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Using first principles to predict bimetallic catalysts for the ammonia decomposition reaction.pdf

Using first principles to predi…

上传者: silly 2010-05-24 评分 0 0 0 0 0 0 暂无简介 简介 举报

简介:本文档为《Using first principles to predict bimetallic catalysts for the ammonia decomposition reactionpdf》,可适用于工程科技领域,主题内容包含MacmillanPublishersLimitedAllrightsreservedUsingfirstprinciplestopredictbim符等。

MacmillanPublishersLimitedAllrightsreservedUsingfirstprinciplestopredictbimetalliccatalystsfortheammoniadecompositionreactionDanielleAHansgen,DionisiosGVlachos*andJingguangGChen*Thefaciledecompositionofammoniatoproducehydrogeniscriticaltoitsuseasahydrogenstoragemediuminahydrogeneconomy,andalthoughrutheniumshowsgoodactivityforcatalysingthisprocess,itsexpenseandscarcityareprohibitivetolargescalecommercializationTheneedtodevelopalternativecatalystshasbeenaddressedhere,usingmicrokineticmodellingcombinedwithdensityfunctionalstudiestoidentifysuitablemonolayerbimetallic(surfaceorsubsurface)catalystsbasedonnitrogenbindingenergiesTheNi–Pt–Pt()surface,withonemonolayerofNiatomsresidingonaPt()substrate,waspredictedtobeacatalyticallyactivesurfaceThiswasverifiedusingtemperatureprogrammeddesorptionandhighresolutionelectronenergylossspectroscopyexperimentsTheresultsreportedhereprovideaframeworkforcomplexcatalystdiscoveryTheyalsodemonstratethecriticalimportanceofcombiningtheoreticalandexperimentalapproachesforidentifyingdesirablemonolayerbimetallicsystemswhenthesurfacepropertiesarenotalinearfunctionoftheparentmetalsTheammoniadecompositionreactionhasrecentlybeensubjecttoanincreasinglevelofattentionduetothepossibilityofammoniabeingusedasahydrogenstoragemediuminapossiblehydrogeneconomyAmmoniacanbeliquefiedeasilyatapressureofatmatK,leadingtohighenergydensitiesItisreadilyavailablebecauseofitsuseinfertilizers,andisaCOfreesourceofhydrogenExperimentalstudiesofsinglemetalcatalystshaveshownthatRuisthemostactivedecompositioncatalyst–,butitisexpensiveandlimitedinsupplyhencethereisaneedtodevelopeitherlessexpensivealternativesorcatalystswithhigheractivityAmmoniadecompositionproceedsbymeansofdehydrogenation,NHHNHHNHHNfollowedbyrecombinationofNandHtoformNandH,respectivelyIthasbeenshownthattheheatofnitrogenchemisorptionisagooddescriptorforammoniasynthesisanddecomposition,ThebindingenergyofthenitrogenatomtothesurfacemustbestrongenoughfordehydrogenationoftheNHxspeciestooccur,butsufficientlyweakthatthenitrogenrecombinestodesorbfromthesurfacetocompletethecatalyticcycleThistradeoffleadstoavolcanotyperelationshipbetweennitrogenbindingenergyandammoniadecompositionactivityAlthoughRuhastheoptimalheatofchemisorptionamongsinglemetals,itispossiblethatbimetalliccatalystswithhigheractivitiesmightexistEncouraginglyfortheammoniasynthesisreaction,abimetalliccatalyst(CoMo)hasbeenfoundthatismoreactivethanRuHowever,becausetheactivityfortheCoMocatalystdecreasessignificantlyinthepresenceofammonia,withconcentrationsaslowas,itsuseasadecompositioncatalystisrestricted,TheCoMobimetalliccatalystforthesynthesisreactionwaspredictedthroughtheconceptofPeriodicTableinterpolation,inwhichthebindingenergyofamixedmetal(alloyed)surfaceistakenasalinearcombinationofthebindingenergiesoftheparentmetalsAmetalwithahighnitrogenbindingenergy(Mo)andonewithalowbindingenergy(Co)werechosentogiveasurfacewithanintermediatebindingenergyAlthoughthisspecificalloycatalystmaybestable,formanybimetallicalloys,onemetaloftensegregatestothesurfaceeithertominimizethesurfaceenergyofthealloyedsystemorduetoadsorbateinducedreconfiguration–Thisresultsinsurfacepropertiesthatarevastlydifferentfromthealloyedsurface,andmakespredictionsfromtheperiodictableinterpolationmethodinvalidMonolayerbimetalliccatalystsconsistofamonolayerofanadmetalinthetoplayersofahostmetal–Thesesurfacescanbeusedtorepresentthesegregatedsurfaceofanalloyorcanbeusedtomodelcore–shellbimetallicnanoparticlesTheadmetalcanbeonthesurfaceofthehostmetal,givingrisetothesurfaceconfiguration,orbelowthesurfacelayer,formingthesubsurfaceconfigurationThesetwoconfigurationshavebeenshown,bothexperimentallyandthroughdensityfunctionaltheory(DFT)calculations,tohavepropertiesthatdifferdrasticallyfromoneanotherandfromtheparentmetalsthesebimetallicsurfacesarealsoverydifferentfromthecorrespondingalloyedsystem,wherethetopmostsurfacelayerisintermixedwiththetwoparentmetalsTypically,themonolayerbimetallicsurfaceshavebindingenergiesthataregreaterorlessthanbothoftheparentmetals,dependingontheconfiguration,andarenotafunctionoftheparentmetalbindingenergies,Owingtothisnonlinearbehaviour,thereiscurrentlynomethodtorationallydesignthesenovelcatalystsInthisstudy,wepresentfullmicrokineticmodelsforNHdecompositiononvarioussinglemetalcatalysts,includingCo,Pt,Pd,Ni,Ru,Rh,Ir,ReandMoThesemodelsincludeadsorbate–adsorbateinteractions,whichhavebeenshowntohaveasignificanteffectoncalculatedsurfacecoverages,theratedeterminingstepandcatalyticactivityTheresults,combinedwithDFTdata,areusedtopredictsuitablemonolayerbimetallic(surfaceorsubsurface)systems,basedonnitrogenbindingenergies,fortheammoniadecompositionreactionThesebimetallicsurfacesarethentestedexperimentallyfortheiractivitytowardsammoniadecompositionusingtemperatureprogrammeddesorption(TPD)andhighresolutionelectronenergylossspectroscopy(HREELS)tovalidatethemodelpredictionsTothebestofourknowledge,ourapproachrepresentsthefirsttimethatfullmicrokineticmodelsandDFTpredictions,togetherwithexperimentalverification,havebeencombinedtoidentifynovelcatalystformulationsandsurfacestructuresCenterforCatalyticScienceandTechnology,DepartmentofChemicalEngineering,UniversityofDelaware,Newark,Delaware,USA*email:vlachosudeledujgchenudeleduARTICLESPUBLISHEDONLINE:APRIL|DOI:NCHEMNATURECHEMISTRY|VOL|JUNE|wwwnaturecomnaturechemistryMacmillanPublishersLimitedAllrightsreservedResultsComputationalresultsDFTcalculationswereperformedtoobtaintheinteractionenergiesfornitrogenandhydrogenadsorbatesonthesurfaceThebindingenergiesweredeterminedatcoveragesoftomonolayer(ML),whichwerethenplottedasafunctionofcoverageAlthoughnotcompletelylinear,approximatingthedatawithalinearfunctionisadequatefortrendsandscreeningstudiesasperformedhereThebindingenergiesextrapolatedtothezerocoveragelimit(QA())andtheinteractionparameters(IP),theslopeoftheline,foreachmetalstudiedarelistedinTablePlotsofthebindingenergiesandthelinearfitscanbefoundintheSupplementaryInformationThezerocoveragebindingenergyandtheinteractionparameterwereusedtoapproximatethebindingenergyatanysurfacecoveragewithinthemicrokineticmodelsthroughthefollowingequation:QA(uA)=QA()IPuA()whereuAisthecoverageofadsorbateATheinteractionparametersforhydrogenarelow,atapproximatelykcalmol(MLhydrogen)Thismaybearesultofthesmallsizeofthehydrogenatom,thelowbindingenergyoracombinationofbothBecausethehydrogeninteractionparametersaresmall,theyhaveverylittleeffectonthereactionmechanismandpredictedammoniaconversionsThenitrogeninteractionparameters,ontheotherhand,aremuchgreater,rangingfromtokcalmol(MLnitrogen)(dependingonthemetal),andhaveasignificanteffectonconversionForeachmetal,themicrokineticmodelwasusedtocalculatetheconversionatthereactorexitTheoverallammoniadecompositionreactionwasmodelledwithelementaryreactionsteps,withnoassumptionofaratedeterminingstepTheelementaryreactionstepsareasfollows:NHNH(R)NHNH(R)NHNHH(R)NHHNH(R)NHNHH(R)NHHNH(R)NHNH(R)NHNH(R)NNN(R)NNN(R)HHH(R)HHH(R)whererepresentsanadsorbedsurfacespeciesThecoveragedependentatomicbindingenergieswereusedtocalculatethemolecularheatsofchemisorption(QNHx)andtheactivationbarriersoftheelementaryreactionsusingthebondorderconservationmethod,resultinginactivationbarriersthatwerecoveragedependentPreexponentialsfortheelementaryreactionstepsweretakenfromapreviousliteraturestudyinwhichthepreexponentialswerefittoRuexperimentaldatawithconstraintsontheoverallentropicconsistencyFigureshowsthepredictedconversionofammoniaversusthenitrogenheatofchemisorption(QN())atareactortemperatureofKAmongthesinglemetalcatalystsstudied,Ruwasfoundtohavethehighestactivity,consistentwithexperimentaldata–Themodelresults(circles)forthefullmicrokineticlibraryareingoodagreementwithanextensiveexperimentalstudyofmetalsfromGanleyandcolleagues(triangles)Figurerevealsavolcanorelationship,andshowsthattheheatofnitrogenchemisorptionisagooddescriptortoidentifysurfaceswithdesirablecatalyticactivity(otherpossibledescriptorsarelistedinSupplementarySection)Thisconclusionisconsistentwithpreviousstudiesonammoniasynthesisanddecompositionreactions,,althoughinthecurrentstudyrepulsiveadsorbate–adsorbateinteractionswereaccountedforandnoassumptionsoftheratedeterminingsteporsurfacecoveragesweremade,asisfrequentlydoneinpreviousliteraturestudiesThroughthemodels,amaximumactivity(peakofthevolcanocurve)ispredictedtobeatanitrogenheatofchemisorptionofkcalmolTodeterminethekineticallysignificantreactionsteps,asensitivityanalysiswasperformedThepreexponentialsofeachTable|MonometallicbindingenergiesandinteractionenergiesMetalZerocoverageNbindingenergyCalculatedNinteractionparameterZerocoverageHbindingenergyCalculatedHinteractionparameterPdPtIrNiRhCoRuReMoBindingenergiesextrapolatedtothezerocoveragelimitandthecalculatedinteractionparameterscanbeusedtoestimatetheatomicbindingenergiesatanysurfacecoveragethroughequation()ThenegativesignindicatesthattheinteractionsarerepulsiveAllenergiesareinkcalmolConversion()(kcalmol–)PtIrPdCoRhRuNiReMoTOF(s–)Figure|AmmoniadecompositionvolcanocurveAmmoniaconversioncalculatedfrommicrokineticmodelling(circles,leftaxis)atKforvarioustransitionmetalcatalystsandexperimentalsupportedcatalystturnoverfrequencies(TOF)(triangles,rightaxis)fromGanleyandcolleaguesatKplottedagainstthenitrogenbindingenergies(QN())Thedottedlineiscalculatedbyassumingaverageinteractionenergies(seeTable)andacorrelationbetweennitrogenandhydrogenbindingenergiestoaidindeterminingthevolcanomaximumThenitrogenbindingenergyisfoundtobeagooddescriptorforthisreactionThepeakofthevolcanocurveisatanitrogenbindingenergyofkcalmol,andthisvalueisusedtoidentifybimetallicsurfaceswithdesirablecatalyticactivityfortheammoniadecompositionreactionNATURECHEMISTRYDOI:NCHEMARTICLESNATURECHEMISTRY|VOL|JUNE|wwwnaturecomnaturechemistryMacmillanPublishersLimitedAllrightsreservedforwardandreverseelementaryreactionsteppair(Ai)(thatis,RandR,RandR,andsoon)wereperturbedsimultaneously,thusensuringthermodynamicconsistencyThenormalizedsensitivitycoefficientforeachreactionpair(NSCi)wascalculatedusingNSCi=D(lnX)D(lnAi)()whereXistheconversionattheendofthereactorThelargest(inabsolutevalue)modelresponsesamongthereactionpairsindicatekineticallysignificantreactionsteps,(seeSupplementaryInformation)Asensitivityanalysisperformedoneachofthesurfacesshowstheratedeterminingsteptobetheremovalofthesecondhydrogen(fromtheNHreactionR)forsurfaceswithanitrogenbindingenergylessthankcalmolForsurfaceswithhighernitrogenbindingenergies,theremovalofthefirstandsecondhydrogens(RandR)andnitrogendesorption(R)arekineticallysignificant,asshowninFigAtthepeakofthevolcanocurve(kcalmol),theremovalofthesecondhydrogenisthemostsignificantelementaryreactionstep,althoughtheremovalofthefirsthydrogenandnitrogendesorptionarebothkineticallysignificantInterestingly,thedominantsurfacecoveragechangesfromthelefttotherightlegofthevolcanocurve(SupplementaryFigS)Thisanalysisunderscoresthefactthatthekineticallysignificantstepanddominantcoveragemaybechangingalongavolcanocurve,themaximuminthevolcanocurvemaybearesultofmultiplephysicalmechanisms,andtheimportanceofperformingafullmicrokineticanalysisratherthanassumingaprioriaratedeterminingstepandadominantsurfacespeciesBecausethenitrogenbindingenergyisagoodactivitydescriptor,aDFTsearchwasperformedtoidentifycatalystsurfacesthathaveasimilarbindingenergytotheoptimalvalueofkcalmolPtbasedmonolayerbimetallicsurfaceswerethefocusofthisstudyThesesurfaceshavebeenshowntoformthesurface(M–Pt–Pt)andsubsurface(Pt–M–Pt)configurations,bothexperimentallyandthroughDFTcalculations,,,BindingenergieswerecalculatedattheMLcoverage,whichisagoodapproximationtothebindingenergiesextrapolatedtozerocoverageTableshowsthecalculatednitrogenbindingenergiesforseveralsurfaceandsubsurfaceconfigurationsAlsoincludedarethemetal–nitrogenbondlengthsoneachsurfaceThebindingenergiesforthesubsurfaceconfigurationsarelowerthanboththeparentmetalsduetoabroadeningofthedband,whereasthebindingenergiesforthesurfaceconfigurationsarestrongerthanboththeparentmetals,duetoacontractionofthedbandForthesubsurfaceconfigurations,thePt–NbondlengthsweresimilartoPt(),withonlyaslightlengtheningofthebondPt–Ni–PtwastheonlysubsurfaceconfigurationinwhichtherewasashorteningofthebondcomparedtothePt()surfaceForthesurfaceconfigurations,thenitrogensurfacebondsshowmuchmorevariation,whichisprobablyaresultofthedifferencesinthemetalstowhichthenitrogenisboundThenitrogenbindingenergiesvaryfromtokcalmolbyaddingasecondmetaltothesurfaceorsubsurfacelayerofthePthostBasedonthetheoreticalpredictionsinFig,theactivityofthesebimetallicsurfacesshouldalsofollowavolcanorelationshipTheNi–Pt–Pt()bimetallicsurfacehasanitrogenbindingenergyofkcalmol,slightlylowerthanthatofRu,andisapotentiallyactivecatalyst(seethemaximuminFig)Thesubsurfaceconfiguration,Pt–Ni–Pt(),andtheparentmetals,Pt()andNi(),areexpectedtohaveloweractivitiesbecauseoftheweakernitrogenbindingenergies(Table)TheNiPtbimetallicsystemwaschosenasatestsystemtobestudiedexperimentallyinthecurrentpapertoassessthemodelpredictionsTablealsoidentifiestheCo–Pt–Pt()andFe–Pt–Pt()surfacesasadditionalpromisingsystemsTheactivityoftheseadditionalsurfaceswillbeevaluatedinfuturestudiesTPDstudiesofammoniadecompositionWetestedtheDFTmicrokineticmodelpredictionsusingtheNi–PtbimetallicsystemsTheNi–Pt–Ptsurface,togetherwithPt–Ni–Pt,Pt()andaNi()film,weretestedfortheiractivitytowardsammoniadecompositionthroughTPDexperimentsDepositingNiatroomtemperatureleadstotheNi–Pt–Ptsurface,whereasdepositingatKleadstothePt–Ni–PtconfigurationNormalizedsensitivitycoefficient(kcalmol–)PtIrPdCoRhRuNiReMoNH**NH**NH*H*NH*H*N*N*Figure|NormalizedsensitivitycoefficientsofthekineticallysignificantelementaryreactionstepsforeachmonometallicmetalPreexponentialsoftheforwardandreversereactionpairswereperturbedsimultaneouslyatatemperatureofKandtheresponseofconversionwasmonitoredThereactionpairsthathavethehighestnormalizedsensitivitycoefficientarekineticallysignificantreactionstepsThesensitivityanalysisshowsthattherearemultiplekineticallysignificantreactionstepsandtheirsensitivitychangesacrossthevolcanocurveOnlyreactionpairsthathaveanormalizedsensitivitycoefficientaboveforanymetalareshownTable|LibraryofDFTbindingenergiesandbondlengthsofnitrogenatomsataMLcoverageonvariousmonolayerbimetallicsurfacesConfigurationMetal()surfaceNitrogenbindingenergy(kcalmol)BondlengthdM–N(Å)SubsurfacePt–Ti–PtPt–V–PtPt–Cr–PtPt–Mn–PtPt–Fe–PtSinglemetalPt–Co–PtPt–Ni–PtPtNiNi–Pt–PtSurfaceCo–Pt–PtFe–Pt–PtMn–Pt–PtCr–Pt–PtV–Pt–PtTi–Pt–PtByaddingametaltothePt()surface,ineitherthesurfaceorsubsurfaceconfiguration,thenitrogenbindingenergyismodified,creatingalargerangeofbindingenergiesonthebimetallicsurfacesThebindingenergiesandbondlengthsonthePt()andNi()surfacesarealsoincludedforcomparisonThedifferentcoloursinthestructuresindicatethedifferentmetalsARTICLESNATURECHEMISTRYDOI:NCHEMNATURECHEMISTRY|VOL|JUNE|wwwnaturecomnaturechemistryMacmillanPublishersLimitedAllrightsreservedDepositingatleastMLofNiachievesasurfacewithchemicalpropertiessimilartoaNi()surfaceAmmonia(L,equaltotorrfors,whereLindicatesLangmuir)wasdosedontoeachofthefoursurfacesatKFigureshowsthedesorptionspectraofthedecompositionproduct,nitrogen,foreachofthesurfacesThemzAMUwasusedtomonitorNdesorption(NatomsfromNcracking)toeliminatetheoverlapbetweenNandanybackgroundCOadsorbedtothesurface(bothseenatmzAMU)DetectionofthepeakatKconfirmsthattheNi–Pt–PtsurfaceisactivetowardsdecompositionTheabsenceofanypeaksontheotherthreesurfacesshowsthatthesesurfacesarenotactivetowardsdecompositionatthesedosingconditionsTheresultsforPt()areconsistentwithpreviousresults,whichshowedthatPt()doesnotdecomposeammoniaunderK(ref)TheinactivityofthePt(),thickNi()filmandthePt–Ni–Ptsurfacescanbeattributedtothelowbindingenergyofnitrogenonthesesurfaces,confirmingthepredictionsfromthemicrokineticmodelsTPDresultsfollowingthedesorptionofhydrogenfromthefoursurfacesaswellasammoniadosedatlowt

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