PerformanceCharacterizationofCu/ZeoliteandFe/ZeoliteCatalystsfortheSelectiveCatalyticReductionofNOxGiovanniCavataio,JamesGirard,JosephPatterson,CliffordMontreuil,YisunCheng,andChristineLambertFordMotorCompany,ResearchandInnovationCenter2101VillageRoad,Dearborn,MI48124USAIntroductionReductionofnitrogenoxides(NOx)intheleanexhaustgasofdieselenginesisnottrivial.Thewell-knownreactionchemistryofatypicalpreciousmetalthree-waycatalyst(TWC)isnoteffectiveforNOxconversioninleanexhaust.Urea-basedSelectiveCatalyticReduction(SCR)hasthepotentialtomeetU.S.DieselTier2Bin5emissionstandardsforNOxin2010[1].ThereductionofNOxwitheitherammonia(NH3)orureahasbeenusedextensivelyforstationarysourceemissioncontrol.NOxreductionispossibleduetothehighselectivityoftheNH3andNOxreactiontoformelementalN2.UreadecomposestoNH3followingawell-knowntwo-stepdecompositionprocessinvolvingthermolysisandhydrolysis.TheU.S.Tier2Bin5NOxstandardrepresentsachallenging90+%reductionfrompreviousstandards.Thevaryingoperatinganddrivingconditionsoflight-dutyandheavy-dutyvehiclesmakeitnecessarytocustomizecatalystfeaturestotheapplication.Generally,light-dutyapplicationsrequireSCRcatalyststofunctionwellatlowtemperatures(<350°C)whereasheavy-dutyapplicationsrequirefunctionalityathighertemperature(>350°C)andhighspacevelocity.OnemajorthermaldurabilityrequirementofSCRformulationsinvolveswithstandingthehightemperatureprocessofregeneratingparticulatefiltersfromaccumulatedsoot.Onthebasisofperformance,threeimportantparametersfortheselectingtheproperSCRformulationinvolvestheexhausttemperatureprofile,theNO2/NOxratio,andtheamountofNH3storedonthecatalystsurface.Inadditiontothermaldurability,thispaperreviewsadetailedcomparisonbetweenthebasiccatalyticactivityofCu/ZeoliteandFe/Zeoliteformulations,andtheirresistancetopoisonssuchassulfurandhydrocarbons.MaterialsandMethodsFundamentalcatalystactivitydatawereobtainedusingalaboratory-scaleflowreactorsystem.Aroundsamplecoreof1"diameterby1"longwastakenfromawashcoatedCu/ZeoliteandFe/Zeolitemonolithobtainedfromasupplier.Thecatalysttemperaturewasmaintainedwithatubefurnace.Simulateddieselexhaustgaswasflowedthroughthesamplecoreconsistingof350ppmNOx,350ppmNH3,14%O2,5%H2O,5%CO2,andbalanceN2.Thetotalgasflowratewas6.44litersperminutesresultinginagashourlyspacevelocity(GHSV)equalto30,000/hr.ANicoletFTIRinstrumentwithaheatsamplecellwasusedattheoutletofthereactortomeasureNO,NO2,N2O,NH3,H2O,andCO2concentrations.Beforeevaluation,allcatalystswerehydrothermallyagedat670°Cfor64hoursinthepresenceof5%H2O,14%O2,5%CO2,andbalanceN2.ResultsandDiscussionPerformanceplotsinFigure1showtheactivetemperaturerangeforCu,Fe,andvanadiumbasedSCRcatalysts,theimpactfromhightemperature1houraging,theinfluenceofstoredNH3onlowtemperatureactivity,andtheNOxconversionsensitivitytoexhaustNO2fraction.Thefollowingmainconclusionsemergefromthisstudy:a)Basemetal/zeoliteSCRformulationswerefoundtobemorethermallystablethanvanadiumbasedcatalysts,b)Thenever-to-exceedtemperatureforCu/Zeolitewasestablishedtobe775°C,whileforFe/Zeoliteitwashigher(925°C),c)NOxconversionforbothCuandFe/ZeoliteSCRcatalystsareastrongfunctionsofNH3storagecapacity,d)lowtemperatureNOxperformanceofbothCu/ZeoliteandFe/ZeolitewasimprovedwiththeincreaseinNO2fraction.However,theFeformulationwasmoresensitivetotheNO2/NOxratio.Figure1.a)NOxconversionoverCu,Fe,andVanadiumbasedSCRformulations,b)Cu/ZeoliteNOxconversionafter1houragingfrom750°C–950°C,c)Cu/ZeoliteNOxconversionasfunctionofexposedNH3,d)Cu/ZeoliteNOxconversionversusNO2/NOxratio.Note:SCRcatalystsagedat670°Cfor64hoursina)–d).SignificanceThetransientdrivingnatureoflight-dutyandheavy-dutyvehiclesmakesitnecessarytounderstandthekeySCRfunctionsanddurabilitylimitationsundervariousoperatingconditions.ResultsinthisstudyhighlightmaindifferencesbetweenCu/ZeoliteandFe/ZeoliteSCRformulationsandprovidesinsightintokeyparametersimpactingNOxconversion.References1.ChristineK.Lambert,PaulM.Liang,andRobertH.Hammerle,UsingDieselAftertreatmentModelstoGuideSystemDesignforTierIIEmissionStandards,SAE2002-01-1868.0102030405060708090100100150200250300350400450500550600InletGasTemperature(oC)

N O x C o n v e r s i o n ( % )950°C900°C800°C750°C64hrsat670°C0102030405060708090100100150200250300350400450500550600InletGasTemperature(oC)

N O x C o n v e r s i o n ( % )0%NO2/NOx20%NO2/NOx50%NO2/NOx80%NO2/NOx0102030405060708090100100150200250300350400450500550600InletGasTemperature(oC)

N O x C o n v e r s i o n ( % )Cu/ZeoliteSCRFe/ZeoliteSCRVanadiumBasedSCR0102030405060708090100100150200250300350400450500550600InletGasTemperature(oC)

N O x C o n v e r s i o n ( % )SteadyState1.00g/LNH30.75g/LNH30.50g/LNH30.25g/LNH30.10g/LNH3a)b)c)d)