Anomalous H/D isotopic effect in the kinetics of polarographic hydrogen evolution

Electrolysis of acidic H2O+D2O mixtures has been performed using the well e stablished mercury drop electrode. The experiment was done in order to stud y, for the first time in a chemical reaction, a possible cooperative behavi our of hydrogen species which is evident from neutron and Raman light scatt ering studies. The electrochemical-kinetic results show a volcano type of d eviation of the rate constants, centered around [D]/[H+D] = 0.8, from behav iour expected from nominal D and H concentrations. It can be fitted assumin g an effective fractionation factor of l* = 0.25, which deviates significan tly from the established (bulk) fractionation factor of l = 0.69. Basically , a different D+/H+-ratio is effective for electron transfer on the mercury drop surface than is present in bulk solution. Various strategies have bee n followed to explain the observed phenomena on a classical basis. The assu mption that a reorganisation of the water I mercury interface may generate the reduced fractionation factor is presently not sustained by convincing e xperimental or theoretical evidence. We therefore propose to consider limit ations of the applicability of simple reversible statistical concepts to th e electrochemical water splitting process being in line with ongoing resear ch on quantum correlation with H/D systems. (C) 1999 Elsevier Science S.A. All rights reserved.