Adsorption and transformations of ethene on hydrogenated rhodium clusters in faujasite-type zeolite. A computational study

Regarding the heterogeneous catalytic hydrogenation of ethene on small rhodium clusters, anchored in faujasite, we computationally studied C₂H_x (x = 2-5) intermediates on zeolite-supported Rh₃ and Rh₄ clusters with pre-loaded hydrogen. According to the calculated Gibbs free energies, at temperatures and hydrogen pressure values representing pertinent experiments, the favored stable species on hydrogen-loaded Rh₃ is ethylidyne which apparently prevents further hydrogenation due to its strong CH₃-CRh_n interaction. In contrast, on bare and hydrogenated Rh₄ clusters, the partially hydrogenated species C₂H₅, a locally stable intermediate, is among the most stable adsorption complexes examined. These "pseudo ethyl" C₂H₅ ligands exhibit a very specific structure, where both carbon atoms and a hydrogen atom of the organic moiety interact with the apical Rh center of Rh₄; previously, such ligand structures have been identified only computationally as transition states in homogeneous catalysis of ethene hydrogenation. According to our analysis based on the thermodynamic phase diagrams, one is led to conclude that only Rh clusters of such specific topology may be appropriate for the catalytic hydrogenation of ethene, in contrast to the zeolite-supported Rh₃ clusters. Suggestions are made for further probing this size-specific catalytic activity of Rh clusters.