Hydrogen Adsorption on Small Zeolite-Supported Rhodium Clusters. A Density Functional Study

Abstract

Using a periodic density functional approach, we modeled the dissociative adsorption of hydrogen on small Rh clusters, supported inside the cavity of a faujasite-type zeolite. H spillover from a zeolite hydroxyl group to the metal cluster is favorable by at least 160 kJ/mol. Therefore, we used the complexes HRh3 and HRh4 in deprotonated zeolite as reference for H adsorption from the gas phase. For the most stable location of the complexes, at three four-membered rings, the average adsorption interaction of hydrogen decreases monotonously with increasing H loading. Adsorption energies are calculated from −70 to −52 kJ/mol for the complexes H3Rh3 and H7Rh3, respectively, and from −67 to −47 kJ/mol for the complexes H3Rh4 and H9Rh4, respectively. The preferred coordination of H ligands changes with the H loading, bridging at Rh–Rh bonds at low loading and terminal at Rh centers at high loading. Concomitantly the average Rh–Rh nearest-neighbor distances increase from 242 to 272 pm (Rh3) and from 247 to 265 pm (Rh4). A thermodynamic model based on the calculated Gibbs free energies of the structures studied suggests that complexes with maximum H loading, H7Rh3 and H9Rh4, dominate in a wide range of H2 pressures and temperature. The calculated atomic charges suggest that the metal moieties are oxidized due to reverse hydrogen spillover and adsorption of hydrogen from the gas phase.