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

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 (VN2)(VN2) and the volume of narrow micropores (VCO2)(VCO2). 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.