New Electrocatalytic Reactions at a Mercury Electrode in the Presence of Homocysteine or Cysteine and Cobalt or Nickel Ions

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 Ni2+ or Co2+ ions. The sulfhydryl function in the mercury thiolate undergoes a slow disintegration resulting in surface‐attached mercury sulfide. During the cathodic scan, Hg2+ substitution by Ni2+ or Co2+ 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 Ni2+ or Co2+, but only Cys (bound in metal complexes) induces typical catalytic hydrogen evolution processes such as the Brdička reaction (with Co2+; pH around 9), or the catalytic hydrogen prewave (CHP) (with Ni2+; pH near 7). On the other hand, Hcy catalyzes the hydrogen evolution in the presence of Co2+ 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.