2013-孔材料比表面与孔结构的表征

ቤተ መጻሕፍቲ ባይዱ• Chemical Adsorption
• Temperature Programmed Analyses
• Temperature Programmed Techniques • Heat of Desorption • Applications
June 2009
Slide 13
Adsorption
June 2009
Slide 20
Physisorption - Hardware
• Stainless Steel manifold • Dedicated vacuum system • 1000, 10, 1 torr transducers • Cryogen level control / long Dewar
June 2009
Slide 18
Adsorption
June 2009
Slide 19
Adsorption
Na A w V N Surface Area = m a A Vg w Surface Area = nm
nm = Monolayer quantity, mol Vm = Monolayer volume, cm3
◆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。
滞后环的种类
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。
Adsorption → Enrichment in an interfacial layer Adsorbate → Substance in the adsorbed state Adsorptive → Adsorbable substance in the fluid phase Adsorbent → Solid material on which adsorption occurs
June 2009
Slide 16
Adsorption
• • •
Surface features change the adsorption potential. Surface area models neglect the effects of localized phenomenon. Curved surfaces or roughness provide enhanced adsorption potential.
※ 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)
Outline
• Definition of Adsorption • Physical Adsorption
116.970 m?g
90.0825 m?g
Pore Volume Single point adsorption total pore volume of pores less than 673.740 ?radius at P/Po = 0.985558180: Single point desorption total pore volume of pores less than 339.969 ?radius at P/Po = 0.970898318: t-Plot micropore volume: BJH Adsorption cumulative volume of pores between 8.500 ?and 1500.000 ?radius: BJH Desorption cumulative volume of pores between 8.500 ?and 1500.000 ?radius:
Surface Area Single point surface area at P/Po = 0.200095567: BET Surface Area: Langmuir Surface Area: t-Plot Micropore Area: t-Plot External Surface Area: BJH Adsorption cumulative surface area of pores between 8.500 ?and 1500.000 ?radius: BJH Desorption cumulative surface area of pores between 8.500 ?and 1500.000 ?radius: 344.4811 m?g 340.2511 m?g 469.4068 m?g 141.0897 m?g 199.1614 m?g
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(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)。
• • • •
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.
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； 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
0.311831 cm?g
0.310266 cm?g 0.065320 cm?g
0.216591 cm?g
0.190212 cm?g
Pore Size Adsorption average pore width (4V/A by BET): Desorption average pore width (4V/A by BET): BJH Adsorption average pore radius (2V/A): BJH Desorption average pore radius (2V/A): 36.6589 36.4750 37.033 42.231
Physisorption → Adsorption without chemical bonding psat or Psat → Saturation pressure (of the cryogen)
p0 or P0 → Saturation pressure of the adsorptive
Chemisorption → Adsorption involving chemical bonding
June 2009
Slide 14
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 equilibration; transport limited.
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
June 2009
Slide 15
Adsorption
• •
• •
Molecules from the gas phase strike 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.
◆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*0.001547＝0.61mL
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N2sorption isotherm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。
June 2009
Slide 17
Adsorption
• • •
As 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 increased and adsorption proceeds gas condenses in the pores and this volume of condensed adsorptive is used for characterizing porosity.
Vg = Molar volume of gas at STP, cm3/mol
Na = Avogadro’s number 6.023 x 1023 molecules/mol w = sample mass, g σA = Cross-sectional area of the adsorbate, m2
life
June 2009
Slide 21
Adsorption
• • • • •
Quantity adsorbed vs. pressure. Pressure is usually varied from vacuum to near atmospheric. Constant temperature. Quantity adsorbed is normalized for adsorbent mass. Six isotherm classifications.
• • • • • • • • • Monolayer Models Standard Isotherms Meso-Porosity Micro-Porosity High-pressure Sorption Characterization Static Chemisorption Pulse Chemisorption Heat of Adsorption