Ethylene hydrogenation on a Ni(100) surface

The kinetics of ethylene hydrogenation on a Ni(100) surface were investigated using a reactor cell directly attached to a UHV chamber under a total pressure of ca. 100 Pa and a temperature range of 270-320 K. An activation energy of 10 kJ/mol and kinetic orders with respect to hydrogen and ethylene partial pressure of 1.3 and -0.3 were obtained, respectively. The rate of ethane formation gradually decreased during the course of the reaction even in the presence of excess hydrogen. RAIR spectra taken at 295 K using IR cell showed vinyl species in the presence of hydrogen and ethylene mixture. Preadsorption of ethylene at 295 K, forming ethylene decomposed fragments, decreased the reaction rate, but half of the hydrogenation rate on the clean surface still remained upon saturated ethylene preadsorption. It indicates that the surface is not covered completely by ethylene decomposed fragments. Then, the rate of ethane production in the very beginning of the reaction was measured under vacuum by introducing pulses of hydrogen and ethylene mixture containing ca. 10(15) ethylene molecules to the hydrogen preadsorbed Ni surface kept at 240-310 K. The reaction probability calculated from the response caused by the first pulse was 10 times larger than that obtained for the initial reaction rate iu the reactor cell. The activation energy of 25 kJ/mol from ethylene hydrogenation in the pulse experiment coincides with that of 24 kJ/mol reported on Pt(111) corresponding to the hydrogenation of weakly adsorbed ethylene. (C) 1999 Elsevier Science B.V. All rights reserved.