Pathways of conversion of nitrogen oxides by nano TiO₂ incorporated in cement-based materials

The ability of photocatalytic cementitious materials to degrade nitrogen oxides (NO_x) has been widely used to evaluate their air-purifying function. However, attention has been predominantly placed on the changes in the NO_x concentration but the fate of the degradation products is seldom investigated. This study presents a preliminary laboratory investigation on the photocatalytic NO_x degradation of nano titanium dioxide (TiO₂) incorporated self-compacting glass mortar (SCGM). Focus was mainly placed on the generation and distribution of the degradation products, nitrite (NO₂ ⁻) and nitrate (NO₃ ⁻), during the NO_x removal process. The results showed that the photocatalytic SCGM induced an apparent decrease of NO together with a slight increase of NO₂ under UVA irradiation. Meanwhile, the amount of the generated NO₂ ⁻ and NO₃ ⁻ on the surface of the tested samples, in the suspended aerosols, and in the flowing air stream increased as the photocatalytic reaction progressed. The accumulation of NO₂ ⁻ and NO₃ ⁻ on the sample surface led to a slight deactivation effect on the photocatalytic NO_x removal ability of the SCGM, which, in turn, slowed down the production of NO₂ ⁻ and NO₃ ⁻. Interestingly, compared with NO₃ ⁻, NO₂ ⁻ could more easily be carried away from the sample surface by the flowing air stream. On-site monitoring of the photocatalytic NO_x removal activity of the paving blocks produced with the incorporation of TiO₂ on the surface layer (Eco-block) confirmed the build-up of NO₂ ⁻ and NO₃ ⁻ with time, which well demonstrated the NO_x removal ability of the photocatalytic blocks in real site applications.