Isosteric heats of adsorption on carbon predicted by density functional theory

Isosteric heats of adsorption (q(st)) of propane and butane adsorbed on carbon were studied by numerical differentiation of nonlocal density functional theory (NDFT) isotherms. The q(st) values of both adsorbates in slit-shaped pores were a weak function of temperature, and decreased with increasing pore width. Using the calculated pore size distribution (PSD) of an activated carbon, it was further determined that the q(st) for both propane and butane decreased with increasing loading, consistent with a heterogeneous adsorbent. The NDFT results utilizing the PSD were also in fair agreement with those obtained from the classical approach using experimental isotherms fitted to a model and applied to the Clausius-Clapeyron-type equation; both models predicted q(st) of butane to be similar to 10 kJ/mol higher than that of propane at the same loading. On a model homogeneous carbon, the q(st) of both adsorbates increased with reduced surface coverage up to similar to 0.5, then they dropped rapidly. The reduced surface coverages corresponding to monolayer completion were 0.61 for propane and 0.65 for butane, in agreement with published experimental results.