Visible-Light Photoactivity of Nitrogen-Doped TiO_2: Photo-oxidation of HCO_2H to CO_2 and H_2O

Nitrogen-doped TiO_2 was synthesized by high-temperature exposure of TiO_2 to ammonia. The catalytic efficiency was tested by monitoring the photocatalytic degradation of formate (HCO_2^-) to CO_2 and H_2O under visible-light irradiation. The N-doped TiO_2 powders were found to be active for the degradation of formic acid under visible light. However, the catalytic efficiency of the N-doped TiO_2 under UV light alone is less than that of the pure TiO_2 starting material. FTIR evidence indicates that the visible-light-active N-doped TiO_2 has defect sites in the form of Ti−N triple bonds and that the increase of these sites leads to a loss of crystallinity that accounts for the reduced photocatalytic activity under UV irradiation. An optimal synthesis temperature of 550 °C was determined as a balance point between catalyst crystallinity and the presence of defect sites that absorb visible-light photons.