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In this work, we report on a single-pot synthesis route based on a polymeric precursor method used for successfully producing undoped and iron-doped CeO2 nanoparticles with iron contents up to 10.0mol. %. The formation of high-crystalline nanoparticles with a cubic fluorite structure is determined for all the studied samples. Meanwhile, the magnetic measurements of the undoped ceria nanoparticles revealed the occurrence of ferromagnetism of bound magnetic polarons of a fraction of Ce3+ at room temperature, and only a paramagnetic behavior of Fe3+ ions was determined for Fe-doped ceria nanoparticles. A monotonous reduction of the effective magnetic moment of the Fe3+ ions was determined. It suggests a change from a high-spin to low-spin state of Fe ions as the Fe content is increased. The 3+ valence state of the iron ions has been confirmed by the Fe K-edge X-ray absorption near-edge structure (XANES) and Mössbauer spectroscopy measurements. X-ray photoelectron spectroscopy data analysis evidenced a coexistence of Ce3+ and Ce4+ ions and a decreasing tendency of the relative fraction of Ce3+ ions in the surface region of the particles as the iron content is increased. Although the coexistence of Ce3+ and Ce4+ is confirmed by results obtained via Ce L3-edge XANES measurements, any clear dependence of the relative relation of Ce3+ ions on the iron content is determined, suggesting a homogeneous distribution of Ce3+ and Ce4+-ions in the whole volume of the particles. Ce L3-edge extended X-ray absorption fine structure revealed that the Ce-O bond distance shows a monotonous decrease as the Fe content is increased, which is in good agreement with the shrinking of the unit cell volume with the iron content determined from XRD data analysis, reinforcing the substitutional solution of Ce and Fe ions in the CeO2 matrix. [ABSTRACT FROM AUTHOR]

Abstract: Silica–cobalt composites with cobalt contents (w/w) around 10% were synthesized by employing the sol–gel method, tetraethylorthosilicate as the silica precursor and either a solution of cobalt nitrate or a colloidal dispersion of Co3O4 or Co(OH)2 nanoparticles as the cobalt precursors. Cobalt oxide was obtained by the precipitation of Co2+ ions in alkaline medium and by subsequent thermal treatment. Stable colloidal dispersions of cobalt oxide were prepared by using high power ultrasound and by covering nanoparticles with lauric acid bilayers. All materials were characterized by X-ray diffraction, infrared and Raman spectroscopy, transmission electron microscopy, and adsorption/desorption of nitrogen. Further, the reducibility of cobalt species was studied using the thermal-programmed reduction technique. All the Co3O4/silica composites were mesoporous (3.9–5.4nm) with considerably high porosity (321–567m2 g−1). The TEM mean size of Co3O4 nanoparticles within the calcined composites varied from 10 to 18nm, according to the cobalt precursor employed in the synthesis. The reducibility of cobalt species also depended on the employed cobalt precursor, as shown by the thermal programmed reduction experiments. The composites which were prepared from the Co3O4 colloidal dispersions were reduced at lower temperatures; 80% of the total Co amount was reduced in the range of 300–500°C. On the other hand, only 53% of total Co in the sample prepared from cobalt nitrate solution was reduced from 200 up to 900°C. The difference on the reducibility was attributed to morphological characteristics of the composite samples. [Copyright &y& Elsevier]