DeNOxing the air in urban spaces by building and construction photocatalytic coverings

A variety of air depolluting TiO₂-based marketed products were applied on bituminous mixtures, sidewalk pavements and facades, giving NO_x oxidation ratios under ISO 22197–1:2007 in the 35–9%, 56–2% and 28–2% ranges, respectively. Correspondingly, DeNO_x toxicity indexes varied from −0.8 to 5.6, 0 to 14 and −4 to 1 μmol.

The three most efficient photocatalytic products were selected: two TiO₂-water dispersions, for road and sidewalk, and a TiO₂-covering, for facade. NO_x purifying ability of these materials were evaluated when key physical parameters were modified. The observed NO_x conversion is positively correlated with UV-A irradiance up to 10 W/m², reaching a plateau, and negatively correlated with relative humidity, with a more pronounced decrease above 35%. Inversely, no dependence with inlet NO_x concentration is observed in the range of 0.14–1 ppm_v.

Further, two first-order kinetic approximations were followed to calculate NO surface deposition rates, giving 2 to 8 10⁻³ m/s on the selected photocatalytic urban surfaces. Subsequently, the potential NO_x sink effect induced in a photocatalytic urban canyon and a NO_x-purifier was modelled taking NO_x surface deposition rates from 10⁻³ to 10⁻¹ m/s. Purifying devices could be utilised as a preferred option to help alleviate local atmospheric NO_x in high-polluted areas.