FIRST-PRINCIPLES STUDY OF Ti-CATALYZED HYDROGEN ADSORPTION O(2)

TheequilibriumlatticeconstantsandtheexperimentaldataofLiBwerelistedinTable1.Comparingwiththeexperimentaldata,theequilibriumlatticeconstantsaandbvaluesincreasedby4.05%,whilethecellparametercvalueincreasedby0.97%.Thiswasattributedtothefollowingtworeasons:(i)theGGA'sinherentreason,whichusuallyoverestimatedtheparametersbyafewpercent36;(ii)therelativelysmallsizeoftheLiBunitcell,whichcontained4atoms.TheinteractionbetweenLi–Batomsmightnotbenegligiblebecauseoftheunitcell,whichwaslimitedbyourcurrentcomputationalpower.Thisinteractionmightalsobethe

Table1.StructureparametersofLiB.

Structure

LiBExperimentsa?b?4:022??A,c?2:796??A??????????90,???120Present(001)a?b?4:185??A,c?2:823??A??????????90,???120

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First-PrinciplesStudyofTi-CatalyzedHydrogenAdsorption

reasonoftheobserveddiscrepancybetweentheexperimentalandcalculationresults.Thusitisjustiˉedthatthismethodiscredibleandcanbeappliedintothefollowingcalculations.

Auniform(2?2)LiB(001)super-cellwasestablished.Althoughdi?erentdopingsiteswereconsidered,ourcalculationsshowthatthemostsuitablepositionforTiisonthetopofLiatom.TounderstandthemechanismoftheenhancementforH2adsorptionintheTi/LiB(001)system,itisimportanttodeterminetheTiinteractedwiththeLiB(001)surface.TheTidopedontheLiB(001)systemwouldinevitablyleadtheelectricchargetotransferamongatoms.ThepopulationoftheTi,Li,BatomsbeforeandaftergeometricequilibriumwasgiveninTable2.OntheTidopedLiB(001)surface,thepopulationofthesurfacelayerLi,Batomshadamaximalchange.Themiddleandsixthlayershadnoobviouschange,indicatingTimainlyinteractedwiththesurfaceLi,Batoms.Then,thenetchargeandbondpopulationofthesurfaceLiandBatomsweremainlydiscussed.ThepositivechargeoftheLiatomsonthesurfacelayerincreased.ThelargestonewasLi(4),from0.81to1.14.ThenegativechargeoftheBatomsonthesurfacelayerdecreased.ThelargestonewasB(3),fromà0:89toà1:10.Thebondpopulationrepresentedtheoverlapdegreeoftheelectroncloudoftwobondingatoms,andcanbeusedtoestimatethecovalentorionicnatureofachemicalbond.Forthebondpopulation,thepositiveandnegativevaluesimpliedthechemicalbondexhibitingcovalenceorionicity.37–39IntheTi/LiB(001)system,thepopulationofB–Tibondisabout0.90,indicatingthehighcovalentinteractionforB–Ti.However,thepopulationofTi–Hbondwasaboutà0:22,whichrevealedthattheTi–Hbondhadanionicnature.

TheEadoftheeH2Tn@Ti/LiB(001)systemwasaroundà0:22eV/H2toà0:31eV/H2,asshowninTable3.However,allEadvalueswereintherangeofidealbindingenergylevelsofthephysisorbedandchemisorbedstates.Meanwhile,thepositivechargeonTiincreasedasthenincreased,andthenegativechargeontheLiB(001)surfaceincreasedaswell.ThechangeonTiwasinducedbytheelectrondonationfromTitotheH2andLiB(001)surface.Consequently,thechargetransfersfromTitotheanti-bondingorbitalofH2elongatedtheH–HbondofH2.Thiselectrontransferwassuggestiveofweakchemisorption,notsimplephysisorption.Importantly,asingleTionLiB(001)canadsorbuptofourH2moleculeswithEad%à0:31eV/H2,whichishigherthanthatreportedinpreviousstudiesonH2/LiB(001)systems.Thistrendimpliesthatthein°uenceofTi-dopedonhydrogenstoragepropertiesofLiB(001)ismoresigniˉcantthanintrinsicLiB(001).

Tounderstandtheenhancemente?ectofthedopedTiontheH2adsorption,thePDOSoftheTi/LiB(001)andH2@Ti/LiB(001)systemswereinvestigated.ThezeropointenergywasalignedattheFermilevel.Figure1(a)showedthePDOSofTi/LiB(001)surface.TheresonancesofTi,B,Liatomsoccurredataroundà10eVto3eV,andthehybridizationwasamongTi3d,4sandB2s,2p.TheTi-3dorbitalwashybridizedstronglywithB-2porbital.ThehybridizationandresonanceresultedinamorestableTi/LiB(001)system.AfterH2adsorbedontheTi/LiB(001)surface,thePDOSofH1selectronswasobviouslychanged,asshowninFig.1(b).The

1350065-5J. Theor. Comput. Chem. Downloaded from by TIANJIN UNIVERSITY on 10/14/13. For personal use only.

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First-PrinciplesStudyofTi-CatalyzedHydrogenAdsorption

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0J. Theor. Comput. Chem. Downloaded from by TIANJIN UNIVERSITY on 10/14/13. For personal use only.Density of StatesEnergy(eV)(a)30303030-60Density of States-50-40-30-20-100Energy(eV)(b)

Fig.1.ThePDOSofH2adsorbedonTi/LiB(001)surface:(a)beforeH2adsorbedontheTi/LiB(001)surface,(b)afterH2adsorbedontheTi/LiB(001)surface.

mainpeakofH1sshiftedfrom0eVtoà8eV.Theresonanceoccurredatà8eVtoà6eVbetweenTi4s,B2sandH1sorbital.ThehybridizationactionmainlyoccurredbetweentheH1sandTi4sandB2sorbital.ItcanbeconcludedthattheinteractionbetweenHatomsandthesurfaceatomsismainlyattributedtothebondingofH1s,Ti4sandB2sstates.Inaddition,thedopedTichangestheelectronicstructuresofbothHandtheLi,Batomsonthesurface.ThePDOSoftheatomsshiftstowardthelowerenergy,whichsuggeststhattheH2@Ti/LiB(001)surfacecon-ˉgurationisamuchmorestablesystem.

TheillustrationsofelectrondensitydistributionfortheTi/LiB(001)andH2@Ti/LiB(001)systemsareshowninFig.2.Thewhiteregionrepresentsthedeletion

1350065-7

W.Zhangetal.

J. Theor. Comput. Chem. Downloaded from by TIANJIN UNIVERSITY on 10/14/13. For personal use only.(a)(b)Fig.2.TheillustrationsofelectrondensitydistributionfortheTi/LiB(001)andH2@Ti/LiB(001)systems.ofelectron,whiletheblackregionrepresentstheregionofelectronenrichment.InthesystemofTi/LiB(001)[Fig.2(a)],theelectronmainlyexistsintheregionbetweenTiandBatom.ItmeansthattheTiatommainlyinteractedwiththeBatoms.InFig.2(b),theelectronsmainlyexistedbetweentheregionamongHandTiatoms,indicatingthattheH2adsorbedontheTi/LiB(001)isweakchemisorption,notsimplephysisorption.TheinteractionbetweenH2andthesurfaceatomsmainlytakeplaceattheH2andTiatom.ThissupportsthenotionthattheH2@Ti/LiB(001)systempossessesamuchstrongerH2adsorptionability.AfterunderstandingthemechanismofenhancementforH2adsorptionontheTi-dopedLiB(001)system,itisimportanttodeterminehowmuchH2moleculescanbeadsorbedonthesurface.Weconstructedanadsorptionconˉgurationwithoneand

moreH2adsorbedonthedi?erentadsorptionpositions.Afterrelaxation,itwasfoundthateachTi-dopedonLiB(001)systemcanadsorbuptofourmolecules.Theadsorptionwithˉvemoleculeswithdi?erentorientationsinthesystemwasalsocalculated,anditisfoundthatoneH2wasreleased.TheinteractionbetweenH2moleculeswouldweakentheadsorptionandthesaturatednumberofH2moleculesadsorptionisfour.ThisadsorptionbehaviorissimilartothatreportedpreviouslyforAl-dopedgraphene.19Figure3showsthetopviewsandsideviewsoftheequilibriumconˉgurationsfortheadsorptionofdi?erentnumberH2moleculesontheTi/LiB(001),respectively.TheEadisaroundà0:22eV/H2toà0:31eV/H2,whichsatisˉestherequirement40–42ofEad?à0:20eV/H2to0:40eV/H2atroomtemperaturesetbyDOE.Thistrendimpliesthatthein°uenceofTi-dopedonhydrogenstoragepropertiesofLiB(001)ismoresigniˉcantthanthatofintrinsicLiB(001).SuchoptimalmolecularhydrogenadsorptionsystemmakesH2adsorptionanddesorptionfeasibleatambientconditions,whichiscriticalforpracticalapplications.1350065-8

First-PrinciplesStudyofTi-CatalyzedHydrogenAdsorption

J. Theor. Comput. Chem. Downloaded from by TIANJIN UNIVERSITY on 10/14/13. For personal use only.(a)(b)(c)(d)(e)Fig.3.TherelaxedhydrogenstoragesystemsbasedontheTi-dopedLiB(001)surface.Theplotsa–e

showtherelaxedmodelswhenhydrogennumberincreasesfrom0to4.

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