The physical properties of two samples of Raney nickel catalysts have been studied. It was found that the hydrogen content was 45 to 100 cc./g. catalyst. The evolution of hydrogen was an exothermic reaction but involved no phase changes and was substantially complete at 400°. X-Ray measurements indicated the predominant phase was face-centered-cubic and was presumably an aluminum-nickel alloy containing for one preparation 3% and for the second 8% aluminum by weight. The surface area of these two catalysts was 76 and 63 m.2/g. and was not reduced by heat treatment below 250 to 300°. Measurements of density and magnetic susceptibility were carried out at room temperature as a function of hydrogen content. These data led to the picture that the hydrogen in Raney nickel is present as atoms in a substitutional solid solution.

No abstract

No abstract

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 (CZH~, 02, 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. IntroductionNearly 30 years ago, Woodman and Taylor' discovered that zinc oxide is an effective catalyst for the hydrogenation of ethylene. Since this discovery, a number of reports'-' have appeared dealing with this aspect of the chemistry of zinc oxide, but the only thorough studies of kinetics have been carried out by Teichner's g r~u p~-~J at elevated temperatures, Le., roughly 100 to 400". Under these conditions, the reaction is quite complex: orders and activation energies change from one temperature range to a n~t h e r ,~ and formation of ethylene residuess with consequent poisoning' is evident. Since these studies span a range of temperatures in which the adsorption of hydrogen is a complex phenomenon,*-1° complexities of the related hydrogenation kinetics are to be expected. Nevertheless, the results are such that they frustrate attempts to put forth a coherent picture for the mechanism of ethylene hydrogenation.

No abstract