In situ FT-IR spectroscopic investigation of gold supported on tungstated zirconia as catalyst for CO-SCR of NOx

CatalysisTodayxxx (2012) xxx–xxx

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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

In

situ

FT-IR

spectroscopic

investigation

of

gold

supported

on

tungstated

zirconia

as

catalyst

for

CO-SCR

of

NO

x

M.

Kantcheva

,

M.

Milanova,

S.

Mametsheripov

DepartmentofChemistry,BilkentUniversity,06800Ankara,Turkey

a

r

t

i

c

l

e

i

n

f

o

InsituFT-IRspectroscopy NOx+COsurfacereaction

CO-SCRofNOx

a

b

s

t

r

a

c

t

Thepossibilityforapplicationofgold supportedon tungstatedzirconiaascatalystintheselective catalyticreductionofNOxwithCO(CO-SCR)hasbeeninvestigatedbymeansofinsituFT-IR

spec-troscopy.Tungstatedzirconiacontaining18wt%ofWO3 waspreparedbyco-precipitation.Goldwas

depositedonzirconiaandtungstatedzirconiabycationicadsorptionusing[Au(en)2]Cl3 asa

precur-sor(en=ethylenediamine).TheFT-IRspectraobtainedduringtheinteractionofCOwithNOxspecies

adsorbedontheAu/ZrO2–WOx andAu/ZrO2 samplesrevealedtheformationofsurfaceisocyanates

attachedtothegoldparticles.ThegenerationofAu–NCOspeciesoccurredatlowtemperature(25–50◦C). TheW-containingsamplewascharacterizedbyhigheractivityinthead-NOx+COreactionthanthe

W-freeone.ItisshownthatthenitratespeciesoradsorbedNO2areessentialforthegenerationofsurface

isocyanatesandtheoxidationofNObyoxygenisanimportantstep.TheobtainedresultssuggestthatAu catalystssupportedontungstatedzirconiamightbeofinterestfortheselectivereductionofNOxwith

COatlowtemperatures.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

TheselectivereductionofNOxinthepresenceofoxygenisa

verypromising technologytocontroltheemissionsfromdiesel andlean-burnengines.ReductionofNOxusingeithertheresidual

hydrocarbonsoron-boardfuelwouldbethemostideal technol-ogy.However,thetraditionalmaterialsdevelopedfortheselective reductionofNOxwithhydrocarbons(HC-SCR)donotshow

suffi-cientactivitiesundertheconditionsofleanexhaustespeciallyat lowtemperatures(<150–250◦C)[1–4].Oneofthetypical reduc-tantpresentinthecarexhaustisCO.RecentlyIr/SiO2,Ir/WO3and

Ir/WO3/SiO2catalystshavebeenreportedtobehighlyactiveinthe

selectivereductionofNOwithCO(CO-SCR)[5–11].However,these catalystsshowactivityin the260–400◦C temperaturewindow. Supportedgoldcatalystsareknowntobehighlyactiveinvariety ofreactionsatlowtemperatures[12,13].Inthepresentstudy,we reporttheresultsofFT-IRspectroscopiccharacterizationofgold catalystsupportedontungstatedzirconia.Inordertoevaluatethe potentialofanewmaterialasacatalystinadesiredprocess,itis importanttoinvestigatetheinteractionofthereactantswiththe surface.ForthispurposewestudiedtheadsorptionofCOandits co-adsorptionwithoxygenandNOoverAu-promotedandAu-free tungstatedzirconia.

∗ Correspondingauthor.

E-mailaddress:margi@fen.bilkent.edu.tr(M.Kantcheva).

2. Experimental 2.1. Samplepreparation

Thesamplepreparationisdescribedindetailelsewhere[14]. Inbrief,tungstatedzirconiawassynthesizedbyco-precipitation [15]withnominalcontentof18wt%ofWO3.Goldwasdeposited

ontungstatedzirconiabycationadsorptionfor2hfromaqueous solutionof[Au(en)2]3+complexatpH=9.6androomtemperature.

Afterdryingofthewashedsolidat80◦Cfor48h,thesamplewas calcinedfor1hat400◦C.Thecationicgoldprecursorwasprepared followingtheprocedureofBlockandBailar[16].Thesamplewas labeledasAu/18WZ-CP.

Inordertostudytheeffectoftungsten,goldwasdepositedon tetragonalzirconiaalsobycationadsorptionof[Au(en)2]3+

com-plexusingthesameconditionsappliedtotheW-containingsample.

2.2. Samplecharacterization

The samples were characterized by XRD, DR-UV–vis spec-troscopy,BETmeasurementsandchemicalanalysis,andtheresults werereportedearlier[14].

TheFT-IRspectrawererecordedusinga BomemHartman& BraunMB-102modelFT-IRspectrometerwithaliquid-nitrogen cooledMCTdetector ata resolutionof 4cm−1 (100 scans).The self-supportingdiscs(∼0.01g/cm2_{)wereactivatedin theIRcell}

byheatingfor1hinavacuumat400◦C,andinoxygen(100Torr, 0920-5861/$–seefrontmatter © 2012 Elsevier B.V. All rights reserved.

Physico-chemicalcharacterizationoftheinvestigatedsamples.

Sample SBET(m2_{/g) WO3(wt%) Auloading(wt%) AverageAuparticle}

size(nm)

ZrO2 152 –

Au/ZrO2 143 – 1.43±0.03 8

18WZ-CP 116 18.0 –

Au/18WZ-CP 118 17.6 2.27±0.01 10

passedthroughatrapcooledinliquidnitrogen)atthesame tem-perature,followedbyevacuationfor1hat400◦C.Thespectraof adsorbedgaseswereobtainedbysubtractingthespectraofthe activatedsamplefromthespectrarecorded.Thesamplespectra werealsogas-phasecorrected.

3. Resultsanddiscussion

3.1. Structuralcharacterization

Table1summarizesthechemicalcompositionandBETsurface areasofthesamples.Theresultsofthechemicalanalysis[14]show thatthegolduptakeontungstatedzirconiaishigherthanthaton zirconia.Itiswellknownthattungstatedzirconiacontainsacidic protons[15,17–21]whichinbasicmediumcanundergo deproto-nation.ThiscanleadtotheformationofgreaternumberofM–O− anchoringsites(M=WandZr)forthe[Au(en)2]3+complex

result-inginhighergolduptake.

AccordingtotheXRDdata[14]thematerialshavethestructure oftetragonalzirconia.Theaveragesizeofgoldparticles(Table1)is calculatedbyusingScherrerequationandthemaingolddiffraction lineof2=38.2◦.TheDR-UV–visspectraofgoldcontaining sam-ples[14]exhibitbroadabsorptionwithmaximumat550–590nm characteristicoftheplasmonicoscillationmodeofnanosizedgold particles[22–24].

3.2. AdsorptionofCOatroomtemperature

TheanalysisoftheFT-IRspectraofCOadsorbedatroom tem-peratureonthesamplescanbeveryusefultoobtainqualitative informationonthenatureofsupportedgoldspecies.Fig.1 dis-plays the FT-IR spectra in the carbonyl region of CO (10Torr) adsorbedontheactivatedAu-containingsamples.Theabsorption at2194–2198cm−1 is assigned toCO adsorbed onZr4+ _surface

sites[18,21,23,25–27].Thehigh-frequencyshiftexhibitedinthe WOx-containingmaterialisassociatedwiththeincreasedacidity

ofthecoordinativeunsaturatedZr4+_{sitescausedbythe}

electron-withdrawing WOx groups [18,21]. The band corresponding to

theZr4+_{–COspeciesand thecomposedbandwithmaximumat}

2113–2116cm−1areremovedupondynamicevacuationatroom temperature. Accordingto data from the literature [23,25–39], theabsorptionwithmaximumat2113–2116cm−1isassignedto COadsorbedonsmallthree-dimensionalgoldclusters, whereas theshoulderat2125–2130cm−1 couldbeattributedtoAuı+–CO species.Formationofpositivelypolarizedgoldis assumedtobe causedbythepresenceofadsorbedoxygenonthegoldparticles ortheirinteraction withthesupport[23,24,26,28,31,32,35–37]. Hadjiivanovandcoworkers[36,37]proposedthattheabsorption inthe2155–2130cm−1regioncouldbeattributedalsotoAu+_–CO

speciesinwhichtheAu+_{cationresidesonthemetallicgold}

parti-cles.Itshouldbenotedthatthelastactivationstepofthesamples consistedofevacuationat400◦Candadsorbedoxygencannotbe expectedundertheseconditions.Moreover,afterthereductionof thesamplesat400◦CwithCO,theabsorptionat2125–2130cm−1 isstillpresentinthespectraofCOadsorbedatroomtemperature. Therefore,itcouldbeproposedthatthefeatureat2125–2130cm−1

couldbeassociatedwiththepresenceofmoredefectivegoldsites locatedmostlikelyattheinterface.Itcannotbeexcludedthatthe absorptionat2125–2130cm−1isassociatedwiththepresenceof largerAuparticles.Thispropositioncouldbesupportedbythefact thatCOadsorbedonsmallgoldparticlesischaracterizedby(CO) modeatlowerwavenumbersandthe(CO)stretchingvibrationis blue-shiftedastheparticlesizeincreases[40].Recently, absorp-tionbandat 2130–2140cm−1 observed upon COadsorption on Au/Nb2O5 hasbeenattributedtoCOcoordinatedtolargergold

nanoparticles[41].

3.3. InsituFT-IRspectroscopicinvestigationoftheNO+O2+CO

reaction

3.3.1. NO+O2surfacereaction

Ingeneral,themechanismofSCRofNOxonvariousoxide

cata-lystsinvolvestheinteractionofstronglyadsorbedNOx−species(x

is2or3)withthereducer[2,42–46].Thereforewestudiedthe ther-malstabilityofNOxspeciesadsorbedonthesurfaceofthe18WZ-CP

andAu/18WZ-CPsamples.

NOxspecies onthesurface oftheAu/18WZ-CPsamplewere

createdby adsorptionof a (10TorrNO+25TorrO2)mixtureat

roomtemperaturefollowedbyevacuationfor20minatthesame temperature(Fig.2A,spectrum(a)).Theabsorptionbandsinthe 1700–1000cm−1 range are typicalof surface nitrates observed ontungstatedzirconia[45–47] and areidentified as monoden-tate(1582and1275cm−1)andbidentate(1629and1219cm−1) NO3− species. The broad absorption centered at 2140cm−1 is

2050 2100 2150 2200 2250 Absorbance Wavenumber (cm-1) Au/ZrO₂ Au/18WZ-CP 0.02 2 116 2113 219 4 2198 2130 2125

Fig.1. FT-IRspectraofCO(10Torr)adsorbedatroomtemperatureonthesamples studied.

M.Kantchevaetal./CatalysisTodayxxx (2012) xxx–xxx 3 1000 1250 1500 1750 2000 2250 2500 d c Absorbance Wavenumber (cm-1₎ a b 0.5 1629 1582 1275 1219 2140

(A)

(B)

Fig.2.PanelA:FT-IRspectraoftheAu/18WZ-CPsampletakenaftertheadditionofa(10TorrNO+25TorrO2)mixturetotheIRcellfor10minatroomtemperaturefollowed

byevacuationfor20min(a),andafterheatingtheisolatedIRcellfor10minat100◦C(b),200◦C(c)and300◦C(d).PanelB:Gasphasespectracollectedat100◦C(b),200◦C (c)and300◦C(d).

characteristicofNO+_{species[45–47].HeatingtheisolatedIRcell}

inthetemperaturerange100–300◦C,leadstogradualdecreasein theintensitiesofthenitratebands.Thespectraofthegasphase (Fig. 2B) showthat themajor productof decomposition ofthe surfacenitratesisNO2.TheAu-free18WZ-CPsamplecontains

anal-ogoustypeofadsorbedNOxspecieswhichweregeneratedbythe

samewayasdescribedabove(thespectraarenotshown).However, thesurfacenitratesontheAu-containingsampledisplayhigher thermalstability.

Therearereportsintheliterature[35,37],showingthata mix-tureof NO+O2 cancause oxidation of supportedmetallic gold

particlestocationicgoldspeciesandthisprocesscanoccuratroom temperature.Inordertofindtheeffectofco-adsorptionofNO+O2

mixtureontheoxidationstateofgoldintheAu/18WZ-CPsample, weinvestigatedtheadsorptionofCOonthecatalystcontaining pre-adsorbedNOxspecies.Itiswellknownthatthespectral

fea-turesofadsorbedCOcanprovideinformationabouttheoxidation stateoftheadsorptionsite[34].Surfacenitratespecies(absorption bandsbetween1700and1000cm−1)weregeneratedbybringing incontacttheAu/18WZ-CPsamplewithamixtureof10Torrof NOand25TorrofO2for10minatroomtemperaturefollowedby

evacuationatthesametemperaturefor20min(Fig.3,spectrum (a)).Tothesampletreatedinthisway,10TorrofCOwereadded. Thespectrumdetectedatroomtemperatureafter5mincontainsa strongbandwithmaximumat2185cm−1(Fig.3,spectrum(b)).The intensityofthissignalincreasessignificantlyafter10minofcontact withtheCO(Fig.3,spectrum(c)).Weakabsorptionat2130cm−1is observedaswellwhoseintensityincreasesinparallelwiththemain bandat2185cm−1.Thelattersignalfallsintherangeofreported (CO)stretchingvibrationsofAu+_{–COspecies(2197–2160cm}−1

[31–37]).However,theincrease intheintensityof thebandat 2185cm−1 duringthecontactwithCO contradictstheobserved instabilityoftheAu+_{–COspeciesinCOatmosphere[31–33,35]due}

toreductionofAu+_{adsorptionsitesbyCO.}

Inordertoensuremoreefficientoxidationofgold,the cata-lystpelletwastreated witha(10Torr NO+25TorrO2)mixture

at300◦Cfor30min.Thenthetemperaturewasloweredto200◦C andthegasmixturewasremovedfromtheIRcellupondynamic

evacuationwhilecoolingtoroomtemperature.Subsequent adsorp-tionofCO(10Torr)atroomtemperatureresultsintheformation ofweakabsorptionbands at2190and2165cm−1 (Fig.4, spec-trum(a)).Thespectruminthenitrateregion(notshown)displays thesamesetofnitratebandsasshowninFig.2A,althoughwith weakerintensity.Thebandat2190cm−1resiststheevacuationat roomtemperature(Fig.4,spectrum(b))anddisappearsupon out-gassingat100◦C(Fig.4,spectrum(d)).Theobservedbehaviorof theabsorptionat2190cm−1istypicalofAu+_{–COspecies[34].The}

COadsorbedonAu+_{sitesisstabilizedbythesynergismbetween}

the␴-dativeand␲back-bondingcomponentsoftheAu+ _CObond

[34–37]andthecarbonylcomplexcanbedestroyedat temper-aturehigherthan25◦C.Theabsorptionat2165cm−1 disappears uponevacuationatroomtemperature(Fig.4,spectrum(b))and couldbeattributedtoAuı+–COspeciesorCOadsorbedonlarge

1000 1200 1400 1600 1800 2000 2200 2400 Absorbance Wavenumber (cm-1₎ a b c 2185 1622 1582 12 75 1227 0. 5 2130

Fig.3. FT-IRspectraoftheAu/18WZ-CPcatalystcollectedaftertheadsorptionof a(10TorrNO+25TorrO2)mixturefor10minatroomtemperaturefollowedby

evacuationfor20minatroomtemperature(a)andadsorptionof10TorrCOatroom temperaturefor5min(b)and10min(c).

2100 2125 2150 2175 2200 2225 2250 2275 Absorbance Wavenumber (cm-1) 21 9 0 216 5 a b c d 0. 02

Fig.4. FT-IRspectraoftheAu/18WZ-CPsamplecollectedaftertheadsorptionofa (10TorrNO+25TorrO2)mixturefor30minat300◦Cfollowedbydynamic

evacu-ationfrom200◦_{Ctoroomtemperatureandsubsequentadsorptionof10TorrofCO}

for10min(a),evacuationfor15minatroomtemperature(b),50◦_C(c)and100◦_C

(d).

goldparticles.Thisbandisfoundathigherfrequencythanthe cor-respondingbandobservedontheactivatedAu/18WZ-CPsample (seeFig.1)indicatingthateithertheAuı+sitesbearagreater pos-itivechargeorthereisagglomerationofthegoldparticlesafter thehigh-temperaturetreatmentwithNO+O2mixture.Itislikely

thatthespeciesgivingrisetothebandat2165cm−1couldbeAu+

adsorptionsitesdispersedonthesurfaceofmetallicgoldparticles asproposedin[36,37].

Spectrum(a)inFig.4differsfromthespectrumofadsorbed COonthesampletreatedwithNO+O2mixtureatroom

temper-ature(Fig.3,spectrum (c)).It seemsthat theoxidationof gold particlesbytheNO+O2 mixturetakesplaceathigher

tempera-ture(300◦C)andtheabsorptionbandat2185cm−1detectedduring theCOadsorptiononthesampletreatedwithNO+O2mixtureat

roomtemperature(Fig.3,spectra(b)and(c))cannotbeascribedto Au+_{–COspecies.Thisconclusionissupportedalsobythefactthat}

thecoloroftheAu/18WZ-CPsampleafterthepre-treatmentwith NO+O2mixtureatroomtemperatureremainedthesameasthatof

theactivatedsample,i.e.gray-black.However,thetreatmentwith NO+O2mixtureat300◦Ccauseschangesinthecolorofthesample

fromgray-blacktopale-violet.

Takingintoaccountthatthecompoundsadsorbedatroom tem-peratureontheAu/18WZ-CPsampleareCOandNOxspecies,it

couldbeproposedthat thestrongbandat 2185cm−1 in Fig.3 couldbeattributedtoNCOspeciesformedonmetallicgoldsites. Accordingtothe literature[38,48,49] theAu–NCO species give rise to absorption bandat 2180–2190cm−1. In order to verify this assumption we performed13_{CO adsorption onthe sample}

treatedwith NO+O2 mixtureat roomtemperature (Fig. 5).The

bandat2185cm−1isshifteddownby57cm−1andispositionedat 2128cm−1on13_{Csubstitution.Theobservedvalueisconsistent}

withthatreportedbyCelioetal.[50]fortheisotopicshiftdetected onsubstituting13_Cfor12_{CinNCOspeciesadsorbedonCu(100).}

Fromtheseresultsitcanbeconcludedthattheweakabsorptionat about2130cm−1inFig.3,spectrum(c)belongstothe13_COsatellite

ofthebandat2185cm−1.

Thebehavioroftheabsorptionat2185cm−1 inthepresence ofwatervaporcanbeusedasadditionalevidencesupportingthe assignmentofthisfeaturetoAu–NCOspecies.Itiswellknownthat theNCOspeciescanreactwithwaterproducingammoniaandCO2

throughtheformationofHNCO[11,51].Thespeciescharacterized

2050 2100 2150 2200 2250 Absorbance Wavenumber (cm-1₎ 2185 2128 a b 0. 5

Fig.5.FT-IRspectraof10TorrofCO(a)and10Torrof13_{CO(b)adsorbedatroom}

temperatureontheAu/18WZ-CPsample.

bytheabsorptionbandat2185cm−1aregeneratedbyadsorption ofCO(10Torr,for10min)onthesampletreatedwithNO+O2

mix-tureatroomtemperatureasdescribedaboveandthenthegaseous COwasevacuatedfor15min(Fig.6A,spectrum(a)).Theaddition of0.1Torrofwatervaporatroomtemperaturecausessignificant decreaseintheintensityofthebandat2185cm−1(Fig.6A, spec-trum(b))andappearanceofCO2andHNCO(as(NCO)at2266cm−1

[51])in thegasphase(Fig.6B, spectrum(b)).The formationof thelattercompoundsconfirmsunambiguouslytheassignmentof thebandat2185cm−1 toAu–NCO species.Increasingthe tem-perature to100◦C resultsin furtherdecreasein theamountof isocyanates(Fig.6A,spectrum(c))andtheydisappearcompletely at200◦C(thespectrumisnotshown).Noammoniawasdetectedin thegasphasemostlikelybecauseofitslowconcentrationand/or adsorptiononthecatalystsurface.ThesamplespectraintheNH stretchingregionarenoisyandarenotrepresentative.The bend-ingmodesofcoordinatedammoniaandNH4+ionfallintheregion

ofstretchingvibrationsof thenitratesand ifformed,they can-notberesolved.ItshouldbenotedthattheadsorptionofCOon theNOx-precoved18WZ-CPsampledoesnotleadtotheformation

ofNCOspecieswhichonoxidessurfacesproduceabsorptionsin the2280–2200cm−1region[38,46,48,49,52].TheabsenceofNCO speciescoordinatedtothesupportinthecaseofAu/18WZ-CP sam-plecanbeexplainedbyunavailabilityofadsorptionsiteswhichare blockedbythenitratespecies.

TheAu–NCOspeciesobservedontheAu/18WZ-CPsampleare characterizedbyhighthermalstabilityandtheyareremovedby dynamicevacuationat350◦C(Fig.7).

3.3.2. NOx+COsurfacereaction

TheaimofthisexperimentistoinvestigatethepossibilityofCO toactasreducingagenttowardtheNOxspeciesadsorbedonthe

M.Kantchevaetal./CatalysisTodayxxx (2012) xxx–xxx 5 2000 2100 2200 2300 2400 Absorbance Wavenumber (cm-1) 0.2 5 ₂₁₈₅

(A)

(B)

Fig.6.PanelA:FT-IRspectraoftheAu/18WZ-CPcatalystcollectedaftertheadsorptionofa(10TorrNO+25TorrO2)mixturefor10minatroomtemperaturefollowed

byevacuationfor20minatroomtemperature,adsorptionof10TorrCOatroomtemperaturefor10minandevacuationofthegaseousCOfor15min(a),andsubsequent additionof0.1Torrofwatervaporfor10minatroomtemperature(b)and100◦C(c).PanelB:Gasphasespectracorrespondingtothesamplespectra(b)and(c). surfaceoftheAu/18WZ-CPcatalyst.Adsorbednitratespecieswere

createdbyadsorptionofmixtureof10TorrNOand25TorrO2atRT

for10minfollowedbyevacuationfor20minatRT(Fig.8A, spec-trum(a)).ThenCO(10Torr)wasadded(Fig.8A,spectrum(b))and

2100 2120 2140 2160 2180 2200 2220 2240 3500C 3000C 2500C 2000C 1500C 1000C Absorbance Wavenumber (cm-1) RT 0. 25 2185

Fig.7. FT-IRspectraoftheAu/18WZ-CPcatalystcollectedaftertheadsorptionofa (10TorrNO+25TorrO2)mixturefor10minatroomtemperaturefollowedby

evacu-ationfor20minatroomtemperature,adsorptionof10TorrCOatroomtemperature for10minandevacuationofthegaseousCOfor15minatvarioustemperatures (RT=roomtemperature).

theisolatedIRcellwasheatedfor10minatvarioustemperatures. Thesharpbandat2185cm−1assignedtoAu–NCOspeciesincreases inintensityat50◦C(Fig.8A,spectrum(c))anddisappearsat200◦C (Fig.8A,spectrum(f))simultaneouslywiththeNO2observedinthe

gasphase(Fig.8B,spectrum(f)).ItcanbeconcludedthattheNCO speciesreactwiththeNO2leadingtotheformationofCO2andN2.

SomeamountsofN2OandNOaredetectedaswell.Thefactthat

intheabsenceofCO,theNO2ispresentinthegasphase(Fig.2)

supportstheassumptionthattheNO2isreducedbyCOthrough

theintermediacyofAu–NCOspecies.

The spectra shown in Fig. 9 illustrate the reactivity of the Au–NCO species generated ontheAu/18WZ-CP sample toward NO2.TheabsenceofCOinthegasphasepreventstheformation

ofadditionalAu–NCOspeciesat50◦C(seeFig.8A,spectrum(c)) and allowsmonitoringtheonsettemperature oftheNCO+NO2

reaction.TheNCOspecies havebeenobtainedbyadsorptionof 10TorrofCOontheNOx-precoveredAu/18WZ-CPsamplefollowed

byevacuationfor20minatroomtemperature(Fig.9,spectrum (a)).Aftertheadmissionof0.4TorrofNO2 atroomtemperature

(Fig.9,spectrum(b)),theisolatedIRcellwasheatedbetween25 and250◦Cfor 10minateach temperature.Theexposureofthe sampletoNO2atroomtemperature(Fig.9,spectrum(b))causes

strongdecreaseintheintensityofthebandat2185cm−1indicating thattheNCO+NO2reactiontakesplacealreadyat25◦C.The

sur-faceconcentrationoftheNCOspeciesdecreasesrapidlywiththe temperatureandtheydisappearat250◦C(Fig.9,spectrum(g)). Thegasphasespectra(notshown)containCO2andNO2thathas

beentakeninexcess.Asshownabove,theAu–NCOspecieshave highthermalstabilityandleavethesurfaceupondynamic evacu-ationat350◦C(Fig.7).Theseresultsconfirmthehighreactivityof theisocyanatesformedonthegoldparticlesandsuggestthatthe Au/18WZ-CPcatalystmayhavethepotentialforlow-temperature reductionofNOxwithCOinthepresenceofoxygen.

Inordertoobtaininformation abouttheroleofWOxspecies

intheNOx+COsurfacereaction,westudiedtheinteractionofCO

withnitratespeciespre-adsorbedontheAu/ZrO2 catalyst.Also

inthiscase,nitratespecies(bandsbetween1700and1000cm−1) werecreatedbyadsorptionofmixtureof10TorrNOand25Torr O2 at RT for 10min followed by evacuation for 20min at RT

1000 1250 1500 1750 2000 2250 2500

(B)

(A)

Fig.8. PanelA:FT-IRspectraoftheAu/18WZ-CPcatalystcollectedaftertheadsorptionofa(10TorrNO+25TorrO2)mixturefor10minatroomtemperaturefollowedby

evacuationfor20minatroomtemperature(a),subsequentadsorptionof10TorrCOatroomtemperaturefor10min(b)andheatingtheisolatedIRcellfor10minat50◦_C

(c),100◦_C(d),150◦_C(e),200◦_C(f)and250◦_{C(g).PanelB:Gasphasespectracollectedat25}◦_C(b),50◦_C(c),100◦_C(d),150◦_C(e),200◦_C(f)and250◦_C(g).

(Fig.10A,spectrum(a)).Aftertheadditionof10TorrofCOatroom temperature,theisolatedIRcellwasheatedfor10minatvarious temperatures.AswiththeAu/18WZ-CPsample,thesharpbandat 2184cm−1 ischaracteristicof NCOspeciesadsorbedonAu.The

2100 2120 2140 2160 2180 2200 2220 2240 Absorbance Wavenumber (cm-1₎ a b c d e f g 0. 2 5 2185

Fig.9. FT-IRspectraoftheAu/18WZ-CPcatalystcollectedafterthegenerationofthe Au–NCOspecies(a)(fortheconditionsseeFig.7,spectrum(RT))andsubsequent additionof0.4TorrofNO2for10minat25◦C(b),50◦C(c),100◦C(d),150◦C(e),

200◦_C(f)and250◦_C(g).

comparisonof thespectrashownin Figs.8and 10leadstothe conclusionthattheWOx-containingcatalystshouldhavehigher

activitybecausetheadsorbedNCOspeciesandNO2 disappearat

200◦Cwhereas inthecase ofAu/ZrO2 sampletheyarepresent

atthistemperature.Despite ofthehigherconcentrationof sur-facenitratesontheAu/ZrO2 sample,theamountofevolvedNO2

islower.Thissuggeststhatthesurfacetungstateslowerthe stabil-ityofthenitrateions.Inaddition,theweakabsorptionsat1450and 1420cm−1observedinthespectrumofAu/ZrO2detectedat250◦C

indicatetheformation of stablepolydentate carbonates [53,54] whichmayhavenegativeeffectonthecatalyticactivity.Nosuch speciesarefoundinthecaseoftheAu/18WZ-CPsample.

Finally,itshouldbenotedthat theAu-free18WZ-CPsample doesnotcatalyzetheinteractionbetweenadsorbedNOxspecies

andCO:theevolvedNO2ispresentinthegasphasebetween25

and350◦C(thedataarenotshown).

3.3.3. Interactionofa(10TorrNO+14TorrCO)mixturewith Au/18WZ-CP

The aim of this experiment is to show the role of NO+O2,

respectivelyNO2,fortheformationofAu–NCOspecies.Thespectra

(Fig.11)arecollectedinthepresenceofNO+COreactionmixtureat varioustemperatures.Thebandat2230cm−1observedinthe spec-trumtakenatroomtemperature(Fig.11A,spectrum(a))istypical ofadsorbedN2OwhichispresentasimpurityinthegaseousNO

(Fig.11B).Theabsorptionat2198cm−1 corresponds toZr4+_–CO

whereasthesignalat 2118cm−1 is assignedtoAu0_–COspecies

(Fig.11A).ThespectrumrecordedatRTcontainsabsorptionbands whichareattributedtobidentateHCO3−(1606and1222cm−1),

bidentate(1555and1305cm−1)andpolydentatecarbonates(1456 and1420cm−1)[53,54].Theassignmentoftheabsorptionsinthe 1700–1000cm−1regiontocarbonatespeciesisconfirmedbythe FT-IRspectraofadsorbedCOtakeninthe25–250◦Ctemperature range.Thedataarenotshownforthesakeofbrevity.The intensi-tiesofthebandsat2230,2198and2118cm−1decreasewiththe temperatureandtheseabsorptions arenolongerpresentinthe spectrumtakenat150◦C(Fig.11A,spectrum(d)).OnlyZr4+_–NO

M.Kantchevaetal./CatalysisTodayxxx (2012) xxx–xxx 7 1000 1250 1500 1750 2000 2250 2500

(B)

(A)

Fig.10.PanelA:FT-IRspectraoftheAu/ZrO2catalystcollectedaftertheadsorptionofa(10TorrNO+25TorrO2)mixturefor10minatroomtemperaturefollowedby

evacuationfor20minatroomtemperature(a),subsequentadsorptionof10TorrCOatroomtemperaturefor10min(b)andheatingtheisolatedIRcellfor10minat50◦C (c),100◦C(d),150◦C(e),200◦C(f)and250◦C(g).PanelB:Gasphasespectracollectedat25◦C(b),50◦C(c),100◦C(d),150◦C(e),200◦C(f)and250◦C(g).

1000 1200 1400 1600 1800 2000 2200 f e d Absorbance Wavenumber (cm-1₎ a b c 22 30 ₂₁98 2118 0.1 160 6 15 5 5 1305 1222 11 78 1937 14 56 142 0

(A)

(B)

Fig.11.PanelA:FT-IRspectraoftheAu/18WZ-CPcatalystcollectedaftertheadsorptionofa(10TorrNO+14TorrCO)mixturefor10minatroomtemperature(a)and subsequentheatingtheisolatedIRcellfor10minat50◦_C(b),100◦_C(c),150◦_C(d),200◦_C(e)and250◦_{C(f).PanelB:Gasphasespectracollectedat25}◦_C(a),50◦_C(b),100◦_C

(c),150◦_C(d),200◦_C(e)and250◦_C(f).

identifiedasNOxspeciesadsorbedatroomtemperature.Nosurface

nitratesandAu–NCOspeciesareformedevenathigher tempera-tures.ThisfactindicatesthatthenitratespeciesoractivatedNO2

areessentialfortheformationofAu–NCOspeciesandthatthe oxi-dationofNObyoxygenisanimportantstep.Thegasphasespectra (Fig.11B)showthattheamountofN2Oincreaseswiththe

tem-peraturebeingthehighestat250◦C(spectrum(f))whentheNO− speciesareremovedcompletelyfromthesurface.TheCO2detected

in thegas phase at 200◦C (Fig.11B, spectrum (e)) most likely appearsasaresultofdecompositionofthesurfacecarbonates.

4. Conclusions

Goldcatalystspreparedbycationicadsorptionofaqueous solu-tionof[Au(en)2]Cl3complexonzirconiaandtungstatedzirconia

havebeeninvestigatedbyinsituFT-IRspectroscopy.The adsorp-tionofCOshowsthepresenceofpositivelycharged(orlargegold clusters)andneutralgoldparticlesonbothsupports.Detailed FT-IRinvestigationoftheinteractionbetweenCOand NOxspecies

pre-adsorbedonAu/ZrO2 andAu/ZrO2–WOx samplesrevealthe

(25–50◦C). The gold isocyanates display highthermal stability. However,theyreactreadilywithNO2atroomtemperature.

Dur-ingthead-NOx+COreactionovertheAu/ZrO2–WOxsample,the

goldisocyanatesdisappearat200◦CsimultaneouslywiththeNO2

thatisgeneratedinthegasphasebydecompositionofthesurface nitrates.ThisprocessontheAu/ZrO2catalysttakesplaceathigher

temperature(250◦C).ItisconcludedthattheincorporationofW tozirconiadecreasesthestabilityoftheadsorbednitrates.The Au-freetungstatedzirconiadoesnotcatalyzetheinteractionbetween adsorbednitratesandCO:theevolvedNO2 ispresentinthegas

phasebetween25and350◦C.

NoformationofsurfacenitratesandAu–NCOspeciesisdetected duringtheNO+CO adsorptiononAu/ZrO2–WOx catalyst inthe

25–250◦Ctemperaturerange.Thisresultimpliesthatthenitrate speciesoradsorbedNO2areessentialforthegenerationofgold

isocyanatesandtheoxidation ofNObyoxygenisanimportant step.IntheabsenceofCO,gaseousNO2overbothAu-containing

catalystsisobservedinthe25–350◦Ctemperaturerange.Thisfact supportstheconclusionthatNO2isreducedbyCOwiththe

inter-mediacyofAu–NCOspeciesleadingtotheformationofN2andCO2.

TheresultsofinsituFT-IRspectroscopicinvestigationpresumethat Ausupportedontungstatedzirconiamightbepotentialcandidate forthedevelopmentoflow-temperaturecatalystfortheselective reductionofNOxwithCO.

Acknowledgments

ThisworkwasfinanciallysupportedbytheScientificand Tech-nologicalResearchCouncilofTurkey(ProjectTBAG-109T854)and BilkentUniversity.ThefinancialsupportofNATOgrantESP.CLG. No.984160isgreatlyappreciated.

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