Partially Decarbonylated Tetrairidium Clusters on γ-Al2O3: Structural Characterization and Catalysis of Toluene Hydrogenation

Abstract

Supported metal clusters were prepared as [Ir4(CO)12] was adsorbed intact fromn-pentane solution onto γ-Al2O3powder that had been partially dehydroxylatedin vacuoat 400°C. The supported clusters were characterized by infrared spectroscopy and extended X-ray absorption fine structure (EXAFS) spectroscopy. The supported [Ir4(CO)12], which was stable after heating in He at temperatures up to 100°C, was decarbonylated to various degrees by treatment in He at temperatures higher than 100°C, with the decarbonylation being complete at 300°C. EXAFS data indicated an average Ir–Ir first-shell coordination number of about 3.0 at an average bond distance 2.67 Å at each stage of the decarbonylation, demonstrating that the decarbonylation proceeded without disruption of the tetrahedral cluster frame, ultimately giving Ir4/γ-Al2O3. Chemisorption of hydrogen on the supported Ir4clusters was characterized by an H/Ir atomic ratio of about 0.13, a value much less than that characteristic of larger iridium clusters, which indicates that the supported clusters have reactivities different from those of bulk metallic iridium or iridium particles large enough to have bulklike properties. The [Ir4(CO)12] clusters were partially reconstructed from Ir4/γ-Al2O3by treatment in CO at 150–200°C. The supported tetrairidium clusters at various stages of decarbonylation were found to be catalytically active for toluene hydrogenation at 60°C and atmospheric pressure. The catalytic activity of supported [Ir4(CO)12] was negligible, and the activity increased with increasing decarbonylation, until the degree of decarbonylation reached about 70%, whereupon the catalytic reaction rate became almost independent of the degree of decarbonylation. The data suggest that the last remaining CO ligands have almost no effect on the toluene hydrogenation reaction because the clusters have attained a sufficient degree of unsaturation to provide bonding sites for the reactant ligands.