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

19December2014;accepted14April2015passivationstudies,andshowthatlocalfluores-cencelifetimeimagingprovidesaroutebywhichchangesinfilmprocessingcanbeevaluatedtoassesstheirinfluenceoncarrierrecombinationinfilms.Byremovingthesenonradiativepath-waystoobtainuniformbrightnesswithhighemissivityacrossallgrains,itislikelythatwewillseetheperformanceofperovskitedevicesapproachthethermodynamiclimitsforsolarcellsandotherlight-emittingdevices.

REFERENCESANDNOTES

1.T.C.Sum,N.Mathews,Energ.Environ.Sci.7,2518–2534(2014).2.M.A.Green,A.Ho-Baillie,H.J.Snaith,Nat.Photonics8,

506–514(2014).

3.J.Burschkaetal.,Nature499,316–319(2013).

4.G.E.Eperon,V.M.Burlakov,P.Docampo,A.Goriely,

H.J.Snaith,Adv.Funct.Mater.24,151–157(2014).5.G.Xingetal.,Nat.Mater.13,476–480(2014).

6.G.E.Eperonetal.,Energ.Environ.Sci.7,982–988(2014).7.J.H.Noh,S.H.Im,J.H.Heo,T.N.Mandal,S.I.Seok,Nano

Lett.13,1764–1769(2013).

8.NationalCenterforPhotovoltaicsattheNationalRenewable

EnergyLaboratory,Researchcellefficiencyrecords,www.nrel.gov/ncpv/(accessed11April2015).9.S.D.Stranksetal.,Science342,341–344(2013).10.H.Zhouetal.,Science345,542–546(2014).

11.O.D.Miller,E.Yablonovitch,S.R.Kurtz,IEEEJ.Photovolt.

2,303–311(2012).

12.N.K.Noeletal.,ACSNano8,9815–9821(2014).13.J.Youetal.,ACSNano8,1674–1680(2014).

14.P.W.Liangetal.,Adv.Mater.26,3748–3754(2014).15.S.D.Stranksetal.,Phys.Rev.Appl.2,034007(2014).16.F.Deschleretal.,J.Phys.Chem.Lett.5,1421–1426(2014).17.C.Wehrenfennig,M.Liu,H.J.Snaith,M.B.Johnston,

naturalgas,hasthehighestH-to-CratiodirectmethaneconversionintoliquidaromaticL.M.Herz,Energ.Environ.Sci.7,2269–2275(2014).ofallhydrocarbons;therefore,itismorehydrocarbonsinasinglestep(dehydroaromati-18.J.S.Manser,P.V.Kamat,Nat.Photonics8,737–743

environmentallyfriendlyintermsofCO2zationwiththemainreaction6CH4(2014).

→C6H6+19.SeethesupplementarymaterialsonScienceOnline.emissionsthanoilorcoal-derivedfuels.

9H2)usingcatalystswithMonanostructures20.G.Xingetal.,Science342,344–347(2013).

However,30to60%ofnaturalgasreservesaresupportedonshape-selectivezeolites(2,8–16).21.Y.Yamada,T.Nakamura,M.Endo,A.Wakamiya,Y.Kanemitsu,

classifiedas“stranded”becauseshippinggasisThistechnologyofferstwoadvantagesoverotherJ.Am.Chem.Soc.136,11610–11613(2014).noteconomical,andthecostsofliquefactionormethaneactivationchemistries:Completeoxida-22.M.Sabaetal.,http://wendang.chazidian.commun.5,5049(2014).23.A.Abateetal.,NanoLett.14,3247–3254(2014).buildingapipelineareusuallyprohibitivelyhightion,aswellasexplosivecombustion,isnotpossi-24.S.Watanabeetal.,Nat.Methods8,80–84(2011).(1–5).Theproblemofnaturalgasutilizationisex-blebecauseoftheabsenceofO2orotheroxidizing25.E.Edrietal.,NanoLett.14,1000–1004(2014).acerbatedbyburningandventingoftheassociatedreagents,andprocessingcanbeperformedat26.W.J.Yin,T.Shi,Y.Yan,Adv.Mater.26,4653–4658

gasproducedinthecourseofcrudeoilproductionremotelocationsbecausenoreagentsareneeded.(2014).

27.M.M.Lee,J.Teuscher,T.Miyasaka,T.N.Murakami,

atremotelocations.ConversionofmethaneintoThebiggestissuesincommercializationarerapidH.J.Snaith,Science338,643–647(2012).

shippableliquidscansolvetheseproblemsbutcatalystdeactivationandcomparativelylowsingle-28.P.Gao,M.Grätzel,M.K.Nazeeruddin,Energ.Environ.Sci.

remainsscientificallychallenging(1–3,6–8).

passconversionlevelsof~10%(2,8,13–16).De-7,2448–2463(2014).

velopmentofimprovedcatalystshasbeenhindered29.K.Munechikaetal.,NanoLett.11,2725–2730(2011).30.X.Wenetal.,J.Phys.Chem.Lett.5,3849–3853(2014).1

DepartmentofChemicalEngineeringandMaterialsScience,http://wendang.chazidian.comingstedtetal.,Sci.Rep.4,6071(2014).

StevensInstituteofTechnology,Hoboken,NJ07030,USA.identityofthezeolite-supportedMonanostruc-32.S.DeWolfetal.,J.Phys.Chem.Lett.5,1035–1039

2

OperandoMolecularSpectroscopyandCatalysisLaboratory,turesandtheirstructuraltransformations.

(2014).

DepartmentofChemicalEngineering,LehighUniversity,WestudiedMonanostructuressupportedon33.C.H.Seager,Annu.Rev.Mater.Sci.15,271–302(1985).Bethlehem,PA18015,USA.3DepartmentofChemical34.J.S.Yunetal.,J.Phys.Chem.Lett.6,875–880(2015).Engineering,NationalChungHsingUniversity,Taichung,ZSM-5zeolitesbycombiningquantumchemical35.Q.Dongetal.,Science347,967–970(2015).Taiwan,RepublicofChina.

calculationsusingdensityfunctionaltheory(DFT)36.W.Nieetal.,Science347,522–525(2015).*Correspondingauthor.E-mail:iew0@lehigh.edu(I.E.W.);withmultiplespectroscopictechniques,includ-37.D.Shietal.,Science347,519–522(2015).

simon.podkolzin@stevens.edu(S.G.P.)

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

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

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

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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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.

REFERENCESANDNOTES

5.InternationalEnergyAgency,“Goldenrulesforagoldenageof

gas-WorldEnergyOutlookspecialreportonunconventionalgas,”(InternationalEnergyAgency,Paris,France,2012);http://wendang.chazidian.com/goldenrules/.

http://wendang.chazidian.combinger,J.E.Bercaw,Nature417,507–514

(2002).

7.X.Guoetal.,Science344,616–619(2014).8.A.Holmen,Catal.Today142,2–8(2009).9.L.Wangetal.,Catal.Lett.21,35–41(1993).

10.L.Y.Chen,L.W.Lin,Z.S.Xu,X.S.Li,T.Zhang,J.Catal.

157,190–200(1995).

11.F.Solymosi,A.Erdöhelyi,A.Szöke,Catal.Lett.32,43–53

(1995).

12.J.Z.Zhang,M.A.Long,R.F.Howe,Catal.Today44,293–300

(1998).

13.Y.Xu,X.Bao,L.Lin,J.Catal.216,386–395(2003).

14.Z.R.Ismagilov,E.V.Matus,L.T.Tsikoza,EnergyEnviron.Sci1,

526–541(2008).

15.T.V.Choudhary,E.Aksoylu,D.W.Goodman,Catal.Rev.Sci.

Eng.45,151–203(2003).

16.J.J.Spivey,G.Hutchings,Chem.Soc.Rev.43,792–803

(2014).

17.SupplementarymaterialsareavailableonScienceOnline.18.H.Tian,C.A.Roberts,I.E.Wachs,J.Phys.Chem.C114,

14110–14120(2010).

19.E.L.Lee,I.E.Wachs,J.Phys.Chem.C111,14410–14425

(2007).

20.E.L.Lee,I.E.Wachs,J.Phys.Chem.C112,20418–20428

(2008).

21.S.Sklenaketal.,Phys.Chem.Chem.Phys.11,1237–1247

(2009).

22.J.Děde?ek,Z.Sobalík,B.Wichterlová,Catal.Rev.Sci.Eng.54,

135–223(2012).

23.J.-P.Tessonnieretal.,J.Phys.Chem.B110,10390–10395

(2006).

24.P.Hoffmann,J.A.Lobo,MicroporousMesoporousMater.106,

122–128(2007).

26.H.Aritani,H.Shibasaki,H.Orihara,A.Nakahira,J.Environ.Sci.

(China)21,736–740(2009).

27.S.Liu,L.Wang,R.Ohnishi,M.Ichikawa,J.Catal.181,175–188

(1999).

28.H.Zhengetal.,J.Am.Chem.Soc.130,3722–3723

(2008).

29.J.Gaoetal.,J.Phys.Chem.C118,4670–4679

(2014).

30.J.P.Tessonnier,B.Louis,S.Rigolet,M.J.Ledoux,

C.Pham-Huu,Appl.Catal.A336,79–88(2008).

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

http://wendang.chazidian.com/content/348/6235/686/suppl/DC1MaterialsandMethodsFigs.S1toS12TablesS1toS6ReferencesMovieS1

16January2015;accepted26March2015Publishedonline9April2015;1.E.McFarland,Science338,340–342(2012).2.J.H.Lunsford,Catal.Today63,165–174(2000).3.S.G.Podkolzin,E.E.Stangland,M.E.Jones,

E.Peringer,J.A.Lercher,J.Am.Chem.Soc.129,2569–2576(2007).

4.R.Khalilpour,I.A.Karimi,Energy40,317–328(2012).

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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