PdO 小颗粒 王克明

ColloidsandSurfacesA:Physicochem.Eng.Aspects325(2008)21–25

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ColloidsandSurfacesA:Physicochemicaland

Engineering

Aspects

journalhomepage:http://wendang.chazidian.com/locate/colsurf

a

Sizecontrolsynthesisofpalladiumoxidenanoparticlesbymicrowaveirradiation

KemingWang,TaoHuang?,HanfanLiu?,YanxiZhao,HaowenLiu,ChuntaoSun

KeyLaboratoryofCatalysisandMaterialScienceoftheStateEthnicAffairsCommission&MinistryofEducation,CollegeofChemistryandMaterialScience,HubeiProvince,South-CentralUniversityforNationalities,Wuhan430074,PRChina

articleinfoabstract

Palladiumoxidenanoparticleswithsmallsizesandnarrowdistributionsweresynthesizedbyhydrol-ysisofpalladiumchlorideinthepresenceofpolyvinylpyrrolidoneundermicrowaveirradiation.Theas-preparedpalladiumoxidenanoparticleswerecharacterizedbytransmissionelectronmicroscopy,X-raypowderdiffractionandX-rayphotoelectronspectroscopy.Thefactorsaffectingthesizesofpalladiumoxidenanoparticleswereinvestigated.Duetotherapidandhomogeneousheatingwithmicrowave,themeansizeoftheas-synthesizedpalladiumoxidenanoparticleschangedslightlywiththeincreaseoftheinitialpHoftheprecursorsolution.Ahigherconcentrationofpolyvinylpyrrolidonecanonlyacceleratethehydrolysisrate.ItwasalsofoundthatthespeciesoftheacidgroupanionhadlittleeffectontheparticlesizeatanapproximatelyequalpH.

©2008ElsevierB.V.Allrightsreserved.

Articlehistory:

Received13November2007

Receivedinrevisedform5April2008Accepted13April2008

Availableonline20April2008Keywords:

PalladiumoxideNanoparticleColloidMicrowave

1.Introduction

Inrecentdecadesconsiderableattentionhasbeenpaidtothenanosizedmetalcolloidsandclustersduetotheiruniqueproper-tiesand/orpotentialapplicationsinseveralareassuchascatalysis,optoelectronics,microelectronics,magneticmaterialsandothers[1–7].Thecatalyticactivityandselectivity,electrical,thermody-namicandchemicalpropertiesofthesenanoparticlesarestronglydependentontheparticlesize[8].Therefore,http://wendang.chazidian.comparedwithalargebodyofsize-controlresearchesontheplatinummetalsnanoparticles[9,10],muchlessisknownconcerningtheircorre-spondingnanosizedmetaloxidesexceptforplatinumoxide,thewell-knownAdamscatalyst[10–13].

Palladiumoxide(PdO)isanimportantplatinumgroupmetaloxide.Itwasusuallyusedasacatalyst.Especially,thesupportedormodi?edPdOwasusedascatalystsincatalyticcombustionofnaturalgas(methane)orliquid-phaseoxidationofalcoholswithoxygeninrecentyears[14–17].PdOwasgenerallypreparedbyheatdecompositionofPd(NO3)2withalongerheatingtimeatahighertemperatureandthecrystallitesizewithca.9nmcouldbeobtained[14].PdOnanoparticleswithadimensionofca.5nmwerepreparedbyoil-bathheating,butparticlesagglomerationwas

observed[18].Sofar,however,fewreportsonsize-controlsynthe-sisofPdOnanoparticleshavebeenmade.AlthoughsomechemicalapproachesareavailableforthepreparationofPdOnanoparticles,itisstillnecessarytodevelopaconvenientmethodfortheprepa-rationofnanoparticleswithdesirablesizes.Microwaveirradiationasafast,simple,uniformandenergyef?cientheatingmethodhasbeenwidelyusedinchemistrysince1986[19,20].Inrecentyears,microwave-assistedsynthesishasbeenwidelyusedintheprepa-rationofnanosizedmaterials[21–30].Ithasbeenveri?edthatmicrowaveirradiationisaneffectivemethodforthesynthesisofsmallandhomogeneousmetalnanoparticlesdispersion.

Inthispaper,polymer-stabilizedcolloidalpalladiumoxideswithsmallersizesandnarrowerdistributionswerepreparedbymeansofmicrowaveirradiation.Theexperimentswereperformedwithasimplehydrolyticdecompositionofpalladiumchlorideaque-oussolutioninthepresenceofpolyvinylpyrrolidone(PVP)undermicrowaveirradiation.Thein?uencesofsomeselectedexperi-mentalparameterssuchastheinitialpHvalue,themolarratioofstabilizingpolymertotheprecursor,andsomeacidgroupanionsonthehydrolysisrate,particlesizeanddistributionwerealsoexam-ined.

2.Experimental2.1.Materials

?Correspondingauthors.Tel.:+862767842752;fax:+862767842752.E-mailaddresses:huangt6628@http://wendang.chazidian.com(T.Huang),h?iu@http://wendang.chazidian.com(H.Liu).

0927-7757/$–seefrontmatter©2008ElsevierB.V.Allrightsreserved.doi:10.1016/j.colsurfa.2008.04.027

Palladiumchloride(PdCl2),whichwaspurchasedfromShang-haiChemicalReagentCo.Ltd.,China,wasusedasastartingmaterial

22K.Wangetal./ColloidsandSurfacesA:Physicochem.Eng.Aspects325(2008)21–25

andconvertedintoH2PdCl4·nH2Obytreatmentwithconcentratedhydrochloricacidbeforeuse.Polyvinylpyrrolidone(averagemolec-ularweight,MW=30,000)waspurchasedfromAcrosandotherchemicalswerepurchasedfromthedomestic.Allreagentswereofanalyticalgradepurityandwereusedasreceivedwithoutfurtherpuri?cation.Deionizedwaterwasusedthroughout.Alltheglass-wareswerecleanedwithchromicacidandrepeatedlywashedwithdeionizedwater.

2.2.Preparationofpalladiumoxidenanoparticles

Inatypicalsynthesis,8×10?5molofH2PdCl4,8×10?3molofPVP(asmonomericunit)wereaddedintoa100-mlround-bottomed?ask.Anaqueoussolutionofsodiumhydroxide(NaOH)(orsodiumacetate,sodiumbenzoate)wasthenaddeddrop-wisewithvigorousstirringtoadjustthepHoftheprecursorsolution.Thetotalvolumeofthereactionsolutionwaskeptat20ml.Thesolutionwasheatedfor8minbyamodi?eddomesticmicrowaveoven(Galanz,900W)with10%outputofthepower.ThecolorofthesolutionturnedfrompaleyellowtobrownishblackandPVP-stabilizedpalladiumoxidecolloidwasobtained.Themicrowaveovenwasmodi?edwithoutchangingthepowerasdescribedasinRef.[22]:awater-cooledcondenseroutsidetheoven’scavitywasconnectedbyaglassjointofaglassround-bottomed?asksetinside.ATe?onstirrerwassetinthe?askandwasdrivenbyamotor.Thepoweroutputwasadjustedbythemicrowavemachinewitharoutineon–offmanner.2.3.Characterization

Transmissionelectronmicroscopy(TEM)measurementswereconductedonaFEITecnaiG220instrument.ThesampleforTEMobservationwaspreparedbyplacingadropofthecolloidaldis-persionontoacoppergridcoatedwithaperforatedcarbon?lm,followedbyevaporatingthesolventatanambienttemperature.Particlesizewasdeterminedfromtheenlargedphotographsand

sizedistributionhistogramswereobtainedonthebasisofthemea-surementofabout300particles.

X-raypowderdiffraction(XRD)wasperformedonaBrukerD8X-raydiffractometeremployingCuK?radiationwith40kVand50mA.

X-rayphotoelectronspectroscopy(XPS)wasrecordedonaVGMultilab2000X-rayphotoelectronspectrometerusingMgK?radi-ationunderavacuumof8×10?7Pa.Allbindingenergyvaluesweredeterminedwithreferencetocarbon,C1s=284.6eV.Thesamplewaspreparedbyconcentratingthepalladiumoxidecolloids,fol-lowedbywashingthoroughlywithacetoneandthenevaporatingtodrynessunderanambientcondition.3.Resultsanddiscussion

3.1.SizecontrolofpalladiumoxidenanoparticlesbyvaryingthepHofprecursorsolution

BasedonthehydrolysisofPdCl42?,thepreparationofPdOnanoparticlesisthoughttobedependentonthepHofthepre-cursorsolution.TheeffectoftheinitialpHontheparticlesizeofPdOcolloidspreparedbymicrowavemethodwasinvestigated.Fig.1showstherepresentativeTEMimagesandhistogramsofPVP-stabilizedPdOnanoparticlespreparedbymicrowaveheatingwhenthesolution’sinitialpHchangedfromweakacidictobasic.Fig.1aandbshowstheTEMimagesofPdOnanoparticlespreparedwithadditionofNaOHandthecorrespondinginitialpHvaluesofthereactionsystemsare5.28and11.88,respectively,withmolarratiosofNaOH/PdCl42?of4/1and6/1,whilePVPconcentrationiskeptconstant.TheresultsshowthatasthepHoftheprecursorsolutionisincreasedfrom5.28to11.88,themeansizesofPdOnanoparti-clesseemtoincreaseslightlyfrom1.80±0.43to3.20±1.05nmasshowninFig.1aandb.

Similarly,PdOnanoparticleswerepreparedwithadditionofNa2CO3insteadofNaOHtoadjustthestartingsolution’spH.Theaveragediametersoftheas-preparedPdO

nanoparticles

Fig.1.TEMimagesandthecorrespondingparticlesizehistogramsofplatinumoxides.(a)pH5.28,adjustedwithNaOH;(b)pH11.88,adjustedwithNaOH;(c)pH5.20,adjustedwithNa2CO3;(d)pH9.68,adjustedwithNa2CO3.Allcontain4mMofH2PdCl4and400mMofPVP.

K.Wangetal./ColloidsandSurfacesA:Physicochem.Eng.Aspects325(2008)21–2523

lookedtoincreasefrom2.40±0.55to3.81±1.10nmasthepHoftheprecursorsolutionchangedfrom5.20to9.68asshowninFig.1candd.

Consideringthestandarddeviation(??),whichwascalculatedfromthedistributionofthecolloidalparticlesizes,however,itcanbeseenthattheaveragediametersofPdOnanoparticleschangedlittlewiththeincreaseoftheinitialpHoftheprecursorsolutionunderweakacidictobasicaqueousconditions.Inotherwords,thestartingsolution’spHhadalittleeffectonthesizeofthePdOnanoparticlespreparedbymicrowaveirradiation.

Ontheotherhand,whentheinitialpHofsolutionwasaslowas3.07or2.03withamolarratioofNaOH/PdCl42?of2/1orwithoutadditionofNaOH,respectively,themeansizeofthePdOnanoparti-cleswas5.44±1.86or5.45±1.34nm,asshowninFig.2aandb,theaverageparticlesizeincreasedcomparativelyunderstrongacidicconditions.Thiscanbeunderstoodthatthehydroxideorhydrousoxideisdif?culttoforminanacidicmediumincomparisonwiththatinabasicsolution,sincethehydrolyticdecompositionprocessisinhibitedatalowerpHmedium.Inotherwords,lesshydrox-idesorhydrousoxidesareformedinanacidicsolution.Hence,

lessFig.2.TEMimagesofpalladiumoxidenanoparticles.(a)pH2.03withoutNaOHand(b)pH3.07with2/1ofNaOH/H2PdCl4.Allcontain4mMofH2PdCl4and400mMofPVP.

nucleusareformedofthemetaloxideclustersandleadstofurthergrowthofnucleuscrystalline.Basedonatwo-stepmechanismfortheformationofplatinumgroupmetalsnanoparticlesviahydroly-sis/condensationandreductionprocesses[31,32],thedependenceofsizesofpalladiumoxidenanoparticleswiththepHofprecur-sorsolutionisfairlyinaccordancewiththeresultsreportedinliterature[33–35].

Totallyspeaking,theseresultsarequitedifferentfromthatofPtO2colloidspreparedbyoil-bathheating[12].PtO2particlesizeschangedremarkablyfrom38.1to0.94nmwhenthestartingpHchangedslightlyfrom6.68to6.92.Apossibleexplanationmaybethatinthecaseofplatinumthehydrolysisproduct—platinumhydroxideisamphoteric(butpalladiumhydroxideisnot),itgrowsinsizeandformsgelsfollowingthemechanismsimilartosilica,thusthehydrolysisismuchmorepHdependentthanthepalladiumdoes.Furthermore,themicrowaveirradiationprovidesauniformenvironmentfornucleation.Duetothefastanduniformnatureofmicrowaveheatingincomparisonwiththeconventionalheatingmethods,itisfavorablefortheformationofhomogeneouscolloidalPdOnanoparticleswithsmallersizes.3.2.Effectofstabilizingpolymer

Intheformationofwater-solublepalladiumoxidenanoparti-cles,stabilizingpolymermolecules,PVP,wereadsorbedonthesurfacesofPdOnanoparticlespreventingtheiraggregation.Gener-ally,ahigherconcentrationofPVPshouldleadtosmallerparticlesandthishasbeenveri?edinmanyotherreports.However,ourresultsshowedthattheconcentrationofPVPhadlittleeffectonthemeanparticlesizeasthemolarratioofPVPtoPdincreased,whilethereactiontimechangedobviouslyasindicatedbytherateofthecolorchangefrompaleyellowtobrownishblack.AtthesamepHwithadditionofthesamebase,whenthemolarratioofPVP/PdCl42?was100/1,thereaction?nishedwiththecolorchangetobrownishblackat8min,whilethemolarratioofPVP/PdCl42?was10/1,20/1or50/1,thecoloronlychangedtobrownordarkbrownevenremainingthereactionfor40min,implyingthatthehydrolysisratewasmuchfasteratahigherconcentrationofPVP.

ItiswellestablishedthattheformationofPVP-stabilizedcol-loidalmetaloxidenanoparticlesgoesthroughtwoprocessesofhydrolyticdecompositionandcondensation[11,12].Firstly,thePdCl42?speciesarehydrolysedtothecorrespondinghydroxideorhydrousoxide,thenthepalladiumoxidemoleculesaccumu-latetoformmetaloxideclustersbyacondensationreaction.Owingtothemicrowaveirradiationacceleratestherateofhydrol-ysis/condensationofmetalsalts,itmasksthein?uenceofPVPonthesizeofnanoparticleproductstoalargeextent.Similarresultshavebeenreportedinliteratures[21–23].

Nevertheless,hugeamountofPVPmayleadtothereduc-tionofPdOwithheating[36,37].Accordingly,themolarratioofPVP/PdCl42?waskeptat100/1throughouttheexperimentswhenotheraffectingfactorswereinvestigated.3.3.Effectofacidgroupanions

Inourpreviousstudy[12]onthepreparationofpolymer-stabilizedwater-solubleplatinumoxidenanoparticles,itwasfoundthattheadditionofsodiumacetatehadasigni?cantin?uenceontherateofhydrolysis,therateofthecondensationreactionincreasednearlylinearlywiththeincreaseoftheconcentrationofsodiumacetate.Theacidgroupanion,acetate(Ac?),mayberespon-sibletothiseffect.Therefore,possiblein?uencesofseveralselectedweakacidsaltssuchassodiumacetateandsodiumbenzoateonthepreparationofPdOnanoparticlesatapproximatelyequalinitialpH(5.20–5.37)wereinvestigatedincomparisonwiththatofNa2CO3

24K.Wangetal./ColloidsandSurfacesA:Physicochem.Eng.Aspects325(2008)

21–25

Fig.3.TEMimagesofpalladiumoxidenanoparticlesatapproximatelyequalinitialpH.(a)pH5.31,themolarratioofsodiumacetatetoH2PdCl4was20:1;(b)pH5.37,themolarratioofsodiumbenzoatetoH2PdCl4was20:1.Bothcontain4mMofH2PdCl4,Pd:NaOH=1:2,Pd:PVP=1:100(molarratio).

andNaOH.Fig.3showstheTEMimagesandthecorrespondingpar-ticlesizehistogramsofthePdOcolloidspreparedwithadditionofsodiumacetateorsodiumbenzoate.Inthetwosystems,themolarratioofsodiumacetateorsodiumbenzoatetoPdCl42?was20:1andthecorrespondingpHwas5.31or5.37,respectively.Allreac-tionsfordifferentsystems?nishedat6–8minandbrownishblackcolloidswereobtained.Nomuchdifferenceinreactiontimewitheachotherfordifferentsaltsinthesesystemswasobserved.More-over,littledifferencesinreactiontimewereobservedwithvaryingthemolarratioofsodiumacetatetotheprecursor,PdCl42?,atthesamepHandthesamePdconcentration.Similarresultsforsodiumbenzoatewerealsoobtained.

AsshowninFig.3,themeansizesofPdOnanoparticlespreparedinsodiumacetateandsodiumbenzoatesystemswere2.44±0.45and2.5±0.44nm,respectively.IncomparisonwiththeNa2CO3andNaOHsystemsasshowninFig.1canda,respectively,themeansizesofnanoparticlesinallcasesweremuchclosetoeachotherconsid-eringthestandarddeviation.Theresultsimplythattheselectedweakacidgroupanionssuchascarbonate,acetateandbenzoateinthereactionsystemhavelittleeffectontheparticlesizeanddisper-sityofthepalladiumoxidenanoparticles.Thedifferentbehaviorsofpalladiumsaltinhydrolysis/condensationreactionarethoughtmostlyduetoitscharacteristics,whilemicrowaveheatingaccel-eratedthenucleationofpalladiumoxideclusterandfavorstheformationofuniform,monodispersednanoparticleswithsmallersizes.

3.4.XRDpatternandXPSspectrogramofPdOnanoparticles

Fig.4showsXRDpatternofatypicalsampleofPdOnanopar-ticles.FromtheXRDpattern,thecharacteristicpeaksforPdOat2Â=27.35?,31.69?,45.43?,56.45?and66.20?correspondingtothe(111),(200),(220),(222)and(400)latticeplanesaccordingtotheJCPDS?leNo.046-1211,respectively,wereobserved,

revealing

Fig.4.XRDpatternofpalladiumoxide

nanoparticles.

Fig.5.XPSspectrogramofpalladiumoxidenanoparticles.

theresultantPdOnanoparticleswithaface-centeredcubic(FCC)

structurewithlatticeparameters5.642A.

?XPSwasemployedtoobtaintheinformationontheelectronicstateofthesurfaceregionofthePdOparticles.AsshowninFig.5,XPSmeasurementindicatesthebindingenergyofPd3d5/2andPd3d3/2at335.9and341.1eV,respectively,refertoC1s=284.6eV.Thebindingenergy335.9eV,correspondingtoPd3d5/2,iscoincidentwiththereferencevalue,336.2eV[38],indicatingtheexistenceofPd(II).BothXRDandXPSdatacon?rmtheformationofpalladiumoxideparticles.4.Conclusions

Insummary,thepalladiumoxidenanoparticleswithFCCstructurecanbesynthesizedbyhydrolyticdecompositionofthecorrespondingmetalchlorideinthepresenceofPVPundermicrowaveirradiation.ThePdOnanoparticleswithsmallsizesandnarrowdistributionswereobtainedunderweakacidictobasicconditions.Themeansizeoftheas-preparedPdOnanoparticleschangedslightlywiththeincreaseoftheinitialpHoftheprecur-sorsolutions.AhigherconcentrationofPVPcanacceleratethehydrolysisrate,butshowslittleeffectonthemeansizeofPdOnanoparticles.Thespeciesandtheconcentrationsoftheacidgroup

K.Wangetal./ColloidsandSurfacesA:Physicochem.Eng.Aspects325(2008)21–2525

anionshadlittleeffectontheparticlesizecontrolatacertainpHandthesameprecursorconcentration.Acknowledgements

ThisresearchwassupportedbytheNationalNatureScienceFoundationofChina(GrantNo.20673146),theNatureScienceFoundationofHubeiProvince(GrantNo.2006ABA366),theScienceFoundationofSouth-CentralUniversityforNationalities(GrantNos.YZY02002andYZZ05002).References

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