孔材料比表面與孔結(jié)構(gòu)的表征

1、孔材料比表面與孔結(jié)構(gòu)的表征注意 我們拿到的數(shù)據(jù)，只有吸脫附曲線是真實(shí)的，比表面積、孔徑分布、孔容等都是通過模型計(jì)算出的數(shù)據(jù)。 我們所講的BET（Brunauer-Emmet-Teller）只是對(duì)N2-sorption isotherm中p/p0之間的一小段用BET公式處理了一下，得到單層吸附量Vm，然后據(jù)此算出比表面積。 吸附等溫線 以相對(duì)壓力p/p0為X軸，氮?dú)馕搅繛閅軸得到的曲線稱為吸附等溫線 可將X軸相對(duì)壓力粗略地分為低壓（）、中壓(0.3-0.6)、高壓（）三段。低壓端偏Y軸則說明材料與氮有較強(qiáng)作用力(型，型，型)，較多微孔存在時(shí)由于微孔內(nèi)強(qiáng)吸附勢(shì)，吸附曲線起始時(shí)呈型；低壓端偏X軸說

2、明作用力弱(型，型)。 中壓端多為氮?dú)庠诓牧峡椎纼?nèi)的冷凝積聚，介孔分析就來源于這段數(shù)據(jù)，包括樣品粒子堆積產(chǎn)生的孔，有序或梯度的介孔范圍內(nèi)孔道。BJH方法就是基于這一段得出的孔徑數(shù)據(jù) 高壓段可粗略地看出粒子堆積程度，如型中若最后上揚(yáng)，則粒子不均勻。平常得到的總孔容通常是取相對(duì)壓力為左右時(shí)氮?dú)馕搅康睦淠?滯 后 環(huán)滯后環(huán)的產(chǎn)生原因 由于毛細(xì)管凝聚作用使N2 分子在低于常壓下冷凝填充了介孔孔道，由于開始發(fā)生毛細(xì)凝聚時(shí)是在孔壁上的環(huán)狀吸附膜液面上進(jìn)行，而脫附是從孔口的球形彎月液面開始，從而吸脫附等溫線不相重合，往往形成一個(gè)滯后環(huán)。 滯后環(huán)的特征對(duì)應(yīng)于特定的孔結(jié)構(gòu)信息H1是尺寸分布較窄的均勻孔模型，

3、如圓柱形孔。H2比較難解釋，一般認(rèn)為是多孔吸附質(zhì)或均勻粒子堆積孔造成的，孔分布較寬，多認(rèn)為是“ink bottle”，當(dāng)小孔徑瓶頸中的液氮脫附后，束縛于瓶體中的液氮?dú)怏w會(huì)驟然逸出；滯后環(huán)的種類H3與H4相比高壓端吸附量大，認(rèn)為是片狀粒子堆積形成的狹縫孔（slit）如粘土H4也是狹縫孔，區(qū)別于粒子堆集，是一些類似由層狀結(jié)構(gòu)產(chǎn)生的孔如活性炭幾個(gè)常數(shù) 溫度77K時(shí),液氮六方密堆積氮分子橫截面積0.162 nm2，形成單分子層鋪展時(shí)認(rèn)為單分子層厚度為0.354 nm 標(biāo)況（STP）下1mL氮?dú)饫淠螅俣勖芏炔蛔儯w積為0.001547mL例：吸附曲線p/p0最大時(shí)氮?dú)馕搅考s為400 mL，則可

4、知總孔容 STP 每毫升氮?dú)夥肿愉伋蓡畏肿訉诱加妹娣e4.354 m2例：BET方法得到的比表面積S(平方米/每克)4.354*Vm，其中Vm為單層吸附量，由BET方法處理可知Vm=1/(斜率截距)June 2009June 2009Slide 13OutlineDefinition of AdsorptionPhysical AdsorptionMonolayer ModelsStandard IsothermsMeso-PorosityMicro-PorosityHigh-pressure SorptionChemical AdsorptionCharacterizationStatic C

5、hemisorptionPulse ChemisorptionHeat of AdsorptionTemperature Programmed AnalysesTemperature Programmed TechniquesHeat of DesorptionApplicationsJune 2009June 2009Slide 14AdsorptionAdsorption Enrichment in an interfacial layerAdsorbate Substance in the adsorbed stateAdsorptive Adsorbable substance in

6、the fluid phaseAdsorbent Solid material on which adsorption occursPhysisorption Adsorption without chemical bondingpsat or Psat Saturation pressure (of the cryogen)p0 or P0 Saturation pressure of the adsorptiveChemisorption Adsorption involving chemical bondingJune 2009June 2009Slide 15AdsorptionGen

7、eral phenomenon with a relatively low degree of specificity.Retains identity; desorbs to fluid phase in its original form.Exothermic adsorption similar to the energy of condensations.Rapid equilibration; transport limited.Dependent on reactivity of adsorbent and adsorptive.Chemisorbed molecule may r

8、eact or dissociate.Energy is similar to energy change for chemical reaction.Activated process at elevated temperature.June 2009June 2009Slide 16AdsorptionMolecules from the gas phase strike the surface.At equilibrium the molecule adsorbs, loses the heat of adsorption (q), and subsequently desorbs fr

9、om the surface.At equilibrium the rate of condensation = rate of desorption.Constant surface coverage at equilibrium.June 2009June 2009Slide 17AdsorptionSurface features change the adsorption potential.Surface area models neglect the effects of localized phenomenon.Curved surfaces or roughness provi

10、de enhanced adsorption potential.June 2009June 2009Slide 18AdsorptionAs the system pressure is increased (gas concentration also increases) multiple layers sorb to the surface.The monolayer coverage, a densely packed single adsorbed layer, is used for determining surface area.As pressure is further

11、increased and adsorption proceeds gas condenses in the pores and this volume of condensed adsorptive is used for characterizing porosity.June 2009June 2009Slide 19AdsorptionJune 2009June 2009Slide 20Adsorptionnm = Monolayer quantity, molVm = Monolayer volume, cm3Vg = Molar volume of gas at STP, cm3/

12、molNa = Avogadros number 6.023 x 1023 molecules/molw = sample mass, gA = Cross-sectional area of the adsorbate, m2June 2009June 2009Slide 21Physisorption - HardwareStainless Steel manifold1000, 10, 1 torr transducersDedicated vacuum systemCryogen level control / long Dewar lifeJune 2009June 2009Slid

13、e 22AdsorptionQuantity adsorbed vs. pressure.Pressure is usually varied from vacuum to near atmospheric.Constant temperature.Quantity adsorbed is normalized for adsorbent mass.Six isotherm classifications.Types I, II and IV most materialsType II uncommonType V rareType VI highly uniform surfaceJune

14、2009June 2009Slide 23June 2009June 2009Slide 24Type I IsothermMono-layer adsorptionMicropore fillingMicropores 50 nm pore widthUniform surfaceMultilayer adsorptionInfinite adsorption as p/p0 approaches 1June 2009June 2009Slide 31Brunauer, Emmett, and TellerStephen BrunauerPaul EmmettEdward TellerJun

15、e 2009June 2009Slide 32BET AssumptionsMulti-layer adsorptionNon-porous, Uniform surfaceHeat of adsorption for the first layer is higher that successive layers.Heat of adsorption for second and successive layers equals the heat of liquefactionLateral interactions of adsorbed molecules are ignoredJune

16、 2009June 2009Slide 33BET ModelJune 2009June 2009Slide 34BET ModelJune 2009June 2009Slide 35BET ModelJune 2009June 2009Slide 36Brunauer, Emmett, and TellerJune 2009June 2009Slide 37Brunauer, Emmett, and TellerJune 2009June 2009Slide 38N2 Adsorption on Macro-Porous SilicaAmorphous1000, poreDesorption

17、Lack of HysteresisJune 2009June 2009Slide 39N2 BET Transformation 25.7 m2/gJune 2009June 2009Slide 40Type IV - IsothermMulti-layer adsorptionMesopores between 2 and 50 nm pore widthReduced saturation pressure in poresHysteresisShapeTortuosityJune 2009June 2009Slide 41N2 Adsorption on Amorphous Silic

18、a-Alumina Amorphous100, poreDesorptionHysteresisJune 2009June 2009Slide 42N2 BET Transformation 215.5 m2/gJune 2009June 2009Slide 43N2 Adsorption on MCM-41Mesoporous Silica40, cylindrical poreDesorptionLack of HysteresisJune 2009June 2009Slide 44N2 BET Transformation 926.8 m2/gJune 2009June 2009June

19、 2009June 2009Slide 47t-Plot “Rules of Thumb”Slope of a linear region corresponds to areaIntercept from a linear region is a pore volumeBased on BET surface area大孔或無孔微孔微孔-介孔June 2009June 2009Slide 48Statistical t-CurveJune 2009June 2009Slide 49Statistical t-CurveJune 2009June 2009Slide 50Statistical

20、 t-CurveJune 2009June 2009Slide 51Statistical t-CurveJune 2009June 2009Slide 52The t-MethodStatistical curvesJune 2009June 2009Slide 53The t-MethodSilica surface with 1000 poresDFT used to determine monolayer capacityJune 2009June 2009Slide 54The t-MethodSilica Surface with 1000 poresBET used to det

21、ermine monolayer capacityJune 2009June 2009Slide 55The t-Method Microporous sample 13XFaujasite 13XSilica surface with 1000 pores DFT used for Vm典型微孔截距-微孔體積斜率-外表面積June 2009June 2009Slide 56The t-Method Mesoporous sample Silica Alumina110 mesoporous silicaSilica surface with 1000 pores DFT used for V

22、m典型介孔斜率-表面積June 2009June 2009Slide 57The t-Method Mesoporous sample MCM 4140 mesoporous silica, cylindrical poresSilica surface with 1000 pores DFT used for Vm微孔-介孔斜率-表面積June 2009June 2009Slide 58The t-Method Mesoporous sample MCM 4140 mesoporous silica, cylindrical poresSilica surface with 1000 por

23、es DFT used for Vm截距-介孔體積斜率-外表面積June 2009要點(diǎn)：大孔或無孔為通過原點(diǎn)的直線微孔為先陡后緩的折線介孔為平緩后轉(zhuǎn)向上升，對(duì)應(yīng)于毛細(xì)管凝聚對(duì)于所有材料，t-曲線的最后部分均為外表面吸附t-曲線是最好的計(jì)算微孔體積的方法June 2009June 2009June 2009June 2009Slide 62Capillary CondensationHydraulic radiusKelvin equationAdsorbed LayerThickness equation or curveJune 2009June 2009Slide 63BJH Calcul

24、ationsJune 2009June 2009Slide 64BJH CalculationsCalculate the hydraulic radius for capillary condensation in Mesopores.Cylindrical geometry is the standard for BJH calculations.Pore size 20 , (reduced precision below 75)June 2009June 2009Slide 65BJH Example DataAmorphous Silica AluminaSurface area 2

25、14 m2/gJune 2009June 2009Slide 66BJH Example DataAmorphous Silica AluminaBET Surface area 214 m2/gJune 2009June 2009Slide 67BJH Example DataAmorphous Silica AluminaBET Surface area 214 m2/gJune 2009June 2009Slide 68BJH Example DataAmorphous Silica AluminaBET Surface area 214 m2/gJune 2009June 2009Slide 69BJH Example DataMesoporous SilicaSurface area 926.8 m2/gJune 2009June 2009Slide 70BJH Example DataMesoporous SilicaSurface area 926.