Photo-oxidation of nitrite anions in aqueous solution for the benchmarking of nano-TiO2 photocatalytic coatings

Photocatalytic coatings have potential as an air purifying technology for removal of pollutants such as NO_x gases, which are converted into and immobilized as nitrate via photo-oxidation. This work is an investigation of the photocatalytic conversion of nitrite into nitrate in the aqueous phase. The study of this reaction is important to understand the mechanism through which the immobilization of the NO_x species as nitrates takes place during NO_x conversion, especially in humid environments. We investigate the photocatalytic performance of TiO₂ nanoparticles (Aeroxide® P25) and the corresponding photocatalytic coatings prepared using acrylic binder (TiO₂ NP-acrylic) towards the photo-conversion of nitrite. The combination of a simple colorimetric method (Griess test) for nitrite quantification and UV–vis spectroscopy for nitrite and nitrate analysis is shown to be a useful method for monitoring the nitrite conversion. Photocatalytic tests with suspended TiO₂ nanoparticles show that the nitrite conversion benefits from lower pH, higher temperature and higher concentration of the photocatalyst. The advantage of lower pH is attributed to electrostatic attraction between TiO₂ with adsorbed H₃O⁺ and NO₂⁻, which facilitates transport of nitrite to the active surface. When the photocatalyst is immobilized in a coating the factor of primary significance is the pigment volume concentration relative to critical pigment volume concentration (λ), as the nitrite conversion increases linearly with λ in the range 0.6–1.2. This is attributed to the creation of a more porous structure that facilitates the access of nitrite and light to the active TiO₂ surface sites.