Equilibrium Adsorption ChE 553 Lecture 6 Equilibrium Adsorption

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

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

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

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.)

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

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

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.

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

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

Isotherm Arises From A Site Balance

Qualitative Features of Langmuir’s Model

Derivation Of Langmuir Isotherm For Competitive Adsorption

Solving Equations Simultaneously

Qualitative Picture

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

Comparison To Data   (4.27) Rearranging  

Real Situation: Interactions Between Molecules Attractive interactions lead to islands

Repulsive Interactions Order Overlayer Continued

Phase Diagrams For Adsorption

Strength of Attraction Qualitative Behavior 1/RT Strength of Attraction

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

Complication: Multilayer Adsorption

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

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

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

Why 5 Types Of Adsorption Isotherms?

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)

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

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.

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

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

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)