The contribution of Pd doping to enhance the hydrogen storage capacity of porous carbon materials is investigated. Using the Density Functional Formalism, we have studied the competition between the molecular adsorption and the dissociative chemisorption of H 2 on Pd clusters anchored on graphene vacancies. The molecular adsorption of H 2 takes place with energies in the range of 0.7 -0.3 eV for adsorption of one to six hydrogen molecules. Six molecules saturate the cluster, and additional hydrogen could only be adsorbed, with much smaller adsorption energies, at farther distances from the cluster. The dissociative chemisorption is the preferred adsorption channel from one to three hydrogen molecules, with adsorption energies in the range of 1.2 -0.6 eV. After the first three molecules are dissociatively quemisorbed, three additional hydrogen molecules can be adsorbed * Corresponding Author: E-mail: maria.lopez@fta.uva.es 1 non-dissociatively onto the Pd cluster with adsorption energies of 0.5 eV. The desorption of Pd-H complexes is prevented in all cases because the Pd clusters are firmly anchored to graphene vacancies. Our results are very promising and show that Pd clusters anchored on graphene vacancies retain their capacity to adsorb hydrogen and completely prevent the desorption of Pd-H complexes that would spoil the hydrogen releasing step of the cycle.

Abstract. Hydrogen adsorption on nanoporous carbon materials is a promising technology for hydrogen storage. However, pure carbon materials do not meet the technological requirements due to the week binding of hydrogen to the pore walls. Experimental work has shown that doping with Pd atoms and clusters enhances the storage capacity of porous carbons. Therefore, we have investigated the role played by the Pd dopant on the enhancement mechanisms. By performing density functional calculations, we have found that hydrogen adsorbs on Pd clusters deposited on graphene following two channels, molecular adsorption and dissociative chemisorption. However, desorption of Pd-H complexes competes with desorption of hydrogen, and consequently desorption of Pd-H complexes would spoil the beneficial effect of the dopant. As a way to overcome this difficulty, Pd atoms and clusters can be anchored to defects of the graphene layer, like graphene vacancies. The competition between molecular adsorption and dissociative chemisorption of H 2 on Pd 6 anchored on a graphene vacancy has been studied in detail.