Surface area and heat of adsorption measurements of a microporous carbon

Abstract

Surface areas, micropore volumes, and isosteric heats of adsorption are reported for a microporous carbon as a function of conversion. The isosteric heats of adsorption were calculated from the temperature dependence of the equilibrium adsorption pressure at temperatures between 112 and 184 K, and are reported for a surface coverage (relative to the BET surface area) up to θ = 0.33 The isosteric heats show a decrease with coverage that is characteristic of an energetically heterogeneous surface. This decrease in adsorption energy with coverage was found to be more marked for the higher conversion chars. The Henry's Law regime isosteric heat, however, showed no significant variation with respect to carbon conversion or the presence of carbon-oxygen surface complexes. The average value for the isosteric heat in the Henry's Law regime is 15.3 kj/mole. Both the heat of adsorption and the surface area data are consistent with a structural model for microporous carbons in which the ultramicropores are formed by slit-like pores of two adjacent lamellae. These carbons have a distribution of pore widths, but the smallest accessible pore is approximately 6.2Å wide.