孔材料比表面与孔结构的表征

孔材料比表面与孔结构的表征第1页/共84页★★注意★★

我们拿到的数据，只有吸脱附曲线是真实的，比表面积、孔径分布、孔容等都是通过模型计算出的数据。我们所讲的BET（Brunauer-Emmet-Teller）只是对N2-sorptionisotherm中p/p0=0.05~0.35之间的一小段用BET公式处理了一下，得到单层吸附量Vm，然后据此算出比表面积。

第2页/共84页第3页/共84页第4页/共84页吸附等温线

以相对压力p/p0为X轴，氮气吸附量为Y轴得到的曲线称为吸附等温线可将X轴相对压力粗略地分为低压（0.0-0.1）、中压(0.3-0.6)、高压（0.80-1.0）三段。第5页/共84页低压端偏Y轴则说明材料与氮有较强作用力(І型，ІІ型，Ⅳ型)，较多微孔存在时由于微孔内强吸附势，吸附曲线起始时呈І型；低压端偏X轴说明作用力弱(ІІІ型，Ⅴ型)。

中压端多为氮气在材料孔道内的冷凝积聚，介孔分析就来源于这段数据，包括样品粒子堆积产生的孔，有序或梯度的介孔范围内孔道。BJH方法就是基于这一段得出的孔径数据高压段可粗略地看出粒子堆积程度，如І型中若最后上扬，则粒子不均匀。平常得到的总孔容通常是取相对压力为0.99左右时氮气吸附量的冷凝值第6页/共84页◆滞后环※滞后环的产生原因

由于毛细管凝聚作用使N2分子在低于常压下冷凝填充了介孔孔道，由于开始发生毛细凝聚时是在孔壁上的环状吸附膜液面上进行，而脱附是从孔口的球形弯月液面开始，从而吸脱附等温线不相重合，往往形成一个滞后环。第7页/共84页

滞后环的特征对应于特定的孔结构信息H1是尺寸分布较窄的均匀孔模型，如圆柱形孔。H2比较难解释，一般认为是多孔吸附质或均匀粒子堆积孔造成的，孔分布较宽，多认为是“inkbottle”，当小孔径瓶颈中的液氮脱附后，束缚于瓶体中的液氮气体会骤然逸出；

滞后环的种类H3与H4相比高压端吸附量大，认为是片状粒子堆积形成的狭缝孔（slit）如粘土

H4也是狭缝孔，区别于粒子堆集，是一些类似由层状结构产生的孔如活性炭第8页/共84页第9页/共84页第10页/共84页◆几个常数※温度77K时,液氮六方密堆积氮分子横截面积0.162nm2，形成单分子层铺展时认为单分子层厚度为0.354nm

※标况（STP）下1mL氮气冷凝后（假定凝聚密度不变）体积为0.001547mL

例：吸附曲线p/p0最大时氮气吸附量约为400mL，则可知总孔容＝400*0.001547＝0.61mL第11页/共84页

※STP每毫升氮气分子铺成单分子层占用面积4.354m2

例：BET方法得到的比表面积S(平方米/每克)＝4.354*Vm，其中Vm为单层吸附量，由BET方法处理可知Vm=1/(斜率＋截距)第12页/共84页June2009June2009Slide13OutlineDefinitionofAdsorptionPhysicalAdsorptionMonolayerModelsStandardIsothermsMeso-PorosityMicro-PorosityHigh-pressureSorptionChemicalAdsorptionCharacterizationStaticChemisorptionPulseChemisorptionHeatofAdsorptionTemperatureProgrammedAnalysesTemperatureProgrammedTechniquesHeatofDesorptionApplications第13页/共84页June2009June2009Slide14AdsorptionAdsorption→EnrichmentinaninterfaciallayerAdsorbate→SubstanceintheadsorbedstateAdsorptive→AdsorbablesubstanceinthefluidphaseAdsorbent→SolidmaterialonwhichadsorptionoccursPhysisorption→AdsorptionwithoutchemicalbondingpsatorPsat→Saturationpressure(ofthecryogen)p0orP0→SaturationpressureoftheadsorptiveChemisorption→Adsorptioninvolvingchemicalbonding第14页/共84页June2009June2009Slide15AdsorptionGeneralphenomenonwitharelativelylowdegreeofspecificity.Retainsidentity;desorbstofluidphaseinitsoriginalform.Exothermicadsorptionsimilartotheenergyofcondensations.Rapidequilibration;transportlimited.Dependentonreactivityofadsorbentandadsorptive.Chemisorbedmoleculemayreactordissociate.Energyissimilartoenergychangeforchemicalreaction.Activatedprocessatelevatedtemperature.第15页/共84页June2009June2009Slide16AdsorptionMoleculesfromthegasphasestrikethesurface.Atequilibriumthemoleculeadsorbs,losestheheatofadsorption(q),andsubsequentlydesorbsfromthesurface.Atequilibriumtherateofcondensation=rateofdesorption.Constantsurfacecoverageatequilibrium.第16页/共84页June2009June2009Slide17AdsorptionSurfacefeatureschangetheadsorptionpotential.Surfaceareamodelsneglecttheeffectsoflocalizedphenomenon.Curvedsurfacesorroughnessprovideenhancedadsorptionpotential.第17页/共84页June2009June2009Slide18AdsorptionAsthesystempressureisincreased(gasconcentrationalsoincreases)multiplelayerssorbtothesurface.Themonolayercoverage,adenselypackedsingleadsorbedlayer,isusedfordeterminingsurfacearea.Aspressureisfurtherincreasedandadsorptionproceedsgascondensesintheporesandthisvolumeofcondensedadsorptiveisusedforcharacterizingporosity.第18页/共84页June2009June2009Slide19Adsorption第19页/共84页June2009June2009Slide20Adsorptionnm=Monolayerquantity,molVm=Monolayervolume,cm3Vg=MolarvolumeofgasatSTP,cm3/molNa=Avogadro’snumber6.023x1023molecules/molw=samplemass,gσA=Cross-sectionalareaoftheadsorbate,m2第20页/共84页June2009June2009Slide21Physisorption-HardwareStainlessSteelmanifold1000,10,1torrtransducersDedicatedvacuumsystemCryogenlevelcontrol/longDewarlife第21页/共84页June2009June2009Slide22AdsorptionQuantityadsorbedvs.pressure.Pressureisusuallyvariedfromvacuumtonearatmospheric.Constanttemperature.Quantityadsorbedisnormalizedforadsorbentmass.Sixisothermclassifications.TypesI,IIandIV–mostmaterialsTypeII–uncommonTypeV–rareTypeVI–highlyuniformsurface第22页/共84页June2009June2009Slide23第23页/共84页June2009June2009Slide24TypeI–IsothermMono-layeradsorptionMicroporefillingMicropores<2nmporewidthFinelydividedsurfaceLimitingamountadsorbedap/p0approaches1第24页/共84页June2009June2009Slide25Langmuir第25页/共84页June2009June2009Slide26Langmuir第26页/共84页June2009June2009Slide27Langmuir第27页/共84页June2009June2009Slide28N2Adsorptionon13xZeoliteCrystalline10(8)Å,poreSilica–aluminaCa+exchangedzeolite第28页/共84页June2009June2009Slide29N2Adsorptionon13xZeolite第29页/共84页June2009June2009Slide30TypeII–IsothermNon-poroussurfaceMacropores>50nmporewidthUniformsurfaceMultilayeradsorptionInfiniteadsorptionasp/p0approaches1第30页/共84页June2009June2009Slide31Brunauer,Emmett,andTellerStephenBrunauerPaulEmmettEdwardTeller第31页/共84页June2009June2009Slide32BETAssumptionsMulti-layeradsorptionNon-porous,UniformsurfaceHeatofadsorptionforthefirstlayerishigherthatsuccessivelayers.HeatofadsorptionforsecondandsuccessivelayersequalstheheatofliquefactionLateralinteractionsofadsorbedmoleculesareignored第32页/共84页June2009June2009Slide33BETModel第33页/共84页June2009June2009Slide34BETModel第34页/共84页June2009June2009Slide35BETModel第35页/共84页June2009June2009Slide36Brunauer,Emmett,andTeller第36页/共84页June2009June2009Slide37Brunauer,Emmett,andTeller第37页/共84页June2009June2009Slide38N2AdsorptiononMacro-PorousSilicaAmorphous1000Å,poreDesorptionLackofHysteresis第38页/共84页June2009June2009Slide39N2–BETTransformation25.7m2/g第39页/共84页June2009June2009Slide40TypeIV-IsothermMulti-layeradsorptionMesoporesbetween2and50nmporewidthReducedsaturationpressureinporesHysteresisShapeTortuosity第40页/共84页June2009June2009Slide41N2AdsorptiononAmorphousSilica-Alumina

Amorphous100Å,poreDesorptionHysteresis第41页/共84页June2009June2009Slide42N2–BETTransformation–215.5m2/g第42页/共84页June2009June2009Slide43N2AdsorptiononMCM-41MesoporousSilica40Å,cylindricalporeDesorptionLackofHysteresis第43页/共84页June2009June2009Slide44N2BETTransformation–926.8m2/g第44页/共84页June2009第45页/共84页June2009第46页/共84页June2009June2009Slide47t-Plot→“RulesofThumb”SlopeofalinearregioncorrespondstoareaInterceptfromalinearregionisaporevolumeBasedonBETsurfacearea大孔或无孔微孔微孔-介孔第47页/共84页June2009June2009Slide48Statisticalt-Curve第48页/共84页June2009June2009Slide49Statisticalt-Curve第49页/共84页June2009June2009Slide50Statisticalt-Curve第50页/共84页June2009June2009Slide51Statisticalt-Curve第51页/共84页June2009June2009Slide52Thet-MethodStatisticalcurves第52页/共84页June2009June2009Slide53Thet-MethodSilicasurfacewith1000ÅporesDFTusedtodeterminemonolayercapacity第53页/共84页June2009June2009Slide54Thet-MethodSilicaSurfacewith1000ÅporesBETusedtodeterminemonolayercapacity第54页/共84页June2009June2009Slide55Thet-Method→

Microporoussample–13XFaujasite–13XSilicasurfacewith1000Åpores–DFTusedforVm典型微孔截距-微孔体积斜率-外表面积第55页/共84页June2009June2009Slide56Thet-Method→Mesoporoussample–SilicaAlumina110ÅmesoporoussilicaSilicasurfacewith1000Åpores–DFTusedforVm典型介孔斜率-表面积第56页/共84页June2009June2009Slide57Thet-Method→

Mesoporoussample–MCM4140Åmesoporoussilica,cylindricalporesSilicasurfacewith1000Åpores–DFTusedforVm微孔-介孔斜率-表面积第57页/共84页June2009June2009Slide58Thet-Method→

Mesoporoussample–MCM4140Åmesoporoussilica,cylindricalporesSilicasurfacewith1000Åpores–DFTusedforVm截距-介孔体积斜率-外表面积第58页/共84页June2009要点：大孔或无孔为通过原点的直线微孔为先陡后缓的折线介孔为平缓后转向上升，对应于毛细管凝聚对于所有材料，t-曲线的最后部分均为外表面吸附t-曲线是最好的计算微孔体积的方法第59页/共84页June2009第60页/共84页June2009第61页/共84页June2009June2009Slide62CapillaryCondensationHydraulicradiusKelvinequationAdsorbedLayerThicknessequationorcurve第62页/共84页June2009June2009Slide63BJHCalculations第63页/共84页June2009June2009Slide64BJHCalculationsCalculatethehydraulicradiusforcapillarycondensationinMesopores.CylindricalgeometryisthestandardforBJHcalculations.Poresize>20Å,(reducedprecisionbelow75Å)第64页/共84页June2009June2009Slide65BJHExampleDataAmorphousSilicaAluminaSurfacearea–214m2/g第65页/共84页June2009June2009Slide66BJHExampleDataAmorphousSilicaAluminaBETSurfacearea–214m2/g第66页/共84页June2009June2009Slide67BJHExampleDataAmorphousSilicaAluminaBETSurfacearea–214m2/g第67页/共84页June2009June2009Slide68BJHExampleDataAmorphousSilicaAluminaBETSurfacearea–214m2/g第68页/共84页June2009June2009Slide69BJHExampleDataAmorphousSilicaAluminaBETSurfacearea–214m2/g第69页/共84页June2009June2009Slide70BJHExampleDataMesoporousSilicaSurfacearea–926.8m2/g第70页/共84页June2009June2009Slide71BJHExampleDataMesoporousSilicaSurfacearea–926.8m2/g第71页/共84页June2009June2009Slide72BJHExampleDataMesoporousSilicaSurfacearea–926.8m