Size-Dependent Catalytic Activity of Zeolite-Supported Iridium Clusters

Iridium carbonyl clusters approximated as Ir 2 (CO) 8, Ir 4 (CO) 12, and Ir 6 (CO) 16 were synthesized by carbonylation under various conditions of Ir(CO) 2 (acac) sorbed in the cages of zeolite NaY. These supported clusters were decarbonylated by treatment in helium and used to catalyze ethene hydrogenation at atmospheric pressure and temperatures up to 80 °C. Extended X-ray absorption fine structure (EXAFS) spectroscopy was used to characterize the decarbonylated clusters, which are approximated as Ir 2, Ir 4, and Ir 6, respectively. The data are consistent with the inference that these clusters are the catalytically active species. The catalytic activities and apparent activation energies for ethene hydrogenation depend significantly on the cluster size, with the identity of the most active of the three catalysts depending on the temperature. There appears to be no simple explanation for the variation in catalytic performance with cluster size.