Density Functional Theory Study on the Reaction Mechanism of Spinel CoFe2O4 with CO during Chemical-Looping Combustion

Owing to the synergistic enhanced performance by incorporating Co atoms into the Fe-based oxide, spinel-type CoFe2O4 is regarded as a promising oxygen carrier. Herein, the reaction mechanism of CoFe2O4 with CO during chemical-looping combustion was comprehensively studied based on density functional theory calculations. The results reveal that the existence of Co atoms in CoFe2O4 is primarily responsible for the improved reactivity. On both the perfect and defective CoFe2O4(100) surfaces, Co atoms are the preferable sites for CO adsorption. In particular, the presence of surface O vacancy will weaken CO adsorption. Three types of surface O coordinated with three different metal atoms show different reactivities. Two reaction channels for CO oxidation were proposed on the perfect surface. For the one-step channel, CO can react barrierlessly with the surface O coordinated with two Co and one Fe atoms to form the CO2 molecule. The three-step channel beginning with the Co site is more kinetically and thermodynamically favorable and includes the elementary steps of CO adsorption, CO diffusion, and CO2 desorption. Noticeably, the lattice O consumption primarily occurs around the Co2+ atoms, indicating that CoFe2O4 is preferentially reduced to Co accompanied by Fe3+ to Fe2+. These results are consistent with the experimental kinetics. Further, O diffusion from sublayers to the surface shows the highest activation energy (165.95 kJ/mol) during the entire reaction, suggesting that O diffusion is inherently a high-temperature process. However, O diffusion in spinel CoFe2O4 is easier than that in Fe2O3. These findings will be an important step in understanding the improved reactivity of spinel CoFe2O4 due to the presence of Co.