Catalytic science daniel duprez, fabrizio cavani (eds ) handbook of advanced methods and processes in oxidation catalysis from laboratory to industry (2014, imperial college press)

Different aspects of total oxidation processes are reviewed in the first part ofthe book: hydrocarbon oxidation Chapter 1 and soot oxidation Chapter 2 formobile applications while oxidat

From Laboratory to Industry

CATA LYSIS

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Duprez, Daniel, 1945–

Handbook of advanced methods and processes in oxidation catalysis : from laboratory to industry / Daniel

Duprez, University of Poitiers, France, Fabrizio Cavani, Universita di Bologna, Italy.

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Preface Advanced Methods and Processes in Oxidation Catalysis

From Laboratory to Industry

edited by Daniel Duprez (University of Poitiers, France) &

Fabrizio Cavani (Universit`a di Bologna, Italy)

Since the discovery by Humphry Davy in 1817 of the flameless combustion ofcoal gas over Pt wires, tremendous progress has been made in the understanding

of complex phenomena occurring in oxidation catalysis In parallel, advanced nologies were developed to make these processes more efficient and safer In thenineteenth century, researchers observed that hydrocarbon oxidation could lead toorganic intermediates on noble metals The huge demand from the chemical indus-try for new compounds prompted them to take advantage of the selective oxidation

tech-to synthesize oxygenated chemicals Synthesis of new compounds required specificoxide catalysts much more selective than noble metals Considerable progress wasmade during the twentieth century while the development of cleaner, greener andsafer catalytic processes remains a permanent objective of the chemical industrytoday

This book offers a comprehensive overview of the most recent developments

in both total oxidation and combustion and also in selective oxidation For eachtopic, fundamental aspects are paralleled with industrial applications The bookcovers oxidation catalysis, one of the major areas of industrial chemistry, outliningrecent achievements, current challenges and future opportunities One distinguish-ing feature of the book is the selection of arguments which are emblematic ofcurrent trends in the chemical industry, such as miniaturization, use of alternative,greener oxidants, and innovative systems for pollutant abatement Topics outlinedare described in terms of both catalyst and reaction chemistry, and also reactor andprocess technology

The book is presented in two volumes The first ten chapters are devoted to totaloxidation while the next eighteen chapters deal with selective oxidation

v

Different aspects of total oxidation processes are reviewed in the first part ofthe book: hydrocarbon oxidation (Chapter 1) and soot oxidation (Chapter 2) formobile applications while oxidation of volatile organic compounds (VOC) is treated

in the next five chapters Chapter 3 provides a general overview of VOC oxidationwhile chlorinated VOCs are specifically discussed in Chapter 4 and persistent VOC

in Chapter 5 Plasma catalysis processes for VOC abatement are reviewed in Chapter

6 Finally, Chapter 7 gives the point of view of industry for the development andapplications of catalysis for air depollution technologies Total oxidation is also usedfor energy production by combustion processes exemplified in Chapter 8 The lasttwo chapters are devoted to oxidation processes in liquid media by electrochemicaltechniques (Chapter 9) or more generally as "advanced oxidation processes" forwater depollution (Chapter 10)

The part devoted to selective oxidation includes chapters aimed at providing anoverview of oxidation technologies from an industrial perspective, with contribu-tions from chemical companies such as eni SpA, Radici Chimica, Polynt, Sabic,DSM, and Clariant (Chapters 11–16) Then, Chapters 17–19 gives an updated view

of experimental tools and techniques aimed at the understanding of catalyst tures and interactions between catalysts and reactants/products Chapters 20–23 arefocussed on specific classes of homogenous and heterogeneous catalysts, such asvanadyl pyrophosphate, polyoxometalates, supported metals and metal complexes.Finally, Chapters 24–28 deal with classes of reactions, reactor configurations andprocess technologies used in selective oxidation, again offering a perspective onrecent developments and new trends, such as oxidation of alkanes, oxidations undersupercritical conditions, use of non-conventional oxidants, membrane and structuredreactors

fea-Daniel Duprez and Fabrizio Cavani

1 Oxidation of CO and Hydrocarbons in Exhaust Gas Treatments 1

Jacques Barbier Jr and Daniel Duprez

1.1 Introduction 1

1.2 The Pioneer Works (1970–1990) 2

1.3 Recent Investigations (After 1990) 11

1.4 Conclusions 19

References 20

2 Soot Oxidation in Particulate Filter Regeneration 25 Junko Uchisawa, Akira Obuchi and Tetsuya Nanba 2.1 Introduction 25

2.2 Method for Evaluation of Catalytic Soot Oxidation Activity 28

2.3 Classification of PM Oxidation Catalyst 30

2.4 Mechanisms and Examples of each Catalyst Type 31

2.5 Practical Application and Improvement of Soot Oxidation Catalysts 39

2.6 Concluding Remarks and Outlook 44

References 44

3 The Catalytic Oxidation of Hydrocarbon Volatile Organic Compounds 51 Tomas Garcia, Benjamin Solsona and Stuart H Taylor 3.1 Introduction 51

3.2 Technology Options for VOC Abatement 52

3.3 Operational Parameters Affecting the Catalytic Combustion of VOCs 53

3.4 Review of VOC Oxidation Catalysts 59

3.5 Conclusions 82

References 83

vii

4 Catalytic Oxidation of Volatile Organic Compounds:

Juan R Gonz´alez-Velasco, Asier Aranzabal, Be˜nat Pereda-Ayo,

M Pilar Gonz´alez-Marcos, and Jos´e A Gonz´alez-Marcos

4.1 Introduction 91

4.2 Catalysts for Chlorinated VOC Oxidation 94

4.3 Kinetic Studies 98

4.4 Influence of Water Vapour and Co-Pollutants in Feed Streams 102

4.5 Chlorinated VOC Catalyst Deactivation and Regeneration 112

4.6 Outlook and Conclusions 120

Acknowledgements 123

References 124

5 Zeolites as Alternative Catalysts for the Oxidation of Persistent Organic Pollutants 132 St´ephane Marie-Rose, Mihaela Taralunga, Xavier Chaucherie, Fran¸cois Nicol, Emmanuel Fiani, Thomas Belin, Patrick Magnoux and J´erˆome Mijoin 5.1 Introduction 132

5.2 Preliminary Study on POP Precursors 138

5.3 Advanced Study: Oxidation of PAHs in the Presence of a Complex Pollutants Matrix 145

5.4 Conclusion 149

References 150

6 Plasma Catalysis for Volatile Organic Compounds Abatement 155 J Christopher Whitehead 6.1 Introduction 155

6.2 Plasma Catalyst Interactions 156

6.3 Plasma Catalysis for the Abatement of Halomethanes 157

6.4 Plasma Catalysis for the Abatement of Hydrocarbons 163

6.5 The Role of Ozone in Plasma Catalysis for VOC Abatement 168

6.6 Cycled Systems for Plasma Catalytic Remediation 168

6.7 Conclusions 169

Acknowledgments 170

References 170

7 Catalytic Abatement of Volatile Organic Compounds:

Pascaline Tran, James M Chen and Robert J Farrauto

7.1 Introduction 173

7.2 Case #1: Catalytic Oxidation of Purified Terephthalic Acid 176

7.3 Case #2: Oxidation of Nitrogen-Containing VOCs: Precious Metal Catalysts vs Base Metal Catalysts 185

7.4 Case #3: Regenerative Catalytic Oxidation Catalysts 188

7.5 Conclusions 196

References 196

8 Hydrocarbon Processing: Catalytic Combustion and Partial Oxidation to Syngas 198 Unni Olsbye 8.1 Introduction 198

8.2 Catalytic Partial Oxidation of Hydrocarbons to Syngas 200

8.3 Catalytic Combustion 209

References 211

9 Oxygen Activation for Fuel Cell and Electrochemical Process Applications 216 Christophe Coutanceau and St`eve Baranton 9.1 Introduction 216

9.2 Thermodynamics 217

9.3 Molecular Oxygen Electroreduction 221

9.4 Atomic Oxygen Activation: Alcohol Electro-Oxidation 235

9.5 Conclusion 242

References 243

10 Advanced Oxidation Processes in Water Treatment 251 Gabriele Centi and Siglinda Perathoner 10.1 Advanced Oxidation Processes 253

10.2 Conclusions 281

References 283

11 Selective Oxidation at SABIC: Innovative Catalysts and Technologies 291 Edouard Mamedov and Khalid Karim References 301

12 Development of Selective Oxidation Catalysts at Clariant 302

Gerhard Mestl

12.1 Introduction 302

12.2 Research in Oxidation Catalysis 303

Acknowledgements 316

References 317

13 The Industrial Oxidation of KA Oil to Adipic Acid 320 Stefano Alini and Pierpaolo Babini 13.1 Introduction 320

13.2 Nitric Acid Oxidation of a Cyclohexanol/Cyclohexanone Mixture to Produce Adipic Acid 322

13.3 Development of Reactors for Adipic Acid Synthesis 326

13.4 Safety Aspects 328

13.5 Materials 330

13.6 Conclusion 331

References 332

14 Selective Oxidation Reactions in Polynt: An Overview of Processes and Catalysts for Maleic Anhydride 334 Mario Novelli, Maurizio Leonardi and Carlotta Cortelli 14.1 Introduction 334

14.2 Maleic Anhydride Market Trends and Production 336

14.3 The Most Consolidated Gas-Phase Selective Oxidation Process for Maleic Anhydride Production: The Oxidation of Benzene 338

14.4 Selective Oxidation ofn-Butane for Maleic Anhydride Production 341

14.5 Gas-Phase Selective Oxidation ofn-Butane to Maleic Anhydride: The ALMA Process 343

14.6 Some Recent Developments in the Fixed-Bed Process for Gas-Phase Selective Oxidation ofn-Butane to Maleic Anhydride 348

14.7 Conclusions 350

References 351

15 Selective Oxidations at Eni 353 Roberto Buzzoni, Marco Ricci, Stefano Rossini and Carlo Perego 15.1 Introduction 353

15.2 TS-1 and Related Materials: A Materialized Dream 354

15.3 Selective Oxidation with Hydrogen Peroxide by TS-1 and Related Materials 355

15.4 Hydrogen Peroxide Production 362

15.5 Other Oxidations 368

References 376

16 Selective Oxidation in DSM: Innovative Catalysts and Technologies 382 Paul L Alsters, Jean-Marie Aubry, Werner Bonrath, Corinne Daguenet, Michael Hans, Walther Jary, Ulla Letinois, V´eronique Nardello-Rataj, Thomas Netscher, Rudy Parton, Jan Sch¨utz, Jaap Van Soolingen, Johan Tinge and Bettina W¨ustenberg 16.1 Polyhydroxy Compounds: Ascorbic Acid 382

16.2 Aromatic Oxidations 389

16.3 Oxidations in Monoterpene Chemistry 394

16.4 Vitamin B5: Ketopantolactone 403

16.5 Cyclohexane Oxidation 405

16.6 Toluene Side-Chain Oxidation 408

References 410

17 In Situ and Operando Raman Spectroscopy of Oxidation Catalysts 420 Israel E Wachs and Miguel Ba˜nares 17.1 Introduction 420

17.2 Methanol Oxidation to Formaldehyde 421

17.3 Methane Oxidation to Formaldehyde 424

17.4 Ethane Oxidative Dehydrogenation (ODH) to Ethylene 425

17.5 Ethylene Oxidation to Ethylene Epoxide 428

17.6 Propane Oxidative Dehydrogenation to Propylene 429

17.7 Propylene Oxidation and Ammoxidation 429

17.8 Propane Oxidation and Ammoxidation 432

17.9 Butane Oxidation to Maleic Anhydride 433

17.10 Isobutane Oxidation 435

17.11 o-Xylene Oxidation to Phthalic Anhydride 436

17.12 SO2oxidation to SO3 438

17.13 Conclusions and Outlook 439

Acknowledgments 440

References 440

18 Infrared Spectroscopy in Oxidation Catalysis 447

Guido Busca

18.1 Introduction 447

18.2 Experimental Techniques 448

18.3 The Bulk Characterisation of Solid Oxidation Catalysts by IR 448

18.4 Surface Characterisation of Oxidation Catalysts by IR Spectroscopy 453

18.5 Studies of Oxidation Reactions Over Solid Catalysts: Methodologies 462

18.6 IR Spectroscopy Studies of Heterogeneously Catalyzed Oxidations: Case Studies 465

Conclusions 485

References 485

19 In Situ Non-Vibrational Characterization Techniques to Analyse Oxidation Catalysts and Mechanisms 496 Angelika Br¨uckner, Evgenii Kondratenko, Vita Kondratenko, J¨org Radnik and Matthias Schneider 19.1 Introduction 496

19.2 Electronic (Resonance) Techniques 498

19.3 X-ray Techniques 509

19.4 Temperature-programmed Reduction, Oxidation and Reaction Spectroscopy (TPR, TPO and TPRS) 529

19.5 Transient Techniques 532

19.6 Concluding Remarks 541

References 542

20 Vanadium-Phosphorus Oxide Catalyst for n-Butane Selective Oxidation: From Catalyst Synthesis to the Industrial Process 549 Elisabeth Bordes-Richards, Ali Shekari and Gregory S Patience 20.1 Introduction 549

20.2 Portrait of a Selective Oxidation Catalyst 551

20.3 Application to VPO Catalysts in n-butane Oxidation to Maleic Anhydride 553

20.4 Transient Regimes 564

20.5 Experiments in Alternative Reactors 569

20.6 Conclusions 577

References 579

21 Polyoxometalates Catalysts for Sustainable Oxidations

Mauro Carraro, Giulia Fiorani, Andrea Sartorel and Marcella Bonchio

21.1 Polyoxometalates 586

21.2 Oxidation Catalysis by POMs 589

21.3 Heterogeneous Polyoxometalate-Based Systems 615

21.4 Conclusions 618

Acknowledgments 619

References 619

22 Supported Metal Nanoparticles in Liquid-Phase Oxidation Reactions 631 Nikolaos Dimitratos, Jose A Lopez-Sanchez and Graham J Hutchings 22.1 Introduction 631

22.2 Oxidation of Alcohols and Aldehydes using Molecular Oxygen 632

22.3 Selective Oxidation of Hydrocarbons 656

22.4 Other Selective Oxidation Reactions 666

22.5 Conclusions and Final Remarks 668

References 669

23 Sustainability Trends in Homogeneous Catalytic Oxidations 679 Alessandro Scarso and Giorgio Strukul 23.1 Introduction 679

23.2 Use of Oxygen and Hydrogen Peroxide 681

23.3 Enantioselective Oxidations 681

23.4 Water as the Reaction Medium 719

23.5 The Use of Less Toxic Metals as Active Ingredients 732

23.6 Heterogenization of Homogeneous Systems 738

References 755

24 Light Alkanes Oxidation: Targets Reached and Current Challenges 767 Francisco Ivars and Jos´e M L´opez Nieto 24.1 Introduction 767

24.2 Oxidative Dehydrogenation of Light Alkanes to Olefins 789

24.3 Partial Oxidation of C2–C4Alkanes 792

24.4 Selective Oxidative Activation of Methane 809

24.5 Conclusions 814

References 815

25 Opportunities for Oxidation Reactions under Supercritical Conditions 835 Udo Armbruster and Andreas Martin 25.1 Introduction 835

25.2 Oxidation in Supercritical Carbon Dioxide 845

25.3 Oxidation in Supercritical Water 851

25.4 Heterogeneously Catalysed Oxidation in Other Supercritical Fluids 863

25.5 Summary and Outlook 864

References 865

26 Unconventional Oxidants for Gas-Phase Oxidations 877 Patricio Ruiz, Alejandro Karelovic and Vicente Cort´es Corber´an 26.1 Nitrous oxide (N2O) 877

26.2 Carbon dioxide (CO2) 894

References 914

27 Membrane Reactors as Tools for Improved Catalytic Oxidation Processes 921 Miguel Men´endez 27.1 Introduction 921

27.2 Dense Membranes 922

27.3 Porous Membranes 925

27.4 Conclusions 933

References 934

28 Structured Catalytic Reactors for Selective Oxidations 943 Gianpiero Groppi, Alessandra Beretta and Enrico Tronconi 28.1 General Considerations on Structured Catalysts 943

28.2 Applications of Structured Catalysts in Short Contact Time Processes 951

28.3 Applications of Monolithic Catalysts Based on Low Pressure Drop Characteristics 965

28.4 Applications of Structured Catalysts Based on Enhanced Heat Exchange 970

28.5 Summary and Conclusions 989

References 990

Chapter 1 Oxidation of CO and Hydrocarbons in Exhaust Gas Treatments

Jacques BARBIER Jr and Daniel DUPREZ ∗

The present chapter aims to describe the kinetics and mechanisms of CO and HC oxidation in exhaust gas treatments Attention will be paid to reactions carried out

on noble metal catalysts (Pt, Pd, Rh) usually employed in three-way catalysts (spark ignition engines) around stoichiometry The effect of ceria, usually employed as

an oxygen storage material, will also be reviewed.

Since 1972 in the United States and 1989 in Europe, regulations have been imposed

on the automobile industry to limit air pollution emitted by vehicles Since thesedates, legislation has been regularly reinforced with more and more severe regu-lations concerning four categories of pollutants: carbon monoxide, hydrocarbons(and other organics), nitrogen oxides (NO and NO2) and soot particulates.1–3 Toachieve abatement of these pollutants, automotive catalytic converters were imple-mented on new cars to eliminate CO, HC and NOx, while particulate filters areintended to be mounted in the exhaust gas pipe of diesel engines Oxidation of

CO and hydrocarbons is an important process occurring over three-way catalysts.These catalysts are currently employed in the catalytic converters of gasoline engines(close-looped engines) while similar formulations are used in diesel oxidation con-verters Three-way (TW) catalysts contain Pt, Pd and Rh deposited on a mixed oxide

made typically of doped alumina (La, Ca, ) and an oxygen storage capacity

com-ponent (CexZr1 −xO2 binary oxides or CeZrXOy ternary oxides, X being anotherrare earth element).4–7The term “oxygen storage capacity” was introduced by Yaoand Yu Yao to qualify the ability of the catalyst to work in cycling conditions: thesolid stores oxygen during the lean phases and releases it during the rich ones.8

With this method, the noble metals continue to be fed with O species when the O2

concentration significantly decreases in the gas phase

∗Laboratoire de Catalyse en Chimie Organique, UMR 6503 CNRS-Universit´e de Poitiers, 40 avenue du Recteur

Pineau, 86022 Poitiers cedex France.

1

Oxidation of CO and hydrocarbons in conditions of exhaust gas conversion(concentrations around 1% for CO and less for HCs) has been widely studied sincethe implementation of catalytic converters Yu Yao was one of the first authors topublish a systematic study of these reactions over Pt, Pd and Rh catalysts in O2

excess.9, 10 Moreover, the effect of ceria was also investigated, making Yao and

Yu Yao’s reports a source of important information Their results will be analyzedand summarized in the first part of this chapter In the second part, more recentstudies will be reviewed with special attention paid to investigation under cyclingconditions

In TW catalysis, an optimal conversion of all the pollutants (reducers like CO and

HC, and oxidants like NO and NO2)is achieved for an S ratio (defined by Schlatter)11close to unity The S ratio is given in Eq 1.1, in which chemical formulae representthe volume percentages of the gases

CO+ H2+ 3nCnH2n+ (3n + 1)CnH2n +1 (1.1)

The numerator represents the number of O atoms available in the oxidants(O2 and NOx) while the denominator represents the number of O atoms requiredfor a total oxidation of the reducers: CO, HC (alkenes and alkanes) and H2 TheSchlatter equation may easily be extended to other HC (aromatics for instance) oroxygenated compounds However, other gases such as H2O and CO2are not sup-posed to react with pollutants, which is not always observed (see Section 1.4) YuYao investigated CO and HC oxidation in O2excess (S = 2) Oxidation reactions

were carried out over Pd, Pt and Rh catalysts of different metal loading and sions and at different temperatures In the publications of Yu Yao,9, 10 the reactionswere carried out over bulk metals (wires), alumina-supported catalysts and finallyover metals supported on ceria-alumina Specific rates (per gram of catalyst) werereported as well as activation energies, metal dispersions of supported catalysts

disper-or metal area of bulk catalysts From this infdisper-ormation it was possible to calculateturnover frequencies (TOF) extrapolated at a given temperature (the same for everymetal catalyst).12Metal catalysts are compared on the basis of their TOF

1.2.1.1 Effect of metal particle size

Oxidation of carbon monoxide (Eq 1.2) is a reaction which can be catalyzed by allthe noble metals usually employed in TW converters but also by many oxides or

Table 1.1. Intrinsic activity of Pd, Pt and Rh for CO oxidation

at 250 ◦C (s−1) Metal dispersion (%) is given in parentheses.

dispersion Oh et al confirmed that rhodium was more active than platinum in

O2 excess.14, 15 However, Rh is more sensitive than Pt to the presence of NO,14

or a hydrocarbon.15 For instance, in a 0.5%CO+ 0.5%O2 + 500 ppm NO or a0.1%CO+ 1%O2 + 0.2%CH4, Pt appears to be more active than Rh Althoughthe reaction was carried out in conditions far from those encountered in catalytic

converters (silica support), the study by Cant et al gives useful information about

the reaction of the stoichiometry (1.3%CO+ 0.65%O2).16The results are given interms of TOF (molecule CO2 per metal atom per hour) At 127◦C, the following

ranking is observed: Ru; 250 > Pt; 30 > Rh; 23 > Pd; 4 > Ir; 0.4 while at 177◦C, the

same metal/silica catalysts exhibit the following activity: Ru; 5900 > Rh; 900 > Pt;

150 > Pd; 110 > Ir; 12 The changes between 127 and 177◦C are due to the lowestactivation energy of Pt (58 kJ mol1)instead of 100 kJ mol1for the other metals Thevery good behavior of Ru in CO oxidation is also observed in many other reac-tions involved in TW catalysts Unfortunately, the volatility of Ru tetroxide madeimpossible the use of this metal in automotive converters.17

1.2.1.2 Effect of ceria

In O2excess, ceria (20% in alumina) changes the activity of Pt and Pd very littlebut significantly increases that of Rh.10 By contrast, the influence of ceria is muchmore marked around the stoichiometry (S= 1) at least for Pt and Rh.18, 19It is clearthat the beneficial effect of ceria can be observed mainly at low O2concentrationand most probably in cycling conditions

Table 1.2. Kinetic orders and activation energies for CO oxidation at 250 ◦C over Pd, Pt and Rhcatalysts From Ref 10.

a Metal dispersion on alumina: 16–65% for Pd, 4–87% for Pt and 7–69% for Rh.

1.2.1.3 Kinetics and mechanisms

Kinetic data reported byYuYao10are summarized in Table 1.2 Rates were expressedaccording to the power law equation:

r = ke

− E RT

The mechanism generally proposed for the reaction on unsupported metals andalumina-supported catalysts is a classical Langmuir–Hinshelwood mechanism with

CO and O2competing for the same metal sites M

Although heat of O2adsorption is higher than that of CO on noble metals, COcoverage is always higher than that of oxygen (see Section 1.3) Under the conditions

of CO oxidation, CO appears to be more strongly adsorbed than O2 so that: KCO

which explains the order−1 with respect to CO Orders +1 in oxygen can be obtained

by modifying the mechanism, supposing either that O2is not dissociatively adsorbed

The hydrocarbon reactivity depends on numerous factors: chain length, unsaturation,

presence of cycles more or less distorted For instance, Bart et al showed that

light alkanes and acetylene were particularly refractory to oxidation over a cial Pt-Rh/CeO2-Al2O3catalyst.20 Table 1.3 gives the light-off temperatures (T50

commer-required to reach a 50% conversion) of some hydrocarbons over this catalyst In thealkane series, HC reactivity increases significantly with the chain length, methanebeing by far the most refractory hydrocarbon with a T50 above 500◦C Alkenesand aromatics are relatively easy to oxidize, their T50 being comprised between

Table 1.3. Light-off temperatures T50(50% conversion) for different hydrocarbons and hols over a Pt-Rh/CeO2-Al2O3commercial catalyst The synthetic gas mixture contains 0.15%

alco-HC (in C1 equivalent) + 0.61%CO + 0.2%H 2 + 480 ppm NO + 10 CO 2 + 10% H 2 O It is at

the stoichiometry (S = 1) Volumic space velocity was 50,000 h−1 From Ref 20.

Benzene : 205 ◦CToluene : 220 ◦C

o-Xylene : 225 ◦C

Methanol : 195 ◦CEthanol : 200 ◦CPropanol : 205 ◦CButanol : 210 ◦C

185 and 225◦C Bart et al also investigated a series of alcohols whose oxidation

seems very easy

Some of the factors influencing the hydrocarbon reactivity have been recentlyreinvestigated on a series of 48 hydrocarbons.21The main results will be reported

in Section 1.3

1.2.3.1 Effect of metal particle size

Yu Yao investigated the oxidation of C1–C4 alkanes over Pd, Pt and Rh catalysts9

(Table 1.4) As for CO oxidation, TOF were calculated on the basis of specificactivities and metal dispersions reported by Yu Yao For each alkane, TOF wereextrapolated to the same temperature (using the activation energy also reported by

Yu Yao), which allowed a direct comparison between the three metals

It was confirmed that oxidation rates strongly depend on the length of themolecule Palladium was the most active metal for methane oxidation, the order

of activity being: Pd Rh > Pt It is still very active in ethane oxidation with an inversion between Pt and Rh (Pd > Pt > Rh) For C3–C4 hydrocarbons, platinum is

definitely the most active catalyst (Pt Pd  Rh) Whatever the alkane molecule,all the metals show high structure sensitivity in oxidation: the greater the particlesize, the higher the TOF As the specific activity Rm(per gram of metal) is propor-tional to the product D× TOF, there exists a value of the metal dispersion D forwhich Rm is maximal Depending on the hydrocarbon, this optimal dispersion isbetween 15 and 40% for Pt, while it is somewhat higher for Pd (about 50%)

Similar size effects were observed by Hicks et al for methane oxidation

(6.5%CH4+ 15%O2) over Pt and Pd catalysts.22, 23 At 350◦C, TOF of 0.005 s−1

Table 1.4. Oxidation of C1 −C4 alkanes over Pd, Pt and Rh catalysts (unsupported or supported on alumina) From Ref 9.

and 0.008 s−1 were found for well-dispersed and sintered Pt, respectively Theyamounted to 0.02 s−1 and 1.3 s−1 for well-dispersed and sintered Pd catalysts.The preferential orientation along more active surfaces during sintering has beenemployed to interpret these results.24, 25

1.2.3.2 Effect of ceria

Except for Rh, ceria has rather a negative effect in alkane oxidation (Table 1.5).This is essentially due to the fact that O2chemisorption is not a limiting factor inalkane oxidation (see Section 1.2.3.3) Rhodium was also the metal most sensitive

to the presence of ceria for CO oxidation (see Section 1.2.1.2) There is certainly aspecificity to the interaction of this metal with ceria

1.2.3.3 Kinetics and mechanisms

Kinetic data relative to propane oxidation are reported in Table 1.6 Contrary to whatwas observed in CO oxidation, the kinetic orders strongly depend on the nature ofthe metal They are nil or positive for Pd and Rh while, on Pt, a negative orderwith respect to O2and an order of+2 with respect to C3H8are recorded Relativeclose orders (around−1 in O2and+1 in HC) were reported by Yu Yao for methaneoxidation over Pt, which tends to prove that the mechanism of oxidation is similarfor both alkanes

Table 1.5. Effect of ceria in alkane oxidation Activity ratio (per

g of metal) between catalysts supported on 20%CeO2-Al2O3and catalysts supported on pure alumina (S = 2) From Ref 9.

The results of Table 1.6 show that oxygen is more strongly bound to the metalsthan propane The difference is much more marked on Pt than on the other twometals, which leads to a negative order with respect to O2 There is no mechanismunanimously accepted for alkane oxidation on Pt For this reaction an “oxygenoly-sis” mechanism comparable to that of hydrogenolysis has been proposed with thefollowing elementary steps:12

Dehydrogenating adsorption of propane:

On Pt, only Eq 1.14 is able to account for the kinetic observations If mechanism

Eq 1.11–Eq 1.12–Eq 1.14 occurs, the kinetic derivation leads to the following rateequation:

equi-O  1 + KCPC Therate equation (Eq 1.15) can then be simplified:

Table 1.7. Propene oxidation at 150 ◦C: turnover frequencies (s−1)of ported metals and metals supported on alumina Metal dispersion is given in parentheses Gas composition: 0.1% C3H6+ 1% O 2 + N 2 From Ref 9.

1.2.4.1 Effect of metal particle size and effect of ceria

Specific activities of Pd, Pt and Rh catalysts in propene oxidation are reported inTable 1.7 Contrary to what was observed in alkane oxidation, propene oxidation

is not very sensitive to the nature of the metal Quite similar TOF were measuredover unsupported metals, while Pt and Pd seemed to be slightly more active than Rhwhen supported on alumina Propene oxidation is not very sensitive to metal particlesize However, intrinsic activity would be rather higher on small particles As TOFare higher or much higher on unsupported metals, it seems that alumina couldplay a negative role in propene oxidation The intermediary formation of partiallyoxidized compounds (acrolein, alcohols, ) is not excluded Alumina might storeand stabilize these intermediates, slowing down the total oxidation

Propene oxidation is much faster than propane oxidation over Pd and Rh Thereverse tendency would occur over Pt However, propene is more strongly adsorbed

on Pt than propane, which explains why, in the oxidation of C3H6/C3H8mixtures,propene oxidizes first; propane oxidation starts when virtually all the propene isoxidized.26

Ceria has a moderate effect in propene oxidation It is rather positive on Pt and

Rh The presence of 20% ceria in alumina can increase the activity by a factor oftwo or three on these metals For Pd, the effect of ceria seems limited and rathernegative

1.2.4.2 Kinetics and mechanisms

Kinetic orders are very different to those observed for alkane oxidation (Table 1.8).They are rather close to those measured in CO oxidation at least for Pd and Pt,rhodium showing a different behavior

Propene appears to be more strongly adsorbed than O2over Pt and Pd: kineticorders are definitely positive in O2 and negative in C3H6 This inhibiting effect ofpropene is not observed on Rh, on which O2appears to be more strongly adsorbedthan propene

Table 1.8. Propene oxidation over Pd, Pt and Rh catalysts Kinetic orders with respect to O2(m) and to C3H6(n) and activation energies From Ref 9.

As a rule, activation energies are close to those measured on alkanes Again,ceria tends to decrease Ea for Pd and Pt while it is virtually unchanged for Rh

and HC oxidation

Activity of Pd, Pt and Rh catalysts for CO and HC oxidation and correspondingrate equations depend first on the relative adsorption equilibrium of the reducer andoxygen on the metals From the kinetic data reported in Sections 1.2.2 to 1.2.4, thescheme represented in Fig 1.1 can be drawn

This scheme allows us to account for the general behavior of Pd, Pt and Rhcatalysts in oxidation Reducers whose adsorption constant is higher than that of

O2(bars on the right of O2)are strongly adsorbed and behave as inhibitors of thereaction (negative orders while that of O2is positive) Conversely, reducers whoseadsorption constants are lower than that of O2(bars on the left of O2)are weaklyadsorbed: O2 acts as an inhibitor of the reaction (negative orders while those ofthe reducers are positive) Ceria significantly changes this picture as it offers newsites for O2adsorption Chlorine has a detrimental effect on propane and propeneoxidation as it blocks hydrocarbon adsorption.26 Fortunately, water produced dur-ing oxidation leads to progressive catalyst dechlorination, which helps in restoringactivity

Figure 1.1. Relative adsorption constant of CO, propane, propene and O2on Pd, Pt and Rh catalysts (unsupported or supported on alumina).

The most recent advances in CO oxidation were detailed in the review paper of Royerand Duprez.13As far as noble metals are concerned, the kinetic data of Yao and YuYao,8reported in Section 1.2.1., remain valid Nevertheless, the intensive works ofErtl and coworkers on this reaction allowed a more detailed and more exact picture ofwhat really occurs at the metal surface There is an apparent contradiction betweenthe data of adsorption heats of O2and CO and the assumption that O2coverage isvery low in the reaction Heat of chemisorption of CO on noble metals was reported

or reviewed in many papers by Engel and Ertl,27 Nieuwenhuys,28Toyoshima andSomorjai,29Bradford and Vannice30and Ge et al.31For Pt, most of the available datashow that the heat of chemisorption of CO, QCO, are comprised between 109 and

138 kJ mol−1 Only Ge et al reported higher values for QCO Heats of chemisorption

of O2(QO) were first measured by Brennan et al for most metals used in catalysis.32

For Pt they found a value of 265 kJ mol−1, in agreement with the heat reported byNieuwenhuys (230 kJ mol−1).28 The significant differences between QO and QCO

on Pt prompted Yeo et al to revisit the question on Pt (111).33 Again, the sameconclusions could be drawn from this new investigation with values of QO and

QCOat zero coverage of 339 and 185 kJ mol−1, respectively The following questionshould then be addressed: why is oxygen coverage so low in CO oxidation in spite

of its high heat of chemisorption? Several phenomena may explain this apparentdiscrepancy First, O2 should be dissociated upon chemisorption which requirestwo adjacent sites Both Q and Q decrease when CO and O coverage increases

but, because of the dual site requirement, QOdecreases more rapidly in the presence

of CO Secondly, while the sticking coefficient of CO is high (0.8 at zero coverageand 0.2 at full coverage), that of O2is very low (0.05 at zero coverage and 0.02 atfull coverage).33Moreover, the platinum surface is not static, a deep reconstructionoccurring upon CO and O2chemisorption.34, 35While CO chemisorption is not veryperturbed by this surface reconstruction, the chemisorption of O2can be drasticallyaltered These phenomena lead to oscillating reactions with highly contrasted COand O coverage changing with time and space, as clearly demonstrated by the group

of Ertl.36, 37 For that reason, hysteresis in the reaction rate when CO or O2partialpressure is varied, it is often observed.38 The Langmuir–Hinshelwood mechanismdescribed in Section 1.2.1.3 is certainly oversimplified, even though it accounts formost of the kinetic observations made on three-way catalysts

The role of ceria has also been studied in many papers published after 1990.39–42

It is now accepted that two types of sites on ceria should be distinguished: thosethat are located at the metal-support interface and those that are located on thesupport, not in interaction with the metal Direct evidence of these specific sites of

ceria were obtained by Johansson et al on model catalysts prepared by electron

beam lithography.43Most authors concluded that sites located at the metal-support

interface would be very active in CO oxidation Serre et al described these sites as a

bridged oxygen ion bonded both to Pt atoms and Ce ions (Pt-O-Ce) Oxygen would

be very labile with a high propensity of O vacancy formation during the rich phases.The doped catalyst loses a great part of its exceptional activity under prolonged

oxidative medium (S > 1) This implies that the promotion by ceria is more marked

in transient conditions around the stoichiometry than under O2excess

The state of the metal during CO oxidation has also been debated in the erature Though it is expected that Pt remains in the metallic form, the rhodiumcould be largely oxidized in reaction.41 CO adsorption would occur on ionic Rhsites surrounded by O vacancies

lit-All these investigations have described the role of ceria, but the original tion of Yu Yao8and Oh and Eickel18that new sites for O2chemisorption are created

assump-on CeO2remains valid for a good description of kinetic observations

1.3.2.1 Light hydrocarbons (C1–C6)

One of the major results of Yu Yao was to demonstrate that oxidation of light alkaneswas extremely sensitive to the particle size of metals, with turnover frequenciesbeing much higher on big particles (see Table 1.4).9This behavior was confirmed

by Gololobov et al for oxidation of C1–C6 alkanes over Pt.44 Their results aresummarized in Table 1.9

Table 1.9. Turnover frequency of Pt in oxidation of light alkanes (C1–C6).aEffect of particle size of metal TOF values are given in 10 −2s−1 HC concen-tration: 5,000 ppm (in C1 equivalent) diluted in air; GHSV (gas hourly space velocity): 60,000 h −1 From Ref 44.

aReaction temperatures were adjusted to obtain the same n-alkane conversion

on the catalyst sample having a mean particle size of 1.3 nm.

It is confirmed that longer alkanes are easier to oxidize: the temperature at whichthe same oxidation rate can be observed decreases with the number of carbon atoms

in the molecule (420◦C for methane vs 172◦C for hexane) The particle size sitivity significantly increases with the chain length: the ratio between maximal

sen-and minimal TOF values is 3.6 for methane while it amounts to 26.5 for n-hexane.

Within the domain of dPtinvestigated here (1.3–11.5 nm), TOF values continuously

increase with the particle size for n-hexane while a maximum is observed at 2.9 nm

for methane and ethane

Although alkanes are not strongly adsorbed over noble metals, it seems thatethane and propane may shift light-off temperatures for CH4 oxidation to highervalues This behavior was clearly observed over Pd catalysts and ascribed to a change

of the reduction state of Pd in the presence of ethane and propane.45 Inhibition ofmethane oxidation by C2–C3 alkanes is virtually not observed over Pt, whereasstrongly adsorbed hydrocarbons such as alkenes and acetylene may strongly affectalkane oxidation.46

Many authors have shown that the support could play a role, not only inchanging particle size but also in modifying adsorption properties of the metals.Ceria could stabilize ionic species of platinum leading to a strong metal-support

interaction Bera et al have compared the behavior of Pt/CeO2 and Pt/Al2O3 in

TW catalysis.47 The enhanced activity observed in several reactions (CO+ O2,

CO+ NO and HC + O2, Table 1.10) has been attributed to the formation of newsites (-O2−Ce4+-O2−Ptn+-O2−with n= 2 or 4) Ceria-supported catalysts are moreactive than alumina ones for all the reactions NO as an oxidant is more sensitive

in nature to support than O2 Moreover, ceria is a better promoter for oxidation of

CO and propane than that of methane Whatever the oxidant (NO or O2), methaneoxidation remains difficult with a modest promotion by ceria

Table 1.10. Support effect on Pt and Pd catalysts for different reactions involved

in three-way catalysis The catalytic activity is given by the temperature at which

a conversion close to 100% is observed Partial pressures of reactants are in the range of 1–20 Torr with molar ratios close to the stoichiometry From Ref 47.

The deep oxidation of C2-C4alkanes has been studied by Garetto et al over Pt

cat-alysts supported on various oxides.48Pt/zeolite catalysts show better performances

in oxidation than Pt/Al2O3and Pt/MgO Support acidity is not a major contributingfactor for this enhancement of activity, the promotion effect being observed both

on acidic (HY, ZSM5, Beta) and rather basic zeolites (KL) The following turnoverfrequencies (h−1)were obtained for propane oxidation at 250◦C (C3H8/O2/N2molarratio= 0.8/9.9/89.3):

Pt/Beta; 39,000 > Pt/ZSM5; 10,000 > Pt/HY; 9,000 > Pt/KL; 1,400 > Pt/Al2O3;

85 > Pt/MgO; 30.

It seems that the zeolites allow a better adsorption of alkanes by using a ment effect A similar effect was observed on hierarchical porous silica membraneswith mesopores of 4 nm.49 These mesopores allowed a better stabilization of Ptparticles and preferential adsorption of the reactants

confine-Less conventional supports were also used for hydrocarbon oxidation For

exam-ple, Postole et al investigated the performance of palladium deposited on boron

nitride.50 This PdO/BN catalyst showed relatively good performances in propeneand methane oxidation even in the presence of moisture

Acidic or basic promoters can affect the activity of Pt and Pd supported on mina or zirconia.51, 52Basic promoters (e.g Na) improve CO and propene oxidationwhile they inhibit NO oxidation By contrast, acidic promoters (e.g sulfates) showthe greatest effect in propane oxidation Carbon monoxide and propene are toostrongly adsorbed (with respect to O2): the effect of basic promoters would be toweaken their adsorption on the metals and thereby to reinforce that of oxygen Thereverse is true for acidic promoters: they strengthen the adsorption of propane, tooweakly adsorbed on Pt and Pd (see Section 1.2.5) The same result can be observed

alu-if sulfates are replaced by SO directly injected with the propane/air mixture.53The

effect of acidic promoters on Rh/Al2O3is somewhat different Lee et al reported

that sulfates increased both the reactivity of propene and propane.54This result can

be explained by the relative strengths of C3H8, C3H6and O2adsorption on Rh onwhich both the alkene and the alkane are weakly adsorbed (Fig 1.1)

The state of palladium can change upon hydrocarbon oxidation Under stoichiometry of O2, PdO is reduced before CO and alkene oxidation starts while

sub-it is reduced during the light-off oxidation of propane.55, 56In methane combustion,even in O2excess, it was proven that the oxidation starts when some PdO sites arereduced by CH4.57, 58 The active sites of palladium for methane oxidation would

be created by association of oxidized Pd2 +and reduced Pd Finally, combining Pd

with other metals like Ni can improve the catalytic activity in CO and propeneoxidation.59Again, the support (CeZrOx/Al2O3)plays a decisive role in favoringinteraction between the two metals

1.3.2.2 Heavy hydrocarbons (>C6)

If oxidation of light hydrocarbons were still the subject of many studies in the1990s, the tendency after 2000 has been to investigate oxidation of heavier hydro-carbons representing the fraction of HC emissions between gaseous hydrocarbonsand soots.21, 60, 61Catalytic oxidation of 48 hydrocarbons from C6 to C20 was inves-

tigated by Diehl et al over a well-dispersed Pt/Al2O3catalyst.21From this study itwas possible to draw some conclusions and rules about hydrocarbon reactivity as afunction of their molecular structure

1.3.2.2a Normal alkanes and alkane isomers

The tendency observed with light alkanes is confirmed up to n-C20: n-alkane

oxid-ability increases with the chain length with a correlative decrease of the light-offtemperature (temperature for a 50% conversion, T50) Nevertheless, the decrease of

T50is weak above 10 carbon atoms (Table 1.11) A correlation was found between

Table 1.11. Light-off temperature of normal alkanes over

a 1%Pt-Al2O3catalyst Reaction conditions: HC tion: 1,500 ppm C diluted in air; air flow rate: 20 cm3min −1;

concentra-35 mg catalyst From Ref 21.

the light-off temperature and the ionization potential of hydrocarbon Normal nes are adsorbed on Pt by C-H bond rupture and C-Pt bond formation O2can formvery reactive oxygen species (O, superoxides, peroxides) by an electron transferfrom the metal to O atoms A likely hypothesis is that these electrons may comefrom the adsorbed hydrocarbon: the easier the hydrocarbon ionization, the higherthe oxygen reactivity.

alka-Oxidation of alkane isomers strongly depends on the number of primary,secondary, tertiary or quaternary carbons in the molecule Secondary carbons and,still more, tertiary carbons are easy to oxidize while primary and quaternary car-bons are much more difficult to oxidize An example is given in Fig 1.2 for selectedhydrocarbons containing eight carbon atoms They can be ranked by decreasing oxi-

dation rate: n-octane ≈ methyl-2-heptane > trimethyl-2,3,4-pentane  2,2-hexane > trimethyl-2,2,4-pentane tetramethyl-2,2,3,3-butane This latterhydrocarbon, which contains only primary and quaternary carbons, is extremelydifficult to oxidize

dimethyl-1.3.2.2b Normal alkenes and alkene isomers

The behavior of alkenes in oxidation significantly differs from that of alkanes mal alkenes are generally easy to oxidize over Pt, Pd and Rh Contrary to normal

Nor-alkanes, their reactivity depends little on the chain length for 7 < n < 10 while the

light-off temperature of shorter alkenes (C2–C6) slightly increases with the chain

0 10

Table 1.12. Light-off temperatures of normal alkenes and alkene isomers with seven and eight carbon atoms Reaction conditions: see Table 1.11 From Ref 21.

Light-off temperature T50(◦C)

length.62Light olefins (ethylene, propene, butenes) oxidize at much lower

tempera-tures than the corresponding alkanes, while n-heptene and n-octene oxidize at around

200◦C, the same temperature as n-heptane and n-octane For longer hydrocarbons, the reverse situation is even observed: n-decene oxidizes at a higher temperature

(200◦C) than n-decane (167◦C) For these hydrocarbons, the superiority of Pt foroxidation reactions is much less marked.63, 64

Contrary to branched alkanes, alkene isomers are rather more reactive than the

corresponding n-alkenes (Table 1.12) As a matter of fact, normal alkenes are too

strongly adsorbed on the metals Substituting H atoms by alkyl groups tends todecrease the adsorption heat of alkene isomers, which contributes to equilibrate theadsorption coefficients of hydrocarbons and oxygen.65

Octadienes were also studied by Diehl et al who showed that the presence of

a second double bond has little effect on the reactivity of the hydrocarbon Forinstance, T50 = 201◦C for oct-1-ene, 208◦C for octa-1,5-diene and 211◦C for

Table 1.13. Effect of CO on hydrocarbon oxidation (T50in ◦C) Reaction conditions: mixture

of hydrocarbons (25 ppm each) + 0.6%O 2 + 1%CO (when present); Gas hourly space velocity: 30,000 h-1 Catalysts: metal/Al2O3/monolith crushed and sieved to 100−300 µm Metal content

(in the whole monolith): 0.306%Pt; 0.305%Pd; 0.246%Rh From Ref 63.

Other molecules present in the gas mixtures like alcohols or ketones may have amoderate effect on aromatic oxidation but the reverse (inhibition of alcohol oxidation

by aromatics) is most often observed.69Different supports of Pt were used for tolueneoxidation: Al2O3,21, 63, 70, 71Al2O3/Al,72 ZnO/Al2O3,73 TiO2,69 mesoporous fibroussilica74 or monoliths.75 Zeolites, generally promoted by platinum, were shown togive excellent catalysts for aromatic oxidation.76 Basic zeolites showed excellent

performances in oxidation of m-xylene even in the absence of platinum.77Palladiumcatalysts, either supported on alumina or ceria-alumina, were also investigated foroxidation of benzene and several alkylbenzenes.78, 79

The nature and the size of the alkyl substituent on the aromatic cycle may have a

great effect on the conversion efficiency Diehl et al showed that the light-off

tem-peratures of monoalkylbenzenes on Pt/Al2O3increased with the number of carbonatoms in the alkyl group.21 This tendency was observed up to four carbon atoms

For very long alkyl groups (as in n-decylbenzene), T50decreases and becomes close

to that of benzene (Table 1.14) Diehl et al also compared the oxidation of several

polyalkylbenzenes It was shown that the monomethylbenzene (toluene) oxidized

at the highest temperature: increasing the number of methyl groups increases thehydrocarbon reactivity A similar tendency is observed for branched alkyl groups(mono and ditertiobutyl)

1.3.2.2d Polycyclic hydrocarbons

Polycyclic aromatic hydrocarbons (PAH) are generally difficult to oxidize lene was generally chosen as a model of these PAHs Pt/Al O 80Pd/Ce-Al O ,81

Naphtha-Table 1.14. Effect of substituents (nature and number) on the reactivity of alkylbenzenes over Pt/Al2O3 Reaction conditions: see Table 1.11.

Diehl et al showed that naphthalene oxidized over Pt/Al2O3at 240◦C far over the

light-off temperature of n-decane (171◦C) However, when the bicyclic molecule ispartially (tetraline) or totally (decaline) hydrogenated, its oxidation becomes mucheasier (T50 = 230◦C for tetraline and 205◦C for decaline).21Oxidation of polycyclichydrocarbons is often not total at moderate temperatures Two kinds of behavior can

be observed:21

— Oxygenated intermediates are formed during the light-off tests and total version to CO2and H2O is achieved only at very high conversions The case offluorene is typical of this behavior The selectivity to CO2exceeds 50% only forconversions greater than 80% There is a significant formation of fluorenone allalong the fluorene conversion

con-— Dehydrogenated intermediates can be formed up to very high conversions Forinstance, acenaphthene oxidation gives rise to acenaphthylene as an intermediate

of reaction The selectivity to acenaphthylene is still 20% at a 40% conversion

Oxidation of CO is fast over noble metals The presence of CO may inhibit theoxidation of the most reactive hydrocarbons

The reactivity of normal alkanes increases with the chain length, methane being

by far the most difficult hydrocarbon to oxidize As a rule, alkane isomers are lessreactive than normal alkanes with the same number of carbon atoms Isomers withquaternary carbons are extremely difficult to oxidize Alkenes are more reactive thanalkanes and, contrary to these hydrocarbons, alkene isomers are easier to oxidizethan normal alkenes

Monocyclic aromatics are generally oxidized around 200◦C over Pt Nature,number and size of the alkyl groups can affect the reactivity of the benzene ring.Polycyclic hydrocarbons are more difficult to oxidize The reaction is often complexand gives rise to oxygenated or dehydrogenated intermediates up to high conver-sions As a matter of fact, the selectivity to CO2should be checked very carefully

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Chapter 2 Soot Oxidation in Particulate Filter Regeneration

Junko UCHISAWA, Akira OBUCHI and Tetsuya NANBA ∗

This chapter deals with the mechanism of soot oxidation for the regeneration of particulate filters Nature of catalysts and effect of process parameters (temperature,

NO 2 partial pressure, intimacy of contact between soot and catalyst, ) are reviewed The role of oxygen mobility and of nature of oxygen species are also discussed.

Generally, diesel engines are operated under lean-burn conditions, with excess air tofuel, so that the exhaust gases contain lower concentrations of CO and hydrocarbonsthan gasoline engines, which are operated near the stoichiometric air-to-fuel ratio.However, the liquid fuel is directly injected and burned in the cylinder; therefore,the air-to-fuel ratio may become very rich locally and part of the fuel is thermallydecomposed to form solid carbonaceous particles, commonly referred to as soot Thesoluble organic fraction (SOF) derived from fuel components with higher boilingpoints, lubricant oil, and sulfate (sulfuric acid mist) produced from sulfur in the fuelare later attached to this soot, which forms the main constituent of diesel particulatematter (PM)

Most of the PM mass from diesel engines ranges from 0.1 to 0.5µm in diameter,1

which is much less than the suspended particulate matter (SPM) standard prescribed

in the Air Quality Standards of Japan (10µm or less), and the PM2.5 (2.5 µm orless) standard prescribed by the United States Environmental Agency To make mat-ters worse, the majority of PM is made up of ultrafine particles with diameters of0.005–0.1µm Ultrafine particles of 0.05 µm or smaller are called nanoparticles, andthese cause concern with regard to invasion of respiratory organs such as bronchialtubes, alveolar cells, and further blood vessels, and are thus detrimental to humanhealth.2

∗National Institute of Advanced Industrial Science and Technology (AIST), Energy-saving System Team, Research

Center for New Fuels and Vehicle Technology, 16-1 Onogawa, Tsukuba, 305-8569, Japan.

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