Homogeneous and heterogeneous micropore structures in carbonaceous adsorbents

Abstract

Microporous carbonaceous adsorbents obtained by moderate activation possess a rather narrow pore-size distribution, and their microporous structure may be considered to be practically homogeneous. In the case of high degrees of burn-off and of extreme activation, micropores of various sizes are formed, including large ones such as supermicropores. The distribution of the micropore volume with respect to their dimensions becomes broader, a characteristic feature of heterogeneous micropore structures. For adsorbents with homogeneous micropore structures close to carbonaceous molecular sieves, the equation of the theory of volume filling of micropores (TVFM) applies with n = 2 over a large range of relative pressures. In the case of slit-shaped micropores, their half-width is proportional to the inverse of the characteristic energy of adsorption, E 0. The constant of proportionality is obtained from the molecular sieve properties of the carbonaceous adsorbents, and in the case of active carbons of industrial origin it may be regarded as a constant, with a good approximation. The development of the adsorption theory for carbonaceous adsorbents with heterogeneous micropore structures has made it possible to express the distribution of the micropore volume according to their dimensions. It is also shown that the adsorption properties of such materials can be approximated with great accuracy by a two-term equation of TVFM, with n = 2. A possible alternative is also considered for carbonaceous adsorbents with heterogeneous micropore structures, and for which the adsorption equation contains a term with exponent n = 3.