Redox Kinetics of Nonstoichiometric Oxides

Abstract

Cerium oxide (CeO2-δ) and its derivatives are the most attractive materials under consideration for solar-driven thermochemical production of chemical fuels. Understanding the rate-limiting factors in fuel production is essential for maximizing the efficacy of the thermochemical process. The rate of response of the porous ceria structured with architectural features typical of those employed in solar reactors was measured via electrical conductance relaxation methods. A transition from behavior controlled by material surface reaction kinetics to that controlled by sweep-gas supply rates is observed on increasing temperature, increasing volume specific surface area, and decreasing normalized gas flow rate. The transition behavior is relevant not only for optimal reactor operation and architectural design of material, but also for accurate measurement of material properties. The redox kinetics of undoped ceria, CeO2-δ at extreme high temperature (1400 °C) was investigated using the electrical conductivity relaxation method, and those of 10% Pr doped ceria at low temperature (700 °C) were done using the mass relaxation method. It was demonstrated under sufficiently high gas flow rates relative to the mass of the oxide, which is required in order to overcome gas phase limitations and access the material kinetic properties. Furthermore, the surface reaction rate constant of undoped ceria, ,kChem, was investigated at high temperature (1400 °C) in humidified gas atmosphere, in consideration of the operating conditions in thermochemical fuel production system. It was demonstrated that H2O potentially plays a role of oxidants as increasing temperature and/or decreasing oxygen partial pressure; thus in such thermodynamic conditions, pH2O, besides temperature and pO2, needs to be carefully considered in surface reaction study. In addition to relaxation experiments under small driving force for redox reaction, the kinetics of surface related oxidation reaction under large chemical driving force (large ΔpO2 change) was investigated by mass relaxation method. Based on the normalized reaction rates of several possible rate determining steps, the relaxation behavior in oxygen concentration for all possible rate determining steps was computed. On the comparison with the experimental results, the most probable rate determining step was suggested (reduction of diatomic oxygen from neutral oxygen molecule to superoxide), and the oxidation kinetics under large driving force was explained.