CatalysisToday176 (2011) 437–440
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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
baDepartmentofChemistry,BilkentUniversity,06800Bilkent,Ankara,Turkey
bInstituteofGeneralandInorganicChemistry,BAS,Sofia1113,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.
© 2010 Elsevier B.V. All rights reserved.
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,weinvestigatedtheeffectofSO2ontheactiv- 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)ofperoxonio- bium(V)complex,[Nb2(O2)3]4+,ensuringZrO2:Nb2O5moleratio of6:1.Detailsaboutthemethodofpreparationandcharacteriza- tionofthesurfaceacidityaregivenelsewhere[9].TheBETsurface areaofthematerialcalcinedat600◦C(denotedas25NbZ-P)was 42m2/g.AccordingtoXRDtheobtainedsamplehasthestructureof 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)wereactivatedin theIRcell by heating for 1h in a vacuumat 450◦C, and in oxygen(100 mbar,passedthroughatrapcooledinliquidnitrogen)atthesame temperature,followedbyevacuationfor1hat450◦C.Theexperi- mentswerecarriedoutunderstaticconditions.Thespectraofthe 0920-5861/$–seefrontmatter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.cattod.2010.10.072
438 M.Kantchevaetal./CatalysisToday176 (2011) 437–440
600 500
400 300
200 100
400 45 50 55 60 65 70 75
SO2 cut SO2: 200 ppm
SO2: 56 ppm
NOx 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.
Thecatalystdisplaysstableactivityatthetemperatureofmaxi- 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)intozir- coniaontheformationofsurfaceSOxspeciesweinvestigatedthe adsorptionof(0.5mbarSO2+10mbarO2)mixtureontheZrO2and 25NbZ-Psamplefor15minatvarioustemperatures.Thepredom- inantsurfaceSOxcompoundsobservedinthespectraofzirconia takenbelow200◦CareadsorbedSO32−species(thespectraarenot shown).Theweakbandat1362cm−1detectedonzirconiaat200◦C (Fig.2,dottedspectrum(a))ischaracteristicofthehigh-frequency componentof thesplit3 modeofhighly covalentsurfacesul- 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
10200.2 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-frequencycom-
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 presenceof0.5mbarSO2(solidline)at25◦C(a),150◦C(b)and200◦C(c).
M.Kantchevaetal./CatalysisToday176 (2011) 437–440 439
Table1
Assignmentoftheabsorptionbandsinthespectraof25NbZ-Pcatalystobservedinthe25–350◦Ctemperaturerangeduringtheinvestigationof 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, thesametypeofsurfaceSOxspeciesaredetectedonthe25NbZ- Psample,howeverwithsignificantlylowerconcentrations(Fig.2, solidtraces).ThisindicatesthattheincorporationofNb5+ionsinto zirconiasuppressestheoxidationofSO2toSO3.
3.3. EffectofSO2ontheC3H6+NO+O2surfacereaction
Theresultsofourpreviousinvestigation[8]haveshownthat overthe25NbZ-Psample,characterizedbystrongBrønstedacid- ity[9],theactivationofpropeneinthepresenceofadsorbedNOx
speciesisquiteeasyatlowtemperatures,producingsurfaceiso- 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.Thesurfaceiso- 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,containweakabsorp- 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.
ThisindicatesthatthenitratespeciesoractivatedNO2lowersig- nificantlytheoxidationtemperatureofSO2.Mostlikely,thesulfate speciesblocktheactivesites(Nb5+ions)fortheoxidationofNO 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◦Cisconsid- 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–niobiasolidsolutioncouldbeofinter- 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 tosur- facesulfates.ThepresenceofsurfaceSO42−groupsdecreasesthe amountofadsorbednitrates,respectivelynitroacetone,andhin- 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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