Fe 3 + and Ce 3 + modi ﬁ ed nano - TiO 2 for degradation of exhaust gas in tunnels

: To solve the environmental pollution caused by automobile exhaust in a tunnel, this study has developed a modi ﬁ ed nano - TiO 2 based on Fe 3 + and Ce 3 + . The mod i ﬁ ed nano - TiO 2 is prepared by the sol – gel method, and the modi ﬁ cation adopts Fe 3 + single - doping, Ce 3 + sing - doping, and co - doping. The properties were also characterized by X - ray di ﬀ raction analysis, UV - vis di ﬀ use re ﬂ ectance ana -lysis, ﬂ uorescence spectroscopy analysis, speci ﬁ c surface area analysis, and paramagnetic resonance popper ana -lysis. The analyses showed that the doping of ions would change the energy band structure of nano - TiO 2 and pro duce crystal defects, thus improving the photocatalytic activity. Then, a self - fabricated exhaust gas degradation device was used to carry out the exhaust gas degradation experiments. The results showed that the modi ﬁ cation improves the catalytic e ﬃ ciency of nano - TiO 2 , and Fe 3 + , Ce 3 + co - doping > Ce 3 + single - doping > Fe 3 + single - doping > pure TiO 2 . At the dosage of 0.5%, the maximum degrada tion e ﬃ ciencies of NO and CO before compensation are 53.85% and 16.39%, respectively, and the maximum degra dation rates are 1.04 and 0.93 ppm · min − 1 . After compensa tion, the maximum degradation e ﬃ ciencies of NO and CO are 20.14% and 6.04%, respectively. The maximum degra dation rate is 0.40 and 0.41 ppm · min − 1 , respectively.


Introduction
In 2017, the number of motor vehicles in China had reached 310 million, and the total emission of motor vehicle exhaust pollutants reached 43,597,000 tons. The substantial exhaust emissions will threaten human beings' living environment if not treated in time. The automobile exhaust mainly includes CO, HC, NO x , CO 2 , and CO and easily binds to hemoglobin which will cause human hypoxia. HC and NO x can form toxic fumes through the action of ultraviolet light, and can also cause discomforts, such as dizziness, chest tightness [1], and even cancer [2][3][4]. Nano-TiO 2 has the following advantages [5][6][7]: (i) low price, simple preparation equipment, and nontoxic; (ii) strong photocatalytic performance and strong redox ability; (iii) excellent chemical stability and corrosion resistance; (iv) suitable forbidden bandwidth (3.2 eV) and can effectively absorb ultraviolet light with wavelengths less than 387 nm in sunlight, etc. Therefore, compared with other catalysts, nano-TiO 2 has obvious advantages that make it the most widely used.
In 1972, Japanese scientists Fujishima and Honda [8] discovered that TiO 2 single-crystal electrodes could decompose water, which started the research on photocatalytic materials. In 1976, Frank [9,29] proposed that semiconductor materials could be used for photocatalytic degradation of organic pollutants, which opened the door to the application of photocatalytic materials. At the end of the 20th century, Japanese scholars [30] proposed the concept of the photocatalytic highway. They used various ways to improve the photocatalytic activity of nano-TiO 2 , among which the main ones are surface noble metal deposition [31], surface photosensitization [32], composite semiconductors [33], ion doping, etc. [34]. Meng and Liu [35] sprayed a lipophilic permeate containing nano-TiO 2 into asphalt pavement. They have used scanning electron microscopy to observe the distribution of TiO 2 after penetrating into the interior of the asphalt pavement voids. The effects of humidity, light intensity, and other factors on the photocatalytic efficiency of the eco-friendly asphalt pavement were investigated. The results showed that the catalytic efficiency first increased and then slightly decreased with the increase in air humidity, and the stronger the light and the shorter the wavelength, the higher the photocatalytic efficiency. Hassan et al. [36] studied the wear resistance of concrete pavements coated with TiO 2 photocatalysts and evaluated the effect of coated photocatalysts on the wear resistance of the concrete pavements by controlling the light intensity and air humidity to simulate different test environments. They have used scanning electron microscopy and energy electron spectroscopy to determine the concentration distribution of ions after TiO 2 coating, and high degradation efficiency of 26.9% was achieved for NO x removal at 5% of TiO 2 doping concentration. Vaiano et al. [37] used the sol-gel method to prepare N-doped nano-TiO 2 and loaded them onto glass microspheres to prepare coatings with photocatalytic functions. Larumbe et al. [38] used the sol-gel method to prepare N and Fe co-doped nano-TiO 2 . They analyzed the effect of ion doping on the structure of nano-TiO 2 , magnetic properties, and photocatalytic properties. Heather et al. [39] studied the degradation effect of concrete pavement coated with TiO 2 photocatalyst on NO x . The results showed that the degradation effect was significant at an air relative humidity of 25%. Shang [10] used hot asphalt as a carrier to load photocatalytic materials so that they could adhere to the road surface or in the void, and achieve the purpose of decomposing NO x under the irradiation of visible light. Tan et al. [11] used doping to study the effect of exhaust gas degradation and investigated various road properties of doped asphalt mixes. The results showed that photocatalysts using doping hardly affect the multiple properties of asphalt mixes. Although nano-TiO 2 has many advantages as a photocatalyst, it has a wide energy bandgap. It can only use ultraviolet light in sunlight to trigger an electron leap. It does not absorb visible light, which accounts for 45% of sunlight [12,13], so the utilization of sunlight is extremely low.
In this study, for the poor ventilation conditions and low light intensity in the tunnel, a high-pressure sodium lamp with high penetrating power and high luminous efficiency is used as the light source, unlike the previous incandescent lamp. Based on the previous studies, Fe 3+ single doping, Ce 3+ single doping, and co-doping are used to modify the nano-TiO 2 to study the modification principle and effect of metal ions to achieve the adjustment of TiO 2 energy band structure, so that it can absorb and utilize visible light. X-ray diffraction (XRD) analysis shows that the doping of ions will reduce the size of nano-TiO 2 . UV-Vis diffuse reflectance analysis shows that the doping of ions causes a significant redshift in the absorption band of nano-TiO 2 . Fluorescence spectroscopy analysis shows that the doping of ions decreases the probability of electron-hole complexation. The specific surface area analysis shows that the doping of ions causes the pore size of nano-TiO 2 to become smaller and the surface area to increase. Paramagnetic resonance popper analysis shows that the doping of ions increases the number of electron-hole pairs and generates more reactive radicals. This experiment used AC-13 asphalt mixture rutting slabs to simulate tunnel pavement and then conducted tail gas degradation tests using a self-fabricated tail gas degradation device, which can simulate the tunnel environment to the maximum extent. The exhaust gas generation device is a standard exhaust gas tank, which can ensure the consistency of the initial concentration to the greatest extent, and compensate for the experimental error by using the blank group experiment. Finally, the best modifier and its optimal doping amount were determined to provide a method and basis for the subsequent photocatalyst modification research.

Experimental steps
The specific steps for the preparation of nano-TiO 2 and modified nano-TiO 2 by the sol-gel method are as follows: 1. 25 mL of anhydrous ethanol and 5 mL of glacial acetic acid was poured into a beaker at room temperature. Then, 17 mL of butyl titanate was slowly added with uniform stirring using a magnetic stirrer and stirred for 30 min to obtain a uniform and transparent light yellow solution A. 2. 20 mL of anhydrous ethanol, 5 mL of glacial acetic acid, and 10 mL of deionized water was thoroughly mixed at room temperature and a few drops of hydrochloric acid was added to control the pH of the solution between 2 and 3 to form solution B. Then, place it in a separatory funnel and set aside. Add 0.1%, 0.5%, and 1% of Fe 3+ , Ce 3+ relative to the molar fraction of Ti atoms to make modified nano-TiO 2 . 3. The solution B was slowly added (1-2 drops per second) to the solution A to obtain the sol under the action of a thermostatic magnetic stirrer at a controlled temperature of 30°C. Attention was paid so that the speed of the dropping was not be too fast to prevent the butyl titanate from hydrolyzing too fast, and the gel was obtained by aging for a while. 4. The gel was dried in a vacuum drying oven at 80°C for 24 h to get a dry gel. 5. The dry gel was crushed in a mortar and then placed in a muffle furnace and calcined at 500°C for 2 h to obtain nano-TiO 2 . Nano-TiO 2 sols and nano-TiO 2 particles are shown in Figure 1.
The yield of nano-TiO 2 in the theoretical case is 4 g. The orthogonal experiments of nano-TiO 2 obtained under different conditions are given in Table 2.
The dissolution speed of butyl titanate is too fast and white flocculent is easily formed, while the dissolution speed is too slow and it is not easy to form a sol. Generally, the sol contains many tiny bubbles after it is made, and a series of chemical reactions will occur subsequently. Hence, it needs to be placed for a while to make the butyl titanate fully hydrolyzed. Calcination temperature and calcination time mainly affect the crystalline structure. When the temperature is 400°C, the crystalline form is mostly anatase phase, but the crystallization is not complete; when the temperature is 500°C, the crystalline form is completely anatase phase; when temperature increases, the material will be transformed from anatase phase to rutile phase, and the photocatalytic activity is reduced. With close to theoretical yield as the standard, the dissolution time of butyl titanate should be selected for 30 min, gel placed for 10 days, and calcination for 2 h at 500°C.

Photocatalytic principle
As a semiconductor material, the electronic energy level of nano-TiO 2 is discontinuous, in which there are empty energy level regions, so there are both full and empty bands. The bottom full band is called the valence band, the top empty band is called the conduction band, and the region from the top of the valence band to the bottom of the conduction band is called the forbidden band or bandgap. When the photoelectrons' energy is greater than or equal to the width of the forbidden band, the electrons in the valence band will be excited by light and jump to the conduction band, while the electrons in the valence band will generate holes due to the jump, forming electron-hole pairs. The electron-hole complex pair will be compounded under the action of an electric field and thus lose its activity. The electron-hole complex pair can react with H 2 O and O 2 , which are on the surface of nano-TiO 2 . (1) It can be seen from the above reactions that the electron-hole complex pair eventually generates superoxide ion radicals and hydroxyl radicals, which can oxidize CO and NO in the air into CO 2 , various carbonates, nitrates, etc., due to the strongly oxidizing properties of these radicals, so as to achieve the purpose of degrading the car exhaust to cure the environment, and the specific catalytic principle is shown in Figure 2 [18].

Test systems
The P750 gas catalytic reaction device, which was developed by the Cusson Company in the United States, calculates the catalytic conversion efficiency by testing the change in the gas concentration (mainly refers to the harmful gas of automobile exhaust gas) flowing through the reactor. However, the equipment could not perform to its full efficiency when it is applied to test the catalytic decomposition performance of asphalt mixture on automobile exhaust gas, and the reason is that the equipment cannot provide the light conditions needed for the photocatalytic reaction [20]. The auto-exhaust reaction test equipment was developed independently by Tohoku Forestry University [21], which contains an automobile exhaust gas supply system, computer-aided test system, gas chamber, and other multi-functional components and structures, with a variety of sensors (such as oxygen sensor, carbon dioxide sensor, hydrocarbon sensor, carbon monoxide sensor, pressure box, temperature sensor, etc.). However, due to the instability of the sensor, the device has the disadvantages of unstable reading and short service life. In this study, the actual situation in the tunnel is combined with the disadvantages of poor lighting conditions and poor ventilation in the tunnel, so a closed  chamber is used to simulate the tunnel, and the light is a high-pressure sodium lamp, which is commonly used in such low-light conditions in the tunnel due to its high luminous efficiency, low power consumption, long life, and high penetration [22]. The test device consists explicitly of three parts: a standard exhaust gas tank, a closed reaction chamber, and an exhaust gas analyzer, as shown in Figure 3.

Photocatalytic material specimen preparation
The coating type in this study uses a water coating solution. In the process of preparing the coating solution, it is easy to precipitate in the process of preparation as the specific gravity of nano-TiO 2 is 4.26, and nano-TiO 2 is easy to disperse unevenly in water and produce agglomeration. Therefore, during the preparation, a certain amount of dispersing stabilizer needs to be added, and the addition amount is 10-20% of the mass of nano-TiO 2 . In the coating solution, water:nano-TiO 2 :dispersion stabilizer = 1%:2%:0.4%. The specific laboratory preparation steps are as follows: (1) Weigh a certain amount of water, dispersant, and stabilizer. Add the dispersant and stabilizer to the aqueous solution, and shear at 500 rpm for 5 min with a high-speed shearer.
(2) Add nano-TiO 2 to the above solution, and the water-based nano-TiO 2 coating solution can be produced by shearing at a high speed of 2,000 rpm for 30 min with a high-speed shearer [23,24].
In this study, relying on the actual needs of the project, the AC-13 asphalt mixture is used, fine aggregate is limestone, coarse aggregate is basalt, oil to stone ratio is 5.0%, 70# matrix asphalt is used, and the gradation design is as shown in Table 3.
The nano-TiO 2 coating solution and the specimens covered with photocatalytic materials are shown in

Evaluation indicators
Among the evaluation indices currently used to evaluate the performance of photocatalytic decomposition of vehicle exhaust, the cumulative decomposition rate (i.e., decomposition efficiency) is the most common evaluation index [25,26]. The cumulative decomposition rate is calculated in Eq. 7:  where ηcumulative decomposition rate, %; M 0initial concentration, ppm; and M 1 -final concentration, ppm. The cumulative decomposition rate is one of the essential evaluation indicators for the reaction photocatalytic decomposition of automobile exhaust gas, as well as a quantitative indicator, but there are shortcomings that the indicator requires a fixed initial gas concentration of the test as well as a fixed closed reaction chamber.
In the process of photocatalytic degradation of automobile exhaust, the rate of gas concentration reduction is not always constant, which means that the gas concentration is not linearly related to time, so the decomposition rate is different at different periods. The average decomposition rate is an important index to react to different decomposition rates at different periods. The average decomposition rate is calculated in Eq. 8 [27]: where vaverage decomposition rate, ppm·min −1 ; and treaction time, min. In this study, the average decomposition rate is differentiated based on the average decomposition rate, i.e., it is more time-sensitive. The concentration-time relationship graph is made with time as the horizontal coordinate and gas concentration as the vertical coordinate, which was then fitted by Origin to obtain the exhaust gas degradation curve, as shown in Figure 12. The slope of this curve indicates the decomposition rate.

Results and discussion
Nanomaterials are materials with at least one dimension in the nanoscale (1-100 nm) at the three-dimensional spatial scale, which is a typical mesoscopic system in the transition region at the junction of atomic clusters and macroscopic objects, neither microscopic nor macroscopic, with a series of specific physicochemical properties [19].

XRD
XRD uses the position and intensity of XRD diffraction angles to analyze the composition of minerals. It explores the phases of the substance based on the spacing of each crystal plane and the relative intensity of diffraction lines against PDF cards. By comparing the diffraction peaks of unknown substances with those of known substances, the various phases in the sample are derived. It can be  The determination of XRD crystal size is related to the peak width of the diffraction peak and the material grain size, which is calculated in Eq. 9: where Dgrain size, nm; K -Scherrer constant, generally taken as 0.89; λwavelength of the rays, nm; Bhalfpeak width of the diffraction peak, rad; and θ -Bragg diffraction angle,°. This study used a DB-type X-ray diffractometer with the following technical parameters: Cu target K-α radiation (λ = 1.5406 nm), operating voltage of 40 kV, operating current of 150 mA, scan range of 5-90°, and scan rate of 10°·min −1 . The calculation results of the relevant lattice parameters for different ion doping are displayed in Table 4.
As can be seen from Figure 5, the doping of Fe 3+ and Ce 3+ did not cause fundamental changes in the positions of the diffraction peaks of nano-TiO 2 , which indicates that the modified nano-TiO 2 is still dominated by anatase. With the doping of ions, the diffraction peaks show specific regular changes. From Table 4, it can be concluded that the average grain size of Ce 3+ , Fe 3+ co-doping is significantly smaller than that of pure TiO 2 . The ionic radius of Fe 3+ is 0.064 nm and that of Ti 4+ is 0.068 nm, and the difference between the ionic radii is not large, indicating that part of Fe 3+ will replace Ti 4+ and exist in the crystal lattice during the doping modification process. The entry of Fe 3+ into the nano-TiO 2 lattice will destroy the periodicity of the crystal plasmon arrangement, thus inducing crystal distortion and inhibiting the growth of TiO 2 crystals. The ionic radius of Ce 3+ is 0.102 nm, which is much larger than that of Ti 4+ , so the possibility of Ce 3+ entering the TiO 2 lattice to replace Ti 4+ is negligible, but it may exist at the TiO 2 grain boundary, i.e., the interface between two grains or microcrystals. On the one hand, it induces crystal distortion and form lattice defects. On the other hand, it will increase the diffusion energy barrier of atoms between grains and prevent direct contact between grains. Both of them will inhibit lattice growth, form lattice defects to trap photogenerated electrons, and reduce the probability of electron-hole complexation, thus improving photocatalytic activity.

UV-visible (UV-Vis) spectroscopy analysis
UV-Vis diffuse reflectance spectroscopy is the UV-Vis spectrum generated by using the degree of absorption of UV or visible light by the sample. This study used a UV 3600 Plus UV-Vis spectrophotometer with a scanning range of 200-900 nm. The wavelengths of various kinds of light are composed of ultraviolet wavelength (below 400 nm), visible wavelength (400-760 nm), and infrared wavelength (above 760 nm). It can be seen from Figure 6 that the absorbance of nano-TiO 2 decreases with the increase in the wavelength of light, which decreases dramatically between the wavelengths of 350-450 nm. In this interval, the rate of decline first increases and then decreases, that is, there is an inflection point, making a tangent line at the point. The intersection of the tangent line and the horizontal coordinate can roughly represent the maximum value of the absorbance wavelength. The tangent line drawn for each spectrum in Figure 6 concludes that the doping of ions will increase the wavelength of TiO 2 response to visible light so that its absorption range is  expanded from ultraviolet light to visible light. The co-doping effect is better than Ce 3+ single doping and Fe 3+ single doping because the doping of ions inhibits the growth of TiO 2 nanocrystals and generates lattice defects, so that electron leap can occur even at low energy. The redshift of the sample's absorbed optical band is beneficial to excite the photocatalytic activity and form an electron-hole, thus improving the utilization of visible light.
To further investigate the effect of different ion doping on the forbidden bandwidth of nano-TiO 2 , the optical band gap energy value of TiO 2 can be calculated after the Kubelka-Munk transformation of the UV-Vis diffuse absorption spectrum of TiO 2 . The specific expression of the change is given by Eq. 10: where R ∞reflectance; sscattering coefficient; and kmolar absorption coefficient. The relationship between the forbidden bandwidth of semiconductor materials and the wavelength of light absorption is given by Eq. 11: where E gbandwidth and λ glight absorption threshold. After the K-M transformation of the UV-Vis diffuse reflectance absorption spectrum, the tangent line intersects with the horizontal axis and then the K-M transformation curve graph was obtained. It can be seen from Figure 7 that the absorption sidebands of the samples gradually move from the shortwave direction (390 nm) to the longwave direction (520 nm) with the doping of different ions, and the forbidden bandwidth decreases from 3.22 to 2.71 eV. This is related to the lattice distortion produced by Fe 3+ and Ce 3+ , which causes defect traps on the TiO 2 surface and achieves the purpose of trapping photogenerated electron pairs. And according to the semiconductor energy band theory, the low-valent Fe 3+ replaces Ti 4+ in the lattice and forms a new d-energy band in the energy band gap of TiO 2 . The d-energy band creates separated impurity energy levels in the forbidden band, leading to a decrease in the position of the bottom of the TiO 2 conduction band. The position of the TiO 2 conduction band bottom decreases so that the less energetic photons are excited to the impurity energy levels to trap electrons. This is consistent with the findings that ion doping can cause the UV-Vis absorption edge of TiO 2 to be redshifted.

PL analysis
Photoluminescence (PL) reflects the separation of electrons and holes. In general, the lower the intensity, the more efficient the separation of electrons and holes and the lower the probability of compounding. In this experiment, an FL4500 type fluorescence photometer was used for characterization and analysis. The PL spectra in the range of 375-525 nm were measured at an excitation wavelength of 365 nm, and as shown in Figure 8, the intensities  Reducing gas exhaust in tunnels  635 of modified nano-TiO 2 were all lower than those of pure nano-TiO 2 , indicating that the doping of modified ions induced crystal distortion and formed lattice defects, which produced traps for trapping photogenerated electrons and reduced the probability of electron-hole complexation. Among them, Fe 3+ and Ce 3+ co-doping has a lower electron-hole complex probability, indicating that the co-doping of two metal ions is more favorable to improve the catalytic efficiency of nano-TiO 2 .

BET analysis
In chemical materials research, the biggest advantage of nanoscale photocatalytic materials over other macroscopic photocatalytic materials is that nanomaterials have a huge surface area and surface structure advantages. The specific surface area test referred to as the BET test method, has become the standard for qualitative analysis of adsorption properties of materials and microscopic particles in the field of materials research at home and abroad. Before the BET test analysis of composite nano photocatalytic materials, the sample needs to be subjected to nitrogen adsorption-desorption isothermal test and then the data are fitted and analyzed according to the BET formula, and the specific surface area and pore size of nano-TiO 2 and modified nano-TiO 2 are derived from the distribution data. As shown in Figure 9a, the modified nano-TiO 2 changed the linear type from type II to type IV compared with the pure TiO 2 . It can be observed that the inflection point of modified nano-TiO 2 occurs between 0.55 and 0.75 relative pressure. After the inflection point, the rise rate decreases, and both show the absorption-desorption hysteresis phenomenon, indicating that the modified nano-TiO 2 belongs to the mesoporous material structure. This hysteresis phenomenon is caused by the presence of a large number of capillary pores on the surface, indicating that the doping of ions makes the catalyst surface more rough and porous with increased capillary structures. Figure 9b also reflects that the average pore size of modified nano-TiO 2 is smaller than that of pure TiO 2 and the specific surface area is larger than that of pure TiO 2 , which further confirms the XRD analysis.

Electron spin resonance (ESR) analysis
In this thesis, ESR was used to monitor the formation of • OH. As shown in Figure 10, the area ratio of the four characteristic peaks was 1:2:2:1 under the high-pressure sodium lamp irradiation for 30 min, which belonged to the typical characteristic signal peaks of • OH, and all samples had obvious • OH signal peaks. The ESR signals of modified nano-TiO 2 are stronger than those of pure TiO 2 , which should be related to the number of separated electron-hole pairs. In general, more reactive radicals are beneficial to photocatalytic performance, which provides the possibility to subsequently enhance the tail gas degradation efficiency.

Exhaust degradation test
The source of exhaust gas for the test was a standard exhaust gas tank, and the composition ratio of the gas mixture was NO (40,560 ppm), CO (9.96%), CO 2 (5%), and N 2 . The volume of the exhaust gas tank is 8 L, the length, width, and height of the sealed chamber are 60 cm, converted to a volume of 216 L. Using a microflow meter to control the volume of gas passing each time, the volume of gas mixture passed in the test is 0.5 L, and the initial gas concentration control range is shown in Table 5.
As can be seen from Table 5, the actual concentrations of CO and NO are less than the theoretical values, but the concentration of CO 2 is greater than the theoretical value. The reason for this is that in the process of the mixed gas entering the pipe, the connection between the pipes is not completely dense and part of the gas is left in the pipe, resulting in the actual concentration being less than the theoretical concentration. The increase in CO 2 concentration is due to the fact that the closed chamber is not a vacuum and contains air, which contains CO 2 in its composition, and when this content is greater than the loss value, it causes an increase in CO 2 content.
Before starting the test, the tightness of the reaction chamber should be checked to prevent large test errors. The airtightness was checked by not putting any specimen in the reaction chamber and then introducing 0.5 L of the gas mixture. The main objects of this test are CO and NO, and the concentration changes in CO and NO are tested within 60 min. To avoid the error brought by one group of tests, two groups of each test are done in this study, and the specific test results are shown in Figure 11.
As can be seen from Figure 11, the concentration of CO slightly decreases with the growth of time, while the concentration of NO significantly reduces with the growth of time. The reason is that part of NO has an oxidation reaction with O 2 in the air, so the concentration of CO slightly decreases and can be concluded that the airtightness of the confinement chamber is good and meets the requirements of the test.
After the airtightness check, debug and preheat the exhaust gas analyzer, then put the photocatalytic specimen into the sealed chamber, close the sealing door, start the exhaust gas analyzer, and open the pressure reducing valve of the exhaust gas tank after the analyzer indicates a stable number. The volume of the gas mixture    is controlled by the micro-flow meter, and the rate chosen by the micro-flow meter is 2 L·min −1 , i.e., the pressure reducing valve is closed after 15 s. Turn on the small fan and wait for 5 min to make the mixture evenly fill the sealed chamber, then turn on the high-pressure sodium lamp, set the parameters of the exhaust gas analyzer for a total duration of 60 min, because the exhaust gas analyzer is equipped with a data export function, the data export interval is set to 6 min per time. At the end of the test, turn off the fan, open the sealed door, and take out the test specimen after the exhaust gas is exhausted from the test system to prepare for the next test.
In the exhaust gas degradation test, the surface of the AC-13 asphalt mixture was pretreated first. Then, the prepared water-based nano-TiO 2 coating solution was sprayed onto the rutting plate at a dosage of 556 mL·m −2 , and it could be used in the exhaust gas degradation test after the water evaporated. A total of 20 groups were tested. The specific experimental data are shown in Table 6.
Because the airtightness of the closed chamber body is not completely sealed as well as the reason that NO will be naturally oxidized in the air, a blank test group is needed to make the correction comparison. The specific compensation method is: the initial concentration of the blank group is C 0 , the concentration of the blank group is C 1 , and the concentration of the test group is C 2 , then the actual concentration of the test group after degradation is given by Eq. 12: After compensating the data of each group, the average value of two groups A and B was taken as the final test data, as shown in Figure 12.
It can be noted that the doping of ions will improve the degradation efficiency of the exhaust gas of nano-TiO 2 under high-pressure sodium lamp irradiation, and the degradation effect of co-doping is better than cerium ion single doping, better than iron ion single doping better than pure TiO 2 , and the degradation efficiencies are shown in Tables 7 and 8.
The concentration curve of exhaust gas degradation was obtained by fitting with Origin software. The maximum value of the slope of the curve was found to represent the maximum value of the degradation rate of exhaust gas, and the categories from left to right in Table 8 were represented by S 0 -S 10 . The exhaust gas degradation rates of NO and CO are shown in Figure 13.
The maximum values of the degradation rates of NO and CO were obtained as shown in Tables 9 and 10. It can be noted from exhaust gas degradation test that the degradation effect of Ce 3+ , Fe 3+ co-doping is better than Ce 3+ single doping, Fe 3+ single doping and pure TiO 2 , where the maximum degradation efficiency and maximum degradation rate are Ce 3+ , Fe 3+ co-doping, and the doping amount is 0.5%. This is because in the process of doping, Fe 3+ with a small ion radius replaces part of Ti 4+ and thus changes the crystal structure, while Ce 3+ with a large ion radius exists at the grain boundary position and increases the diffusion energy barrier between the grains, both of which can induce lattice distortion, form lattice defects, reduce the probability of electron-hole complexation, improve photocatalytic activity, and thus improve the exhaust gas degradation efficiency. But too much ion doping will cause the remaining ions to accumulate on the particle surface of the catalyst, resulting in a reduced contact area between nano-TiO 2 and visible light, thus decreasing the catalytic activity.

Conclusion
In conclusion, the efficiency of exhaust gas degradation efficiency of nano-TiO 2 with different ion doping for use in tunnels is investigated. The two ions differ in the principle of increasing photocatalytic activity: Fe 3+ is in the form of substituted Ti 4+ , while Ce 3+ is present at the grain boundary sites, increasing the diffusion energy barrier. The doping of different ions will improve the degradation efficiency of nano-TiO 2 exhaust gas under a high-pressure sodium lamp, and the degradation efficiency is: Fe 3+ , Ce 3+ co-doping > Ce 3+ single doping > Fe 3+ single doping > pure TiO 2 . The maximum degradation efficiency and maximum degradation rate were of Fe 3+ , Ce 3+ codoped, and the doping amount was 0.5%. The maximum degradation efficiencies of NO and CO before compensation were 53.85% and 16.39%, respectively, and the maximum degradation rates were 1.04 and 0.93 ppm·min −1 . The maximum degradation efficiencies of NO and CO after compensation were 20.14% and 6.04%, with top degradation rates of 0.40 and 0.41 ppm·min −1 .
The use of the in-tunnel exhaust gas degradation materials prepared in this study for practical engineering purposes requires further research. In this study, an aqueous solution coating is used, and further research is needed on how to make a highly durable and waterproof coating subsequently. The optimal amount of co-doping is 0.5%, and the molar fraction ratio of 1:1 is used for co-doping. The specific molar fraction ratio of the two to achieve the best photocatalytic performance of TiO 2 nanomaterials remains to be investigated.  Figure 13: Exhaust gas degradation rate of NO and CO.
Reducing gas exhaust in tunnels  641 Data availability statement: All data generated or analyzed during this study are also available from the corresponding author on reasonable request.