工业催化原理-英文课件chapter32

§3.4Adsorptionkineticsandadsorptionequilibrium•§3.4.1Rateequationofadsorptionanddesorption•thenumberofmolecularcollisionsonthesurfaceperunittimeandperunitareauno§3.4.1Rateequationofadsorptionanddesorption•N=¼·nouno=P/kTandu=(8kT/m)/2getN=p/(2mkT)1/2•effectivecollision:Boltzmannfactorp/(2mkT)1/2·e-Ea/RT•condensationcoefficientandcertainfunctionofcoveragef()ra=p/(2mkT)1/2·e-Ea/RT·f()similarlyrd=kd’·e-Ed/RT·f’()§3.4.1Rateequationofadsorptionanddesorption•ingeneral,both,EaandEdarefunctionsofcoverage•However,inidealcasebothEaandEdareindependentoncoverage,threemainassumptions:–1.Surfaceisuniform,alladsorptionsitesareofsamecharacteristic–2.Withoutinteractionbetweenadsorbedspecies–3.Monolayeradsorption§3.4.2Rateequationfortheidealadsorbedlayer•adsorptionanddesorptionontheuniformsurface,and,EaandEdareindependentwithcoverage•ra=p/(2mkT)1/2·e-Ea/RT·f()rd=kd’·e-Ed/RT·f’()•Itcanbewrittenas:ra=kaPf()ka=/(2mkT)1/2·e-Ea/RTrd=kdf‘()kd=kd’·e-Ed/RT§3.4.2Rateequationfortheidealadsorbedlayer•Innon-dissociateadsorption,onemoleculeadsorbedononesitef()=o=1-f‘()=thenatequilibriumwegetra=rdkaP(1-)=kd/(1-)=ka/kdP=P=P/(1+P)•ThisisthefamousLangmuirisotherm§3.4.2Rateequationfortheidealadsorbedlayer•Inthecaseofdissociateadsorptionf()=(1-)2f‘()=2atequilibriumkaP(1-)2=kd2=1/2P1/2/(1+1/2P1/2)•atlowpressurenon-dissociateadsorptionPPdissociateadsorption1/2P1/2P1/2•Itisacriterionfornon-dissociateordissociateadsorption§3.4.2Rateequationfortheidealadsorbedlayer•MixingadsorptiontwoadsorbatesAandBcorrespondingtoAandBrespectivelyf()=o=1-A-BatequilibriumkaAP(1-A-B)=kdAAkaBP(1-A-B)=kdBBA=APA/(1+APA+BPB)B=BPB/(1+APA+BPB)§3.4.3Rateequationfortherealadsorbedlayer•Adsorptionoccursonanon-uniformsurfaceandEa,Edarethefunctionofcoverage•Elovichrateequation:linearrelationshipEa=Eao+Ed=Edo-ra=P/(2mkT)1/2·(1-)e-(Eao+)/RT§3.4.3Rateequationfortherealadsorbedlayer•Whenatlowormediumcoverage,thedependenceofraonf()canbeneglectedcomparedwithitsdependenceonexponentialtermra=kaPe-/RTsimilarly,wehavethedesorptionequationrd=kde/RTaboveisElovichequationwithka=/(2mkT)1/2·(1-)e-Eao/RTkd=kd’e-Edo/RT§3.4.3Rateequationfortherealadsorbedlayer•IntegralformofElovichequationifthechangeofpressurealsocanbenegligibleandattheearlystageofadsorptiond/dtraa·e-/RTintegrateaboveequationtoget=RT/·ln(t+to)/tohereto=RT/a§3.4.3Rateequationfortherealadsorbedlayer•TemkinisothermatequilibriumforElovichadsorptionrateequationkaPe-/RT=kde/RT=RT/(+)·ln(kaP/kd)=1/·lnCoP•applicationmainlyinlowandmiddlerangeofcoverageandforchemisorptiononly,suchasfordatatreatmentofammoniadecompositionHydrogenadsorbedontungstenwireapplicationofTemkinisotherm§3.4.3Rateequationfortherealadsorbedlayer•rateequationforlogarithmicalrelationEa=Eao+│ln(1-)│Ed=Edo-│ln(1-)│ra=kaPe-│ln(1-)│/RT=kaP(1-)-/RTrd=kd’·e-Ed/RT·f’()=kd/RTthiscalledas管孝男equationhereka=/(2mkT)1/2f()e-Eao/RTkd=kd’f‘()e-Edo/RT§3.4.3Rateequationfortherealadsorbedlayer•Freundlichisothermatequilibriumra=rdkaP(1-)-/RT=kd/RT=(kaP/kd)RT/(+)=CP1/nwhereC=(ka/kd)RT/(+)n=(+)/RTHydrogenadsorbedontungstenpowderApplicationofFreundlichisotherm§3.4.3Rateequationfortherealadsorbedlayer•NotetheFreundlichisothermgivesnolimitingvalueof,indicatingitsuitsforbothchemicalandphysicaladsorption.Italsojustcanbeusedinthemiddlerangeofcoverage.•Atheoreticalinterpretationofnisasaconstantrepresentingthemutualinteractionofadsorbedspecies.Avalueofgreaterthanunityisinterpretedtomeanthatadsorbedmoleculesrepulseoneanother.Summary•Idealcase:Eaindependenton•Langmuirisotherm=P/(1+P)•Non-idealcase:•Ea=Eao+,Temkinisotherm=1/·lnCoP•Ea=Eao+│ln(1-)│,Freundlichisotherm=CP1/n§3.4.4BETequation•BETequationisanisothermformultilayeradsorption.Itisbasedonfollowingtwoassumptions:1.Thesurfaceofsolidisuniform,andthemolecularadsorptionordesorptionarenotaffectedbyotheradsorbedspecies.2.TheinteractionbetweensurfaceandgasmoleculesisthroughVanderWaalsforcewhichmakestheadsorptionphysicalandmultilayer.§3.4.4BETequationModelofmultilayeradsoption§3.4.4BETequation•DifferenttypeisothermduetomultilayeradsorptionⅠmonolayeradsorptionⅡ,Ⅲ,Ⅳ,ⅤmultilayeradsorptionIsothermofmultilayeradsorption•Ⅱ:adsorptionofpowderorlargeporematerials,theturningpointdenotestheadsorptionchangesfrommonolayertomultilayer.•Ⅲ:stronginteractionbetweenadsobateswhichenhancesthefurtheradsorbing.•ⅣⅤ:capillarycondensingoccurs§3.4.4BETequation•WeomitthedetaileddeductionofBETequationandonlygivethefinalexpressionP/V(P-Po)=1/CVm+(C-1)/CVm·P/PowhereV/Vm=,VmrepresentthevolumeofadsorbatetoformafullmonolayerandPoisthesaturatedvaporpressureattheexperimenttemperature.§3.5Surfaceareaofcatalystanditsmeasurement•Thetotalexposedsurfaceareaofonegramcatalystistermedasspecificsurfacearea.•Heterogeneouscatalysisoccursonthesurfaceofcatalyst,sothatthespecificsurfaceareaisofgreatimportancetothecatalyticactivity.However,itisusuallynotastrictproportionalrelationship.Estimatesurfacearea•measuretwodifferentequilibriumpressurePandthecorre