co变换催化剂

Y.Maetal./ChemicalEngineeringJournal287(2016)155–161157

60%N2,30%H2Oand10%COatatmosphere.Thecatalystwasini-tiallyheatedinCO/N2atmospherefromroomtemperatureto300°Cbeforesteamwasaddedtothemixture.Thereactionwas rstlyconductedat300°Cforabout5htoobtainstablecatalyticactivitybeforethetemperaturewasdecreasedupto200°Ctostudytheeffectoftemperatureoncatalystactivity.

3.Resultsanddiscussion

3.1.EffectsofpotassiumadditiononactivityforWGSreactionInitially,theRu/ACcatalyststreatedwithdifferentkindsofpotassiumsalts(KOH,K2CO3,KOAc)werestudiedforWGSreac-tions.COconversionsatdifferentreactiontemperatureoverthesecatalystsweredepictedinFig.1.Thereactionwas rstlyconductedat300°Cforabout5htoobtainstablecatalyticactivitybeforethetemperaturewasdecreasedupto200°Ctostudytheeffectoftem-peratureoncatalystactivity.Theactivitytestwaskeptforatleast2hateverytemperature.Noobviouslydeactivationwasfoundatourexperimentcondition.Forallofthecatalysts,paredwiththeparentRu/ACcatalyst,thethreekindsofpotassiumsaltdopedRu/ACcat-alystsshowedanenhancedactivityatthesamereactiontempera-ture.TheseresultsindicatedthatthedopingofalkaliimprovedtheactivityofRu/ACcatalysttowardsWGSreaction.Atlowreactiontemperaturebelow220°C,thesealkalidopedRu/ACcatalystsexhibitedonlyamarginalincreaseinCOconversion.However,thesepotassiumdopedRu/ACcatalystsshowedmuchhighercat-alyticactivitythantheparentRu/ACcatalystbeyond220°C.Itisalsonotedthatthekindofpotassiumsaltshowedagreatin uenceontheactivityofthesealkalidopedRu/ACcatalysts.Ru-K2CO3/ACandRu-KOAc/ACcatalystsdemonstratedmuchhigherWGSactiv-itythanRu-KOH/ACcatalystwiththesameratioofRutoKof1:10.Forinstance,COconversionwasattainedin44.8%withRu-KOH/ACcatalystat300°C,whilethosereached51.8%and56.1%underthesamereactionconditionsfortheRu-KOAc/ACandRu-K2CO3/AC,respectively.Thereasonsofthedifferentcatalyticactivityofthesecatalystswillbeillustratedinthefollowingpart.ThemethaneselectivityovervariouscatalystsasafunctionofreactiontemperaturewasshowninFig.S1.Therewasnomethaneinproductionbelow275°Cforallthetestedcatalystsinour

experimentcondition.Themethaneselectivityincreasedwiththeincreaseofthereactiontemperature,butstilllessthan2%at300°C.TheadditionofKOHdecreasedthemethaneselectivity,forinstance,theSCH4decreasedfrom1.7%to0.7%afterKOHdopingat300°C.Theadditionofpotassiumsalt(K2CO3,KOAc)didn’thaveanobviouseffectontheselectivityofmethaneatourtestedcondition.

ThehighestTOFobtainedfromourcatalystswas76hÀ1for2%Ru-K2CO3/ACat300°C.AsshowninTable1,theWGSactivitywasmuchhigherthanthe3%Ru-K2CO3/SiO2catalystsinRef.[34],whichTOFwas35hÀ1.OurcatalystWGSactivitywasalittlelowerthanthePt/KOH/Al2O3catalyst22,however,theactivitywastestedatmuchhigherpressure(5bar)intheirworkthanours(atmosphere).

Fig.2showstheTEMimagesofRu/ACandK2CO3-Ru/ACcatalysts.AsobservedinFig.2,Runanoparticleswereuniformlydistributedonthesupportsofthetwokindsofcatalysts,andnodistinctaggregationofRunanoparticleswasobservedintheTEMimagesofthetwocatalysts.TheparticlessizedistributionofRunanoparticleswasalsoestimatedbythemeasurementoftheRuparticlesfromthegivenareaoftheTEMimage.TEManalysispro-videddirectinformationonthesizeoftheRunanoparticlesintherangefrom1to5nm,andtheaverageofparticlesizewasesti-matedtobe2.4nm.AfterdopingwithK2CO3,theaverageparticlesizeofRunanoparticlesincreasedto3.2nmforK2CO3-Ru/ACcat-alyst,indicatingthedopingofK2CO3ontheactivecarboncausedthegrowthofRunanoparticles.Similarphenomenonwasalsoobservedbyotherresearchers[22].Kuscheandco-workersfoundthatthesizeofPtnanoparticlesinPt/Al2O3catalystincreasedfrom3.2nmto4.4nmafterdopingAl2O3withKOH.Xiongetal.foundalkalipromoters(alkali:Li,Na,KorCs)alsoledtoanincreaseincrystallitesizeoftheironoxideinFe/CNTcatalyst[23].ThepossiblereasonfortheslightincreaseoftheRunanoparticlessizewasthattheas-preparedRu/ACcatalystafterthedopingofK2CO3wassubjectedtobeheatedat160°C,whichresultedinthemove-mentofRunanoparticles,leadingtoagrowingsizeofRunanoparticles.

BETmeasurementswereusedtocharacterizethetexturestruc-tureofsupportandcatalysts,andtheresultsareshowninTable2andFig.S2.TheaverageporediameteroftheRu/ACandK2CO3-Ru/ACcatalystswasclosetothesupportactivecarbon.However,thesurfaceareaandporevolumedecreasedofthetwocatalystsweremuchlowerthanthesupport,indicatingRuorpotassiumsaltentersintotheporecanalofactivatedcarbon.Interesting,theBETsurfaceandporevolumeoftheK2CO3-Ru/ACcatalystshowedaveryslightdecreaseascomparedwithRu/ACcatalyst,whichfur-therindicatedthatthepotassiumsalt(K2CO3)wasdopedonthesurfaceoftheRu/ACcatalyst.

H2-TPRmeasurementsareusedtoprobethenatureandthereducibilityofthecatalysts.TheTPRpro lesoftheRu/ACcatalystandthepotassiumdopedRu/ACcatalystsaredepictedinFig.3.AlthoughRunanoparticlesareinitsmetallicformunderreducingpreparationconditions,theRunanoparticlescanbeoxidizedtohighvalencestatewhenexposedtooxygenduringstorage[24].XPSresultscon rmedmetallicRu(0)andRuO2wereco-existedintheRu/ACandRu-K2CO3/ACcatalysts.Twopeakswereclearlyobservedinthereductionpro leofRu/ACcatalystfrom180to

Table1

WGSactivitycomparisonwithliteratures.CatalystsTOF(hÀ1)ReactionconditionReferences2%Ru-K2CO3/AC76T=300°C,P=atmosphereOurworkPt-KOH/Al2O3

95T=230°C,P=5bar

Ref.[22]3%Ru-K2CO3/SiO2

35

T=250°C,P=atmosphere

Ref.[34]