Micropore Adsorption Dynamics in Synthetic Hard Carbons

Abstract

The kinetics of the adsorption of CO₂ at 296 K in the micropores of sol-gel derived, monolithic carbons indicates two different types of micropores. While the micropore volume as well as the micropore width of both species turns out to be very similar, their accessibility is significantly different. To study the adsorption kinetics in more detail a set of sol-gel derived hard carbons was prepared with average macropore sizes ranging from about 50 nm to 5 micron at a total porosity of about 85%. To characterize the morphology of the samples small-angle X-ray scattering and N₂ sorption at 77 K were applied. Evaluation of the structural characteristics and the adsorption kinetics of the carbons investigated reveals that the two different types of micropores are homogeneously spread throughout the carbon backbone. The adsorption kinetics in the readily accessible type of micropores is dominated by the gas phase transport through the macropores combined with transport along the inner surface. In contrast, the access to the second category of micropores is highly restricted even at 296 K; this is reflected in equilibration times of about 500 s. The characteristics of the slow adsorption component is almost independent of the macroscopic size and the morphology of the specimen, suggesting that the slow kinetics is controlled by the local accessibility of the micropores. Surprisingly, the equilibrium data of the two adsorption components as a function of the relative pressure reveal that the micropores that are only slowly filled are actually characterized by a lower (Dubinin-Raduskevich) energy and thus a larger micropore width than the ones that are readily filled. This can be interpreted in terms of micropores with a very narrow entrance (restricted diffusion) or a widening of the micropores due to swelling of the carbon upon CO₂ adsorption.