# Evaluation of NOx Reduction Effect and Impact on Asphalt
## 3 Experimental Results and Analysis
### Evaluation of NOx Removal Performance of the TiO\({}_{2}\) Surface Treated Asphalt Specimens  
To evaluate the efficiency of the NOx removal performance of TiO\({}_{2}\) surface treated asphalt specimens, an experiment was conducted using a mixed tank photoreactor. The specimen used for surface treatment was an asphalt specimen manufactured in the form of a slab with a width of 300 mm, length of 300 mm, and thickness of 50 mm. A mixed tank photoreactor test was performed at a flow rate of 3 L/min, UV intensity of 10 W/m\({}^{2}\), temperature of 25 \(\pm\) 5 \({}^{\circ}\)C, and humidity of 45 \(\pm\) 5%. Similar to the experimental results in Section 3.1, the NOx and NO concentrations gradually decreased until 2 h and 30 min after the UV light was turned on, and then, a parallel state was maintained. Figure 9 show the change in NOx concentration in the mixed tank photoreactor experiment on the surface treated asphalt specimen, and Table 8 show the experimental results according to the type of TiO\({}_{2}\).  
Figure 8: Variation of NOx concentrations during the mixed tank photo reactor experiment.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Test ID**} & **Total** & **Total** & **Total** \\  & **NO (umol)** & **Removed NO (umol)** & **Removed NO (\%)** \\ \hline Type-1 & 38.74 & 16.92 & 43.66 \\ Type-2 & 37.62 & 11.82 & 31.41 \\ Type-3 & 38.27 & 15.94 & 41.65 \\ \hline \hline \end{tabular}
\end{table}
Table 7: NO reduction and reduction efficiency of mixed tank photo reactors.  
As a result of the experiment, each TiO\({}_{2}\) surface treated specimen exhibited a photocatalytic reaction. The NO removal efficiencies of the surface treated asphalt specimens were 2.54% for Type-1, 5.94% for Type-2, and 7.83% for Type-3. Surface treated specimens exhibited a tendency to significantly decrease NOx removal performance compared to the TiO\({}_{2}\) powder test result in Section 3.1, which can be explained by two reasons.  
The first is the reduction in the specific surface area of TiO\({}_{2}\). In the case of TiO\({}_{2}\) in powder form in the previous experiment, all surfaces of TiO\({}_{2}\) can undergo photocatalytic reactions. However, in the case of TiO\({}_{2}\) mixed with a surface treatment agent, the viscosity of the asphalt binder in the specimen is restored by the rejuvenator, and the TiO\({}_{2}\) fixed to the asphalt binder does not photocatalytically react. As the specific surface area that can be reacted to becomes smaller than that of the powder, the ability to reduce NOx decreases [41; 42]. The second is the particle change due to the aggregation of TiO\({}_{2}\), as shown in Figure 10. Nano-sized TiO\({}_{2}\) has the property of aggregating by interparticle attraction due to its negative charge action. Similar to the first reason, the specific surface area of TiO\({}_{2}\) capable of a photocatalytic reaction was reduced. TiO\({}_{2}\) in the agglomerated state forms flocs, and the particle size and sedimentation rate are relatively increased [43]. These TiO\({}_{2}\) are precipitated in the asphalt binder, whose viscosity is restored and softened, preventing the photocatalytic reaction. Therefore, it is estimated that the NO reduction rate of the surface treated asphalt specimen is lower than that of TiO\({}_{2}\) in powder form.