Hydrogen adsorption on palladium dimer decorated graphene: A bonding study

We report a density-functional theory study of dihydrogen adsorption on a graphene sheet functionalized with palladium dimers considering different adsorption sites on the carbon surface and both molecular and dissociative Pd2H2 coordination structures. Our results show that a (PdH)2 ring without an H–H bond and not dissociative Pd2(H2) complexes are stable adsorbed systems with more elongated Pd−Pd and Pd–H bonds compared to the unsupported configurations caused by C–Pd interactions. In contrast, individual Pd atoms supported on graphene react with H2 to form only a Pd(H2) complex with a relaxed but not dissociated H–H bond. We also performed the Mulliken analysis to study the bonding mechanism during the adsorption process. In most cases, we found donor-acceptor C−Pd and Pd−H interactions in which C 2p, Pd 5s, and H 1s orbitals played an important role. We also found that the adsorption of a second Pd atom close to a PdH2 system destabilizes the H−H bond. In this work we contribute to shed more light on the relation between Pd clustering and the possibility of hydrogen storage in graphene-based materials.