Hydrogen adsorption on small zeolite-supported rhodium clusters. A density functional study

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. (Chemical Equation Presented).