7/29/2019 Ch20Adsorption
1/12
Lecture 20.
Adsorption Phenomena
7/29/2019 Ch20Adsorption
2/12
Adsorption is the accumulation of atomsor molecules on the surface of a material.This process creates a film of theadsorbate (the molecules or atoms beingaccumulated) on the adsorbent's surface.It is different from absorption, in which asubstance diffuses into a liquid or solid toform a solution. The term sorption encompasses both processes, whiledesorption is the reverse process of "adsorption ".
7/29/2019 Ch20Adsorption
3/12
Isotherms
Adsorption is usually described throughisotherms, that is, the amount of adsorbateon the adsorbent as a function of itspressure (if gas) or concentration (if liquid)at constant temperature.some of these isotherms are:
Langmuir BET
7/29/2019 Ch20Adsorption
4/12
Langmuir
Langmuir derived a relationship for q; weightadsorbed per unit wt of adsorbent and C;concentration in fluid based on some quitereasonable assumptions. These are: auniform surface, a single layer of adsorbedmaterial, and constant temperature. The rateof attachment to the surface should beproportional to a driving force times an area.
The driving force is the concentration in thefluid, and the area is the amount of baresurface. If the fraction of covered surface isF , the rate per unit of surface is:
7/29/2019 Ch20Adsorption
5/12
Langmuir rate go ing on = k 1 C ( 1 - )
The evaporation from the surface isproportional to the amount of surfacecovered:
rate leav ing = k 2
where k 1 and k 2 are rate coefficientsC = concentration in the fluid= fraction of the surface covered
7/29/2019 Ch20Adsorption
6/12
Langmuir
At equilibrium, the two rates are equal,and we find that:
A plot of versus should indicate astraight line of slope and anintercept of . The graph shows datapoints and lines fitted to both Freundlichand Langmuir equations.
7/29/2019 Ch20Adsorption
7/12
Langmuir Isotherm
7/29/2019 Ch20Adsorption
8/12
BET Theory
BET theory is a rule for the physicaladsorption of gas molecules on a solidsurface and serves as the basis for animportant analysis technique for themeasurement of the specific surface areaof a material. The concept of the theory is
an extension of the Langmuir theory,which is a theory for monolayer molecular adsorption, to multilayer adsorption with
the following hypotheses:
7/29/2019 Ch20Adsorption
9/12
BET Theory
(a)gas molecules physically adsorb on asolid in layers infinitely;
(b) there is no interaction between eachadsorption layer; and
(c) the Langmuir theory can be applied toeach layer.
7/29/2019 Ch20Adsorption
10/12
BET Theory
The resulting BET equation is expressedby:
P and P 0 are the equilibrium and thesaturation pressure of adsorbates at thetemperature of adsorption,v is the adsorbed gas quantity (for example, in volume units), and
7/29/2019 Ch20Adsorption
11/12
BET Theoryv m is the monolayer adsorbed gasquantity.
c is the BET constant.
The BET method is widely used in surfacescience for the calculation of surface areasof solids by physical adsorption of gasmolecules.
7/29/2019 Ch20Adsorption
12/12
BET Isotherm