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.