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Catalysis
Today
jo u rn al h om ep a g e :w w w . e l s e v i e r . c o m / l o c a t e / c a t t o d
Spectroscopic
investigation
of
sulfur-resistant
Pt/K
2
O/ZrO
2
/TiO
2
/Al
2
O
3
NSR/LNT
catalysts
Z.
Say,
M.
Tohumeken,
E.
Ozensoy
∗DepartmentofChemistry,BilkentUniversity,06800Ankara,Turkey
a
r
t
i
c
l
e
i
n
f
o
Articlehistory:
Received6October2015
Receivedinrevisedform
25November2015
Accepted4December2015
Availableonline18January2016
Keywords: NSR/LNT
DeNOxAl2O3/ZrO2/TiO2
FTIR TPD
a
b
s
t
r
a
c
t
AnalternativeternarysupportoxidematerialanditsK2OandPtfunctionalizedcounterpartsintheform
ofPt/K2O/Al2O3/ZrO2/TiO2withdifferentK2Oloadingsweresynthesized.Structuralandmorphological
propertiesofthecatalystswerecharacterizedviaXRDandBETtechniquesincomparisontoa
conven-tionalPt/20Ba/AlbenchmarkNSR/LNTcatalyst.Comprehensivein-situFTIRandTPDanalysisrevealed
thatincreasingtheK2OloadinginthePt/K2O/AZTsystemleadstoanincreaseinNOxStorageCapacity
(NSC)attheexpenseoftheformationofbulk-likesulfatesrequiringhighertemperatureforcomplete
sulfureliminationwithH2(g).Observeddelicatetrade-offbetweenNSCandsulfurpoisoning
tenden-ciesofthecurrentlyinvestigatedfamilyofAZT-basedNSR/LNTcatalystsimpliesthatPt/5.4K2O/AZTisa
promisingcatalystrevealingcomparableNSCwithinthetemperaturerangeof473–673Ktothatofthe
conventionalPt/20Ba/Albenchmarkcatalyst,whileexhibitingsuperiorsulfurtoleranceandregeneration
characteristics.
©2016ElsevierB.V.Allrightsreserved.
1. Introduction
NOx emitted from mobile sources have serious destructive effectsontheatmosphere,globalecosystemandespeciallyonthe humanhealth.AboutonehalfofthetotalNOxemissionsresults frommobilesources[1].Whiletheveryfirstregulationsfordiesel engineemissionswereprimarilyfocusing onparticle emissions, otherhazardouspollutantssuchasCO,SO2,NOxand unburned
hydrocarbonsfrommobilesourcesarecurrentlybeingregulated withincreasingly stringentlimitations. Thisleadstoa constant pressureontheglobalautomotiveindustrytodevelopnoveland innovativeaftertreatmenttechnologiesthatcansatisfythe con-tinuously evolvingenvironmentallegislations and to lowerthe exhaustemissionlevels[2–4].Recently,itwasreportedthatNOx emissionsofsomeofthecurrentlyexistingdiesel-enginepassenger carsequippedwithmodernDeNOxaftertreatmentsystemsonthe highwaywereupto20–35timeshigherthanthatoftheallowed emissionlimits[5].Furthermore,averyrecentstudypublishedby theEuropeanEnvironmentAgency(EEA)[6]reportedthatwithout
∗ Correspondingauthor.
E-mailaddress:ozensoy@fen.bilkent.edu.tr(E.Ozensoy).
anyexception,eachEuropeanUnion(EU)memberstateviolates atleastoneormoreoftheexistingannualemissionlimitations associatedwithNOx,SOx,NH3andnon-methanevolatileorganic
compounds(NMVOC).AmongtheseEUmemberstates,particularly Germany,AustriaandIrelandwerefoundtofailmeetingannual EuropeanNOxemissionstandardsin2014.Thesestrikingexamples clearlycallforthedesignanddevelopmentofmoreefficient,more stableandmoreaffordableheterogeneouscatalyticarchitectures thatcanbeusedinmodernDeNOxaftertreatmenttechnologies.
Forlean-burnengines,apromisingaftertreatmentmethodfor thecatalyticNOxreductionisthesocalledNOxstorage/reduction
(NSR)/LeanNOxTrap(LNT)technology[7,8].AtypicalNSR/LNT
cat-alystiscomprisedofbasicoxides(e.g.BaO,K2O),redoxsites(e.g.
Pt,Pdand/orRh)and ahighsurfaceareasupportmaterial(e.g. -Al2O3)[2,3,9].
The conventional NSR/LNT catalyst, Pt/BaO/␥-Al2O3, exhibits
efficientNOxconversionandstorageperformancewithinthe oper-ationaltemperaturewindowofthedieselemissiontailpipe(i.e. 473–673K)[10–18].However,recentengineapplicationssuchas thefuel-efficientgasoline directinjection(GDI)engines require catalyticaftertreatmentsolutionswhichshouldbeableto oper-ateattemperaturesabove400◦C,wheretheconventionalNSR/LNT catalystscannotfunctioneffectively[19].ToyotaMotorCompany http://dx.doi.org/10.1016/j.cattod.2015.12.013
reportedthatK2OandBaOaretwoofthemostpromisingNOx
stor-agecomponentstobeusedinNSR/LNTcatalysts[20].Amongthese twodifferenttypesofbasicmetaloxides,theuseofK2Oattracted
particularinterestduetoitssuperiorNOxstoragecapacity(NSC)at elevatedtemperatures[21].OthernoteworthyadvantagesofK2O
domainsareassociatedtotheirstrongerbasicityandthelackof unfavorablesolid-stateinteractionsbetweenK2Oandthe-Al2O3
supportmaterial,unlikethatofBaOwhichmayleadtothe for-mationofundesiredBaAl2O4athightemperatures[22].Luoetal.
investigatedtheeffectofK2Oloading(within2–20wt.%)onthe
NSCofthePt/K2O/␥-Al2O3system.Itwasfoundoutthatthe
cat-alystformulationcontaining10wt.%K2Oresultedinthehighest
NSCvalueswithinawidetemperaturewindowof523–823K[22]. InadditiontothepromisingNSCofK2O-functionalized
mate-rialsin high temperature DeNOx applications, sulfur-poisoning tolerances as well as the sulfur regeneration characteristics of such systems should be also taken into consideration. It is knownthatK2Odomainsdispersedona␥-Al2O3 support
mate-rialarehighlypronetosulfurpoisoning,experiencingrapidand ratherirreversiblecatalyticdeactivation.AclassofnovelAl/Ti/Zr mixedoxides hasemerged in recent years with enhanced sur-faceandstructuralpropertiesassupportforK2O-basedNSR/LNT
catalysts [23–25]. In recent studies, ZrO2/TiO2, TiO2/Al2O3 and
Al2O3/ZrO2/TiO2-supportedNSR/LNTcatalystscanrevealsuperior
sulfurregenerationandNOxrecoveryperformancesascomparedto thatof␥-Al2O3-basedsystems(i.e.Pt/BaO/Al2O3vs.Pt/K2O/Al2O3)
[26,27].Takashietal.reportedthataZrO2:TiO2supportmaterial
withamassratioof70:30(whichalsorevealedthehighestSSA amongtheinvestigatedmaterialstherein)exhibitedthebest per-formanceintermsofsulfurresistance,thermaldurabilityandNOx abatement[28].TheirstudieswhichalsoincludedPt/Rh/Ba/K/AZT catalyst with nano-composite ternary oxide Al2O3/ZrO2/TiO2
-supportshowedexcellentNOxstoragecapacity(NSC)comparedto thatof␥-Al2O3/ZrO2/TiO2-support,where␥-Al2O3wasphysically
mixedwithZrO2/TiO2[29,30].Inaddition,Zouetal.[27]performed
adetailedanalysisontheeffectofAl2O3dopingintotheZrO2/TiO2
matrix,suggestingthattheAl:(Ti+Zr)atomicratioof3:1exhibited thehighestNSCforfreshandsulfur-regeneratedcatalyst.Inamore recentwork,Zouetal.studiedtheeffectofKloadingontheNSCand sulfurregenerationperformanceofPt/K/Al2O3/ZrO2/TiO2catalyst
underrealisticflowconditions[31].
However, these aforementioned comprehensive studies includedalimitednumberofspectroscopicinvestigationsonthe interactionsbetweenSOxspeciesandthecorrespondingcatalyst surfaces.Thus, inthecurrent work,we focusonthemolecular levelinvestigationofthefundamentalinteractionsthattakeplace betweenSOxspeciesandK-basednovelNSR/LNTcatalystsurfaces in a qualitative and a semi-quantitative manner. Along these lines, we investigate the SOx adsorption/uptakeas well as the
SOxreduction/regeneration/releasepropertiesofAl2O3/ZrO2/TiO2
(AZT)supportedPt/K/AZTcatalystsincomparisontoabenchmark NSR/LNTcatalyst (i.e.Pt/BaO/Al2O3)byutilizing in-situ
spectro-scopictechniques.GenerationofS-containingsurfacefunctional groups,theirthermalevolution,reductionandreleasesafunction oftemperatureandK2Oloadingaresystematicallymonitoredby
meansofin-situFourierTransformInfraredSpectroscopy(in-situ FTIR)andTemperatureProgrammedDesorption(TPD).Moreover, structuralandmorphologicalpropertiesofthesynthesized mate-rialsarealsoanalyzedviaX-rayDiffraction(XRD)andBrunauer, Emmettand Teller(BET)surface areaanalysis techniques. Cur-rent resultsprovide valuable molecular level insight regarding the interaction of SOx species withK-based NSR/LNT catalysts supportedonnovelAZT mixed oxidesurfaces and thedelicate trade-offbetweentheNSCandsulfurpoisoningphenomena.
2. Experimental
2.1. Materialsynthesis
2.1.1. SynthesisofPt/Al2O3/ZrO2/TiO2
Al2O3/ZrO2/TiO2 (AZT) supportmaterial was synthesized as
describedinoneofourformerpublicationswheretherelative com-positionoftheternaryoxidesystem(i.e.Al2O3/ZrO2/TiO2)bymass
was50:35:15[25].1wt. %platinum-incorporatedternary oxide materials were synthesized by incipient wetness impregnation methodusingasolutionofPt(NH3)2(NO2)2 (Aldrich,
diammine-dinitritoplatinum(II),3.4wt.%solutionindiluteNH3(aq)).Priorto
thePtaddition,Al2O3/ZrO2/TiO2wasinitiallycalcinedinairat973K
for150mininordertoremovetheorganicfunctionalitiesinthe precursor.AfterthePt-incorporation,Pt/AZTmaterialwas subse-quentlycalcinedin airat973Kfor 150minin ordertoremove nitrite/nitrateoriginatingfromthePtprecursorandtostructurally stabilizethecatalystsurface.
2.1.2. SynthesisofPt/K2O/Al2O3/ZrO2/TiO2
K2O-based catalysts were also prepared via wetness
impregnation. Pt/K2O/Al2O3/ZrO2/TiO2 catalysts with 2.7, 5.4
and 10.0wt. % K2O loading (i.e. Pt/2.7K2O/Al2O3/ZrO2/TiO2,
Pt/5.4K2O/Al2O3/ZrO2/TiO2 and Pt/10K2O/Al2O3/ZrO2/TiO2;
respectively)werepreparedviaimpregnationofAl2O3/ZrO2/TiO2
support(initially calcined at 973K for 150min) withan aque-ous solution of potassium nitrate (KNO3·6H2O, >99.0 %, Fluka,
France)followedbycalcinationat873Kfor150mininorderto thermallyremovethenitratecontentpresent intheprecursors. Finally, K2O/Al2O3/ZrO2/TiO2 structure was impregnated with
thePt(NH3)2(NO2)2 precursorandcalcined at973Kfor150min
under ambient conditions in order to attain 1wt. % nominal preciousmetalloading.Throughoutthecurrenttext,synthesized Pt/K2O/Al2O3/ZrO2/TiO2 catalysts with 2.7, 5.4 and 10.0wt. %
K2Oand 1wt.%PtloadingswillbeabbreviatedasPt/2.7K/AZT,
Pt/5.4K/AZTandPt/10K/AZT,respectively.
2.1.3. SynthesisofPt/BaO/-Al2O3
For the synthesis of the Pt/20BaO/Al benchmark catalyst, ␥-Al2O3supportmaterial(SASOLPuralox,210m2/g)was
impreg-natedwithanaqueoussolutionofbariumnitrate(Ba(NO3)2,ACS
Reagent,≥99%,Riedel-deHäen,Germany) whichwasfollowed bycalcination at873Kin air for 150min. Finally,20BaO/Al2O3
was impregnated with the Pt(NH3)2(NO2)2 precursor (Aldrich,
diamminedinitritoplatinum(II),3.4wt.%solutionindiluteNH3(aq))
toobtain1wt.%nominalpreciousmetalloading,followedby cal-cinationat973Kfor150min.Thiscatalystwillbeabbreviatedas Pt/20Ba/Althroughoutthecurrenttext.
2.2. Experimentalsetup
Comprehensivedescription ofthecustom-made batch-mode in-situ FTIR and TPD spectroscopic setup used in the current measurementscanbefoundelsewhere[10,14,25,32].Briefly,an FTIR spectrometer (Bruker Tensor 27) and a quadruple mass spectrometer(QMS,StanfordResearch Systems,RGA200)were simultaneouslyconnectedtoabatch-typespectroscopicreactor. FTIRexperimentswereperformedintransmissionmode.TPD pro-fileswereobtainedundervacuumbyusingacomputer-controlled lineartemperaturerampof12K/minwithamaximumsample tem-peratureof1173K.
2.3. Experimentalprocedures
2.3.1. MonitoringSOxadsorptionviain-situFTIR
Sulfuradsorption/poisoningcharacteristicsofeachmaterialwas investigatedbyexposingthecatalystsurfacestoa2.0TorrSO2+O2
gasmixture (SO2:O2=1:10, v/v) at 323K (SO2 purity>99%, Air
Products;O2purity>99.999%,LindeGmbH).Afterthe
introduc-tionofSOxmixtureat323K,sampleswereannealedto373,473, 573and673Kfor5mininthepresenceoftheSOxmixture.FTIR spectraofthesesulfatedsurfaceswereacquiredaftercoolingto 323Kinthepresenceofthegasmixtureandsubsequentevacuation to<10−3Torr. It shouldbenoted that theeffective concentra-tionofSO2usedinthecurrentpoisoningexperimentscorresponds
toca.263ppm (inabalancecarriergasunderflow conditions), whichtranslatesintoextremelyseverepoisoningconditions con-sideringthetypicalsulfurcontent(15ppm) ofUltraLowSulfur Diesel(ULSD)fuel.Thus,thecurrentpoisoningexperimentscanbe assessedasacceleratedandextremesulfurpoisoningexperiments, wherethenovelK-AZTbasedcatalystswereexposedtoparticularly challengingconditions,wheretheycandemonstratetheirultimate sulfur-regenerationcapabilities.
2.3.2. MonitoringSOxdesorptionviain-situFTIR
PriortoSOx desorptionexperiments,materialsweresulfated asdescribed aboveby collectinga series ofin-situFTIR spectra inthepresenceoftheSOxmixtureasafunctionoftemperature until673K.Afterthesaturationofthesurfaceswithsulfurat673K, thereactorwasevacuatedtoapressureof<10−2Torr,followedby theintroductionof15.0TorrofH2(g)(H2purity>99.999%,Linde
GmbH) at323K.Next, poisonedcatalystswere annealedunder hydrogenatmosphereat473,673,773,873and973Kfor5min. In-situFTIRspectrawereobtainedaftereachH2exposureandby
coolingthesampleto323KinthepresenceofH2.
2.3.3. SOxdesorptionviaTPD
BeforetheSOx-TPDexperiments,materialsurfaceswere ini-tiallyexposedtoa2.0TorrSO2+O2 gasmixture(SO2:O2=1:10)
at673Kfor30min.ThentheIRspectroscopicreactorwas evacu-atedtoapressurelowerthan10−3Torrfollowedbyheatingunder vacuumto1173Kwithalinearheatingrateof12K/min.IntheTPD experiments,m/z=32(correspondingtoO2(g)desorptionandS(g)
formationduetotheimpactionization-inducedfragmentationof desorbedSO2(g)speciesinQMS)andm/z=64(correspondingto
SO2(g)desorption)channelsweremonitoredviaQMS.
3. Resultsanddiscussion
3.1. Materialcharacterization 3.1.1. X-raydiffractionanalysis(XRD)
Fig. 1 illustrates the XRD patterns of Pt/AZT, Pt/2.7K/AZT, Pt/5.4K/AZT,Pt/10K/AZTandPt/20Ba/Al.Apartfromthepresence ofstructurallywell-orderedmetallicplatinum(JCPDS001-1190), Pt/AZTandPt/K/AZTcatalystsgiveninFig.1exhibithighly amor-phouscharacteristics.On theotherhand,materialwith10.0wt. % K2O (i.e. Pt/10.0K/AZT) reveals additional poorly discernible
diffractionsignalsat2=30.48◦,50.50◦,60.91◦correspondingto tetragonal ZrO2 (JCPDS 80-2155) together with some
suppres-sion of Pt diffraction features. XRD analysis of the benchmark Pt/20Ba/AlNSR/LNT catalyst reveals␥-Al2O3 (JCPDS 001-1303),
BaAl2O4(JCPDS017-0306)andmetallicPt(JCPDS001-1190)
fea-tures.OnthePt/20Ba/Alcatalyst,BaOdomainsinteractwiththe ␥-Al2O3supportatelevatedtemperaturesyieldingtheformation
ofundesiredBaAl2O4phaseasaresultofthermalaging[2,3,33].
10 20 30 40 50 60 70 80 Pt/AZT Pt/20Ba/Al Pt/2.7K/AZT Pt/5.4K/AZT Pt/10K/AZT XR D I n te n sit y ( ar b . u .) 2θ(degree)
t-ZrO2 Pt γ-Al2O3 BaAl2O4
Fig.1.XRDpatternscorrespondingtoPt/AZT,Pt/2.7K/AZT,Pt/5.4K/AZT,Pt/10K/AZT
andPt/20Ba/Almaterialsuponcalcinationat973K.
Fig.2.BETspecificsurfacearea(SSA)valuesoftheinvestigatedmaterials.
3.1.2. BETspecificsurfacearea(SSA)measurements
Fig.2illustratesSSAvaluesforPt/AZT,Pt/2.7K/AZT,Pt/5.4K/AZT, Pt/10K/AZTandPt/20Ba/Al.SSAvaluesforPt/AZTisslightlyhigher than2.7wt.%K2O-modifiedcounterpart(i.e.Pt/2.7K/AZT).
How-ever,increaseintheK2Oloadingfrom2.7wt.%to5.4and10.0wt.
%monotonicallydecreasesSSAvaluesform177m2/gto155and
125m2/g;respectively.ItshouldbenotedthattheSSAvalueofthe
catalystwiththehighestK2Oloadingwascomparabletothatofthe
benchmarkPt/20Ba/Alcatalyst(134m2/g),whileSSAvaluesofall
oftheothercatalystswererelativelyhigher.
3.2. SOxUptake/adsorptionviain-situFTIRspectroscopy
Fig.3representstemperature-dependentadsorbedSOxspecies on Pt/AZT, Pt/2.7K/AZT, Pt/5.4K/AZT and Pt/10K/AZT material surfaces upon exposure to 2.0Torr of SO2+O2 gas mixture
(SO2:O2=1:10).Whiletheblack-coloredspectraineachpanel
cor-respondtothesurfaceSOxspeciesgeneratedwithinatemperature rangeof323–573K,thetopmostredspectracorrespondtosulfur poisoningat673K.
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Fig.3. FTIRspectrademonstratingtheSOxuptake/adsorptionpropertiesof(a)Pt/AZT,(b)Pt/2.7K/AZT(c)Pt/5.4K/AZTand(d)Pt/10K/AZTsurfaces.Blacksetofspectra
ineachpanelwereacquiredafterSOxexposure(2.0Torr,SO2:O2=1:10)at323K,followedbyannealingat373,473and573KintheSOxgasmixturefor15minand
subsequentevacuation.RedspectraineachpanelwererecordedafterSOxexposureat673Kandsubsequentevacuation.Allspectrawererecordedat323Kinvacuum.(For
theinterpretationofthereferencestocolourinthisfigure,thereaderisreferredtothewebversionofthisarticle.)
Kimetal.reportedthatanincreaseinK2Oloadingfrom2wt.
%to 30wt. % ledto a boost in NSC of Pt/K2O/Al2O3 materials
within600–800K[34].Aswillbedemonstratedlatter,although suchextremelyhighK2Oloadingscouldbebeneficialtoenhance
NSCintheabsenceofSOx,theymayalsoleadtoirreversiblesulfur poisoninginthepresenceofSOx.Therefore,inthecurrentstudy, welimitedtheK2OloadingoftheAZT-basedNSR/LNTmaterialsto
10wt.%.
Fig.3ashowsthatSOxadsorptiononPt/AZTatrelativelylower temperatures(i.e.323and373K)leadstotwomain vibrational featureslocatedat1027and975cm−1whichcanbeassignedto sulfate(SO42−)andsulfite(SO32−)functionalgroups,respectively
[35–39].Absorbanceintensitiesofthesetwoparticularvibrational frequenciesarecomparableatlowtemperatures,whilethesulfate featurestartstodominatethesulfitefeatureat higher temper-atures.Thermally-triggeredcatalyticoxidation of sulfitespecies tosulfatesonthePt/AZTsurfacecanalsobefollowedinFig.3a
bymonitoringthegrowthoftheantisymmetricstretchingmode ofsurfacesulfategroupslocatedat1391and1306cm−1 [14,40]. Asillustrated inFig. 3b–d,additionof basic K2Odomains onto
AZTternaryoxidesystemresultsinalterationofthespectralline shapes.Pt/2.7K/AZT(Fig.3b)presentsfivemajorvibrational fea-tureslocatedat1306,1178,1105,1048and1019cm−1.WhileIR stretchingsat1306,1105,1048and1019cm−1canbeattributedto thesurfacesulfate(SO42−)groupsonK2Oand/orontheAZT
sup-port,vibrationalfeaturelocatedat1178cm−1canbeattributedto bulk-likesulfategroupsonK2O[41,42].Thislatterfeaturebecomes
morediscerniblewithanincreaseintheK2Oloading(i.e.5.4wt.%
and10wt.%)evidentbytheincreasingrelativeabsorbance inten-sityof the1178cm−1 signalinFig.3candd.SOxadsorptionon Pt/5.4K/AZTandPt/10K/AZTmaterialsat673Kleadstovibrational featuresat1281and1238cm−1alongwiththeabsenceofany sig-nificantvibrationalbandslocatedatca.>1300cm−1 (Fig.3cand d).Thevibrationalsignalsat1281and1238cm−1canbeassigned
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Fig.4. FTIRspectraassociatedwithSOxreductionandregenerationof(a)Pt/AZT,(b)Pt/2.7K/AZT(c)Pt/5.4K/AZTand(d)Pt/10K/AZTmaterialsviaH2(g).Catalystswere
initiallysulfated(2.0Torr,SO2:O2=1:10for15minat673K)followedbyevacuationandsubsequentexposuretoH2(g)(15.0Torr)at323,473,673,773,873and973Kfor
5min.Allspectrawererecordedat323K.
topredominantlysulfatesonK2Odomains[14,41–43].Itcanbe
arguedthatwiththeincreasingK2Oloading,anincreasinglylarger
portionof the AZT surface is covered by K2O islands/domains,
decreasingtheextentofexposed/uncoveredAZTsurface.Itisalso likelythattheincreaseintheK2Oloadingalsoresultsinthegrowth
oftheK2Oparticlesizeandformationof3Dagglomerates,enabling
thestorageofSOxintheformofbulk-likesulfatesinthesub-surface ofthese3Dnanoparticles.Thisargumentisalsoingoodagreement withthemeasuredSSAvaluespresentedinFig.2suggestingthat theincreaseintheK2Oloadinginthecatalystformulationleads
toamonotonicdecreaseintheSSAasexpectedbysinteringofthe K2Odomainsandparticlesizegrowth.
3.3. SulfurregenerationwithH2(g)viain-situFTIRspectroscopy
Asmentionedabove,SOxreduction/regenerationperformance hasasignificantinfluenceonthecatalystlifetimeandNOxstorage
capacity.Therefore, SOx reductioncharacteristicsofsynthesized materialswerealsostudiedasafunctionoftemperaturebymeans of in-situ FTIR spectroscopy. Fig. 4 illustrates the evolution of theS-containingsurfacefunctionalgroupsonPt/AZT,Pt/2.7K/AZT, Pt/5.4K/AZTandPt/10K/AZTcatalystsurfacesasafunctionof tem-peraturewithin323–773Kinthepresenceofanexternalreducing agent,H2(g).
Inthesesetofexperiments,catalystswereinitiallysaturated witha2.0TorrofSO2+O2gasmixture(SO2:O2=1:10)at673Kfor
5minandthencooledto323Kfollowedbytheevacuationofthe spectroscopicreactor,introductionof15.0TorrH2(g)at323Kand
annealinginH2(g)atthegiventemperatureswithin323–773K.
This particularly chosen initialsulfation/poisoning temperature (i.e.673K)isnot onlyrelevant torealisticNSR/LNT operational temperatures,but isalsohighenoughtoactivateSO2 oxidation
tosulfitesandsulfatesinacomprehensivemanner.Intheseries ofexperimentsgivenin Fig.4,spectral lineshapesdo not
typi-300 400 500 600 700 800 900 1000
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O2 SO2Fig.5. TPDprofilesfor(a)Pt/AZT,(b)Pt/2.7K/AZT,(c)Pt/5.4K/AZTand(d)Pt/10K/AZTcatalystsafter2.0TorrSOx(2.0TorrSO2+O2,SO2:O2=1:10)adsorptionat673Kfor
30minandsubsequentevacuation.
callychangeinanoteworthymanneratreductiontemperatures ≤473K.However,increasingthereductiontemperatureto673K (grayspectrainFig.4a–c)leadstonoticeablealterationsintheFTIR spectra,wherebulkandsurfacesulfate/sulfitespeciessignificantly attenuateforPt/AZT,Pt/2.7K/AZTandPt/5.4K/AZT.Increasingthe temperatureto773KinthepresenceofH2 leadstothealmost
complete elimination of the SOx-related vibrational signatures onPt/AZT, Pt/2.7K/AZTandPt/5.4K/AZTsurfaces(redspectrain
Fig.4a–c). It canbe seenin Supporting information Fig.1 that whenPt/20Ba/AlbenchmarkNSR/LNT catalystis exposed toan identicalsetofsulfationandsubsequentreductiontreatments,a significantlygreaterportionofsulfate/sulfitespeciescontinueto existonthePt/20Ba/Alcatalystevenat773Kinthepresenceof 15.0Torr H2(g).This comparativeanalysis clearly suggests that
Pt/AZT,Pt/2.7K/AZTandPt/5.4K/AZTcatalystsexhibit asuperior sulfurregenerationperformancethanthatofthePt/20Ba/Al
cata-lyst,asthisformersetofmaterialscanbefullyde-sulfatedat773K inthepresenceofH2.
However, AZT-based catalystswith thehighest K2O loading
usedinthecurrentstudy(i.e.Pt/10K/AZT)notonlyshowedunique sulfur uptake characteristics as presented in Fig. 3d which is dominated by bulk-like sulfates, but also revealed a fairly dif-ferent SOx-reductionprofilein thepresence of H2 (Fig. 4d).As
can be seen in Fig. 4, at relatively low temperatures (i.e.T ≤ 673K), a significantportion of the SOx species can already be eliminated from Pt/AZT, Pt/2.7K/AZT and Pt/5.4K/AZT surfaces. However,atT≤673K,almostalloftheS-relatedsurfacefunctional groupsremainintactonPt/10K/AZT.Evenatareduction temper-atureof773K,althoughPt/AZT,Pt/2.7K/AZTsurfacescanbefully regenerated(Fig.4a–c),Pt/10K/AZTsurfacestillremainspartially blocked/poisonedbysulfur-containingfunctionalgroupsand com-pletelyeliminatedatonlyat≥873K(Fig.4d).
3.4. SulfurregenerationundervacuumviaTPDanalysis
TPDexperimentswerealsocarriedoutinvacuuminorderto investigatethethermalregenerationabilityofthesynthesized cat-alystsaftersulfurpoisoningintheabsenceofareducingagent,as wellastocomparetherelativeadsorptionstrengthsofSOxspecies
residingonthepoisoned catalystsurfaces.Prior toTPD experi-ments,eachcatalystwasexposedto2.0TorrSO2+O2gasmixture
(SO2:O2=1:10)at673Kfor30min.
Fig. 5 shows the TPD spectracorresponding to the thermal decomposition of sulfates and sulfites on Pt/AZT, Pt/2.7K/AZT, Pt/5.4K/AZTandPt/10K/AZTcatalystsurfaces.IntheseTPD exper-iments,onlyO2 and SO2 desorptionchannels(correspondingto
masstochargeratiosofm/z=32and64;respectively)revealed sig-nificantsignalsandotherSOxorH2Sspecieswerenotdetectable.As
inthecaseoftheBaO-basedconventionalNSR/LNTcatalyst (Sup-portinginformationFig.2),SOx-relatedspeciesadsorbedonPt/AZT anditsK2O-incorporatedcounterpartsrevealhighthermal
stabil-itywhichisevidentbytheappearanceofSOxdesorptionsignalsat T>700K.AnalysisofthegeneralTPDlineshapesgiveninFig.5a–c suggeststhatatleasttwodifferentSOxdesorptionsignalsexistfor Pt/AZT,Pt/2.7K/AZTandPt/5.4K/AZTcatalystsatT<1050K, reveal-ingdesorptionmaximalocatedatca.800–820Kandat900–975K. Furthermore,SO2desorptiononthesesurfaceswithin700–1050K
isaccompaniedbyO2 desorption,suggestingthat sulfate/sulfite
decompositionoccursintheformofsimultaneousSO2+O2release.
It should benoted that thecontribution of theSO2 gastothe
m/z=32 signalduetoelectron-impactinducedfragmentationof SO2intheQMSionizerchamberislessthan10%,suggestingthat
m/z=32signalcanbealmostexclusivelyattributedtotheevolution ofO2(g)fromthecatalystsurfaces.
ItisvisibleinFig.5a–cthattheTPDdesorptionmaximatendto shifttowardshighertemperatureswithincreasingK2Oloadingin
thecatalystformulation.Itcanalsobenoticedthatwith increas-ingK2Oloadingto5.4wt.%(seeFig.5c),relativeintensityofthe
820KdesorptionfeaturewhichcanbemostlyassociatedwithSOx speciesonAZTsurfaceissuppressed,alongwiththegenerationof ahigh-temperaturedesorptionshoulderat975K.Thisisinperfect agreementwiththein-situFTIRresultspresentedinFig.3 suggest-ingthatwithincreasingK2Osurfacecoverage,extentofexposed
(uncovered)AZTsurfacedecreasesalongwithanincreaseinthe K2Oparticlesize,facilitatingtheformationofbulk-likesulfatesthat
arealsothermallymorestablethanthatofthesulfatesonAZT. ItisalsoimportanttonotethattheSOxdesorptionisnot com-pleteinFig.5b–devenat1050K(i.e.thehighestexperimentally attainabletemperature inthecurrent TPDsetup)as evidentby thepresenceof adesorptiontailatT=1050Kwhichis presum-ablyextendingwell-beyondthistemperature(assupportedbythe in-situFTIRresultsthatwillbeprovided later inthetext).This observationimpliesthatwhilesurfacesulfates/sulfitespresenton AZTsupportandK2Odomainsfullydecomposeattemperatures
below920K,bulk-like potassium sulfate species requirehigher desorptiontemperaturesforcompletethermaldecompositionand desorption.Inotherwords,presenceofbasicK2Odomainsyields
strongbindingsitesforSO2,leadingtotheformationofthermally
stablesurfaceandbulk-likeSOxspecies.
On theotherhand,a furtherincrease in theK2Oloadingto
10wt.%illustratesratherdifferentSOxdesorptioncharacteristics (Fig.5d).ItisevidentthattheSOxdesorptionfeaturesatT<1050K are suppressedto a great extent, leading toa relatively minor desorptionfeaturelocatedat910Kwithashoulderatca.810K. ConsideringthesignificantSOxuptakeofthePt/10K/AZTcatalyst surfacedemonstratedbythein-situFTIRdatagiveninFig.3d,itis clearthatmostofthesulfate/sulfitespeciesonPt/10K/AZTremain intactevenaftervacuumannealingupto1050K.Thisisalso quan-titativelypresentedinFig.6,whichpresentstheintegratedSO2TPD
Fig.6.AnalysisofrelativeSOxreleasefrominvestigatedcatalystscalculatedvia
integratedTPDsignalsgiveninFig.5.
desorptionsignalsfortheinvestigatedAZT-basedcatalystswithin 323–1050K.Fig.6illustratesthattheintegratedSOxdesorption signalofPt/AZTisroughlytwicegreaterthanthatofPt/2.7K/AZT and Pt/5.4K/AZTand alsoaboutfourtimes greaterthan thatof Pt/10K/AZT.
Figs.3,5and6,suggestthatafterthesulfationoftheAZT-based catalystsat673Kandasubsequentvacuumannealingupto1050K duringtheTPDexperiments,asignificantfractionofthesulfateand sulfitespeciesremainintactontheK-containingsamplesurfaces. Thus,itiscrucialtoinvestigatetheresidualSOxspeciesremaining ontheK-containingAZTsystemsaftertheTPDruns.Fig.7shows suchin-situFTIRexperimentscorrespondingtoallofthe investi-gatedsulfurpoisonedAZT-basedcatalystsbefore(blackspectra) andafter(redspectra)TPDexperiments.Fig.7aclearlyindicates thatintheabsenceofK2O,sulfur-poisonedPt/AZTcatalystcanbe
almostfullyregeneratedviavacuumannealingupto1050K dur-ingtheTPDexperiments.Ontheotherhand,Pt/2.7K/AZTcatalyst which releasesabout%50lesser amountof SOx speciesduring TPD(Fig.6),stillrevealsaminor,yetreadilydetectablequantityof SOx(Fig.7b).Ontheotherhand,Pt/5.4K/AZTcatalystwhichhasa comparableintegratedSOxdesorptionsignaltothatofPt/2.7K/AZT (Fig.6),revealsastrongerresidualSOxsignalintheFTIRspectrum obtainedaftertheTPDrungiveninFig.7c.Thisobservationisin linewiththefactthat Pt/5.4K/AZTsurfacestores asignificantly greateramountofSOxspecies(whicharealsothermallymore sta-ble)as compared tothatof Pt/2.7K/AZT.Finally, residualsulfur analysisofthePt/10K/AZTsurface(Fig.7d)indicatesthatalmost alloftheSOxspeciesgeneratedduringinitialpoisoningprocess remainintactaftertheTPDrunandvacuumannealingat1050K. Thus,itisapparentthattheminoramountofSOxreleaseduringthe TPDexperimentforPt/10K/AZT(Fig.6)correspondstoatiny frac-tionoftheoverallsulfurthatisstoredonthissurface.Thislatter resulthassomeresemblancetotheTPDdatacorrespondingtothat ofthePt/20Ba/AlbenchmarkcatalystgiveninSupporting informa-tionFigs.2and3andalsoinFig.6whichalsorevealanincomplete thermalregenerationuponvacuumannealingupto1050Kduring theTPDrun.
WealsoperformedacomprehensiveinvestigationoftheNOx
storage,releaseandreductioncharacteristicsofPt/K/AZTsystems viain-situFTIR,TPDaswellasquantitativeflow-reactor experi-ments[44].Adetailedaccountoftheseadditionalexperimentswill
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Fig.7.FTIRspectracorrespondingtoSOxcontentof(a)Pt/AZT,(b)Pt/2.7K/AZT,(c)Pt/5.4K/AZTand(d)Pt/10K/AZTcatalystsbefore(black)andafter(red)SOx-TPDruns.(For
interpretationofthereferencestocolourinthisfigurelegend,thereaderisreferredtothewebversionofthisarticle.)
bediscussedthoroughlyinaforthcomingreport.Nevertheless,itis instructivetopresentrelativeintegratedTPDNOxdesorption sig-nalsobtainedaftersaturationofthefreshlypreparedAZT-based catalystswithNO2(5.0TorrNO2at323Kfor10min)intheabsence
ofsulfurascomparedtothatofthePt/20Ba/Albenchmark cata-lyst(Fig.8).AscanbeseeninFig.8,relativeNOxstorageamounts oftheAZT-basedcatalystsincreasemonotonicallywithincreasing K2Oloadinguntil5.4wt.%K2O,afterwhichitconvergestoavalue
thatiscomparabletothatofthePt/20Ba/Albenchmarkcatalyst. ItisworthmentioningthatNSCofthesynthesizedmaterialswere investigatedinaflow-modetubularreactorwheretheinletgasfeed wascomposedof500ppmNO,v.%5O2,v.%5CO2andv.%5H2O
balancedwithAr(g),revealingsimilarNSCvaluesforPt/5.4K/AZT (0.165mmol/gcat)andconventionalPt/20Ba/Al(0.171mmol/gcat)
at573K[44].
Acombinedanalysisofthestructuralcharacterizationresults aswellas thespectroscopicprobe moleculeadsorption experi-mentsgiveninthecurrentstudyallowsustoshedlightonsulfur poisoning,regeneration and NOxstorage characteristicsof AZT-basedNSR/LNTcatalystsfunctionalizedwithK2O.Intheabsence
oftheK2O,Pt/AZTsystemrevealshighSSA(191m2/g)and
rela-tivelyweaklyboundsulfates/sulfiteswhichcanreadilyberemoved fromthesurfaceina completefashioneitherbyreductionwith H2(g)at 773Kor simplybythermalregeneration invacuumat
ca.950K.However,duetolackofbasicK2Odomains,Pt/AZT
suf-fersfromrelativelylow NSC.IncreasingtheK2Oloadingto2.7,
5.4and10.0wt.%leadstoaprogressivelyincreasingNOx adsorp-tionwhereNSCseemstobeconvergingtoavaluesimilartothat ofPt/20Ba/AlbenchmarkcatalystforPt/5.4K/AZTandPt/10K/AZT. Furthermore,Pt/5.4K/AZTsampleallowscompleteremovalofSOx
Fig.8.RelativeintegratedNOxdesorptionsignalsobtainedfromNOx-TPD
experi-ments.
catalystwhosecompleteregenerationrequiresmuchhigher tem-peratures(i.e.973K)underidenticalreducingconditions.Although increasingtheK2Oloadingfrom5.4to10.0wt.%doesnotseem
tohaveatremendousenhancementinNOxadsorptionproperties ofthePt/K/AZTsystem,itdoesresultinunfavorableSOxuptake, releaseandregenerationcharacteristics.TPDandFTIRdata sug-gestthatK2OdomainstendtoagglomeratewithincreasingK2O
loadingand form 3D clusterswith growingK2O particle sizes.
Thesephenomena alsoexpeditetheformation of bulk-like sul-fatefunctionalitiesinthesubsurfaceofK2Odomainswithmuch
higherthermalstabilityandmuchstrongerresistanceagainst ther-maldecompositionandreductionwithhydrogen.Consequently, Pt/5.4K/AZTsystemappearsasapromisingalternativewhichcan alsobeusedinconjunctionwithconventionalPt/20Ba/AlNSR/LNT catalysts.
4. Conclusion
Inthecurrentstudy,advancedternaryandquaternarymixed oxidematerialsintheformofPt/K2O/Al2O3/ZrO2/TiO2were
syn-thesizedwithdifferentK2Oloadings.Synthesizedmaterialswere
structurallycharacterizedviaXRDandBETincomparisontoa con-ventionalPt/20Ba/AlbenchmarkNSR/LNTcatalyst.Interactionof thesecatalystsurfaceswithSOx(i.e.SO2+O2)mixturewere
moni-toredspectroscopicallyusingin-situFTIRandTPD.Ourfindingscan besummarizedasfollows:
• BesidesthepresenceoforderedmetallicPt,Pt/AZT,Pt/2.7K/AZT and Pt/5.4K/AZT materialsrevealed disordered structures. On theotherhand,Pt/10K/AZTexhibitedadditionaldiffraction sig-nals corresponding to tetragonal ZrO2 domains. Unlike the
AZT-supported materials,conventional Pt/20Ba/Albenchmark catalystwascomposedoforderedphasesincluding␥-Al2O3and
BaAl2O4.
• Increasein K2Oloadingfrom2.7 to5.4and10.0wt.%
mono-tonicallydecreasestheSSA values from177m2/gto 155and
125m2/g,respectively.ApartfromthePt/10K/AZTcatalyst,SSA
valuesofthecorrespondingPt/K/AZTcatalystsarehigherthan thatofthebenchmarkPt/20Ba/Alcatalyst(134m2/g).
• IncreasingtheK2OloadinginthePt/K/AZTsystemleadstothe
growthoftheK2Odomainsize(i.e.sintering),covering ofthe
AZTsurface withK2Oand anincreasein thebulk-likesulfate
functionalgroups requiringhigher temperaturesfor complete sulfurelimination viathermal decompositionorviareduction withH2(g).
• Increasein K2Oloadingin thePt/K/AZTformulationincreases
theNOxadsorptionupto5.4wt.%ofK2O.HoweverK2Oloadings
higherthanthisvaluedonothaveasignificantpositiveinfluence onNOxadsorption.
• Thereis a delicate trade-offbetweenNSC and sulfur adsorp-tion/release/regenerationcharacteristics.NSCandSOxtolerance ofAZTbasedNSR/LNTcatalystscanbeoptimizedsimultaneously bycarefullyfine-tuningtheK2Oloading.
• Among the investigated catalysts, Pt/5.4K/AZT was found to revealsuperiorsulfurregenerationperformancethanthatofthe conventionalPt/20Ba/Albenchmarkcatalystalongwitha com-parableNSC(0.165vs.0.171mmol/gcat,repsectively).
Acknowledgements
AuthorsacknowledgethefinancialsupportfromtheScientific andTechnologicalResearchCouncilofTurkey(TUBITAK)(Project Code:111M780).AuthorsalsoacknowledgeProf.LouiseOlssonand OanaMihai(ChalmersUniversityofTechnology)forquantitative flow-modeNSCmeasurements.
AppendixA. Supplementarydata
Supplementarydataassociatedwiththisarticlecanbefound, intheonlineversion,athttp://dx.doi.org/10.1016/j.cattod.2015.12. 013.
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