photocatalytic Catalytic and reactions of H + CO on supported Aucatalysts Applied Catalysis A: General

AppliedCatalysisA:General506(2015)85–90

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Applied Catalysis A: General

j o u r n a l h o m e p a g e :w w w . e l s e v i e r . c o m / l o c a t e / a p c a t a

Catalytic and photocatalytic reactions of H ₂ + CO ₂ on supported Au catalysts

Gyula Halasi, Andrea Gazsi, Tamás Bánsági, Frigyes Solymosi

MTA-SZTEReactionKineticsandSurfaceChemistryResearchGroup,RerrichBélatér1,H-6720Szeged,Hungary

a r t i c l e i n f o

Articlehistory:

Received2July2015

Receivedinrevisedform3August2015 Accepted27August2015

Availableonline29August2015

Keywords:

Aucatalysts ReactionofCO2

IRspectroscopy Photocatalysis

a b s t r a c t

TheinteractionandthereactionbetweenH2+CO2havebeeninvestigatedonsupportedAucatalysts.By meansofinfraredspectroscopy,theformationofformatespecieswasdetected.Thereactionbetween H2+CO2occurredabove475–500K.ThemainreactionpathwaywastheformationofCO.Thecatalytic efficiencyofAusensitivelydependeduponthenatureofthesupport.HighestconversionofCO2was foundonAuparticlesdispersedonn-typeoxides,TiO2,ZnO,andCeO2.Audepositedoninsulatingoxides exhibitedmuchlessactivity.Athigherpressure,at8.5atm,asmallamountofCH4andCH3OHwerealso produced.Illuminationoftheactivecatalystsinducedthereactionevenatroomtemperatureresulting intheformationofCH4.ThehighactivityofAuparticlessupportedbyn-typesemiconductingoxideswas ascribedtotheelectronicinteractionbetweenAuandoxidesleadingtotheactivationofCO2.

©2015ElsevierB.V.Allrightsreserved.

1. Introduction

TheactivationofCO₂anditsconversionintomorevaluablecom- poundsisagreatchallengeforheterogeneouscatalysis[1–5].We becameinvolvedinthissubjectinearly80s[6–9].Ourmainfind- ingswereasfollows:incontrasttotheearlierbeliefCO₂doesnot dissociateonsupportedRhat 300K,but additionof H2 toCO2

inducesitsdissociation even atroomtemperatureand leadsto theformationof formate species.Thisresultwasconfirmed on Rh(111)inUHVbyseveralelectronsspectroscopicmeasurements [10].Itwasanimportantfindingthatthecatalyticperformance ofthePtmetalsinthehydrogenationofCO₂isstronglyaffected by thenature ofthe supports. TiO₂ was foundtobe themost effectiveone,which wasattributedtotheelectronicinteraction betweenmetalsandTiO₂ [9].Thisexplanationwasbasedonour earlyfindingnamelythatvariationoftheworkfunctionofTiO₂ appreciablyaffectedthecatalyticperformanceof Nievaporated onTiO₂[11–13].Itappearsthattheinterestinthecarriereffect becamemoreintensivenowadaysthankstothemodernsurface sciencemethods,andthepossibleroleofoxygenvacancyofTiO2

cameintoprominence[14–16].

∗Correspondingauthor.Fax:+3662544106.

E-mailaddress:fsolym@chem.u-szeged.hu(F.Solymosi).

AfterdiscoverythatAuinnanosizeexhibitsasurprisinglyhigh catalyticactivityinmanyreactions[17–19],itsinfluencehasbeen alsotestedinthecatalyticchemistryofCO₂[20–23].Audeposited onTiO2andtitanatenanotubewerefoundtobeinactiveinCO2

hydrogenation[21].Athighpressure(50atm)andatrelativelylow spacevelocity(3000h⁻¹mlg⁻¹),however,asignificantamountof methanolwasfound[22,23].

Inthepresentwork,weexaminethesurfaceinteractionand reactionbetweenH₂andCO₂onAucatalysts.Attentionispaidto theeffectsofsupports,promotorsandillumination.Weexpectthat theAuinnanosize(1–3nm)depositedonn-typesemiconducting oxidesmightbeabletoactivatetheratherinertCO₂andinducesits thermalandphotoreactionswithH₂evenatlowpressure.Recently, itwasfoundtheAu/TiO2 isaveryeffectivecatalystevenforthe photoinducedreactionsofNO+C₂H₅OHandinthedecomposition ofseveralorganiccompoundsatroomtemperature[24].Asregards theactivationofCO2,recentlywefoundthatthedepositionofKon Au(111)singlecrystalleadstotheformationofnegativelycharged CO₂anditsdissociationevenabove300K[25].Illuminationofthe CO2+K/Au(111)systemgreatlypromotedthedissociationofCO2

[26].

http://dx.doi.org/10.1016/j.apcata.2015.08.035 0926-860X/©2015ElsevierB.V.Allrightsreserved.

Fig.1. SizesofAuparticlesondifferentsupports(A),XPSspectraofdifferentAucatalysts(B).

Fig.2.SelectedIRspectraobservedatroomtemperatureafterH2+CO2(4:1)reactionatdifferenttemperatureonAu/TiO2(A)a,300K(15m);b,373K(30m);c,423K(30m);

d,523K(30m);e,573K(30m);f,afterevacuationat300K;andfollowing0.5TorrHCOOHadsorptiononAu/SiO2(B)a,adsorptionat300K(5m);b,300K(10m);c,373K (10m);d,473K(10m);e,573K(10m).

G.Halasietal./AppliedCatalysisA:General506(2015)85–90 87 2. Experimental

2.1. Materials

SupportedAucatalystswithagoldloadingof1and5wt%was preparedbyadeposition-precipitationmethod.Chloroauricacid (HAuCl₄×aqp.a.49%Au,FlukaAG)wasfirstdissolvedintriply distilledwater.AfterthepHoftheHAuCl4 aqueoussolutionwas adjustedto7.5pHbyadding25%ammoniasolution,thefinepow- derofoxidicsupportwassuspendedandkeptat343Kfor1hwith continuousstirring.Thesuspensionwasagedfor24hatroomtem- peratureandwashedwithdistilledwaterrepeatedly,driedat353K, calcinedinairandreducedat673Kfor4h.Thefollowingsupports wereused:Al₂O₃(Degussa);SiO₂(Aerosil380);CeO₂(ALFAAESAR) andMgO(DAB).Inaddition,weusedcommercial1%Au/TiO2and 1%Au/ZnO(AUROlite)samplespurchasedbyStremChem.

2.2. Methodsandcharacterizationtechniques

X-rayphotoelectronspectra(XPS) weretakenwitha Kratos XSAM800instruments,using non-monochromaticAlK˛radia- tion(hv=1486.6eV)anda180^◦ hemisphericalanalyseratabase pressure of 1×10⁻⁹mbar.Binding energies werereferenced to theC 1sbinding energy (BE) (285.1eV), withthe exception of Au/SiO₂,wheretheSi2pcorelevelat103.4eVwasusedasrefer- ence.Transmissionelectronmicroscopy(TEM)imagesweretaken withaPhilipsCM20andaMorgagni268Delectronmicroscopeat 300K.Approximately1mgofcatalystwasplacedonaTEMgrid.

InfraredspectrawererecordedwithaBiorad(Digilab.Div.FTS 155)withawavenumberaccuracy±4cm⁻¹.Typically,128scans werecollected.Allsubstractionsofthespectraweretakenwithout theuseofascalingfactor(f=1.0).Thesamplecanbeheatedand cooledinsitu.Thechambercanbeevacuatedto10⁻⁶mbar.

2.3. Catalytictest

ThereactionofH₂+CO₂ wasinvestigatedina flowmicrore- actoratatmosphericpressure.Theinnerdiameterofthereactor was8mm,and itslengthwas250mm.Thereactorwasheated by anexternal oven.The weight of thecatalyst was0.3g.The spacevelocitieswere3000–6000h⁻¹.Inthekineticmeasurements theconversionofCO2waskeptlessthan10–15%.Experimentsat 9.5barpressurewerecarriedoutinstainlesssteeltube.Analyses ofthegaseswereperformedwithanAgilent4890Dgaschromato- graph.A2mlong0.25indiametercolumnpackedwithPorapakQS allowedcompleteseparationanddeterminationofreactantsand products.

Thephotocatalyticreactionwasfollowedinathermallycontrol- lablephotoreactor(volume:1650cm⁻¹)equippedwitha 500W medium pressuremercury vapor lamp(TQ 718, Heraeus Noble light,Germany)asalightsource.Thislampemitspredominantly inthewavelengthrangeof250–440nm.Itsmaximumintensityis at254nm.

3. Results

3.1. Characterizationofthecatalysts

AsshowninFig.1A,theparticlesizedistributiondependedon thenatureofthesupport.InthecaseofAu/TiO2 andAu/ZnOthe mainsizeoftheAuparticlesissmall,1.5–2.5nm,anduniform.This valueforAu/SiO₂andAu/Al₂O₃fellsintherangeof2–10nm.The averagesizesofAuparticlesondifferentoxidicsupportsaregiven inTable1.TheXPSspectraofsomeselectedAusamplesareshown inFig.1B.IntheanalysisofXPSspectra,weacceptedtheBEsofthree Austates:84.0eVforAu⁰,84.6eVforAu¹⁺and85.9eVforAu³⁺

[31–33].BindingenergiesarealsogiveninTable1.Accordingly, theAuinthereducedsamplesisintheformofAu⁰.However,we cannotexcludethepresenceofAu¹⁺intheAu/TiO₂andAu/CeO₂in averysmallconcentration.

3.2. FTIRstudies

TheprimaryaimoftheIRspectroscopicmeasurementsisthe detection of formate species formed in the surface interaction betweenH₂ and CO₂.This surfacegroup wasvery easilyiden- tified in the case of supported Pt metals [6–9,27–30]. As was established beforetheasymmetric stretch of adsorbedformate isat155–1591cm⁻¹ andthesymmetriconeat1351–1390cm⁻¹ [6–9,27–30].AsCO₂ formingdifferentcarbonatespeciesonTiO₂ andotheroxidesgavesimilarspectralfeatures,itisnot easyto establish thepresence ofa very small amountofformate gen- eratedintheH₂+CO₂ interactiononsupportedAucatalysts.In Fig.2A,wepresentedtheselectedregionofthespectrafollowing thecoadsorptionofH₂+CO₂overAu/TiO₂sample.Itshowsthatno absorptionbandisdevelopedat1384cm⁻¹at300K,butitclearly appearedat373K.Itsintensityslightlyincreasedupto423K,then startedattenuatingathighertemperature.Inordertoestablishthe locationformatespecies,HCOOHwasadsorbedonAu/SiO₂ sam- ple.AsseeninFig.2B,intenseabsorptionbandsappearedat1727, 1598,1380and1361cm⁻¹.Theirintensitiesgraduallydecreased withtheraisingoftheadsorptiontemperature.

3.3. Catalyticstudy

Onthemostactive1%Au/TiO₂catalystthereactionofH₂+CO₂ wasmeasurableevenaround450K.TheconversionofCO2reached 50%at773K.Thisvaluewas∼35%onthe1%Au/CeO₂and28%on Au/ZnO.MuchlessactivitywasexhibitedbyAu/SiO₂andAu/MgO.

Allthecatalystsexhibitedaremarkablestabilityat773K.Themain productofthereactionwasCOandH₂O.Methaneandmethanol formedonlyintracequantitiesevenontheactivesamples.Increas- ingtheamountofAufrom1%to5%didnotleadtohighercatalytic performance.Notethatthepureoxidesexhibitednoorverylit- tleactivityevenat773K.ThecatalyticbehaviorofCeO₂nanowire andnanotubewasalsotestedwithoutanypositiveresults.Some importantdataarepresentedinFig.3A.

Theeffectofpotassiumpromotorwasexaminedinthecaseof Au/CeO₂.AsshowninFig.3B,adding1%K₂OtoAu/CeO₂appre- ciablyenhanceditscatalyticperformance.TheconversionofCO2

increased from 38 to 50% and a slight formation of CH₄ also occurred.Notethatpotassiumexertednopromotoreffectinthe caseofpureCeO2support.

ThedependenceoftherateofCOformationonthepartialpres- suresofH₂andCO₂wasdeterminedonAu/TiO₂fromtheslopes oflog–logplotsoftherateagainstthereactantpartialpressures (Fig.3C). It wasfound that thekinetic orderfor CO2 was0.62 and thatfor H₂ 0.57. Theactivationenergyof thereactionwas determinedfromthetemperaturedependenceoftherateofCOfor- mationinthesteadystate.TheArrheniusplotisshowninFig.3D.

Weobtainedavalueof64.8kJ/mol.

Somemeasurementwerecarriedoutathigherpressure,9.5bar.

On Au/TiO2, beside CO a significant amount of methane and methanolalsoformed.Lessamountofthesecompoundswerepro- ducedonAu/CeO₂andAu/ZnO.Importantresultsarepresentedin Fig.4.

3.4. Effectofillumination

Illuminationoftheactivecatalystsinitiatedthereactionevenat roomtemperature.ThemostefficientsamplewasAu/TiO₂followed byAu/ZnOandAu/CeO2.ResultsareplottedinFig.5.Themajor

Fig.3.ConversionofCO2intheH2+CO2(4:1)reactiononvariousAucatalystsatdifferenttemperatures(A)and(B).DependenceofrateofCOformationonthepartial pressureofCO2andH2onAu/TiO2catalyst(C).ArrheniusplotofH2+CO2reactionforAu/TiO2(D).

productinthiscasewasCH4,COformedonlyinatraceamount.

TheconversionofCO₂onAu/TiO₂reached3.5%in220m.

4. Discussion 4.1. IRstudies

BeforediscussingtheresultsofIRspectroscopicmeasurements, wementionthatintheinteractionofH2+CO2onsupportedPtmet- als,theformationofadsorbedCOandformatespecieswasdetected [6,7,26–29].Asthenumberof formategroups onRh/Al₂O₃ and Rh/MgOwas5–7timeshigherthanthatofsurfaceRhatoms,itwas concludedthatformateresidesonthesupportandnotontheRh [6–9].Thisconclusionwassupportedbythefindingthatnoformate wasdetectedonSiO2-supportedPtmetalsabove300K[6–9].AsCO adsorbsweaklyonAuparticlesatandabove300K[17–19],itisnot surprisingthatwefoundnoabsorptionbandsduetoCObondedto

Aumetal.Itisanopenquestion,whetheradsorbedformateisbeing formedinthecatalyticreactionofH₂andCO₂

H₂+CO₂= HCOO_(a)+H_(a)(1)

on supported Au catalyst. The difficulty of the detection of a verysmall amountof formatein thepresence of largeamount of CO₂ is described in chapter 1. After several unsuccessful attempts,weidentifiedthesymmetricstretchofformatespeciesat 1384cm⁻¹generatedbytheH₂andCO₂reactiononAu/TiO₂cata- lyst(Fig.2A).ThefindingthatadsorptionHCOOHonAu/SiO₂gave intenseabsorptionbandsat1598cm⁻¹ (asymmetricstretch)and 1380cm⁻¹ (symmetricstretch)suggeststhattheformategroup existsonAuparticles (Fig.2B).Theseabsorption featureswere detectedevenafterheatingtheadsorbedlayerto573Kindicat- ingthehighstabilityofformatespeciesonAumetal,andtheless reactivityofAuparticlescomparedtothatofPtmetals.Theintense

Table1

SomecharacteristicdataforthesupportedAucatalysts.

Catalyst Surfaceareaofthe support(m²/g)

Averagesizeofthe Auparticles(nm)

Calculatedsurface areaofAuparticles (nm²)

D(%) Bindingenergyof theAuondifferent supports(eV)

1%Au/TiO2 50 2 12 70 84.2

1%Au/Al2O3 100 5,5 35 26 –

1%Au/ZnO 40–50 2 12 70 –

1%Au/MgO 170 2,1 13 67 84.1

1%Au/SiO2 380 6,5 133 21 84.0

1%Au/CeO2 50 1,9 11 74 84.2

G.Halasietal./AppliedCatalysisA:General506(2015)85–90 89

Fig.4.ConversionofCO2intheH2+CO2(4:1)reactionoverAu/TiO2at9.5barasafunctionoftemperature(A)andintimeat773K(B).FormationofCO(C)andCH4and CH3OH(D).

Fig.5.EffectofilluminationonthephotocatalyticreactionofH2+CO2(4:1)ondifferentcatalystsat300K.

absorptionbandat1727cm⁻¹isverylikelyduetothevibrationof molecularlyadsorbedHCOOHonsilicasurface.

4.2. Catalyticstudies

AsthemethanationofCO₂onsupportedPtmetalsalsoinvolves thedissociationof COand thehydrogenationofsurface carbon [7–9],itisnotsurprisingtheabsenceofmethaneinthecatalytic reactionofH₂+CO₂onsupportedAu(seeFig.3).Theweakbond- ingbetweenCOandAumetalisprobablythereasonthatH2+CO2

reactiondidnotproduceCH4evenathightemperatures.

Accordingtoourpreviousresults,CO₂alonedoesnotdissoci- ateoneitherAu(111)orsupportedAu[25].Inharmonywiththis finding,thereactionbetweenH2 andCO2 isverylimitedonAu depositedoninsulatingSiO₂,Al₂O₃andMgO.However,whenAu isdepositedonn-typesemiconductingoxides,TiO₂,ZnOandCeO₂, apartialactivationofCO2occurredattheAu/oxideinterface,which reactedwithhydrogenleadingtotheproductionofCO.Asthework functionofAu(5.31eV)ishigherthanthatofTiO₂ (4.6eV),ZnO (3.9–4.2eV)andCeO2(2.5–2.7eV),weexpectanelectrontransfer fromtheseoxidestothedepositedAuparticles.Thisassiststhe activationofCO₂onAu,e.g.theformationofnegativelycharged CO2−,whichreactsmoreeasilywithhydrogentogiveCO

CO2+e⁻=CO2ı−(2)

CO_2(a)^ı−+H₂=CO+H₂O+e⁻(3)

IntheexplanationofthehighefficiencyofTiO2 asasupport recently,agreatattentionispaidtotheroleofitsoxygenvacancy [34–36].AstheZnO,whichcontainsexcessZnintheinterstitial position,wasalsoaneffectivesupport,webelievethatintheelec- tricpropertiesofthesupportingoxidesinthisreactionaremore importantthanthenatureoftheirdefectstructure.

ThepositiveeffectofpotassiumadditiononAu/CeO₂(Fig.3B) canbealsoattributedtothedonationof anelectrontotheAu particles,whichcontributestotheactivationofCO₂[25].Itisto bementionedthatPraliaudetal.[37]disclosedthatanelectron donationcanalsooccurfromK⁺–O^2–overlayerstothemetal.The electrondonatingcharacterofthisoverlayerwasalsoconsidered byothersaswell[38,39].

Asregardstheeffectofillumination,wemayassumethegen- erationofanelectroninaphotoexcitationprocess,

TiO₂+h=h⁺+e⁻(4)

whichenhancesthechargetransferbetweenn-typeoxidesandAu particlesleadingalsototheactivationofCO(adsorbedmolecules) andtotheslightformationofCH₄

CO+3H2= CH4+H2O(5)

Athigherpressure(9.5atm)asmallamountofmethanolwas alsoproduced,

3H2+CO2= CH3OH+H2O(6)

butthedominantpathwayoftheH₂+CO₂ reactionwasstillthe formationofCO.

5. Conclusions

(i)Infraredspectroscopicmeasurementsrevealedaslightforma- tionofformatespeciesintheH₂+CO₂reactiononAu/TiO₂.

(ii)Auparticlesdepositedonn-typesemiconductingoxides(ZnO, TiO2,CeO2)catalysestheH2+CO2reactiontogiveCOabove 500K,whichwasexplainedbytheoccurrenceofanelectronic interactionbetweenAuandtheoxides.

(iii)Athigherpressure,9.5barasmallamountofmethanolalso formed.

(iv)IlluminationofAu/TiO₂inducedtheH₂+CO₂reactionevenat roomtemperature.

Acknowledgements

ThisworkwassupportedbyTÁMOPundercontract number 4.2.2.A-11/1/KONV-2012-0047andbyOTKAundercontractnum- berPD115769.

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