1. Equilibrium Adsorption
ChE 553 Lecture 6
Equilibrium Adsorption

2. Objective Introduce Adsorption Isotherms
How much gas adsorbs at equilibrium as a function of pressure
Qualitative features of isotherms
Types 1-5
More complex phase behavior

3. Topics Introduction to Adsorption Isotherms Five types
Langmuir Adsorption Isotherm
Simple Adsorption Isotherm
Finite number of surface spaces to hold gas
More complex behavior: surface phase transitions due to adsorbate/adsorbate interactions

4. Introduction To Adsorption Isotherms
Adsorption Isotherm – amount adsorbed as a function of pressure
S-shaped curve typical at T>300K

5. Typical Behavior At Low Temperature
Figure 4.2 Isotherms for adsorption of krypton, argon at 77 K, argon at 91.3 K, and ammonia on graphitized carbon black. (Data of Putnam and Fart [1975], Ross and Winkler [1955], Basset, Boucher, and Zettlemoyer [1968], and Bomchil et al. [1979], respectively.)

6. S-Shaped Curve At Low Pressure
Figure 4.3 A blowup of the low-pressure part of the krypton data in Figure 4.2.

7. General Adsorption Isotherms
Figure 4.4 The five types of adsorption isotherms described by Brunauer [1945].

8. Next Langmuir’s Model Of Adsorption:
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.

9. Key Features Of Langmuir’s Model
Finite sites to adsorb gas
Ideal behavior in surface phase (no interactions between adsorbed molecules?
At low pressures coverages proportional to pressure (or p1/2)
Eventually surface fills up
Adsorption limited by availability of sites
If multiple species – competition for sites
Maximum coverage 1ML

10. Kinetic Derivation Of Langmuir’s Model
Assume equilibrium
At equilibrium rd=rad. Solving
(4.4)
(4.1)
(4.2)
(4.3)

11. Isotherm Arises From A Site Balance

12. Qualitative Features of Langmuir’s Model

13. Derivation Of Langmuir Isotherm For Competitive Adsorption

14. Solving Equations Simultaneously

15. Qualitative Picture

16. Derivation Of Langmuir Adsorption For Dissociative Adsorption
Qualitatively the same as non-dissociative

17. Comparison To Data
  (4.27)
Rearranging

18. Real Situation: Interactions Between Molecules
Attractive interactions lead to islands

19. Repulsive Interactions Order Overlayer Continued

20. Phase Diagrams For Adsorption

21. Strength of Attraction
Qualitative Behavior
1/RT
Strength of Attraction

22. Universal Curve: Monolayer Adsorption: Only Nearest Neighbor Interactions
Equilibrium constant at half coverage

23. Complication: Multilayer Adsorption

24. The BET Equation Assume Random distribution of sites with
1, 2, 3… adsorbed molecules
No lateral interactions between molecules
Does not work that well in practice

25. A Comparison Of The Krypton Data In Figure 4
A Comparison Of The Krypton Data In Figure 4.12 To The Best Fit With The BET Equation

26. Comparison Continued Used anyway
Figure 4.45 A blowup of the portion of the date in Figure 4.44 below 4 torr.
Used anyway

27. Why 5 Types Of Adsorption Isotherms?

28. Type I 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)

29. Type III
Type III arises when there are strong attractive interactions leading to condensation
Initially, no adsorption
Pressure increases lead to nucleation and growth of islands
Eventually liquids condense on the surface

30. Type II 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.

31. Type V Type V is another case for attractive interactions
Initially no adsorption
Next nucleation and growth of islands or liquid drops
Coverage saturates when no more space to hold adsorbates

32. Type IV
Type V 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

33. Summary Introduction to Adsorption Isotherms
Five types
Langmuir Adsorption Isotherm
Simple Adsorption Isotherm
Finite number of surface spaces to hold gas
More complex behavior: surface phase transitions due to adsorbate/adsorbate interactions
Attractive Interactions: Classical two phase regions i.e. solids and gases
Repulsive interactions - adsorbate ordering
Leads to universality (general phase diagrams that do not depend on gas and solid)