Oxygen Vacancy Creation Energy in Mn-Containing Perovskites: An Effective Indicator for Chemical Looping with Oxygen Uncoupling

Chemical looping with oxygen uncoupling (CLOU) is a novel process for carbon dioxide capture from coal combustion. Designing a metal oxide oxygen carrier with suitable oxygen release and uptake (redox) properties represents one of the most critical aspects for CLOU. The current work aims to correlate oxygen vacancy creation energy of metal oxide oxygen carriers with their redox properties. Oxygen vacancy creation energies of CaMnO3-delta, Ca0.75Sr0.25MnO3-delta, CaMn0.75Fe0.25O3-delta, and BaMnO3-delta were determined through density functional theory (DFT) calculations. The effect of the Hubbard U correction on the ground state magnetic configurations and vacancy creation energies was investigated, along with the effect of lattice oxygen coordination environment. It was determined that Hubbard U only slightly changes the relative differences in vacancy creation energies between the Mn-containing perovskites investigated. Therefore, ranking of oxygen vacancy creation energies among the various oxides can be determined using a simplified method without using Hubbard U. Comparisons with experimental data confirmed that vacancy creation energy is an effective indicator for oxygen release properties of the perovskites investigated: oxygen carrier materials with lower vacancy creation energies can release their lattice oxygen more readily. Thermogravimetric analysis indicated increased oxygen release with decreasing oxygen vacancy creation energy at temperatures below 700 degrees C. Higher activities for coal char combustion were also observed. The simplified DFT strategy also satisfactorily predicted the effects of iron and strontium doping on lattice distortions as well as the crystal volume changes upon oxygen vacancy creation. These findings indicate that oxygen vacancy creation energies in Mn-containing perovskites can potentially be used as an effective design parameter for oxygen carrier development and optimizations.