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Preparation of TiO2 photocatalyst loaded with V2O5 for O2 evolution

By Dustin Spencer,2014-02-18 12:20
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Preparation of TiO2 photocatalyst loaded with V2O5 for O2 evolutionof,O2,TiO2,with,V2O5,for

    Preparation of TiO2 photocatalyst loaded

    with V2O5 for O2 evolution

    J.Cent.SouthUniv.Techno1.(2009)16:09190925

    Dol:lO.1007/sl177l-009-0l53-9

    Springer

    PreparationofTiO2photocatalystloadedwithV205for02evolution

    GAOYou.1iang(高友良)-,CHENQiyuan(陈启元),TONGHai.xia(童海

    ),HUHui.ping(胡慧萍),

    QIANDong(~),YANGYa_hui(杨亚辉),ZHOUJianliang0~)

    (1.SchoolofChemistryandChemicalEngineering,CentralSouthUniversity,Changsha410083,China;

    2.SchoolofMaterialsScienceandEngineering,JingdezhenCeramicInstitute,Jingdezhen333403,China)

    Abstract:Ti0,photocatalystsIoadedwithV,Owerepreparedviaamodifiedhydrolysisprocess.andcharacterizedbyXray

    di

    raction.transmissionelectronmicroscopy,RamanspectraanddiflusereflectanceUv_Visspectrameasurements.The

    photocatalyticactivityofV2Os/TiO2wasinvestigatedbyemployingsplittingofwaterforO2evolution.Theresultsindicatethatv2O5

    1oadingcanpronouncedlyimprovethephotocatalyticactivityofTiO,wifhFejasanelectronacceptorunderUVorvisiblelight

    irradiation.TheoptimummassfractionoftheloadedV,Ois8%,andthelargestspeedofO,evolutionfor8%V2Os(massfraction)

    loadedTiO,catalystisll8.2Bmol/(L.h)underUVirradiation,and83.7lamol/(L_h1undervisiblelightirradiation.

    Keywords:TiO2;V205;photocatalyticactivity;02evolution

1Introduction

    Nowadays,moreandmoreattentionispaidtothe researchonphotocatalyticdecompositionofwater becauseofthehigheciency.cleannessandother

    advantagesofhydrogenenergy.FUJISHIMAand HONDA[1usedn.typesemiconductorTiO2tosplit waterandproduceH,inthe20thcentury.Astitaniawith largebandgapischeap,nontoxic,stableand reclaimable[2],itisaverypromisingkindof semiconductormaterialthatcanbephoto.excitedto generateelectronholepairsonitssurfaceandcreatea strongoxidationabilitywhenitabsorbslightwithenergy greaterthanitsbandgap,andtheelectronsinthevalence bandareexcitedtotheconductionband.TiO,isan effectivephotocatalystunderUVirradiationfor decomposingvolatileorganiccompounds[3], smartwindows[4andwaterelectrolysisforhydrogen production[5].However.puretitaniahaslow photo-activityandslowphotolysisspeedfor photocatalysisofwaterandisverydifficulttobe practicallyused.Soitisnecessarytoimproveits photoactivityandenhancethephotolysisspeedfor watersplitting.Generally,thephotocatalyticactivityof semiconductorcanbeimprovedthroughdopingor loadingsemiconductor,suchasRu+/TiO2[6,

    TiO2A1203,V2Os/TiO2A120317],TiO2?La20318],TiO2- WO319],V205WO3-TiO2[10].Andmostofthesedoped semiconductorswereusedtophoto.decomposeorganic pollutants,andsomeofthemwereusedtosplitwater withlowphotocatalyticefficiency[6].WhenTiO2is

    combinedwithotheroxidessuchasV2O,thespectral rangeofabsorptionisincreasedduetothesmallband gapofV205(2.2eV,2=564nln)comparedwiththatof TiO,(3.0eV,2=413nm).Thisallowslightwich?

    564nmtobeabsorbedbytheheterogeneousstructure. andthephotocatalyticactivitycanbeextendedtoa largerspectralrange.Inaddition,V205canactasan electronscavengerandpullthephotogenerated

    conductionbandelectronsofTiO,toitsownconduction band,whichkeepstheelectron-holepairsapartlonger (chargeseparation),andimprovestheoverall photocatalyticperformance.Vanadiumsupportedon titaniaiscommonlyusedasacatalystforanumberof industriallyimportantreactions.includingtheselective oxidationreactionsofoxylenel11,ammoxidationof

    hydrocarbons112],aswellasselectivereductionof NwithNH3inthepresenceofO2113].JIANGetal [14reportedthatthespreadingofvanadiumoverTiO2 supportleadstoamodificationofthechemicalphysical

    peculiaritiesoftheformerandtoanenhancementofits catalyticproperties.However,therearefewreportsabout V2Os/TiO2usedasacatalystforsplittingwater.Asyou knowthereactionofphotocatalyticsplittingwateriS carriedoutviatwohalfreactions,ofwhich,oneisthe photoreductionwithH,evolution.theotheristhe photooxidationwithO,evolution.Andthelatterismore diculttoachieve.Inthiswork.byusingFeas electronacceptors,V205?TiO2catalystspreparedbya modifiedhydrolysisprocesswereemployedforthe photocatalyticoxidationofwaterwimO,evolution.

Foundationitem:Project(08JJ3022)supportedbytheNaturalScienceFoundationofHunan

    Province,China

    Receiveddate:20090326;Accepteddate:200906O6

    Correspondingauthor:GAOYou-liang,Professor;Tel:+86798

    8499697;E-mail:gaocsu@163corn

    920J.Cent.SouthUniv.Techno1.(2009)16:09190925

    2Experimental

    2.1SynthesisofrutileTi02andV2os-Ti02catalysts TiO2colloidsolutionwaspreparedbythe

    hydrolysisoftetrabutyltitanate[Ti(0C4I-I9)4].Allthe chemicalreagentswereanalyticallypureandused withoutfurtherpurification.Ti(OC4H9)dwasdissolvedin ethanol,andthenthemixturewasaddedtodistilled waterdropbydropundermagneticstirring.Themixture wasstirredfor2handthendepositedfor6-8hbefore beingdriedat373Ktoobminawhiteprecursor.The whiteprecursorwasgroundinacarnelianmortarand loadedinanavicularquartzvesseltobecalcinedat l223Kfor5h.Thewhiteprecursorwasgroundagainto obtainfutileTi02RuffleTiO2wasdippedinammonium matavanadate(NH4VO3)solution,thendried,and calcinedat723Kfor5h.Aseriesofcatalystswere obtainedwithconcentrationsof5%,8%,1O%,20%and 40%ofloadedV2O.respectively.

    2.2CharacterizationofV2OsTi02catalysts

    Xraydiffraction(XRD1analysiswasusedtocheck thecoexistenceofdifferentcrystalphasesofthecatalyst byaHATCHID/max2250powderXraydiffractometer.

    ThediffractionprofileswererecordedwithCuK radiationr0.154056nm)overa20rangefrom10.to90..

    Aplumbaginouscounterwithmonochromatorwasused. TheXraytubewasoperatedat40kVand300mA.

    DiffusereflectanceUv.Visspectra(UVDRS1

    measurementswerecarriedoutonaBeijingPurkinje TU1901UV,visspectrophotometerequippedbya diffusereflectanceaccessorywithanIS19-1integrating sphere,andBaSO4powderwasusedasthereference. RamanspectrawererecordedonaRT1-30

    spectrophotometer.Thelineat4888amfromanargon ionlaserwasusedastheexcitationsource.Thelaser powerappliedwas326mW,andtherasterwidthwas 201.Thescanwasarrangedinthewavenumber

    rangeof100-1100cm-'.

    TheresultswereexaminedbyaHATCHISP-2305 gaschromatographyequippedwithathermal conductivitydetectonArgonwasusedasthecarriergas andthefixedphasewasmoleculesievef0.5nm). 2.3Photo-activitytests

    Thephotocatalyticreactionwasperformedusinga closedgas.circulatingsystemwithaninnerirradiation reactor.Alightsource(250Whigh-pressureHglampor 250WXelamp),wascoveredwithaglassjacketmade ofquartz(cut-offwavelength1essthan200nm).The reactortemperaturewaskeptconstantat293Kusing coolingwatchAmixtureofcatalyst(2g),distilledwater (640mL)andarequiredamountofFe2(804)3inthe reactorwascompletelydegassed,andthenargongaswas introducedintothesystem.Thecatalystpowderwas suspendedbyusingamagneticstirrer.Theproductgas oppressedtheliquidinthereactorintothegraduated

    cylinderthroughtheoutlet.Sothevolumeofproductgas canbereadthroughthegraduatedcylinderindirectly. Theevolutionof02wasdetectedbygas

    chromatography.

    3Resultsanddiscussion

    3.1Characterizationofcatalyst

    Fig.1showsXRDpatternsforcatalystsV2O5-TiO,. FromthepaRernssomedistinctpeaksforTiO2canbe observed.Fromthelefttoright20is27.56..36.21., 41-38..54.44..56.74.and69.O9.correspondingtothe diffractionpeakofruffleTi02crystalfaces(110),(101), (111),(211),(220),and(3l1),respectively,andwhich areconsistentwithliteraturereports15.XRDanalysis

    doesnotpresentanycharacteristicdiffractionpeakof V,Ophaseorothervanadiumoxideunti1theloading concentrationofV,O5ismorethan8%(massfraction). TheXRDpatternsforthecatalystswith10%.20%and 40%ofV,O5-loadedTiO,showthecharacteristic diffractionpeakofV205(20=-20.26.).Therefore,V205 willbewel1dispersedontheTiO,phasewhenits concentrationissmall;andonthecontrary,whenthe concentrationofloadingV205isenhanced,V205is gatheredandcrystallized

    Fig.1XRDpatternsforV205TiO2catalysts:(a)5~~V205TiO2;

    (b)8%V2O5TiO2;(c)10%V205TiO2;(d)20%V205TiO2;

    (e)40%V205TiO2

    RamanspectraofV205TiO2catalystssynthesized

    underdifferentdehydratedconditions,arepresentedin Fig.2.TherutileTiO2possessesRamanbandsat440and 612cm-[161,andV2O5possessesRamanbandsat

143,280,486,695and995cml7.Ramanbandat

    J.Cent.SouthUniv.Techno1.(200916:09190925921

    Ramanshift/cmi

    Fig.2RamanspectraofV205TiO2catalysts:(a)5%V205TiO2;

    (b)8%V2O5TiO2;(c)10%V205-TiO2;(d)20%V205TiO2;

    (e)40%V2OsTiO2

    995cmisassignedtothestretchingmodeofV0in

    thespectraof10%V205TiO2catalystsfromFig.2(c),

    whichindicatesthatV2Ohasdispersedonthesurfaceof TiO,inmonolayer.WhentheconcentrationofV,Ois beyonditsdispersionability,themicrocrystalliteofV205 isformed.WiththequantityincreaseofloadingV2o5, theintensityofthecharacteristicpeakofV.__——Obecomes

    moreobvious.WACHSandWECKH【?ENf181

    reportedthatwhenV2O5dispersedonthesurfaceofTiO2 inmonolayer,theatomicityofvanadiumwouldbe 7-8nm.Inthespectraof8%V2O5-TiO,catalysts.there isnopeakforV,Oandtheatomicityofvanadiumonthe surfaceofTiO,isabout6.8nm2.whichisclosetothe atomicityofthemonolayer.However,whenthecoverage isbeyond8%,thepeakforV0appears,whichisina

    goodagreementwithXRDpatterns.

    Fig.3showsanobviousabsorptionbandinthe wavelengthrangeof200-380nm.Itisthecharacteristic absorptionofthechargetransferof02D}Ti3dandthe

    chargeisfromoxygenatomcoordinatedwithtitaniumto theemptyorbitofthecentertitaniumatoml91.As

    showninFig.3,theabsorptionbandinthewavelength rangeof200-600nmresultsfromelectrontransitionof

    02pV3d,andtheabsorptionbandinthewavelength rangeof300-350nmresultsfromelectrontransitionof octahedronCO0rdinatevanadium.Buttheabsorption bandinthewavelengthrangeof200-300amisfrom utterlyisolatedoctahedronvanadium.Sotheabsorption bandinthewavelengthrangeof200-350nmshouldbe attributedtotheelecontransitioninteractionbetween O2pTi3dandO2pV3d[20].Meanwhile,the

    absorptionofvisiblelightforV2OsTiO2catalystsis

    enhancedobviously.SoV205loadingcanimprovethe absorptionofvisiblelightforTiO2photocatalyst markedly.

    )l/nm

    Fig.3DiffusereflectanceUV-Visspectra(UV-DRS)of catalysts:(a)5%V205TiO2;(b)8%V2OsTiO2;(c)10%V205?

    TiO2;(d)20%V205?TiO2;(e)40%V205TiO2

    Thereareseveralexistingformsforvanadium. oxygenspeciesonthesurfaceofTiO2onaccountofthe loadingquantityofV205,suchasisolated?VOx, poly.VOandcrystalloid.V2O5.Soitisreasonableto considerthatisolated-VOxisdominantwhenloading quantityisverysmall,whichislikelytobeconcerned withTiO2byVoTibond(showninFig.4(a)).wjth

    theincreaseofloadingquantityofV205,vacanciesnear theisolatedVOxaregraduallyoccupiedandthe proportionofpolyVOxthatisformedbyV0V

    chemistrybondconnectionincreasesgradually,leading toincreasingthepolymerizationdegreeofVOThe proportionofV——0——Tibonddeclineswitllthe

proportionofV0Vbondmountingup[21](shown

    inFigs.4(b)and(c)).

    OO0

    0/

    O0

    II

    (a)(b)(c)

    Fig.4BondgraphsofV205andTiO2onsurfaceofTiO2 Fig.5showsthetransmittancespectraofche suspendingliquidofTiO2powderwithdifferentV2O5 loadings.Itisclearfromthetransmittancecurvesthat withtheincreaseoftheconcentrationofloadingV2O, theoptica1.cransmittancegraduallydecreases.The averagetransmiRanceinthevisibleregionislargerthan thatinUV.1ightregionforallsamples.andthe transmissionintheUVlightregionislessthan5%.Tl1is meansthatal1catalystscanabsorbalmostallthe UVlightinthephotocatalyticreactionsaccordingto Uv-DRSinFig.3andthetransmittedspectraofthe suspendingliquidofV2o5TiO2catalystsinFig.5.Inthe wavelengthregionof400-580nm,forallthesamples therefractionandthetransmittanceincreasegradually .Ic01.0

    /.

    VO\.

    OV?O?n

    O.

    V?O?n\.

    V,O?n/O?n

    0

VOn

    /0,n?

    922J.

    Cent.SouthUniv.Techno1.(2009)l6:0919-0925 commonsemiconductorssuchasGaAsandInSbin III.VandII.?subgroupsarecalleddirectband

    semiconductorsbecausetheirminimumconductband andmaximumvalencebandarecorrespondingtothe samewavevector.Theoreticalcalculationofelectronsof thiskindofsemiconductorsfromtheirintrinsic absorptionshowsthatifthetransitionforanyKis permittedinthedirecttransitiontherelationshipbetween theabsorptioncoecientandphotonenergycanbe

    denotedasfollows:

    fromFig.3andFig.5.Thisindicatesthatthecatalystscan absorbvisiblelight.Butobviouslywhentheredshiftof thewavelengthoccurstheabsorptionwil1decrease.That istosay,thecatalystshaveagoodabsorptionto UV-lightandcanalsoabsorbvisiblelight.Butthe transmittancedoesnotchangeobviouslywiththe increaseofloadingV,O.

    ,nm

    Fig.5UV-VistransmittancespectraofV2O5-TiO2catalysts: (a)5%V205TiO2;(b)8%V2O5TiO2;(c)10%v2o5-TiO2;(d)

    20%V205TiO2;(e)40%V205-TiO2

    Undertheillumination,thetransitionforelectrons absorbingphotonsmustkeeDconservationofmomentum besidesconservationofenergy,whichsatisfiesthe selectionrule.Itisassumedthattheprimarywavevector oftheelectronisjwhichwilltransittothestatewith

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