FT-IR spectroscopic investigation of the effect of SO2 on the SCR of NOx with propene over ZrO2–Nb2O5 catalyst

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CatalysisToday176 (2011) 437–440

ContentslistsavailableatScienceDirect

Catalysis

Today

j o ur na l 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 / c a t t o d

FT-IR

spectroscopic

investigation

of

the

effect

of

SO

2

on

the

SCR

of

NO

x

with

propene

over

ZrO

2

–Nb

2

O

5

catalyst

M. Kantcheva

a,∗

_,

_I.

_Cayirtepe

a

_,

_A.

_Naydenov

b

_,

_G.

_Ivanov

b

a_Department_of_Chemistry,_Bilkent_University,₀₆₈₀₀_Bilkent,_Ankara,_Turkey b_Institute_of_General_and_Inorganic_Chemistry,_BAS,_Soﬁa_1113,_Bulgaria

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received15September2010

Receivedinrevisedform19October2010 Accepted20October2010

Available online 24 November 2010 Keywords:

ZrO2–Nb2O5solidsolution

C3H6-SCRofNOx

EffectofSO2

InsituFT-IRspectroscopy MechanismofSO2poisoning

a

b

s

t

r

a

c

t

TheSO2toleranceofacatalystbasedonZrO2–Nb2O5 solidsolution(moleratioZrO2:Nb2O5=1:6)in

thecatalyticactivityforNOxreductionwithpropeneinexcessoxygenhasbeenstudied.Nolossinthe

C3H6-SCRactivitywasobservedfor2haftertheadditionof56ppmofSO2 tothereactionmixture.

WhentheconcentrationofSO2hasbeenincreasedto200ppm(theso-calledfastpoisoningexperiment)

theactivityofthecatalystdecreasedby13%andremainedunchangedformorethan5hunderthese conditions.TheeffectofSO2onthesurfacereactionoftheSCRreactantshasbeeninvestigatedbyinsitu

FT-IRspectroscopyandmechanismforthesuppressionofthecatalyticactivityathighconcentrationof thepoisonhasbeenproposed.

1. Introduction

SelectivecatalyticreductionofNOxbyhydrocarbons(HC-SCR) hasbeenextensively studied for removalof NOx fromexhaust gasesgenerated bydieselandlean-burngasolineengines[1–4]. Animportant factor determining the efﬁciencyof DeNOx cata-lysts is their SO2 tolerance. It is assumed that under reaction conditions, the SO2 (which is present in the exhaust) reacts withoxygentoformthermodynamically stablesurfacesulfates. Thelatterspecies coverthecatalystsurface andblockthesites forNOx adsorption[5].Therefore, thedevelopmentofcatalysts resistant to SO2 poisoning is an important task. It has been reportedthatthedepositionofniobiumonoxidesurfacessuch asalumina[6]orzirconia[7] lowerstheconcentrationofbasic siteswhichshouldresultinimprovedresistancetoSO2 poison-ing.Recently,wehave foundthatzirconia–niobiasolidsolution (mole ratioZrO2:Nb2O5=6:1)is activein theselectivecatalytic reductionofNOxwithpropene(C3H6-SCR)inexcessoxygen[8]. The conversion of NOx reaches maximum at 220◦C. Based on in situ FT-IR results, we proposed a reaction mechanism with nitroacetone and NCO species as the key intermediates [8]. In thepresentwork,weinvestigatedtheeffectofSO2onthe activ-ity of the ZrO2–Nb2O5 solid solution for NO reduction with propeneinexcessoxygen.ThesurfacereactionoftheSCRreactants

∗ Correspondingauthor.Tel.:+903122902451;fax:+903122664068. E-mailaddress:margi@fen.bilkent.edu.tr(M.Kantcheva).

in the presence of SO2 was monitored by in situ FT-IR spec-troscopy.

2. Experimental

Mixedzirconium–niobiumoxidewaspreparedbyimpregnation ofhydratedzirconiawithacidicsolution(pH∼0.5)of peroxonio-bium(V)complex,[Nb2(O2)3]4+,ensuringZrO2:Nb2O5moleratio of6:1.Detailsaboutthemethodofpreparationand characteriza-tionofthesurfaceacidityaregivenelsewhere[9].TheBETsurface areaofthematerialcalcinedat600◦C(denotedas25NbZ-P)was 42m2_/g._According_to_XRD_the_obtained_sample_has_the_structure_of Zr6Nb2O17.Theadsorptionof2,6-dimethylpyridineonthe25NbZ-P samplerevealedthepresenceofstrongBrønstedacidity.

The equipment and the conditions of catalytic tests of NOx reductionbypropeneweredescribedearlier[8].Theconversion degreesofNOxweretakenasameasureofthecatalyticactivity. TheSO2poisoningexperimentwasperformedwith56ppmofSO2 intheinletandunderfastpoisoningconditions(200ppmofSO2) attemperaturecorrespondingtothemaximumNOxconversion.

TheFT-IRspectrawererecordedusinga BomemHartman& BraunMB-102modelFT-IRspectrometerwithaliquid-nitrogen cooledMCTdetector ata resolutionof 4cm−1 (128 scans).The self-supportingdiscs(∼0.01g/cm2₎_were_activated_in _the_IR_cell by heating for 1h in a vacuumat 450◦C, and in oxygen(100 mbar,passedthroughatrapcooledinliquidnitrogen)atthesame temperature,followedbyevacuationfor1hat450◦C.The experi-mentswerecarriedoutunderstaticconditions.Thespectraofthe 0920-5861/$–seefrontmatter © 2010 Elsevier B.V. All rights reserved.

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438 M.Kantchevaetal./CatalysisToday176 (2011) 437–440 600 500 400 300 200 100 0 40 45 50 55 60 65 70 75 SO 2 cut SO 2 : 200 ppm SO 2 : 56 ppm NO x conversion, % Time, min

Fig.1. ResultsoftheSO2poisoningexperimentat220◦Cusing56and200ppmof

SO2intheinlet.(Reactionconditions:245ppmNOx(NO/NO2=1.77),504ppmC3H6,

9vol.%O2,GHSV=10,000h−1).

adsorbedcompoundswereobtainedbysubtractingthespectraof theactivatedsamplefromthespectrarecorded.Thesamplespectra werealsogas-phasecorrected.ThegasesNO(99.9%),C3H6(99.9) andSO2(99.9%)usedintheinsituFT-IRexperimentsweresupplied byAirProducts.

3. Resultsanddiscussion

3.1. CatalyticactivityinthepresenceofSO2

Weshowedearlier[8]thattheNOxconversionintheC3H6-SCR reactionoverthe25NbZ-Pcatalystreachesmaximum of62%at 220◦Candthandecreasesasthecombustionofpropenebecomes predominant.TheconversionofC3H6 iscloseto100%at250◦C. Thecatalystdisplaysstableactivityatthetemperatureof maxi-mumNOxconversion(thedurationofcatalyticactivitytestswas limitedto10h).The25NbZ-Pcatalystshows goodresistanceto SO2 poisoning(Fig.1).ThereisnolossintheC3H6-SCRactivity for2haftertheadditionof56ppmofSO2tothereactionmixture. WhentheconcentrationofSO2hasbeenincreasedto200ppm(the so-calledfastpoisoningexperiment)theactivityofthe25NbZ-P catalystdecreasestoapproximately54%of NOx conversionand remainsunchangedformorethan5hundertheseconditions. 3.2. InsituFT-IRspectroscopicinvestigations

3.2.1. Co-adsorptionofSO2+O2overZrO2and25NbZ-Psamples InordertoﬁndouttheeffectofincorporationofNb(V)into zir-coniaontheformationofsurfaceSOxspeciesweinvestigatedthe adsorptionof(0.5mbarSO2+10mbarO2)mixtureontheZrO2and 25NbZ-Psamplefor15minatvarioustemperatures.The predom-inantsurfaceSOxcompoundsobservedinthespectraofzirconia takenbelow200◦CareadsorbedSO32−species(thespectraarenot shown).Theweakbandat1362cm−1detectedonzirconiaat200◦C (Fig.2,dottedspectrum(a))ischaracteristicofthehigh-frequency componentof thesplit3 modeofhighly covalentsurface sul-fates[10,11].Thestrongabsorptionbetween1100and900cm−1 (presentalsointhespectradetectedbelow200◦C)isattributedto thevibrationalmodesofSO32−ionscoordinatedthroughtheSatom tothesurface[12,13].Thebandat1000cm−1isassignedtothe1 mode,whereasthebandsat1020and935cm−1correspondtothe split3stretchingvibrationsofSO32−ions[12,13].Thisassignment issupportedbythefactthatat25◦C,theadsorptionof0.5mbarof SO2onthezirconiasamplecausestheappearanceofabsorptions at1020,1000and930cm−1whoseintensitiesarenotaffectedby

960 1080 1200 1320 1440 Absorbance Wavenumber [cm-1 ]

a

b

c

1362 1020 0.2 1000 935 1060

Fig.2.FT-IRspectracollectedduringtheexposureofzirconia(dottedline)and 25NbZ-Psample(solidline)toa(0.5mbarSO2+10mbarO2)mixturefor10minat

200◦_C_(a),₂₅₀◦_C_(b)_and₃₀₀◦_C_(c).

theincreaseinthetemperatureoftheisolatedIRcellupto350◦C. Inaddition,thebandat1362cm−1hasnotbeenobservedunder theseconditions(thespectraarenotshown).

Theheatingat300◦Ccausessigniﬁcantincreaseintheintensity ofthesulfatebandat1362cm−1attheexpenseoftheabsorption correspondingtothesulﬁtespecies(Fig.2,dashedspectrum(c)). Theshoulderat1060cm−1isassignedtothelow-frequency

com-1000 1200 1400 1600 1800 2000 2200 2400 Absorbance Wavenumber [cm-1 ] a b c 2270 1900 1740 1680 1544 1450 1346 _{1044 1013} 1610 1570 1655 1277 1245 0.2 1415

Fig.3. FT-IRspectraofthe25NbZ-Psamplecollectedduringtheexposurefor10min toa(2mbarC3H6+6mbarNO+4mbarO2)mixtureintheabsence(dottedline)and

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M.Kantchevaetal./CatalysisToday176 (2011) 437–440 439

Table1

Assignmentoftheabsorptionbandsinthespectraof25NbZ-Pcatalystobservedinthe25–350◦_C_temperature_range_during_the_{investigation}_of

thereactivityofsurfacespeciesformeduponroom-temperatureadsorptionofNO+C3H6+O2mixtureintheabsenceandpresenceofSO2.

Species Bandposition(cm−1) Vibration

NCO 2270 as(NCO)

N2O3(ads) 1900 (N O)

Nitroacetone(ads) 1740 (C O)

Acetone(ads) 1680 (C O)

BridgedNO3−(twotypes) BidentateNO3−

1655,1610,1245

1570,1275

(N O),as(NO2) (N O),as(NO2)

CH3COO−(twotypes) 1570,1544,1450,

1415,1346,1315

as(COO),s(COO),ı(CH3)

SO42− 1346,1044–1013 (S O),(S−O)

ponentofthesplit3 modeof theSO42− groups[10,11].These

resultsshowthatnoticeableoxidationofSO2overzirconiastarts at300◦C.Inthisprocess,mostlikely,surfaceoxygenvacanciesare involvedfacilitatingtheactivationofO2.Between200and300◦C, thesametypeofsurfaceSOxspeciesaredetectedonthe 25NbZ-Psample,howeverwithsigniﬁcantlylowerconcentrations(Fig.2, solidtraces).ThisindicatesthattheincorporationofNb5+_ions_into zirconiasuppressestheoxidationofSO2toSO3.

3.3. EffectofSO2ontheC3H6+NO+O2surfacereaction

Theresultsofourpreviousinvestigation[8]haveshownthat overthe25NbZ-Psample,characterizedbystrongBrønsted acid-ity[9],theactivationofpropeneinthepresenceofadsorbedNOx speciesisquiteeasyatlowtemperatures,producingsurface iso-propoxides.Theinteractionofthelatterspecieswiththesurface nitrate complexes leads to the formation of nitroacetone. It is proposed[8]that nitroacetonetransformsthroughtwo parallel reactions. Path(1)involvesthebasicoxidesitesof thecatalyst producingacetatespeciesandaci-nitromethane.Path(2)proceeds throughoxidationofnitroacetonetoacetatesandCOx/H2Owith release ofNO2.The latterprocessis importantat temperatures higherthan200◦C.Theaci-nitromethanegeneratesNCOspecies coordinatedtothecationicsitesofthemixedoxide.Thesurface iso-cyanatesaredetectedalreadyatroomtemperature.Itisproposed that theisocyanates reactwiththeNO3−/NO2 surface complex formedbybothoxidationofNOandoxidationofnitroacetone[8]. Fig.3comparesthespectraofthe25NbZ-Psampleobtainedat varioustemperaturesduringtheadsorptionfor15minofgaseous mixturecontaining(2mbarC3H6+6mbarNO+4mbarO2)inthe absence(dottedlines)andpresenceof0.5mbarSO2(solidlines). Table1givestheassignmentoftheabsorptionbands.

Thespectratakenatroomtemperature,containweak absorp-tionat2270cm−1indicatingtheformationofNCOspecies[8]in bothcases,inthepresenceandabsenceofSO2(Fig.3,spectra(a)). Thebroadbandat1900cm−1 ischaracteristicofadsorbedN2O3 [8,11].Someamountsof adsorbednitroacetone(1740cm−1 [8]) andacetone(1680cm−1[8])areobservedaswell.Thedifference betweenbothspectraisintheconcentrationofthesurfacenitrates (bandsat1655,1610,1570,1277and1245cm−1[8,11]),whichis lowerinthepresenceofSO2.Inaddition,theappearanceofbands at1346and1044–1013cm−1correspondingtothesplit3mode ofmultidentateSO42−groupscoordinatedtocationicsurfacesites [10,11],revealsformationofsurfacesulfatesatroomtemperature. ThisindicatesthatthenitratespeciesoractivatedNO2lower sig-niﬁcantlytheoxidationtemperatureofSO2.Mostlikely,thesulfate speciesblocktheactivesites(Nb5+_ions)_for_the_oxidation_of_NO toNO2[8]leadingtodecreaseintheconcentrationofthesurface nitrates.Asaconsequence,theamountofnitroacetoneformedat 150◦CinthepresenceofSO2islowerthanthatintheabsenceof thepoison(Fig.3,spectra(b)).Thespectrumtakenat200◦Cinthe

presenceofSO2 (Fig.3,spectrum(c),solidtrace) showsfurther increaseintheamountofsulfatespecies,whichisevidentbythe enhancementoftheabsorptionsat1346and1044–1013cm−1.The decreaseinthesurfaceconcentrationsofacetoneandnitroacetone at200◦Cmeasuredagainsttheconcentrationsat150◦Cis consid-erablysmallerthanthecorrespondingdecreaseobservedinthe absenceofSO2(Fig.3,comparespectra(b)and(c)).Thisfactleadsto theconclusionthatthesurfacesulfateshinderthetransformation ofnitroacetonetoNCOspecies.Mostlikely, thelow concentra-tionofnitroacetoneandhindranceofitstransformationresultin lowering theconcentrationoftheNCOspecies and decreasein thecatalystactivity.Itshouldbepointedoutthatthemechanism proposedforSO2poisoningshouldoperateathighconcentration ofSO2whichisevidentfromthecatalyticactivitymeasurements (seeFig.1).Theresultsoftheinvestigationshowthatthecatalytic propertiesofthezirconia–niobiasolidsolutioncouldbeof inter-estregardingthedevelopmentofsulfur-tolerant,low-temperature catalystsfortheSCRofNOxwithhydrocarbons.

4. Conclusions

TheincorporationofNb(V)intozirconialeadingtotheformation ofzirconia–niobiasolidsolution(moleratioZrO2:Nb2O5=6:1) sup-pressestheoxidationofSO2toSO3.Themixedoxidedisplaysgood resistancetowardSO2poisoningintheC3H6-SCRofNOx.Nolossin theSCRactivityisobservedatlowconcentrationofSO2(56ppm) inthefeedgas.Theactivitydeclinesby13%athighconcentration ofthepoison (200ppm)and remainsunchangedfor morethan 5h. Insitu FT-IRinvestigations showthat thenitratespeciesor activatedNO2 lower(s)theoxidationtemperatureofSO2 to sur-facesulfates.ThepresenceofsurfaceSO42−groupsdecreasesthe amountofadsorbednitrates,respectivelynitroacetone,and hin-dersthetransformationof thelattercompound toNCOspecies consideredasreactionintermediates.Thiscausesdecreaseinthe catalyticactivityobservedathighconcentrationsofSO2inthefeed gas.

Acknowledgements

ThisworkwasﬁnanciallysupportedbyBilkentUniversityand theScientiﬁcandTechnicalResearchCouncilofTurkey(TÜBITAK), Project TBAG-106T081.I.C.and M.K.gratefully acknowledge the supportbytheEU7FrameworkprojectUNAM-REGPOT(Grantno 203953).

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