Oxygen exchange kinetics over Sr- and Co-doped LaFeO3
Résumé
Oxygen exchange constants and oxygen diffusion coefficients were determined by relaxation (TGA-DSC and in situ XRD) and steady-state isotopic transient kinetic analysis (SSITKA) techniques as a function of oxygen partial pressure, temperature, and Co content for La0.6Sr0.4CoyFe1-yO3-delta (y = 0.1, 0.2, and 0.3). When y = 0.2, oxygen exchange constants and oxygen diffusion coefficients appeared to be enhanced once above the temperature at which the unit cell symmetry increased from rhombohedral to cubic. Oxygen partial pressure demonstrated minimal influence upon these properties over the higher range (near ambient) studied, but showed a major impact when results obtained near ambient oxygen partial pressures are compared to those obtained under N-2. The direction of oxygen transport was also important. Oxygen exchange constants and oxygen diffusion coefficients calculated from oxidation steps were greater than values for the same properties calculated from reduction steps under all conditions examined. Explanations for this behavior are proposed with use of kinetic, transport, and ESR spectroscopy data. While the oxygen exchange constant increased with Co content, the oxygen diffusion coefficient progressed through a maximum for the formations studied. The nonlinear trend for oxygen diffusivity with Co content is explained by an electronic structure change over this compositional range, which aligned with differences in Mossbauer spectra.