Electrochemical Promotion of Catalysis: II: The Role of a Stable Spillover Species and Prediction of Reaction Rate Modification

Abstract

A simple model to describe the reversible electrochemical promotion of supported metal catalysts has been formulated. This model is derived from first principles and allows for the presence of two forms of catalyst surface oxygen, one ionic and one neutral. Both forms of surface oxygen are able to undergo charge-transfer reactions with the solid–electrolyte ion-conducting support. The ionic oxygen is assumed to be the only ion present with a significant surface coverage and, as such, dictates the properties of the catalyst surface double layer. Using the adsorption isotherm for oxygen ions it is easy to show that in the presence of significant lateral interactions the change in catalyst surface potential must be less than the overpotential applied to the catalyst/support interface. By considering a simple reaction mechanism that involves a charged transition state and accounting for the effect of long-range electrostatic interactions it is possible to express reaction rates as a function of applied overpotential. It is shown that under certain conditions reaction rates are predicted to increase on application of both negative and positive overpotentials (for both positively charged and negatively charged transition states), in agreement with some experimental studies, as a result of electron donation and acceptance from both the metal catalyst and the surface O/O2− redox couple. In addition the enhancement factor (the ratio of change in reaction rate to electrochemical oxygen flux) is shown to depend upon the selectivity for oxygen ion formation from charge-transfer and adsorption processes.