蒋世伟2111Hydrogen generation by hydrolysis of sodium borohydride on CoBSiO2 catalyst.docx
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蒋世伟2111Hydrogen generation by hydrolysis of sodium borohydride on CoBSiO2 catalyst.docx
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蒋世伟2111HydrogengenerationbyhydrolysisofsodiumborohydrideonCoBSiO2catalyst
HydrogengenerationbyhydrolysisofsodiumborohydrideonCoB/SiO2catalyst
Chin-ChangYanga, 1,Miao-ShengChena,Yu-WenChenb,
a
GraduateInstituteofManagementSciences,DepartmentofBusinessAdministration,NanhuaUniversity,ChiayiCounty622,Taiwan
b
DepartmentofChemicalandMaterialsEngineering,NationalCentralUniversity,Jhong-Li32050,Taiwan
Received25July2010;revised29October2010;Accepted1November2010.Availableonline4December2010.
Abstract
Generationofhydrogenbyhydrolysisofalkalimetalhydrideshasattractedattention.UnsupportedCoBcatalystdemonstratedhighactivityforthecatalytichydrolysisofNaBH4solution.However,unsupportedCoBnanoparticleswereeasytoaggregateanddifficulttoreuse.Toovercomethesedrawbacks,CoB/SiO2waspreparedandtestedforthisreaction.Cobalt(II)acetateprecursorwasloadedontotheSiO2supportbyincipient-wetnessimpregnationmethod.Afterdryingat100 °C,Cocationsweredepositedonthesupport.Thedriedsamplewasthendispersedinmethanol/watersolutionandthenfullyreducedbyNaBH4atroomtemperature.ThecatalystwascharacterizedbyN2sorption,XRDandXPS.TheresultsindicatedthattheCoBonSiO2possessedamorphousstructure.BandCoexistedbothinelementalandoxidizedstates.SiO2notonlyaffectedthesurfacecompositionsofCoB,butalsoaffectedtheelectronicstatesofCoandB.B0coulddonatepartialelectrontoCo0.ThestructureeffectcausedbytheSiO2supporthelpedtopreventCoBnanoclusterfromaggregationandthereforetheactivityincreasedsignificantlyonhydrolysisofalkalineNaBH4solution.TheCoB/SiO2catalystshowedmuchhigheractivitythantheunsupportedCoBcatalyst.At298 K,thehydrogengenerationrateonCoB/SiO2catalystwas4timesmorethanthatontheunsupportedCoBcatalyst.Thehydrogengenerationratewasashighas10,586 mL min−1 g−1catalystat298 K.CoB/SiO2isaverypromisingcatalystforthisreaction.
Keywords:
CoB;SiO2support;Hydrogengeneration;Sodiumborohydride;Hydrolysis
ArticleOutline
1.Introduction
2.Experimental
2.1.Chemicals
2.2.Catalystspreparation
2.3.Catalystcharacterization
2.4.Catalyticactivity
3.Resultsanddiscussion
XRD
3.2.N2sorption
3.3.TEM3.3透射电子显微镜
3.4.XPS
3.5.HydrolysisreactionofNaBH4
4.Conclusions
Acknowledgments
References
1.Introduction
Hydrogenisconsideredasapromisingalternativeenergyinthenextgenerationduetoitsnon-toxic,environmental-friendly,avarietyofsourcestoobtainandtherelativelylowcost.Manyresearchershavedevotedtotheworkofhydrogengeneration.Therearemanymethodstoproducehydrogen.Amongthem,themostattractiveoneisthegenerationofhydrogenbyhydrolysisofalkalinemetalhydrides.Sodiumborohydrideshowsthesuperiorqualitiesforprovidingasafeandpracticalwaytogeneratehydrogen.GeneratinghydrogenbyhydrolyzingNaBH4aqueoussolutionhasmanyadvantages[1]highhydrogenstoragecapacity(10.7 wt.%);stabilityunderalkalinestate;hydrogengenerationoccursonlyinthepresenceofselectedcatalysts;thereactionbyproduct,sodiumborate(NaBO2),canberecycled,andtheotherbyproductingasstreamiswater;reactionrateiseasytocontrol;hydrogencanbegeneratedevenat273 K,andthehydrogengeneratedhashighpurity.
Atroomtemperatureandwithoutacatalyst,onlyanegligibleamountofhydrogenisreleasedinaqueoussolutionofNaBH4,especiallyfortheinstanceforaddingalkaliasNaOHintothesolution.Byaddingasuitablecatalyst,thehydrogengenerationrateissignificantlyenhanced.Thehydrolysisreactionisexothermicandproceedsaccordingtothefollowingequation[2]:
(1)
Recently,moreandmoreeffortshavebeenmadeonresearchofthecatalystsonhydrolysisofNaBH4.Manydifferentmetalsoralloyshavebeenreportedtoshowgoodcatalyticactivitiesonthishydrolysisreaction.Schlesingeret al.[3]reportedthatcolloidalplatinum,platinizedasbestos,platinumoxidationcatalyst,copper-chromicoxide,activatedcharcoalsupportedthefactthatRaneynickelcouldacceleratethehydrolysisreactionapparently.NoblemetalsincludingRu[4]andPt[5]arewidelyemployedascatalystsduetothehighcatalyticactivities,butthescarcityandthehighcostofthesenoblemetalslimitedtheirapplication.Appropriatesubstitutematerialsforthenoblemetalsbasedcatalystsarekeenlyexpected.
Sincethe1980s,metalborideamorphousalloycatalystspreparedbyreductionofmetalsaltswithborohydridehaveattractedmuchattentionincatalyzingthehydrolysisreactionowingtotheiruniqueproperties,suchastheisotropicstructureandthehighconcentrationofthecoordinationofunsaturatedsites.IthasbeenreportedthatcobaltboridecatalystshowedthecompetitiveactivitieswithPtandRubasedcatalysts.Inourlab,amorphousCoB,NiBcatalystshavebeenusedforthehydrolysisreaction.TheresultsshowedthatCoBcatalystillustratedhigheractivitythanNiBcatalystinNaBH4hydrolysisreaction,whichisingoodaccordancewiththeliterature.Daiet al.[6]synthesizednickelfoam-supportedcobalt–boroncatalystsbyhydrogenationwithsodiumborohydride.ThesecatalystsshowedsimilarcatalyticactivitiesasRuandPt-basedcatalysts.Thestabilityofthecatalystisoneofthemostimportantfactorstolimittheapplicationofthesekindofcatalysts.Itwasimprovedapparentlyonthesupportedcatalysts.Thereareseveralpapersonthesethatsupporthydrogenproduction.Xuet al.[7]preparedγ-alumina-supportedCocatalysts,whichexhibitedexcellentactivitiesonhydrolysisofNaBH4.Simaginaet al.[8]foundthattheRh/TiO2catalystsshowedthebestperformanceforNaBH4hydrolysis.Demirciet al.[9]studiedtheRu-promotedsulphatedzirconia(ZrO2-SO42-)catalystandinvestigatedthekineticsoftheNaBH4hydrolysis.UnsupportedCoBcatalystdemonstratedhighactivityforthecatalytichydrolysisofNaBH4solution.However,unsupportedCoBnanoparticleswereeasytoaggregateanddifficulttoreuse.
Inthisstudy,SiO2-supportedCoBcatalystwasprepared.UnsupportedCoBwaspreparedforcomparison.ItisknownthatCoBhasaverystronginteractionwithAl2O3.SiO2isinertandwaschosenasthesupportinthisstudy.ThesampleswerecharacterizedbyN2sorption,XRD,TEMandXPS.Thecatalystsweretestedforhydrogengenerationbyhydrolysisofsodiumborohydrideinabatchreactor.
2.Experimental
2.1.Chemicals
Cobaltacetatetetrahydrate(>98%)wassuppliedbyShowaChemicals(Tokyo,Japan).Sodiumborohydride(>98.5%)waspurchasedfromSigma–Aldrich.SiO2wasobtainedbyHayashiPureChemicalIndustries.Methanol(>99.9%)wasfromTedia.Doubledistilledwaterwasusedinthepreparation.
2.2.Catalystspreparation
Allthereagentsusedinthisresearchwereanalyticalgradewithoutfurtherpurification.Doubledistilledwaterwasusedforthecatalysts’preparation.CoB/SiO2catalystswerepreparedbyatwo-stepimpregnation-reductionmethod.0.21 g(0.001 mol)Cobalt(II)acetatetetrahydrate(Co(CH3COO)2·4H2O)wasdissolvedindistilledwater.AsuitableamountofSiO2supportwasputintothesolutionundermagneticstirringatroomtemperatureforabout30 min,andthenthematerialwasdriedinanovenat373 Kfor4 h.Thedriedpowderwasputintoaconicalflask.5 mL1 mol L−1sodiumborohydridesolution(methanol–watermixtureassolute,volumeratio = 1:
1)wasaddedintotheconicalflaskinadropwisemannerusingmicro-tubingpumptoreduceCo2+.Thesuspensionwaskeptundermagneticstirring.Thereductionwasconductedundernitrogenstream,whichwasusedasasheltergastopurgeoutoxygeninsolution.TheloadingofCoBwas5 wt.%.Excessamountofborohydride(NaBH4/Co2+molarratio = 3/1)wasusedinordertofullyreducecobaltcations.Theresultantblackparticleswerecentrifugedandwashedwithwaterandmethanolthoroughly.SinceCoBnanoclusteronsupportwaseasilytobeoxidized,itwaskeptinmethanolsolution.
Forcomparison,anunsupportedCoBcatalystwasalsosynthesizedusingsimilarmethodwithoutSiO2support.SinceunsupportedCoBwasveryfineandwouldcatchfireindryformduetorapidoxidationbyair,itwasalsokeptinmethanolforfurtheruse.
2.3.Catalystcharacterization
Themorphology,crystallinestructure,surfacecompositionsandtheelectronicstatesofthecorrespondingelementswerecharacterizedbyvarioustechniques.
N2sorptionisothermsweremeasuredat77 KusingaMicromeriticsASAP2010.Priortotheexperiments,thesamplesweredegassedat393 Kuntilthevacuumpressurewasbelow10−4 Pa.ThespecificsurfaceareasofthesampleswerecalculatedbyBrunauer–Emmett–Teller(BET)methodforrelativepressuresintherangep/p0 = 0.05–0.2.
X-raydiffraction(XRD)patternswereobtainedusingaSiemensD500powderdiffractometer.TheXRDpatternswerecollectedusingCuKαradiation(0.15405 nm)atavoltageandcurrentof40 kVand30 mA,respectively.Thesampleswerescannedintherangeof2θ = 20-70°atascanningrateof0.05°s−1.
Themorphologyofthesampleswasinvestigatedbytransmissionelectronmicroscopy(TEM)usingaJEOLJEM-2000FXIImicroscopeoperatedat160 kV.Thesamplewasdispersedinmethanolandthendippedontoacarboncoatedcoppergrid(300#)(TedPellaInc.,CA,USA),anddriedinvacuumovernight.
XPSspectrawerecollectedusingAlKαradiationatavoltageandcurrentof20 kVand30 mA,respectively.Thebasepressureintheanalyzingchamberwasmaintainedontheorderof10−7 Pa.Thespectrometerwasoperatedat23.5 eVpassenergy.Thesurfacecompositionofthesampleswasdeterminedfromthepeakareasmu
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