Adsorption and Dissociation of Molecular Hydrogen on Palladium Clusters Supported on Graphene

Doping of porous carbon materials with metallic atoms, clusters and nanoparticles is viewed as a way to enhance the hydrogen storage in those materials. Transition metals are dopants of interest. For this reason we present a theoretical study of the interaction of molecular hydrogen with small palladium clusters (Pd n, n = 1–6) supported on a graphene layer. The adsorption of H 2 on those supported Pd clusters leads to two types of adsorption states. The simplest one is an activated state of the hydrogen molecule, with the H–H distance stretched and the H–H bond weakened, but not broken. Adsorption in the activated states occurs with no barriers and the binding energies are in a range of values of interest for achieving a favorable reversible hydrogen storage in the doped material. The second type is a dissociated and chemisorbed state, with the separated hydrogen atoms attached to the Pd cluster. The dissociative chemisorption states are more stable than the activated states. However, starting with Pd 4, there are barriers for the dissociative chemisorption of H 2, and the heights of those energy barriers are a few tenths of an electronvolt.