Probing the limits of structure insensitivity: Size-dependent catalytic activity of Al2O3-supported iridium clusters and particles for toluene hydrogenation

Supported iridium cluster catalysts prepared from [Ir-4(CO)(12)] supported on gamma-Al2O3 were decarbonylated to give Ir-4/gamma-Al2O3. This was treated in H-2 under various conditions to give a family of samples with aggregated iridium having average cluster or particle diameters up to about 70 Angstrom on average, as determined by extended X-ray absorption fine structure (EXAFS) spectroscopy. The samples were used as catalysts for toluene hydrogenation at 1 arm and 60 degrees C. The data show that the catalytic activity per exposed Ir atom increased by more than an order of magnitude as the iridium cluster or particle size increased from that of Ir-4 to about 20 Angstrom in diameter. The results indicate that the rate of a structure-insensitive reaction is significantly dependent on the cluster size in the range < 20 Angstrom in diameter, but there is still too little information to attribute causes of the dependence to, for example, changes in the metal-metal distance, effects of supports, or cluster shapes.