When relatively high pressure methane isotherms at room temperature are measured on carbons, differences in the overall uptake are very obvious. They are believed to be due to differences in the total pore volume and, also, the pore size distributions (PSDs) of these carbons. A recently developed [l] method for determining a PSD) of a carbon adsorbent based on the high pressure methane isotherm was used to analyze two series of PVDC carbons, progressively activated, one with steam, and the other with CO,.
Notable similarities in the properties of both series were observed. Methane uptakes (at 600 psi) per unit mass rose continuously from -90 mg CH,/g in unactivated samples to -190 mg CHJg at 65% weight loss. Considered on the "per unit mass of the original sample" basis, the uptakes exhibited a maximum of -98 mg CHJl g original sample, at about 15% weight loss, after which they decreased steadily down to -63 mg CH$l g original sample, at 65% weight loss. This would suggest that, in samples activated beyond 15% weight loss, the extent of changes to the porous structure is outweighed by the loss of adsorbent material during activation. The size of the average micropore also increased during activation, from an init@ value of -6.5 to -14 A at 65% weight loss. Interestingly, at 15% weight loss it was found to be -8 A, which is close to the optimum determined by Matranga et al. [2]. The analysis also suggests that all the changes were due to alterations to the existing pores and that no new pores were created during the activation.