Catalysis by oxide-supported clusters of iridium and rhodium: hydrogenation of ethene, propene, and toluene

The hydrogenation reactions of ethene, propene, and toluene were used as probes of the catalytic properties of small clusters of rhodium (Rh6) and of iridium (Ir4 and Ir6) (as well as of larger aggregates of these metals) on oxide supports (gamma-Al2O3, MgO, and La2O3). The catalysts were characterized in the working state by extended X-ray absorption fine structure (EXAFS) spectroscopy, providing evidence of the cluster structures and cluster-support interactions; by infrared spectroscopy, providing evidence of hydrocarbon adsorbates and possible reaction intermediates on the clusters; and by kinetics of the hydrogenation reactions. The EXAFS data indicate that the metal clusters, while remaining intact and maintaining their bonding to the support during catalysis, underwent slight rearrangements to accommodate reactive intermediates. As the concentrations of reactive intermediates such as pi-bonded alkenes and alkyls on the clusters increased, the cluster frames swelled, and the clusters flexed away from the support. The data indicate self-inhibition of reaction by adsorbed hydrocarbons and differences between ethene hydrogenation and propene hydrogenation that may arise primarily from different adsorbate-adsorbate interactions.