Add time:07/27/2019         Source:sciencedirect.com

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.

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