Encapsulation of Cobalt Oxide into Metal-Organic Frameworks for an Improved Photocatalytic CO2 Reduction

Article abstract of DOI:10.1002/cssc.202002656

The increased emission of CO₂ has negative impacts on the environment. Among the strategies, photocatalytic reduction is promising to convert the CO₂ into chemicals. In this report, CoO_x nanoparticles were loaded in the channels of MIL-101(Cr), a kind of metal-organic frameworks (MOF), to construct a novel CoO_x/MIL-101(Cr) system to facilitate CO₂ photoreduction. Under the optimal conditions, the CoO_x/MIL-101(Cr) showed a significantly enhanced performance for photocatalytic CO₂ reduction compared with bare CoO_x and MIL-101(Cr). Our findings provide a pathway for a rational design of efficient MOF systems for the photocatalytic reduction of CO₂.

Full text of DOI:10.1002/cssc.202002656

Chemistry–Sustainability–Energy–Materials
Accepted Article
Title: Encapsulation of cobalt oxide into metal-organic frameworks for
an improved photocatalytic CO2 reduction
Authors: Yiwen Ma, Juan Du, Yuanxing Fang, and Xinchen Wang
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To be cited as: ChemSusChem 10.1002/cssc.202002656
Link to VoR: https://doi.org/10.1002/cssc.202002656
0
1/2020

ChemSusChem
10.1002/cssc.202002656
FULL PAPER
Encapsulation of cobalt oxide into metal-organic frameworks for
an improved photocatalytic CO
2
reduction
[
a]
[a]
[a]
[a]
Yiwen Ma, Juan Du, Yuanxing Fang * and Xinchen Wang *
[
a]
Y. Ma, J. Du, Prof. Y. Fang, Prof. X. Wang
State Key Laboratory of Photocatalysis on Energy and Environment
College of Chemistry
Fuzhou University
Fuzhou 350116, P. R. China.
E-mail: yxfang@fzu.edu.cn; xcwang@fzu.edu.cn
Website: http://wanglab.fzu.edu.cn
Supporting information for this article is given via a link at the end of the document.
Abstract: The increased emission of CO
environment. Among the strategies, photocatalytic reduction is
promising to convert the CO into chemicals. In this report, CoO
nanoparticles are loaded in the channels of MIL-101(Cr), a kind of
metal organic frameworks (MOF), to construct a novel CoO /MIL-
photoreduction. Under the optimal
2
has negative impacts on
x
Cobalt oxide species (CoO ) is a typical one that has been widely
investigated for CO
2
to CO conversion, and its low cost and high
stability have generated broad scientific interesting.[
11]
It is
2
x
therefore promising to encapsule CoO
x
nanoparticles
x
homogeneously in the channels of the MOF as a co-catalytic
1
01(Cr) system to facilitate CO
2
system for surface CO
2
reduction reaction, and sufficiently
promote the efficiency of photocatalytic CO
2
conversion.[
12]
conditions, the CoO
x
/MIL-101(Cr) has showed
a
significantly
enhanced performance for photocatalytic CO
with bare CoO and MIL-101(Cr). Our findings provide a pathway for
a rational design of efficient MOF system for the photocatalytic
reduction of CO
2
reduction compared
In this study, MIL-101(Cr) is acquired with encapsulated CoO
nanoparticles as a co-catalytic system for photocatalytic CO
reduction. The chemical absorption of MIL-101(Cr) is investigated,
x
x
2
2
.
and strong adsorption property is presented to uptake CO
Simultaneously, homogeneously dispersed CoO
presented an improved performance for photocatalytic CO
2
.
x
nanoparticles
2
conversion. As a result, this system presented a significant
improvement for photocatalytic CO reduction with respect to the
pristine MIL-101(Cr) and CoO . Our findings provide pathway for
a rational design of an efficient MOF system for the photocatalytic
reduction of CO
Introduction
2
x
2
The increasing emission of the CO is considered as one of the
main factors for global warming, and resulting an irreversible
damage of the environment by climate change.[1] In the past years,
numbers of strategies have been developed to reduce the
2
.
atmospheric concentration of CO
physical absorbing CO for storage and chemical conversing CO
to other chemicals. Among them, photocatalytic conversion of
CO by primarily sunlight energy is an ideal approach to
immobilize and recycle CO
.[3] However, the efficiency is still far-
away to the state-of-the-art. This phenomenon can be attributed
to a few key factors in photocatalytic system, such as light
harvesting, charge separation and catalytic surface reaction.[4]
Among them, catalytic surface-reaction is particularly important
2
, and they mainly include
Results and Discussion
2
2
[2]
MIL-101(Cr) was synthesized according to the reported
2
x
procedures and the CoO nanoparticles were loaded by in-situ
2
double solvent method.[ By this approach, Co2+ ions were
incorporated into the pores of MIL-101(Cr) driving by capillary
force. Subsequently, the thermal oxidization was carried out to
13]
convert Co2+ ions into CoO
nanoparticle. The crystallinities of the
x
as-prepared samples were characterized by powder X-ray
diffraction (PXRD). As shown in Figure 1a, the diffraction patterns
of MIL-101(Cr) loading with 1.4 wt% and 2.6 wt% CoO
nearly the same as the pure MIL-101(Cr), indicating the crystal
structure of MIL-101(Cr) was retained after loading CoO . When
the concentration of the CoO becomes 3.9 wt%, the
crystallization of MIL-101 (Cr) reduces, which is reflected by the
reduced intensities of the PXRD peaks. This phenomenon can be
attributed to the formation of disordered crystal structure with
for CO
CO with the energy, ca. 750 kJ·mol , for the C=O cleavage.
While, most of the photocatalysts present poor capability of
physical adsorbing and chemical activate CO , therefore a
reduction reaction has
2
reduction reaction because of the chemical inertness of
−1
[5]
2
x
were
2
x
rational design of the co-catalyst for CO
received increasing attention.[6]
2
x
Metal-organic frameworks (MOFs) have emerged as a mediate
for the activate CO gas and selective photo-reduce CO gas due
to their large capacity and functional tenability of its surface. As
a noble example, MIL-101(Cr) can stably uptake CO gas by
dispersion-dominated interaction through delocalize π electrons
of the benzene ring for the quadrupole of CO
molecules.[8]
Meanwhile, Cr(III) functionalizes as Lewis acid sites that
2
2
[
7]
increasing the particle size of CoO
The diffraction peaks of MIL-101(Cr) cannot be clearly observed
when CoO is 4.9 wt%. However, we cannot detect any signal of
CoO from PXRD. Therefore, X-ray photoelectron spectroscopy
XPS) was further carried out to understand the chemical
composition of the samples (Figure S1-S3). The Co 2p spectrum
is shown in Figure 2a when 2.6 wt% of CoO is loaded into MIL-
01(Cr). The CoO can be deconvoluted into four peaks,
x
into MIL-101(Cr) channels.
2
x
2
x
(
3+
[9]
coordinate with CO
2
molecules via O=C=O•••Cr to active CO
2
.
In addition to the surface adsorption and CO
2
activation, reducing
x
the over-potential for the reduction reaction is the subsequent
step, and it relies on the activity of the co-catalytic species.[10]
1
x
1
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ChemSusChem
10.1002/cssc.202002656
FULL PAPER
corresponding to the mixed divalent and trivalent Co element,[14]
Table 1. The research of reaction conditions and reference experiments. [a]
indicating it is a mixture of CoO and Co
MIL-101(Cr).
3 4
O to encapsulate into the
Entry
1
CO [μmol]
28.7
13.6
0.7
H2 [μmol]
12.1
6.8
Sel. CO [%][b]
70.3
2[c]
3[d]
4[e]
5[f]
66.7
0.2
77.8
n.d.
0.2
-
-
-
n.d.
21.5
n.d.
6[g]
n.d.
[
a] Reaction conditions: [Ru(bpy)
solvent (5 mL, acetonitrile:H O=3:2), triethanolamine (TEOA, 1 mL), CO
atm), λ>420 nm. [b] Sel. CO=mol CO/mol (H +CO). [c] Using CoO /MIL-101
instead of CoO . [d] Using CoO /MIL-101 instead of MIL-101. [e] Without using
3
]Cl
2
·H
2
O (10 μmol), CoO
x
/MIL-101 (1 mg),
2
2
(1
2
x
x
x
any co-catalyst. [f] Using Ar instead of CO
condition.
2
. [g] Experiment conducted in dark
We also conducted a series of control experiment as shown in
Table 1. Compared with the optimal performance (entry 1, Table
Figure 1. (a) PXRD patterns of MIL-101(Cr), CoO
different loading concentration of the CoO . (b) The diameter distribution of the
CoO in MIL-101(Cr). (c) TEM and (d) HRTEM of 2.6 wt% CoO /MIL-101(Cr).
e, f) EDX elemental mapping results of Co.
x x
and CoO /MIL-101 with
x
1), it was not able to detect any CO, but trace amount of H
detected in the absence of 2.6 wt% CoO /MIL-101(Cr) (entry 4,
Table 1). When 2.6 wt% CoO /MIL-101(Cr) was used, the CO
evolution rate decreased to 13.6 μmol·h-1 (entry 2, Table 1). In
addition, negligible CO and H were found when MIL-101(Cr) was
used (entry 3, Table 1). The results indicate that MIL-101 acts as
a mediate to chemisorb CO gas for activation, but charge must
be injected in CoO as active sites for the reduction reaction. We
have also conducted the experiment by replacing CO gas with Ar
gas (entry 5, Table 1), but no CO was detected and only H gas,
confirming the produced CO come from induced CO gas, but not
by decomposing acetonitrile or other substrates. Without light
illumination, neither H nor CO are detected (entry 6, Table 1),
2
was
x
x
x
(
x
2
The samples were studied by transmission electron microscopy
TEM) and scanning electron microscopy (SEM) (Figure S4a and
(
2
S4b). As shown in Figure 1c, the structure of MIL-101(Cr) can be
clearly observed. Figure 1d presents the high-resolution TEM
x
2
(
HR-TEM) image of the compound, in which, CoO
were found to be homogeneously loaded into the channel of MIL-
01(Cr). The particle size of CoO obtained from statistical
x
nanoparticles
2
2
1
x
analysis ranges from 1.7 to 3.1 nm (Figure 1b), and a dominant
peak locates at 2.3 nm, which is smaller than the pore size of MIL-
2
and the result also confirm that the reduction reaction is driven by
light by photoredox catalysis.
1
1
01(Cr) (2.9 to 3.4 nm).[15] In addition, the pore windows of MIL-
01(Cr) is around 1.2 to 1.6 nm in diameters, and this size is
2+
appropriate to diffuse Co
into the channel. The energy-
dispersive X-ray spectroscopy (EDX) elemental mapping was
further conducted to identify the chemical composition (Figure 1e
and 1f), and C, O and Cr can be clearly observed from the EDX
(
Figure S5). Meanwhile, we can confirm that the CoO
x
nanoparticles are homogeneous distributing along the MOF
channels.
Photocatalytic CO
study the performance of CoO
by using [Ru(bpy) ]Cl as photosensitizer. The quantity of the
CoO
2
reduction is conducted subsequently to
x
/MIL-101(Cr) co-catalytic system
3
2
x
that loading in MIL-101(Cr) was studied as shown in Figure
3
CoO
1
. The rate of CO evolution was 21.5 μmol·h-1 by loading 1.4 wt%
Figure 2. (a) XPS Co 2p spectrum of 2.6 wt% CoO
x
/MIL-101(Cr). (b) CO
2
-
x
in MIL-101(Cr). The performance improves to 28.7 μmol·h
adsorption isotherms (273 K) of MIL-101(Cr) and 2.6 wt% CoO
x
/MIL-101(Cr).
x
for CO production when the loading amount of CoO is increased
to 2.6 wt%. However, further increasing the loading amount of
CoO decreases CO evolution rate. It is most likely that high
loading content of CoO would lead to the aggregation of
nanoparticles, which decreased the porosity of support and
number of active sites, and thus reduced the CO evolution rate.
The observation reveals that it is important to homogeneous
x
The co-catalytic system was studied by the N
2
and CO
2
x
adsorption-desorption isotherms. The BET surface area of MIL-
3
-1
1
01(Cr) was 3255 cm ·g (Figure S6 in the Supporting
Information), which provides sufficient space to load CoO
nanoparticles. After loading CoO the BET surface area
decreased to 2408 cm ·g . In addition, CO absorption was also
measured as it is one of the key steps to determine the activity in
CO photoreduction reaction. As shown in Figure 2b, the
maximum CO
x
x
,
distribute CoO
exposed active site for CO
x
with trace amount of the quantity to ensure the
photoreduction.
3
-1
2
2
2
3
-1
2
uptake of MIL-101(Cr) is 105 cm ·g and 2.6 wt%
2
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3
-1
CoO
x
/MIL-101(Cr) is 78 cm ·g at 1 atm and 273 K. This
shown in Figure S8. When aprotic hybrid-atom-containing used,
performance is better than many reported materials,[16] and
confirmed the intrinsic properties of MIL-101(Cr) for chemisorb
an improved performance for photoreduction of CO
received. On the contrary, when the reactions were performed in
TCM as solvent, only trace amount of CO and H can be observed
since this solvent possesses weak chemical affinity toward uptake
CO The studies demonstrated that selecting appropriate
reaction medium is important for photocatalytic CO conversion.
The effects of H O additive on the catalytic performance were also
studied. As listed in Table S1, there is only small amount of CO
and H can be received when using pure MeCN as solvent (entry
, Table S1). Adding H O into the reaction solution can
2
can be
and activate CO
2
gas.[17]
2
2
.
2
2
2
1
2
significantly improve the photocatalytic activity and the selectivity
of CO (entries 2, Table S1). Further increasing volume ratio of
MeCN/H
were obtained, while the CO selectivity was 54.1%. However,
excess H O led to decreased evolution rates of CO and H
entries 3, 4 and 5, Table S1). There was no CO but only a small
2 2 2
O to 4/1, the maximum evolution rate of the CO and H
2
2
(
2
amount H produced when using pure water as solvent (entry 6,
Table S1). Therefore, appropriate water is indispensable to
promote the performance in this photoreduction system. Water
may induce a small amount of dissociation for Ru(bpy) 2+
and
3
produced the ligand bpy, which is also benefit for CO
reduction.[
2
20]
Figure 3. Productions of CO and H
2 x
by different amounts of CoO loaded into
MIL-101(Cr) under visible light illumination.
To understand the working mechanism of this photocatalytic
CO reduction, cyclic voltammetry (CV) was also acquired. The
CV curves of CoO , MIL-101(Cr) and 2.6 wt% CoO /MIL-101(Cr)
in CO -saturated MeCN electrolyte were shown in Figure 4a. The
CV of CoO showed two irreversible reduction peaks at -0.9 V and
1.3 V, which can be assigned to the reduction of Co ions from
2
x
x
2
x
-
III
II
II
0
II
Co to Co and Co to Co . The third reduction peak after the Co
to Co was the reduction of CO
Figure 4. (a) Cyclic voltammograms of CoO
101(Cr) in CO atmosphere. (b) Cyclic voltammograms of CoO
CO and Ar atmosphere.
x
, MIL-101(Cr) and CoO
x
/MIL-
0
2
x
/MIL-101(Cr) in
2
and located at around -2.0 V. As
2
for the support MIL-101(Cr), its CV curve showed a weak CO
reduction peak located at the more negative -2.3 V, indicating its
weak CO activation and reduction ability. Furthermore, the co-
catalyst system 2.6 wt% CoO /MIL-101(Cr) possessed a CO
reduction peak at -2.1 V and the catalytic current was much higher
than that of the other two samples, which illustrated that over-
2
2
The temperature is also important to affect the reaction rate.
Figure S9 presents the evolution rate of CO and H at different
reaction temperatures. The evolution rate of CO and H increases
by increasing the reaction temperature from 10ºC to 30ºC. The
increased temperature is favorable to active CO and therefore an
improved performance. Further increasing the temperature to
x
2
2
2
potential of 2.6 wt% CoO
The absence of the first two reduction peaks was due to the very
low loading of CoO . The 2.6 wt% CoO /MIL-101(Cr) was also
x x
/MIL-101(Cr) system is lower than CoO .
2
x
x
5
0ºC led to the reduction of CO yield, which is probability due to
performed under Ar conditions as shown in Figure 4b. This
observation reveals that the reduction peak is owing to the
the decreased solubility of CO
2
in the solvent.[ Therefore, the
21]
reaction temperature at 30ºC is the optimal temperature to
balance between CO adsorption and thermodynamic equilibrium.
The thermal stability of the 2.6 wt% CoO /MIL-101(Cr) was also
interaction between CO
impedance spectrscopy (EIS) was also acquired for MIL-101(Cr)
and 2.6 wt% CoO /MIL-101(Cr), as shown in Figure S7. The
electron transmission capacity is enhanced after MIL-101 is
loaded with CoO , indicating the improved intraction between 2.6
wt% CoO /MIL-101(Cr) and CO and thus increased
performance for photocatalytic CO reduction.
2 x
and CoO /MIL-101(Cr). Electrochemical
2
x
x
studied by thermal gravimetric analysis (TGA, Figure 5a). A rapid
weight-loss up to 100ºC is owing to the leaving of the guest
molecules. A weak structure response was observed from 100ºC
to 270ºC, and seriously structural damage was presented up to
x
x
2
,
2
3
50ºC. Therefore, the CoO
photoreduction reaction. The weight-dependent performance of
CoO /MIL-101(Cr) was also carried out for the photocatalytic
reaction, and the result is shown in Figure S10. The relatively high
evolution rate of CO and H were achieved with 0.5 mg of 2.6 wt%
CoO /MIL-101(Cr). When the amount of 2.6 wt% CoO /MIL-
01(Cr) increases to 1.0 mg, the maximum production rate of CO
x
/MIL-101(Cr) is stable for the
The influences of the reaction conditions on the photocatalytic
performance was further studied.[18] Generally, solvent plays an
important role in the reaction, especially when it contains nitrogen
x
or oxygen, it would be favor to interact with CO
2
by Lewis acid-
base interactions.[19] In this study, solvents including MeCN, DMF
2
x
x
and THF, were investigated for CO photoreduction reaction as
2
1
3
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ChemSusChem
10.1002/cssc.202002656
FULL PAPER
was obtained with the selective ca. 70.3%. Further increasing the
forming Ru(bpy) 3
CoO
electron acceptor and meanwhile as active sites to convert the
3
+. Electrons migrate from the excited dye to
/MIL-101(Cr), and CoO in CoO /MIL-101(Cr) acts as
x x x
amount to 3.0 mg resulted the increased H
decreased the yield of CO reduction and selectively conversion
of CO production. Generally, the high amount of the 2.6 wt%
CoO /MIL-101(Cr) would reduce illumination on the catalyst, and
2
evolution rate but
2
activated CO
2
. Finally, TEOA acts as the sacrificial electron donor
3 3
to quench Ru(bpy) ] .
3+, and re-produce [Ru(bpy)
2+
x
thus a non-liner relationship between the activity and the weight
Conclusion
In summary, photocatalytic CO evolution by reduction of CO
achieved using CoO /MIL-101(Cr) system with a photosensitizer.
The MIL-101(Cr) presented superior capability for CO
chemisorbing. In addition, we had encapsuled CoO
nanoparticles in the channels of MIL-101(Cr) to reduce the over-
potentials of the photoreduction of CO . As a result, the CoO /MIL-
01(Cr) system had presented an improved performance for
photocatalytic CO reduction compared with bare CoO and MIL-
2
was
x
2
x
2
x
1
of the co-catalyst system.
2
x
1
01(Cr). This work demonstrates the MIL-101(Cr), a chemically
Figure 5. (a) Thermal gravimetrical analysis of the as-prepared 2.6 wt%
stable MOFs, is a promising support to uptake CO , and with
encapsulation of a proper photocatalyst, the system offers
opportunities for CO
2
photocatalytic reduction.[
2
x
CoO /MIL-101(Cr). (b) Four-hour measurement of 2.6 wt% CoO
x
/MIL-101(Cr)
for photocatalytic CO
2
reduction.
23]
The prolong-time experiment was carried out to verify the
stability of 2.6 wt% CoO /MIL-101(Cr), and the CO and H
evolutions of every hour is shown in Figure S11. In the first half
hour, the produced CO and H were sharply increased. After that,
x
2
Experimental Section
2
Materials. Cr(NO
3
)
3
·9H
2
O, HF, NH
4 3 2 2
F, Co(NO ) ·6H O were purchased
the evolution rates were gradually decreased. This observation is
.[22]
from Alfa Aesar. [Ru(bpy)
3
]Cl ·6H O was purchased from Tokyo Chemical
2
2
3 2
likely due to the photo-blenching dye of [Ru(bpy) ]Cl
Industry (TCI, Shanghai). The organic solvents, including acetonitrile, N,N-
dimethylformamide, triethanolamine, trichloromethane (TCM) and
tetrahydrofuran (THF) were purchased from China Sinopharm Chemical
reagent Co., Ltd. All the chemicals were used without further purification.
Therefore, the dye was added every hour for this four-hour
experiment. Results are shown in Figure 5b, and in this test, the
co-catalyst system is preserved. The production rate of CO was
nearly the same as the first hour, suggesting the deactivation was
resulted from the photobleaching of [Ru(bpy)
3 2
]Cl rather than the
Synthesis of MIL-101(Cr). Typically, terephthalic acid (1.6 g, 10.0 mmol),
degeneration of CoO /MIL-101(Cr). Furthermore, the used
x
3 3 2
Cr(NO ) ·9H O (4.0 g, 10.0 mmol), HF solution (0.5 mL, 46 wt%) and
sample was characterized by PXRD (Figure S12), Fourier
transformed infrared (FTIR, Figure S13), UV-Vis diffuse
reflectance spectra (UV-Vis DRS, Figure S14), and XPS Co 2p
deionized water (70 mL) were mixed in a 100 mL Teflon-liner autoclave
and heated at 220ºC for 8 h. After cooling down to the room temperature,
the as-prepared solid was separated by filtration and washed with DMF
(
150 ml) to remove the unreacted chemicals. The powder was further
purified by using ethanol at 100ºC for 24 h. Ion exchange was carried out
by dispersing the solid in 37.5 mmol of NH F aqueous solution with stirring
(
(
Figure S15), C 1s (Figure S16), O 1s (Figure S17) and Cr 2p
Figure S18). Negligible differences are observed from those
4
experiments and confirmed the stability of the 2.6 wt% CoO
x
/MIL-
at 70ºC for 10 h. The green powder was collected by filtration and then
washed with hot water (80ºC) for several times. The products were dried
1
01(Cr) for this CO photoreduction reaction.
2
in vacuum oven. As
a result, 1.6 g terephthalic acid and 4.0 g
Cr(NO ·9H O yields 0.8 g MIL-101(Cr).
3
)
3
2
x
Load CoO in MIL-101(Cr). A double solvent approach was used to load
Co2+ cations into the channel of MIL-101(Cr). Normally, the as-prepared
MIL-101(Cr) (150 mg, 0.13 mmol) was dispersed in 30 mL anhydrous
hexane. The solution was sonicated and stirred vigorously for 30 min. 200
μL Co(NO
3 2
) aqueous solutions with different concentration (0.2 M, 0.4 M,
0.6 M and 0.8 M) was slowly added to the prepared solution by syringe
pump, and the mixture was stirred for 2 h. The production was collected
by filtration and then washed with hexane for several times. The obtained
product was dried in vacuum. The sample was calcined at 250ºC for 3 h in
argon atmosphere followed by heating at 150ºC for 2 h. The as-obtained
2
Figure 6. A plausible mechanism of the CO photoreduction by this system.
sample was referred to as y wt% CoO
percentage of CoO , depending on the concentration of the cobalt nitrate
aqueous solution). If there were no special instructions, the loading amount
of CoO on MIL-101(Cr) was 2.6 wt%.
x
/MIL-101(Cr) (y is the weight
x
The plausible mechanism of the CO
photocatalytic system was illustrated in Figure 6. With pumping
CO gas into the system, it could be chemisorbed by the MOF part
in CoO
photosensitizer, Ru(bpy)
2
photoreduction by this
x
2
Characterization. Powder X-ray diffraction measurements were
performed on Bruker D8 Advance diffractometer with Cu Kα radiation. X-
ray photoelectron spectroscopy were acquired by a Thermo ESCALAB250
x
/MIL-101(Cr). Meanwhile, under light illumination, the
2+, is excited to generate electrons by
3
instrument with
a monochromatized Al Kα line source (200 W).
4
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Transmission electron microscopy and energy-dispersive X-ray
spectroscopy were performed on the FEI Tencai 20 microscope. HITACHI
SU8010 field emission scanning electron microscopy was used to
investigate the morphologies of the samples. The UV-vis diffuse
reflectance spectra of samples were conducted on a Varian Cary 500 Scan
UV-vis spectrophotometer with barium sulfate as the reference. A Netzsch
Thermoanalyzer STA 449 F3 was used for thermal gravimetrical analysis
with a heating rate of 10ºC·min-1 from 20ºC to 1000ºC under a He
Matsunaga, T. Nakashima, K. Wada, A. Yamakata, O. Ishitani,
K. Maeda, J. Am. Chem. Soc. 2016, 138, 5159-5170.
[
[
6] a) J. Qin, L. Lin, X. Wang, Chem. Commun. 2018, 54, 2272-2275;
b) J. Wu, H. Lu, X. Zhang, F. Raziq, Y. Qu, L. Jing, Chem.
Commun. 2016, 52, 5027-5029; c) Y. Hou, B. L. Abrams, P. C.
K. Vesborg, M. E. Björketun, K. Herbst, L. Bech, A. M. Setti, C.
D. Damsgaard, T. Pedersen, O. Hansen, J. Rossmeisl, S. Dahl,
J. K. Nørskov, I. Chorkendorff, Nat. Mater. 2011, 10, 434-438.
7] a) D. Wang, R. Huang, W. Liu, D. Sun, Z. Li, ACS Catal. 2014, 4,
atmosphere. N
Micromeritics ASAP3020 equipment and CO
2
adsorption-desorption isothermal were measured on
adsorption isotherms were
4254-4260; b) X. Li, S. Liu, K. Fan, Z. Liu, B. Song, J. Yu, Adv.
2
Energy Mater., 2018, 8, 1800101; (c) Z. Jiang, X. Xu, Y. Ma, H.
S. Cho, D. Ding, C. Wang, J. Wu, P. Oleynikov, M. Jia, J. Cheng,
Y. Zhou, O. Terasaki, T. Peng, L. Zan, H. Deng, Nature, 2020,
586, 549-554.
measured on Micromeritics ASAP2020 equipment. Fourier transformed
infrared (FTIR) spectra were obtained by a Nicolet Magna 670 Fourier
transform infrared spectrometer.
[
[
8] Q.-L. Zhu, J. Li, Q. Xu, J. Am. Chem. Soc. 2013, 135, 10210-
10213.
Photocatalytic reaction. The photocatalytic reaction was carried out in a
9] P. L. Llewellyn, S. Bourrelly, C. Serre, A. Vimont, M. Daturi, L.
Hamon, G. De Weireld, J.-S. Chang, D.-Y. Hong, Y. Kyu Hwang,
S. Hwa Jhung, G. Férey, Langmuir 2008, 24, 7245-7250.
10] H. Liu, X. Meng, T. D. Dao, H. Zhang, P. Li, K. Chang, T. Wang,
M. Li, T. Nagao, J. Ye, Angew. Chem., Int. Ed. 2015, 54, 11545-
11549.
5
0 mL reactor at 1 atm CO
was maintained at 30ºC by water cooling system. The CoO
1mg) and [Ru(bpy) ]Cl ·6H O (10 μmol) were added to the reaction
solution (5 mL, acetonitrile: H O = 3: 2), and 1 ml TEOA was added in the
2
partial pressure and the reaction temperature
x
/MIL-101(Cr)
(
3
2
2
[
2
reaction as hole scavenger. The solution was stirred vigorously and
irradiated under a 300 W Xe lamp with a 420 nm cut-off filter. An Agilent
[11] a) A. Iwase, S. Yoshino, T. Takayama, Y. H. Ng, R. Amal, A.
Kudo, J. Am. Chem. Soc. 2016, 138, 10260-10264; b) S. Wang,
Y. Hou, X. Wang, ACS Appl. Mater. Interfaces 2015, 7, 4327-
7820A gas chromatography, equipping with a thermal conductivity
detector (TCD) and a TDX-01 packed column, was employed to analyse
the products using Argon as the carried gas.
4335.
[
[
12] G. Sahara, H. Kumagai, K. Maeda, N. Kaeffer, V. Artero, M.
Higashi, R. Abe, O. Ishitani, J. Am. Chem. Soc. 2016, 138,
Electrochemical measurements. The working electrode was prepared
14152-14158.
x
by 1 mg CoO /MIL-101(Cr) ink, the powder of which was dispersed in 1 ml
13] Y.-Z. Chen, Y.-X. Zhou, H. Wang, J. Lu, T. Uchida, Q. Xu, S.-H.
ethanol followed by bath-sonication. 0.1 mL CoO
x
/MIL-101(Cr) ink was
Yu, H.-L. Jiang, ACS Catal. 2015, 5, 2062-2069.
dropped on FTO glass and dried at ambient condition. Pt plate was used
[14] a) G. Zhang, Z.-A. Lan, L. Lin, S. Lin, X. Wang, Chem. Sci. 2016,
7, 3062-3066; b) M. Higashi, K. Domen, R. Abe, J. Am. Chem.
Soc. 2012, 134, 6968-6971.
+
as counter electrode. For cyclic voltammograms test, Ag/Ag was used as
reference electrode, and the electrolyte is 0.1 M TBAPF
test, Ag/AgCl was used as reference electrode and the electrolyte is 0.5 M
KHCO solution.
6
in MeCN. For EIS
[
15] G. Férey, C. Mellot-Draznieks, C. Serre, F. Millange, J. Dutour,
S. Surblé, I. Margiolaki, Science 2005, 309, 2040-2042.
16] a) Y. Liao, Z. Hu, Q. Gu, C. Xue, Molecules 2015, 20, 18847; b)
R. Li, J. Hu, M. Deng, H. Wang, X. Wang, Y. Hu, H.-L. Jiang, J.
Jiang, Q. Zhang, Y. Xie, Y. Xiong, Adv. Mater. 2014, 26, 4783-
3
[
4
788.
Acknowledgements
[
17] H. Zhang, J. Wei, J. Dong, G. Liu, L. Shi, P. An, G. Zhao, J. Kong,
X. Wang, X. Meng, J. Zhang, J. Ye, Angew. Chem., Int. Ed. 2016,
This work was financially supported by the National Key
Technologies R & D Program of China (2018YFA0209301), the
National Natural Science Foundation of China (21425309,
55, 14310-14314.
[18] S. G. Kazarian, M. F. Vincent, F. V. Bright, C. L. Liotta, C. A.
Eckert, J. Am. Chem. Soc. 1996, 118, 1729-1736.
[
[
19] K. H. Kim, Y. Kim, J. Phys. Chem. A 2008, 112, 1596-1603.
20] a) S. Wang, X. Wang, Appl. Catal., B 2015, 162, 494-500; b) Y.
Ma, Y. Fang, X. Fu, X. Wang, Sustain. Energ. Fuels, 2020, 4,
5812-5817.
2
1961142019, 22075047, 21761132002 and 21861130353), the
Natural Science Foundation of Fujian Province of China
2020J01446), the 111 Project, and Chang Jiang Scholars
Program of China ( T2016147).
(
[
[
[
21] J. Schneider, H. Jia, J. T. Muckerman, E. Fujita, Chem. Soc. Rev.
2012, 41, 2036-2051.
22] S. Wang, W. Yao, J. Lin, Z. Ding, X. Wang, Angew. Chem., Int.
Ed. 2014, 53, 1034-1038.
23] a) Y. Lei, H. Jia, W. H, Y. Zhang, H. Hou, G. Zhu, Z. Zheng, J.
Am. Chem. Soc. 2012, 134, 17392-17395; b) Y. Fang, X. Li, X.
Wang, ChemSusChem 2019, 12, 2605-2608.
2
Keywords: Photocatalytic CO reduction • Co-catalyst system •
MIL-101(Cr) • Cobalt oxide • Metal-organic frameworks
[
[
1] a) W. Tu, Y. Zhou, Z. Zou, Adv. Mater. 2014, 26, 4607-4626; b)
K. Maeda, Adv. Mater., 2019, 31, 1808205; c) H. Lee,
ChemSusChem 2020, 13, 2799-2081.
2] a) J. Lin, Z. Pan, X. Wang, ACS Sustain. Chem. Eng. 2014, 2,
353-358; b) L. Yu, G. Li, X. Zhang, X. Ba, G. Shi, Y. Li, P. K.
Wong, J. C. Yu, Y. Yu, ACS Catal. 2016, 6, 6444-6454; c) L. Zou,
R. Sa, H. Lv, H. Zhong, R. Wang, ChemSusChem 2020,
DOI:10.1002/cssc.202001796.
[
[
3] a) J. Qin, S. Wang, H. Ren, Y. Hou, X. Wang, Appl. Catal., B
2015, 179, 1-8; b) F. Raziq, Y. Qu, M. Humayun, A. Zada, H. Yu,
L. Jing, Appl. Catal., B 2017, 201, 486-494; c) Y. Fang, X. Fu, X.
Wang, ACS Mater. Lett., 2020, 2, 975-980.
4] a) J. Lin, Z. Ding, Y. Hou, X. Wang, Sci. Rep. 2013, 3, 1056; b)
Y. Fang, X. Wang, Chem. Commun. 2018, 54, 5674-5687; c) Y.
J. Jang, J.-W. Jang, J. Lee, J. H. Kim, H. Kumagai, J. Lee, T.
Minegishi, J. Kubota, K. Domen, J. S. Lee, Energy Environ. Sci.
2015, 8, 3597-3604.
[
5] a) Y. Fang, Y. Ma, M. Zheng, P. Yang, A. M. Asiri, X. Wang,
Coord. Chem. Rev. 2018, 373, 83-115; b) R. Kuriki, H.
5
This article is protected by copyright. All rights reserved.

ChemSusChem
10.1002/cssc.202002656
FULL PAPER
Table of Contents
In this paper, MIL-101(Cr) is acquired with encapsulated CoO
x
nanoparticles as a co-catalytic system for photocatalytic CO
2
reduction.
The strong chemical absorption of MIL-101(Cr) is presented to uptake CO
for photocatalytic CO conversion.
2
, and an improved performance is presented by this system
2
6
This article is protected by copyright. All rights reserved.