Molecular-Understanding-of-Reactivity-and-Selectiv

SelectiveAlcoholOxidationDOI:10.1002/anie.201301868MolecularUnderstandingofReactivityandSelectivityforMethanolOxidationattheAu/TiO2Interface**MatteoFarnesiCamellone,*JianliZhao,LanyingJin,YueminWang,*MartinMuhler,andDominikMarxGold-basedcatalystshaveattractedgreatinterestsincethediscoveryoftheexceptionalcatalyticactivityofoxide-supportedAunanoparticlesforalargenumberofoxidationreactionsincludinglow-temperatureCOoxidation.[1]Despiteextensiveexperimentalandtheoreticalstudies,[2–10]however,theoriginofthehighcatalyticactivityofAu/oxidecatalystshasnotbeenclarified.Manyimportantissuessuchasthenatureoftheactivesitesandtheactivationofmolecularoxygenarestillunderdebate.Inparticular,littleisknownaboutthemicroscopicmechanismofcomplexreactionssuchastheselectiveoxidationofalcoholmoleculesonoxide-supportedAunanoparticles.[11]Methanol(CH3OH)isoneofthemostimportantindustrialchemicals[12]andtheselectiveoxidationofalcoholswithoxygenisakeyissuefortheproductionoffineandspecialtychemicalsusing“green”processes.[13]Herein,wepresentasurface-sciencestudyoftheselectiveoxidationofmethanolonaAu/TiO2(110)modelcatalyst.Thecombineddata,obtainedfromextensivedensityfunctionaltheory(DFT)calculationsandexperimentsusinghigh-resolutionelectronenergylossspectroscopy(HREELS)andthermaldesorptionspectroscopy(TDS),allowustoachieveanatomic-levelunderstandingofthelow-temperatureoxidationofmethanolwithmolecularoxygentoformaldehyde.WedemonstratehereinthatthehighcatalyticactivityandselectivityofAu/TiO2originatefromthepresenceofactiveinterfacialsites,wheretheactivationofmolecularoxygenoccursthroughchargetransferfromAu/TiO2formingO2dspecies.OxygenactivationisfollowedbytheformationofaCH3OHO2intermediate,whichundergoesoxidativedehydrogenationyieldingCH2OandH2O.ThelatterisproducedbyrecombinationofhydroxygroupsincludinghydrogentransferfromtheAuclustertoanOatomattheperimetersite.WefirstinvestigatedthemolecularreactionmechanismoftheselectiveoxidationcycleofCH3OHbyO2onanAu11/TiO2(110)modelsurfacebymeansofGGA+Ucalculations.Ithasbeenreportedthatverysmallunsupportedandoxide-supportedAuclusters(3–10atoms)alsoshowhighcatalyticactivitytowardsCOoxidationandvariousorganicreac-tions.[14]WehavestudiedindetailtheelectronicstructureofasetofAunclusters(n=11–16)supportedonTiO2(110)(seeSupportingInformation,SectionIII.H).ItwasdemonstratedthatthechargeoftheactivatedO2dspeciesisalmostconstant(TableS3),indicatingthatthebehaviorofAunnanoclustersofdifferentsize,shape,andmorphologyisqualitativelythesamewithrespecttoO2activation.Therefore,theAu11model,althoughcertainlybeingasimplificationofnanometer-sizedclustersonTiO2(110),wasshowntoappropriatelymimictheactivesiteslocatedatthenanogold/oxideinterfacewheretheoxidationprocesstakesplace.[15]AsshowninFigure1A,aAu11goldclusterwasanchoredatasinglesurfaceoxygenvacancyontheTiO2(110)surface.Oxygenvacancies,whicharealwayspresentontherutilesurface,[16]resultinstableanchoringsitesforAuadatoms.InthecaseofanisolatedoxygenvacancyonthestoichiometricTiO2(110)surface,thechargeneutralityismaintainedbythepresenceoftwoTi3+ions.[17]ThestablebindingoftheAu11clusteronanoxygenvacancysiteentailsastrongchargerearrangementatthegold/oxidecontact(centralpanelinFigure1).ThebondingchargeanalysisrevealsthatchargetransferoccursfromtheAu11clustertothereducedoxidesurface,thusleavingareducedsurfacewiththreeTi3+ionsandapositivelychargedAu11d+cluster.[18]ThisfindingisfurthersupportedbytheHREELSdataasdiscussedbelow(seealsoFigureS1).AllthestepsinvolvedintheproposedmechanismforCH3OHoxidationattheactiveAu11/TiO2(110)interfacearedepictedinFigure1;allunderlyingdataarecompiledintheSupportingInformationincludingconnectingpathwaysandmuchadditionalanalyses(FigureS3–S14).ThecatalyticcyclestartswiththeadsorptionofanO2moleculeattheinterfaceonadualperimetersitebetweentheAu11clusterandtheTiO2(110)surface(Figure1B)withastrongbindingenergyof2.15eV.TheadsorbedO2moleculehasanOObondlengthof1.44,closetothetypicalvalueforaperoxide(O22:[*]Dr.M.FarnesiCamellone,[+]Prof.Dr.D.MarxLehrstuhlfrTheoretischeChemie,Ruhr-UniversittBochum44780Bochum(Germany)E-mail:[e-mail],[+]L.Jin,Dr.Y.WangLehrstuhlfrPhysikalischeChemieIandLehrstuhlfrTechnischeChemie,Ruhr-UniversittBochum44780Bochum(Germany)E-mail:[e-mail]rTechnischeChemie,Ruhr-UniversittBochum44780Bochum(Germany)[+]Theseauthorscontributedequallytothiswork.[**]ThisworkwassupportedbytheDeutscheForschungsgemeinschaft(DFG)withinSFB558andbytheClusterofExcellence“RESOLV”(EXC1069).PartialfinancialsupportfromtheResearchDepartment“InterfacialSystemsChemistry”fundedbyRUBisgratefullyacknowledged.ComputationalresourceswereprovidedbyNIC(Jlich),Bovilab@RUB(Bochum),RV-NRWaswellasPRACE(FERMIatCineca).Supportinginformationforthisarticleisavailableonthe2013Wiley-VCHVerlagGmbH&Co.KGaA,WeinheimAngew.Chem.Int.Ed.2013,52,5780–57841.49).EachOatombindstoonesurfaceTi5catom.Acloseexaminationofthespindensity,projecteddensity