The synergistic catalysis effect within a dinuclear nickel complex for efficient and selective electrocatalytic reduction of CO2 to CO

Article abstract of DOI:10.1039/c7gc03451b

Developing cheap and earth-abundant catalysts for an efficient and selective reduction of CO₂ is a promising approach to cut down the increasing emissions of CO₂ and obtain valuable fuels/chemicals simultaneously. Here, we present a

Full text of DOI:10.1039/c7gc03451b

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The synergistic catalysis effect within a dinuclear nickel complex
for efficient and selective electrocatalytic reduction of CO2 to CO
Li-Ming Cao,+a Hai-Hua Huang,+a Jia-Wei Wang,a Di-Chang Zhong,*b and Tong-Bu Lu*ab
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Developing cheap and earth-abundant catalysts for efficient and attracted extraordinary attention due to their adjustable
selective reduction of CO2 is a promising approach to cut down the structures, remarkable catalytic activities and available
rising emission of CO2 and obtain valuable fuels/chemicals intermediates for mechanism studies.13,20 Therefore,
simultaneously. Here, we present a dinuclear nickel complex, tremendous efforts have been devoted to the design of
[Ni2L1](ClO4)4
(1,
L1
=
1,2-bis((5,7-dimethyl-1,4,8,11- molecular catalysts for CO2 reduction, and a large number of
tetraazacyclotetradecan-6-yl)methyl)benzene), which shows molecular catalysts have been reported, including precious
excellent performance for electrocatalytic reduction of CO2 to CO, metal-based catalysts,21-30 and earth-abundant metal-based
with a Faradaic efficiency of 95%, and a turnover number (TON) catalysts. 31-47 Up to now, the reduction of CO2 to CO has been
and turnover frequency (TOF) values of 4.1 × 106 and 190.0 s-1, garnered particular interest, since the pathway is beneficial for
respectively. Electrochemical experiments and density functional producing syngas (CO and H2), which can be applied in
theory (DFT) calculations revealed that the excellent catalytic manufacturing various fuels by the Fischer-Tropsch
performance of 1 attributes to the synergistic catalysis effect technologies.48,49 In addition, CO is also a crucial industrial
between two Ni centers within 1.
feedstock in chemical synthesis.50,51 As a consequence, it is
particularly desired to develop applicable electrocatalysts for
efficiently and selectively reducing CO2 to CO.
Carbon dioxide (CO2), originating from industrial processes and
biological activities, is a type of greenhouse gas threatening
the sustainable development of humanbeings.1 In the past
several decades, due to the excessive utilization of traditional
fossil fuels, the concentration of atmospheric CO2 has reached
a record level and is going up steadily, leading to global
warming and sea level rising. Therefore, developing renewable
energy and reducing CO2 emissions are vital to the sustainable
development of human beings.2-8 The electrocatalytic
reduction of CO2 to valuable fuels/chemicals is a promising
access to both targets.9-13 However, due to the high
thermodynamic stability of CO2, the reduction of CO2 driven by
electricity needs a high overpotential.14 To facilitate the
appealing CO2 reduction, the delicate design of
electrocatalysts that can lower the overpotential is
required.1,15-20 Among them, molecular catalysts have
To design suitable molecular catalysts for high-performance
CO2-to-CO conversion, nature has given us inspiration.52 A vivid
example in biological system is the carbon monoxide
dehydrogenase (CODH), in which nickel acts as a Lewis base to
transfer a net electron to the CO2 molecular and the iron
center serves as a Lewis acid to stabilize the negative partial
charges at the oxygen atom, facilitating the effective
transformation between CO2 and CO through the dinuclear
metal synergistic catalysis (DMSC).53,54 Enlightened by this
great observation, many dinuclear complexes with desired
synergistic catalysis effect have been designed and employed
for
a variety of molecular activation and conversion
reactions.55-61 We have also devoted to the investigation of
molecular recognition and activation by dinuclear metal
complexes during the past decade,59-61 and recently, we
a.MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, School of
Chemistry, Sun Yat-Sen University, Guangzhou 510275, China. E-mail:
lutongbu@mail.sysu.edu.cn.
b.Institute for New Energy Materials
& Low Carbon Technologies, School of
Materials Science & Engineering, Tianjin University of Technology, Tianjin 300384,
China. E-mail: zhong_dichang@hotmail.com.
† These two authors contributed equally to this paper.
Electronic Supplementary Information (ESI) available: The syntheses and
characterization of the catalyst, DFT calculation, supplementary Figures and Tables.
See DOI: 10.1039/x0xx00000x
Fig. 1 Molecular structures of 1, 2 and 3.
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reported a dinuclear cobalt complex [Co2(OH)L1](ClO4)3 (L1
=
and -1.37 V, respectively (Fig. S1). The former is attributed to
N[(CH2)2NHCH2(m-C6H4)CH2NH(CH2)2]3N), which displays much the NiII/I reduction, with the reduction potential positively
high selectivity and efficiency for the photocatalytic reduction shifting in contrast to the one in Ar (-1.18 V), which suggests
of CO2 to CO in CH3CN/H2O (v/v = 4:1), with CO2 to CO the binding of CO2 to the NiI species of 1 65
. The latter, a newly-
selectivity reaching as high as 98%, and turnover number appeared wave, may result from a quick, irreversible chemical
(TON) and turnover frequency (TOF) reaching as high as 16896 step following the binding of CO2.65
and 0.47 s-1, respectively.55
As shown in Fig. S1, in anhydrous acetonitrile, the CVs of 1
To further make use of the DMSC effect for boosting the show no obvious catalytic current density enhancement under
CO2-to-CO conversion, herein, we designed and synthesized a CO2 compared with that under Ar, suggesting that the CO2
dinuclear nickel complex, [Ni2L1](ClO4)4
dimethyl-1,4,8,11-tetraazacyclotetradecan-6-
yl)methyl)benzene, Fig. 1).
CO2 reduction is based on the following two considerations: (1) that with the increase of H2O, the reduction wave of NiII/I
The mononuclear [Ni(cyclam)]2+ (cyclam
1,4,8,11- shows a marked current enhancement with a positive shifting
tetraazacyclotetradecane) possesses good eletrocatalytic (Fig. 2), indicative of a proton-dependent electron transfer
activity for the reduction of CO2 to CO.44 with two process.65,66 This result indicates that CH3CN/H2O mixture is a
[Ni(cyclam)]2+ connected by phenyl group may exhibit DMSC good solvent system for electrocatalytic reduction CO2 to CO
effect beneficial for electrocatalytic CO2-to-CO conversion. (2) by , where CH3CN with a wide electrochemical window can
The separation between two catalytic centers of is suitable serves as a solvent for absorbing enough carbon dioxide for
for the generation of synergistic catalysis effect, which may the subsequent CO2 reduction, and H2O can acts as a proton
greatly boost the electrochemical CO2-to-CO conversion. Based source to improve the catalytic performance of . When the
on the above considerations, was synthesized and estimated concentration of H2O increases to 14 M (corresponding to the
as catalyst for electrochemical CO2 reduction. For 4:1 volume ratio of CH3CN/H2O), the current density reaches
(
1
, L1 = 1,2-bis((5,7- reduction in the absence of a proton reagent is unfavorable.62
Therefore, we tentatively added water as proton source to the
1
was selected as electrocatalyst for system to improve the catalytic performance. It was noticed
=
1
1
1
1
1
a
comparison,
[NiL2](ClO4)4
a
mononuclear NiII-macrocycle complex, the largest value of 2.2 mA/cm2 and the reduction potential of
L2 6-benzyl-5,7-dimethyl-1,4,8,11- NiII/I couple positively shifts to the largest value of -1.00 V (Fig.
(
2
,
=
tetraazacyclotetradecane, Fig. 1) and another dinuclear NiII- 2). These observations indicate that the 4:1 CH3CN/H2O (v/v) is
macrocycle complex, [Ni2L3](ClO4)4
dimethyl-1,4,8,11-tetraazacyclotetradecan-6-
yl)methyl)benzene, Fig. 1) with a farther separation between (v/v) solvent system was employed as a catalytic medium in
two NiII catalytic centers were synthesized. As expected,
the subsequent electrochemical analysis. Further control
shows excellent catalytic activity and selectivity for experiments showed that without or CO2, the CVs display no
electrocatalytic reduction of CO2 to CO in CH3CN/H2O (v/v = current enhancement, indicating that the catalytic activity
4:1) solution, with Faradaic efficiency of 95%, and turnover originates from rather than the blank GCE or proton
(
3,
L3
=
1,4-bis((5,7- the most appropriate solvent system where
1 can exhibit the
best electrocatalytic property. Therefore, the 4:1 CH3CN/H2O
1
1
1
number (TON) and turnover frequency (TOF) values of 4.1 × reduction (Fig. S4). To well understand the catalysis of
106 and 190.0 s-1, respectively. These values are much higher
1 for
than those of
2 and 3 under the same conditions. The
experimental observations, coupled with the DFT calculations
demonstrated that the DMSC effect between two NiII catalytic
centers in
electrochemical CO2-to-CO conversion.
were synthesized according to the literature methods.62
1 contributes to its high catalytic activity for
1-3
Except as otherwise noted, all potentials are referenced to the
normal hydrogen electrode (NHE). Cyclic voltammograms (CVs)
of
1
(0.5 mM) measured in anhydrous acetonitrile using a
quasi-reversible
glassy carbon electrode (GCE) show
a
reduction peak at E1/2 = -1.18 V under an argon atmosphere,
corresponding to the NiII/I reduction (Fig. S1). Further
controlled-potential electrolysis (CPE) experiment at -1.21 V
exhibits the charge reaching a plateau at 0.96 Coulombs after a
Fig. 2 CVs of 0.5 mM 1 in a 0.1 M TBAPF6 MeCN solution under
CO2 in the presence of different concentration of H2O (scan
rate: 100 mV/s).
3 h of CPE with 0.5 mM
reversible redox couple of
1
(Fig. S2), indicating that the
is a 2e- reduction/oxidation
1
process.63 This result illustrates that two NiII centers can be
simultaneously reduced to NiI state. On the basis of the 2e-
NiII/I couple, the diffusion coefficient (D) of
1
can be calculated
by the Randles-Sevcik equation (Fig. S3).64 The calculated value
of D is 1.3 × 10-6 cm2/s. Under a CO2 atmosphere, the CV of
1
(0.5 mM) exhibits two irreversible reduction waves at -1.08 V
2
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of
catalytic current density of
those of and
(0.42 and 0.4 mA/cm2, respectively, Fig. S9
and S10). These results reveal that possesses much higher
catalytic performance than and Actually, to our
knowledge, the catalytic performance of for CO2-to-CO
2
and
3
, with 38 and 45-fold, respectively. In addition, the
1
(0.60 mA/cm2) is larger than
2
3
1
2
3.
1
conversion is also better than most reported electrocatalysts
with mononuclear metal catalytic centers (Table S1),38,44,66,70-74
including those state-of-the-art ones.38,44,66,70 The advantage of
1
over mononuclear metal molecular catalyst for the CO2-to-
CO conversion may be the synergistic catalysis effect between
two Ni centers, where one Ni center could act as a Lewis base
to transfer electron to the CO2 molecule and the other could
serve as a Lewis acid to stabilize the partial negative charges at
the O atom, increasing the kinetics of the reduction reaction
and facilitating the effective transformation from CO2 to CO.
Fig. 3 CVs of 0.5 mM 1, 1.0 mM 2 and 0.5 mM 3 in 0.1 M
TBAPF6 CH3CN/H2O (v:v = 4:1) under Ar and CO2 atmosphere,
respectively (scan rate: 100 mV/s).
CO2 reduction, the correlation between the catalytic currents
and the concentrations of 1 was investigated. As shown in Fig.
S5, the catalytic current density is enhanced linearly with the
concentrations of catalyst, indicating that the electrocatalytic
Considering the excellent catalytic activity of
conversion in CH3CN/H2O (v/v = 4:1) mixture, we further
evaluated the catalytic performance of in pure H2O. As
shown in Fig. S11, although not as high as in CH3CN/H2O (v/v =
4:1), the catalytic current density of in H2O also clearly
enhanced under CO2 in contrast to that under Ar, indicating
that may also possess good catalytic activity for CO2-to-CO
conversion in H2O. This speculation was confirmed by the CPE
result. The CPE experiment of 0.5 mM carried out in a 20 mL
1 for CO2-to-CO
1
CO2 reduction by
The electrochemical properties of
mM) were also investigated in CH3CN/H2O (v/v = 4:1) solvent
system under Ar or CO2. As shown in Fig. 3, for both and
1
is a first order reaction.
2
(1.0 mM) and
3 (0.5
1
2
3,
1
the current densities show an obvious enhancement in CO2
compared with that in Ar, and the reduction potentials of the
NiII/I couple in CO2 also exhibit an obvious positive-shift in
contrast to that in Ar. These observations suggest that the NiI
1
H2O solvent containing 0.1 M KCl under a CO2 atmosphere at -
1.16 V showed that after electrolyzed for 6 h, 9.6 coulombs of
charge were accumulated at a relatively stable current density
of 0.32 mA/cm2. 43.1 μmol of CO was obtained, corresponding
to a Faradaic efficiency of 87% (Fig. S12). The remarkable
species of
potential electrocatalysts for CO2 reduction. In contrast to
the reduction potentials of NiII/I couple of
and appear at -
1.08 and -1.10 V in CO2 atmosphere, respectively, more
negative than that of (-1.00 V) under the same conditions
(Fig. 3), which suggests that the NiII/I reduction is easier in
than those in and and may possess better
electrocatalytic activity for CO2 reduction than and
The CPE experiment of 0.5 mM was carried out in a 20 mL
2 and 3 can also bind with CO2, and 2 and 3 are
1,
2
3
1
Faradaic efficiency suggests that
1 also possesses good
1
electrocatalytic performance for reduction of CO2 to CO in
pure H2O solvent system.
2
3,
1
2
3.
Energy dispersive X-ray spectroscopy (EDX) measurements
for the GCEs after CPE showed that there was no nickel residue
on the surface of GCEs (Fig. S13), indicating that all the
1
CH3CN/H2O (v/v = 4:1) containing 0.1 M TBAPF6 under a CO2
atmosphere. After electrolyzed at -1.16 V for 6 h, 18.8
coulombs of charge were accumulated at a relatively stable
current density of 0.60 mA/cm2 (Fig. S6). The gas
chromatography (GC) analyses revealed that the amounts of
generated gas products were 92.6 μmol for CO and 4.9 μmol
for H2, corresponding to a Faradaic efficiency of 95% for CO
and 5% for H2 (Table 1, and Fig. S7). The total TON
(molCO/molcat.) was calculated as 9.26. This value is significantly
undervalued due to limited catalyst molecules interacting with
the surface of the electrode.67 Based on the catalyst molecules
in the diffusion layer of the cathode, an intrinsic catalytic rate
(kcat) of 603.0 s-1, a remarkable TON of 4.1 × 106 and a
noticeable TOF of 190.0 s-1 were obtained (see the details in
electrocatalytic reactions of CO2 reduction by 1, 2 and 3 are
homogeneous, and these catalysts have perfect durability
during the catalytic process of CO2-to-CO conversion. The
robustness, together with the high selectivity and efficiency
reveal that
1 is a potential electrocatalyst for CO2-to-CO
Table 1 The results of electrocatalytic reduction of CO2 by
1-
3a
n(CO)
n(H2)
F(CO)
TOF(CO) TON
Cat.
Q(C)
(μmol) (μmol) (%)
(s-1)b (CO)b
1
18.8
13.0
12.3
92.6
42.0
16.2
4.9
95
62
25
190.0
4.1 × 106
1.1 × 105
the SI).68,69 The CPE analyses for
were also performed under the same conditions as those of
For , 42.0 μmol of CO was obtained, corresponding to a
Faradaic efficiency of 62% and a TON value of 1.1 × 105 for CO
(Table 1; Fig. S8); For , 16.2 μmol of CO was afforded,
2 (1.0 mM) and 3 (0.5 mM)
2
3
11.5
24.2
5.0
4.2
1.
9.2 × 104
2
aCPE experiments were operated with 0.5 mM
and 0.5 mM
1, 1.0 mM 2,
in a 0.1 M TBAPF6 CH3CN/H2O (v/v = 4:1)
3
3
corresponding to a Faradaic efficiency of 25% and a TON value
of 9.2 × 104 for CO (Table 1; Fig. S8). Obviously, the Faradaic
solution at Ecat = -1.16 V vs NHE for 6 h under CO2 atmosphere.
bThe TON and TOF values were calculated based on the
number of the catalyst molecules in the diffusion layer.
efficiency of
1
(95%) is much higher than those of
2 (62%) and
3
(25%), and the TON value of
1
is also much higher than those
3
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The results of DFT calculations have illustrated that the
electrocatalytic reduction of CO2 to CO by either dinuclear
complex of
consistent with the experimental observations. Moreover, the
energy barriers of rate-determining CO2 reduction step for
(8.6 kcal/mol) is much lower than that for (11.7 kcal/mol),
indicating that the reduction of CO2 to CO by is much more
. This result solidly evidenced the
with a dinuclear catalytic center
1 or mononuclear complex of 2 is feasible,
1
2
1
favored in comparison with
experimental result, that is,
2
1
shows much higher electrocatalytic activity for CO2-to-CO
conversion than with a single catalytic center. The dramatic
decrease of the energy barriers by in contrast to clearly
reveals that there is really a synergistic catalysis effect
between two NiII within
. The significant enhancement of the
electrocatalytic activity of , together with the excellent
2
1
2
1
1
photocatalytic performance for CO2-to-CO conversion of our
previously reported dinuclear CoII complex,55 clearly
demonstrates that dinuclear molecular catalysts with DMSC
effect are beneficial for CO2 reduction.
Fig. 4 Proposed mechanism for CO2 to CO conversion catalyzed
by a dinuclear nickel complex 1.
conversion. The higher catalytic performance of
dinuclear Ni(II) catalytic centers than with mononulcear
catalytic center, together with the much higher catalytic
performance of with the close Ni⋅⋅⋅Ni separation than with
far Ni⋅⋅⋅Ni separation, solidly evidenced that there is synergistic
catalysis effect between two NiII catalytic centers within
which endows with excellent catalytic performance in
1 with
2
Conclusions
In summary, we report a dinuclear nickel complex which can
1
3
serve as
a highly efficient and selective catalyst for
electrochemical CO2-to-CO conversion. The TON and TOF
values reach as high as 4.1 × 106 and 190.0 s-1, respectively,
and the Faradaic efficiency reaches as high as 95%. The
experimental studies and theoretical calculations reveal that
the synergistic catalytic effect between two Ni centers plays a
key role in enhancing the catalytic activity and selectivity for
CO2-to-CO conversion. The presented studies may provide new
insights for designing of high-performance electrocatalysts for
CO2-to-CO conversion.
1,
1
electrochemical CO2 reduction.
The synergistic catalysis effect between two Ni centers in
1
for CO2-to-CO conversion has been further studied by the DFT
calculations. Combined with the experimental observations
and previous mechanism investigation on CO2-to-CO
conversion by molecular catalysts,75 a reasonable catalytic
mechanism has been proposed. As shown in Fig. 4, firstly,
II,II
1
I,I
with Ni2 undergoes a 2e- reduction to generate 1a with Ni2
according to the electrochemical results. The calculated redox
I,I
potential for Ni2II,II/Ni2 is -1.15 V vs. NHE, consistent with the
Conflicts of interest
experimental result (-1.18 V). Secondly, 1a binds with a CO2
molecule to yield 1b. The bonded CO2 undergoes a 2e-
reduction to obtain 1c through transition state TS1-1. The total
energy barrier calculated is 8.6 kcal/mol. In this process, the
transition state and intermediate may be stabilized by
through-space interactions between the partial negative
charge on the COO species bound to one Ni and the positive
charges of the other Ni. 76 Thirdly, 1c is protonated to form 1d
with a comproportionation process. Finally, the C-OH bond
There are no conflicts to declare.
Acknowledgements
We are grateful for the financial support by the 973 program
of China (2014CB845602), the NSFC (21331007,,21401026),
and the NSF of Guangdong Province (S2012030006240).
within 1d is cleaved with the assistance of H3O+ to yield 1e
After the release of CO, is regenerated and the catalytic cycle
restarts. The pathway for the reduction of CO2 to CO catalyzed
by was also investigated by the DFT calculations. As shown in
Fig. S14, firstly, the NiII in is reduced to NiI (2a). Then a
molecular CO2 bonds to 2a and undergoes a 2e- reduction by
NiI via transition state TS1-2 to generate 2b, with the total
calculated energy barrier of 11.7 kcal/mol. The resulted 2b
undergoes a proton-coupled electron transfer (PCET) process
to form 2c, and the C-OH bond in 2c is cleaved assisted by H3O+
.
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A dinuclear nickel complex was found to be excellent catalyst for electrocatalytic reduction of
CO2 to CO. This excellent catalytic performance can be attributed to the synergistic catalysis
effect between two Ni centers.
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