BET
(rus. метод БЭТ abbr., БЭТ)
—
method of mathematical description of physical adsorption based on the theory of polymolecular (multilayer) adsorption.
Description
This method was proposed by Brunauer, Emmett and Taylor. It makes the following assumptions: the adsorbent surface is homogeneous, the adsorbent-adsorbate interaction is stronger than the adsorbate-adsorbate interaction, the interaction of adsorbed molecules is considered only in the direction perpendicular to the surface, and is regarded as condensation. The linear form of the adsorption isotherm (BET equation) is 
where
is the ratio of system pressure to condensing pressure, 
is the value of adsorption, 
is the volume of the monolayer on the surface of the adsorbent, and C is the ratio of the adsorption equilibrium constants in the first layer to the condensation constant.
The main practical objective of the BET method is to find the surface area of porous solids. This requires one to obtain the experimental dependence of the adsorption on the pressure at a constant temperature (adsorption isotherm), following which the BET equation is used to calculate the value , and then the number of molecules in the monolayer is calculated. Knowing the area occupied by one molecule, one can calculate the total surface area of the adsorbent of any shape and porosity. It is believed that the BET method can be used for determining surface area with an accuracy of 5-10% in the range of relative pressures () from 0.05 to 0.35. A more detailed analysis of the porous structure of solids (to calculate the fractions of pores of different diameters in the total porous structure) on the basis of adsorption isotherms requires the use of additional computational models (see, for example, BJH).
where
is the ratio of system pressure to condensing pressure, is the value of adsorption, is the volume of the monolayer on the surface of the adsorbent, and C is the ratio of the adsorption equilibrium constants in the first layer to the condensation constant. The main practical objective of the BET method is to find the surface area of porous solids. This requires one to obtain the experimental dependence of the adsorption
on the pressure at a constant temperature (adsorption isotherm), following which the BET equation is used to calculate the value , and then the number of molecules in the monolayer is calculated. Knowing the area occupied by one molecule, one can calculate the total surface area of the adsorbent of any shape and porosity. It is believed that the BET method can be used for determining surface area with an accuracy of 5-10% in the range of relative pressures () from 0.05 to 0.35. A more detailed analysis of the porous structure of solids (to calculate the fractions of pores of different diameters in the total porous structure) on the basis of adsorption isotherms requires the use of additional computational models (see, for example, BJH).Authors
- Smirnov Andrey V.
- Tolkachev Nikolay N.
Sources
- Brunauehr S. The adsorption of gases and vapors (in Russian). V. 1. — Moscow: IL, 1948. — 783 p.
- Poltorak O.M. Thermodynamics in Physical Chemistry. — Moscow: Vysshaja shkola, 1991. — 319 p.
- Karnaukhov A.P. Adsorption. The texture of dispersed and porous materials (in Russian). - Novosibirsk: Nauka, 1999. - 470. p.