Evaluation of Reducing NO and SO2 Concentration in Nano SiO2-TiO2 Photocatalytic Concrete Blocks

The use of titanium dioxide in concrete block pavements is a promising approach to reduce air pollution in the roadside. When TiO2 is used as an additive of cement concrete or mortar, it is not dispersed uniformly due to agglomeration between particles causing the degradation of photocatalytic reaction. To improve the photocatalytic performance of TiO2, the Nano SiO2-TiO2 (NST) has been developed by coating TiO2 with SiO2 as a support using the sol-gel method. The environmental performance of concrete blocks incorporating NST as an additive was evaluated using both laboratory and full-scale chamber experiments. It was observed from laboratory environment chamber testing that the NO reduction efficiency of concrete blocks with 4% NST ranged from 16.5 to 59.1%, depending on the UV intensity. Results of the full-scale chamber test on NST concrete blocks indicated that the NO and SO2 reduction efficiencies were 22.3% and 14.4% at a 564 W/m2 of solar radiation, respectively. It was found that the increase in UV intensity and solar radiation had a positive effect on decreasing NO and SO2 concentration. In the future, the NST will be applied at in-service photocatalytic block pavements to validate the environmental performance in field conditions.


Introduction
With economic development led by rapid industrial growth, the air pollution has been steadily increasing, due to soot and sulfur oxides from factories and vehicle exhaust emissions, and it has become a rising common interest around the world [1,2]. As for the primary air pollutant NO x , road and non-road mobile pollutant sources account for the most proportion at 59%. NO x emitted from road mobile sources such as vehicles and stationary sources including industrial factories power generation facilities is a harmful air pollutant; NO emitted from vehicles is especially a serious problem in metropolitan areas [3][4][5]. In this regard, it is necessary to develop measures to reduce the NO x , including NO and NO 2 , from on-road mobile sources for the entire society.
The titanium dioxide (TiO 2 ), one of the photocatalytic compounds, has various strengths, including physical and chemical stability, excellent acid resistance, alkali resistance, UV protection, dispersibility, durability, and high reactivity and activity [6]. TiO 2 absorbs ultraviolet rays with a wavelength of 400 nm or less and is separated into highenergy electrons (e − ) and holes (h + ), which react with surface adsorbed oxygen and water to form active species, such as superoxide anions (O 2 − ) and hydroxyl radicals (OH − ), respectively. Due to the strong oxidizing power of the active species, TiO 2 can decompose air pollutants, have a sterilization effect, and remove and absorb harmful gases [7]. It is expected that concrete products with excellent performance can be produced by mixing TiO 2 with mortar and concrete. When it comes to road structures, it will be very effective in preventing air pollution, as TiO 2 directly adsorbs and removes harmful gases emitted from vehicles. Its very large, specific surface area will maximize photocatalytic efficiency [8][9][10][11]. When the concrete material with TiO 2 is applied to the road facilities, it will be very effective in reducing air pollution by directly absorbing and removing the harmful gases from vehicle emissions [12,13]. For this reason, Japan, Hong Kong, and some European countries use TiO 2 for road structures and concrete blocks in removing air pollutants. In Italy, when photocatalytic (TiO 2 ) cement was applied to the concrete block pavement of Borgo Palazzo Street in Bergamo, air pollution in the area was reduced by 30-40%. In Belgium, TiO 2 block pavement was constructed over an area of 10,000 m 2 in the Antwerp area, and air pollution decreased by about 20% one year after the construction. In Japan, TiO 2 concrete block pavement was applied on an area of 50,000 m 2 in Osaka, Chiba, Chigasaki, and Saitama-Shintoshin. As a result of the study, it was found that the photocatalytic pavements can remove 15% of NO x emitted from vehicles in motion and they have a higher NO x decomposition effect than roadside trees [12,14]. The use of higher content TiO 2 in concrete material can interrupt the hydration reaction of cement, thus degrading the concrete's strength [4]. When the TiO 2 particles inside the concrete is not exposed to light sources or exhaust gases, it is difficult to trigger a photocatalytic reaction [15,16]. In particular, TiO 2 of nano size with a large specific surface area was not dispersed due to agglomeration between particles, but rather was present as aggregates on the surface of the concrete block, thereby reducing the photocatalytic reaction [17].
In recent years, the nanotechnology has been dramatically developed through the continuous improvements in the production and characterization of solid materials [18][19][20][21][22]. The performance of solid materials is greatly affected by its surface and electronic properties when size is reduced to the nanoscale [23][24][25][26]. In particular, the aggregates are easily formed since nanomaterials have a large specific surface area and great unsaturated bonds when its size gets smaller [27,28]. Therefore, nano-sized TiO 2 materials could not produce a sufficient effect with mortar and concrete as it exists as aggregates.
In order to improve the aggregation of nano-sized TiO 2 in the binder, the nanosized TiO 2 interface should be deformed. Coating SiO 2 as a support can create Ti-O-Si bonds on the nano TiO 2 interface [29,30]. This type of chemical bonding can bring more negative charges to the surface of TiO 2 , which helps dispersion in water-based bonds on the electrostatic repulsion reaction. Accordingly, the dispersing force in the binder can be improved by decreasing the reduction of agglomerates. In addition, adding SiO 2 as a support to cement is expected to create a pozzolanic reaction, improving concrete properties [31][32][33][34][35].
In order to solve the problems mentioned above, the SiO 2 -TiO 2 material has been developed by coating TiO 2 with SiO 2 as a support using the sol-gel method. The developed Nano SiO 2 -TiO 2 material was used to fabricate the concrete blocks. The environmental performance of the concrete blocks was evaluated based on the ability to remove air pollutants using laboratory and full-scale environmental chamber experiments.

Background Photocatalyst
The utilization of TiO 2 on pavement and road facilities has a significant effect on reducing NO, a primary source of fine particulate matter [36][37][38][39][40]. NO is considered the primary pollutant, which is mainly introduced into the atmosphere directly from hightemperature combustion in transport and industrial activities, whereas NO 2 is considered a secondary pollutant, since it is mostly formed in the atmosphere due to the interaction between NO with O 2 or O 3 and/or sunlight. Photocatalysts are capable of decomposing various numbers of oxides and organic compound pollutants that cause health and environmental problems. The governing decomposition mechanism involves the generation of radicals due to the irradiation to the photocatalyst substance, and subsequently, converting pollutants into harmless compounds [36].
The photocatalytic process of TiO 2 is described in nine reactions [41]. This process begins with irradiating UV light to the TiO 2 . When the TiO 2 absorbs an equal or higher energy than the band gap, an electron is transferred from the valence band to the conduction band. The band gap energy of TiO 2 in its anatase phase, which is mainly used as a photocatalyst, is 3.2 eV. (Equation (1)).
The mechanism by which O 2 − and OH − produced by the above reaction react with NO x and are removed is as follows.
According to Equations (5) and (6), OH· produced by the photocatalytic reaction reacts with NO x , NO, NO 2 , etc. to finally produce HNO 3 . The HNO 3 is water-soluble and can be easily removed from the photocatalyst surface by an external environment, such as rain. The O 2 − generates HO 2 · by the reaction of Equation (4), and HO 2 reacts with NO according to Equations (7) and (8) to finally produce HNO 3 .
In addition, in the NO x environment, as shown in Equation (9), NO x molecules react with O 2 − to generate NO 3 − , which effectively affects the removal of nitrogen oxides [42]. Similarly, purification of SO 2 is as follows: As shown in Equations (10)-(12), sulfuric acid (H 2 SO 4 ) production from SO 2 oxidation proceeds through a series of radical reactions. The HSO 3 radical then rapidly reacts with molecular oxygen (O 2 ) to yield SO3. SO 3 reacts with atmospheric moisture (H 2 O) to form H 2 SO 4 . The finally produced H 2 SO 4 can be easily removed from the photocatalytic surface by external environments, such as rain. In order to improve the photocatalysis efficiency of TiO 2 , the TiO 2 material had been developed as an anatase crystal phase with a larger specific surface area and excellent photodissociation. Various attempts had been made, including sputtering, which was a process in which TiO 2 was directly and physically coated on a proper solid support.

Materials and Experiment Method
Although the specific surface area of TiO 2 was large, it was difficult to achieve a proper photocatalytic effect due to aggregation of nano-sized particles in the mixture with a binder. To solve these problems, the Nano SiO 2 -TiO 2 (NST) had been developed using the Sol-gel method in this study.
TEOS and EtOH were stirred at a molar ratio of 1:1, at 600 rpm or higher, for one hour. 2.
HNO 3 and distilled water were stirred at a molar ratio of 1:150, at 600 rpm or higher, for one hour. 3.
The solutions prepared in steps 1 and 2 were stirred at 600 rpm or higher for five hours.

4.
TIP and IPA were stirred at a molar ratio of 1:1 at 600 rpm or higher for one hour.

5.
The solutions prepared in steps 3 and 4 were mixed and stirred at 800 rpm for 24 h. 6.
The mixture prepared in step 5 was refluxed at 80 • C for six hours. 7.
The prepared NST slurry was washed and filtered for neutralization. 8.
The NST slurry was dried at 80 • C for 48 h. 9.
Heat treatment was applied to the dried NST at 450 • C for six hours. 10. The heat-treated NST was ground.  The main performance of NST developed according to the existing research results were as follows [43]: Figures 2-4 presented the results of the SEM (Akishima, Tokyo, Japan), XRD (Bruker-AXS, Shibuya, Tokyo, Japan), and UV-Vis (Gangnam, Seoul, Korea) analysis for NST with optimum mix proportion. It was found from Figure 2 that there was only the peak of anatase phase with excellent photocatalysis. However, the peaks of the rutile and brookite phases were not observed, because SiO 2 with relatively better thermal properties interrupted the phase transition during the heat treatment process. Results of the SEM analysis in Figure 3 showed no aggregation of NST or single phases of TiO 2 or SiO 2 .
As shown in Figure 4, it was well known that TiO 2 was activated in the wavelength range below 380 nm to act as a photocatalyst. The UV/Vis spectrophotometer testing was conducted to analyze the absorption spectra of NST. Test results showed that UV absorption peak of NST was found in the ultraviolet range below 380 nm, with much higher absorption than general TiO 2 .

Cement
The cement used for this study was ordinary Portland cement (OPC) (Hanil, Seoul, Korea), which had a density of 3.14 g/cm 3 and Blaine fineness of 3,492 cm 2 /g. The physical and chemical properties of OPC were as shown in Table 2.

Silica Sand
The fine aggregate used in this study, silica sand (Hanil-Eco, Gongju, Korea), was used instead of general natural fine aggregate, and the physical properties of silica sand were shown in Table 3. The coarse aggregate used in this study was crushed stone (Hanil-Eco, Gongju, Korea). The grain size of the aggregate was 5 to 8 mm, and the physical properties of the coarse aggregate were shown in Table 4.

Production of Concrete Block
As a result of testing the mortar mixed with NST, the flow rate of the mortar decreased, and the strength increased as the NST replacement rate increased. The increase in the strength could be attributed to the pozzolan reaction of SiO 2 , the nucleation and filling effects of NST, and the formation of pores smaller than 50 nm within the mortar matrix. Accordingly, 4% was found to be the optimal amount of NST to mix in order to obtain the target strength and flow value without using excessive chemical admixture. Table 5 showed the mix proportions of the concrete blocks prepared in this study. The concrete block consisted of surface and concrete layers. As for the surface layer of the concrete block, which was 8 mm high, 4% of NST was applied to the mixture. For the 52 mm concrete layer, 5-8 mm aggregates were used. As shown in Figure 5, they were made into a standard square of 200 × 200 × 60 mm using equipment for manufacturing concrete blocks.

Laboratory Experiment
A laboratory experiment was conducted to assess the capacity of the concrete block with NST to remove the NO x pollutants. The laboratory testing device (Ecotech, Knoxfield, Australia) with environment chamber was manufactured to evaluate the photocatalytic efficiency of concrete block specimens. The test setup was adapted from ISO 22197-1 (2007): "Test method for air-purification performance of semiconducting photocatalytic materials-Part1: Removal of Nitric Oxide" and UNI 11247: 2010: "Determination of the degradation of nitrogen oxides in the air by inorganic photocatalytic materials: continuous flow test method". The developed experimental setup consisted of a pollutant source (gas cylinder of NO), zero air source, adjustable valves, humidifier, calibrator, photoreactor, and chemiluminescent NO x analyzer, as shown in Figures 6 and 7.
The calibrator controlled the NO x concentration flowing into the environment chamber, and a pressure gauge and a valve were in place to control NO x inflow. A T-shaped connector before the photoreactor was linked to the NO x analyzer to measure the concentration in the inlet. A gas mixture of NO x and zero air filled the photoreactor at the controlled humidity, flow, and NO x concentration.
The chamber's photoreactor (500 × 500 × 500 mm) was fully sealed to maintain the controlled environment. All tests were performed at a temperature of 25 ± 2 • C and a humidity level of 40 ± 5%. UV light was placed above the photoreactor to simulate for photocatalysis.
The concrete block was placed in the center of the photoreactor, and the concentration of NO x was set to be 1.00 ppm by adding a gas mixture of NO x and zero air before the testing. The light source of the UV lamp was irradiated under three conditions of 10, 20, and 30 W/m 2 for more than five hours to measure changes in the NO x concentration at one-minute intervals.

Full-Scale Environment Chamber Experiment
The full-scale environment chamber (KICT, Yeoncheon, Korea) had been constructed to evaluate the air pollutant reduction technologies on the roadside at the Korea Institute of Civil Engineering and Building Technology (KICT) near Yeoncheon, Korea ( Figure 8). The chamber was a tunnel in shape, with a width of 11.4 m, a length of 22.8 m, and a height of 6.0 m. The total volume of this chamber was 1000 m 3 . This chamber was fabricated from transparent ETFE (Ethylene Tetrafluoroethylene) film to enable natural sunlight and ambient temperatures to govern the photochemical reactions occurring inside the chamber. The ETFE film was selected as the membrane material for the full-scale chamber structure because of its lightweight, high tensile strength, good durability, and high solar transmittance [44,45]. A previous study proved that ETFE films could supply sufficient natural sunlight for the photochemical reaction in the environment chambers, at an average of approximately 89% natural sunlight transmission at 300-1000 nm [46].
The environment chamber was divided into reference and testing chambers. The membrane layer was installed in the middle of chamber for perfectly blocking the transportation of aerosol and gas materials between the two chambers. The test data measured in the reference chamber was used to determine the wall loss rates and leakage rates of gaseous reactants and particles. It was applied to calibrate the measurement data in the testing chamber for the analysis. The temperature and humidity control systems were implemented at each chamber to maintain the designed environment condition. A total of 16 sensors at different heights were installed to monitor the temperature and relative humidity data inside the chamber during the testing.
The testing was conducted to determine NO/SO2 reduction

Full-Scale Environment Chamber Experiment
The full-scale environment chamber (KICT, Yeoncheon, Korea) had been constructed to evaluate the air pollutant reduction technologies on the roadside at the Korea Institute of Civil Engineering and Building Technology (KICT) near Yeoncheon, Korea (Figure 8). The chamber was a tunnel in shape, with a width of 11.4 m, a length of 22.8 m, and a height of 6.0 m. The total volume of this chamber was 1000 m 3 . This chamber was fabricated from transparent ETFE (Ethylene Tetrafluoroethylene) film to enable natural sunlight and ambient temperatures to govern the photochemical reactions occurring inside the chamber. The ETFE film was selected as the membrane material for the full-scale chamber structure because of its lightweight, high tensile strength, good durability, and high solar transmittance [44,45]. A previous study proved that ETFE films could supply sufficient natural sunlight for the photochemical reaction in the environment chambers, at an average of approximately 89% natural sunlight transmission at 300-1000 nm [46].
The environment chamber was divided into reference and testing chambers. The membrane layer was installed in the middle of chamber for perfectly blocking the transportation of aerosol and gas materials between the two chambers. The test data measured in the reference chamber was used to determine the wall loss rates and leakage rates of gaseous reactants and particles. It was applied to calibrate the measurement data in the testing chamber for the analysis. The temperature and humidity control systems were implemented at each chamber to maintain the designed environment condition. A total of 16 sensors at different heights were installed to monitor the temperature and relative humidity data inside the chamber during the testing.
The testing was conducted to determine NO/SO 2 reduction efficiency of concrete blocks with TiO 2 application using a full-scale environment chamber. The chamber was purged with rural background air prior to the experiment. The diesel and gasoline exhaust gas were generated from a 2010 Hyundai Star Rex (Hyundai, Ulsan, Korea) and 2019 Sonata engine (Hyundai, Ulsan, Korea), which were run under idling conditions. An air fan was used to uniformly diffuse the exhaust gas inside the chamber within a short period of time.
A total of 50 concrete blocks were placed at the center of the testing chamber with 4m 2 of coverage area. The concrete blocks were exposed to natural sunlight for photocatalytic reaction by removing the aluminum cover after ensuring an equilibrium condition. The environment parameters such as temperature, relative humidity, and solar radiation were monitored during the testing at one-minute intervals.

2
A total of 50 concrete blocks were placed at the center of the testing chamber with 4m 2 of coverage area. The concrete blocks were exposed to natural sunlight for photocatalytic reaction by removing the aluminum cover after ensuring an equilibrium condition. The environment parameters such as temperature, relative humidity, and solar radiation were monitored during the testing at one-minute intervals. 2 A total of 50 concrete blocks were placed at the center of the testing chamber with 4m 2 of coverage area. The concrete blocks were exposed to natural sunlight for photocatalytic reaction by removing the aluminum cover after ensuring an equilibrium condition. The environment parameters such as temperature, relative humidity, and solar radiation were monitored during the testing at one-minute intervals.   Figure 9 illustrated the variation of NO x concentration during the laboratory environment chamber test for the concrete block sample treated with 4% of NST replacement rate under 30W/m 2 of UV intensity. The inlet concentration reached equilibrium at 1 ppm before the light was turned on. After turning on the UV light, a fast drop of NO and NO x concentration in the outlet was exhibited, and the NO 2 was produced from the NO oxidation. The light and gas supply were turned off after five hours of testing. For the test condition shown in Figure 9, the use of NST photocatalyst had a NO reduction of 59.1%, and the overall NO x reduction was 51.8%. Figure 10 presented the effects of UV light intensity on NO x reduction efficiency. Results presented in Figure 10 showed that the NO/NO x reduction efficiency doubled with an increase of 10 W/m 2 in UV intensity. As a result of the experiment, the concrete blocks using NST were capable of reducing the NO x concentration through photocatalysis. This fact proved that NST does not penetrate the cement pores and was evenly dispersed in the surface layer of the concrete blocks, since negative charges were formed due to the Ti-O-Si bond and activated interface.   Figure 11 illustrated the variation of NO concentration measured in reference and testing chambers during the full-scale experiment for the concrete block specimens treated with TiO 2 material. As shown, the NO concentration rapidly increased in both chambers, by injecting the exhaust gas for roughly 40 min, until NO concentration inside the chamber reached approximately 800 ppb. After stopping the gas injection, it took 20 min to ensure equilibrium conditions inside the chambers. Once the concrete blocks were exposed to natural sunlight in the testing chamber, a fast drop of NO concentration was exhibited due to the photocatalytic reaction of TiO 2 material. To determine NO/SO 2 reduction efficiency of TiO 2 concrete blocks in the environment chamber, two hours of measured data were compared between the reference and testing chambers. The measured data in the reference chamber was used to calculate the absolute reduction of NO/SO 2 concentration by considering wall loss and leaking phenomenon. The testing was conducted at different weather conditions and times to investigate the effects of solar radiation on NO/SO 2 reduction efficiency.  Figure 12 compared the nitrogen oxide (NO) concentration data measured from the reference and testing chambers for different levels of solar radiation. For the reference chamber, it was observed from test results that the NO concentration decreased linearly with time at 0.5~1 ppb/min. As shown in Figure 12, the NO concentrations in the testing chamber were significantly reduced after the application of natural sunlight at high levels of solar radiation. The difference in NO concentration measured in the testing and reference chambers gradually increased with time due to the continuous photochemical reaction. However, there were no differences in NO concentration between the testing and reference chambers at very low solar radiation, indicating that the average 67 W/m 2 of solar radiation was not enough for activating a photocatalytic reaction of concrete blocks with TiO 2 application. Table 6 presented the absolute reduction and reduction efficiency of NO measured after two hours in three different solar radiation levels. At high levels of solar radiation, the absolute reduction and reduction efficiency of NO were 0.145 ppm and 22.3%, respectively. It was found from this table that NO reduction efficiency tended to increase as the level of solar radiation increased.  Figure 13 presented the comparison of SO 2 concentrations measured in the testing and reference chambers. It was observed from this figure that the SO 2 concentration in the testing chamber could be reduced using TiO 2 at 358 and 564 W/m 2 solar radiation. Similar to the NO measurement results, the SO 2 reduction efficiency of concrete blocks with TiO 2 applications tended to increase with an increase of solar radiation. It was also found from the test results that the reduction efficiencies of SO 2 were slightly lower than those of NO at high solar radiation levels.   Table 7 presented the measured SO 2 reduction efficiencies for the concrete block that was treated with TiO 2 . It was found that the use of TiO 2 photocatalyst materials had an SO 2 reduction of 14.4% at 564 W/m 2 of average solar radiation.

Conclusions
This study evaluated the environmental effectiveness of incorporating NST as an additive to concrete blocks. The NST coated with SiO 2 as a support was prepared using the sol-gel method and blended with a conventional concrete binder at 4% of replacement rate for concrete blocks. Prepared concrete block samples were evaluated for the NO x reduction performance using laboratory and full-scale chamber experiments. Based on the results of the experimental programs, the following conclusions may be drawn: (1) Based on the results of the laboratory chamber test, the concrete blocks using NST were effective in reducing NO x pollutants in the air stream. The NO reduction efficiency ranged from 16.5 to 59.1%, and the highest NO reduction efficiency was achieved with a UV intensity of 30 W/m 2 . The increase in UV intensity positively affected the effectiveness of the NO x reduction capacity.
(2) Results of the full-scale environmental chamber test showed that the concrete blocks with NST was effective in reducing NO and SO 2 pollutants. The maximum environmental performance was achieved at a 564 W/m 2 with NO and SO 2 reduction efficiencies of 22.3% and 14.4%, respectively.
(3) In the future, the field application of NST as a road material is required to validate the NO x and SO 2 reduction efficiencies by considering various influencing parameters. In order to improve the photocatalysis efficiency of TiO 2 , the TiO 2 material has been developed as an anatase crystal phase with larger specific surface area and excellent photodissociation. Various attempts have been made, including sputtering, a process in which TiO 2 is directly and physically coated on a proper and solid support.

Conflicts of Interest:
The authors declare that there is no conflict of interests regarding the publication of this article.