IRIDIUM CLUSTERS SUPPORTED ON GAMMA-AL2O3 - STRUCTURAL CHARACTERIZATION AND CATALYSIS OF TOLUENE HYDROGENATION

Iridiun clusters were prepared on gamma-Al2O3 that had been calcined a t 400 degrees C. When supported [Ir-4(CO)(12)], the precursor, was tre ated in H-2 at various temperatures, it was decarbonylated, with aggre gation taking place to various degrees to give a family of supported c lusters and aggregates with average diameters in the range of about 9 to 33 Angstrom, as determined by extended X-ray absorption fine struct ure (EXAFS) spectroscopy. Similarly, samples were decarbonylated in He , and the degree of aggregation of the iridium was less than in H-2 un der comparable conditions. Infrared spectra show that decarbonylation of the supported [Ir-4(CO)(12)] in the presence of H-2 was not reversi ble and was accompanied by aggregation of the iridium; infrared spectr a of CO adsorbed on the decarbonylated samples were characterized by a band at 2058 cm(-1), typical of terminal CO adsorbed on iridium parti cles. The gamma-Al2O3-supported iridium clusters and aggregates were u sed to catalyze toluene hydrogenation at 60 degrees C and 1 atm. Sampl es with nearly the same Ir-Ir first-shell coordination number (nearly the same dispersion) were characterized by essentially the same cataly tic activity, independent of whether the decarbonylation was carried o ut in He or H-2. The catalytic activity for the structure-insensitive toluene hydrogenation reaction was found to increase with increasing a verage cluster or aggregate diameter and to be greater for iridium on the calcined gamma-Al2O3 than for iridium on uncalcined gamma-Al2O3. ( C) 1998 Academic Press