BaCoO3−δ perovskite-type oxygen carrier for chemical looping air separation, part Ⅰ: Determination of oxygen non-stoichiometry and cyclic stability of oxygen carrier

Abstract

Chemical looping air separation (CLAS) is a cost-effective high-temperature air separation technology, and finding suitable oxygen carriers with high oxygen selectivity and capacity is of vital importance to it. Perovskite-type oxides have been extensively studied as oxygen carriers due to their high oxygen selectivity. In this work, BaCoO3-δ perovskite oxide with large oxygen capacity was proposed as an excellent oxygen carrier for CLAS. BaCoO3-δ perovskite powder was synthesized through a sol–gel method, and the determination of oxygen non-stoichiometry and cyclic stability of BaCoO3-δ was investigated via thermogravimetric analysis (TGA), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS). Results indicated that the oxygen non-stoichiometry of BaCoO3-δ was a function of temperature and oxygen partial pressure, and the empirical formula was determined by linear fitting in the range of δ less than 0.5. The standard enthalpy change and standard entropy change of the oxygen desorption reaction were 169.43 kJ/mol and 156.19 kJ/(mol·K) at δ of 0.38, which increased to 292.58 kJ/mol and 298.81 kJ/(mol·K) at δ of 0.47, respectively. The oxygen desorption amount and desorption rate of BaCoO3-δ remained extremely stable during 50 cycles at 875℃, with an oxygen capacity of approximately 0.018 g O2/g BaCoO3 of each cycle. The kinetics of the oxygen desorption process of BaCoO3-δ perovskite was described using the 3-D diffusion Jander model, and the obtained values of apparent activation energy (Ea) and pre-exponential factor (A) were 209.79 kJ/mol and 1.93E06, respectively. Further results indicated that BaCoO3-δ demonstrated a hexagonal perovskite phase, the desorbed oxygen was provided by lattice oxygen in perovskite, the crystal structure remained stable during cycles, and the micro-morphology slightly changed after 50 cycles.