Copyright 2003 by Taylor & Francis Group, LLCLibraryofCongressCataloging-in-PublicationDataAcatalogrecordforthisbookisavailablefromtheLibraryofCongress.ISBN:0-8247-0879-2Thisbookisprintedonacid-freepaper.HeadquartersMarcelDekker,Inc.270MadisonAvenue,NewYork,NY10016tel:212-696-9000;fax:212-685-4540EasternHemisphereDistributionMarcelDekkerAGHutgasse4,Postfach812,CH-4001Basel,Switzerlandtel:41-61-260-6300;fax:41-61-260-6333WorldWideWebhttp://www.dekker.comThe publisher offers discounts onthis bookwhenorderedinbulkquantities. For moreinformation, write to Special Sales/Professional Marketing at the headquarters address above.Copyright # 2003byMarcelDekker,Inc.AllRightsReserved.Neither thisbooknor anypart maybereproducedortransmittedinanyformor byanymeans,electronicormechanical,includingphotocopying,microlming,andrecording,orbyany information storage and retrieval system, without permission in writing fromthepublisher.Currentprinting(lastdigit):10987654321PRINTEDINTHEUNITEDSTATESOFAMERICACopyright 2003 by Taylor & Francis Group, LLCForewordCatalysis andElectrocatalysis at Nanoparticle Surfaces reects many of the newdevelopments of catalysis, surface science, and electrochemistry. The rst threechaptersindicatethesophisticationof thetheoryinsimulatingcatalyticprocessesthat occur at the solidliquidandsolidgas interface inthe presence of externalpotential. The rst chapter, by Koper and colleagues, discusses the theory ofmodelingof catalyticandelectrocatalyticreactions. Thisisfollowedbystudiesofsimulationsofreactionkineticsonnanometer-sizedsupportedcatalyticparticlesbyZhdanovandKasemo. Thenal theoretical chapter, byPacchioni andIllas, dealswith the electronic structure and chemisorption properties of supported metalclusters.Chapter4,byBatzillandhiscoworkers, describesmodernsurfacecharacter-ization techniques that include photoelectron diffraction and ion scattering as well asscanning probe microscopies. The chapter by Hayden discusses model hydrogen fuelcell electrocatalysts, and the chapter by Ertl and Schuster addresses theelectrochemical nanostructuring of surfaces. Henry discusses adsorption andreactions onsupportedmodel catalysts, andGoodmanandSantradescribesize-dependentelectronicstructureandcatalyticpropertiesofmetal clusterssupportedon ultra-thin oxide lms. In Chapter 9, Markovic and his coworkers discuss modernphysical and electrochemical characterization of bimetallic nanoparticle electro-catalysts.Bo nnemannandRichardsleadoffthesectiononsyntheticapproacheswithadiscussionof nanomaterials as electrocatalysts totailor structure andinterfacialproperties. Teranishi andToshima as well as Simonov andLikholobov discusspreparation and characterization of supported monometallic and bimetallicnanoparticles.Van der Klink and Tong cover NMR studies of heterogeneous andelectrochemical catalysts, andX-rayabsorptionspectroscopystudiesarethefocusofthechapterby Mukerjee.In Chapter15,StimmingandCollinsdiscussSTMandinfraredspectroscopyinstudiesoffuelcellmodelcatalyst.Copyright 2003 by Taylor & Francis Group, LLCLambert reviews the role of alkali additives on metal lms and nanoparticles inelectrochemicalandchemicalbehaviormodications.Metal-supportinteractionsisthe subject of the chapter by Arico and coauthors for applications in lowtemperature fuel cellelectrocatalysts,and Haruta and Tsubota look at the structureand size effect of supported noble metal catalysts in low temperature CO oxidation.Promotionof catalytic activityandthe importance of spillover are discussedbyVayenas and coworkers in a very interesting chapter, followed by Verykiossexaminationofsupporteffectsandcatalyticperformanceofnanoparticles. InsituinfraredspectroscopystudiesofplatinumgroupmetalsattheelectrodeelectrolyteinterfacearereviewedbySun.Watanabediscussesthedesignofelectrocatalystsforfuel cells, andCoqandFiguerasaddressthequestionofparticlesizeandsupporteffectsoncatalyticpropertiesofmetallicandbimetalliccatalysts.In Chapter 24, Duoand coworkers discuss metal oxide nanoparticlereactivityonsyntheticborondopeddiamondsurfaces.LamyandLe gertreatelectrocatalysiswith electron-conducting polymers in the presence of noble metal nanoparticles, andnewnanostructurematerialsforelectrocatalysisarethesubjectofthenalchapter,byAlonso-Vante.Thereisnodoubtthatthisbookwillbeavaluableadditiontothelibraryofsurfacescientists, electrochemists, andthoseworkingininterfacescienceswhoareinterestedinresearchthat isbeingcarriedout at thefrontiersof solidliquidandsolidgas interfaces as well as nanomaterials. The contributors are outstandingsenior researchers and the volume gives a glimpse of the present and the future in thisfrontier area of physical chemistry and interface chemistry: molecular-level catalysisandelectrochemistry, andtheirapplicationsinthepresenceof nanoparticles. Theeditorsshouldbecommendedfordoingsuchanexcellentjobincollectingsuperbchaptersforthisoutstandingvolume.GaborSamorjaiUniversityofCaliforniaatBerkeleyBerkeley,California,U.S.A.Copyright 2003 by Taylor & Francis Group, LLCPrefaceCatalysis andElectrocatalysis at NanoparticleSurfaces is amodern, authoritativetreatise that provides comprehensive coverage of recent advances in nanoscalecatalytic and electrocatalytic reactivity. It is a newreference on catalytic andelectrochemical nanotechnology, surface science, andtheoretical modelingat thegraduatelevel.Heterogeneous catalysis and aqueous or solid-state electrochemistry have beentreated traditionally as different branches of physical chemistry, yet similar conceptsare usedinthese elds and, over the past three decades, similar surface sciencetechniques and ab initio quantummechanical approaches have been used toinvestigate their fundamental aspects at the molecular level. The growingtechnological interest in fuel cellsboth high-temperature solid oxide fuel cells(SOFC)andlow-temperaturepolymerelectrolytemembrane(PEM)fuelcellshasdrawn the attention of the electrochemical community to three-dimensionalelectrodeswithnelydispersedactivecomponents(metals,alloys, semiconductors)anchored to appropriate conducting supports. This has raised the question of specicelectrochemical properties of small-dimensionsystems. Utilizationof nano-sizedmaterials introduces a number of phenomena specic for nanoscale, includingparticle size effects, metal-support interactions, and spillover, which are common forheterogeneous catalysis andelectrocatalysis, andcalls for joint efforts fromtheexperts in these elds. This has brought the catalytic and electrochemicalcommunitiescloser,astheneedforheterogeneouscatalyticknowledgeindesigningand operating efcient fuel cell anodes and cathodes is being more widely recognized.Theelectricalpotentialdependenceoftheelectrochemical(electrocatalytic,i.e.,netcharge transfer) reaction rate has traditionally been considered a characteristicfeature that distinguished it from the catalytic (no net charge transfer) reaction rate.Yetrecentadvanceshaveshownthatthisdistinctionisnotsorigid,astheratesofcatalytic processes at nanoparticles in contact with solid or aqueous electrolytes alsodepend profoundly on catalyst potential, similar to the electrochemical reaction rate,andthat aclassicelectrochemical double-layerapproachisalsoapplicabletothekineticsofcatalyticreactions.Copyright 2003 by Taylor & Francis Group, LLCThecollectionof26invitedchaptersbyprominentscholarsoriginatingfromboththeelectrochemicalandthecatalyticschoolsofthoughtportraysthisgrowingand mutually enriching merge of the elds of heterogeneous catalysis andelectrocatalysis to address new technological and fundamental challenges of catalyticandelectrocatalyticreactivityatnanoparticlesurfaces.Comprehensiveness is the key qualier for this book. Its unique feature is thatitprovidesnearlycompletecoverageofthemaintopicsincontemporarynanoscalecatalysisandelectrocatalysis,withcoherent,state-of-the-artpresentationoftheory,experimentation, andapplications. This is the rst volume of its kindtobringtogether suchabroadanddiverseproleof surfacescienceandelectrochemistryfrom the perspective of unique nanoscale surface properties. Similarly, it emphasizestheemergenceandprogress of knowledgeof nanoparticlesurfaceproperties, thenewest subeld of surface science and electrochemistry, which has not been reviewedbeforeexceptinselectedproceedingsvolumes. Surfacescienceandelectrochemicalsurfacescience,jointlypresentedinthisvolume,haveenabledscientiststodevelopatomic- andmolecular-level perspectives of reactions occurringat gassolidandliquidsolidinterfaces. Suchperspectives, bothcurrentandforecastforthefuture,areamplydemonstrated.The strength of the eld lies in its employment of basic science fortechnological applications, inrelationtomaterials, catalysis, andenergy efforts.Themainthrustofthisbookisthatitpresentsanduniestwomaininterfacesforuse in heterogeneous catalysis and electrocatalysis, for both basic science andapplications. The contributors were asked to make their chapters accessible toadvanced graduate students in surface science, catalysis, electrochemistry, andrelated areas. The tutorial feature of the volume adds to its strength and educationalvalue.The book is divided into six parts: theory of nanoparticle catalysis andelectrocatalysis; model systems fromsingle crystals to nanoparticles; syntheticapproaches innanoparticle catalysis andelectrocatalysis; advancedexperimentalconcepts; particle size, support, andpromotional effects; andadvanced electro-catalytic materials. This facilitates access tothe general readers interests. Eachchapter begins with a summary and a table of contents to provide an overview of itsscope.Part I presents the state of the art of the theory of catalysis and electrocatalysisat clusters and nanoparticles. This section provides the current frame of modeling oftheinteractionof clusters withsubstrates as well as catalyticandelectrocatalytickinetics on clusters and nanoparticles, including ab initio quantummechanicalcalculations,andepitomizesrecentadvancesinunderstandingtherelationbetweenelectronicstructureandcatalytic/electrocatalyticactivity.PartIIaddressesthekeyquestionofhowelectronicandgeometricpropertiesofcatalyticsurfaceschangeuponmovingfromsinglecrystalstonanoparticlesandhow this inuences their catalytic/electrocatalytic activity. The use of model catalystsandelectrocatalystsinconjunctionwithpowerful surfacespectroscopictechniquesprovidesnewinsightintotheuniquephenomenaobservedupondecreasingthesizeof catalyst particles downtonanoscale andbuilds alinkbetweencatalysis andelectrocatalysis at perfect single crystals andnanoparticles. The chapters inthissectionscrutinize approaches toelectrochemical nanostructuringof surfaces andprovideinsightintouniquebehaviorofsmall,connedsystems.Copyright 2003 by Taylor & Francis Group, LLCPartIIIpresentsthestateoftheartofthesynthesisofnanoparticlecatalystsand electrocatalysts,including bimetallic nanoparticles. Particularemphasis is giventocarbon-supportednanoparticles due totheir technological signicance inthefabricationofelectrodesforPEMfuelcells.Part IVdescribesrecent breakthroughsintheuseof advancedexperimentaltechniques for the in situ study of nanoparticle catalysts and electrocatalysts,includingX-rayabsorptionspectroscopy,NMR,andSTM.PartVaddressesthecritical issuesofparticlesizeeffects, alloying, spillover,classic and electrochemical promotion, and metalsupport interactions on thecatalytic andelectrocatalytic performance of nanoparticles. Recent experimentalevidenceis presentedonthefunctional similarities andoperational differences ofpromotion,electrochemicalpromotion,andmetalsupportinteractions.Part VI discusses novel advanced electrocatalytic materials, including polymer-embedded nanoparticle electrodes for PEMfuel cells and synthetic diamondsupportedelectrocatalystnanoparticlesfortoxicorganiccompoundtreatment.This bookwill be anindispensable source of knowledge inlaboratories orresearch centers that specialize in fundamental and practical aspects of hetero-geneous catalysis, electrochemistry, and fuel cells. Its unique presentation of the keybasicresearchonsuchtopics inarichinterdisciplinarycontext will facilitatetheresearchers task of improving catalytic materials, in particular for fuel cellapplications, basedonscientic logic rather thanexpensive Edisoniantrial-and-errormethods.Thehighlightofthevolumeistherichandcomprehensivecoverageof experimental andtheoretical aspects of nanoscale surface science andelectro-chemistry. We hope that readers will benet fromits numerous ready-to-usetheoretical formalisms andexperimental protocols of general scientic value andutility.AndrzejWieckowskiElenaR.SavinovaConstantinosG.VayenasCopyright 2003 by Taylor & Francis Group, LLCContentsForeword GaborSamorjaiPrefaceContributorsI. THEORYOFNANOPARTICLECATALYSISANDELECTROCATALYSIS1. TheoryandModelingofCatalyticandElectrocatalyticReactionsMarcT.M.Koper,RutgerA.vanSanten,andMatthewNeurock2. SimulationsoftheReactionKineticsonNanometer-SizedSupportedCatalystParticlesV.P.ZhdanovandB.Kasemo3. ElectronicStructureandChemisorptionPropertiesofSupportedMetalClusters:ModelCalculationsGianfrancoPacchioniandFrancescIllasII. MODELSYSTEMS:FROMSINGLECRYSTALSTONANOPARTICLES4. State-of-the-ArtCharacterizationofSingle-CrystalSurfaces:AViewofNanostructuresMatthiasBatzill,SantanuBanerjee,andBruceE.Koel5. Single-CrystalSurfacesasModelPlatinum-BasedHydrogenFuelCellElectrocatalystsBrianE.Hayden6. ElectrochemicalNanostructuringofSurfacesRolfSchusterandGerhardErtl7. AdsorptionandReactionatSupportedModelCatalystsClaudeR.HenryCopyright 2003 by Taylor & Francis Group, LLC8. Size-DependentElectronic,Structural,andCatalyticPropertiesofMetalClustersSupportedonUltrathinOxideFilmsA.K.SantraandD.W.Goodman9. PhysicalandElectrochemicalCharacterizationofBimetallicNanoparticleElectrocatalystsN.M.Markovic,V.Radmilovic,andP.N.Ross,Jr.III. SYNTHETICAPPROACHESINNANOPARTICLECATALYSISANDELECTROCATALYSIS10. NanomaterialsasPrecursorsforElectrocatalystsHelmutBonnemannandRyanRichards11. Preparation,Characterization,andPropertiesofBimetallicNanoparticlesToshiharuTeranishiandNaokiToshima12. PhysicochemicalAspectsofPreparationofCarbon-SupportedNobleMetalCatalystsP.A.SimonovandV.A.LikholobovIV. ADVANCEDEXPERIMENTALCONCEPTS13. NMRInvestigationsofHeterogeneousandElectrochemicalCatalystsY.Y.TongandJ.J.vanderKlink14. In-SituX-RayAbsorptionSpectroscopyofCarbon-SupportedPtandPt-AlloyElectrocatalysts:CorrelationofElectrocatalyticActivitywithParticleSizeandAlloyingSanjeevMukerjee15. STMandInfraredSpectroscopyinStudiesofFuelCellModelCatalysts:ParticleStructureandReactivityJ.A.CollinsandU.StimmingV. PARTICLESIZE,SUPPORT,ANDPROMOTIONALEFFECTS16. ElectrochemicalandChemicalPromotionbyAlkaliswithMetalFilmsandNanoparticlesRichardM.Lambert17. Metal-SupportInteractioninLow-TemperatureFuelCellElectrocatalystsA.S.Arico,P.L.Antonucci,andV.Antonucci18. EffectsofSizeandContactStructureofSupportedNobleMetalCatalystsinLow-TemperatureCOOxidationMasatakeHarutaandSusumuTsubota19. Promotion,ElectrochemicalPromotion,andMetalSupportInteractions:TheUnifyingRoleofSpilloverConstantinosG.Vayenas,C.Pliangos,S.Brosda,andD.TsiplakidesContentsCopyright 2003 by Taylor & Francis Group, LLC20. SupportEffectsonCatalyticPerformanceofNanoparticlesXenophonE.Verykios21. AbnormalInfraredEffectsofNanometer-ScaleThinFilmMaterialofPlatinumGroupMetalsandAlloysatElectrodeElectrolyteInterfacesShi-GangSun22. DesignofElectrocatalystsforFuelCellsMasahiroWatanabe23. EffectsofParticleSizeandSupportonSomeCatalyticPropertiesofMetallicandBimetallicCatalystsBernardCoqandFranc oisFiguerasVI. ADVANCEDELECTROCATALYTICMATERIALS24. ConductiveMetal-OxideNanoparticlesonSyntheticBoron-DopedDiamondSurfacesI.Duo,SergioFerro,AchilleDeBattisti,andC.Comninellis25. ElectrocatalysiswithElectron-ConductingPolymersModiedbyNobleMetalNanoparticlesC.LamyandJ.-M.Leger26. NovelNanostructuredMaterialBasedonTransition-MetalCompoundsforElectrocatalysisNicolasAlonso-VanteContentsCopyright 2003 by Taylor & Francis Group, LLCContributorsNicola sAlonso-Vante Universite dePoitiers,Poitiers,FranceP.L.Antonucci UniversityofReggioCalabria,ReggioCalabria,ItalyV.Antonucci InstituteforTransformationandStorageofEnergy,Messina,ItalyA.S.Arico InstituteforTransformationandStorageofEnergy,Messina,ItalySantanuBanerjee UniversityofSouthernCalifornia,LosAngeles,California,U.S.A.MatthiasBatzill UniversityofSouthernCalifornia,LosAngeles,California,U.S.A.HelmutBo nnemann MaxPlanckInstitutfu rKohlenforschung,Mu lheimanderRuhr,GermanyS.Brosda UniversityofPatras,Patras,GreeceJ.A.Collins TechnicalUniversityofMunich,Munich,GermanyC.Comninellis SwissFederalInstituteofTechnologyEPFL,Lausanne,SwitzerlandBernardCoq ENSCM-CNRS,Montpellier,FranceAchilleDeBattisti Universita` diFerrara,Ferrara,ItalyI.Duo SwissFederalInstituteofTechnologyEPFL,Lausanne,SwitzerlandGerhardErtl Fritz-Haber-InstitutderMax-Planck-Gesellschaft,Berlin,GermanySergioFerro Universita` diFerrara,Ferrara,ItalyFranc oisFigueras InstitutdeRecherchesurlaCatalyse,Villeurbanne,FranceD.W.Goodman TexasA&MUniversity,CollegeStation,Texas,U.S.A.Copyright 2003 by Taylor & Francis Group, LLCMasatakeHaruta NationalInstituteofAdvancedIndustrialScienceandTechnology,Tsukuba,JapanBrianE.Hayden TheUniversityofSouthampton,Southampton,EnglandClaudeR.Henry CRMC2-CNRS,Marseille,FranceFrancescIllas UniversitatdeBarcelona,Barcelona,SpainB.Kasemo ChalmersUniversityofTechnology,Go teborg,SwedenBruceE.Koel UniversityofSouthernCalifornia,LosAngeles,California,U.S.A.MarcT.M.Koper EindhovenUniversityofTechnology,Eindhoven,TheNetherlandsRichardM.Lambert CambridgeUniversity,Cambridge,EnglandC.Lamy Universite dePoitiers-CNRS,Poitiers,FranceJ.-M.Le ger Universite dePoitiers-CNRS,Poitiers,FranceV.A.Likholobov BoreskovInstituteofCatalysisattheRussianAcademyofSciences,Novosibirsk,RussiaN. M. Markovic University of California at Berkeley, Berkeley, California, U.S.A.SanjeevMukerjee NortheasternUniversity,Boston,Massachusetts,U.S.A.MatthewNeurock UniversityofVirginia,Charlottesville,Virginia,U.S.A.GianfrancoPacchioni Universita` diMilano-Bicocca,Milan,ItalyC.Pliangos UniversityofPatras,Patras,GreeceV.Radmilovic UniversityofCaliforniaatBerkeley,Berkeley,California,U.S.A.Ryan Richards* Max Planck Institut fu r Kohlenforschung, Mu lheim an der Ruhr,GermanyP.N.Ross,Jr. UniversityofCaliforniaatBerkeley,Berkeley,California,U.S.A.A.K.Santra TexasA&MUniversity,CollegeStation,Texas,U.S.A.RolfSchuster Fritz-Haber-InstitutderMax-Planck-Gesellschaft,Berlin,GermanyP. A. Simonov Boreskov Institute of Catalysis at the Russian Academy of Sciences,Novosibirsk,RussiaU.Stimming TechnicalUniversityofMunich,Munich,GermanyShi-GangSun XiamenUniversity,Xiamen,ChinaToshiharuTeranishi JapanAdvancedInstituteofScienceandTechnology,Ishikawa,JapanY.Y.Tong GeorgetownUniversity,Washington,D.C.,U.S.A.NaokiToshima TokyoUniversityofScience,Yamaguchi,Japan* Currentafliation:InternationalUniversityofBremen,Bremen,GermanyCopyright 2003 by Taylor & Francis Group, LLCD.Tsiplakides UniversityofPatras,Patras,GreeceSusumuTsubota NationalInstituteofAdvancedIndustrialScienceandTechnology,Ikeda,JapanJ.J.vanderKlink EcolePolytechniqueFederaledeLausanne,Lausanne,SwitzerlandRutgerA.vanSanten EindhovenUniversityofTechnology,Eindhoven,TheNetherlandsConstantinosG.Vayenas UniversityofPatras,Patras,GreeceXenophonE.Verykios UniversityofPatras,Patras,GreeceMasahiroWatanabe UniversityofYamanashi,Kofu,JapanV.P.Zhdanov ChalmersUniversityofTechnology,Go teborg,Sweden,andBoreskovInstituteofCatalysisattheRussianAcademyofSciences,Novosibirsk,RussiaCopyright 2003 by Taylor & Francis Group, LLCCopyright 2003 by Taylor & Francis Group, LLCLibraryofCongressCataloging-in-PublicationDataAcatalogrecordforthisbookisavailablefromtheLibraryofCongress.ISBN:0-8247-0879-2Thisbookisprintedonacid-freepaper.HeadquartersMarcelDekker,Inc.270MadisonAvenue,NewYork,NY10016tel:212-696-9000;fax:212-685-4540EasternHemisphereDistributionMarcelDekkerAGHutgasse4,Postfach812,CH-4001Basel,Switzerlandtel:41-61-260-6300;fax:41-61-260-6333WorldWideWebhttp://www.dekker.comThe publisher offers discounts onthis bookwhenorderedinbulkquantities. For moreinformation, write to Special Sales/Professional Marketing at the headquarters address above.Copyright # 2003byMarcelDekker,Inc.AllRightsReserved.Neither thisbooknor anypart maybereproducedortransmittedinanyformor byanymeans,electronicormechanical,includingphotocopying,microlming,andrecording,orbyany information storage and retrieval system, without permission in writing fromthepublisher.Currentprinting(lastdigit):10987654321PRINTEDINTHEUNITEDSTATESOFAMERICACopyright 2003 by Taylor & Francis Group, LLCForewordCatalysis andElectrocatalysis at Nanoparticle Surfaces reects many of the newdevelopments of catalysis, surface science, and electrochemistry. The rst threechaptersindicatethesophisticationof thetheoryinsimulatingcatalyticprocessesthat occur at the solidliquidandsolidgas interface inthe presence of externalpotential. The rst chapter, by Koper and colleagues, discusses the theory ofmodelingof catalyticandelectrocatalyticreactions. Thisisfollowedbystudiesofsimulationsofreactionkineticsonnanometer-sizedsupportedcatalyticparticlesbyZhdanovandKasemo. Thenal theoretical chapter, byPacchioni andIllas, dealswith the electronic structure and chemisorption properties of supported metalclusters.Chapter4,byBatzillandhiscoworkers, describesmodernsurfacecharacter-ization techniques that include photoelectron diffraction and ion scattering as well asscanning probe microscopies. The chapter by Hayden discusses model hydrogen fuelcell electrocatalysts, and the chapter by Ertl and Schuster addresses theelectrochemical nanostructuring of surfaces. Henry discusses adsorption andreactions onsupportedmodel catalysts, andGoodmanandSantradescribesize-dependentelectronicstructureandcatalyticpropertiesofmetal clusterssupportedon ultra-thin oxide lms. In Chapter 9, Markovic and his coworkers discuss modernphysical and electrochemical characterization of bimetallic nanoparticle electro-catalysts.Bo nnemannandRichardsleadoffthesectiononsyntheticapproacheswithadiscussionof nanomaterials as electrocatalysts totailor structure andinterfacialproperties. Teranishi andToshima as well as Simonov andLikholobov discusspreparation and characterization of supported monometallic and bimetallicnanoparticles.Van der Klink and Tong cover NMR studies of heterogeneous andelectrochemical catalysts, andX-rayabsorptionspectroscopystudiesarethefocusofthechapterby Mukerjee.In Chapter15,StimmingandCollinsdiscussSTMandinfraredspectroscopyinstudiesoffuelcellmodelcatalyst.Copyright 2003 by Taylor & Francis Group, LLCLambert reviews the role of alkali additives on metal lms and nanoparticles inelectrochemicalandchemicalbehaviormodications.Metal-supportinteractionsisthe subject of the chapter by Arico and coauthors for applications in lowtemperature fuel cellelectrocatalysts,and Haruta and Tsubota look at the structureand size effect of supported noble metal catalysts in low temperature CO oxidation.Promotionof catalytic activityandthe importance of spillover are discussedbyVayenas and coworkers in a very interesting chapter, followed by Verykiossexaminationofsupporteffectsandcatalyticperformanceofnanoparticles. InsituinfraredspectroscopystudiesofplatinumgroupmetalsattheelectrodeelectrolyteinterfacearereviewedbySun.Watanabediscussesthedesignofelectrocatalystsforfuel cells, andCoqandFiguerasaddressthequestionofparticlesizeandsupporteffectsoncatalyticpropertiesofmetallicandbimetalliccatalysts.In Chapter 24, Duoand coworkers discuss metal oxide nanoparticlereactivityonsyntheticborondopeddiamondsurfaces.LamyandLe gertreatelectrocatalysiswith electron-conducting polymers in the presence of noble metal nanoparticles, andnewnanostructurematerialsforelectrocatalysisarethesubjectofthenalchapter,byAlonso-Vante.Thereisnodoubtthatthisbookwillbeavaluableadditiontothelibraryofsurfacescientists, electrochemists, andthoseworkingininterfacescienceswhoareinterestedinresearchthat isbeingcarriedout at thefrontiersof solidliquidandsolidgas interfaces as well as nanomaterials. The contributors are outstandingsenior researchers and the volume gives a glimpse of the present and the future in thisfrontier area of physical chemistry and interface chemistry: molecular-level catalysisandelectrochemistry, andtheirapplicationsinthepresenceof nanoparticles. Theeditorsshouldbecommendedfordoingsuchanexcellentjobincollectingsuperbchaptersforthisoutstandingvolume.GaborSamorjaiUniversityofCaliforniaatBerkeleyBerkeley,California,U.S.A.Copyright 2003 by Taylor & Francis Group, LLCPrefaceCatalysis andElectrocatalysis at NanoparticleSurfaces is amodern, authoritativetreatise that provides comprehensive coverage of recent advances in nanoscalecatalytic and electrocatalytic reactivity. It is a newreference on catalytic andelectrochemical nanotechnology, surface science, andtheoretical modelingat thegraduatelevel.Heterogeneous catalysis and aqueous or solid-state electrochemistry have beentreated traditionally as different branches of physical chemistry, yet similar conceptsare usedinthese elds and, over the past three decades, similar surface sciencetechniques and ab initio quantummechanical approaches have been used toinvestigate their fundamental aspects at the molecular level. The growingtechnological interest in fuel cellsboth high-temperature solid oxide fuel cells(SOFC)andlow-temperaturepolymerelectrolytemembrane(PEM)fuelcellshasdrawn the attention of the electrochemical community to three-dimensionalelectrodeswithnelydispersedactivecomponents(metals,alloys, semiconductors)anchored to appropriate conducting supports. This has raised the question of specicelectrochemical properties of small-dimensionsystems. Utilizationof nano-sizedmaterials introduces a number of phenomena specic for nanoscale, includingparticle size effects, metal-support interactions, and spillover, which are common forheterogeneous catalysis andelectrocatalysis, andcalls for joint efforts fromtheexperts in these elds. This has brought the catalytic and electrochemicalcommunitiescloser,astheneedforheterogeneouscatalyticknowledgeindesigningand operating efcient fuel cell anodes and cathodes is being more widely recognized.Theelectricalpotentialdependenceoftheelectrochemical(electrocatalytic,i.e.,netcharge transfer) reaction rate has traditionally been considered a characteristicfeature that distinguished it from the catalytic (no net charge transfer) reaction rate.Yetrecentadvanceshaveshownthatthisdistinctionisnotsorigid,astheratesofcatalytic processes at nanoparticles in contact with solid or aqueous electrolytes alsodepend profoundly on catalyst potential, similar to the electrochemical reaction rate,andthat aclassicelectrochemical double-layerapproachisalsoapplicabletothekineticsofcatalyticreactions.Copyright 2003 by Taylor & Francis Group, LLCThecollectionof26invitedchaptersbyprominentscholarsoriginatingfromboththeelectrochemicalandthecatalyticschoolsofthoughtportraysthisgrowingand mutually enriching merge of the elds of heterogeneous catalysis andelectrocatalysis to address new technological and fundamental challenges of catalyticandelectrocatalyticreactivityatnanoparticlesurfaces.Comprehensiveness is the key qualier for this book. Its unique feature is thatitprovidesnearlycompletecoverageofthemaintopicsincontemporarynanoscalecatalysisandelectrocatalysis,withcoherent,state-of-the-artpresentationoftheory,experimentation, andapplications. This is the rst volume of its kindtobringtogether suchabroadanddiverseproleof surfacescienceandelectrochemistryfrom the perspective of unique nanoscale surface properties. Similarly, it emphasizestheemergenceandprogress of knowledgeof nanoparticlesurfaceproperties, thenewest subeld of surface science and electrochemistry, which has not been reviewedbeforeexceptinselectedproceedingsvolumes. Surfacescienceandelectrochemicalsurfacescience,jointlypresentedinthisvolume,haveenabledscientiststodevelopatomic- andmolecular-level perspectives of reactions occurringat gassolidandliquidsolidinterfaces. Suchperspectives, bothcurrentandforecastforthefuture,areamplydemonstrated.The strength of the eld lies in its employment of basic science fortechnological applications, inrelationtomaterials, catalysis, andenergy efforts.Themainthrustofthisbookisthatitpresentsanduniestwomaininterfacesforuse in heterogeneous catalysis and electrocatalysis, for both basic science andapplications. The contributors were asked to make their chapters accessible toadvanced graduate students in surface science, catalysis, electrochemistry, andrelated areas. The tutorial feature of the volume adds to its strength and educationalvalue.The book is divided into six parts: theory of nanoparticle catalysis andelectrocatalysis; model systems fromsingle crystals to nanoparticles; syntheticapproaches innanoparticle catalysis andelectrocatalysis; advancedexperimentalconcepts; particle size, support, andpromotional effects; andadvanced electro-catalytic materials. This facilitates access tothe general readers interests. Eachchapter begins with a summary and a table of contents to provide an overview of itsscope.Part I presents the state of the art of the theory of catalysis and electrocatalysisat clusters and nanoparticles. This section provides the current frame of modeling oftheinteractionof clusters withsubstrates as well as catalyticandelectrocatalytickinetics on clusters and nanoparticles, including ab initio quantummechanicalcalculations,andepitomizesrecentadvancesinunderstandingtherelationbetweenelectronicstructureandcatalytic/electrocatalyticactivity.PartIIaddressesthekeyquestionofhowelectronicandgeometricpropertiesofcatalyticsurfaceschangeuponmovingfromsinglecrystalstonanoparticlesandhow this inuences their catalytic/electrocatalytic activity. The use of model catalystsandelectrocatalystsinconjunctionwithpowerful surfacespectroscopictechniquesprovidesnewinsightintotheuniquephenomenaobservedupondecreasingthesizeof catalyst particles downtonanoscale andbuilds alinkbetweencatalysis andelectrocatalysis at perfect single crystals andnanoparticles. The chapters inthissectionscrutinize approaches toelectrochemical nanostructuringof surfaces andprovideinsightintouniquebehaviorofsmall,connedsystems.Copyright 2003 by Taylor & Francis Group, LLCPartIIIpresentsthestateoftheartofthesynthesisofnanoparticlecatalystsand electrocatalysts,including bimetallic nanoparticles. Particularemphasis is giventocarbon-supportednanoparticles due totheir technological signicance inthefabricationofelectrodesforPEMfuelcells.Part IVdescribesrecent breakthroughsintheuseof advancedexperimentaltechniques for the in situ study of nanoparticle catalysts and electrocatalysts,includingX-rayabsorptionspectroscopy,NMR,andSTM.PartVaddressesthecritical issuesofparticlesizeeffects, alloying, spillover,classic and electrochemical promotion, and metalsupport interactions on thecatalytic andelectrocatalytic performance of nanoparticles. Recent experimentalevidenceis presentedonthefunctional similarities andoperational differences ofpromotion,electrochemicalpromotion,andmetalsupportinteractions.Part VI discusses novel advanced electrocatalytic materials, including polymer-embedded nanoparticle electrodes for PEMfuel cells and synthetic diamondsupportedelectrocatalystnanoparticlesfortoxicorganiccompoundtreatment.This bookwill be anindispensable source of knowledge inlaboratories orresearch centers that specialize in fundamental and practical aspects of hetero-geneous catalysis, electrochemistry, and fuel cells. Its unique presentation of the keybasicresearchonsuchtopics inarichinterdisciplinarycontext will facilitatetheresearchers task of improving catalytic materials, in particular for fuel cellapplications, basedonscientic logic rather thanexpensive Edisoniantrial-and-errormethods.Thehighlightofthevolumeistherichandcomprehensivecoverageof experimental andtheoretical aspects of nanoscale surface science andelectro-chemistry. We hope that readers will benet fromits numerous ready-to-usetheoretical formalisms andexperimental protocols of general scientic value andutility.AndrzejWieckowskiElenaR.SavinovaConstantinosG.VayenasCopyright 2003 by Taylor & Francis Group, LLCContentsForeword GaborSamorjaiPrefaceContributorsI. THEORYOFNANOPARTICLECATALYSISANDELECTROCATALYSIS1. TheoryandModelingofCatalyticandElectrocatalyticReactionsMarcT.M.Koper,RutgerA.vanSanten,andMatthewNeurock2. SimulationsoftheReactionKineticsonNanometer-SizedSupportedCatalystParticlesV.P.ZhdanovandB.Kasemo3. ElectronicStructureandChemisorptionPropertiesofSupportedMetalClusters:ModelCalculationsGianfrancoPacchioniandFrancescIllasII. MODELSYSTEMS:FROMSINGLECRYSTALSTONANOPARTICLES4. State-of-the-ArtCharacterizationofSingle-CrystalSurfaces:AViewofNanostructuresMatthiasBatzill,SantanuBanerjee,andBruceE.Koel5. Single-CrystalSurfacesasModelPlatinum-BasedHydrogenFuelCellElectrocatalystsBrianE.Hayden6. ElectrochemicalNanostructuringofSurfacesRolfSchusterandGerhardErtl7. AdsorptionandReactionatSupportedModelCatalystsClaudeR.HenryCopyright 2003 by Taylor & Francis Group, LLC8. Size-DependentElectronic,Structural,andCatalyticPropertiesofMetalClustersSupportedonUltrathinOxideFilmsA.K.SantraandD.W.Goodman9. PhysicalandElectrochemicalCharacterizationofBimetallicNanoparticleElectrocatalystsN.M.Markovic,V.Radmilovic,andP.N.Ross,Jr.III. SYNTHETICAPPROACHESINNANOPARTICLECATALYSISANDELECTROCATALYSIS10. NanomaterialsasPrecursorsforElectrocatalystsHelmutBonnemannandRyanRichards11. Preparation,Characterization,andPropertiesofBimetallicNanoparticlesToshiharuTeranishiandNaokiToshima12. PhysicochemicalAspectsofPreparationofCarbon-SupportedNobleMetalCatalystsP.A.SimonovandV.A.LikholobovIV. ADVANCEDEXPERIMENTALCONCEPTS13. NMRInvestigationsofHeterogeneousandElectrochemicalCatalystsY.Y.TongandJ.J.vanderKlink14. In-SituX-RayAbsorptionSpectroscopyofCarbon-SupportedPtandPt-AlloyElectrocatalysts:CorrelationofElectrocatalyticActivitywithParticleSizeandAlloyingSanjeevMukerjee15. STMandInfraredSpectroscopyinStudiesofFuelCellModelCatalysts:ParticleStructureandReactivityJ.A.CollinsandU.StimmingV. PARTICLESIZE,SUPPORT,ANDPROMOTIONALEFFECTS16. ElectrochemicalandChemicalPromotionbyAlkaliswithMetalFilmsandNanoparticlesRichardM.Lambert17. Metal-SupportInteractioninLow-TemperatureFuelCellElectrocatalystsA.S.Arico,P.L.Antonucci,andV.Antonucci18. EffectsofSizeandContactStructureofSupportedNobleMetalCatalystsinLow-TemperatureCOOxidationMasatakeHarutaandSusumuTsubota19. Promotion,ElectrochemicalPromotion,andMetalSupportInteractions:TheUnifyingRoleofSpilloverConstantinosG.Vayenas,C.Pliangos,S.Brosda,andD.TsiplakidesContentsCopyright 2003 by Taylor & Francis Group, LLC20. SupportEffectsonCatalyticPerformanceofNanoparticlesXenophonE.Verykios21. AbnormalInfraredEffectsofNanometer-ScaleThinFilmMaterialofPlatinumGroupMetalsandAlloysatElectrodeElectrolyteInterfacesShi-GangSun22. DesignofElectrocatalystsforFuelCellsMasahiroWatanabe23. EffectsofParticleSizeandSupportonSomeCatalyticPropertiesofMetallicandBimetallicCatalystsBernardCoqandFranc oisFiguerasVI. ADVANCEDELECTROCATALYTICMATERIALS24. ConductiveMetal-OxideNanoparticlesonSyntheticBoron-DopedDiamondSurfacesI.Duo,SergioFerro,AchilleDeBattisti,andC.Comninellis25. ElectrocatalysiswithElectron-ConductingPolymersModiedbyNobleMetalNanoparticlesC.LamyandJ.-M.Leger26. NovelNanostructuredMaterialBasedonTransition-MetalCompoundsforElectrocatalysisNicolasAlonso-VanteContentsCopyright 2003 by Taylor & Francis Group, LLCContributorsNicola sAlonso-Vante Universite dePoitiers,Poitiers,FranceP.L.Antonucci UniversityofReggioCalabria,ReggioCalabria,ItalyV.Antonucci InstituteforTransformationandStorageofEnergy,Messina,ItalyA.S.Arico InstituteforTransformationandStorageofEnergy,Messina,ItalySantanuBanerjee UniversityofSouthernCalifornia,LosAngeles,California,U.S.A.MatthiasBatzill UniversityofSouthernCalifornia,LosAngeles,California,U.S.A.HelmutBo nnemann MaxPlanckInstitutfu rKohlenforschung,Mu lheimanderRuhr,GermanyS.Brosda UniversityofPatras,Patras,GreeceJ.A.Collins TechnicalUniversityofMunich,Munich,GermanyC.Comninellis SwissFederalInstituteofTechnologyEPFL,Lausanne,SwitzerlandBernardCoq ENSCM-CNRS,Montpellier,FranceAchilleDeBattisti Universita` diFerrara,Ferrara,ItalyI.Duo SwissFederalInstituteofTechnologyEPFL,Lausanne,SwitzerlandGerhardErtl Fritz-Haber-InstitutderMax-Planck-Gesellschaft,Berlin,GermanySergioFerro Universita` diFerrara,Ferrara,ItalyFranc oisFigueras InstitutdeRecherchesurlaCatalyse,Villeurbanne,FranceD.W.Goodman TexasA&MUniversity,CollegeStation,Texas,U.S.A.Copyright 2003 by Taylor & Francis Group, LLCMasatakeHaruta NationalInstituteofAdvancedIndustrialScienceandTechnology,Tsukuba,JapanBrianE.Hayden TheUniversityofSouthampton,Southampton,EnglandClaudeR.Henry CRMC2-CNRS,Marseille,FranceFrancescIllas UniversitatdeBarcelona,Barcelona,SpainB.Kasemo ChalmersUniversityofTechnology,Go teborg,SwedenBruceE.Koel UniversityofSouthernCalifornia,LosAngeles,California,U.S.A.MarcT.M.Koper EindhovenUniversityofTechnology,Eindhoven,TheNetherlandsRichardM.Lambert CambridgeUniversity,Cambridge,EnglandC.Lamy Universite dePoitiers-CNRS,Poitiers,FranceJ.-M.Le ger Universite dePoitiers-CNRS,Poitiers,FranceV.A.Likholobov BoreskovInstituteofCatalysisattheRussianAcademyofSciences,Novosibirsk,RussiaN. M. Markovic University of California at Berkeley, Berkeley, California, U.S.A.SanjeevMukerjee NortheasternUniversity,Boston,Massachusetts,U.S.A.MatthewNeurock UniversityofVirginia,Charlottesville,Virginia,U.S.A.GianfrancoPacchioni Universita` diMilano-Bicocca,Milan,ItalyC.Pliangos UniversityofPatras,Patras,GreeceV.Radmilovic UniversityofCaliforniaatBerkeley,Berkeley,California,U.S.A.Ryan Richards* Max Planck Institut fu r Kohlenforschung, Mu lheim an der Ruhr,GermanyP.N.Ross,Jr. UniversityofCaliforniaatBerkeley,Berkeley,California,U.S.A.A.K.Santra TexasA&MUniversity,CollegeStation,Texas,U.S.A.RolfSchuster Fritz-Haber-InstitutderMax-Planck-Gesellschaft,Berlin,GermanyP. A. Simonov Boreskov Institute of Catalysis at the Russian Academy of Sciences,Novosibirsk,RussiaU.Stimming TechnicalUniversityofMunich,Munich,GermanyShi-GangSun XiamenUniversity,Xiamen,ChinaToshiharuTeranishi JapanAdvancedInstituteofScienceandTechnology,Ishikawa,JapanY.Y.Tong GeorgetownUniversity,Washington,D.C.,U.S.A.NaokiToshima TokyoUniversityofScience,Yamaguchi,Japan* Currentafliation:InternationalUniversityofBremen,Bremen,GermanyCopyright 2003 by Taylor & Francis Group, LLCD.Tsiplakides UniversityofPatras,Patras,GreeceSusumuTsubota NationalInstituteofAdvancedIndustrialScienceandTechnology,Ikeda,JapanJ.J.vanderKlink EcolePolytechniqueFederaledeLausanne,Lausanne,SwitzerlandRutgerA.vanSanten EindhovenUniversityofTechnology,Eindhoven,TheNetherlandsConstantinosG.Vayenas UniversityofPatras,Patras,GreeceXenophonE.Verykios UniversityofPatras,Patras,GreeceMasahiroWatanabe UniversityofYamanashi,Kofu,JapanV.P.Zhdanov ChalmersUniversityofTechnology,Go teborg,Sweden,andBoreskovInstituteofCatalysisattheRussianAcademyofSciences,Novosibirsk,RussiaCopyright 2003 by Taylor & Francis Group, LLC1TheoryandModelingofCatalyticandElectrocatalyticReactionsMARCT.M.KOPERandRUTGERA.VANSANTENEindhovenUniversityofTechnology,Eindhoven,TheNetherlandsMATTHEWNEUROCKUniversityofVirginia,Charlottesville,Virginia,U.S.A.CHAPTERCONTENTS1.1 Introduction1.2 OverviewofComputationalMethods1.3 ReactivityandCatalystElectronicStructure1.4 TheSolventinElectrodeReactions1.5 MonteCarloSimulationsofCatalyticReactions1.6 ConclusionsSUMMARYAbriefoverviewisgivenofthemaintheoreticalprinciplesandcomputermodelingtechniques to describe catalytic and electrocatalytic reactions. Our guiding principlesin understanding the relationship between electronic structure and catalystperformance are the periodic table and the Sabatier principle. We discuss thedifferent modeling tactics in describing quantum-mechanically adsorbate-surfaceinteractions (i.e., cluster vs. slabmodels) andtheir accuracyinmodeling(small)catalyst particles. Theroleof thesolvent inelectrocatalyticprocessesisdescribedboth fromthe point of view of quantumchemical densityfunctional theorycalculations, and frommore classical Marcus theory-type considerations usingmolecular dynamics simulations.Furthermore, we discuss how kinetic Monte CarloCopyright 2003 by Taylor & Francis Group, LLCmethods can be used to bridge the gap between microscopic reaction models and thecatalystsmacroscopicperformanceinanaccuratestatisticalandmechanicalway.1.1 INTRODUCTIONWith the incessant boom in computational power of present-day computers, the roleof theory and modeling in the understanding and development of catalytic processesis becoming increasingly important. Theory and computational modeling havealready played a signicant role in heterogeneous gas-phase catalysis for quite someyears, but comparabledevelopments intheelectrocatalysis of reactions at metalliquidinterfaceshavebeenlessforthcoming.However,newtheorydevelopmentsinelectrochemistry, the emergence of newcomputational possibilities withmodernmachinesandcodes,aswellastheincreasingoverlapbetweenelectrochemistryandsurfacesciencepromiseasimilarlyprominentroleofcomputationalelectrochem-istryintheelectrocatalysiscommunity.The aimof this chapter is toprovide the reader withanoverviewof thepotential of modern computational chemistry in studying catalytic and electro-catalytic reactions. This will take us fromstate-of-the-art electronic structurecalculations of metaladsorbate interactions, through (ab initio) molecular dynamicssimulations of solvent effects in electrode reactions, to lattice-gas-based Monte Carlosimulations of surface reactions taking place on catalyst surfaces. Rather thanextensively discussing all the different types of studies that have been carried out, wefocus on what we believe to be a few representative examples. We also point out themoregeneraltheoryprinciplestobedrawnfromthesestudies,aswellasrefertosome of the relevant experimental literature that supports these conclusions.Examples are primarily takenfromour ownwork; other recent reviewpapers,mainlyfocusedongas-phasecatalysis,canbefoundin[13].Thenextsectiongivesabriefoverviewofthemaincomputationaltechniquescurrently applied to catalytic problems. These techniques include ab initio electronicstructurecalculations, (abinitio) moleculardynamics, andMonteCarlomethods.Thenext threesectionsare devoted toparticular applicationsof thesetechniquestocatalytic and electrocatalytic issues. We focus on the interaction of CO and hydrogenwith metal and alloy surfaces, both from quantum-chemical and statistical-mechanical points of view, as these processes playanimportant role infuel-cellcatalysis. We alsodemonstrate the role of the solvent inelectrocatalytic bond-breaking reactions, using molecular dynamics simulations as well as extensiveelectronic structure andabinitiomolecular dynamics calculations. Monte Carlosimulations illustrate the importance of lateral interactions, mixing, andsurfacediffusion in obtaining a correct kinetic description of catalytic processes. Finally, wesummarizethemainconclusionsandgiveanoutlookoftheroleofcomputationalchemistryincatalysisandelectrocatalysis.Copyright 2003 by Taylor & Francis Group, LLC1.2 OVERVIEWOFCOMPUTATIONALMETHODS1.2.1 ElectronicStructureCalculationsElectronicstructurecalculationsaregenerallyconcernedwithsolvingtheelectronicgroundstate of a certainmolecular ensemble basedonrst-principles quantummechanics. The techniques are called ab initio if they require only the atomicnumbers of atoms involvedintheensemble, andnoadjustableparameters takenfrom experiment or other calculations. A good introduction into the various ab initioandotherquantum-chemicalmethodsisgiveninJensensbook[4].By far the most popular method currently used in the quantum-chemicalmodeling of adsorbatesubstrate interactions is that based on the density functionaltheory (DFT). Contrary to the more classical quantum-chemical techniques, DFTisnotdirectlybasedonelectronicwavefunctionsbutratheronelectronicdensity.TheformalcornerstoneofDFTisatheoremderivedbyHohenbergandKohn[5],whichstatesthat theground-stateelectronicenergyis auniquefunctional of theelectronic density n(r), with r the space coordinate. In other words, there exists a one-to-one correspondence between the r-dependent electronic density of the system andthe energy. However, the exact functional giving the exact energy is not known, andinpracticeonemustthereforeresorttooneofthemanyapproximateexpressionsavailable. The quality of these functionals is now such that one may calculate overallenergies to within an accuracy of about 510%of the exact result. For manypurposes,thisaccuracyisquitesufcient.Inpractical calculations, theelectronicdensityis still calculatedfromwavefunctions, using a method originally devised by Kohn and Sham [6]. These so-calledKohnShamorbitalscorrespondtoavirtual systemofnoninteractingelectrons,similartoHartreeFockmethods.EventhoughtheseKohnShamorbitalshavenoclear physical meaning, they may still be quite useful in interpreting the results fromaDFTcalculationintermsofmolecularorbitaltheory.The great advantage of DFT over other, say HartreeFock-based, methods, isthat it does not require toomuchcomputational effort toinclude the effects ofelectroniccorrelationinthecalculation, whichis knowntobeveryimportant inobtainingreliableenergiesof chemical bonds. Allit takesis an improvedfunctionalusing the same set of KohnShamorbitals, whereas the HartreeFock-basedmethodscanonlyaccountforelectroniccorrelationbyconsiderablyincreasingthenumber of wave functions, leadingtoverylongcomputationtimes for eventhesmallest systemsizes. This makes DFTthe methodof choice for handlinglargesystems,suchasadsorbatesinteractingwithsurfaces.Themost popularseriesof functionalsusedincomputational chemistryarethosebasedonthegeneralizedgradient approximation(GGA). Thesefunctionalsincludenotonlytheelectronicdensity, butalsoitsspatial derivative(gradient), inorder toaccount more accuratelyfor longer-rangedelectronic correlationeffects(though very long-ranged electronic correlation effects, such as Van der Waalsforces, are notaccurately treatedwithintheGGA).This typeof calculationis oftenreferred to as DFT-GGA. Various DFT-GGAfunctionals are available; somepopular ones have been tested in [4,7,8]. As mentioned, the accuracy of thesemethodsisroughly510%.Becauseextendedsurfacesandevennanoparticlesarelargesystemsfromtheatomisticpointof view, thequestion stillarises ofwhatkind of molecularensembleCopyright 2003 by Taylor & Francis Group, LLCwouldconstituteareasonablemodel fortheadsorbatecatalyst interaction. Here,essentiallytwodifferenttypesofapproacheshavebeentaken. Oneapproachistomodel thesurfaceasasmall clusterofatoms, whereatypical clustersizeis1050atoms. It is important tonotethat thissizeis still smaller thanthat of atypicalcatalytic nanoparticle, whichconsists of at least several hundreds of atoms (andusually more)[9]. A second approach that has become popular in the last 10 years istomodel thesurfaceasaperiodicslab, inwhichacertainensembleisperiodicallyrepeated in three dimensions by applying periodic boundary conditions in thecomputational setup. A surface, and a molecule-surface ensemble, is then modeled asa number of layers (typically 35 layers) of atoms in one part of the periodic cell, andavacuumintheotherpartofthecell. Thevacuumregionischosenlargeenoughsuchthatthedifferentperiodicimagesofthesurfacehaveanegligibleinteractionwith each other. One nal comment to be made about clusters and slabs concerns thedifferent types of basis sets in which the KohnSham orbitals are expanded. Clustercalculations are usually carriedout witha standardquantum-chemical packageemployingso-calledlocalizedbasis sets. These localizedbasis sets enable one tointerprettheKohnShamorbitalsasmolecularorbitalsasintroducedinquantum-chemical molecular orbital theory [10]. This may have great advantages in obtainingamorechemistry-basedunderstandingofbindingtrends. Slabcalculations, ontheotherhand, usuallyemployso-calledplanewavesastheirbasisset, becauseoftheperiodicityof the simulationcell [11,12]. Plane waves are not localized,and hence itis more difcult (though not impossible) to obtain molecular orbital-typeinformation. However, it isnowgenerallyagreedthat slabcalculationsgivemorereliable quantitative results whenthe binding energetics of the molecule-surfaceinteractionis concerned. More local information, suchas bindingdistances andvibrational properties, may still be calculated with good accuracy using clustermodels.WediscusstheseissuesinsomemoredetailinSection1.3.1.2.2 AbInitioMolecularDynamicsOne very prominent development in DFThas been the coupling of electronicstructurecalculations (which, whenthe groundstateis concerned, applytozerotemperature) with nite-temperature molecular dynamics simulations. The foundingpaper inthis eldwas publishedbyCarr andParrinelloin1985[13]. Carr andParrinelloformulate effective equations of motionfor the electrons tobe solvedsimultaneously with the classical equations of motion for the ions. The forces on theionsarecalculatedfromrstprinciplesbyuseoftheHellmanFeynmantheorem.An alternative to the CarrParrinello method is to solve the electronic structure self-consistently at every ionic time step. Bothmethods are referredtoas abinitiomoleculardynamics(AIMD)[14].AIMD is still a very time-consuming simulation method and has so far mainlybeenusedtostudythe structure anddynamics of bulkwater [14,15], as well asproton transfer [16] and simple SN2 reactions in bulk water [17]. AIMDsimulationsare as yet limited to small system sizes and real simulation times of not more thanafewpicoseconds.However,somerstapplicationsofthistechniquetointerfacialsystems of interest toelectrochemistry have appeared, suchas the watervaporinterface[18]andthestructureofthemetalwaterinterface[19].ThereisnodoubtCopyright 2003 by Taylor & Francis Group, LLCthat AIMDsimulations of electrochemical interfaces will become increasinglyimportantinthefuture,andsomerstresultsaredescribedinSection1.4.2.1.2.3 MolecularDynamicsInclassicalmoleculardynamics(MD)methods[20]theatomsandmoleculesinthesystemofinterestinteractthrougheffectivepairpotentials,whichmaybeobtainedfromprevious abinitioelectronic structure calculations. However, the quantumnatureofthesystemisnotexplicitlytakenintoaccount,andthetimeevolutionofthe system is obtained by solving Newtons classical equations of motion. Averagingof the MDtrajectories over a sufciently long simulationperiodallows one toextract thermodynamic, dynamical, andother macroscopicproperties. Dependingon the system size (which can include up to several thousands of atoms) and the typeofparticlesinvolved,typicalrealsimulationtimesinclassicalMDstudiescanbeaslongasseveralnanoseconds.AswediscussinSection1.4.1,MDsimulationsareofgreatvalueininvestigatingtheroleofsolventreorganizationinelectron-transferreactions.1.2.4 MonteCarloAnalternative tothe MDmethodinobtainingstatistical averages is the MonteCarlo(MC) simulationmethod, whichis basedonanefcient samplingof low-energy congurations rather than on generating a dynamical trajectory [20]. Adrawbackof the standardMCalgorithmis that it does not give anydynamicalinformation, though for certain purposes it may be a more efcient way ofcalculatingthermodynamicproperties.A method closely related to the standard Metropolis-type MC algorithm is asimulation technique known as kinetic Monte Carlo or dynamic Monte Carlo(DMC).Thismethodisespeciallyusefulforstudyingprocessesonlattices,suchas,for instance, catalytic reactions taking place onthe reaction sites of a catalystsurface.Onerstdenestheadsorptionrates,desorptionrates,reactionrates,anddiffusionratesofthevariousreactionsandprocessesassumedtotakeplaceonthesurface, quantities that may infact be estimatedfromrst-principles electronicstructure calculations. Next, the evolution of the entire system is obtained by solvingthe so-called master equation using an MC-type algorithm [2124]. The algorithm isdesignedsuchthattheexacttimedependenceisobtained, i.e., thatthesubsequentcongurationsgeneratedsatisfythecorrectdetailedbalance.AsSection1.5shows,thismethodisveryuseful inobtainingthecorrectdynamicbehaviorofareactivesystemfromthe elementary processes. There is noneedtoresort tostatistical-mechanical approximationschemes suchas the mean-eldapproximation[9] inwhichall rates areexpressedinterms of averagecoverages. Suchapproximationschemes may break down severely if the catalystsurface is very inhomogeneous, forinstanceduetopoor mixingof thereactants onthecatalyst surfaceor toalowconcentration of catalytically active sites, or when strong interactions betweenadsorbates exist. DMCis themethodof choicefor themicrokineticmodelingofcatalyticreactions onsurfaceswhenordering, islandformation, andslowsurfacemobility are deemed important. Lattice sizes in DMC can be as large as 1000 61000sites (corresponding to 1 million surface sites), whereas the real computation timesCopyright 2003 by Taylor & Francis Group, LLCobviously depend on the (lowest) rate constants but can span time scales from a fewmillisecondstoseveralminutes.1.3 REACTIVITYANDCATALYSTELECTRONICSTRUCTURE1.3.1 ModelingSmallParticlesTheroleofparticlesizeincatalysisandelectrocatalysisisasubjectoflongstandinginterest. It is not our intention here to discuss in detail the available experimental andtheoretical literature. Extensive reviews on particle-size effects in gas-phase catalysisandelectrocatalysiscanbefoundinthepapersof Henry[25] andKinoshita[26],respectively. Also, several monographs, reviews, and conference proceedings discussparticle-size effects from experimental, theoretical, and computational points of view[9,27,28].Itiswellknownthattheelectronicpropertiesofsmallmetalparticlesdependonthe particle size. The workfunctionor cluster ionizationpotential is amostdramatic example of this effect. Knickelbeinet al. [29] have shownthat for Niclusters consistingof 3to90atoms the cluster ionizationpotential continues tochangeandforthelargest particlesizeis still about 0.2 eVhigherthantheworkfunction of an extended surface. There is, however, a growing awareness thatchangesinreactivityandchemisorptionpropertieswithparticlesizereectamuchmorelocal phenomenon. Smallerparticleshavemoreedgesandkinksthanlargerparticles, and these local sites usually possess a reactivity that is substantiallydifferent from those of the terraces. Surface-science studies in UHV andelectrocatalysis have convincingly demonstratedthe special reactivity of defects,showingthat incertaincases thecatalyticreactiontakes placeonlyonstepanddefectsites.These facts obviously raise the question of what constitutes the bestcomputational model of a small catalytic particle. As catalysis is often a localphenomenon,aclustermodelofthereactiveorchemisorptionsitemaygivequiteareasonabledescriptionofwhathappensattherealsurface[1,3,30].However,theclustershouldstill belargeenoughtoeliminateclusteredgeeffects, andeventhenone must bear in mind that the cluster sizes employed in many computational studiesarestillmuchsmallerthanrealcatalyticparticles(say1050versus501000atoms,respectively). Hence, aslabmodel ofasteppedsurfacemayprovideamuchmorerealisticmodel of theactivesiteof acatalyticnanoparticle. Hammer [31,32] hascarriedoutquiteextensiveDFT-GGAslabcalculationsofN2andNOdissociationatsteppedRuandPdsurfaces,showinghowthedissociationenergyissignicantlylower at the low-coordination step sites compared to terrace sites. The specialreactivityofstepsitesforthedissociationofNOandN2hasbeendemonstratedinseveral recent surface-science studies [33,34]. Also, the preferential adsorption of COonstepsiteshasbeendemonstratedinUHV[35],underelectrochemicalconditions[36],aswellasbymeansofDFT-GGAslabcalculations[37].1.3.2 CatalyticReactivityandthePeriodicTableBecause a catalytic reaction generally consists of a series of elementary steps, one ofthe key questions in understanding catalytic reactivity is which of these steps is rate-limiting. The rate-limiting step determines not only the overall catalytic activity, butCopyright 2003 by Taylor & Francis Group, LLCoften also the selectivity of the reaction. In search of an optimal catalyst, the overallrate of the reaction is often studied as a function of a certain reactivity parameterofthecatalystmaterial. Atypical reactivityparameter istheheatofcompoundformation, e.g., surfaceoxideformation, ortheenthalpyofadsorptionofwhatisbelievedtobeakeyintermediate. Plotsof catalyst reactivityversusthereactivityparameter usually yieldthe well-knownvolcanoplots, the rationalizationforwhich was rst given by Sabatier [38]. The Sabatier principle essentially states that anactive catalyst should adsorb a key intermediate neither too weakly nor too strongly.A weakly adsorbed intermediate has a low reactivity because of its lowconcentration, and a strongly adsorbed intermediate leads to a lowreactivitybecause of the difculty with whichit can desorb. An intermediate interactionstrengthoftenleadstotheoptimumactivityandselectivity.There are innumerable examples of volcano plots and the Sabatier principle incatalysis, as we have recently elaboratedinrelationtoheterogeneous gas-phasecatalysis [2]. An illustrative recent example of the Sabatier example in electrocatalysisistheammoniaoxidationinalkalinesolution[39]. Theselectivityoftheammoniaelectro-oxidationtowardN2is determinedbythe stabilityandconcentrationofhydrogenated nitrogen adsorbates NHx (x 1,2), as the atomic nitrogen adsorbate isinactiveinproducingN2at roomtemperature. Asaresult, metalsthat adsorbNstrongly, such as Ru, Rh, and Pd, showno selective NH3oxidation, as thedehydrogenation capacity of these metals is too high, leading to a fully inactive Nad-covered surface. Coinage metals such as Cu, Ag, and Au show very littledehydrogenation capacity, and hence no activity for ammonia oxidation underelectrochemical conditions. OnlyPt andIr, whichshowanintermediatedehydro-genation capacity, are able to oxidize ammonia to N2 with a reasonable steady-stateactivity.Thedehydrogenationcapacitymayberelatedtothechemisorptionenergyofatomicnitrogen:ahighadsorptionenergywouldimplyahighdehydrogenationcapacity.Given the importance of the Sabatier principle in catalytic science, the questionarisesaboutwhetherthereactivityparameter, usuallythechemisorptionenergyofsome key intermediate, can be related to a more fundamental property of the catalystsurface. Inparticular, onewouldliketomakeaconnectionbetweenthereactivityparameter and the properties of a metal or alloy surface as they can be deduced fromtheperiodictable. Animportant development alongtheselineshasbeenamodelproposedbyHammer andNrskov[3,40]. Their model singles out threesurfacepropertiescontributingtotheabilityofthesurfacetomakeandbreakAdsorbatebonds:(1)thecenter edofthed-band;(2)thedegreeofllingfdofthed-band;and(3)thecouplingmatrixelementVadbetweentheadsorbatestatesandthemetal d-states.ThesequantitieshavebeencalculatedfromextensiveDFTcalculationsforasignicant portionof theperiodictable. Thebasicideaof theHammerNrskovmodel is that trends in the interaction and reactivity are governed by the coupling oftheadsorbatestateswiththemetal d-states, sincethecouplingwiththemetal sp-statesisessentiallythesameforthetransitionandnoblemetals.HammerandNrskovhavetestedtheirmodel quiteextensively[3] andhaveshown a good semiquantitative agreement between their model for the dcontribution of the adsorption energy and the DFT-computedtotal adsorptionenergy of atomic adsorbates such as oxygen and hydrogen and molecular adsorbatessuchascarbonmonoxide. Also, theactivationenergy, andhencethereactivity, ofCopyright 2003 by Taylor & Francis Group, LLChydrogen dissociation onto various metal surfaces is described semiquantitatively bytheir model. Signicantly, many of the observedvariations among the differentmetals can be traced to variations in ed, and the hence total adsorption energy variesas Eads*ged. Shifts in the (local)d-bandenergy,and hence variationsin reactivity,canbebroughtaboutinanumberofdifferentways.Variationsin edoccuratstepsand defects (see previous section), by alloying, or by making an overlayer system. Wemakeuseofthisd-bandshiftmodelinthenextsection.1.3.3 HydrogenandCarbonMonoxideAdsorptionon(Bi)metallicSurfacesAlloy surfaces are of substantial importance in catalysis, such as in thehydrogenation of unsaturated hydrocarbons, FisherTropsch synthesis, steamreforming of methane, and many other processes [41]. In electrocatalysis, theyhaverecentlyreceivedattentioninrelationtothedevelopmentofCO-tolerantfuel-cell catalysts [42]. In many of these processes, atomic hydrogen and carbonmonoxide are the most important intermediates or poisons; therefore, these twoadsorbateshavereceivedagreatdealofattentionintheoreticalandcomputationalstudies.Pallassana and Neurock [4344], for example, use periodic DFT calculations toexamine the chemisorption of hydrogen over different PdxRe1x surfaces in order tounderstandthepropertiesthatcontrolchemisorptionoverthesealloys.Theresultsfor hydrogen adsorption over the bare Pd and Re surfaces along withpseudomorphic overlayers of PdonRe(0001) andRe onPd(111) are presentedhere in Figure 1 [43]. The binding energy for atomic hydrogen is signicantlyFigure1 AdsorptionofatomichydrogenonPdRemodel surfaces. (a)purePd(111), (b)pure Re(0001), (c) monolayer of Re onPd(111), and(d) monolayer of PdonRe(0001).(AdaptedfromRef.[43].)Copyright 2003 by Taylor & Francis Group, LLCstronger on Re than it is on Pd. Re lies much further to the left in the periodic tableandthereforehasamuchmoreopend-band. Re, therefore, containsmorevacantstates near the Fermi level that can accommodate electron transfer fromtheadsorbedhydrogen, thusleadingtoastrongerbindingenergy. Thed-bandcentermodel proposedbyHammer andNrskov[40] suggests that thereshouldbeanincreaseinorbital overlapsincethecenterofthed-bandshiftsclosertotheFermilevel. In addition, Pauli repulsion is reduced as we move from Pd to Re since Re hasfewer lled states. Depositing an atomic layer of Re over Pd(111) or Pd overRe(0001) createsideal pseudomorphicReML/Pd(111) andPdML/Re(0001) surfacesthat haveuniquepropertiesduetothedegreeof chargetransferbetweenthetwometals. Thedensityof statesprojectedontothed-statesat thetopmetal layerofPd(111), Re(0001), PdML/Re(0001), andReML/Pd(111)werecalculatedinordertoprobe the electronic properties for each of these surfaces [43]. The results, which areshowninFigures2and3,indicatethatPdandReformaverystrongbondatthePdReinterfaceofthePdMLRe(0001)surface.Ananalysisofthedensityofstatesshowsasignicantshiftofthed-bandwellbelowtheFermienergy.Thedensityofstates near the Fermi level looks quite similar tothat of bulkgold. This wouldsuggest that adsorbatesshouldbindmuchmoreweaklyonathePdMLRe(0001)surfacethanthoseonthepurePdorpureResurfaces. Indeed, thebindingenergydecreasesfrom2.77to2.35 eVaswemovefromPdML/Pd(111)toPdML/Re(0001).AswemovetotheResurfaces, wendthatthereislittlechangeinthehydrogenadsorption energies. The binding energy of hydrogen on Re is sufciently strong thatthere is basically no change as we move from ReML/Re(0001) to ReML/Pd(111). Thecalculatedbindingenergiesforhydrogenon thesesurfacescorrelatedverywellwiththed-bandcenterofthesurfacelayer.TheresultsareshowninFigure3.All the trendsfrom these pseudomorphicstudies readily fall out of the d-bandcentermodel.PallassanaandNeurockhaveshownthatthismodelcanbeextendedtopredict what will happenoverthealloyedsurfacesaswell [44]. Ingeneral, thehydrogenbinding energy increases as the level of Re inthe Pdsurface layer isincreased over the Pd(111) slab and decreases as the amount of Pd in the Re surfaceincreases over the Re(0001) slab. Modeling the effect of the alloy, however, requiresthatwedistinguishbetweenPdandReadsorptionsitessincetheenergiesonthesemetalsarequitedifferent. Aweightedd-bandmodel of thesurfacewasthereforedened in order to account for differences between Pd and Re. The weighted d-bandmodelisgiveninEq.(1):[dweighted V2Re ? [Red? NReV2Pd ? [Pdd V2Re ? NReV2Pd ? NPd1V2herereferstothed-bandcouplingmatrixelementforthesurfacemetal atoms,whereasNReandNPdrefertothenumberofReHandPdHbonds, respectively[44].HydrogenadsorptionwasexaminedonaseriesofalloyedsurfacesincludingReML/Pd(111), Pd33Re66ML/Pd(111), Pd66Re33ML/Pd(111), PdML/Pd(111), PdML/Re(0001), Pd66Re33ML/Re(0001), Pd33Re66ML/Re(0001), andReML/Re(0001). Thecalculatedhydrogen-bindingenergiesonthesesurfacesaregiveninFigure4. Theweightedd-bandmodel appears tocapturemost of thechanges totheelectronicstructure. A plot of the weighted d-band against the DFT calculated binding energiesCopyright 2003 by Taylor & Francis Group, LLConthesealloyedsurfacesindicatesthatthismodel worksquitewell forpredictingbinding energies over the PdRe alloys (Fig. 5). Some of the outlier points are systemswhere the most favored binding site changed due to a change in the surfacecomposition. OnthePd33Re66surface, forexample, PdactstoisolatesmallerRedimers structures. These dimers are unique. They behave like site-isolated Re clustersand therefore bind hydrogen more strongly. This would readily explain thedeviationsfromthesimpled-bandcentermodel.The adsorptionof carbonmonoxide onmetal surfaces canbequalitativelyunderstoodusing a model originally formulatedby Blyholder [45]. AsimpliedmolecularorbitalpictureoftheinteractionofCOwithatransitionmetalsurfaceisgiveninFigure6.TheCOfrontierorbitals5sand2p*interactwiththelocalizeddmetal states by splitting into bonding and antibonding hybridized metal-Figure2 Thedensityofstatesprojectedtothed-bandofPdRemodelsurfaces.Solidlinerefers to the DOS projected to the d-band of the top layer. The dotted line refers to the secondlayer. (a) pure Pd(111), (b) PdML/Re(0001), (c) Re(0001), and (d) ReML/Pd(111). The center ofthed-bandforeachsurfaceisdepictedbythearrow.(AdaptedfromRef.[43].)Copyright 2003 by Taylor & Francis Group, LLCchemisorbate orbitals, which are in turn broadened by the interaction with the muchmoredelocalizedspmetalstates.Hammer et al. [46] show how this simple picture emphasizing the interaction oftheCOfrontierorbitalswiththemetal d-statescanexplaintrendsinthebindingenergies of CO to various (modied) metal surfaces. Generally, CO binds stronger tothe lower transition metals (i.e., toward the left in the periodic table), mainly due tothe center of the d-band ed moving up in energy, leading to a stronger back donation.The same Blyholder model may also be used to explain why on metals in the upper-right corner of the periodic table (Pd, Ni) COprefers multifold coordination,whereas toward the lower left corner (Ru, Ir) COpreferentially adsorbs atop(althoughstericconsiderationsalsoplayanimportantroleintheargument)[47].Of special interest tofuel-cell catalysis is theadsorptionof COtoPt-basedbimetallic surfaces, in particular PtRu. These surfaces have long been known to beconsiderablymoreactiveintheelectrochemical COoxidationthanpurePtorRu.TheyarealsomoreCO-toleranthydrogenoxidationcatalystsandbettermethanoloxidationcatalystsforuseinlow-temperaturefuelcells.WatanabeandMotoo[48]suggest a bifunctional mechanism to explain the unique reactivity of PtRu surfaces.In their mechanism, the oxygen species with which CO reacts, presumablychemisorbedOHformedfromthe dissociative water oxidation, is preferentiallyformedontheRusites.Theactivesiteis a PtRupairwithCOonPtreactingwithOHonRu. Dynamic Monte Carlosimulations of this mechanism, showingtheimportanceofCOmobility[49],arediscussedinSection1.5.2.Althoughthehigheroxophilicity of Ru compared to Pt renders the bifunctional mechanism a reasonableassertion, it does not take into account any changes in the CO binding properties tothePtRusurfacecomparedtopurePtorRu.Figure 3 The binding energy of atomic hydrogen at the 3-fold fcc site on the PdRe surfacesasafunctionofthed-bandcenterrelativetotheFermienergy.(AdaptedfromRef.[43].)Copyright 2003 by Taylor & Francis Group, LLCWe have recently carried out extensive slab calculations of COand OHinteractingwith PtRusurfaces [50,51].Resultsfromboth ourgroups show that onthe surface of a bulk alloy the CO binding to the Pt site weakens whereas that to theRusitegetsstronger. Forexample, onthesurfaceof ahomogeneousPt2Ru(111)alloy, theCOatopbindingtoPtisweakenedbyabout0.20.3 eVandthattoRustrengthened by about 0.1 eV. However, real catalytic surfaces are not homogeneousbutmayshowthetendencytosurfacesegregate. Figure7 showsthebindingenergyand vibrational properties of CO to an atop Pt site on a series of surfaces with a purePt top layer, but for which the bulk composition changes from pure Pt, to Pt:Ru 2:1,toPt:Ru1:2, topureRu. It isobservedthat ahigherfractionof Ruinthebulkcauses aweakeningof theCObondtothePt overlayer. This electronicalloyingeffectcanbeunderstoodonthebasisoftheHammerNrskovd-bandshiftmodel.AlloyingPtwithRucausesadownshiftofthelocal d-bandcenteronthePtsitesbecause the ow of electrons from Pt to Ru lowers the local d-band on Pt in order tomaintainlocal electroneutrality. Interestingly, Figure7alsoillustratesthatthereisnocorrelationbetweenthebindingenergyandtheCOstretchingfrequency,eventhoughsuchacorrelationisoftenassumedintheliterature.Ontheotherhand,thePtC stretching mode correlates well with the binding energy: a stronger bond causestheexpectedincreaseinthePtCfrequency.Figure4 Thebindingenergyof atomichydrogenat themost favorablesites onPdRealloyedsurfaces: (a) ReML/Pd(111), (b) Pd33Re66ML/Pd(111), (c) Pd66Re33ML/Pd(111), (d)Pd(111), (e) PdML/Re(0001), (f) Pd66Re33ML/Re(0001), (g) Pd33Re66ML/Re(0001), and (h)ReML/Re(0001).(AdaptedfromRef.[43].)(a)(b)(c)(d)(e)(f)(g)(h)Copyright 2003 by Taylor & Francis Group, LLCFigure5 Acomparisonof thebindingenergyof atomichydrogenat themost favorableadsorptionsitesonthePdRealloyedsurfacesandthed-bandcenterrelativetotheFermienergy.(AdaptedfromRef.[44].)Figure6 Model of theorbital interactiondiagramfor COadsorbedontransition-metalsurfaces.(AdaptedfromRef.[47].)Copyright 2003 by Taylor & Francis Group, LLCTheoppositeeffect is observedwhenasurfacewithapureRuoverlayerisconsidered, andthebulkisstepwiseenrichedwithPt. TheCObindingtotheRuoverlayerisstrengthenedwithahighercontentofPtinthebulk. Infact, theRu/Pt(111)surfaceistheonethatshowsthestrongestCObinding. Asimilareffectisalso observed when the top layer of a Pt(111) surface is mixed with Ru, leading to asurface alloy. As the Ru content of the surface layerincreases, the bindingto the PtsurfacesitesisweakenedandtotheRusurfacesitesisstrengthened.ThebindingofOHtoPtRusurfacesshowstrendsthatarequitesimilartoCO.Ingeneral,thosesurfacesitesthatbindCOstrongly(whichareusuallyrichinRu)alsoshowastrongOHbinding, andthosesurfacesitesthatbindCOweakly(which are usually rich in Pt) also show a weak OH binding. This raises the questionof whether PtRu is a good example of a truly bifunctional catalyst with one surfacecomponent adsorbing one reactant and the other surface component the otherreactant. Infact, onthe basis of their extensive experimental investigations, theFigure 7 The binding energy, CO stretching frequency oC-O, and PtC stretchingfrequencyoPt-Confourdifferentsurfaces, all withasurfacelayerofPt, butdifferent bulkcompositions(Pt,Pt2Ru,PtRu2,Ru).(AdaptedfromRef.[51].)Copyright 2003 by Taylor & Francis Group, LLCBerkeleygrouphassuggestedthat PtSnandPtMoalloysmayconstitutebetterexamples of bifunctional catalysts [52]. Indeed, our most recent DFT-GGAslabcalculations [53] showthat onthese surfaces COmaypreferentiallybindtoPt,whereasOHpreferstobindtoSnorMo.1.3.4 ElectricFieldEffectsonCOAdsorptionOne effect of special interest to electrochemists is the potential-dependentchemisorptionof ions andmolecules onelectrode surfaces. Aparticularly well-studiedexampleistheadsorptionof COonplatinumsingle-crystal electrodes. Incollaboration with the Weaver group at Purdue, we have recently undertakendetailed DFTcalculations of the potential-dependent chemisorption of COonplatinum-group(111)surfaces[47,54,55], modeledasclusters, forcomparisonwiththe extensive vibrational characterizationof these systems as carriedout bythePurdue group [56,57]. The electrode potential in these studies is modeled as avariableexternal electriceldappliedacrossthecluster, anapproachmanyothershavetakeninthepast.Two issues are of interest in relation to the potential-dependent bonding of COto transition-metal surfaces: the potential-dependent binding energy and theresulting potential-dependent site preference; andthe potential-dependent vibra-tionalproperties,in particular theinternalCOstretching frequency and themetal-chemisorbatePtCOstretchingmode.Figure8showstheeffectoftheappliedeldon the binding energy of CO in a onefold (atop) and multifold (hollow) coordinationon a 13-atom Pt cluster. What is particularly signicant in this gure is the change insitepreferencepredictedbythesecalculations, fromatopcoordinationat positiveFigure8 Field-dependentbindingenergyplots,andconstituentsteric,donation,andbackdonationcomponents,forCOadsorbedatopandhollowona13-atomPt(111)-typecluster.The orbital components are plottedwithrespect totheir zero-eldvalues. 0.01elda.u.correspondsto0.514 V/A .(AdaptedfromRef.[54].)Copyright 2003 by Taylor & Francis Group, LLCelds tothreefoldcoordinationat negativeelds. Qualitatively, suchapotential-dependent site switch is indeed observed experimentally [56,57]. By decomposing thebindingenergyinstericandorbital contributions, anddeconvolvingthecontribu-tions of the different KohnShamorbitals according tothe different symmetrygroups, it can be appreciated that the main factor driving this preference formultifoldcoordinationtowardanegativeeldisthebackdonationcontribution.This increasingimportance of backdonationwithamore negative eldcanbeunderstoodonthebasis of theclassical Blyholder picture. Amorenegativeeldimplies an upward shift of the metal electronic levels with respect to thechemisorbate2p* orbital,leadingtoan enhancedbackdonationofmetal electrons.Sincetheinteractionofthemetalwiththe2p*orbitalisabondinginteraction,andbondinginteractionstendtofavormultifoldcoordination,COwillfavormultifoldadsorptionsitesatnegativeelds.The change in the intramolecular CO stretching mode with electrode potentialhas also been a subject of longstanding theoretical interest and has become known asthe interfacial Stark effect. Important theoretical contributions in this eld have beenmadebyLambert,Bagus, Illas, Curulla, andHead-GordonandTully[5862]. WehaverecentlyreexaminedthisissueinthecontextofadetailedcomparisonofDFTcalculationswiththespectroscopicdataof COandNOadsorbedonavarietyofsingle-crystallinetransition-metalelectrodes[47].Thesecalculationshaveconrmedthat NOgenerally possesses a higher Stark tuning slope (i.e., change in COstretchingmodewithpotential) thanCOandthat multifoldCOexhibitsahigherStark tuning slope than atop CO. In agreement with earlier calculations, adecompositionanalysis shows that the lowering (blueshift) of the internal COstretching mode upon adsorption is mainly due to the back donation contribution, asmetal electrons occupy the 2p* antibonding orbital of CO[63,64]. This backdonationcontributionisoffsettingthestericcontribution,alsoknownasthewalleffect, which by itself leads to a redshift upon chemisorption. Although the positiveStarktuningslopeisalsomainlytheresultofthebackdonationcontribution, i.e.,the negative eld leading to enhanced back donation and hence a lowering of the CO stretching mode, the different slopes found for the different metals andcoordinationgeometriesaregenerallyfoundtobetheresult of asubtleinterplaybetweenbackdonation,donation,andstericeffects[47].Theelddependenceof thesubstrate-chemisorbateMCOstretchingmodehas received much less attention as its low frequency makes it difcult to be observedby in-situ IR spectroscopy. However, recent advances in the preparation oftransition-metal surfaces for surface-enhancedramanspectroscopy (SERS) havemade it possible to study in some detail the potential dependence of this vibrationalmode [65,66]. Our recent DFTcalculations suggest that, if a sufciently widepotential windowis available, the MCOmode on Pt, Pd, and Ir exhibits amaximum[55].TheMCOStarktuningslopeispositivefor(very)negativeelds,and negative for positive elds. A charge and dynamic dipole analysis indicates thatthis behavior is at least partially related to the ionicity of the surface bond.Negativelychargedadsorbates, suchas chemisorbedCl or COat negativeelds,exhibit a positive Stark tuning slope, whereas positively charged adsorbates, such aschemisorbedNaandCOat positiveelds, exhibit anegativeStarktuningslope.Covalentlybondedadsorbates, suchas COat intermediateelectricelds, donotexhibit a strong dependence of surface bond vibration on the electric eld, and in factCopyright 2003 by Taylor & Francis Group, LLCfortheseadsorbatestherelationshipbetweenionicityandStarktuningslopeisnotonetoone[67].Hence,thepotentialdependenceofthesurfacebondvibrationmayserveas a(semiquantitative) measureof thebondionicity, providedthebondisindeedionicenough.Experimentally,thePtCOvibrationexhibitsanegativeStarktuningslope, andthemaximumpredictedbytheDFTcalculationshasyet tobeobservedexperimentally.1.4 THESOLVENTINELECTRODEREACTIONS1.4.1 MarcusTheoryandMolecularDynamicsofElectrodeReactionsOnemajorcomplicationthatdistinguisheselectrocatalyticreactionsfromcatalyticreactionsat metalgasormetalvacuuminterfacesistheinuenceof thesolvent.Modelingtheroleof thesolvent inelectrodereactionsessentiallystartedwiththepioneering work of Marcus [68]. Originally these theories were formulated todescribe relativelysimple electron-transfer reactions, but more recentlyalsoion-transfer reactions and bond-breaking reactions have been incorporated [6971].Moreover, extensive molecular dynamics simulations have been carried out to obtainamore molecular pictureof the role of the solvent incharge-transfer processes,eitherinsolutionoratmetalsolutioninterfaces.In the Marcus theory, the electron-transfer act between a donor and anacceptor molecule is a radiationless event accommodated by a suitable non-equilibriumcongurationof surrounding solvent molecules. Hence, the relevantreaction coordinate in electron-transfer reactions is a kind of collective solventcoordinate, more precisely dened as the electrostatic potential at the location of theacceptingor donatingredoxspecies due tothe prevailingcongurationof polarsolvent molecules. Clearly, the energy associated with creating these nonequilibriumcongurations is a free energy, as a multitude of solvent congurations maycorrespond to one-and-the-same value of this electrostatic potential. The idea of thiscollective or generalizedsolvent coordinate is hence tomapall different solventcongurationsontoasinglereactioncoordinate.Moreexplicitly, wecanconsidertheprototypeof asimpleelectron-transferredoxreaction,theferri/ferrocouple:Fe3e $Fe22atsomeinertelectrodematerial suchasgold.Figure9showsatypical free-energysurface. Inthe Marcus theory, the free-energysurface consists of twoparaboliccurveswithminimaattheequilibriumsolventconguration(s). Attheequilibriumelectrode potential, the free energies of the twominima are equal. The solventuctuates around the minimum, and when it reaches the intersection point of the twocurves, an electron may be exchanged with the metal in a radiationless fashion. Thisis theelectron-transfer equivalent of theFranckCondonprinciple, as duringtheelectronexchangeat thecrossingpoint thenuclei donot changetheir positions.Whethertheelectrontransferreallyoccursisdeterminedbytheadiabaticityoftheprocess. Usually, if thereactiontakesplacesufcientlyclosetotheelectrode, theelectrontransferwillalwaystakeplaceandthereactionisadiabatic.Copyright 2003 by Taylor & Francis Group, LLCThe activationenergy for electrontransfer is thus determinedby the freeenergyattheintersectionpoint(i.e.,thetransitionstate),whichinturndependsonthecurvatureoftheparaboliccurves. Thiscurvature, traditionallydenotedbythesymbol l (see Figure 9), is knownas the solvent reorganizationenergy. IntheformulaoriginallygivenbyMarcus, thesolventreorganizationenergydependsonthe static and optical dielectric properties of the solvent, eopt and es, and the radius aof the reactant(s). For a single ion reacting at an electrode, the Marcus formula readsl e20a1eopt1es 3wherewehaveneglectedtheimageinteractionoftheionwiththemetalelectrode.Thisexpressionassumesalinearresponsebetweenthechargeontheionandthepolarizationinducedinthesurroundingsolvent,resultinginthepredictionthatthereorganizationenergydependsonlyonthesizeof theionandnot onitscharge.However, recent detailed MDsimulations [72] have shown that this is not anaccurate description, and the reorganization energy of multivalent ions is lower thanthat of uncharged species or ions of low valency with the same size. The main reasonforthisisthedielectricsaturationoccurringnearstronglychargedions,whichmaybe interpreted in an effective lowering of the static dielectric constant es near the ion,leading to a lowering of l according to Eq. (3). Strongly nonlinear solvent responsesalso occur in the transition froma neutral species to a singly charged species, as theiondipole interactions lower the effective radius a in Eq. (3). The effect isparticularly strong for a negatively charged species as the water molecule canapproachclosertoanegativelychargedspeciesthantoapositivelychargedspecies[72].Manytechnologicallyimportant electrochemical processes aremorecompli-catedthanthesimpleelectrontransferintheFe3/Fe2couple,inwhichneitherofthereactantsissupposedtoadsorbontotheelectrodesurface.Typically,however,reactants or products are adsorbed onto the electrode surface, or the electron-transfer act induces the breaking of a bond in the reacting molecule. It is quite clearthatinthesemorecomplexsituationstheMarcustheoryisexpectedtobreakdownand a more detailed description of the solventreactantmetal interaction is required.Figure 9 Free-energy curves for simple outer-sphere electron-transfer reactions. l/4 is theactivationenergyatequilibrium, lbeingthesolventreorganizationenergy.Copyright 2003 by Taylor & Francis Group, LLCIt is in the understanding of these processes that molecular dynamics (MD) currentlyplayavitalrole.Theelectronexchangebetweentherelevantenergylevel onthereactantandthe metal levels canbe treatedbyaso-calledAndersonNewns model [73]. Theinteraction with the water solvent is treated by extensive MD simulations. In a sense,thisrepresentsakindofhighlysimpliedtight-bindingMD.Theenergybarrierforelectrontransferiscalculatedby so-calledumbrellasamplingtechniques,whichisasystematicwayof samplingtrajectoriesawayfromtheequilibriumcongurations[20].Theelectrontransferitselfistreatedadiabatically.We have applied this formalism to the simplest type of bond-breaking electron-transfer reaction at a metal electrode [71,74]. The type of reaction we have in mind isthereductivecleavageofanRXmolecule:R-Xe?R

X4wheretypicallyR

issomecarbon-centeredradical fragmentandXahalideion.The RX molecule is separated from the metal electrode surface by at least one layerof water molecules, in agreement with the experimental deductions from the Save antgroupforthemethylchloridereduction[75].Assuming a simple Marcus-type model for the interaction with the solvent, onecanderive ananalytical expressionfor the potential (free) energysurface of thebond-breaking electron-transfer reaction as a function of the collective solventcoordinate q and the distance r between the fragments Rand X[71,75]. Theactivationenergyofthereactioncanalsobecalculatedexplicitly:DGact l DeZ24l De5where l is the solvent reorganization energy, De the bond dissociation energy, and Zthe free energy of the reaction, i.e., the distance fromthe equilibriumpotential(overpotential). Save ant, whooriginally derivedEq. (5), carriedout extensiveexperimentstotestitsqualitativevalidity[75].Inorder totest the (in)correctness of the Marcus solvent model, we havecarriedoutextensiveMDsimulationsofabond-breakingelectron-transferreactionin water at a platinum electrode. Figure 10a shows the computer simulated potentialenergy surface obtained by a two dimensional umbrella sampling technique. AnalysisoftheresultsinFigure10abringstolighttwoimportanteffectsofthesolventtheMarcusmodeldoesnotaccountfor.First, the presence of an additional minimum at r 3 Aalong the reaction pathis clearly discernible in the computer-simulated contour plot. The small energybarrier between this minimum and the minimum at r 6 Ais due to the molecularityof thesolvent: thereis afree-energycost associatedwiththemakingof theholebetweenthetwofragmentsinordertotinawatermolecule.Second, wehaveanalyzedthecurvatureofthecomputer-simulatedpotentialenergy surface by a parabolic tting of the energy surface along the q solventcoordinateatvariousseparationdistancesr,inordertoinvestigateanypossibler-dependenceof theeffectivesolvent reorganizationenergy. Theresult is showninFigure 10b and clearly shows that the local solvent reorganization energy dependsbothonthewell (i.e., RXorR

andX) andtheinterfragment distancer. TheCopyright 2003 by Taylor & Francis Group, LLCdashedlineinFigure10bshows thesolvent reorganizationenergyasdeterminedfroma local harmonic t to the reactant well. Hence, the comparison of theanalytical theory[71] andtheMDcomputersimulations[74] clearlydemonstratesthe important role of solvent molecularity and nonlinearity in the breaking of bondsatelectrodesurfaces. Bothfeaturesarenotincorporatedinthetraditional Marcustheory.1.4.2 EffectofWateronCatalyticReactionsfromDFTandAIMDCalculationsThe presence of a protic solvent at the surface of a metal signicantly alters the localenvironmentatthesurface. Thiscanimpactbothchemisorptionaswell assurfacereactivity.Modelingtheeffectsofsolution,however,presentsa majorchallengeforrst-principlemethods.Avarietyofsemiempiricalmodelsexistthatusecontinuummethods to describe solvation effects directly in the Hamiltonian. Amovilli et al. [76]provideanelegantreviewoftherecentadvancesofpolarizablecontinuummodels.Thesemethodsleadtothemostefcientanalysesofsolventeffects.Thisapproachhas been used extensively in the area of modeling enzymes and proteins. Thedifculty with this approach, however, lies in the empirical description of the solventcavity. A second method for treating solvent effects involves the introduction of oneor two explicitsolventmoleculesinto the calculations. This approach has proventobe quite valuable in the area of modeling homogeneous catalytic systems andreactions in zeolites. Catalysis on surfaces, however, presents a more difcultchallenge.Figure 10 (a) Computer-simulatedcontour plot of the free-energy surface obtainedbymolecular dynamics for the methylchloride reduction on Pt(111) electrode. (b) Theinterfragment distance-dependent solvent reorganization energy extrapolated from themolecular dynamics simulations. Thedashedlinegives theMarcus solvent reorganizationenergyobtainedbyalocalharmonictofthereactantwell.(AdaptedfromRef.[74].)Copyright 2003 by Taylor & Francis Group, LLCDesai et al. [77] extend this idea by adding between 826 explicit watermolecules into a unit cell of rst-principles DFT slab calculations. The vacuum layeressentially is nowcompletely lled with solvent molecules. The layer thicknessbetweentheslabs, theunit cell size, andthe number of explicit water moleculesaddedaremanipulatedtomaintaintheappropriatedensityof liquidwaterat thesurface. The water molecules that ll the vacuum region effectively form an extensivehydrogen-bondingnetwork.TheadsorptionofwateronPd(111)inthepresenceofliquidwater was examinedtoshowthe effect of solutiononthe chemisorptionproperties. The calculated structure and energetics are shown in Figure 11. Althoughthereis onlyasmall changeinthestructureof water molecules adsorbedat thesurface, thereis amuchmoredramaticchangeinheat of adsorption. Inthegasphase, water adsorbs to Pd through the lone pair of electrons on oxygen.Watersitsat anangleof 17%tiltedfromthesurfacenormal. Thisisconsistent withknownexperimental resultsforwateradsorbedonPd(100). Watermoleculesthat adsorbdirectly onthe Pdsurface lie at along the surface layer. The surface layer issubsequently stabilized by the formation of a hydrogen-bonding network thatorganizes in the water multilayers above the surface. Water rapidly begins to lose itsorientation and structure (i.e., crystallinity) with respect to the metal in the layers ofmoleculesthat arefartherremovedfromthesurface. ThechangesinstructureofwaterareshowninFigure11.TheresultingadsorptioncongurationfoundhereisquitesimilartothebilayermodelproposedbyDoeringandMadey[78].Figure11 Theadsorptionof liquidwater onPd(111). Thebindingenergyfor water onPd(111)inthevaporphaseis30 kJ/mol.Thebindingenergyforliquidwateris 2.5 kJ/mol.(AdaptedfromRef.[77].)Copyright 2003 by Taylor & Francis Group, LLCThe formation of a hydrogen-bonded network weakens the metaloxygenbonds at the expense of forming hydrogen bonds with the neighboring watermoleculesinsolution. TheadsorptionenergyofwateronPd(111)isreducedfrom30 kJ/mol to 2.5 kJ/mol as we move from the vapor phase to solution.Experimental results showasimilar trend. The experimental binding energy forwater on Pd(100) is estimated to be43 kJ/mol in the vapor phase [79]. Theexperimental TPDresultsformultilayersofwateronAg(110)byBangeetal. [80]suggest that the strength of the multilayer waterwater interaction is nearly the sameaswatermetalinteraction.ThiswouldindicateanetbindingenergyofaboutzeroonAg(110)inthepresenceofwatermultilayers.The presence of solution can dramatically affect dissociative chemisorption. Inthe vapor phase, most metal-catalyzed reactions are homolyticlike, whereby theintermediates that formare stabilized by interactions with the surface. Proticsolvents, ontheotherhand, canmoreeffectivelystabilizecharge-separatedstatesandthereforeaidinheterolyticactivationroutes.Heterolyticpathsca