First-Principles Insights into the Oxidation States and Electronic Structures of Ceria-Based Binary, Ternary, and Quaternary Oxides

Ceria and ceria-based materials have
versatile technological and industrial applications physically ascribed
to the flexible fluctuation of the Ce oxidation state between Ce4+ and Ce3+. Considerable multidisciplinary
research has been carried out to obtain the Ce oxidation state, which
is crucial for their application; however, a rigorous and physically
correct determination of the oxidation state is still lacking. Here,
we conduct first-principles DFT + U calculations
to unambiguously determine the physical oxidation state of Ce in ceria-based
materials, such as homogenous CenO2n–2 (n = 7, 9, 10,
11, and 12), ceria doped by multivalent Ti and V, Ce–Ti­(V)–O
ternary compounds, and Ce–Ti–V–O quaternary compounds.
The results show that the Ce oxidation state depends on the local
structure and chemical surrounding: oxygen vacancy facilitates the
transition from Ce4+ to Ce3+, which is consistent
with the localization of Ce 4f electrons; Ti and V with the 3d energy
levels higher than 4f energy levels of Ce generally tend to reduce
Ce4+ to Ce3+, particularly under an oxygen-deficient
condition. The atom-resolved determination of the Ce oxidation states
in complicated compounds offers great promise for understanding the
physical and chemical behavior of ceria-based materials and for rational
design of novel ceria-based materials for application potentials.