科学杂志 Identification of molybdenum oxide nanostructures on zeolites for natural gas conversion
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科学杂志 Identification of molybdenum oxide nanostructures on zeolites for natural gas conversion
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time-of-flightsecondaryionmassspectrometry38.M.Grätzel,Nat.Mater.13,838–842(2014).
inScienceandEngineering.Theauthorsgratefullyacknowledge(ToF-SIMS)(fig.S11F)(19)andfoundhigher39.G.Grancinietal.,J.Phys.Chem.Lett.5,3836–3842
fundingfromtheNationalInstituteforBiomedicalImagingandClcontentinCH3NH3PbI3(Cl)filmsthanin(2014).
Bioengineering(NIHgrantEB-002027)supportingtheNationalESCACH40.Y.Tidharetal.,J.Am.Chem.Soc.136,13249–13256
andSurfaceAnalysisCenterforBiomedicalProblemsandToF-SIMS3NH3PbI3filmswithoutCl.Thistechnique(2014).
instrumentation.D.W.D.thanksI.Braly,S.Braswell,D.Moerman,andprobesthetop2nmofthefilm.
41.S.T.Williamsetal.,ACSNano8,10640–10654(2014).
B.Millerforvaluableassistance.S.M.V.gratefullyacknowledgesAlthoughperovskitesolarcellshavebetterra-D.GrahamforassistancewithToF-SIMS.Additionaldata,includingdiativeefficienciesthanmanyothertypes,suchACKNOWLEDGMENTS
materials,methods,andkeycontrols,areavailableonlineasasdye-sensitized,organic,orevencadmiumtel-ThismaterialisbasedinpartonworksupportedbytheStateofsupplementarymaterials(19).
luridesolarcells,theystillsufferfromgreaternon-WashingtonthroughtheUniversityofWashingtonCleanEnergyInstitute.D.W.D.acknowledgessupportfromanNSFGraduateradiativelossesthaninorganicmaterialssuchasResearchFellowship(DGE-1256082).S.M.V.acknowledgessupportSUPPLEMENTARYMATERIALS
http://wendang.chazidian.com/content/348/6235/683/suppl/DC1ingtheradiativeefficienciesofcopperindiumFellowship.TheresearchleadingtotheseresultshasreceivedMaterialsandMethodsgalliumselenide(CIGS)(31).OurresultsidentifyfundingfromtheEuropeanUnionSeventhFrameworkProgramSupplementaryText(FP7/2007-2013)underGrantAgreementNo.604032oftheasubpopulationofdarkgrainsandgrainbound-Figs.S1toS11
MESOproject.G.E.E.issupportedbytheEngineeringandPhysicalariesasspecifictargetsforperovskitegrowthandSciencesResearchCouncilandOxfordPhotovoltaicsthrougha
内容需要下载文档才能查看19December2014;accepted14April2015passivationstudies,andshowthatlocalfluores-cencelifetimeimagingprovidesaroutebywhichchangesinfilmprocessingcanbeevaluatedtoassesstheirinfluenceoncarrierrecombinationinfilms.Byremovingthesenonradiativepath-waystoobtainuniformbrightnesswithhighemissivityacrossallgrains,itislikelythatwewillseetheperformanceofperovskitedevicesapproachthethermodynamiclimitsforsolarcellsandotherlight-emittingdevices.
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inginsituultraviolet-visiblediffusereflectance
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spectroscopy(UV-visDRS),insituinfrared(IR)spectroscopy,andoperandoRamanspectroscopyatelevatedreactiontemperatureswithsimulta-neousonlinemassspectrometryofreactionpro-ducts.WedeterminedtheidentityandanchoringsitesoftheinitialMooxidenanostructuresandestablishedstructure-activityrelationships.Thecatalyticactivitycanbefullyrestoredbyregener-atinginitialMooxidenanostructureswithagas-phaseO2treatment.Furthermore,theactivitycanevenbeenhancedbycontrollingthedistri-butionofMooxidenanostructuresbyadjustingconditionsofsuchanO2regenerationtreatment.Molybdenumnanostructuressupportedonze-oliteswereinitiallypresentinanoxideformafterModepositionandanoxygentreatmentatele-vatedtemperatures(oursampleswerecalcinedat773K)(17).ThenumberofMoOxunitsinanaverageindividualnanostructurewasevaluatedusingtheedgeenergy(Eg)oftheinsituUV-visDRSspectra.TheEgvaluesforthefollowingwell-definedMooxidereferencecompoundsarepre-sentedinFig.1A:(i)MoO6-coordinatedMo7-Mo12clusters,(ii)linearchainsofalternatingMoO4andMoO6units,(iii)infinitelayeredsheetsofMoO5units,(iv)Mo2O7dimerasMoO3-O-MoO3,(v)iso-latedMoO4andMoO6monomers,and(vi)aque-ousmolybdateanionsasafunctionofthesolutionpH(18).TheEgvaluesinFig.1AexhibitalinearcorrelationwiththenumberofbridgingMo-O-MocovalentbondsaroundthecentralMocationand,correspondingly,withthenumberofMoOxunits
Fig.1.Spectroscopicmeasurements.(A)ElectronicedgevaluesbasedoninsituUV-visspectraofreferenceMooxidecompoundsexhibitalinearcorrelationwiththenumberofbridgingMo-O-MocovalentbondsaroundthecentralMocation.Thevalueof4.8eVfor2wt%Mo/ZSM-5(Si/Al=15)correspondstoMooxidespecieswithasingleMoatom.(BandC)InsituRamanspectraofMo/ZSM-5catalystsunderoxygenflowat773Kasafunctionof(B)MoloadingforconstantSi/Al=15and(C)Si/Alratioforconstant1.3wt%MoloadingwithbandassignmentstoMooxidespeciesbasedonDFTcalculations.a.u.,arbitraryunits.
inananostructure.TheEgvalueforarepresenta-tivecatalystsamplewith2weightpercent(wt%)MosupportedonaZSM-5(Si/Al=15)zeolite,whichisthemostcommonzeoliteevaluatedformethanedehydroaromatization,was4.8eV,whichfallsintherangeofisolatedMoOxnanostructureswithasingleMoatom.
ThenatureoftheMooxidenanostructureswasfurtherexaminedwithinsituRamanspectros-copybyvaryingtheconcentrationofMofrom0.7to3.3wt%onaZSM-5zeolitesupportwithaconstantSi/Alratioof15(Fig.1B)andbyvaryingtheSi/Alratiofrom15to140ataconstantMoconcentration
内容需要下载文档才能查看of1.3wt%(Fig.1C).Thespectrumfor1.3wt%MoonZSM-5withSi/Al=15isshowninbothsetsinFig.1,BandC,andasimilarspectrumisshowninoperandoRamanmeasure-mentswithmethaneflowinfig.S1(17).Theab-senceofsharpRamanbandsfromcrystallineMoO3nanoparticles(NPs)at996,815,and666cm?1(19)orcrystallineAl2(MoO4)3at~1004and1045cm?1(18,20)indicates,inagreementwiththeUV-visresultsinFig.1A,thatMooxidewascompletelydispersed;anyamorphousMooxidespecieswouldcrystallizeattheelevatedpretreatmenttemper-atureof773K.Somespectraexhibitedweakshoulderfeaturesat950cm?1fromMooxidespeciesinzeoliteframeworkvacancydefectsandat1026cm?1fromMooxidespeciesonextra-frameworkaluminaNPs(17).
FortheZSM-5(Si/Al=15)zeoliteinFig.1B,asingleRamanbandat993cm?1wasobservedin
theMo-OstretchingregionforallMoconcen-trations.However,athigherSi/AlratiosinFig.1C,anewbandat975cm?1wasobserved,andanad-ditionalbandappearedat984cm?1atthehighestSi/Al=140(Fig.1C).ThesethreebandscannotbeattributedtoasingleMooxidenanostructurebecausetheirrelativeintensitieschangewiththeSi/Alratio.Todeterminetheidentityandanchor-ingsitesoftheseMooxidestructuresintheZSM-5zeoliteframework,variousmonomericMooxidespecieswereevaluatedwithDFTcalculations,andthecalculatednormalvibrationalmodeswerecomparedwiththeexperimentalRamanspectra.
Aftercalcinationat773K,Mowaspresentinitshighestoxidationstateof+6,asevidencedbytheabsenceofd-dtransitionsforreducedMointheinsituUV-visspectra.OurDFTcalculationsshowthatneutralMoO3speciesonframeworkSisitesareunstableandthatframeworkAlsitesarerequiredforanchoring(17).Thisresultisinagree-mentwithchangesintheinsituIRspectraforsurfaceOHgroupsasafunctionoftheMoload-inginfig.S2(17)thatshowedpreferentialelim-inationofBrønstedacidsites(H+on[AlO4]–)afterModeposition.OnasitewithtwoadjacentframeworkAlatoms,thestoichiometryoftheMooxidespeciesshouldbeMo(=O)22+asdioxospeciestocounterbalancethe2–chargeof2[AlO4]–andmaintainMointhe+6oxidationstate.ThesizeofisolatedModioxospeciesservesasageometricrestriction,whichdeterminestheacceptablerangeofseparationdistancesbetweenthetwoanchor-ingframeworkAl-atomsites.BecauseZSM-5isaSi-richzeolite,Lowenstein’sruleprohibitsoneAlatomtobethefirstneighborofanotherAlatomintheframeworkasAl-O-Al.AnarrangementofAl-O-Si-O-AlwithtwoAlatomsassecondneigh-borswasnotfoundexperimentally,basedon27Alnuclearmagneticresonance(NMR)andaddi-tionalcharacterizationforZSM-5sampleswithSi/Al>8(21,22).Finally,anarrangementofAl-O-(Si-O)2-AlwithtwoAlatomsasthirdneigh-borsmustbetheonlypossibledoubleAl-atomanchoringsitesforModioxospecies.OurDFTresultsconfirmthattwoAlatomsasfourthneigh-borsinAl-O-(Si-O)3-Alcanserveonlyastwoin-dividualsingleanchoringsites(17).
AlthoughtheexactdistributionofAlatomsamongdifferentframeworksitesinZSM-5zeo-litesiscurrentlynotwellunderstood,itcanbevariedbyadjustingthezeolitesynthesisproce-dure.Forexample,thenumberofAlatomsasdoubleanchoringsitesinthearrangementAl-O-(Si-O)2-Alcanbevariedfrom4to46%forZSM-5sampleswithSi/Al=~20,basedoncharacteri-zationwithhydratedCocations(22).Thefrac-tionofAlatomsasdoubleanchoringsitestypicallydecreases,butnotproportionally,withtheincreasingSi/Alratioforthesamesynthesisprocedure(22).OurevaluationofAl-O-(Si-O)2-AlarrangementsinZSM-5showsthatthesesitescanserveasdoubleAl-atomanchoringsitesiftheyarelocatedinthesamechannel,butnotinthesameplane.AdditionalclassificationofdoubleAl-atomanchoringsitesisprovidedinfig.S4(17).ArepresentativeMo(=O)22+dioxostructureon
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ananchoringsitewithapairofAlatomsinT8andT12frameworkpositionsisshowninFig.2.Inthisnanostructure,theMoatomisbridge-bondedtotwoframeworkAlatomsthroughtwoneighboringframeworkoxygenatomsandter-minatedwithtwoadditionaloxygenatoms.ThenormalvibrationalmodesobtainedwithDFTcalculationsfortheseterminaloxygenatomsinMo(=O)22+aresummarizedinTable1.Thesym-metricstretch(ns)iscalculatedtobeat992cm?1.Thecalculatedgeometriesandnormalvibration-almodesfortheMo(=O)22+nanostructureonotherdoubleAl-atomanchoringsiteswithtwobridgingframeworkOatomsaresimilar(tableS2andfig.S6)(17).OnasitewithasingleframeworkAlatom,thestoichiometryofMooxidespeciesshouldbeMo(=O)2(OH)+tocounterbalancethe1–chargeof[AlO4]–andmaintainMointhe+6oxidationstate.Thevibrationalmodeforthesym-metricstretchoftheterminaloxygenatomsintheseMospeciesispredictedtobeat975cm?1,basedonevaluationofgeometriesandvibra-tionalmodesoftheMo(=O)2(OH)+nanostructureanchoredonsingleAl-atomsitesinT8(Table1)andotherZSM-5frameworkpositions(tableS1andfig.S5)(17).
Ramanspectroscopygivesrisetostrongbandsofsymmetricstretches(ns)andweakerbandsofasymmetricstretches(nas),withthelattersome-timesbeingundetectable.InourpreviousstudiesofMoO3/SiO2(19,20),nasforMo(=O)2wasnotobservedforMoloadingsbelow4wt%.Therefore,onlynsisexpectedtobeobservedforlowerMoloadings.AcomparisonofthedominantRamanbandsat975and993cm?1inFig.1withthecalculatedsymmetricstretchvalues(ns)inTable1(975and992cm?1)allowedustoassignthesebandstotwodistinctisolatedModioxospeciesanchoredon,respectively,singleanddoubleAl-atomframeworksites.
TheidentificationoftheisolatedMooxidestructuresprovidedinsightastohowtheywereaffectedbythemaincatalystformulationparam-eters:theMoloadingandSi/Alratio.AtalowSi/Al=15,MooxidespeciespreferentiallyanchoredonsiteswithtwoAlatoms(bandat993cm?1inFig.1B).EvenatthehighestMoloadingof3.3wt%,theAl/Moatomicratiois2.8,whichallowedallMoatomstobeanchoredondoubleAl-atomsites.However,whentheSi/Alratioincreased,thenumberofAlatomsperunitvolumeofthezeolitedecreased,andthenumberofsiteswithtwoAlatomsshouldhavedecreasedmorerapidlythantheoverallnumberofAlatoms.Asaresult,athigherSi/Alratiosof25and40inFig.1C,thedominantbandwasat975cm?1,arisingfromMo(=O)2OHspeciesanchoredonsiteswithoneAlatom.TheidentificationofsingleanddoubleAl-atomanchoringsitesisinagreementwithpreviousfindingsthateachMoatomdisplacesoneH+fromframework[AlO4]–sitesinZSM-5withSi/Al=40andtwoH+inZSM-5withSi/Al=15(23).AtthehighestSi/Al=140showninFig.1C,whenthecorrespondingAl/Moratiofellbelowunityto0.8,therewerenotevenenoughsingleAl-atomsitesforstabilizingallMoatoms.Forthiscatalystformulation,Mooxidespecieswereforced
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tobestabilized,notinthezeoliteporesbutontheleastpreferableSisitesontheexternalsurfaceofthezeolite.Anewbandat984cm?1forSi/Al=140inFig.1CisconsistentwithourpreviousRamanspectraforMooxidespeciessupportedonamor-phousSiO2(19,20).OurDFTcalculationscon-firmedthatModioxospeciesdidnotstabilizeinzeoliteporesintheabsenceofAlsitesandthatthestructureofisolatedMo(=O)2dioxospeciesas(Si-O-)2Mo(=O)2ontheexternalsurfaceofthezeolite(Fig.1;fulldetailsinfig.S8andtableS4)(17)issimilartothatonSiO2.ThesefindingsarealsosupportedbytheinsituIRspectraofthesurfaceOHregionforZSM-5(Si/Al=15)asafunctionoftheMoloadinginfig.S2(17).Theintensityofthepeakat3608cm?1forOHgroupsonframeworkAlsites(24)decreasedthroughre-placementbyMooxidespeciesatlowMoload-ings,followedbyadecreaseintheintensityofthepeakat3745cm?1forOHgroupsontheexternalsurfaceSisites(24)athigherMoloadings.TheisolatedMooxidestructurespreferentiallyanchoredondoubleAl-atomframeworksites,thensingleAl-atomframeworksites,andfinallySisitesontheexternalsurfaceofthezeolite.TheisolatedMooxidenanostructuresanchoredonthesethreetypesofzeolitesitesareshownschematicallyinFig.1andwith3DanimationinmovieS1.
DynamicchangesofMonanostructuresunderreactionandregenerationconditionswereeval-uatedbysimultaneouslycollectingoperandoRamanspectroscopyandonlinemassspectrom-etrymeasurements,firstwithCH4flowat953to1053K(fig.S1)(17)andthenunderregeneration
内容需要下载文档才能查看conditionswithgas-phaseO2flowat773K(figs.S10andS11)(17).UponCH4introduction,CO2wastheonlyinitialcarbon-containingproduct,andtheRamanbandat993cm?1fortheisolatedMooxidestructuresgraduallydisappeared.Be-causeCH4wastheonlyreactant,Mooxidenano-structuresreducedtooxycarbideorcarbidespecies.Severalstudieswithdifferenttechniques,suchasx-rayabsorptionfinestructure,MoLIIIedgex-rayabsorptionnear-edgestructure,and95MoNMR,providedirectevidencethatthereducedMophaseisacarbidewiththestoichiometryofMoCxorMoCxOyandthattheinitialoxidespeciesag-glomerateintoparticleswithasizeof~0.6nm(25–28).Aftertheinductionperiod,CO2forma-tionstopped,theRamanbandat993cm?1fortheinitialMooxidespecieswasnolongerobserved(Fig.2Bandfig.S1)(17),andthecatalystper-formedCH4dehydroaromatizationwithC6H6asthemainhydrocarbonproduct.
OurresultsdemonstratethatanO2treatmentcanreverseboththecarbideformationandtheagglomerationofMonanostructures.TheRamanspectraat753Kfortheinitialcatalystwithiso-latedMooxidestructuresandfortheregeneratedcatalystafterreactioninFig.2aresimilar,withasinglebandat993cm?1andashoulderfeatureat950cm?1.ThesimilarityintheRamanbandpo-sitionsandintensitiesbeforereactionandafterregenerationindicatesthattheregenerationcon-vertscarbidedMoNPsintoanoxidephase,redispersesthisphaseintoisolatedoxidenano-structureswithasingleMoatom,andallowstheseMooxidespeciestodiffuseandthenstabilizeonsubstantiallythesamezeoliteanchoringsitesasintheinitialcatalystbeforethereaction.
Fig.2.OperandoRamanspectraof2wt%Mo/ZSM-5(Si/Al=15).Spectra(A)afterinitialpretreatmentwith
gas-phaseoxygen,(B)duringreactionwithmethane,and(C)afterregenerationwithgas-phaseoxygenareshown.ThespectrademonstratethattheinitialMo(=O)22+nanostructuresanchoredondoubleAl-atomframeworksites(shownschematicallyontherightandinazeoliteporebelow)withavibrationalmodeat993cm?1arerecoveredafterregeneration.
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EffectsofregenerationtimewithO2ontheiden-tityofMonanostructuresandoncatalyticperform-ancewithCH4afterregenerationwereevaluatedbycombiningadditionalRamanspectroscopicmeasurementswithreactiontesting.RamanspectrawerecollectedinO2flowat773Kfortwo1.3wt%Mo/ZSM-5(Si/Al=15and25)catalystsaftertheirdeactivationinreactionwithCH4.TheevolutionofRamanspectraasafunctionofregenerationtimeinfigs.S10AandS11A(17)showsthatisolatedMooxidenano-structureswereregeneratedsequentially.IsolatedMo(=O)2speciesanchoredondoubleAl-atomframeworksiteswereregeneratedfirst,asevi-dencedbyasingleinitialRamanbandat993cm?1.Withincreasedregenerationtime,asecondRamanbandat975cm?1causedbyMo(=O)2OHspeciesanchoredonsingleAl-atomsitesappearedandgrewinintensity.Finally,athirdRamanbandat984cm?1duetoMo(=O)2speciesanchoredonSisitesontheexternalsurfaceofthezeoliteap-pearedandgrewinintensityforthecatalystwithalowerAlconcentrationinthezeolite(Si/Al=25insteadof15).Thesedirectspectroscopicobserva-tionsdemonstratethatexposuretogas-phaseO2firstregeneratesisolatedMooxidenanostruc-turesanchoredonsiteswithtwoAlatoms,thenforcesthesespeciestomigratetositeswithoneAlatomand,eventually,toSisitesontheexternalsurfaceofthezeolite.
AcomparisonofC6H6formationratesinCH4conversionasafunctionoftimeonstreamforafresh1.3wt%Mo/ZSM-5catalyst(Si/Al=25)versusthesamecatalystafterdeactivationinthe
内容需要下载文档才能查看for120min(Fig.3A)demonstratesthatthecat-alyticperformancecanbefullyrestored.TheC6H6formationratesafterregenerationmatchedthoseforthefreshcatalyst.AdditionalreactionresultsforC6H6andH2formationratesfortwo1.3wt%Mo/ZSM-5(Si/Al=15and25)catalystsasafunctionofregenerationtime(figs.S10andS11)(17)showthatboththeoverallactivityandselectivitytoC6H6fullyrecoveredafterregener-ation.Thus,rapidcatalystdeactivationcanbesuccessfullyaddressedbyregenerationwithgas-phaseO2,andthecatalystlifetimecanbeex-tendedbyrepeatedregenerationcycles.
Correlationsbetweenthestructureoftheini-tialMooxidespeciesandcatalyticperformancecanbeestablishedbycomparingtheevolutionoftheRamanspectrawithchangesinreactionratesasafunctionofregenerationtimeinfigs.S10andS11(17).ThecatalyticactivitywasrestoredonceMooxidenanostructuresondoubleAl-atomframeworksiteswereregenerated(after~20min).Withincreasedregenerationtime,theseisolatedMooxidespeciesmigratedfromdoubletosingleAl-atomzeoliteframeworksites,andthecatalyticperformancewithCH4remainedunchanged.Fur-thermore,thecatalyticperformanceofaregen-eratedcatalystcanbeoptimizedandmayexceedthatofafreshcatalystiftheregenerationtreat-mentisstoppedbeforeMooxidenanostructuresareforcedtomigratetoSianchoringsitesontheexternalsurfaceofthezeolite.Specificallyforthe1.3wt%Mo/ZSM-5(Si/Al=25)catalyst,MooxidenanostructureswereregeneratedandmovedfromdoubletosingleAl-atomzeoliteframeworksites(fig.S11A)(17).Notably,Monanostructuresre-mainedanchoredonzeoliteframeworkAlsiteswhentheregenerationwaslimitedtothisdura-tion,andtheratesofC6H6formationforsuchregeneratedcatalystsamplesactuallyexceededthoseforthefreshcatalyst.TheC6H6formationratesforacatalystregeneratedfor100mininFig.3Aexceededthoseforthesamecatalystbe-foredeactivationduringtheinitialtimeonstreamperiod.Incontrast,whentheregenerationtimewasextendedbeyond100min,Mooxidenano-structureswereforcedtomigratefromAlframe-worksitestoSianchoringsitesontheexternalsurfaceofthezeolite.Thischangeintheanchor-ingsitescausedthecatalyticactivitytodecreasetothelevelofthefreshcatalyst,andtheC6H6formationratesforthecatalystregeneratedfor120min(Fig.3A)matchedthoseforthefreshcat-alyst.WithtimeonstreamwithCH4,thecatalyticactivitydeclinedlikelybecauseofmigration,growth,andcokingofMoNPs,andtheperformanceforallregeneratedcatalystseventuallybecamein-distinguishable.However,inthefirst60minoftimeonstream,thebenzeneformationratesinFig.3Aandfig.S11C(17)weredependentontheidentityoftheinitialMooxidenanostructures.Forunderstandingtheseinitialactivitydiffer-ences,transition-stateDFTcalculationswereusedforcomparingCH4activationovercatalyticMocarbidenanostructuresanchoredontheidenti-fiedthreetypesofanchoringsites:doubleandsingleAl-atomzeoliteframeworksitesandSisitesontheexternalsurfaceofthezeolite.Thecalculationscomparedthefirststep(http://wendang.chazidian.com
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RESEARCH|REPORTS
zeolite(Fig.3,BtoD).TheCH4initiallyapproachesanexposedMoatom,anatomthatisnotdirectlybondedtothezeolite.Inthetransitionstate(Fig.3,CandE),CH4formsaMo-CH3-H-CcycleinwhichtheCatomofCH4bindstotheexposedMoatomand,simultaneously,oneoftheHatomsofCH4bindstoaCatominthecarbide.Thus,aMo-CpairofatomsintheMocarbidenanostructureservesasasinglecatalyticactivesite.ThisdualMo-CsiteactivatesCH4inascissoringmotionthatproducesaCH3groupbondedtoMoandanHatombondedtoCofthecarbide(Fig.3,DandF).AlthoughthemechanismofCH4activationissimilar,differencesingeometriesandelectronicpropertiesofMocarbidenanostructuresanchoredonAlandSisitesleadtodifferencesintheircatalyticproperties.TheCH4activationenergyovertheMocarbideanchoredonthedoubleAl-atomsiteof112kJ/molinFig.3Eislowerthan140kJ/molfortheSisiteinFig.3C.ThetransitionstateforthesingleAl-atomanchoringsiteisanalogoustothatforthedoubleAl-atomsiteinFig.3E,withacomparableactivationenergyof117kJ/mol(tableS6)(17).TheCH4reactionisthereforepredictedtobedominatedbytheactivityofMonanostructuresanchoredonframeworkAlsites.Thiscomputa-tionalresultisconsistentwithknownexperimen-talobservationsthatthecatalyticactivityofMonanostructuresdependsstronglyontheSi/Alratioofthesupportingzeoliteanddeclinessub-stantiallywhenAlframeworksitesarelostthroughdealumination(2,8,13–15,30).
TheobtainedinformationontheidentityofMostructures,theirregeneration,andtheirin-fluenceoncatalyticactivityopensnewoppor-tunitiesforrationaldesignofimprovedcatalystformulationsandforoptimizingreactioncondi-tionsfordirectconversionofnaturalgasintoliquidtransportationfuelsandvaluablefeed-stocksforthechemicalindustry.ItisimportanttocontrolthedistributionofMooxidespeciesandlimittheiranchoringtoframeworkAlsitesbecauseinitialMooxidenanostructuresanchoredonAlsitesofthezeoliteframeworkareconvertingintocarbidedMoNPswithhighercatalyticac-tivitythanthoseproducedbyinitialMooxidespeciesanchoredonSisites.ThenumberanddistributionofsingleanddoubleAl-atomanchor-ingsitescanbeoptimizedbyadjustingazeolitesynthesisprocedure.ThenumberofSianchoringsitesontheexternalsurfaceofthezeolitecanbereduced,ortheseSisitescanbeeliminatedcom-pletelybyadjustingtheModepositionpro-cedure.Furthermore,thecatalyticperformanceofMospeciesandtheirperiodicregenerationcanbeoptimizedbyadjustingcatalystformula-tions(forexample,withpromotermetals)andchangingthetemperaturesofthereactionandregeneration,flowrates,andotherreactionconditions.
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ACKNOWLEDGMENTS
TheworkinS.G.P.’sgroupatStevensInstituteofTechnologywassupportedbytheNSFundergrantCBET-1133987.TheworkinI.E.W.’sgroupatLehighUniversitywassupportedbytheNSFundergrantCBET-1134012.TheMaterialsStudiosoftwarewasusedunderacollaborativeresearchlicensefromBIOVIACorp.inSanDiego,California.Authorcontributions:J.G.andY.Z.obtainedthe
computationalandreaction-testingresultsanddiscussedtheoverallresults;J.-M.J.andY.T.obtainedtheexperimentalspectroscopicdataanddiscussedtheoverallresults;I.E.W.conceivedandsupervisedthespectroscopicexperimentsandinterpretedtheresults;andS.G.P.conceivedandsupervisedthecalculationsandreactiontesting,interpretedtheresults,andpreparedtheinitialmanuscript.
SUPPLEMENTARYMATERIALS
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systemII(PSII)ofplants,algae,andcyano-bacteriafacilitatessplittingofwaterintoO2,protons,andelectrons(1–4).Crystallo-graphicstructures(5–8)revealthatthecoreoftheOECconsistsofaMn3CaO4cubanemotifanda“dangler”Mnlinkedviatwobridgingox-ides,formingadistinctasymmetricMn4Ca-cluster(Fig.1A).Thisclusteriscoordinatedtofourwater
groupsoftheaminoacidresiduesofthePSIIpolypeptides(Fig.1C).ThestructureoftheOECaswellastheoxidationstatesofthefourmanga-neseionsundergochangesduringthewater-oxidationreactioncycle,orS-statecycle(4,9,10).Spectroscopicresultsandcomputationalchem-istryhaveprovidedinsightinreactioninterme-diatesandmechanisms(4,9–16).Thelabilityof
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