Activity and selectivity of pure manganese oxides

ApplaudCatalystBEnvuonmentul,3(1994)173-169ElsewerScienceBV,Am&rdam173APCATB64ActivityandselectivityofpuremanganeseoxidesintheselectivecatalyticreductionofnitricoxidewithammoniaF.Kapteljn,L.SmgoredjoandAAndremlDepartmentofChemwalEngmneenng,UnwersttyofAmsterdam,Amsterdam(Netherlands)andJ.A.MouhJnDepartmentofChemrcalEnguwenng,DelftUnwersrtyofTechnology,Delft(Netherlands)(Recewed11June1993,revisedmanuscnptrecewed25August1993)AbstractManganeseoxidesofdfierentcrystallm~ty,oxldatlonstateandspecficsurfaceareahavebeenusedmthet&&wecatalyticreduction(SCR)ofmtncox&wthammomabetween385and575KMnO,appearstoexhibitthehighestactlvltyperunitsurfacearea,followedbyMnsOs,Mn,O,,Mn,O,andMnO,mthatorderThisSCRactlvltycorrelatesw&htheonsetofreductionmtemperature-pro-grammedreduction(TPR)expenmenta,mtllcatmgarelationbetweentheSCRprocessandactivesurfaceoxygenMn,OaIIIpreferredmSCRsmceltaselectlntytowardsmtrogenformationdurmgthmprocessItheh&e&InallcasestheselectlvltydecreasesanthmcreasmgtemperatureTheoxldatlonstateofthemanganese,thecrystalhmtyandthespsclficsurfaceareaaredeclswefortheperformanceoftheoxidesThespecficsurfaceareacorrelateawellwrththemtncoxidereductionactlmtyThemtrousox&ongmateafromareactionbetweennltncoxideandammomabelow475Kandfrom0x1-datlonofammomaathqhertemperaturss,provenbyusmglsNHBPartlapatlonofthebulkoxygenofthemanganeseox&ecanbeexcluded,smceTPRrevealsthatthebulkoxldatlonstateremainsun-changeddurmgSCR,exceptforMnO,whichXItransformedmtoMnsO,undertheapphedcondltlonsIntheoxldatlonofammomathedegreeofoxtitlonofthenitrogencontaunngproducta(N2,NzO,NO)mcreaaeswdhmxeasmgtemperatureandwkhmcreasmgoxldatlonstateofthemanganeseAreactionmodelPproposedtoaccountfortheobservedphenomenaKeywordsammonmoxldatlon,manganeseoxides,selectivecatalyticreductionINTRODUCTIONManganeseoxidesarewellknownforthemactmtymoxldatlonreactions[l-4]Thmactmty1sascribedtotheeasebywhichtheyundergochangesmCorrespondencetoDrFKaptqnathispresentaddressDepartmentofChemicalEngmeenng,DelftUnlversltyofTechnology,Julmnalaan136,2628BLDelft,NetherlandsFax([phone]-3373/94/$070001994ElsevlerScienceBVAlInghtareservedSSDZ0926-3373(93)E0034-W174FKapteynetal/ApplCatalB3(1994)173-189oxldatlonstate.Recently,itwasreportedthatsupportedmanganeseoxldesarehighlyactivefortheselectivecatalyticreductionofnitricoxidemthammonia[SCR,eqn(1)1,mthelowtemperaturerangeof389-570K[5,6]4NO+4NH3+02+4N2+6Hz0(1)Thisa&v&yhasbeenreportedbeforeforpureMnOzbyMarkvartandPour[71.Severaloxldesofmanganeseareknown,VIZMnOz,Mn608,Mnz03,MnaOaandMnO,whmhareallstableatamblentconditions.Thethermodynamicstabilityoftheseoxidesmcreasesinthegrvenorderwithincreasingtempera-tureanddecreasingpartialpressure[81,afactwhichcanbemadeuseofwhilepreparmgthedifferentoxidesArecentstudy[51vnthalummasupportedmanganeseoxrde,preparedfromnitrateandfromacetateprecursors,revealedsignificantdifferenceswrthre-specttotheirSCRperformance.Forthesealummasupportedcatalyststwotypesofoxldeswereldentlfied,onewithanaveragestorchiometryofMn203,preparedfrommanganeseace-tate,andtheotherwithastoichiometryapproachmgthatofMnO,,preparedfrommanganesenitrateApartfromnitrogenalsonitrousoxidewasobservedasaproduct,especiallyattemperaturesabove470K(2)Themtrogenselectlvrty,definedmeqn(2))decreasedcontmuouslywrthm-creasingloadingfortheex-acetatecatalyst,whereastheselectivityremamedconstantabove3wt-%Mnloadmgfortheex-mtratecatalystTheresultsofthesestu&es[5,9]mdrcatedthattheSCRactrvrtyandselectlvltywasrelatedtothedegreeof&persionandoxidationstateofthemanganese.Tluspromptedustomvestlgatethebehaviourofthedifferentpuremanganeseoxldesforthelow-temperatureSCR.ThepureoxrdesturnedouttohavegoodactlvltlesforSCR,partlyduetothefactthattheycanbepreparedw&hsign&cantspecificsurfaceareas,whichmakesthemfairlyuniquemodelsystemstomvestlgateSeveralaspects,hketheactivity,theeffectofoxygenontherutrlcoxidecon-versron,theselectlvltytowardsnitrogenformation,theammomaoxldatlon,togethermththecharactenzatlonbytemperature-programmedreduction(TPR)andX-rayhffractlon(XRD)havebeenmvestlgatedmordertoes-tabhshwhethercorrelationsexistwiththeoxldatlonstateofthemanganeseandotherfactorshkethecrystalhmtyandthespecificsurfaceareaEXPERIMENTALGases040vol.-%NO/He,0.40“NH3/He,042vol-%NH,/He,O2(2.6)andHe(46)wereuseddurmgthestudy(UCAR)Theoxygenwasdriedbeforeusewithmolecularsieves(5A,JanssenChlmlca)FKaptevnetal/ApplCatalB3(1994)173-189175catazystsTheunsupportedmanganeseoxides,usedascatalysts,werepreparedm&f-ferentways,usingthree&fferentprecursors:manganeseacetatetetrahydrate(Mn(CHBC02)x*4Hz0,Aldrrch),manganesenitratetetrahydrate(Mn(NOs)2*4Hz0,Merck)andmanganesecarbonate(MnCO,,Aldrich).InTable1thepreparationmethodsaregivenforthemodelcompoundsused,aswellastheirspecificsurfacearea,determinedbynitrogenadsorptionat77K(BETmethod)Theletterbetweenbracketsmdlcateswhethertheprecursorusedwasmanganeseacetate(A),manganesenitrate(N)ormanganesecarbonate(C)MnOz(M),purchasedfromMerck,wasalsousedMn,Oawaspreparedfrommanganesecitrate[121bycalcnungitmoxygenfor8hat666K.ItcouldalsobepreparedbyoxidationofMn304[8,15],butmthiscaset