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 H-2 adsorption is analyzed by the comparison with the results obtained from the adsorption of N-2 at 77 K and CO2 at 273-798 K and an approximated value of the upper limit of H-2 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 H-2. For H-2 adsorption at temperature higher than 200 K, the H-2 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 H-2 adsorbs at room temperature mainly in the narrow microporosity, whereas micropores larger than 7 angstrom are not effective to adsorb H-2 because in them only takes place monolayer adsorption. The theoretical maximum H-2 adsorption in an idealized narrow microporous carbon is around 3.5 wt%. (C) 2007 Elsevier Inc. All rights reserved.