Concept
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: (a) gas molecules physically adsorb on a solid in layers infinitely; (b) there is no interaction between each adsorption layer; and (c) the Langmuir theory can be applied to each layer. The resulting BET equation is expressed by (1):
and are the equilibrium and the saturation pressure of adsorbates at the temperature of adsorption, is the adsorbed gas quantity (for example, in volume units), and is the monolayer adsorbed gas quantity. is the BET constant, which is expressed by (2):
is the heat of adsorption for the first layer, and is that for the second and higher layers and is equal to the heat of liquefaction.
Equation (1) is an adsorption isotherm and can be plotted as a straight line with on the y-axis and on the x-axis according to experimental results. This plot is called a BET plot. The linear relationship of this equation is maintained only in the range of . The value of the slope and the y-intercept of the line are used to calculate the monolayer adsorbed gas quantity and the BET constant . The following equations can be used:
The BET method is widely used in surface science for the calculation of surface areas of solids by physical adsorption of gas molecules. A total surface area and a specific surface area are evaluated by the following equations:
where is in units of volume which are also the units of the molar volume of the adsorbate gas
| : Avogadro's number, |
| : adsorption cross section of the adsorbing species, |
| : molar volume of adsorbate gas |
| : mass of adsorbent (in g) |
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