MolecularCatalysis440(2017)19–24
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Molecular Catalysis
jou rn al h om ep a g e :w w w . e l s e v i e r . c o m / l o c a t e / m c a t
Effects of H 2 O on the thermal and photocatalytic reactions of ethane on supported Au
Anita Tóth, Tamás Bánsági, Frigyes Solymosi
∗MTA-SZTEReactionKineticsandSurfaceChemistryResearchGroup,RerrichBélatér1,H-6720,Szeged,Hungary
a r t i c l e i n f o
Articlehistory:
Received30May2017
Receivedinrevisedform4July2017 Accepted9July2017
Keywords:
Photo-induceddecompositionofethane EffectsofH2O
ProductionofH2andC2H4
TiO2-supportedPtmetals
a b s t r a c t
TheeffectofilluminationontheH2O+C2H6reactionwasinvestigatedonAudepositedonTiO2,ZnOand CeO2samples.Whereasthephotocatalyticdecompositionofethaneisverylimitedevenonthemost activepureTiO2:theconversionwasonly∼4%at300Kin210min,thedepositionofAumetalontoTiO2
markedlyenhancedtherateofphoto-induceddecompositionofC2H6.AdditionofH2Otoethanefurther enhancedtheconversionandledtotheproductionofH2.Thehighestconversionofethane,23.5%was measuredforAu/TiO2.Ethylenewasnotdetectedevenintraces,indicatingthecompletedegradation ofC2H6toH2andcarboncontainingdeposit.Temperatureprogrammedreaction(TPR)measurements revealedthatthecarbonaceousdepositonthecatalystsisverystable.OnAu/TiO2itreactedwithH2
togiveCH4atTp=453and604KandC2H6andC3H8withTp=602K.Thepromotingeffectofmetals wasexplainedbyabetterseparationofchargecarriersinducedbyilluminationandbytheenhanced electronicinteractionbetweenmetalsandTiO2.
©2017ElsevierB.V.Allrightsreserved.
1. Introduction
Agreatattemptisbeingmadetoobtainhydrogenineconomical ways.Besidesthewater-gasshiftprocess,thecatalyticdecomposi- tionofC HcompoundsisthemostsuitablesourceofH2production [1–8]. Although dispersed metals areeffective catalystsfor the decompositionofmethaneandethane,theirreactionsneedhigh temperature,andwecancountwiththedepositionofcarbonlead- ingtothedeactivationofthecatalysts.Asinothercasesthereaction canbeacceleratedbyillumination[8–19].Therateofthephotocat- alyticreactioncanbefurtherincreasedbyusinganotherreaction partner,CO2orH2O,whichmayreactwiththehydrocarbonsand/or withthecarbondeposition,therebyenhancingtheproductionof H2[20].
There is a growing interest in establishing how the defect structureandelectricpropertiesofoxidicsupportsinfluencethe catalytic performanceof deposited metals [21,22]. Thisinterest canbeledbacktotheearlyfindings, namelythatchangingthe electricconductivityofn-typeTiO2markedlyinfluencesthecat- alyticefficiencyof metalsdeposited onitssurface[23–27].The electricstructureofthesupportwasfoundtobedecisiveinthe differenttypesofreactions,suchasphotocatalyticdecomposition
∗Correspondingauthor.
E-mailaddress:fsolym@chem.u-szeged.hu(F.Solymosi).
ofdimethylether[16],inthereactionofethanewithCO2 [7],in thehydrogenationofCO2[20],andintheproductionofH2inthe photocatalyticreactionsofethane[15].
InthepresentworkanaccountisgivenontheH2O-promoted thermalandphotocatalyticdecompositionofethaneat300K.The catalystchosenissupportedAu,whichwasdepositedonvarious semiconductionoxides,suchasTiO2,ZnOandCeO2.
2. Experimental 2.1. Methods
Catalyticmeasurementswerecarriedoutinafixedbedcontin- uousflowreactormadeofaquartztube.Theflowrateofreactant gases was 12ml/min. The exit gas was analysed by gas chro- matograph(Hewlett-Packard5890)onaPorapakQScolumn.The carriergaswasArwhichcontained12.5%ofC2H6.Inthestudyof H2O+C2H6reactionweappliedagasmixtureof1:1molratio.
Thephotocatalyticreactionwasfollowedinthesamewayas described in ourprevious paper[7]. The reactorwas equipped witha500Wmediumpressuremercuryvaporlamp(TQ718,Her- aeus Noblelight, Germany) as a light source.The approximate lightintensityatthecatalystfilmsis59.4mW/cm2.Thephotore- actor (volume: 670ml) consistsof two concentric quartz glass tubesfittedoneintotheotherandacentrallypositionedlamp.It isconnectedtoa gas-mixingunitservingfortheadjustmentof http://dx.doi.org/10.1016/j.mcat.2017.07.008
2468-8231/©2017ElsevierB.V.Allrightsreserved.
20 A.Tóthetal./MolecularCatalysis440(2017)19–24
Fig.1. (A)EffectofilluminationontheinfraredspectraofC2H6onAu/TiO2.(a)0min;(b)5min;(c)30min;(d)60min;(e)90min;(f)120minafterdegassing.(B)Interruption ofexperimentanddegassingthesample(a)after15(min):(b)30min;(c)60min;(d)120min.
thecompositionofthegasorvapormixturestobephotolyzedin situ.ThecarriergaswasArwhichwasmixedwithC2H6(∼1.5%, 330mol).InthestudyoftheeffectsofH2O,itsamountwasvaried between∼1.5-4.5%.Thegas-mixturewascirculatedbyadiaphragm pump.ThereactionproductswereanalysedwithanAgilent4890 gaschromatographequippedwithPORAPAK1/2Q+PORAPAK1/2S packedandEquity-1capillarycolumns.Thevolumeofthesam- plingloopoftheGCwas500l.Theamountofallproductswas relatedtothisloop.TheconversionofC2H6wascalculatedtaking intoaccounttheamountofC2H6consumed.Thisvalueagreedwell withthatbasedontheHbasis,e.g.takingintoaccounttheHcontent oftheC2H6andtheamountofH2formed.
ForFTIRstudiesamobileIRcellhousedinametalchamberwas used[15].InfraredspectrawererecordedwithaBiorad(Digilab.
Div.FTS155)instrument.Sampleswereilluminatedbythefullarc ofaHglamp(100WLPS-220,PTI)outsidetheIRsamplecompart- ment.Thefilteredlightpassedthroughahigh-purityCaF2window intothecell.Allthespectrapresentedinthisstudyaredifference spectra.
Inthetemperatureprogrammeddesorption(TPD)studiesthe heatingratewas5K/mlandtheflowrateofArwas20ml/min.
2.2. Materials
1% Au/TiO2 and 1% Au/ZnO catalysts were purchased from STREM Chem. Inc. Average gold crystallite size is ∼2–3nm.
Other supported Au catalysts were prepared by a deposition- precipitationmethod.HAuCl4×aq(p.a.,49%Au,FlukaAG)wasfirst dissolvedin triply distilled water. Afterthe pHof theaqueous HAuCl4solutionhadbeenadjustedto7.5bytheadditionof1M NaOHsolution,asuspensionwaspreparedwiththefinelypow- deredoxidicsupport,andthesystemwaskeptat343K for1h undercontinuousstirring.Thesuspensionwasthenagedfor24hat roomtemperature,washedrepeatedlywithdistilledwater,dried at353Kandcalcinedinairat573Kfor4h.Theotheroxidesused assupportswere:CeO2(AlfaAesar)andMgO(Reanal).Theaverage
goldcrystallitesizeinthesesamplesis5–8nm.Thesurfaceareaof thecatalystswasdeterminedbyBETmethodwithN2adsorptionat
∼100K.Thedispersionofmetalswasdeterminedbytheadsorption ofH2atroomtemperature.
3. Resultsanddiscussion 3.1. IRspectroscopicstudies
InapreviouspaperwedemonstratedthatdepositionofPtmet- alsontoTiO2onlyslightlyinfluencedtheabsorptionspectraofC2H6 attheroomtemperature[15].InthecaseofAu/TiO2theadsorp- tionofH2O+C2H6producedabsorptionbandsat3001,2963,2952, 2928,2890,2870and1636cm−1,which agreedwellwiththose obtainedfor pureTiO2 [15]. Illumination only slightly affected thepositions of theabove absorptionbands.After flushingthe C2H6+H2Omixtureabsorptionbandsat2972,2935and2868cm−1 remainedinthehighfrequencyregion.Newspectralfeatureswere detectedat1662,1533,1442,1380and1344cm−1inthelowfre- quencyrange,which we attribute tothedifferentvibrationsof adsorbedC2H4andC2Hx fragmentsformedinthephotocatalytic reaction(Fig.1A).Astheseabsorptionbandswereseenonlyafter evacuatingthegasphase,inordertoseethedevelopmentofthese bandsthecellwasevacuatedatvarioustimes.Spectraobtainedare showninFig.1B.Accordingly,theabovebandsdevelopedatvery earlystageofthephotoreactionandtheybecamesomewhatlarger withtheincreaseofilluminationtime.
3.2. TPDmeasurements
Following the adsorption of C2H6 on Au/TiO2 a very small amountofC2H4desorbedfirstwithTp∼363K,followedbyC2H6 (Tp=753K) and CH4 (Tp=723K). A larger amount of H2 was releasedabove600Kwithapeakof663and993K(Fig.2).Adsorb- ingH2O+C2H6gasmixtureoverAu/TiO2 gaveCH4(Tp=753and 903K)andasmalleramountofC2H4(Tp=753K).H2desorbedwith
A.Tóthetal./MolecularCatalysis440(2017)19–24 21
Fig.2.TemperatureprogrammeddesorptionofC2H6followingitsadsorptionoverAu/TiO2(a)intheabsenceofH2O;(b)inthepresenceofH2O.
Tp=753and 1023K.Averysmallamountof C2H6 continuously releasedabove300Kupto∼850K.
ThefactthattheamountofH2desorbedisincomparablylarger thanthatofC2H4suggeststheoccurrenceofthefollowingreactions
C2H6= C2H4+H2 (1)
C2H4= CH4+C (2)
C2H4= 2C+2H2 (3)
ThedecompositionofC2H6startsaround550K.Thedesorption ofweaklybondedC2H6andC2H4occursslightlyabove300K.The mainfeatureofTPDcurvesremainthesamewhenH2Owasadded toC2H6.QualitativelysimilarTPDcurveswereobtainedforAu/ZnO.
3.3. Thermalcatalyticreaction
Fig. 3 shows the results obtained for the thermal decom- positionof ethane as a functionof temperature on variousAu catalysts.Au/ZnOprovedtobethemostactive:theconversionof ethanereached10%at773K,whereasonothersamplesthisvalue requiredhighertemperaturestobereached.Theproductdistribu- tiondependsonthenatureofthesupports.OnAu/TiO2thesame amountofH2 and C2H4 wasformed inthewholetemperature range,onanotherAusampletheH2/C2H4ratiowaslargerthan1, andawellmeasurableamountofCH4wasalsoproduced.Adding H2OtoC2H6(H2O/C2H6∼1)markedlyincreasedtheconversionof C2H6onallsamples(Fig.4).Itseffectwasmoreexplicitinthefor- mationofH2.OnAu/TiO2theamountofH2increasedbyafactorof 5at950K.Interestingly,muchlessenhancementoccurredinthe productionofC2H4.
3.4. Photocatalyticreactions
ThephotoactivityofpreviouslystudiedAusampleswasinves- tigated at 300K. In the absence of H2O the illumination over supportedmetalsledonly toa verylittle,less than1% decom- position of ethane. Adding H2O to the C2H6 (H2O/C2H6∼1:1) inducedonlyaslightphotoreactioneitheronAu/ZnO,Au/MgOor
onAu/CeO2.ThesituationwascompletelydifferentoverAu/TiO2. AsignificantamountofH2evolvedandalsoasmalleramountof CO2.Thisis shownin Fig.5A. Theslight formationof methane alsooccurred.OnthebasisofH2formedtheconversionofethane approached25%in200min.Theconsumptionofethanegavesome- whatlowervalues.Forcomparisonwementionthatillumination exertedonlyveryslighteffectonthedecompositionofH2Oand C2H6alone.InthecaseofH2Oweregisteredonly1–2%decomposi- tionin200min,whereasalargerphotoeffectwasfoundforC2H6: in200mintheconversionreached∼5%.
IntheexplanationofthehighphotoactivityofAu/TiO2wehave totakeintoaccountthatTiO2isasemiconductingoxide.Thefact thatonpureTiO2theilluminationcausedonlyaslightincreasein theconversionisprobablyduetotherecombinationofthecharges inducedbyillumination
TiO2+h= h++e− (4)
h++e−= h (5)
isveryfastonTiO2.ThedepositionofAumetalontoTiO2,how- ever,markedlyenhancedtheextentofphoto-effectofTiO2.This promotingeffectofmetalsinphotocatalyticprocessesisgenerally explainedbythebetterseparationofthechargecarriersgenerated intheprimaryprocess[8,9],whichprovidesagreaterpossibility fortheactivationofC2H6
C2H6+e−=C2H6␦− (6)
C2H6␦−= C2H5(a)+H(a)+e− (7) It can bealsoassumed that the Schottkybarrier at Au/TiO2
interfacecanfunctionasanefficientbarrierpreventingelectron- holerecombination[10,14].In thecase ofAucatalystWuetal.
[28]pointedoutthatsmallermetalparticlesinducemorenegative Fermilevelshiftthanthelargerparticles.Inadditionthesurface plasmon resonance absorptionmayalsocontribute tothetotal absorptiontherebytotheenhancedphotoactivityofAu/TiO2cat- alyst[17,23].AstheworkfunctionofAumetalishigher(5.16eV) thanthatofTiO2(4.6eV),wecanexpectthetransferofelectrons fromTiO2tometalattheAu/TiO2interfacecontributesalsotothe
22 A.Tóthetal./MolecularCatalysis440(2017)19–24
Fig.3. EffectofsupportonthethermaldecompositionofC2H6fromAu/ZnOAu/CeO2,andAu/TiO2.
Fig.4.EffectofH2OonthedecompositionofC2H6catalyzedbysupportedAucatalysts.
A.Tóthetal./MolecularCatalysis440(2017)19–24 23
Fig.5.PhotocatalyticreactionofC2H6inthepresenceofH2OonAu/TiO2catalyst.
enhancedactivationofadsorbedmolecules.Theroleoftheelectron transferintheenhancedcatalyticeffectofTiO2supportedmetals catalystshasbeenassumedandconfirmedlongtimeago[24–27]
andthisideahasbeengenerallyusedsincethen.
Asregardsthefurtherstepswecouldassumethedecomposition ofC2H5radicaltoC2H4
C2H5(a)=C2H4(g)+H(a) (8)
oraswasfoundonmanymetalsurfaces[29,30],itsrecombina- tion
2C2H5(a)= C4H10(a) (9)
ThefactthatneitherC2H4norC4H10wasdetectableintheprod- uctssuggeststhatthelifetimeoftransientlyformedC2H5orCxHy isveryshort,andinsteadoftheircouplingreactionstheyunder- wentfastphoto-generateddegradationresultinginsomekindof carbonaceousdepositontothecatalyst.
C2H5(a)=C(s)+2.5H2 (10)
ItisanopenquestionhowtheH2Oparticipatesinthephoto- catalyticprocess.AseparatestudyonthephotolysisofH2Oonthe sameAu/TiO2catalystindicatedthatilluminationinducesonlya veryslightdecompositionofH2O,lessthan2%.Thepossiblerea- sonisthattheproductofdissociationofH2O,verylikelyOatoms formedintherecombinationofOHgroups
2OHa=H2O+O (11)
remainthesurfaceoccupyingtheactivecenter.Weassumethat inthecaseofTiO2-containingsampletheoxygenvacanciesarethe activecentersforthisprocess.ThefactthatZnOdoesnotcontain Ovacancy,butZnexcessoccupiesinterstitialpositioninthelattice maybeoneofthereasonsofthelessactivityofAu/ZnOcompared toAu/TiO2.
5. Conclusions
(i)n-typeoxides(TiO2,ZnO,CeO2)onlyslightlycatalysesthether- malandthephotoreactionofH2O+C2H6.
(ii)DepositionofAuontheaboveoxidesenhancesthethermal reactionofH2O+C2H6atveryhightemperature;theconver- sionexceeds40%onthemostactivecatalystat900K (iii)Illumination,however,inducestheH2O+C2H6 reactionover
Au/TiO2evenatroomtemperatureresultingin20%conversion ofC2H6in200minyieldingH2andsomeCHxfragments.
Acknowledgement
ThisworkwassupportedbytheHungarianAcademyofSciences.
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