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1、孔材料比表面与孔结构的表征,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-sorption isotherm中p/p0=0.050.35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。,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。,低压端偏Y轴则说明材料与氮有较强作用力(型，型，型)，较多微孔存在时由
2、于微孔内强吸附势，吸附曲线起始时呈型；低压端偏X轴说明作用力弱(型，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,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。,滞后
3、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 “ink bottle”，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,几个常数, 温度77K时,液氮六方密堆积氮分子横截面积0.162 nm2，形成单分子层铺展时认为单分子层厚度为0.354 nm, 标况（STP）下1mL氮气冷凝后（假定凝聚密度不变）体积
4、为0.001547mL 例：吸附曲线p/p0最大时氮气吸附量约为400 mL，则可知总孔容400*0.0015470.61mL, STP 每毫升氮气分子铺成单分子层占用面积4.354 m2 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),June 2009,June 2009,Slide 13,Outline,Definition of Adsorption Physical Adsorption Monolayer Models Standard Isotherms Meso-Porosity Micro-Po
5、rosity High-pressure Sorption Chemical Adsorption Characterization Static Chemisorption Pulse Chemisorption Heat of Adsorption Temperature Programmed Analyses Temperature Programmed Techniques Heat of Desorption Applications,June 2009,June 2009,Slide 14,Adsorption,Adsorption Enrichment in an interfa
6、cial layer Adsorbate Substance in the adsorbed state Adsorptive Adsorbable substance in the fluid phase Adsorbent Solid material on which adsorption occurs,Physisorption Adsorption without chemical bonding psat or Psat Saturation pressure (of the cryogen) p0 or P0 Saturation pressure of the adsorpti
7、ve,Chemisorption Adsorption involving chemical bonding,June 2009,June 2009,Slide 15,Adsorption,General 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 equilibrat
8、ion; transport limited.,Dependent on reactivity of adsorbent and adsorptive. Chemisorbed molecule may react or dissociate. Energy is similar to energy change for chemical reaction. Activated process at elevated temperature.,June 2009,June 2009,Slide 16,Adsorption,Molecules from the gas phase strike
9、the surface. At equilibrium the molecule adsorbs, loses the heat of adsorption (q), and subsequently desorbs from the surface. At equilibrium the rate of condensation = rate of desorption. Constant surface coverage at equilibrium.,June 2009,June 2009,Slide 17,Adsorption,Surface features change the a
10、dsorption potential. Surface area models neglect the effects of localized phenomenon. Curved surfaces or roughness provide enhanced adsorption potential.,June 2009,June 2009,Slide 18,Adsorption,As the system pressure is increased (gas concentration also increases) multiple layers sorb to the surface
11、. The monolayer coverage, a densely packed single adsorbed layer, is used for determining surface area. As pressure is further increased and adsorption proceeds gas condenses in the pores and this volume of condensed adsorptive is used for characterizing porosity.,June 2009,June 2009,Slide 19,Adsorp
12、tion,June 2009,June 2009,Slide 20,Adsorption,nm = Monolayer quantity, mol Vm = Monolayer volume, cm3 Vg = Molar volume of gas at STP, cm3/mol Na = Avogadros number 6.023 x 1023 molecules/mol w = sample mass, g A = Cross-sectional area of the adsorbate, m2,June 2009,June 2009,Slide 21,Physisorption -
13、 Hardware,Stainless Steel manifold 1000, 10, 1 torr transducers,Dedicated vacuum system Cryogen level control / long Dewar life,June 2009,June 2009,Slide 22,Adsorption,Quantity adsorbed vs. pressure. Pressure is usually varied from vacuum to near atmospheric. Constant temperature. Quantity adsorbed
14、is normalized for adsorbent mass. Six isotherm classifications. Types I, II and IV most materials Type II uncommon Type V rare Type VI highly uniform surface,June 2009,June 2009,Slide 23,June 2009,June 2009,Slide 24,Type I Isotherm,Mono-layer adsorption Micropore filling Micropores 2nm pore width Fi
15、nely divided surface Limiting amount adsorbed a p/p0 approaches 1,June 2009,June 2009,Slide 25,Langmuir,June 2009,June 2009,Slide 26,Langmuir,June 2009,June 2009,Slide 27,Langmuir,June 2009,June 2009,Slide 28,N2 Adsorption on 13x Zeolite,Crystalline 10 (8) , pore Silicaalumina Ca+ exchanged zeolite,
16、June 2009,June 2009,Slide 29,N2 Adsorption on 13x Zeolite,June 2009,June 2009,Slide 30,Type II Isotherm,Non-porous surface Macropores 50 nm pore width Uniform surface Multilayer adsorption Infinite adsorption as p/p0 approaches 1,June 2009,June 2009,Slide 31,Brunauer, Emmett, and Teller,Stephen Brunauer Paul Emmett Edward Teller,June 2009,June 2009,Slide 32,BET Assumptions,Multi-layer adsorption Non-porous, Uniform surface Heat of adsor
