Transition-Metal-Modified Vanadoborate Clusters as Stable and Efficient Photocatalysts for CO2Reduction

Article abstract of DOI:10.1021/acs.inorgchem.1c00499

Photocatalytic carbon dioxide reduction (CO2RR) is considered to be a promising sustainable and clean approach to solve environmental issues. Polyoxometalates (POMs), with advantages in fast, reversible, and stepwise multiple-electron transfer without changing their structures, have been promising catalysts in various redox reactions. However, their performance is often restricted by poor thermal or chemical stability. In this work, two transition-metal-modified vanadoborate clusters, [Co(en)2]6[V12B18O54(OH)6]·17H2O (V12B18-Co) and [Ni(en)2]6[V12B18O54(OH)6]·17H2O (V12B18-Ni), are reported for photocatalytic CO2 reduction. V12B18-Co and V12B18-Ni can preserve their structures to 200 and 250 °C, respectively, and remain stable in polar organic solvents and a wide range of pH solutions. Under visible-light irradiation, CO2 can be converted into syngas and HCOO- with V12B18-Co or V12B18-Ni as catalysts. The total amount of gaseous products and liquid products for V12B18-Co is up to 9.5 and 0.168 mmol g-1 h-1. Comparing with V12B18-Co, the yield of CO for V12B18-Ni declines by 1.8-fold, while that of HCOO- increases by 35%. The AQY of V12B18-Co and V12B18-Ni is 1.1% and 0.93%, respectively. These values are higher than most of the reported POM materials under similar conditions. The density functional theory (DFT) calculations illuminate the active site of CO2RR and the reduction mechanism. This work provides new insights into the design of stable, high-performance, and low-cost photocatalysts for CO2 reduction.

Full text of DOI:10.1021/acs.inorgchem.1c00499

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Article
Transition-Metal-Modified Vanadoborate Clusters as Stable and
Efficient Photocatalysts for CO Reduction
2
§
§
Xiang Yu, Cong-Cong Zhao, Jian-Xia Gu, Chun-Yi Sun,* Hai-Yan Zheng, Li-Kai Yan,* Min Sun,
Xin-Long Wang,* and Zhong-Min Su
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sı Supporting Information
ABSTRACT: Photocatalytic carbon dioxide reduction (CO RR) is considered to be a
2
promising sustainable and clean approach to solve environmental issues. Poly-
oxometalates (POMs), with advantages in fast, reversible, and stepwise multiple-
electron transfer without changing their structures, have been promising catalysts in
various redox reactions. However, their performance is often restricted by poor thermal
or chemical stability. In this work, two transition-metal-modified vanadoborate clusters,
[
(
Co(en) ] [V B O (OH) ]·17H O (V B −Co) and [Ni(en) ] [V B O -
2
6
12 18 54
6
2
12 18
2
6
12 18 54
OH) ]·17H O (V B −Ni), are reported for photocatalytic CO reduction.
6
2
12 18
2
V B −Co and V B −Ni can preserve their structures to 200 and 250 °C,
respectively, and remain stable in polar organic solvents and a wide range of pH
1
2
18
12 18
solutions. Under visible-light irradiation, CO can be converted into syngas and
2
−
HCOO with V B −Co or V B −Ni as catalysts. The total amount of gaseous
1
2
18
12 18
−
1
−1
products and liquid products for V B −Co is up to 9.5 and 0.168 mmol g h .
12 18
−
Comparing with V B −Co, the yield of CO for V B −Ni declines by 1.8-fold, while that of HCOO increases by 35%. The AQY
1
2
18
12 18
of V B −Co and V B −Ni is 1.1% and 0.93%, respectively. These values are higher than most of the reported POM materials
1
2
18
12 18
under similar conditions. The density functional theory (DFT) calculations illuminate the active site of CO RR and the reduction
2
mechanism. This work provides new insights into the design of stable, high-performance, and low-cost photocatalysts for CO2
reduction.
1
. INTRODUCTION
tion, while the tunability in structures facilitates the
modification of active centers and incorporation of functional
Excessive carbon dioxide emissions from fossil fuel combustion
have led to numerous energy and environmental problems.
Converting CO into valuable fuels and chemical feedstocks
through photochemical reduction is an appealing approach to
promote CO fixation and sustainable energy generation.
Two-electron-reduction products of syngas (made of CO and
H ) and formic acid are important raw materials and liquid fuel
in the organic industry or oxidized fuel cells. Transition
metals with multiple redox couples, which can facilitate
electron transfer and overcome the unfavorable high-energy
intermediates, have presented a unique activity toward CO
photoreduction.
1
−3
groups to adjust the yield and selectivity in CO
2
2
,24−27
reduction.
However, most POM clusters have a major
2
limitation in catalytic durability, as structural stability is the
prerequisite for POM chemistry to be applied in any
4
,5
2
28−30
catalysis.
Herein, two transition-metal-modified wheel-type vanado-
2
6
,7
borate clusters, [Co(en)
2
]
6
[V12
B
18
O
54(OH)
6
]·17H
2
O
(
(
V B −Co) and [Ni(en) ] [V B O (OH) ]·17H O
1
2
18
2
6
12 18 54
6
2
V B −Ni), were reported. Both of them have excellent
1
2 18
thermal and chemical stabilities. Under visible-light irradiation,
CO
and V B −Ni. The syngas product (CO and H ) of V B −
Co reaches up to 9.5 mmol g
2
−
8
−10
2
can be converted into syngas and HCOO by V12B18−Co
It is worth noting that not only the
1
2
18
2
12 18
reduction reactivity but also the kinds of products are strongly
affected by the transition-metal sites.
−
1
−1
11,12
h
that is 2.7-fold higher than
that of V B −Ni, while for the liquid products, the yield of
1
2 18
Currently, transition-metal- (such as Co, Ni, Zn, and Fe)
−
1
−1
V B −Co is 0.168 mmol g
h
that is lower than that of
1
2 18
modified polyoxometalates (POMs) have received increasing
attention for their potential applications in various fields.
Especially, the properties of fast, reversible, and stepwise
multiple-electron transfer without changing of structures
endow POMs’ highly catalytic activity for numerous redox
13,14
Received: February 23, 2021
15−23
reactions, such as CO2 photoreduction.
Besides, the
specific crystalline structures will help to understand the
structure−activity relationship in photochemical CO reduc-
2
©
XXXX American Chemical Society
https://doi.org/10.1021/acs.inorgchem.1c00499
A
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
Figure 1. (a) Schematic representation of the V O cluster. (b) Ball-and-stick representation of the V O cluster connected by the B18 ring. (c)
6
18
12 60
Ball-and-stick view of the whole structural arrangement of V B −Co showing the situation of six Co(en) connected to the V B cluster. (d)
1
2
18
2
12 18
Polyhedral representation of V B −Co.
1
2 18
V B −Ni, by 35%. The AQY of them is 1.1% and 0.93%,
high-purity CO gas was bubbled into the mixed solution for 15 min
2
1
2 18
until the CO concentration became saturated and the oxygen in the
respectively. The value of V B −Co climbs to the top of the
2
12 18
device was removed. After reacting under visible light (420 ≤ λ ≤ 780
reported POM materials that are under similar conditions. The
calculations of charge decomposition analysis (CDA) and
density functional theory (DFT) illuminate the active site of
nm) for 3 h, the generated gases (CO and H ) were collected by a 1
2
mL syringe, then immediately tested, and analyzed by a GC-9800. To
ensure data reliability, all photocatalytic experiments were repeated
three times.
CO RR and the reduction mechanism.
2
2
.3.2. Isotopic Experiment. Isotope experiments were performed in
2
. EXPERIMENTAL SECTION
a 50 mL quartz tube with a catalyst (1 mg), acetonitrile (4 mL),
triethanolamine (1 mL), and water (1 mL) as the solvent. Before the
2
.1. Syntheses of V B −Co. NaVO (0.085 g, 0.7 mmol),
13
12
18
3
photocatalytic reaction, CO (98%) gas was bubbled into the mixed
2
H BO (1.625 g, 0.026 mol), and Co(CH COO) ·4H O (0.623 g, 2.5
mmol) were added to water (10 mL), the formed solution was stirred
uniformly at room temperature for 2 h, and then ethylenediamine
3
3
3
2
2
solution for 20 min until the dissolved oxygen was removed
completely. A 300W xenon lamp with a CUT420 filter (420 ≤ λ ≤
13
7
80 nm) was used as the light source. After 3 h, the C-labeled
(
en) (1 mL) was slowly added to the stirring solution. After that, the
product was tested by a gas chromatography−mass spectrometer
solution was transferred into a Teflon-lined reactor, heated at 160 °C
for 60 h, and gradually cooled to room temperature. Red cubic
(
(
GC-MS) and a 500 MHz nuclear magnetic resonance spectrometer
NMR), respectively.
crystals were obtained by ultrasound, washed with H O, and dried in
2
air (yield: ∼40%, based on NaVO ). Elemental analysis (%) for
3
[
Co(en) ] [V B O (OH) ]·17H O: Calcd: C, 9.14; H, 4.31; N,
3. RESULTS AND DISCUSSION
.1. Structural Description and Characterization.
Single-crystal X-ray diffraction reveals that V B −Co and
2
6
12 18 54
6
2
−
1
1
2
7
0.65. Found: C, 9.13; H, 4.30; N, 10.64. IR (KBr, cm ): 3259(w),
890(w), 1642(m), 1396(s), 1353(s), 1066(s), 950(s), 792(m),
27(m), 663(m).
3
1
2 18
V B −Ni are isostructural and crystallize in trigonal form
2
.2. Syntheses of V B −Ni. The preparation of V B −Ni was
12 18
12
18
12 18
with the R3
̅
space group (Table S4). Therefore, only the
similar to the method of V B −Co except for replacing cobalt
1
2 18
acetate with nickel acetate (yield: ∼40%, based on NaVO ).
crystal structure of V12B18−Co is described here. The
asymmetric unit of V B −Co includes 1 Co ion, 2 V ions,
3
Elemental analysis (%) for [Ni(en) ] [V B O (OH) ]·17H O:
2
6
12 18 54
6
2
12 18
Calcd: C, 9.14; H, 4.32; N, 10.66. Found: C, 9.13; H, 4.31; N,
3 B ions, and 2 ethylenediamine ligands. Two V atoms have
the same coordination environment bridged by five O atoms
with square-pyramidal geometries, and six VO5 square
pyramids are edge-sharing to form a V O cluster (Figure
−
1
1
1
0.65. IR (KBr, cm ): 3268(w), 2960(w), 1627(m), 1389(s),
350(s), 1064(s), 962(s), 789(m), 724(m), 669(m).
2
.3. Photocatalytic Reaction. 2.3.1. Photocatalytic Test. The
6
18
experiment of photocatalytic reduction of carbon dioxide was
performed in a 50 mL quartz tube with a cover and irradiated
under a 300W xenon lamp with a 420 nm cut off filter. The reaction
was kept at about 20 °C by condensate circulating water. Specifically,
the as-prepared catalyst (1 mg) was dispersed in deionized water (1
mL), acetonitrile (4 mL), and triethanolamine (1 mL). After that, a
1
a). Two V clusters are bridged by boron and oxygen atoms
6
to form a V O cluster, as shown in Figure 1b. The bond
12 60
lengths of V−O are in the range of 1.607−2.007 Å. Outside the
V
12 cluster, there is a B18 ring. B atoms in the B18 ring show
two different coordination geometries. One type of B atom
B
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Figure 2. PXRD patterns of V B −Co in different manual variable temperatures (a), solvents (b), and aqueous solutions of pH = 5−13 (c) and
1
2 18
PXRD patterns of V B −Ni in different manual variable temperatures (d), solvents (e), and aqueous solutions of pH = 5−13 (f).
12 18
3
7
coordinates with four oxygen atoms and forms tetrahedral
coordination with the B−O distance of 1.425−1.527 Å.
Another type of B atom is connected to three oxygen atoms
showing a triangular geometry with B−O lengths from 1.352 to
transition. In addition, V B −Co and V B −Ni clusters
1
2
18
12 18
both absorb ultraviolet and visible light.
Bond valence sum (BVS) calculations (Tables S1 and S2)
show the oxidation states of Co atoms of V B −Co and Ni
1
2 18
1
.368 Å. As shown in Figures 1c and 1d, six cobalt atoms are
atoms of V B −Ni are +2. The V and B atoms in V B −Co
1
2
18
12 18
modified on the outer ring of the V B cluster. Each Co
and V B −Ni are +4 and +3, respectively, which is consistent
12
18
12 18
cation is coordinated with four N atoms from two independent
ethylenediamine ligands (Co−N 2.036−2.141 Å) and two O
atoms with a distorted octahedral geometry (Co−O 2.08−2.26
Å).
with X-ray photoelectron spectroscopy (XPS) (Figures S4 and
S5). The peaks at 191.9 eV can be assigned to the B 1s signals
of V B −Co and V B −Ni, indicating that the B species in
1
2
18
12 18
3+
38
the V B −Co and V B −Ni exist as B . The V 2p peak
1
2
18
12 18
The phase purities of V B −Co and V B −Ni are
can be fitted with two peaks at 515.9 and 523.5 eV, which are
12
18
12 18
4+
39
characterized by powder X-ray diffraction (PXRD) patterns
assignable to V . The peaks located at 856.0 and 873.5 eV
2
+
2+
40
(
Figure S1). The PXRD patterns of the synthesized V B −
correspond to Ni (2p3/2) and Ni (2p1/2) of V B −Ni.
The weak peaks are located at approximately 802.5 and 789.7
eV, and the strong peaks located at 796.2 and 781.3 eV
originate from the Co (2p1/2), Co (2p3/2), Co (2p1/2),
and Co (2p3/2) spectra, respectively.
1
2
18
12 18
Co and V B −Ni match well with the simulated patterns,
1
2 18
indicating the synthetic samples are pretty pure. The catalysts
are characterized by Fourier transform infrared (FTIR)
spectroscopy using KBr pellets in the range of 4000−400
2
+
2+
2+
2
+
41,42
−
1
cm on a Nicolet 6700 FT/IR spectrophotometer (Figure
Excellent thermal and chemical stabilities are crucial to
S2). In the IR spectrum of V B −Co and V B −Ni, there
catalytic applications. The superior thermal stability of V B −
1
2
18
12 18
12 18
−
1
are strong bands at 950 and 962 cm attributable to the V−O
Co and V B −Ni is evidenced by the high decomposition
1
2 18
3
1
bond stretching. Due to the symmetrical and asymmetrical
temperature and variable-temperature powder X-ray diffraction
(VT-PXRD). Thermogravimetric analyses (TGA) of V B −
stretching of V−O−V, there are strong peaks at 792, 727, and
1
2 18
−
1
−1
6
63 cm for V B −Co and 789, 724, and 669 cm for
Co and V B −Ni were carried out, and the curve is shown in
1
2
18
12 18
3
2
V B −Ni. In addition, two peaks located at 1350−1400
Figure S6. The TGA curve shows that the mass loss of V B −
1
2
18
12 18
−
1
−1
cm and 1060−1070 cm are ascribed to the B−O stretching
Co is 10.9% at 25−110 °C, which is mainly due to the free
water molecules in the crystal, and the subsequent multistep
weight loss at 218−420 °C is attributed to the decomposition
of the ligand. Thus, we believe that V B −Co can be
33,34
of [BO ] and [BO ] units, respectively.
The peaks at
3
4
−1
35
1
650−1600 and 3250−3300 cm are the stretch of N−H.
The symmetric and asymmetric stretching of the C−H bond
1
2 18
−
1
results in peaks at 2960 for V B −Ni and 2890 cm for
stabilized up to 200 °C. For V B −Ni, the TGA curve shows
12
18
12 18
3
6
V B −Co. The light absorption capabilities of V B −Co
that the mass loss is divided into two steps, from 25 to 200 °C
and 300 to 450 °C, and the corresponding mass loss is 9.8 and
8.4%, respectively. The former is due to the loss of physical
water in the crystal, and the latter is attributed to the
decomposition of the ligand. The thermal stability of V B −
1
2
18
12 18
and V B −Ni are studied by UV−vis absorption spectrosco-
1
2 18
py, which displays three intense absorption bands (Figure S3).
The higher energy bands are at 233 (5.32 eV) and 312 nm
(
3.97 eV), which can be assigned to O → V and O → B charge
1
2 18
transfers of the [V B ] cluster. The weak absorption at 555
Co and V B −Ni has been further explored by the manual
1
2
18
12 18
43
nm (2.23 eV) presumably arises from the d−d electronic
VT-PXRD measurements. The manual VT-PXRD patterns
C
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Article
−
Figure 3. (a) Amount of CO, H , and HCOO produced from V B −Co, V B −Ni, and V B under visible-light irradiation. Recyclability of
2
12 18
12 18
12 18
1
3
V B −Co (b) and V B −Ni (c) photocatalysts in five consecutive runs. (d) GC-mass result of the isotopic experiment under a CO2
12
18
12 18
1
3
12
atmosphere of V B −Co and (e) the C NMR and C NMR spectra of the liquid phase after catalysis. PXRD patterns of V B −Co (f) and
12
18
12 18
V B −Ni (g) for simulated and after catalysis, presenting its stable structure during the reaction.
12 18
−
1
−1
−1
from 100 to 200 °C and 100 to 250 °C at intervals of 50 °C of
crystals V B −Co and V B −Ni, respectively, are matched
amount of CO (0.47 mmol g h ) and H (0.24 mmol g
2
−
1
−
h ) was detected, and almost no HCOO was detected
1
2
18
12 18
well with the simulated one, which suggests that these two
catalysts retain excellent thermal stability (Figures 2a and 2d).
For chemical stability, we studied the stability of the sample in
different organic solvents and aqueous solutions with different
pH values. The PXRD patterns of V B −Co and V B −Ni
(Figure 3a). In contrast to V B −Co and V B −Ni, the gas
1
2
18
12 18
products of V B decrease by an order of magnitude. These
1
2 18
results suggest the transition-metal sites might be active centers
for CO photoreduction.
2
The apparent quantum yield (AQY) of products could
evaluate the utilization efficiency of one material to the
irradiated light in photocatalysis. The AQY of V B −Co and
12
18
12 18
after 12 h of immersion in different solutions remained similar
to the simulated PXRD patterns, which illustrate these two
crystal samples possess good chemical stability in polar
solvents, acids, and bases (Figures 2b,e and 2c,f).
1
2 18
V B −Ni is estimated at the wavelength at 420 nm (blue
1
2 18
LED) and calculated based on eq S1 presented in the
Supporting Information, and the value is up to 1.1% for
V B −Co and 0.93% for V B −Ni. The value of V B −Co
3
.2. Photochemical CO Reduction Properties. Photo-
2
catalytic carbon dioxide reduction activity of V B −Co and
12
18
12 18
12 18
12 18
V B −Ni was studied under a pure CO atmosphere in the
climbs to the top of the reported POMs that are under similar
conditions (Table S3). These results reflect the outstanding
light utilization ability of transition-metal-modified vanadobo-
1
2
18
2
solution containing triethanolamine (TEOA) as a sacrificial
reagent and [Ru(bpy) ]Cl ·6H O as a photosensitizer. Under
3
2
2
visible-light irradiation, gaseous products of syngas (CO and
rate clusters in photocatalytic CO reduction.
2
H ) and liquid products of formic acid were detected for both
To detect the origin of the products, isotope tracer analysis
2
1
3
V B −Co and V B −Ni. After 3 h of illumination, the
is performed with CO . The gas products produced by the
1
2
18
12 18
2
amount of CO and H for V B −Co reaches 5.7 and 3.8
chemistry are analyzed by gas chromatography−mass spec-
2
12 18
−
1
−1
13
mmol g h , respectively (Figure 3a). The selectivity of CO
over H is 60%. For the liquid HCOO , the yield is 0.168
(Figure 3a). With V B −Ni as a catalyst, the
generation amount of CO, H , and HCOO is 3.2, 0.3, and
trometry (GC-MS), and the liquid product is analyzed by
C
−
NMR spectroscopy. The signal at m/z = 29 is observed rather
2
−
1
−1
mmol g
h
than at m/z = 28 for both V B −Co and V B −Ni as
1
2
18
12 18
12 18
−
catalysts (Figure 3d and Figure S7), verifying that the CO
2
2
−
1
−1
0
.260 mmol g h , respectively, under the same reaction time
molecules in these systems are the sources of generated CO.
1
3
(
Figure 3a). Comparing with that of V B −Co, the total gas
The results of C NMR spectroscopy present the peaks at
164.8, 160.6, and 159.3 ppm (Figure 3e and Figure S8), which
12 18
reduction product declines by 2.7-fold, but the selectivity of
CO rises to 91.43%. The high selectivity of CO in Ni-based
materials over Co-based ones has been reported, which may be
ascribed to stronger CO adsorption energy and weaker H O
1
3
−
13
2−
13
−
are determined to be H COO , CO3 , and H CO ,
3
−
45
respectively, revealing HCOO also originating from CO .
2
As photocatalytic stability is crucial for practical application,
meanwhile, the stability of POM under photocatalytic CO2
reduction has been a huge challenge. It is particularly
2
2
1
1
adsorption energy. These results unambiguously illuminate
the transition metals in these two POMs play an important role
in CO photoreduction. To further demonstrate this issue, the
important to investigate the stability of V B −Co and
12 18
2
12 18
kernel of V B −Co without transition metals, that is V B
V B −Ni in photocatalysis. Recycling experiments were
1
2
18
12 18
44
(
[V B O (OH) ]), was synthesized. When it was used as a
performed under the supply of fresh [Ru(bpy) ]Cl in each
1
2
18 54
6
3
2
catalyst under the same CO reduction conditions, only a small
round. Five cyclic experiments (Figure 3b,c) show no
2
D
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Article
a
Table 1. Various Experimental Conditions of V B −Co and V B −Ni
12
18
12 18
b
CO
c
d
b
CO
c
d
entry V B −Co TON
V B −Co TON(H )
V B −Co TON
12 18
V B −Ni TON
12 18
V B −Ni TON(H )
V B −Ni TON
12 18 HCOOH
12
18
12 18
2
HCOOH
12 18
2
1
2
3
4
5
6
7
47.77
3.72
n.d.
n.d.
n.d.
32.39
1.48
n.d.
n.d.
n.d.
n.d.
6.4
1.4
26.68
3.72
n.d.
n.d.
n.d.
n.d.
6.2
2.51
1.48
n.d.
n.d.
n.d.
n.d.
1.82
2.22
trace
trace
trace
trace
trace
0.49
e
f
trace
trace
trace
trace
trace
8.28
g
h
i
n.d.
6.68
j
a
Conditions: [Ru(bpy) ]Cl ·6H O (7 mg), V B −Co or V B −Ni (1 mg), acetonitrile (4 mL), TEOA (1 mL), H O (1 mL), CO (1 atm), λ ≥
3
2
2
12 18
12 18
2
2
b
c
d
4
20 nm, 20 °C, 3 h. Turnover number (molCO/mol
). Turnover number (mol /mol
). Turnover number (molHCOOH)/(molcatalyst).
catalyst
g
H
i
2
catalyst
e
f
h
j
Without the catalyst. Without [Ru(bpy) ]Cl ·6H O. Without TEOA. In the dark. Without CH CN. Without H O.
3
2
2
3
2
Figure 4. (a) Charge decomposition analysis (CDA) of V B −Co. The value represents the charge transfer amount. (b) Calculated free energy
12 18
diagram by the CO conversion to V B −Co.
2
12 18
−
noticeable decline of CO, H , and HCOO in each cycle. In
3.3. Mechanism for Photocatalytic CO Reduction. To
2
2
addition, PXRD patterns and IR spectra for V B −Co and
reveal the rooted reasons behind the high activity of CO
photoreduction, the energy level of the catalysts is analyzed.
2
1
2 18
V B −Ni after the photocatalysis experiment are nearly the
1
2 18
The optical band gap (E ) was measured by UV−vis diffuse
same as that of the fresh sample (Figure 3f,g and Figures S9
g
1
/2
reflectance spectroscopy and Tauc plots of (αhν) versus
and S10). These results confirm that V B −Co and V B −
12
18
12 18
photon energy (hν). The values of E evaluated from the
Ni are pretty stable during the photocatalytic reduction of
carbon dioxide.
g
absorption edge are 1.42 eV for V B −Co and 1.26 eV for
1
2 18
V B −Ni (Figure S11). Meanwhile, their valence band (VB)
1
2 18
In order to illuminate the importance of each component in
energy (E ) was examined through the VB XPS spectrum
VB
the CO photocatalytic reduction system, a series of reference
2
(
0
Figure S12), and the EVB of V B −Co and V B −Ni is
12
18
12 18
experiments are conducted and summarized in the results in
Table 1. According to Table 1, we found that in the absence of
V B −Co or V B −Ni (entry 2, Table 1), the turnover
.28 and 0.62 eV, respectively. The conduction band (CB)
energy (E ) of V B −Co and V B −Ni was calculated by
CB
12 18
12 18
46
1
2
18
12 18
the formula (E = E −E ), and the values of E for
CB
VB
g
CB
number decreases strongly to 5.2, suggesting the POM might
be a cocatalyst in the photocatalysis. Without [Ru(bpy) ]Cl
V B −Co and V B −Ni are determined to be −1.14 eV and
1
2
18
12 18
3
2
−0.64 eV (Table S5). The E of V B −Co and V B −Ni is
CB
12 18
12 18
(
entry 3, Table 1) or TEOA (entry 4, Table 1) in the
higher than the redox potential of E
= −0.52 V vs
(
CO
2
/CO)
48
photocatalytic system, no CO or H2 was detectable,
47
−
NHE and E(CO2
= −0.61 V vs NHE, revealing that
/HCOO )
demonstrating [Ru(bpy) ]Cl2 as a photosensitizer and
3
the transition-metal-modified VB-based POMs are a suitable
TEOA as a sacrificial reagent are vital to the reduction.
When the system is in the dark, no gas or liquid product was
−
catalyst for converting CO into high-yield CO and HCOO .
2
The exhibited high ECB value for V B −Co rather than
12
18
detected (entry 5, Table 1), indicating that the CO reduction
2
V B −Ni may contribute to the superior catalytic activity of
1
2 18
reaction is powered by light. It is found that the reaction
medium has a sharp influence on the catalytic effect. When
only water is used as the reaction medium, the reaction does
V B −Co. According to the previous literature, the highest
1
2 18
occupied molecular orbital (HOMO) and the lowest
unoccupied molecular orbital (LUMO) energy levels of the
[Ru(bpy) ]Cl photosensitizer were 1.24 and −1.25 eV (vs
not occur (entry 6, Table 1). When only CH CN is employed
3
3
2
49
as the reaction medium, reduction products are detected, but
NHE). Since the E value of −1.14 eV (vs NHE) for
CB
the amount is quite lower than that with CH CN and water as
3
V B −Co and −0.64 eV (vs NHE) for V B −Ni is lower
1
2
18
12 18
the medium. These results illuminate CH CN as the favorable
than that of the LUMO energy levels of [Ru(bpy) ]Cl , the
3
3 2
medium for reducing carbon dioxide (entry 7, Table 1), and
electrons on [Ru(bpy) ]Cl preferentially transfer to the
3 2
H O might be the crucial donor of H protons.
2
transition-metal-modified VB-based POMs.
E
https://doi.org/10.1021/acs.inorgchem.1c00499
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
Next, in order to determine the active site of CO RR, the
2
charge decomposition analysis (CDA) was carried out using
5
0
Multiwfn3.8, taking V B −Co as the example, and the
12 18
results are summarized in Figure 4a. CDA calculation results
show that the transfer of the injected electrons from the
2+
C H (NH ) (en) ligand to Co is favorable, with a number
2
4
2 2
of 2.6853. Meanwhile, almost no electrons flow back to the en
ligand. These processes are consistent with the classical
chemical theory that the electrons of the coordination bond
51
are contributed to by a pure ligand. It is unfavorable for the
electrons to continue to transfer from the Co2 to the inner
V B core. It is worth noting that the amount of charge
+
1
2 18
transferred from en to the inner V B core is very small and
12 18
can be ignored. Therefore, it suggests that the photoinduced
electrons from the photosensitizer move to the en ligand and
then transfer to Co2 instead of the V B core, so Co
+
2+
12 18
probably serves as the active site of CO RR.
2
Further, the deep understanding of the catalytic mechanism
of CO RR is revealed by density functional theory (DFT)
2
calculations, and V B −Co is also taken as an example.
12 18
Under light excitation, the adsorption energy of CO on
2
Figure 5. Possible mechanism of the photocatalytic reduction of CO2.
V B −Co is −0.43 eV. The first step of hydrogenation of
1
2 18
CO can produce both HCOO* and COOH* intermediates as
2
+
−
calculations illustrate CO is the favorable product contrasted
with HCOOH. This work provides novel insights into the
design of stable, high-performance, and low-cost photocatalysts
that convert carbon dioxide to high-value chemical feedstock.
the proton−electron pairs (H + e ) can attack both C and O
atoms of CO . As can be seen from Figure 4b, the ΔG (−1.06
2
eV) for the generation of HCOO* from attacking the C atom
is significantly lower than that for COOH* (0.61 eV).
However, the ΔG for the hydrogenation process of HCOO*
−
+
ASSOCIATED CONTENT
sı Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.1c00499.
+
e + H → HCOOH* is very large (1.38 eV), which is
■
*
thermodynamically unfavorable (the gray lines in Figure 4b).
For COOH* hydrogenation, the second proton−electron pair
can either attack the O atom to obtain CO* + H O (l) or
2
attack the C atom to produce the HCOOH* intermediate,
Materials and methods, computational details, synthesis,
characterization, and crystallographic data (PDF)
with a ΔG of −1.10 eV and −0.29 eV, respectively. It indicates
that the CO RR in the present work is more inclined to
2
Accession Codes
produce CO*, which is consistent with the experimental result
that the CO yield is much higher than that of HCOOH. The
subsequent desorption processes of CO* and HCOOH*
obtain CO and HCOOH products, respectively, leaving the
catalyst for the next catalytic cycle. In conclusion, the possible
CCDC 2060466−2060467 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
mechanism of VB-based POMs in the CO photocatalytic
2
system is illustrated in Figure 5. Under visible light, the ground
state photosensitizer [Ru(bpy) ]Cl2 stimulates the excited
3
AUTHOR INFORMATION
Corresponding Authors
■
state, and then the electron sacrificial agent TEOA combines
with it to form a reduced photosensitizer. Next, the electrons
from the reduced photosensitizer transfer to the transition
Chun-Yi Sun − Key Laboratory of Polyoxometalate Science of
Ministry of Education, School of Chemistry, Northeast
Normal University, Changchun, Jilin 130024, P. R. China;
Email: suncy009@nenu.edu.cn
metals of VB-based POMs and trigger the CO reduction
2
reaction.
Xin-Long Wang − Key Laboratory of Polyoxometalate Science
of Ministry of Education, School of Chemistry, Northeast
Normal University, Changchun, Jilin 130024, P. R. China;
orcid.org/0000-0002-5758-6351; Email: wangxl824@
nenu.edu.cn
Li-Kai Yan − Key Laboratory of Polyoxometalate Science of
Ministry of Education, School of Chemistry, Northeast
Normal University, Changchun, Jilin 130024, P. R. China;
orcid.org/0000-0002-1352-4095; Email: yanlk924@
nenu.edu.cn
4
. CONCLUSION
In summary, two isolated transition-metal-modified vanado-
borate clusters, V B −Co and V B −Ni, have been
1
2
18
12 18
synthesized under hydrothermal conditions. Both of them
possess good thermal and chemical stabilities in a wide range
of pH’s of the aqueous solutions and polar solvents. As
catalysts for CO RR, V B −Co shows high efficiency under
2
12 18
−
visible light with the yield of syngas and HCOO up to 9.5 and
−
1
−1
0
.168 mmol g h , while the corresponding yield for V B −
−1 −1
1
2 18
Ni is 3.5 and 0.260 mmol g h . The AQY of V B −Co and
1
2 18
V B −Ni is estimated to be 1.1% and 0.93%. The value of
Authors
1
2 18
V B −Co climbs to the top of the reported POM materials
Xiang Yu − Key Laboratory of Polyoxometalate Science of
Ministry of Education, School of Chemistry, Northeast
Normal University, Changchun, Jilin 130024, P. R. China
1
2 18
which are under similar conditions. The CDA suggests that
transition metal is used as the active site of CO RR, and DFT
2
F
https://doi.org/10.1021/acs.inorgchem.1c00499
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
Cong-Cong Zhao − Key Laboratory of Polyoxometalate
Science of Ministry of Education, School of Chemistry,
Northeast Normal University, Changchun, Jilin 130024, P.
R. China
Boosted by a Partially Delocalized Charge in Molybdenum Sulfide
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Jian-Xia Gu − Key Laboratory of Polyoxometalate Science of
Ministry of Education, School of Chemistry, Northeast
Normal University, Changchun, Jilin 130024, P. R. China
Hai-Yan Zheng − Key Laboratory of Polyoxometalate Science
of Ministry of Education, School of Chemistry, Northeast
Normal University, Changchun, Jilin 130024, P. R. China
Min Sun − Key Laboratory of Polyoxometalate Science of
Ministry of Education, School of Chemistry, Northeast
Normal University, Changchun, Jilin 130024, P. R. China
Zhong-Min Su − Key Laboratory of Polyoxometalate Science
of Ministry of Education, School of Chemistry, Northeast
Normal University, Changchun, Jilin 130024, P. R. China;
Jilin Provincial Science and Technology Innovation Center of
Optical Materials and Chemistry, School of Chemistry and
Environmental Engineering, Changchun University of Science
and Technology, Changchun, Jilin 130022, P. R. China;
orcid.org/0000-0002-3342-1966
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Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.1c00499
(
Wang, X. P.; Tung, C. H.; Sun, D. pH-Controlled assembly of two
novel Dawson-sandwiched clusters involving the in situ reorganization
Author Contributions
12‑
of trivacant alpha-[P W O ]( ) into divacant alpha-[P W O ]-
2
15 56
2
16 57
§
8‑
X.Y. and C.-C.Z. contributed equally.
(
). Dalton Trans. 2016, 45, 8404−8411.
14) Wang, M.-Y.; Ma, R.; He, L.-N. Polyoxometalate-based ionic
(
Notes
liquids-promoted CO conversion. Sci. China: Chem. 2016, 59, 507−
2
The authors declare no competing financial interest.
516.
(
15) Du, J.; Lang, Z.-L.; Ma, Y.-Y.; Tan, H.-Q.; Liu, B.-L.; Wang, Y.-
ACKNOWLEDGMENTS
■
H.; Kang, Z.-H.; Li, Y.-G. Polyoxometalate-based electron transfer
modulation for efficient electrocatalytic carbon dioxide reduction.
Chem. Sci. 2020, 11, 3007−3015.
This work was financially supported by the NSFC of China
Nos. 21971032, 21801038, 21771035), the Fundamental
Research Funds for the Central Universities (Nos.
412016KJ041, 2412019QD009), and the Jilin Provincial
Department of Science and Technology (No.
(
(
16) Lei, J.; Yang, J.-J.; Liu, T.; Yuan, R.-M.; Deng, D.-R.; Zheng, M.-
S.; Chen, J.-J.; Cronin, L.; Dong, Q.-F. Tuning Redox Active
Polyoxometalates for Efficient Electron-Coupled Proton-Buffer-
Mediated Water Splitting. Chem. - Eur. J. 2019, 25, 11432−11436.
2
2
0190103018JH).
(
17) MacDonald, L.; McGlynn, J. C.; Irvine, N.; Alshibane, I.; Bloor,
L. G.; Rausch, B.; Hargreaves, J. S. J.; Cronin, L. Using earth abundant
materials for the catalytic evolution of hydrogen from electron-
coupled proton buffers. Sustain Energ Fuels. 2017, 1, 1782−1787.
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