The Characterization of Non-Porous Surfaces by a Combination of the BET and the Dubinin-Radushkevich-Kaganer (DRK) Theories

The characterization of non-porous surfaces by the BET and the DRK techniques should be complementary. However, the monolayer capacities N_am obtained by these techniques may be different, with trends for the adsorption of simple molecules on a given surface. Assuming that the BET monolayer is correct, its use in the DRK equation leads to exponents n different from the standard value n = 2. This exponent is directly related to the width of χ(ε), the distribution of the gas–solid adsorption energy ε for a given system. Therefore, the analysis based on an extended DRK equation leads to information on the heterogeneity of the surface. For example, the data for adsorption of simple molecules physisorbed on a non-porous silica, on the (111) face of rhombic sulphur and on graphitised carbon blacks lead to average exponents of 1.2, 2.0 and 2.2 respectively, with specific scaling factors β. In the case of nitrogen adsorbed on SiO₂, the value n = 1.8 suggests the presence of specific interactions.