Understanding of Transition from Gas-Phase to Material-Kinetic Limitations for Nonstoichiometric Oxides

Abstract

Oxygen nonstoichiometric oxides are important materials in technologies such as fuel cells, permeation membranes, and solar-driven thermochemical fuel production owing to their reversible redox characteristics. The material kinetic properties of the oxides, the surface reaction rate constant at the solid/gas interface and the chemical diffusion in the bulk, have been widely investigated for the understanding of chemical redox kinetics. However, under certain conditions, insufficient oxidant/reductant supply hinders the overall redox kinetics in which the oxides accompany relatively large nonstoichiometry (δ) deviations during the reaction. Furthermore, since the oxides are usually structured as mesoporous or microporous forms to enhance the effective surface reaction rate and thereby enhance kinetics, understanding the correlation among the material kinetic properties, the morphology of the oxides, and the oxidant/reductant gas supply is essential for establishing the strategy for maximizing global reaction rates. We will present the effects of morphology, temperature and gas flow rate on the global redox kinetics of porous undoped ceria (CeO2-δ) as a model system. Using the reticulated porous ceramic (RPC) approach, and the morphology of the samples was controlled by modifying the starting ceria slurry with/without pore-formers. Kinetic behavior was examined as a function of temperature and gas flow rate by the electrical conductance relaxation technique, which is attractive due to its ease for extracting the kinetic parameters. The validity of the electrical conductance relaxation technique for assessing the kinetic behavior of the porous oxides in the limits of the gas-phase supply and the surface reaction of material will be discussed, and the theoretically anticipated transition behavior from the gas-phase to the material kinetic limits will be shown in comparison to the experimental result.