Adsorption and Desorption of H2 on Graphite by Molecular Dynamics Simulations

We report equilibrium properties and rate constants for adsorption and desorption of hydrogen on graphite as a function of temperature, from 70 to 390 K, using equilibrium molecular dynamic simulations. Below 170 K, we found that isotherms can be modeled with Langmuir isotherms, while at higher temperature, Henry’s law applied. The isosteric adsorption enthalpies and entropies were calculated for different loadings and showed a decrease as the loading increased, compatible with a nonideal adsorbate. These results are in good agreement with previous results from the literature. The observed adsorption and desorption rate constants were, however, not described by Langmuir kinetics. We propose a set of rate equations which follow from the law of mass action with activities as variables rather than concentrations. The unidirectional rates combine at equilibrium to give the Langmuir isotherm with the same result for the Langmuir constant. This kinetic behavior is interpreted as the consequence of a mobile adso...