Characterization of carbonaceous materials for their accessibility to methane-a concept of accessible pore size distribution

摘要

Structural determination of coal and carbon based materials has been a subject of great interest to adsorption and coal scientists for many decades. The structural parameters, such as void volume, surface area, pore-size and pore-size distribution, have been determined from various theories or approaches. For example, void volume is usually determined by the helium expansion method, surface area from the BEt theory and the pore size from various classical theories such as the Hovarth-Kawazoe (HK), SaitoFoley (SF), Dubinin-Stoeckli (DS), Barrett-JoynerHalenda (BJH), Broekhoff-de Boer (BdB) theories, etc. More recently, tools such as Density Functional theory and Monte Carlo molecular simulation, have been applied to these problems, and these have helped to remove many of the difficulties associated with the limited applicability of the classical theories, for example to microporous solids (HK, SF, DS) or to mesoporous solids (BJH and BdB). Other problems associated with the classical approach are (i) the possibility of helium adsorption in fine pores, (ii)the improper assumptions inherent in the BET theory and (iii) the assumption of a specific geometry for the pore in the determination of pore size distribution, for example the HK and DS equations are designed for slits, SF theory is for cylinders, and BJH and BdB were developed for slits and cylinders (extension to spherical pores is also possible). Real solids do not conform to these geometries and therefore by making these assumptions about pore shape, the pore size distribution is treated qualitatively and can only be used as an indicator to compare one sample against the others in the same family. With these limitations inherent either in the theories or in the way the procedure is carried out, the derived information on pore volume, surface area and pore size distribution will have a limited value. Here we present a development of a consistent and coherent means to derive pore volume, surface area and pore size distribution. Recognizing the fact that these structural parameters depend on the molecular probe, we give examples of both methane and argon as molecular probes. By assuming known atomic configurations for the carbonaceous solids, we show how the void volume, geometrical area and pore size can be obtained as accessible quantities using Monte Carlo integration. The inaccessible solid volume is simply the difference between the total system volume and the accessible void volume. The challenge that faces us ahead is the experimental determination of these accessible parameters.