The use of new photocatalysts active under visible light in cement-based building materials represents one interesting alternative to improve the air quality in the urban areas. This work undertakes the feasibility of using BiOX (X = Cl and I) as an addition on mortars for visible-light-driven NOx removal. The interaction between BiOX photocatalysts and cement matrix, and the influence of their addition on the inherent properties of the cement-based materials was studied. The NO removal by the samples ranking as follows BiOCl-cem > BiOI-cem > TiO 2 -cem. The higher efficiency under visible light of BiOCl-cem might be ascribed to the presence of oxygen vacancies together with a strong oxidation potential. BiOI-cem suffers a phase transformation of BiOI in alkaline media to an I-deficient bismuth oxide compound with poor visible light absorbance capability. However, BiOI-cem showed considerably higher nitrate selectivity that resulted in the highest NOx global removal efficiency. These results can make its use more environmentally sustainable than TiO 2 and BiOCl cement composites.Catalysts 2020, 10, 226 2 of 21 toxicity. Nevertheless, the limited visible light absorption and high electron-hole recombination rate are considered the drawbacks of its wide application [9,10].Recently, there has been considerable research interest in layered composite materials, such as silicates [11,12], graphene [13][14][15][16], perovskites [17], graphitic carbon nitrides [18,19] and layered double hydroxides [20]. These layered materials possess several extraordinary advantages, such as high surface area, more surface-active sites, superior electron mobility, and good electron transfer, endowing them with promising potential for photocatalytic applications. Of these layered materials, bismuth oxyhalides (BiOX, X = Cl, Br, and I) belong to a new class of promising layered materials because of their unique layered-structure-mediated fascinating physicochemical properties and suitable band-structure, as well as their high chemical and optical stability, nontoxicity, low cost, and corrosion resistance [21,22]. Among all of BiOX materials, BiOI with band gap energy of 1.7 eV exhibits the highest visible light-driven photocatalytic activity attributed to its smaller band gap [21,23,24]. Many studies have investigated the removal of nitric oxide gas-phase through pure BiOX photocatalysts, but most are focused on the performance based on the decrease of NO concentration [10,[25][26][27][28]. Only a few reports focus on the final or by-products of NO removal process [9,23,29]. The mechanisms of NO removal remain unclear in the literature. Generally, NO can be oxidized by either photogenerated hole or other active species [30][31][32] to different kinds of products, such as NO 2 , HNO 2 , and HNO 3 . However, some of these products, such as NO 2 , is considerably more toxic than NO [31].Moreover, very scarce references have been found in relation to the addition of BiOX oxides into construction materials. For example, Wang et al.[33] prepare...

Cement-based materials modified with 3D BiOX (X = I, Cl) microspheres at different percentages (1, 5 and 10% by weight of the cement binder) were prepared to investigate the durability of the photocatalytic NOx removal under outdoor conditions. Weathering—corresponding to a period of 13 months outdoors—was studied in terms of NO removal efficiency under visible and UVA light irradiation for BiOI and BiOCl mortars, respectively. Following this period, the samples were protected from the environment for four years, and NOx removal and selectivity to nitrates were assessed. BiOI and BiOCl mortar samples were initially photocatalytically active; NOx removal performance increased as BiOX content increased. There was good photocatalyst dispersion, and compressive strength was not significantly impacted. The BiOI mortars had nearly completely lost their activity after 5 years from casting, whereas mortars containing 10% BiOCl had maintained about 7% of initial performance. The results suggest that mortar deactivation is due to surface dirt and nitrates accumulation from NOx oxidation on the surface rather than carbonation. An internal self-deactivation mechanism that affects BiOI in mortar matrix has also been postulated.