Phase composition, morphology, properties and improved catalytic activity of hydrothermally-derived manganese-doped...

Manganese-doped ceria nanoparticles were prepared by hydrothermal synthesis and the prepared samples were thermally treated at 500 °C for 2 h. The samples were investigated using x-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM), energy-dispersive x-ray spectroscopy (EDS), N₂adsorption and x-ray photoelectron spectroscopy (XPS). XRD revealed that nanocrystalline ceria is the main phase in all samples, while a romanechite-like phase (Na₂Mn₅O₁₀) appears in the sample doped with 30% of Mn. TEM coupled with EDS exposed the presence of the same phase in the sample doped with 20% Mn. While ceria particles have spherical morphology and particle size ranging from 4.3 to 9.2 nm, the rare crystals of the romanechite-like phase adopt a tubular morphology with a length of at least 1μm. However, the decrease in the ceria lattice constant and the EDS spectra of the ceria nanoparticles clearly indicate that a substantial amount of manganese entered the ceria crystal lattice. Manganese doping has a beneficial impact on the specific surface area of ceria. XPS measurements reveal a decrease in the Ce³⁺/Ce³⁺ + Ce⁴⁺content in the doped samples which is replaced by Mn³⁺. Moreover, a drastic increase in adsorbed oxygen is observed in the doped samples which is the consequence of the increase in Mn³⁺species that promotes oxygen migrations to the surface of the sample. Compared to the pure sample, the doped samples showed significantly higher catalytic activity for the process of toluene oxidation.