Crossing-link of experimental reducibility tests, XPS characterizations and DFT estimates on ferrite oxygen carriers in CLC

Abstract

Chemical looping combustion (CLC) is an efficient technology of thermal conversion of carbonaceous fuels with CO2 being concentrated. Oxygen carriers, which are generally metal oxides or their composites, are critical in the development of different CL processes, addressing combustion, gasification and H2 production. Iron-based oxide, rich in natural resource and inexpensive, is one of ideal candidates as oxygen carriers in the CL process. Understandings toward the general principles of the selection on iron-based oxygen carriers are highly demanded. A unique approach was applied in the present study to investigate the relationship between the reducibility and the electronic structure of selected three typical ferrites as oxygen carriers, via combining experimental reducibility tests using H2-TPR and CO-TGA, the material characterizations using XRD (X-ray Diffraction) and XPS (X-ray Photoelectron Spectroscopy), and electronic structures using DFT (Density Functional Theory) calculations. Proportions of the lattice oxygen and Fe2+ in ferrites positively and negatively influence their reduction reactivity, respectively. Varieties of oxygen components in ferrites determine their reduction temperatures and reactivity. Both the approaching degrees of the O-2p and M-3d valance bands to Fermi level of ferrites positively influence the initial reducing temperature and the reduction rate, respectively. Using these deeper prediction insights into structure-property relations will greatly increase the success of our subsequent synthetic effort.