Low-temperature synthesis of tunable mesoporous crystalline transition metal oxides and applications as Au catalyst supports

Mesoporous transition metal oxides are of great potential as catalyst supports, shape-selective catalysts, photocatalysts, and sensor materials. Previously stable crystalline mesoporous oxides were mostly obtained by thermally induced crystallization or by segregating the nanocrystals with an amorphous phase. Here we report a novel direct approach to crystalline mesoporous frameworks via the spontaneous growth and assembly of transition metal oxide nanocrystals (i.e., rutile TiO₂, fluorite CeO ₂, cassiterite SnO₂, and anatase Sn_xTi _1-xO₂) by oxidative hydrolysis and condensation in the presence of anionic surfactants. The influences of synthesis time, surfactants with different chain lengths, concentrations of the oxidant (i.e., hydrogen peroxide), and synthesis temperatures on the composition and morphologies of the resulting materials were investigated by X-ray diffraction, N ₂-sorption, transmission electron microscopy, selected area electron diffraction, scanning electron microscopy, and X-ray photoelectron spectroscopy. A mechanism for the templated synthesis of crystalline mesoporous metal oxides was tentatively proposed. To demonstrate the catalytic applications of these materials, gold nanoparticles were loaded on mesoporous rutile TiO₂ and fluorite CeO₂ supports, and their catalytic performance in CO oxidation and water-gas shift was surveyed. Au nanoparticles supported on the mesoporous crystalline metal oxides exhibit higher reactivity and excellent on-stream stability toward CO oxidation and water-gas shift reaction compared with Au nanoparticles supported on commercial TiO₂ and CeO ₂.