Evaluation of the photocatalytic performance of construction materials for urban air depollution

Abstract

The photocatalytic degradation of trichloroethylene and nitric oxide (model volatile organochloride and inorganic compounds) by two commercial TiO₂-based cementitious materials at the laboratory scale was studied. TiO₂ P25 was selected as the benchmark photocatalyst. The materials were characterized by N₂ adsorption–desorption, TGA, SEM–EDX, and XRD, and the hydrophobic/hydrophilic nature of the cement surface was determined through contact angle measurements. Preliminary photoactivity tests in which a glass plate was covered with photocatalytic material were performed. Moreover, the photocatalytic activities of asphalt plates (approximately 1 cm thick) covered with either of the photoactive cementitious materials were also evaluated. A notable improvement in the photocatalytic activity from the first to the second photocatalytic run was observed because sample conditioning with airflow and UV-A irradiation led to the elimination of species (carbonates) that were adsorbed at active TiO₂ sites, ensuring stability under operating conditions. The cementitious samples completely photooxidized the organochloride compound but presented reduced photoefficiency for nitric oxide oxidation. The results obtained when the materials were deposited on glass and asphalt showed the crucial influences of the sample loading and the nature of the substrate on nitric oxide photooxidation. Using TiO₂ P25 led to the complete conversion of nitric oxide but also to the formation of a large amount of nitrogen dioxide. On the other hand, one of the photoactive asphalts exhibited a nitric oxide conversion rate of 32%, which was lower than that achieved with pure TiO₂-P25, but using this material did not lead to the release of nitrogen dioxide. When tested according to the ISO 22197-1:2007 standard, the selected photocatalytic asphalt removed 29% of the NO and 22% of the NO_x under more demanding operating conditions.