Bonding analysis of Pd2H2 systems adsorbed on graphene layers

IGNACIO LÓPEZ CORRAL; ESTEFANÍA GERMÁN; MARÍA ALICIA VOLPE; GRACIELA BRIZUELA; ALFREDO JUAN

La Plata

Conferencia; Humboldt Kolleg International Conference on Physics (Argentina-Alemania); 2011

Instituto de Física La Plata (CONICET-UNLP)

In this work we report a bonding study of the adsorption of Pd2H2 systems on a graphene layer, using the density-functional theory and the SIESTA code [1,2]. We modelled the graphene sheet by means of a periodically repeated tetragonal unit cell, consisting of 24 C atoms in a plane honeycomb structure. Different adsorption sites on the carbon surface and both molecular as dissociative Pd2H2 coordination structures were considered. Our results show that the adsorption of the Pd2 dimer in a parallel approach respect to the graphene sheet is only 260 meV stronger than the adsorption of two individual Pd atoms located on remote sites, so palladium shows less preference to cluster than other transition metals adsorbed on carbon surfaces [3]. According to previous studies [4], we verified by means of Mulliken analysis that the C-Pd bonding involves a small donation from Pd 4d to C 2s orbitals and a higher back-donation from C 2p to Pd 5s orbitals. In presence of dihydrogen, both dissociative (PdH)2 and not dissociative Pd2(H2) systems are originated, developing similar binding energies for the bridge (at the midpoint of a C-C bond) or top (above a C atom) sites. On the contrary, adsorbed single Pd atoms originate preferably a molecular Pd(H2) structure, with a relaxed but not dissociated H-H bond [5]. As a result of the support interaction, Pd-Pd and Pd-H bonds are elongated regarding the un-supported Pd2H2 coordination structures, and the C-Pd interactions results weakened with respect to the free-H graphene/Pd2 system, reducing both donation from Pd 4d to C 2s orbitals as back-donation from C 2p to Pd 5s orbitals. Finally, we found that the Pd-H bonding in the not dissociative adsorbed PdH2 and Pd2H2 systems comes from two cooperatively coupled intermolecular donor-acceptor delocalizations: the s donation from H2 s orbital to Pd 5s orbitals, and the p back-donation from Pd 4d orbital to H2 s* orbitals. The analysis of the hydrogen molecular orbital occupation in these structures reveals that a second Pd atom adsorbed nearly to the PdH2 system enhances the s donation and the p back-donation, destabilizing the H-H interaction, but these changes are small in all cases. In this way, we could assume that the dimerization effect for carbon-supported Pd atoms is limited and could not impact negatively on the hydrogen uptake of Pd-decorated graphene-based materials.