日本ゼオライト学会　刊行物 Publication of Japan Zeolite Association

Gas adsorption is a prominent method to obtain a comprehensive characterization of porous materials with respect to the specific surface area, pore size discribution and porosity. This requires, however, a detailed understanding of the fundamental processes associated with the sorption and phase behavior of fluids in porous materials and their influence on the shape of sorption isotherm, which serves as a basis for surface and pore size analysis.

So-called classical macroscopic, thermodynamic concepts are based on the assumption of a certain pore filling mechanism. Methods based on the Kelvin equation (e.g. BJH method) are linked to the pore condensation phenomena, i.e., they are applicable for mesopore size analysis, but they fail to describe the pore filling of micropores and even narrow mesopores in a correct way. Other classical theories, like for instance the Dubinin-Radushkevich approach, and semi empirical treatments such those of Horvath-Kawazoe (HK), and Saito-Foley (SF) are dedicated to describe micropore filling but cannot be applied for mesopore size analysis. Hence, in case of a materials consist both micro- and mesopores, at least two different methods have to be used to obtain the pore size distributions from such an adsorption/desorption isotherm.

Non-Local Density Functional Theory (NLDFT) and Grand Canonical Monte Carlo simulation (GCMC) methods provide a microscopic and accurate description of fluinds in confined geometries in contrast to macroscopic, thermodynamic approaches like the methods of Barrett-Joyner-Halenda, Dubinin-Radushkevich etc. In contrast to the classical, macroscopic approches, the NLDFT and GCMC methods allow to obtain a more accurate pore size analysis for narrow micro- and mesopores. No assumptions are necessary concerning the nature of pore filling. The equilibration density profiles of the confined fluid at given temperature and pressure can be directly obtained by NLDFT and GCMC calculations. NLDFT and GCMC methods can be applied to obtain a pore size analysis over the complete micro- and mesopore diameter range by using a single method.