The number of active sites for the disproportionation of ethylene on supported rhenium oxide catalysts

Abstract

The adsorption and disproportionation of deuterated ethylene on an alumina-supported rhenium oxide catalyst has been studied. The average heat of adsorption of ethylene was found to be 6.02 kcal/mole and the adsorption entropy corresponded to that consistent with a mobile adsorbate. The activation energy for the surface reaction was found to be 9.45 kcal/mole, and, based on the available surface area, the preexponential factor was estimated to be 5 × 10 −11 cm 2/molec-sec. Based on these kinetic and thermodynamic parameters, models for a pairwise and nonpairwise chain reaction with both a mobile and an immobile transition state have been considered. Although the disproportionation of ethylene is not sufficiently intricate enough to distinguish between a four-centered pairwise exchange and a nonpairwise sequence with a carbene chain carrier, the results suggest that, for either case, the reaction proceeds through an immobile transition state with an active site concentration on the order of 7 × 10 11 sites/cm 2. For a mobile transition state, the calculated preexponential factor was at least six orders of magnitude greater than that observed experimentally.