Photocatalytic characterization of BiOCl and BiOBr prepared by polyol-assisted coprecipitation for the elimination of NOx from air

The BiOCl and BiOBr oxyhalides were prepared by simple coprecipitation in a rich polyol medium at 100 °C where glycerol was used as solvent and mannitol as additive to increase the concentration of –OH groups in the reaction medium. The characterization of the samples was performed by X-ray powder diffraction, scanning electron microscopy, diffusion reflectance spectroscopy, and the specific surface area was studied by \(\hbox {N}_2\) ${\text{N}}_2$ adsorption–desorption isotherms following BET protocol. The photocatalytic activity of BiOCl and BiOBr was determined in the photooxidation of nitric oxide (NO) in air, obtaining values of nitric oxide conversion of 77% and 90%, respectively. The photocatalytic activity is associated to the higher concentration of –OH groups in the medium of reaction that induce a preferential orientation of the crystalline plane (110) in BiOCl and in the formation of the heterojunction \(\hbox {BiOBr/B}_{24}\hbox {O}_{31}\hbox {Br}_{10}\) ${\text{BiOBr/B}}_{24}{\text{O}}_{31}{\text{Br}}_{10}$ in BiOBr. The highly selective reaction of NO photooxidation to innocuous \(\hbox {NO}_3^-\) ${\text{NO}}_3^{-}$ ions was confirmed with values of 96% BiOCl and 93% BiOBr. Electron paramagnetic resonance measurements determined that the highly oxygen reactive hydroxyl ( \(\cdot \hbox {OH}\) $·\text{OH}$ ) and superoxide ( \(\cdot \hbox {O}_2^-\) $·{\text{O}}_2^{-}$ ) radicals rule the NO oxidation photocatalyzed by BiOCl and BiOBr oxyhalides.