AstudyontheactivationofmesoporousPt–MnOx–Al2O3catalystfordehydrogenationofperhydrobenzyltoluene

Abstract

무분별한화석연료의사용으로현재여러환경오염문제들이대두되고있다.따라서지속가능한에너지원에대한관심도가높아지고있는데,재생에너지의가장큰단점은에너지생산량이일정하지못하다는점이다.따라서에너지를저장하는기술이필수적이게되었는데,본연구에서는수소를사용해에너지를저장하는기술에대해다루려고한다. 액상유기수소운반체(LOHC)는수소를화학적으로저장하거나방출할수있는상온에서액체상태인유기물질을통칭한다.물의전기분해를통해생산된수소를수소가부족한LOHC에수소화시킨다.수소화된LOHC상태로저장하거나운송하고,에너지를필요로하는곳에서탈수소화시킨다.방출된수소로물의전기분해의역반응을사용해전기에너지를생산하고,다시수소가부족해진LOHC는재활용이가능해진다. 퍼하이드로벤질톨루엔(Perhydrobenzyltoluene)은LOHC물질중하나로,탈수소화반응에는주로백금-알루미나촉매가사용된다.하지만귀금속을사용하기때문에촉매의가격이비싸다는점과탈수소화효율이낮다는점이문제점으로꼽히고있다. 이전연구에서는용매-결핍침전법(Solvent-DeficientPrecipitation)을통해합성한백금-알루미나촉매에열처리조건을바꾸거나첨가물을넣어활성을향상시켰다.하지만백금단일원자의활성화는아니라는한계점을갖고있었다.하지만본연구에서는망간을프로모터(Promoter)로도입하여백금-망간클러스터(Cluster)를형성하고,환원조건변경을통하여분산도를향상시켰다.이를통해퍼하이드로벤질톨루엔의탈수소화정도(DegreeofDehydrogenation,DoDH)를향상시키고자하였다. 망간의함량과합성법을최적화해본결과,백금과망간의몰비율을1:0.33으로맞춘상태로백금과망간전구체를한번에넣고용매-결핍침전법으로합성한촉매가가장탈수소화정도가높았다.망간을백금-알루미나촉매에첨가한다고해서,기본적인촉매구조가변화하지는않았다.하지만,망간을넣으며금속-지지체상호작용이향상되었고,백금이전자적으로결핍되도록만들었다. 가장탈수소화정도가좋은mPtMnA에대해환원승온속도를조절하며백금-망간클러스터의재건을꾀한결과,94.1%라는DoDHgas결과를얻을수있었다.환원승온속도를조절하며백금을더욱전자적으로결핍되도록만들었고,분산도또한향상시켰다.CO-DRIFT분석으로확인해본결과,mPtMnA_r10은약2070cm−1으로백금-망간클러스터로이루어져있었다. 환원승온속도조절을통한백금-망간클러스터의재건이가능하게된이유는용매-결핍침전법으로합성한알루미나의특이성때문이었다.합성법의특성상보에마이트-유사결함부위(Boehmite-likedefectsites)가존재하게되는데,이를통해상용감마-알루미나보다AlO(OH)나5배위결합알루미늄(Penta-coordinatedAl)이더많이존재하게된다.그중,5배위결합알루미늄은담지된금속이빠르게결합하는것으로알려져있다.하지만백금이없는망간-알루미나의경우양이줄어들지않았다. 즉,백금-알루미나나,백금-망간-알루미나만5배위결합알루미늄의양이줄어들게되는데,환원승온속도가느린경우,높은이동성(mobility)을가지게되어,표면에서많이움직이게된다.반대로환원승온속도가빠른경우,낮은이동성을가지게되어,주변에가까운5배위결합알루미나부위로결합하게된다. 본연구에서는재건방식을재활용실험(Recycletest)을통하여증명하였다.mPtMnA_r10에대해5번의재활용실험을거친결과,반응물의전환율/생성물의선택도/탈수소화정도가모두일정하였다.그리고,환원승온속도가느린상태로환원된촉매를빠른환원승온속도로환원할경우탈수소화정도가향상되는것을알수있었다.CO-DRIFT분석으로확인해본결과,백금단일원자가사라지고백금클러스터로변화하는것을찾아낼수있었다.|Variousenvironmentalpollutionproblemsarecurrentlyemergingduetotherecklessuseoffossilfuels.Therefore,interestinsustainableenergysourcesisincreasing,andthebiggestdisadvantageofrenewableenergyisthattheenergyproductionisnotconstant.Therefore,thetechnologytostoreenergyhasbecomeessential,andthisstudyisgoingtodealwiththetechnologyforstoringenergyusinghydrogen. Liquidorganichydrogencarrier(LOHC)collectivelyreferstoorganicsubstancesthatareliquidatroomtemperaturethatcanchemicallystoreorreleasehydrogen.Hydrogenproducedthroughtheelectrolysisofwaterishydrogenatedinahydrogen-poorLOHC.ItisstoredortransportedashydrogenatedLOHCanddehydrogenatedwhereenergyisrequired.Thereleasedhydrogenisusedtoproduceelectricalenergyusingthereversereactionofwaterelectrolysis,andtheLOHC,whichhasrunoutofhydrogenagain,canberecycled. PerhydrobenzyltolueneisoneoftheLOHCmaterials,andaplatinum-aluminacatalystismainlyusedforthedehydrogenationreaction.However,becauseoftheuseofpreciousmetals,thecostofthecatalystishighandthedehydrogenationefficiencyislow. Inpreviousstudies,theactivitywasimprovedbychangingtheheattreatmentconditionsoraddingadditivestothePt-Al2O3catalystsynthesizedthroughthesolvent-deficientprecipitationmethod.However,ithadalimitationthatitwasnottheactivationofaPtsingleatom.However,inthisstudy,MnwasintroducedasapromotertoformaPt-Mncluster,anddispersionwasimprovedbychangingthereducingconditions.Throughthis,itwasattemptedtoimprovethedegreeofdehydrogenation(DoDH)ofperhydrobenzyltoluene. AsaresultofoptimizingtheMncontentandsynthesismethod,thecatalystsynthesizedbythesolvent-deficientprecipitationmethodinwhichPtandMnprecursorswereaddedatatimewiththePtandMnmolarratioadjustedto1:0.33hadthehighestdegreeofdehydrogenation.TheadditionofMntothePt-Al2O3catalystdidnotchangethebasiccatalyststructure.However,theadditionofMnimprovedthemetal-supportinteractionandmadethePtelectronicallydeficient. FormPtMnA,whichhasthebestdegreeofdehydrogenation,theDoDHgasresultof94.1%wasobtainedasaresultofreconstructingthePt-Mnclusterbycontrollingthereductiontemperatureincreaserate.Controllingthereductiontemperatureincreaseratemadetheplatinummoreelectronicallydeficient,andthedispersionwasalsoimproved.AsaresultofCO-DRIFTanalysis,mPtMnA_r10wasabout2070cm−1andconsistedofPt-Mnclusters. ThereasonthatthePt-Mnclusterwasabletobereconstructedbycontrollingtherateofreductionintemperaturerisewasduetothespecificityofthealuminasynthesizedbythesolvent-depletedprecipitationmethod.Duetothenatureofthesynthesismethod,noehmite-likedefectsitesexist.Throughthis,AlO(OH)orpenta-coordinatedAlismorepresentthancommercialγ-alumina.Amongthem,penta-coordinatedAlisknowntoberapidlybondedtothesupportedmetal.However,inthecaseofPt-freeMn-Al2O3,theamountdidnotdecrease. Thatis,onlyPt-Al2O3andPt-Mn-Al2O3reducetheamountofpenta-coordinatedAl.Ifthereductiontemperatureincreaserateisslow,ithashighmobilityandmovesalotonthesurface.Conversely,ifthereductiontemperatureincreaserateisfast,ithaslowmobility,andiscoupledtothepenta-coordinatedbondingaluminasiteclosetotheperiphery. Inthisstudy,thereconstructionmethodwasverifiedthroughtherecyclingtest.AsaresultoffiverecyclingexperimentsformPtMnA_r10,theconversionofthereactant/selectivityoftheproduct/degreeofdehydrogenationwereallconstant.Inaddition,itwasfoundthatthedegreeofdehydrogenationwasimprovedwhenthecatalystreducedinastatewithaslowreductiontemperatureincreaseratewasreducedatafastreductiontemperatureincreaserate.AsaresultofconfirmationbyCO-DRIFTanalysis,itwasfoundthatPtsingleatomsdisappearedandchangedintoPtclusters.