Doping with palladium has been proposed as a means to enhance the hydrogen storage capacity of nanoporous carbon materials. Palladium atoms and clusters attach strongly to defects on the walls of nanoporous carbons, which can be mimicked as graphene layers with vacancies. On the other hand, atomic hydrogen also binds strongly to the dangling bonds of defects and edges of graphitic carbon. Therefore, hydrogen adsorbed on Pd-doped nanoporous carbons could compete with the Pd dopant to saturate the vacancies. In this work we have performed density functional calculations to investigate the competition between palladium atoms and clusters, on one hand, and hydrogen, on the other hand, to saturate graphene vacancies. We find that palladium binds stronger than hydrogen to graphene vacancies and, therefore, hydrogen can not replace the palladium atoms or clusters attached to the vacancies. Instead, hydrogen adsorbs on the palladium. Thus, hydrogen adsorption on Pd-doped carbons does not destroy the stability of the material. Moreover, our study shows that graphene vacancies decorated with Pd just on one side of the graphene layer are not fully saturated. The other side of the vacancy remains quite reactive and therefore Pd atoms and clusters can be attached, simultaneously, to both sides of the vacancy. Interestingly, the hydrogen adsorption mechanisms and energies do not depend on whether Pd atoms and clusters are decorating one side or both sides of the vacancies.
