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 Pd-2(H-2) 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 H-2 to form only a Pd(H-2) 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. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.