/Tb02 mixed oxide without occurrence of any detectable phase segregation phenomena [103].
6.3.2. Doped Ce02-Zr02 Materials In the case of Ce0 2 -Zr0 2 mixed oxides, lanthana and yttria have been mostly investigated as dopants [62,77,104-106]. Mn and Cu were also added to Ce0 2 -Zr0 2 with the aim to increase the oxidation capability of these systems, however, it should be noted that the solubility of these elements in Ce02-containing materials is generally low, accordingly their incorporation may occur only at low concentrations [107,108]. Generally speaking the main reason for adding a low-valent dopant to Ce0 2 Zr0 2 is to prevent the undesirable phase separation, which may occur upon high temperature ageing. In analogy to the Zr02-containing systems (PSZ and FSZ), the low-valent dopant stabilises the fluorite type of structure, presumably by the same mechanism discussed above for doped-Zr02, i.e. by releasing the stress around zirconium generated by the eight-fold co-ordination in the fluorite lattice, which is accomplished by preferably localising the oxygen vacancy near to Zr [21]. Accordingly, upon doping of Ceo.3Zro.7O2 with 10 mol% of yttria, phase transition from t to a cubic phase was detected [77]. In another investigation of ceramic type of materials of the composition 0.9(Zri_xCexO2)—0.1(Y2O3) (0 < x < 1) cubic fluorite type solid solutions were detected over the whole range of Ce0 2 -Zr0 2 compositions as indicated by a linear type of dependence of the lattice parameter upon the Ce0 2 content [109]. Interestingly, although the effective ionic radius increased by ~ 13% passing from 0.9(ZrO2)—0.1(Y2O3) to 0.9(CeO2)—0.1(Y2O3), the effective increase of the lattice parameter was about 5 %, suggesting some disorder of the oxygen sublattice to occur [109]. An interesting observation is that in addition to the stabilisation of the cubic phase, use of dopants may favour both
Structural properties and thermal stability of ceria-zirconia
233
phase purity and surface area [62]. The choice of the dopant and its amount may critically affect phase stability of the product. Under reducing conditions it was observed that phase transformation occurs, presumably leading to long range ordering as detected by appearance of new peaks at 10-15° 29 in the XRD patterns. This behaviour is consistent with that above discussed for undoped Ce0 2 -Zr0 2 . The extent of this possible ordering appeared related to the nature and amount of the trivalent dopant (Fig.6.8.).
o o
or
2
4
6
10
Dopant Loading (% mol)
Figure 6.8. Degree of long-range ordering in doped Ceo^Zro^MxCh.*^ as detected from the ratio of integrated intensities of the XRD peaks in the range 10-25° (29) relative to the (111) reflection: M = Y (•), La (•) and Ga (-»- ).
The scarcity of the data available in the literature does not allow a rationale for the role of the dopant in modifying the textural and phase stability to be derived, even though some general indication such as that addition of La 2 0 3 favours surface area stability in Ce02-containing moieties may be found [110]. In fact, as above discussed, the synthesis plays a critical role in determining the properties of the Ce02-Zr02 mixed oxides, which, of course, applies to the doped systems too. Very often, the effects of dopants can be masked by the fact that samples with different textural properties are compared. Thus appropriate tuning of pore structure allowed to achieve a surface area of 22 m2 g"' after a calcination at 1273 K in a Ceo.2Zr0.802
234
Catalysis by ceria and related materials
single solid solution [111], whereas the surface area of most of the reported systems, both doped and undoped, typically drops below 10 m2 g"1.
6.4. Effects of Addition of A1203 to Ce0 2 -Zr0 2 Mixed Oxides Investigation of effects of addition of A1203 to Ce0 2 -Zr0 2 on thermal and structural stability of these mixed oxides is an area which attracted interest only recently [112118]. A possible reason for this is that it has long been shown in the production of the TWCs, which is the major catalytic application of the Ce02-based materials, that a close interaction between Ce0 2 and A1203 unfavourably affects the OSC property due to formation of CeA103 [119-121]. Accordingly, it is usual practice to employ pre-formed Ce0 2 or Ce0 2 -Zr0 2 particles to make the TWCs. These particles are then supported (wash-coated) on AI2O3 (see for example US patent n° 5,945,369, issued on August 31st, 1999). From a structural point of view, it should be noted that one of the first issues when such systems are considered is to ensure that homogeneous solid solutions are formed in the synthesis. In fact, for intermediate Ce0 2 -Zr0 2 compositions, there is the highest statistical probability of compositional fluctuations, which may favour compositional inhomogeneites when a wet-impregnation synthesis method is employed [117]. Accordingly, it is important to employ appropriate methodology to achieve and check the homogeneity of the supported Ce0 2 -Zr0 2 solid solution. Notably, the above discussed (section 6.2.1.) criterion for detecting the homogeneity by XRD technique can profitably be employed also for this systems [82]. When the structural properties of these systems are considered, it appears that the major effect of addition of A1203 is that of favouring high dispersion of the Ce0 2 -Zr0 2 particles on the A1203 surface [112-114,118]. Fernandez-Garcia et al. [118] have indeed detected two dimensional and three dimensional patches of Ce0 2 -Zr0 2 on the A1203 surface, suggesting that there is some kind of interaction between the solid solution and the alumina, leading to a stabilisation of highly dispersed Ce0 2 -Zr0 2 particles. Accordingly, no appreciable increase of particle size was detected by increasing the loading of the mixed oxide from 10 to 33 wt%. The ability of A1203 to thermally stabilise the Ce0 2 -Zr0 2 mixed oxides is remarkable, thus no phase separation could be detected in Ce0.6Zr0.4O2(10 wt%)/Al 2 0 3 after calcination for 100 h at 1273 K [115]. The stabilisation effect between the Ce0 2 Zr0 2 mixed oxide and A1203 is a mutual one, since the thermal transformation of yA1203 into a-Al 2 0 3 is also prevented. Both Zr0 2 [122] and Ce0 2 [120] are known
Structural properties and thermal stability of ceria-zirconia
235
to thermally stabilise A1203; this stabilisation being particularly effective under reducing conditions [120,123], due to a structural coherence of the transitional aluminas with the LnA103 systems (Ln = lanthanide) [124]. Table 6.3 Oxygen storage and textural characterisation of the Ce0 2 -Zr0 2 and Ce0 2 -Zr0 2 /Al 2 0 3 catalysts
Sample"
Calcination
BET area/Ce0 2 -Zr0 2 crystallite mV size / nm
400°C 500°C 600°C
Temp./°C Time / h Ceo.2Zra802/Al203
Ce0 2 /Al 2 0 3
Ceo.2Zro.jjO2
osc'/ mlO 2 Ssolid solution
500 1000 1100 1100 500
5 5 5 24 5
180 115 68 63 180
7 11 11
-
0.2 0.6 0.1 0.3 3.2
2.2 2.5 1.1 1.3 9.3
5.8 4.6 3.5 3.6 13.8
1100 1100 500 1100
5 24 5 5
60 32 24 1
20 28 6 35
0.3 0.4 0.3 0
0.9 1.0 1.6 0
1.6 1.9 3.4 0.1
" 13 wt% of Ce0 2 or Ceo.2Zr0.g02 were loaded on Al203j * For sake of consistency OSC (0 2 uptake) is given per g of Ce0 2 -Zr0 2 mixed oxide. Dynamic-OSC measured at the indicated temperatures, using alternating pulses of CO and 0 2 [114],
In summary, the major effect of addition of alumina is that of stabilising highly dispersed Ce02-Zr02; as a consequence the inhibition of the particle growth seems to be responsible for the lack of separation after calcination at 1273 K. As shown in Table 6.3, the addition of both A1203 and Zr02 to Ce0 2 plays a key role in improving the thermal stability of these systems, which is due to the fact that: each component, i.e. AI2O3, and Zr02, is able to retard particle sintering of the Ce0 2 based oxide, however, the combination of the two factors is particularly effective, giving comparable dynamic-OSC in Ce02/Al203 and Ceo.2Zro.gO2/Al2O3 even if much less Ce0 2 is contained in the latter sample. High content of Zr02 also plays an important role in determining the thermal stability of these systems. Consistently, when the calcination temperature for a comparable Ceo.6Zro.4O2C 13 wt%)/Al203 sample is increased up to 1373 K, the sintering becomes more effective and, accordingly, phase separation is detected [114]. Acknowledgments. Drs. Roberta Di Monte and Neal Hickey, University of Trieste are gratefully acknowledged for helpful discussions. Financial support from University of Trieste, CNR (Roma) Programmi Finalizzati "Materiali Speciali per Tecnologie Avanzate II, Contract n. 97.00896.34, Regione
Catalysis by ceria and related
236
materials
Friuli Venezia-Giulia, Fondo regionale per la ricerca L.R. 3/1998, and Fondo Trieste - 1999 is acknowledged.
6.5. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.
Duwez, P. and Odell, F., J.Amer.Ceram.Soc. 33 (1950), 274-283. McHale, A.E. Phase diagrams for ceramists. Annual 1991 (Amer.Ceram.Soc, Columbus, Ohio, 1991), 20. Leonov, A.I., Andreeva, A.B. & Keler, E.K., Izv.Akad.Nauk SSSR, Neorg.Mater. 2 (1966), 137-144. Leonov, A.I., Keler, E.K. and Andreeva, A.B., Ogneupory 3 (1966), 42-48. Pal'guev, S.F., Alyamosvkii, S.I. and Volchenkova, Z.S., Russ.J.Inorg.Chem. 4(1959), 1185-1188. Longo, V. and Roitti, S., Ceramurgia Int. 1 (1971), 4-10. Pepin, J.G. and Vance, E.R., Phys.Stat.Sol.(a) 67 (1981), K167-K169. Tani, E., Yoshimura, M. and Somiya, S., J.Amer.Ceram.Soc. 66 (1983), 506510. Yashima, M., Takashina, H., Kakihana, M. and Yoshimura, M., J.Amer.Ceram.Soc. 77(1994), 1869-1874. Meriani, S., Mater Sci Eng 71 (1985), 369-373. Meriani, S., J.De Physique 47 (1986), Cl-485. Meriani, S. and Spinolo, G., Powder Diffraction 2 (1987), 255-256. Meriani, S., Mater Sci Eng A-Struct Mater 109 (1989), 121-130. Duran, P., Gonzales, M., Moure, C , Jurdo, J.R. and Pascal, C., J.Mater.Sci. 25 (1990), 5001. Yashima, M., Kakihana, M. and Yoshimura, M., Solid State Ionics 86-8 (1996), 1131-1149. Yashima, M., Hirose, T., Kakihana, M., Suzuki, Y. and Yoshimura, M., J Amer.Ceram Soc 80 (1997), 171-175. Smith, D.K. and Newkirk, H.W., Acta Crystallographica 18 (1965), 983-991. Li, P., Chen, I.W. and Penner-Hahn, J.E., Phys.Rev.B 48 (1993), 1006310073. Ho, S.M., Mater.Sci.Eng.A-Struct.Mater. 54 (1982), 23. Shannon, R.D., Acta Crystallographica A32 (1976), 751. Li, P., Chen, I.W. and Penner-Hahn, J.E., J.Amer.Ceram.Soc. 11 (1994), 118128.
Structural properties and thermal stability of ceria-zirconia
237
22. Yashima, M., Morimoto, K., Ishizawa, N. and Yoshimura, M., J.Amer.Ceram.Soc. 76(1993), 1745-1750. 23. Yashima, M., Morimoto, K., Ishizawa, N. and Yoshimura, M., J.Amer.Ceram.Soc. 76 (1993), 2865-2868. 24. Yashima, M., Ohtake, K., Arashi, H., Kakihana, M. and Yoshimura, M., J.Appl.Phys. 74 (1993), 7603-7605. 25. Yashima, M., Arashi, H., Kakihana, M. and Yoshimura, M., J.Amer.Ceram.Soc. 77(1994), 1067-1071. 26. Fornasiero, P., Balducci, G, Di Monte, R., et al. J.Catal. 164 (1996), 173183. 27. Garvie, R.C., J.Phys.Chem. 69 (1965), 1238. 28. Garvie, R.C., J.Phys.Chem. 82 (1978), 218. 29. Garvie, R.C. and Goss, M.F., J.Mater.Sci. 21 (1986), 30. Mitsuhasi, T., Ichihara, M. and Tatsuke, U., J.Amer.Ceram.Soc. 57 (1974), 97. 31. Livage, J., Doi, K. and Mazieres, C , J.Amer.Ceram.Soc. 51 (1968), 349. 32. Keramidas, V.G. and White, W.B., J.Amer.Ceram.Soc. 57 (1974), 22-24. 33. Chatterjee, A., Pradhan, S.K., Datta, A., De, M. and Chakravorty, D., J.Mater.Res. 9 (1994), 263-265. 34. Yoldas, B.E., J.Amer.Ceram.Soc. 65 (1982), 387. 35. Stefanic, G., Popovic, S. and Music, S., Thermochim.Acta 303 (1997), 31-39. 36. Longo, V. and Minichelli, D., J.Amer.Ceram.Soc. 56 (1973), 1186-1187. 37. Hirano, S., Kawabata, A., Yoshinaka, M., Hirota, K. and Yamaguchi, O., J.Amer.Ceram.Soc. 78(1995), 1414-1416. 38. Kawabata, A., Hirano, S., Yoshinaka, M., Hirota, K. and Yamaguchi, O., J.Mater.Sci. 31 (1996), 4945-4949. 39. Otsuka-Yao-Matsuo, S., Omata, T., Izu, N. and Kishimoto, H., J.Solid.State.Chem. 138 (1998), 47-54. 40. Izu, N., Omata, T. and Otsuka-Yao-Matsuo, S., J.Alloys Comp. 270 (1998), 107-114. 41. Izu, N., Kishimoto, H., Omata, T. and OtsukaYaoMatsuo, S., J.Solid State Chem. 151 (2000), 253-259. 42. Vlaic, G., Fornasiero, P., Geremia, S., Kaspar, J. and Graziani, M., J.Catal. 168 (1997), 386-392. 43. Vlaic, G., Di Monte, R., Fornasiero, P., Fonda, E., Kaspar, J. and Graziani, M., J.Catal. 182 (1999), 378. 44. Hirata, T., JPhys Chem Solids 56 (1995), 951-957. 45. Yashima, M., Sasaki, S., Yamaguchi, Y., Kakihana, M., Yoshimura, M. and Mori, T., Appl.Phys.Lett. 72 (1998), 182-182.
238
Catalysis by ceria and related materials
46. Enzo, S., Delogu, F., Frattini, R., Primavera, A. and Trovarelli, A., J.Mater.Res. 15(2000), 1538-1545. 47. Colon, G., Pijolat, M., Valdivieso, F., et al., J.Chem.Soc, Faraday Trans. 94 (1998), 3717-3726. 48. Colon, G., Valdivieso, F., Pijolat, M., Baker, R.T., Calvino, J.J. and Bemal, S., Catal.Today 50 (1999), 271-284. 49. Chiang, H.W., Blumenthal, R.N. and Fournelle, R.A., Solid State Ionics 66 (1993), 85-95. 50. Yashima, M., Ishizawa, N. and Yoshimura, M., J.Amer.Ceram.Soc. 75 (1992), 1550-1557. 51. Yashima, M., Ishizawa, N. and Yoshimura, M., J.Amer.Ceram.Soc. 75 (1992), 1541-1549. 52. Kim, D.J., J.Amer.Ceram.Soc. 72 (1989), 1415-1421. 53. Glushkova, V.B., Hanic, F. and Sazonova, L.V., Ceramurgia Int. 4 (1978), 176-178. 54. Shannon, R.D. and Prewitt, C.T., Acta Cryst. B26 (1970), 1046-1048. 55. Hori, C.E., Permana, H., Ng, K.Y.S., et al. Appl.Catal.B Environ. 16 (1998), 105-117. 56. Jen, H.W., Graham, G.W., Chun, W., et al., Catal.Today 50 (1999), 309-328. 57. Egami, T., Dmowski, W. and Brezny, R., SAE 970461 (1997), 58. Fornasiero, P., Di Monte, R., Kaspar, J., Montini, T. and Graziani, M., Stud.Surf.ScLCatal. 130 (2000), 1355-1360. 59. Fornasiero, P., Fonda, E., Di Monte, R., Vlaic, G., Kaspar, J. and Graziani, M., J.Catal. 187(1999), 177-185. 60. Torng, S., Miyazawa, K., Suzuki, K. and Sakuma, T., Phil Mag A 70 (1994), 505-517. 61. Mamontov, E., Egami, T., Brezny, R., Koranne, M. and Tyagi, S., J.Phys.Chem.B 104 (2000), 11110-11116. 62. Vidmar, P., Fornasiero, P., Kaspar, J., Gubitosa, G. and Graziani, M., J.Catal. 171 (1997), 160-168. 63. Ranga Rao, G., Kaspar, J., Di Monte, R., Meriani, S. and Graziani, M., Catal.Lett. 24 (1994), 107. 64. Ranga Rao, G., Fornasiero, P., Di Monte, R., et al., J.Catal. 162 (1996), 1-9. 65. Daturi, M., Finocchio, E., Binet, C , et al., J.Phys.Chem.B 104 (2000), 91869194. 66. Magistris, A., Chiodelli, G., Sergo, V. and Meriani, S., Thermochim.Acta 133 (1988), 113-118.
Structural properties and thermal stability of ceria-zirconia
239
67. Balducci, G., Fomasiero, P., Di Monte, R., Kaspar, J., Meriani, S. and Graziani, M., Catal.Lett. 33 (1995), 193-200. 68. Otsuka-Yao-Matsuo, S., Izu, N., Omata, T. and Ikeda, K., J.Electrochem.Soc. 145(1998), 1406-1413. 69. Omata, T., Kishimoto, H., OtsukaYaoMatsuo, S., Ohtori, N. and Umesaki, N., J.Solid.State.Chem. Ul (1999), 573-583. 70. Thomson, J.B., Amstrong, A.R. and Bruce, P.G., J.Solid State Chem. 148 (1999), 56-62. 71. Thomson, J.B., Amstrong, A.R. and Bruce, P.G., J.Am.Chem.Soc. 118 (1996), 11129-11133. 72. Thomson, J.B., Amstrong, A.R. and Bruce, P.G., J.Chem.Soc.Chem.Commun. (1996), 1165-1166. 73. Baker, R.T., Bernal, S., Blanco, G., et al., Chem.Commun. (1999), 149-150. 74. Vidal, H., Kaspar, J., Pijolat, M., et al., Appl.Catal.B Environ. 27 (2000), 4963. 75. Vidal, H., Kaspar, J., Pijolat, M., et al., Appl.Catal.B Environ. 30 (2001), 7585. 76. Otsuka-Yao, S., Morikawa, H., Izu, N. and Okuda, K., J.Japan Inst.Metals 59 (1995), 1237-1246. 77. Ikryannikova, L.N., Aksenov, A.A., Markaryan, G.L., et al., Appl.Catal.A.Gen. 210 (2001), 225-235. 78. Heussner, K.H. and Claussen, N., J.Amer.Ceram.Soc. 72 (1989), 1044-1046. 79. Zhu, H.Y., Hirata, T. and Muramatsu, Y., J.Amer.Ceram.Soc. 75 (1992), 2843-2848. 80. Miki, T., Ogawa, T., Haneda, M., et al., J.Phys.Chem. 94 (1990), 6464-6467. 81. Cho, B.K., J.Catal. 131 (1991), 74-87. 82. Kaspar, J., Graziani, M. and Fomasiero, P. in Handbook on the Physics and Chemistry of Rare Earths: The Role of Rare Earths in Catalysis., eds. Gschneidner, Jr.K.A. and Eyring, L. (Elsevier Science B.V., Amsterdam, 2000), 159-267. 83. Sorensen, O.T. in Nonstoichiometric Oxides, ed. Sorensen, O.T. (Academic Press, New York, 1981), 1-59. 84. Martin, R.L., J.Chem.Soc.Dalton.Trans. (1974), 1335-1350. 85. Hoskins, B.F. and Martin, R.L., Aust.JChem. 48 (1995), 709. 86. Kang, Z.C. and Eyring, L., J.Alloys Comp. 249 (1997), 206-212. 87. Kang, Z.C. and Eyring, L., Aust.JChem. 49 (1997), 981-996. 88. Kang, Z.C. and Eyring, L. in Oxides, ed. Boulesteix, C. (Trans Tech Publicati, W-3392 Clausthal Zel, 1998), 301-358.
240
Catalysis by ceria and related materials
89. Etsell, T.H. and Flengas, S.N., Chem.Rev. 70 (1970), 339-376. 90. Tuller, H.L. and Nowick, A.S., J.Electrochem.Soc. 122 (1975), 255-259. 91. Butler, V., Catlow, C.R.A., Fender, B.E.F. and Harding, J.H., Solid State Ionics 8 (1983), 109-113. 92. Li, C , Xin, Q. and Guo, X.X., Catal.Lett. 12 (1992), 297-306. 93. Sayle, T.X.T., Parker, S.C. and Catlow, C.R.A., J.Chem.Soc.Chem.Commun. (1992), 977-978. 94. Chen, P.L. and Chen, I.W., J.Amer.Ceram.Soc. 77 (1994), 2289-2297. 95. Hong, S.J. and Virkar, A.V., J.Amer.Ceram.Soc. 78 (1995), 433-439. 96. Yahiro, H., Eguchi, K. and Arai, H., Solid State Ionics 21 (1986), 37-47. 97. Thompson, J.G., Rae, A.D., Bliznyuk, N. and Withers, R.L., J.Solid State Chem. 144(1999), 240-246. 98. Thompson, J.G., Withers, R.L. and Brink, F.J., J.Solid State Chem. 143 (1999), 122-131. 99. Pijolat, M., Prin, M., Soustelle, M., Touret, O. and Nortier, P., JChem.Soc, Faraday Trans. 91 (1995), 3941-3948. 100. Dauscher, A., Wehrer, P. and Hilaire, L., Catal.Lett. 14 (1992), 171-183. 101. Rocchini, E., Trovarelli, A., Llorca, J., et al., J.Catal 194 (2000), 461-478. 102. Zamar, F., Trovarelli, A., de Leitenburg, C. and Dolcetti, G., J.Chem.Soc.Chem.Commun. (1995), 965-966. 103. Bernal, S., Blanco, G., Botana, F.J., et al. J.Alloys Comp. 207 (1994), 196200. 104. Tanaka, H. and Yamamoto, M., SAE 960794 (1996), 233-240. 105. Kirichenko, O.A., Graham, G.W., Chun, W. and McCabe, R.W, Stud.Surf.Sci.Catal. 118 (1998), 411-420. 106. Markaryan, G.L., Ikryannikova, L.N., Muravieva, G.P., et al. Colloid Surface ^151 (1999), 435. 107. Terribile, D., Trovarelli, A., de Leitenburg, C , Primavera, A. and Dolcetti, G., Catal.Today 47 (1999), 133-140. 108. Liu, W. and Flytzani-Stephanopoulos, M., J.Catal. 153 (1995), 317-332. 109. Ananthapadmanabhan, P.V., Venkatramani, N., Rohatgi, V.K., Momin, A.C. and Venkateswarlu, K.S., J.Eur.Ceram.Soc. 6 (1990), 111-117. 110. Bernal, S., Blanco, G., Cifredo, G.A., Perez-Omil, J.A., Pintado, J.M. and Rodriguez-Izquierdo, J.M., J.Alloys Comp. 250 (1997), 449-454. 111. Kaspar, J., Di Monte, R., Fornasiero, P., Graziani, M., Bradshaw, H. and Norman, C , Top.Catal. 000 (2001), 112. Nunan, J.G., SAE 970467 (1997), 77-86.
Structural properties and thermal stability ofceria-zirconia
241
113. Di Monte, R., Kaspar, J., Fomasiero, P., Ferrero, A., Gubitosa, G. and Graziani, M., Stud.Surf.Sci.Catal. 116 (1998), 559-569. 114. Di Monte, R., Fomasiero, P., Kaspar, J., et al., Chem.Commun. (2000), 21672168. 115. Di Monte, R., Fomasiero, P., Kaspar, J., Rumori, P., Gubitosa, G. and Graziani, M., Appl.Catal.B Environ. 24 (2000), 157-167. 116. Masui, T., Fujiwara, K., Peng, Y.M., Machida, K. and Adachi, G.Y., Chem.Lett. (1997), 1285-1286. 117. Yao, M.H., Baird, R.J., Kunz, F.W. and Hoost, T.E., J.Catal. 166 (1997), 6774. 118. FernandezGarcia, M., MartinezArias, A., IglesiasJuez, A., et al., J.Catal 194 (2000), 385-392. 119. Yao, H.C. and Yu Yao, Y.F., J.Catal. 86 (1984), 254-265. 120. Shyu, J.Z., Weber, W.H. and Gandhi, H.S., J.Phys.Chem. 92 (1988), 49644970. 121. Miki, T., Ogawa, T., Ueno, A., Matsuura, S. and Sato, M., Chem.Lett. (1988), 565-568. 122. Horiuchi, T., Teshima, Y., Osaki, T., Sugiyama, T., Suzuki, K. and Mori, T., Catal.Lett. 62(1999), 107-111. 123. Piras, A., Trovarelli, A. and Dolcetti, G., Appl.Catal B Environ. 28 (2000), L77-L81. 124. Oudet, F., Courtine, P. and Vejun, A., J.Catal. 114 (1988), 112-120.
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CHAPTER 7 OXYGEN STORAGE/REDOX CAPACITY AND RELATED PHENOMENA ON CERIA-BASED CATALYSTS
DANIEL DUPREZ and CLAUDE DESCORME Laboratoire de Catalyse en Chimie Organique, UMR 6503 CNRS, Universite de Poitiers, 40 Avenue du Recteur Pineau, 86022 Poitiers Cedex, France.
7.1. Introduction Ceria-based catalysts are widely used for gasoline-fuelled engines exhaust gas treatment. The so-called Three-Way Catalysts (TWC) must simultaneously reduce NOx and oxidize CO and hydrocarbons. An optimum conversion of these pollutants is only obtained around the stoichiometry ("operating window"). Nevertheless, under real driving conditions, the exhaust gas composition may vary drastically. Consequently, if one wants to maintain constant the overall efficiency of TWC, materials used as supports will be required to store oxygen under lean conditions (excess oxygen) and to release oxygen under rich conditions (excess fuel). If so, post-treatment catalysts will be efficient even under oscillatory conditions generated by the electronic control of the Air/Fuel ratio (= 1 Hz oscillations) as well as by the rapid changes in both driving and traffic conditions (s 0.1 Hz oscillations). This property of oxides supports is evaluated via Oxygen Storage Capacity (OSC) measurements.
7.2. Oxygen Storage Capacity Measurements Oxygen storage measurements consist in the determination of the amount of a reducing gas (H2, CO) which is oxidized after passing through an oxygen presaturated catalyst. It deals with the study of a reducer oxidation under transient conditions and in the absence of gaseous oxygen (anaerobic oxidation). OSC measurements may be carried out in many different ways, depending essentially on the experimental conditions. The first option depends on how fast the gas composition is perturbed. One may distinguish lab-scale low frequency 243
244
Catalysis by ceria and related materials
measurements and high frequency measurements under more realistic operating conditions. The second option comes from the reducer selected as a probe to test oxygen lability.
7.2.1. OSC Measurements at Low Frequency Oxygen Storage Capacity may be determined under dynamic conditions according to the method introduced by Yao and Yu Yao [1] and further adapted by Duprez [2]. Two different measurements of the oxygen storage capacity may be distinguished: - the OSC (Oxygen Storage Capacity) is related to the most reactive oxygen species and the most readily available oxygen atoms. OSC may characterize the dynamic of the system. - the OSCC (Oxygen Storage Capacity Complete) is the total or maximum oxygen storage capacity. OSCC contains information about the overall reducibility of the solid. OSC and OSCC measurements can be carried out in a pulse chromatographic system which was described earlier [2]. The experimental setup is schematized below on Fig. 7.1.
Figure 7.1. Flow-sheet of the apparatus used for OSC measurements at low frequency.
The sample (typically 0.01 to 0.05g) is introduced in a U-shaped fixed bed
Oxygen storage/redox capacity and related phenomena
245
reactor and heated up to the temperature of measurement (4°C.min1) under flowing He (30cm3 .min"). The catalyst is further pretreated in situ under O2. Oxygen Storage Capacities may be measured in many different ways depending on the operating conditions but also on the nature of the reducer and the oxidant. Four different combinations are reviewed hereafter.
7.2.1.1. Transient CO Oxidation by 0 2 The most common oxidant/reducer system is probably the one where CO is used as the reducer and 0 2 as the oxidant. In our experimental setup, oxygen storage measurements take place in three steps. After 10 min outgassing, CO pulses (typically 0.2 to 0.3 cm3) are injected every 2 min up to a maximum reduction of the sample: phase 1. Ce0 2 + xCO <-• CeO(2.x) + x C 0 2
(7.1)
Then, after another 10 minutes outgassing, five pulses of 0 2 are injected every 2 minutes up to a maximum re-oxidation of the solid: phase 2. 2 CeO(2-x) + x 0 2 <-> 2 CeOj
(7.2)
In all cases the fraction of 0 2 or CO consumed by the catalyst is smaller than 50%. Finally, to simulate lean and rich working conditions of an engine, alternate pulses of CO or 0 2 are injected: phase 3. OSC and OSCC are expressed either in umolCO.g'1 from the CO consumption or in umolO.g"1 from the oxygen consumption. Many studies aimed to the formulation of post-combustion catalysts with higher OSC. Ceria proved to be a good candidate as a TWC support with enlarged OSC and satisfactory behavior under dynamic conditions [1,3-7] but a poor thermal stability. The improvement of the thermal stability of ceria by introduction of zirconium has been widely studied [8-10] and "estimated" by measuring the surface area of mixed oxides after high temperature thermal treatment. In the case of mixed oxides prepared by co-precipitation for example, an increase of the "preserved" surface area of aged oxides was observed even with the introduction of only 10 at.-% of
Catalysis by ceria and related materials
246
zirconium. A maximum stabilization was obtained with 60 at.-% Zr [11]. The same kind of results were obtained by Madier [12] with surface areas after calcination at 900°C for 6 hours of 26, 40, 41, 44, 27 and 12 m2.g-' for Ce0 2 , Ceo.6sZro.32O2, Ceo.63Zro.37O2, Ceo.5Zro.5O2, Ceo.15Zro.85O2 and Zr0 2 respectively. The OSC at 400°C of this series of cerium-zirconium mixed oxides pre-calcined at 900°C was measured. Fig. 7.2 represents the variations of the OSC vs. cerium content.
0
20
40
60
80
100
Ce (%)
Figure 7.2. Oxygen storage capacity at400°C of CexZr(,.x)02 samples [13].
The incorporation of zirconium into the Ce0 2 lattice greatly improves the OSC. An optimum was observed for CexZr(i.X)02 oxides with x between 0.6 and 0.8 [1118]. All these oxides have a fluorite-type structure, which appears to be an important parameter. The largest OSC is obtained for Ceo.63Zro.37O2 with 219 umolO.g"1 (5.5 umolO.m"2), which OSC was calculated to be 4 times larger than the OSC of pure ceria. In fact, similar results are found in the literature. Trovarelli et al. [16] studied oxygen storage (OSC) on CexZr(1.x)02 solid solutions prepared by mechanical milling of the parent oxides (ceria and zirconia). The maximum 0 2 uptake was observed for solids with 0.6 < x < 0.8 and an OSCC at 377°C of approximately 185 umol02.g"'. The same trend was also observed by Cuif et al. [11,15] with an optimum OSC for Ceo.6Zro.4O2. After some simple calculations one can see that in the case of Ce0 2 the storage of oxygen at 400°C is restricted to the surface. On the opposite, in the case of Ceo.63Zro.37O2, oxygen storage takes place not only at the surface but also in the bulk. The specificity of mixed oxides for oxygen storage was shown to be the
Oxygen storage/redox capacity and related phenomena
247
participation of bulk oxygen. In fact, at least one sub-surface oxygen layer is involved in the storage process [12,13]. It was further proposed that the introduction of zirconium induces the formation of structural defects [19-29] associated with oxygen vacancies [30]. All these results also showed that the introduction of small amounts of zirconium into the Ce0 2 lattice was responsible for such a high oxygen mobility. Furthermore, it was clearly shown that noble metals, rhodium in particular, play an active role in promoting the OSC of the support [1,3-6,31]. It was shown for example that only Rh can really promote OSC on alumina catalysts [32,33]. Nevertheless the situation changes when ceria is added to alumina. A comparative study of alumina and ceria-alumina supported bimetallic catalysts [32,33] demonstrated the differences between those systems. Ceria-alumina catalysts were shown to have higher OSC values which do not depend on the composition of the bimetallic (Fig. 7.3). 220 B
200
s, 1 18° »
160
|8
140
*S
120
™
100
o O |
80 60 40 0
10
20
30
40 SO 60 X at.%(Rh/Pt+Rh)
70
80
90
100
Figure 7.3. Effect of the metallic phase composition on the OSC at 450°C for alumina (triangles) and ceria-alumina (squares) supported bimetallic PtRh catalysts.
Furthermore, a closer look to experimental data and a tentative balance between CO consumption, C0 2 formation and 0 2 uptake showed the irreversible loss of oxygen on Pt/Al 2 0 3 as well as the significant retention of carbon on ceria-alumina supports. In fact surface carbonated species were clearly identified by FT-IR spectroscopy (1200-1600 cm"1) upon CO adsorption [34-36]. An even more detailed study of Pt/Al203 and Pt/Ce0 2 systems [37] evidenced that the OSC of alumina systems mainly originates from the WGS reaction with alumina surface OH groups. Furthermore, in the case of ceria-supported catalysts, three contributions to the OSC could be distinguished: the OSC related to the metal particle (M/MO), the OSC of ceria and the OSC originating from the metal-ceria contact.
Catalysis by ceria and related materials
248
Finally a study on the effect of temperature on the OSC was performed on Pt/Al203, Rh/Al203, Pt/Ce0 2 -Al 2 0 3 and Rh/Ce0 2 -Al 2 0 3 catalysts [32,33]. For Pt/Al 2 0 3 almost no influence of the temperature could be detected (Fig. 7.4).
JS.-
"•'
• - - ' *•
'm\s^
s
A
s
**
*'/
tf
**
.'* ^ . D
••'/ ' * flT D-^CS-.--*
D-' '
~a-
100
^ 150
,
,
250
,
300
.-
,
400
,
450
= CC)
Figure 7.4. Effect of temperature on the OSC of alumina (open symbols) and ceria-alumina (closed symbols) supported Rh (squares) and Pt (triangles) catalysts.
For the other three catalysts it was shown that below 250°C the OSC is approximately equal to the OSCC and that the storage process is certainly restricted to the metal itself or to its close perimeter. Above 250°C, farther oxygen atoms of the support become activated and may react. This oxygen mobilization step is easier on ceria-containing supports so that the increase in OSC with temperature is faster even at lower temperature than on alumina. The influence of temperature on the OSC was also examined on four different ceria- or ceria-zirconia-supported Rh or Ir catalysts. The results are presented on Fig. 7.5.
Figure 7.5. Evolution of the oxygen storage capacity (OSC) as a function of temperature. Effect of the metal ( • : Ir, • : Rh) and the support (Ce0 2 : broken lines, Ceo.63Zro.37O2: full lines).
Oxygen storage/redox
capacity and related
phenomena
249
These results evidence that OSC is larger for Ir-based catalysts than for Rh catalysts in the whole temperature range from 200 to 500°C. Additionally, OSC for ceria-supported catalysts varies slightly with temperature and reaches a maximum after full reduction of the surface. In this case, OSC appears to be limited by surface diffusion. On the opposite, for ceria-zirconia supported catalysts, OSC is multiplied by a factor of 3 to 4 - depending on the metal - between 200 and 500°C. Bulk reduction is then responsible for such a large increase of the OSC. In that case, oxygen storage would be limited by bulk diffusion. The influence of the nature of the metal particles was also analyzed. Many noble metal/ceria-containing supports combinations were tested [12,38,39]. 1200 -
.>.
1000 -
3
800 -
\» cT 2
600 -
8
400-
O
E
11
200 0 no metal
Rh
Pt
Pd
Ru
Ir
Figure 7.6. Evolution of the OSC at 400°C as a function of both the support (Ce0 2 in white or Ceo.63Zro.37O2 in black) and the metal (Rh, Pt, Pd, Ru or Ir).
These results confirmed that the presence of metal particles clearly enhance the OSC of these materials. OSC measured at 400°C could be increased by a factor of 3 to 7. With both supports, iridium and ruthenium were shown to be the most effective metal to enlarge OSC. 7.2.1.2. Transient H2 Oxidation by O2 H2 may also replace CO as a reducer. Different methods have been developed for such measurements. Yamada et al. [40] measured the OSCC of Ce-Zr-Y oxides from the change in weight between the reduced (20%H2/N2 - 7min) and the oxidized (50%O2/N2 - 7 minutes) sample. Weight variations were evaluated using a thermobalance. OSCC at 500°C of fresh Ce-Zr-Y bare oxides appeared to be a function of the cerium content. OSCC decreases as the concentration in cerium decreases. Surprisingly, no real
250
Catalysis by ceria and related materials
influence of the nature of the metal (Rh, Pt, Pd) was observed. Measurements performed on aged catalysts (reach-lean perturbations - 1000°C - 1 hour) evidenced the positive effect of Y on the stability of oxygen storage performances of such CeZr-Y systems. In fact the OSCC of Pt/Ce0 2 was reduced to zero while for yttriumdoped samples the OSCC remained almost unchanged or even increased for highly substituted oxides (Ceo.2Zro.6Y02). These authors concluded that the improvement of the OSCC correlates with the absence of crystallite growth, that is no sintering. Using a pulse method (5 ml pulses of 1%02/He and 10 ml pulses of 1%H2/He), Miki et coll. [41] investigated the OSC of a full series of La203/Ce02/Al203 samples. When precious metal were present at the surface of these oxides, addition of La 2 0 3 had a positive effect on OSC. An optimum was found for a (La203/La203+Ce02) ratio equal to 0.25. Furthermore the influence of precious metal was confirmed by the absence of synergism when an alumina-supported precious metal catalyst was simply physically mixed with a La203/Ce02/Al203 oxide. The influence of the synthesis route to prepare Ce02-based solid solutions was also evaluated using OSCC measurements at 427°C after pre-reduction of the samples at 527°C for lh [42]. Introduction of isovalent cations (Zr, Hf, Tb) in the ceria lattice appeared to have a positive effect on oxygen storage. Such effect was tentatively interpreted by the creation of defects in the oxide structure and by the stress induced by high energy mechanochemical milling during the preparation. Both would result in an enhanced oxygen mobility. Finally Trovarelli et al. [16] studied nanophased fluorite-structured Ce0 2 -Zr0 2 catalysts prepared by high-energy mechanical milling using OSC measurements at 377°C. Experiments consisted in alternating 250 ul pulses of 1%02/He and hydrogen every 3 minutes. OSC was shown to be almost constant when the concentration in ceria exceeded 50%. Nevertheless, a maximum was observed for Ceo.9Zro.1O2 with an OSC of 6.5 itmol^.g" 1 . These observations were correlated with both structural features and the presence of highly reducible ions favoring oxygen mobility.
7.2.1.3. Transient HC Oxidation by 0 2 Hydrocarbons may also be used as reducers. Both the OSC and OSCC of a PtRh/Ce0 2 -Al 2 0 3 catalyst was measured by pulsing propane and oxygen at 450°C [43]. The results are presented in Table 7.1.
Oxygen storage/redox
capacity and related
phenomena
251
Table 7.1. OSC measurement by propane transient oxidation at 450°C and subsequent reoxidation by oxygen at 450°C on a PtRh/Ce02-Al203 catalyst [43].
Pulse # C3H8 1 2 3 4 5 Total
37.2 15.8 7.9 3.0 2.8 64.9
C3H8 pulses H2 CO CH4 C0 2 C2H4
0 2 pulses 0 / C0 2
C producedb consumed0 7.5 11.8 18.2 12.1 18.0 78.1 111.6 68.4 26.3 8.5 6.7 8.6 13.0 0.3 38.9 47.5 1.7 0 6.1 6.1 9.1 0.2 4.4 24.2 23.7 0.5 0 5.8 5.2 7.9 0.1 3.2 19.6 9.0 0 0 16.2 8.4 5.5 4.6 7.0 0 2.3 0 0 31.6 36.3 55.8 12.2 36.4 177.0 200.2 70.7 26.3
* consumed b Cpr^ced = I C in the products (CO + CH4 + C0 2 + 2xC2H4) c Consumed = 3 x amount of C3H8 converted
First of all, one can easily see that propane is not as efficient as CO as a reducer. After a sequence of 5 pulses of propane, 60.7 umoles of oxygen atoms (for the solid) per gram of solid are consumed while after the 5 following oxygen pulses, almost 89 umolO.g"1 are used for the reoxidation of the sample. This means that the catalyst is fully reoxidized. Furthermore, looking at the carbon balance, we observe the formation of a carbon deposit during C3H8 pulses (CCOnsumed>CProduced) which is reoxidized upon 0 2 pulses with the formation of CO2. Comparable results were also obtained with CH4 as a reducer [44]. In the case of two preoxidized PtRh/Al203 and PtRh/Ce02 catalysts, the authors could show that the oxygen stored in the sample may oxidize methane at temperatures as low as 350°C. As previously seen, CH4 is also much less efficient as a reducer than CO and only 1/4 of the CO2 is produced compared to CO. Moreover, ceria is confirmed as a good oxygen provider as twice as much methane is oxidized on PtRh/Ce02 compared to the alumina supported catalyst (Table 7.2). Table 7.2. OSC measurements using methane transient oxidation at 450°C on PtRh/Al203 and PtRh/Ce02-Al203 catalysts.
Catalyst PtRh/Al203 PtRh/Ce0 2 -Al 2 0 3
O2 consumption (umolQ2.g"') 39
CH4 consumption CO2 production (umol.g'1) (umol.g'1) 33 33 72 69
252
Catalysis by ceria and related
materials
7.2.1.4. Transient CO Oxidation by NO In real working conditions and during rich phases, up to 0.4% NO is emitted and may act as an oxidant. So, it appeared interesting to carry out OSC measurements using NO instead of 0 2 and to see how far NO may substitute itself to oxygen during the storage process [33]. On RhPt/Al203 and RhPt/Ce02-Al203, it was shown that at 450°C NO is selectively reduce to N2 and that its efficiency to reoxidize the sample pre-reduced by CO is equal to 0 2 (Fig. 7.7).
0
in 10
25
100
% Rh in the bimetallic
Figure 7.7. Effect of the nature of the oxidant on the OSC at 450°C (0 2 in black, NO in white).
The same conclusions had already been obtained from the study of the transient response of alumina and ceria-supported rhodium catalysts in the NO+CO reaction under symmetric composition-cycling experiments at 500°C [45]. NO decomposes and the oxygen produced is stored on the catalyst. This oxygen may further react with CO to give C0 2 . It was shown that ceria-supported catalysts can tolerate longer cycling period for a given performance requirement. This improvement is attributed to the larger OSC of ceria. 7.2.2. OSC Measurements at High Frequency The apparatus used for these experiments was dedicated to the study of the CO+0 2 reaction which was carried out under transient conditions at any given frequency close to 1 Hz. This system consists in two parts: the gas manifold and the mass spectrometer. The global experimental setup is schematized on Fig. 7.8.
Oxygen storage/redox capacity and related phenomena
253
Figure 7.8. Experimental setup used for OSC measurements at high frequency.
Reactants (CO and 0 2 ) flows are regulated using mass flow controllers. Before the reactor entrance, reactants were mixed with a l%Ar/He carrier gas (100 cm3.min"'). Any kind of CO/(l%Ar/He) and 0 2 /(l%Ar/He) mixtures could be obtained. For this study 2%CO and 1%02 mixtures were used. Reactant injections were controlled using automated injection valves. Alternate CO and 0 2 injections may take place at a frequency as high as 1 Hz. To optimize the response time, the dead volume was reduce to less than 2.5 cm3, including the whole reaction circuit from the automated valve up to the capillary tube before the mass spectrometer. The catalyst, diluted in cordierite, is placed in a straight Pyrex reactor and pretreated under pure helium at 450°C (ramp rate = 2°C.min"1) for 15 minutes. Reactions are carried out at 450°C. Experiments consist in 5 different steps, as presented on Fig. 7.9. (i) OSC measurement at 450°C under transient conditions: alternate pulses of 2%CO or 1%02 at a frequency of 1 or 0.5 Hz for 10 minutes. Carrier gas is 1% Ar/He. The last pulse is 0 2 .
Catalysis by ceria and related
254
materials
2%CO
l%Ar
co2 50
100
i%o.
r V
15 150 ' ' 266' 256' ' 366' ' 350 ' 400 t(s)
Figure 7.9. General scheme of the experimental sequence.
(ii) Outgassing under pure He for 30 seconds at 450°C (iii) Step change from 0 to 2%CO (2 minutes) at 450°C. Carrier gas is He/Ar. The Ar signal is used as a reference to deduced the dead volume of the reactor and to recalculate the inlet CO step change from 0 to 2% (iv) Outgassing under pure He for 30 seconds at 450°C (v) Step change from 0 to 1%02 (2 minutes) at 450°C. Carrier gas is again He/Ar. The Ar signal is used as a reference. The evolution of the gas phase composition is followed by mass spectrometry. Masses m/e = 4 (He), 28 (CO), 32 (0 2 ), 44 (C0 2 ) and 40 (Ar) are monitored every 0.1 second All the results presented below will be expressed per gram of solid. OSC measurements under transient conditions were carried out at 450°C. In all cases, CO2 formation occurs after every gas change. One CO2 formation peak (peak 1) appears going from 0 2 to CO. In this case the catalyst is partially oxidized before the introduction of CO. This C0 2 formation peak includes C0 2 coming from both the catalytic oxidation of CO in the presence of gaseous oxygen and the oxygen storage reverse reaction. The second C0 2 formation peak (peak 2) forms when CO is replaced by 0 2 in the gas feed. In that case, the catalyst is partially reduced. Here, the only reaction responsible for the formation of C0 2 is the catalytic oxidation of CO. These observations indicate that CO oxidation kinetics are governed by the catalyst state: oxidized or reduced. In fact, C0 2 formation related to the oxygen storage process only occurs on the oxidized catalyst when 0 2 is replaced by CO (peak 1). In the course of a switch from CO to 0 2 , oxygen storage occurs: an oxygen uptake is observed but no C0 2 forms. As an example, the evolution of C0 2 formation as a function of time is presented in Fig. 7.10 for Pd-based catalysts.
Oxygen storage/redox capacity and related
,
0 0 ! produced (vol-0/Q
255
CO; produced (vol-Vij
F=aSHz
F=lHz IUIi/ , 1
(180.6-
phenomena
-*^-WA|
Pteakl V
0.4-
Peak2
0.2-
n. 3
m
o
as
l
Figure 7.10. Evolution of C0 2 production upon reaction under transient conditions (0.5Hz and 1Hz) at 450°C on Pd-based catalysts.
Quantitative information may be obtained from the integration of CO, 0 2 and C0 2 profiles. From all those results we could conclude that: (i) mass balance is complete and no carbon is left over the surface (ii) oxygen storage is more sensitive to the catalyst state than the catalytic CO oxidation reaction. In fact, OSC depends on both the number and the nature of active sites. In that case, the nature of the metal (Pt, Pd or Rh) as well as the support (alumina or ceria-alumina) are important factors. Looking at OSC values, catalysts may be classified in descending order as follows: RhCA>PtRhCA>PdRhCA>PdCA>PtCA>RhA=PtRhA>PdRhA>PtAsPdA. Relative reactivities could be explained both by the introduction of ceria on the alumina support or by the substitution of Rh to Pt or Pd. The presence of ceria would induce a new reaction pathway through a bifunctional mechanism. C0 2 formation would derive from the reaction between an adsorbed CO molecule on the metal surface and an oxygen atom coming from the ceria surface in the vicinity of the metal particles. As a result, cerium oxide would reduce deactivation effects linked to metal sintering. Furthermore, at 450°C, Rh-based catalysts are more active than Pt and Pd-based catalysts. The activity of aged Rh-based catalysts would derive from both a high stability of Rh 2 0 3 particles towards sintering compared to Pt0 2 and PdO [7] and a better oxygen transfer from Ce0 2 to Rh [32,46]. In all cases, step changes (iii and v) showed that the sample re-oxidation by 0 2 (v) is much faster than the sample reduction by CO (iii).
256
Catalysis by ceria and related materials
7.2.3. Oxygen Buffering capacity More recently, Oxygen Buffering Capacity (OBC) measurements were presented as a new way to look at oxygen storage. OBC corresponds to the capability of any given material for attenuating fast oscillations (0.1 Hz) of the oxygen partial pressure. The results are expressed as a percentage corresponding to the ratio between the amplitude of the buffered oxygen concentration oscillations (outlet) and the amplitude of the "original" oxygen concentration variations (inlet). The first developments of this type of measurements were first introduced in 1974 by Keramidas et coll. and applied to the characterization of ceramic materials [47]. Later, this method was specifically applied to the characterization of oxygen storage materials for TWC and fully described by Bernal et al. [48]. Briefly, experiments consist in injecting 5%02/He pulses (0.25 ml) every 10 seconds into the inert gas flowing on the 200 mg sample (60 ml.min"1). Gas composition is followed using a gas chromatograph equipped with a TCD detector. Vidal et coll. [49] applied this technique to the characterization of Ceo.68Zro.32O2 mixed oxides (CZ-68/32). Three samples were characterized: (i) the high surface area (HS) starting material, (ii) the low surface area (LS) sample obtained after calcination under wet air at 900°C for 140 hours and (iii) the SR sample prepared by severe reduction under H2 at 850°C for 5 hours, evacuation under He at 950°C for 1 hour and re-oxidation at 550°C with 5%02/He for 1 hour. The results are presented in Table 7.3. Authors evidenced the positive effect on OBC of both temperature (400-900°C) and a severe reduction pretreatment. Table 7.3. OBC values (%) for Ceo.68Zro.32O2 samples pretreated under various conditions [Fig. 3 in ref. 49].
Samples CZ-68/32-HS CZ-68/32-LS CZ-68/32-SR
400°C 1 1 4
650°C 34 21 34
900°C 82 69 84
Furthermore the influence of mixed oxides composition and redox cycling on OBC was evaluated for two complete series of HS [50] and LS [51] samples (Tables 7.4 & 7.5). While pure ceria was completely inactive at almost any temperature, the best materials for oxygen buffering were shown to be CZ-68/32 and CZ-50/50 for both series. Additionally, redox cycling was shown to have little effect on HS samples
Oxygen storage/redox capacity and related phenomena
257
and a positive effect on LS samples, particularly for zirconium-rich samples. Considering these two sets of experiments along with the strong decrease of the surface area upon pretreatment, the authors concluded that redox cycling could compensate for the deterioration of the textural properties by making bulk and subsurface oxygen atoms more readily available for oxygen buffering. Table 7.4. OBC values (%) for Ceo.6sZro.32O2 samples pretreated in various conditions [Table 3 inref.50].
Samples CZ-80/20-HS CZ-80/20-HS CZ-68/32-HS CZ-68/32-HS CZ-50/50-HS CZ-50/50-HS
Number of redox cycles 0 3 0 3 0 3
400°C 2 5 1 3 5 4
650°C 28 23 34 36 36 46
900°C 78 76 82 85 83 89
Table 7.5. OBC values (%) for Ceo.6gZro.32O2 samples pretreated in various conditions [Table 3 inref.51].
Samples CZ-80/20-LS CZ-80/20-LS CZ-68/32-LS CZ-68/32-LS CZ-50/50-LS CZ-50/50-LS
Number of redox cycles 0 3 0 3 0 3
400°C 4 9 1 6 0 5
650°C 24 24 21 33 15 36
900°C 75 75 69 81 65 81
7.3. Elementary Steps Involved in OSC Processes
7.3.1. Inventory of Elementary Steps In the course of the oxygen storage process, different steps could be distinguished: - Oxygen activation on the metal particles (1) - Oxygen direct activation on the support (2) - Oxygen surface migration towards the "reaction" site (3) - Bulk oxygen migration towards the "reaction" site (4)
258
Catalysis by ceria and related materials
Such a reaction sequence may be schematized as follows:
© It
o2
Figure 7.11. Schematic of the different steps potentially involved in the oxygen storage process.
Consequently, a better understanding of the oxygen storage process will derive from a better understanding of both oxygen activation and migration on these samples. To tackle theses questions, the study of isotopic exchange reactions has been developed.
7.3.1.1. Principle of Oxygen Isotopic Exchange Measurements On oxide-supported metals, 18 0/ 16 0 exchange occurs through a sequence of well differentiated steps: 1. dissociative adsorption of 18 0 2 on the metal particle 2. transfer of 18 0 atoms from the metal to the support 3. surface migration of 18 0 atoms on the support 4. exchange of 18 0 atoms with 16 0 atoms of the surface 5. finally, every step i is coupled with step -i corresponding to the reverse route for the exchanged species. Depending on reaction temperature, two other steps may be involved: 6. internal (bulk) migration and exchange of 18 0 with 16 0 atoms of the support 7. direct exchange of 1802(g) with oxygen atoms of the support Strictly looking to oxygen surface migration on oxides, that is, assuming no bulk diffusion and no direct exchange, three types of exchange can occur according to either Boreskov [52], Winter [53] orNovakova [54]:
Oxygen storage/redox capacity and related phenomena
259
(a) - Homoexchange (Type I, R3 or R), without participation of oxygen atoms of the solid [52,55-59]: 18
02(g) +
16
02(g) -* 2 18 0 16 0(g)
(7.3)
(b) - Simple Heteroexchange (Type II, R1 or R'), between one oxygen atom of a dioxygen molecule and one oxygen atom of the solid: 18
02(g) +
16
0 (s) - •
18 16
0 0(g) +
18
0(s)
(7.4)
(c) - Multiple Heteroexchange (Type III, R2 or R"), between a dioxygen molecule and two oxygen atoms of the solid: 18
02(g) +
16 16
0 0(s) ->
16
02(g) +
18 18
0 0(s)
(7.5)
As a result, two types of reactions may be studied: homoexchange and heteroexchange and two different levels of information are accessed. (a) - homoexchange, also denoted "equilibration", when the adsorption-desorption on the metal particle is rate limiting. This type of exchange corresponds to an equilibration of two oxygen isotopomers at the catalyst surface. In most cases, this reaction is much more rapid on the metal than on the support. Later on, only the homoexchange reaction on the metal will be considered. Experiments are carried out with an initial equimolar mixture of 16 0 2 and 18 0 2 and the partial pressure in 16 0 18 0 is followed as a function of time. Equilibration experiments give useful information on the oxygen activation process at the surface of metallic particles. (b) - heteroexchange, simply denoted "isotopic exchange". In that case, pure 18 0 2 is initially introduced in the reactor and the formation of 18 0 16 0 and 16 0 2 is monitored. These measurements provide a direct estimation of the oxygen surface migration kinetics. Nevertheless, three conditions must be fulfilled to get reliable data: exchange must occur via the metal particle (the rate of direct exchange is negligible), surface migration must be the rate-determining step and exchange must exclusively occur with surface atoms.
7.3.1.2. Determination of Oxygen Diffusivities (Surface and Bulk) (g)
:
refers to the gas phase
260
ag (s) Ds Db Io A P
Ne Ng Po re t T
Catalysis by ceria and related materials
atomic fraction of 18 0 in gas phase at time t denotes atoms of the oxide surface concentration of 18 0 atoms on the metal particles (atoms.m2) surface diffusion coefficient of oxygen on the support (m2.s_1) bulk diffusion coefficient of oxygen in the oxide (m2.s_1) specific perimeter of the metal particles (m.g"1) surface area of the solid density of the solid number of atoms of the support exchanged at time t number of 1 8 0+' 6 0 atoms in the gas phase total pressure rate of ls O exchange with the support (atoms.s"'.m"2meta|) rate of ( 18 0 2 + 16 0 2 ) equilibration time temperature of exchange
Isotopic exchange experiments were carried out in a recirculated batch reactor coupled with a Quadrupole Mass Spectrometer (BALZERS QMS 420). The home made apparatus used for these studies was described in earlier publications [60-63]. A recirculating pump is necessary to avoid any diffusion and mass transport problems in the gas phase that would limit the changes in partial pressure measured by mass spectrometry. Before reaction, samples may be pretreated in situ in any conditions and at any temperature. For the reaction, pure gases were used. 16 0 2 , 99.5+% pure, was delivered by ALPHAGAZ and 18 0 2 , provided by ISOTEC INC., was 99+% pure. During the isotopic exchange reaction, the evolution of the oxygen isotopomer partial pressures above the catalyst was monitored by mass spectrometry. Mass 28 was routinely recorded to detect any possible leak in the reactor. For a thorough interpretation of these measurements, several parameters may be calculated. From homoexchange experiments, the initial rate of equilibration, used to compare the activity of the metal in the activation of oxygen, is systematically given by: dP180160 'q(t = 0) = ^ f dt 2Pn Nn
(7.6) Jt=o
Oxygen storage/redox capacity and related phenomena
261
Nevertheless the rate equation depends on the mechanism of homoexchange. Different methods were in fact developed for the purpose of more fundamental and mechanistic studies [52,53,55,56,64-66]. On the other hand, the rate of isotopic heteroexchange may be accessed from the mass balance in 18 0 atoms during the experiments. Neglecting the 18 0 accumulation in metal particles, we get: da„
re=-Ng-^dt da„
i
dt
P0
dPI80160 dP160160 +• 2dt dt
(7.7)
(7.8)
The rate of exchange is then calculated from the initial slopes of the plots "partial pressure" versus "time". The number of atoms of the oxide support exchanged is given by: Ne=Ng(l-ag)
(7.9)
Furthermore, information on the mechanism of exchange may be obtained from the relative evolution of the oxygen isotopomers partial pressure at the beginning of the reaction. The type of mechanism also give indication on the nature of mobile oxygen species. Finally, when exchange is controlled by surface diffusion on the support, a good measurement of the surface mobility is obtained. Metal particles are then in equilibrium with the gas phase and there is no gradient of concentration across the metal/support interface. The driving force is the labeled-oxygen concentration gradient at the support surface along the x axis. Surface diffusivities can be determined from the model developed by Kramer and Andre [67], assimilating metal particles, randomly distributed on the support, to circular sources of diffusing oxygen species. The amount of oxygen species diffusing on the support is then given by: (7.10)
Ds is derived from the initial part of the curve Ne vs. t
Catalysis by ceria and related materials
262
Furthermore, when the support exhibits a significant internal mobility of oxygen, the bulk diffusion coefficient Db may be calculated using a model developed by Kakioka [68]: (
-Ln
t
0>
4D h t aoog - a s0 = PA]
V 8
s
)
(7.11)
No
Db is calculated from the slope (S), determined after full exchange of the surface, of the plot: -Ln(
(7.12)
7.3.2. Oxygen Activation and Equilibration Reliable information on oxygen activation could be obtained from the study of the 16 18 0/ 0 homoexchange reaction (or equilibration reaction) on supported metal catalysts. In fact, 1 6 0/ l 8 0 homoexchange is an overall measurement of the adsorption/desorption rate. Recent results from our Group are presented hereafter.
7.3.2.1. Relative Activities of Metals in the Oxygen Activation Process According to Table 7.6, the relative activities in the 16 0/ 18 0 equilibration reaction at 300°C of solids prepared using chlorine-free precursors would be: Ru>Rh>Pt> Pd. Additional measurements on a full series of ceria and cerium-zirconium mixed oxides supported noble metals showed that Ru was at least 10000 times more active than Pd and about 20 times more active than Rh for the activation of oxygen [70]. Up to now, most results were obtained with Rh catalysts but Ru could be a good candidate for surface diffusion measurements.
Oxygen storage/redox
capacity and related phenomena
263
Table 7.6. , 8 0 2 + 16 0 2 equilibration at 300°C - Effect of the nature of the metal [69,70].
Catalyst 0.5Rh/CeAl(a) lPt/CeAl (b) 0.9Pd/Ce,(c) 0.8Ru/Ce!(d) 0.9Pd/CeZri (c) 0.9Ru/CeZr,(d)
Metal dispersion (%) 84 84 65 4 60 7
(a) impregnated using Rh(N03)3 (c) impregnated using Pd(N03)2
Rate of equilibration (at0.s"'.m"2metai) 3xl0 18 3xl0 17 lxlO 16 lxlO 20 5xl0 15 7xl0 19
(b) impregnated using Pt(NH3)2(N02)2 (d) impregnated using Ru(C5H702)3
7.3.2.2. Effect of the Support on the Activation of Oxygen To evaluate the influence of the support on the equilibration reaction on the metal particle a wide number of oxides supported rhodium catalysts have been studied (Table 7.7). Looking at Table 7.7, one can see that all oxygen equilibration rates at 300°C are in the 1018-1019 range. A factor of only 4 is observed between the most and the less active Rh-catalyst. Everything proceeds without the participation of the support. Table 7.7. I 8 0 2 + l 6 0 2 equilibration over Rhodium catalysts at 300°C- Support effects [61,71].
Catalyst 0.3Rh/Ce, (a'b) 0.6Rh/Ce2(a'c) 0.3Rh/CeZr,(a,d) 0.3Rh/CeZr2(a'e)
Metal dispersion (%) 56 32 85 65
(a) impregnated using Rh(N03)3 (b) Ce, : Ce0 2 , SBET=26m2g1 (c) Ce 2 : Ce0 2 , SBET=60m2g-'
Rate of equilibration (at0.s"1.rn"2metai) 2xl0 19 5xl0 18 lxlO 19 lxlO 19
(d) CeZr, : Ce0.63Zro.3702, SBEr=41m2g-' (e) CeZr2: Ce0 .sZro.gsOj, SBET=27m2g-1
Some recent studies dealing with CexZr(1.x)02 (0<x
Catalysis by ceria and related materials
264
7.3.2.3. Sensitivity of the Oxygen Activation Process to the Metal Dispersion l6
0/ 1 8 0 equilibration is a structure-sensitive reaction. If one compares the results obtained on fresh and sintered ceria-alumina supported catalysts to look at particle size effects, all metal supported catalysts are not affected in the same way (Table 7.8). Table 7.8. 18 0 2 + catalysts [69].
16
0 2 equilibration at 200°C - Particle size effect on cerium-based
Catalyst 0.5Rh/CeAl(a'c) 0.5Rh/CeAl(a'c) 0.5Rh/CeAl(a'c) lPt/CeAl ( M lPt/CeAl ( M !Pt/CeAl(b'c)
Metal dispersion (%) 84 35 14 84 31 4
Rate of equilibration (at0.s"'.m"2metai) 3xl0 18 4xl0 18 6xl0 18 3xl0 17 5xl0 16 lxlO 18
(a) impregnated using Rh(N03)3 (b) impregnated using Pt(NH3)2(N02)2 (c) CeAl: 12%Ce02-Al203, S B ET= 93 m2.g-'
In general, Pt catalysts are much more sensitive to particle size variations than Rh catalysts. It clearly appears that the 1602 + 1802 equilibration reaction is only slightly sensitive to the Rh particles size. In fact the rate of equilibration varies by a factor of only 1 to 2 when the dispersion decreases by a factor of 6. On platinum the variations are much more important with a factor of 20 on the rate of equilibration when the dispersion of the metallic phase varies by a factor of 21. The reaction is faster on larger Pt particles [33,69,72] while the opposite occurs with rhodium [33,69,72,73].
7.3.3.
Oxygen Species Involved in OSC
FT-IR Spectroscopy can be used (i) to identify surface oxygen species formed under oxygen pressure and (ii) to study the reactivity of such entities during oxygen exchange reactions. 0 2 adsorption infrared studies were performed using self-supported oxide wafers (~50 mg). In order to eliminate surface pollutants, mainly carbonates, all
Oxygen storage/redox capacity and related phenomena
265
samples are pretreated in situ in flowing O2 at 400°C over 12 hours. After the "cleaning" procedure, the IR cell is evacuated for 1 hour at 400°C before adsorption studies. Spectra were collected at a resolution of 4 cm"' on a Nicolet Magna 550 FTIR spectrometer. Two types of measurements were carried out: the study of oxygen adsorption and the study of the pre-adsorbed surface oxygen species exchange. Upon oxygen adsorption on CexZr(1.X)02 at room temperature, the formation of superoxides characterized by a band at 1126 cm"1 is systematic. Moreover it was clearly observed that the OSC at 400°C roughly varies as the population in 0 2 species estimated from FT-IR measurements (Fig. 7.12). It was then postulated that these species may be involved in the oxygen storage [74],
7
ii
k /1
l:i .-ji.i C«15
Ce50
Ce63
O100
Figure 7.12. Correlation between the OSC at 400°C of CexZr(i.X)02 samples and the amount of superoxides formed upon oxygen adsorption (0.5mbar) at room temperature.
Furthermore, it was concluded from isotopic exchange experiments that mixedoxides predominantly exchange their oxygen via a multiple exchange process. So, it appeared interesting to parallel the predominance of multiple exchange during isotopic exchange reaction with the presence of such binuclear oxygen species at the surface. In fact, we observed that non-labeled superoxides species directly convert to fully labeled superoxides species (Fig. 7.13). One can easily observe that both oxygen atoms are exchanged simultaneously, accordingly to what was observed in the course of isotopic exchange experiments. It appears that binuclear oxygen species must be involved in the whole process of oxygen mobility on ceria-based oxides.
Catalysis by ceria and related
266
1110
110O
1090
materials
1080
Wavenumber (cm"1)
Figure 7.13. Exchange at room temperature on Ce0.63Zr0 37O2, followed by FT-infrared, between surface species (formed upon oxygen adsorption) and 1802 from the gas phase.
16
02
7.3.4. Oxygen Diffusion Quite a lot of work has been dedicated to the study of oxygen mobility on oxides [33,62,72,75,76]. Some recent studies [63,72] on a series of simple oxides demonstrated that surface diffusion coefficients (Ds) may vary by three orders of magnitude going from silica to ceria (Table 7.9). Table 7.9 - Evolution of the oxygen surface diffusion coefficient at 400°C as a function of the support.
Catalysts 0.6%Rh/SiO2 0.5%Rh/y-Al2O3 0.5%Rh/CeO2-Al2O3 0.6%Rh/ZrO2 0.3%Rh/CeO2a 0.5%Rh/MgO 0.6%Rh/CeO2 a
Ds ( m y ) at 400°C 3xl0"iU 2xl0"18 4xl0"18 6xl0~18 8xlfJ18 lxlO"17 6xlfJ-16
the ceria support was precalcined at 900°C before impregnation of the metal
Nevertheless, important deviations may be observed for a given support depending on the surface area and the pretreatments. For ceria for instance, a tremendous
Oxygen storage/redox capacity and related phenomena
267
decrease of Ds is observed if the ceria support is pretreated at 900°C in air before impregnation of the metal. Finally, comparing surface diffusion and storage capacities, one can conclude that OSC values vary in the same way as surface diffusion coefficients. In fact, ceria has the best OSC and the highest surface mobility for oxygen. At some point, those two characteristics must be linked. Nevertheless no direct relation may be derived since on two different ceria surface mobility may vary by a factor of almost 80 while the OSC only changes by a factor of 3. This could be explained by the fact that OSC is not only a surface phenomena but also involves bulk oxygen atoms.
7.4. OSC and Catalysis
7.4.1. Effect of Additives and Poisons on OSC As seen before, ceria is the major OSC component in three-way catalysts. Noble metals play a specific role in activating redox sites of ceria, which allows to record very high O storage at low temperatures. OSC properties are currently improved by 4+
4+
substituting Ce with Zr (or rare-earth) ions (see previous sections). However, OSC can also be affected by certain other elements incorporated during the preparation (additives) or getting adsorbed during catalysis (poisons). Kacimi et al. [7] have studies the effect of various additives (V, Cr, Mn, Fe, Co, Ni, Cu and Pb) on the OSC of Ce0 2 and Rh/Ce0 2 catalysts between 350 and 550°C. Co, Ni and Cu were found to promote OSC both of ceria and of Rh/ceria while Mn, V and Pb were found to be strong inhibitors. Fe and Cr showed an intermediary behavior, with either a slight promoter or a slight inhibitor effect. Among all these additives, Cu showed an exceptional effect on Rh/Ce0 2 improving both OSC and catalyst stability (Table 7.10). Table 7.10. Effect of aging in 0 2 (2%) at 900°C on the OSC at 450°C (nmolO.g 1 ) of RhCu/ceria catalysts (0.36 wt-% Cu and 0.67 wt-% Rh; f = fresh and s900 = sintered at 900°C).
Catalyst
Ce0 2
Rh/Ce0 2
Cu/Ce0 2
Rh-Cu/Ce02
OSCf
123
323
158
337
OSCs900
49
231
108
359
Catalysis by ceria and related materials
268
The effect of K , CI , S0 4 ions on the OSC properties and oxygen mobility (measured by 1 8 0/ l 6 0 exchange) over Rh/Al 2 0 3 (RhA) and Rh/Ce0 2 -Al 2 0 3 (RhCA) was investigated by Martin et al. [32]. On alumina, the main OSC component is the metal itself. Chlorine and sulfate ions are strong poisons both of OSC and of 0 mobility. A moderate loading of the catalyst in potassium can improve the OSC and, in parallel, increase the coefficient of oxygen surface diffusion. On RhCA, a poisoning effect of CI was also observed but contradictory results were recorded with sulfates: SO4" ions significantly increase OSC values while they still decrease surface mobility. These results can be explained by a better reducibility of sulfates ions over ceria-containing catalysts [77], which increases CO and 0 2 uptakes in OSC measurements. The metal plays a decisive role in reducing these sulfate ions 3+ but it was also proven that Ce sites were able to reduced directly S0 2 into sulfide species at low temperatures (at 25°C for surface species and at 320°C for bulk species) [78].
7.4.2. Role of OSC in Catalytic Reactions Ceria may have a great impact on CO and HC oxidations and NOx reduction. Yu Yao [79,80] has compared the activity of noble metals (Pt, Pd, Rh) for oxidation of CO and hydrocarbons when these metals are supported on A1203 or on 20%CeO2A1203. The reactions were carried out in 0 2 excess (5=2). The results are expressed in activity ratio A (per gram) between the metal deposited on 20%CeO2-Al2O3 and the same metal deposited over alumina. For each pair of catalyst, A values are compared at different temperatures by using kinetic equations reported by Yu Yao (Table 7.11). Table 7.11. Effect of Ce0 2 in oxidation reactions over Pt, Pd and Rh catalysts: A ratios for oxidation of CO and various hydrocarbons.
Metal
0.5% CO + 0.5% 0 2
0.1% CH 4 + 1% 0 2
0.1%C 3 H 8 + 1% 0 2
0.1%C 3 H 6 + 1% 0 2
0.22% Pt
0.8 (250°C)
0.05 (500°C)
0.5 (250°C)
2 (300 °C)
0.15% Pd
1.4(250°C)
0.3 (400°C)
0.2 (350°C)
0.5 (250°C)
0.15% Rh
7 (250°C)
1.2(500°C)
3(400°C)
2 (300°C)
Oxygen storage/redox capacity and related phenomena
269
There is a limited effect of ceria on Pt and Pd catalysts, rather negative in alkane oxidation. On the contrary, there is a beneficial effect of ceria on rhodium catalysts, particularly marked in CO oxidation. This promoter effect of ceria on Rh in CO oxidation has been the object of several investigations [81-83]. However, most authors found a promoter effect more pronounced around the stoichiometry than in 0 2 excess. The results obtained by Oh and Eickel [82] clearly illustrate this tendency (Fig. 7.14). The rate of CO oxidation dramatically decreases at low 0 2 pressure over RI1/AI2O3. Ceria allows to maintain a good activity even in rich conditions. To explain these results, Oh and Eickel proposed a mechanism involving CO2 formation via a reaction between CO adsorbed on Rh and surface oxygen derived from the neighboring ceria particles. This mechanism implies that an oxygen vacancy is formed by reaction of CORJ, on Oce, this vacancy being thereafter filled by 0 2 adsorption. Further kinetic studies have confirmed that there existed a bifunctional mechanism for CO oxidation over M/Ce0 2 catalysts (M = Rh, Pt) implying two types of sites, one for CO adsorption and one another for 0 2 adsorption [84-86]. The nature and localization of O species in CO oxidation over ceria containing catalysts is still a matter of debate. 10-5
It)"7 0.1
1 0 2 concentration (vol %)
10
Figure 7.14. Effect of ceria on CO oxidation over Rh catalysts. From Oh and Eickel [82].
Most authors accept the idea that these species are located just around the metal particles, which suggests that a limited number of O stored on ceria be involved in CO oxidation. In certain studies, the sites for O adsorption is thought to be restricted
Catalysis by ceria and related materials
270
to those just localized at the metal/support interface [87,88]. Serre et al. [87] have shown that the beneficial role of ceria was mostly observed after a reductive treatment. They propose the model (7.13) for active sites over prereduced Pt-ceria while model (7.14) could explain the deactivating effect of a preoxidation: CO-Pt° CO-Pt°
Pt0-O-Ce-O Pt0 2 -0-Ce-0
(7.13) (7.14)
In this latter case, migration of O atoms from ceria to platinum would be blocked by O strongly bonded to Pt as Pt0 2 species. A quite different explanation was proposed by Bera et al [88]. Actually, Pd and Pt would be ionically dispersed on ceria while they cannot on alumina. These high Pt or Pd dispersion would lead to a strong-metal interaction in the form of solid solution Cei_xMx02-(4-n)x/2; having linkages of the type: -0 2 ~-Ce 4 + -0 2 "-M n + -0 2 ~ (n = 2 or 4)
(7.15)
However, these catalysts were prepared by a combustion method and used without further reduction by H2. It is thus conceivable that ceria retains highly dispersed ionic form of Pt and Pd. Kinetic data obtained by Bera et al. for CO oxidation (Table 7.12) show remarkable differences in activity and activation energy between alumina and ceria supported catalysts. Table 7.12. Intrinsic activity for CO oxidation and activation energy of Pt and Pd catalysts supported on ceria or alumina. Adapted from Bera et al. [88].
Catalyst
TON (s"1) at 175°C
E (kJ.mol"1)
l%Pt/Ce0 2
0.99
31
l%Pd/Ce0 2
0.98
25
l%Pt/Al 2 0 3 l%Pd/Al 2 0 3
0.029
54
0.015
65
Extremely high activity of Pd/Ce0 2 and Pd/Ce0 2 -Al 2 0 3 catalysts for CO oxidation was reported by Fernandez-Garcia et al. [89]. Compared to alumina, ceria decreases the onset temperature of reaction by about 130°C (stoichiometric conditions: l%CO + 0.5%O2). CO conversion can be observed even at ambient
Oxygen storage/redox capacity and related phenomena
271
temperature on Pd/Ce02-Al203. The catalysts were characterized by DRIFT, ESR and electron diffraction. A dual role of ceria in promoting CO oxidation was proposed by the authors: ceria would facilitate Pd reduction by CO (and thus CO adsorption) while it provides sites for 0 2 adsorption. Curiously, however, pure ceria was shown to give less active catalysts than Ce02-Al203 above ca 100°C. At these 2+
temperatures, Pd seems to be reoxidized at the Pd-Ce interface into less active Pd species. ESR studies reveal that superoxide 0"2 could be involved as active oxygen species in CO oxidation. Ceria is also a very good promoter of the water-gas shift reaction (WGSR) [90,91], which can be linked to the fact that hydroxyl groups are extremely mobile on this oxide [63]. Barbier Jr. and Duprez showed there was a beneficial effect of ceria both on CO oxidation and on WGSR, more marked over Rh than over Pt catalysts [92]. Moreover, a promoter effect of H 2 0 can be observed in CO oxidation while 0 2 was rather a poison of WGSR. The fact that ceria-zirconia mixed oxides possess a much higher OSC than pure ceria prompted several authors to verify if these supports can lead to higher performances in CO oxidation. Fernandez-Garcia et al [93] found extremely high activity for their l%Pd/Ceo.5Zro.502 catalyst, CO conversion being total at ambient temperature. Nevertheless, contrary to ceria, ceria-zirconia catalysts showed inferior catalytic properties when they were dispersed on alumina. It seems that the highest performances are obtained when Pd° particles are surrounded by a high number of vacancies, which can occur in Pd deposited on bulk ceria-zirconia. From these results, we may conclude that the higher the OSC of the catalyst, the higher its activity for CO oxidation. However, it is likely that a limited number of oxygen species stored on the catalyst (those just in the vicinity of metal particles) may intervene in the catalytic process. More than the total OSC, the local concentration of sites for 0 2 adsorption would be a determining factor in CO oxidation. A close correlation between OSC and catalytic activity may however be found since a high concentration in anionic vacancy is a prerequisite for a high OSC value of sample. Under conditions close to the stoichiometry, most authors report a beneficial effect of Ce0 2 in NO reduction. Kinetic data of Bera et al [88] for CO+NO are given in Table 7.13. Pd was found to be more active than Pt in NO reduction by CO. A significant promotion by ceria was also observed for both metals, Ce0 2 decreasing the reaction temperature of more than 100°C. Similar results were obtained by Noronha et al. [94] over Pd/MeOx/Al203 catalysts (MeOx = M0O3, Ce0 2 , Nb 2 0 5 ). Ceria shows
Catalysis by ceria and related
272
materials
better promotion (activity and selectivity) than molybdena and niobia even though NO dissociation is better on Pd/Mo03/Al203 (Table 7.14.). The authors conclude that CO is primarily adsorbed on Pd and NO, on MeOx. NO is dissociated above 127°C according to a redox mechanism involving oxygen vacancies of the promotor and charge transfer to the cations (Ce , Mo , Nb ). Table 7.13. Intrinsic activity for NO reduction by CO and activation energy of Pt and Pd catalysts supported on ceria or alumina. Adapted from Bera et al. [88].
TON (s "') at 225°C at 325°C 0.47 21.4
Catalyst l%Pt/Ce0 2 l%Pd/Ce0 2
0.19 0.24
l%Pt/Al 2 0 3 l%Pd/Al 2 0 3
E (kJ.mol"1) 48 52 50 60
Table 7.14. NO dissociation and NO reduction by CO on Pd catalysts. From Noronha et al. [94].
Catalyst (~l%Pd + -10% MeOx)
Pd dispersion (%) (from CO chem.)
Gas yield (%) from NO TPD NO
N20
60 42
Pd/Mo03/Al203
31 13 22
1
9 12 21
Pd/Nb205/Al203
30
43
11
Pd/Al203 Pd/Ce02/Al203
NO+CO reaction at 220°C
N2 31 46
NO conv. %
N2 sel. %
18 39.5
12 41
78
12 12
26 0
46
A different picture was proposed by Mullins et al. [95] for Rh/Ce0 2 (lll) on the basis of a surface science study. After these authors, CO and NO would be adsorbed on the metal. The promotor role of Ce0 2 would be due to a spillover of O atoms from the metal to reduced ceria. This explanation is consistent with the kinetic results of Loof et al. [96]: when it is physically mixed with a Pt/Al 2 0 3 or a Rh/Al 2 0 3 catalyst, ceria slows down the inhibition effect of O adsorbed species on the reaction. Whatever the mechanism (primary dissociation of NO on ceria or on the metal), O mobility and storage can play a decisive role in cleaning metal surface from adsorbed O species. Nevertheless, we may also imagine that the reverse
Oxygen storage/redox capacity and related phenomena
273
phenomenon can occur under lean conditions. Oxygen species are then able to invade the metallic surface by a back-spillover process, which can explain the relatively poor results obtained by Cho in NO reduction by CO over Rh catalysts and under low-frequency cycling conditions [45]. 4+
The possible effect of Zr ions, substituted to Ce ions in cubic ceria, on the performances of noble metal catalysts in NOx reduction has been evaluated by several authors. Di Monte, Fornasiero et al. showed that both Rh [97,98] and Pd [99] exhibited excellent performances in NO reduction by CO when they were supported on Ceo.5Zro.5O2 or on Ceo.6Zro.4O2. These supports are known to be extremely resistant to thermal sintering; they seems also to play a decisive role in activating the NO molecule. The same group of authors showed that there was a close correlation between the support reducibility (Rh/Ce02-Zr02) and its ability to decompose NO into N 2 0 and N2 [100]. Rh is thought to be zero valent under reaction conditions close to the stoichiometry. A different explanation was given by Fajardie et al. [101] on the basis of TPR and FTIR spectroscopy: ceria-zirconia supports could maintain X+
the rhodium as very small Rh clusters, extremely active in NO reduction but inactive in benzene hydrogenation (a reaction occurring exclusively over Rh particles). In conclusion, noble metals deposited on ceria-zirconia supports show both increased OSC and increased activity for NOx conversion, the clearer effect being obtained in dynamic conditions [102]. They are also more resistant than pure ceria to thermal sintering, since they are able to maintain a good activity even in a 1000°C durability test [102]. 7.4.3. On-Board Diagnostic Continuous measurement of OSC of the catalyst inserted in the converter is one of the simplest tool to monitor catalyst efficiency. This is known as the "On-board diagnostic" (OBD). Sideris has recently reviewed the OBD technologies on the basis of the corresponding patent literature [103]. Even though other techniques are available (temperature sensors, specific sensors for CO, HC and NOx), OBD determined by means of 0 2 sensors remains the technique the most widely employed in car industry. This technique is based on the generation of periodic rich/lean perturbations monitored by two lambda sensors, one located upstream and the second downstream the converter. Catalysts that have good OSC considerably attenuate the upstream perturbation (with a significant time lag) while virtually dead
274
Catalysis by ceria and related materials
catalysts transmit the perturbation through the converter without any attenuation nor any time-lag. By comparing the signals generated by the two sensors, it is possible to calculate the instantaneous OSC of catalyst. However, the threshold value for OSC below which the catalyst should be replaced depends on many factors: nature of catalyst, type of pollutant, type and sensitivity of 0 2 sensor. Taha et al. [104] have compared the effect of thermal aging on both OSC values and catalytic performances. Two commercially available catalysts were selected: Cat 1 "Pt-Rh" (0.152%Pt-0.031%Rh on a wash-coated monolith containing 6.3% Ce and 0.66% La) and Cat.2 "Pd-Rh" (0.19%Pd-0.022%Rh on a wash-coated monolith containing 4.6% Zr, 1.9% Ce and 0.9% Ba). These catalysts were aged in N 2 +H 2 0 at 900,1000 and 1100°C or in an engine-bench for 25, 50 or 200 h. OSC of the fresh catalysts were 91.8 (Pt-Rh) and 59.7 umolO.g"1 (Pd-Rh) respectively. OSC changes with aging parameters are shown in Fig. 7.15. There is a severe deactivation in OSC properties of Pt-Rh while Pd-Rh is moderately affected by aging treatments. A parallelism can be found between the temperature of laboratory aging under N 2 +H 2 0 and the duration of the engine-bench test. However, contrary to Pt-Rh, the Pd-Rh catalyst resists rather better to the engine-bench test than to the high temperature treatment. There is no apparent correlation between changes in OSC values and decreases in BET area of catalysts. In fact, OSC seems to follow the variations of ceria area determined by XRD, which confirms that ceria is the major OSC component and that sites for oxygen storage are mainly localized at the ceria surface.
Figure 7.15. OSC of Pt-Rh and Pd-Rh catalysts aged under N 2 +H 2 0 at 900,1000 and 1100°C (grey bars) or in an engine-bench for 25,50 and 200 h (checked bars). From Ref. [104].
Oxygen storage/redox capacity and related phenomena
275
The catalytic activity for CO, HC and NO abatement was measured at 450°C in a synthetic gas which simulated reactions conditions around the stoichiometry (1 Hz, amplitude, ± 0.05). Conversions versus OSC are shown for the two catalysts on Fig. 7.16.
Figure 7.16. Correlation between CO, HC and NOx conversions and OSC values of Pt-Rh (left) and of Pd-Rh catalysts (right). Full lines: laboratory aging; dashed lines: engine-bench aging. From Ref. [104].
It is confirmed that OSC measurements can be used to follow catalyst deactivation. Nevertheless, there is no one-to-one relationship between a given OSC value and the extent of deactivation which depends largely on the mode of aging. Long-duration engine-bench tests lead to a severe deactivation on both catalysts. This is in line with the low OSC measured on Pt-Rh but does not fit well with the relatively high OSC values still recorded on Pd-Rh. It seems clear that the OSC threshold should be adjusted for a given type of engine and a given type of catalyst. Moreover, NOx conversion is particularly sensitive to deactivation and should be the determining factor in fixing the threshold level of OBD systems. Similar results concerning the sensitivity of NOx conversion to aging were obtained by Permana et al. [105] who showed, however, that the use of a wash-coat loaded with Ceo.75Zro.25O2 permitted to maintain a much better performance than with pure ceria. Many attempts have been made to improve OSC properties and catalytic activity in modern TW catalysts. It seems that the incorporation of an extra stabilizer allows to reduce the ceria content while maintaining very high
Catalysis by ceria and related materials
276
performance and durability of Pd-only or Pd-Rh catalysts [106]. In conclusion, the use of ternary Ce-Zr-X compounds (may be quaternary oxides) is certainly a way to a better control of OSC properties and to an increased stability of catalysts over longer periods on-stream in the converters.
7.5. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.
Yao H. C , Yu Yao Y. F., J. Catal. 86 (1984), 254 Duprez D., J. Chim. Phys. 80 (1983), 487 Su E. C , Montreuil C. N., Rothschild W. G., Appl. Catal. 17 (1985), 75 Su E. C , Rothschild W. G., J. Catal. 99 (1986), 506 Harrison B., Diwell A. F., Hallett C , Platinum Metals Rev. 32 (1988), 73 Engler B., Koberstein E., Schubert P., Appl. Catal. 48 (1989), 71 Kacimi J., Barbier Jr. J., Taha R., Duprez D., Catal. Lett. 22 (1993), 343 Janvier C , Pijolat M., Valdivieso F., Soustelle M., Zing C , J. European Ceram.Soc. 18(1998), 1331 Colon G., Valdivieso F., Pijolat M., Baker R. T., Calvino J. J., Bernal S., Catal. Today 50(1999), 21'1 Colon G., Pijolat M., Valdivieso F., Baker R. T., Bernal S., Adv. Sci. Tech. 16 (1999), 605 Cuif J.-P., Blanchard G., Touret O., Seigneurin A., Marczi ML, Quemere E., in SAE Technical Paper Series (1997), # 970463 Madier Y., Ph-D Thesis, Poitiers (1999) Madier Y., Descorme C , Le Govic A.-M., Duprez D., J. Phys. Chem. B 103/50 (1999), 10999 Murota T., Hasegawa T., Aozasa S., Matsui H., Motoyama M., J. Alloys and Comp. 193 (1993), 298 Cuif J.-P., Blanchard G., Touret O., Marczi M., Quemere E., in SAE Technical Paper Series (1996), # 961906 Trovarelli A., Zamar F., Llorca J., de Leitenburg C , Dolcetti G., Kiss J. T., /. Catal. 169 (1997), 490 Permana H., Belton D. N., Rahmoeller K. M., Schmieg S. J., Hori C. E., Brenner A., Ng K. Y. S., in SAE Technical Paper Series (1997), # 970462 Finocchio E., Daturi M., Binet C , Lavalley J.-C, Fally F., Perrichon V., Vidal H., Kaspar J., Graziani M., Blanchard G., Sci. Tech. Catal. (1998), 257
Oxygen storage/redox capacity and related phenomena
277
19. Fornasiero P., Di Monte R., Ranga Rao G., Kaspar J., Meriani S., Trovarelli A., Graziani M.,J. Catal. 151 (1995), 168 20. Hirata T., Zhu H., Furubayashi T., Nakatani I., J. Am. Ceram. Soc. 76/5 (1995), 1361 21. Torng S., Miyazawa K., Sakuma T., Mat. Sci. Tech. 11(1995), 130 22. Yashima M., Morimoto K., Ishizawa N., Yoshimura M., J. Am. Ceram. Soc. 76/11 (1993), 2865 23. Yashima M., Ohtake H., Kakihana M., Yoshimura M., J. Am. Ceram. Soc. 77/10 (1994), 2773 24. Tsukuma K., Am. Ceram. Soc. Bull. 65/10 (1986), 1386 25. Yashima M., Morimoto K., Ishizawa N., Yoshimura M., J. Am. Ceram. Soc. 76/7(1993), 1745 26. Meriani S., Mater. Sci. Eng. A109 (1989), 121 27. Kim D. J., Jung H. J., Kim H. J., J. Mater. Sci. Lett. 14 (1995), 285 28. Zhu H. Y., J. Met. Sci. 29 (1994), 4351 29. Vlaic G., Fornasiero P., Geremia S., Kaspar J., Graziani M., J. Catal. 168 (1997), 386 30. Rossignol S., Micheaud-Especel C , Duprez D., Stud. Surf. Sci. Catal. 130D (2000), 3327 31. Loof P., Kasemo B., Heck K.-E., J. Catal. 118 (1989), 339 32. Martin D., Taha R., Duprez D., Stud. Surf. Sci. Catal. 96 (1995), 801 33. Taha R., Ph-D Thesis, Poitiers (1994) 34. Li C , Sakata Y., Arai T., Domen K., Maruya K.-L, Onishi T., J. Chem. Soc. Faraday Trans. 85/4 (1989), 929 35. Binet C , Badri A., Boutonnet-Kizling M., Lavalley J.-C, J. Chem. Soc. Faraday Trans. 90/7 (1994), 1023 36. Binet C , Jadi A., Lavalley J.-C, J. Chim. Phys. 89 (1992), 1779 37. Holmgren A., Andersson B., Duprez D., Appl. Catal. B 11 (1999), 215 38. Descorme C., Madier Y., Duprez D., Birchem T., Stud. Surf. Sci. Catal. 130A (2000), 347 39. Bedrane S., Descorme C , Duprez D., submitted for publication in Catal. Today 40. Yamada K., Tanaka H., Yamamoto M., in SAE Technical Paper Series (1997), # 970464 41. Miki T., Ogawa T., Haneda M., Kakuta N., Ueno A., Tateishi S., Matsura S., Sato M., J. Phys. Chem. 94 (1990), 6464
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42. Zamar F., Trovarelli A., de Leitenburg C , Dolcetti D., Stud. Surf. Sci. Catal. 101 (1996), 1283 43. Maillet T., Madier Y., Taha R., Barbier Jr. J., Duprez D., Stud. Surf. Sci. Catal. 112 (1997), 267 44. Kacimi S., Duprez D., unpublished results 45. Cho B. K., J. Catal. 131 (1991), 74 46. Taha R., Martin D., Kacimi S., Duprez D., Catal. Today 29 (1996), 89 47. Keramidas V. G., White W. B., J. Am. Ceram. Soc. 57 (1974), 22 48. Bernal S., Blanco G., Cauqui M. A., Corchado P., Pintado J. M., RodriguezIzquierdo J. M., Chem. Commun. (1997), 1545 49. Vidal H., Bernal S., Kaspar J., Pijolat M., Perrichon V., Blanco G., Pintado J. M., Baker R. T., Colon G., Fally F., Catal. Today 54 (1999), 93 50. Vidal H., Kaspar J., Pijolat M., Colon G., Bernal S., Cordon A., Perrichon V., Fally F., Applied Catal. 11 (2000), 49 51. Vidal H., Kaspar J., Pijolat M., Colon G., Bernal S., Cordon A., Perrichon V., Fally F., Applied Catal. 30 (2001), 75 52. Boreskov G. K., Adv. Catal. 15 (1964), 285 53. Winter E. R. S., J. Chem. Soc. 1 (1968), 2889 54. Novakova J., Catal. Rev. 4 (1970), 77 55. A. Ozaki, in Isotopic Studies of Heterogeneous Catalysis (Kodansha, Tokyo and Academic Press, New York, 1977) 56. Klier K., Novakova J., Jiru J., J. Catal. 2 (1963), 479 57. Muzykantov V. S., Popovski V. V., Boreskov G. K., Kin. Katal. 5 (1964), 624 58. G.M. Schwab, E. Killman, in Proc. 2nd. Int. Congr. Catal. (1960), 1047 59. Hirota K., Chono M., J. Catal. 3 (1964), 196 60. Abderrahim H., Ph-D Thesis, Poitiers (1986) 61. Martin D., Ph-D Thesis, Poitiers (1994) 62. Abderrahim H., Duprez D., Stud. Surf. Sci. Catal. 30 (1987), 359 63. Martin D., Duprez D., J. Phys. Chem. 100 (1996), 9429 64. Courbon H., Pichat P., C. R. Acad. Sci. Paris 285C (1977), 171 65. Courbon H., Formenti M., Pichat P., J. Phys. Chem. 81 (1977), 550 66. Courbon H., Herrmann J.-M., Pichat P., J. Phys. Chem. 88 (1984), 5210 67. Kramer R., Andre M., J. Catal. 58 (1979), 287 68. Kakioka H., Ducarme V., Teichner S. J., J. Chim. Phys. 68 (1971), 1715 69. Taha R., Duprez D., J. Chim. Phys. 92 (1995), 1506 70. Roux M., Descorme C , Duprez D., unpublished results 71. Madier Y., Descorme C , Duprez D., in preparation
Oxygen storage/redox capacity and related phenomena
72. 73. 74. 75. 76. 77. 78. 79. 80. 81. 82. 83. 84. 85. 86. 87. 88. 89. 90. 91. 92. 93. 94. 95. 96. 97. 98. 99.
279
Duprez D., Stud. Surf. Sci. Catal. 112 (1997), 13 Maillet T., Ph.D thesis, Poitiers (1998) Descorme C , Madier Y., Duprez D., J. Catal. 196 (2000), 167 Duprez D., Abderrahim H., Kacimi S., Riviere J., in Proc. 2nd Int. Conf. on Spillover (Leipzig, 1989), 127 Martin D., Duprez D., Stud. Surf. Sci. Catal. 11 (1993), 201 Bazin P., Saur O., Lavalley J. C , Le Govic A. M., Blanchard G., Stud. Surf. Sci. Catal. 116(1998), 571 Overbury S. H., Mullins D. R., Huntley D. R., Kundakovic L., J. Phys. Chem. 5103(1999), 11308 Yu Yao H. C , Ind. Eng. Chem. Prod. Res. Dev. 19 (1980), 293 Yu Yao, H. C , J. Catal. 87 (1984), 152 Herz R. K., Kiela J. B., Sell J. A., Ind. Eng. Chem. Prod. Res. Dev. 22 (1983), 387 Oh S. H., Eickel C. C., J. Catal. 112 (1988), 543. Nunan J. G., Robota H. J., Cohn M. J., Bradley S. A., J. Catal. 133 (1992), 309 Zafiris G. S., Gorte R. J., J. Catal. 143 (1993), 86 Bunluesin T., Putna E. S., Gorte R. J., Catal. Lett. 41 (1996), 1 Nibbelke R. H., Campman M. A. J., Hoebink H. B. J., Marin G. B., J. Catal. Ill (1997), 358 Serre C , Garin F., Belot G., Maire G., J. Catal. 141 (1993), 9 Bera P., Patil K. C , Jarayam V., Subbanna G. N., Hedge M. S., J. Catal. 196 (2000), 293 Fernandez-Garcia M., Martinez-Arias A., Salamanca L. N., Coronado J. M., Anderson J. A., Conesa J. C , Soria J., J. Catal. 187 (1999), 474 Barbier Jr. J., Duprez D., Appl. Catal. B 4 (1994), 105 Trovarelli A., in "Catalytic properties of ceria and ceria-containing materials", Cat. Rev., Sci. Eng. 38 (1996), 439 Barbier Jr. J., Duprez D., Appl. Catal. B 3 (1993), 61 Fernandez-Garcia M., Martinez-Arias A., Iglesias-Juez A., Hungria A. B., Anderson J. A., Conesa J. C , Soria J., Appl. Catal. B 31 (2001), 39 Noronha F. B, Baldanza M. A. S., Monteiro R. S., Aranda D. A. G., Ordine A., Schmal M., Appl. Catal. A 210 (2001), 275 Mullins D. R., Kundakovic L., Overbury S. H., J. Catal. 195 (2000), 169 Loof P., Kasemo B., Andersson S., Frestad A., J. Catal. 130 (1991), 181 Di Monte R., Fornasiero P., Graziani M., Kaspar J., J. Alloys Compounds 275277 (1998), 877 Fornasiero P., Ranga Rao G., Kaspar J., L'Erario F., Graziani M., J. Catal. 175 (1998), 269 Di Monte R., Fornasiero P., Kaspar J., Rumori P., Gubitosa G., Graziani M., Appl. Catal. B 24 (2000), 157
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Catalysis by ceria and related materials
100. Ranga Rao G., Fornasiero P., Di Monte R., Kaspar J., Vlaic G., Balducci G., Meriani S., Gubitosa G., Cremona A., Graziani M., J. Catal. 162 (1996), 1 101. Fajardie F., Tempere J. F., Manoli J.-M., Touret O., Blanchard G., DjegaMariadassou G., J. Catal. 179 (1998), 469 102. Ozawa M., J. Alloys Compounds 275-277 (1998), 886 103. Sideris M., in "Methods for monitoring and diagnosing the efficiency of catalytic converters", Stud. Surf. Sci. Catal. 115 (1998) 104. Taha R., Duprez D., Mouaddib-Moral N., Gauthier C , Stud. Surf. Sci. Catal. 116 (1998), 549 105. Permana H., Belton D. N., Rahmoeller K. M., Schmieg S. J., Hori C. E., Brenner A., Ng K. Y. S., SAE Technical Paper Series (1997) # 970462 106. Van Yperen R., Lindner D., Mussmann L., Lox E. S., Kreuzer T., Stud. Surf. Sci. Catal. 116 (1998), 51
CHAPTER 8 COMPUTER SIMULATION STUDIES OF CERIA-BASED OXIDES
M. SAIFUL ISLAM Department of Chemistry, University of Surrey Guildford, Surrey GU2 7XH, U.K. m.islam @surrey.ac. uk GABRIELE BALDUCCI Dipartimento di Scienze Chimiche Universita' di Trieste via L. Giorgieri, 1 34127 Trieste, Italy balducci @ univ.trieste.it
8.1.
Introduction
Computer modeling techniques are now well-established tools in the field of solid state chemistry, and have been applied successfully to studies of structures, energetics and dynamics of a wide variety of materials at the atomic level.' -3 A key aspect of modeling work has been the strong connection with experimental studies, which is evolving in the direction of increasingly complex systems. This review addresses recent trends and progress in the use of such computational techniques in the investigation of the defect, transport and surface properties of ceria (Ce02) and ceria-based materials. Ceria is an important material in the framework of three-way catalysts which are used in automobile catalytic converters for the treatment of exhaust gases.4-15 One of the key functions of this material is the ability of cerium to switch between the C e 4 + and C e 3 + oxidation states, depending on the oxygen partial pressure of the environment. This allows the reversible addition and removal of oxygen, the so-called Oxygen Storage Capacity (OSC). Ceria catalysts can therefore work in both oxidizing and reducing conditions, converting carbon monoxide, nitrogen oxides (NOx) and unburned hydrocarbons to non-toxic products. In addition to catalytic applications, ceria-based materials (especially Gd-doped CeO~2) have attracted considerable 281
282
Catalysis by ceria and related materials
attention as possible alternative solid electrolytes to the conventional yttria-stabilized zirconia system in solid oxide fuel cells (SOFCs); this is due to the highly favorable oxygen ion conductivity exhibited by ceria-based electrolytes at lower operating temperatures.15-20 It has been recently established that addition of zirconia (Zr02) to ceria to form a mixed-oxide solid solution greatly enhances the reducibility of C e 4 + in the catalyst material,21-23 which has generated considerable interest in the CeOi — ZrOi system.10'24-30 The energetics of the C e 4 + / C e 3 + reduction step and the corresponding formation of oxygen vacancies are likely to be involved, but difficult to probe at the atomic level by purely experimental techniques. Indeed, it is clear that fundamental solid state properties such as the precise role of structural defects and dopants, the nature of redox reactions to create electronic species (both within the bulk and at the surfaces), as well as the mechanism of oxygen ion migration, are crucial to the greater understanding of these important oxide materials. In this article, we review contemporary computational studies on ceria-based catalytic and fuel-cell materials using atomistic simulation, molecular dynamics (MD) and ab initio techniques to illustrate the breadth of information that can be obtained. Indeed, the main aims of computer modeling have been to interpret and stimulate experimental work, and to have a predictive role in the design of improved materials. Since the body of experimental information concerning ceria is vast, a comprehensive survey would be impractical and the reader is referred to the reviews of Trovarelli10 and Mogensen19 and to the other chapters in this collection. Emphasis here is placed on highlighting computational investigations of reduction energetics, dopant-defect clustering, oxygen ion transport and surface structures, which have assisted in the further understanding of these materials. First, this work is prefaced by a summary of the main computational techniques used in current studies.
8.2.
Computational Techniques
The present account of the computational techniques will be brief since comprehensive reviews are given elsewhere.1>31,32 In general, three classes of techniques have been employed in the study of solid state ionic materials: atomistic (static lattice), molecular dynamics (MD) and ab initio quantum mechanical methods. First, atomistic (static lattice) methods determine the lowest energy configuration of the crystal structure by employing efficient energy minimization procedures. The simulations rest upon the specification of an interatomic potential model which ex-
Computer simulation studies of ceria-based oxides
283
presses the total energy of the system as a function of the nuclear coordinates. For ceramic oxides, the Born model framework is commonly employed which partitions the total energy into long-range Coulombic interactions, and the following short-range term to model the repulsions and Van der Waals attractions between electron charge clouds:
4>ij
=
Aijexp(-^-)-^-
(8.1)
where ions i and j are at distance r,j and A^, pij and C^ are potential parameters. Because charge defects will polarize other ions in the lattice, ionic polarizability must be incorporated into the potential model. The shell model33 provides a simple description of such effects and has proven to be effective in simulating the dielectric and lattice dynamical properties of ceramic oxides. It should be stressed, as argued previously,1 that employing such a potential model does not necessarily mean that the electron distribution corresponds to a fully ionic system, and that the general validity of the model is assessed primarily by its ability to reproduce observed crystal properties. In practice, it is found that potential models based on formal charges work well even for some semi-covalent compounds such as silicates and zeolites. An important feature of these calculations is the treatment of lattice relaxation about the point defect, defect cluster or migrating ion. The Mott-Littleton approach is to partition the crystal lattice into two regions so that ions in a spherical inner region surrounding the defect are relaxed explicitly. In contrast, the remainder of the crystal, where the defect forces are relatively weak, is treated by more approximate quasi-continuum methods. In this way local relaxation is effectively modeled and the crystal is not considered simply as a rigid lattice. These methods are embodied in the GULP simulation code34 and have been applied to a range of mixed-metal oxides including oxygen-ion conductors,3,35 lithium insertion compounds36 and high Tc superconductors.37 The atomistic modeling of surface structures uses similar methodology to bulk calculations, but describes the crystal as a stack of planes periodic in two dimensions. The structure and energy of a surface are then obtained by relaxing the ions to their mechanical equilibrium positions. This is achieved via a two region approach in which the ion co-ordinates in the surface region are adjusted so that they experience zero net force, whilst those ions which are distant from the surface are kept fixed at their bulk equilibrium positions. The calculation of point defect energies at the surfaces uses a version of the Mott-Littleton approach but with hemispherical regions around the
284
Catalysis by ceria and related materials
defect species. The second main type of simulation method is the molecular dynamics (MD) technique, which consists of an explicit dynamical simulation of the ensemble of particles for which Newton's equations of motion are solved numerically. Interatomic potentials are also used to treat the forces, but repetition of the integration algorithm yields a detailed picture of the evolution of ion positions and velocities as a function of time. This technique allows the inclusion of the kinetic energy for an ensemble of ions (to which periodic boundary conditions are applied) representing the system simulated. The analysis of ion positions and velocities from the MD simulations generates a wealth of dynamical detail. Atomic transport parameters (such as diffusion coefficients) are extracted using the time-dependent mean square displacements (MSD). Mechanistic information can be obtained by analysis of particle trajectories. In this way, point defects in the system {e.g. oxygen vacancies in doped ceria), can now be "observed" to migrate between equilibrium sites, thus revealing the mechanism directly. MD methods (embodied in the DLPOLY code 38 ) have been applied previously to ionic conductors, such as Y/2V02, 39 and to molecular diffusion in zeolites.40,41 Finally, we note that there is an expanding role for quantum mechanical (QM) or ab initio methods in solid state studies. In general, such techniques attempt, at some level of approximation, to solve the Schrodinger equation for the system. They are thus able to provide detailed information on the electronic structure of materials and have been also employed in the derivation of potential parameters. Hartree-Fock (HF) methods have been used in which all integrals (above a threshold) are evaluated analytically or numerically.32 Periodic HF methods (implemented in the CRYSTAL package 42 ) have been used to study the electronic structure of transition metal oxides43'44 indicating the importance of high-quality basis sets. In addition, there has been recent progress in the study of defects in solids using "embedded" cluster techniques; here a QM cluster is surrounded by some point charge representation of the rest of the crystal or by an embedding scheme using effective core potentials.45 Techniques based on density functional theory (DFT) are now increasingly viewed as an important approach in materials science with exchange-correlation energy being treated by the local density approximation (LDA) or the generalized-gradient approximation (GGA). A widely used implementation of DFT combines a plane-wave basis set with the "pseudopotential" method, in which the pseudopotential represents the interaction between valence electrons and the atomic cores (incorporated in the CASTEP package46 ). In general, ab initio studies assist in confirming the validity of the defect simulations, but provide additional details on the electronic structure that are inaccessible
Computer simulation studies of ceria-based oxides
285
to atomistic simulations. The latter simulation methods of course have the benefit of being computationally cheaper by several orders of magnitude.
8.3.
Bulk Defect Chemistry
Catalytic applications of ceria and ceria-based mixed oxides depend primarily upon the nature and concentration of the defects present in the material. Although experimental techniques are available for the study of these defects, the characterization of their physical properties at the atomic level is often very difficult. The most important point defects in ceria are oxygen vacancies, reduced C e 3 + centers and dopant impurities. The formation energy of such defects and the energetics of their mutual interactions within the bulk oxide have been the subject of several computational studies. 8.3.1.
Ce4+ /Ce3+ Reduction Energetics
As mentioned in the introduction, the ability of cerium to switch between the +4 and + 3 oxidation states determines many important applications of ceria-based materials. For this reason, the energy change associated with the Cei+/Ce3+ bulk reduction in different environments has been investigated using computer simulation techniques. Balducci et a/.47,48 have calculated the energy change for the following reduction process (using the Kroger-Vink notation for defects):
2Ce£ e + 0 *
=
2Ce'Ce + V6 + \02{g)
(8.2)
where Ce'Ce is the Ce3+ species and VQ an oxygen vacancy. The reduction energy was calculated for ceria solid solutions Ce\-XMXC>2 with M = Zr,Hf,Th as a function of the dopant concentration. The results of these calculations are presented in Fig. 8.1. and show that the reduction energy decreases substantially with dopant concentration for Zri+ and Th4+, the effect being larger for Th4+, while it increases slightly for Hfi+. The trend can be explained only in part by the different ionic size of the dopants: Thi+ is larger than C e 4 + , while Zri+ is smaller. Since the size of cerium increases upon reduction, the resulting lattice strain is better accommodated in the case of the larger dopant. However, the different behavior of Zr4+ and Hfi+ cannot be accounted for on these grounds, as the two ions have nearly the same size. These reduction energies are consistent with the observation that the catalytic activity
286
Catalysis by ceria and related materials
1
1
1
1 _.-
8 -
>
"8
N
6 -
M = Zr
1
0.0
N
0.2
1
i
^
0.4 0.6 xin Ce(1_x)Mx02
0.8
1.0
Figure 8.1. Ce 4+ /Ce 3+ reduction energy as a function of composition for three isovalent dopants in the Cei-XMXC>2 system (from ref. 48)
is enhanced by the introduction of Zr into ceria, which is associated with increased reducibility of the bulk oxide. Temperature Programmed Reduction (TPR) experiments also indicate a large bulk participation in the reduction process.22,49 The Cez+ and the oxygen vacancy formed in reaction (8.2) are oppositely charged leading to possible association. These interactions can influence the overall energetic balance for the C e 4 + / C e 3 + reduction. In order to clarify this point, Balducci et al.41 have considered the possibility that Ce'Ce defects can associate with an oxygen vacancy to form a charged pair or a neutral trimer as follows:
ICel^Ol
=
Ce'Ce + (Ce'CeVoy
2Ce*e + OZ = (Ce'CeV6Ce'Ce)x+
+
-02(g)
\02{g)
(8.3) (8.4)
In every case the interaction between oxygen vacancies and reduced cerium centers produces bound states which lower the energy of the overall C e 4 + / C e 3 + reduction, with the formation of {Ce'CeVQ Ce'Ce)x trimers being energetically more favorable than (V 0 'Ce^ e )' pairs. This suggests that defect association assists in promoting
Computer simulation studies of ceria-based oxides
287
reduction. However, one must also consider that a high binding energy effectively "traps" the oxygen vacancies, hindering oxygen ion diffusion through the bulk lattice (a topic we return to in Sec. 8.4. below). There is considerable experimental evidence that the reducibility of ceria is a function of the structure of the material.50 This has particular relevance to applications in catalysis, since, depending on the operating conditions, aging of the catalyst can cause structural modifications {e.g. phase transitions). Gorte et al. have studied the dependence of the reducibility of pure ceria upon the crystallite size.51 They used the "simulated annealing" technique in which small ceria clusters of variable size (Cen02n, n = 2 — 50) are simulated atomistically. For every cluster, the starting structure was a random collection of Ce 4 + and 02~~ ions constrained in a sphere using reflecting boundary conditions. NVT Monte Carlo simulations were performed starting at a high temperature (8000 — 10000 K) which was progressively decreased. At each stage of the cooling program, care was taken so as to ensure that thermodynamic equilibrium was attained: the goal of this methodology is to find the global energy minimum of the cluster, bypassing the many local minima present in the complex energy hypersurface. The results of the simulations show that for every n value, the C e 3 + ions tend to segregate at the cluster surface after energy minimization. The reduction energy is also strongly dependent upon the size and structure of the clusters with a general trend of more favorable reduction energies for smaller n values. The large fluctuations found in the plot of Ecei+/Ce3+ versus n are in line with the high variability of catalytic performance observed for ceria-based catalysts of the same composition.
8.3.2. Defect Clustering An important aspect concerning the defects in ceria-based materials is their tendency to associate to form clusters of various complexity, which was mentioned in the previous section. The most studied defect clusters have been those formed by lower-valent (acceptor) dopant cations and oxygen vacancies. The reason for this is that these associations are widely considered to be responsible for the observed variation of the electrical conductivity of ceria-based oxides with dopant size, concentration and temperature.19 Since the oxidation state of the acceptor dopant is lower than that of cerium, the cationic sites occupied by the guest cation are negatively charged. In order to maintain electroneutrality, oxygen vacancies are created, so that the solution process may be represented by the following equations:
288
Catalysis by ceria and related
materials
M203 + 2Ce£e + 0*
=
2M'Ce + Vo+2Ce02
(8.5)
MO + Ce*e + 0£
=
M'ie + Vo+Ce02
(8.6)
where M20z and MO are trivalent and divalent dopant oxides, respectively. In general, the stability of a defect cluster relative to its components is measured by the binding energy (BE), which can be defined as the difference between the energy of the cluster and the sum of the energies of the isolated component point defects:
BE
=
Eciuster
- I
2__,
Edefect J
\ isolated defects
(8-7)
J
where, in this case, negative BE values indicate relative stability of the cluster. Early work of Butler et al.52 carried out computer simulations of dopant-vacancy clusters in ceria-based systems, in which the binding energy was calculated for a series of divalent and trivalent dopants. These authors evaluated the energy change for the following processes: V0+nM'Ce V0+nM'ie
= =
(nMl,eVS)2'n (nM'ieVo?-
2n
(8.8) (8.9)
in which the vacancy and the dopant cations are at nearest-neighbor positions. Calculations were performed for n = 1 - 4 and M = Sc3+, Y3+, Gd3+, Ce3+, La3+, Mg2+,Ca2+. A few key points emerge from this study: first, the interpretation of the observed conductivity data53 is confirmed (as shown in Fig. 8.2.). Second, the results, both theoretical and experimental, show that ionic size may have a large effect on ionic conductivity and this factor should clearly be born in mind in designing solid electrolytes. The third point is that the results show the quantitative success of this class of defect calculation in treating a subtle effect. Subsequently, Grimes and co-workers extended the calculations to a wider range of dopants and more complex cluster configurations.54'55 These studies have addressed the dependence of the binding energy and configuration of various dopant-vacancy associates upon the dopant size and charge. There are two main contributions to the binding energy. The first is the Coulomb interaction between the component defects: this is directly determined by the oxidation state of the dopant. The second is the
Computer simulation studies of ceria-based oxides
l ^ >0) >> CD 1_
ing
in 1
l
0.4 _
—
\
0.2 _ 1 Sc3+
\
\
^ \
1
0.9
\
X V^Y 3+
l
0.8
1 -
0.6
0.0 -
289
1
1 Gd 3 +
— 1 La 3 + -
1
1.0 1.1 Ionic radius (A)
1.2
Figure 8.2. Binding energy for the [M'Ce VQ J cluster as a function of the dopant ionic size in ceria: calculated points are connected by the full line, experimental points53 are connected by the dashed line (from ref. 52)
lattice relaxation around the defect cluster which depends primarily upon the dopant size. The relaxation of the lattice around a dopant-vacancy defect cluster is influenced by two factors. First, the general preference of small cations for lower coordination numbers favors the first nearest-neighbor configuration when the ionic size of the dopant is much smaller than that of the host C e 4 + . Second, when the vacancy and the dopant are in second nearest-neighbor positions, the adjacent C e 4 + cation can relax away from the vacant site while maintaining a favorable interaction distance with the dopant; this is expected for larger dopants. The subtle interplay of the above factors in determining the total BE of a given dopant-vacancy cluster is illustrated in Table 8.1., which lists the calculated binding energies of ( C ^ c e ^ o ) * an^ (^n'ce^6) clusters at first, second and third nearest54 neighbor distances. For the divalent dopant Cd2+, the trend is that expected on the basis of the Coulomb attraction. But it can be seen that the cluster involving the trivalent dopant In3+ has the same binding energy for both the first and second nearestneighbor configurations. This is explained based on the lattice relaxation arguments mentioned above, which become competitive with the electrostatic attraction, owing
Catalysis by ceria and related materials
Table 8.1. Binding energies (eV) of (Cd'^V^ n
* and
c u s t e r s at
V Ce^o) ' increasing separation. Note that the sign notation has been made consistent with definition 8.7 (from ref. 54) Relative position
(Cd£eVQ)x
First neighbor
-1.14
-0.44
Second neighbor Third neighbor
-0.88 -0.31
-0.42 -0.10
{In'CeV0)'~
to the lower charge of the In'Ce defect. Although the values presented in Table 8.1. were revised in a subsequent paper,55 the general concept of the lattice relaxation effect was confirmed and found applicable to more complex cluster geometries. 8.4.
Oxygen Ion Migration
Oxygen ion migration in ceria-based materials has attracted considerable attention largely due to possible applications as the electrolyte within solid oxide fuel cells (SOFCs);15"20 certain systems (especially Gd doped Ce0 2 ) exhibit highly favorable oxygen ion conductivity in comparison to the conventional Y/ZrC>2 solid electrolyte. Ceria-based electrolytes may allow lower temperature operation of SOFCs, although there are problems associated with electronic leakage currents.20 Applications in threeway catalyst technology are also dependent upon oxygen mobility, since the oxygen uptake/release process must necessarily involve a diffusion step from the surface to the bulk and vice versa. Again, computational techniques have proved particularly useful in rationalizing the underlying mechanistic factors. It is widely accepted that the migration of oxygen in ceria and ceria-based materials takes place via a vacancy hopping mechanism.56 The activation energy for oxygen migration can be estimated using atomistic (static lattice) calculations. It is assumed that an oxide ion migrates to a nearest-neighbor vacant site along a linear path. An energy profile can be determined by simulating the oxide ion at a number of intermediate positions along this migration path. The activation energy is finally obtained as the difference between the maximum energy ("saddle-point") experienced by the migrating oxide ion and the energy of the starting configuration (Fig. 8.3.). Using this type of calculation, Balducci et al. evaluated an activation energy for oxygen migration of 0.63 eV41 and 0.57 eV48 in pure and thoria-doped ceria, respectively. These
Computer simulation studies of ceria-based
oxides
291
o 0*o or O'l D Vo
o c LU
Distance along the migration path Figure 8.3. site
Schematic illustration of the energy profile for an oxygen ion migrating to an adjacent vacant
values are in reasonably good accord with corresponding available experimental data of 0.49 eV51 and 0.75 eV.58 Other computational determinations52'54,59 compare similarly with experimental values.60'61 The main experimental techniques used to study oxygen migration in doped cerias are based on the AC impedance analysis of the measured electrical conductivity. It is found that the oxygen ion conductivity of ceria-based oxides depends strongly upon the dopant size and concentration. Both these factors are related to defect association between oxygen vacancies (the charge carriers) and other defects (mainly dopant substitutionals) which were discussed in section 8.3.2.. Results obtained with static calculations have shown the strong dependence of the binding energy of dopant-vacancy clusters upon the dopant size. It appears that a minimum association energy (and hence a maximum oxygen mobility) is obtained for an "optimum" size of the dopant. For instance, introduction of gadolinia into ceria causes only negligible changes in the lattice parameter; accordingly, Cei-xGdx02-f 62,63 is known to be one of the best oxide ion conductors. It is known that as the dopant content increases, the conductivity rises, but reaches a maximum followed by a steep decrease.16 The exact dopant concentration for maximum conductivity depends to some extent upon the dopant type, but is generally around 10 — 15% mol for trivalent dopants.
292
Catalysis by ceria and related materials
At very low dopant concentration (less than 1%) oxygen migration is adequately modeled by taking into account the formation equilibrium of simple pair clusters (equations (8.8) or (8.9) with n = 1), so that the concentration of charge carriers (oxygen vacancies) is regulated by the following equations (in the case of a trivalent dopant):
[V6\ Wee [WbeVSY]
[V6] + [(MbeV6Y] =
K c
(8.10)
°(^l)
<8-n>
where K is the equilibrium constant, x is the mole fraction of the dopant oxide in ceria, (Ce02) 1 _ a . (M2C>3)X, and C0 is the concentration of cationic sites. However, both experimental and theoretical considerations point out that such a simple model breaks down at higher dopant contents. For instance, in an 1% yttria-ceria solid solution, only 70% of the YQC substitutionals do not have another defect of the same type in nearestneighbor or next nearest-neighbor positions.60 This means that even at concentrations of a few percent, more complex cluster configurations must be taken into account for a reasonable description of the system. Murray et al. have simulated oxygen migration in yttria-doped ceria by a combination of static and Monte Carlo methods.59 First, they evaluated the activation energy for the hop of an oxide ion into an available vacant site. In order to take into account the interaction of the migrating oxygen with dopant substitutionals, they considered the presence of a number (0 to 6) of dopant cations in nearest-neighbor positions with respect to the anion sites involved in the migration. This resulted in a total of 30 possible energetically different configurations. In a subsequent Monte Carlo simulation, each oxygen migration attempt was given a probability proportional to the calculated activation energy. The plots of the resulting conductivity versus dopant concentration reproduce qualitatively the main features of the experimentally observed maximum at 833 and 455 K64 (Fig. 8.4.). In the assumed model, the decrease of conductivity with dopant concentration is explained with the increase of YQ6 centers, which trap oxygen vacancies more efficiently. In the Monte Carlo method of Murray etal. the long-range Coulombic interaction between charged defects was neglected. By using the same Monte Carlo techniques, Adler et al. tested a model for oxygen transport in yttria-doped ceria that includes
Computer simulation studies of ceria-based
oxides
293
-2 -
T = 833 K
T = 455 K
0.00
0.04
0.08
fraction of V
0.12 o
Figure 8.4. Experimental (full lines) and calculated (dashed lines) d.c. ionic conductivities at two different temperatures for yttria-doped ceria as a function of the composition (adapted from ref. 59)
such effects.57'65 For each possible oxygen vacancy jump, the Coulombic contribution of the whole lattice to the activation energy was computed and the jump probability was assessed. The diffusion coefficient was also calculated, from which the electrical conductivity was evaluated at different temperatures for low dopant levels (0.019 - 0.21%), showing good agreement with available experimental data.60 From the results of these Monte Carlo simulations, a model based on Debye-Hiickel theory was put forward: the concentrations of vacancies and dopant cations are corrected by activity coefficients for the effect of charge clouds that form as a result of long-range Coulombic forces.57 However, the variation in conductivity with increasing dopant concentration based on attractive interactions between vacancies and dopant substitutionals has been recently questioned by Meyer et al.66 These authors performed Monte Carlo sim-
294
Catalysis by ceria and related materials
ulations on generic {MOi)l_x (DOr)x systems with the fluorite structure (where M = host cation, D = guest cation, r = 1 for divalent dopants, r = 3/2 for trivalent dopants). In order to take into account the effects of dopant concentration, the sites of the anionic sublattice in the fluorite structure were classified according to the number t of nearest-neighboring dopant cations (t values range from 0 to 4). Then, three different possible models were considered for the migration of oxide ions: (a) nearestneighbor attraction, in which the energy of an anion site is lowered by an amount proportional to t; (b) nearest-neighbor repulsion, in which the energy of an anion site is increased by an amount proportional to t; (c) barrier model, in which the activation energy for migration is increased by a fixed amount whenever the hop of the oxide ion takes place between two sites of which at least one is of type t > 1. Meyer et al.66 find that model (a) gives the worse results, being unable to produce any maximum in electrical conductivity. Model (b) gives rise to a peak in the expected region, but a second peak originates at higher dopant concentrations, not observed by experiment. Model (c) is the only one which produces a reasonable accord with experimental data. These results are interpreted in terms of percolation theory,67 in which a high oxygen migration rate is expected if the sample sides are connected by a continuous path of neighboring sites of the given type. At low dopant concentrations, there are many percolating paths, so that oxygen ions can easily circumvent the "obstacles" due to the presence of dopant cations. As the dopant level increases, more diffusion paths are blocked, leading to a decrease in oxygen mobility. Oxygen migration in doped cerias has been recently studied with molecular dynamics methods by Inaba et a/.68 and Hayashi et al.69 These authors examined the systems {CeC>2)l_x (M203)x,2, (where M = Y,Gd,La) with dopant contents in the range 0 — 15% mol. The diffusion coefficient of oxygen at 1273 K was evaluated from the simulations at different compositions with a maximum at around 10% mol of dopant. Different interpretations are given for the behavior of the oxygen diffusion coefficient as a function of composition. First, it turns out that trimers of type (M'Ce VQ M'Ce)x are formed with increased probability at increasing dopant content. Since the local structure of these associates is distorted with a shorter M'Ce — VQ' distance, the migration of oxygen ions into the vacant sites of the anion sublattice becomes more difficult. A second possible explanation is given in terms of Coulombic repulsion between the vacancy defects, which becomes more appreciable as the dopant content increases. Finally, another factor which could decrease the oxygen mobility is the increase of dopant-dopant pairs, similar to that advanced by Shimojo et al.10 for the mechanism of oxygen migration in yttria stabilized zirconia (YSZ).
Computer simulation studies of ceria-based oxides
8.5.
295
Surface Properties
The preceding sections have focused on the properties of the bulk oxide. However, computer simulation techniques are also well established tools in the study of the structural and defect chemistry of oxide surfaces, which are often difficult to characterize by experiment alone. 8.5.1.
Surface Structures of Ce02
Using atomistic (static lattice) methods, Sayle et al.lx first modeled the (110), (310) and (111) surfaces of CeO^. The (110) and (310) surfaces are known as type I surfaces: i.e. they are charge neutral with stoichiometric proportions of anions and cations in each plane (parallel to the surface). The potential for each plane is exactly zero due to the cancellation of the effects of the positive and negative charges and therefore there is no dipole moment perpendicular to the surface. The (111) surface of ceria is a type II surface, i.e. the surface terminates with a single anion plane and consists of a neutral three-plane repeat unit. Surface energies were calculated by the following expression:
75
=
^
^
(8.13)
where Es is the energy of the surface region, EB is the energy of the perfect crystal with the same number of ions as in the surface region and A is the surface area. Results of Sayle et al.lx for the three surfaces are reported in Table 8.2.. Based on energetic criteria, the relative stability of the surfaces of CeC>2 is in the order (111) > (110) > (310) which remains the same before and after relaxation. We note that using similar simulation methods, Conesa72 finds the same order of surface stability. Table 8.2. shows, however, that relaxation energies are substantial for these surfaces, which emphasizes the point that surface relaxation cannot be omitted in any quantitative study of surface energies and structures. Vyas et alP investigated an extensive set of ceria surfaces using four different interatomic potential models; they find that while there are differences in the absolute surface energies the relative energies do not vary, producing an octahedral-type crystal morphology. In addition to atomistic (static lattice) methods, MD techniques have been used by Baudin et al.74 to study 20 - 30 A thick CeOi slabs with 2-D periodicity of the three low index surfaces (111), (011) and (001). The simulations were performed within a
Catalysis by ceria and related materials Table 8.2. Calculated surface energies of CeO-z surfaces (from ref. 71) Surface (111) (110) (310)
Energy (J/m2) Unrelaxed Relaxed 1.707 1.195 3.597 1.575 11.577 2.475
NPT ensemble and used the shell-model to describe polarizability. All simulation runs were performed at atmospheric pressure and in the temperature range 10 — 1100 K. For all three surfaces at both 300 and 1100 K it was found that the surface mean square displacements are generally larger for the oxide ions than for the cations and that the out-of-plane surface motion is usually larger than the in-plane surface motion. At room temperature, the oxygen mean square displacements at the (111) surface are a factor 1.2 larger than in the bulk, a factor 1.6 for the (011) surface and approximately five times larger at the metastable (001) surface compared to the bulk. The effect of the presence of a surface on the ion dynamics (and on the structure for (Oil)) persists all the way to the slab centers, even for these rather thick slabs. Extending static lattice and MD simulations (based on interatomic potentials), Gennard et al.15 have presented high quality QM calculations at a periodic HartreeFock level on CeOz surfaces. These calculations have established the high degree of ionicity with net charges very similar to the formal values of +4 and —2 on the metal and oxygen ions, respectively. Gennard et al. have calculated the surface energies of the (011) and (111) faces of both ceria and zirconia. They find that interatomic potential-based methods provide a correct estimate of the surface relaxations and the correct order of stability of the two faces examined, with the energy difference between the (011) and the (111) surfaces being approximately 1 J/m2, as found in the QM study. However, interatomic potential-based methods do not discriminate adequately between the properties of the two materials. It was also found that geometric and electronic relaxations in the (111) surface are confined to the outermost oxygen ions, while in the (011) slabs they are more important and extend to the subsurface layers in a columnar way. The unsaturation of the surface ions in the (011) face may have important implications for catalytic activity.
Computer simulation studies of ceria-based oxides
8.5.2. Surface Structures
297
ofCe02-Zr02
It was observed recently that incorporation of zirconia into ceria to form a solid solution gives a material in which the reducibility of C e 4 + is greatly enhanced.21'23 These considerations prompted Balducci et al.16 to extend their bulk simulations47 to the modeling of the surfaces of CeC>2 — ZrC>2 solid solutions. In Fig. 8.5. the surface energy calculations for the (110), (111) and (310) surfaces of the Ce\-xZrx02 system as a function of zirconia content (x) are reported, both before and after relaxation. It is clear that the surface energies of the fully relaxed structures are lower than those for the unrelaxed structures. It is also found that the (111) surface is the most stable at all compositions and will probably dominate the low temperature crystal morphology in the absence of dopants or surface irregularities. The same result has already been found in other computational studies of pure ceria.51,71,72 The relaxed energies of the (110) and (111) surfaces exhibit a maximum at a zirconia fraction of about 0.5. Interestingly, the best performance of this material with regard to oxygen storage capacity has been observed for cubic samples of this composition.77 Since these two surfaces are expected to be present in polycrystalline samples, the above correlation may be explained in terms of a higher activity due to a lower stability of the surface. However, it must be noted that the increase in the relaxed surface energy at the 0.5 zirconia fraction is not very large suggesting that this could be only one among several factors. The (310) surface of the fluorite structure is a stepped surface with a high coordinative unsaturation which explains both the high unrelaxed energy and the large amount of relaxation (Fig. 8.5.). Balducci et al.16 find that upon energy minimization, the (111) and (110) surfaces show only slight relaxation, whereas the (310) surface undergoes extensive reconstruction (Fig. 8.6.). In addition, the relaxed energy of (310) decreases with increasing zirconia content and reaches values comparable with those of the (110) surface. This can be explained by the smaller ionic size of Zri+ (0.84 A) in comparison with C e 4 + (0.97 A), together with the preference of zirconium for a lower coordination number. Both factors should relieve the elastic strain in the relaxed surface. It has been observed that ceria films vapor deposited on zirconia and zirconiabased substrates (such as yttrium-stabilized zirconia) are more easily reduced than films supported on a - Al20z?6 In this context, Maicaneanu et al.n employed a simulated amorphization and recrystallization methodology79 to explore the structural changes that evolve within ZrC>2 (HI) supported CeC>2 • This method involves straining the C e 0 2 thin film under considerable pressure and placing it on top of a ZrC>2
298
Catalysis by ceria and related materials
(111)
> LU
0.0 0.2 0.5 0.8 1.0 x in Cei_xZrx02
(310)
0.0 0.2 0.5 0.8 1.0
0.0 0.2 0.5 0.8 1.0
x in Cei_xZrx02
x in Cei_xZrx02
Figure 8.5. Energy of the (111), (110) and (310) surfaces of the Ce.\-xZrxOi function of x, before (o) and after (•) relaxation (from reference 76)
mixed oxide as a
support. MD techniques are then applied to the system at high temperature upon which the CeOi amorphises. Under prolonged dynamical simulation, the Cz02 recrystallises revealing a wealth of structural modifications that evolve as the system endeavors to accommodate the lattice misfit, whilst maximizing interfacial interactions. The simulations of Maicaneanu et al.19 find that the final ceria thin film structure exposes the (111) plane at both the interface and surface. This consists of ca. five Ce02 repeat units with an incomplete (ca. 25% occupancy) surface layer, which comprises small clusters (e.g. Ce2C>4 and CeiOs) and larger clusters of up to 500 A in size. A detailed graphical analysis revealed that the system comprises cerium
Computer simulation studies of ceria-based
oxides
299
[1111
ototototo ototototo OtOtOtOtO OtOtOtOtO
[110]
OtOtOtOtO OtOtOtOtO
[310]
Figure 8.6. Side view of the surface structures before (left) and after (right) relaxation for the (111), (110) and (310) surfaces of the Ce\-xZrxC>2 system. Empty circles are oxygen ions, filled circles are cations (adapted from reference 76)
(ca. 0.8%) and zirconium (ca. 0.3%) vacancies which are charge compensated by associated oxygen vacancies. In addition, dislocations including pure edge and mixed screw-edge dislocations have evolved in both the CeO? thin film and within the Zr02 support. Experimentally, dislocation arrays with periodicity of ca. 44 A were observed to accommodate the lattice misfit for ceria supported on YSZ.80
300
Catalysis by ceria and related materials
Table 8.3. Energies of redox reactions in CtOi (from ref. 71)
Defect equilibria (a) 01 + 2Ce*e = \02{g) + V0 + 2Ce'Ce (b) CO + \02{g) = C02{g) (c) CO{g) + Ce02 = C02(g) + VQ + 2Ce'Ce (d) Binding energy (e) Cluster effect
Bulk 6.58 -2.93 3.65 -0.60 3.05
Energy (eV) (111) (110) 2.71 -0.47 -2.93 -2.93 -0.22 -3.40 -0.40 0.57 -0.62 -2.83
(310) -6.25 -2.93 -9.18 7.25 -1.93
(a) Formation of oxygen molecule from cerium oxide (b) Enthalpy of carbon monoxide oxidation (c) Overall reaction (d) The difference between the formation energies of the neutral clusters and the corresponding isolated defects (e) Energy of reaction (c) with addition of binding energy
8.5.3. Surface Redox Behavior ofCe02 and
Ce\-xZrx02
As in the bulk oxide, the redox behavior of ceria surfaces was examined on the basis of the energetics of reaction 8.2, which involves reduction of cerium species (Ce 4 + to Ce 3 + ) and formation of oxygen vacancies. Sayle et al?x first calculated these energies of reduction (and of CO oxidation) in the bulk and at the surfaces of pure ceria (which we summarize in Table 8.3.). The results reveal that the energy of reduction of Ce02 (equation (a)) is more exothermic for oxygen abstraction from the surfaces than from the bulk. Moreover, the energies of reduction on the (110) and (310) surfaces are more exothermic owing to the lower reduction energy. This behavior may be correlated with the fact that the (110) and (310) surfaces are less stable and hence more active. While no direct comparison can be made with the calculated energies, experimental values for the relative partial molar enthalpy of oxygen atoms in the bulk of ceria have been reported to be 4.98 ± 0.33 eV at 1073 - 1273 K.81 Sayle et al.lx then considered the role of clustering in stabilizing defects in the bulk and at the surfaces of the material. The strength of the interactions between the components of a defect cluster is measured by its binding energy, in this case between an oxygen vacancy and two Ce 3 + substitutionals. Clusters in the bulk and at the (111) surface are found to be bound whereas those at the (110) and (310) surfaces are unbound: aggregation is not therefore expected on the latter surfaces. Simulations of Conesa72 also show a tendency towards defect association at the (111)
Computer simulation studies of ceria-based oxides
301
surface, but not at (110) or (100). This suggests that the redox reactivities of ceria materials with different proportions of these surfaces might follow different ordering, depending on whether the redox reaction involves two closely located vacancies {e.g. for adsorption/desorption of O2). The most significant result of the study of Sayle et al.11 is the demonstration that the oxidation of carbon monoxide using oxygen from CeOi is exothermic at the (110) and (310) surfaces: the enthalpy for the same reaction in the bulk is endofhermic. Thus they predict that any processing conditions which favor the formation of the (110) and (310) surfaces will result in enhanced activity towards oxidation. Balducci el al.16 extended these simulations on pure ceria by modeling the surface defects of the mixed Ce\-xZrx02 system across the whole composition range. The formation of neutral (Ce'M VQ Ce'M)x clusters on the relaxed surfaces was considered, so that the calculated reduction energies include a binding energy term. Fig. 8.7. reveals that an increase in the zirconia content of the solid solution favors the reduction process on both (110) and (111) surfaces. On the latter surface, a sharp decrease is calculated for a zirconia fraction of about 0.8. However, the reduction process is still unfavorable on the (111) surface in comparison with the other surfaces and with the bulk. It is noted that recent Temperature Programmed Desorption (TPD) studies on CeC>2 find no significant desorption of oxygen on the (111) surface.82 Balducci et al.76 also note that the reduction energies ( « 1.5 eV) from their previous bulk calculations47 for zirconia fractions greater than about 0.1 are not significantly higher than those found here for the (110) surface, whilst they are more favorable than the values for the (111) surface. This is consistent with the experimental findings of a large bulk participation in the reduction process during temperature programmed reduction experiments.83 The behavior of the reduction energy as a function of zirconia content (a;) for the (310) surface seems to be opposite to that found for the (110) and (111) surfaces. At a fixed composition, the reduction energy heavily depends on the particular surface cluster. This suggests a complex topography with a wide variety of different reduction sites. At high zirconia contents the (310) surface stability becomes comparable with that of the most stable surfaces (Fig. 8.7.). The results of Balducci et al. therefore suggest that reconstruction with consequent creation of low energy routes toward C e 4 + / C e 3 + reduction may be one of the key factors in determining the high oxygen storage capacity observed for these materials. Computational studies of de Carolis et al.S4 have combined MD simulations of both Ca-doped and undoped Ce02 with QM electronic structure calculations of embedded cluster models built from the MD structures. Several interesting aspects were
302
Catalysis by ceria and related
materials
(111) l
l
4.0 # - * - • 3.5 3.0 2.5
>
LU
i
I
1
1
' *-*-»-«
\
\
i
i
-
V
0.0 0.2 0.5 0.8 1.0
x in Cei_xZrx02
(110) I
I
1.00 > 0.75 0.50 III 0.25
(310)
1 1 %
-
> (LI
red
UJ i
i
i
i
0.0 0.2 0.5 0.8 1.0
0.0 0.2 0.5 0.8 1.0
x in Cei_xZrx02
x in Cei_xZrx02
Figure 8.7. C e 4 + / C e 3 + reduction energy on the (111), (110) and (310) surfaces of the Cei-xZrxC>2 mixed oxide as a function of x. The plot for the (310) surface was obtained by a polynomial fit of the calculated reduction energy values (from reference 76)
examined: in particular, the finding of coordinatively unsaturated cerium ions at all depths in the doped crystal leads to interesting possibilities for charge-transfer (CT) reactions in which the Ce 4 + may accept an electron from a neighboring oxygen anion or some other electron donor to become Ce3+. The computed CT energies leading to a C e 3 + species are found to be significantly reduced, but still insufficient to explain the observed reactivity of the material; the energy cost is still of the order of 2 - 3 eV which is too high to lead to exothermic reactions. It is suggested that additional temperature-controlled distortions could be important for a complete understanding of chemical reactions involving doped and undoped CeO-z.
Computer simulation studies of ceria-based oxides
303
8.5.4. Surface Segregation of Oxygen Vacancies and Metal Ions Previous surface modeling work on metal oxides has demonstrated the importance of the variation of the defect energy as the defect penetrates from the surface into the bulk, an effect that can lead to the surface segregation of defects. Balducci et al. have therefore studied the oxygen vacancy energy as a function of the distance from the surface of the Cei-xZrx02 system.76 Results are displayed in Fig. 8.8. for the (110) and (111) surfaces. As can be seen, there are energy barriers to the penetration of the oxygen vacancy from the surface into the bulk. This suggests that oxygen vacancies tend to segregate to the (110) surface at all compositions and this tendency increases with increasing zirconia content, particularly for zirconia fractions greater than about 0.5. The tendency for segregation of the oxygen vacancies on the (111) surface is much less pronounced than that on the (110) surface. Automobile exhaust catalysts typically contain noble metals such as Pt, Pd and Rh with a ceria promoter supported on alumina.7'10,51'85"89 Traditionally, the principal function of the Rh is to control emissions of nitrogen oxides (NOx) by reaction with carbon monoxide, although the increasing use of Pd has been proposed. For example, recent X-ray absorption spectroscopy studies of Holies and Davis88 show that the average oxidation state of Pd was affected by gaseous environment with an average oxidation state between 0 and +2 for a stoichiometric mixture of NO and CO. Exposure of Pd particles to NO resulted in the formation of chemisorbed oxygen and/or a surface oxide layer. In this context, Sayle et al.90 have used atomistic simulation methods to investigate the interaction of ceria with impurities, particularly rhodium, palladium and platinum. The energetics of the most common valence states for the metal atoms were investigated, as well as the variation of the energy of the impurities with depth below the surface and the tendency of defects to segregate to the surfaces. Fig. 8.9. shows the substitutional defect formation energies as a function of the distance from the (111) and (110) surfaces of Ce02 for Ce3+, Rh3+, Pd?+ and Pt2+. Sayle etal. found that for both surfaces, the energies increase with depth monotonically toward bulk values. Thus all defects will segregate to the surface under equilibrium conditions, with no significant barrier to segregation. The results also suggest that substitutional formation is most favorable on the (110) surface. This supports the view that the (110) surface will be more catalytically active than the (111) surface, as impurities segregate preferentially to this surface. Sayle et al.90 note that the segregation energies (i.e. the differences between bulk and surface energies) are larger for the M 2 + cations than for M 3 + cations due to elec-
304
Catalysis by ceria and related materials
x in Cei_xZrx021 .(T^T)
x in C e i _ x Z r x 0 2 U T ^ ^ *
Depth
Depth
Figure 8.8. Oxygen vacancy energy in the Ce\-xZrx02 system as a function of both x and depth (expressed in lattice parameter units) for the (111) and (110) surfaces (from reference 76)
trostatic factors. The results show that Rh3+ (ionic radius of 0.680 A) has a lower segregation energy than C e 3 + (1.034 A) indicating a substantial force for segregation from the reduction in elastic strain, i.e., the larger the defect the higher the tendency to migrate to the surface as segregation is largely promoted by the release of strain obtained by incorporating large ions at the surface rather than in the bulk.
Computer simulation studies of ceria-based
oxides
5 10 15 20 25 Depth (A)
5 10 15 20 25
0 5 10 15 20 25 Depth (A)
0 5 10 15 20 25 Depth (A)
305
Depth (A)
Figure 8.9. Formation energies as a function of depth for the (111) (o) and (110) (•) surfaces of ceriafor C e 3 + , Rh3+, Pd2+ and Pt2+ defects. The dashed horizontal lines show the defect energy in the bulk (from reference 90)
8.6.
Conclusion
This survey has aimed to demonstrate that computational techniques can play a valuable role in contemporary studies of ceria-based oxides, which complement related experimental work, and provide information that is relevant to catalytic and fuel cell applications. Materials that were investigated include pure CeOi, M 3 + doped Ce02 (where, for example, M = 5c, Y, Gd) and the mixed Ce\-xZrxOi system. These simulation studies, based on either static lattice, MD or ab initio methods, have been able to provide deeper insight as to the fundamental solid state properties at the atomic level, particularly in the following key areas: (a) defect chemistry and dopant-vacancy association, (b) mechanisms of oxygen ion migration, (c) structures and stability of surfaces (mainly (110) and (111)), (d) energetics of redox reactions
Catalysis by ceria and related materials
306
involving oxygen vacancies and C e 4 + / C e 3 + reduction, and (e) surface segregation of oxygen vacancies and metal ions. Future developments in this field are likely to include the growing use of ab initio QM techniques to study, for example, the reactions of molecules at ceria surfaces, which will be assisted by the continuing growth in computer power. Acknowledgments: We are grateful for useful discussions with J. Gale and J. Harding. For financial support, we are grateful to the University of Trieste, MURST PRIN 2000 "Catalysis for the reduction of the environmental impact of mobile source emissions", CNR (Roma) Programmi Finalizzati "Materiali Speciali per Tecnologie Avanzate II", Contract n. 97.00896.34, Fondo Trieste 1999 and CNR (Roma), "Short Term Mobility Program".
8.7.
References
1. Catlow, R. C. A., Ed., Computer Modelling in Inorganic Crystallography, (Academic Press, London, 1997). 2. Bernholc, J., Phys.Today 52 (1999), 30-35. 3. Islam, M. S., J.Mater.Chem. 10 (2000), 1027-1038. 4. Summers, J. C , Ausen, S. A., J.Catal. 58 (1979), 131. 5. Yao, H. C , Yao, Y. F. Y, J.Catal. 86 (1984), 254-265. 6. Gandhi, H. S., Shelef, M , Stud.Surf.Sci.Catal. 30 (1987), 199. 7. Taylor, K. C , Catal.Rev.-Sci.Eng. 35 (1993), 457-481. 8. Shelef, M., Graham, G. W., Catal.Rev.-Sci.Eng. 36 (1994), 433-457. 9. Mc Cabe, R. W., Kisenyi, J. M., Chem.Ind. (1995), 605-608. 10. Trovarelli, A., Catal.Rev.-Sci.Eng. 38 (1996), 439-520. 11. Kreuzer,T., Lox,E. S., Lindner, D., Leyrer, J., Catal.Today 29 (1996), 17-27. 12. Holmgren, A., Duprez, D., Andersson, B., J.Catal. 182 (1999), 441-448. 13. Daturi, M., Finocchio, E., Binet, C , Lavalley, J. C , Fally, F , Perrichon, V, J.Phys.Chem.B 103 (1999), 4884-4891. 14. Mamontov, E., Egami,T., Brezny,R., Koranne,M., Tyagi, S.,J.Phys.Chem.B 104 (2000), 11110-11116. 15. Sharma, S., Hilaire, S., Vohs, J. M., Gorte, R. J., Jen, H. W., J.Catal. 190 (2000), 199-204. 16. Steele, B. C. H., Kilner, J. A., in Non-Stoichiometric Oxides, Toft Sorensen, O., Ed., (Academic Press, N.Y., 1981). 17. Tuller, H. L., J.Phys.Chem. Solids 55 (1994), 1393-1404. 18. Inaba, H., Tagawa, H., Solid State Ionics 83 (1996), 1-16. 19. Mogensen, M , Sammes.N. M„ Tompsett, G. A., Solid State Ionics 129 (2000), 63-94. 20. Steele, B. C. H., Solid State Ionics 129 (2000), 95-110. 21. Murota, T, Hasegawa, T, Aozasa, S., Matsui, H., Motoyama, M., J.Alloys Compd. 193 (1993), 298-299.
Computer simulation studies of ceria-based oxides
307
22. Balducci, G., Fornasiero, P., Di Monte, R., Kaspar, J., Meriani, S., Graziani, M., Catal.Lett. 33 (1995), 193. 23. Zamar, F., Trovarelli, A., de Leitenburg, C., Dolcetti, G., J.Chem.Soc.Chem.Comm. (1995), 965. 24. Overbury, S. H., Huntley, D. R., Mullins, D. R., Glavee, G. N., Catal.Lett. 51 (1998), 133-138. 25. Kaspar, J., Fornasiero, P., Graziani, M., Catal.Today 50 (1999), 285-298. 26. Putna.E. S., Bunluesin, T., Fan.X.L., Gorte.R. J., Vohs,J. M., Lakis,R. E., Egami.T., Catal.Today 50 (1999), 343-352. 27. Descorme.C, Madier, Y, Duprez, D., J.Catal. 196 (2000), 167-173. 28. Janvier, C , Pijolat, M., Valdivieso, F, Soustelle, M., Solid State Ionics 127 (2000), 207222. 29. Fally, F, Perrichon, V., Vidal, H., Kaspar, J., Blanco, G., Pintado, J. M., Bernal, S., Colon, G., Daturi, M., Lavalley, J. C , Catal.Today 59 (2000), 373-386. 30. Daturi, M., Bion, N., Saussey, J., Lavalley, J. C , Hedouin, C., Seguelong, T., Blanchard, G., Phys.Chem.Chem.Phys. 3 (2001), 252-255. 31. Meyer, M., Pontikis, V, Eds., Computer simulation in materials science, (Kluwer, Netherlands, 1991). 32. Dovesi, R., Pisani, C., Roetti, C , Eds., Hartree-Fock ab initio treatment of crystalline systems, volume 48 of Lecture Notes In Chemistry (Springer, Berlin, 1988). 33. Dick, B. G., Overhauser, A. W., Phys.Rev. 112 (1958), 90. 34. Gale, J. D., J.Chem.Soc,Faraday Trans. 93 (1997), 629-637. 35. Khan, M.S., Islam, M. S., Bates, D. R., J.Phys.Chem.B 102 (1998), 3099-3104. 36. Ammundsen, B., Roziere, J., Islam, M. S., J.Phys.Chem.B 101 (1997), 8156-8163. 37. Allan, N. L., Baram, P. S., Gormezano, A., Mackrodt, W. C., J.Mater.Chem. 4 (1994), 817-824. 38. Smith, W., Forester, T. R., J.Mol.Graphics 14 (1996), 136-141. 39. Khan, M. S., Islam, M. S., Bates, D. R., J.Mater.Chem. 8 (1998), 2299-2307. 40. Sastre, G., Catlow, C. R. A., Corma, A., J.Phys.Chem.B 103 (1999), 5187-5196. 41. Demontis, P., Suffritti, G., Chem.Rev. 97 (1997), 2845. 42. Dovesi, R., Saunders, V., Roetti, C., Causa, M., Harrison, N., Orlando, R., Apra, E., "CRYSTAL95" (University of Torino, Italy, 1996). 43. Harrison, N. M., Saunders, V. R., Dovesi, R., Mackrodt, W. C , Phil.Trans.R.Soc.Lond. A 356 (1998), 75-87. 44. Cora, R, Catlow, C. R. A., Solid State Ionics 112 (1998), 131-135. 45. Triguero, L., de Carolis, S., Baudin, M., Wojcik, M., Hermansson, K., Nygren, M., Pettersson, L., Faraday Discuss. 114 (1999), 351. 46. Payne, M. C , Teter, M. P., Allan, D. C , Arias, T. A., Joannopoulos, J. D., Rev.Mod.Phys. 64 (1992), 1045-1097. 47. Balducci, G., Kaspar, J., Fornasiero, P., Graziani, M., Islam, M.S., Gale, J. D., J. Phys. Chem. B 101 (1997), 1750-1753. 48. Balducci, G., Islam, M. S., Kaspar, J., Fornasiero, P., Graziani, M., Chem.Mater. 12 (2000), 677-681.
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49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69. 70. 71. 72. 73. 74. 75. 76. 77. 78. 79. 80.
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Kundakovic, L., Flytzani-Stephanopoulos, U.J.Catal. 179 (1998), 203-221. Trovarelli, A., de Leitenburg, C , Dolcetti.G., Lorca, J. L.,J.Catal. 151 (1995), 111-124. Cordatos, H., Pord, D., Gorte, R. I , J.Phys.Chem. 100 (1996), 18128. Butler, V., Catlow, C. R. A., Pender, B. E. R, Harding, J. H., Solid State Ionics 8 (1983), 109-113. Gerhardt-Anderson, R., Nowick, A. S., Solid State Ionics 5 (1981), 547-550. Pryde, A. K. A., Vyas, S., Grimes, R. W., Gardner, J. A., Wang, R. P., Phys.Rev.B 52 (1995), 13214-13222. Minervini, L., Zacate, M. O., Grimes, R. W., Solid State Ionics 116 (1999), 339-349. Catlow, C. R. A., J.Chem.Soc,Faraday Trans. 86 (1990), 1167-1176. Adler, S. B., Smith, J. W., J.Chem.Soc.Faraday Trans. 89 (1993), 3123. Giordano, N., Antonucci, V., Bart, J., Maggiore, R., Z.Anorg.Allg.Chem. 484 (1982), 195-202. Murray, A. D., Murch, G. E., Catlow, C. R. A., Solid State Ionics 18-9 (1986), 196-202. Wang, D. Y., Park, D. S., Griffith, J., Nowick, A. S., Solid State Ionics 2 (1981), 95-105. Tuller, H. L., Nowick, A. S., J.Electrochem.Soc. 122 (1975), 255. Kilner.J. A., Brook, R. J., Solid State Ionics 6 (1982), 237-252. Kilner, J. A., Solid State Ionics 8 (1983), 201-207. Nowick, A. S., in Diffusion in Crystalline Solids, Murch, G. E., Nowick, A. S., Eds., (Academic Press, N.Y., 1984). Adler, S. B., Smith, J. W., Reimer, J. A., J.Chem.Phys. 98 (1993), 7613-7620. Meyer, M., Nicoloso, N., Jaenisch, V, Phys.Rev.B 56 (1997), 5961-5966. Stauffer, D., Aharony, A., Introduction to Percolation Theory, (Taylor & Francis, London, 1992). Inaba,H„ Sagawa,R., Hayashi, H., Kawamura, K., Solid State Ionics 122 (1999), 95-103. Hayashi, H., Sagawa, R., Inaba, H., Kawamura, K., Solid State Ionics 131 (2000), 281290. Shimojo, P., Okabe, T., Tachibana, R, Kobayashi, M., Okazaki, H., J.Phys.Soc.Jpn. 61 (1992), 2848-2857. Sayle, T. X. T, Parker, S. C , Catlow, C. R. A., Surf.Sci. 316 (1994), 329. Conesa, J. C , Surf.Sci. 339 (1995), 337. Vyas, S., Grimes, R. W., Gay, D. H., Rohl, A. L., J.Chem.Soc.Faraday Trans. 94 (1998), 427-434. Baudin, M., Wojcik.M., Hermansson, K., Surf.Sci. 468 (2000), 51-61. Gennard, S., Cora.R, Catlow, C. R. A., J.Phys.Chem.B 103 (1999), 10158-10170. Balducci, G., Kaspar, J., Fornasiero, P., Graziani, M., Islam, M. S., J. Phys. Chem. B 102 (1998), 557-561. Fornasiero, P., Di Monte, R., Ranga Rao, G., Kaspar, J., Meriani, S., Graziani, M., J.Catal. 151 (1995), 168. Maicaneanu, S. A., Sayle, D. C , Watson, G. W., Chem.Commun. (2001), 289-290. Sayle, D. C , Catlow, C. R. A., Harding, J. H., Healy, M. J. R, Maicaneanu, S. A., Parker, S. C , Slater, B., Watson, G. W., J.Mater.Chem. 10 (2000), 1315-1324. Wang, A. C , Belot, J. A., Marks, T. J., Markworth, P. R., Chang, R. P. H., Chudzik, M. P.,
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Kannewurf, C. R., Physica C 320 (1999), 154-160. 81. Chang, E. K., Blumenthal, R. N., J.Solid State Chem. 72 (1988), 330-337. 82. Putna, E. S„ Vohs, J. M., Gorte, R. J., J.Phys.Chem. 100 (1996), 17862. 83. RangaRao.G., Kaspar,J., Di Monte, R., Meriani,S., Graziani, M., Catal.Lett. 24 (1994), 107. 84. de Carolis, S., Pascual, J. L., Pettersson, L. G. M., Baudin, M., Wojcik, M., Hermansson, K., Palmqvist, A. E. C , Muhammed, M., J.Phys.Chem.B 103 (1999), 7627-7636. 85. Fornasiero, P., KaSpar.J., Sergo, W., Graziani, M., J.Catal. 182 (1999), 56-59. 86. Yang, L., Kresnawahjuesa, O., Gorte, R. J., Catal.Lett. 72 (2001), 33-37. 87. Smirnov, M. Y, Graham, G. W., Catal.Lett. 72 (2001), 39-44. 88. Holies, J. H., Davis, R. J., J.Phys.Chem.B 104 (2000), 9653-9660. 89. Holmgren, A., Duprez, D., Andersson, B., J.Catal. 182 (1999), 441-448. 90. Sayle, T. X. T., Parker, S. C , Catlow, C. R. A., J.Phys.Chem. 98 (1994), 13625.
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CHAPTER 9 CERIA SURFACES AND FILMS FOR MODEL CATALYTIC STUDIES USING SURFACE ANALYSIS TECHNIQUES
STEVEN H. OVERBURY and DAVID R. MULLINS Oak Ridge National Laboratory, Oak Ridge, TN 37831-6201 USA
9.1. Introduction and Scope During the last few decades there has been increasing use of the "surface science approach" to obtain fundamental information about surface properties and processes relevant to catalytic systems. It is the intent of this Chapter to review the use of this approach to obtain a clearer understanding of processes occurring at surfaces of cerium oxide and model catalysts based upon cerium oxide supported metals. To define the scope of this Chapter it is helpful to state three defining characteristics of the surface science approach. These are 1) control of the environment of the sample, achieved through ultra-high vacuum (UHV) techniques and controlled dosing of gases and surface modifiers, 2) use of well-defined, flat samples of uniform structural characteristics and 3) use of compatible surface analysis techniques, such as electron or ion spectroscopies, to probe the surface atomic and molecular composition, the geometric and electronic structure, and the dynamics and energetics of adsorption, desorption and surface reactions. These characteristics are enlisted to limit the scope of this Chapter. In particular results of studies of highly dispersed materials will not be considered, i.e. practical catalysts, even if they were analysed using a technique such as XPS that is surface sensitive and traditionally thought of as a surface analysis technique. Such results are included within the subject matter of Chapter 4. With a few exceptions, most research applying the surface science approach to study ceria based systems has been performed in about the last five years. As evidence for this statement consider that in a 1995 book which presents a comprehesive review of surface science of oxide surfaces only a single reference to cerium oxide surfaces is cited;1 or that in a 1997 review of structural, electronic and chemisorptive properties of metal films and particles on oxide surfaces there are six references to ceria surfaces;2 or that in a 1998 review of surface studies of supported model catalysts there are only six references to ceria as a support.3 The 311
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driving motivation for most of the work cited in this Chapter is an interest in ceriabased catalysts, especially as used for oxidation and reduction type reactions.4 Other motivating factors however have been an interest in the use of ceria as a potential electrode material for use in fuel cells, as a sensor material,5 for use as an oxidation resistant coating for metals and alloys, in optical devices,6 as an insulator for Siinsulator interfaces,7 or as a buffer layer material in the growth of superconducting films.8 There are a wide variety of surface analysis techniques that can be applied to studies of this type, and books and reviews are available which describe them.9,10 It is appropriate to mention those techniques that are used most frequently in the research described below. Various electron spectroscopies, especially Auger electron spectroscopy (AES), x-ray photoelectron spectroscopy (XPS) and a variant called soft x-ray photoelectron spectroscopy (SXPS), which uses x-rays below 1000 eV as an excitation source, are useful for providing the atomic composition of the surface in the top 0.5-2 nm. SXPS and XPS are useful for providing information about the oxidation state of ceria, and often the identity and adsorption environment of adsorbed molecular species can be determined. Low energy electron diffraction (LEED) provides an indication of surface ordering, but to date it appears that no full dynamic LEED analysis has been carried out in any ceria surface system. Low energy ion scattering (LEIS) using alkali ions and mass spectroscopy of recoiled ions are additional methods that have been applied to ceria surfaces to obtain average short-range surface structural information. Scanning probe techniques, both scanning tunnelling microscopy (STM) and atomic force microscopy (AFM) have been used for ceria films to get information about surface defects, ordering and site structure, but not yet to identify metal particle morphology. Temperature programmed desorption (TPD), involving monitoring of gas phase species during a controlled sample temperature ramp, is useful to identify reaction pathways of surface reactions and give energetics of desorption. To date the surface science approach and techniques such as those described above have been used to study structure of ceria surfaces, the adsorption and desorption of several molecular species on ceria and model ceria supported catalysts, and the co-adsorption and reaction of certain of these molecular species. The results provide a basis for clarifying the elementary reaction steps underlying catalytic processes occurring on ceria based catalysts. In this Chapter it is attempted to review and summarize this research.
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9.2. Techniques for Preparation of Ceria Films and Model Catalysts
9.2.1. Preparation of Model Ceria Supports The first challenge to surface studies of model ceria surfaces is to produce a suitable Ce0 2 surface that can be mounted and manipulated within a UHV system. Typically bulk polished or cleaved single crystals, wafers or polycrystalline foils of the desired material are used for surface studies. This approach has been used in the case of CeC>2. Ceria single crystals can be obtained commercially, and can be cut and polished to a desired orientation. Several studies on the (111) surface of bulk single crystal Ce0 2 have been reported.""13 More commonly thin or ultra-thin films of cerium oxide have been prepared on dissimilar substrates either in situ or ex situ and used for subsequent surface studies. This approach permits the possibility of using substrates that may be easier to mount or handle and which are electrically conductive, an important advantage for studies by analysis techniques using charged particles. It also opens the possibility of varying the extent of reduction of the film or of codeposition of another metal to produce a mixed oxide. There has been considerable research in developing new methods for growing ceria films and characterization of the resulting films. This research has derived from interest in developing chemically stable buffer layers as substrates for growth of high temperature superconductors. Production of sharp metal oxide semiconductor interfaces is also of interest. Ce0 2 is a good candidate for both applications.14'15 Several of these methods are mentioned below.
9.2.1.1.Vapor Deposited and Oxidized (VDO)Films Growth by vapor deposition and oxidation (VDO) of Ce onto a substrate has been used successfully. The simplicity of this approach and its ability to be integrated into UHV systems designed for multiple surface diagnostic methods makes this a common technique for surface studies of chemisorption and surface reaction studies on model catalytic surfaces. Many of the ceria films used in work described below were produced in this way. Ce deposition and oxygen exposure (oxidation) may be performed simultaneously16 or sequentially.17'18 Single crystal metals (Pt,17'19 Cu,18 Pd,20 Ni,16and Ru16) and oxides, including yttrium-stabilized zirconia (YSZ),21 and sapphire,22 have been used as substrates for this approach. Such films have been
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checked by "bulk" ex situ techniques such as XRD and SEM, but more typically in situ LEED patterns or ion scattering have been employed to show the symmetry of the surface, and XPS or AES is used to monitor the ceria growth. In most of the cases cited above, it seems that the ceria film is thought of as continuous and thick enough that it completely covered the substrate surface, as opposed to isolated islands of ceria with exposed substrate. However, for most of the metal substrates (Ni, Pt and Pd) there are strong interactions resulting in reduction or decomposition upon annealing to high temperatures. On Pt it is believed that Pt-Ce surface alloys are formed following annealing to 1000 K.24 Of these, Ru seems the most stable substrate. It can be heated to 1000 K without measureable decomposition, reduction or changes in film morphology.16 Dmoski et al.21 have grown films on three different orientations of YSZ by depositing Ce followed by subsequent oxidation in air. They find epitaxial growth of Ce0 2 and propose that lattice mismatch is relieved by islands which are each orientationally matched to the substrate.
9.2.1.2. Pulsed Laser Deposition This method has been shown to produce highly oriented cerium oxide surfaces Pulsed laser deposition (PLD) is based upon using a high power pulsed laser to ablate a plume of cerium oxide from a ceria target onto the nearby substrate. Rapid crystallization evidently occurs upon deposition and can yield very high quality ceria films. The substrate temperature, ambient oxygen pressure and growth rates are controlled to optimize epitaxy and crystallinity. The crystalline structure and orientation of the substrate is also a controlling factor in determining the structure of the resulting ceria film. Ce0 2 films have been grown on Ge(100),7 amorphous silica,14 single crystal Si(l 11) and Si(100),25"27 bi-axially textured Ni 8,28 ' 29 and on a variety of other oxides including LaA103, SrTi03, sapphire, YBCO and yttriastabilized zirconia (YSZ).15'30"32
9.2.1.3. OPA-MBE and Ion Assisted Growth Another method for production of ceria films is by oxygen-plasma-assisted molecular beam epitaxy (OPA-MBE). This method has been reviewed by Chambers.33 The advantage of the oxygen assisted plasma technique is that for certain films the dissociation of 0 2 is limiting in the growth of fully oxidized films. By use of a plasma source of oxygen, O atoms and ions are provided to the
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substrate during vapor deposition of metal atoms. The oxygen atoms react rapidly with metal atoms permitting production of fully oxidized films. Similarly, in oxygen ion assisted film growth, energetic oxygen ions are supplied with the intent that the additional energy leads to improved film properties. OPA-MBE has been used to grow CeO2(001) and Ce,.xZrxO2(001) on single crystal SrTiO 3 (001). H 35 This method leads to highly oriented, epitaxial CeO2(100) films as indicated by the sharp (002) reflection in the XRD 2 0 scan and by very sharp x-ray pole figures (cp scans) as illustrated in Fig. 9.1.34 Deposition at 973 K leads to flat, continuous films as indicated by AFM. Attempts to grow films on Si (111) and MgO(OOl) gave poorer film quality, attributed to amorphous Si oxides at the interface in the case of Si(l 11) and to lattice mismatch in the case of MgO.
Phi (degree) Figure 9.1. XRD pole figures indicate that a ceria film grown by OPA-MBE on SrTi0 3 , with (001) epitaxy, is highly oriented azimuthally (top and middle). A small portion of the film is oriented to (111) epitaxy (bottom). From Kim et al.
9.2.1.4. Spray Pyrolysis Polycrystalline films can be prepared by spray pyrolysis. In this technique a spray of an aqueous solution of cerium salt is nebulized and directed by a stream of compressed gas onto a heated substrate. Various parameters are important for determining the resulting structure of the ceria film. The effects of the spray solution and of substrate temperature for films deposited upon silica substrates have
316
Catalysis by ceria and related materials
been examined. Non-porous films of cubic Ce0 2 with preferential (100) orientation can be obtained from cerium nitrate solution. Wang et al. ' have studied the effects of rate of spray deposition of cerium acetylacetonate upon film structure as determined by XRD, AFM and Raman. By adjusting the duration of spray pulses and of the interval between pulses they were able to prepare Ce0 2 films on Si(100) at 725 K which were fairly flat (rms roughness not quoted) and with crystallite sizes of 20 nm. There was no indication of columnar growth, but there appears to be no evidence to conclude that the films are highly oriented or epitaxial with the substrate structure. Gorte et al. have used spray pyrolysis as a means to make films expressly to be used for catalytic studies. They have used cerium nitrate solutions sprayed onto NaCl40 and a-alumina41 substrates. For the alumina substrate, deposition and annealing extensively to 970 K leads to flat regions with cracks and crystallite sizes of 9 to 12 nm. Subsequent annealing to 1720 K leads to the growth of a distinctly non-continuous film of faceted crystallites of size 30 to 35 nm.
9.2.1.5. Oxidation of Bulk Ce Metal Another approach is to use metallic Ce foils or single crystals as a substrate upon which to grow the oxide under controlled (e.g. UHV) conditions. This approach is complicated by the difficulty of handling highly reactive Ce metal in air prior to mounting in the surface apparatus. Nevertheless there have been studies of metallic Ce surfaces and of its oxidation.42"45 Contaminations are removed by sputtering, although complete removal of bulk carbon is problematic. One difficulty with this approach is that a thin oxide overlayer grown by controlled exposure of metallic Ce to oxygen is unstable to subsequent thermal treatments. Annealing induces redistribution of the oxygen into the bulk, causing spontaneous auto-reduction of the surface.
9.2.1.6. MO-CVD Thin films of Ce0 2 have been obtained using molecule based metal-organic chemical vapor deposition (MO-CVD). This method is attractive because of the possibility to coat complex shapes and to use lower growth temperatures. The actual application of this technique was accomplished for the first time using a newly developed Ce precursor molecule46 to grow Ce0 2 on YSZ(100). The method
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produced dense, highly oriented epitaxial films that exhibited columnar growth with stacking dislocations of periodicity about 4.4 nm.
9.2.1.7. Other Techniques Ozer has described a method for making ceria films using sol-gel solutions that are aged and then spin coated onto a substrate.6 The films were uniform and crack-free. Electron beam evaporation of Ce metal has been used to grow Ce0 2 onto rolled Ni films for high temperature superconductivity applications.29 Films have been produced using electron beam evaporation of CeC>2, i.e. using Ce02 as a source, onto a Pt metal foils23, 47 or onto a Pt single crystal24 and on sapphire.48 This technique has been incorporated in a UHV system allowing subsequent analysis without removing the sample from the UHV environment.24 Films have also been grown by sputter deposition, especially using rf magnetron sputtering of CeC>2 in a reactive oxygen plasma.49'50
9.2.2. Preparation of Model Ceria Supported Catalysts In most cases studied to date, UHV based studies of model ceria supported catalysts have been based upon in situ vapor deposition of a metal onto a ceria surface or film. This methodology is widely used to produce model catalysts on a variety of oxide surfaces2 and is usually fairly easily implemented in a UHV system. It has been used to deposit Rh, Pt, Pd and Cu onto ceria single crystals or films. The metal vapor deposition rate is often monitored with a quartz crystal monitor or mass spectrometer, while the properties of the metal particles are typically probed by AES or XPS. This combination can provide information about the morphology of the metal overlayer.11' 40 Typically for metals deposited on oxides, the deposited metal first nucleates into small particles that grow with subsequent deposition (Volmer-Weber growth).2 High temperature annealing usually causes further particle coarsening. The morphology of the metal islands, their size distribution and density, where they nucleate on the ceria and the dependence of these factors upon the temperature, oxidation state and structure of the ceria film are of obvious interest, but still little is known about these aspects for metals on ceria. The assumption in this method is that the ceria is "infinitely" thick and the catalytic system is defined by the interaction of the gases with the deposited metal and the exposed ceria (but including its bulk oxygen reservoir) and at the interface between
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Catalysis by ceria and related materials
them. The support upon which the ceria film is grown is not supposed to affect the catalytic processes. In fact, however, it has been shown that oxygen transport between the support and the ceria film can occur (see below). Another approach to preparing model catalysts is the preparation of "inverse supported catalysts". In this approach, the catalytically active metal (usually single crystal) is used as a substrate upon which an oxide is deposited, presumably leaving patches of exposed metal. This approach has been used to study reduction of ceria, and methanation kinetics on Rh as promoted by deposited ceria, and chemisorption of various molecules.51"56 As stated above, it is generally assumed that thick enough ceria layers will continuously cover the metal substrate, placing a limit on the thickness of the ceria islands that can be achieved for an inverse supported catalyst. The different procedures used for the inverse and metal particle on bulk oxide model catalysts is expected to produce differences in thermal stability, morphology and surface structure which may have consequences for the reactivity of the model catalyst.
9.3. Structure of Ceria Surfaces In order to understand the reactivity of ceria surfaces and the interaction of it with metal particles or adsorbates, it is of fundamental interest to know its surface structure and the extent or type of defects present. Even though the film may be an oriented single crystal, there is still the question of whether the surface is terminated in oxygen anions, Ce cations, a mixture or in defects associated with the termination. Charge neutrality, interfacial relaxation and dielectric discontinuities may modify the properties of an oxide surface.57 Also the ability of the surface to adsorb or give up oxygen, as well as the structure, clustering and reactivity of defects may be expected to depend upon the surface orientation and structure.
9.3.1. Theoretical Studies of Structure and Defects on Clean Ceria Surfaces The structure and stability of free Ce0 2 surfaces has been considered theoretically using simulation techniques based upon interatomic potentials and molecular dynamics.58"61 For surfaces examined, it is found that the stability increases as (310) < (110) < (111). The (111) surface is predicted to terminate in an anionic layer of a neutral three plane repeat unit while (110) terminates in a neutral plane of mixed cations and anions. Minor inward relaxation of the outermost anionic layer (0.005
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to 0.09 A) are predicted for (111) but more significant relaxation and buckling of the (110) and the (310) surfaces are expected. Sayle et al concluded that the dipolar (100) face should be inherently unstable with respect to reconstruction or impurity adsorption, and therefore they could not consider its termination. In the bulk crystal, (100) planes are composed alternately of oxygen and cerium ions, leading to an unbalanced dipole at the surface and offering two possible surface terminations. Conesa concluded that the (100) surface can be stabilized by adjusting the amount of outer layer (capping) oxygen. Subsequent work suggested that at high temperature the cationic and anionic terminations are of comparable stability.6'
Figure 9.2. Left: Structure of a CeO,(l 11) surface as relaxed on a cubic ZrO,(l 11) substrate, generated by dynamic simulation. Zr (light blue), Ce(magenta), oxygen in ZrO,(red), oxygen in CeO.(green). Right: Stick representation of a screw-edge dislocation threading through the CeO, layer and the first ZtO, sub-layer. From ref. 62, reproduced by permission of the Royal Society of Chemistry.
The formation and clustering of oxygen vacancies and corresponding Ce'* cations were also considered. The enthalpy for oxygen vacancy formation is more exothermic for the (110) and (310) surfaces compared to (111) suggesting that these surfaces are more easily reduced (e.g by CO) or are more unstable to auto-reduction (i.e. oxygen loss) by thermal anneal in vacuum. Resulting oxygen vacancies are more stable at surface sites than in the bulk, leading to stabilization of vacancies at the surface and promoting segregation of bulk oxygen vacancies to the surface. Conesa examined the energetics of configurations of clusters of oxygen vacancies and Ce" cations. Pairing of oxygen vacancies appeared to be favorable on the (111) surfaces, but not on (100) or (110). The structure and stability of ceria films grown on mismatched substrates have been studied computationally.57'62 A 36% lattice mismatch in growth of Ce0 2 on YSZ is found to be accommodated by the formation of islands and dislocation arrays such as edge and screw dislocations. A configuration obtained by simulation is shown in Fig. 9.2 and demonstrates the complexity of defect structures that might
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Catalysis by ceria and related materials
occur in a supported film. Charge neutrality is maintained by oxygen vacancies throughout the bulk. It seems clear that such defect structures may contribute to variations in the reducibility and therefore catalytic activity in redox type reactions.
9.3.2. Experimental Studies of Surface Structure Experimental studies of structure and defects on ceria surfaces has been undertaken by STM, LEED and ion techniques. In agreement with theoretical predictions, there is evidence that the (100) face is unstable. STM studies of single crystal CeO2(001) revealed a V2/2(3 x 2)R45° surface reconstruction, but following oxygen removal and subsequent reoxidation and annealing a c(3 x 3) was observed. Based upon direct recoil scattering cross sections and angle resolved mass spectroscopy of recoil ions (MSRI) Herman concluded that the surface of a Ce0 2 film, grown by OPA-MBE onto SrTi03, had a nominal (100) orientation and approximately 50% oxygen termination.64'65 However, the MSRI results were consistent only with a structure exhibiting ordered rows of oxygen vacancies. Although the proposed ordering was consistent with the observed ( l x l ) LEED pattern,34 variations in this ordering might be consistent with the STM results. Low energy alkali ion scattering was used to analyze samples of CeO2(100), grown by PLD onto SrTi0 3 , which exhibited ( l x l ) LEED patterns.30 The incident angle dependence was consistent with that expected for unreconstructed (100) but with a mixture of oxygen terminated and cerium terminated surface regions. The relative amounts of each termination varied from sample to sample, but each was relatively stable to annealing and gas treatments. Taken together these results suggest that CeO2(100) terminates in a structure "close" to the expected structure, but that the surface dipole is balanced by variable oxygen content, although the amount and ordering may be difficult to stabilize. In contrast to (100), experimental evidence indicates that the (111) surface is stable. STM micrographs of single crystal Ce0 2 (lll) demonstrate the expected lattice constant and hexagonal symmetry, and the tunneling conditions suggest oxygen termination.66"68 A hexagonal ( l x l ) LEED pattern is also observed.11 The measured step height was consistent to that expected for a neutral three plane repeat unit.66 There appear to be no published measurements of the outerlayer spacings. The stability of the (111) termination is also indicated by the fact that it is preferentially formed on a variety of metal substrates including Pt(lll), 2 4 Cu(lll), 1 8 Pd(lll), 2 0 Ru(0001),16 Ni(lll), 1 6 and even polycrystalline Pt.23 Ceria films grown on Ru(0001) substrate were analyzed by LEIS using alkali ions for
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both oxidized and reduced films. Results demonstrated oriented Ce0 2 (lll) film growth terminating in an unreconstructed oxygen layer for the fully oxidized surface. The data for the reduced surface were most consistent with a fluoritic type lattice containing large numbers of oxygen vacancies and Ce cations in the top layer.
Figure 9.3. STM micrograph of the surface of single crystal C e 0 2 ( i n ) showing atomic and defect structure after annealing in UHV to a) 1225 K b) 1300 K and c) extended anneal at 1300 K. Reprinted from ref. 68 with permission from Elsevier Science.
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Oxygen vacancies have also been imaged on the (111) surface by STM and micrographs from this work are shown in Fig. 9.3,68 Vacancies could be induced by annealing to about 1250 K in vacuum, leading to rows and clusters of oxygen vacancies. A common pattern were holes caused by clusters of three vacancies. Although this configuration was not considered by Conesa, it is in agreement with the general conclusion that vacancy clustering is favorable on the (111) surface. Less work has been carried out on the (110) or higher index surfaces. STM studies on single crystal CeO2(110) indicate that this surface reconstructs to a predominantly ( 2 x 1 ) structure upon annealing to 1215 K,69 possibly due to "missing rows" of both oxygen and Ce. Annealing to 1300 K leads to widening of these missing rows indicative of {111} facet formation, an interpretation confirmed by RHEED. This reconstruction sequence is similar to that observed on (110) surfaces of some metal surfaces. It is proposed that the thermally induced reconstruction and faceting may occur without substantial oxygen loss, and is instead due to diffusional reconstruction. Inoue et al have found {llljfacets when CeO2(110) is grown on Si(100) miscut toward (HO).70 They find that the surface morphology changes with crystallinity of the Ce0 2 film and the number of irregular features increasing with decreasing crystallinity. Besides the growth morphology, the surface atomic structure may also be affected when Ce0 2 is grown on substrates with a large lattice mismatch. For example, Ce0 2 films grown by magnetron sputtering onto R-cut sapphire exhibited (100) orientation, but upon inspection by cross section TEM showed massive faceting to {111} orientation.71 Interestingly, extensive annealing in oxygen induced flattening of the facet tips, resulting in (100) oriented regions. It is speculated that strain due to lattice mismatch may induce island growth during film formation, but the relation to faceting is not clear. Formation of {111} facets and periodic dislocation arrays are also observed for (100) films grown by MO-CVD on YSZ for low growth temperatures.46
9.4. Chemisorption Studies on Clean Ceria Surfaces In identifying and differentiating the roles of the active metal and the ceria support in a model catalyst, it is helpful to study the reactivity and chemisorptive properties of the ceria surface by itself. Generally, ceria and other oxide surfaces are less active than metals to most adsorbates, but substantial uptake occurs in some cases. Most studies performed to date, and summarized below, reflect interest in molecular adsorbates related to emission control and redox type reactions, i.e. small molecule
Ceria surfaces andfilmsfor model catalytic studies
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oxidants and reductants. Some of the important goals are to identify the binding sites, clarify the nature of the chemisorptive interaction and to understand the effect of surface defects, e.g. vacancies and Ce3+ cation sites, upon chemisorptive uptake and subsequent reaction.
9.4.1. CO and C02 It has been reported that CO does not adsorb on clean, single crystal Ce0 2 (l 11) or on CeO2(100) (sputter grown onto r-plane sapphire) under "UHV conditions."11 These surfaces were prepared by annealing at 850 K in 10"7 torr oxygen, intended to fully oxidize it and heal vacancies. It is also reported that little or no CO desorbs in TPD following exposure at room temperature from Ce0 2 films grown by spray pyrolysis onto sapphire surfaces.40 These surfaces are composed of small crystallites (9-12 nm), displaying 1 um sized islands and possible porosity.41 It is also found that no CO adsorption is detected by TPD for surfaces "reduced" by exposure to CO or H2. However, Mullins et al. has reported evidence from SXPS of a tenaciously adsorbed carbonaceous species from CO exposure at low temperature on a reduced ceria surface.72 This surface, oriented to (111), was prepared by VDO in low oxygen ambient. The carbonaceous species was assigned to a carbonate or carboxylate, and it decomposed above 600 K, but was not known to desorb as CO. A lack of CO desorption observed for surfaces with variable structures is difficult to reconcile with work on dispersed ceria powders which are known from FTIR to adsorb CO and retain it in vacuo.171 It would seem that CO adsorption must occur at many structurally distinct sites on the ceria surface. Appearance of surface species on reduced ceria, may indicate that in fact some CO does adsorb, but is not seen in TPD because CO dissociates and the C and O diffuse into the bulk, rather than desorbing. A lack of CO adsorption also implies inability of CO to reduce ceria surfaces, although CO readily reduces ceria powder.4 It is possible that the low pressure conditions of UHV experiments is partly responsible for the general lack of CO adsorption and surface reduction. Li et al have shown that CO species linearly bound to Ce4+ are unstable in vacuum and these may be partly responsible for the reduction of Ce0 2 . 73 C0 2 is known to form from CO adsorption when Pt group metals are present on the surface of ceria. It is also presumed that the C0 2 may be adsorbed on the ceria. In spite of this important observation, there appears to be no published studies of the chemisorption of C0 2 on ceria surfaces. It is known that C0 2 adsorbs readily on dispersed ceria.73
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Catalysis by ceria and related materials
9.4.2. NO, N20 and N02 The adsorption of NO was studied on various ceria samples including PLD CeO2(100)/SrTiO3 and VDO CeO 2 (lll)/Ru(0001) by Overbury et al.74 and on single crystal Ce0 2 (lll), polycrystalline Ce0 2 on sapphire and CeO2(100) on YSZ(100)byFerrizzetal. 75 Ferrizz et al find that annealed, fully oxidized single crystal Ce0 2 (lll) adsorbs little or no NO at room temperature.75 Sputter reduced ceria, or reduced films grown at decreased oxygen pressure do adsorb NO, which dissociates to yield N2 during TPD. They find that for Ce0 2 grown on YSZ, there is always an N2 yield. The yield of N2 increases with increasing annealing of the YSZ supported ceria, an effect attributed to thermally induced reduction of the ceria. They see N 2 desorption in more than one peak in TPD. Overbury et al used SXPS to monitor surface species following NO adsorption. Following NO exposure, various N containing surface species were observed by N Is SXPS and the distribution of these species depended upon surface oxidation state, exposure and adsorption temperature. Sample spectra are shown in Fig. 9.4.74 These species included N 2 0, N0 2 , NO~ and three states believed to be associated with atomic or anionic forms of N, including nitride. N 0 2 and N 2 0 are seen on a fully oxidized surface at low temperature, desorbing as NO and N 2 0 below 400 K. NO", N 2 0 and dissociation products are observed on a reduced surface. The primary reaction of NO with reduced ceria is re-oxidation of the ceria, both by NO" formation and by NO dissociation leading to immediate and thermally induced N2 desorption. The N2 desorption occurs in multiple peaks associated with decomposition of NO and recombination of surface and bulk nitride near 350, 500 and 700 K, respectively. Adsorption of NO at 150 K is predominantly molecular while exposure to NO at 400 K leads to thermally activated nitride formation. Continued exposure of NO at 400 K eventually leads to the replacement of nitride by oxide resulting in N2 formation and desorption. N 2 0 was found to adsorb at 100 K but desorb at low temperatures from the oxidized surface. N 2 0 reacted with reduced ceria at low temperature to oxidize the ceria and evolve N2. Adsorption of N0 2 on ceria surfaces grown on Pt(lll) was studied by Rodriguez et al.using XPS.' 9 They find that on the oxidized surface a nitrate is formed which is stable at 300 K. For a sputter reduced surface, N0 2 partially dissociates yielding states assigned to N, NO and NO3.
Ceria surfaces and films for model catalytic
160
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Figure 9.4. N Is SXPS spectra differentiate N containing states obtained upon exposure of a reduced VDO Ce0 2 (l 11) surface to NO at different temperatures. From ref. 74 with permission.
9.4.3. 02 There is actually little known about the adsorption of this important molecule, in part because its adsorption state is elusive, it presumably reacts with oxygen vacancies, and it is difficult to differentiate from lattice oxygen in surface spectroscopic methods. Evidence for desorbable 0 2 , which can be repopulated by oxidizing in 0 2 above 400 K, has been reported.76'77 Isotopic studies indicate that this state is due to recombination of surface lattice oxygen, rather than adsorbed molecular oxygen, peroxide or superoxide. This state was sensitive to the structure of the ceria as indicated by the fact that it is not seen on single crystal Ce0 2 (l 11), but is seen for films of ceria grown on sapphire and zirconia. Bulk exchange with surface oxygen is seen for the zirconia supported film.
9.4.4. H20,H2and -OH Water has been studied on OPA-MBE grown (100) and VDO grown (111) surfaces of CeC>2.78'79 On both surfaces water is weakly chemisorbed, and following low
326
Catalysis by ceria and related materials
temperature adsorption it desorbs mostly between 150 and 200 K. No evidence of dissociation, yielding H2, was observed for the completely oxidized surfaces. On the (111) surface chemisorbed and condensed H 2 0 could be clearly differentiated in the O photoemission by peaks located 1.6 and 3.7 eV to higher binding from the lattice oxygen peak, respectively. Surprisingly, these photoemission features were not clearly observed on the (100) surfaces in spite of comparable desorption profiles,79 although curve fitting indicated a small peak at 1.5 eV above the lattice peak. The absence of visible condensed water in this case was attributed to 3D ice island formation which thickens at low exposures and cover so small a surface area as to be unobservable by XPS. Evidently, a more continuous growth mode occurs on the (111) surfaces. The 1.5 eV peak observed for the (100) surface was assigned to hydroxyls which recombine to desorb as water at 275 K. This assignment disagrees with that by Kundakovic et al. for the (111) surface. In that case the 1.6 eV peak is assigned to chemisorbed water, based upon the fact that hydroxyls desorb as H2, and no H2 was observed in desorption. Exposure of H 2 0 to a reduced ceria surface does lead to water dissociation as indicated by evolution of H2 upon subsequent TPD. The photoemission peak resulting from adsorption on reduced ceria was not clearly distinguishable from chemisorbed water in binding energy, but it was stable to about 500 K, at about which temperature H2 desorbed. This stable feature was therefore assigned to hydroxyl and this was the basis used by Kundakovic et al. to distinguish chemisorbed water from hydroxyls. Exposure of a Ce0 2 film to H2 at 120 K results in no uptake of H2 as indicated by the absence of either H2 or H 2 0 in subsequent TPD.80
9.4.5. SO2 Adsorption of S0 2 has been studied on Ce0 2 (lll) films using SXPS, TPD and XANES.81' 82 Unlike CO or methanol, S0 2 adsorbs readily on fully oxidized or reduced ceria surfaces. The principle interaction is chemisorption of S0 2 at oxygen anions to form a sulfite which reversibly desorbs as S0 2 over a broad temperature range from 200 to 600 K. This interaction is characterized as a Lewis acid-base interaction and occurs with approximately equal facility if the surface is hydroxylated or sulfided. On reduced ceria the adsorption is more heterogeneous suggesting a variety of local bonding environments, including a small, distinguishable amount of adsorption at Ce3+ sites. A significant portion of the chemisorbed S0 2 oxidizes the ceria, converting to sulfide above 300 K. The
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resulting S2" cations equilibrate between bulk and surface sites above 600 K. Overbury et al.82 claim that thermal conversion of chemisorbed S0 2 to sulfate is not observed, especially on the fully oxidized surface, except as a result of photochemical conversion by x-ray exposure. This interpretation disagrees with Rodriquez et al.81 who assigned the room temperature adsorbate to sulfate. Formation of sulfate implies oxidation of the S and corresponding reduction of the ceria. Evidence against sulfate formation is the reversible adsorption, the failure to observe the reduction of the ceria, expected if a sulfate was formed, and general reluctance of ceria films to be reduced by low temperature and pressure gas exposures.
9.4.6. Alcohols and Carboxylic Acids Adsorption of methanol has been studied on various ceria surfaces, including single crystal Ce0 2 (lll), and ceria films grown on YSZ, sapphire and Cu(lll). 1 8 , 8 3 The results are sensitive to structure or more probably the amount of anion vacancies on the surface. Ferrizz et al find that annealed single crystal C e 0 2 ( l l l ) is relatively unreactive to methanol.83 "Saturation" exposure at 300 K leads to a very small amount of methanol uptake, estimated at less than 0.08 ML. Sputtering the surface, a process known to remove oxygen, increases the methanol uptake by an order of magnitude. The adsorbed methanol desorbs as methanol and a small amount of formaldehyde. Apparently, methanol is only adsorbed if oxygen vacancies or reduced Ce are present. This conclusion is supported by results for methanol adsorption on films. CeO2(100) grown on YSZ, and to a lesser extent Ce0 2 (lll) on sapphire, are reduced upon annealing. In both cases the amount of methanol uptake increases with the extent of pre-annealing. Adsorption of methanol is presumed to be dissociative to methoxy. Although some methoxy recombines during TPD to desorb as methanol, this pathway competes with dehydrogenation to H2CO and CO while the hydrogen desorbs as H2 or H 2 0. Increased thermal reduction of the CeO2(100) / YSZ surface causes increased selectivity to the CO and H2 channels. However, for the C e 0 2 ( l l l ) / sapphire surface, relatively less CO and more H2CO are formed compared to (100) even for comparable methanol uptakes. TPD studies of 10 ML thick Ce0 2 (lll) grown on Cu(l 11) exhibited TPD distributions similar to the results for CeO2(100) / YSZ, i.e. high selectivity to CO and H2_ indicating that the former may be fairly
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Catalysis by ceria and related materials
extensively reduced.18 RAIRS from this surface identifies three types of adsorbed methoxy, two of which are assigned to adsorption at vacancy sites. Stubenrauch et al. have studied adsorption of formic and acetic acids on single crystal Ce0 2 (lll) and sputter deposited CeO2(100).84 These surfaces were sputter cleaned and oxidized in 10"7 torr oxygen. Extra features in the HREELS spectra suggested that the (111) surface may have contained a significant number of oxygen defects. Both surfaces exhibited primarily dissociative adsorption of these acids, leading to monodentate carboxylates and hydroxyls at 300 K, identified by HREELS on the (111) surface. However, it was noted that small amounts of molecular formic acid desorbed near 200 K following low temperature saturation exposure. Subsequent annealing during TPD leads to nearly simultaneous production of decomposition products (C0 2 , H 2 0 and CO or ketene) between 600 and 700 K. Formation of CO was favored to C0 2 on both surfaces for formate, while ketene was favored to C0 2 for acetate, suggesting preference for dehydration rather than dehydrogenation. Apparently no H2 is desorbed which is surprising in view of reports that hydroxyls formed on reduced VDO Ce0 2 (lll) decompose to form H2 at 600 K.78 Small amounts of formaldehyde (from formate) and acetone (from acetate) desorption products were also observed. Although HREELS and LEED could not be applied to the (100) surface, the only apparent structure sensitivity reported is the formation of acetone on the (111) but not on the (100) surfaces.
9.4.7. Hydrocarbons In the absence of metal particles, ethylene is found to chemisorb very weakly on reduced ceria, desorbing molecularly below 150 K without evidence of dissociation.85 There appears to be no other work on any hydrocarbons.
9.5. Reducibililty of Ceria Surfaces Of major concern for ceria based catalysts is the reducibility of the ceria support. Several papers mention experiments related to this subject. For model surfaces it is possible to quantitatively determine an average oxidation state of the ceria film, i.e. the ratio of Ce 3+ / Ce 4+. This is most commonly done using XPS of the Ce 3d photoemission features. The 3d spectrum is complicated by satellite features which are well described and provide useful variation with Ce oxidation state. 23 ' 42,43 ' 86
Ceria surfaces and films for model catalytic
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Valence band photoemission, the Ce 4d photoemission and the 0 Is x-ray absorption edge are also very sensitive to oxidation state as illustrated in Fig. 9.5.87 These techniques provide a depth weighted average of the oxidation state making it difficult to precisely distinguish between surface and below surface Ce3+. Nevertheless, they can be used to precisely monitor the effectiveness of reduction/oxidation treatments. - i — . — | — i — i — . — i —
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It is well known that ion sputtering reduces oxides in most cases, and this is observed for ceria.23'30'88 Since sputtering may introduce structural damage it is not a desireable method, and it is irrelevant to catalytic applications. CeC>2 single crystal surfaces are very stable to thermally induced oxygen loss. In vacuum, a single crystal Ce02(lll) surface shows no 0 2 evolution upon heating to 1300 K.76 However on ceria films there is evidence for a reversibly removable oxygen species that is active for CO oxidation in the presence of Rh particles.76'77 Larger amounts of this active oxygen could be obtained and its exchange with the bulk could be
330
Catalysis by ceria and related materials
enhanced if the ceria film was grown on a YSZ support. Similarly, in the case of mixed ceria-zirconia films there is evidence of oxidation of freshly-deposited Pd particles, indicative of facile transfer of oxygen from the ceria-zirconia, when the surface is heated above about 473 K.89 This transfer may be an essential step in CO oxidation and in reduction of ceria films by CO. Reduction of pure ceria by chemisorption is generally sluggish. The lack of CO adsorption on ceria, discussed above, implies that ceria films are not readily reducible by CO. Even addition of a metal, which adsorbs CO, only leads to a small amount of CO2 formation on highly crystalline ceria (see below). H2 is also ineffective for reducing model ceria surfaces. Following room temperature exposure and subsequent TPD, no reduction or H 2 0 formation is observed for Pt or Pd on ceria (polycrystalline grown by spray pryolysis).40,80 Cordatos et al. report that high temperature exposures to H2 (or CO) induced changes in subsequent TPD behavior suggesting that ceria reduction occurs. However, for Rh on VDO Ce0 2 (l 11), fairly high exposures of H2 at various temperatures did not lead to reduction observable by spectroscopic techniques.87 In fact, water will oxidize reduced ceria, yielding H2 in TPD due to decomposition of hydroxyls.78 H2 induced reduction has been observed for very high exposures on a "reverse" ceria on Pt surface.23 Ethylene is effective at reducing model ceria catalysts, when an active metal is present. In the presence of Rh, dissociative chemisorption of C2H4 on Rh leads to CO formation (but not H 2 0) in subsequent TPD.85 Even the atomic H, resulting from ethylene decomposition is not effective in reduction, since it recombines to form H2.
9.6. Studies of Chemisorption on Metal Loaded Ceria Surfaces Following deposition of an active metal upon a ceria surface, it is possible to study chemisorption on a surface that models many of the important aspects expected for actual ceria supported catalysts. Surface techniques offer the possibility to identify where the adsorbates are located and to identify intermediates that are formed in their interaction. By comparison of ceria surfaces, with and without metal, the synergisms between metal and support can be deduced. By controlled metal deposition, it is possible to study the effects of loading and particle size. By selected preparation of the ceria substrate it is possible to vary factors which may affect the interaction between the metal particle and the ceria, such as structure, defect concentration or oxidation state of the ceria. The goal of chemisorption studies, summarized below, is to relate all these factors to the interaction of the model catalyst with particular adsorbates.
Ceria surfaces andfilms for model catalytic studies
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9.6.1. CO There have been many studies of the adsorption of CO on ceria surfaces with deposited particles of Rh, Pt, Pd.1 '• 22 ' 40 ' 4I - 72,80,90 " 94 As mentioned above, CO does not adsorb readily on ceria, but chemisorbs strongly on the metal particles. The major conclusions from these studies is that the nature of the ceria support controls the fate of CO adsorbed on the metal, in part through oxygen transfer between the ceria and the metal particle and partly by other interactions between the metal and the support. There are two important manifestations of these interactions. The first is the well known ability of the ceria to contribute oxygen to oxidize CO to C0 2 , i.e. the oxygen storage capacity of ceria. Product CO2 is observed following room temperature CO exposure during subsequent TPD as shown in Fig. 9.6.94 It is demonstrated that the "extra" oxygen is not due to incidental oxygen chemisorbed on the metal or to CO disproportionation on the metal but derives from the ceria.80 It is found however, that the amount of C0 2 produced depends, not surprisingly, on the extent to which the ceria is reduced and also on the structure of the ceria film. For well ordered single crystal Ce0 2 (l 11) or CeO2(100) (film on sapphire) surfaces less than 2% of the CO adsorbed on Rh is oxidized to C0 2 upon subsequent TPD.11 If the ceria is more polycrystalline the yield of C0 2 may be much larger, on the order of 50%.41 If the ceria is grown on YSZ and is fully oxidized the yield of C0 2 can be as much as 70%.94 The second manifestation was probably not known prior to these surface experiments, namely the CO can be dissociated on Rh and the extent of the dissociation depends upon the extent of reduction of the ceria film. The dissociation is suggested by a second peak in the TPD spectrum observed at a temperature above that associated with simple desorption from Rh 93' 94 as illustrated in Fig. 9.6. Adsorption of 13C180 results in a preponderance of 13C160 in the high temperature peak indicating that CO adsorbed on Rh dissociates and the C atoms recombine with O from the ceria support.93 Further, using C Is SXPS it is possible to distinguish adsorbed CO from the adsorbed C dissociation product permitting the ability to monitor the thermally induced dissociation process on the Rh.72 Using SXPS on in situ grown ceria films with controllable oxidation state, it was demonstrated that CO dissociates on Rh between 400 and 500 K and that the extent of the dissociation is strongly dependent upon how reduced the ceria is prior to CO adsorption.
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Catalysis by ceria and related
i
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Figure 9.6. A comparison of CO TPD from (a) oxidized and (b) reduced ceria loaded with Rh. Oxidation of CO to CO2 is decreased for a reduced support. Reduction of ceria opens a new channel for desorption at high temperatures. Isotope experiments (c) show that CO is recombinative at higher temperatures. Adapted from ref. 94.
9.6.2. NO, NO2 The adsorption of NO on metal loaded ceria has been examined for Rh, 75 ' 85 ' 95 ' 96 Pt,40,53 and Pd.80 As known from work on single crystals, NO dissociates to some extent on each of these metals. The amount of dissociation is dependent upon the structure of the metal surface. Gorte considered Pt and Pd particles deposited on rough, polycrystalline ceria films grown by spray pyrolysis.40 For Pt they found variation in the TPD results (amount of NO uptake and shape of N2 desorption profile) that varied with the size of the Pt particles. However, the results were comparable to NO TPD results from Pt grown on sapphire. It was concluded that no unusual interaction existed between Pt and the (oxidized) ceria. For Pd it was found that a pronounced difference in the TPD product ratio, NO/N2, occurred for Pd on ceria compared to Pd on sapphire. They attributed the difference to NO adsorption on reduced ceria.80 These results were clarified by subsequent studies of Rh on ceria in which both TPD product and surface species could be monitored, and the oxidation state of the ceria could be systematically modified.96 Using SXPS, the extent of NO dissociation on the Rh particles could be measured directly as a function of the extent of reduction of the ceria support. Importantly, the N and NO species on Rh could be distinguished from N-containing species on ceria. Similar to CO adsorption, described above, it was found that the extent of NO dissociation on Rh was significantly enhanced by increasing reduction of the ceria support as shown in Fig. 9.7.96 On a fully oxidized ceria support the extent of dissociation was comparable to that observed for single crystal Rh(lll), while for highly reduced
Ceria surfaces and films for model catalytic
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ceria complete dissociation is obtained. The oxygen of dissociation is presumed to diffuse to the ceria. Aspects of this interpretation are supported by Ferrizz et al. who in addition investigated the effect of the substrate underlying the ceria film.75 For ceria grown on YSZ, interfacial interactions affect the oxygen transport capability of the ceria, and thereby increase the extent of NO dissociation from a Rh/ceria surface. 1.0 z
A
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Berner has studied adsorption of N0 2 and NO on an inverse system of substoichiometric Ce0 2 . x (x=0.21) on Pt(lll). 5 3 They find that N 0 2 effectively oxidizes ceria, by adsorbing on exposed Pt, dissociating to NO + O and subsequent diffusion of O atoms to the ceria islands. Using RAIRS, bridged and linear NO can be distinguished on the Pt depending upon coverage. Interestingly, the NO does not further dissociate, consistent with their observation that exposure to NO does not oxidize the ceria islands. This seemingly conflicts with other studies where NO is observed to cause oxidation of (reduced) ceria and dissociate on Rh (or Pt). An explanation is that in the inverse system, the Ce3+ is preferentially located at the buried Pt/ceria interface and is kinetically inhibited from reduction by molecular NO.
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Catalysis by ceria and related materials
9.6.3, C2H4
Ethylene adsorbs dissociatively on Rh, undergoing a sequence of dehydrogenations leading to hydrocarbon fragments (ref97 and references therein). For Rh on ceria model catalysts the ethylene dehydrogenation and H2 evolution mimic that of Rh single crystals, indicating these steps occur only on the metal particles with no influence or contribution from the ceria surface. However, the resulting C atoms are oxidized by the ceria and evolve as CO. Either the C diffuses to react with oxygen at the island edge or oxygen migrates onto the Rh. The CO evolution temperature (TPD peak maximum) increases from about 540 to 690 K as the extent of reduction of the ceria substrate increases.85 Similarly, since ethylene thus reduces the ceria, successive ethylene exposures and TPD experiments leads to an increasing CO desorption temperature.22'97 There is no C0 2 formation, since CO desorbs before further oxidation can occur. Ferrizz et al. argue that the CO evolution is kinetically limited by the C + O reaction if the ceria is highly oxidized, but the rate limiting step shifts to oxygen transfer to the Rh if the ceria is highly reduced.
9.7. Coadsorption and Reaction Studies on Ceria Model Catalysts Although there have been several studies of chemisorption of certain molecules of interest, there are fewer studies of co-adsorption. This possibly arises from the increasing complexities of controlling coverage in situations where competitive adsorption may exclude or alter the coverage of the reactants. The goal of coadsorption studies is to learn about interactions between surface coadsorbates, a subject of obvious importance to their catalytic reaction. It is especially of interest from the view of emission control catalysis to study coadsorption of an oxidant and a reductant.
9.7.1. Coadsorption of CO and NO Mullins et al. have performed studies of the interaction of CO and NO using a model surface consisting of a low coverage of Rh and a ceria film of controlled oxidation state.95 Although the co-adsorption of these species is competitive, coadsorption by sequential exposures was performed using controlled sub-saturation doses of CO and NO. Since the adsorption of either CO or NO is strongly affected by the oxidation state of the ceria, it is not surprising that their interaction with each
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other is also affected. When the ceria is fully oxidized, the surface species and desorption products mimic results on Rh surfaces, and the only indication of interaction is the formation of C0 2 , a product of reaction between CO and O atoms from dissociated NO. If the ceria support is reduced, then new features are observable by SXPS, as illustrated in Fig. 9.8.95 These were assigned to isocyanate,
Binding Energy
Binding Energy
Figure 9.8. Cls and N Is SXPS spectra show effect of annealing a Rh loaded Ce0 2 (l 11) film following sequential exposure to CO and NO at low temperature. Changes in features indicate formation and disappearance of OCN, CN, nitride on ceria and carbon nitride species. Adapted from ref. 95.
cyanide and a stable carbon-nitride phase, all of which are believed to form on the Rh. The conditions leading to formation of these species was explored. Isocyanate was formed from reaction of CO and atomic N and not from reaction of NO and atomic C. Evidently the reasons isocyanate forms when the ceria is reduced but not when it is oxidized are that 1) atomic N is increased on the Rh surface due to enhanced NO dissociation and 2) O from NO dissociation is rapidly transferred from the Rh to the ceria thereby decreasing the probability of C0 2 formation. Cyanide is also believed to form but disappears above about 600 K, although no cyanogen desorption occurs. Above 600 K there is an interaction between remaining C and N that stabilize them with respect to N2 formation or C oxidation to CO. New peaks in the C Is spectra suggest the formation of a carbon-nitride phase. These results demonstrate how reactions on ceria supported Rh may differ
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Catalysis by ceria and related materials
between reducing and oxidizing conditions. Important by-products such as OCN and N2O may be strongly affected in this way.
9.7.2. Coadsorption ofC2H4 and NO Similar studies of co-adsorption of ethylene and NO have also been reported.85 Many of the interactions observed for N and C deriving from NO and CO are seen in this case. However, since no oxygen derives from dissociation of ethylene (as it does for CO), the observed formation of CO must be due to a reaction with ceria derived oxygen. The presence of H also opens the possibility of NH3 formation, which is seen in small amounts.
9.7.3. Coadsorption of CO and Water This coadsorption has been studied for Rh loaded onto Ce0 2 (l 11) films.78 As with the systems mentioned above, the interaction between water and CO depends strongly upon the oxidation state of the ceria. The origin of most of the oxidation effects are the result of the interaction of the individual components, e.g. hydroxyls form from H 2 0 only on a reduced ceria surface. Rh catalyses the decomposition of hydroxyls, leading to a decrease in the resulting recombinative H2 peak desorption temperature from 580 K in the Rh-free surface to about 500 K when Rh is present on the surface. This effect is presumably due to H spillover onto the Rh, facilitating recombination and desorption. There are subtle interactions between the coadsorbates also. The coadsorption of CO somewhat inhibits this process of recombinative H2 desorption, but the presence of the hydroxyls slightly lowers the temperature of recombinative CO desorption.
9.7.4. Reactor Studies The further step of monitoring a dynamic catalytic reaction on a surface is barely approachable with existing surface science method by using molecular beam techniques, or by isolating a model catalysts prepared and analysed in UHV in a reactor appended to the UHV system. Although such studies apparently have not been performed for ceria supported model catalysts, it is appropriate to mention reactor studies of CO oxidation 91 ' 92 ' 98, " and water gas shift100 reactions performed
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in the group of R.J. Gorte. In this work, similar methods were used to prepare ceria model catalysts both for catalytic measurements in an ex situ flow reactor and for UHV characterization, adsorption and TPD. This work was ground breaking in that it demonstrated the onset of the redox mechanism for CO oxidation and linked it to evolution of C0 2 oxidation product in CO adsorption and TPD. Using this method they were able to study the effect of Rh loading on ceria, to demonstrate that the redox mechanism occurs similarly for different metals (Pd, Rh and Pt) and to compare the results for ceria vs mixed ceria-zirconia oxide.
9.8. Overview It is important to consider what research directions and outstanding issues remain. Many questions arise from the apparently different chemisorption characteristics between dispersed and single crystal surfaces. Answering them will require a better , understanding of the relationships between surface bonding and ceria surface structure, especially defect structure. Additional adsorption and coadsorption studies are needed to get quantitative rates of elementary steps. Techniques to bridge the "pressure gap" may be helpful, i.e. in situ measurement of chemisorbates from low to high pressure conditions as a function of temperature. Dynamic monitoring of products during gas exposure, rather than a TPD approach, could yield a clearer picture of surface reactions. There is a need to characterize the structure of metal islands on ceria and how they are affected by structure, defects and changes in oxidation state of the underlying support. High quality experiments of this type have been successfully conducted on other model oxides.101 Since ceria is clearly involved in oxygen transfer in many catalytic redox reactions, it is important to clarify mechanisms and factors that are responsible for oxygen transport and transferability between ceria support and metal particles. These factors include the role of structure and doping. Acknowledgements: The authors acknowledge support of the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy, under contract DE-AC05-00OR22725 with Oak Ridge National Laboratory, managed and operated by UTBattelle, LLC. The authors wish to thank John Vohs for critically reading the manuscript.
Catalysis by ceria and related materials
338 9.9. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28.
Henrich V. E. and Cox P. A. The Surface Science of Metal Oxides (Cambridge University Press, New York, 1994), Campbell C. T. Surf Sci. Rep. 27 (1997), 1-111. Henry C. R. Surf. Sci. Rep. 31 (1998), 231. Trovarelli A. Catal. Rev.-Sci. Eng 38 (1996), 439. Bene R., etal. Sens. Actuator B-Chem. 71 (2000), 36-41. Ozer N. Solar Energy Mat. Solar Cells 68 (2001), 391-400. Norton D. P. Budai J. D. and Chisholm M. F. Appl. Phys. Lett. 76 (2000), 1677-1679. Sun E. Y., et al. Physica C 321 (1999), 29-38. Hubbard A. The Handbook of Surface Imaging and Visualization (CRC Press New York 1995) Duke C. B. Surf Sci. 299/300 (1994). Stubenrauch J. and Vohs J. M. J. Catal. 159 (1996), 50-57. Mamontov E. Egami T. Dmowski W. and Kao C. C. J. Phys. Chem. Solids 62(2001), 819-823. Mamontov E. Dmowski W. Egami T. and Kao C. C. J. Phys. Chem. Solids 61 (2000), 431-433. Zhu S. Lowndes D. H. Budai J. D. and Norton D. P. Appl. Phys. Lett. 65 (1994), 2012-2014. Wu X. D., et al. Appl. Phys. Lett. 58 (1991), 2165-2167. Mullins D. R. Radulovic P. V. and Overbury S. H. Surf. Sci. 429 (1999), 186198. Hardacre C. Roe G. M. and Lambert R. M. Surf. Sci. 326 (1995), 1 -10. Siokou A. and Nix R. M. J. Phys. Chem. B 103 (1999), 6984-6997. Rodriguez J. A., et al. J. Chem. Phys. 112 (2000), 9929-9939. Alexandrou M. and Nix R. M. Surf. Sci. 321 (1994), 47-57. Dmowski W., et al. Physica B 248 (1998), 95-100. Ferrizz R. M. Egami T. and Vohs J. M. Surf. Sci. 465 (2000), 127-137. Pfau A. and Schierbaum K. D. Surf Sci. 321 (1994), 71-80. Schierbaum K. D. Surf. Sci. 399 (1998), 29-38. Yoshimoto M. Nagata H. Tsukahara T. and Koinuma H. Jpn. J. Appl. Phys. 29 (1990), LI 199. Koinuma H., et al. Appl. Phys. Lett. 58 (1991), 2027. Yoshimoto M., et al. Jpn. J. Appl. Phys. Part 2 - Lett. 34 (1995), L688-L690. Cui X., et al. Physica C 316 (1999), 27-33.
Ceria surfaces andfilms for model catalytic studies
339
29. Yang C. Y., et al. Physica C 307 (1998), 87-98. 30. Overbury S. H., et al. J. Vac. Sci. Technol. A-Vac. Surf. Films 15 (1997), 1647-1652. 31. Denhoff M. W. Grant P. D. and Mccaffrey J. P. Can. J. Phys. 70 (1992), 1124-1132. 32. Denhoff M. W. and Mccaffrey J. P. J. Appl. Phys. 70 (1991), 3986-3998. 33. Chambers S. A. Surf. Sci. Rep. 39 (2000), 105-180. 34. Kim Y. J., et al. J. Vac. Sci. Technol. A-Vac. Surf. Films 17 (1999), 926-935. 35. Gao Y., et al. J. Vac. Sci. Technol. A-Vac. Surf. Films 17 (1999), 961-969. 36. Konstantinov K., et al. Int. J. Inorg. Mater. 2 (2000), 277-280. 37. Elidrissi B., et al. Thin Solid Films 379 (2000), 23-27. 38. Wang S. Y., et al. Solid State Ion. 133 (2000), 211-215. 39. Wang S. Y. Wang W. Zuo J. and Qian Y. T. Mater. Chem. Phys. 68 (2001), 246-248. 40. Zafiris G. S. and Gorte R. J. Surf. Sci. 276 (1992), 86-94. 41. Cordatos H., et al. J. Phys. Chem. 100 (1996), 785-789. 42. Mullins D. R. Overbury S. H. and Huntley D. R. Surf.Sci. 409 (1998), 307319. 43. Praline G., et al. J. Electron Spectr. Rel. Phen 21 (1980), 17-30. 44. Strasser G. and Netzer F. P. J. Vac. Sci. Technol. A-Vac. Surf. Films 2 (1984), 826-830. 45. Strasser G. Rosina G. Bertel E. and Netzer F. P. Surf. Sci. 152 (1985), 765775. 46. Wang A. C., et al. Physica C 320 (1999), 154-160. 47. Pfau A. Schierbaum K. D. and Gopel W. Surf. Sci. 333 (1995), 1479-1485. 48. Kotelyanskii I. M., et al. Thin Solid Films 280 (1996), 163-166. 49. Guo S., et al. J. Appl. Phys. 11 (1995), 5369. 50. Wang L. P., et al. Mater. Sci. Eng. A-Struct. Mater. Prop. Microstruct. Process. 308 (2001), 17'6-179. 51. Belton D. N. and Schmieg S. J. J. Vacuum Sci Technology 11 (1993), 2330. 52. Hayek K. Jenewein B. Klotzer B. and Reichl W. Top. Catal. 14 (2001), 2533. 53. Berner U., et al. Surf. Sci. 467 (2000), 201-213. 54. Warren J. P. Zhang X. Andersen J. E. T. and Lambert R. M. Surf. Sci. 287 (1993), 222-227. 55. Hardacre C. Ormerod R. M. and Lambert R. M. J. Phys. Chem. 98 (1994), 10901-10905. 56. Hardacre C. Rayment T. and Lambert R. M. J. Catal. 158 (1996), 102-108.
340
Catalysis by ceria and related materials
57. Sayle D. C, et al. Surf. Sci. 334 (1995), 170-178. 58. Sayle T. X. T. Parker S. C. and Catlow C. R. A. Surf. Sci. 316 (1994), 329336. 59. Sayle T. X. T. Parker S. C. and Catlow C. R. A. J. Chem. Soc.-Chem. Commun. (1992), 977-978. 60. Conesa J. C. Surf. Sci. 339 (1995), 337-352. 61. Baudin M. Wojcik M. and Hermansson K. Surf. Sci. 468 (2000), 51-61. 62. Maicaneanu S. A. Sayle D. C. and Watson G. W. Chem. Commun. (2001), 289-290. 63. Norenberg H. and Harding J. H. Surf. Sci. All (2001), 17-24. 64. Herman G. S. Phys. Rev. B 59 (1999), 14899-14902. 65. Herman G. S. Surf. Sci. 437 (1999), 207-214. 66. Norenberg H. and Briggs G. A. D. Phys. Rev. Lett. 79 (1997), 4222-4225. 67. Norenberg H. and Briggs G. A. D. Surf. Sci. 404 (1998), 734-737. 68. Norenberg H. and Briggs G. A. D. Surf. Sci. 424 (1999), L352-L355. 69. Norenberg H. and Briggs G. A. D. Surf. Sci. 435 (1999), 127-130. 70. Inoue T., et al. J. Vac. Sci. Technol. A-Vac. Surf. Films 18 (2000), 1613-1618. 71. Jacobsen S. N., et al. Surf. Sci. 429 (1999), 22-33. 72. Mullins D. R. and Overbury S. H. J. Catal. 188 (1999), 340-345. 73. Li C, et al. J. Chem. Soc. Faraday Trans. 1 85 (1989), 929. 74. Overbury S. H. Mullins D. R. Huntley D. R. and Kundakovic L. J. Catal. 186 (1999), 296-309. 75. Ferrizz R. M. Egami T. Wong G. S. and Vohs J. M. Surf. Sci. 476 (2001), 921. 76. Putna E. S. Vohs J. M. and Gorte R. J. J. Phys. Chem. 100 (1996), 1786217865. 77. Putna E. S. Vohs J. M. and Gorte R. J. Catal. Lett. 45 (1997), 143-147. 78. Kundakovic L. Mullins D. R. and Overbury S. H. Surf. Sci. 457 (2000), 5162. 79. Herman G. S. Kim Y. J. Chambers S. A. and Peden C. H. F. Langmuir 15 (1999), 3993-3997. 80. Cordatos H. and Gorte R. J. J. Catal. 159 (1996), 112-118. 81. Rodriguez J. A., et al. Catal. Lett. 62 (1999), 113-119. 82. Overbury S. H. Mullins D. R. Huntley D. R. and Kundakovic L. J. Phys. Chem. B 103 (1999), 11308-11317. 83. Ferrizz R. M. Wong G. S. Egami T. and Vohs J. M. Langmuir 17 (2001), 2464-2470. 84. Stubenrauch J. Brosha E. and Vohs J. M. Catal. Today 28 (1996), 431 -441.
Ceria surfaces and films for model catalytic studies
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85. Mullins D. R. and Zhang K. J. Phys. Chem. 105 (2001), 1374. 86. Burroughs P. Hamnett A. Orchard A. F. and Thornton G. J. Chem. Soc. Dalton Trans. 17 (1976), 1686. 87. Overbury S. H. and Mullins D. R.,unpublished data 88. Holgado J. P. Munuera G. Espinos J. P. and Gonzalez-Elipe A. R. Appl. Surf. Sci. 158(2000), 164-171. 89. Smirnov M. Y. and Graham G. W. Catal. Lett. 72 (2001), 39-44. 90. Zafiris G. S. and Gorte R. J. J. Catal. 139 (1993), 561-567. 91. Bunluesin T. Putna E. S. and Gorte R. J. Catal. Lett. 41 (1996), 1-5. 92. Zafiris G. S. and Gorte R. J. J. Catal. 143 (1993), 86-91. 93. Stubenrauch J. and Vohs J. M. Catal. Lett. 47 (1997), 21-25. 94. Putna E. S. Gorte R. J. Vohs J. M. and Graham G. W. J. Catal. 178 (1998), 598-603. 95. Mullins D. R. Kundakovic L. and Overbury S. H. /. Catal. 195 (2000), 169179. 96. Overbury S. H. Mullins D. R. and Kundakovic L. Surf. Sci. 470 (2001), 243. 97. Ferrizz R. M. Egami T. and Vohs J. M. Catal. Lett. 61 (1999), 33-38. 98. Bunluesin T. Cordatos H. and Gorte R. J. J. Catal. 157 (1995), 222-226. 99. Bunluesin T. Gorte R. J. and Graham G. W. Appl. Catal. B-Environ. 14 (1997), 105-115. 100. Bunluesin T. Gorte R. J. and Graham G. W. Appl. Catal. B-Environ. 15 (1998), 107-114. 101. Baumer M. and Freund H.-J. Prog. Surf. Sci. 61 (1999), 127-198.
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CHAPTER 10
CERIA AND OTHER OXYGEN STORAGE COMPONENTS IN AUTOMOTIVE CATALYSTS MORDECAI SHELEF, GEORGE W. GRAHAM, and ROBERT W. McCABE Ford Research Laboratories, Dearborn, Michigan 48121, USA
10.1. Origin and Evolution of "Oxygen Storage" in Automotive Catalysts The introduction of catalytic treatment of automotive exhaust in the United States in the first part of the 1970s began with the removal of the products of incomplete combustion, carbon monoxide and residual hydrocarbons. This task can be accomplished by using a simple oxidation catalyst, where, in the presence of excess air, noble metal catalysts promote the additional oxidation of the products of incomplete oxidation from the IC engine. The regulations necessitating the catalytic removal of nitrogen oxides, formed in the combustion chamber, kicked-in in 1980. It initially seemed that the simplest way to accomplish this would be to use a "dual" system where the upstream catalyst bed is fed by exhaust resulting from combustion of a slightly rich mixture of fuel and air. Under these conditions the reduction of NOx is fast and nearly complete. Secondary air is then injected ahead of a downstream oxidation catalyst to remove the CO and hydrocarbons. This seemingly straightforward approach was found to contain a hidden flaw: the reduction of the NOx in the upstream catalyst resulted in a majority of the product being ammonia, due to the presence of hydrogen in the exhaust. When re-oxidized on the downstream catalyst the ammonia reverted back to NOx, vitiating the whole approach [1]. Another solution to the problem was called for. It was proposed by Gross et al [2] that if one could catalytically equilibrate an exhaust resulting from the combustion of an exactly stoichiometric combustion mixture it is thermodynamically possible to remove all three pollutants, leaving only water, C0 2 and nitrogen. This is a single three-way catalyst (TWC). In the same time frame the other systems needed for tight combustion control have matured technologically, such as affordable computerized electronic engine controls and exhaust composition sensors (electrochemical solid-state oxygen sensors). (See Fig. 10.1)
343
344
Catalysis by ceria and related
materials
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Figure 10.1. Typical TWC conversion efficiency plot for hydrocarbons (HC), CO, and NO„ as a function of air-fuel ratio. Also shown are representative air-fuel ratio vs. time traces for 1986 and 1990 vehicles with control bandwidth mapped onto the catalyst efficiency plot. [3]
Nevertheless, the required tight stoichiometry constraints in a randomly oscillating dynamic combustion were not easily attainable. There was a need for a composition smoothing device/material akin to a surge tank in a hydraulic system or
Ceria and other oxygen storage components in automotive catalysts a capacitor in an electrical circuit. This is where the incorporation of such a material into the catalyst came into consideration. The existence of oxides with varying oxygen stoichiometrics is well known. In particular, among the rare-earth oxides, those of Ce, Pr and Tb contain, under a wide range of conditions, metal ions of different valence and are able to incorporate more or less oxygen into their crystal structure depending on various parameters such as the gaseous atmosphere with which they are in contact, temperature, and pressure. The release of the oxygen can be accomplished in some cases without the use of a reducing agent, by switching to a lower oxygen pressure or higher temperature or both. In practice, however, the release of the oxygen is enhanced by a reductant such as CO, H2 or a hydrocarbon, which itself undergoes oxidation in the process. The rate of the uptake of the oxygen is accelerated by the presence of catalysts capable of dissociatively adsorbing dioxygen, which is a necessary step in the incorporation of oxygen ions from diatomic gaseous oxygen into a solid. The same catalysts also strongly promote the reduction of the solid by the reductant gaseous molecules. Since the scission of the oxygen bond at the surface is energetically demanding, the reduction half of the overall cycle tends to be rate limiting. The first description in the open literature of the use of "oxygen storage" to buffer the lean-rich swings of the exhaust gas composition was in 1976 [4]. Initially, the main role of the "oxygen-storage component" was to extend the threeway "window" on the lean side of stoichiometry by acting as a sink for gas-phase oxygen during rich-to-lean transients. This uptake of oxygen allowed NOx conversion to continue for an interval of time proportional to the oxygen storage capacity (OSC), as shown in Fig. 10.2 (top). On the other hand, the oxygen storage component could also promote oxidation of reductants, like CO, during lean-to-rich transients, as shown in Fig. 10.2 (bottom). Early on, ceria was recognized as a promising storage material because of its combination of facile redox cycling between the trivalent and tetravalent oxidation states of the Ce ions, good thermal stability, ease of impregnation onto alumina, compatibility with noble metals and, most importantly, availability and affordability. In addition, while other, distinct, components were at first identified for their water-gas shift (WGS) and steamreforming activities, further extending the "window" on the rich side under steadystate conditions, ceria was found to perform these functions, as well. For many years, ceria has been the chief oxygen storage component for three-way catalysts, and the mechanisms by which it works have been the subject of many studies and excellent review papers, including a recent detailed survey by Trovarelli [5].
345
346
Catalysis by ceria and related
materials
511 513 Time (seconds)
585
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591
593
595
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Time (seconds)
Figure 10.2. Dynamometer data showing the response of two TWCs, A and B, to lean (top) and rich (bottom) transients in normalized air-to-fuel ratio, Lambda. Both exhibit oxygen storage, but the oxygen storage capacity of B is greater than A, as can be seen by the longer delay in the response of the tailpipe exhaust gas oxygen sensor to the changes in the feedgas. Correspondingly, the amounts of NO and CO breaking through B are less than A. (D. Gregory and B. Campbell)
The TWC has undergone significant advances during the past 20 years or so, a number of which have involved major changes in the way ceria is deployed. In the early years (roughly 1981-1985), the TWC formulation was relatively simple and consisted of noble metals co-impregnated with ceria onto alumina. Even this
Ceria and other oxygen storage components in automotive catalysts
1A1
rudimentary addition of ceria dramatically improved TWC performance, as evidenced by the laboratory study of Kim, for example [6]. Beginning in the mid-1980s a second generation of ceria-containing TWCs emerged known as "high-tech" catalysts. High-tech, in this case, was synonymous with strategies to increase both the amount and dispersion of ceria in the catalyst to maximize the interaction between the ceria and the noble metals. Through a combination of surface impregnation of high concentrations of ceria onto alumina together, in some catalysts, with admixtures of ceria and alumina particles, ceria concentrations approaching 50% of the total washcoat loading were obtained in some formulations. Again, the improvements in emission control were dramatic, although the high ceria concentrations contributed to hydrogen sulfide (H2S) formation - a problem addressed by incorporating Ni or other sulfur scavengers into the catalyst formulation. A general consensus existed during the mid- to late-1980s in the U.S. that the "catalyst part" of automotive pollution control was essentially finished with the development of the high-tech ceria-containing TWC formulations. Greater efforts were being directed toward improvements in air-fuel control (e.g., mass-air sensors, multi-point fuel injection, sequential fuel injection timing, etc.). However, a series of regulatory changes - including stricter emission standards in the U.S. (both federal and California) and in Europe, as well as increased durability requirements in the U.S. (100,000 miles vs 50,000 miles) - revealed shortcomings in high-tech catalyst performance, especially thermal durability. The latter was compounded by a trend toward close-coupling of exhaust catalysts to the exhaust manifold to obtain faster light-off. The performance deficiencies of the high-tech formulations were manifested by breakthrough emissions of CO and NOx (and to some extent hydrocarbons) during transient driving, and pointed directly to loss of oxygen storage with time in service. Characterization of thermally deactivated high-tech TWCs associated the loss of oxygen storage with sintering of both the noble metals and ceria and concomitant loss of contact area between the two [7]. Catalyst technology that was thought to be mature was suddenly found wanting. Moreover, significant advances had been made in engine controls, sensors, and fuel injection hardware by the early '90s, which served to turn the tables such that catalysts were again the weak link in achieving high-mileage emissions targets. Attempts to stabilize the interactions between the noble metals and ceria were only partly successful. True success came in the form of a fundamentally different approach to oxygen storage, still employing ceria but in solid solution with other metal oxides - most notably zirconia. Development of these mixed oxide materials began in the late 1980s and ultimately achieved widespread incorporation
348
Catalysis by ceria and related materials
into TWCs by the mid-1990s under the name of "advanced" TWC formulations. The advanced TWC formulations are capable of much higher temperature operation than their high-tech predecessors and have dramatically improved long-term emissions performance. These catalysts are more complex than their predecessors, both in composition and structure; some have multiple washcoat layers, containing various combinations of active noble metals, ceria, and ceria-containing mixed oxides, as shown in Fig. 10.3.
Figure 10.3. Cross-sectional SEM backscattering image (lower left) and compositional maps showing the structure of a double-layer washcoat found in a current TWC. Both pure ceria (dispersed throughout an alumina-rich inner layer) and ceria-zirconia (outer layer) are present. (J. Hangas)
Most of the emphasis of this chapter is on the mixed-oxide solid solution oxygen storage materials that comprise the advanced catalyst formulations in use today and are still under development. In particular, we focus on their durability, both with respect to thermal and chemical deactivation, while also briefly reviewing special uses of these and other oxygen storage materials in automotive applications. 10.2. Interaction of Ceria with the Active Noble Metals Interaction of ceria with the active noble metals is fundamental to the provision of oxygen storage, or buffering, in three-way catalysts, and it depends first and
Ceria and other oxygen storage components in automotive catalysts
349
foremost upon having (and maintaining) a ceria-metal contact. Details of the interaction are covered in the chapter by Bernal et al. For reactions in which ceria is a source of oxygen (as opposed to being a sink for oxygen), the detailed mechanism is thought to involve adsorption of the reductant on the noble metal, extraction of oxygen from the ceria, and reaction between the reductant and oxygen on (or in the periphery of) the metal. This view is consistent with the results of numerous model studies conducted in both UHV, such as CO and H2 TPD/TPR, as well as at normal pressure, such as transient and steadystate CO oxidation, WGS, and steam-reforming reactions (e.g., [8]). The fact that these reactions can proceed in the presence of large concentrations of C0 2 and/or H 2 0, both in actual use and during simple laboratory measurements (such as the evaluation of OSC using CO-pulse techniques), appears surprising in view of thermodynamic considerations, according to which the free energy changes, as calculated for crystalline solids, are uphill. For example, the free energy change in the simultaneous reduction of Ce0 2 to Ce 2 0 3 and oxidation of CO (H2) to C0 2 (H 2 0) is +15 kcal/mol-CO (+22 kcal/mol-H2) at 300 K, falling only to +9 kcal/mol-CO (+7 kcal/mol-H2) at 1000 K. A recent study by Yang et al. [9] has shown that the binding of oxygen in freshly-prepared ceria (and ceria-zirconia) is actually much weaker than in well-crystallized solids, ~200 kJ/mol (Fig. 10.4) vs. 760 kJ/mol, lending support to the possibility that the oxygen involved in oxygen storage is associated with defects (see Section 10.3.1.1 below). The observation that 300 -i L A 250 •
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Figure 10.4. Calorimetnc results for O2 adsorption at 510 K on reduced 1 wt% Pd on alumina (o), ceria (A), and ceria-zirconia (D). [9]
350
Catalysis by ceria and related materials
Pd films may be oxidized by their ceria-zirconia supports merely by heating to 470 K in UHV reinforces Yang et al.'s result [10].
10.3. Deactivation of Oxygen Storage Deactivation of oxygen storage occurs by both thermal and chemical modes. 10.3.1. Thermal Deactivation Thermal deactivation involves processes such as diffusion and solid-state reaction. In early three-way catalysts, where both the active metal and ceria were dispersed onto high-surface-area y-Al203, loss of contact between them, due to sintering of either one or both, could effectively eliminate oxygen storage. The temperature required for ceria to sinter, somewhat above 800°C, was typically not attained under normal operating conditions, although relatively harsh conditions, with temperatures well in excess of 800°C under rich exhaust gas, did exist in heavyduty truck operation, and in this case, reaction between ceria and alumina at times produced stable, inert cerium aluminate. Higher operating temperatures were inevitable, however, and it was apparent that even if the metal/ceria contact could be maintained, loss of ceria surface area would dramatically reduce the amount of oxygen easily extractable from ceria. Attempts to stabilize the dispersion, or surface area, of ceria by doping with elements such as La or Si led to the realization that the bulk reducibility of ceria could be altered by alloying, and eventually, the mixed oxide, ceria-zirconia, in which oxygen storage capacity and surface area are not strongly correlated, was developed and implemented, as noted before. The study of its properties has led to new insights into the behavior of ceria and the identification of new factors determining thermal stability of oxygen storage in current three-way catalysts. 10.3.1.1. Loss of "Active Oxygen" As mentioned in Section 10.2 above, both ceria and ceria-zirconia contain relatively weakly-bound oxygen when freshly prepared, e.g., in high-surface-area form. The thermal stability of this oxygen may differ in the two materials, however, as shown in steady-state CO-oxidation measurements performed by Bunluesin et al. [11] on model planar catalysts. In these experiments, films of ceria and ceria-zirconia were subjected to calcination treatments over a wide range of temperature before noble
Ceria and other oxygen storage components in automotive catalysts
351
metals were applied, ensuring that the metal dispersion was constant in all cases. For the ceria films, the process yielding a zeroth-order dependence of C0 2 production on CO partial pressure at high [CO]/[02] ratios, indicative of ceriasupplied oxygen, was suppressed by treatments above 800°C and entirely lost by 1000°C, as shown in Fig. 10.5. For the ceria-zirconia films, on the other hand, this process persisted at much higher temperatures, well in excess of 1000°C, where the mixed oxide began to undergo phase separation. Previously, the loss of oxygen storage capacity by ceria at such temperatures was simply attributed to the loss of surface area, but this explanation clearly cannot account for these results.
Figure 10.5. Steady-state CO oxidation rates as a function of CO partial pressure at 515 K for 2 x 10 ,s Pd/cm2 on ceria films annealed at 570 K (•), 1070 K (A), 1170 K (+), 1270 K (o), and 1670 K (•). The O2 partial pressure was fixed at 0.3 torr. [11]
Recently, Mamontov et al. [12,13] have found that Frenkel defects exist in ceria and ceria-zirconia, as prepared in high-surface-area form. Also known to occur in U0 2 at high temperature due to thermal activation, these vacancyinterstitial pairs present an intriguing possible explanation for the "active oxygen" involved in oxygen storage, according to these authors. Significantly, these defects have been observed to annihilate in ceria above 800°C but persist, virtually unaffected, to at least this temperature in ceria-zirconia (Fig. 10.6).
Catalysis by ceria and related materials
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Ceria and other oxygen storage components in automotive catalysts
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Irrespective of the underlying reason, ceria-zirconia certainly retains a much higher oxygen storage capacity than ceria in model Pd catalysts after hightemperature "redox" aging, intended to simulate automotive exhaust, as shown in Table 10.1. Table 10.1. Steady-state oxygen storage capacities of fresh and "redox" aged model Pd automotive catalysts made with ceria and ceria-zirconia [14]. Oxygen Storage Capacity (^mol O/g) Catalyst Pd/C2
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10.3.1.2. Phase Separation in Ceria-zirconia The behavior of ceria-zirconia as an oxygen-storage material apparently derives from its existence as a solid solution. At high temperature, the solid solution may become unstable with respect to separation into a mixture of phases. The precise temperature depends on mixed-oxide composition and gas atmosphere. Ceria-rich solid solutions, for example, begin to undergo phase separation by 1050°C in air [14], but they are stable at 1150°C under redox conditions, as shown in Fig. 10.7. Zirconia-rich solid solutions, on the other hand, are relatively less stable, separating into mixtures of phases by 1150°C under the same redox conditions [15]. The oxygen storage capacity of the mixture of phases generally differs from that of the original solid solution. For compositions near the peak value of OSC, for example, phase separation lowers the overall oxygen storage capacity.
10.3.1.3. Loss of Noble-metal Dispersion As already emphasized above, the ability of a three-way catalyst to utilize its OSC depends critically on the noble metal. Not only does the metal need to be in contact with the oxygen-storage material, but it must also be sufficiently well dispersed in
Catalysis by ceria and related materials
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order to access all the oxygen that is available for extraction. For ceria, the stored oxygen comes mostly from the near-surface region, but for ceria-zirconia, due to its enhanced oxygen diffusivity [16], oxygen comes from the bulk, as well. This leads to a strikingly different dependence of CO oxidation rate on noble-metal dispersion, according to Hori et al [17]. As shown in Fig. 10.8, the rate falls steadily with decreasing Pt dispersion, induced by thermal aging, in the case of ceria, but there is a distinct threshold in the case of ceria-zirconia, above which the rate is constant. A similar threshold effect, consistent with there being a relatively large (compared with ceria) but finite range of oxygen extraction about a noble-metal particle supported on ceria-zirconia, has also been observed in the noble-metal loading dependence of OSC measured for Pd, Pt, and Rh (Jen/unpublished).
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10.3.1.4. Migration of Noble Metal The processes underlying noble-metal sintering can also lead to large-scale migration of noble metal from one oxide support (e.g., the oxygen storage material) to another (e.g., the alumina washcoat binder) [18]. To the extent that non-oxygenstorage-materials present a significant fraction of the total surface area of the washcoat, such redistribution can lower the concentration of noble metal in contact with the oxygen storage material, compounding the effect of noble-metal sintering alone. Thermal stability of surface area is thus still important, even in the case of ceria-zirconia, so as to minimize its dilution by other washcoat components.
355
356
Catalysis by ceria and related materials
10.3.1.5. Encapsulation of Noble Metal In addition to the dilution effect described above, the surface area of the oxygenstorage material also figures into a potentially more serious deactivation mode, loss of noble metal by deep encapsulation [19,20]. As ceria-zirconia sinters, some of the noble metal particles supported on it may become trapped, either within single grains or at grain boundaries of the dense ceramic, as shown by the TEM micrograph in Fig. 10.9 [20].
Figure 10.9. TEM image showing Pd particles (some of which are labeled: 1, 2, 3, and B) encapsulated in ceria-zirconia. [20]
This phenomenon has been observed for Pt (Graham and Shigapov/unpublished), Pd, and Rh supported on ceria-rich mixed oxide as well as for Pd supported on a variety of other materials with composition ranging from zirconia-rich mixed oxide to pure ceria [15,21]. Generally, encapsulation commences when the specific surface area of the support falls below a few m2/g. The extent to which the noble metal becomes encapsulated depends on additional factors, including metal loading. Table 10.2 lists results obtained for a number of 2 wt% Pd catalysts made with a
Ceria and other oxygen storage components in automotive catalysts
357
variety of commercial-grade ceria-zirconias upon redox aging at 1050 or 1150°C. The wide variation in amount of Pd encapsulated from one catalyst to another reflects subtle differences in quality of the various support materials owing to differences in synthesis procedures. Table 10.2. Extent of Pd encapsulation in catalysts made with a variety of commercial-grade ceriazirconias upon redox aging at 1050 or 1150°C. CeO? content (wt%)
%Pd encapsulated 1150°C
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The thermal stability of three-way catalysts, at one time limited by that of the oxygen-storage material, ceria, has been greatly improved with the introduction of ceria-zirconia. Consequently, catalyst temperature limits have risen to around 1000°C, allowing the catalyst to be placed closer to the manifold, resulting in faster light-off. Further, over-fueling (the practice of occasionally adding excess fuel in order to cool the exhaust gas through vaporization of the liquid) is no longer needed in order to protect the catalyst from thermal deactivation, which results in better fuel economy. Greater catalyst durability, made possible by the better performance and stability of ceria-zirconia, may also permit the use of lower noble-metal loadings.
10.3.2. Chemical Deactivation Chemical deactivation of oxygen storage materials occurs primarily by two modes: sulfur poisoning and poisoning from oil additives. Each of these is discussed in turn below.
358
Catalysis by ceria and related materials
10.3.2.1. Sulfur Poisoning Sulfur is contained in fuel as a contaminant at various levels depending on the source of petroleum feedstock and subsequent extent of hydrotreating during gasoline production. Typical gasoline sulfur levels in the U.S. and Europe are currently in the 100-300 ppm range, but regulations have been enacted in both markets that will decrease fuel sulfur levels to 30-50 ppm over the next five years or so. California already operates under a gasoline sulfur average mandated at 30 ppm that will decrease to 15 ppm by the end of 2002. When combusted in an IC engine, the various organo-sulfur compounds in the fuel are largely converted to S0 2 . Sulfur is also contained in lubricating base oil components and in lubricating oil additives (zinc dialkyldithiophosphate (ZDDP) and Ca and Mg detergents). Currently the sulfur contribution due to oil consumption is estimated to be roughly equivalent to 2 ppm sulfur in fuel (assuming 10,000 miles per quart oil consumption) [22]. Hence, the sulfur content of oil will only become a significant issue in the event that fuel sulfur levels are regulated to 5-10 ppm or less. When contacted with the catalyst, SO2 can poison both the noble metal and the ceria-containing OSC materials. That S0 2 interacts with the catalyst to form cerium sulfate has been recognized since the early 1980s when the introduction of "high-tech" ceria-containing TWCs led to an associated problem with tailpipe emissions of H2S (i.e. rotten egg odor). The sulfate would store on the ceria under lean engine operating conditions and then convert to H2S (with the aid of the noble metal) during fuel-rich excursions. The problem was most pronounced on fresh catalysts, reflecting high ceria surface area and close proximity of noble metal to ceria sites. Various approaches have been employed to solve the problem; the most widely used in the U.S. is to incorporate Ni into the three-way catalyst to "scavenge" H2S (i.e. to form NiS under rich conditions and release the sulfur as S0 2 under lean conditions) [23]. Of greater concern than the potential odor associated with sulfur and ceria is the detrimental impact of sulfur on OSC. Details of the interaction of sulfur compounds with ceria are the subject of the chapter by Gorte. Consequently, our discussion is limited to a few brief examples illustrative of automotive exhaust catalysis. Sulfur affects both the noble metals and the ceria (as well as other promoters and stabilizers in the catalyst formulation). Hence, it is often difficult to ascribe a precise cause to sulfur poisoning of automotive catalysts, and the extent of poisoning can vary dramatically depending on operating conditions (i.e. temperature, air-fuel ratio, space velocity) and catalyst formulation. In general, sulfur poisoning is most severe on Pd-based catalysts, and this is attributed to a
Ceria and other oxygen storage components in automotive catalysts
359
greater sensitivity of Pd to reduced sulfur species [24-26]. Direct sulfur poisoning of ceria may or may not adversely affect oxygen storage depending on the temperature range of operation and magnitude/frequency of the air-fuel perturbations. Evidence of this is given in Fig. 10.10, comparing laboratory oxygen storage experiments over redox-aged 2%Pd/70wt%ceria-30wt%zirconia model catalyst powders in the absence (a) and presence (b) of 10 ppm S0 2 in the feed. At 350 and 500°C, S0 2 clearly has a poisoning effect on the catalyst as evidenced by the smaller areas of the C0 2 peaks (dashed curves) in the case of the sulfurcontaining feed. At 700°C, however, the area under the C0 2 peak is greater in the case of the sulfur-containing feed. These results appear consistent with the argument that at relatively high temperatures, cerium can provide OSC by a redox cycle involving conversion of cerium sulfate to cerium sulfite. Even though the capacity for storing oxygen is considerable in the presence of S0 2 at 700°C, the kinetics are slow, as best seen by comparing the delay time before CO breakthrough in the presence and absence of sulfur. Such data may explain a curious result often observed with on-board catalyst diagnostic (OBD) systems; high-sulfur fuels degrade the emission performance of catalyst systems but in many cases the emission performance degradation is not detected by the OBD system used to monitor catalyst activity. This phenomenon is illustrated in section 10.5.1 below. CO~_
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Catalysis by ceria and related materials
360
Another example of the effect of S0 2 on OSC of the same model Pd/ceriazirconia catalyst (in this case in the fresh state) is given in Fig. 10.11. Here one sees amounts of stored oxygen associated with CO titration of successive 0.5% 0 2 pulses following introduction of 10 ppm S0 2 continuously in the feed. The OSC drops in the span of 3 CO/0 2 cycles from about 700 u-moles O/g to less than 500 u-moles O/g and then holds fairly constant. Upon removing the S0 2 from the feed, the OSC returns to nearly the fresh state, albeit quite slowly. Similar sulfur poisoning effects and reversibility characteristics of Pd-based automotive catalysts have been reported elsewhere [27,28]. The slow (and sometimes incomplete) recovery from sulfur poisoning often leads emission engineers to carry out so-called sulfur purge cycles on catalyst systems in order to avoid slow test-to-test changes in emission performance. The sulfur purge cycles involve operating the vehicle on low-sulfur fuel at high speeds and loads to obtain catalyst temperatures in the range of 600700°C.
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Looking to the future, sulfur poisoning of three-way catalysts is not expected to be a major issue, given that fuel sulfur levels will be mandated at much
Ceria and other oxygen storage components in automotive catalysts lower levels and that catalyst suppliers have been successful in mitigating the impact of sulfur poisoning by utilizing multi-component/multi-layer washcoat designs with various additives [29]. The situation with regard to catalysts for leanburn applications (both gasoline and diesel) is much different, of course, due to the problem of forming very stable sulfates on materials that store NOx, such as barium oxide. In the lean-burn cases, however, ceria does not play as important a role as in TWCs, as noted in section 10.6 below.
10.3.2.2. Phosphorus Poisoning Increasingly stringent emission standards, together with the significant advances in catalyst thermal durability in recent years (see 10.3.1.6 above) have triggered increasing concern over catalyst poisoning from the lubricating oil anti-wear/antioxidant compound, ZDDP. The phosphorus compounds produced during the decomposition and/or combustion of ZDDP can poison the catalyst in a number of ways. In contrast to sulfur poisoning, phosphorus poisoning is essentially irreversible and the emissions impact increases at high mileage as contaminant levels build up, often at accelerated rates due to engine wear. We have recently reported on two major modes of catalyst deactivation in taxi-cab service: a layer of largely crystalline mixed Ca/Zn or Mg/Zn phosphates (with the Ca and Mg deriving from detergent sulfonates in the oil), and aluminum phosphate within the washcoat [30]. These species have been identified by both 27A1 and 31P solid-state NMR (see Figs. 10.12 and 10.13) and both are believed to deactivate the catalyst by physical means - pore blockage in the case of the overlayer, and densification and associated pore collapse (and noble metal occlusion) in the case of aluminum phosphate. The same study provided indirect evidence for a small amount of amorphous cerium phosphate (undetectable by 31P-NMR because of the unpaired electron in Ce(3+) which destabilizes the NMR P signal). Other work [31], and indeed our own subsequent studies of catalyst poisoning at temperatures higher than those normally experienced in taxi-cab operation, have revealed large amounts of CeP0 4 (monazite) and other mixed oxide cerium phosphate compounds (e.g. bariumcerium phosphate and strontium cerium phosphate). Fig. 10.14, for example, shows an X-ray diffraction pattern for a front-brick catalyst from a dynamometer aged Pdonly catalyst formulation that had seen peak catalyst temperatures near 1000°C. Note the correspondence of diffraction features to those of the simulated CeP0 4 (monazite) pattern.
361
Catalysis by ceria and related
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Ceria and other oxygen storage components in automotive catalysts
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Unlike sulfur, phosphorus compounds are not believed to directly poison the noble metals. This may owe to the extreme difficulty of reducing phosphorus below the +3 state. Also unlike sulfur, cerium phosphate chemistry does not have a corresponding redox chemistry that can provide a secondary source of OSC. Cerium phosphates, once formed (likely from P2Os or H 3 P0 4 precursors), trap the ceria in the 3+ oxidation state and irreversibly poison OSC. Moreover, one cannot rule out the possibility that this may be facilitated by the noble metals thus leading to preferential poisoning of ceria in the vicinity of the noble metal particles.
10.4. Other Materials Providing Oxygen Storage To our knowledge, ceria and modified ceria compounds are used exclusively within the automotive industry to provide oxygen storage. Other materials are certainly capable of storing and releasing oxygen under conditions of automotive exhaust operation, but none thus far demonstrates the combination of surface area stability, reversibility to sulfur poisoning, and rapid kinetics of oxygen storage and release
Catalysis by ceria and related
364
materials
shown by ceria-containing compounds. One exception is worthy of note, however, and that is palladium. Although often overlooked as a source of oxygen storage, heavily loaded Pd-containing catalyst formulations provide significant oxygen storage in their own right under certain ranges of operation. This is illustrated in Fig. 10.15 from data obtained in our laboratory on various Pd-containing catalysts some with and some without ceria [32]. The plot shows the amount of oxygen titrated in the first CO pulse (0.3%CO for 15s) following a 120s exposure to 0 2 (0.87%) at 500°C. Key to the plot is the solid line which is a fit through the origin to the amount of oxygen titrated from three Pd-containing catalyst formulations that have no oxygen storage component. This curve lies well above the Pd surface oxygen capacity data shown in the lower part of the figure; thus, the stored oxygen cannot be accounted for by chemisorption. Furthermore, on all of the Pd-containing catalysts examined, the amount of titrated oxygen following the pre-oxidation at 500°C is a significant fraction of the total amount of stored oxygen (and represents almost all of the oxygen storage for the 100,000 mile-equivalent dynamometer-aged catalysts).
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Ceria and other oxygen storage components in automotive catalysts The conditions under which these experiments were run certainly favor the formation of PdO, and subsequent work in our laboratory has shown that ceria-free Pd catalysts are somewhat limited in their oxygen storage contributions due to relatively slow re-oxidation compared to ceria-containing OSC materials. Nevertheless, Pd is a significant source of OSC, and the highly-loaded Pd-only formulations employed during the mid-1990s offered a means of adding a thermally stable source of OSC (i.e Pd/PdO) during the transition period from pure ceria to ceria-zirconia. Today, with the widespread use of ceria-zirconia materials, it is possible in many applications to decrease noble metal loadings substantially from levels used only a few years ago, and much effort is being directed toward the development of such low-loaded formulations.
10.5. Special Uses of Oxygen Storage 10.5.1. Role of Ceria in On-board Catalyst Diagnostics. The oxygen-storage function of three-way catalysts provides a means of performing on-board diagnostics of catalyst performance. In general, regulations in both the U.S. and Europe to monitor the performance of the catalytic converter over its lifetime have led to on-board monitoring systems based on comparison of signals from exhaust gas oxygen sensors located upstream and downstream of one or more catalyst bricks. The setup is illustrated in Fig. 10.16. The monitoring strategy is based on comparing the switching characteristics of the upstream and downstream sensors during warmed-up catalyst operation. Switching occurs quickly (1-2 Hz) and with relatively high amplitude in the upstream sensor as part of the normal three-way closed loop engine control. Switching occurs much more slowly and with lower amplitude in the rear sensor due to the dampening effect produced by OSC. This can be seen, for example, in the feedgas vs. tailpipe Lambda traces in Fig. 10.2. The exact details of how the sensor signals are compared has evolved with time and varies from manufacturer to manufacturer. However, the basic approach is simple: active catalysts give large differences between upstream and downstream sensor response characteristics whereas deactivated catalysts show less difference. In the limit of a totally deactivated catalyst with no oxygen storage, for example, the response characteristics of upstream and downstream sensors would differ only by the transport delay.
365
Catalysis by ceria and related materials
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The biggest challenge in implementing a catalyst monitoring strategy based on oxygen sensors is in the link between OSC and catalyst efficiency. For hydrocarbon oxidation, especially, this link is rather weak and has a "hockey stick" shape [33]. In other words, the catalyst monitor system does not show significant change in response characteristics until hydrocarbon (HC) emissions have increased to levels close to the threshold level for activation of the malfunction indicator light (MIL) - currently 1.75 times the 100,000 mile standard. The differing responses of the catalyst monitor and the catalyst system to poisons can be seen with the aid of experiments that were run on catalyst systems from high-mileage 4.6L Crown Victoria and Grand Marquis vehicles to assess the extent to which deactivation is reversible. The reversibility studies were conducted with emission systems that were demonstrating catalyst monitor switch ratios very near the MIL activation threshold. Testing involved stepwise actions to "undo" the deactivation, thereby assessing the relative contributions of the heated exhaust gas oxygen (HEGO) sensors, sulfur poisoning, and phosphorus poisoning to the high switch ratios observed. Techniques employed were 1) switching to new sensors for the HEGO sensor effect, 2) employing a brief high-temperature driving schedule to purge sulfur from the catalyst, and 3) acid-washing the catalyst with oxalic acid to remove phosphorus. Fig. 10.17 shows the results of the stepwise reversibility study on HC ineff-secs for three high mileage taxi catalyst systems (where HC ineff-secs is defined as the cumulative departure of the catalyst from 100% conversion efficiency over the first 300 seconds of the U.S. Federal Test Procedure). Reading from left to right, the first cluster gives the baseline (i.e. as-received) measure of HC ineff-secs (except for one of the CV systems, for which no baseline data were
Ceria and other oxygen storage components in automotive catalysts
367
obtained since the original HEGO sensors were not available). The second cluster shows the effect of replacing the original upstream HEGO sensors with new ones. For both systems, this had negligible effect on the HC ineff-secs. The sulfur purge cycle resulted in small but significant reductions in the HC ineff-secs (3 rd bar cluster). By far, the largest reductions were observed after acid washing to remove phosphorus as shown by the bar cluster on the right. In general, reductions in HC ineff-secs (i.e. restoration of catalyst activity) should be accompanied by decreases in the switch ratio recorded by the catalyst monitor (i.e. restoration of oxygen storage). For the most part this is true, as evidenced by the trends observed when the average switch ratio is plotted as a function of the stepwise reversibility actions in Fig. 10.18, however, there is one important difference. In Fig. 10.18, it can be seen that the sulfur purge step had little affect on switch ratio, actually increasing the switch ratio in two of the three cases (indicating a decrease in OSC), while the HC ineff-secs data of Fig. 10.17 shows that the sulfur purge restored activity to all of the catalysts. The implication is that removing strongly adsorbed sulfur from these catalysts slightly decreases oxygen storage, consistent with the data presented above on model Pd/ceria-zirconia catalysts (Figs 10.10a and 10.10b) where sulfur poisoning increases OSC at relatively high temperatures, presumably via sulfite to sulfate conversion. The acid washing results indicate that, of the three factors examined, oil contamination has the strongest deactivating effect on both catalyst efficiency and OSC (as probed by catalyst switch ratio).
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Figure 10.17. Stepwise reversibility experiments (i.e. effect on HC Ineff-secs) on catalyst systems from high-mileage Crown Victoria (open and hashed bars) and Grand Marquis (solid bar) vehicles. Note that no baseline data (with the original HEGO sensors) were available in the case of one of the Crown Victoria catalyst systems.
368
Catalysis by ceria and related
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10.5.2. Light-off Enhancement ofTWCs and Hydrocarbon Traps 10.5.2.1. Light-off Enhancement by Pre-oxidation of Pd Although oxygen storage is normally viewed as facilitating CO and NOx conversion under warmed-up engine operating conditions, it can also be of value in promoting catalyst light-off. In the case of light-off enhancement, however, the role of ceria is less pronounced than at warmed-up operating temperatures, presumably due to slower kinetics of oxygen transport from the ceria phase to the noble metal particles. Also, the main challenge during light-off is often to decrease cold-start emissions of hydrocarbons, and hydrocarbon conversion is not strongly promoted by oxygen storage materials. Given this situation, there are potential advantages to pre-oxidizing the noble metal (i.e. pre-storing oxygen) prior to light-off. A typical scenario would involve exposing the catalyst to oxygen during cool-down from warmed-up conditions, thus oxidizing the noble metal and providing a source of oxygen to enhance light-off during the subsequent cold-start (typically carried out at slightly rich A/F conditions to improve driveability). Such an effect has been demonstrated for Pd catalysts [34] as shown in Fig. 10.19, and may explain to some extent the much better light-off seen with highly loaded Pd catalysts compared to catalysts based on Pt (not readily oxidized) and Rh (oxidizable, but deployed at much lower loadings than Pt or Pd). Two effects are shown in Fig. 10.19: preoxidation (O) versus pre-reduction (R) and light-off in a lean gas mixture
Ceria and other oxygen storage components in automotive catalysts
369
containing excess oxygen (L) versus light-off in a stoichiometric gas mixture (S), In the case of the Pd-only catalyst formulation of Fig. 10.19, the effects are additive, with the combination of pre-oxidation followed by lean light-off resulting in the lowest light-off temperatures. In contrast, Fig. 10.20 shows the same experimental sequence carried out over a Pt/Rh catalyst formulation. In this case, pre-oxidation versus pre-reduction of the catalyst has no effect, whereas light-off under lean conditions produces some decrease in light-off temperatures compared to light-off under stoichiometric conditions. 1o0
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Catalysis by ceria and related materials
370
10.5.2.2. Low-temperature OSC via Ceria-praseodymia Mixed Oxide Support Phases Another possibility for enhancing low-temperature oxygen availability is to employ an oxygen storage component that releases oxygen at lower temperatures than typical of materials such as ceria-zirconia. Praseodymia is such a candidate [35]. It can undergo a +4 <=> +3 redox cycle, but only under conditions of relatively low temperatures and/or high 0 2 pressures. Under normal warmed-up operating conditions in a vehicle, the redox equilibrium of praseodymia is toward the +3 oxidation state and consequently praseodymia does not contribute significantly to oxygen storage. Praseodymia also tends to sinter and react with alumina under automotive conditions. We have shown, however, that when praseodymia is combined with ceria (or ceria-zirconia), the thermal stability of the resultant mixed oxide is only marginally worse than that of the best ceria-zirconia materials [36]. Furthermore, when used as a support phase for Pd, it shows excellent lowtemperature OSC behavior, as evidenced in Fig. 10.21 comparing the OSC of 2%Pd on a Pr-Ce mixed oxide support material versus a Ce-Zr mixed oxide support material. The advantage of Pr-Ce relative to Ce-Zr is lost at temperatures of 500°C and higher in Fig. 10.21, in keeping with the tendency of praseodymia towards auto-reduction at elevated temperatures. Never-the-less, the excellent low temperature OSC characteristics raise the possibility that if suitably pre-oxidized i.e., similar to the operating strategy proposed for Pd pre-oxidation above - Pr-Ce mixed oxide support phases could be used to enhance light-off. 1200 ,
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Figure 10.21. OSC (u-mole O/g) measured in alternating 5%CO/2.5% 0 2 pulses on 2% Pd/Ce-Zr (open bar) and 2%Pd/Pr-Ce (solid bar) model catalysts after 1050°C redox aging. The Ce-Zr support is a 70wt% ceria - 30wt% zirconia mixed oxide while the Pr-Ce support is a 55wt% praseodymia - 45 wt% ceria mixed oxide. (H. Jen)
Ceria and other oxygen storage components in automotive catalysts Low-temperature OSC catalysts could also play a role in HC trap/catalysts. Here, low-temperature light-off is essential, as it is necessary to initiate light-off prior to, or simultaneous with, desorption of stored hydrocarbons from the zeolitebased trapping phase. A "brute-force" way of doing this is by relying on an electrically-heated catalyst behind a HC trap. However, automakers are looking for solutions that do not require the complexity and energy requirements of an electrically-heated catalyst. Low-temperature OSC materials, such as those based on Pr-Ce, offer the possibility of directly lowering the light-off temperature and/or eliminating the need for air injection to promote low-temperature light-off.
10.6. Oxygen Storage and NOx Traps The demand for better fuel economy and lower C0 2 emissions is currently driving renewed efforts to develop lean-combustion technology, a critical element of which is the control of NOx emissions. A novel approach, developed by Toyota and used on some vehicles in Japan since 1994, relies on a trap to accumulate NOx in the form of a surface barium nitrate during extended periods of engine operation under lean conditions (e.g., [37]). Periodically, the trap undergoes regeneration by having engine operation shifted briefly to stoichiometric or slightly rich conditions. The NOx that is released during these intervals is then converted to N2 in the usual way. Because of the need to employ these distinct modes of operation (and for other reasons as well), the complete catalyst system consists of a conventional TWC in combination with the NOx trap. Various implementations, ranging from serial configurations to a physical blend of the two entities, have been considered, but incompatibilities, some involving interactions with oxygen storage components, exist. In the most direct application, the TWC is placed upstream from the NOx trap, providing for both the usual means of emissions control under start-up and stoichiometric conditions and a cooler environment for the trap, which has a relatively narrow, low-temperature window of efficient operation under lean conditions. If the TWC stores oxygen, the shift from lean to stoichiometric conditions, initiating the release of NOx from the trap, will precede the breakthrough of reductants, allowing some of the NOx to escape. Limiting the OSC of the TWC mitigates this effect, but it also compromises TWC performance. Poisoning of the NOx trap by fuel sulfur, which as a relatively stable sulfate impedes the formation of the nitrate, presents another problem, but in this case, oxygen storage may be useful. The accumulation of sulfates can be diminished
371
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Catalysis by ceria and related materials
periodically by exposing the trap to rich conditions at a temperature of at least 600°C. This can be accomplished by rapid air fuel modulation if the trap contains an oxygen storage component, which will react exothermically with both the reductants and oxygen [38]. An additional benefit of having an oxygen storage component, like ceria, is that it may also act as a sink for sulfates, providing some protection for the trapping material. On the other hand, incorporation of an oxygen storage component into the NOx trap also leads to the generation of an exotherm during normal NOx regeneration, which has the effect of lowering the upper temperature limit of operation.
10.7. Outlook The chapter summarizes some of the key developments in the use of ceria in automotive catalysts over the last quarter-century or so, with emphasis on the dramatic advances related to solid-solution ceria-containing mixed-oxide materials during the last decade. Given the success of these materials and their ubiquitous presence in today's TWC formulations, together with the absence of any serious competition to ceria as the OSC material of choice for automotive applications, the overall outlook is that ceria-containing materials will continue to be a major component of automotive exhaust catalysts. As noted in section 10.6, this conclusion would appear to hold for lean-burn engine emission control as well as stoichiometric engines as long as NOx trapping is utilized with its requirement of either stoichiometric or rich engine operation during the regenerative mode. Although it is true that after twenty-five years of development ceriacontaining OSC materials have progressed to a mature state, further advances are needed in a number of areas, and we end this review with a brief listing of some possible areas for additional research and development: For the conventional automotive TWC, continued improvements will be needed in thermal durability of OSC materials, especially given that upcoming regulations, such as those for Partial Zero Emission Vehicles in California, allow for no increase of emissions above the extremely low mandated levels over 150,000 miles of vehicle life. Such requirements can only be met if there is almost no deterioration in emission performance over the useful life of the vehicle. Hence, extremely durable oxygen storage materials - even better than those on vehicles today - will be needed. Similarly, the impact of chemical deactivation, particularly phosphorus poisoning, will need to be minimized. More understanding is needed of
Ceria and other oxygen storage components in automotive catalysts the mechanisms by which phosphorus species in automotive exhaust interact with oxygen storage components; it may be possible to change the formulation of the oxygen storage material to make it less susceptible to phosphorus poisoning without sacrificing other characteristics. Along the same lines, ceria's dual role as an oxygen storage component in the TWC and as the basis for on-board catalyst monitoring, suggest that there may be opportunities to "tune" these characteristics separately i.e., to incorporate ceria in various forms in the catalyst, some optimized for oxygen storage and some for on-board diagnostics. The same can probably be said of the dual roles of the oxygen storage material in promoting both oxidation reactions and NOx reduction. Many TWCs already contain more than one ceria-containing phase (Fig. 10.3), each generally associated with a specific noble metal or combination of noble metals, but there appears to be no general consensus within the industry as to the optimum way to deploy the various ceria-containing materials and noble metals. Recent trends toward the development of low-cost, reduced-loading, noble metal catalysts, have also spurred efforts to "extend" the capability of the noble metals by placing more of the oxygen storage burden on the OSC material rather than the noble metal. This creates a need for deeper understanding of the interactions between the noble metal and the oxygen storage material and ways of maintaining those interactions in the face of long term exposure to thermal sintering processes. Efforts to develop low-cost catalyst systems have also brought to the forefront the need to deploy the noble metals and OSC material efficiently throughout the entire catalytic converter system. Changes in temperature and reactant concentrations through the catalyst system present opportunities for catalyst staging and zone-coating. Undoubtedly, further developments in these areas will be forthcoming. Finally, as noted in section 10.5, special uses of oxygen storage materials will likely emerge in response to lower emission standards and increased fuel economy requirements. Particularly intriguing is the possibility of controlling the amount of oxygen storage as a function of catalyst operating temperature by relying on mixtures of OSC materials having different redox potentials at different temperatures. Our experience with ceria-praseodymia versus ceria-zirconia certainly hints at this possibility. Acknowledgments: The authors are grateful to David Gregory and Bruce Campbell (Cambustion Ltd.) for Fig. 2, Jon Hangas for Fig. 3, Hung-Wen Jen for Figs. 10, 11, and 21, Charlotte Lowe-Ma for Fig. 14, Joe Theis for discussions about NOx traps, and Carrie Graham for assistance in preparing the figures.
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374 10.8. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22.
Shelef M., Catal. Rev. Sci. Eng. 11 (1975), 1-40. Gross et al, U. S. Patent 3,370,914. Shelef M. and McCabe R. W., Catal. Today 62 (2000), 35-50. Gandhi H. S., Piken A. G., Shelef M., and Delosh R. G., SAE Paper 760201 (1976), 55-66. Trovarelli A., Catal. Rev. Sci. Eng. 38 (1996), 439-520. Kim G., Ind. Eng. Chem. Prod. Res. Dev. 21 (1982), 267. Usmen R. K., McCabe R. W., Graham G. W., Weber W. H., Peters C. R., and Gandhi H. S., SAE Paper 922336 (1992). Cordatos H., Bunluesin T., Stubenrauch J., Vohs J. M., and Gorte R. J., J. Phys. Chem. 100 (1996), 785-789. Yang L., Kresnawahjuesa 0., and Gorte R. J., Catal. Lett. 72 (2001), 33-37. Smirnov M. Yu. and Graham G. W., Catal. Lett. 11 (2001), 39-44. Bunluesin T., Gorte R. J., and Graham G. W., Appl. Catal. B 14 (1997), 105115. Mamontov E. and Egami T., J. Phys. Chem. Solids 61 (2000), 1345-1356. Mamontov E., Egami T., Brezny R., Koranne M., and Tyagi S., J. Phys. Chem. B 104 (2000), 11110-11116. Jen H.-W., Graham G. W., Chun W., McCabe R. W., Cuif J.-P., Deutsch S. E., and Touret O., Catal. Today 50 (1999), 309-328. Graham G. W., Jen H.-W., McCabe R. W., Straccia A. M., and Haack L. P., Catal. Lett. 67 (2000), 99-105. Boaro M., de Leitenburg C , Dolcetti G., and Trovarelli A., J. Catal. 193 (2000), 338-347. Hori C. E., Brenner A., Ng K. Y. S., Rahmoeller K. M., and Belton D., Catal. Today 50 (1999), 299-308. Yu-Yao Y.-F. and Kummer J. T., J. Catal. 106 (1987), 307-312. Graham G. W., Jen H.-W., Chun W., and McCabe R. W., Catal. Lett. 44 (1997), 185-187. Jiang J. C , Pan X. Q., Graham, G. W., McCabe R. W., and Schwank J., Catal. Lett. 53 (1998), 37-42. Graham G. W., Jen H.-W., Chun W., and McCabe R. W., J. Catal. 182 (1999), 228-233. Korcek S., Johnson M. D., and Jensen R. K., 2nd World Tribology Congress, Vienna, Austria, September 3-7, 2001 (to be published in conference proceedings).
Ceria and other oxygen storage components in automotive catalysts 23. Gandhi H. S. and Shelef M., Appl. Catal 11 (1991), 175-186. 24. Thoss J. E., Rieck J. S., and Bennett C. J., SAE Paper 970737 (1997). 25. Lafyatis D. S., Bennett C. J., Hales M. A., Morris D., Cox J. P., and Rajaram R. R., SAE Paper 1999-01-0309 (1999). 26. Beck D.D. and Sommers J. W., Stud. Surf. Sci. Catal, 96, eds. Frennet, A. and Bastin, J.-M. (Elsevier, 1995), 721-748. 27. Beck D. D., Sommers J. W., and DiMaggio C. L., Appl Catal B: Environ. 3 (1994), 205-227. 28. Beck D.D. and Sommers J. W., Appl. Catal B: Environ. 6 (1995), 185-200. 29. Dettling J., Hu Z., Lui Y. K., Smaling R., Wan C. Z., and Punke A., in Stud. Surf. Sci. Catal, 96, eds. Frennet, A. and Bastin, J.-M. (Elsevier, 1995), 461472. 30. Rokosz M. J., Chen A. E., Lowe-Ma C. K., Kucherov A. V., Benson D., Paputa Peck M. C , and McCabe R. W., Appl Catal. B: Environ, in press (2001). 31. Smedler G., Eriksson S., Lindblad M., Bernier H., Lundgren S., and Jobson E., SAE Paper 930944 (1993). 32. McCabe R. W. and Usmen R. K., in Stud. Surf. Sci. Catal, 101, eds. Hightower J. W., Delgass W. N., Iglesia E., and Bell A. T. (Elsevier, 1996), 355-368. 33. Hepburn J., Chanko T., and Zimlich G., SAE Paper 972847 (1997). 34. McCabe R. W., U.S. Patent 6,187,709 (Feb. 13, 2001). 35. Logan A. D. and Shelef M., J. Mater. Res. 9 (1994), 468-475. 36. Shigapov A. N., Jen H.-W., Graham G. W., Chun W., and McCabe R. W., in Stud. Surf. Sci. Catal, 130, eds. Corma A., Melo F. V., Mendioroz S., and Fierro J. L. G. (Elsevier, 2000), 1373-1378. 37. Matsumoto S., Ikeda Y., Suzuki H., Ogai M., and Miyoshi N., Appl Catal B 25(2000), 115-124. 38. Asik J. R., Meyer G. M., and Dobson D., SAE Paper 2000-01-1200 (2000).
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CHAPTER 11 S 0 2 POISONING OF CERIA-SUPPORTED, METAL CATALYSTS
RAYMOND J. GORTE and TIAN LUO 311 Towne Building, 220 S. 33rd Street, University of Pennsylvania, Philadelphia, PA 19104 USA; e-mail: [email protected]
11.1. Introduction Automobiles have been equipped with catalytic converters since 1975 in the US and since 1986 in Europe [1]. The emissions limits for CO, NO, and hydrocarbons have been steadily decreasing since that time in both regions of the world, requiring improved catalysts and emissions-control systems. A major advance in the development of three-way emissions-control catalysts was the addition of ceria, which is added primarily to provide oxygen-storage capacity (OSC) to allow the oxidation of hydrocarbons and CO to occur simultaneously with the reduction of NO [2]. While the OSC properties in early ceria-containing catalysts were prone to deactivation during high-temperature aging [3], the redox properties of ceria in modern catalysts are stabilized by the addition of zirconia and are very durable, even in the very harsh, high-temperature, hydrothermal environment of the catalytic converter [4]. The current generation of catalysts would be sufficient to meet some of the most stringent requirements if sulfur were not present in essentially all hydrocarbon fuels. The combustion of sulfur-containing fuels produces S0 2 in the engine exhaust. In high concentrations and for reducing environments, S0 2 poisons sites on the precious-metal catalysts by forming adsorbed sulfur atoms, which in turn block a portion of the active metal sites and reduce the activity of adjacent metal sites [5]. However, for the relatively low sulfur concentrations to which three-way catalyst are typically exposed, the precious metals are usually unaffected. Poisoning of ceria is much more serious than poisoning of the precious metal at the levels of 5 to 20 ppm S0 2 currently present in the typical automotive exhaust. At these concentrations, S0 2 interacts primarily with the ceria-containing component of in the catalytic converter and it is this poisoning of ceria that appears to be the primary problem associated with sulfur poisoning [6-11]. The evidence for this is strong. For 377
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Catalysis by ceria and related materials
example, it has been reported that the improvement in activity found with Pd/ceria/alumina catalysts compared to Pd/alumina alone is cancelled by the addition of 30 ppm of S0 2 at 450°C [6]. Interestingly, sulfur poisoning is reported to be partially reversible above 650°C and its impact, at least in the short term, appears to be minimal for operation at 700°C [8]. Clearly, understanding the interaction between S0 2 and ceria is extremely important in three-way, emissions-control catalysis. This chapter will review what is known about the nature of sulfur poisoning of ceria and how this affects the catalytic properties.
11.2. Effect of S0 2 on Catalytic Activity
11.2.1. Oxygen Storage Capacity As stated above, the primary role of ceria in three-way, emissions-control catalysis is to provide OSC. At first examination, OSC is a very simple concept, suggesting the effect of sulfur is also very simple. Because precious-metal catalysts are not selective, CO and hydrocarbons in the exhaust will first be oxidized by any 0 2 that remains in the gas phase. In the absence of 0 2 , CO and hydrocarbons will be oxidized by NO. Therefore, if the exhaust stoichiometry is properly matched, all three pollutants can be removed very effectively [1], Most gasoline-fuelled cars attempt to control the exhaust stoichiometry with a feedback control system that adjusts the air-to-fuel ratio entering the engine. Since oscillations about the set point prevent the possibility of perfectly matching the fuel-air stoichiometry, an additional level of control is required within the catalyst itself. In the conventional view, the role of OSC is to widen the three-way conversion window by storing oxygen during lean excursions and releasing oxygen during rich excursions. The primary effect of S0 2 in the automotive exhaust is a severe loss in OSC. Indeed, the loss of OSC upon the introduction of S0 2 is well documented. For example, Beck, et al. reported that just 5 ppm of S0 2 in the exhaust, a level which corresponds to 75 ppm sulfur in the fuel, changed the OSC of an aged catalyst by a factor of two [7]. Similar losses in OSC due to sulfur poisoning were reported by Gandhi and coworkers [12,13]. Furthermore, it has been demonstrated that the large loss in OSC only occurs when ceria is present, showing that precious metals, which provide some OSC, are not as severely affected as the ceria-containing component [14]. Finally, OSC is measured continuously by the sensors required for monitoring
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catalyst performance according to US Federal regulations (OBD-II). The effect of sulfur on this monitoring is well documented [7,13,15]. A closer examination of OSC and the effect of sulfur on OSC suggests that these are actually more complex phenomena. If the role of ceria were simply to store and release oxygen, it should be possible to determine OSC simply by titration with CO and 0 2 . However, there is strong evidence that simple titration methods do not properly measure OSC. For example, Hepburn, et al. reported "that the CO/0 2 titration method was unable to differentiate between catalysts which displayed greatly different transient performances on a vehicle." [16]. In a more recent demonstration that titration methods do not properly account for OSC, it has been shown that Pd/ceria and Pd/ceria-zirconia catalysts are capable of reversibly releasing more oxygen after they have been poisoned with S0 2 than they are before being poisoned with S0 2 [17]. An example of this is given in the pulse data from a Pd/ceria-zirconia catalyst at 773 K, reported in Figure 11.1, with data for the unpoisoned catalyst shown on left and data for the poisoned catalyst on the right.
—i
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Figure 11.1. 02/CO pulse-reactor results for a Pd/ceria catalyst before (left half) and after (right half) exposure to S02. From bottom to top, the curves are the 0 2 (m/e=32), CO (m/e=28), and COz (m/e=44) partial pressures. The measurements were performed at 773 K and were reported in reference [17].
Here, we only show two pulses of 0 2 (m/e=32) and CO (m/e=28) from a long string of essentially identical, alternating pulses. A complete discussion of this data is given in the original reference; however, simple observation indicates that much more C0 2 (m/e=44) is formed upon the introduction of CO at -1500 sec on the S0 2 poisoned catalyst compared to the unpoisoned catalyst. In this example, approximately 1000 |J.mol/g of oxygen could be removed from the poisoned catalyst compared to only 700 |Hmol/g on the unpoisoned catalyst. This increased, reversible oxygen uptake on the S02-poisoned catalyst is due to oxidation and reduction of
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Catalysis by ceria and related materials
sulfate species, as will be discussed shortly. For our purposes here, it should be noted that, if OSC were simply a "capacitance", the data in Figure 11.1 would imply that sulfur poisoning should increase OSC. Clearly, this is not the case. It is useful to examine how OSC is measured in those situations where a decrease in OSC was reported due to the presence of S0 2 . Beck and Sommers measured oxygen uptake and release in a gaseous mixture that simulated a real exhaust environment [8]. A key point to notice is that the gas mixture in their study oscillated from oxidizing to reducing conditions by switching from 0.2% 0 2 to 1.0% 0 2 every 0.5 sec, while maintaining a steady concentration of 10% H 2 0 and 10% C0 2 . Likewise, Hepburn, et al. used a pulse-flame-combustor method in which H 2 0 and C0 2 are again major gas-phase components above the catalyst [16]. Based on the fact that ceria-supported metals are active for a number of reactions that are important in the catalytic converter, including water-gas shift (WGS) [18,19], steam reforming [19,20], and C0 2 reforming [21], it seems very likely that H 2 0 and C0 2 are key components in the reaction mixture and that understanding their role is necessary for understanding OSC.
11.2.2. Steady-State Reactions Even though OSC is an inherently transient phenomenon, it appears that there is a relationship between steady-state reaction rates and OSC [3,17]. For the COoxidation, WGS, and steam-reforming reactions, it has been shown that rates can be enhanced by contact between the precious metals and ceria. Furthermore, hightemperature treatments, which are known to deactivate the OSC properties of pure ceria, also remove the promotional effects associated with ceria [3,20,22]. Given that S0 2 affects OSC, one should expect S0 2 to influence the steady-state behavior of ceria-supported catalysts, if OSC is related to these reactions. As will be discussed shortly, ceria enhances reactions primarily by supplying oxygen under reducing conditions. Therefore, in atmospheres that are highly oxidizing, the promotional effects of ceria are less important since oxidation of the catalyst can occur directly from the gas phase. Not surprisingly, the poisoning of reaction rates by low levels of S0 2 are found to be negligible under oxidizing conditions [18]. Some reports even suggest that hydrocarbon oxidation can be enhanced by the presence of S0 2 [23,24]. The effect of having S0 2 present in the reaction mixture is dramatic under reducing conditions [18]. Small amounts of S0 2 poison the steam-reforming and WGS reactions [17,25]. For CO oxidation, the enhancement normally found by
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including ceria in the catalyst is lost after exposure to S0 2 [11,17]. Clearly, it is necessary to understand why ceria promotes these reactions in order to understand how S0 2 poisons the reaction. It has been suggested that ceria influences the steam-reforming and WGS reactions through a redox mechanism similar to that shown below for WGS [22,26]: CO + a -> COad
(11.1)
H 2 0 + Ce 2 0 3 -> 2 Ce0 2 + H2
(11.2)
COad + 2 Ce0 2 -> C0 2 + Ce 2 0 3 + a
(11.3)
Here, a represents an adsorption site on the metal catalyst. The key point in this mechanism is that ceria is oxidized by H 2 0, allowing the oxygen from steam to be used in the oxidation of CO or other reducing agents. There is significant support for this mechanism, both from kinetic and spectroscopic data. For example, the oxidation of CO adsorbed on a precious metal by oxygen from the ceria has been observed to occur below 400 K in temperatureprogrammed-desorption (TPD) measurements [27]. Indeed, recent spectroscopic data has shown that Pd particles are oxidized by their ceria-zirconia support, beginning at -470 K [28]. Evidence for the other important step in the reaction, the oxidation of reduced ceria by steam, has also been presented [29]. Finally, the kinetic rate expression for the WGS reaction over ceria-supported precious metals, in which the reaction is zeroth order in CO, agrees with expected rate expression for the mechanism shown above [29]. If one accepts the above mechanism for the WGS reaction, poisoning of the catalyst by S0 2 must either block the oxidation of reduced ceria by water (Reaction (2)) or decrease the rate at which adsorbates on the metal can be oxidized by ceria (Reaction (3)). In either case, it is necessary to understand how S0 2 interacts with ceria in any attempt to synthesize sulfur-tolerant catalysts.
11.3. Chemistry of S0 2 Poisoning of Ceria
11.3.1. Thermodynamic Considerations Before reviewing work on the characterization of species formed by sulfur poisoning of ceria-supported catalysts, it is helpful to examine what is known from thermodynamic investigations of sulfur-containing Ce compounds. Cerium, in a
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Catalysis by ceria and related materials
mixture with other rare-earth metals known as "mischmetal", is used as an additive for steel treatment to tie up sulfur impurities [30]. There is also a great deal of interest in using ceria as an adsorbent for removing H2S and S0 2 from dilute streams [31,32]. Since the temperatures associated with the above applications are very high, thermodynamic equilibrium can be assumed. Therefore, thermodynamic properties for sulfur-containing compounds with cerium have been measured and are reasonably well known [33-36]. The particular compounds that form depend on the fugacities (or partial pressures) of 0 2 and of S2, H2S, or S0 2 . Under the conditions relevant to automotive catalysis, it is probably appropriate to consider the S2 fugacity to be fixed, with the 0 2 fugacity varying with the amount of reductant in the exhaust stream. Under neutral and mildly reducing conditions, the stable species in the presence of sulfur is reported to be Ce2(S04)3 [36]. Notice that Ce, unlike the other rare-earth metals, is not known to form an oxysulfate of the form Ce 2 0 2 S0 4 [35]. With decreasing 0 2 fugacity, Ce2(S04)3 decomposes, first to Ce0 2 and then to Ce202S [33-36]. Reaction can proceed all the way to Ce2S3 for very low 0 2 fugacities, but these conditions are probably not approached in normal catalytic environments.
11.3.2. Surface Investigations Under catalytic reaction conditions, one should not necessarily expect species to proceed to the thermodynamic final state. An additional complication comes from the fact that the redox properties of catalytically active ceria and of ceria-zirconia mixed oxides appear to be quite different from the bulk thermodynamic values for ceria [37,38]. For example, ceria films calcined above 1270 K no longer promote the WGS [22] or steam-reforming reactions [20] and are much more difficult to reduce upon heating in vacuum [39]. These observations appear to be explained by calorimetric studies, which have shown that the heat of reoxidation for reduced Pd/ceria and Pd/ceria-zirconia catalysts is much lower than bulk thermodynamics would suggest [38]. Therefore, bulk thermodynamic information may not be entirely relevant for describing the nature of sulfur-containing species on catalytically active materials. There is surprisingly little information on the nature of species formed by contacting ceria with H2S or S0 2 at the more moderate temperatures and pressures of the automotive exhaust environment. Some of the more comprehensive studies include those performed by Lavalley and coworkers [40-43], using primarily IR
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spectroscopy. The main features of this work are supported by a Ramanspectroscopic study performed under similar conditions [44]. For room-temperature adsorption of S0 2 on ceria [40], only surface species, which have been tentatively identified as sulfites or hydrogen sulfites, were observed. For adsorption of S0 2 at 400°C with 0 2 , sharp bands between 1300 and 1400 cm"1 appeared in the spectrum and were assigned to sulfate groups on the surface of ceria. For larger exposures, the main feature in the IR spectrum was a broad band centered near 1160 cm'1, which has been assigned to bulk sulfate formation. In agreement with the interpretation of bulk and surface sulfates, gravimetric results showed weight increases in excess of that expected for a monolayer sulfate at the same conditions which lead to bulk sulfates in the IR spectra [42]. It is interesting to consider which factors are important in the formation of cerium sulfates. The presence or absence of Pt had little effect, showing that ceria is able to oxidize S0 2 without an additional catalyst [42]. Furthermore, the oxidation of S0 2 to S042" occurs on ceria to some extent without the addition of gas-phase 0 2 , obviously with the simultaneous reduction of ceria [40]. Finally, bulk sulfates were only formed when the ceria samples were exposed to S0 2 at temperatures above 250°C. Some of the essential conclusions of these earlier investigations were also observed in a TPD study performed in our laboratory, the results of which are shown in Figures 11.2 and 11.3. These measurements were performed in the same pulse reactor described in reference [21]. In this case, pure He was passed through the catalyst bed and the reactor effluent monitored using a mass spectrometer. The set of curves in Figure 11.2 were obtained on a bulk sample of Ce(S04)2xH20. Decomposition of the sulfate to S0 2 (m/e = 64) and 0 2 (m/e = 32), in a ratio of 2:1, is clearly observed at ~700°C. The amount of S0 2 removed from the sample, 5.5 mmol/g, is within experimental error of the amount expected based on the stoichiometry of the compound. X-ray diffraction patterns of the samples after TPD showed only peaks associated with the fluorite structure of Ce0 2 . It is not exactly clear how to reconcile these results with literature reports that indicate Ce(S0 4 ) 2 decomposes at 195°C, leaving Ce2(S04)3 as the stable sulfate at higher temperatures [45]. The curves in Figure 11.3 were obtained after exposing a Pd/ceria catalyst to a flowing gas with 1000 ppm of S0 2 in He for 2 hrs at 400°C. The results are virtually identical to those for the bulk sulfate decomposition with the following exceptions. First, the quantity of sulfate was calculated to be -0.50 mmol/g, a coverage that is only slightly higher than that expected for a monolayer on the 30-m2/g catalyst assuming that a monolayer coverage corresponds to a surface coverage of -10"1
Catalysis by ceria and related materials
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mmol/m2. It seems likely that bulk sulfate formation was limited in our case because of the absence of gas-phase 0 2 in our experiments, while Lavalley and coworkers reported significant bulk formation because they introduced S0 2 with a large excess of 0 2 . Second, the S0 2 peak from poisoned Pd/ceria has a low-temperature shoulder
3
<
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Temperatire (°Q
Figure 11.2. TPD measurements showing simultaneous desorption of S 0 2 (m/e=64) and 0 2 (m/e=32) from a bulk sample of Ce 2 (S0 4 ) 2 xH 2 0 obtained from Alpha Aesar. The quantity of S0 2 leaving the sample matched the bulk value of the sulfate, and XRD patterns of the sample after making the TPD measurements showed that it had converted to CeO,.
3
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Temperature (°C) Figure 11.3. TPD measurements showing simultaneous desorption of S0 2 (m/e=64 and 48) and 0 2 (m/e=32) from a Pd/ceria catalyst after exposure to flowing S0 2 .
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metal catalysts
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with no corresponding 0 2 desorption. We suggest that this feature may be due to decomposition of a more reduced form of ceria, with some 0 2 consumed in the oxidation of Ce. In agreement with this, we found that including 0 2 with the He carrier in the TPD measurements completely eliminated the low-temperature shoulder. Up to this point, we have only discussed sulfur poisoning under oxidizing or mildly reducing conditions. As predicted from thermodynamic considerations, the sulfates of cerium are not stable under more harshly reducing environments. Figure 11.4 shows the TPD curves for an S02-poisoned, Pd/ceria catalyst, identical to the sample used in Figure 11.3, with 7% H2 in the carrier gas. Under these conditions, the sulfate decomposes and the sulfur desorbs as H2S below 500°C [11,41]. However, the amount of sulfur leaving the sample accounts for only about one third of the sulfur initially on the sample. Reoxidation, followed by another TPD experiment, gave additional H2S, suggesting that a significant fraction of the sulfur remained in the sample, probably in the form of Ce 2 0 2 S.
M/e = 34
3 I
<
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\
,M/e = 32
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Temperature (°C) Figure 11.4. TPD measurements on a Pd/ceria catalyst after exposure to flowing S0 2 , using 93% He and 7% H2 as the carrier gas. The sulfate species decomposes to H2S (m/e=32, 34) and Ce202S under reducing conditions.
All of this agrees with chemistry that can be inferred from data on Ce0 2 as an adsorbent for H2S. For adsorption at 700°C, Ce0 2 is completely converted to Ce202S [31]. There are also indications that S0 2 can react to form oxysulfides at room temperature on reduced ceria in ultra-high-vacuum experiments [46]. Interestingly, Ce202S is oxidized back to Ce0 2 or Ce 2 0 3 upon exposure to S0 2 at 600°C, producing S0 2 [31]. The above discussion examined results aimed at understanding the final products obtained after high-temperature adsorption or reduction. In order to
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Catalysis by ceria and related materials
develop strategies for controlling the formation of sulfur-containing species, fundamental information about the nature of the initial adsorption states for S0 2 and H2S on ceria is necessary. Unfortunately, there is relatively little data on this. Based on spectroscopic studies on model catalysts, it has been reported that sulfates are formed following adsorption of S0 2 on ceria films at 300 K [47]. However, FTIR studies on ceria powders suggest that only sulfites or hydrogen sulfites are formed under these conditions [40]. Given that surface species in the model studies were observed to decompose between 390 and 670 K, while the sulfates are clearly more stable on powders, as shown in Figure 11.3, one must conclude that more work is needed to clarify the initial adsorption states.
11.4. Lessons from Catalysts for S 0 2 Reduction While not the subject of this paper, it is relevant to point out that ceria can be used as a catalyst for the reduction of S0 2 to elemental sulfur under steady-state conditions and considerable effort has been devoted towards understanding these reactions. Comprehensive reviews of this literature are available [48,49]. Briefly, S0 2 can be reduced by CH4 or CO in the temperature range 450°C to 750°C [48]. It is interesting to notice that the S0 2 reduction reaction lights off at roughly the same temperature at which TPD measurements of a sulfated catalyst show a rapid reduction to H2S and Ce202S [41]. Indeed, reduction of the sulfate by CO is proposed as a necessary step for reaction light off [48]. This information, along with the observation that S0 2 can oxidize Ce202S back to CeOx, with the formation of S2 [41], would seem to explain the essential features of the S0 2 reduction reactions.
11.5. Future Directions While much is known about the interactions of S0 2 with ceria, especially in regards to the thermodynamic species that are formed, a number of important issues are still unresolved. The incentive of finding sulfur-tolerant materials for OSC in three-way catalysis is very large, since it would lessen the need for increasingly lower sulfur contents in fuels. Finding OSC materials that do not adsorb S0 2 , or that release sulfur more readily, would be of great commercial interest. To develop strategies for this, fundamental information about the nature of the initial adsorption states for S0 2 and H2S would be very helpful. As discussed above, most characterization work to date has involved the final states, such as the sulfates
SO2 poisoning of ceria-supported, metal catalysts
387
and the oxysulfides, and not the species leading up to the final states. The natures of intermediate adsorption states are less well understood. The effect of promoters, such as zirconia, on the surface species are also uncertain. There are indications that the composition of the OSC materials can lead to improved sulfur tolerance [50]. It is possible that some combinations of oxides might block sulfate formation or destabilize the sulfate, in the same way that practical OSC materials require the presence of zirconia to maintain reducibility in ceria. Finally, just as the reducibility of ceria is structure sensitive [37], there may be structure sensitivity to adsorption of sulfur compounds as well. Clearly, understanding sulfur chemistry on ceria is an interesting subject with important implications. Hopefully, major advances will be forthcoming. 11.6. Subject Index Ceria; Cerium Sulfate; Cerium Sulfide; Supported-Metal Catalysts; Oxygen-Storage Capacity; Water-Gas Shift; Steam Reforming; S0 2 ; H2S.
Acknowledgements: The authors gratefully acknowledge support from the DOE, Basic Energy Sciences, Grant # DE-FG03-85-13350 and from the Coordinating Research Council, Inc.
11.7. References 1. 2. 3. 4. 5. 6. 7. 8. 9.
McCabe, R. W., and Kisenyi, J. M., Chemistry & Industry, 15 (1995) 605-608. Yao, H. C, and Yao, Y. F. Y., J. Catal., 86 (1984) 254-265. Bunluesin, T., Gorte, R. J., and Graham, G. W., Appl. Catal. B, 14 (1997) 105115. Yao, M. H., Baird, R. J., Kunz, F. W., and Hoost, T. E., /. Catal., 166, (1997) 67-74. Bartholomew, C. H., Agrawal, P. K., and Katzer, J. R., Adv. in Catal., 31 (1982) 135-242. D. D. Beck, J. W. Sommers, and C. L. Dimaggio, Appl. Catal. B, 3 (1994) 205-227. Beck, D. D., Sommers, J. W., and DiMaggio, C. L., Appl. Catal. B, 11 (1997) 273-290. Beck, D. D., and Sommers, J. W., Appl. Catal. B., 6 (1994) 185-200. D. D. Beck, Catalyst Deactivation, 111 (1997) 21.
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Catalysis by ceria and related materials
10. D. D. Beck, J. W. Sommers, Catalyst and Automotive Pollution Control III, 96 (1995)721. 11. Boaro, M., de Leitenburg, C , Dolcetti, G., and Trovarelli, A., Topics in Catalysis, in press. 12. Gandhi, H. S., presented at the CRC Auto/Oil Symposium, Dearborn, MI, September 11, 1997. 13. Hepburn, J. S., Dobson, D. A., Hubbard, C. P., Guidberg, S. O. Thanasiu, E., Watkins, W. L., Burns, B. D., and Gandhi, H. S., SAE paper 942057, 1994. 14. Beck, D. D., Monroe, D. R., DiMaggio, C. L., and Sommers, J. W., SAE paper 952416,1995 15. Rieck, J. S., Collins, N. R., and Moore, J. S., SAE paper 980665,1998. 16. Hepburn, J. S., Dobson, D. A., and Gandhi, H. S., SAE paper 942071 (1994). 17. Hilaire, S., Sharma, S., Gorte, R. J., Vohs, J. M., and Jen, H.-W., Catalysis Letters, 70 (2000) 131-135. 18. Barbier, J., and Duprez, D., Appl. Catal. B, 4 (1994) 105-140. 19. Whittington, B. I., Jiang, C. J., and Trimm, D. L., Catal. Today, 26 (1995) 4145. 20. Craciun, R., Shereck, B., and Gorte, R. J., Catal. Lett, 51 (1998) 149-153. 21. Sharma, S., Hilaire, S., Vohs, J. M., Gorte, R. J., and Jen, H.-W., /. Catal, 190 (2000) 199-204. 22. Bunluesin, T., Gorte, R. J., and Graham, G. W., Appl. Catal. B, 15 (1998) 107114. 23. Bart, J. M., Pentenero, A., and Prigent, M. F., ACS Symposium Series, 495 (1992) 42-60. 24. Ansell, G. P., Golunski, S. E„ Hatcher, H. A., and Rajaram, R. R., Catal. Lett., 11(1991)183-190. 25. Whittington, B. I., Jiang, C. J., and Trimm, D. L., Catal. Today, 26 (1995) 4751. 26. Li, Y., Fu, Q., and Flytzani-Stephanopoulos, M., Appl. Catal. B, 27 (2000) 179-191. 27. Zafiris, G. S., and Gorte, R. J., J. Catai, 139 (1993) 561-567. 28. Smirnov, M. Yu., and Graham, G. W., Catal. Lett., 70 (2001) 39-44. 29. Hilaire, S., Wang, X., Luo, T., Gorte, R. J., and Wagner, J., Appl. Catai, in press. 30. Kilbourn, B. T., Cerium: A Guide to its Role in Chemical Technology, (Molycorp, Inc., Fairfield, NJ, 1992) pp. 10-17. 31. Zeng, Y., Kaytakoglu, S., and Harrison, D. P., Chem. Eng. Sci., 55 (2000) 4893-4900.
S02 poisoning of ceria-supported, metal catalysts
389
32. Akyurtlu, J. F., and Akyurtlu, A., Chem. Eng. Sci., 54 (1999) 2991-2997. 33. Fruehan, R. J., Metall. Trans. B, 10B (1979) 143-148. 34. Akila, R., Jacob, K. T., and Shukla, A. K., Metall. Trans. B, 18B, (1987) 163168. 35. Dwivedi, R. K., and Kay, D. A. R., Metall. Trans. B, 15B (1984) 523-528. 36. Kay, D. A. R., Wilson, W. G., and Jalan, V., J. Alloys & Comp., 192 (1993) 11-16. 37. Cordatos, H., Bunluesin, T., Stubenrauch, J., Vohs, J. M., and Gorte, R. J., Journal of Physical Chemistry, 100 (1996) 785-789. 38. Yang, L., Kresnawahjuesa, O., and Gorte, R. J., Catal. Lett, 72 (2001) 33-37. 39. Putna, E. S., Vohs, J. M., and Gorte, R. J., J. Phys. Chem., 100 (1996) 1786217865. 40. Waqif, M., Bazin, P., Saur, O., Lavalley, J. C., Blanchard, G., and Touret, O., Appl. Catal. B, 11 (1997) 193-205. 41. Bazin, P., Saur, O., Lavalley, J. C., Blanchard, G., Visciglio, V., Touret, O., Appl. Catal. B., 13 (1997) 265-274. 42. Bazin, P., Saur, O., Lavalley, J. C , Le Govic, A. M., and Blanchard, G., Stud. Surf. Sci. Catal, 116 (1998) 571-579. 43. Waqif, M., Pieplu, A., Saur, O., Lavalley, J. C , Blanchard, G., Sol. State Ionics, 95 (1997) 163-167. 44. Twu, J., Chuang, C. J., Chang, K. I., Yang, C. H., and Chen, K. H., Appl. Catal. B., 12 (1997) 309-324. 45. Handbook of Chemistry and Physics, 54th Edition, CRC Press, Cleveland, (1973) pg. B-81. 46. Overbury, S. H., Mullins, D. R., Huntley, D. R., Kundakovic, L., J. Phys. Chem. B, 103 (1999) 11308-11317. 47. Rodriguez, J. A., Jirsak, T., Freitag, A. Hanson, J. C, Larese, J. Z., and Chaturvedi, S., Catal. Lett., 62 (1999) 113-119. 48. Flytzani-Stephanopoulos, M., Zhu, T., and Li, Y., Catal. Today, 62 (2000) 145-158. 49. Liu, W„ Sarofim, A. F., Flytzani-Stephanopoulos, M., Appl. Catal. B, 4 (1994) 167-186. 50. Andersen, P. J. and Rieck, J. S., SAEpaper 970739, 1997.
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CHAPTER 12 CERIUM AND PLATINUM BASED DIESEL FUEL ADDITIVES IN THE DIESEL SOOT ABATEMENT TECHNOLOGY
MICHIEL MAKKEE, SYTSE J. JELLES, and JACOB A. MOULIJN Delft University of Technology, Section Industrial Catalysis, Julinanalaan 136, NL 2628 BL Delft, The Netherlands, e-mail: [email protected]
12.1.
Introduction
Since the invention by Rudolf Diesel in 1893, the application of the diesel engine has become very widespread across the world. The popularity of the diesel engine is a result of its attractive characteristics, such as fuel economy, durability, low maintenance requirements, and large indifference to fuel specification. It is applied in various fields. Transport applications of the diesel engine can be found in light passenger cars, trucks, construction equipment, and ships. Another large field of application is that of stationary power-sources. Many electricity- and hydraulic power plants are equipped with diesel engines. With increasing concern about the environment, the emissions of diesel engines have come into focus; in the beginning of the 1970's emission control legislation was introduced in the US and Europe. Since then the legislation has become more stringent and the effort needed to meet the legislation has become more intense. A major part of the reduction of harmful engine emissions can be accomplished by either engine adjustments or fuel changes. All new passenger cars equipped with a diesel engine apply to the EURO III standard without aftertreatment devices. The currently proposed EURO IV emission standards, that are expected to be effective from 2005, can probably be achieved without aftertreatment. The focus for the reduction of harmful diesel emissions is mainly on particulate matter (PM) and NOx. Both components are harmful to health and environment and are present in relatively large quantities. The other regulated harmful emissions, hydrocarbons and carbon monoxide, can be removed with relatively simple measures, such as flow-through monoliths with an oxidation catalyst. Some of the techniques used for removal of particulate matter and/or NOx 391
392
Catalysis by ceria and related materials
can also lead to a reduction of hydrocarbons and carbon monoxide. The SOx content of the exhaust gas is directly proportional to the fuel sulfur content. The occurrence of NOx and particulate in the exhaust gas is coupled. NOx is formed as a result of the high combustion temperature and pressure in combination with a superstoichiometric amount of oxygen. Particulates are formed due to local shortage of oxygen in the burning diesel spray. If better combustion is pursued to reduce the emission of particulates, this often leads to better oxidation conditions and an increased NOx emission can be the result. If on the other hand the oxygen partial pressure is reduced to reduce the NOx emissions, this will lead to more soot formation. This effect is called the NOx-soot trade-off. For emission reduction the first option to be considered is the prevention of harmful emission rather than aftertreatment. Since the introduction of the emission standards the engine-out emissions have been significantly reduced. Passenger-car engines that apply to the EURO-IV standard without any form of aftertreatment are currently being developed and tested. Several engine adjustments can result in the reduction of engine-out emissions. Particulate reduction can be achieved by improving the availability of oxygen for the combustion, for example with a turbocharger, or by improving the fuel-air mixture with higher injection pressures or staged fuel injection. Although the emitted particulate mass can be reduced with high injection pressure, the number of emitted particulates is hardly affected. Some of these techniques may result in an increased combustion temperature, leading to a higher NOx emission. Staged fuel injection can be used to control the in-cylinder temperature and can also lead to reduced NOx emissions. The production of NOx can be reduced with exhaust-gas recirculation. This leads to a decreased oxygen concentration in the cylinder, partially preventing the formation of NOx. The lower oxygen content can, however, lead to an increased formation of particulates. Water injection can be used to lower the in-cylinder temperature, leading to a reduced NOx formation. This can also be achieved by using a fuel that contains water. Elf-Aquitane started distribution of water-containing diesel fuel early in 1999 [1]. Main users of this product are cities for their bus fleet and garbage collection vehicles. The soot emissions are reported to be reduced with 30 - 80%, the NOx emissions with 15 - 30% [1]. Although engine adjustments and improved fuel quality have significantly contributed to the reduction of diesel engine emissions, the obtained reductions will probably not be sufficient. Further it is speculated that in certain countries, for
Cerium in the diesel soot abatement technology
393
example Germany, aftertreatment by use of a particulate filter is required in the near future for new diesel cars, regardless of the engine-out emissions. From the methods and applications to improve the oxidation rate of diesel particulate matter at low temperatures, the combination of a metal fuel additive and a particulate filter is one of the most successful options tested in practice until this moment. Its success is a result of the intimate contact between the soot and the catalyst. All additive-filter combinations known up to this moment operate at temperatures from 625 K and higher [2]. The three metals studied most frequently are, in order of activity, copper, iron, and cerium. Recently, PSA Peugeot-Citroen announced the introduction of diesel passenger cars equipped with a combination of a particulate filter and a cerium additive dosage system. To initiate the regeneration of the filter, delayed injection of diesel fuel is applied, in combination with an oxidation catalyst placed upstream from the filter [3]. This system can be further optimised by, for example, a more active fuel additive. Because the initiation of the regeneration of the filter takes a significant amount of fuel, an additive system that results in a higher soot oxidation rate at a lower temperature could prove attractive. Several metal additives have been tested for their activity for soot oxidation during the last decade. A recent overview of all the studied additives is given by Neeft et al. [4]. Three additives have been extensively studied and are considered to be the most effective and applicable in practical applications. These additives are cerium, copper, and iron. The cerium additive is thoroughly discussed by, among others, Summers et al. [5] and Pattas et al. [6]. The copper additive was discussed recently by Daly et al, [7], and Gantawar et al, [8]. All three additives were compared by Bloom et al. [9], Ladegaard et al. [10], and Lepperhoff et al. [11]. The activity of these three metals decreases in the following order: copper > iron > cerium Recently, a sodium/strontium additive was tested by Associated Octel [12]. The authors of this work give no indication of the relative activity of this additive combination. The minimum temperature, where the additive filter systems can operate, is in the range of 625 - 650 K. The additive dosage rates to the diesel fuel generally are 30 - 50 ppm metal content on a weight basis. The resulting ash from cerium and iron does not plug the filter, in contrast to copper, where serious filter pluggings are reported. When 25 ppm of cerium additive is used for a typical heavy duty truck, the filter will be 50 % filled after 75,000 to 150,000 miles [5]. Copper is reported to
394
Catalysis by ceria and related materials
deteriorate ceramic fibre-wound filters [9]. Furthermore, when copper is used, the filter temperatures reached during regeneration of the filter generally are too high [10]. All metal additives potentially can be emitted in due time. Especially copper can be a threat to the environment and catalysts based on copper are known for their catalytic activity for dioxin formation [13]. Another successful application is the so-called Continuously Regenerating Diesel Particulate Filter (CR-DPF), where a catalytic flow-through monolith is placed upstream of a wall-flow monolithic filter. In this catalyst the NO present in the exhaust gas is oxidised to N0 2 [14], which successively oxidises the soot that is collected on the filter, forming NO, CO, and C0 2 . This system operates well between 575 K and 675 K and preferably between 625 and 675 K, but is limited to virtually sulfur free fuel (< 10 ppm) and Euro II engines [14]. A combination of a fuel additive and a CR-DPF could possibly cover the temperature window from 575 K and higher and preferably at temperatures lower than 575 K. The combination of a platinum fuel additive for NO oxidation with a cerium additive for catalysed soot oxidation with 0 2 was studied as a possible application in which the above mentioned two mechanisms are incorporated. In this chapter the performance of platinum/base metal fuel additive-filter systems is discussed with studies on a pilot engine as a basis. It will be compared to the performance of cerium, iron, and copper base metal additives, the latter two also in combination with platinum. The background of the difference in performance of the platinum/base metal combinations is discussed with results from flow-reactor experiments as a basis.
12.2.
Experimental
12.2.1. Engine Experiments 12.2.1.1. Engine Set-up and Materials During the research programme a Lister-Petter LPW2, a two cylinder, direct injected, naturally aspired engine was used. It was fitted with a Stamford generator with a maximum power output of 5.0 kW. Load was applied to the generator with a 4 kW resistor. Figure 12.1 shows a simplified scheme of the engine set-up that was used. The major part of the exhaust gas was vented through a silencer. Part of the exhaust gas was pumped through the test filter with a membrane vacuum pump (KNFVerder).
Cerium in the diesel soot abatement
technology
395
ELPI sample points monolith filter I
D80-
K
filter
«-r-C8G
C3 •Ji-CSJ-
engine
-MZ) $ heat condensation membrame exchanger vessel .—| ppump ump/
proportional control valve
o
open/close valve pressure difference transmitter pressure transmitter
condensation vessel \ ii II
( \/ ]
) electronic rotameter
gas expansion vessel
catalytic filter
M
wall-flow monolith filter electronic rotameter heat exchanger
F i g u r e 12.1. Simplified scheme of the engine set-up
The gas temperature before and after the filters was controlled with temperature controllers (models West 6100 and 4400). The pressure drop over both filters was measured with pressure-difference transmitters (model Valydine DP-15), and the pressure in the exhaust pipe with a pressure transmitter (model Druck PTX-1400). The flow rate was measured with an electronic rotameter (model Brooks MT-3809) and adjusted by a PID controller (model West 6100) that controlled a proportional valve (model Kammer 20037-I/P-PN400). The diesel fuel used was obtained from Van Gelder Aardolie, Nijmegen, the Netherlands. This diesel fuel was in conformance with the EN590:1996 specifications, containing less than 500 ppm of sulfur. The fuel was purchased in batches of 2000 litres to ensure constant quality during the programme. In total, four batches were used. Further specifics concerning the engine set-up can be found in [14, 15].
396
Catalysis by ceria and related materials
12.2.1.2. Preparation of Monolithic Filters Corning EX80 was used as the monolithic base substrate. Samples of 20 mm in diameter and 40 mm in length were cut out of a standard filter. Platinum impregnated filters were prepared by dipping into an aqueous platinum solution (8 mg/g tetraammineplatinum(II)chloride hydrate, Aldrich 27,590), followed by drying at 365 K and subsequent calcination at 1025 K for 30 minutes. From the filter segments the alternate channels were plugged using Ceramabond 503 from Aremco. The filters were then glued to the quartz tube, using Ceramabond 503, followed by drying at 360 K for 30 minutes and subsequent calcination at 675 K for 30 minutes. Platinum aged filters were prepared by placing the quartz tube with a clean monolithic filter glued to it in the filter test set-up, heating it to 923 K, and subsequently pumping 10 1/min of exhaust gas through the filter for 24 hours, while the engine was running on fuel containing 2 ppm of platinum. This way, all the soot collected on the filter is burnt, while the platinum present in the soot is collected on the filter. The platinum treated EX80 filters will be addressed as PtEX80. Further specifics concerning the filter preparation can be found in [15]. The platinum additive studied was Platinum Plus 3100, supplied by Clean Diesel Technologies, Inc. The copper additive was OS96401, a gift from Lubrizol. The cerium additive was DPX9 from Rhodia, and the iron additive was ferrocene from Aldrich.
12.2.1.3. Procedure for Filter Experiments Before an experiment with a new fuel additive formulation, the engine was conditioned to the new fuel for 24 hours. After this period, a filter was placed in an oven that was attached to a side stream of the exhaust system. Exhaust gas was pumped through the filter with a vacuum pump, at a controlled rate of typically 8 10 lSTP/min. The pressure drop over the filter was measured and recorded, together with other data such as filter exhaust temperature, fuel consumption, etc. The soot collected on the filter leads to an increased pressure drop over the filter, that requires adjustment of the flow rate. When a certain amount of soot is collected on the filter, the pressure drop over the filter stabilises. At this point, there is an equilibrium between the rate of soot collected on the filter and the rate of soot oxidised on the filter. This equilibrium can be achieved at different temperatures, a lower temperature leading to a higher stable pressure drop over the filter. When the temperature is too low, it is not possible to achieve an equilibrium: the filter is
Cerium in the diesel soot abatement technology
397
overloaded. The lowest temperature where an equilibrium can be found is defined as the balance point.
12.2.2. Flow-reactor Experiments Flow-reactor experiments were performed in the "sixflow micro-reactor" which is described in detail in [15], as well as the procedures followed. A short summary of an oxidation experiment will be given here. Soot (20 mg) was placed in a quartz reactor tube, if appropriate mixed with a supported platinum catalyst (6 mg, 1 wt% Pt on a silica-alumina support). Five tubes were placed in an oven and while flushing with argon (200 mlSTP/min) they were heated to the experimental temperature. At this point, oxygen (and, if appropriate, NO) were admitted to the reactor tube. The CO and C0 2 concentration in the outlet gas of each reactor were measured using an NDIR (Hartmann & Braun URAS 10E). Using these concentrations and the applied flow rate the oxidation rate was calculated. Soot samples collected from the engine running on fuel with additive were studied. Printex-U, a flame soot supplied by Degussa, was used as a reference material.
12.3.
Results
12.3.1. Engine Experiments In general, the engine responded well to the different fuel additives. The fuel consumption was hardly affected, although the copper additive resulted in an increase in fuel consumption with 5%. The engine exhaust temperature was also somewhat higher when the copper additive was used (600 K instead of 570 K). Both are thought to be the result of a delayed combustion caused by the copper additive. The minimum temperature or balance temperature (BT) where the filter/additive system can achieve a stable pressure drop is listed in table 12.1. In case of an untreated EX80 filter, the effect of 50 ppm of cerium in the fuel is clear. The minimum temperature is 45 K lower when cerium is used. From literature data [1] and data from Table 12.1, it is concluded that the effect of iron (BT of 650 K) and copper (BT of 640 K) on the minimum temperature is somewhat larger.
398
Catalysis by ceria and related materials
Table 12.1. Minimum operation temperature for several fuel additive and filter combinations. additive
cone.
min.temp.
(ppm wt) EX80
(K)
None Cerium Cerium Copper Iron
750 50 100 50 22
705 700 640 650
cone.
additive
(ppm wt) Platinum EX80 none Platinum Platinum/cerium Platinum/cerium Platinum/cerium + foam Platinum/copper Platinum/iron
2 2/30 0.5/5 0.5/5 2/50 1/22
min.temp. (K) 675 675 590 595 550 620 630
The effect of platinum on the trap and in the fuel, in absence of any other additive, is significant. The minimum temperature is approximately 75 K lower as a result of the platinum. If the platinum additive is used in combination with an untreated EX80 filter the balance point decreases from an initial 750 K to 675 K after several days, provided that the temperature in the filter has been occasionally high, for example during regenerations. If the temperature in the filter was kept low, the filter was less active. Study of several filters with an electron microscope revealed that the platinum particles on high-temperature treated filters were large (> 100 nm) compared to the particles found on filters that had not been heated at high temperature, which were smaller than 50 nm. Similar effects were observed when a platinum additive was used in combination with a base metal additive. Platinum collected on the filter enhances the activity of the system after a high-temperature treatment. All combinations of platinum with other additives lead to a decreased balance point temperature. The largest effect of platinum in the fuel and on the filter is observed when it is used in combination with a cerium additive. The minimum temperature is lowered with a total of 160 K. Copper and iron are more active as an individual additive, whereas the minimum temperature is higher when these additives are used, instead of cerium, in combination with the platinum additive and filter. The lowest additive concentration studied with this equipment was platinum/cerium 0.5/5 ppm/ppm. Field tests showed that the platinum concentration can be reduced to 0.25 ppm. A typical back pressure of the system at balance temperature is around 80 mbar. The lowest balance point temperature was measured with a combination of platinum and cerium fuel additives with a platinum treated ceramic foam filter placed
Cerium in the diesel soot abatement technology
399
upstream of a platinum treated EX80 filter. This system was stable at 550 K for a time interval of at least 2 months continuously running. The NOx reduction was measured when the platinum/cerium additive was used and it ranged from 15% in the beginning of the experimental programme. After the programme, the engine-out NOx emission was measured. This was approximately 20% below the engine-out emission before the start of the programme.
12.3.2. Flow-reactor Experiments The influence of the inlet concentration of NO was studied with four soot samples mixed with a supported platinum catalyst (1 wt% Pt on ASA) at 650 K. The oxidation rate at 50% soot conversion is plotted as a function of the NO inlet concentration in Figure 12.2.a. From this figure it is clear that the influence of the NO concentration on the oxidation rate of the synthetic Printex-U and the diesel soots activated with copper or iron is comparable. There is a first order relation between the NO inlet concentration and the oxidation rate. For cerium activated soot, there is also a first order relation between the NO inlet concentration and the oxidation rate. In this case, however, the effect of NO is approximately twice as large as is the case with Printex-U, Printex-U with a physical mixture of a cerium catalyst (not shown), and copper- or iron-activated soot. In Figure 12.2.b, the effect of the total amount of platinum catalyst mixed with the soot is presented for cerium activated soot. Without supported platinum catalyst, the oxidation rate is comparable with the rate without NO. It is clear that with more platinum catalyst, the oxidation rate is higher. In this case, there is a first order relation between the amount of platinum catalyst and the oxidation rate. Only when 24 mg of platinum catalyst is used, the oxidation rate does not follow this relation and the oxidation rate levels off. The sum of the CO and C0 2 concentration is plotted on the right axis in Figure 12.2.b. The value of this sum is approximately 620 ppm at a conversion level of 50% when 24 mg of platinum catalyst is present. This is 540 ppm more than the same number without platinum catalyst, which exceeds the concentration of NO (300 ppm) that is led in the reactor. It can, therefore, be concluded that each NO molecule is used more than once (at least 1.8 times, at most 3.6 times) during the oxidation process. Because in all experiments a platinum catalyst is used, significant formation of CO is not observed. Almost all CO formed is oxidised to C0 2 , probably with 0 2 . It is, therefore, not possible to make an exact calculation of the number of NO-oxidation cycles.
400
Catalysis by ceria and related
* • • •
materials
cerium-activated soot T 300 ppm NO and 10% O. in argon, 650 K
cerium-activated soot copper-activated soot iron-activated soot printex-u
e
500
1000
1500
NO concentration (ppm )
Figure 12.2.a. Oxidation rate at 50 % conversion; Influence of NO inlet concentration on the isothermal oxidation rate of the three systems investigated and PrintexU. In all experiments the temperature was 650 K and the used catalyst was platinum on ASA (1 wt.%).
0
5
10
15
20
2
m a s s of p]athum c a t a V s t ftn g)
Figure 12.2.b. Oxidation rate at 50 % conversion. Influence of the amount of platinum catalyst on the isothermal oxidation rate of cerium-activated soot. In all experiments the temperature was 650 K and the used catalyst was platinum on ASA (1 wt.%).
During the oxidation experiments with NO in the gas phase and a supported platinum catalyst mixed with the soot, the outlet NO concentration did not decrease very much. A typical example of the NO outlet concentration is shown in Figure 12.3. Up to 60% soot conversion the NO concentration decreased with typically 25 ppm (approximately 10%). At higher soot conversion levels, the NO concentration decreases more and stabilises around 200 ppm. This coincides with the thermodynamic conversion of NO to N0 2 , which is approximately 30% under these conditions. Because at that point no soot is present in the reactor, it can be
Cerium in the diesel soot abatement technology
401
concluded that the decreased NO concentration is mainly a result of formation of N0 2 .
cerium-activated soot mixed with 6 mg of platinum catalyst 350 potlt f oxygen adm issim
— ti]etNO concentration
300 -
- - ' » S3 250 -
/
200 -
/
150 -
/
/
/ outtetNO blank outlet NO soot conversion
/ 0
\
500
100 0
tin e frn in)
Figure 12.3. Typical course of the outlet NO concentration during a flow-reactor experiment. In this case cerium-activated soot mixed with 6 mg of platinum-ASA was oxidised at temperature of 650 K in a gas phase of 300 ppm NO and 10 % 0 2 in argon.
12.4.
Discussion
12.4.1. Function of Platinum From Figures 12.2.a and 12.2.b it is clear that in flow-reactor experiments NO does hardly influence the soot oxidation rate in absence of a supported platinum catalyst. From results not shown here it is clear that the same holds for the effect of a supported platinum catalyst: the oxidation rate in the absence of NO is the same, with or without supported platinum catalyst. This leads to the conclusion that only
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Catalysis by ceria and related materials
the combination of a supported platinum catalyst and NO in the gas phase results in a higher soot oxidation rate. From flow-reactor experiments with stacked beds of supported platinum catalyst and Printex-U, discussed in [16], it was concluded that the platinum catalyst does not need to be mixed with the soot to be effective. The combination of platinum catalyst and gas phase NO is effective through the gas phase. In engine experiments, the platinum is particularly effective after some period of use or in combination with a platinum treated filter. This leads to the assumption that the role of the platinum in the soot is small and that the platinum collected on the filter is more important for the oxidation of soot. This is confirmed by flowreactor experiments, that showed that platinum present in the soot did not influence the oxidation rate in any case (not shown). The platinum particles in the soot are too small (atomic dispersion) to contribute to the oxidation of NO and the concentration of platinum is very low. The results from both types of experiments lead to the same conclusion: platinum agglomerates on the filter act as an NO oxidation catalyst. The resulting N0 2 subsequently oxidises the soot. The observation that platinum treated filters are more effective after a high-temperature treatment is supported by studies reported by Xue et al. [17]. There is a direct relation between the size of platinum particles and the effectiveness as an NO-oxidation catalyst. Platinum present in the soot does not influence the oxidation rate of soot, either in presence or absence of NOx. The influence of the platinum additive on the engine-out emissions cannot be neglected and platinum may contribute to the soot oxidation on other places than on the filter, such as the exhaust manifold and in the exhaust pipes.
12.4.2. Performance of the Base Metal In engine experiments, the effect of platinum addition to either copper or iron additives is limited to a decrease in balance point of approximately 50 K (estimated), whereas this decrease is more than 100 K when platinum is used in combination with cerium. In total, the addition of platinum and cerium to the fuel results to a balance point that is 155 K lower than with untreated fuel. In flow-reactor experiments, the effect of NO in the gas phase and a supported platinum catalyst mixed with the soot is twice as large for cerium when compared to that of copper, iron, and Printex-U. All other conditions are similar and, therefore, it is concluded that cerium catalyses the oxidation of soot with N0 2 . Because there is
Cerium in the diesel soot abatement technology
403
no significant difference between the acceleration by NO for copper, iron, and Printex-U, it is concluded that copper and iron do not catalyse the oxidation of soot with N0 2 . The comparison between cerium-activated soot with a cerium catalyst physically mixed with Printex-U in the presence pf both a platinum catalyst and NOx on the gas phase very clearly showedthat only in the case of the ceriumactivated soot the oxidation rate of soot is strongly enhanced.
12.4.3. Reaction Network The amount of CO and C0 2 that is formed is high compared to the amount of NO used in the flow-reactor experiments with cerium activated soot. It is possible to achieve a situation where each NO molecule participates more than once in the oxidation reaction. This confirms an oxidation cycle that was proposed by Mul et al. [18]. This cycle is visualised in Figure 12.4. When platinum and cerium additives are applied, there is a synergistic effect resulting in a high oxidation rate. This synergy can enhance the use of the proposed oxidation cycle, because the reactions involving NO are kinetically coupled. If the rate at which N0 2 oxidises soot is high, the N0 2 concentration is lowered, which facilitates the formation of N0 2 from NO. At high N0 2 concentrations, this formation is limited by thermodynamics. In the NOx-trap, for example, this coupling is not possible because the two reactions are spatially separated. The effect of a foam placed upstream of the EX80 filter, a decrease of the minimum temperature with another 40 K, makes clear that it is possible to optimise the oxidation with NOx by creating the possibility for NO to participate in the oxidation-reduction cycle more than once in the exhaust system. This concept is visualised in Figure 12.5.
12.4.4. NOx Reduction In engine experiments, significant NOx reduction, up to 20%, has been observed. Also in flow-reactor experiments, some reduction was measured, although this was less pronounced. The outlet concentration of NO in the oxidation experiments was only 10 % lower than the inlet concentration of NO during the first 60 % of the oxidation experiments, whereas the oxidation rate was high. This indicates that the
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Catalysis by ceria and related materials
Platinum Particles Figure 12.4. Oxidation cycle with NO as an oxygen carrying intermediate
Figure 12.5. Optimal use of NO, on a platinum treated foam.
NO is not consumed in large portions during the soot oxidation. While oxidising soot, N0 2 is reduced to mainly NO. Some reduction of NOx to N2 or N 2 0 does take place. More on these reactions can be found in Yamashita et al. [19] and Matsuoka etal. [20]. These reactions are, however, not very significant for the soot
Cerium in the diesel soot abatement technology
405
oxidation process. From a NOx reduction point-of-view, these reactions may be very interesting and could possibly be optimised to achieve substantial NOx reduction.
12.5. •
•
• •
• • • •
•
Conclusions
A combination of platinum and cerium fuel additives with a platinum treated filter results in continuous filter regeneration at the lowest temperature known for fuel additives (595 K). If a platinum treated foam is placed upstream of the filter, the temperature can be even lower (550 K). Better utilisation of the NOx in the exhaust gas stream is observed. Cerium-activated soot has a lower soot oxidation activity than the copper and/or iron based counterpart. Platinum in the soot does not influence the oxidation of soot. It is very effective when present as large (100 nm and larger) particles on the filter. The platinum additive may be necessary to maintain substantial activity on the filter. The total additive dose rate can be low, less than 6 ppm, resulting in less ash formation and, hence, longer maintenance free operation. The oxidation of soot with N0 2 is catalysed by cerium present in the activated soot, and not by copper- or iron-activated soot. The addition of a separate cerium catalyst has only a minor effect on the soot oxidation rate both in the presence of 0 2 and 0 2 /N0 2 . An oxidation cycle can be achieved in which NO is used as an oxygen carrying intermediate. The reactions in this cycle are kinetically coupled, so that fast oxidation of soot with N0 2 (catalysed by cerium) leads to fast (re)oxidation of NO (catalysed by platinum). With the system studied, NOx reduction up to 20% was measured. This reduction could possibly be optimised using the right filter geometry, for example with a ceramic foam.
12.6. 1. 2.
References
Langer,D. A., Petek, N. K. and Schiferl,E. A. SAE Paper 2001-01-0513 (2001). Liiders, H., Stommel, P. and Backes, R. SAE Paper 970470 (1997).
406 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.
Catalysis by ceria and related materials Salvat, O., Marez, P. and Belot, G. SAE paper 2000-01-0473 (2000). Neeft, J. P. A., Makkee, M. and Moulijn, J. A. Fuel Processing Technology 74 (1997) 1. Summers, J. C , Van Houtte, S. and Psaras, D. Appl.Catal.B 10 (1996) 139 . Pattas, K. N., Kyriakis, N., Samaras, Z., Mustel, W., and Rouveirolles, P., SAE Paper 970184 (1997). Daly, D. T., McKinnon, D. L., Martin, J. R., and Pavlich, D. A. SAE Paper 930131 (1993). Gantawar, A. K., Opris, C. N., and Johnson, J. H., SAE Paper 970187 (1997). Bloom, R. L. Brunner, N. R. and Schroer, S. C. SAE Paper 970180 (1997). Ladegaard,N., Sorenson, S. C , and Schramm, J., SAE Paper 970181 (1997). Lepperhoff, G., Liiders, H., Barthe, P., and Lemaire, J. SAE Paper 950369 (1995). Richards, P., Terry, B., Vincent, M. W , and Cook, S. L. SAE Paper 1999010112 (1999). Luijk, R., Akkerman, D. M., Slot, P., Olie, K., and Kapteijn, F. Environ.Sci.Technol. 23 (1994) 312. Cooper , B. J., and Thoss, J. E., SAE Paper 890404 (1989). Jelles, S. J., Makkee, M., Moulijn, J. A., Acres, G. J. K., and Peter-Hoblyn, J. D. SAE Paper 1999-01-0113 (1999). Jelles, S. J., Krul, R. R., Makkee, M., and Moulijn, J. A. Catal. Today 5 3 (1999) 623. Xue, E., Seshan, K., and Ross, J. R. H., Appl.Catal.B; 11 (1996) 65. Mul, G., Zhu, W. D„ Kapteijn, F., and Moulijn, J. A. Appl.Catal.B; 17 (1998) 205. Yamashita, H., Tomita, A., Yamada, H., Kyotani, T., and Radovic, L. Energy and Fuels, 7, (1993), 85. Matsuoka, K., Orikasa, H., Itoh, Y., and Tomita, A. Preprints of symposia 216th ACS National Meeting, Boston, MA, August 23-27, 1998, (1998), 852.
CHAPTER 13
FUNDAMENTALS AND APPLICATIONS OF CERIA IN COMBUSTION REACTIONS MICHEL PRMET and EDOUARD GARBOWSKI Lab. d'application de la chimie a Venvironment, Univ. Claude Bernard Lyon 1 43 boulevard du 11 novembre 1918, Building 303, F 69622 Villeurbanne cedex -FRANCE
13.1. Introduction
13.1.1. Catalytic Combustion Combustion of hydrocarbons is widely used for energy production either thermally or in electrical power plants [1]. This combustion may occur conventionally in burners leading to a flame whose temperature is difficult to control and which can reach 1800°C according to the fuel used [2]. Fine tuning of stoichiometry is necessary to avoid either flame blowing or carbon deposition. Presence of carbon monoxide due to partial reversal of equilibrium concerning CO and C0 2 , presence of unburned hydrocarbons due to locally bad air/fuel mixing, presence of NOx due to partial nitrogen oxidation according to the Zeldovich mechanism [3] cannot be totally avoided. These substances are emitted in air during the combustion with a flame. In order to have an environmental friendly energy production, catalytic combustion is one alternative according to the best achievable technique. As previously noted "catalysis takes on pollution" [4] and is certainly the only way to protect the environment [5]. What occurs in a flame at high temperature is radicals propagation which is limited by mass transfer only. On a catalyst the same occurs but radicals are trapped on the catalyst surface: this has been verified experimentally in the case of platinum [6]. Thus with a catalyst combustion may be strictly limited on the solid surface, and reaction may occur at much lower temperatures due to the easiness of forming such radicals: reaction is limited by kinetics only leading to an Arrhenius behaviour of process. In that sense it can occur at very low temperature provided catalyst is sufficiently active in contrast with an homogeneous process by a flame needing a very high temperature. With a catalyst one has absolutely to 407
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Catalysis by ceria and related systems
activate one of the reactants and this occurs preferentially with oxygen. Thus in catalytic combustion molecular oxygen is generally dissociated in atoms which are strong oxidiser and strong electrophile as well [7]. Solids able to do that belong to two classes: i) noble metals [8,9,10,11,12] having the unique ability to dissociate 0 2 and C-H bond in atoms even at room temperature [13] ii) and transition metal oxides [3, 13, 14, 15] able to release oxygen atoms at some higher temperature leading to suboxide and/or oxygen vacancies. The more covalent the oxide, the stronger the bond and the less the process to occur due to insulating character of the oxide [16]. In order to obtain a good catalyst made of oxide, two ionic valence states must occur with sufficient electrochemical potential (thermodynamics aspect), the switching between them must occur very promptly (kinetics aspect). A lot of simple or mixed oxides do have these properties: CuO, Mn0 2 , Fe 2 0 3 , AB 2 0 4 spinels like C03O4, CuCr204 [17] AB0 3 perovskites like LaMn0 3 [18, 19]. All these oxides are semiconductors of both n and p types [20], Amongst all catalytically active oxides Ce0 2 appears quite apart due to its exceptional properties.
13.1.2. Ceria Structure During the last two decades ceria-based materials have been intensively used in automobile exhaust gas conversion (see Chapter 10) along with in electrochemical systems such as fuel cell (see Chapter 15). Cerium is the unique rare earth for which dioxide is the normal stable phase contrary to the others for which Ln 2 0 3 is the normal stoichiometry [21]. This is due to the [Xe]4f' electronic structure of Ce3+ which loose easily its last valence electron. Ceria has the CaF2 fluorite structure [22] consisting of a network of cerium ions arranged in the fee array leading to octahedral and tetrahedral holes, the latter only being filled with oxygen ions. As such each cerium ion is at the centre of a cube formed by 8 oxygen ions. It is a pale yellow solid easily synthesised from cerium salts decomposition (nitrate, carbonate, hydroxide) followed by an air calcination at 500°C to 1000°C. The colour results from UV charge-transfer band extending down to 450 nm: there is a small absorption of the violet colour [23]. After salts calcination at 500°C X-ray diffraction already shows the peaks corresponding to the Ce0 2 structure. The higher the temperature the better the crystallinity and the lower the surface area. The second oxide is Ce 2 0 3 , having the so-called A rare-earth sesquioxide structure [22]. It is a blue black coloured solid prepared with difficulty by extensive reduction of ceria [24, 25]. This structure is related to the preceding CaF2 one and is obtained by removing one quarter of the oxygen ions and slightly moving of the metallic
Fundamentals and applications of ceria in combustion reaction
409
cations. Cerium ions are now 7-coordinated, an unusual symmetry. The local M0 7 geometry derives in fact from the normal cubic M0 8 in Ce0 2 : a cube is distorted and six oxygen ions form an octahedron and an additional ion oxygen is located above the centre of the face of the octahedron, which is distorted by separating the atoms at the corners of this face [26]. However when reduced partially (not to bulk Ce 2 0 3 ) ceria can retain the fluorite structure despite numerous oxygen vacancies. This explain the strong reactivity of reduced ceria towards oxygen even at low temperature [27].
13.1.3. Ceria in Catalysis Ceria has some activity in isomerisation and dehydrogenation [8]. However one of the main use of ceria, up to now, is the three-way catalysis. It is generally assumed that ceria acts an oxygen storage system, being reduced in rich conditions (furnishing plenty of oxygen) and recovering a lot of oxygen in lean conditions [28]. Due to the lambda probe maintaining the air/fuel ratio very close to the stoichiometry, ceria acts as a buffer for strong deviation of oxygen concentration (see Chapter 10). When prepared from salts decomposition like nitrate, ceria shows considerable superficial OH groups on surface, the number of which decreasing with calcination temperature [29]. These OH group have normal basic character [30], but due to the presence of surface Ce4+ ions, both Bronsted and Lewis type sites are revealed by basic probes [31]. Thus specific reactions like alcohol (isopropanol) dehydration/dehydrogenation has been experienced [30]. For several years ceria along with other rare-earth oxides have been studied for their ability to decompose nitrogen oxides [21]. On the other hand a lot of new results appears concerning S0 2 recovery using Ce0 2 [32]
13.1.4. Ceria and Total Oxidation One of the important catalytic property of ceria is its ability to oxidise totally hydrocarbons along with carbon monoxide and hydrogen. For these later reasons ceria may be used as electrode in the fuel cell process, one of the most difficult challenge for next years to come (see Chapter 15). Generally speaking the oxidation properties of ceria are related to the Ce47Ce3+ electrochemical potential. According to the Nernst equation the normal potential is 1.61 V meaning that Ce4+ species are strong oxidising agents. Of course for oxygen containing solids, application of
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Catalysis by ceria and related systems
Nernst equation is not straightforward: however whit thermodynamics data one can demonstrate that Ce0 2 can act as an efficient oxidation catalyst [33]. According to its ability to "deliver" oxygen, Ce0 2 is consider as a n-type semiconductor. This type of oxide is generally assumed to have good oxidation properties [7,20].
13.2. Catalytic Combustion on Ceria
13.2.1. CO Oxidation Informations obtained from CO oxidation are more important as it might be thought. All hydrocarbon combustions produce carbon dioxide which may act as a poison or at least as an inhibitor due to some basic character of the ceria surface. Pioneering work on catalytic CO oxidation on ceria seems to be assigned to Rienacker [34]. Careful physicochemical studies on the same subject has been done more extensively by Claudel et al.: 0 2 , CO and C0 2 adsorptions, electrical conductivity, microcalorimetry and thermogravimetry measurements have been experienced to better understand the mechanism on the surface [35]. The authors confirmed the participation of lattice oxygen ions, the zero-order relative to 0 2 deduced from kinetics and they proposed a mechanism. Moreover they explain the self poisoning effect by C0 2 adsorption, the latter being always observed [36]. Similar results were previously observed by L.A Sazonov et al. during their study on CO oxidation on rare-earth oxides at 350°C [37]. They found effectively that ceria is by far the most efficient catalyst. In a more recent study on ceria performing CO transient oxidation, Descorme et al. measured the reactivity of surface oxygen atoms by CO TPR [38] onto ceria-zirconia solid solutions. They found that Ce0 67Zr0 33 0 2 has the large oxygen storage capacity involving dioxygen species like peroxides or superoxides.
13.2.2. Oxidation of Hydrocarbons Catalytic combustion of hydrocarbons on ceria has been studied for a long time. Both paraffins and unsaturated hydrocarbons are totally oxidised on ceria between 300°C and 500°C. Previous results of Morooka and co-workers have clearly evidence the catalytic properties of ceria compared to other oxides in total oxidation of acetylene, ethylene, propene, isobutene and propane [39]. They noted
Fundamentals and applications ofceria in combustion reaction correlations between reaction order in hydrocarbon, reaction order in oxygen, rates of oxidation at 300°C activities and heat of formation of oxides [40]. Although ceria was active, and supposed to be the most active rare-earth oxides in hydrocarbons oxidation, it was reported that it is still "10 to 100 times less active than typical transition metal oxides like CuO, Mn0 2 , Cr 2 0 3 and almost 10000 less active than platinum" [21]. Oxidation of butane has been performed on rare earth oxides at 400°C to 500°C [41]. Ceria had the highest activity, the better selectivity in C0 2 production and the lowest one in butenes formation. A nice correlation was found between C0 2 yield and the rare earth fourth ionisation potential I4: the lower the I4 value the higher the activity. According to the authors the value is in connection with the rate limiting step. Similar results were previously observed with butane oxidation also at 550°C, [42]. All these findings were summarised in a short review published by Pomonis [43]. Oxidation of methane on ceria has been studied by C. Bozo [44]. Ceria prepared by a precipitation method developed a BET area of 80 m2/g after 500°C calcination and only lm2/g when treated at 1000°C. Specific activities calculated at 500°C and 700°C were found to be 15.10"6 moKCH^.h-'m"2 and 5.10"4 mol(CH4).hW 2 respectively. When aged at 1000°C ceria had a very low activity due to dramatic loss of area: however specific activity was almost not modified. Thus sintering ofceria changes only area and not nature and density of active sites [45]. Activation of oxygen on ceria is a necessary perequisite to understand catalytic properties. In a work published by C.Li et al. it has been demonstrated that on ceria surface there are at least two dioxygen species, the superoxide (0 2 ) and peroxide (0 2 2 ) ions [46]. Both are well known and definitively assigned to total oxidation [7]. The authors studied the reactivity of these species towards CH4, C2H6 and C2H4 [47]. By using in-situ FTTR along with 18 0 2 from room temperature up to 400°C the authors hypothesised that only one species 022" is able to oxidise hydrocarbon, whereas the other 02~ is almost inert towards hydrocarbon below 300CC. Moreover they detected intermediate formate species that are converted to carbonate at temperature exceeding 300°C. In a more recent work Long et al. have studied activation of oxygen on Ce0 2 and reactivity of oxygen species towards CH4 and C2H6 by Raman spectroscopy [48]. They found only diatomic species like 022", 02" and 025" with 0<8<1. These species are quite stable and are adsorbed for temperature lower than 150°C. For higher temperatures these species decompose themselves except 02" which is still present at 750CC. When contacted with methane at 100°C there is some reactivity but methane activation with superoxide ion 02"
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and oxygen ions from the bulk do not occur until 750°C. Thus according to the authors participation of oxygen ions of ceria network does not proceed until 750°C. On the other hand Otsuka et al. found that for ceria both surface and bulk oxygen ions participate to the partial oxidation of methane [49], When oxygen is missing there is syngas H2 + CO formation. However this reaction occurs only at high temperature close to 750°C.
13.2.3. H2 Oxidation Hydrogen combustion on ceria, up to now, is not used for energy production except in fuel cell. Thus only Ln0 2 solids, as expected, and more precisely Ce0 2 , Pr0 2 and TbOz, may react with hydrogen. In fact hydrogen oxidation by oxides has been widely used as a thermal programmed method for checking oxygen atom reactivity. For ceria this has been done and reported in several papers [51-56]: In summary, it can be said that surface oxygens react first at ~ 600°C leading to superficial Ce3+ ions, whereas bulk reduction occurs at ~ 1000°C.
13.3. Catalytic Combustion on Ceria Containing Oxides
13.3.1. Ceria Associated with Zirconia On of the major drawback of ceria is its poor resistance towards sintering. Ceria having a cubic related structure is able to sinter equally along the three x, y and z axis. According to some rule established several years ago [57] oxides sintering is favoured for metals having a high atomic weight and this is the case for cerium. According to the fact that ceria in catalysis must be associated with a specific surface area as high as possible, there has been a lot of research to hinder or retard sintering. Thus doping or mixing ceria with other oxides made of divalent, trivalent or tetravalent ions has been encountered. One of the main breakthrough was the use of zirconia as a dopant. Now this choice may appear no fortuitous. Zr0 2 is a refractory oxide having the same fluorite (8:4) structure. Despite slightly different ionic radii i.e. 0,97 A (Ce4+) and 0.84A (Zr4+), zirconium ions can enter the ceria network without too much stress and solid solution of ceria and zirconia whatever the composition has been synthesised [28] and used with success. This has been
Fundamentals and applications ofceria in combustion reaction valuable for enhancing thermal stability [58-60], for oxygen storage capacity [58,60], for sintering resistance [61], for electrochemical properties like for fuel cell [62] and for catalytic properties enhancement and amongst them catalytic combustion. Preparation of this mixed oxide is rather easy. Coprecipitation of cerium and zirconyle nitrates by ammonia, followed by washing, drying and calcination at 500CC leads to the right structure with an area of 85 m2/g [44]. Addition of a cationic surfactant like cetyltrimethylammoniurn bromide allows to obtain the fluorite structure having a BET area of 56 m2/g after calcination at 900°C [63]. Other precipitating bases like hydrazine has been also used [64]. Sol-gel has been experienced using Ce(III)(Acac)3 and Zr(0-But)4 as organometallic precursor in alcohol. After hydrolysis the gel is dried and calcined and the solid solution is obtained [65]. Recently Duprez et al. use Zr(0-Pr)4 and Ce(IV)(N03)4 in so-called "soft chemistry". After gelification and calcination at 600°C they obtained a new orthorhombic phase leading to a BET area of 60 m2/g [66].
13.3.1.1. Catalytic Combustion of Hydrocarbons Numerous studies concerning hydrocarbons combustion have been experienced and reported in catalysis literature. Trovarelli et al. used the introduction of Zr (and Hf as well) for methane combustion [67]. For solids prepared by coprecipitation and calcined at 930°C BET area increases from 6 m2/g (ceria) to 26-29 m2/g for ceriazirconia (Ce08Zr02O2). For the catalytic methane combustion T50 was lowered by 130°C. Rate calculated at 450°C was multiplied by a factor of 8 when using solid solution. Moreover authors noted no CO formation for either ceria or solid solution. Finally it was concluded that catalytic activity was related, at least partially, to higher oxygen mobility at lower temperature for solid solution and easier Ce4+/Ce3+ switching. A recent and similar study has been performed by C. Bozo concerning Ce0 2 and Ce0 2 -Zr0 2 solid solutions [44]. All solids showed similar activity whatever the zirconia content. Specific activities, calculated at 500°C, were in the range of 7 to 15xl0"6 moKa-y.lr'.m" 2 . When aged at 1000°C in a water + oxygen atmosphere, severe loss of BET areas was observed (80-104 m2/g to 1-8 m2/g depending on ceria zirconia ratio) for Ce1/3Zr2/302. Ageing at 1200°C leads to complete collapse of porosity and loss of BET area whereas activity was almost nil (5% conversion at 750°C). However from calculations it was concluded that intrinsic surface activity was constant whatever the ceria concentration and whatever the BET area.
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Comparable results were obtained by Trovarelli et al. using Ce0 2 and Ce08Zr02O2 (or Ce 0 8 Hf 0 2 O 2 ) calcined at 900°C [67]. In methane catalytic combustion, activities calculated at 450°C were 1.2xl0"6 (for ceria) and 9.3xl0"6 mol (CH4) g'.s"1 (for Ce08Zr02O2) whereas a value of 0.4xl0"6 mol (CH4) g'.s"1 was obtained by Bozo [44]. According to the measured BET areas (6 m2/g for ceria and 29 m2/g for ceria-zirconia) it could be deduced that that activities are roughly proportional to the BET area. However in their case, both the BET area and the catalytic activity were superior for ceria containing either Zr or Hf as an iso-element substituting for cerium Thus light-off temperature was lowered by 130°C from 670°C to 540°C when zirconium or hafnium were present.
13.3.1.2. CO Oxidation Like for hydrogen combustion, CO combustion on ceria containing solids has not been extensively studied. CO oxidation has been studied for solid solutions prepared by the microemulsion method [68,69]. The technique allows to obtain nanosized ceria-zirconia particles having a uniform shape and a BET area of 285m2/g at 150°C. However BET area decreases tremendously to 2 m2/g with increasing calcination temperature up to 1000°C. Deposition of the particles on a y-alumina by the same method leads to an enhanced activity in CO oxidation: thus after calcination at 1000°C during 5 h, supported ceria-zirconia catalyst showed a lightoff temperature 140°C lower than that for a similar catalyst prepared by a classical impregnation method. Moreover for this new catalyst T50 was still 40°C lower than that for non calcined catalyst prepared by impregnation. On the contrary for three-way catalysis CO oxidation has been widely studied as a reaction model for exhaust gas removal (see Chapter 10).
13.3.2. Ceria Associated with a Support Due to low sintering resistance of ceria containing solids it has been thought to preserve BET area by supporting ceria and ceria-zirconia solid solutions on a support. In most cases alumina is chosen because solid state reaction between Ce0 2 and A1203 does not occur. CeA103 perovskite formation is very difficult to realise even at very high temperature and needs a preliminary cerium reduction or decomposition [70,71].
Fundamentals and applications of ceria in combustion reaction
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CH4 oxidation has been experienced for ceria supported on a barium hexaaluminate, an heat resistant support. Preparation by a new reverse microemulsion method leads to ceria nanoparticles deposited on support and having a BET area close to 100 m2/g after calcination at 1000CC [72]. Such ultrahigh disperse nanoparticles show exceptional thermal resistance: the authors mentioned that ceria particles prepared with a size of 6 nm sinters only to 18 nm after a calcination at 1100°C under a water containing atmosphere. Of course excellent activity in methane combustion has been observed. According to their experimental conditions calculated specific activity expressed as mol(CH4).h"1.m"2 was estimated to 6.4xl0"5 at 500°C whereas Bozo [44] reported a value of 1.5xl0'5 at the same temperature: both values look similar. Thus the difference in methane conversion may be related to BET area only which is spectacularly preserved using the reverse micro-emulsion method for synthesis.
13.3.3 Ceria Associated with Transition Metal Oxides Transition metal oxides have been widely used as catalysts for total oxidation of CO and hydrocarbons. Although they are less active than precious metals they are sufficiently stable to sustain higher temperature. According to Zwinkels [2] there are two approaches to obtain a good high temperature catalytic combustion temperature: - incorporation of an active phase made of transition metal oxide in a stable matrix like hexaaluminates [73,74] use of a stable active phase deposited on a thermostable oxide De Leitenburg et al. have studied the effect of doping solid solutions Ce08Zr02O2 by either Mn or Cu [63]. They used low metal loadings i.e. 2.5%, 5% and 10% as atomic ratio in order to maintain homogeneous solid solutions as checked by XRD. Introduction of copper or manganese strongly modified redox properties of solids obtained and so activity (Fig. 13.1). Thus Ce076Zr019M005O2 was used in combustion of light alkanes (ethane, propane, butane and isobutane) and authors concluded that higher catalytic activity of ceria results from an higher oxygen mobility by creation of defect sites. Presence of Zr is important for stabilising surface area, but catalytic activity is greatly enhanced by the presence of Mn or Cu. The latter metals issued from elements having a different charge, induces charge compensation and so defects and associated catalytic activity.
Catalysis by ceria and related systems
416 900 c o
> c o o
oin 3
70 80 90 Ce02 content (mol %) Figure 13.1. Light-off temperature (left scale) and surface area after calcination at 800°C (right scale) of Ce0 2 -Zr0 2 solid solutions: T50 for solids containing Mn or Cu are indicated also (Reprinted from Catalysis Today, Ref. 63).
A study concerning solid solutions of Ce0 2 -Zr0 2 containing 7% in weight of Mn, Cr or Fe has been also performed by Bozo [44]. Metals were either supported on ceria-zirconia or mixed during the synthesis steps. Intrinsic activity (mol(CH4).h' \m" 2 ) calculated at 450°C was 10 times higher for the most active solid compared to the support alone. However when aged at 1000°C under a water + oxygen atmosphere deactivation occurred leading to catalysts less active than the support. Thus at 700°C intrinsic activities were roughly 100 times lower than that for non aged solid calculated at 450°C only and were all alike irrespective of the presence or not of transition metal ions. According to the values of intrinsic activities it was concluded by authors, similarly to the studies of de Leitenburg et al [63] that the presence of ions modify the oxygen mobility and so activity.
13.4. Ceria Associated with Noble Metals A lot of studies have been published concerning ceria, pure or doped, associated with a noble metal for boosting activity in catalytic combustion. The main metals studied are Pt and Pd essentially, but some studies concern Rh and Ag.
Fundamentals and applications of ceria in combustion reaction
13.4.1. Silver Associated with Ceria Generally silver oxide is considered as a partial oxidation catalyst, utilized for oxidation of ethylene into ethylene oxide. Preparation of Ag 2 0 supported ceria has been reported with the idea that Ce0 2 can maintain silver in an oxidised state [75]. Normally Ag 2 0 decomposes itself roughly at 400°C leading to metallic particles that aggregated promptly to large particle having metallic character. By use of ceria it was observed that silver oxide loose oxygen at lower temperature (250-350°C), some oxygen transfer to ceria has been observed maintaining activity at 200°C in CO oxidation. This is an unusual fact contrary to what is generally admitted. On the contrary pure silver obtained from silver oxide decomposition deactivates promptly within a few hours. The authors concluded that the function of ceria was to maintain the dispersed state of silver and so activity in CO oxidation.
13.4.2. Rhodium Associated with Ceria Numerous studied have been published concerning interaction of ceria with rhodium in the case of three-way catalysis (see Chapter 4). Studies are still vigorous in order to better understand interaction of Rh towards NO in the presence of Ce0 2 This is due to the fact that there is a strong interaction between rhodium and ceria modifying oxygen storage capacity, an important phenomenon studied particularly by Kaspar et al. [76]. More recently Bernal et al. studied the metal dispersion in the case of Rh/Ce0 2 by HREM [77]. The authors concluded that under oxidising conditions Rh sintered at 500°C as oxide, whereas at 900°C strong redispersion is observed. However there is no study strictly devoted to catalytic combustion of hydrocarbons using ceria supported rhodium.
13.4.3. Platinum Associated with Ceria A recent study of Bozo et al concerning platinum deposited onto ceria-zirconia solid solution has been published [45]. Pt/CeO067ZrO033 was a most attractive catalysts which showed an activity much higher than for platinum deposited onto alumina: T50 for methane combustion was lowered from 470 to 300°C according to their experimental conditions. This activity was attributed to enhanced oxygen species mobility onto ceria containing solids. Unfortunately a continuous
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Catalysis by ceria and related systems
deactivation was observed, first during test, second after a test at full conversion at 600°C, and finally after an ageing test at 1000°C. The authors suggested that platinum was in an oxidised state in strong interaction with support (Fig. 13.2).
Temperature / (°C) Figure 13.2. Catalytic combustion of methane onto and Pt/CeO0 66ZrO0 33 according to the state of catalyst (reprinted from Catalysis Today, Ref. 45).
Such interactions between platinum and ceria have been previously hypothesised by Shyu et al. [78], and by Tiernan et al. [79]. When such oxide interaction O-Pt-O-Ce-0 exists, recovering back platinum in metallic state able to activate oxygen is not further possible. In the same study it was also reported that ageing at 1000°C has a dramatic effect. Comparison with traditional Pt/Al 2 0 3 showed that catalyst was no longer active due a demixion of the support along with a tremendous sintering from 79 m2/g to 7 m2/g: activity was comparable to that of support alone. As a final conclusion the use of platinum with ceria-zirconia solid solution is not suitable as far as temperature as high as 1000°C are expected. And for use at lower temperature in the 200-400°C where activity is high, further efforts to avoid continuous poisoning have to be undertaken.
13.4.4. Palladium Associated with Ceria Amongst all noble metals palladium represents the most attractive active phase for methane combustion. It is now well established that it works as oxide PdO having a limited stability range from 400°C to roughly 750/800°C, and so a powerful oxygen
Fundamentals and applications of ceria in combustion reaction donor capacity. Yu-Yao et al [80] studied the interaction of noble metal on alumina doped with ceria in association with CO combustion, which occurs between 200°C and 400°C. Concerning CO oxidation with palladium it was reported than the T50 was almost not modified with some ceria loading; the opposite was observed for platinum or rhodium. On the contrary, for propane combustion on Pd/Al 2 0 3 modified by ceria addition, Shyu and co-workers observed that ceria allows to maintain Pd in a more stable oxidised state less prompt to react with propane [78]. Catalytic oxidation which occurred from 200°C to 350°C is now delayed to higher temperature by 50°C to 100°C from 250°C to 450°C. More dramatically is the decrease of the conversion at 350°C from 100% to 20% as the partial pressure of oxygen increases beyond the stoichiometric ratio up to 8 times. They concluded from their studies that ceria and palladium are in close interaction, may be in a Pd-O-O-Ce-0 model via a 02" species. In a well known article, Groppi et al; published their results in the combustion of methane on Pd/Al203 boosted by ceria [81]. The aim of the work was to study the effect of an oxygen donor oxide on the stability and performance of PdO. It is now recognised that PdO is the true active phase when using alumina supported palladium. However the latter decomposes completely at temperature higher than 800°C leading to metallic palladium whose activity in oxidation is much lower. By using Ce0 2 PdO <=> Pd equilibrium is shifted to higher temperature by 50-60°C: ceria seems to stabilise PdO by hindering palladium reduction. However in methane combustion no effect was really observed at low temperature: T50 was not changed. Only when high temperature is considered ceria have a beneficial effect on PdO stabilisation and activity. Similar results were found by Bozo [44]. Palladium deposited onto ceriazirconia Ce067Zr033O2 solid solution showed very high activity in methane combustion (T50 close to 300°C) but similar to that of palladium deposited onto alumina. Like for the case of platinum a deactivation is observed during tests at temperatures comprised between 200°C and 400°C (Fig. 13.3). However when aged at 1000°C under an air+water mixture this catalysts showed superior resistance compared to classical catalysts as far as activity is considered. Despite a severe sintering of both metal (dispersion is now 1 %) and support, whose surface area is close to 4 m2/g, T50 was shifted to 420°C, i.e. 120°C only, still much lower for platinum deposited on the same support which showed a T50 close to 620°C. Calculation of specific activities in the 200-300°C range have clearly evidenced that ceria-zirconia support does not have any influence upon performance of PdO in
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Catalysis by ceria and related systems
methane combustion. In accordance with work of Shyu [78] it is concluded that interaction of PdO with ceria, if any, is limited.
150
200
250
300
350
400
450
500
550
600
650
700
750
800
Temperature / (°C) Figure 13.3. Catalytic combustion of methane onto CeO0 66ZrO0 33 and Pd/CeO0 66ZrO0 33 (reprinted from Catalysis Today, Ref. 45)
13.4.5. Conclusion on Noble Metals Associated with Ceria All studies concerning noble metals deposited onto ceria or ceria containing solid solutions reveal some important behaviours always observed in oxidising conditions similar to those encountered in catalytic combustion: - There is some strong interactions between noble metals and cerium proved by spectroscopy like Raman diffusion [82], photoluminescence and diffuse reflection [83], FTIR of probe molecules [84]. Interaction may form species like MO-Ce where both M and Ce are in oxidised states. - Several studies showed clearly the participation of oxygen atoms from the bulk of ceria for both combustion of CO [85] or hydrocarbons [86]. - There is some activation of catalysts by a reducing treatment disappearing with oxidising conditions like for catalytic combustion [87, 88]. - Solid solution of ceria-zirconia contains always some reduced Ce3+ species [89]. - Ceria stabilises noble metal in high oxidation states [70, 84, 87] leading to the superior interaction in the case of O-Pt-O-Ce- [78]. - There are some surface oxygen anionic vacancies [83, 90, 91, 92]. These vacancies induce formation of surface oxygen peroxide or superoxide close to the
Fundamentals and applications ofceria in combustion reaction metal-ceria interface and might be the true active species. So the role of the metal might be only that of donor/acceptor of electrons.
13.5. The Future of Ceria in Catalytic Combustion Contrary to the other rare-earth oxides which do have some highly desirable physical properties for electronics, optics, opto-electronics, magnetism, etc., the main use of ceria remains in chemistry, and for catalysis essentially. Apart the important use in three-way-catalysis, use of ceria in catalytic combustion has been rather limited up-to-now: there are several reasons for that. Major drawbacks for the use of Ce0 2 alone are its poor textural resistance at high temperature, its relatively reduced catalytic properties, its poor resistance to chemical poisoning by SOx. All these have been at least partially circumvented by the use of a support, doping with zirconia or transition metal oxide, use of noble metal combined to ceria, both being supported, etc. As calculated and reported by Trovarelli et al. the number of papers and patents appearing has regularly increased from 1980 up to now by a factor 6 [93]. On the other hand when high power is needed, catalytic fuel combustion requires solids having two antagonistic characteristics that are i) high thermal and structural resistance that is related to strong M-0 bond ii) high activity that is related to labile oxygen species i.e. to a labile M-0 bond: a subtle balance has yet to be found. On the whole one can forecast that catalytic combustion does posses a volcano curve "Activity vs Bond energy" like all other catalytic process. This has been recently suggested by Trimm [94]. Amongst the landmark work of Egon Matijevic [95], there is a second way: may be to use ceria partially doped with praseodymium oxide (Pr02) because the structure are the same and ionic radii are identical. One recent study concerning oxygen buffer capacity on mixed oxides has demonstrated that only one homogeneous phase is obtained when mixing both precursors. The bulk mixed oxide presents an enhanced oxygen buffer capacity due to an increase of the surface Ce/Pr [96]. But the more important may be the electrochemical standard potential of Pr4+/Pr3+ (2.86 V) which is one of the strongest oxidiser known up to now, comparable to fluorine. Thus by fine (and probably low) tuning of Pr0 2 concentration in ceria, by careful preparation, enhanced oxidising properties of ceria should be obtained highly valuable for methane combustion for example. This has been reported recently: specific activity calculated and compared to others reported in the literature has been estimated to be enhanced by two orders of magnitude [97] (Fig. 13.4). However by hydrogen TPR measurements authors suggested that
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Catalysis by ceria and related systems
surface reducibility has almost nothing to cope with catalytic methane combustion, contrary to what it might be thought. One solution for maintaining high catalytic activity is to prepare nanophase material. Very recently some new intriguing results have appeared in the open •
o
Ox
onversion
70Ce/25La/5Pr
— • — 75Ce/18La/5Pr
80
—*— 77Ce/18La/5Pr 80Ce/15La/Pr5 — • — 82Ce/13La/5Pr
g «°#
A
90Ce/5La/5Pr
/ r/
20
,
0 450
500
r 550
^
-
600
T
^
650
^
J /*
U?J-°
700
750
800
850
900
950
Reaction Temperature / K Figure 13.4. Catalytic combustion of methane on CeCVI^CyPrO^ mixed oxides prepared with 5% molar ratio of Pr0 2 after a calcination at 800°C (Reprinted from Catalysis Today, Ref. 97)
550
600
650
700
Temperature / °C Figure 13.5. Catalytic methane combustion of methane on: a) BaAl12019 issued from sol-gel process, b) BaAl12019 issued from reverse microemulsion method and c) Ce0 2 -BaAl 12 0 19 composite issued from microemulsion method (Reprinted from Letters to Nature, Ref. 72).
literature and have been also patented by J.Y. Ying [72]. Such extraordinary results (Fig. 13.5) have to be confirmed by other laboratories. As noted by these authors in
Fundamentals and applications ofceria in combustion reaction Nature "Ce02-base materials have not be widely used as combustion catalysts since other possess higher specific activities".
13.6. Conclusion For use in catalytic combustion where high temperature are encountered, where huge amount of water and carbon dioxide are produced, all ceria based catalysts must meet the following requirements: poor sinterability. Some studies concerning ceria have clearly evidence the crucial role of the synthesis method to preserve small particle size and so a good BET area. In particular it has been shown that sintering is independent of the particle size. Direct formation of ceria from precipitates is a key step to favourable sintering. On the contrary solid bridges between particles hinder sinterability [98]. For use in VOC destruction this may not be critical: for use in energy production this is one of the main requirement. enhanced carbon dioxide insensitivity. At low temperature C0 2 adsorption may be hazardous: in fact as soon as temperature exceeds 400°C decomposition of carbonates occurs leading the surface free of inhibitor adsorption. Thus only in case of destruction of VOC one has to deal with a minimum temperature to ensure a continuous catalytic process. Hardening in the Lewis sense by some acidic ions like Cr3+ may be valuable. enhanced sulphur resistance. Natural gas contains tiny amounts of sulphide as odoriser. The latter leads to SOx by combustion which may compete with oxygen or hydrocarbon for adsorption: poisoning by sulphite or sulphate can then occur. This is a critical point which is now well documented (see Chapter 11). However in the case of catalytic combustion some acidic resistance has to be found. stabilisation of mixed oxides. Previous works concerning ceria-zirconia solid solutions have definitively proved that the better active mixed oxide is not the more stable due to demixion at high temperature [44]. Further works dealing with structural stabilisation by careful selection of dopes is needed. lowered thermal expansion coefficient. Pure ceria has a high thermal expansion coefficient close to 12.10"6K"': it seems that partial substitution increases the value up to 14-15.10"6 K"1 in the case of Eu or La. [99]. Such a too high value precludes completely any industrial use for high temperature catalysis like combustion due to its low thermal resistance. During stop-and-go processes thermal shocks are likely to occur: cracks, failures, weakening unavoidably appear for monoliths or other bodies made of ceria based materials.
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Catalysis by ceria and related systems
In conclusion future efforts in structural bulk and surface modifications are needed urgently unless ceria will stay an academic subject for research. This is illustrated by the following figure (Fig. 13.6) showing percentage of conversion (hydrocarbon combustion) versus temperature using a catalyst containing ceria as support or ceria-other oxide solid solution. Two dashed temperature zones are clearly evidenced: - at low temperature for which using platinum for example very active catalyst are found: deactivation occurs due to some oxidation of platinum stabilised by ceria. Such interaction has to be circumvented by some ceria "hardening" limiting mixed platinum cerium oxide surface formation. Moreover carbon dioxide adsorption may inhibit either oxygen or hydrocarbon or both activation. - at high temperature sintering of ceria occurs tremendously limiting high temperature excursion. Only limited feed may be used leading to less value of that type of catalyst - between the two zones there is a limited temperature zone for which ceria may be used as a catalyst or as a support. But in that case one has to control temperature by feed composition and hydrocarbons conversion: efficient feedback has to occur. Conversion %
F-ioo '/////S/////S
'///////////£
'///s/s//ys/s,
50
'S////4/S///S
'//A/s////// '/////s/s/s/s Jr/s/ss//////
* '//////////// '////////////
Temperature 0.. '/////S//S/SS 200°C 400°C 600°C 800°C 1000°C Poisoning by Irreversible deactivation by M-O-Ce-O formation sintering and demixing Too active catalysts already poisoned by C0 2 adsorption Deactivated catalysts for which use of ceria is of no value Temperature range useful for of ceria-based catalysts. Figure 13.6. Temperature range of ceria based catalysts in catalytic combustion
Fundamentals and applications ofceria in combustion reaction
13.7. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
17.
18. 19.
Forzatti P. and Groppi G., Catal. Today, 54 (1999) 165-180. Zwinckels M.F.L., Jaras S.G. and Menon P.G., Catal. Rev. Sc. Eng. 35(3) (1993), 319-358. Ismagilov Z.R. and Kerzhentsev M.A. Catal. Rev. Sc. Eng. 32(1-2) (1990), 51103. Jacoby M., Chem. Eng News, June 30 (1997) 32-35. Bell A.T. Manzer L.E., Chen N.Y., Weekman V.W. Hegedus L.L. and Pereira C.J., Chem. Eng. Proc. February 1995 (26-34). Berlowitz P., Driscoll D.J., Lunsford J.H., Butt J.B. and Kung H.H., Comb. Sc. Tech., 40(1984)317-321. Bielanski A. and Haber J, Oxygen in Catalysis (Chemical Industries Vol 43 Marcel Dekker, New York 1991), 132-139. Hicks R.F., QIH., Young M.L and Lee R.G., J. Catal. 122 (1990) 280-294. Baldwin T.R. and Burch R. Catal. Letters, 6 (1990) 131-138. Farrauto R.J., Hobson M.C., Kenelly T. and Waterman E.M., Appl. Catal. A General 81 (1992) 227-237. Garbowski E., Feumi-Jantou C , Mouadibb N. and Primet M. Appl. Catal. A General 109(1994)227-291. Labalme V., Benhamou N., Guilhaume N., Garbowski E. and Primet M., Appl. Catal. A General 133 (1995) 351-366. Prasad R., Kennedy L.A. and Ruckenstein E., Catal Rev. Sc. Eng., 26 (1) 1984 1-58. Spivey J.J. Catalysis Vol 8, A Specialist Periodic Report, Bond G.C.and Webb G. Eds, (1989) 157-203. Marion M.C., PhD Thesis N°90-02, Universite Claude Bernard Lyon 1, 1990. Masel R.I. Principles of adsorption and reaction on solid surfaces, Wiley Series in Chemical Engineering ( John Wiley & Sons New York 1996) 769776. Golodets G.I., Heterogeneous Catalytic Reactions involving Molecular Oxygen Studies in Surface Science and Catalysis, Vol 15, (Elsevier Amsterdam 1991) 437-469. Voorhoeve R. J. H. in Advanced Material in Catalysis, Burton J.J and Garten R.L. Eds (Academic Press, New York 1977) 129-180. Tejuca L.J., Fierro J.L.G. and Tascon J.M.D., Adv. Catal. 36 (1989) 237-467.
425
426
Catalysis by ceria and related systems
20. Krylov O., Catalysis by nonmetals, Rules for catalyst selection, (Physical Chemistry, Vol 17, Academic Press New York 1970) 168-185. 21. RosynekM.P. Catal. Rev. Sc. Eng. 16(1) 1977) 111-154. 22. Wells, A.F., Structural Inorganic Chemistry, Oxford University Press, Oxford 1961, 3rd edition, 457. 23. Garbowski E., Unpublished results. 24. Yao H.C. and Yu Yao Y.F., J. Catal. 86 (1984) 254-265. 25. Perrichon V., Laachir A., Abouarnadasse S., Touret O. and Blanchard G., Appl. Catal,A General, 129 (1995) 69-82. 26. Wells A.F., Structural Inorganic Chemistry (Oxford University Press, Oxford 1961, 3rd edition) 465. 27. Perrichon V., Laachir A., Bergeret G., Frety R., Tournayan L. and Touret L. J. Chem. Soc. FaradayTrans., 90 (1994) 773-781. 28. Trovarelli A., Catal. Rev. Sc. Eng. 38 (1996) 439-520. 29. Tanabe K., Mismo K., Ono Y. and Hattori H., New solid acids and bases (Kodansha Tokyo/ Elsevier New-York, (1989) 41-47. 30. Zaki M.I. and Sheppard N., J. Catal, 80 (1983) 114-122. 31. Zaki M.I., Hussein G.A.M., Mansour S.A.A., El-Ammaway H.A., Poltz P. and Knozinger H., J. Mol. Cat. 57 (1990) 367-378. 32. Trovarelli A., de Leitenburg C., Boaro M. and Dolcetti G., Catal. Today, 50 (1999) 353-367. 33. Ackermann R.J. and Rauh E.G., J. Chem. Therm. 3 (1971) 609-624. 34. Rienacker G, Z. Anorg. Alleg. Chem., 59 (1949) 280. 35. Breysse M., Guenin M., Claudel B., Latreille H. and Veron J., J. Catal. 27 (1972) 275-280. 36. Breysse M., Guenin M., Claudel and Veron J., J. Catal. 28 (1973) 54-62. 37. Sazonov L.A and Logvinenko M.G., Kinet. Ratal. 3 (1962) 761-767. 38. Madier Y., Descorme C , Le Govic A.M. and Duprez D., J. Phys. Chem. 103 (1999) 10999-11006. 39. Morooka Y. and Osaki A., /. Catal. 5 (1966) 116-124. 40. Morooka Y. Morikawa Y. and Osaki A., J. Catal. 7 (1967) 23-32. 41. Hattori T., Inoko I.J. and Murakawi J., /. Catal. 42 (1976) 60-72. 42. Minachev K.M. and Khodatov E.V., Kinet. Katal. 6 (1965) 89. 43. Pomonis P., React. Kinet. Catal. Letters, 18 (1981) 247-251. 44. Bozo C; PhD Thesis N° 43-00, Universite Claude Bernard LYON1 (2000). 45. Bozo C , Guilhaume N., Garbowski E. and Primet M. Catal. Today, 59 (2000) 33-45.
Fundamentals and applications ofceria in combustion reaction
427
46. Li C , Domen K., Maruya and Onishi T. J. Am. Chem. Soc 111 (1989) 76837687. 47. Li C., Xin Q., Guo X. and Onishi T. New Frontiers in Catalysis, Guczi et al Ed., Studies in Surface Science and Catalysis, Elsevier Amsterdam, Vol 75 (1993) 1955-1958. 48. Long R. Q . and Wan H. L. J. Chem. Soc. Faraday Trans. 93 (1997) 355-358. 49. Otsuka K. Wanng Y. and Nakamura M., Appl. Catal. A General. 183 (1999) 317-324. 50. De Leitenburg C , Goi D., Primavera A. Trovarelli A. and Dolcetti G., Appl. Catal. B Environmental, 11 (1996) L29-L35. 51. Yao H.C. and Yu Yao Y.F., J. Catal. 86 (1984) 254-265. 52. Fierro J.L.G, Soria J., Sanz J. and Rojo J.M., J. Solid. State Chem. 66 (1987) 154-162. 53. Badri A., Binet C. and Lavalley J.C., J. Chem. Soc. Faraday Trans. 92 (1996) 4469-4473. 54. El Fallah J., Boujana S., Dexpert H., Kiennemann A., Majerus J., Touret O., Villain F. and Le Normand F., J. Phys. Chem. 98 (1994) 5522-5533. 55. Salasc.S., PhD thesis, Universite Claude Bernard LYON1, n° 215-98 (1998). 56. Vlaic G., Fornasiero P., Geremia M. Kaspar J. and Graziani M., J. Catal. 182 (1999) 378-389. 57. Boreskov G.K., Preparation of Catalyst I: Scientific bases for the preparation of heterogeneous catalysts. Study in surface science and catalysis, Delmon B., Jacobs P.A. and Poncelet G. Ed. Elsevier Amsterdam. Vol 1, 1975, Paper C I . 58. Cuif J.P., Blanchard G., Touret O., Marczi M. and Quemere E. SAE Paper 961906 (1996) 73-81 59. Cuif J.P., Blanchard G., Touret O., Marczi M. and Quemere E. SAE Paper 970463 (1997) 1-13. 60. Trovarelli A. de Leitenburg C. and Dolcetti G., Chemtech 27 (1997) 32-37. 61. Pijolat M., Prin M., Soustelle M., Touret O. and Nortier P., J. Chem. Soc. Faraday Trans. 91 (1995) 3941-3948. 62. Steele B.C.H. Solid State Ionics, 129 (2000) 95-110. 63. Terribile D., Trovarelli A., de Leitenburg C , Primavera A. and Dolcetti G., Catal. Today 47 (1999) 133-140. 64. Hirano S., Kawabata A., Yoshinaka M., Hirota K. and Yamaguchi O., J. Am. Ceram. Soc, 78 (1995) 1414-1416. 65. Meriani S. and Soraru G., Ceramics Powders, Vicenzini P. Ed. (1983) 547-554. 66. RossignolS., Madier Y. and Duprez D., Catal. Today, 50 (1999) 261-270.
428
Catalysis by ceria and related systems
67. Zamar F., Trovarelli A., de Leitenburg C. and Dolcetti G. J. Chem. Soc. Chem. Com. (1995) 965-966. 68. Masui T., Fujiwara K., Peng Y. Machida K. and Adachi G., Chem. Letters (1997) 1285-1286. 69. Masui T., Fujiwara K., Peng Y. Sakata T., Machida K. Mori H. and Adachi G., J. Alloys Comp. 269(1998)116-1226. 70. Shyu J.Z., Otto K. Watkins W.L.H., Graham G.W. Belitz R.K. and Gandhi H.S.,7. Catal. 114 (1988) 23-33. 71. Djuricic B., Pickering S. MacGarry D., Tambuyser P. and Thomas P., J. Mat. Sc. 34(1999)1911-1919. 72. Zarur A.J. and Ying J. Y. Nature, 403 (2000) 65-67. 73. Machida M., Eguchi K. and Arai H., J. Catal. 103 (1987) 385-393. 74. Artizzu P., PhD thesis Universite Claude Bernard Lyon 1, n° 261-96 (1996). 75. Inamura S., Yamada H. and Utani K., Appl. Catal. A: General, 192 (2000) 221-226. 76. Fornasiero P., DiMonte R., Ranga Rao G., Kaspar J., Meriani S. Trovarelli A. and Graziani M., J. Catal. 151 (1995) 168-177. 77. Bernal S., Calvino J.J., Cauqui M.A., Perez Omil J.A., Pintado J.M. and Rodriguez-Izquierdo J.M. Appl. Catal. B : Environmental, 16 (1998) 127-128 78. Shyu J.S. and Otto K„ /. Catal. 115 (1989) 16-23. 79. Tiernan M.J. and Finlayson O.E., Appl. Catal. B., Environmental, 19 (1998) 2335. 80. Yu-Yao Y.F. and Kummer J.T, J. Catal. 106 (1987) 307-31. 81. Groppi G., Cristiani C , Lietti L., Ramella C , Valentini M. and Forzatti P., Catal. Today 50 (1999) 399-412. 82. Brogan M.S. Dines T.J. and Cairns J.A., J. Chem. Soc, Faraday Trans. 90 (1994) 1461-1466. 83. Sass A.S., Kuznetsov A.V., Shvets V.A., Saveleva G.A., Popova N.M. and Kazanskii V.B. Kinet. Catal. 28 (1987) 1217-1220. 84. Summers J.C. and Ausen S.A. J. Catal. 58 (1979) 131-143. 85. Jin T., Okuhara T., Mains G.J. and White J.M., J. Phys. Chem. 91 (1987) 33103315. 86. Mendelovici L. and Steinbberg M., J. Catal. 139 (1993) 561-567. 87. Diwell A.F., Rajaram R.R., Shaw H.A. and Truex T.J., Catalysis and Automotive Pollution Control II (CAPoC 2), Crucq A. Ed, Studies in surface science and catalysis, Elsevier Amsterdam, Vol. 71, (1991) 139-152. 88. Serre C , Garin F., Belot G. and Maire G., J. Catal. 141 (1993) 9-20.
Fundamentals and applications ofceria in combustion reaction
429
89. Fally F., Perrichon V., Vidal H., Kaspar J., Blanco G., Pintado J.M., Bernal S., Colon G., Daturi M. and Lavalley J.C., Catal. Today 59 (2000) 347-363. 90. Li C., Domen K., Maruya K. and Onishi T., J. Am. Chem. Soc. I l l (1989) 7683-7687. 91. Ibid, J. Catal., 123 (1990) 436-442. 92. Rossignol S., Gerard F. and Duprez D., J. Mater. Chem., 9 (1999) 1615-1620. 93. Trovarelli A., de Leitenburg C , Boaro M. and Dolcetti G., Catal. Today, 50 (1999) 353-367. 94. Trimm D.L., Catal. Today 26 (1995) 231-238. 95. Matijevic E. and Hsu W.P., /. Coll. Inter/. Sc, 118 (1987) 506-523. 96. Bernal S., Blanco G., Cauqui M.A., Pintado J.M., Galteyries A. and Sporken R., Surf, lnterf. Anal. 30 (2000) 85-89. 97. O'Connell M. and Morris M.A., Catal. Today, 59 (2000) 387-393. 98. Chen P.L and Chen I.W., J. Am. Ceram. Soc, 76 (1993) 1577-1583. 99. Dikmen S., Shuk P. and Greenblatt M., Solid State Ionics, 126 (1999) 89-95.
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CHAPTER 14
CERIA BASED WET-OXIDATION CATALYSTS SEIICHIROIMAMURA Department of Chemistry, Kyoto Institute of Technology Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan
14.1. Introduction — Background of Wet-Oxidation Wet-oxidation is carried out under a high pressure of oxygen at elevated temperatures to decompose organic pollutants in wastewaters. This process has been applied for the treatment of wastewaters discharged from chemical industries which are toxic or refractory for biological treatment [1]. Energy recovery is possible by this process [2]; when a wastewater contains more than 30 g of COD/L, spontaneous oxidation is maintained while incineration by a kiln requires more than 300 g of COD/L to sustain self-combustion. Useful organic compounds are also recovered by the wet-oxidation of biomass [3]. Thus, this technique has a highly promising future from the standpoint of both environmental and energy related issues. The author began the investigation on wet-oxidation process nearly two decades ago. Although only a limited amount of data were available when he started this project, much literature is being published every year recently, showing the increased interest in this process. As this process requires relatively severe reaction conditions, the development of active catalysts is one of the main research objects. Copper(II) salts are the only effective homogeneous wet-oxidation catalyst, which are used practically in the petrochemical industries [4,5]. However, recovery of copper salts is necessary to avoid contamination of the water by toxic copper ion after treatment. Thus, many kinds of solid catalysts have been developed [6-15]. Among the heterogeneous catalysts, ceria based catalysts began to attract attention these days because of their superior action. This chapter deals with the past and present research activity for the development of ceria-based wet-oxidation catalysts. Catalysts used in supercritical wet-oxidation are not taken up for discussion here, because it is in a little different category from that of ordinary wet-oxidation. Interested readers should refer to the fine review papers to understand more about wet-oxidation and to see the recent activities in this field [16,17]. A review paper which also deals with the ceria based wet-oxidation catalysts is available [18]. 431
432
Catalysis by ceria and related materials
14.2. Catalysts 14.2.1. Mn/Ce Composite Oxide Ceria (Ce02) or cerium based catalysts first appeared in 1975 [19]. Chowdhury and Ross treated a highly contaminated wastewater with various catalysts. However, they focused their attention mainly on iron(III) and copper(II) ions coupled with the action of hydrogen peroxide (HOOH), and reported that cerium(IV) sulfate or Ce0 2 had rather retarding effect. Although cerium(III) hydroxide exhibited catalytic action on the wet-oxidation of some organic compounds, it was used in combination with perchloric acid plus nitric acid and the main oxidizing reagents were these acids [20]. For a while since then, no research paper concerning ceria based catalyst was published except patent data. Wheaton et al. claimed the usefulness of various manganese based catalysts, and among the various elements added to manganese, cerium was also used as one component [21,22]. Thiel et al. treated off-gas stream after wet-oxidation using supported catalysts. The active components are various transition metals including manganese, cerium, and precious metals [23]. Martin et al. also prepared manganese based catalyst including cerium-manganese composite oxide [24]. In 1985 Imamura et al. developed manganese-cerium composite catalyst independently without noticing aforementioned patent data [25]. They started from modifying Co/Bi composite oxide which was effective for the wet-oxidation of various carboxylic acids [26]. They tried to see the effect of the addition of various oxides of lanthanide; at the beginning they didn't appreciate the extraordinary superior function of CeC>2. The series of experiments led to the combination of Ce0 2 and manganese oxide as the best choice. Figure 14.1 shows the effect of the composition of Mn/Ce composite oxides calcined at 350 °C on the activity for the oxidation of ammonia. Ammonia is one of the potent pollutants and causes eutrophication of the receiving water. The nitrogen components contained in various organic compounds are converted to ammonia by wet-oxidation, and its further oxidation is quite difficult [27]. The removal of total nitrogen (ATN) reached more than 70 % after 1 h at a Ce content region from 20 to 50 mol %. It was found that the ATN using Cu(N03)2 under the same reaction condition ([Cu]: 20 mM) was 54.7 %, indicating the superiority of the Mn/Ce catalyst. In this case, an addition of Ce0 2 increased the surface area (Sw) of the catalysts, and the specific activity (ATN/Sw) decreased monotonically with an increase in the Ce content. However, the apparent activity, as judged from ATN shows that the Mn/Ce catalysts are much more useful in the actual treatment
Ceria based wet-oxidation catalysts
433
150
-100
5 -50
50 100 Ce/(Mn+Ce) (mol%) Figure 14.1. Wet-oxidation of ammonia at 263°C. Reaction time: lh. [Mn/Ce]: 20mM (total metal concentration). D: ATN, A: Sw, O: ATN/Sw [25].
of NH3 than Mn oxides. It was found that no NO and a trace amount of N0 2 were detected in the vapor phase, and only a small amount of N0 3 (about 1.5 % based upon ATN) was present in the solution, showing that most of NH3 was converted into N2. Deiber et al. also notice the effectiveness of Mn/Ce composite oxide for the removal of ammonia in a wastewater [28]. The Mn/Ce catalyst was found to be effective for the wet-oxidation of various compounds [29,30], Table 14.1 shows the activity of the Mn/Ce catalyst (Mn/Ce molar ratio of 7/3) in comparison with those of Cu(N03)2 and Co/Bi(5/l). Judging from ATOC and the initial reaction rate (Ri), the Mn/Ce catalyst exhibits higher activity than the other two for the oxidation of almost all compounds. When the composition of the Mn/Ce catalysts was varied, the tendency in the change of the activity in the wet-oxidation and that in the decomposition of HOOH roughly coincided. Therefore, it may well be deduced that the redox property plays an important role in the catalyst activity. The rate of the TOC decrease in the wet-oxidation of PEG-200 catalyzed by Mn/Ce(4/6) was expressed by -d[TOC]/dt = k[Cat]062[TOC]°
(14.1)
The observed reaction order with respect to the catalyst concentration less than unity (0.61) suggests that a radical chain reaction (Eqs. 14.2 - 14.5) occurs at least partly.
Catalysis by ceria and related materials
434
RH R + 02 ROO- + RH 2ROO-
Cat —> —> —» —»
RROOROOH + Rinactive products
(14.2) (14.3) (14.4) (14.5)
k, k2 k3 k4
Table 14.1. Wet-oxidation catalyzed by Cu(NQ3)2, Co/Bi(5/l) and Mn/Ce(7/3)" [29], reactant
catalyst
acetic acid
none
n-butylamina
PEG-200
pyridine
ammonia
Cu(NO,)2 Co/Bi(5/l) Mn/Ce(7/3) none Cu(NO,)2 Co/Bi(5/l) Mn/Ce(7/3) none Cu(NO,)2 Co/Bi(5/l) Mn/Ce(7/3) none Cu(N03)2 Co/Bi(5/l) Mn/Ce(7/3) none Cu(N03)2
temp(°C) 247 247 247 247 220 220 220 220 220 220 220 220 270 270 270 270 263 263
ATOCb,% 42.0 87.2 95.5 99.5 3.5 16.6 5.3 35.4 4.6 30.7 62.2 59.4 10.7 16.3 17.1 22.1 7.7" 54.7"
R^ppm/min 7.0 63.7 50.0 90.0 0.3 2.0 0.8 9.0 0.7 6.7 20.3 20.3 1.8 2.0 1.0 2.0
Co/Bi(5/l)
263 13.0" Mn/Ce(7/3) 263 69.9" °[TOC],[TN]: 2000ppm; [Cat]:20mM (total metal concentration). PO2:1.0MPa; PN2:2.5MPa. Sw=49.8 rnVg [Co/Bi(5/l)], 94.6 mVg [Mn/Ce(7/3)]. Co/Bi(5/l) and Mn/Ce(7/3) were calcined at 350°C in air for 3h. "After lh. Initial rate of TOC decrease. ''Percentage decrease in TN after 60 min.
Different from the usual liquid-phase oxidation in organic solvents, the radical chain cannot be long enough to sustain an efficient autoxidation mode in polar media like water, especially when the concentration of the organic pollutants is small. This is the most serious problem in the wet-oxidation process, and therefore, the role of the catalysts is very important. This point will be addressed later again.
Ceria based wet-oxidation catalysts
435
Although exact forms of Ce oxide and, especially, Mn oxide in Mn/Ce0 2 composite catalyst calcined at 350 °C are not clear from XRD data [25], Ce is basically present in the form of Ce0 2 [31]. The interaction of Ce0 2 with manganese oxide is not known well. However, Ce0 2 affects the valence state of Mn. Fig. 14.2 shows the change of the valence state of Mn in Mn-Ce composite oxide (5 molar content of Ce) during calcination [32]. In the low temperature region, the valence state of Mn in Mn/Ce composite oxide exceeds 3+, whereas that in the single component oxide is below 3+. As the temperature increases above 800K, the
3.5
c 2 o 3.0
8 C
Mn/Ce
2.5
J
" 0
I
I
200
I
I
I
I
400 600 Temp.fC)
I
l_
800
1000
Figure 14.2. Change in the valence of Mn during heat treatment in air. Mn/Ce=95/5 (molar ratio) [32].
valence of the Mn in the former oxide decreases to the value correspondent to Mn304, while that in the single component oxide increases gradually and, then, decreases above 900 K. This kind of action of Ce0 2 to increase the valence of Mn (that is, to provide oxygen to Mn) at low temperature region and, on the contrary, to decrease it in the higher temperature region, is probably related to the well-known oxygen storage function of Ce0 2 . Although details are not known, the action of Ce0 2 may be to increase the valence of Mn during catalyst calcination procedure (usually 350 - 500 °C ) and increase its oxidation ability to effectively initiate the reaction (Eq. 2). De Leitenburg et al. see the action of Ce0 2 based catalyst in a different way, focusing Ce0 2 as the main active component [33]. They investigated the action of CeO/ZrO/CuO and CeO/ZrO/MnO,. Fig. 14.3 shows the behavior of these composite oxides against reduction with hydrogen. Addition of Zr0 2 increases the reducibility of Ce02, and further addition of CuO or MnOx results in the further
436
Catalysis by ceria and related materials
increment in the reduction extent. Thus they conclude that the replacement of the large Ce** ion with smaller cations increases the mobility of oxygen of Ce0 2 and the surface defect sites which accelerate the surface redox process during wet-oxidation.
0.0006-
0.0004-
1(4) (3)/
0.0002-
(2)/
'(D 0300
400
500 600 Temp. (K)
700
800
Figure 14.3. Amount of H2 consumed. (1) Ce02 (2) 80CeO/20ZrO2 (mol%) (3) IdCsOJ^ZrOpUnO^ (4) VeCeO/igZrO^CuO [33].
There is another information on the strong affinity and interaction between Ce0 2 and Mn 2 OJ29]. When Ce0 2 and Mn203 are allowed to contact, they interact easily even at a temperature as low as 150 °C, and isolated Mn2+ species (sextet) begins to appear (Fig. 14.4). This means that Mn, although in a small amount, migrates quite easily into Ce0 2 matrix and is reduced. However, it is not known now how this phenomenon is related to the catalytic action of Mn/Ce composite oxide. Thus, the interaction between Mn oxides and Ce0 2 is very complex and exact reason for the high activity of this catalyst is not clear. Work on Mn/Ce composite oxide catalysts is abundant [31,34-41]. However, deactivation phenomenon is often observed, and deposition of carbonaceous compounds is the main cause for deactivation [31,35,38-40,42]. This situation is shown in Fig. 14. 5 for the oxidation of phenol; the higher the reaction temperature, the faster the deposition of carbonaceous compounds [34]. As will be addressed later, deposition of carbonaceous compounds is suppressed by supporting a small amount of Ag and/or Pt. Larachi investigated the regeneration of the used Mn/Ce composite oxide catalyst by combustion [43].
Ceria based wet-oxidation
437
catalysts
Figure 14.4. ESR spectra of Mn 2 0, and Mn203 + Ce0 2 . (1) Mn 2 0, (2) Mn/Ce: 150°C, 5rnin (3) Mn/Ce: 250°C, 20min (4) Mn/Ce: 350°C, 20min [29].
600
' 400
200
30
60
90
Time (min)
30
60 Time (min)
90
Figure 14.5. Wet-oxidation of phenol. A: phenol conversion, B: carbon deposition [MnCyCeOj]: 5g/l, [phenol]: lg/1, Mn/Ce molar ratio =7/3 [34].
Concerning the oxidation of ammonia, there are data which show that Pt/Ti02, Pd/Ti02, Ru/Al203, and Pd/Al203 are superior to Mn/Ce composite catalyst [44,45].
14.2.2. Modification of Mn/Ce Composite Catalyst Hamoudi et al. modified Mn/Ce composite oxide by supporting Ag and/or Pt (each 1 wt%) [34,40]. As shown in Fig. 14.6, un-promoted catalyst shows two reduction
438
Catalysis by ceria and related materials
peaks at 250 and 375°C attributable to the reduction of Mn and Ce oxides. Addition of Pt and/or Ag results in the shift of peaks at lower temperature.
1
0
100
i
200
1
300
i
400
1
500
i
600
700
Temp.(°C)
Figure 14.6. TPR profiles of MnO/CeO, and Pt(Ag)Mn02/CeOr (1) MnCyCe0 2 (2) Pt/MnO/CeO, (3) Pt/Ag/MnCyCe02 (4) Ag/MnCyCeO,. Mn/Ce molar ratio=7/3, Pt=Ag=lwt% [34].
The shift toward lower reduction temperatures shows the improvement of the low temperature redox properties of the MnO/CeOj catalyst, which indicates the presence of metal-metal as well as metal-support interactions. These interactions are enhanced by the presence of Ce0 2 that induces the well-known spillover of hydrogen from Pt/and or Ag to Ce0 2 or an increase in the oxygen mobility within Ce0 2 . The promoted catalysts showed higher activity than un-promoted one in the oxidation of phenol, and the amount of carbon deposit on them was small. For example, oxygen uptake by the combustion of carbonaceous compounds after the oxidation of phenol at 80°C for 1 h was 3535^mol/g-cat, 2660|^mol/g-cat, and 2941|^mol/g-cat for MnCyCe02, Pt-MnO/CeO,, and Ag-Mn(yCe0 2 , respectively. Larachi also promoted Mn/Ce composite oxide with lwt% of Pt [43]. Yoon et al. tried to improve Mn/Ce based catalyst. They used various support such as Si0 2 , Ti0 2 , ZrSi04, Zr0 2 , and ZrC)2-Ti02 and found that Ti0 2 is the best support. They also investigated the effect of the additives on Mn/Ce composite oxide supported on Ti0 2 . As Table 14. 2 shows, the best result is obtained with the catalyst modified with a small amount of Co: i.e. Mn (2.7 wt%)-Ce(6.8 wt%)Co(0.5wt%)onTiO 2 [36].
Ceria based wet-oxidation
catalysts
439
Table 14.2. Wet-oxidation of acetic acid at 200°C. Support Ti0 2 , AcOH:2500 ppm, catalyst: 50mmol. [36]. active component (wt%)" Mn
Ce
Ru
2.8 2.7 2.7 2.7 2.7 2.5 2.5 2.5 2.5 2.5 2.5 2.4 2.4 2.4
7.2 6.8
0 0.5
Co
6.8 6.8 6.8 6.5 6.5 6.5 6.5 6.5 6.5 6.1 6.1 6.1
-
0.5
0.5 0.5 0.5 0.5 0.5 0.5
0.5
Sn
Ag
BET
conv.
Reaction rate
(mVg)
(%) 28 59 75 40 59 66 33 44 54 42 55 69 48 40
g/gc„-h 1.20xl02
0.5 0.5 1.0 1.0 1.0 0.5 0.5 1.0 1.0 1.0
16 14 27 17 21 24 18 23 14 13 15 17 12 13
2.56xl02 3.32xl02 1.72xl0'2 2.55xl02 2.98xl02 1.41x102 1.90xl02 2.39xl02 1.81xl02 2.39xl02 3.15xl0"2 2.07xl02 1.75xl0'2
based on metals
14.2.3. Ceria-promoted Precious Metal Catalysts Ce0 2 is an inevitable component in the automobile exhaust purification catalysts in which precious metals are the main active elements. In the wet-oxidation catalysts based on precious metals, Ce0 2 also plays a very important role in activating them. Table 14. 3 shows the result of the oxidation of PEG-200 over precious metals supported on Ce0 2 , together with the activity of Cu(N03)2 and Mn/Ce composite oxide [Mn/Ce(l/1)]. The activities of Ru, Pt, and Rh were much higher than those of Cu(N03)2 and Mn/Ce(l/l) [46]. It was found that Ru exhibited the highest activity among the three precious metals (ATOC at 45 min: 99.1 % for Ru, 95.7 % for Pt, and 82.8 % for Rh). Examination on the effect of supports (Ce02, y-Al203, NaY-Zeolite, Zr0 2 , and Ti02) revealed that Ce0 2 was the best. Ru was deduced to be in the form of Ru0 2 on the basis of ESCA and XRD analyses. Table 14.4 shows the comparison between the activity of Ru/Ce02 and that of Cu(N03)2. Ru/Ce02 exhibited almost the same or even higher activity than Cu(N03)2 for the oxidation of the compounds listed in the table. Although acetic acid was not decomposed completely by the both catalysts, it was found that its complete removal was attained by Mn/Ce(l/1) under the same reaction condition.
440
Catalysis by ceria and related
materials
Table 14.3. Wet-oxidation of PEG-200" [46]. Catalyst" catalyst' DTOC,c % ATOC;% none 9.4 Ir/Ce02 74.8 Ru/Ce02 100 Pd/Ce02 49.7 Rh/Ce02 100 Cu(NO,)2 12.3 Pt/CeO, 100 Mn/Ce(l/1) 43.8 a [TOC]:2000ppm. [CAT]:12mM (total metal concentration). P02: IMPa. PN2: 2Mpa. Temperature 200°C. "Precious metal loaded is 5wt.%. 'After lh. Table 14.4. Wet-oxidation catalyzed by Ru/Ce02 and Cu(N03)2 at 200°C' [46], ATOC," % Reactant Ru/CeO,' Cu(NO,), n-propyl 47.2 28.3 alcohol n-butyl alcohol 27.8 40.1 Phenol 94.8 93.5 Acetamide 51.6 18.1 PPG-1000' 54.3 29.5 acetic acid 44.5" 32.6' "[TOC]:2000ppm. [CAT]:12mM (total metal concentration). P02: IMPa. PN2: 2Mpa. "After lh. °Ru:5wt%. dpH:2.7. cpH:2.5. 'polypropylene glycol).
These facts may suggest that Ru/Ce02, different from Mn/Ce, has selectivity toward specific substances. PEG-200 and its degradation products were oxidized over Ru/Ce02 and Cu(N03)2. (Table 14.5) The former is especially effective for those oxygen-containing substances; especially stable ethylene glycol can be readily decomposed. The Ru/Ce02 is effective also for the vapor phase combustion of toxic formaldehyde [47]. Barbier et al. extensively investigated the action of Ru/Ce02 catalyst in the oxidation of acetic acid [48]. When Ru was loaded on different supports (Ce02, Ti0 2 , Zr0 2 , activated charcoal), a high-surface area Ce0 2 (Sw: 163m2/g) exhibited the highest support performance, activated charcoal having the second largest effect. For the components supported on Ce02, following activity order was obtained: Ru > Ir > Pd ~ Fe ~ Cu > Ag ~ Ni « Co = Cr ~ un-promoted Ce0 2 . The activity of prereduced Ru/Ce02 is higher than that of pre-oxidized Ru/Ce02. On the basis of the electrochemical potential data for Ru, it was shown that metallic state of Ru is stable under the reaction condition (0.083 M acetic acid). In fact, Ru maintained metallic state even after a reaction at 200 °C for 3 h.
Ceria based wet-oxidation catalysts
441
Table 14.5. Wet-oxidation catalyzed by Ru(5wt.%)/Ce02 and Cu(NO3) 2 atl50°C'[46]. ATOC,''% Ru/CeO,e Cu(NOJ, Reactant PEG 200 48.3 9.1 ethylene glicol 98 6.2 formic acid 100 64.7 Formaldeyde 96.4 24.1 *[TOC]:2000ppm. [CAT]:12mM (total metal concentration). PQ2: IMPa. PN2: 2Mpa. "After lh. 'Ru:5wt%.
The working state of Ru/Ce02 catalyst is assumed to be composed of large Ru° particles (hexagonal phase) with surface Run+ species surrounded by small clusters of ceria. The mean Ru metal particle is in the range of 20 to 30 nm. The effect of the dispersion of Ru° particle was investigated and it was found that the larger the particle, the larger the turnover frequency; the turnover frequency increased by a factor of 5 when the metal dispersion decreased by a factor of 8; e.g. from 57 to 7 %. The rate is expressed by Ri = 1.13 x 109exp(-96600/RT)[CH3COOH]"05PO205
(14.6)
The negative rate dependence on acetic acid concentration (-0.5) indicates its strong adsorption on the catalyst, which would inhibit the access of oxygen to the catalyst surface. They assumed that the catalytic action is the formation of superoxide ion (0 2 ) followed by a production of hydroperoxy radical as an active species under the reaction condition (pH = 2.9) Mnn+ +
02
-»
O;
H+
<-> H O O
+
M(n+1)+
+ 02
(14.7) (14.8)
where Mn"+ is Ru(III), Ru(IV), or Ce0 2 . Initiation step in the oxidation of a compound ZH is assumed to be one of the followings: ZH + M(n+" -*
Z» + NT
H+
ZH + 0 2
-*
Z« + H02«
(14.10)
ZH + H02« -^
Z» + H 2 0 2
(14.11)
(14.9)
In the case of acetic acid, the part of the cleavage of Z-H bond is assumed to be the O-H bond of carboxyl group. Reaction (14.9) occurs in the adsorbed phase on
442
Catalysis by ceria and related materials
the surface Ru"*. Reactions (14.10) and (14.11) can occur either in an adsorbed phase or in homogeneous phase. The action of the catalysts will be again discussed later. Importance of metal-support interaction in Ru(metal)/Ce02 catalyst is emphasized [49]. Ru-loaded high surface area Ce0 2 (Sw: 163 m2/g) [denoted as Ru/CeO2(160)] exhibited a much higher activity than Ru supported low surface area Ce0 2 (40 m2/g) [Ru/CeO2(40)]. The mean particle size of Ru metal and that of Ce0 2 in the former are 20 - 30 nm and 7 nm, respectively, whereas the latter has smaller (8-12nm) Ru particle and larger (25 nm) Ce0 2 . This situation is schematically depicted in Fig. 14.7, in which the contact points of Ru with Ce0 2 in Ru/CeO2(40) are restricted on the Ru particle periphery while they are regularly distributed all around the metal particle for Ru/Ce02 (160). Thus, for each metal particle, only about 125 Ru atoms interact with ceria in the former catalyst but as much as 780 Ru atoms participate in the Ru-Ce interaction in the latter. This kind of metal support interaction is important and the abundance in Ru-Ce interaction in Ru/CeO2(160) is the cause for the accelerated oxygen transfer from gas phase (liquid) to the active metal sites. Moreover, the crystallographic orientation of Ru metal may be the cause for the activity difference, although details are not known. All of the Ru metal particles in Ru/CeO2(160) grows along the (111) axis while tfiat in Ru/CeO2(40) along (131)and (010) axes. Different from the above detailed investigation of Barbier et al., there is some information obtained by using two kinds of Ru(3 wt%)/Ce02 catalysts on the state of Ru and its activity [50]. One catalyst (Ru/Ce02-A) was prepared as follows.
A
B
Figure 14.7. Interaction of Ru and Ce0 2 in (A) Ru/CeO2(160) and (B) Ru/CeO2(40) [49].
Cerium hydroxide was precipitated from cerium(III) nitrate solution under stirring for 24 h, followed by drying at 80 °C overnight. RuCl3 and formaldehyde
Ceria based wet-oxidation catalysts
443
was added to the deionized water containing the above cerium hydroxide under stirring at 90°C for 1 h, followed by subsequent stirring for another 23 h at room temperature. The solid portion thus obtained was calcined at 500°C for 3 h in air to obtain Ru/Ce02-A. The amount of Ru loading was 3 at% as metal. The other catalyst (Ru/CeOz-B) was obtained in the same procedure except that the stirring time is both 1 h for cerium hydroxide formation and Ru loading. Ru/Ce02-B had much higher activity than Ru/Ce02-A; the percentage decrease in TOC after 2 h in the oxidation of acetic acid at 200°C was 89 % for the former and 20 % for the latter. TPR experiment on both catalysts is shown in Fig. 14. 8.
Ru/Ce0 2 -A
Ru/Ce0 2 -B
100 200 300 400 500 600 700 800 900 Temp.(°C)
Figure 14.8. TPR experiment. Ru: 3wt% [50]. For Ru/Ce02-A, H2 was consumed in a relatively wide temperature range between 100 and 200 °C, while the peak for Ru/Ce02-B splitted into two and shifted to much lower temperature region of around 100°C. The amount of consumed H2 in both cases exactly corresponded to the stoichiometric reaction, Ru0 2 + 2H2 -> Ru° + 2H 2 0. Thus behavior of oxygen on Ru plays a critical role determining the catalytic activity, and the above result shows the importance of the oxygen transfer to the reactant through Ru. Even if the working state of Ru is metallic as Barbier et al. pointed out, oxygen is probably activated directly on Ru metal or transferred to Ru metal from Ce0 2 and Ru will be temporarily in an oxidized state. Then this oxygen is transferred to the reactants, and Ru returns to its original reduced state. The action of Ce0 2 surely affects the state of Ru and, also, may act as oxygen carrier as discussed later.
444
Catalysis by ceria and related materials
Oliviero et al. compared the activity of Ru/C(activated charcoal, Sw: 961 m2/g), Ru/CeO/C, and Ru/Ce02 in the oxidation of phenol and acrylic acid [51]. Carbon supported catalysts are very active for the oxidation of phenol (Ru/CeO/C > Ru/C > Ru/Ce02) but not for acrylic acid (Ru/Ce02 > Ru/CeCyC > Ru/C). The difference in the support effects can be explained by the difference in the adsorption tendency of the organic compounds and in the oxygen transfer ability of the catalysts. Acrylic acid is adsorbed more strongly on Ru than phenol or oxygen. They assume that Ce0 2 acts as an oxygen donor toward Ru when strongly adsorbing organics (acrylic acid) are treated. Thus they also emphasize the importance of the contact points between Ce0 2 and Ru. Zhang and Chuang carried out the catalytic wet oxidation of wastewater from paper and pulp mills (black liquor) on Pt(lwt%)y-Al203 and Pd(lwt%)/y-Al203[52]. Addition of Ce(4 wt%) increased the activity of the former but decreased that of the latter, although the reason for the difference is not known. Interestingly, Pt(l wt%)/Pd(l wt%)/Ce(4 wt%)/y-Al 2 0 3 exhibited even higher activity than the former two catalysts [53]. They deduced that the action of ceria is to modify the redox property of the active components (precious metals). Fe(10wt%)/Ce(4 wt%)/yA1203 and Ce(4 wt%)/y-Al203 rather retarded the reaction. They also investigated the stability of Pd/Pt/Ce/ Y"A1203 catalyst [54]. Imamura et al. tried to improve the performance of Mn/Ce0 2 and Ru/Ce02 catalyst by combining these three elements [55]. As an attempt to apply wetoxidation process for the treatment of domestic wastewater treatment, a model wastewater was used which contains dextrin (91.8 ppm), peptone (196.2 ppm), yeast extract (196.2 ppm), beef extract (223.8 ppm), NaCl (20.1ppm), MgS0 4 (12.1 ppm), KH2P04 (55.8 ppm), and KCl (40.2 ppm). Figure 14.9 compares the activity of Ru(3 wt %)/Mn/Ce02 and Mn/Ce02 with varying the Mn/Ce ratio. Although an addition of Ru to Mn/Ce02 improves the performance of the latter, the effect is very small for the Mn/Ce02 with a low Ce content. The effect of Ru is more remarkable in the higher Ce content region. Thus the interaction of Ru with Mn is not important, so the trial to utilize the asset of the Mn/Ce02 by incorporating Ru failed. The result also suggests the importance of Ce0 2 in the catalyst development. Although Ce0 2 is a good support and Ru seems to be the best active component among precious metals, there is work which shows that Ce0 2 and/or Ru is not necessarily the best choice as catalyst components. Dobrynkin et al. carried out the wet-oxidation of N-containing compounds such as acetonitrile, carbamide,
Ceria based wet-oxidation
catalysts
445
100 90
g 80
8 %
70 60 50 0
20
40
60
80
100
Ce/(MnCe) (mol%)
Figure 14.9. Wet-oxidation of a model domestic wastewater over (A) Ru(3wt%)/Mn/Ce and ( • ) Mn/Ce at 200°C. Reaction time: 3h, [catalyst]: 20mM (total metal concentration) [55].
dimethyl formamide, and found that Ru/graphite-like carbon is better than Ru/graphite-like carbon promoted by ceria concerning C0 2 yield and N2 selectivity [56]. Qin et al. investigated the wet-oxidation of p-chlorophenol over noble metal catalysts and also found that catalysts supported on activated charcoal showed higher activity for TOC reduction than those supported on alumina or Ce0 2 [57]. Platinum is the most active among noble metals (Pt > Pd > Ru), and the high performance of activated charcoal is deduced to be due to its high surface area and its significant adsorption capacity for p-chlorophenol. The reaction proceeds most likely via a free radical reaction as well as hydrolysis and decarboxylation.
14.2.4. Other Ceria Based Catalysts There is not much work on Ce0 2 based catalysts containing elements other than Mn or precious metals. Imamura et al. reported that Co/Ce composite oxide with 13 mol% of Ce [Co/Ce(87/13)] is active for the oxidation of ammonia, although details are not known [25], Hocevar et al. investigated the action of Ce!.xCux02.85 (0.05 < x < 0.20) for phenol oxidation (reaction temperature: 150°C) encouraged by the fact that this kind of catalyst is active in the vapor-phase oxidation of organic pollutants and also by the fact that de Leitenburg et al. [33] used Ce0 2 -Zr0 2 -CuO catalyst in the wetoxidation of acetic acid [58], The activity and stability of this catalyst depend much
Catalysis by ceria and related materials
446
on the preparation condition: the concentration of the mixed metal salt solution, the rate of co-precipitation, and the stirring speed during co-precipitation. The amount of leached Cu was small (less than 100 ppm after 5 h reaction), and only a very low quantity of carbonaceous deposits was formed on the catalyst surface (0.6 wt%). Xray diffraction analysis revealed that Ce and Cu are in the form of Ce0 2 and CuO, respectively. However, addition of Cu gradually reduces the lattice constant of the unit cell of pure Ce0 2 (afcc = 0.54113 nm), indicating the incorporation of Cu in the Cerianite lattice. ESCA analysis revealed that the concentration of Ce3+ increases with an increase in the amount of Cu incorporated. They also report that lower valence state of Cu (Cu1*) is formed with an interaction of the inherently present Ce3* as follows. Ce3*
+ Cu2+
<-> Ce4*
+
Cu*
(14.12)
However, there is much that remains unknown and in particular it is not known how the above phenomenon (Eq. 14.12) reflects the catalyst performance. Fig. 14.10 shows the change in the rate constant k (mol1 s'1) of the oxidation of phenol with the change in the Cu content. The rate constant (k) linearly increases with the Cu content regardless of the calcination temperature of the catalysts. Thus, the active component in Ce, xCux02.s catalysts is Cu. In the subsequent work, they reported that sol-gel technique is better than co-precipitation method, and high dispersion of copper oxide phase on the cerium oxide causes high catalytic activity. They emphasized the importance of the combination of the mixed valence state of Ce3* and Ce4* caused by an incorporation of Cu, which causes the reversible addition and removal of oxygen in this inherently defect structure. Here, the defect structure in Ce0 2 is also emphasized. However, their discussion is rather sophisticated and their claim on the inherent function of this catalyst is difficult to be understood. The clear point is that copper in this catalyst is considerably active and durable in the wet-oxidation condition due to the effect of Ce0 2 .
14.3. Summary — Role of the Catalysts The action of the catalysts in the wet-oxidation has not been explicitly clarified. In the usual liquid-phase oxidation of hydrocarbons, the most important function of the catalyst (metal ions) is to decompose hydroperoxide (ROOH) formed during the reaction (known as Haber-Weiss mechanism) and accelerate chain propagation step by producing active radicals (ROO- and R O ) according to eq. 14.13 and 14.14.
Ceria based wet-oxidation catalysts
447
X ^
1.0-
0.8-
A
^
•
•
1
O
k (Imol
> °-6"
0,2-
•
^ / *
0.0-
()
i
i
6
10
15
•
1 20
'
Cu content in the catalyst (mol%)
Figure 14.10. Rate constant (k) vs. Cu content in the wet-oxidation of phenol over Ce^Cu.Oj.j. Catalyst calcination temperature (K): ( • ) 773, (•) 933, (A) 1033, ( • ) 1133 [58].
Mnt M"1
+ ROOH -> + ROOH ->
RO- + OH ROO- + H+
+ M + M
(14.13) (14.14)
However, this kind of action is never required for wet-oxidation catalysts because hydroperoxide is decomposed as soon as it is formed under the severe condition of wet-oxidation with the aid of the catalytic action of the reactor wall [59,60]. Basically the mechanism involves free radical reaction. As stated before, however, the chain length is very short due to the relatively low concentration of the pollutants and the polar nature of water. Therefore, RO- or ROO- is converted easily to inert end-products before carrying out chain propagation step efficiently. Thus, the addition of hydrogen peroxide, which is often tried, is useless [59,60]. This fact is important in designing active catalysts. The sole function of the we-oxidation catalyst is to produce active radicals via an interaction with the pollutants in the first step of the reaction. Although the details of this step are not known, it is surely one important strategy to design catalysts which have a high redox ability to carry out an efficient electron transfer with the pollutants. However, the action of copper ion (the only effective homogeneous catalyst) poses ambiguity. Its redox potential is lower than those of Mn3+ and Fe3* (redox potential: 0.17 volt for Cu2+, 0.771 volt for Fe3t, and 1.488 volt for Mn5* for one electron reduction) [61]. This fact implies the involvement of other factors, among which activation of oxygen is considered as one possibility. Despite much effort of the researchers, the clear evidence for the activation of oxygen in the usual liquid-phase oxidation in organic media has not
448
Catalysis by ceria and related materials
been attained during the past few decades. However, a work by Okamura et al. may explain the peculiar action of copper ion in the wet-oxidation [62]. They oxidized benzene to phenol with molecular oxygen in an aqueous acetic acid at room temperature. The Cu loaded MCM-41 (mesoporous silicate) was the catalyst and ascorbic acid was needed to reduce Cu2+. Hydrogen peroxide was produced during the reaction, and it was assumed to be the active species to oxidize benzene. This suggests that the reduced state of Cu (Cu1*) activates oxygen and produces HOOH perhaps via superoxide ion (0 2 ) formation. In the highly severe wet-oxidation condition in the presence of organic pollutants, Cu2+ can be more readily reduced and may activate oxygen. Thus the activation of oxygen can be considered as one important factor for developing efficient wet-oxidation catalysts. As early as 1974, Sadana and Katzer proposed, perhaps intuitively, the activation of oxygen by 10 wt%CuO/y-Al203 to produce superoxide ion (0 2 ) which plays a role as one active species [63]. Recently Barbier et al. also proposed activation of oxygen to form superoxide over Ru/Ce02 catalyst as shown in Eq (14.7) [48], Based on the above discussion, the function of the wet-oxidation catalysts should be confined to (i) activation of oxygen and (ii) direct electron transfer with the reactants (redox reaction) in the first step of the reaction. Ce0 2 seems to effectively contribute to both factors. Ce0 2 behaves quite differently from other oxides of lanthanide and is always a constituent of automobile-exhaust purification catalysts. It stabilizes supports and keeps high surface area [64,65], prevents the sintering of precious metals and, thus, stabilizes their dispersed state [66,67], and acts as an oxygen reservoir [68,72]. When combined with precious metals, it works in various reactions other than the purification of vehicle exhausts: e.g., detoxification of N 2 0, methanol decomposition, methanol synthesis, combustion of formaldehyde, etc [47,73-75]. Precious metals are remarkably activated and behave quite differently on Ce0 2 compared with their action on other supports. Among various functions of Ce0 2 , oxygen storage action (oxygen reservoir function) is the most important, which is caused by the highly mobile nature of its lattice oxygen. This action inherently produces oxygen defect sites in itself. In addition, when combined with other elements, it provides oxygen to or withdraws oxygen from them. Whether Ce0 2 provides oxygen or withdraws oxygen depends upon the nature of the counter elements and the reaction conditions and is very difficult to estimate as is shown in the case of Mn/Ce composite oxide (Fig. 14.2). Other examples of the direction of oxygen flow (shown by an arrow) ascertained in the separate experiments are Ag -> Ce [76], Pt <- Ce [74], Pd <- Ce [75], and Ni 4Ce. Although these oxygen migration phenomena are observed in the vapor-phase reaction or during the procedure of preparing Ce02-containing catalysts, and thus,
Ceria based wet-oxidation catalysts
449
may not necessarily be related to the action of the working state of Ce02-containing catalysts in the wet-oxidation, this facile oxygen-migration surely creates active sites in the catalysts preparation procedure. If oxygen flows from Ce0 2 to the counter element, the defect sites of Ce0 2 further increase and they themselves may activate oxygen as Barbier et al. suggested (Eq. 14.7) [14]. Oxygen migration to the counter element will also increases the redox property of the element as Imamura et al. pointed out for Mn/Ce composite oxide [32]. In addition, Ce0 2 can act as an oxygen donor to the counter elements which act as main catalyst components during the reaction. If oxygen migration occurs from the counter elements to Ce0 2 , counter elements are reduced and may activate oxygen; this may be the case for Ru/Ce02 (Fig. 14.8). It can be concluded that the very mobile nature of the oxygen on Ce0 2 is one of the critical causes for the high performance of Ce02-containing wet-oxidation catalysts, whether oxygen migration creates defect sites for oxygen activation or increases the redox property of the active components as de Leitenburg et al. [33] and Hamoudi et al. [34,40] deduced. Precious metals supported on Ce0 2 seems to be one promising catalyst group (e.g. Ru/Ce02). However, the interaction of precious metals with Ce0 2 or the state of precious metals on Ce0 2 surface is very complex, and this point is out of the scope of this chapter. Interested readers should refer to the abundant appropriate literature [77]. Whether heterogeneous catalysts are practically used widely or not depends sorely on the their stability. If the problems concerning poisoning by carbonaceous deposits and leaching of the catalyst components are solved, catalytic wet-oxidation (CWO) process will have a highly promising future.
14.4. References 1. 2. 3. 4. 5. 6.
Keckler, K. P., Brandenburg, B. L., Momont, J. A. and Lehman, R. W., U.S. Patent 5192453, March 1993; Chem. Abstr. 118(1993), 197432. Versar, Inc. Report, DOE/ID 10368-1, Vol. 2, 1989; Chem. Abstr. 114(1991), 68424. McGinnis, G. D., Wilson, W. W., Prince, S. E. and Chen, C. C. Ind. Eng. Chem. Prod. Res. Dev. 22(1983), 633. Akitsune, K. Nikkakyo Geppo 29(1976), 9. Imamura, S., Sakai, T. and Ikuyama, T. Sekiyu Gakkaishi 25(1982), 74. Gallezot, P., Chaument, S., Perrard, A. and Isnard, P. /. Catal. 168(1997), 104.
450
7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35.
Catalysis by ceria and related materials Beziat, J. C , Besson, M., Gallezot, P. and Durecu, S. J. Catal. 182(1999), 129. Gallezot, P., Laurain, N. and Isnard, P. Appl. Catal. B 9(1996), LI 1. Chollier, M. J., Epron, F., Lamy-Pitara, E. and Barbier, J. Catal. Today 48(1999), 291. Maugans, C. B. and Akgerman, A. Water Res. 31(1997), 3116. Alejandre, A. etal. Appl. Catal. B 18(1998), 307. Fortuny, A., Font, J. and Fabregat, A. Appl. Catal. B 19(1998), 165. Fajerwerg, K. and Debellefontaine, H. Appl. Catal. B 10(1996), L229. Centi, G., Perathoner, S., Torre, T. and Verduna, M. G. Catal. Today 55(2000), 61. Barrault, J. et al. Appl. Catal. B 15(1998), 269. Mishra, V. S., Mahajani, V. V. and Joshi, B. Ind. Eng. Chem. Res. 34(1995), 2. Matatov-Meytal, Y. and Sheintuch, M. Ind. Eng. Chem. Res. 37(1998), 309. Trovarelli, A., de Leitenburg, C. Boaro, M. and Dolcetti, G. Catal. Today 50 (1999), 353. Chowdhury, A. K. and Ross, L. W. AIChE Symp. Ser. 151(1975), 46. Martinle, G. D. and Schilt, A. A. Anal. Chem. 48(1976), 70. Wheaton, R. B. and Van Kirk, J. W. U.S. Patent 4229296, October 1980. Wheaton, R. B., Nelson, J. A. and Scherpereel, D. E. U.S. Patent 4460628, July 1984. Thiel, R. et al. U.S. Patent 4141829 February 1979. Martin, W. J. and Yang, K. T. U.S. Patent 4211174, July 1980. Imamura, S., Doi, A. and Ishida, S. Ind. Eng. Chem. Prod. Res. Dev. 24(1985), 75. Imamura, S., Hirano, A. and Kawabata, N. Ind. Eng. Chem. Prod. Res. Dev. 24(1982), 570. Ottengraf, S. P. and Lotens, J. P. Water Res. 12(1978), 171. Deiber, G., Foussard, J. N. and Debellefontaine, H. Environ. Pollut. 96(1997), 311. Imamura, S. et al. Ind. Eng. Chem. Prod. Res. Dev. 25(1986), 34. Imamura, S., Nishimura, H. and Ishida, S. Sekiyu Gakkaishi 30(1987), 199. Hamoudi, S., Larachi, F., Adnot, A. and Sayari, A. J. Catal. 185(1999), 333. Imamura, S. et al. Appl Catal. A 142(1996), 279. de Leitenburg, C. et al. Appl. Catal. B 11(1996), L29. Hamoudi, S. et al. Catal. Today 62(2000), 379. Belkacemi, K., Larachi, F., Hamoudi, S. and Sayari, A. Appl. Catal. A 199(2000), 199.
Ceria based wet-oxidation catalysts
451
36. Yoon, W. L. et al. ENERGEX 2000: Proceedings of the 8th International Energy Forum, Las Vegas 23(2000), 23, 170. 37. Lee, B. N. and Lou, J. C. Water Sci. Technol. 42(2000), 131. 38. Belkacemi, K., Larachi, F., Hamoudi, S. and Turcotte, G. Ind. Eng. Chem. Res. 38(1999), 2268. 39. Hamoudi, S., Larachi, F., Cerrella, G. and Cassanello, M. Ind. Eng. Chem. Res. 37(1998), 3561. 40. Hamoudi, S., Larachi, F. and Sayari, A. J. Catal. 177(1998), 247. 41. Duprez, D. et al. Catal. Today 29 (1996), 317. 42. Belkacemi, K. et al. Making Bus. Biomass Energy. Environ., Chem., Fibers Mater., Proc. Biomass Conf. Am., 3rd, 2, 1105. 43. Larachi, F.Appl. Catal. B 30(2001), 141. 44. Qin, J. and Aika, K. Appl. Catal. B 16(1998), 261. 45. Hayashi, H., Uno, M., Chiyo, M. and Sugiyama, S. Nippon Kagaku Kaishi 9(1999), 589. 46. Imamura, S., Fukuda, I., and Ishida, S. Ind. Eng. Chem. Res. 27(1988), 718. 47. Imamura, S., Uematsu, Y„ Utani, K. and Ito, T. Ind. Eng. Chem. Res. 29(1991), 18. 48. Barbier, J., Jr. et al. J. Catal. 177(1998), 378. 49. Oliviero, L., Barbier, J. Jr., Labruqere, S. and Duprez. D. Catal. Lett. 60(1999), 15. 50. Imamura, S., Taniguchi, Y. and Ikeda, Y. unpublished data. 51. Oliviero, L. et al. Appl. Catal. B 25(2000), 267. 52. Zhang, Q. and Chuang, K. T. Ind. Eng. Chem. Res. 37(1998), 3343. 53. Zhang, Q. and Chuang, K. T. Appl. Catal. B 17(1998), 321. 54. Zhang, Q. and Chuang, K. T. Environ. Sci. Technol. 33(1999), 3641. 55. Imamura, S. et al. Ind. Eng. Chem. Res. 37(1998), 1136. 56. Dobrynkin, N. M., Batygina, M. V. and Noskov, A. S. Catal. Today 45(1998), 257. 57. Qin, J., Zhang, Q. and Chuang, K. T. Appl. Catal. B 29(2001), 115. 58. Hocevar, S., Batista, J. and Levee, J. J. Catal. 184(1999), 39. 59. Imamura, S. and Okuda, K. Mizushori Gijyutsu 22(1981), 9. 60. Imamura, S. Ind. Eng. Chem. Res. 38(1999), 1743. 61. Dean, J. A. in Lange's Handbook of Chemistry, McGraw-Hill, New York (1985), 6-2. 62. Okamura, J., Nishiyama, S., Tsuruya, S. and Masai, M. J. Mol. Catal. A 135(1998), 133. 63. Sadana, A. and Katzer, J. R. J. Catal. 35(1974), 140.
452
Catalysis by ceria and related materials
64. Harrison, B., Diwell, A. F. and Hallet. C. Plat. Met. Rev. 32(1988), 73. 65. Ozawa, M. and Kimura, M. J. Mater. Sci. Lett. 9(1990), 291. 66. Cook, A., Fitzgerald, A. G. and Cairns, J. A. in: Dines, T. J., Rochester, C. H. and Thomson, J. (Eds.) Catalysis and Surface Characterization, Royal Society of Chemistry, Cambridge 1992, 249. 67. Normand, F. L„ Hilaire, L., Kili, K. and Maire, G. J. Phys. Chem. 92(1988), 2561. 68. Miki, T. et al. J. Phys. Chem. 94(1990), 339. 69. Padeste, C , Cant, N. W. and Trimm, D. L. Catal. Lett. 18(1993), 305. 70. Kacimi, S., Barbier, J. Jr., Taha, R. and Duprez, D. Catal. Lett. 22(1993), 343. 71. Zafiris, G. S. and Gorte, R. J. J. Catal. 143(1993), 86. 72. Zafiris, G. S. and Gorte, R. J. J. Catal. 139(1993), 561. 73. Imamura, S. et al. Appl. Catal. A 201(2000), 121. 74. Imamura, S. et al. Catal. Today 50(1999), 369. 75. Imamura, S. et al. Sekiyu Gakkaishi, in presss. 76. Imamura, S., Yamada, H. and Utani, K. Appl. Catal. A 192(2000), 221. 77. Trovarelli, A. Catal. Rev. Sci. Eng. 38(1996), 489.
CHAPTER 15
CERIA-BASED ELECTRODES MOGENS MOGENSEN Materials Research Department, Ris0 National Laboratory DK-4000 Roskilde, Denmark; e-mail:[email protected]
15.1. Background The solid oxide fuel cell (SOFC) have been under development during several decades1 since it was discovered by Baur and Preis2 in 1937. In order to commercialise this high temperature (600 - 1000°C) fuel cell it is necessary to reduce the costs of fabrication and operation. Here ceria-based materials are of potential interest because doped ceria may help to decrease the internal electrical resistance of the SOFC by reducing the polarisation resistance in both the fuel and the air electrode. Further, the possibility of using less pre-treatment and lower water (steam) partial pressure in the natural gas feed due to lower susceptibility to coke formation on ceria containing fuel electrodes (anodes) may simplify the balance of plant of the fuel cell system, and finally it is anticipated that ceria based anodes will be less sensitive to poising from fuel impurities such as sulphur. The electrolyte in an SOFC must consist of a good ion conductor, which has essentially no electronic conductivity. Otherwise the cell will be internally shortcircuited. An often-used electrolyte material is yttria-stabilised zirconia (YSZ). The electrodes must possess good electron conductivity in order to facilitate the electrochemical reaction and to collect the current from the cell. The fuel electrode usually contains metallic nickel for this purpose. The anodic oxidation of the fuel (H2 or CO) can only take place in the vicinity of the so-called three-phase boundary (TPB), where all reactants (oxide ions, gas molecules and electrons) are present. Thus, it is advantageous to extend the length and width of the TPB zone as much as possible. One way to do this is by making a composite of Ni and YSZ called a NiYSZ-cermet. Another way is to use a mixed ionic and electronic conductor, which in principle can support the electrochemical reaction all over the surface as illustrated in Fig. 15.1. Partially reduced ceria is a mixed ionic and electronic 453
454
Catalysis by ceria and related materials
conductor, and this is a common argument in favour of using ceria as a part of the SOFC anode.
Pore CHCKMft&IC&Xr
.d~ YSZ
Figure 15.1. Illustration of the difference in location of the electrode reaction on two different SOFC electrode types. Upper: In an electrode where the electrode material is exclusively an electronic conductor, the reaction zone isrestrainedto the vicinity of the triple phase boundary (TPB). Lower: In a mixed ionic-electronic conductor (MIEC) the electrode reaction can take place on the entire electrode surface. However, mixed conductivity is not sufficient to make up an efficient SOFC fuel electrode. An electrode must also possess a sufficient high electrocatalytic activity.
Ceria-based electrodes
455
In this context it should be noted that all electrodes are catalysts (good or bad) as the electrochemical processes take place on solid surfaces even though the concept of electrocatalysis is not quite as straight forward as the concept of catalysis. Usually, an electrocatalyst means an addition to or a component of an electrode, which increases the rate of an electrochemical reaction without being consumed. The special feature of electrocatalysis is that there cannot be any non-catalised pathway in the absence of an electrode. As the electrocatalyst may be an addition to or a part of a composite electrode it need not necessarily be an electronic conductor, but the electrode as such must naturally be electronic conducting. Thus, doped ceria is believed by some authors to have a positive electrocatalytic effect in SOFC oxygen electrodes even though ceria under these oxidising conditions is close to being a pure ionic conductor. Furthermore, the electrode material must be reasonably stable and not change its volume as a result of reduction or oxidation because such a volume change may cause the electrode to peel off the electrolyte. Also, the thermal expansion coefficient (TEC) of the electrode material must be close to the electrolyte material. Thus, in order to select a proper composition of the doped ceria for a specific electrode application, it is necessary to have knowledge of the ceria chemistry and its relation to the thermal, crystallograhical and electrical properties of the doped ceria. Therefore, the next sections briefly describe the ceria chemistry and the related properties.
15.2. The Chemistry of Ceria This section gives a brief description of the defect chemistry of ceria. Especially, thermodynamic data on x (in Ce02.x) as a function of oxygen partial pressure, P0 2 , and temperature are discussed together with the effects of substituents and particle size. 15.2.1. Types of Defects and Reactions When Ce0 2 is reduced to Ce02.x, defects are created in the form of Ce3+, which in the Kroger-Vink-notation is written as Ce'Ce as the Ce3+ has one negative charge compared to the normal lattice In early works3"6, it was discussed whether these substitutional negative defects were balanced by some of the Ce3+ going on interstitial sites as Ce;*** or by oxide ion vacancies, V**0. Later works7"9 appear to show that the behaviour of Ce0 2 is consistent with the oxide vacancy model, which
Catalysis by ceria and related materials
456
is now generally accepted. Faber et al.10 examined the electron density distribution using X-ray diffraction and concluded that the amount of interstitial Ce is less than about 0.1 % of the total defect concentration in CeO; 91. The process of ceria reduction may be written as: 0 0 + 2CeCe = V" 0 + 2Ce'Ce + 1/2 0 2 (gas)
(15.1)
to the extent it is reduced simply by heating. In case it is reduced by a gas like hydrogen it is: 0 0 + 2CeCe + H2(gas) = V" 0 + 2Ce'Ce + H20(gas)
(15.2)
Oxide vacancies may also be introduced by doping with oxides of metals with lower valences, e.g. by dissolution of CaO or Gd 2 0 3 : CaO = Ca"Ce + V" 0 + 0 0
(15.3)
Gd 2 0 3 = 2Gd'Ce + V" 0 + 3 0 0
(15.4)
Already existing oxide vacancies may be removed by doping with oxides of higher valency than 4: Nb 2 0 5 + V" 0 = 2Nb-Ce + 4 0 0
(15.5)
Reaction (15.1) may also take place in doped ceria, and doping will naturally affect the equilibrium of reaction (15.1) by changing the concentration of oxide vacancies and by decreasing the concentration of CeCe. Assuming no interaction between the various defects (only valid for very low concentrations) the law of mass action is valid. Applied on eq.(15.1) it gives: [ V"0][Ce'Ce]2 P021/2 = constant
(15.6)
as the concentrations of 0 o and CeCe are approximately constant. In the case of undoped ceria [V" 0 ] = x/2 and [Ce'Ce] = 2x (site fractions). Insertion in (15.5) and rearrangement gives: x = constant P02"
(15.7)
Ceria-based electrodes
457
For doped ceria the concentration of vacancies may also be regarded as constant, and thus the result is x = constant P02"1/4
(15.8)
where x still is defined by 2x = [Ce'Ce ].
15.2.2. Thermodynamic Properties Extensive studies of the thermodynamics of reaction (eq.15.1) have been carried out on pure ceria in which x was measured as a function of oxygen partial pressure11"20. The methods used were thermogravimetry, coulometric titration and electrical conductivity. It was found possible to divide the P02-range into intervals where x is proportional to P02"1/n. Near stoichiometry (x < 0.006) an n-value of 5 has been found in many cases. In the composition range 0.006 < x < 0.1 it is reported that 2 < n < 5. The region 0.1 < x < 0.3 must be divided into smaller intervals with n increasing rapidly with x, up to n-values of approximately 30. The oxygen partial pressure dependence of -1/6 (eq. 15.7) is only valid for Ce0 2 free of oxide vacancies21. The strong increase in n-values with increasing x above x = 0.1 is explained by strong defect interaction. Riess et al.22 measured the chemical potential of oxygen in non-stoichiometric Ce02.x as a function of x (10 4 < x < 10"2) and temperature (873K-1073K), by coulometric titration using an electrochemical cell based on yttria-stabilised zirconia (determined by measuring the EMF of the cell, using air as a reference). The results of their work and other previous work 23"24 are summarised in Fig. 15.2, which shows x (in Ce02.x) as a function of the oxygen equilibrium pressure. Similar curves have been measured for a variety of doped ceria.25"32 A discussion of these literature results has been given elsewhere.33 The partial free energy of formation (per mol 0 2 ) of a given nonstoichiometric ceria, AG02, is given by AG02 = RTlnPQ2
(15.9)
Catalysis by ceria and related
458
materials
where P0 2 is the oxygen partial pressure which is in equilibrium with this ceria. Note that eq. (15.1) only operates with 1/2 a mol 0 2 . Combining this with AG02 = AH02 - TAS02 and rearranging it gives (15.10)
lnP0 2 = (AH02/R)(l/T) - AS02/R
io°
KT' —
. i « * . V o . V *. * • • *
• •
•
•
.o
• • 10" "* • »• * • • • • • icr — • •
to-
,0
/
i icr*
'
'
•
A
-
o 0 •o
A*
<<*
<^
**
J .•*
* y ''• ^ ' '* /* yy
•
V-'io°
o
•
/
e
k£ io'*
JL icr"
Zl 1 w* io**° P
l
io"*4
ic"
io -M
Figure 15.2. Oxygen equilibrium pressure of Ce02.„ vs oxygen deficiency x from several reports. After Riess et al.22. Dashed line: Tuller and Nowick24, • Panlener et al15, o Bevan and Kordis23, + Riess et al. experimental. Solid line: Riess et al. theoretical
From eq. (15.10) it can be seen that AH02 (the partial free enthalpy of formation) and AS02 (the partial free entropy of formation) may be derived from curves of the types shown in Fig. 15.2. AH02/R may be obtained from the slopes of the curves and AS02/R from the intercepts. Results of AH02 are shown in Fig. 15.3 as a function of log(x)for pure ceria and for two levels of Gd 2 0 3 substitution. It is seen that the data fall in three groups according to the doping level. Fig. 15.4 shows the same data as a function of log(x+y), i.e. the of the oxide vacancy concentration. This brings the AH02 values onto one common curve.
Ceria-based
•
electrodes
459
•
• A. •
•
• PureCe02;#1 APureCe02;#2 • CeO.8GdO.201.9-x • CeO.9GdO.101.9S-x
1.00E-04
1.00E-03
1.00E-02
1.00E-01
400 1.00E+00
Figure 1S.3. The partial free enthalpy of formation, -AH02, as a function of log(x) in Ce|.2yGd2y02_(X+y) for y = 0, 0.1 and 0.2. The data are taken from a literature review.33
• • 900
•
• (lout
800
J
700
t •
•
ri
•
600
*PureCe02;#1 500
A PureCeQ2;#2
•
,Ce0.8Gd0.2O1.9-*
•
aCe0.9Gd0.1O1.95-x
. 1.00E-03
Figure 15.4. The partial free enthalpy of formation, -AH02, as a function of log(x+y) in Ce1.2yGd2y02.(X+y) for y = 0, 0.1 and 0.2. The data are taken from a literature review.33
460
Catalysis by ceria and related materials
Data for AS02 are given in Fig. 15.5, which shows a plot of -AS02 versus log(x). Fig. 15.6 shows AS02 as a function of log(x+y). From this it is seen that AS 02 is rather dependent on the oxide vacancy concentration up to about x + y = 0.1 above which it seems less influenced. It is occasionally claimed that it is in general easier to reduce doped ceria than undoped ceria.34 This is, however, not the full truth. It depends on x (the actual degree of reduction), the temperature and the type of dopant. Some of the interaction parameters of Ce0 2 and other metal oxides like Zr0 2 , Y 2 0 3 and Ce 2 0 3 are positive. Tools for estimating actual thermodynamic values are available.35,36 Using the data given in Figs. 15.4 - 6 in eq. 15.10 will give the relation between temperature and P0 2 for given x-values with a fair approximation, especially for Gd 2 0 3 doped and pure Ce0 2 in the temperature range from 700 - 1000°C AH02 and AS02 are probably not exactly temperature independent. Here, a need for further work is seen in order to make better thermodynamic data available, especially for highly substituted ceria and for lower temperatures where short and/or long range ordering might occur. Furthermore, the redox chemistry of doped ceria is dependent on the ionic radius of the doped cation. The energy due to lattice stresses resulting from mismatch of the host and dopant cation sizes will also go into the free energy balance as briefly described below. Recently, it has been reported by Chiang et al.37,38 that nanocrystalline (ca. lOnm crystallite size) undoped and doped ceria are very easily reduced. The heat of reduction was found to be less than one-half of the value for conventional polycrystalline and single crystal samples. Ceria may be dissolved in YSZ, which also has the fluorite structure. Then a driving force for reduction of Ce4+ to Ce3+ exist because the radius of Ce3+ is big enough to facilitate the formation of the pyrochlore compound, Ce2Zr207, which has a crystal structure similar to the fluorite but with fully ordered oxide vacancies. This tends to segregate to the grain boundaries and induces electronic conductivity into the YSZ.39
Ceria-based
electrodes
461
•
•V. • Pure Ce02; #1 APure Ce02; #2 «Ce0.8Gd0.2O1.9-x •Ce0.9Gd0.1O1.95-x
A •
Figure 15.5. The partial free entropy of formation, -AS02, as a function of log(x) in Ce1.2yGd2y02.(x+y) for y = 0, 0.1 and 0.2. The data are taken from a literature review.33
• »PureCe02;#1 APureCe02;#2 • Ce0.8Gd0.2O1.9-x • Ce0.9Gd0.1O1.95-x 200 I 1.00E-04
•
1.00E-02
I 200 1 .OOE+00
x+y
Figure 15.6. The partial free entropy of formation, -AS02, as a function of log(x+y) in Ce1.2yGd2y02.(;(+jr) for y = 0,0.1 and 0.2. The data are taken from a literature review.33
Catalysis by ceria and related materials
462
15.2.3. Lattice Parameters of Pure, Doped and Reduced Ceria This section describes relations between lattice parameter, temperature, substituent cation radius, solubility limit of a substituent oxide and x (degree of reduction).
15.2.3.1. Thermal Expansion The lattice parameter at room temperature is 0.541134 nm.40 A reasonable agreement between the results of measured thermal expansion of pure stoichiometric Ce0 2 up to 1000°C is found,16, 41"47 and my conclusion is that the linear thermal expansion coefficient (TEC) is: At room temperature: (11.0 ± 0.5) 106 K"1 Room temperature to 500 °C: (11.5 ± 0.5) 10'6 K"1 Room temperature to 1000°C: (12.1 ± 0.5) 106 K"1 A doped ceria will have a slightly different TEC dependent on the ion radius and charge (oxidation state) of the dopant cation. Larger radius and lower charge than Ce4+ will increase the TEC and vice versa.
15.2.3.2. Solubility of Oxides and Change in Lattice Parameter For a simple solid solution the lattice parameter usually follow Vegard's rule with good approximation, i.e. a linear relationship exists between lattice parameter and the concentration of the solute. The slope of this straight line is termed Vegard's 48
slope. Kim published an empirical relation between ionic radius of the metal ion of oxides (dopants) dissolved in ceria (and other fluorite structured oxides) a = 0.5413 + Z(0.00220Ark + 0.00015 Azk)mk
(15.11)
where a (in nm) is the lattice constant of the ceria solid solution at room temperature, Ark (in nm) is the difference in ionic radius (rk - rCe) of the kth dopant and the Ce4+-radius, which in eightfold coordination is 0.097 nm according to 49
Shannon , Azk is the valence difference, (z k - 4), and mk is the mole percent of the kth dopant in the form of MOx. It is noted that the Kim's lattice constant of the pure ceria is 0.000166 nm higher than the now accepted value of 0.541134 nm. Kim
Ceria-based electrodes
463
verified his relations against a considerable amount of data, and his analysis of the uncertainty gave a standard error of 3-10"4 nm. Hong and Virkar analysed the relationship between concentration of LnO L5 in ceria (and zirconia) and lattice parameter of the fluorite lattice (Ln = lanthanide). When we reduce their formula we get for the ceria case (using the symbols described above) a = 0.54112 nm + 0.023027 (mol%Ln01.5)-1(rk - 0.1024 nm) m
(15.12)
Kim argues that the solubility limit of a solute depends on the elastic energy, W, which is introduced in the lattice due to differences in ionic radius. The larger the elastic energy per substituted ion, the lower is the solubility. The relation between W and the change in the lattice parameter, Aa, due to formation of a substitutional solid solution is given by: W = 6Gao(Aa)2
(15.13)
where G is the shear modulus of ceria. Aa is governed by Vegard's slope for the given solute. This implies that the highest solubilities should be achieved for a Vegard's slope of Sv = 0. Based on analogy with Th0 2 , it seems reasonable to assume that the solubility limit at 1500 -1600CC is about 60 mol% for rare earth cations with ionic radius near rc and close to zero if (Sv)2 is above ca. 6 10 s (nm/mol%)2. From eq. 15.11 the following expressions for Vegard's slopes can be derived: SV3,K = 0.0022(mol%)1(rk -0.097nm) -1.5 10-4nm(mol%)-1
(15.14)
for trivalent dopants, and SV2K = 0.0022(mol%)"'(rk -0.097nm) -3.0 10'4nm(mol%)-'
(15.14)
for divalent dopants. From eq.15.12 another type of expression for the trivalent dopants is obtained SV3,H = 0.023027 (mol%)'1(rk - 0.1024 nm)
(15.15)
464
Catalysis by ceria and related materials
The different formulae give different values of Vegard's slopes and also imply different values of the critical radius, rc, which is the radius giving a Vegard's slope of zero. Kim's expression implies that rc = 0.1106 nm for divalent ions and 0.1038 nm for trivalent ions, whereas Hong and Virkar's expression gives rc = 0.1024 nm. As mentioned elsewhere33 a value of rc = 0.101 nm may be derived from other sources. The experimental literature shows that ceria fluorite structure is in fact very tolerant to dissolution of lower valent metal oxides with cation radii close to rc to high concentrations above 40 % in some cases51,52 whereas oxides with cations much smaller or bigger than rc are less soluble, but there is a lot of contradicting information in the literature about the solubility limits.28,53,54 Fair predictions of solubility limits are possible using the Vegard's slope criteria. For example, the solubilities of CoO and NiO in ceria were estimated to be below ca. 1 %55 in good agreement with later experimental findings.56 Thermodynamic tools by which the solubility values may be quantitatively estimated exists.35
15.2.3.3. Expansion of Ceria on Reduction As the Ce ion is bigger than rc ceria will expand on reduction. If the value for r^e3+ = 0.1143 nm is inserted in eq. 15.14, it gives: a = 0.5413 nm + m-2.306-10-4nm/mol% Ce0 1 5
(15.16)
ie a Vegard's slope of 2.306-10"4nm/mol% Ce0 15 . Inserting the Ce3+-radius in eq. 15.15 we get a = 0.54112 nm + m 2.740 10'4 nm/mol% CeC^ 5
(15.17)
giving a Vegard slope of 2.740-10"4 nm/mol% CeC^ 5. Or, as m (in %) = 200x (x in Ce02.x), we get for homogeneous samples at room temperature: a = 0.5413 nm + x-5.48-10"2 nm).
(15.18)
Fig. 15.7 shows a plot of percentage of expansion versus O/M - ratio at 900°C. Using the lattice parameter of 0.54665nm at 900°C and the slope of the upper full line in Fig. 15.7 we find a Vegard's slope of 2.71 10"4 nm/mol % CeOj 5. The corresponding value at room temperature is 2.68 10"4 nm/mol% CeC^ 5. This is in
Ceria-based
electrodes
465
good agreement with the radius relations, i.e. with eq. 15.16 and in particular with eq. 15.17. In spite of this, the detailed reason for the expansion on reduction of ceria is often discussed. The problem may be phrased as: 1) is it because the Ce3+-ion is bigger than the Ce4+-ion? or 2) is it because the oxide vacancy volume is bigger than the oxide ion volume? One reason for the questions is that the effective ion radius is dependent on how many nearest neighbours the ion has (its coordination number). Hong and Virkar50 analysed this problem, and concluded that the reason is that of point 1). They introduced the concept of an oxide vacancy radius. For ceria the oxide vacancy radius was found to be 0.1164 nm at room temperature which is significantly smaller than the radius of the oxide ion of 0.138 nm.
2DOO
1350
1900
1B50
1300
N in CeO^ Figure 15.7. Percent expansion of ceria versus N = 2-x at 900°C33. Main data are from Chiang et al.4 - is after Mobius57, • are from Mogensen and Mogensen58, and — is best linear fit.
Expansion of reduction data for ceria with several Gd 2 0 3 doping levels are given in Table 15.1.33 Results for pure ceria are included for comparison. The expansion on reduction is of great importance for the practical applications at temperatures
Catalysis by ceria and related materials
466
above ca. 800°C, and more work is needed here especially for dopants other than Gd 2 0 3 .
Table 15.1. Expansion on reduction of pure and gadolinia doped ceria at 1000°C. Sv,ceo2 is t n e Vegard's slope for the CeO, 5-component produced by reduction.
Mol% GdC*! 5 0 0 18 18 40 40
X
% expansion
Sv,Ce02
0.105 0.200 0.060 0.120 0.022 0.045
1.14 2.10 0.77 1.49 0.28 0.67
2.7 10" 2.7 10"" 3.5 10" 3.4 10" 3.5 10" 4.1 10"
15.3. Electrical Conductivity 15.3.1. Electronic Conductivity Ce02.x is both an n-type semi-conductor and an oxide ion conductor, i. e. a mixed conductor5963even though the ionic conductivity of pure reduced ceria at 1000°C is less than 3% of the total conductivity at a P0 2 of 106atm and even lower at lower P0 2 and at lower temperatures.47 The maximum ionic conductivity at 1000°C is estimated to be ca. 7-102 S/cm for CeO, 9 . The total conductivity for this condition is 2.5 S/cm. The n-type conductivity takes place by small polaron transport.64'67 A small polaron is a defect created when an electronic carrier becomes trapped at a given site as a consequence of the displacement of adjacent atoms or ions in the crystal lattice. The entire defect, ie the electron (or electron hole) plus distortion, migrates by a thermally activated hopping. The small polaron in Ce02.x may thus be identified as Ce'Ce. The electronic conductivity, c e , may be expressed by: o e = [Ce'Ce]eix
(15.19)
Ceria-based
467
electrodes
where [Ce'Ce] is the concentration of Ce3+-ions/cm3, e is the charge of the electron (in As) and (X is the mobility in (cm2/Vs). The mobility has the form H = (B/T)exp(-EH/kT)
(15.20)
where EH is the activation energy for the small polaron hopping and B is a constant. This implies that c e = (A/T)exp(-EH/kT)
190 y in C«Oy
(15.21)
iao
Figure 15.8. Conductivity at 1000°C as a function of nonstiochiometry. Data are from Tuller and Nowick (O) with earlier data from Bumenthal et al.5 (x). After Tuller and Nowick66.
Fig. 15.8 shows an example of the conductivity, o, as a function of y =2 - x at 1000°C. It shows that even though o e is known to be proportional to x in the vicinity of stoichiometry (up to x = 10"2) a saturation already takes place at x = 0.05 and ae goes through a broad maximum at about x = 0.1. Also the activation energy vary with x from about 0.2 eV near stoichiometry68 and increasing to just above 0.5 eV at a vacancy concentration of x + y = 0.25. The grain boundaries do not form any barrier to the electron migration.69 Results on nano-crystalline ceria37,38 have indicated that the grain "boundary phase" is more easily reduced than the bulk of the crystal. The activation energy of the electron hopping seems to be similar to that of the bulk material.
468
Catalysis by ceria and related materials
Electron hole (p-type) conductivity have been reported in ceria at P0 2 around 1 atm in the temperature range of 600 - 800°C.70 This was envisaged to be caused by vacancies induced by lower valent impurities according to the reaction V" 0 + 1/2 02(g) = 0 0 + 2h'
(15.22)
An electron hole mobility of approximately 10"5 cm2/Vs found at 800°C in the high oxygen partial pressure region may be compared to an electron mobility of 6.1xl0"3 cnvYVs in the low oxygen partial pressure region.19 This means that only by using very special conditions and extremely careful measurements, electron hole conductivity may be observed in pure ceria even at high oxygen pressure.
15.3.2. Ionic Conductivity Whereas the ionic conductivity is always much lower than the electronic conductivity in pure reduced ceria, the situation is quite different in ceria doped with oxides of two- or three-valent metals due to the introduction of oxide ion vacancies, cf eqs. 15.2 and 15.3. A high vacancy concentration will shift eq. 15.1 to the left. This means that the ionic domain extended down to 10"13 atm or even lower in the temperature range of 600 - 1000°C.71"74 The electronic conductivity in air may be very low, and the doped cerias are under these conditions excellent electrolytes. The conductivity mechanism is the hopping of oxide ions to vacant sites, and the ionic conductivity, a{, may be expressed as (Tj = (a0 /T)exp(-EH/kT)
(15.23)
It has been found that the total conductivity consists of a contribution from the bulk of the grains (crystals or crystallites) and one from the grain boundaries, which are often very badly conducting. The two contributions are in series, i.e. a low grain boundary conductivity results in a low total conductivity. The grain boundary resistance may occur due to an amorphous glassy phase in the grain boundaries caused by impurities, micro-porosity in the boundaries or segregation of the dopant ions.75'76 The total conductivity may be separated in bulk and grain boundary contributions.77 Even in very pure material the intrinsic limit of the grain boundaries remain 100 - 1000 times lower than the bulk material.38,78 It was also found that very small grains might affect the bulk conductivity adversely because the dopant is
Ceria-based electrodes
469
extracted to the grain boundaries to an extent that the grains become practically undoped. In materials with grain sizes of a micron and above the grain boundary phase constitutes a very tiny fraction of the material, and therefore the total conductivity is not necessarily affected significantly. It is possible to fabricate dense samples with very low contribution from the grain boundaries to the total resistivity.79 A large number of studies of the ionic conductivity of ceria doped with alkaline and rare earth metal oxides have been reported.80"94 Maximum ionic conductivities in the range of 0.1 - 0.2 S/cm at 1000°C and the activation energies of 0.6 - 1.0 eV were reported. A maximum in ionic conductivity was consistently observed for a nominal oxide vacancy concentration of 1 - 3%. The maximum conductivities as a function of lattice parameter (or dopant type) at a given temperature in the range of 800 - 1000°C varied within a factor of about 3. For a given dopant type the variation in the activation energy with concentration was followed by a corresponding variation in the values of o 0 (at least in the intermediate temperature range), which more or less compensated the changes in activation energy, and hence the magnitudes of the conductivities were similar at certain temperatures. 1
'
I
'
I
'
1
'
I Z
3 2 I O
T 2 3 4 5
6 ~" 10
14 18 IOVT [K"1]
22
Figure 15.9. Ionic conductivity of Ce(Gd)02.y. Dopant concentrations as indicated (xlOO). After Hohnke.92
Figs. 15.9 and 15.10 shows examples for Gd 2 0 3 doped ceria. Fig. 15.11 shows the ionic conductivity of (Ce02)o.8(LnO, 5)0.2 as a function of radius of dopant ion. The ionic conductivity increases with increasing ionic radius, from Yb to Sm, but decreased at rd0Dant > 0.109nm.
470
Catalysis by ceria and related materials
*
124.5 _ 115.8
-
106.2 96.5
-
86.9 77.2 67.6 57.9
\
t
f 1
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9 l
1
*•
]
0.02 0.04 0.06 0.08 0.10 ANION VACANCY CONCENTRATION Figure 15.10. Activation enthalpies for Ce(Gd)02.,, • (low temp.) • (high temp.). After Hohnke.9:
_.,_, T .—-g.,,,
i
2.0
r
• Sm
-
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£ u in 1.8 • Yb
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0.10 0.11 0.12 Radius of dopant cation /nm Figure 15.11. Dependence of ionic conductivity for (Ce02)o8(Ln01.5)o.2 at 800°C on a radius of dopant cation. After Eguchi et al.53
Based on a number of studies it seems clear that the most important parameter for oxide ion conductivity in fluorites is the cation match with the critical radius, rc, as defined in sub-section 15.3.2.3.95"97 This means that the highest conductivity is obtained when the ionic radius of the dopant is as close to rc as possible. However, according to Steele34,98 the highest ionic single crystal (bulk grain) conductivity of
Ceria-based electrodes
All
the doped cerias is exhibited by the Ce08Gd0.2O19 in contradiction with the Fig. 15.11 data. Comparison of recent data99"102 indicates that the reason for this might be that doping with samaria for some reason tends to induce a lower grain boundary resistance.
15.4. Ceria Based Fuel Electrodes for SOFC A simple electrode design would appear to involve only a single material, but it is very difficult to find one material, which will fulfil all the requirements of an electrode.103 For instance, the requirement for a low electrode polarisation resistance points to the use of a mixed conductor with electronic and ionic conductivity in order to get as high active electrode area as possible as already mentioned and illustrated in Fig. 15.1. Takahashi et al.104 showed that doped ceria is an excellent hydrogen electrode at 1000°C and explained this to be due to the mixed ionic and electronic conductivity of reduced ceria. However, the data presented in the previous sections indicate that there may be problems in using ceria as an anode material. In the temperature range of 700 - 1000°C ceria undergoes a change of volume when the oxygen partial pressure, P0 2 , is varied from air (usual sintering atmosphere) to that of the operating SOFC anode. Furthermore, the electronic conductivity of doped ceria is not sufficient to take care of the current collection in an SOFC stack, and the sintering of a doped ceria anode on a YSZ electrolyte involves formation of a reaction (diffusion) zone with limited oxide ion conductivity due to the radii misfit of Ce*4 and Zr"4.105106 The ceria expansion and reaction problems may be minimised by a composite solution where YSZ particles protrude into the ceria layer at short distances for anchoring, providing sufficient adhesion at low sintering temperature.107 Sintering of the ceria on YSZ at temperatures below about 1200°C prevents any significant diffusion of ceria and YSZ into each other. To limit the mechanical implications of the volume instability during redoxing, the ceria layer must be thin. This makes the problem of the low electronic conductivity even worse, and therefore, it is necessary to add a current collection layer of another material with high electronic conductivity.103 A sketch of this kind of advanced composite is shown in Fig 15.12. It appears that instead of a simple electrode a complicated one emerged, and a suitable (cheap and redox stable) current collector material has not yet been demonstrated. Nevertheless, ceria based anodes have important advantages over conventional Ni-based anodes, namely the ability to endure repetitive oxidation and reduction (redoxing) and the ability to avoid (or tolerate) carbon deposition from
472
Catalysis by ceria and related materials
hydrocarbon fuels. These properties strongly encourage continued efforts of developing this electrode.
Figure 15.12. Electrode structure with two elements of composite structure. 1) The electrode adhesion on the interface towards the dense electrolyte is improved by a physical anchoring (YSZ-scales), and 2) the electrode functions are divided on two layers taking care of the electrochemical oxidation of hydrogen (ceria) and current collection, respectively.
15.4,1. Hydrogen/Ceria Electrodes Back in 1964, Mobius and Rohland108 showed that ceria could work as an anode in an SOFC fuelled with H2 and CO, and shortly after Takahashi et al.104 described Ce 06 Y 04 O 18 a n d Ceo^Lao^C1! 8 as being excellent anode materials for SOFC. Since then, a number of studies have been carried out.109"114 Cells with a ceria anode may have a performance similar to cells with Ni-YSZ- electrodes in hydrogen at 1000°C. Ceria has also been used as the ceramic part in nickel - or ruthenium - cermet anodes for hydrogen oxidation.115"122 Beneficial effects have been reported and interpreted as being probably due to the broadening of the three-phase boundary zone width. At temperatures below 1000°C the doped ceria has a considerable larger polarisation resistance, Rp, even in case of cermet with nickel.123 (The reaction rate is proportional to 1/Rp). At the lower temperatures the reaction may be accelerated by addition of fine Ni particle to the surface of the doped ceria. Impedance spectra at 850°C of Ni/Ce09Gd0jO, 95 cermet electrodes with and without small amounts of fine Ni particles (probably of nanometer size) are given in Fig. 15.13. The impedance is significantly affected. The dominant low frequency arc (fSuramit = 1 Hz)
Ceria-based
473
electrodes
of the spectrum is reduced by a factor of 3 by addition of 3 w/o Ni. A switch of the fuel from hydrogen with 3% water to deuterium with 3% heavy water affected the polarisation resistance. The low frequency part of the electrode impedance was increased about 30%. This indicates that hydrogen uptake by the surface is a major rate limiting step for SOFC anodes based on Gd-doped Ce0 2 in H2/H20 mixtures at 850°C. This is in good agreement with the catalytic literature about ceria,124 in which studies of the rate of ceria reduction by hydrogen has been reported to increase dramatic by addition of only a few percent of noble metals. With other words: the ceria itself has a relatively low electrocatalytic activity for hydrogen oxidation at temperatures below about 1000°C.
0.2CN
£
• •
T
Ni/CG1 Ni/CG1 + 3% Ni
1
r
~i
•
1
r_
5 Hz
-0.1 -
N o.o
0.0
0.1
0.2
—I—
—i—
0.3
0.4
0.5
0.6
Z'./Qcm2
Figure 15.13. Impedance spectra for Ni/Ce09Gd0 [0, 9 5 (CGI) cermet electrodes on YSZ foils at 850°C in H2 + 3% H 2 0 after correction for active electrode area, as sintered, and after addition of 3 w/o fine Ni.
15.4.2. Oxidation of Hydrocarbons on Ceria Based Electrodes Work with the purpose of using ceria based materials as an anode for direct oxidation (i.e. without steam reforming) of methane and other hydrocarbons have been performed by several authors.109"114 In methane good performances were reported at 1000°C when Pt or Ni metals were present. Originally, it was believed that reduced ceria was a good electrocatalyst for direct methane oxidation.109,111 This was based on the observation that in blind tests with the Pt current collector directly on the YSZ electrolyte, a very high polarisation resistance was obtained, and thus it was concluded that Pt was almost inert for the reactions in question as also reported by Steele et al.125 The
474
Catalysis by ceria and related materials
tricky point is, however, that platinum is a good methane cracking catalyst. Thus, it was later shown126 that if an inert gold current collector is used, then the ceria-based anode is equally good in hydrogen at 1000°C but almost inert in methane, and hence a suitable cracking catalyst must be present. This is illustrated in Fig. 15.14, which shows that at the reducing conditions necessary for an SOFC anode only a limiting current density of about 0.1 A/cm2 (oxidation of H2 from simple cracking of CH4) can be obtained at relevant electrode potentials at 1000°C. The current density does not increase significantly until the polarisation reaches the oxygen evolution regime. This finding is supported by measurements of bond breaking rates of CH4 on Ce09Gd0 ,Oi.95 in the temperature range of 700 - 900°C.127128 If these data are extrapolated to 1000°C a turnover frequency of 5 CH4 molecules per site per min is obtained corresponding to a current density of 30 p.A/cm2. This is insignificant compared to the ca. 100 mA/cm2 due to the oxidation of hydrogen formed by cracking on the walls of the electrode measurement system as shown in Fig. 15.14. In spite of this, we believe that there is a real potential in ceria as an anode for conversion of hydrocarbon fuels, because ceria can tolerate carbon precipitation and is able to oxidise the carbon. In this context it should be remembered that one of the oldest applications of ceria has been as a carbon oxidation catalyst, and still today it is used as a catalyst in self-cleaning ovens and for the oxidation of diesel soot in automobiles.129 Recently, sensational papers about direct oxidation of methane and hydrocarbon in solid oxide fuel cells (SOFC) at relative low temperatures about 700°C were published.130"133 Even though the conversion of almost dry CH4 on ceramic anodes were demonstrated more than 10 years ago109,111,125 at 1000°C, the reports about high current densities for methane oxidation at such low temperatures are indeed surprising. A power density of 0.37 W/cm2 at 650°C for an SOFC using a 2 |am NiYSZ cermet anode on top of a 0.5 |0.m functional layer of (Y2O3)015(Ce02)o.85 (YDC) on a 8 p.m YSZ electrolyte.131 It was also shown that the polarisation resistance of the YSZ cermet without the YDC-layer was about 6 times higher. The result was interpreted as a direct electrochemical oxidation of CH4 facilitated by the YDC. This interpretation is in strong contrast to the findings that doped ceria in itself is about inert to direct oxidation of CH4126, and that Ni-YSZ anodes will be quickly destroyed by carbon precipitation from CH4 cracking.109134 Therefore, these results should be taken with some reluctance until they have been reproduced under condition where it is clear that no oxygen is leaking through the very thin electrolyte
Ceria-based
475
electrodes
or the seals. If a small amount of oxygen is leaking into the anode compartment then hydrogen will be produced by partial oxidation of the methane. Potential vs. Pt/air (V) -1 0,4
r—'
'a
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"S
0 r
!-
s*"
-01 -0,2
-0,8 1
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1
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1
0
0,2
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1
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1
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1
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.
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0,8
1
1,2
Overpotential (V) Figure 15.14. Current density versus overpotential and electrode potential against a Pt/air reference for a Ceo.6Gdo.4O1.8 (CG4) electrode on an 8YSZ electrolyte. The current collector was of Au mixed with CG4 to assure sufficient porosity. Partial pressures pCH4 = 9kPa, pH 2 0 = 3kPa, bal. N2, 1000°C.126
Other workers report that the reactivity of CH4 and higher hydrocarbons on Cu -YSZ anodes is relativity low, but the addition of ceria is claimed to increases the reactivity substantially.132134135 Again, the interpretation is in contrast to the above findings, and in a strict sense any appreciable direct electrochemical oxidation of CH4 on reduced ceria seems unlikely. The reported high reactivities are rather based on a cracking of the CH4 on the metals followed by electrochemical oxidation of hydrogen and maybe also of carbon. The fact that ceria is very resistant to carbon precipitation is probably also of great importance for keeping a high reactivity of the anode. However, much more work is needed and is under way to through light on the actual electrode mechanism of hydrocarbon oxidation on ceriacopper-cermet anodes.
476
Catalysis by ceria and related materials
15.5. References 1. Minh, N. Q. and Takahashi, T., Science and Technology of Ceramic Fuel Cell Cells (Elsevier, 1995). 2. Baur, E. und Preis, H., Zeitschrift fur Elektrochemie, 43 (1937) 727. 3. Kofstad, P., and Hed, A. Z., J. Am. Ceram. Soc. 50 (1967) 681. 4. Kevane, C. J., Phys. Rev., 133 (1964) A1431. 5. Blumenthal, R.N., Lee, P.W., and Panlener, R.J., J. Electrochem. Soc, 118 (1971) 123. 6. Iwasaka, B., and Katsura, T., Bull. Chem. Soc. Japan, 44 (1971) 1297. 7. Steele, B.C.H., and Floyd, J.M., Proc. Brit. Ceram. Soc, 19 (1971) 55. 8. Vinokurov, I.V., Neorganischekie Material, 6 (1970) 31. 9 Vinokurov, I.V., and V.A. Ioffe, Soviet. Physics-Solid State, 11 (1969) 207. 10. Faber, J., Seitz, M.A., Mueller, M.H., J.Phys. Chem. Solids, 37 (1975) 903. 11. Noddack W., and Walch, H., Z. Physik. Chem., 211 (1959) 194. 12. Greener E.H., Wimmer, J.M., and Hirthe, W.M., Rare Earth Research III, Edited by Karl S. Vorres (Gordan and Breach, Inc., New York) (1964). 13. Blumenthal, R.N. and Hofmaier; R.L., J. Electrochem. Soc, 121 (1974) 126. 14. Blumenthal, R.N., and Sharma, R.K., J. Solid State Chem., 13 (1975) 360. 15. Panlener R.J., Blumenthal R.N., and Gamier J.E., J. Phys. Chem. Solids., 36 (1975) 1213. 16. Toft S0rensen, O., J. Solid State Chem., 18 (1976) 217. 17. Campserveux J., and Gerdanian D., J. Solid State Chem., 23 (1978) 73. 18. Wang, D.Y., Park, D.A., Griffith, J. and Nowick, A.S., Solid state Ionics, 2 (1981)251. 19. Panhas, M.A., and Blumenthal, R.N., Solid State Ionics, 60 (1993) 279. 20. Dawicke, J.W., and Blumenthal, R.N., J. Electrochem. Soc, 133 (1986) 905. 21. Chang, E.K., and Blumenthal, R.N., J. Solid State Chemsitry, 72 (1988) 330. 22. Riess, I., Janczikowski, H., and Nolting, J., J. Appl. Phys., 61 (1987) 4931. 23. Bevan, D.J.M., and Kordis, J., /. Inorg. Nucl. Chem., 26 (1964) 1509. 24. Tuller, H.L., and Nowick, A.S., J. Electrochem. Soc, 126 (1979) 209. 25. Gamier, J.E., Blumenthal, R.N., Panlener, R.J., and Sharma, R.K., J. Phys. Chem. Solids., 37 (1976) 369. 26. Park, J.H., Blumenthal, R.N. and Panhans, M.A., J.Electrochem. Soc. 135 (1988) 855. 27. Zachau-Christiansen, B., Jacobsen, T., West, K., and Skaarup, S., in:Proc 3rd Int. Symp. on SOFC,eds. Singhal, S.C. and Iwahara, H., Electrochemical Soc. Proc. Vol. 93-5(1993)p.l04.
Ceria-based electrodes
All
28. Zachau-Christiansen, B., Jacobsen T., and Skaarup, S., Solid State Ionics, 86 88 (1996) 725. 29. Schneider, D., Godickemeier, M., and Gauckler, L., J. Electroceramics, 1 (1997) 165. 30. Wang, S., Inaba, H., Tagawa H., and Hashimoto, T., J. Electrochem. Soc, 144 (1997) 4076. 31. Wang, S., Inaba, H., Tagawa, H., Dokiya, M. and Hashimoto,T., Solid State Ionics, 107 (1998) 73. 32. Yasuda, I. and Hishinuma, M., in: Ionic and Mixed Conducting Ceramics III. Eds. Ramanarayanan, T.A., Worrell, W., Tuller, H.L., Khandkar, A.C., Mogensen, M., Gopel, W., Electrochem. Soc. Proc. Vol. 97 - 24, 1998, p. 178. 33. Mogensen, M., Sammes, N.M. and Tompsett, G.A., Solid State Ionics, 129 (2000) 63. 34. Steele, B.C.H., Solid State Ionics, 129 (2000) 95. 35. Yokokawa, H., J. Phase Equilibria, 20 (1999) 258. 36. Yokokawa, H., et al., Solid Oxide Fuel Cells VII, Proc. vol. 2001-16, The Electrochem. Soc, NJ, (2001) p. 339. 37. Chiang, Y.-M., Lavik, E.B., Kosacki, I., Tuller, H.L., and Ying, J.M., Appl. Phys. Letter, 69 (1996) 185. 38. Chiang, Y.-M., Lavik, E.B., and Blom, D.A., NanoStructured Materials, 9 (1997) 633. 39. Ramanarayanan, T.A., Ling, S., Anderson, M.P., and Ozekcin, A., in 14th Ris0 International Symposium on Materials Science, Eds. F.W. Poulsen et al., Ris0 National Lab., Denmark (1993) p. 381. 40. JCPDS #43-1002. 41. Chiang, H.-W., Blumenthal, R.N., and Fournelle, R.A., Solid State Ionics, 66 (1993) 85. 42. Brauer, G. and Gingerich, K.A., J. Inorg. Nucl. Chem., 16 (1960) 87-99. 43. Sims, J.R. and Blumenthal, R., High Temp. Sci., 8 (1976) 99. 44. Korner, R., Ricken, M., Notling, J., and I. Riess, J. Solid State Chem., 78 (1989) 136. 45. Wilfong R.L., Dominiques, L.P., Furlong L.R.R. and Findlayson, J.A., U.S. Bur. Mines Report, Invest. No.6180 (as quoted by Korner et al.). 46. Schwab, R.G., Steiner, R.A., Mages, G. and Beie, H.-J., Thin Solid Films, 207 (1992) 288. 47. Mogensen, M., Lindegaard, T., Hansen, U.R. and Mogensen, G., J.Electrochem. Soc, 141 (1994) 2122.
478
48. 49. 50. 51. 52.
53. 54. 55. 56.
57. 58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69. 70. 71.
Catalysis by ceria and related materials
Kim, D.-J., /. Am. Ceram. Soc, 72 (1989) 1415. Shannon, R.D. Acta Cryst. A32 (1976) 751. Hong, S.J. and Virkar, A.V., J. Am. Ceram. Soc, 78 (1995) 433. Etsell, T.H. and Flengas, S.N., Chemical Reviews, 70 (1970) 339. Bevan, D. J.M. and Summerville, E. in Handbook on the Physics and Chemistry on Rare Earth's, Vol. 4, Eds. K.A. Gschneider and L. Eyring, North-Holland, Amsterdam, 1979. Eguchi, K., Setoguchi, T., Inoue, T.,and Arai, H. Solid State Ionics, 52 (1992) 165. Keler, E.K., Godina, N.A., and Kalinina, A.M., Zh. Neorgan. Khim., Moscow, 1 (1956) 2557. Ranl0v, J., Poulsen, F.W. and Mogensen, M., Solid State Ionics, 61 (1993) 277. Sirman, J.D., Waller, D. and Kilner, J.A., in: Solid Oxide Fuel Cells V, Eds. Slimming, U., Singhal, S.C., Tagawa, H., Lehnert, W„ Electrochem. Soc. Proc. Vol. 97 - 40, 1997, p.1159. Mbbius, H.H., Z. Chem., 4 (1964) 81 (as cited by Chiang et al.). Mogensen, G. and Mogensen, M., Thermochimica Acta, 214 (1993) 47. Ruloph, V.J., Z Naturforschung, 14a (1959) 727-737. Noddack, W., and Walch, H., Z Physik. Chem., 211 (1959) 194. Greener, E.H., Wimmer, J.M., and Hirthe, W.M., Rare Earth Research III, Edited by Karl S. Vorres (Gordan and Breach, Inc., New York) (1964). Blumenthal, R.N., and Pinz, B.A., J. Appl. Phys., 38 [5] (1967) 2376. Vinokurov, I.V., Zonn, Z.N., and loffe, V.A., Soviet Physics-Solid State 9 [12] (1968) 2659. Blumenthal, R.N., and Panlener, P.J., J. Phys. Chem. Solids, 31 (1970) 1190. Blumenthal, R.N., and Hofmaier, R.L., J. Electrochem. Soc, 121 (1974) 126. Tuller, H.L., and Nowick, A.S., /. Phys. Chem. Solids., 38 (1977) 859. Naik, I.K., and Tien, T.Y., J. Phys. Chem. Solids., 39 (1978) 311. Ananthapadmanabhan, P.V., Menon, S.B., Patil, D.S., Venkatramani, N., /. Mater. Sci. Letters, 11 (1992) 501. Brugner F.S, and Blumenthal, R.N., J. Am. Ceram. Soc-Discussions and Notes, 54 (1971) 57. VanHandel, G.J., and Blumenthal, R.N., J. Electrochem. Soc, 121 (1974) 1198. Takahashi, T., in "Physics of Electrolytes", Vol.2, Ed. J. Hladik, Acdemic Press, London (1972) p.989.
Ceria-based electrodes
479
72. Takahashi, T., Ito, K., and Iwahara, H., in Proc Journees Int. d'Etude des Piles a Combustible III, p.42 S.E.R.A.I., Bruxelles (1965). 73. Tuller, H.L., and Nowick, A.S., J. Electrochem. Soc, 122 (1975) 255. 74. Kudo, T., and Obayashi, H., J. Electrochem. Soc, 123 (1976) 415. 75. Kilner, J.A. and Steele, B.C.H., in "Non-stoichiometric oxides" Ed. O. Toft S0rensen, 1981, Academic Press, N.Y.,1981, p.233. 76. Riess, I., Braunshtein, D„ and Tannhauser, D.S., J. Am. Ceram. Soc, 64 (1981) 479. 77. Wang, DaYu, and Nowick, A.S., J. Solid State Chem., 35 (1980) 325. 78. Christie, G.M., and van Berkel, F.P.F., Solid State Ionics, 83 (1996) 17. 79. Van herle J., Horita, T., Kawada, T., Sakai, N., Yokokawa, H. and Dokiya, J. M. Eur. Ceram. Soc, 16 (1996) 961. 80. Kevane, C.J., Holverson, E.L., and Watson, R.D., J. Appl. Phys., 34 (1963) 2083. 81. Meyer, E., and Marincek, B., Schweizer Archiv., 36 (1970) 194. 82. Yushina, L.D., and Pal'guev, S.F., in Electrochemistry of Molten and Solid Electrolytes, Volll, (edited by M.V. Smirnov), Consultants Bureau, New York, (1969) p74. 83. Blumenthal, R.N., Brugner, F.S., and Garnier, J.E., J. Electrochem. Soc, 120 (1973) 1230. 84. Seitz, M.A. and Holliday, T.B., J. Electrochem. Soc, 121 (1974) 122. 85. Arai, H., Kunisaki, T., Shimizu, Y., and Seiyama, T., Solid State Ionics, 20 (1986) 241. 86. Eguchi, K., Kunisaki, T., and Arai, H., J. Am. Ceram. Soc, 69 (1986) C282. 87. Blumenthal, R.N. and Garnier, J.E., J. Solid State Chem., 16 (1976) 21. 88. Yahiro, H., Eguchi, K., and Arai, H., Solid State Ionics, 21 (1986) 37. 89. Kudo, T. and Obayashi, H., /. Electrochem. Soc, 122 (1975) 142. 90. Kudo, T., and Obayashi, H., /. Electrochem. Soc, 123 (1976) 415. 91. Dirstine, R.T., Blumenthal, R.N., and Kuech, T.F., J. Electrochem. Soc, 126 (1979) 264. 92. Hohnke, D.K., Solid State Ionics, 5 (1981) 531. 93. Yahiro, H., Eguchi, Y., Eguchi, K., and Arai, H., J. Appl. Electrochem. (1988) 527. 94. Yahiro, H., Eguchi, K., and Arai, H., Solid State Ionics, 36 (1989) 71. 95. Kilner, J.A. and Faktor, J.D., in Progress in Solid electrolytes (Eds. T.A. Wheat, A.K. Kuriakose), Publication ERP/MSL 83-94 (TR) (Energy, Mines and Resources, Ottawa, Canada, p.347, (1983).
480
Catalysis by ceria and related materials
96. Kilner, J., Solid State Ionics, 8 (1983) 201. 97. Kilner, J., and R.J. Brook, Solid state Ionics 6 (1982) 237. 98. Steele, B.C.H., in "High Conductivity Solid Ionic Conductors, Recent Trends and Applications", Ed. Takahashi, World Scientific Publishing Co., London, 1989, p.402. 99. Huang, W., Shuk, P., Greenblatt, M., Solid state Ionics, 100 (1997) 23. 100. Balazs, G.B. and Glass, R.S., Solid State Ionics, 76 (1995) 155. 101. Steele, B.C.H., Zheng, K., Rudkin R.A., Kiratzis, N. and Cristie M., in Electrochem. Soc. Proc. vol 95-1, Eds.M. Dokiya, O. Yamamoto, T. Tagawa, S.C. Sinhal, (1995) pl028. 102. Ralph, J.M., Przydatek J., Kilner J.A. and Seuelong T., Ber. Bunsenges.Phys. Chem., 101 (1997) 1403. 103. Mogensen, M., Primdahl, S., J0rgensen, M. J., Bagger, C , J. Electroceramics, 5 (2000) 141 104. Takahashi, T., Iwahara, H., and Suzuki, Y., in Proc. Third International Symposium on Fuel Cells, Bruxelles 16-20 - VI (Presses Academiques Europeennes, Bruxelles B 1969) p. 113. 105. Bentzen, J.J., and Schwartzback, H., Solid State Ionics, 40/41, (1990)942. 106. Clausen, C , "Electron Microscopical Characterisation of Interfaces in SOFC Materials", PhD-thesis, Ris0-R-626(EN) (1992). 107. Marina, O., Bagger, C , Primdahl, S., and Mogensen, M., Solid State Ionics, 123, (1999)199. 108. Mobius, H.H. and Rohland, B., US Patent no. 3,377,203, April 9, 1968, filed Nov. 18, 1964. 109. Mogensen, M. and Bentzen, J.J., in Proc. 1st Internat. Symp. on SOFC, Ed. S.C. Singhal, Electrocem. Soc. Proceedings Vol. 8 9 - 1 1 (1989), p.99. 110. Steele, B.C.H., Middleton, P.H., and Rudkin, R.A., Solid State Ionics, 40-41 (1990) 388. 111. Mogensen M., Kindl B. and Malmgreen-Hansen B., in Program and Abstracts of 1990 Fuel Cell Seminar, Courtesy Associates, Inc., Washington, D.C., 1990, p.195. 112. Putna, E.S., Stubenrauch, J., Vohs, J.M. and Gorte, R.J., Langmuir, 11 (1995) 4832. 113. Marina, O.A., Bagger, C , Primdahl, S.and Mogensen, M., in Proc. 3rd Europ. SOFC Forum, Oral Presentations Vol., Ed. P. Stevens, European Fuel Cell Forum, Oberrohrdorf, Switzerland, 1998, p.427.
Ceria-based electrodes
481
114. Marina, O.A., Bagger C , Primdahl, S. and Mogensen, M., Solid State Ionics, 123 (1999) 199. 115. Schmidberger, R., Deutsches Patentamt, 2747467,1979. 116. Eguchi, K., Setoguchi, T., Okamoto, K.and Arai, H., in Proc. Internal Fuel Cell conference, Makuhari, Japan, 1992, IV - F - 4. 117. Watanabe, M., Uchida, H. and Yoshida, M., J. Electrochem. Soc, 144 (1997) 1739. 118. Uchida, H., Mochizuki, N. and Watanabe M., J. Electrochem. Soc, 143 (1996) 1700. 119. Uchida, H., Suzuki, H. and Watanabe, M., J. Electrochem. Soc, 145 (1998) 615. 120. Tsai, T. and Barnett, S.A., J. Electrochem. Soc. 145 (1998) 1696. 121. Schouler, E.J. and Kleitz, M., J.Electrochem. Soc, 134 (1987) 1045. 122. Horita, T., Yamaji, K., Sakai, N., Ishikawa, M., Yokokawa, H., Kawada, T. and Dokiya, M., Electrochem. and Solid State Letters, 1 (1998) 4. 123. Primdahl, S., Mogensen M., Solid State Ionics, submitted (2001). 124. Trovarelli A., Catal. Rev. - Sci. £ng.,38 (1996) 439. 125. Steele, B.C.H., Kelly, I., Middleton, H. and Rudkin, R., Solid State Ionics, 28 30 (1988) 1547. 126. Marina, O., Mogensen M., Appl. Catal. A, 189, (1999)117. 127. Ramirez-Cabrera E., Atkinson, A. and Chadwick D., in Proc. 4th European SOFC Forum, Ed. A. J. McEvoy, p. 49 (2000). 128. Ramirez-Cabrera E., Atkinson, A. and Chadwick, D., in Solid Oxide Fuel C VII, Eds. Yokokawa and Singhal, p.703 (2001). 129. Kilbourn, B. T., "Cerium, A guide to its role in chemical technology", Molycorp, Inc., N.Y., USA, 1992. 130. Perry Murray, E., Tsai, T., Barnett, S.A., Nature, 400, (1999)649. 131. Perry Murray, E., Barnett, S.A., in Solid Oxide Fuel Cells VI, S.C. Singhal and Dokiya M., Editors, PV 99-19, The Electrochemical Society Proceedings Series, Pennington, NJ (1999)p. 1001. 132. Park, S., Vohs, J.M., Gorte, R.J., Nature, 404, (2000)265. 133. Xia,C, Chen, F. and Liu, M., Electrochem. and Solid-State Lett., 4 (2001), 152. 134. Park, S., Craciun, R., Vohs, J.M. and Gorte, J., J. Electrochem. Soc, 146 (1999), 3603. 135. Gorte, R.J., Park, S., Vohs, J.M. and Wang, C , Adv. Mater., 12, (2000)1465.
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CHAPTER 16 THE USE OF CERIA IN FCC, DEHYDROGENATION AND OTHER CATALYTIC APPLICATIONS
MARTA BOARO, ALESSANDRO TROVARELLI CARLA de LEITENBURG and GIULIANO DOLCETTI Dipartimento di Scienze e Tecnologie Chimiche Via Cotonificio 108, 33100 Udine, Italy.
16.1.
Introduction
The main use of Ce0 2 and ceria based compounds is in depollution of noxious species from gaseous stream originating either from stationary or mobile source. The majority of studies deals with the application of ceria as an oxygen storage in TWCs and in the development of novel technologies for the treatment of emissions from diesel and spark-ignited internal combustion engines [1,2]. In these applications ceria plays a variety of catalytic roles [3], but one of its primary functions is to enhance the oxidation activity of noble metal-based emissions control catalysts [3,4]. In fact cerium has multiple stable oxidation states (Ce3+/Ce4+) and mediates the amount of available oxygen in the catalytic converter, releasing oxygen under reducing conditions and taking up oxygen under oxidizing conditions. Several studies have appeared in the last decade on this matter and the main findings have been summarised in previous chapters. All this "popularity" has stimulated the study of ceria-based materials for several other applications. In the present chapter, the main industrial processes and some potential applications where Ce0 2 is a key ingredient in catalyst formulation are reviewed. In particular, we will deal with well established industrial processes such as fluid catalytic cracking (FCC) and ethylbenzene dehydrogenation [5], as well with emerging processes such as the applications of ceria in desulfurization processes and in redox reactions for syn-gas production. Our description, which does not claim to be exhaustive, will focus mainly on the assessment of the role of ceria in these applications and in the identification of some common points that link the various utilizations. Although oxygen storage is a key feature of the auto exhaust 483
484
Catalysis by ceria and related materials
treatment catalysts, it is an important function also in other applications, and this will be emphasized throughout. In all these applications and related studies, Ce0 2 is seldom used alone but it often employed in combination with other oxides or in conjunction with active metals (nobles metals or Ni, Ag, Cu) and thermally stable supports. Therefore strictly speaking, ceria may function either as structural/electronic promoter or as cocatalyst, depending on the type of application, but not as a true catalyst, although the amount of cerium oxide which is used in certain catalyst formulations may easily exceed 20-30 wt. %.
16.2. Treatment of SOx Air pollution arising from the emissions of sulfur and nitrogen oxides as a results of combustion taking place in boilers, furnaces and engines, has increasingly been recognized as a problem. There is a number of stack gas desulfurization processes being used commercially and all of these methods produce waste material that must be collected, stored and disposed. New and less costly methods to significantly remove SOx emissions have been recently investigated and the use of ceria as sorbent or catalyst additive seems to be promising for the development of new solutions to de-SOx technologies [6,7]. Ceria is in fact an important component in the formulation of desulfurization catalysts and sulfur sorbing material because it reacts quite easily with sulfur compounds (S0 2 , H2S, COS, etc., see Chapter 11). According to the operating conditions, the sulfation of ceria causes the formation of sulfates, sulfites, oxysulfides, sulfides. The nature of the formed compounds depends on the state of oxidation of ceria and on the nature of sulfurizing agents [8]. However, generally speaking, the sulfation of ceria is thermodynamically but not kinetically favored; the presence of metals (such as Ni and Cu) increases the rate of reaction [6,7] and what actually makes interesting the use of ceria in the desulfurization processes is the easy regeneration, and therefore, a profitable recovering of sulfur. This ability to store and release sulfur compounds, is related to the redox properties of the oxide. For example in the processes involving S0 2 , ceria oxidizes S0 2 to S0 3 and the latter forms sulfate and sulfite species with the reduced oxide. Other oxides present in the catalyst or in the adsorbent formulation or even as support could participate to the sulfation process [9,10]. The direct reaction between S0 2 and hydrocarbons or CO to give sulfur catalyzed by ceria is also of potential interest in this regard [7,11,12].
The use of ceria in other catalytic applications
485
16.2.1. Fluid Catalytic Cracking (FCC) Catalytic cracking is probably one of the most important catalytic processes economically. It enables the transformation of the heavy hydrocarbons to the desired gasoline-range hydrocarbons. The typical cracking catalyst is a mixture of zeolite and Si02-A102, which is most commonly prepared by mixing a silica solution, as a binder, with zeolite and alumina. Since after a contact time of only a few seconds in the cracking section the catalyst is largely deactivated by coke, at a level of 0.8-1 wt. %, the catalyst is routed to a regenerator, where the coke is burned off to a level of less than 0.1%. The temperature in the catalyst regeneration section of the plant is in excess of 973 K whereas the cracker operates at around 820K [13]. Increasingly restrictive emissions standards, together with the incorporation of heavier FCC feeds with higher sulfur content have required a higher control of SOx emissions from catalytic crackers. In this process nearly 40% of incoming sulfur remains in the liquid products and 50% is converted to H2S which is then treated downstream in a Claus plant. The rest (<10%; the amount is highly dependent on feedstock compositions) remains trapped in the coke which builds up on the catalyst. It is this sulfur that must be oxidized to S0 2 /S0 3 in the catalyst regeneration step and that needs to be treated before release into the atmosphere. De-sulfurization at this stage can be done by conventional flue gas desulfurization, however, a highly effective and less costly approach involves incorporation into the catalyst of an SOx adsorption/reduction additive. The function of this additive is to transform SOx back to H2S which can then be treated directly in the Claus plant. One commercial catalytic system is composed of a ceria containing magnesium aluminate spinel-MgO solid solution of the type Ce02/MgAl205-MgO [9]. Several modification of the basic system have recently appeared containing V and Fe either deposited or incorporated in the solid solution [14,15]. Such type of catalyst has basic sites for adsorption of S0 2 /S0 3 , active centers for oxidation of S0 2 to S 0 3 and redox properties for the reductive decomposition of sulfates to H2S under reducing condition. A possible scheme for the reaction is depicted in Fig. 16.1. The role of ceria in this catalyst formulation derives from its basic/redox character. Its reducibility facilitates the oxidation of S0 2 to S0 3 under FCC regeneration condition by reacting with S0 2 to give sub-stoichiometric cerium oxide, which is then reoxidized by oxygen. The S0 3 is then adsorbed as Ce2(S04)3 and Mg(SO)4. The best catalyst support for S0 3 adsorption is a solid solution of MgO and the aluminate spinel MgAl 2 0 5 [9]. This has sufficient S 0 3 adsorption capacity and facilitates removal of sulfates.
486
Catalysis by ceria and related
materials
Ce02/Mg2Al^)5 MgO =
CeQ2
MgO =
Ce0 2
S02
Figure 16.1. A mechanism proposed for the action of Ce0 2 -MgO based catalyst in the treatment of S0 2 in FCC plants [9]. Left: action performed by MgO: right: action performed by Ce0 2 .
Although the adsorption of S0 2 to give sulfate is a specific function carried out by MgAl205, oxidative adsorption of S0 2 on ceria with its concomitant reduction is possible, and it was confirmed by TPD and TG thermo-desorption studies [16]. This redox mechanism involves the formation of vacancies. A correlation between the oxygen vacancies formed and the de-S0 2 activity of Ce02-supported MgAl 2 0 5 spinel has been recently found. It is shown that de-SOx activity reaches a maximum as the amount of Ce0 2 loaded onto the spinel increases, and falls exactly in the region of monolayer Ce0 2 dispersion , when the cerium oxide content is in the range 7-10 wt. %. In this region of optimum activity a maximum number of oxygen vacancies was also detected [17]. Ce0 2 can also have an important role in the reduction of sulfates to give H2S. The efficiency of several metal oxides supported on Mg-La-Al-0 oxide is reported in Table 16.1. It was demonstrated that reduction to H2S of sulfates species adsorbed on ceria is fairly achieved by using propane as reducing agents. Among the materials tested, oxides with strong redox character like Ce0 2 and V2Os were the most active, both in terms of ignition temperature for H2S formation and total H2S released. It seems that the principal role of the metal oxide is to provide active hydrogen from hydrocarbons under condition typical of FCC, facilitating the C-H bond cleavage. The rate determining step is the direct reaction between S0 2 and hydrocarbons to give H2S. The use of catalyst composition similar to those of FCC has also been reported to be effective for the treatment of tail gases (S0 2 , COS, CO, and CS2) from a Claus plant [18]. In this case, tail gases are oxidatively adsorbed over the
The use of ceria in other catalytic applications
487
catalyst in one step and reductively de-sorbed with H2 to give H2S which is recycled to the main Claus plant for additional processing. Tablel6.1. Performance of supported oxides in the reduction of sulfated samples by propane-TPR [14]. Catalyst
Loading (wt.%)
Support
-
v2o5
2.49 2.14 2.13 4.22
Cr 2 0 3 Fe 2 0 3 CeO,
Surface Area (m2/g) 151 167 193 186 186
Ignition Temp. (K) 873 733 853 793 773
total H2S released (limoles) 118 235 212 171 229
Under FCC conditions, ceria is also able to reduce NOx emission originating from cracking unit [19]. Here the role of ceria is to provide oxygen vacancies for the reduction of NO to N2. Ceria containing material can be reduced by H2 or hydrocarbons in the riser reactor and in the regenerator they serve as reducing agents for NO. A substantial amount of reducing agent is also required in the regenerator to maintain the redox process of cerium between Ce3+ and Ce4+. CO originating from incomplete combustion of coke can be used for this purpose and a direct inverse relationship exist between CO concentration in the regenerator and NO emitted by the cracking unit.
16.2.2. de-SOxde-NOx Processes The redox properties of ceria and its ability to form sulfur compounds in the presence of S0 2 or H2S has concurred to its use in developing sorbents and catalysts for desulfurization processes. For instance, Zeng et al. [6] studied the use of ceriumbased sorbents in the integrated gasification combined cycle for electric power generation. A H2S concentration lower than 100 ppm is required in this process. A two stage desulfurization process using Ce0 2 for bulk H2S removal followed by zinc adsorbents polishing step has been proposed and it was compared with the singlestage desulfurization process using conventional zinc-based adsorbents. The primary advantage of the cerium-based process is the potential to produce directly elemental sulfur avoiding the problem of control of residual S0 2 . Details on the comparisons of the two processes are reported in [6]. In the cerium-based process the main reaction are sulfidation (eq. 16.1) and regeneration of Ce0 2 with S0 2 to give direct production of elemental sulfur (eq. 16.2):
488
Catalysis by ceria and related
2Ce0 2 (s)+H2S (g)+ H2 Ce 2 0 2 S (s) +S0 2 (g)
o
materials
<-> Ce 2 0 2 S+2H 2 0
(16.1)
2Ce0 2 (s) + S2
(16.2)
Rapid and complete regeneration is possible over the range 773-973K, and only elemental sulfur is formed. Recently, several investigators have considered cerium oxide as a possible alternative sorbent/catalyst to CuO for the simultaneous removal of SOx and NOx [20-22]. Akyurtlu et al. developed a CuO/Ce02/Al203 sorbent able to remove both S0 2 and NOx within a relatively wide range of temperature. This formulation should improve de-SOx and SCR processes widening the temperature window of operation. In fact the SCR process (high-dust configuration) is effective only in a narrow range of temperature due to side reactions such as the oxidation of NH3 and the oxidation of S0 2 to S0 3 at high temperature, and the formation ammonium sulfates at low temperature. It was pointed out that the combined system CeCyCuO on alumina has specific adsorption capacity and sulfation rate higher than that of the pure oxides. The sulfation of the support as well the specific sulfur capacity is a function of the ratio Cu/Ce. The best sorbent composition in the 723-823K temperature range is that containing copper and cerium in 1:1 molar ratio. This suggests that there is a synergism between the two oxides: the ceria improves the specific sulfur capacity of sorbent whereas CuO effects the limiting step of the sulfation of ceria, allowing its rapid sulfation even at low temperature. One probable mechanism may be the adsorption of S0 2 on the copper oxide and subsequent S0 2 oxidation to S0 3 on an adjacent ceria sites making the copper oxide sites free for further S0 2 adsorption. The synergism between ceria and the other components play a fundamental role also in the system MnOx/Ce02 for the NO adsorption/storage under oxidizing atmosphere [23,24]. Even in this case, a mixed oxide with the fluorite structure (1:1 molar Mn:Ce) shows a higher adsorption capacity than the single oxides. This composition produces a large number of adjacent sites for oxidative adsorption: the oxidation of NO to nitrite and nitrate is catalyzed by Mn, then, the N0 2 adsorbates are coordinated to a Ce4+—O2" pair in an adjacent site to produce monodentate and bidentate nitrate species. The oxygen present in the atmosphere oxidize readily the Mn sites for further adsorption. To summarize, the use of ceria with its double functionality (redox material with basic site) represents a more versatile solution suitable for various operating conditions. Concerning de-sulfurization processes, ceria is succesfully applied in the direct catalytic reduction of S0 2 to sulfur using various reductants (CO, CH4, syngas)
The use of ceria in other catalytic
489
applications
[25,26]. This technology, which has not yet been commercialized, is another promising and cost effective solution for desulfurization problems in industrial processing. Using CO as reductant, the overall reactions involved in the process are: S0 2 +2CO -» 2C0 2 + 1/x Sx
(16.3)
CO+l/x Sx -> COS
(16.4)
2COS+S0 2 -> 2C02+(3/x)Sx
(16.5)
where x=2-8 according to the reaction conditions. COS is a more toxic compound than S0 2 and its production should be minimized in a sulfur recovery process. Ceria-based catalyst containing small amount of transition metal oxides such as NiO, CuO, Co 2 0 3 , are among the most active and selective catalysts for this reaction both in dry and/or wet gas stream [25]. c o
°
Ce" O Ce" O Ce'* O Ce" Ce" O Ce" 0 Ce4* O Ce"
CO,
Ce"
SO,
S,
Ce'* 0 Ce" 0 Ce"* Ce" O Ce" O Ce" O Ce"
Figure 16.2. Redox mechanism for direct conversion of S 0 2 to sulfur [11].
Flytzani-Stephanopoulos and co-workers deeply investigated these type of catalysts under a variety of operating conditions [11,12,25]. In the temperature range of 673-773K ceria based systems show excellent redox activity, producing sulfur as a primary product, and avoiding the production of COS as intermediate. They are effective even with trace amount of S0 2 . It was reported that S0 2 reduction by CO on ceria-based catalysts follows a redox mechanism (Fig. 16.2) [11,26]. CO creates surface oxygen vacancies that are taken up by S0 2 , which is then reduced to sulfur. With methane as reductant, the presence of ceria influences negatively the S0 2 conversion due to the formation of sulfate species on the surface [12,25]. The reaction occurs only at temperature higher than 600°C after decomposition of sulfates. The presence of metal, especially Cu and Ni, significantly increases the performance of these catalysts at low temperature. This may be attributed to the promotion effect of the metal on the redox activity of ceria. Moreover the presence
490
Catalysis by ceria and related materials
of metal favors the decomposition of sulfate species and decreases the breakthrough temperature of the reaction. The addition of Cu or Ni into ceria has different effect on the sulfur selectivity of the catalyst under fuel-rich conditions. Cu promotes the complete oxidation and it is selective for S2. Conversely, on Ni/ceria catalysts, side reactions favor H2S production over elemental sulfur. It is worth to note that both the catalysts suppress carbon formation. The high carbon resistance shown by the metal/ceria based catalysts may be attributed to a higher dispersion of metals into this kind of matrices. Moreover, the high mobility of oxygen ions in ceria allows a rapid supply of oxygen to the metal interfaces speeding up the surface oxidation of carbon species and thus inhibiting deposition of carbon on the catalyst surfaces [25,26].
16.3. Ethylbenzene
Dehydrogenation
Ceria is added as a promoters in the K/Fe-based catalyst used for the dehydrogenation of Ethylbenzene [27-29], which is the most important process for the production of styrene (20.000.000 tons/y) [30]. The reaction (eq. 16.6) is highly endothermic (AH= 124.9 KJ/mol"1) and thermodinamically limited: C6H5-C2H5 <-> C6H5-C2H3 + H2
(16.6)
Therefore the commercial process is a two step process: conversions not higher than 50-70 % are obtained using two reactors in series that operate at a temperature ranging from 540 to 640°C and pressure from sub atmospheric to 2 atm. The selectivity to styrene is higher than 90%, but a large quantity of high temperature steam (steam/EB= 4-20) is necessary as heat vector and to limit poisoning of the catalyst by coke. In the last two decades, alternative processes, such as oxidative dehydrogenation with C0 2 or 0 2 have been proposed. However, the scaling up of these processes has not shown reasonable economic vantages to change the old technology [31-34], Several studies have been also carried out to develop new and efficient catalysts, not only based on K-Fe oxides [35-37]. One of the strategy to improve the selectivity and activity of the catalyst is to add promoters such as oxides of V, Ce, W, Li, Mg, Ca, Ti, Zr, Ni, and Co. Although at present only a few basic studies have been carried out to understand the role of promoters, it has been suggested that they could influence the number, organization and nature of the reactive centers and contribute either to stabilize the active phase or to accelerate its formation under
The use of ceria in other catalytic
491
applications
reaction conditions [38]. Cerium oxide is usually added in percentages ranging from 5 to 60 wt% as carbonate, oxide, nitrate and hydroxide along with other promoters, such as NiO, CuO, CaO, Mo0 3 , Bi 2 0 3 , Ti0 2 [39-42]. A highly selective catalyst is commercialized by the Siid Chemie catalyst group containing 58 wt % Fe 2 0 3 , 23 wt. % K 2 C0 3 , 5 wt. % Ce0 2 and 2.5 wt. % Mo0 3 [43]. Concerning this composition it was observed that ceria alone improves the activity of catalyst decreasing the activation energy for the dehydrogenation, while the molibdenum oxide increases the selectivity. If added together both the selectivity and the activity were enhanced. Table 16.2 reports data taken from [44]. Ce-Mo-K-Fe catalyst shows a higher activity and selectivity than the single K-Fe oxide catalyst A synergism is suggested to occur between ceria and molibdenum oxides. Speculations on the role if ceria have been already reported [5, 44]. It was found that cerium oxide has a positive effect on the nature of the active sites. Cerium ions may enhance the polarization of the Fe-0 bond and increase the basicity of this reaction center, and in addition they may hinder the reduction of Fe 2 0 3 . Table 16.2. Performance of Ce and Mo containing catalysts in the dehydrogenation of ethylbenzene' [43]. Catalyst
S.A. (m2/g)
Rate b
selectivity to styrene
Activation Energy (Kcal/mol)
Styrene benzene toluene Fe-K 6.3 26.6 96.9 30.0 36.2 50.9 Fe-Ce-K 7.0 34.6 96.7 25.6 35.8 48.7 Fe-Mo-K 6.1 • 20.2 97.2 30.4 35.8 50.0 Fe-Mo-Ce-K 6.9 30.5 97.6 23.6 36.2 49.3 a: Reaction conditions: T=823-900K, steam/EB molar ratio=11.8, Composition: Fe2O3:ca.60-70wt.%, K 2 C0 3 : 28wt.%, Ce02:5wt.%, Mo03=2.5wt.%. b: mol g"1 min'1 * 105 measured at 853K.
In accordance with a basic mechanism, the dehydrogenation occurs by abstraction of P-hydrogen to form a n-adsorbed complex with a scheme of reaction similar to that reported for the ammoxidation of propylene to acrylonitrile [45]: R-—-H+ + O2- - > R + O H (acid-base)
(16.7)
R" + Mn+ -> R+M(n-2)+ (redox)
(16.8)
In this mechanism the reduced Fe(II) cation could be reoxidized by migration of lattice oxygen from the cerium (IV) oxide which in turn is reoxidized by water. Other mechanisms where the ceria is itself an active site have been suggested [5]. In a conventional basic mechanism not only Fe3+ but even Ce4+ could be a center of oxidation for ethylbenzene. Instead, in a concerted mechanism close to that suggested
492
Catalysis by ceria and related
materials
by Wu et al. [46], cerium is supposed to be present as Ce3+. Figure 16.3 shows a schematic representation of a mechanism in which the a-hydrogen of ethylbenzene attacks the acid site of catalyst (Fe3+) while simultaneously the fJ-hydrogen attacks the basic site (Ce3+). This model can be constructed by assuming that ironcontaining species, like a mixture of KFe0 2 , K2Fe22034, and Fe 3 0 4 , are the active sites [47,48]. This assumption has been recently objected by Kuhurs et al. [49]. They suggested that the working catalyst is not an iron-containing mixture, but carbon plus oxygen [49], A defective model mechanism was proposed where EB is adsorbed via the phenyl ring over acidic Fe sites next to a defect, while dehydrogenation occurs at the basic oxygen sites exposed to the defect. In this mechanism the active phase should be the carbon plus oxygen coming from the polymerization of styrene. It was suggested that potassium serves as catalyst for the polymerization of styrene. At the reaction temperature the polymer species decompose to a coke layer which represent the active phase. The activity is based on the equilibrium between formation and decomposition of the coke layer. In that case ceria, if present, could contribute as an oxygen storage material to keep an optimal thickness of the coke layer [50].
H -H
H
C5*
I . •. •. F e J + •. •.
Ce".
C5--
I
•. •. FeJ+'. •. • C e ^
•Fe'V.-.Ce*
Figure 16.3. A schematic representation of a mechanism for the reaction catalyzed by Ce/Fe containing catalyst.
Ce0 2 is also used as a component in new catalyst formulations for the oxidative dehydrogenation of paraffins and EB [51]. Hagemeyer et al. claimed a redox process involving the dehydrogenation of ethylbenzene by contact with a catalyst containing a reducible metal oxide (Bi203, Ce02) and Cr 2 0 3 in the absence of oxygen and simultaneous reduction of the catalyst, followed by oxidation of reduced catalyst with an oxidizing agent [52].
The use of ceria in other catalytic applications
493
16.4. Other Catalytic Reactions Even if the main use of ceria is related to develop of TWCs, ceria participates as a component in the formulation of catalyst for several reactions. A few selected examples are reported in the following sections. 16.4.1. Environmental Applications Ceria is studied as a component in the formulation of catalysts for several environmental processes. The most important are reviewed in this book (see. Chapters 10, 12, 14). The oxygen storage properties of ceria play a key role in all these applications. A catalyst formulation containing noble metals supported on allumina ceria washcoat has been developed and commercialized in California by Engelhard for the removal of volatile organic compounds which are emitted from chain driven charbroilers to reduce VOC emissions from food cooking in restaurants [53]. In this case it is believed that oxygen storage helps oxidation of heavy hydrocarbons molecules which are generated during cooking. These big molecules are deposited on ceria surface and their oxidation is facilitated by oxygen coming from ceriacontaining support. Ceria was also proposed as a component of catalysts for the removal of chlorinated hydrocarbons [54]. The process is based on the destructive adsorption of the chlorinated hydrocarbons on metal oxides [55]. It was demonstrated that CaO and MgO were able to convert CC14, CHC13 and C2C14 to C0 2 and COCl2 and the corresponding metal chlorides at temperature around 400-500°C in the absence of an oxidant, such as oxygen. Ceria has shown comparable properties; CC14 destruction started at around 450°C, and was accompanied by the reduction of Ce(IV) to Ce(III) and by the formation of CeOCl as intermediate product. 16.4.2. Syn-gas Production One of the most important reactions to produce syn-gas is the steam reforming of hydrocarbons. The reactions involved in this process are the following: CnHra + nH 2 0 -» nCO + (n+m/2)H2 CO+H20->C02+H2
(16.9) (16.10)
494
Catalysis by ceria and related materials
CO+3H2-*CH4+H20
(16.11)
The feedstock usually used is light nafta or methane depending on the country of production. The process has found applications for the production of hydrogen for ammonia synthesis, for the production of synthesis gas and for the synthesis of methanol and oxo-alcohols. Moreover, in the last decade, the reforming reactions have received great attention due to the application in the fuel cell technology [56]. Recently, alternative processes for syn-gas production have been developed. They are based on C02-methane reforming reaction (eq. 16.12) [57] and on the partial oxidation of methane (eq. 16.13) [58]. These processes permit to obtain a variable CO-H2 ratio and therefore they are more suitable for specific applications such as Fisher-Tropsch synthesis, chemical energy transmission and storage systems [5961]. CH 4 + C0 2 -> 2CO + 2H2
(16.12)
C H 4 + 0 . 5 O 2 - > C O + 2H2
(16.13)
Commercial reforming catalyst are generally based on supported nichel. Because the operating conditions in the reforming are severe (T=850-1100°C, P=2030 bar) the carriers must be thermally and mechanically resistant. Moreover the deactivation due to coke formation must be limited. Common supports are aalumina, magnesia, aluminum spinel or zirconia. Several studies pointed out that the support has a great influence not only on the activity of catalyst, but also on the reaction pathways and on the catalyst stability. In fact, the catalytic stability seems to depend on the nature of the carrier on which the metal is dispersed and on the interaction between the metallic phase and the support [57, 62-63]. The addition of promoters, especially rare earth oxides and ceria, results in more active and stable catalysts for the reforming processes, but the mechanism of action strongly depends on the methodology of catalyst preparation [63-65]. The majority of recent studies focused on ceria-based catalysts deals with C0 2 reforming reaction [66-73], where ceria has been studied as promoter and support, although methanol decomposition to syn-gas has been studied as well [65]. As a support ceria does not always contribute to improve the activity and the resistance of catalyst because of a strong interaction between the metal and the support [64,66]. However, an enhancement of the activity and more stable catalysts have been obtained if ceria is added in the form of solid solutions or if it can form solid solutions with the support [64-70]. In these systems
The use of ceria in other catalytic applications
495
ceria contributes to metal dispersion making the catalyst more resistant to poisoning by coke and to the sintering process. In addition ceria participates to the reaction mechanism as an oxygen storage component. A two pathway mechanism can be proposed for the reaction: the first step is the hydrocarbon/CH4 decomposition into carbon; then, the carbon atoms react with oxygen coming from the ceria-based support, which is continuously replenished by the dissociation of C0 2 in the dry reforming [71-73], or by H 2 0 in the steam reforming [69]. In ceria-zirconia solid solutions, where the rate of oxygen transfer is greatly enhances compared to ceria, this step is accelerated [67-70]. This is mainly ascribed to the synergism between zirconia, ceria and the metal. Zirconia and the metal promote the reducibility of ceria and therefore its oxygen storage capacity. A scheme of the catalytic behaviour of these systems is illustrated in Fig. 16.4. CH4
CO + 2H2
Ce-Zr02 Figure 16.4. Scheme of reforming of methane on M/Ce02-Zr02 catalyst. Adapted from [69]. The top layer is constituted of relatively free metal particles; an intermediate layer where metal is strongly interacting with Ce-Zr02 is then sandwiched between the top layer and the support. The reduced ceria sites in the solid solution produce active oxygen species by reacting with H 2 0 or C0 2 molecules. These species then react with the deposited carbon via oxygen spillover from the support onto the metal. In addition, the high reducibility and oxygen storage capacity of ceria-zirconia provide highly mobile species via a redox cycle. This helps the oxidation of carbon through the participation of lattice oxygen which is then replenished by oxygen from water. The role of ceria as oxidant in the conversion of methane to syn-gas under anaerobic conditions has been investigated in recent studies [74-79]. Otsuka et al. have shown that the reaction of methane with ceria in the absence of gaseous oxygen selectively produced a synthesis gas with a H2/CO ratio of 2, while C0 2 and H 2 0
Catalysis by ceria and related materials
496
were formed as main products in the presence of gaseous oxygen [74,76,77], Therefore, the use of an oxygen storage components as oxidant in the absence of gas-phase oxygen, may be a promising approach to the direct partial oxidation of methane to syn-gas, which is not easy to achieve in the presence of oxygen due to the over-oxidation of methane [79]. CH4 pulse experiments, where ceria is the only oxygen source, has been carried out to clarify the oxidation mechanism [74]. It has been found that ceria does not act as a true catalyst but as an oxidant, and during reaction is reduced to Ce02.x. This reduced phase can then be reoxidized back to Ce0 2 with air in a second step (Fig. 16.5).
Air
CHA
I
I
"^"'J&>
•mfriffi.
CeOz T$r$
i
CO * 2H 2
CeOz-
*ftft :VA,a
i
Figure 16.5. A two stage reaction for the production of syn-gas by anaerobic oxidation of CH4 [74],
However, temperatures higher than 650°C are needed for the oxidation of methane by ceria. The incorporation of zirconia into ceria lattice improves the reactivity of the lattice oxygen and thus the formation of hydrogen and CO is possible at lower temperature (ca. 500°C). It has been reported that the formation of hydrogen and CO was enhanced by the incorporation of zirconia into ceria up to a Zr0 2 content of 20%. Further increases in the content of Zr0 2 decreased the rates of both hydrogen and CO formation [78,79]. The reaction between methane and ceria to produce syngas in a pulse apparatus was also studied by Fathi et al. at 700°C. [81]. Ceria was supported on y-Al203 and promoted by impregnation with Pt or Rh. The presence of metal lowered the temperature necessary to reduce ceria and drastically enhanced the conversion of methane. Studies of the reaction between methane and promoted ceria showed that the selectivity to syngas depends on the degree of reduction of the cerium oxide. The main problem in these application is related to
The use of ceria in other catalytic applications
497
the formation of carbon during the reduction step. The use of mobile lattice oxygen in this kind of reactions has been also demonstrated for the quaternary Ce07Zr025Tb005O2_x and related compounds [82]. All these results suggest that the ceria has a fundamental role as an oxygen storage material also in the promising technologies for the production of H2 via syngas.
16.5.
Conclusions
In summary it appears that the redox characteristics of ceria play a key role in many applications in which Ce0 2 is used as a component of the catalyst formulation. The enhancement of the redox features (obtained by careful modification of ceria with dopants, by interaction with metals and by using new methods for catalyst preparation) in many cases contribute to an increase of the overall performance of the catalyst and at present is one of the main direction of research in this field.
16.6. 1. 2. 3. 4.
References
Ferrauto R.J., and Heck, R.M., Catal. Today 51 (1999), 351. Kaspar J., Fornasiero P. and Graziani M., Catal.Today 50 (1999), 285. Trovarelli A. Catal. Rev. Sci-Eng. 38 (1996), 439. Putna E. S., Bunluesin T., Fan X. L„ Gorte R. J., Vohs J. M., Lakis R. E. and Egami T., Catal. Today 50 (1999), 343. 5. Trovarelli A., de Leitenburg C., Boaro M., Dolcetti G., Catal. Today 5 0 (1999), 353. 6. Zeng S., Zhang S., Groves F. R., Harrison D. P., Chem. Eng. Sci. 5 4 (1999), 3007. 7. Flytzani-Stephanopoulos M., Zhu., T. L. and Li Y., Catal. Today 50 (1999), 353. 8. Rodriguez G. A., Gilsak T., Freitag A., Hanson G. C., Larese G. Z. and Chaturvedi S., Catal. Lett. 62 (1999), 113. 9. Bhattacharyya A. K., Woltermann G. M., Yoo G. S., Karch G. A. and Kormier W.E., Ind. Eng. Chem. Res. 27 (1988), 1356. 10. Wakif M., Basin P., Saur O., Lavalley G.C., Blanchard G. and Tourette,0., Appl. Catal. B: Env. 11 (1997), 193.
498
Catalysis by ceria and related materials
11. Liu W„ WadiaC. and Flytzani-Strphanopulos M., Catal. Today 28 (1996), 391. 12. Zhu T., Kundakovic D. A. and Flytzani-Stephanopoulos M., Catal. Today 5 0 (1999), 381. 13. Ferrauto R. G. and Bartholomew C. H., in Fundamental of Industrial Catalytic Processes, Blackie Academic and Professional, London 1997. 14. Kim G. and Juskelis M.V., Stud. Surf. Sci. Catal. 101 (1996), 137. 15. Yoo G. S., Bhattacharyya A. A., Radlowski C. A. and Karch J. A., Stud. Surf. Sci. Catal. 75 (1993), 1391. 16. Wakif M., Saad A. M., Bensitel M., Bechelier J., Saur O. and Lavalley G. C., J. Chem. Soc. Faraday Trans. 88 (1992), 2931. 17. Wang J-A., Chen L-F. and Li C.L., J.Mater. Sci. Lett. 17 (1998), 533. 18. Buchanan G. S., Stern D. L., Nariman K. E., Teitman G. J., Sodomin J.F. and Johnson D. L., Ind. Eng. Chem. Res. 35 (1996), 2495. 19. Zhao X., Peters A. W. and Weatherbee G. W., Ind. Eng. Chem. Res. 3 6 (1997), 4535. 20. Akyurtlu J. F. and Akyurtlu A., Chem. Eng. Sci. 54 (1999), 2991. 21. Ginger E. A., U.S. Patent n° 4.192.855 (1980). 22. Longo J. M., U.S. Patent n° 4.251.496 (1981). 23. MachidaM., Uto M., Kurogi D. and Kijima T., Chem. Mater. 12 (2000), 3158. 24. Machida M., Uto M., Kurogi D. and Kijima T., Chem. Mater. 12 (2000), 3165. 25. Flytzani-Stephanopoulos M., Zhu T. L., Li Y., Catal. Today 62 (2000), 145. 26. Liu W., Sarofim A. F. and Flytzani-Stephanopoulos M., Appl. Catal. B.: Env. 4 (1994), 167. 27. Hirano T., Bull. Chem. Soc. Jpn. 59 (1986), 1653. 28. Hirano T., Appl. Catal. 26 (1986), 81. 29. Hirano T., Appl. Catal. 26 (1986), 65. 30. James D. H. and Castor, W.M., Ullman's Encycl. Ind. Chem. 5th ed. 25 (1994), 329. 31. Cavani F., and Trifiro F., Appl. Catal. A: Gen. 133 (1995), 219. 32. Strofer E., 4th G. M. Schwab Symposium 8-12th July, Berlin, (2001), p.30. 33. Minura N. and Saito M. Catal. Today 55 (2000), 173. 34. Tiscareno F., Hill C. G., and Anderson M. A., Appl. Catal. A: Gen. 9 6 (1993), 33.
The use of ceria in other catalytic applications
499
35. Forni L., Azzolini S., and Pernicone N., 4th European Congress on Catalysis Europacat IV, Rimini (1999), p.665. 36. Wu J. C , Chung C. S. and Wang I., Appl. Catal. 18 (1985), 295. 37. Park J. N., Noh J., Chang J.-S. and Park S.E., Catal. Lett. 65 (2000), 75. 38. Muhler M., Scholg R. and Ertl G., J. Catal. 138 (1992), 413. 39 Murakamy A., Unei H., Teranishi M. and Ohta M., US patent n°5190906 (1993). 40. Hara F. J. 0., US Patent n°3904552 (1975). 41. Imanari M. and Takiguchi M., US patent n°4460706 (1984). 42. Sherrod F. A. and Smith A. R., US patent n°4758543, (1988). 43. General Catalog, Sud Chemie Catalyst Group, Munich, Germany. 44. Hirano T., Appl. Catal. 28 (1986), 119. 45. Grasselli R. K., J. Chem. Ed. 63 (1986), 216. 46. Wu J. C , Chung C. S„ Ay C. L. and Wang, I., /. Catal. 87 (1984), 98. 47 Muhler M., Schiitze J., Wesemann M ., Rayment T., Dent A., Scholgl R. and Ertl G., J. Catal. 126 (1990), 339. 48. Muhler M., Scholg R. and Ertl G., /. Catal. 138 (1992), 413. 49. Kuhurs C , Arita Y., Weiss W., Ranke W., and Schlogl R., Topics Catal. 14 (2001), 111. 50. Ranke W., 4th G. M. Schwab Symposium 8-12th July, Berlin, (2001), p. 5. 51. Valenzuela R. X., Bueno G., Corberan V. C , Xu Y., and Chen C , Catal. Today 61 (2000), 43. 52. Hagemeyer A., Poepel W. J., Buechele W., Hofstadt O., Deimling A. and Hoffman W., German patent, DE 4422770 (1996). 53. Fu J. C , Czarnecki L., Patellis C , Bouney A., Whittenberger W. A. "Catalytic technology for restaurant emission control" presented at Air and Waste Management Association 92nd annual Meeting 1999, St. Louis, Missouri (U.S.A.). 54. Weckhusen M. B., Rosynek M. P., and Lunsford J. H., Phys. Chem. Chem. Phys. 1 (1999), 3157. 55. Jiang Y., Decker S., Mohs C , Klabunde K. J., J.Catal. 180 (1998), 2450. 56. Hutchings G. J., and Burstein, G. T. ed. "Fuel Cells and Catalysis" Catalysis Today 38 (4) (1997). 57. Bradford M. C. J., and Vannice M. A., Catal. Rev. Sci. Eng. 41, (1999), 59. 58. Tsang S.C., Claridge J. B., and Green M. L. H. Catal. Today 23 (1995), 3. 59. Richardson J. T. and Paripatyador S. A., Appl. Catal. A: Gen. 61, (1990), 293. 60. Meirovitch E. and Segal A. Sol. Energy 46, (1991), 219.
500
Catalysis by ceria and related materials
61. Ross J. R. H., van Keulen A. N. J., Hegarty M. E. S. and Seshan K., Catal. Today 30, (1996), 193. 62. Erdoheley A., Cserenyi J., Papp E„ Solymosi F; /. Catal. 141 (1993), 287. 63. Ferreira-Aparicio P., Guerrero-Ruiz A. and Rodriguez-Ramos I., Appl. Catal. A: Gen. 170 (1998), 177. 64. Wang H.Y. and Ruckenstein E., Appl. Catal. A: Gen. 204 (2000), 143. 65. Liu Y., Hayakawa T., Ishii T., Kumagai M., Yasuda H., Suzuki K., Hamakawa S. and Murata K„ Appl. Catal. A: Gen. 210 (2001), 301. 66. Montoya J.A., Romero-Pascual E., Gimon C , Del Angel P. and Monzon A., Catal Today 63 (2000), 71. 67. Wang S. and Lu G.Q., Appl. Catal. B: Env., 19 (1998), 267. 68. Montoya J. A., Romero-Pascual E., Monzon A. and Guimon C, Stud. Surf. Sci. Catal. 130D (2000), 3669. 69. Roh H.-S., Jun K.-W., Dong W.-S., Park S. E. and Baek Y.-S. Catal. Lett. 74 (2001), 31. 70. Noronha F. B., Fendley E. C., Soares R. R., Alvarez W. E. and Resasco D.E., Chem. Eng. J. 82 (2001), 21. 71. Stagg-Williams S. M.., Noronha F. B., Fendey G. and Resasco D.E. J. Catal. 194 (2000), 240. 72. Sharma S., Hilaire S. and Gorte R. J., Stud. Surf. Sci. Catal. 130 (2000), 677. 73. Xu G. L., Ski K. Y., Gao Y, Xu H. Y. and Wei Y.D. /. Mol. Catal. A: Chem. 147 (1999), 47. 74. Otsuka K., Wang Y., Sunada E. and Yamanaka I., J. Catal. 175 (1998), 152. 75. Nakamura J., Kubushiro K. and Uchijima T., Stud. Surf. Sci. Catal. 11 (1993), 373. 76. Otsuka K., Ushiyama T. and Yamanaka I., Chem. Lett. (1993), 1517. 77. Otsuka K., Sunada E., Ushiyama T., Yamanaka I., Stud. Surf. Sci. Catal., 107 (1997), 531. 78. Otsuka K., Wang Y. and Nakamura M., Appl. Catal. A: Gen. 183 (1999), 317. 79. Pantu P. K. K. and Gavalas G. R., Appl. Catal. A: Gen. 193 (2000), 203. 80. Hickman D. A. and Schmidt L. D., Science 259 (1993), 343. 81. Fathi M., Bjorgum E., Viig T. and Rokstad O. A., Catal. Today 63 (2000), 489. 82. Kang Z.C., Eyring L., J. Solid State Chem. 154 (2000), 129.
INDEX Ab initio techniques, 284 AB 2 0 4 , 408 AB0 3 ,408 Acetic acid, 434 Acetone, 328 Activation energy, 467 Additives, 393, 398 Adsorbent, 382 Adsorption, 331 -H2,184, 326 -H 2 0, 326 -N 2 0, 324 -NO, 172, 176,180, 324 -N0 2 , 324 -0 2 , 171-173, 258, 324, 349 -S0 2 , 198, 205 Advanced TWC formulation, 348 AES, Auger electron spectroscopy, 312 AFM, atomic force microscopy, 316 Ag/Ce0 2 ,417 Ag 2 0,417 Air-fuel ratio, 243 Alcohol, 327 Ammonia, 336, 432 Anode, 472 Anodic oxidation, 453 Apparatus for OSC, 25.3 Atomistic simulation, 282 Atomization process, 68 A-type Ce 2 0 3 , 23 Bastnasite, 2, 9 Binding energy, 40, 289 Born model, 283
Bronsted site, 409 CaF2,408 Calorimetric studies, 349, 382 Carbon dioxide, 323 Carbon monoxide, 245, 252, 272, 323, 369, 489 Carbon, 444 Carboxylic acids, 327 Catalytic cracking, 485 Ce 2 0 2 S, 382, 488 Ce 2 0 2 S0 4 , 382 Ce 2 0 3 , 22, 205, 349 Ce2S3, 382 Ce2Zr207, 74,221,228,460 Ce2Zr3O10, 221 Ce3ZrOg, 221 Ce7012, 29 Ce„ 3 (Si0 4 ) 6 0 2 , 231 CeA103, 192, 211, 234, 235, 414 CeO,.714, 18 Ce0 2 -Zr0 2 , -catalytic studies, 413 -EPR studies, 178 -EXAFS studies, 204, 223 -phase diagram, 74, 219, 353 -Raman studies, 222 -redox behavior, 73, 229, 246, 256, 264, 300, 370 -structural defects, 222, 352 -supported noble metals, 111,149-152,273 -supported on A1203, 178, 234
502
Catalysis by ceria and related materials
-synthesis, 53, 55, 58 -t,t',t" phases, 219-221 -XPS studies, 193 CeOCl, 90, 194 CeOx, 17, 19,464 -free energy of formation, 458 -free enthalpy of formation, 458 Ceria film deposition, 313 -MO-CVD,316 -oxygen plasma assistedmolecular beam epitaxy, 314 -pulsed laser deposition, 314 -spray pyrolysis, 315 -vapor deposition, 313 Ceria, -as adsorbent, 382, 409 -films, 313,382 -hydrogen-electrodes, 472 -interaction with noble metals, 96,106,348 -oxidation, 190 -phase diagram, 21 -single crystal, 93, 313 -supported on A1203,176, 191,197,203,235,248, 250 -supported on Si0 2 ,180, 231 Ceria-yttria, 56 Ceria-zirconia, 298, 315, 337, 217235,285,297,301,412 Cerium acetates, 54 Cerium acetylacetonate, 413 Cerium ammonium nitrate, 67
Cerium carbonate, 4,9,408 Cerium carboxylates, 7, 54 Cerium chloride, 9, 55 Cerium citrates, 54 Cerium fluoride, 7, 9 Cerium halides, 54 Cerium hydroxide, 4, 54, 408,432 Cerium interstitial compensation, 37 Cerium nitrate, 9, 54, 55,408,413 Cerium oxalate, 4, 54 Cerium oxysulfate, 382 Cerium sulfates, 54, 55, 383 Cerium sulfite, 382,485 Cerium, additive, 393 CeTi0 3 ,192 CeV0 4 , 198 Chemical filing process, 75 Chemical oxygen demand, (COD), 431 Chemisorption, 95, 318, 322 -CO, 112 -H2, 110 -low temperature, 110 Chlorinated hydrocarbons, 493 Chlorine, 90, 118,194 Clusters, 287 Co, 439 Coadsorption, 334 Cobalt/bismuth, oxidation catalysts, 432 Combustion, 92 Coordination defect model, 27 Coordination number, 464 Copper, additive, 393 COS, 484 Cr 2 0 3 ,411 C-type Ce 2 0 3 , 22 Cu(N0 3 ) 2 ,432,439
Index Cu, 313,415,431,445,489 Cu/Ce02, 181,261 CuO, 408,411, 488 Current density, 474 CVD, 71 Cyanide, 335 D 2 ,114 Decomposition reaction, 87 -CH3OH, 87 -Ethanol, 87 -N 2 0, 87 Defect association, 37 Defects, 23, 286, 319, 352,455 Dehydrogenation of ethylbenzene, 483,490 Deposition, 91 Desorption, S0 2 , 384 Desulfurization, 488 Diffusion coefficients, 262, 266 Dislocations 319 Dopant interstitial compensation, 37 Doped ceria, -CeNb04,231 -Ce02-BaAl12019,422 -CeO r CaO, 178, 456 -Ce0 2 -CuO,71,197,445 -Ce0 2 -Gd 2 0 3 ,456,472 -Ce0 2 -Hf0 2 , 53, 285 -Ce02-LaOx, 71, 232, 249, 294, 350,472 -Ce0 2 -Ln 2 0 3 , 54, 55,470 -Ce02-MnOx, 432,438, 488 -Ce0 2 -Nb 2 0 5 , 456 -Ce02-NiO, 196 -Ce02-PrOx, 67, 370,421 -Ce0 2 -Si0 2 , 231, 350
503 -Ce02-TbOx, 53,103, 123, 129, 149,193, 232 -Ce0 2 -Ti0 2 ,63,231 -CeOx-Fe203,490 -CeTa04, 231 -Gd, 292 -ternary compounds, 232, 250,422,435 -Th, 285 -various additives, 261 -Y, 294, 472 -synthesis, 57-59, 60, 62, 445 Doped ceria-zirconia, 232,415, 435, 497 Dydimium, 5 Dynamometer, 346 EDTA, 6 EELS, 141, 145 Electrical conductivity, 32, 39, 466 Electrode, 471 Electronic conductivity, 464 Electronic mobility, 34 Emission standards, 347 EPR, 170-183 Ethylene, 334 Euro III, 391 Euro IV, 391 EXAFS, 200, 223 Experimental set-up, OSC, 254 Extraction, 4 Faraday magnetic balance, 97 Fe 2 0 3 ,408,492 Federal test procedure, 366 Filter, -ceramic fibre-wound, 394
504
Catalysis by ceria and related materials
-continuously regenerating, 394 -diesel, 393 -monolithic, 396 Fisher-Tropsch synthesis, 494 Flow reactor, 397 Fluid catalytic cracking, 6,485 Fluorite, 15, 408, 443 Formaldehyde, 328 Free energy, 458 Free enthalpy, 458 Free entropy, 464 Frenkel,25, 351 FT-IR, 112, 247, 264, 383, 411 Ge, 314 Glass decolorization, 7 Growth, Volmer-Weber, 317 Heated exhaust oxygen sensors, (HEGO), 367 Homoexchange reaction, 262 HREM, 123-149 Hydrogen sulfide, 347, 358, 382,484 Hydrogen, 249 Hydrogenation reaction, 87 -alkanes, 87 -CO, 87, 318 -C0 2 , 87 -N2, 87 -unsaturated organics, 87 Hydrogen-ceria electrodes, 472 Hydroperoxy radical, 441 Hydrothermal crystallization, 61 Hydroxyl, OH, 328 Impregnation, 89
Intermetallic compounds, 93, 140143 Inverse catalysts, 318 Ionic conductivity, 40,468 Ionic mobility, 34 Ionic radius, 224, 462 Ir/Ce0 2 ,91,248 Iron, additive, 393 Isocyanate, OCN, 335 Ketene, 328 Kroger-Vink, 24, 217, 455 LaA103, 314 Lattice expansion, 18, 464 Lattice parameters, -ceria, 18,462 -ceria-zirconia, 225 -reduced ceria, 19 LEED, low energy electron diffraction, 314, 320 LEIS, low energy ion scattering, 320 Lewis, 409 Light-off, 368 Loparite, 1 Lubricating oil, 358 Luminescence, 8 Magnesium sulfate, 485 Matt-Littleton, 283 Mechanical alloying, 53 Metal decoration, 95, 137, 143 Metal dispersion, 124, 195, 264, 354 Methanation, 318 MgAl 2 0 5 .485 Mg0,315 Microemulsion, 66 Migration, noble metals, 355
505
Mischmetal, 8, 382 Mixed ionic electronic conductor, 454 Mn, 250, 415, 439 Mn 3 0 4 , 435 MnO 2 ,408,411 Molecular dynamics, 284, 311, 319 Monazite, 3, 361 Monoclinic phase, Zr0 2 , 220 Monte Carlo methods, 292 MoO x /Ce0 2 , 170, 181, 184,197 MSRI, mass spectroscopy of recoil ions, 320 N 2 0, 176 N 2 0, 448 n-butylamina, 434 Nernst, 409 Nernst-Einstein relation, 43 Neutron diffraction, 227 Neutron scattering, 225 Ni, 313,320, 358,489 Ni/Ce02, 107, 196 Nitrate, 324 Nitride, 324 Nitrite, 324 NM/Ln02.x, 94 NM/Ti02, 138, 143 NMR, 183-185, 362 NO, -reduction, 87, 197, 252, 272, 403, 487 -traps, 371,488 N0 2 , 403 Noble metals, -effect on OSC, 248 -encapsulation, 356 -migration, 355
-poisoning by sulfur, 377386 n-type semiconductor, 410 On-board diagnostic, (OBD), 273, 359, 364 Operating window, TWC, 243, 344 Oxidation reaction, 87, 245 -anaerobic, 496 -CH4, 87,251,411 -CO, 87, 197, 245, 268, 349,351,369,410 -electrochemical, 475 -H2, 249, 349, 412 -hydrocarbons, 87, 250, 268,410,413,473 -soot, 400 -wet oxidation, 431-449 Oxygen buffering capacity, (OBC), 256 Oxygen storage capacity (OSC), 103105, 243, 345, 378 -complete, OSCC, 244 -effect of sulfur, 379 -effect of thermal aging, 274, 350 -low temperature, 370 Oxygen, -activation, 263 -chemical potential, 457 -defects, 352 -diffusion, 43, 259, 266 -isotopic exchange measurements, 258 -migration, 257, 290, 294 -sensors, 366 Particle size, 196, 203
506
Catalysis by ceria and related materials
Particulate matter, (PM), 391 Pd, 313, 320, 364 Pd/Ce02, 107, 214 -CO oxidation, 270 -CO/NO reaction, 272 -dispersion, 195 -EPR studies, 179 -interaction with S0 2 , 383 -preparation, 92 -reduction, 92 Pd/CeO r Zr0 2 , 353, 357 PdO, 365,419 PEG-200, 434 Perovskite, 408 Peroxide species, 411 Phase segregation, 226 Phenol, 437 Phosphates, 361 Phosphorus, 361 Platinum, additive, 398 Point defect, 27 Poisoning, NMR studies, 362 Poisoning, phosphorus, 361 Poisoning, sulfur, 358, 379 Poisonong, Pd catalysts, 359 Polarization, 474 Polaron, 466 Power density, 474 Pr 7 0 12 , 29 PrOCl, 91 PrOx, 15, 94 Pt, 247, 313,320 Pt, promotion of OSC, 247 Pt/Ce02, 108 -CO oxidation, 270 -CO/NO reaction, 272 -dispersion, 100, 264, 355 -preparation, 92
-reduction, 100 Pt/Ce02-Zr02, 355,417 Pt-Ce alloys, 93,140 Pulse chromatographic system, 244 Pyridine, 434 Pyrochlore, 228 RAIRS, reflection adsorption infrared spectroscopy 333 Raman spectroscopy, 222 Reducibility, 300, 328 Reduction with CO, 122 Reduction, 17, 91, 190, 285, 323, 330 Reforming reaction, 494 Restaurant emissions, 493 Rh, 318,331 -promotion of OSC, 247 Rh/Ce0 2 ,108 -dispersion, 125, 264 -OSC, 261 -preparation, 90, 97 -reduction, 100 -XPS studies, 194 Rh 2 0 3 ,104 Ru, 313, 320,439 Ru/C, 444 Ru/Ce0 2 ,109,441,443 Sapphire, 313 Schottky, 24 Selective catalytic reduction, 198 Separation, 5 Si, 314 Silver, 417, 436, 439 Si0 2 ,438 Sn, 439 SnCe0 4 ,183 S0 2 , reduction by CO, 489
Index
507
S0 3 , 485 Sol-gel, 62,413 Solid oxide fuel cell, (SOFC), 282, 290,453 Solvothermal methods, 60 Soot, 396, 400 Spillover, 99,110 Spinel, 408,485 Spray-ICP, 68 Sputtering, 329 SrTi0 3 , 320, 324 Steam reforming reaction, 87, 345, 380 STM, scanning tunneling microscopy, 320 Strong metal support interaction (SMSI), 96,106 -reversibility, 143 Sulfation, 484 Sulfur dioxide, 326, 358,484 Sulfur, 378,423, 484 Superoxide species, 171, 265, 411, 441,448 Surface diffusion, 249, 258 Surface structure, 295, 318 Surfaces, 318 Surfactants, 64,413 SXPS, soft x-ray photoelectron spectroscopy, 323, 326 Syn-gas, 493
Temperature programmed reduction (TPR), 100, 286, 487 Tetragonal phase, Zr0 2 , 220 Thermal expansion coefficient, 423, 455,462 Thermal reaction, 52 Thermodynamic properties, 16, 349, 381,457 Three-way catalysts, (TWC), 243, 343 Ti0 2 ,438 Total organic carbon, (TOC), 439 Transient reactions ,CO oxidation, 245 Transient reactions, -CO/NO, 252 -H2 oxidation, 249 -hydrocarbon oxidation, 250 Triple phase boundary, 454
Tb 7 0 12 , 29 TbOCl, 91 TbOx reduction, 102 TbOs, 15 Temperature programmed desorption (TPD), 113, 120, 326, 332, 383
Wastewaters, 431 Water gas shift reaction, 87, 271, 345, 380 W0 3 /Ce0 2 , 197
UHV, ultra high vacuum, 311 U0 2 , 351 V 2 0 5 ,486 Vacancy compensation, 37 Vacancy, 17, 26, 319, 322 Vegard rule, 224,462 VOC, 493 VO x /Ce0 2 ,170,181
XAFS, 199-206 XANES, 200
508
Catalysis by ceria and related materials
Xenotime, 1 XPS, 186-199, 312,446 X-ray diffraction (XRD), 17, 313, 221 YSZ, yttria stabilized zirconia, 121, 313,316,324,453,471 Zeldovich, 407 Zeolites, 195, 439 Zinc, 358 Zirconia ceramics, 217 Zr0 2 , 438 Zyrconyl nitrate, 413
Catalytic Science Series - Vol. 2
Catalysis by Ceria and Related Materials "... this book represents an excellent review of the applicability and the full potential of ceria in catalysis, but it is also recommended as a starting platform for non-experts in order to become acquainted with important aspects of environmental catalysis." Applied Catalysis B: Environmental, Nov 2002 "... this book is an excellent compendium on the science, technology, and applications of ceria-based catalysts. It provides useful overviews, both to graduate students beginning their scientific careers in the field of catalysis and to industrial researchers working in the fields of industrial, environmental, and automotive catalysts." Journal of the American Chemical Society, 2002 The use of Ce02-based materials in catalysis has attracted considerable attention in recent years, particularly in applications like environmental catalysis, where ceria has shown great potential. This book critically reviews the most recent advances in the field, with the focus on both fundamental and applied issues. The first few chapters cover structural and chemical properties of ceria and related materials, i.e. phase stability, reduction behaviour, synthesis, interaction with probe molecules (CO, O2, NO), and metal-support interaction — all presented from the viewpoint of catalytic applications. The use of computational techniques and ceria surfaces and films for model catalytic studies are also reviewed. The second part of the book provides a critical evaluation of the role of ceria in the most important catalytic processes: three-way catalysis, catalytic wet oxidation and fluid catalytic cracking. Other topics include oxidation-combustion catalysts, electrocatalysis and the use of cerium catalysts/additives in diesel soot abatement technology.
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Imperial College Press www.icpress.co.uk
781860"94299C
