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1、SurfaceScience138(1984)505-523505North-Holland,AmsterdamCATALYTICOXIDATIONOFCOONPt(ll1):THEINFLUENCEOFSURFACEDEFECTSANDCOMPOSITIONONTHEREACTIONDYNAMICSJ.SEGNER*,C.T.CAMPBELL**,G.DOYENandG.ERTLInstiiut/iirPhysikalischeChemie,UniuersrriitMiinchen,D-8000Miinchen2,Fed.Rep.ofGermanyReceived14July1983;acc
2、eptedforpublication1November1983AngulardistributionsofCO,,I(y,),formedbycatalyticreactionbetweenCOand0,onasteppedPt(ll1)surface,havebeenmeasuredbymeansofmolecularbeamtechniquesasafunctionofCOand0coverages,inthepresenceofsubsurfaceoxide,andatvaryingtemperatures.TheresultingdatacanwellbefittedbyI(y,)=
3、acosy,+(l-a)cosy,withabeinganadjustableparameterwhichvarieswiththesurfaceconditions.Thecosyrpartisascribedtoparticleswhichwerecompletelyaccommodatedwiththeirtranslationalenergytothesurfacetemperaturebeforeleavingthesurface,whilethecosy,channelcorrespondstomoleculescarryingexcesstranslationalenergyfr
4、omcrossingtheactivationbarrierofthesurfacereaction.Thefractionofthermallyaccommodatedparticlesincreasesbythepresenceofstepsorsubsurfaceoxide,anddecreaseswith0andCOcoverageaswellaswithincreasingtemperature.Theeffectofcoveragecanqualitativelybeattributedtothevaryingparticipationofdefectsitesinproductf
5、ormation,whiletheinfluenceofsurfacetemperatureiswellreproducedintheframeworkofageneraltheoreticalmodel.ScatteringexperimentswithCO,revealedthatthetrappingprobabilityforthismolecule(inthetemperaturerangeinterestinghere)isoftheorderof0.5,thusconfirmingtheconclusionofincompletethermalaccommodationofaCO
6、,moleculeinteractingwithaPt(ll1)surface.1.IntroductionInachemicalreaction,transformationofenergyoccursthroughelementaryexcitationsofthereactingsystems.Withgasphasereactions,theseinvolvevariationsofthetranslationalenergyandofthepopulationsofquantizedinternal(i.e.rotational,vibrationalandelectronic)le
7、velswhichcanmostdirectlybestudiedbycrossed-beamexperiments.Inthecaseofgas/surfacereactionsanadditionalcomplicatingfactorarisesfromtheexistenceofquasi-continuousphononic(andelectronic)excitationsofthes