HYDROGENATION OF ETHYLENE BY ZINC OXIDE .2. MECHANISM AND ACTIVE SITES

The rate of hydrogenation of ethylene over zinc oxide at room temperature has been determined for hydrogen pressures ranging from 23 to 583 mm and ethylene pressures ranging from 5 to 330 mm with the ratio of hydrogen to ethylene ranging from 1 to 50. Initial rates were proportional to the hydrogen pressure to the one-half power and showed a slight dependence on ethylene pressure ascribable to the extent of the surface covered with ethylene. The kinetic isotope effect was also determined for a fixed set of conditions. These measurements, supplemented by infrared studies of the sample either with chemisorbed gases (C2H4, O2, CO) or under reaction conditions and an examination of poisoning effects of water vapor provide the basis for a model of the active site consisting of zinc ions imbedded in a close-packed oxide layer. This tentative model leads to a mechanism consistent with the kinetics and provides a plausible explanation for most of the experimental observations. It further suggests that there should be striking similarities between hydrogenation over oxides and hydrogenation over homogeneous catalysts.