Hierarchically structured ceria-silica : synthesis and thermal properties

A one-pot method for the synthesis of hierarchically structured ceria-silica composite materials is reported along with the results of their characterization by a variety of physical techniques. Crystallization of ceria in an aqueous mixture of anionic, cationic, and neutral surfactants, namely Pluronic P123 ($(EO_{20}PO_{70}EO_{20})$), CTAB (hexadecyltrimethylammonium bromide), and SDS (sodium dodecyl sulfate) leads to the formation of a suspension of capped ceria nanoparticles. Subsequent addition of tetraethoxysilane followed by aging at 40-80 $^{o}\textrm{C}$ leads to the condensation of silica. After thermal removal of the organic species, the formation of high surface area composites directed by the interaction of the capped nanoparticles and the remaining surfactants is possible. The thermal stability and redox activity of the composite materials have been studied by in situ powder X-ray diffraction, TGA/DSC, transmission electron microscopy, Ce $L_{III}$-edge XANES, and temperature-programmed reduction under $H_{2}/N_{2} $. Encapsulation of the ceria nanoparticles in the templated silica matrix leads to high thermal stability with the nanocrystalline nature of the ceria retained upon heating to 900 $^{o}\textrm{C}$ in air with no annealing evident by in situ thermodiffractometry. Temperature-programmed reduction shows large hydrogen uptake at around 600 $^{o}\textrm{C}$, corresponding to complete reduction of all Ce(IV) to Ce(III) in the case of a cerium-rich sample (Ce:Si = 5:12). This reduction leads to amorphization of the ceria followed by the collapse of the hierarchical structure with formation of $Ce_{2}Si_{2}O_{7}$ crystallites embedded in amorphous silica. For a sample of lower cerium content, crystalline $Ce_{6}(Si_{4}O_{13})(SiO_{4})_{2}$ is formed under reductive conditions