Study on the synergistic effect and oxygen vacancy of CeO2/Fe2O3 oxygen carrier for improving reactivity in carbon monoxide chemical looping combustion

Chemical looping combustion (CLC) is a promising way for CO2 separation and capture. The oxygen carrier (OC) plays a key role in CLC by delivering lattice oxygen for oxidation reaction. Herein, CeO2/Fe2O3 oxygen carrier with oxygen vacancies is successfully synthesized, and the synergistic effect of Ce and Fe in CeO2/Fe2O3 OC is investigated by thermogravimetric analysis (TGA) tests and theoretical calculations. The reactivity of OCs is also tested in a fixed bed. Experimental results show that the CeO2/Fe2O3 oxygen carrier exhibits higher CO oxidation efficiency than CeO2 and Fe2O3, which may be caused by the synergistic effect of Ce and Fe. Moreover, the CeO2/Fe2O3 composite OC shows a downward trend and achieved a weight loss rate of 4.1% which is higher than that of Fe2O3 and CeO2 in the TGA test. In addition, the electron paramagnetic resonance (EPR) spectra of oxygen carrier demonstrates that the typical signal peaks of oxygen vacancies around at g = 2.003 of CeO2, Fe2O3 and CeO2/Fe2O3 OC were observed, which can rapidly facilitate migration of lattice oxygen from interior to the surface of OC due to the existence of oxygen vacancies. Further, the oxygen vacancy formation energy was calculated by DFT, showing that the figure of the CeO2/Fe2O3 OC (3.95 eV) is lower than Fe2O3 (9.56 eV), which indicates that the addition of CeO2 can efficiently reduce the oxygen vacancy formation energy. Finally, based on the results of reactivity and characterization test of OCs, it is demonstrated that the CO CLC reaction with CeO2/Fe2O3 oxygen carrier follows Mars-van Krevelen (MvK) mechanism. This work reveals new insights into the synergistic improvement of the performance of Ce-Fe oxygen carriers with oxygen vacancies.