WOx/TiO2 Catalysts via Titania Nanotubes for the Oxidation of Dibenzothiophene

WOx/TiO2 catalysts were synthesized by impregnating aqueous (NH4)2WO4 on hydrous titania nanotubes. The materials were annealed in air at 500 °C and characterized by X-ray diffraction (XRD), Raman spectroscopy, high-resolution transmission electron microscopy (HRTEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), and X-ray photoelectron spectroscopy (XPS); their catalytic activity was evaluated in the oxidation reaction of dibenzothiophene (DBT). After annealing at 500 °C, the structure of the support transformed from orthorhombic, with nanotubular morphology, to tetragonal, yielding anatase nanoparticles decorated by tungsten nanoparticles on its surface. During this transformation, the nanotubes released residual Na+ ions from the interlayer space, which reacted with tungstate species to change the W coordination from octahedral to tetrahedral, yielding ≤1 nm Nax(WO4) nanoparticles on the surface of anatase TiO2. These nanoparticles were highly active for the DBT oxidation, showing a linear dependence on the W surface density at concentrations below 6.9 W/nm2. In TiO2 oversaturated with tungsten nanoparticles, the intrinsic kinetic velocity (rDBT) of the DBT oxidation reached its maximum value; 6.9 W /nm2 is then the optimum surface density concentration to attain a high catalytic activity in the DBT oxidation.