B-sites for NO and adsorption behavior via in-situ spectroscopy Palladium doped perovskite-based NO oxidation catalysts: The role ofPd Applied Catalysis B: Environmental

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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

aDepartmentofChemistry,BilkentUniversity,06800Ankara,Turkey

bBoreskovInstituteofCatalysis,630090Novosibirsk,RussianFederation

cDepartmentofMetallurgical&MaterialsEngineering,MiddleEastTechnicalUniversity,06800Ankara,Turkey

dGeneralMotorsGlobalR&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.Perovskitemateri- alswerefoundtobecapableofactivatingN–Olinkagestypicallyatca.550K(evenintheabsenceofan externalreducingagent)formingN2andN2OasdirectNOxdecompositionproducts.H2-pretreatment yieldedadrasticboostintheNOoxidationandNOxadsorptionofallsamples,particularlyfortheCo- 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-poweredenginesandhasasignifi- cantlynegativeinfluenceontheenvironment.Duringthelasttwo decades,emission regulationshavebecometighter,andnumer- oustechnologiesforNOxafter-treatmenthavebeendevelopedand commercialized.Fortheconventionalgasolineengines,three-way catalysts(TWC)canreducetoxicgaseseffectively undera stoi- chiometricairtofuelratio(14.5);howeverTWCisnoteffective inNOxaftertreatmentindieselenginesoperatingunderoxygen- richleanconditions(wheretheairtofuelratiogreaterthan14.5).

NOxstoragereduction(NSR)catalystsweredevelopedasanalter- nativetechnology[1–3].ConventionalNSRcatalystsarecomposed

∗ Correspondingauthor.Tel.:+903122902121;fax:+903122664068.

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

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

PerovskitesintheformofABO3havebeenconsideredaspromis- 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 playanefficientroleinthecatalyticNOoxidationunderleancon- 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 tobehighlysensitivetowardpretreatmentunderreducingcondi- 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 NO_x 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].Supportedpalladiumcatalystswerepreparedusingclassi- 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.

NO2saturationofthesynthesizedmaterialswascarriedouttyp- icallybydosing5.0TorrNO2(g)overthesamplefor10minat323K.

NO₂(g)usedintheexperimentswaspreparedbymixingNO(g) (AirProducts,99.9%)andO₂(g)(LindeGmbH,Germany,99.999%) followedbymultiplefreeze-pump-thawcyclesforfurtherpurifi- cation.

2.2. Instrumentation

AlloftheFTIRspectroscopicexperimentswereconductedin transmissionmodeusingaBrukerTensor27spectrometercoupled toabatch-typecatalyticreactorwhosedetailshavebeendescribed elsewhere[20,21].AlloftheFTIRspectrawerecollectedat323K.

Alloftheadsorption/pre-treatmentstepswerealsoperformedin batchmode.

PriortoTPDexperiments,sampleweremountedonthespec- 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)undergoesfrag- 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,usingthefragmentationcharacteris- ticsofpureNO₂(g),relativecontributionsofpureNO(g)andpure NO2(g)totheoverallm/z=30signalcanbereadilydistinguished.

Ex-situXPSanalysiswasperformedusinga SPECSXPSspec- 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/LaCoO₃materialsobtainedaftersynthesisandcal- cinationat973K.XRDpatternsverifythatsynthesisofperovskite structureswasachievedasindicatedbythepresenceofthechar- 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 revealadoubletindicatingatransformationfromcubicintorhom- 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 thesynthesizednanoparticleshavearelativelylowsurfacemobil- itywhichenablesarelativelygoodPdO/PdOxdispersionevenafter calcinationat773K.AssumingatomicdispersionforPdOxparticles andaLaMnO₃ surfaceatomdensityof10¹⁵atoms/cm²,asimple estimationrevealsthatsurfacedispersionofPdOxisca.38%.How- ever,TEMimagesinFig.2suggestthataveragePdOxparticlesize is∼4nmindicatingthattheactualPdOxdispersionissmallerthan theestimatedvalue.TheoxidiccharacterofthePdparticles(i.e.the presenceofPd^x+/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 toacrystallinestructuretakesplaceatarelativelylowertemper- atureforLaCoO_3.WhileLaCoO₃hasacubiccrystallinestructure at 873K, LaMnO3 is still amorphous at the same temperature.

30 32 34 36 LaCoO₃

Pd/LaMnO₃

LaMnO₃ Pd/LaCoO₃

10 20 30 40 50 60 70 80

LaCoO₃

Pd/LaMnO₃

LaMnO₃ Pd/LaCoO₃

Cubic LaMnO₃ Rhombohedral LaCoO₃

2 thet a (degrees )

Intensity (a.u.) Intensity (arb. u.)

(a) (b)

32.56o

32.84 33.21

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,thisassignmentisalsoconsis- tentwiththepreviouslypublishedRamandataonPd/LaMnO3[28]

whichrevealedamajorsignalat653cm⁻¹.Thereisalsoanother weakfeaturelocatedatca.520cm⁻¹correspondingtotheLaMnO₃ andPd/LaMnO3samples.Xietal.reportedthatthisweakfeature mightberelatedtofluorescencebands[29].Moreover,Ilievetal.

assigned thisweakfeaturetotheAg Ramanactivemodeofthe perovskitestructure[30].

Fig.2.RepresentativeTEMimagesfor((a)and(b))Pd/LaMnO3and(c)Pd/LaCoO3thatwereinitiallycalcinedat973Kfor5h.

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.5illustratestherelativesurfaceatomicratiosofthesyn- thesizedmaterialsobtainedvia XPSmeasurements.The atomic ratioswerecalculatedfromthecorrespondingPd3d,La3d,Mn2p, andCo3pspectrabytakingthecorrespondingXPsensitivityfac- torsintoaccount.ForbothPd-freeandPd-containingperovskites, XPSdatarevealthatthesamplesurfacesareenrichedbyLa(i.e.

Mn/Laand Co/Lasurfaceatomicratiosarelessthan1).Laaccu- mulationonthesurfacemostprobablyoccursduetoformation ofLa₂(CO₃)₃,La(OH)₃and/oramorphousLa₂O₃domainstogether

200 400 600 800 1000 (a) (b) (c) (d)

Intensity (arb. u.) 100

Raman Shift (cm^-1)

644

421

205 612

409 530

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,whoreportedthattheLasurfaceter- 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 nitriteswithdif- ferentsurfaceadsorptiongeometries[34–40].Vibrationalfeatures appearing at 1649 and 1009cm⁻¹ are assigned to asymmetric and symmetric stretchingmodes of bridging nitratespecies on theperovskitesurface,respectively[34].Itisevidentthatbridg- ingnitratesarereadilyvisibleontheLaMnO₃ perovskite,while thesespeciesarelesspronouncedontheLaCoO3surface.Another typeofadsorbednitrate,revealingvibrationalfeaturesat1530and 1269cm⁻¹canbeassignedtomonodentatenitrates[37].Twochar- 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 transformedintosurfacenitritospecieswhilebridgingnitratesig- nalscontinuetogrowtogetherwithmonodentateandbidentate nitrates.FurtherNO₂exposureleadstothesimultaneousgrowth ofthenitratesandmonodentatenitrite(nitrito)speciesuntilthe surfacesaresaturatedwithNOx.Alongtheselines,itisapparent thatontheinvestigatedperovskitesurfaces,NO₂adsorptionispre- 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)3domainsorLa- terminatedperovskitesurfaces.Inordertohaveabetterinsight intosurfacefunctionalgroups,benchmarkNOxadsorptionexper- imentswerealsoperformedonpureLa2O3 asshown inFig.6c whichrevealssimilarvibrationalfrequenciestothatofLaMnO₃and LaCoO₃.ThismaysuggestthatalthoughB-sitesmayplayacrucial roleduringtheinitialstages ofNOx adsorption,finaladsorption sitesofthestoredNO_xspeciesonLaMnO₃andLaCoO₃surfacesare moresensitivetotheA-sitesoftheABO₃structure,ratherthanthe B-sites.

AnotherimportantobservationregardingtheFTIRdatagiven inFig.6whichisworthemphasizingisthattotalIRsignalinten- sitiesrelatedtoadsorbedNOxspeciesarequitedissimilarforCo- andMn-basedmaterials.AssessmentoftheIRabsorbancevalues fortheNO₂-saturatedLaMnO₃andLaCoO₃surfaces(i.e.redspectra

inFig.6aandb),immediatelyrevealsthatLaMnO₃adsorbssignif- icantlyhigheramountofNOxthanLaCoO₃,inaccordancewithits considerablyhigherspecificsurfacearea(Fig.5).

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

FurtherinsightregardingtherelationshipbetweenNOoxida- 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 12691246 1194 1090 1009 839804

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

16321603 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^-1)

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

Wavenumber (cm^-1) (b)

(a)

1648 1575 1526 1442 1323 1250 1012 1561 1523 1446 1334 1271 1243 1014

LaMnO₃ Pd/LaMnO₃

LaCoO₃ Pd/LaCoO₃

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

LaMnO₃

LaCoO₃

Pd/LaMnO₃

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

Temperature (K)

Temperature (K)

Temperature (K)

Temperature (K)

QMS Intensity (arb. u.)QMS Intensity (arb. u.)

Pd/LaCoO₃

(a) (b)

(c) (d)

NO N₂O

O₂

NO₂ N₂

NO N₂O O₂

NO₂ N₂

NO N₂O

O₂ NO₂ N₂

NO N₂O O₂

NO₂ N₂

395 460 565 740 395 460 540 790

395 490 550 740 395 635 690 840

440

2E-7 2E-7

2E-7 2E-7

Fig.8.TPDprofilesobtainedfrom(a)LaMnO3,(b)Pd/LaMnO3,(c)LaCoO3and(d)Pd/LaCoO3samplesaftersaturationwith5TorrNO2(g)at323Kfor10min.

measurements.Moreover,integratedNO+NO₂+N₂+N₂OTPDsig- nalsofLaMnO3wasfoundtobe48%higherthanthatofLaCoO3(see theSupportinginformationsectionfordetails).Adetailedanalysis ofthedesorbingspeciesobservedintheTPDdatagiveninFig.8 suggeststhatNOdesorptionoccursinabroadtemperaturewindow (i.e.400–800K)yieldingmultipleandconvoluteddesorptionfea- turesandaglobaldesorptionmaximumatca.650K.Theseconvo- lutedNOdesorptionfeaturescanbebetterinterpretedbyanalyzing thesimultaneouslyrecordedadditionaldesorptionchannels.

NO₂(m/z=46)desorptionprofilesgiveninFig.8aandbcorre- spondingtoLaMnO3andPd/LaMnO3samplespossessalineshape where at leasttwo desorption states arediscernible at ca. 460 and565K.Itiswell-knownthatpureNO₂(g)canbereadilyfrag- mentedinsideaQMSduetoelectronimpactionizationyieldingNO andNO2asthemajorsignals,whereNO:NO2ratioisfoundtobe typicallybetween3:1and4:1.Thus,thecharacteristicNOdesorp- 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/PdO_xsitesintoPdO.

Anotherclearlyvisible signalappearing simultaneouslywith theNOandNO2 desorptionchannelsintherangeof490–570K inFig.8aandbisN₂O(m/z=44).N₂Odesorptionchannelhasa desorptionmaximum atca. 565Kwhich is coincidingwiththe high-temperatureNO2desorptionsignal.DetectionofN2Oisquite noteworthyasitindicatesthatevenintheabsenceofanexternal reducingagent,LaMnO₃andPd/LaMnO₃surfacescandirectlyacti- 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.ThisstrongO₂desorp- tion signallocated within750–800Kis detectedtogether with anintenseNOdesorptionsignalappearingatthesametemper- ature.In otherwords,inthis temperatureinterval,NOx species remainingonthesurfacedecomposeasNO+O2withalesssignif- icantcontributionfromN₂andN₂O(notethatNO₂desorptionis notdetectedatthistemperature).Temperature-programmedFTIR studies(SupportinginformationFig.2)suggestthatatthistemper- aturenitritospeciesaretheprominentlyexistingNO_xspecieson LaMnO₃andPd/LaMnO₃withasmallercontributionfromnitrates.

Thus,itis apparentthatthesethermallystablebridgingnitrites decomposemostlyintheformofNO+O₂.Itisworthmentioning thatthehightemperatureO₂desorptionfeatureappearingwithin 750–800Kcanalsobeassociatedtothedirectoxygenevolution

fromLaMnO₃andPd/LaMnO₃andtheformationofoxygenvacan- ciesintheperovskitestructureorduetothedecompositionofPdO [41,42].Asa finalnoteonFig.8aandb, itisworthmentioning thatthetotalNOxadsorptionisnotsignificantlyaltereduponPd additiontotheMn-basedperovskitesystemwherepresenceofPd increasestheintegratedtotal NO_x(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,Pdadditionalsoincreasesthethermalsta- bilityofthestoredNOxspeciesshiftingtheirdesorptionsignals tohighertemperatures.Inparticular,Fig.8dalsoillustratesthat Pdadditionleadstoasharplow-temperatureNOdesorptionsig- nallocatedat400K.Correspondingtemperature-dependentFTIR dataindicatethat,atthesetemperaturesmonodentateandbridg- ingnitratesarepartiallydestroyed.Itisalsoworthemphasizing thatthisenhancedlow-temperatureNOdesorptionsignaliscon- comitanttoN₂OandN₂signalsappearingatthesametemperature.

Thus,itisclearthatPdincorporationhasaminorbutdetectable positiveinfluenceonthelow-temperaturedirecttotal/partialNOx

reductionontheCo-basedperovskitesystems.Thisisconsistent withthefactthatunlikeRh-basedcatalysts,itiswellknownthat Pd-basedcatalystsarenotveryactiveindirectNOdissociation.

CombinationofXPSandTPDdatarevealsaninterestingcorrela- tionbetweentheB-sitecation/La³⁺ratio(i.e.therelativenumberof B-sitecationsandLa³⁺cations)intheperovskitestructureandthe correspondingNOxadsorptionquantities.XPSdata(Fig.5)suggests thatincorporationPdintoCo-basedperovskitestructurecausesan increaseinCo/Laatomicratioby16%whichisfollowedbyasim- ilarrelativeincrease(i.e.16%)inthetotalNOxadsorption(Fig.9).

Inasimilarfashion,PdadditiontotheMn-basedperovskitestruc- tureleadstoanincreaseinMn/Laratioby8%,whichleadsto11%

increaseintotalNOxadsorption.ThesetrendssuggestthatB-site cationsplayacrucialroleintheinitialNO₂adsorptionandoxida- tionintheformofnitratesandnitrites.Itisfeasiblethattheinitial adsorptionandoxidationofNO₂isgovernedbyB-sitecationswhile uponformationofnitrates/nitrites,theseNOxspeciesspilloveron thesurfaceLa–Osites.

3.3. Effectofreductivepre-treatmentonNOxuptakeandcatalyst structure

Structuralintegrityanddurabilityaresomeofthemostcriti- calcharacteristicsofalonglastingcatalyticsystem.Preservation ofthestructuralandfunctionalpropertiesofperovskitesisrather challengingduetothestoichiometricflexibilityofthesesystems allowinga vast number of compositional variationsoriginating fromthealterationsintheoxygendefectdensityandthechanges intheoxidationstatesofB-sitecationsintheABO₃structure.Since manycatalyticprocessesincludesequentialoxidationandreduc- tioncycles,wehavealsoinvestigatedtheNOxadsorptionbehavior of the currently synthesized LaMnO₃, Pd/LaMnO₃, LaCoO₃ and Pd/LaCoO₃ samplesupontheirexposuretoreducingconditions.

ItisknownthattreatmentofCoandMn-basedperovskiteswith anaggressivereducingagentsuchasH₂(g)athighenoughtem- peraturesmaycausereversibleorirreversiblestructuralchanges [6,43].ThiswasfoundtobeparticularlyvalidforLaCoO3,where

Fig.9.IntegratedtotalNOxTPDdesorptionsignalsobtainedforvariousNO2-saturatedperovskitematerials(seetextfordetails).

DacquinandDujardinshowedthatdestructionoftheperovskite structureduetothetwo-stepreductionoftheB-sitecation(via Co³⁺→Co⁺²→Co⁰)ledtotheformationofmetallicCoandLa2O3

[44–46].

Thus,inthecurrentstudy,wehavealsoinvestigatedtheNOx

uptakeandreleasepropertiesofLaMnO3,Pd/LaMnO3,LaCoO3and Pd/LaCoO₃surfacesafterpretreatingthemwithH₂.Itshouldbe notedthat pre-reduction wasperformedwith 5.0Torr H₂(g) at 623Kfor10mininbatchmodeconsistentwithsimilarformerstud- 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.Theseobser- vationscanbeexplainedbythereductionoftheLaCoO3 sample andtheformationofCo⁰/CoO/La₂O₃phases[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

PdsitesontheNOxadsorptionafterH₂treatmentcanbeassoci- 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 H₂ canalso directlyfunction asactive redoxsites providing additionalsites forNOx adsorp- tion.

It is interesting tonote that although H₂-pretreatmentalso booststheNOxuptakefortheLaMnO₃surface(Fig.10a,blackspec- trum)toacertainextent,itisnotaccompaniedbyaseverebaseline changeintheFTIRspectrasuggestingthelackofa drasticelec- 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-situXPSandXRDmeasure- mentsobtainedimmediatelyaftertheH2-pretreatmentdidnot revealanyindications ofthepresence ofreduced La,CoorMn species.Thus,itislikelythatduringthetransferofthesamples fromtheFTIR/TPDreactor(wherein-situH₂-pretreatmentwasper- formed)totheXPSorXRDsetup,sampleswereexposedtoambient atmosphericconditions whichresultedinthere-oxidation.This suggeststhatthein-situreductionphenomenaandthestructural changesobservedfortheperovskitesamplesindirectlyviaFTIRand TPDexperimentsoccurmostprobablyonthesurfacesofthemate- rialsratherthaninthebulk.Furthermore,itisapparentthatthe reducedsurfacescanreadilybere-oxidizedinaratherreversible fashionunderambientconditions.

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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 NO_xdesorptionsignalinTPDincreasesby101%inthecaseofpre- 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/La₂O₃[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 H₂-pretreatment are responsiblefortheenhanceddirectNOxreduction.Theseintense low-temperaturedesorptionsignalscanalsobeassociated with thedesorption/decompositionofweaklybounddinitrosylspecies (illustratedintheFTIRresultsgiveninFig.10)whichareformed onthereducedCo⁰/Co²⁺species.Howeveritisworthemphasizing thatduetotherelativelylowdesorptiontemperatureofallofthe NOxspecies,pre-reducedLaCoO₃sampledoesnotseemtopossess asignificanthigh-temperatureNOxadsorptioncapability.

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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.Asamat- teroffact,comparisonofFig.11bandcimmediatelyrevealsthat theamountofN₂/N₂OgeneratedviadirectNOxreductionissig- nificantlyenhanceduponPdadditionandpre-reduction.Third,Pd additiontendstopreservetheABO₃perovskitestructuretoacer- tainextent,sincethedesorptionprofilesofpre-reducedPd/LaCoO₃ system(Fig.11c)somewhatresemblesthedesorptioncharacter- isticsoffreshLaCoO3 (Fig.11a)whilethis isclearlynottruefor thepre-reducedLaCoO₃(Fig.11b).Itislikelythatinthepresence ofPd,H₂ canreadilybeactivatedanddissociatedonthePdsites [51].H-speciesgeneratedthiswaycaneitherreducethelocalPdO speciesandformmetallicPd[52]orspillovertheperovskiteto generateoxygendefects.Suchoxygendefectsitescanfunctionas strongadsorptionsitesforNO2(g)species,formingadsorbednitro- sylsandnitriteswhichboosttheoverallNO_xuptake.Inasimilar fashion,metallicPdsitesformeduponH₂-pretreatment(whichare wellknowntobeefficientoxidationcatalysts)mayalsoassistthe oxidationofNO₂intonitratesandenhancetheNO_xadsorption.

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-basedperovskitesareconfirmedbybothFTIRandTPDexperi- ments.Althoughtheseexperimentsdonotrevealaclearevidence fortheoriginofthisbehavior,itisplausiblethattheincreasein theNOxuptakeuponH₂-pretreatmentcanbeassociatedwiththe reduction-inducedmorphologicalchangessuchassurfacerecons- tructions,whichmayincreasethespecificsurfaceareaandhence theNOxadsorption.Alternatively,theincreaseintheNOxuptake uponreductioncanalsobecloselyrelatedtotheelectronicand compositionalchangesoccurringonthesesurfacessuchasthepar- 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 manyheteroge- neouscatalyticapplicationsrelevanttoautomotiveindustrysuch asNSR/LNTprocessesconsistsofa NOxstorage(lean)cyclefol- 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,