Upper limit of hydrogen adsorption on activated carbons at room temperature: A thermodynamic approach to understand the hydrogen adsorption on microporous carbons

This paper analyzes the adsorption of hydrogen on a microporous carbon from the point of view of the fundamentals of adsorption with basis on the Dubinin–Radushkevich equation. The mechanism of H2 adsorption is analyzed by the comparison with the results obtained from the adsorption of N2 at 77 K and CO2 at 273–798 K and an approximated value of the upper limit of H2 adsorption in carbon micropores has been tried to find. A detailed analysis is made to obtain values with physical sense of the saturation pressure, the density and the affinity coefficient of adsorbed H2. For H2 adsorption at temperature higher than 200 K, the H2 characteristic curves superimpose over the CO2 ones obtained at 273 K, whereas for low adsorption potentials they show a downward deviation. This result reveals that H2 adsorbs at room temperature mainly in the narrow microporosity, whereas micropores larger than 7 Å are not effective to adsorb H2 because in them only takes place monolayer adsorption. The theoretical maximum H2 adsorption in an idealized narrow microporous carbon is around 3.5 wt%.