工业催化原理-英文课件chapter51

§5PrincipleofheterogeneouscatalysisHeterogeneouscatalystgenerallycanbeclassifiedas1.Metalsorsupportedmetals2.Oxides3.Solidacidandbase4.Zeolite§5.1MetalliccatalystMostmetalcatalystsaresupportedtransitionmetalornoblemetal,theyarewidelyusedinhydrogenation,dehydrogenation,andoxidationreactions.examples:OCH2=CH2+O2CH2CH2(ethyleneoxide)AgcatalystCH4+H2OCO+H2Ni/Al2O3cyclohexanebenzene+3H2Pt/SiO2§5.1MetalliccatalystTherearethreeimportantfactorswhichwillaffectthecatalyticperformanceofametalcatalyst.1.Electronicfactor2.Geometricfactor3.Energycorrespondence4.Strongmetalsupporterinteraction§5.1.1Electronicfactorinmetalliccatalystdcharacter:d%Themetallicbondsoftransitionmetalconsistofd-electrons,d%expressesthepercentageofd-electrontakenpartinthemetallicbond.d%=bondingdelectrons/(totalbondingorbital+vacantorbital)100%§5.1.1ElectronicfactorinmetalliccatalystNi:10outshellvalanceelectrons,3d84S2.Butonly6ofthemwilltakepartinmetallicbond.Therest4arenon-bondingelectrons.Accordingtomagnetizationmeasurements,therearetwotypesofhybridsinnickelatom.Theconfigurationsareasfollowing:A:d2SP3••••••3d4s4pB:d3SP2••••••§5.1.1ElectronicfactorinmetalliccatalystNi(A):d%=2/6100%=33.3%Ni(B):d%=3/(6+1)100%=42.9%theprobabilitiesofthetwotypesappearinmetalnickelare0.3and0.7respectively,sothatthedcharacterofNiatomis0.333.3%+0.742.9%=40%§5.1.1Electronicfactorinmetalliccatalystsomeothermetaldcharacter:Fe:39.7%,Pt:44%,Pd:46%,Rh:50%therelationshipbetweend-characterandcatalyticactivity:Insomecasethereisagoodcorrelation,forexample,inhydrogenationofethylene,itwasfoundthehigherd%thehigheractivitycatalysthas.However,thisisnotalwaysthecase.Therealsituationismorecomplicated.§5.1.1ElectronicfactorinmetalliccatalystThereactionactivityagainstd%factorforethylenehydrogenation§5.1.2GeometricfactorandenergycorrespondenceIncatalyticreaction,itisimportantthatthecatalystshouldcorrespondsgeometricallytothereactantmoleculethismeansthegroupingofadsorbingsitesonthecatalystsurfaceshouldmatchthegeometricalatomarrangementthecatalyzedmolecule.§5.1.2GeometricfactorC2H4+H2=C2H6dualsitesadsorbedactivationHHH-C-C-H**requirementforcertaindistancebetweenthetwositesAdsorptionofethyleneonnickelsuface§5.1.2Geometricfactorbonddistance:C-Ca=0.153nm,C-Nib=0.182nm,Ni-Nic1=0.248nmc2=0.351nmhencefor∠CCN=arccos[(a-c)/2b]=105o4’and122o6’forc1andc2sinceCatomhassp3hybridwiththeangleof109o28’sothatthedualsitedistancec1ismoresuitfortheethyleneadsorptionbutlesssuitforactivation.ThestructureofdifferentcrystalfacesforFCCnickelcrystal§5.1.2Geometricfactortherearethreekindsofcrystalfacesinnickelstructureandmoredualsiteswithc2distanceonface(110)thanthatonface(100)and(111).ItfollowsthattheNi(110)facehashighercatalyticactivityinethylenehydrogenation.Thisconclusionwasexperimentallyproved.§5.1.2EnergycorrespondenceEnergycorrespondingprinciple:Agoodcatalystneedsnotonlytomatchthegeometricalrequirementbutalsotoagreewiththeenergycorrespondence.ForasurfacereactionA-B+C-DA..B+C..D**E’EA..C+B..DA-C+B-D**§5.1.2EnergycorrespondenceHereE’istheenergychangeintheformationofthesurfaceintermediateE’=(QA*+QC*+QB*+QD*)-QAB-QCDQisthecorrespondingbondenergyAndEistheenergychangeoftheintermediatetoformtheproductE=QAC+QBD-(QA*+QC*+QB*+QD*)§5.1.2EnergycorrespondenceAlso,thetotalenergychangeinreactionu=QAC+QBD-QAB-QCDWholeenergyofbondbreakingandformations=QAB+QCD+QAC+QBDTheheatofadsorptionq=QA*+QC*+QB*+QD*thenE’=q-s/2+u/2E=-q+s/2+u/2§5.1.2Energycorrespondenceforagivenreaction,s,uarefixingbutE’(q),E”(q)aredependentoncatalystproperty.ForcurveE’q,slopeis+1andE”q,slopeis–1SothatcurveofE’andE”vsqisavolcanoshapewithtwotypesofendothermicorexothermicrespectively.Atthecrosspointp,E’=E”henceq=s/2andEa=u/2,itcorrespondstooptimumcatalystEnergycorrespondingprinciple.§5.1.2Energycorrespondenceendothermicexothermic§5.1.2EnergycorrespondenceEnergycorrespondingprinciple:Theheatofadsorptionofanoptimizedcatalystshouldapproximatelyequaltohalfofthesumofthebondenergyofformationandbondenergyofbreaking,whileitsactivationenergyisapproximatelyhalfofthereactionthermaleffect(energychangeinreaction).EstimationoftheactivationenergySincethemolecularbondsinvolvedinthereactiondonotbreakcompletelybutmorelikelyweredeformed.Amodifiedformulahasbeenproposed:Ea=A–bErforexothermicreactionb=0.25A=46kJ/molforendothermicreactionb=0.75A=0DecompositionofformicacidonthemetalsurfacesExampleforenergycorrespondence:DecompositionofformicacidaplotofthevolcanocurveasshownbelowwhereHfistheformationheatofformateandTisthetemperatureatwhichformicacidhas90%decomposition.HfandTarecorrelatedtotheabovementionedqandE’respectively.Onthevolcanotopmetalssuchasplatinum,iridium,ruthenium,palladiumgivehighactivityforformicaciddecompositionwhilemetalsatthevolcanobottomsuchasgoldandtungstenhaslowactivity.DecompositionofformicacidonthemetalsurfacesΔHfistheheatofformationofcorrespondingformatesTisthereactiontemperaturetoensurea90%decomposition§5.1.3StrongmetalsupporterinteractionAtfirstthesupporterisonlyconsideredasani