Spectroscopic characterization of gold supported on tungstated zirconia

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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

Spectroscopic

characterization

of

gold

supported

on

tungstated

zirconia

M.

Kantcheva

, M.

Milanova

_{, I.}

_Avramova

_{, S.}

_{Mametsheripov}

b_{InstituteofGeneralandInorganicChemistry,BulgarianAcademyofSciences,Sofia1113,Bulgaria}

a

r

t

i

c

l

e

i

n

f

o

Received30June2011

Receivedinrevisedform1December2011 Accepted12February2012

Available online 18 March 2012 Keywords:

Goldsupportedontungstatedzirconia XRD

DR-UV–visspectroscopy XPS

InsituFT-IRspectroscopyofadsorbedCO andCO+O2

a

b

s

t

r

a

c

t

Goldcatalystssupportedontungstatedzirconia(containing5–20wt%WO3)arepreparedbycationic

adsorptionfromaqueoussolutionof[Au(en)2]Cl3complex.ThematerialsarecharacterizedbyXRD,

DR-UV–visspectroscopyandXPS.ThenatureofthedepositedgoldspeciesisstudiedbyFT-IRspectroscopy ofadsorbedCO.ItisconcludedthatthegoldparticlesoccupypreferentiallytheWOx-freezirconiasurface

andthedispersionofgolddependsontheamountofcoodinativelyunsaturated(cus)Zr4+_ions.

Modi-ficationofzirconiabytungstenincreasesthegolduptakebutatthesametimecausesdecreaseinthe concentrationof(cus)Zr4+_{ionsthusloweringthedispersionofgoldclusters.Accordingtotheresultsof}

insituFT-IRspectroscopy,theAu/WOx ZrO2catalystsdisplayhigheractivityintheCOoxidationinthe

low-temperaturerange(upto150◦C)thantheWOx-freeAu/ZrO2sample.Thelow-temperature

activ-ityoftheW-containingcatalystscouldbeassociatedwithdecreasedbasicityofthesupportoxideions resultinginreducedaccumulationofstablecarbonatespecies.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Supportedgoldcatalystsarehighlyactiveinreactionsimportant for the environmental catalysis [1], in particular for low-temperatureCOoxidation[1–9].Crucialfactorsthat controlthe activityofgoldcatalystsarethemetalparticlesize,thenatureand particlesizeoftheoxidesupport,andthestructureofAu-oxide contacts[1].Thepracticalapplicationofgoldcatalysts,however,is hinderedbecauseoftheirtendencyfordeactivationwiththe opera-tiontime.Themostprobablereasonforthedecreaseinthecatalytic activityistheformationofstablecarbonatespecieswhichblockthe perimeterofgold–supportinterfaceconsideredasactivereaction zone[3–6].TheaccumulationofCOxspeciesonthecatalystsurface

duringthecourseofCOoxidationtakesplacebyreadsorptionof CO2productmolecules[5].SinceCO2isanacidicmolecule,

lower-ingofthebasicityofthesupportsurfacebyintroductionofanacidic componentwouldpreventtheformationorreducetheamountof stablecarbonatespecies.

Highlydispersed(2–5nm)goldparticlesonoxidesupportscan beobtainedbydeposition–precipitation(DP)usingHAuCl4 asa

goldprecursor[1].However,thismethodcannotbeappliedinthe caseofacidicoxidesupportswithapointofzerocharge(PZC)below 5suchassilicaorWO3 [1].Thisisbecausethesupportsurface

isnegativelychargedabovethePZCwhichdoesnotallowstrong interactionbetweenthesupportand theanionicgoldprecursor

∗ Correspondingauthor.

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

(AuCl4−).Cationicexchangeispresumedtobeaneffectivewayof

introducinggoldintoacidicoxides,e.g.SiO2-basedsupports[8–12].

Herein,wereporttheresultsofcharacterizationofgold cata-lystsupportedontungstatedzirconias.TheAu/WOx ZrO2samples

werepreparedbycationicadsorptionusing[Au(en)2]Cl3asthe

pre-cursor(en=ethylenediamine).Thismethodwasdevelopedfirstby Guillemotetal.[10] fortheintroductionofgoldintoYzeolites andusedalsobyothers[8,9,11]fordepositionofgoldonsilica.In ordertoevaluatethepotentialofanewmaterialasacatalystin theprocessofCOoxidation,itisimportanttoinvestigatethe inter-actionofthereactantswiththesurface.Forthatpurposeweused insituFT-IRspectroscopytostudytheadsorptionofCOandits coadsorptionwithoxygenoverAu-promotedandAu-freezirconia andtungstatedzirconias.

2. Experimental

2.1. Samplepreparation

Twodifferentprocedureswereusedforpreparationofhydrated zirconia.Accordingtoprocedure1,hydratedzirconia(denotedas HZ-1) wasprepared byhydrolysis of 0.3Msolution ofzirconyl chloride(ZrOCl2·H2O,Aldrich)withconcentrated(25%)ammonia

solutionatpH9.Theslurryoftheprecipitatedmaterialwaskept foragingatroomtemperaturefor12h.Thentheproductwas sepa-ratedbyvacuumfiltration,redispersedindeionizedwater,washed thoroughlytoremovethechlorideionsanddriedat100◦C.The crystallographicstructureofthematerialobtainedafter calcina-tionat600◦Cwaspredominantlymonocliniczirconia.Tungstated 0920-5861/$–seefrontmatter © 2012 Elsevier B.V. All rights reserved.

Table1

Samplenotation,BETsurfaceareas,nominaltungstenandanalyticalgoldcontents.

Sample SBET(m2/g) WO3(wt%) Wa(at/nm2) Auloading(wt%) Auloading(at%) Auparticlesize(nm)b

Au/ZrO2 143 – – 1.43±0.03 0.30 8

Au/5WZ-I 116 5.0 1.2 1.27±0.01 0.27 8

Au/12WZ-I 115 11.8 3.0 1.83±0.01 0.40 9

Au/20WZ-I 140 19.4 4.5 2.06±0.03 0.44 10

Au/18WZ-CP 118 18.0 4.8 2.27±0.01 0.48 10

a_{TheWsurfacedensityiscalculatedbasedontheweightofzirconia.} b _{AccordingtoXRD.}

zirconiawas synthesized by coprecipitation [13] withnominal contentof18wt%WO3andbyimpregnationofHZ-1with

aque-oussolutionofammoniummetatungstate(AMT,Fluka)usingthe methodproposedbyMartinezetal.[14].Thisprocedureconsisted ofimpregnationofHZ-1withaqueoussolution(3mL/ghydrated zirconia)containingtherequiredamountofAMTtoobtainWO3

loadings corresponding to 5, 12 and 20wt%. The final calcina-tiontemperatureofalltungstatedzirconiaswas600◦C.Goldwas depositedbycationadsorptionfor2hfromaqueoussolutionof [Au(en)2]3+complex(5.85× 10−3M)atpH=9.6androom

tempera-ture.Aftertheseparationofthegoldcomplexsolutionbyfiltration, thesolidwaswashedbydeionizedwateruntilnegativetestfor chlorideions.Thesamplesweredriedat80◦Cfor48handcalcined for1hat400◦C.Thecationicgoldprecursorwaspreparedfollowing theprocedureofBlockandBailar[15].Theobtainedmaterialswere labeledasAu/xWZ-CPorI,wherexstandsfortheWO3nominal

contentinwt%.CPandIdenoteco-precipitationandimpregnation, respectively(seeTable1).

Inorder tostudytheeffect oftungsten,gold wasdeposited alsoonzirconiabycationadsorptionof[Au(en)2]3+complexusing

thesameconditionsappliedtotheW-containingsamples.Since theincorporationofWOxspeciesstabilizesthetetragonalphase

ofzirconia[14,16–22],WOx-freetetragonalzirconiawasusedfor

thepreparationofZrO2-supportedgoldsample.Thegoalwasto

eliminate theeffect of crystal phase onthe interactionof gold precursorwiththesupportbecausethere aredifferencesinthe acid–baseproperties[23],andtypesandconcentrationofthe sur-facehydroxylgroups[24]ofzirconiapolymorphs.Inaddition,the COadsorptioncapacityofmonocliniczirconiaislargerthanthat ofthetetragonalphase[23]whichmayaffecttheCO+O2surface

reaction.Lietal.[25]havefoundthatAusupportedonmonoclinic zirconiaexhibitedmuch higheractivityin thelow-temperature WGSreactionthanthecatalystsupportedontetragonalzirconia. Tetragonalphaseofzirconiawaspreparedbyamethodsimilarto thatdescribedbyJungandBell[24]denotedasprocedure2.The synthesisconsistedofhydrolysisof0.6MsolutionofZrO2Cl2·8H2O

withconcentratedammoniasolution(25%)atpH=9androom tem-perature.Theobtainedprecipitatewaswashedseveraltimeswith deionizedwateruntilnegativetestforCl−ions.Thenthewashed precipitatewasagedinaqueoussolutionofammonia(withpH=9) at100◦Cfor48hunderrefluxandperiodicalsupplementofNH_3(aq) inordertokeepconstantpHof9.Theresultanthydratedzirconia wasdriedat100◦C for24h.ThematerialwasdenotedasHZ-2. Tetragonalzirconiawasobtainedbycalcinationofthehydrated zirconiaHZ-2at600◦Cfor6h.

2.2. Samplecharacterization

XRDanalysiswasperformedonaRigakuMiniflex diffractome-terwithNi-filteredCuK␣radiation(=1.5405 ˚A).TheDR-UV–Vis spectrawereobtainedunderambientconditionswithafiberoptic spectrometerAvaSpec-2048(Avantes)usingWS-2asareference. TheBETsurfaceareameasurementswereperformedwithaTriStar 3000automatedgasadsorptionanalyzer(Micrometrics).The sam-plesweredehydratedundervacuum(10−2Torr)for3hat250◦C

beforethemeasurementsinordertoremoveadsorbedwaterand volatilecompounds.

ThecontentofgoldwasdeterminedbyICP-MSanalysis.The tungsten content in the samples was calculated as difference betweenthenominalcontentinthesolidandtheconcentration oftungsteninthefiltrateproducedduringthedepositionofgold fromthegoldprecursor.Thetungstencontentinthefiltratewas determinedspectrophotometricallybythethiocyanatemethodat =410nm[20,26,27].

The X-ray photoelectron spectra were obtained using un-monochromatizedAlK␣(1486.6eV)radiationina VGESCALAB MKIIelectronspectrometerunderbasepressure of1_{× 10}−8Pa. ThespectrometerresolutionwascalculatedfromtheAg3d5/2line

withtheanalyzertransmissionenergyof20eV.Thehalf-widthof thislinewas1eV.ThespectrometerwascalibratedagainsttheAu 4f7/2 line(84.0eV)andthesamplechargingwasestimatedfrom

C 1s(285eV)spectrafromnatural hydrocarboncontaminations onthesurface.TheaccuracyoftheBEmeasuredwas0.2eV.The photoelectronspectraofC 1s,O1s,Zr3d,W4fandAu4fwere recordedandcorrectedbysubtractingaShirley-typebackground andquantifiedusingthepeakareaandScofield’sphotoionization cross-sections.TheAuparticlesizewasobtainedfromtheXPSpeak intensitytreatedaccordingtotheKerkhof–Moulijnmodel[28,29]. TheFT-IRspectrawererecordedusingaBomem Hartman& BraunMB-102modelFT-IRspectrometerwithaliquid-nitrogen cooledMCTdetectorat a resolutionof 4cm−1 (100scans).The self-supportingdiscs(∼0.01g/cm2_{)wereactivatedintheIRcell}

byheatingfor1hinavacuumat400◦C,andinoxygen(100Torr, passedthroughatrapcooledinliquidnitrogen)atthesame tem-perature,followedbyevacuationfor1hat400◦C.Thespectraof adsorbedgaseswereobtainedbysubtractingthespectraofthe activatedsamplefromthespectrarecorded.Thesamplespectra werealsogas-phasecorrected.

3. Resultsanddiscussion

3.1. Structuralcharacterization

Accordingto the XRDdata (Fig.1) all of thesamples stud-ied(exceptAu/5WZ-I)havethestructure oftetragonalzirconia (ICDDCartNo.04-005-4479).TheAu/5WZ-Isamplecontainssmall amountofmonocliniczirconia(ICDDCartNo.00-013-030).

Withincreaseinthetungstenloading,theanalyticalcontentof goldincreases(Table1).Itiswellknownthattungstatedzirconia containsacidicprotonsandtheiramountincreaseswiththe sur-facedensityoftheWOxspecies[13,16–20,30].Itisreasonableto

proposethatashighertheconcentrationof acidichydroxylson thesupportsurfaceaslargertheamountofdeprotonatedM O− surface sites(M=W and Zr)thus leadingtogreater number of anchoringsites for the[Au(en)2]3+ complex resultingin higher

surfaceconcentrationofgold.Theaveragesizeofgoldparticles, calculatedbyusingScherrerequationandthemaingold diffrac-tionlineof2=38.2◦(ICDDCartNo.00-004-0784),increaseswith theWcontent(Table1).

Fig.1.XRDpatternsofthesamplesstudied(t:tetragonal;m:monoclinic).

Thebasicmediumusedforthe[Au(en)2]3+adsorptioncaused

someleachingofWOxspeciesonlyinthecaseofAu/12WZ-Iand

Au/20WZ-Isamples.Afterthedepositionofgold,thetungstenloss amountsto0.20±0.05and0.61±0.02wt%ofWO3forAu/12WZ-I

andAu/20WZ-I,respectively. 3.2. DR-UV–visspectra

Fig. 2 compares the optical spectraof the samples studied. Theabsorptionbandat293–270nmwitha shoulderat 260nm observedinthespectraoftheAu-freesamples(Fig.2,spectra(a)) correspondstoLMCT(O2−_→W6+_{)transitioninoligomericWO}

x

specieswithdifferentdegreeofpolymerization[14,17–21].The introductionofgoldcausesdrasticchangeintheopticalspectra (Fig.2,spectra(b)).Thebroadabsorptionwithmaximumat550nm observedonthegold-containingsamplesischaracteristicofthe plasmonicoscillationmodeofnanosizedgoldparticles[11,31,32]. Thestrongbandat270–293nmhasdisappearedandweaksignals at225–230and260–275nmareobservedinstead.Basedonthe spectrumofAu/ZrO2 sample(Fig.2A,spectrum(c)),theformer

bandisattributedtotheO2−→Zr4+_{CTtransition.Thespectraof}

theAu-containingsamplesindicatethatgoldhinders the detec-tionofLMCTtransitions.Duetothehighabsorptioncoefficientof goldparticles[33],thesupportcannotinteractwiththeradiation. Consequently,thefundamentalbandsoftungstatedzirconiaare detectedwithsignificantlylowerintensities.

3.3. XPSanalysis

The resultsof XPS analysisfor all samples are summarized in Table 2. Binding energy of Au 4f7/2 is around 82.8–83.5eV

which, in agreementwith theliterature dataon Au/ZrO2

cata-lysts[7,25,34–36],isassigned tometallicgold.Thefullwidthat halfmaximum(FWHM)ofAu4fphotoelectronlinedecreaseswith

increasingtheWO3content.Thisisasignofmoreuniform

distribu-tionofAuparticlesontheW-modifiedsamplesurfacesmostlikely duetotheincreasedgolduptake.Thereasonfortheformationof metallicgoldwithoutadditionalreductionstepisthelowthermal stabilityofthe[Au(en)2]3+precursorcomplexwhenadsorbedon

oxidesurfaces[10,11,37].Theinitialcolorofthesampleswaslight yellow,however,duringthedryingat80◦Cthesamplesbecame gray-black. The changein thecolor wasfaster for the samples withhigherWcontent,whichsuggeststhattungstenassistsinthe decompositionofgoldprecursorcomplex.

SincetheXRDdataprovideinformationaboutthesizeoflarge goldclusters(largerthan5nm[38]),thesizeofthegoldparticles wasestimatedbyXPSintensityratiosusingthemodelproposedby KerkhofandMoulijn[28].Thismodelisbasedonthemetalloading andspecificsurfaceareaofthecatalystsandisusefulforthe char-acterizationofverysmallmetalparticles[38].Theaverageparticle sizecalculatedfromXPSforallgoldcontainingsamplesisaround 3nm.Theseresultsindicatethatlarge(8–10nm)andsmall(∼3nm) goldparticlesarepresentonthesurfacesofthesamplesprepared bycationicadsorptionofthe[Au(en)2]3+precursor.

ThespectraofthesamplesintheW4fregioncontainintense doubletwithW4f7/2 lineat35.1–36.3eVcorrespondingtoW6+

[22,30].TheZr3dphotoelectronlineforallAucatalystsexhibits peakforZr3d5/2at182.0–182.9eV,closetothatobservedforZr4+

ions[25,30,34,36,39–41].Fig.3showsthattheW/Zrsurfaceratios forthegoldcatalystssupportedontungstatedzirconiapreparedby impregnationincreaseslinearlyasafunctionofthetungsten con-tent.ThissuggeststhatthedispersionoftheWOxspeciesonthe

surfaceofthegoldcatalystsisuniform[19,22].FortheAu/WZ-CP samplethecalculatedW/Zrsurfaceatomicratiodeviatesfromthe establishedlineardependence.Ithasbeenshownthatalltungsten islocatedonthesurfacewhentungstatedzirconiawasprepared byimpregnationwhereasusingco-precipitationresultsin incor-porationofWatomsintoZrO2 lattice,stabilizingthetetragonal

structure[16,30].

IntheO1sXPSregionanintensivepeakat∼530.4eVisobserved forallinvestigatedcatalysts.Smallasymmetryathigherbinding energysideisdetectedtoo.Thissecondpeakcanberelatedtothe existenceofO−ions[42].Thissuggeststhepresenceinthe subsur-faceofoxygenionsthatbearlowerelectrondensitythanthe“O2−” ions;formallytheseoxideionscouldbedescribedas“O−”species. Theycouldbeassociatedwithsiteshavinghighercovalenceofthe M Obondsandsmallercoordinationnumberofoxygenionsthan aregularsite.Areasonablehypothesisistoconsidertheexistence, invariableproportions,ofdefectsinthesubsurface.

TheresultsinTable2showthatthemethodofintroductionof WOxspeciestozirconia(coprecipitationversusimpregnation)does

notaffectthesurfaceconcentrationofgold.However,in agree-mentwiththechemicalanalysis,theamountofgoldonthesurface increaseswiththeWO3loading.

3.4. InsituFT-IRspectroscopy

3.4.1. FT-IRspectraoftheactivatedsamples

Fig.4comparesthespectraoftheactivatedAu-freeand Au-containingxWZ-Isamples.IntheOHstretchingregionallsamples oftungstatedzirconiacontainabandat3640–3635cm−1whichis attributedtoW OHgroups[20].Thebroadabsorptionat approx-imately3445cm−1indicatesthepresenceofH-bondedhydroxyls. Thespectra inthefundamental W Ostretching regionshowa sharpbandat1008–1002cm−1typicalofW Ospecies[17,19–21]. ThedepositionofgolddoesnotcauseperturbationoftheW O band.However,comparedtothexWZ-Isupports,allAu-containing samples display bands in theOH stretching region withlower intensities.Thisisassociatedwiththeinvolvementofthesurface

Fig.2.(PanelA)Opticalspectraof18WZ-CP(a),Au/18WZ-CP(b)andAu/ZrO2(c).(PanelB)Opticalspectraof20WZ-I(a)andAu/20WZ-I(b).(PanelC)Opticalspectraof 12WZ-I(a)andAu/12WZ-I(b).(PanelD)Opticalspectraof5WZ-I(a)andAu/5WZ-I(b).

Table2

Bindingenergies,surfacecompositionandgoldparticlesizeforthesamples.

Sample O1s Zr3d5/2 W4f7/2 Au4f7/2 Auparticlesize(nm)

BE(eV) at% BE(eV) at% BE(eV) at% BE(eV) at% FWHMa

Au/ZrO2 530.2 65.2 182.5 34.6 – – 82.8 0.2 2.03 2.6 Au/5WZ-I 530.0 63.3 182.1 35.3 35.1 1.2 82.8 0.2 1.87 2.9 Au/12WZ-I 530.5 63.5 182.6 33.9 35.8 2.3 83.0 0.3 1.88 2.8 Au/20WZ-I 530.6 63.0 182.6 33.0 35.9 3.7 83.4 0.3 1.71 3.1 Au/18WZ-CP 530.4 64.7 182.9 32.4 36.3 2.6 83.5 0.3 1.68 3.3 a_{FWHMfortheAu4f}

Fig.3.W/ZrsurfaceratiosasafunctionofWO3contentinAu/xWZ-I(x=0,5,12and 20wt%WO3)()andAu/18WZ-CPsamples().

hydroxylsofW-containingsupportsinthedepositionprocessof goldprecursor.

3.4.2. AdsorptionofCOatroomtemperature

TheanalysisoftheFT-IRspectraofCOadsorbedatroom tem-peratureonthesamplescanbeveryusefultoobtainqualitative informationonthenatureofsupportedgoldspecies.Fig.5A dis-plays the FT-IR spectra in the carbonyl region of CO (10Torr) adsorbedonthezirconiasamplesobtainedafterthecalcination at600◦C ofhydratedzirconiaspreparedbyprocedures1and2. ThefigureshowsalsothespectraofCOadsorbedonthecalcined supportspreparedbyimpregnatingtheHZ-1precursorwithAMT solutionandbycoprecipitation.Asmentionedabove,thesamples

ofzirconiaobtainedusingasprecursorshydratedzirconiasHZ-1 andHZ-2crystallizeafterthecalcinationat600◦Cinmonoclinic andtetragonalstructure,respectively.Astrongbandat2196cm−1 withapoorly resolvedshoulder isobserved inthespectrumof COadsorbedonmonocliniczirconia(Fig.5A)which correspond totwotypesofZr4+ _{COcarbonyls[2,17,20,23,31,43–45].Forthe}

puretetragonalzirconiasampletheCOadsorptionyieldsalsoone asymmetricbandat2194cm−1withshoulderataround2188cm−1 whichareascribedtotwo typesofLewisacidsites[23,46].The spectra clearly show that the population of the coordinatively unsaturated(cus)Zr4+_{sitesonmonocliniczirconiaislargerthan}

onthetetragonalzirconia.Thisexperimentalfactisinagreement withtheresultsofMorterraandcoworkers[23]whoconcluded thatmorphologicalandstructuralreasonsareresponsibleforthe differentconcentrationof(cus) Zr4+_{sitesonthesurfacesofthe}

two crystallographicmodifications ofzirconia.Comparedtothe m-ZrO2,thetungstatedsamplespreparedfromtheHZ-1precursor

arecharacterizedbylowerintensityoftheZr4+ _{COband(Fig.5B)}

duetosaturationofcoordinativepositionsofZr4+_ionsinthe

sur-facelayerbytheWOxspecies.TheintensityoftheZr4+ COband

decreases graduallywith increase in the tungstenloading.The high-frequencyshiftofthecarbonylbandexhibitedintheWOx

-containingzirconiasamplesisassociatedwiththeincreasedacidity ofthe(cus)Zr4+_{sitescausedbytheelectron-withdrawingWO}

x

groups [17,20].The intensityof theZr4+ _{CO bandin allxWZ-I}

samples(havingatetragonalstructure)issignificantlyhigherthan thatinthetetragonalzirconia.Thisdifferencecanbeascribedto theapplication of two differentprocedures for thepreparation of the hydratedzirconia usedas precursor leading to different morphologyof thet-ZrO2 andxWZ-Isamples,i.e.differentsize

and shape ofthe particles, and differentamounts of structural defects.Inaddition,theacidityofthe(cus)Zr4+_{ionslocatedinthe}

proximityoftheWOxdomainsisenhancedresultinginincreased

amountof(cus) Zr4+_{ions detectableby COadsorptionat room}

temperature.

Fig.5.FT-IRspectraofCO(10Torr)adsorbedatroomtemperatureonthesamplesstudied(t:tetragonal;m:monoclinic).

As with the Au-free materials, the absorption at 2195–2198cm−1observedforallAu-promotedsamplesisassigned toCOadsorbedonZr4+_{surfacesites(Fig.5C).Theintensityofthe}

Zr4+ _{CO bandinthe W-containingsamplesdecreasesafterthe}

depositionofgold(comparewithFig.5B)indicatingthatthereare goldnanoparticleslocatedonthezirconiasurface.Lower concen-trationofthe(cus)Zr4+_{ionshasbeendetectedalsoonthesurface}

ofAu/ZrO2 sampleascomparedwiththeAu-freetetragonal

zir-conia.Accordingtodatafromtheliterature[2,10,31,43,44,47–51], theabsorptionwithmaximumat2113–2116cm−1isassignedto COadsorbedonsmallthree-dimensionalgoldclusters, whereas theshoulderat2128–2135cm−1isusuallyattributedtoAuı+ CO species.FormationofpositivelypolarizedgoldonZrO2isassumed

tobecausedbythepresenceofadsorbedoxygenonthegold par-ticlesortheirinteractionwiththesupport[2,31,43,47].Itshould benotedthatthelastactivationstepoftheinvestigatedsamples consistedofevacuationat400◦Candadsorbedoxygencannotbe expectedundertheseconditions.Moreover,afterthereductionof thesamplesat400◦CwithCO,theabsorptionat2128–2135cm−1 isstillpresentinthespectraofCOadsorbedatroomtemperature. Recently, absorption bandat 2130–2140cm−1 observed during theCOadsorptiononAu/Nb2O5hasbeenattributedtoCO

coor-dinatedtolargergoldnanoparticles[52].Therefore,basedonthe latterinterpretation thefeatureat 2128–2135cm−1 (Fig.5C) is assignedtentativelytoCOcoordinatedtolargergoldnanoparticles. Thisproposition is supported bythe estimates of goldparticle sizesfromXRDand XPSdatashowingthatthesampleshaveat least two fractions of crystallites, large (∼8–10nm) and small (∼3nm). For convenience, the absorptions at 2128–2135cm−1 and 2113–2116cm−1 are denoted as high-frequency (HF) and low-frequency(LF)goldcarbonyls,respectively.Thebands corre-spondingtotheZr4+ _{COandAu COspeciesareremovedupon}

dynamicevacuationatroomtemperature.

AccordingtothechemicalanalysistheAuloadingonAu/ZrO2

andAu/5WZ-Isamplesisveryclose,1.43and1.27wt%,respectively. However,theintensitiesofthebandsduetoCOadsorbedonthe goldsitesarehigherfortheW-containingsample(Fig.5C) suggest-inghighergolddispersion.Usingthesamearguments(Auloading

andintensitiesoftheAucarbonylbands),bettergolddispersioncan bededucedfortheAu/20WZ-I(2.06wt%Au)whencomparedwith theAu/18WZ-CPsample(2.27wt%Au).ThespectraofCOadsorbed ontheAu-freesupportsshowthattheamountof(cus)Zr4+_ionsis

higheronthe5WZ-Isamplethanont-ZrO2.Likewise,thesurface

concentrationof(cus)Zr4+_{ionsonthe20WZ-Isampleishigherthan}

thatonthe18WZ-CPsample.Itcanbeproposedthatthedispersion ofgolddependsontheamountof(cus)Zr4+_{ions.Thisassumption}

canbesupportedbytheresultsofChenandGoodman[53]who showedbyusingHREELSandCOadsorptionthatAubondsdirectly tocoordinativelyunsaturatedTiatomonTiO2(110).

FortheAu/xWZ-Isampleseries,theincreaseintheintensitiesof Aucarbonylbandswiththeamountoftungstenisassociatedwith theincreaseintheAuloading.Accordingtotheresultsofcurve fittingofthegoldcarbonylbands(Table3),thefractionoflarger goldparticlesgivingrisetotheHFcarbonylbandincreaseswith theWloadingbylargerextentthanthefractionofthesmallergold particlescharacterizedbytheLFcarbonylband.Thisexperimental factcouldbeexplainedbytheassumptionthatinthecaseof W-containingsamplesthegoldparticlesformedduringthecalcination occupypreferentiallytheWOx-freezirconiasurface.The

modifica-tionofzirconiabytungstenfacilitatesthegolduptakebutatthe sametimecausesdecreaseintheconcentrationof(cus)Zr4+_ions.

Thedecreaseintheamountofnucleationsitesforgoldparticles withincreaseintheWloadinglowersthedispersion.

Table3

Integratedareasofthegoldcarbonylbandsrecordedatroomtemperatureand PCO=10Torr(seeFig.5).

Sample HFbanda_(cm−1_{) IHF(a.u.) LFband}b_(cm−1_{) ILF(a.u.) IHF/ILF}

Au/ZrO2 2128 0.78 2113 0.97 0.80

Au/5WZ-I 2132 1.28 2115 1.35 0.95

Au/12WZ-I 2136 1.79 2116 1.38 1.30

Au/20WZ-I 2136 2.66 2116 1.53 1.74

Au/18WZ-CP 2134 0.84 2115 0.73 1.15

a_{HFbandcorrespondstoCOadsorbedonlargeAuparticles.} b_{LFbandcorrespondstoCOadsorbedonsmallgoldparticles.}

Fig.6.FT-IRspectracollectedduringtheexposureofthesamplesAu/ZrO2(PanelA)andAu/12WZ-I(PanelC)toa(10TorrCO+10TorrO2)gasmixturefor10minatvarious temperatures.(PanelsBandD)GasphasespectraabovetheAu/ZrO2andAu/12WZ-Isamples,respectively(RT=roomtemperature).

3.4.3. FT-IRspectroscopicinvestigationofCOoxidationover Au/xWZ-Isamples

Inordertoevaluatethepotentialofanewmaterialasa cata-lyst,westudiedbyFT-IRspectroscopytheCOoxidationasasize

sensitiveprobereaction.Inthisinvestigationweusedthegold cat-alystssupportedontungstatedzirconiapreparedbyimpregnation. Inordertofindinformationabouttheeffectoftungsten,theCO+O2

reactionhasbeenstudiedontheAu/ZrO2 sampleaswell.Fig.6

Table4

AssignmentoftheIRabsorptionbandsobserveduponCO+O2adsorptiononAu/ZrO2andAu/12WZ-Isamplesatvarioustemperatures.

IRband(cm−1_{) Assignment IRband(cm}−1_{) Assignment}

2352,1344 Zr O C O 1660–1650,1375 CO2− 2193–2197 Zr4+ _{CO 1615,1220 BidentateHCO} 3− 2135,2115 Auı+ _CO,Au0 _{CO 1555,1320 BidentateCO} 32− 1755 BridgedCO32− 1452–1450,1425–1418 PolydentateCO32−

showsthespectraobtainedduringthecontactoftheAu/ZrO2and

Au/12WZ-Icatalystswitha(10TorrCO+10TorrO2)mixtureinthe

isolatedIRcellatvarioustemperaturesfor10min.Thespectrum

ofAu/ZrO2catalystdetectedatroomtemperature(spectrumRTin

Fig.6A)containsabsorptionsat2193,2135and2115cm−1the for-merbandbeingassignedtoZr4+ _{COwhereasthelattertwobands}

correspondtoHFand LFgoldcarbonyls,respectively.The spec-truminthe1620–1000cm−1regionissimilartothatreportedby Bachiller-Baezaetal.[54],Pokrovskietal.[55]andBolisetal.[56] forCO2 adsorbedontetragonalzirconia.Accordingly,thepeaks

at1615and 1220cm−1 revealthepresenceofbidentate hydro-gencarbonates(b-HCO3−).Thebandsat1555and1320cm−1 are

attributedtobidentatecarbonatespecies(b-CO32−)[54–56]and

thepairofbands at1450and 1425cm−1 areassigned to poly-dentatecarbonates(p-CO32−)[54,55]ormonodentatecarbonate

speciespeculiarofthet-ZrO2phase[56].Thelattertwobandsare

detectedupontheadsorptionofCO(10Torr)atroomtemperature althoughwithmuchweakerintensities.Theabsorptioncentered at1755cm−1 canbeassignedtobridgedCO32−[56,57],whereas

theweakbandsat1660–1650and1375cm−1(seealsoFig.6C)are attributedtocarboxylate,CO2−,species[57–59].Inthegasphase,

inadditiontothesignalofCO,veryweakabsorption correspond-ingtoCO2isdetected(Fig.6B,spectrumRT).Theexperimentaldata

showthatsomeoxidationofCOtakesplaceatroomtemperature. However,theproductofoxidation isretainedonthesurfaceof thecatalystmainlyascarbonate–carboxylatestructures.Increasing thetemperatureto50◦Ccausesenhancementoftheabsorptions between1800and1000cm−1anddecreaseintheintensitiesofthe carbonylbands(Fig.6A,spectrum50◦C).Atthesametime,the for-mationofanewsharpbandat2352cm−1isobserved.Thissignal isattributedtothe␯3modeofCO2moleculelinearlyadsorbedon

thesurfaceZrcationicsitesthroughoneOatom[56,58,59],i.e.to theZr O C Oconfiguration.Increasingthetemperatureto100◦C doesnotaffecttheintensitiesofthebandsofthep-CO32−speciesat

1450–1425cm−1(Fig.6A,spectrum100◦C).However,thereis con-siderabledecreaseintheabsorptionscorrespondingtoadsorbed CO2 (2352cm−1), b-HCO3− (1615 and 1220cm−1)and b-CO32−

species(1555and1320cm−1).Decreaseintheintensitiesofthe broadsignalcenteredat1755cm−1andtheshoulderat1660cm−1 isobservedaswell.Thebandsat2193(Zr4+ _CO)and2115cm−1

(LFAucarbonyl)arenolongerpresent.However,thespecies char-acterizedbytheHFAucarbonylbanddisplayhigherstabilityand areobservedinthespectrumtakenat150◦C(notshownhere).This indicatesthatCOadsorbedonthelargergoldparticles(givingrise tothebandat2135cm−1)islessreactivethanthatcoordinatedto thesmallergoldclusters.Lowreactivityofgoldspecies character-izedbycarbonylbandsat2125–2140cm−1hasbeenreportedfor othergoldcontainingsamples[2,31,43].

Thegasphasespectrumtakenat100◦C(Fig.6B)showsdecrease in theintensity of theCO band and significantincrease in the amountofCO2produced.Furtherraiseinthetemperatureto200

and300◦Ccausesvanishingofthebandsat1755and1660cm−1 andtheabsorptionsduetoadsorbedCO2andCO,andloweringof

thesurfaceconcentrationofthehydrogencarbonateand carbon-atespecies.UndertheseconditionstheCOinthegasphasehas almostdisappearedandtheamountofCO2formedhasincreased

significantly.

The spectra obtained during the interaction of a (10Torr CO+10TorrO2)mixtureinthe25–300◦CwiththeAu/12WZ-I

cat-alyst(Fig.6CandD)containthesametypeofabsorptionbandsas thoseobservedontheAu/ZrO2sample.However,thereare

differ-encesthatshouldbenoted:(i)CO2adsorbedontheAu/12WZ-I

sampleisobservedalreadyatroomtemperatureandtheamount ofCO2producedat25◦Cissignificantlyhigherthanthatinthecase

oftheAu/ZrO2 sample.ThelargerconcentrationofCO2adsorbed

ontheAu/12WZ-Isample allows thedetectionof the␯1 mode

Fig.7.COconversionovertheAu/xZW-IandAu/ZrO2catalystsasafunctionof thetemperature.TheCOconversion(in%)isestimatedfromtheintegratedarea oftheIRabsorptionofgaseousCOobtainedatagiventemperatureinthe pres-ence(A)andabsenceofacatalyst(A0)accordingtotheequation:COconversion (%)=(A0−A)100/A0.

oftheadsorbedmoleculeat1344cm−1,otherwiseIRinactivefor gas-phaseCO2;(ii)between25and100◦Cthereisapproximately

2.5-fold increase in the surface concentration of adsorbed CO2

whereastheamountofsurfaceHCO3−andCO32−increasesonly

slightlyandismuchlowerthanthatontheAu/ZrO2sample.This

factreflectsthereducedbasicityoftheoxideionsofzirconiacaused bythedepositedWOx species.CO2 isacidic andit isused asa

probemoleculeforbasicsurfacesites[54,56].Theassignmentof theabsorptionbandsissummarizedinTable4.

ItshouldbepointedoutthatAu-freeWOx ZrO2samplesdonot

catalyzetheoxidationofCOinthe25–300◦Ctemperaturerange underthesamepartialpressuresofthereactinggases.Fig.7shows theCOconversionovertheAu/xZW-IandAu/ZrO2catalystsasa

functionofthetemperature.TheCOconversion(in%)isestimated fromtheintegratedareaoftheIRabsorptionofgaseousCOobtained atagiventemperatureinthepresence(A)andabsenceofa cata-lyst(A0)accordingtotheequation:COconversion=(A0−A)100/A0.

Thecurvesclearly indicatethatundertheconditionsofthe FT-IRexperiment, theAu/ZrO2 catalystdisplaysthelowestactivity

inthelow-temperaturerange(upto150◦C).Thebehaviorofthe lattersamplecanbeexplainedbyextensiveformationofsurface HCO3−/CO32−structures(seeFig.6A)thatblocktheactivesites

forthereaction.Theamountofcarbonatesretainedonthesurface dependsonthebasicityoftheoxideions,i.e.ontheW concentra-tion.ThespectrainFig.6AandCshowthatthemostresistantto decompositionarethep-CO32−speciesandtheirsurface

concen-trationdecreaseswiththeincreaseintheWcontent.

TheAu/5WZ-Isamplehasthebesthigh-temperatureactivity (above150◦C)amongthesamplesstudied.Thepoorerperformance ofthegoldcatalystswithWO3loadingof12and20wt%underthese

conditionscouldbeexplainedbythepresenceofsignificantamount oflargegoldparticles(seeTable3).ThereactivityofCOadsorbed onlargegoldclustersislowresultinginlowerCOoxidation activ-ity. Asmentioned above, both Au/ZrO2 and Au/5WZ-Icatalysts

havesimilarAuloadingandthefractionofsmallgoldparticlesis comparable(Table3).TheloweractivityoftheAu/ZrO2 catalyst

thanthatofAu/5WZ-Icouldbeattributedtothepresenceofstable HCO3−/CO32−structureswhichareaccumulatedattemperatures

below150◦Candcannotberemovedevenat300◦C.Theresults obtainedindicatethatgoldcatalystssupportedontungstated zirco-niashowlow-temperatureactivityintheoxidationofCO.However,

thedeposition of gold ontungstated zirconiaby adsorption of [Au(en)2]3+complexleadstotheformationofnon-uniforminsize

goldcrystallitesi.e.largeandsmall.Sincethesizeofgold parti-clesisakeyfeaturedeterminingtheactivityinCOoxidation,our futuretaskistooptimizetheprocessofgolddepositionby appli-cationofothermethodssuchasusingcolloidalgoldprecursorsor post-modificationofAu/ZrO2byWOxspecies.

4. Conclusions

Wehave shownthat goldcatalystssupportedontungstated zirconia(containing5–20wt%WO3)canbepreparedbycationic

adsorptionfromaqueoussolutionof[Au(en)2]Cl3complex.

Accord-ingtoXRDandXPSdatalarge(8–10nm)andsmall(∼3nm)gold particlesarepresentonthecatalystsurfaces.TheFT-IRspectraof adsorbedCOshowtheformationoftwo typesofAu CObands, at2128–2135cm−1 (high-frequencyband)and2113–2116cm−1 (low-frequencyband),whichareattributedtoCOcoordinatedto largeandsmallgoldparticles,respectively.Itisconcludedthatthe goldparticlesoccupypreferentiallytheWOx-freezirconiasurface

andthedispersionofgolddependsontheamountofcoodinatively unsaturated(cus)Zr4+_{ions.Modificationofzirconiabytungsten}

increasesthegolduptakebutatthesametimecausesdecrease in theconcentration of (cus) Zr4+ _{ions.Consequently, the}

frac-tionoflargegoldparticlesincreases.Undertheconditionsofthe FT-IR experiments, the gold catalysts supported on tungstated zirconiapreparedbyimpregnationdisplayhigheractivityinthe CO oxidationin thelow-temperaturerange(upto150◦C)than theWOx-freeAu/ZrO2catalyst.Theimprovedperformanceofthe

Au/WOx ZrO2 catalystsisassociatedwithreducedformationof

stableHCO3−/CO32−structuresthatblocktheactivesitesforthe

reaction.

Acknowledgments

ThisworkhasbeenperformedintheframeworkofaD36/003/06 COSTprogram.ThefinancialsupportofTBAG–109T854projectand NATOGrantESP.CLG.No.984160isgreatlyappreciated.

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