The characterization of microporosity in carbons with molecular sieve effects

The apparent and the real micropore size distributions (PSDs) of molecular sieve carbons can be assessed by combining the adsorption of CO₂ at 273 K with immersion calorimetry into liquids of increasing molecular dimensions. On the basis of model isotherms resulting from computer simulations, the adsorption of carbon dioxide, a relatively small probe, leads to the overall PSD of the carbon (essentially the internal micropore system). Immersion calorimetry, on the other hand, reveals the distribution of the pores accessible directly from the liquid phase, that is without constrictions. Liquid CS₂ probes the same volume as CO₂ and can be used as a reference. The paper describes the case of an industrial molecular sieve carbon obtained by blocking partly the entrance to a relatively broad micropore system, thus limiting its accessibility to molecules with diameters below 0.5–0.6 nm. It is shown how activation by steam at 900 °C removes the constrictions and leads to a gradual overlap of the two PSDs. The distribution of the pore widths on the surface, observed directly by scanning tunnelling microscopy, is also given.