Titanium dioxide surface modified with both palladium and fluoride as an efficient photocatalyst for the degradation of urea

Abstract

TiO2 surface modified with both Pd nanoparticles and fluorides (F-TiO2/Pd) was prepared and applied as a photocatalyst in the degradation of urea. Various surface analysis techniques, including X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, and energy-dispersive X-ray spectroscopy, were used to verify the coexistence of Pd nanoparticles and fluorides on the surface of TiO2 in F-TiO2/Pd. F-TiO2/Pd showed a higher photocatalytic activity than those of bare TiO2 and single-component-modified TiO2 photocatalysts such as fluorinated TiO2 (F-TiO2) and Pd-loaded TiO2 (Pd/TiO2). The higher urea degradation efficiency of F-TiO2/Pd is ascribed to the enhanced production of hydroxyl radicals (●OH) by the synergistic action of the surface Pd and fluoride. Pd nanoparticles and fluorides facilitate the transfer of valence band holes (hvb+) and their reaction with water molecules, respectively, synergistically enhancing the production of ●OH. The photocatalytic activity of F-TiO2/Pd for the degradation of urea increased upon increasing the fraction of the fluorinated TiO2 surface, which is higher at higher fluoride concentrations and lower pH. Although Pt/TiO2 showed higher photocatalytic activity for the degradation of urea than those of Pd/TiO2 and Au/TiO2, the strong positive effect of fluoride complexation was only exhibited by Pd/TiO2 (a slight positive effect and a negative effect were observed for Au/TiO2 and Pt/TiO2, respectively). As a result, the degradation of urea proceeded more rapidly in a UV-irradiated suspension of F-TiO2/Pd than when any of other photocatalysts (i.e., bare TiO2, Pd/TiO2, F-TiO2, Au/TiO2, F-TiO2/Au, Pt/TiO2, and F-TiO2/Pt) were used under the same conditions. The first-order degradation rate constants (k) of urea depending on the type of TiO2 were as follows: 0.097 h？1 for bare TiO2, 0.158 h？1 for Pd/TiO2, 0.151 h？1 for F-TiO2, 0.351 h？1 for F-TiO2/Pd, 0.173 h？1 for Au/TiO2, 0.223 h？1 for F-TiO2/Au, 0.240 h？1 for Pt/TiO2, and 0.165 h？1 for F-TiO2/Pt, respectively. In addition, F-TiO2/Pd proved to be stable in repeated urea degradation cycles.