Ethylene hydrogenation on a Ni(100) surface

The kinetics of ethylene hydrogenation on a Ni(100) surface were investigat ed using a reactor cell directly attached to a UHV chamber under a total pr essure of ca. 100 Pa and a temperature range of 270-320 K. An activation en ergy 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 e thane formation gradually decreased during the course of the reaction even in the presence of excess hydrogen. RAIR spectra taken at 295 K using IR ce ll showed vinyl species in the presence of hydrogen and ethylene mixture. P readsorption of ethylene at 295 K, forming ethylene decomposed fragments, d ecreased 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 fragment s. Then, the rate of ethane production in the very beginning of the reactio n was measured under vacuum by introducing pulses of hydrogen and ethylene mixture containing ca. 10(15) ethylene molecules to the hydrogen preadsorbe d Ni surface kept at 240-310 K. The reaction probability calculated from th e 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 o f 25 kJ/mol from ethylene hydrogenation in the pulse experiment coincides w ith that of 24 kJ/mol reported on Pt(111) corresponding to the hydrogenatio n of weakly adsorbed ethylene. (C) 1999 Elsevier Science B.V. All rights re served.