Pure and Fe-doped CeO2 nanoparticles obtained by microwave assisted combustion synthesis: Physico-chemical properties ruling their catalytic activity towards CO oxidation and soot combustion

A sample of pure CeO2 and two samples of Fe-doped CeO2 containing either 3 or 6 at.% Fe were obtained by microwave assisted combustion synthesis. The powders were extensively characterized by several techniques and tested as catalysts for both CO oxidation and soot combustion. As-synthesized CeO2 nanoparticles have a mostly squared shape and size well below 100 nm; they are characterized by the presence of surface Ce³⁺ species likely due to the occurrence of very defective ceria crystalline phases, as revealed by HR-TEM. Oxidation at 400 °C leads to the formation of a hydroxyls-rich surface, with several types of OH groups related to both Ce⁴⁺ and Ce³⁺ species; reduction in H2 at mild temperature (200 °C) leads both to reduction of surface Ce⁴⁺ to Ce³⁺ and formation of new OH groups. With respect to CeO2 nanoparticles, Fe-doped ones have, as a whole, a larger size and less abundant surface OH species. A core-shell structure is inferred where Fe is mostly present in the shell, both in a secondary phase (CeFeO3) and as a dopant, finally lowering the band gap of the material. The presence of Fe improved samples reducibility, as shown by the lowering of the onset of temperature programmed reduction. Catalytic tests of CO oxidation showed that surface Fe species significantly improve the catalytic performance of the samples, by lowering the onset of CO conversion to CO2 especially at low Fe content (i.e. 3 at.%), whereas at 6 at.% Fe loading, the preferential formation of the secondary phase CeFeO3 occurs, finally lowering the CO conversion with respect to the sample containing 3 at.% Fe. Conversely, the soot combustion activity was higher for pure CeO2 nanoparticles, likely due to their smaller size, which increases the amount of solid–solid contact points between soot and the catalyst. However, a positive effect of the presence of surface Fe species on the catalytic activity towards soot combustion was observed, as well.