Density functional modeling of reverse hydrogen spillover on zeolite-supported tetrairidium clusters

Stepwise reverse hydrogen spillover from bridging OH groups of the support to Ir4 clusters in a zeolite cavity was calculated exothermic, in total −421 kJ/mol for the three steps studied. In contrast to analogous modeling of supported Rh6, hydrogenation does not entail a notable extension of the metal–metal distances (here at most 2 pm). Thus, despite reverse spillover from the surrounding of the cluster, there remains a large difference, 21 ± 6 pm, between calculated and EXAFS derived Ir–Ir distances. Hydrogen migration onto Ir4 results in partial oxidation of the metal atoms, particularly those interacting with zeolite oxygen centers.