Heat of adsorption and binding affinity for hydrogen on pitch-based activated carbons

Abstract

The distribution of the binding site affinity and isosteric heat of adsorption of hydrogen at (i) 77 K and 100 kPa and (ii) 298 K and 10 MPa on five pitch-based activated carbons, four of which contain different heteroatoms (Si, B, Ti or Fe) were studied using a Sips isotherm and the Clausius–Clapeyron expression, respectively. Based on the studies with the Sips isotherm, the hydrogen was adsorbed at 77 K on binding sites of pitch based activated carbons having different binding affinity (heterogeneous), whereas at 298 K the hydrogen was deposited only on a single class of binding sites (homogeneous). The binding site affinity of the activated carbons at 77 K was unimodally distributed, exhibiting a quasi-Gaussian shape at the studied experimental conditions (100 kPa). The binding affinity of activated carbons for hydrogen at both 77 and 298 K was found to be linearly proportional to the total volume of micropores ( V N 2 ) and the volume of narrow micropores ( V C O 2 ) . The isosteric heat of the activated carbons was obtained by applying the Sips isotherm in the Clausius–Clapeyron equation and was found to be 3.05–6.9 kJ/mol. The values calculated suggest that the addition of heteroatoms to activated carbons has a minor effect on the isosteric heat at the experimental conditions used.