and TiO (1 1 0) Structure and reactivity bimetallic of Au–Rh clusters on titanate nanowires,nanotubes Catalysis Today

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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

Structure and reactivity of Au–Rh bimetallic clusters on titanate nanowires, nanotubes and TiO 2 (1 1 0)

J. Kiss

, L. Óvári

, A. Oszkó

, G. Pótári

, M. Tóth

, K. Baán

, A. Erdóhelyi

aDepartmentofPhysicalChemistryandMaterialScience,UniversityofSzeged,Aradivértanúkt.1,H-6720Szeged,Hungary

bReactionKineticsResearchLaboratory,InstituteofNanochemistryandCatalysis,ChemicalResearchCenterofHungarianAcademyofSciences, P.O.B.168,Aradivértanúkt.1,H-6701Szeged,Hungary

a r t i c l e i n f o

Articlehistory:

Received29March2011

Receivedinrevisedform25May2011 Accepted1June2011

Available online 30 June 2011

Keywords:

Core–shellstructure Titaniananowire Titaniananotube Gold

Rhodium

Ethanoldecomposition

a b s t r a c t

AuandRhclusters,aswellasAu–Rhbimetallicnanoparticleswerepreparedontitanatenanowires, nanotubesandonTiO2(110).TheywerecharacterizedbyX-rayphotoelectronspectroscopy(XPS),low energyionscatteringspectroscopy(LEIS)andFouriertransforminfraredspectroscopy(FTIR).Byper- formingcarefulLEISexperiments,itwasfoundthatforappropriateAuandRhcoverage,athinAulayer almostcompletelycoverstheRhnanoparticles,aRhcore–Aushellstructurewasdetected.Theforma- tionofthisstructurewasnotaffectedbyalkali(K)adatoms.LEISandFTIRmeasurementsdisclosedthat adsorbedCOat300KcausesthesegregationofRhatomstothesurfaceofmetalclustersinordertobind toCO.UponCOadsorptiononRh/titanatenanostructurestheIRstretchingfrequenciescharacteristicof thetwinformweredominant,whereasbimetallicnanosystemsfeaturedapronouncedlinearstretching vibrationaswell.InspiteofthisstructureadsorbedCOisdetectableduringtheethanoladsorptionon gold–rhodiumbimetallicclusterandtheethanoldecompositionrateistwicehigherthanonAu/TiO2.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Highlydispersedgoldparticleshaveincreasinglygainedatten- tion in the catalytic community due totheir unusual catalytic propertiesinavarietyofoxidationandhydrogenationreactions [seee.g.Refs.[1–3]andreferencestherein].Supportedbimetallic catalystshave been shown toexhibit superior catalytic perfor- manceswhencomparedtotheirmonometalliccounterparts[4,5].

Dependingonthenatureofthesecondmetal,differentkindsof mechanismswereobservedintheinteractionswithbimetals.Both ligandeffects(i.e.anelectronicfactorduetothechangeinelectron density)andensembleeffects(i.e.surfacestructurefactordueto thepositioningofdifferentatomtypes)cancontributetosyner- geticeffectsoftenobservedforbimetallicsurfaceparticles[6–8].

Enhanceddispersionand stabilityofgoldnanoparticlesonstoi- chiometricandreducedTiO2(110)wereobservedinthepresence ofmolybdenum[9].IthasalsobeenobservedthatRhsignificantly changedthemorphologyandtopologyofAuonTiO₂(110)sur- face[10,11].STMandlowenergyionscatteringspectroscopy(LEIS) experimentsrevealedthatatproperAuandRhcoveragethepost- depositedAucompletelyanduniformlycoverstheRhnanoparticles

∗Correspondingauthorat:DepartmentofPhysicalChemistryandMaterial Science,UniversityofSzeged,Aradivértanúkt.1,H-6720Szeged,Hungary.

Tel.:+3662343638;fax:+3662546482.

E-mailaddress:erdohely@chem.u-szeged.hu(A.Erdóhelyi).

(core–shellstructure).InthecaseoftheAu–Pdsystemtheresults showsegregationofgoldtothesurface.Densityfunctionaltheory calculationconfirmsthatAupreferstobeattheedgesofAuPdalloy particlesundervacuumconditions[12].ThepresenceofPtinthe bimetallicPt–Auclustersinhibitssintering,andtheaveragesizeof theclustersafterannealingdecreaseswithincreasingPtcontent.

BasedonLEISandSTMexperiments,performedonTiO2(110),it wasstatedthatthedepositionofAuonPtclustersresultsinthe formationofbimetallicclustersduetotheseedingofmobileAu atomsatexistingPtnuclei,butthedepositionofAuonPtdoes notproducecore–shellstructureswithAuontopatsmallcover- ages[13].LaterontheformationofPtcore–Aushellstructureswas demonstratedathighercoverages(0.25–0.5ML).AdsorptionofCO onthePtclusterscoveredbythegoldcappinglayerpromotedthe diffusionofPtatomstotheclustersurfaceinordertobindtoCO [14].

Inadditiontotheabovefindingsitturnedoutthatthenature ofthesupport(i.e.Fe₂O₃(111),MgO(100),andCeO₂(111))often playsacriticalroleinreactivity[4,5,12].Theobservedvibrational frequencyofCOadsorbedonthesurfacecanbeusedtoidentify itsspecificbindingsitesandthustoelucidatethesurfacecomposi- tion.DesorptionofCOasafunctionoftemperatureprovidessimilar informationandsupportstheidentificationofbindingsitesonthe surface.Thisapproachhasbeensuccessfullyappliedinthepastto investigateAu–Pdalloysurfaces[15],Au–Pdparticlessupportedon SiO2[16]andAl2O3film[17],Au–Rhbilayersonsupportedpowder TiO₂[18]andontitanatenanowireandnanotubes[19,20].

0920-5861/$–seefrontmatter© 2011 Elsevier B.V. All rights reserved.

doi:10.1016/j.cattod.2011.06.002

Lowenergyion scatteringspectroscopy(LEIS),besidesother surfacesensitivetechniques,wassuccessfullyappliedinthechar- acterizationofbimetallicnanoclustersonTiO₂(110)[9–11,13,14], becauseithasveryhighsensitivityinthetopmostlayer.In this paperweusethistechniquewithXPSandFTIRmethodstoinvesti- gatetheeffectofpotassiuminthearrangementofAu–Rhclusters onTiO₂(110)surfaceandontitanatenanowireandnanotubessup- ports.

One-dimensionalnanostructureshavebeeninthefocusofthe material sciencecommunity for well more thana decade now [21].Althoughcarbonnanotubesmaysolvemanychallenges of materials engineering in the long run, at present their practi- calapplicabilityappearstobelimitedbythelackofadequately selective synthesis technologies. On the other hand, inorganic nanostructures(e.g.metallicnanowires,oxides,sulfides,selenides, etc.)canbepreparedinarathercontrolledmanner,andtherefore theirindustrial-scaleapplicationisveryclosenow.

Titanatenanotubesarealsoofgreatinterestforcatalyticappli- cations,since their highcation exchange capacity providesthe possibilityofachievingahighmetaldispersion[22].Inthisregard, thereareseveraldemonstratedexamplesofsuccessfulutilization oftitanatenanotubesasmesoporouscatalystsupportsfordiffer- entnanoparticles.High-aspect-ratioTiO₂andtitanatenanoobjects areintensivelystudiedatpresentbecauseoftheirpromisingphoto- electrical[23,24],biomedical[25],andhydrogenstorageproperties [26–28]. Gold-containing titania nanotubes were found to dis- playhigheractivityinthephoto-oxidationofacetaldehyde[29], thewater–gasshiftreaction[30],andCOoxidation[31]thanthe DegussaP-25catalyst.

In this paper, we shall use the term “nanotube” for high- aspect-ratioobjectswithahollowinnerchannelbelow50nmand

“nanowire”forhigh-aspect-ratioobjectswithoutaninnervoidand adiameterbelow200nm.

The catalytic activity of titanate structures was tested in theethanol decomposition. Catalytic conversion of bio-ethanol receivedconsiderableattentioninthelastdecade[32–37]andthe searchforthemostactive,stableandselectivecatalysthasresulted inmanyheterogeneouscatalyticsystems.

SupportedRh isoneof thebestsamplesfor thesereactions, butonlyafewpapers dealwiththeefficiencyofsupportedAu.

Idrissetal.[38]studiedtheoxidationofethanolonAu/CeO2,and theadsorptionand desorption of it onAu/TiO₂ [39]. Guan and Hensen[40]recentlyexaminedthedehydrogenationofethanol onAunanoparticlesdepositedonvariousSiO2supports.Astrong influenceoftheAuparticlesizewasobserved.Itisnoteworthy that,inthepresenceof oxygen,theintrinsicactivityofAu/SiO₂ increasedconsiderably.LippitsandNieuwenhuys[41]investigated theconversionofethanolintoethyleneoxideongold-basedcata- lysts.ItwasfoundthatwhengoldwasaddedtotheAl₂O₃support thecatalystproducedethyleneoxidewhichwasnotobservedon thesupportalone.Thepresenceofgoldnanoparticlesisnecessary fortheformationofethyleneoxide.Solymosiandco-workers[37]

studiedtheethanoldecompositiononAunanoparticlesdeposited onvariousoxides.Theformationofhydrogenandtheproductdis- tributiondependedsensitivelyonthenatureofthesupport.These samplescatalyzemainlythedehydrogenationofethanol,topro- ducehydrogenand acetaldehyde. Thehighest rateofhydrogen evolutionwasobservedonAu/CeO₂.

2. Experimental

Titanatenanowiresandnanotubeswerepreparedbyhydrother- malconversionofanataseTiO2 as describedelsewhere[42,43].

Briefly,thenanostructureswerepreparedbymixing2gofanatase into140cm³10MaqueousNaOHsolutionuntilawhitesuspen- sionwasobtained,agingthesuspensioninaclosed,cylindrical,

Teflon-lined autoclave at 400K for 1–72h while rotating the wholeautoclaveintensivelyat60rpmarounditsshortaxis,and finally washingthe productwith deionized water and neutral- izingwith0.1MHClacidsolutiontoreachpH=7;atthispoint, theslurrywasfilteredandtheresiduewasdriedinairat353K.

Au,Rhand theircoadsorbedlayers withdifferentcompositions wereproducedbyimpregnatingtitaniananowiresandnanotubes withthemixturesof calculated volumesof HAuCl4 (Fluka) and RhCl₃·3H₂O(JohnsonMatthey)solutionstoyield1wt%metalcon- tent.InthebimetalliccatalysttheAu/Rhatomicratiowas1.The impregnatedpowdersweredriedinairat383Kfor3h,thanoxi- dizedat573K.Thefinalpre-treatmentwasat573Kinhydrogen atmosphere[18].

Titanatenanotubesareopenendedhollowtubularobjectsmea- suring7–10nminouterdiameterand50–170nminlength.They featureacharacteristicspiralcrosssectioncomposedof4–6wall layers.Thetypicaldiameteroftheirinnerchannelis5nm.Titanate nanowirerepresentsthethermodynamicallymoststableformof sodiumtrititanateundertheappliedalkalinehydrothermalcon- ditions(notethatthepost-syntheticneutralizationstepconverts theoriginal Na2Ti3O7 into its hydrogenform withoutaffecting thenanowiremorphology).Theirdiameteris45–110nmandtheir lengthisbetween1.8and5␮m.Thespecificsurfaceareaoftitanate nanotubesisratherlarge(∼185m²g⁻¹)duetotheirreadilyaccessi- bleinnerchannelsurface,whereasthatofsolidtitanatenanowires is∼20m²g⁻¹.Theimpuritylevelofproducednanocompositeswas lessthan1%.TheforeignelementsdeterminedbyXPSwereK,Ca, MgandNaremainedfromthepreparationprocess.

Theultrahighvacuumexperimentsweredonein twosepa- ratechambers.Oneofthem(1)wasusedtoanalyzethehigharea titanatesamples,whilemeasurementsrelatedtoTiO2(110)were conductedintheother(2).

1.XPspectraweretakenwithaSPECSinstrumentequippedwith aPHOIBOS150MCD9hemisphericalanalyzer.Theanalyzerwas operatedintheFATmodewith20eVpassenergy.TheAlK_␣radi- ation(h=1486.6eV)ofadualanodeX-raygunwasusedasan excitationsource.Thegunwasoperatedatthepowerof150W (12.5kV,12mA).Theenergystepwas25meV,electronswerecol- lectedfor100msinonechannel.Typicallyfivescansweresummed togetasinglehighresolutionspectrum.Forbindingenergyrefer- encetheTi2p_3/2 maximum(458.9eV)wasused.Thesamedata wereobtainedwhenC1s(adventitiouscarbonat285.1eV),orO1s latticeoxygen(530.4eV)wasusedasreferences.Asampleprepa- rationchamberwasdirectlyconnectedtothemeasuringchamber toavoidthecontaminationofsamplesbetweeneachstep.Forspec- trumacquisitionandevaluationbothmanufacturer’s(SpecsLab2) andcommercial(CasaXPS,Origin)softwarepackageswereused.

ASPECSIQE12/38ionsourcewasusedforgenerationoflow- energyscattering(LEIS)spectra.He⁺ionsof800eVkineticenergy wereappliedatalowionfluxequalto0.03␮A/cm²,whichwasnec- essarytoavoidthesputteringofsurface.Theincidentangleswas 55^◦(withrespecttosurfacenormal),ionsejectedalongthesurface normalweredetected.Theionenergies(LEIS)weremeasuredby thesamehemisphericalenergyanalyzerasmentionedabove.

2.Experiments related totheTiO₂(110)single crystalwere conductedina separateUHVchamber.Thecrystal wasaprod- uctofPI-KEM.Itstemperaturecouldbechangedbetween150and 1000K.Thesamplewascleanedat1000K,applyingAr⁺ionsputter- ing(2.5␮A,1.5keV,10min)andvacuumannealing(5min)cycles.

ThefinaltreatmentwasAr⁺ionsputteringat300K(30min).Met- alsweredepositedfromane-beamevaporatorofOxfordApplied Research. The chamber had facilities for Auger electron spec- troscopy(AES),XPSandLEIS.ElectronsaswellasHe⁺ionswere detectedbyaLeyboldEA10/100hemisphericalanalyzer.During LEISmeasurements,theincidentanddetectionangleswere50^◦ (withrespecttosurfacenormal),andthescatteringanglewas95^◦.

AnAlK_␣anodewasusedasanX-raysource,andthebindingenergy scalewasreferencedtothe4f_7/2peakofa thickAulayersetto 83.8eV.

IRstudieswereperformedinahighvacuumsystem.Thesam- ples werepressed onto a Ta-mesh.The mesh wasfixedtothe bottomofaconventionalUHVsamplemanipulator.Itwasresis- tivelyheatedandthetemperatureofthesamplewasmeasured withaNiCr–Nithermocouplespotweldeddirectlytothemesh.IR spectrawererecordedwithaGenesis(Mattson)FTIRspectrometer.

ThewholeopticalpathwaspurgedbyaBalston75-62FTIRpurge generator.

DRIFTspectrawereregisteredduringtheethanoladsorption withBio-RadFTIRspectrometerwithwavenumber accuracyof

±4cm⁻¹.Theinstrumentwasequippedwithadiffusereflectance attachment(Spectra-Tech)withBaF2windows.Typically32scans wereregistered.Thecatalystswerepretreatedasmentionedabove and the sample was cooled down to room temperature. Then ethanolwasintroducedintothecellbybubblingArgasthrough ethanolat273Kwhilethesamplewasheatedlinearlywithaheat- ingrateof10K/minupto573K.TheIRspectrawereregistered continuously.

Thecatalyticexperimentswerecarriedoutinafixedbedcon- tinuousflowreactor(8mmo.d.quartztube),which washeated externally.Thedeadvolumeofthereactorwasfilledwithquartz beads.Forcatalyticstudiessmallfragments(1–3mm)ofslightly compressedpelletswereused.Typically50mgof catalystswas used.Ethanolwasintroducedintothereactorbybubblingthecar- rier gas(Ar 80ml/min) throughliquid ethanol cooledto 273K.

Analysis of the products and reactants was performed with a Chrompack9001gaschromatographusingPorapackQScolumn.

ThegasesweredetectedsimultaneouslybyTCandFIdetectors.

3. Resultsanddiscussion

3.1. LEISexperimentsobtainedonapotassiumcontaining TiO₂(110)surface

InourpreviouspapersitwasdemonstratedthatRhcore–Au shellclusterscanbepreparedonTiO₂(110)ifAuispostdeposited byphysicalvapordeposition(PVD)ontheoxidesurfacecontaining Rhclusters[10,11].NoseparateAuclustersformedintheprocess atasubstratetemperatureof500K.TheexistingRhclustersact asnucleationcentersforgoldatomsdepositedsubsequently.The highdiffusionrateofAuatomsontheoxideduringevaporation probablyplaysanimportantroleinthisprocess.Earlier[11]we demonstratedthattheformationofcore–shellRh–Auparticlesdid notdependonthestateofreductionofthesurface:itproceeded bothonunreconstructedTiO₂(110)andalsoonthemorereduced TiO₂(110)−(1×2).

ThethermodynamicdrivingforcethatAuatomstendtobeout- sidewithinbimetallicclusterscomesfromthesignificantlylower surfacefreeenergyofAucomparedtoRh[44].Whenthereverse depositionsequence was applied,i.e. Rh wasdeposited onthe Au/TiO2(110)surface,averyefficientplaceexchangebetweenRh andAuatomswasobservedbyLEISevenatroomtemperature, movingRhatomsintosubsurfaceregionsofbimetallicclustersand theclustersurfaceremainedcoveredbygold[10].Thisindicates thatdiffusionprocesseswithinsmallmetalclustersareratherfacile comparedtobulkmaterials.

Consideringtheimportantroleofalkalipromotersincatalysis andthatalkalineatoms/ionsaretypicalconstituents/impuritiesin titanatenanotubesandnanowiresitseemedworthinvestigating whethertheformationofcore–shellRh–Auclustersproceedsona potassiumcontainingTiO₂(110)surfaceaswell.

The potassium overlayer was prepared by thermal segrega- tion:annealingthecleanedsputteredsurfaceatT≥800K(5min)

Fig.1.AugerspectraobtainedafterannealingtheTiO2(110)surface–previously Ar⁺sputteredat300K– athighertemperaturesinUHVfor5min.

resultedintheappearanceofpotassiumonthesurfacedetected byAESandXPS.Generally,thepotassiumcontentofaTiO₂crystal isexhaustedbysputtering-annealingcycleswithinafewweeks, butwiththissampletheKcoverageobtainedbythe5minanneal- ingat1000Kwasstableduringtheperiodoftheseexperiments (∼2month),characterizedbyaK/TiAESratioof0.4monitoring theKLMM(252eV)andTiLMM(387eV)peaks(Fig.1).Othertypical impurities(Ca,Na)werenotpresentonthissurface.

ThechemicalstateofthepotassiumcontainingTiO₂(110)sur- faceobtainedafter5minannealingat1000KwasanalyzedbyXPS.

TheTi2p_3/2 peakwasobservedat458.7eV(notshown),which istypicalforTi⁴⁺ionsin thetitaniasurface.Apparentlynodra- maticshiftsorincreasedreductionwasfoundduetothepresence ofpotassium.TheTi2pfeature,collectedatadetectionangleof16^◦ (withrespecttosurfacenormal)containedasmallcontributionof Ti³⁺sites,withanareaofca.6.5%comparedtotheareaoftheTi⁴⁺

doublet.ThisisrathertypicalforausedTiO2(110)crystal,already coloreddarkblue.Thepeakpositionswereidentical,whentheTi2p regionwasmonitoredatagrazing(71^◦)detectionangle,resulting inasmallerinformationdepth.However,theextentofreduction wassomewhatlarger,resultinginaTi³⁺componentwithanareaof ca10%oftheareacorrespondingtotheTi⁴⁺component.Evidently thisdifferenceisaclearsignthatmoredefectsitesexistinthetop surfaceregion.TheK2p_3/2peak(notshown)wasfoundat294.0eV indicatingthatKatomsatthesurfacearepartiallyionized[45].

The possible formation of core–shell Rh–Au clusters on the potassiumcoveredTiO₂(110)surfacewasinvestigatedbyLEIS.The scatteringangleinthatchamberwasrelativelysmall(95^◦),which isadvantageousforthesensitivity,butresultsinalowerresolu- tion.Consequently,theLEIScontributionsofKandTiaremerged intoonepeak,becauseofthesimilarmassesoftheseelements.

First,0.5MLofRhwasevaporatedonK/TiO₂(110)atasubstrate temperatureof330K.Annealingto500Kdidnotcauseanyappre- ciablechangemonitoredbyLEIS.Goldwassubsequentlydeposited at500KandthechangesintheLEISintensitiesasafunctionofAu coverageareshowninFig.2.

TheRhpeakdecreasedsteeplywithincreasingamountsofgold, whiletheintensitiesoftheOpeakandofthepeakcorresponding toKandTiwereonlymildlyattenuated.EvidentlyAuatomsevap- oratedonthesamplenucleatedonexistingRhclusters.IfonlyAu atomslandedontopofRhclustersduringevaporationhadbeen stabilizedontopofRh,thanthecompletedisappearanceofRhsig- nalwouldhaveoccurredapproximatelyatthecoverageof1ML orevenabove,consideringthecurvatureoftheclusters.However, theRhsignalinourmeasurementsdecreasedtozeroatamuch

Fig.2.ChangesinLEISintensitiesduetoAudepositionat500KontheK/TiO2(110) surfaceprecoveredwith0.5MLofRh.

smallercoverage(0.35ML).Thisindicatesthatamajorfractionof goldatoms–impingedonthefreeoxidesurfaceduringevaporation – migratedtoRhclustersandwasstabilizedthere,similarlyasit wasdemonstratedforthepotassium-freeTiO₂(110)surfaceinour previouspapers[10,11].Atasubstratetemperatureof450Kana- logueresultswereobtained,buttheprocesswasnotinvestigated atevenlowerTvaluesinordertoavoidtheeffectofbackground COonLEISintensities.

3.2. Resultsobtainedontitanatenanowiresandnanotubes

ThesurfacecompositionofAu–Rhclustersontitanatenanocom- positewasalsoinvestigatedbyLEIS.Aswasobservedinthecase ofTiO₂(110)substrate,withincreasinggoldcontenttheRhLEIS intensitydecreaseddramatically.Themostpregnantfeaturewas observedinthe0.5%Au+0.5%Rhcase.Onthemonometallicsys- temsthegoldandrhodiumHe⁺scatteringsignalsappearedat753 and707eV,respectively.Onbimetallicnanocomposite,however, onlythegoldsignalshowedup(Fig.3)[20].Therhodiumpeak wasjustabithigherthanthenoiselevel.Itisveryinterestingthat whilethegoldcontentisless(0.5%)inbimetallicsystem,theLEIS intensityofAuishigherthaninthemonometallic(1%)case.

OnthemonometallicRh/TiO2nanowirethedominantXPpeak forRh 3d_5/2 appeared at307.1eVafter reductionat 573K(not shown).Thetailingatthehigherbindingenergysideofthisemis- sionmayconsistoftheoriginalasymmetryandtraceofforeign element(MgKLL Augerpeak)thatremainedfromtheprepara- tionprocess.Acarefuldeconvolutionrevealedsomeemissionat 309.3eV,presumablyduetomoredispersednanoparticles.Very similarrhodiumXPspectrawererecordedontitanatenanotubes.

Inthecaseofthe1%Au/TiO₂nanowiretwopeakswereobserved onthereducedsampleforAu4f_7/2at83.7eV(metallicstate)and 85.6eV.Theemissionat85.6eVcannotbeattributedtoakindof higheroxidationstate,becauseitdevelopedafterhydrogentreat- mentat 573K.We may attribute this featuretogold atoms in verysmallsizednanoparticles(“final-state”effect)[19,20].Under drasticconditionsofanoxygenplasmagoldoxideformationwas observed;theAu4f_7/2wasobservedabove85eV[46].Wefound thatthefeatureat85.6eVappearedonlyafterreductionandnot afteroxidationsoweexcludetheoxidizedAuformation.

TheXPspectraofthebimetallicAu+Rhlayersupportedontita- niananowiresareshowninFig.4.Surprisingly,theemissionforthe higherenergypeakofAu4f_7/2at85.6eVcorrespondingtotheatom- icallydispersedstateisverysmallinthepresenceofRh(Fig.4A), andatthesametimetheemissionforRh3d_5/2ataround309.3eV

780 760

740 720

700 680

Rh

Rh Au

Kinetic energy [eV]

100 a.u.

a

b

c LEIS spe ctra t aken on titanate na nowire

Au

Fig.3. LEISspectraof1%Au/TiO2(b),1%Rh/TiO2nanowire(c),0.5%Au+0.5%

Rh/TiO2nanowire(a)[20].

alsodiminished.TheadsorptionofCOat300Kdidnotalterthe positionofgoldemissions.TheRhXPSfeatureat308.9–309.3eV appearedorintensifiedalittlebitafter60minCOadsorptionmay beduetotheappearanceofmoredispersedparticles(Fig.4B).The COpressurewas1.3mbarinthepreparationchamber.

The morphology of Au, Rh and Au+Rh supported on TiO₂ nanowires and nanotubes wasinvestigated by FTIR,employing adsorbedCO asa probe moleculesensitivetothe localsurface structure. AdsorbedCO exhibitsatleastthree differentstretch- ing frequencies belonging tocertain adsorption sites of Rh on oxidesupports[46].Thebandat2070–2030cm⁻¹ isdue toCO adsorbedlinearlytoRh⁰(depending onthecoverage),theband at∼1855cm⁻¹representsthebridgebondedCO(Rh2–CO),andthe featureat∼2100cm⁻¹andat∼2020cm⁻¹correspondstothesym- metricandasymmetricstretchingsofRh⁺(CO)₂(twinCO).These latterIRsignalsweredetectedwhenthecrystallitesizewasvery small[47].COadsorptionongoldsupportedontitaniananostruc- turewasnotobserved.OnmonometallicRhsupportedonnanowire practicallythetwinformwaspresent(2027and2097cm⁻¹),the linearformbetweenthesetwopeakswasmuchsmallerandthe bridgeformwashardlyobservable(Fig.5A).Onnanotubesthelin- earlyadsorbedCOfeaturesshowedupat2075cm⁻¹betweenthe peaksat2100and2036cm⁻¹(twinform)(notshown).Fromthese IRstudieswemayconcludethatasignificantpartofRhexistsin smallparticlesizesonbothnanowiresandtubes.

Itisplausibleifthegoldcompletelyanduniformlycoversthe Rhnanoparticles(core–shellstructure)adsorbedCOcouldnotbe detectedbyFTIR.However,thesituationisthatrelativelystrongCO bandsappearedat300Katapressureof1.3mbar.Inthepresence

80 82 84 86 88 90 92

Binding energy [eV]

200 cps

Binding energy [eV]

B A

83.7

85.6

H

306 308 310 312 314

307.0

H

200 cps

80 82 84 86 88 90 92

85.6

83.7

CO CO

200 cps

306 308 310 312 314

Binding energy [eV]

Binding energy [eV]

200 cps

307.0

308.9

Fig.4. XPspectraofAu4f(A)andRh3d(B)of0.5%Au+0.5%Rh/TiO2nanowirebeforeandafterCOtreatment.TheCOpressurewas1.3mbar.

ofgold,thepeakcorrespondingtothelinearformbecamestronger at 2070–2071cm⁻¹ and the twin CO stretching frequencies decreased(Fig.5B).Seeminglythere isa contradictionbetween theresultsofLEISandCOadsorptioninfraredexperiments.Onthe topmostlayertherearenoRhatoms(Fig.3),butadsorbedCOwas detectedbyFTIRonthissurface(Fig.5B).Thisdiscrepancycanbe explainedbyaCOinducedsurfacereconstruction.Theadsorption ofCOonAu–RhclustersmaypromotethediffusionofRhtothe surfaceofthecluster.Similarphenomenawasobservedrecentlyin thecaseofbimetallicPt–AuclustersonTiO₂(110)[14].Thesame effect was also observed on Pd–Au bimetallic model catalysts, synthesizedeitherasthinfilmsonMo(110)orasnanoparticleson TiO₂thinfilminCOoxidationatelevated(8–16Torr)COpressures [48].Pdpreferentiallysegregatestothesurfacetoformcontiguous PdsitesandCO oxidationreactivityisregained.Thedifferences obtainedonFTIRspectrabetweenmonometallicRhandbimetallic Au–Rhnanoclustersontitanatenanowirearenotsurprising.The presence of the core–shell composites and theseparate highly dispersedgold clustersmay significantlyaffectthemorphology

andelectronicstructureofRhnanoparticlesthereforethebonding modesofCOadsorbedonRhsites.TheobservedFTIRfeaturemay be related to theXPS result detected afterCO adsorption.The morphologyandcrystallitesizeofsegregatedRhcouldbedifferent in surrounding of Au–Rh core–shell and in the inside of the core–shellstructure.

InordertogetclosertotheunderstandingofCO-inducedmor- phologicalchangessomelowenergyionscatteringmeasurements (LEIS)werecarriedoutbeforeandafterCOadsorptiononAu–Rh bimetallicclustersupportedontitanatenanotubes.Intheseexper- imentstwocompositionswereused:0.25%Au+0.75%Rh/TiO₂and 0.5%Au+0.5%Rh%/TiO2nanotubes.Theresultsarerepresentedin Fig.6.InordertoavoidorminimizethesputteringeffectoftheHe ionsweappliedrelativelylowionflux(0.03␮A/cm²).Atthelower goldcontentrhodiumscatteringappearedat707eVafterhydro- gentreatmentat573K.AfterCOadsorptionat300Kitsintensity increasedbyabout20%(Fig.6A).TheeffectofadsorbedCOwas morepronouncedinthe0.5%Au+0.5%Rhcase.Practicallynoor verysmallRhscatteringsignalwasdetectedindicating thatthe

1800 1950 2100 2250

2097 2027

1845

Wavenumber (cm^-1) 0.1

1800 1950 2100 2250

0.05

Wavenumber (cm^-1) 1843 2033 2070 2098

B A

Fig.5. FTIRspectraofadsorbedCOat300K:(A)1%Rh/TiO2nanowireand(B)0.5%Au+0.5%Rh/TiO2nanowire.TheCOpressurewas1.3mbar.

goldcoveredtherhodiumparticles(Fig.6B),similarlythenanowire case(Fig.3).WhenCOwasaddedtothebimetalliccomposite,the rhodiumionscatteringsignalshowedup.Theseexperimentaldata stronglysupporttheabovementionedphenomena,namelythatCO inducesareconstructionofthesurface.Rhpreferentiallysegregates tothesurfacetoformRh–CObond.

ThisprocessmayimplythemigrationofRhorAuatomswithin theclustersoronthesurfaceoftheclusters.ConsideringthatAu–Au andAu–Rhbondsareweaker thanRh–Rhbonds,itmayappear moreprobablethatthemovementofAuatomsismoreimportant

780 760 740 720 700 680

R h

K inetic energy [eV ] K inetic energy [eV ]

B A

200 a.u.

C O

H2

C O

H₂ A u

780 760 740 720 700 680

R h

A u

200 a u.

Fig.6. LEISspectraof0.25%Au+0.75%Rh/TiO2nanotube(A),0.5%Au+0.5%Rh/TiO2

nanotube(B)beforeandafterCOadsorptionat300K.TheCOpressurewas1.3mbar.

(theshell“opens”), whileRhatoms moveless. Theinfluenceof COgasinthispossibleprocessisanimportantquestion.Though theAu–COinteractionisratherweakcomparedtoRh–CObond,in therelativelyhighCOpressureapplied,theCOcoverageonthe Aucappinglayerisprobablysignificant.TheadsorbedCO layer presumablydecreasesthemetal-metalbondsbothforAuandRh, leadingtoanincreaseddiffusionrate.ItisalsopossiblethatCO, adsorbedinsomehollowsiteoftheAulayercanformsomekindof bondwithsubsurfaceRhatomsandthisinteractionpromotesthe displacementofAuatomsandthesegregationofRh.

Anotherpossiblescenarioisthatthereisacontinuousthermal fluctuationofmetalatomswithinthebimetallicclustersindepen- dentofthepresenceofCO,andfor shortperiodsRhatomscan befoundontheclustersurface,whicharesubsequentlytrapped byCO.Theexistenceofthisfluctuationmaybepossible,sincesig- nificantmovementofmetalatomswithinsmallclustersatroom temperaturewasobservedinseveralcases[10,49,50]characterized bydiffusioncoefficientsmanyordersofmagnitudehigherthanthe correspondingbulkvalues.Anyhow,furtherstudiesareneededto understandindetailtheCOinducedrestructuringofRh–Auclus- ters.

3.3. Ethanoladsorptionontitanatenanowiresupportedcatalysts

Duringethanoladsorptionatroomtemperaturepredominantly thecarbon–carbonbonddissociatesandthecarbon–oxygenbond ispreservedleadingtoadsorbedCObutnotoadsorbedatomicOon Rh(111)[51]surfaces.InadditiontoCO,alsosignificantamounts ofmethylidyneandethylidynespeciesareformedonRh(111),the latterpresumablyviaareformingreaction.GongandMullins[52]

reportedthatonAu(111)ethanoladsorbsonlyweaklyanddesorbs molecularly.EthanoladsorptionhasbeenextensivelystudiedbyIR spectroscopyoverTiO₂andTiO₂supportedmetals[53,54]among othersonRh/TiO₂[55]andAu/TiO₂[39],too.

Fig. 7 shows the DRIFT spectra of ethanol adsorbed on 1%

Au/TiO₂,0.5%Au+0.5%Rh/TiO₂and1%Rh/TiO₂catalystatdifferent temperatures.Similarspectrawereregisteredinallcasesexcepting theCOregion.Theseresultssupportthatthesespeciesadsorbed onthe nanowire. At 300K absorption bands were observed at 2974,2929and2875cm⁻¹ in theC–Hstretchingregion.Inthe lowfrequencyrangeabsorptionbandsweredetectedat1448–1450

1500 2000

3000 0.0

0.5 1.0 1.5 2.0

2036

145014481448 114211401144

138313831384

16331633

2929 2875

2974

2038

2028

Absorbance (a.u.)

Wavenumber (cm^-1) 1

2 3 4 5 6 7 8 9 10 11 12

2028

2974 2929 2875

2974 2929 2875 1633

1847

Fig.7.Infraredspectraofethanoladsorbedatroomtemperature(1,5,9),at373(2, 6,10),423(3,7,11),473K(4,8,12)on1%Au/TiO2nanowire(1–4),0.5%Au+0.5%

Rh/TiO2nanowire(5–8),and1%Rh/TiO2nanowire(9–12).

and 1383–1384cm⁻¹, both could be assigned as ␦CH₃ vibra- tionsofethanol.Thebandsobservedat1140–1144,1124–1121, 1069–1074, 1045–1047cm⁻¹ could be attributed to C–O and C–C vibrations of monodentate and bidentate ethoxide species [39,55].Theintensitiesofthesebandsdecreasedasthetemperature increasedbutdramaticchangeswerenotdetected.

HerewefocusontheCOregionwheresignificantdifferences wereobserved.OnAu/TiO2wedidnotobserveanyabsorbance between2100and1800cm⁻¹ duringethanoladsorptionaswas foundafterCO adsorption [20]. WhentheRh/TiO2 wastreated with ethanol above 373K new peaks were detected at 2028 and 1847cm⁻¹. The intensities of these bands increased with increasingthetemperatureandthepeakobservedat2028cm⁻¹ at room temperature shifted to higher wave numbers. The absorbanceat 1847cm⁻¹ could beattributedto bridgebonded COonRhsites[47,55].ThefeatureoftheadsorbedCO detected at2028–2038cm⁻¹issignificantlydifferingfromthatdepictedin Fig.5. Thisdiscrepancy couldbe explained bythe Rhcarbonyl hydrideformation(H–Rh–CO)[55,56].

When0.5%Au+0.5%Rh/TiO₂ wasthecatalysttooursurprise nearlythesamespectrawereobservedasinthecaseof1%Rh/TiO₂ onlytheintensitiesofthebandswereweaker;at373Kasmallpeak wasdetectedatabout2028cm⁻¹attributedtoadsorbedCO.

AccordingtotheXPSresultsbindingenergiesofRhandAudid notalterduringtheethanoladsorptionevenathighertempera- ture.TheC1sspectrashowedformationofdifferentcarbonoxygen containingspecies(∼289and286.3eV).

120 100

80 60

40 20

0 0 5 10 15 35 40 45 50

Conversion [%]

Time [min]

Fig.8. Theconversionofethanolinthedecompositionreactionat603KonTiO2

nanowire(),1%Au/TiO2nanowire(),0.5%Au+0.5%Rh/TiO2nanowire()and1%

Rh/TiO2nanowire(䊉).

3.4. Ethanoldecompositionontitanatenanowiresupported catalysts

Theethanol decompositionwasinvestigatedat603KonAu, Rhand Au–Rhsupportedontitanatenanowire.Themainprod- ucts were in all cases carbon monoxide, methane, hydrogen, acetaldehyde, but ethylene, diethyl ether and acetic acid were alsodetectable.Theethanolconversionontitanatenanowirewas slightlyhigher(7.1%)thanonAu/TiO2(6%)(Fig.8)butinthelat- tercasetheacetaldehydeselectivitywasmorethantwicehigher (37%) than ontheclean support.Earlierit wasalsofoundthat thesupportedAuparticlescatalyzemainlythedehydrogenation ofethanol,toproducehydrogenandacetaldehyde[37].Whenthe catalystcontainsRhtheconversionsignificantlyincreased;on0.5%

Au-0.5%Rh/TiO₂itwas12.2%andonRh/TiO₂theconversionwas aboutfivetimeshigher(36.9%)thanonAu/TiO2(Fig.8).Theproduct distributionsweresimilaronbothsamples,mainlyCOwasformed theacetaldehydeselectivitywasonly4–6%.Itshouldbenotedthat inthesecasestheethanolconsumptiondecreasedintimewhilein othercasesitwasstable.

FromDRIFTandcatalyticmeasurementsweconcludethatgold doesnotblockentirelytheadsorptionandtheactivesitesofRhina core–shellstructure.TheethanolortheCOproducedintheethanol decompositionmayalsoinducethesegregationofRhandthe0.5%

Au+0.5%Rhsupportedontitanatenanowireactsasaneffective catalystinethanoldecomposition.

4. Conclusions

1.Au and Rh clusters, as well as Au–Rh bimetallic nanoparti- cleswerepreparedontitanatenanowires,nanotubesand on TiO₂(110).ThesampleswerecharacterizedbyX-rayphotoelec- tronspectroscopy(XPS),lowenergyionscatteringspectroscopy (LEIS)and Fourier transforminfraredspectroscopy (FTIR).By performing careful LEIS experiments, it was found that, for appropriate Au and Rh coverage, the Au almost completely covers theRh nanoparticles. Rhcore–Au shell structure was detected.Theformationofthisstructure wasnot affectedby alkali(K)adatoms.LEISandFTIRshowedthatadsorbedCOat 300KcausesthesegregationofRhtothesurfaceinordertobind toCO.

2.Rhsupportedontitanatenanowireexhibitahighcatalyticactiv- ityinethanoldecomposition.Inbimetallicformitpreservesits activitybecausetheethanolsimilarlytoCOmayalsoinducethe segregationofRhfromtheAu-shellRh-corestructure.

Acknowledgements

The financial supports of the Hungarian Scientific Research Fund (OTKA) through projects K69200, K76489 and TÁMOP- 4.2.1/B-09/1/KONV-2010-0005are acknowledged.Wethankthe preparation of titanate nanocomposites to Ákos Kukovecz and AndrásSápi.

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