Preparation of Ordered Mesoporous Carbons Containing Well-dispersed and Highly-alloying Pt-Co Bimetallic Nanoparticles toward Methanol-resistant Oxygen Reduction Reaction

AppliedCatalysisB:Environmental108–109 (2011) 81–89

ContentslistsavailableatScienceDirect

Applied

Catalysis

B:

Environmental

j ou rna 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 b

Preparation

of

ordered

mesoporous

carbons

containing

well-dispersed

and

highly

alloying

Pt–Co

bimetallic

nanoparticles

toward

methanol-resistant

oxygen

reduction

reaction

Shou-Heng

Liu

,

Feng-Sheng

Zheng,

Jyun-Ren

Wu

DepartmentofChemicalandMaterialsEngineering,NationalKaohsiungUniversityofAppliedSciences,Kaohsiung80778,Taiwan

a

r

t

i

c

l

e

i

n

f

o

Received28February2011

Receivedinrevisedform9August2011 Accepted11August2011

Available online 19 August 2011 Keywords:

PtCoalloy

Orderedmesoporouscarbons Oxygenreductionreaction Methanoltolerance XANES/EXAFS

a

b

s

t

r

a

c

t

Asimplerouteisdescribedforthesynthesisofwell-dispersedandhighlyalloyingPtCobifunctional nanoparticlessupportedonorderedmesoporouscarbons(Pt100−xCox/OMC)bythesimultaneous

pyrol-ysisofcarbonandmetalprecursorsinamesoporoussilicaasthehardtemplate.Avarietyofdifferent spectroscopicandanalyticaltechniqueswasusedtothoroughlycharacterizethePt100−xCox/OMC

sam-ples.ByX-raydiffraction,N2 adsorption/desorptionisothermsandtransmissionelectronmicroscopy,

itwasfoundthatPt100−xCox/OMCpossessedwell-dispersedPt/PtConanoparticles(2–3nm)supported

onhighsurfacearea(∼1000m2_g−1_{)andregularporechannels(∼2.8nm).AmongPt100}

−xCox/OMC

cata-lysts,thePt50Co50/OMCwasfoundtohavesuperiorelectrocatalyticactivityandthetolerancetomethanol

crossoverduringoxygenreductionreactionascomparedtotypicalcommercialelectrocatalysts.Thismay beattributedtothedispersionanduniquenanostructureofhighlyalloyedPtConanoparticlessupported onPt50Co50/OMCevidencedbyX-rayabsorptionspectroscopy.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Direct methanol fuel cells (DMFCs) and polymer electrolyte membrane fuelcells (PEMFCs) have beenconsidered astwo of next-generationelectricalpower sourcesfor light-dutyvehicles and stationary or portable applications [1–3] as an alternative toconventionalpowersources,forexample,internalcombustion engines and secondary batteries [4–6].For thepast few years, much effortfromgovernment,industry, andacademy hasbeen devotedtodeveloping DMFCs/PEMFCsand greatadvanceshave beenachieved;however,theremainingbarrierstowidespreaduse areincluding(i)thehighcostsofPt-basedelectrocatalysts[7–9], (ii)thepoorkineticsofbothanodeandcathodereactions[10,11], (iii)durabilityofcarbonsupportedcatalysts[12–14]and(iv)the crossoverofmethanolfromtheanodetothecathodethroughthe protonexchange membranes[15–17].In terms ofthefirst two problems,progressive increase onpriceof Ptthat is theactive speciesinthemostofthepresentlyusedelectrocatalystsrequires adecreaseintheusageofPtand/oranincreaseinthemass spe-cificactivityoftheactivespecies.Thus,supportingmaterialswith highsurfaceareaconducivetoPtdispersion,propertextural prop-ertiesfavortokineticsofbothanodeandcathodereactions,and

∗ Correspondingauthor.Tel.:+88673814526x5152;fax:+88673830674. E-mailaddress:[email protected](S.-H.Liu).

highelectronicconductivityarehighlydesirable[18].Withrespect tothelasttwoones[19–22],structuralstabilityandsurface proper-tiesofthecatalystsupportsaswellasthemethodologiesinvokedin incorporatingPtcatalystontothesupportarecrucialforthe disper-sionandstabilityofthemetalnanoparticlesduringDMFCs/PEMFCs operations in terms of eliminating CO-poisoningand methanol crossover[23,24]atanodeandcathode,respectively.

Recently,alloyingofPtwithtransitionmetalssuchasFe[25,26], Co[27–29],Ni[30,31]andCr[32]catalystshasbeenintensively studiedand shown enhanced activitytoward oxygenreduction reaction(ORR)andhightolerancetowardmethanolcrossover.In general,alloyedcatalystsshouldbeperformedathigh tempera-turestoassisttheformationofalloymetalcomponents.PtCoalloy supportedelectrocatalystsforORRhavebeenalsopreparedviathe additionofCospeciesonto carbon-supportedPtmetalcatalysts andthenalloyingathightemperaturesmorethan973Kininertgas orhydrogen[33,34].However,thealloyedparticlessofabricated easilyaggregatetoformlargerparticlesduringtreatmentathigh temperatures.Tocircumventthisproblem,Pt-basedalloycatalysts fortheORRinDMFCs/PEMFCsarethenpreparedbythereduction ofmetalprecursorswithchemicalreductantssuchasNaBH4atlow temperaturesinplaceofcatalysttreatmentathightemperatures underhydrogenflow[35–37].Nevertheless,thealloyingdegreeof themetalcatalystspreparedinthiswayisgenerallyrelativelylow. Itiswell-knownthatthecatalyticperformanceofthealloyed cat-alystsstronglydependsontheirmetalparticlesizeandalloying 0926-3373/$–seefrontmatter © 2011 Elsevier B.V. All rights reserved.

82 S.-H.Liuetal./AppliedCatalysisB:Environmental108–109 (2011) 81–89

degree.Thus,therearestillhighdemandstoexplorenew prepara-tionmethodstoattainthealloyedparticleswithauniformlysmall sizeandahighalloyingdegree.

Here we report a novel procedure for the synthesis of an orderedmesoporouscarbonwithwell-dispersedandhighly alloy-ingPtConanoparticles(denotedasPt100−xCox/OMC,x=0,30and 50)based on thesimultaneous pyrolysis offurfuryl alcohol, Pt andCoprecursorswithdifferentPt/Coatomicratiosina meso-poroussilica,namelySBA-15.AmongPt100−xCox/OMCsamples,the Pt50Co50/OMCcatalystssofabricatedpossessnotonlyhigh activ-itybutalsodurability(tolerancetomethanolcrossover)favorable forORRandhenceshouldrenderfuturepracticalapplicationsas supportedcathodicelectrocatalystsforDMFCsandPEMFCs.

2. Experimentalmethod

2.1. Preparationofcatalysts

The template (SBA-15 mesoporous silica) was synthesized accordingtotheprocedurereportedintheliterature[38]. Subse-quentdirectreplicationofSBA-15templateintomono(Pt100/OMC) andbifunctional(Pt100_−xCox/OMC)sampleswithvariousrelative metalloadingwasaccomplishedbythestepwisemethodasbelow. Typically,ca.0.5gofcalcinedSBA-15wasfirstdehydratedat673K for4hundervacuumwhilevariousamountsofplatinum acetylace-tonate(Pt(CH(COCH3)2)2),denotedasPt(acac)2(98%,Acros)and cobaltacetylacetonate(Co(CH(COCH3)2)2),denotedasCo(acac)2 (99%,Acros)wereco-dispersedinfurfurylalcohol(FA;98%,Acros) andtrimethylbenzene(TMB;98%,Acros)underultrasonication.In addition,theFAandTMBsolutionwerepolymerizedbythe addi-tionof oxalicacid(98%,Acros). Themixturesolution wasthen infiltratedinSBA-15atroomtemperature(298K)byanincipient wetnessimpregnationmethod,followedbypolymerizationfirstat 333Kthenat353Keachfor12hinair.Theresultantcompositewas increasedthetemperatureto1073Kwithaheatingrateof1K/min andeventuallymaintainedatthesametemperaturefor3hunder vacuum.Finally,theresultantblackpowderswereleachedwithHF (1wt.%)aqueoussolutionforatleast24htoremovethesilica tem-plate,washedwithdistilledwaterandalcohol,thendriedat373K toobtainthePt100/OMCandPt100−xCox/OMCsamples.

2.2. Characterizationmethods

X-raydiffraction(XRD)patternsofallsampleswererecorded ona PANalytical(X’PertPRO)instrumentusingCu-K_␣radiation (=0.1541nm).Thecompositionsofcatalystsweremeasuredby energydispersiveX-rayanalysis(EDX,JEOLJEM-2100F).X-ray pho-toelectronspectra(XPS)wereacquiredthroughanenergyanalyzer witha constant pass energy of 20eV followed byirradiating a samplepellet(6mmin diameter)witha monochromatic Al-K␣ (1486.6eV)X-rayunderultra-highvacuumcondition(10−10Torr). Nitrogen adsorption isotherms were measured at 77K on a MicromeriticsASAP2020analyzer.Forthehigh-resolution trans-missionelectronmicroscopy(TEM),sampleswerefirstsuspended inacetone(99.9vol.%)byultrasonication,followedbydeposition ofthesuspensiononalacey carbongrid,then theTEMimages wereobtainedatroomtemperatureusinganelectronmicroscope (JEOLJEM-2100F)operatingatanelectronaccelerationvoltageof 200kV.ThePtLIII-edgeandCoK-edgeXANESandEXAFSspectraof thePt100−xCox/OMCsampleswerecollectedattheWiggler beam-lines17CoftheNationalSynchrotronRadiationResearchCenter (NSRRC)inTaiwan.ASi(111)double-crystalmonochromatorwas usedforselectionofenergywitharesolutionof2_×10−4eV/eV. Twogas-filledionizationchamberswereusedinseriestomeasure theintensitiesoftheincidentbeam(Io)andthebeamtransmitted

throughthesample(It)onareferencefoil(Ir).Athirdion cham-berwasusedinconjunctionwithareferencesample(Ptfoilor Copowder forPtLIII-edge orCoK-edgemeasurements, respec-tively).Standardprocedureswereemployedtoanalyzethespectra acquiredbyX-rayabsorptionspectroscopy(XAS).EachEXAFS func-tion()wasobtainedbysubtractingthepost-edgebackground fromtheoverallabsorptionandthennormalizedwithrespectto theedgejumpstep.Subsequently,k2_{-weighted(k)spectrainthe} k-space,rangingrespectivelyfrom3.6to13.8 ˚A−1forPtLIII-edge andfrom3.6to11.5 ˚A−1forCoK-edge,wereFouriertransformed (FT)tother-spacetoseparatetheEXAFScontributionsfrom differ-entcoordinationshells.Anonlinearleast-squaresalgorithmwas appliedtofit(withoutphasecorrection)theEXAFSspectrainthe r-spacebetween1.5and3.2 ˚AforPtandbetween1.3and3.1 ˚Afor Co,respectively.ThePt–Coreferencefilewasdeterminedby the-oreticalcalculation.Allcomputerprogramswereimplementedin anUWXAFS3.0package [39]withthebackscatteringamplitude andthephaseshiftforthespecificatompairsbeingtheoretically calculatedusingtheFEFF7code[40].

2.3. Electrochemicalmeasurements

Theelectrocatalyticmeasurementswereperformedinasingle compartmentglasscellwithastandardthree-electrode configu-ration. Aglassycarbon electrode witha diameterof 5mm was usedasaworkingelectrodeandasaturatedAg/AgClelectrodeand aplatinumwirewereusedasreferenceandcounterelectrodes, respectively.Theglassycarbonthin-filmelectrodewasprepared bythefollowingsteps:firstly,ca.5mgofPt/PtCo-loadedcarbon samplewasaddedinto2.5mLdeionizedwater,followedby ultra-sonictreatmentfor0.5h.Then,ca.20␮Loftheresultantsuspension mixturewaswithdrawnandinjectedontotheglassycarbon elec-trode,followedbydryinginairat333Kfor1h.Finally,20␮Lof5% Nafion®_{(DuPont)solutionwasaddedasabinderunderN}

2 environ-ment.Electrocatalyticactivitymeasurementsofvarioussamples and a commercialJohnson-Matthey Pt/Csample (20wt.%Pt on VulcanXC-72,denotedasJM-Pt/C)wereperformedona galvanos-tat/potentiostat(CHIInstruments,727D).Cyclicvoltammetry(CV) experimentsweredonetocleanandactivatetheelectrode sur-face.PriortoeachCVmeasurement,theelectrolyticsolutionwas purgedwithhigh-purityN2 (99.9%)for atleast0.5htoremove thedissolvedoxygen,subsequentlytheexperimentwasconducted between−0.2and1.0Vvs.Ag/AgClunderpurgingN2 condition. ORRwasevaluatedbyalinearsweepvoltammetry(LSV)technique. The0.5MH2SO4electrolytewassaturatedwithultrahighpurity oxygenfor atleast0.5h.Thepolarizationcurves wereobtained between−0.1and0.8Vvs.Ag/AgClatascanningrateof5mV/s andarotatingspeedof1600rpmunderroomtemperature con-dition. Accelerated durability tests (ADT) of Pt50Co50/OMC and JM-Pt/Csampleswereperformedbycyclingtheelectrodepotential between−0.2and1.0Vvs.Ag/AgClatascanningrateof50mV/s for1000cyclesinanitrogen-saturatedatmosphereoveraperiod of13h.

Single celltests were carried out in a 5cm2 _{cross-sectional} catalyst area(2.25cm×2.25cm). Variouscatalyst inks contain-ing Nafion ionomer (DuPont, 5wt.% solution), electrocatalysts, and isopropyl alcohol were applied oncarbon cloth substrates (APWOS1002).Ptloadingwas0.2and0.4mg/cm2 _fortheanode andcathode,respectively.ANafion212membrane(DuPont)was insertedbetweentheanodeandcathodelayersbyhotpressingat 398Kunderapressureof50kg/cm2_{.Thehydrogenandairwere} usedas thefuels withtheflow rates of 300and 2000mL/min, respectively.Thegaseswerehumidifiedatarelativehumidityof 100%.Thecellswereoperatedat353Kandtheoperatingpressure wasatmospheric.Priortothemeasurement,cellswereactivated

S.-H.Liuetal./AppliedCatalysisB:Environmental108–109 (2011) 81–89 83

1

2

3

4

5

6

7

8

2θ (deg

ree)

(a)

Pt

/OMC

Pt

Co

/OMC

Pt

Co

/OMC

OMC

20

30

40

50

60

70

80

90

2θ (deg

ree)

(b)

Pt

/OMC

Pt

Co

/OMC

Pt

Co

/OMC

OMC

Fig.1. (a)Small-and(b)large-anglepowderedXRDpatternsofvarioussamples.

bypolarizationataconstantcurrentuntilstableperformancewas obtained.

3. Resultsanddiscussion

AsdisplayedinFig.1a,thesmall-angleXRDpatternofOMC sam-plesshowamain(100)diffractionpeakat2=∼1.0◦_andtwoweak features (110)and (200)at 2=∼1.6◦ _{and ∼1.8}◦_{,respectively,} indicatingtheexistenceof amesoporousstructurewitha long-rangeorderandatwo-dimensionalhexagonalsymmetrysimilar tothatoftheorderedmesoporouscarbon(CMK-3)[41].However, upon incorporationof Ptand Pt/CointothepristineOMC sam-ple,lessresolvedhigherorder(110)and(200)diffractionpeaks wereobserved,implyingthatthepresenceofmetalshasresulted inlackingalong-rangestructuralorderingofmesopores.Also,it isnotedthatvariationsinPt/Coloadingindeedhavesome influ-enceontheoverallstructureand thephysicalpropertiesofthe catalystsinceadecreaseintheunitcellparameter(a)ofthe2-D hexagonallatticeofPt_100−xCox/OMCsampleswasfound(Table1) comparedtothepristineOMC.Inaddition,thelarge-angleXRD patternof Pt100/OMC (seeFig.1b)showsdistinct (111),(200), (220),and(311)diffractionpeaksat2=39.8◦,46.2◦,67.8◦,and 81.3◦,respectively,indicatingthatthePtmetalparticlehasa

face-centeredcubic(fcc)structure.UponincreasingtheColoading,all thePt100_−xCox/OMCsamplesexhibitdiffractionpatternsthatare similartothatofthePt100/OMCcatalyst,exceptthatthe2 val-uesoftheirmainpeakwasslightlyshiftedtowardahighervalue, suggestingareductionoftheirrespectivelatticeconstants(afcc; seeTable1)[42]andhenceanincreasingextentofthealloying ofthePtandCoparticlesintheOMCsupports[43].Theaverage metalparticlesizes(Dp)ofPt100_−xCox/OMCsamplesdeducedfrom theScherrerformulaareshowninTable1.Increasingsubstitution ofPtbytheCoatomsinPt_100−xCox/OMCsampleswasfoundto haveinsignificantimpactsonthePtCoalloyedparticlesize. More-over,theXRDpatternobservedforPt100_−xCox/OMCsamplesdid notmatchwiththetypicalhexagonalclose-packed(hcp)structure ofpureCoandCooxide,indicatingthatsomecobaltoxidesmaybe presentasanamorphousstateinPt100_−xCox/OMC.Nonetheless,the absenceofthehcpphaseofCoinPt_100−xCox/OMCsamplesconfirms theformationofPtCoalloysinthesupportedcatalysts.

All the N2 adsorption/desorption curves of OMC and Pt100−xCox/OMC samples showed the typical type-IV isotherm withawell-definedhysteresisloop(seeFig.S1ainthe support-inginformation).Consequently,theBETsurfacearea,porevolume, andporesizedistribution(byBJHmethod)ofvarioussamplescan bederived.AscanbesummarizedinTable1,allsupportedcatalyst sampleswerefoundtopossessahighsurfacearea(∼1000m2_/g) and auniformporesizedistribution(2.6–2.9nm; Fig.S1b).The averageporesizeobservedforvariousPt_100−xCox/OMC samples iscomparabletothepristineOMCwhichisduetothefactthat they arise from the skeleton of the SBA-15 mesoporous silica template,whichwassubsequentlyremovedbyHFwashingafter carbonization.Nevertheless,thesurfaceareasandporevolumesof Pt100_−xCox/OMCsamplesaresmallerthanthoseofpristineOMC, whichmaybeowingtopartialexposureofthePtConanoparticles (aftersilicatemplateremoval)andthehigherdensityofthealloyed metalthanOMC.

The structure and metal dispersionof Pt100−xCox/OMC sam-pleswerefurtherconfirmedbyTEMmeasurements.Fig.2presents theTEMimagesforthreePt100_−xCox/OMCsamples,togetherwith theparticlesizedistributionhistograms.ThePt/PtCoparticlesize distributionwasestimatedbymeasuringthesizeofatleastone hundredrandomlychosenparticlesinthemagnifiedTEMimages. AsshowninFig.2,thePt100/OMC,Pt70Co30/OMCandPt50Co50/OMC samplesexhibitanorderedarrayofmesoporouscarbonnanorods dispersedwithuniformPtandPtConanoparticleswithaverage sizesof2.2,2.5and3.0nm,respectively.Theaverageparticlesizes calculatedbyTEMareinagreementwiththoseobtainedbyXRD,as showninTable1.Thehistogramsofmetalparticlesizedistribution shown in Fig.2alsoindicate a uniformand narrower distribu-tionofPtandPtCo particlemorphology.Thisindicatesthat the well-definedmesostructuresofPt_100−xCox/OMCsamplesfacilitate aconfinedeffectforthePt/PtConanoparticles,mostlikelydueto therestrictedOstwaldripeningand/ormigration-coalescenceof Ptparticlesinthesestructures,asobservedforthesystemsofAu nanoparticlesincorporatedmesoporoussilicas[44,45].

The structuralfeatures and thechemical oxidation states of C, O, Pt and Co, in the Pt_100−xCox/OMC samples with various Pt/Coatomic ratioswere characterizedbyXPS analysis.Ascan beseen inFig.S2a,alltheC 1sXPS spectraobtainedfromthe Pt100−xCox/OMC samples revealeda broad peak centered at ca. 284.5eV,whichcouldbeattributedtothesp2 _{graphiticcarbon} species.Comparedtographitized carbonblack(0.82eV)[46,47], asimilarfull-widthhalf-maximum(FWHM)linewidthofca.1.3eV wasobservedinalltheC1sspectraforPt_100−xCox/OMCsamples, suggesting thepresenceofless orderedgraphene layers.Again, thismaybeattributedtoperturbationoftheembeddedPtorPtCo alloysintheOMCframeworks.Similarly,alltheO1sspectra(see Fig.S2binthesupportinginformation)showingsinglebroadpeaks

S.-H.Liuetal./AppliedCatalysisB:Environmental108–109 (2011) 81–89 89 relatedareas,forexample,aselectrocatalystsforDMFCsand

PEM-FCs.

Acknowledgements

ThesupportofthisworkbytheNationalScienceCouncil,Taiwan (NSC 99-2221-E-151-044-MY2 and 99-2221-E-151-023-MY2) is gratefullyacknowledged.TheauthorswishtothankMr.Ding-Goa LiuandDr.Jyh-FuLee(NationalSynchrotronRadiationResearch Center,Taiwan)fortheirassistanceandhelpfuldiscussionsonthe X-rayabsorptionmeasurements.

AppendixA. Supplementarydata

Supplementarydataassociatedwiththisarticlecanbefound,in theonlineversion,atdoi:10.1016/j.apcatb.2011.08.011.

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