A non-BET adsorption and its LgBET model

Abstract

Abstract The paper proposes new mathematical tools for interpretation of adsorption isotherms at relative pressures ranging to 0.95. Adsorption of small (nearly spherical) molecules in microporous adsorbents containing hole-like micropores is considered. In such materials formation of adsorbate clusters more compact than BET stacks is possible that leads to non-BET configurational effects. Additionally, geometrical and energetic constraints for the cluster size may be of significance. The paper shows that such effects can be handled with a rigorous mathematical model based on the BET approach. The derived general formulas may be useful in theoretical studies of effects of pores geometry on the adsorption isotherm shape. The neglecting of the geometrical constraints leads to a BET-like analytical formula (gBET equation). Moreover, an approximating formula is proposed (LgBET equation), taking into account the cluster size constraints represented by an exponential distribution function. The gBET and LgBET models may be used to draw semi-quantitative information on the shape of pores by the examination of adsorption isotherms at high relative pressures (ranging to 0.9). They provide also more reliable information on the adsorbent surface area and adsorption energy than the BET equation. Exemplary results of the new model application to analysis of N2 adsorption on activated carbons are discussed.