Evaluation of the influence of environmental conditions on the efficiency of photocatalytic coatings in the degradation of nitrogen oxides (NOx)

The incorporation of titanium dioxide (TiO2) into cement mortars generates materials with photocatalytic properties, that is, a cement matrix able to capture and degrade atmospheric pollutants like nitrogen oxides (NOx). The chemical phenomenon of NOx degradation requires the activation of TiO2 through ultraviolet radiation (UV-A), as well as water and oxygen molecules necessary for the generation of hydroxyl radicals (OH
• ), responsible for the degradation of NOx. While photocatalytic materials are in service they are subject to different environmental conditions with regard to the incidence of UV-A radiation, relative humidity of the air and movement of the pollutant masses by the wind. This study aims to evaluate the influence of these different environmental conditions on the efficiency of photocatalytic mortar in the degradation of NOx. Using a specifically-developed test apparatus, the efficiency of a photocatalytic mortar was tested under 27 different environmental conditions, obtained by varying the relative air humidity (30, 50 and 70%), UV-A radiation (10, 25 and 40 W/m2) and pollutant mass flow rate (1, 3 and 5 l/min). All tests were carried out with an initial concentration of nitric oxide (NO) of 20 ppmv. It was observed that the higher the levels of UV-A radiation the better the performance of the mortar in the degradation of NOx. On the other hand, higher percentages of relative humidity and flow rate caused a decrease in photocatalytic activity. The experimental results verified that the environmental conditions have a considerable influence on the efficiency of photocatalytic mortar in the degradation of NOx.

► Environmental changes are crucial for the performance of the paving blocks in the degradation of NOx.
► Higher the levels of UV-A radiation, the better the performance of the mortar in the degradation of the pollutant.
► Higher percentages of relative humidity and flow rate cause a decrease in photocatalytic activity.