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Influence of ceria on the NOx reduction performance of NOx storage reduction catalysts

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ContentslistsavailableatSciVerseScienceDirect

Applied

Catalysis

B:

Environmental

j ou rn a l h om epa g e : w w w . e l s e v i e r . c o m / l o c a t e / a p c a t b

Influence

of

ceria

on

the

NO

x

reduction

performance

of

NO

x

storage

reduction

catalysts

Zafer

Say

a

,

Evgeny

I.

Vovk

a,b

,

Valerii

I.

Bukhtiyarov

b

,

Emrah

Ozensoy

a,∗

aDepartmentofChemistry,BilkentUniversity,06800Ankara,Turkey bBoreskovInstituteofCatalysis,630090Novosibirsk,RussianFederation

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received23February2013

Receivedinrevisedform26April2013 Accepted30April2013

Available online 8 May 2013 Keywords: CeO2 BaO Pt,NOx NSR.

a

b

s

t

r

a

c

t

InfluenceofceriaontheNOxstorageandreductionbehaviorofNSRcatalystswasinvestigatedina

systematicmannerover␥-Al2O3,Ba/Al,Ce/Al,Ba/Ce/Al,Pt/Al,Pt/Ce/AlandBa/Pt/Ce/AlsystemsusingBET,

XRD,RamanspectroscopyandinsituFTIR.Althoughceriapromotiondoesnotseemtohaveasubstantial

influenceontheoverallNOxstoragecapacity,itdoeshaveaclearlypositiveeffectontheNOxreduction

viaH2(g)duringcatalyticregenerationunderrichconditionswhichisassociatedwiththeenhancement

inthetotalamountofactivatedhydrogenonthecatalystsurfaceandloweringofthethermalthreshold

forhydrogenactivation.Astrongmetalsupportinteraction(SMSI)betweenPtsitesandtheBaOx/CeOx

domainsleadstoacomplexredoxinterplayincludingoxidationofthepreciousmetalsites,reductionof

ceria,formationofBaO2speciesaswellastheformationofPt–O–CeinterfacialsitesontheBa/Pt/Ce/Al

surface.CeriadomainsalsoactasanchoringsitesforPtspecies,limittheirsurfacediffusion,enhance

dispersionandhindersinteringatelevatedtemperatures.OntheBa/Pt/Ce/Alcatalystsurface,reduction

ofthestorednitratesunderrelativelymildconditionsviaH2(g)initiallyleadstotheformationofsurface

–OHand–NHxspeciesandgasphaseN2O,aswellasthedestructionofsurfacenitratespecies,leaving

bulknitratesmostlyintact.ReductionproceedswiththeconversionofN2O(g)intoN2(g)alongwiththe

partiallossofsurface–OHand–NHxgroups,dehydrationandthelossofbulknitrates.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

NOxemissionsfrommobilesourceshaveseriouslydestructive effects on the atmosphere, global ecosystem and especially on thehumanhealth.Duetotherigorousenvironmentalregulations, automotiveindustryiscompelledtosearchfornewtechnologies tominimizetheexhaustemissions.Three-waycatalysts(TWC)are commonlyusedfor thereductionoftoxicgasesinconventional gasolineengineswhichoperatewithanairtofuelratios(A/F)close to14.5.Forabetterfueleconomy,dieselandleanburngasoline engineshavebeenconsideredasattractivealternativesthatcan operateat higherA/F. For leanburn engines, apromising after treatmentmethodforthecatalyticNOxreductionistheNOx stor-age/reduction(NSR)catalysttechnology[1,2].NSRcatalystsconsist ofthreemaincomponents.ThesearetheNOxstoragecomponent basedonalkalineandalkalineearthoxides(e.g.K,Ba,Sr),precious metalsites(e.g.Pt,Pd,Rh)whichareresponsiblefortheoxidation andreduction,andthehighsurfaceareasupportmaterial(e.g. ␥-Al2O3)[3].␥-Al2O3isthemostcommonlyusedoxidesupportin NSRformulationsduetoitsporousstructure,highsurfaceareaas wellasitsdistinctchemical,mechanicalandthermalproperties

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

[4,5].FormerstudieshaveshownthatCeO2canbeusedeitherasa promoter[6]orasasupportmaterial[7]inTWCandNSRsystems. Theprimaryfunctionofceriastemsfromitsfavorableredox prop-ertiesaswellitshighoxygenstorageandtransportcapacity,which isassociatedwiththefaciletransitionbetweenthetwodifferent Ceoxidationstates(i.e.Ce3+Ce4+)andthepresenceofoxygen vacanciesinitslatticestructure.Ceriaisalsoaveryefficient pro-moterinthewater-gasshift(WGS)andsteamreformingreactions thatarealsorelevantforexhaustemissioncontrolsystems[8–11]. Inaddition,ceriahasbeenreportedtoenhancethepreciousmetal dispersion[12].

Theprimaryaimofthecurrentworkistostudyceria-promoted NSRcatalystsinordertoelucidatetheinfluenceofceriaontheNOx storageandreductionpathwaysatthemolecularlevel.Inorderto achievethisgoal,wefocusourattentionontheBa/Pt/Ce/AlNSR catalystsandelucidatethestructureandthecatalyticbehaviorof thiscomplexsystembysystematicallyanalyzingitsstructural sub-components,suchas␥-Al2O3,Ba/Al,Ce/Al,Ba/Ce/Al,Pt/Al,Pt/Ce/Al.

2. Experimental

2.1. Catalystsynthesis

BinaryCeO2/Al2O3(Ce/Al)materialsweresynthesizedby incip-ientwetnessimpregnationof␥-Al2O3(PURALOX,200m2/g,SASOL 0926-3373/$–seefrontmatter © 2013 Elsevier B.V. All rights reserved.

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GmbH,Germany)withCe(NO3)3·6H2O(>99.0%,Fluka,France) fol-lowedbycalcinationat823Kfor2hinair.Ce/Almaterial(denoted as20Ce/Al)waspreparedwitha20wt%CeO2loading.

The BaO/CeO2/Al2O3 (Ba/Ce/Al) ternary oxides with 8 and 20wt%BaOloadingweresynthesized bythefurther impregna-tionofthe20Ce/Alsamples(whichwereinitiallycalcinedat823K for2hinair)withaqueoussolutionsofbariumnitrate(Ba(NO3)2, ACS Reagent, ≥99%, Riedel-de Häen, Germany). Next, resulting Ba/Ce/Almaterialswere annealedinAr(g) flow atvarious tem-peratures within 323–1273K as described in the text. For the synthesisof 1wt%Pt-containingPt/AlandPt/20Ce/Almaterials, ␥-Al2O3 (asis)or20Ce/Al(whichwasinitiallycalcinedat823K for2hinair)wasimpregnatedwithasolutionofPt(NH3)2(NO2)2 (Aldrich,diamminedinitritoplatinum(II),3.4wt.%solutionindilute NH3(aq)).Afterimpregnation,Pt/AlandPt/20Ce/Alsampleswere annealedinAr(g)flowatvarioustemperatureswithin323–1273K. BaO/Pt/Al2O3 (20Ba/Pt/Al)samples with20wt% BaOloading wassynthesizedbytheimpregnationofthePt/Alsamples(which wereinitiallycalcinedat823Kfor2hrinair)withaBa(NO3)2 solu-tionfollowedbyannealinginAr(g)flowatvarioustemperatures within323–1273K.

Similarly,BaO/Pt/CeO2/Al2O3(Ba/Pt/20Ce/Al)sampleswith8or 20wt%BaOloadingweresynthesizedbytheimpregnationofthe Pt/20Ce/Alsamples(whichwereinitiallycalcinedat823Kfor2h inair)withaBa(NO3)2solutionfollowedbyannealinginAr(g)flow atvarioustemperatureswithin323–1273K.

2.2. Instrumentation

TheXRDpatternswererecordedusingaRigakuMiniflex diffrac-tometer,equippedwithaMiniflexgoniometerandanX-raysource withCuK␣radiation(=1.54 ˚A,30kV,15mA).Thepowder sam-ples were pressed and affixed onto standard-sized glass slides and scanned within the 10–80◦, 2 range with a scan rate of 0.01◦s−1.DiffractionpatternswereassignedusingJoint Commit-teeonPowderDiffractionStandards(JCPDS)cardssuppliedbythe InternationalCentreforDiffractionDatabase(ICDD).

TheRamanspectrawererecordedunderambientconditions usingaHORIBAJobinYvonLabRamHR800instrument,equipped witha confocalRamanBX41microscope,spectrographwithan 800mmfocallengthandaCCDdetector.TheRaman spectrome-terwasequippedwithaNd:YAGlaser(=532.1nm).Whilethe Ramanexperimentswereconducted,thelaserpowerwastuned to20mW.Allofthepowdersamplesweremechanicallydispersed ontoasingle-crystalSiholder.Theincidentlightsourcewas dis-persedbyaholographicgratingwith600grooves/mmandfocused ontothesampleusinga×50objective.Theconfocalholeandslit entranceweresetat1100and200␮m,respectively.The spectrom-eterwasregularlycalibratedbyadjustingthezero-orderpositionof thegratingandcomparingthemeasuredSiRamanbandfrequency withthetypicalreferencevalueof520.7cm−1.AllRamanspectra wereacquiredwithin100–4000cm−1withanacquisitiontimeof 213sandaspectralresolutionof4cm−1.

BET specific surface area (SSA) data were recorded usinga MicromeriticsTristar3000surfaceareaandporesizeanalyzervia N2(g)adsorptionat77K.PriortotheBETmeasurements,materials wereoutgassedinvacuumat623Kfor2h.

FTIRspectroscopicmeasurementswere carriedout in trans-missionmode,inabatch-typecatalyticreactorcoupledtoaFTIR spectrometer(BrukerTensor27).AlloftheFTIRspectrawere col-lectedat323K.BeforetheFTIRexperiments,allofthesamples wereactivatedintheIRcellbyexposingthesamplesurfacesto 2TorrNO2(g)for5minat323Kfollowedbyannealinginvacuum (<10−3Torr)at973K.NO2 adsorptiononthesynthesized mate-rialswascarriedouttypicallybydosing5.0TorrNO2(g)overthe samplesurfacefor10minatrelevanttemperatures(i.e.323,473,

523,573or623K).AftertheNO2exposure,thereactorwas evac-uatedandthesamplewascooledto323K.FortheNOxreduction experimentsviaH2(g)(LindeGmbH,Germany,>99.9%),15.0Torrof H2(g)wasintroducedintothereactorat323Kandthesamplewas heatedusingaconstantheatingrateof12K/minuntilthedesired temperaturewasreached.Next,sampleswerekeptinH2(g)atthis temperaturefor30mininordertoletthereductionproceed.

NO2(g)usedintheexperimentswerepreparedbymixingNO(g) (AirProducts,99.9%)andO2(g)(LindeGmbH,Germany,99.999%) followedbymultiplefreeze-pump-thawcyclesforfurther purifi-cation.

Temperatureprogrammedreduction(TPR)experimentswere performedatGeneralMotorsTechnicalCenter(Warren,MI)usinga MicromeriticsAutochemII2920analyzerequippedwithathermal conductivitydetector(TCD).PriortotheTPRmeasurements,each catalystsample(c.a.100mg)waspretreatedin10%O2inHefor 1hat400◦C.ThegasmixtureforTPRwascomprisedof10%ofH2 balancedwithAr,andthegasflowratewasfixedat50ml/min.The temperaturewasincreasedfromroomtemperatureto600◦Cwith aheatingrateof10◦C/minandtheeffluentstreamwasmonitored usingaTCD.

3. Resultsanddiscussion

3.1. Structuralcharacterizationofthesynthesizedsamplesvia BET,RamanspectroscopyandXRD

Fig.1showsBETspecificsurfacearea(SSA)valuesofthesamples aftercalcinationatvarioustemperatures.ItisvisibleinFig.1athat incorporationof␥-Al2O3(havingatypicalSSAof210m2/g)with CeO2and/orBaOdecreasestheSSA.Calcinationofthesynthesized samplesathighertemperaturesmonotonicallydecreasestheSSA whichcanbeassociatedwithorderingofthesurfacedomainsand sintering.ItisapparentinFig.1bthatSSAofPt/Alsamplerevealsa relativelyhigherthermalstabilitywithrespecttoallothersamples thatcontainBaOand/orCeO2.ComparisonoftheSSAvaluesofthe 20Ba/Pt/Alsamplewiththatofthe20Ba/Pt/20Ce/Alsamples sug-geststhatceriapromotionleadstoanoticeabledecreaseintheSSAs oftheNSRcatalystsaftercalcinationatelevatedtemperatures.

Fig. 2 illustrates the ex situ Raman spectra acquired after the calcination of the synthesized 20Ce/Al, Pt/20Ce/Al and 20Ba/Pt/20Ce/Alsamplesatvarioustemperaturesinordertofollow thestructuralevolutionofthesematerialsasaresultof composi-tionalvariationsandthermaleffects.ThemostdominantRaman bandobservedinFig.2appearsat460–465cm−1.This character-isticbandhasalsobeenpreviouslyobservedforpureCeO2 and assignedtotheRamanactiveF2gmodeoftheCeO2fluorite struc-ture[13].ItisalsoworthmentioningthatthemainCeO2 Raman signalalsodisplaysanasymmetrytowardlowfrequencies revea-lingashoulderatc.a.400cm−1whichcanbeattributedtodefects [14].On theotherhand,very weaksecondorder (A1g+Eg+F2g) Ramanscatteringfeaturesof CeO2 arebarelyvisibleat594and 1170cm−1forthe20Ce/Alsample(Fig.2a).Ramansignalslocated at740and∼1050cm−1inFig.2aandccorrespondtothebending andthesymmetricstretchingmodesofNO3−speciesoriginating fromthemetalnitrateprecursors(Ce(NO3)3·6H2OandBa(NO3)2 usedinthesynthesis,respectively[15].Thesenitratebands grad-uallydisappearafterelevatedtemperaturetreatmentsasaresult ofthedecompositionofmetalnitratesyieldingperoxide(O22−) specieswithacharacteristicRamansignalatc.a.830cm−1 [16]. Comparisonofthe623KspectrainFig.2a–crevealsthatinthe absenceofPtandBa,nitratesontheCe/Alsamplehavemoderate stabilityandonlypartiallydecomposeat623K.Ontheotherhand, onthePt/20Ce/Alsample(Fig.2b),presenceofPtsitesfacilitate thenitratedecompositionwhereallofthenitratespeciesonthe

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600 700 800 900 1000 1100 1200 1300 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 600 700 800 900 1000110012001300 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 20Ce/Al 20Ba/20Ce/Al Pt/Al 20Ba/Pt/Al 20Ba/Pt/20Ce/Al Temperature (K) Temperature (K) SBET (m 2/g) SBET (m 2/g)

a

b

Fig.1.BETspecificsurfaceareavaluesfortheanalyzedsamplesuponannealinginAr(g)flowwithin623-1273K.(a)20Ce/Al(black)and20Ba/20Ce/Al(red),(b)Pt/Al(black), 20Ba/Pt/Al(blue)and20Ba/Pt/20Ce/Al(red).(Forinterpretationofthereferencestocolorinthisfigurelegend,thereaderisreferredtothewebversionofthisarticle.)

Pt/20Ce/Alsurfacecanbecompletelydestroyedat623K. Mean-while,uponBa(NO3)2incorporationtothesystem(Fig.2c),dueto thehigherthermalstabilityofbariumnitrates,eveninthepresence ofPt,completion ofthenitratedecompositionprocess requires temperaturesgreaterthan1073K.Decompositionbehaviorofthe metalnitrateprecursorsobservedviacurrentRamanexperiments isinexcellentagreementwiththeXRDdatagiveninFig.3,where thedecomposition of theordered Ba(NO3)2 phase and the for-mationofanamorphousBaO/BaO2phaseonthe20Ba/Pt/20Ce/Al surface(Fig.3d)isclearlyvisible[17–19].Itisworthmentioning thatalthoughXRDdatasuggestthatallofthenitratespeciesonthe 20Ba/Pt/20Ce/Alsurfaceseemtobedecomposedat873K(Fig.3d), owingtothehighersensitivityoftheRamanspectroscopy, corre-spondingRamandata(Fig.2c)revealstheexistenceofnitratesat T≤1073K.

Probably,themostimportantsetofRamanfeaturesobservedin Fig.2arethebandsthatariseaftertheadditionofPttothe syn-thesizedmaterials.ItisseeninFig.2bandcthattheadditionof PtfollowedbyannealinginAr(g)flowatmoderatetemperatures (T≤873K)leadstotheappearanceofa broadandaconvoluted setofbandslocatedwithin500–700cm−1.Thesefeaturesbecome relatively sharper after elevatedtemperature treatments yield-ingsignalsat205,544, 582,and689cm−1.Basedontheformer Ramanspectroscopicstudieson␤-PtO2[20],205cm−1featurecan beassignedtotheRamanactiveAg(␻1)modeofPtOxand/orPtO2. Similarly,inthelightofthepreviousstudiesonPt/CeO2[21],544 and689cm−1 featurescanbeattributedtothePt–O–Cespecies (locatedatthePt/CeOxinterface)andaPt–Osignalassociatedwith surfacePtOx/PtO2 species,respectively.Theremaining582cm−1 featurecanbetentativelyassignedtoPt–O–Cespeciesand/orthe

200 400 600 800 1000 1200 1400 200 400 600 800 1000 1200 1400 200 400 600 800 1000 1200 1400

Raman Shift (cm-1) Raman Shift (cm-1) Raman Shift (cm-1)

In tens ity (a rb. u .) In tens ity (a rb. u .) In tens ity (a rb. u .) 25 8 39 8 117 0 105 0 72 3 59 4 20 5 31 7 39 3 54 4 58 2 68 6 83 0 20 5 25 2 39 3 54 4 58 2 692 83 0 105 4 1273K 1073K 873K 623K 1273K 1073K 873K 623K 1273K 1073K 873K 623K

a

b

c

50 50 50

20Ce/Al Pt/20Ce/Al 20Ba/Pt/20Ce/Al

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Fig.3.ExsituXRDpatternscorrespondingto(a)Pt/Al,(b)Pt/20Ce/Al,(c)20Ba/Pt/Aland(d)20Ba/Pt/20Ce/AlsamplesuponannealinginAr(g)flowwithin623–1273K.

secondorder(A1g+Eg+F2g)RamanscatteringfeaturesofCeO2.Itis worthmentioningthatmetallicPtisnotRamanactive[21].Thus, metallicPtspecieswerenotdetectedinthecurrentRaman spectro-scopicmeasurements;althoughpresenceofsuchspeciesareshown incomplimentaryXRDmeasurementsgiveninFig.3.

CurrentRamanspectroscopicresultsclearlypointoutastrong interactionbetweenthepreciousmetal(Pt)sitesandthe underly-ingreduciblemetaloxidesupport(CeO2),evidentbytheexistence ofPt–O–Cespecieswhichareformedduetothestrongmetal sup-portinteraction (SMSI)[22–28].SMSIis knowntobetriggered bythepartialreductionofthereducibleoxidesupport(i.e.CeO2)

whichisinitiatedbytheactionofapreciousmetal(i.e.Pt)[29]. Thisleadstotheformationofapartiallyreducedoverlayerofthe supportmaterial,demonstratinganunusuallyhighaffinitytoward thepreciousmetal.Asaresult,partiallyreducedmetaloxidelayer bindsstronglytothepreciousmetalsitesandcanevenmigrate overthesesitestocoverthem[8–10,30].SMSIcanhavecontrasting implicationsonthecatalyticperformance[29].Inthecasewhere thepreciousmetalsitesarecompletelycoveredandburiedbythe metaloxideoverlayer,catalyticactivityoftheoverallsystemcan belostinasignificantmanner[8].Suchalossinthecatalytic activ-ityalsorendersitselfasadrasticdecreaseinthetypicalCO(g)and

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H2(g)adsorptioncapacitiesofthecorrespondingcatalystsurfaces. Alternatively,ifthereducedmetaloxideoverlayeronlypartially coversthepreciousmetalsites,newactivesitescanbegenerated atthemetal/metaloxideboundary.Thesenewlyformedinterfacial sitescanradicallyboostthecatalyticactivityofthesystem[8],as inthecaseofthewater-gasshift(WGS)reactionwherethe forma-tionofsuchsiteshasbeenrecentlyreportedtoincreasetheWGS performancebyupto20folds[9]asaresultoftheunusualactivity ofthePt/CeOxinterfacialsystemtowardH2Odissociation accom-paniedbythestabilizationof thedissociation products.Clearly, currentlyinvestigatedPt/20Ce/Aland20BaPt/Ce/Alsystemsdoalso revealpotentPt–O–Cesitesthatmaypresentimportantcatalytic implicationsontheNOxstorageandNOxreductionviaH2(g).

In a recent Raman and Infrared spectroscopic study onthe Pt/CeO2 system[21],ithasbeenshownthat inthepresenceof H2(g),PtOx/PtO2 sitesarereducedfasterthanCeO2(whereCeO2 reductionhasbeenindirectlyfollowedbymonitoringthelossof the–OH(Type-I)vibrationsat3695cm−1 inFTIR),leadingtothe formationofreducedPtsiteswhichcanfurthertriggerCeO2 reduc-tionbyactivationofhydrogenandhydrogenspill-over.Thesame studyhasalsoelegantlydemonstratedthatduringthereoxidation oftheinitiallyreducedPt/CeO2systemwith18O2(g),oxidationof theceriasupportprecedestheoxidationofthesupportedPtphase. Thus,subsequentoxidationofthesupportedPtphaseisachieved bythe16Ospeciesofthecerialatticeandnotdirectlyby18O2(g).In otherwords,theprimaryfunctionoftheO2(g)duringtheoxidation processwassuggestedtobehealingoftheoxygenvacanciesinthe CeOx/CeO2lattice.

Inthelightofthesefindings,exsituRamanspectroscopicdata correspondingtoPt/20Ce/Al(Fig.2b)and20Ba/Pt/20Ce/Al(Fig.2c) samplescanbeanalyzedinacomparativemannerinordertogain furtherinsightregardingtheinteractionbetweendifferentmetal oxidedomainsonthesurface.Itisapparentthattheincorporation ofBaOxdomainsintothePt/20Ce/Alsystem(forinstance,compare 623KspectrainFig.2band2c)resultsinthestrengtheningofthe Pt–O–Ce(544cm−1)andPt–O(692cm−1)signals.Itislikelythat theperoxidespeciesthatareformedduringthedecompositionof themetalnitrateprecursors[16,31]canassisttheoxidationofthe Ptsites.Thiscanalsobeaccompaniedbyotheradditionaloxidation routes,inwhichceriacanactasanoxygenreservoirandoxidizePt sites.Alternatively,nitratescanalsodirectlytakepartinthePt oxi-dationduringthethermalnitratedecomposition.Inoverall,current Ramandataindicatethepresenceofvarioussurfacespeciessuch asPtOx,Pt–O–Ce,CeOxandO22−whichcandirectlytakepartin NOxsurfacechemistryandareratherelusivetodetectbyother experimentaltechniques.

Fig. 3 shows ex situ XRD data for the Pt/Al (a),Pt/20Ce/Al (b),20Ba/Pt/Al(c)and20Ba/Pt/20Ce/Al(d)samplesobtainedafter thermaltreatmentstepsatdifferenttemperatures.XRDpatterns in Fig.3 showdiffractionsignals correspondingto the␥-Al2O3 (JCPDS001-1303),CeO2 (JCPDS001-0800),Ba(NO3)2 (JCPDS 24-0053),BaAl2O4(JCPDS017-0306)andmetallicPt(JCPDS001-1190) phases.It isimportanttonotethatalthough␥-Al2O3→␣-Al2O3 (corundumJCPDS001-1296)phasetransitioncanbeobservedat 1273KforotherbinarymixedoxidesofaluminasuchasTiO2–Al2O3 [17,18]orFe2O3–Al2O3 [19],corundumphasewasnotobserved inthepresenceofceria(Fig.3).Inasimilarfashion,XRDsignals correspondingtotheoxidicphasesofPtsuchasPtO2 (e.g.JCPDS 021-0613)wasnotvisibleinthecurrentmeasurements.

ComparisonofFig.3aandbindicatesthatthepresenceofceria hindersthepreciousmetalsinteringandfacilitatesthedispersion ofthePtsites,asthemajorXRDsignalofmetallicPt(2=39.65◦) speciesisbroaderandweakeronPt/20Ce/Alwithrespecttothat ofPt/Al.ThesameobservationisalsovalidfortheBa-containing samples (Fig. 3c and 3d). For Ba-containing samples, a crys-tallineBa(NO3)2phasewasdetectedatlowertemperatureswhich

decomposesintoamorphousBaO/BaO2atT≥873K[16]. Stabiliza-tionorimmobilizationofthePtparticlesbyaceriasubstratehas beenreportedinformerstudiesintheliterature[23],wheresuch anobservationhasbeenattributedtoastrongPt–CeO2interaction, anchoringthePtsitesontheceriasurfaceandpreventingtheir sin-tering.FortheBa-containingsamplesinFig.3candd,formationof aBaAl2O4isalsoobservedatelevatedtemperatures,whichisone ofthecharacteristicindicatorsassociatedwiththethermalaging ofconventionalNSRcatalystsatelevatedtemperatures[17–19].

3.2. ReductionofadsorbedNOxspeciesonPt-freeceriapromoted NSRmaterialsviaH2(g)

Thelow-temperatureNOxuptake(viaNO2(g)adsorption)and thesubsequentNOxreductionbehavior(viaH2(g))oftheprepared Pt-freeNSRmaterialswereinvestigatedbyinsituFTIR.Inorder todemonstratethedirecteffectoftheceriadomainsontheNOx reduction,intheabsenceofPtandBaOsites,␥-Al2O3and20Ce/Al surfaceswereinvestigated.Fig.4showstheFTIRspectraafterthe saturationofthe␥-Al2O3and20Ce/Alsurfaceswith5TorrNO2(g) for10min(blackspectra)at473K,523Kand573Kaswellastheir subsequentreductionwithH2(g)at473K,523Kand573K(red spectra).NOxadsorption(black)spectrawereacquiredinvacuum whileH2reduction(red)spectrawereacquiredinthepresenceof 15.0TorrH2(g).IRbandsobservedaftersaturationofthe␥-Al2O3 withNO2(g)havebeendiscussedinpreviousreports[17,32–34] and canbeassigned tovarious typesofnitrates suchas bridg-ing(1650,1620and1257cm−1),bidentate(1595and1290cm−1) andmonodentate(1565and1298cm−1)nitrates.Lineshapesof theFTIRspectracorrespondingtoNO2-saturated20Ce/Alsurface (Fig.4b)demonstrateasignificantresemblancetothatofthe ␥-Al2O3surface(Fig.4a).Reductionofthestorednitrateson␥-Al2O3 byH2(g)at473Kresultsinaminordecreaseinthenitratesignals, whileamorenoticeabledecreasewasobservedfor20Ce/Alunder identicalconditions.Afterreductionat523K,lossinthenitrate sig-nalsbecomepronouncedforthe␥-Al2O3surface,whilereduction stillseemstooccuratamuchlargerextenton20Ce/Al.Reductionat 573Kleadstothecompletelossofthenitratespeciesonthe20Ce/Al surfacewhereasnitratebandsarenotcompletelyeliminatedfor ␥-Al2O3.Thus,Fig.4demonstratesthatCeO2domainscandirectly facilitateNOxreductionviaH2(g)onthe20Ce/Alsystemthrough theformationofoxygenvacancies[35].Furtherinsightregarding thispointcanbeobtainedbyinvestigatingthe–OH/–NHstretching regionofthecorrespondingFTIRspectra(Fig.5).

–OH/–NHstretchingregionoftheFTIRspectracorresponding to the ␥-Al2O3 and 20Ce/Alsamples upon NOx saturation and subsequentreductionwithH2(g)isgiveninFig.5.Thesespectra presentacomplexsetofconvolutedIRfeatureswhicharelocated atthree differentfrequency windowsnamely,3789–3695cm−1, 3695–3510cm−1 and3510–3000cm−1.UponNO2adsorptionon ␥-Al2O3and20Ce/Alsamplesat473–573K,agroupof character-isticallysharpand negativeIRsignalsweredetectedwithinthe firstfrequencywindowat3789,3755,3742,and3695cm−1.These featuresrevealrelativelyinvariantvibrationalfrequenciesforboth sampleswithin473–573Kandcanbereadilyassignedtotheso calledTypeIandTypeIIisolatedhydroxylgroups[34,36].In par-ticular,3695cm−1featuredeservesspecialattentionasthelossof thisparticularIRfeature(i.e.observationofanegativepeakatthis frequency)wasreportedtobeanindicationofthereductionofthe ceriadomains[21].Howevercareshouldbetakenregardingthe analysisofthisparticularvibrationalfeature,since␥-Al2O3 sur-facealsohasisolated–OHspeciesrevealinganIRsignatureatthis frequency.Thus,weanalyzed␥-Al2O3and20Ce/Alsamplesina comparativefashioninordertomonitortheevolutionofthis par-ticularband.WithinthesecondfrequencywindowinFig.5,very broadand convolutedIRfeatures canbenoticed(e.g.3649and

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1800 1600 1400 1200 1800 1600 1400 1200 1650 1620 1595 1565 1298 1257 1257 1298 1565 1595 1620 1650 0.5 0.5 20Ce/Al Al2O3

a

b

573K

473K

523K

573K

473K

523K

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

Absorbance (arb. u.) Absorbance (arb. u.)

Fig.4. InsituFTIRspectracorrespondingtoNO2adsorptionandsaturation(5.0TorrNO2(g),10min,blackspectra)followedbysubsequentreductionwithH2(g)(15.0Torr

H2(g),30min,redspectra)atvarioustemperatureson(a)␥-Al2O3and(b)20Ce/Alat473,523and573K.Allspectrawereacquiredat373K.Blackspectrawereacquiredin

vacuumwhileredspectrawereobtainedinH2(g).(Forinterpretationofthereferencestocolorinthisfigurelegend,thereaderisreferredtothewebversionofthisarticle.)

3558cm−1).Thesepoorlydefinedfeaturescanbeattributedto H-bondedhydroxylspecies[34,36].Finallywithinthethirdfrequency window,additionalpoorlyresolvedand broadfeaturescanalso bediscernedat3384and3266cm−1whichcanbeassignedto H-bondedhydroxylspeciesaswellas–NHmodesassociatedwith –NH2,–NH3,–OH···NHxor–OH···NOxspecies[37,38].

ItisvisibleinFig.5athatreductionofthe␥-Al2O3surfacewith H2at473Kleadsonlytominorrecoveryoftheisolated–OHspecies locatedat3789,3755and3742cm−1.Ontheotherhand,inaddition tothepartialrecoveryoftheisolated–OHspecies,onsetofthe–NH stretchingsignals(at3384and3264cm−1)isalsovisibleonthe 20Ce/Alsampleuponreductionat473K(Fig.5d).Thisobservation isinagreementwiththeFTIRdatapresentedinFig.4,suggesting thatceriadomainsstarttofacilitateNOxreductionevenundermild reductionconditions.Reductionathighertemperaturesresultsin afurtherincreaseinthe–NHsignals(at3384and3264cm−1)for bothsamples.Inaddition,reductionofthe20Ce/Alsample(Fig.5e andf)athighertemperaturesalsoleadstotheprogressivegrowth ofthenegativebandsat3695–3711and3662cm−1whichcanbe attributedtothereductionofthecerialatticeandtheformationof oxygenvacancies.Itcanbearguedthatduringthereductionofthe NO2-saturated␥-Al2O3and20Ce/Alsurfaces,H2activationoccurs morereadilyontheceriapromotedsurface.Duringtheinitialstages ofthereductionprocess, itis likelythat basicsurfacesites(i.e. O2−)areprotonatedformingisolated–OHspecies.Thisis accom-paniedbytheinteractionofsurface–OHspecieswiththeadsorbed nitratestoyieldsurfacespecieshaving–NHxstretchings. Further-more,hydrogenexposureatelevatedtemperaturesalsoresultsin thepartialreductionofthecerialatticeandtheformationofoxygen vacanciesonthe20Ce/Alsample.

Asthenextstep,thestructuralcomplexityoftheinvestigated systemsisincreasedfurtherbytheincorporationofNOxstorage domains. Along these lines, NOx adsorption and subsequent reductionviaH2(g)wereinvestigatedonBa-containingsamples (i.e.20Ba/Al,20Ba/20Ce/Aland8Ba/20Ce/Al)intheabsenceofPt (Fig.6).Theseparticularsetofsampleswereanalyzedinorderto shedlightontheinfluenceoftheceriapromotiononPt-freeNSR systemsasafunctionofBa-loading.FTIRspectracorrespondingto theNO2(g)adsorptiononBaO/Al2O3surfacehasbeenthoroughly

analyzedintheliterature[39–45].Inthelightoftheseformer stud-ies,NO2adsorptionsignals(upon5.0TorrNO2exposurefor10min at473, 523, 573or 623Kand subsequentevacuationat 323K) in theFTIRexperiments (Fig.6a) correspondingtothe 20Ba/Al samplecanbeassignedtobulk(ionic)Ba-nitrates(∼1320,∼1440 and ∼1480cm−1) and surface (bidentate) Ba-nitrate features (1585, 1565,1300cm−1). Remaining nitratebands at1583 and ∼1630cm−1 areassociatedwithbidentateandbridgednitrates, respectively.

Incorporationofceriatothe20Ba/Alsystem(Fig.6aand6b)does notsignificantlyalterthegenerallineshapesoftheFTIRspectra correspondingtotheNO2adsorptionat473,523and623K.Upon reductionwithH2(g)(5.0Torr,30minexposureforeachreduction step)at473Kand523Kthemostprominentlyobservedchange istheattenuationofthesurfacenitratefeatures.AtT≤523K(i.e. undermildreductionconditions),reductionoccursmostlyonthe surfaceoftheNOxstoragedomains,leavingbulkstoragedomains ratherintact.Theseobservationssuggestthatunderthese condi-tions,wherethepromotionaleffectofceriaisnotoperationalinits fullextent,relativelysimilarnitratereductionpathwaysseemtobe underwayforthe20Ba/Aland20Ba/20Ce/Alsurfaces.Ontheother hand,reductionat623Krevealsadifferentbehavior(Fig.6aand b).AlthoughmostoftheadsorbedNOxsignalsintheFTIRspectrum arelostforthe20Ba/20Ce/Alsampleat673K,anoticeablylarger NOxsignalisvisibleforthe20Ba/Alsample.Thus,uponactivation oftheceriadomains,NOxreductionprocessissignificantly facili-tated.Activationoftheceriadomainsatelevatedtemperatureson the20Ba/20Ce/Alsurfaceislikelytobeassociatedwiththe forma-tionofoxygenvacanciesinthecerialatticeasdiscussedearlierfor the20Ce/Alsurface(Fig.5f).

InfluenceoftheBa-loadingontheNOxstorageandreductionon Pt-freeBa/Ce/Alternaryoxidesystemscanbediscussedby com-paringFig.6bandc.ItisclearthatalowerBaloadingresultsin weakerbulk-nitratebandsandstrongersurfacenitratesignals, con-sistentwiththepresumablysmallerparticlesizeoftheBadomains onthe8Ba/20Ce/Alsurface.SurfaceBa-nitratesareknowntohave lowerstabilitywithrespecttothatofthebulkBa-nitratespecies [18].Along theselines,Fig.6c clearlydemonstratesthat onthe 8Ba/20Ce/Alsurface,allofthenitratespeciescanbereducedalmost

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4000 3800 3600 3400 3200 3000 4000 3800 3600 3400 3200 3000 4000 3800 3600 3400 3200 3000 4000 3800 3600 3400 3200 3000 4000 3800 3600 3400 3200 3000 4000 3800 3600 3400 3200 3000

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

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

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a

b

c

d

e

f

0.05

0.05

0.05

0.05

0.05

0.05

Fig.5.–OH/–NHstretchingregionoftheinsituFTIRspectracorrespondingtoNO2adsorptionandsaturation(5.0TorrNO2(g),10min,blackspectra)followedbysubsequent

reductionwithH2(g)(15.0TorrH2(g),30min,redspectra)on␥-Al2O3at(a)473K,(b)523K,(c)573Kandon20Ce/Alat(d)473K,(e)523K,(f)573K.Blackspectrawere

acquiredinvacuumwhileredspectrawereobtainedinH2(g).(Forinterpretationofthereferencestocolorinthisfigurelegend,thereaderisreferredtothewebversionof

thisarticle.)

fullywithH2at573K,whileonthe20Ba/20Ce/Alsurface,someof thebulknitratespeciessurviveevenat623K.Thisobservationcan beattributedtothelargerNOxstoragecapacityandthegreater sta-bilityofnitratespeciesonthe20Ba/20Ce/Alsurface,aswellasthe suppressionofthepromotionalceriasitesduetohighBasurface concentration.

3.3. ReductionofadsorbedNOxspeciesonPt-containingceria promotedNSRmaterialsviaH2(g)

Havinginvestigatedtheinfluenceoftheceriadomainsonthe NOxadsorptionandsubsequentreductionviaH2(g)overbinary

(Ce/AlandBa/Al)andternary(Ba/Ce/Al)oxidesystems,wefocused our attention on Pt-containing ceria promoted NSR systems. Ratherthandirectlytacklingwiththecomplex20Ba/Pt/20Ce/Al system, we performed initial benchmark studies on Pt/Al and Pt/20Ce/Alsamplesinordertogainsystematicinsightregarding the catalytic sub-components of the system of interest. Our preliminaryNOx saturationand reductionexperiments onPt/Al andPt/20Ce/AlatT≥473K(datanotshown)revealedthatnitrate reduction with 15Torr H2(g) is extremely fastover Pt/Al and Pt/20Ce/Al surfaces, readily removing nitrates from these sur-faces and rendering their comparison rather difficult. Thus, in order todecelerate thereduction kineticsanddemonstrate the

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20Ba/Al 20Ba/20Ce/Al 8Ba/20Ce/Al

16 20 1580 15 65 14 80 14 20 13 50 13 25 13 00 12 45 16 20 15 80 14 80 14 20 14 45 13 25 12 45 13 00 1565 12 45 13 00 13 25 14 20 14 45 14 80 15 65 15 80 16 00 1620 623K 473K 523K 623K 473K 523K 573K 473K 523K

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

Absorbance (arb. u.) Absorbance (arb. u.) Absorbance (arb. u.)

0.5 0.5 0.5

a

b

c

Fig.6. InsituFTIRspectracorrespondingtoNO2saturation(5.0TorrNO2(g),10min,blackspectra)followedbysubsequentreductionwithH2(g)(15.0TorrH2(g),30min,

redspectra)of(a)20Ba/Alat473K,523Kand623K,(b)20Ba/20Ce/Alat473K,523Kand623K,(c)8Ba/20Ce/Al473K,523Kand573K.Allspectrawereacquiredat373K. BlackspectrawereacquiredinvacuumwhileredspectrawereacquiredinH2(g).(Forinterpretationofthereferencestocolorinthisfigurelegend,thereaderisreferredto

thewebversionofthisarticle.)

differencesbetweentherelativereductionperformancesofPt/Al andPt/20Ce/Alsurfaces,NO2adsorptionandsubsequentH2 reduc-tionexperimentswereperformedatalowertemperature(i.e.at 323K)andthereductionprocesswasmonitoredasafunctionof time.

Fig.7illustratestheFTIRspectraobtainedduringthese experi-ments.BlackspectrainFig.7aand7bwereacquiredimmediately aftertheNO2 adsorptiononfreshPt/AlandPt/20Ce/Alsurfaces at323K,respectively. Afterthe saturationof thesurfaceswith NO2(g), reduction process wasinitiated by the introduction of 15.0TorrH2(g)at323Kandthetime-dependent(red)FTIR spec-trawereacquiredoverthecourseof4hr.Bluespectrarepresent thefinalconditionsattheendofthe4hrperiod.Blackspectrum forthePt/AlsampleinFig.7aischaracterizedbynitratesignals locatedat1630and1255cm−1(bridgingnitrates)and1570and 1300cm−1 (monodentate nitrates). It is interesting that imme-diatelyaftertheintroductionofH2(g),anoticeablealterationin theadsorptiongeometryofthesurfacenitratespeciesisobserved wherebridgingnitratesarepartiallyconvertedintomonodentate nitrates.Corresponding–OH/–NHstretchingregion(Fig.7c)clearly showsthatthetransformation inthenitrateadsorption geome-tryis accompaniedbytherecoveryoftheisolated–OH species (3789, 3737 and 3696cm−1) and the growth of theH-bonded –OH(3531cm−1)and–NH(3450,3384,3282cm−1)species.This observationis consistentwiththecompetitionfortheavailable surface adsorptionsites betweenthe adsorbedbridging nitrate species(requiringtwovacantadsorptionsitesperadsorbate)and thereductionspeciesthatareproducedaftertheintroductionof H2(g)enforcingadsorbednitratesgeometrychangefrombridging tomonodentateconfiguration.Itisalsoworthmentioningthatthe broadshoulderat1500cm−1appearinginthepresenceofH2canbe attributedtonitritespecieswhichisformedasanintermediate dur-ingthenitratereduction[46]whilethefeatureat1410cm−1canbe assignedtobulknitrates[32,46].Increaseinthebulknitratesignal canbeassociatedwiththeconversionofsurfacenitratesintobulk nitrates,inducedby–OH/–NHspeciesformedduringthereduction process.Suchatransformationhasalsobeenobservedinformer studiesuponreductionorhydrationofnitratedsurfaces[42,46,47]. Fig.7aandcshowsthatafter4hreductionat323K,signals associ-atedwithH-bonded–OHspeciesandthe–NHsignalscontinuously

growin intensitywhile nitratesignals attenuate.Clearly,these observationsareconsistentwiththereductionofadsorbedNOx speciesviaH2.Itisapparentthatunderthesemildreduction con-ditions,onlyacertainportionoftheisolatedhydroxylspeciesis recoveredonthePt/Alsurface.Similarly,adsorbednitratesareonly partiallyreduced.

Ontheotherhand,identicalsetofexperimentsperformedon thePt/20Ce/Alsamplepresentsnoteworthydifferences(Fig.7band d).Althoughasimilarsetofvibrationalfeaturesassociatedwith NOxspeciesareobservedonthePt/20Ce/Alsurface(Fig.7b), time-dependentevolutionoftheintensitiesofthesefeaturesarequite different.Inoverall,it isapparentthatnitratesignalsattenuate muchfasteronthePt/20Ce/Alsurfacerevealingthepromotional effectofceriaintheNOxreduction.AlsoFig.7canddshowsthatthe negative3695cm−1featurewhichisassociatedwiththereduced ceriadomains,ismorepronouncedforthePt/20Ce/Alsurfacethan thePt/Alsample.Thissuggeststhatthepromotionaleffectofceria canbeassociatedwiththeoxygenvacancyformationinthe par-tiallyreducedceriadomains.

Asdiscussedabove,ourpreliminaryNOxstorageandreduction experimentsonthe␥-Al2O3,20Ba/Al,20Ba/20Ce/Al,8Ba/20Ce/Al, Pt/Al,20Ce/AlandPt/20Ce/Alsurfaces(Figs.4-7)providea valu-ableinsightregardingthecatalyticbehaviorofthekeycomponents constitutingthecomplex20Ba/Pt/Aland20Ba/Pt/20Ce/Alsystems. In thelight of thesepreliminaryexperiments,NOx storage and reductionphenomenaonthe20Ba/Pt/20Ce/Alsystemcannowbe elucidatedindetail.

Fig.8presentsNO2 adsorption(5.0TorrNO2(g)exposurefor 10minat323K,followed byevacuation)and reduction(15Torr H2(g)exposurefor4hrat323K)experimentsonthe20Ba/Pt/Aland 20Ba/Pt/20Ce/Alsurfaces.Fig.8aand8bindicatesthatafterNO2 adsorption,nitratefeaturesaredetectedat1620and1260cm−1 (bridgingsurfacenitrates),1560and1330cm−1(bidentateand/or monodentate surface nitrates) and 1420cm−1 (bulk nitrates) [42,46,47].Time-dependentFTIRspectraobtainedafterthe intro-ductionofH2(g)onthe20Ba/Pt/Aland20Ba/Pt/20Ce/Alsurfaces at323Kdemonstrateanimmediatechangeintherelative intensi-tiesofthesurfaceandbulknitrates,wheresurfacenitratesignals decreaseandthebulknitratesignalsincrease.Thisobservationisin agreementwiththeformerexperimentsperformedonceria-free

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+NO

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Absorbance (arb. u.) Absorbance (arb. u.)

Absorbance (arb. u.) Absorbance (arb. u.)

Fig.7.Time-dependentinsituFTIRspectracorrespondingtoNO2saturation(5.0TorrNO2(g),10min,blackspectra)followedbysubsequentreductionwithH2(g)(15.0Torr

H2(g),3,10,30,60,120,240min,redspectra)onPt/AlandPt/20Ce/Alsamplesat323K.Bluespectrawereobtainedattheendofthe240minreductionperiod.Panels(a)

and(c)correspondtoPt/Alsamplewhilepanels(b)and(d)representPt/20Ce/Alsample.Allspectrawereacquiredat373K.Blackspectrawereacquiredinvacuumwhile redandbluespectrawereacquiredinH2(g).(Forinterpretationofthereferencestocolorinthisfigurelegend,thereaderisreferredtothewebversionofthisarticle.)

NSRcatalystssuggestingaggregationoftheBadomains[32,46]. Aftertheinitialminorspectralchangesoccurringinthefirst60min ofreduction,theintensitiesofsurfaceandbulknitratespecieson 20Ba/Pt/Alsurface remainpractically invariantuntil theend of 240minreductionprocess.Meanwhiletheintensityofsurfaceand bulknitratesonceria-containingmaterialmonotonicallydecreases overthecourseofthe240minreductionperiodat323K. Further-more,theintensitiesofthenitraterelatedfeaturessubstantially decreaseafterheatingbothmaterialsat473Khowevertheceria promoted20Ba/Pt/20Ce/Al materialdemonstratesasignificantly strongerattenuationinnitratefeatures.Afterheatinginhydrogen at573Kthenitratesarealmostcompletelyeliminatedfromboth materials.Thepresenteddataclearlypointoutthatceriapromotion enhancesthenitratereductionperformanceoftheconventional NSRcatalyst.

–OH/–NH regionof the correspondingFTIR spectragiven in Fig.8band8calsoindicatethatduringthereduction,isolated–OH species areslightly recovered.Concomitant tothis observation, intensitiesoftheH-bonded–OHspeciesaswellas–NHspeciesalso increaseduringthereductionperiod.Afterthesereduction exper-imentsundermildconditions(i.e.at323K),surfacetemperature

ofthecatalystwasincreasedto473Kandsubsequentlyto573Kin thepresenceofH2(g)inordertomonitorthereductionphenomena understronglyreducingconditions(seebluespectrainFig.8).Itis clearlyvisibleinFig.8athatafterreductionat473K,almostall ofthesurfacenitratesignalsdisappearand thebulknitrate sig-nalsattenuatesignificantly,indicatingthefacileeliminationofthe storednitrates.–OH/–NHregionofthecorrespondingFTIRspectra revealasignificantlossof–OH,–NHsignalsindicatingtheremoval ofthereductionintermediates/productsanddehydroxylation.This isaccompaniedbythegrowthofanegativesignalat3695cm−1 suggestingthereductionoftheceriadomains.It isworth men-tioningthatamuchweaker,yetvisiblenegativefeaturewasalso observedinanalogousexperimentsperformedon20Ba/Pt/Ba/Al catalyst(datanotshown)indicatingthatthe3695cm−1featurein Fig.8isassociatedwithbothaluminaandreducedceriadomains. Howevernoticeablystrongernegative3695cm−1 featureforthe 20Ba/Pt/20Ce/Alsurfaceindicatesthatthisfeaturecanstillbeused tomonitortheextentofceriareductioninaqualitativemanner. AftertheH2reductionat573K,almostallofthenitratesignals van-ish(Fig.8a),alongwithfurtherdehydroxylationandceriareduction (Fig.8c).

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c

0.1 60 min 240 min 473K 573K

Absorbance (arb. u.)

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3260 3540 0.1 30 min 10 min 3 min +NO2 1620 1560 1435 1420 1340 1330 1260 573K 473K

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20Ba/Pt/20Ce/Al 0.5 3695 473K 573K 0.5 20Ba/Pt/Al H2reduction 0-30min H2reduction 60-240 min

d

+NO2 1561 1435 1422 1342 1330 1624

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

Fig.8.Time-dependentinsituFTIRspectracorrespondingtoNO2adsorptionandsaturation(5.0TorrNO2(g),10min,blackspectra)followedbysubsequentreductionwith

H2(g)(15.0TorrH2(g),3,10,30,60,120,180,240min,redspectra)on(a)20Ba/Pt/Al(b)20Ba/Pt/20Ce/Alsamplesat323K.Bluespectrawereobtainedafterthe240min

reductionperiodbyincreasingthesampletemperatureto473Kand573KinH2(g),respectively.Panel(a)and(b)presenttheNOxregionsoftheFTIRspectraof20Ba/Pt/Al

and20Ba/Pt/20Ce/Al,respectively.Panels(c)and(d)correspondto–OH/–NHstretchingregionof20Ba/Pt/20Ce/Al.Allspectrawereacquiredat373K.Blackspectrawere acquiredinvacuumwhileredandbluespectrawereacquiredinH2(g).(Forinterpretationofthereferencestocolorinthisfigurelegend,thereaderisreferredtotheweb

versionofthisarticle.)

3.4. DirectevidencefortheenhancementofH2activationupon ceriapromotion:H2-TPRanalysis

Inordertoprovideadirectevidencefortheenhancementof theH2activationbyceriapromotedNSRcatalystsunderflow con-ditions,wealsoperformedH2TPRexperimentson20Ba/Pt/Ba/Al and20Ba/Pt/20Ce/Alsamplesinacomparativemanner(Fig.9).

400 500 600 700 800

BaO/Pt/Al2O3

BaO/Pt/CeO2/Al2O3

Temperature (K) TCD Signal (a.u.) 393 423 465 496 661 709 (a) (b) 4.31 cc/g 6.13 cc/g

Fig.9.TPRprofilesofBaO/Pt/Al2O3(red)andBaO/Pt/CeO2/Al2O3(black)under10%

H2balancedwithArwhilethetemperaturewasraisedfrom323Kto873Katarate

of10K/min.(Forinterpretationofthereferencestocolorinthisfigurelegend,the readerisreferredtothewebversionofthisarticle.)

Ceria is known to have two characteristic TPR features at 673 and 973K. While the former TPR feature is attributed to reduction of surface oxygen, latter feature is related to reduc-tionofthelatticeoxygen[48–50].Ceria-promoted NSRcatalyst (i.e.BaO/Pt/20CeO2/Al2O3)inFig.9revealsthree mainTPR sig-nalsat423,496and661K.The423Ksignalcanbeattributedto thereductionofPtOxspeciesonceriaandthereductionofceria inthecloseproximityof thePtsites[51,52].Thelow tempera-tureshoulder at<393Kcanbeassociated withthereductionof PtOxlocatedonaluminasurface.The496Ksignalsaretentatively assignedtothereductionof bridgingoxygen(Pt–O–Ce)sitesin thePt/CeOxheterojunctions.Furthermore,theTPRsignalforthe BaO/Pt/20CeO2/Al2O3samplelocatedat661Kcanbeassignedto thereductionofceriadomainsonthesurfacewhicharenotindirect contactwiththePtsites.ThepresenceofintenseTPRfeaturesfor thissampleatT<550KisconsistentwithahighPtdispersionon theceriapromotedsample.

TPRtracefortheconventionalNSRcatalyst(i.e.BaO/Pt/Al2O3) inFig.9,displaystwoweakfeaturesat393and465Kaswellas astrongerfeatureat709K.TPRsignalsat393and465Kcanbe attributedtothereductionofoxidizedPtspecies.Ontheotherhand, thehightemperaturesignalat709Kcanbeassociatedwith reduc-tionofBaO2speciesand/ordecompositionofBaCO3,whereCO2 evolvingfromcarbonatedecompositioncanconsumehydrogenvia thereversewater-gasshiftreactioncatalyzedbyPt[52].

ComparisonofthetotalH2consumptionduringtheTPR exper-imentsforthe20Ba/Pt/Ba/Aland20Ba/Pt/20Ce/Alsamplesreveals thatwhiletheconventionalNSRcatalystconsumed4.31cc/gH2 duringtheTPR,ceria-promotedNSRcatalystconsumed6.13cc/g. Theseresultsarein perfectagreementwiththepreviously dis-cussedstructuralcharacterizationandinsituspectroscopicdata suggestingthatceria-promotedNSRcatalystsrevealnovelcatalytic

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0 min 1min 3 min 10 min 60 min 30 min 120 min 240 min

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Absorbance (arb. u.)

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

Fig.10.Time-dependentinsitugasphaseFTIRspectracorrespondingtoNO2adsorptionandsaturation(5.0TorrNO2(g),10min,blackspectra)followedbysubsequent

reductionwithH2(g)(15.0TorrH2(g),redspectra)on(a)Pt/20Ce/Aland(b)20Ba/Pt/20Ce/Alat323K.Bluespectruminpanel(b)wasobtainedafterthe240minreduction

periodbyincreasingthesampletemperatureto573KinH2(g).Allspectrawereacquiredat373K.Blackspectrawereacquiredinvacuumwhileredandbluespectrawere

acquiredinH2(g).(Forinterpretationofthereferencestocolorinthisfigurelegend,thereaderisreferredtothewebversionofthisarticle.)

siteswhichdonotonlyboostthetotalamountofactivated hydro-genonthesurface but alsodecreasethethermal threshold for nitratereduction.

3.5. MonitoringreductionproductsviagasphaseFTIR spectroscopy

Finally,timedependentevolutionofthegasphaseproducts dur-ingthereductionofNOx-saturated(andsubsequentlyevacuated) Pt/20Ce/Aland20Ba/Pt/20Ce/AlviaH2(g)wasalsomonitoredby usinggasphaseFTIRspectroscopyat323K(Fig.10aandb, respec-tively).Duringtheseexperiments,catalystsamplesinsidetheFTIR reactorwasmovedabovetheIRbeam,preventingthedetection ofsurface IRsignalsoriginatingfrom thecatalystsamples.It is seeninFig.10athatN2O(g)(i.e.2225cm−1feature)immediately becomesvisibleastheonlydetectablegasphasereaction prod-uctviaFTIRoverthePt/20Ce/Alsampleinthefirst3minofthe reductionperiod.N2O(g)signalcontinuestoincreaseinthefirst 10minofthereductionperiodandthenstartstodecrease.Itis likelythattheattenuationoftheN2O(g)signalisaccompaniedby thefurtherreductionofN2O(g)toN2(g),aspecieswhichis invis-ibleingasphaseFTIR.Thisobservationisinagreementwiththe correspondingsurfaceFTIRdatapresentedinFig.7b,showingthat afterthe4hrreductionperiodat323KonthePt/20Ce/Alsample, alargeportionofthestoredNOxspeciesareeliminated.Thus,it isapparentthattheeliminatednitratespeciesfromthePt/20Ce/Al surfaceat323KarefirstconvertedintoN2O(g)andthentoN2(g). Inaddition,formationofH2O(g)(1620cm−1)isalsovisibleinthe FTIRspectraacquiredbetween10–240minofthereductionperiod (Fig.10a).Besidesthesefeatures,otherspectralartifactsarealso observedinFig.10a,namelytheprogressivelygrowingnegative

featureslocatedat1530,1265and1000cm−1whichareassociated withtheeliminationofthemonodentatenitrateschemisorbedon theIR-transparentBaF2 windowsoftheFTIRreactorduringthe reductionprocess.

Fig.10brevealsthatN2O(g)andH2O(g)aretheonlydetectable reduction products during the reduction of the NOx-saturated 20Ba/Pt/20Ce/Al NSR catalyst at 323Kvia H2(g). It is also visi-bleinFig.10bthatevenaftera4hreductionperiod,N2O(g)still existsinthereactorandnotcompletelyconvertedintoN2.This observationisingoodaccordancewiththesurfaceFTIRdatagiven inFig.8a revealingthatafterthe4hreductionperiod at323K, asignificantlylargeportionofthenitratespeciesarepresenton the20Ba/Pt/20Ce/AlsurfacewhichcanconstantlysupplyN2O(g) intothe reactor.Thus,for a complete removal/reductionof the N2O(g)species, strongerreductionconditionsare required.This wasalsoaccomplishedbyincreasingthe20Ba/Pt/20Ce/Alsample temperaturequicklyto573KinthepresenceofH2(g)afterthe4 hrreductionperiod(topmostspectruminFig.10b).Clearly,these stronglyreducingconditionscanreadilyeliminatealloftheN2O(g) signalbypresumablyproducingN2(g).Notethatthesharpspectral artifactlocatedat1350cm−1inFig.10b,whichisformed immedi-atelyafterincreasingthesampletemperatureto573Kisduetothe formationofbulknitratesontheBaF2windowsoftheFTIR reac-torasaresultoftherapidNOxdesorption/breakthroughfromthe catalystsurface.

4. Conclusions

Inthecurrent work,NOx storageandthesubsequent reduc-tionofthestored NOxspeciesviaH2(g)wereinvestigatedover ceria-promotedNSRcatalysts(i.e.20Ba/Pt/20Ce/Al).Asystematic

(12)

approachwasfollowedinordertoelucidatethecatalyticfunctionof theindividualstructuralcomponentsconstitutingthecatalyst for-mulationbyinvestigating␥-Al2O3,20Ba/Al,20Ce/Al,20Ba/20Ce/Al, 8Ba/20Ce/Al, Pt/Al, Pt/20Ce/Al, 20Ba/Pt/Al and 20Ba/Pt/20Ce/Al samplesinacomparativemanner.

SMSIbetweenPtsitesandtheBaO/BaO2/CeOx/CeO2 domains wereobservedleadingtoacomplexredoxinterplayincludingthe oxidationofthepreciousmetalsites,reductionofceria,formation ofBaO2speciesaswellastheformationofPt–O–Ceinterfacialsites onthe20Ba/Pt/20Ce/Alsurfacewhichseemtohaveavitalrolein theNOxstorageandreductionchemistry.Ceriadomainsalsoactas anchoringsitesforPtspecies,limittheirsurfacediffusion,enhance dispersionandhindersinteringatelevatedtemperatures.

On the Ba/Pt/Ce/Al catalyst surface, reduction of the stored nitratesunderrelativelymildconditionsviaH2(g)initiallyleads toformationofsurfacenitrites,–OHand–NHxsurfacespeciesand gasphaseN2O,aswellasthedestructionofsurfacenitratespecies, leavingbulknitratesmostlyintact.Reductionproceedswiththe conversionofN2O(g)intoN2(g)alongwiththepartiallossofsurface –OHand–NHxgroups,dehydrationandthelossofbulknitrates.

Formationofoxygenvacanciesinthecerialattice accompan-iestheNOxreductionprocess.Althoughceriapromotiondoesnot seemtohaveasubstantialinfluenceontheoverallNOxstorage capacity,itdoeshaveaclearlypositiveimpactontheNOx reduc-tionwhichisassociatedwithenhancementinthetotalamount ofactivatedhydrogenonthecatalystsurfaceandloweringofthe thermalthresholdforhydrogenactivation.

Acknowledgements

AuthorsgratefullyacknowledgeDr.ChangHwanKimandDr. WeiLiofGeneralMotorsGlobalResearchandDevelopment (War-ren, MI) for performing the TPR experiments and for fruitful discussions about the experimental results. E.O. acknowledges supportfromTurkish Academyof Sciences (TUBA)throughthe “OutstandingYoungInvestigator”grant.E.V.andV.B.acknowledge RFBR(Russia)#12-03-91373-CTafor financialsupport.Authors alsothankMargaritaKantchevaforfruitfuldiscussions.

AppendixA. Supplementarydata

Supplementary data associated with this article can be found,intheonlineversion,athttp://dx.doi.org/10.1016/j.apcatb. 2013.04.075.

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Şekil

Fig. 1. BET specific surface area values for the analyzed samples upon annealing in Ar(g) flow within 623-1273 K
Fig. 3. Ex situ XRD patterns corresponding to (a) Pt/Al, (b) Pt/20Ce/Al, (c) 20Ba/Pt/Al and (d) 20Ba/Pt/20Ce/Al samples upon annealing in Ar(g) flow within 623–1273 K.
Fig. 4. In situ FTIR spectra corresponding to NO 2 adsorption and saturation (5.0 Torr NO 2 (g), 10 min, black spectra) followed by subsequent reduction with H 2 (g) (15.0 Torr H 2 (g), 30 min, red spectra) at various temperatures on (a) ␥-Al 2 O 3 and (b)
Fig. 5. –OH/–NH stretching region of the in situ FTIR spectra corresponding to NO 2 adsorption and saturation (5.0 Torr NO 2 (g), 10 min, black spectra) followed by subsequent reduction with H 2 (g) (15.0 Torr H 2 (g), 30 min, red spectra) on ␥-Al 2 O 3 at
+5

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