Use and mis-use of x-ray photoemission spectroscopy Ce3d spectra of Ce2O3 and CeO2

X-ray photoemission spectroscopy (XPS) work on Ce₂O₃ and CeO₂ oxides has been an active topic of research over the past four decades or so. Such research aimed to find the reasons for the unusual complexity of Ce3d spectra of the two oxides, and it studied catalytic properties of materials that contained them. I discuss how theoretical and experimental studies exploited the diagnostic potential of XPS to reach our current knowledge of the electronic properties of the two oxides. A part of these studies provided peak-fitting guidelines to resolve Ce3d spectra produced by the co-existence of both oxides into the individual spectral components arising from Ce³⁺ and Ce⁴⁺ ions. Basing myself on the analysis of several peak-fittings of Ce3d spectra carried out in studies of the catalytic applications of CeO₂-based materials, I show that more often than not they largely ignore the findings of theoretical, experimental, and methodological XPS work. I discuss typical problems that flaw Ce3d peak-fittings, and how they affect their accuracy. I argue that, although several XPS studies do list primary literature of Ce3d spectra in their bibliography, they often do so for decorative purposes, rather than practical purposes.