Atomic origins of the high catalytic activity of(3)

highercatalyticperformance

ofgoldcatalysts.

Thecomplex3DmorphologyofNPG

possesses

rgecurvature

gradientsbetweenconcaveand

convexregionsgeometricallyrequirestepsandkinkswithahighconcentrationoflow-coordinationatomicsitesthatare

778

Figure4|HRTEMcharacterizationandtheoreticalmodellingofNPG

surfacestrain.a,HRTEMmicrographofaconvex(111)surfaceplaneviewedalong[01¯1].Prede neddirections(xandy)werechosentobeparallelandperpendiculartothesurfaceplane,respectively.b,c,Strainmapsofεxx(b)andεyy(c)withaspatialresolutionof0.6nmobtainedbyGPA.Colour

representsvariationofstrainfrom 10.0to10.0%.d,Structuralmodelwitha(111)surfaceplane,basedonFig.4a,fornear-surfacestrainestimation.e,SimulatedHRTEMimageofthesurfacecon gurationafterstructuralrelaxationtoevaluatethesurfacestrain.Averagestrainsrelativetothe(111)latticespacingofthebulksamplearecalculatedforthetopfourlayers(1–4).Dottedlinesrepresentupwarddisplacement.Parametersfortheimagesimulationwereasfollows:acceleratingvoltage,200kV;sphericalaberrationcoef cient,1µm;focusspread,80Å;beamconvergence,0.1Å 1;defocus,10nm(overfocus).Scalebar,5Å.

chemicallyactive.Suchvicinalsurfacesarerelativelystableagainstfacetingintolow-indexsurfacesasaresultofkineticandentropicfactors31,whichisunlikethebehaviourofsmallnanoparticlesthathavegreaterfreedomforstructuraltransformationtoformlow-indexfacets32.Asidefromthehighconcentrationoflow-coordinationsurfaceatoms,wealsofound5–10%out-of-planepositivestrainnearthesurfaceinHRTEMimages.Previousstudiesofsurfacestrainforclose-packedflatsurfacesgenerallyshowedsmalleroutwardrelaxations(andeveninwardrelaxationforsomesurfaceorientations)33.Theoreticalandexperimentalworkshaveshownthatstraincancontributetoenhancedmolecularinteractionandhencereduceenergybarriersfor

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a

b

ba

(111)

cd

–cd

Figure5|HRTEMobservationviewedalong[01¯1]undercatalytic

reactionsforCOoxidation.Thereactionenvironmentis1vol%COinanairgasmixtureat30Paatroomtemperature.a,Low-AgNPGcontaining1.2at%Ag.b,Enlargedimagefromtheselectedareaofashowing{111}facetsalonga(011)surface.c,High-AgNPGcontaining20at%Ag.d,Enlargedimagefromtheselectedareaofcshowingthat{111}facetsarenotobviousincomparisonwithlow-AgNPG.

Figure6|HRTEMobservationoflow-AgNPGviewedalong[01¯1]inpuregasenvironmentsusingenvironmentalTEM.a,PureN2(>99.9995%,45Pa).b,PureO2(>99.999%,6Pa).c,PureCO(>99.95%,10Pa).d,COdecompositionintocarbonpromotedbyelectronbeamirradiationafterafewsecondsofc.Ineachcase,nomolecule-inducedsurfacereconstructionsareobserved.

about6hinvacuumtoproduceamoreuniformandhomogeneousnanoporousalloystructure.Furtherdetailscanbefoundelsewhere41.ThedealloyedNPGsampleswereplacedoncopperTEMgridswithoutcarbonlayersforsubsequentmicrostructuralcharacterization.

TEMcharacterizationofNPG.Weuseda200kVJEM-2100Felectronmicroscope(JEOL)equippedwithtwoaberrationcorrectors(CEOSGmbH)fortheimage-andprobe-forminglenssystemsandX-rayenergy-dispersivespectroscopy(JED-2300T,JEOL)forchemicalcompositionanalysis.BothCscorrectorsareoptimizedforimageobservationsandthepoint-to-pointresolutionsofTEMandSTEMare1.3and1.0Å,respectively.Beforetheatomicresolutionimageswererecorded,thelensaberrationsweremeasuredbyevaluatingtheZemlintableauofanamorphousarea.Accordingtothesemeasurements,theresiduallensaberrationsareasfollows:sphericalaberrationCS=300nm±1.2µm(95%certificate),twofoldastigmatismA1<2nm,threefoldastigmatismA2<10nmandaxiscomaB2<15nm.Aprobeconvergenceangleof29mradandanHAADFdetectorwithaninneranglegreaterthan100mradwereusedforHAADF-STEM.TheprocedureforelectrontomographyofNPGisdescribedelsewhere17.

ForenvironmentalTEM,weusedanewlydedicated1000kVJEM-1000KRSelectronmicroscope(JEOL)equippedwithaspeciallydesignedenvironmentalcellatNagoyaUniversity.Thepoint-to-pointresolutionofTEMis1.5Å.WeobservedtheNPGsamplesina1vol%CO/airgasmixture(1vol%CO,20vol%O2,79vol%N2),pureN2gas(>99.9995%),pureO2gas(>99.999%)andpureCOgas(>99.95%)overawiderangeoftotalpressures(1–50Pa).Theobservationswereconductedatroomtemperature.Todecreasetheknock-ondamagebyelectronirradiationandtoincreasetheimagecontrast,thebeam

intensityandtotalpressurewerecarefullytuned.Thecurrentfluxwasmeasuredtobe27.7Acm 2byaFaradaygauge.

Abinitiosimulation.TheatomicsimulationwasperformedusingtheViennaabinitiosimulationpackage42.TheprojectedaugmentedwavemethodandthePerdew–Wanggeneralizedgradientapproximationwereusedtodescribeelectron–ioninteractions.Athermalbroadeningof0.2eVwasappliedtoimprovetheconvergenceoftheself-consistentprocedureandnon-spinpolarizationwasemployedforallcalculationswithaplane-wavecut-offenergyof50Ry.ForbulkAu,a20×20×20Monkhorst–Packgridwasusedfortheintegrationoftheelementalface-centred-cubicunitcellovertheBrillouinzone.Thecalculatedequilibriumlatticeconstantforface-centred-cubicAuwas4.08Å,whichagreeswellwiththeexperimentalvalue(4.078Å).Toinvestigatetheatomicconfigurationofasteppedsurface,thesupercellcontainedaslabofAu(111)layerswith15×1periodicity.Thetopandbottomsurfaceswereseparatedbyavacuumregionwithathicknessabout1.2nm.StableconfigurationswereobtainedthroughrelaxationaccordingtotheHellmann–Feynmanforceswithtwofixedbottomlayers.

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chemicalreactions.TheuniquestructureoffersdealloyedNPGahighintrinsiccatalyticactivityasdemonstratedbytheNPG-catalysedhydrosilylationreactionevenintheabsenceofoxygen(seeSupplementaryInformation).Inadditiontothesestructuralorigins,thecatalyticactivityofNPGtowardsCOoxidationhaschemicaloriginsfromtheresidualAgthatplayanimportantroleinacceleratingcatalyticoxidationratesbysuppressingthesurfacereconstructiondynamicsandbyprovidingmoreactiveAgsitesfordissociativeO2adsorption.

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