A first principles study of water dissociation on small copper clusters

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1、PAPERwww.rsc.org/pccp

2、PhysicalChemistryChemicalPhysicsAfirstprinciplesstudyofwaterdissociationonsmallcopperclustersaaaaaLeiChen,QingfanZhang,YunfengZhang,WinstonZ.Li,wBoHan,aabcChenggangZhou,JinpingWu,*RobertC.Forrey,DiwakarGargandadHansongCheng*Received15thJanuary2010,Accepted2ndJune2010DOI:10.

3、1039/c001006eWaterdissociationoncopperisoneoftherate-limitingstepsinthewater-gas-shift(WGS)reaction.CopperatomsdispersedevenlyfromfreshlymadecatalystsegregatetoformclustersundertheWGSoperatingconditions.Usingdensityfunctionaltheory,wehaveexaminedwateradsorptionanddissociationonthesmalleststable

4、3-dimensionalcoppercluster,Cu7.Watermoleculesareadsorbedontheclustersequentiallyuntilfullsaturationatwhichnodirectwater–coppercontactisstericallypossible.Theadsorptionisdrivenmainlybytheoverlapbetweenthep-orbitalofOatomoccupiedbythelonepairandthe3d-orbitalsofcopper,fromwhichafractionalchargeisp

5、romotedtothe4s-orbitaltoaccommodatethechargetransferfromwater.WaterdissociationontheCu7clusterwasinvestigatedatbothlowandhighwatercoverage.ItwasfoundthatwaterdissociationintoOHandHisexothermicbutisinherentlyahightemperatureprocessatlowcoverage.Athighcoverage,thereactionbecomesmoreexothermicwith

6、fastkinetics.Inbothcases,watercancatalyzethereaction.ItwasfoundthatdirectdissociationoftheOHspeciesisendothermicwithasignificantlyhigherbarrieratbothlowandhighcoverage.However,theOHspeciescanreadilyreactwithanotheradjacenthydroxylgrouptoformanOadatomandwatermolecule.Ourstudiesindicatethatthebasi

7、cchemicalpropertiesofwaterdissociativechemisorptionmaynotchangesignificantlywiththesizeofsmallcopperclusters.Similaritiesbetweenwaterdissociationoncopperclustersandoncoppercrystallinesurfacesarediscussed.1.IntroductionadsorptionwaslargelyattributedtotheelectrontransferfromtheHOMOofCuclusterstoth

8、eO2antibondingp*-orbitalWatergasshift(WGS)reaction(CO+H2O=CO2+H2)isbuttheOmolecularnatureremainsessentiallyintact.132animportantindustryprocessforhydrogenproductionandStrongCOchemisorptiononsmallCuclusterswasalso1,2COremovalandhas