ADSORPTION AND STRUCTURE IN MICROPOROUS CARBONS

The microporous structure of carbons consists of a tangled network of defective carbon layer planes in which micropores are the spaces between the layer planes. Adsorption of gases in micropores is characterized by: (1) strong adsorption at low pressure due to overlap of force fields from opposite pore walls, (2) activated diffusion effects caused by constrictions in the microporous network, and (3) molecular size and shape selectivity (molecular sieving). The surface fractal dimension of activated carbons decreases from three to two with increasing activation, indicating that activation smoothens pore surfaces. Calculated adsorption potentials for slit-shaped model micropores show that adsorption potentials are enhanced by a factor of up to 2 and enable critical dimensions for diffusion of gases through micropores to be estimated. The Brunauer-Emmett-Teller equation is unsuitable for analyzing adsorption with a significant microporous contribution but may be used to estimate the nonmicroporous surface area, provided that the microporous contribution can be removed. The Dubinin-Radushkevich and Dubinin-Astakhov equations have been more successful when applied to microporous carbons because they reflect the influence of adsorbent heterogeneity, as they result from an approximation to the generalized adsorption isotherm (GAI). More exact solutions of the GAI enable adsorption energy distribution functions to be obtained. The possibility of extracting micropore size distributions from adsorption measurements is briefly considered.