Oxygen Storage Properties of Double Perovskite Oxides Containing Manganese
Jeamjumnunja, Kannika 1986-
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Non-stoichiometric oxides which are capable of reversibly storing and releasing large amounts of oxygen are called oxygen-storage materials (OSMs). The A-site double-perovskite LnBaMn2O5+δ oxides have attracted a lot of interest as good OSM candidates for various oxygen-related applications, such as chemical looping, combustion and oxygen separation. In this work, the oxygen uptake/release behavior of the LnBaMn2O5+δ (Ln = Y, Gd, Eu, Sm, Nd, and Pr) compounds were investigated by thermogravimetry. The results show good oxygen uptake/release ability at moderate temperatures with oxygen-storage capacity exceeding 3 wt. % for all compounds. Oxygen uptake begins at lower temperatures in both air and oxygen in the compounds with larger Ln3+ ions. These oxides show almost complete and reversible oxygen uptake/release between LnBaMn2O5 and LnBaMn2O6 during switching between oxygen (or air) and 1.99% H2/Ar. The oxygen non-stoichiometries of the LnBaMn2O5+δ (Ln = Y, Gd, and Pr) oxides were determined as a function of pO2 at different temperatures by Coulometric titration. These materials exhibit two distinct phases (with δ ≈ 0, 0.5) and a third phase with a range of composition with the δ value approaching to ~1 during oxidation/reduction. The phase transition occurs at higher pO2 with increasing temperature. Isothermal experiments show that the larger the Ln3+ cation the lower pO2 for phase conversion. The thermodynamic quantities corresponding to the phase transition were investigated. At some temperatures and pO2 conditions, the LnBaMn2O5+δ compounds are unstable with respect to decomposition to BaMnO3-δ and LnMnO3. The electrical conductivity of YBaMn2O5 was studied as a function of temperature by using the four-probe DC method in an electrochemical cell in the low-pO2 range of 10-16 to 10-23 atm. The electrical conductivity is relatively low with an average activation energy (E_a) of 0.87 eV in 500-700 ºC. Barium migration and segregation on the sample surface were observed after the long-period experiments in low-pO2 atmosphere at high temperatures. The linear thermal expansion for the three phases (O5, O6, and O5.5) of YBaMn2O5+δ was investigated by dilatometry measurements and high-temperature XRD studies under different atmospheres. The phase transitions of LnBaMn2O6 were examined by DSC measurements.