NOx-Reduction Performance Test for TiO2 Paint

Abstract

In South Korea, the gradual increase in particulate matter generation has received significant attention from central and local governments. Exhaust gas, which contains nitrogen oxides (NOx), is one of the main sources of particulate matter. In this study, the reduction of NOx using a coating material mixed with a titanium dioxide (TiO2) photocatalyst was demonstrated. The NOx reduction performance of the TiO2 photocatalyst-infused coating was evaluated by applying the ISO 22197-1: 2007 standard. Subsequently, the performance was evaluated by changing the NO gas concentration and ultraviolet (UV)-A irradiance under standard experimental conditions. It was determined that NOx reduction can be achieved even if the NO gas concentration and UV-A irradiance are lower than those under the standard conditions when the TiO2 photocatalyst-infused coating was used. This study revealed that NOx reduction can be realized through TiO2 photocatalyst-infused coating in winter or cloudy days with a low solar altitude. It was also confirmed that compared with the UV-A irradiance, the NO gas concentration has a greater effect on the NOx reduction performance of the TiO2 photocatalyst-infused coating. These findings can be used to evaluate a variety of construction materials with TiO2 photocatalysts in the future.

Full text

molecules
Article
NOx-Reduction Performance Test for TiO2Paint
Yong Woo Song 1, Min Young Kim 2, Min Hee Chung 2, Young Kwon Yang 2and
Jin Chul Park 2, *
1Graduate School, Chung-Ang University, Seoul 06794, Korea; yongma0930@cau.ac.kr
2School of Architecture and Building Science, Chung-Ang University, Seoul 06974, Korea;
kmyhg@naver.com (M.Y.K.); mhloveu@cau.ac.kr (M.H.C.); dora84@naver.com (Y.K.Y.)
*Correspondence: jincpark@cau.ac.kr; Tel.: +82-2-823-2221
Received: 25 July 2020; Accepted: 3 September 2020; Published: 7 September 2020


Abstract:
In South Korea, the gradual increase in particulate matter generation has received significant
attention from central and local governments. Exhaust gas, which contains nitrogen oxides (NO
x
),
is one of the main sources of particulate matter. In this study, the reduction of NO
x
using a coating
material mixed with a titanium dioxide (TiO
2
) photocatalyst was demonstrated. The NO
x
reduction
performance of the TiO
2
photocatalyst-infused coating was evaluated by applying the ISO 22197-1:
2007 standard. Subsequently, the performance was evaluated by changing the NO gas concentration
and ultraviolet (UV)-A irradiance under standard experimental conditions. It was determined that
NO
x
reduction can be achieved even if the NO gas concentration and UV-A irradiance are lower
than those under the standard conditions when the TiO
2
photocatalyst-infused coating was used.
This study revealed that NO
x
reduction can be realized through TiO
2
photocatalyst-infused coating in
winter or cloudy days with a low solar altitude. It was also confirmed that compared with the UV-A
irradiance, the NO gas concentration has a greater effect on the NO
x
reduction performance of the
TiO
2
photocatalyst-infused coating. These findings can be used to evaluate a variety of construction
materials with TiO2photocatalysts in the future.
Keywords:
titanium dioxide (TiO
2
); nitrogen oxides (NO
x
); particulate matter; secondary source;
reduction test
1. Introduction
Since the gradual increase in particulate matter (PM) concentration has been recognized as a
national problem in South Korea, central and local governments have made considerable efforts to
reduce such concentration. According to data from the World Health Organization (WHO), the annual
average PM concentration of Seoul, South Korea in 2016 was 46
µ
g/m
3
, which is 1.2–3.5 times higher
compared with Tokyo, Japan (28
µ
g/m
3
), Paris, 1‘France (28
µ
g/m
3
), and Washington DC, USA
(16 µg/m3) [1].
The causes of PM are divided into primary and secondary sources. Primary sources are pollutants
that are directly generated by pollution sources, and their particle sizes are less than 10
µ
m (PM
10
).
The introduction of primary sources into indoor spaces can be reduced by applying high-efficiency
particulate air (HEPA) filters to indoor air purifiers and heating, ventilating, and air-conditioning
(HVAC) systems [2–4].
Secondary sources are generated through chemical bonding between gaseous pollutants in primary
pollutants and precursors in the atmosphere, and their particle sizes are generally less than 2.5
µ
m
(PM
2.5
). These pollutants vary in size, weight, color, and their internal components. Their main source
is the exhaust gas that is generated through transportation, which includes automobiles. This is due to
the incomplete combustion of fossil fuels; thus, their concentrations are high in downtown areas with
Molecules 2020,25, 4087; doi:10.3390/molecules25184087 www.mdpi.com/journal/molecules

Molecules 2020,25, 4087 2 of 14
large traffic volumes [
5
]. Sulfur oxides (SOx) and nitrogen oxides (NO
x
) account for approximately
58% of all substances causing secondary sources, which is the highest proportion [6].
In particular, the representative substance that generates secondary sources in South Korea is
NO
x
, which represents 42.3% of all substances that can cause secondary sources [
7
,
8
]. Therefore, it is
expected that NOxreduction will decrease the overall PM concentration in South Korea.
The representative materials that can reduce secondary sources through chemical reactions are
TiO
2
photocatalysts. They can achieve self-cleaning [
9
–
16
], antibacterial resistance [
16
], air-cleaning,
deodorization [
16
–
23
], and water-purification [
23
–
27
] effects by absorbing light in the 350–380 nm
range of the wavelengths of light. Among them, air cleaning was used in this study because it can
remove the cause of PM generation by oxidizing NO
x
. This can reduce PM through photochemical
reaction with ultraviolet (UV) rays. The photochemical reaction between TiO
2
and UV rays is given in
Equation (1) [28]. The photocatalytic oxidation mechanism of nitrogen oxides is given by
Activation : TiO2+∗hv →h++e−(1)
H2O(g)+Site∗∗ →H2Oads (2)
O2(g)+Site∗∗ →O2ads (3)
NO(g)+Site∗∗ →NOads (4)
NO2(g)+Site∗∗ →NO2ads (5)
Hole trapping : H2O+h+→ •OH +H+(6)
Electron trapping : O2(g)+e−→O2
−(7)
Hydroxyl attack : NOads +2•OH →NO2ads +H2O NO2ads +•OH →HNO3(8)
*hv: light (ultraviolet radiation), **Site: surface of TiO2,•OH: hydroxyl radical
As shown in Equations (1)–(8), when TiO
2
on the material surface absorbs energy from UV rays in
sunlight, holes (h
+
) and electrons (e
−
) are generated (Equation (1)).
•
OH and O
2−•
radicals with a
strong oxidizing power are generated on the surface of TiO
2
through reactions with H
2
O and O
2
in the
atmosphere (Equations (2)–(7), electron trapping).
This can lead to the hydroxyl attack step (Equation (8)) in which NO
x
, a representative precursor,
is decomposed [
29
,
30
]. Through this mechanism, secondary sources such as NO
x
are decomposed on the
photocatalyst surface, resulting in PM reduction effect. With the development of light source technology,
concrete and cement mixed with TiO
2
photocatalysts have recently been actively developed. From 2000
to 2010, only two studies were conducted in which TiO
2
photocatalysts were mixed with concrete and
cement [
31
,
32
]. However, seven studies were conducted from 2010 to 2020 [
29
,
33
–
38
]. These studies
mainly focused on road pavement materials, exterior materials, and roof finishing materials.
A review of previous studies on the application of TiO
2
is presented below. The most
representative applications are cement road materials, which include pavement blocks [
29
],
asphalt
pavement [33,36,37]
, and mortar [
34
,
35
]. The reduction in NO
x
concentration was examined
by adding TiO
2
photocatalysts to these cement road materials, and it was observed that the NO
x
concentration can be reduced by up to 60–80% [
31
]. Yu et al. investigated NOx concentration reduction
performance based on automobile exhaust gas concentration through field measurement and verified
the optimal mass ratio [
36
]. Wang et al. investigated the effect of exhaust gas decomposition by setting
the UV irradiance as 26.7 W/m
2
, which is the average outdoor illumination, and it was confirmed that
exhaust gas decomposition can be achieved when the nano-TiO
2
content is 8% [
37
]. Witkowski et al.
conducted NO removal experiment using a photocatalytic pavement block used for 7 years on a silver
bicycle road, and the NO gas reduction performance was investigated by applying 300 W of light;
scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) mapping analyses
were performed to confirm TiO
2
mixing in the pavement block [
38
]. The irradiance of UV applied

Molecules 2020,25, 4087 3 of 14
in the experiment was very high compared with the atmosphere; therefore, the actual environment
should be considered while conducting the experiment.
Cassar et al. conducted a study on the application of TiO
2
photocatalysts to build an exterior
other than road pavement. As a representative case, the Italcementi group in Italy installed an exterior
material mixed with TiO
2
photocatalysts in the Roman Jubilee Cathedral and road pavements on a
trial basis. They confirmed an antifouling performance and air pollution reduction of 30–40% [32].
In addition, Luna et al. applied the TiO
2
photocatalyst coating to limestone and granite surfaces.
They found that granite is better than limestone in terms of its antifouling performance and smoke
removal [
39
]. Lettieri et al. applied a TiO
2
photocatalyst coating to a limestone surface and investigated
its NO
x
reduction performance. They also found that the antifouling performance was lost after
8 months [40].
Ramirez et al. applied a TiO
2
photocatalyst coating and particle injection to porous cementitious
materials. They determined that the use of the particle injection method on porous materials results in
a high toluene removal efficiency [
20
]. Tang et al. coated TiO
2
photocatalysts in the form of granules
on building roofs in addition to the exterior materials to investigate their self-cleaning characteristics.
They found that these photocatalysts were effective, even for the urban heat island phenomenon,
owing to the observed lower surface temperatures [41].
As described above, most recent studies on TiO
2
focused on the evaluation of air-cleaning
performance using cement and stone. In addition, a few studies have been conducted on the quantitative
evaluation of the air-cleaning performance of TiO2combined with other building materials.
In this study, two tests were conducted to examine the air-cleaning performance of building
materials that utilized the photochemical properties of TiO
2
photocatalysts (activation in the 350–380 nm
range). The building material used in the tests was a coating material mixed with TiO
2
photocatalysts;
this was utilized in this study because it can be applied to a variety of building materials or the
structures of existing buildings.
In the first test, the ISO 22197-1:2007 test method was used with the coating material mixed with
TiO
2
photocatalysts to examine its NO
x
reduction performance in secondary sources [
42
]. The second
test was conducted to examine the quantitative performance change of TiO
2
photocatalysts when the
NO concentration and UV-A irradiance based on the ISO 22197-1:2007 test conditions were changed.
The results of each test are analyzed in Section 2, and they are combined and used to analyze the NO
x
reduction performance of TiO
2
photocatalysts as presented in Section 3. It is expected that the results
of this study can be used to reduce PM concentrations in buildings in the future.
2. Materials and Methods
2.1. Materials and Experimental Overview
In this study, a TiO
2
coating material was used to evaluate its NO
x
reduction performance. To this
end, the TiO
2
coating material (ZT-01 from Bentech Frontier, Jeollanam-do, South Korea) was applied
on a Pyrex glass specimen along with a primer. The primer was applied before the coating to make the
back surface of the specimen opaque and block external light that may enter the back surface.
The TiO
2
coating material was made of anatase-based titanium dioxide. The detailed physical
properties are listed in Table 1.
Table 1. TiO2coating composition.
Contained Chemicals Proportion
Titanium dioxide (anatase) 1.75%
Silicone compound 5.6%
Ethanol 41.6%
Water 51.0%
Other 0.05%

Molecules 2020,25, 4087 4 of 14
FESEM (field emission scanning electronic microscope, Gwangju, Republic of Korea) and EDS
mapping analyses were (Gwangju, Republic of Korea) performed on the ZT-01 material used in the
experiment. The contents of the equipment used for FESEM and EDS analysis are as in Table 2.
The analyses were performed on pre- and postcoating STUB (sample holder)s, and the STUBs before
and after the application were analyzed. The materials in the STUB before the application of TiO
2
coating material were composed of C (12.84%), O (3.04%, Al (79.34%), and Cu (4.79%) (See Table 3,
Figure 1).
Table 2.
Field emission scanning electronic microscope (FESEM) and energy dispersive spectroscopy
(EDS) measuring equipment.
Classification Contents
Model SIGMA 500 (Carl Ziess)
Detector SE 2
EDS detector X-MaxN50 (Oxford)
Acceleration voltage 18.0 kV
Working distance 8.5 mm
Magnification 500×to 1000×
Time-resolution 0.8 nm
Table 3. STUB (Sample Holder) composition.
Element wt%
C 12.84
O 3.04
Al 79.34
Cu 4.79
Total 100.00
Molecules 2020, 25, x FOR PEER REVIEW 4 of 15
4
FESEM (field emission scanning electronic microscope, Gwangju, Republic of Korea) and EDS
mapping analyses were (Gwangju, Republic of Korea) performed on the ZT-01 material used in the
experiment. The contents of the equipment used for FESEM and EDS analysis are as in Table 2. The
analyses were performed on pre- and postcoating STUB(sample holder)s, and the STUBs before and
after the application were analyzed. The materials in the STUB before the application of
TiO
2
coating
material were composed of C (12.84%), O (3.04%, Al (79.34%), and Cu (4.79%) (See Table 3, Figure 1).
Table 2. Field emission scanning electronic microscope (FESEM) and energy dispersive spectroscopy
(EDS) measuring equipment.
Classification Contents
Model SIGMA 500 (Carl Ziess)
Detector SE 2
EDS detector X-Max
N
50 (Oxford)
Acceleration voltage 18.0 kV
Working distance 8.5 mm
Magnification 500× to 1000×
Time-resolution 0.8 nm
Table 3. STUB(Sample Holder) composition.
Element Wt%
C 12.84
O 3.04
Al 79.34
Cu 4.79
Total 100.00
Figure 1. STUB(Sample Holder) field emission scanning electronic microscope (FESEM) image.
After applying the TiO
2
coating material to the STUB, FESEM, and EDS mapping analyses
showed that it was the same as Table 4 and Figure 2, and TiO
2
was included in coating material
approximately 21.05%.
Figure 1. STUB (Sample Holder) field emission scanning electronic microscope (FESEM) image.
After applying the TiO
2
coating material to the STUB, FESEM, and EDS mapping analyses showed
that it was the same as Table 4and Figure 2, and TiO
2
was included in coating material approximately
21.05%.

Molecules 2020,25, 4087 5 of 14
Table 4. TiO2coating composition.
Element wt%
C 3.47
O 65.09
Al 10.39
Ti 21.05
Total 100.00
Molecules 2020, 25, x FOR PEER REVIEW 5 of 15
5
Figure 2. TiO
2
coating FESEM and energy dispersive spectroscopy (EDS) mapping image.
Table 4. TiO
2
coating composition.
Element wt%
C 3.47
O 65.09
Al 10.39
Ti 21.05
Total 100.00
Through the analysis of FESEM and EDS mappings, sufficient Ti and O components were
present inside the TiO
2
coating agent used in this experiment to determine their effect on the
reduction of NO
x
.
The size of the specimen that was used (10 cm
2
) was small; hence, the TiO
2
coating was applied
using a brush. The amount of coating used was calculated from the precoating brush weight, the
postcoating brush weight, the precoating specimen weight, and the postcoating specimen weight
difference; approximately 12 g of coating was applied. In this study, we focused on measuring the
volume of coating rather than the coating thickness.
Coating was applied one to 10 times, and preliminary experiments were conducted to confirm
the NOx reduction in efficiency. This study confirmed that the same efficiency can be achieved seven
times. After applying the coating, the specimen surface was confirmed to be cured and smooth.
The size of the prepared specimen was 10 mm × 100 mm (width × height), which is based on the
flow rate of 3 lpm specified in the ISO 22197-1:2007.
The NO
x
reduction performance test was conducted using the following two methods. First, it
was conducted by applying NO 1.00 ppm and UV-A 10 W/m
2
, which are the standard conditions in
the ISO 22197-1:2007. Second, changes in the NO
x
concentration were measured, while the NO gas
and UV-A concentrations were reduced at a constant rate. The UV-A irradiance was reduced by
adjusting the vertical distance between the specimen and the lamp. The UV-A lamps used in this
experiment consisted of TL-D 18 W BL lamps (P Company), irradiating light from 315–400 nm. The
UV-A includes the active wavelengths of the TiO
2
photocatalysts, and it represents 95% of the UV
rays that reach the ground surface [43]. The interexperimental TiO
2
photocatalyst coating specimen
was used with distilled water for cleaning.
To measure the temperature and humidity, we used KIMO’s multipurpose measuring
instrument with a SOM 900 probe. Serinus 40 (Ecotech’s NO
x
Gas Analyzer) was used to measure the
NO, NO
2
, and NO
x
concentrations (Figure 3 and Table 5).
Figure 2. TiO2coating FESEM and energy dispersive spectroscopy (EDS) mapping image.
Through the analysis of FESEM and EDS mappings, sufficient Ti and O components were present
inside the TiO
2
coating agent used in this experiment to determine their effect on the reduction of NO
x
.
The size of the specimen that was used (10 cm
2
) was small; hence, the TiO
2
coating was applied
using a brush. The amount of coating used was calculated from the precoating brush weight,
the postcoating brush weight, the precoating specimen weight, and the postcoating specimen weight
difference; approximately 12 g of coating was applied. In this study, we focused on measuring the
volume of coating rather than the coating thickness.
Coating was applied one to 10 times, and preliminary experiments were conducted to confirm
the NOx reduction in efficiency. This study confirmed that the same efficiency can be achieved seven
times. After applying the coating, the specimen surface was confirmed to be cured and smooth.
The size of the prepared specimen was 10 mm
×
100 mm (width
×
height), which is based on the
flow rate of 3 lpm specified in the ISO 22197-1:2007.
The NO
x
reduction performance test was conducted using the following two methods. First, it
was conducted by applying NO 1.00 ppm and UV-A 10 W/m
2
, which are the standard conditions in the
ISO 22197-1:2007. Second, changes in the NO
x
concentration were measured, while the NO gas and
UV-A concentrations were reduced at a constant rate. The UV-A irradiance was reduced by adjusting
the vertical distance between the specimen and the lamp. The UV-A lamps used in this experiment
consisted of TL-D 18 W BL lamps (P Company), irradiating light from 315–400 nm. The UV-A includes
the active wavelengths of the TiO
2
photocatalysts, and it represents 95% of the UV rays that reach
the ground surface [
43
]. The interexperimental TiO
2
photocatalyst coating specimen was used with
distilled water for cleaning.
To measure the temperature and humidity, we used KIMO’s multipurpose measuring instrument
with a SOM 900 probe. Serinus 40 (Ecotech’s NO
x
Gas Analyzer) was used to measure the NO, NO
2
,
and NOxconcentrations (Figure 3and Table 5).

Molecules 2020,25, 4087 6 of 14
Molecules 2020, 25, x FOR PEER REVIEW 6 of 15
6
Figure 3. Test chamber diagram.
Table 5. Specifications of measuring equipment.
Classification
AMI 310, SOM 900(KIMO, Montpon, France)
Measurement
Range
Accuracy Resolution
Temperature −20 to +80 °C ±3% of the leading value ±
0.25 °C 0.1 °C
Relative
humidity 0–100% RH
Accuracy: ±1.8% RH
Calibration
Uncertainty: ±0.88% RH
0.1% RH
Velocity 0.00–5.00 m/s
±3% of the leading value ±
0.05 m/s 0.01 m/s
Classification
Serinus 40(ECOTECH, Melbourne, Austraila)
Range
Automatic 0–20 ppm
USEPA approval 0.0–0.5 ppm
TUVEN certified Less than NO (0–1000 ppm),
NO
2
(0–260 ppm)
Accuracy/pre
cision
Precision 0.4 ppb or 0.5% of reading (the lesser of
the two)
Linearity ±1% of the total scale
Reaction time 90% in 15 s
Sample flow rate 0.3 slpm (total flow rate of 0.6 slpm for
the NO and NO
x
flow path)
2.2. Experimental Methods on NO
x
Reduction Using Titanium Oxide Photocatalyst
In the ISO 22197-1 standard condition test, changes in the NO, NO
2
, and NO
x
concentrations
were measured according to the on/off status of the UV-A lamp, whereas the NO 1.00 ppm and UV-
A 10 W/m
2
conditions in the chamber were continuously maintained for 3 h. The average values of
three measurements were used to reduce the errors.
The UV-A irradiance used in the test was 10 W/m
2
. This is similar to the annual winter level (13.4
W/m
2
) UV irradiance in South Korea according to the statistics of the comprehensive climate change
monitoring information from the Korea Meteorological Administration (KMA) [44]. The statistics
Figure 3. Test chamber diagram.
Table 5. Specifications of measuring equipment.
Classification AMI 310, SOM 900(KIMO, Montpon, France)
Measurement
Range Accuracy Resolution
Molecules 2020, 25, x FOR PEER REVIEW 6 of 15
6
Figure 3. Test chamber diagram.
Table 5. Specifications of measuring equipment.
Classification
AMI 310, SOM 900(KIMO, Montpon, France)
Measurement
Range
Accuracy Resolution
Temperature −20 to +80 °C ±3% of the leading value ±
0.25 °C 0.1 °C
Relative
humidity 0–100% RH
Accuracy: ±1.8% RH
Calibration
Uncertainty: ±0.88% RH
0.1% RH
Velocity 0.00–5.00 m/s
±3% of the leading value ±
0.05 m/s 0.01 m/s
Classification
Serinus 40(ECOTECH, Melbourne, Austraila)
Range
Automatic 0–20 ppm
USEPA approval 0.0–0.5 ppm
TUVEN certified Less than NO (0–1000 ppm),
NO
2
(0–260 ppm)
Accuracy/pre
cision
Precision 0.4 ppb or 0.5% of reading (the lesser of
the two)
Linearity ±1% of the total scale
Reaction time 90% in 15 s
Sample flow rate 0.3 slpm (total flow rate of 0.6 slpm for
the NO and NO
x
flow path)
2.2. Experimental Methods on NO
x
Reduction Using Titanium Oxide Photocatalyst
In the ISO 22197-1 standard condition test, changes in the NO, NO
2
, and NO
x
concentrations
were measured according to the on/off status of the UV-A lamp, whereas the NO 1.00 ppm and UV-
A 10 W/m
2
conditions in the chamber were continuously maintained for 3 h. The average values of
three measurements were used to reduce the errors.
The UV-A irradiance used in the test was 10 W/m
2
. This is similar to the annual winter level (13.4
W/m
2
) UV irradiance in South Korea according to the statistics of the comprehensive climate change
monitoring information from the Korea Meteorological Administration (KMA) [44]. The statistics
Temperature −20 to +80 ◦C±3% of the leading
value ±0.25 ◦C0.1 ◦C
Relative humidity 0–100% RH
Accuracy: ±1.8% RH
Calibration
Uncertainty: ±0.88% RH
0.1% RH
Velocity 0.00–5.00 m/s±3% of the leading
value ±0.05 m/s0.01 m/s
Classification Serinus 40 (ECOTECH, Melbourne, Austraila)
Range
Automatic 0–20 ppm
Molecules 2020, 25, x FOR PEER REVIEW 6 of 15
6
Figure 3. Test chamber diagram.
Table 5. Specifications of measuring equipment.
Classification
AMI 310, SOM 900(KIMO, Montpon, France)
Measurement
Range
Accuracy Resolution
Temperature −20 to +80 °C ±3% of the leading value ±
0.25 °C 0.1 °C
Relative
humidity 0–100% RH
Accuracy: ±1.8% RH
Calibration
Uncertainty: ±0.88% RH
0.1% RH
Velocity 0.00–5.00 m/s
±3% of the leading value ±
0.05 m/s 0.01 m/s
Classification
Serinus 40(ECOTECH, Melbourne, Austraila)
Range
Automatic 0–20 ppm
USEPA approval 0.0–0.5 ppm
TUVEN certified Less than NO (0–1000 ppm),
NO
2
(0–260 ppm)
Accuracy/pre
cision
Precision 0.4 ppb or 0.5% of reading (the lesser of
the two)
Linearity ±1% of the total scale
Reaction time 90% in 15 s
Sample flow rate 0.3 slpm (total flow rate of 0.6 slpm for
the NO and NO
x
flow path)
2.2. Experimental Methods on NO
x
Reduction Using Titanium Oxide Photocatalyst
In the ISO 22197-1 standard condition test, changes in the NO, NO
2
, and NO
x
concentrations
were measured according to the on/off status of the UV-A lamp, whereas the NO 1.00 ppm and UV-
A 10 W/m
2
conditions in the chamber were continuously maintained for 3 h. The average values of
three measurements were used to reduce the errors.
The UV-A irradiance used in the test was 10 W/m
2
. This is similar to the annual winter level (13.4
W/m
2
) UV irradiance in South Korea according to the statistics of the comprehensive climate change
monitoring information from the Korea Meteorological Administration (KMA) [44]. The statistics
USEPA approval 0.0–0.5 ppm
TUVEN certified Less than NO (0–1000 ppm),
NO2(0–260 ppm)
Accuracy/precision
Precision 0.4 ppb or 0.5% of reading (the lesser of
the two)
Linearity ±1% of the total scale
Reaction time 90% in 15 s
Sample flow rate
0.3 slpm (total flow rate of 0.6 slpm for the
NO and NOxflow path)
2.2. Experimental Methods on NOxReduction Using Titanium Oxide Photocatalyst
In the ISO 22197-1 standard condition test, changes in the NO, NO
2
, and NO
x
concentrations
were measured according to the on/offstatus of the UV-A lamp, whereas the NO 1.00 ppm and UV-A
10 W/m
2
conditions in the chamber were continuously maintained for 3 h. The average values of three
measurements were used to reduce the errors.
The UV-A irradiance used in the test was 10 W/m
2
. This is similar to the annual winter level
(13.4 W/m
2
) UV irradiance in South Korea according to the statistics of the comprehensive climate change
monitoring information from the Korea Meteorological Administration (KMA) [
44
]. The statistics from
KMA, however, were values measured on horizontal surfaces. These values are expected to be lower
when the actual TiO2photocatalysts are utilized owing to vertical surfaces and other obstacles.

Molecules 2020,25, 4087 7 of 14
The changes in the NO, NO
2
, and NO
x
concentrations measured in the test were analyzed
using the amount of NO reduced (a) and the amount of NO
2
generated (b) according to the on/off
status of the UV-A lamp to calculate the amount of NO
x
reduced (a
−
b). Table 6lists the detailed
measurement conditions.
Table 6. ISO standard condition test values.
Classification Value
UV-A irradiance 10 W/m2
NO gas concentration 1.00 ppm
Temperature 25 ±2.5 °C
Relative humidity 50%
Test time/measurement interval 3 h/1 min
Number of experiments 3 times
2.3. Experimental Methods Based on the UV-A Irradiance and Changes in the Concentration of NO
The test for changes in the condition was conducted to examine the changes in the NO
x
reduction
performance of the TiO2photocatalysts by varying the UV-A irradiance and the NO concentration.
The irradiance was set to 7.50 W/m
2
for 25% cloud cover, 5.00 W/m
2
for 50% cloud cover,
and 2.50 W/m
2
for 75% cloud cover. This was to simulate low cloud cover situations in winter based
on the ISO standard of 10 W/m
2
. In this experiment, changes in the NO
x
concentration were measured.
In addition, changes in the NO
x
concentration were measured when the NO concentration was
reduced by 25%, 50%, and 75%, which are 0.75, 0.50, and 0.25 ppm, respectively, according to the ISO
standard of 1.00 ppm (Table 7).
Table 7. Changes in condition test value.
Classification
UV-A Irradiance (W/m2)
2.50 5.00 7.50 10.0
NO concentration (ppm)
CASE 1
0.25
CASE 5
0.25
CASE 9
0.25
CASE 13
0.25
CASE 2
0.50
CASE 6
0.50
CASE 10
0.50
CASE 14
0.50
CASE 3
0.75
CASE 7
0.75
CASE 11
0.75
CASE 15
0.75
CASE 4
1.00
CASE 8
1.00
CASE 12
1.00
CASE 16
1.00
The amount of NO reduced (a), the amount of NO
2
generated (b), and the amount of NO
x
reduced
(a
−
b) were calculated using the measurement results. For the conditions other than the irradiance
and concentration, the standard conditions were applied.
3. Results and Discussion
3.1. Reactivity at ISO 22197-1 Standard Condition
The results of the test that applied the ISO 22197-1:2007 standard conditions (1.00 ppm and
10.0 W/m
2
) are presented in Table 8and Figure 4. In the test, changes in the NO
x
concentration were
investigated according to the UV-A on/offstatus after maintaining the standard condition concentration
of 1.00 ppm in the chamber.
Table 8. ISO standard condition test result.
Classification Start Concentration End Concentration Reduction Rate (Reduction Amount)
NO 1.015 ppm 0.653 ppm 35.67% (12.06 µmol/10 cm2·3 h)
NO20.000 ppm 0.143 ppm 3.11 µmol/10 cm2·3 h (generated)
NOx1.015 ppm 0.796 ppm 21.58% (8.95 µmol/10 cm2·3 h)

Molecules 2020,25, 4087 8 of 14
Molecules 2020, 25, x FOR PEER REVIEW 8 of 15
8
Table 8. ISO standard condition test result.
Classification
Start Concentration End Concentration Reduction Rate (Reduction Amount)
NO 1.015 ppm 0.653 ppm 35.67% (12.06 µmol/10 cm
2
·3 h)
NO
2
0.000 ppm 0.143 ppm 3.11 µmol/10 cm
2
·3 h (generated)
NO
x
1.015 ppm 0.796 ppm 21.58% (8.95 µmol/10 cm
2
·3 h)
Figure 4. NO
x
reduction results for the ISO standard condition.
As shown in Figure 4, the NO
x
concentration reduced from 1.00 ppm to approximately 0.8 ppm
when the UV lamp was turned on. However, it returned to 1.00 ppm when the lamp was turned off.
This confirmed that the operation of the UV lamp reduces the NO
x
concentration.
It can be observed from Table 8 that the NO concentration decreased by 35.67% (12.06 µmol/10
cm
2
·3 h) and the NO
x
concentration by 21.03% (8.95 µmol/10 cm
2
·3 h) in 3 h, which confirms the NO
x
concentration reduction performance of the TiO
2
coating material. µmol/10 cm
2
·3 h is a unit using
which the concentration value measured in parts per million is expressed as a quantity. The 10 cm
2
·3
h expressed behind the unit represents 10 cm
2
, the size of the experimental specimen, and 3 h, the
experimental time.
In addition, it can be observed from Figure 4 and Table 8 that NO
2
was generated when the UV
lamp was turned on and, approximately, 3.11 µmol/10 cm
2
·3 h was generated until the lamp was
turned off. This, in addition to the difference in the NO
x
concentration, confirms the occurrence of
oxidation reactions through the combination of TiO
2
photocatalysts and UV rays.
3.2. Reactivity at Different UV-A and NO Concentrations
Although the above test was conducted by applying the ISO standard conditions (UV-A: 10
W/m
2
, NO: 1.00 ppm), the test to evaluate changes in the NO
x
concentration was conducted by
changing the UV-A irradiance and NO concentration.
The test results are presented in Table 9, Table 10, and Table 11 and Figure 5. This test was also
conducted according to the on/off status of the UV-A lamp while a certain concentration was
maintained in the chamber.
Figure 4. NOxreduction results for the ISO standard condition.
As shown in Figure 4, the NO
x
concentration reduced from 1.00 ppm to approximately 0.8 ppm
when the UV lamp was turned on. However, it returned to 1.00 ppm when the lamp was turned off.
This confirmed that the operation of the UV lamp reduces the NOxconcentration.
It can be observed from Table 8that the NO concentration decreased by 35.67% (12.06
µ
mol/10
cm
2·
3 h) and the NO
x
concentration by 21.03% (8.95
µ
mol/10 cm
2·
3 h) in 3 h, which confirms the
NO
x
concentration reduction performance of the TiO
2
coating material.
µ
mol/10 cm
2·
3 h is a unit
using which the concentration value measured in parts per million is expressed as a quantity. The 10
cm
2·
3 h expressed behind the unit represents 10 cm
2
, the size of the experimental specimen, and 3 h,
the experimental time.
In addition, it can be observed from Figure 4and Table 8that NO
2
was generated when the UV
lamp was turned on and, approximately, 3.11
µ
mol/10 cm
2·
3 h was generated until the lamp was
turned off. This, in addition to the difference in the NO
x
concentration, confirms the occurrence of
oxidation reactions through the combination of TiO2photocatalysts and UV rays.
3.2. Reactivity at Different UV-A and NO Concentrations
Although the above test was conducted by applying the ISO standard conditions (UV-A: 10 W/m
2
,
NO: 1.00 ppm), the test to evaluate changes in the NO
x
concentration was conducted by changing the
UV-A irradiance and NO concentration.
The test results are presented in Table 9, Table 10, and Table 11 and Figure 5. This test was
also conducted according to the on/offstatus of the UV-A lamp while a certain concentration was
maintained in the chamber.
Table 9. Test for changes in the condition that resulted in NO reduction.
UV-A Irradiance (W/m2)
NO Concentration (ppm) Reduction in Concentration (a)
0.25 ppm 0.50 ppm 0.75 ppm
2.50 W/m20.119 ppm 0.179 ppm 0.189 ppm
5.00 W/m20.108 ppm 0.136 ppm 0.234 ppm
7.50 W/m20.113 ppm 0.201 ppm 0.327 ppm

Molecules 2020,25, 4087 9 of 14
Table 10. NO2generation results based on NO concentration and UV-A irradiance.
UV-A Irradiance (W/m2)
NO Concentration (ppm) Increase in Concentration (b)
0.25 ppm 0.50 ppm 0.75 ppm
2.50 W/m20.053 ppm 0.105 ppm 0.091 ppm
5.00 W/m20.028 ppm 0.060 ppm 0.088 ppm
7.50 W/m20.024 ppm 0.076 ppm 0.094 ppm
Table 11. Test results of changes in condition to achieve NOxreduction.
UV-A Irradiance (W/m2)
NO Concentration (ppm) NOxReduction Concentration (c =a−b)
0.25 ppm 0.50 ppm 0.75 ppm
2.50 W/m20.067 ppm 0.074 ppm 0.098 ppm
5.00 W/m20.080 ppm 0.076 ppm 0.121 ppm
7.50 W/m20.089 ppm 0.126 ppm 0.202 ppm
Molecules 2020, 25, x FOR PEER REVIEW 10 of 15
10
In addition, when the NO concentration was changed under the same UV-A irradiance, the NO
x
concentration reduction increased as the NO concentration increased. However, at 0.25 and 0.5 ppm,
which are low NO concentrations, there is a difference in the reduction amount according to the
change in the light amount for 60 min. As the concentration decreased, the difference became less.
Figure 5d–f show the results of the corresponding tests. The results confirm that there were certain
differences in the concentration reduction as the NO concentration changed under the same
irradiance.
(a) (b)
(c) (d)
(e) (f)
Figure 5. Test results of changes in condition to achieve NO
x
reduction. NO
x
reduction according to
change in the irradiance based on a concentration of (a) 0.25 ppm, (b) 0.50 ppm, and (c) 0.75 ppm.
NO
x
reduction according to the change in the reference concentration at a light rate of (d) 2.50 W/cm
2
,
(e) 5.00 W/cm
2
, and (f) 7.50 W/cm
2
.
4. Discussion
The NO
x
reduction performance test of the TiO
2
photocatalysts was conducted using the
photocatalyst coating material. The test was conducted by applying the ISO standard conditions and
changing the irradiance and NO concentration. The test results are presented in Table 12, Figure 6,
and Figure 7.
Figure 5.
Test results of changes in condition to achieve NO
x
reduction. NO
x
reduction according
to change in the irradiance based on a concentration of (
a
) 0.25 ppm, (
b
) 0.50 ppm, and (
c
) 0.75 ppm.
NO
x
reduction according to the change in the reference concentration at a light rate of (
d
) 2.50 W/cm
2
,
(e) 5.00 W/cm2, and (f) 7.50 W/cm2.

Molecules 2020,25, 4087 10 of 14
Table 9presents the NO reduction results according to the UV-A irradiance and NO concentration.
The applied UV-A irradiance ranged from 2.5 to 7.5 W/m
2
, and the applied NO concentration ranged
from 0.25 to 0.75 ppm. It was determined that the NO concentration reduction increased as the NO
concentration increased under a constant UV-A irradiance. This is because the molecular weight of
NO increased as the NO concentration increased when the UV-A wavelength energy that affects the
TiO2photocatalysts is the same.
Under a constant NO concentration, however, there was no significant trend in the cases of 0.25
and 0.50 ppm as the irradiance increased. Meanwhile, the 0.75 ppm case showed an increase in NO
concentration reduction with increasing irradiance. This result shows that the NO reduction effect can
be obtained by the TiO
2
photocatalyst at concentrations of 0.5 ppm or less; however, the effect is not
proportional to the amount of light. The above results indicate that the NO concentration affects the
NO reduction rather than the UV-A irradiance.
Table 10 presents the NO
2
generation results according to the NO concentration and UV-A
irradiance. NO
2
is an indicator of oxidation reactions because it is generated through photochemical
reactions between UV-A and TiO
2
photocatalysts. The NO
2
generation results confirmed that there
were photochemical reactions between UV-A and TiO
2
photocatalysts during the test, with changes in
the UV-A irradiance and NO concentration. However, NO
2
was generated through a combination
with NO and oxygen in the atmosphere. Thus, the amount of NO
2
generated did not exhibit a constant
trend with an increase in the UV-A irradiance and NO concentration.
Table 11 presents the NO
x
reduction results according to the NO concentration and UV-A irradiance.
When the UV-A irradiance was changed under the same NO concentration, the NO
x
concentration
reduction increased in proportion to the reduction rate. This is because the amount of
•
OH and O
2−•
radicals that were generated on the surface of the coating material changed according to the UV-A
irradiance, thereby reducing the amount of oxidized NO
x
. Figure 5a–c show the results according
to changes in the corresponding conditions. These were calculated for 1 h after stabilization of the
concentration reduction. The results confirmed that the concentration reduction varied depending on
the irradiance.
In addition, when the NO concentration was changed under the same UV-A irradiance, the NO
x
concentration reduction increased as the NO concentration increased. However, at 0.25 and 0.5 ppm,
which are low NO concentrations, there is a difference in the reduction amount according to the change
in the light amount for 60 min. As the concentration decreased, the difference became less. Figure 5d–f
show the results of the corresponding tests. The results confirm that there were certain differences in
the concentration reduction as the NO concentration changed under the same irradiance.
4. Discussion
The NO
x
reduction performance test of the TiO
2
photocatalysts was conducted using the
photocatalyst coating material. The test was conducted by applying the ISO standard conditions and
changing the irradiance and NO concentration. The test results are presented in Table 12, Figure 6,
and Figure 7.
Table 12. NOxreduction amount.
UV-A Irradiance (W/m2)
NO Concentration (ppm) 0.25 0.50 0.75 1.00
Reduction Amount (µmol/10 cm2·3 h)
2.50 2.81 3.68 4.32 5.41
5.00 2.99 3.23 5.34 5.45
7.50 3.24 5.05 8.18 7.99
10.0 4.45 6.08 8.07 8.95

Molecules 2020,25, 4087 11 of 14
Molecules 2020, 25, x FOR PEER REVIEW 11 of 15
11
Table 12. NO
x
reduction amount.
NO Concentration (ppm)
UV-A Irradiance (W/m
2
)
0.25 0.50 0.75 1.00
Reduction Amount (µmol
/
10 cm
2
·3 h)
2.50 2.81 3.68 4.32 5.41
5.00 2.99 3.23 5.34 5.45
7.50 3.24 5.05 8.18 7.99
10.0 4.45 6.08 8.07 8.95
Figure 6. Results of NO
x
reduction rate.
Figure 7. Total NO
x
reduction test results.
According to the results of the test under the standard conditions according to the ISO 22197-
1:2007 (NO concentration: 1.00 ppm, UV-A irradiance: 10 W/m
2
), the amount of NO
x
that was reduced
was 8.95 µmol/10 cm
2
·3 h, as presented in Table 12 and Figure 7. In addition, when the NO
concentration and UV-A irradiance were low, the amount of NO
x
that was reduced ranged from 2.81
to 8.18 µmol/10 cm
2
·3 h.
Moreover, when the reduction amount confirmed that the standard condition test was assumed
to have 100% reduction, a NO
x
reduction efficiency of 90% or higher could be obtained under the
conditions that exceed Section 1, as shown in Figure 6. The NO
x
reduction efficiency was reduced to
approximately 50% under the conditions between Sections 1 and 2. Under the conditions of Section
Figure 6. Results of NOxreduction rate.
Molecules 2020, 25, x FOR PEER REVIEW 11 of 15
11
Table 12. NO
x
reduction amount.
NO Concentration (ppm)
UV-A Irradiance (W/m
2
)
0.25 0.50 0.75 1.00
Reduction Amount (µmol
/
10 cm
2
·3 h)
2.50 2.81 3.68 4.32 5.41
5.00 2.99 3.23 5.34 5.45
7.50 3.24 5.05 8.18 7.99
10.0 4.45 6.08 8.07 8.95
Figure 6. Results of NO
x
reduction rate.
Figure 7. Total NO
x
reduction test results.
According to the results of the test under the standard conditions according to the ISO 22197-
1:2007 (NO concentration: 1.00 ppm, UV-A irradiance: 10 W/m
2
), the amount of NO
x
that was reduced
was 8.95 µmol/10 cm
2
·3 h, as presented in Table 12 and Figure 7. In addition, when the NO
concentration and UV-A irradiance were low, the amount of NO
x
that was reduced ranged from 2.81
to 8.18 µmol/10 cm
2
·3 h.
Moreover, when the reduction amount confirmed that the standard condition test was assumed
to have 100% reduction, a NO
x
reduction efficiency of 90% or higher could be obtained under the
conditions that exceed Section 1, as shown in Figure 6. The NO
x
reduction efficiency was reduced to
approximately 50% under the conditions between Sections 1 and 2. Under the conditions of Section
Figure 7. Total NOxreduction test results.
According to the results of the test under the standard conditions according to the ISO 22197-1:2007
(NO concentration: 1.00 ppm, UV-A irradiance: 10 W/m
2
), the amount of NO
x
that was reduced
was 8.95
µ
mol/10 cm
2·
3 h, as presented in Table 12 and Figure 7. In addition, when the NO
concentration and UV-A irradiance were low, the amount of NO
x
that was reduced ranged from 2.81
to 8.18 µmol/10 cm2·3 h.
Moreover, when the reduction amount confirmed that the standard condition test was assumed
to have 100% reduction, a NO
x
reduction efficiency of 90% or higher could be obtained under the
conditions that exceed Section 1, as shown in Figure 6. The NO
x
reduction efficiency was reduced to
approximately 50% under the conditions between Sections 1and 2. Under the conditions of Section 3,
the NO
x
reduction efficiency sharply decreased compared with those of Sections 1and 2. This confirms
that it was reduced to less than 40% compared with those achieved under the ISO standard conditions.
Thus, it was confirmed that the NO
x
reduction efficiency is less than 40% compared with those
achieved under the ISO standard conditions that can be obtained if the TiO
2
photocatalysts are applied
to vertical surfaces. In this case, the UV-A irradiance is reduced owing to the influence of reflected
light other than the roofs where the UV rays reach directly. It was also confirmed that a similar NO
x
reduction effect can be obtained in winter when the solar altitude is low or on days when the cloud
coverage is high.

Molecules 2020,25, 4087 12 of 14
5. Conclusions
The purpose of this study is to reduce the concentration of PM precursors. In this study, the NO
x
concentration reduction performance of TiO
2
photocatalysts through their reactions with UV-A was
analyzed as a method for reducing NO
x
in secondary sources. The findings of this study can be
summarized as follows.
First, the conditional test results that meet the international standard test standard of TiO
2
photocatalyst performance evaluation showed that the TiO
2
coating material used in this study had a
NO
x
reduction effect. The presence of Ti and O in the coating agent was confirmed through FESEM
and EDS analysis.
Second, the NO
x
reduction effect of TiO
2
coating material was confirmed through changes in
UV-A irradiance and NO concentration. The UV-A irradiance and concentration were reduced to 25%,
50%, and 75% compared to the ISO standard experimental method, and the NO
x
reduction effect was
also changed. It is expected that the NO
x
reduction effect can be obtained even in the actual NO
x
concentration and the UV-A irradiance in the atmosphere.
Third, according to the changed UV-A irradiance and NO concentration, it was confirmed that
the factor that significantly affects NO
x
reduction is the NO concentration rather than the UV-A
irradiance. In addition, when UV-A irradiance is 7.5 W/m
2
and NO concentration is 0.75 ppm or more,
the reduction effect is confirmed to increase significantly.
Fourth, it was discovered that the NO
x
reduction efficiency is less than 40% compared with those
achieved under the ISO standard conditions that can be obtained if the TiO
2
photocatalysts are applied
to vertical surfaces where the UV-A irradiance is reduced. It was also confirmed that a similar NO
x
reduction effect can be obtained in winter when the solar altitude is low compared with in summer or
days when the cloud coverage is high.
Therefore, it is expected that the coating material mixed with TiO
2
photocatalysts used in this
study can be applied to existing buildings and structures. In addition, it is also highly applicable
to building materials in which the direct application of TiO
2
photocatalysts is difficult. It was also
confirmed that the coating material can be used to reduce PM precursors because it can reduce the NO
x
concentration in atmospheric environments. Therefore, the findings of this study are expected to be
used to evaluate the performance of various building materials that are mixed with TiO
2
photocatalysts
in the future.
Author Contributions:
Conceptualization, Y.W.S. and J.C.P.; methodology, Y.W.S.; validation, Y.W.S., M.Y.K.,
and J.C.P.; formal analysis, Y.W.S. and M.H.C.; investigation, Y.W.S. and Y.K.Y.; resources, Y.W.S.; data curation,
Y.W.S.; writing—original draft preparation, Y.W.S.; writing—review and editing, J.C.P. and M.H.C.; visualization,
Y.W.S. and M.Y.K.; supervision, J.C.P.; project administration, Y.W.S. and J.C.P. All authors have read and agreed to
the published version of the manuscript.
Funding:
This research was funded by a grant from the Korea Agency for Infrastructure Technology Advancement
(KAIA) that was funded by the Ministry of Land, Infrastructure and Transport (grant number 20SCIP-B146254-03).
Conflicts of Interest: The authors declare no conflict of interest.
References
1.
World Health Organization, “Global Urban Ambient Air Pollution Database” Summary results. Available
online: https://www.who.int/airpollution/data/cities-2016/en/(accessed on 26 May 2020).
2.
Cai, J.; Yu, W.; Li, B.; Yao, R.; Zhang, T.; Guo, M.; Wang, H.; Cheng, Z.; Xiong, J.; Meng, Q.; et al. Particle
removal efficiency of a household portable air cleaner in real-world residences: A single-blind cross-over
field study. Energy Build. 2019,203, 109464. [CrossRef]
3.
Dai, X.; Liu, J.; Li, X.; Zhao, L. Long-term monitoring of indoor CO
2
and PM
2.5
in Chinese homes:
Concentrations and their relationships with outdoor environments. Build. Environ.
2018
,144, 238–247.
[CrossRef]
4.
Ao, C.H.; Lee, S.C. Indoor air purification by photocatalyst TiO
2
immobilized on an activated carbon filter
installed on an air cleaner. Chem. Eng. Sci. 2005,60, 103–109. [CrossRef]

Molecules 2020,25, 4087 13 of 14
5.
Cunha-Lopes, I.; Martins, V.; Faria, T.; Correia, C.; Almeida, S.M. Children’s exposure to sized-fractioned
particulate matter and black carbon in an urban environment. Build. Environ.
2019
,155, 187–194. [CrossRef]
6.
Wang, Z.; Liu, J. Spring-time PM
2.5
elemental analysis and polycyclic aromatic hydrocarbons measurement in
High-rise residential buildings in Chongqing and Xian, China. Energy Build.
2018
,173, 623–633. [CrossRef]
7.
National Air Pollutants Emission 2010, National Institute of Environmental Research, Ministry of Environment.
Available online: http://www.me.go.kr/mamo/web/index.do?menuId=588 (accessed on 26 May 2020).
8.
National Institute of Environmental Research, Amount of Emissions by Air Pollutant Sector in 2016. Available
online: http://airemiss.nier.go.kr/mbshome/mbs/airemiss/index.do (accessed on 26 May 2020).
9.
Ohko, Y.; Saitoh, S.; Tatsuma, T.; Fujishima, A. Photoelectrochemical anticorrosion and self-cleaning effects of
a TiO2coating for type 304 stainless steel. J. Electrochem. Soc. 2001,148, B24–B28. [CrossRef]
10.
Wu, Z.; Lee, D.; Rubner, M.F.; Cohen, R.E. Structural color in porous, superhydrophilic, and self-cleaning
SiO2/TiO2bragg stacks. Small 2007,3, 1445–1451. [CrossRef]
11.
Paz, Y.; Luo, Z.; Rabenberg, L.; Heller, A. Photooxidative self-cleaning transparent titanium dioxide films on
glass. J. Mater. Res. 1995,10, 2842–2848. [CrossRef]
12.
Cedillo-Gonzalez, E.I.; Ricco, R.; Montorsi, M.; Falcaro, P.; Siligardi, C. Self-cleaning glass prepared from a
commercial TiO
2
nanodispersion and its photocatalytic performance under common anthropogenic and
atmospheric factors. Build. Environ. 2014,71, 7–14. [CrossRef]
13.
Chabas, A.; Alfaro, S.; Lombardo, T.; Verney-Carron, A.; Da Silva, E.; Triquet, S.; Cachier, H.; Leroy, E. Long
term exposure of self-cleaning and reference glass in an urban environment: A comparative assessment.
Build. Environ. 2014,79, 57–65. [CrossRef]
14.
Chabas, A.; Lombardo, T.; Cachier, H. Behaviour of self-cleaning glass in urban atmosphere. Build. Environ.
2008,43, 2124–2131. [CrossRef]
15.
Nuño, M.; Ball, R.J.; Bowen, C.R.; Kurchania, R.; Sharma, G.D. Photocatalytic activity of electrophoretically
deposited (EPD) TiO2coatings. J. Mater. Sci. 2015,50, 4822–4835. [CrossRef]
16.
Hamidi, F.; Aslani, F. TiO
2
-based Photocatalytic Cementitious Composites: Materials, Properties, Influential
Parameters, and Assessment Techniques. Nanomaterials 2019,9, 1444. [CrossRef] [PubMed]
17.
Kolarik, J.; Toftum, J. The impact of a photocatalytic paint on indoor air pollutants: Sensory assessments.
Build. Environ. 2012,57, 396–402. [CrossRef]
18.
Hüsken, G.; Hunger, M.; Brouwers, H.J.H. Experimental study of photocatalytic concrete products for air
purification. Build. Environ. 2009,44, 2463–2474. [CrossRef]
19.
Ramakrishnan, G.; Orlov, A. Development of novel inexpensive adsorbents from waste concrete to mitigate
NOxemissions. Build. Environ. 2014,72, 28–33. [CrossRef]
20.
Ramirez, A.M.; Demeestere, K.; De Belie, N. Titanium dioxide coated cementitious materials for air purifying
purposes: Preparation, characterization and toluene removal potential. Build. Environ.
2010
,45, 832–838.
[CrossRef]
21.
Nuño, M.; Ball, R.J.; Bowen, C.R. Study of solid/gas phase photocatalytic reactions by electron ionization
mass spectrometry. J. Mass Spectrom. 2014,49, 716–726. [CrossRef]
22.
Li, R.; Li, T.; Zhou, Q. Impact of Titanium Dioxide (TiO
2
) Modification on Its Application to Pollution
Treatment—A Review. Catalysts 2020,10, 804. [CrossRef]
23.
Pichat, P. Are TiO
2
Nanotubes Worth Using in Photocatalytic Purification of Air and Water? Molecules
2014
,
19, 15075–15087. [CrossRef]
24.
Fujishima, A.; Zhang, X.; Tryk, D.A. TiO
2
photocatalysis and related surface phenomena. Surf. Sci. Rep.
2008,63, 515–582. [CrossRef]
25.
Lee, S.-A.; Choo, K.H.; Lee, C.H.; Lee, H.I.; Hyeon, T.; Choi, W.; Kwon, H.H. Use of ultrafiltration membranes
for the separation of TiO
2
photocatalysts in drinking water treatment. Ind. Eng. Chem. Res.
2001
,40,
1712–1719. [CrossRef]
26.
Ryu, J.; Choi, W. Substrate-specific photocatalytic activities of TiO
2
and multiactivity test for water treatment
application. Environ. Sci. Technol. 2007,42, 294–300. [CrossRef] [PubMed]
27.
Suzuki, N.; Okazaki, A.; Kuriyama, H.; Seizawa, I.; Hara, A.; Hirano, Y.; Nakabayashi, Y.; Roy, N.;
Terashima, C.; Nakata, K. Synthesis of Mesoporous TiO
2
/Boron-Doped Diamond Photocatalyst and Its
Photocatalytic Activity under Deep UV Light (
λ
=222 nm) Irradiation. Molecules
2018
,23, 3095. [CrossRef]
[PubMed]

Molecules 2020,25, 4087 14 of 14
28.
Ballari, M.M.; Hunger, M.; Hüsken, G.; Brouwers, H.J.H. NOx photocatalytic degradation employing concrete
pavement containing titanium dioxide. Appl. Catal. B 2010,95, 245–254. [CrossRef]
29.
Yoon, I.H.; Lee, K.B.; Kim, J.S.; Kim, S.D. A study on the Estimation of NO
x
reduction in Ambient by
Photocatalyst (TiO2) Block. Korea Soc. Urban Environ. 2017,17, 433–443.
30.
Takeuchi, K.; Murasawa, S.; Ibusuki, T. The World of Photocatalytic, 1st ed.; Daeyoungsa: Seoul, Korea, 2000;
pp. 62–87.
31.
Choi, Y.J.; Park, J.Y.; Lee, S.J.; Huh, N.I.; Kim, H.J. A Fundamental Study on the Properties of NO
x
Removal
on Cement Mortar With TiO
2
Powder as Photocatalyst. J. Archit. Instit. Korea Struct. Constr.
2002
,18, 43–50.
32.
Cassar, L.; Pepe, C.; Tognon, G.; Guerrini, G.L.; Amadelli, R. White cement for architectural concrete,
possessing photocatalytic properties. In Proceedings of the 11th International Congress on the Chemistry of
Cement, Durban, South Africa, 11–16 May 2003.
33.
Guerrini, G.L.; Beeldens, A.; Crispino, M.; D’Ambrosio, G.; Vismara, S. Environmental benefits of innovative
photocatalytic cementitious road materials. In Proceedings of the 10th International Conference on Concrete
Pavements, Quebec City, QC, Canada, 8–12 July 2012.
34.
Guo, M.-Z.; Maury-Ramirez, A.; Poon, C.S. Photocatalytic activities of titanium dioxide incorporated
architectural mortars. Build. Environ. 2015,94, 395–402. [CrossRef]
35.
Janus, M.; Madraszewski, S.; Zajac, K.; Kusiak-Nejman, E.; Morawski, A.W.; Stephan, D. Photocatalytic
Activity and Mechanical Properties of Cements Modified with TiO2/N. Materials 2019,12, 3756. [CrossRef]
36.
Yu, H.; Dai, W.; Qian, G.; Gong, X.; Zhou, D.; Li, X.; Zhou, X. The NOx degradation performance of nano-TiO
2
coating for asphalt pavement. Nanomaterials 2020,10, 897. [CrossRef]
37.
Wang, H.; Jin, K.; Dong, X.; Zhan, S.; Liu, C. Preparation technique and properties of nano-TiO
2
photocatalytic
coatings for asphalt pavement. Appl. Sci. 2018,8, 2049. [CrossRef]
38.
Witkowski, H.; Jackiewicz-Rek, W.; Chilmon, K. Air purification performance of photocatalytic concrete
paving blocks after seven years of service. Appl. sci. 2019,9, 1735. [CrossRef]
39.
Luna, M.; Mosquera, M.J.; Vidal, H.; Gatica, J.M. Au-TiO
2
/SiO
2
photocatalysts for building materials:
Self-cleaning and de-polluting performance. Build. Environ. 2019,164, 106347. [CrossRef]
40.
Lettieri, M.; Colangiuli, D.; Masieri, M.; Calia, A. Field performances of nanosized TiO
2
coated limestone for
a self-cleaning building surface in an urban environment. Build. Environ. 2018,147, 506–516. [CrossRef]
41.
Tang, X.; Ughetta, L.; Shannon, S.K.; Houz
é
de l’Aulnoit, S.; Chen, S.; Gould, R.A.T.; Russell, M.L.; Zhang, J.;
Ban-Weiss, G.; Rebecca, L.A. De-pollution efficacy of photocatalytic roofing granules. Build. Environ.
2019
,
160, 106058. [CrossRef]
42.
DIN German Institute for Standardization. Fine Ceramics (Advanced Ceramics, Advanced Technical
Ceramics)—Test Method for Air-Purification Performance of Semiconducting Photocatalytic Materials—Part 1:
Removal of Nitric Oxide; Beuth Verlag GmbH: Berlin Germany, 2007.
43.
Lin, Y.; Yin, F.; Liu, Y.; Wu, K. Influence of vulcanization factors on UV-A resistance of silicone rubber for
outdoor insulators. IEEE T. Dielect. El. In. 2020,27, 296–304. [CrossRef]
44.
Korea Meteorological Administration, Comprehensive Climate Change Monitoring Information. 2017.
Available online: http://www.climate.go.kr/home/09_monitoring/uv/uva (accessed on 26 May 2020).
Sample Availability: Samples of the TiO2Coating are available from the author Y.W.
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2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
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