Oxygen transport kinetics in mixed conducting oxide membranes

This thesis describes the studies of oxygen transport kinetics in dense mixed ionic electronic conducting (MIEC) oxide membranes particularly with respect to the importance of the surface exchange kinetics at the gas-solid interface relative to the bulk diffusion rate inside a membrane. The main focus is on better understanding the surface exchange kinetics in such membranes and developing a theoretical model for the surface exchange flux crossing the interface.;The analytical model for the surface exchange flux previously developed in our laboratory is experimentally verified by measuring the pressure dependence of oxygen flux through SrCo0.8Fe0.2O3-delta and Sm0.5Sr0.5CoO3-delta membranes. The determination of the ambipolar diffusion and the surface exchange coefficients of those membranes by analyzing the experimental data using the model is discussed. Emphasis is on the permeation measurement apparatus newly developed for tubular membranes.;The development of an advanced surface exchange model which takes into account the effect of vacancies at the gas-solid interface is discussed. This effect was neglected in the previous model since the assumption of first order reaction rate is still valid under the condition in which the permeation experiments were carried out. Based on the new model, the difference between the surface exchange coefficients determined from different experimental techniques such as isotope exchange experiments, permeation experiments and conductivity relaxation experiments is elucidated. The model is applied to interpret the experimental results obtained from permeation and conductivity relaxation measurements on La0.5Sr0.5Fe0.8Ga0.2O 3-delta. Analyses of the experimental data show how the new model differs from the old one.;The surface exchange kinetics under large oxygen partial pressure gradients corresponding to those encountered under actual operating conditions in methane conversion reactors is studied. A model is used to estimate the oxygen flux enhancements under large gradients made by the CO oxidation reaction at the permeate side of the membrane. The observed increase in permeation flux is compared with the enhancement predicted by the model.;Intergrowth structures in which perovskite-type blocks or layers alternates with non-perovskites have been of particular interest as possible candidates for membrane reactor applications because of their structural stability under very reducing environments. The studies of oxygen permeation through an oxide membrane with bulk composition SrFeCo0.5Ox, which is a derivative of a non-perovskite Sr4Fe6O13 are described. Detailed studies of phase stability of SrFeCo0.5O x are also discussed.;A perovskite La0.2Sr0.8Cu0.4Co 0.6O3-x was reported to have an unusually large concentration of oxygen vacancies which are randomly orientated. Accordingly, the fundamental transport properties including electric conductivity and oxygen permeability, as well as the phase stability of La0.2Sr0.8Cu 0.4Co0.6O3-x were investigated. Possible causes of the observed decrease in permeation flux with time due to vacancy ordering and chemical decomposition in a reducing atmosphere are discussed.