Abstract
Homocysteine (Hcy) and cysteine (Cys) mercury thiolate layers were prepared by anodic polarization of a mercury electrode in amino acid containing solutions and then investigated in the cathodic regime in the presence of Ni^2+^ or Co^2+^ ions. The sulfhydryl function in the mercury thiolate undergoes a slow disintegration resulting in surface‐attached mercury sulfide. During the cathodic scan, Hg^2+^ substitution by Ni^2+^ or Co^2+^ yields minute amounts of the relevant metal sulfide. Such a species catalyzes hydrogen evolution at −1.3 V vs. Ag|AgCl|KCl(3 M). Hcy experiences a faster decomposition and, consequently, displays a stronger catalytic effect. Each compound catalyzes the reduction of Ni^2+^ or Co^2+^, but only Cys (bound in metal complexes) induces typical catalytic hydrogen evolution processes such as the Brdička reaction (with Co^2+^; pH around 9), or the catalytic hydrogen prewave (CHP) (with Ni^2+^; pH near 7). On the other hand, Hcy catalyzes the hydrogen evolution in the presence of Co^2+^ at −1.5 V in the same way than sulfur derivatives with no amine function do. Metal sulfide formation does not interfere with CHP and Brdička processes. Correlations between the physical state of the metal sulfide (adsorbed molecule or aggregate form) and its catalytic properties are discussed and possible analytical applications suggested.