Mechanism study on the high-performance BaFe2O4 during chemical looping gasification

As an attractive oxygen carrier, BaFe2O4 shows an outstanding chemical looping gasification (CLG) performance with a high CO yield, but the reaction mechanisms of BaFe2O4 during CLG process are still not clear. The present work aims to reveal the reaction mechanisms of BaFe2O4 with solid carbon (C) and carbon monoxide (CO) by means of experiments and computational methods based on the density functional theory (DFT). The performance of Fe2O3 was also investigated for comparison. Results of fixed-bed experiments show that BaFe2O4 has a better reactivity in solid-solid reaction with high CO yield, while Fe2O3 shows a better performance in solid-gas reaction with high CO2 yield. Components of Fe and FeO were detected in the products of BaFe2O4-C reaction, but not found in the products of Fe2O3-C reaction, indicating that BaFe2O4 is more active in the solid-solid reaction. Pure Fe and FeO phases in the products of Fe2O3-CO reaction suggest that Fe2O3 is more active in the solid-gas reaction. Combining the experimental and calculation results, it can be found that both the solid-solid reaction and the solid-gas reaction can be divided into the steps of C/CO adsorption, formation of CO*/CO2* complex, CO*/CO2* desorption and lattice oxygen migration. The rate-limiting-steps in solid-solid and solid-gas reactions are the formation of CO* and CO2* complex, respectively. The adsorption energy of C/CO on BaFe2O4 surface is lower than that on Fe2O3 surface, and there is a large amount of adsorbed CO on BaFe2O4 surface, indicating that BaFe2O4 surface is more active than Fe2O3 surface. The BaFe2O4-C reaction is more favorable with a lower energy barrier (0.545 eV) than that Fe2O3-C reaction (0.960 eV). In the solid-gas reaction, Fe2O3-CO reaction has a lower reaction energy barrier of 0.791 eV than BaFe2O4-CO reaction (0.997 eV). The large amount of Fe2+ ion in Fe2O3 bulk indicates the strong oxygen migration ability. Therefore, BaFe2O4 displays good potential in CLG process due to its relative stronger surface activity and weaker migration ability of lattice oxygen.