Microscopic modelling of the reduction of a Zn(II) aqua-complex on metal electrodes

The mechanism of the Zn(II) reduction from acid aqueous solution at a metal electrode surface has been elucidated at a microscopic level. The Anderson–Newns model was employed in order to construct the adiabatic potential energy surfaces along the solvent coordinate for several reactions as a function of the electrode–reactant distance and the overpotential. A quantum chemical approach was employed to treat the coupling of the reactant to the metal electrode within a cluster model. The reduction of a Zn(H₂O)₆]²⁺ complex was found to proceed in the adiabatic regime, the transfer of the first electron being rate-determining. Main attention is focused on effects of a qualitative nature, which are discussed in the light of available experimental data. The electrode charge excess and distance of maximal approach were found to affect significantly the Frank–Condon barriers of the reaction. An experimentally observed dependence of the rate constant of Zn(II) reduction upon the electrode material has been interpreted.