Atomic origins of the high catalytic activity of

Atomicoriginsofthehighcatalyticactivityofnanoporousgold

TakeshiFujita1,2*,PengfeiGuan1,KeithMcKenna1,3,XingyouLang1,AkihikoHirata1,LingZhang1,TomoharuTokunaga4,ShigeoArai4,YutaYamamoto4,NobuoTanaka4,YoshifumiIshikawa1,NaokiAsao1,YoshinoriYamamoto1,JonahErlebacher5andMingweiChen1,6,7*

Distinctfrominertbulkgold,nanoparticulategoldhasbeenfoundtopossessremarkablecatalyticactivitytowardsoxidationreactions.Thecatalyticperformanceofnanoparticulategoldstronglydependsonsizeandsupport,andcatalyticactivityusuallycannotbeobservedatcharacteristicsizeslargerthan5nm.Interestingly,signi cantcatalyticactivitycanberetainedindealloyednanoporousgold(NPG)evenwhenitsfeaturelengthsarelargerthan30nm.HerewereportatomicinsightsoftheNPGcatalysis,characterizedbyspherical-aberration-correctedtransmissionelectronmicroscopy(TEM)andenvironmentalTEM.AhighdensityofatomicstepsandkinksisobservedonthecurvedsurfacesofNPG,comparableto3–5nmnanoparticles,whicharestabilizedbyhyperboloid-likegoldligaments.InsituTEMobservationsprovidecompellingevidencethatthesurfacedefectsareactivesitesforthecatalyticoxidationofCOandresidualAgstabilizestheatomicstepsbysuppressing{111}facetingkinetics.

tiswellknownthatgoldistheonlymetalthathasanendother-micoxygenchemisorptionenergy.Inprinciple,itdoesnotbindoxygenatallandthusisinerttowardsoxidationreactions1.Forthisreason,thediscoverythatgoldnanoparticlesareacatalystforcarbonmonoxide(CO)oxidation2stimulatesextensiveresearchenthusiasmandconsiderabledebate1,3–5.Ascatalyticallyactivegoldnanoparticlesareusuallysupportedonhigh-surface-areaoxides,theircatalyticperformancemaybeaffectedbythesupports6,whichhaslimitedtheunderstandingoftheintrinsiccatalyticactivityofgold.NPGfabricatedbyelectrochemicaldealloyingpossessesaself-organized,self-supportingthree-dimensional(3D)nanoarchi-tecture,andhasbeengainingincreasingattentionasamultifunc-tionalmaterialusefulinawiderangeofapplications7–14.DealloyedNPGwithoutanysupportexhibitsasimilaractivitytowardsoxi-dationreactionsassmall,oxide-supportedgoldnanoparticles,butpossessesmuchlargercharacteristiclengths,bothinligamentandnanoporediameters7–10.Unlikegoldnanoparticles,NPGpossessesacomplexbicontinuousporousstructureandalwayscontainsresidualless-nobleelementsthatcannotbecompletelyremovedbydealloying.However,asfornanoparticulategold,thecatalyticactivityofNPGhasbeenfoundtostronglydependonnanoporesizeandresidualelements7,8,15.UnderstandingthecatalyticmechanismsoftheunsupportedNPGmayshedlightonthemechanismsofgoldcatalysis.Tohelpsettlethisquestion,experimentalcharacterizationofthemorphologyofNPGiscriticalwithquantificationofsurfacedefectconcentrations(forexample,step,kinkandterracedensi-ties),latticestrain,anditscorrespondencetolocalsurfacecurva-ture.Inthisstudy,wecharacterizedthesurfaceatomicstructureofNPGusingspherical-aberration-correctedTEM(Cs-correctedTEM)inascanningTEM(STEM)modewithahigh-angleannulardark-field(HAADF)detector.Thearrangementofsurfaceatoms

I

wasalsomeasuredaccuratelybyCs-correctedhigh-resolutionTEM(HRTEM),whichprovidesphase-contrastimageswithhighatomicdisplacementsensitivity16.Inadditiontothestaticobservationinaninertvacuumenvironment,wealsocharacterizedthesurfaceatomicstructureevolutionduringCOoxidationinareactiveatmosphereusinganewlydedicatedenvironmentalHRTEM.

Figure1ashowsarepresentativebright-fieldTEMimageofNPGatalowmagnification.Theaveragediameterofbothgoldligamentsandnanoporesis～30nm.TEMtomography(Fig.1b)revealsthatNPGhasabicontinuous3Dstructureandmostgoldligamentshaveahyperboloid-likeshapewithanegativecurvatureaccommodatingnanoporesandpositivecurvatureformingcolumnarligaments17.Figure1cshowsaHAADF-STEMimageofagoldligamentalongwithchemicalmapsofAu-Mα(Fig.1d)andAg-Lα(Fig.1e)measuredbyX-rayenergy-dispersivespectroscopy.ThemeasuredresidualAgconcentrationinthegoldligamentis～1.2at.%.Thelinechemicalprofile(Fig.1f)suggeststhattheligamenthasacylindricalshape,andtheAg-Lα/Au-Mαcountratioof～0.05isnearlyconstantacrosstheligament.

Asgoldligamentsandnanoporechannelsarerandomly

¯]-oriented,wearbitrarilyselectananoporesurroundedby[011orientedgoldligaments(Fig.2a).HAADF-STEMimagesfromfour

¯]direction,differentsitesareshowninFig.2b–e.Fromthe[011

exposedfacetswithMillerindicesof{100}and{111}canbefrequentlyobservedontherelativelyflatregions.Inthecurvedregions,thesurfacesareterminatedbyhigh-indexfacetswithahighdensityofatomicsteps,asrevealedbyintensityvariation.Distinctfromnanoparticleswithwell-definedfacets18,theorientationofhigh-indexfacetsinNPGisrandomanddependsonlocalcurvature.Forexample,Fig.2bshowsanegativelycurvedregionformedbyflat(100)terraces.Theintensityprofile(Fig.2binset)shows

AdvancedInstituteforMaterialsResearch,TohokuUniversity,Sendai980-8577,Japan,2PRESTO,JST,4-1-8HonchoKawaguchi,Saitama332-0012,Japan,3DepartmentofPhysics,UniversityofYork,YorkYO105DD,UK,4EcotopiaScienceInstitute,NagoyaUniversity,Nagoya464-8603,Japan,5DepartmentofMaterialsScienceandEngineering,JohnsHopkinsUniversity,Baltimore,Maryland21218,USA,6StateKeyLaboratoryofMetalMatrixComposites,SchoolofMaterialsScienceandEngineering,ShanghaiJiaoTongUniversity,Shanghai200030,China,7CREST,JST,4-1-8HonchoKawaguchi,Saitama332-0012,Japan.*e-mail:tfujita@wpi-aimr.tohoku.ac.jp;mwchen@wpi-aimr.tohoku.ac.jp.

NATUREMATERIALS|VOL11|SEPTEMBER2012|/naturematerials石墨产业联盟交流群：335495551

1WPI

技术，文献，行情 请访问石墨论坛

775

a

b

cde

Integrated X-ray intensity (a.u.

)

f

Distance (nm)

Figure1|TEMmicrographsandchemicalanalysisofNPG.

a,Low-magni cationbright- eldTEMimage.b,3Dtomographicreconstructionbyelectrontomography.c,HAADFimageofagold

ligament.d,Au-Mαchemicalmapofthegoldligament.e,Ag-Lαchemicalmapofthegoldligament.f,Linepro leacrosstheligamentbyintegratingtheX-rayintensityofAu-MαandAg-Lαfromtheselectedareasindande.

agradualdecreaseinintensityoverseveralatomiccolumns,suggestingthatthe(100)atomicstepsarenotstraight(that …… 此处隐藏：6047字，全部文档内容请下载后查看。喜欢就下载吧 ……