Structural and thermodynamic study of Ca A- or Co B-site substituted SrFeO3-δ perovskites for low temperature chemical looping applications

Perovskite-structured materials, owing to their chemical-physical properties and tuneable composition, have extended their range of applications to chemical looping processes, in which lattice oxygen provides the oxygen needed for chemical reactions omitting the use of co-fed gaseous oxidants. To optimise their oxygen donating behaviour to the specific application a fundamental understanding of the reduction/oxidation characteristics of perovskite structured oxides and their manipulation through the introduction of dopants is key. In this study, we investigate the structural and oxygen desorption/sorption properties of Sr1-xCaxFeO3-delta and SrFe1-xCoxO3-delta (0 <= x <= 1) to guide the design of more effective oxygen carriers for chemical looping applications at low temperatures (i.e. 400-600 degrees C). Ca A- or Co B-site substituted SrFeO3-delta show an increased reducibility, resulting in a higher oxygen capacity at T <= 600 degrees C when compared to the unsubstituted sample. The quantitative assessment of the thermodynamic properties (partial molar enthalpy and entropy of vacancy formation) confirms a reduced enthalpy of vacancy formation upon substitution in this temperature range (i.e. 400-600 degrees C). Among the examined samples, Sr0.8Ca0.2FeO3-delta exhibited the highest oxygen storage capacity (2.15 wt%) at 500 degrees C, complemented by excellent redox and structural stability over 100 cycles. The thermodynamic assessment, supported by in situ XRD measurements, revealed that the oxygen release occurs with a phase transition perovskite-brownmillerite below 770 degrees C, while the perovskite structure remains stable above 770 degrees C.