Surfactant-assisted synthesis of mesoporous silica/ceria-silica composites with high cerium content under basic conditions

Ordered mesoporous silica/ceria-silica composites were synthesized using cerium(iv) hydroxide and tetraethyl orthosilicate (TEOS) as co-precursors in the presence of hexadecyltrimethylammonium bromide (CTAB) under basic conditions. These composites consisted of Ce-doped mesoporous silica particles (about 500 nm) with highly ordered 2D hexagonal (p6mm) and 3D bicontinuous cubic (Ia3d) structures and irregular ceria-rich silica-ceria particles. Wide angle XRD, diffuse reflectance UV-vis, and XPS analyses showed that 10-24% of cerium was at the Ce³⁺ oxidation level and the remaining predominant fraction of Ce was at the Ce⁴⁺ oxidation level. The cerium loading was varied in these composite materials up to 43 wt% (∼3.1 mmol g^-1). The specific surface areas of the mesoporous silica/ceria-silica composite samples obtained on the basis of nitrogen adsorption isotherms were higher than 350 m² g^-1 and their pore widths were between 3.3 and 3.5 nm. The mesoporous silica/ceria-silica samples were reduced at 850 °C under flowing H₂ in a N₂ environment. The crystal structure of the reduced samples changed to a hexagonally structured phase with the oxidation state of Ce³⁺, while the ordered mesostructure of silica was preserved.