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Article
Empirical NOx Removal Analysis of Photocatalytic
Construction Materials at Real-Scale
Miyeon Kim 1,2, Hyunggeun Kim 1and Jinchul Park 2, *
Citation: Kim, M.; Kim, H.; Park, J.
Empirical NOx Removal Analysis of
Photocatalytic Construction Materials
at Real-Scale. Materials 2021,14, 5717.
https://doi.org/10.3390/ma14195717
Academic Editor: Yeonung Jeong
Received: 25 August 2021
Accepted: 23 September 2021
Published: 30 September 2021
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Attribution (CC BY) license (https://
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4.0/).
1SH Urban Research Center, Seoul Housing and Communities Corporation, Seoul 06336, Korea;
mykim1983@naver.com (M.K.); hgkim@i-sh.co.kr (H.K.)
2Department of Architectural Engineering, Chung-Aug University, Seoul 06974, Korea
*Correspondence: jincpark@cau.ac.kr; Tel.: +82-2-820-5261; Fax: +82-2-816-7740
Abstract:
The NOx removal performance of photocatalytic construction materials is demonstrated us-
ing two experiments under indoor and outdoor environments: (1) A photoreactor test was conducted
to assess the NO removal performance of construction materials (e.g., coatings, paints and shotcrete)
using a modified ISO 22197-1 method; (2) A water washing test was conducted using two specimens
enlarged to the size of actual building materials and artificially exposed to NOx in a laboratory to
analyze NOx removal performance. For (1), the UV irradiation of the outdoor environment was
analyzed and the experiment was conducted in an indoor laboratory under UV irradiation identical
to that of the outdoor condition. Photoreactor tests were conducted on construction materials applied
to actual buildings located in Seoul, South Korea. In (2), the enlarged specimen was used for a field
experiment by applying a modified method from the ISO 22197-1 standard. On sunny days, the NOx
removal performance (3.12–4.76
µ
mol/150 cm
2·
5 h) was twice as much as that of the ISO 22197-1
standard specification (2.03
µ
mol/150 cm
2·
5 h) in the real-world. The washing water test results
indicated that general aqueous paint achieved a NOx removal of 3.88
µ
mol, whereas photocatalytic
paint was superior to 14.13 µmol.
Keywords:
photocatalysis; construction materials; ISO22197-1; titanium dioxide (TiO
2
); nitrogen
oxides (NOx)
1. Introduction
Recently, the emission of nitrogen oxide (NOx) has been considered a major envi-
ronmental concern. NOx not only causes respiratory disorders in the human body, but
also destroys the ozone layer in the stratosphere, thereby promoting climate change [
1
,
2
].
Further, NOx is a precursor substance of particulate matter generated via chemical reac-
tions with other precursors [
3
–
5
]. Thus, NOx abatement regulation based on various air
pollution policies established by the Gothenburg and Kyoto Protocols has contributed to a
decrease in the worldwide emissions of NOx [
6
]. In addition to regulating NOx generation
from automobiles, power plants and manufacturing facilities [
7
], technologies that remove
NOx in the atmosphere play a vital role in NOx abatement [
8
–
12
]. Heterogeneous photo-
catalysis is a promising NOx removal technology that has been applied as a construction
material [
13
,
14
]. Titanium dioxide (TiO
2
) is among the most widely used photocatalytic
nanoparticles for such applications [
15
]. The NOx removal process using TiO
2
involves
the illumination of the surface TiO
2
, which produces two types of carriers: an electron
(e
−
) and a hole (h
+
), followed by oxidation of a donor molecule adsorbed on TiO
2
by the
photo-induced hole. The strong oxidation power of the holes enables the production of
hydroxyl radicals (
·
OH) via the oxidation of water. Then, the adsorbed oxygen can be
reduced by the promoted electron to form a superoxide ion (
·
O
2−
). NOx oxidation is a
complex process involving several steps and intermediate species including the reduction
of previously oxidized species. The NOx oxidation mechanism is summarized in Table 1
and Figure 1(where hv: ultraviolet radiation, Siteˆ(**): surface of TiO
2
and OH: hydroxyl
Materials 2021,14, 5717. https://doi.org/10.3390/ma14195717 https://www.mdpi.com/journal/materials
Materials 2021,14, 5717 2 of 18
radical). Extensive studies have been conducted to analyze the oxidation mechanism of
TiO2under various experimental conditions [16–21].
Table 1. Mechanism for oxidation of NOx on the surface TiO2.
Process Formula
Activation
TiO2+hv →h++e−
H2O(g)+Site ∗∗ →H2Oads
O2(g)+Site ∗∗ →O2ads
NO(g)+Site ∗∗ →NOads
Hole trapping H2O+h+→ ·OH +H+
Electron trapping O2(g)+e−→O−
2
Hydroxyl attack NOads +2·OH →NO2ads +H2O NOads +·OH →HNO3
Site **: surface of TiO2and OH hydroxyl radical.
Materials 2021, 14, x FOR PEER REVIEW 2 of 18
and Figure 1 (where hv: ultraviolet radiation, Site^(**): surface of TiO
2
and OH: hydroxyl
radical). Extensive studies have been conducted to analyze the oxidation mechanism of
TiO
2
under various experimental conditions [16–21].
Table 1. Mechanism for oxidation of NOx on the surface TiO
2
.
Process Formula
Activation
TiOh
v
→h
e
HOgSite
∗∗ → HO
OgSite
∗∗ →O
NOgSite
∗∗ →NO
Hole trapping HOh
→ ∙ OH H
Electron trapping Oge
→O
Hydroxyl attack NO 2∙OH→NO
H
O NO ∙ OH → HNO
Site **: surface of TiO2 and OH: hydroxyl radical).
Figure 1. Mechanism for oxidation of NOx on the wall coated with TiO
2
.
In the past few decades, TiO
2
has been used as a building material for atmospheric
NOx removal by exploiting the NOx oxidation mechanism of TiO
2
[22–25]. Chen and Poon
[26] provided an overview of the development of photocatalytic construction materials in
terms of academic achievements and practical applications. They determined that photo-
catalytic substances can be successfully applied to commercialized construction materials
such as concrete, glass, paints and various types of cementitious materials. Seo and Yun
[27] studied the nitric oxide (NO) removal performance of TiO
2
cementitious materials
under wet conditions. They found that the NO removal and absorption rates under dry
conditions is higher as TiO
2
particles come smaller. The recovery rate changed during
evaporation under wet conditions, and three distinct phases (rapid recovery, stationary
reaction and final recovery) were observed. In addition, they determined that the rate of
NO removal was reduced when the photocatalytic cementitious material was rewetted.
Song et al. [28] studied the NOx removal performance of TiO
2
-based coating materials
used as paint. They demonstrated that sufficient NOx removal can be achieved although
the NO gas concentration and UV-A irradiance are smaller than the experimental condi-
tions of the ISO 22197-1: 2007 standard when their TiO
2
photocatalyst-infused coating is
used. Further, they determined that sufficient NOx removal can be achieved under ad-
verse climatic conditions such as winter and gloomy days using TiO
2
photocatalyst-in-
fused coating materials. Based on the ISO 22197-1: 2007 standard, empirical studies on
NOx removal efficiency and durability settings were conducted comprehensively in an
indoor laboratory.
Figure 1. Mechanism for oxidation of NOx on the wall coated with TiO2.
In the past few decades, TiO
2
has been used as a building material for atmospheric
NOx removal by exploiting the NOx oxidation mechanism of TiO
2
[
22
–
25
]. Chen and
Poon [
26
] provided an overview of the development of photocatalytic construction ma-
terials in terms of academic achievements and practical applications. They determined
that photocatalytic substances can be successfully applied to commercialized construction
materials such as concrete, glass, paints and various types of cementitious materials. Seo
and Yun [
27
] studied the nitric oxide (NO) removal performance of TiO
2
cementitious ma-
terials under wet conditions. They found that the NO removal and absorption rates under
dry conditions is higher as TiO
2
particles come smaller. The recovery rate changed during
evaporation under wet conditions, and three distinct phases (rapid recovery, stationary
reaction and final recovery) were observed. In addition, they determined that the rate of
NO removal was reduced when the photocatalytic cementitious material was rewetted.
Song et al. [
28
] studied the NOx removal performance of TiO
2
-based coating materials used
as paint. They demonstrated that sufficient NOx removal can be achieved although the
NO gas concentration and UV-A irradiance are smaller than the experimental conditions
of the ISO 22197-1: 2007 standard when their TiO
2
photocatalyst-infused coating is used.
Further, they determined that sufficient NOx removal can be achieved under adverse
climatic conditions such as winter and gloomy days using TiO
2
photocatalyst-infused
coating materials. Based on the ISO 22197-1: 2007 standard, empirical studies on NOx
removal efficiency and durability settings were conducted comprehensively in an indoor
laboratory.
Recently, the extensive studies on the NOx removal efficiency and durability of TiO
2
-
based construction materials have resulted in the development of methods for deriving
Materials 2021,14, 5717 3 of 18
quantitative results in outdoor environments for more practical purposes [
29
–
34
]. Guer-
rini [
35
] analyzed the NOx removal performance of photocatalytic cement-based paint
practically by demonstrating the findings of two environmental monitoring campaigns
involving the study of NOx levels measured before and after the reconstruction of Rome’s
“Umberto I” tunnel. The results obtained by Guerrini indicate that the photocatalytic
treatment of the Umberto I tunnel vault with cementitious paint resulted in effective pol-
lution abatement, as shown by the lower concentrations found after the renovation. Yu
et al. [
36
] investigated the performance of a mineral-based transparent air-purifying paint.
The efficiency of this photocatalytic paint for removing air pollutants was determined in
a laboratory setting using the ISO 22197-1 procedure. Subsequently, outdoor monitoring
over a 20-month period was conducted to determine the air pollution removal efficiency
under realistic conditions. They proposed a new protocol using the measured nitrate nitro-
gen (NO
3−
) produced by the photocatalytic oxidation of NO to monitor the air pollutant
removal efficiency. Cordero et al. [
37
] evaluated the NOx removal efficiency of 10 selected
materials using two pilot-scale demonstration platforms installed at two separate locations.
The materials were exposed outdoors for measuring ground-level NO and nitrogen dioxide
(NO
2
) concentrations over a one-year period. The pollutant removal efficiency of the
materials was determined by comparing them to simultaneously measured concentrations
of reference, nonactive materials.
Even though current studies on practical photocatalytic materials on a real scale have
proposed their own distinctive methodology, there remain several challenges to providing
an accurate assessment of NOx removal efficiency. The modified methodology based on
the ISO 22197-1 standard addresses the following challenges. It is necessary to prove that
there is a similar trend of quantitative NOx outcomes derived from indoor and outdoor
experiments to confirm the NOx removal performance of photocatalytic materials in an
outdoor environment. Further, the size of a real construction material, which is a trend
indicating an increase in the amount of NOx observed as the size changes, should be
demonstrated to certify the NOx removal efficiency of photocatalytic materials.
To this end, the NOx removal performance of photocatalytic construction materials is
demonstrated in this study using two different types of experiments in both indoor and
outdoor environments.
(1) A photoreactor test was conducted to assess the NO removal performance of construc-
tion materials such as coatings, paints and shotcrete using a modified ISO 22197-1
method. The UV irradiation of the outdoor environment was analyzed, and the
experiment was conducted in an indoor laboratory under UV irradiation identical to
that of the outdoor condition. Subsequently, photoreactor tests were conducted on
construction materials applied to actual buildings located in Seoul, South Korea. The
NO removal performance at the real scale was demonstrated by confirming that the
trends of the indoor and outdoor environments showed comparable results.
(2)
In this experiment, the NOx removal performance was analyzed by assessing the
amount of NOx ions remaining in the water after washing the surface of the spec-
imen artificially exposed to NOx in the laboratory. The preliminary test used two
same-sized specimens according to the specification in the ISO 22197-1 standard;
the specimens were enlarged to the size of building materials. The experimental
conditions were confirmed when the NOx removal performance was found to in-
crease similarly to the tendency of the specimen to increase in size. The enlarged
specimen was used for a field experiment by applying a modified method from the
ISO 22197-1 standard.
2. Preliminary Test for Measuring NOx Removal
2.1. Experimental Setup
Various types of photocatalytic materials were used to analyze NOx removal perfor-
mance (e.g., paint, coating material and shotcrete). Each photocatalytic material was pre-
pared by mixing anatase-based TiO
2
powder (NP-400 (Product Overview of Photoctalytic
Materials 2021,14, 5717 4 of 18
Powder NP-100. http://www.btfgreen.com/bbs/board.php?bo_table=product1&wr_id=4
(accessed 23 May 2021))from Bentech Frontier, Jeollanam-do, South Korea) with a com-
mercial material. The mixing ratio of anatase-based TiO
2
powder for each photocatalytic
material is summarized in Table 2. In the table, composition of each photocatalytic material
is specified in order to enhance the understanding of the results of this study. TiO
2
powder
was added to each commercial material. A fluid ceramic binder was used in advance
to prevent substances in the paint from interfering with the photocatalytic reaction by
chemically affecting the TiO
2
powder. Other components of the paints are similar to those
of common paint. The optimal photocatalytic reaction was elicited by adjusting the mix-
ing ratio. Synthetic resin emulsions are used as a binder to increase the photocatalytic
performance of shotcrete.
Table 2. Composition of photocatalytic materials.
Material no.
Photocatalytic Coating Photocatalytic Paint Photocatalytic Shotcrete
Contained
Chemicals
Proportion
(%)
Contained
Chemicals Proportion (%) Contained
Chemicals Proportion (%)
1 TiO21.75 TiO23.5 TiO23.5
2Silicone
compound 5.6 Fluid ceramic
binder 7.2 Synthetic resins
emulsions 2.2
3 Water 51.0 Others 89.3 Cements 17.2
4 Others 41.65 - - Others (Aggregates
and water, etc.) 77.1
2.2. Photoreactor Test
We performed a preliminary test in the laboratory to measure the amount of NO
removal from various types of construction materials such as coatings, paints and shotcrete
using the photocatalyst before conducting the field application experiment. A prelimi-
nary test was conducted following ISO 22197-1 to verify the NO removal performance
of the photocatalytic product specimens quantitatively. The NO injections ranged from
37.8–38.6 micromole
(
µ
mol) on a 5 h basis and 8.61
µ
mol on a 1 h basis in shotcrete mea-
surement. A test condition of high NO gas concentration was prepared by connecting a
hose line for gas injection and a bag containing NO gas while the flow rate of 1 ppm of NO
gas was set to 1 L/min. Temperature was set to be 25
±
2.5
◦
C and relative humidity was
set to be 50% according to the ISO 22197-1 standard.
Figure 2shows the apparatus used for NO removal analysis (CM2041, Casella, London,
UK). In the ISO standard, the requirements of the analysis apparatus are clearly stated,
and the apparatus used in this study is suitable for the ISO standard. For this test, UV-A
with a wavelength range of 300 nm to 400 nm was used as the light source. This study
followed the ISO standard’s content to use black light lamps with wavelength ranges of up
to
351 nm
. For the test, the ultraviolet light irradiance (UV irradiation) was kept constant at
10 W/m
2
. The NO removal performance of the photocatalytic coating and photocatalytic
paint was measured for 5 h each, and that of photocatalytic shotcrete for 1 h each.
Table 3lists the NO removal results for each photocatalytic construction material.
Even with a 1-h measurement, the performance level of NO removal from photocatalytic
shotcrete was higher than that of other materials.
Even though the ISO 22197-1 standard officially recommends setting UV irradiation
as 10 W/m
2
, this study performed an additional test to determine UV irradiation identical
to outdoor UV conditions for a more empirical experiment. We intended to determine
the level of actual UV light incident on a vertical wall where the photocatalyst is applied
because the UV light for photocatalytic action varies in the field whereas it is constant in
the laboratory. As the photocatalyst is applied to the wall to which the reactor is attached,
UV light on the southern wall, eastern wall and floor is measured on clear winter days.
In the preliminary test, the NO removal amount on the eastern wall was measured in the
Materials 2021,14, 5717 5 of 18
field to determine the amount of NO injection (see Figure 3). A self-made photoreactor
with a length of 300 mm and a width of 500 mm was used for the field measurements;
this photoreactor is three times larger than the reactor prescribed in the ISO-22197-1 test
method. The transparent window material was composed of quartz to facilitate the passage
of ultraviolet light. The photoreactor was installed on the rooftop of the building with an
NO injection of 3 ppm. Further, the NO removal amount was measured in the preliminary
test to determine the appropriate level of NO injection for use in the field test.
Materials 2021, 14, x FOR PEER REVIEW 5 of 18
Figure 2. Apparatus used to measure NO removal as per ISO 22197-1 specifications.
Table 3 lists the NO removal results for each photocatalytic construction material.
Even with a 1-h measurement, the performance level of NO removal from photocatalytic
shotcrete was higher than that of other materials.
Table 3. Photoreactor test results for NO removal rate at the preliminary test.
Materials NO Injection
(
μ
mol)
NO Removal
(
μ
mol/50 cm2)
Measuring Time
(Hou
r
)
UV Irradiation
(W/m2)
Photocatalytic coating 38.57 7.29 5 10
Photocatalytic paint 37.78 5.76 5 10
Photocatalytic shotcrete 8.61 11.28 1 10
Even though the ISO 22197-1 standard officially recommends setting UV irradiation
as 10 W/m
2
, this study performed an additional test to determine UV irradiation identical
to outdoor UV conditions for a more empirical experiment. We intended to determine the
level of actual UV light incident on a vertical wall where the photocatalyst is applied
because the UV light for photocatalytic action varies in the field whereas it is constant in
the laboratory. As the photocatalyst is applied to the wall to which the reactor is attached,
UV light on the southern wall, eastern wall and floor is measured on clear winter days. In
the preliminary test, the NO removal amount on the eastern wall was measured in the
field to determine the amount of NO injection (see Figure 3). A self-made photoreactor
with a length of 300 mm and a width of 500 mm was used for the field measurements; this
photoreactor is three times larger than the reactor prescribed in the ISO-22197-1 test
method. The transparent window material was composed of quartz to facilitate the
passage of ultraviolet light. The photoreactor was installed on the rooftop of the building
with an NO injection of 3 ppm. Further, the NO removal amount was measured in the
preliminary test to determine the appropriate level of NO injection for use in the field test.
Figure 2. Apparatus used to measure NO removal as per ISO 22197-1 specifications.
Table 3. Photoreactor test results for NO removal rate at the preliminary test.
Materials NO Injection
(µmol)
NO Removal
(µmol/50 cm2)
Measuring
Time
(Hour)
UV
Irradiation
(W/m2)
Photocatalytic coating 38.57 7.29 5 10
Photocatalytic paint 37.78 5.76 5 10
Photocatalytic shotcrete 8.61 11.28 1 10
Figure 4shows the UV irradiation analysis results performed on the southern, eastern
and floor surfaces between 9 a.m. and 4 p.m. at on 21 March. On the southern wall, the UV
irradiation reached the highest level at noon and decreased gradually. On the eastern wall,
the UV irradiation was relatively lower than that on the southern wall and floor surface,
which indicates that the highest irradiation level was 6 W/m
2
. The UV irradiation is the
highest on the floor because the construction materials receive sunlight when they are
attached horizontally towards the sky. Thus, the installation strategy for photocatalytic
materials needs to consider solar insolation efficiency.
2.3. Washing Water Experiment
This experiment involves the quantitative analysis of NO
2
and NO
3-
ions contained
in washing water after washing the surface coated with photocatalytic paint with distilled
water after the specimen is exposed to UV light for a certain period.
Materials 2021,14, 5717 6 of 18
Materials 2021, 14, x FOR PEER REVIEW 6 of 18
(a) (b)
Figure 3. The experimental setup of photoreactor experiment on the wall. (a) Photoreactor with NO gas injection hoses on
the eastern wall; (b) portable analysis device for NOx removal.
Figure 4 shows the UV irradiation analysis results performed on the southern,
eastern and floor surfaces between 9 a.m. and 4 p.m. at on 21 March. On the southern wall,
the UV irradiation reached the highest level at noon and decreased gradually. On the
eastern wall, the UV irradiation was relatively lower than that on the southern wall and
floor surface, which indicates that the highest irradiation level was 6 W/m
2
. The UV
irradiation is the highest on the floor because the construction materials receive sunlight
when they are attached horizontally towards the sky. Thus, the installation strategy for
photocatalytic materials needs to consider solar insolation efficiency.
Figure 4. UV irradiation analysis results for southern, eastern and roof surfaces.
Figure 3.
The experimental setup of photoreactor experiment on the wall. (
a
) Photoreactor with NO gas injection hoses on
the eastern wall; (b) portable analysis device for NOx removal.
Materials 2021, 14, x FOR PEER REVIEW 6 of 18
(a) (b)
Figure 3. The experimental setup of photoreactor experiment on the wall. (a) Photoreactor with NO gas injection hoses on
the eastern wall; (b) portable analysis device for NOx removal.
Figure 4 shows the UV irradiation analysis results performed on the southern,
eastern and floor surfaces between 9 a.m. and 4 p.m. at on 21 March. On the southern wall,
the UV irradiation reached the highest level at noon and decreased gradually. On the
eastern wall, the UV irradiation was relatively lower than that on the southern wall and
floor surface, which indicates that the highest irradiation level was 6 W/m
2
. The UV
irradiation is the highest on the floor because the construction materials receive sunlight
when they are attached horizontally towards the sky. Thus, the installation strategy for
photocatalytic materials needs to consider solar insolation efficiency.
Figure 4. UV irradiation analysis results for southern, eastern and roof surfaces.
Figure 4. UV irradiation analysis results for southern, eastern and roof surfaces.
This quantitative experiment is conducted to determine the amount of NOx removed
by the actual photocatalytic reaction. The experiment based on the washing water recovery
method specified in Item 8.3 of ISO 22197-1 was conducted (Figure 5). Distilled water
(
50 mL
) was used, and the washing time was set to 1 h. Elution operations were performed
twice, and ions were measured via chromatography (Figure 6). Following ion measurement,
the removed amount of NOx was calculated based on the number of ions eluted by the
formulas specified in Item 9.7 of the ISO 22197-1 test method.
n=nw1+nw2(1)
nw1+nw2=Vw1pNO−
3,w1÷62 +pNO−
2,w1÷46+Vw2pNO−
3,w2÷62 +pNO−
2,w2÷46(2)
Materials 2021,14, 5717 7 of 18
where n,v,
pNO−
3
and
pNO−
2
denote the number of moles of nitric acid eluted from the
specimen, amount of distilled water recovered for washing, concentration of nitric acid
eluted from the specimen and concentration of nitrous acid eluted from the specimen,
respectively.
Materials 2021, 14, x FOR PEER REVIEW 7 of 18
2.3. Washing Water Experiment
This experiment involves the quantitative analysis of NO
2
and NO
3-
ions contained
in washing water after washing the surface coated with photocatalytic paint with distilled
water after the specimen is exposed to UV light for a certain period.
This quantitative experiment is conducted to determine the amount of NOx removed
by the actual photocatalytic reaction. The experiment based on the washing water
recovery method specified in Item 8.3 of ISO 22197-1 was conducted (Figure 5). Distilled
water (50 mL) was used, and the washing time was set to 1 h. Elution operations were
performed twice, and ions were measured via chromatography (Figure 6). Following ion
measurement, the removed amount of NOx was calculated based on the number of ions
eluted by the formulas specified in Item 9.7 of the ISO 22197-1 test method.
𝑛𝑛
𝑛
(1)
𝑛
𝑛
𝑉
𝑝
,
62𝑝
,
46 𝑉
𝑝
,
62𝑝
,
46
(2)
where n, v, 𝑝
and 𝑝
denote the number of moles of nitric acid eluted from the
specimen, amount of distilled water recovered for washing, concentration of nitric acid
eluted from the specimen and concentration of nitrous acid eluted from the specimen,
respectively.
Figure 5. Overview of the washing water experiment of the specimen specified in ISO 22197-1.
Figure 6. 930 Compact IC Flex ion chromatography (940 Compact IC Flex, Metrohm, FL, USA).
In this study, several experimental conditions were reviewed to modify the existing
test methods of ISO 22197-1 for outdoor field test conditions. First, the amount of water
washing was analyzed. If more washing water than necessary is used, it can dilute the ion
concentration and make the measurement difficult; separate enrichment work is required
Figure 5. Overview of the washing water experiment of the specimen specified in ISO 22197-1.
Materials 2021, 14, x FOR PEER REVIEW 7 of 18
2.3. Washing Water Experiment
This experiment involves the quantitative analysis of NO
2
and NO
3-
ions contained
in washing water after washing the surface coated with photocatalytic paint with distilled
water after the specimen is exposed to UV light for a certain period.
This quantitative experiment is conducted to determine the amount of NOx removed
by the actual photocatalytic reaction. The experiment based on the washing water
recovery method specified in Item 8.3 of ISO 22197-1 was conducted (Figure 5). Distilled
water (50 mL) was used, and the washing time was set to 1 h. Elution operations were
performed twice, and ions were measured via chromatography (Figure 6). Following ion
measurement, the removed amount of NOx was calculated based on the number of ions
eluted by the formulas specified in Item 9.7 of the ISO 22197-1 test method.
𝑛𝑛
𝑛
(1)
𝑛
𝑛
𝑉
𝑝
,
62𝑝
,
46 𝑉
𝑝
,
62𝑝
,
46
(2)
where n, v, 𝑝
and 𝑝
denote the number of moles of nitric acid eluted from the
specimen, amount of distilled water recovered for washing, concentration of nitric acid
eluted from the specimen and concentration of nitrous acid eluted from the specimen,
respectively.
Figure 5. Overview of the washing water experiment of the specimen specified in ISO 22197-1.
Figure 6. 930 Compact IC Flex ion chromatography (940 Compact IC Flex, Metrohm, FL, USA).
In this study, several experimental conditions were reviewed to modify the existing
test methods of ISO 22197-1 for outdoor field test conditions. First, the amount of water
washing was analyzed. If more washing water than necessary is used, it can dilute the ion
concentration and make the measurement difficult; separate enrichment work is required
Figure 6. 930 Compact IC Flex ion chromatography (940 Compact IC Flex, Metrohm, FL, USA).
In this study, several experimental conditions were reviewed to modify the existing
test methods of ISO 22197-1 for outdoor field test conditions. First, the amount of water
washing was analyzed. If more washing water than necessary is used, it can dilute the ion
concentration and make the measurement difficult; separate enrichment work is required
to solve this problem. Second, a long washing time can increase the time required to
process the specimen, which can result in an excessively long experimental schedule. The
ISO-based specimens were washed following the test method specified in ISO 22197-1.
Before the field test, a few experimental variables were modified to examine that-the
impact of the simplification of ISO 22197-1 method may have a significant effect on deriving
the experimental results. Thus, the amount of washing water was adjusted to 20 mL for
each washing considering the quantity of samples to be measured via ion chromatography;
the washing time was limited to 5 min. The specific experimental conditions are addressed
in Table 4.
Materials 2021,14, 5717 8 of 18
Table 4. The conditions of chromatographic ionic analysis.
Model 930 Compact IC Flex
Column Metrosep A Supp 5, 250 ×4 mm
Eluent 3.2 mM Sodium carbonate,
1.0 M Sodium bicarbonate
Flow rate 0.7 mL/min
Inj. Volume 20 µL
Detection Conductivity Detector
The outdoor preliminary tests were conducted to check whether the test results
showed a similar tendency to that observed in the laboratory experiments. A wall with
photocatalytic paint was presumed, and common aqueous paint and photocatalytic paint
were applied to ordinary concrete specimens. These specimens were then ex-posed to the
same outdoor conditions and washed under the same conditions as the ones used in the
preceding experiments to check the state of ion detection. For the pre-liminary outdoor
experiments, specimens were installed on the roof of an apartment in Seoul, Korea. The
specimens were exposed to natural conditions for a certain period, collected and washed
by dipping in 20 mL of washing water for 5 min. The washing water was then collected to
analyze the ion number using ion chromatography The size of the specimen is
49.5 mm
in
width and 99.0 mm in length, and the thickness of the specimen is 5 mm. The specimen
is made by cutting the large plate with photocatalytic paint to the size specified above.
The Specimen A and B were made by applying photocatalytic paint (containing about
3.5% of TiO
2
component) to the surface of a plate made of concrete. In this study, the
following pre-process is conducted on the specimens. First, in order to remove organic
matter remaining on the surface, UV rays are irradiated for 16 h. The irradiated UV rays
are10 W/m2which can sufficiently decompose organic matter on the surface.
Two photocatalytic specimens, A and B, were used to conduct the ISO 22197-1 experi-
ment to verify the activity for the preliminary test. Figure 7shows the test results for the
two types of specimens. In order to sufficiently supply NO gas in the reactor and react
sufficiently with UV-A, lag occurred at the beginning. Since the experiment time is 5 h, it
shows constant removal performance up to 300 min and shows the following results after
300 min. Specimen A showed an active photocatalytic reaction, with an NO removal rate
of approximately 12%; specimen B showed a removal rate of approximately 5%, which is
approximately half of that of specimen A.
Materials 2021, 14, x FOR PEER REVIEW 9 of 18
(a) (b)
Figure 7. NO removal rate results for 6 h 30 min while UV-A is illuminated. (a) and (b), Ion chromatography result of
specimen A and specimen B.
Table 5. NO removal for each specimen, nitric acid ion weight and ratio analysis results.
Division Removed NO
(μmol)
Nitric Acid Ionic Weight
(μmol)
Ratio
(%)
Specimen A 4.74 0.96 48
Specimen B 2.03 0.34 41
NO removal results and recovered nitric acid ionic weights from each result showed
different trends. However, the recovery rate was found to be less than 50%, which differs
from the calculated NO removal amount. Based on this measurement method, it can be
concluded that the conditions for washing the specimen presented above are not corre-
lated with the amount of NOx attached to the surface by photocatalytic activity. Therefore,
only performing the test method specified in ISO 22197-1 can produce an overall error. In
the above experiment, we used cured concrete specimens as the base materials to be
painted; however, it is difficult to use them for panels exposed to outdoor environment.
Even in the case of ISO specimens, it is expected that using mass-produced base materials
for various experiments will yield more objective results. To sum up, the amount of wash-
ing water was adjusted from 20 mL to 50 mL for each washing considering the quantity
of samples to be measured via ion chromatography; the washing time was also re-adjusted
from 5 min to 1 h.
Considering the experimental conditions above, a cellulose fiber reinforced cement
(CRC) board (from Byuksan corporation (Product Overview of CRC board used in this
study. http://www.byucksan.com/01_product/product.asp?cate=001004002) Korea) was
selected as the base material that satisfies the above conditions. The CRC board was fab-
ricated by mixing natural pulp such as cellulose fiber, Portland cement and silica sand
with water. The CRC board is a non-flammable (flame-retardant grade 1) construction
material produced through an autoclave curing process after pressurizing to 10,000 tons.
It is an eco-friendly asbestos-free construction material that exhibits only slight changes
in length due to temperature variation, water resistance and noncombustibility (flame re-
tardant grade 1). The experiment was conducted according to the method specified in ISO
22197-1 because washing the specimen under the conditions set previously does fully re-
flect the NO amount on the surface. The results are summarized in Table 6. The CRC-A
and CRC-B were made by applying photocatalytic paint (containing about 3.5% of TiO
2
component) to the surface of CRC board.
Figure 7.
NO removal rate results for 6 h 30 min while UV-A is illuminated. (
a
) and (
b
), Ion chromatography result of
specimen A and specimen B.
Table 5shows the results of analyzing NO removal of the NO attached to the surface
during the test performed using the ISO 22197-1 measurement method and the nitric acid
Materials 2021,14, 5717 9 of 18
ionic weight detected in the washing water. For this experiment, the amount of washing
water was 20 mL, and specimens were eluted five times for 5 min.
Table 5. NO removal for each specimen, nitric acid ion weight and ratio analysis results.
Division Removed NO
(µmol)
Nitric Acid Ionic Weight
(µmol)
Ratio
(%)
Specimen A 4.74 0.96 48
Specimen B 2.03 0.34 41
NO removal results and recovered nitric acid ionic weights from each result showed
different trends. However, the recovery rate was found to be less than 50%, which differs
from the calculated NO removal amount. Based on this measurement method, it can be
concluded that the conditions for washing the specimen presented above are not correlated
with the amount of NOx attached to the surface by photocatalytic activity. Therefore, only
performing the test method specified in ISO 22197-1 can produce an overall error. In the
above experiment, we used cured concrete specimens as the base materials to be painted;
however, it is difficult to use them for panels exposed to outdoor environment. Even in the
case of ISO specimens, it is expected that using mass-produced base materials for various
experiments will yield more objective results. To sum up, the amount of washing water
was adjusted from 20 mL to 50 mL for each washing considering the quantity of samples to
be measured via ion chromatography; the washing time was also re-adjusted from 5 min to
1 h.
Considering the experimental conditions above, a cellulose fiber reinforced cement
(CRC) board (from Byuksan corporation (Product Overview of CRC board used in this
study. http://www.byucksan.com/01_product/product.asp?cate=001004002, accessed
on 22 September 2021) Korea) was selected as the base material that satisfies the above
conditions. The CRC board was fabricated by mixing natural pulp such as cellulose fiber,
Portland cement and silica sand with water. The CRC board is a non-flammable (flame-
retardant grade 1) construction material produced through an autoclave curing process
after pressurizing to 10,000 tons. It is an eco-friendly asbestos-free construction material
that exhibits only slight changes in length due to temperature variation, water resistance
and noncombustibility (flame retardant grade 1). The experiment was conducted according
to the method specified in ISO 22197-1 because washing the specimen under the conditions
set previously does fully reflect the NO amount on the surface. The results are summarized
in Table 6. The CRC-A and CRC-B were made by applying photocatalytic paint (containing
about 3.5% of TiO2component) to the surface of CRC board.
Table 6. NO removal results for CRC board panel experiments.
Division NO Amount with Washing Once
(µmol)
Accumulated Removed NO Amount
(µmol)
CRC-A 0.81 3.57
CRC-B 1.11 2.84
CRC boards are supplied through various processes such as manufacturing and
storage, and therefore, it is difficult to verify if a significant amount of nitrogen oxides
is already present. Therefore, removing nitrogen oxides contained in the base material
is deemed necessary to use the CRC board as the base material. For the preliminary
experiment, the experimental method with the modified specimens was conducted based
on ISO standard methods established for outdoor field experiments. The experimental
conditions and methods were as follows:
1. Preparation of specimen
•The base material of the specimen was unified as the CRC board.
Materials 2021,14, 5717 10 of 18
•
The CRC board used as the base material was washed sufficiently to remove
pre-exiting nitrogen oxides.
•
The test specimens are prepared with CRC boards; the photocatalytic paint was
applied to these boards, and the general aqueous paint specimens were prepared
as a control group.
2. Methods of analyzing NO removal and washing water.
•NO removal was conducted in accordance with the ISO 22197-1 method.
•
Concentrations other than those specified in ISO 22197-1 were not significant in
the experiment; hence, the experiments were conducted only at the prescribed
concentration of 1 ppm.
•
The washing of the specimens that have undergone the ISO 22197-1 experiment
are in accordance with the washing method specified in ISO 22197-1.
•
The washing of ISO specimens exposed to the outdoor environment should be
performed according to the washing method specified in ISO 22197-1.
3. Methodology for the Field Test
3.1. Photoreactor Field Test Method
In this study, photocatalytic materials applied to a real building were used to conduct
field tests (Figure 8). After applying primers all over the southern wall of Building A, the
photocatalytic coating agent was applied using rollers; the coating area was 297 m
2
. In
Building B, we applied the photocatalytic paint by brushing and spraying onto the eastern
wall, and the area of application was 889 m
2
. In Building C, the photocatalytic shotcrete
was applied to a site of 61 m
2
, up to the second floor on the east side of the building. In the
same manner as the preliminary test method, a reactor was installed in the area with the
photocatalytic material applied to each building to measure the NO amount of injection
and removal for more than 5 h. We simultaneously measured the UV irradiation. In the
case of outdoor experiments, since it is impossible to adjust experimental variables such as
illuminance, temperature and humidity, the experimental variables were carried out under
natural experimental conditions.
Materials 2021, 14, x FOR PEER REVIEW 11 of 18
(a) (b) (c)
Figure 8. Photocatalytic materials applied on the real buildings in the field assessment. (a) Building A, photocatalyst coat-
ing; (b) Building B, photocatalyst paint; (c) Building C, photocatalyst shotcrete.
Figure 9 shows installed photoreactor and UV radiometer used in the field experi-
ments. The size of the UV radiometer is 50 × 300 mm, which is three times larger than the
specimen defined in the ISO method. Considering that the gas could evenly reach the
surface of specimen, the space between the transparent plate and the inner specimen was
set to 5 mm. The photoreactor was set to 3 ppm as the specimen size was increased by a
factor of three. We conducted a comparative evaluation of NO removal using three pho-
tocatalytic products based on the minimum criteria for photocatalytic product certifica-
tion set by the Photocatalysis Industry Association of Japan.
Figure 9. Installed photoreactor and UV radiometer on the rooftop.
3.2. ISO-Based Washing Water Field Test Method for Real-Scale Construction Materials
As the ISO-based specimen size is smaller than the size of the actual building mate-
rial, a modified experimental method is used to perform practical and empirical analyses.
To this end, the established methods are summarized as follows.
It is difficult to fabricate panels made for outdoor exposure using concrete. Hence,
the CRC board reviewed in the ISO specimen tests was used as the base material. The size
of the CRC board was set to 800 × 900 mm. The panel simulates the photocatalyst-coated
surface for the convenience of the experiment because collecting washing water from the
wall where photocatalytic paint is applied is difficult; this facilitates the identification of
the amount of NOx removed from the surface by exposing it to the outdoor environment.
Therefore, considering the field situation, we applied a washing method before measuring
instead of dipping the specimens as prescribed in the ISO method. Figure 10 shows that
the surface was evenly washed using a sprayer containing 2 L of distilled water. The quan-
tity of distilled water is determined based on the empirical experiment, which indicates
that 2 L of distilled water can fully wash the ions attached on the surface of CRC board.
Figure 8.
Photocatalytic materials applied on the real buildings in the field assessment. (
a
) Building A, photocatalyst coating;
(b) Building B, photocatalyst paint; (c) Building C, photocatalyst shotcrete.
Figure 9shows installed photoreactor and UV radiometer used in the field experiments.
The size of the UV radiometer is 50
×
300 mm, which is three times larger than the specimen
defined in the ISO method. Considering that the gas could evenly reach the surface of
specimen, the space between the transparent plate and the inner specimen was set to
5 mm
. The photoreactor was set to 3 ppm as the specimen size was increased by a factor of
three. We conducted a comparative evaluation of NO removal using three photocatalytic
products based on the minimum criteria for photocatalytic product certification set by the
Photocatalysis Industry Association of Japan.
Materials 2021,14, 5717 11 of 18
Materials 2021, 14, x FOR PEER REVIEW 11 of 18
(a) (b) (c)
Figure 8. Photocatalytic materials applied on the real buildings in the field assessment. (a) Building A, photocatalyst
coating; (b) Building B, photocatalyst paint; (c) Building C, photocatalyst shotcrete.
Figure 9 shows installed photoreactor and UV radiometer used in the field
experiments. The size of the UV radiometer is 50 × 300 mm, which is three times larger
than the specimen defined in the ISO method. Considering that the gas could evenly reach
the surface of specimen, the space between the transparent plate and the inner specimen
was set to 5 mm. The photoreactor was set to 3 ppm as the specimen size was increased
by a factor of three. We conducted a comparative evaluation of NO removal using three
photocatalytic products based on the minimum criteria for photocatalytic product
certification set by the Photocatalysis Industry Association of Japan.
Figure 9. Installed photoreactor and UV radiometer on the rooftop.
3.2. ISO-Based Washing Water Field Test Method for Real-Scale Construction Materials
As the ISO-based specimen size is smaller than the size of the actual building
material, a modified experimental method is used to perform practical and empirical
analyses. To this end, the established methods are summarized as follows.
It is difficult to fabricate panels made for outdoor exposure using concrete. Hence,
the CRC board reviewed in the ISO specimen tests was used as the base material. The size
of the CRC board was set to 800 × 900 mm. The panel simulates the photocatalyst-coated
surface for the convenience of the experiment because collecting washing water from the
wall where photocatalytic paint is applied is difficult; this facilitates the identification of
the amount of NOx removed from the surface by exposing it to the outdoor environment.
Therefore, considering the field situation, we applied a washing method before measuring
instead of dipping the specimens as prescribed in the ISO method. Figure 10 shows that
the surface was evenly washed using a sprayer containing 2 L of distilled water. The
quantity of distilled water is determined based on the empirical experiment, which
indicates that 2 L of distilled water can fully wash the ions attached on the surface of CRC
board.
Figure 9. Installed photoreactor and UV radiometer on the rooftop.
3.2. ISO-Based Washing Water Field Test Method for Real-Scale Construction Materials
As the ISO-based specimen size is smaller than the size of the actual building material,
a modified experimental method is used to perform practical and empirical analyses. To
this end, the established methods are summarized as follows.
It is difficult to fabricate panels made for outdoor exposure using concrete. Hence, the
CRC board reviewed in the ISO specimen tests was used as the base material. The size of
the CRC board was set to 800
×
900 mm. The panel simulates the photocatalyst-coated
surface for the convenience of the experiment because collecting washing water from the
wall where photocatalytic paint is applied is difficult; this facilitates the identification of
the amount of NOx removed from the surface by exposing it to the outdoor environment.
Therefore, considering the field situation, we applied a washing method before measuring
instead of dipping the specimens as prescribed in the ISO method. Figure 10 shows that the
surface was evenly washed using a sprayer containing 2 L of distilled water. The quantity
of distilled water is determined based on the empirical experiment, which indicates that
2 L of distilled water can fully wash the ions attached on the surface of CRC board.
Materials 2021, 14, x FOR PEER REVIEW 12 of 18
(a) (b)
Figure 10. Spraying and collecting process of distilled water. (a) Spraying on the CRC panel; (b) collecting process of
washing water.
The experiment was conducted from 25 October to 6 December. The reason why the
outdoor experiment of this study was conducted during above period is to prevent the
experimental results from being biased due to weather conditions such as excessive
rainfall or insufficient rainfall. This experiment was conducted in Seoul, the capital of
Korea. Seoul’s meteorological characteristics are that the rainfall is higher than other
periods due to the influence of rainy seasons and typhoons in summer season (June to
September). In winter season (December to February), snow falls and the temperature is
low, making the walls coated with photocatalytic paint to freeze. These seasonal
characteristics are expected to have an impact on the experimental results of this
experiment. Therefore, in order to prevent bias in the experimental results due to the
previous seasonal characteristics, the experimental period was set from 25 October to 6
December.
We performed washing up to five times as in the preliminary test to check the
concentration of ions contained in the collected washing water; the results showed that
the concentration was significantly lowered as the number of washings increased and that
the concentration after the second washing was not significant. Therefore, the panels were
only washed twice in the actual experiment. The experiments were conducted on the
rooftop of a building adjacent to the roadway because measurements immediately next to
the roadway were not feasible. The following field experimental conditions determined
through preliminary experiments were used.
1. Panels with photocatalytic paint applied
• The material of the panel was CRC board.
• The paint was applied to the panel after washing and drying the CRC board.
• The control group was prepared with a CRC board with general aqueous paint
applied.
2. Methods of washing and analyzing the panels
• A total of 2 L of washing water was used for each panel.
• The number of washes was limited to two; the washing intervals were
determined by considering the weather conditions.
• The panel was washed by spraying.
• A separate holder and recovery bin was designed to collect rainwater.
• The number of ions in the washing water were measured via ion chromatography.
Figure 10.
Spraying and collecting process of distilled water. (
a
) Spraying on the CRC panel; (
b
) collecting process of
washing water.
The experiment was conducted from 25 October to 6 December. The reason why the
outdoor experiment of this study was conducted during above period is to prevent the
experimental results from being biased due to weather conditions such as excessive rainfall
or insufficient rainfall. This experiment was conducted in Seoul, the capital of Korea.
Seoul’s meteorological characteristics are that the rainfall is higher than other periods due
to the influence of rainy seasons and typhoons in summer season (June to September). In
winter season (December to February), snow falls and the temperature is low, making the
walls coated with photocatalytic paint to freeze. These seasonal characteristics are expected
to have an impact on the experimental results of this experiment. Therefore, in order to
Materials 2021,14, 5717 12 of 18
prevent bias in the experimental results due to the previous seasonal characteristics, the
experimental period was set from 25 October to 6 December.
We performed washing up to five times as in the preliminary test to check the con-
centration of ions contained in the collected washing water; the results showed that the
concentration was significantly lowered as the number of washings increased and that the
concentration after the second washing was not significant. Therefore, the panels were
only washed twice in the actual experiment. The experiments were conducted on the
rooftop of a building adjacent to the roadway because measurements immediately next to
the roadway were not feasible. The following field experimental conditions determined
through preliminary experiments were used.
1. Panels with photocatalytic paint applied
•The material of the panel was CRC board.
•The paint was applied to the panel after washing and drying the CRC board.
•
The control group was prepared with a CRC board with general aqueous paint
applied.
2. Methods of washing and analyzing the panels
•A total of 2 L of washing water was used for each panel.
•
The number of washes was limited to two; the washing intervals were deter-
mined by considering the weather conditions.
•The panel was washed by spraying.
•A separate holder and recovery bin was designed to collect rainwater.
•
The number of ions in the washing water were measured via ion chromatography.
4. Results of NO Removal Analysis Experiment
4.1. Photoreactor Test Results
The results for the three experimental construction materials in each field are listed in
Table 7. The NO removal amount on a clear day was more than twice that on the cloudy
day for all three buildings. Assuming that the NO removal was converted to the ISO test
reactor size of 50 cm
2
, Building C with photocatalytic shotcrete applied had the lowest
NO removal amount of 0.87
µ
mol/50 cm
2
. However, it is still higher than that specified
by the Photocatalysis Industry Association of Japan performance standard (
0.5 µmol)
.
Building C received less UV irradiation compared to buildings A and B because of the
shade formed for a certain period of time. The preliminary test showed the highest NO
removal performance of the photocatalytic shotcrete; however, the field measurement
showed the lowest performance. As the NO removal performance is highly influenced by
weather conditions, conditions such as the bearing of the building and its relationship with
adjacent buildings need to be considered when applying photocatalytic products as exterior
construction materials. Compared to preliminary test results measured in the laboratory,
the field test results of Buildings A and B showed higher NO removal performance. For
Building C, lower NO removal occurred in the field test than in the laboratory, which can
be attributed to the difference in UV irradiation, which was 10 W/m
2
in the preliminary
test and less than 10 W/m2in the field measurement.
4.2. Washing Water Field Test Results of Specimens
A specimen experiment was conducted according to the ISO 22197-1 method to
determine how the amount of NOx removed by the photocatalyst is reflected in the
washing water. The experiment was conducted with the specimen applied with aqueous
paint as a comparison group and four types of specimens applied with photocatalytic paint.
The specimens were classified into aqueous and photocatalytic paint specimens to facilitate
the distinction of experimental specimens.
Materials 2021,14, 5717 13 of 18
Table 7. Outdoor NO removal results for construction materials.
Materials NO Injection
Rate (µmol)
NO Removal
(µmol/50 cm2)
NO Removal
(µmol/50 cm2)
UV Irradiation
(W/m2)
Measurement
Date
(Weather)
Building A: Photocatalytic coating 118.97 11.06 3.68 2–13
25 October 2018
(Sunny)
Building B:
Photocatalytic
paint
1st 107.82 10.03 3.34 6–14 14 November
2018 (Sunny)
2nd 122.79 4.87 1.62 2–16 5 December
2018 (Cloudy)
Building C:
Photocatalytic
shotcrete
1st 95.84 6.78 2.26 1 13 November
2018 (Sunny)
2nd 114.52 2.62 0.87 2–9 6 December
2018 (Cloudy)
The size of the specimen here is 800 mm in width and 900 mm in length, and the
thickness of the specimen is 5 mm. Photocatalytic paint containing the identical amount of
TiO
2
powder (about 3.5%) was applied to the surface of CRC boards to create Specimen A,
B, C and D. For the photocatalytic paint specimens, newly prepared specimens A and B for
this experiment and specimens C and D with photocatalytic paint applied approximately a
month ago were used. These CRC specimens also went through the pre-process to remove
organic matter remaining on the surface of CRC boards, UV rays are irradiated for 16 h.
The irradiated UV rays are10 W/m
2
which can sufficiently decompose organic matter on
the surface. Each specimen was washed four times; however, only the ions in the first and
second washing water were added and analyzed as prescribed in ISO 22197-1. Table 8
summarizes the results of the ISO 22197-1 experiment for the five specimens used for NOx
removal with the results of washing twice.
Table 8. Washing water field test results of five specimens.
Specimen Type NOx Removal Amount
(µmol)
Eluted NOx Amount
(µmol)
Aqueous paint specimen 0.08 0.53
Photocatalytic paint specimen A 7.90 7.86
Photocatalytic paint specimen B 10.80 10.38
Photocatalytic paint specimen C 2.40 3.62
Photocatalytic paint specimen D 2.45 3.25
Refer to the preliminary test calculation formula for analyzing and calculating NO elution from ISO specimens.
Experimental results with the aqueous paint showed that the NOx removal amount
was 0.08
µ
mol, which resulted in a weak removal effect; the washing water test results
showed a small NOx removal amount of 0.53
µ
mol. The results obtained using specimens
A and B for this experiment show that the calculated NOx removal amount and the
amount of eluted nitrogen oxides are similar; in the case of existing specimens C and D,
the results show a slightly more eluted NOx than the removed NOx. Thus, the results
indicate that the amount removed by the ISO evaluation method is similar to the amount of
elution measured from the analysis of the washing water. However, for existing specimens
previously exposed to the outdoor environment, the NOx contained—whether through
adsorption or reaction—was not completely removed during the pretreatment process.
This is believed to have a partial effect on the elution into the washing water. When the
specimen was washed sufficiently before the experiment, the error shown in the existing
specimen is expected to decrease. In addition, it can be inferred that nitrogen oxides were
sufficiently removed through the washing water experiment.
The specimen was placed outside the building from 3 June 2019 to 12 June 2019;
during this period, the specimen was moved indoors on rainy days to prevent the surface
Materials 2021,14, 5717 14 of 18
from being washed by rainwater. The comparison results of the values obtained from
experiments and the specimens exposed under the above conditions according to the ISO
test method are listed in Table 9.
Table 9. Washing water field test results of specimens for which the surface was washed sufficiently.
Specimen Type
ISO NOx Removal
Performance
(µmol/50 cm2·5 h)
Elution Amount after ISO
Experiment
(µmol)
Elution Amount after Outdoor
Exposure for Five Days
(µmol)
Aqueous paint 0.08 0.53 3.88
Photocatalytic paint sample
C2.40 3.62 14.13
The specimens were exposed outdoors for five days in the apartment after washing
was completed for the ISO experiment. For the general aqueous paints, 3.88
µ
mol of
NOx was removed, whereas photocatalytic paints showed significantly better NOx elution.
Some elution was found in the case of general aqueous paints; however, this was attributed
to the result from surface adsorption rather than photocatalytic reaction. Further, for the
photocatalytic paint with photocatalytic activity, we detected a large amount of elution in
a definite contrast. Thus, we confirmed that the ISO NOx removal amount was equal to
the value of the nitrogen oxide elution and that the photocatalytic performance showed a
significant difference when the photocatalytic specimens were exposed in the field.
4.3. Washing Water Field Test Results of Photocatalytic Panel
An ISO-based washing water field experiment was conducted for real-scale construc-
tion materials under the same conditions as the specimen experiments.
Figure 11 shows panels installed on the roof of an apartment. The installation period
is the same as that for which the ISO specimens were exposed outdoors; further, the panels
were moved and kept indoors on rainy days to prevent the surface from being washed by
rainwater. All panels were washed together under the same conditions, and all washing
water was collected thoroughly and labeled (see Figure 12).
Materials 2021, 14, x FOR PEER REVIEW 15 of 18
An ISO-based washing water field experiment was conducted for real-scale
construction materials under the same conditions as the specimen experiments.
Figure 11 shows panels installed on the roof of an apartment. The installation period
is the same as that for which the ISO specimens were exposed outdoors; further, the panels
were moved and kept indoors on rainy days to prevent the surface from being washed by
rainwater. All panels were washed together under the same conditions, and all washing
water was collected thoroughly and labeled (see Figure 12).
Figure 11. Specimens placed on the rooftop of the apartment.
(a) (b)
Figure 12. Collecting process and storage procedure of washing water. (a) collecting process of washing water; (b)
collected washing water in seperate bottles.
Table 10 summarizes the results of measuring the ionic value of the washing water
of panels exposed on the roof of apartments in Seoul, Korea. In this study, three different
types of panels are used for field test. First one is panel coated with aqueous paint. S1 is
the panel that is newly coated with photocatalytic paint before the experiment day. S2 and
S3 photocatalytic panels are created before two weeks before the experiment day. After
exposing the panel to the outside, the surface was washed with water and reused.
Table 10. Washing water field test results of photocatalytic panels.
Photocatalytic Panels Eluted NO2−, NO3− Ions
(μmol)
Aqueous paint 210
Photocatalytic panel (S1) 1469
Figure 11. Specimens placed on the rooftop of the apartment.
Table 10 summarizes the results of measuring the ionic value of the washing water
of panels exposed on the roof of apartments in Seoul, Korea. In this study, three different
types of panels are used for field test. First one is panel coated with aqueous paint. S1 is
the panel that is newly coated with photocatalytic paint before the experiment day. S2 and
S3 photocatalytic panels are created before two weeks before the experiment day. After
exposing the panel to the outside, the surface was washed with water and reused.
Materials 2021,14, 5717 15 of 18
Materials 2021, 14, x FOR PEER REVIEW 15 of 18
An ISO-based washing water field experiment was conducted for real-scale
construction materials under the same conditions as the specimen experiments.
Figure 11 shows panels installed on the roof of an apartment. The installation period
is the same as that for which the ISO specimens were exposed outdoors; further, the panels
were moved and kept indoors on rainy days to prevent the surface from being washed by
rainwater. All panels were washed together under the same conditions, and all washing
water was collected thoroughly and labeled (see Figure 12).
Figure 11. Specimens placed on the rooftop of the apartment.
(a) (b)
Figure 12. Collecting process and storage procedure of washing water. (a) collecting process of washing water; (b)
collected washing water in seperate bottles.
Table 10 summarizes the results of measuring the ionic value of the washing water
of panels exposed on the roof of apartments in Seoul, Korea. In this study, three different
types of panels are used for field test. First one is panel coated with aqueous paint. S1 is
the panel that is newly coated with photocatalytic paint before the experiment day. S2 and
S3 photocatalytic panels are created before two weeks before the experiment day. After
exposing the panel to the outside, the surface was washed with water and reused.
Table 10. Washing water field test results of photocatalytic panels.
Photocatalytic Panels Eluted NO2−, NO3− Ions
(μmol)
Aqueous paint 210
Photocatalytic panel (S1) 1469
Figure 12.
Collecting process and storage procedure of washing water. (
a
) collecting process of washing water; (
b
) collected
washing water in seperate bottles.
Table 10. Washing water field test results of photocatalytic panels.
Photocatalytic Panels Eluted NO2−, NO3−Ions
(µmol)
Aqueous paint 210
Photocatalytic panel (S1) 1469
Photocatalytic panel (S2) 475
Photocatalytic panel (S3) 504
A clear difference is observed between general aqueous paint and photocatalytic paint
in terms of washing water for the CRC panels installed in the apartment. NOx removed by
photocatalytic effects remains in an ionic state on the surface of the photocatalyst paint,
and they are eluted by water washing and released into ionic nitric acid and nitrous acid.
The amount of NOx ions eluted by water washing, which was investigated based on the
NOx removal performance in the ISO test method, was almost the same as the amount
of NOx removed by photocatalytic action. These results indicate that the amount of NOx
ions in the washing water can be analyzed to quantify the amount of NOx removed by the
photocatalyst. The NOx ions were observed in the washing water of general aqueous paint;
however, only a few were detected in the washing water of photocatalytic paints. When
tested and exposed to a real outdoor environment, the results indicated that photocatalysts
are effective for removing NOx in the real world.
The results of s1 and s2 have the following implications for the NOx reduction per-
formance of the photocatalytic paint used in this study. First, it shows that contaminants
adhering to the surface are sufficiently removed just by washing the surface of the wall
coated with photocatalytic paint with rainwater. Photocatalytic paint can maintain its
NOx removal performance even after a certain period of time has passed since it is actu-
ally installed on the exterior wall of a building. Through this experiment, the durability
of the performance when the photocatalytic paint was applied to an actual building is
demonstrated.
Table 11 shows the amount of NOx that can be removed per day; it is calculated from
the panel test results above. The average daily removal of the panel was calculated by
averaging the installation days, excluding the rainy days when there was almost no UV
light irradiation. Further, the estimated annual removal of the photocatalytic panel was
calculated by excluding the average annual rainfall days in Korea. Moreover, the annual
removal amount corresponds to the amount of NOx that can be removed per photocatalytic
panel unit area.
Materials 2021,14, 5717 16 of 18
Table 11. Washing water field test results of photocatalytic panels.
Division Daily Average NOx Removal
Amount of Panel (1) (µmol)
Estimated Annual NOx Removal
Amount of Panel (2) (µmol)
Estimated Annual NOx
Removal Amount (3) (g/m2)
Aqueous paint 42.00 10,920 0.46
Photocatalytic panel (S1)
293.80 76,388 3.19
Photocatalytic panel (S2)
95.00 24,700 1.03
Photocatalytic panel (S3)
100.80 26,208 1.09
(1)
Total nitrogen oxides washed off from the panel/5 (number of days the panel was installed).
(2)
Panel daily average removal amount
×
260 days (excluding the average number of rainfall days per year).
(3)
Estimated annual removal amount (
µ
mol)/106 (calculated as mol)
×
1.39 (area multiplier, m2)×30 g/mol (nitrogen molecular weight).
5. Conclusions
A modified ISO 22197-1 standard method to analyze the NOx removal performance
of photocatalytic construction materials in both indoor and outdoor environments was
presented. The first experiment involved a photoreactor test conducted to assess the NO
removal performance of construction materials such as coatings, paints and shotcrete. In the
preliminary analysis, the impact of UV irradiation according to the direction at the outdoors
was analyzed. For sunny days, the NOx removal performance (
3.12–4.76 µmol/150 cm2·5 h
)
was twice as much as the ISO 22197-1 standard specification (2.03
µ
mol/150 cm
2·
5 h) in the
real world. Even on cloudy days, it is slightly lower than that of the ISO 22197-1 standard
specification; the NOx removal performance ranging from 0.68–1.89
µ
mol/5 cm
2·
5 h is
shown, which is 63% of the indoor experiment results.
The second experiment was a washing water experiment, wherein the NOx removal
performance was analyzed by assessing the amount of NOx ions remaining in the water
after washing the surface of the specimen artificially exposed to NOx in the laboratory.
The preliminary test employed two specimens, one of which was of the same size as that
defined in the ISO 22197-1 standard, and the other was expanded to a size comparable to
that of the construction materials. The experimental conditions were validated when it
was discovered that the NOx removal efficiency improved in tandem with the specimen’s
propensity to grow. The expanded specimen was then used in a field experiment using an
adapted approach from the ISO 22197-1 standard. The washing water test was performed
after sufficient washing of the ISO test specimen and exposure to the outdoors for 5 days in
natural light. The washing test results indicated that general aqueous paint showed a NOx
removal of 3.88
µ
mol, whereas that for photocatalytic paint was higher at 14.13
µ
mol. The
amount of NOx removed by the washing water test was estimated to be 3.19 g/m
2
. This
was demonstrated by the results of outdoor exposure, which is almost similar to the ISO
standard test. Thus, it is possible to prove the direct correlation between the photocatalytic
activity according to the ISO standard and the NOx removal performance.
The results of this study are expected to encourage the application of photocatalytic
construction materials to real buildings with obvious NOx removal effectiveness. Em-
pirically demonstrating that the real NOx removal effect increases when the small-sized
specimen stated in the ISO standard is expanded to a size close to that of the actual construc-
tion material is a significant contribution from both academic and practical standpoints.
Future work will include more extensive analysis using various types of photocatalytic
construction materials at various sites for a longer experiment time.
Author Contributions:
Conceptualization, M.K. and H.K.; methodology, M.K. and H.K.; software,
M.K.; validation, M.K. and H.K.; formal analysis, M.K.; resources, M.K. and H.K.; data curation, M.K.;
writing—original draft preparation, M.K.; writing—review and editing, H.K.; visualization, M.K.;
supervision, J.P.; project administration, H.K.; funding acquisition, H.K. All authors have read and
agreed to the published version of the manuscript.
Funding:
This research was supported by a grant (21SCIP-B146966-04) from construction technol-
ogy research project funded by Ministry of Land, Infrastructure and Transport (MOLIT) of Korea
government and Korea Agency for Infrastructure Technology Advancement (KAIA).
Materials 2021,14, 5717 17 of 18
Conflicts of Interest: The authors declare no conflict of interest.
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