Oxygen surface exchange kinetics of SrTi1−xFexO3−δ mixed conducting oxides

Abstract

The oxygen surface exchange kinetics of mixed conducting perovskite oxides SrTi_1−xFe_xO_3−δ (x = 0, 0.01, 0.05, 0.35, 0.5) has been investigated as a function of temperature and oxygen partial pressure using the pulse-response ¹⁸O–¹⁶O isotope exchange (PIE) technique. Arrhenius activation energies range from 140 kJ mol⁻¹ for x = 0 to 86 kJ mol⁻¹ for x = 0.5. Extrapolating the temperature dependence to the intermediate temperature range, 500–600 °C, indicates that the rate of oxygen exchange, in air, increases with increasing iron mole fraction, but saturates at the highest iron mole fraction for the given series. The observed behavior is concomitant with corresponding increases in both electronic and ionic conductivity with increasing x in SrTi_1−xFe_xO_3−δ. Including literature data of related perovskite-type oxides Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ, La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ, La_0.6Sr_0.4CoO_3−δ, and Sm_0.5Sr_0.5CoO_3−δ, a linear relationship is observed in the log–log plot between oxygen exchange rate and oxide ionic conductivity with a slope fairly close to unity, suggesting that it is the magnitude of the oxide ionic conductivity that governs the rate of oxygen exchange in these solids. The distribution of oxygen isotopomers (¹⁶O₂, ¹⁶O¹⁸O, ¹⁸O₂) in the effluent pulse can be interpreted on the basis of a two-step exchange mechanism for the isotopic exchange reaction. Accordingly, the observed power law dependence of the overall surface exchange rate on oxygen partial pressure turns out to be an apparent one, depending on the relative rates of both steps involved in the adopted two-step scheme. Supplementary research is, however, required to elucidate which of the two possible reaction schemes better reflects the actual kinetics of oxygen surface exchange on SrTi_1−xFe_xO_3−δ.