A model for electrochemical proton-transfer reactions

A a new model for electrochemical proton-transfer reactions on non-catalytic metals is presented, which is similar to the familiar Grotthus mechanism for proton transport in the bulk. The rate-determining step is assumed to be the transfer of a proton from a hydronium ion in the second water layer to a water molecule in contact with the metal, from where another proton is passed on to the metal surface. Monte Carlo simulations show that a considerable negative surface charge on the metal is required to obtain a configuration in which the proton can be transferred to the metal surface. The transfer rate is governed by the reorientation of the water molecule that accepts the proton from the hydronium ion. The change in free energy required for this step is calculated by means of umbrella sampling techniques. Our results do not conform to the Butler-Volmer law for electrochemical reactions. (C) 1998 Elsevier Science B.V.