Mesoporous Co3O4 and AuCo3O4 Catalysts for Low-Temperature Oxidation of Trace Ethylene

MesoporousCo3O4andAu/Co3O4CatalystsforLow-TemperatureOxidationofTraceEthylene

ChunYanMa,†ZhenMu,†JinJunLi,†YongGangJin,‡JieCheng,†GaoQingLu,‡ZhengPingHao,*,†andShiZhangQiao*,‡StateKeyLaboratoryofEnVironmentalChemistryandEcotoxicology,ResearchCenterforEco-EnVironmentalSciences,ChineseAcademyofSciences,Beijing100085,P.R.China,andARCCentreofExcellenceforFunctionalNanomaterials,AustralianInstituteforBioengineering

andNanotechnology,TheUniVersityofQueensland,QLD4072,Australia

ReceivedAugust2,2009;E-mail:zpinghao@rcees.ac.cn;s.qiao@uq.edu.au

Abstract:Low-temperaturecatalystsofmesoporousCo3O4andAu/Co3O4withhighcatalyticactivitiesforthetraceethyleneoxidationat0°Carereportedinthispaper.Thecatalystswerepreparedbyusingthenanocastingmethod,andthemesostructurewasreplicatedfromthree-dimensional(3D)cubicKIT-6silicas.Highresolutiontransmissionelectronmicroscopy(HRTEM)studiesrevealedthat{110}facetsweretheexposedactivesurfacesinthemesoporousCo3O4,whereastheCo3O4nanosheetspreparedbytheprecipitationmethodexhibitedthemostexposed{112}facets.WefoundthatthemesoporousCo3O4wassignificantlymoreactiveforethyleneoxidationthantheCo3O4nanosheets.Theresultsindicatedthatthecrystalfacet{110}ofCo3O4playedanessentialroleindeterminingitscatalyticoxidationperformance.ThesynthesizedAu/Co3O4materials,inwhichthegoldnanoparticleswereassembledintotheporewallsoftheCo3O4mesoporoussupport,exhibitedstable,highlydispersed,andexposedgoldsites.GoldnanoparticlespresentonCo3O4readilyproducedsurface-activeoxygenspeciesandpromotedethyleneoxidationtoachievea76%conversionat0°C,whichisthehighestconversionreportedyet.

1.Introduction

Chemistsandmaterialscientistsaredriventoimprovetheperformanceofmaterialsfortechnologicalapplications.Materi-als’functionsdependontheircompositions,crystalstructures,andmorphologies.Thepropertiesofmaterialswiththesamecompositionbutdifferentstructureormorphologiescanvarysubstantially.1-4Scientistsdirectedincreasinglymoreeffortonnanostructuralorganizationtodesignfunctionalmaterials.5Thefundamentalunderstandinghasshowedthatstructureandmorphologycontrolofbasetransition-metaloxidesallowspreferentialexposureofcatalyticallyactivesites.6Co3O4hasbeenreportedtobeaneffectivecatalystintheoxidationreactionandhasalsobeenusedasasupportfornoblemetals.Co3O4isaversatileoxidethatisinvolvedinmanyadvancedphysicalapplications(magneticproperties)andinvariousheterogeneouscatalysisprocesses,forexample,NOxreduction,7COoxidation(with8,9orwithoutgoldpromoter10),

ChineseAcademyofSciences.TheUniversityofQueensland.

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MesoporousCo3O4andAu/Co3O4Catalystsand{001}facetsofCo3O4nanocubesformethanecatalyticcombustion.WhileXieetal.9reportedthattheCo3O4nanorods,whichpredominantlyexposedtheir{110}facets,favoringthepresenceofactiveCo3+speciesatthesurface,exhibitedamuchhigheractivityforCOoxidationthanthatofconventionalnanoparticleswhichmainlyexposedthe{001}and{111}facets,containingonlyinactiveCo2+sites.Therefore,theselectivesynthesisofnanostructuredCo3O4catalystswithhighlyreactivecrystalfacetsundernanoscaleisakeytoexploringdifferentcatalyticpropertiesandapplications.

ManyeffortshavebeenmadetoobtainporousCo3O4particleswithcontrolleddimensionsviananocasting,i.e.,bycrystallizinginsidetheporesofbothaperiodicandperiodicsilicas(com-mercialmonoliths,MCM,SBA).10,11,20-25Basicissuesonthenanocastingstrategyhavebeenrecentlyreviewed.26However,veryfewpublishedresultsareavailableonthereactivecrystalplanesofCo3O4materialspreparedusingthenanocastingmethodandtheircorrelationwithcatalyticreactionactivity.HighlydispersedAunanoparticlessupportedonmetaloxidescanshowexceptionallyhighactivitiesforseveralkindsofreactionsincludingCOoxidation,ozonedecomposition,oxida-tionofhydrogen,andtheCO+NOreaction.27-30Manystudieshavefocusedontheunusuallow-temperatureactivityandtheactivationmechanismofgoldcatalysts.31-34TheefficiencyofsupportedAunanoparticlesforalow-temperaturereactiondependsonavarietyoffactorsincluding,forexample,thesizesofAuparticles,thepropertiesofsupports,andtheirpreparationproceduresandpretreatmentconditions.AlthoughthesupporthasactivityinsomereactionssuchasCOoxidation,thehighlydispersedAunanoparticlesappeartoberesponsibleforthehighactivityofsupportedcatalysts.35,36Ethylene,alowmolecularweightvolatileorganiccompound(VOC)thatpossessesbothC-CσandC-Cπbonds,isharmful,causinganestheticillnessandenhancingphotochemicalpollution.Itisthusworthwhiletoremovetraceethylenefromsomeenvironments.Forexample,infruitstores(suchasrefrigeratedwarehouses),ethylenereleasedfromfruitscan

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ARTICLES

accelerateagingandspoilingofproduce.37,38Inordertomaintainfreshness,theeliminationoftraceethyleneatlowtemperature(0°C)isrequired.Althoughtheoxidationofethyleneisathermodynamicallyfavoredprocess,ethyleneisthermallystableandusuallyoxidizedinthepresenceofcatalysts.Untilnow,onlylimitedsolidthermalcatalystshavebeenreported,buttheircatalyticactivitiesarenotsufficientandtheoperatingtemperaturesarehigherthan100°C.38Tothebestofourknowledge,therehasnotbeenanyreportonethyleneoxidationoversolidthermalcatalystsat0°C.ItisverydifficulttoactivateandbreaktheC-Cσbondofethylene(unlikeHCHOandCO)atalowreactiontemperatureof0°C;thus,morepowerfulcatalyticmaterialsaredesirable.

OurrecentworkrevealedthattheCo3O4catalystpreparedbyaprecipitationmethodhadnocatalyticactivityoftraceethyleneoxidationat20°C,andtheAu/Co3O4catalyst,with2%goldloadingpreparedbyadeposition-precipitationmethod,converted7.4%oftraceethyleneat20°C.30Thus,thiscatalystisnotsuitableintheeliminationoftraceethylenebecauseofitsinsufficientactivityatlowtemperature,asitcannotefficientlyactivateethyleneandbreakitsC-Cσbondandproducecarbondioxide.Furthermore,thecatalyticactivationmechanismofCo3O4andAu/Co3O4catalystsinethyleneoxidationwasstillnotclearlydiscussed.

Herein,wereportthedevelopmentoflow-temperaturecatalystsofmesoporousCo3O4andAu/Co3O4andtheirexcellentcatalyticperformanceontheoxidationeliminationoftraceethyleneatlowtemperatures.WefoundmoreenhancedcatalyticactivityforethyleneoxidationovermesoporousCo3O4preparedbythenanocastingmethodcomparedwithCo3O4nanosheetspreparedbytheprecipitationmethod.Therolesoftheactivecrystalfacets{110}ofCo3O4catalystsonethyleneoxidationarediscussedinthispaper.Furthermore,wefoundthatthemesoporousAu/Co3O4catalystexhibitedwell-distributedAunanoparticlesontheporousCo3O4matrix,whichprovidedagreaternumberofactivegoldsitesandledtohighcatalyticactivityforethyleneoxidation(76%conversion)at0°C.Thisisthehighestreportedconversionofethyleneoxidationatalowtemperature.Thestudyofthecatalyticactivationmechanismpresentedhereiscriticallyimportanttoenhancinglow-temper-aturecatalyticactivitiesinvariousoxidationreactions.

2.ResultsandDiscussion

2.1.SynthesisandCharacterizationofCo3O4andAu/Co3O4Materials.ThesynthesisofmesoporousCo3O4material

involvesthreesubsteps:(1)theimpregnationandthermaldecompositionofacobaltprecursorintheporeofthree-dimensional(3D)cubicKIT-6silicawithania3dmesostructure(hardtemplate,seeFigureS1-S3intheSupportingInformationforrelativecharacterizations);(2)therepeatedimpregnationanddecompositionsteps;(3)theremovalofthehardtemplatebyNaOHsolutionetching.TheobtainedCo3O4growsalongthedirectionoftheKIT-6channelsandreplicatesthe3Dmeso-porousstructureofKIT-6.ThepreparationofthemesoporousAu/Co3O4samplefollowsthesamestepsasinthesynthesisofmesoporousCo3O4abovebutalsoinvolvesimpregnationofcobaltandgoldprecursorstogether.Itiswidelyrecognizedthatactivephasedispersionisgreatlyinfluencedbytheaffinityof

(37)Park,D.R.;Ahn,B.-J.;Park,H.-S.;Yamashita,H.;Anpo,M.Korean

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6,3599–3603.

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ARTICLESFigure1.(A)Low-angleXRDpatternsand(B)N2adsorption/desorption

isotherms(inset:poresizedistributioncalculatedfromdesorptionbranch)ofthemesoporousCo3O4andAu/Co3O4materialswithdifferentgoldloadings:(a)Co3O4,(b)1.0%Au/Co3O4,(c)2.5%Au/Co3O4.TheisothermcurvesbandcinpanelBareshiftedby15and30cm3·g-1,STP,respectively,forclarity.

theprecursortothesupport,andthatthisstrongaffinityfacilitatesgooddispersion.39WhenmesoporousAu/Co3O4issynthesized,hydrolysisandagingofthegoldchloride(HAuCl4)solutionatdifferentpHconditionsmayvarytheconcentrationofvariousgoldspecies[Au(OH)nCl4-n]-(n)1-3).ThestronglyadsorbingspeciesAu(OH)3Cl-canbeobtainedatpH7.Therefore,inthisworkthewell-dispersedgoldnanoparticlesattachedtotheCo3O4supportweresynthesizedatpH7.Intheimpregnationprocess,thesolutionmixturecontainingAu(OH)3Cl-andCo2+precursorwasintroducedintotheporesofKIT-6.AfterthesampleswerecalcinedandthehardtemplateKIT-6wasremoved,orderedmesoporousCo3O4wasobtainedasafaithfulreplicaofthetemplateKIT-6andthegoldnanoparticleswerestronglyadsorbed/embeddedonthemeso-porousCo3O4base.

Thelow-angleX-raydiffraction(XRD)patternsofCo3O4andAu/Co3O4samples,withdifferentAucontents,synthesizedusingKIT-6astemplatesareshowninFigure1A.Figure1A(a)showswell-resolveddiffractionpeaks(211)and(332),andtheshoulderpeak(220),whicharecharacteristicofacubicia3dmesostructure,indicatingahighdegreeoforderingofthemesoporousCo3O4material.MesoporousCo3O4materialiswell-resolvedincomparisontothecharacteristicdiffractionpeaksfromitstemplateKIT-6(seeFigureS1inSupportingInforma-tion),indicatingthatthemesostructureoftheCo3O4samplesisanegativereplicaofKIT-6.AdecreaseofthestructuralorderingwithanincreaseofthegoldcontentcanalsobefoundforvariousAu/Co3O4catalysts.ItshowsthattheadjustmentofthepHvalueandtheproductionof[Au(OH)nCl4-n]-mayinterrupttheimpregnationanddecompositionofacobaltprecursorinthesilicapore.LargeangleXRDpatternsofmesoporousCo3O4andAu/Co3O4materialsexhibitpeaksat31.3°,36.9°,38.2°,44.5°,55.6°,59.4°,and65.3°(2θ),40indicatingthatthecobaltprecursoristurnedcompletelyintocrystallinecobaltoxide(Figure2).Thediffractionpeakat38.2°correspondstotheCo3O4(222)plane.BecausetheAu0(111)diffractionpeakoverlapswiththeCo3O4(222)diffractionpeaks,itisdifficultandnotmeaningfultocalculatethegoldparticlesizeusingtheScherrerequation.

(39)Lee,S.-J.;Gavriilidis,A.J.Catal.2002,206,305–313(40)Li,J.J.;Xu,.

X.Y.;Hao,Z.P.;Zhao,W.J.PorousMater..

2008,15,

163–1692610

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Maetal.

Figure2.Large-angleXRDpatternsofthemesoporousCo3O4andAu/Co3O4materials.(a)Co3O4,(b)1.0%Au/Co3O4,(c)2.5%Au/Co3O4.Table1.EthyleneOxidationActivitiesandPhysicalPropertiesof

Co3O4andAu/Co3O4Catalystsacatalysts

C2H4conversion(%)SBETb(m2/g)Vpc(cm3/g)Dpd(nm)

Co3O4(3D)

30(0°C)840.143.6/11.71.0%Au/Co3O4(3D)50(0°C)940.173.4/11.42.5%Au/Co3O4(3D)76(0°C)1000.183.4/11.5Co3O4e(P)

0(20°C)---2%Au/Co3O4e(DP)

7.4(20°C)---

aInitialconcentrationofethyleneis50ppm.bBETspecificsurfaceareasdeterminedfromthelinearpartoftheBETequation(P/P0)0.05-0.25).cTotalporevolumesobtainedatP/P0)0.99.dPoresizedeterminedfromthedesorptionbranchusingtheBJHmethod.eReproducedwithpermissionfromref7.P:precipitationmethod,DP:deposition-precipitationmethod.

TheN2sorptionresults(Figure1B)showthatallofthemesoporousCo3O4andAu/Co3O4materialsexhibittypeIVisotherms(IUPACclassification),indicatingthepresenceofmesopores.41,42However,thecapillarycondensationstepofmesoporousAu/Co3O4isnotverypronounced,indicatingtherelativelysmallsizesofordereddomains.43Nitrogenphysisorp-tiondataareingoodagreementwiththelow-angleXRDresults.TheBarrett-Joyner-Halenda(BJH)poresizedistributionsofthemesoporousCo3O4andAu/Co3O4materialsareshownintheinsetofFigure1B.TheCo3O4andAu/Co3O4materialshavetwoprimarymesoporediametersof3.4and11.5nm,respectively.Abroadpeakofporesizedistributionat11.5nmisprobablyattributedtointerspaceofthesamples.Table1showsstructureparametersoftheCo3O4andAu/Co3O4catalysts.ThemesoporediametersofAu/Co3O4samplesdonotdecreaseremarkablywiththeincreaseofgoldloading,indicatingthatmostAuparticlesarestuddedonthesurfaceorembeddedintheporewall.TheBrunauer-Emmett-Teller(BET)surfaceareas(Table1)ofthemesoporousCo3O4andAu/Co3O4materialssynthesizedbythenanocastingmethodaremuchlargerthanthoseoftheCo3O4samplespreparedbytheprecipitationmethod(about15m2·g-1).30Highangularannulardark-fieldscanningtransmissionelec-tronmicroscopy(HAADF-STEM)imagesofthemesoporous2.5%Au/Co3O4materialsareshowninFigure3.ThesupportCo3O4hasIa3dsymmetryandorderedmesostructures.Thegoldnanoparticles(smallerthan5nm),imagedaswhiteluminousdots,arepresentedaspseudosphericalparticlesandincorporatedintheCo3O4matrix.Suchamesoporousstructurecanthus

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MesoporousCo3O4andAu/Co3O4CatalystsFigure3.HAADF-STEMimagesofthemesoporous2.5%Au/Co3O4

materialsobservedalongdifferentdirections.

Figure4.O2-TPDprofilesofmesoporousCo3O4andAu/Co3O4with

differentgoldloadings.

effectivelypreventgoldnanoparticlesfrombothaggregationandleachingbecauseoftheconfinementofthechannels.Wecanalsonotethatthemesoporouschannelsarenotblockedbythegoldnanoparticles,thusgreatlyfacilitatingthetransportofbothreactantandproduct.FormesoporousAu/Co3O4materials,goldparticlesenterpores,embedintoporewalls,anddrillthroughtheCo3O4walls,whichmayoffermoreactivesitesforthecatalyticreaction.

Figure4showstheO2-temperature-programmeddesorption(O2-TPD)profilesofthefreshcatalysts.Thepeakbelow300°suchCisascribedasOtothedesorptionofsurface-activepeakaboveoxygen350species2-andO-,andthedesorption°CisattributabletothedesorptionoflatticeoxygeninCo3O4.44Generallyspeaking,thelargerthecorrespondingdesorptionpeakareaofsurface-activeoxygenspeciesatthelow-temperaturerange,thehigherthecatalyticabilityforoxidationreaction.29Abroadandstrongdesorptionpeakfrom50to300°Cforthe2.5%Au/Co3O4sampleisevident.Incontrast,asmalldesorptionpeakcanbedetectedinCo3O4inasimilartemperaturerange.Bothsurface-activeoxygenspeciesandlatticeoxygendesorptionpeakscanalsobeobservedintheO2-TPDprofilesofthe1.0%Au/Co3O4samples.Itisclearthatthesurface-activeoxygenspeciesoftheAu/Co3O4catalystsincreasewithincreasinggoldcontent.

Figure5showsthediffusereflectanceinfraredFouriertransform(DRIFT)spectraofsurfacespeciesonmesoporousCo3O4andAu/Co3O4samplesat25°Cinaflowof50ppmC2H4/22%O2/He.SeveralIRpeaksareobservedintherangeof1500to4000cm-1.Thebandsappearingat2340and2360

(44)Xue,L.;Zhang,C.;He,H.;Teraoka,Y.Appl.Catal.,B2007,75,

167–174.

ARTICLES

Figure5.DRIFTspectraofethyleneoxidationonmesoporousCo3O4and2.5%Au/Co3O4catalysts.

cm-1areascribedtotheasymmetricstretch,νasym(OCO),ofCO2moleculesadsorbedonthecatalysts.45Thebandsobservedaround11and3246cm-1areattributedtothestretchingvibrationofcarbon-carbondoublebonds,ν(CdC),andthevibrationofcarbon-hydrogenbonds,ν(C-H),ofC2H4mol-ecules,respectively.45,46ComparedwithCo3O4,theintensityofpeakat11cm-1onAu/Co3O4increases,whichsuggestsarelativelystrongadsorptionofreactantC2H4.ApartfromC2H4andCO2,nootherethyleneoxidesaredetectedinDRIFTmeasurementsonmesoporousCo3O4andAu/Co3O4catalysts,thusindicatingthatonlythecompleteoxidationofethylenetoCO2occurs.

2.2.Low-TemperatureActivityofEthyleneOxidation.Anethyleneconcentrationof50ppmintheinitialgaswasusedtoinvestigatecatalystactivityinthisstudy.Table1alsoexhibitsanethyleneconversionoverCo3O4andAu/Co3O4catalysts.Itisevidentthatthepreparationmethodhasasignificantinfluenceoncatalyticperformance.The2.0%Au/Co3O4catalystpreparedbythedeposition-precipitationmethodwaslessactiveat0°Candcanoxidizeonly7.4%ethyleneat20°C.TheCo3O4catalyst(withoutnanogoldloading)preparedbytheprecipitationmethoddoesnothaveanyethyleneoxidationactivityevenat20°C.Incontrast,themesoporousCo3O4catalystspreparedusingthenanocastingmethoddrasticallyenhancetheactivityofethyleneoxidation,withanethyleneconversionof30%at0°C.Inaddition,theactivityofmesoporousAu/Co3O4catalystspreparedusingthenanocastingmethodisfurtherenhancedwhennan-ogoldisintroduced.Thepercentageofethyleneconversionis50%overmesoporous1.0%Au/Co3O4catalystat0°C.Themesoporous2.5%Au/Co3O4catalystshowsmuchhigheractivityat0°C(76%conversionofethylene),whichisaroundtentimestheactivityofthe2.0%Au/Co3O4catalystat20°Cpreparedbythedeposition-precipitationmethod.

2.3.CorrelationbetweenCatalystStructureandActivity.Inthiswork,themesoporousCo3O4samplesreplicatethe3DmesostructureoftheKIT-6.MesoporousCo3O4showsaround30%ethyleneconversiontoCO2at0°C.However,theCo3O4nanosheetspreparedbytheprecipitationmethodhavenoactivityofethyleneoxidationat20°C.ThisindicatesthatstructuralcontrolofCo3O4allowsthepreferentialexposureofcatalyticallyactivesites.Figures6and7showHRTEMimagesoftheCo3O4nanosheetspreparedbytheprecipitationmethodandthemesoporousCo3O4andAu/Co3O4synthesizedviananocasting,respectively.TheCo3O4nanosheetsarehexagonalinshapewithsizeca.20-50nm.Bothasetof{220}planeswithalattice

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X.-M.;Yang,X.-F.;Zhu,A.-M.;Fan,H.-Y.;Wang,X.-K.;Xin,

Q.;Zhou,X.-R.;Shi,C.Catal.Commun.2009,10,428–432.

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ARTICLESFigure6.(a)TEMimageand(b)HRTEMimageofCo3O4nanosheets

preparedbytheprecipitationmethod.

Figure7.HRTEMimagesofmesoporousCo3O4(a)andAu/Co3O4(b)preparedbythenanocastingmethod.TheinsertinpanelaistheFFTdiffractogramofthecorrespondingHRTEMimage.

spaceof0.286nmandasetof{111}planeswithasquarecrossinglatticespaceof0.467nmwereobservedforCo3O4nanosheets(Figure6b).ThedominantexposedplanesofCo3O4nanosheetsare{112},whichareperpendiculartoboththefirstsetof(220)planesandthesecondsetof(222)planes.12Figure7showsHRTEMimagesoftypicalmesoporousCo3O4andAu/Co3O4.TheupperleftinsetinFigure7ashowsthefastFouriertransform(FFT)diffractogramfromthemesoporousCo3O4with[110]direction.Figure7ashowsthatthelatticeplanesofmesoporousCo3O4areboth{111}withalatticespaceof0.467nmand{220}withalatticespaceof0.286nm.The{111}planesarebelievedtobeinactivecrystalplanes.9Furthermore,{220}planeswithalatticespaceof0.286nmwerealsoobservedformesoporousAu/Co3O4(Figure7b).ForthemesoporousCo3O4andAu/Co3O4,theexposedactiveplanesare{110}planesparallelto{220}.The{110}planesarecomposedmainlyofCo3+cations,andtheabundantCo3+cationsonthe{110}planesprovidesufficientsitesforethyleneandoxygenadsorptionandareregardedastheactivesitesforethyleneoxidation.WhenthereactantspassedthroughandadsorbedintotheporesofmesoporousCo3O4,theywereactivatedbytheexposedfacets.Thus,wecanconcludethatthe{110}planesarethemainlyactiveplanesforethyleneoxidation.Thisisinaccordwithrecentlypublishedsignificantresearch,inwhichlow-temperatureoxidationofCOonCo3O4nanorodswassystematicallystudied.9InthepreparationofmesoporousAu/Co3O4,theprecursorAuCl(OH)3-isadsorbedintotheporesofsilicatemplate,andtheporechannelslimitthegrowthofAuparticlesduringthecalcinationprocess.Followingremovalofthehardtemplate,theAuparticlesareembeddedorpartlyentertheCo3O4porewalls.Auparticlesoffermoreactivesitesbyenteringpores,embeddingintoporewalls,anddrillingthroughthewallsofCo3O4(Figure3).Seemingly,thewell-dispersedgoldnanopar-ticlesmayberesponsiblefortheenhancementofcatalyticactivity.Itiswell-knownthatactivesurfaceoxygenspeciesalwaysexertagreatinfluenceonthecatalyticactivityinlow-2612

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temperatureoxidation.47TheactivityoftheAu/Co3O4catalystcorrelateswiththesurface-activespecies,i.e.,themoreasurface-activeoxygenspeciesoccursonthecatalyst,thehigherthecatalystactivity.AunanoparticlesdispersedonCo3O4playanimportantroleinpromotingtheproductionofactiveoxygenspeciesonthecatalystsurface,whichapparentlyleadstotheenhancedoxidationactivityofethylene.ItisobviousthatthecompleteoxidationofethylenetoCO2occursonbothCo3O4andAu/Co3O4catalysts,whichisevidencedbytheDRIFTmeasurements.Consequently,thegoldnanoparticlesimprovetheproductionofsurface-activeoxygentoincreasecatalyticactivitybutdonotalterthereactionpathofethyleneoxidation.

3.Conclusions

Insummary,mesoporousCo3O4andAu/Co3O4catalystshavebeensuccessfullysynthesizedusingthenanocastingapproachandfoundtobehighlyactivetowardeliminatingtraceethyleneat0°C.Ourresultsshowthat30%ethyleneconversiononmesoporousCo3O4occursat0°C,buttheCo3O4nanosheetspreparedbytheprecipitationmethoddonothaveanycatalyticactivityat20°C.FurtherinvestigationrevealsthatthestructurecontrolofCo3O4allowsimprovementofthecatalyticactivity,i.e.,mesoporousCo3O4,withreactiveplanes{110},aremoreactivethanCo3O4nanosheetswith{112}planes.Furthermore,thehighestactivityisfoundovermesoporous2.5%Au/Co3O4catalyst(76%)becauseofthesurface-activeoxygenspeciesproducedeasilybytheactivesitesofnanogoldonporousCo3O4.TheobtainedmesoporousCo3O4andAu/Co3O4materialsareeffectivelow-temperaturecatalysts,whichcanbreaktheC-CσandC-Cπbondsatalowreactiontemperature,evenat0°forC.theTheseefficientcatalyticeliminationmaterialsofhavetracegreatethylenepotentialin,forapplicationsexample,warehousestorage.Theactivationmechanismstudyinthisworkhelpstounderstandtheimprovementofoxidecatalystactivity.

4.ExperimentalSection

SynthesisofKIT-6Silicas.KIT-6mesoporoussilicasweresynthesizedusingtetraethoxysilane(TEOS)asasilicasourceandPluronicP123(EO20PO70EO20)asastructure-directingagent.48Inatypicalsynthesis,P123(0.17mmol,1.0g),n-butanol(13.5mmol,1.0g),andHCl(35mL,0.6M)werestirredat35°Cuntilahomogeneousmixtureformed.TEOS(2.08g)wasthenaddedandstirredforanother24hatthesametemperature,followedbyahydrothermaltreatmentinanautoclaveat100°Cfor24h.Followingsynthesis,themixturewaswashedwithdistilledwateruntiltherewasnofoaminthewaterused.Thesamplewasthendriedandcalcinedat550°Cfor3htoabsolutelyremovetheP123template.Theresultingwhitepowderis3DcubicKIT-6mesopo-roussilica.

NanocastingPreparationofMesoporousAu/Co3O4andCo3O4.PorousAu/Co3O4materialswerepreparedusing3DcubicKIT-6as·3Hahardtemplate.Typically,anaqueoussolutionofHAuCl42O(16.09mgAu/mL)wasadjustedby1MNaOHsolutiontopH7andwasthendispersedin21.5gof7wt%solutionofCo(NO3)2·6H2Oinethanolandstirredatroomtemperaturefor2h.Subsequently,theKIT-6wasaddedintothesolutioncontaininggoldandcobaltprecursors.Ethanolwasremovedbytheevaporationat70°Cfor12h,andtheresultingpowderwascalcinedat300°Cfor3htocompletelydecomposethenitratespecies.Theimpregna-tionanddecompositionstepswererepeatedonceinordertoachieve

(47)Chavadej,S.;Saktrakool,K.;Rangsunvigit,P.;Lobban,L.L.;

Sreethawong,T.Chem.Eng.J.2007,132,345–353(48)Shi,Y.;Meng,Y.;Chen,D.;Cheng,S.;Chen,P.;.

Yang,H.;Wan,

Y.;Zhao,D.Y.AdV.Funct.Mater.2006,16,561–567.

MesoporousCo3O4andAu/Co3O4Catalystsperfectnanocasting,buttheamountofHAuCl4·3H2OandCo(NO3)2·6H2Ousedintherepeatedstepwastwo-thirdsofthatusedinthefirststep.Thesilicatemplatewasthenremovedbyetchingin65mLof2MNaOHaqueoussolutionat80°C.TheblackAu/Co3O4materialwasrecoveredbycentrifugationanddriedatroomtemperature.ThepreparedAu/Co3O4materialwasnamedasmesoporousx%Au/Co3O4(x)1.0,2.5),wherexisthecalculatedloadingamountofgold,assumingthatallthegoldandCo3O4wereincorporatedinthefinalproduct.ThemesoporousCo3O4sampleswerepreparedbyimpregnatingonlyCo(NO3)2·6H2Ointhenanocastingprocess.

MaterialCharacterization.Wide-angleXRDpatternsweremeasuredonX’pertPROequipmentusingCuKRradiation(λ)0.15418nm)inthe2θrangeof10-70°withascanningstepsizeof0.006°.Small-angleXRDwasrecordedonaX’pertPROpowderdiffractionsystemusingCuKRradiationinthe2θrangeof0.7-6.0°withascanningstepsizeof0.0016°.ThetexturalpropertiesofthesamplesweremeasuredbyN2sorptionatliquidnitrogentemperature,usingagasadsorptionanalyzerNOVA1200.HAADF-STEMandHRTEMmicrographswereobtainedwithaTecnaiG2F20u-TWINinstrumentatanacceleratingvoltageof200kV.Thespecimenswerepreparedbyultrasonicdispersioninethanol,evaporating-TPDadroptestsofthewereresultantcarriedsuspensionoutinMicromeriticsontoacarbonsupportgrid.O2Chemisorb2720apparatus.PriortoeachTPDrun,thecatalystwaspretreatedintheHeflowat300°Cinaquartzreactor.Afterthereactortemperaturewasloweredtoroomtemperature,thecatalystadsorbedO2for30minunderO2flowof50mL·min-1.ThenHegaswasfedintothereactorat50mL·min-1for30mintopurgeanyresidualoxygen.Thecatalystwas-then1heatedto750°Cataconstantheatingrateof10°C·minunderHeflowof50mL·min-1.ThedesorbedoxygenwasmonitoredbytheTCDdetector.InfraredspectraofthesampleswererecordedonaBrukerTensor27usingtheDRIFTtechniqueandscannedfrom4000to

ARTICLES

600cm-1with256scansataresolutionof4cm-1.BeforetheDRIFTspectrawererecorded,thesamplewassweptwithHegasat150°Cfor30min,andthenthecatalystbedtemperaturewasloweredto25°C.Themixedgas(50ppmC2H4+22%O2inHe),preparedusing1massflowcontrollerswithatotalgasflowof60mL·min-,wasthenpassedthroughthesamplecellat25°Cfor20min.

ActivityMeasurementofCatalystsforEthyleneOxidation.Catalytictestswereperformedusingafixedbedreactorloadingwith0.25gcatalyst(20-40mesh).Thereactionfeedconsistedof50ppmethyleneinsyntheticair(O2:22%,N2:balance).Thecombinedgasflowratewasmaintainedat60mL·min-1.Thecatalystbedtemperaturewasmaintainedat0°C,andthereactantsandproductswereanalyzedbyusingagaschromatographequippedwithanFIDdetector(Porapak-Rcolumn).ThegaschromatographwasdirectlyconnectedtoaNicatalystconverter.Conversionwascalculatedonthebasisofethyleneconcentrationintheeffluent.Acknowledgment.ThisworkwasfinanciallysupportedbyNation-alNaturalScienceFundsforDistinguishedYoungScholar(20725723),NationalBasicResearchProgramofChina(2010CB732300),andtheNationalHighTechnologyResearchandDevelopmentProgramofChina(2006AA06A310).S.Z.Q.acknowledgesUQMiddleCareerResearchFellowshipandfinancialsupportbytheAustralianResearchCouncil(ARC)throughDiscovery(DP1095861,DP0987969)andLinkage(LP0882681)programs.

SupportingInformationAvailable:Low-angleXRDpatterns,

nitrogenphysisorptionisotherms,andTEMimagesofthemesoporoussilicaKIT-6.ThismaterialisavailablefreeofchargeviatheInternetathttp://pubs.acs.org.

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