Ln Dependence of the Large-Capacity Oxygen Storage/Release Property of Ln Oxysulfate/Oxysulfide Systems

Abstract

In a series of large-capacity oxygen storage materials using the redox between Ln2O2SO4(S6+) and Ln2O2S(S2-), the Pr system can work at as low as 600 °C, compared to ≥650 °C required for Ln = La, Nd, and Sm. The exceptional character of the Pr system has been studied from physicochemical points of view by means of thermogravimetric analysis, X-ray photoelectron spectroscopy, X-ray diffraction, Rietveld analysis, Fourier transform infrared spectroscopy, and Raman spectroscopy. Unlike the other Ln oxysulfates/oxysulfides, the Pr system contained a considerable amount of tetravalent cation (Pr4+) on the surface. The smooth redox between Pr3+ and Pr4+ would promote the oxidation of bulk Pr2O2S to Pr2O2SO4. On the other hand, the smooth reduction of Pr2O2SO4 to Pr2O2S appears to be associated with a local structure of sulfate. The X-ray and spectroscopic analysis predicted that the tetrahedral SO4 unit of Pr2O2SO4 is more distorted than that of La2O2SO4. Instability caused by the stronger distortion of SO4 would lead to the easier reduction to S2- species. A synergy of these two different effects on the redox process of the Pr system seems to be a possible reason for the successful oxygen storage/release cycles at lowest possible temperatures.