物理化学() PHYSICAL CHEMISTRY(22),11.7 固体表面的吸附Adsorption on solid surface,1. Surface energy of solids and adsorption on surface of solids,Solid Na Ag NaCl MgO 石蜡 聚乙烯 云母 /(mJ m-2) 200 800 190 1200 25.4 33.1 2400,大多数固体比液体具有更高的表面能,（4） 表面组成,合成氨催化剂（Fe-Al2O3-K2O)Fe K Al Ca O 体相组成：mol% 40.5 0.35 2.0 1.7 53.2 表面组成：mol% 8.6 36.1 10.7 4.7 40.0,（5）表面Gibbs函数表面张力个性异性 不同方法测出的数值不同,固体表面的气体与液体有在固体表面自动聚 集，以求降低表面能的趋势。,2. Basic concepts(1) adsorbent(吸附剂) and adsorbate (吸附质),固体表面的气体或液体的浓度高于其本体 浓度的现象，称为固体的表面吸附。,广泛的应用：,干燥剂、防毒面具、脱色剂、色谱、污水处 理、催化剂、,(2) adsorption equilibrium,S + G,At equilibrium : ra = rd ra : rate of adsorption rd : rate of desorption,(3) Amount adsorbed (吸附量）,SG,adsorption,desorption,(4) Adsorption curves,(a) 吸附等温线,For a given adsorbent and adsorbate,p,T1T2T3,p,p,p,p,(b) 吸附等压线,(c) 吸附等量线,p1 p2 p3,T,p,T,1 23,3. Adsorption isothermal equations,(1) Langmuir equation (1916),(a) Adsorption is monolayer.,(b) The surface of solid is uniform.,(c) There is not interacting force amongthe molecules adsorbed.,Basic assumptions:,Considered adsorption as a reaction,G + S SG,ka,kd,ka , kd : 分别为吸附和解吸(脱附)过程的速率系数,吸附速率：,p ：被吸附气体的压力 ：表面覆盖率(fraction of the surface covered) 1- : 表面空白率(fraction of the surface vacancy),脱附速率：,或,Langmuir isothermal adsorption equation,b= ka/kd : 吸附系数(adsorption coefficient)(吸附平衡常数）,Discussion: (a) isotherm:at low pressure , bp1,bp, pat high pressure, bp1, 1, = max max: saturated amount of adsorption(即固体表面全部铺满一层气体分子时的吸量),p,max,(b) 方程式可改写为:,以p/(或p/V)对p作图,可求max(Vmax) 和a.,p,P/V,Ha : 等压吸附热(adsorption heat at constant pressure),In general,HabB,P 0,First - order,P,Zeroth- order,r,pA,(2) 反应物为两种不同的物质,A + B P, L - H (Langmuir - Hinsheluood) Mechanism,在反应过程中,两种物质同时被吸附,A + B + 2S AS + BS PS P + S,kr,If pB keeps constant,