Study on the mechanism of CO chemical looping combustion over Fe-Based oxygen carriers with unsaturated coordination environments

It is crucial to study the Fe-O coordination environment due to oxygen defects for elucidating the dynamic active center during CO chemical looping combustion with hematite. In this work, Fe-based oxygen carriers with varying oxygen vacancy concentrations were firstly synthesized by direct thermal decomposition of ferric nitrate and subsequently evaluated through fixed-bed experimentation. It was showed that the reactivity of oxygen carriers produced by calcination at 700 °C exhibited a higher CO conversion rate due to the moderate oxygen vacancy concentration. Then, density-functional theory (DFT) calculations were conducted to examine the impact of oxygen vacancies. The results indicated that the formation of oxygen vacancies exhibited a proximity effect, resulting in the emergence of various Fe-O coordination environments. The low coordination number of Fe-O enhances the reactivity of the lattice oxygen and significantly lowers the activation energy barrier for the oxidation of CO to CO₂. Furthermore, the effect of oxygen vacancies on the migration of bulk phase oxygen was also investigated. It was shown that the migration barriers of bulk oxygen increased with the concentration of oxygen vacancies, resulting in a reduction in the oxygen supply rate. Moderate concentration of oxygen vacancies facilitates CO oxidation by aligning surface catalysis with the oxygen migration rate. This evidence suggests that the Fe-O coordination environment and oxygen vacancy concentration serve as key factors in controlling in controlling CO oxidation over Fe-based oxygen carriers.