METHODS OF CHARACTERIZATION OF ADSORBENTS-A REVIEW VII

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METHODS OF CHARACTERIZATION OF ADSORBENTS-A REVIEW VII

chemonaire
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2.4 Brunauer-Emmet-Teller (BET) Gas Sorption Method

The method is widely used to assess surface area and porous characteristics of materials with technological importance using N2 adsorption at 77 K. In fact, it has been recognised as a standardised method (Mohamed, 2011)
BET adsorption is a monolayer adsorption in which layers of the adsorbate molecules are adsorbed on those already on the surface of the adsorbate. It is assumed that, the molecules or atoms of the adsorbate in successive layers are in dynamic equilibrium and the amount adsorbed in each layer when full is the same. Also, the heat of adsorption for all but the first layer are equal to each other and to the heat of liquefaction of the adsorbate (Thomas, 2007).
The BET equation is given as

The surface area of the adsorbent can be evaluated from the equation

Where:
Vm= volume of the monolayer
P/Po= relative pressure
C= Constant = exp(∆H1-∆H2)
∆H1= heat of adsorption of the first layer
∆H2= heat of adsorption of the other layers
A= surface area of the adsorbent
P’= standard pressure
To= standard temperature
R= molar gas constant
L= Avogadro’s constant

S’= area of the Nitrogen molecule (adsorbate)

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