Partially decarbonylated tetrairidium clusters on MgO: structural characterization and catalysis of toluene hydrogenation

Supported [HIr4(CO)(11)](-), prepared by adsorption of [Ir-4(CO)(12)] from n-pentane solution onto MgO powder that had been partially dehydroxylated i n vacuum at 400 degreesC, was identified by infrared and extended X-ray abs orption fine structure (EXAFS) spectroscopies. The clusters were decarbonyl ated to various degrees by treatment in He or H-2 and characterized by infr ared and EXAFS spectroscopies. When the decarbonylation was carried out in H-2, the iridium aggregated; infrared data show that the CO ligands reacted with H-2, resulting in the formation of water and hydrocarbons. When the d ecarbonylation was carried out in He, the tetrahedral cluster frame remaine d unchanged; the fury decarbonylated clusters, modeled as Ir-4 tetrahedra, had an Lr-Ir first-shell coordination number of 3.0 +/- 0.1 at a distance o f 2.67 +/- 0.01 Angstrom. The partially decarbonylated tetrairidium cluster s chemisorbed hydrogen. Temperature-programmed desorption (TPD) data show t hat the strength of bonding of hydrogen to the clusters increased with incr easing cluster decarbonylation. The completely decarbonylated clusters were characterized by low chemisorption capacities (H/Ir and CO/Ir values, e.g. 0.27 and 0.39, respectively). Chemisorption and EXAFS data show that the a mount of hydrogen or CO chemisorbed on Ir-4 increases with increasing size of the iridium clusters and aggregates, consistent with a support effect th at is maximized for the smallest clusters, Ir-4. Data representing the cata lytic hydrogenation of toluene show that the rate depends on the amount of hydrogen on the clusters - and not just on the degree of decarbonylation - consistent with the suggestion that iridium atoms free of CO Ligands are no t sufficient for catalysis to proceed. Toluene adsorbed on the support is i mplicated in the catalysis. (C) 2000 Elsevier Science B.V. All rights reser ved.