A hexanuclear cobalt metal-organic framework for efficient CO2 reduction under visible light

Article abstract of DOI:10.1039/c7ta02611k

Increasing global challenges including climate warming and energy shortage have stimulated worldwide explorations for efficient materials for applications in the capture of CO₂ and its conversion to chemicals. In this study, a novel pillared-layer porous metal-organic framework (Co₆-MOF) with high nuclearity Co^II clusters has been synthesized. This material exhibited a CO₂ adsorption capacity of up to 55.24 cm³ g^-1 and 38.17 cm³ g^-1 at 273 K and 298 K, respectively. In a heterogeneous photocatalytic system of CO₂ reduction, this material, co-operated with a ruthenium-based photosensitizer, can efficiently realize CO₂ to CO conversion. Under visible-light irradiation for 3 hours, 39.36 μmol CO and 28.13 μmol H₂ were obtained. This result is higher than those of most of the reported MOF materials under similar conditions and to the best of our knowledge, this is the first example of a high nuclear MOF used in CO₂ reduction. The rooted reasons behind the high reactivity were studied through theoretical calculation studies. The results showed that electrons on reduced [Ru(bpy)₃]Cl₂·6H₂O (bpy = 4,4′-bipyridine) could transfer to the Co₆-MOF and the adsorbed CO₂ molecule on the charged Co₆-MOF could be activated more facilely. This work not only clarifies the reasons for high efficiency of the CO₂ photoreduction system but also points out to us the direction for designing more effective MOF materials as photocatalysts for artificial CO₂ photoreduction.

Full text of DOI:10.1039/c7ta02611k

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
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DOI: 10.1039/C7TA02611K
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ARTICLE
Hexanuclear Cobalt Metal–Organic Framework for Efficient CO2
Reduction under Visible Light
Received 00th January 20xx,
Accepted 00th January 20xx
Jiao Zhao†, Qi Wang†, Chunyi Sun*, Tiantian Zheng, Likai Yan*, Mengting Li, Kuizhan Shao, Xinlong
Wang* and Zhongmin Su
DOI: 10.1039/x0xx00000x
Increasing global challenges including climate warming and energy shortage have stimulated worldwide explorations of
efficient materials for applications in capture of CO2 and conversion it to chemicals. In this study, a novel pillared-layer
porous metal–organic framework (Co6-MOF) with high nuclearity CoII clusters has been synthesized. This material
exhibited CO2 adsorption capacity up to 55.24 cm3g-1 and 38.17 cm3g-1 at 273 K and 298 K, respectively. In a heterogeneous
photocatalytic system of CO2 reduction, this material, cooperated with a ruthenium-based photosensitizer, can efficiently
realize CO2 to CO conversion. Under visible-light irradiation for 3 hours, 39.36 μmol CO and 28.13 μmol H2 was obtained.
This result is higher than most of reported MOF materials under similar condition and to the best of known, this is the first
example of high nuclear MOF used in CO2 reduction. Rooted reasons behind the high reactivity were studied through
www.rsc.org/
.
theoretic calculation studies. Results showed electrons on reduced [Ru(bpy)3]Cl2 6H2O (bpy = 4,4'-bipyridine) could transfer
to Co6-MOF and adsorbed CO2 molecule on charged Co6-MOF could be activated more facilely. This work not only clarifies
the reasons for high efficiency in CO2 photoreduction system but also points out the direction for us in designing more
effective MOF materials as photocatalysts for artificial CO2 photoreduction.
into value-added chemicals could be more suitable alternative and
sustainable methods as CO2 represents abundant and inexpensive
carbon source.11-13 Owing to chemically inert of CO2 molecule,
Introduction
In recent years, climate warming has become
a global
reducing it into chemical feedstocks needs high energy and
appropriate catalysts.14-17 Utilizing clean and renewable solar
energy as the power resource to accomplish this conversion could
be the most economical and environmentally friendly choice.18
Therefore, the exploration of high-performance photocatalytic
systems for CO2 reduction has been fundamental goal of experts. In
photocatalytic systems, there are three essential steps to achieve
the transition from solar energy to chemical energy,19-22 including
light harvest, generation of electron–hole pairs, transferring redox
equivalents to reactive centers for redox reactions. Noble metal
complex, such as Ru and Re complexes are widely utilized as light-
harvester in various photocatalytic systems, including water
splitting,23-25 and CO2 reduction.26-29 Transition metal with multiple
redox states and organic ligands are essential to forming electron-
transport chains together with light harvesters accelerating CO2
reduction. For instance, cobalt complexes and oxides have been
reported serve as cocatalysts enhancing CO2 photoconversion
through boosting charge-separation and surface reaction.30-31 In
addition to the three aspects above, for the gaseous reactions,
adsorption and activation of CO2 molecules is especially crucial in
practical applications since effective charge transfer between
catalytic centers and CO2 molecules lies on their intimate and stable
binding.31 Therefore, in the process of CO2 reduction, it is important
to combine the adsorption and activation of CO2 with transition-
metal catalysis. Transition-metal MOFs with excellent capability for
CO2 adsorption might be incomparable advantage materials in CO2
environmental problem which primarily results from excessive
emissions of CO2 from fossil fuel combustion.1-3 To solve this issue,
tremendous efforts have been devoted to reducing CO2
concentration in air. CO2 capture and sequestration (CCS)
technologies using porous materials as sorbent materials are an
accepted and working approach.4-6 One kind of the outstanding
porous materials in this field is metal–organic frameworks (MOFs)
which is constructed from organic ligands and metal ions or metal
clusters. Benefiting from large surface areas and tailorable
structure, this hybrid porous materials show intriguing applications
in various fields including chemical separation,7 catalysis,8 drug
delivery,9 optical sensing detection,10 and gas storage,11,12
particularly in CO2 capture.1 The CO2 adsorbance of MOFs materials
is related to their pore size and shape, surface characteristic and
porous functionality. One promising strategy for increasing CO2
adsorption is to introduce N-rich aromatic ligand.13
Beyond CCS route, catalytic transformation of the captured CO2
Institute of Functional Material Chemistry, National
& Local United Engineering
Laboratory for Power Batteries, Key Laboratory of Polyoxometalate Science of
Ministry of Education, Northeast Normal University, Changchun, 130024 Jilin,
People’s Republic of China. *E-mail: qinc703@nenu.edu.cn, zmsu@nenu.edu.cn;
Fax: + 86 431-85684009; Tel: + 86 431-85099108
† Electronic Supplementary Information (ESI) available: Schemes, figures and CIF
files giving selected Bonds and angles, PXRD, TGA and crystallographic data (CCDC
1458565). See DOI: 10.1039/b000000x/
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reduction. Employing a classical cobalt-containing MOF material, 1096.91(w), 1174.40(w), 1220.58(w), 1275.12(m), 1313.54(s),
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Co-ZIF-9 (ZIF = zeolitic imidazolate framework), as cocatalyst,27 and 1398.86(s), 1505.31(m), 1541.36(s), 1595.64(s), 1664.80(s),
DOI: 10.1039/C7TA02611K
.
[Ru(bpy)3]Cl2 6H2O as photosensitizer, Wang et al. reported an 2927.58(w), 3394.84(w), 3742.00(w), 3858.87(w).
efficient CO2 reduction system under visible light. In spite of this,
the design and synthesis of a kind MOF material with excellent
X-ray crystallography study
capability for CO2 adsorption meanwhile maintaining highly Single-crystal X-ray diffraction data for Co6-MOF were recorded by
matched energy band with photosensitizer is
challenge.32-35 What’s more, the mechanism in such CO2 reduction monochromated MoKα radiation (λ = 0.71069 Å) at 293 K.
system needs to be revealed. Absorption corrections were applied using multi-scan technique. All
a
significant using
a
Bruker Apex CCD diffractometer with graphite-
Herein, we reported the synthesis of a hexanuclear cobalt-clust the structures were solved by Direct Method of SHELXS-2014 and
MOF [Co3(OH)3(NTB)(4,4’-bpy)1.5]·DEF (Co6-MOF, NTB = 4,4',4''- refined by full-matrix least-squares techniques using the SHELXL-
nitrilotribenzoic acid ligands, 4,4’-bpy = 4,4'-bipyridine, DEF = N, N- 2014 program within WinGX. Non-hydrogen atoms were refined
diethylformamide ), in which 2D layer is constituted by Co6(μ3-OH)6 with anisotropic temperature parameters. CCDC 1533801 for Co6-
clusters and NTB and the pillar is 4,4’-bpy ligand. This novel porous MOF contains the supplementary crystallographic data for the work.
material acting as heterogeneous co-catalyst exhibits efficient CO2 These data can be obtained free of charge from the cambridge
photochemical reduction by cooperating with a ruthenium-based crystallographic data centre.
photosensitizer. Under visible-light irradiation for 3 hours, 39.36
μmol CO and 28.13 μmol H2 was obtained. Based on theoretical
Gas sorption experiments
calculation, the reaction mechanism was studied which indicated The N2 and CO2 sorption measurements were performed on
that the excited electrons can be efficiently transferred from the automatic volumetric adsorption equipment (Belsorp mini II). The
lowest unoccupied molecular orbital (LUMO) of ruthenium-based solvent-removal assay has been employed before the gas
photosensitizer to the LUMO of hexanuclear cobalt-clust MOF. And adsorption measurements. The fresh as-synthesized samples were
after accepting electrons, the activation of CO2 molecular is immersed in CH2Cl2 for 48 h, and then transferred these samples to
facilitated by the charged Co6-MOF.
CH3OH for 48 h to remove the DEF. Subsequently, the samples were
gathered by decanting CH2Cl2 similarly to remove CH3OH. When the
CH2Cl2 was removed by decanting, these samples were activated in
a heated manner under a dynamic vacuum at 100 °C for 24 h to
form the activated samples. Before the gas adsorption test, the
EXPERIMENTAL SECTION
Materials
Co(CH3CHO)2·6H2O, NTB and 4,4'-bpy for the synthesis were samples needed to be dried again by using the ‘outgas’ function of
purchased from Alfa, and they were used as received. Purity of all the surface area analyzer for 12 h at 150 °C. After activation, the
reagents was analytical grade.
samples were tested for N2 and CO2 sorption measurements.
Instrumentation
Photocatalytic Test
The C, H and N elemental analyses were performed on a Perkin- Photocatalytic reactions were carried out in a quartz tube with a
Elmer 2400 CHN elemental analyzer. The FT-IR spectra were cap at 1 atm CO2 partial pressure. The reaction system contained
.
examined in the range of 4000-400 cm-1 on a Mattson Alpha- [Ru(bpy)3]Cl2 6H2O (0.01 mmol), Co6-MOF (0.005 mmol), acetonitrile
Centauri spectrometer using KBr pellets. The UV-Vis absorption (4 mL), H2O (1 mL), triethanolamine (TEOA 1 mL). After the solution
spectra
were
conducted
on
a
Shimadzu
UV-2550 was purified by CO2 for 10 min, it was beamed by a 150 W Xenon
spectrophotometer in the wavelength range of 200-800 nm. lamp at 420 ≤ λ ≤ 780 nm in a carousel irradiation apparatus. The
Powder X-ray diffraction (PXRD) patterns were collected with an X- reaction temperature was maintained at 35 °C by using a water
ray diffractometer of Rigaku, Rint 2000. Thermogravimetric bath. The result by PXRD can show that at the end of the reaction,
analyses (TGA) were carried out on a Perkin-Elmer TG-7 analyzer the crystal keeps the original structure(Fig. S11). After the catalytic
heated from room temperature to 1000 °C under nitrogen gas reaction completed, the gases (CO and H2) were tested and
atmosphere at a heating rate of 5 °C/min.
analyzed by GC instrument. To detect the formation of carbon
monoxide from the reaction mixture, 500 μL of the middle of the
test tube was taken out by a syringe and injected into a GC with FID
Synthesis of [Co3(NTB) (4,4'-bpy)1.5(OH)3]·DEF (Co6-MOF)
A solid mixture of NTB (30.0 mg, 0.080 mmol), 4,4'-bpy (15.6 mg, detector, using argon as carrier gas and reference gas. The volume
0.100 mmol), Co(CH3CHO)2·6H2O (37.4 mg, 0.150 mmol) was of the carbon monoxide produced was calculated by comparing the
suspended in DEF (diethylformamide, 6 mL) in a 25 mL Teflon-lined integrated area of the signals of carbon monoxide with a calibration
stainless steel container. The mixture was heated at 120 °C for 2 curve. The injector and detector temperatures were set to be 60 °C.
days resulting in redness crystals, which were washed and isolated Retention times of hydrogen were 1.9 min. To detect the formation
using DMF. Subsequently, it was dried in air at room temperature. of hydrogen from the reaction mixture, 1000 μL of the headspace of
The yield of the Co6-MOF violet crystals were 78% based on NTB. the test tube was taken out by a syringe and injected into a GC with
Elemental analyses: Anal. Calcd (%) for C41H38N5O10 Co3, TCD detector, using a 5Å molecular sieve column and argon as
calculated(%): C 52.52; H 4.09; N 7.47. Found (%): C 52.55; H 4.11; N carrier gas and reference gas. The volume of the hydrogen
7.43.Selected IR (KBr pellet, cm-1): 532.29(w), 582.05(w), 633.28(w), produced was calculated by comparing the integrated area of the
667.37(w), 709.96(w), 784.19(m), 816.01(w), 1008.66(w), signals of hydrogen with a calibration curve. The injector and
2 | J. Name., 2012, 00, 1-3
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detector temperatures were set to be 60 °C. Retention times of with the dimensions of 11.31  12.56 Å (a  c or b Vcie)w(FAirgti.cleS4On(lain)e)
DOI: 10.1039/C7TA02611K
hydrogen were 0.7 min. All the photocatalytic reactions were and 8.89  8.89 Å (a  b) (Fig. S4 (b)). Topological analysis shows
repeated five times to confirm the reliability of the data.
that the Co6(μ3-OH)6 SBUs can be regarded as 8-connected nodes,
and the NTB ligand was defined as 3-connected nodes, so the
architecture of the Co6-MOF can be reduced to a 3,8-connected tfz-
d (α-UO3) type topology with the point symbol of (43)2(46.618.84)
(Fig. 1 (d)). Many MOFs with the tfz-d type topology have been
RESULTS AND DISCUSSION
Crystal structures
Co6-MOF was prepared through admixing 4,4'-bpy, NTB ligands and reported.36-38 In this type topology, 3-connected and 8-connected
Co(CH3CHO)2·6H2O in DEF heated at 120 °C for 2 days. The reaction exist simultaneously, and they will form a double connected 3D
produced violet crystals with polyhedral morphology. The structure. PLATON calculation shows that the effective pore volume
structures of the Co6-MOF were confirmed by single X-ray of the Co6-MOF is about 63.9% (4682.8 Å3) per unit cell (7324.9 Å3),
crystallography. Single-crystal X-ray diffraction analysis reveals that which is occupied by DEF molecules.
the Co6-MOF crystallized in the hexagonal P63/mcm space group
(Table S2). In the asymmetric unit, there are a half Co ion, one-sixth
Chemical and thermal stability
NTB ligands and one-fourth 4,4'-bpy ligands. The solvent DEF The purity of the synthesized Co6-MOF was verified by high
molecules in the channels are not crystallographically well defined. similarities between simulated and experimental powder X-ray
Six Co cations and six μ3-OH groups construct Co6(μ3-OH)6 clusters
diffraction (PXRD) patterns. In order to determine the possibility of
serving as secondary building units (SBUs) in the structure. Each Co practical application of Co6-MOF, we conducted the stability toward
ion in the Co6(μ3-OH)6 clusters adopts six coordination connecting moisture. The XRPD patterns still keep the high similarities
with one nitrogen atom of 4;4’-bpy ligand and five oxygen atoms compared with simulated PXRD patterns after soaking in water for
provided by two carboxylates of different NTB ligands and three μ3- 24h. In addition, the acid resistance and alkali resistance of Co6-
OH groups (Fig. 1 (a)). Besides each 4,4'-bpy ligand binds to two Co MOF were also investigated. Surprisingly, after placing it in a
atoms in two Co6(μ3-OH)6 SBUs through N atoms in 4,4’-position hydrochloric acid solution (pH = 2) and sodium hydroxide solution
(Fig. S1). Each NTB ligand act as a 3-connecting nodes linked with (pH = 12) for 24 h, it exhibits excellent acid and alkali resistance
three Co6(μ3-OH)6 SBUs forming two-dimensional layers (Fig. S2). revealing from the similar shape and intensity of the PXRD (Fig. S6).
The adjacent Co6(μ3-OH)6-NTB layers are further pillared by the bpy Such high acid resistance and alkali resistance is quite rare in
triangular prism (Fig. 1 (a)), constituting an overall 3D columnar reported MOFs.39 Thermal stability of Co6-MOF is revealed through
supporting structure (Fig. S5). It is worth noting that such a the TG curve. As shown in Fig. S7, an evident weight loss of 11.2%
triangular prism pillared-layer structure enhanced the stability of was observed in the step from 270 to 285 °C, which may ascribe to
architecture. The pillared layer structure contains three different the release of solvent molecules. After 285 °C, the framework of
types channels continuity along a, b and c axes (Fig. 1 (b), (c), and Co6-MOF starts to decompose.
S3)
Gas adsorption properties
To study the porosity and CO2 adsorption capability of Co6-MOF, we
performed the N2 and CO2 adsorption experiments (Fig. 2). The
samples were adequately activated by the method represented in
experimental section. The PXRD of the activated sample showed
similar peaks with the as-synthesized sample, indicating the
maintenance of the framework.
N2 adsorption of the activated Co6-MOF was carried out at 77 K.
The adsorption isotherm of N2 unfolds a characteristic type I
behavior with good reversibility for microporous solids. The
Brunauer–Emmett–Teller (BET) and Langmuir surface areas are
1957.5 m2g-1 and 2257.1 m2g-1, respectively, calculated by the N2
adsorption isotherms, and the total pore volume of the Co6-MOF is
0.87 cm3g-1. The N2 adsorption amounts of the Co6-MOF at 1 bar is
approximately 568.1 cm3 g-1, which is equivalent to 42.1 N2 per unit
cell, lower than the theoretical pore volume calculated by PLATON
based on the single crystal X-ray diffraction data. In order to prove
that the Co6-MOF remains intact structure in water, we performed
N2 sorption of the Co6-MOF which was soaked in water for 24h.
Fig. 1 (a) The coordination environment of Co6(μ3-OH)6 SBUs. (b)
Corresponding adsorption isotherm of this sample is presented in
and (c) Ball-and-stick model of the carbon nanotubes channel along
Fig. S9 and the result shows the BET of it is similar to activated
the[a  b] and [a  c or b  c] direction. (d) Topological nets of the
sample indicating structure of Co6-MOF maintains well after soaked
Co6-MOF. All hydrogen atoms and solvent molecules have been
in water.
omitted for clarity. Color coding: Co: Violet; C: gray; O: red; N: Sky
blue.
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DOI: 10.1039/C7TA02611K
Fig. 3 The effect of the time of reaction on the evolution of CO and H2 from
the CO2 photoreduction system.
In order to determine the importance of the components in the
photocatalytic CO2 reduction system, we provided a series of
reference experiments and the results are summed up in Table 1.
As we have seen, in total darkness, CO or H2 was undetectable in
the photocatalytic system (entry 2, Table 1). When the system was
.
short of [Ru(bpy)3]Cl2 6H2O, no gases were detected in the reaction
system (entry 3, Table 1). Replacing [Ru(bpy)3]Cl2·6H2O with Tris(2-
phenylpyridinato)iridium(III), CO gas wasn’t detected in the reaction
while only a trace amounts of H2 produced (entry 13, Table 1).
These results mentioned above indicate the necessity of the energy
band matching of photosensitizer and MOFs in the photocatalytic
CO2 reduction reaction. This was further ascertained by the truth
that no CO or H2 was detected when the photosensitizer didn’t exist
or the band mismatch of photosensitizer and MOFs.
In the experimental study, we also explored the influence of the
amount of Co6-MOF on the photocatalytic property in the system
(Fig. 4). From this Figure, you can see that the yield of CO and H2
changed obviously when the added amount of Co6-MOF changed a
little. The optima amount of Co6-MOF is 3 mg with a maximum
value of both CO and H2. Overall, these results have been proved
the major role of Co6-MOF by fastening CO2 acting as a CO2 redox
promoter and absorber in photocatalytic reduction system.
Fig. 2 (a) N2 sorption isotherms of the Co6-MOF; (b) CO2 sorption isotherms of
the Co6-MOF.
The CO2 sorption behaviors for the Co6-MOF were measured at
273 K and 298 K, respectively and the sorption isotherms are shown
in Fig. 2 (b). At 1 atm and 273 K, the Co6-MOF has CO2 uptakes at a
saturation of 55.24 cm3g-1, based on the total weight. And at 298 K,
the CO2 uptakes decreased slightly, up to 38.17 cm3g-1 (Fig. 2 (b)).
This capacity is comparable to classical MOF, Uio-66 and NH2-Uio-
66 which were employed as catalyst for CO2 reduction.40
Photochemical CO2 reduction properties
The photocatalytic CO2 reduction system consists of Co6-MOF,
.
[Ru(bpy)3]Cl2 6H2O, and TEOA (triethanolamine). The solvent of the
photocatalytic system filled with atmospheric CO2 is MeCN and
H2O. After irradiation with visible light (λ ≥ 420 nm) for 30 min, the
reduction system generated CO (3.73 μmol) and H2 (2.81 μmol)
gases. In this reduction process, CO2 was split into CO at a reaction
rate of 0.13 μmol min-1, along with the evolution of H2 with a rate of
0.10 μmol min-1 (Fig. 3). Trace CH3OH was detected in the solution.
With reaction time increasing, the evolution amount of CO and H2
enhance consistently (Fig. 3). After 3 hours’ irradiation, total
amount of CO and H2 reaches 39.35 and 28.13 μmol (entry 1, Table
1). This activity is much higher than that of many classical MOF
27
materials for converting CO2 to CO listed in Table S1.20,
Photochemical quantum yield of CO (ΦCO) of Co6-MOF under
irradiation of 420 nm light was calculated as 0.758 % (The detailed
calculation was given in supporting information).
Fig. 4 The effect of the amount of Co6-MOF on the evolution of CO
and H2 from the CO2 photoreduction system.
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In absence of Co6-MOF, the turnover number decreases 3.37, result shows the HOMO and LUMO of Co6-MOF is -6.22 and -4.56
View Article Online
significantly
lower
than
catalytic
system
containing eV (Fig. S14). Since the LUMO of the Co6-MODFOisI:lo10w.1e0r3t9h/Can7TtAh0e26th11aKt
.
[Ru(bpy)3]Cl2 6H2O and Co6-MOF (entry 4, Table 1). To further of [Ru(bpy)3]Cl2, the electrons in the LUMO of [Ru(bpy)3]Cl2 can be
confirm the role of the formation of MOFs as a co-catalyst in transferred to the LUMO of Co6-MOF (Fig. 5).
photocatalytic reaction system, we destroyed the Co6-MOF by
calcination at 900°C in N2 gas, and then the residues were applied in
Table 1 The research of reaction conditionsa)
the reaction system to instead of Co6-MOF (entry 5, Table 1). The
result showed that CO and H2 production decreased dramatically.
This indicates the necessity of the complete framework structure of
Co6-MOF in the CO2 splitting, possibly by the promotion of
substrate concentration and carrier transfer. To further confirm the
role of the formation of MOFs as a co-catalyst in photocatalytic
reaction system, we measured the photocatalytic efficiency of
physical mixtures including the Co2+ and ligand in a homogeneous
system. We use 4,4'-bpy, NTB, and Co(CH3CHO)2·6H2O to replace
Co6-MOF, and the result reflected that the physical mixture of them
also plays the catalytic role in photocatalytic reaction system, but
with lower TON than those with Co6-MOF (entry 6, Table 1). These
results noted above confirmed that as a heterogeneous cocatalyst,
the Co6-MOF effectively promotes the efficiency of photochemical
reaction.
Entry
CO [μmol]
H2 [μmol]
TONb)
1
39.36
n.d.d)
28.13
n.d.
13.50
n.d.
2c)
3e)
4f)
n.d.
10.46
27.64
13.6
n.d.
6.39
13.97
5.61
2.37
n.d.
_
3.37
8.29
3.84
0.47
_
5g)
6h)
7i)
n.d.
8j)
n.d.
In the experiment of photocatalytic reaction, the participation of
CO2 was investigated by replacing CO2 with N2 (entry 7, Table 1).
Once CO2 was replaced by N2, the evolution of CO was not observed,
and only 2.37 μmol H2 was detected under the same reaction
conditions. It is noticeable that when there was no CO2 in the
system, light-induced electrons just can promote H2 generation.
In the course of the experiment, we found the reaction medium
has a great influence to the catalytic effect of Co6-MOF on CO2
photocatalytic reduction reaction. There was no reaction occurred
when only using H2O as the reaction medium which might due to
the weak chemical affinity towards CO2 molecules (entry 10, Table
1). Meanwhile, MeCN and DMF was a favorable reaction medium
for the CO2 reduction (entry 11 and entry 12, Table 1). They
possessed nitrogen or oxygen atoms can interact with and solubilize
CO2 via Lewis acid−base interactions.41-42 In addition, we replaced
Co6-MOF with Co3O4-Uio-66, the turnover number decreases 3.87,
significantly lower than original catalytic system(entry 14, Table 1
Fig. S13).
In order to unambiguously determine the source of the C atom of
CO, we conducted an isotopic tracing experiment by replacing CO2
by 13CO2 under the same photocatalytic reaction conditions, and we
used gas chromatography mass spectrometry to analysis the
evolution of CO. After irradiation with visible light for 30 min, the
peak at 1.99 min and m/z 29 was assigned to 13CO (Fig. S12) and no
sign at m/z 28 was detected. It turns out to be the case that Co6-
MOF can indeed stimulate the deoxygenative reduction of CO2 into
CO.
9k)
29.23
n.d.
9.98
n.d.
7.84
_
10l)
11m)
12n)
13p)
14q)
15.11
11.74
n.d.
63.96
20.53
0.61
2.80
15.81
6.45
0.12
3.87
16.56
a) Reaction conditions: [Ru(bpy)3]Cl2·6H2O (0.01 mmol), Co6-MOF
(0.005 mmol, activated), acetonitrile (MeCN, 4 mL), H2O (1 mL),
TEOA (1 mL), CO2 (1 atm), λ ≥ 420 nm, 35 °C, 3 h. b) Turn over
number (mol amount of CO and H2) / (mol amount of Co6-MOF). c)
In the dark. d) Not detectable. e) Without [Ru(bpy)3]Cl2·6H2O. f)
Without Co6-MOF. g) the Co6-MOF was destroyed by calcination at
900 °C in N2 gas. h) Using 4,4'-bpy, NTB, and Co(CH3CHO)2·6H2O to
replace Co6-MOF. i) Using N2 to replace CO2. j) Without TEOA. k)
Using TEA instead of TEOA. l) Without MeCN. m) Without H2O. n)
Using DMF (N,N-dimethylformamide) to replace MeCN. p) Using
Tris(2-phenylpyridinato)iridium(III) instead of [Ru(bpy)3]Cl2·6H2O. q)
Using Co3O4-Uio-66 to replace Co6-MOF.
The results above clearly demonstrate the effective role of Co6-
MOF in the photocatalystic system. The next important target is to
understand the root reason in such system. Inspired by Tang’s
work,43 the highest occupied molecular orbital (HOMO) and the
lowest unoccupied molecular orbital (LUMO) energy levels of
[Ru(bpy)3]Cl2 photosensitizer and Co6-MOF is studied firstly.
According to the reference,43 the HOMO and LUMO of [Ru(bpy)3]Cl2
is -5.68 and -3.19 eV, respectively. Theoretic calculation is
introduced to reveal the HOMO and LUMO of Co6-MOF. Calculation
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Fig. 5 (a) Proposed mechanism for the photosensitized catalytic reduction of CO2 to CO. (b) Schematic energy-level diagram showing
electron transfer from [Ru(bpy)3]Cl2 to Co6-MOF. (c) Potential energy surfaces along the O–C bond length for the activation of the CO2
molecule adsorbed on Co6-MOF in the neutral state or the states charged with one or two electrons, from which the EB of CO2 were
deducted.
On the basis of above results, a possible rooted reason for CO2
As described above, Co6-MOF has the capability of CO2 reduction in Co6-MOF participated system was proposed. Under
adsorption, the adsorption energy was investigated, as well. As irradiation, the photosensitizer [Ru(bpy)3]Cl2 is excited to the
density functional theory (DFT) calculation shown, adsorption excited state. This excited state then quenched reductively by
energy of Co6-MOF in CO2 adsorption is 0.275 eV. Science the sacrificial electron donor, TEOA40 (Fig. 5 (a)) and formed a reduced
activation of CO2 molecule is usually triggered by two-electron photosensitizer. Then the electrons in the reduced photosensitizer
charges.44 From first-principles simulations, we calculated the transferred to Co6-MOF. The adsorbed CO2 molecule on Co6-MOF
adsorption energy changes after Co6-MOF accepting one- or two was activated after Co6-MOF obtaining electrons. In the end, CO2 is
electrons. The result shows when receiving one electron, the reduced to CO and released from Co6-MOF.
energy increases from 0.275 to 0.361 eV. When obtaining two
electrons, the energy further enhances to 0.907 eV. What’s more, Conclusions
we simulated the potential energy surfaces with the variation of O–
In conclusion, based on highly symmetrical Co6(μ3-OH)6 cluster SBUs
C bond of CO2 adsorbed on Co6-MOF. From the simulation results,
and organic ligands of NTB and 4,4'-bpy,
a pillared-layer
we can see that one-electron charge can hardly change the
activation-energy barrier (EB) of CO2 while two-electron charge can
effectively reduce the EB from 5.66 to 4.98 eV (Fig. 5 (c) Fig. S15).
Therefore, upon receiving electrons from [Ru(bpy)3]Cl2
photosensitizer, Co6-MOF can turn into an active photocatalyst
material for the CO2 activation and further facilitating CO2 reduction.
hexanuclear cobalt MOF, Co6-MOF, was obtained containing
nanoporous structure. With this merit, Co6-MOF displays a satisfied
amount of CO2 uptake, up to 55.24 cm3g-1. Under irradiation with
visible light, Co6-MOF can be used as stable co-catalyst coupled to
.
an appropriate photosensitizer ([Ru(bpy)3]Cl2 6H2O) realizing
6 | J. Name., 2012, 00, 1-3
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16 Y. H. Fu, D.R. Sun, Y. J. Chen, R. K. Huang, Z. X. VDieiwngA,rtiXcl.e ZO.nlFinue
photocatalytic reduction of CO2 into CO. Around 39.36 μmol CO and
28.13 μmol H2 was produced after 3 hours irradiation. This result is
higher than most of reported classic MOF materials under similar
condition. To the best of known, this is the first example of high
nuclear MOF used in CO2 reduction. Possible mechanism was
proposed through theoretic calculation studies. The results showed
and Z. H. Li, Angew. Chem., 2012, 124, 3420-3423.
DOI: 10.1039/C7TA02611K
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that electrons on reduced [Ru(bpy)3]Cl2 6H2O could transfer to Co6-
MOF and the adsorbed CO2 molecule on charged Co6-MOF could be
activated more facilely. Rooted reasons behind the high reactivity
were revealed through theoretic calculation studies which showed
.
electrons on reduced [Ru(bpy)3]Cl2 6H2O could transfer to Co6-MOF
and the adsorbed CO2 molecule on charged Co6-MOF could be
activated more facilely. This work not only clarifies the reasons for
the high efficiency in the CO2 photoreduction system but also points
out the direction for us in designing more effective MOF materials
as photocatalysis for artificial photochemical CO2 reduction.
23 Y. Baia, L.Q. Yea, L. Wang, X. Shia, P. Q. Wang, W. Baia and P.
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Acknowledgements
25 L. N. Li, S. Q. Zhang, L. J. Xu, J. Y. Wang, L. X. Shi, Z. N. Chen,
M. C. Hong and J. H. Luo, Chem. Sci., 2014, 5, 3808-3813.
This work was financially supported by the NSFC of China (No.
21601032, 21671034, 21471027), National Key Basic Research
Program of China (No. 2013CB834802), the Fundamental Research
Funds for the Central Universities (2412016KJ021), Changbai
Mountain Scholars of Jilin Province, Foundation of Jilin Educational
Committee (No. 2016498). †These authors contributed equally to this
work.
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Journal of Materials Chemistry A
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DOI: 10.1039/C7TA02611K
Hexanuclear Cobalt Metal-organic framework with excellent properties for CO2 reduction under
visible-light irradiation is reported and the mechanism is revealed through density functional
theory calculation.