催化剂研究方法及表征外教课件

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1、SurfaceenrichmentThecompositionofthesurfaceofmostmaterialsisdifferentfromthatofthebulkandthishaslargeconsequences,notonlyincatalysis,butinallareaswherethesurfaceplaysanimportantrole.Thereasonforthesurfaceenrichmentisthatatomsatthesurfacehaveadifferentsurr

2、oundingthaninthebulkandthereforethermodynamicsfavourssomeatomstobeatthesurfaceandotherstobeinthebulk.Wewillstudythiswiththeexampleofalloys,buttheprinciplesarealsovalidforothermaterials,likemetaloxidesandsulfides.LetustakealookatthesurfaceofabinaryalloyAB.

3、Wewillassumethatanatominthebulkhaspdirectneighboursintheplaneparalleltothesurfaceandmdirectneighboursperpendiculartothatplane.Foranfccmetalp+m=12andforabccmetalp+m=8.Anatomatthesurfacewillhavethesamenumber(p)ofneighboursinthesurfaceplane,butonlym/2neighbo

4、ursperpendiculartotheplane,becausetheneighboursabovetheplanearemissing.Wewillassumethatthebroken-bondmodelapplies,whichassumesthatonlybondsbetweendirectneighbouringatomscontributetotheenergyofthealloy,andthatsurfaceenrichmentonlyoccursinthetopsurfacelayer

5、.Allotherlayershavethebulkcomposition,alsothesubsurfacelayer.WhenxsistheconcentrationofatomAatthesurfaceandxbitsconcentrationinthebulk,thentheenergyEAsofatomAatthesurfaceisEAs=p[xsEAA+(1-xs)EAB]+m/2[xbEAA+(1-xb)EAB]EAs=(pxs+mxb/2)EAA+[p(1-xs)+m(1-xb)/2]EA

6、BwhereEAAisthebindingenergybetweentwoneighbouringAatomsandEABisthebindingenergybetweentwoneighbouringAandBatoms.TheenergyofanatomAinthebulkisEAb=(p+m)[xbEAA+(1-xb)EAB]Similarly,foratomBatthesurfaceandinthebulkwegettheenergiesEBsandEbb,respectively:EBs=(px

7、s+mxb/2)EAB+[p(1-xs)+m(1-xb)/2]EBBandEBb=(p+m)[xbEAB+(1-xb)EBB]TheenergydifferenceDEbetweenatomAatthesurfaceandatomBinthebulkontheonehandandatomBatthesurfaceandatomAinthebulkontheotherhand,isDE=(EAs+EBb)–(EBs+EAb)=(EAs-EAb)–(EBs-EBb)==[p(xs–xb)–mxb/2]EAA–

8、[2p(xs–xb)–mxb+m/2]EAB+[p(xs–xb)-mxb/2+m/2]EBBThebindingenergiesEAAandEBBcanbeexperimentallydeterminedfromtheheatsofsublimationofthepuremetalsAandB,becauseinthebrokenbondmodelthemolarheatofsublimationofmetalAisequal