Hydrogenation of ethylene on metal electrodes. Part 1.—Reduction of ethylene with hydrogen at a platinum electrode on open circuit

Abstract

Reduction of ethylene with hydrogen was studied at a Pt electrode on open circuit in H₂SO₄ and HClO₄ solutions. The reaction rate and the open circuit potential were followed as a function of the gas composition (C₂H₄+ H₂+ He, 1 atm) and the acid concentration. The rate, v, is independent of the acid concentration and is expressed as v=k_Hk_EP_HP_E/(k_HP_H+k_EP_E), where the k values are constants and P_H and P_E the partial pressures of H₂ and C₂H₄. The ratio k_E/k_H is 5–6, which is the same value obtained for the ratio of the diffusion rate constant of C₂H₄ in solution to that of H₂. The rate-determining step is the mass transfer step of H₂ or of C₂H₄ depending on the condition, P_H/P_E < 5–6 or > 5–6, respectively. This conclusion is supported by the dependence of the open circuit potential on the gas composition. Time variation of the catalytic activity of the activated electrode is discussed in terms of the activity change of the adsorbed hydrogen atom. Newly formed adsorbed hydrogen atoms are reactive, but after remaining on the surface, some of them become stable and have a retarding effect on the reaction.