Influence of the sol – gel preparation method on the photocatalytic NO oxidation performance of TiO2/Al2O3 binary oxides

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ContentslistsavailableatScienceDirect

Catalysis

Today

jo u rn al h om ep a g e :w w w . e l s e v i e r . c o m / l o c a t e / c a t t o d

Inﬂuence

of

the

sol–gel

preparation

method

on

the

photocatalytic

NO

oxidation

performance

of

TiO2/Al2O3

binary

oxides

Meryem

Polat

a

_,

_Asli

_M.

_Soylu

a

_,

_Deniz

_A.

_Erdogan

a

_,

_Huseyin

_Erguven

a

_,

_Evgeny

_I.

_Vovk

a,b

_,

Emrah

Ozensoy

a,∗

a_Department_of_Chemistry,_Bilkent_University,₀₆₈₀₀_Ankara,_Turkey b_Boreskov_Institute_of_Catalysis,₆₃₀₀₉₀_Novosibirsk,_Russian_Federation

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received10January2014

Receivedinrevisedform20March2014 Accepted2April2014

Availableonline13May2014 Keywords: TiO2 Al2O3 Airpuriﬁcation Photocatalysis NO NO2

a

b

s

t

r

a

c

t

Inthecurrentwork,TiO2/Al2O3binaryoxidephotocatalystsweresynthesizedviatwodifferentsol–gel

protocols(P1andP2),wherevariousTiO2toAl2O3moleratios(0.5and1.0)andcalcinationtemperatures

(150–1000◦C)wereutilizedinthesynthesis.Structuralcharacterizationofthesynthesizedbinaryoxide

photocatalystswasalsoperformedviaBETsurfaceareaanalysis,X-raydiffraction(XRD)andRaman

spectroscopy.ThephotocatalyticNO(g)oxidationperformancesofthesebinaryoxidesweremeasured

underUVAirradiationinacomparativefashiontothatofaDegussaP25industrialbenchmark.TiO2/Al2O3

binaryoxidephotocatalystsdemonstrateanovelapproachwhichisessentiallyafusionofNSR(NOx

stor-agereduction)andPCO(photocatalyticoxidation)technologies.Inthisapproach,ratherthanattempting

toperformcompleteNOxreduction,NO(g)isoxidizedonaphotocatalystsurfaceandstoredinthesolid

state.CurrentresultssuggestthataluminadomainscanbeutilizedasactiveNO_xcapturingsitesthat

cansigniﬁcantlyeliminatethereleaseoftoxicNO2(g)intotheatmosphere.Usingeither(P1)or(P2)

protocols,structurallydifferentbinaryoxidesystemscanbesynthesizedenablingmuchsuperior

photo-catalytictotalNO_xremoval(i.e.upto176%higher)thanDegussaP25.Furthermore,suchbinaryoxides

canalsosimultaneouslydecreasethetoxicNO2(g)emissiontotheatmosphereby75%withrespectto

thatofDegussaP25.Thereisacomplexinterplaybetweencalcinationtemperature,crystalstructure,

compositionandspeciﬁcsurfacearea,whichdictatetheultimatephotocatalyticactivityina

coordina-tivemanner.Twostructurallydifferentphotocatalystspreparedviadifferentpreparationprotocolsreveal

comparablyhighphotocatalyticactivitiesimplyingthattheactivesitesresponsibleforthephotocatalytic

NO(g)oxidationandstoragehaveanon-trivialnature.

1. Introduction

Airpollutionintheurbansettingsleadstosigniﬁcantly detri-mentalimplicationsonhumanhealth.Someofthemainairborne contaminantsintheatmospherearenitrogenoxides(NOx),sulfur oxides (SOx), volatile organic compounds (VOCs) and particu-latematter(PM)[1].NOx-basedcontaminantsareemittedtothe atmospherethroughvariousanthropological,industrialor natu-ral combustion processes [1–3]. Under atmospheric conditions, NO(g)canbehomogeneouslyoxidizedtoNO2(g)viathermal (non-catalytic)chemicalpathways.NO2(g)isconsideredtobeevenmore toxicthanNO(g),asitcancauseasthmaandmanyother respira-toryillnesses[4].Inthelastfewdecades,varioustechnologieshave

∗ Correspondingauthor.Tel.:+903122902121. E-mailaddress:ozensoy@fen.bilkent.edu.tr(E.Ozensoy).

beendevelopedinordertoreduceairbornetoxicNO_xspecieswhich includeselectivenon-catalyticreduction(SNCR),selectivecatalytic reduction (SCR),NOx Storage and Reduction (NSR)(also called LeanNOxTraps,LNT)andThreeWayCatalysis(TWC)technologies [4–13].However,almostwithoutexception,thesetechnologiesare effectiveonlyatelevatedtemperatures(i.e.T≥250◦C).

Heterogeneousphotocatalyticoxidation(PCO)isanalternative approachthatcanbeutilizedunderambientconditions(i.e.room temperatureandatmosphericpressures)forairandwater purifi-cation[14–18].Alargevarietyofphotocatalyticmaterialsthatcan provideairpurificationunderUVorvisiblelightexcitationhave beenreportedintheliterature[15,19–28].TiO2-basedmaterialsare amongthemosteffectivephotocatalystsoperatingunderambient conditionsforairpurificationapplications[15,22,25,26,29]. How-ever,TiO2 isalsoknowntohavesomedrawbacks,suchaspoor mechanicalpropertiesandlowspecificsurfacearea(SSA),which limititscatalyticperformance[30–32].

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In a recent study, we have reported a TiO2/Al2O3 binary oxidesystemdemonstratingapromisinggas-phasephotocatalytic DeNOxperformance, whichwasfoundtobesuperiortoa com-mercialTiO2(DegussaP25)benchmarkphotocatalyst[33,34].This materialwasdesignedtodemonstrateanovelapproachwhichis essentiallyafusionofNSRandPCOtechnologies.Inthisapproach, ratherthanattemptingtoperformcompleteNOxreduction,NOx isoxidizedonaphotocatalystsurfaceandstoredinthesolidstate intheformofnitratesandnitritesonastoragecomponent.Unlike thephotocatalyticmetal-oxidesurface,whichisnotwater-soluble, storednitratesandnitrites(ortheirprotonatedsurfacederivatives) canbereadilywashedoffthephotocatalystsurface(e.g.viarainor wetscrubbing),restoringthephotocatalyticactivityofthesurface andregeneratingthephotocatalyst.

Chemicalandstructuralpropertiesofthebinaryoxidesystems stronglyinfluencetheNOxoxidationandstoragecapacity[16,18]. Alongtheselines, inthecurrent work,wefocus onTiO2/Al2O3 binaryoxidephotocatalystswhicharesynthesizedusingtwo differ-entsol–gelroutes(i.e.P1andP2)andthermallytreatedatvarious temperatures.ThefirstfamilyofTiO2/Al2O3binaryoxide materi-alspreparedviaP1manifestsitselfasinhomogeneouslydispersed TiO2crystallitesdepositedonalumina,whilethesecondone pre-paredviaP2isamostlyamorphoussponge-likeAlxTiyOz mixed oxidewithamorehomogenousmorphology[35].Thus,inthe cur-rentcontribution,theinfluenceofthephotocatalystbinaryoxide structureonthephotocatalyticperformanceandNOxstorage capa-bilityareinvestigated.Photocatalyticperformancesofthesetwo differentfamiliesofTiO2/Al2O3binaryoxidesarealsocomparedto acommercialbenchmarkphotocatalyst(i.e.DegussaP25)inorder toshowthatphotocatalyticNOx(g)abatementcanbesignificantly improvedbyutilizingTiO2/Al2O3binaryoxidesystems.

2. Experimental

2.1. PreparationandstructuralcharacterizationofTiO2/Al2O3 binaryoxides

TiO2/Al2O3binaryoxidematerialswerepreparedbytwo dif-ferent sol–gelsynthesis protocolsdenoted as P1 and P2 which weredescribedindetailinourearlierreports[11,12,33,35].Brieﬂy, in theﬁrst synthetic protocol(P1), ␥-Al2O3 (PURALOXS Ba200, 200m2_/g,_SASOL_GmbH,_Germany)_and_TiCl

4(Fluka,titanium(IV) chloridesolution)wereusedasstartingmaterials.Inordertoform agel,25vol%NH3 wasaddedtotheaqueous␥-Al2O3 andTiCl4 mixtureunderconstantstirring.Inthesecondprotocol(P2), tita-nium(IV)isopropoxide(TIP,97%,Sigma–Aldrich)andaluminum tri-sec-butoxide(ASB,97%,Sigma–Aldrich)wereusedas precur-sors.SynthesizedP1andP2materialsweresubsequentlycalcined inairfor2hattemperaturesrangingbetween150and1000◦C.In thesynthesizedmaterials,TiO2toAl2O3moleratioswereadjusted tobeeither0.5or1.0.Theseratioswerechosenbasedonour pre-viousstudies,whichindicatedthatfor(P2)Ti/Almaterials,TiO2to Al2O3moleratiosof0.5and1.0yieldedsomeofthebest perform-ingphotocatalysts[33].Currentlysynthesizedsamplesarelabeled as“(P#)nTi/Al-T”,whereP#denotestheutilizedsynthesis proto-col(i.e.P1orP2),nstandsfortheTiO2toAl2O3moleratio(i.e.0.5 or1.0)andTcorrespondstothecalcinationtemperatureinCelsius scale.AcommerciallyobtainedTiO2photocatalyst(DegussaP25, 99.5%,Sigma–Aldrich)comprisedofapproximately80–85%anatase and15–20%rutilebymass;wasalsousedasabenchmarksample inordertocomparephotocatalyticactivitiesofdifferentsamples underidenticalphotocatalyticconditions[36].

BETspeciﬁcsurfaceareameasurementswereperformedusing a Micromeritics Tristar 3000 surface area and pore size ana-lyzer via low-temperatureisothermal adsorption–desorption of

N2.Before thesurfaceareameasurements, materialswere out-gassedinvacuumat350◦Cfor4h.ThepowderX-raydiffraction (XRD)patternswererecordedusingaRigakupowder diffractome-ter, equipped witha Miniﬂex goniometer and an X-ray source withCuK␣radiation,=1.5418 ˚A,30kV,and15mA.TheXRD pat-ternswererecordedinthe2rangeof10–60◦ withascanrate of0.02◦s−1.DiffractionpatternswereassignedusingJoint Com-mitteeonPowderDiffractionStandards(JCPDS)cardssuppliedby theInternationalCentrefor DiffractionDatabase(ICDD). Raman spectrawererecordedusingaHORIBAJobinYvonLabRamHR800 spectrometer,equippedwithaconfocalRamanBX41microscope andaCCDdetector.TheRamanspectrometerwasalsoequipped withaNd:YAGlaser(=532.1nm);thelaserpowerwasadjusted to20mWfordataacquisition.

2.2. Gasphasephotocatalyticactivitymeasurements

Acustom-designphotocatalyticflowreactorsystemwasusedin thephotocatalyticactivitymeasurements[33,37].Theflowreactor systemconsistedofagasmanifoldsystem,massflowcontrollers (MKS1479A),acapacitancepressuregauge(MKSBaratron622B), acustom-madephotocatalyticreactorandachemiluminescence NOxanalyzer(HoribaAPNA370).Thegasmanifoldsystemwas con-nectedtogascylinderscontainingN2(g)(99.998%,LindeGmbH), O2(g)(99.998%,LindeGmbH),and100ppmNOdilutedinN2(g) (LindeGmbH).Massflowcontrollers(MFC)calibratedforN2andO2 gaseswereusedtocontrolthevolumetricflowratesofthegases. Flowrateofthegaseswereadjustedto0.750standardlitersper minute(SLM)forN2(g),0.250SLMfor O2(g)and0.010SLMfor themixtureof100ppmNO(g)dilutedinN2(g).Themixedgases werebubbledthroughathermostatichumidifierfilledwith deion-izedwaterwhichwaskeptat25◦C.Therelativehumidityofthe totalgasmixturewas70%whichwasmeasuredatthesample posi-tionintheflowreactorwithaHannaHI9565humidityanalyzer. Foratypicalphotocatalyticperformancemeasurement,950mgof apowdersamplewasplacedona2mm×40mm×40mm poly-methylmethacrylate(PMMA)planarsampleholder andthegas mixture was allowedto sweep over the photocatalyst powder which was packed into the sample holder. Before the photo-catalyticperformance measurements,theembeddedTiO2/Al2O3 sampleswereirradiatedwithUVAlight(F8W/T5/BL368,Sylvania orF8W/T5/BL350,Sylvania)underatmosphericconditionsfor18h inordertoremovethesurfacecontaminationsandtoactivatethe photocatalysts.Afterthisinitialexsitupretreatmentstep,activated photocatalystpowderswereplacedintheflowreactorwhichwas equippedwithan8WUVAlamp.Duringtheperformance analy-sisexperiments,(P1)Ti/Alsampleswereirradiatedwitha368nm wavelengthlightsource(F8W/T5/BL368,Sylvania,Germany)and (P2)Ti/Alsampleswereirradiatedwitha350nmwavelengthlight source(F8W/T5/BL350,Sylvania,Germany).Forbothcases, photo-catalyticperformancesoftheTi/Alphotocatalystswerenormalized usingthephotocatalyticperformancevaluesoftheDegussaP25 benchmarksamples, which weremeasuredunder the identical photocatalyticconditionsusingthecorrespondingUVAlightsource utilizedintheTi/Alsamplemeasurements.

Forthedeterminationoftherelative photocatalytic perform-ances,percentphotonicefﬁciencies(%)wereutilizedwhichare describedinEqs.(1)and(2).

%=

nNO_x nphoton

×100 (1)

where,nNO_xrepresentseitherthedecreaseinthetotalnumberof molesofallgaseousNO_xspeciesorthenumberofmolesofNO2(g) generated in a60minphotocatalytic activitytest.Furthermore, nphotoncorrespondstothetotalnumberofmolesofincidentUVA

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photonsimpingingonthephotocatalystsurfacein60min,which iscalculatedviaEq.(2)as:

nphoton=ISt

Nhc (2)

whereIstandsforthepowerdensityoftheUVAlamp experimen-tallymeasuredatthesamplepositioninthephotocatalyticreactor (typically,7.5Wm−2),istherepresentativeemissionwavelength oftheUVAlamp(i.e.350nmor368nm),Sisthesurfaceareaofthe planarPMMAsampleholderinthereactorthatisexposedtothe UVAirradiation(i.e.40mm×40mm=1600mm2_);_t_is_the_duration oftheperformancetest(i.e.3600s),NistheAvogadro’snumber,h isPlanck’sconstantandcisthespeedoflight.

Inthecurrentwork,experimentallymeasuredpercentphotonic efficienciesof(P1)Ti/Alsampleswerenormalizedbyusingtheper centphotonicefficiencyvaluesoftheDegussaP25benchmark pho-tocatalyst(SupportingInformationFig.S1)whosephotocatalytic performancewasmeasuredunderidenticalflowandillumination conditionswiththe(P1)Ti/Alsamples.Thus,normalized photo-catalyticperformancesofthe(P1)Ti/Alsampleswerereportedby dividingthepercentphotonicefficiencyofthe(P1)Ti/Alsampleto thepercentphotonicefficiencyofDegussaP25benchmark photo-catalystandbymultiplyingthisratioby100,asshowninEq.(3): %relativephotonicefficiencyof(P1)Ti/AlwithrespecttoP25

=

%(P1)Ti/Al %P25

×100 (3)

Thesameapproachwasalsofollowedforthe(P2)Ti/Al fam-ily, where thenormalized photocatalytic activity values of the (P2)Ti/Alsampleswerereportedusingthepercentphotonic efﬁ-ciencyvaluesoftheDegussaP25benchmarksample(Supporting InformationFig.S1)whichwasanalyzedunderidenticalﬂowand illuminationconditionswiththe(P2)Ti/Alfamily.

Notethatallofthereportedphotocatalyticperformance mea-surements in this workcorrespondto950mg ofphotocatalyst. Inotherwords,thesevaluesareintentionallyneithernormalized usingthephotocatalystspecificsurfaceareanorusingthe pho-tocatalystmass.Drawbacksassociatedwiththenormalizationof thephotocatalyticreactionratesusingphotocatalystmasswas dis-cussed recentlyin detail by Wachs et al.[38].We believe that normalizationofthephotocatalyticactivityvaluesinthecurrent studyviaSSAcouldalsobequitemisleading;astheoverallSSA ofthecurrentlyemployedcomplexmixed oxideheterogeneous catalystsmaynotbesimplyrelatedtothespecificsurfaceareas oftheactivephase(s).Furthermore,tothebestofourknowledge, theexactchemical/electronic/morphologicalnatureoftheactive site(s)responsibleforthephotocatalyticNOxoxidationandstorage onTiO2/Al2O3binaryoxidesarealsonotclear[39–46].This compli-cationalsopreventsanunambiguousexperimentaldetermination ofthesurfacecoverageoftheactivesites,precludingthe unequiv-ocalcalculationofaturnoverfrequency(TOF)value.Furthermore, anotheradditionalcomplicationregardingthecomparisonof abso-lutephotocatalyticactivitiesofdifferentpowderphotocatalystsin agas/solidphotochemicalreactionisassociatedwiththe macro-scopic packing of the micron/millimeter-sized grains that are formingthepowdersamples.Duetothemacroscopicgrainsizeand graindensitydistributiondifferences,lightpenetration/scattering towardthegrainswhich arephysicallylocatedfartherfromthe PMMAsampleholdersurfacelevelvariesina complexmanner. Thissuggeststhatmacroscopicgrainsizeandgrainpacking den-sityofdifferentphotocatalystpowdersareimportantparameters that play anactive role in themagnitude of theabsolute pho-tonicefficiencyvalues.Consideringallofthesecomplications,we believethatreportingphotocatalyticactivityvaluesasdescribedin

0 20 40 60 80 0.0 0.2 0.4 0.6 0.8 1.0 C oncent ration ( ppm ) Time(min) Light-on Light-oﬀ Adsorpon NO_{x (g)} NO(g) NO2 (g)

Fig.1. Concentrationversustimeplotforarepresentativephotocatalytic perfor-mancetest.Blue,blackandredcurvescorrespondtotheconcentrationsoftotal NOx(g),NO(g)andNO2(g),respectively(seetextfordetails).(Forinterpretationof

thereferencestocolorinﬁgurelegend,thereaderisreferredtothewebversionof thearticle.)

Eqs.(1)–(3)providesarelativelyunambiguouscomparisonofthe (P1)Ti/Al,(P2)Ti/AlandDegussaP25samples.Inthelightofthis approach,byconsideringtherelativeperformanceofTi/Alsamples withrespecttoP25;onecanalsoreadilycompareTi/Albinaryoxide sampleswithmanyotherexistingphotocatalyticsystemsreported intheliterature.

Inthephotocatalyticperformanceanalysistests,concentrations versustimeplotsacquiredbythechemiluminescenceNO_xanalyzer wereexploited.Fig.1illustratesatypicalconcentrationversustime plotrecordedduringaphotocatalyticperformancetest.Inthisplot, (blue-colored)totalNO_xconcentration(i.e.sumofthe concentra-tionsofalloftheNOxspeciesexistinginthereactor)aswellas NO(g)(blackcurve)andNO2(g)(redcurve)concentrationsinthe photocatalyticreactorarepresentedasafunctionoftime.Inthe ﬁrst∼20minoftheanalysis,agasmixturecontainingN2(g),O2(g), H2O(g)and1ppmNO(g)issuppliedtothephotocatalystsurface underdarkconditions,wheretheUVAlampisoffandany back-groundlightexposureofthephotocatalystsurfaceisprevented.In thisperiod(i.e.att∼7min),gasfeedisswitchedfromtheby-pass linetothereactorwhichisaccompaniedbyatransientdecreasein thetotalNO_x(g)andNO(g)concentrations.Thiscanbeattributed tothedilutionofthegasinthereactorandtheadsorptionofNOx speciesonthegaslines,reactorwallsaswellasadsorptiononthe photocatalystsurface.SinceundertheseconditionsnoUVorVIS radiationisallowedtoimpingeonthephotocatalystsurface,no photocatalyticactivityisobservedduringthisinitialstage,evident bythepresenceofaminoramountofNO2(g)productionwhichis mostlyduetothermalcatalyticoxidationprocessesoccurringon thecatalystsurface.Afterthisinitialdarkperiod,reactorwallsand thephotocatalystsurfacearesaturatedwithNOx,whichis appar-entbythereturnoftheNOx(g)andNO(g)tracestotheoriginalinlet concentrationvalueofc.a.1ppm,signifyingtheendofthethermal catalyticactivity.

Afterthisinitialtransientperiodthatisatt∼25min,UVA irra-diationisturnedonandthephotocatalyticreactionisstarted.Due totheUVAillumination,asigniﬁcantandapermanentdecreasein theNO(g)andtotalNO_x(g)concentrationsconcomitanttoavisible permanentriseintheNO2(g)levelareobserved.Thisindicatesthe conversionofNO(g)intoNO2(g)viaPCOprocess.Furthermore, pro-ducedNO2(g)canadsorbonthephotocatalystsurfaceintheform ofchemisorbedNO2,nitritesandnitrates[11,12,35]andstoredin thesolidstate,resultinginafurtherfallintheNO(g)andtotalNOx

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300 400 500 600 700 800 900 1000 1100 0 100 200 300 400 500 Temperature(o_C) Spe cific Sur fac e Area (m 2 /g ) (b) (P2) 0.5T_{(P2) 1.0Ti/Al}i/Al 300 400 500 600 700 800 900 1000 1100 0 100 200 300 400 500 S pe cific S ur fa ce A rea (m 2 /g ) Temperature(o_C) (P1) 0.5Ti/Al (P1) 1.0Ti/Al (a) 243 175 ₁₆₁ 101 ₉₉ 216 148 124 86 79 424 393 131 64 9 9 25 86 390 470

Fig.2.Speciﬁcsurfaceareavaluesforthesynthesized(a)(P1)Ti/Aland(b)(P2)Ti/Albinaryoxidesamplesaftercalcinationatvarioustemperatures.

signals.Itshouldbenotedthat,decreaseintheNO(g)concentration mightalsohavesomecontributionfromthedirectphotocatalytic decompositionandphoto-reductionofNO(g)formingN2(g)and/or N2O(g)[47]. However,sincethedirect photocatalytic reduction isarelativelyinefﬁcientpathway,thisreactionchannelmaybe expectedtobeaminorphotochemicalroute.Consequently,the totalNOxconcentration(blue)curve(whichismostlycomprised ofthesumofNO(g)andNO2(g)signals)inFig.1remainsmostly below1ppmduringtheUVA-activatedregime,illustratingthe con-tinuousphotocatalyticactivityandphotochemicalNOxstoragein thesolidstate.

3. Resultsanddiscussion

Thermalevolutionandthestructuralchangesofthe(P1)Ti/Al and(P2)Ti/Alsampleswerestudiedasafunctionofthecalcination temperatures.Thespeciﬁcsurfaceareas(SSA)ofthesynthesized Ti/Albinaryoxidescalcinedatdifferenttemperaturesarepresented inFig.2.Beforecalcination(P2)Ti/Alsampleshavesigniﬁcantly (almosttwotimes)higherSSAvaluesthanthecorresponding(P1)

Ti/Alsamples.Thisdifference canbeassociated withdissimilar morphologiesof P1 andP2 samples.TEM-EDXanalysis ofTi/Al samplessynthesizedviaP1andP2suggeststhat(P1)Ti/Al mate-rialconsistsofTiO2crystallitesdispersedratherinhomogeneously on_␥-Al2O3,while(P2) Ti/Almaterialpresentsamore homoge-nousandanamorphousAlxTiyOzmixedoxidesystemwithahigher porosity[35].TheSSAvaluesof both(P1) and(P2)Ti/Albinary oxidesdrasticallydecreaseinamonotonicfashionwith increas-ingcalcinationtemperature.AswillbeshownbelowviaXRDdata (Fig.3),thismonotonicdecreaseinSSAvaluesisassociatedwith theorderingandcrystallizationofthemixedoxidesystemand for-mationofnewphases.Anotherimportantbehavior observedin Fig.2isthatforlowercalcinationtemperaturestheTi/Almixed oxidespreparedviaP2revealmuchhigherSSAvaluescompared tothat of P1;for higher calcination temperaturesthis trend is reversedandP2samplescontinuetoloseSSAinaradical man-nerwhileSSAlossforP1samplesremainatamoderatelevel.This isduetothefactthatintheP1materials,titaniaisdispersedonto the␥-Al2O3supportmaterialwherealuminaandtitaniadomains existasseparatephases.Inthissystem,␥-Al2O3servesasastrong

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backboneenablingarelativelyhighintrinsicsinteringresistance, where␥-Al2O3→␣-Al2O3(corundum)andTiO2(anatase)→TiO2 (rutile)phasetransitionsarehindered(Fig.3).Incontrast,P2 sys-temiscomprisedofaAlxTiyOzmixedoxidewhichcanreadilyform low-surfaceareaphasessuchascorundumandrutileatelevated temperatures(Fig.3).

Structuralalterationsofthe(P1)and(P2)Ti/Alphotocatalysts werealsoinvestigatedasafunctioncalcinationtemperaturevia XRDmethod(Fig.3).Fig.3showstherepresentativeXRDpatterns ofthesebinaryoxidesampleswithatitaniatoaluminamoleratio of0.5.ItcanbeseeninFig.3athatthe(P1)0.5Ti/Al-700 sam-pleyieldsbroaddiffractionpeakscorrespondingtoanatasephase (JCPDS21-1272)andevenbroaderdiffractionsignalsassociated with␥-Al2O3 (JCPDS 29-0063)indicating smallcrystallite sizes. Furthercalcinationat800◦C,doesnotresultinasigniﬁcantchange intheXRDpatternofthe(P1)Ti/Alsystem.However,calcination at900◦Cleadstothesharpeningoftheanatasediffractionsignals suggestingcrystallographicorderingofthisparticularphase. Fur-thermore,atthiscalcinationtemperature,sharpandintenserutile diffractionsignals(JCPDS04-0551)alsobecomeapparent;clearly demonstratingthebeginningofananatase→rutilephase trans-formation.Calcinationofthe(P1)0.5Ti/Alsampleat1000◦Cleads tosigniﬁcantsuppressionofanatasediffractionsignals,alongwith theformationofweakbutdiscernible␣-Al2O3(corundum)phase (JCPDS10-0173).Itisworthmentioningthatthecorresponding XRDpatternsofthe(P1)1.0Ti/Alsamplerevealidentical crystal-lographictrendstothatofthe(P1)0.5Ti/Alsampleandhenceare notbeshownhere(SupportingInformationFig.S2).

Fig.3b shows XRD patternsof the(P2) 0.5 Ti/Al photocata-lysts calcinedat various temperatures. It is apparentin Fig.3b thatuponcalcinationattemperaturesbelow900◦C,(P2)0.5Ti/Al systemrevealsamostlyamorphousstructureandrelatively dis-orderedphaseswithsmallparticlesizes.Calcinationofthe(P2) 0.5 Ti/Al sample at 900◦C leadsto the formation of the rutile phase.Itshouldbenotedthatinthe(P2)0.5Ti/Alsystem, amor-phousAlxTiyOzmixedoxidetransformsreadilyintotherutilephase withoutrevealingasubstantialamountofanatase.Increasingthe

calcinationtemperatureto1000◦C,resultsinthesharpeningand strengtheningoftherutilesignalsandformationofintense␣-Al2O3 signals.XRDpatternsofthe(P2)1.0Ti/Alsample(datanotshown) revealanalogoustrendstotheonesgiveninFig.3bwheretheonly noticeable differenceis theshiftintheonset ofthe crystalliza-tion/orderingtemperatureto800◦C;asaresultoftheincreasing titanialoadinginthebinaryoxidemixture.

Comparisonofthethermallyinducedstructuralchangesof(P1) Ti/Aland(P2) Ti/Alsamplesrevealsthatforthe(P1)Ti/Al sam-ple,anatasephaseexistsinalargerthermalwindowincontrastto the(P2)Ti/Alsystem.Inaddition,althoughcorundumformationis observedinaratherlimitedfashiononthe(P1)Ti/Alsystemwithin theinvestigatedthermalwindow,presenceofcorundumismuch moresigniﬁcantforthe(P2)Ti/Alsystem.Furthermore,XRDdata giveninFig.3areinverygoodagreementwiththeBETresults pre-sentedinFig.2.ItisapparentthatthedrasticfallintheSSAvalues ofthe(P2)Ti/Alsystematelevatedcalcinationtemperaturescan belinkedtotheformationofhighlycrystallineand low-surface areaphasessuchasrutileandcorundum,whilesuchphasesare notexpressedinastrongfashionforthe(P1)Ti/Alsystem.

Fig.4presentsRamanspectraofthesynthesized(P1)and(P2) 0.5Ti/Alsamplesaftercalcinationatvarioustemperatures.Raman spectrumofthe(P1)0.5Ti/Alsamplecalcinedat700◦Cgivenin Fig.4ashowsweakRamanfeaturesat144,400,519and639cm−1 whichcanbereadilyassignedtotheanatasephase[11].Increasing thecalcinationtemperatureto800◦Cleadstosharperandmore intense anataseRaman signals suggesting ordering and further crystallizationofthisparticularphase.Calcinationofthe(P1)0.5 Ti/Alsampleat900◦Cresultsinadecreaseintherelative intensi-tiesofanatasesignalswhilenewRamanfeaturesassociatedwith rutilephasestarttoappearat236,447and612cm−1[11]. Fur-thercalcinationat1000◦Cleadstoalmostcompletesuppression oftheanatasefeatureswhilerutilesignalssigniﬁcantlyintensify (Fig.4a)inverygoodagreementwiththecorrespondingXRDdata presentedinFig.3a.

Forthe(P2)0.5Ti/Alsamples,noRamanbandsweredetected forcalcinationtemperatureslessthanorequalto600◦C(Fig.4b).

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Fig.5. RelativephotocatalyticperformancesofthesynthesizedTi/AlbinaryoxidesamplesnormalizedwithrespecttothephotocatalyticactivityofDegussaP25industrial benchmark(seetextfordetails).

Ontheotherhand,calcinationat800◦Cyieldsacomplicatedand convolutedsetofRamanbandswhichcontainbothanataseand rutilefeatures.Itshouldbenotedthatsuchfeaturesareelusive todetectinthecorrespondingXRDdatapresentedinFig.3b sug-gestingthe presenceof relativelysmalland disorderedanatase andrutile crystallitesat this temperature.Athighercalcination temperaturessuchas900◦Cand1000◦C,(P2)0.5Ti/Alsampleis dominatedbytherutilephasealongwiththegradual disappear-anceoftheanatasephase;inaccordancewiththecorresponding XRDdatapresentedinFig.3b.

PhotocatalyticNOoxidation and storageperformancesof all ofthesynthesizedbinaryoxidesamplesaswellastheDegussa P25benchmarkphotocatalystwerealsomeasuredinthe custom-designphotocatalyticflowreactor.Percentphotonicefficiencies for “Photocatalytic total NOx(g) decrease” and “Photocatalytic NO2(g)generation”werecalculatedbyintegratingthetimeversus concentrationplotssimilartotheonegiveninFig.1andusingEqs. (1)and(2).Next,percentphotonicefficiencyvaluesoftheTi/Al sampleswerenormalizedusingthecorrespondingvaluesoftheP25 DegussabenchmarkcatalystasdescribedinEq.(3).Thesevalues aredenotedas“%RelativephotonicefficiencywithrespecttoP25”. Fig.5presentsthesenormalizedpercentphotonicefficiencyvalues for“PhotocatalyticTotalNOxDecrease”(bluebars)andfor “Photo-catalyticNO2(g)Generation”(redbars).Inthisnormalizedvertical axis,100%correspondstoaphotocatalyticactivitythatisidentical tothatofDegussaP25benchmarkphotocatalyst(illustratedbythe horizontalflatlineinthehistogram).

NotethatforanultimatephotocatalystwithasupremeDeNOx performance,bluebarsshouldbemaximized(toobtainmaximum photocatalyticNOx(g)storage/conversion);whileredbarsshould besimultaneouslyminimized(toreleaseminimumamountoftoxic NO2(g)intotheatmosphere).DegussaP25benchmark photocata-lysthasahighphotocatalyticNO(g)oxidationabilityleadingtothe generationoflargequantitiesofunwanted NO2(g)alongwitha verylimitedNO_xstoragecapability(SupportingInformationFig. S1).Keepinginmind thatNO2(g)isa much moretoxic chemi-calthanNO(g),highphotocatalyticNOoxidationactivityofthe

DegussaP25industrialbenchmarkphotocatalystwithouta signif-icantNOxstoragecapabilityrendersthismaterialanon-idealNOx removalphotocatalyst.

AnalysisoftheperformanceresultspresentedinFig.5reveals interesting observations.For all of thesynthesized Ti/Albinary oxidesystems,NO2(g)sliptotheatmosphere(i.e.redbarsinFig.5) is alwaysless than thatof theDegussa P25 benchmark photo-catalyst.Thisobservationmayhavetwodifferentorigins.Firstly, for a largenumber of theanalyzed Ti/Al samples,overall pho-tocatalyticNOoxidation andfurtherNO2 storageintheformof nitrates/nitritesaregreaterthanthatofP25(thisisevidentbythe bluebarsabove100% mark,concomitanttothered barsbelow the100%mark). Forthesesystems, althoughalargeamountof NO2isgeneratedviaphotocatalyticNOoxidation,theseoxidized speciescannotfindtheopportunitytoslipintotheatmospheredue tothehighNO_xstoragecapacityofthebinaryoxidesystem pro-videdbythealuminadomainswhichareabletorapidlycapture theoxidizedNOxspeciesinthesolidstate.Thisisalsoconsistent withthefactthatNO_xadsorptionenergyonaluminaistypically higherthanthatoftitania[12].Thesecondoriginoftherelatively lowNO2 slipofasmallgroupofTi/Alsamples(suchas(P2)0.5 Ti/Aland(P2) 1.0Ti/AlsamplescalcinedatT<800◦C)are asso-ciatedwiththefactthatthesephotocatalystshaveanextremely poorphotocatalyticNOoxidationcapability.Sincethese photocata-lystscannotgenerateNO2(g)viaphotocatalyticoxidation(verified by smallblue bars in Fig. 5), NO2 slip is accordingly also very limited.Thusingeneral,performancedatainFig.5indicatethat forhighlyactivephotocatalystswhichcanefficientlyconvertNO(g) intoNO2(g),aluminadomainscanbeutilizedasactiveNOx captur-ingsitesthatcansignificantlyeliminatethereleaseoftoxicNO2(g) intotheatmospherebyadsorptionandsolidstatestorage.Aswill bediscussedinmoredetailbelow,aluminaadditionalsotypically improvestheSSAofthebinaryoxidesystems(Fig.2)andenables themtohave generallyhigher SSAvalues thanthat ofDegussa P25(i.e.55m2_/g)._On_the_other_hand,_it_is _also_worth mention-ingthatintheabsenceofaphotocatalyticoxidationcomponent suchasTiO2,pure␥-Al2O3hasanextremelylimitedphotocatalytic

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NO(g)storage/adsorptionandNO2(g)generationcapability(data notshown).Thisobservationclearlydemonstratestheneedforthe co-existenceofphotocatalyticNO(g)→NO2(g)oxidation function-alitiestogetherwithNO2(g)adsorption/storagefunctionalitiesin anultimatephotocatalyst.

Inordertoinvestigatetheeffectofcalcinationtemperatureon thephotocatalyticperformanceoftheTi/Alsystems,onecanfocus onthecorrespondingdataforthe(P2)0.5Ti/Alphotocatalystfamily giveninFig.5.Assessmentofthissetofdataclearlyrevealsthatfor calcinationtemperatureslessthan900◦C,(P2)0.5Ti/Alsamples do notpresent anysigniﬁcantphotocatalyticactivity.However, calcinationat900◦Cleadstoamajorboostinthephotocatalytic activity,whilethisincreasedactivityfallsdrasticallyuponfurther calcinationat 1000◦C. Structural characterizationdata given in Figs.2–4indicatethatforcalcinationtemperaturesbelow900◦C, (P2)0.5Ti/AlsystemiscomposedofamostlyamorphousAlxTiyOz mixedoxide,whichalsocontainspoorlyorderedandsmallanatase andrutilecrystallites(Figs.3band4b)renderingarelativelyhigh SSA (i.e. ≥131m2_/g)₍_Fig. ₂_b)._Upon _calcination _of _the _(P2) _0.5 Ti/Alphotocatalystat900◦C,formationofanorderedrutilephase, togetherwitharelativelyminorcontributionfromanataseparticles (Figs.3band4b)areobserved;concomitanttoadrasticdecrease inSSAto64m2_/g₍_Fig.₂_b).

Note that analogous temperature-dependent performance trendsarealsovalidforthe(P2)1.0Ti/AlfamilygiveninFig.5, wheretheonsetofthephotocatalyticactivitiesobservedatalower temperatureof800◦Cwithincreasingtitanialoadinginthebinary oxidesystem.Thisminortemperatureshiftintheonsetof pho-tocatalyticactivityfor thehigher titanialoadingis in linewith theshiftintheanatase/rutilecrystallizationtemperaturesforthe (P2)1.0Ti/Alsamples,discussedabove.Furthermore,thedecline in the photocatalytic activity of the Ti/Al systems at T>900◦C canbeexplained withthealmost completeloss ofthe anatase phase(Figs.3 and4).It isknownthattheco-existenceof both anataseandrutilephasesiscriticalforobtainingahigh photocat-alyticNOoxidationandstorageactivity[34,48,49].Thusinoverall, theseobservationssuggestthatalthough Ti/Alsystemscalcined atlowertemperaturesrevealrelativelyhigherSSA,thesepoorly orderedoramorphous systemsdonot possesstheproper crys-tallographic/electronic/morphological features required for the generationofthephotocatalyticactivesiteswithsufﬁcient qual-ity/quantity.

Comparison of thephotocatalytic performance results given in Fig. 5 for the(P1) Ti/Al family with that of (P2) Ti/Al fam-ilyimpliesthatthereisacomplexinterplaybetweencalcination temperature,crystal structure,composition and SSAwhich dic-tatethefinalphotocatalyticactivityinacoordinativemanner.In ordertoillustratethispoint,onecancomparethebest perform-ingphotocatalystintheP1family(e.g.(P1)0.5Ti/Al-800)tothat of P2 (e.g.(P2) 0.5 Ti/Al-900). It can readilybeseen that pho-tocatalytictotal NOxdecrease (i.e.bluebarsin Fig.5)forthese samplesare about 161–176%greater than that of Degussa P25 benchmarkphotocatalystwhiletheirNO2sliptotheatmosphere (i.e.redbarsinFig.5)aresignificantlylowerthanthatofDegussa P25(i.e.75%lowerfor(P1)0.5Ti/Al-800and55%lowerfor(P2) 0.5Ti/Al-900). Inotherwords,bothof thesephotocatalystscan simultaneouslyoxidize NOinto NO2 and captureoxidized NOx speciesinthesolidstateina muchmoreefficientmannerthan Degussa P25.However, comparison ofthestructural properties of thesetwo efficient photocatalysts presents stark dissimilari-ties.Firstly,XRD(Fig.3a)andRaman(Fig.4a)dataforthe(P1) 0.5Ti/Al-800sample revealthat this materialis predominantly comprisedof ananatasephase withaminorcontributionfrom rutile,whilecorrespondingdataforthe(P2) 0.5Ti/Al-900 sam-ple(Figs. 3b and4b)suggesta composition whererutile isthe dominantphasewithanatasebeingtheminorityphase.Thesetwo

structurally different photocatalysts revealing comparably high photocatalyticactivitiesimpliesthatthephotocatalyticallyactive site(s)haveacomplexnaturewhichmayinvolveanamorphous phase,a particularanatase(orrutile)adsorptionsite,a particu-laradsorptionsiteontheanatase/rutilehetero-junctionand/ora particular OHfunctionalityonanyofthesedomainsetc. How-ever,itisclearthatordinaryXRDandRamanmeasurementsdonot revealunambiguousinformationregardingneitherthenaturenor thequantityofsuchactivesites.

It canalsobeemphasizedthatthesetwo differentbut com-parablyactivephotocatalysts(i.e.(P1)0.5Ti/Al-800and(P2)0.5 Ti/Al-900)havequitedissimilarSSAvalues.While(P1)0.5 Ti/Al-800samplehaveaSSAof161m2_/g,_(P2)_0.5_Ti/Al-900_sample_has_a SSAof64m2_/g._This_observation_demonstrates_that_using_absolute SSAvaluesasthesoleparameterforcomparingthephotocatalytic activitiesof dissimilar materials canbe misleading(Supporting InformationFig.S3).

Consequently, results presented in the current work indi-cate that Ti/Al binary oxide photocatalysts can be synthesized usingdifferentsyntheticprotocolspossessingquitedifferent struc-tural features detectedin BET, Ramanand XRDmeasurements yet,revealingsimilarphotocatalyticactivities.Hence,neitherthe nature,northequantityofthephotocatalyticactivesitescanbe readilyinferreddirectlyfromsuchconventional andlong-range characterizationtechniqueswhichlackspatial/energeticresolution inthenanometerscalethatcanlocallyprobetheelectronicand structuralpropertiesoftheparticularactivesite(s).

4. Conclusions

In thecurrent work,TiO2/Al2O3 binary oxidephotocatalysts were synthesized via two different sol–gel protocols (P1 and P2)where various TiO2 to Al2O3 moleratios(0.5 and 1.0)and calcinationtemperatures(150–1000◦C)wereutilizedinthe syn-thesisprocedures.Structuralcharacterizationofthesynthesized binaryoxidephotocatalysts wasalsoperformedviaBETsurface areaanalysis,X-raydiffraction (XRD)and Ramanspectroscopy; and the photocatalytic NO(g) oxidation performances of these binaryoxides were measuredunderUVA irradiation in a com-parative fashion to that of Degussa P25 industrial benchmark. TiO2/Al2O3binaryoxidephotocatalystsweredesignedto demon-strate a novel approach which is essentially a fusion of NSR (NOxStorageReduction)andPCO(PhotocatalyticOxidation) tech-nologies. In this approach, rather than attempting to perform completeNOxreduction,NO(g)isoxidizedonaphotocatalyst sur-faceandstoredinthesolidstateintheformofnitrates/nitrites (ortheirprotonatedsurfacederivatives)onastoragecomponent. Currentresultssuggestthataluminadomainscanbeutilizedas active NOx capturing sites that can signiﬁcantly eliminate the release of toxic NO2(g)into theatmosphere by adsorptionand solid statestorage.Using eitherP1or P2protocols, structurally different Ti/Al binary oxide systems can be synthesized which enable much superiorphotocatalytic total NOx removal(i.e.up to 176% higher) than Degussa P25 industrial benchmark. Fur-thermore, suchTi/Al binary oxides can also decrease thetoxic NO2(g) emission into theatmosphere (which is formed due to the photocatalytic oxidation of NO(g)) by 75% with respect to that of Degussa P25. It is apparent that there is a complex interplaybetweenthecalcinationtemperature,crystalstructure, composition and SSA which dictate theultimatephotocatalytic activityinacoordinativemanner.Itwasobservedthattwo struc-turallydifferentphotocatalystspreparedviadifferentpreparation protocols may reveal comparably high photocatalytic activities implying that thephotocatalyticallyactive sites responsiblefor thephotocatalyticNO(g)oxidationandstoragehaveanon-trivial nature.

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Acknowledgements

Theauthorsacknowledgethefinancialsupportfromthe Scien-tificandTechnicalResearchCouncilofTurkey(TUBITAK)(Project Code:109M713). E.O.alsoacknowledgesfinancialsupportfrom TurkishAcademyofSciencesthroughthe“TUBA-GEBIP Outstand-ingYoung ScientistPrize”and fromFevziAkkayaScienceFund (FABED)throughEserTümenScientificAchievementAward. AppendixA. Supplementarydata

Supplementary data associated with this article can be found,intheonlineversion,athttp://dx.doi.org/10.1016/j.cattod. 2014.04.001.

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