Hydrogenation of ethylene on metal electrodes. Part 5.—Reduction of light ethylene on Pt in deuteroperchloric acid solution and the dual-pathway mechanism

Abstract

Electroreduction of light ethylene on a platinum electrode was conducted in a heavy-water solution of deuteroperchloric acid. Deuterium-atom distributions in the product, ethane, support the previous conclusion that ethylene diffusion is rate-controlling at potentials less positive than ca. 100 mV, whereas the surface reaction is rate-controlling at more positive potentials where the Tafel line holds. The D-atom distribution in the latter potential region reveals double maxima at [2H2]- and [2H6]-ethanes. This distribution is explained by the dual-pathway mechanism which assumes two reaction rates for the step C2H4(a)+ H(a)⇌ C2H5(a). The difference in the reaction rate will be attributed to the difference in the adsorption state of C2H4(a) but not of H(a), since only the weakly adsorbed hydrogen atoms are active in the hydrogenation.Reduction of light ethylene with D2 on platinum in deuteroperchloric acid solution gives the same results.A computer simulation based on the above mechanism can reproduce quantitatively not only the present distributions but also others given in the literature, even those observed for the gas-phase heterogeneous reduction.