FIRST-PRINCIPLES STUDY OF Ti-CATALYZED HYDROGEN ADSORPTION O

FIRST-PRINCIPLES STUDY OF Ti-CATALYZED HYDROGEN ADSORPTION ON LiB (001) SURFACE

JournalofTheoreticalandComputationalChemistry

Vol.12,No.7(2013)1350065(11

pages)

cWorldScientiˉcPublishingCompany#DOI:10.1142/S021963361350065X

FIRST-PRINCIPLESSTUDYOFTi-CATALYZED

HYDROGENADSORPTIONONLiB(001)SURFACE

WEIBINZHANG*,?,AILINGWU?,YIDINGLIU?,**,SHAOLINZHANG*,

JIANHONGGONG§,LANCHANG?,JIANLI?,HUIZHANG?,§§HAIFENGLIU?,KEHUALI?,KAIHUANG?andWOOCHULYANG*,||,**,

J. Theor. Comput. Chem. Downloaded from by TIANJIN UNIVERSITY on 10/14/13. For personal use only.ofPhysicsDonggukUniversity,Seoul100-715,Korea?The*DepartmentEngineering&TechnicalCollegeofChengduUniversityofTechnologyLeshan614099,P.R.China?SchoolofSpaceScienceandPhysicsShandongUniversityatWeihai,Weihai264209,P.R.China§SchoolofMechanical,ElectricalandInformationEngineeringShandongUniversityatWeihaiWeihai264209,P.R.ChinaCollegeofPhysicsandElectronicEngineeringLeshanNormalUniversity,Leshan614004,P.R.China||wyang@dongguk.edu§§newonenail@sina.comReceived15June2013Accepted15July2013Published25September2013?Ti-dopedLiB(001)isapromisingmaterialforhydrogenstorage.TheadsorptionofH2isgreatly

enhancedbydopingTiintoLiB(001),changetheelectronicstructuresofthesurfaceLi,Batoms.AfterH2isadsorbedonthesurface,theEadoftheeH2Tn@Ti/LiB(001)systemisconsidered.Itisaroundà0:22eV/H2toà0:31eV/H2,whichisclosetothetargetspeciˉedbyU.S.DepartmentofEnergy.ThenatureofthebondingbetweenTiandH2isduetotheH1s,Ti4sandB2sorbitalhybridization.Inaddition,Ti3dorbitalishybridizedstronglywithB-2porbital,resultinginmorestableTi/LiB(001)system.Theseresultsareveriˉedbytheelectrondensitydistributionintuitively.ItisfoundthatthesystemcanadsorbuptofourH2atambienttemperatureandpressure.Therefore,theTi-dopedLiB(001)wouldbeapromisinghydrogenstoragematerial.SuchoptimalmolecularhydrogenadsorptionsystemmakesH2adsorptionfeasibleatambientconditions,whichiscriticalforpracticalapplications.

Keywords:Densityfunctionaltheory;adsorptionenergy;Ti-doped;electronicstructure.**Correspondingauthors.

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1.Introduction

Hydrogenisanalmostideal,environmentalfriendlyenergycarrier.1TheU.S.DepartmentofEnergy(DOE)'stargetforgravimetricstoragedensityis5.5wt%forcompetitiveconsumervehiclesthatusehydrogenasthefuel.2However,howtostorehydrogene?ectivelyandsafelyisanissueforthehydrogeneconomy.Eachofthecurrentavailablehydrogenstoragemodalities,includingcompressedgaseoushydrogen,cryogenicliquidhydrogen,metalhydridesandcomplexhydrides3–7cannotmeetalltherequirements.Therefore,developingacompactande±cienthydrogenstoragetechnologyisthekeytothehydrogen-basedpropulsion.

ManyattemptshavebeenmadetodevelopnewmaterialswithhighhydrogenstoragecapacitiesinordertomeetthedemandofcommercialvehiclespoweredbyH2/O2protonexchangemembranefuelcells.8–10Lithiumborohydride(LiBH4)hasbeenanattractivecandidateduetoitsintrinsicallyhighgravimetricandvolumetrichydrogencapacities(18.2wt%,121kg/m3).9Nowadays,LiBH4ismainlysynthesizedbyLiBcompoundexposedunderhighhydrogenpressure.11–15Friedrichs11andCetin12foundthatLiBcompoundscane?ectivelyadsorbhydrogenunderalowpressure.Usingpulsedlaserdeposition(PLD)methodwithalowhydrogenpressureatambienttemperature,WangHaipingetal.14synthesizedLiBH4ˉlmsfromexperiment.Toyodaetal.15obtainedthehydrogenadsorptionrateonLiBˉlmfromexperiment.TheirresultsshowthatLiBˉlmcanadsorbhydrogenatomswithdensityof6:5?1020atomámà2in28s.Morerecently,itwasfoundthatthetransitionmetalatoms,suchasAl,Ti,a±xingtocarbonnanostructuresorcomplexmetalhydride,canstronglyadsorbseveralhydrogenmolecules.16–19ItwasalsorevealedthatslightlydopingTiintocomplexmetalhydridesincreasedthereactionratesdramatically,bringingthesystemclosetowhatisrequiredforpracticalappli-cations.20–22Theinteractionbetweenhydrogenmoleculesandtransitionmetalsisveryunique,lyingbetweenchemi-andphysisorption,withabindingenergyof0.4eV,compatiblewithroomtemperaturedesorption/absorption.CarboneMarilenaetal.23havecalculatedtheenergybarriersconnectingthephysisorbedtothechemisorbedstates.Theysuggestedthatapossiblecontrolovertheadsorptionconˉgurationsiseasierexertedontheconversionofthephysisorbedstates.Theoriginofthisunusual\molecularchemisorption"isexplainedbywell-knownDewarcoordinationandKubasinteraction.MuDanetal.24showedthatasinglepoly-dimethylsiloxane(PDMS)chaincouldbeadsorbedontothesilicon(111)surfacewellinvacuumorinsolutionenvironmentwhilethegoodsolventenvironmentwasgoodfordesorption.Somestudiesofdi?erentsystemshaveinvestigatedtheadsorptionprocessandpropertiesinthesolution.25,26TheH2adsorptiononLiB(001)insolventsystemwillbeinvestigatedinthefuture.

Accordingtoourprevioustheoreticalprediction,27theLiBH4ˉlmshavebeensynthesizedandimprovedsuccessfully.14However,thereisnoreportaboutthetheoreticalorexperimentalresearchoftheH2adsorbedonTi-dopedLiB(001)surfaceuptonow.ThisintricateH2adsorptionprocessisanimportantpartinthe

1350065-2J. Theor. Comput. Chem. Downloaded from by TIANJIN UNIVERSITY on 10/14/13. For personal use only.

First-PrinciplesStudyofTi-CatalyzedHydrogenAdsorption

J. Theor. Comput. Chem. Downloaded from by TIANJIN UNIVERSITY on 10/14/13. For personal use only.LiBhydrogenstoragesystems.TheinteractionsbetweentheTiatomandLiB(001)surfaceareconsidered.ThereishighcovalentinteractionforB–Ti,whiletheionicnatureexistsinLi–Tibond.ThismeansthatTiclusteringcanbesuppressedbypreferentialbindingofTiatomsonLiB(001).Then,theeH2Tn@Ti/LiB(001)systemisconsidered.TheelectrontransfersfromtheTiatomtotheH2andLiB(001)surface,whichissuggestiveofweakchemisorption,notsimplephysisorption.Theprojectedelectronicdensityofstates(PDOS)andtheillustrationsofelectrondensitydistributionareconsidered.Finally,itisfoundthatTi-dopedonLiB(001)systemcanadsorbuptofourmolecules.Thebindingenergyisaroundà0:22eV/H2toà0:31eV/H2,whichisclosetothetargetspeciˉedbytheU.S.DOEwithabindingenergyofà0:2eV/H2toà0:4eV/H2atambienttemperatureandmodestpressureforcommercialapplications.SuchoptimalmolecularhydrogenadsorptionenergiesmakeH2adsorptionanddesorptionfeasibleatambientconditions,whichiscriticalforpracticalapplications.2.ComputationalDetailsTheDFT28programCambridgeSerialTotalEnergyPackage(CASTEP)29wasusedtoinvestigatetheeH2Tn@Ti/LiB(001)system.Thegeneralizedgradientapproxi-mation(GGA)30withaplane–wavepseudopotentialbasiswasperformed.ThePerdewWang(1991)(PW91)31,32functionwasemployedtoexplaintheexchange-correlatione?ect.GeometryoptimizationwasperformedbytheBroydenFletcherGoldfarbShanno(BFGS)routineandanultrasoftpseudopotentialwasusedtocalculate.Thein°uencesofdi?erentk-pointsamplingandplane-wavecut-o?energywerestudiedinaseriesoftestingcalculations.Allthegeometryoptimizationcal-culationswereperformedwithacuto?energyof270eV,andtheconvergencecriteriawas2?10à5eVáatomà1.Thek-pointoftheLiBbulkandeH2Tn@Ti/LiB(001)systemswas2?2?1inthiswork.Testcalculationsshowedthattheresultswerenotsigniˉcantlyaltereduponincreasingthek-point.Reciprocal-spaceintegration

overtheBrillouinzonewasapproximatedwithacarefulsamplingwithaˉnitenumberofk-pointsintheMonkhorst-Pack33,34k-pointmesh.

Electronicstructurecalculationswerestartedwithatompositionsfrompublishedcrystallographicreˉnementresultsandtheunitcellparameterswerevalidatedbyoptimizingtheunitcell.Thereisnomorethan5%deviationinthecalculatedunitcellparametersforthesystemsstudied.Thisisimportantforestablishingconˉdenceinthefunctional(PBE)usedinthisstudy.Thisfunctioniswell-knownforaccuratepredictionsofsurfacereactionsandchemisorptionenergiesinmetals,alloysandothersimilarsystems.Therelaxedunitcellwereusedtoformsuper-cellanddopedwithanappropriateconcentrationofTiatoms.Then,thelowestenergysurfacewascleavedinawaytoexposetheTiatomforfurtherstudyofstructuralrearrangementandreactivity.Thesurfaceswererelaxedtoattainenergy-minimizedstablestruc-turesandweresubsequentlyusedtostudyabsorptionandreactivityofmolecularhydrogen.Theresultspresentedherewerebasedonperiodicsystems,astudyofthe

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J. Theor. Comput. Chem. Downloaded from by TIANJIN UNIVERSITY on 10/14/13. For personal use only.LiB(001)2?2surfacewasessentiallyaninˉnitelyperiodicsurfacecomposedofthese2?2repeatingunits.TheLiB(001)surfacewasmodeledbyaslabofsixatomiclayers,whichwasseparatedfroma15??Athickvacuumregion.Thethicknessofslabsandvacuumregionhadbeensetlargeenoughtoavoidtheinteractionbetweenthereplicas.H2wasoptimizedinalargeperiodiccubicboxwithacellparameterof10?10?10??A3.ThebondlengthofH–Hwasabout0.752??A.Forcomputationale±ciency,thebot-tomthreelayersofLiB(001)wereˉxed,onlythetopthreelayersandTi,H2wereallowedtorelaxuntiltheirresidualforceswerelessthan0.100eVá??Aà1.HuanwenWuetal.35haveinvestigatedthegeometric,electronicstructuresandthevacancyfor-mationenergiesoftwokindsofCu2O(111)oxygen-vacancysurfaces.Theresultsshowedthattherelaxationmainlyhappensinthetoptrilayer.ThebindingenergyEboftheTiatomdopedontheLiB(001)isdeˉnedas:Eb??mETitELiBe001TàEmTitLiBe001T??=m;wheremindicatesthenumberofTiatoms.ETi,ELiBe001T,EmTitLiBe001TarethetotalenergiesofafreeTiatom,thepureLiB(001)andthehybridsystem,respectively.ForH2adsorptiononTi/LiB(001)system,theaverageadsorptionenergyEadperH2tothesystemisdeterminedby:Ead??EtàEmTitLiBe001TànEH2??=n;wherenindicatesthenumberofH2molecules.Thesubscriptst,mTitLiBe001TandH2denotethetotalenergyoftheconsideredsystem,thecorrespondingstoragematerialwithnumberofTiatomsm,andthefreeH2molecules,respectively.ThenegativevalueofEadindicatedthattheadsorptionwasexothermic(stable)reaction,andpositivevalueindicatedstableadsorptionwasendothermic(unstable)reaction.3.ResultsandDiscussion