SO x uptake and release properties of TiO 2 /Al 2 O 3 and BaO/TiO 2 /Al 2 O 3 mixed oxide systems as NO x storage materials

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Catalysis Today

jo u r n al h om epa ge :w w w . e l s e v i e r . c o m / l oc a t e / c a t t o d

SO _x uptake and release properties of TiO ₂ /Al ₂ O ₃ and BaO/TiO ₂ /Al ₂ O ₃ mixed oxide systems as NO x storage materials

Göksu S. S¸ entürk

, Evgeny I. Vovk

, Vladimir I. Zaikovskii

, Zafer Say

, Aslı M. Soylu

, Valerii I. Bukhtiyarov

, Emrah Ozensoy

aChemistryDepartment,BilkentUniversity,06800Bilkent,Ankara,Turkey

bBoreskovInstituteofCatalysis,630090Novosibirsk,RussianFederation

a r t i c l e i n f o

Articlehistory:

Received15September2011 Receivedinrevisedform 23November2011 Accepted1December2011 Available online 30 December 2011

Keywords:

Al2O3

BaO TiO2

Anatase Sulfurpoisoning DeNOx

NOx

SOx

Sulfation NSR LNT HDS ClausProcess

a b s t r a c t

Titaniawas used as a promoter toobtain novel materials in the form of TiO2/Al2O3 (Ti/Al)and BaO/TiO2/Al2O3(Ba/Ti/Al,containing8wt%or20wt%BaO)thatarerelevanttoNOxstoragereduction (NSR)catalysis.Twodifferentprotocols(P1,P2)wereutilizedinthesynthesis.Ti/Al(P1)manifestsitselfas crystallitesofTiO2on␥-Al²O3,whileTi/Al(P2)revealsanamorphousAlxTiyOzmixedoxide.Thestructures ofthesynthesizedmaterialswereinvestigatedviaTEM,EDX,BETanalysisandXPSwhilethecatalytic functionality/performanceofthesesupportmaterialsuponSOxandsubsequentNOxadsorptionwere investigatedwithTPDandinsituFTIRspectroscopy.Ti/Al(P1,P2)revealedahighaffinitytowardsSOx. OverallthermalstabilitiesoftheadsorbedSOxspeciesandthetotalSOxuptakeoftheBa-freesamples increaseinthefollowingorder:TiO2(anatase)␥-Al2O3<Ti/Al(P1)<Ti/Al(P2).ThesuperiorSOxuptake ofTi/Al(P1,P2)supportmaterialscanbetentativelyattributedtotheincreasingspecificsurfacearea uponTiO2promotionand/orthechangesinthesurfaceacidity.PromotionofBaO/Al2O3withTiO2leads totheattenuationoftheSOxuptakeandasignificantdecreaseinthethermalstabilityoftheadsorbed SOxspecies.TherelativeSOxadsorptioncapacitiesoftheinvestigatedmaterialscanberankedasfollows:

8Ba/Ti/Al(P1)<8Ba/Ti/Al(P2)<8Ba/Al∼20Ba/Ti/Al(P1)<20Ba/Al<20Ba/Ti/Al(P2).

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Controllingthesurfacechemistryofmixed-oxidesurfacesisof vitalimportancetodesignnovelcatalyticmaterialswithunprece- dentedfunctionalities.Sulfur poisoningonmetaloxidesurfaces isoneof thefrequentlyobservedsurface deactivationphenom- enainheterogeneouscatalysis.AccumulationofSOxspecies on metaloxidesurfaceshavebeenverycommonlystudiedintheliter- atureinrelevancetothree-waycatalysis(TWC),selectivecatalytic reduction(SCR),NO_xstoragereduction(NSR),hydrodesulfuriza- tion(HDS)andothercatalyticprocesses,whereAl₂O₃isutilizedas themainsupportmaterial[1–7].Twomajordeactivationphenom- enaareoftenreportedfortheNSRcatalysts.Thefirstrouteinvolves thermaldegradationofthestructuralintegrityofthecatalystmate- rialduetosolidstatereactionsbetweenthecatalystcomponents

∗ Correspondingauthor.

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

andsinteringwhilethesecondrouteisassociatedwiththesulfur poisoning [8,9].The latterdeactivationphenomenon is particu- larlyachallengingproblem.InNSRapplications,NOx(g)andSOx(g) adsorbates,competeforthesamebasicadsorptionsitesonoxide surfaces.Consequently,maximizationoftheNOxstoragecapac- ity(NSC)whileminimizingtheirreversibleSOxuptakeoftheNOx

storagesitesimpliescarefuloptimizationofthesurfaceproperties ofthecatalyticsupportmaterialssuchasthesurfacecomposition, acidity,morphologyandspecificsurfacearea(SSA).

SulfurpoisoningofNSRcatalyststypicallyleadstotheforma- tionofalkalineearth/preciousmetalsulfates,sulfitesorsulfides [8]. For a large number of oxide substrates, the stability of some of the common adsorbates increases in the following order:NO₂⁻∼CO₃²⁻<NO₃⁻<SO₄²⁻ [10–15]. Thus, sulfur effec- tively blocks the catalytic sites for NOx storage and gradually reducestheoverallNOxstoragecapacityoftheNSRcatalysts[16].

Effortstowardsimprovingthecatalytictoleranceagainstsulfur poisoninganddesigninghighlyactiveandstablenovelcatalysts arevitalfortheglobalizationoftheNSRtechnology[14,15,17–20].

0920-5861/$–seefrontmatter © 2011 Elsevier B.V. All rights reserved.

doi:10.1016/j.cattod.2011.12.006

Scheme1.SynthesisofTiO2-promotedNOxstoragematerials:P1andP2.

Misonoand Inui[21],Fritzand Pitchon [22]and severalothers [12,13,23–27]reporteddetailedstudiesontheimprovementand thedurabilityenhancementofNSRcatalysts.Thecommercialcat- alystsarealsosuccessfullyusedinlimitedmarketssuchasJapan wherethesulfurcontentofthedieselfuelisrelativelylow(below 10ppm)[28].AccordingtoYamazakietal.,Fewasfoundtodecrease thesulfuruptakeandpromotethedecompositionofBaSO₄ [29].

Fansonetal.observedthatFefacilitatedtheformationofabulk nitratespecieswhichweresuggestedtoberesilientagainstsulfur poisoning[30].Kayhanetal.reportedthattheinitialNO_xuptake mechanismandtheratiobetweenthesurfaceandbulkBaOsites weresignificantlyinfluencedbyFeloading[20].

TiO₂ containingsupportmaterialswerealsoreportedtosup- pressthesulfurpoisoning[31,32].Thiseffectwasassociatedwith thesurfaceacidityofTiO2,inhibitingtheadsorptionofacidicSOx

species[33].Decompositiontemperatureofthesulfatesonpure TiO₂ isknowntobelowerthanthatof␥-Al2O₃[34,35].Further- more,activesitesonthesulfatedTiO₂surfacecanalsobereadily regeneratedunderreactionconditions[36].Recently,itwasalso reportedthatTiO₂sitesonthesupportsurfacecanfunctionaseffi- cientanchoringsitesforBaO,whichmaybeexploitedtoenhance thesurfacedispersionofBaOdomainsandfine-tunethesurface morphology[37].

Inordertocircumventsomeoftheunfavorablepropertiesof pureTiO2suchasthelimitedthermalstability,lowsurfacearea, andpoormechanicalproperties,itscombinationwithsecondary oxidesareutilizedtodesignnovelsupportmaterialswithenhanced properties.Alongtheselines,␥-Al2O3[14,19,32,36,38–42]and/or ZrO₂[43–45]arepromisingchoicesforthesecondaryoxidesthat can beused in combination withTiO₂. It was reported that if sulfur-poisonedBa/Pt/Al₂O₃catalystwasblendedwithnonsulfur- poisonedPt/TiO2catalyst,sulfurdesorptionfromtheBa/Pt/Al2O3

catalystunderrich conditionsisimproved[14].Furthermore,it wassuggestedthat theinterfacebetweenAl₂O₃ andTiO₂ plays animportantroleinthesulfate decompositionandthedesorp- tionprocesses[32].Matsumotoandco-workers[14,46]reported thatacombinationofTiO₂ andlithium-doped␥-Al2O₃ presents anoptimumsurfaceaciditytowardssulfurpoisoning.Themacro- scopicgeometricalstructure ofthe catalytic monolithwasalso foundtobeeffectivein limitingthesizeofthesulfate particles andcontrollingthesulfatedecomposition/desorptiontemperature [47,48].Itwasshownthatsulfatedecompositionisfacilitatedon

thesmallersulfatedomains.Therealsoexistanumberofsurface sciencestudiesonplanarmodelNSRcatalysts[49–53]focusingon thestructureandtheoperationalprinciplesofNSRsystemsatthe molecularlevel.Despitetheestablishedsulfurresistingeffectof TiO₂asanadditiveinthecompositionofthePt/Ba/␥-Al2O₃cata- lysts,anumberofcrucialaspectsregardingtheinfluenceofTiO2

ontheinteractionbetweentheNOxstoragecomponentandthe supportmaterialhavenotyetbeenelucidated.

Thus,inthecurrentwork,wefocusourattentionontheinter- action ofSO_xwiththeTiO₂/Al₂O₃ and BaO/TiO₂/Al₂O₃ surfaces aswellastheinfluenceofTiO₂domainsontheNOxuptakeafter deactivationbySO2(g)+O2(g).

2. Experimental

TiO₂/Al₂O₃binaryoxidesupportmaterials(whichwillbehere- afterreferredasTi/Alinthetext)werepreparedviatwodifferent syntheticprotocolsP1[23,37]andP2[37]whichweredescribed in detail in ourformer reports. These syntheticprotocolswere schematically described in Scheme 1. Briefly, in the first syn- theticprotocol(P1),␥-Al2O₃(PURALOX,200m²/g,SASOLGmbH, Germany)andTiCl₄(Fluka,titanium(IV)chloridesolution∼0.1M in20%hydrochloric acid)wereusedasstartingmaterials.TiCl4

wasdilutedincooleddeionizedwaterundercontinuousstirring atatemperaturebelow333K.Then,␥-Al2O₃powderwasslowly added to theprepared solution. Next, 30vol% NH3 was slowly addedtothesolutionuntilpH≥9.0wasachievedandagelwas formed.Thiswhite gelwasagedfor 24hunderambientcondi- tions,filtered,washedwithdistilledwater,andcalcinedat873K for 2hin air. In thesecond syntheticprotocol (P2), thebinary Ti/Aloxidesupportmaterialwassynthesizedbyasolgelmethod.

In this synthetic protocol (P2), titanium and aluminum alkox- ides were used as precursors. First, aluminum-tri-sec-butoxide (97%,Sigma–Aldrich)wasdissolvedinthemixtureofpropan-2- ol(99.5+%,Sigma–Aldrich)andacetylacetone(99.3%,Fluka).Then, titanium(IV)isopropoxide(97%,Sigma–Aldrich)wasaddedtothe solutionatroomtemperature. Next,0.5MHNO₃ wasgradually addedtothesolutionin ordertoinitiatethegelformation.The resulting gel was aged for 10 days under ambient conditions, ground, and baked at873Kfor 2hin air. The molefractionof TiO₂ (i.e. TiO₂=nTiO₂/(nTiO₂+nAl₂O₃))intheTiO₂/Al₂O₃binary mixture wasequal to 0.3 (based onthe amounts of Ti and Al

precursorsusedinthesynthesis)inbothofthesyntheticprotocols.

BaO/TiO2/␥-Al2O3 (whichwillbehereafterreferred asBa/Ti/Al) sampleswithdifferentBaloadings(8and20wt%BaO)weresyn- thesizedbyconventionalincipientwetnessimpregnationofthe Ti/AlbinaryoxidesupportmaterialswithBa(NO3)2.In thecur- renttext,sampleswith8and20wt%BaOloadingwillbedenoted as 8Ba/Ti/Al(P1, P2) and 20Ba/Ti/Al(P1, P2), respectively. For a comprehensivediscussionofthestructuralcharacterizationofthe synthesized materials via SEM, EDX, BET, Raman spectroscopy andXRD,theanalysisofthesurfaceacidityoftheTi/Al(P1)and 8/20Ba/Ti/Al(P1)samplesviapyridineadsorption,aswellasNOx

adsorption/releasepropertiesviaFTIRandTPD,readerisreferred toourpreviouslypublishedwork[23,37]whichwillalsobeused frequentlyinthediscussionofthecurrentresults.␥-Al2O₃(Puralox, 200m²/g,SASOLGmbH,Germany)usedin(P1)syntheticprotocol wasalsoutilizedasareferencematerialinsomeexperiments.

TEMimageswere obtainedwitha resolution of 0.14nm on a JEM-2010 (200keV) microscopeequipped withan EDX spec- trometerwitha Si (Li) detectorhavinganenergy resolution of 130eV. The analyzed area in a typical EDX measurement was about10×10nm².ThesamplesfortheTEManalysiswerepre- paredby dispersing thepowders in anultrasonic ethanol bath andsubsequentdepositionofthesuspensionupona“holey”car- bonfilmsupportedonacopperTEMgrid.XPSdatawererecorded using a SPECS spectrometer with a PHOIBOS-100 hemispheri- calenergyanalyzerandamonochromaticAlK_␣X-rayirradiation (h=1486.74eV, 200W). FTIRspectroscopic measurementswere carriedoutintransmissionmodeinabatch-typecatalyticreac- tor[20]coupledtoanFTIRspectrometer(BrukerTensor27)and a quadrupole mass spectrometer(QMS,Stanford Research Sys- tems,RGA200)usedforTPDexperiments.AllFTIRspectrawere acquiredat323K.IntheTPDexperiments,alineartemperature rampwithaheatingrateof12K/minwasutilizedtoheatthesam- plewithin323–1023K.TheQMSsignalswithm/zequalto18(H2O), 28(N₂/CO), 30(NO),34(H₂S),32(O₂), 44(N₂O/CO₂), 46(NO₂)and 64(SO₂)weremonitoredduringtheTPDmeasurements.BETSSA andporesizedistributionmeasurementswereperformedusinga MicromeriticsTristar3000surfaceareaandporesizeanalyzerby low-temperatureisothermaladsorption–desorptionofN₂.

Thesulfurexposureexperimentswereperformedthroughfour differentconsecutivesteps.In thefirstspectralacquisitionstep, the sample was exposed to 0.6Torr of SO₂(g)+O₂(g) mixture (SO₂:O₂=1:10)for1hat323Kand thefirstFTIRspectrum was obtainedinthepresenceoftheSO2(g)+O2(g)mixtureoverthe sample.Inthesecondstep,thesamplewasannealedto473Kin thepresenceoftheSO₂(g)+O₂(g),andaftercoolingto323K,the secondFTIRspectrumwasacquired.Inthethirdstep,beforethe spectrumacquisition,thesamplewasannealedto673K(inthe presenceofSO₂(g)+O₂(g)),thencooledto323Kandsuccessively evacuatedat323Kfor20min(Preactor<1×10⁻⁴Torr).Inthefourth step,thesamplewasflashedto673Kinvacuumandaftercool- ingthesampleto323K,aFTIRspectrumwasobtained.ForNO₂ adsorptionexperiments,thefreshsampleswerepre-poisonedby anexposureof0.6TorrofSO2(g)+O2(g)(SO2:O2=1:10)mixture at323Kandwerefurtherheatedinthegasmixtureat473Kfor 30min.Afterhavingpumpedoutthereactor,poisonedsamplewas exposedto8TorrofNO2(g)at323Kfor20mininordertosaturate thesurfacewithNOx.ThisisfollowedbytheevacuationandFTIR spectraacquisitionat323K.

3. Resultsanddiscussion

3.1. TEMcharacterizationofTi/Almaterials

Fig.1representstheTEMmicrographsoftheTi/Al(P1,P2)sam- plesshowingthemorphologyofthesynthesizedsupportmaterials.

EDXanalysisofarea1giveninFig.1a,whichisabundantindarker domains,revealsthepresenceofTiO2andAl2O3containingspecies withaTi:Alatomicratioequalto70:30.EDXanalysisofthebright domainssuchasarea2inFig.1a,demonstratedthattheseareas exclusivelycontainAl2O3.However,theFourieranalysisofarea2 inFig.1adidnotrevealwell-definedspots,mostlikelyduetothe smallparticlesizesandthedefectivenatureofthesealuminacrys- tallites.TheimageinFig.1bshowsahigh-resolutionTEM(HRTEM) micrographofsuchabrightdomain.

ItisvisibleinFig.1athatthedarkerdomainscontainaggregates withsizesofabout10nm.Fig.1cshowsaHRTEMimageofone ofthesedarkerdomains.InterplanarspacingmeasuredonHRTEM micrographsofthesedarkerdomainsareinverygoodagreement withtheanatasephaseofTiO₂.FastFourierTransform(FFT)picture obtainedfromthisimageisalsoshownasaninsetinFig.1cand isingoodagreementwiththereflexesofanatasephase(fileno.

21-1272inPDF-2Database,JCPDS-ICDD).

Fig.1d–fshowsTEMimagesoftheTi/Al(P2)sample.Itisclearly visibleintheseimagesthatthissampledisplaysaratherhomoge- nousmorphologyexhibitingauniformsponge-likefinestructure whichisduetothemesoporouscharacterofthemixedTixAlyOz

oxidephasethatisabundantinTiO_x/AlO_xinterfaces.Area1given in Fig. 1e highlights a characteristic region where this rather homogenousandporousstructureisapparent.Ti:Alelementalratio obtainedfromtheEDXspectrumofarea1inFig.1discloseto 15:85which isinvery goodagreementwiththenominalcom- positionexpectedfrom therelative concentrationsof TiandAl precursorsusedin thematerialsynthesis (20:80).On theother hand,EDX elementalanalysisof a lesscharacteristic (minority) regioninFig.1d(labeledasarea2)revealsthepresenceofalmost exclusivelyAl2O3 particles.TheTEMimagegivenin Fig.1eand HRTEMimagegiveninFig.1fshowthecharacteristicsponge-like structureofTi/Al(P2).FFTimageoftheTEMimagegiveninFig.1f isalsopresentedasaninset.FuzzycharacteroftheFFTimageis inagreementwiththeamorphouscharacteroftheTi_xAl_yO_zoxide phase withsmall anddisorderedAlOxand TiOx domains.How- ever,the steepintensitychanges near0.45nm fromthecenter showsthat disorderedstructure isprobablyduetothealumina spinelstructure(thisdistancecanbeassociated withd-spacing between(111)closepackedoxygenplanesinthelatticeof␥-Al2O₃ (fileno.29-0063inPDF-2Database,JCPDS-ICDD).FFTfilteredpic- ture(Fig.1g)supportsthelackofanorderedcrystallinestructure.

TheseobservationsindicatethattheTi/Al(P2)structureismostly composedofa TixAlyOz mixedoxideexhibitinga largeconcen- tration ofTiO_x/AlO_x interfacialsites,while theTi/Al(P1) reveals aninhomogeneousdistributionoflargerAl₂O₃andTiO₂agglom- erates witha relativelysmaller concentrationof suchinterface sites.

3.2. Poresizedistribution

BET SSA analysis demonstrates that Ti/Al(P1) and (P2) sam- pleshavesurfaceareavaluesof167.0and393.2m²/g,respectively [37]. SSA values of Ti/Al(P1, P2) support materials as well as 8(20)Ba/Ti/Al(P1,P2) materialsafter thermal treatments within 623–1023Kcanbefoundinoneofourformerreports[37].Thepore sizedistributionsofthesesupportmaterialsmonitoredduringN2

desorptionarepresentedinFig.2.TheTi/Al(P1)sampleshowsa relativelybroadpeakwithanaverageporesizeabout80 ˚Awhichis probablyduetotheconvolutionoftheporesizedistributionsofthe discreteTiO2andAl2O3domains,whileTi/Al(P2)sampledemon- stratesarelativelynarrowdistributionwithanaverageporesizeof 30 ˚A,correspondingtotheTixAlyOzmixedoxidenetwork.Ascanbe seeninFig.2,theporesizeanalysisisconsistentwiththedissimilar surfacestructuresoftheTi/Al(P1)andTi/Al(P2)supportmaterials.

Fig.1.(a–c)TEMimagesofTi/Al(P1),region1:TiO2-richdomains,region2:␥-Al2O3-richdomains,and(d–g)Ti/Al(P2),region1:sponge-likestructureoftheTiOx AlOx

mixedoxide,region2:(dark)Al2O3domains.EDXspectraobtainedfromselectedregionsarealsogivenasinsets.

3.3. SOxinteractionwithTi/AlandBa/Ti/Almaterials:FTIR

Itshouldbenotedthatthereisnotaclearconsensusonthe vibrationalspectroscopicassignmentsoftheSOxspeciesonmetal oxides(particularlyonAl2O3)duetotheheavilyconvolutednature oftheFTIRsignalsoftheadsorbedSO_xspecies.Assignmentsofthe FTIRsignalsforvariousSOxspeciesdiscussedintheliteratureare summarizedinTable1.

WeperformedFTIRinvestigationsofSO2(g)adsorptionon␥- Al₂O₃.Theresults(datanot shown)are inagreementwiththe formerinvestigations,revealingtheformationofsurfacesulfites (SO32−)(notethatbulkAl2(SO3)3doesnotexist[60]),withoutany additionaloxidationtosulfates.

300 250 200 150 100 50 0 0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5

4,0 Ti/Al (P1)

Ti/Al (P2)

pore size (Å) pore volume (cm3/g)

Fig.2.PoresizedistributionplotsforTi/Al(P1)andTi/Al(P2)supportmaterials.

TheadsorptionofSO₂(g)+O₂(g)onthe␥-Al2O₃surfaceat323K (Fig. 3a, spectrum i) revealsa major broad band at 1073cm⁻¹ aswellasweakerandpoorlydefinedadditionalfeaturesaround 1350,1250,1180and1005cm⁻¹.ThepresenceofSO₃²⁻species areapparentdue totheintense characteristic sulfitefeatureat c.a.1073cm⁻¹(3)andtheshouldersat1049cm⁻¹(3)aswellas 1005cm⁻¹(1)[58–60].Minorfeaturesat1350(3)and1180cm⁻¹ (1)canbeattributedtochemisorbedSO2 onabasic(O²⁻)sur- facesites[59,60]whilethefeaturelocatedat1250cm⁻¹ canbe associatedwiththeSO₂adsorbedonAl³⁺Lewisacidsitesand/or bidentatesulfates[59].AveryminorcontributionfromtheSO₂(g) speciesmayalsobepresentat1130(3),1150(1)and2499cm⁻¹ (1+3,notshown)[60].

As the adsorption temperature increases to 473K (Fig. 3a, spectrumii)the1368,1102signalsbecomemore visiblewhich correspondtothe3 and1 modesofsurfacesulfatesonAl₂O₃, [60],respectively.Furthermore,theshoulderlocatedat1170cm⁻¹ whichcanbeassociatedwithbulkAl2(SO4)3[56]startstobemore discernible.Inaddition,arelativelyminorcontributionfromsul- fatespeciesthatareinteractingwiththesurface–OHgroupswhich revealtypicalsignalsat1290and1080cm⁻¹cannotbeexcluded [58].Increasingthetemperatureto673K(Fig.3a,spectraiiiandiv) resultsinthegrowthofthesurfaceandbulkaluminasulfatesin expenseofthesurfacesulfiteandchemisorbedSO₂species.

Theseresultsindicatethatsulfitesareinitiallyformedonthe aluminasurfaceduringtheSO₂+O₂ adsorptionat323Kwhileat elevatedtemperatures(473–673K),thesespeciesare converted intorelativelystablesurfacesulfates(SO42−/Al2O3)aswellasbulk Al₂(SO₄)₃.Theformationofsulfatesonaluminasurfacewascon- sideredinnumerousformerstudies[56,57,60,68].Typically,SO₂ bindstotheacidicsites(i.e.coordinatelyunsaturatedaluminum cations,Al³⁺)formingphysisorbedSO₂.TheadsorptionofSO₂on thebasicadsorptionsitesisfollowedbyacleavageofanAl Obond onthesurface(primarilyatOHsitesoratexposedoxygenatoms,

Table1

TypicalFTIRsignalsassociatedwithcommonSOxspecies[7,54–67].

Species Symmetry v1(cm⁻¹) v3(cm⁻¹) References

SO2(gas) C2v 1151 1362 [54]

SO32−(freeion) C3v 961 1010 [54,55]

SO42−(freeion) Td 1104 [54]

S

O O

Al³⁺

(physisorbed) C2v 1135–1150 1300–1370 [7,55–58]

S

O O

Al

(chemisorbed) C2v 1255(typeI),1189(typeII) [59]

S

O O

OH Al

Cs 1148–1150 1330–1334 [55,59]

O S

O O

Al Al

C2v 1140 1320–1326 [55,59,60]

S

O O

Al O

C3v 1050–1065 1135 [56,59]

O S O O O Al Al

Al ^{C3v 1045–1130 1380 [58,61]}

O S O

O O Ti Ti

Ti ^{C3v 1005,1045 1370 [61]}

S

O O

O O

C C

(organicsulfates) C2v 1230–1150 1440–1350 [62,63]

S

O O

S

O O

C2v ∼910 960–1000

S O O Fe

O O

C2v 1180,968 1375, 1025 [64]

(NH4)2SO4 C2v 1090 1390 [65]

BaSO4(surface) 1060 1120 [66]

Ce(SO4)2(surface) 980 1340–1400 [67]

Al2(SO4)3(bulk) 1190 [56]

BaSO4(bulk) 1155,1248 [66]

Ce(SO4)2(bulk) 1145–1240 [67]

O²⁻)leadingtotheformationofchemisorbedSO₃²⁻.Theoxidation ofadsorbedSO32−/SO2inoxygenatrelativelyhightemperatures (673–773K)leadstotheformationofsurfacesulfatespecieswhich arecoordinatedtothemetalcationsoftheoxidesurfacethrough threeoxygenatoms[60].

In order to examine the sulfur accumulation on the TiO₂ (anatase) surface, SO₂+O₂ adsorption experiments were also performed on TiO2 (Fig. 3b). At 323K (Fig. 3b, spectrum i)

acomplexandaconvolutedgroupofsignalswereobservedwithin 1150–950cm⁻¹whicharelikelytobeassociatedwithSO₃²⁻and/or HSO3− species[60].At473K,thefeatureat1137cm⁻¹ startsto growtogetherwithanintensefeatureat1350cm⁻¹.In analogy withthesimilarbehaviorobserved fortheAl₂O₃ surface, these twobandsareassignedtoweaklyadsorbedmolecularSO2species.

This assignment is also consistent with the attenuationof the 1137cm⁻¹ signalafterevacuationat673K(Fig.3b,spectrumiv).

1000 1100 1200 1300 1400

1049

1005

1350

1250

absorbance (arb.u.)

γ-Al2O3

ii 1368

1170 995

1102 1073 0.05

(a)

i iii iv

900 1000 1100 1200 1300

1137 0.05

ii i iii iv

TiO2 (anatase) (b)

10461002

1083 1160 1287 13661356

1000 1100 1200 1300 1400

(c) Ti/Al (P1)

wavenumber(cm

)

1353

1173 1032

1235 1384

i ii iii iv 0.05

900 1000 1100 1200 1300

1060 Ti/Al (P2) (d)

1360

1080 1350 1260 1160

0.05

i ii iii iv

Fig.3. FTIRspectrafor0.6TorrSO2(g)+O2(g)(SO2:O2=1:10)co-adsorptionon(a)␥-Al2O3,(b)TiO2(anatase),(c)Ti/Al(P1),(d)Ti/Al(P2).(i)After1hexposuretoSO2(g)+O2(g) at323K(spectrumwasobtainedinthepresenceofthegasmixture),(ii)aftersubsequentflashingofthesampleto473KinSO2(g)+O2(g)mixtureandcoolingto323K(spec- trumwasobtainedinthepresenceofthegasmixture),(iii)aftersubsequentflashingto673KinSO2(g)+O2(g)mixtureandfurtherevacuationat323K(Preactor<1×10⁻⁴Torr), (iv)aftersubsequentflashingthesampleto673Kinvacuumandcoolingto323K.

Notethatitisdifficulttofollowthefateofthe1350cm⁻¹atele- vatedtemperaturessincenewbandsstartstogrowinthesame spectralregionat673K.Thefeaturelocatedat1287cm⁻¹together withthe1083cm⁻¹featurecanbeattributedtobidentatesulfates interactingwiththeadsorbedwaterat473–673K[58]andortho- chelatingbidentatesulfateswhichrevealvibrationalfeaturesina similarrange[64,69].Aftertheevacuationat673K,relativelywell- resolvedfeaturesareobservedintheFTIRdata(Fig.3b,spectrum iv).Thusthebandsat1366–1356,1160,1046and1002cm⁻¹are assignedtosurfacesulfatesontheTiO₂surfacewhichareformed aftertheoxidationoftheadsorbedSOxspeciesat673K[61,70].

Fig.3canddshowstheIRspectraoftheTi/Al(P1)andTi/Al(P2) supportmaterialsaftertreatmentinSO₂+O₂atdifferenttemper- atures(323–673K),followedbyevacuation.TheIRspectradueto

theSO₂+O₂adsorptiononTi/Al(P1,P2)at323and473K(spectra iandiiinFig.3candd)exhibitastrongresemblancewherethe mostprominentfeatureisthebroadIRsignalat∼1050cm⁻¹cor- respondingtosulfitespecies.Themaindifferencebetweenthese twospectraisthepresenceoftheshoulderat1130cm⁻¹forthe Ti/Al(P1) sample which maybe likely due toweakly adsorbed SO₂species,similartotheonesthatwereobservedforbothpure anataseand ␥-alumina surfaces.Increasing the temperatureto 673KinthepresenceofSO2+O2leadstotheformationofaddi- tionalfeaturesassociatedwithsurfacesulfatesat1350–1360and 1160–1175cm⁻¹onbothTi/Al(P1,P2)samples.Hightemperature SO2+O2 exposure alsoseem to triggerthegrowth of the con- volutedbandat 1230–1260cm⁻¹ on both samples,which may indicatetheformationofbidentatesulfates.Thepresenceofthe

1160–1170cm⁻¹signalinFig.3canddalsosuggeststhepossible formationofbulkAl2(SO4)3species.Ageneralcomparisonofthe lineshapesoftheFTIRspectragiveninFig.3canddrevealsthat sulfatecontainingsurfacedomainsontheTi/Al(P1)samplearerela- tivelymoreorderedandwell-crystallizedwithrespecttothatofthe Ti/Al(P2)sample,whichisconsistentwiththesharperandbetter resolvedsulfatebandsvisibleinFig.3c.

Analogous experiments were also performed with Ba con- taining samples. In these studies, conventional 8(20)Ba/Al benchmark samples that do not contain TiO₂, were compared with 8(20)Ba/Ti/Al(P1,P2) samples containing TiO2. After the introductionofa SO2+O2 mixtureonto the8Ba/Al and 20Ba/Al samplesat323K(Fig.4aandc,spectrai)amajorbroadbandat 1080cm⁻¹,whichcanbeattributedtosulfitespecies,isobserved.

Aftersubsequentheating at473K (intheSO2+O2 gasmixture) no substantial changes are observed in the case of the 8Ba/Al sample(Fig.3a, spectrumii),while in thecaseof 20Ba/Al,two differentfeaturesat1250and1160cm⁻¹becomeapparent.These featurescanbeassignedtobulkBaSO₄[66].Subsequentheating at673KinSO₂+O₂mixture(Fig.4a,spectrumiii)andinvacuum (Fig.4a,spectrumiv)leadstotheappearanceofthebulkBaSO4

bandsforthe8Ba/Alsample.Theadditionalsignalat1350cm⁻¹ is associated with surface sulfates on alumina domains. This bandis morepronouncedforthe8Ba/Alsample, incomparison to the 20Ba/Al surface due to the larger number of exposed alumina sites in the former case as a result of the lower BaO loading.

Thesulfationexperimentsof8Ba/Ti/Al(P1)and20Ba/Ti/Al(P1) samplesarepresentedinFig.4bandd. AfterSO₂+O₂ exposure at 323K, a set of broad and overlapping bands appear within 1100–980cm⁻¹indicatingtheformationofvarioussulfitespecies.

Interestingly,onlyinsignificantlyminorchangesareobservedfor the8Ba/Ti/Al(P1)sampleevenafterhightemperaturetreatment (Fig. 4b, spectra ii–iv) indicating that the formation of surface aluminum sulfate and bulkBaSO₄ species is rather suppressed.

The changes observed for the 20Ba/Ti/Al(P1) sample are more pronounced (Fig. 4d): bulk BaSO4 related features at 1260 and 1160cm⁻¹appearafterheatingat473Kandthesebandsbecome slightlymoreintenseaftersubsequentheatingat673K.Itisclear thatsuppressionofbulk BaSO₄ formation byTiO₂ promotionis effectiveforthe8Ba/Ti/Al(P1)samplewhileitislessefficientfor 20Ba/Ti/Al(P1)sample.Wehavereportedinourpreviousstudies thatTiO₂domainsfunctionasanchoringsitesforBaOsitesandlimit thesurfacediffusionofBaOclusters[23,37].Thus,itislikelythat forlowBaOloadings,mostoftheBaOdomainsonthe8Ba/Ti/Al(P1) samplecaneffectivelybindtoTiO₂domains,andpreventthesin- teringofBaOdomains.Thus,forlowBaOloadings,TiO2domains canefficientlysuppresstheformationoflargeBaOclusterswhere thermallystablebulkBaSO₄canform.Ontheotherhand,forhigher BaOloadings,asinthecaseof20Ba/Ti/Al(P1),alargerfractionof theBaOdomainsarelocatedonthealuminasurfacewheretheycan diffusefasterandformlarger3DBaOclusterswhichcanenablethe formationofbulkBaSO₄ asinthecaseof20Ba/Al(Fig.4c). Itis worthmentioningthatsimilarexperimentswerealsoperformed on8(20)Ba/Ti/Al(P2)samples(datanotshown)andaqualitatively similarbehavior wasobservedindicatingthesignificanceofthe Ba/TiratiointhepromotionaleffectofTiinSOxuptake.

Inordertoquantitatively comparetheamountofSOxstored ontheinvestigated surfaces,XPSanalysisofthepoisoned sam- pleswasperformed.Becauseoftherelativelylowsensitivityofthe XPStechniquetowardssulfur,poisoningprocedurewasmodified toobtainabettersignaltonoiseratioandmoreaccurateatomic ratiovalues.Thus,beforetheatomicratiodeterminationbyXPS,the sampleswerepoisonedin10TorrofSO_x(SO₂:O₂=1:10)at673K for30min.TheshapeoftheFTIRspectrarecordedafterthispoi- soningprocedure(datanotshown)wereinverygoodagreement

withthecorrespondingspectrapresentedinFigs.3and4.Binding energy(BE)ofS2p(168–170eV)XPSsignalindicatedthepresence ofmainlysulfates(i.e.S⁶⁺states)onallsamples.Itisworthmen- tioningthatalthoughinsituFTIRdatagiveninFig.4bsuggestthe presenceofmostlysulfitespeciesonthe8Ba/Ti/Al(P1)sample,XPS dataindicatesthepresenceofpredominantlysulfatespecies.This maybeassociatedwiththeexposureofthe8Ba/Ti/Al(P1)sample toatmospherebeforetheXPSanalysisresultingintheoxidation ofsulfitesintosulfates.Fig.5demonstratesthatunderidentical poisoningconditions,Ti/Al(P1,P2)samplesaccumulateupto3–5 timesmoresulfurthan␥-Al2O3.Thisbehaviormightbeoriginat- ingfromthelargerSSAof theTi/Almaterialsincomparison to

␥-Al2O₃aswellasthedecreasingnumberoftotalLewisacidsites inmixedTi/Aloxides.ThelessacidicTi/Almixedoxidesurfaces candemonstrateahigheraffinitytowardsacidicSOx.Theacidityof TiO₂ Al₂O₃mixedoxidesaswellas␥-Al2O₃hasbeeninvestigated inpreviousstudies[23,71]bypyridineadsorption.Itwassuggested thatadditionofTiO2toAl2O3introducessomemediumstrength Lewisacidsites,[23]howeveritwasdemonstratedinanotherstudy thatthetotalamountofLewisacidsitesisdecreasedby30%for TiO2 Al2O3incomparisontopurealumina[71].

Analysis of Fig. 5 also reveals that for an identical mass of eachmaterial(i.e.20mg),8Ba/Ti/Al(P1,P2)samplesaccumulate alesserquantityofsulfurthanthe8Ba/Alsample,demonstrating thefavorablepromotionaleffectofTionlimitingsulfuraccumula- tion.Furthermore,8Ba/Ti/Al(P2)sampleseemstostoremoreSOx

(perunitsampleweight)thanthe8Ba/Ti/Al(P1)samplewhichcan beexplainedbythelargerSSAoftheformermaterial(185cm²/g vs.150cm²/g,respectively)[37].However,itisimportanttomen- tion that the promotion of 8Ba/Al sample with TiO₂ favorably decreasesthetotalSOxuptake(perunitsampleweight)regard- lessofthepreparationmethod(i.e.P1orP2)(notethattheSSA ofthefresh8Ba/Al sampleis185cm²/g[20]).Thistrendispar- tiallyreversedwhentheBaOloadingisincreasedto20wt%.Fig.5 revealsthatalthough20Ba/Ti/Al(P1)sample(SSA=79cm²/g[37]) storesalesserquantityofSOx(perunitsampleweight)thanthat of20Ba/Alsample(SSA=126cm²/g[20]);20Ba/Ti/Al(P2)sample (SSA=173cm²/g[37])storesagreatertotalamountofSOx.These resultssuggestthatforhigherBaOloadings,thepromotionaleffect ofTiO₂isweaker.BesidestheseSSAtrends,thisobservationmay alsoarisefromthefactthatnotalloftheBaOdomainsarelocated ontheTiO₂sitesforhighBaOloadingsandalargefractionofthe BaOdomainsdirectlyinteractwiththeunderlyingaluminasitesas inthecaseof20Ba/Alsystem.

3.4. InfluenceofSOxpoisoningontheNOxadsorption

NO_xadsorptiononfreshTi/Al(P1,P2)andfresh8(20)Ba/Ti/Al(P1, P2)samplesviaFTIRtechniqueandthecorrespondingassignments oftheobservedvibrationalbandswerethoroughlydiscussedinone ofourrecentreports[37]andthuswillnotbereiteratedhere.In thelightofthesepreviousstudies,similarNOxadsorptionexperi- mentswerealsoperformedonthesulfur-poisonedmaterialsand theresultswereanalyzed.

Fig.6a comparestheNOxuptake characteristicsof thefresh and poisoned ␥-Al2O₃ surface at 323K.After the saturationof thefresh␥-Al2O3 surfacewithNO2(g),vibrationalfeaturesasso- ciatedwithdifferenttypesofnitratespecieswereobserved.These nitratespeciesadsorbedon␥-Al2O₃wereintheformofbridged (1258,1628cm⁻¹),bidentate(1300,1604cm⁻¹)andmonodentate nitrates(1300, 1564cm⁻¹)[37,72].Additionally,theweakband locatedat∼1958cm⁻¹ thatwasformedafterNOxadsorptionon thepoisonedsurfaceisassociatedwiththeadsorbedNO⁺and/or weaklyadsorbedN₂O₃[73].Thepoisoningofthe␥-Al2O₃surface bySO₂+O₂decreasestheintensitiesofthenitratesignals,indicat- ingadeclineintheNOxuptakeofthe␥-Al2O3 surface.Itisseen

1000 1100 1200 1300 1400

1043

1350 1250

absorbance (arb.u.)

8Ba/Al

ii 1160

995 1080 0.05

(a)

i iii iv

900 1000 1100 1200 1300

1250 0.05

ii i iii iv

8Ba/Ti/Al (P1) (b)

1046995 1100 1160 1350

1000 1100 1200 1300 1400

1080

(c) 20Ba/Al

wavenumber (cm

)

1350

1160

1040

1250

i ii iii iv 0.05

900 1000 1100 1200 1300

980 1120

1046 20Ba/Ti/Al (P1) (d)

1100 1160

1350 1260 0.05

i ii iii iv

Fig.4.FTIRspectrafor0.6TorrSO2(g)+O2(g)(SO2:O2=1:10)co-adsorptionon(a)8Ba/Al,(b)8Ba/Ti/Al(P1),(c)20Ba/Al,(d)20Ba/Ti/Al(P1).(i)After1hexposureto SO2(g)+O2(g)at323K(spectrumwasobtainedinthepresenceofthegasmixture),(ii)aftersubsequentflashingofthesampleto473KinSO2(g)+O2(g)mixtureand coolingto323K(spectrumwasobtainedinthepresenceofthegasmixture),(iii)aftersubsequentflashingto673KinSO2(g)+O2(g)mixtureandfurtherevacuationat323K (Preactor<1×10⁻⁴Torr),(iv)aftersubsequentflashingthesampleto673Kinvacuumandcoolingto323K.

inFig.6athatSOxpoisoningdecreasestherelativeratioofmon- odentateandbidentatenitrates(1300cm⁻¹)tothebridgednitrates (1258cm⁻¹).Thisobservationsuggeststhatsulfatespeciesonthe

␥-Al2O₃surfacesuppresstheformationofalltypesofnitrates,par- ticularlythemonodentateandbidentatenitratesbyoccupyingthe correspondingadsorptionsites.

Theappearanceofthebandsat1375and1100cm⁻¹inFig.6a forthepoisonedsampleindicatestheformationofsurfacesulfates.

Althoughsulfateformationonthe␥-Al2O3surfaceintheabsence ofNO_xspecies(Fig.3a)requiresarelativelyhightemperature(i.e.

673K),sulfateformationcanreadilyoccurbyexposingthealumina surfacetoSO₂(g)+O₂(g)at473KfollowedbyNO₂(g)exposureat 323K.Itisapparentthat,NOxspeciesfunctionasefficientoxidizing agentsintheoxidationoftheSO₂ and SO₃²⁻tosulfatesonthe

␥-Al2O₃surface.Thefollowingreactionpathwayscanbesuggested fortheinteractionbetweensurfaceSOxspeciesandtheadsorbed NOxspecieson␥-Al2O3:

I:

SO32−(ads)+NO2(ads,g)→ SO42−(ads)+NO(g) (1) SO₃²⁻(ads)+2NO₂(ads,g) →SO₄²⁻(ads)+N₂O₃(ads) (2) N₂O₃(ads)→ NO₂⁻(ads)+NO⁺(ads) (3)

II:

3NO₂(g)+O²⁻(s)→2NO₃⁻(ads)+NO(g) (4) SO₃²⁻(ads)+NO₃⁻(ads)→SO₄²⁻(ads)+NO₂⁻(ads) (5)

Fig.5. Relativesulfurcontent(i.e.percentileofsulfuratomsonthesurfacewith respecttoallotheratoms)oftheinvestigatedsamplesaftersulfurpoisoning(as describedinthetext)determinedbyexsituXPSanalysis.

NO₂⁻(ads)+NO₂(ads,g) →NO₃⁻(ads)+NO(g) (6) NO₂⁻(ads)+2NO₂(ads,g) →NO₃⁻(ads)+N₂O₃(ads) (7) PathwayIsuggeststhatthesulfitespeciesonthe␥-Al2O₃surface thatareformedintheprocessofpoisoningcanbedirectlyoxidized tosulfateswiththehelpofNO2(g)orweaklyadsorbedmolecular NO2.Ontheotherhand,pathwayIIproposesanalternativeroute forthesulfateformation,wheresurfacenitratespeciesfacilitate theoxidationofsurfacesulfites(orweaklyadsorbedmolecularSO₂ species).Inthislatterroute,duringthesulfateformation,nitrates areconsumed at theexpense ofnitrite and NO(NO⁺)or N₂O₃ generation.Theobservationof1958cm⁻¹featureintheFTIRspec- trumcorrespondingtothepoisonedsampleinFig.6a,which is notpresentinthecorrespondingspectrumforthefreshsample, supportstheformationofN₂O₃/NO⁺(reactions(2)and(7)).Itis difficulttoassesswhichofthesetwopathwaysisfavoredonthe surfaceunderthecurrentexperimentalconditions.However,by takingintoaccountthefacileformationofnitratespeciesonthe

␥-Al2O3surface,itcanbearguedthatpathwayIImaypresumably beoccurringmorereadilythanpathwayI.

Fig. 6b, presents analogous poisoning and subsequent NOx

uptakeexperimentsconductedonpureTiO₂(anatase).TheLewis acidsitesontheanatasesurfacerevealfourandfive-coordinated Ti⁴⁺ions(refereedas␣and␤sites,respectively)where␣-Lewis acid sites (with two oxygen vacancies) favor bidentate (1578 andshoulder at∼1220cm⁻¹)and bridge(1627 and1236cm⁻¹) nitrateformationwhilethe␤-sites(Ti⁴⁺withoneoxygenvacancy) favortheformationofmonodentatenitrates(groupoffeaturesat 1550–1500cm⁻¹andpeakat1282cm⁻¹)[37,74–77].SimilarFTIR signals,thoughwithreducedintensitiesofallNO_xvibrationalsig- nalsarealsoobservedforthepoisonedanatasesamplemarkedin redinFig.6b.Thepoisonedanatasesamplealsoshowssomeaddi- tionalbandsat1359and1030–1070cm⁻¹thatareassociatedwith thesulfiteandsulfatespecies.

SimilarexperimentswerealsoperformedonTi/Al(P1,P2)sur- facesasshowninFig.6candd.ItisreadilyseenthattheNOxuptake issuppressedbySOxpoisoningonbothsurfaces.ItisvisibleinFig.6 thattheextentofthenitratesignalsuppressionismorepronounced forTi/Al(P1,P2)samplesincomparisonwithpure␥-Al2O3andTiO2. ThisresultisinagreementwiththeXPSdatagiveninFig.5,indi- catingasignificantlyhigheraffinityofTi/Al(P1,P2)towardsSOx

thanthatofpure␥-Al2O3.Sucha behaviorcanbeexplainedby anincrease intheSSAvalues and adecreasein thetotal num- berofLewisacidsitesuponTiO₂promotion.Asimilarinteresting behaviorwasalsoreportedintheliteratureforTiO2–ZrO2mixed oxidesystems.Forinstance,itisknownthatpureTiO₂(anatase)or pureZrO₂ (monoclinic/tetragonal)haveverylimitedNOxstorage capacities[78].Ontheotherhand,whenthesetwodifferenttypes

ofoxidesarecombinedtoobtainanamorphousTiO₂–ZrO₂mixed oxide,NOxstoragecapacitycanbeimprovedbymorethananorder ofmagnitude,whichwasattributedtotheformationofnewLewis basicsitesattheTiO₂–ZrO₂hetero-junctions[78].Inanalogywith theseobservations,Ti/Al(P1,P2)surfacesmayalsocontainnewand strongSO_xadsorptionsites,whicharenotpresentoneitherpure TiO₂or␥-Al2O₃surfaces.

WhentherelativepoisoningofTi/Al(P1)iscompared tothat ofTi/Al(P2) (Fig.6cand d),itis visiblethatnitratefeaturesare suppressedtoalesserextentonTi/Al(P2),althoughtheintensities ofsulfatefeatures(1360and1100cm⁻¹)arestrongeronTi/Al(P2) (Fig.5d).Thisobservationsuggeststhattheporousanddisordered surfacemorphologyoftheTi/Al(P2)sampleresultsinalargerover- allSOxuptake,howeverduetoitslargesurfacearea(SSAoffresh Ti/Al(P1)and Ti/Al(P2)are167and393m²/g,respectively [37]), Ti/Al(P2)surfacestillpossessesalargernumberofNOxbindingsites withrespecttoTi/Al(P1).Ontheotherhand,suchacomparison basedsolelyonIRintensitiesshouldbeconsideredwithcaution, astheIRabsorptioncross-sectionsofadsorbednitratespecieswith dissimilaradsorptionconfigurationscanbesignificantlydifferent, whichmayrenderadirectIRintensitycomparisonrelativelyinac- curate.

Fig. 7 compares the NOx adsorption properties of 8(20)Ba/Ti/Al(P1, P2), samples before and after SOx adsorp- tion at323K.The FTIRresultscorrespondingtotheadsorption of NO₂ onthefreshsampleswerealready discussed elsewhere [37].Briefly,uponNO₂ adsorption,thefreshsamplesurfacesare characterized by adsorption bands due to the presence of the bulk(ionic)Ba-nitrateslocatedat∼1320,∼1440and∼1480cm⁻¹, surface (bidentate) Ba-nitrate features located at 1585, 1565, 1300cm⁻¹andadditionalnitratebandsat1583and∼1630cm⁻¹ thatareassociated withnitratesadsorbedontheTiO₂ domains havingbidentateandbridgedconfigurations.AfterSOxpoisoning ofthesampleswithSO2+O2at473K,itisclearlyvisiblethatthe NO_x storage capacities decrease significantly. Additionally, the presenceofminorbandsat∼1150and∼1245cm⁻¹ impliesthe existenceofsulfatespeciesonallofthesamples.Formationofsul- fatespeciesonallofthesamplesaftersubsequentNO2adsorption at 323Kdemonstrates theefficient oxidizingcapability of NO₂ whichcanreadilyoxidizesulfitespeciesthatareformedduring the SO2+O2 exposure. It is worth mentioning that the similar experimentsperformedon8(20)Ba/Alsamples(datanotshown) revealedqualitativelysimilarresultsindicatingthesuppressionof NOxuptakeuponsulfurpoisoning.

3.5. ThermalstabilityoftheadsorbedNOxspeciesonthe sulfur-poisonedmaterials

In order to have a betterunderstanding of theinfluence of SO2+O2 treatmenton thethermal stability and thedesorption propertiesoftheNOxspecies,TPDexperimentswerepreformed.

Fig.8showstheNOxdesorptionprofilesfromthefreshandSOx

poisoned␥-Al2O₃andTiO₂(anatase)benchmarksamples.Forclar- ity,onlythem/z=30,m/z=32andm/z=46desorptionchannelsare shown.Forthefreshaluminasurface,twomajorNOandNO₂des- orptionfeaturesareobserved,thatisingoodagreementwiththe literaturedata[20,79].ThefirstNOxdesorptionfeatureat387K is associated withthe desorption of monodentate nitrates and weaklyboundN₂O₃and/orNO⁺specieswhichdesorbintheform ofNO2andNO(almostwithoutO2).Thesecondmajordesorption bandhasitsmaximumat625Kandisassociatedwiththedesorp- tion/decompositionofbridgedandbidentatenitrateswhichyield agreaterNO2desorptionsignalalongwithNOandO2[20,79].

TheSO₂-poisoning ofthealuminasurfacehasastronginflu- enceontheNOxdesorptionfeatures.AsitisseeninFig.8b,sulfur poisoningaltersboththetypeandthequantityofadsorbedNOx

1000 1200 1400 1600 1800 2000 2200

absorbance (arb.u.)

γ-Al2O3 (a)

1958 1628

1604

1375 1300

1259

1102 10411003

Fresh sample

1564 0.2

Poisoned sample

1200 1400 1600 1800 2000

TiO2 (anatase) (b)

Poisoned Sample Fresh Sample

1627

1578

1550

1359 12821236

1070 1028 0.2

1000 1200 1400 1600 1800 2000 2200

(c) Ti/Al (P1)

wavenumber (cm^-1)

Fresh Sample Poisoned Sample

1631

1580

1294 1258

1040 0.2

1200 1400 1600 1800 2000 Ti/Al (P2) (d)

Fresh Sample Poisoned Sample

1640

1584

1237 1212

1360 1100

1040 0.2

Fig.6. FTIRspectramonitoredafterNO2adsorptionat323Konfresh(blackspectra)andpoisoned(redspectra)(a)␥-Al2O3,(b)TiO2(anatase),(c)Ti/Al(P1),(d)Ti/Al(P2) samples.Poisoningwasperformedbyexposingthesamplesto0.6TorrSO2(g)+O2(g)(SO2:O2=1:10)at323K,followedbyheatinginthegaseousmixtureat473Kfor30min andafinalevacuationat323K(Preactor<1×10⁻³Torr).NOxuptakewasperformedbyexposingthesamplesto8TorrofNO2(g)at323Kfor20min,followedbyevacuation to<1×10⁻³Torr.Allspectrawereacquiredinvacuumat323K.(Forinterpretationofthereferencestocolorinthisfigurelegend,thereaderisreferredtothewebversion ofthisarticle.)

species.Forthepoisonedsample,themajorfeaturearound380K (associatedwiththemonodentatenitratesorweaklyboundN2O3

desorption)appearstoalargerextentthandesorptionofbiden- tate/bridgednitrateat675K.Thisfindingisalsoconsistentwiththe FTIRresultsobtainedforNO2adsorptiononthesulfated␥-Al2O3

surface,yieldingthebandcorrespondingtoadsorbedNO⁺/N₂O₃ speciesat 1958cm⁻¹ (Fig.6a).Relativelylower intensityofthe 1958cm⁻¹ band(Fig.5a)incomparisontotheotherNOxbands couldbeassociatedwithitslowIRabsorptioncross-section.Thus, SOx species seem tocompete withthe nitratespecies for sur- faceadsorptionsites.Furthermore,inthepresenceofSOxspecies,

stronglyadsorbednitratesarepartiallyconsumedandconverted intoweaklyboundN2O3(reactions(2)and(7))orNO⁺ (reaction (3))whilesomeofthesulfitesandchemisorbedSO₂speciesarecon- vertedintosulfatesthroughreactionssimilartotheonesproposed above(pathwayII).

Fig.8canddshowstheTPDprofilesforNO_xdesorptionfrom freshandSOxpoisonedTiO₂(anatase)samples.FreshTiO₂sample presentsaweakshoulderat455Kandtwobroadfeatureslocated at610and900KinNOdesorptioncurve.Inthelightoftheformer studies[74–77]theshoulderat455Kcanberelatedtomolecu- larlyboundNO₂ speciesandmonodentatenitrates.Thebandat

1000 1200 1400 1600 1800 2000 2200

8Ba/Ti/Al (P2) 8Ba/Ti/Al (P1)

absorbance (arb.u.)

(a)

1630 1440 1310

1240

1150 1040

Fresh sample

1570 0.2

Poisoned sample

1200 1400 1600 1800 2000

1450

(b)

Poisoned Sample Fresh Sample

1635

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1485 13051260

1150 1040 0.2

1000 1200 1400 1600 1800 2000 2200

1040 1440

20Ba/Ti/Al (P1) ^(c)

wavenumber (cm

)

Fresh Sample Poisoned Sample

1630 1570

1320

1250

1150 0.2

1200 1400 1600 1800 2000

1320

1440

1570 1630

20Ba/Ti/Al (P2) ^(d)

Fresh Sample Poisoned Sample

1250

1155 1040 0.2

Fig.7.FTIRspectramonitoredafterNO2adsorptionat323Konfresh(blackspectra)andpoisoned(redspectra)(a)8Ba/Ti/Al(P1),(b)8Ba/Ti/Al(P2),(c)20Ba/Ti/Al(P1),(d) 20Ba/Ti/Al(P2)samples.Poisoningwasperformedbyexposingto0.6TorrSO2(g)+O2(g)(SO2:O2=1:10)at323K,followedbyheatinginthegaseousmixtureat473Kfor 30minandafinalevacuationat323K(Preactor<1×10⁻³Torr).NOxuptakewasperformedbyexposingthesamplesto8TorrofNO2(g)at323Kfor20min,followedby evacuation.Allspectrawereacquiredinvacuumat323K.(Forinterpretationofthereferencestocolorinthisfigurelegend,thereaderisreferredtothewebversionofthis article.)

610Kisattributedtothedesorption/decompositionofthebridged andbidentatenitrates.Thehightemperaturedesorptionfeatureat 900Kcanbeattributedtothedesorptionofstronglyboundnitrates.

Itisknown[23]thatphasetransitionfrombulkanatasetobulk rutilestartsatT>800K.ThereforethestrongNOxdesorptionsig- nalat900KinFig.8ccouldbeassociatedwiththedrasticdecrease inthesurfaceareaofTiO₂asaresultofthephasetransitionfrom anatasetotherutilephase.Notethatduringtheactivationofthe freshandpoisonedanatasesurfacesbeforetheTPDexperiments, TiO2sampleswerenotexposedtotemperatureshigherthan623K inordertopreservetheanatasephasepuritythus,theactivation wasperformedviaO₂(g)ratherthanNO₂(g).

The poisoned TiO₂ sample shows an additional minor des- orption feature at 345K (Fig. 8d). We attribute this feature to thedesorptionofweaklyadsorbedN2O3/NO⁺ (desorptionoccurs mainlyintheformofNO+NO₂).Thedesorptionfeatureat450Kis associatedwithweaklyadsorbedmolecularspeciesand/orwith monodentatenitrates. TPDprofileof thepoisoned TiO2 sample (Fig.8d)revealsa decreasein the450K desorptionsignalwith respecttothatofthefreshsample(Fig.8c)inagreementwiththe FTIRdata(Fig.6b)indicatinga suppressionofthemonodentate nitratesignalsat1550and1282cm⁻¹ uponpoisoning.Themain desorptionbandforthepoisonedTiO₂sampleoccursat615Kindi- catingthatonthepoisonedanatase,NO₂adsorbspredominantlyin