Catalytic hydrogen and nickel prewaves in deuterium oxide solutions

Abstract

In order to investigate the proton-transfer reactions involved in catalytic hydrogen or nickel dischare, the limiting currents measured in isotopically different solutions, for otherwise identical conditions, were compared. The catalytic hydrogen prewave shows a strong direct isotope effect. Analysis of phenomena related to the solvent properties, chemical equilibria and reaction rates in isotopic media demonstrates the kinetic nature of this effect. Conversely, the catalytic nickel prewave presents an apparently inverse isotope effect at pH (pD)>6.3. To account for this effect one must consider two possible reactions of cysteine molecules released by the decomposition of the zero-valent nickel complex: (1) protonation of the amino group; (2) reaction with nickel ions to give a reducible complex. The former reaction occurs slowly in D 2O, thus stimulating the second reaction which determines in part the value of the limiting catalytic nickel current.