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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,∗aDepartmentofChemistry,BilkentUniversity,06800Ankara,Turkey
bKUYTAMSurfaceScienceandTechnologyCenter,Koc¸University,34450Istanbul,Turkey
cDepartmentofChemistry,Koc¸University,34450Istanbul,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).OurresultsindicatethattheonsetofthephotocatalyticNOxabatementactiv- ityisconcomitanttotheswitchbetweenamorphoustoacrystallinephasewithanelectronicbandgap within3.05–3.10eV;wherethemostactivephotocatalystrevealedpredominantlyrutilephasetogether andanataseastheminorityphase.
©2014ElsevierB.V.Allrightsreserved.
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))aregeneratedduringthefossilfuelcombus- tionprocessesviathehomogenousreactionofnitrogenandoxygen gasesathightemperatureswherethemajorcontributioncomes fromNO(g).NOxabatementcanbeperformedinaveryefficient mannerusingthermalcatalytictechnologiessuchasselectivecat- 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,asthesesystemscanbetail- oredtoefficientlyclean/purifyairunderambientconditionswith thehelpofultraviolet(UV)and/orvisible(VIS)light[13].Among thesesystemsTiO2-basedmaterialsarethemosteffectivephoto- 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/Al2O3binaryoxidephotocatalystsur- faceanditsstorageinthesolidstateintheformofnitratesand nitrites.Thisalternativestrategywasinspiredbyourrecentstud- ies onNSR technology which is used for thethermal catalytic aftertreatmentofautomotiveNOxemissions[12,16–21].Incou- pleof theseformer studies,we spectroscopicallydemonstrated http://dx.doi.org/10.1016/j.apsusc.2014.02.065
0169-4332/©2014ElsevierB.V.Allrightsreserved.
that[12,16–21]ontheTiO2/Al2O3binaryoxidesurface,oxidized NOx species suchasNO2(g)canreadilyundergoa thermaldis- 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,itcannotbeefficientlyachievedunderambientcondi- 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-surfaceareaAl2O3componentenablesboththedis- 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 obtainedhydrox- 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
Determinationofthecrystalstructureofthesynthesizedmate- 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)spectrawereuti- lizedinordertoobtainelectronicbandgapvalues.Thesespectra wererecordedwithaShimadzuUV-3600UV-Vis-NIRspectropho- 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 thepho- tocatalyticflow reactor.Thisgasmixturerepresentsasynthetic polluted air sample. Before the performance tests, synthesized powdersampleswereplacedona2mm×40mm×40mmpoly- 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
analyzer.
Scheme1. Descriptionofthecustom-designedphotocatalyticflowreactorsystem.
Gasphasephotocatalyticactivitymeasurementsarereportedin termsofpercentphotonicefficiencies(%)asdescribedinEqs.(1) and(2).
%= nNOx
nphoton×100 (1)
wherenNOx correspondstoeitherthedecreaseinthetotalnum- berofmolesofallgaseousNOxspeciesorthenumberofmoles ofNO2(g)generatedina 60min(i.e.3600s)photocatalyticper- formancetest.Ontheotherhand,nphotoncorrespondstothetotal numberofincidentUVAphotonsimpingingonthecatalystsurface in3600s,whichcanbecalculatedthroughEq.(2)as:
nphoton=ISt
Nhc (2)
whereIrepresentsthephotonpowerdensity oftheUVAlamp, experimentally measuredat the sample positionin thephoto- catalytic reactor (typically, 7.5Wm−2), is the representative emissionwavelengthoftheUVAlamp(i.e.350nm),Sisthesur- faceareaofthephotocatalystsampleholderinthereactorthatis exposedtotheUVAirradiation(i.e.40mm×40mm=1600mm2);
tisthedurationoftheperformancetest(i.e.3600s),NistheAvo- 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 samplespossessedarela- tivelyhighsurfaceareaafterpreparationandcalcinationatlow temperatures(e.g.≥420m2/g).ThesehighSSAvalueswerepre- servedtoalargeextentupto600◦C.Thisobservationisinverygood accordancewiththecurrentXRDandRamanresults(Figs.2and3) suggestingapredominantlyamorphousstructureforallTiO2/Al2O3 binaryoxidesamplesbelow600◦C.Athighertemperatures,adras- 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, SSAval- uesfor0.25Ti/Al-900,0.5Ti/Al-900,1.0Ti/Al-900samplesdecreased to108,64and25m2/g,respectively.Theseparticularvaluesare
ratherclosetotheSSAofthecommercialDegussaP25catalyst(i.e.
55m2/g)whichisusedasthebenchmarkphotocatalystinthecur- rentstudy.Athighercalcinationtemperaturessuchas1000◦C,SSA valuesforalloftheTiO2/Al2O3 binaryoxidesamplesdrastically decreasetoc.a.9–17m2/gwhichisinperfectagreementwiththe increasedcrystallinityandtheformationofthelowsurfacearea phasessuchasrutileand␣-Al2O3(corundum)observedintheXRD andRamanexperiments(Figs.2and3).
3.2. XRDandRamanspectroscopyexperiments
Fig.2presentsXRDprofilesobtainedfortheTiO2/Al2O3samples withdifferentmolarcompositionsthatwerecalcinedatvarious temperatureswithin150–1000◦C.Itisapparentthatforallsam- ples,calcinationattemperatureslessthanorequalto600◦Cyields amorphous structures. Calcination at 800◦C resultsin the first discernibleindicationsofcrystallinity,where␥-Al2O3(JCPDS29- 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
aerAecafruScificepS (m2 /g)
Temperature (oC)
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◦Cinair.
Fig.2. XRDpatternsfortheTiO2/Al2O3binaryoxidesampleswithdifferentmolarcompositionsthatwerecalcinedatvarioustemperatureswithin150–1000◦Cinair.
theobservedphasesincreases.Thisisinlinewiththefactthatpure (bulk)TiO2hasmuchlowerphasetransitiontemperaturesbetween amorphous,anataseandrutilephasesthantheTiO2 domainson theTiO2/Al2O3 surface[16,18].Thus atlowTiO2 toAl2O3 mole ratios,thereexistsastronginteractionbetweentheTiO2minor- itydomainsandtheAl2O3majoritydomains,whichisdecreasing
thesurfacemobilityoftheTiO2domainsandhinderingthenuclea- tionandgrowthofanataseandrutilephasesatlowtemperatures.
HoweverathigherTiO2toAl2O3moleratios,interactionbetween theTiO2andAl2O3domainsweakenstoacertainextentasTiO2 convergestoa morebulk-like configuration,pushingthephase transitiontemperaturestolower(bulk-like)values.
Fig.3. RamanspectrafortheTiO2/Al2O3binaryoxidesampleswithdifferentmolarcompositionsthatwerecalcinedatvarioustemperatureswithin150–1000◦Cinair.
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,inverygoodhar- monywiththedrasticSSAdecreasesobservedforthesesamplesin Fig.1.
Raman spectra of the synthesized TiO2/Al2O3 binary oxide sampleswithdifferentmolarcompositionsthatwerecalcinedat varioustemperatureswithin150–1000◦CaregiveninFig.3.These Ramanspectralfeaturescanbereadilyexplainedinthelightofthe XRDresultsgiveninFig.2,aswellastheformerRamanspectro- 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).Atcalcinationtemper- atureshigherthan600◦C,anatasephaseappearsasthedominant phasetogetherwithaminorcontributionfromrutile.Withincreas- ingtemperature,anatasetorutileratiointhesamplesdecreases whereat900◦Crutilebecomesthepredominantphasedetectedin theRamanspectra.Forthe0.5Ti/Al-900sample,anatasephaseis stillvisibleintheRamanspectra(Fig.3b),althoughrutileisdefi- 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)aswellassep- 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,nophotocat- 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,sincethedirectphoto- catalyticreductionisknowntobearelativelyinefficientpathway, thisreactionchannelmaybeexpectedtobeaminorphotochem- icalroute.Consequently,thetotalNOxconcentration(blue)curve (whichismostlycomprisedofthesumofNO(g)andNO2(g)signals) inFig.4remains mostlybelow1ppmduringtheUVA-activated regime,illustratingthecontinuousphotocatalyticactivityandNOx
storageinthesolidstate.
Photochemical NO oxidation and storage performance tests wereperformedforallofthesynthesizedsamplesandthesum- maryoftheseperformancetestswerepresentedintermsofpercent photonicefficienciesinFig.5,alongwiththecorrespondingdatafor theDegussaP25industrialbenchmark.Inthehistogramgivenin Fig.5,blueandredbarsrepresentthepercentphotonicefficien- ciesfortotalNOx(g)decreaseandNO2(g)production,respectively.
Thesevalueswereobtainedbyintegratingthecorrespondingareas undertheconcentrationversustimecurvesforthedatasimilarto theonesgiveninFig.4.
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).WhenthebehavioroftheDegussaP25indus- 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 doesnotqualifytobeaveryefficientphotocatalyticDeNOxsys- temforNOxabatement.Anotherbenchmarksampleusedinthe controlexperimentswas␥-Al2O3.Fig.5unambiguouslyindicates that,␥-Al2O3hasneithersignificantphotocatalyticNOxstoragenor photocatalyticNO2(g)productioncapabilities.
Ontheotherhand,whenthephotocatalyticperformancedata fortheTiO2/Al2O3 binaryoxidesamplesare examined,onecan immediatelynotetheremarkableimprovementinthephotocat- alyticDeNOxperformancecomparedtotheDegussaP25industrial benchmark.InFig.5,performanceresultsfortheTiO2/Al2O3binary oxidesamplesareassembledinthreegroupsbasedonTiO2toAl2O3 moleratio(i.e.0.25,0.5and1.0)inthephotocatalyststructure.It isvisiblethatforthe0.25Ti/Alsamplescalcinedatvarioustem- peratures,catalystscalcinedbelow900◦CrevealverylowDeNOx
performance,wheretheperformancereachesanoptimumvalue between900and950◦Candstartstofallat1000◦C.
Asimilarperformancetrendisobservedfor0.5Ti/Alcatalysts calcinedatvarious temperatures(Fig.5).For thisfamily ofcat- alysts,althoughnosignificantactivityisobservedatcalcination temperatureslessthan900◦C,photocatalyticDeNOxperformance presentsaveryradicalenhancementat900◦C, revealingvalues thataremuchbetterthananyofthephotocatalystsinthe0.25Ti/Al family.Itisworthmentioningthatafurtherincreaseinthecalci- nationtemperatureto1000◦CresultsinthephotocatalyticDeNOx
performanceofthe0.5Ti/Alsystem.
Fig.5indicatesthatforthe1.0Ti/Alphotocatalystfamily,no significantphotocatalyticactivityisdetectedupto800◦C,whileat thiscalcinationtemperaturearemarkableincrease intheactiv- 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)slipintotheatmo- sphereaswellasdecreasingNOxstorageinthesolidstate.Thus, ageneralanalysisoftheperformanceresultspresentedinFig.5 revealsthat,0.5Ti/Al-900binaryoxidecatalystshowsthehighest NOxabatementperformanceamongalloftheanalyzedphotocata- lysts,whereitperforms160%higherNOxstorageand55%lower NO2(g)releasetotheatmospherecompared totheDegussaP25 industrialbenchmark.
PhotocatalyticperformanceoftheTiO2/Al2O3binaryoxidesam- plescanbereadilyinterpretedinthelightofcurrentstructural characterization experiments (Figs. 1–3) which reveal valuable insightregardingthespecificsurfaceareasaswellasthecrystal- lographicphasesthatarepresentontheTi/Alsamples.Firstly,itis apparentinFig.5thatforthebestperformingphotocatalystfamily (i.e.0.5Ti/Al),onsetofactivityisobservedinaverydrasticmanner asthecalcinationtemperatureisincreasedfrom800◦Cto900◦C.
BET,XRDandRamanmeasurementsgiveninFigs.1–3suggests thatthis thermalwindowdirectlyoverlapswiththecrystalliza- tionoftheamorphousTiO2toformamixtureofanataseandrutile phaseswherethelatteristhedominantphase.Inotherwords,itis apparentthatinordertoachievethebestphotocatalyticNOxabate- mentperformance,auniquecrystallographicmixtureofanatase andrutilephaseshastobeobtained.
Secondly,Fig.5alsosuggeststhatforTi/Alfamilieswithdifferent TiO2loadings,ultimateperformanceisobservedfortheinterme- diateloadingandtheperformancewasseentodecreaseforvery loworveryhighTiO2loadings.Thiscanbeexplainedbythefact thatatlowTiO2loadings,itislikelythatTiO2loadingisnothigh enoughtobedispersedonalloftheAl2O3 surface.Thusnot all oftheNOxadsorption/storage(i.e.Al2O3)sitescanbeutilizeddue tolimitedphoto-oxidationcapabilityoftheinadequatenumberof TiO2oxidationsitesonthesurface.Ontheotherhand,atveryhigh TiO2loadings,TiO2coversmostoftheAl2O3surfaceanduponcal- cinationabove800◦C,SSAofthecatalystsamplefallsdrastically togetherwiththeformationofcrystallineanataseandrutilemix- 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 within150–1000◦Cinair.
Fig.6. ElectronicbandgapvaluesderivedfromDR-UV–visspectroscopicresultsfortheTiO2/Al2O3binaryoxidesampleswithdifferentmolarcompositionsthatwere calcinedatvarioustemperatureswithin150–1000◦Cinair.
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.Inverygoodagree- mentwiththediscussiongivenabove,electronicbandgapvalues fortherelativelyinactiveamorphousTi/Alsampleswhicharecal- cinedatlowertemperatures,revealacharacteristicallyhighvalue within3.4–3.6eV.Ontheotherhand,withtheonsetofthepho- tocatalyticactivity,a very sharpfallin theelectronicbandgap valueswereobserved,where thebandgapdecreasestoatypi- 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.Thisisduetothefactthatoncethephotocatalyt- 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-900photocat- alystshowedremarkablephotocatalyticNOxoxidationandstorage performancewhichwasfoundtobemuchsuperiortothatofa DegussaP25industrialbenchmarkphotocatalyst(i.e.160%higher NOx storageand 55%lower NO2(g)release totheatmosphere).
OurresultsindicatethattheonsetofthephotocatalyticforNOx
abatementactivityisconcomitanttotheswitchbetweenamor- phoustoacrystallinephase withanelectronicbandgapwithin 3.05–3.10eVwherethemostactivephotocatalystrevealedpre- dominantly rutile phase together withanatase as theminority phase.
Acknowledgments
AuthorsacknowledgeZaferSayforperformingBETmeasure- 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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