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

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 (^[rad]OH) by the synergistic action of the surface Pd and fluoride. Pd nanoparticles and fluorides facilitate the transfer of valence band holes (h_vb ⁺) and their reaction with water molecules, respectively, synergistically enhancing the production of ^[rad]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.