Enhanced photocatalytic conversion and selectivity of nitrate reduction to nitrogen over AgCl/TiO₂ nanotubes

TiO2 nanotubes (TNTs) and AgCl/TiO2 nanotubes (AgCl/TNTs) were synthesized by a hydrothermal method and used in the reduction of nitrate with formic acid as a hole scavenger. As-synthesized photocatalysts were well characterized by field emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), X-ray photoelectron spectra (XPS), N2 adsorption-desorption and UV-vis diffuse reflectance spectroscopy (UV-vis DRS). 5 wt% AgCl/TNTs showed excellent performance in the reduction of nitrate (94.5% conversion of nitrate and 92.9% selectivity to N2, respectively) under UV light (365 nm) irradiation for 30 min. After four reduction cycles of nitrate the 5 wt% AgCl/TNTs also exhibited steady photoactivity. The enhanced photoreductive ability and stability attributed to the nanotube structures provided a higher specific surface area and more active sites. For the combination of AgCl and TNTs, the SPR effect of Ag0 formed by UV light irradiation improved the separation of electron-hole pairs which was proved by the electrochemistry impedance spectra. The effects of AgCl content, hole scavengers, and concentration of formic acid were systematically investigated. Based on the above results a mechanism was proposed. This work provides a novel method for high conversion and selectivity of nitrate reduction to nitrogen by photocatalytic technology.