Deciphering the Ce3+ to Ce4+ Evolution: Insight from X-ray Raman Scattering Spectroscopy at Ce N4,5 Edges.

Abstract

Cerium oxide, or ceria, (CeO₂) is one of the most studied materials for its wide range of applications in heterogeneous catalysis and energy conversion technologies. The key feature of ceria is the remarkable oxygen storage capacity linked to the switch between Ce⁴⁺ and Ce³⁺ states, in turn creating oxygen vacancies. Changes in the electronic structure occur with oxygen removal from the lattice. Accordingly, the two valence electrons can be accommodated by the reduction of support cations where the electrons can be localized in empty f states of Ce⁴⁺ ions nearby due to small polaron hopping resulting in the formation of Ce³⁺. Quantifying the different oxidation states in situ is crucial to understand and model the reaction mechanism. Beside the different techniques that have been used to quantify Ce³⁺ and Ce⁴⁺ states, we discuss the use of X-ray Raman Scattering (XRS) spectroscopy as an alternative method. In particular, we show that XRS can observe the oxidation state changes of cerium directly in the bulk of the materials under realistic environmental conditions. The Hilbert++ code is used to simulate the XRS spectra and quantify accurately the Ce³⁺ and Ce⁴⁺ content. These results are compared to those obtained from in situ X-ray Diffraction (XRD) collected in parallel and the differences arising from the two different probes are discussed.