X-ray photoelectron and Raman spectroscopy of nanostructured ceria in soot oxidation under operando conditions

Ceria nanoshapes exhibiting different amounts of {100}, {110} and {111} facets (cubes, rods, octahedra and polyhedra) were prepared, characterized, tested in the carbon soot oxidation reaction, and studied by operando near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) and operando Raman spectroscopy up to 550 degrees C. The specific soot oxidation reaction rate (mg(C).m(-2).min(-1)) clearly indicated that the {110} and {100} crystallographic planes of ceria were more active than the {111} ones for the oxidation of carbon soot. As deduced from the Ce 3d and O 1s signals recorded using different photon energies, all ceria nanoshapes experienced progressive reduction upon increasing the temperature under Ar, which was accompanied by the formation of oxygen vacancies. Raman studies revealed that, at this stage, isolated graphene layers in carbon soot reacted with ceria lattice oxygen atoms following a Mars-Van Krevelen mechanism. After exposure to O-2 at 550 degrees C, a broad signal in the O 1s region at 531.2-532.5 eV was ascribed to surface active oxygen species, such as peroxide (O-2(2-) ) and superoxide (O-2(-)) species, generated from the interaction of molecular O-2 with oxygen vacancies, which proved to be highly reactive to oxidize the graphitic structures of carbon soot. (C) 2021 Elsevier Ltd. All rights reserved.

Automated summary

Ceria nanoshapes with different crystallographic planes exhibited varying activity in carbon soot oxidation, with {110} and {100} facets being more active than {111}. Raman studies showed interaction between isolated graphene layers in carbon soot and ceria lattice oxygen atoms. Exposure to O2 generated surface active oxygen species which were highly reactive in oxidizing the graphitic structures of carbon soot.