Mechanisms and Rate Laws for Oxygen Exchange on Mixed-Conducting Oxide Surfaces

Abstract

Although the thermodynamic and transport properties of mixed-conductors are generally understood, a consensus has not emerged regarding the mechanisms (and appropriate rate laws) governing exchange of oxygen with the bulk at the gas-exposed surface. One difficulty is that mass-action rate expressions typically used in developing kinetic models are at odds with the thermodynamic behavior, which often deviates substantially from mass-action. We have developed an alternative framework for deriving O2 reduction rate laws that is rigorously consistent with thermodynamics, yet allows rates of some individual steps to be developed in terms of traditional mass-action laws, where appropriate. We have used this framework to re-examine equilibrium oxygen exchange kinetics reported for electron-rich perovskite mixed conductors La1-xSrxCoO3-δ (LSC) and La1- xSrxFeO3-δ (LSF). Our analysis suggests that the metallic band structure of LSC may play an important role in the catalysis by stabilizing physisorbed O2 on the surface.