Beneficial effect of order-disorder phase transition on oxygen sorption properties of perovskite-type oxides

A beneficial effect of oxygen vacancy disorder-order phase transition on oxygen sorption properties of perovskite-type oxides is reported in this paper. Three perovskite-type oxides, La_0.1Sr_0.9Co_0.9Fe_0.1O_{3 - δ}, SrCo_0.8Fe_0.2O_{3 - δ} and La_0.5Sr_0.5CoO_{3 - δ} were prepared by liquid citrate method. The phase transition temperatures from the brownmillerite structure to perovskite structure in He are 780 and 860 °C for SrCo_0.8Fe_0.2O_{3 - δ} and La_0.1Sr_0.9Co_0.9Fe_0.1O_{3 - δ}, respectively. In the sorption process (He-air), SrCo_0.8Fe_0.2O_{3 - δ} and La_0.1Sr_0.9Co_0.9Fe_0.1O_{3 - δ} exhibit larger oxygen sorption capacity than La_0.5Sr_0.5CoO_{3 - δ} below the phase transition point temperatures_. Oxygen sorption kinetics on all the samples are fast, however, desorption kinetics on the samples are complex, depending on both the temperature and sorbent structure. An enhanced desorption rate is found for SrCo_0.8Fe_0.2O_{3 - δ} and La_0.1Sr_0.9Co_0.9Fe_0.1O_{3 - δ} at low temperatures with disorder-order phase transition involved in the process. The enhancement results from the lower isosteric heat of sorption on the brownmillerite structure, which implies that oxygen is easier to be released from the lattice structure as compared to the perovskite structure.