Colloid chemistry - Chapter 5: Adsorption on G-S surface - Ngo Thanh An

Nội dung text: Colloid chemistry - Chapter 5: Adsorption on G-S surface - Ngo Thanh An

1.   =  g P o   =  g = g + RT ln Po o Where g is the standard chemical potential of the vapor phase, that is, the chemical potential at the reference pressure po. Differentiating at constant adsorbed phase concentration and applying Gibbs – Helmholtz relation   T H = − T T 2
2. Ⅰ Microporous (Active carbon, Zeolite) Ⅱ Non-porous （Metal powder) Ⅲ Non porous and weak adsorption interaction Ⅳ Mesoporous （Silica gel） Ⅴ Porous and weak adsorption interaction Ⅵ Energetically uniform surface Micropore ～2nm Mesopore 2nm～50nm Macropore 50nm～
3. • Type I arises when only one type of site: • Initially surface fills randomly • Eventually saturates when surface filled (or pores filled with a porous material) 24
4. • Type II arises when the is more than one adsorption site • Initial rapid adsorption • Saturates when first site filled • Second rise when second site fills • Second site could be a second monolayer, a second site on the surface. In porous materials, it can also be a second type of pore. 26
5. • Type IV occurs when there are multiple phase transitions due to a mixture of attractive and repulsive interactions • Can also arise in multilayer adsorption where adsorption on second layer starts before first layer saturates 28
6. Adsorption on Mesoporous Samples: Capillary Condensation n n n 0 0 0 0 p/p 1 0 p/p 1 0 p/p 1 n n n 0 0 0 0 p/p 1 0 p/p 1 0 p/p 1
7. The relation of equilibrium vapor pressure to the saturation vapor pressure can be thought of as a relative humidity measurement for the atmosphere. As Pv/Psat increases, vapor will continue to condense inside a given capillary. If Pv/Psat decreases, liquid will begin to evaporate into the atmosphere as vapor molecules System A → Pv=0, no vapor is present in the system System B → Pv=P1<Psat, capillary condensation occurs and liquid/vapor equilibrium is reached System C → Pv=P2<Psat, P1<P2, as vapor pressure is increased condensation continues in order to satisfy the Kelvin equation System D → Pv=Pmax<Psat, vapor pressure is increased to its maximum allowed value and the pore is filled completely This figure is used to demonstrate the concept that by increasing the vapor pressure in a given system, more condensation will occur. In a porous medium, capillary condensation will always occur if Pv ≠ 0.
8. Figure 4.5 Langmuir’s model of the structure of the adsorbed layer. The black dots represent possible adsorption sites, while the white and mauve ovals represent adsorbed molecules. 40
9. • Molecules in gas and surface are in dynamic equilibrium G (g) +  (surface) ↔ G- • Isotherm describes pressure dependence of equilibrium • Langmuir isotherm proposed by Irving Langmuir • (1932 Noble Prize) • Adsorption saturates at 1 monolayer • All sites are equivalent • Adsorption is independent of coverage * ratea= k a P A N rated= k d N A Site conservation Equilibrium KPA +  ==, K k k θ + θ* = 1 rate = rate A a d A ads des 1+ KPA 42
10. Assumptions １ Surface is energetically homogenious. ２ There is no lateral interface between adsorbed molecules. The adsorption energies in the second and all higher layers are equal ３ to condensation energy of adsorptive. y x
11. Assignments 1. Check the fit of Langmuir equation and determine K (equilibrium constant of adsorption) and V : P, Torr 100 200 300 400 500 600 700 V, ml 10.2 18.6 25.5 31.5 36.9 41.6 46.1 2. The data given below are for the adsorption of nitrogen on alumina at 77.3 K. Show that they fit in a BET isotherm in the range of adsorption and 2 find Vm and hence surface area of alumina (m /g). At 77.3 K, saturation pressure, Po = 733.59 torr. The volumes are corrected to STP and refer to -20 2 1g of alumina. Given: contact area of one N2 molecule =16.2x10 m ) P, Torr 37.67 74.20 114.54 142 185.34 V, ml/g 23.14 28.1 33.1 36.35 41.49