Determination of the Enthalpy of Adsorption of Hydrogen in Activated Carbon at Room Temperature*

A very important quantity for adsorption performance is the enthalpy of adsorption ΔH. The determination of ΔH for weakly adsorbing gases (e.g., H₂) in carbonaceous porous materials is difficult, normally requiring measuring adsorption isotherms at two cryogenic temperatures and calculation of ΔH using Clausius-Clapeyron’s (CC) equation. Here we demonstrate a calculation of ΔH based on ca. room temperature isotherms at 273K, 296K. CC requires absolute isotherms; however, excess adsorption is measured; conversion between these requires the adsorbed film volume. We show that the film volume can be estimated by fitting the excess adsorption with an Ono-Kondo model and the auxiliary use of a fixed point corresponding to the saturation film density (estimated 100±20 g/L) which seems to be remarkably sample- and temperature-independent, i.e., an adsorbate property. We find that for high-quality porous carbons the film volumes are ~40%, ~12% of the total pore volume at 77K, 296K, respectively. Using these, we calculate ΔH = 8.3±0.4 kJ/mol at room temperature, an excellent agreement with the low-coverage cryogenic determination. The methodology proposed facilitates reliable calculations of the ΔH at room temperatures for weakly-adsorbing gases.