Production and Evaluation of Porous Photocatalyst Concrete Filter (deNOx PCF) for NOx Reduction

Department of Structural Engineering Research, Korea Institute of Civil Engineering and Building Technology, Gyeonggi-do 10223, Republic of Korea Department of Urban, Civil and Environmental Engineering, Shinhan University, Gyeonggi-do, Republic of Korea Department of Civil and Environmental Engineering, Hanbat National University, Daejeon, Republic of Korea Department of Urban and Real Estate Development Engineering, Shinhan University, Gyeonggi-do, Republic of Korea


Introduction
Fine dust pollution is air pollution caused by fine particles (PM) which can be classified into PM2.5 (ultrafine dust) and PM10 (fine dust) based on the diameter of particles with a diameter of 2.5 μm and 10 μm or less, respectively [1]. Although the annual average concentration of PM2.5 in Seoul (South Korea) in 2018 decreased from 25 μg/m 2 to 23 μg/m 2 compared to 2017, potential human health risks due to environmental exposure to fine dust is still significant and stricter environmental regulation is required. erefore, there is a need to develop reduction technologies and management plan for ultrafine dust. While primary fine dust, generated by direct emission because of human activity or natural processes, accounts for 30% of the total fine dust generated, secondary fine dust, generated by chemical reaction with primary fine dust under certain conditions in the air, contributes to 70% of the total fine dust generated [2]. e major secondary pollutants include NOx, SOx, VOCs, and CO; in particular, NOx contributes to the formation of fine particles and ground level ozone, both of which are associated with adverse health effects [3]. While NOx can be emitted naturally in the air without any human intervention, but it is mainly caused by artificial combustion activities such as exhaust gas from automobiles and factories [4]. NOx is also well known for causing respiratory diseases such as asthma and photochemical smog and acid rain [5].
Recent research has been actively conducted in decomposing secondary pollutants such as NOx and SOx to remove precursor materials of ultrafine dust and to apply photocatalyst directly into construction materials [6][7][8].
Since titanium dioxide (TiO 2 ), one of the photocatalyst, is a functional substance that catalyzes the reduction of NOx in the atmosphere through photolysis [9], NOx in the air can be substantially reduced due to the photocatalysis reactions when TiO 2 is coated on the surface of the concrete [10]. e removal efficiency of NOx by photocatalysis can be maximized when the TiO 2 -coated cement is used in the road and the buildings through the greater surface area exposure to direct sunlight [11]. Since the lightweight aerated concrete with continuous pores inside the specimen by simultaneously mixing water, cement, and air bubbles of fine particle size is different from ordinary concrete [12], lightweight aerated concrete has been used as insulation or soundproofing materials and is not widely used as a filter to remove air pollutants [13].
In this study, a porous photocatalyst concrete filter (deNOx PCF) is manufactured to reduce NOx by mixing TiO 2 photocatalyst with present lightweight aerated concrete. As both porosity and volume of continuous pores inside the porous concrete are determined by the types and the amount of foaming agent, the optimal conditions should be determined to manufacture high-quality porous concrete filters. en, porous photocatalyst concrete filter are prepared according to the derived optimal condition of both type and amount of forming agents and mixing amount of TiO 2 photocatalyst. e surface characteristics of continuous pores formed in the manufactured deNOx PCF were analyzed, and the NO removal efficiencies of various deNOx PCF were evaluated by conducting a nitric oxide removal experiment based on ISO-22197-1 [14].

Materials.
Normal Portland cement (Class 1, Ssan-gYong Cement, Korea) was used as a main material for the porous concrete filter specimen, P-25 (Degussa) was used as a TiO 2 photocatalyst, and three different types of foaming agent (A, B, C, Korea) were used to form pores inside the concrete filter. e types and properties of foaming agents are summarized in Table 1. P-25 is nanosized TiO 2 (100 nm on average) photocatalyst with an anatase crystal and a rutile crystal structure with a ratio of 7 : 3 and a specific surface area of approximately 55-70 m 2 /g. Porous photocatalyst concrete filter specimens (deNOx PCF) were manufactured in rectangle with the size of 10 cm (W) × 5 cm (L) × 1 cm (H) required by NO removal performance evaluation apparatus manufactured based on NO ISO-22197-1 [14].

Evaluation of Optimal Bubble Generation Conditions.
To adsorb and photodegrade NOx stably using deNOx PCF, development of numerous pores inside and outside of deNOx PCF is significant and the pores in deNOx PCF can be generated when water, air, and foaming agents are properly mixed. e bubble generator consists of an air controller, a flow rate controller, an inlet of foaming agent, a revolving type bubble generator, and an outlet of bubbles (NBG-Nano Bubble Generator, SBE&E, Korea). e detail procedures are as follows. e generator can produce up to 0.1 m 3 /min of air bubbles with the size of less than 500 nm. A consistent amount of air, water, and foaming agents was flowed into the generator through air, flow, and air bubble controllers installed inside the generator. Air, water, and foaming agents were mixed at 1 : 1:1 ratio (by volume) to produce uniform size of nanobubbles less than 500 nm. As both type and amount of foaming agent affected the density and the retention time of the air bubble generated, the experiment was performed with varying conditions of types and the infusion rate of the foaming agent injected into the bubble generator.

Producing Method of Porous Photocatalyst Concrete Filter (deNOx PCF).
e optimal nanosized bubbles produced using the bubble generator were mixed into the porous concrete specimen, taking up 7% of the total weight of the specimen, to manufacture deNOx PCF specimen before mixing with TiO 2 photocatalyst (P-25). As summarized in Table 2, TiO 2 photocatalyst were mixed with 0%, 3%, and 5% of the total weight of the porous concrete specimen. To manufacture the deNOx PCF specimens, photocatalyst was mixed into a cement paste composed of W/C (water/cement weight ratio) � 0.3, and then, the dry mixing was performed for 5 minutes. e pregenerated air bubbles were inserted into cement paste and mixed. Finally, the specimen preparation was completed by pouring cement paste containing photocatalyst and bubbles into molds for NOx removal.

Analysis of Internal and External Surface Characterization of deNOx PCF.
Well-developed pores inside and outside of the structure are significant since deNOx PCF not only works as a photocatalyst but also works as a filter, NOx in the air is removed through pores formed inside and outside. TiO 2 particles on the outer surface of the structure directly reduce NOx in the air, but pores inside and outside the structure adsorb NOx in the air. Figure 1 shows the plausible mechanism of reducing hazardous substances (NOx and NH3) in air through the deNOx PCF. To analyze the degree of generation of pores inside and outside the specimen based on the different mixing conditions, scanning electron microscope (SEM) images were applied at 5,000 and 10,000 magnifications. Under the vacuum condition, microscopic electromagnetic waves of 100 nm were injected to the surface of the deNOx PCF specimen to observe the form and microstructure of the specimen by analyzing the signals of various wavelengths such as secondary electrons, reflective electrons, transmission electrons, visible light, infrared rays,

Measurement of NO Reduction Rate by a Different Mixing
Amount of TiO 2 Photocatalyst. An experiment was conducted to compare and analyze the NO reduction performance of deNOx PCF containing 0%, 3%, and 5% of TiO 2 photocatalyst of the total porous concrete specimen, in accordance with the Nitrogen Oxide Removal Performance Evaluation (ISO-22197-1). e reaction apparatus consisting of gas inlet, reactor, UV lamp, and reactive gas analysis part is displayed in Figure 2. A deNOx PCF with the size of 10 cm (W) × 5 cm (L) × 1 cm (H) was injected into the NO reduction experimental device manufactured in accordance with ISO standard, as represented in Figure 2. To compare and analyze the NO reduction rate of deNOx PCF specimens, four specimens containing different ratios of photocatalyst (i.e., 0%, 3%, and 5% and regular concrete specimen) were analyzed and compared. e temperature inside the generator was maintained at 25°C, and nitrogen gas was injected through gas inlet to keep the nitrogen concentration to 1 mg/L for 90 minutes. For the first 30 minutes of reaction, experiments were conducted under a darkroom without UV irradiation to induce the adsorption of specimens; then, UV lamps (10 W/m 2 ) were irradiated for one hour to evaluate the NO reduction efficiency of photocatalyzed porous specimen (i.e., deNOx PCF). e experimental conditions are summarized in Table 3. e concentrations of NO x and NO 2 were measured every 1 minute in real time, and the NO reduction rate and the amount were analyzed by converting NO 2 volume rate and NO concentration.
ree specimens were analyzed for each condition, and the mean values of NO removal efficiencies were obtained.

Determination.
To form the pores inside and outside the deNOx PCF effectively, an experiment was conducted to determine the optimal bubble generation condition according to both type and infusion rate of foaming agents. e generated bubbles under the different conditions were monitored over 6 hours, and the results are displayed in Figure 3. For foaming agent A, the height of the foaming agent was significantly reduced over time, as displayed Figure 3(a). e height of foaming agent C was not reduced notably, as illustrated in Figure 3(c), but the volume of the bubbles was significantly reduced, suggesting that foaming agent C was considered to be of poor quality.
Based on the results of the experiment, the optimal infusion rate is 4% as the formation of the bubbles were stable under such condition. When 5% of foaming agent was injected, water, air, and the foaming agent were not mixed properly compared to 4% of the foaming agent was injected. When the infusion amount was lower than 3%, the amount of water was greater than the foaming agent, leading to the poor quality of bubbles. As summarized in Figure 3(d) for 4% infusion rate of each foaming agent, the height of foaming agent B was reduced from 14.7 cm to 9.1 cm, which provided the smallest change of the height, except for the predetermined poor foaming agent C. Consequently, it is considered that the optimal quality of the air bubbles can be produced by using foaming agent B with 4% of the infusion rate.

Surface Characterization of deNOx PCF.
e pictorial view of surface for deNOx PCF specimen was revealed in Figure 4. When the 0% of photocatalyst was mixed with the porous concrete filter, the irregular and heterogeneous pores were formed on the surface with the concrete textures. When 3% of photocatalyst was mixed, less irregular and relatively homogeneous pores were formed on the surface with white color due to the proper amount of photocatalyst applied. When 5% of photocatalyst was mixed, the photocatalyst interrupted the formation of pores and the surface was white due to the excessive amounts of photocatalyst.
As is evident in Figure 5, SEM images indicated that the irregular and heterogeneous pores less than 500 nm were observed at 5,000-10,000 magnifications of porous deNOx PCF specimen manufactured with 0% of photocatalyst injected. However, less irregular and relatively

Advances in Civil Engineering
Under 500 nm  Advances in Civil Engineering homogeneous pores less than 500 nm were observed at 5,000-10,000 magnifications of porous deNOx PCF specimen manufactured with 3% of photocatalyst injected. When 5% of photocatalyst is mixed, the pores were not formed due to the interruption and clogging caused by excessive photocatalyst, leaving the internal part of the specimen to be filled. ese results suggest that optimal deNOx PCF with regular and homogeneous pores less than 500 nm can be manufactured when 3% of catalyst is mixed to the total weight of the specimen. Figure 6 displays the results of permeability and compressive strength test of deNOx PCF. In the case of PCF injected with 0% and 3% photocatalyst, the permeability was measured to be high while the compressive strength was measured to be low. ese results are mainly attributed to the fact that pores of 500 nm or less are continuously formed inside the PCF, so the permeability is high while the compressive strength is low. On the contrary, in the case of PCF injected with 5% photocatalyst, few pores were formed inside the deNOx PCF. erefore, general concrete with high compressive strength but low water permeability was manufactured. Figure 7 and Table 4 show the results of comparing the NOx removal efficiency according to the TiO 2 photocatalyst injection amount in the deNOx PCF specimen. For the regular concrete without forming agent (control), the reduction amount of NOx per hour was 0.09 μmol/hr equivalent to the 1.06% of total amount of NO. When 0% of TiO 2 photocatalyst was mixed into the specimen, 0.32 μmol/hr was reduced equivalent to the 3.88% of NO reduction. ese results can be mainly due to the fact that the continuous pores inside the specimen adsorbed significant amount of NO compared to the regular concrete. When 3% of photocatalyst was mixed to the specimen, 1.03 μmol/hr of NO was reduced equivalent to 10.99% of NO reduction. e TiO 2 photocatalyst dispersed in the continuous and well-developed pores inside the specimen successfully performed the removal of NO flowing through deNOx PCF using synergistic effects of adsorption and photodegradation reaction. When 5% of TiO 2 photocatalyst was mixed into deNOx PCF specimen, 0.58 μmol/hr of NO was reduced equivalent to 6.28% of NO reduction. Compared to the 3% of photocatalyst, NO reduction efficiency of 5% of TiO 2 photocatalyst is lower mainly due to the failure in forming continuous pores resulting in the limited adsorption and photodegradation reaction. From the aforementioned results, the optimal deNOx PCF can be produced when photocatalyst can be injected 3% out of the total weight of the specimen.

Comparison of Reaction Rate Constant.
e NO reduction results of deNOx PCF specimen manufactured with different amounts of photocatalyst injection were described using the primary reaction of Langmuir-Hinshelwood (L-H) to analyze the reaction rate constant (K app ). e NO reduction reaction using deNOx PCF specimen is comparable by pseudoprimary L-H rate equation [15]: where C 0 is the initial concentration of NO in the atmosphere (ppmv), C is the concentration of NO after the experiment (ppmv), and K app is the constant of reaction rate (min −1 ). As summarized in Table 5, the reaction rate of deNOx PCF specimens with 3% and 5% of photocatalyst resulted in reaction rate constant of 0.0555 min −1 and 0.032 min −1 , respectively. e higher reaction rate of the specimen containing 3% of TiO 2 is mainly attributed to the relatively homogeneous dispersion of photocatalyst in the continuous pores inside the specimen, causing active adsorption and photodegradation reactions. e regular concrete specimen with 0% photocatalyst injection was Advances in Civil Engineering excluded as it did not show sufficient reduction of NO concentration.

Conclusions
In this study, a porous photocatalyst concrete filter (deNOx PCF) is manufactured to reduce NOx by mixing TiO 2 photocatalyst with lightweight aerated concrete. en, the surface characteristics of continuous pores formed in the manufactured deNOx PCF were analyzed, and the NO removal efficiencies of various deNOx PCF were evaluated by conducting a nitric oxide removal experiment based on ISO-22197-1. From the results, 4% infusion rate of each foaming agent provided the smallest change of the height, and the optimal quality of the air bubbles can be produced by using foaming agent B with 4% of the infusion rate. When 3% of photocatalyst was mixed, less irregular and relatively homogeneous pores were formed on the surface with white color due to the proper amount of photocatalyst applied. However, when 5% of photocatalyst was mixed, the  photocatalyst interrupted the formation of pores and the surface was white due to the excessive amounts of photocatalyst. ese results suggest that optimal deNOx PCF with regular and homogeneous pores less than 500 nm can be manufactured when 3% of catalyst is mixed to the total weight of the specimen. For 3% of photocatalyst mixed with deNOx PCF, 1.03 μmol/hr of NO was reduced equivalent to 10.99% of NO reduction, suggesting that the TiO 2 photocatalyst dispersed in the continuous and well-developed pores inside the specimen successfully performed the removal of NO flowing through deNOx PCF using synergistic effects of adsorption and photodegradation reaction. Also, the higher reaction rate of the specimen containing 3% of TiO 2 is mainly attributed to the relatively homogeneous dispersion of photocatalyst in the continuous pores inside the specimen, causing active adsorption and photodegradation reactions. e aforementioned results, the optimal deNOx PCF can be produced when photocatalyst can be injected 3% out of the total weight of the specimen. Finally, the specimen of porous deNOx PCF for reducing NOx developed in this study can be applied to various construction sites and the air quality can be solved by reducing NOx contributing to the formation of fine particles [16].

Data Availability
e data used to support the findings of this study are included within the article.

Conflicts of Interest
e authors declare that there are no conflicts of interest regarding the publication of this paper.