Ch20Adsorption

7/29/2019 Ch20Adsorption

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Lecture 20.

Adsorption Phenomena


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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 ".


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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


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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:


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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


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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.


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Langmuir Isotherm


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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:


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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.


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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


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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.


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BET Isotherm

Ch20Adsorption

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