Experimental and theoretical insights into the mechanism of spinel CoFe2O4 reduction in CO chemical looping combustion

The reaction mechanism of CoFe2O4 with CO during chemical looping combustion (CLC) was studied via thermogravimetric analysis (TGA) experiments and density functional theory (DFT) calculations. The CO temperature-programmed reduction revealed that the reduction of CoFe2O4 is a one-step reaction process. CoFe2O4 can be directly reduced into the Co-Fe alloy. The existence of Co evidently improves the reactivity of CoFe2O4 as compared to Fe2O3. Two types of reaction kinetics are involved in the isothermal reduction of CoFe2O4. DFT calculations were performed to study the adsorption and oxidation of CO on two terminated surfaces (surface A and surface B) of CoFe2O4 (110). The results indicated that CO preferentially chemisorbs at Co/Fe-O bridge sites on surface A and at Co/Fe atop sites on surface B. CO oxidation on surface A was more thermodynamically and kinetically favorable than that on surface B. Furthermore, the oxygen vacancy formation on surface B needs more energy consumption than that on surface A. The different reactivity between surface A and surface B may be responsible for the two reaction rate peaks observed in isothermal experiments. The synergistic effect of Co and Fe atoms on the reactivity of CoFe2O4 can be mainly ascribed to the oxygen atoms in different Co/Fe coordination environments. These results can provide fundamental insights for further improving the performance of spinel CoFe2O4 oxygen carrier.