**3. Laboratory Results: Parameter Evaluation**

Different test methods have been developed to determine the efficiency of photocatalytic materials towards air purification. An overview is given in [11]. A distinction can be made by the type of air flow; in the flow-through method according to ISO 22197-1 [19], the air, with a concentration of 1 ppmV of NO, passes once-only over the sample which is illuminated by a UV-lamp with light intensity equal to 10 W/m² in the range between 300 and 400 nm, as illustrated in Figure 2. Afterwards, the NO*<sup>x</sup>* (= sum of NO and NO2) concentration is measured at the outlet and the reduction (in %) is calculated. It is also worth to note here that within Europe actions are underway to harmonize and develop new standards for photocatalyis [20]. In any case, the test procedure used for the current results is still based on the existing ISO standard.

**Figure 2.** (**a**) Schematic and (**b**) photo of measurement set-up based on ISO 22197-1:2007 [19] at Belgian Road Research Center (BRRC).

The pre-treatment of the samples in the laboratory can be important to obtain reproducible results and mainly depends on the type of base material (e.g., concrete or paints). A typical test scheme according to the ISO standard is presented in Figure 3, where the following steps are applied to the sample: 0.5 h at 1 ppmV NO concentration, no light—5 h exposure to an air flow of 3 L/min with 1 ppmV NO and UV-illumination—0.5 h with UV-illumination and no exposure. A small increase with time of the NO*x* concentration is visible due to the deposit of the NO– 3 at the surface.

**Figure 3.** Typical result obtained in the laboratory following the standard ISO test procedure.

The influence of different important test parameters affecting the photocatalytic reaction has been investigated before [2] such as temperature, light intensity, relative humidity, contact time (controlled by surface area, flow velocity, height of air flow, *etc*.). For instance, the effect of relative humidity of the ingoing air is illustrated in Figure 4 for different materials including cementitious (concrete, mortar) and other (paint) substrates. Clearly, for cementitious materials the reduction of the NO*x* concentration in the outlet air decreases with increasing relative humidity (RH, %), an observation which was also found by other authors [21]. This probably has to do with the fact that the water in the atmosphere plays a role in the adhesion of the pollutants at the surface and with the competition effect that can arise between water molecules and NO*x* in the ambient air with increasing RH. For paints (acidic environment) though, it has been noticed that there is an optimum in RH where a maximal efficiency is obtained. Anyway, relative humidity proves to be an important limiting factor for photocatalytic applications in humid areas like Belgium. Temperature on the other hand, was found to have no significant influence on the NO*x* reduction within the ambient range (5–25 °C).

In general, it can be stated that the efficiency towards the reduction of NO*x* (in %) increases with a longer contact time (larger surface area, lower air velocity, smaller height of air flow, higher turbulence at the surface), a lower relative humidity (for cementitious materials) and a higher intensity of incident light. These are the conditions at which the risk of ozone formation in summer is the largest: higher sun light intensity, no wind and no rain. At these days, the photocatalytic reaction will be more pronounced.
