Understanding the promotion of Cu in CuFe2O4 composite oxygen carrier for CH4 oxidation in chemical-looping combustion

Chemical-looping combustion offers a novel and energy-efficient way for CO2 capture. However, it is still essential to identify efficient oxygen carriers and elucidate their reaction mechanisms. Herein, the promotion mechanism of Cu in spinel CuFe2O4 for CH4 oxidation was fully understood by combining thermogravimetric tests and density functional theory (DFT) method. CuFe2O4 reduced in CH4 reveals two reaction stages relevant to the processes of CuFe2O4 → Cu, Fe3O4 → Cu, FeO/Fe. In the initial reaction stage, the existence of Cu in CuFe2O4 markedly enhances the reaction rate compared to Fe2O3. DFT results demonstrate that CH4 dehydrogenation, CO2 and H2O formation via surface Cu atoms is more favorable in kinetics. This is primarily because the radicals from CH4 dehydrogenation show lower adsorption strength on Cu site, which is conducive to the next migration and final oxidation. DFT results can well explain the better reactivity of CuFe2O4 than Fe2O3 observed in TGA tests. CH4 → CH3 + H is found to be the rate-controlling step during the CH4 oxidation process, indicating that the catalytic property of oxygen carrier is very important for CH4 dissociation. This work contributes to a fundamental understanding of the promotion of Cu in CuFe2O4 for CH4 oxidation.