**2. Photocatalytic Concrete: Purifying the Air through the Pavement**

A solution for the air pollution by traffic can be found in the treatment of the pollutants as close to the source as possible. Therefore, photocatalytically active materials can be added to the surface of pavement and building materials [4]. Air purification through heterogeneous photocatalysis consists of different steps: under the influence of UV-light, the photoactive TiO2 at the surface of the material is activated. Subsequently, the pollutants are oxidized due to the presence of the photocatalyst and precipitated on the surface of the material. Finally, they can be removed from the surface by the rain or cleaning/washing with water, see Figure 1.

**Figure 1.** Schematic of photocatalytic air purifying pavement.

Heterogeneous photocatalysis with titanium dioxide (TiO2) as catalyst is a rapidly developing field in environmental engineering, as it has a great potential to cope with the increasing pollution. Besides its self-cleaning properties, it is known since almost 100 years that titanium dioxide acts as a photo-catalyst that can decompose pollutants under UV radiation [5]. The impulse for the more widespread use of TiO2 photocatalytic materials was further given in 1972 by Fujishima and Honda [6], who discovered the hydrolysis of water in the presence of light, by means of a TiO2-anode in a photochemical cell. In the 1980s, organic pollution in water was also decomposed by adding TiO2

and under influence of UV-light [7]. The application of TiO2, in the photo-active crystalline phase anatase, as air purifying material originated in Japan in 1996 (e.g., [8]). Since then, a broad spectrum of products appeared on the market for indoor use as well as for outdoor applications. Regarding traffic emissions, it is important that the exhaust gases stay in contact with the active surface during a certain period. The street configuration, the speed of the traffic, the speed and direction of the wind, all influence the final reduction rate of pollutants *in situ*.

In the case of concrete pavement blocks [9,10], the anatase is added to the wearing layer of the pavers which is approximately 8 mm thick. In the case of cast-in-place concrete pavements, the TiO2 is added in the top layer (40 mm thick). The fact that the TiO2 is present over the whole thickness of this layer means that even if some surface wear takes place, for example by traffic or weathering, new TiO2 will be present at the surface to maintain the photocatalytic activity (in contrast to the abrasion of a coating or dispersion layer for instance). The use of TiO2 in combination with cement leads to a transformation of the NO*x* into NO– <sup>3</sup> , which is adsorbed at the surface due to the alkalinity of the concrete [11]. Thus, a synergetic effect is created in the presence of the cement matrix, which helps to effectively trap the reactant gases (NO and NO2) together with the nitrate salt formed. Subsequently, the deposited nitrate can be washed away by rain or washing with water. In addition, these nitrates pose no real threat towards pollution of body waters because the resulting concentrations in the waste water are very low, even below the current limit values for surface and ground water [12].

Special attention is given here to the NO and NO2 content in the air, since they are for almost 50% caused by the exhaust of traffic and are at the base of smog, secondary ozone and acid rain formation as indicated above. The photocatalytic oxidation of NO is usually assumed to be a surface reaction between NO and an oxidizing species formed upon the adsorption of a photon by the photocatalyst, e.g., a hydroxyl radical, both adsorbed at the surface of the photocatalyst [13]. It has been shown by various authors that the final product of the photocatalytic oxidation of NO in the presence of TiO2 is nitric acid (HNO3) while HNO2 and NO2 have been identified as intermediate products in the gas phase over the photocatalyst [2,4,11,13,14]. The resulting reaction pathway of the photocatalytic oxidation of NO has been discussed in several publications e.g., [2,4,13–16] most of which proposed the photocatalytic conversion of NO via HNO2 to yield NO2, which is subsequently oxidized by the attack of a hydroxyl radical to the final product HNO3:

$$\rm{NO\_{ads}} + \rm{OH\_{ads}} \rightarrow \rm{HNO\_{2ads}} \tag{1}$$

$$\rm HNO\_{2ads} + \rm OH\_{ads} \longrightarrow NO\_{2ads} + \rm H\_2O\_{ads} \tag{2}$$

$$\rm NO2ads + OH\_{ads} \longrightarrow HNO\_{3ads} \tag{3}$$

Here, all nitrogen compounds adsorbed at the photocatalyst surface are assumed to be in equilibrium with the gas phase.

Until now, UV-light (in the UV-A spectrum) was necessary to activate the photocatalyst. However, recent research indicates a shift towards the visible light [17], for instance by doping the TiO2 with transition metal ions or non-metallic anionic species, or forming reduced TiO*x*. These techniques introduce impurities and defects in the band gap of TiO2 thereby increasing the amount of visible light that can be absorbed and used in the photocatalytic process. This means that applications in tunnels and indoor environments become more realistic. Especially the application in tunnels is worth looking at due to the high concentration of air pollutants at these sites. One of the projects in Belgium is focusing on this subject [18].
