systems and ternary binary oxides as NO storage and reduction(NSR) NO storage and reduction pathways on zirconia and titaniafunctionalized Catalysis Today

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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

NO _x storage and reduction pathways on zirconia and titania

functionalized binary and ternary oxides as NO x storage and reduction (NSR) systems

Zafer Say, Merve Tohumeken, Emrah Ozensoy

DepartmentofChemistry,BilkentUniversity,06800Ankara,Turkey

a r t i c l e i n f o

Articlehistory:

Received22October2013 Receivedinrevisedform 17December2013 Accepted19December2013 Availableonline28January2014

Keywords:

Zirconia Titania Pt NOx

LNT NSR

a b s t r a c t

Binaryand ternaryoxidematerials, ZrO2/TiO2 (ZT)andAl2O3/ZrO2/TiO2 (AZT),aswellastheirPt- functionalizedcounterpartsweresynthesizedandcharacterizedviaXRD,Ramanspectroscopy,BET, insituFTIRandTPDtechniques.IntheZTsystem,astronginteractionbetweenTiO2andZrO2domains athightemperatures(>973K)resultedintheformationofalowspecificsurfacearea(i.e.26m²/gat 973K)ZTmaterialcontainingahighlyorderedcrystallineZrTiO4phase.IncorporationofAl2O3inthe AZTstructurerendersthematerialhighlyresilienttowardcrystallizationandordering.Aluminaactsas adiffusionbarrierintheAZTstructure,preventingtheformationofZrTiO4andleadingtoahighspecific surfacearea(i.e.264m²/gat973K).NOxadsorptionontheAZTsystemwasfoundtobesignificantly greaterthanthatofZT,duetoalmostten-foldgreaterSSAoftheformersurface.WhilePtincorporation didnotalterthetypeoftheadsorbednitratespecies,itsignificantlyboostedtheNOxadsorptiononboth Pt/ZTandPt/AZTsystems.ThermalstabilityofnitrateswashigherontheAZTcomparedtoZT,mostlikely duetothedefectivestructureandthepresenceofcoordinativelyunsaturatedsitesontheformersur- face.Ptsitesalsofacilitatethedecompositionofnitratesintheabsenceofanexternalreducingagentby shiftingthedecompositiontemperaturestolowervalues.PresenceofPtalsoenhancespartial/complete NOxreductionintheabsenceofanexternalreducingagentandtheformationofN2andN2O.Inthe presenceofH2(g),reductionofsurfacenitrateswascompletedat623KonZT,whilethiswasachieved at723KforAZT.NitratereductionoverPt/ZTandPt/AZTviaH2(g)undermildconditionsinitiallyleads toconversionofbridgingnitratesintomonodentatenitrates/nitritesandtheformationofsurface OH and NHxfunctionalities.N2O(g)wasalsocontinuouslygeneratedduringthereductionprocessasan intermediate/byproduct.

©2014ElsevierB.V.Allrightsreserved.

1. Introduction

Automobileindustryhasbeenforcedbynew,morestringent regulationstoinventnoveltechnologiesfortheeliminationofthe environmentalimpactofexhaustemissions.Sincethree-waycat- alystsare not efficient under lean conditions, NO_x storage and reduction(NSR)catalystshavebeendevelopedbyToyotaMotor Companyasapromisingafter-treatmentprocess[1,2].Theoper- ationalprincipleofNSRcatalystsreliesonthefactthatNO(g)is initiallyoxidizedtoNO₂(g)onthepreciousmetalsiteunderlean conditionsfollowedbystorageintheformofnitritesandnitrates onaNOxstoragedomainsuchasBaOorK2O[3–8].Finally,stored NOxspeciesarereducedtoN₂(g)intherichoperationalcycle[3].

ForadetaileddiscussiontheNOxstorageandreductiontechnology,

∗ Correspondingauthor.Tel.:+903122902121.

E-mailaddress:ozensoy@fen.bilkent.edu.tr(E.Ozensoy).

readerisreferredtotwocomprehensivereviewsbyRoyetal.[3]

andEplingetal.[4].

However,NSR materialshavetwo majordrawbacksnamely, sulfurpoisoning[9,10]andthermalaging[5,11,12].Senturketal.

investigatedtheeffectofTiO2promotionontheBaO/Al2O3binary oxideanddemonstratedthatthesulfuruptakeandreleaseproper- tiesoftheTiO₂-promotedBaO/Al₂O₃materialsweresignificantly enhanced[13].Matsumotoetal.[14]alsoreportedthatTiO2could beusedasapromoteragainstsulfurpoisoningduetoitshighacid- ity.However,ithasbeenalsoreportedthattitaniacanreadilylose itsfunctionalityduetothermaldeteriorationathightemperatures andvarioussolid-phasereactionsbetweenNOxstoragedomains, promotersandthesupportmaterial[11,12,15].Therefore,ZrO₂is typicallyusedtogetherwithTiO₂inanattempttostabilizethetita- niacomponent[16,17].Anothercriticalfactorthatfavorstheusage ofZrO₂/TiO₂asamixedmetaloxidecomponentisitshighersurface acidityascomparedtoeitherZrO₂orTiO₂,alone[18,19].

ZrO2/TiO2 and Al2O3/ZrO2/TiO2 mixed oxides have recently beenthoroughlystudiedwithaparticularemphasisonthesulfur 0920-5861/$–seefrontmatter©2014ElsevierB.V.Allrightsreserved.

http://dx.doi.org/10.1016/j.cattod.2013.12.037

tolerance,materialpreparation,NOxstoragecapacityandthermal stabilityaspects.Takahashietal.[20]investigatedtheinfluenceof therelativeabundanceofTiO₂andZrO₂componentsandfoundthat theNSRsystemthatwascomprisedof70wt%ZrO₂and30wt%TiO₂ exhibitedthebestsulfurtolerance.Imagawaetal.[21,22]reported adetailedcharacterizationstudyrelatedtotheAl₂O₃/ZrO₂/TiO₂ ternaryoxidesystememphasizingtheimpactofAl₂O₃incorpora- tionintotheZrO2/TiO2 mixedoxidewhereitwasindicatedthat nano-compositeAl₂O₃/ZrO₂/TiO₂hadahigherNO_xstoragecapac- ityandahigherthermalresistanceascomparedtothephysically mixedAl2O3andZrO2/TiO2.

Thus,inthecurrentstudy,weaimedtoprovideamechanistic viewintotheNOxstorageandreductionpathwaysofbinaryand ternarymixedoxidesbymonitoringthenitratereductionviaH₂at themolecularlevelbymeansofinsituFTIRandTPD.

2. Experimental 2.1. Materialpreparation

ZrO₂/TiO₂ binary and Al₂O₃/ZrO₂/TiO₂ ternary oxides were synthesizedusingtheconventionalsol–gelmethod.Forthesyn- thesisofthebinaryoxides,zirconiumpropoxide(SigmaAldrich, ACSReagent,70wt%in1-propanol)andtitanium(IV)isopropoxide (SigmaAldrich,ACSReagent,97%)precursorswereinitiallydis- solvedin100mlof2-propanol(SigmaAldrich,ACSReagent>99.5%) andstirred for40minunder ambientconditions.Thisstepwas followedby thedrop-wiseadditionof3mLof0.5Mnitricacid solution(SigmaAldrich,ACSReagent,65%)in ordertoobtaina gel.Similarly,thesynthesisoftheternaryoxidewascarriedout bymixingzirconiumisopropoxide,titaniumisopropoxideandalu- minumsec-butoxide(SigmaAldrich,ACSReagent,97%)followed by the addition of 100mL of 2-propanol. Next, theslurry was stirredfor 60min underambient conditions and gel formation wasachievedbydrop-wiseadditionof9mLof0.5Mnitricacid solution.Inthebinaryoxide,thecompositionratioofZrO₂:TiO₂ was70:30bymass.Thisspecificratiohasbeenreportedasthe optimumcandidateforthehighestNOxremovalabilityandthe highesttoleranceagainstsulfur-poisoning[20].Finally,themate- rialswere driedunderambientconditions for48hfollowed by calcinationinairwithin323–1173K.Relativecompositionofthe ternaryoxidesystem(i.e.Al2O3/ZrO2/TiO2)bymasswas50:35:15 [20].1wt%platinum-incorporatedbinaryandternaryoxidemate- rialsweresynthesizedbyincipientwetnessimpregnationmethod usingasolutionofPt(NH3)2(NO2)2 (Aldrich,diamminedinitrito- platinum(II),3.4wt% solutionin diluteNH₃(aq)). PriortothePt addition,ZrO₂/TiO₂andAl₂O₃/ZrO₂/TiO₂wereinitiallycalcinedin airat773Kfor150mininordertoremovetheorganicfunction- alitiesintheprecursors.Finally,eachmaterialwassubsequently calcinedinairat973Kfornitrite/nitratecontenteliminationand structuralstabilization.Inthecurrenttext,synthesizedZrO2/TiO2, Al₂O₃/ZrO₂/TiO₂,Pt/ZrO₂/TiO₂,Pt/Al₂O₃/ZrO₂/TiO₂sampleswillbe abbreviatedasZT,AZT,Pt/ZTandPt/AZT,respectively.

2.2. Instrumentation

Detaileddescriptionoftheinstrumentationused inthecur- rentlypresented experimentsinvolvingX-raydiffraction(XRD), Ramanspectroscopy,BETsurfaceareaanalysis,temperaturepro- grammeddesorption(TPD)andinsituFTIRcanbefoundelsewhere [8].Briefly, in situ FTIRspectroscopic measurementswere per- formed in transmission mode using a Bruker Tensor 27 FTIR spectrometerwhichwasmodifiedtohouseabatch-typespectro- scopicreactorcoupledtoaquadrupolemassspectrometer(QMS, StanfordResearchSystems,RGA200)forTPDmeasurements.

EachsynthesizedmaterialwasexposedtoNO₂(g)whichwas preparedbymixingNO(g)(AirProducts,99.9%)andexcessO2(g) (LindeGmbH,Germany,99.999%).Freeze–thaw-pumpcycleswere appliedfortheremoval ofcontaminations andunreactedgases intheNO2(g).The materialsurfaceswereinitiallyflushed with 1.0TorrofNO₂(g)for5minandsubsequentlyannealedto973K witha12K/minheating rateundervacuum.Then, fortheFTIR analysis,material surfaces wereexposed to NO2(g) at 323K in a stepwise fashion from low to higher pressures where each exposure takes 1min. Finally, surface saturation was achieved by the introduction of 5.0Torr NO2(g) over the samples for 10minat323Kfollowedbyevacuationtoapressurelowerthan 10⁻²Torr.

Nitratereductionexperiments forthePt-freematerialswere carriedoutbyexposingtheNO2(g)-saturatedmaterialsurfaceto 15.0TorrofH₂(g)(LindeGmbH,Germany,>99.9%)at323K,fol- lowedbygradualheatingataconstantrateof12K/minuntilthe desiredtemperature.ForthePt-containingmaterials,15.0Torrof H₂(g)wasintroducedovertheNO₂-saturatedmaterialat323Kand thetime-dependentFTIRspectrawereacquiredfor2h.After2hof reductionat323K,samplewasheatedupto473Kinthepresence ofH₂(g)forthecompletereductionandremovalofadsorbedNO_x. AlloftheFTIRspectragiveninthisstudywereobtainedat323K.

InTPDexperiments,eachsamplewassaturatedbyNO2(g)as describedabove.Subsequently,saturatedmaterialswereheated upto973Kwithalinearheatingrateof12K/mininvacuum.FTIR spectraofthecorrespondingsurfaceswerealsorecordedbefore andaftertheTPDexperiments.

3. Resultsanddiscussion

3.1. Structuralcharacterizationofthesynthesizedmaterials

Fig.1illustratesexsituXRDanalysisfortheZTandAZTmateri- alsrecordedaftercalcinationatvarioustemperaturesintherange of323–1173K.AsshowninFig.1a,theamorphousstructureof ZTbinaryoxidepersistsupto773Kfollowed bytheformation of crystalline phases above 773K, namely tetragonal ZrO₂ and ZrTiO₄. The structural evolution of AZT is also shown by XRD inFig.1bwhere theternary oxidesystembehavesquitediffer- entlycomparedtothebinaryoxide.AZTpreserveditsamorphous natureupto973K withoutrevealing anywell-resolveddiffrac- tionsignals.Alongtheselines,AZTshowedonlypoorlydiscernible diffractionsignalsat2=30.48^◦,50.50^◦,60.91^◦correspondingto tetragonalZrO₂(JCPDS80-2155)at1173K.Apparently,theaddi- tion of an alumina component to the ZT system elevates the ordering/crystallizationtemperaturesandsuppressestheforma- tionofZrTiO₄ (JCPDS 34-415).It isworthmentioningthatXRD analysiswasalsoperformedforthePt/ZTand Pt/AZT materials (SupportingInformationFig.1),whereitwasobservedthatPtaddi- tiondidnotleadtoamajorcrystallographicchangeovertheZTand AZTsystemsbesidesthepresenceofdiffractionsignalsassociated withmetallicPtparticles.

Thetemperature-dependentRamanspectraofZTandAZTare illustrated in Fig.2. ZrTiO₄,one of the main crystallinephases detectedforZTsamplesinXRD,hasanorthorhombicsymmetry withaPbcnspacegroupandammmpointgroup.Thisphasehas33 opticallyactivemodes,18ofwhichareRamanactive[23,24].How- ever,intheRamandatacorrespondingtotheZTsamplepresented inFig.2a,onlysixofthesevibrationalfeaturesat135,259,320,391, 565and778cm⁻¹arediscernible.Thiscanbeassociatedwiththe bandbroadeningandsignaloverlapduetotherandomdistribution ofZr⁴⁺andTi⁴⁺ionsinthecrystallattice[23,24].Ramanshiftsat 467and622cm⁻¹inFig.2acorrespondingtotheZTsampleare mostlikelyassociatedwiththetetragonalZrO2phase,whilethe

10 20 30 40 50 60 70 80 10 20 30 40 50 60 70 80 623K

773K 973K 1173K

2 theta 2 theta

Intensity (arb. u.)

ZrO₂/TiO₂ (b) Al₂O₃/ZrO₂/TiO₂ )

a (

2500 2500

323K

623K 773K 973K 1173K

323K ZrTiO₄

t-ZrO₂

ZrTiO₄ t-ZrO₂

Fig.1. XRDpatternscorrespondingto(a)ZrO2/TiO2and(b)Al2O3/ZrO2/TiO2materialsuponcalcinationattemperatureswithinof323–1173K.

featureat391cm⁻¹canbeattributedtothepresencezirconium titanatephase[25,26].

AscomparedtotheZTbinaryoxide,Ramanspectracorrespond- ingtotheAZTternaryoxidesample(Fig.2b)revealedmuchweaker signalsduetothepoorcrystallinityandthelackofatomicorderin thelattersample.InFig.2b,threeweakRamanfeatureslocatedat 320,465and625cm⁻¹arevisiblewhichcanbeattributedtothe tetragonalzirconiaphase.Thisrevealsthatbesidesthetetragonal ZrO₂phase,XRDandRamandatadonotprovideanyclearindi- cationsforthepresenceofadditionalorderedphasesintheAZT system,suchasZrTiO4.Theseobservationsareinlinewithstud- iesreportedbyEscobaretal.[27]whosuggestedthatintheZT binarysystemZrO₂canprovideZr⁴⁺ionswhichmaydiffuseinto theTiO₂latticeintheabsenceofaluminumoxide,formingZrTiO₄. However,intheAZTternarysystem,XRDandRamandatagiven inFigs.1and2suggestthatAl₂O₃actsasadiffusionbarrierpre- ventingdiffusionofZr⁴⁺ionsintotheTiO₂lattice,preventingthe orderingofthecrystallatticeandtheformationZrTiO4.

Fig.3presentstheBETspecificsurfacearea(SSA)valuesforthe synthesizedmaterialswhichwerecalcinedat773,973and1173K.

Fig.3indicatesthattheAZTternaryoxidesystemhasamuchhigher

surfaceareacomparedtotheZTbinaryoxidesystemforallcalci- nationtemperatures.Whiletheternaryoxidesystemhasalmost two-foldgreatersurfaceareaat773K,thisgapwasobservedto extenduptoaten-fold differenceat973K.Theseresultsarein very good agreementwiththecurrent XRD and Ramanresults suggestingamoredisorderedstructurefortheAZTsystem.Crys- tallizationoftheZTbinaryoxideleadstorelativelyorderedand largerparticlesat973K,whiletheternary systempreservesits ratheramorphousstructureandsmallparticlesizeevenat1173K.

ItisworthmentioningthatonaccountofthedifferentTiandZr precursorsused inthecurrent work,synthesizedternary oxide material(i.e.Al2O3/ZrO2/TiO2)hasahighersurfacearea(i.e.SSA 264m²/g)comparedtothenano-compositeternaryoxidemate- rialwithaSSA∼200m²/gwhichwasreportedinarecentstudy [20].BETSSAanalysiswasalsoperformedforPt-containingsam- pleswhichwerepreparedasdescribedintheexperimentalsection.

ThesemeasurementsshowedthatalthoughPtadditionandsubse- quentcalcinationat973Kdidnothaveasignificantinfluenceon theSSAvaluesoftheZTsystem(26m²/gversus37m²/gforZT andPt/ZT,respectively),SSAvaluesoftheAZTsystemsignificantly decreasedinthepresenceofPt(264m²/gversus191m²/gforAZT

200 400 600 800

x3

200 400 600 800

135 259 320 391 467 622 778

200

Raman Shift (cm^-1)

465 625

320

Raman Shift (cm^-1)

5

973K 1173K

Intensity (arb. u)

ZrO₂/TiO₂ Al₂O₃/ZrO₂/TiO₂

(a) (b)

ZrTiO₄ t-ZrO₂

ZrTiO₄ t-ZrO₂

Fig.2. ExsituRamanspectracorrespondingto(a)ZrO2/TiO2and(b)Al2O3/ZrO2/TiO2materialsuponcalcinationattemperatureswithin773–1173K.

1200 1100

1000 900

800 0 50 100 150 200 250 300 350 400

264

342 113

177 26 3

SBET(m2/g)

Temperature (K) ZrO₂/TiO₂ Al₂O₃/ZrO₂/TiO₂

Fig.3. BETspecificsurfaceareavaluesfor(a)ZrO2/TiO2(black)andAl2O3/ZrO2/TiO2

(red)aftercalcinationattemperatureswithin773–1173K.

andPt/AZT,respectively).Alongtheselines,itcanbearguedthatPt sitesfacilitatetheoxidationoftheAZTmatrixduringthecalcina- tioncarriedoutat979K,whichresultsinamoreorderedternary oxidesystemwithlargerZrO₂andTiO₂crystallites.Howeverthese crystallitesstillseemtobesmallenoughtobeelusiveinXRD(Sup- portingInformationFig.1).Furtherevidenceforthisargumentwill beprovidedalongwiththediscussionoftheTPDresultsfortheAZT andPt/AZTsamplesgiveninthenextsection.

3.2. NO₂(g)adsorptionanddesorptiononZrO₂/TiO₂and Al₂O₃/ZrO₂/TiO₂

Fig. 4 represents the FTIR spectra corresponding to NO₂(g) adsorption on both of the binary and ternary oxide materials (alreadycalcinedat973K)at323Kforincreasingexposures.Five particularvibrationalfeaturesweredetectedforZTwhichwere locatedat1644,1578,1550,1280and1209cm⁻¹ asillustrated in Fig.4a. The spectral regionbetween1700 and 1200cm⁻¹ is

characteristicforthenitrateandnitritespeciescoordinatedonTiO₂ andZrO2[28–30].Theobservedfrequenciesat1644and1209cm⁻¹ canbeattributedtobridgingnitrates,whilethefeaturesat1578, 1550and1280cm⁻¹canbeassignedtobidentatenitrates[8,31].

SimilarabsorptionfeaturesalsoappearedfortheAZTternaryoxide systemas shown in Fig. 4b. However, theposition of bridging nitrates(1639and1244cm⁻¹)ontheAZTternaryoxideisdifferent whencomparedtotheZTbinaryoxidesystem.Probably,themost prominentaspectofFig.4isthedissimilarityintherelativeFTIR intensitiesofAZTandZTsystems.ItisclearthattheNOxadsorption ontheAZTternaryoxideissignificantlyhigherthanthatoftheZT binaryoxideduetoaten-foldhigherspecificsurfaceareaandthe significantlygreaterIRabsorptionintensityuponNOxadsorption oftheformersurfaceat973K.

MorequantitativeinsightintothetotalNOxadsorptioncapabil- ityandthermalstabilityofadsorbednitratespeciescanbeobtained bycombining FTIRand TPDresults. TPD profilesrelated tothe ZTandAZTsystemsobtainedafterNO2(g)saturation(5.0Torrfor 10min)at323KarepresentedinFig.5aandb,respectively.Com- parisonoftheTPDlineshapesforZTandAZTsystemsimmediately revealsthedissimilarNOxdesorptioncharacteristicsoneachmate- rial.WhilemostoftheNO_xdesorptioniscompletedbelow800K fortheZTsample,thisisnottrueforAZTwhereevenat1000K, NOxdesorptionisstillincomplete.NO(m/z=30)desorptionsignal fortheZTsample(Fig.5a)showstwomajorfeaturesatof640and 750Krelatedtothenitratedecomposition.Inthefirstdesorption stateof640K,nitratedecompositiontakesplacebysimultaneous evolutionofNO,O2,N2OandNO2correspondingtom/zsignalsat 30,32,44and46,respectively.Ontheotherhand,thehightem- peraturenitratedesorptionstate(750K)oftheZTsystemreveals primarilyNOandN2OwithalessercontributionfromO2andNO2. NO(m/z=30)desorptionsignalfortheAZTsysteminFig.5b indicatesthatwithinthetemperaturewindowofthecurrentTPD experiments(i.e.323–973K),NOxdesorptionwasnotcompleted.

Thistrendindicatesthattherelativethermalstabilityofthestored NOxspecies(i.e.nitrates)ontheAZTsurfaceismuchhigherthan that of the ZT surface. Thiscan at least bepartially attributed totheexistenceofa largeconcentrationofsurface defectsand coordinativelyunsaturatedadsorptionsiteswhicharepresenton thedisordered/poorlycrystallineAZTsurfacerevealinga higher SSA. Furthermore, comparison of the TPD signalintensities for ZTandAZTsamplesrevealsthattheNOxadsorptionoftheAZT ternaryoxideisfargreaterthanthatoftheZTbinaryoxide(i.e.

thetotal integrated NOx-uptakerelated desorptionsignal com- prisedofNO+NO₂+N₂+N₂Odesorptionchannelsis%143greater forAZT.ReaderisreferredtotheSupportingInformationsection forthedetailsoftheTPDintegratedsignalanalysis).Inaddition, Fig.5balsosuggeststhatthenitratedecomposition ontheAZT

1800 1700 1600 1500 1400 1300 1200 1100 1000 1800 1700 1600 1500 1400 1300 1200 1100 1000

1644 1578 1550 1280 1209 1639 1582 1555 12441283

) b ( )

a

( ZrO₂/TiO₂ Al₂O₃/ZrO₂/TiO₂

Absorbance (arb. u.)

Wavenumber (cm^-1) Wavenumber(cm^-1)

0.5 0.5

Fig.4.FTIRspectracorrespondingtothestepwiseNO2(g)adsorptionat323Kon(a)ZrO2/TiO2and(b)Al2O3/ZrO2/TiO2surfaces.Thebold(red)spectrumineachpanel correspondstotheNO_x-saturated(5.0TorrNO2(g)for10minat323K)surface.(Forinterpretationofthereferencestocolorinthisfigurelegend,thereaderisreferredtothe webversionofthearticle.)

300 400 500 600 700 800 900 1000 300 400 500 600 700 800 900 1000 )

K ( e r u t a r e p m e T )

K ( e r u t a r e p m e T

QMS Intensity (arb. u.)

Fig.5.TPDprofilesobtainedfrom(a)ZrO2/TiO2and(b)Al2O3/ZrO2/TiO2samplesaftersaturationwith5TorrNO2(g)at323Kfor10min.Theinsetineachpanelpresentsthe FTIRspectraofthesurfacesbefore(black)andafter(red)TPDanalysis.(Forinterpretationofthereferencestocolorinthisfigurelegend,thereaderisreferredtotheweb versionofthearticle.)

surfaceoccursviaevolutionofmostlyNO,O2andNO2witharel- ativelyinsignificantcontributionfromotherdesorptionchannels.

TheseTPDresultsareinperfectagreementwiththecorrespond- inginsituFTIRdatapresentedasinsetsinFig.5aandb.Inthese insets,blackcurvescorrespondtoNO2-saturatedsurfacesbefore theTPDexperiments,whiletheredcurvescorrespondtotheZTand AZTsurfacesaftertheTPDanalysis(i.e.afterannealingundervac- uumat973K).FTIRdataclearlyshowthatwhilecompletenitrate eliminationwasachievedonZrO₂/TiO₂aftertheTPDexperiment, asignificantamountofnitratespeciesobservedtoberemainingon theAZTsurfaceaftertheTPDrun.

TheeffectofPtincorporationonNO_xadsorptiongeometryand storagecapacitywasalsoinvestigatedviainsituFTIRspectroscopy and TPD. FTIR studies involving NOx adsorption experiments onPt/ZrO2/TiO2andPt/Al2O3/ZrO2/TiO2(SupportingInformation Fig.2)leadustoconcludethatthepresenceofPthasnosignif- icantinfluence ontheNOxadsorptiongeometries.Fig.6 shows theTPD profilesobtainedafter NO2 saturationonPt/ZrO2/TiO2

and Pt/Al₂O₃/ZrO₂/TiO₂ at 323K. InFig. 6a, two NOdesorption statesthatbelongtoPt/ZrO₂/TiO₂at630and755Karevisible,as inthecaseofZrO2/TiO2(Fig.5a).However,thehightemperature desorptionstatelocatedat755Kissignificantlysuppressedinthe presenceofplatinum.ThisimpliesthatonthePt/ZrO₂/TiO₂ sur- face,Ptsitesfacilitatethedecompositionofstronglyboundnitrate species,shiftingtheirdecompositiontemperaturestolowervalues.

Furthermore,comparisonoftheTPDdatagiveninFigs.5aand6a suggeststhatPtadditionsignificantlyincreasestherelativedesorp- tionofN2andN2OforthePt/ZrO2/TiO2.Thiscanpossiblybedue tothePt-catalyzeddirectpartial/totalreductionoftheadsorbed nitratesonthePt/ZrO₂/TiO₂sampleintoN₂andN₂O,particularly at755K.LackofasignificantO2desorptionsignalat755K,indi- catesthattheoxidizingspeciessuchasatomicoxygenordiatomic oxygengeneratedduringthepartial/totalreductionofnitratesare possiblycapturedbythePt/ZrO2/TiO2systemwheretheymayoxi- dizePtsitesand/ortitrateoxygendefectsinthetetragonalZrO₂or ZrTiO₄phases.Itisalsoworthmentioningthatcomparisonofthe integratedTPDdesorptionsignalsfortheZrO2/TiO2(Fig.5a)and Pt/ZrO₂/TiO₂(Fig.6a)indicatesthatPtadditiontotheZTsystem significantlybooststheNOxadsorption.Numerically,Ptaddition totheZTsystemincreasestheintegratedNOdesorptionsignalby about32%andincreasesthetotalintegratedNOx-uptakerelated desorptionsignalassociatedwithNO+NO₂+N₂+N₂Oby101%(see SupportingInformationsectionfordetails).Notethatthesenum- bersdonotcorrespondtoNOxstoragecapacity(NSC)valuesand usedmerelyforthesemi-quantitativecomparisonoftherelative NOxuptakesoftheinvestigatedsurfaces.

Investigationof theTPD datagiven in Fig.6b indicatesthat theinfluenceofthePtsitesonthenitratedecompositionismuch moreprominentfortheAZTsystem.Firstly,Ptincorporationseems to promote more facile thermal nitrate decomposition on the Al2O3/ZrO2/TiO2 ternary oxidesystem,wheretheNO (m/z=30) desorptionmaximaaresignificantlyshiftedtolowertemperatures.

Moreinterestingly,NOdesorptionmaximaforthePt/AZTsystem (640and730K,Fig.6b)arealmostidenticaltothatofZT(640and 750K,Fig.5a)andPt/ZT(630and730K,Fig.6a)andquiteunlike AZT(740,800,855K,Fig.5b).Combiningthisobservationwiththe SSA valuesobtainedforthesesystems discussedearlierimplies thatalthoughAZTsystemiscomprisedofaquitedisorderedand aratherdefectiveternaryoxidesystem,Pt/AZTsystemconsistsof moreorderedfinelydispersedsmallparticlesofZrO2,TiO2,ZrTiO4

andAl2O3.Theseparticlesarepossiblyformedduringthecalci- nationstep,wherePtsitesoxidizetheAZTsurface,increasingthe crystallographicorderofthesurfacetoacertainextent,whichis elusivetodetectviabulkcharacterizationtechniquessuchasXRD (SupportingInformationFig.1).PresenceofAl₂O₃domainsonthe Pt/AZTsurfaceisalsosupportedbytheexistenceoftheNOdesorp- tionshoulderat400–450KinFig.6b(concomitanttoaNO2signal whichalsoappearsatthesametemperaturewindow)whichisa characteristicfeatureofNO₂TPDfrom␥-Al2O₃[32]Althoughthis 400KNOxdesorptionfeatureisabsentforZTandPt/ZTsamples (Figs.5aand6),itisvisible(thoughwithamuchsmallerintensity) fortheAZTsample,suggestingthatthealuminadomainsontheAZT samplearemuchlessfullyoxidized/ordered.SincetheNOxdesorp- tion/decompositionwasincompletefortheAZTsamplewithinthe thermalwindowoftheTPDanalysis,itisdifficulttomakeaquanti- tativecomparisonfortheNOxuptakeofAZTversusPt/AZTsystems.

Howeveranalysisof theintegratedTPD signalsassociated with AZT(Fig.5b)andPt/AZT(Fig.6b)samplesshowsthatPtaddition totheAZTsystemincreasestheintegratedNOdesorptionsignal more than 288% and increasesthetotal integratedNOx-uptake relateddesorptionsignalassociatedwithNO+NO₂+N₂+N₂Oby morethan50%.Itcanalsobenotedthatalthoughthe640KNOx

desorptionfromthePt/AZTsurfaceoccursintheformofNO,O2

andNO₂;NO_xdesorptionat730Ktakesplacewiththeevolutionof NOandO₂only.

Ontheotherhand,relativeNOxadsorptionamountsshouldbe alsoassessedconsideringtherelativespecificsurfaceareavalue foreachmaterial.OurcalculationsindicatethatPtincorporation totheZTbinaryoxidesystemincreasesthetotalintegratedNOx- uptake/m²by41%.Moreover,intheAZTternaryoxidesystem,Pt playsamuchmoreeffectiveroleinincreasingthetotalintegrated NOxuptake/m²by109%.

300 400 500 600 700 800 900 1000 300 400 500 600 700 800 900 1000

) K ( e r u t a r e p m e T )

K ( e r u t a r e p m e T

Fig.6.TPDprofilesobtainedfrom(a)Pt/ZrO2/TiO2and(b)Pt/Al2O3/ZrO2/TiO2samplesaftersaturationwith5TorrNO2(g)at323Kfor10min.Theinsetineachpanelshows theFTIRspectraofthesurfacesbefore(black)andafter(red)TPDanalysis.(Forinterpretationofthereferencestocolorinthisfigurelegend,thereaderisreferredtotheweb versionofthearticle.)

TrendsobservedfortheTPDdatagiveninFig.6areinverygood agreementwiththecorrespondingFTIRspectraobtainedbefore andaftertheTPDrunswhicharegivenasinsetsofFig.6.Particu- larly,theinsetofFig.6bshowshowPtplaysasignificantroleinthe nitratedecompositionperformanceoftheAZTternaryoxidesys- tem.AsshownintheinsetofFig.6b,Pt/AZTsurfacewascompletely freeofadsorbedNOxspeciesaftertheTPDrun(i.e.afterannealing at973K)whilethiswasnotthecaseforthePt-freeAZTcounterpart (insetofFig.5b).

CurrentresultsdiscussedaboveallowsustoconsidertheAZT systemasapromisingcandidatematerialforlow-temperatureNSR applications.ComparedtotheconventionalPt/20wt%BaO/Al₂O₃ system,Pt/AZT materialcalcined at973Khasa specificsurface areaof191m²/g, whileconventional Pt/20wt%BaO/Al₂O₃ hasa SSAof125m²/g.Ontheotherhand,althoughPt/20wt%BaO/Al₂O₃ hasalowersurfacearea,ithas19%greaterintegratedtotalNOx

desorptionsignalthanPt/AZT70obtainedviaidenticaladsorption experiments(datanotshown).InfluenceoftheBaOdomainsonthe NOxadsorptionandreleasepropertiesofZTandAZTsystemsisan interestingaspectwhichwillbediscussedindetailinaforthcoming report[33].

3.3. NOxreductiononbinaryandternaryoxidesystemsviaH₂(g)

AswellastheNO_xstoragecharacteristicsofZTandAZTmateri- als,theirNOxreduction/regenerationperformancesunderreducing conditionsshouldalsobetakenintoconsiderationforthesakeof NSRapplications.ReductionresistanceofnitratespeciesonPt-free

ZTandAZTmaterialswasinvestigatedviaFTIRspectroscopyinthe presenceofanexternalreducingagent(i.e.H₂(g)),asillustratedin Fig.7aandb,respectively.Priortonitratereduction,thematerial surfacesweresaturatedwith5.0TorrNO₂(g)for10minat323K.

AfterthesaturationofthesurfacewithNO₂(g),15.0TorrofH₂(g) wasintroducedoverthesamplesurface.Temperature-dependent FTIRspectrawereobtainedafterannealingtheNO2-saturatedsur- facesat373,473, 573,623,673,723KinthepresenceofH₂(g).

AdsorbednitratesonZT(Fig.7a)fullysurvivedunderreducingcon- ditionsupto473KwithonlyminorIRintensitychanges.However, theIRsignalintensitiesstartedtodecreaseat573K,wheresig- nificantamountsofnitrateswerelostfromthesurface.Itisalso apparentinFig.7athatbidentatenitrates(1580cm⁻¹)arerelatively more stablethan bridging nitrates (1649cm⁻¹)under reducing conditionsonthebinaryoxidesystem.Finally,completeremovalof nitratesontheZTsurfaceunderH2(g)environmentwasachievedat 623K(correspondingtotheredspectruminFig.7a);atemperature thatiscompatiblewiththethermalwindowofrealisticexhaust emissioncontrolsystems.

SimilarexperimentswerealsoperformedontheAZTternary oxidesystem(Fig.7b).Asdiscussedabove,adsorbednitratesonAZT revealaveryhighthermalstabilityinaH₂-freeenvironment.Even underreducingconditions,nitratesonAZTsurfacewerefoundto bequitestableevenat573K(Fig.7b).FTIRanalysisclearlyindicates thatunderreducingconditions,allofthenitratespeciesonAZTcan becompletelyreducedat723K.

Our preliminary experiments (data not shown) related to elevated-temperature nitrate reduction on Pt-containing

1800 1700 1600 1500 1400 1300 1200 1100 1000 1800 1700 1600 1500 1400 1300 1200 1100 1000

1649 1582 1554 1280 1206 1647 1583 1554 12381281

) b ( )

a

( ZrO₂/TiO₂ Al₂O₃/ZrO₂/TiO₂

Absorbance (arb. u.)

Wavenumber (cm^-1) Wavenumber(cm^-1)

0.5 0.5

373K 623K

373K 723K

573K 673K

Fig.7.FTIRspectracorrespondingtothetemperature-dependentnitratereductionvia15.0TorrofH2(g)on(a)ZrO2/TiO2and(b)Al2O3/ZrO2/TiO2surfaces.Thetopmost spectrumineachpanelcorrespondstotheNOx-saturated(5.0TorrNO2(g)for10minat323K)surface.Theseriesofblackspectraineachpanelwerecollectedatdifferent temperatureswithin323–723K.Thebottommost(red)spectrumineachpanelcorrespondstothehighestreductiontemperatureexploited(i.e.623KforZTand723Kfor AZT).(Forinterpretationofthereferencestocolorinthisfigurelegend,thereaderisreferredtothewebversionofthearticle.)

1800 1600 1400 1200 1000 1800 1600 1400 1200 1000

1646 1580 1511 1291 1205

(a) Pt/ZrO₂/TiO₂

Absorbance (arb. u.)

Wavenumber (cm^-1) Wavenumber (cm^-1)

1422

1st

2nd

3rd

0-40 min

50-120 min

373-473K

0.25 1643 1582 1511 1299 1232

(b) Pt/Al₂O₃/ZrO₂/TiO₂

1411

1st

2nd

3rd

0-80 min

90-120 min

373-473K

0.75

Fig.8. FTIRspectracorrespondingtothetime-dependentnitratereductionvia15.0TorrofH2(g)on(a)Pt/ZrO2/TiO2and(b)Pt/Al2O3/ZrO2/TiO2surfaces.Thetopmost spectrumineachpanelcorrespondstotheNOx-saturated(5.0TorrNO2(g)for10minat323K)surface.Theseriesofspectrainthe“1st”and“2nd”intervalswerecollectedas afunctionoftimeduringtheinitial120minofreductionat323K.“1st”intervalcorrespondstothe0–40minor0–80minoftheinitialreductionperiodsforPt/ZrO2/TiO2and Pt/Al2O3/ZrO2/TiO2,respectively;whilethe“2nd”intervalcorrespondstotheremainingtimeevolutionuntil120minofreduction.FTIRspectrainthe“3rd”intervalwere collectedaftertheinitial120minreductionat323Kbyincreasingtotemperatureto373,423and473KinthepresenceofH2(g).

materials (i.e. Pt/ZT and Pt/AZT) revealed extremely fast reac- tionevenatrelativelylowertemperatures,renderingmechanistic comparisonofthesetwosystemsratherdifficult.Therefore,time- dependentexperimentswerecarriedoutwithslowerkineticsat lowtemperatures(i.e.323K)inordertomonitorandelucidatethe temporalchangesinthesurfacefunctionalgroupsonmaterialsur- facesuponreductionbyH₂(g).Fig.8illustratesthecorresponding FTIRspectraforPt/ZTandPt/AZTrecordedasafunctionoftime afterNOxsaturated(5.0TorrNO2for10min)materialsurfacewas exposedto15.0TorrofH₂(g).Forthesakeofclarity,eachpanelin Fig.8isdividedintothreetimeintervals.Thefirstintervalofthe datasetinFig.8aisrelatedtotheinitialreductionperiodof40min at323K.Inthefirstperiod,whiletheintensitiescharacteristicfor bridgingnitrates(1646and1205cm⁻¹)diminish,othervibrational features at1511and 1291cm⁻¹ correspondingtomonodentate nitratespeciessimultaneouslystarttoemerge[31].Anothergrow- ingfeaturein thisperiodobservedat1422cm⁻¹ (together with 1291cm⁻¹)canbeassignedtonitrites[34].Forthesecondtime intervalbetween50 and120min,initiallyformedmonodentate nitratesandnitritesconcurrentlyattenuateuponinteractionwith H₂(g)togetherwithallothernitratespeciessuchasbridgingand bidentatenitrates.Since theNOx reductiononPt/ZThasmostly ceasedafter120min(Fig.8a),inthethirdstage,thematerialwas annealedtohigher temperaturesinH₂(g)inorder toeliminate anyremainingNOxspeciesonthesurface.Thespectrainthethird intervalcorrespondingtothereductionofnitritesandnitrateson thematerialsurfacewerecollectedat373,423and473K.These resultsallowustoobtainabetterinsight regarding thenitrate reductionmechanismonthePt/ZTsurfaceunderH₂(g)atmosphere whichinitiallytakesplaceviatransformationofbridgingnitrates intomonodentatenitratesandnitritesfollowedbythecomplete removalofallNO_xspeciesaround473K.

Anidentical set of experimentswas also performedfor the Pt/AZTsurface(Fig.8b).AsthereducingagentH2(g)isintroduced

overtheNOx-saturatedsurface,theintensitiesofthevibrational features at 1643 and 1232cm⁻¹ (bridging nitrates) attenuate togetherwithincreaseintheintensitiesof1511and1299cm⁻¹ (monodentate nitrates)as wellas 1411cm⁻¹ (nitrites).A note- worthydifferencebetweenthebinaryandternarysystemsisthat whileallnitrate-relatedstretchingsignalsdiminishinthesecond timeintervalat323KonPt/ZT(Fig.8a),nospectralchangeswere observedinthesecondtimeinterval(90–120min)ofthePt/AZT system(Fig.8b).Theseobservationsareinharmonywithprevious TPDandFTIRresultsindicating thatadsorbedNOxspecies typi- callypossessahigherstabilityontheAZTsurfacethanZT.Similar totheZTsystem,almostalloftheadsorbedNOxspeciescanbe eliminatedfrom thePt/AZT surfacein thepresence ofH₂(g)at 473K.ItisworthmentioningthatcomparisonoftheNOxreduction capabilitiesofthePt/AZTsystemwithconventionalNSRsystems thatwehaveinvestigatedinthepastsuchasPt/20wt%BaO/Al₂O₃ andPt/20wt%BaO/20wt%CeO₂/Al₂O₃[8],revealsthatreductionof adsorbedNOxspecieswithH2(g)at473Koccursinamorefacile manneronthePt/AZTsurface.

Fig.9illustratestheregionoftheinsituFTIRspectraassociated withthe OHand NHx stretchingregionsofthePt/ZTsystem, whichwereacquiredduringthetime-dependentreductionexper- imentsdescribed abovealong withthedatapresentedinFig.8.

Fig.9aandbshowstheevolutionoftheinsituFTIRspectraforthe first(0–40min)andthesecond(50–120min)timeintervalsofthe nitratereductionviaH₂(g),respectively.UponNO₂(g)introduction tothePt/ZTsurface(5.0TorrNO₂(g)for10min),negativefeaturesat 3722and3678cm⁻¹wereobserved(Fig.9a).Whiletheformerfea- turehasbeenassignedtolinear(type-I) OHspecies,thelatterone ischaracteristicsforthree-fold(type-III)hydroxyls[35–42]which disappear from the surface upon interaction/coordination with adsorbednitratesand nitrites.Anotherbroadandhighlyconvo- lutedfeaturecenteredat3512cm⁻¹canbeattributedtoH-bonded surfacehydroxylspecies[38,39].InFig.9a,theinteractionofH2

3800 3600 3400 3200 3000 3800 3600 3400 3200 3000

3722 3678 3512 3350 3256

0.1

(a)

0-40 minutes

3722 3678 3512 3350 3256

(b)

50-120 minutes

Absorbance (arb. u.)

Wavenumber (cm^-1) Wavenumber (cm^-1)

0.05

Pt/ZrO₂/TiO₂ Pt/ZrO₂/TiO₂

Fig.9. OH/ NHstretchingregionoftheinsituFTIRspectracorrespondingtoNO2adsorptionandsaturation(5.0TorrNO2(g)for10minat323K)followedbysubsequent reductionwith15.0TorrofH2(g)onPt/ZrO2/TiO2at323Kduring(a)0–40minofreductionand(b)50–120minofreduction.Bold(red)spectrumineachpanelcorresponds tothelastspectrumobtainedattheendofthegiventimeinterval.(Forinterpretationofthereferencestocolorinthisfigurelegend,thereaderisreferredtothewebversion ofthearticle.)

withnitratespeciesonthePt/ZrO₂/TiO₂ surfaceinthefirsttime intervalcanbefollowedbythegraduallyemergingvibrationalfre- quenciesat3350and3256cm⁻¹ relatedto NHstretchingsand thegrowing featureat 3512cm⁻¹ related toH-bonded surface

hydroxyls[43,44].Thereforeinthefirsttimeinterval(0–40min), itisapparentthatthenitratereductionmechanisminvolvescon- versionofbridgingnitratesintomonodentatenitratesandnitrites togetherwiththeformationofH-bondedsurfacehydroxylgroups

3800 3600 3400 3200 3000 3800 3600 3400 3200 3000

3719 3678 3515 3365 3278

0.1

(a)

0-80 minutes

3719 3515 3365 3278

0.1

(b)

90-120 minutes

Absorbance (arb. u.)

Wavenumber (cm^-1) Wavenumber (cm^-1)

Pt/Al₂O₃/ZrO₂/TiO₂ Pt/Al₂O₃/ZrO₂/TiO₂

Fig.10.OH/–NHstretchingregionoftheinsituFTIRspectracorrespondingtoNO2adsorptionandsaturation(5.0TorrNO2(g)for10minat323K)followedbysubsequent reductionwith15.0TorrofH2(g)onPt/Al2O3/ZrO2/TiO2at323Kduring(a)0–80minofreductionand(b)90–120minofreduction.Bold(red)spectrumineachpanel correspondstothelastspectrumobtainedattheendofthegiventimeinterval.(Forinterpretationofthereferencestocolorinthisfigurelegend,thereaderisreferredto thewebversionofthearticle.)

222 5

(a) (b)

Absorbance (arb. u.)

Wavenumber (cm

) Wavenumber (cm

) Pt/ZrO

/TiO

Pt/Al

O

/ZrO

/TiO

222 5

1 min 120 min

1 min 120 min

0.005 0.005

2225 2125 2225 2125

Fig.11. Time-dependentinsitugasphaseFTIRspectracorrespondingtonitrate reductionvia15.0TorrofH2(g)on(a)Pt/ZrO2/TiO2and(b)Pt/Al2O3/ZrO2/TiO2at 323Kfor120min.Redspectrumineachpanelcorrespondstothelastspectrum obtainedattheendofthegiventimeinterval.(Forinterpretationofthereferences tocolorinthisfigurelegend,thereaderisreferredtothewebversionofthearticle.)

and NH functionalitieswhich mightbeassociatedwith NH₂, NH₃, OH···NHxor OH···NOxspecies[8,43,44].

InsituFTIRdatapresentedinFig.9bshowsadditionalsignif- icantspectralchangesin thecourseofthesecondtime interval (50–120min)of reduction, during which a largeportionofthe nitrategroupswerereducedbyH2.Duringthisperiod,whilethe isolatedterminalandbridging OHspecies(initiallypresentnega- tivefeaturesat3722and3678cm⁻¹)wereregenerated,thefeature at3512cm⁻¹relatedtoH-bondedsurfacehydroxylspeciesdimin- ished.Inotherwords,asthenitrateswereeliminatedfromthe surface, OH···NHxor OH···NOxinteractionsseizedtoexistand someofthesurface OHfunctionalitieswereconvertedintoiso- latedhydroxyls.

Fig.10illustratesthesamesetofexperimentsperformedonthe Pt/AZTsystem.SimilartoPt/ZT,inthefirsttimeintervalofH₂(g) interaction(0–80min) withtheNOx-saturated surfacerevealsa gradualincreaseofthevibrationalintensitiesat3515cm⁻¹related toH-bondedhydroxylspeciesandnewlyformedfeaturesat3365 and3278cm⁻¹relatedto NHxstretchingsindicatingamechanism ofNOxreductionoverPt/AZTwhichissimilartothatofPt/ZT.We shouldalsonotethattheformationof NHxspecies,whichisan indicationofnitratereduction,occursatalatertimeonthePt/AZT system(Fig.10a)comparedtoPt/ZT(Fig.9a).Thisisinverygood agreementwiththecurrentFTIRandTPDresultsdiscussedearlier indicatingthatnitratespeciesonthePt/AZTternaryoxidesystem possessahigherstabilityandahigherresilienceagainstreduction withrespecttothatofthePt/ZTbinaryoxidesurface.Thisargument isalsoinlinewiththeobservationthatunlikethePt/ZTbinaryoxide

system(Fig.9b),nospectralchangeswereobservedforthePt/AZT ternaryoxidesurface(Fig.10b)after80minofreduction.

3.4. MonitoringtheNOxreductionproductsinthegasphase

GasphaseproductsformedduringtheNOxreductionviaH₂(g) overPt/ZTandPt/AZTsurfaceswerealsomonitoredbymeansof gas-phaseinsituFTIRspectroscopy.Inthesesetofexperiments, catalyst sampleswerelifted abovetheIRbeamin thespectro- scopicreactor,sothattheincomingIRphotonswereonlyabsorbed bythegasphase species.Abackgroundspectrumwasobtained immediatelyaftertheintroductionoftheH₂(g)intothereactor overtheNO₂-saturatedcatalystsurfaces.Allgas-phaseinsituFTIR spectragiveninFig.11wereacquiredusingtheaforementioned backgroundspectra.Theevolutionofgas-phasereductionproducts fromPt/ZTandPt/AZTsurfacesover120minisshowninFig.11a andb,respectively.Thegasphasespectraforbothmaterialsreveal theimmediateformationofN₂O(g)(i.e.2225cm⁻¹feature)even afterthefirstminuteofH₂(g)exposure,indicatingthatN₂Oisan earlyintermediate/byproductofthenitratereductionmechanism overPt/ZTandPt/AZTsystems.Itisworthmentioningthatother possiblegasphasereductionfeaturesinadditiontoN₂O(g)suchas NH3(g),waselusivetodetectingas-phaseFTIRspectroscopydueto theoverwhelmingintensityoftheH₂O(g)absorptionenvelopeat 1620cm⁻¹whichappearedinthespectraastheIRbeamtraveled throughambientconditionsafterexitingthespectroscopicreactor.

4. Conclusions

In the current work, binary and ternary oxide materials, ZrO2/TiO2 (ZT) and Al2O3/ZrO2/TiO2 (AZT), as wellas their Pt- functionalized counterparts were synthesized by the sol–gel methodandcharacterizedbymeansofXRD,Ramanspectroscopy andBETtechniques.TheNOxstoragecapacity(NSC)andreduction performanceofeachmaterialwereinvestigatedandthecharac- teristicbehaviorsofsurfacenitratefunctionalgroupsuponH₂(g) interactionweremonitoredviainsituFTIRandTPDanalysis.Our findingscanbesummarizedasfollows:

• IntheZTbinaryoxidesystem,astronginteractionbetweenTiO₂ andZrO2domainswereobservedathightemperatures(>973K), whichresultedintheformationofahighlyorderedcrystalline ZrTiO₄phaseandalowspecificsurfacearea(i.e.26m²/gat973K).

• IncorporationofAl2O3 intheAZTstructurerendersthemate- rialhighlyresilienttowardcrystallization andorderingwhere AZTmaterialwasfoundtobemostlyamorphousevenat1173K.

Moreover,aluminaactsasadiffusionbarrierintheAZTstruc- ture,preventingtheformationofZrTiO4andleadingtoavery highspecificsurfacearea(i.e.264m²/gat973K).

• NOxadsorptioncapabilityoftheAZTternaryoxidesystemwas foundtobesignificantlygreaterthanthatofZT,inlinewiththe almostten-foldgreaterSSAoftheformersurface.

• The interactionof NO₂(g) withZT and AZT surfacesrevealed adsorbednitratespecieswithsimilargeometries.WhilePtincor- porationdidnotalterthetypeoftheadsorbednitratespecies,it significantlyboostedtheNOxadsorptionamountonbothPt/ZT andPt/AZTsystems.Thermalstabilityofnitrateswashigheron theAZTcomparedtoZT,mostlikelyduetothedefectivestruc- tureandthepresenceofcoordinativelyunsaturatedsitesonthe formersurface.

• Ptsiteswerefoundtoassistthepartialordering/crystallization oftheAZTsystem.Ptsiteswerealsoobservedtofacilitatethe decompositionofnitratesintheabsenceofanexternalreduc- ingagentbyshiftingthedecompositiontemperaturestolower valuesandbyboostingtheformationofN₂andN₂O.