ContentslistsavailableatScienceDirect
Catalysis
Today
jo u rn al h om ep a g e :w w w . e l s e v i e r . c o m / l o c a t e / c a t t o d
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),aswellastheir
Pt-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).NO
xadsorptionontheAZTsystemwasfoundtobesignificantly
greaterthanthatofZT,duetoalmostten-foldgreaterSSAoftheformersurface.WhilePtincorporation didnotalterthetypeoftheadsorbednitratespecies,itsignificantlyboostedtheNOxadsorptiononboth
Pt/ZTandPt/AZTsystems.ThermalstabilityofnitrateswashigherontheAZTcomparedtoZT,mostlikely duetothedefectivestructureandthepresenceofcoordinativelyunsaturatedsitesontheformer sur-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-way cat-alystsare not efficient under lean conditions, NOx storage and
reduction(NSR)catalystshavebeendevelopedbyToyotaMotor Companyasapromisingafter-treatmentprocess[1,2].The oper-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
oxideanddemonstratedthatthesulfuruptakeandrelease proper-tiesoftheTiO2-promotedBaO/Al2O3materialsweresignificantly
enhanced[13].Matsumotoetal.[14]alsoreportedthatTiO2could
beusedasapromoteragainstsulfurpoisoningduetoitshigh acid-ity.However,ithasbeenalsoreportedthattitaniacanreadilylose itsfunctionalityduetothermaldeteriorationathightemperatures andvarioussolid-phasereactionsbetweenNOxstoragedomains,
promotersandthesupportmaterial[11,12,15].Therefore,ZrO2is
typicallyusedtogetherwithTiO2inanattempttostabilizethe
tita-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.
tolerance,materialpreparation,NOxstoragecapacityandthermal
stabilityaspects.Takahashietal.[20]investigatedtheinfluenceof therelativeabundanceofTiO2andZrO2componentsandfoundthat
theNSRsystemthatwascomprisedof70wt%ZrO2and30wt%TiO2
exhibitedthebestsulfurtolerance.Imagawaetal.[21,22]reported adetailedcharacterizationstudyrelatedtotheAl2O3/ZrO2/TiO2
ternaryoxidesystememphasizingtheimpactofAl2O3
incorpora-tionintotheZrO2/TiO2 mixedoxidewhereitwasindicatedthat
nano-compositeAl2O3/ZrO2/TiO2hadahigherNOxstorage
capac-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.Forthe syn-thesisofthebinaryoxides,zirconiumpropoxide(SigmaAldrich, ACSReagent,70wt%in1-propanol)andtitanium(IV)isopropoxide (SigmaAldrich,ACSReagent,97%)precursorswereinitially dis-solvedin100mlof2-propanol(SigmaAldrich,ACSReagent>99.5%) andstirred for40minunder ambientconditions.Thisstepwas followedby thedrop-wiseadditionof3mLof0.5Mnitricacid solution(SigmaAldrich,ACSReagent,65%)in ordertoobtaina gel.Similarly,thesynthesisoftheternaryoxidewascarriedout bymixingzirconiumisopropoxide,titaniumisopropoxideand alu-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,the mate-rialswere driedunderambientconditions for48hfollowed by calcinationinairwithin323–1173K.Relativecompositionofthe ternaryoxidesystem(i.e.Al2O3/ZrO2/TiO2)bymasswas50:35:15 [20].1wt%platinum-incorporatedbinaryandternaryoxide mate-rialsweresynthesizedbyincipientwetnessimpregnationmethod usingasolutionofPt(NH3)2(NO2)2 (Aldrich,
diamminedinitrito-platinum(II),3.4wt% solutionin diluteNH3(aq)). PriortothePt
addition,ZrO2/TiO2andAl2O3/ZrO2/TiO2wereinitiallycalcinedin
airat773Kfor150mininordertoremovetheorganic function-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 inthe cur-rentlypresented experimentsinvolvingX-raydiffraction(XRD), Ramanspectroscopy,BETsurfaceareaanalysis,temperature pro-grammeddesorption(TPD)andinsituFTIRcanbefoundelsewhere [8].Briefly, in situ FTIRspectroscopic measurementswere per-formed in transmission mode using a Bruker Tensor 27 FTIR spectrometerwhichwasmodifiedtohouseabatch-type spectro-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)forthecompletereductionandremovalofadsorbedNOx.
AlloftheFTIRspectragiveninthisstudywereobtainedat323K. InTPDexperiments,eachsamplewassaturatedbyNO2(g)as
describedabove.Subsequently,saturatedmaterialswereheated upto973Kwithalinearheatingrateof12K/mininvacuum.FTIR spectraofthecorrespondingsurfaceswerealsorecordedbefore andaftertheTPDexperiments.
3. Resultsanddiscussion
3.1. Structuralcharacterizationofthesynthesizedmaterials Fig.1illustratesexsituXRDanalysisfortheZTandAZT materi-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 oxidesystembehavesquite differ-entlycomparedtothebinaryoxide.AZTpreserveditsamorphous natureupto973K withoutrevealing anywell-resolved diffrac-tionsignals.Alongtheselines,AZTshowedonlypoorlydiscernible diffractionsignalsat2=30.48◦,50.50◦,60.91◦correspondingto tetragonalZrO2(JCPDS80-2155)at1173K.Apparently,the
addi-tion of an alumina component to the ZT system elevates the ordering/crystallizationtemperaturesandsuppressesthe forma-tionofZrTiO4 (JCPDS 34-415).It isworthmentioningthatXRD
analysiswasalsoperformedforthePt/ZTand Pt/AZT materials (SupportingInformationFig.1),whereitwasobservedthatPt addi-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 ( 250 0 250 0 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,Ramanspectra correspond-ingtotheAZTternaryoxidesample(Fig.2b)revealedmuchweaker signalsduetothepoorcrystallinityandthelackofatomicorderin thelattersample.InFig.2b,threeweakRamanfeatureslocatedat 320,465and625cm−1arevisiblewhichcanbeattributedtothe tetragonalzirconiaphase.Thisrevealsthatbesidesthetetragonal ZrO2phase,XRDandRamandatadonotprovideanyclear
indi-cationsforthepresenceofadditionalorderedphasesintheAZT system,suchasZrTiO4.Theseobservationsareinlinewith
stud-iesreportedbyEscobaretal.[27]whosuggestedthatintheZT binarysystemZrO2canprovideZr4+ionswhichmaydiffuseinto
theTiO2latticeintheabsenceofaluminumoxide,formingZrTiO4.
However,intheAZTternarysystem,XRDandRamandatagiven inFigs.1and2suggestthatAl2O3actsasadiffusionbarrier
pre-ventingdiffusionofZr4+ionsintotheTiO
2lattice,preventingthe
orderingofthecrystallatticeandtheformationZrTiO4.
Fig.3presentstheBETspecificsurfacearea(SSA)valuesforthe synthesizedmaterialswhichwerecalcinedat773,973and1173K. Fig.3indicatesthattheAZTternaryoxidesystemhasamuchhigher
surfaceareacomparedtotheZTbinaryoxidesystemforall calci-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-compositeternaryoxide
mate-rialwithaSSA∼200m2/gwhichwasreportedinarecentstudy [20].BETSSAanalysiswasalsoperformedforPt-containing sam-pleswhichwerepreparedasdescribedintheexperimentalsection. ThesemeasurementsshowedthatalthoughPtadditionand subse-quentcalcinationat973Kdidnothaveasignificantinfluenceon theSSAvaluesoftheZTsystem(26m2/gversus37m2/gforZT
andPt/ZT,respectively),SSAvaluesoftheAZTsystemsignificantly decreasedinthepresenceofPt(264m2/gversus191m2/gforAZT
200 400 600 800 x3 200 400 600 800 13 5 259 32 0 39 1 46 7 62 2 77 8 20 0 Raman Shift (cm-1) 46 5 62 5 32 0 Raman Shift (cm-1) 5 973K 1173K Intensity (arb. u)
ZrO2/TiO2 Al2O3/ZrO2/TiO2
(a) (b)
ZrTiO4 t-ZrO2
ZrTiO4 t-ZrO2
1200 1100 1000 900 800 0 50 100 150 200 250 300 350 400 264 342 11 3 17 7 26 3 SBE T (m 2/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 sitesfacilitatetheoxidationoftheAZTmatrixduringthe calcina-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.
MorequantitativeinsightintothetotalNOxadsorption
capabil-ityandthermalstabilityofadsorbednitratespeciescanbeobtained bycombining FTIRand TPDresults. TPD profilesrelated tothe ZTandAZTsystemsobtainedafterNO2(g)saturation(5.0Torrfor
10min)at323KarepresentedinFig.5aandb,respectively. Com-parisonoftheTPDlineshapesforZTandAZTsystemsimmediately revealsthedissimilarNOxdesorptioncharacteristicsoneach
mate-rial.WhilemostoftheNOxdesorptioniscompletedbelow800K
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,thehigh tem-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 164 4 157 8 155 0 128 0 120 9 163 9 158 2 155 5 124 4 128 3 ) 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
correspondstotheNOx-saturated(5.0TorrNO2(g)for10minat323K)surface.(Forinterpretationofthereferencestocolorinthisfigurelegend,thereaderisreferredtothe
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,O2andNO2witha
rel-ativelyinsignificantcontributionfromotherdesorptionchannels. TheseTPDresultsareinperfectagreementwiththe correspond-inginsituFTIRdatapresentedasinsetsinFig.5aandb.Inthese insets,blackcurvescorrespondtoNO2-saturatedsurfacesbefore
theTPDexperiments,whiletheredcurvescorrespondtotheZTand AZTsurfacesaftertheTPDanalysis(i.e.afterannealingunder vac-uumat973K).FTIRdataclearlyshowthatwhilecompletenitrate eliminationwasachievedonZrO2/TiO2aftertheTPDexperiment,
asignificantamountofnitratespeciesobservedtoberemainingon theAZTsurfaceaftertheTPDrun.
TheeffectofPtincorporationonNOxadsorptiongeometryand
storagecapacitywasalsoinvestigatedviainsituFTIRspectroscopy and TPD. FTIR studies involving NOx adsorption experiments
onPt/ZrO2/TiO2andPt/Al2O3/ZrO2/TiO2(SupportingInformation Fig.2)leadustoconcludethatthepresenceofPthasno signif-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 suggeststhatPtadditionsignificantlyincreasestherelative desorp-tionofN2andN2OforthePt/ZrO2/TiO2.Thiscanpossiblybedue
tothePt-catalyzeddirectpartial/totalreductionoftheadsorbed nitratesonthePt/ZrO2/TiO2sampleintoN2andN2O,particularly
at755K.LackofasignificantO2desorptionsignalat755K,
indi-catesthattheoxidizingspeciessuchasatomicoxygenordiatomic oxygengeneratedduringthepartial/totalreductionofnitratesare possiblycapturedbythePt/ZrO2/TiO2systemwheretheymay
oxi-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).Notethatthese num-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.Theseparticlesarepossiblyformedduringthe
calci-nationstep,wherePtsitesoxidizetheAZTsurface,increasingthe crystallographicorderofthesurfacetoacertainextent,whichis elusivetodetectviabulkcharacterizationtechniquessuchasXRD (SupportingInformationFig.1).PresenceofAl2O3domainsonthe
Pt/AZTsurfaceisalsosupportedbytheexistenceoftheNO desorp-tionshoulderat400–450KinFig.6b(concomitanttoaNO2signal
whichalsoappearsatthesametemperaturewindow)whichisa characteristicfeatureofNO2TPDfrom␥-Al2O3[32]Althoughthis
400KNOxdesorptionfeatureisabsentforZTandPt/ZTsamples
(Figs.5aand6),itisvisible(thoughwithamuchsmallerintensity) fortheAZTsample,suggestingthatthealuminadomainsontheAZT samplearemuchlessfullyoxidized/ordered.SincetheNOx
desorp-tion/decompositionwasincompletefortheAZTsamplewithinthe thermalwindowoftheTPDanalysis,itisdifficulttomakea quanti-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;NOxdesorptionat730Ktakesplacewiththeevolutionof
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 nitratedecompositionperformanceoftheAZTternaryoxide sys-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/Al
2O3 hasa
SSAof125m2/g.Ontheotherhand,althoughPt/20wt%BaO/Al 2O3
hasalowersurfacearea,ithas19%greaterintegratedtotalNOx
desorptionsignalthanPt/AZT70obtainedviaidenticaladsorption experiments(datanotshown).InfluenceoftheBaOdomainsonthe NOxadsorptionandreleasepropertiesofZTandAZTsystemsisan
interestingaspectwhichwillbediscussedindetailinaforthcoming report[33].
3.3. NOxreductiononbinaryandternaryoxidesystemsviaH2(g)
AswellastheNOxstoragecharacteristicsofZTandAZT
materi-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-saturated
sur-facesat373,473, 573,623,673,723KinthepresenceofH2(g).
AdsorbednitratesonZT(Fig.7a)fullysurvivedunderreducing con-ditionsupto473KwithonlyminorIRintensitychanges.However, theIRsignalintensitiesstartedtodecreaseat573K,where sig-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 164 9 158 2 155 4 128 0 120 6 164 7 158 3 155 4 123 8 128 1 ) 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
1800
160
0
140
0
120
0
100
0
180
0
160
0
140
0
120
0
100
0
164 6 158 0 15 11 129 1 120 5(a) Pt/ZrO2/TiO2
Absorbance (arb. u.) Wavenumber (cm-1) Wavenumber (cm-1) 142 2 1st 2nd 3rd 0-40 min 50-120 min 373-473K 0.25 164 3 158 2 15 11 129 9 123 2
(b) Pt/Al2O3/ZrO2/TiO2
14 11 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 temporalchangesinthesurfacefunctionalgroupsonmaterial sur-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].Another grow-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 removalofallNOxspeciesaround473K.
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-dependentreduction exper-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).Whiletheformer fea-turehasbeenassignedtolinear(type-I) OHspecies,thelatterone ischaracteristicsforthree-fold(type-III)hydroxyls[35–42]which disappear from the surface upon interaction/coordination with adsorbednitratesand nitrites.Anotherbroadandhighly convo-lutedfeaturecenteredat3512cm−1canbeattributedtoH-bonded surfacehydroxylspecies[38,39].InFig.9a,theinteractionofH2
3800
360
0
340
0
320
0
300
0
3800
360
0
340
0
320
0
300
0
372 2 367 8 351 2 335 0 325 6 0.1 (a) 0-40 minutes 372 2 367 8 351 2 335 0 325 6 (b) 50-120 minutes Absorbance (arb. u.) Wavenumber (cm-1) Wavenumber (cm-1) 0.05Pt/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
intervalcanbefollowedbythegraduallyemergingvibrational fre-quenciesat3350and3256cm−1 relatedto NHstretchingsand thegrowing featureat 3512cm−1 related toH-bonded surface
hydroxyls[43,44].Thereforeinthefirsttimeinterval(0–40min), itisapparentthatthenitratereductionmechanisminvolves con-versionofbridgingnitratesintomonodentatenitratesandnitrites togetherwiththeformationofH-bondedsurfacehydroxylgroups
3800
360
0
340
0
320
0
300
0 3800
360
0
340
0
320
0
300
0
371 9 367 8 351 5 336 5 327 8 0.1 (a) 0-80 minutes 371 9 351 5 336 5 327 8 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
222
5
(a)
(b)
Absorbance
(arb.
u.)
Wavenumber
(cm
-1)
Wavenumber
(cm
-1)
Pt/ZrO
2/TiO
2Pt/Al
2O
3/ZrO
2/TiO
2222
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.9bshowsadditional signif-icantspectralchangesin thecourseofthesecondtime interval (50–120min)of reduction, during which a largeportionofthe nitrategroupswerereducedbyH2.Duringthisperiod,whilethe
isolatedterminalandbridging OHspecies(initiallypresent nega-tivefeaturesat3722and3678cm−1)wereregenerated,thefeature at3512cm−1relatedtoH-bondedsurfacehydroxylspecies dimin-ished.Inotherwords,asthenitrateswereeliminatedfromthe surface, OH···NHxor OH···NOxinteractionsseizedtoexistand
someofthesurface OHfunctionalitieswereconvertedinto iso-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 the spectro-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
performanceofeachmaterialwereinvestigatedandthe charac-teristicbehaviorsofsurfacenitratefunctionalgroupsuponH2(g)
interactionweremonitoredviainsituFTIRandTPDanalysis.Our findingscanbesummarizedasfollows:
• IntheZTbinaryoxidesystem,astronginteractionbetweenTiO2
andZrO2domainswereobservedathightemperatures(>973K),
whichresultedintheformationofahighlyorderedcrystalline ZrTiO4phaseandalowspecificsurfacearea(i.e.26m2/gat973K).
• IncorporationofAl2O3 intheAZTstructurerendersthe
mate-rialhighlyresilienttowardcrystallization andorderingwhere AZTmaterialwasfoundtobemostlyamorphousevenat1173K. Moreover,aluminaactsasadiffusionbarrierintheAZT struc-ture,preventingtheformationofZrTiO4andleadingtoavery
highspecificsurfacearea(i.e.264m2/gat973K).
• NOxadsorptioncapabilityoftheAZTternaryoxidesystemwas
foundtobesignificantlygreaterthanthatofZT,inlinewiththe almostten-foldgreaterSSAoftheformersurface.
• The interactionof NO2(g) withZT and AZT surfacesrevealed
adsorbednitratespecieswithsimilargeometries.WhilePt incor-porationdidnotalterthetypeoftheadsorbednitratespecies,it significantlyboostedtheNOxadsorptionamountonbothPt/ZT
andPt/AZTsystems.Thermalstabilityofnitrateswashigheron theAZTcomparedtoZT,mostlikelyduetothedefective struc-tureandthepresenceofcoordinativelyunsaturatedsitesonthe formersurface.
• Ptsiteswerefoundtoassistthepartialordering/crystallization oftheAZTsystem.Ptsiteswerealsoobservedtofacilitatethe decompositionofnitratesintheabsenceofanexternal reduc-ingagentbyshiftingthedecompositiontemperaturestolower valuesandbyboostingtheformationofN2andN2O.
• In the presence of H2(g), the complete reduction of surface
nitrateswasachievedat623KonZT,whilethis wasachieved at723KforAZT.
• Nitratereduction overPt/ZT andPt/AZT viaH2(g)under mild
conditionsinitiallyleadstotheconversionofbridgingnitrates intomonodentatenitratesandnitritestogetherwiththe forma-tionofsurface OHand NHxfunctionalities.N2O(g)wasalso
observedintheveryearlystagesofthereductionprocessasan initialintermediate/byproduct.
AppendixA. Supplementarydata
Supplementarymaterialrelated tothis article canbefound, in the online version, at http://dx.doi.org/10.1016/j.cattod. 2013.12.037.
References
[1]N.Miyoshi,S.Matsumoto,K.Katoh,T.Tanaka,J.Harada,N.Takahashi,K.Yokota, M.Sugiura,K.Kashahara,SAEPaper950809,1995.
[2]N.Takahashi,H.Shinjoh,T.Iijima,T.Suzuki,K.Yamazaki,K.Yokota,H.Suzuki,N. Miyoshi,S.Matsumoto,T.Tanizawa,T.Tanaka,S.Tateishi,K.Kasahara,Catalysis Today27(1996)63.
[3]S.Roy,A.Baiker,ChemicalReviews109(2009)4054.
[4]W.S.Epling,L.E.Campbell,A.Yezerets,N.W.Currier,J.E.Parks,CatalysisReviews 46(2004)163.
[5]E.Ozensoy,J.Szanyi,C.H.F.Peden,JournalofCatalysis243(2006)149. [6]E.I.Vovk,E.Emmez,M.Erbudak,V.I.Bukhtiyarov,E.Ozensoy,JournalofPhysical
ChemistryC115(2011)24256.
[7]E.I.Vovk,A.Turksoy,V.I.Bukhtiyarov,E.Ozensoy,JournalofPhysicalChemistry C117(2013)7713.
[8]Z.Say,E.I.Vovk,V.I.Bukhtiyarov,E.Ozensoy,AppliedCatalysisB: Environmen-tal142–143(2013)89.
[9]R.Hummatov,D.Toffoli,O.Gulseren,E.Ozensoy,H.Ustunel,JournalofPhysical ChemistryC116(2012)6191.
[10]Z.Say,E.I.Vovk,V.I.Bukhtiyarov,E.Ozensoy,TopicsinCatalysis56(2013)950. [11]S.M.Andonova,G.S.Senturk,E.Kayhan,E.Ozensoy,JournalofPhysical
Chem-istryC113(2009)11014.
[12]E.Emmez,E.I.Vovk,V.I.Bukhtiyarov,E.Ozensoy,JournalofPhysicalChemistry C115(2011)22438.
[13]G.S.Senturk,E.I.Vovk,V.I.Zaikovskii,Z.Say,A.M.Soylu,V.I.Bukhtiyarov,E. Ozensoy,CatalysisToday184(2012)54.
[14]S.Matsumoto,Y.Ikeda,H.Suzuki,M.Ogai,N.Miyoshi,AppliedCatalysisB: Environmental25(2000)115.
[15]S.M.Andonova,G.S.Senturk,E.Ozensoy,JournalofPhysicalChemistryC114 (2010)17003.
[16]T.Kanazawa,CatalysisToday96(2004)171.
[17]K.Ito,S.Kakino,K.Ikeue,M.Machida,AppliedCatalysisB:Environmental74 (2007)137.
[18]K.Arata,Akutagawa,K.Tanabe,BulletinoftheChemicalSocietyofJapan49 (1976)390.
[19]I.Wang,W.F.Chang,R.J.Shiau,J.C.Wu,C.S.Chung,JournalofCatalysis83(1983) 428.
[20]N.Takahashi,A.Suda,I.Hachisuka,M.Sugiura,H.Sobukawa,H.Shinjoh,Applied CatalysisB:Environmental72(2007)187.
[21]H.Imagawa,T.Tanaka,N.Takahashi,S.Matsunaga,A.Suda,H.Shinjoh,Journal ofCatalysis251(2007)315.
[22]H.Imagawa,N.Takahashi,T.Tanaka,S.Matsunaga,H.Shinjoh,AppliedCatalysis B:Environmental92(2009)23.
[23]M.A.Krebs,R.A.Condrate,JournalofMaterialsScienceLetters7(1988)1327. [24]F.Azough,R.Freer,J.Petzelt,JournalofMaterialsScience28(1993)2273. [25]V.Lughi,D.R.Clarke,JournalofAppliedPhysics101(5)(2007)053524. [26]M.T.Colomer,M.Maczka,JournalofSolidStateChemistry184(2011)365. [27]C.A.Ulin,J.A.deLosReyes,J.Escobar,M.C.Barrera,M.A.Cortes-Jacome,Journal
ofPhysicsandChemistryofSolids71(2010)1004.
[28]K.Hadjiivanov,V.Bushev,M.Kantcheva,D.Klissurski,Langmuir10(1994)464. [29]M.Kantcheva,AppliedCatalysisB:Environmental42(2003)89.
[30]M.Kantcheva,V.S.Vakkasoglu,JournalofCatalysis223(2004)352. [31]M.Kantcheva,JournalofCatalysis204(2001)479.
[32]E.Kayhan,S.M.Andonova,G.S.Senturk,C.C.Chusuei,E.Ozensoy,Journalof PhysicalChemistryC114(2010)357.
[33]Z.Say,M.Tohumeken,E.Vovk,E.Ozensoy,inpreparation.
[34]T.Szailer,J.H.Kwak,D.H.Kim,J.C.Hanson,C.H.F.Peden,J.Szanyi,Journalof Catalysis239(2006)51.
[35]M.Kantcheva,E.Z.Ciftlikli,JournalofPhysicalChemistryB106(2002)3941. [36]K.T.Jung,A.T.Bell,JournalofMolecularCatalysisA163(2000)27.
[37]G.Cerrato,S.Bordiga,S.Barbera,C.Morterra,AppliedSurfaceScience115 (1997)53.
[38]E.Ozensoy,D.Herling,J.Szanyi,CatalysisToday136(2008)46. [39]G.Busca,CatalysisToday27(1996)457.
[40]G.Busca,CatalysisToday41(1998)191.
[41]D.Dautzenberg,H.Knozinger,JournalofCatalysis33(1974)142. [42]H.Knozinger,K.Kochloefl,W.Meye,JournalofCatalysis28(1973)69. [43]A.Davydov,MolecularSpectroscopyofOxideCatalystSurfaces,Wiley,USA,
2003,pp.81–83.
[44]T.A.Gordymova,A.A.Davydov,JournalofAppliedSpectroscopy39(1983) 621.
