**Preparation Technique and Properties of Nano-TiO<sup>2</sup> Photocatalytic Coatings for Asphalt Pavement**

**Hui Wang 1,2,\*, Ke Jin <sup>1</sup> , Xinyu Dong <sup>1</sup> , Shihao Zhan <sup>1</sup> and Chenghu Liu <sup>1</sup>**


Received: 17 September 2018; Accepted: 22 October 2018; Published: 25 October 2018

**Featured Application: Due to having the same color with the asphalt, the prepared nano-TiO<sup>2</sup> photocatalytic coating could be applied directly onto the pavement surfaces, located in densely-populated areas, environmentally-sensitive areas, service areas and parking lots etc., to further play a better role in the degradation of exhaust gas.**

**Abstract:** According to the characteristics of asphalt pavement, a kind of nano-TiO<sup>2</sup> photocatalytic coating was prepared by using the emulsified asphalt as the carrier. All of its properties met the technical requirements. An exhaust gas degradation test device and its test steps were developed. The evaluation indexes, cumulative degradation rate, and degradation efficiency, were put forward. From the two aspects of the nano-TiO<sup>2</sup> content in photocatalytic coatings and the spraying amount of photocatalytic coatings in the surface of slabs (300 mm × 300 mm), the exhaust gas degradation effects, the performances of skid resistance, and the water permeability of asphalt mixture were analyzed. The test results showed that the cumulative degradation rate of exhaust gas was better when nano-TiO<sup>2</sup> content was increased in the range of 0–8% and the spraying amount was changed in the range of 0–333.3 g/m<sup>2</sup> . In practical engineering applications, the anti-skid performance of asphalt pavement can be satisfied when the spraying amount of photocatalytic coating was limited to under 550 g/m<sup>2</sup> . The spraying amount of nano-TiO<sup>2</sup> photocatalytic coating had little effect on the water permeability of the asphalt mixture. Therefore, 8% nano-TiO<sup>2</sup> content in the coating and a 400 g/m<sup>2</sup> spraying amount were finally recommended based on the photocatalytic properties, as well as for economic reasons.

**Keywords:** preparation technique; emulsified asphalt; nano-TiO2; photocatalytic coating; degradation of exhaust gas

### **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 [1–4]. TiO<sup>2</sup> 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, TiO2-coated materials can significantly decompose a variety of organic (e.g., volatile organic compounds) and inorganic (e.g., NO<sup>x</sup> and SO2) pollutants due to its unique photocatalytic property, which contributes to its ability toward air-purifying and self-cleaning. Nano-TiO<sup>2</sup> 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<sup>2</sup> have been developed and applied in different situations [6,9–11]. 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<sup>2</sup> 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 [14–16].

Currently, there are two ways to apply nano-TiO<sup>2</sup> on the road, which are a blending method (i.e., nano-TiO<sup>2</sup> was directly added to the aggregate and mixed with the asphalt to form a nano-TiO<sup>2</sup> asphalt mixture) and a spraying method (i.e., coatings containing nano-TiO<sup>2</sup> were sprayed on the surface of the pavement or other road-affiliated facilities) [17]. Tan et al. [18] applied nano-TiO<sup>2</sup> to an asphalt pavement using both methods to analyze the photocatalytic degradation of automobile exhaust gas. Test results showed that nano-TiO<sup>2</sup> under the two kinds of applications had a good photocatalytic degradation effect. However, for the blending method, the amount of nano-TiO<sup>2</sup> used was large and the cost was high. Marwa, Hassan, and co-workers [19–21] studied a sustainable photocatalytic asphalt pavement by using TiO<sup>2</sup> 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<sup>2</sup> coating in photodegrading mixed NO<sup>2</sup> and NO gases from the atmosphere was evaluated. David et al. [22] studied the performance of asphalt pavement with a TiO<sup>2</sup> photocatalyst. They developed and tested a new photocatalytic method to quantify the short-term durability of TiO<sup>2</sup> spraying on two kinds of pavements: concrete and asphalt. Clement 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 NOx. 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. Usually photocatalytic compounds used only as a coating are painted on the roadside or crash barrier. Therefore, based on the characteristics of the sprayed asphalt pavement, a photocatalytic coating of nano-TiO<sup>2</sup> and emulsified asphalt as a carrier was prepared in this paper. The effect of the degradation performance of a photocatalytic coating on the exhaust gas was evaluated, and the pavement performance of the asphalt pavement with a photocatalytic coating was analyzed.

#### **2. Materials and Experimental Preparation**

### *2.1. Selection of Nano-TiO<sup>2</sup> Crystals*

TiO<sup>2</sup> was divided into three different crystal structures, namely anatase, rutile, and brookite. Table 1 shows the basic properties of the three types of crystal structures. The common ones are anatase and rutile. The internal structures of these crystals composed of TiO<sup>2</sup> are all octahedral and connect with each other. However, they have different material properties and internal electronic structures due to the differences of the degree of deformation of the octahedral and the connection forms. Despite the valence band of the phase, the electron transition produces the same cavitation oxidation performance. However, the band width of anatase is 3.2 eV, while rutile is only 3.0 eV. Therefore, the rutile is more negative, and the reducibility of transitional electrons is stronger. In addition, the electron (e−)–hole (h<sup>+</sup> ) pairs produced by the lower-bandwidth rutile electron transition are easier to recover, reducing the redox activity. Therefore, the anatase TiO<sup>2</sup> has better photocatalytic performance than the rutile

one. In this study, the anatase TiO<sup>2</sup> was selected. The technical indicators of anatase nano-TiO2, which were provided by the manufacturer Xuancheng Jingrui New Material Co., Ltd. (Xuancheng, China), are shown in Table 2.


**Table 1.** The physical properties of nano-TiO<sup>2</sup> crystals.


### *2.2. Materials for TiO<sup>2</sup> Photocatalytic Coating*

Besides nano-TiO2, the materials for the preparation of nano-TiO<sup>2</sup> photocatalytic coatings include:





### **Table 4.** Technical indicators of emulsifiers.


**Table 5.** Basic physical properties of asphalt.

#### *2.3. Asphalt Mixture for Base Sample*

A kind of styrene-butadiene-styrene block copolymer (SBS) modified asphalt binder with 8% high viscosity modified additive by the weight of asphalt was used in this paper. The main technical indexes were tested according to the test methods in the Chinese Standard Test Methods of Bitumen and Bitumen Mixtures for Highway Engineering (JTG E20-2011) [24]. Its basic properties met the Chinese Specifications for Construction of Highway Asphalt Pavements (JTG F40-2004) [25], as shown in Table 6. The aggregate used was taken from a basalt quarry (Qiangyuan Basalt Development Co., Ltd., Zhuzhou, China). It was completely crushed to give larger particles with an angular shape and rough surface texture. The test methods were based on the Chinese Test Methods of Aggregate for Highway Engineering (JTG E42-2005) [26]. Its basic properties met the Chinese Specifications for Construction of Highway Asphalt Pavements (JTG F40-2004) [24], which are listed in Table 7. In order to make nano-TiO<sup>2</sup> play a better photocatalytic role, open-graded friction course (OGFC) gradation was selected owing to its large air voids and deep structural texture. The design gradation of the aggregate met the requirements of OGFC-10 according to the Chinese Technical Specifications for Construction of Highway Asphalt Pavements (JTG F40-2004) [25], as shown in Figure 1. The designed asphalt aggregate ratio was 4.5%. The results of OGFC design are shown in Table 8.






**Table 8.** Results of OCFG-10 design. *Appl. Sci.* **2018**, *8*, x FOR PEER REVIEW 5 of 15 **Table 8.** Results of OCFG-10 design. **Table 8.** Results of OCFG-10 design.

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**Figure 1.** OGFC-10 aggregate gradation curve. **Figure 1.** OGFC-10 aggregate gradation curve. **Figure 1.** OGFC-10 aggregate gradation curve.

#### *2.4. Exhaust Gas Degradation Test Device and Evaluation Indexes 2.4. Exhaust Gas Degradation Test Device and Evaluation Indexes 2.4. Exhaust Gas Degradation Test Device and Evaluation Indexes*

#### 2.4.1. Exhaust Gas Test Devices and Test Steps 2.4.1. Exhaust Gas Test Devices and Test Steps 2.4.1. Exhaust Gas Test Devices and Test Steps

The test devices for measuring the degradation efficiency of exhaust gas were developed, including the exhaust gas analyzer, degradation reaction chamber with build-in two parallel UVA-340 ultraviolet lamps and a small fan, voltage regulator, and a gas cylinder filled with CO, HC, and NOx. The composition and concentration of the gas mix in the cylinder were similar to automobile exhaust. The test devices are illustrated in Figure 2. The test devices for measuring the degradation efficiency of exhaust gas were developed, including the exhaust gas analyzer, degradation reaction chamber with build-in two parallel UVA-340 ultraviolet lamps and a small fan, voltage regulator, and a gas cylinder filled with CO, HC, and NOx. The composition and concentration of the gas mix in the cylinder were similar to automobile exhaust. The test devices are illustrated in Figure 2. The test devices for measuring the degradation efficiency of exhaust gas were developed, including the exhaust gas analyzer, degradation reaction chamber with build-in two parallel UVA-340 ultraviolet lamps and a small fan, voltage regulator, and a gas cylinder filled with CO, HC, and NOx. The composition and concentration of the gas mix in the cylinder were similar to automobile exhaust. The test devices are illustrated in Figure 2.

**Figure 2.** Exhaust gas degradation test device. **Figure 2. Figure 2.** Exhaust gas degradation test device. Exhaust gas degradation test device.

Test steps: Test steps:

• The nano-TiO<sup>2</sup> photocatalytic coating was prepared (the detailed procedures are shown in Section 3.2). The nano-TiO2 photocatalytic coating was prepared (the detailed procedures are shown in Section 3.2).

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nano-TiO2 content or spraying amount of photocatalytic coating.

**Figure 3.** Painting the emulsified asphalt coating on the slab sample. **Figure 3.** Painting the emulsified asphalt coating on the slab sample.

#### 2.4.2. Evaluation Indexes 2.4.2. Evaluation Indexes

#### 1. Cumulative degradation rate 1. Cumulative degradation rate

Under a certain initial concentration of the exhaust gas and light intensity, the cumulative degradation rate is defined as a ratio of the decreasing value of the exhaust gas concentration at the time *t* caused by the nano-TiO2 photocatalytic coating to the initial concentration of the exhaust gas: Under a certain initial concentration of the exhaust gas and light intensity, the cumulative degradation rate is defined as a ratio of the decreasing value of the exhaust gas concentration at the time *t* caused by the nano-TiO<sup>2</sup> photocatalytic coating to the initial concentration of the exhaust gas:

$$e = \frac{\mathcal{C}\_0 - \mathcal{C}\_l}{\mathcal{C}\_0} \times 100\%,\tag{1}$$

where *e* is the cumulative degradation rate, %; *C0* is the initial concentration of exhaust gas, ppm; and *Ct* is the exhaust gas concentration at the time *t*, ppm. where *e* is the cumulative degradation rate, %; *C<sup>0</sup>* is the initial concentration of exhaust gas, ppm; and *C<sup>t</sup>* is the exhaust gas concentration at the time *t*, ppm.

#### 2. Degradation efficiency 2. Degradation efficiency

Degradation efficiency is defined as a ratio of the photocatalytic degradation mass of the exhaust gas caused by the photocatalytic coating with a certain nano-TiO2 content in the test sample to the maximum photocatalytic degradation mass of the exhaust gas caused by the same mass of no carrier nano-TiO2 powder that was evenly distributed on the slab within time 0–*t*: Degradation efficiency is defined as a ratio of the photocatalytic degradation mass of the exhaust gas caused by the photocatalytic coating with a certain nano-TiO<sup>2</sup> content in the test sample to the maximum photocatalytic degradation mass of the exhaust gas caused by the same mass of no carrier nano-TiO<sup>2</sup> powder that was evenly distributed on the slab within time 0–*t*:

$$
\sigma = \frac{m\_t}{m\_{\text{max}}} \times 100\% \,\tag{2}
$$

max

,

where *r* is the degradation efficiency, %; *m<sup>t</sup>* is the degraded mass of exhaust gas under nano-TiO<sup>2</sup> with emulsified asphalt as the carrier during the time *t*, *mg*; and *m*max is the maximum photocatalytic degradation of the exhaust gases under nano-TiO<sup>2</sup> without the carrier during the time *t*, *mg*. with emulsified asphalt as the carrier during the time *t*, *mg*; and *m*max is the maximum photocatalytic degradation of the exhaust gases under nano-TiO2 without the carrier during the time *t*, *mg*. **3.** P**reparation of the Nano-TiO2 Photocatalytic Coating** 

where *r* is the degradation efficiency, %; *mt* is the degraded mass of exhaust gas under nano-TiO2

### **3. Preparation of the Nano-TiO<sup>2</sup> Photocatalytic Coating**

#### *3.1. Materials Selection 3.1. Materials Selection*

#### 3.1.1. Dispersant 3.1.1. Dispersant

Two dispersants were mixed with nano-TiO<sup>2</sup> in the ratios (dispersant versus nano-TiO2) of 0, 1:5, 1:10, 1:15, and 1:20. Then, the high purity water was added into the mixture to ensure that the content of nano-TiO<sup>2</sup> in the slurry was 20%. Taking the preparation of the 300 g slurry as an example, the amount of each material in the slurry is shown in Table 9. After that, the conventional mechanical stirring device of DF-101 was used to stir the slurry for 10 min. Then, 50 mL of slurry was poured into the cylinder immediately to observe the stratification states of the slurry at different times. The stratification states of nano-TiO<sup>2</sup> after 72 h are shown in Figure 4. Two dispersants were mixed with nano-TiO2 in the ratios (dispersant versus nano-TiO2) of 0, 1:5, 1:10, 1:15, and 1:20. Then, the high purity water was added into the mixture to ensure that the content of nano-TiO2 in the slurry was 20%. Taking the preparation of the 300 g slurry as an example, the amount of each material in the slurry is shown in Table 9. After that, the conventional mechanical stirring device of DF-101 was used to stir the slurry for 10 min. Then, 50 mL of slurry was poured into the cylinder immediately to observe the stratification states of the slurry at different times. The stratification states of nano-TiO2 after 72 h are shown in Figure 4.

**Table 9.** The amount of each material in 300 g slurry. **Table 9.** The amount of each material in 300 g slurry.


(The ratios from left to right in the figures are 0, 1:5, 1:10, 1:15, and 1:20) (**a**) (**b**)

**Figure 4.** The stratification states of nano-TiO2 after 72 h with (**a**) S-5040, and (**b**) SHP. **Figure 4.** The stratification states of nano-TiO<sup>2</sup> after 72 h with (**a**) S-5040, and (**b**) SHP.

As shown in Figure 4, the dispersion effect of S-5040 to nano-TiO2 is better than that of SHP. When the proportion of S-5040 to nano-TiO2 was 1:5, the mixed slurry had no obvious stratification and kept better dispersibility and stability after 72 h. All mixtures of SHP and nano-TiO2 were stratified and separated after 72 h. As a result, S-5040 was chosen to disperse nano-TiO2, and the ratio was chosen to be 1:5, that is, the amount of dispersant was 20% of the amount of nano-TiO2. As shown in Figure 4, the dispersion effect of S-5040 to nano-TiO<sup>2</sup> is better than that of SHP. When the proportion of S-5040 to nano-TiO<sup>2</sup> was 1:5, the mixed slurry had no obvious stratification and kept better dispersibility and stability after 72 h. All mixtures of SHP and nano-TiO<sup>2</sup> were stratified and separated after 72 h. As a result, S-5040 was chosen to disperse nano-TiO2, and the ratio was chosen to be 1:5, that is, the amount of dispersant was 20% of the amount of nano-TiO2.

#### 3.1.2. Emulsifier 3.1.2. Emulsifier

Nano-TiO2 photocatalytic coatings were prepared with three types of emulsifiers for performance comparison. Test conditions: the content of nano-TiO2 was 8%; the content of dispersant S-5040 was 20% of that of nano-TiO2; the content of 70# penetration grade asphalt was 50%; the contents of three kinds of emulsifiers (SY-QCE, KZW-802, SY-CME) were 0.5%, 0.8%, and 2%; the temperature of soap solution was 70 °C; the temperature of hot asphalt was 140 °C; the content of the stabilizer (CaCl2) was 0.2%; and a colloid mill (Denimo TECH A/S, Aarslev, Denmark) was adopted Nano-TiO<sup>2</sup> photocatalytic coatings were prepared with three types of emulsifiers for performance comparison. Test conditions: the content of nano-TiO<sup>2</sup> was 8%; the content of dispersant S-5040 was 20% of that of nano-TiO2; the content of 70# penetration grade asphalt was 50%; the contents of three kinds of emulsifiers (SY-QCE, KZW-802, SY-CME) were 0.5%, 0.8%, and 2%; the temperature of soap solution was 70 ◦C; the temperature of hot asphalt was 140 ◦C; the content of the stabilizer (CaCl2) was 0.2%; and a colloid mill (Denimo TECH A/S, Aarslev, Denmark) was adopted as an emulsifying

device. The pH value of the SY-QCE solution was adjusted to a range of 3–4 using hydrochloric acid, and the others did not require acid value adjustment. The basic performances are shown in Table 10.


**Table 10.** Basic performances of nano-TiO<sup>2</sup> photocatalytic emulsion coatings.

The color of emulsified asphalt was brown, which indicated that asphalt emulsification was in good condition. However, the standard viscosity of SY-QCE emulsified asphalt was only 8 s, approaching the lower limit of the technical requirement of PC-1 (sprinkling cationic rapid-setting emulsified asphalt) in the Chinese Technical Specifications for Construction of Highway Asphalt Pavements (JTG F40-2004) [25]. Therefore, it is improper to spray SY-QCE emulsified asphalt in a practical project.

The standard viscosities of KZW-802 and SY-CME emulsified asphalt met the technical requirements, but the storage stability of KZW-802 was poor. As a result, SY-CME was selected as the carrier of the nano-TiO<sup>2</sup> photocatalytic coating.

### *3.2. Preparation Process*

Taking 1 kg of photocatalytic coating with 8% nano-TiO<sup>2</sup> as an example, the preparation process is given below with the content of each material (by weight) in the coating shown in Table 11. The temperature of the soap solution was 70 ◦C, the temperature of the hot asphalt was 140 ◦C, and the colloid mill was adopted as an emulsifying device.


**Table 11.** The content of each material (by weight) in the photocatalytic coating.

### 1. Soap solution


### 2. Asphalt

The asphalt was heated to the flowing state. Then, 500 g of heated asphalt was weighed and the temperature of the asphalt was kept on standby at 140 ◦C.

### 3. Emulsification

The prepared soap solution and heated asphalt were added into the colloid mill successively. After 3 min of emulsification, the emulsified asphalt was cooled. Finally, the emulsified asphalt was placed in a plastic bucket as a reserve.

For the preparation of photocatalytic coatings with different nano-TiO<sup>2</sup> contents, only the amounts of nano-TiO<sup>2</sup> and dispersant needed to be adjusted.

### *3.3. Performance of the Nano-TiO<sup>2</sup> Photocatalytic Coating*

The photocatalytic coatings with different nano-TiO<sup>2</sup> contents were prepared and their basic properties met the Chinese Technical Specifications for Construction of Highway Asphalt Pavements (JTG F40-2004) [25], as shown in Table 12.



**Table 12.** Basic properties of photocatalytic coatings with different nano-TiO<sup>2</sup> content.

### **4. Evaluation of Exhaust Gas Degradation Effect**

### *4.1. Effects of the Nano-TiO<sup>2</sup> Content on Exhaust Gas Degradation*

A total of 30 g of photocatalytic coatings with nano-TiO<sup>2</sup> content of 3%, 5%, 8%, 10%, and 15% were uniformly applied to the surface of the 300 × 300 mm OGFC-10 slab samples to perform photocatalytic degradation of the exhaust gas test. The test specimens are shown in Figure 5. The effect of degradation of exhaust gas by photocatalytic coating with different nano-TiO<sup>2</sup> contents was studied. The test results are shown in Figures 6 and 7.

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**Figure 5.** Test specimens: (**a**) without photocatalytic coating, and (**b**) with sprayed photocatalytic coating. **Figure 5.** Test specimens: (**a**) without photocatalytic coating, and (**b**) with sprayed photocatalytic coating. **Figure 5.** Test specimens: (**a**) without photocatalytic coating, and (**b**) with sprayed photocatalytic coating.

**Figure 6.** Changes of the cumulative degradation rate of the exhaust gas at different nano-TiO2 contents. **Figure 6.** Changes of the cumulative degradation rate of the exhaust gas at different nano-TiO<sup>2</sup> contents. **Figure 6.** Changes of the cumulative degradation rate of the exhaust gas at different nano-TiO2 contents.

**Figure 7.** Changes of the degradation efficiency of the exhaust gas at different nano-TiO2 contents. **Figure 7.** Changes of the degradation efficiency of the exhaust gas at different nano-TiO2 contents. **Figure 7.** Changes of the degradation efficiency of the exhaust gas at different nano-TiO<sup>2</sup> contents.

**Figure 7.** Changes of the degradation efficiency of the exhaust gas at different nano-TiO2 contents. As can be seen from Figure 6, with the increase of nano-TiO2 content, the performance of photocatalytic coatings for degradation of exhaust gas was better. When the content of nano-TiO2 increased from 0 to 8%, the cumulative degradation rate of CO, HC, and NO increased rapidly. However, the cumulative degradation rate of the CO, HC, and NO grew slowly when the content of As can be seen from Figure 6, with the increase of nano-TiO2 content, the performance of photocatalytic coatings for degradation of exhaust gas was better. When the content of nano-TiO2 increased from 0 to 8%, the cumulative degradation rate of CO, HC, and NO increased rapidly. However, the cumulative degradation rate of the CO, HC, and NO grew slowly when the content of As can be seen from Figure 6, with the increase of nano-TiO2 content, the performance of photocatalytic coatings for degradation of exhaust gas was better. When the content of nano-TiO2 increased from 0 to 8%, the cumulative degradation rate of CO, HC, and NO increased rapidly. However, the cumulative degradation rate of the CO, HC, and NO grew slowly when the content of nano-TiO2 was above 8%. Figure 7 illustrates that the nano-TiO2 content had a good quadratic As can be seen from Figure 6, with the increase of nano-TiO<sup>2</sup> content, the performance of photocatalytic coatings for degradation of exhaust gas was better. When the content of nano-TiO<sup>2</sup> increased from 0 to 8%, the cumulative degradation rate of CO, HC, and NO increased rapidly. However, the cumulative degradation rate of the CO, HC, and NO grew slowly when the content of nano-TiO<sup>2</sup> was above 8%. Figure 7 illustrates that the nano-TiO<sup>2</sup> content had a good quadratic nonlinear

nano-TiO2 was above 8%. Figure 7 illustrates that the nano-TiO2 content had a good quadratic

nonlinear correlation with the degradation efficiency of CO, HC, and NO. With the increase of nano-

nano-TiO2 was above 8%. Figure 7 illustrates that the nano-TiO2 content had a good quadratic

correlation with the degradation efficiency of CO, HC, and NO. With the increase of nano-TiO<sup>2</sup> content, the degradation efficiency of three kinds of exhaust gas increased gradually. However, the degradation efficiency of nano-TiO<sup>2</sup> to three kinds of exhaust gas tended to decrease when the content of nano-TiO<sup>2</sup> was above 8%. TiO2 content, the degradation efficiency of three kinds of exhaust gas increased gradually. However, the degradation efficiency of nano-TiO2 to three kinds of exhaust gas tended to decrease when the content of nano-TiO2 was above 8%. The exhaust gas degradation performance of the coating was related to the size of the effective contact area between the nano-TiO2 and exhaust gas, and ultraviolet intensity. When the nano-TiO2 the degradation efficiency of nano-TiO2 to three kinds of exhaust gas tended to decrease when the content of nano-TiO2 was above 8%. The exhaust gas degradation performance of the coating was related to the size of the effective contact area between the nano-TiO2 and exhaust gas, and ultraviolet intensity. When the nano-TiO2 content exceeded 8%, the entire surface of the slab sample was covered by a saturated nano-TiO2 thin

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TiO2 content, the degradation efficiency of three kinds of exhaust gas increased gradually. However,

The exhaust gas degradation performance of the coating was related to the size of the effective contact area between the nano-TiO<sup>2</sup> and exhaust gas, and ultraviolet intensity. When the nano-TiO<sup>2</sup> content exceeded 8%, the entire surface of the slab sample was covered by a saturated nano-TiO<sup>2</sup> thin layer. Therefore, there was no substantial increase in the effective number of nano-TiO<sup>2</sup> particles touching with the exhaust gas per unit area with the continuous increase of the content of nano-TiO2. The degradation performance of the photocatalytic coating was not significantly improved. Furthermore, the degradation efficiency of nano-TiO<sup>2</sup> gradually decreased. content exceeded 8%, the entire surface of the slab sample was covered by a saturated nano-TiO2 thin layer. Therefore, there was no substantial increase in the effective number of nano-TiO2 particles touching with the exhaust gas per unit area with the continuous increase of the content of nano-TiO2. The degradation performance of the photocatalytic coating was not significantly improved. Furthermore, the degradation efficiency of nano-TiO2 gradually decreased. *4.2. Effects of the Spraying Amount on Exhaust Gas Degradation*  layer. Therefore, there was no substantial increase in the effective number of nano-TiO2 particles touching with the exhaust gas per unit area with the continuous increase of the content of nano-TiO2. The degradation performance of the photocatalytic coating was not significantly improved. Furthermore, the degradation efficiency of nano-TiO2 gradually decreased. *4.2. Effects of the Spraying Amount on Exhaust Gas Degradation*  Separately, 10 g, 20 g, 30 g, 40 g, and 55 g of photocatalytic coatings with 8% nano-TiO2 were

#### *4.2. Effects of the Spraying Amount on Exhaust Gas Degradation* Separately, 10 g, 20 g, 30 g, 40 g, and 55 g of photocatalytic coatings with 8% nano-TiO2 were uniformly sprayed on the slab samples of OGFC-10 to perform the photocatalytic degradation of

Separately, 10 g, 20 g, 30 g, 40 g, and 55 g of photocatalytic coatings with 8% nano-TiO<sup>2</sup> were uniformly sprayed on the slab samples of OGFC-10 to perform the photocatalytic degradation of exhaust gas test. The performances of nano-TiO<sup>2</sup> photocatalytic coatings with different spraying amounts on the degradation of exhaust gas were studied. The test results are shown at Figures 8 and 9. uniformly sprayed on the slab samples of OGFC-10 to perform the photocatalytic degradation of exhaust gas test. The performances of nano-TiO2 photocatalytic coatings with different spraying amounts on the degradation of exhaust gas were studied. The test results are shown at Figures 8 and 9. exhaust gas test. The performances of nano-TiO2 photocatalytic coatings with different spraying amounts on the degradation of exhaust gas were studied. The test results are shown at Figures 8 and 9.

**Figure 8.** Changes of the cumulative degradation rate of the exhaust gas at different nano-TiO2 spraying amounts. **Figure 8.** Changes of the cumulative degradation rate of the exhaust gas at different nano-TiO<sup>2</sup> spraying amounts. spraying amounts.

**Figure 9.** Changes of the degradation efficiency of the exhaust gas at different nano-TiO2 spraying amounts. **Figure 9.** Changes of the degradation efficiency of the exhaust gas at different nano-TiO2 spraying amounts. **Figure 9.** Changes of the degradation efficiency of the exhaust gas at different nano-TiO<sup>2</sup> spraying amounts.

As can be seen from Figure 8. The cumulative degradation rate of CO, HC, and NO increased gradually with the increase of the amount of photocatalytic coating. However, when the spraying amount was over 333.3 g/m<sup>2</sup> , the cumulative degradation rate of CO, HC, and NO gas remained unchanged with the continuous increasing of spraying amount. When the spraying amount of nano-TiO<sup>2</sup> photocatalytic coating was less than 333.3 g/m<sup>2</sup> , with the increase of spraying amount, the number of nano-TiO<sup>2</sup> particles on the surface of the slab sample and the effective contact area with the exhaust gas and the ultraviolet light gradually increased and the effect of degradation improved. When the nano-TiO<sup>2</sup> photocatalytic coating spraying amount reached 333.3 g/m<sup>2</sup> , the effective contact area and quantities of nano-TiO<sup>2</sup> with the car exhaust and ultraviolet also achieved a peak value. After that, the increase of the spray volume only resulted in an even thicker coating that had little effect on improving the effect of degradation on vehicle exhaust gas.

As can be seen from Figure 9, with the continuous increase in the spraying amount of photocatalytic nano-TiO2, the degradation efficiency of CO, HC, and NO decreased gradually. This was mainly because a small amount of coating being sprayed on the surface of the slab samples can form a very thin film layer. At this time, most of the nano-TiO<sup>2</sup> could act in its photocatalytic role. However, with the increase of the spraying amount, the thickness of the coating film also increased. Those nano-TiO<sup>2</sup> particles at the bottom of film could not effectively contribute to photocatalytic reactions, resulting in the decrease of degradation efficiency.

After comprehensive consideration, when the added content of nano-TiO<sup>2</sup> in the coating reached 8%, the photocatalytic coating had the best performance of degradation of the automobile exhaust gas. When the dosage of nano-TiO<sup>2</sup> increased to more than 8%, the ability of photocatalytic degradation exhaust was no longer a definite improvement with the increase of nano-TiO2. In addition, the photocatalytic coating containing 8% of nano-TiO<sup>2</sup> was evenly sprayed on the slabs for the photocatalytic degradation of the exhaust gas test. It indicated that with the continuous increase of the spraying amount of coating, the photocatalytic degradation property of exhaust gas was gradually improved. However, the performance of the photocatalytic degradation property of exhaust gas basically kept at the same level after the spraying amount was over 333.3 g/m<sup>2</sup> .

#### **5. Pavement Performance Evaluation**

#### *5.1. Skid Resistance*

The friction coefficient and texture depth were used to evaluate the skid resistance of asphalt pavement at the different spraying amounts of nano-TiO<sup>2</sup> photocatalytic coatings. The friction coefficient of the sprayed nano-TiO<sup>2</sup> asphalt mixture was measured using the pendulum friction meter (Shuyang highway equipment plant, Suqian, China) according to the test method of T0964 in the Chinese Field Test Methods of Subgrade and Pavement for Highway Engineering (JTG E60-2008) [27] and the surface texture depth was determined using the test method of T0731 in the Chinese Standard Test Methods of Bitumen and Bituminous Mixtures for Highway Engineering (JTG E20-2011) [24]. The test results are shown in Figure 10.

**Figure 10.** Effect of spraying amount on the skid resistance of asphalt pavement. **Figure 10.** Effect of spraying amount on the skid resistance of asphalt pavement.

It can be seen from Figure 10, both the friction coefficient and the texture depth of the asphalt mixture were reduced with the increase of the spraying amount of photocatalytic coating. It indicated that the skid resistance of the asphalt mixture was decreased due to spraying the nano-TiO2 photocatalytic coating on the sample surface. When the nano-TiO2 photocatalytic coating spraying amount was greater than 550 g/m2, the friction coefficient was close to the critical lower limit value (58BPN) of the technical requirement in the Chinese Specifications for Design of Highway Asphalt Pavement (JTG D50-2006) [28]. Therefore, it is recommended that the spraying amount of nano-TiO2 photocatalytic coating should not exceed 550 g/m2 in practical projects. It can be seen from Figure 10, both the friction coefficient and the texture depth of the asphalt mixture were reduced with the increase of the spraying amount of photocatalytic coating. It indicated that the skid resistance of the asphalt mixture was decreased due to spraying the nano-TiO<sup>2</sup> photocatalytic coating on the sample surface. When the nano-TiO<sup>2</sup> photocatalytic coating spraying amount was greater than 550 g/m<sup>2</sup> , the friction coefficient was close to the critical lower limit value (58BPN) of the technical requirement in the Chinese Specifications for Design of Highway Asphalt Pavement (JTG D50-2006) [28]. Therefore, it is recommended that the spraying amount of nano-TiO<sup>2</sup> photocatalytic coating should not exceed 550 g/m<sup>2</sup> in practical projects.

#### *5.2. Water Permeability 5.2. Water Permeability*

The water permeability of asphalt pavement with different nano-TiO2 photocatalytic coatings was investigated. The corresponding technical indicator met the requirements of Chinese Technical Specifications for Construction of Highway Asphalt Pavements (JTG F40-2004) [25]. The test results are summarized in Table 13. The water permeability of asphalt pavement with different nano-TiO<sup>2</sup> photocatalytic coatings was investigated. The corresponding technical indicator met the requirements of Chinese Technical Specifications for Construction of Highway Asphalt Pavements (JTG F40-2004) [25]. The test results are summarized in Table 13.


**Table 13.** Effect of spraying amount on permeability of asphalt mixture.

It can be seen from the Table 13 that the water permeability performance of the asphalt mixture was basically not affected by the spraying amount of nano-TiO2 photocatalytic coating. This was because the amount of photocatalytic coating that was sprayed was so little that it cannot effectively fill the void in the surface of the asphalt mixture. Therefore, the influence of the spraying amount of nano-TiO2 photocatalytic coating on the water permeability performance of the asphalt pavement can be ignored in practical projects. It can be seen from the Table 13 that the water permeability performance of the asphalt mixture was basically not affected by the spraying amount of nano-TiO<sup>2</sup> photocatalytic coating. This was because the amount of photocatalytic coating that was sprayed was so little that it cannot effectively fill the void in the surface of the asphalt mixture. Therefore, the influence of the spraying amount of nano-TiO<sup>2</sup> photocatalytic coating on the water permeability performance of the asphalt pavement can be ignored in practical projects.

#### **6. Conclusions 6. Conclusions**

According to the application characteristics of the spraying method, the emulsified asphalt was selected to act as the carrier, and the materials for preparing nano-TiO2 photocatalytic coating were determined by comprehensive comparison and selection. The test results indicated that the properties of prepared nano-TiO2 photocatalytic coating met the Chinese specifications technical According to the application characteristics of the spraying method, the emulsified asphalt was selected to act as the carrier, and the materials for preparing nano-TiO<sup>2</sup> photocatalytic coating were determined by comprehensive comparison and selection. The test results indicated that the properties

requirements. Meanwhile, the color of the coating (seen in Figure 5) was the same as that of the

of prepared nano-TiO<sup>2</sup> photocatalytic coating met the Chinese specifications technical requirements. Meanwhile, the color of the coating (seen in Figure 5) was the same as that of the asphalt after setting, having offset the deficiency of different colors between traditional photocatalytic coating and the pavement. Therefore, the coating could be applied directly onto the surface of the pavement using an asphalt distributor, just like fog seal, to play a better role in the degradation of exhaust gas. The coating could be applied to pavement surfaces that are located in densely-populated areas, environmentally-sensitive areas, service areas and parking lots, and so on.

An exhaust gas degradation test device was developed, and its test steps were described in detail. The evaluation indexes, cumulative degradation rate, and degradation efficiency were put forward.

The test results of the performance of exhaust gas degradation of nano-TiO<sup>2</sup> photocatalytic coating showed that, when nano-TiO<sup>2</sup> content was changed in the range from 0–8% and the spraying amount was changed in range from 0–333.3 g/m<sup>2</sup> , the performance of the photocatalytic coating on the degrading exhaust gas was significantly improved with the increase of nano-TiO<sup>2</sup> content and photocatalytic coating spraying amount. When over those dosages, the efficacy of photocatalytic coating was not significantly improved regarding exhaust gas degradation.

In practical projects, the amount of spraying should be controlled strictly because the nano-TiO<sup>2</sup> photocatalytic coating can influence the skid resistance of the asphalt pavement to a certain extent. It is recommended that the spraying amount of nano-TiO<sup>2</sup> photocatalytic coating should not exceed 550 g/m<sup>2</sup> . The nano-TiO<sup>2</sup> photocatalytic coating had little effect on the water permeability performance of asphalt pavement. As such, excessive dosage cannot effectively degrade exhaust gas but thickens the oil membrane, which leads to an inadequate skid resistance of the pavement, which is unsafe and uneconomical. Therefore, 8% nano-TiO<sup>2</sup> in coating and a 400 g/m<sup>2</sup> spraying amount (considering the loss of construction, etc.) were finally chosen to be the optimum parameters based on the photocatalytic properties as well as economic reasons.

**Author Contributions:** H.W. designed the experiment and wrote the manuscript; K.J. and X.D. conducted experiments, analyzed experimental data and wrote the manuscript; S.Z. and C.L. helped conducting tests.

**Funding:** This research was funded by "the National Natural Science Foundation of China, grant number 51478052", and "Guangdong Transportation Department, grant number 2013-01-002".

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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