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

• Published in: Journal of catalysis Vol. 245; no. 1; pp. 91 - 109
• Main Authors: Adler, S.B., Chen, X.Y., Wilson, J.R.
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier Inc 2007; Elsevier; Elsevier BV
• Subjects: Applied sciences; Catalysis; Catalyst; Catalysts; Cathode; Chemistry; Electrocatalysis; Electrochemistry; Energy; Energy. Thermal use of fuels; Equipments for energy generation and conversion: thermal, electrical, mechanical energy, etc; Exact sciences and technology; Fuel cells; General and physical chemistry; Kinetics and mechanism of reactions; Mechanism; Mixed conductor; Oxide; Oxygen; Perovskite; Rate law; Reduction; Solid oxide fuel cell; Solid oxide fuel cells; Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry; Thermodynamics; Vacancy; Oxide; Electrocatalysis; Perovskite; Oxygen; Reduction; Mixed conductor; Cathode; Vacancy; Rate law; Solid oxide fuel cell; Catalyst; Mechanism; Perovskite type compound; Oxides; Chemical reduction; Heterogeneous catalysis
• ISSN: 0021-9517; 1090-2694
• DOI: 10.1016/j.jcat.2006.09.019

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 O
2 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−
x
Sr
x
CoO
3−
δ
(LSC) and La
1−
x
Sr
x
FeO
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 O
2 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.