Rhodium cluster ultradispersed catalysts of high and low activity

Abstract

Catalysts were prepared using RH 6(CO) 16 adsorbed or slurried on a silica substrate. For these catalysts the rate constants for the reduction of benzene and in some cases cyclohexene were determined. The catalyst particle size distributions were determined by electron microscopy. These results were analyzed to provide varying rate constants assigned to catalyst particles of various sizes. Clusters of rhodium atoms of 0.8 to 1.2 nm corresponding to about Rh 6 were found to have very low, possibly zero, activity for hydrogenation reactions. Clusters of atoms above 1.4 nm diameter were found to be active catalysts for hydrogenation under the same conditions. Some evidence indicating a very high activity for catalyst particles between 1.2 and 2.0 nm was obtained. Below 2.0 nm only two-dimensional rafts are found. This high activity is not therefore attributable to terrace edge sites although evidence of such enhanced activity was found at lower dispersions, 2.0 to 3.0 nm diameter.