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.