Surface coordination chemistry of noble-metal electrodes. Hydrogen/iodine ligand (adsorbate) substitution at smooth polycrystalline platinum

Abstract

Adsorbate (ligand) displacement/substitution has been found to occur between iodine and hydrogen at smooth polycrystalline platinum electrodes in aqueous acid, neutral, or base electrolyte; experimental measurements were based upon thin-layer electrochemical methods. The iodine/hydrogen substitution reaction is a two-electron redox-activated process, the mechanism of which appears to depend upon the pH of the solution. In the absence of an applied potential, iodine is the only species present on the Pt surface. When a negative potential is applied in the vicinity of the hydrogen evolution reaction, chemisorbed iodine is progressively desorbed as iodine ions accompanied by the immediate chemisorption of protons as hydrogen atoms; when the potential is subsequently made positive, the reverse displacement reactions occurs in which hydrogen is oxidatively desorbed as protons accompanied by the oxidative chemisorption of iodine as iodine. In acid media, iodine desorption occurs after evolution of molecular hydrogen, in basic solutions, iodine removal occurs before electrogeneration of hydrogen gas.