Studies of hydrogen surface processes by the multipulse potentiodynamic method. Part 4.—Effect of potential on ethylene and acetylene adsorption on platinum

Abstract

At 30–60°C, the adsorption from 1 N HClO₄ of both ethylene and acetylene over the approximate potential range 0.2–0.4 V (measured against a reversible hydrogen electrode in the same solution) results in an ad-layer of constant composition (probably C₂H₂). The rate of adsorption of additional hydrocarbon on a surface partially covered with this low-potential species is rapid. At higher potentials, the composition varies such that saturation coverage is obtained at relatively low molar surface concentrations of the ad-species, suggesting further de-hydrogenation of the ad-layer. The rate of adsorption on a surface partially covered with the high-potential species is relatively slow. Steady-state coverage was studied as a function of potential, temperature and concentration for ethylene and acetylene. Decreasing coverage at low potentials may be ascribed to rapid electro-chemical reduction of the hydrocarbon. At high potentials, the equivalents of hydrocarbon adsorbed decrease in part because of increased multiple bonding to the surface and increased adsorption of “oxygen”. Additional decrease in coverage is brought about by electrochemical oxidation (to CO₂ and H₂O) which though slow, may effectively oppose the slow rates of adsorption encountered at high potentials. The results are also pertinent to ethane adsorption which involves, in part, similar surface species.