This article presents an XPS study of Ce 3d emission spectra dominated by atomic multiplet effects in core level spectroscopy of rare earth compounds (Ce oxides). Core level spectroscopy has been used to study the electronic states of Ce 3d _5/2 and Ce 3d _3/2 levels in Ce ⁴⁺ and Ce ³⁺ states. The well‐resolved components of Ce 3d _5/2 and Ce 3d _3/2 spin‐orbit components, due to various final states (4f ⁰ , 4f ¹ , 4f ² configurations), were determined on 3d XPS spectra from commercial powders (CeO ₂ , CePO ₄ ). These results were used to study the 3d spin‐orbit component of mixed cerium‐titanium oxide. This compound was prepared by co‐melting commercial powders of CeO ₂ and TiO ₂ at 1800 K under air using a solar furnace with a flux density of 16 MW.m ⁻² at the focal point of the parabolic concentrator. The mixed oxide Ce ₂ Ti ₂ O ₇ was produced and contained Ce(III) species which may be reactive with water to give back the initial metal oxides and generate hydrogen, a valuable product considered as a promising energy carrier in the future in replacement of oil. The 3d photoemission spectra revealed the presence of mixed components attributed to mainly Ce(III) and Ce(IV) species. Copyright © 2008 John Wiley & Sons, Ltd.

Currently, hydrogen is mainly generated by steam methane reforming, with significant CO₂ emissions, thus exacerbating the greenhouse effect. This environmental concern promotes methane cracking, which represents one of the most promising alternatives for hydrogen production with theoretical zero CO/CO₂ emissions. Methane cracking has been intensively investigated using metallic and carbonaceous catalysts. Recently, research has focused on methane pyrolysis in molten metals/salts to prevent both reactor coking and rapid catalyst deactivation frequently encountered in conventional pyrolysis. Another expected advantage is the heat transfer improvement due to the high heat capacity of molten media. Apart from the reaction itself that produces hydrogen and solid carbon, the energy source used in this endothermic process can also contribute to reducing environmental impacts. While most researchers used nonrenewable sources based on fossil fuel combustion or electrical heating, concentrated solar energy has not been thoroughly investigated, to date, for pyrolysis in molten media. However, it could be a promising innovative pathway to further improve hydrogen production sustainability from methane cracking. After recalling the basics of conventional catalytic methane cracking and the developed solar cracking reactors, this review delves into the most significant results of the state-of-the-art methane pyrolysis in melts (molten metals and salts) to show the advantages and the perspectives of this new path, as well as the carbon products’ characteristics and the main factors governing methane conversion.