Assessment of (La1-xSrx)MnO3±δ perovskites as oxygen- carrier materials in chemical-looping processes

Abstract

(La1-xSrx)MnO3±δ (x = 0, 0.3, 0.5, 0.7, 1) perovskites prepared by co-precipitation of nitrate salts have been evaluated for a chemical looping process with fuel CH4 and CO2 or O2 as oxidants. The CH4 conversion is initially, at small deficiencies, high and is directed to total oxidation products. Subsequently, and as the available oxygen is being reduced, the CH4 conversion decreases and the main reaction products become CO and H2. The higher the Sr content the higher the CH4 conversion and the concentration of created oxygen vacancies. When the CO2 splitting reaction is used for the oxidation of the reduced solid, only a part of the created deficiency is used up, which decreases as Sr content increases. SrMnO3 oxidates CH4 almost exclusively to CO2 and, even when strongly oxygen deficient, does not react with CO2. The oxygen transport capacity with CO2 as oxidant decrease as Sr content increases and attain average values of approximately 1 wt% for LaMnO3 and 0 wt% for SrMnO3. Instead, the oxygen transport capacity with O2 as oxidant increase as Sr content increase and attain average values of approximately 1 wt% for LaMnO3 and 6.5 wt% for SrMnO3. The experimental results have been explained on basis of proposed defect models for (La1-xSrx)MnO3±δ according to which the Mn4+ → Mn3+ reduction creates oxygen vacancies the concentration of which increases with Sr content and do not participate in the CO2 splitting reaction. On the other hand, the subsequent Mn3+ → Mn2+ reduction creates oxygen vacancies the concentration of which increases as Sr content decreases and their elimination, upon oxidation, participates in the CO2 splitting reaction.