## 1 Introduction  
Vehicles have brought convenience to people but also more and more serious air pollution. A lot of studies on the purification technology of automobile exhaust have been conducted in many countries around the world [1; 2; 3; 4]. TiO\({}_{2}\) is generally considered as one of the most effective photoinduced catalysts and is frequently used to oxidize organic and inorganic compounds in the air and water due to its strong oxidative ability and long-term photostability, as well as being a non-expensive and non-toxic material. Therefore, titania-mediated photocatalysis is a very promising approach to the increasing crucial issues associated with environmental pollution [5; 6]. Indeed, among the possible solutions, TiO\({}_{2}\)-coated materials can significantly decompose a variety of organic (e.g., volatile organic compounds) and inorganic (e.g., NO\({}_{\text{x}}\) and SO\({}_{2}\)) pollutants due to its unique photocatalytic property, which contributes to its ability toward air-purifying and self-cleaning. Nano-TiO\({}_{2}\) has been proved to be a good photocatalytic material, which can degrade harmful substances in waste gas [7; 8]. A series of new photocatalytic materials related to TiO\({}_{2}\) have been developed and applied in different situations [6; 9; 10; 11]. The emissions of harmful air pollutants associated with highway operations often surpass the concentrations from industrial sources, rendering traffic emissions the primary source of urban air pollution [12; 13]. Therefore, TiO\({}_{2}\) is widely used in road engineering due to its excellent photocatalytic properties in many countries such as China, Italy, and so on. The air purification effect on both sides of the road area has been improved to a greater extent [14; 15; 16]. Currently, there are two ways to apply nano-TiO\({}_{2}\) on the road, which are a blending method (i.e., nano-TiO\({}_{2}\) was directly added to the aggregate and mixed with the asphalt to form a nano-TiO\({}_{2}\) asphalt mixture) and a spraying method (i.e., coatings containing nano-TiO\({}_{2}\) were sprayed on the surface of the pavement or other road-affiliated facilities) [17]. [PERSON] et al. [18] applied nano-TiO\({}_{2}\) to an asphalt pavement using both methods to analyze the photocatalytic degradation of automobile exhaust gas. Test results showed that nano-TiO\({}_{2}\) under the two kinds of applications had a good photocatalytic degradation effect. However, for the blending method, the amount of nano-TiO\({}_{2}\) used was large and the cost was high. [PERSON], [PERSON], and co-workers [19; 20; 21] studied a sustainable photocatalytic asphalt pavement by using TiO\({}_{2}\) as a photocatalytic material to reduce nitrogen oxides and sulfur dioxide. The development of this new sustainable road has the potential to reduce the high pollution level caused by traffic vehicles. The environmental effectiveness of a TiO\({}_{2}\) coating in photodegrading mixed NO\({}_{2}\) and NO gases from the atmosphere was evaluated. [PERSON] et al. [22] studied the performance of asphalt pavement with a TiO\({}_{2}\) photocatalyst. They developed and tested a new photocatalytic method to quantify the short-term durability of TiO\({}_{2}\) spraying on two kinds of pavements: concrete and asphalt. [PERSON] et al. [23] applied three commercially-available photocatalytic coatings to roadside concrete to elucidate how environmental parameters, exposure, and real roadside conditions impact the degradation of NO\({}_{\text{x}}\). The test results for a 20-month period indicated that the efficacy diminished over time. This research shows that the application of photocatalysis technology has a good effect in dealing with automobile exhaust and is rather promising. Under the effect of light, the photocatalyst can react with the automobile exhaust to form water, carbon dioxide, and salt, which prevents secondary pollution to the environment. In spite of these promising findings, the application of this technology to pavements is still in its infancy.