ABSORPTION AND ADSORPTION OF H IN THE H-2 EVOLUTION REACTION AND THE EFFECTS OF COADSORBED POISONS

Conditions under which absorption of H into cathode metals arises are examined in relation to coverage, theta(H), by adsorbed H. theta(H) is determined by potential and the mechanism of the H-2 evolution reaction, which thus determines the driving force for H entry into cathodes. By means of lattice occupancy thermodynamic equations, it is shown how the lattice occupancy fraction X(H) depends on overpotential. The application of a Nernst-type equation for evaluation of pressures of sorbed H is shown to be inappropriate, as is also the assumption that sorption of H is driven by H coverage arising in the H + H recombination step. The latter is inapplicable at most cathode metals except active Pt, and also inapplicable at poisoned electrodes where theta(H) is diminished substantially by poisons at transition metals. Potential-relaxation and frequency response experiments have been conducted at Pt in the presence and absence of thiourea as a poison and clearly demonstrate the role of the latter in diminishing coverage by ''overpotential-deposited'' H. Finally, kinetic simulation calculations are given for various reaction pathways and the effects of a co-adsorbed poison giving rise to interaction effects in the adsorbed H film.