Deciphering the Ce to Ce Evolution: Insight from X-ray Raman Scattering Spectroscopy at Ce N Edges.

Cerium oxide, or ceria, (CeO) 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 Ce and Ce 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 Ce ions nearby due to small polaron hopping resulting in the formation of Ce. 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 Ce and Ce 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 Ce and Ce 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.