Mechanisms and rate laws for oxygen exchange on mixed-conducting oxide surfaces

Transition metal oxides with mobile electrons and oxygen ions (mixed conductors) constitute a broad class of materials that include selective oxidation catalysts, high-temperature electrocatalysts, and ion-transport membrane materials. Although the thermodynamic and transport properties of mixed conductors are generally understood, a consensus has not yet emerged regarding the mechanisms and rate laws governing exchange of oxygen with the bulk at the gas-exposed surface. To aid interpretation of existing kinetic data, and generate testable hypotheses for further research, this paper outlines a framework for predicting O₂ reduction rate laws based on specific reaction mechanisms. Based on nonequilibrium thermodynamics and transition state theory, this framework yields rate laws that are rigorously consistent with thermodynamics, yet allow rates of individual steps to be developed in terms of simple mass action laws, where appropriate. This framework is used to reexamine equilibrium oxygen-exchange kinetics reported for electron-rich perovskite mixed conductors La_1?xSr_xCoO_3?δ (LSC) and La_1?xSr_xFeO_3?δ (LSF), which have a metallic and a semiconducting band structure, respectively. Our analysis suggests that metallic band structure may play an important role in catalysis by stabilizing physisorbed O₂ on the surface. We also show that equilibrium surface exchange rates (as measured by ¹⁸O/¹⁶O isotopes, concentration steps, impedance, etc.) are generally only weak indicators of mechanism, and emphasize the need for kinetic data involving moderate to large displacements from equilibrium.