Adsorption in slit-like and cylindrical micropores in the henry's law region. A model for the microporosity of carbons

Abstract

Calculations are presented of the potential energy profiles for atoms adsorbed in slit-like pores, and of the enhancement of the depth of the potential energy well compared with that for adsorption by a single surface. The vibrational frequency normal to the wall is calculated as a function of slit width and methods of analysing experimental data are suggested. A comparison is made with similar expressions for adsorption in cylindrical pores. Recently published data on the adsorption of noble gases by a series of microporous carbons is analysed and shown to be best represented in terms of slit-like pores having an average width (between the nuclei of carbon atoms) of about 0.77 nm. The width available to an adsorbed molecule is about 0.52 nm which is in quantitative agreement with the known molecular sieve properties of these carbons. A well-activated carbon has pores with a somewhat larger mean width. The calculated vibration frequencies lie in the range 5–10 × 1011 Hz. The calculated surface areas and micropore volumes are 3–4 times smaller than the “monolayer equivalent areas” and pore volumes derived from micropore adsorption of nitrogen and benzene. It is concluded, in agreement with Dubinin, that the micropore volume can be divided into ultra-micropores, consisting of 25–30 % of the total micropore volume having free widths of about 0.52 nm, together with supermicropores which are larger and are filled with adsorbed vapour only at higher relative pressures.