Research Progress on Nano-TiO₂ Photocatalytic Degradation of Automobile Exhaust

Abstract

Nano-TiO₂ is widely used in many industrial fields due to its unique physical and chemical properties. In recent years, it has become a core material in the research of road engineering for degrading automobile exhaust. Under ultraviolet irradiation, it can excite electron-hole pairs and use its strong redox capacity to decompose automobile exhaust and improve air quality. From the perspectives of materials, performance and engineering application, this paper briefly describes the structure and physicochemical properties of nano-TiO₂, reviews the recent research progress of nano-TiO₂ in the photocatalytic degradation of automobile exhaust, systematically compares the effects of various strategies such as incorporation methods and modified materials on exhaust degradation efficiency, and conducts a quantitative analysis of performance differences. It is pointed out that insufficient road durability, poor compatibility with pavement materials and limited adaptability to unconventional environments are the main current problems and challenges in this research direction. The future development directions such as developing self-healing composite systems and constructing machine learning prediction models are also prospected.

1. Introduction

Nano materials are ultra-fine materials with many excellent properties not found in traditional materials, such as surface effect, small size effect, enhanced specific surface area and magnetism [1]. Titanium dioxide (TiO₂) is a white and non-toxic oxide with many good properties such as simple preparation process, environmental friendliness, photocatalysis, antibacterial property, corrosion resistance and weather resistance [2]. It is also rich in resources and low in price. Nano-TiO₂ is widely used in catalytic materials [3], environmental protection [4,5,6], ceramic coatings [7], medical antibacterial [8], cosmetics [9], superhydrophobic materials [10] and other fields, and it is one of the most widely used nano materials at present. Since the 20th century, the transportation industry has developed rapidly and the number of motor vehicles has increased significantly. The accompanying problems have also increased. The increasing number of automobiles release more and more air pollutants, which aggravate the degree of environmental pollution. Traditional subgrade and pavement materials cannot actively reduce the environmental pollution caused by automobile exhaust. We need higher quality and more environmentally friendly pavement materials, which have important practical significance for the future technological development of road engineering and the improvement of residents’ living environment. At present, photocatalytic environmental protection pavement is regarded as one of the most promising functional facilities in road infrastructure. Therefore, the photocatalytic degradation of automobile exhaust has become a research hotspot in pavement materials in recent years, aiming to reduce the pollution level by degrading the polluting gases in motor vehicle exhaust through pavement materials. Nano-TiO₂ can degrade some organic and inorganic pollutants in automobile exhaust through photocatalytic reaction [11], and reduce the impact of automobile exhaust on urban air and residents’ physical health.

Existing reviews mainly focus on two aspects. One is the crystal form regulation, surface modification and photocatalytic principle of nano-TiO₂ [12], which do not analyze the performance stability combined with the complex pavement environment. The other is the application of photocatalytic technology in atmospheric governance [13], which mentions the pollutant degradation effect but does not correlate with the engineering characteristics of pavement. A small number of recent reviews related to photocatalytic pavement only focus on engineering cases [14], without systematically comparing the advantages and disadvantages of nano-TiO₂ incorporation methods or analyzing common problems such as insufficient visible light response. Compared with existing studies, this paper focuses on the application scenario of pavement engineering and combines material characteristics with pavement service requirements. It integrates the research results of the whole process of materials, performance and application, so as to provide support for the engineering practice guidance of photocatalytic environmental protection pavement. Aiming at the current technical bottlenecks, it puts forward future directions combined with recent data to provide reference for the large-scale application of the technology.

2. Structure and Physicochemical Properties of Nano-TiO₂

2.1. Structure of Nano-TiO₂

Nano-TiO₂ is a polycrystalline semiconductor compound. The common crystal forms include rutile, anatase and brookite. Their crystal structures are shown in Figure 1, which show different properties due to the differences in crystal structure [15]. Among them, rutile belongs to the tetragonal crystal system. One Ti⁴⁺ is at the center of the crystal lattice, and six O₂⁻ are at the edges of the TiO₆ octahedron. Two TiO₂ molecules form a unit cell, which is the most stable crystal form under most conditions. The structure of anatase is similar to that of rutile, but each unit cell contains four TiO₂ molecules, which are stable at low temperature and begin to transform into rutile when the temperature reaches 500 °C. Brookite belongs to the orthorhombic crystal system, and six TiO₂ molecules form a unit cell. It only exists in natural minerals with a small content and directly transforms into rutile when the temperature is above 650 °C, with low application value. At present, the commonly used nano-TiO₂ is mostly composed of 80% anatase and 20% rutile [16,17].

2.2. Physicochemical Properties of Nano-TiO₂

Nano-TiO₂ is a white amphoteric oxide in solid or powder form, with morphologies including spherical, rod-like, tubular and porous structures. These different morphologies have a significant impact on its specific surface area and the distribution of active sites, thus affecting its photocatalytic performance. The average particle sizes of anatase and rutile are 11 nm and 50 nm respectively [1]. Rutile nano-TiO₂ has high hardness and wear resistance, so it is often used as a coating reinforcement material to improve the mechanical strength and durability of the substrate [21]. Nano-TiO₂ nanoparticles have high specific surface area which provides abundant active sites and enhances adsorption, and reaction capacity and surface hydroxyl characteristics which are easy to bond with other molecules and improve surface activity [1]. In terms of optics, nano-TiO₂ shows strong absorption characteristics in the ultraviolet region with a wavelength less than 387 nm, which is transparent in the visible light region [22]. In terms of chemical stability, although it has excellent acid and alkali corrosion resistance and oxidation resistance, it may still dissolve in strong acid or strong alkali environments [1], which limits its application in complex environments. At the same time, as a typical n-type semiconductor, it has high electron mobility, and the photoelectric conversion efficiency in dye-sensitized solar cells can reach more than 10% [23].

In summary, nano-TiO₂ has a significant prospect in the development of visible light catalysis, environmental friendliness and multi-functional composite materials by virtue of its strong photocatalytic activity, chemical stability, ultraviolet shielding ability and adjustable surface properties.

3. Mechanism of Nano-TiO₂ Photocatalytic Degradation of Automobile Exhaust

TiO₂ is a semiconductor material with photocatalytic redox capacity. The principle of nano-TiO₂ photocatalytic reaction is as follows. Under the irradiation of light with energy higher than the band gap energy, the valence band electrons are excited to jump to the conduction band, leaving holes at the original valence band positions. The conduction band electrons jump back to the valence band and recombine with holes to release energy. The surface holes act as oxidants to oxidize hydroxyl ions into hydroxyl radicals, and electrons act as reducing agents to reduce molecular oxygen into superoxide anion radicals [24]. The above reactions are shown in Formulas (1)–(10), and the reaction process is shown in Figure 2.

$${\mathrm{TiO}}_2+\mathrm{hv} \to { \mathrm{TiO}}_2{ + \mathrm{h}}^{+}+{\mathrm{e}}^{-}$$

(1)

$${\mathrm{h}}^{+}+{\mathrm{e}}^{-}\to \mathrm{Energy}$$

(2)

$${\mathrm{OH}}^{-}{+ \mathrm{h}}^{+}\to \cdot \mathrm{OH}$$

(3)

$${\mathrm{H}}_2{\mathrm{O}+\mathrm{h}}^{+}\to { \mathrm{H}}^{+}+\cdot \mathrm{OH}$$

(4)

$${\mathrm{O}}_2+{\mathrm{e}}^{-} \to \cdot { \mathrm{O}}_2^{ -}$$

(5)

$${\mathrm{H}}_2\mathrm{O}+\cdot { \mathrm{O}}_2^{ -}\to {\mathrm{HO}}_2\cdot +\mathrm{OH}\cdot$$

(6)

$${2\mathrm{HO}}_2\cdot \to { \mathrm{O}}_2{ + \mathrm{H}}_2{\mathrm{O}}_2$$

(7)

$${\mathrm{HO}}_2\cdot {+ \mathrm{H}}_2\mathrm{O}+{\mathrm{e}}^{-}\to {\mathrm{H}}_2{\mathrm{O}}_2+\mathrm{OH}\cdot$$

(8)

$${\mathrm{H}}_2{\mathrm{O}}_2{ + \mathrm{e}}^{-} \to \cdot \mathrm{OH} + {\mathrm{OH}}^{-}$$

(9)

$${\mathrm{H}}_2{\mathrm{O}}_{2 }+\cdot {\mathrm{O}}_2^{ -} \to \cdot \mathrm{OH}+{\mathrm{OH}}^{-}{+\mathrm{O}}_2$$

(10)

In the actual road application process, under ultraviolet conditions, nano-TiO₂ acts as a catalyst, which can degrade CO, HC, and NO_x in automobile exhaust into carbonate and nitrate, which can then be washed away by rain or street sprinkling to avoid biological inhalation. The degradation reaction formula is shown in Formulas (11)–(18) [25,26,27,28,29,30,31], and the reaction process is shown in Figure 3. At the macro level, nano-TiO₂ photocatalytic environmental protection pavement has significant advantages. The large-scale and long-mileage characteristics of road construction provide a broad application scenario for the nano-TiO₂ photocatalytic degradation of exhaust. As a close contact of vehicle exhaust, the pavement can realize real-time treatment of the exhaust. At the same time, ultraviolet irradiation is an essential condition for photocatalytic reaction, and most traffic pavements can naturally obtain sunlight, which provides a free and sustainable energy source for photocatalytic reaction [32].

CO degradation produces carbonates and CO₂:

$$\mathrm{CO} +\cdot \mathrm{OH} \to { \mathrm{CO}}_2 +{ \mathrm{H}}^{+}$$

(11)

$${\mathrm{CO}}_2+{ \mathrm{H}}_2\mathrm{O} \to { \mathrm{HCO}}_3^{ -}+{\mathrm{H}}^{+}$$

(12)

HC (using methane CH₄ as an example) is degraded into CO₂ and H₂O:

$${\mathrm{CH}}_4 +\cdot \mathrm{OH} \to \cdot {\mathrm{CH}}_3 +{\mathrm{H}}_2\mathrm{O}$$

(13)

$$\cdot {\mathrm{CH}}_3{+\mathrm{O}}_2{+\mathrm{H}}_2\mathrm{O}\to { \mathrm{CO}}_2{+\mathrm{HO}}_2\cdot {+\mathrm{H}}^{+}$$

(14)

NO_x degradation produces nitrate:

$$\mathrm{NO} +\cdot \mathrm{OH} \to { \mathrm{HNO}}_2$$

(15)

$${\mathrm{HNO}}_{2 }+\cdot \mathrm{OH} \to { \mathrm{NO}}_2{ + \mathrm{H}}_2\mathrm{O}$$

(16)

$${\mathrm{NO}}_2+\cdot \mathrm{OH} \to {\mathrm{HNO}}_3$$

(17)

$${\mathrm{HNO}}_3{ + \mathrm{H}}_2\mathrm{O} \to {\mathrm{NO}}_3^{ -}{ + \mathrm{H}}_3{\mathrm{O}}^{+}$$

(18)

4. Research on the Application of Nano-TiO₂ Photocatalytic Degradation of Automobile Exhaust

Although nano-TiO₂ shows great potential in the pavement application of photocatalytic degradation of automobile exhaust, its actual popularization still faces multiple technical bottlenecks. Aiming at the existing problems, researchers have carried out research from multiple dimensions such as durability improvement, carrier structure design, the expansion of visible light response range and analysis of influencing factors, and formed a series of research results with application value.

4.1. Optimization Methods to Improve Durability and Engineering Adaptability

Nano-TiO₂ is easy to lose due to abrasion, rain scouring, freeze–thaw cycles and other factors in pavement use, and the traditional incorporation method has insufficient adaptability with pavement substrates, which directly affects the long-term photocatalytic effect and road performance. Existing studies have realized the balance between the durability and engineering practicability of nano-TiO₂ photocatalytic pavement through the combined scheme of incorporation method improvement and material modification design.

The traditional incorporation methods are mainly divided into mixing method and coating method. The mixing method mixes nano-TiO₂ powder into asphalt mixture to replace part of mineral powder. Although it has strong wear resistance and basically does not affect the various properties of the asphalt mixture, a large amount of incorporation is needed to ensure the degradation efficiency, which not only increases the cost, but also may reduce the pavement uniformity due to particle agglomeration [33,34]. The coating method mixes nano-TiO₂ with water or binder to prepare a catalyst solution, which is then coated on the road surface to play its role. A small amount of incorporation can achieve a high degradation rate, but the bonding force between the coating and the pavement surface is weak, and mass loss is easy to occur due to abrasion, thus affecting the exhaust degradation rate [33]. To break this limitation, it is necessary to improve and innovate on the basis of traditional methods. Adding nano-TiO₂ to the fog seal material of asphalt pavement not only retains the advantage of the low incorporation amount of the coating method, but also improves the durability by virtue of the permeation combination between the fog seal and the pavement. The actual measurement shows that its abrasion loss rate is about 50% lower than that of water-based coating, and the impact on the pavement skid resistance is within the scope permitted by the specification [35]. However, the degradation rate of this method is slightly poor, which may be related to the fact that some TiO₂ particles are covered by asphalt. Considering that traffic marking is an essential requirement for the normal operation of roads, taking advantage of the characteristics of marking coatings with less contact with tires and low wear rate, nano-TiO₂ is mixed into marking coatings. When the additional amount is 4%, the NO degradation rate under natural light can reach 71%, and the degradation efficiency attenuation is small during the service cycle of the marking [36]. However, this method only acts on the marking area, cannot realize the whole area degradation of the pavement, and has limited application scenarios. In terms of material modification design, some studies have cross-linked nano-TiO₂ on the surface of fine rubber powder to obtain a three-dimensional network for adsorbing and degrading exhaust gas as much as possible. At the same time, the chemical cross-linking reaction between nano-TiO₂ and rubber powder makes the composite degradation particles have stronger wear resistance, compression resistance and water washing resistance. Under the same conditions, the degradation efficiency of composite degradation particles is higher than that of pure nano-TiO₂ powder [37]. To solve the compatibility between degradation materials and pavement carriers, some studies have developed a cellulose polymer stabilizer with polyacrylamide chain grafting. This stabilizer can significantly improve the compatibility and stability between degradation materials and carriers. At the incorporation amount of 2% and storage time of 180 h, the stability coefficient is not more than 5.5%, which meets the requirements of engineering application [38,39].

In summary, the core of improving durability and adaptability is to balance the relationship among degradation efficiency, material loss and road performance. Although the existing methods have made various breakthroughs, the coordination of global performance, long-term stability and cost control still needs to be optimized, which provides a direction for the subsequent carrier structure innovation and composite modification.

4.2. Efficiency Improvement Strategies of Porous Carrier and Structural Design

On the basis of solving the durability problem, how to improve the degradation efficiency has become the key. The degradation efficiency of nano-TiO₂ is directly related to its specific surface area, the number of active sites and the contact opportunity of pollutants. The traditional particles have insufficient exposure of active sites due to agglomeration. The porous carrier and special structural design can effectively solve this problem by optimizing the spatial distribution and interface characteristics of materials, and become the key path to improve the degradation efficiency.

The porous pavement structure makes nano-TiO₂ more exposed to light and exhaust through large voids, and at the same time improves the drainage performance in rainy days and reduces the scouring loss of catalysts by rainwater. Some researchers have integrated photocatalytic materials into porous pavement to endow the pavement with additional exhaust degradation capacity [40,41], and determined the key indicators for evaluating photocatalytic performance according to experimental data [42]. Some studies have also sprayed nano-TiO₂ on the road surface to prepare photocatalytic environmental protection pervious concrete, combining two new pavement research technologies to exert greater environmental benefits [43]. The microporous membrane embedding technology, such as preparing honeycomb-like microporous structure on the asphalt surface by breath figure method and fixing TiO₂ particles in the holes, not only expands the sunlight contact area, but also reduces the direct effect of abrasion [44,45]. However, the pore membrane size is easily affected by the humidity of the construction environment, resulting in insufficient performance stability in batch application. High specific surface area carrier is also the core means to improve efficiency. SiO₂ aerogel carrier is a typical representative. The TiO₂-SiO₂ aerogel has a specific surface area of 200 m²/g, with a continuous three-dimensional network and uniform pore size. The rich pore structure not only provides a large number of active sites, but also can adsorb and enrich exhaust pollutants. Its photonic efficiency of NO_x is higher than that of ordinary nano-TiO₂ [46]. However, the aerogel has low mechanical strength, and it is easy to break when directly mixed into asphalt mixture, so it is recommended to be used in combination with resin for reinforcement.

On the whole, the porous carrier design needs to balance the high specific surface area and engineering strength. Although aerogel and mesoporous materials have significant advantages in specific surface area, their mechanical properties are insufficient. In the future, the coordination of structure and performance can be further optimized through carrier surface modification and in situ growth of TiO₂.

4.3. Modification Methods and Efficiency Comparison for Expanding Visible Light Response

Although a variety of incorporation methods and carrier structure designs have been developed, the spectral response limitation and easy loss of nano-TiO₂ itself limit its degradation efficiency. Traditional TiO₂ has a large band gap and can only respond to photons in the ultraviolet region, which accounts for only about 4% of sunlight, while visible light, which accounts for about 45% of sunlight, is not effectively utilized. In addition, the recombination rate of electron-hole pairs in TiO₂ is relatively high, leading to low quantum yield and poor degradation efficiency in photocatalytic reaction [47]. Existing studies have effectively narrowed the band gap, inhibited carrier recombination and significantly improved the catalytic performance under visible light through modification methods such as ion doping, semiconductor composite and carbon material composite [48,49]. The technical characteristics, advantages, and disadvantages of each method are shown in

Table 1. Modification methods for extending visible-light response of nano-TiO₂ photocatalytic materials.

Modification Type	Typical Solution	Core Mechanism	Limitations
Nonmetallic element doping [50,51,52,53]	S, N, C, I, B, F	Nonmetallic atoms replace O atoms in the TiO2 lattice or dope into the lattice interstitially. This introduces new impurity levels and reduces the band gap so that the material can absorb visible light.	High concentration doping easily forms carrier recombination centers and reduces quantum efficiency. Doped elements tend to desorb at high temperatures and show poor long-term stability.
Nonmetallic material loading [54,55,56,57,58,59]	Porous nonmetallic minerals, Glass, Carbon materials, Polymer materials	A porous nonmetallic TiO2 composite system is formed to solve the agglomeration of nano TiO2 particles. Porous mineral composites have strong adsorption ability for pollutants.	Most nonmetallic supports have no catalytic activity and only play a physical supporting role. Some supports have poor light transmittance and affect light absorption efficiency.
Metal ion doping [60,61,62,63,64,65]	Fe3+, Mo5+, Os3+, Rh3+	After metal ions enter the TiO2 lattice, they cause lattice distortion and form local energy levels. Metal ions can also act as electron trapping centers to restrain the recombination of photogenerated electron hole pairs.	Metal ions easily undergo photochemical migration and cause agglomeration of active sites. Noble metal ions have high cost and are hard to be used on a large scale. High doping content easily leads to excessive lattice defects and reduces material stability.
Multi element co doping [66,67,68,69]	Co doping of different metal elements, Doping of metal elements with nonmetallic elements, Co doping of different nonmetallic elements	The synergistic effect among multiple dopants makes co doped TiO2 show higher visible light absorption than single doped TiO2 and effectively improves photocatalytic performance.	The type and proportion of doped elements are difficult to control accurately. Uneven doping and excessive lattice defects may become recombination centers of photogenerated carriers and reduce catalytic efficiency.
Semiconductor composite [70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85]	Traditional types (type I cross band gap, type II staggered band gap and type III broken band gap), Z scheme and S scheme	Heterojunctions are constructed to realize the separation and transfer of photogenerated carriers by using the energy level difference in different semiconductors. The light response range can also be broadened synergistically.	Poor interface compatibility of heterojunctions easily increases the resistance of interface charge transfer. The composite ratio is difficult to control and excessive compounding may lead to decreased activity.
Noble metal deposition [61,86,87,88]	Pt, Ag, Au, Pd	Electrons transfer from the TiO2 surface with higher energy levels to the noble metal surface with lower energy levels. When the energy levels of the two surfaces are equal, electron transfer stops and a Schottky barrier is formed. This effectively separates photogenerated electron hole pairs and improves the photocatalytic activity of TiO2.	The cost of noble metal deposition is very high. TiO2 modified by noble metal deposition has high selectivity in photocatalytic degradation of organic matters, which limits the application of these materials in pollution control to a certain extent.
Dye sensitization [89,90,91,92]	organometallic dyes containing transition metals and organic dyes composed of organic chromophores	Sensitizer dyes bind to the TiO2 surface through chemical or physical adsorption, which shifts the absorption wavelength of visible light to long waves. Thus the wavelength response range of TiO2 is expanded and solar energy utilization efficiency is improved.	Organic dye molecules degrade gradually under photocatalysis. Most sensitizers have weak absorption in the near-infrared band and easily compete with pollutants for adsorption.

.

In recent years, the research on the modification of nano-TiO₂ for visible light response has gradually shifted from early single doping to multi-component synergistic strategies such as semiconductor composite and carbon material composite, and the degradation efficiency has been improved compared with single modification. However, the current research still has the following core problems. The analysis of micro mechanisms is insufficient, focusing on the improvement of macro performance, and the explanation of key mechanisms such as lattice interaction and charge transfer path is inadequate, resulting in the difficulty of the precise regulation of modification effects. The engineering applicability is insufficient. Most laboratory studies are based on ideal conditions such as single pollutant and constant environment, which are disconnected from the actual pavement conditions such as abrasion scouring and the coexistence of multiple pollutants, and the long-term stability of materials lacks verification. The contradiction between cost and large-scale application is prominent. The cost of modifiers such as graphene and precious metals is high, and some multi-step synthesis processes are complex, which are difficult to adapt to the large-scale construction requirements of pavement engineering. In the future, research should rely on characterization technologies such as in situ XRD and XPS to establish a quantitative relationship between modification parameters, microstructures, and photocatalytic performance to consolidate the theoretical support. Develop an integrated scheme of modification and protection and verify the environmental adaptability of materials through long-term outdoor exposure tests. At the same time, use industrial wastes such as steel slag and fly ash as low-cost modifiers, optimize simplified processes such as one-step hydrothermal method, and promote the transformation of laboratory results into engineering applications.

4.4. Analysis of Influencing Factors of Photocatalytic Degradation Reaction

The nano-TiO₂ photocatalytic reaction is affected by various surrounding conditions. Although extensive photocatalytic experiments have been carried out in the laboratory to verify the feasible degradation of nano-TiO₂, in situ monitoring shows that the purification performance is highly dispersed [93]. Therefore, it is crucial to determine the influence mechanism of photocatalytic reaction from different factors before large-scale popularization.

Table 2. Main influencing factors on the photocatalytic degradation efficiency of TiO₂ [93,94,95,96,97,98,99,100,101,102,103].

Influencing Factor	Relationship Trend with Degradation Efficiency	Core Influence Mechanism
Crystal structure	Mixed crystal (anatase: rutile = 3:1) > anatase > rutile	The anatase crystal structure is open with many active sites. The 3:1 mixed crystal form has intercrystalline synergy to accelerate the separation of electron-hole pairs.
Material physical properties	The increase in specific surface area, the decrease in particle size with the optimal range of 10 to 20 nm and the rich surface hydroxyl groups all lead to the improvement of efficiency	Specific surface area and particle size determine the number of active sites, and surface hydroxyl groups are the main source of ·OH free radicals.
Relative humidity	The efficiency decreases significantly with the increase in humidity when the humidity is higher than 40%	Under high humidity, water competes with nitrogen oxides for adsorption sites on the catalyst surface and forms a water film to prevent pollutants from contacting the active center.
Temperature	It shows a parabolic relationship, and the efficiency decreases when the temperature is lower or higher than the optimal temperature	Low temperature inhibits reaction kinetics, and high temperature destroys the adsorption balance of pollutants. Both lead to the increase in efficiency first and then decrease. The performance is more stable in warm seasons.
Ultraviolet irradiance	Positive correlation at low irradiance and negative correlation at high irradiance	At the low irradiance stage, the generation of electron-hole pairs is dominant, and the higher the irradiance, the higher the efficiency. At the high irradiance stage, irradiance causes temperature rise and humidity reduction, which in turn inhibits the generation of ·OH free radicals, forming a linear to nonlinear transformation.
Additive incorporation amount	Positive correlation when less than the optimal amount and the efficiency tends to be stable when more than the optimal amount	The number of active sites increases with the increase in incorporation amount at low amount, and the marginal benefit decreases due to particle agglomeration at high amount.
Mixture form	The efficiency of porous type is higher than that of dense type	The porous structure expands the contact area between photocatalyst and ultraviolet light and exhaust gas and promotes the diffusion of pollutants at the same time.
Carbon deposition on TiO2 surface	Negative correlation: the more carbon deposits, the lower the degradation efficiency	Carbon deposits formed by incomplete oxidation of HC during redox reaction cover the active sites of TiO2, block the contact between TiO2 and ultraviolet light, lead to catalyst poisoning and deactivation, and reduce long-term photocatalytic efficiency. The degree of deactivation is affected by reaction conditions and TiO2 material properties.
Construction process	The efficiency of dry method is significantly higher than that of wet method	The dry method can reduce the agglomeration of nano particles, improve their dispersion uniformity in the mixture and avoid the active sites being wrapped.

systematically sorts out the core influencing factors of nano-TiO₂ photocatalytic degradation efficiency and clarifies the correlation trend between each factor and efficiency.

In the actual pavement environment, the coupling effect between factors often significantly affects the degradation efficiency, among which the antagonistic effect of temperature and humidity is the most critical. The combination of the two leads to the difficulty of achieving optimal efficiency through the optimization of a single factor. The coupling of ultraviolet irradiance with temperature and humidity is more complex and there are controversial points. The root cause may lie in the difference in experimental conditions. In the laboratory environment with constant temperature and humidity, irradiation only reflects the leading role of electron-hole generation, showing a positive correlation. In outdoor experiments, the temperature and humidity fluctuations caused by irradiation cover up the direct promoting effect, leading to the decrease in efficiency under high irradiation. At the same time, the type of pollutants will also affect the results. Pollutants such as NO_x that compete with water for adsorption sites are more susceptible to the negative effects of dehumidification under high irradiation. On the whole, the influences of crystal structure, material physical properties, humidity, temperature, mixture form, construction process and additive content on degradation efficiency have formed a strong consensus, which can directly guide engineering practice, while the influence of ultraviolet irradiation needs to be judged dynamically in combination with the environment. Future research should focus on the construction of a quantitative model of multi-factor coupling, incorporate the interaction effects of temperature, humidity, and irradiance into the prediction system, avoid the actual performance deviation caused by single factor optimization, and provide accurate support for the parameter design of photocatalytic pavement in different climate zones.

5. Conclusions and Prospects

The application of nano-TiO₂ in road engineering for degrading automobile exhaust is of great significance to environmental protection and sustainable development, and conforms to the development concept of green and environmental protection roads and low-carbon cities in the new era. Due to the late start of the research on nano-TiO₂ photocatalytic degradation of automobile exhaust, the technical feasibility has been verified through laboratory research in this field at present. However, from the achievement transformation to the large-scale engineering application, there are still core bottlenecks, and targeted breakthrough directions need to be put forward as follows.

(1)

The nano-TiO₂ photocatalyst itself does not participate in the redox reaction and is not consumed with time, which is theoretically permanent. However, in the actual application process, it will be lost due to the influence of pavement wear and surrounding environmental factors, and there is no substrate-catalyst matching scheme that takes into account strong adhesion and catalytic activity, which is difficult to meet the pavement durability cycle. In the future, we can focus on developing adhesives with self-healing functions to extend the service life of catalysts. For example, microcapsule repair agents are compounded with nano-TiO₂. When microcracks appear in the coating due to abrasion, the microcapsules rupture and release the repair agents to realize the secondary bonding of the interface and reduce the annual loss rate. At the same time, explore new carriers such as porous ceramics and basalt fibers, and lock nano-TiO₂ particles through pore structure to take into account high specific surface area and wear resistance.

(2)

The compatibility between nano materials and asphalt is generally poor, and nano particles have agglomeration effect. How to effectively disperse nano materials into asphalt mixture in the production process is still a great challenge, and how to maximize the role of nano-TiO₂ in the pavement. In the future, ultrasonic assistance, silane coupling agent surface modification and other technologies can be used to reduce the particle agglomeration effect and improve the dispersion uniformity. At the same time, carry out research on the synergistic effect of multiple additives, screen anti-rutting agents, anti-aging agents and other additives compatible with nano-TiO₂, and realize the double improvement of degradation efficiency and road performance.

(3)

Most of the existing studies are based on laboratory conditions such as high concentration exhaust and constant temperature and humidity, which are far from the actual road conditions. There is also a lack of efficiency prediction models under the coupling of multiple factors such as temperature, humidity and irradiance, which cannot accurately predict the actual degradation effect. In the future, it is necessary to carry out special degradation experiments for unconventional environments such as low temperature, high humidity and high dust. At the same time, introduce machine learning methods, integrate multi-dimensional data to build a high-precision efficiency prediction model, and provide support for the parameter design of different climate zones such as the cold region in Northeast China and the high humidity region in South China.

(4)

Current research focuses on exhaust degradation efficiency, but ignores the potential impacts of the whole technology chain, such as the pollution risk of nano-TiO₂ particle shedding on soil or water bodies, and the recycling path of materials after pavement abandonment. A practical engineering technology should not only emphasize the advantages of improving the environment and prospering the economy, but also consider the relevant adverse effects from a long-term perspective. An ecological risk early warning model can be developed at the same time to ensure that the technology meets the requirements of ecological security while exerting environmental benefits, and promoting the nano-TiO₂ photocatalytic pavement from laboratory verification to large-scale engineering application.