Pd-Al2O3的DFT研究

AppliedCatalysisB:Environmental126(2012)108–120ContentslistsavailableatSciVerseScienceDirectAppliedCatalysisB:Environmentaljournalhomepage:-Al2O3catalyst:AcomputationalstudyRiguangZhang,HongyanLiu,BaojunWang∗,LixiaLingKeyLaboratoryofCoalScienceandTechnologyofMinistryofEducationandShanxiProvince,TaiyuanUniversityofTechnology,Taiyuan030024,Shanxi,People’sRepublicofChinaarticleinfoArticlehistory:Received18February2012Receivedinrevisedform8July2012Accepted16July2012Availableonline23July2012Keywords:Pd/-Al2O3CO2hydrogenationHydroxylationSupportDensityfunctionaltheoryabstractTheelementarystepsleadingtotheformationofHCOOandCOviaCO2hydrogenation,twoimportantintermediatescontributingtomethanolandmethaneformation,respectively,havebeenexploredtoidentifyhydroxylationeffectoftheoxidesupportontheselectivityinCO2hydrogenationonPd/-Al2O3catalystbythedensityfunctionaltheorytogetherwithslabmodelcalculations.Twomodels:tetramerPd4clustersupportedonthedry-Al2O3(110)surface,D(Pd4),andonthehydroxylated-Al2O3(110)surface,H(Pd4),havebeenemployedtomodelPd/-Al2O3catalyst.Meanwhile,Pd(111)surfaceisusedtomodeltheunsupportedlargePdparticlesizes.OnD(Pd4),theformationofCOispreferredbothkineti-callyandthermodynamically.OnH(Pd4),HCOOformationbecomesmorefavorablekineticallywhileCOformationismorefacilethermodynamically.However,Pd(111)surfacehasnotshownstrongselectivityandactivityforCO2hydrogenationtoHCOOorCO.Theseresultsshowthatvaryingthepropertiesof-Al2O3supportcanaltertheselectivityofCO2hydrogenation,moreover,thepresenceandnumberoflow-coordinatedPdparticlesisofgreatimportancetoimprovetheoverallactivityandselectivityofCO2hydrogenation.OurresultsalsoshowthattoachievehighselectivityofCO2hydrogenation,Pd/-Al2O3catalysthastogethelpfromadditives,whichshouldbeabletoimproveitsdispersionortocontrolthehydroxylation.ThepresentstudyprovidesthebasisandoneofthedirectionstothedesignofimprovedcatalystsinCO2hydrogenationformethanol,methaneandotherproducts.©2012ElsevierB.V.Allrightsreserved.1.IntroductionCatalyticrecyclingofCO2,themajorman-madegreenhousegas,tomoreusefulcompoundsiscurrentlyunderconsiderationamongthepossibleapproachesformitigatingitsemissionstotheatmo-sphere[1–3].Inthisregard,theselectivehydrogenationofCO2tomethanolormethanehasemergedinthelastdecadeasapromisingandviableoption[4–8],formateandCOaretwoimportantinter-mediatesintheselectivehydrogenationofCO2,whichhavebeenproposedasprecursorsfortheformationoffinalproductsmethanolandmethane,respectively.EvidencefortheexistenceofthekeyintermediateformatespeciesinCO2hydrogenationtomethanolhasbeenderivedfrombothinfrared(IR)[9]andtemperature-programmeddesorption(TPD)[10]spectra.Ontheotherhand,COisaproductofreverseWGSreaction,andithasbeensuggestedasthekeyintermediateformethaneformation[11–15].Thesestud-iessuggestthattheproductsoftheinitialCO2hydrogenationstepswilldeterminethedistributionofthefinalproducts.∗Correspondingauthorat:No.79YingzeWestStreet,Taiyuan030024,People’sRepublicofChina.Tel.:[phone];fax:[phone].E-mailaddresses:[e-mail],[e-mail](B.Wang).Nowadays,Cu/ZnO-Al2O3catalystistypicallyusedtosynthe-sizemethanolonanindustrialscalefrommixturesofCO/CO2/H2(syngas)[3,16],andNi-basedcatalystshavebeenwidelychosenasanactivemethanationcatalysttodissociateCOinthepresenceofhydrogenandsubsequentlyformmethane[17–20].However,thesecatalystsareeasilysusceptibletosulfurpoisoning[21].There-fore,ahighpotentialofmetalPdhasbeenemployedasanactivecatalystforthehydrogenationofcarbonoxides(COandCO2)[22–29,4,30–40],andPd-basedcatalystshavebeenreportedtobemoreeffectivethantheconventionalCu-basedcatalysts.Forexam-ple,Fujitanietal.[28]preparedanactivePd/Ga2O3catalystformethanolsynthesisfromCO2hydrogenation,whichcanwellcom-petewiththeclassicalCu/ZnOcatalyst.Lateron,Shenetal.[29]reportedthatthecatalyticactivityofPd/CeO2inmethanolsynthe-sisfromCOandH2issignificantlyhigherthanthatofCucatalyst.Naitoetal.[36,37]studiedtheCOhydrogenationoverPd/CeO2cat-alyst,suggestingthatbothmethaneandmethanolcanbeformedonthiscatalyst.Importantly,Erdöhelyiandco-workers[34,35]inves-tigatedthehydrogenationofCO2overPdsupportedbyAl2O3,TiO2,SiO2andMgO,respectively,insituIRrevealedthatbondedCOandformatespecieswerepresentonthecatalystsurfaceduringthereaction,andformatespecieswasmoreeasilyformed.However,VanniceandGarten[38]reportedthatPd/Al2O3catalystwasanactiveandselectivecatalystformethane,whichwasalsosupported0926-3373/$–seefrontmatter©2012ElsevierB.V.Allrightsreserved.:Environmental126(2012)108–120109byRothaemeletal.’sstudies[39].Thus,onthebasisofaboveexperi-mentalfacts,wecanseethatalthoughtherearesomediscrepanciesfortheactivityandselectivityofPd,Pdisfoundtobeagoodcata-lystofCO2hydrogenation.Asaresult,developingcatalystsforCO2hydrogenationefficientlyandselectivelyhasbeenoneofthemajorchallengesincatalysis.Ontheotherhand,amongaboveoxide-supportedPdca