Visible light-induced decomposition of monoethanolamine in water using graphitic carbon nitride as a photocatalyst

Article abstract of DOI:10.1016/j.jphotochem.2017.10.015

Photochemical decomposition of monoethanolamine (MEA) in the presence of graphitic carbon nitride (g-C₃N₄) in water under visible-light irradiation was investigated. When aqueous MEA was irradiated by visible light (>389 nm) in the presence of either TiO₂ (P25) and O₂ or g-C₃N₄ and argon, virtually no decomposition of MEA was observed. In contrast, when aqueous MEA was irradiated by visible light in the presence of g-C₃N₄ and O₂, the MEA decomposed to produce HCOOH, NO₂^?, NO₃^?, and NH₃. Although metal (Ag or Pt)-loading onto g-C₃N₄ enhanced the photocatalytic MEA decomposition slightly, it accelerated the formation of NO₂^? and NO₃^?. After the g-C₃N₄ catalyzed reactions, the recovery of carbon atoms was almost completely accounted for by the remaining MEA and HCOOH formed in the reaction solution, indicating that the carbon atoms in the reacted MEA were predominantly transformed to HCOOH.

Full text of DOI:10.1016/j.jphotochem.2017.10.015

Journal of Photochemistry and Photobiology A: Chemistry 351 (2018) 162–169
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Visible light-induced decomposition of monoethanolamine in water
using graphitic carbon nitride as a photocatalyst
a
a
b
Hisao Horia, , Shohei Wachi , Kentaro Iwamura , Taizo Sano
*
a
Department of Chemistry, Faculty of Science, Kanagawa University, 2946 Tsuchiya, Hiratsuka 259-1293, Japan
National Institute of Advanced Industrial Science and Technology (AIST), AIST Tsukuba West, 16-1 Onogawa, Tsukuba 305-8569, Japan
b
A R T I C L E I N F O
A B S T R A C T
Article history:
Photochemical decomposition of monoethanolamine (MEA) in the presence of graphitic carbon nitride
(g-C3N4) in water under visible-light irradiation was investigated. When aqueous MEA was irradiated by
visible light ( >389 nm) in the presence of either TiO2 (P25) and O2 or g-C3N4 and argon, virtually no
decomposition of MEA was observed. In contrast, when aqueous MEA was irradiated by visible light in the
presence of g-C3N4 and O2, the MEA decomposed to produce HCOOH, NO2À, NO3À, and NH3. Although
metal (Ag or Pt)-loading onto g-C3N4 enhanced the photocatalytic MEA decomposition slightly, it
accelerated the formation of NO2À and NO3À. After the g-C3N4 catalyzed reactions, the recovery of carbon
atoms was almost completely accounted for by the remaining MEA and HCOOH formed in the reaction
solution, indicating that the carbon atoms in the reacted MEA were predominantly transformed to
HCOOH.
Received 31 July 2017
Received in revised form 18 September 2017
Accepted 9 October 2017
Available online 16 October 2017
Keywords:
Amine
Degradation
Graphitic carbon nitride
Formic acid
Post-combustion CO2 capture
© 2017 Elsevier B.V. All rights reserved.
1. Introduction
and nitramines [4–8]. Nitrosamines are mostly derived from
secondary amines [5]. However, they can also be generated from
Alkanolamines, which have an amino group and a hydroxyl
group on an alkane backbone, have been used as components for
surfactants, emulsifiers, lubricants, polishes, corrosion inhibitors,
pharmaceuticals, and so forth [1]. In addition to these traditional
usages, alkanolamines are increasingly used for post-combustion
CO2 capture technology aimed at mitigating climate change [2]. In
CO2 capture plants, aqueous amines are employed as capture
solvents of CO2 from combustion flue gases. Monoethanolamine
(HOCH2CH2NH2, MEA), a primary amine, is the most common
amine used for this purpose, although secondary and tertiary
amines are also used or planned to be used in CO2 capture [2–4].
With the blossoming of post-combustion CO2 capture technol-
ogy, the sites where the technology is introduced (e.g., coal-fired
power plants) are potentially major sources of the amines and their
degradation products in the environment [4–6]. These amines are
lost from the CO2 absorber column through evaporation and
entrainment in the plants; aqueous and solid wastes from the
plants also contain these amines and their degradation products
[6]. Consequently, the presence of amines in surface waters has
been reported, although available data are limited so far [4]. These
amines can degrade to form potentially carcinogenic nitrosamines
primary and tertiary amine solutions used for CO2 capture because
these amines transform to secondary amines in the capture solvent
[7]. In fact, nitrosamines were detected in regenerated MEA solvent
from a post-combustion CO2 capture plant at a coal-fired power
station [8]. MEA is also used as a stripper in the manufacturing of
thin-film transistor liquid crystals and as a corrosion inhibitor in
metal working fluid; the wastewater from these processes is also a
cause for concern [9–12]. Accordingly, there is a need to develop
techniques to decompose MEA in water to harmless species under
mild conditions to protect against emissions from stationary
sources. Among the techniques aimed at decomposition of MEA in
water or solid, biodegradation has been extensively investigated
[9–18]. Photocatalytic approaches are also attractive. However, we
found only two reports about the photocatalytic decomposition of
MEA in water: use of titanium silicalite-1 (TS-1) zeolite [19] or
TiO2-loaded clinoptilolite [20]. In both reports, the decomposition
of MEA was driven by UV light irradiation, and the common
photocatalyst TiO2 plus UV light irradiation served as a comparator,
although TiO2 showed less photocatalytic activity than TS-1 zeolite
and TiO2-loaded clinoptilolite.
* Corresponding author.
E-mail address: h-hori@kanagawa-u.ac.jp (H. Hori).
https://doi.org/10.1016/j.jphotochem.2017.10.015
1010-6030/© 2017 Elsevier B.V. All rights reserved.

H. Hori et al. / Journal of Photochemistry and Photobiology A: Chemistry 351 (2018) 162–169
163
Recently, graphitic carbon nitride (g-C3N4) has received much
attention because it can be easily prepared by heating nitrogen-
high-pressure mercury lamp, as reported previously [37]. Likewise,
HT-g-C3N4-Pt was prepared by photodeposition of platinum onto
HT-g-C3N4 by use of aqueous H2PtCl6 and UV light irradiation. The
amount of silver and platinum in HT-g-C3N4-Ag and HT-g-C3N4-Pt,
respectively, was 1.0 wt%. The specific surface areas of Std-g-C3N4
and HT-g-C3N4, measured by using the BET method with nitrogen
absorption, were 7.7 and 65 m2 gÀ1, respectively.
rich organic compounds, such as melamine, and acts as
a
photocatalyst when induced by visible-light irradiation [21–23].
Since g-C3N4 was reported to have photocatalytic water splitting
activity under visible-light irradiation [24], there have been
numerous studies on water purification using g-C3N4 or
a
composite as a photocatalyst. Although many of these reports
focused on the degradation of model contaminants such as organic
dyes [25–33], there have been an increasing number of reports on
the degradation of real contaminants such as phenols [30,34,35].
Studies focused on air purification, for example, the removal of NO
[36] and methyl mercaptan [37] induced by the photocatalytic
activity of g-C3N4, have also been reported.
In the present work, we report the photocatalytic decomposi-
tion of MEA in water by use of g-C3N4 or metal (Ag or Pt)-loaded g-
C3N4. To the best of our knowledge, this is the first report on the
visible light-induced photocatalytic decomposition of MEA in
water.
2.2. Photocatalytic reactions
g-C3N4 powder (Std-g-C3N4, HT-g-C3N4, HT-g-C3N4-Pt, or HT-g-
C3N4-Ag; 30 mg), an aqueous solution of MEA (5.00–5.18 mM,
22 mL), and a polytetrafluoroethylene stirring bar were introduced
into a gold vessel (25-mL volume, 3.8-cm o.d.), which was stable to
corrosive MEA solutions. The vessel was inserted into a column-
shaped pressure-resistant photochemical reactor made of Inconel
alloy (205-mL volume, 3.8-cm i.d.). The reactor had a sapphire
window on the top to receive the light and had gas inlet and outlet
ports on the side. After the reactor was pressurized to 0.5 MPa with
oxygen gas through the gas inlet port, the mixture in the vessel was
irradiated with visible light ( >389 nm) while stirring. The reason
for using pressurized oxygen is ease of gas collection after the
reaction. Under the pressurized conditions, the gas in the reactor
after the reaction can be spontaneously transferred to the gas
sampling bag by opening the valve which connects the reactor and
the sampling bag. The irradiation light was generated by a
combination of a long-pass filter (LU0400, Asahi Spectra, Tokyo,
Japan) and a 500 W xenon short-arc lamp (UXL 500D, Ushio, Tokyo,
Japan). The light from the lamp was introduced to an optical fiber,
passed through a water filter, the long-pass filter, the sapphire
window of the reactor, and then reached the reaction mixture. The
irradiation light intensity of this system was low: the irradiance at
2. Materials and procedures
2.1. Materials
MEA (>99%) and other chemicals were purchased from Wako
Pure Chemical Industries (Osaka, Japan). Powdered TiO2 (Degussa
P25, BETsurface area, 50 m2 gÀ1) was obtained from Nippon Aerosil
(Tokyo, Japan). We used four kinds of g-C3N4: g-C3N4 prepared by a
standard method (Std-g-C3N4), g-C3N4 obtained from hydrother-
mal treatment of Std-g-C3N4 (HT-g-C3N4), silver-loaded g-C3N4
(HT-g-C3N4-Ag), and platinum-loaded g-C3N4 (HT-g-C3N4-Pt). Std-
g-C3N4 was synthesized from melamine by heating at 550 ꢀC for
2 h; additional details are described elsewhere [36]. HT-g-C3N4 was
prepared from Std-g-C3N4 by hydrothermal treatment with 0.1 M
NaOH at 110 ꢀC for 18 h [36]. The synthesis scheme for all catalysts
is shown in Scheme S1 in Supplementary materials. Detailed
characterization of these species was described in the reference
[36]. HT-g-C3N4-Ag was obtained by photodeposition of silver onto
HT-g-C3N4 by use of aqueous AgNO3 and UV light irradiation from a
the center of the light spot on the sample position was 120 mW
cmÀ2 at 405 nm, as measured by a power meter (FieldMaster,
detection head, LM-2VIS, Coherent, Santa Clara, CA, USA) with a
band-pass filter (HB0405, Asahi Spectra, half bandwidth, 10 nm).
During the photochemical reaction, the temperature of the
reaction mixture was kept constant at 25 ꢀC by use of a circulation
water bath. Control reactions using TiO2 instead of g-C3N4 were
also carried out.
At the end of the irradiation time period, a gas-sampling bag
(CEK-1, GL Sciences, Tokyo, Japan) was connected to the gas outlet
port of the reactor. The gas in the reactor was transferred to the
sampling bag by releasing the pressure, and the collected gas was
subjected to gas chromatography-mass spectrometry (GC–MS).
The reaction mixture was separated by centrifugation, and the
collected reaction solution was analyzed by ion chromatography
and high-performance liquid chromatography (HPLC). The collect-
ed solid was dried under vacuum and subjected to X-ray
photoelectron spectroscopy (XPS) to examine the g-C3N4 for
degradation.
2.3. Analytical procedures
An ion chromatograph (IC 2010, Tosoh, Tokyo, Japan) with an
automatic sample injector (injection volume, 30 mL), a separation
column (Tosoh TSKgel SuperIC-CR, i.d., 4.6 mm; length, 15 cm), a
column oven, and a conductivity detector in cationic mode was
used to quantify the MEA in the reaction solution. The temperature
of the column oven was kept constant at 40 ꢀC. The mobile phase
was an aqueous solution containing methanesulfonic acid
(1.0 mM) and 18-crown-6 (1.0 mM). A second ion chromatograph
(Tosoh IC 2001) was used to quantify the NO2 and NO3À. In this
system, analyte separation was performed by a combination of a
guard column (Tosoh TSKgel Super IC-A, i.d., 4.6 mm; length,1 cm),
a separation column (Tosoh TSKgel Super IC Anion, i.d., 4.6 mm;
À
Fig.1. (a) Diffuse reflectance UV–visible absorption spectra of Std-g-C3N4 and HT-g-
C3N4, and transmittance of the LU0400 long-pass filter, and (b) UV–visible
absorption spectrum of MEA in water (5.0 mM; path length, 1.0 cm). In panel b, the
MEA concentration is the same as that used in the photochemical reactions.

164
H. Hori et al. / Journal of Photochemistry and Photobiology A: Chemistry 351 (2018) 162–169
Table 1
Reactivity of MEA under various conditionsa
Entry
Initial MEA (mM)
Catalyst
Light irradiation
Gas
Remaining MEA (mM) [ratio (%)]
TOC (mM) [remaining ratio (%)]b
1
2
3
4
5f
6
7
4.91
5.16
absent
present
absent
O2
O2
argon
O2
O2
4.75 [97]
4.89 [95]
4.78 [94]
3.63 [73]
9.8 [100]
9.6 [93]
c
c
c
c
HT-g-C3N4
HT-g-C3N4
HT-g-C3N4
HT-g-C3N4
d
5.08
5.00
absent
4.94
4.94
present
present
present
present
present
–
10.0 [100]
d
–
n.d.e (<0.02)
g
d
TiO2
TiO2
O2
O2
4.86 [98]
4.75 [96]
–
h
d
–
a
Light irradiation time, 17 h; volume of reaction solution, 22 mL.
b
c
d
e
f
Remaining TOC ratio was calculated according to the equation: [TOC]/(initial [MEA] Â 2).
30 mg.
Not measured.
n.d = not detected.
Aqueous NaOH solution (pH = 10) was used.
3.0 mg.
30 mg.
g
h
length, 15 cm), and a mobile phase consisting of aqueous Na2B4O7
(6 mM), H3BO3 (15 mM), and NaHCO3 (0.2 mM). The conductivity
detector was operated with a suppressor device in anionic mode.
Total organic carbon (TOC) concentration in the reaction solutions
was determined by means of a TOC analyzer (N/C 3100 BU, Analytik
Jena, Jena, Germany). In this system, the TOC concentration was
obtained by subtracting the inorganic carbon concentration from
the total carbon concentration.
environment (O2 or argon) (Table 1). When the reaction was
performed under visible-light irradiation and in the presence of O2
but in the absence of g-C3N4, virtually no reaction occurred: 97% of
the initial MEA remained (entry 1). When the reaction was
performed in the presence of HT-g-C3N4 and O2 but in the absence
of light irradiation, 95% of the initial MEA remained (entry 2).
Likewise, when the reaction was performed in the presence of HT-
g-C3N4 and light irradiation under argon, 94% of the initial MEA
remained (entry 3). In contrast, when the reaction was performed
in the presence of HT-g-C3N4, light irradiation, and O2, the
concentration of MEA markedly decreased to 73% of the initial
value (entry 4). These results clearly demonstrate that the
combination of HT-g-C3N4, light irradiation, and O2 was required
for efficient decomposition of MEA. This finding is consistent with
our knowledge that O2 can act as an acceptor of the electron
generated at the conduction band edge of HT-g-C3N4 by light
irradiation. When TiO2 was used, little decrease in MEA was
observed (the remaining ratios were 98 and 96% at 3.0 and 30 mg
TiO2, entries 6 and 7, respectively), reflecting the lack of light
absorption ability at the selected irradiation wavelength.
The HPLC system (Infinity 1200, Agilent, Santa Clara, CA, USA)
consisted of an automatic sample injector (injection volume,
20 mL), a separation column (Tosoh TSKgel ODS–100 V, i.d.,
4.6 mm; length, 15 cm), a column oven (40 ꢀC), and a UV detector
(detectionwavelength, 210 nm) was used to quantify the HCOOH in
the reaction solution. The mobile phase was an aqueous solution
containing acetonitrile (0.38 M) and H3PO4 (15 mM).
To investigate whether any harmful gaseous species formed
during the reactions, gas collected in the sampling bag was
analyzed by GC–MS (QP2010 SE, Shimadzu, Kyoto, Japan). Analytes
were separated by use of a fused silica capillary column (Rt-Q-
BOND, Restek, Bellefonte, PA, USA) with helium as the carrier gas.
The injection temperature was kept constant at 120 ꢀC. The sample
gas was injected into the GC instrument in split mode (ratio, 20/1),
and analyses were conducted in full-scan mode (m/z 2.0–200). The
oven temperature was kept at 30 ꢀC for 5 min, raised to 200 ꢀC at a
rate of 20 ꢀC minÀ1, and kept at that temperature for 20 min.
XPS spectra of g-C3N4 before and after the reactions were
Because the concentration of MEA significantly decreased with
the combination of HT-g-C3N4, O2, and light irradiation, we next
investigated the photochemical decomposition of MEA in detail by
using four kinds of g-C3N4. The irradiation-time-dependent
degradation of MEA induced by each g-C3N4 under O2 is shown
measured with Al-Ka radiation using a Fisons Escalab220i-XL
instrument (Thermo Fisher Scientific, Waltham, MA, USA). The
binding energy was calibrated using C(1s) peak at 248.8 eV.
3. Results and discussion
3.1. Photochemical decomposition of MEA
We irradiated a mixture of g-C3N4 powder (Std-g-C3N4, HT-g-
C3N4, HT-g-C3N4-Pt, or HT-g-C3N4-Ag) and an aqueous solution of
MEA with light from a xenon short-arc lamp through a LU0400
long-pass filter. Under these conditions, the reaction mixture can
receive light at wavelengths above 389 nm (Fig. 1a, see transmit-
tance of LU0400 filter: the transmittance at 389 nm is 0.09%). g-
C3N4 absorbs light up to approximately 480 nm, as shown by the
diffuse diffraction spectra of Std-g-C3N4 and HT-g-C3N4 (Fig. 1a).
MEA has no light absorption above 340 nm (Fig. 1b). Therefore, g-
C3N4 is the only species that can absorb light in our photochemical
reaction system.
Fig. 2. Irradiation-time dependence of the MEA concentration. A mixture of g-C3N4
powder (Std-g-C3N4, HT-g-C3N4, HT-g-C3N4-Pt or HT-g-C3N4-Ag; 30 mg) and an
aqueous solution (22 mL) containing MEA (5.00–5.18 mM) was irradiated
( >389 nm) under O2 (0.5 MPa). After the indicated time, the reaction solution
was collected and subjected to ion chromatography to quantify the MEA
concentration.
We performed the photochemical reactions of MEA with
various combinations of catalyst, light irradiation, and gas

H. Hori et al. / Journal of Photochemistry and Photobiology A: Chemistry 351 (2018) 162–169
165
Table 2
Photocatalytic decomposition of MEA by use of four kinds of g-C3N4 after 34 h of irradiation: fate of carbon
Entry
g-C3N4
Initial MEA (mM)
Remaining MEA (mM)[ratio (%)]
TOC(mM) [remaining ratio (%)]
HCOOH(mM)
Carbon recoverya (%)
1
2
3
4
Std-g-C3N4
HT-g-C3N4
HT-g-C3N4-Pt
HT-g-C3N4-Ag
5.03
5.00
5.00
5.18
3.21 [64]
2.89 [58]
2.59 [52]
2.76 [53]
11.5 [114]
11.0 [110]
11.5 [115]
11.7 [113]
4.1
5.2
3.8
5.7
105
109
90
108
a
Carbon recovery (%) was calculated using the formula: [(sum of the molar concentration of carbon atoms in the remaining MEA and in the formed HCOOH)/(the molar
concentration of the carbon atoms in the initial MEA)]Â100.
in Fig. 2. All four kinds of g-C3N4 led to a decrease in the MEA
concentration over similar time courses. The concentration of MEA
after 34 h of treatment is shown in Table 2.
presence of O2 for 17 h, the TOC concentration after the reaction
was below the detection limit (<0.02 mM, Table 1, entry 5). This
result indicates that carbon atoms did not elute from g-C3N4 in the
absence of MEA.
To identify the organic species formed in the reaction solutions,
we analyzed the reaction solutions by use of HPLC. HPLC analysis
revealed that the major product was HCOOH. The time courses of
When Std-g-C3N4 was used, the concentration of MEA
decreased to 3.21 mM, which corresponds to 64% of the initial
concentration (entry 1). When HT-g-C3N4 was used, the remaining
proportion of MEA (58%, entry 2) was slightly lower than that for
Std-g-C3N4 (64%). Although the specific surface area of HT-g-C3N4
(65 m2 gÀ1) was one order of magnitude higher than that of Std-g-
C3N4 (7.7 m2 gÀ1), this difference had no drastic effect on the
reactivity. When HT-g-C3N4-Pt was used, the remaining MEA
further decreased slightly (52%, entry 3). When HT-g-C3N4-Ag was
used, the remaining MEA (53%, entry 4) was almost the same as
that when HT-g-C3N4-Pt was used. Although we initially expected
that the loading of metal particles (Pt or Ag) on g-C3N4 would
increase the photochemical reactivity of g-C3N4 due to enhance-
ment of charge separation and suppression of electron-hole
recombination, adding metal to g-C3N4 had little effect on the
decomposition of MEA.
To examine whether g-C3N4 degraded during reactions with
MEA, we measured the XPS spectra of HT-g-C3N4-Pt before and
after a reaction with MEA for 34 h in the presence of O2 (Fig. 3). The
spectra for the C(1s) and N(1s) regions after the reaction (panels b
and d, respectively) were very similar to the corresponding spectra
before the reaction (panels a and c, respectively). This result
indicates that HT-g-C3N4-Pt did not degrade markedly during the
reaction, which proves that HT-g-C3N4-Pt acted as a photocatalyst
for MEA decomposition.
3.2. Fate of carbon atoms in MEA
We performed photochemical reactions using four kinds of g-
C3N4 in the presence of O2, and analyzed the gas phase by means of
GC–MS. Our analysis revealed that no gaseous products were
formed except for trace amounts of CO2. Before light irradiation,
the pH of the sample solutions was 10, which indicates that even if
the MEA decomposed to CO2, most of the CO2 molecules were
À
2–
present as HCO3 and CO3 in the reaction solution [38]. To
evaluate the organic compounds present in the reaction solution,
we measured the TOC concentration of the reaction solutions. The
irradiation-time dependence of the TOC concentration during the
photochemical reaction with each g-C3N4 in the presence of O2 is
shown in Fig. 4. Surprisingly, while the MEA concentration clearly
decreased with increasing light irradiation (Fig. 2), the TOC
concentration was almost unchanged for each g-C3N4. After 34 h,
the TOC concentrations were 11.0–11.7 mM, which indicated that
the proportions of remaining TOC were 110–115% (Table 2). These
values were somewhat higher than the ideal TOC concentration in
the initial MEA (the initial MEA concentrations were 5.00–
5.18 mM, which translates to a 10.0–10.4 mM initial concentration
of carbon atoms because each MEA molecule as two carbon atoms).
This fact suggests that carbon atoms may be eluted from g-C3N4
during photoreactions. However, when a mixture of aqueous NaOH
(pH = 10) and HT-g-C3N4 (without MEA) was irradiated in the
Fig. 3. XPS spectra of HT-g-C3N4-Pt for the C(1s) region before (a) and after the
reaction for 34 h (b) and for the N(1s) region before (c) and after the reaction for 34 h
(d).

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H. Hori et al. / Journal of Photochemistry and Photobiology A: Chemistry 351 (2018) 162–169
Fig. 4. Irradiation-time dependence of the TOC concentration in the reaction
solution. The reaction conditions were the same as those described in the caption
for Fig. 2.
Fig. 6. Irradiation-time dependence of the (a) NO2À and (b) NO3À concentrations in
the reaction solution. The reaction conditions were the same as those described in
the caption for Fig. 2.
Fig. 5. Irradiation-time dependence of the HCOOH concentration in the reaction
solution. The reaction conditions were the same as those described in the caption
for Fig. 2.
HCOOH concentration during decomposition of MEA with each g-
C3N4 are shown in Fig. 5. All reactions that included g-C3N4
produced HCOOH, and the HCOOH concentration increased with
increasing irradiation time. When Std-g-C3N4 was used, the
HCOOH concentration reached 4.1 mM after 34 h. At the same
time, the sum of the molar concentration of carbon atoms in the
residual MEA and the molar concentration of carbon atoms in
HCOOH was calculated to be 10.5 mM (= 3.21 mM Â 2 + 4.1 mM, see
Table 2, entry 1). This value was similar to the initial concentration
of carbon atoms in MEA (10.1 mM = 5.03 Â 2, Table 2, entry 1).
Likewise, the carbon recovery calculated from the equation: [(sum
of the molar concentration of carbon atoms in the remaining MEA
and the molar concentration of carbon atoms in HCOOH)/(the
molar concentration of carbon atoms in the initial MEA) Â 100],
was 105% [=10.5 mM/(5.03 mM Â 2) Â 100]. The other g-C3N4 also
showed high values for carbon recovery (90–109%, Table 2, entries
2–4). These results indicate that the carbon atoms in the reaction
Fig. 7. Irradiation-time dependence of the NH3 concentration in the reaction
solution. The reaction conditions were the same as those described in the caption
for Fig. 2.
solutions after 34 h of light irradiation can be accounted for by the
remaining MEA and the formed HCOOH.
3.3. Fate of nitrogen atoms in MEA
To identify nitrogen-containing products in the reaction
solutions, we performed ion chromatography in anionic mode.
À
Our analysis indicated that the reaction solutions involved NO2À
and NO3À. The irradiation-time dependences of the NO2À and NO3
concentrations are shown in Fig. 6a and b, respectively. As
mentioned above, the four kinds of g-C3N4 gave similar time
courses for decreasing the MEA concentration (Fig. 2). In contrast,
the different kinds of g-C3N4 showedÀdifferent time courses for thÀe
À
concentrations of NO2 and NO3
. The formation of NO2

H. Hori et al. / Journal of Photochemistry and Photobiology A: Chemistry 351 (2018) 162–169
167
Table 3
Photocatalytic decomposition of MEA by use of four kinds of g-C3N4 after 34 h of irradiation: formation of nitrogen compounds.
Nitrogen recoverya (%)
À
À
m
Entry
g-C3N4
Initial MEA (mM)
Remaining MEA (mM) [ratio (%)]
NO2
(m
M)
NO3
(
M)
NH3
(m
M)
1
2
3
4
Std-g-C3N4
HT-g-C3N4
HT-g-C3N4-Pt
HT-g-C3N4-Ag
5.03
5.00
5.00
5.18
3.21 [64]
2.89 [58]
2.59 [52]
2.76 [53]
24.7
74.2
115
3.3
6.5
12.9
14.3
69.3
112
221
225
66
68
59
62
194
a
Nitrogen recovery (%) was calculated using the formula: [(sum of the molar concentration of nitrogen atoms in the remaining MEA and in the formed NO2–, NO3–, and
NH3)/(the molar concentration of the nitrogen atoms in the initial MEA)] Â 100.
Scheme 1. Proposed mechanism of MEA decomposition.
increased in the order Std-g-C3N4 < HT-g-C3N4 < HT-g-C3N4-Pt <
HT-g-C3N4-Ag (Fig. 6a). The formation of NO3À, the concentration
of which was one order of magnitude lower than that of NO2À, also
increased in the order Std-g-C3N4 ꢁ HT-g-C3N4 < HT-g-C3N4-Pt ꢁ
HT-g-C3N4-Ag (Fig. 6b). These results indicate that the loading of
metal particles (Ag, Pt) on g-C3N4À enhanced the decomposition
respectively (Table 3, entries 1–3). As described in Section 3.2, the
fate of carbon atoms in the initial MEA was well accounted for by
the HCOOH formed in the reaction solutions. In contrast, the
recovery of nitrogen atoms was low. This difference may indicate
that the rest of the nitrogen atoms were mainly present on the
surface of the g-C3N4, although XPS measurements were not
sensitive to this change (Fig. 3c and d).
À
step of the intermediate(s) to NO2 and the oxidation of NO2 to
NO3À, rather than the initial decomposition step of MEA.
Of the four kinds of g-C3N4, HT-g-C3N4-Ag led to the highest
À
À
À
3.4. Proposed reaction mechanism
NO2 concentration. The concentrations of NO2 and NO3 after
34 h were 194 and 14 M, respectively. At the same irradiation
m
In the present study, MEA decomposed to form HCOOH and
NH3. The formation of HCOOH and NH3 has often been observed
during the decomposition of MEA under CO2 capture conditions
[39–41] and when MEA is decomposed using the TS-1 zeolite
photocatalyst driven by UV light irradiation [19]. However, in many
of these cases, NH3 was a major product and HCOOH was a minor
product. HCHO was also a major product [39,40,42,43]. Consistent
with the literature [42,43] and our findings in this study, we
propose a mechanism for the decomposition of MEA (Scheme 1).
First, an electron is abstracted from the nitrogen atom in MEA. The
formed aminium radical undergoes deprotonation, which results
in the formation of an imine radical. This radical also loses an
electron and is deprotonated to form an imine. This imine then
reacts with water to form NH3 and HCHO. In our system, O2 and g-
C3N4 are present. Although the active oxidizing species is not clear,
time, the concentration of MEA was 2.76 mM and the initial
concentration of MEA was 5.18 mM (Table 2, entry 4). According to
these concentrations, the total concentration of nitrogen atoms
after 34 h was calculated to be 2.97 mM (=194 mM + 14 mM + 2.76
mM), and the concentration of nitrogen atoms in the initial MEA
was 5.18 mM. That is, the recovery of nitrogen atoms after the
reaction was 57% (=2.97/5.18 Â 100). This value suggested the
presence of other nitrogen-containing products in addition to
NO2À and NO3À. In fact, when the reaction solution was subjected
to ion chromatography in cationic mode, a peak corresponding to
NH4+ appeared. This observation indicated that NH3 was present in
the reaction solution because the pH of the reaction solution was
ꢂ10. The irradiation-time dependence of the NH3 concentration is
shown in Fig. 7. NH3 was detected in all of the reaction solutions foÀr
each g-C3N4 at concentrations comparable to those of NO2
(Fig. 6a). When HT-g-C3N4-Ag was used, the NH3 concentration
O2 and/or OHꢃ may be generated on g-C3N4 upon visible-light
ꢃ–
irradiation. These oxidizing specÀies would then likely oxidize
was 225 mM after 34 h (Table 3, entry 4). This value indicates that
HCHO to HCOOH and NH3 to NO2 and NO3À, as observed.
the total concentration of nitrogen atoms was 3.20 mM and the
recovery of nitrogen atoms was 62% (Table 3, entry 4). Likewise, the
recovery of nitrogen atoms for the reactions with Std-g-C3N4, HT-
g-C3N4, and HT-g-C3N4-Pt were calculated to be 66, 68, and 59%,

168
H. Hori et al. / Journal of Photochemistry and Photobiology A: Chemistry 351 (2018) 162–169
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We investigated the photochemical decomposition of MEA in
the presence of g-C3N4 in water under visible-light irradiation.
When aqueous MEA was irradiated by visible light, the MEA
concentration did not change in the presence of either TiO2 (P25)
with O2 or g-C3N4 with argon. In contrast, visible-light irradiation
in the presence of g-C3N4 with O2 led to significant decomposition
of MEA to form HCOOH, NO2À, NO3À, and NH3. Although the TOC
concentration in the reaction solution was not decreased, this
result is consistent with the predominant formation of HCOOH.
When g-C3N4 prepared by using a hydrothermal treatment (HT-g-
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lower than that for g-C3N4 prepared by a standard method. Loading
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À
NO2 and NO3À. The recovery of carbon atoms was accounted for
by the remaining MEA and HCOOH in the reaction solution. This
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Recently, HCOOH has been evaluated as a hydrogen storage
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MEA waste from post-combustion CO2 capture technology could
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a source of HCOOH in the HCOOH utilization
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Acknowledgment
This work was supported by a grant from the Japan Society from
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