工业催化原理-英文课件Chapter41

Chapter4HeterogeneouscatalyzedreactionkineticsAheterogeneouscatalyzedreactionmaybegenerallyconsideredtoinvolvefollowingconsecutivesteps1.diffusionofreactantstothesurfaceofcatalyst(externaldiffusion)2.diffusiontotheinnersurface(internaldiffusion)3.adsorptionofreactants4.reactiononthesurface5.desorptionofproducts6.diffusionofproductsawayfromthesurface§4.1specialfeaturesofheterogeneousreactionkineticsreactionoccursinthemonolayeradsorbedonthesurface,theratedependsonthereactantcoverageuponthesurface.reactionratedependsontheratedeterminestep,thesloweststepinthereactionsequence.reactionisstronglyaffectedbymassand/orheattransfer.However,forconvenient,weusuallyassumethemasstransferrateismuchfastersothatitsinfluencetotheoverallreactionratecanbeexcluded.§4.2ReactionkineticsinidealadsorptionTherateofheterogeneouscatalyzedreactionisdeterminedbythesurfacecoverageofadsorbedreactants.However,sofarthecoveragecannotbedirectlymeasured.Ageneralmethodisusingsomekindsofisothermtocorrelatethecoveragetothebulkconcentrationwhichcanbedeterminedexperimentally.§4.2.1SurfacemassactionlawForasurfacereactionaA+bBcC+dDr=kAaBbo=c+d-a-bItdenotesthatthevacancyofsurfaceisimportantwhenisnotequaltozero.Alsowealwaystakeequaltozerowhenisnegative.§4.2.1SurfacemassactionlawForexample,2AB=-1r=kA2A2B=1r=kAo§4.2.2EstablishmentofkineticsequationsUnimolecularsurfacereactionsCase1surfacereactionisratedeterminingstepA+A*A*B*rdsB*B+hereA*andB*meantheadsorbatesforthefastequilibriumstep,LangmuirisothermcanbeappliedUnimolecularsurfacereactionsr=-dPA/dt=kAA=APA/(1+APA+BPB)r=kAPA/(1+APA+BPB)ifadsorptionofAisstrongwhileadsorptionofBisweakAPABPBandAPA1thenr=kzeroorderUnimolecularsurfacereactionsCase2adsorptionisratedeterminingstepkaA+A*rdskdr=kaPAo-kdATheadsorptionanddesorptionhasnotreachedequilibriumyet,thereforeLangmuirisothermcannotbeused.UnimolecularsurfacereactionsForthewholereactionsystematequilibriumABKP=PB/PA’herePA’isahypotheticalequilibriumpressureA=APA’/(1+APA’+BPB)o=1/(1+APA’+BPB)r=kaPAo-kdA=ka(PA-PB/KP)/(1+APB/KP+BPB)inabove,weusingA=ka/kdUnimolecularsurfacereactionsCase3desorptionisratedeterminingstepr=kBKP=PB’/PAherePB’isalsoahypotheticalequilibriumpressureforspeciesBB=BPB’/(1+APA+BPB’)r=kBKPPA/(1+APA+BKPPA)BimolecularsurfacereactionsA+BPCase1:Langmuir-HinshelwoodmodelDualsiteadsorptionA+A*B+B*A*+B*P+2rdsr=kABA=APA/(1+APA+BPB)B=BPB/(1+APA+BPB)r=kABPAPB/(1+APA+BPB)2BimolecularsurfacereactionsCase2:Rideal-EleymodelSinglesiteadsorptionA+A*A*+BP+rdsr=kAPBA=APA/(1+APA)r=kAPAPB/(1+APA)ReversiblereactionsBothadsorptionandreactionarereversiblewithequilibriumconstantKA+A*KAB+B*KBkrA*+B*C*+D*rdsk-rC*C+1/KCD*D+1/KDr=krAB-k-rCDReversiblereactionsKA=A/PAoKB=B/PBo1/KC=PCo/C1/KD=PDo/DKr=kr/k-ro=1/(1+KAPA+KBPB+KCPC+KDPD)krKAKB(PAPB-KCKDPCPD/KAKB)r=──────────────(1+KAPA+KBPB+KCPC+KDPD)2withoutratedeterminingstepk1A+A*k2A*B+inthiscase,onlysteadystateapproximationcanbeapplieddA/dt=k1Ao-k2A0o=1-AA=(k1PA/k2)/(1+k1PA/k2)r=-dPA/dt=k1PAo=k2A=k1PA/(1+k1PA/k2)§4.2.3Temperatureeffecttheeffectoftemperaturetothesurfacereactionistwofolds1totherateconstantArrheniusequation:k=koexp(-Ea/RT)2tothechemisorptionequilibrium=oexp(q/RT)hereqistheheatofadsorption§4.2.3TemperatureeffectForexample,inRidealmodel,ifPAisverylowthenAPA1,hencer=kAPAPB/(1+APA)≈kAPAPB=k’PAPBdlnk’/dT=dlnk/dT+dlnA/dT=(Ea-q)/RT2sothattheapparentactiveenergyisEa’=Ea-q§4.3Determinationofreactionmechanismcyclohexane(C6H12)dehydrogenationtobenzene(C6H6)onPt/Al2O3.Experimentalresultsshowthatthereactionrateisindependentonpressureofhydrogenandincreasewiththecyclohexanepressurebutdecreasewiththepressureofbenzene.AlsothechemicalequilibriumconstantKPisfairlylarge.§4.3DeterminationofreactionmechanismKPA(C6H12)B(C6H6)+3H2PAPBPH2KP=PH23·PB/PApossiblereactionmechanism1A(g)+*A*2A*B*+3H22’A*+*’B*’+*+3H23B*B+*PossiblereactionmechanismCase1adsorptionofcyclohexaneisrdsr=k1PAotheequilibriumpressure:PA’=PH23·PB/KPr=k1PA/(1+APH23·PB/KP+BPB)k1PA/(1+BPB)itcantransformtoPA/r=1/k1+B/k1·PBnamelyR=A+CPBhereR=PA/rA=1/k1C=B/k1PossiblereactionmechanismCase2surfacereactiononsinglesiteisrds:equation2r=k2A=k2APA/(1+APA+BPB)similarlytransformtoR=A+BPA+CPBhereR=PA/rA=1/k2AB=1/k2C=B/k2APossiblereactionmechanismCase2’surfacereactionondualsitesisrds:equation2’r=k2Ao=k2’APA/(1+APA+BPB)2alsocanbewrittenasR=A+BPA+CPBhereR=(PA/r)1/2A=(1/k2’A)1/2B=(’A/k2’)1/2C=(B/k2’A)1//2PossiblereactionmechanismCase3desorptionofbenzeneisrdsr=k3Btheequilibriumpressure:PB’=PAKP/PH23r=(k3BPAKP/PH23)/(1+APA+BPAKP/PH23)SinceKPisfairlylargeBPAKP/PH231+APAthenr=k3§4.3DeterminationofreactionmechanismThisleadstoazerothorderreaction.ItmeansthereactionrateisindependentonbothPAandPB.Itisobviouslyconflictwiththeexperimentaldatasothatcase3canbeeliminated.Accordingtotheexperimentaldata,usingLeastsquarefittingtocalculateconstantsA,B,andC.Resultsareshownasinfollowingtable.Theexperimental