ZIF-67/CoOOH cocatalyst modified g-C3N4 for promoting photocatalytic deep oxidation of NO

Article abstract of DOI:10.1016/j.jallcom.2021.160318

The removal of nitrogen oxides (NO_X) by semiconductor photocatalysis is an emerging technology in recent years. However, due to incomplete oxidation, the photocatalytic oxidation of NO_X is usually accompanied by the generation of toxic intermediate by-products nitrogen dioxide (NO₂), which causes secondary pollution and seriously limits its practical application. To tackle the issue, ZIF-67/CoOOH (ZIF-CH) cocatalyst was constructed via flexible strongly alkali oxidation treatment to modify g-C₃N₄, in which ZIF-67 was selected as a cobalt source of CoOOH. XRD, SEM, TEM and XPS demonstrated the ZIF-CH was successfully synthesized and anchored on CN. UV–vis, PL, EIS, transfer photocurrent response and DFT indicated that the introduction of ZIF-CH enlarged the response range to visible light, favored the separation and transfer of carriers and improved NO/NO₂ adsorption ability. Consequently, the optimized ZIF-67/CoOOH/g-C₃N₄ (ZIF-CH/CN) exhibited a superior NO removal efficiency of 52.5% without any generation of toxic by-product NO₂, and the cycling tests indicated the high stability of ZIF-CH/CN was obtained. In-suit DRIFTS and ESR were used to investigate the reaction pathway by comparing adsorption energy and detecting the reaction intermediates and products. More importantly, this result reveal that amount of hydroxyl radical (·OH) increased after introducing ZIF-CH cocatalyst, which promotes the deep oxidation of NO. These findings could supply a convenient and effective strategy for the design of a cocatalyst to enhance the photocatalytic oxidation performance of NO and inhibit the production of toxic by-product NO₂.

Full text of DOI:10.1016/j.jallcom.2021.160318

Journal of Alloys and Compounds 882 (2021) 160318
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
ZIF-67/CoOOH cocatalyst modified g-C N for promoting photocatalytic
3
4
deep oxidation of NO
a,b
a,⁎
a
a
a
a
Guangzhi Du , Qian Zhang , Wenyan Xiao , Zeyu Yi , Qian Zheng , Hongtao Zhao ,
a
a
a
c
c
Yanzhao Zou , Bangxin Li , Zeai Huang , Dajun Wang , Lin Zhu
a
The Center of New Energy Materials and Technology, School of New Energy and Materials, Southwest Petroleum University, Chengdu 610500, PR China
State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu 610500, PR China
State Key Laboratory of Industrial Vent Gas Reuse, The Southwest Research & Design Institute of the Chemical Industry, PR China
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
The removal of nitrogen oxides (NO ) by semiconductor photocatalysis is an emerging technology in recent
X
Received 7 March 2021
Received in revised form 30 April 2021
Accepted 5 May 2021
years. However, due to incomplete oxidation, the photocatalytic oxidation of NO
the generation of toxic intermediate by-products nitrogen dioxide (NO ), which causes secondary pollution
and seriously limits its practical application. To tackle the issue, ZIF-67/CoOOH (ZIF-CH) cocatalyst was
constructed via flexible strongly alkali oxidation treatment to modify g-C , in which ZIF-67 was selected
X
is usually accompanied by
2
Available online 11 May 2021
3 4
N
as a cobalt source of CoOOH. XRD, SEM, TEM and XPS demonstrated the ZIF-CH was successfully synthe-
sized and anchored on CN. UV–vis, PL, EIS, transfer photocurrent response and DFT indicated that the
introduction of ZIF-CH enlarged the response range to visible light, favored the separation and transfer of
Keywords:
ZIF-CH cocatalyst
G-C
Photocatalysis
Toxic NO inhibition
NO removal
3 4
N
carriers and improved NO/NO
2 3 4
adsorption ability. Consequently, the optimized ZIF-67/CoOOH/g-C N (ZIF-
2
CH/CN) exhibited a superior NO removal efficiency of 52.5% without any generation of toxic by-product NO ,
2
and the cycling tests indicated the high stability of ZIF-CH/CN was obtained. In-suit DRIFTS and ESR were
used to investigate the reaction pathway by comparing adsorption energy and detecting the reaction in-
termediates and products. More importantly, this result reveal that amount of hydroxyl radical (·OH) in-
creased after introducing ZIF-CH cocatalyst, which promotes the deep oxidation of NO. These findings could
supply a convenient and effective strategy for the design of a cocatalyst to enhance the photocatalytic
oxidation performance of NO and inhibit the production of toxic by-product NO
2
.
©
2021 Elsevier B.V. All rights reserved.
1. Introduction
photocatalysis has attracted much more attention because of its
green and sustainable utilization [6–10].
The emission of nitrogen oxides (NO
increasing greatly due to the rapid development of industrialization
and the huge consumption of fossil fuels [1]. The increased con-
X
) in atmosphere has been
So far, many photocatalysts have been investigated for NO
moval, such as TiO , BiOX (X˭Cl, Br, I), SrTiO3, g-C , et al. [11–15].
Among them, g-C is attractive in photocatalytic NO removal
X
re-
2
3 4
N
3
N
4
X
centration of NO
ozone pollution, resulting in immeasurable damages to plants, soil
and serious threats to human health [2–5]. Thus, various techniques
X
will cause acid rain, photochemical smog and
because of its appropriate visible light absorption capacity and
simple preparation method [16–18]. However, there are still some
problems need to be solved over g-C
photocatalytic NO removal efficiency. Firstly, active species such as
superoxide radical (·O
easily generated under visible light on the surface of g-C
3 4
N for obtaining enhanced
and methods for NO
X
removal have been explored, such as physical
X
-
adsorption, selective catalyst reduction and wet scrubbing [2,3].
However, not only high energy is consumed, but also secondary
pollution is usually caused through the above methods. Therefore,
scientists were committed to develop sustainable and efficient
2
) and hydrogen peroxide (H
2
O
2
) could be
. But it’s
3 4
N
difficult to produce the stronger oxidizing species hydroxyl radical
(·OH) [19], resulting in the massive production of nitrogen dioxide
methods for NO
X
removal. Among these methods, semiconductor
(NO
2
). As well known, NO
can cause a variety of respiratory disorders
emphysema and bronchitis) and even syncope and shock
6,10,19,20]. In addition, the adsorption and deep oxidation capacity
to NO is not desired, which leads to the high content of
2
is much more harmful to human than NO.
Even ppb level of NO
(
[
2
⁎
Corresponding author.
of g-C
3
N
4
2
E-mail address: zhangqian@swpu.edu.cn (Q. Zhang).
https://doi.org/10.1016/j.jallcom.2021.160318
0925-8388/© 2021 Elsevier B.V. All rights reserved.

G. Du, Q. Zhang, W. Xiao et al.
Journal of Alloys and Compounds 882 (2021) 160318
NO
still a challenge to improve the photocatalytic NO
mance of g-C -based photocatalyst.
Up to now, many tactics have been applied to improve the pho-
tocatalytic oxidation capacity of g-C by doping g-C with
metals (Au@CN) [22] or nonmetals (C -gCN) [23], coupling with
other semiconductors to form heterojunction (Bi CO /g-C
SnO /g-C ) [20,24], and cocatalyst modification. It’s worth noting
2
in the final products [21]. Considering the above problems, it is
2.1.4. Synthesis of ZIF-CH/g-C
ZIF-CH/g-C photocatalysts were prepared by magnetic stir-
ring at room temperature. ZIF-CH and g-C were dissolved in
40 mL ethanol at a certain mass ratio and then stirred for 3 h. The
mass ratios of ZIF-CH to g-C (CN) were kept in 10%, 20%, 30% and
3 4
N
X
removal perfor-
3 4
N
3
N
4
3 4
N
3
N
4
3
N
4
3 4
N
v
40%, the obtain samples were respectively marked as ZIF-CH(10%)/
CN, ZIF-CH(20%)/CN, ZIF-CH(30%)/CN and ZIF-CH(40%)/CN.
2
O
2
3
3 4
N ,
2
3 4
N
that cocatalyst modification has attracted much attention because it
can combine the advantages of both cocatalyst and photocatalyst.
Usually, the photocatalytic performance of semiconductors could be
improved by loading the noble metals co-catalyst such as Pt [25], Pd
2.2. Characterization
Powder X-ray diffraction (XRD) was carried out to study the
phase structure of the catalyst in PANalytical X′pert diffractometer
using Cu Kα radiation. The microstructure of the samples was stu-
died by scanning electron microscope (SEM) and transmission
electron microscope (TEM). X-ray photoelectron spectroscopy (XPS
Thermo Scientific Escalab 250Xi spectrometer) was used to study the
chemical composition and state of the samples. UV–vis spectroscopy
(Shimadzu 2600 spectrophotometer) utilizing barium sulfate
[
26], Ag [27], et al. However, the practical applications of noble
metals are limited because of the scarce reserves and high prices. To
solve this problem, the noble-metal-free co-catalysts such as Ti
17], Ni-based [28,29] co-catalysts and so on have been studied.
Recently, Zhang, et al. [28] prepared the layered material g-C /α-
Ni(OH) . The g-C /α-Ni(OH) exhibited outstanding NO removal
rate and inhibited the generation of NO . Therefore, transition metal
hydroxides may well inhibit the production of NO . As we all know,
3 2
C
[
3 4
N
2
3
N
4
2
2
2
4
(BaSO ) as the reflectance standard sample and photoluminescence
the CoOOH has been widely used in electrochemistry because of its
rapid transmission of electrons, which may improve the efficiency of
electron-hole separation. However, there is still a problem for
CoOOH to overcome, which is that CoOOH could not provide enough
active sites for photocatalytic reactions due to the small specific
surface area [30]. Recently, metal-organic frameworks (MOFs) have
attracted significant attention for photocatalysis because of their
well-defined crystalline structure and high surface area [31,32].
Meanwhile, previous studies suggest that Zeolitic Imidazolate Fra-
mework-67(ZIF-67) has a uniform distribution of cobalt ions and
controllable structure. Hence, ZIF-67 could be used as a precursor to
increase the specific surface area of CoOOH [33].
(PL Hitachi F-7000 fluorescence spectrometer) using the light source
of MVL-210 were recorded to test optical properties of samples.
Electron spin resonance (ESR) spectroscopy was tested to research
the main active species on a JES-FA200 spectrometer.
3. Results and discussion
3.1. Crystal structure
To investigate the crystal structures and phase compositions of
the CN, ZIF-CH, and different mass ratios of ZIF-CH/CN nano-
composites, XRD was tested as shown in Fig. 1. The two peaks at
13.1° and 27.4° were attributed to the (100) and (002) planes of CN
In this study, ZIF-67 was selected as a cobalt source to construct a
monolithic co-catalyst ZIF-67/CoOOH (ZIF-CH) via strong alkali oxi-
dation treatment. Then ZIF-CH were anchored on the surface of g-
.
[16] As for ZIF-67, all the diffraction peaks was consisted with pre-
C
3
N
4
via facile magnetic stirring. It can be found that ZIF-CH could
vious study [34,35]. Compared to ZIF-67, the three main character-
istic peaks at 20.2°, 36.9°, 38.8° were corresponded to (003), (101),
(012) of CoOOH (JCPDS 07-0169) in ZIF-CH [30], illustrating that ZIF-
CH monolithic co-catalyst was successfully prepared. For ZIF-CH/CN
nanocomposites, the diffraction peaks of both ZIF-CH and CN were
clearly detected, which indicated that ZIF-CH/CN nanocomposites
were successfully fabricated.
not only inhibit the recombination of photo-generated carriers, but
also promotes the production of the stronger oxidizing species·OH.
Benefiting from these, NO could be deeply oxidized and the forma-
2 3 4
tion of NO could be inhibited over ZIF-CH/g-C N composites, de-
monstrating its great potential for air purification. This finding
supplies an effective strategy for the better design of MOFs derived
photocatalysts in the area of deep NO oxidation by photocatalysis.
2
2
2
. Experimentation
.1. Preparation
3 4
.1.1. Synthesis of g-C N
g-C
28]. Urea was positioned in an aluminum oxide crucible with a
cover. Then the crucible was heated to 550 °C and held for 1 h. Fi-
nally, the light-yellow g-C powder was obtained.
3 4
N was synthesized by the thermolysis of 15 g urea precursor
[
3 4
N
2.1.2. Synthesis of ZIF-67
As reported earlier [34], 5.820 g of Co(NO ) and 6.568 g of 2-
3 2
Methylimidazole were melted in 500 mL methanol, and the obtained
mixed solution was aged for 24 h. The violet product was collected
by centrifugation and then dried at 60 °C.
2
.1.3. Synthesis of ZIF-67/CoOOH
00 mg of ZIF-67 was added in 40 mL deionized water, then
00 mg NaOH and 2 mL NaClO (Active content ≥5.5%) were dis-
2
6
persed into the above solution and stirred. The dark-purple sample
was washed and collected by centrifuged, then dried at 60 °C to
obtain ZIF-67/CoOOH (ZIF-CH).
3 4
Fig. 1. XRD of g-C N , ZIF-CH, and different ratio of ZIF-CH/CN.
2

G. Du, Q. Zhang, W. Xiao et al.
Journal of Alloys and Compounds 882 (2021) 160318
2 2
Fig. 2. The NO removal of different samples (a); NO generation of different samples (b); The NO removal ratio (c) and NO generation (d) of ZIF-CH(20%)/CN in recycle
experiment.
3
.2. Photocatalytic NO oxidation performances
stability over ZIF-CH(20%)/CN in combination with the XRD of
samples after reaction (Fig. S3). These results revealed that the in-
troduction of CoOOH on the surface ZIF-67 co-catalyst not only ef-
fectively improved the NO oxidation capability of CN, but also
To study the effect of ZIF-CH on the photocatalytic activity, ZIF-
CH/CN was applied for photocatalytic oxidation of NO. Meanwhile,
the generation rate of toxic by-product NO was recorded during the
NO oxidation reaction. As depicted in Fig. 2(a) and (b), CN showed
NO oxidation ratio of 34.4%, and the generation rate of NO reached
2
2
completely inhibited the generation of NO .
2
to 15.5% after visible light irradiation for 30 min. While ZIF-CH
showed little activity for photocatalytic NO oxidation. Meanwhile, as
shown in Fig. S1, the concentration of NO did not obviously decrease
over CoOOH, which illustrates that ZIF-CH is mainly used as a co-
catalyst. And the photocatalytic stability of ZIF-67(20%)/CN ob-
viously increased compared with CH/CN (Fig. S2), indicating ZIF-67
could improve the stability of photocatalytic NO oxidation. Ob-
viously, the photocatalytic performances of ZIF-CH/CN nanocompo-
sites were better than pure CN and ZIF-CH. As shown in Fig. 2(a), the
oxidation ratio of NO first increased and then decreased with the
increased mass ratio of ZIF-CH. Among these sample, ZIF-CH(20%)/
CN exhibited the highest NO oxidation ratio of 52.5%. As shown in
Fig. 2(a), the photocatalyst activity of ZIF-67(20%)/CN and ZIF-CH
3.3. Morphology and surface compositions
The microstructure of ZIF-67, ZIF-CH, ZIF-CH(20%)/CN were
tested by SEM and TEM. As displayed in Fig. 3(a) and (d), ZIF-67 was
well-dispersed dodecahedron structure with a size of roughly
500 nm and a fairly smooth surface. After strong alkali oxidation
treatment, the original morphology of ZIF-67 was not changed
drastically except new nanorods microstructure emerged as shown
in Fig. 3(c). Specifically, the lattice spacing observed in Fig. 3(e) was
0.438 nm, corresponding to the (003) crystal plane of CoOOH [36]. It
could be illustrated that CoOOH and ZIF-67 were successfully com-
bined to form a ZIF-CH cocatalyst. From the observation of Fig. 3(c)
and (f), it was clearly that the ZIF-CH was successfully anchored on
CN, which may increase the active sites of ZIF-CH(20%)/CN. It is
proving that ZIF-CH/CN nanocomposites were prepared, which was
consistent with XRD results. In addition, the nitrogen adsorption
isotherms and BJH (Barrett-Joyner-Halenda) pore diameter dis-
tribution were measured to obtain the specific surface area (SSA) of
(
20%)/CN were investigated in order to reveal the effect of ZIF-67 co-
catalyst and ZIF-CH co-catalyst. Compared with ZIF-67(20%)/CN, the
performance of ZIF-CH(20%)/CN was increased by 12.6%. Re-
markably, no NO was tested during the NO photocatalytic oxidation
2
over ZIF-CH/CN in Fig. 2(b). Further cyclic illumination test was
performed on ZIF-CH(20%)/CN, as show in Fig. 2(c) and (d). After four
recycles, the NO oxidation ratio of ZIF-CH(20%)/CN could be kept at
2
the different sample (Fig. S4). The SSA of ZIF-CH(20%)/CN (241.9 m /
g) was 4.1 times than that of CN (58.4 m /g), which illustrated that
2
around 47% without any NO
2
generation, which prove the striking
the combination of ZIF-CH cocatalyst could obviously increase SSA
3

G. Du, Q. Zhang, W. Xiao et al.
Journal of Alloys and Compounds 882 (2021) 160318
Fig. 3. SEM and TEM images of (a), (d) ZIF-67 and (b), (e) ZIF-CH and (c), (f) ZIF-CH (20%)/CN.
and provide more active sites for the photocatalytic oxidation re-
action.
XPS spectra of ZIF-67, ZIF-CH and ZIF-CH(20%)/CN were mea-
sured to study the surface chemical compositions and chemical
status in Fig. 4. For ZIF-67, there were two peaks located at 781.4 eV
and 796.97 eV corresponding to Co 2p3/2 and Co2p1/2, as well as the
other two peaks at 802.45 and 786.6 eV were relevant to their sa-
tellite peaks [37], respectively. Two emergent peaks appeared at
Fig. 4. XPS of ZIF-67, ZIF-CH and ZIF-CH(20%)/CN: (a) Co 2p; (b) C 1s; (c) N 1s; (d) O 1s.
4

G. Du, Q. Zhang, W. Xiao et al.
Journal of Alloys and Compounds 882 (2021) 160318
7
[
79.9 eV and 795.1 eV, which was assign to Co3+ of CoOOH in ZIF-CH
30]. The disappearance of the peak at 795.1 eV in ZIF-CH(20%)/CN
3.4. Optical and electrochemical properties
might be due to the low anchoring amount of ZIF-CH. Moreover, the
relative content of Co was determined by XPS and the results were
shown in Table S1. The actual amount of Co in ZIF-CH sample is 6.7%
and the content in ZIF-CH(20%)/CN was 1.2%. For C 1s spectra, two
peaks at 285.8 eV and 284.8 eV were associated with C-N and C-C/
C＝C in ZIF-CH, respectively [33]. The positions of C 1s of ZIF-CH
were in line with that of ZIF-67, indicating that the CoOOH only grow
on the framework of ZIF-67 without changing its skeleton. And for
ZIF-CH(20%)/CN, the peak of 288.04 eV was attributed to carbon of
UV-Vis absorption spectrum was used to evaluate the light-ab-
sorption ability of the photocatalyst. The UV–vis absorption spectra
of CN, ZIF-67, ZIF-CH and ZIF-CH(20%)/CN were shown in Fig. 5(a). A
strong absorption peak at the 580 nm were observed in ZIF-CH and
ZIF-67, demonstrated that alkali oxidation treatment had no affect
3 4
on the light absorption properties of ZIF-67. Compared with g-C N ,
an weak absorption peak was also appeared at 580 nm in ZIF-CH
(20%)/CN, which enhanced its visible light response. The photo-
luminescence (PL) spectrum is an effective method to investigate the
2
sp hybridization (C-N˭C) in CN [16], while the other peaks did not
change, indicating that ZIF-CH/CN was successfully combined.
Moreover, for the XPS spectra of N 1s, the two peaks at 398.8 eV and
3 4
separation of the electron-hole pairs. The PL spectra of g-C N and
ZIF-67(20%)/CN ZIF-CH(20%)/CN were presented in Fig. 5(b). CN has
higher electron-hole recombination due to its higher π bond con-
jugated interface [21], but the fluorescence intensities of the com-
posites decreased obviously after doping ZIF-67. The ZIF-CH(20%)/CN
possessed the weakest fluorescence intensities, proved that the
CoOOH further increased interfacial charge transfer and inhibited
electron-hole recombination. Meanwhile, transient photocurrent
response (TPC) and electrochemical impedance spectra (EIS) also
were applied to explore photogenerated carrier transfers perfor-
mance. As shown in Fig. 5(c), it was apparent that CN has lowest
4
3 2
00.1 eV in ZIF-67 and ZIF-CH were attributed to N-C and H-N-C
peaks. For the N 1s of ZIF-CH(20%)/CN, the peak at 398.0 eV was
2
resulted in the sp hybrid nitrogen (C-N˭C) [25,38]. Compared with
3 2
N-C and H-N-C bond peak of ZIF-CH and ZIF-67, the corresponding
peak shifted to 399.4 eV and 400.4 eV respectively [39], which also
showed that there was a strong interaction between ZIF-CH and CN.
For O 1s spectra, the only one peak located at 531.3 eV could be
corresponding to Co-OH in ZIF-67 [40]. After the in-situ growing of
CoOOH, a new characteristic peak was observed at 529.3 eV, de-
monstrated the existence of Co-O in CoOOH and the successful
preparation of ZIF-CH [41].
2
photocurrent density (less than 0.03 uA/cm ), which was attributed
to the quick recombination of photo-generated electron-hole pairs.
But the photocurrent density significantly increased to 0.15 uA/cm2
Fig. 5. UV–vis absorption (a) PL of different samples (b), photocurrent transient responses (c) and (d) EIS spectra of the catalysts.
5

G. Du, Q. Zhang, W. Xiao et al.
Journal of Alloys and Compounds 882 (2021) 160318
2 3 4
Fig. 6. Adsorption energy of NO and NO over both CoOOH and g-C N .
after doping ZIF-67. ZIF-CH(20%)/CN shows the strongest photo-
current intensity (0.3 uA/cm ) that is twice as strong as ZIF-67(20%)/
illustrated that NO adsorbed on ZIF-CH(20%)/CN could combine to-
gether. According to previous research, the N is easily oxidized to
NO in the air. But no characteristic peak of NO was observed. Be-
sides, the adsorption peak of N was observed at 893 cm , which
demonstrated that the NO could rapidly transform into N
Meantime, the adsorption bands of NO
2
2 2
O
CN, indicating the recombination of carriers was lagged. The efficient
separation of electron-hole pairs was further confirmed with EIS
shown in Fig. 6(d). Compared with pristine CN, the arc radius of the
ZIF-67(20%)/CN, ZIF-CH(20%)/CN composites obviously were shor-
tened, and ZIF-CH(20%)/CN possessed the smallest arc radius in the
EIS plots both in light and dark conditions. The above result sug-
gested that a rapidly interfacial charge transfer occurred and an ef-
fectively separation of photogenerated charge carrier after the
comprise of ZIF-CH/CN, which was consistent with the above PL
results. Undoubtedly, ZIF-CH was an effective co-catalyst which
significantly inhibited the recombination of electrons and holes to
produce more photo-generated carriers.
2
2
−1
2 4
O
2
2
O
4
.
,
−
−1
3
appeared at 994 cm
which illustrate that a slowly NO removal process could be con-
ducted under dark condition [24]. After reaching the adsorption
equilibrium, visible light was exposed to measure the FTIR spectra of
ZIF-CH(20%)/CN photocatalysts for NO removal dynamically
−1
(Fig. 7b). Noticeably, the adsorption peak of NO (1089 cm ) declined
−
−1
and the adsorption peak of NO
creasing visible-light irradiation time [17]. Similarly, no adsorption
peak of NO was observed. These results indicate that ZIF-CH(20%)/
CN photocatalyst can effectively inhibit the production of NO
3
(994 cm ) increased with the in-
2
2
.
3.5. The process of photocatalytic NO oxidation
In addition, ESR of ZIF-67, ZIF-CH and ZIF-CH(20%)/CN were
measured to study the generation of hydroxyl radical (•OH) and
-
In the general NO oxidation process, NO was firstly adsorbed on
superoxide radical (•O
2
) in Fig. 7. The ESR of ZIF-67, ZIF-CH and ZIF-
the surface of catalysts, and then oxidized to NO
dized to nitrate by free radicals [25]. Therefore, adsorption of NO and
NO was the key step to ensure the final products were non-toxic
nitrate. Here, DFT calculation was employed to investigate the ad-
sorption of NO and NO over CoOOH and g-C . Based on the XRD
2
and finally oxi-
CH(20%)/CN and CN showed no obvious signal peaks for •OH and
-
•O
2
under dark condition (Fig. S6). As shown in Fig. 8(a) and (b), ZIF-
-
2
CH(20%)/CN has the strongest signal peak of •O
2
and •OH due to
more electron holes were generated and separated effectively, which
endowed ZIF-CH(20%)/CN strongest oxidation performance. Nota-
2
3 4
N
and TEM characterization, the (003) surface was mainly exposed
over CoOOH. Hence, the atomic structure of CoOOH (003) surface
was built as shown in Fig. S5. Calculation results showed that both
blely, the signal peak intensity of •OH in ESR over ZIF-CH(20%)/CN
-
was significantly higher than that of •O
to NO , thus reducing the production of NO
3 2
2
，and •OH could oxidize NO
[19]. The ESR results
might explain the •OH source of ZIF-CH and ZIF-67. As shown in
-
2 3 4
NO and NO were more easily adsorbed on CoOOH than g-C N ,
-
because of the more negative adsorption energy over CoOOH (−2.02
Fig. 7(a) and (b), a strong •O
2
and a weak •OH signal peaks were
and −3.18 eV) than that of g-C
Importantly, the adsorption of NO
hibit the generation of NO
is obviously lower than that of CoOOH, indicating that NO
to desorption from g-C surface and result in a toxic NO
production. Conversely, the high adsorption energy of NO on the
CoOOH surface was beneficial for NO further oxidation to nitrate.
3
N
4
(−1.46 and −1.64 eV) in Fig. 6.
was one of the key steps to in-
observed in ZIF-67 under light illumination. While for ZIF-CH, the
weaker •O signal and a stronger •OH signal peak were found, which
2
-
2
2
. The adsorption energy of NO
2
on g-C
is easier
by-
3
N
4
were attributed to the reaction of surface hydroxyl ions and photo-
generated holes to produce •OH. Moreover, active species capture
experiments indicated •OH is the mainly active specie (Fig. S7).
Based on the above calculation and analysis, a possible NO oxi-
dization process could be expressed in Eqs. (1)–(6). Under visible
2
3
N
4
2
2
2
To study the intermediates and products in the photocatalytic NO
process, in situ DRIFTS was carried out (Fig. 7). As shown in Fig. 7(a),
the absorb peak of NO (1089 cm−1) were obtained meaning physical
absorption rather than chemical absorption was realized [24]. The
light illumination, the electron-hole pairs were generated, and
-
oxygen was activated photo-electrons by to form •O
2
. Meanwhile，
•OH were generated by hydroxyl ions reacting with the photo-holes.
+ h+
adsorption bands at 1048 cm−1 was ascribed to
ZIF OH/CN + hv
e
(1)
2 2
N O , which
6

G. Du, Q. Zhang, W. Xiao et al.
Journal of Alloys and Compounds 882 (2021) 160318
Fig. 7. In situ IR spectra of NO adsorption (a) and visible light reaction processes (b) over ZIF-CH(20%)/CN photocatalysts.
Fig. 8. ESR profile for (a) •O -
and (b) •OH for ZIF-CH(20%)/CN,CN, ZIF-CH and ZIF-67.
2
O2 +e
•O2
•OH
(2)
(3)
(4)
(5)
(6)
the combination of ZIF-CH and CN. Meanwhile, the toxic by-product
NO was almost completely inhibited. This work could supply a
novel insight into the design of photocatalyst for air purification.
2
OH + h+
NO + 2H2O + 3h+
NO3 +4H+
Declaration of Competing Interest
NO + •O2
NO3
NO2 + •OH
HNO3
The authors declare that they have no known competing fi-
nancial interests or personal relationships that could have appeared
to influence the work reported in this paper.
4. Conclusion
In summary, a ZIF-67/CoOOH cocatalyst was successfully con-
structed by simply alkali oxidation treatment. Then ZIF-CH were
loaded on g-C surface by magnetic stirring. The successful com-
bination of ZIF-CH/CN not only enlarged its visible light response
range, but also boosted the separation of electron-hole pairs. Most
important of all, much more highly oxidizing·OH generated on ZIF-
CH/CN could react directly with NO to remove rapidly and steadily
NO and improve its deep oxidation capacity. As a result, the NO
Acknowledgement
3
N
4
We gratefully acknowledge financial support from the National
Natural Science Foundation of China (21403172, 51102245), the
Innovative Research Team of Sichuan Province (2016TD0011), the
International Collaboration Project of Chengdu city (2017-GH02-
00014-HZ), State Key Laboratory of Industrial Vent Gas Reuse Fund
Project (SKLIVGR-SWPU-2020-06) and College Students Innovation
Fund Project (X151518KCL09; X151518KCL27).
3 4
removal efficiency of g-C N was improved from 34.4% to 52.5% after
7

G. Du, Q. Zhang, W. Xiao et al.
Journal of Alloys and Compounds 882 (2021) 160318
Appendix A. Supporting information
visible light irradiation-induced NO removal, Appl. Catal. B: Environ. 234 (2018)
70–78.
[
[
21] X. Yang, X. Cao, B. Tang, B. Shan, M. Deng, Y. Liu, rGO/Fe-doped g-C N visible-
3 4
Supplementary data associated with this article can be found in
the online version at doi:10.1016/j.jallcom.2021.160318.
light driven photocatalyst with improved NO removal performance, J.
Photochem. Photobiol. A: Chem. 375 (2019) 40–47.
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reaction pathway of photocatalytic NO oxidation process to control the sec-
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