Partially decarbonylated tetrairidium clusters on gamma-Al2O3: Structural characterization and catalysis of toluene hydrogenation

摘要

Supported metal clusters were prepared as [Ir-4(CO)(12)] was adsorbed intact from n-pentane solution onto gamma-Al2O3; powder that had been partially dehydroxylated in vacuo at 400 degrees C. The supported clusters were characterized by infrared spectroscopy and extended X-rap absorption fine structure (EXAFS) spectroscopy. The supported [Ir-4(CO)(12)], which was stable after heating in He at temperatures up to 100 degrees C, was decarbonylated to various degrees by treatment in He at temperatures higher than 100 degrees C, with the decarbonylation being complete at 300 degrees C. EXAFS data indicated an average II-Ir first-shell coordination number of about 3.0 at an average bond distance 2.67 Angstrom at each stage of the decarbonylation, demonstrating that the decarbonylation proceeded without disruption of the tetrahedral cluster frame, ultimately giving Ir-4/gamma-Al2O3. Chemisorption of hydrogen on the supported Ir-4 clusters was characterized by an H/Ir atomic ratio of about 0.13, a value much less than ?hat 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 [Ir-4(CO)(12)] clusters were partially reconstructed from Ir-4/gamma-Al2O3 by treatment in CO at 150-200 degrees C. The supported tetrairidium clusters at various stages of decarbonylation were found to be catalytically active for toluene hydrogenation at 60 degrees C and atmospheric pressure. The catalytic activity of supported [Ir-4(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. (C) 1998 Academic Press. [References: 45]