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

The ability of photocatalytic cementitious materials to degrade nitrogen oxides (NOx) has been widely used to evaluate their air-purifying function. However, attention has been predominantly placed on the changes in the NOx concentration but the fate of the degradation products is seldom investigated. This study presents a preliminary laboratory investigation on the photocatalytic NOx 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 NOx 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 NOx 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 NOx 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 NOx removal ability of the photocatalytic blocks in real site applications.