(459a) Dopant Effect on Perovskite Based Mixed Metal Oxides for Chemical Looping with Oxygen Uncoupling (CLOU)

As an alternative to conventional combustion, chemical looping combustion (CLC) has the potential to achieve higher 2^nd law efficiency with inherent CO₂ capture capabilities. Although CLC has been extensively demonstrated for gaseous fuels, extension of this process to solid fuels, such as coal, is challenging due to the slow kinetics of solid-solid reactions. Chemical Looping with Oxygen Uncoupling (CLOU) provides a possible solution to this challenge. In a CLOU process, lattice oxygen from the oxygen carrier is released into the gas phase for facile combustion of solid fuels. The oxygen deprived oxygen carrier is then regenerated with air while producing heat. In the current study, perovskite-structured oxygen carriers with a general formula of A_xA’_1-xMnO₃ (A/A’ = Sr, Ca, Ba) are investigated for CLOU applications. Results show CaMnO₃ provides excellent oxygen uncoupling ability; however, irreversible phase decomposition is observed. The effects of Sr and Ba as A-site dopants on CaMnO₃ are explored with the primary intention to prevent irreversible phase decomposition. These materials are tested for oxygen release properties, redox stability, and reactivity. It was determined that both barium and strontium doping lead to decreased oxygen release at high temperatures. Furthermore, Sr dopant exhibits excellent compatibility with the CaMnO₃ parent structure at a wide range of concentrations. Ca_1-xSr_xMnO₃ is also able to reduce the initial oxygen release temperature by over 400^oC, enabling facile oxygen donation. With the goal of understanding the effect of A-site cation in perovskite based oxygen carriers, DFT calculations were performed to determine the vacancy formation energy for these perovskite materials. DFT results indicate CaMnO­₃ requires less energy to create an oxygen vacancy whereas Ba stabilizes lattice oxygen in the perovskite structure. The finding is used to explain CLOU properties of the aforementioned oxygen carriers and to guide future efforts to improve oxygen carriers.