Palladium doped perovskite-based NO oxidation catalysts: the role of Pd and B-sites for NOx adsorption behavior via in-situ spectroscopy

ContentslistsavailableatScienceDirect

Applied

Catalysis

B:

Environmental

jo u r n al ho me p a 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

Palladium

doped

perovskite-based

NO

oxidation

catalysts:

The

role

of

Pd

and

B-sites

for

NO

x

adsorption

behavior

via

in-situ

spectroscopy

Zafer

Say

_,

_Merve

_Dogac

_,

_Evgeny

_I.

_Vovk

_,

_Y.

_Eren

_Kalay

_,

_Chang

_Hwan

_Kim

_,

Wei

Li

_,

_Emrah

_Ozensoy

a_{DepartmentofChemistry,BilkentUniversity,06800Ankara,Turkey} b_{BoreskovInstituteofCatalysis,630090Novosibirsk,RussianFederation}

c_{DepartmentofMetallurgical&MaterialsEngineering,MiddleEastTechnicalUniversity,06800Ankara,Turkey} d_{GeneralMotorsGlobalR&DChemicalSciences&MaterialsSystemsLab,30500MoundRd.,Warren,MI48090,USA}

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received1November2013

Receivedinrevisedform13January2014 Accepted20January2014

Availableonline28January2014

Keywords: LaCoO3 LaMnO3 Pd NOx DeNOx

a

b

s

t

r

a

c

t

Perovskite-basedmaterials(LaMnO3,Pd/LaMnO3,LaCoO3andPd/LaCoO3)weresynthesized,

character-ized(viaBET,XRD,Ramanspectroscopy,XPSandTEM)andtheirNOx(x=1,2)adsorptioncharacteristics

wereinvestigated(viain-situFTIRandTPD)asafunctionofthenatureoftheB-sitecation(i.e.MnvsCo),

Pd/PdOincorporationandH2-pretreatment.NOxadsorptiononofLaMnO3wasfoundtobesignificantly

higherthanLaCoO3,inlinewiththehigherSSAofLaMnO3.IncorporationofPdOnanoparticleswithan

averagediameterofca.4nmdidnothaveasignificanteffectontheamountofNO2adsorbedonfresh

LaMnO3andLaCoO3.TPDexperimentssuggestedthatsaturationoffreshLaMnO3,Pd/LaMnO3,LaCoO3

andPd/LaCoO3withNO2at323KresultedinthedesorptionofNO2,NO,N2OandN2(withoutO2)below

700K,whileabove700K,NOxdesorptionwaspredominantlyintheformofNO+O2.Perovskite

materi-alswerefoundtobecapableofactivatingN–Olinkagestypicallyatca.550K(evenintheabsenceofan

externalreducingagent)formingN2andN2OasdirectNOxdecompositionproducts.H2-pretreatment

yieldedadrasticboostintheNOoxidationandNOxadsorptionofallsamples,particularlyforthe

Co-basedsystems.PresenceofPdfurtherboostedtheNOxuptakeuponH2-pretreatment.Increaseinthe

NOxadsorptionofH2-pretreatedLaCoO3andPd/LaCoO3surfacescouldbeassociatedwiththeelectronic

changes(i.e.reductionofB-sitecation),structuralchanges(surfacereconstructionandSSAincrease),

reductionofthepreciousmetaloxide(PdO)intometallicspecies(Pd),andthegenerationofoxygen

defectsontheperovskite.Mn-basedsystemsweremoreresilienttowardB-sitereduction.Pd-addition

suppressedtheB-sitereductionandpreservedtheABO3perovskitestructure.

©2014ElsevierB.V.Allrightsreserved.

1. Introduction

NOx(i.e.predominantlyNO,NO2)isoneofthemainpollutants emittedbydieselandgasoline-poweredenginesandhasa signifi-cantlynegativeinfluenceontheenvironment.Duringthelasttwo decades,emission regulationshavebecometighter,and numer-oustechnologiesforNOxafter-treatmenthavebeendevelopedand commercialized.Fortheconventionalgasolineengines,three-way catalysts(TWC)canreducetoxicgaseseffectively undera stoi-chiometricairtofuelratio(14.5);howeverTWCisnoteffective inNOxaftertreatmentindieselenginesoperatingunder oxygen-richleanconditions(wheretheairtofuelratiogreaterthan14.5). NOxstoragereduction(NSR)catalystsweredevelopedasan alter-nativetechnology[1–3].ConventionalNSRcatalystsarecomposed

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

ofthreemaincomponents,Pt,Al2O3,and BaOwhereBaO func-tionsastheNOxstoragecomponent.AlthoughthepresenceofPt is critical forNO oxidation (animportant stepin NOx storage), itcontributessignificantlytothetotalcostofthecatalyst.These importantdrawbacksledthedirectionofresearchtowardnoble metal-freematerials.

PerovskitesintheformofABO3havebeenconsideredas promis-ingalternativesforlow-costautomotivecatalystswithexcellent redoxpropertiesandhighthermaldurability[4,5].Thechemical properties ofthe perovskite materialsarealso knownfor their flexiblecharacteristicswhichareassociated withtheirA and/or B site substitution capabilities and availability of a wide vari-etyofsub-stoichiometricstructures[6].Thecatalyticefficiencies ofperovskitematerialsforNOand N2OreductionintoN2 were demonstratedbyseveralstudies[7–13].Moreover,thesematerials playanefficientroleinthecatalyticNOoxidationunderlean con-ditionsevenintheabsenceofanoblemetal[14,15].Recently,Kim etal.[16]reportedthatSr-promotedLa-basedperovskitecatalysts

0926-3373/$–seefrontmatter©2014ElsevierB.V.Allrightsreserved. http://dx.doi.org/10.1016/j.apcatb.2014.01.038

(La_1−xSrxCoO3)exhibithighercatalyticNOtoNO2conversionrates ascomparedtoPt-basedcatalysts.

LaCoO3andPd/LaCoO3basedperovskitematerialsareknown tobehighlysensitivetowardpretreatmentunderreducing condi-tionsleadingtosignificantstructuralchangesandreconstruction

[12,17,18]wherethereductionoftheperovskitelatticeisclosely linked to the nature of the B-site cation in the ABO3 struc-ture.In thecurrent work,the effectof H2(g) pre-treatmenton NOx oxidation and adsorptionover LaCoO3, LaMnO3 as wellas theirPd-enriched forms were investigated by meansof in-situ Fouriertransforminfrared(FTIR)spectroscopy andtemperature programmeddesorption(TPD) in anattempttoprovide funda-mentalknowledgeassociatedwiththeinfluenceofthenatureof theB-sitecationandthePdincorporationontheDeNOxcatalytic chemistryofperovskites.

2. Experimental

2.1. Catalystpreparation

LaCoO3andLaMnO3,weresynthesizedviacitricacidmethod involving a citrate route as prescribed in a recent GM patent

[19].Supportedpalladiumcatalystswerepreparedusing classi-cal incipientwetness impregnation method utilizing palladium nitratetogether with LaCoO3 or LaMnO3. Pd wasimpregnated ontothesynthesizedperovskitesamplesafterthecalcinationof theperovskites at 973K for 5hin air. The nominalloading of palladiumwasadjusted to1.5wt%Pd. Pd incorporated materi-alsweresubsequentlycalcinedat773Kfor5hinair.TheLa2O3 benchmarkmaterialwassynthesized viadirect calcinationof a La(NO3)3·6H2O(s)(SigmaAldrich)precursorat973Kinanopen-air ovenfor12h.

Priortoin-situFTIRanalysis,perovskitesamplesmountedon thespectroscopicbatchreactorwereinitiallytreatedandactivated withanexposureof2TorrNO2(g)for5minat323Kfollowedby annealingandsurfacecleaninginvacuum(<10−3Torr)at973K. Finally, samples were cooled to 323K for subsequent NO2(g) adsorptionexperiments.Theperovskitematerialsreferredinthe textaspre-reducedweretreatedwith5.0TorrH2(g)(LindeGmbH, Germany,>99.9)at623Kfor10min.

NO2saturationofthesynthesizedmaterialswascarriedout typ-icallybydosing5.0TorrNO2(g)overthesamplefor10minat323K. NO2(g)usedintheexperimentswaspreparedbymixingNO(g) (AirProducts,99.9%)andO2(g)(LindeGmbH,Germany,99.999%) followedbymultiplefreeze-pump-thawcyclesforfurther purifi-cation.

2.2. Instrumentation

AlloftheFTIRspectroscopicexperimentswereconductedin transmissionmodeusingaBrukerTensor27spectrometercoupled toabatch-typecatalyticreactorwhosedetailshavebeendescribed elsewhere[20,21].AlloftheFTIRspectrawerecollectedat323K. Alloftheadsorption/pre-treatmentstepswerealsoperformedin batchmode.

PriortoTPDexperiments,sampleweremountedonthe spec-troscopicbatchreactorandthenwereexposedto5.0TorrNO2(g) for10min.TPDexperimentswerecarriedoutinvacuum,witha heatingrateof12K/min.ForTPDexperimentsaquadrupolemass spectrometer(QMS, Stanford Research Systems, RGA 200) was used.TheQMSsignalswithm/zequalto18(H2O),28(N2/CO),30 (NO/NO2),32(O2),44(N2O/CO2),and46(NO2)weremonitored duringtheTPDmeasurements.Itis wellknownthatduetothe hardionizationprocessintheQMS,pureNO2(g)undergoes frag-mentationyieldingtwomajorcomponentsnamely,NO(m/z=30)

andNO2 (m/z=46).TheNO:NO2 ratiointheNO2 fragmentation patternvariesbetween3.0and4.0asafunctionoftheionization conditions.Underthecurrentexperimentalconditions, fragmen-tationratioofNO:NO2 wasdeterminedtobe3.23.Inthecurrent work,astrictlyquantitateanalysisoftheTPDdesorptionchannels willnotbegiven.However,usingthefragmentation characteris-ticsofpureNO2(g),relativecontributionsofpureNO(g)andpure NO2(g)totheoverallm/z=30signalcanbereadilydistinguished.

Ex-situXPSanalysiswasperformedusinga SPECSXPS spec-trometerwithaPHOIBOS-100hemisphericalenergyanalyzerand a DLD detectorutilizing monochromatic AlK␣X-rayirradiation (h=1486.7eV,400W).Instrumentaldetailsregardingtheother utilized experimental techniques (i.e. XRD, TEM, BET, TPD and Ramanspectroscopy)weredescribedelsewhere[20,22].

3. Resultsanddiscussion

3.1. CharacterizationofthesynthesizedsamplesviaXRD,TEM, BET,XPS,andRamanspectroscopy

Fig.1illustratestheex-situXRDprofilesofLaMnO3,Pd/LaMnO3, LaCoO3andPd/LaCoO3materialsobtainedaftersynthesisand cal-cinationat973K.XRDpatternsverifythatsynthesisofperovskite structureswasachievedasindicatedbythepresenceofthe char-acteristic diffractionlinesat 32.56◦ (LaMnO3)as wellas32.84◦ and33.21◦ (LaCoO3)[23].Thismajordiffractionsignalpossesses additional information aboutthe latticestructure of perovskite materialsasshownindetailinFig.1b.WhileMn-basedperovskite structureshaveonlyasinglemajorpeakrelatedtothecubiclattice orpoorlycrystallinerhombohedralstructure,Co-basedperovskites revealadoubletindicatingatransformationfromcubicinto rhom-bohedralstructure[24].AnothercriticalfeaturepresentedinFig.1b is theshiftin 2-thetavalue from32.56◦ to32.84◦. Thiscanbe explainedbythesmallerlatticespacingofCo-basedmaterialsdue tosmallerionicsizeofCo3+_{.XRDdatagiveninFig.1pointoutthat} afterPdimpregnation,theonlydetectablephaseswereperovskite phasesandnootherorderedphasessuchasLa2O3,MnOx,CoOx,Pd orPdOwerevisible.Lackofsuchsignalsislikelyduetothesmall particlesize,lowvolumepercentileorlackofcrystallographicorder insuchphases.AsillustratedintheTEMimagesofPd-impregnated materialsgiveninFig.2,PdO/PdOxparticlesareclearlyvisibleand aredispersedontheperovskitesurfacewithanaverageparticlesize ofca.4nm.ItisworthmentioningthatPdO/PdOxnanoparticleson thesynthesizednanoparticleshavearelativelylowsurface mobil-itywhichenablesarelativelygoodPdO/PdOxdispersionevenafter calcinationat773K.AssumingatomicdispersionforPdOxparticles andaLaMnO3 surfaceatomdensityof1015atoms/cm2,asimple estimationrevealsthatsurfacedispersionofPdOxisca.38%. How-ever,TEMimagesinFig.2suggestthataveragePdOxparticlesize is∼4nmindicatingthattheactualPdOxdispersionissmallerthan theestimatedvalue.TheoxidiccharacterofthePdparticles(i.e.the presenceofPdx+_/Pd2+_{species)isevidentbyaca.+1.5eVshiftinthe} Pd3dXPSbindingenergy(B.E.)valuesobservedforthePd/LaMnO3 andPd/LaCoO3samples(seetheSupportinginformationsection) comparedtothetypicalmetallicPd3dB.E.of335.3eV.

TheresultsoftheBETspecificsurfacearea(SSA)measurements ofthesynthesizedsamplesarepresentedinFig.3.TheMn-based samplescalcinedat973KhavecomparativelyhigherSSAswhich areapproximately20m2_{/g.Ontheotherhand,SSAsofCo-based} samplesare ca. 8m2_{/g.Temperature-dependent structural} evo-lutionbymeansofXRDanalysis(datanotshown)ofperovskite materialsrevealsthatstructuraltransformationfromamorphous toacrystallinestructuretakesplaceatarelativelylower temper-atureforLaCoO3.WhileLaCoO3hasacubiccrystallinestructure at 873K, LaMnO3 is still amorphous at the same temperature.

30

32

34

36

10

20

30

40

50

60

70

80

Cubic LaMnO3 Rhombohedral LaCoO3

2 thet

a

(degrees

)

(a)

(b)

Fig.1. (a)Ex-situXRDpatternscorrespondingtoLaMnO3,Pd/LaMnO3,LaCoO3andPd/LaCoO3samplesaftercalcinationat973Kfor5h.(b)DetailedXRDpatternbetween 29◦_<2<37◦_.

Therefore,itisapparentthatMn-basedperovskitesrevealhigher SSAvaluesandarelessorderedthantheirCo-basedcounterparts. Interestingly,Co-basedcatalystwasreportedtobesignificantly more active than Mn-based catalyst [16] suggesting that the intrinsicrateofNOoxidationoverLaCoO3catalystcouldbemuch greaterthanthatofLaMnO3.Moreover,Fig.3illustratesthatPd additioncausesonlyaminorincreaseintheSSA(i.e.∼1–2m2_/g) forbothMn-andCo-basedperovskites.

Ramanspectroscopycouldbeausefultechnique inorderto identifyvariousphasesinthematerialstructurewhichmightbe elusivetodetectinXRDduetopoorcrystallinityorsmallparticle size.Thus,Ramanspectroscopiccharacterizationofthesynthesized materialshasalsobeenperformedinordertocheckthepresenceof additionalphasesotherthantheperovskitephases.Ramanspectra ofLaMnO3,Pd/LaMnO3,LaCoO3 andPd/LaCoO3arepresentedin

Fig.4.LaMnO3andPd/LaMnO3samplesrevealthreemajorRaman

featuresatca.205,421and644cm−1asillustratedinFig.4aand b,respectively.Theformerfeaturesat210and421cm−1 canbe associated withA1g and Eg modes ofoxygencagerotation and vibrationinLaMnO3perovskite,respectively[25].AnotherRaman featureat644cm−1 canbeattributedtoaMn–O–Mnstretching mode(␯Mn–O–Mn)intheperovskitestructureinaccordancewithLi etal.[26]whoreportedasimilarpeakat654cm−1forMn3O4,as wellasAmmundsenetal.[27]whoalsoreportedaRamansignal at647cm−1forMnO2.Furthermore,thisassignmentisalso consis-tentwiththepreviouslypublishedRamandataonPd/LaMnO3[28] whichrevealedamajorsignalat653cm−1.Thereisalsoanother weakfeaturelocatedatca.520cm−1correspondingtotheLaMnO3 andPd/LaMnO3samples.Xietal.reportedthatthisweakfeature mightberelatedtofluorescencebands[29].Moreover,Ilievetal. assigned thisweakfeaturetotheAg Ramanactivemodeofthe perovskitestructure[30].

Fig.3. BETspecific surface areavalues forLaMnO3, Pd/LaMnO3, LaCoO3 and Pd/LaCoO3aftercalcinationat973Kfor5h.

AsshowninFig.4spectra(c)and(d),LaCoO3andPd/LaCoO3 materialsexhibitaRamanscatteringfeatureatca.409cm−1 and aweakbroadfeatureatca.612cm−1.The409cm−1featurecan beassociatedwiththeperovskite structure.Tanget al. investi-gatedtheRamanspectraofdifferentcobaltcontainingcompounds suchasCoOOH,Co3O4andCoO;andobservedasharpfeatureat ca.650cm−1[31].Thus,itislikelythatthefeatureat615cm−1in spectra(c)and(d)isassociatedwithCo–Ospecies.

Fig.5illustratestherelativesurfaceatomicratiosofthe syn-thesizedmaterialsobtainedvia XPSmeasurements.The atomic ratioswerecalculatedfromthecorrespondingPd3d,La3d,Mn2p, andCo3pspectrabytakingthecorrespondingXPsensitivity fac-torsintoaccount.ForbothPd-freeandPd-containingperovskites, XPSdatarevealthatthesamplesurfacesareenrichedbyLa(i.e. Mn/Laand Co/Lasurfaceatomicratiosarelessthan1).La accu-mulationonthesurfacemostprobablyoccursduetoformation ofLa2(CO3)3,La(OH)3and/oramorphousLa2O3domainstogether

200 400 600 800 1000 (a) (b) (c) (d) Intensity (arb. u.) 10 0 Raman Shift (cm-1₎ 64 4 42 1 20 5 61 2 40 9 53 0 LaMnO3 Pd/LaMnO3 Pd/LaCoO3 LaCoO3

Fig.4.Ex-situRamanspectracorrespondingto(a)LaMnO3,(b)Pd/LaMnO3,(c) LaCoO3and(d)Pd/LaCoO3samplesaftercalcinationat973Kfor5h.

withthe perovskite. As illustrated in a recent study [32], it is possibletocontroltheCo/Lasurfaceatomic ratio,bymodifying thesynthesisparameters.However,thecurrentlyusedsynthesis parameterswerechoseninordertomimicthesyntheticconditions reportedinarecentstudybyGM[16]whichdemonstratedhighly activeperovskitecatalysts.Thepresenceofsurfacecarbonateand hydroxidegroupsisconfirmedbytheC1sandO1sXPspectra(data notshown).Laenrichmentofthesurfacescanalsobeassociated withaLa-terminatedperovskitestructurewhichisconsistentwith recentDensityFunctionalTheory(DFT)calculationsofSchneider etal.,ontheLaCoO3surface,whoreportedthattheLasurface ter-minationisenergeticallythemostfavorableterminationforthe LaCoO3unitcell[33].ItisworthmentioningthatPdloadingdoes notaffecttheMn/LaandCo/Lasurfaceatomicratios.

3.2. NO2adsorption/desorptionbehaviorofperovskitesurfaces

Fig.6presentstheFTIRspectraobtainedforthestepwiseNO2(g) adsorptiononLaMnO3(Fig.6a),LaCoO3(Fig.6b)andLa2O3(Fig.6c) samplesat 323K. FTIRspectra yield a convoluted setof bands correspondingtovarioustypesofnitratesand nitriteswith dif-ferentsurfaceadsorptiongeometries[34–40].Vibrationalfeatures appearing at 1649 and 1009cm−1 are assigned to asymmetric and symmetric stretchingmodes of bridging nitratespecies on theperovskitesurface,respectively[34].Itisevidentthat bridg-ingnitratesarereadilyvisibleontheLaMnO3 perovskite,while thesespeciesarelesspronouncedontheLaCoO3surface.Another typeofadsorbednitrate,revealingvibrationalfeaturesat1530and 1269cm−1canbeassignedtomonodentatenitrates[37].Two char-acteristicvibrationalmodesofbidentatenitratesarealsoobserved onthesurfacewhicharelocatedat1568and1246cm−1[35,36].The vibrationalbandsat1434,1322and804cm−1inFig.6a(andsimilar bandsinFig.6b)canbeassignedto␯N=O,␯N–Oand␦ONOvibrationsof monodentatesurfacenitrites(–O–N=O),respectively[37–40].The weakabsorptionbandsat1090and1194cm−1whichareapparent onlyattheinitialstageofNO2(g)exposureanddisappearathigher exposurescan beassigned tobridging and/orchelating nitrites

[11].Thesimultaneousgrowthofnitrateandnitritespeciescanbe associatedwiththedisproportionationofNO2ontheperovskites surfaces:

2NO2(ads)+O2−(surf)→ NO3−(ads)+NO2−(ads)

Fig.6 suggeststhat duringthe initialstages ofNO2 adsorp-tion,bridgingnitratesandbridging/chelatingnitritesareformed. With further NO2 exposure, the bridging/chelating nitrite are transformedintosurfacenitritospecieswhilebridgingnitrate sig-nalscontinuetogrowtogetherwithmonodentateandbidentate nitrates.FurtherNO2exposureleadstothesimultaneousgrowth ofthenitratesandmonodentatenitrite(nitrito)speciesuntilthe surfacesaresaturatedwithNOx.Alongtheselines,itisapparent thatontheinvestigatedperovskitesurfaces,NO2adsorptionis pre-dominantlyaccompaniedwithitsoxidationtonitratespecies.Thus, thesurfacecoverageofnitratesontheinvestigatedperovskitesnot onlyrevealstherelativeamountofNOxadsorptionbutitcanalso beanindirectindicatorassociatedwiththeinherentNOoxidation capabilitiesofthesesurfaces.

It is visiblethat thegeneralaspects of thevibrational spec-tracorrespondingtoNOxadsorptiongeometriesoftwodifferent perovskitesurfacesshowsignificantresemblances.Thisbehavior canbeexplainedbyconsideringtheXPSresultsgiveninFig.5, which clearlyshowthat both LaMnO3 and LaCoO3 surfacesare La-enriched.Asdiscussedearlier,thiscaneitherbeattributedto La-terminatedperovskite surfacesorthepresence ofadditional amorphousLa2O3 (and/orLa2(CO3)3,La(OH)3)domainsonboth surfacesrevealingsimilaradsorbedNOxspeciesforLaMnO3 and

Fig.5.QuantitativedeterminationofthesurfaceatomicratiosviaXPSforthesynthesizedLaMnO3,Pd/LaMnO3,LaCoO3andPd/LaCoO3samplesaftercalcinationat973Kfor 5h.

LaCoO3.Inotherwords,onbothLaMnO3andLaCoO3,adsorbedNOx speciesaremostlyintheformofnitrateswhicharecoordinatedto eitheramorphousLa2O3and/orLa2(CO3)3,La(OH)3domainsor La-terminatedperovskitesurfaces.Inordertohaveabetterinsight intosurfacefunctionalgroups,benchmarkNOxadsorption exper-imentswerealsoperformedonpureLa2O3 asshown inFig.6c whichrevealssimilarvibrationalfrequenciestothatofLaMnO3and LaCoO3.ThismaysuggestthatalthoughB-sitesmayplayacrucial roleduringtheinitialstages ofNOx adsorption,finaladsorption sitesofthestoredNOxspeciesonLaMnO3andLaCoO3surfacesare moresensitivetotheA-sitesoftheABO3structure,ratherthanthe B-sites.

AnotherimportantobservationregardingtheFTIRdatagiven inFig.6whichisworthemphasizingisthattotalIRsignal inten-sitiesrelatedtoadsorbedNOxspeciesarequitedissimilarfor Co-andMn-basedmaterials.AssessmentoftheIRabsorbancevalues fortheNO2-saturatedLaMnO3andLaCoO3surfaces(i.e.redspectra

inFig.6aandb),immediatelyrevealsthatLaMnO3adsorbs signif-icantlyhigheramountofNOxthanLaCoO3,inaccordancewithits considerablyhigherspecificsurfacearea(Fig.5).

NO2uptakebehaviorsofPd-containingandPd-freeperovskite materialswerealsoinvestigatedinacomparativefashionbymeans ofFTIRspectroscopy.Fig.7showsthecorrespondingFTIR spec-traobtainedafterthesaturationofLaMnO3,Pd/LaMnO3,LaCoO3 andPd/LaCoO3surfaceswith5TorrNO2(g)for10minat323K.It isclearlyvisibleinFig.7thatincorporationofPdspeciesdoesnot significantlyalterthenatureoftheadsorbedNOxspeciesonboth oftheperovskitesurfaces.Inaddition,thepresenceofPdspecies resultinaminorincreaseintheIRabsorptionintensitiesforboth Mn-andCo-basedmaterialswhichisingoodagreementwiththe slightincreaseinthespecificsurfaceareavaluesofthesesurfaces asshowninFig.5.

FurtherinsightregardingtherelationshipbetweenNO oxida-tionandNOxadsorptionaswellasthedesorption/decomposition

800 1000 1200 1400 1600 1800 2000 800 1000 1200 1400 1600 1800 2000 Wavenumber (cm-1₎

Absorbance (arb. u.)

1649 1568 1530 1434 1322 1269 1246 1194 1090 1009 839 804 LaMnO₃ 0.1 800 1000 1200 1400 1600 1800 2000 1565 1526 1441 1327 1272 1242 1194 ₁₀₁₀ 839 803 0.1 0.2 (a) (b)LaCoO₃ (c)La2O3 1632 1603 1578 1520 1448 1355 1254 1016 ₈₀₆ Wavenumber (cm-1_{) Wavenumber (cm}-1₎

Fig.6. FTIRspectracorrespondingtothestepwiseNO2adsorptionat323Konthe(a)LaMnO3(b)LaCoO3and(c)La2O3samples.ThespectracorrespondingtotheNO2-saturated (via5.0TorrNO2(g)overthesamplesurfacefor10minat323K)samplesurfacesaremarkedwithredspectra.

1000

1200

1400

1600

1800

1800

1600

1400

1200

1000

Wavenumber (cm

₎

Absorbance (arb. u.)

Absorbance (arb. u.)

Wavenumber (cm

₎

(b)

(a)

1648

1575

1526

1442

1323

1250

1012

1561

1523

1446

1334

1271

1243

1014

LaMnO

Pd/LaMnO

LaCoO

Pd/LaCoO

0.1

0.1

Fig.7.FTIRspectracorrespondingtotheNO2-saturated(a)LaMnO3andPd/LaMnO3and(b)LaCoO3andPd/LaCoO3at323K.BlackspectraineachpanelrepresentthePd-free materialsandredspectracorrespondtoPd-containingmaterials.

pathwaysoftheadsorbedNOxspeciescanbeobtainedviaTPD.

Fig.8illustratestheTPDprofilesforLaMnO3,Pd/LaMnO3,LaCoO3 and Pd/LaCoO3 samplesobtained after saturationof these sur-faceswithNO2at323K(5.0TorrNO2exposurefor10min)where someof themajordesorptionchannels(m/z=28, 30,32,44, 46

correspondingtoN2,NO,O2,N2OandNO2,respectively)areshown. WiththehelpoftheTPDdata,NOxadsorptiononLaMnO3 and LaCoO3canbecompared.Suchacomparisonclearlyindicatesthat LaMnO3adsorbsagreateramountofNOxthanLaCoO3inexcellent agreementwiththecurrentBET(Fig.3)andin-situFTIR(Fig.6)

300 400 500 600 700 800 900 1000 28 32 30 44 46 300 400 500 600 700 800 900 1000 28 32 30 44 46 300 400 500 600 700 800 900 1000 28 32 30 44 46

LaMnO

LaCoO

Pd/LaMn

O

Temperature (K)

Temperature (K)

Temperature

(K)

Temperature

(K)

QMS

Intensity

(arb.

u.)

QMS

Intensity

(arb.

u.)

Pd/LaCoO

(a)

(b)

(c)

(d)

39

5

46

0

56

5

74

0

39

5

46

0

54

0

79

0

39

5

49

0

55

0

74

0

39

5

63

5

69

0

84

0

44

0

2E-7

2E-7

2E-7

2E-7

measurements.Moreover,integratedNO+NO2+N2+N2OTPD sig-nalsofLaMnO3wasfoundtobe48%higherthanthatofLaCoO3(see theSupportinginformationsectionfordetails).Adetailedanalysis ofthedesorbingspeciesobservedintheTPDdatagiveninFig.8

suggeststhatNOdesorptionoccursinabroadtemperaturewindow (i.e.400–800K)yieldingmultipleandconvoluteddesorption fea-turesandaglobaldesorptionmaximumatca.650K.These convo-lutedNOdesorptionfeaturescanbebetterinterpretedbyanalyzing thesimultaneouslyrecordedadditionaldesorptionchannels.

NO2(m/z=46)desorptionprofilesgiveninFig.8aandb corre-spondingtoLaMnO3andPd/LaMnO3samplespossessalineshape where at leasttwo desorption states arediscernible at ca. 460 and565K.Itiswell-knownthatpureNO2(g)canbereadily frag-mentedinsideaQMSduetoelectronimpactionizationyieldingNO andNO2asthemajorsignals,whereNO:NO2ratioisfoundtobe typicallybetween3:1and4:1.Thus,thecharacteristicNO desorp-tionfeaturesinFig.8aandbappearingat460and565Kcanbe attributedpredominantlytoNO2desorptionfromtheLaMnO3and Pd/LaMnO3surfaceswithsomecontributionfromNOdesorption.It isimportanttonotethatthesedesorptionstatesdonotrevealany simultaneousO2(g)desorptionsignal.Temperature-programmed FTIRexperiments (Supporting information Fig. 1)performed in a parallelfashion tothecurrent TPDexperiments indicatethat nitratesand nitritospecies disappearin agradual anda simul-taneous mannerduring theheating ramp withouta significant changeinthelineshapeoftheFTIRspectra.Thus,forthedesorption windowwithin460–570K,itcanbearguedthatontheLaMnO3and Pd/LaMnO3surfaces,nitritoandnitratespeciesdesorb/decompose intheformofNO2+NO(withoutO2)wheretheOspeciesgenerated duringnitratedecompositioniscapturedbytheperovskite cur-ingoxygendefectsorinthecaseofPd/LaMnO3,possiblyoxidizing Pd/PdOxsitesintoPdO.

Anotherclearlyvisible signalappearing simultaneouslywith theNOandNO2 desorptionchannelsintherangeof490–570K inFig.8aandbisN2O(m/z=44).N2Odesorptionchannelhasa desorptionmaximum atca. 565Kwhich is coincidingwiththe high-temperatureNO2desorptionsignal.DetectionofN2Oisquite noteworthyasitindicatesthatevenintheabsenceofanexternal reducingagent,LaMnO3andPd/LaMnO3surfacescandirectly acti-vateN–Olinkagesinaratherefficientmannerandpartiallyreduce nitrate(NO3−)andnitrite(NO2−)speciesabove500K.AstheN2O formationimpliesthepresenceofatomicNspeciesgeneratedon thecatalystsurface,recombinativedesorptionofatomicNinthe formofN2asthedirecttotalreductionproductisalsolikely.Asa matteroffact,m/z=28desorptionchannelinFig.8aandbclearly revealsthepresenceofN2desorptioninaverybroadtemperature windowwithalocalmaximumatca.565K.ObservationofN2O andN2 intheNO2TPDofLaMnO3 andPd/LaMnO3 clearlyshows thatthesesurfacescanbepromisingmaterialsasDeNOxcatalysts, astheycandirectlyreducestoredNOx species(i.e.nitratesand nitrites)evenintheabsenceofanexternalreducingagent.

Athighertemperatures,anadditionalTPDfeatureinthem/z=32 channelbecomesapparentinFig.8aandb.ThisstrongO2 desorp-tion signallocated within750–800Kis detectedtogether with anintenseNOdesorptionsignalappearingatthesame temper-ature.In otherwords,inthis temperatureinterval,NOx species remainingonthesurfacedecomposeasNO+O2withaless signif-icantcontributionfromN2andN2O(notethatNO2desorptionis notdetectedatthistemperature).Temperature-programmedFTIR studies(SupportinginformationFig.2)suggestthatatthis temper-aturenitritospeciesaretheprominentlyexistingNOxspecieson LaMnO3andPd/LaMnO3withasmallercontributionfromnitrates. Thus,itis apparentthatthesethermallystablebridgingnitrites decomposemostlyintheformofNO+O2.Itisworthmentioning thatthehightemperatureO2desorptionfeatureappearingwithin 750–800Kcanalsobeassociatedtothedirectoxygenevolution

fromLaMnO3andPd/LaMnO3andtheformationofoxygen vacan-ciesintheperovskitestructureorduetothedecompositionofPdO

[41,42].Asa finalnoteonFig.8aandb, itisworthmentioning thatthetotalNOxadsorptionisnotsignificantlyaltereduponPd additiontotheMn-basedperovskitesystemwherepresenceofPd increasestheintegratedtotal NOx(NO+NO2+N2+N2O) desorp-tionsignalbyonlyabout11%(Fig.9).Forthedetailedcalculation oftheintegratedtotalNOxTPDsignals,readerisreferredtothe Supportinginformationsection.

GeneraldesorptioncharacteristicsobservedfortheNO2 TPD experimentsperformedonLaMnO3andPd/LaMnO3surfacesare translatedtoalargeextentontheLaCoO3andPd/LaCoO3samples aswell(Fig.8candd).Asdiscussedearlier,totalintegratedNOx desorptionsignalisabout48%smallerfortheLaCoO3withrespect tothatofLaMnO3 (Fig.9).Ontheotherhand,Pd-incorporation seems to have a larger positive influence on the NOx adsorp-tion for the Co-based systems where the integrated total NOx (NO+NO2+N2+N2O) desorption signalincreases by about 16% (Fig.9).Furthermore,Pdadditionalsoincreasesthethermal sta-bilityofthestoredNOxspeciesshiftingtheirdesorptionsignals tohighertemperatures.Inparticular,Fig.8dalsoillustratesthat Pdadditionleadstoasharplow-temperatureNOdesorption sig-nallocatedat400K.Correspondingtemperature-dependentFTIR dataindicatethat,atthesetemperaturesmonodentateand bridg-ingnitratesarepartiallydestroyed.Itisalsoworthemphasizing thatthisenhancedlow-temperatureNOdesorptionsignalis con-comitanttoN2OandN2signalsappearingatthesametemperature. Thus,itisclearthatPdincorporationhasaminorbutdetectable positiveinfluenceonthelow-temperaturedirecttotal/partialNOx reductionontheCo-basedperovskitesystems.Thisisconsistent withthefactthatunlikeRh-basedcatalysts,itiswellknownthat Pd-basedcatalystsarenotveryactiveindirectNOdissociation.

CombinationofXPSandTPDdatarevealsaninteresting correla-tionbetweentheB-sitecation/La3+_{ratio(i.e.therelativenumberof} B-sitecationsandLa3+_{cations)intheperovskitestructureandthe} correspondingNOxadsorptionquantities.XPSdata(Fig.5)suggests thatincorporationPdintoCo-basedperovskitestructurecausesan increaseinCo/Laatomicratioby16%whichisfollowedbya sim-ilarrelativeincrease(i.e.16%)inthetotalNOxadsorption(Fig.9). Inasimilarfashion,PdadditiontotheMn-basedperovskite struc-tureleadstoanincreaseinMn/Laratioby8%,whichleadsto11% increaseintotalNOxadsorption.ThesetrendssuggestthatB-site cationsplayacrucialroleintheinitialNO2adsorptionand oxida-tionintheformofnitratesandnitrites.Itisfeasiblethattheinitial adsorptionandoxidationofNO2isgovernedbyB-sitecationswhile uponformationofnitrates/nitrites,theseNOxspeciesspilloveron thesurfaceLa–Osites.

3.3. Effectofreductivepre-treatmentonNOxuptakeandcatalyst structure

Structuralintegrityanddurabilityaresomeofthemost criti-calcharacteristicsofalonglastingcatalyticsystem.Preservation ofthestructuralandfunctionalpropertiesofperovskitesisrather challengingduetothestoichiometricflexibilityofthesesystems allowinga vast number of compositional variationsoriginating fromthealterationsintheoxygendefectdensityandthechanges intheoxidationstatesofB-sitecationsintheABO3structure.Since manycatalyticprocessesincludesequentialoxidationand reduc-tioncycles,wehavealsoinvestigatedtheNOxadsorptionbehavior of the currently synthesized LaMnO3, Pd/LaMnO3, LaCoO3 and Pd/LaCoO3 samplesupontheirexposuretoreducingconditions. ItisknownthattreatmentofCoandMn-basedperovskiteswith anaggressivereducingagentsuchasH2(g)athighenough tem-peraturesmaycausereversibleorirreversiblestructuralchanges

Fig.9.IntegratedtotalNOxTPDdesorptionsignalsobtainedforvariousNO2-saturatedperovskitematerials(seetextfordetails).

DacquinandDujardinshowedthatdestructionoftheperovskite structureduetothetwo-stepreductionoftheB-sitecation(via Co3+_→Co+2_→Co0_{)ledtotheformationofmetallicCoandLa}

2O3

[44–46].

Thus,inthecurrentstudy,wehavealsoinvestigatedtheNOx uptakeandreleasepropertiesofLaMnO3,Pd/LaMnO3,LaCoO3and Pd/LaCoO3surfacesafterpretreatingthemwithH2.Itshouldbe notedthat pre-reduction wasperformedwith 5.0Torr H2(g) at 623Kfor10mininbatchmodeconsistentwithsimilarformer stud-iesintheliterature[46].Fig.10presentssuchexperimentswhere fresh(redspectra)andH2pre-reduced(blackspectra)perovskite surfacesweresaturatedwith5.0TorrNO2(g)for10minat323K.

Fig.10illustratesthatonthepre-reducedsurfaces,abundance of nitrite/nitrito species increases as evident from the relative strengtheningofthevibrationalfrequenciesat1487,∼1450and 1330cm−1whilenitrate-relatedvibrationalfeatures(e.g.1567and 1270cm−1)attenuateinarelativefashion[35,36].

Moreimportantly,itisvisibleinFig.10thatH2-pretreatment leadstoa drastic increase in theNOx uptake of both Mn- and Co-basedperovskites.BlackspectruminFig.10cshowsthatthe radicalincreaseintheNOxadsorptionontheLaCoO3surfaceupon H2pretreatmentisaccompaniedbyacharacteristicchangeinthe spectralbaselineintheFTIRdatawhichmaybeassociatedwithan alterationintheelectronicstructureofthesample.These obser-vationscanbeexplainedbythereductionoftheLaCoO3 sample andtheformationofCo0_/CoO/La

2O3phases[17].Implicationsof suchstructuralalterationsarealsoevidentbytheformationoftwo additionalspectralfeatureslocatedat1897and1818cm−1 cor-respondingtodinitrosylspeciesonCo+2_{[47–49](Fig.10c,black} spectrum).Furthermore,itcanbeseenthatPd-incorporationboth enhancestheNOxoxidation(therebyfacilitatingNOxadsorption) afterH2-pretreatment(Fig.10d,blackspectrum)andhindersthe completeB-sitereduction bypreservingthestructuralintegrity oftheperovskitelatticetoacertainextent,evidentbyasmaller changeintheIRspectralbaseline(forinstance,compareFig.10c, blackspectrumwithFig.10d,black spectrum).Influence ofthe

PdsitesontheNOxadsorptionafterH2treatmentcanbe associ-atedwithmultiplereasons.Forinstance,PdsitesinthePd/LaCoO3 systemcanfacilitateH2 activationandleadtotheformationof reactive CoOx surface species which can present high activity toward NO oxidation and NOx uptake without forming a fully reducedformofCo(i.e.Co0_{).Alternatively,metallicPdsites(i.e.} Pd0_{) createdafterreduction with H}

2 canalso directlyfunction asactive redoxsites providing additionalsites forNOx adsorp-tion.

It is interesting tonote that although H2-pretreatmentalso booststheNOxuptakefortheLaMnO3surface(Fig.10a,black spec-trum)toacertainextent,itisnotaccompaniedbyaseverebaseline changeintheFTIRspectrasuggestingthelackofa drastic elec-tronicstructuralmodification.Furthersupportforthisargument willbealsoprovidedtogetherwiththecorrespondingNO2 TPD datafortheH2-pretreatedsurfaces(Fig.11).Inotherwords, Mn-basedsamplesseemlesspronetoB-sitereductionandtheboost intheNOxadsorptionuponH2-pretreatmentand/orPdaddition islessprominentinMn-basedsystems.Yet,indicationsofB-site reductionstillexistsasseenfromthepresenceofthe1773cm−1 givenintheinsetsofFig.10aandb(blackspectra),corresponding tonitrosylspeciescoordinatedtoMn2+_{siteswhicharegenerated} uponreduction[50].

Itisworthmentioningthattheex-situXPSandXRD measure-mentsobtainedimmediatelyaftertheH2-pretreatmentdidnot revealanyindications ofthepresence ofreduced La,CoorMn species.Thus,itislikelythatduringthetransferofthesamples fromtheFTIR/TPDreactor(wherein-situH2-pretreatmentwas per-formed)totheXPSorXRDsetup,sampleswereexposedtoambient atmosphericconditions whichresultedinthere-oxidation.This suggeststhatthein-situreductionphenomenaandthestructural changesobservedfortheperovskitesamplesindirectlyviaFTIRand TPDexperimentsoccurmostprobablyonthesurfacesofthe mate-rialsratherthaninthebulk.Furthermore,itisapparentthatthe reducedsurfacescanreadilybere-oxidizedinaratherreversible fashionunderambientconditions.

800 1000 1200 1400 1600 1800 2000

1897 1818

1567

1487 1441

1330

1270

1031

839

0.2

1773

1649

1567

1487 1453 1330

1270

1029

803

0.2

1901 1818

1561

1470

1334

1271

1028

804

0.2

1771

1571

1488

1334

1251

1023

804

0.2

(a)

(b)

(d)Pd/LaCoO

(c)

LaMnO

LaCoO

Pd/LaMnO

Wavenumber (cm

₎

_{Wavenumber (cm}

₎

Absorbance (arb. u.)

Absorbance (arb. u.)

Fig.10. FTIRspectracorrespondingtotheNO2saturatedsurfacesof(a)LaMnO3,(b)Pd/LaMnO3,(c)LaCoO3and(d)Pd/LaCoO3samplesat323K.Redspectraineachpanel correspondtofreshperovskitesurfacesandblackspectracorrespondtopre-reduced(via5.0TorrH2(g)at623Kfor10min)perovskitematerials.(Forinterpretationofthe referencestocolorinthisfigurelegend,thereaderisreferredtothewebversionofthisarticle.)

Complementary TPD experiments were also performed in orderto havea clearunderstandingof theeffectof a reducing treatmentonNOxuptake.TPDprofilesobtainedbysaturatingthe pre-reducedLaCoO3,Pd/LaCoO3,LaMnO3andPd/LaMnO3surfaces withNO2 at 323K are illustratedin Fig. 11. Although theTPD profilescorrespondingtofreshLaMnO3andLaCoO3(Fig.11aand d)werealreadypresentedinFig.8aandc,thesedataarerevisited (and plotted using a different scale) in Fig. 11 for thesake of comparisonwiththepre-reducedsamples.TPDprofilesinFig.11

areinvery goodagreementwiththeFTIRdatagiven inFig.10

confirmingthe drasticincrease in theNOx uptake of all ofthe H2-pretreatedsamples.Quantitativelyspeaking,incomparisonto thefreshLaMnO3sample,integratedtotalNOxdesorptionsignal inTPDincreasesby46%inthecaseofpre-reducedLaMnO3 and by82%inthecaseofpre-reducedPd/LaMnO3(Fig.9).Inasimilar fashion,incomparisontothefreshLaCoO3sample,integratedtotal NOxdesorptionsignalinTPDincreasesby101%inthecaseof pre-reducedLaCoO3andby196%inthecaseofpre-reducedPd/LaCoO3 (Fig. 9). Furthermore, indications of the radical compositional

andelectronicchangesinferredbytheFTIRdatagiveninFig.10

arealsoapparentintheTPDprofileof thepre-reducedLaCoO3 (Fig.11b),wheretheTPDlineshapeandthedesorptionmaxima undergoasignificantalteration.Thisisinlinewiththedestruction of the perovskite ABO3 lattice to a certain extent upon H2(g) pre-treatmentandtheformationofCo0_/CoO/La

2O3[17].Another very important aspect of the pre-reduced LaCoO3 TPD results (Fig.11b)isthestronglowtemperature(i.e.420K)N2 andN2O evolution,indicatingthecapabilityofthispre-reducedsurfaceto directlyreducestored-nitrate/nitritespecies.Itispossiblethatthe reduced Co0_/Co2+ _{species generated upon H}

2-pretreatment are responsiblefortheenhanceddirectNOxreduction.Theseintense low-temperaturedesorptionsignalscanalsobeassociated with thedesorption/decompositionofweaklybounddinitrosylspecies (illustratedintheFTIRresultsgiveninFig.10)whichareformed onthereducedCo0_/Co2+_{species.Howeveritisworthemphasizing} thatduetotherelativelylowdesorptiontemperatureofallofthe NOxspecies,pre-reducedLaCoO3sampledoesnotseemtopossess asignificanthigh-temperatureNOxadsorptioncapability.

1000 900 800 700 600 500 400 300 32 30 44 46 1000 900 800 700 600 500 400 300 28 14 32 30 44 46 1000 900 800 700 600 500 400 300 28 14 32 30 44 46 1000 900 800 700 600 500 400 300 32 30 44 46 1000 900 800 700 600 500 400 300 28 14 32 30 44 46 1000 900 800 700 600 500 400 300 28 14 32 30 44 46

420

NO

N

N

O

N

LaCoO

LaCoO

(pre-reduced)

1.25E-6

580

430

NO

N

N

O

N

1.25E-6

Pd/LaCoO

(pre-reduced

)

NO

N

O

470

LaMnO

(pre-reduced

)

510

NO

Pd/LaMnO

(pre-reduced

)

1.25E-6

1.25E-6

Temperature (K)

Temperature (K)

QMS Intensity (arb. u.)

QMS Intensity (arb. u.)

Temperature (K)

Temperature (K)

Temperature (K)

Temperature (K)

LaMnO

(a)

(b)

(c)

(d)

(e)

(f)

1.25E-6

1.25E-6

Fig.11.TPDprofilesobtainedafterNO2saturation(via5TorrNO2(g)at323Kfor10min)of(a)LaCoO3,(b)pre-reducedLaCoO3,(c)pre-reducedPd/LaCoO3,(d)LaMnO3,(e) pre-reducedLaMnO3and(f)pre-reducedPd/LaMnO3surfaces.

ComparisonoftheTPDdatafor thepre-reducedLaCoO3 and pre-reducedPd/LaCoO3samples(Fig.11bandc,respectively) sug-gestthatPdadditionhasthreemajoreffectsontheNOxuptake behavioroftheCo-basedperovskitesystems.First,Pdadditionand H2-pretreatment(Fig.11c)furtherbooststheNOxuptakeofthe pre-reducedLaCoO3(Fig.11b)byabout52%basedonthetotalNOx desorptionsignalinTPD(Fig.9).Second,throughPdadditionand H2-pretreatment(Fig.11c),pre-reducedPd/LaCoO3systemgains bothlow-temperature(i.e.430K)aswellashigh-temperature(i.e. 580K)NOxstoragecapabilitieswithoutcompromisingitsability towarddirectNOxreduction andN2/N2Oproduction.Asa mat-teroffact,comparisonofFig.11bandcimmediatelyrevealsthat theamountofN2/N2OgeneratedviadirectNOxreductionis sig-nificantlyenhanceduponPdadditionandpre-reduction.Third,Pd additiontendstopreservetheABO3perovskitestructuretoa cer-tainextent,sincethedesorptionprofilesofpre-reducedPd/LaCoO3 system(Fig.11c)somewhatresemblesthedesorption character-isticsoffreshLaCoO3 (Fig.11a)whilethis isclearlynottruefor thepre-reducedLaCoO3(Fig.11b).Itislikelythatinthepresence ofPd,H2 canreadilybeactivatedanddissociatedonthePdsites

[51].H-speciesgeneratedthiswaycaneitherreducethelocalPdO speciesandformmetallicPd[52]orspillovertheperovskiteto generateoxygendefects.Suchoxygendefectsitescanfunctionas strongadsorptionsitesforNO2(g)species,formingadsorbed nitro-sylsandnitriteswhichboosttheoverallNOxuptake.Inasimilar fashion,metallicPdsitesformeduponH2-pretreatment(whichare wellknowntobeefficientoxidationcatalysts)mayalsoassistthe oxidationofNO2intonitratesandenhancetheNOxadsorption.

AsimilaranalysiscanalsobeperformedfortheNO2TPDprofiles offreshLaMnO3,pre-reducedLaMnO3andpre-reducedPd/LaMnO3 (Fig.11d–f,respectively).Suchananalysisbringsthreemainpoints intoattention.First,bothpre-reductionandPdadditionprocesses haveanoticeablypositiveinfluenceonNOxuptakeofMn-based

perovskitesystems.Second,pre-reductionprocess(inthepresence orabsenceofPd)doesnotsignificantlyaltertheTPDdesorptionline shapesorshiftthedesorptionmaxima,suggestingthattheABO3 structureispreservedtoagreaterextentforMn-basedsystems. Third,relative amountof direct NOx reduction and the forma-tionofN2/N2OislesspronouncedinMn-basedperovskitesystems (Fig.11d–f).

Inoverall,thesignificantboostinNOoxidationandimproved NOx adsorption over Pd promoted and H2 pretreated Co- and Mn-basedperovskitesareconfirmedbybothFTIRandTPD experi-ments.Althoughtheseexperimentsdonotrevealaclearevidence fortheoriginofthisbehavior,itisplausiblethattheincreasein theNOxuptakeuponH2-pretreatmentcanbeassociatedwiththe reduction-inducedmorphologicalchangessuchassurface recons-tructions,whichmayincreasethespecificsurfaceareaandhence theNOxadsorption.Alternatively,theincreaseintheNOxuptake uponreductioncanalsobecloselyrelatedtotheelectronicand compositionalchangesoccurringonthesesurfacessuchasthe par-tial/total reductionof theB-site cation(particularly in thecase ofCo),reductionofPdOspeciesintoPdand/ortheformationof surface oxygen defects which may act as additional anchoring sitesfor NOxspecies. Considering thefactthat many heteroge-neouscatalyticapplicationsrelevanttoautomotiveindustrysuch asNSR/LNTprocessesconsistsofa NOxstorage(lean)cycle fol-lowedbyareduction(rich)cycle;MnandCo-basedsystemscanbe consideredaspromisingcatalyticmaterialswhoseNOxuptakecan beboosted/regeneratedviaonboardcyclicreduction(rich) treat-ments.

4. Conclusions

In the current work, perovskite-based materials (LaMnO3, Pd/LaMnO3, LaCoO3 and Pd/LaCoO3) were synthesized,

characterizedandtheirNOxuptakebehaviorwasinvestigatedas a functionofthenatureof theB-sitecation (i.e.MnvsCo), Pd incorporationandH2-pretreatment.Someofthemajorfindingsof thecurrentstudycanbesummarizedasfollows:

• LaMnO3andPd/LaMnO3revealedacubicperovskitecrystal struc-ture,whereasLaCoO3 and Pd/LaCoO3 exposed bothcubic and rhombohedralphasesaftersynthesisandcalcinationat973K. • SSAvaluesforLaMnO3andPd/LaMnO3weresignificantlyhigher

comparedtothatofLaCoO3andPd/LaCoO3(20.6and21.9m2/gvs 7.1and8.9m2_{/g,respectively).Alongtheselines,theamountof} NOxadsorptiononLaMnO3wasfoundtobesignificantlyhigher thanLaCoO3,inlinewiththehigherSSAofLaMnO3.

• SurfacesofallsampleswerefoundtobeLa-enrichedeitherdue tothepresenceofamorphoussurfaceLa2O3,La2(CO3)3,La(OH)3 domainsorLa-terminatedABO3structure.

• IncorporationofPdO/PdOxnanoparticleswithanaverage diam-eterof ca. 4nmdid not have a significanteffect onthe NOx adsorptionbehavioroverfreshLaMnO3andLaCoO3.

• PerovskitematerialswerefoundtobecapableofactivatingN–O linkagestypicallyatca.550K(evenintheabsenceofanexternal reducingagentsuchasH2)formingN2 andN2OasdirectNOx reductionproducts.

• H2-pretreatmentyieldedadrasticimprovementintheamount ofNOxadsorptionforallsamples,particularlyfortheCo-based systems.PresenceofPdfurtherboostedtheinteractionbetween NOxandthesurfaceoftheH2-pretreatedperovskite.

• Increase in the NOx adsorption of H2-pretreated LaCoO3 and Pd/LaCoO3 surfaces could be associated with the changes in theiroxidationstates(i.e.reductionofB-sitecation),structural changes(surfacereconstructionandSSAincrease),reductionof thepreciousmetaloxide(PdO)intometallic species(Pd)and thegenerationofoxygendefectsontheperovskite.Mn-based systemswerefoundtobemoreresilienttowardB-sitereduction. • Pd-additionwasfoundtosuppresstheB-sitereductionand

pre-servedtheABO3perovskitestructure.

AppendixA. Supplementarydata

Supplementarydataassociatedwiththisarticlecanbefound, intheonline version,athttp://dx.doi.org/10.1016/j.apcatb.2014. 01.038.

References

[1]K.Kato,H.Nohira,K.Nakanishi,S.Iguchi,T.Kihara,H.Muraki,EuroPatent Application0,573,672A1,1993.

[2]N.Myioshi,S.Matsumoto,K.Katoh,T.Tanaka,K.Harada,N.Takahashi,K. Yokota,M.Sugiura,K.Kasahara,SAETechnicalPapersSeriesNo.950809,SAE, 1995.

[3]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.

[4]S.Rousseau,S.Loridant,P.Delichere,A.Boreave,J.Deloume,P.Vernoux,Applied CatalysisB:Environmental88(2009)438.

[5]N. Li, A. Boreave, J. Deloume,F. Gaillard,Solid State Ionics 179 (2008) 1396.

[6]M.A.Pe ˜na,J.L.Fierro,ChemicalReviews101(2001)1981.

[7]J.Zhu,D.Xiao,J.Li,X.Xie,X.Yiang,Y.Wu,JournalofMolecularCatalysisA: Chemical233(2005)29.

[8]Z.Zhao,X.Yang,Y.Wu,AppliedCatalysisB:Environmental8(1996)281. [9]Y.Zhu,D.Wang,F.Yuan,G.Zhang,H.Fu,AppliedCatalysisB:Environmental

82(2008)255.

[10]H.Iwakuni,Y.Shinmyou,H.Yano,H.Matsumoto,T.Ishihara,AppliedCatalysis B:Environmental74(2007)299.

[11]Y.Wu,C.Cordier,E.Berrier,N.Nuns,C.Dujardin,P.Granger,AppliedCatalysis B:Environmental140–141(2013)151.

[12]I.Twagirashema,M.Engelmann-Pirez,M.Frere,L.Burylo,L.Gengembre,C. Dujardin,P.Granger,CatalysisToday119(2007)100.

[13]M.Yung,E.Holmgreen,U.Ozkan,JournalofCatalysis247(2007)356. [14]M.Irfan,J.Goo,S.Kim,AppliedCatalysisB:Environmental78(2008)267. [15]Y.Wen,C.Zhang,H.He,Y.Yu,Y.Teraoka,CatalysisToday126(2007)400. [16]C.Kim,G.Qi,K.Dahlberg,W.Li,Science(2010)1624–1627.

[17]I.Twagirashema,M.Frere,L.Gengembre,C.Dujardin,P.Granger,Topicsin Catalysis42–43(2007)171.

[18]M.Engelmann-Pirez,P.Granger,L.Leclercq,G.Leclercq,TopicsinCatalysis 30–31(2004)59.

[19]C.H.Kim,W.Li,K.A.Dahlberg,USPatent7,964,167B2.

[20]E.Kayhan,S.M.Andonova,G.S.Senturk,C.C.Chusuei,E.Ozensoy,Journalof PhysicalChemistryC114(2010)357.

[21]P.Basu,T.H.Ballinger,J.T.YatesJr.,ReviewofScientificInstruments59(1988) 1321.

[22]Z.Say,E.I.Vovk,V.I.Bukhtiyarov,E.Ozensoy,AppliedCatalysisB: Environmen-tal142–143(2013)89.

[23]M.Popa,M.Kakihana,SolidStateIonics151(2002)251.

[24]X.Zhoua,Y.Zhao,X.Cao,Y.Xue,D.Xua,L.Jiang,W.Su,MaterialsLetters62 (2008)470.

[25]E.Granado,N.O.Moreno,A.Garcia,J.A.Sanjurjo,C.Rettori,I.Torriani,Physical ReviewB:CondensedMatter58(1998)11435.

[26]W.Li,G.V.Gibbs,S.T.Oyama,JournalofAmericanChemicalSociety120(1998) 9041.

[27]B.Ammundsen,G.R.Burns,M.S.Islam,H.Kanoh,J.Rozière,JournalofPhysical ChemistryB103(1999)5175.

[28]J.M.Giraudon,A.Elhachimi,F.Wyrwalski,S.Siffert,A.Aboukais,J.F.Lamonier, G.Leclercq,AppliedCatalysisB:Environmental75(2007)157.

[29]L.Xi,Z.Peng,W.Fan,K.Guo,J.Gu,M.Zhao,J.Meng,MaterialsChemistryand Physics46(1996)50.

[30]M.N.Iliev,M.V.Abrashev,H.G.Lee,V.N.Popov,Y.Y.Sun,C.Thomsen,R.L.Meng, C.W.Chu,PhysicalReviewB:CondensedMatter57(1998)5.

[31]C.W.Tanga,C.B.Wang,S.H.Chien,ThermochimicaActa473(2008)68. [32]Y.Wu,X.Ni,A.Beaurain,C.Dujardin,P.Granger,AppliedCatalysisB:

Environ-mental125(2012)149.

[33]Z.Chen,C.H.Kim,L.T.Thompson,W.F.Schneider,SurfaceScience(2013), http://dx.doi.org/10.1016/susc.2013.09.012.

[34]F.E.López-Suárez,M.J.Illán-Gómez,A.Bueno-López,J.A.Anderson,Applied CatalysisB:Environmental104(2011)261.

[35]B.Klingenberg,M.A.Vannice,AppliedCatalysisB:Environmental21(1999)19. [36]A.Martinez-Arias,J.Soria,J.C.Conesa,X.L.Seoane,A.Arcoya,R.Cataluna,Journal

ofChemicalSociety,FaradayTransactions91(11)(1995)1679.

[37]K.I.Hadjiivanov,CatalysisReviews—ScienceandEngineering42(2000)71. [38]F.Prinetto,G.Ghiotti,I.Nova,L.Lietti,E.Tronconi,P.Forzatti,JournalofPhysical

ChemistryB105(2001)12732.

[39]G.Ghiotti,A.Chiorino,SpectrochimicaActaA49(1993)1345. [40]M.J.D.Low,R.T.Yang,JournalofCatalysis34(1974)3479. [41]Y.Yokoi,H.Uchida,CatalysisToday42(1998)167.

[42]V.A.Bondzie,P.Kleban,D.J.Dwyer,SurfaceScience347(1996)319. [43]M.Crespin,W.K.Hall,JournalofCatalysis69(1981)359.

[44]J.P.Dacquin,C.Dujardin,P.Granger,JournalofCatalysis253(2008)37. [45]J.P.Dacquin,C.Dujardin,P.Granger,CatalysisToday137(2008)390. [46]C.Dujardin,I.Twagirashema,P.Granger,JournalofPhysicalChemistry112

(2008)17183.

[47]T.Montanari,O.Marie,M.Daturi,G.Busca,AppliedCatalysisB:Environmental 71(2007)216.

[48]K.Hadjiivanov,D.Klissurski,G.Ramis,G.Busca,AppliedCatalysisB: Environ-mental7(1996)251.

[49]K.Hadjiivanov,E.Ivanova,M.Daturi,J.Saussey,J.C.Lavalley,ChemicalPhysics Letters370(2003)712.

[50]X.Wang,M.Zhou,L.Andrews,JournalofPhysicalChemistryA104(2000)7964. [51]J.A.Rodriguez,D.W.Goodman,JournalofPhysicalChemistry95(1991)4196. [52]M.Blanco-Rey,D.J.Wales,S.J.Jenkins,JournalofPhysicalChemistryC113