Theoretical and experimental study of the catalytic hydrogen evolution reaction in the presence of an adsorbed catalyst by means of square-wave voltammetry

Surface catalytic electrode mechanism based on the hydrogen evolution reaction is analyzed both theoretically and experimentally under conditions of square-wave voltammetry (S W). The electrode mechanism involves preceding chemical reaction in which the adsorbed catalyst (Cat((ads))) is undergoing protonation at the electrode surface, i.e., Cat((ads)) + H-(aq)(+) reversible arrow CatH((ads))(+). The protonated form of the catalyst (CatH((ads))(+)) is irreversibly reduced yielding the initial form of the catalyst and atomic hydrogen, i.e., CatH((ads))(+) + e -> Cat((ads)) + H-(aq). The concentration of protons is assumed to be constant in the course of the voltammetric experiment due to the buffered solution, whereas the current is controlled by the variation of the surface concentration of both unprotonated and protonated forms of the catalyst. The overall voltammetric behavior of the system is a specific combination of a simple surface irreversible electrode reaction, surface catalytic reaction and surface reaction preceded by a chemical reaction (CE mechanism). The effect of the thermodynamics and kinetics of the preceding chemical reaction, as well as the kinetics of the irreversible electrode reaction, to the SW voltammetric response is examined in detail. The overall catalytic effect is predominantly controlled by the ratio of the protonation rate constant and the time window of the voltammetric experiment that is represented by the frequency of the potential modulation. Theoretical predictions are illustrated by experiments with the hydrogen evolution reaction at the hanging mercury drop electrode catalyzed by adsorbed famotidine. (c) 2005 Elsevier B.V. All rights reserved.