However, actual adsorbents usually contain weak H2 binding sites and pore space for free H2, even if strong H2 binding sites are also present. Although it is difficult to describe the adsorption properties of heterogeneous materials because the usable capacity varies with the degree of heterogeneity, a weighted average of adsorption enthalpy of multiple binding sites (∆H°w.ave) and a heterogeneity parameter, s = (∆H°max-∆H°min)/2, where ∆H°max and ∆H°min are the maximum and minimum values of adsorption enthalpy, respectively, can be a good measure. Ideal adsorbent materials are expected to have the weighted average of adsorption enthalpy within the optimal range (i.e., from -15 to -25 kJ/mol) and a small heterogeneity parameter so that extreme adsorption and desorption conditions are not required.