A Highly Selective and Robust Co(II)-Based Homogeneous Catalyst for Reduction of CO2 to CO in CH3CN/H2O Solution Driven by Visible Light

Article abstract of DOI:10.1021/acs.inorgchem.7b00566

Visible-light driven reduction of CO₂ into chemical fuels has attracted enormous interest in the production of sustainable energy and reversal of the global warming trend. The main challenge in this field is the development of efficient, selective, and economic photocatalysts. Herein, we report a Co(II)-based homogeneous catalyst, [Co(NTB)CH₃CN](ClO₄)₂ (1, NTB = tris(benzimidazolyl-2-methyl)amine), which shows high selectivity and stability for the catalytic reduction of CO₂ to CO in a water-containing system driven by visible light, with turnover number (TON) and turnover frequency (TOF) values of 1179 and 0.032 s^-1, respectively, and selectivity to CO of 97%. The high catalytic activity of 1 for photochemical CO₂-to-CO conversion is supported by the results of electrochemical investigations and DFT calculations.

Full text of DOI:10.1021/acs.inorgchem.7b00566

Communication
pubs.acs.org/IC
A Highly Selective and Robust Co(II)-Based Homogeneous Catalyst
for Reduction of CO2 to CO in CH3CN/H2O Solution Driven by Visible
Light
Ting Ouyang,†,‡ Cheng Hou,‡ Jia-Wei Wang,‡ Wen-Ju Liu,‡ Di-Chang Zhong,*,†,§ Zhuo-Feng Ke,‡
and Tong-Bu Lu*,†,‡
†Institute of New Energy Materials & Low Carbon Technology, School of Material Science & Engineering, Tianjin University of
Technology, Tianjin 300384, China
‡MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, School of Chemistry and Chemical Engineering, Sun Yat-Sen
University, Guangzhou 510275, China
§School of Chemistry and Chemical Engineering, Gannan Normal University, Guanzhou 341000, China
S
* Supporting Information
based complex reported by the Lau group exhibiting a TON
value of 2660 in CH3CN12a (see Table S1). However, the good
ABSTRACT: Visible-light driven reduction of CO2 into
chemical fuels has attracted enormous interest in the
production of sustainable energy and reversal of the global
warming trend. The main challenge in this field is the
development of efficient, selective, and economic photo-
catalysts. Herein, we report a Co(II)-based homogeneous
catalyst, [Co(NTB)CH3CN](ClO4)2 (1, NTB = tris-
(benzimidazolyl-2-methyl)amine), which shows high
selectivity and stability for the catalytic reduction of CO2
to CO in a water-containing system driven by visible light,
with turnover number (TON) and turnover frequency
(TOF) values of 1179 and 0.032 s−1, respectively, and
selectivity to CO of 97%. The high catalytic activity of 1
for photochemical CO2-to-CO conversion is supported by
the results of electrochemical investigations and DFT
calculations.
performances of most catalysts are achieved in nonaqueous
solvent systems, as in water-containing systems, the easy
occurrence of the photocatalytic proton reduction reaction is
competitive with the CO2 reduction reaction, which may
inevitably lead to a decrease of the efficiency and selectivity for
CO2-to-CO conversion.8f,12g Considering that a water-contain-
ing system is necessary for building an artificial photosynthesis
cycle by combination of a CO2 reduction with a water
oxidation, it is meaningful to develop catalysts that can exhibit
high activity and selectivity for the photocatalytic CO2
reduction in a water-containing reaction system.
During the past two decades, tripodal ligands have been
widely used for preparing metal-based homogeneous molecular
catalyts,12a,15 as they are polydentate and can strongly bind with
a metal center to form robust molecular complexes. The
tetradentate tripodal ligands, usually impose C3 symmetry and
bond to a five-coordinated metal center, leaving one position
available on the trigonal-bibyramidal metal center to combine
with the solvent molecule. This solvent molecule is readily
exchanged by the substrate molecule, facilitating the activation
and catalytic transformation of the substrate molecule. Herein,
we report a mononuclear cobalt(II) complex, [Co(NTB)-
CH3CN](ClO4)2 (1, NTB = tris(benzimidazolyl-2-methyl)-
amine, Scheme S1a), which can perform as a highly selective
and robust catalyst for photocatalytic reduction of CO2 to CO
in acetonitrile/water (v/v = 4:1). The TON and TOF values
reach as high as 1179 and 0.032 s−1, respectively, and the
selectivity to CO reaches as high as 97%. Electrochemical
investigations and DFT calculations have revealed that the high
CO2 reduction activity of 1 can be attributed to its lower onset
CO2 reduction potential, which originates from the existence of
electron-rich benzimidazol groups of NTB in 1.
he use of sunlight and catalysts to convert carbon dioxide
T
(CO2) into chemical fuels and feedstocks has attracted
tremendous scientific interest for two main reasons: the
shortage of energy and the problem of global warming.1 To
achieve both purposes, the development of artificial photo-
synthetic systems from which solar fuels can be produced from
CO2 and H2O is essential.2,3 The past investigations have
shown that the reduction products of CO2 usually include
CO,3a formic acid,4 methane,5 methanol,3b and oxalate,6 of
which getting a single reduction production of CO has attracted
particular attention.7 Over the past several decades, a number
of metal complexes have been investigated as homogeneous
electrochemical catalysts for reduction of CO2 to CO,8,9 some
of which have shown high electrocatalytic activities,9 even in a
pure water catalytic system.9b−g This has been supported by
Saveant et al.9g,h and Koper et al.,5 who proposed that
electrocatalysts could remain highly selective toward CO2-to-
CO conversion in water after mechanistic studies. In the
photochemical CO2 reduction, a series of catalysts based on
Ir,3a,b Ru,4a Mn,4b Re,10 Os,11 Co,12 Fe,13 and Ni14 complexes
have been created, some of which have also displayed good
performance for CO2-to-CO conversion, such as a Co(II)-
The crystals of 1·3CH3CN·1.25H2O were obtained from the
reaction of Co(ClO4)2·6H2O and NTB in acetonitrile (see the
Supporting Information). The result of single X-ray structural
Received: March 3, 2017
© XXXX American Chemical Society
A
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a
Table 1. Results of Control Experiments of Photocatalytic Reduction of CO2 to CO
entry
catalyst
CO (μmol)
H2 (μmol)
selectivity to CO (%)
TON (CO)
TOF (s−1
)
ΦCO (%)
1
2
3
4
5
6
7
8
1
11.79
0.37
0.049
1.93
0
97%
0
1179
0
0.032
0.22
blank
0
0
0
1
0
0
0
0
0
1
0
0
0
0
0
1
0
0.21
0
0
0
0
0
1
0
0
0
0
0
CoCl2
4.1
6.86
0.12
0.16
97%
97%
410
686
0.011
0.019
0.077
0.13
2
a
Entries 1 with and 2 without catalyst 1 in CO2; 3, with catalyst 1 in Ar; 4, without [Ru(phen)3](PF6)2; 5, without TEOA; 6, without light; 7, with
b
catalyst CoCl2 in CO2, 8: with catalyst 2 in CO2. Reaction conditions: [Ru(phen)3](PF6)2 (0.4 mM), TEOA (0.3 M), LED light (450 nm, 100 mW·
cm−2, irradiation area 0.8 cm2), 25 °C. TON and TOF values are averaged over three reactions, with deviations below 5%. Calculation details for
TON, TOF and ΦCO are illustrated in the Supporting Information.
analysis has revealed that 1·3CH3CN·1.25H2O crystallizes in
the monoclinic space group P21/n (Table S2). The central
metal Co(II) is five-coordinated to four N atoms from an NTB
ligand and one acetonitrile molecule, forming a slightly
distorted trigonal bipyramid geometry (Figure S1). The lattice
CH3CN and H2O molecules in 1·3CH3CN·1.25H2O can be
removed by evacuation at 25 °C for 10 h to get 1. The UV−vis
spectrum of 1 in CH3CN/H2O (v/v = 4:1) shows four
absorption peaks at 213, 272, 279, and 528 nm (ε = 54 560,
37 108, 34 440, and 196 M−1·cm−1 respectively; Figure S2a),
the former three peaks can be assigned to the π → π*
transitions of the NTB ligand, and the fourth peak can be
assigned to the d−d transition of the Co(II) ion. After being
irradiated by 450 nm LED light with an intensity of 100 mW·
cm−2 for 10 h, 3.7% of 1 degrades, illustrating that 1 is stable in
Figure 1. CO and H2 evolution catalyzed by 1 (2 μM) under visible-
light irradiation (450 nm LED light, light intensity of 100 mW·cm−2)
in the presence of [Ru(phen)3](PF6)2 (0.4 mM) and TEOA (0.3 M)
in CH3CN/H2O (v/v = 4:1) solution at 25 °C under 1 atm of CO2.
CH3CN/H2O under irradation (Figure S2b). The electro-
chemical behavior of 1 in CH3CN/H2O (v/v = 4:1) was
investigated by cyclic voltammograms (CVs; see the Supporting
Information). As shown in Figure S3, under an Ar atmosphere,
the CV of 1 shows an irreversible reduction wave at −1.09 V vs
NHE, corresponding to the reduction of [CoII(NTB)-
CH3CN]2+ to [CoI(NTB)CH3CN]+. Under a CO2 atmos-
phere, the CV of 1 exhibits enhanced current at Eonset = −0.65
V, and the current intensity increases over 3 times that under
an Ar atmosphere. These results indicate that 1 can act as a
catalyst for the reduction of CO2.8
for CO of 1179 and 0.032 s−1, respectively (Table 1, entry 1).
The quantum yield of 1 to CO was determined to be 0.22%
(see the Supporting Information), significantly lower than that
of the most efficient reported catalyst, which can be attributed
to the low light collection of this catalytic system, that is, the
low efficiency of oxidative quenching of the sensitizer by 1 due
to the low concentration of 1. However, the TON and TOF
values based on 2 μM cobalt catalyst are comparable to those of
the most efficient CO2 reduction catalyst (Table S1), revealing
that 1 behaves highly efficiently for the reduction of CO2 to
CO. The high catalytic activity of 1 for CO2-to-CO conversion
can be attributed to its low reduction potential of Eonset = −0.65
V under a CO2 atmosphere, which leads to the easy reduction
of the CoII in 1 by [RuI(phen)3]+, strongly accelerating the
process of CO2-to-CO conversion. It is worth noting that the
high selectivity and activity of 1 for CO2-to-CO conversion are
achieved in CH3CN/H2O solution, a water-containing reaction
system where almost all the reported molecular catalysts display
inactivity for the photoreduction of CO2 to CO.8f,12g
Obviously, these outstanding catalytic performances will greatly
enhance the practical application of 1 for CO2 reduction in
combining with a water oxidation, to achieve a carbon neutral
artificial photosynthesis cycle.
The photocatalytic experiments of CO2 reduction were
carried out using 1 as a catalyst, [Ru(phen)3](PF6)2 as a
photosensitizer, and triethylolamine (TEOA) as a sacrificial
reductant (see the Supporting Information). The UV/vis
absorption spectrum depicted in Figure S4 shows that the
[Ru(phen)3](PF6)2 exhibits a broad absorption band in the
visible region. Typically, under 1 atm of CO2 and 25 °C, a glass
reactor containing a mixture of 5 mL of CH3CN/H2O (v/v =
4:1), 1, [Ru(phen)3](PF6)2, and TEOA was irradiated by a 450
nm LED light, with a light intensity of 100 mW·cm−2. The
generated gases were analyzed by gas chromatography (GC).
The result showed that the visible-light photoredox cycle
produced a significant amount of CO and a very small amount
of H2 (Table 1, entry 1; Figure 1), along with a trace amount of
formate detected in the liquid phase by ion chromatograph
(IC). Kinetic investigations have shown that the amount of CO
production increases linearly with the concentration of 1
(Figure S5), suggesting a single site cobalt catalysis process.
Using 2 μM cobalt catalyst, 11.79 μmol of CO and 0.37 μmol
of H2 were generated within 10 h, corresponding to the
selectivity of 1 to CO of 97%, and the TON and TOF values
A series of control experiments were further performed to
thoroughly investigate the photocatalytic CO2-to-CO reduction
reaction. First, the photocatalytic reaction was carried out in the
absence of 1. The result showed that no CO was detected, and
a tiny amount of H2 was recorded (Table 1, entry 2), indicating
B
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that the production of CO originates from the catalytic
reduction of CO2 by 1 rather than by [Ru(phen)3](PF6)2.
Second, the photocatalytic reaction was performed under an Ar
atmosphere (Table 1, entry 3). The result showed that no CO
was generated, suggesting that the CO formed in entry 1 comes
from the reduction of CO2 rather than the decomposition of 1
and/or photosensitizer/sacrificial reductant. This result was
further confirmed by the isotope tracer experiment. Using
13CO2 instead of CO2, the generated gas is 13CO rather than
CO (Figure S6), strongly evidencing that the generated CO
originates from the reduction of CO2 by 1. Third, the
photocatalytic reaction was performed without [Ru(phen)3]-
(PF6)2, TEOA, or visible light irradiation (Table 1, entries 4−
6). The results showed that no CO was detected, suggesting
that photosensitizer, sacrificial reductant, and visible-light
irradiation are indispensible to the CO2-to-CO conversion.
Fourth, the photocatalytic reaction was operated using CoCl2
instead of 1. The results showed that the production of CO and
H2 dramatically decreased (Table 1, entry 7), indicating that
formation of complex is significant for enhancing the catalytic
activity of Co(II).
To intensively study the effect of the molecular structure of
catalyst on the catalytic activity, a typical tetradentate tripodal
ligand tris(2-pyridylmethyl)amine (TPA, Scheme S1b) and its
Co(II) complex [Co(TPA)]Cl2 (2) have been designed and
synthesized.12a Under the same conditions, 2 also exhibits
activity for catalytic reduction of CO2 to CO, with TON and
TOF values of 686 and 0.019 s−1, respectively (Table 1, entry
8). Obviously, 1 exhibits a better catalytic activity than 2, which
is consistent with the electrochemical results; that is, 1 has a
more positive onset potential for electrochemical CO2
reduction (−0.65 V vs NHE, Figure S3) than 2 (−0.92 V vs
NHE, Figure S7). Complex 1 possessing higher catalytic
activity than 2 can be attributed to the fact that the
benzimidazol group of NTB in 1 is more electron donating
than the pyridyl group of TPA in 2. This result clearly reveals
that the electron-rich groups of a ligand will enhance the
catalytic activity of the corresponding metal complex for
photochemical CO2 reduction.
Besides activity and selectivity, durability of 1 has also been
investigated. It has been found that over 10 h, the rate of CO
generation apparently slows down (Figure S8). It is interesting
to find that the production of CO can be resumed by an
addition of equivalent [Ru(phen)3](PF6)2 (0.4 mM, 2 μmol) to
this nearly completed catalytic system (Figure S8a), while it
cannot be recovered by an injection of equivalent TEOA (0.3
M, 0.015 mmol; Figure S8b), demonstrating that the
deactivation of the system is the photodegradation of the
photosensitizer rather than the deactivation of 1 or insufficiency
of TEOA. This conclusion was further confirmed by the
obvious hypochromism of the UV/vis absorption spectra of
[Ru(phen)3](PF6)2 after irradiation with an LED light (Figure
S9). The result of dynamic light scattering (DLS) showed that
no significant nanoparticle was generated in the solution after
the reaction, further illustrating that 1 did not decompose
during the photocatalytic process. The above observations
demonstrate that 1 possesses good durability and can serve as a
stable homogeneous catalyst for photochemical CO2 reduction
to CO.
emission peak at 610 nm upon excitation at 450 nm.1e As
the fluorescence intensity of the deaerated [Ru(phen)3]2+
solution (CH3CN/H2O; v/v = 4:1) dramatically decreased
with the gradual addition of 1 (Figure S10a and b), rather than
TEOA (Figure S10c and d), the quenched mode of the excited
[Ru(phen)3]2+ can be assigned to an oxidatively quenched
pathway.1d The catalytic mechanism of 1 was further studied at
a molecular level. DFT calculations were carried out to evaluate
the reaction processes and related active species (Figure S11).
With detailed calculations, a reasonable reaction pathway is
presented in Figure S12: (i) With the assistant of the
photosensitizer [Ru(phen)3](PF6)2 and visible light,
[CoII(NTB)(CH3CN)]2+ is reduced to [CoI(NTB)-
(CH3CN)]+. (ii) [CoI(NTB)(CH3CN)]+ undergoes CO2
reduction via transition state TS1 to generate [CoIII(NTB)-
(CO22−)]+ after a ligand exchange reaction. The activation free
energy of TS1 is 14.3 kcal/mol relative to [CoI(NTB)(CO2)]+.
A similar mechanism was also suggested by Fujita et al. in their
[CoIIHMD]2+ systems.16 (iii) A proton-coupled electron
transfer (PCET) reduction of CoIII in [CoIII(NTB)(CO22−)]+
to CoII in [CoII(NTB)(CO2H−)]+ occurs before the C−O
cleavage to generate CO. The calculated redox potential of the
PCET from [CoIII(NTB)(CO22−)]+ to [CoII(NTB)(CO2H−)]+
is 0.93 V, indicating that [RuI(phen)3]+ is able to promote this
reduction. (iv) After the PCET reduction from [CoIII(NTB)-
(CO22−)]+ to [CoII(NTB)(CO2H−)]+, the C−O bond is
cleaved at a CoII center via the transition state TS2 (G⧧
=
21.8 kcal/mol) to yield CO and OH−. In contrast, without
PCET, the cleavage via the CoIII center is verified to be very
high in free energy (G⧧ = 46.9 kcal/mol, Figure S13). Thus, the
C−O cleavage process should occur via the CoII center after the
PCET reduction. Similar one-electron reduction of the metal
center before C−O cleavage has also been proposed by Fujita
et al.16 and Song et al.17 After the release of CO and OH−, the
catalyst [CoII(NTB)(CH3CN)]2+ is regenerated, and the
catalytic cycle restarts. As depicted in the overall mechanism,
the C−O cleavage is the rate-determining step with G⧧ = 21.8
kcal/mol, suggesting first-order kinetics with respect to the
concentrations of 1. This calculation result strongly supports
the experimental observation that the catalytic mechanism is a
single site cobalt catalysis process (Figure S5). Furthermore,
the importance step (iii) of protonation revealed by the
calculation also strongly supports that water, a protic solvent, is
crucial for the high catalytic activity of 1 in the experiment
(Table 1, entry 1).
In summary, we have demonstrated here that a mononuclear
cobalt complex, [Co(NTB)CH3CN](ClO4)2 (1), can act as a
stable homogeneous photocatalyst for CO2 reduction to CO,
with selectivity to CO as high as 97% and TON and TOF
values as high as 1179 and 0.032 s−1, respectively (Table S1).
The high CO2 reduction activity of 1 can be attributed to its
lower onset CO2 reduction potential at Eonset = −0.65 V vs
NHE, which originates from the existence of the electron-rich
benzimidazol groups of NTB in 1. The results of DFT
calculations have revealed that during the process of CO2
reduction to CO, the CoI species promotes the reduction of
CO2, and a subsequent PCET reduction leads to the CoII-
mediated C−O cleavage to CO. The results presented here
pave a new way toward the development of efficient, robust,
and economic CO2 reduction catalysts.
To reveal the catalytic mechanism of 1 in the photochemical
reduction of CO2 to CO, the quenching mode of the
[Ru(phen)3]2+ in the excited state was initially investigated.
As shown in Figure S10, the [Ru(phen)3]2+ exhibits an
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b00566.
■
S
Experimental and computational details and additional
data (PDF)
Accession Codes
CCDC 1545986 contains the supplementary crystallographic
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.
AUTHOR INFORMATION
Corresponding Authors
*E-mail: zhong_dichang@hotmail.com.
*E-mail: lutongbu@mail.sysu.edu.cn.
■
ORCID
Jia-Wei Wang: 0000-0003-1966-7131
Di-Chang Zhong: 0000-0002-5504-249X
Zhuo-Feng Ke: 0000-0001-9064-8051
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the 973 program of China
(2012CB821706, 2014CB845602), the NSFC (Grant Nos.
21331007, 21363001, 21401026), and the NSF of Guangdong
Province (S2012030006240).
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