On the Competition of Homogeneous and Heterogeneous Chain Terminations in Heterogeneous Photooxidation Catalysis by Zinc Oxide

Abstract

Abstract The photooxidation catalysis of isopropyl alcohol on zinc oxide has been investigated with special reference to the chain termination, mechanism. It has been proposed that homogeneous and heterogeneous chain terminations, compete in this system. At a low concentration of zinc oxide, the homogeneous termination predominates, while at a high concentration the heterogeneous termination may come into play. As a result, a maximum may appear in the relation of the rate of oxygen uptake to the amount of zinc oxide present. With excess zinc oxide, chain reactions are suppressed, so no maximum may appear in the corresponding plot and the temperature coefficient should be very small. The experimental results have been found to be consistent with the expectations. The effect of doping zinc oxide on the photo-oxidation kinetics has been investigated and discussed, mainly on the basis of the data available on the formation and decomposition of hydrogen peroxide on doped zinc oxide samples. It has been suggested that lithia-doped zinc oxide is more active than alumina-doped zinc oxide at higher temperatures because of the; accompanying hydrogen peroxide-initiated oxidation of isopropyl alcohol on the former, while it is less active at lower temperatures because, of the efficient decomposition of hydrogen peroxide, giving oxygen and water.