Oxygen Nonstoichiometry and Defect Models of Brownmillerite-Structured Ca2MnAlO5+delta for Chemical Looping Air Separation

Brownmillerite-structured Ca2MnAlO5+delta has demonstrated excellent oxygen storage capacity that can be used for chemical looping air separation (CLAS), a potentially efficient approach to produce high-purity oxygen from air. To effectively utilize this material as an oxygen sorbent in CLAS, it is necessary to comprehensively understand its thermodynamic properties and the structure-performance relationships in the operating range of interest. In this work, the oxygen nonstoichiometry (delta) of Ca2MnAlO5+delta was systematically measured by thermogravimetric analysis (TGA) in the temperature ranging from 440 to 660 degrees C and under an oxygen partial pressure ranging from 0.01 to 0.8 atm. The partial molar enthalpy and entropy for the oxygen-releasing reaction were calculated using the van't Hoff equation with an average value of 146.5 +/- 4.7 kJ/mol O-2 and 162.7 +/- 5.1 J/K mol O-2, respectively. The experimentally measured nonstoichiometry (delta) was well fitted by a point defect model applied in two regions divided by the predicted equilibrium P-T curve. The equilibrium constants for appropriate defect reactions were also determined. The thermochemical parameters, molar enthalpy and entropy for the main reaction, obtained from the defect model were 136.9 kJ/mol O-2 and 225.3 J/K mol O-2, respectively, showing reasonable agreement with the aforementioned values. The applicability of the defect model was also verified at a higher oxygen partial-pressure environment of up to 4 atm and exhibited reasonable prediction of the trend. The experimental studies on oxygen nonstoichiometry combined with the defect modeling provide useful insights into oxygen sorbents' redox performances and helpful information for the design and optimization of oxygen sorbents in CLAS.