碱性介质中异丙醇在铂电极表面的吸附和电化学氧化

1、1280物理化学学报(WuliHuaxueXuebao)ActaPhys.鄄Chim.Sin.,2005,21(11)：12801284NovemberTheAdsorptionandOxidationofIsopropanolatPlatinumElectrodeinAlkalineMedia*LIN,Heng1CHEN,GuoLiang1,2ZHENG,ZiShan1CHEN,ShengPei2LIN,ZhongHua2ZHOU,JianZhang2(1DepartmentofChemistry,ZhangzhouNormalCollege,Zhangzhou363000；2StateKe

2、yLaboratoryofPhysicalChemistryoftheSolidSurfaces,CollegeofChemistryandChemicalEngineering,XiamenUniversity,Xiamen361005)AbstractTheadsorptionandoxidationofisopropanolinalkalinemediaatplatinumelectrodehavebeeninvestigatedbyusingelectrochemicalquartzcrystalmicrobalance(EQCM)andinsituFTIRspectroscopy.T

3、heresultsshowthatthereisnoselfpoisoningintheelectrooxidationofisopropanolinalkalinemedia.Thoughnopoisonspecies,suchasCO,areevidencedbyinsituFTIRspectroscopy,theadsorptionofisopropanoloritsdissociativeproductsonPtsurfaceissuggestedbyEQCMdata.Thefinalproductofisopropanoloxidationisonlyacetoneunderexpe

4、rimentalcondition,whichsuggeststhattheoxidationofisopropanolintoacetonetakesplaceviadehydrogenationstep.TheEQCMstudiesprovidequantitativeresultsofsurfacemassvariationandhavethrownnewlightintheelucidatingisopropanoloxidation.Keywords:Ptelectrode,Electrooxidation,Isopropanol,InsituFTIR,EQCM,Alkalineme

5、diaTheelectrochemicaloxidationofsmallorganicmoleculesisoffundamentalimportanceinelectrocatalysis,andisalsoakeyfactorintheapplicationoftheoxidationasanodicprocessindirectfuelcells.Isopropanolisthesmallestsecondaryalcoholmolecule.Duetotheparticularityofthemolecularstructure,isopropanolistheonlymolecul

6、eamongC3alcoholthatfailstodissociateintoCOatthesurfaceofplatinumelectrodesinacidicmedia12.Thisfeaturemakesisopropanolsuitabletobeemployedinfundamentalstudies,suchaselectrochemicalreactionkinetics34andtheeffectsofmolecularstructure1.Theoxidationofisopropanolinacidicmediaonplatinumsinglecrystalandpoly

7、crystallineelectrodehasbeeninvestigatedextensivelybyusingtraditionalelectrochemicalmethods56,massspectroscopy7,andinsituspectroscopy14.2528251).mentionedabovearecarriedoutinacidicmedia.Inthelastdecade,theapplicationofthequartzcrystalmicrobalance(QCM)techniqueinanelectrochemicalcell,underpotentialcon

8、trol,ledtotheEQCM.Theattractivenessofthisapproachisbasedonthehighsensitivityofthequartzcrystalresonatortomasschange9.Underfavorableconditions,atypicalEQCMcanmeasureamasschangeof0.11ng·cm-2atelectrode/electrolyteinterfaces,sohaswideapplicationsinthecharacterizationsoftheinterfacialelectrochemica

9、lprocessessuchasthetransportofionsandsolventsduringelectrochemicalreactions,detectionofchemicalandbiochemicalspecies,depositionofmetalsonsurfacesandthegrowthofmolecularlayers.Althoughadirectfuelcellbasedonalkalineelectrolyteismorepromising10,littleattentionhasbeenpaidtoinsituIRspectroscopicandEQCMin

10、vestigationsoftheoxidationofsmallorganicmoleculesinalkalinemediabecauseoftheprogressive“carbonation”ofthesolutionasaresultofCO2retention11.TheaimofthestudyistoquantitativelyinvestigatetheelectrochemicaloxidationmechanismofisopropanolonPtelectrodeinalkalinemediaatmolecularlevelviainsituFTIRandEQCM.Be

11、sides,theresultsarecomparedwiththoseofitsoxiReceived：April20,2005;Revised：June22,2005.Correspondent：LIN,Heng(Email：chengl259;Tel：05962591445;Fax：0596*TheProjectSupportedbyNSFC(20433040)andNSFofFujianProvince(E0310026)LIN,Hengetal.：TheAdsorptionandOxidationofIsopropanolatPlatinumElectrodeinAlkalineMe

12、dia1281dationinacidicmedium1inordertoelucidatethereactionmechanismofisopropanolinalkalinemedia.1ExperimentsTheinsituelectrochemicalFTIRexperimentswerecarriedoutusingaNicolet870FTIRapparatusequippedwithaGopherIRsourceandaMCTBdetectorcooledwithliquidnitrogen.Theinstrumentwaspurgedthroughouttheexperime

13、ntwithcleanairfreeCO2andH2ObyaBalston745301cleanairpackage.ACaF2diskwasusedastheIRwindow,andanIRcellofthinlayerconfigurationbetweenelectrodeandIRwindowwasapproachedbypushingtheelectrodeagainstthewindowduringFTIRmeasurements.Theincidentinfraredbeamwasalignedatabout60°tothenormalofelectrodesurfac

14、e.ThemultistepFTIRspectroscopy(MSFTIRS)technique12wasappliedfortheinvestigationofisopropanoladsorptionandoxidation,andthespectrumwasreportedastherelativechangeinreflectivitythatiscalculatedasR/R(Eis)=R(Eis)-R(ER)/R(ER),whereistheresultingspectrumoftheithsamplepotentialEsi.R(RE/R(Esi)thesinglesi)isth

15、esinglebeamspectrumacquiredatEbeamspectrumrecordedatthereferencesi,andR(ER)ispotentialER.Inthedifferencespectra,thepositivegoingbandsindicatethattheIRadsorptionatERisgreaterthanthatatEsandthenegativegoingbandscorrespondtothegreaterIRabsorptionatEsthanthatatER.Foreachspectrum400interferogramswerecoll

16、ectedat16cm-1resolutionandcoaddedintoeachsinglebeamspectrum.TheEG&GPARCelectrochemicaldeviceconsistedofamodel263ApotentiostatandamodelQCA917Seikoquartzcrystalanalyzer13,bothcontrolledbyM270software,usinganmicrocomputer.ATcutquartzcrystalswithafundamentalfrequencyof10MHzwereusedastheworkingelectr

17、odes.Eachcrystalhad0.2cm2circularareaoneachside,coveredbyabimetalliclayerconsistingofaTisubstratecoatedwithaPtsurface.Electrochemicalmeasurementsweremadeinacompartmentglasswith30mLcapacity.ThecellhadaspecialTefloncoverwithholesdesignedtohosttheauxiliaryandworkingelectrodesandthedegassingtubes.TheLug

18、gincompartmentwasconnecteddirectlytothecellsbody.TheworkingelectrodeusedinIRexperimentswasaPtdiskofca.5.0mmdiameter.Itwaspolishedwithaluminapowderof1,0.3and0.05m,cleanedinanultrasonicbath,washedagainwithMilliQwater.Itwassubjectedfinallytopotentialcyclingin0.1mol·L-1mV·s-1untilastablevoltam

19、mogramwasachieved.Thispotentialcyclingwascarriedoutasapretreatment(electrochemicalcleaning)inbothcyclicvoltammetryandIRexperiments.Thereferenceelectrodewasasaturatedcalomelelectrode(SCE),separatedfromthecellbyabridgemadeofastopcockwithtwoendscontainingthesolution.ThecounterelectrodewasaPtfoilwithage

20、ometricareaof1cm2.AlltheglasswarewascleanedbeforeusebyimmersioninCrO3+H2SO4solution,followedbythoroughrinsinginMilliQwater.TheelectrolyteswerepreparedwithNaOH,isopropanolofanalyticalgradeandwaterpurifiedbyMilliporesMilliQsystem.Thesolutionsweredeaeratedbybubblingnitrogengasofhighpurity.Theexperiment

21、swerecarriedoutataround20,andunderatmosphericpressure.2ResultsandDiscussionSmallorganicmoleculeoxidationinacidicmediaatPtelectrodegenerallycontainsselfpoisoningphenomenon1,i.e.poisonspeciesisgeneratedduringtheelectrooxidation.However,evenasprefacestated,isopropanolistheonlymoleculeamongC3alcoholthat

22、doesnotdissociateintoCOatthesurfaceofplatinumelectrodesinacidicmedia.Fig.1presentsthecyclicvoltammogramofisopropanoloxidationinalkalinemediaatanATcutquartzcrystalPtelectrode.ItisclearfromFig.1thateveninthepotentialregionofhydrogenadsorptiondesorption,isopropanoloxidationinthepositivegoingpotentialsw

23、eep(PGPS)couldtakeplaceimmediatelyandproducetwocurrentpeaks(IandII)at-0.56and-0.31V,whichissimilartothatofisopropanoloxidationinacidicmedia1.Theresultsimplythatthereisnoselfpoisoninginthereaction,whichisinaccordanceE/V(vsSCE)Fig.1CyclicvoltamogramsofPtelectrodein0.1mol·L-1CH3CHOHCH3+0.1mol·

24、;L-1NaOHsolution(a),0.1mol·L-1NaOHsolution(b)sweeprate50mV·s-11282ActaPhys.鄄Chim.Sin.(WuliHuaxueXuebao),2005Fig.2presentsaseriesofspectraobtainedbyfixingERat-0.90V,whichisinthepotentialregionofhydrogenadsorptiondesorptionandislowerthantheoxidationonsetpotentialofisopropanol,andbyvaryingESf

25、rom-0.8to0.4V.WhenESislowerthanthethresholdoxidationpotentialofisopropanol(near-0.7V,seeFig.1)therearenosignificantfeaturesinthespectrum,butwhenESisat-0.7V,anegativegoingbandsituatedat1699cm-1appearsimmediately.Thisfeaturecoincidesroughlywiththeoxidationonsetpotentialofisopropanol.Thesespectrahaveth

26、efollowingfeatures.(a)Intherangefrom2100to1800cm-1,noIRbandscorrespondingtoadsorbedCOcanbeobservedinFig.2.ThisisadirectproofthatisopropanolcannotdissociateintoCOspeciesduringitsinteractionwiththePtsurface,whichisinaccordance滓/cm-1·L-1NaOHsolutionThereferencepotentialERwasat-0.90V,andthevaluesof

27、potentialESforeachspectrumareindicatedinthefigure.withtheobservationfromcyclicvoltammogramofPtelectrodeinFig.1.ThesimilarsituationhasbeenreportedinitsoxidationonbothPtpolycrystallineandsinglecrystalelectrodeinacidicmedia14.Intheisopropanolmolecule,theOHgroupisconnectedtothesecondcarbonatomofthechain

28、,withamethylateitherside,soitmaybetoodifficultforthiscarbonatomtoapproachthePtsurfacefordissociationintoCO.(b)Afewpositivegoingbandsnear947,1127,1116,1166,1468cm-1denotesthedecreaseofisopropanolconcentrationinthethinlayerduringtheoxidationofisopropanolatES.(c)NonegativegoingbandcorrespondingtotheCO2

29、asymmetricalstretchat2340cm-1appearsinthespectrumuntilESisat0.4V,whichindicateseffectivelythatisopropanolisnotoxidizedintoCO2underpresentconditions1.Thisresultdiffersfromthatreportedinacidsolutions,whereisopropanolcaneasilybeoxidizedintoCO21.(d)Inthefingerprintregionafewnegativegoingbandsmaybeseenon

30、thespectra1,15.Thebandsnear1426and1369cm-1canbeassignedtothethesymmetricalandasymmetricaldeformationofCHintheCOCH3grouprespectively;thebandnear1238cm-1canbeattributedtothestretchingplusbendingoftheCCOCchain;thebandnear1699cm-1canbeassignedtotheCOstretching.Itisworthpointingoutthattheintensityofthese

31、bandsfirstincreasessteadilywiththeelectrodepotentialtoreachamaximumbetween-0.30-0.10V,andthenstartstodecreaseatmorepositivepotentials.(f)Thebaselineshiftandthebroadbandwhichappearsat1600cm-1inthespectraofFig.2astheelectrodepotentialincreasescouldberelatedtochangesintheconcentrationofH+andOH-inthethi

32、nlayerasthereactionproceeds16.Infact,H+isproducedasisopropanolisoxidized,givingrisetochangeinpHwhichmaybecompensatedbydiffusionofOH-andH2Ointothethinlayer.CH3COCH3+2H2O+2e-Fig.3showsthemvariationinasolutioncontaining0.1mol·L-1CH3CHOHCH3and0.1mol·L-1NaOHmeasuredwiththeATcutquartzcrystalPtel

33、ectrodeduringtheCVscanning.Nearlyaconstantvalueofmismaintainedfrom-0.90to-0.78VinPGPS.Comparedwiththeisopropanolfreeelectrolyte,LIN,Hengetal.：TheAdsorptionandOxidationofIsopropanolatPlatinumElectrodeinAlkalineMedia1283E/V(vsSCE)Fig.3mvariationofPtelectrodein0.1mol·L-1CH3CHOHCH3+0.1mol·L-1N

34、aOHsolution(a)and0.1mol·L-1NaOHsolution(b)sweeprate50mV·s-1wherethemincreasedcontinuouslyinthesamepotentialregion,thisresultmayimplythatisopropanoloritsadsorptiveproductsarepossiblyadsorbedpreferentiallyontheelectrodesurfaceinthispotentialregion1718.Sincenochemicallyadsorbedspeciesareevide

35、ncedbyinsituFTIRspectroscopy,thispointcannotexcludethepossibilityofisopropanolanditsdissociativeadsorptiononPtsurface,whichindicatesEQCMisapowerfultoolforstudyingtheinterfacialelectrochemicalprocesses.From-0.4Von,themincreasesquickly,whichisascribedtotheelectrooxidation,readsorptionofisopropanolando

36、xygenadsorption(orPtsurfaceoxidation).WhenEvariesfrom-0.4to0.6V,themincreasesby87.9ng·cm-2.Itisinterestingtoseethatthemalmostremainsconstantbetween0.6and-0.20VinNGPS,then,itdecreasesquicklytojointthemcurverecordedinthePGPSwhenelectrodepotentialissweptbelow-0.20V,implyingthatthemremainsconstanta

37、fteroneCVcycle.3ConclusionsTheCV,EQCMandinsituFTIRspectroscopyareusedtorevealthereactionmechanismofisopropanoloxidationinalkalinemediaonPtelectrodeatamolecularlevelandquantitatively.Themainconclusionscanbedrawnfromtheexperimentalresultsasfollows:(1)thereisnoselfpoisoningintheelectrooxidationofisopro

38、panolinalkalinemedia.(2)Sincenochemicallyadsorbedspecies,suchasCO,areevidencedbyinsituFTIRspectroscopy,theadsorptionofisopropanoloritsdissociativeproductsonPtsurfacearesuggestedbyEQCMdata.(3)IsopropanoloxidationinPGPScouldtakeplaceimmediatelyeveninthepotentialregionofhydrogenadsorptiondesorption,and

39、twocurrentpeaks(IandII)at-0.56and-0.31Varedetected.(4)Thefinalproductofisopropanoloxidationisonlyacetoneundertheexperimentalcondition,itmaythereforebesuggestedthattheoxidationofisopropanolintoacetonetakesplaceviadehydrogenationstep.(5)TheEQCMstudiesprovidequantitativeresultsofsurfacemass1Sun,S.G.;Yang,D.F.;Tian,Z.W.J.Electroanal.Chem.,1990,289:1772Sun,S.G.;Lin,Y.J.Electroanal.Chem.,1994,375:4013Sun,S.G.;Lin,Y.Electrochim.Acta,1996,41:6934Sun,S.G.;Lin,Y.Electrochim.Acta,1998,44:11535Horanyi,G.;Vertes,G.;Konig,P.ActaChim.Acad