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Journal of Photochemistry and Photobiology A:
Chemistry
j o u r n a l ho me p ag e :w w w . e l s e v i e r . c o m / l o c a t e / j p h o t o c h e m
Photocatalytic decompositions of methanol and ethanol on Au supported by pure or N-doped TiO 2
Andrea Gazsi, Gábor Schubert, Tamás Bánsági, Frigyes Solymosi
∗MTA-SZTEReactionKineticsandSurfaceChemistryResearchGroup,RerrichBélatér1,H-6720Szeged,Hungary
a r t i c l e i n f o
Articlehistory:
Received6June2013
Receivedinrevisedform16July2013 Accepted6August2013
Available online xxx
Keywords:
Methylalcohol Ethylalcohol Methylformate Photolysis Au/TiO2catalyst EffectofN-doping
a b s t r a c t
TheeffectsofAuparticlesofdifferentsizeswereinvestigatedonthephotocatalyticdecompositionsof methanolandethanolonpureorN-dopedTiO2.IRstudiesrevealedthatthedepositionofAupromoted thedissociationofbothcompoundsduringilluminationandalsoresultedintheformationofformate species.Whereasthephoto-induceddecompositionsofmethanolandethanoloccurredtoonlyalimited extentonpureTiO2,thedepositionofAu,particularlyasnanosizedparticles,markedlyenhancedthe rateandtheextentofthephotocatalyzedreactions.Aninterestingfeatureofthephotodecompositionof methanolwasthat,besidesH2,CO2andCO,asignificantamountofmethylformatewasalsoproduced.
AdditionofH2OorO2tothealcoholinbothcasesdecreasedthelevelofCOformed,andinthecaseof methanolCOwascompletelyeliminated.AuparticlesonN-dopedTiO2withalowerbandgapcatalyzed thephotodecompositionsofbothcompoundseveninvisiblelight.
© 2013 Published by Elsevier B.V.
1. Introduction
Following the pioneering work of Haruta et al. [1,2], who demonstratedtheunexpectedlyhighcatalyticactivityofnanosized supportedAuparticles,greateffortshavebeenmadetoexploitthis propertyofAuinseveralareasofcatalysis[3–5].Asthegeneration ofH2freeofCOisoneofthechallengesinheterogeneouscatal- ysis,thecatalyticbehaviorofAuhasalsobeentestedfromthis aspect.ItwasfoundthatsupportedAuparticleseffectivelycatalyze theproductionof H2 inthethermal decompositionsofHCOOH [6–10],CH3OH [11–18],C2H5OH [19–23] and CH3OCH3 [24] at 423–573K.Pure,CO-freeH2wasobtained,butonlyinthecatalytic decompositionofHCOOHathighertemperatures[6–8].Afurther developmentinthistopicwastheproductionofH2byphotocat- alyticdecompositionoftheabovecompoundsoversupportedAu samplesatroomtemperature[25–32].Inthecontinuationofthis researchprograminthepresentworkweinvestigatedthephoto- catalyticdecompositionsofCH3OHandC2H5OHonvariousAu/TiO2
catalysts.Theoverallaimistoelaborateexperimentalconditions forthegenerationofH2withthelowestachievableCOcontent,to identifysurfacespeciesformedinthephotoreactionandtoproduce H2invisiblelightbynarrowingthebandgapofTiO2byN-doping.
Thedevelopmentofaneffectivephotocatalystusingvisiblelightis achallengingproject,asvisiblelightaccountsfor50%oftotalsolar
∗Correspondingauthor.Tel.:+3662544107;fax:+3662544106.
E-mailaddress:fsolym@chem.u-szeged.hu(F.Solymosi).
energyincontrasttoUVlight,whichaccountsonly∼5%oftotal solarenergy.
2. Experimental
2.1. Materialsandpreparationofthecatalysts
Thefollowingcompoundswereusedassupports.TiO2(Hom- bikat, 200m2/g and P25, 51m2/g), SiO2 (CAB-O-SiL, 198m2/g).
SupportedAucatalystswithanAuloadingof1,2or5wt%werepre- paredbyadeposition-precipitationmethod.HAuCl4·aq(p.a.,49%
Au,FlukaAG)wasfirstdissolvedintriplydistilledwater.Afterthe pHoftheaqueousHAuCl4solutionhadbeenadjustedto7.5bythe additionof1MNaOHsolution,asuspensionwaspreparedwith thefinelypowderedoxidicsupport,andthesystemwaskeptat 343Kfor1hundercontinuousstirring.Thesuspensionwasthen agedfor24hatroomtemperature,washedrepeatedlywithdis- tilledwater,driedat353Kandcalcinedinairat573Kfor4h.1%
Au/TiO2wasalsopurchasedfromSTREMChem.Inc.Thissampleis marked“Aurolite”.ForthepreparationofN-dopedTiO2weapplied thedescriptionofXuetal.[33].Titaniumtetrachloridewasused asaprecursor.AfterseveralstepstheNH3-treatedTiO2slurrywas vacuumdriedat353Kfor12h,followedbycalcinationat723Kin flowingairfor3h.Thissampleisnotedwith“SX”.
ThesizesoftheAunanoparticlesweredeterminedwithanelec- tron microscope.We obtainedthefollowingvalues: 1.5–2.0nm for 1% Au/TiO2 (Aurolite), 10–15nm for 1% Au/TiO2 (Hombi) and 6.0–7.0nm for 1% Au/SiO2 (Cabosil). For photocatalytic 1010-6030/$–seefrontmatter© 2013 Published by Elsevier B.V.
http://dx.doi.org/10.1016/j.jphotochem.2013.08.009
measurementsthesample(70–80mg)wassprayedontotheouter sideoftheinnertubefromaqueoussuspension.Thesurfaceofthe catalystfilmwas168cm2.Thecatalystswereoxidizedat573Kand reducedat573Kinsitu.
ForIRstudiesthedriedsampleswerepressedinself-supporting wafers (30mm×10mm ∼10mg/cm2). For photocatalytic mea- surements the sample (70–80mg) was sprayed onto the outer sideoftheinnertubefromaqueoussuspension.Forphotocatalytic studiesthesamplewassprayedontotheoutersideoftheinner tubefromaqueous suspension.The surfaceof thecatalyst film was168cm2.Thecatalystswereoxidizedat573Kandreducedat 573KintheIRcellorinthecatalyticreactorfor1h.Methanoland ethanolweretheproductsofScharlauwithpurityof99.98and 99.7%,respectively.
2.2. Methods
For the determination of bandgap of solids,we appliedthe sameproceduresasdescribedinpreviouspapers[34,35].Diffuse reflectancespectraofTiO2sampleswereobtainedusinganUV/Vis spectrophotometer(OCEANOPTICS,Typ.USB2000)equippedwith a diffuse reflectance accessory. In the calculation we followed theprocedureofBeranekandKisch[34],whousedtheequation
˛=A(h−Eg)n/h,where˛istheabsorptioncoefficient,Aisacon- stant,histheenergyoflight,andnisaconstantdependingonthe natureoftheelectrontransition.Assuminganindirectbandgap (n=2)forTiO2,with˛proportionaltoF(R∞),thebandgapenergy canbeobtainedfromtheplotsof[F(R∞)h]1/2vs.h,astheinter- ceptat[F(R∞)h]1/2=0oftheextrapolatedlinearpartoftheplot.
Table1
SomecharacteristicdataforpureandN-modifiedTiO2.
Sample Pretreatment temperature(K)
Surface area(m2/g)
Bandgap (eV)
Notation
TiO2 Asreceived 200 3.17 Hombikat
TiO2 723 135
TiO2 723 265 3.00 SX
TiO2+N 723 79 1.96
ThesurfaceareaofthecatalystsweredeterminedbyBETmethod withN2adsorptionat∼100K.DataarelistedinTable1.
Photocatalytic reaction was followed in the same way as describedinourpreviouspaper[35].Brieflythephotoreactor(vol- ume:970ml)consistsoftwoconcentricPyrexglasstubesfitted oneintotheotherandacentrallypositionedlamp.Itisconnected toagas-mixingunitservingfortheadjustmentofthecomposi- tionof thegasor vapormixtures tobephotolyzed in situ.The lengthoftheconcentrictubeswas250mm.Thediameterofouter tubewas 70mm, and that of theinside tube28mmlong. The widthofannulusbetweenthemwas42mm,andthatofthepho- tocatalystfilmwas89mm.Weuseda15Wgermicidelamp(type GCL307T5L/CELL,LighttechLtd.,Hungary),whichemitspredom- inantly in the wavelength range of 250–440nm, its maximum intensityisat254nm.Forthevisiblephotocatalyticexperiments anothertypeoflampwasused(LighttechGCL307T5L/GOLD)with 400–640nmwavelengthrangeandtwomaximumintensitiesat 453and 545nm.Theapproximatelightintensityatthecatalyst filmsare3.9mW/cm2forthegermicidelampand2.1mW/cm2for theotherlamp.Theincidentlightintensitiesweredeterminedby
Fig.1.EffectsofilluminationtimeontheFTIRspectraofadsorbedCH3OHon1%Au/TiO2(Aurolite)(A),1%Au/TiO2(Hombikat)(B),TiO2(P25)(C)and1%Au/SiO2(D).
Fig.2. PhotocatalyticdecompositionofCH3OHon1%Au/TiO2and1%Au/SiO2samples.
anactionometry.Methanol(∼1.2%,227mol)andethanol(∼1.3%, 252mol)wereintroducedinthereactorthroughanexternally heatedtubeavoidingcondensation.ThecarriergaswasAr,which was bubbled through alcohols at room temperature. The gas- mixturewascirculatedbya pump.Thereactionproductswere analyzedwithaHP5890gaschromatographequippedwithPORA- PAKQandPORAPAKSpackedcolumns.Thesamplingloopofthe GCwas500l.Theamountofallproductswererelatedtothisloop.
Forinfrared(IR)studiesamobileIRcellhousedinametalcham- berwasused.Thesamplecanbeheatedandcooledinsitu.TheIR cellcanbeevacuatedto10−5Torrusingaturbomolecularpumping system.ThesampleswereilluminatedbythefullarcofaHglamp (LPS-220,PTI)outsidetheIRsamplecompartment.TheIRrangeof thelightwasfilteredbyaquartztube(10cmlength)filledwith triplydistilledwaterappliedattheexitofthelamp.Thefiltered lightpassedthroughahigh-purityCaF2windowintothecell.The lightofthelampwasfocusedontothesample.Theoutputproduced bythissettingwas300mWcm−2atafocusof35cm.Themaximum photonenergyatthesampleisca.5.4eV.Afterillumination,theIR cellwasmovedtoitsregularpositionintheIRbeam.Infraredspec- trawererecordedwithaBiorad(Digilab.Div.FTS155)instrument withawavenumberaccuracyof±4cm−1.Allthespectrapresented inthisstudyaredifferencespectra.
3. Resultsanddiscussion
3.1. Adsorptionandreactionsofmethanol
TheadsorptionofCH3OHonAu/TiO2 (Aurolite)at300Kpro- ducedabsorptionbandsat2940,2919,2888,2838and2815cm−1 inthehigh-frequencyrange, andat∼1563,1452,∼1358,1158,
1132and1055cm−1inthelow-frequencyregion(Fig.1A).Illumi- nationoftheCH3OHvaporcatalystsystemresultedinonlyslight attenuationinthehigh-frequencyrange,butledtoasignificant intensification of thevery weakbandsat 1563and 1358cm−1. In light of the IR spectroscopic results of previous studies [16,30,35,36],thebandsat2940and∼2838cm−1canbeassignedto theasymmetricandsymmetricstretchingfrequenciesofadsorbed CH3OHandthoseat∼2919and∼2815cm−1toadsorbedmethoxy (CH3O).Thepeaksintheinterval1000–1200cm−1areduetothe C Ostretchingofthetwoadsorbedspecies.Approximately,same features were registered for Au/TiO2 (Hombi) sample (Fig.1B).
Accordingly,theoccurrence ofthefollowing stepsmaybepre- sumed:
CH3OH(g)= CH3OH(a) (1)
CH3OH(a)=CH3O(a)+H(a) (2)
Theappearanceofnewabsorptionbandsat1563and1358cm−1 suggeststhatadsorbedformatespeciesarealsoformedduringthe photolysis[35,36].Withregardtotheresultsofphotocatalyticstud- ies(nextchapter),adsorbedformateisveryprobablyformedinthe dissociationofHCOOCH3(methylformate)producedbythephoto- conversionofCH3OH[30–32].SimilarlytoHCOOH,HCOOCH3does notexhibitavibrationat1558–1578cm−1[31,32].Asnearlythe samespectralfeatureswereobservedforthepureTiO2(P25)sam- ple(Fig.1C),itmaybeconcludedthatallthesespecieslocateon theTiO2surface.
WeobtaineddifferentresultsonAu/SiO2.Followingtheadsorp- tionofCH3OH,absorptionbandsat∼2957,∼2850,1470,1451and 1389cm−1predominatedinthespectrum,indicatingthatCH3OHis mainlyadsorbedmolecularlyonthiscatalyst.Illuminationexerted
Fig.3.EffectsofH2OandO2additiononthephotocatalyticdecompositionofCH3OHover1%Au/TiO2(Aurolite)catalyst.
onlyveryslightalterationsintheIRspectrum.However,thevery weakabsorption at around 1587cm−1, due to the asymmetric stretchofformate,wasclearlystrengthened.IRspectraareshown inFig.1D.AsnoformatespeciesexistsonSiO2,itfollowsthata proportionoftheprocessesinvolvedoccurontheAuparticles.
Fig.2depictstheconversionofCH3OHandtheformationof variousproductsondifferentAu/TiO2catalystsasafunctionofthe durationofillumination.Themosteffectivecatalystwasclearly 1%Au/TiO2(Aurolite),onwhichalmostcompletephotodecompo- sitionoftheCH3OHwasattainedin∼100min.Themainproducts wereH2andHCOOCH3:COandCO2wereformedinonlyrelatively smallamounts.WhenCH3OHhasbeencompletelydecomposed, theamount of HCOOCH3 started decreasing. At the same time CO appeared in the products. As reported previously [30], the photocatalyticdecompositionofCH3OHalsooccursonpureTiO2
(Hombi):theconversionofCH3OHreachedonly2–3%in240min.
Asthephotoactivity ofpureTiO2 dependsonitsorigin,forthe reliableestablishmentoftheeffectsofAuweexaminedthephoto- catalyticdecompositionofCH3OHonthesameTiO2(P25)asused forthepreparationofAu/TiO2(Aurolite).TheactivityofthisTiO2 (P25)washigherthanthatofTiO2(Hombi),butevenonthissample theextentofphotodecompositionofCH3OHreachedonly6–7%in 210min(Fig.3).Notethatmethylformatewasalsoproducedon thisTiO2.InordertoassesstheimportanceoftheTiO2supportand thatofthemetal/TiO2contact,thephotolysisofCH3OHwasalso carriedoutona2%Au/SiO2catalyst.AscanbeseeninFig.2,only slightdecompositionoccurred;theconversionapproached15%in 240min.
Agreateffortwasmadetoeliminateorfundamentallyreduce the formation of CO. The addition of H2O to the CH3OH (a H2O/CH3OHratioof5:1)enhancedtheproductionofH2andcom- pletelyeliminatedtheCOfromtheproductsduringthecompletion ofreaction,∼90minandevendecreasedtheCOcontentafterwards (Fig. 3).Similarresultswere foundwhen O2 was addedtothe CH3OH.AtanO2/CH3OHratioof1:1,theformationofCOceased completely.Atthesametime,theproductionofH2andHCOOCH3 alsodecreased,whiletheamountofCO2generatedincreased.This clearlyindicatestheoxidationofCH3OHand/ortheproducts.The mainresultsoftheeffectsofH2OandO2arepresentedinTable2.
TheeffectsofNincorporationintoTiO2wereexaminedbyusing theTiO2(SX)sample,whichwasconsiderablylessactivethanthe Au/TiO2/Aurolite.AsshowninFig.4,thephotoactivityofAu/TiO2 (SX)wasenhancedsignificantlybyN-doping.Whenthephotolysis wasperformedinvisiblelight,theextentofphotodecomposition waslower,butthepositiveeffectofN-dopingwasclearlyexhibited (Fig.5).
For comparison, we studied the thermal decomposition of CH3OHonthemostactiveAu/TiO2(Aurolite)catalyst.Nodecom- positionwasobservedat300–423Kin60min.Thedecomposition startedat448K,andreached∼3%in60min.Itisimportanttopoint outthatnoHCOOCH3wasformedinthethermalreaction.
Intheinterpretationoftheeffectsofillumination,itshouldbe takenintoaccountthattherate-determiningstepinthethermal decompositionofCH3OHisthecleavageofoneoftheC Hbonds intheadsorbedCH3Ospecies.TheoccurrenceofthissteponTiO2 at300Krequiresactivation,asotherwisenoreactionsoccuratall.
Table2
EffectofH2OandO2additiononthephotocatalyticdecompositionofmethanolandethanolon1%Au/TiO2(Aurolite).
Conversion(%) CO(%) CO/H2ratio
60min 180min 60min 180min 60min 180min
CH3OH 91.1 100 1.5 3.9 0.03 0.07
H2O/CH3OH(5:1) 98.2 100 0 0.5 0 0.007
O2/CH3OH(1:1) 100 100 0 0 0 0
C2H5OH 100 100 6.1 11.9 0.16 0.3
H2O/C2H5OH(5:1) 96.8 100 1.9 4.3 0.05 0.08
O2/C2H5OH(1:1) 100 100 2.0 3.6 0.09 0.1
Illumination,however,initiatedthedecompositionofthissurface speciesevenonpureTiO2at300K,whichcanbeexplainedbythe donationofphotoelectronsformedinthephoto-excitationprocess
TiO2+h=h++e− (3)
totheCH3Ospecies:
CH3O(a)+e−=CH3O(a)␦− (4)
whichdecomposestoH2andCO:
CH3O(a)␦−= CH2O(a)␦−+H(a) (5)
CH2O(a)␦−= CO(g)␦−+H2(g) (6)
However,eventhephoto-inducedreactionoccurredtoonlya verylimitedextentonpureTiO2,afindingwhichcanbeattributed
tothefast recombinationof theelectronsand holes formedin the photo-excitation process. The incorporation of N into the TiO2appreciablyincreasedtheextentofphotodecomposition,very likelyasaconsequenceofthepreventionofelectron-holerecom- bination[30–32].ThedepositionofAuontoTiO2greatlyimproved thephotocatalyticeffectoftheTiO2.We assumethattheCH3O speciesformedontheAuparticlesorattheAu/TiO2interfaceare muchmorereactivethanthatlocatedonTiO2.
Aninterestingfeatureofthephotocatalyticdecompositionof methanolistheformationofmethylformate.Thiscompoundhas been consideredas a precursor in the synthesis of formamide, dimethylformamide,aceticacid,propionicacid,cyanhydricacid and several other materials [37]. It is mainly synthesized by dehydrogenation of methanol over Cu-based catalyst at higher temperatures. However, recent works showed that it is also formedinthephotocatalyticoxidation[38–41]anddecomposition of methanol onpolycrystalline TiO2 at roomtemperature [30].
Fig.4.EffectsofNdopingofTiO2(SX)onthephotocatalyticdecompositionofCH3OHon1%Au/TiO2and1%Au/TiO2+N.
Fig.5.EffectsofNdopingofTiO2(SX)onthephotocatalyticdecompositionofCH3OHinthevisiblelight.1%Au/TiO2(A)and1%Au/TiO2+N(B).
Fig.6.EffectsofilluminationtimeontheFTIRspectraofadsorbedC2H5OHonTiO2(P25)(A),1%Au/TiO2(Hombikat)(B),1%Au/TiO2(Aurolite)(C)and1%Au/SiO2(D).
Fig.7.PhotocatalyticdecompositionofC2H5OHon1%Au/TiO2(Aurolite)andTiO2(P25)samples.
Its production was markedly increased when Pt metals were depositedonTiO2[30].Thehighestyieldofmethylformatewas measuredforPt/TiO2(62.2)andthelowestoneforRu/TiO2(26.0).
TheCO/H2ratiovariedbetween0.017and0.023.Aunanoparticles alsoenhancedtheproductionofmethylformate(Fig.2).Theyield ofmethylformateonthemostactiveAu/TiO2(Aurolite)was78.0 at the maximum (60min). The formation of HCOOCH3 can be ascribedtotherecombinationofCH2Oformedintheprocessof CH3Odissociation(Eq.(4)and(5)):
2CH2O(a)= HCOOCH3(a) (7)
orbythereactionofCH2OwithafurtherCH3Ospecies:
CH2O(a)+CH3O(a)=HCOOCH3(a)+H(a) (8) RecentstudiesperformedunderUHVconditionsonpreoxidized TiO2(110)disclosedthat methylformate is produced fromthe photo-oxidationof methanolevenat∼200K[42].Itsformation requiredthatbothmethoxyandformaldehydebepresentonthe surface,indicatingthatthephotochemicalactivationofformalde- hydeisfasterthanthemethoxyphotooxidationtoformaldehyde.
Incontrasttoabovereactionstepsit wasassumed thatmethyl formateisformedinthecouplingofHCOwithmethoxyspecies.
ThefactthattheconcentrationofHCOOCH3 startsdecreasing when CH3OH almostcompletely consumedsuggeststhe occur- rence of the photocatalytic decomposition of HCOOCH3. This featureappearedonlyonthemosteffectiveAu/TiO2(Fig.2).Itwas notobservedevenontheTiO2-supportedPtmetals[30]indicating theexceptionallyhighreactivityofAuinnanosizeonTiO2.AsIR spectrashowthepresenceofadsorbedformateverylikelyformed
inthedissociationofHCOOCH3,itsphoto-induceddecomposition canbedescribedasfollows:
HCOO(a)+e−=HCOO(a)␦− (9)
HCOO(a)= CO2␦−+H(a) (10)
CO2␦−+h+= CO2(g) (11)
TheformationofCOinthisstageofphotodecomposition(Fig.2) suggeststheoccurrencethereaction
2HCOO(a)␦−= 2CO2+2OH␦−(a) (12)
ThepromotingeffectofAudepositedonTiO2canbeexplained bythebetterchargecarrierseparationinducedbyilluminationand bytheoccurrenceofanelectronicinteractionbetweentheAupar- ticlesandn-typeTiO2[43,44].Theroleoftheelectronicinteraction betweenmetalsandTiO2hasbeenfirstdemonstratedinthecat- alyticdecompositionofformicacidonNidepositedonpureand dopedTiO2[45].Asfarasweareaware,TiO2 wasfirstusedasa supportinthiscase[45,46].AstheworkfunctionofTiO2(∼4.6eV) islessthanthatofAu(5.31eV),electrontransferisexpectedto occurfromTiO2tothedepositedAu,whichincreasestheactiva- tionofadsorbedmolecules.Weassumethatilluminationenhances theextentofelectrontransferfromTiO2toAuattheinterfaceof thetwosolids,leadingtoincreaseddecomposition.TheSchottky barrierformedatAuandTiO2interfacecanalsoserveasaneffi- cient barrierpreventingelectron-holerecombination [28,47,48].
AswaspointedoutbyLietal.[28]smallergoldparticlesinduce morenegativeFermilevelshiftthanthebiggerparticles.Onthe
Fig.8.EffectsofH2OandO2additiononthephotocatalyticdecompositionofC2H5OHover1%Au/TiO2(Aurolite)catalyst.
basisofthisconsiderationweexpectthatthecatalystwithsmaller goldnanoparticlesiscatalyticallymoreactivethanthatwithlarger goldparticles.
3.2. Adsorptionandreactionsofethanol
Inharmonywithourpreviousstudies[23],theadsorptionof C2H5OHonAu/TiO2(Aurolite)producedintenseabsorptionbands intheC-HstretchingregionoftheIRspectrumat2969,2932and 2866cm−1andbandsofdifferentintensitiesat1448,1379,1269, 1118and∼1072cm−1(Fig.6).Virtuallyidenticalspectraweremea- suredfollowingtheadsorptionofC2H5OHonthepureTiO2 and onotherAu/TiO2samples.Inviewoftheresultsofpreviousstud- ies[19,23],themajorbandsat2969and2866cm−1cancertainly beassignedtotheasymmetricandsymmetricstretches,andthe peaksat1118and1072cm−1tothe(OC)vibrationsoftheethoxy group.Thepresenceofmolecularlyadsorbedethanolisindicated bytheabsorptionbandat 1279cm−1,due tothe␦(OH),and at 1379cm−1,dueto(␦CH3)ofethanol.AccordinglyC2H5OHreadily dissociatesonTiO2andAu/TiO2evenatroomtemperaturewithout illumination:
C2H5OH(g)=C2H5OH(a) (13)
C2H5OH(a)= C2H5O(a)+H(a) (14) As a result of irradiation, the very weak absorption at 1550–1564cm−1 wasconvertedintoawell-detectablepeak,the intensityofwhichincreasedwiththedurationofillumination.This absorptionbandisveryprobablyduetotheasymmetricstretch
offormatespecies.Itshouldbenotedthattherewasnopeakat 1718–1723cm−1duetoCH3CHO.Absorptionbandsidentifiedon Au/SiO2sample(Fig.6D)suggestthatthedissociationofC2H5OH didnot occuron this catalyst to detectableextentby IR spec- troscopy.
Whereasthe conversionof C2H5OH onthe pureTiO2 (P25) usedforthepreparationofAu/TiO2(Aurolite)waslessthan20%
in60min,inthepresenceof1%Auitwasalmost100%(Fig.7).
TheprimaryproductswereH2andCH3CHO,theamountsofwhich increasedasthedurationofilluminationwaslengthened.Whenthe totalconversionoftheC2H5OHwasattained,theconcentrationof CH3CHOdecreased,indicatingtheoccurrenceofitsphoto-induced degradation (Fig. 7).In thecase of TiO2 (Hombikat) we exam- inedtheeffectofAuloadingonthephotocatalyticdecomposition of ethanol. The extentof theconversion is graduallyincreased withtheriseofAucontentfrom∼20%(pureTiO2)to∼100%on 5%Au/TiO2in210min.TheformationofCOinthephotocatalytic decompositionofC2H5OHwasmoreextensivethaninthephotore- actionofCH3OH.TheadditionofH2OtotheC2H5OHexertedonly slighteffectontheconversion,butmarkedlyloweredtheCOcon- tentontheAu/TiO2 (Aurolite)catalyst(Fig.8).AtaH2O/C2H5OH ratioof5:1,theamountofCOdecreasedfrom6.1%to1.9%,andthe CO/H2ratiofrom0.16to0.05at60min.ThequantityofCH3CHO also decreased. The addition of O2 to the C2H5OH also ledto loweramountsofallproducts,withtheexceptionofCO2.Atan O2/CH3OH ratio of1:1, theformation of CO decreased to2.0%, andtheCO/H2 ratioto0.09%.Inthiscasethephoto-oxidationof C2H5OHistobeexpected.Somecharacteristicdataarepresented inTable2.
Fig.9.EffectsofNdopingofTiO2(SX)onthephotocatalyticdecompositionofC2H5OH.1%Au/TiO2(A)and1%Au/TiO2+N(B).
Fig.9depictsthephotocatalyticeffectsofAudepositedonpure and N-modifiedTiO2 (SX). Thephotoactivity of theN-modified catalystsisseentobemarkedlyhigherthanthatofAu/TiO2 free ofnitrogen.ThisisreflectedintheconversionofC2H5OHandin theamountsof theproductsformed.Theamountof H2 gener- atedincreasedbyafactorof6.TheeffectsofN-dopingofTiO2(SX) werealsoinvestigatedinvisiblelight.WhereasAu/TiO2exhibited relativelylittleactivity,thephotoactivityofAu/TiO2+N(SX)was clearlyhigher(Fig.10).
Someexperimentswerealsodevotedtothethermaldecomposi- tionofC2H5OHontheAu/TiO2(Aurolite)catalyst.At323K,merely veryslightreactionwasdetected(<1%in60min).Amoreconsid- erabledegreeofdecomposition(∼3%in60min)wasobservedat
448K.Theresultsofthesecontrolexperimentsledustoexclude thecontributionofthermaleffectstothedecompositionofC2H5OH inducedbyillumination.
TheeffectsofilluminationonthedecompositionofC2H5OHcan bedescribedanalogouslyasinthecaseofCH3OH.Thefirststepis theactivationofC2H5Oinvolvingthedonationofaphotoelectron formedinthephoto-excitationprocesstothissurfacespecies:
C2H5O(a)+e−= C2H5O(a)␦− (15) Thisstepisfollowedbythephoto-induceddecompositionof C2H5OtoCH3CHOandH2:
C2H5O(a)␦−=CH3CHO(a)␦−+H(a) (16)
Fig.10.EffectsofNdopingofTiO2(SX)onthephotocatalyticdecompositionofC2H5OHinthevisiblelight.1%Au/TiO2(A)and1%Au/TiO2+N(B).
ThelevelofphotolysisonpureTiO2waslow,mostlikelybecause ofthereadyrecombinationbetweenelectronsandholesgenerated bylight.ThepresenceofAuonTiO2,however,markedlyenhanced thephotocatalyticperformanceofTiO2.Afterthecompletecon- versionofC2H5OH,thephotocatalyzeddecompositionofCH3CHO cameintoprominence
CH3CHO(a)␦−=CH4+CO(a)␦− (17)
CO(a)␦−+h+= CO(g) (18)
ThepromotingeffectofAuappearstobethesameasthatdis- cussedforthephotocatalyticdecompositionofCH3OH.Itshouldbe borneinmindthatnanosizedAuisanactivecatalystforthethermal decompositionofC2H5OHatelevatedtemperature[19–23].Thisis attributedtopromotionoftheruptureofaC HbondintheC2H5O speciesadsorbedontheAuorattheAu/TiO2interface.
4. Conclusions
(i)IRspectroscopicstudiesrevealedthattheilluminationofpure orAu-containingTiO2promotesthedissociationofCH3OHand C2H5OHtoCH3OandC2H5Ospecies.
(ii)NanosizedAuparticlesmarkedlyenhancethephotocatalytic decompositionsofbothCH3OHandC2H5OH.
(iii)BesidestheproductionofCOandH2,HCOOCH3isformedfrom CH3OHandCH3CHOisformedfromC2H5OH.
(iv)ThroughtheadditionofH2OorO2tothesealcohols,thequan- tityofCO releasedcanbesignificantlydecreased and even completelyeliminatedinthephotodecompositionofCH3OH.
(v)LoweringthebandgapofTiO2byNincorporationincreasesthe photoactivityoftheAu/TiO2catalystandleadstothephotode- compositionofCH3OHandofC2H5OHeveninvisiblelight.
Acknowledgements
Thisworkwassupportedbythegrant OTKAunder contract number K 81517 and TÁMOP under contract numbers 4.2.2/B- 10/1-2010-0012and4.2.2.A-11/1/KONV-2012-0047.Theauthors expresstheirthankstoDr.D.Seb ˝okforsomespectroscopicexper- iments.A loanofTiO2 used forAu/TiO2 (Aurolite)fromSTREM Chemicals,Inc.isgreatlyacknowledged.
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