co变换催化剂

156Y.Maetal./ChemicalEngineeringJournal287(2016)155–161

SomenoblemetalsespeciallyPt,Pd,AuandRucatalystshavebeenusedasalternativesforWGSreactionstocircumventthenamedlimitationsofthecommercialWGScatalysts[7–10].Amongthem,paredwithPtcatalysts,Rucatalystsaremuchcheaper,thustheuseofRucatalystswillbepromisingasaneco-nomicalwayintheindustrialWGSreactions.Infact,homogeneousRucatalystshavebeenusedforWGSreactionsforalongtime,whichcanbeoperatedatlowtemperature,achievinghighequilib-riumconversion.However,WGSreactionoverhomogeneouscata-lystswasmainlycarriedoutunderhighCOpressures(>10bar)ineetal.preparedahomogeneousRu3(CO)12catalystandapplieditinWGSreaction.Aratioof150molofH2producedpermoleofRu3(CO)12over30daysbythissystem[11].Comparedwiththehomogeneouscat-alysts,theRubasedheterogeneousWGScatalystssometimesshowedevenmuchhigherhighcatalyticactivitytowardsWGSreaction[12].Forexample,Shindeetal.[13]preparedhighlyactiveandcokeresistantZr0.93Ru0.05O2catalyst,andapplieditinWGSreaction.Thiscatalystcanafforded99%conversionofCOwith100%H2selectivitybelow290°C.AlackofCOmethanationactivityisattributedtotheionicnatureofRuspecies(Ru4+).

Inrecentyears,someresearchersfoundthatthecatalystactiv-ityofheterogeneouscatalystsoverWGSreactioncouldbeimprovedbythemodi cationofthecatalystsurfacewithalkaliadditives,especiallypotassiumsalts[14–17].Forexample,thecat-alyticactivitiesofseveralkindsofPtcatalystscanbeenhancedbythedopingofNa+,K+,Li+,Rb+,Cs+[16,18].ThemainroleofalkalicationswasclaimedthatitcanweakentheC–Hbondofformate,thusfacilitatingCO2desorptionfromthecatalystsurface[15].ItisalsobelievedthattheintroductionofalkaligeneratednewactivesitessuchasPt–alkali–Ox(OH)yclustertopromotetheWGSreac-tion[18].Wateriseasilydissociatedontheseclusterstoafford–OH,whichisthenreactedwithCOatlowtemperatures.TheactiveenergyofwaterdissociationishigherthanotherstepsinWGSreaction[19].TheformationofPt–alkali–Ox(OH)yclustercanlowertheactiveenergyofwaterdissociationstep.Besidestheimprove-mentofthecatalystactivity,thealkaliadditionwasalsofoundtoimprovethePt@SiO2catalyststabilityincyclicoperations[20].Inourpreviouswork,wehavealsofoundthatthecatalyticactivityofRunanoparticlessupportedonactivecarbon(Ru/AC)wasalsogreatlyenhancedbytheadditionofK2CO3[21].AlthoughthehighercatalyticactivityofRu/ACafterthedopingofK2CO3wasobtained,manyissuesarestilldeservedtostudysuchasthesourceofpotassiumsaltsandthecatalystpreparedmethodsontheactiv-ityofRu/ACcatalyst,aswellasthedeepinsightintothedifferencesinthesecatalyststowardWGSreaction.Herein,wehavepreparedseveralkindsofpotassiumsaltsdopedRu/ACcatalystsbytwodif-ferentmethodsandsystematicallystudiedtheircatalyticactivitiestowardswater–gasshift(WGS)reaction.Moreimportantly,thedifferenceinthecatalyticactivitiesoftheseas-preparedcatalystswasalsotriedtobeunderstoodbyvarioustechnologies.

2.Experimentalsection2.1.Materialsandmethod

30wt.%HNO3waspurchasedfromAladdinChemicalReagentCo.,Ltd(ShanghaiChina).Ethanol(99.5%),K2CO3(99.9%)KOAc(99.9%),KOH(99.9%)andethyleneglycol(EG)werepurchasedfromSinopharmChemicalReagentCo.,Ltd(ShanghaiChina).Ru(NO)(NO3)3(10wt.%)andactivecarbon(AC)werepurchasedfromAlfaAesarchemicalsCo.,Ltd(Shanghai,China).AllofthechemicalreagentsexceptACwereusedasreceivedwithoutfurtherpuri cation.

2.2.Catalystsynthesis

Ru/ACcatalystwaspreparedaccordingtothemethodasdescribedinourpreviousworkwithaslightmodi cation[21].ACwas rstlytreatedwith30wt.%HNO3at90°Cfor4h.ForthepreparationofRu/ACcatalyst,Ru(NO)(NO3)3(0.2g,Ru:10wt.%)inEGsolution(30mL)andAC(1g)waswelladdedanddispersedinthesolutionwithultrasonic-assistancefor30min.Then,themixturewasstirredat160°Cfor6h.Aftercoolingtoroomtemper-ature,themixturewas ltrated,washedseveraltimesbyethanolanddriedatroomtemperature.TheRucontentwas2wt.%inthe-oryandthecatalystwasnamedasRu/AC.

Thealkalidopingcatalystswerepreparedbytwodifferentmethods.Onmethodisthepost-modi cationoftheas-preparedRu/ACcatalysts.Brie y,theRu/ACcatalyst(1.0g)wasimpregnatedwithofacertainamountofaqueousK2CO3for24hatroomtem-perature,anddriedat160°Cfor4h.ThecatalystwasabbreviatedasRu-K2CO3/AC.Methodforthemodi cationoftheRu/ACcatalystwithKOHandKOAcwasthesameasthepreparationofRu-K2CO3/AC,whichwerenamedasRu-KOH/ACandRu-KOAc/AC,respec-tively.Ineachcase,themolratioofRutoKwassettobe1:10.ThecontentofKwas7.6%,7.4%and7.3%forRu-KOH/AC,Ru-K2CO3/ACandRu-KOAc/ACcatalystsdetectedbyICP.ThecontentofRuwasabout1.8%forthethreecatalystsdetectedbyICP.

Ontheotherhand,thesupportwas rsttreatedwithK2CO3beforethedepositionofRunanoparticles.ACwas rstimpreg-natedwiththesameamountofK2CO3,andthendriedat160°Cfor4h.Then,theK2CO3treatedACwasaddedinRu(NO(NO3)3EGsolution,andotherstepswerethesamefortheRu/ACcatalystasdescribedabove.Theas-preparedcatalystwasdenotedasRu/K2CO3-AC.

2.3.Catalystcharacterization

Fouriertransforminfrared(FT-IR)spectraofthesampleswerecollectedusingaNicoletFouriertransforminfraredspectrometer(NEXUS470).Fortheanalyses,thepowdersamplesweremixedwithpotassiumbromide(KBr)powderandpressedintodiskswithoutanypretreatment.

H2-temperatureprocessedreduction(H2-TPR)wasexperimentswerecarriedoutusingAMI-200fromZetonAltamiraCompany.Thesample(50mg)inaquartzreactorwaspurgedwith30ml/minArwhileheatingataramprateof10°/minto150°Candmain-tainingthattemperaturefor1htoremovetracesofwater.Aftercoolingto50°thesamplewasreducedina owof10vol.%H2/Ar(30ml/min)whileheatingfrom50°to500°atarateof10°/min.BETsurfaceareaofthepreparedmaterialswasdeterminedbyphysisorptionofN2at77KbyusingaquantachromeAutosorb-1-C-MSinstrument.ThetotalporevolumesandtheaverageporesizeswereobtainedbyusingtheBarrett–Joyner–Halendamethod.Transmissionelectronmicroscopy(TEM)imagesofthecatalystsampleswereobtainedwithaFEITecnaiG20instrument.Thesam-pleswerepreparedbydirectlysuspendingthecatalystinethanolwithultrasonictreatment.Acoppermicroscopegridcoveredwithperforatedcarbonwasdippedintothesolutionandthendried.TheICP-AESanalysiswasperformedonOptima4300DV,Perkine-ElmertodeterminethecontentofRuandKintheiongelcatalysts.

2.4.TypicalprocedureforWGSreaction

Theactivityofallpreparedcatalystswasevaluatedinacontin-uoustestrigwithon-lineanalysisoftheef uentgasesviaAgilentMicroGC3000AGC.Inatypicalrun,0.4goftheas-preparedcata-lystwasplacedinastainless-steeltubular xedbedreactorandcontactedwithacontinuousgas ow(40mL/min)consistingof