Preparation of La2NiO4-ZSM-5 catalyst and catalytic performa

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http://www.77cn.com.cn/locate/apcata

PreparationofLa2NiO4/ZSM-5catalystandcatalyticperformance

inCO2/CH4reformingtosyngas

W.D.Zhang,B.S.Liu*,C.Zhu,Y.L.Tian

DepartmentofChemistry,SchoolofScience,TianjinUniversity,Tianjin300072,PRChinaReceived4January2005;receivedinrevisedform16May2005;accepted24May2005

Availableonline1July2005

Abstract

ALa2NiO4/ZSM-5catalystwaspreparedbythesol–gelmethod,whichensuredhighdispersionofthemetalandhighactivityintheCO2reformingofmethane.Thecatalyticperformance,catalyststabilityandcokedepositionaswellasthein uenceofthedopedLaspeciesontheNi/ZSM-5catalystwereinvestigated.TheresultsshowthattheconversionofCO2andCH4tosyngasandtheselectivityofCOandH2observedovertheLa2NiO4/ZSM-5catalystweresigni cantlyhigherthanthoseoveraNi/ZSM-5orLa2NiO4/g-Al2O3catalyst.Suchresultssuggestthatthedopingoftherareearthion(La3+)intheZSM-5notenhancedthestabilityofthecatalyst,butalsoincreasedthecatalyticactivity.Inaddition,thecatalystswerecharacterizedbyX-raydiffraction(XRD),BETandthermogravimeticanddifferentialthermalanalysis(TG/DTA).TheX-raydiffraction(XRD)studiesrevealedthatthepreparedcatalyst,La2NiO4,exhibitedatypicalspinelstructureandwasuniformlydispersedbothonthesurfaceandintheZSM-5zeolitechannels.DuringtheCO2/CH4reforming,theLa2O3generatedfromtheLa2NiO4decompositionwastransformedintoLa2O2CO3inanatmosphereofCO2,whichpromotedCO2adsorptiononthesurfaceofthecatalyst.TheTG/DTAresultsindicatethattheamountofcarbondepositionontheLa2NiO4/ZSM-5catalystwassmallerthantheamountontheNi/ZSM-5catalyst,andthattherewereatleasttwokindsofcarbondepositionontheusedLa2NiO4/ZSM-5catalyst.However,onlyonekindofcarbondepositionexistedontheusedNi/ZSM-5catalyst.#2005ElsevierB.V.Allrightsreserved.

Keywords:La2NiO4/ZSM-5catalyst;DispersionofNi;Cokeformation;Sol–geltechnique;CO2/CH4reforming

1.Introduction

Inrecentyears,therehasbeenanincreasinginterestinthecatalytictransformationofcarbondioxide,agreenhousegas,intomoreusefulorvaluablechemicals;especially,importantistheCO2reformingofmethane,thecheapestcarbon-containingfeedstock,intosynthesisgas(i.e.CO+H2),whichcanbeusedinchemicalenergytransmis-sionsystemsorutilizedintheFisher–Tropschsynthesistoproduceliquidhydrocarbons[1,2].TheCO2/CH4reforminghasbeenstudiedovernumeroussupportedmetalcatalysts,suchasNi-based[3–7]andnoblemetalcatalysts[8–10].However,whenoneconsidersthehighcostandlimitedavailabilityofnoblemetal,itismorepractical,fromtheindustrialstandpoint,todevelopNi-basedcatalyststhatnot

*Correspondingauthor.Tel.:+862227892471;fax:+862227403475.E-mailaddress:bingsiliu@http://www.77cn.com.cn(B.S.Liu).

0926-860X/$–seefrontmatter#2005ElsevierB.V.Allrightsreserved.doi:10.1016/j.apcata.2005.05.018

onlypossesshighstabilityandactivity,butalsoareabletoresistcarbondepositionduringthereaction.AnumberofstudiesonCO2/CH4reformingoversupportedNi[3,11,12]catalystshaveindicatedthatmetal-dopedsupports,suchasLa-dopedNi/Al2O3[3]andNi/yttria-dopedceriacatalyst[11]playanimportantroleinthecatalyticactivityandinthecokingresistanceofthecatalysts.TheeffectoflanthanumdopedsupportsonnickelcatalyststosuppresscarbondepositionhasbeenattributedtothedispersionofthedopedspeciesontheNisurface,whichsegregatestheactivecentersatthemetal-supportinterfaces[13,14].Althoughtheeffectofadopedsupportonnickelcatalystshasbeenrecognized,therehavebeenveryfewreportsofCO2/CH4reformingoverrareearth-dopedNi/ZSM-5catalysts.Becauseofitslargespeci csurfaceareaanditswell-de nedstructure,ZSM-5isanidealcandidateforacatalystsupport,sinceitallowsahighdispersionofactivemetalonthesurface.Metalatomsonthesurfaceofthesupportare

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W.D.Zhangetal./AppliedCatalysisA:General292(2005)138–143139

segregatedintotinyislands.Thedrivingforceforthisistominimizethesurfacefreeenergyandtomaximizespreadingonthesupport.

Previously,wereportedthecatalyticperformanceofLa2NiO4/g-Al2O3[3,15]andLa2NiO4[6]catalystsforCO2/CH4reforming.ItwasfoundthattheLa2NiO4catalystexhibitedexcellentcatalyticactivityandstability.However,thespeci csurfaceareaofthesecatalystsdidnotmeettherequirementsforindustrialization.TheaimofourpresentworkwastoinvestigatetheactivityandstabilityofLa2NiO4/ZSM-5andNi/ZSM-5catalystsbyintroducingZSM-5zeolitefortheCO2/CH4reformingtosyngas.Inaddition,theeffectofLadopingontheNi/ZSM-5catalystaswellastheoptimalreactionconditions(gashourspacevelocity(GHSV)andreactiontemperature)wereinvestigated.Thermogravimetry(TG)wasusedtostudythenatureofthecarbondepositiononLa2NiO4/ZSM-5(sol–gel)andNi/ZSM-5(imp)catalystsaftertheCO2/CH4reforming.2.Experimental

2.1.Preparationofthecatalysts

TheLa2NiO4/ZSM-5catalystwaspreparedfromNi(NO3)2Á6H2O,La(NO3)3Á6H2OandZSM-5bymeansofasol–geltechnique.Nickelnitrate(1.22g)andlanthanumnitrate(3.44g)weredissolvedinde-ionizedwater,andthencitricacidandethyleneglycolwereaddedtothesolution.Themolaramountsofcitricacidandethyleneglycolwere1.5timesthatofthetotalmetalions.Thesolutionwasthenheatedto333Kwithconstantstirring,andZSM-5zeolite(2.0g)wasaddedtothesolution.Aftertheremovalofthewaterbyevaporation,atranslucentgreengelwasformed.Next,theco-gelobtainedwasagedanddriedinabeakeratroomtemperature(RT)for3days.Thiswasfollowedbyatwo-stepdecomposition: rstheatingat773Kfor5handthencalcinatingat1073Kinairfor5h,accordingtothemethodreportedintheliterature[6].Thissol–gelgeneratedcatalystwillbereferredtoas‘‘La2NiO4/ZSM-5’’hereafter.Forcomparison,Ni/ZSM-5catalystswerepreparedbyimpregnatingZSM-5supportswithanappropriateamountofanaqueoussolutionofNi(NO3)2Á3H2O(1.22g).Aftertheexcesswaterwasevaporatedat353K,thecatalystwascalcinatedusingthemethodmentionedabove.Inboththesol–gel-andimp-generatedcatalysts,theloadedamountofnickelonthesupportwasca.9%.Allcatalystswere nallypressed,crushedandsievedthrough40–60meshes.

2.2.Characterizationofthecatalyst

ThestructureofthecatalystswascharacterizedusingX-raydiffraction(XRD),BETandthermogravimeticanddifferentialthermalan …… 此处隐藏：18480字，全部文档内容请下载后查看。喜欢就下载吧 ……