Kinetic isotope effect in the hydrogen evolution reaction

In the framework of the medium reorganization-proton-tunneling mechanism, the dependence of the kinetic isotope effect (KIE) on the electrode potential for poorly adsorbing hydrogen cathodes has been considered. Two experimentally accessible quantities are discussed: the ratio of the rates of hydrogen evolution in light and heavy water (K-H/D) and the isotopes' separation factor S-H/D. Their opposite dependencies on the electrode potential have been explained. With increasing polarization, many factors decrease the KIE. The opposite trend should result, in the normal (alpha approximate to0.5) region, from the effect of the electrode potential on the relative contribution of the products formation in the ground and vibrationally excited states. This explains the increase of K,l, with polarization. For the barrierless (alpha = I) and the activationless (alpha = 0) processes, where the true Value of c! is constant in the whole range of potentials, the effect of vibrationally excited states is predicted to be independent of potential. In accordance with this prediction, the KIEs for the barrierless hydronium discharge and for activationless electrochemical desorption were found to decrease with increasing polarization. This presents the first experimental evidence of a substantial role for the formation of vibrationally excited final states in the total process of hydrogen evolution. (C) 2001 Elsevier Science Ltd. All rights reserved.