Light-Intensity-Responsive Changes of Products in Photocatalytic Reduction of Nitrous Acid on a Cu-Doped Covalent Triazine Framework–TiO2 Hybrid

Article abstract of DOI:10.1002/cssc.202000687

In the design of solar-energy conversion electrochemical systems, it is important to consider that natural sunlight fluctuates. By taking nitrous acid photoreduction as an example, this study has shown that the reaction pathway, and hence the reaction products, dynamically respond to variations in light intensity. Under irradiation, the photooxidation of methanol (as sacrificial agent) on TiO₂ and the reduction of HNO₂ on a Cu-modified covalent triazine framework (Cu-CTF) are electrically coupled, which leads to the photoreduction of HNO₂ without an external bias. The major product of the reaction changes from N₂O to NH₄⁺ with an increase in the light intensity. The operating potential also shifts negatively (or positively) when the light intensity is increased (or decreased). These results indicate that a change in the reaction pathway is triggered by a change in the operating potential of the Cu-CTF catalyst under varying light intensity. Such a light-intensity-dependent change in the reaction pathway is particularly important in systems that use photoresponsive electrodes and where multiple products can be obtained, such as the solar-driven reduction of carbon dioxide and nitrogen oxides.

Full text of DOI:10.1002/cssc.202000687

Chemistry–Sustainability–Energy–Materials
Accepted Article
Title: Light intensity responsive changes of products in photocatalytic
reduction of nitrous acid on a Cu-doped covalent triazine
framework/TiO2 hybrid
Authors: Kazuhide Kamiya, Rino Sugimoto, Tomomi Tatebe, Takashi
Harada, and Shuji Nakanishi
This manuscript has been accepted after peer review and appears as an
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To be cited as: ChemSusChem 10.1002/cssc.202000687
Link to VoR: https://doi.org/10.1002/cssc.202000687
01/2020

10.1002/cssc.202000687
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Light intensity responsive changes of products in photocatalytic
reduction of nitrous acid on a Cu-doped covalent triazine
framework/TiO2 hybrid
Kazuhide Kamiya*,[a,b] Rino Sugimoto,[a] Tomomi Tatebe,[a] Takashi Harada[b], and Shuji Nakanishi*[a,b]
[a]
[b]
Dr. K. Kamiya, R. Sugimoto, T. Tatebe, Dr. T. Harada,Prof. Dr. S. Nakanishi
Graduate School of Engineering Science, Osaka University
1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan
E-mail: nakanishi@chem.es.osaka-u.ac.jp
E-mail: kamiya@chem.es.osaka-u.ac.jp
Dr. K. Kamiya, Dr. T. Harada, Prof. Dr. S. Nakanishi
Research Center for Solar Energy Chemistry, Osaka University
1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan
Supporting information for this article is given via a link at the end of the document
Abstract: In the design of solar-energy conversion electrochemical
systems, it is important to consider that natural sunlight fluctuates. By
taking nitrous acid photoreduction as an example, the present work
reaction intermediate on the electrocatalysts that influence the
selection of the reaction pathway is dependent on the electrode
potential, i.e., the operating potential of the electrocatalysts
has shown that the reaction pathway, and hence the reaction products, themselves.
dynamically responded to the variation of light intensity. Under
irradiation, the photooxidation of methanol (as sacrificial agent) on
TiO2 and the reduction of HNO2 on Cu-modified covalent triazine
framework (Cu-CTF) were electrically coupled, which led to the
photoreduction of HNO2 without an external bias. The dominant
The operating potential of an electron transfer co-catalyst on a
photocatalyst is generally fixed at a certain potential that is
intrinsic to the co-catalyst or the semiconductor particles.[16-19]
However, for light-energy conversion systems using
photoelectrodes, the operating potential of the catalyst changes
depending on the balance of the absolute value of the anodic and
cathodic currents. Such a modulation of the operating potential
might also occur on some sheet-like photocatalysts on which both
semiconductor and electron transfer co-catalysts are loaded on a
single conductive substrate.[20-23] The magnitude of the
photocurrent is dependent on the light intensity; therefore, in
principle, the operating potential of the catalyst could be
influenced by the light intensity, as schematically shown in Figure
1a. Thus, for energy-conversion systems with photoelectrodes,
fluctuation of the environmental light intensity would lead to a
change of the reaction pathway and eventually a change of the
reaction products. However, to our best knowledge, there have
been no studies that have focused on the light intensity
dependence of reaction pathway selection.
+
reaction product changes from N2O to NH4 with an increase in the
light intensity. The operating potential also shifted negatively (or
positively) when the light intensity was increased (or decreased).
These results indicate that a change of the reaction pathway was
triggered by a change of the operating potential of the Cu-CTF
catalysts under varying light intensity. Such
a light-intensity
dependent change of the reaction pathway is particularly important in
systems that use photoresponsive electrodes and where multiple
products can be obtained, such as the solar-driven reduction of
carbon dioxide and nitrogen oxides.
Introduction
In the present work, we attempted to experimentally verify the
light intensity dependence of the reaction pathway selection for
the light-driven reduction of nitrous acid (HNO2). Since nitrogen
oxides (such as nitrate and HNO2) contamination due to
overfertilization and industrial drainage is becoming a significant
issue, the denitrification of wastewater is an urgent need to
maintain the global nitrogen cycle. Thus, it is important to
understand factors to determine the selectivity of HNO2 reduction.
Here, for HNO2 photoreduction, we electrically coupled anatase
TiO2 with a Cu-modified covalent triazine framework (Cu-CTF)[24]
using two systems: a two-electrodes photoelectrochemical cell
without an external bias and a sheet-like photocatalytic device.
TiO2 particles can serve as reaction sites for the photooxidation
of methanol, a sacrificial reagent used to provide electrons to the
electrical circuit. On the other hand, Cu-CTFs were connected
with the circuit as the catalyst for the reduction of HNO2.
Light-driven conversion of chemical substances has attracted
much attention with respect to the photogeneration of valuable
chemicals and the photodegradation of pollutants.[1-4] Direct
photochemical energy conversion can be achieved through the
use of photocatalysts or photoelectrodes. In both cases,
appropriate electron transfer co-catalysts that accelerate a target
reaction must be loaded on the semiconductor particles and/or
electrode.[5-8] In most cases, materials that are already known to
be active as electrocatalysts are used as co-catalysts. For
example, platinum and iridium oxide, which are excellent
electrocatalysts for hydrogen and oxygen generation reactions,
respectively, are widely used as co-catalysts for water splitting
with photocatalysts or photoelectrodes.[9-11] In electrocatalytic
reaction systems that can generate multiple chemical species, the
composition of the reaction products is generally dependent on
the electrode potential.[12-15] This is because the stability of the
1
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Accordingly, the reduction of HNO2 on Cu-CTF would, in principle,
proceed by coupling with the photooxidation of methanol on TiO2
via an electrical connection without an external bias, as illustrated
in Figure 1b. The reaction products of HNO2 on the Cu-CTF
knowledge obtained through the present work can be generalized
because CTF-based materials can serve as electrocatalysts for
various reactions simply by replacement of the modified metal
species.[24, 30-38]
catalyst are expected to depend on the operating potential[24-25]
;
therefore, this reaction system is suitable to verify the light-
intensity dependent selection of the reaction pathway.
s
Scheme 1. Major reaction pathways for the electrochemical reduction of nitrous
acid in acidic electrolytes.
Physical and morphological characterizations of Cu-CTF.
Our CTF was obtained through the polymerization of 2,6-
pyridinedicarbonitrile in the presence of conductive carbon
particles (please see the synthesis part) to add electron
conductivity. [24, 34-38] The resulting CTF was completely insoluble
in any common solvents.[28, 39] Then, the CTF was impregnated in
1 mM CuCl2 aqueous solutions to dope Cu atoms. Although the
detailed characterizations were reported in our previous reports,
we again explain those to help readers to understand the detail of
28]
our material.[23-24,
The SEM image of a Cu-CTF electrode
(Figure 2a) shows the primary particle size of 20–60 nm,
corresponding to the size of the added carbon particles. The
powder X-ray diffraction (XRD) pattern revealed that our Cu-CTF
has an amorphous structure, which is consistent with the reported
CTF from 2,6-pyridinedicarbonitrile (Figure S1).[40-41] The atomic
concentration of Cu and N atoms were 0.19 and 4.9 %,
respectively, which was determined by semi-quantitative X-ray
photoelectron spectroscopy (XPS). The narrow N-1s spectra of
CTF and Cu-CTF are shown in Figure S2a and S2b, respectively.
These spectra could be deconvoluted into C2NH (399.2 eV) and
C3N (401.0 eV), as reported by Thomas and coworkers.[42]
Notably, the peak assignable to the cyano group in the monomer
at 400.5 eV was very small, indicating that the polymerization
efficiently proceeded.[34] The N2 isotherms and the pore size
distribution of Cu-CTF are shown in Figure S3. Cu-CTF has a
hierarchical porous structure consisted of micro- and mesopores.
The BET surface area was 731 m2/g.
Next, we investigated the Cu atoms in Cu-CTF. Figure 2b
shows the Cu-2p XPS spectrum of Cu-CTF. The satellite peaks
on the higher binding energy side of the main peak were clearly
observed, characterizing the existence of Cu(II).[43] The Cu-K
edge X-ray absorption near edge structure (XANES, Figure 2c)
analysis showed that the absorption edge for Cu-CTF located at
8980 eV, which is consistent with that for CuO and Cu(II)-
tetraphenylporphyrin (TPP). The XPS and XANES results
indicated that the Cu(II) valence state was dominant in Cu-CTF.
The coordination environment of the Cu site in Cu-CTF was
investigated by the extended X-ray absorption fine structure
(EXAFS, Figure 2d). The Cu-N peak at 0.16 nm was clearly
Figure 1. Schematic diagrams for (a) the relationship between the operating
potential and oxidation/reduction currents, and (b) the photoreduction of nitrous
acid via an electrical connection with TiO2 and Cu-CTF.
Results and Discussion
Model reaction system. Here, we explain the detail of HNO2
reduction on Cu-CTF and why it could be a suitable model system
for the present work. In acidic solution,[26-27] HNO2 is
-
spontaneously disproportionated into nitrate (NO3 ) and nitric
oxide (NO). NO can exist either as surface-bound or soluble
species, depending on the adsorption energy of NO. When the
adsorption energy is sufficiently weak, the soluble NO is dominant,
and the catalyst surface can be free from adsorbates. In this case,
HNO2 can be electrochemically reduced on the catalyst, which
+
results in the direct generation of ammonium ions (NH4 ) from
HNO2. On the other hand, when the adsorption energy is
moderate, reductive dimerization of the surface-bound and
soluble NO proceed, which results in the formation of nitrous
oxide (N2O). The adsorption energy of the reaction intermediate
is generally dependent on the potential; therefore, the potential
also influences the composition of the reaction products.[15, 26-28]
The reaction pathway for HNO2 reduction is summarized in
Scheme 1. Cu-CTF was selected as the electrocatalyst for HNO2
reduction for the following reasons. (1) The adsorption energy of
NO on Cu-CTF is appropriately moderate[24, 28-29], and (2) the
2
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observed, whereas the peaks assignable to Cu-Cu peak (0.22
nm) or Cu-O-Cu (0.27 nm) were not detected.[28] Therefore, the
Cu atoms in Cu-CTF were almost atomically dispersed. The peak
intensity of Cu-N for Cu-CTF was lower than that for Cu-TPP,
suggesting that the coordination number of N atoms in Cu-CTF is
smaller than 4. In fact, the coordination number had been
calculated to be 3.4 based on the EXAFS fitting, as reported in
our previous manuscript.[24] This unsaturated coordination
structure of Cu sites leads to the strong adsorption of adsorbate
because of a low steric hindrance and more accessible d-
orbitals.[29] In the case of nitrogen oxides reduction reactions, the
Cu site in Cu-CTF strongly binds and effectively reduces NO, as
mentioned above and in our previous works.[23-24]
reaction on TiO2 without an external bias, which will be verified
later.
DEMS analyses were performed simultaneously with
voltammetry measurements in 0.1 M HClO4 containing 0.1 M
methanol (Figure 3b). The onset potentials for the increase of the
15N2O signal (m/z = 46) and the cathodic current correlated well
with each other, which indicates that the reductive dimerization of
NO is the dominant process in this potential region. Although the
15N2O signal initially increased with the negative potential shift, the
signal began to decrease after exhibiting a peak at around −0.05
V. The cathodic current then continued to increase, even after
15N2O began to decrease, which indicates that the contribution of
the reductive dimerization reaction to the total reduction current
gradually decreases, and other reactions become dominant with
the negative potential shift. Thus, the reaction mechanisms (i.e.,
the reaction pathway) for HNO2 reduction change depending on
the potential. To clarify the dominant reaction pathway in the
negative potential region, the reaction products were analyzed
using chromatography. NH4+ was the main products (Figure 3c),
which suggests that HNO2 can be directly reduced on Cu-CTF in
this potential region.
The reaction pathway for HNO2 reduction can be
summarized as follows. First, HNO2 is partially disproportionated
−
to NO and NO3 . Under positive potentials, the surface-bound NO
and soluble NO are then reductively dimerized to form N2O. On
the other hand, under negative potentials, HNO2 can be directly
+
reduced on Cu-CTF, which leads to the generation of NH4 . It was
confirmed that the onset potential of the NO3− reduction current -
0.70 V (Figure S5), which was out of the potential region of
interest in this study.[23-24] The complete results described here
are essentially consistent with that shown in Scheme 1. However,
it should be noted that the reaction pathway is changed according
to the potential of Cu-CTF.
Figure 2. (a) The representative SEM image of a Cu-CTF electrode. Scale bar:
200 nm. (b) Cu-2p XPS spectrum of Cu-CTF (c) The Cu-K XANES spectra for
Cu (gray), Cu2O (green), CuO (brown), Cu(II)-TPP (blue) and Cu-CTF (red). (d)
k3-weighted FT-EXAFS spectra of the Cu-K edge for Cu-CTF (red), Cu(II)-TPP
(blue), and Cu metal (gray).
Half-cell electrochemical measurements. Current density (j) vs.
potential (U) curves for the TiO2 electrode are shown in Figure 3a,
in which 0.1 M methanol was added as a sacrificial reagent to
generate photoexcited electrons for other reactions. The onset
potential of the anodic photocurrent was −0.34 V, which
corresponds to the flat band potential of anatase TiO2. The j vs. U
curves obtained for the Cu-CTF on FTO in a perchloric acid
(HClO4) electrolyte with (blue curve) and without (black curve)
dissolved HNO2 are also shown in Figure 3a. A comparison of the
two curves in this figure reveals that the onset potential for the
reduction current on Cu-CTF located at +0.05 V vs. Ag/AgCl. We
also performed chronoamperometry at -0.3 V vs. Ag/AgCl with
and without HNO2 (Figure S4). The cathodic current largely
increased on Cu-CTF by the addition of HNO2 at -0.3 V. Thus, the
onset potential of the photoelectrochemical oxidation of methanol
on TiO2 (-0.34 V) was more negative than that of HNO2 reduction
on Cu-CTF, which indicates that HNO2 reduction can
spontaneously proceed when coupled with the photooxidation
Figure 3. (a) Polarization curves obtained with (upper) TiO2 and (lower) Cu-CTF
acquired at a scan rate of 10 mV s-1. Electrolyte: (upper) 0.1 M HClO4 containing
0.1 M methanol at a light intensity of 1 mW cm-2 (red), and (downer) 0.1 M HClO4
containing 0.1 M methanol with/without 10 mM HNO2. (b) Mass signals assigned
to 15N2O and the corresponding currents on Cu-CTF in 0.1 M HClO4 containing
3
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0.1 M methanol with 10 mM HNO2. (c) Faradaic efficiency during the HNO2
reduction reaction using Cu-CTF in 0.1 M HClO4 containing 0.1 M methanol with
10 mM HNO2 at -0.4 V.
onset potential of the anodic photocurrent was not influenced by
the addition of HNO2, which indicates that the electrons excited
on TiO2 were not transferred to HNO2 but to the FTO electrodes,
despite the presence of HNO2. Therefore, the reduction of HNO2
occurred only on Cu-CTF in this device.
Light-intensity dependent selection of the reaction pathway.
Here, we evaluate the photo-driven nitrous acid reduction activity
of the couple of TiO2/Cu-CTF by fabricating a sheet-like
photocatalytic device and a two-electrodes electrochemical cell.
The sheet-like photocatalytic device incorporating both Cu-CTF
and TiO2 were prepared on a single FTO (Figure S6). As shown
in Figure 3a, the onset potential for the photooxidation current on
TiO2 was more negative than that for the cathodic HNO2 reduction
current on Cu-CTF. Therefore, HNO2 reduction on the devices is
expected to occur spontaneously based on the principle of local
cell reactions under irradiation conditions.[23, 44] The device was
irradiated with UV light (350–400 nm) at light intensities of 0.6, 2,
and 20 mW cm-2. The generation of N2O was confirmed to
increase with time by in situ mass analysis (Figure 4a), which
clearly indicates that the photo-driven reduction of N2O proceeded
on the device. Although the operating potential was monotonically
shifted to the negative side with an increase in the light intensity,
the generation of N2O decreased under the highest light intensity
(20 mW/cm-2). Considering that the operating potential of this
device was about -0.3 V at 20 mW/cm-2, this result is well
consistent with the relationship between the potential and the N2O
generation rate that was obtained by DEMS measurements
(Figure 3b).
The product distribution depending on light intensity was
investigated a double-chamber electrochemical cell in which the
two chambers were separated by a Nafion membrane (Figure S7)
to focus on only HNO2 reduction reaction at Cu-CTF. It is noted
here that this cell was operated in a short-circuit condition in a
similar manner to the sheet-like photocatalysts on a single
conductive substrate. The ratio and amounts of reaction products
after 6 h of light irradiation are summarized in Figure 4b and Table
Figure 4. (a) Nitrous acid reduction yield 6 h after initiation of the reaction in 0.1
M HClO4 containing 0.1 M methanol with 10 mM HNO2. (b) Time course of the
operating potential at light intensities of 1 mW cm-2 (red), and 9 mW cm-2 (blue).
(c) Changes in operating potential (blue) and mass signals assigned to 15N2O
(m/z=46) generated by HNO2 reduction on Cu-CTF (red) with changes in the
light intensity. Electrolyte: 0.1 M HClO4 containing 0.1 M methanol with 10 mM
HNO2.
+
S1a, respectively. The major products were N2O and NH4 , which
were detected by using gas chromatography mass spectrometry
(GCMS, Figure S8) and ion chromatography, respectively. The
amounts of other products, such as N2 and hydroxylammonium
ions were almost below the detection limit. The time courses of
the device potential during light irradiation are also shown in
Figure 4c. At a light intensity of 1 mW cm-2, N2O was the dominant
Reaction selectivity under naturally fluctuating sunlight. The
intensity of natural sunlight fluctuates dynamically due to the
Earth’s rotation and changes in the weather. In view of the above
conclusion, the composition of the reaction products would thus
fluctuate under irradiation with natural sunlight. This was verified
by quantifying the amounts of reaction products obtained under
natural sunlight using the same double-chamber electrochemical
cell (Figure 5a). The area of TiO2 coated FTO (36 cm2) for this
experiment was larger than that used under artificial light (2 cm2)
(for the detail, see experimental section). Figure 5b shows the
amount of N2O generated every 2 h. N2O generation decreased
at the third period (12:30-14:30) compared with that at the second
period (10:00-12:00), although the intensity of sunlight increased
(Figures 5b and c). Considering the measured operating potential,
this result is consistent with the results using artificial light. In
addition, Figure S11 shows this relationship among N2O
production, sunlight intensity, and operating potentials obtained
on other days. These results indicate that the light-energy
conversion systems used under natural sunlight should be
designed with consideration of the dynamic change in the catalyst
operating potential.
+
product, and a small amount of NH4 was detected (ca. 10 %).
+
When increasing the light intensity to 9 mW cm-2, the ratio of NH4
drastically increased to over 40 %. The external quantum
efficiency (QEs) for each product are summarized in Table S1b
(for the calculation detail, see Supporting Information). When
changing the light intensity from 1 mW cm-2 to 9 mW cm-2, the QE
of N2O decreased from 5% to 1%. Notably, the XANES spectrum
of Cu-CTF did not at all change after the catalyst test under 1 mW
cm-2, indicating that our catalysts are stable in these reaction
conditions (Figure S9). Although the reason why the potential
gradually shifted to the negative side during the photoreaction is
still unclear (Figure 4c), the decrease in HNO2 substate might
cause the lower cathodic current and the negative shift of the
operating potential. We would like to discuss this issue as future
work quantitatively.
Notably, the anodic photocurrent of TiO2 with and without
HNO2 was also measured using the half-cell setup to confirm if
HNO2 reduction proceeded directly on TiO2 (Figure S10). The
4
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Experimental Section
Synthesis of Cu-CTFs. CTF was prepared by the same method
as described in our previous reports.[23-24] Briefly, 2,6-
pyridinedicarbonitrile (64.5 mg, Aldrich) and KetjenBlack (64.5 mg,
EC600JD, Lion Corp.) were mixed with ZnCl2 (6.82 g, Wako) in a
vacuum glass tube, and then heated at 400 °C for 40 h. After
cooling down to room temperature, the resultant power was
washed with deionized water, tetrahydrofuran (THF), and HCl (1
M, Wako), and was then modified with Cu atoms by stirring for 2
h at 60 °C in 1 mM CuCl2 aqueous solution. The products were
subsequently collected by centrifugation and washed thoroughly
with deionized water to remove residual metal salt.
Electrode Preparation and Characterization. The Cu-CTF
electrode was fabricated as follows. 5 mg Cu-CTF catalyst and
100 μL Nafion (5 wt%, Du Pont, Corp.) were homogeneously
dispersed in 200 μL ethanol. 50 μL of the obtained catalyst ink
was dropped onto a glassy carbon plate (2 cm2), and was then
left to dry in air at room temperature to yield a catalyst layer with
a loading weight of 0.4 mg cm-2. For in situ differential
electrochemical mass spectrometry (DEMS) measurements, 30
μL of catalyst ink was dropped onto a glassy carbon electrode
(0.07 cm2).[49]
The TiO2 electrode was prepared according to the reported
protocol.[50] TiO2 powder (3.2 g, anatase form, surface area 52 m2
g-1; AMT-600, TAYCA) was homogeneously dispersed into a
solution of distilled water (35.2 mL), acetyl-acetone (0.64 mL),
nitric acid (60%, 0.8 mL), and polyoxyethylene-p-isooctylphenol
(0.835 mL, Triton X-100). The solution was applied onto a
fluorine-doped tin oxide (FTO) substrate (resistance of ca. 7 Ω
sq−1, Aldrich) using a spin-coater at 2000 rpm, which was heated
at 150 °C. This was repeated four times and was followed by
calcination at 500 °C for 3 h.
Figure 5. (a) Photograph and schematic diagram of the reactor. (b) Amount of
N2O generated every 2 h. (c) Time course of the operating potential (red) and
sunlight intensity (black) under natural sunlight. Date and place: 16/10/2019 in
Toyonaka, Osaka, Japan.
Conclusion
The reaction pathway for photo-driven HNO2 reduction on
substrates with TiO2 and Cu-CTF was demonstrated to be
dependent on the light intensity. The potential of the device was
determined by the balance of the absolute photooxidation and
reduction currents. Therefore, when the light intensity is increased
(or decreased), the balanced potential shifts negatively (or
positively). According to the shift of the potential, the reaction
pathway is altered, which results in a change of the composition
of the reaction products. This study has thus shown
experimentally that it is necessary to consider the light-intensity
dependent change of the catalyst potential when constructing
light-energy conversion systems with photoelectrodes. This is
especially important for systems where multiple products can be
generated under fluctuating light intensity, such as the solar-
driven reduction of carbon dioxide and nitrogen oxides.
We must note that the operating potential in this system is
limited by a conduction band bottom of n-type semiconductors
(e.g., -0.3 V vs. RHE for anatase TiO2). Considering that the
operating potential for CO2 reduction is generally more negative
than -0.6 V vs. RHE, TiO2 cannot be utilized for the CO2 reduction
reaction. Recently, several n-type semiconductors with more
negative conduction band bottoms, such as TaON, Ta2O5 and g-
C3N4, have been reported as photocatalysts for CO2 reduction.[1,
45-48] These n-type semiconductors can also oxidize water to O2 in
the presence of appropriate co-catalysts. When we utilize these
semiconductors for CO2 reduction reactions, the insight provided
by this work will become quite important.
XPS spectra were measured using monochromatic Al Kα X-
rays at hν = 1486.6 eV. X-ray absorption fine structure (XAFS)
measurements were performed using the hard X-ray BL01B01
beamline at SPring-8, Japan. SEM was performed on a field
emission scanning electron microscope (FE-SEM; S-5000,
Hitachi). A powder XRD pattern was obtained using a PANalytical
X'Pert PRO diffractometer with Cu K radiation. The N2 isotherm
was recorded by a Micromeritics 3 Flex at 77 K.
Half-cell electrochemical measurements. Electrochemical
measurements were undertaken using an electrochemical station
(HZ-5000, Hokuto Denko). A Ag/AgCl (saturated KCl as inner
solution) electrode and the titanium wire were used as reference
and counter electrodes, respectively. HNO2 reduction activities
were measured in 0.1 M HClO4 solutions (pH 1) containing 0.1 M
methanol. 15N-labeled sodium nitrite (Cambridge Isotope
Laboratories) was used as a HNO2 source to exclude the
possibility of detecting products from the degradation of the CTF.
Details of the in situ DEMS analysis are described in the previous
[49]
study.
DEMS signals of 30 (fragment of 15N2O and 15N2), 31
(15NO and fragment of 15N2O), and 46 (15N2O) were recorded.
Photocatalytic test by the sheet-like devices incorporating
both Cu-CTF and TiO2. After preparing the TiO2 layer on half the
area of the FTO substrate (4 × 5 cm), a graphite sheet (1 × 1 cm
×17 μm, EYGS091202, Panasonic,) was pasted next to the TiO2
5
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layer using conductive silver paste (Dotite) and epoxy resin. 15
μL of Cu-CTF catalyst ink was dropped on the paper and left to
dry at room temperature to obtain a photocatalytic device (Figure
S6). The reaction was started by UV light irradiation
(Lightningcure UV-LED LC-LIVB, Hamamatsu, 350–400 nm). A
Ag/AgCl (saturated KCl as inner solution) was used as a
reference electrode. The in situ mass analysis was conducted
using the same setup with DEMS.
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Entry for the Table of Contents
The Cu-doped covalent triazine framework/TiO2 hybrid photocatalytically reduced HNO2 without an external bias under artificial light
and natural sunlight. The dominant reaction product changes from N2O to NH4+ with an increase in the light intensity due to the shift of
operating potentials.
8
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