Dual Ce⁴⁺/Fe³⁺ Redox Phenomena into Nanocrystalline Ce1–xFexO2–x/2 Solid Solution

The Ce1–xFexO2–x/2 solid solution was synthesized using a microwave-assisted hydrothermal route. The solubility limit corresponds to a Fe (x) content equal to 0.15. Our previous works showed also that isolated Fe3+ distorted octahedral sites and Fe3+ clusters are randomly distributed into the ceria network. Thermogravimetric analysis (TGA) under Ar/5%H2 revealed a higher reduction rate for Fe-substituted ceria with changes of slope and the appearance of pseudoplateaus around 400 and 550 °C. The first Fe K-edge X-ray absorption near-edge spectroscopy (XANES) spectra recorded at several temperatures up to 700 °C shift gradually to lower energies as the temperature increases, with the formation of metallic iron starting at 550 °C, as confirmed by X-ray diffraction analysis. Moreover, on the basis of the principal component factor analysis, three K-edge structures associated with three oxidation states of iron (Fe3+, Fe2+, and Fe0) have been distinguished during this temperature-programmed reduction. The second component attributed to Fe2+ is the most intense one around 500 °C. Beyond this temperature, the coexistence of the three oxidation states Fe3+/Fe2+/Fe0 has to be mentioned up to 650 °C, the temperature at which metallic iron is mainly stabilized. Ce LIII-edge and Fe K-edge XANES spectra recorded in quick extended X-ray absorption fine structure mode and at various temperatures show the progressive reduction of Ce4+ and Fe3+ starting beyond 250 °C, which is in good agreement with the change of slope observed on the TGA curves. The Ce LIII-edge XANES spectrum of pure CeO2 recorded at 700 °C under reducing atmosphere exhibits much less Ce3+ stabilized into the fluorite network than Fe-substituted ceria. Fe3+ ions randomly distributed into ceria strongly contribute to enhancing the Ce4+ reducibility properties. The Ce4+ reduction seems to appear at lower temperatures and is slower than the Fe3+ reduction, whose speed decreases around 400 °C. At this temperature, the Fe2+ and Fe3+ components dominate and the Ce3+ content remains high. However, a plateau with the stabilization of Ce4+/Ce3+ mixed valences around 500 °C appears where the Fe2+ contribution is also a maximum and the Fe reduction is slower. Because of the large content of either Ce3+ and Fe2+, a charge-transfer equilibrium Ce4+ + Fe2+ ⇔ Ce3+ + Fe3+ may occur at this temperature at which a change of slope on TGA curve also is observed with the appearance of a plateau at 550 °C associated with the Fe0 demixtion phenomenon. Then, after reaching the maximum Fe2+ rate, the Ce4+ reduction rate increases again up to 700 °C, the temperature at which all Fe atoms have been transformed into metallic iron.