TiO2-Al2O3 binary mixed oxide surfaces for photocatalytic NOx abatement

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ContentslistsavailableatScienceDirect

Applied

Surface

Science

j o u r n a l ho me p ag e :w w w . e l s e v i e r . c o m / l o c a t e / a p s u s c

TiO

2

–Al

2

O

3

binary

mixed

oxide

surfaces

for

photocatalytic

NO

x

abatement

Asli

Melike

Soylu

a

_,

_Meryem

_Polat

a

_,

_Deniz

_Altunoz

_Erdogan

a

_,

_Zafer

_Say

a

_,

Cansu

Yıldırım

b

_,

_Özgür

_Birer

b,c

_,

_Emrah

_Ozensoy

a,∗

a_Department_of_Chemistry,_Bilkent_University,₀₆₈₀₀_Ankara,_Turkey

b_KUYTAM_Surface_Science_and_Technology_Center,_Koc¸_University,₃₄₄₅₀_Istanbul,_Turkey c_Department_of_Chemistry,_Koc¸_University,₃₄₄₅₀_Istanbul,_Turkey

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received15November2013

Receivedinrevisedform10February2014 Accepted12February2014

Availableonline22February2014 Keywords: TiO2 Al2O3 Photocatalysis NOxabatement DeNOx

a

b

s

t

r

a

c

t

TiO2–Al2O3binaryoxidesurfaceswereutilizedinordertodevelopanalternativephotocatalyticNOx

abatementapproach,whereTiO2siteswereusedforambientphotocatalyticoxidationofNOwithO2and

aluminasiteswereexploitedforNOxstorage.Chemical,crystallographicandelectronicstructureofthe

TiO2–Al2O3binaryoxidesurfaceswerecharacterized(viaBETsurfaceareameasurements,XRD,Raman

spectroscopyandDR-UV-VisSpectroscopy)asafunctionoftheTiO2loadinginthemixtureaswellasthe

calcinationtemperatureusedinthesynthesisprotocol.0.5Ti/Al-900photocatalystshowedremarkable photocatalyticNOxoxidationandstorageperformance,whichwasfoundtobemuchsuperiortothatof

aDegussaP25industrialbenchmarkphotocatalyst(i.e.160%higherNOxstorageand55%lowerNO2(g)

releasetotheatmosphere).OurresultsindicatethattheonsetofthephotocatalyticNOxabatement

activ-ityisconcomitanttotheswitchbetweenamorphoustoacrystallinephasewithanelectronicbandgap within3.05–3.10eV;wherethemostactivephotocatalystrevealedpredominantlyrutilephasetogether andanataseastheminorityphase.

1. Introduction

Indoorand outdoorair pollutants suchasNOx,SOx,volatile

organic compounds (VOCs) and particulate matter (PM) result insigniﬁcantly adverse effects onhumanhealth. Further nega-tiveimplicationsofair pollutioncanalsobeobservedonwater resources,agricultureandbiologicalhabitat[1–6].Amongthese airbornetoxicspecies,particularlynitrogenoxides(NOx)presenta

majorchallengeforairpuriﬁcation.NOxspecies(i.e.mostlyNO(g),

NO2(g)andN2O(g))aregeneratedduringthefossilfuel

combus-tionprocessesviathehomogenousreactionofnitrogenandoxygen gasesathightemperatureswherethemajorcontributioncomes fromNO(g).NOxabatementcanbeperformedinaveryefﬁcient

mannerusingthermalcatalytictechnologiessuchasselective cat-alyticreduction(SCR)[7–9]andNOxstorageandreduction(NSR)

(whichis alsocalled LeanNOx Traps, LNT)[10–12] atelevated

temperatures(i.e.T>300◦C).Inthesethermallyactivatedcatalytic DeNOx technologiesalthough SCRapproach requires utilization

∗ Correspondingauthor.Tel.:+903122902121;fax:+903122664068. E-mailaddress:ozensoy@fen.bilkent.edu.tr(E.Ozensoy).

URL:http://www.fen.bilkent.edu.tr/ozensoy(E.Ozensoy).

ofureaas anexternal reducing agent,NSR/LNTtechnologycan be used in the absenceof an additional reducing agent. How-ever,animportantchallengeinairpuriﬁcationistheabatement of gaseous NOx species under ambientconditions (i.e. atroom

temperatureandunderregularatmosphericconditions). Photocat-alyticsystemsofferpromisingopportunitiesinordertotacklethis importantenvironmentalchallenge,asthesesystemscanbe tail-oredtoefﬁcientlyclean/purifyairunderambientconditionswith thehelpofultraviolet(UV)and/orvisible(VIS)light[13].Among thesesystemsTiO2-basedmaterialsarethemosteffective

photo-catalystsforair/waterpuriﬁcationapplications[14,15].Howeverit hasbeenreportedthatcompletephotocatalyticreductionoftoxic NOxspeciesintoharmlessN2occursonlywitharelativelylimited

performanceforthesesystems[13].

Inthecurrentwork,ratherthanattemptingtoperformcomplete photocatalytic reduction of NOx,an alternative NOx abatement

strategy hasbeen demonstrated, which includes photocatalytic oxidationofNOxonaTiO2/Al2O3binaryoxidephotocatalyst

sur-faceanditsstorageinthesolidstateintheformofnitratesand nitrites.Thisalternativestrategywasinspiredbyourrecent stud-ies onNSR technology which is used for thethermal catalytic aftertreatmentofautomotiveNOxemissions[12,16–21].In

cou-pleof theseformer studies,we spectroscopicallydemonstrated

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that[12,16–21]ontheTiO2/Al2O3binaryoxidesurface,oxidized

NOx species suchasNO2(g)canreadilyundergoa thermal

dis-proportionationreactionforming adsorbednitrites and nitrates allowingeffectivesolidstateNOxstorage.HoweverNO(g)hasa

limitedadsorptionenergyonmanymetaloxidesurfacescompared tothatofNO2,hinderingthestorageofNOinthesolid(adsorbed)

state.Thus,forsolidstateNOxstorage,NOshouldbeﬁrstoxidized

toNO2 andthensubsequentlystoredontheavailableadsorption

sitesofthecatalystsurfaceintheformofnitrites/nitrates.Although thiscan bedonereadilyatelevatedtemperaturesusinga plat-inumgroupmetal(PGM)promotedmetaloxidecatalystsuchas Pt/Al2O3,itcannotbeefﬁcientlyachievedunderambient

condi-tions(i.e.atroomtemperature)duetokineticlimitations.However thislimitationcanbeovercomebydesigningacatalytic system includinga photocatalyticNO(g)oxidation componentwhich is coupledtoa NOx storage component. Along theselines, inthe

currentwork,weshowthatTiO2/Al2O3binaryoxidesurfacescan

beexploitedtoperformphotocatalyticNOxoxidationandstorage,

whereTiO2surfacedomainsprovideNOoxidationcapabilityunder

ambientconditions,convertingNO(g)+O2(g)intonitrites/nitrates

whilethehigh-surfaceareaAl2O3componentenablesboththe

dis-persionofthephotocatalyticTiO2domainsaswellasthecreation

ofadditionalstoragesitesforoxidizedNOx.Oncesaturatedwith

NOx,suchaphotocatalyticNOxoxidationandstoragecatalystcan

readilyberegeneratedbytreatmentwithwater,whichcandissolve theadsorbednitrites/nitratesandrestoretheNOxadsorptionsites

[22].

In order to demonstrate this alternative strategy, in the current study, a set of TiO2/Al2O3 binary oxide

photocata-lysts were synthesized and characterized. A sol–gel synthesis method was used to co-precipitate titania with alumina. The inﬂuences of the surface structure on the photocatalytic NO oxidation and storage was investigated by modifying the sur-face structure via calcination. Photocatalytic performances of thisnewfamilyofTiO2/Al2O3 binaryoxidephotocatalystswere

alsocomparedwitha commerciallyavailablephotocatalyst(i.e. DegussaP25)inordertodeterminetherelative performanceof theTiO2/Al2O3 system against a widelyused industrial

bench-mark.

2. Experimental

2.1. Samplepreparation

Titanium (IV) isopropoxide (TIP, 97%, Sigma–Aldrich) and aluminum-tri-sec-butoxide(ASB,97%,Sigma–Aldrich)wereused as the main ingredients in the preparation of the TiO2/Al2O3

binaryoxides via sol–gelmethod [16,18].Three series of sam-ples were prepared by varying therelative molar composition of the TiO2 component in the TiO2/Al2O3 binary oxide. These

samples are labeledas “xTi/Al-y”, where x represents theTiO2

to Al2O3 mole ratio (i.e. 0.25, 0.5 and 1.0) and y represents

the calcination temperature (150–1000◦C) of the sample. In thesynthesis,dependingonthecorrespondingTiO2–Al2O3mole

ratio, an appropriate amount of ASB was mixed with propan-2-ol(99.5%,Sigma–Aldrich)andacetylacetone(99.3%,Fluka)for 30min. Subsequently, TIP was added in a drop wise fashion to the mixture over the course of another 30min. All of the synthesis steps were carried out at room temperature under vigorous stirring. The co-precipitation of the obtained hydrox-ides was accomplished after the gradual addition of 0.5M HNO3(aq) to the solution which led to the formation of a

gel. The resulting yellow gel was aged under ambient condi-tions for 2 days and the dried sample was ground to form a ﬁne powder. Next, synthesized TiO2/Al2O3 binary oxides were

calcinedinairfor2hatvarioustemperaturesrangingfrom150 to1000◦C.

2.2. Structuralcharacterizationmeasurements

Determinationofthecrystalstructureofthesynthesized mate-rialswerecarriedoutwithaRigakuMiniﬂexX-raydiffractometer (XRD)equippedwithCuK␣radiationoperatedat30kV,15mA, and 1.54 ˚A (wavelengthof copper X-raysource). The XRD pat-ternswererecordedinthe2rangeof10–60◦withastepwidth of0.02s−1.Ramanspectraofthesampleswerecollectedin the rangeof200–1500cm−1witharesolutionof4cm−1usingaHoriba JobinYvonLabRAMHR800spectrometerequippedwithaconfocal RamanBX41microscope.TheRamanspectrometerwasequipped witha Nd:YAGlaser (=532.1nm)where thelaser powerwas 20mW. Thespeciﬁc surfacearea(SSA) valuesof theTiO2

sam-plesweredeterminedbyconventionalBrunauer–Emmett–Teller (BET)N2 adsorptionmethodusinga MicromeriticsTristar 3000

surface areaand pore sizeanalyzer. Priorto theBET measure-ments,all ofthe sampleswereoutgassed in vacuumfor 2hat 150◦C.DiffuseReﬂectanceUV–vis(DR-UV–vis)spectrawere uti-lizedinordertoobtainelectronicbandgapvalues.Thesespectra wererecordedwithaShimadzuUV-3600UV-Vis-NIR spectropho-tometerusingtheISR-3100integratingsphereattachmentinthe specularreﬂection(8◦)mode.Bariumsulfate(BaSO4)wasusedas

thereferencematerialintheDR-UV–vismeasurements.Obtained DR-UV–visspectrawereﬁnallycorrectedusingtheKubelka-Munk transformation.

2.3. Photocatalyticactivitymeasurements

The custom-designed photocatalytic ﬂow reactor system (Scheme1)wasusedtomeasure thephotocatalyticNOx

oxida-tionandstorageperformancesofTiO2/Al2O3binaryoxidesunder

UVA excitation. The photocatalytic ﬂow reactor system mainly consistedofa gasmanifoldsystem,a samplecompartmentand a chemiluminiscenceNOx analyzer(Horiba APNA-370).The gas

manifoldsystemwasconnectedtogascylinderscontainingN2(g)

(99.998%,LindeGmbH),O2(g)(99.998%,LindeGmbH)and100ppm

NO diluted in N2 (Linde GmbH). Mass ﬂow controllers (MFCs,

MKS 1479A)wereused tocontrolthe volumetric flowrates of gasesandacapacitancepressuregauge(MKSBaratron)wasused tomeasure total pressure of theflowing gaswhich wasset to 1atm. The following flow rates were used to prepare the gas mixture,0.750SLM(standardlitersperminute)forN2(g),0.250

SLM for O2(g),and 0.010SLM for NO(g) witha total gas ﬂow

rate of 1.010 SLM. Prior to mixing, N2(g) and O2(g) were also

bubbledthroughahumidifier.Therelativehumidityofthetotal gas mixture was 70% RH which was measured with a Hanna HI 9565 humidity analyzerat the sample position in the pho-tocatalyticflow reactor.Thisgasmixturerepresentsasynthetic polluted air sample. Before the performance tests, synthesized powdersampleswereplacedona2mm×40mm×40mm poly-methyl methacrylate (PMMA) sample holder and subsequently irradiated with UVA (350nm) light bulbs (F8W/T5/BL350, Syl-vania/Germany) under ambientconditions for 18houtside the flow reactor in order to remove the surface contaminations and toactivatethephotocatalysts. For each measurement, typ-ically a 950mg activatedphotocatalyst samplewasplaced into the flow reactor. The photocatalytic flow reactor was illumi-nated with8W UVA lamps (F8W/T5/BL350, Sylvania/Germany) whoseemissionwavelengthwas350nm.ConcentrationsofNO(g), NO2(g)andtotalNOx(g)speciesinthephotocatalyticreactorwere

quantitativelymeasuredonlinewiththechemiluminiscenceNOx

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Scheme1. Descriptionofthecustom-designedphotocatalyticﬂowreactorsystem.

Gasphasephotocatalyticactivitymeasurementsarereportedin termsofpercentphotonicefﬁciencies(%)asdescribedinEqs.(1) and(2).

%= nNOx

nphoton×

100 (1)

wherenNOx correspondstoeitherthedecreaseinthetotal

num-berofmolesofallgaseousNOxspeciesorthenumberofmoles

ofNO2(g)generatedina 60min(i.e.3600s)photocatalytic

per-formancetest.Ontheotherhand,nphotoncorrespondstothetotal

numberofincidentUVAphotonsimpingingonthecatalystsurface in3600s,whichcanbecalculatedthroughEq.(2)as:

nphoton=ISt

Nhc (2)

whereIrepresentsthephotonpowerdensity oftheUVAlamp, experimentally measuredat the sample positionin the photo-catalytic reactor (typically, 7.5Wm−2), is the representative emissionwavelengthoftheUVAlamp(i.e.350nm),Sisthe sur-faceareaofthephotocatalystsampleholderinthereactorthatis exposedtotheUVAirradiation(i.e.40mm×40mm=1600mm2_);

tisthedurationoftheperformancetest(i.e.3600s),Nisthe Avo-gadro’snumber,hisPlanck’sconstantandcisthespeedoflight.

3. Resultsanddiscussion

3.1. Speciﬁcsurfaceareameasurements

Thermal evolution and the structural variations of the TiO2/Al2O3binaryoxidesampleswithvaryingmolarcompositions

wereinvestigatedaftercalcinationstepsatdifferenttemperatures (Fig.1).Fig.1revealsthatTiO2/Al2O3 samplespossesseda

rela-tivelyhighsurfaceareaafterpreparationandcalcinationatlow temperatures(e.g.≥420m2_/g)._These_high_SSA_values_were

pre-servedtoalargeextentupto600◦C.Thisobservationisinverygood accordancewiththecurrentXRDandRamanresults(Figs.2and3) suggestingapredominantlyamorphousstructureforallTiO2/Al2O3

binaryoxidesamplesbelow600◦C.Athighertemperatures,a dras-ticand a monotonic decreasein theSSA values wereobserved in line withthe enhanced crystallinity and structural ordering ofthesamplesatelevatedtemperatureswhicharealsoevident in the current XRD and Ramanmeasurements (Figs. 2 and 3). Itisworthmentioningthatuponcalcinationat900◦C, SSA val-uesfor0.25Ti/Al-900,0.5Ti/Al-900,1.0Ti/Al-900samplesdecreased to108,64and25m2_/g,_{respectively.}_These_particular_values_are

ratherclosetotheSSAofthecommercialDegussaP25catalyst(i.e. 55m2_/g)_which_is_used_as_the_benchmark_{photocatalyst}_in_the

cur-rentstudy.Athighercalcinationtemperaturessuchas1000◦C,SSA valuesforalloftheTiO2/Al2O3 binaryoxidesamplesdrastically

decreasetoc.a.9–17m2_/g_which_is_in_perfect_agreement_with_the

increasedcrystallinityandtheformationofthelowsurfacearea phasessuchasrutileand␣-Al2O3(corundum)observedintheXRD

andRamanexperiments(Figs.2and3). 3.2. XRDandRamanspectroscopyexperiments

Fig.2presentsXRDproﬁlesobtainedfortheTiO2/Al2O3samples

withdifferentmolarcompositionsthatwerecalcinedatvarious temperatureswithin150–1000◦C.Itisapparentthatforall sam-ples,calcinationattemperatureslessthanorequalto600◦Cyields amorphous structures. Calcination at 800◦C resultsin the ﬁrst discernibleindicationsofcrystallinity,where␥-Al2O3(JCPDS

29-0063)phasestartstobevisiblefor0.25Ti/Aland0.5Ti/Alsamples. Forthe1.0Ti/Alsample,inadditiontothe_␥-Al2O3phase,

forma-tionofanatase(JCPDS21-1272)andrutile(JCPDS04-0551)phases ofTiO2alsobecomesvisible.ItisclearthatwithincreasingTiO2to

Al2O3moleratiointhephotocatalystcomposition,crystallinityof

300 400 500 600 700 800 900 1000 1100 0 100 200 300 400 500 600 a er A ec afr u S cifi c e p S (m 2 /g ) Temperature (o_C) 0.25 Ti/Al 0.5 Ti/Al 1.0 Ti/Al 470487 285 256 108 17 424 393 131 64 9 390 86 25 ₉

Fig.1. SpeciﬁcsurfaceareavaluesfortheTiO2/Al2O3binaryoxidesampleswith

differentmolarcompositionsthatwerecalcinedatvarioustemperatureswithin 150–1000◦_C_in_air.

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Fig.2. XRDpatternsfortheTiO2/Al2O3binaryoxidesampleswithdifferentmolarcompositionsthatwerecalcinedatvarioustemperatureswithin150–1000◦Cinair.

theobservedphasesincreases.Thisisinlinewiththefactthatpure (bulk)TiO2hasmuchlowerphasetransitiontemperaturesbetween

amorphous,anataseandrutilephasesthantheTiO2 domainson

theTiO2/Al2O3 surface[16,18].Thus atlowTiO2 toAl2O3 mole

ratios,thereexistsastronginteractionbetweentheTiO2

minor-itydomainsandtheAl2O3majoritydomains,whichisdecreasing

thesurfacemobilityoftheTiO2domainsandhinderingthe

nuclea-tionandgrowthofanataseandrutilephasesatlowtemperatures. HoweverathigherTiO2toAl2O3moleratios,interactionbetween

theTiO2andAl2O3domainsweakenstoacertainextentasTiO2

convergestoa morebulk-like conﬁguration,pushingthephase transitiontemperaturestolower(bulk-like)values.

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Uponcalcinationat900◦C,although␥-Al2O3seemstobethe

onlydiscerniblecrystallinephaseonthe0.25Ti/Alsurface(where TiO2isstillinamorphousstate),anataseandrutilephasesbecome

clearlyvisibleonthe0.5 Ti/Aland1.0 Ti/Alsurfaceswherethe crystallinityofthelatterissigniﬁcantlygreater.Thisisinperfect agreementwiththeSSAvalues presentedinFig.1,suggestinga muchlowerSSAfor the1.0 Ti/Al-900samplecompared to0.25 Ti/Al-900and0.5Ti/Al-900samples.Itisalsoworthmentioning thatalthough_␣-Al2O3(JCPDS10-0173)phaseisnotsigniﬁcantly

visibleat900◦CforlowerTiO2toAl2O3moleratios;thisphaseis

noticeablydiscernibleforthe1.0Ti/Al-900sample.Furthermore, ␣-Al2O3(corundum)phasestartstoappearduringtheanataseto

rutilephasetransition.Asdiscussedinoneofourformerreports

[16],this canbeexplained bytheformationof asolid solution betweenanataseand alumina.In this solid solution,when the anatasephase isconvertedintorutileatelevatedtemperatures, aphasesegregationoccurswhichtriggersaphasetransitioninthe aluminacomponentfrom␥to␣-phase.Finally,aftercalcinationat 1000◦C,allsamplesseemtobehighlyordered,wherecorundum andrutilearetheonlyvisiblecrystallinephases,inverygood har-monywiththedrasticSSAdecreasesobservedforthesesamplesin

Fig.1.

Raman spectra of the synthesized TiO2/Al2O3 binary oxide

sampleswithdifferentmolarcompositionsthatwerecalcinedat varioustemperatureswithin150–1000◦CaregiveninFig.3.These Ramanspectralfeaturescanbereadilyexplainedinthelightofthe XRDresultsgiveninFig.2,aswellastheformerRaman spectro-scopicstudiesintheliterature[16,18,23,24].Itisknownthatthe RamanspectrumofanatasephaseshowssixRamanfeatures(1A1g,

2B1g,and3Eg)at144(Eg),197(Eg),399(B1g),516(A1g+B1g),639

(Eg)and796cm−1 (Eg)[23].Ontheotherhand,therutilephase

canbecharacterizedbyaRamanspectrumwithfourmajorRaman activefeatures(A1g+B1g+B2g+Eg)at143(B1g),447(Eg),612(A1g),

826cm−1(B2g)andalsoatwo-phononscatteringbandat236cm−1

[24].InverygoodagreementwiththeXRDresultsgiveninFig.2,up to600◦C,allsamplesrevealanamorphousstructurewithnosharp Ramanfeatures.Itisworthmentioningthatsample1.0Ti/Al-600 revealsverybroadandconvolutedRamansignalscorresponding tosmallandpoorlycrystallineanataseandrutiledomainswhich seemtobeelusivetodetectinXRD(Fig.2c).Atcalcination temper-atureshigherthan600◦C,anatasephaseappearsasthedominant phasetogetherwithaminorcontributionfromrutile.With increas-ingtemperature,anatasetorutileratiointhesamplesdecreases whereat900◦Crutilebecomesthepredominantphasedetectedin theRamanspectra.Forthe0.5Ti/Al-900sample,anatasephaseis stillvisibleintheRamanspectra(Fig.3b),althoughrutileis deﬁ-nitelythemajorityphase.InperfectharmonywiththeXRDresults (Fig.2),RamanspectrainFig.3alsosuggestthatincreasingTiO2

toAl2O3moleratioenhancesthecrystallinityofthephasesonthe

TiO2/Al2O3binaryoxidesurfaceswhichisevidentbythesharper

andstrongerRamanscatteringfeatures. 3.3. Photocatalyticperformanceexperiments

Fig.4shows atypicalconcentrationversustime plotthat is obtainedduringaphotocatalyticperformancetest.InFig.4,the totalNOxconcentration(i.e.sumoftheconcentrationsofallofthe

NOxspeciesexistinginthereactor,i.e.bluecurve)aswellas

sep-arateNO(g)(blackcurve)andNO2(g)(redcurve)concentrations

inthephotocatalyticreactormeasuredbythechemiluminiscence NOxanalyzerarepresented.Duringtheinitialc.a.20minofthe

analysis,asyntheticpollutedairgasmixturecomprisedofN2(g),

O2(g),H2O(g) aswell as 1ppm NO(g) is fed to the

photocata-lystsurfaceunderdarkconditionswheretheUVAlampisoffand anybackgroundexposuretosunlightisprevented. Underthese conditions(i.e.intheﬁrst15min), aminortransientfallinthe

0 20 40 60 80 0.0 0.2 0.4 0.6 0.8 1.0

0.5 Ti/Al-900

Concentration (ppm) Time(min) Light-on Light-oﬀ Thermal Adsorpon NOx(g) NO(g) NO2 (g)

Fig.4.Concentrationversustimeplotforthephotocatalyticperformancetestofthe 0.5Ti/Al-900sample.Blue,blackandredcurvescorrespondtotheconcentrationsof totalNOx(g),NO(g)andNO2(g),respectively(seetextfordetails).(Forinterpretation

ofthereferencestocolorinthisﬁgurelegend,thereaderisreferredtotheweb versionofthisarticle.)

totalNOx(g)andNO(g)concentrationswasobserved.Thiscanbe

attributedtothedilutionofthegasinthereactorpipelineandthe thermaladsorptionofNOxspeciesonthegaslines,reactorwallsas

wellasadsorptiononthephotocatalystsurface.Sincethereactor iskeptincompletedarknessundertheseconditions,no photocat-alyticactivityisobservedduringthisinitialstageevidentbythe presenceofaminoramountofNO2(g)productionduetothermal

catalytic disproportionation processesoccurring onthecatalyst surface.Followingthisinitialtransientperiod,reactorwallsandthe photocatalystsurfacearesaturatedwithNOx,afterwhichNOx(g)

andNO(g)tracesquicklyreturntotheoriginalinletconcentration valueofc.a.1ppm,signifyingtheendofthermalcatalyticactivity. Afterthispreliminarytransientperiod,UVAexcitationsource isturnedonandthephotocatalyticreactionisstarted.UponUVA illumination,adrasticandapermanentfallintheNO(g)andtotal NOx(g)concentrationsconcomitanttoaquickandsigniﬁcantjump

in theNO2(g)level, were observed.This behavior suggeststhe

conversionofNO(g)intoNO2(g)viaphotocatalyticoxidation.

Fur-thermore,producedNO2(g)canadsorbonthephotocatalystsurface

intheformofchemisorbedNO2,nitritesandnitrates[16,18]and

storedin thesolidstate,resultingin afurtherfallintheNO(g) and total NOx signals. It is worth mentioning that, fall in the

NO(g)concentrationmightalsohavesomecontributionfromthe directphotocatalyticdecompositionandphoto-reductionofNO(g) formingN2(g)and/orN2O(g)[25].However,sincethedirect

photo-catalyticreductionisknowntobearelativelyinefﬁcientpathway, thisreactionchannelmaybeexpectedtobeaminor photochem-icalroute.Consequently,thetotalNOxconcentration(blue)curve

(whichismostlycomprisedofthesumofNO(g)andNO2(g)signals)

inFig.4remains mostlybelow1ppmduringtheUVA-activated regime,illustratingthecontinuousphotocatalyticactivityandNOx

storageinthesolidstate.

Photochemical NO oxidation and storage performance tests wereperformedforallofthesynthesizedsamplesandthe sum-maryoftheseperformancetestswerepresentedintermsofpercent photonicefﬁcienciesinFig.5,alongwiththecorrespondingdatafor theDegussaP25industrialbenchmark.Inthehistogramgivenin

Fig.5,blueandredbarsrepresentthepercentphotonic efﬁcien-ciesfortotalNOx(g)decreaseandNO2(g)production,respectively.

Thesevalueswereobtainedbyintegratingthecorrespondingareas undertheconcentrationversustimecurvesforthedatasimilarto theonesgiveninFig.4.

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It is worth mentioning that for an ideal catalyst with an utmostphotocatalyticDeNOxperformance,bluebars(i.e.NOx(g)

storage/conversion) should be maximized; while red bars are simultaneouslyminimized(i.e.minimumslipoftoxicNO2(g)into

theatmosphere).WhenthebehavioroftheDegussaP25 indus-trialbenchmarkphotocatalystgiveninFig.5isinvestigated,itis immediatelyseenthatthisindustrialphotocatalysthasaveryhigh NO(g)photo-oxidation capabilitygeneratinga largequantityof NO2(g),whilethesamecatalyst hasa verylimited NOxstorage

capability(bluebar).ConsideringthefactthatNO2(g)isamuch

moretoxicpollutantthanNO(g),althoughDegussaP25industrial benchmarksystemisveryactiveinphoto-oxidation,thismaterial doesnotqualifytobeaveryefﬁcientphotocatalyticDeNOx

sys-temforNOxabatement.Anotherbenchmarksampleusedinthe

controlexperimentswas␥-Al2O3.Fig.5unambiguouslyindicates

that,␥-Al2O3hasneithersigniﬁcantphotocatalyticNOxstoragenor

photocatalyticNO2(g)productioncapabilities.

Ontheotherhand,whenthephotocatalyticperformancedata fortheTiO2/Al2O3 binaryoxidesamplesare examined,onecan

immediatelynotetheremarkableimprovementinthe photocat-alyticDeNOxperformancecomparedtotheDegussaP25industrial

benchmark.InFig.5,performanceresultsfortheTiO2/Al2O3binary

oxidesamplesareassembledinthreegroupsbasedonTiO2toAl2O3

moleratio(i.e.0.25,0.5and1.0)inthephotocatalyststructure.It isvisiblethatforthe0.25Ti/Alsamplescalcinedatvarious tem-peratures,catalystscalcinedbelow900◦CrevealverylowDeNOx

performance,wheretheperformancereachesanoptimumvalue between900and950◦Candstartstofallat1000◦C.

Asimilarperformancetrendisobservedfor0.5Ti/Alcatalysts calcinedatvarious temperatures(Fig.5).For thisfamily of cat-alysts,althoughnosigniﬁcantactivityisobservedatcalcination temperatureslessthan900◦C,photocatalyticDeNOxperformance

presentsaveryradicalenhancementat900◦C, revealingvalues thataremuchbetterthananyofthephotocatalystsinthe0.25Ti/Al family.Itisworthmentioningthatafurtherincreaseinthe calci-nationtemperatureto1000◦CresultsinthephotocatalyticDeNOx

performanceofthe0.5Ti/Alsystem.

Fig.5indicatesthatforthe1.0Ti/Alphotocatalystfamily,no signiﬁcantphotocatalyticactivityisdetectedupto800◦C,whileat thiscalcinationtemperaturearemarkableincrease inthe activ-ity is observed, though this catalyst is not as effective as the 05 Ti/Al-900 catalyst in total NOx abatement, due to the

signiﬁcantNO2(g)generationoftheformer.ItcanbeseeninFig.5

that for calcinationtemperaturesabove 800◦C, NOx abatement

startstofall,evidentbytheincreasedNO2(g)slipintothe

atmo-sphereaswellasdecreasingNOxstorageinthesolidstate.Thus,

ageneralanalysisoftheperformanceresultspresentedinFig.5

revealsthat,0.5Ti/Al-900binaryoxidecatalystshowsthehighest NOxabatementperformanceamongalloftheanalyzed

photocata-lysts,whereitperforms160%higherNOxstorageand55%lower

NO2(g)releasetotheatmospherecompared totheDegussaP25

industrialbenchmark.

PhotocatalyticperformanceoftheTiO2/Al2O3binaryoxide

sam-plescanbereadilyinterpretedinthelightofcurrentstructural characterization experiments (Figs. 1–3) which reveal valuable insightregardingthespeciﬁcsurfaceareasaswellasthe crystal-lographicphasesthatarepresentontheTi/Alsamples.Firstly,itis apparentinFig.5thatforthebestperformingphotocatalystfamily (i.e.0.5Ti/Al),onsetofactivityisobservedinaverydrasticmanner asthecalcinationtemperatureisincreasedfrom800◦Cto900◦C. BET,XRDandRamanmeasurementsgiveninFigs.1–3suggests thatthis thermalwindowdirectlyoverlapswiththe crystalliza-tionoftheamorphousTiO2toformamixtureofanataseandrutile

phaseswherethelatteristhedominantphase.Inotherwords,itis apparentthatinordertoachievethebestphotocatalyticNOx

abate-mentperformance,auniquecrystallographicmixtureofanatase andrutilephaseshastobeobtained.

Secondly,Fig.5alsosuggeststhatforTi/Alfamilieswithdifferent TiO2loadings,ultimateperformanceisobservedforthe

interme-diateloadingandtheperformancewasseentodecreaseforvery loworveryhighTiO2loadings.Thiscanbeexplainedbythefact

thatatlowTiO2loadings,itislikelythatTiO2loadingisnothigh

enoughtobedispersedonalloftheAl2O3 surface.Thusnot all

oftheNOxadsorption/storage(i.e.Al2O3)sitescanbeutilizeddue

tolimitedphoto-oxidationcapabilityoftheinadequatenumberof TiO2oxidationsitesonthesurface.Ontheotherhand,atveryhigh

TiO2loadings,TiO2coversmostoftheAl2O3surfaceandupon

cal-cinationabove800◦C,SSAofthecatalystsamplefallsdrastically togetherwiththeformationofcrystallineanataseandrutile mix-ture;limitingtheavailablenumberofNOxstoragesitesthatare

availableafterphoto-oxidation.

Thirdly, Fig.5 indicates that onset of photocatalytic activity is observed in a rather sharp manner at 950, 900 and 800◦C for the0.25Ti/Al, 0.5Ti/Al and 1.0Ti/Al samples,respectively. In

Fig.5.PhotocatalyticDeNOxperformanceresultsfortheTiO2/Al2O3binaryoxidesampleswithdifferentmolarcompositionsthatwerecalcinedatvarioustemperatures

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Fig.6. ElectronicbandgapvaluesderivedfromDR-UV–visspectroscopicresultsfortheTiO2/Al2O3binaryoxidesampleswithdifferentmolarcompositionsthatwere

calcinedatvarioustemperatureswithin150–1000◦_C_in_air.

otherwords,astherelativeTiO2loadingintheTiO2/Al2O3binary

oxidesamplesincreases,onsettemperatureforthephotocatalytic activityshiftstolowertemperatures.Thiscanalsobeexplained bytheonsettemperatureforthecrystallizationofTi/Alsamples (and hence the formation of photo-active TiO2 sites) observed

inXRDandRamanmeasurements(Figs.2and3)whichsuggest thatincreasingTiO2 loadingincreasesthetemperaturerequired

to switch form an amorphous TiO2 structure to a crystalline

structure.

3.4. DR-UV–vismeasurementsandelectronicbandgap

Inordertoinvestigatetherelationshipbetweentheelectronic structure and the photocatalytic NOx abatement performance,

electronicband gap values were calculated from the currently performed(notshown)DR-UV–visspectroscopicmeasurements. ThesebandgapvaluesarepresentedinFig.6.Inverygood agree-mentwiththediscussiongivenabove,electronicbandgapvalues fortherelativelyinactiveamorphousTi/Alsampleswhichare cal-cinedatlowertemperatures,revealacharacteristicallyhighvalue within3.4–3.6eV.Ontheotherhand,withtheonsetofthe pho-tocatalyticactivity,a very sharpfallin theelectronicbandgap valueswereobserved,where thebandgapdecreasestoa typi-calvalueof3.05–3.10eV,in linewiththeformationof ordered anataseandrutilephases.Typicalbandgapvaluesforbulkanatase andrutilephasesarec.a.3.2and3.0eV,respectively[26].Thus,for theactivephotocatalystsamples,thebandgapvalueisinbetween thatofanataseandrutile,beingclosertothelatter,inaccordance withthefactthatin themostactivephotocatalyst,rutileexists asthepredominantphasetogetherwithanataseastheminority phase.

Itisalsoworthnotingthatalthoughonsetofthephotocatalytic activityasafunctionofcalcinationtemperaturecanbefollowed withtheelectronicbandgapvalues,electronicbandgapcannot beusedasasoleindicatorfortheestimationofthephotocatalytic activitytrends.Thisisduetothefactthatoncethe photocatalyt-icallyactivestructureisobtainedleadingtoadrasticdecreasein theelectronicbandgap,bandgapvaluesceasetochangeathigher calcinationtemperaturesalthoughphotocatalyticactivitystartsto decline.

4. Conclusions

TiO2–Al2O3 binary oxide surfaces were utilized in order to

develop analternative photocatalyticNOx abatement approach,

where TiO2 sites were used for ambient photocatalytic

oxida-tion of NO with O2 and alumina sites wereexploited for NOx

storage.Chemical,crystallographicandelectronicstructureofthe TiO2–Al2O3 binaryoxidesurfaceswerecharacterizedasa

func-tionoftheTiO2loadinginthemixtureaswellasthecalcination

temperatureusedinthesynthesisprotocol.0.5Ti/Al-900 photocat-alystshowedremarkablephotocatalyticNOxoxidationandstorage

performancewhichwasfoundtobemuchsuperiortothatofa DegussaP25industrialbenchmarkphotocatalyst(i.e.160%higher NOx storageand 55%lower NO2(g)release totheatmosphere).

OurresultsindicatethattheonsetofthephotocatalyticforNOx

abatementactivityisconcomitanttotheswitchbetween amor-phoustoacrystallinephase withanelectronicbandgapwithin 3.05–3.10eVwherethemostactivephotocatalystrevealed pre-dominantly rutile phase together withanatase as theminority phase.

Acknowledgments

AuthorsacknowledgeZaferSayforperformingBET measure-ments. E.O. also acknowledges financial support from Turkish AcademyofSciencesthroughthe“TUBA-GEBIPOutstandingYoung Scientist Prize” and from Fevzi Akkaya Science Fund (FABED) throughEserTümenScientificAchievementAwardaswellasthe Scientific and Technical Research Council of Turkey (TUBITAK) (ProjectCode:109M713).

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