Filling of Micropore Volume in Adsorption

The constants B and W of the DR equation (Equation 2.121) are structural parameters and depend upon the microporous structure of activated carbons. When B is sufhciently small, the micropore volume W becomes almost equal to the limiting micropore volume The constant B expresses that property of the microporous structure that determines the effect of the increase in adsorption energy in micropores. Evidently, it determines the micropore size parameters. The smaller the value of constant B, the steeper the rise of the isotherm in the lower region of equilibrium pressures. 

Source Dubinin, M.M., in Chemistry and Physics of Carbon, P.L. Walker, Jr., Ed., Marcel Dekker, New York, 2, 56, 1966. Reproduced with pmnission from Marcel Dekker. 

For relatively poorly adsorbed vapors and at small equilibrium relative pressures, the values of the filling factor F are low, differing by orders of magnitude. These values strongly depend on the value of constant B so that in snch cases the value of B is the determining factor for adsorption of these vapors, the development of micropore volume playing a secondary role. 

FIGURE 2.29 Adsorption of vapors by a heterogeneous carbon sample F-02 with n = 1.5. (Source Dubinin, M.M Zaverina, D.E. and Radushkevich, L.V., Zh. Fiz. Khim., 1351, 1947. With permission.) 

Although the DR plot was linear, about 70% of the pores are found within the 0.4 to 0.6 nm range. For typical activated carbons, as the degree of bum-off increases and the characteristic energy decreases from 35 to 30 kJ/mol to approximately 20 kJ/mol, the classical DR approximation with n = 2 holds with a variable degree 

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